BOTH CELLULAR AND SOLUBLE FORMS OF THROMBOMODULIN INHIBIT FIBRINOLYSIS BY POTENTIATING THE ACTIVATION OF THROMBIN-ACTIVABLE FIBRINOLYSIS INHIBITOR

Thrombin-activable fibrinolysis inhibitor (TAFI) is a recently describ ed plasma zymogen that can be activated by thrombin to an enzyme with carboxypeptidase B-like activity, The enzyme, TAFIa, potently attentua tes fibrinolysis, TAFI activation, like protein C activation, is augme nted about 1250-fold by thrombomodulin (TM), In this work, the effects of both soluble and cellular forms of TM on TAFI activation-dependent suppression of fibrinolysis were investigated, Soluble TM included in clots formed from purified components, barium citrate-adsorbed plasma , or normal human plasma maximally increased the tissue plasminogen ac tivator-induced lysis time PS-fold, with saturation occurring at 5, 10 , and 1 nM TM in the three respective systems. Soluble TM did not effe ct lysis in the system of purified components lacking TAFI or in plasm as immunodepleted of TAFI, In addition, the antifibrinolytic effect of TM was negated by monoclonal antibodies against either TAFI or TM, Th e inhibition of fibrinolysis by cellular TM was assessed by forming cl ots in dialyzed, barium citrate-adsorbed, or normal plasma over cultur ed human umbilical vein endothelial cells (HUVECs), Tissue plasminogen activator-induced lysis time was increased 2-fold, with both plasmas, in the presence of HUVECs, The antifibrinolytic effect of HUVECs was abolished 66% by specific anti-TAFI or anti-TM monoclonal antibodies, A newly developed functional assay demonstrated that HUVECs potentiate the thrombin-catalyzed. TM-dependent formation of activated TAFI, Thu s, endothelial cell TRI, in vitro at least, appears to participate in the regulation of not only coagulation but also fibrinolysis.