Influence of factor XIIIa activity on human whole blood clot lysis in vitro

We studied the influence of Factor XIIIa (F XIIIa) activity on the lysis rate of fresh whole human blood clots, without using anticoagulants. Clotting was induced by exogenous thrombin, lysis by tissue-type Plasminogen Activator (t-PA) added before clotting. After various periods of time, lysis rates were determined by measuring the radioactivity in the supernatant of the clot originating from 125I-Fibrinogen added before clotting. Lysis rates were determined in the presence of endogenous F XIIIa and compared with those obtained after specific inhibition of F XIIIa activity. We used an IgG fraction of an antiserum quenching the F XIIIa activity. Addition of increasing amounts of the antibodies to normal blood resulted in a dramatic increase in clot lysis rate, concomitant with loss of F XIII activity. Lysis of blood clots from a patient with a congenital, homozygous, functional alpha 2-Antiplasmin (alpha 2-AP) deficiency (alpha 2-AP-Enschede) was not or slightly increased by the anti F XIII antibodies indicating that fibrin-fibrin crosslinking per se does not contribute essentially to resistance of the blood clot against fibrinolysis. Both active alpha 2-AP and F XIIIa are required for the major part of the F XIII-dependent resistance of whole blood clots against lysis.