title: Detection of autoantibodies to citrullinated BiP in rheumatoid arthritis patients and pro-inflammatory role of citrullinated BiP in collagen-induced arthritis.
abstract: BACKGROUND
  Anti-citrullinated protein/peptide antibodies (ACPAs) are highly specific to rheumatoid arthritis (RA) patients and are thought to have a close relationship with the pathogenesis of arthritis. Several proteins, including fibrinogen, vimentin, and alpha-enolase, were reported as ACPA-target antigens, and their importance in RA pathogenesis was widely proposed. We identified citrullinated immunoglobulin binding protein (citBiP) as another ACPA target in RA patients and examined its pro-inflammatory role in arthritis.
  
  
  METHODS
  We measured the levels of anti-citBiP, anti-BiP, and anti-cyclic citrullinated peptide (CCP) antibodies in the serum of RA patients (n = 100), systemic lupus erythematosus (SLE) patients (n = 60), and healthy controls (n = 30) using ELISA and immunoblotting. Epitope mapping was performed using 27 citBiP-derived peptides. In the mouse study, after DBA/1J mice were immunized with BiP or citBiP, serum titers of ACPAs were measured by ELISA and immunohistochemistry. The development of collagen-induced arthritis (CIA) was observed in BiP- or citBiP-pre-immunized mice.
  
  
  RESULTS
  The serum levels of anti-BiP and anti-citBiP antibodies were significantly increased in RA patients, although only anti-BiP antibodies were slightly increased in SLE patients. Interestingly, anti-citBiP antibody levels were higher than anti-BiP antibody levels in 72% of RA patients, whereas no significant increase in anti-citBiP antibody levels was detected in SLE patients and healthy controls. The serum levels of anti-CCP antibodies were correlated with those of anti-citBiP antibodies in RA patients (R2 = 0.41). Several citrulline residues of citBiP were determined to be major epitopes of anti-citBiP antibodies, one of which showed cross-reactivity with CCP. Immunization of DBA/1J mice with citBiP induced several kinds of ACPAs, including anti-CCP and anti-citrullinated fibrinogen antibodies. Pre-immunization with citBiP exacerbated CIA, and anti-CCP antibody levels were increased in citBiP-pre-immunized CIA mice.
  
  
  CONCLUSIONS
  CitBiP is a newly described ACPA target that may play a pro-inflammatory role in arthritis.
authors: Department of Allergy and Rheumatology, Graduate School of Medicine, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan.
authors: Hirofumi
authors: H
authors: Shoda
authors: Keishi
authors: K
authors: Fujio
authors: Mihoko
authors: M
authors: Shibuya
authors: Tomohisa
authors: T
authors: Okamura
authors: Shuji
authors: S
authors: Sumitomo
authors: Akiko
authors: A
authors: Okamoto
authors: Tetsuji
authors: T
authors: Sawada
authors: Kazuhiko
authors: K
authors: Yamamoto
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001169:Arthritis, Experimental / Q000097:blood / Q000276:immunology*; D001172:Arthritis, Rheumatoid / Q000097:blood / Q000276:immunology*; D001323:Autoantibodies / Q000097:blood / Q000276:immunology*; D001324:Autoantigens / Q000276:immunology; D002956:Citrulline / Q000378:metabolism; D024043:Collagen Type II / Q000276:immunology; D000091342:Endoplasmic Reticulum Chaperone BiP; D018604:Epitope Mapping / Q000379:methods; D000939:Epitopes / Q000737:chemistry / Q000276:immunology; D005260:Female; D005455:Fluorescent Antibody Technique; D006360:Heat-Shock Proteins / Q000737:chemistry / Q000276:immunology* / Q000378:metabolism; D006801:Humans; D015151:Immunoblotting; D008180:Lupus Erythematosus, Systemic / Q000097:blood / Q000276:immunology; D051379:Mice; D008811:Mice, Inbred DBA; D008969:Molecular Sequence Data
chemical_list: D001323:Autoantibodies; D001324:Autoantigens; D024043:Collagen Type II; D000091342:Endoplasmic Reticulum Chaperone BiP; D000939:Epitopes; D006360:Heat-Shock Proteins; D002956:Citrulline