It is well recognized that the function of the intestinal immune system depends on the fine balance between effector, tolerogenic, and regulatory mechanisms. This balance is important not only in preventing disease, but also in providing flexibility and thus instructing the appropriate immune response. Dysregulation in any components of this homeostasis has been known to contribute to IBD. Intestinal commensal bacteria are known to affect multiple immune mechanisms and mouse models of IBD depend on the presence of microbiota. However, the role of the composition of the microbiota ihas not been investigated and whether different taxons of commensal bacteria activate specific branches of the immune system is not known. Th17 cells are a newly discovered helper T cell population that has been proposed to play pro-inflammatory role in the context of IBD. At steady state Th17 cells are exclusively present in the small intestinal lamina propria where they co-exist in a well-regulated balance with Foxp3+ regulatory T cells (Treg). The role of different components of the intestinal microbiota in regulating this balance is currently unknown. We have discovered that the Th17:Treg ratio depends on the composition of the intestinal microbiota. The focus of the current proposal will be to further investigate the role of gut bacteria in this process. Our main hypothesis is that specific components of the commensal intestinal microbiota can be identified to specifically induce Th17 cells with protective function in the intestine. To test this hypothesis, we propose to 1) identify bacterial taxons that specifically induce Th17 cell differentiation;2) examine the mechanisms of their action in vivo and 3) test their involvement in both mucosal and systemic immunity. We will perform most of the experiments in 1) during the K99 phase and the bulk of the experiments in 2) and 3) in the independent phase of the award. Information gathered from these studies will provide major advances in our understanding of the regulation of intestinal immune responses by commensal bacteria. The ultimate goal will be to provide knowledge for the rational design of therapeutic strategies for IBD, based on modulations of the composition or function of the intestinal microbiota.