IgG subclass distribution of the rheumatoid arthritis‐specific autoantibodies to citrullinated fibrin
In the rheumatoid synovium, deiminated (‘citrullinated’) forms of fibrin are the major targets of IgG autoantibodies to citrullinated proteins (ACPA), the most specific serological markers of rheumatoid arthritis (RA). To further the characterization of ACPA, we determined their subclass distribution. From a previously validated highly sensitive and specific enzyme‐linked immunosorbent assay (ELISA) onto in vitro deiminated human fibrinogen − antihuman fibrin(ogen) autoantibodies (AhFibA)‐ELISA − we derived and calibrated four ELISAs, using monoclonal antibodies to each of the four IgG subclasses, to determine the proportions of AhFibA subclasses in the sera. A series of 186 serum samples from RA patients was analysed. All AhFibA‐positive sera contained IgG1‐AhFibA, which reached the highest titres and accounted for more than 80% of AhFibA in three‐quarters of the sera. One or two other subclasses were associated with IgG1 in 39% of the sera, IgG4‐AhFibA being observed much more frequently and at higher titres than IgG3‐ or IgG2‐AhFibA. IgG1 alone or IgG(1 + 4)‐AhFibA were the AhFibA subclass profiles found in more than 80% of patients. AhFibA are mainly IgG1 and, to a lesser extent, IgG4. Such IgG subclass profiles may influence the effector phases of the immunological conflict between ACPA and deiminated fibrin that takes place specifically in the rheumatoid synovium and therefore may play a critical role in the self‐maintenance of rheumatoid inflammation.