Influence of HLA–DR genes on the production of rheumatoid arthritis–specific autoantibodies to citrullinated fibrinogen

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Abstract
Objective Antibodies directed against citrullinated fibrinogen are highly specific for rheumatoid arthritis (RA). This study was undertaken to test whether RA‐associated HLA–DR alleles are associated with anti–citrullinated fibrinogen in RA patient sera and whether replacement of arginyl by citrullyl residues on fibrinogen peptides modifies their binding to HLA–DR molecules and their recognition by T cells. Methods Antikeratin, antifilaggrin, and anti–citrullinated fibrinogen antibodies were assayed in RA patients who had undergone HLA–DR typing. Direct assays were performed to investigate binding of citrullinated or native fibrinogen peptides (encompassing the entire α‐ and β‐chains of fibrinogen) to purified HLA–DR molecules. T cell proliferative responses to citrullinated or native fibrinogen peptides were measured in RA patients and controls. Results HLA–DRB1*0404 was associated with anti–citrullinated fibrinogen in RA sera (P = 0.002). For the RA‐associated alleles HLA–DRB1*0401 and HLA–DR1, there was a nonsignificant trend toward association (P = 0.07). Multiple peptides from the α‐ and β‐chains of fibrinogen bound many HLA–DR alleles; DRB1*0404 was the best fibrinogen peptide binder. Citrullination did not influence fibrinogen peptide binding to HLA–DR or fibrinogen peptide recognition by T cells. Peripheral blood T cells that recognized native or citrullinated fibrinogen peptides were common in RA patients but not in healthy controls. Conclusion The RA‐associated HLA–DRB1*0404 allele is also associated with production of antibodies to citrullinated fibrinogen. DRB1*0401 and DRB1*01 tend to be associated with anti–citrullinated fibrinogen, but this is not statistically significant. Citrullination of fibrinogen peptide does not influence peptide–DR–T cell interaction. Finally, T cell proliferation in response to citrullinated or uncitrullinated fibrinogen peptides is frequent in RA patients and very infrequent in controls.

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