Thrombin-activatable fibrinolysis inhibitor is activated in vivo in a baboon model of Escherichia coli induced sepsis.

Activated thrombin-activatable fibrinolysis inhibitor (TAFIa or CPU) is a carboxypeptidase that is able to attenuate fibrinolysis. Although its role in fibrinolysis and inflammation has been studied extensively in vitro, its levels and subsequent effect in vivo has not been studied to the same extent. Using our recently developed assay that is specific for TAFIa, we were able to quantify its levels in plasma samples obtained from an Escherichia coli (E. coli) challenged baboon sepsis model. TAFIa levels accumulated appeared to be E. coli dose dependent, where the lethal dose of 10(10) CFU/kg generated a peak TAFIa level of 24 nM by 2 h, which represents almost 32% of total plasma level of its precursor, thrombin-activatable fibrinolysis inhibitor (TAFI or proCPU). Furthermore, our data suggest that there is continual TAFI activation under lethal level of E. coli as the apparent half-life of TAFIa is increased from 8 min to 2.2 h. Two sublethal doses of 10(8) and 10(6) CFU/kg generated peak TAFIa levels of 1.1 and 0.4 nM, respectively, both by 6 h. Taken together, our data show that TAFIa is generated at systemic levels, in a dose-dependent manner, that can substantially affect both fibrinolysis and inflammatory response in the E. coli challenged baboon sepsis model.

An assay for measuring functional activated thrombin-activatable fibrinolysis inhibitor in plasma.

Thrombin-activatable fibrinolysis inhibitor (TAFI), also called procarboxypeptidase U (proCPU), is a plasma zymogen that can be activated by thrombin, the thrombin-thrombomodulin complex, or plasmin. The activated form of TAFI (TAFIa, CPU) removes C-terminal lysine residues of plasmin-modified fibrin (FN') that mediates a positive feedback mechanism in plasminogen (Pg) activation, thereby attenuating fibrinolysis. The plasma concentration of TAFI is approximately 75 nM. Because the half-maximal effect of TAFIa occurs at 1 nM, only approximately 1.3% of TAFI needs to be activated to exert an effect on clot lysis. The assay is performed by mixing soluble FN' covalently attached to a quencher and fluorescein-labeled Pg. The sample containing TAFIa is then added, and the rate of fluorescence increase due to removal of C-terminal lysine from FN' and loss of Pg binding is measured with a fluorescence plate reader. The assay was shown to be sensitive for TAFIa at a concentration as low as 12 pM. The intraassay variability and interassay variability of the assay were 6.3 and 8.3%, respectively. This assay was not confounded by the naturally occurring TAFI Thr325Leu polymorphism that affects the thermal stability of TAFIa or endogenous plasminogen in plasma.