TAFIa, PAI‐1 and α 2 ‐antiplasmin: complementary roles in regulating lysis of thrombi and plasma clots

Summary.  PAI‐1 and α 2 ‐antiplasmin (α 2 AP) are the principal direct inhibitors of fibrinolytic proteases. Thrombin activatable fibrinolysis inhibitor (TAFI), a plasma procarboxypeptidase activated by thrombin–thrombomodulin to form TAFIa, also regulates fibrinolysis by modulating fibrin. In this study, the relative contributions of PAI‐1, α 2 AP and TAFIa to inhibition of lysis were assessed. In platelet‐poor plasma clots, α 2 AP, TAFIa and PAI‐1 all inhibited lysis, as shown by the addition of neutralizing antibodies to α 2 AP and PAI‐1 ± CPI, a potato carboxypeptidase inhibitor. α 2 AP played the largest role in regulating plasma clot lysis, but neutralization of inhibitors in combinations was more effective in shortening lysis times, with a maximal effect when all three inhibitors were neutralized. In platelet‐rich clots, a larger contribution of PAI‐1 was evident. Tissue plasminogen activator induced lysis of model thrombi, made from whole blood, was approximately doubled on incorporation of CPI, illustrating a substantial contribution of TAFIa to inhibition of thrombus lysis. Similar increases in thrombus lysis were observed on inclusion of neutralizing antibodies to PAI‐1 and α 2 AP, with α 2 AP playing the dominant role. Maximal thrombus lysis occurred upon neutralization of all three inhibitors. These observations suggest that, despite the differences in concentrations and activities of inhibitors, and the different modes of action, the roles of the three are complementary in both plasma clot lysis and thrombus lysis.