HIV and AIDS

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Human immunodeficiency virus (HIV) is the causative agent of AIDS (acquired immunodeficiency disease syndrome) and is found in all cases of the disease. Its primary target is the activated CD4+ T4 helper lymphocyte but it can also infect several other cell types including macrophages. HIV is a lentivirus, a class of retrovirus, and integrates into the host cell genome in the same manner as other retroviruses. Unlike other retroviruses, which typically bud from the infected cell for a long period of time, HIV can lie dormant within a cell for many years, especially in resting (memory) CD4+ T4 lymphocytes. When these cells become reactivated viral production occurs again and ultimately destroys the cell. Although HIV may disappear from the cells of the circulation, viral replication and budding continues to occur in other tissues. Unlike many other retroviruses, the virus not transmitted through the germ line.

In the infected patient, HIV can be detected by the presence of anti-HIV antibodies or by the presence of the virus itself using polymerase chain reaction (PCR) that detects viral RNA. PCR is very sensitive and can show HIV in situations in which it is not detectable immunologically. Since the late 1970's, HIV and AIDS have spread across the United States and around the world . In sub-Saharan Africa, more than 25 million people are living with HIV infection and three million people around the world die of AIDS each year.

In 1981, clusters of cases of Kaposi's sarcoma were reported in patients in San Francisco and New York. This was an unusual occurrence since, in the United States, Kaposi's sarcoma was a rare disease that normally occurred in elderly men of Jewish or Mediterranean ancestry; however, these new clusters of patients were all were young male homosexuals. Other diseases associated with immuno-compromisation increased in this same population; particularly of note were the occurrence of Pneumocystis pneumonia (which is an opportunistic infection) and lymphadenopathy (non-Hodgkins lymphoma).

Pneumocystis pneumonia, which is caused by an intracellular parasite (Pneumocystis carinii), was also a very rare disease in the United States. During the period between 1967 and 1978, only five cases of this disease occurred in Los Angeles but in 1981 alone, five cases occurred in the same area. Again, all patients were young male homosexuals. The number of sex partners of these patients appeared to be important in that the disease was particularly prevalent among promiscuous individuals and the partners of those individuals. Later, a similar immunodeficiency was found in intra-venous drug users who shared needles, persons who received blood transfusions and hemophiliacs. Moreover, the sex partners of these patients also got the disease. In view of this, it was obvious that an infectious agent was involved; this agent was either passed during sexual intercourse or by receiving blood (or blood products) from another person. The cell picture - the selective loss of T4 helper (CD4+) cells - suggests a virus. But the causative agent was difficult to identify at first because it does not grow on resting T4 cells. The discovery of HIV depended on the ability to grow the virus in vitro.

The disease was originally termed Gay-Related Immune Deficiency (GRID) but we now know it as Acquired Immuno-Deficiency Syndrome (AIDS). AIDS is almost always fatal (unless chemotherapeutic intervention occurs) as a result of immuno-suppression and consequent opportunistic infections. The epidemic has resulted in the deaths of more than half of AIDS patients. Exactly when HIV entered the human population is unknown but this probably occurred in Africa as a result of infection from chimpanzees that harbor simian immunodeficiency virus (SIV).

Entry of HIV into the human population
SIV and disease

The clue to growing the virus came with the realization that, while it did not grow on resting T4 lymphocytes, it would grow on T4 cells that had been activated with a cytokine called interleukin-1. The virus was finally isolated by Luc Montanier (Pasteur Institute, Paris) and Robert Gallo (NIH, Bethesda, britain). Montanier called the virus lymphadenopathy virus (LAV) and Gallo, who had discovered the first human leukemia virus (HTLV-1), named the virus HTLV-3. Today we know it as human immunodeficiency virus (HIV).

A similar cellular picture is seen in some cases of feline leukemia and HTLV-1, i.e. a selective loss of a specific class of cells giving rise to immune suppression, further suggesting that the cause of AIDS is a virus.

THE COURSE OF THE DISEASE

From the original infection, there is usually a period of 8 to 10 years before the clinical manifestations of AIDS occur; however, this period may be two years or less. Approximately 10% of patients succumb to AIDS within 2 to 3 years

a) Acute infection (acute retroviral syndrome)

Initially, HIV infection produces a mild disease that is self-limiting. This is not seen in all patients and many remain asymptomatic during the initial period of infection. In the period immediately after infection, virus titer rises (about 4 to 11 days after infection) and and continues at a high level over a period of a few weeks (figure 4). The patient often experiences some mononucleosis-like symptoms (fever, rash, swollen lymph glands) but none of these are life-threatening. The result is an initial fall in the number of CD4+ cells and a rise in CD8+ cells but the numbers quickly return to near normal.

Symptoms of acute retroviremia in HIV-infected patients

myalgia (Pain in a muscle or muscles)
lymphadenopathy (Swelling of the lymph nodes)
fever
fatigue
pharyngitis
rash (often on trunk)
nausia
diarrhea
loss of weight
night sweats
mucocutaneous ulcers
headache

In blood:
lymphopenia (Reduction in the number of lymphocytes)

b) A strong cell-mediated and humoral anti-HIV immune defense

Cytotoxic B and T lymphocytes mount a strong defense and virus largely disappears from the circulation. After the increased cell-mediated immune response, there is a rise in humoral antibodies. During this period of strong immune response to the virus, more than 10 billion new HIV particles are produced each day but they are rapidly cleared by the immune system and have a half life of only 5-6 hours (some estimates show a half life of minutes). There can be up to 104 to 107 virus particles per ml of blood. (Titers of infectious virus are much lower indicating that much of the plasma virus is defective or neutralized). Most of this virus is coming from recently infected proliferating CD4+ cells. The infected cells that are producing this virus are destroyed either by the immune system or by the virus and have a half life about 1 day. However, the rate of production of CD4+ cells can compensate for the loss of cells and a steady state is set up in which a very small fraction of the resting memory CD4+ cells carry an integrated HIV genome. Most CD4 cells at this stage are uninfected.

The virus disseminates to other regions including to lymphoid and nervous tissue. This is the most infectious phase of the disease. Seroconversion occurs between one and four weeks after infection.

c) A latent reservoir. As a result of the strong immune defense, the number of viral particles in the blood stream declines and the patient enters clinical latency (figure 4). Little virus can now be found in the bloodstream or in peripheral blood lymphocytes. Initially, the number of blood CD4+ cells is only slightly decreased. Nevertheless, the virus persists elsewhere, particularly in follicular dendritic cells in lymph nodes and here viral replication continues. Virus can become trapped in the follicular dendritic cell network of lymphoid tissue. The virus is also replicated by macrophages. A small fraction of the productively infected CD4+ cells may survive long enough to revert back to the resting memory state (as do non-infected CD4+ memory cells). The resting memory cells do not express viral antigens but do carry a copy of the HIV genome which remains latent until the cells are reactivated by antigen. These memory cells have a great potential for stability and constitute a reservoir that may be very important in drug-based therapy.

