Category Archives: Part 1 – The Basics

HIV Testing
Pathogenesis of HIV-1 Infection
Preventive HIV-1 Vaccine
Acute HIV-1 Infection


– Jürgen Kurt Rockstroh –

Acquired Immune Deficiency Syndrome (AIDS) was first described as a new clinical entity in 1981. Initial reports were based on an unusual increase in the incidence of Kaposi sarcoma (KS) and Pneumocystis pneumonia (PCP), diseases that were considered at that time to occur rarely. While both diseases are occasionally observed in different populations (e.g., KS in older men from the Mediterranean region or PCP in patients with leukemia after intensive chemotherapy), the occurrence of these diseases as indicators for severe immunodeficiency had not been observed before in otherwise healthy young individuals. Because the initially affected population were men who had sex with men (MSM) the disease as well as those with the disease were highly stigmatized. Though initially lifestyle and behavioral factors were hypothesized to be causally related, finally in 1983 the human immunodeficiency virus (HIV) was identified as the true cause of AIDS.

In 1987 the first antiretroviral agent, AZT (zidovudine, Retrovir®) was licensed for the treatment of HIV. Despite the failure of this therapeutic concept in terms of monotherapy in achieving long-term suppression of HIV replication, symptoms and clinical manifestations of HIV infection were temporarily relieved with AZT (at 1500 mg/day) and the occurrence of AIDS was slightly delayed. What happened next is unprecedented in medicine to date – within a few years of its discovery an inevitably deadly disease turned into a disease with durable and effective treatment options. The rapid introduction of additional antiretroviral drug classes and the concept of highly active antiretroviral therapy (HAART, an acronym that will be replaced in this book by ART) enabled a durable suppression of viral replication thereby preventing disease progression – as long as the antiretroviral drugs were tolerated and regularly taken. Long-term toxicities and the emergence of resistance led to a search for and identification of further promising drugs with other therapeutic mechanisms of action or better resistance profiles. In parallel, administration modalities and tolerability of antiretroviral drug regimens improved significantly. In 2010 several HIV therapies are available that only require an intake of 1–3 tablets a day mostly resulting from the introduction of fix-dose combinations.

All these advances should not be confused with the fact that lifelong medical therapy will probably lead to substantial problems, especially in terms of adherence to therapy and possible long-term toxicities. With only 10 years of experience so far, the latter aspect has thus far only been captured in part. Infection with HIV should still be avoided at all costs. Apart from further improvement of ART and development of new therapeutic concepts such as eradication, a main focus of our endeavors must be the prevention of HIV in order to contain the further spread of disease.

The HIV epidemic

In 1981 the first three clinical descriptions of AIDS were published in the Morbidity and Mortality Weekly Report and later the New England Journal of Medicine. These reports described an epidemic of community-acquired Pneumocystis pneumonia, in most cases combined with oral thrush in previously healthy homosexual men, as well as chronic ulcerating perianal herpes infections (Gottlieb 1981a, Gottlieb 1981b, Masur 1981, Siegal 1981).

A little later, in June 1982, a notice from the Centers for Disease Control and Prevention (CDC) on three PCP cases among hemophiliacs was issued (CDC 1982a). In the same year a case of cryptosporidiosis in a hemophiliac patient from Pennsylvania (Eyster 1982) and an AIDS manifestation in an infant after a blood transfusion were reported (CDC 1982b). The occurrence of AIDS among hemophiliacs triggered a discussion of whether a viral infection could cause AIDS (Marx 1982). In particular, the similarity of populations at risk for AIDS and hepatitis B led to the hypothesis of a viral agent causing AIDS.

Studies on AIDS patients comprising different populations at risk quickly revealed common characteristics: compared to healthy controls, all AIDS cases had diminished counts of CD4-positive T lymphocytes. Conversely, a relative and absolute increase in CD8-positive T lymphocytes and a reduced mitogen-induced proliferative capacity of lymphocytes was observed (Gottlieb 1981, Masur 1981, Siegal 1981, Mildvan 1982, Stahl 1982). It became quickly clear, however, that the manifestation of AIDS was not a prerequisite for developing an immune deficiency. A defect of cellular immunity, associated with a generalized lymphadenopathy, had already been described very early in otherwise asymptomatic men who had sex with men (Kornfeld 1982, Stahl 1982). In January 1983 two cases of hemophiliacs with a lymphadenopathy syndrome were reported, both with significant dysfunction of cellular immunity (Ragni 1983). This led to the assumption that the lymphadenopathy syndrome and the observed cellular immune defects may have been precursors to AIDS and that a transmission of the AIDS causative agent via blood products was probable. Subsequently numerous studies on altered states of cellular immunity among hemophiliac patients were published. The main finding was a reduced CD4/CD8 ratio, the result of a relative and/or absolute decrease of CD4 lymphocyte counts together with elevated CD8 T cell counts. Only those patients who had been treated with small amounts of blood-clotting factors or where blood-clotting factors had been derived from small donor pools showed normal lymphocyte subpopulations (Luban 1983, Rasi 1984).

The altered immunological findings among hemophiliacs were discussed contraversially. In part they were attributed to a chronic antigen exposure due to the blood-clotting factor substitution. Other groups considered this hypothesis unlikely, given the fact that, prior to the advent of AIDS, no enhanced risk for infections was observed among hemophiliacs compared to other populations (except for viral infections, in particular hepatitis B and non-A-non-B-hepatitis via receipt of blood products). Overall, at that time no indication was seen to call into question the concept of blood-clotting substitution therapy among hemophiliacs (Anonymous 1983, Goldsmith 1983). As an alternative explanation of AIDS, particularly among the transmission group of men who have sex with men, coinfection with human cytomegalovirus, use of injection drugs, inhalation of amylnitrate (poppers) and exposure to foreign proteins (spermatozoa) were discussed (Essex 1997).

In 1983 different working groups raised the hypothesis that a variant of the T-lymphotropic retrovirus (HTLV-I), which had been discovered in 1980 by Gallo and colleagues, could be the causative agent of AIDS (Essex 1983, Gallo 1983). Several arguments were in favor of this hypothesis. At that time HTLV-I was the only known virus with the potential to infect human CD4-positive T lymphocytes (Poiesz 1980). In addition HTLV-I shared the same transmission routes with the potential AIDS agent, i.e., sexual contacts, blood-to-blood and perinatal transmission (Essex 1982).

First experiments to isolate virus related to HTLV-I or -II were only partially successful. Though cross-reactive antibodies with HTLV-related genome sequences were found in a small subset of AIDS patients, the overall assay reactivity was weak and suggested a coinfection with HTLV. The observations led to the assumption of a genetically more distant virus, one with weaker assay reactivity, as a putative etiologic agent. Indeed, only a short time later HTLV-III, later renamed Human Immunodeficiency Virus type I (HIV-1), was discovered as the causative agent of AIDS (Barré-Sinoussi 1983, Popovic 1984). In 2008 the French research group of Luc Montagnier and Francoise Barré-Sinoussi received the Nobel Prize in Medicine for their discovery of HIV-1.

Transmission routes

The main transmission routes of HIV are

  1. unsafe sex with an HIV-infected partner
  2. sharing injection paraphernalia with an HIV-infected partner
  3. vertical transmission of HIV from the HIV-infected mother to the newborn (before or at birth; or later, due to breastfeeding)

All other transmission routes, for the most part case reports, are notably rare. Among these are transmissions due to transfusion of blood or blood products in countries where blood donations are not routinely screened for HIV.

Extremely rare are transmissions due to contact with HIV-positive blood through open wounds or mucosa, or transmission of HIV after a bite (Bartholomew 2008). Recently three cases were reported where mothers infected their newborns probably via pre-chewed food (Gaur 2008). These transmission routes however are of a casuistic nature. Large case registries, in particular from the CDC, which have investigated into other transmission routes of HIV, clearly show that daily contacts of everyday life, such as the shared use of toilets or drinking from the same glass, cannot transmit HIV. Case registries in the health care setting, which analyze contact via saliva, urine, or infectious blood with intact skin, did not find a single transmission of HIV (Henderson 1990).

Potentially favorable factors and risks


The most important transmission route for HIV is sexual contact. The prerequisite for sexual transmission is direct exchange of infectious body secretions / fluids. The highest viral concentrations are found in blood and seminal fluid. A study investigating heterosexual transmission of HIV in female partners of HIV-positive hemophiliacs in Bonn found a seroconversion rate of HIV of 10% (Rockstroh 1995). The risk for sexual transmission was significantly higher if the HIV-positive partner suffered from advanced immunodeficiency or an advanced clinical stage of HIV infection. It is important to note that a precise calculation of transmission risk after one individual exposure is not possible. Various environmental factors have an influence on the actual transmission risk, such as specific sexual practices, concurrent sexually transmitted diseases, skin lesions, circumcision and mucosal trauma, that are difficult to take into account. The average transmission risks according to different sexual practices are shown in Table 1.

The correlation of transmission risk with the level of HIV viremia has important epidemiological implications. In environments where body fluids like blood and seminal fluid are exchanged with many persons over days or weeks, the risk of meeting people who have been recently infected, and thus who are highly infectious, is high. Likewise, the probability of infecting someone else between the transmission event and the detection of HIV antibodies is high. The later stage of disease is also a highly infectious,period, as HIV infection progresses and higher viral loads are again observed as one gets closer to falling below 200 CD4 T cells or AIDS. Sexually transmitted diseases and infections disrupt physiological skin and mucosal barriers and enhance the risk for HIV transmission. This is particularly true for endemic areas with a high prevalence of other sexually transmitted diseases. Primarily genital herpes lesions have been identified as a potential co-factor facilitating HIV transmission in endemic areas (Mahiane 2009).

Table 1. Likelihood for HIV transmission. (Modified from the guidelines  of the German and Austrian AIDS Society; please see also

Type of contact / partner

Probability of infection per contact

Unsafe receptive anal intercourse with HIV-positive partner

0.82% (95% CI 0.24 – 2.76)

Range 0.1 – 7.5%

Unsafe receptive anal intercourse with partner of unknown HIV serostatus

0.27% (95% CI 0.06 – 0.49)

Unsafe insertive anal intercourse with partner of unknown HIV serostatus

0.06% (95% CI 0.02 – 0.19)

Unsafe receptive vaginal intercourse

0.05 – 0.15%

Unsafe insertive vaginal intercourse

0.03 – 5.6%

Oral sex

No known probability, although case reports have been described, in particular after reception of seminal fluid into the mouth (Lifson 1990)

Note: 95% CI = Confidence Interval according to a large US HIV seroconverter Study (Vittinghoff 1999).

The observation that the level of HIV RNA is obviously critical in the infectiousness of an HIV-positive person, has recently initiated a controversial discussion regarding the possibility of a seropositive person having “safe” unprotected sex. The Swiss Commission for AIDS (Eidgenössische Kommission für AIDS-Fragen, EKAF) proposed to classify HIV-infected persons who are on ART with a plasma HIV RNA below the level of detection for at least 6 months, if they are adherent to therapy, regularly come to medical examinations, and if they do not have any signs of any other sexually transmitted diseases, as persons who most likely do not transmit HIV via sexual contact and therefore may have unprotected sex if they want (Vernazza 2008). The intention of the EKAF recommendation is to manage fears of HIV transmission and to enable a normal sex life, as far as possible, in persons with and without HIV. The EKAF recommendation is not agreed to by all HIV experts. Recently a case report from Frankfurt raised questions (Stürmer 2008), where HIV transmission occurred, though HIV viral load was not detectable and the HIV-positive partner was on successful ART (see chapter on Prevention).

Sharing injection paraphernalia

Sharing injection paraphernalia is the most important HIV transmission route for persons who use drugs intavenously. Due to the usually quite large amount of blood that is exchanged when sharing needles, the transmission risk is high. The aspiration of blood to control the correct intravenous position of the needle constitutes the reservoir for transmission. With the introduction of needle exchange programs, the installation of needle vendors, methadone substitution and multiple other preventive measures and social programs, HIV transmission rates have significantly decreased within intravenous drug users in Western Europe. In Eastern Europe, where intravenous drug use constitutes a criminal offence and no clean needles are provided, one sees an unyielding continual increase of HIV transmissions in this population. One can only hope that the success of prevention efforts in Western Europe will lead to a more liberal management and implementation of prevention programs in Eastern Europe.

Vertical transmission

Without intervention up to 40% of newborns born to HIV-1-positive mothers are infected with HIV-1. The most important risk factor is viral load at the time of delivery. Since 1995 the mother-to-child transmission rate of HIV-1-infected mothers has been reduced to 1 – 2%. These low transmission rates were reached through the combination of antiretroviral therapy / prophylaxis for the pregnant woman, elective caesarian section prior to the start of labor, the antiretroviral post-exposition prophylaxis for the newborn and substitution for breast feeding. For details refer to the chapter “HIV and Pregnancy” as well as to the European AIDS Clinical Society (EACS) guidelines for the clinical management and treatment of HIV-infected adults (HIV Med. 2008 Feb;9(2):65-71; please also visit the website


The transmission of HIV via blood and blood products has been largely reduced on a global scale, though the risk is not completely eliminated. In Germany blood and blood products are considered safe. Since 1985 all blood donations are tested for HIV-1 via antibody tests, and since 1989 also against HIV-2. For a few years now blood donations are additionally tested via PCR to identify donors who may be in the window of seroconversion and where the HIV ELISA is still negative. Persons with so-called risk behavior, i.e., active injection drug users, sexually active men and women as well as immigrants from high-prevalence countries are excluded from blood donations

Occupationally-acquired HIV infection

The overall risk for HIV infection after a needlestick injury is estimated to be around 0.3%. The risk for HIV transmission is significantly higher if the injury occurred using a hollow needle – e.g., during blood withdrawal – than with a surgeon’s needle. For details on post-exposure prophylaxis (PEP) please refer to the respective chapter in this book. On the other hand, the risk of infecting a patient with HIV when the medical personnel is HIV-positive is extremely low. In 1993 19,036 patients of 57 HIV-infected physicians, dentists or medical students were screened for HIV infection (CDC 1993a). While 92 patients tested HIV-positive, none of the transmissions was related to the health practitioner.

Non-suitable transmission routes

In general, HIV-transmission due to day-to-day contact between family members is unlikely. It is important to avoid blood-to-blood contacts. Thus, razor blades or tooth brushes should not be commonly shared. In cases of cannula or needle usage, these should be safely deposited in appropriate sharps-containers and not be placed back into the plastic cover.


All studies that have investigated the possible transmission of HIV via insects have come to the same conclusion, that it is not possible. This holds true as well for studies performed in Africa with a high AIDS prevalence and large insect populations (Castro 1988).

The natural course of HIV infection

The natural course of HIV – in the absence of antiretroviral therapy – is shown in Figure 1. Shortly after infection a so-called acute retroviral syndrome is observed in some patients. This syndrome is characterized mainly by lymphadenopathy, fever, maculopapular rash, myalgia and usually does not last longer than four weeks (see chapter on Acute HIV Infection).

The symptoms are unspecific and variable so that the diagnosis of HIV infection is rarely made without additional testing. A period of several years follows where most patients are clinically asymptomatic.

Thereafter symptoms or diseases may occur, classified according to the CDC classification as category B (Table 2). Among these, oral thrush, oral hairy leukoplakia and herpes zoster are particularly noteworthy, and HIV infection as an underlying diagnosis should always be taken into account. Diseases of category B are not AIDS-defining, however their occurrence is defined as symptomatic of HIV infection and hints to a disturbed cellular immune system.

Still later in the course of HIV infection AIDS-defining illnesses occur at a median of 8–10 years after infection. Without highly active antiretroviral therapy these illnesses eventually lead to death after a variable period of time.

The level of HIV RNA, which reaches extremely high values shortly after primary infection, usually decreases to less than 1% of the maximum value at the time of first HIV antibodies and remains on a relatively stable level for a number of years. This level is called the viral set point. The level of the viral set point determines the speed of disease progression. While most patients with less than 1000 HIV RNA copies/ml are usually not affected by AIDS even 12 years after primary infection, more than 80% of patients have developed AIDS only 2 years after infection if the viral load remains at levels above 100,000 copies/ml (O’Brien 1996).

