PROJECT SUMMARY Accumulating evidence support the role of bacterial members of the gut microbiota in host defense and a variety of immune disorders, such as inflammatory bowel disease (IBD). The mammalian gastrointestinal tract also harbors diverse animal viruses. Despite their ubiquitous presence, the functional and mechanistic consequences of intestinal colonization by viruses beyond their role as diarrheal pathogens has been unclear. Over the previous funding periods of this award, we developed the murine norovirus (MNV) model as a tractable experimental system to address this major gap in our knowledge. We found that MNV can function as a symbiotic virus by providing benefits to the host, such as protection against secondary bacterial infection. At the same time, MNV induces intestinal abnormalities in mice with a mutation in Atg16L1, an IBD susceptibility gene that participates in the cellular degradative pathway of autophagy. The main objective of this proposal is to use the MNV model to define the mechanisms that determine whether intestinal colonization by a viral symbiont leads to beneficial or adverse outcomes for the host. We introduce data showing that MNV infection can significantly improve survival of newly weaned mice from lethal infection by the Gram-negative bacterial pathogen Citrobacter rodentium. We propose to use this model to elucidate how RNA sensing pathways are induced by a viral symbiont to mediate cross-protection during early-life infection by an enteric bacterial pathogen, a leading cause of mortality. In parallel, we will investigate how Atg16L1 mutation makes an otherwise beneficial virus become harmful. Our preliminary data suggests a novel mechanism in which MNV infection replaces anti-inflammatory lymphocytes in the gut with cytotoxic subsets that induce necrotic death of Paneth cells, a key secretory epithelial cell implicated in human IBD. Defining the mechanistic details of these processes will yield new insight into the nature of a viral symbiont in the gut, and may reveal new pathways involved in the resolution of tissue injury that are distinct from those previously examined during bacterial colonization.

PROJECT NARRATIVE Although diverse animal viruses inhabit the intestines of healthy individuals and patients with chronic diseases, how they impact our physiology is unclear. We developed animal models in which a viral member of the gut microbiota defends against bacterial infections, but triggers inflammatory bowel disease in a genetically susceptible host. We will apply this model system towards uncovering the mechanisms that determine whether intestinal colonization by a virus leads to a beneficial or adverse outcome.