PROJECT ABSTRACT Tumor necrosis factor (TNF) is a pleiotropic cytokine that promotes host defense, cell survival and tissue regeneration under homeostatic condition However, if dysregulated and overexpressed, TNF is a major driver of chronic inflammation. Excessive TNF production in the gastrointestinal tract targets the epithelium, drives increased cell death, and is sufficient to elicit substantial tissue inflammation and chronic disease. Blockade of TNF is a widely utilized biologic that provides therapeutic benefit in a subset of inflammatory bowel disease (IBD) patients. Despite this knowledge, the mechanisms that control the beneficial versus detrimental roles of TNF in the intestine are poorly defined. The fundamental focus of this proposal is to mechanistically define a novel pathway that protects the intestine from TNF-driven damage and inflammation. In recently published and new preliminary data, we have determined that group 3 innate lymphoid cells (ILC3s) are essential to protect the intestinal epithelium from TNF-driven damage and inflammation. Surprisingly, this did not occur via traditional effector pathways, and rather involved production of prostaglandins and growth factors. These data provoke a fundamental hypothesis that ILC3s are essential to shape the protective versus pathologic roles of TNF in the intestine, and this balance is disrupted in human IBD where ILC3s are known to become dysregulated. We will mechanistically test this hypothesis by asking the following specific questions: (1) How does ILC3 production of sensing of prostaglandins impact intestinal health and inflammation? And (2) What are the cellular and molecular mechanisms by which ILC3-derived HB-EGF augments intestinal immunity and protects the gut from TNF? Finally, we will directly translate our findings from basic mouse models into samples from IBD patients. Results from these experiments will pave the way for a greater understanding of TNF-driven intestinal damage and inflammation, which could provoke novel preventative, therapeutic or curative strategies for multiple chronic inflammatory diseases.