The higher the viral set point the faster the decrease of CD4 T cells. CD4 T cells usually drop considerably during acute primary infection. Subsequent CD4 counts recover after a few months to values within the normal range, though pre-infection values are rarely reached. Normal values for CD4 T cell counts vary from laboratory to laboratory, however these are usually in the range of absolute CD4-positive T lymphocytes in adults of 435–1600/µl or relative percentage between 31–60% of total lymphocytes. For children other values apply (see chapter on Pediatrics).

During the progressive course of HIV infection a gradual decrease of CD4 T cells is observed. The risk for AIDS-defining illnesses increases with time when CD4 T cells decrease below 200. To ascertain the level of immunodeficiency the relative percentage of CD4 T cells should also be taken into account.

Figure 1: The natural course of HIV infection
Table 2. Clinical categories of HIV infection according to CDC Classification

Category A

Asymptomatic HIV infection

·   Acute, symptomatic (primary) HIV ­in­fec­tion

·   Persistant generalized lymphade­nopa­thy (LAS)

Category B

Symptoms or signs of diseases that do not fall into Category C but are associated with a disturbed cellular immunity. Among these are:

·   Bacillary angiomatosis

·   Infections of the pelvis, in particular complications of fallopian tube or ovarian abscesses

·   Herpes zoster in the case of more than one dermatome or recurrence in the same dermatome.

·   Idiopathic thrombocytopenic pur­pura

·   Constitutional symptoms like fever or diarrhea lasting >1 month

·   Listeriosis

·   Oral hairy leukoplakia (OHL)

·   Oropharyngeal candidiasis (oral thrush)

·   Vulvovaginal candidiasis, either chronic (>1 mo­nth) or difficult to treat

·   Cervical dysplasia or car­cinoma in situ

·   Peripheral neuropathy

Category C

AIDS‑defining diseases

·   Candidiasis of the bronchia, trachea, or lungs.

·   Oesophageal candidiasis

·   CMV infections (except liver, spleen and lymph nodes)

·   CMV retinitis (with loss of vision)

·   Encephalopathy, HIV-related

·   Herpes simplex infections: chronic ulcer (>1 month); or bronchitis, pneumonia, oesophagitis

·   Histoplasmosis, disseminated or extrapulmonary

·   Isosporiasis, chronic, intestinal, duration >1 month

·   Kaposi sarcoma

·   Coccidioidomycosis, disseminated or extrapulmonary

·   Cryptococcosis, extrapulmonary

·   Cryptosporidiosis, chronic, intestinal, duration >1 month

·   Lymphoma, Burkitt

·   Lymphoma, immunoblastic

·   Lymphoma, primary CNS

·   Mycobacterium avium complex or M. kansasii, disseminated or extrapulmonary

·   Mycobacterium, other or not identified species

·   Pneumocystis pneumonia

·   Pneumonia, bacterial, recurrent (>2 within a year)

·   Progressive multifocal leukoencephalopathy

·   Salmonella Sepsis, recurring

·   Tuberculosis

·   Toxoplasmosis, cerebral

·   Wasting Syndrome

·   Cervix carcinoma, invasive

Under certain conditions (e.g., under myelosuppressive interferon therapy) low absolute CD4 T cell counts are observed in the context of leuko- and lymphopenia, while the immune status assessed by the relative CD4 T cell count remains normal. 200 CD4 T cells/µl correspond to approximately 15% of CD4 positive lymphocytes. Conversely, the absolute CD4 T cell count may suggest false high values, e.g., after a splenectomy.

Patients can be categorized depending on the speed of the CD4 T cell decrease (Stein 1997) to those with a high risk of disease progression (loss of more than 100 CD4 T cells/µl within 6 months), those with a moderate risk of disease progression (loss of 20–50 cells/µl per year) and those with a low risk of disease progression (loss of less than 20 cells/µl per year).

While the overall risk for AIDS increases if the CD4 T cell count drops below 200 cells/µl, considerable differences exist for the risk of individual AIDS manifestations (see chapter on AIDS). As an example, opportunistic infections usually occur at far lower CD4 T cell counts than AIDS-associated malignancies (Schwartländer 1992). Apart from the level of HIV RNA and CD4 T cell count, the age of the patient is another important risk factor for progression to AIDS (Figure 2). A 55-year-old patient with a CD4 T cell count of 50 cells/µl and an HIV RNA of 300,000 copies/ml has an almost twice as high risk of developing AIDS within six months as a 25-year-old patient. This explains why the latest antiretroviral treatment guidelines for HIV have included individual factors such as age and level of HIV viral load into their algorithms regarding when to start treatment.

In the pre-ART era the average time between the first manifestations of AIDS and death was 2–4 years. Without therapy probably more than 90% of all HIV patients die from AIDS. Today, the progression of HIV infection to AIDS can be halted with treatment. After reaching a maximal suppression of HIV RNA, CD4 T cell counts usually recover and patients regain an almost normal life expectancy.

The level of HIV RNA or the viral set point is dependent on a variety of host-specific factors such as HLA-type and other, as yet unidentified, factors. In addition, virus-related factors associated with HIV disease progression have to be taken into account.

Figure 2

It is important to visualize that the level of plasma viral load represents an equilibrium between new and dying HIV.

Disease progression stage

In order to classify the progression of HIV infection in most clinical settings the 1993 CDC classification is still being used that takes the clinical presentation and CD4 T cell count into account (Table 3).

Table 3: Classification of HIV disease according to the 1993 CDC classification

Symptoms /

CD4 T cells

Asymptomatic or acute HIV disease

Symptomatic but not stage A or C

AIDS-defining illness*





200 – 499/µl








*for AIDS-defining conditions please refer to Table 2.

In 2008 a revised version of the CDC classification of HIV disease was presented. This revised version has been combined into a single case definition for adolescents ≥ 13 years and adults and is summarized in Table 4. Aim of the revised version was to introduce a simplified classification for continued epidemiological monitoring of HIV and AIDS, which refelected the improved diagnostics and treatment possibilities in the HIV field. In addition to the three stages listed below a fourth new stage (HIV infection, stage unknown) was introduced for patients in whom no CD4-counts or patient history were available.

Table 4: Classification of HIV-disease according to the 2008 classification


AIDS-defining illness*

CD4 T cell count



> 500/µl or ≥ 29%



200 – 499/µl or 14-28%

3 (AIDS)

Documented AIDS –defining illness

or < 200/µl or <14%


No information available

No information available

*the AIDS-defining illnesses have remained unchanged and are listed in table 2.

As a general rule for the classification of a patient, the stage is always adapted according to progression of disease (e.g., someone who is previously asymptomatic, CD4 T cell count 530/µl, they are a Category A; but if they develop oral thrush, their CD4 T cell count drops to 320/µl, they are Category B2). Reclassification upward upon improvement is not considered. If we take the same example as before and the patient has received fluconazole therapy and ART, and at present is asymptomatic and their CD4 T cells have returned to 550/µl the CDC stage remains at B2. The case definitions of the revised 2008 CDC classification are intended for public health surveillance and not as a guide for clinical diagnosis. Whereas in Europe the term AIDS is only used in cases of clinically manifest AIDS, in the US a CD4 T cell count below 200 cells/µl is also considered AIDS.


The Human Immunodeficiency Virus probably emerged in the 1920s or ‘30s when the Simian Immunodeficiency Virus (SIV) jumped host from the chimpanzee to the human in Western Africa (Worobey 2008). The oldest HIV-positive human blood sample was found in Kinshasa (Zaire, now the Democratic Republic of Congo) and dates back to 1959 (Zhu 1998). After the first description of AIDS in 1981 by now almost all countries in the world have been affected by HIV.

The first to be infected are usually persons from so-called high-risk groups (intravenous drug users, professional sex workers, men who have sex with men) and subsequently other population groups are infected via unsafe sex. In industrialized countries homosexual sex is frequently the most common mode of transmission, whereas in countries of the former Soviet Union intravenous drug use (sharing injection paraphernalia) is the most common mode of transmission. In Africa most infections occur due to heterosexual intercourse.

Table 4: AIDS epidemic according to UNAIDS, 2010 (

HIV-infected adults and children

HIV prevalence among adults in 2009

New infections 2008

Yearly deaths due to AIDS 2009






Middle East and North Africa





South and Southeast Asia





East Asia










Latin America










Eastern Europe and Central Asia





Western and
Central Europe





North America










The prevalence and subsequent implications on the epidemic are markedly different from country to country. Whereas HIV/AIDS constitutes a rather marginal health care problem in industrialized countries, in Sub-Saharan Africa AIDS has become the most common cause of death: every 5th death in Africa is due to AIDS. The overall life expectancy has decreased in some African states by more than 20 years. More than 10 million children have been orphaned. The economies of hard-hit states have and are continuing to suffer from dramatic slumps. According to UNAIDS, in 2009 around 33.3 million people were infected with HIV/AIDS worldwide (30.8 million adults, 16. million women ≥15 years and 2.5 million children under 15 years of age), approximately 2.6 million new HIV infections occurred and 1.8 million people died of AIDS (Table 4). Most profoundly affected countries are the regions of Sub-Saharan Africa, where more than 22,5 million people are infected with HIV. The highest dynamic of spread and incidence rates are currently observed in countries of the former Soviet Union, in particular Estonia, Latvia, Russia and the Ukraine, as well as in South and South-East Asia.

In Germany in 2010, around 70,000 people were HIV-positive (Table 6).

Table 6: Epidemiology of HIV/AIDS in Germany (modified according to


Total numbers (lower and upper estimate)

People with HIV/AIDS in 2010

70,000 (60,000–83,000)


57,000 (49,000–68,000)


13,000 (11,000–16,000)



According to transmission group    

Men who have sex with men

42,000 (36,000–49,000)

Persons infected via heterosexual contacts

10,000 (8,700–12,000)

Persons from high prevalence regions / countries

7,300 (6,900-9,200)

Intravenous drug users

10,000 (8,500–12,000)

Hemophiliacs and received blood transfusions


Mother to child transmission



The first serological evidence for HIV infection was found in human sera from Zaire dating to 1959, Uganda dating back to1972 and Malawi to 1974 – evidence that HIV was circulating in Africa at those times. The first cases of AIDS were than described in the US in 1981. The discovery of HIV as the cause of AIDS was made in 1983. Since then HIV/AIDS has emerged as a worldwide epidemic which continues to spread today – 30 years later – with some 2.6 million new infections each year. In particular the high infection rates in Eastern Europe and Asia demonstrate the immense challenges that need to be met in current and future implementations of prevention measures. Even though the success of antiretroviral therapy in the treatment of HIV infection appears to enable a normal life expectancy for HIV-infected patients, knowledge on the natural course of HIV infection remains important. Not only in order to make the correct decision on when to start ART in an individual patient, but also to correctly diagnose HIV in patients with first symptoms of HIV infection who have not previously shown AIDS manifestations, this knowledge is important. In light of the fact that in Europe about 50% of all HIV-infected persons do not know their HIV status, tremendous challenges remain in the area of early diagnosis of HIV infection. Joint efforts are being made ( in order to diagnose HIV infection earlier and thus enable physicians and patients to start ART on time, as well as to lower new infection rates by counseling patients on transmission modes and prevention.


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HIV Testing

(Christian Noah)

Despite extensive testing possibilities and recommendations HIV infection continues to be recognized and diagnosed more often than not at a later stage. At the time of initial diagnosis approximately one third of HIV patients already have an immunodeficiency with a CD4 T cell count below 200/μl. In half of these late-diagnosed infections an AIDS-related illness is simultaneously diagnosed (RKI 2007). Rapid diagnosis of HIV infection is also important in order to avoid further transmission. Every pregnant woman should be offered an HIV test. HIV testing also plays an important security role in blood and organ donation.

The basics of HIV diagnostics

The diagnosis of HIV infection is primarily based on a laboratory screening test. A reactive result of a screening test has to be confirmed by an alternative assay (confirmatory test). Due to its relatively high sensitivity, the 4th generation test (“Combo test”) which simultaneously detects both HIV-specific antibodies and p24 antigen should be used (Breast 2000, Weber 2002, Sickinger, 2004, Skidmore 2009). A “seronegative” chronic HIV infection is an absolute rarity and irrelevant in practice (Spivak 2010). Any approved screening test detects all known HIV types (HIV-1 and -2), HIV groups and HIV subtypes.

There are numerous commercial systems available for screening. However, the basic technological principle is the same for all and is based on antigen-antibody binding. The prototype assay is the ELISA (enzyme linked immunosorbent assay). Its central element is a plastic plate with 96 wells (microtiter plate). The surface of each cavity is coupled with HIV antigens and HIV antibodies. When a patient’s serum or plasma containing HIV antibodies is placed into one cavity, antibodies bind to the coupled antigen. An enzyme-labelled second antibody is then added, which recognizes and binds to human antibodies. Finally a substrate is added that is converted by the enzyme at the second antibody. The result is a colour change, measured photometrically. The optical density correlates with the HIV antibody concentration in the sample of the patient – the higher the intensity, the more antibodies present in the sample.

Based on this prototype several advances have improved the efficiency and effectiveness of the screening test (Perry 2008). Modern test systems are highly automated to achieve a very high degree of standardization and generate a result in significantly less than one hour. In these systems, the solid phase consists of microparticles coupled with the virus antigens and antibodies. Accordingly, the method is referred to as a “microparticle enzyme immunoassay” (MEIA).

The measured value is usually an index without dimensions, calculated from the ratio of the measured value of the patient sample and the negative control (Sample/Control, S/Co). Values below 1 are considered negative, values above 1 as reactive. It should always be called “reactive” and not a “positive” result to document that this result needs to be confirmed by a second test.

With the screening test, sensitivity has the highest priority (this way, no infection should be missed), while a high specificity is preferred for the confirmatory test. Screening tests approved in Germany require a specificity of 99.5%. That means that one in 200 HIV-negative samples could have a false-reactive test result. False-reactive results are caused for example by stimulation of the immune system (e.g., viral infections, pregnancy, vaccinations, autoimmune diseases). Thus, in certain patient groups (e.g., pregnant women, dialysis patients) an increased proportion of false reactive test results can occur.

To confirm a reactive screening test result a Western Blot (immunoblot) analysis is typically carried out. Viral proteins (antigens) are separated by their molecular weight via electrophoresis and transferred to a membrane, which is then used as a test strip. An advancement in terms of standardization is the so-called line blot which is produced by spraying recombinant HIV antigens directly onto a test membrane. The test strip is incubated with the serum or plasma of the patient. If HIV-specific antibodies are present, they bind to the antigen. Analogous to the ELISA (see above) the resulting antigen-antibody complex will become visible on the test strip using an enzyme-labeled second antibody and a corresponding substrate. According to the antibody specificities present in the sample a corresponding band spectrum occurs on the test strip.

The various HIV proteins are assigned to three functional groups (“p” – protein, “gp” – glycoprotein. The numbers refer to the molecular weight):

● envelope proteins (env):                 gp41, gp160, gp120
● polymerase proteins (pol):              p31/p34, p39/p40, p51/p52, p66/p68
● core proteins (gag):                          p17/p18, p24/p25, p55

The formation of antibodies after infection follows a specific kinetic: while p24 and gp120 antibodies are detectable early, the p31 band usually occurs later in the course of infection (Fiebig 2003). A Western Blot is considered positive when at least two or three bands are visible. With regard to the antibody specifics, the criteria for a positive result are internationally not uniformly defined. According to the German guidelines, based on the DIN 58 969 Part 41 (“serodiagnosis of infectious diseases – immunoblot”), a test result is considered positive when antibodies against an env protein and also against a gag protein and/or a pol protein are detected. According to WHO criteria a Western Blot is positive when antibodies against at least 2 env proteins are detectable. According to the guidelines of the American Red Cross a gag, pol and env-band is required. The US FDA demands a p24, a p34 and a gp41 or gp120/160-band. For example, a Western Blot with a gp120 and p24 band would be interpreted borderline according to the WHO and positive to the German criteria. A weak band spectrum may indicate an early phase of an HIV infection and further tests such as PCR should be carried out (see below).

