IgG antifilaggrin autoantibodies (AFA) are the most specific serological markers of rheumatoid arthritis. In epithelial tissues, they recognize citrulline-bearing epitopes present on various molecular forms of (pro) filaggrin. Histological analysis of rheumatoid synovial membranes with an Ab to citrulline showed labeling of interstitial amorphous deposits and mononuclear cells of various types. Immunochemical analysis of exhaustive sequential extracts of the same tissues showed that they contain several deiminated (citrulline containing) proteins. Among them, two proteins, p64–78 and p55–61, present in urea-DTT and guanidine extracts, were shown by immunoblotting to be specifically targeted by AFA. By amino-terminal sequencing the proteins were identified as deiminated forms of the α-and β-chains of fibrin, respectively. Their identity was confirmed using several Abs specific for the Aα-and/or to the Bβ-chain of fibrin (ogen). Moreover, AFA-positive rheumatoid arthritis (RA) sera and purified AFA were highly reactive to the Aα-and Bβ-chains of human fibrinogen only after deimination of the molecules by a peptidylarginine deiminase. Autoantibodies affinity purified from a pool of RA sera onto deiminated fibrinogen were reactive toward all of the epithelial and synovial targets of AFA. This confirmed that the autoantibodies to the deiminated Aα-and Bβ-chains of fibrinogen, the autoantibodies to the synovial proteins p64–78 and p55–61, and, lastly, AFA, constitute largely overlapping autoantibody populations. These results show that deiminated forms of fibrin deposited in the rheumatoid synovial membranes are the major target of AFA. They suggest that autoimmunization against deiminated fibrin is a critical step in RA pathogenesis.

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, capable of eroding adjacent cartilage and bone, subsequently leading to joint destruction. Both cellular and humoral autoimmune mechanisms have been implicated in the still poorly understood initiating and perpetuating events of RA pathogenesis. Reflecting the humoral autoimmune processes occurring during RA is the presence of autoantibodies of diverse specificities in the serum of patients. Among these, antifilaggrin autoantibodies (AFA) are the most disease specific and are increasingly recognized as a useful diagnosis tool (1, 2, 3, 4, 5). AFA were originally described as IgG labeling the cornified layer of rat esophagus epithelium by indirect immunofluorescence and formerly called “anti-keratin Abs”(AKA)(6). The recognized Ag is also present in the cornified layer of human epidermis (7). For the past 10 years, our group has gathered proofs that the epithelial Ags recognized by AKA are all related to (pro) filaggrin, that is either to filaggrin, a protein putatively involved in the aggregation of cytokeratin intermediate filaments, or to its precursor, profilaggrin. In human epidermis, the Ag was identified as a 37-to 40-kDa neutral/acidic isoform (isoelectric point (pI) from 5.8 to 7.4) of filaggrin (8), and, in the rat esophagus epithelium, the Ags were characterized as three (pro) filaggrin-related proteins of 210, 120–90, and 130–60 kDa, respectively (9). Antiperinuclear factor (APF), designating serum IgG that decorate perinuclear granules in the superficial cells of the human buccal mucosa epithelium by indirect immunofluorescence, had also been demonstrated to be specifically associated to RA (10). We showed that APF detect (pro) filaggrin-related proteins of buccal epithelial cells and provided direct immunological …