Abstract: The overall goal of this grant is to elucidate the specificity, function and role in disease pathogenesis of an innate immune receptor (CLEC2d) that we have discovered recognizes cellular histones and HMGB1. The importance of this project is that histones and HMGB1, when released from injured cells, at as damage- associated molecular patterns (DAMPS) that have been implicated in the pathogenesis of a number of important diseases, including sepsis, ischemia-reperfusion injury, toxic organ damage and autoimmunity. The study of CLEC2d will provide insight into how these DAMPs mediate their effects and whether CLEC2d could be a therapeutic target to block the contribution of these DAMPs to disease. The first aim will elucidate the function of CLEC2d in innate immune cells. We will explore the hypotheses that this CLEC is important for: (a) the direct stimulation of innate immune cells by histones and HMGB1; (2) stimulation of innate immune cells by histoneHMGB1-associated immunostimulatory molecules (PAMPs and DAMPs), and/or (3) the clearance of extracellular histones/HMGB1. The second aim will elucidate the role of the CLEC2d in histone/HMGB1- mediated diseases in vivo. We will explore the hypothesis that the CLECs are needed for histone/HMGB1- mediated pathology in vivo and the pathogenic sequelae of tissue injury and sepsis. The third aim will define the specificity of CLEC2d, how histone modifications alter recognition and responses, and whether this receptor recognizes other poly-basic DAMPs. Our hypotheses are that CLEC2d recognized a poly-basic degenerate motif(s) that is shared between 5 histones, HMGB1 and some other DAMPs and is sensitive to alternations in post-translational modifications of these DAMPs that occur in some diseases.