Role of viruses in the etiopathogenesis of multiple sclerosis.

Epidemiological studies performed to identify the possible cause of Multiple Sclerosis (MS) suggest that an environmental agent could be involved in its etiopathogenesis. For a long time it has been hypothesized that this agent was a virus, but until now no virus specific to MS has been consistently identified. Animal models indicate that the demyelination of the central nervous system can be induced by certain families of viruses, but the implication of this in the etiopathogenesis of MS has not been clearly demonstrated. Morbilliviruses were the most studied. Research on this subject arose from the observation of the similarity between the brain lesions occurring in MS and encephalitis caused by the measles virus (Subacute Sclerosing Panencephalitis). Antimeasles, antirubella, antiherpes zoster antibodies have been found in the blood and the cerebrospinal fluid of MS patients, but the relationship between this finding and the disease is not clear. It has recently been proposed that the pathogenetic immune response in the brain of MS patients might be directed predominantly towards antigens of a DNA virus, such as JCV. This preferentially infects glial cells and causes a demyelinating syndrome in immunodeficient subjects, called Progressive Multifocal Leucoencephalopathy. The target JC viral antigens in MS could be synthesized during transient viral reactivation. A recent hypothesis is that retroviruses may intervene in the etiopathogenesis of MS. Strong interest has been taken in HTLV-I after its identification in Tropical Spastic Paraparesis, a disease with certain similarities to MS. Serologic and polymerase chain reaction findings from various authors have suggested an association between this human retrovirus and MS. However more recent data are not consistent with this. Current experiments aimed at detecting retroviral particles in long-term cultured peripheral blood monocytes and cerebrospinal fluid mononuclear cells in MS could clarify whether these cells provide a reservoir for such viruses, with a latency of many years without expression at brain level.