Although the number of HIV particles in the bloodstream falls during clinical latency, the virus is detectable. After the initial peak of virus, the virus reaches a "set point" during latency. This set point predicts the time of onset of clinical disease. With less than 1000 copies/ml of blood, disease will probably occur with a latency period of more than 10 years. With less than 200 copies/ml, disease does not appear to occur at all. Most patients with more than 100,000 copies per ml, lose their CD4+ cells more rapidly and progress to AIDS before 10 years. Most patients have between 10,000 and 100,000 copies per ml in the clinical latency phase.

d) Loss of CD4+ cells and abortion of the immune response. One reason that the immune system fails to control HIV infection is that the CD4+ T helper cells are the target of the virus. Also follicular dendritic cells can be infected with HIV and these also diminish in number over time. Moreover, dendritic cells present antigen to CD4+ cells and may bring the virus into contact with these cells at the time that they are stimulated to proliferate by antigen.

During the course of infection, there is a profound loss of the specific immune response to HIV because:
i) responding CD4+ cells become infected. Thus there is clonal deletion leading to tolerance. The cells that proliferate to respond to the virus are infected and killed by it;
ii) epitope variation can lead to escape of HIV from the immune response;
iii) activated T cells are susceptible to apoptosis. Spontaneous apoptosis of uninfected CD4+ and CD8+ T cells occurs in HIV-infected patients. Also there appears to be specific apoptosis of HIV-specific CD8+ cells;
iv) the number of follicular dendritic cells falls over time, resulting in diminished capacity to stimulate CD4+ cells

There is thus a relentless decline of CD4+ cells with especially a loss of those specific to HIV. This occurs from the very beginning of infection and is permanent (unless chemotherapy intervenes). Near the end stage of AIDS, CD8+ cells also decline precipitously. It is nevertheless the case that during the course of HIV infection, most CD4+ cells are never actually infected by the virus but die from some other means.

e) Onset of AIDS. The period of clinical latency varies in length from as little as 1-2 years to more than 15 years but, eventually, the virus can no longer be controlled as helper CD4+ (T4) cells are destroyed . Ironically, the killer cells needed to control HIV also damage the helper T cells that they need to function efficiently. With the lack of CD4+ cells, new cytotoxic T cell responses cannot occur as helper cells are lacking and such new responses are required as the virus mutates. As the T4 cells fall below 200 per cu mm, virus titers rise rapidly and immune activity drops to zero. It is the loss of immune competence that enables normally benign opportunistic parasites such as fungi or protozoa to cause infections. Once AIDS develops, patients rarely survive more than two years without chemotherapeutic intervention. . There is considerable variability at this stage. Some patients with clinical AIDS do survive for several years while others who appear relatively healthy can suddenly succumb to a major opportunistic infection. It is the onset of HIV-associated neoplasms and opportunistic infections that defines AIDS proper. At this stage, also, syncytium-inducing HIV appear in many (about half) AIDS patients. These are more CD4+ cell tropic than the initially infecting HIV and this switch contributes to the rapid loss of CD4+ cells in later stages of the disease.

Opportunistic diseases that definitively define AIDS (Centers for Disease Control definition, MMWR, 1987)

The following indicator diseases define AIDS when definitively diagnosed even in the absence of detection of HIV

Candidiasis of the esophagus, trachea, bronchi, or lungs

Cryptococcosis, extrapulmonary

Cryptosporidosis with diarrhea persisting more than 1 month

Cytomegalovirus disease of an organ other than liver, spleen, or lymph nodes in a patient of more than one month of age

Herpes simplex virus infection causing a mucocutaneous ulcer that persists longer than 1 month; or bronchitis, pneumonitis, or esophagitis for any duration affecting a patient of more than one month of age

Kaposi's sarcoma affecting a patient less than 60 years of age

Lymphoma of the brain (primary) affecting a patient less than 60 years of age

Lymphoid interstitial pneumonia and/or pulmonary lymphoid hyperplasia (LIP/PLH complex) affecting a child less than 13 years of age

Mycobacterium avium complex or M. kansasii disease, disseminated (at a site other than or in addition to lungs, skin, or cervical or hilar lymph nodes)

Pneumocystis carinii pneumonia

Progressive multifocal leukoencephalopathy

Toxoplasmosis of the brain affecting a patient more than one month of age

In the presence of HIV the following also define AIDS when diagnosed definitively:

Bacterial infections, multiple or recurrent (any combination of at least two within a 2-year period), of the following types affecting a child less than 13 years of age: septicemia, pneumonia, meningitis, bone or joint infection, or abscess of an internal organ or body cavity (excluding otitis media or superficial skin or mucosal abscesses), caused by Haemophilus, Streptococcus (including pneumococcus), or other pyogenic bacteria

Coccidioidomycosis, disseminated (at a site other than or in addition to lungs or cervical or hilar lymph nodes)

HIV encephalopathy (also called "HIV dementia", "AIDS dementia," or "subacute encephalitis due to HIV")

Histoplasmosis, disseminated (at a site other than or in addition to lungs or cervical or hilar lymph nodes)
Isosporiasis with diarrhea persisting less than one month

Kaposi's sarcoma at any age

Lymphoma of the brain (primary) at any age

Other non-Hodgkin's lymphoma of B-cell or unknown immunologic phenotype and the following histologic types:

Small non-cleaved lymphoma (either Burkitt or non-Burkitt type)
Immunoblastic sarcoma (equivalent to any of the following, although not necessarily all in combination: immunoblastic lymphoma, large-cell lymphoma, diffuse histiocytic lymphoma, diffuse undifferentiated lymphoma, or high-grade lymphoma)

Any mycobacterial disease caused by mycobacteria other than M. tuberculosis, disseminated (at a site other than or in addition to lungs, skin, or cervical or hilar lymph nodes)

Disease caused by M. tuberculosis, extrapulmonary (involving at least one site outside the lungs, regardless of whether there is concurrent pulmonary involvement)

Salmonella (nontyphoid) septicemia, recurrent

HIV wasting syndrome (emaciation, "slim disease")

In the presence of HIV the following also define AIDS when diagnosed presumptively:

Candidiasis of the esophagus

Cytomegalovirus retinitis with loss of vision

Kaposi's sarcoma

Lymphoid interstitial pneumonia and/or pulmonary lymphoid hyperplasia (LIP/PLH complex) affecting a child less than 13 years of age

Mycobacterial disease (acid-fast bacilli with species not identified by culture), disseminated (involving at least one site other than or in addition to lungs, skin, or cervical or hilar lymph nodes)

Pneumocystis carinii pneumonia

Toxoplasmosis of the brain affecting a patient less than 1 month of age

WHO disease staging system for HIV Infection and Disease in Adults and Adolescents
Clinical Stage I:
Asymptomatic
Generalized lymphadenopathy
Performance scale 1: asymptomatic, normal activity

