MyD88-Mediated Signaling in Protective Immunity against an Attenuated West Nile Virus Infection

West Nile virus (WNV) nonstructural (NS) 4B-P38G mutant is highly attenuated in mice. Interestingly, it induces strong immune responses and protects mice from subsequent lethal wild-type WNV NY99 infection. These features have important applications in flavivirus vaccine development. The goal of my dissertation is to understand the underlying mechanisms of WNV NS4B-P38G induced protective immunity. Toll-like receptor (TLR) 7/myeloid differentiation factor 88 (MyD88)-mediated signaling pathways protect host against wild-type WNV infection. Both MyD88-/- and TLR7-/- mice had reduced effector T cell functions compared to wild-type mice following NS4B-P38G mutant infection. TLR7-/- mice displayed normal memory T cell functions and were fully protected from secondary challenge lethal WNV NY99. MyD88-/- mice had reduced memory T cell responses and were partially protected. These results suggest that TLR7-dependent-MyD88 signaling is required for T cell priming during NS4B-P38G vii mutant infection. Whereas the TLR7-independent-MyD88 signaling pathways are involved in memory T cell development, which may contribute to host protection during secondary challenge with NY99. Aging is a risk factor for WNV encephalitis. Similar to TLR7-/- mice, old mice had reduced effector T cells and were partially protected from primary WNV NS4B-P38G mutant infection, but had normal memory T cell response and were all protected from re-challenge with WNV NY99. An impaired TLR7 signaling in old DCs led to lower innate cytokine response and a reduced antigen-presenting capacity compared to young DCs. I also used two human cell lines-THP-1 and THP-1 macrophages to study the immune response following NS4B-P38G infection. NS4B-P38G mutant produced more viral RNA than parental NY99 in both cell types and boosted higher innate cytokine responses with no detectable infective virus. NS4B-P38G mutant infection in THP-1 cells led to more diverse and robust innate cytokine responses than that seen in THP-1 macrophages, which were mediated by TLR7 and retinoic acid-inducible gene 1 (RIG-I) signaling pathways. Thus, a defective viral life cycle during NS4B-P38G mutant infection in human monocytic and macrophage cells leads to more potent cell intrinsic innate cytokine responses. In summary, my dissertation studies suggest that MyD88-mediated signaling pathway regulates protective immune response to WNV NS4B-P38G mutant infection.