<jats:title>Abstract</jats:title> <jats:p>Theiler’s murine encephalomyelitis virus (TMEV) infection in the central nervous system (CNS) of susceptible mice results in an immune-mediated demyelinating disease, which is considered as a relevant viral model for human multiple sclerosis. We previously showed that the expression of positive costimulatory molecules is higher on microglia of TMEV-resistant B6 mice compared to TMEV-susceptible SJL mice. In this study, we analyzed the expression levels of negative costimulatory molecules, PD-1 and PDL-1, in the CNS of TMEV infected SJL and B6 mice. Our results indicated that TMEV infection induces the expression of both PD-1 and PDL-1 on microglia and macrophages in the CNS but not in the periphery. The expression of only PD-1 on CNS-infiltrating macrophages, but not on resident microglia, was considerably higher in TMEV-infected SJL mice compared to TMEV-infected B6 mice. We further showed that the presence of IL-6 is necessary to induce the upregulated expression of PDL-1 after TMEV infection. TMEV infection appears to provide both IL-6 and other undefined signal(s) for the upregulation. The expression of PD-1 and PDL-1 in the CNS of TMEV-infected mice was increased in the absence of signaling by type I IFNs, TLR3 or MDA5-mediated pathway. Taken together, these results strongly suggest that the expression of PD-1 and PDL-1 in the CNS is upregulated by IL-6 plus an unknown signal(s) following TMEV infection, and downregulated type I IFNs induced via TLRs and MDA5 pathways.</jats:p>