Tissue factor pathway inhibitor is an inhibitor of factor VII‐activating protease

Background:  Factor VII‐activating protease (FSAP) is a serine protease that circulates in plasma in its inactive single‐chain form and can be activated upon contact with dead cells. When activated by apoptotic cells, FSAP leads to the release of nucleosomes. The serpins C1‐inhibitor and α₂‐antiplasmin are reported to be the major inhibitors of FSAP. However, regulation of FSAP activity by Kunitz‐type inhibitors is not well studied. Objectives:  To compare the inhibition of FSAP activity and FSAP‐induced nucleosome release from apoptotic cells by tissue factor pathway inhibitor (TFPI) with that of C1‐inhibitor and α₂‐antiplasmin. Methods:  Apoptotic cells were incubated with plasma or FSAP in presence of the inhibitor, and nucleosome release was analyzed with flow cytometry. Monoclonal antibodies against TFPI and altered forms of TFPI were used to investigate which domains of TFPI contribute to FSAP inhibition. Results and Conclusions:  We show that TFPI abrogates FSAP activity and nucleosome release from apoptotic cells. TFPI is a much more efficient inhibitor than C1‐inhibitor or α₂‐antiplasmin. The active site of K2 is required for inhibition of FSAP. A direct binding interaction between FSAP and the C‐terminal domain of TFPI is also required for efficient inhibition. Inhibition of FSAP‐induced nucleosome release by recombinant TFPI might, in part, explain the anti‐inflammatory effects of recombinant TFPI infusion observed in animal and human sepsis.