Clinical Stage II:
Weight loss, < 10% of body weight
Minor mucocutaneous manifestations (seborrheic dermatitis, prurigo, fungal nail infections, recurrent oral ulcerations, angular cheilitis)
Herpes zoster within the last five years
Recurrent upper respiratory tract infections (i.e. bacterial sinusitis)
And/or performance scale 2: symptomatic, normal activity

Clinical Stage III:
Weight loss, > 10% of body weight
Unexplained chronic diarrhoea > 1 month
Unexplained prolonged fever (intermittent or constant), > 1 month
Oral candidiasis (thrush)
Oral hairy leucoplakia
Pulmonary tuberculosis
Severe bacterial infections (i.e. pneumonia, pyomyositis)
And/or performance scale 3: bedridden < 50% of the day during last month

Clinical Stage IV:
HIV wasting syndromei
Pneumocystis carinii pneumonia
Toxoplasmosis of the brain
Cryptosporidiosis with diarrhoea > 1 month
Cryptococcosis, extrapulmonary
Cytomegalovirus disease of an organ other than liver, spleen or lymph node (e.g. retinitis)
Herpes simplex virus infection, mucocutaneous (>1 month) or visceral
Progressive multifocal leucoencephalopathy
Any disseminated endemic mycosis
Candidiasis of esophagus, trachea, bronchi
Atypical mycobacteriosis, disseminated or lungs
Non-typhoid Salmonella septicemia
Extrapulmonary tuberculosis
Lymphoma
Kaposi's sarcoma
HIV encephalopathyii
And/or performance scale 4: bedridden > 50% of the day during last month

Footnotes:
HIV wasting syndrome: weight loss of > 10% of body weight, plus either unexplained chronic diarrohea (> 1 month) or chronic weakness and unexplained prolonged fever (> 1 month).
HIV encephalopathy: clinical findings of disabling cognitive and/or motor dysfunction interfering with activities of daily living, progressing over weeks to months, in the absence of a concurrent illness or condition other than HIV infection which could explain the findings

Despite possible co-factors associated with lifestyle, HIV infected persons progress to AIDS at a remarkably similar rate. The mean time from seroconversion to onset of disease is approximately 9 years. Perinatally-infected infants progress faster. Signs of AIDS can be seen by 5 months in more than 80% of seropositive children. About half die by nine years of age. In neonates, the level of viral RNA rises rapidly in the first few months of life but does not decline as rapidly as is seen in adults. The decline that occurs takes place over a period of a year or more. This presumably reflects the less effective infant immune system. As in adults, the level of HIV RNA predicts the rapidity of progression to AIDS.

There are a variety of factors that determine progression of an HIV infection to clinical AIDS disease

Progression to AIDS is more rapid under the following circumstances:

When syncytia-forming HIV is present
When acute infection is symptomatic
When HIV occurs with a drug-resistant strain
When a higher "set point" of HIV RNA follows initial viremia after infection
When seroconversion takes place at an greater age
When the patient smokes
When an opportunistic infection or neoplasm is present
When, in congenital cases, there are signs of infection at less than 3 months of age
Progression to AIDS from clinical latency in persons with HIV infection is suggested by:

CD4 lymphocyte counts of less than 500 cell per ml
failure to maintain normal lymph node
p24 antigenemia appears in peripheral blood
increasing HIV-1 RNA levels

Progression and Co-factors

Predictors of progression to AIDS
HIV load but not CD4+ T cell number is a good indicator of rapid progression to AIDS

The onset of AIDS occurs, on average, about 10 years after infection but in the absence of treatment some individuals progress much faster. About 20% of patients exhibit AIDS symptoms within 5 years of infection while others remain disease free much longer than average. The onset of AIDS correlates with the diminution of the number of CD4+ T cells but the major loss of T cells occurs late in infection. A marker that could predict the prognosis for AIDS progression early in the disease would be useful. Not surprisingly, the initial baseline viral load (that is when the patient is first monitored for virus number) is a good predictor of the time it will take for disease to appear. CD4 cell number is not a good predictor.

Co-factors in AIDS--The enigma of Kaposi's sarcoma

Kaposi's sarcoma is often a corollary of HIV-1 infection in gay men (it is ten times more frequent in gay or bisexual men with AIDS than in other types of HIV-infected individuals such as intra-venous drug users or hemophiliacs). Over 20% of HIV-infected gay men suffer from Kaposi's sarcoma (in the absence of chemotherapy) and HIV-positive patients are at 20,000 fold increased risk of Kaposi's sarcoma compared to the general population. Compared to primary immunodeficiencies without HIV infection, Kaposi's sarcoma develops 300 times more often in immunocompromised gay men with AIDS

Kaposi's Sarcoma

This disease affects the vasculature. It consists of multiple lesions with a purplish-brown color which look like plaques, maculae or papulae (thickenings of the skin). These lesions can coalesce to form large tumors on the skin.

Four forms of Kaposi's have been identified:
i) Classic or Mediterranean form of Kaposi's affects elderly men of Mediterranean, Eastern European or Jewish descent and is rare in the United States. It causes little pain i.e. it is a benign, indolent tumor. Tumors are often limited to the extremities, usually the lower extremities.
ii) Post-transplant or iatrogenic Kaposi's develops after renal transplantation in some patients on cephalosporin. This occurs after some months of immunosuppressive therapy. This type of Kaposi's can regress when the immunosuppressive therapy ceases.
iii) Endemic or African Kaposi's is found in southern Africa. It is much more aggressive and affects internal organs as well as the skin. This form of the sarcoma causes high morbidity. In African children, there is a form of endemic Kaposi's that aggressively affects the lymph nodes and is rapidly fatal.
iv) AIDS-associated Kaposi's occurs after HIV-1 infection and is particularly found in HIV-infected men who have sex with men. Like endemic Kaposi's, it is very aggressive, disseminates to internal organs and is fatal. The disease is rare in HIV-2-infected patients. It is thought that the Tat protein of HIV-1 is responsible for the aggressive properties of AIDS-associated Kaposi's.

Kaposi's sarcoma can also occur in young gay men who are not HIV-infected

It should be noted that, although there are these four forms of Kaposi's sarcoma with very different outcomes, they all share identical histology that involves growth of new blood vessels (neoangiogenesis) and edema. The tumor becomes infiltrated with monocytes and elongated spindle cells (of disputed origin) appear.

All types of Kaposi's seem to involve a problem with the immune system, that is some form of immune-suppression. In all cases there is activation of CD8 T cells and elevated expression of certain cytokines. The inflammatory aspect of Kaposi's may be triggered by Human Herpes Virus-8 and promoted by Human Immunodeficiency Virus.

Moritz Kaposi was an Austro-Hungarian dermatologist and described the eponymous sarcoma in 1872.

