Antibodies against 4 Atypical Post-Translational Protein Modifications in Patients with Rheumatoid Arthritis

Abstract

Patients with rheumatoid arthritis (RA) show autoantibodies against post-translational protein modifications (PTMs), such as anti-citrullinated protein antibodies. However, the range of recognized PTMs is unknown. Here, we addressed four PTMs: chlorination, non-enzymatic glycation, nitration, and homocysteinylation, identified as targets of atypical RA autoantibodies in studies whose protocols we have followed. The modified antigens included collagen type II, an extract of synovial proteins and a selection of peptides. We interpreted the results according to the optical density (OD) obtained in an enzyme-linked immunosorbent assay ( ELISA) with the modified antigen and the corrected OD obtained after subtracting the reactivity against the unmodified antigen. The results showed evidence of specific antibodies against glycated collagen type II, as the corrected ODs were higher in the 182 patients with RA than in the 164 healthy controls (p = 0.0003). However, the relevance of these antibodies was doubtful because the magnitude of the specific signal was small (median OD = 0.072 vs. 0.027, respectively). There were no specific antibodies against any of the other three PTMs. Therefore, our results showed that the four PTMs are not inducing a significant autoantibody response in patients with RA. These results indicated that the repertoire of PTM autoantigens in RA is restricted.

Significance of Type II Collagen Posttranslational Modifications: From Autoantigenesis to Improved Diagnosis and Treatment of Rheumatoid Arthritis

“…Antibody responses against COL2 modified by hypochlorous acid were evaluated in this study, together with antibodies against two other collagen oxidative modifications that can be present in inflamed RA joint—glycation and nitration. The obtained results led to the conclusion that antibodies targeting these PTMs are absent or inconsequential in RA patients [ 162 ].…”

“…These findings could be related to joint COL2 supported by the well-known anti-CarB immunity, while evidence indicating the existence of anti-glycated collagen in RA development are scarce, despite the fact that glycation was proven in RA despite the absence of hyperglycemia [ 166 ] and increased levels of advanced glycation end products were detected in the serum, synovial fluid and urine of RA patients [ 167 , 168 ]. A recent report by Rodriguez-Martinez et al showed evidence for the presence of antibodies specific to glycated COL2 in the sera of RA patients [ 162 ], but the magnitude of the detected signal was relatively small to conclude that immunity against this type of COL2 modification plays a significant role, suggesting a more restricted posttranslationally modified autoantigens repertoire in RA.…”

“…Homocysteinylation is another PTM type that could be involved in RA pathogenic reactions and inflammatory processes [ 172 ]. COL2 modification by L-homocysteine-thiolactone has not been confirmed as a significant collagen PTM in assays with RA patients’ serum samples [ 162 ].…”

Glucose as a Potential Key to Fuel Inflammation in Rheumatoid Arthritis

“…Such modifications include both enzymatic (e.g., phosphorylation, methylation, and ubiquitination) and nonenzymatic (e.g., glycation, oxidation/reduction, and acetylation) changes [ 16 ]. Several post-translationally modified proteins have already been identified as autoantigens in the RA sera [ 17 ]. For example, autoantibodies to post-translationally carbamylated or acetylated proteins have been discovered in the RA [ 17 , 18 , 19 , 20 ].…”

“…Several post-translationally modified proteins have already been identified as autoantigens in the RA sera [ 17 ]. For example, autoantibodies to post-translationally carbamylated or acetylated proteins have been discovered in the RA [ 17 , 18 , 19 , 20 ]. The citrullinated proteins are the representative example of such modified proteins because the antibodies against the citrullinated proteins (anticitrullinated protein antibodies or ACPA) are a well-established hallmark of RA diagnosis and disease assessment [ 18 ].…”