ABSTRACT Extracellular histones are nuclear proteins released to the extracellular environment during tissue destruction or injury. Emerging evidence implicates them as danger associated molecules with immunostimulatory capability. The receptor-signaling mechanisms responsible for their tissue or cell-specific effects are poorly understood. In recent studies, we detected elevated plasma levels of histones in burn patients as well as animals. Administration of histones caused microvascular leakage and endothelial hyperpermeability-characteristic pathology underlying multiple organ dysfunction following burns, whereas histone inhibitors attenuated burn-induced barrier leakage. Moreover, we obtained novel evidence for the critical role of C-type lectin receptor 2d (Clec2d)-mediated tyrosine kinase signaling in endothelial response to histones. Built on these intriguing findings, this study will characterize the release, pathophysiological function, and molecular mechanisms of histones as an important contributor to burn-induced endothelial barrier injury in edema-prone tissues, including lungs and gut/mesenteric microvessels. We propose three aims: Aim 1 to characterize circulating histones in burn patients and animals correlated with organ dysfunction; Aim 2 to determine the causal effects of histones on microvascular hyperpermeability during burns; Aim 3 to explore the molecular mechanisms by which histones induce endothelial barrier breakdown. The specific mechanistic hypothesis to be tested is that following thermal destruction of tissues, injured cells release histones into the circulation where they directly interact with the vascular endothelium by binding to Clec2d and activating downstream intracellular signaling mediated by Syk/Src-FAK; these tyrosine kinases phosphorylate proteins that constitute cell-cell junctions and cell-matrix focal adhesions, thereby triggering their conformational changes and leading to increased permeability. This novel pathway will be tested in innovative experiments that incorporate newly developed imaging techniques and molecular assays into a comparative analysis of burn patients, human organs, and animal/cell models. Through this translational study, we expect to gain new insights that will not only shift the current paradigms in vascular endothelial cell biology, but also fill the gaps of knowledge in understanding burn pathophysiology. Identification of circulating histones as a key mediator of burn-induced tissue/organ injury may lead to the development of new diagnostics and therapies for thermal trauma.