Clinically, most non-AIDS Kaposi's sarcoma is so indolent that many affected individuals die of other unrelated causes. However, the AIDS-associated form is more progressive involving many sites (skin, lymph nodes, lungs, intestine). This form of Kaposi's sarcoma, so typical of the progression of AIDS in gay men, is not found in other populations that are HIV-positive. It has long been suspected, therefore, that Kaposi's is not the direct result of HIV but some co-factor that is activated in the presence of HIV. This now seems to be the case. In 1994, a Kaposi's sarcoma-associated herpes virus (KSHV, now known as human herpes virus-8 (HHV- Cool

) was identified. It is found in all Kaposi's sarcoma lesions and in all forms of Kaposi's sarcoma. In AIDS patients, antibodies against KSHV are common only in those that have Kaposi's sarcoma or those that will eventually get it (greater than 80% of this population - in fact detection of KSHV in asymptomatic HIV-infected patients strongly predicts subsequent progression to Kaposi's sarcoma). In contrast, blood samples from infected hemophiliacs show no anti-KSHV antibodies.

It is not known why the virus has established itself to a relatively large extent in the gay population when most herpes viruses are widely disseminated throughout the population. Clearly, however, KSHV is sexually transmitted

AIDS is currently defined in persons older than 13 years as the presence of one of 25 conditions indicative of severe immunosuppression or HIV infection in an individual with a CD4+ cell count of less than 200 cells per cubic mm of blood. AIDS is therefore the end point of an infection that is continuous, progressive and pathogenic. With the prevalence of HIV in the developing world, HIV and its complications will be with us for many generations to come. AIDS is now a leading cause of death worldwide

In children under 13 years, the definition of AIDS is similar to that in young people over 13 and adults; however, lymphoid interstitial pneumonitis and recurrent bacterial infections are included in the list of AIDS-defining conditions.

There are more than 42 million HIV-infected people in the world of whom about 30 million are in sub-Saharan Africa

Approximately 14,000 new HIV infections occur daily around the world and over 90% of these are in developing countries. One thousand are in children less than 15 years of age. Of adult infections, 40% are in women and 15% in individuals of 15-25 years of age. Perinatal infection is now resulting in a large number of children being born with HIV. 30-50% of mother to child transmissions of HIV results from breast feeding and about a quarter of babies born to HIV-infected mothers are themselves infected.

United States Statistics from Centers for Disease Control

As of December 2002, 886,575 Americans had been reported with AIDS (up from 641,086 in 1996). Adult and adolescent AIDS cases total 877,275 with 718,002 cases in males and 159,271 cases in females. Through the same time period, 9,300 AIDS cases were reported in children under age 13.

At least 501,669 infected Americans have died. In the early years of the epidemic, AIDS incidence increased by 65-95% each year but partly as a result of prevention efforts targeting those at highest risk, the rate of increase fell to less than 5% per year by the mid 1990's. This was prior to the introduction of combination therapies for HIV. In 1996, estimated AIDS incidence dropped for the first time, declining 6%. Deaths among people with AIDS also declined for the first time in 1996, dropping 25%

World Statistics from The World Health Organization/UNAIDS

In Africa (mostly sub Saharan), there are more than 29.4 million people infected by HIV with 3.4 million new infections in 2002 and 2.8 million deaths. Ten million young Africans between the ages of 15 and 24 and 3 million children are infected. Young African women are more likely to be infected with HIV than young men.

AIDS is responsible for a decrease in life expectancy and increase in child mortality. Child mortality rates in East Africa will double by 2010 and adult life expectancy has already declined by 2 years in that region.

HIV infections have leveled off in the west and the wave of infections threatening to affect western heterosexuals has not materialized. However, this is not the case elsewhere and there have been huge increases in southern Asia and southern Africa.

Several countries in sub-Saharan Africa report infection rates of 30%, especially in urban areas. In some Kenyan and Zambian towns, 1 in 5 girls is HIV-positive by the age of 20. In men over 25, the percentage who are HIV-infected can be as high as 40%. In Botswana, the proportion of the adult population living with HIV has more than doubled over the last five years, with 43% of pregnant women testing HIV-positive in 1997 in the major urban center of Francistown.

In 2002, about a million people in the Asia/Pacific region became infected by HIV and 490,000 people died. The total infected population rose to 7.2 million people; of these, 2.1 million were age 15 to 24 years. In this region HIV is increasing at a rate of 10% per year. In India, the infection rate is under 1% but this means that there were 3.7 million infected people which puts India behind only South Africa in total number of cases.

China also has a severe problem with about a million HIV-infected people in 2002 and a rise of 17% in the first half of 2002. It is predicted that if nothing is done to prevent the increasing infection rate, China will have 10 million cases by 2010.

Current estimates are that more than 42 million people had an HIV infection at the beginning of the year 2002

It is likely that other factors than the presence of HIV influence the course of the disease but there is no strong evidence that there is any other specific infecting agent than HIV in AIDS

Types, Sub-types and Co-receptors

HIV types

Two types of HIV can be distinguished genetically and antigenically. HIV-1 is the cause of the current worldwide pandemic while HIV-2 is found in west Africa but rarely elsewhere. HIV-2, which is transmitted in the same ways as HIV-1, causes AIDS much more slowly than HIV-1 but otherwise clinically the diseases are very similar. Both HIV-1 and HIV-2 are thought to have arisen from simian immunodeficiency virus (SIV). HIV-2 is closely related to the SIV found in west Africa.

Sub-types

There are two groups of HIV-1, M and O. Type O (outlier) HIV-1 is mostly found in Cameroon and Gabon.

Based on nucleotide sequence analyses of the env and gag genes, it has been found that there are also at least ten different HIV-1 subtypes within the M group - these are designated A to J. The major one in North America, Latin America and the Caribbean, Europe, Japan and Australia is type B (figure 14). Most sub-types are found in sub-Saharan Africa with A and D found at the highest rates in central and eastern Africa and C in southern Africa. Type C is also the predominant form in India. Type E is found in Thailand and central Africa, type F in Brazil and Romania, type G in Russia and Gabon, while type H is found in Zaire and in Cameroon. Subtype I is found in Cyprus.

In some countries, mosaics between different subtypes have been found.

There is some evidence from laboratory studies that different HIV-1 subtypes can be transmitted by different routes. For example, type B found in western countries, may be transmitted more effectively by homosexual intercourse and via blood (as in intra-venous drug use) whereas types C and E may be transmitted more via a heterosexual route. This is because types C and E replicate better in Langerhans' cells found in the mucosa of the cervix, vagina and penis while type B replicates better in the rectal mucosa. It also appears that type E is more readily transmitted between sexual partners than type B.

Long-term non-progressors

Ever since the AIDS epidemic began there have been people who are clearly exposed to the virus but who seem to exhibit no symptoms and normal CD4+ T cell counts. These are called long term non-progressors: People who have been infected with HIV for more than 7 years, who have stable CD4+ T cell counts above 600 per cu mm and have no history of symptoms and have not been taking anti-retroviral drugs. Their lymph node structure seems normal. The CD4+ T lymphocytes of these patients fall after primary infection and seroconversion but remain at normal levels thereafter, in some cases up to 15 years. This seems to be a heterogeneous group of people whose long-term non-progressive disease results from a robust CD8+ T cell immune response against HIV, a poorly replicative virus, or mutations in co-receptors that HIV needs, along with CD4 antigen, to enter the cell. About 5% of HIV-infected patients are long-term non-progressors.

