Anti-cyclic Citrullinated Peptide Antibodies Are a Collection of Anti-citrullinated Protein Antibodies and Contain Overlapping and Non-overlapping Reactivities

Objective: Anti-citrullinated protein antibodies (ACPA) and anti-cyclic citrullinated peptide (anti-CCP) antibodies are a hallmark of rheumatoid arthritis and are believed to play a role in disease pathogenesis. These antibodies are typically detected in ELISA with citrullinated peptides (eg, CCP2) or proteins as antigens. The absolute concentration of anti-CCP antibodies in serum is unknown. Although antibodies to several citrullinated proteins can mainly be detected within anti-CCP-positive sera, it is currently unknown whether anti-CCP antibodies are in fact ACPA. Likewise, it is unknown to what extent antibody responses to different citrullinated antigens are crossreactive.

Methods: An affinity purification method was established in which citrullinated antigen-specific antibodies were eluted from ELISA plates and then used for detection of other citrullinated antigens in ELISA or western blot. For additional crossreactivity studies, ELISA-based inhibition assays were performed with citrullinated or control peptides as inhibitors.

Results: The concentration of anti-CCP IgG antibodies was estimated to be at least 30 μg/ml in patients with high anti-CCP levels (>1600 μg/ml). Affinity-purified anti-CCP antibodies were able to recognise citrullinated fibrinogen (cit-fib) and citrullinated myelin basic protein (cit-MBP) on western blot. Furthermore, antibodies specific for cit-fib and cit-MBP were crossreactive. However, additional crossreactivity studies indicated that non-overlapping antibody responses to citrullinated peptides can also exist in patients.

Conclusions: This report shows for the first time that anti-CCP antibodies recognise multiple citrullinated proteins and are thus a collection of ACPA. More importantly, the data indicate that different ACPA responses are crossreactive, but that crossreactivity is not complete, as distinct non-crossreactive responses can also be detected in patients with RA.