The HIV Virus

Human Immunodeficiency Virus (HIV) is an enveloped human retrovirus of the lentivirus family. Two strains of HIV have been described, HIV-1 and HIV-2, the latter found mostly in Western Africa. HIV-1 is the more virulent strain.

The viral core contains two identical single strands of genomic RNA and three enzymes: integrase, protease and reverse transcriptase. The envelope, which is derived from the host cell membrane displays viral glycoproteins, including gp120 and gp41, which are critical for infection.

1.  gp120 has high affinity for CD4, therefore cells expressing CD4, such as CD4+ T cells, and macrophages/monocytes and dendritic cells which also express low levels of surfaces CD4, are potential targets for the virus.

After binding to the CD4, gp120 must also bind a coreceptor. Two chemokine receptors can fulfil this role depending on the tropism of the HIV. This tropism is determined by the variant of the gp120 molecule. Macrophage–tropic HIV uses CCR5 and requires only a low level of CD4 on the host cell. CCR5 is expressed by macrophage, dendritic cells and CD4+ T cells. Lymphotropic HIV uses CXCR4 found on T cells and requires a high density of CD4 on the cell surface. Both coreceptors are G-coupled proteins with seven transmembrane spanning domains. CCR5 normally binds RANTES, MIP-1a and MIP-1B. CXCR4 binds stromal derived factor-1.

CCR5 seems to be the major coreceptor for establishing primary infection since individuals with mutations in CCR5 appear to be at least partially protected. Development of drugs directed at chemokine receptors is thus an active area of research. It is suggested that if an individual is first infected by sexual contact with a macrophage tropic variant, the virus will be established in the mucosal associated lymphoid tissue where macrophage and dendritic cells will then provide a reservoir. Exposure to antigen promotes viral replication, a switch to the lymphotropic form and further rapid dissemination in the body. The tropism of the virus changes within the infected individual.

2.  Following binding of gp120 to CD4 and its coreceptor, gp41 allows fusion of the viral envelope and cell membrane with viral entry.

3.  In the host cell, viral RNA is replicated to a cDNA copy by its enzyme, reverse transcriptase.

4.  cDNA then enters the nucleus where it is integrated into the host genome as a provirus with the help of the viral enzyme, integrase. The virus may remain in this relatively latent form for years.

5.  The HIV genome has a long terminal repeat region (LTR) at each end. The LTR is required for viral integration and has binding sites for regulatory proteins. When the T cell is activated by antigen, a cascade of reactions results in an increase in NF-kB transcription factor. NF-kB binds to a promoter region in LTR activating transcription of the provirus by host RNA polymerase.

The long mRNA transcript is spliced at alternative sites for protein synthesis. The first two proteins made are Tat and Rev. Tat returns to the nucleus, acting as a transcription factor itself, binding to LTR and increasing the rate of viral transcription. Rev also acts in the nucleus, binding to the Rev responsive element in the viral mRNA transcript and increasing RNA transport rate to the cytoplasm. When the mRNA is transported more rapidly, less splicing occurs in the nucleus. Thus, the second wave of proteins made are structural components of the viral core and envelope. In the third wave, unspliced RNA serves as the RNA for the new viral particles and for the translation of gag and pol. Pol codes for the viral protease, which cleaves the product of env to produce gp120 and gp41, reverse transcriptase, and integrase.

Release of virus from CD4+ T cells frequently results in lysis of the cell, whereas the macrophage serves as a reservoir, transporting virus to other parts of the body (lymphoid tissue and central nervous system) and producing a small number of particles without cytopathic consequences.

Azidothymidine (AZT), also called Zidovudin, is an inhibitor of reverse transcriptase and was the first promising drug used in HIV infection. Protease inhibitors form a second class of agents in use. HIV is capable of an amazing rate of spontaneous mutation during the course of an infection in a single individual. This is as a result of lack of fidelity of the reverse transcriptase and RNA polymerase. In consequence, drug resistance develops rapidly.

