Suppressive Role of Tissue Factor Pathway Inhibitor-alpha in Platelet-Dependent Fibrin Formation under Flow Is Restricted to Low Procoagulant Strength

Tissue factor pathway inhibitor-alpha (TFPI-alpha) is a Kunitz-type serine protease inhibitor, which suppresses coagulation by inhibiting the tissue factor (TF)/factor VIIa complex as well as factor Xa. In static plasma-phospholipid systems, TFPI- thus suppresses both factor Xa and thrombin generation. In this article, we used a microfluidics approach to investigate how TFPI- regulates fibrin clot formation in platelet thrombi at low wall shear rate. We therefore hypothesized that the anticoagulant effect of TFPI-alpha in plasma is a function of the local procoagulant strengthdefined as the magnitude of thrombin generation under flow, due to local activities of TF/factor VIIa and factor Xa. To test this hypothesis, we modulated local coagulation by microspot coating of flow channels with 0 to 100 pM TF/collagen, or by using blood from patients with haemophilia A or B. For blood or plasma from healthy subjects, blocking of TFPI- enhanced fibrin formation, extending from a platelet thrombus, under flow only at <2 pM coated TF. This enhancement was paralleled by an increased thrombin generation. For mouse plasma, genetic deficiency in TFPI enhanced fibrin formation under flow also at 100 pM TF microspots. On the other hand, using blood from haemophilia A or B patients, TFPI-alpha antagonism markedly enhanced fibrin formation at microspots with up to 100 pM coated TF. We conclude that, under flow, TFPI-alpha is capable to antagonize fibrin formation in a manner dependent on and restricted by local TF/factor VIIa and factor Xa activities.