Abstract Abstract 2313 Proteolytic cleavage of ultra large von Willebrand factor (ULVWF) released from endothelial cells by ADAMTS13 metalloprotease is critical for maintaining normal hemostasis. However, the effect of infusing ADAMTS13 on thrombus composition remains poorly understood. In this study, we determined the morphology and composition of thrombi formed in carotid arteries after topical application of FeCl3 in Adamts13−/− mice receiving PBS, recombinant human full-length ADAMTS13 (FL) and carboxyl-terminal truncated variant after spacer domain (S), using scanning electron microscopy and quantitative image analysis. We showed that in Adamts13−/− mice 5 min after FeCl3 injury, formed arterial thrombi were comprised ∼39% platelets, ∼26% red blood cells, and ∼35% fibrin. The arterial thrombi in these mice were structurally deformed. An infusion of recombinant FL (final concentration of 10 nM) significantly reduced the accumulation of platelets (∼18%) but increased the fibrin network (57%) without affecting the composition of red blood cells (∼25%) in the arterial thrombi formed at the same time point after FeCl3 injury. Similar effects on the morphology and composition of FeCl3-induced arterial thrombi were observed after infusion of recombinant S (10 nM) into Adamts13−/− mice. Kinetic analysis showed that there was a decrease in platelet accumulation over the time of 30 min during thrombus formation with a slight increase in accumulation of red blood cells and formation of fibrin in Adamts13−/− mice. But, the infusion of recombinant FL and S into Adamts13−/− mice restored the kinetics of platelet/red blood cell accumulation and fibrin formation to those observed in wild-type mice. Our findings, revealing the apparent difference in thrombus composition, provide novel insight into the mechanism of ADAMTS13 function in vivo, which may shed more light on the pathogenesis of thrombotic thrombocytopenic purpura and other arterial thrombotic disorders associated with deficiency of plasma ADAMTS13 activity. Disclosures: No relevant conflicts of interest to declare.