PROJECT SUMMARY Noroviruses are the leading global cause of acute gastroenteritis, responsible for 685 million total cases, 200 million pediatric cases, and up to 200,000 deaths in children annually. The long-term goal of this research project is to define the mechanisms by which intestinal microbiota influence norovirus pathogenesis and host immunity, and thus identify critical targets for novel therapeutics. Bile acids are a class of metabolites regulated by microbiota that play opposing roles in norovirus infections: Host-derived bile acids promote human and murine norovirus infections whereas microbiota-derived bile acids inhibit infection. Considering that neonates are more susceptible to norovirus infections and they have a distinct bile acid pool compared to adults that is dominated by host-derived (proviral) species, our central hypothesis is that altered homeostatic bile acid metabolism is the central determinant of the increased vulnerability of neonates to norovirus disease. This will be tested by pursuing three specific aims: 1) Test the effect of perturbing the neonatal bile acid pool on norovirus disease; 2) Determine the mechanism by which microbiota- metabolized bile acids protect neonates from norovirus disease; and 3) Determine the role of maternal breast milk bile acids in regulating neonatal susceptibility to norovirus diarrhea. The proposed research is significant because it will provide mechanistic insight into the role of bile acid metabolism in norovirus pathogenesis and test therapeutic strategies to reduce disease severity in infected infants based on this insight. Our studies will also explore the innovative concept that maternal gut metabolites delivered to neonates via breastfeeding determine neonatal susceptibility to norovirus diarrhea, laying the groundwork for developing novel infant-specific therapeutics that target the entero-mammary pathway in the lactating mother. Finally, bile acids are being increasingly recognized as contributing to other pathologic conditions so understanding basic aspects of their metabolism and trafficking in the mammalian host has broad applicability.