PROJECT SUMMARY Stroke is the 5th leading cause of death in the United States and the number one leading cause of long- term disability in adults. Progress in the development of effective pharmacotherapies for acute stroke has been limited and centered on neuronal preservation despite extensive and robust data implicating glial cells, including microglia, in the pathophysiology of ischemic injury. Ischemic preconditioning (IPC) is a brief period of ischemia that confers robust neuroprotection against subsequent ischemic events. Elucidating cellular and molecular mediators of IPC is considered a critical challenge in stroke research with promising therapeutic applications. Research by our group and others has implicated innate immune signaling pathways as being critical for IPC-mediated neuroprotection, and we have specifically identified a requirement for these innate immune pathways in microglia. Research from other groups have suggested possible roles for infiltrating peripheral immune cells in supporting IPC- mediated protection in the central nervous system (CNS), however individual immune cell contributions are not well understood. Furthermore, a growing body of data suggest differential responses to injury between white and grey matter microglia. In this study we will collect single cell RNA sequencing (scRNA-seq) data from the myeloid cells of the cortex after animals have been exposed to an IPC stimulus, prolonged cerebral ischemia (stroke), or both. We will use this dataset to identify differential clusters of cells based on gene expression profiles in these different experimental paradigms to determine neuroimmune and peripheral immune cell responses to IPC and cerebral ischemia, and how the latter is modulated to a neuroprotective milieu by a prior IPC stimulus. Additionally, we will perform these experiments in a white matter model of ischemia/reperfusion injury to elucidate differential responses between white matter and grey matter injury after prolonged cerebral ischemia. Gathering data on white matter predominant tracts is important due to the large volume of white matter impacted in human stroke patients compared to in vivo mouse models, which are grey matter predominant. These studies will help us better understand the contribution of immune cells to ischemia/reperfusion injury and identify potential cellular and molecular targets for therapeutic intervention in stroke patients.