Clinical Course

1.  Acute Infection

Upon initial infection with HIV, many patients are asymptomatic. Others show a flulike illness, characterized by fever, sore throat, and general malaise starting 2 to 4 weeks after infection and lasting 1 to 2 weeks. During this time there is viremia (virus in peripheral blood) and a precipitous drop in circulating CD4+ T cells. The immune system responds by generating cytotoxic T cells and antibodies specific for the virus. the CTL are partially responsible for the drop in CD4+ T cells, killing virally infected cells. At this point the patient has seroconverted, expressing detectable antibody to HIV proteins. The number of CD4+ T cells in the peripheral blood then recovers.

2.  Chronic Latent Phase

Although the immune response seems standard for a viral infection, it merely contains rather than eradicates the virus. The extremely high rate of mutations may explain the ineffectiveness of the immune response. A latent phase is established which may last as long as 15 years.

During this relatively asymptomatic period, a low level of viral replication continues, associated with a gradual decline in CD4+ T cell number. HIV, therefore, never has a truly latent phase. The number of virally infected T cells in the peripheral blood is extremely low during this phase. The lymph nodes are the predominant location of infected cells. Macrophages act as reservoir. Follicular dendritic cells function not only as reservoir but also to present virus to T and B cells, resulting in the intense follicular (germinal center) hyperplasia and lymphadenopathy typical of this phase.

The T cells are undergoing a slow rate of lysis, which eventually results in involution of the lymph node. This T cell death seems to be the result of a combination of factors. First, production of virus in the cells themselves causes lysis. Second, the infected cell seems to be more susceptible to apoptosis. Third, CTLs kill some of the infected cells. Finally, uninfected T cells may be killed in an ADCC like mechanism due to binding of soluble gp120 and anti-gp120 antibody to CD4 molecules expressed on their surface.

Patients have traditionally been followed during this phase by their peripheral CD4+ T cell counts and CD4/CD8 cell ratio. The ratio, which is normally approximately 2, is reversed with CD8 cells outnumbering CD4 cells. This can be seen in other viral infections; however, in those cases the reversal is often due to an increase in CD8+ cells. In HIV, CD4+ T cells are diminished. As these cells reach progressively lower values, the patient becomes symptomatic, entering the final phase, AIDS.

3.  Crisis Phase

What initiates this symptomatic, or crisis phase appears to be several factors acting concurrently. The gradual drop in CD4+ T cells eventually results in an immunodeficient state, leaving the individual susceptible to opportunistic infections, as is seen in patients with primary immunodeficiency and in immunosuppressed transplant patients. Activation of virally infected T cells by antigen results in stimulation of viral transcription and pyrogeny formation. This leads to accelerated T cell death, exacerbating the immunodeficient state. Rapid viral replication also increases the mutation rate, allowing escape from any immune controls that might remain.

The patterns of infection and malignancies may partially reflect the risk factors for a particular patient. This is suggested by differences seen between AIDS patients and other immunosuppressed individuals and among AIDS patients with different modes of exposures. Thus, some individuals have not only been infected with HIV but also with other sexually transmitted diseases. Human papillomavirus (HPV) is associate with the development of cervical cancer in women. Exposure to HPV, combined with the individual's immunodeficient state, may be responsible for a markedly increased incidence of invasive cervical cancer in HIV+ women. The CDC has now included invasive cervical cancer in the AIDS-associated malignancies.

The aggressive form of Kaposi's sarcoma (KS) is virtually unique to AIDS patients, particularly male homosexuals, in whom it may occur early in the course of the disease. This abnormal proliferation of small blood vessels normally presents as a slow-growing tumor on the lower extremity skin of elderly men. Human Herpes Virus 8 (HHV-8) has been identified in KS from AIDS patients. Whether these vessels grown in response to the virus, or the virus is directly oncogenic is unknown. This virus is also associated with an unusual form of aggressive lymphoma seen in AIDS patients, primary effusion lymphoma. Again, this malignancy is more common in male homosexual AIDS patients, and some have both the lymphoma and KS concurrently.