Compared to a 4th generation screening test the p24 antigen is not included in the confirmatory test. In the case of “reactive screening test – negative confirmatory test”, acute HIV infection where HIV-specific antibodies are not yet formed although the p24 antigen is present cannot be excluded. Such a result should be checked after 2-3 weeks. If a patient is concerned regarding an acute infection (acute retroviral syndrome, recent exchange of bodily fluids with an HIV-infected person) the implementation of an HIV PCR is useful. The PCR is also recommended in case of a highly positive screening and negative confirmatory test result. It is recommended to consult the laboratory to discuss the adequate procedure.

Ideally, the laboratory will use a Western Blot, which also covers antibodies for HIV-2. In general, a synthetic HIV-2 peptide is used for this purpose. In case of a reactive HIV-2 band, this result must be confirmed by an HIV-2-specific Western Blot.
As an alternative to the Western Blot for confirmation of a reactive screening test an immunofluorescence assay (IFA) is available, although less common.

To exclude sample confusion each first positive test result should be confirmed by examination of a second sample. If a patient is suspected to have an HIV infection, the result of viral load measurement can be used for confirmation (see chapter on HIV monitoring). In this case, a second serological test is not necessary.

In addition to the serological test systems, molecular methods for detection of HIV RNA (PCR, bDNA) are available. The quantitative detection of HIV RNA (a viral load determination) is one of the essential components of the monitoring of HIV infection (Berger 2002, Wittek 2007).

In the context of primary HIV diagnosis however, HIV PCR, is reserved only for specific issues such as the suspicion of acute infection or vertical transmission (see below). For the general exclusion of HIV transmission, HIV PCR is only conditionally suitable and cannot replace the serological HIV test. Furthermore, the commercially available test systems have not been validated yet by the manufacturers for primary diagnosis.

Rapid tests

Rapid HIV tests functionally correspond to a screening test, i.e., a reactive result must be confirmed by a Western Blot analysis. Rapid tests can be carried out quickly, easily and without any equipment expense and can therefore be used as so-called “point of care” tests. In addition to plasma and serum, full or capillary blood (from the fingertip or the ear lobe) is suitable as test material, so that no centrifuge is required. In some test systems urine or oral transsudate (not saliva) may be used. However, rapid tests exhibit less sensitivity if specimens others than serum or plasma are used (Pavie 2010). Results are available within 15 to 30 minutes. Most frequently, rapid tests are based on immuno-chromatographic methods. Other techniques such as particle agglutination, ImmunoDOT and immunofiltration are also used (Giles 1999, Branson 2003, Greenwald 2006).

Almost all currently available rapid tests only detect HIV antibodies but not p24 antigen, corresponding to an (outdated) HIV test of the 3rd generation. Since 2009 a certified 4th generation rapid test (Determine HIV-1/2 Ag/Ab Combo, Inverness Medical) is available for the first time which not only detects but can also differentiate HIV antibodies and p24 antigen. However, in a comparative study the test exhibited deficiencies regarding the recognition of primary HIV infections. About one third of the samples of patients with acute HIV infection tested falsely negative. Reactivity was delayed by one week compared to a reference test (Mohrmann 2009).

Rapid tests are particularly suitable for use in emergency situations where the test result has immediate consequences. These include emergency operations and needlestick injuries. Also in pregnant women with unknown HIV status at delivery a rapid test can be useful. However, the cooperating laboratory should be contacted to indicate the need for a rapid HIV result. When necessary, the result of a conventional HIV test can be available within one hour upon receipt of the sample. Rapid tests are also useful in countries with poor medical infrastructure (WHO 1998, 2004) and in the context of low-threshold testing for individuals who would otherwise not be tested.

Rapid tests should be used only for initial orientation. The results of the testing should be confirmed at the earliest opportunity in a routine laboratory with a standard HIV test. Studies have shown that some rapid tests exhibit a lower sensitivity in comparison to conventional HIV tests. In one study in Cape Town, South Africa, a significant proportion of HIV-infected children had false-negative rapid test results (Claassen 2006).

The diagnostic window

The “diagnostic gap” or “window” indicates the time period between transmission of a pathogen and the onset of biochemical measurable infection markers such as antibodies, antigen or nucleic acids (Busch 1997).

At the earliest, HIV antibody production begins two weeks after transmission. HIV-specific antibodies can be detected after four weeks in 60-65% of cases, after six weeks in 80%, after eight weeks in 90% and after twelve weeks in 95% of cases. 4th generation diagnostic tests can shorten the diagnostic gap by simultaneous detection of p24 antigen (Gürtler 1998, Ly 2001). The p24 antigen is detectable about five days before seroconversion (the first occurrence of specific antibodies). The earliest lab marker is HIV RNA that is detectable approximately seven days before the p24 antigen (Fiebig 2003). In many cases HIV RNA can be detected by the second week after transmission. However, a negative result at this time point cannot exclude an infection.

A negative result in the HIV screening test precludes the existence of HIV antibodies and p24 antigen at the time of testing. The security of this result, however, depends particularly on the time interval from the possible transmission event. This has important consequences:

1. HIV testing immediately after a possible transmission is not meaningful, as no HIV antibodies are yet formed. An HIV test should therefore be carried out at the earliest in the 3rd week after exposure. Exception: If it needs to be documented for legal reasons (e.g., needle stick injury) that at the time of transmission no existing HIV infection was present.

2. An HIV infection can not be ruled out until three months after possible transmission with sufficient certainty. Check-ups should be performed two and six weeks and three months after exposure. A further test (after six months) is appropriate only in exceptional cases, for example, if there is suspicion of acute retroviral syndrome.

3. A negative test result is dependable only in the case of no re-exposure within the past three months (from the time of the original exposure).

HIV diagnostics in newborns

In newborns of HIV-infected mothers maternal antibodies may remain detectable until the age of 18 months (Moodley 1997, European collaborative study 1991). The antibodies are transplacentally transferred from the 32nd week of gestation although they do not have any protective effect. A serological HIV test for the detection or exclusion of vertical transmission of HIV is therefore not enough as a positive result will be expected in any case.

According to the German-Austrian recommendations for HIV therapy in pregnancy and in HIV-exposed infants (2008) at least two negative PCR results are required to exclude HIV transmission. The first HIV PCR should be performed after the first month of life (sensitivity 96%, specificity 99%), then again because of the nearly 100% sensitivity and specificity after the third month. Vertical transmission can be ruled out, however, only if there was no renewed risk of transmission in the meantime through breastfeeding.

Even with negative PCR results, the disappearance of maternal antibodies should be documented at least once. In the case of positive results, these must be confirmed by examination of a second sample.

HIV diagnostics after occupational exposure

After a needlestick injury or other occupational exposure a hepatitis B and C and HIV infection of the index patient should be excluded (of course, consent of the index patient is required). With regard to the potential necessary rapid start of post-exposure prophylaxis (PEP) a needle stick injury should always be considered an emergency. The earlier PEP is initiated, the better the chances of success (Puro 2004, Panlilio 2005). PEP should preferably be started within 24 hours of HIV exposure. If a rapid result of an HIV screening test is not available for logistical reasons, an HIV rapid test should be considered. To save time, PEP can be initiated immediately and terminated at any time in the case of a negative result.

If the index patient has no symptoms consistent with acute retroviral syndrome the negative result of the screening test excludes HIV infection with a high level of security. An HIV PCR test should be considered only if there is evidence of acute HIV infection of the index patient.

Conversely, if the index patient is infected with HIV or if the HIV status is unknown, HIV screening should be performed in the exposed person. For legal reasons, the first HIV test should take place immediately after the needlestick injury to document that no HIV infection was present at the moment of the accident. Check-ups should be carried out at 6 weeks, at 3 and at 6 months. If the index patient is infected with HIV, testing at 12 months is recommended (Ridzon 1997, Ciesielski 1997).

What is relevant in practice?

● The legal situation: Because of possible medical, social and legal consequences, an informed consent of the patient is required before performing an HIV test. Testing against the wishes of the patient is an invasion of privacy, potentially corresponding with legal consequences for the doctor. A written consent is not required, but the consent should be documented. In children or infants, the patient’s parents or legal guardians must agree. The special status of the HIV test is currently being debated. There is a demand for “destigmatization” and a “re-medicalization” of the HIV test (Manavi 2005, Beckwith 2005). The background is that the accompanying requirements of the informed consent are often discouraging, so it is often easier to omit doing the HIV test. Current CDC recommendations allow for the so-called “opt-out” in many situations: the patient is informed about the HIV test, but it will be performed without further detailed guidance, provided the patient does not explicitly reject testing (CDC 2006).

● Advice: There should not be any HIV testing without counseling and education. The patient should be informed about the testing algorithm and the possibilities and limitations of HIV testing. Particularly, the limitations of the (frequently demanded) HIV PCR in primary diagnostics should be addressed: while a sensitive method for detection, it is only conditionally suitable for the rapid exclusion of HIV infection or transmission. Due to the distress caused to the patient, the high cost of the PCR as a counter argument against the method is a rare deterrent for the patient. During the consult, all the possibilities of the test result and in particular the “diagnostic window” should be noted. A desired HIV test could also be an occasion to discuss the risk of transmission in general (also for other sexually transmitted diseases) and appropriate prevention methods with the patient.

● Reporting: A negative test result can possibly be reported by telephone if the patient has been previously advised of its value. The diagnosis of HIV, however, has to be given in a personal counseling interview by a physician (or expert virologist) only (in many places, the result can be given by a registered nurse or counselor). The response of a patient cannot be assessed adequately when reporting is done by telephone. Sometimes patients can develop suicidal thoughts. Similarly, the negative result of a confirmatory test following a reactive screening test should be personally discussed with regard to the possibility of an acute infection. Patients should be directed to an HIV-focused practice. In addition, the patient should be advised of regional counseling and care centers. The result of a reactive HIV screening test should never be reported before the result of the confirmatory test is available.


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Filed under 2. HIV Testing, Part 1 - The Basics

Pathogenesis of HIV-1 Infection

(Andrea Rubbert, Georg Behrens and Mario Ostrowski)

Since the initial description of the Human Immunodeficiency Virus type I (HIV-1) in 1983 (Barré-Sinoussi 1983, Gallo 1983) and HIV-2 in 1986 (Clavel 1986), these two viruses have been identified as the primary cause of Acquired Immunodeficiency Syndrome (AIDS). As HIV-1 is the major cause of AIDS in the world today, our discussion will be primarily limited to HIV-1 infection. Worldwide, the number of HIV-1 infected persons exceeds 33 millions (according to UNAIDS), the majority of whom live in the developing countries of Sub-Saharan Africa, Asia and South America.

Despite all the therapeutic advantages achieved over the last decade, including the evolution of “HAART”, once an individual has become infected, eradication of the virus is not possible. In addition, new problems relating to the short- and long-term toxicity of drug treatments and the occurrence of resistance mutations in both circulating and transmitted viruses are emerging. In most countries in South East Asia and Africa, the incidence and prevalence of HIV-1 infection continues to increase and surpass that of Europe and North America.

However, due to the high costs of drug regimens and the lack of healthcare infrastructure in these developing countries, the widespread use of ART is currently still partial at best. The further course of the HIV-1 pandemic, therefore, mainly depends on how and to what degree developing countries with a high HIV-1 prevalence are able to take advantage of the medical progress achieved in Europe and North America, and whether an effective prophylactic vaccine will become available in the near future (see also chapter on Preventive HIV-1-Vaccine).

An understanding of the immunopathogenesis of HIV-1 infection is a major prerequisite for rationally improving therapeutic strategies, developing immunotherapeutics and prophylactic vaccines. As in other virus infections, the individual course of HIV-1 infection depends on both host and viral factors.

The course of infection with HIV-1 in HIV-infected humans may vary dramatically, even if the primary infection comes from the same source (Liu 1997). In individuals with a long-term non-progressive HIV-1 infection (i.e., lack of decline in CD4 T cell counts, or chronic infection for at least 7 years without the development of AIDS), a defective virion has been identified (Kirchhoff 1995). Thus, infection with a defective virus, or one that has a poor capacity to replicate, may prolong the clinical course of HIV-1 infection. However, in most individuals, HIV-1 infection is characterized by a replication-competent virus with a high daily turnover of virions.

Host factors may also determine whether or not an HIV-1-infected individual rapidly develops clinically overt immunodeficiency, or whether this individual belongs to the group of long-term non-progressors that represent about 5% of all infected patients. The identification and characterization of host factors contributing to the course of HIV infection, including immunological defense mechanisms and genetic factors, will be crucial for our understanding of the immunopathogenesis of HIV infection and for the development of immunotherapeutic and prophylactic strategies.

The structure of HIV-1

HIV-1 is a retrovirus and belongs to the family of lentiviruses. Infections with lentiviruses typically show a chronic course of disease, a long period of clinical latency, persistent viral replication and involvement of the central nervous system. Visna in sheep, simian immunodeficiency virus (SIV) in monkeys, or feline immunodeficiency virus (FIV) in cats are typical examples of lentivirus infections.

Using electron microscopy, HIV-1 and HIV-2 resemble each other strikingly. However, they differ with regard to the molecular weight of their proteins, as well as having differences in their accessory genes. HIV-2 is genetically more closely related to SIV found in sootey mangabeys (SIVsm) rather than HIV-1 and it is likely that it was introduced into the human population via monkeys. Both HIV-1 and HIV-2 replicate in CD4 T cells and are regarded as pathogenic in infected persons, although the immune deficiency may be less severe in HIV-2-infected individuals.

The morphologic structure of HIV-1

HIV-1 viral particles have a diameter of 100 nm and are surrounded by a lipoprotein membrane. Each viral particle contains 72 glycoprotein complexes, which are integrated into this lipid membrane, and are each composed of trimers of an external glycoprotein gp120 and a transmembrane spanning protein gp41. The bonding between gp120 and gp41 is only loose and therefore gp120 may be shed spontaneously within the local environment. Glycoprotein gp120 can be detected in the serum as well as within the lymphatic tissue of HIV-infected patients. During the process of budding, the virus may also incorporate different host proteins from the membrane of the host cell into its lipoprotein layer, such as HLA class I and II proteins, or adhesion proteins such as ICAM-1 that may facilitate adhesion to other target cells. The matrix protein p17 is anchored to the inside of the viral lipoprotein membrane. The p24 core antigen contains two copies of HIV-1 RNA. The HIV-1 RNA is part of a protein-nucleic acid complex, which is composed of the nucleoprotein p7 and the reverse transcriptase p66 (RT). The viral particle contains all the enzymatic equipment that is necessary for replication: a reverse transcriptase (RT), an integrase p32 and a protease p11 (Gelderbloom 1993) (Fig. 1).

The organization of the viral genome

Most replication competent retroviruses depend on three genes: gag, pol and env: gag means “group-antigen”, pol represents “polymerase” and env is for “envelope” (Wong-Staal 1991) (Fig. 2). The classical structural scheme of a retroviral genome is: 5’LTR-gag-pol-env-LTR 3’. The LTR (long terminal repeat) regions represent the two end parts of the viral genome, that are connected to the cellular DNA of the host cell after integration and do not encode for viral proteins. The gag and env genes code for the nucleocapsid and the glycoproteins of the viral membrane; the pol gene codes for the reverse transcriptase and other enzymes. In addition, HIV-1 contains six genes (vif, vpu, vpr, tat, rev and nef) in its 9kB RNA that contribute to its genetic complexity. Nef, vif, vpr and vpu were classified as accessory genes in the past, as they are not absolutely required for replication in vitro. However, the regulation and function of these accessory genes and their proteins have been studied and characterized in more detail over the past few years. The accessory genes nef, tat and rev are all produced early in the viral replication cycle.

Tat and rev are regulatory proteins that accumulate within the nucleus and bind to defined regions of the viral RNA: TAR (transactivation-response elements) found in the LTR; and RRE (rev response elements) found in the env gene, respectively. The tat protein is a potent transcriptional activator of the LTR promoter region and is essential for viral replication in almost all in vitro culture systems. Cyclin T1 is a necessary cellular cofactor for tat (Wei 1998). Tat and rev stimulate the transcription of proviral HIV-1 DNA into RNA, promote RNA elongation, enhance the transportation of HIV RNA from the nucleus to the cytoplasm and are essential for translation. Rev is also a nuclear export factor that is important for switching from the early expression of regulatory proteins to the structural proteins synthesized later on.

