The BeAn strain of Theiler's murine encephalomyelitis virus (TMEV) persists in the
CNS and produces a chronic inflammatory demyelinating disease that is an animal model
for human multiple sclerosis (MS). The mechanisms leading to TMEV-induced demyelination
are still under study but most likely involve both immune-mediated and virus induced
damage to cells in the CNS, both depending on viral persistence. It is therefore important
to identify the cells in which continued virus production is permitted. In this study,
we looked at virus infection in primary astrocytes, microglia and oligodendrocytes,
derived from brains of neonatal susceptible SJL/J mice. As evidenced by Western blots
and immunocytochemistry, we were able to detect viral antigens in all these brain-derived
cells. In addition, we extended the study to spinal cord tissues from mice suffering
TMEV-induced disease. Immunohistochemistry staining with anti-TMEV sera and antibodies
to specific cell markers detected viral antigens in all these cells. We then asked
the question whether viral antigen present in these cells, particularly in microglia/macrophages,
represented true viral replication or not. By using different techniques, including
immunoprecipitation experiments and the very sensitive method of negative RNA detection
through RNase protection assay, we show that both astrocytes and oligodendroglia permit
de novo viral replication and viral protein synthesis but with only minimal cytopathic
effects. Of these two cell types, astrocytes carry the brunt of viral replication.
In microglia, on the other hand, viral replication is restricted since only minimal
amounts of negative RNA copies can be demonstrated, while there are clear signs that
some of these cells undergo apoptosis. These findings show that the main cell for
viral replication is the astrocyte, rather than the microglia/macrophage. Most of
the viral antigen present in macrophages, therefore, is probably the result of phagocytosis,
rather than actual viral replication. In view of the demonstrated presence of viral
replication in astrocytes and of great amounts of viral antigens in microglia/macrophages,
it is possible that both types of cells act as antigen presenting cells during this
immunopathological disease.