Increased number of crosslinking sites for factor XIII counteracts the mechanical and lytic destabilization of fibrin caused by citrullination of fibrinogen.

Introduction: In thrombi, fibrin formation is accompanied by neutrophil activation, which leads to the release of neutrophil extracellular traps (NETs). Citrullination is a post-translational modification of arginine catalyzed by peptidylarginine deiminases, which are released during NET formation and modify extracellular proteins. Citrullinated fibrin has been detected within thrombi, but its impact on clot stabilization by activated Factor XIII (FXIIIa) has not been investigated. Objective: To characterize the effect of FXIIIa on the mechanical strength and lytic susceptibility of citrullinated fibrin. Methods: FXIII cross-linking sites in fibrin were identified by mass spectrometry. Fibrinolysis was investigated by turbidimetry and viscoelastometry in clots containing plasminogen and tissue plasminogen activator (tPA). Mechanical and viscoelastic parameters of the same clots were determined by oscillatory rheometry and viscoelastometry. Results: Non-crosslinked fibrin formed from citrullinated fibrinogen showed lower stability in the applied biomechanical and fibrinolytic assays. However, citrullination increased the number of heterochain crosslinks formed by FXIIIa from 27 to 36, and that of homochain crosslinks from 29 to 30. In line with this finding, the antifibrinolytic effect of FXIII crosslinking was consistently more pronounced in citrullinated fibrin. While FXIIIa increased the maximal clot firmness of native fibrin by 10% in the viscoelastometric assay, this FXIII-related rise in mechanical strength was 38% in citrullinated fibrin. Similarly, in the rheometric assays, the FXIII-associated increment in the critical shear stress required to disassemble the clot was higher in citrullinated fibrin than in the native one. Conclusion: FXIIIa is essential to counteract the weakening effects of citrullination in fibrin clots.