Antibodies to citrullinated α‐enolase peptide 1 are specific for rheumatoid arthritis and cross‐react with bacterial enolase

Abstract Objective To map the antibody response to human citrullinated α‐enolase, a candidate autoantigen in rheumatoid arthritis (RA), and to examine cross‐reactivity with bacterial enolase. Methods Serum samples obtained from patients with RA, disease control subjects, and healthy control subjects were tested by enzyme‐linked immunosorbent assay (ELISA) for reactivity with citrullinated α‐enolase peptides. Antibodies specific for the immunodominant epitope were raised in rabbits or were purified from RA sera. Cross‐reactivity with other citrullinated epitopes was investigated by inhibition ELISAs, and cross‐reactivity with bacterial enolase was investigated by immunoblotting. Results An immunodominant peptide, citrullinated α‐enolase peptide 1, was identified. Antibodies to this epitope were observed in 37–62% of sera obtained from patients with RA, 3% of sera obtained from disease control subjects, and 2% of sera obtained from healthy control subjects. Binding was inhibited with homologous peptide but not with the arginine‐containing control peptide or with 4 citrullinated peptides from elsewhere on the molecule, indicating that antibody binding was dependent on both citrulline and flanking amino acids. The immunodominant peptide showed 82% homology with enolase from Porphyromonas gingivalis , and the levels of antibodies to citrullinated α‐enolase peptide 1 correlated with the levels of antibodies to the bacterial peptide (r 2 = 0.803, P < 0.0001). Affinity‐purified antibodies to the human peptide cross‐reacted with citrullinated recombinant P gingivalis enolase. Conclusion We have identified an immunodominant epitope in citrullinated α‐enolase, to which antibodies are specific for RA. Our data on sequence similarity and cross‐reactivity with bacterial enolase may indicate a role for bacterial infection, particularly with P gingivalis, in priming autoimmunity in a subset of patients with RA.