A new pattern of citrullinated peptides improves the sensitivity for diagnosing rheumatoid arthritis

Anti-citrullinated protein antibody (ACPA) is found almost exclusively in patients with rheumatoid arthritis (RA). Commercial cyclic citrullinated peptide (CCP) assays that target ACPAs yield specificities as high as 95% for RA diagnoses, but their sensitivities only reach 50-70%. To improve the sensitivity of the CCP assay, a new pattern of citrullinated peptide was identified and named the MCSM (multiple citrulline-similar motif).The MCSM comprised a citrulline core surrounded by citrulline-similar amino acids. A series of peptides with or without the MCSM was synthesized to evaluate the function of the citrulline core and citrulline-similar amino acids. These peptides were used in the enzyme-linked immunosorbent assay to compare samples from 94 RA patients, 117 non-RA patients and 116 healthy subjects. Additionally, the MCSM assay was compared with a commercial CCP assay.When the cutoff value was set at 0.274, the sensitivity and specificity of the MCSM assay were 79.6% and 96.6%, respectively. When one citrulline was substituted in the citrulline core, the sensitivity of the assay decreased from 79.6% to 61%. If all three citrulline-similar amino acids were substituted in the backbone, the sensitivity of the MCSM assay decreased from 79.6% to 58.5%. The coincidence rate of the MCSM assay to the commercial CCP assay was 97.6%.The citrulline core and citrulline-similar amino acids are crucial components of the MCSM pattern. This new MCSM assay could be used to diagnose RA.