Figure 1: Structure of an HIV virion particle. For detailed explanations see text.


Figure 2: HIV and its genes. For detailed explanations see text.


Nef has been shown to have a number of functions. It may induce downregulation of CD4 and HLA class I molecules (Collins 1998) from the surface of HIV-1-infected cells, which may represent an important escape mechanism for the virus to evade an attack mediated by cytotoxic CD8 T cells and to avoid recognition by CD4 T cells. Nef may also interfere with T cell activation by binding to various proteins that are involved in intracellular signal transduction pathways (Overview  in: Peter 1998). In SIV-infected rhesus macaques, an intact nef gene was essential for a high rate of virus production and the progression of disease. HIV-1, with deletions in nef, was identified in a cohort of Australian long-term non-progressors (Kirchhoff 1995). However, more recent reports indicate that some of these patients are now developing signs of disease progression including a decline of CD4 T cells. Thus, although deletions of the nef gene may slow viral replication, they cannot always prevent the eventual development of AIDS.

Vpr seems to be essential for viral replication in non-dividing cells such as macrophages. Vpr may stimulate the HIV LTR in addition to a variety of cellular and viral promoters. More recently, vpr has been shown to be important for the transport of the viral pre-integration complex to the nucleus (Overview in: Miller 1997) and may arrest cells in the G2 phase of the cell cycle.

Vpu is important for the virus “budding” process, because mutations in vpu are associated with persistence of viral particles at the host cell surface. Membrane molecules such as tetherin (CD317) can bind vpu-deficient HIV-1 and prevent viral release. Thus, vpu can be considered as a viral escape mechanism in order to antagonise this effect (Neil 2009) and appears to be of great importance for the evolution of the pandemic virus (Sauter 2009). Vpu is also involved when CD4-gp160 complexes are degraded within the endoplasmic reticulum and therefore allows recycling of gp160 for the formation of new virions (Cullen 1998).

Some recent publications have highlighted a new and important role for vif in supporting viral replication (Mariani 2003). Vif-deficient HIV-1 isolates do not replicate in CD4 T cells, some T cell lines (non-permissive cells) or in macrophages. Vif-deficient isolates are able to enter a target cell and initiate reverse transcription, but synthesis of proviral DNA remains incomplete. In vitro fusion of permissive and non-permissive cells leads to a non-permissive phenotype, suggesting that the replication of HIV depends on the presence or absence of a cellular inhibitor. This endogenous inhibitory factor was identified as APOBEC3G (Sheehey 2002). APOBEC3G (apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3G) belongs to a family of intracellular enzymes that specifically deaminate cytosine to uracil in mRNA or DNA resulting in an accumulation of G-to-A mutations that lead to degradation of viral DNA. By forming a complex with APOBEC3G, vif blocks the inhibitory activity of APOBEC3G (Fig. 3a).

Of interest, the antiviral activity of APOBEC3G is highly conserved among various species, whereas the blockade of APOBEC3G by vif is highly specific for HIV. HIV-1 vif does not complex to murine or rhesus APOBEC3G. In the absence of vif, APOBEC3G is incorporated into newly formed viral particles and in subsequently infected target cells synthesis of proviral DNA is blocked (Fig 3b). In contrast, in the presence of vif, APOBEC3G is complexed, degraded and not incorporated in newly formed virions. APOBEC3G is expressed in lymphocytes and macrophages representing the primary target cells of HIV infection. In DC, the activation status of the cells influences the amount of APOBEC3G. Upon DC maturation there is an increase of APOBEC3G expression (Pion 2006).

Figure 3a: HIV wild-type infection: vif interacts with APOBEC3G, binds to APOBEC3G and prevents its incorporation in newly formed viruses.
Figure 3b: Vif-deleted HIV isolates fail to inhibit intracellular APOBEC3G, which is then incorporated into new viruses and interferes with reverse transcription in the target cell.

There are still a lot of open questions regarding the regulation of intracellular APOBEC3G. For example, whether there is a critical amount of intracellular APOBEC3G that restricts HIV infection in the presence of vif, or whether genetic polymorphisms of APOBEC3G exist that may potentially affect the course of disease, is not clear. In addition, the enzymatic function of intracellular APOBEC3G in lymphocytes may depend on the cellular activation status (Chiu 2005). Meanwhile, the epitopes by which vif and APOBEC3G interact with each other have been characterized and the pathway of intracellular degradation of the APOBEC3G-vif complex explored. Of note, specific inhibitors that block the interaction of vif and APOBEC3G or that interfere with the intracellular degradation of APOBEC3G could represent promising future treatments. In principle, blockade of cellular structures will likely be associated with a minimal risk that the development of resistance might compromise the efficacy of an antiviral agent. Therefore, targeting vif and APOBEC3G probably represents an interesting therapeutic track.

In summary, these data explain not only why vif is essential for HIV replication but also show why HIV replication is species-specific. Another cellular factor (see below) has also been discovered which explains species specificity of HIV replication.

The crucial role for APOBEC3G or other cytidine deaminases might not be restricted to HIV-1. An accumulation of G-to-A mutations was also demonstrated in various HBV isolates. In vitro, the accumulation of HBV DNA is dramatically reduced in the presence of APOBEC3G but co-transfection with vif can revert this inhibition.

The HIV replication cycle

HIV entry

CD4 as a primary receptor for HIV

CD4 is a 58 kDa monomeric glycoprotein that can be detected on the cell surface of about 60% of T lymphocytes, on T cell precursors within the bone marrow and thymus, and on monocytes and macrophages, eosinophils, dendritic cells and microglial cells of the central nervous system. The extracellular domain of the CD4 on T cells is composed of 370 amino acids; the hydrophobic transmembrane domain and the cytoplasmic part of CD4 on T cells consist of 25 and 38 amino acids, respectively. Within the extracellular part of CD4, four regions D1-D4 have been characterized that represent immunoglobulin-like domains. Residues within the V2 region of CD4 (amino acids 40-55) are important for the bonding of gp120 to CD4 and this region overlaps the part of the CD4 where its natural ligands, HLA class II molecules, bind.

The identification of the gp120 binding site on the CD4 receptor of CD4 T cells stimulated attempts to use soluble CD4 (sCD4) to neutralize the circulating virus in patients, the aim being the inhibition of viral spread. However, it became evident that even though laboratory viral isolates were easily neutralized by sCD4, neutralization of primary patient-derived isolates was not achieved.

In contrast, sCD4 was able to induce conformational changes within the viral envelope that promoted the infection of target cells.

CD4 attaches to the T cell receptor complex (TCR) on CD4 T cells and binds to HLA class II molecules on antigen-presenting cells. The binding of gp120 to CD4 is not only a crucial step for viral entry, but also interferes with intracellular signal transduction pathways and promotes apoptosis in CD4 T cells (Banda 1992). In the past couple of years, the idea of blocking CD4 as the primary cellular receptor of HIV has regained an interest. PRO542 represents a genetically engineered tetravalent CD4-IgG2 fusion protein that not only inhibits viral replication in vitro, but also shows an impressive antiviral efficacy in patients with high viral load that were in the initial clinical trials (see the chapter on ART).

CD4, as a primary and necessary receptor for HIV-1, HIV-2 and SIV, was already characterized in 1984 (Dalgleish 1984). However, experiments using non-human cell lines transfected with human CD4, showed that expression of human CD4 on the cell surface of a non-human cell line was not sufficient to allow entry of HIV. Therefore the existence of additional human co-receptors necessary for viral entry was postulated. On the other hand, some laboratory HIV-1 isolates, as well as some HIV-2 and SIV isolates are able to infect human cells independently from CD4. Interestingly, monoclonal antibodies against CD4-induced conformational (CD4i) epitopes to bind to the gp120 of CD4-independent viruses. This observation suggests that the gp120 of CD4-independent viruses already exposes the regions that are necessary for co-receptor recognition and binding and therefore binding to CD4 is not a prerequisite of entry for these viruses. CD4-independent viruses are easy to neutralize using the serum of HIV-infected patients, suggesting that the immune response selects against CD4-independent viruses (Edwards 2001).

Chemokine receptors as co-receptors for HIV entry

A milestone for the characterization of the early events leading to HIV-1 entry was an observation by Cocchi and co-workers in 1995. CD8 T cells from HIV-infected patients are able to suppress viral replication in co-cultures with HIV-infected autologous or allogenic CD4 T cells, and this is independent from their cytotoxic activity (Levy 1996). Cocchi identified the chemokines MIP-1a, MIP-1ß and Rantes in supernatants from CD8 T cells derived from HIV-infected patients, and was able to show that these chemokines were able to suppress replication in a dose-dependent manner of some, but not all, viral isolates tested (Cocchi 1995). MIP-1a, MIP-1ß and Rantes are ligands for the chemokine receptor CCR5, and a few months later several groups were able to show that CCR5 is a necessary co-receptor for monocytotropic (M-tropic) HIV-1 isolates (Deng 1996, Doranz 1996, Dragic 1998). A few weeks earlier, the chemokine receptor CXCR4 (fusin) was described as being the co-receptor used by T cell-tropic (T-tropic) HIV isolates (Feng 1996). Monocytotropic (M-tropic) HIV-1 isolates are classically those viruses that are most easily propagated in macrophage cultures, are unable to infect T cell lines (i.e., immortalized T cells), but are able to easily infect primary T cells from peripheral blood samples. Conversely, T cell-tropic HIV-1 isolates have classically been identified as being those that are easily propagated in T cell lines, and grow poorly in macrophages, but are also able to easily infect primary T cells from peripheral blood samples. It should be noted that both M-tropic and T-tropic HIV-1 variants can easily infect primary human non-immortalized T cells in vitro.

Chemokines (Chemotactic cytokines) and their receptors have been previously characterized with regard to their role in promoting the migration (chemotaxis) of leukocytes and their pro-inflammatory activity. They are proteins of 68-120 amino acids which depend on the structure of their common cysteine motif, and may be subdivided into C-X-C (a-chemokines), C-C (ß-chemokines) and C-chemokines. Chemokines typically show a high degree of structural homology to each other and may share the receptors they bind to. Chemokine receptors belong to the group of receptors with seven transmembranic regions (7-transmembrane receptors) and are intracellularly linked to G-proteins.

SDF-1 (stromal cell-derived factor 1) was identified as the natural ligand of CXCR4 and is able to inhibit the entry of T-tropic HIV-1 isolates into activated CD4 T cells. Rantes (regulated upon activation T cell expressed and secreted), MIP-1a (macrophage inhibitory protein) and MIP-1ß represent the natural ligands of CCR5 and are able to inhibit the entry of M-tropic HIV-1 isolates into T cells. A schematic model is depicted in Fig. 4. T-tropic HIV-1 isolates mainly infect activated peripheral blood CD4 T cells and cell lines and use CXCR4 for entry into the CD4-positive target cell. M-tropic isolates are able to infect CD4 T cells, monocytes and macrophages, and depend on the use of CCR5 and CD4 for viral entry.

The interaction of gp120 and the cellular receptors is now understood in more detail. Gp120 primarily binds to certain epitopes of CD4. Binding to CD4 induces conformational changes in the gp120 that promote a more efficient interaction of the V3 loop of gp120 with its respective co-receptor. Membrane fusion is dependent on gp120 co-receptor binding. Gp41, as the transmembrane part of the envelope glycoprotein gp160, is crucial for the fusion of the viral and host cell membrane. Similar to influenza hemagglutinin, it was postulated that consequent to the binding of gp120 to CD4, a conformational change is induced in gp41 that allows gp41 to insert its hydrophobic NH2 terminal into the target cell membrane. Gp41 has been compared to a mouse trap and a crystallographic analysis of the ectodomain of gp41 seems to confirm that (Chan 1997). The identification of crucial amino acid sequences for this process was used to synthesize peptides that bind to gp41 within the domains, are critical for the induction of conformational changes, and that may inhibit membrane fusion.

T-20 is the first of several peptides that bind to gp41 and has been tested in clinical trials for suppressing viral replication (see chapter on ART). T-20 is available as a therapeutic option for patients with advanced HIV. One disadvantage of T-20 is that it must be taken subcutaneously twice a day.

Using transfected cell lines, besides CCR5 and CXCR4, other chemokine receptors, such as CCR3, CCR2, CCR8, CCR9, STRL33 (“Bonzo”), Gpr 15 (“Bob”), Gpr 1, APJ and ChemR23, were identified and shown to be used for entry by certain HIV isolates (Deng 1997, Liao 1997). HIV-1 may also bind to certain integrins such as α4β7 and perturb cell function and migration (Arthos 2008). APJ may represent a relevant co-receptor within the central nervous system. Despite this broad spectrum of potentially available co-receptors, CCR5 and CXCR4 seem to represent the most relevant co-receptors for HIV-1 in vivo.

Figure 4: Inhibition of viral entry of CCR5-utilizing (monocytotropic) and CXCR4-utilizing (T cell tropic) HIV isolates by the natural ligands of the chemokine co-receptors CCR5 and CXCR4.

The importance of CCR5 as the predominant co-receptor for M-tropic HIV isolates is underscored by another observation. The majority of individuals with a genetic defect of CCR5 are resistant to infection with HIV-1 (Liu 1996). In vitro experiments show that lymphocytes derived from these individuals are resistant to HIV-1 infection using M-tropic isolates but not to infection with T-tropic isolates. Lymphocytes from these individuals do not express CCR5 on their cell surface and genetically have a 32-basepair deletion of the CCR5 gene. Worldwide, a few patients have been identified that have acquired HIV-1 infection despite a homozygous deletion of the CCR5. As expected, all of them were infected with CXCR4-using HIV-1 isolates. In epidemiological studies, the allelic frequency of the CCR5 gene deletion is 10-20% among Caucasians, particularly amongst those of Northern European descent. The frequency of a homozygous individual is about 1% in Caucasians (Dean 1996). Studies conducted on African or Asian populations, however, do not find this 32-basepair deletion, suggesting that this mutation arose after the separation of these populations in evolutionary history.

Individuals that are heterozygous for the 32-bp deletion of the CCR5 show a decreased expression of CCR5 on the cell surface and are more frequently encountered within cohorts of long-term non-progressors compared to patients who have a rapid progression of disease (Dean 1996). In addition, HIV-infected individuals who are heterozygous for the 32-bp deletion, have a slower progression to AIDS, a better treatment response to ART and lymphoma incidence is decreased. These data demonstrate that the density of CCR5 on the cell surface is not only a limiting factor for replication of HIV in vitro but in vivo as well.

In addition to the 32-bp deletion, other genetic polymorphisms with regard to chemokine receptors (CCR2) or their promoters (CCR5) have been described. Based on the occurrence of these polymorphisms within defined patient cohorts, they are associated with a more rapid or a more favorable course of disease, depending on the particular polymorphism (Anzala 1998, Winkler 1998). More recent studies shed light on the impact CCL3L1-CCR5 genotypes have not only on disease progression but also on response to antiretroviral therapy (Ahuja 2008). The mechanism appears to be independent of viral entry mechanisms but rather related to the quality of the cell-mediated immune response (Dolan 2007).

In patients who have a rapid progression of disease (rapid drop in CD4 T cell count), virus isolates that use CXCR4 as a predominant co-receptor tend to be frequently isolated from their cells in comparison to patients with a stable CD4 T cell count. The expression of co-receptors on CD4 lymphocytes depends on their activation level.

CXCR4 is mainly expressed on naive T cells, whereas CCR5 is present on activated and effector/memory T cells. During the early course of HIV-1 infection, predominantly M-tropic HIV-1 isolates are detected. Interestingly, M-tropic HIV-1 isolates are preferentially transmitted regardless of whether or not the source predominantly harbors T-tropic isolates. At present, it remains unclear whether this in vivo preference of M-tropic HIV-1 isolates is determined by selected transportation of M-tropic isolates by sub-mucosally located dendritic cells or whether the local cytokine/chemokine milieu favors the replication of M-tropic viruses. Intriguing studies suggest that M-tropic HIV-1 viruses are able to ‘hide’ more easily from the immune system by replicating in macrophages, in comparison to T-tropic viruses, thus giving them a survival advantage in the infected individual.

