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Background: Autoimmune responses can occur when antigens from the central nervous system (CNS) are presented to lymphocytes in the periphery or CNS. However, whether autoimmune responses emerge after brain ischemia remains controversial. Similarly, it is debated whether genesis of autoimmune responses can impact outcome of ischemic stroke. We hypothesized that that brain ischemia can diversify T cell responses against CNS antigens, and that expanded CNS antigen-specific T cells can promote ischemic brain injury.Methods: We quantified the presence and status of T cells from brain slices of ischemic patients. Using a mouse strain that harbor a transgenic T cell receptor (TCR) to a CNS antigen, the myelin oligodendrocyte glycoprotein (MOG35-55) epitope (2D2), we determined the anatomic location and involvement of antigen presentation cells in the development of T cell reactivity after brain ischemia and how T cell reactivity impacts stroke outcome. Transient middle cerebral artery occlusion (MCAO) and photothrombotic stroke models were used in this study.Results: By coupling transfer of labeled myelin oligodendrocyte glycoprotein (MOG)35-55-specific (2D2) T cells with tetramer tracking, we show an expansion in reactivity of 2D2 T cells to MOG97-108 and MOG103-125in transient MCAO and photothrombotic stroke models. This reactivity and T cell activation occur in the brain and not in the peripheral lymphoid organs after ischemia. Also, microglia act as antigen-presenting cells (APCs) that effectively present MOG antigen, and depletion of microglia ablates expansion of 2D2 reactive T cells. Notably, the adoptive transfer of neuroantigen-experienced 2D2 T cells exacerbates Th1/Th17 responses and brain injury. Finally, T cell activation and MOG-specific T cells can be seen in the brain of ischemic stroke patients.Conclusion: Our findings suggest that brain ischemia activates and diversifies T cell responses locally, which exacerbates ischemic brain injury.