ABSTRACT Gastrointestinal diseases caused by enteric viral pathogens such as norovirus and rotavirus impose a significant global health burden and can be lethal in vulnerable populations. For these and other viral pathogens, interferon (IFN) family cytokines are among the most potent elements of the antiviral immune response. Thus, IFN pathways are clinically relevant targets for treatment of disease, and are an important area for continued foundational research. We and others have shown that the most recently discovered type of IFN (type III IFN, IFN-λ) promotes innate antiviral immunity in intestinal epithelial cells (IECs) while minimizing inflammatory damage that can accompany a type I IFN response. Our most recent work has now identified a homeostatic IFN-λ response, stimulated by bacterial microbiota, that elicits antiviral genes within pockets of intestinal epithelium prior to viral exposure. Our studies of mouse rotavirus infection suggest that homeostatic IFN-λ preemptively limits viral infection of IECs. The objective of this research proposal is to define the cellular source of homeostatic IFN-λ production in the intestine (aim 1), the mechanistic basis for its expression (aim 2), and its impact on enteric viral disease (aim 3). Based on our prior work and preliminary studies, our central hypothesis is that intestinal dendritic cells produce homeostatic IFN-λ during transient exposure to bacterial products, promoting preemptive antiviral responses in epithelial cells. Through the studies of this research project, we will identify the basis of the localized homeostatic IFN-λ response in IECs, thereby advancing our fundamental understanding of this IFN type in the antiviral immunity to enteric viral pathogens.