Mechanisms of West Nile virus pathogenesis in the central nervous system

West Nile virus (WNV) a neurotropic flavivirus that has emerged globally as a significant cause of viral encephalitis. The central aims of this thesis were to test whether type I IFN and interferon induced effectors restricted West Nile virus pathogenesis and to determine the mechanisms of West Nile virus-induced neuronal death. Type I IFN acts as a key innate defense against virus infection by upregulating an array of antiviral effector molecules and modulating cell survival. Mice lacking the type I IFN receptor were acutely susceptible to WNV infection and developed high viral loads associated with enhanced infection in normally resistant cell populations and peripheral tissues. The type I IFN response also independently modulated WNV pathogenesis in the CNS and increased survival of primary neurons. To begin to examine the downstream mechanisms by which IFN functions to inhibit WNV infection, studies were performed utilizing mice lacking two key IFN induced effectors, RNA-activated protein kinase (PKR) and the endoribonuclease RNase L (PKR -/- x RL-/-). PKR-/- x RL-/- mice were highly susceptible to subcutaneous WNV infection and had increased viral loads in the peripheral tissues. A deficiency in PKR and RNase L resulted in decreased IFN-mediated inhibition of WNV in primary macrophages and some neuronal cell types. These data demonstrate that type I IFN and IFN induced effectors control WNV infection by restricting tropism in a cell-specific manner, reducing viral burden, and preventing death of infected neurons. To examine the mechanisms of WNV-induced neuronal death and the importance of apoptosis in pathogenesis, we evaluated the role of a key apoptotic regulator, caspase-3. WNV infection induced caspase-3 activation and apoptosis in the brains of wild-type mice. Caspase-3-/- mice were more resistant to lethal WNV infection, and this was associated with decreased neuronal death in the CNS. Analogously, primary CNS-derived neurons demonstrated caspase-3 activation and apoptosis after WNV infection, and treatment with caspase inhibitors or a genetic deficiency in caspase-3 significantly decreased virus-induced death. These studies establish that caspase-3 dependent apoptosis contributes to the pathogenesis of lethal WNV encephalitis. Taken together, these findings suggest novel therapeutics that bolster type I IFN responses or inhibit apoptotic processes may restrict WNV-induced CNS injury.