Evaluation of thrombomodulin/TAFI function in plasma using tPA-induced plasma clot lysis time.

Backgrounds and Objects Thrombin activatable fibrinolysis inhibitor (TAFI) is one of the most important physiological fibrinolysis inhibitors. Its inhibitory efficacy under physiological condition remains uncertain. Methods Since thrombin is required to generate activated TAFI (TAFIa) which targets the C-terminal lysine of partially digested fibrin, a clot lysis assay is suitable for evaluating its function. Using tissue plasminogen activator-induced plasma clot lysis time (tPA-PCLT) together with TAFIa inhibitor and recombinant soluble thrombomodulin (TM), we evaluated the specific function of TM/TAFI in plasma milieu. Results TPA-PCLT values were significantly shortened by the TAFIa inhibitor. Recombinant soluble TM (rsTM) supplementation prolonged tPA-PCLT, which was shortened by the TAFIa inhibitor to a time similar to that obtained without rsTM and with the TAFIa inhibitor. Plasma obtained from patients treated with rsTM showed prolonged tPA-PCLT which was shortened by the TAFIa inhibitor but not further prolonged by rsTM. However, no significant correlation was observed between tPA-PCLT and parameters of TM/TAFI system in the plasma. Conclusion The role of the TM/TAFI system in regulating fibrinolysis was successfully evaluated using TAFIa inhibitor and rsTM. Trace amounts of soluble TM in normal plasma appeared sufficient to activate TAFI and inhibit fibrinolysis. Further, a therapeutic dose of rsTM appeared sufficient to activate TAFI and regulate fibrinolysis in the plasma milieu.