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

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).

We aimed to investigate whether anti-FXIII-B antibodies affect the catalytic function of FXIII.

FXIII activation and fibrin crosslinking were examined in the presence of patient plasma, isolated patient IgG, or rat anti-FXIII-B monoclonal antibodies.

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. Furthermore, 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.

Anti-FXIII-B antibodies binding to the A₂B₂ heterotetramer and FXIII-B inhibited FXIII activation and its crosslinking function despite being directed against its noncatalytic subunit (FXIII-B).