Factor XI dependent and independent activation of Thrombin Activatable Fibrinolysis Inhibitor (TAFI) in plasma associated with clot formation

Thrombin Activatable Fibrinolysis Inhibitor (TAFI) also known as plasma pro carboxypeptidase B is activated by relatively high concentrations of thromb in in a reaction stimulated by thrombomodulin. In plasma an intact factor X I-dependent feed back loop via the intrinsic pathway is necessary to genera te sufficient thrombin for TAFI activation. This thrombin generation takes place after clot formation with consequent down-regulation of fibrinolysis. We developed a specific and sensitive assay for activated TAFI (TAFIa) and studied its factor XI-dependent generation during clot formation. In the a bsence of thrombomodulin, addition of 20 nM thrombin to normal plasma gener ated 5-10% of the amount of TAFIa generated by 20 nM thrombin in the presen ce of 8 nM thrombomodulin. Minimal activation of TAFI was detected in facto r II deficient plasma when clotting was initiated by 20 nM thrombin. Additi on of 320-640 nM of thrombin to factor II deficient plasma resulted in the same amount of TAFIa as in normal plasma, suggesting that similar to 50% of factor II has to be converted to thrombin for extensive activation of TAFI . A Mab that neutralizes activated factor XII had no effect on TAFI activat ion indicating that an intact contact system is not necessary for the activ ation of TAFI. The dependency of TAFI activation of factor XI was tested us ing a Mab that neutralizes activated factor XI. When plasmas from 13 health y individuals were tested, this Mab reduced TAFI activation by 65% (range 3 5-89%). Our results indicate that activation of TAFI in serum after clot fo rmation can be quantitated and that it takes place in both factor XI-depend ent and factor XI-independent mechanisms.