The antifibrinolytic function of factor XIII is exclusively expressed through α₂-antiplasmin cross-linking.

Factor XIII (FXIII) generates fibrin-fibrin and fibrin-inhibitor cross-links. Our flow model, which is sensitive to cross-linking, was used to assess the effects of FXIII and the fibrinolytic inhibitor, α₂-antiplasmin (α₂AP) on fibrinolysis. Plasma model thrombi formed from FXIII or α₂AP depleted plasma lysed at strikingly similar rates, 9-fold faster than pooled normal plasma (PNP). In contrast, no change was observed on depletion of PAI-1 or thrombin activatable fibrinolysis inhibitor (TAFI). Inhibition of FXIII did not further enhance lysis of α₂AP depleted thrombi. Addition of PNP to FXIII or α₂AP depleted plasmas normalized lysis. Lysis rate was strongly inversely correlated with total cross-linked α₂AP in plasma thrombi. Reconstitution of FXIII into depleted plasma stabilized plasma thrombi and normalized γ-dimers and α-polymers formation. However, the presence of a neutralizing antibody to α₂AP abolished this stabilization. Our data show that the antifibrinolytic function of FXIII is independent of fibrin-fibrin cross-linking and is expressed exclusively through α₂AP.