Antifilaggrin autoantibodies (AFA) are a population of IgG autoantibodies associated to rheumatoid arthritis (RA), which includes the so-called “antikeratin” Abs and antiperinuclear factor. AFA are the most specific serological markers of RA. We previously showed that they recognize human epidermal filaggrin and other profilaggrin-related proteins of various epithelial tissues. Here, we report further characterization of the protein Ags and epitopes targeted by AFA. All the Ags that exhibit numerous neutral/acidic isoelectric variants were immunochemically demonstrated to be deiminated proteins. In vitro deimination of a recombinant human filaggrin by a peptidylarginine deiminase generated AFA epitopes on the protein. Moreover, two of three filaggrin-derived synthetic peptides with a citrulline in the central position were specifically and widely recognized by AFA affinity-purified from a series of RA sera. These results indicate that citrulline residues are constitutive of the AFA epitopes, but only in the context of specific amino acid sequences of filaggrin. In competition experiments, the two peptides abolished the AFA reactivity of RA sera, showing that they present major AFA epitopes. These data should help in the identification of a putative deiminated AFA-inducing or cross-reactive articular autoantigen and provide new insights into the pathogenesis of RA. They could also open the way toward specific immunosuppressive and/or preventive therapy of RA.

Rheumatoid arthritis (RA) 4 is the most frequent of human systemic autoimmune diseases. It is characterized by the formation, in synovial membranes, of an inflammatory and invasive tissue, the rheumatoid pannus, that leads to the destruction of synovial joints. Both cellular and humoral autoimmune mechanisms have been implicated in its still poorly understood pathogenesis. The presence of circulating IgM rheumatoid factor in RA patients is widely considered as a diagnosis criterion (1). Nevertheless, a large variety of autoantibodies have been described in the sera of these patients, such as antiperinuclear factor (APF)(2),“antikeratin”(AKA)(3), anticollagen (4), antinuclear (5), anti-Sa (6), anticalpastatin (7, 8) Abs, and also autoantibodies directed against chondrocyte (9) or synovial membrane (10) proteins. Among these Abs, including rheumatoid factor, AKA have been largely demonstrated to be the most specific serological marker of RA because their diagnosis specificity was reported to be from 95 to 100%(11, 12, 13, 14). They are increasingly recognized as being a major diagnosis tool.