HIV primarily infects cells with CD4 cell-surface receptor molecules, using them to gain entry. Many cell types share common epitopes with this protein, though CD4 lymphocytes play a crucial role. In macrophages and in some other cells lacking CD4 receptors, such as fibroblasts, an Fc receptor site or complement receptor site may be used instead for entry of HIV. Cells of the mononuclear phagocyte system, principally blood monocytes and tissue macrophages, T lymphocytes, B lymphocytes, natural killer (NK) lymphocytes, dendritic cells (Langerhans cells of epithelia and follicular dendritic cells in lymph nodes), hematopoietic stem cells, endothelial cells, microglial cells in brain, and gastrointestinal epithelial cells are the primary targets of HIV infection.
HIV Structure and Function
The mature virus consists of a bar-shaped electron dense core containing the viral genome--two short strands of ribonucleic acid (RNA) about 9200 nucleotide bases long--along with the enzymes reverse transcriptase, protease, ribonuclease, and integrase, all encased in an outer lipid envelope with 72 surface projections containing an antigen, gp120, that aids in the binding of the virus to the target cells with CD4 receptors. By electron microscopy, the plasma membrane of an infected CD4+ lymphocyte exhibits budding virus particles approximately 90 to 100 n in diameter. The genome of HIV, similar to retroviruses in general, contains three major genes--gag, pol, and env.
The major structural components coded by env include the envelope glycoproteins, including the outer envelope glycoprotein gp120 and transmembrane glycoprotein gp41 derived from glycoprotein precursor gp160. Major components coded by the gag gene include core nucleocapsid proteins p55, p40, p24 (capsid, or "core" antigen), p17 (matrix), and p7 (nucleocapsid); the important proteins coded by pol are the enzyme proteins p66 and p51 (reverse transcriptase), p11 (protease), and p32 (integrase). Although most of the major HIV viral proteins, which include p24 (core antigen) and gp41 (envelope antigen), are highly immunogenic, the antibody responses vary according to the virus load and the immune competence of the host. The antigenicity of these various components provides a means for detection of antibody, the basis for most HIV testing.
HIV has the additional ability to mutate easily, in large part due to the error rate of the reverse transcriptase enzyme, which introduces a mutation approximately once per 2000 incorporated nucleotides. This high mutation rate leads to the emergence of HIV variants within the infected person's cells that can resist immune attack, are more cytotoxic, can generate syncytia more readily, or can resist drug therapy. Over time, different tissues of the body may harbor differing HIV variants.
HIV-2
A second HIV designated HIV-2 has been isolated. Most cases have appeared in West Africa and have appeared only sporadically in other parts of the world. The genetic sequences of HIV-1 and HIV-2 are only partially homologous. HIV-2, or other as yet uncharacterized members of the HIV-group of viruses, will not necessarily be detected by using the various laboratory tests for HIV-1 antibody. HIV-2 is genetically more closely related to simian immunodeficiency virus (SIV) than HIV-1.
The transmission of HIV-2 is similar to that for HIV-1, though perinatal transmission is much less frequent. HIV-2 infection has a longer latent period before the appearance of AIDS, a less aggressive course of AIDS, and a lower viral load with higher CD4 lymphocyte counts than HIV-1 infection until late in the course of the disease when clinical AIDS is apparent. This may explain the limited spread of HIV-2, both in West African countries and elsewhere, due to less efficient transmission, particularly via heterosexual and perinatal modes. The mortality rate from HIV-2 infection is only two-thirds that for HIV-1.
Establishment of HIV Infection
After entering the body, the viral particle is attracted to a cell with the appropriate CD4 receptor molecules where it attaches by fusion to a susceptible cell membrane or by endocytosis and then enters the cell. The probability of infection is a function of both the number of infective HIV virions in the body fluid which contacts the host as well as the number of cells available at the site of contact that have appropriate CD4 receptors.
