During viral encephalitis, T-cell infiltration of the CNS is essential for host protection.86–89 Studies have shown that major histocompatibility complex (MHC) class I and II proteins and costimulatory molecules CD40 and CD86 are expressed on the surface of activated microglia following viral infection, and thus, the peptides in the MHC class I and MHC class II complexes are presented to CD8+ and CD4+ T cells, respectively.86,90,91 However, whether microglia are essential antigen-presenting cells in the CNS for host protection is not clear from the evidence obtained thus far. This evidence concerns the gene CD8A and viral encephalitis.