HIV infects and kills CD4+ T lymphocytes in vitro, although scientists have developed immortalized T-cell lines in order to propagate HIV in the laboratory (Popovic et al., 1984; Zagury et al., 1986; Garry, 1989; Clark et al., 1991). Several mechanisms of CD4+ T cell killing have been observed in lentivirus systems in vitro and may explain the progressive loss of these cells in HIV-infected individuals (reviewed in Garry, 1989; Fauci, 1993a; Pantaleo et al., 1993a) (Table 2). These mechanisms include disruption of the cell membrane as HIV buds from the surface (Leonard et al., 1988) or the intracellular accumulation of heterodisperse RNAs and unintegrated DNA (Pauza et al., 1990; Koga et al., 1988). Evidence also suggests that intracellular complexing of CD4 and viral envelope products can result in cell killing (Hoxie et al., 1986).

Table 2. Potential Mechanisms of the Functional and Quantitative Depletion of CD4 T Lymphocytes

Direct HIV-mediated cytopathic effects (single-cell killing)

HIV-mediated formation of syncytia

Virus-specific immune responses HIV-specific cytolytic T lymphocytes Antibody-dependent cellular cytotoxicity Natural killer cells

Autoimmune mechanisms

Anergy caused by inappropriate cell signaling through gp120-CD4 interaction

Superantigen-mediated perturbation of T-cell subgroups

Programmed cell death (apoptosis)

Reference: Pantaleo et al., 1993a.

In addition to these direct mechanisms of CD4+ T cell depletion, indirect mechanisms may result in the death of uninfected CD4+ T cells (reviewed in Fauci, 1993a; Pantaleo et al., 1993a). Uninfected cells often fuse with infected cells, resulting in giant cells called syncytia that have been associated with the cytopathic effect of HIV in vitro (Sodroski et al., 1986; Lifson et al., 1986). Uninfected cells also may be killed when free gp120, the envelope protein of HIV, binds to their surfaces, marking them for destruction by antibody-dependent cellular cytotoxicity responses (Lyerly et al., 1987). Other autoimmune phenomena may also contribute to CD4+ T cell death since HIV envelope proteins share some degree of homology with certain major histocompatibility complex type II (MHC-II) molecules (Golding et al., 1989; Koenig et al., 1988).

A number of investigators have suggested that superantigens, either encoded by HIV or derived from unrelated agents, may trigger massive stimulation and expansion of CD4+ T cells, ultimately leading to depletion or anergy of these cells (Janeway, 1991; Hugin et al., 1991). The untimely induction of a form of programmed cell death called apoptosis has been proposed as an additional mechanism for CD4+ T cell loss in HIV infection (Ameisen and Capron, 1991; Terai et al., 1991; Laurent-Crawford et al., 1991). Recent reports indicate that apoptosis occurs to a greater extent in HIV-infected individuals than in non-infected persons, both in the peripheral blood and lymph nodes (Finkel et al., 1995; Pantaleo and Fauci, 1995b; Muro-Cacho et al., 1995).

It has also been observed that HIV infects precursors of CD4+ T cells in the bone marrow and thymus and damages the microenvironment of these organs necessary for the optimal sustenance and maturation of progenitor cells (Schnittman et al., 1990b; Stanley et al., 1992). These findings may help explain the lack of regeneration of the CD4+ T cell pool in patients with AIDS (Fauci, 1993a).

Recent studies have demonstrated a substantial viral burden and active viral replication in both the peripheral blood and lymphoid tissues even early in HIV infection (Fox et al., 1989; Coombs et al., 1989; Ho et al., 1989; Michael et al., 1992; Bagnarelli et al., 1992; Pantaleo et al., 1993b; Embretson et al., 1993; Piatak et al., 1993). One group has reported that 25 percent of CD4+ T cells in the lymph nodes of HIV-infected individuals harbor HIV DNA early in the course of disease (Embretson et al., 1993). Other data suggest that HIV infection is sustained by a dynamic process involving continuous rounds of new viral infection and the destruction and replacement of over 1 billion CD4+ T cells per day (Wei et al., 1995; Ho et al., 1995).

Taken together, these studies strongly suggest that HIV has a central role in the pathogenesis of AIDS, either directly or indirectly by triggering a series of pathogenic events that contribute to progressive immunosuppression.