Within the cell, the viral particle uncoats from the envelope to releases its RNA. The enzyme product of the pol gene, reverse transcriptase that is bound to the HIV RNA, provides for reverse transcription of RNA to proviral DNA. It is this HIV proviral DNA which is then inserted into host cell genomic DNA by the integrase enzyme. Once the HIV proviral DNA is within the infected cell's genome, it cannot be eliminated or destroyed except by destroying the cell itself. The HIV provirus is then replicated by the host cell. The infected cell can then release virions by surface budding, or infected cells can undergo lysis with release of new HIV virions which can then infect additional cells. Antibodies formed against HIV are not protective, and a viremic state can persist despite the presence of even high antibody titers.
Dynamics of HIV Infection
After initial entry of HIV and establishment of infection, replication may at first occur within inflammatory cells at the site of infection or within peripheral blood mononuclear cells, but then the major site of replication quickly shifts to lymphoid tissues of the body, including those in lymph nodes, spleen, liver, and bone marrow. Besides lymph nodes, the gut associated lymphoid tissue provides a substantial reservoir for HIV.
Macrophages and Langerhans cells in epithelia such as in the genital tract are important both as reservoirs and vectors for the spread of HIV in the body. Langerhans cells (a subset of blood dendritic cells) act as antigen presenting cells for CD4 lymphocytes. Both macrophages and Langerhans cells can be HIV-infected but are not destroyed themselves. HIV can then be carried elsewhere in the body. Within lymph nodes, HIV virions are trapped in the processes of follicular dendritic cells (FDC's), where they may infect CD4 lymphocytes that are percolating through the node. The FDC's themselves become infected, but are not destroyed.
Viral replication is stimulated by a variety of cytokines such as interleukins and tumor necrosis factor which activate CD4 lymphocytes and make them more susceptible to HIV infection. Primary HIV infection is followed by a burst of viremia in which virus is easily detected in peripheral blood in mononuclear cells and plasma. In the period of clinical latency of HIV infection, there is little detectable virus in peripheral blood, but viral replication actively continues in lymphoid tissues.
Subsets of the CD4+ lymphocyte population are important in determining the host response to infection. The subset known as TH1 (T helper 1) is responsible for directing a cytotoxic CD8 lymphocyte (CTL) response, but the TH2 (T helper 2) subset of CD4 and CD8 T-lymphocytes diminishes the CTL response while increasing antibody production. HIV-infectons accompanied by a dominant TH1 response tend to proceed longer. The switch from a TH1 to a TH2 response has been suggested as a factor in the development of AIDS, but not all cytokines in HIV-infected persons at different stages of disease corroborate this hypothesis. Production of interleukin-5 and interferon-gamma by CD4 and CD8 lymphocytes expressing CD30, however, is associated with promotion of B-lymphocyte immunoglobulin production.
Immunodeficiency
The primary target of HIV is the immune system itself, which is gradually destroyed. Viral replication actively continues following initial HIV infection, and the rate of CD4 lymphocyte destruction is progressive. Clinically, HIV infection may appear "latent" for years during this period of ongoing immune system destruction. During this time, enough of the immune system remains intact to provide immune surveillance and prevent most infections. Eventually, when a significant number of CD4 lymphocytes have been destroyed and when production of new CD4 cells cannot match destruction, then failure of the immune system leads to the appearance of clinical AIDS.
Infection with HIV is sustained through continuous viral replication with reinfection of additional host cells. Both HIV in host plasma and HIV-infected host cells appear to have a short lifespan; and late in the course of AIDS the half-life of plasma HIV is only about 2 days. Thus, the persistent viremia requires continuous reinfection of new CD4 lymphocytes followed by viral replication and infected host cell turnover. This rapid turnover of HIV and CD4 lymphocytes promotes origin of new strains of HIV within the host from mutation of HIV.
Genetic Variability of HIV
Presence or emergence of different HIV subtypes may also account for the appearance of antiretroviral drug resistance as well as the variability in pathologic lesions as different cell types are targeted or different cytopathic effects are elicited during the course of infection. Phylogenetic studies can identify genetic clusters of HIV-1 env genes which are known as subtypes, or clades, that have arisen with progression of the AIDS epidemic worldwide. The V3 loop amino acid sequences of these genetic variants influence HIV phenotype and immune response. Thus, the biologic properties of HIV can vary, even within an individual HIV infected person, where variants of HIV may arise that are "neurotropic" or "lymphocytotropic" for example.
