Crohn's disease (CD) is a chronic and debilitating disease affecting more than 500,000 individuals in the United States and millions worldwide. Both environmental (particularly enteric bacteria) and genetic factors are known to play a critical role in the development of CD. Recent studies have revealed that genetic variation in NOD2 is associated with susceptibility to CD and specifically with ileal disease. NOD2 is an intracellular pattern recognition receptor that is activated by muramyl dipeptide (MDP), a conserved motif in peptidoglycan (PGN) that is known to elicit potent adjuvant activity for antibody production in vivo. Upon activation, NOD2 binds the RICK/RIP2 kinase and activates NF-:B and MAPK signaling pathways leading to the induction of multiple host defense molecules. Homozygosity or compound heterozygocity for NOD2 mutations increases the risk 20-40-fold while heterozygosity lead to minimal increase in risk, indicating that loss of NOD2 function is important for disease development. Consistent with the genetic observations, all three CD-associated NOD2 mutations result in proteins that are deficient in inducing NF-?B activation in response to bacterial components including PGN and MDP. The ability of NOD2 to activate NF-?B in response to bacteria suggests that defective sensing of intestinal bacteria somehow promotes susceptibility to disease. However, many questions remain including how NOD2 mutations contribute to CD and why NOD2 mutations are associated with ileal disease. Furthermore, the role of NOD2 in CD pathogenesis is unclear because animal studies have suggested that the CD- associated L1007insC NOD2 mutation functions as a gain-of-function mutation. Notably, Nod2-null mice are highly susceptible to oral inoculation but not intravenous or intra- peritoneal administration of Listeria, indicating that Nod2 plays a role in intestinal immunity to control pathogenic bacteria. Furthermore, NOD2 is expressed in Paneth cells located at the base of the crypts of Liberk|hn in the small intestine providing a possible link between NOD2 and ileal disease. Consistent with the latter, recent results have implicated Nod2 in the regulation of 1-defensins, a class of anti-microbial peptides produced by Paneth cells. The goal of this proposal is to address the role of Nod2 in intestinal immunity and in the pathogenesis of CD using genetically manipulated mice that are deficient in Nod2 or expressed the common CD-associated L1007insC Nod2 mutation.Both environmental (particularly enteric bacteria) and genetic factors are known to play a critical role in the development of Crohn's disease (CD). Recent studies have revealed that genetic variation in NOD2, an intracytoplasmic pattern recognition receptor, is associated with susceptibility to CD, but the mechanism involved is poorly understood. This proposal will address the role of Nod2 in intestinal immunity and in the pathogenesis of CD using genetically manipulated mice that are deficient in Nod2 or expressed the common CD-associated L1007insC Nod2 mutation.