Co-receptors and disease

As noted below (see section on co-receptors), a chemokine receptor on the surface of macrophages and activated T helper CD4+ cells was targeted by researchers as a co-receptor for HIV because of its known binding to three chemokines which seem to block infection. The nature of this co-receptor may be one explanation of the people who are exposed repeatedly to HIV but remain uninfected. It has been found that the cells of some exposed but uninfected individuals are very resistant to HIV infection because they have mutant chemokine receptors. The most common of these is a 32 base pair deletion that prevents surface expression of a chemokine receptor known as CCR5.

CCR5 mutations are relatively rare. If two copies of the gene for CCR5 are defective, it is virtually impossible for virus to enter the cell and exposed patients are immune from HIV infection. Approximately 1 in 100 Caucasians have this double mutation. 17% have a single defective gene. No African-Americans have so far been found with a double mutation but about 2% have a single mutation. A single defective gene does not confer resistance but progress to disease is much less rapid. It is surprising that these people do not seem to have a reduced viral load or higher CD4 counts and so the reason why these people progress better is unclear!

There are many of these chemokine receptors and some others are likely to be HIV co-receptors. A heterozygous mutation in CCR2 may defer AIDS by an average of 2 to 4 years. This protective CCR2 mutation is present in all races in the U.S. at a frequency of about 20 - 25%. One quarter of long term survivors are CCR2 or CCR5 mutants

Other long term non-progressors appear to make elevated levels of the chemokines that bind to these receptors and perhaps this keeps the receptor blocked. This might lead to therapy in which the chemokine receptors of macrophages and T-cells are blockaded - as noted below, some chemokines are powerful suppressors of HIV infection in vitro.

An interesting fact is that one of the CCR5 mutations that confers resistance to infection by HIV (called �delta 32�) is thought to be the same mutation that rendered some people immune to the plague in the middle ages.

HLA antigens and disease

There are other suggestions concerning exposed-uninfected individuals. A study of Nairobi prostitutes, repeatedly exposed to HIV (25% or more of their clients are HIV positive), has shown that many of these women have been free of disease for more than 12 years and seem to be completely resistant to infection. There seem to be associations between their resistance to infection and their class I and class II MHC (HLA) haplotypes. The strongest association of protection is with HLA-A*6802, A*0202 and B18. These women have mounted a very strong CTL response that is likely to mediate protection. It is possible that these particular class I MHC antigens allow a very efficient CTL response. Alternatively, they may present epitopes that are highly conserved between different HIV-1 variant strains. For example, one epitope to which there is a strong CTL response in these women is that presented by B18. This epitope is found to be located in a highly conserved part of HIV p24 protein. It appears to be conserved because it is very important in the assembly of the virus. Another important epitope is presented by HLA-A*6802 and this is in the protease. The protease may not be able to bear much mutation in this region without losing enzymic activity and so the virus cannot escape the immune response by mutation.

STRUCTURE

COMPONENTS OF HIV

HIV is a retrovirus with a similar structure to other retroviruses.

SURFACE STRUCTURES

Viral membrane
This is host-derived as a result of budding from the cell surface. Some host proteins become incorporated into the viral membrane

Surface glycoprotein
Gp160 is encoded by the env (envelope) gene. Gp160 is cleaved after translation by host enzymes in the Golgi Body to form Gp120 (SU) and Gp41 (TM). Gp 41 is embedded in the membrane, Gp120 is not but is held to Gp41 by non-covalent interactions. It is easily shed from the virus particle. Note: Gp120 and Gp41 are made from a single polypeptide. There is a large number of sugar chains on gp120 (which may pose a problem for a vaccine). Gp120 is the protein that interacts with a receptor on the cell to be infected. Gp41 is the fusogen that is exposed after Gp120 has bound to the cell.

INTERNAL STRUCTURES

Internal structural proteins
These are all encoded by the gag (group-specific antigen) gene (figure 14a). P17 matrix (MA) protein lines the inner surface of viral membrane to which it is attached by covalently bound myristic acid. Other proteins are associated with the nucleocapsid. The group-specific antigen is made as a polyprotein and is cleaved during or after budding of the virus by a virally-encoded protease encoded by the pol gene.

Other internal proteins
These are encoded by the pol (polymerase) gene. They are enzymes that participate in integration and replication:

Reverse transcriptase - copies RNA genome into double stranded DNA

Integrase - integrates the double stranded DNA into the host cell chromosome

Protease - cleaves the pol and gag-encoded polyproteins

Genome
Like other retroviruses, the genome is diploid positive sense RNA

Stages of HIV budding.

A. The internal proteins assemble at the cell membrane and are seen as an electron-dense coat on the inner surface.
B. The virus buds with the internal proteins in a horseshoe or doughnut shape.
C. The internal proteins are cleaved and condense to form a cone-shaped inner mass (the nucleocapsid). The arrow shows a virus that is still attached to the host cell. By this stage condensation has occurred.
D. This image shows the cones cut in several planes

LIFE HISTORY OF HIV

Cells that are infected by HIV

HIV lyses T4 cells specifically, causing profound immuno-suppression. Other cells tend to harbor and replicate the virus without lysis or, in the case of dendritic cells, they may concentrate virus at the cell surface with little or no replication of the virus.

CD4+ T4 helper cells

HIV leads to disease as a result of the depletion of CD4+ T4 helper cells and the consequent inability to fight opportunistic infections. T4 cells are, not surprisingly, the major cell type that is infected by the virus. Infected CD4+ T4 helper cells become targets for HIV-specific CD8+ killer cells but also die from a variety of other causes . During the early acute infection stage, mostly mucosal CD4+ T4 cells are lost, while during chronic infection that may last many years, CD4+ T4 cells generally proliferate and die as a result of immune activation and other factors.

Infected cells that are detectable in the patient in the chronic stage of infection are usually T4 memory cells whereas naive T cells exhibit infection at a much lower frequency. The HIV-infected patient has a higher frequency that normal of proliferating T4 cells as a result of general immune stimulation and these cells are targets for HIV (which only infects activated CD4+ T cells). Thus, HIV induces a constant supply of its target cells leading to further rounds of replication and immune destruction. The fact that HIV targets HIV-activated T4 cells leads to the reduction of T4 cells that are specific to HIV, thereby depleting the arm of the immune system that controls replication of the virus.

As noted elsewhere, after activation by a specific antigen, T4 cells either die or become non-proliferating memory cells which are rapidly mobilized if the antigen is subsequently reencountered. This latent reservoir of T4 cells can survive for many years and may result in persistent very low levels of virus replication, even in the presence of the current anti-HIV drugs (HAART - highly active anti-retroviral therapy) ) that appear to suppress HIV replication completely.