Transmission of HIV
HIV infects definable population subgroups ("risk groups"). The transmission of HIV is a function of where the virus appears in the body and how it is shed. HIV can be present in a variety of body fluids and secretions, but the presence of HIV in genital secretions and in blood, and to a lesser extent breast milk, is significant for spread of HIV. However, the appearance of HIV in saliva, urine, tears, and sweat is of no major clinical importance, as transmission of HIV through these fluids does not routinely occur, primarily because of the low concentration of HIV in these fluids.
HIV is primarily spread as a sexually transmissible disease. Transmission of HIV can occur from male to male, male to female, and female to male. Female to female transmission remains extremely rare, though women with same-sex contact are often bisexual and have additional risk factors for HIV infection. The rate of HIV transmission with sexual intercourse is much lower than with other sexually transmitted diseases-approximately 0.3% per sexual contact with an HIV-infected person. However, some persons have become HIV-infected after a single sexual contact, while other persons have remained uninfected after hundreds of contacts.
Sexual contact with persons whose HIV viral load is greater, either with early infection or in the late stage of clinical AIDS, increases the transmission risk. The presence of cervical ectopia, oral contraceptive use, or pregnancy in women, intact foreskin in men, and genital ulcer disease in either sex increases the risk for HIV infection. Genital ulcers provide a more direct route to lymphatics draining to lymph nodes containing many CD4 lymphocytes and follicular dendritic cells. Tissue trauma during intercourse does not appear to play a role in HIV transmission.
HIV can be transmitted by parenteral exposure, which is the most highly efficient method of HIV transmission--close to 90%. There are many more peripheral blood mononuclear cells capable of either harboring or becoming infected by HIV in blood than are present in other body fluids or secretions. The primary risk group for HIV transmission via blood is intravenous drug users sharing infected needles. Less common practices of blood comingling or use of instruments such as tattoo needles not properly disinfected also carries a potential risk. Health care workers with percutaneous exposures to HIV-containing blood, however, are infected fewer than 1 in 300 times. Screening of blood products for HIV has almost eliminated HIV transmission by this means.
HIV infection can also be acquired as a congenital infection perinatally or in infancy. Mothers with HIV infection can pass the virus transplacentally, at the time of delivery through the birth canal, or through breast milk. Congenital AIDS occurs, on average, in about one fourth of babies born to HIV-1 infected mothers, with actual rates of transmission varying from 7 to 71%, depending upon the presence of risk factors for transmission during the course of HIV infection and pregnancy.
Primary HIV Infection
Primary HIV infection may go unnoticed in at least half of cases or produce a mild disease which quickly subsides, followed by a long clinical "latent" period lasting years. Prospective studies of acute HIV infections show that fever, lymphadenopathy, pharyngitis, diffuse erythematous rash, arthralgia/myalgia, diarrhea, and headache are the commonest symptoms seen with acute HIV infection. These symptoms diminish over 1 to 2 months. The symptoms of acute HIV infection resemble an infectious mononucleosis-like syndrome. Symptomatic acute HIV infection is more likely to occur in persons who acquired HIV infection through sexual transmission.
Generally, within 3 weeks to 3 months the immune response is accompanied by a simultaneous decline in HIV viremia. Both humoral and cell mediated immune responses play a role. The CD4 lymphocytes rebound in number, but not to pre-infection levels. Seroconversion with detectable HIV antibody by laboratory testing accompanies this immune response, sometimes in as little as a week, but more often in two to four weeks. Prolonged HIV-1 infection without evidence for seroconversion, however, is an extremely rare event.
Onset of AIDS
Unlike most infections in past epidemics, AIDS is distinguished by a very long latent period before the development of any visible signs of infection. During this phase, there is little or no viral replication detectable in peripheral blood mononuclear cells and little or no culturable virus in peripheral blood. The CD4 lymphocyte count remains moderately decreased. However, the immune response to HIV is insufficient to prevent continued viral replication within lymphoid tissues. Tests for HIV antibody will remain positive during this time but p24 antigen tests are usually negative. There is no evidence to suggest that seroreversion, or loss of antibody, occurs in HIV infected persons.
