Relatedness of Antibodies to Peptides Containing Homocitrulline or Citrulline in Patients with Rheumatoid Arthritis

Antibodies that target citrullinated protein/peptide (ACPA) and homocitrullinated/carbamylated protein/peptide (AHCPA) are associated with rheumatoid arthritis (RA). The relationship between ACPA and AHCPA remains unclear. We examined the expression and cross-reactivity of these antibodies using citrulline- and homocitrulline-containing synthetic peptides, CitJED and HomoCitJED, respectively, which have equal numbers of citrulline or homocitrulline residues on the same peptide backbone.

Serum from healthy subjects (n = 51) and patients with RA (n = 137), systemic lupus erythematosus (SLE; n = 37), and psoriatic arthritis (PsA; n = 37) were screened for IgG anti-CitJED and anti-HomoCitJED antibodies by ELISA. Cross-reactivity of these antibodies was examined by inhibition with various concentrations of CitJED and HomoCitJED.

Out of 137 patients with RA, antibodies to CitJED and HomoCitJED were detected in 69 (50%) and 78 (57%), respectively. Anti-CitJED and HomoCitJED antibodies were 77% concordant and their levels were strongly correlated [Spearman correlation coefficient (r_s) = 0.6676]. Sera from 25/27 patients (93%) with RA were inhibited by both CitJED and HomoCitJED with equal or higher affinity for the cognate (homologous) peptide.

Antibodies to CitJED and HomoCitJED frequently occurred in RA, but were not found in SLE or PsA, suggesting that these antibodies are specific to RA. Cross-reactivity between anti-HomoCitJED and anti-CitJED antibodies suggests that ACPA and AHCPA are derived from the same B cell population and both may contribute to the pathogenesis of RA.