The antigen specificity of the rheumatoid arthritis-associated ACPA directed to citrullinated fibrin is very closely restricted

The major targets of the disease-specific autoantibodies to citrullinated proteins (ACPA) in synovium of rheumatoid arthritis (RA) patients are borne by the citrullinated α- and β-chains of fibrin. We demonstrated that ACPA target a limited set of citrullinated fibrin peptides and particularly four multicitrullinated peptides which present the major epitopes.In this study, we established the clear immunodominance of the peptides α36–50Cit38,42 and β60–74Cit60,72,74 which were recognised by 51/81 (63%) and 61/81 (75%) of ACPA-positive patients, respectively, more than 90% recognising one, the other or both peptides. We also identified the citrullyl residues αCit42, βCit72 and βCit74 as essential for antigenicity, and at a lesser degree αCit38. Then, we assayed on overlapping 7-mer peptides encompassing the sequences of the two peptides, 3 series of sera recognising either α36–50Cit38,42 or β60–74Cit60,72,74 or both peptides. In each series, the reactivity profiles of the sera, largely superimposable, allowed identification of the two 4/5-mer overlapping epitopes (α: VECit42HQ and α′: Cit38VVE), and the single 5-mer epitope (β: GYCit72ACit74), all located to a flexible globular domain of fibrin on a topological 3D model.In conclusion, we demonstrated that only 3 immunodominant epitopes are targeted by ACPA on citrullinated fibrin stressing their actual oligoclonality. However, the reactivity to the 3 epitopes distinguishes three subgroups of patients. The closely restricted antigen specificity suggests that the autoimmune reaction to citrullinated fibrin is antigen-driven. The accessibility of the epitopes reinforces the hypothesis of a pathogenic role for ACPA via immune complexe formation in the synovial tissue.