The blockade of CCR5, therefore, seems to represent a promising target for therapeutic intervention (Fig. 5). In vitro, monoclonal antibodies to CCR5 (2D7 and others) are able to block the entry of CCR5-using HIV isolates into CD4 T cells and macrophages. Small molecule inhibitors of CCR5 have been designed and preliminary clinical trials (see chapter on ART) demonstrate a significant reduction of plasma viremia in HIV-infected patients (Fätkenheuer 2005). In vitro studies as well as experiments using SCID mice do suggest that blockade of CCR5-using isolates may alter their tropism towards increased usage of CXCR4 (De Clercq 2001).

Small molecule inhibitors such as T-22, ALX40-4C or AMD3100 are able to inhibit CXCR4 and are also subject to preclinical and clinical trials (see chapter on ART). Other CCR5 inhibitors have been used as mucosal microbicides in monkey models and could represent a future preventive approach (Veazey 2005).

Strategies are currently being developed to modulate expression of chemokine receptors. Intrakines are chemokines that stay within the cytoplasm and are able to capture and bind to their corresponding receptor on its way to the cell surface (Chen 1997). Short interfering RNA (siRNA) represents a new molecular tool that is able to selectively inactivate target genes. Double-stranded RNA is split by the enzyme dicer-1 into short pieces (21-23-mers). These oligomers may complementarily bind to longer RNA sequences that are subsequently degraded. This strategy is currently employed in plants and used for its antiviral activity. The use of siRNA against CCR5 can prevent the expression of CCR5 in vitro and targeting of gag can effectively block viral replication (Song 2005).

Figure 5: Strategies to block infection by CCR5-tropic HIV. Blockage of CCR5 on the cell surface by non-agonistic ligands (A) or monoclonal antibodies (B). Alternatively, CCR5 cell surface expression can be reduced by siRNA or intrakine. For further details see text.

Although the therapeutic use of chemokine receptor blockers seems promising, a lot of questions still remain unanswered. In knockout mice it was demonstrated that the absence of CXCR4 or SDF-1 is associated with severe defects in hematopoiesis and in cerebellar development (Zou 1997). Currently, it remains unclear whether the blockade of CXCR4 in postnatal or adult individuals may affect other organ systems.

Post-fusion events

Following membrane fusion the viral core uncoats into the cytoplasm of the target cell. Alternatively, receptor-mediated endocytosis and dynamin-dependent fusion with intracellular compartments (Miyauchi 2009) can lead to viral inoculation. HIV can enter into rhesus lymphocytes but replication is stopped before or during early reverse transcription. This intracellular blockade is mediated by a cellular factor, TRIM5α, a component of cytoplasmic bodies whose primary function is not yet understood. TRIM5α from various species exhibits differential inhibition on various retroviruses. For example, TRIM5α from rhesus macaques, TRIM5αrh, more profoundly inhibits HIV replication than human TRIM5α, whereas SIV (simian immunodeficiency virus) which naturally infects Old World monkeys, is less susceptible to either form of TRIM5α, thus explaining in part the species specificity of HIV for human cells (Stremlau 2004). TRIM5α from human cells or non-human primates is able to inhibit replication of other lentiviruses and represents a novel cellular resistance factor whose definitive biological significance has yet to be fully characterized. TRIM5α serves as a mechanism for intracellular recognition and activation of the unspecific immune response (Pertel 2011), but it is unclear how exactly TRIM5α blocks reverse transcription and it has been hypothesized that TRIM5α interferes with the incoming virus capsid protein, targeting it for ubiquitination and proteolytic degradation.

HIV-1 entry into quiescent T cells is comparable to HIV-1 entry into activated T cells, but synthesis of HIV-1 DNA remains incomplete in quiescent cells (Zack 1990). The conversion of viral RNA into proviral DNA, mediated by the viral enzyme reverse transcriptase (RT), occurs in the cytoplasm of the target cell and is a crucial step within the viral replication cycle (Fig. 6). Blockade of the RT by the nucleoside RT inhibitor AZT was the first attempt to inhibit viral replication in HIV-1 infected patients. Today, numerous nucleoside, nucleotide and non-nucleoside RT inhibitors are available for clinical use and have broadened the therapeutic arsenal substantially since the mid-eighties.

Life cycle of HIV

Reverse transcription occurs in multiple steps. After binding of the tRNA primers, synthesis of proviral DNA occurs as a minus-strand polymerization starting at the PBS (primer binding site) and extending to the 5’ repeat region as a short R/U5 DNA. The next step includes degradation of RNA above the PBS by the viral enzyme RNAase H and a template switch of the R/U5 DNA with hybridization of the R sequence at the 3’ RNA end. Now the full length polymerization of proviral DNA with degradation of the tRNA is completed. Reverse transcription results in double-stranded HIV DNA with LTR regions (long terminal repeats) at each end.

HIV-1 enters into quiescent T cells and reverse transcription may result in the accumulation of proviral, non-integrating HIV DNA. However, cellular activation is necessary for integration of the proviral HIV DNA into the host cell genome after transportation of the pre-integration complex into the nucleus. Cellular activation may occur in vitro after stimulation with antigens or mitogens. In vivo, activation of the immune system is observed after antigen contact or vaccination or during an opportunistic infection. In addition, evidence is emerging that HIV-1 gp120 itself may activate the infecting cell to enhance integration. Besides monocytes, macrophages and microglial cells, latently infected quiescent CD4 T cells that contain non-integrated proviral HIV DNA represent important long-lived cellular reservoirs of HIV (Chun 1997), and cellular microRNAs contribute to HIV-1 latency in resting primary CD4 T lymphocytes (Huang 2007). Since natural HIV-1 infection is characterized by continuing cycles of viral replication in activated CD4 T cells, viral latency in these resting CD4 T cells likely represents an accidental phenomenon and is not likely to be important in the pathogenesis of HIV. This small reservoir of latent provirus in quiescent CD4 T cells gains importance, however, in individuals who are treated with ART, since the antivirals are unable to affect non-replicating proviruses – the virus will persist in those cells and be replication-competent to start new rounds of infection if the drugs are stopped. It is the existence of this latent reservoir that has prevented ART from entirely eradicating the virus from infected individuals (Chun 2005).

Until recently it was not clear why HIV replicates poorly in quiescent CD4 T cells. The cellular protein Murr1 that plays a role in copper metabolism is able to inhibit HIV replication in unstimulated CD4 T cells. Murr1 was detected in primary resting CD4 T cells and interferes with activation of the transcription factor NFkB by inhibiting the degradation of IkBa. IkBa prevents NFkB from migrating to the nucleus, especially after cytokine stimulation (e.g., TNFa). Because the HIV LTR region has multiple sites for NFkB, preventing NFkB migration to the nucleus should inhibit HIV replication. Inhibition of Murr-1 by siRNA is associated with HIV replication in quiescent CD4 T cells (Ganesh 2003). Persistence of HIV in quiescent CD4 T cells and other cellular reservoirs seems one of the main reasons why eradication of HIV is not feasible and why current therapies fail to achieve viral eradication (Dinoso 2009, Lewin 2011). If it is ever possible to achieve, a more detailed knowledge of how and when cellular reservoirs of HIV are established and how they may be targeted is of crucial importance for the development of strategies aiming at HIV eradication.

Cellular transcription factors such as NF-kB may also bind to the LTR regions. After stimulation with mitogens or cytokines NF-kB is translocated into the nucleus where it binds to the HIV LTR region, thereby initiating transcription of HIV genes. Transcription initially results in the early synthesis of regulatory HIV-1 proteins such as tat or rev. Tat binds to the TAR site (transactivation response element) at the beginning of the HIV-1 RNA in the nucleus and stimulates transcription and the formation of longer RNA transcripts. Rev activates the expression of structural and enzymatic genes and inhibits the production of regulatory proteins, therefore promoting the formation of mature viral particles. The proteins coded for by pol and gag form the nucleus of the maturing HIV particle, while the gene products coded for by env form the gp120 spikes of the viral envelope. The gp120 spikes are synthesized as large gp160 precursor molecules and are cleaved by the HIV-1 protease into gp120 and gp41. The gag proteins are also derived from a large 53 kD precursor molecule, from which the HIV protease cleaves the p24, p17, p9 and p7 gag proteins. Cleavage of the precursor molecules by the HIV-1 protease is necessary for the generation of infectious viral particles, and therefore the viral protease represents another interesting target for therapeutic blockade. The formation of new viral particles is a stepwise process: a new virus core is formed by HIV-1 RNA, gag proteins and various pol enzymes and moves towards the cell surface. The large precursor molecules are cleaved by the HIV-1 protease, which results in the infectious viral particles budding through the host cell membrane. During the budding process, the virus lipid membranes may incorporate various host cell proteins and become enriched with certain phospholipids and cholesterol. In contrast to T cells, where budding occurs at the cell surface and virions are released into the extracellular space, the budding process in monocytes and macrophages results in the accumulation of virions within cellular vacuoles.

The replication of retroviruses is prone to error and is characterized by a high spontaneous mutation rate. On average, reverse transcription results in 1-10 errors per genome and per round of replication. Mutations can lead to the formation of replication-incompetent viral species. Mutations that cause drug resistance may also accumulate, which, provided that there is selective pressure due to specific antiretroviral drugs and incomplete suppression of viral replication, may become dominant.

In addition, viral replication is dynamic and turns over quickly at an average rate of 109 new virus particles produced and subsequently cleared per day. Thus, within any individual, because of the extensive viral replication and mutation rates, there exists an accumulation of many closely-related virus variants within the population of viruses, referred to as a viral quasispecies. Selection pressure happens not only due to certain drugs, but also due to components of the immune system such as neutralizing antibodies or cytotoxic T cells (CTL).

HIV and the immune system

The role of antigen-presenting cells

Dendritic cells as prototypes of antigen-presenting cells

Dendritic cells, macrophages and B cells represent the main antigen-presenting cells of the immune system. Dendritic cells (DC) are the most potent inducers of specific immune responses and are considered essential for the initiation of primary antigen-specific immune reactions. DC precursors migrate from the bone marrow towards the primary lymphatic organs and into the submucosal tissue of the gut, the genitourinary system and the respiratory tract. They are able to pick up and process soluble antigens and migrate to the secondary lymphatic organs, where they activate antigen-specific T cells. Because DC have a crucial role in adaptive immunity, there is an increasing interest in using dendritic cells to induce or expand HIV-specific T cells. DC from HIV-infected patients have been purified, incubated with inactivated non-infectious HIV particles and subsequently used for vaccination (Lu 2004).

DC represent a heterogeneous family of cells with different functional capacities and expression of phenotypic markers depending on the local microenvironment and the stage of maturation. Immature DC have the capacity to pick up and process foreign antigens, but do not have great T cell stimulatory capacities. Mature DC show a predominant immunostimulatory ability. DC in tissues and Langerhans cells, which are specialized DC in the skin and mucosal areas, represent a more immature phenotype and may take up antigen. Once these DC have taken up the antigen, they migrate to the lymphoid tissues where they develop a mature phenotype.

The stimulation of CD8 T lymphocytes and the formation of antigen-specific cytotoxic T cells (CTL) depend on the presentation of a peptide together with MHC class I antigens. DC may become infected with viruses, for instance influenza. Viral proteins are then produced within the cytoplasm of the cell, similar to cellular proteins, then degraded to viral peptides and translocated from the cytosol into the endoplasmic reticulum, where they are bound to MHC class I antigens. These peptide-MHC class I complexes migrate to the DC surface. Alternatively, DC take up antigens for infected cells for presentation via MHC class I (Maranon 2004). Interestingly, the efficacy of this presentation of viral antigens is comparable whether or not DC themselves are productively infected. DC are rather resistant against productive HIV infection and intracellular recognition by DC contributes to innate immune repones following inoculation (Manel 2010).

The number of specific antigen-MHC class I complexes is usually limited and must eventually be recognized by rare T cell clones to a ratio of 1:100,000 or less. The T cell receptor (TCR) may display only a low binding affinity (1 mM or less). The high density of co-stimulatory molecules on the DC surface, however, enhances the TCR-MHC: peptide interaction allows efficient signaling to occur through the T cell and results in proliferation (clonal expansion) of the T cell. Virus-infected cells or tumor cells often do not express co-stimulatory molecules, and thus may not be able to induce a clonal expansion of effector cells. This underscores the importance of having a highly specialized system of antigen-presenting cells, i.e., DC, to prime T cells to expand and proliferate rapidly.

The interaction of dendritic cells and B/T cells

B and T lymphocytes may be regarded as the principle effector cells of antigen-specific immune responses. However, their function is under the control of dendritic cells. DC are able to pick up antigens in the periphery. These antigens are processed and expressed on the cell surface together with co-stimulatory molecules that initiate T cell activation. B cells may recognize antigen after binding to the B cell receptor. Recognition of antigen by T cells requires previous processing and presentation of antigenic peptides by DC and the intracellular half life time of peptides impacts on the CD8 T cell response (Lazaro 2011). T cells express different T cell receptors (TCR) that may bind to the peptide: MHC class I on the surface of dendritic cells to allow activation of CD8 T cells, or to the peptide: MHC class II molecules, to activate CD4 T cells. The ability of DC to activate T cells also depends on the secretion of stimulatory cytokines such as IL-12, which is a key cytokine for the generation and activation of TH1 and natural killer (NK) cells.

Only a few DC and small amounts of antigen are sufficient to induce a potent antigen-specific T cell response, thus demonstrating the immunostimulatory potency of DC. The expression of adhesion molecules and lectins, such as DC-SIGN, support the aggregation of DC and T cells, promote the engagement of the T cell receptor (TCR) and mutual infection and therby distribution of the virus within the host (Gringhuis 2010). DC-SIGN is a type C lectin that has also been shown to bind to lentiviruses such as SIV and HIV-1 and -2 by interaction of gp120 with carbohydrates (Geijtenbeek 2000). Mycobacteria and Dengue virus may also bind to DC-SIGN. In vivo, immunohistochemical studies show expression of DC-SIGN on submucosal and intradermal DC, suggesting an implication of DC-SIGN in vertical and mucosal transmission of HIV. The expression of DC-SIGN was shown to enhance the transmission of HIV to T cells and allows utilization of co-receptors if their expression is limited. Thus DC-SIGN may be a mechanism whereby HIV-1 is taken up by DC in the mucosal tissues. It is then transported by the DC to the lymphoid tissues where HIV-1 can then infect all the residing CD4 T cells (Lore 2005).

Lymphatic tissue as the site of viral replication

Viral replication within the lymphatic tissue is extensive in the early stages of the disease even in the hematopoietic system (Carter 2010, Embretson 2003, Pantaleo 1993).

During the initial phase of HIV-1 infection, there is a burst of virus into the plasma, followed by a relative decline in viremia. During this time, a strong HIV-1 specific cytotoxic T cell response is generated, which coincides with the early suppression of plasma viremia  in most patients. Virions are trapped by the follicular dendritic cell (FDC) network within the lymphoid tissue. Macrophages and activated and quiescent CD4 T cells are the main targets of infection. Permanent viral reservoirs, mainly in macrophages and latently infected CD4 T cells are established in the early phase of infection and probably represent the major obstacle so far to successful  eradication of HIV. During the whole course of infection with HIV-1, the lymphoid tissue represents the principle site of HIV-1 replication. The frequency of cells containing proviral DNA is 5-10 times higher in lymphoid tissue than in circulating peripheral mononuclear cells in the blood, and the difference in viral replication in lymphoid tissue exceeds that in the peripheral blood by about 10-100 times.

