The overall objective of this proposal is to provide a training vehicle that will allow the applicant to be prepared for a successful career as an independent investigator in biomedical research. The research is focused on the bacterial pathogen Helicobacter pylori, which persists in the human stomach for the life of the host despite a vigorous innate and adaptive mucosal immune response. The long-term goal is to elucidate the role of the enzyme arginase in inducible nitric oxide (NO) synthase (iNOS)-dependent host defense against H. pylori. The hypothesis of this proposal is that L-arginine (L-Arg) metabolism by arginase II is pathogenic in H. pylori infection by restricting iNOS-dependent host defense and by contributing to the inflammation by generation of polyamines. The specific aims are: Aim 1, in vitro: To demonstrate that L-Arg metabolism by arginase II has important biological effects in the macrophage response to H. pylori. We propose that arginase II effectively competes with iNOS for L-Arg and inhibits iNOS translation and NO synthesis by decreasing the intracellular concentration of L-Arg. Using RAW 264.7 murine macrophages treated with arginase inhibitors and arginase II-specific siRNA/shRNA, and gastric macrophages isolated from arginase ll-/- and wild-type mice, we will assess: A.) i. iNOS expression, protein translation, and activity;and ii. killing of H. pylori;and B.) apoptosis and polyamine levels. Aim 2, in vivo: To determine the biological importance of L-Arg metabolism by arginase II in H. pylori infection. We will use two models of H. pylori infection, a chronic model in which mice are challenged with H. pylori for four months, and an acute model in which mice are challenged for 48 hours. Using both systems, wild-type mice will be treated with arginase inhibitors, and arginase ll-/- and wild-type mice will be compared. At these time points, the following will be assessed: A.) H. pylori colonization, histology, and iNOS expression in gastric tissues;B.) iNOS expression, NO generation, and apoptosis in isolated gastric macrophages. It is expected that these studies will provide new insights into the immunopathogenesis of H. pylori infection. Relevance to public health: H. pylori infects approximately 50% of the world's population and 20-40% of Americans. It causes chronic gastritis in all individuals and is the major cause of gastric adenocarcinoma in the world and the primary cause of peptic ulcer disease that is not drug-induced. Because 10-20% of all of those infected will present with clinical manifestations, the disease burden is enormous. Not all patients can be successfully treated with antibiotics, and there is no screening strategy in place;therefore, greater understanding of the reasons for the failed host immune response, as is being pursued in this study, are greatly needed.