Fibrin Deimination in Synovial Tissue Is Not Specific for Rheumatoid Arthritis but Commonly Occurs during Synovitides

Abstract

Autoantibodies to deiminated (citrullinated) proteins are the most specific serological markers of rheumatoid arthritis (RA). Deimination is critical in generating the peptidic epitopes they recognize. In the synovial tissue (ST), deiminated forms of the α- and β-chains of fibrin are their major autoantigenic targets (anti-human fibrin(ogen) autoantibodies (AhFibA)). We investigated whether the presence of deiminated fibrin in the ST was specific for RA, because this could explain why AhFibA are RA specific. In 13 patients with RA and 19 patients with various other rheumatological disorders, knee ST biopsies were collected in macroscopically inflamed areas identified under arthroscopy. Synovitis was histopathologically confirmed in all of the biopsies. By immunoblotting, using antisera to fibrin, Abs to citrullyl residues, and AhFibA purified from RA sera, deiminated fibrin was evidenced in ST extracts from all of the patients. Moreover, variations in the degree of fibrin deimination were observed that were not related to the disease. Immunohistochemical analysis, using Abs to citrullyl residues and an antiserum to fibrin on adjacent serial sections of ST, confirmed the results because deiminated proteins colocalized with fibrin in RA as well as in control patients. Therefore, fibrin deimination in the ST is a general phenomenon associated to any synovitis, which does not necessarily induce a B autoimmune response with production of AhFibA.

Synovial intracellular citrullinated proteins colocalizing with peptidyl arginine deiminase as pathophysiologically relevant antigenic determinants of rheumatoid arthritis–specific humoral autoimmunity

“…These observations are consistent with findings of a recent study using different phage display antibodies and purified human ACPAs (49) as well as with our findings on deiminated fibrin using an antibody recognizing chemically modified citrulline epitopes independently of the amino acid context (20). The detection of different synovial deiminated proteins, some of which are RA specific, by distinct antibodies is not contradictory but rather indicates that not only citrulline, but also neighboring amino acids, determine the specific epitopes recognized by the different anticitrulline antibodies, mimicking the in vivo situation with ACPAs (16).…”

“…The relevance of this finding is emphasized by several facts. First, previous studies failed to demonstrate a link between ACPAs and non-RA-specific citrullinated proteins, including fibrin (20,49). Second, our findings indicated that the presence of intracellular citrullinated proteins is independent of the degree of synovial inflammation and that the demonstrated link with ACPAs is thus not merely a secondary phenomenon due to higher ACPA levels leading to enhanced inflammation.…”

“…Biochemical studies are ongoing to determine if the RA-specific intracellular citrullinated proteins are distinct intracellular molecules or rather epitopes processed from deiminated extracellular proteins such as fibrin. Considering the presence of several deiminated proteins in RA synovium (18,20,24,26), we continued to investigate the repeated finding that only the intracellular citrullinated proteins described in the present study are highly specific for RA synovium (20,26,27). In sharp contrast to iNOS, we demonstrated that both the number of samples containing PAD-2 and the degree of PAD-2 expression were significantly higher in RA than in control synovium and that intracellular citrullinated proteins colocalized with the citrullinating enzyme PAD-2.…”

“…Third, we demonstrated the presence of intracellular citrullinated proteins that colocalized with ACPA reactivity in RA synovium (26). Although the biochemical nature of these intracellular citrullinated proteins remains ill-defined, the major interest in comparison with both above-mentioned potential antigenic targets is that they are specifically present in RA synovium (20,26,27).…”

“…Moreover, in vitro deiminated fibrinogen is a highly specific and sensitive substrate for the detection of serum ACPAs (19). However, the mere presence of deiminated fibrin in synovium does not fully explain the RA-specific induction of ACPAs, since citrullinated fibrin was detected equally in inflamed synovium in non-RA patients and in animal arthritis, where deiminated fibrin does not induce ACPAs (20)(21)(22). Second, Despres and coworkers described a 50-kd antigen (Sa antigen) in extracts from human placenta, spleen, and RA synovium (23).…”