After entry of HIV-1 into a quiescent CD4 T cell and after completion of reverse transcription, the viral genome is represented by proviral unintegrated HIV DNA. In vitro experiments have shown that HIV-1 preferentially integrates into active genes (hot spots) (Schroder 2002). The activation of CD4 T cells is necessary for the integration of the HIV DNA into the host cell genome and is therefore a prerequisite for the synthesis of new virions. In this regard, the micromilieu of the lymphoid tissue represents the optimal environment for viral replication. The close cell-cell contact between CD4 T cells and antigen-presenting cells, the presence of infectious virions on the surface of the FDC, and an abundant production of pro-inflammatory cytokines such as IL-1, IL-6 or TNFα (Stacey 2009), promotes the induction of viral replication in infected cells and augments viral replication in cells already producing the virus. During later stages of the disease, micriobial translocation from the gut (Brenchley 2006, Klatt 2010, Estes 2010) contribute to systemic immune actiation, which in turn lead to loss of CD4 T cells (Ciccone 2010) and disease progress. It should be noted that both IL-1 and TNFα induce NF-kb, which binds to the HIV-1 LTR to promote proviral transcription. The importance of an antigen-induced activation of CD4 T cells is underlined by several in vivo and in vitro studies that demonstrate an increase in HIV-1 replication in association with tetanus or influenza vaccination or an infection with Mycobacterium tuberculosis (O’Brian 1995). Even though the clinical benefit of vaccination against common pathogens (e.g., influenza and tetanus) in HIV-1-infected patients outweighs the potential risk of a temporary increase in viral load, these studies indicate that in every situation where the immune system is activated, enhanced viral replication can also occur.

Patients on ART demonstrate a dramatic decrease in the number of productively infected CD4 T cells within the lymphoid tissue (Tenner-Racz 1998). However, in all patients so far, there persists a pool of latently infected quiescent T cells despite successful suppression of plasma viremia. It is these latently infected cells that may give rise to further rounds of viral replication if the antiviral drugs are stopped.

During the natural course of HIV-1 disease, the number of CD4 T cells slowly decreases while plasma viremia rises in most patients. If sequential analysis of the lymphoid tissue is performed, progression of the disease is reflected by destruction of the lymphoid tissue architecture and a decreased viral trapping. Various immunohistological studies indicate that the paracortex of the lymph nodes represents the primary site where HIV replication is initiated (Embretson 1993, Pantaleo 1993). Infection of the surrounding CD4 T cells, as well as the initiation of T cell activation by DC, contributes to the spreading of HIV-1 within the lymphoid environment. Similar to SIV infection in rhesus macaques, HIV infection at all stages of disease is associated with preferential replication and CD4 T cell destruction in the gut lamina propria and submucosa rather than in lymph nodes (Veazey 1998, Brenchley 2004). This is likely because the gut is predominantly populated by CCR5-expressing effector memory CD4 T cells, which are ideal targets for HIV replication compared to the mixed populations of CD4 T cells found in the lymph nodes. In addition, HIV-1 nef appears to have lost a function which can be found in SIV, where it leads to a down-regulation of CD3 and the T cell receptor, reducing chronic cell activation which would otherwise render them susceptible for apotosis (Schindler 2006). Several studies have demonstrated that, during acute infection, depletion of CD4+CCR5+ memory cells within the mucosa-associated lymphatic tissue is a hallmark of both HIV and SIV infection. Genome analysis of viral RNA in patients with acute HIV infection revealed that in about 80% of infected patients, viremia results from infection of a single virus particle (Keele 2008). In the early phase of SIV infection, up to 60% of all CD4 T cells within the intestinal lamina propria were shown to express viral RNA. Most of these cells are destroyed by direct and indirect mechanisms within a few days. Further disease progression seems to depend largely on the capacity of the host to reconstitute the pool of memory cells within the mucosa-associated lymphoid tissue. In view of this data, some researchers argue that initiation of ART during acute HIV infection is crucial in order to limit long-term damage to the immune system. However, in patients with exhaustion of lymphpoiesis despite effective long-term HIV therapy disease progression might occur (Sauce 2011).

Recent studies have also examined the effect of HIV infection on the thymus and its role in CD4 T cell depletion and homeostasis. Recent work has suggested that thymic output of CD4 T cells is decreased during HIV infection, particularly with older patients, and that this defect is due to abnormalities of intra-thymic proliferation of T cells, whose mechanism is still undefined, as thymocytes do not express CCR5 and should not necessarily be targets of HIV (Douek 2001).

The HLA system and the immune response to HIV

CD8 T cells recognize specific antigens (peptides) in context with HLA class I molecules on antigen-presenting cells, whereas CD4 T cells require the presentation of antigenic peptides in context with HLA class II molecules. The generation of an HIV-specific immune response is therefore dependent on the individual HLA pattern.

Antigen-presenting cells may bind HIV peptides in different ways on the HLA class I molecules. Therefore, CD8 T cells can be activated in an optimal or suboptimal way or may not be activated at all. Using large cohorts of HIV-1 infected patients, in whom the natural course of disease (fast versus slow progression) is known, HLA patterns were identified that were associated with a slow versus fast disease progression (Pereyra 2010). These studies suggest that the HLA type may be responsible for a benign course of disease in about 40% of patients with a long-term non-progressive course of disease. Homozygosity for HLA Bw4 is regarded as being protective. Patients who display heterozygosity at the HLA class I loci characteristically show a slower progression of immunodeficiency than patients with homozygosity at these loci (Carrington 1999). An initial study by Kaslow in 1996 demonstrated that HLA B14, B27, B51, B57 and C8 are associated with a slow disease progression, while the presence of HLA A23, B37 and B49 were associated with a rapid development of immunodeficiency (Kaslow 1996). All patients with HLA B35 had developed symptoms of AIDS after 8 years of infection. More recent studies suggest that discordant couples with a “mismatch” regarding the HLA class I have a protective effect towards heterosexual transmission (Lockett 2001).

In vitro studies in HLA B57-positive patients demonstrate that these patients display HLA B57-restricted CTL directed against HIV-1 peptides. There is evidence that the better immune responses in these patients is already determined during thymic selection (Kosmrlj 2010). However, it is possible that the identification of protective HLA alleles or HLA-restricted peptides in HIV-1-infected patients with a benign course of disease does not necessarily indicate that the same alleles or peptides are crucial for the design of a protective vaccine. CD8 T cells from HIV-1-exposed but uninfected African women recognize different epitopes than CD8 T cells from HIV-1-infected African women (Kaul 2001). This suggests that the epitopes that the immune system is directed against during a natural infection might be different from those that are protective against infection. In addition, the individual HLA pattern may affect the adaptive immune response and the evolution of viral escape mutations (Friedrich 2004, Leslie 2004). CTL from patients with HLA B57 and B58 may force the virus to develop certain mutations in gag that enable the virus to escape the CTL response, but these mutations result in a reduced replicative competence and thus lower viral load (Goepfert 2008). If such a virus is transmitted to another individual with a different HLA background, the virus may back-mutate to the original genotype and regain its full replicative competence because of the absence of CTL-mediated immune pressure.

HLA class II antigens are crucial for the development of an HIV-1-specific CD4 T cell response. Rosenberg (1997) was the first to show that HIV-1-infected patients with a long-term non-progressive course of disease had HIV-1-specific CD4 T cells that could proliferate against HIV-1 antigens. The identification of protective or unfavorable HLA class II alleles is less well elaborated than the knowledge about protective HLA class I alleles. Cohorts of vertically infected children and HIV-infected adults demonstrate a protective effect of HLA DR13 (Keet 1999).

KIR receptors (Killer cell immunoglobulin like receptors) represent ligands that bind to HLA class I antigens and by functioning as either activating or inhibiting receptors they regulate the activation status of NK cells. Polymorphisms of KIR genes were shown to correlate with slow or rapid progression of HIV disease, especially when the analysis includes known HLA class I polymorphisms (Fauci 2005). During HIV infection NK cells may not only be decreased but may also show a diminished cytolytic activity. Preliminary results suggest that low numbers of NK cells are associated with a more rapid disease progression .

In summary, various genetic polymorphisms have been identified that have an impact on the course of HIV disease. However, there is currently no rationale to recommend routine testing of individual patients or to base therapeutic decisions on genetic testing.

The HIV-specific cellular immune response

Cytotoxic T cells (CTL) are able to recognize and eliminate virus-infected cells. A number of studies clearly demonstrate that CTL are crucial for the control of HIV replication and have a substantial impact on disease progression once infection is established. However, there is little evidence to assume that CTL play a major role in primary protection.

In comparison to HIV-1-infected patients with a rapid decline in CD4 T cell numbers, patients with a long-term non-progressive course of disease (LTNP = long-term non-progressors) have high quantities of HIV-1-specific CTL precursors with a broad specificity towards various HIV-1 proteins. The different capacities of certain HLA alleles to present viral particles more or less efficiently and to induce a generally potent immune response may explain why certain HLA alleles are associated with a more rapid or a slower progressive course of disease (see above).

Individuals have been described, who developed CTL escape mutants after years of stable disease and the presence of a strong CTL response. The evolution of CTL escape mutants was associated with a rapid decline in CD4 T cells in these patients, indicating the protective role of CTL (Goulder 1997).

HIV-specific CTL responses have been detected in individuals exposed to, but not infected by HIV-1. Nef-specific CTL have been identified in HIV-1-negative heterosexual partners of HIV-infected patients and env-specific CTL have been found in seronegative healthcare workers after exposure to HIV-1 by needle stick injuries (Pinto 1995). Unfortunately, patients with a broad and strong CTL response do not seem to be protected from superinfection by a different but closely related HIV isolate (Altfeld 2002).

The presence of a CTL response is not correlated just with the suppression of plasma viremia during the initial phase of HIV infection. Patients who underwent structured therapy interruptions, especially when ART was initiated early following infection, demonstrated the appearance of HIV-specific CTL during the pauses. Goonetilleke (2009) demonstrated that the initial HIV-1 specific CTL response contributes significantly to the control of viremia during acute infection. Novel imaging techniques and animal models enabled for the first time the visualization of the interplay of immune cells with infected target cells and the contribution of this response for the infection control (Li 2009).

However, it is still unclear in most patients who exhibit a potent temporary CTL response why this CTL response diminishes later on (Pantaleo 2004). The appearance of viral escape mutants might explain why previously recognized epitopes are no longer immunodominant.

The nef protein may downregulate HLA class I antigens and therefore counteract the recognition of infected cells by CTL. In addition, the majority of infected individuals show detectable CTL responses. It is unclear why they are unable to control the virus. Interestingly, CTL from HIV-infected patients shows a lack of perforin and an immature phenotype in comparison to anti-CMV-directed effector cells (Harari 2002) even though the ability to secrete chemokines and cytokines is not impaired (Appay 2000). Another study provided evidence that the killing capacity of HIV-specific CTL was associated with the ability to simultaneously produce interferon-g and TNFa (Lichtenfeld 2004). Surface molecules such as PD-1 on CTL, which are transiently upregulated upon cell activation, may persist due to prolonged antigen presence. Persistent PD-1 expression, however, can result in CTL and helper cells (Said 2010) dysfunction and this effect can be restored by blockade of PD-1 and PD-1L interaction on DC by administration of anti-PD-1 antibodies. This restores CTL functions such as cytokine production and killing capacity (Trautmann 2006, Velu 2009).

CD8 T cells may also become infected with HIV (Bevan 2004) although this has not been demonstrated for HIV-specific CD8 T cells. It is unclear whether CD8 T cells temporarily express CD4 and which chemokine co-receptors mediate infection of these CD8 T cells.

Proliferation and activation of CTL is dependent on antigen-specific T cell help. Initiation of ART during primary HIV infection was associated with persistence of an HIV-specific CD4 T cell response that was not detected in patients analyzed during the chronic stage of disease (Rosenberg 1997). HIV-specific CD4 T cells are mainly directed against gag and nef-derived epitopes (Kaufmann 2004). HIV preferentially infects pre-activated CD4 T cells and as HIV-specific CD4 T cells are among the first cells to be activated during HIV infection, their preferential infection has been demonstrated (Douek 2002). Therefore, it is currently unclear whether the loss of HIV-specific CTL activity during the course of disease reflects an instrinsic defect of CTL or develops secondary to a loss of specific CD4 T cell help.

Various therapeutic vaccine strategies have been developed during the last few years and mostly tested in SIV-infected rhesus macaques aiming at inducing an SIV-specific CTL response that may alter the natural course of disease (McElrath 2010). Recently, a promising approach was published on a vaccine trial using autologous dendritic cells in SIV-infected rhesus macaques that were pulsed with inactivated SIV (Lu 2003). In contrast to the unvaccinated control group, monkeys that were vaccinated showed a dramatic decrease in viral load, and the development of anti-SIV-directed humoral and cellular immune responses. Meanwhile, a pilot trial has been initiated in a cohort of 18 HIV-infected antiretroviral-naive patients with stable viral load. The patients were vaccinated with autologous monocyte-derived dendritic cells that were pulsed with inactivated autologous virus. During the following 112 days, a median decrease of 80% of the viral load was observed and maintained for more than one year in 8 patients. In parallel, gag-specific CD8 T cells and HIV-specific CD4 T cells producing IFNg and/or interleukin-2 were detected (Lu 2004). Therapeutic vaccination using autologous dendritic cells appears to be a potential immunotherapeutic, but more controlled clinical studies are definitely needed.

In addition to the cytotoxic activities directed against HIV-infected cells, CD8 T cells from HIV-1 infected patients exhibit a remarkable, soluble HIV-1 inhibitory activity that inhibits HIV-1 replication in autologous and allogeneic cell cultures (Walker 1986). Despite multiple efforts, the identity of this inhibitory activity (CAF) has not been clarified, although chemokines such as MIP-1α, MIP-1ß or Rantes (Cocchi 1995), IL-16 (Baier 1995), the chemokine MDC (Pal 1997) and defensins may account for at least some of the inhibition.

The TH1/TH2 immune response

Depending on the secretion pattern of cytokines, CD4 T cells may be differentiated into TH1 and TH2 cells. TH1 CD4 T cells primarily produce interleukin-2 (IL-2) and IFNg, which represent the cytokines that support the effector functions of the immune system (CTL, NK-cells, macrophages). TH2 cells predominantly produce IL-4, IL-10, IL-5 and IL-6, which represent the cytokines that favor the development of a humoral immune response. Since TH1 cytokines are critical for the generation of CTLs, an HIV-1-specific TH1 response is regarded as being a protective immune response. Studies on HIV-exposed but non-infected individuals have shown that following in vitro stimulation with HIV-1 env antigens (gp120/gp160) and peptides, T cells from these individuals secrete IL-2 in contrast to non-exposed controls (Clerici 1991). Similar studies were undertaken in healthcare workers after needlestick injuries and in newborns from HIV-infected mothers. Although these observations may indicate that a TH1-type immune response is potentially protective, it should be considered that similar immune responses might have been generated after contact with non-infectious viral particles and therefore do not necessarily imply a means of protection against a replication-competent virus.

HIV-1 specific humoral immune responses

The association between an HIV-1-specific humoral immune response and the course of disease is less well characterized.

In an SIV model injection of an antibody cocktail consisting of various neutralizing antibodies is able to prevent SIV infection after a mucosal virus challenge (Ferrantelli 2004) indicating that primary protection is mainly dependent on a broad humoral immune response. Additional animal experiments suggest antibody concentrations in mucosal sites required to confer protection (Hessell 2009). This data suggests that HIV-specific antibodies are necessary for a preventive vaccine strategy. In contrast, B cell depletion by a monoclonal antibody directed against B cells in monkeys with already established SIV infection does not affect the course of plasma viremia (Schmitz 2003).

A slow progression of immunodeficiency was observed in patients with high titers of anti-p24 antibodies (Hogervorst 1995), persistence of neutralizing antibodies against primary and autologous viruses (Montefiori 1996) and lack of antibodies against certain gp120 epitopes (Wong 1993).

Long-term non-progressors with HIV tend to have a broad neutralizing activity towards a range of primary isolates and show persistence of neutralizing antibodies against autologous virus. At present, it is unclear whether the presence of neutralizing antibodies in LTNP represents part of the protection or whether it merely reflects the integrity of a relatively intact immune system. Individuals that have a substantial risk for HIV-1 infection, but are considered exposed yet non-infected by definition represent individuals with a lack of a detectable antibody response to HIV-1. This definition implies that a systemic humoral immune response may not represent a crucial protective mechanism. It has been shown that these individuals may demonstrate a local (mucosal) IgA response against HIV-1 proteins that are not detected by the usual antibody testing methods (Saha 2001). Thus, local IgA, rather than systemic IgG, may be associated with protection against HIV-1 infection. There is also some evidence that some anti-HIV-1 antibodies can enhance infection of CD4 T cells.

