Our findings suggest that the high level of cross reactivity amongst the RA autoreactive B cells is the result of different antigen encounters, possibly at different sites and at different time points. This is in line with the notion that RA is initiated in one context, such as in mucosal organs, and thereafter target other sites, such as joints. This article is protected by copyright. All rights reserved.

Antibodies targeting citrullinated proteins (ACPAs [anticitrullinated protein antibodies]) are commonly found in patients with rheumatoid arthritis (RA), strongly associate with distinct HLA-DR alleles, and predict a more aggressive disease course as compared with seronegative patients. Still, many features of these antibodies, including their site of production and the extent of MHC class II–driven T cell help, remain unclarified. To address these questions, we have used a single B cell–based cloning technology to isolate and express immunoglobulin (Ig) genes from joint-derived B cells of active RA patients. We found ∼25% of synovial IgG-expressing B cells to be specific for citrullinated autoantigens in the investigated ACPA+ RA patients, whereas such antibodies were not found in ACPA− patients. The citrulline-reactive monoclonal antibodies did not react with the unmodified arginine peptides, yet several reacted with more than one citrullinated antigen. A role for active antigen selection of the citrulline-reactive synovial B cells was supported by the strong bias toward amino acid replacement mutations in ACPA+ antibodies and by their loss of reactivity to citrullinated autoantigens when somatic mutations were reverted to the corresponding germline sequences.

Objectives To increase understanding of the aetiology and pathogenesis of rheumatoid arthritis (RA), genetic and environmental risk factors for RA subsets, defi ned by the presence or absence of different anticitrullinated protein/peptide antibodies (ACPAs) targeting citrullinated peptides from α-enolase, vimentin, fi brinogen and collagen type II, were investigated. Methods 1985 patients with RA and 2252 matched controls from the EIRA case-control cohort were used in the study. Serum samples were assayed by ELISA for the presence of anticyclic citrullinated peptides (anti-CCP) antibodies and four different ACPA fi ne specifi cities. Cross-reactivity between ACPAs was examined by peptide absorption experiments. Genotyping was performed for HLA-DRB1 shared epitope (SE) alleles and the PTPN22 gene, while information regarding smoking was obtained by questionnaire. The association of genetic and environmental risk factors with different subsets of RA was calculated by logistic regression analysis. Results Limited cross-reactivity was observed between different ACPA fi ne specifi cities. In total, 17 RA subsets could be identifi ed based on their different ACPA fi ne specifi city profi les. Large differences in association with genetic and environmental determinants were observed between subsets. The strongest association of HLA-DRB1 SE, PTPN22 and smoking was identifi ed for the RA subset which was defi ned by the presence of antibodies to citrullinated α-enolase and vimentin. Conclusion This study provides the most comprehensive picture to date of how HLA-DRB1 SE, PTPN22 and smoking are associated with the presence of specifi c ACPA reactivities rather than anti-CCP levels. The new data will form a basis for molecular studies aimed at understanding disease development in serologically distinct subsets of RA.