Project Summary/Abstract Stroke is a prevalent and devastating disease with limited therapeutic options. Inflammation and immune cells are major components in the pathophysiology of ischemic stroke and contribute to acute and delayed tissue injury. However, our incomplete understanding of the factors regulating the immune responses triggered by cerebral ischemia remains a significant obstacle to the development of effective therapeutic interventions based on modulating post-ischemic inflammation. Besides activation of brain resident immune cells, ischemic stroke is characterized by the recruitment of peripheral innate and adaptive immune cells that participate in the inflammatory response and contribute to the damage. Commensal microbiota that populate epithelial surfaces play a defining role in shaping the immune system, the development, maintenance and function of which depends critically on the relative abundance and composition of the different microbial species. In particular, intestinal commensal bacteria, the most abundant symbiotic compartment in the body, have emerged as a potent regulator of the immune response to stroke. The long-term goal of this research program is to elucidate the role of intestinal microbiota in stroke pathobiology and develop the experimental framework for new preventative and therapeutic approaches for ischemic stroke. In the present application, we will test the hypothesis that the interaction of commensal intestinal microbiota with dendritic cells is a critical determinant of stroke outcome by modulating the immune system and inflammatory response to cerebral ischemia. Supported by relevant preliminary results, this application will test the hypothesis that commensal intestinal microbiota modulate stroke outcome by acting on intestinal dendritic cells to either induce a tolerogenic or pro-inflammatory phenotype affecting T cell differentiation. These intestinal immune changes propagate to the brain and meninges after stroke by increased immune cell trafficking. To this end, we will determine (a) the roles of different dendritic cell populations in mediating the neuroprotective effects of altered microbiota and (b) the cellular and molecular targets of microbiota that lead to altered intestinal immunity and their importance for stroke outcome. These goals will be achieved using a mouse model of transient focal cerebral ischemia with assessment of histological and neurological outcome, a model of altered gut bacteria, cell tracking of intestinal immune cell, and in vitro immune cell coculture models. This proposal may open the way to new avenues for stroke prevention and therapy based on modulation of the immune system by the gut microbiota.