ANTI-ACETYLATED PROTEIN IgM-ANTIBODIES AS THE STARTING POINT OF AUTOANTIBODY FORMATION IN RHEUMATOID ARTHRITIS?

Background: Anti-modified protein antibodies (AMPA) are an important hallmark of rheumatoid arthritis (RA) and the fact that they have consistently been found to develop years before disease onset, has provided important insights into the immunopathology underlying RA. In (auto)antibody development, IgM is the first isotype to be produced. However, it is unclear whether IgM-autoimmunity differs between AMPA targeting different post-translational modifications (citrulline, homocitrulline and acetylated residues), which could provide clues about the starting point of the AMPA response. Objectives: We therefore investigated IgM-levels of anti-citrullinated protein antibodies (ACPA), anti-carbamylated protein antibodies (anti-CarP) and anti-acetylated protein antibodies (AAPA) in healthy individuals, non-RA and RA patients. Methods: Autoantibodies were investigated in 2 cohorts: 1) a Japanese cohort of healthy individuals (community based Nagasaki Island study) known to be ACPA-positive (n=65) or ACPA-negative (n=197) were compared to Dutch healthy donors (n=30) and ACPA-positive RA patients (n=29). ACPA, anti-CarP and AAPA IgG were measured by ELISAs using CCP4, CHcitP4 and CAcetylP4 peptides with sequences similar to the commercial CCP2 antigen and native control peptides. 2) early arthritis patients from the Leiden Early Arthritis Clinic who had RA (n=648) or another form of arthritis (non-RA, n=555) and healthy controls (n=80). ACPA and AAPA were determined by ELISAs using CCP2 and cACP2 peptides and their native control peptides, while anti-CarP was measured on homocitrullinated versus native fetal calf serum. Mann-Whitney U-tests were performed for statistical comparisons. Results: ACPA IgM reactivity was mainly present in established ACPA-positive RA patients ( Figure 1A ) and to a lesser extent in ACPA-positive healthy Japanese individuals, and non-RA arthritis patients ( Figure 1D ). A similar picture was observed for anti-CarP IgM reactivity, for which again highest levels were found in established RA patients (1B and 1E) and ACPA-positive compared to ACPA-negative healthy Japanese individuals (1B). Intriguingly, AAPA IgM reactivity-levels displayed a different pattern as these were comparable between healthy individuals and ACPA-positive RA patients (1C and 1F). Likewise, AAPA IgM reactivity-levels were also not increased in ACPA-positive healthy Japanese individuals, who instead had lower levels compared to their ACPA-negative counterparts. Furthermore, the AAPA IgM-reactivity levels did not differ between non-RA arthritis patients, healthy controls and RA patients (1F). AAPA IgG-levels on the other hand were clearly elevated in RA patients compared to healthy controls and non-RA arthritis patients (1G). Conclusion: AAPA are exceptional compared to other AMPA because IgM AAPA-levels are similar among healthy individuals, non-RA arthritis and RA patients. This suggests that AAPA IgM is part of the “normal” immune repertoire and could constitute the starting point for RA-associated AMPA responses, with isotype switching and epitope spreading to other post-translational modifications leading to the typical RA-associated mature AMPA response. Disclosure of Interests: None declared Citation: , volume 81, supplement 1, year 2022, page 1168Session: Rheumatoid arthritis - aetiology, pathogenesis and animal models (Publication Only)