PROJECT SUMMARY Powassan virus (POWV) is an emerging tick-borne neuroinvasive flavivirus. Similar to mosquito-borne flaviviruses such as West Nile virus and Japanese encephalitis virus, POWV causes neuroinvasive disease, including encephalitis, meningitis, paralysis, and death. While the majority of flavivirus infections result in asymptomatic infection, a subset of symptomatic flavivirus infections progress to neuroinvasive disease, although the factors influencing susceptibility to severe disease are not fully understood and little is known about the pathogenic mechanisms of POWV. We hypothesize that variation in host antiviral response genes contributes to differential disease outcome following flavivirus infection. We propose to use Collaborative Cross (CC) mice to characterize features of POWV pathogenesis and identify host genes that contribute to POWV susceptibility. The CC is a mouse genetic reference population of recombinant inbred mice generated by crossing eight founder lines that represent three wild-derived and five classical laboratory mouse lines. The CC captures the genetic diversity of laboratory mice in a reproducible manner, since each of the ~80 lines has a known and fixed genome, providing a valuable tool for mapping complex traits. In preliminary studies, we infected a panel of Oas1b-null CC mouse lines with POWV and observed a range of susceptibility phenotypes, suggesting there are host factors other than the well-characterized flavivirus restriction factor Oas1b that modulate POWV pathogenesis in mice. We identified multiple highly susceptible lines (100% lethality), including CC071, and a single resistant line (0% lethality), CC045. Building on this preliminary data, we propose to i) determine viral and immunologic features of POWV pathogenesis in susceptible and resistant CC lines, and ii) map Quantitative Trait Loci (QTL) associated with POWV pathogenesis using F2 progeny of susceptible and resistant lines. In Aim 1, we will evaluate viral loads and infiltrating leukocytes in the CNS following POWV infection, characterize POWV replication in primary cells, and assess blood-brain barrier permeability, using susceptible (CC071) and resistant (CC045) CC lines. In Aim 2, we will evaluate POWV lethality, viral loads, and infiltrating leukocytes in the CNS of F2 mice and map QTL associated with these phenotypes to identify host genetic elements that contribute to POWV susceptibility. The proposed studies will further our understanding of flavivirus neuroinvasive disease through identification of host genetic factors that modulate POWV susceptibility in mice and provide insights into factors that impact neuroinvasive flaviviruses more broadly. The proposed training plan will focus on building a foundation in quantitative genetics techniques to investigate viral and immunologic mechanisms of POWV pathogenesis, and will prepare me for a career as an independent investigator using quantitative genetics techniques to study infec...