Aggressive B-cell lymphomas, mostly EBV-associated, are seen at an incidence similar to that seen in immunosuppressed transplant patients. These lymphomas are usually Burkitt's lymphoma or diffuse large B cell and are often found outside the lymph nodes (extranodal). In AIDS, the CNS is a frequent site of primary lymphoma.

The infectious diseases in AIDS reflect the patient's markedly depressed cell mediated immune system. As in any T cell-immunodeficient patient, Pneumocystis carinii pneumonia (PCP) is a major infectious complication. Candidiasis is seen relatively early in AIDS. In addition, the B cells are chronically stimulated, resulting in polyclonal hypergammaglobulinemia (elevated serum immunoglobulins), circulating immune complexes, and markedly increased plasma cell production. Despite this apparent B-cell activity, patients are unable to mount an effective antibody response to new antigens, perhaps due to the T-cell defect, however they also have difficulty with encapsulated organisms. Normally not a human pathogen, Mycobacterium avian can cause overwhelming infection.

The CNS is infected with HIV presumably via transport by macrophage. The virus infects microglia cells, which are of the macrophage lineage, oligodendrocytes, and astrocytes. The CNS is susceptible to infection by cryptococcus, toxoplasmosis and CMV. In addition, AIDS-related dementia and progressive encephalopathy has been frequently documented. In total, up to 50% of AIDS patients show CNS symptoms and over 70% have CNS changes at autopsy.

Prevention and Control

HIV is best accomplished by avoiding unprotected contact with blood and body fluids from infected individuals. Education and public awareness of both what to avoid and what is safe (casual contact) is required to control the disease and contain possible panic.

HIV+ pregnant women are placed on AZT to decrease viral load and thereby diminish risk of placental virus transfer. Caesarean section is performed to eliminate infection during passage through the birth canal. finally, exposure to breast milk is avoided.

Immediately following accidental exposure to infected products, AZT is administered to prevent establishment of infection.

Diagnosis

HIV infection is generally by detection of antibodies by ELISA and confirmed by Western blot analysis. Patients have been monitored by following their absolute CD4+ T-cell count in the peripheral blood. Correlations have shown that opportunistic infections generally are not seen with CD4 counts above 500/ul. The CDC has designated CD4 counts below 200/ul as an indicator of full-blown AIDS.

Therapy with AZT has been used for individuals whose CD4 counts were decreasing below the 500 marks. Currently, individuals who are HIV-positive, but asymptomatic, are placed on triple-agent antiviral therapy with inhibitors directed against reverse transcriptase, protease and nucleoside. This therapy prevents infection of new cells; however, infected cells remain until lysed. Patients are now monitored for viral load by quantitative analysis of viral RNA following PCR. The fall in viral titers is rapid and dramatic, but a small baseline titer almost always remains. Discontinuation of the drugs results in a resurgence of virus as one would expect. Unfortunately, mutations allow escape from control by these agents, thus the push to develop an extensive armory of drugs. The therapy is not without side effects, particularly suppression of hematopoietic cells. Additional time is needed to see how successful this approach will be.

HIV presents a serious challenge to vaccine development. The ability of the virus to escape eradication despite both an antibody and cytotoxic T-cell response in recently infected individuals is evident. both the virus' ability to hide out and its high mutation rate need to be overcome. The question of which arm of the immune system (cell mediated immunity or antibody production) is desirable has not been answered.

The greatest hope would be to have the immune response ready before exposure, as is done in most other vaccines. However, animal models are limited, the best being in the simian monkey, which develops a similar disease after infection with simian immunodeficiency virus (SIV). Human testing is fraught with many ethical difficulties. Understanding the molecular biology and structure of all viral components will be essential for development of a safe vaccine.