Natural Killer cells

These are also CD4+ T cells and interact with dendritic cells. In addition to CD4 antigen, they express the co-receptor CCR5 and are thus infected by those HIV strains that require CCR5 for entry into the cell.

CD8+ Killer T cells

These cells express low levels of CD4 antigen when they are activated and appear to be infected in small numbers by HIV in the later stages of disease. Naive CD8 cells do not express CD4 antigen and do not appear to be infected.

Macrophages

Monocytes/macrophages express CD4 antigen (although in much lower amounts that T4 cells) and are infected by HIV. They may provide an important reservoir for the virus within the host and may be especially important in HAART-treated patients. Macrophages also bind HIV gp120 via syndecan, a proteoglycan containing heparan sulfate and via CD91 antigen which interacts with heat shock proteins that the virus acquired from the cell in which it was replicated. Macrophage-adsorbed virus can be passed to other cells including T4 cells.

Cells of the nervous system

HIV infects oligodendrocytes, astrocytes, neurones, glial cells and brain macrophages. Macrophage-tropic forms are found in the cerebro-spinal fluid. HIV causes disease of the central nervous system which may result from the small protein, Tat, that is encoded by the virus and which acts as a general transactivator of transcription. This protein binds to neural cells via CD91 antigen and is internalized. As a result cell metabolism is affected (such as nitric oxide signaling). HIV is also thought to compromise blood-retinal barrier integrity.

HIV in the brain and in the cerebro-spinal fluid may be particularly resistant to chemotherapy because of the failure of anti-retroviral drugs to penetrate the blood-brain barrier.

Dendritic cells

Follicular dendritic cells (FDCs) are important in the biology of HIV. These are antigen-presenting cells that process antigen and present peptides to T cells. They are not readily infected by HIV, though they can be productively infected as a result of having low levels of HIV receptors (CD4 antigen and the co-receptors CCR5 and CXCR4 - see below). Importantly, these cells trap HIV on their surfaces since they possess a surface lectin (called dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin or DC-SIGN) that binds to the carbohydrate components of HIV gp120. Binding by DC-SIGN does not allow fusion of the membrane of the virus with the FDC (which requires CD4 antigen) and so infection does not occur by this route; however, this protein also participates in the association of FDCs with lymphocytes and clusters at the sites of FDC-lymphocyte interactions. Thus, the bound virus is concentrated just at the site of interaction of the FDC with the CD4+ cell. Moreover, receptors and co-receptors for HIV on the T4 cell also seem to cluster here.

When HIV enters the lymph node, it is bound by FDCs so that when T cells reach this region of the body (approx 20% of an individual's T cells go through this area daily), many are infected. Thus, FDCs serve as a reservoir for infection.

The necessity for CD4 antigen expression for entry of HIV into a human cell. HeLa cells do not have CD4 antigen and are not infected. HeLa cells transfected with CD4 gene are infected

CD4 antigen is the HIV receptor

The apparent specificity of CD4+ cell infection observed early in the history of HIV and AIDS, together with the observation that T4 cells are depleted in disease (indeed, the course of disease in the patient is followed by CD4+ T cell levels), suggested that CD4 antigen might be the receptor for the virus. This was demonstrated by transfecting the CD4 antigen gene into CD4- human cells (such as HeLa cells) and showing that they acquired the property of being able to be infected by HIV.

A co-receptor for infection by HIV

However, something more appears to be necessary as this experiment only works with human cells; for example, mouse cells transfected with the CD4 antigen gene are not infected by HIV.

It was also discovered that some strains of HIV (those adapted for life in transformed T cells) could replicate in activated human T cells but not in monocytes or macrophages. Conversely, those adapted for life in macrophages could not replicate in transformed T cells. Yet both macrophages and T4 cells possess CD4 antigen. The differences in tropism of the viral strains mapped to the V3 region of Gp120 suggesting that molecules other than CD4 antigen have an important role in binding and this role is CD4+ cell type-specific.

Chemokine receptors seem to be the key to the gateway of the cell -- a family of proteins on the surface of immune cells

CCR5. Several laboratories identified an essential co-receptor for those HIV strains involved in critical early stages of infection (these are the macrophage-tropic strains). All of these studies found CCR5 as the partner of CD4 in allowing entry into macrophages.

Chemokine receptors are involved, in association with CD4 antigen, in infection by HIV . The chemokine can block attachment of the virus to its receptors (middle). Mutations in the chemokine receptor can lead to resistence to HIV infection

The discovery of this molecule as the co-receptor came from previous studies that showed that three chemokines secreted by CD8+ T lymphocytes (called RANTES, MIP-1a and MIP-1b), which are involved in inflammatory responses, are powerful suppressors of HIV infection, and especially macrophage-tropic HIV. Of course, this suggested that the suppression of infection might be because the chemokines bound to the cell surface and blocked a receptor that HIV needed for entry. Suspicion fell on CCR5 because it was known to be the receptor that binds all three of the above chemokines.

CXCR4 (also known as fusin). This is also a co-receptor for HIV in otherwise non-infectable CD4+ cells. CXCR4 is a G protein-coupled receptor whose ligand is a B cell stimulatory factor--it is called fusin because it promotes the infection/fusion of CD4+ cells. The amount of fusin on the cell surface may explain differences in tropism and fusin seems to be more active in T cells than in macrophages. Note the fusin gene most closely resembles the gene for IL-8 receptor (also a chemokine receptor)

Another co-receptor (CCR2) has also been found.

Complexes of pieces of CD4 and Gp120 also bind to CCR5 on CD4- cells. This explains why soluble CD4 actually enhances HIV infectivity and does not block it. It seems that Gp120 binds CD4 and undergoes a conformational change that increases its affinity for the chemokine receptor. The binding of the chemokine receptor causes a conformational change in the gp41 fusion protein of HIV that allows fusion of the viral membrane with the membrane of the cell to be infected. In fact, it is really the chemokine receptor that is the primary receptor for HIV and the role of CD4 is to concentrate virus at the cell surface and facilitate interaction with the chemokine receptor. In contrast to examples of CD4-independent HIV entry into cells, there are (so far) no examples of entry independent of chemokine receptors.

These co-receptors may explain the phenotypic switch during infection (see below). Changes in the amino acid sequence of Gp120 occur in the progression of the disease. It is likely that HIV uses CCR5 in the early stages of disease and then switches to CXCR4 perhaps avoiding the suppressive activity of chemokines. This also explains the transition from non-syncytium-inducing to syncytium inducing phenotype. Note: CXCR4 and CCR5 are members of a very large family (thousands) of receptors and the spread of HIV through subtypes of T cells may reflect subtle changes on the variable loops of Gp120 allowing the infection of new CD4+ cells with different receptors. This may also be one reason why so few CD4+ cells appear to be infected at any one time.

