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 alpha- and beta-chains of fibrin. We demonstrated that ACPA target a limited set of citrullinated fibrin peptides and particularly four multi-citrullinated peptides which present the major epitopes. In this study, we established the clear immunodominance of the peptides alpha 36-50Cit(38,42) and beta 60-74 Cit(60,72,74) 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 alpha Cit(42), beta Cit(72) and beta Cit(74) as essential for antigenicity, and at a lesser degree alpha Cit(38). Then, we assayed on overlapping 7-mer peptides encompassing the sequences of the two peptides, 3 series of sera recognising either alpha 36-50Cit(38,42) or beta(60-74)Cit(60,72,74) or both peptides. In each series, the reactivity profiles of the sera, largely superimposable, allowed identification of the two 4/5-mer overlapping epitopes (alpha: VECit(42)HQ and alpha': Cit(38)VVE), and the single 5-mer epitope (beta: GYCit(72)ACit(74)), 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. (C) 2011 Elsevier Ltd. All rights reserved.