Tissue factor‐independent inhibition of thrombin generation by tissue factor pathway inhibitor‐α. (29th November 2014) - Thomassen, M. C. L. G. D.
Summary Background Tissue factor pathway inhibitor‐α (TFPIα) inhibits factor Xa by forming a binary TFPI–FXa complex in a reaction that is stimulated by protein S. TF–FVIIa forms a quaternary complex with TFPIα and FXa, which shuts off the initiation of coagulation via the extrinsic pathway. Aim To investigate whether direct inhibition of FXa by TFPIα independently of TF plays a role in downregulating coagulation. Methods Inhibition of FXa by TFPIα 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 TFPIα and its cofactor, protein S, were quantified: (i) after neutralization of TFPIα and protein S with anti‐TFPI or anti‐protein S antibodies; and (ii) in TFPI‐depleted or protein S‐depleted plasmas supplemented with varying amounts of TFPIα or protein S. Results Both anti‐TFPI and anti‐protein S antibodies enhanced thrombin generation in plasma triggered with RVV‐X, FXa, FIXa, or FXIa. Anti‐TFPI and anti‐protein S antibodies decreased the lag time and increased the peak height of thrombin generation to the same extent, indicating that inhibition of FXa by TFPIα requires the presence of protein S. TFPIα and protein S titrations in TFPI‐depleted or protein S‐depleted plasma in which thrombin formation was initiated with triggers other than TF also revealed TF‐independent anticoagulant activity of TFPIα, which was completely dependent on the presence of protein S. Conclusion Direct inhibition of FXa by TFPIα contributes to the downregulation of coagulation.