Regulation of extravascular fibrinolysis by factor XIII

Abstract

The stability of fibrin clots in the extravascular compartment was examined using samples of 125I-labelled fibrin implanted subcutaneously into Balb/c mice. These samples of 125I-labelled fibrin were prepared in vitro. Crosslinking of fibrin in the presence of a high level of factor XIIIa resulted in complete conversion of γ-chain monomers to γ-γ dimers and extensive covalent polymerisation of the α-chain monomers. This fibrin, categorised as fully crosslinked (FXL), showed a 50% clot lysis time (LT-50) of 9.6±0.8 days. In contrast, fibrin, termed as partially crosslinked (PXL), prepared with a low level of Factor XIIIa, had all of its γ-chain monomers converted γ-γ dimers, but only a portion of its α-chain monomer polymerised. This PXL fibrin showed an LT-50 of 4.8±1.1 days. Non-crosslinked (NXL) fibrin displayed an LT-50 of 5.2±1.2 days. Fibrin produced by the addition of thrombin to plasma was of the PXL type and gave an LT-50 of 4.2±1.0 days. Fibrin produced by the addition of thrombin to whole blood, on the other hand, was found to be FXL and showed an LT-50 of 13.0 ± 2.0 days. Western blot analysis of fibrin samples prepared from plasma and whole blood with the use of antibody to α 2-plasmin inhibitor (α 2-PI) provided evidence that this inhibitor was covalently crosslinked to fibrin from both sources. FXL fibrin proved far more stable than either PXL fibrin or NXL fibrin to fibrinolysis by leukocyte elastase in vitro (p<0.001). Inclusion of the specific elastase inhibitor Elastatinal in the implanted NXL fibrin clot extended the LT-50 from 5 to 11.7 days (p<0.001), whereas a plasmin inhibitor, ϵ-amino caproic acid caused no change in lysis time. These results suggest that extravascular fibrinolysis is mediated chiefly by leukocyte proteases and that factor XIIIa-catalysed α-chain crosslinking is essential for protection of extravascular clots against fibrinolysis by these proteases.