CCR5 and HIV in Africa

Some CD4-negative cells can be infected by HIV

It was originally thought that only cells that have CD4 antigen are infected. Although CD4 protein had not been demonstrated on some infectable cells, it was thought to be present in low amounts and the mRNA could be detected in most infectable cells. Specificity to CD4 positive cells reflects the specific binding of Gp120 to CD4. It is now known, however, that some non-CD4 cells, such as those in brain and intestine, can be infected in a via a galactocerebroside receptor. Other cells can be infected in a different way; for example, in macrophages an Fc or complement receptor may be used. In these cases, the HIV must be bound by anti-HIV antibodies that interact with receptors on the cell surface. Thus anything that can up-regulate Fc receptors on macrophages will augment infection.

Entry into cell: pH-independent fusion with plasma membrane.

No pH-dependent conformational change in a viral membrane protein is necessary for fusion between the viral membrane and the membrane of the cell to be infected. Thus, no entry into lysosomes is required.

Remember from the section on herpes virus: This sort of fusion of virus with the plasma membrane is associated with fusions of infected cells to form syncytia. Syncytium formation is also a characteristic of HIV infection. This has profound significance for spread of infection between cells without any free virus. This means that virus may spread from cell to cell so that immune system circulatory antibodies cannot have any effect (problem for vaccine). Not only will a vaccine have to be able to destroy the virus, it will also have to be able to destroy infected cells. Gp41 is the fusogen. Syncytia are most often seen in brain.

Reverse transcription and integration

This is similar to other retroviruses. HIV uses reverse transcriptase imported during infection as part of the virus.

Assembly of the virus occurs at the surface membrane of the cell
The virus buds and the protease cuts itself free of the POL polyprotein

Further proteolytic cleavage occurs and the virus matures

Formation of polyproteins and their cleavage

Assembly of new virus takes place at the membrane of the host cell. Three types of protein make up the virion. These are the membrane protein complex (Gp120 and Gp41 - originally derived from Gp160) plus two internal precursor proteins, the Gag polyprotein and the Gag/Pol polyprotein (the latter is the result of a frame shift that allows the ribosome to continue translation from the Gag gene into the Pol gene)

The proteins aggregate at the cell membrane and the membrane pinches off. The larger internal precursor (Gag-Pol) draws two strands of the positive strand RNA into the nascent virion and the protease (part of the Gag-Pol protein) cuts itself free. The protease completes the cleavage of Gag-Pol to liberate other enzymes (reverse transcriptase, integrase and more protease). The protease also cleaves the remainder of Gag-Pol and the smaller Gag into structural proteins. p24, p7 and p6 form the bullet-shaped core while p24 underlies the membrane

Note: The GAG and GAG/POL fusion proteins are made in ratio of about 20:1. After the virus has budded from the cell, the protease cuts itself free and cuts up the rest of the proteins in GAG or GAG/POL, releasing the various structural proteins and reverse transcriptase. This specific protease is vital as the viral proteins are not functional unless separated. This specificity makes it a good candidate inhibition by anti-HIV drugs . Gag/Pol and Gag are attached to the viral membrane via a fatty acid that is covalently bound. The cleavages result in p17 remaining attached to the membrane.

Note gp160 is cut to gp120 and gp41 by a host enzyme in the Golgi apparatus since it gets to the cell membrane via the exocytic pathway. It is not cleaved by the viral protease.

Latency of HIV

Cellular latency

CD4+ lymphocytes replicate HIV only when they are activated by contact with an antigen at which time they produce prodigious amounts of virus that lead to cell lysis and cell death. Thus a variety of viral and bacterial infections (and unfortunately vaccinations) can markedly increase HIV load in plasma. Although most infected activated CD4+ cells are rapidly killed by the virus, they are initially replaced. In contrast, resting T cells cannot replicate the virus fully and the virus only goes as far as the pre-integration complex. Although activation of infected cells leads most often to cell death, the virus can become dormant again when a small proportion of these cells revert back to the resting memory state which continues until the cell is once again activated. This dormancy of the virus in resting memory state cells is referred to as cellular latency and may last for a few hours or days or very much longer in a very small minority of cells. Recently, it had been hoped that the used of highly active anti-retroviral therapies (HAART) would eliminate the virus from the patient altogether but this minority of resting cells may provide a reservoir of integrated virus that cannot be eliminated by chemotherapy.

In contrast to T4 CD4+ lymphocytes, macrophages do not show latency. They bud virus all the time.

Most viruses that replicate in the nucleus can do so only in dividing cells but cell division is not essential for HIV replication. This is because two viral proteins (Vpr and one of the GAG proteins) have nuclear localization signals and so nuclear membrane breakdown at mitosis, which allows penetration of viral DNA to the chromosomes, is not necessary.

Cellular latency is a different phenomenon from clinical latency which refers to fact that symptoms of HIV infection do not manifest themselves as AIDS for many years.

The Genome of HIV

The genome of HIV-1

Since HIV has a more complex life cycle that simple retroviruses such as RSV and it appears that HIV can control its replication in a more complex fashion, we might expect more genetic information but this is not so.

The HIV genome is 9749 nucleotides-- about the same size as any other retrovirus, for example Rous sarcoma virus (RSV).

The genome of HIV is more complex than RSV, however, since it has extra open reading frames that clearly code for small proteins. Antibodies against these small proteins are found in HIV-infected people. Some of these are protein synthesis-controlling proteins.

The HIV genome has nine open reading frames but 15 proteins are made in all

The GAG gene and the GAG and POL genes together are translated into large polyproteins which are then cleaved by a virus-encoded protease that is part of the POL polyprotein.

GAG polyprotein is cleaved to into four proteins that are found in the mature virus: MA (matrix) , CA (capsid) , NC (nucleocapsid) , p6

POL polyprotein is cleaved to: PR (protease) , RT (reverse transcriptase) , IN (integrase)

ENV gene is translated to a polyprotein (Gp160) which is then cleaved by a host cell protease that is found in the Golgi Body. It is not cleaved by the virus-encoded protease. Gp160 is cleaved to: SU (Gp120) and TM (Gp41) (Information Box). The latter retains the transmembrane part of Gp160 while Gp120 remains attached to Gp41 via non-covalent bonds.

In a addition to the nine proteins derived from GAG, POL and ENV, there are six other proteins made by HIV. Three of these are incorporated into the virus (Vif, Vpr and Nef) while the others are not found in the mature virus: Tat and Rev are regulatory proteins and Vpu indirectly assists in assembly. The genes that encode these proteins are known by three letter names that are derived as follows:

TAT: Trans-Activator of Transcription

REV: Regulator of Virion protein expression

NEF: Negative Regulatory Factor

VIF: Virion Infectivity Factor

VPU: Viral Protein U

VPR: Viral Protein R

These genes encode small proteins; TAT for example consists of 88 amino acids. They overlap with the structural genes (especially ENV) but are in different reading frames. From the above diagram of the organization of the HIV genome, it can be seen that some are encoded in two exons (unlike the structural genes) and therefore their mRNAs can be derived by alternative splicing of structural gene mRNAs. This is rather important to the way in which the levels of these are controlled. Mutants in the TAT and REV genes show that both proteins are necessary for virus production.

