PROGRAM SUMMARY Crohn's disease (CD) is a debilitating condition with no known cure. The precise cause(s) of CD remain undefined. Increasing evidence suggests that CD may be initiated by a dysregulated innate immune response against unknown antigens in a genetically-susceptible host. The major strengths of this competitive renewal are the unique mouse models of CD-like ileitis, which have been extensively investigated by our group, and the highly synergistic nature of the program. During the last funding period, this Program Project Grant (PPG) has made important discoveries and provided evidence of a key role for innate immunity and NOD2 signaling in the pathogenesis of CD-like experimental ileitis. The objective of this competitive renewal application is to continue our efforts to define the role of innate immunity in CD by using a multidisciplinary approach that takes advantage of the complementary and synergistic expertise of the three PIs and their collaborators, as well as the scientific cores that provide unique services that are otherwise unavailable to PPG investigators. The Program Project will continue to be directed by Dr. Fabio Cominelli and will consist of three projects and three cores. Project 1, headed by Dr. Fabio Cominelli, will extend his focus on NOD1 and RIP2 signaling, and investigate the hypothesis that the interaction(s) between NOD receptors, intestinal dysbiosis and IL-1 play a key role in the pathogenesis of CD-like ileitis. Project 2, headed by Dr. Derek Abbott, will test the hypothesis that aberrant ubiquitination patterns influence the pathogenesis of IBD using CRISPR technologies, retroviral reconstitution experiments, and cutting edge mass spectrometry methodologies. Project 3, headed by Dr. Theresa Pizarro, will study the role of IL-33 and ILCs, and their interactions with the gut microbiome in the pathogenesis of CD-like ileitis. These projects are supported by an Administrative Core, which provides administrative support, monitors scientific progress, and coordinates the enrichment program. The Mouse Congenics Core will centralize production and breeding of mice with experimental CD and the development of congenic substrains for the individual projects. The Germ- free and Gut Microbiome Core will provide germ-free mice, as well as sequencing- and culture-based analysis of the gut microbiota to the individual projects. The long-term goal of this Program Project is to understand key pathogenic mechanisms of innate immunity in experimental CD, which can be immediately translated to the human condition, to aid development of a cure for at least a subgroup of patients with this devastating disease.