Striking similarities between HIV-1 Env protein and the apoptosis mediating cell surface antigen Fas. Role in the pathogenesis of AIDS

HIV-1 viral genes and mitochondrial apoptosis

The mitochondrion is an organelle that regulates various cellular functions including the production of energy and programmed cell death. Aberrant mitochondrial function is often concomitant with various cytopathies and medical disorders. The mitochondrial membrane plays a key role in the induction of cellular apoptosis, and its destabilization, as triggered by both intracellular and extracellular stimuli, results in the release of proapoptotic factors into the cytosol. Not surprisingly, proteins from the human immunodeficiency virus type 1 (HIV) have been implicated in exploiting this organelle to promote the targeted depletion of key immune cells, which assists in viral evasion of the immune system and contributes to the characteristic global immunodeficiency observed during progression of disease. Here we review the mechanisms by which HIV affects the mitochondrion, and suggest that various viral-associated genes may directly regulate apoptotic cell death.

Exclusion of HIV epitopes shared with human proteins is prerequisite for designing safer AIDS vaccines

BACKGROUND

The safety of a potential AIDS vaccine is an issue that will become critical at later stages of product development and needs to be addressed before it is too late.

OBJECTIVE

In order to design safer vaccine, the HIV antigens, to be deployed in it, should be free of regions that are either present in human proteins or exhibit pronounced structural similarity to proteins responsible for important physiological functions.

STUDY DESIGN

The approach is based on the use of an original matrix predicting the antigenic similarity of amino acids. This mathematical approach developed by us was applied for identification of fragments with similarity to human proteins within potentially immunodominant regions of HIV proteins. A potential self-sensitization by viral quasispecies with variants of hypervariable V3 region, generated as a result of immune pressure on the immunodominant region of envelope, was considered in detail.

RESULTS

Viral fragments occurring in normal human proteins as well as regions exhibiting high similarity to proteins responsible for physiological homeostasis were identified in every HIV protein at a frequency higher than expected. Most such regions contained either T-cell (CD8(+) CTL or CD4(+) Helper) or B-cell epitopes, or both of them simultaneously. The gained knowledge was applied in designing a synthetic immunogen containing multiple CTL epitopes. The synthesis of series of chimeric peptides representing hypervariable region of V3 loop of HIV envelope, to be used as a multi-epitope or mixotope vaccine candidate, has been achieved. Such a vaccine could theoretically pre-empt any escape mutant borrowing from antigenic diversity of hypervariable region of V3 loop of HIV envelope.

CONCLUSIONS

The epitopes shared by HIV and its host are likely to be implicated in the immunopathogenesis of AIDS through induction of cross-reacting effectors of the immune system. The prospect that 'house-keeping' immune mechanism can be foiled by molecular mimicry of HIV with physiologically important human proteins should be taken into consideration in safer vaccine design.

Calmodulin antagonists inhibit apoptosis of CD4+ T-cells from patients with AIDS

Recent studies indicate that Fas and Fas ligand are involved in apoptosis of T-cells in HIV-infected patients. We have demonstrated that calcium/calmodulin is involved in Fas-mediated apoptosis in human T-cell lines transfected with HIV recombinant cDNA. In the present study, we examined spontaneous apoptosis of T-cells in vitro in peripheral blood obtained from 11 patients with AIDS and 8 HIV-seronegative normal donors and the effect of the calmodulin antagonists, trifluoperazine (TFP) or tamoxifen (TMX), on apoptosis. The results show that: (1) levels of spontaneous apoptosis were higher in PBMCs obtained from patients with AIDS than HIV-negative normal controls and the levels of apoptosis correlated with the severity of disease. (2) The accelerated apoptosis occurred predominantly in CD4+ cells in patients with AIDS. (3) Calmodulin antagonists inhibited the spontaneous apoptosis of CD4+ T-cells from patients with AIDS, which resulted in an increase in the ratio of CD4+ to CD8+ T-cells. (4) The inhibitory effect of calmodulin antagonists on apoptosis was more significant in patients with advanced disease (CDC category C) compared to less severe disease (CDC category B). These results indicate that calmodulin antagonists inhibit HIV-associated apoptosis of CD4+ T-cells, and imply that the calcium/calmodulin play important roles in mediating apoptosis of CD4+ T-cells induced by HIV infection.

