Splenic macrophages are required for protective innate immunity against West Nile virus

Although the spleen is a major site for West Nile virus (WNV) replication and spread, relatively little is known about which innate cells in the spleen replicate WNV, control viral dissemination, and/or prime innate and adaptive immune responses. Here we tested if splenic macrophages (M?s) were necessary for control of WNV infection. We selectively depleted splenic M?s, but not draining lymph node M?s, by injecting mice intravenously with clodronate liposomes several days prior to infecting them with WNV. Mice missing splenic M?s succumbed to WNV infection after an increased and accelerated spread of virus to the spleen and the brain. WNV-specific Ab and CTL responses were normal in splenic M?-depleted mice; however, numbers of NK cells and CD4 and CD8 T cells were significantly increased in the brains of infected mice. Splenic M? deficiency led to increased WNV in other splenic innate immune cells including CD11b^- DCs, newly formed M?s and monocytes. Unlike other splenic myeloid subsets, splenic M?s express high levels of mRNAs encoding the complement protein C1q, the apoptotic cell clearance protein Mertk, the IL-18 cytokine and the Fc?R1 receptor. Splenic M?-deficient mice may be highly susceptible to WNV infection in part to a deficiency in C1q, Mertk, IL-18 or Caspase 12 expression.