Tissue factor-independent inhibition of thrombin generation by tissue factor pathway inhibitor-alpha

Background Tissue factor pathway inhibitor- (TFPI) inhibits factorXa by forming a binary TFPI-FXa complex in a reaction that is stimulated by proteinS. TF-FVIIa forms a quaternary complex with TFPI and FXa, which shuts off the initiation of coagulation via the extrinsic pathway. AimTo investigate whether direct inhibition of FXa by TFPI independently of TF plays a role in downregulating coagulation. MethodsInhibition of FXa by TFPI in plasma was determined by measuring thrombin generation triggered with FXa, the FX activator from Russell's viper venom (RVV-X), FXIa, or FIXa. TF-independent anticoagulant activities of TFPI and its cofactor, proteinS, were quantified: (i) after neutralization of TFPI and proteinS with anti-TFPI or anti-proteinS antibodies; and (ii) in TFPI-depleted or proteinS-depleted plasmas supplemented with varying amounts of TFPI or proteinS. ResultsBoth anti-TFPI and anti-proteinS antibodies enhanced thrombin generation in plasma triggered with RVV-X, FXa, FIXa, or FXIa. Anti-TFPI and anti-proteinS antibodies decreased the lag time and increased the peak height of thrombin generation to the same extent, indicating that inhibition of FXa by TFPI requires the presence of proteinS. TFPI and proteinS titrations in TFPI-depleted or proteinS-depleted plasma in which thrombin formation was initiated with triggers other than TF also revealed TF-independent anticoagulant activity of TFPI, which was completely dependent on the presence of proteinS. ConclusionDirect inhibition of FXa by TFPI contributes to the downregulation of coagulation