Leukocyte urokinase plasminogen activator receptor and PSGL1 play a role in endogenous arterial fibrinolysis

Fibrin is an integral component of arterial thrombi. Using a mouse model of arteriolar thrombosis, high-speed fluorescence microscopy reveals that, within minutes, the fibrin content of thrombi rapidly increases and then decreases. The decrease in fibrin coincides with leukocyte binding to the thrombi, a process mediated by the interaction of leukocyte P-selectin glycoprotein ligand-1 (PSGL-1) with P-selectin on the surface of activated platelets. Because leukocytes possess urokinase-type plasminogen activator (uPA) activity, we used mice deficient in uPA or the uPA receptor (uPAR) to explore the contribution of leukocyte-associated uPA to the loss of fibrin from these thrombi. Fibrin loss in both uPA-deficient mice and uPAR-deficient mice was reduced compared with that in wild-type controls. Transfusion of leukocytes from wild-type mice into uPAR-deficient mice restored fibrin loss to levels similar to that in wild-type mice. In contrast, transfusion of leukocytes from mice deficient in uPAR or PSGL-1 did not enhance fibrin loss. Thus, fibrin loss from microarteriolar thrombi is mediated, at least in part, by leukocyte-associated uPA in a process that requires leukocyte uPAR and PSGL-1.