Association of autoimmunity to peptidyl arginine deiminase type 4 with genotype and disease severity in rheumatoid arthritis

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Abstract
Objective: Protein citrullination is an important posttranslational modification recognized by rheumatoid arthritis (RA)–specific autoantibodies. One of the citrullinating enzymes, peptidyl arginine deiminase type 4 (PAD‐4), is genetically associated with development of RA in some populations, although the mechanism(s) mediating this effect are not yet clear. There have been descriptions of anti–PAD‐4 autoantibodies in different rheumatic diseases. This study was undertaken to investigate whether anti–PAD‐4 antibodies are specific to RA, are associated with disease phenotype or severity, and whether PAD‐4 polymorphisms influence the anti–PAD‐4 autoantibody response.Methods: Sera from patients with established RA, patients with other rheumatic diseases, and healthy adults were assayed for anti–PAD‐4 autoantibodies by immunoprecipitation of in vitro–translated PAD‐4. The epitope(s) recognized by PAD‐4 autoantibodies were mapped using various PAD‐4 truncations. PAD‐4 genotyping was performed on RA patients with the TaqMan assay. Joint erosions were scored from hand and foot radiographs using the Sharp/van der Heijde method.Results: PAD‐4 autoantibodies were found in 36–42% of RA patients, and were very infrequent in controls. Recognition by anti–PAD‐4 autoantibodies required the 119 N‐terminal amino acids, which encompass the 3 nonsynonymous polymorphisms associated with disease susceptibility. Strikingly, the anti–PAD‐4 immune response was associated with the RA susceptibility haplotype of PADI4. Anti–PAD‐4 antibodies were associated with more severe joint destruction in RA.Conclusion: Our findings indicate that anti–PAD‐4 antibodies are specific markers of RA, independently associated with more severe disease, suggesting that an anti–PAD‐4 immune response may be involved in pathways of joint damage in this disease. Polymorphisms in the PADI4 gene influence the immune response to the PAD‐4 protein, potentially contributing to disease propagation.

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