Rebalancing the hemostatic system by targeting endogenous anticoagulant pathways, such as the protein C (PC) system, was being studied as a technique to improve hemostasis in patients with hemophilia. Recent intravital hemostasis investigations found that thrombin activity is sequestered in the extravascular compartment in some vascular situations. Because PC activation takes place on the surface of endothelial cells, the findings raised serious concerns regarding the context-dependent contribution of activated PC (APC) to the hemostatic response. For the study, researchers investigated the links between thrombin spatial distribution, PC activation, and APC anticoagulant action using a variety of pharmacologic, genetic, imaging, and computational techniques. 

In a microvascular injury model, inhibiting APC activity in animals with the factor V Leiden mutation or infused with an APC-blocking antibody dramatically increased fibrin formation and platelet activation, consistent with APC’s role as an anticoagulant. Computational studies revealed that variations in blood velocity, injury size, and arterial geometry influenced the localization of thrombin formation and, as a result, the degree of PC activation. In vivo testing of computational predictions revealed that when thrombin production occurred intravascularly, without penetration of the vessel wall, inhibiting APC dramatically enhanced fibrin formation in the jugular vein. The findings demonstrated the significance of thrombin spatial distribution in affecting PC activation during hemostasis and thrombosis.