BackgroundFine specificity of anti-citrullinated protein antibodies (ACPAs), in which cross-reactivity exists, varies among patients with rheumatoid arthritis (RA), but it is unclear whether the mechanism of ACPA production is same or different among individuals. Since avidity of serum antibody reflects the direction of immune response, we compared the levels of avidity and cross-reactivity between various ACPAs in a cohort of RA patient.MethodsSera from 180 RA patients positive for anti-cyclic citrullinated peptide (CCP) 2 antibody were screened for positivity of antibodies against CCP1, and citrullnated fibrinogen (cFib), enolase (cEno), and vimentin (cVim) peptides. Avidity of the four ACPAs, and some autoantibodies and antibodies against foreign antigens was determined by an elution assay using sodium thiocyanate solution. Cross-reactivity between different ACPAs was estimated by measuring the inhibition of binding by competitor peptides. ResultsThe prevalence of anti-CCP1, anti-cFib, anti-cEno, and anti-cVim antibodies in the anti-CCP2-positive RA cohort were 37.7%, 38.3%, 15.6%, and 23.9%, respectively. The avidity of ACPAs, except for anti-cVim antibody, was significantly lower than that of antibodies against foreign antigens, while there was a large variety in the avidity of other autoantibodies. At individual levels, the avidity of anti-cVim was significantly higher than that of other ACPAs, and there was a significant correlation in the avidity of anti-CCP and anti-cFib antibodies. Substantial extent of cross-reactivity was seen between different ACPAs, which also showed a fixed hierarchy.ConclusionThe fixed hierarchy in the avidity and cross-reactivity between different ACPAs suggests that the mechanism underlying ACPA production is common to all RA patients. Presence of a dominant antigen that induces whole ACPA response is speculated.