Background Anticitrullinated protein antibodies (ACPA) are one of the best predictors for the development of rheumatoid arthritis. Nonetheless, relatively little information is present on the absolute concentration of ACPA in relation to total immunoglobulin (Ig) concentrations. Such information would be of relevance to compare ACPA levels to other antibody levels. Here, we estimated the relative abundance of ACPA Ig in serum and synovial fluid using a quantitative approach.

Methods ACPA were purified using HiTrap Streptavidin columns coupled with biotinylated cyclic citrullinated peptide (CCP2). Total Ig and anti-CCP2 isotype reactivities were measured by ELISA.

Results ACPA were successfully isolated as substantial antibody amounts were eluted from sera of ACPA-positive patients and neglectable antibody amounts were eluted from sera of ACPA-negative patients. Up to 1 in 80 IgG-molecules were estimated to be ACPA. Strikingly, IgM-ACPA was most abundant in synovial fluid (with the highest enrichment in the range of one IgM-ACPA for every eight IgM-antibodies).

Conclusions ACPA-IgG levels are estimated to be within the range of peak levels of protective antibody responses against recall antigens. IgM-ACPA is abundantly present in synovial fluid, suggesting the presence of a continuous ongoing autoimmune response in the synovial compartment.