Background/Purpose: Anti-Citrullinated Peptides Antibodies (ACPA) aid in rheumatoid arthritis (RA) diagnosis. The nature of autoantigens specifically bound to ACPA immune complex in vivoremains unclear. We therefore sought to identify citrullinated proteins bound to ACPA immune complexes in RA synovial fluids (SFs) and to delineate their immunodominant epitopes.

Methods: ACPA were isolated from RA and non-RA SFs by immunoaffinity using immobilized cyclic citrullinated peptides-3 (CCP3). The composition of the antigen-antibody immune complexes was determined using mass spectrometry. Synthetic peptides containing citrulline residues derived from the target protein were used in ELISA assays to determine the immune reactions of sera from RA patients, controls, and a cohort of first degree relatives of RA patients.

Results: Histone family members were the most frequent autoantigens bound to anti-CCP3 antibodies in RA SFs. H4, H3, H2B, and H2A were identified in 90%, 68%, 53%, and 42% tested samples respectively. A histone 4 peptide containing citrulline at position R39 (Cit39) was a specific ACPA autoantigen to all anti-CCP3+ RA SFs (n=34). Another citrulline at R40 (Cit40) proved to be minor antigenic. Anti-H4 Cit39-40 antibodies were found in sera from 93% (158/170) of anti-CCP3+ RA patients, 77% (20/26) anti-CCP3+ individuals without arthritis, and 2.5% (2/80) controls.

Conclusion: Histones are prominent endogenous autoantigens binding to anti-CCP3 antibodies from RA patients. Anti-citrullinated histone 4 accounts an important part of all ACPA detected by anti-CCP3 standard.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/identification-of-endogenous-autoantigens-bound-to-anti-citrullinated-peptide-antibodies-in-rheumatoid-arthritis-a-key-role-for-citrulline-residues-in-histone-4/