Phospholipid-bound tissue factor modulates both thrombin generation and APC-mediated factor Va inactivation

Hemostasis is initiated by tissue factor (TF) exposed on cellular phospholi pid (PL) membranes, leading to thrombin generation. The binding of thrombin to thrombomodulin (TM), activates the protein C pathway, resulting in the inactivation of factors Va and VIIIa by activated protein C (APC) and a neg ative feedback effect on thrombin generation. A new assay system was develo ped for simultaneous measurement of thrombin and APC generation in defibrin ated plasma induced by large unilamellar PL vesicles complexed with full-le ngth recombinant TF (TF:PL). TF:PL preparations with a low TF concentration induced an initial rate of thrombin generation below 100 nM/min, and resul ted in less thrombin formation in the presence of TM than in its absence. I n contrast, TF:PL preparations with a high concentration of TF induced a hi gher rate of thrombin generation, and APC-mediated feedback inhibition did not occur, despite maximal APC generation. We used the same TF:PL surfaces to study factor Va inactivation by APC in a non-plasma reaction system, and found an inverse correlation between TF surface density and the rate of fa ctor Va inactivation. This observation suggests a previously unrecognized h emostatic effect of TF, namely a non-enzymatic surface density-based inhibi tion of the anticoagulant effect of APC. In this model, high concentrations and surface density of TF exert complementary effects by promoting the reg ular procoagulant cascade and by inhibiting the protein C pathway, thereby maximizing hemostasis after vascular injury.