A number of older as well as recent studies have shown that neutralizing antibodies do exist in HIV-1-infected individuals, although there is a time lag in their appearance. That is, individuals will develop neutralizing antibodies to their own viruses over time, although by the time these antibodies develop the new viruses circulating in the individual’s plasma will become resistant to neutralization, even though the older ones are now sensitive to the current antibodies in the patient’s serum. Thus, the antibody response appears to be constantly going after a moving target, allowing viruses to escape continuously. Further understanding the mechanisms of humoral escape will likely lead to potential new therapies.

A few years ago, selected patients with advanced HIV infection were treated with plasma from HIV-infected patients at an earlier stage of the disease. No significant effect on the course of disease was notable (Jacobson 1998). The therapeutic application of neutralizing antibodies with defined specificity looked more promising, since a few acute and chronically infected patients were able to control their viral load at least temporarily after stopping antiretroviral therapy (Trkola 2005). Functionally, Fc receptors but not complement binding are important in antibody protection against HIV (Hessell 2007) and some neutralising antibodies recognise the CD4-binding site of gp120 (Li 2007). Neutralizing antibodies frequently recognize conserved epitopes with relevance for viral fitness (Pietzsch 2010) and research during the last years has revealed some of the specific structures and surface glycoproteins, which are targeted by neutralizing antibodies (Wu 2010, Zhou 2010). This more detailed understanding is very likely to advance vaccine strategies aiming to treat or to protect against HIV-infection.


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Filed under 3. Pathogenesis of HIV-1 Infection, Part 1 - The Basics

Preventive HIV-1 Vaccine

– Thomas Harrer –

Although great progress has been achieved in the field of treatment and prevention of HIV-1 infection, the HIV-1 pandemic will ultimately be controlled only by an effective HIV-1 vaccine. Unfortunately, despite intense research for over two decades, it has not yet been possible to develop an effective protective HIV-1 vaccine. The following chapter will give a short overview of the current status of HIV-1 vaccine development.

Induction of neutralizing antibodies

Similar to successful vaccination strategies in other infections such as hepatitis B, initial HIV vaccine research focussed on the development of vaccines with the capability of inducing neutralizing antibodies. A variety of studies examined the safety and efficacy of vaccines such as gp120, gp160, parts of gp160 and peptides of gp160 to induce antibodies against HIV-1 envelope proteins. These immunogens stimulated the production of specific antibodies that were able to neutralize HIV-1 strains in vitro, but they failed to induce broadly neutralizing antibodies in HIV-1 variants derived from patients (Mascola 1996).

Two gp120-based vaccines were tested in two large Phase III trials in healthy volunteers: a clade B gp120 from HIV-1 MN and a gp120 from the CRF01_AE HIV-1 isolate were used in the VAX 003 Study in Thailand (Pitisuttithum 2006), while clade B gp120 proteins from HIV-1 MN and HIV-1 GNE8 were tested in the VAX 004 Study in the USA and the Netherlands (Flynn 2005). Despite induction of antibodies against gp120, the incidence of new infections was not lowered in either trial. These studies and others demonstrate that it is difficult to neutralize the biological activity of the envelope molecule gp160 via antibodies. Prior to the binding of gp120 to the CD4 receptor, the conserved and functionally important epitopes are hidden in grooves of the gp120 molecules that are additionally masked by glycan shields and variable sequence loops (Kwong 2002). Therefore, it is difficult for antibodies to block the binding of gp120 to the CD4 molecule.

The binding of the gp120 trimer to CD4 induces a conformational change of the V3 loop that exposes a conserved high-affinity coreceptor binding site on the gp120 molecule. The subsequent binding to the coreceptors CCR5 or CXCR4 triggers structural modifications of the viral transmembrane molecule gp41 and starts the fusion of the virus with the host cell membrane. Antibodies against the V3 loop can neutralize the process, although these activated binding sites on the V3 loop are recognizable by antibodies only for a short period of time. Therefore, high antibody concentrations are required for an efficient neutralization. Another problem for antibody-mediated neutralization is the shielding of the V3 loop-coreceptor interaction site by the gp120 trimer, which also inhibits the binding of antibodies to the V3 loop (Labrijn 2003).

HIV-1 infected patients generate neutralizing antibodies. However, in the majority of patients they are directed against the gp120 variable sequences. Due to the high sequence variability in gp120, HIV-1 can evade antibodies by a rapid generation of escape mutants. Thus, the majority of patients generate antibodies recognizing their own strain of HIV-1, but they neutralize HIV-1 variants from other patients poorly. There are only a few patients able to produce broadly cross-reacting neutralizing antibodies. These exceptional antibodies recognize the conserved binding sites for CD4 and coreceptors in gp120 and an important fusion domain in gp41. Vaccination with a recombinant gp120 molecule is not able to induce such antibodies as the V3 loop epitopes in the native gp120 molecule are not accessible to antibodies. To improve the induction of antibodies targeting the V3 loop, attempts are currently in progress to develop fusion molecules consisting of gp120 and CD4 that simulate the conformational changes in gp120 after binding to the CD4 molecule (Kwong 1998).

An innovative approach is the passive genetic immunization by the transfer of genes encoding highly active neutralizing antibodies or antibody-like immunoadhesins. In Rhesus monkeys, the intramuscular injection of a recombinant adeno-associated virus (AAV) vector encoding such SIV-specific antibody genes could induce the in vivo production of SIV-envelope-specific neutralizing antibody constructs that provided protection from intravenous challenge with SIV (Johnson 2009). This new exciting development stimulated a worldwide search for those few HIV-1 infected individuals that were able to generate unique highly active neutralizing antibodies that could be used for genetic immunization against HIV-1.

Induction of HIV-1-specific T cells

With all these hurdles regarding the induction of neutralizing antibody responses, the focus of vaccine development turned to vaccines that could elicit HIV-1-specific T cell responses. Cytotoxic T cells (CTL) play an important role in the control of HIV-1 in humans (Koup 1994, Harrer 1996b, Pantaleo 1997) and also for the control of SIV in SIV models. Experimental depletion of CD8 T cells in SIV-infected monkeys abrogated immune control of SIV infection and was associated with a strong increase of viral replication (Schmitz 1999). In contrast to neutralizing antibodies, CTLs do not exert a sterilizing immunity as they can only recognize cells that are already infected.

However, the observation of HIV-1-specific CTLs in HIV-1 exposed but uninfected subjects raised the hope that a T cell-based HIV-1 vaccine could prevent an ongoing HIV-1 infection by containment and eradication of small foci of viral infection (Herr 1998, Rowland-Jones 1998). Even if a T cell-based vaccine could not prevent infection of the host, there is the chance that it could influence the course of infection by reducing the extent of viremia after infection, as seen in the SIV monkey models (Letvin 2006). The viral load four months of infection, also known as the viral setpoint, may be one of the most important prognostic parameters for the course of HIV-1 infection. A vaccine could provide a clinical benefit if it could reduce the viral setpoint by half a log (Johnston 2007). In addition, such a vaccine could possibly exert positive effects on the spread of the HIV epidemic, as a lower viremia probably diminishes the infectivity of the patients. The clinical evaluation of these vaccines that do not prevent infection, but rather influence the course of disease, is difficult to achieve as large numbers of patients must be followed for extended periods of time.

HIV-1 can evade CTL recognition via development of CTL escape mutants in T cell epitopes or in proteasome cleavage sites (Maurer 2008). At least in conserved proteins such as gag or protease CTL-mediated immune selection is a major driving force for the development of polymorphisms (Mueller 2007). Our observations in long-term non-progressors showed that the quality of the CTL response with recognition of conserved CTL epitopes is very important (Harrer 1996a, Wagner 1999). It is essential for an effective vaccine to contain enough highly conserved CTL epitopes for the individual HLA alleles.

CTLs can be induced only by vaccines that are able to load viral peptides on HLA class I molecules of dendritic cells that present these peptides to the CTLs. Live attenuated viruses are effective against several infectious pathogens such as measles and they were protective against SIV in rhesus monkeys, but they are unlikely to be used in humans due to safety concerns. DNA vaccines alone are not very immunogenic in humans, but in DNA prime/vector boost strategies DNA priming could increase the immunogenicity of subsequent vaccinations with viral vectors. Lipopeptides allow the induction of CTL, but they can present only a limited repertoire of epitopes.

A new concept is the genetic immunization by transfer of genes encoding highly effective HIV-1-specific T-cell receptors (TCR) into CD8+ cytotoxic T-cells. In contrast to the transfer of antibody genes, transfer of TCR has to consider the HLA-restriction of the targeted CTL epitope and the HLA-I-type of the recipient. Recently, it could be shown in in-vitro experiments, that it is even possible to transfer two different exogenous HIV-1-specific TCRs into the same cell. If such techniques could be applied also in vivo, this could reduce the risk of selection of CTL escape mutations (Hofmann 2011).

Recombinant viral vectors

Recombinant viral vectors can achieve the induction of CTLs without the safety risks of attenuated live viruses. Several vectors have been tested in clinical studies: Adenovirus 5 (Ad5) vectors, ALVAC canarypox viruses, MVA (modified Vaccinia Virus Ankara), NYVAC (Gomez 2007a+b), adenovirus-associated virus and fowlpox vectors.

A great disappointment was the termination of two placebo-controlled Phase IIb trials, the HVTN 502 study (STEP trial) (Buchbinder 2008) and the HVTN 503 study (Phambili Study) ( Both studies were testing Merck’s trivalent MRK Ad5 vaccine (V520), a mixture of Ad5 vectors expressing HIV-1 gag, pol and nef. 3000 volunteers from North America, South America, the Caribbean, and Australia participated in the STEP trial that started in December 2004. The vaccine was immunogenic and induced HIV-1-specific CD8+ T cells in 73% and HIV-1-specific CD4+ T cells in 41% of the vaccinees (McElrath 2008). Nevertheless, the study was terminated ahead of schedule in September 2007 because of lack of efficacy. The vaccine neither prevented HIV-1 infection nor did it lower the viral setpoint in those who were infected. 83 volunteers became infected during the trial. As only one female was infected, the post hoc analyses were restricted to the 82 male newly-infected subjects. There was a non-significant trend towards a greater number of infections in the vaccine recipients (49 new infections in 914 subjects) versus the placebo recipients (33 new infections in 922 subjects). Interestingly, subjects with high pre-existing Ad5-specific neutralizing antibody titers (titer of >200) at enrolment showed a higher infection rate in those who got the vaccine (21 infections) versus those in the placebo arm (9 infections). In contrast there were no significant differences in subjects with absent or low Ad5-specific neutralizing antibody titers of <200 (28 infections in the vaccine arm, 24 infections in the placebo arm). Because of the potential risk of the MRK Ad5 vaccine in subjects with a strong immune response against adenovirus 5, the parallel Phambili trial in South Africa was terminated as well. In Phambili, the MRK Ad5 vaccine showed no efficacy, with 33 new HIV-1 infections (4.54 infections per 100 person-years) in patients receiving at least one vaccination versus 28 HIV-1 infections (3.70 infections per 100 person-years) in the placebo arm (non-significant difference) (Gray 2011).

The STEP trial raises important questions that can be answered only by further examination of infected subjects and transmitted viruses. The fact that the increased infection risk was only seen in subjects with high antibody titers against the Ad5 vector argues against a general risk of immunizing against HIV-1, but it demonstrates the important issue of pre-existing vector immunity. The optimal priming of the immune response by a vaccine seems to be a key element determining the success or failure of a vaccine. Therefore more basic research is needed for a better understanding of the mechanisms of HIV-1 immunological control. Because of the unfavourable effects of pre-existing immunity against the adenovirus 5-vector, other adenoviral vectors are currently developed from less frequent adenovirus serotypes. So far, two phase-1 studies in healthy volunteers could demonstrate the immunogenicity of new HIV-1 vaccines based on the adenovirus serotypes AD26 (AD26.ENVA.01) and AD35 (AD35-GRIN/ENV) (Keefer MC 2010; Barouch D 2010).

In contrast to the STEP trial, the recently published RV144 study (Rerks-Ngarm 2009) involving more than 16,000 volunteers in Thailand showed a modest protective effect with a significant reduction of new HIV-1 infections by about 31%. The vaccine was a Sanofi Pasteur’s canarypox vector-based ALVAC HIV (vCP1521) expressing HIV-1 subtype B gag and protease and subtype E envelope in combination with AIDSVAX B/E gp120 proteins (MN rgp120/HIV-1 plus A244 rgp120/HIV-1). Among the 8198 subjects receiving placebo, 74 new HIV-1 infections were observed during the three years follow-up in comparison to 51 infections among the other half of volunteers that had received four immunizations with the ALVAC HIV and two immunizations with AIDSVAX B/E gp120 glycoproteins within a six months period. The vaccine had no effect on viral setpoints in the subjects infected despite vaccination, probably due to the fact that the vaccine induced only gp120-specific CD4 T cells (in 33% of the vaccinees), but almost no gag-specific CD4 T cells (in 1% of vaccines) and no HIV-1-specific CD8 T cells (measured by intracellular cytokine staining ICS). In contrast, almost every vaccinee developed high titer antibodies, although these antibodies only had a weak to moderate capacity to neutralize various HIV-1 strains. The mechanisms of the protective effect of the vaccine are still unresolved and it has been hypothesized that antibody-dependent cellular cytotoxicity (ADCC) may have played a role.

Another efficacy trial, the HVTN 505 study, started enrolment in 2009. This study is testing a prime boost vaccination regimen using DNA and a rAd5 vector containing env/gag/pol/nef.

A promising approach for the development of more effective HIV-1 vaccines is the therapeutic immunization of HIV-1-infected patients on ART who then undergo a treatment interruption (Harrer 2005). The analysis of a vaccine’s ability to control HIV-1 replication during treatment interruption may be a good instrument in identifying vaccines that are also effective in prevention.


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Filed under 4. Preventive HIV-1 Vaccine, Part 1 - The Basics

Acute HIV-1 Infection

– Hendrik Streeck and Marcus Altfeld –


Within days of HIV-1 acquisition, a transient symptomatic illness associated with high levels of HIV-1 replication and rapid loss of CD4+ T cells occurs. This highly dynamic phase of the infection is accompanied by clinical symptoms similar to mononucleosis. However, despite an estimate of 7,000 new HIV-1 transmissions per day(UNAIDS 2010 Global Report), the diagnosis is missed in the majority of cases. Most commonly other viral illnesses (i.e., flu) are often assumed to be the cause of the symptoms, and there are no HIV-1-specific antibodies detectable at this early stage of infection. The diagnosis therefore requires a high degree of awareness and clinical knowledge based on clinical symptoms and history of exposure, in addition to specific laboratory tests (detection of HIV-1 RNA or p24 antigen and negative HIV-1 antibodies) confirming the diagnosis.

An accurate diagnosis of HIV-1 infection during this early stage of infection is particularly important as about 50% of new sexual transmissions are estimated to happen while a person is in this primary phase of infection (Brenner 2007). Indeed, phylogenetic analyses demonstrate a clustering of infections during primary HIV-1 infection, and the catalytic effect of acute HIV-1 infection on the HIV pandemic could be prevented or at least slowed by early diagnosis and immediate antiretroviral therapy intervention (see below).

Definition and classification

Acute HIV-1 infection (AHI) is defined by high levels of plasma HIV-1 RNA in the presence of a negative anti-HIV-1 ELISA and/or negative/indeterminant Western Blot (<3 bands positive) documenting the evolving humoral immune response; whereas early HIV-1 infection (EHI) includes anyone with documentation of being HIV-1–antibody negative in the preceding 6 months and is therefore broader than the definition of acute HIV-1 infection. Both are included in the term primary HIV-1 infection (PHI) (see Figure 1). Recently, a more detailed classification system of the early phases of HIV infection has been implemented (Fiebig 2003), which has rather little relevance for clinical decisions but is more important for scientific purposes.  The definition used influences the methods needed to make the diagnosis and any considerations regarding the pathogenic implications of this stage of disease. Acute HIV-1 infection is often associated with an acute “retroviral syndrome” that usually includes fever with a variety of nonspecific clinical and laboratory abnormalities. In contrast, subjects with early HIV-1 infection can be asymptomatic. The time from exposure to symptomatic disease is typically 2 to 4 weeks, and the duration of illness is generally days to weeks. Identifying patients with this syndrome requires a thorough risk assessment, recognition of the variable clinical and laboratory manifestations, and understanding what tests need to be performed in order to make the diagnosis.


