Toll-like Receptor 7 Mitigates Lethal West Nile Encephalitis via Interleukin 23-Dependent Immune Cell Infiltration and Homing

West Nile virus (WNV), a mosquito-transmitted single-stranded RNA (ssRNA) flavivirus, causes human disease of variable severity. We investigated Toll-like receptor 7-deficient (Tlr7^-/-) and myeloid differentiation factor 88-deficient (Myd88^-/-) mice, which both have defective recognition of ssRNA, and found increased viremia and susceptibility to lethal WNV infection. Despite increased tissue concentrations of most innate cytokines, CD45⁺ leukocytes and CD11b⁺ macrophages failed to home to WNV-infected cells and infiltrate into target organs of Tlr7^-/- mice. Tlr7^-/- mice and macrophages had reduced interleukin-12 (IL-12) and IL-23 responses after WNV infection, and mice deficient in IL-12 p40 and IL-23 p40 (Il12b^-/-) or IL-23 p19 (Il23a^-/-), but not IL-12 p35 (Il12a^-/-), responded similarly to Tlr7^-/- mice, with increased susceptibility to lethal WNV encephalitis. Collectively, these results demonstrate that TLR7 and IL-23-dependent WNV responses represent a vital host defense mechanism that operates by affecting immune cell homing to infected target cells.