Naoki Oyaizu

Role of apoptosis in HIV disease pathogenesis

After approximately one and a half decades of intensive studies, the exact mechanisms to explain HIV-mediated cytopathicity are still enigmatic and need closer scrutiny. There has been a dichotomy between virological and immunological viewpoints in understanding HIV-mediated cytopathicity, the former emphasizing a killing of infected cells by HIV-1 and the latter emphasizing indirect mechanisms wherein HIV or its soluble component(s) alter CD4 T-cell function and induce susceptibility to apoptosis. Accumulating evidence points to the notion that apoptosis might be a major contributor to the depletion of CD4 T-cells in HIV infection. This review summarizes current information about the regulatory mechanisms of T-cell apoptosis and the role of apoptosis in HIV pathogenesis with the goal of providing an integrated view of HIV cytopathicity.

Mechanism of Apoptosis in Peripheral Blood Mononuclear Cells Of HIV-Infected Patients

Lymphocytes from patients with HIV-infection have been shown to undergo accelerated spontaneous apoptosis. Binding of CD4 molecules by HIV envelope protein gp120 and anti-gp120 antibodies can lead to crosslinking of CD4 molecules (CD4XL) in vitro and conceivably in vivo. We have recently shown that CD4XL in vitro, when performed in unfractioned peripheral blood mononuclear cells (PBMC) on normal HIV seronegative donors, is by itself sufficient to induce T cell apoptosis (Blood 82:3392, 1993). To further examine the mechanisms involved in apoptosis, we have examined the expression of Fas antigen (Fas) using 3 color flow cytometry. Fas is a cell surface molecule known to mediate apoptosis-triggering signals. We induced CD4XL in PBMC obtained from normal donors, either by anti-CD4 mAb Leu3a or by HIV-1 envelope protein gp160. PBMC subpopulations were examined for Fas Ag expression and for apoptosis induction by flow cytometry. CD4XL was found to result in increased Fas expression as well as Fas mRNA in lymphocytes and the up-regulated Fas Ag was closely correlated with apoptotic cell death. CD4XL in PBMC also resulted in induction of the cytokines INF-tau and TNF-alpha in the absence of IL-2 and IL-4 secretion. Both these cytokins contributed to Fas Ag up-regulation and antibodies to TNF-alpha and INF-tau abrogated CD4XL-induced Fas up-regulation and T-cell apoptosis. These findings suggest that CD4XL occurring in vivo might play an important role in inducing an abberant cytokine profile (which has been observed in HIV infected individuals) and also in triggering of T-cell apoptosis.

Requirement of the T cell receptor for antigen presentation by T lymphocytes. Effect of envelope glycoproteins of HIV-1 on antigen presentation by T cells.

We have developed CD4+, tetanus antigen‐specific T cell clones that proliferate in the presence of tetanus antigen and autologous irradiated peripheral blood leucocytes (PBL) as antigen‐presenting cells (APC). There have been several reports that T cells can present antigen themselves. We have used tetanus antigen‐specific T cell clones to examine the effects of envelope glycoproteins of HIV‐1 on processing and presentation of antigen to T cells. Cloned T cells were pre‐incubated with soluble crude preparation of tetanus antigen for 4 h at 37°C, irradiated, and used as APC (T‐APC). These cells could present antigen, as assessed by the ability of the autologous cloned T cells to proliferate. Resting T cells and phytohaemagglutinin‐activated T cell blasts from autologous PBL could not present tetanus antigen to the responder cloned T cells. Antigen presentation by T‐APC was abrogated by treating cells with anti‐HLA‐DR but not by anti‐HLA‐DQ monoclonal antibodies; treatment of tetanus antigen‐pulsed T‐APC with anti‐tetanus antibody also blocked the ability of these cells to induce proliferation in responder T cells. Antigen presentation by cloned T cells was by a chloroquine‐resistant pathway. Pretreatment of T‐APC with envelope glycoprotein of HIV‐1, gp120. did not affect the proliferative responses of the responder T cells. These data suggest that gp 120 does not inhibit the antigen‐presenting function while suppressing antigen‐specific responses.

Analyses of development of insulitogenic T lymphocytes in NOD mice by transplantation of bone marrow, thymus, and pancreas.

To estimate the functional participation of the insulitogenic genes in the development of insulitogenic lymphocytes, we attempted to induce insulitis in normal allogeneic BALB/c hosts by bone marrow transplantation from NOD mice. The development of the insulitogenic lymphocytes was histologically examined using the hosts pancreas and pancreatic tissue from NOD mice which had been grafted under the renal capsules 2 weeks before sacrifice. Adult-thymectomized NOD mice that had been reconstituted with the thymus and bone marrow from NOD mice showed insulitis in both the hosts pancreas and grafted pancreatic tissue from newborn NOD mice. When BALB/c mice were lethally irradiated and then reconstituted with NOD bone marrow cells, no insulitis developed in the pancreatic grafts from NOD and BALB/c mice or in the hosts pancreas. Since the specificities and functions of T lymphocytes are controlled by the thymic microenvironment during the differentiation within the thymus, the thymus of BALB/c genotype may be responsible for the failure to induce the insulitogenic lymphocytes. Therefore, athymic BALB/c mice were reconstructed with bone marrow cells and thymus of NOD genotype. No insulitis developed, however, in either the hosts pancreas or grafted pancreatic tissue. These results suggest that the bone marrow cells and thymus of NOD genotype are not sufficient to induce insulitogenic lymphocytes in the allogeneic BALB/c environment.

LYMPHOCYTE APOPTOSIS IN HIV INFECTION

Human immunodeficiency virus (HIV-1) is the etiologic agent of acquired immunodeficiency syndrome (AIDS). Our understanding of the complexities of pathogenic mechanisms of HIV disease is still evolving; however, the mechanism whereby HIV-1 infection leads to profound depletion of CD4 T cells remains one of the central unsolved problems in AIDS research. In the past several years, there has been a dichotomy between virological and immunological viewpoints in understanding HIV-mediated cytopathicity, the former emphasizing killing of infected CD4 cells by HIV and the latter emphasizing indirect mechanisms wherein HIV or its soluble component(s) alter CD4 T-cell function and induce susceptibility to apoptosis.

Correlation of maternal cytophilic human immunodeficiency virus (HIV)-1 V3 loop peptide-specific antibodies in infants with vertical HIV transmission.

ABSTRACT: Approximately 70% of human immunodeficiency virus (HIV)-1-infected pregnant women do not transmit HIV to their offsprings. The identification of factors involved in maternal-child transmission of HIV is important for the design and implementation of therapeutic and prevention strategies. Recently we have developed a modified peripheral mononuclear cell (PBMC) culture system for in vitro antibody production (IVAP) by which we can distinguish serum-derived cytophilic anti-HIV-1 antibody from de novo synthesized antibody. In this study, we analyzed the presence of antibodies directed to HIV-1 gpl60, gpl20, gp41, and V3 loop synthetic peptides (C51, C53, and C57 from MN and HIB strains) utilizing the grid-blot method in PBMC cultures of 52 mother-child pairs. Among the mothers (39 nontransmitters and 13 transmitters), presence of serum-derived cytophilic antibodies or de novo synthesized HIV V3 loop peptide-specific antibodies did not correlate with vertical transmission. However, PBMC-associated, cytophilic antibodies of maternal origin reactive with C51, C53, and C57 from MN and HIB strains were identified in cultures of uninfected infants, but not in infected infants. These observations suggest that cytophilic antibodies of maternal origin directed to HIV-I V3 loop peptides which are bound to infant cells might play a role in preventing vertical transmission.