The average HIV-infected person may have an initial acute self-limited illness, may take up to several weeks to become seropositive, and then may live up to 8 or 10 years, on average, before development of the clinical signs and symptoms of AIDS. Persons infected with HIV cannot be recognized by appearance alone, are not prompted to seek medical attention, and are often unaware that they may be spreading the infection. There has been no study to date that shows a failure of HIV-infected persons to evolve to clinical AIDS over time, though the speed at which this evolution occurs may vary.
At least 10% of persons infected with HIV-1 are "long survivors" who have not had significant progressive decline in immune function. Findings include: a stable CD4 lymphocyte count, negative plasma cultures for HIV-1, a strong HIV-1 neutralizing antibody response, and a strong virus-inhibitory CD8 lymphocyte response. In addition, the lymph node architecture is maintained without either the hyperplasia or lymphocyte depletion common to progression to AIDS. Though peripheral blood mononuclear cells contain detectable HIV-1 and viral replication continues in long survivors, though their viral burden is low.
The development of signs and symptoms of AIDS typically parallels laboratory testing for CD4 lymphocytes. A decrease in the total CD4 count below 500/microliter presages the development of clinical AIDS, and a drop below 200/microliter not only defines AIDS, but also indicates a high probability for the development of AIDS-related opportunistic infections and/or neoplasms. Plasma HIV-1 RNA increases as plasma viremia becomes more marked. The risk for death from HIV infection above the 200/microliter CD4 level is low.
Persistent Generalized Lymphadenopathy (PGL)
There is loss of normal lymph node architecture as the immune system fails with emergence from latency of HIV infection. It is marked by development of generalized lymphadenopathy. This condition, described by the term persistent generalized lymphadenopathy (PGL), is not life-threatening. Lymph nodes throughout the body are large but usually do not exceed 3 cm in size and they may vary in size over time.
AIDS-related Complex (ARC)
Another phase of HIV infection described clinically but no longer commonly diagnosed in practice, is the condition known as AIDS-related complex (ARC), which is not necessarily preceded by PGL. ARC lacks only the opportunistic infections and neoplasms which define AIDS. ARC patients usually show symptoms of fatigue, weight loss, and night sweats, along with superficial fungal infections of the mouth (oral thrush) and fingernails and toenails (onychomycosis). It is uncommon for HIV infected persons to die at the stage of ARC. The staging of HIV disease progression through the use of CD4 lymphocyte counts has made use of the terms PGL and ARC obsolete.
Clinical AIDS
The stage of clinical AIDS that is reached years after initial infection is marked by the appearance of one or more of the typical opportunistic infections or neoplasms diagnostic of AIDS by definitional criteria. The progression to clinical AIDS is also marked by the appearance of syncytia-forming (SI) variants of HIV in about half of HIV infected patients. These SI viral variants, derived from non-syncytia-forming (NSI) variants, have greater CD4 cell tropism and are associated with more rapid CD4+ cell decline. The SI variants typically arise in association with a peripheral blood CD4 lymphocyte count between 400 and 500/microliter, prior to the onset of clinical AIDS. However, appearance of the SI phenotype of HIV is a marker for progression to AIDS that is independent of CD4 cell counts.
Other laboratory findings which indicate progression to AIDS include HIV p24 antigen positivity, increased serum beta2-microglobulin, elevated serum IgA, or increased neopterin levels in serum, cerebrospinal fluid, or urine. The p24 antigen is a highly specific predictor of progression, but only about 60% of HIV-infected persons develop p24 antigenemia prior to onset of clinical AIDS. Beta2-microglobulin is increased with lymphocyte activation or destruction associated with HIV disease progression. Neopterin, as measured in serum or urine, is also a measure of immune system activation and can predict HIV disease progression. The information provided by these tests is similar, so no advantage accrues from performing all of them simultaneously.
For perinatally acquired HIV infection, the time to development of clinical AIDS may be shorter than in adults. Signs associated with HIV infection appear in over 80% of seropositive infants by the age of 5 months. Infants in whom such signs appear at 3 months tend to have decreased survival. About half of children with perinatally acquired HIV infection are alive at 9 years.
