Epitope-Specific Thyroid Peroxidase Antibodies Reveal Functional Heterogeneity Within a Patient-Derived Monoclonal Repertoire.

INTRODUCTION: Thyroid peroxidase (TPO) autoantibodies are defining features of autoimmune thyroid disease (AITD), yet the relationship between epitope specificity and antibody function remains incompletely understood. METHODS: In this study, we generated patient-derived human monoclonal anti-TPO antibodies from a single donor with AITD (T-011) and characterized their competition-defined epitope relationships, reporter-based Fcγ receptor activity, and direct effects on TPO enzymatic function. RESULTS: Although the antibodies exhibited comparable FcγRIIIa reporter activation, FcγRIIa reporter responses varied according to epitope specificity. Notably, TPO036 demonstrated enhanced FcγRIIa reporter signaling and, following conversion to an F(ab')2 format, competitively inhibited Fc-mediated reporter activation. Direct TPO activity assays demonstrated that representative antibodies produced either no or only minimal inhibition of TPO enzymatic activity under the conditions tested. These findings indicate that competition-defined anti-TPO antibody groups within a single-donor repertoire can exhibit distinct functional profiles, particularly with respect to Fc-mediated reporter responses, without evidence of broad inhibition of TPO catalytic activity. CONCLUSION: Overall, our results support the existence of intra-individual, epitope-associated functional heterogeneity among anti-TPO antibodies and provide a proof-of-concept foundation for future studies evaluating epitope-targeted and Fc-modulating strategies in physiologically relevant models.