PROJECT SUMMARY - ABSTRACT The overall goal of this proposal is to characterize Factor XII (FXII) functions in sickle cell disease (SCD) and their contribution to venous thrombosis (VT), vaso-occlusive crisis (VOC) and end-organ injury. SCD is a hematologic disorder caused by a single nucleotide mutation of the β-globin gene. Hypoxia-induced sickling of red blood cells (RBCs) results in painful recurrent VOC, hemolytic anemia and cumulatively, in multi-organ damage. A chronic hypercoagulable state with increased risk of venous thrombosis contributes to increased morbidity and mortality among patients with SCD. New targeted anticoagulant therapies are still associated with increased rates of bleeding. We previously demonstrated that FXII contributes to the development of VT through both its enzymatic (FXIIa) functions as well as the thromboinflammatory effects of zymogen FXII-mediated activation of urokinase plasminogen activator receptor (uPAR) on neutrophils. Recently, we found that patients with SCD exhibit chronically enhanced FXII activation compared to healthy controls. Moreover, we showed that FXII expression is elevated in SCD neutrophils, and that FXII contributes to neutrophil activation and adhesion in SCD. Importantly, treatment with an anti-FXII antibody that blocks both zymogen and enzymatic functions of FXII(a) significantly attenuated experimental VT and microvascular stasis in a murine model of SCD. These findings support the central hypothesis that FXII contributes to vaso-occlusion and VT in SCD through distinct cellular and molecular mechanisms. Using clinical samples and well-established murine model of SCD together with novel pharmacologic and genetic approaches, our goals in this application are to: i) identify disease-specific triggers of FXII activation; ii) characterize FXII effects on neutrophil functions; iii) investigate the relative contributions of different cellular sources, and compare the zymogen versus protease functions of FXII in vascular stasis and VT; iv) determine the effect of long-term FXII deficiency on disease progression and early mortality associated with SCD. Our goal is to delineate the mechanisms by which FXII and its downstream effectors drive SCD pathologies, which will lay the foundation for future therapeutic approaches to inhibit FXII.