LAUSR

Autoimmune coagulation factor XIII (FXIII) deficiency (AiF13D) is a bleeding disorder caused by anti‐FXIII autoantibodies. Recently, we generated human monoclonal antibodies (mAbs) from the peripheral blood of an AiF13D patient and classified them into three groups: FXIII‐dissociation inhibitor, FXIII‐assembly inhibitor, and non‐neutralizing/inhibitory mAbs. However, the epitope region and molecular inhibitory mechanism of each mAb remain unknown. Here, we localized the epitope regions of the representative inhibitory mAbs A69K (dissociation inhibitor) and A78L (assembly inhibitor) to the β‐barrel‐2 domain and boundary of β‐barrel‐1&2 domains, respectively, of the FXIII‐A subunit, by combining a binding assay using its synthesized peptides and a protease‐protection assay. Our findings suggest that A69K inhibits the activation‐related conformational changes and dissociation of FXIII and that A78L competitively inhibits FXIII‐assembly.