Project Summary: Hyperlipidemia and thromboinflammation are among the unifying drivers of cardiovascular disease—the leading cause of morbidity and mortality worldwide. Vascular inflammation results in part from lipid accumulation within the blood vessel lumen, prompting endothelial cell dysfunction and localized loss of endothelial barrier function. Concomitant activation of the coagulation cascade culminates in thrombin generation, inciting platelet activation and pathological clot formation. Our work has defined the role of coagulation factor (F)XI in propagating thrombin generation by way of both activating downstream coagulation factors as well as proteolyzing inhibitors of thrombin generation. Yet, the crosstalk between the coagulation cascade, FXI, and the pathways mediating inflammation and endothelial cell barrier function remain ill-defined. We made the novel observations that the endothelium selectively inhibits activated FXI by binding, inactivating, and internalizing FXIa. Moreover, we found that pharmacological targeting of FXI protected endothelial barrier function in a mouse model of hyperlipidemia. In a non-human primate model of diet-induced hyperlipidemia, our preliminary studies show that pharmacological targeting of FXI reduced platelet sensitization and markers of inflammation including C-reactive protein (CRP). This reduction in CRP is akin to our observations in prospective clinical trials of FXI therapeutics where we found that FXI inhibition reduced levels of CRP in patients on dialysis or following placement of an indwelling catheter. Herein we will define the mechanistic link between FXI activation and activity, inflammation, and endothelial cell barrier function. This program will build on our ability to develop tools for molecular imaging of cellular functions coupled with the creation and use of novel inhibitors of FXI and the contact pathway coagulation factors, with a focus on translation from in silico models, in vitro studies to in vivo mouse and nonhuman primate models. In Aim 1 we will test the hypothesis that inactivation of FXIa by endothelial cells signals to reduce vascular endothelial barrier function. Aim 2 will focus on defining the role of FXI activation and FXIa activity in platelet activation and thromboinflammation associated with hyperlipidemia. We will employ three complementary mouse models and a non-human primate model of hyperlipidemia to improve rigor and translational relevance of our results. This work will provide rationale for use of FXI inhibitors for the treatment and prevention of thromboinflammation.