The authors wish to thank members of the Artis laboratory for the critical reading of this manuscript. Research in the Artis laboratory is supported by the National Institutes of Health (AI061570, AI074878, AI095466, AI095608, AI102942, and AI097333 to D.A.), the Burroughs Wellcome Fund Investigator in Pathogenesis of Infectious Disease Award (D.A.) and Crohn’s & Colitis Foundation of America (D.A.). Additional funding was provided by NIH F30-AI112023 (J.R.B.), T32-AI060516 (J.R.B.), T32-AI007532 (L.A.M.), KL2-RR024132 (B.S.K.), DP5OD012116 (G.F.S.), P01AI06697 (D.L.F.), F31AG047003 (J.J.T.) and DP2OD007288 (P.S.) and by the Searle Scholars Award (P.S.). We thank M. A. Lazar for scientific and technical advice, D. E. Smith for providing Il33−/− mice, A. Goldrath for providing Id2−/− chimaeras, and A. Bhandoola for providing Tcf7−/− mice. We also thank the Mouse Phenotyping, Physiology & Metabolism Core at the Diabetes Research Center (DRC) of the Institute for Diabetes, Obesity & Metabolism (IDOM) as well as the Penn Diabetes Endocrine Research Center Grant (P30DK19525). In addition, we thank the Matthew J. Ryan Veterinary Hospital Pathology Laboratory, the Penn Microarray Facility, and the Mucosal Immunology Studies Team (MIST) of the NIH NIAID for shared expertise and resources. The authors would also like to thank the Abramson Cancer Center Flow Cytometry and Cell Sorting Resource Laboratory for technical advice and support. The ACC Flow Cytometry and Cell Sorting Shared Resource is partially supported by NCI Comprehensive Cancer Center Support Grant (no. 2-P30 CA016520). This work was supported by the NIH/NIDDK P30 Center for Molecular Studies in Digestive and Liver Diseases (P30-DK050306), its pilot grant program and scientific core facilities (Molecular Pathology and Imaging, Molecular Biology, Cell Culture and Mouse), as well as the Joint CHOP-Penn Center in Digestive, Liver and Pancreatic Medicine and its pilot grant program. In addition, we would like to acknowledge and thank the New York Organ Donor Network, the Cooperative Human Tissue Network-Eastern Division and especially the donors and their families. We apologize to colleagues whose work we were unable to quote owing to space constraints.