title: Antibodies to native and citrullinated RA33 (hnRNP A2/B1) challenge citrullination as the inciting principle underlying loss of tolerance in rheumatoid arthritis.
abstract: BACKGROUND
  Anti-citrullinated protein antibodies (ACPAs) are the hallmark of rheumatoid arthritis (RA). Protein citrullination is believed to drive autoantigen selection in RA. Nonetheless, several autoantigens in RA are targeted as native (unmodified) proteins. Here, the study of hnRNP A2/B1 (RA33) provides a framework to understand the humoral response to native and citrullinated autoantigens in RA.
  
  
  METHODS
  RA synovial fluid (SF) cells were analysed by immunoblotting and mass spectrometry. RA33 was cloned from RASF cells and splice variants expressed as recombinant proteins. Antibodies against native and citrullinated RA33 were characterised by ELISA, immunoblotting and immunoprecipitation.
  
  
  RESULTS
  RA33 is citrullinated in the rheumatoid joint and targeted either as a citrullinated or native protein in distinct patient subsets with RA. A novel splice variant (hnRNP B1b) previously associated with disease initiation in experimental arthritis was identified in the RA joint and acts as the major target of the anti-RA33 response. Antibodies exclusively targeting citrullinated RA33 were positively associated with disease duration and erosive disease. In contrast, anti-(native) RA33 antibodies were detected almost exclusively in early RA and identified patients with low radiographic erosion scores. Finally, a unique subset of double-reactive patients demonstrated intermediate severity, but rapid disease progression, suggesting a transitional disease phase in the evolution of an anti-native protein antibody to ACPA response in RA.
  
  
  CONCLUSIONS
  These data suggest that native and citrullinated proteins targeted by autoantibodies in RA may be part of a single antibody system and challenge the paradigm of citrullination as the unifying principle underlying loss of tolerance in RA.
authors: Division of Rheumatology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
authors: Maximilian F
authors: http://orcid.org/0000-0001-5045-5255
authors: MF
authors: Konig
authors: Division of Rheumatology, Columbia University, College of Physicians and Surgeons, New York, New York, USA.
authors: Jon T
authors: JT
authors: Giles
authors: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, Boston, Massachusetts, USA Division of Immunology, Boston Children's Hospital, Boston, Massachusetts, USA.
authors: Peter A
authors: PA
authors: Nigrovic
authors: Division of Rheumatology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
authors: Felipe
authors: F
authors: Andrade
mesh_terms: D000328:Adult; D001172:Arthritis, Rheumatoid / Q000097:blood / Q000276:immunology* / Q000473:pathology; D001323:Autoantibodies / Q000276:immunology* / Q000378:metabolism; D016022:Case-Control Studies; D002956:Citrulline / Q000276:immunology; D005260:Female; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B / Q000276:immunology*; D006801:Humans; D007108:Immune Tolerance; D008137:Longitudinal Studies; D008297:Male; D010456:Peptides, Cyclic / Q000276:immunology*; D013582:Synovial Fluid / Q000276:immunology / Q000378:metabolism
chemical_list: D001323:Autoantibodies; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B; D010456:Peptides, Cyclic; C487763:cyclic citrullinated peptide; C439768:hnRNP A2; D002956:Citrulline
keywords: Ant-CCP; Autoantibodies; Rheumatoid Arthritis