PROJECT SUMMARY: West Nile virus (WNV) is a highly virulent human pathogen of the central nervous system (CNS) and the most common cause of epidemic encephalitis in the United States. There are no vaccines or specific antiviral treatments available for WNV-infected individuals, and development of such an arsenal requires a more complete understanding of its pathogenesis. WNV is primarily transmitted through the bit of an infected mosquito. Once deposited into the skin, the virus replicates in a local draining lymph node prior to viremia, which allows the virus to spread to various organs, including the CNS, where neurons are the major targets of infection. Currently, it is unclear which cells are infected outside of the CNS, but numerous in vitro studies suggest that myeloid cells, such as monocytes/macrophages, are likely the primary targets. To begin understanding this in a more physiologically relevant model, we utilized an ex vivo human lymphoid tissue model, where WNV replicates robustly. We immunophenotyped the WNV-infected cells and found that a significant proportion were CD4+ T cells. To further validate these findings, we generated WNV mutant strains that contain cell-specific microRNA (miR) targets (miR-Ts) to restrict WNV replication in a cell-specific manner. We engineered lymphoid- or myeloid-specific miR-Ts into the genome of WNV and found that WNV strains unable to replicate in myeloid cells were able to replicate nearly identically to wild-type WNV strains, while WNV strains incapable of replicating in lymphoid cells were unable to replicate in this same model. Therefore, we hypothesize that lymphoid cells are essential initial cellular targets for WNV replication in the periphery. In this application, we seek to identify the cells that are essential for WNV replication in the human lymphoid tissue ex vivo (Aim 1) as well as in mice (Aim 2).