Cross-reactivity of IgM anti-modified protein antibodies in rheumatoid arthritis despite limited mutational load

• Main Authors: Sanne Reijm, Theresa Kissel, Gerrie Stoeken-Rijsbergen, Linda M. Slot, Corrie M. Wortel, Hugo J. van Dooren, Nivine E. W. Levarht, Arieke S. B. Kampstra, Veerle F. A. M. Derksen, Pleuni Ooijevaar-de Heer, Holger Bang, Jan W. Drijfhout, Leendert A. Trouw, Tom W. J. Huizinga, Theo Rispens, Hans U. Scherer, René E. M. Toes
• Format: Article
• Language: English
• Published: BMC 2021-09-01
• Series: Arthritis Research & Therapy
• Subjects: Rheumatoid arthritis; AMPA; IgM; Somatic hypermutation; B cells
• Online Access: https://doi.org/10.1186/s13075-021-02609-5

Abstract Background Anti-modified protein antibodies (AMPA) targeting citrullinated, acetylated and/or carbamylated self-antigens are hallmarks of rheumatoid arthritis (RA). Although AMPA-IgG cross-reactivity to multiple post-translational modifications (PTMs) is evident, it is unknown whether the first responding B cells, expressing IgM, display similar characteristics or if cross-reactivity is crucially dependent on somatic hypermutation (SHM). We now studied the reactivity of (germline) AMPA-IgM to further understand the breach of B cell tolerance and to identify if cross-reactivity depends on extensive SHM. Moreover, we investigated whether AMPA-IgM can efficiently recruit immune effector mechanisms. Methods Polyclonal AMPA-IgM were isolated from RA patients and assessed for cross-reactivity towards PTM antigens. AMPA-IgM B cell receptor sequences were obtained by single cell isolation using antigen-specific tetramers. Subsequently, pentameric monoclonal AMPA-IgM, their germline counterparts and monomeric IgG variants were generated. The antibodies were analysed on a panel of PTM antigens and tested for complement activation. Results Pentameric monoclonal and polyclonal AMPA-IgM displayed cross-reactivity to multiple antigens and different PTMs. PTM antigen recognition was still present, although reduced, after reverting the IgM into germline. Valency of AMPA-IgM was crucial for antigen recognition as PTM-reactivity significantly decreased when AMPA-IgM were expressed as IgG. Furthermore, AMPA-IgM was 15- to 30-fold more potent in complement-activation compared to AMPA-IgG. Conclusions We provide first evidence that AMPA-IgM are cross-reactive towards different PTMs, indicating that PTM (cross-)reactivity is not confined to IgG and does not necessarily depend on extensive somatic hypermutation. Moreover, our data indicate that a diverse set of PTM antigens could be involved in the initial tolerance breach in RA and suggest that AMPA-IgM can induce complement-activation and thereby inflammation.