Complement Regulator Factor H is a Cofactor for Thrombin in both Pro- and Anticoagulant Roles

BackgroundComplement FH (FH) is a key regulator of complement activity whereas thrombin (FIIa) is central to hemostasis with both pro- and anticoagulant functions. Both have separately been shown to have auxiliary activities across the two systems. The purpose of this study was to determine the effect of FH on pro- and anti-coagulant functions and investigate the interaction between FH and thrombin. MethodsTail bleeding time and hemolysis were measured in FH-deficient mice (CFH-/-). Activated partial thromboplastin time (aPTT) was determined in FH-depleted human plasma. FH effect on fibrin clot generation was investigated in turbidity assays and on activated protein C (APC) generation. Binding affinity of thrombin with FH was determined using surface plasmon resonance (SPR). ResultsTail bleeding time in CFH-/- mice was significantly prolonged compared to wild type mice. The aPTT in FH-depleted human plasma was elevated compared to normal plasma and restored by adding back FH to depleted plasma. Accordingly, FH enhanced thrombin-mediated fibrin clot generation by shortening lag time, increasing rate of clot formation and maximum turbidity, and affected clot structure. Despite this, FH also increased the rate of thrombin-mediated protein C (PC) activation, both in the presence and absence of soluble recombinant thrombomodulin (TM). Nanomolar affinity binding of FH with thrombin, but not prothrombin, was confirmed. ConclusionComplement FH binds thrombin with strong affinity and acts as a novel cofactor that enhances both pro- and anticoagulant actions of thrombin. These data highlight an important role for FH in hemostasis. Key pointsO_LIAbsence of FH prolongs tail bleeding time in CFH-/- mice and activated partial thromboplastin time (aPTT) is elevated in human FH-depleted plasma. C_LIO_LIFH acts a cofactor for thrombin by enhancing fibrin generation, altering fibrin clot structure and enhancing TM-thrombin mediated protein C activation C_LI