ObjectiveAntibodies targeting post-translationally modified proteins, such as anti-carbamylated protein antibodies (anti-CarP antibodies) are present in sera of rheumatoid arthritis (RA) patients.... Show moreObjectiveAntibodies targeting post-translationally modified proteins, such as anti-carbamylated protein antibodies (anti-CarP antibodies) are present in sera of rheumatoid arthritis (RA) patients. These autoantibodies associate with increased risk of RA development and with severity of joint destruction. It is not known which proteins in the RA joint are recognised by anti-CarP antibodies. Therefore, we investigated the presence and identity of carbamylated proteins in the human (inflamed) joint.MethodsWe obtained synovium, cartilage and synovial fluid from RA joints. Cartilage and synovium were obtained from controls. Samples were processed and used for immunohistochemistry or mass-spectrometric analysis to investigate the presence of carbamylated proteins. Anti-CarP antibody reactivity towards identified carbamylated proteins was tested by ELISA.ResultsImmunohistochemistry showed extensive staining of RA and control synovial tissue. Whole proteome analyses of the joint tissues revealed a large number of carbamylated peptidyllysine residues. We identified many carbamylated proteins in cartilage and were also able to detect carbamylation in synovial tissue and synovial fluid. Carbamylation was not exclusive to the RA joint and was also present in the joints of controls. Anti-CarP antibodies in the sera of RA patients were able to recognise the identified carbamylated proteins.ConclusionWe conclude that numerous carbamylated proteins are present in the RA joint. These carbamylated proteins can be recognised by anti-CarP antibodies, substantiating the notion that anti-CarP antibodies may play a role in the pathogenesis of RA. Show less
