A West Nile virus NS4B-P38G mutant strain induces adaptive immunity via TLR7-MyD88-dependent and independent signaling pathways.

Prior work shows that an attenuated West Nile virus (WNV), the nonstructural (NS)4B-P38G mutant infection in mice induced strong immune responses and protected host from subsequent lethal wild-type WNV infection. Here, we investigated NS4B-P38G mutant infection in myeloid differentiation factor 88-deficient (MyD88-/-) and Toll-like receptor 7-deficient (TLR7-/-) mice and found they had enhanced susceptibility compared to wild-type mice. Both groups had lower WNV-specific IgM response and reduced effector T cell functions. Dendritic cells (DCs) also exhibited a reduced maturation and impaired antigen-presenting functions compared to wild-type DCs. Moreover, infection with NS4B-P38G mutant in TLR7-/- and MyD88-/- mice provided full and partial protection respectively from subsequent challenge with lethal wild-type WNV. There were reduced T cell responses in MyD88-/- and interleukin-1 receptor deficient (IL-1R-/-) mice during secondary challenge with wild-type WNV. In contrast, TLR7-/- mice displayed normal T cell functions. Collectively, these results suggest that TLR7-dependent MyD88 signaling is required for T cell priming during NS4B-P38G 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 wild-type WNV.