An ongoing immune response to HIV envelope gp120 in human CD4-transgenic mice contributes to T cell decline upon intravenous administration of gp120

The mechanisms accounting for T cell depletion in AIDS patients are not yet fully understood, nor are the roles of host factors in HIV pathogenesis. We show here that an ongoing humoral immune response to HIV gp120 can sensitize non-infected cells towards apoptosis. Thus, i.v. injection of 1 microg recombinant(r) gp120 into gp120-immunized human CD4-transgenic mice (huCD4 Tg), which express huCD4 on both T and B cells, results in T and B cell depletion in peripheral blood and lymphoid tissues. On day 6 after a bolus injection of gp120, the numbers of peripheral T cells and B cells in gp120-immunized huCD4 Tg decreased sevenfold and two- to threefold, respectively. Annexin V staining revealed a higher percentage of early apoptotic cells on day 1 of gp120 i.v. injection from gp120-primed huCD4 Tg spleens compared to gp120-primed controls. Boosting the primed huCD4 Tg mice with soluble gp120 and hen egg-white lysozyme led to lower secondary titers to both antigens than found in controls. Furthermore, splenocytes from gp120-pretreated immunized huCD4 Tg had a lower level of stimulation in response to anti-CD3 treatment. These in vivo results are consistent with in vitro data demonstrating that cross-linking CD4 on splenocytes of huCD4 Tg by rgp120SF2 and anti-gp120 not only sensitizes T cells for apoptosis, but also induces apoptosis per se, and suggest that anti-gp120 responsiveness can contribute to T cell depletion in AIDS.

Can evolutionary principles help resolve the acquired immune deficiency syndrome crisis?

The population of retroviruses inside each infected host can quickly adapt its genes in response to survival threats. For this reason, therapeutics attempting to eliminate or inhibit the human immunodeficiency virus have been unsuccessful. However, it may be possible to apply evolutionary principles to the problem of human immunodeficiency virus infection/acquired immune deficiency syndrome. The rapid evolution capabilities of this pathogen might assist researchers in evolving human immunodeficiency virus toward the goal of virus-host coexistence. In this relationship, infecting viruses do not harm their host. Two different examples illustrate the theoretical application of evolutionary principles to this problem. First, by viewing the virus as a new gene of the immune system, it is possible to evolve this gene toward the goal of improving immune function. Second, the retroviral population of each host is maladapted for long-term survival. Retroviral genes could be evolved with respect to continued existence in the host.

Role of immune activation and cytokine expression in HIV-1-associated neurologic diseases

Central nervous system (CNS) involvement is common during human immunodeficiency virus type-1 (HIV-1) infection. The neurologic disease of the CNS most frequently observed during acquired immunodeficiency syndrome (AIDS) is HIV-1-associated cognitive/motor complex or AIDS dementia complex (ADC), which is most likely a direct consequence of HIV-1 infection of the CNS. The peripheral nervous system (PNS) is also affected in HIV-1-infected individuals and there are several features of immune- and cytokine-related pathogenesis in both the CNS and PNS that are reviewed. Several lines of evidence demonstrate aspects of immune activation in the CNS and peripheral nervous system (PNS) of HIV-1-infected individuals. The relative paucity of HIV-1 expression in contrast to widespread functional and pathologic changes in the CNS and PNS of AIDS patients, and the lack of evidence of productive infection of HIV-1 in neuronal cells in vivo lead to the possibility of indirect or immunopathogenic mechanisms for HIV-1-related neurologic diseases. Proposed mechanisms of neuronal and glial cell damage are injury of oligodendrocytes by tumor necrosis factor-alpha (TNF-alpha) released from activated macrophage/microglia, calcium-dependent excitoneurotoxicity induced by gp120 HIV-1 envelope protein, N-methyl-D-aspartate (NMDA) receptor-mediated neurotoxicity by quinolinic acid (a product of activated macrophages), cell injury by HIV-1-specific cytotoxic T cells, and apoptosis of oligodendrocytes or neurons triggered by interaction between cell surface receptors and HIV-1 gp120 protein. Common to those mechanisms is the dependence on cellular activation with expression of proinflammatory cytokines (TNF-alpha, interleukin-1). Amplification of activation signals through the cytokine network by macrophage/astrocyte/endothelial cell interactions, and cell-to-cell contact between activated macrophages and neural cells by upregulation of adhesion molecules dramatically enhances the toxic effect of macrophage products. Expression of immunosuppressive cytokines such as interleukin-4, interleukin-6, and transforming growth factor-beta is also increased in the CNS and PNS of HIV-1-infected patients. This may serve as neuroprotective and regenerative mechanism against insults to nervous system tissue.

Screening of protein sequences databanks by Automat for search of host sequences integration and/or autoimmune disorders induction by retroviruses

We have designed an efficient algorithm in order to detect systematically all the oligopeptides shared by a given protein with all the protein sequences in a databank. This software, Automat, also makes statistics on the number of shared oligopeptides. In the present study, we apply Automat on HIV-1 proteins to detect putative critical sites and to identify candidate viral antigens that may trigger autoimmune disorders. A list of pertinent similarities between HIV-1 proteins and human proteins, as detected by Automat, is reported.

Effect of purified IgGs from HIV-1-infected and non infected individuals on immune activation

The purification and analysis of IgGs from sera of HIV-1-infected and non infected individuals are reported. The effect of antibodies purified from sera of infected individuals on antigen-induced T cell proliferation was investigated in relation to their possible involvement in an autoimmune reaction in AIDS, in view of the previously unravelled striking peptide similarities between HIV-1 gp120 and the immunoregulatory CD4 and Fas molecules. However, our data do not allow definite conclusions to be drawn. The necessity of purifying antibodies against specific peptides to show their direct effect on T-cell activation is further stressed.