HIV infection leads to the progressive collapse of two systems within the human body: the immune system, and the central nervous system, AIDS results from the loss, in each organ, of a selective cell population: CD4 positive T cells in the immune system, leading to immune incompetence; neurons in the brain, leading to dementia (Ameisen, 1995).
The asymptomatic phase of the disease is characterized by the inability of CD4 positive T cells to proliferate as shown in vitro by the response to MHC class II restricted recall antigens (Shearer and Clerici, 1991). This functional impairment is followed by the decline of CD4 positive T cells and the development of AIDS.
A number of virological and immunologic mechanisms have been proposed to account for the CD4 positive T cell defect and depletion, including syncytium formation between infected and non-infected cells, selective infection and destruction of memory T help cells, inappropriate immune killing of uninfected cells, and autoimmune responses (Fauci, 1988; Banda et al., 1992; Habeshaw et al., 1992). Recently, new developments in this area of research have suggested that the loss of CD4 cells in HIV-1 infected patients is associated with lymphocyte activation. Activation, however, does not result in cell proliferation, but rather in cell death, through a mechanism known as programmed cell death (PCD) (Wyllie et al., 1980; 1984).
PCD is characterized by disappearance of individual cells and is an active cell suicide mechanism. It represents the endpoint of a genetically determined programme which
requires de novo gene expression and protein synthesis. The morphological characteristics of PCD have been called apoptosis. Apoptosis differs from necrosis which is an accidental event triggered by factors external to the dying cells. Moreover, whereas in cell necrosis the event involves large groups of cells and there is loss of membrane integrity, swelling and degeneration of cell organelles and eventually lysis, in apoptosis single cells appear to die: organelles are normal and nuclei show dense chromatin, which is degraded into single and multiple oligonucleosomes, clumped against the nuclear membrane. The DNA is cleaved in the intemucleosomal linker region, where it is relatively weakly associated with histone H I, and electrophoretic separation of DNA of apoptotic cells reveals a "ladder" pattern of bands averaging about 200, 400, 600bp, etc., corresponding to oligonucleosomal fragments (Arends et al., 1990; Walker and Sikorska, 1994). This fragmentation of DNA is enzymatic and generally occurs after activation of a calcium-dependent endogenous endonuclease (Arends et al., 1990).
1.8.1 T lymphocytes
Apoptosis has been recognized as a possible way by which immune cells (Cameron et al., 1994; Schnittman and Fauci, 1994; Carbonari et al., 1994; Howie et al., 1994; Lu et al., 1994; Martin et al., 1994; Sandstrom et al., 1993; Shearer et al., 1993; Lewis et al., 1994) are eliminated in AIDS, and more recently it has been detected among CD4 and CDS lymphocytes in HIV-1 positive individuals at the asymptomatic stage of the infection (Groux et al., 1992; Jaleco et al., 1994; Meyaard et al., 1992; 1994). A study by Gougeon et al (1993a)quantified the apoptotic peripheral blood lymphocytes (PBLs) from 29 asymptomatic HIV-1 infected individuals and 28 seronegative controls. Up to
25% of patients’ cells were apoptotic after stimulation with ionomycin, compared to 9% in cultures from seronegative donors. The apoptotic process could be accelerated by increasing the intracellular Ca^^ mobilization which was supposed to activate the endogenous endonuclease; the latter destroys the chromatin structure and induces apoptosis. Cyclosporin A, a powerful suppressor of the immune system and an inhibitor of activation, and Zn"*"^ ions, known to inhibit endonuclease and DNA fragmentation in PCD induced cell death, reduce the ionomycin-induced DNA fragmentation to control level. A similar mechanism, whereby infected cytotoxic T-lymphocytes are eliminated, has been reported in HTLV-1 infection (Umehara et al., 1994).
An observation by Gougeon and Montagnier (1993) suggested that the T cell tropism of the activator used to induce apoptosis in patients’ lymphocytes will determine which subpopulation will die: ionomycin (Gougeon et al., 1993a,b) and anti-CD3 antibodies (Meyaard et al., 1992) activate the death of both CD4 and CDS positive T cells, whereas superantigens induce preferentially the death of CD4 positive T cells (Groux et al., 1992).
