The characterization of la expression during sindbis virus encephalitis in normal and athymic nude mice

To characterize the expression of la systemically and locally on mononuclear cells during acute viral encephalitis, weanling mice were inoculated intracerebrally with Sindbis virus (SV), an alphavirus. Peripheral blood mononuclear cells, splenocytes and perivascular inflammatory cells in frozen brain sections were examined immunocytochemically for the presence of la. la expression increased in the spleen, blood and brain during SV encephalitis. The majority of the cells in the central nervous system (CNS) expressing la were perivascular mononuclear cells but la was also found on stellate parenchymal cells. Using one micrometer cryopreserved serial sections we identified these parenchymal cells as macrophages and microglia but not astrocytes. We also identified rare la-positive cells resembling endothelial cells. Frozen brain sections of SV-infected T cell-deficient nu/nu mice were also examined for la expression. The number and percentage of la-positive cells in perivascular inflammatory cells were markedly decreased compared to normal mice and la-positive stellate parenchymal cells were less numerous. This suggests that immunocompetent T cells are necessary for “normal” infiltration of inflammatory cells and for la expression in the CNS during SV encephalitis.