Isotype distribution of anti-cyclic citrullinated peptide antibodies in undifferentiated arthritis and rheumatoid arthritis reflects an ongoing immune response

Objective. The evolution of the rheumatoid arthritis (RA)-specific anti-cyclic citrullinated peptide (antiCCP) antibody response, as measured by the isotypes of anti-CCP, has not been described. This study was undertaken to determine anti-CCP isotype usage in patients with undifferentiated arthritis (UA), patients with recent-onset RA, and patients with RA of long duration. Methods. IgA, IgM, and IgG subclasses of anti-CCP were measured by enzyme-linked immunosorbent assay in serum samples that were obtained from IgG anti-CCP antibody-positive patients with UA (n = 110) and IgG anti-CCP antibody-positive patients with RA (n = 152) early after the onset of arthritis. Patients with UA in whom RA developed within 1 year (UA -> RA) were compared with patients with UA in whom RA did not develop within 1 year (UA -> UA). In addition, baseline serum samples obtained from a subset of patients with RA (n = 64) were compared with sera obtained from the same patients a median of 7 years later. Results. IgM anti-CCP was present in early samples from both patients with UA and patients with RA and in followup samples from patients with RA. Several IgG anti-CCP antibody-positive patients who did not have IgM anti-CCP early after disease onset did display IgM anti-CCP later in the course of the arthritis. A diverse pattern of isotype usage was detected in early samples, with a trend toward lower frequencies of all isotypes of anti-CCP in patients with UA compared with patients with RA and in UA -> UA patients compared with UA -> RA patients. Levels of all isotypes except IgG1 had decreased after 7 years. Conclusion. These data indicate development of the anti-CCP isotype repertoire into full usage early in the course of arthritis. The sustained presence of IgM anti-CCP indicates ongoing recruitment of new B cells into the anti-CCP response, reflecting a continuous (re)activation of the RA-specific anti-CCP response during the course of anti-CCP-positive arthritis.