Antibodies against Noncatalytic B Subunit of Factor XIII Inhibit Activation of Factor XIII and Fibrin Crosslinking

Abstract

Background: Coagulation factor XIII (FXIII) is a proenzyme of plasma transglutaminase. It comprises two catalytic A subunits (FXIII-A) and two carrier B subunits (FXIII-B). We previously reported that alloantibodies against FXIII-B could promote FXIII clearance in a patient with congenital FXIII-B deficiency who had received infusions of plasma-derived human FXIII (A2B2 heterotetramer). Objectives: We aimed to investigate whether anti-FXIII-B antibodies affect the catalytic function of FXIII. Methods: FXIII activation and fibrin crosslinking were examined in the presence of patient plasma, isolated patient IgG, or rat anti-FXIII-B monoclonal antibodies. Results: Alloantibody levels were increased by repeated infusions of plasma-derived A2B2 heterotetramer, which enhanced binding to the functionally important FXIII-B Sushi domains. The patient plasma strongly inhibited cleavage of the FXIII-A activation peptide, amine incorporation, and fibrin crosslinking in normal plasma. Further, anti-FXIII-B alloantibodies blocked the formation of the complex of FXIII-B with FXIII-A, and fibrinogen. Rat monoclonal antibodies against the 10th Sushi domain of FXIII-B inhibited the incorporation of FXIII-B to fibrin, FXIII activation (i.e., cleavage of FXIII-A activation peptide), and ultimately fibrin crosslinking in normal plasma, independent of their effect on heterotetramer assembly with FXIII-A. Alloantibody binding to the A2B2 heterotetramer blocked the access of thrombin to the FXIII-A cleavage site, as indicated by the reaction of the alloantibodies to the A2B2 heterotetramer and FXIII-B, but not to FXIII-A. Conclusion: Anti-FXIII-B antibodies binding to the A2B2 heterotetramer and FXIII-B inhibited FXIII activation and its crosslinking function despite being directed against its non-catalytic subunit (FXIII-B).

Functional Consequences of Antibodies against Noncatalytic B Subunit of Factor XIII

“…2 In this issue of Thrombosis and Haemostasis, Souri and colleagues present a detailed biochemical characterization of an anti-FXIII-B alloantibody, which was developed in an FXIII-B-deficient patient as a result of repeated plasmaderived FXIII concentrate substitution therapy. 3 Alloantibody of the patient turned out to be polyclonal as it exhibited binding to several individual Sushi domains of FXIII-B. The alloantibodies did not affect the activity of previously activated FXIII-A, however, inhibited the cleavage of the activation peptide from FXIII-A and fibrin crosslinking in an A 2 B 2 heterotetramer-dependent manner.…”