The Folly of Modeling HIV Vaccine Efficacy in Chimpanzees

HIV Vaccine Research in Chimpanzees

Chimpanzees are the only non-human primates in which HIV replicates. Consequently, they have been used in attempts to stimulate a vaccine response involving the production of neutralizing antibodies and the generation of cell-mediated immunity. Despite having 99% of their DNA in common with humans, chimpanzees do not develop AIDS. Upon inoculation with HIV, they mount long-lasting neutralizing antibody responses to viral envelope and core proteins and cytotoxic T lymphocytes specific for viral proteins. They do not experience apoptosis and follicular disruption in the lymph nodes following HIV exposure. These observations suggest that chimpanzees are genetically different from humans in their response to HIV infection.

Early Immune Responses and Th1/Th2 Differentiation

When the immune system encounters an infective agent, it mounts an immediate response in which TNF-alpha and IL-12 are released. TNF-alpha activates macrophages, and IL-12 stimulates the secretion of IFN-gamma and the differentiation of Th1 CD4 cells. IL-12 simultaneously inhibits Th2 differentiation. In contrast, patients with AIDS show a predominance of Th2 CD4 cells in peripheral blood, lymph nodes, and intestinal mucosa. The mechanism by which this reversal of CD4 subsets occurs remains unknown. The hypothesis has been advanced that the immune response is influenced by genetic and environmental factors present at the time of initial exposure to HIV. If the immune system can mount a strong inflammatory response to the virus, clearance is possible. If the virus enters during a period of immunosuppression, the response is muted, and viral reservoirs are established in macrophages and other cells.

TNF-alpha, RANTES, and CCR5 Regulation

TNF-alpha targets infected cells that express high levels of viral antigens on their surfaces. It is also known to inhibit HIV-1 replication by stimulating the secretion of RANTES and downregulating CCR5 expression. In this way, TNF-alpha restricts the dissemination of monocytotropic strains of HIV that preferentially infect macrophages and dendritic cells. Immunosuppressive mediators, including IL-10, TGF-beta, and PGE2 block TNF-alpha and IL-12 expression. If HIV-1 enters the host when the immune system is under the influence of these mediators, the virus escapes detection and clearance.

Genetic Factors in HIV-1 Infectivity

Four factors have been proposed to influence the infectivity of HIV-1: sensitivity to complement-mediated lysis, vulnerability to the suppressive effects of Tat, IL-10 promoter polymorphisms, and autoimmune predisposition. Of these, the role of the complement system has received the most attention. Neutralizing antibodies to gp120/gp160 should activate, complement, and destroy the virus. However, complement-mediated lysis does not occur in HIV-infected humans. Complement activation by antibodies to gp120 and gp41 is blocked by Factor H, a plasma protein that binds to gp120 and gp41 and inactivates C3b. gp120 and gp41 contain amino acid sequences that resemble the Factor H binding regions on C3b. These observations imply that the virus has evolved a means of escaping complement-mediated lysis by mimicking the binding site for Factor H on C3b.

Viral Incorporation of Host Complement Control Proteins

Other viruses with envelopes, including human herpes virus, Epstein-Barr virus, and cytomegalovirus, also escape complement-mediated destruction. These viruses incorporate CD55 and CD59 into their membranes during budding. HIV does the same, but unlike the others, it has the ability to bind Factor H, which may protect it from complement activation. In vitro studies have shown that HIV-infected cells are lysed by untreated serum from horses, cows, sheep, dogs, and baboons but not by heat-inactivated serum, suggesting that the activity resides in complement. In human serum, complement is activated, but lysis does not occur. There is no information on whether chimpanzee serum will destroy HIV-infected cells. If Factor H from chimpanzees fails to bind gp120 and gp41, then HIV virions may be removed by the complement system before they can enter lymphoid tissue.

Complement Receptor Binding and Immune Evasion

Once C3b is deposited on the surface of a virus or cell, it is quickly degraded into smaller fragments, C3dg and iC3b. These fragments bind to three different cell surface receptors: CR1, CR2, and CR3. These receptors are found on red cells, B cells, follicular dendritic cells, monocytes, macrophages, and neutrophils. The CR1 receptor binds C3b and iC3b; CR2 binds iC3b and C3dg; CR3 binds iC3b.

Under normal circumstances, viral particles are destroyed by complement and removed from the blood. Intact viruses do not accumulate in lymphoid tissues. However, in HIV-infected patients, infectious virus is found in lymph nodes. This implies that virions remain intact while traveling to lymphoid tissues and are not destroyed by complement. Their transport may be facilitated by C3b fragments, which enhance infectivity and promote viral replication in follicular dendritic cells, monocytes, and macrophages. Complement receptor engagement activates NF-kB and other factors that promote viral replication. C3b fragments suppress the release of IL-12 and IFN-gamma, which generally act to clear the virus. There is no indication that this occurs in chimpanzees. The ability of chimpanzee serum to kill virions and virally infected cells has not been investigated.

Neutralizing Antibodies and the gp41 Epitope

A highly conserved epitope, ELDKWA, on gp41, has been identified as a target for neutralizing antibodies. These antibodies may act by blocking the binding of Factor H to gp41 and restoring the cytolytic activity of complement.

Immunosuppressive Effects of HIV Tat Protein

The Tat protein is responsible for many of the immunologic abnormalities observed in AIDS. It is released early in infection and is taken up by uninfected cells in the vicinity. It inhibits mRNA translation, initiates apoptosis, suppresses MHC class I expression, activates calcium channels, and stimulates the production of RANTES and the transcription factor NF-kB. Many of these effects are due to Tat’s ability to stimulate the production of TNF-alpha and prostaglandins. Tat increases macrophage expression of Fas ligand, which induces apoptosis in Fas-bearing cells. Soluble Tat has been shown to induce the migration of macrophages and dendritic cells to lymphoid tissue, where they can destroy uninfected cells. In humans, these cells exhibit increased oxidative stress and increased susceptibility to Fas-mediated apoptosis. In chimpanzees, Tat does not exert these effects. The mechanism responsible for this resistance is not known.

IL-10, TGF-beta, and Immunosuppressive Cytokine Responses

Infection with HIV is characterized by increased production of IL-10 and TGF-beta. These cytokines block the secretion of IL-12 and the development of Th1 CD4 cells. Both genetic and environmental factors influence IL-10 expression. Several polymorphisms have been identified in the promoter region of the IL-10 gene, and these polymorphisms may influence susceptibility to HIV. A number of studies have shown that IL-10 and TGF-beta are increased in response to marijuana, heroin, cocaine, alcohol, corticosteroids, and psychogenic stress. These factors are associated with suppression of Th1 activity and an increase in Th2 responses. The adrenal glands secrete corticosteroids in response to stress, and they function in part by increasing IL-10 and suppressing TNF-alpha production. TNF-alpha, in turn, blocks IL-10 and shifts the immune response toward inflammation.1999