Although known to play a critical role in thrombosis, the precise role of factor XIII in other processes like wound healing and gestation remains to be elucidated. Because a specific, potent inhibitor could help define the function of factor XIII in these processes, or determine the potential benefits of factor XIII suppression in thrombotic disease, we have derived and characterized a monoclonal antibody inhibitor of factor XIII activation. This immunoinhibitor, 9C11, reacts specifically with both plasma and platelet forms of factor XIII. When added to human plasma, either as whole immunoglobulin or as Fab fragments, 9C11 completely prevented thrombin-activated factor XIII activity with small molecular weight substrates like 14C-putrescine. In addition, when clotted with plasma, 9C11 ablated the gamma chain crosslinking of fibrin catalyzed by factor XIII and markedly accelerated the fibrinolysis of plasma clots by urokinase. Studies of the mechanism of action showed that 9C11 inhibited the cleavage and activation of factor XIII by thrombin, but did not affect the catalytic function of previously activated factor XIII. Although it inhibited thrombin activation, experiments indicated that it bound comparably to both factor XIII zymogen and the thrombin-cleaved zymogen, and did not bind to the thrombin cleavage site of the molecule. Taken together these experiments indicated that 9C11 acted as an inhibitor of the thrombin cleavage and activation of the factor XIII A-subunit. Its potency and specificity make it a useful agent for studying thrombin-activatable factor XIII function in biological systems.
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