The structure of aspirated coronary thrombus in ST-segment elevation myocardial infarction (STEMI) is still being studied. Our aims were to characterize coronary thrombi of different ages, focusing on the appearance of activated protein C (APC/PC) and its relation to the elements of neutrophil extracellular traps (NETs), and the factors closely related to fibrin as factor XIII (FXIII) and α2 plasmin inhibitor (α2-PI). The thrombi of n = 24 male patients with atherosclerotic coronary plaque rupture related to native coronary artery occlusion were selected for histopathology analysis. Thrombus age was distinguished as fresh, lytic, and organized, and then analyzed by immunofluorescent staining and confocal microscopy. FXIII was present at a high level and showed a high degree of co-localization with fibrin in all stages of thrombus evolution. The amount of α2-PI was low in the fresh thrombi, which increased significantly to the lytic phase. It was evenly distributed and consistently associated with fibrin. APC/PC appeared in the fresh thrombus and remained constant during its evolution. The presence of NET marker and CD66b was most dominant in the lytic phase. APC/PC co-localization with the elements of NET formation shows its role in NET degradation. These observations suggest the importance of searching for further targeted therapeutic strategies in STEMI patients.

Alpha2-plasmin inhibitor (α2PI) has a heterogeneous structure due to proteolytic cleavages in the circulation. The C-terminally cleaved form loses the plasminogen binding site and is, therefore, a slow plasmin inhibitor (NPB-α2PI). As FXIII primarily crosslinks the plasminogen-binding intact form (PB-α2PI) to fibrin, the effect of NPB-α2PI on fibrinolysis has been less studied. Herein, we investigated the effect of C-terminal truncation. Total-, PB-, and NPB-α2PI antigen levels and α2PI incorporation were measured by ELISAs from samples of 80 healthy individuals. Clot lysis parameters of the same subjects were investigated using an in vitro clot lysis assay. α2PI incorporation into the clot was demonstrated by Western blotting. Clot lysis and clot structure were also analyzed using an α2PI-deficient plasma substituted with recombinant PB- and NPB-α2PI. Both plasma and clot-bound levels of total- and NPB-α2PI showed a significant positive correlation with clot lysis parameters. NPB-α2PI was detected in the clot due to non-covalent binding. Regardless of the type of binding, both forms affected the clot structure by increasing the thickness of the fibrin fibers and reducing the pore size. In conclusion, we found that NPB-α2PI can bind non-covalently to fibrin, and this binding contributes to changes in clot structure and inhibition of fibrinolysis.