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Whether MSRV proves to be an exogenous retrovirus or a replication-competent endogenous one, there are numerous possible mechanisms (Mims, 1985) by which such an agent may be involved in the pathogenic process; antigenic mimicry (Perl

and Banki, 1993; Banki et a l 1994; Wucherpfennig and Strominger, 1995), retrovirally encoded superantigens (Rudge, 1991; Skov and Baadsgaard, 1995), direct cytotoxicity (Rasmussen et a l 1993), trans-activation of host genes (Perl and Banki, 1993; Rasmussen et a l 1993) and interactions with herpesviruses (Haahr et a l 1994; Haahr et a l 1992; Perron et a l 1993; Challoner et a l 1995) can all be plausibly invoked.

4.3.1 Possible Autoimmune Mechanisms

Multiple sclerosis is generally regarded as a disease which has an autoimmune pathology and there are several mechanisms by which MSRV could be implicated in this. It could act simply as a trigger for the development o f the autoimmune condition, either by antigenically mimicking self proteins and giving rise to a cross reactive immune response, or by destruction of infected cells leading to an immune response to host cell proteins which are not normally targeted by the immune system. It has recently been demonstrated that bystander damage is responsible for causing the autoimmune diabetes experimentally induced in mice by Coxsackie B4 virus (Horwitz et a l 1998). A similar phenomenon is theoretically possible in human autoimmune diseases including multiple sclerosis.

Enveloped viruses may take up cellular membrane components such as MHC and HLA molecules during the budding process. These in turn may carry peptides which

could then be presented to the immune system in an aberrant fashion, which could result in an autoimmune response (Dalgleish, 1987).

The location of an endogenous related counterpart o f MSRV in the T-cell receptor alpha delta locus could conceivably lead to aberrant expression of T-cell receptor genes. The established genetic susceptibility conferred by the T-cell receptor locus is intriguing in the context of the recent finding o f an endogenous homologue of MSRV in this location (Alliel et a l 1998; Blond et a l 1999).

A recently published hypothesis is that multiple sclerosis is due to a failure of activation-induced apoptosis of autoreactive T-cells (Pender, 1998). It is conceivable that MSRV might be exerting such an effect by immortalising T-cells. Some of the data from the in-vitro MSRV culture experiments does suggest that MSRV has an immortalising effect on infected cells, as indicated by their tumourigenic potential in nude mice and ability to achieve high passage number (Perron et a l 1997b). Although this is a theoretical possibility, it has not yet been established whether MSRV can or does infect and replicate in T-cells.

4.3.2 Gliotoxin

MSRV could possibly exert a directly toxic effect on the glial cells o f the CNS, although such an effect, or infection of these cells, remains to be demonstrated. MSRV has been shown to produce almost synchronous massive apoptosis in

leptomeningeal and choroid plexus cultures (Perron et a l 1997). Also, a gliotoxic factor has recently been identified in the CSF of multiple sclerosis patients (Menard

et a l 1997b; Menard et a l 1998a; Menard et a l 1998b). This gliotoxin was shown to cause apoptosis in-vitro in astrocytes and oligodendrocytes, but not in fibroblasts, Schwann cells, endothelial cells or neurones (Menard et a l 1998a). Monocyte/macrophage cell culture supernatants from multiple sclerosis patients were shown to possess this gliotoxic activity, which also correlated with reverse transcriptase activity and the presence of the MSRV genome (Menard et a l 1997a).

The gliotoxic factor has been partially characterised and shown to be a stable 17 kDa glycoprotein. The sequence of this glycoprotein remains to be determined, but if it is encoded by the MSRV genome, or its production is induced by MSRV, this would provide a direct mechanism for the demyelination observed in MS. Matrix Assisted Laser Desorption/Ionisation Time O f Flight (MALDI-TOF) mass spectrometry analysis of trypsin digested fragments of the purified gliotoxin has demonstrated that the protein is compatible with a region of the Pol protein o f MSRV (Alliel et a l

1998). However the resolution achieved in this analysis was far from optimal and the relationship of the gliotoxin with MSRV Pol currently remains speculative (J. Belli veau personal communication). If MSRV does in fact encode this gliotoxin or causes it to be expressed then a possible direct mechanism for the pathogenesis of MSRV in multiple sclerosis will have been established.

Injection of approximately lOng of the partially purified 17kDa gliotoxin into the CSF of rats produced widespread apoptosis and also some necrosis within the CNS

(Perron, personal communication). Ten days after injection of the gliotoxin the majority o f the cells to have undergone apoptosis were astrocytes, but also included oligodendrocytes. However, three months after injection of the gliotoxin demyelination and death of oligodendrocytes was a far more important feature o f the damage caused to the CNS (Perron, personal communication). Astrocytes maintain the structure and function o f the blood brain barrier (Stewart and Wiley, 1981), the loss o f integrity of which is one of the primary features of an acute MS lesion, with oligodendrocyte depletion occurring secondarily (Hawkins et al. 1991; Gay and Esiri, 1991; Poser, 1993). These observations provide tantalising evidence for a potential animal model of multiple sclerosis in which the gliotoxin acts as a trigger for the development of disease.

Most recently this gliotoxic factor has been shown to be present in the urine from 32/35 (91%) of multiple sclerosis patients but 0/35 (0%) of healthy controls (Marcus Vocanson et al. 1998). The gliotoxic activity causes a form o f apoptosis to occur in a human glial hybrid cell line (cell line CLTT 1-1 (Galiana et al. 1990)).