Progressive cross-linking of fibrin gamma chains increases resistance to fibrinolysis.

In the presence of plasma transglutaminase (factor XIIIa) fibrin first undergoes intermolecular covalent cross-linking between its gamma chains to create gamma dimers followed by slower cross-linking among its alpha chains to form alpha polymers. Progressive cross-linking of gamma chain dimers occurs at the slowest rate, resulting in gamma trimers and gamma tetramers ("gamma multimers"). Most studies indicate that cross-linked fibrin clots become resistant to fibrinolysis, but the basis for this event is not clear. In this study, we explored the role of gamma chain multimerization compared with alpha polymerization as causal factors in time-dependent development of resistance to fibrinolysis. Fibrin clots prepared from native (intact) fibrinogen were incubated for up to 120 h at near physiological ionic strength and a factor XIIIa level approximating that in plasma. These clots were lysed by plasmin at rates that were inversely proportional to the level of gamma multimers, which increased progressively with the time of incubation. In contrast, fibrin cross-linked at high ionic strength (a condition under which only gamma dimers and alpha polymers form) or fibrin formed in the absence of factor XIII showed no time-dependent decrease in lysis rates. Fibrin cross-linked for a fixed time period with increasing amounts of factor XIIIa contained gamma multimer levels that were proportional to the factor XIIIa concentration and lysed at rates that were inversely proportional to the gamma multimer level. Furthermore, cross-linked fibrin formed from fibrinogen fraction I-9, which has limited potential for alpha polymerization, showed the same reduction in the lysis rate as native cross-linked fibrin. These findings indicate that development of resistance to fibrinolysis of cross-linked fibrin is not measurably dependent upon gamma dimer or alpha polymer formation but develops solely as a function of gamma multimerization.