Signs and symptoms

Table 1: Main symptoms of acute HIV-1 infection (from Hecht 2002).
Symptom Frequency Odds ratio (95% CI)
Fever 80% 5.2 (2.3-11.7)
Rash 51% 4.8 (2.4-9.8)
Oral ulcers 37% 3.1 (1.5-6.6)
Arthralgia 54% 2.6 (1.3-5.1)
Pharyngitis 44% 2.6 (1.3-5.1)
Loss of appetite 54% 2.5 (1.2-4.8)
Weight loss >2.5 kg 32% 2.8 (1.3-6.0)
Malaise 68% 2.2 (1.1-4.5)
Myalgia 49% 2.1 (1.1-4.2)
Fever and rash 46% 8.3 (3.6-19.3)

After an incubation period ranging from a few days to a few weeks after exposure to HIV, most infected individuals present with an acute flu-like illness. Acute HIV-1 infection is a very heterogeneous syndrome and individuals presenting with more severe symptoms during acute infection and a longer duration of the acute infection syndrome tend to progress morerapidly to AIDS (Vanhems 1998, Pedersen 1989, Keet 1993). The clinical symptoms of acute HIV-1 infection were first described in 1985 as an illness resembling infectious mononucleosis (Cooper 1985). Several non-specific signs and symptoms have been reported in association with acute infection. Fever in the range of 38 to 40ºC is almost always present; in addition lymphadenopathy concomitant with the emergence of a specific immune response to HIV occurs. A generalized rash is also common in symptomatic acute HIV infection. The eruption typically occurs 48 to 72 hours after the onset of fever and persists for five to eight days. The upper thorax, collar region, and face are most affected with well-circumscribed, red colored macules or maculopapules. In addition, painful mucocutaneous oral, vaginal, anal or penal ulcerations are one of the most distinctive manifestations of the syndrome. Further common symptoms (see Table 1) are arthralgia, pharyngitis, malaise, weight loss, aseptic meningitis and myalgia (Kahn 1998). Although none of these findings are specific, several features, combinations of symptoms and prolonged duration are suggestive of HIV. The highest sensitivity for a clinical diagnosis of acute HIV-1 infection are fever (80%) and malaise (68%), whereas weight loss (86%) and oral ulcers (85%) had the highest specificity (Hecht 2002). In this study, the symptoms of fever and rash (especially in combination), followed by oral ulcers and pharyngitis had the highest positive predictive value for diagnosis of acute HIV-1 infection. In another study, fever, rash, myalgia, arthralgia and night sweats were the best predictors of acute infection (Daar 2001).


Currently, four different HIV tests are commercially available, but they have limited sensitivity in detecting acute HIV-1 infection. In order to be able to correctly interpret a positive or negative result in the presence (or absence) of acute HIV infection symptoms and corresponding history, it is important to understand differences in the sensitivities of the available tests.

The 1st and 2nd generation EIA tests are able to detect HIV-1 infection with both high specificity and sensitivity, but only after HIV-1 seroconversion, as decent levels of anti-p24 IgG antibodies need to be present to give a positive result (see Figure 1). 3rd generation EIA tests can now detect IgM antibodies and therefore are able to detect a recent infection with HIV earlier than the 1st or 2nd generation tests (Hecht 2002). The newly developed 4th generation EIA test now combines the detection of p24 antigen and p24 antibodies and therefore is able to detect HIV infection prior to seroconversion (Ly 2007). However, although this test is able to detect HIV-1 infection much earlier than all previously developed tests, a second diagnostic false negative window can occur when equal levels of p24 antigen and anti-p24 antibody are present. The most substantiated diagnosis of acute HIV-1 infection is based on the detection of HIV-1 replication in the absence of HIV-1 antibodies  (pre-seroconversion). The most sensitive test is therefore based on detection of plasma HIV-1 RNA.

All assays for HIV-1 RNA that have been compared (branched chain DNA, PCR and GenProbe) have a sensitivity of 100%, but occasionally (in 2-5% of cases) lead to false positive results (Hecht 2002). False positive results from these tests are usually below 2,000 copies HIV-1 RNA per ml, and therefore are far below the high titers of viral load normally seen during acute HIV-1 infection (in our own studies subjects average 13 x 106 copies HIV-1 RNA/ml with a range of 0.25-95.5 x 106 copies HIV-1 RNA/ml). Repetition of the assay for HIV-1 RNA from the same sample with the same test led to a negative result in all false positive cases. Measurement of HIV-1 RNA from duplicate samples therefore results in a sensitivity of 100% with 100% specificity. In contrast, detection of p24 antigen has a sensitivity of only 79% with a specificity of 99.5-99.96%. The diagnosis of acute infection must be subsequently confirmed with a positive HIV-1 antibody test (seroconversion) within the following weeks.

During acute HIV-1 infection, there is frequently a marked decrease of the CD4 cell count, which later increases again, but usually does not normalize to initial levels. In contrast, the CD8 cell count rises initially, which may result in a CD4/CD8 ratio of <1. Infectious mononucleosis is the most important diagnosis to be aware of, but the differential diagnosis also includes cytomegalovirus, toxoplasmosis, rubella, syphilis, viral hepatitis, disseminated gonococcal infection, other viral infections and side effects of medications.

Figure 2: Algorithm for the diagnosis of acute HIV-1 infection.

In summary, the most important step in the diagnosis of acute HIV-1 infection is to keep it in mind during diagnosis. The clinical hypothesis of acute infection requires performance of an HIV-1 antibody test and possibly repeated testing of HIV-1 viral load, as shown in the algorithm in Figure 1 (adapted from Hecht 2002).

Immunological and virological events during acute HIV-1 infection

Transmission of HIV-1 generally results from viral exposure at mucosal surfaces followed by viral replication in submucosal and locoregional lymphoid tissues, and subsequently through overt systemic infection. The virus exponentially replicates in the absence of any detectable adaptive immune response, reaching levels of more than 100 million copies HIV-1 RNA/ml. It is during this initial cycle of viral replication that important pathogenic processes are thought to occur. These include the seeding of virus to a range of tissue reservoirs and the destruction of CD4+ T lymphocytes, in particular within the lymphoid tissues of the gut. Early on in infection, the very high levels of HIV-1 viremia are normally short-lived, indicating that the host is able to generate an immune response that can control viral replication. Over the following weeks, viremia declines by several orders of magnitude before reaching a viral setpoint. This setpoint following resolution of the acute infection is a strong predictor of long-term disease progression rates (Mellors 1995, 2007). It is therefore of critical importance to characterize and understand the immune responses induced in the initialstages of HIV-1 infectionas these first responses appear responsible for the initial control of viral replication.

In contrast to hepatitis B and C infection, acute-phase HIV replication is associated with the activationof a dramatic cytokine cascade, with plasma levels of some ofthe most rapidly induced innate cytokines peaking 7 days afterthe first detection of plasma viremia and many other cytokinesbeing upregulated as viral titers increase to their peak. Although some of the cytokines/chemokines produced inacute HIV infection may contribute to the control of viral replication,the exaggerated cytokine response likely also contributes tothe early immunopathology of the infection and associated long-termconsequences (Stacey 2009). Also, a specific activation and expansion of natural killer cells has been noted during the acute phase of infection (Alter 2007) as well as significant changes in the B cell compartment.

Several factors can influence viral replication during acute infection and the establishment of a viral setpoint. These include the fitness of the infecting virus, host genetic factors and host immune responses. While it has been shown that the transmitted/founder virus population has intact principal gene open reading frames and encodes replication-competent viruses (Salazar-Gonzalez 2009), the envelope (env) gene of elite controllers has been demonstrated to mediate less efficient entry than the envelope protein of chronic progressors (Troyer 2009). Interestingly, acute infection envs exhibit an intermediate phenotypic pattern not distinctly different from chronic progressor envs. These findings imply that lower env fitness may be established early and may directly contribute to viral suppression in elite controllers.

Antibodies against HIV-1 with neutralizing capacities are rarely detectable during primary HIV-1 infection and are therefore less likely to be major contributors to the initial control of viral replication. However, broadly neutralizing antibodies develop over time in a rare subset of HIV-infected individuals and the expression of specific markers on CD4 T cells is modestly associated with the development of these responses (Mikell 2011). In addition, several studies have demonstrated a crucial role of HIV-1-specific cellular immune responses for the initial control of viral replication. A massive, oligoclonal expansion of CD8 T cell responses has been described during acute HIV-1 infection (Pantaleo 1994), and the appearance of HIV-1-specific CD8 T cells has been temporally associated with the initial decline of viremia (Koup 1994, Borrow 1994). These CD8 T cells have the ability to eliminate HIV-1-infected cells directly by MHC class I-restricted cytolysis or indirectly by producing cytokines, chemokines or other soluble factors, thus curtailing the generation of new viral progeny (Yang 1997). The biological relevance of HIV-1-specific cytotoxic T cells (CTL) in acute HIV-1 infection was highlighted in recent in vivo studies demonstrating a dramatic rise of SIV viremia and an accelerated clinical disease progression in macaques after the artificial depletion of CD8 T cells (Schmitz 1999, Jin 1999). Additional evidence for the antiviral pressure of HIV-1-specific CTLs during primary HIV-1 infection was provided by the rapid selection of viral species with CTL epitope mutations that were detected within a few weeks of HIV-1 infection (Price 1997). A study assessing the impact of early HIV-1-specific CD8 T cell responses on the early viral set point in a cohort of over 420 subjects was able to demonstrate that the ability to mount a strong early CD8 T cell response during primary HIV-1 infection is moderately associated with a lower viral setpoint (Streeck 2009). Furthermore, the assessment of the CD8 T cell responses against autologous patient-virus-derived peptides in three subjects suggest that even more, yet undetectable, responses are present during the acute phase of the infection contributing up to 15% each to the initial control of viral replication (Goonetilleke 2009).

Many of the early immunodominant CD8 T cell responses have been shown to be restricted by HLA class I alleles, which have been previously associated with slower disease progression such as HLA-B57 or –B27. Moreover, these HLA-restricted responses preferentially target epitopes within a short highly conserved region of p24/Gag (Streeck 2007). This region encodes the HIV-1 capsid, which has been shown to be crucial for the stability of HIV-1 (Schneidewind 2007). The preservation of the early CD8 T cell responses has been associated with slower disease progression (Streeck 2009), which might be linked by the presence of HIV-1-specific CD4 T helper responses during the CTL priming process. During acute infection, the number of CD4 T cells decline, occasionally to levels that allow the development of opportunistic infections (Gupta 1993, Vento 1993). Even though the CD4 T cell count rebounds with the resolution of primary infection, it rarely returns to baseline levels in the absence of antiretroviral therapy. In addition to the decline in CD4 T cell counts, qualitative impairments of CD4 T cell function are perhaps the most characteristic abnormalities detected in HIV-1 infection. The impairment of HIV-1-specific CD4 T cell function occurs very early in acute infection (Rosenberg 1997, Lichterfeld 2004), potentially due to the preferential infection of virus-specific CD4 T cells by HIV (Douek 2002). This is followed by a functional impairment of CD4 T cell responses to other recall antigens, as well as a reduced responsiveness to novel antigens (Lange 2003). The impairment of HIV-1-specific CD4 T helper cell function in acute HIV-1 infection subsequently results in a functional impairment of HIV-1-specific CD8 T cells (Lichterfeld 2004). In addition, it has been recently shown that the induction of HIV-specific IL21+CD4+ T cells preserves and maintains the function of HIV-specific CD8 T cells (Chevalier 2011).

The contribution of the CD4 T helper response against HIV-1 is not well understood, butreports have suggested that HIV-1 can escape from CD4 T cell targeted epitopes during primary HIV-1 infection (Rychert 2009). However, as the antiviral action of CD4 T cells is most likely indirect through other effector cells, their exact contribution is difficult to measure. First studies in the lymphocytic choriomeningitis virus (LCMV-) mouse model have indicated that a strong and long-lived CD8 T cell memory response is dependent on the presence of a CD4 T cell response (Janssen 2003, Williams 2006). However, it is unknown which signals of the CD4 T cells are required for the generation and maintenance of effective HIV-1-specific CD8 T cell responses. Lack of CD4 T helper cells and chronic antigenic stimulation have been described to be the major cause of the functional deficits CD8 T cells undergo soon after the early phase of infection. This hierarchical loss of CD8 T cell function has been linked to the expression of inhibitory molecules on the cell surface of HIV-1-specific CD8 T cells such as PD-1 and several others (Day 2006, Trautmann, 2006, Blackburn 2009). The identification of such receptors might help in the generation of potential immune therapeutics to boost HIV-1-specific CD8 T cell function.

In addition to host immune responses, host genetic factors play an important role in both susceptibility and resistance to HIV-1 infection and speed of disease progression following infection. The most important of these is a deletion in the major co-receptor for entry of HIV-1 into CD4 T cells, a chemokine receptor called CCR5 (Samson 1996). Homozygotes for this 32 base pair deletion (CCR5Δ32) do not express the receptor at the cell surface and can only be infected with HIV strains that are able to use other co-receptors such as CXCR4. Thus, although CCR5delta32 homozygotic individuals show a significant degree of resistance to HIV-1 infection (Samson 1996), a number of cases of infection with CXCR4-using HIV-1 strains have been described (O’Brien 1997, Biti 1997). Heterozygotes for this deletion exhibit significantly lower viral setpoints and slower progression to AIDS. In addition to mutations in the chemokine receptor genes, a number of HLA class I alleles, including HLA-B27 and -B57, have been described to be associated with both lower viral setpoints and slower disease progression (O’Brien 2001, Kaslow 1996). Studies demonstrate that individuals expressing HLA-B57 present significantly less frequently with symptomatic acute HIV-1 infection and exhibit a better control of viral replication following acute infection (Altfeld 2003). A number of further polymorphisms have been identified that have a potential impact on HIV-1 disease progression. Here especially, the axis between detrimental immune activation and beneficial immune responses is largely unknown and part of ongoing research. For example, it has been demonstrated that polymorphisms in the IL-10 promotor region directly inhibit HIV replication, but may also promote viral persistence through the inactivation of effector immune function (Naicker 2009). These data demonstrate that host genetic factors can influence the clinical manifestations of acute HIV-1 infection and can have an important impact on the subsequent viral setpoint and the speed of disease progression.


The hypothesis of antiretroviral therapy during acute HIV-1 infection is to shorten the symptomatic viral illness, reduce the number of infected cells, preserve HIV-1-specific immune responses and possibly lower the viral setpoint in the long term. Several studies have sugggested that treatment of acute HIV-1 infection allows long-term viral suppression and might lead to a preservation and even increase of HIV-1-specific T helper cell responses.

Pilot studies in patients who are treated during acute HIV-1 infection and subsequently start treatment interruptions show that the HIV-1-specific immune response can be boosted (Rosenberg 2000, Vogel 2006, Grijsen 2011), and they experience at least temporal control of viral replication. However, other studies were not able to confirm this theoretic benefit (Markowitz 1999, Streeck 2006) and viral load rebounded during longer follow-up, requiring the eventual initiation of therapy. The theoretic benefits of early treatment must be balanced against the possible and known risks of prolonged antiretroviral therapy. These include a higher risk of accumulated long-term antiretroviral drug toxicities due to a considerable increase in the duration of antiretroviral exposure and the possibility of drug resistance if therapy fails to completely suppress viral replication.

The long-term clinical benefit of early initiation of therapy has not been demonstrated. It is also not known how long the period between acute infection and initiation of therapy can be without losing immunological, virological and clinical benefit. Thus, no recommendation for or against the initiation of antiretroviral therapy during primary HIV-1 infection can be given and needs to be decided case by case. In view of all these unanswered questions, it would be beneficial to design and enroll patients with acute HIV-1 infection in a controlled clinical trial. As a treatment option, a standard first-line treatment could be considered. It is important during counseling to clearly indicate the lack of definitive data on the clinical benefit of early initiation of antiretroviral therapy and to address the risks of antiretroviral therapy and treatment interruptions, including drug toxicity, development of resistance, acute retroviral syndrome during viral rebound and HIV-1 transmission and superinfection during treatment interruptions.


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