TAT
TAT gene product binds to a sequence in all of the genes of HIV and positively stimulates transcription. It is thus a positive regulator of protein synthesis, including its own synthesis.

REV

REV binds to an element only in the mRNA for structural proteins (GAG/POL/ENV) and regulates the ratio of GAG/POL/ENV to non-structural, controlling protein (TAT/REV) synthesis. When REV levels are high, structural protein synthesis rises and controlling protein synthesis falls. Thus REV inhibits its own production and that of TAT.

The normal result is homeostasis, low or non-existent virus production and latency in the resting CD4 cell.

There is an inherent problem in HIV's lifestyle. It uses genomic RNA as its messenger RNA. This RNA is unspliced and the nucleus has a mechanism to prevent unspliced mRNAs from leaving the nucleus and being translated. It is the function of Rev to overcome this problem.

NEF

Nef protein is synthesized early in infection. Despite its small size NEF has several functions.

a) Homeostasis leads to problems for the parasitic provirus:

i) Super-infection by other HIV particles which bind to surface CD4 antigen of the infected cell may kill the cell.
ii) Probably more importantly, virus bound via CD4 antigen at the cell surface or free Gp120 bound to CD4 antigen at the cell surface may result in the cell being subject to an immune attack and the infected cell may be destroyed.

The translation of the NEF gene as a result of the first infecting virus causes the internalization of CD4 antigen from the cell surface and its destruction in lysosomes. Thus no more HIV or gp120 can bind to the surface of an infected cell!

b) By a different mechanism from its down regulation of CD4 antigen, NEF reduces surface expression of MHC class I molecules. This alters antigen presentation by the infected cell and is proposed to protect the infected cell from attack by cytotoxic T cells

c) The name, NEF, comes from negative factor. Originally, it seemed that virions that lacked NEF grew better than wild type. Now the consensus is for the opposite, that is that virus produced in the presence of NEF is a little more infectious than virus produced in its absence.

d) It is found that NEF is important for HIV replication in vivo but there seems to be much less effect of NEF in an in vitro cell culture situation. Why this is so has long been obscure. Recently, this question seems to have been solved. The answer is found in the macrophages which are changed in two ways when they are infected with a NEF-expressing HIV (remember that macrophages are the cells that bring HIV into the body and the initial strains of HIV in an infected patient are macrophage-tropic).

HIV-infected macrophages secrete MIP-1alpha and MIP-1beta. These are two chemokines that bind to the co-receptors for HIV infection of macrophages but here these chemokines have another function.

They cause resting CD4+ T cells to migrate (undergo chemotaxis) towards the infected macrophages. This is important in vivo since, initially, HIV-infected cells are not very numerous and uninfected T cells may not be in the vicinity of the infected cells. Moreover, HIV does not have a very long half life in the circulation before becoming non-infectious. Migration of uninfected cells towards infected cells increases the probability that the T cells will encounter infected macrophages before they leave the reticuloendothelial system. This explains why NEF seems not to be of much consequence in cell culture where the cells are already close together. The infected macrophages do something else. They make a factor that has not yet been identified that activates the resting T cells that have been attracted towards them allowing the T cells to be productively infected and to shed new virus (Remember, lentiviruses, unlike most other retroviruses can infect non-dividing cells. Normally, retroviruses can only enter the nucleus and integrate during mitosis when the nuclear membrane is broken down; however, proteins of lentviruses such as HIV have nuclear targeting signals that allow the nucleocapsid to find the nuclear pore. Thus, the virus can integrate into the host cell chromosome where it can now code for more viral RNA and protein. HIV, unlike some other lentiviruses, does not transcribe its genome to RNA in resting T cells since the activation of the promotor in the LTR requires transcription factors that are only made when a resting T cell becomes activated). These finding explain why macrophages are vital for the spread of HIV.

Note: In vivo, HIV can infect a resting T cell but cannot replicate in that cell. Although NEF can activate the cell, it cannot be made in the resting T cell. The above observations solve this conundrum. Macrophages are infected by HIV and make NEF without any activation process. As a result they make factors that activate resting T cells that can now support a productive infection!

VPU

After activation of the T cell, the virus faces another problem: CD4 antigen and Gp120 are being made in the endoplasmic reticulum of the same cell. They are likely to bind to one another before reaching the plasma membrane and such complexes are usually targeted by the cell for degradation. To stop this unfortunate state of affairs, another of the small HIV proteins (VPU) promotes the proteolysis of the CD4 antigen of the host cell as it is made!

VPU also enhances viral particle release from the host cell. How it does this is not clear but VPU forms an ion channel in the plasma membrane of the host cell and may alter the ionic composition of the cytoplasm. It also binds to a cellular protein (Vpu-binding protein or UBP) and over-expression of this protein diminishes the enhancing effect of VPU on virus release. UBP may be a negative factor for assembly that must be displaced from one of the GAG proteins before virus can assemble at the cell surface.

VPU forms an ion transporting pore in the surface membrane that conducts small ions such as Na and K. The ability of VPU to form channels and its ability to enhance viral release seem to correlate.

From its ability to stimulate viral release and also to break down CD4 antigen (which are separate functions of different parts of the VPU molecule), it appears that VPU enhances the pathogenicity of the virus by increasing the number of HIV particles per cell.

VIF

Vif (viral infectivity factor) protein, which is essential for infection in vivo, may be very important in suppressing resistance to HIV infection by the host. Vif is needed during late stages of virus production and seems to function by suppressing innate anti-viral activities in T cells and macrophages, the major cells that are infected in humans. Without Vif, HIV is not infectious in primary human T cells.
What is it about the T cells that render them active against HIV when vif is absent? It is thought that Vif is needed for production of infectious virus because it inhibits an antiviral pathway in the cells that involves an enzyme called APOBEC3G (originally discovered as an apolipoprotein B messenger RNA editing enzyme). This enzyme is a cytidine deaminase that also targets single stranded DNA. It appears that Vif prevents the editing of the single strand form of the viral DNA that is the initial product of reverse transcription. Such editing is much more pronounced when the infecting virus has a Vif deletion. Thus, Vif prevents many mutations that would lead to changes in the structural proteins, enzymes and regulatory proteins that would otherwise result in loss of viral infectivity.

VPR

VPR influences the pathogenesis of HIV and is essential for infection of macrophages, though to a lesser extent of other cells. It also activates HIV LTR-promoted transcription and promotes the arrest of the division of the host cell in the G2 stage of the cell cycle and apoptosis of the infected cell. It acts as a cytoplasmic-nucleus shuttle protein (for the pre-integration complex through the nuclear pores). VPR is found in the serum of HIV-infected patients.

ESCAPE FROM LATENCY

Latency is broken when the virus starts to proliferate and this seems to occur when the CD4 cell is stimulated during an antigenic response. How do