Anti-carbamylated protein (anti-CarP) antibodies are associated with the risk and severity of rheumatoid arthritis (RA) and are primarily directed against fibrinogen. The lack of understanding of anti-CarP antibody reactivity has limited analysis of the immunopathogenic associations in RA. To address this shortcoming, we mapped anti-CarP antibody epitope reactivity in RA patient sera.
Immunoblotting identified a patient serum sample with specific reactivity to the carbamylated human fibrinogen β-chain. Liquid chromatography mass spectrometry (LC-MS) identified sites of homocitrullines (carbamylated lysines) present in the human fibrinogen β-chain. The reactivity of an anti-CarP antibody-positive cohort to specific peptides containing carbamylated lysines was determined by enzyme-linked immunosorbent assay, through direct binding (n = 63 sera) and by competition assays (n = 40 sera).
Serum with specific reactivity to carbamylated, but not citrullinated, fibrinogen β-chain was identified in a specimen obtained from an RA patient. LC-MS identified carbamylation of 9 of 34 lysines in the human fibrinogen β-chain. Mapping of immunoreactivity to tryptic peptide fragments demonstrated several candidate carbamylated epitopes that were confirmed by competition experiments. Peptides containing a homocitrulline at position 83 appeared to be an immunodominant epitope in some RA patient sera, with additional reactivity to peptides containing homocitrullines at positions 52, 264, 351, 367, and 374.
Anti-CarP antibodies appear to preferentially target specific regions of the human fibrinogen β-chain that contain homocitrullines. Interestingly, humoral immunoreactivity appears to be relatively restricted in some patients, which may enable detection of specific relationships with disease phenotype.