title: Immune Recognition of Citrullinated Proteoglycan Aggrecan Epitopes in Mice with Proteoglycan-Induced Arthritis and in Patients with Rheumatoid Arthritis.
abstract: BACKGROUND
  Rheumatoid arthritis (RA) is an autoimmune inflammatory disease affecting the joints. Anti-citrullinated protein antibodies (ACPA) are frequently found in RA. Previous studies identified a citrullinated epitope in cartilage proteoglycan (PG) aggrecan that elicited pro-inflammatory cytokine production by RA T cells. We recently reported the presence of ACPA-reactive (citrullinated) PG in RA cartilage. Herein, we sought to identify additional citrullinated epitopes in human PG that are recognized by T cells or antibodies from RA patients.
  
  
  METHODS
  We used mice with PG-induced arthritis (PGIA) as a screening tool to select citrulline (Cit)-containing PG peptides that were more immunogenic than the arginine (R)-containing counterparts. The selected peptide pairs were tested for induction of pro-inflammatory T-cell cytokine production in RA and healthy control peripheral blood mononuclear cell (PBMC) cultures using ELISA and flow cytometry. Anti-Cit and anti-R peptide antibodies were detected by ELISA.
  
  
  RESULTS
  Splenocytes from mice with PGIA exhibited greater T-cell cytokine secretion in response to the Cit than the R version of PG peptide 49 (P49) and anti-P49 antibodies were found in PGIA serum. PBMC from ACPA+ and ACPA- RA patients, but not from healthy controls, responded to Cit49 with robust cytokine production. High levels of anti-Cit49 antibodies were found in the plasma of a subset of ACPA+ RA patients. Another PG peptide (Cit13) similar to the previously described T-cell epitope induced greater cytokine responses than R13 by control (but not RA) PBMC, however, anti-Cit13 antibodies were rarely detected in human plasma.
  
  
  CONCLUSIONS
  We identified a novel citrullinated PG epitope (Cit49) that is highly immunogenic in mice with PGIA and in RA patients. We also describe T-cell and antibody reactivity with Cit49 in ACPA+ RA. As citrullinated PG might be present in RA articular cartilage, Cit PG epitope-induced T-cell activation or antibody deposition may occur in the joints of RA patients.
authors.affiliation: Section of Molecular Medicine, Department of Orthopedic Surgery, Rush University Medical Center, Chicago, Illinois, United States of America.
authors.forename: Adrienn
authors.initials: A
authors.lastname: Markovics
authors.affiliation: Section of Molecular Medicine, Department of Orthopedic Surgery, Rush University Medical Center, Chicago, Illinois, United States of America.
authors.forename: Tímea
authors.initials: T
authors.lastname: Ocskó
authors.affiliation: Rheumatology Associates, Rush University Medical Center, Chicago, Illinois, United States of America.
authors.forename: Robert S
authors.initials: RS
authors.lastname: Katz
authors.affiliation: Department of Genetics, Cell and Immunobiology, Semmelweis University, Budapest, Hungary.
authors.forename: Edit I
authors.initials: EI
authors.lastname: Buzás
authors.affiliation: Section of Molecular Medicine, Department of Orthopedic Surgery, Rush University Medical Center, Chicago, Illinois, United States of America.
authors.forename: Tibor T
authors.initials: TT
authors.lastname: Glant
authors.affiliation: Section of Molecular Medicine, Department of Orthopedic Surgery, Rush University Medical Center, Chicago, Illinois, United States of America.
authors.forename: Katalin
authors.identifier: http://orcid.org/0000-0003-1506-4296
authors.initials: K
authors.lastname: Mikecz
mesh_terms: D053674:Aggrecans / Q000276:immunology / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001169:Arthritis, Experimental / Q000139:chemically induced / Q000276:immunology*; D001172:Arthritis, Rheumatoid / Q000276:immunology*; D001402:B-Lymphocytes / Q000166:cytology / Q000276:immunology; D016022:Case-Control Studies; D049109:Cell Proliferation; D002478:Cells, Cultured; D002956:Citrulline / Q000378:metabolism*; D016207:Cytokines / Q000096:biosynthesis / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D000939:Epitopes / Q000737:chemistry / Q000276:immunology*; D005434:Flow Cytometry; D006801:Humans; D051379:Mice; D011509:Proteoglycans / Q000737:chemistry / Q000633:toxicity*; D013154:Spleen / Q000166:cytology / Q000378:metabolism; D013601:T-Lymphocytes / Q000166:cytology / Q000276:immunology
chemical_list: D053674:Aggrecans; D016207:Cytokines; D000939:Epitopes; D011509:Proteoglycans; D002956:Citrulline