The level of viral infection of antigen-presenting cells correlates with the level of development of theiler's murine encephalomyelitis virus-induced demyelinating disease

Intracerebral infection with Theiler's murine encephalomyelitis virus (TMEV) induces immune-mediated demyelinating disease in susceptible SJL/J mice but not in resistant C57BL/6 mice. Previous studies have indicated that the major histocompatibility complex (MHC) genes play the most prominent role in the development of TMEV-induced demyelinating disease. In this study, we used C57BL/6.S (B6.S) congenic mice, which carry H-2^s MHC genes instead of H-2^b MHC genes in conjunction with the C57BL/6 (B6) background genes. Our data show that virus-infected B6.S mice are free from disease and have significantly lower viral loads than susceptible SJL mice, particularly in the spinal cord. A strong protective Th1-type T helper response with virtually no pathogenic Th17 response was detected in B6.S mice, in contrast to the reduced Th1- and robust Th17-type responses in SJL mice. Notably, lower levels of viral infectivity in B6.S antigen-presenting cells (APCs) correlated with the disease resistance and T-cell-type response. In vitro studies using APCs from B6.S and SJL mice show that TLR2, -3, -4, and -7, but not TLR9, signaling can replace viral infection and augment the effect of viral infection in the differentiation of the pathogenic Th17 cell type. Taken together, these results strongly suggest that the viral replication levels in APCs critically affect the induction of protective versus pathogenic Th cell types via the signaling of pattern recognition receptors for innate immune responses. Our current findings further imply that the levels of viral infectivity/replication and TLR-mediated signaling play critical roles in the pathogenesis of chronic viral diseases.