T cell PCD may exert a beneficial role in the control of viral infection as seen during acute benign Epstein-Barr virus induced mononucleosis in children (Uehara et a l., 1992). However, correlation between T cell PCD and AIDS pathogenesis suggests that, in HIV infection, this deletional mechanism is not beneficial and may contribute to CD4 positive T cell depletion and to development of AIDS (Gougeon et al., 1993b).
Apoptosis of these cells is thought to be implicated not only during developmental but also in the post-maturation stages of the peripheral nervous system (PNS) and CNS neurons, as recent studies have suggested that apoptotic neuronal death occurs in the mature nervous system and that it may be even involved in a number of human diseases such as neuro-degenerative disorders including Alzheimer’s and Parkinson’s disease or in ageing (Chen et al., 1995).
Neuropathological studies in AIDS have revealed that brains of patients suffering from the AIDS dementia complex with the typical encephalitis show, in addition to the well known features, variable amounts of neuronal loss in cortical and subcortical regions (Everall et al., 1993; Ketzler et al., 1990; Wiley et al., 1991). Quantitative studies showed that the closer to the microglial nodules, the greater the neuronal loss is. However, this effect disappeais300 ixm away from the nodule, suggesting the possible diffusion of neurotoxins from the nodule (Masliah et al., 1994a). It has been postulated that, in AIDS, cell loss would take place through a process of apoptosis and indeed, apoptosis has been unquestionably demonstrated in the CNS of adult (Adle-Biassette et al., 1995; Petito and Roberts, 1995) as well as paediatric AIDS patients (Gelbard et al.,
1995).
The regulation of apoptosis is rather complex. Reports showing that some genes induce apoptosis (Rabizadeh et al., 1993; Itoh et al., 1991), and other genes inhibit it (Mah et al., 1993; Zhong et al., 1993), suggest a parallel between the system that modulates the propensity of cells to undergo neoplastic transformation - oncogenes and tumour suppressor genes - and the system that modulates the propensity of cells to undergo
apoptosis - presumably, necrogenes and cellular death suppressor genes. Among them expression of bcl-2 oncogene, which functions as death suppressor gene, is associated with a marked inhibition of neuronal cell death (Bredesen, 1994). M cl-l (Kozopas et al., 1993), which has some similarity to bcl-2, and bcl-x (Boise et al., 1993), are also thought to be involved in the inhibition of apoptosis in the nervous system whereas p53, a tumour suppressor gene induces the occurrence of apoptosis (Yonish-Ronach et al.,
1991).
Neurotrophic factors and cytokines are also thought to be involved in apoptosis. Nerve growth factor (NGF) and its low-affinity receptor p75 n g freceptor(n g f r) al., 1991),
TNF and TNF receptor (TNFR) I (a widely expressed gene whose product may induce apoptosis or necrosis when bound by its ligand, TNF) appear to be involved in cell death. In contrast, TNFR II does not induce cell death when bound by TNF (Tartaglia et al., 1991). As the possible mechanism is still incompletely understood it would be interesting to know by which mechanism the binding of TNFR I by TNF induces cell death (Clement and Stamenkovic, 1994). TNF binds to TNFR I and releases arachidonic acid intracellularly. Reaction oxygen molecules increase intracellularly, possibly because of both arachidonic acid metabolism and a poorly understood block at mitochondrial complex III (Lancaster et al., 1989). The second member of the TNFR superfamily shown to induce cell death is fas antigen, which is a cell surface protein that belongs to the TNFR and NGFR and can mediate apoptosis (Itoh et al., 1991). However, it has not been demonstrated to be expressed in the CNS (Watanabe-Fukunaga et al., 1992). Among other neurotrophic factors ciliary neurotrophic factor (CNTF) is found in astrocytes and is known to protect oligodendrocytes from death induced by TNFs
(apoptosis) (Louis et al., 1993).