The viral hypothesis in multiple sclerosis: role of Epstein-Barr virus and human endogenous retroviruses

Epstein Barr Virus (EBV) is a major risk factor in Multiple Sclerosis (MS), via as yet unclear mechanisms. Several hypotheses have been proposed to explain how EBV infection could cause MS and the aim of this thesis was to better understand the mechanisms of action of EBV in the context of MS studying a) the role of EBV in myelin antigen presentation by B cells and b) the association of HERVs with MS. In a non-human primate experimental autoimmune encephalomyelitis (EAE) model, an EBV-related lymphocryptovirus enables B cells to protect a proteolysis-sensitive immunodominant myelin oligodendrocyte glycoprotein (MOG) peptide (residues 35- 55) against destructive processing. This facilitates its cross-presentation to autoaggressive cytotoxic MHC-E-restricted cytotoxic T cells. The present study extends these observations to human B cells and identifies a key role of autophagy. EBV infection upregulated antigen presentation-related markers on B cells and activated the cross-presentation machinery. Although human MOG protein was degraded less in EBV-immortalized B-lymphoblastoid cell lines (LCL) than in uninfected B cells, induction of cathepsin G activity by EBV led to total degradation of the immunodominant peptides. Inhibition of cathepsin G or citrullination of the arginine residue within a LC3-interacting regions (LIR) motif of immunodominant MOG peptides abrogated their degradation. Internalized MOG co-localized with autophagosomes, which may protect it from destructive processing. Thus, EBV infection switched MOG processing in B cells from destructive to productive possibly facilitating cross-presentation of disease-relevant epitopes to CD8+ T cells. This mechanism could facilitate presentation of myelin autoantigens that may be involved in MS induction and progression. The first part of this thesis shows a possible EBV-mediated mechanism involved in MS pathogenesis, but it is likely that different mechanisms act alternatively or cumulatively in different individuals based on environmental and genetic differences. A further mode of action of EBV is through the activation of Human Endogenous Retroviruses (HERVs). In normal conditions HERVs are silenced or expressed at low levels, but in some pathological cases, like MS, their expression is higher than in the healthy population. We performed a systematic review and meta-analysis of the literature on the association between HERVs and MS. The systematic review suggested a strong association between HERV expression and MS, in particular with the HERV-W family. The meta-analysis showed odds ratios of 22, 44, and 6 for the expression of MSRVpol in serum/plasma, MSRVenv in PBMC and MSRVpol in CSF respectively. Furthermore, we confirmed the association experimentally. An increased expression of MSRV/HERV-Wenv and TLR4 RNA was detected in blood of MS patients compared with control groups and the viral protein Env was expressed mainly by B cells and monocytes, but not by T cells. Our finding that EBV infection can induce the expression of MSRV/HERV-Wenv is consistent with previous reports in the literature. We also established that such increased expression was not due to a repression of retroviral restriction factors in LCL. A further connection between HERVs and MS is supported by the observation that people infected by HIV may have a lower risk of developing MS than the HIV non- infected, healthy population. We found that the expression of MSRV/HERV-Wenv RNA in HIV-infected people was lower than in MS patients and similar to healthy controls. Nevertheless, there was no difference in MSRV/HERV-Wenv expression between antiretroviral drug -treated and -untreated HIV patients. The expression of MSRV/HERV-Wenv was also detected in vitro in LCL treated with different classes of antiretroviral treatments (ART) and only Efavirenz (NNRI) reduced MSRV/HERV- Wenv expression. In conclusion, taking in consideration the multifactorial aetiology of MS, it is likely that EBV infection and increased expression of MSRV/HERV-W are significant contributing factors in genetically predisposed individuals. This thesis helps to better understand the mechanisms of action of EBV and HERVs in the context of MS