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.

Our model demonstrates the inflammatory potential of ACPA-containing ICs via engagement of FcgammaRIIa at the surface of macrophages, strongly supporting their pathophysiologic involvement. Continuing dissection of these molecular pathways could open the way to new therapeutic approaches in patients with RA.

Formation of the epitopes recognized by the rheumatoid arthritis (RA)-specific autoantibodies to citrullinated proteins (ACPA) on filaggrin and on the a-and b-chains of fibrin, their synovial target, requires conversion of their arginyl residues into citrullyl residues, but is also affected by their amino-acyl environment. Using competition with five citrullinated filaggrin-derived peptides bearing major ACPA epitopes, we confirmed the close cross-reactivity between filaggrin and citrullinated fibrin. To identify the sequential epitopes recognized on fibrin by ACPA, 71 citrullinated 15-mer peptides derived from all the sites of the a-and b-chains of fibrin harboring arginyl residues were tested by ELISA using ACPA-positive RA sera exhibiting different reactivity profiles to the five filaggrin peptides. We identified 18 fibrin-derived peptides bearing ACPA epitopes. Regarding the ability of fibrinogen arginyl residues to be citrullinated in vitro, 11 of the peptides likely correspond to in vivo targeted epitopes. Two out of them bear major epitopes and are located in the central globular domain of the protein. In the synovial tissue, fibrin citrullination and ACPA binding could impair fibrin degradation by plasmin. The immunological conflict between ACPA and fibrin could therefore sustain synovial inflammation not only via pro-inflammatory effector mechanisms but also via impairment of fibrinolysis.