Immune Recognition of Citrullinated Proteoglycan Aggrecan Epitopes in Mice with Proteoglycan-Induced Arthritis and in Patients with Rheumatoid Arthritis

• Published in: PloS one 2016-07, Vol.11 (7), p.e0160284-e0160284
• Main Authors: Markovics, Adrienn, Ocskó, Tímea, Katz, Robert S, Buzás, Edit I, Glant, Tibor T, Mikecz, Katalin
• Format: Article
• Language: English
• Subjects: Aggrecan; Aggrecans - immunology; Aggrecans - metabolism; Amino Acid Sequence; Analysis; Animals; Antibodies; Antigenic determinants; Antigens; Arginine; Arthritis; Arthritis, Experimental - chemically induced; Arthritis, Experimental - immunology; Arthritis, Rheumatoid - immunology; B-Lymphocytes - cytology; B-Lymphocytes - immunology; Biology and Life Sciences; Blood plasma; Bone surgery; Cartilage; Cartilage (articular); Case-Control Studies; Cell activation; Cell Proliferation; Cells, Cultured; Citrulline; Citrulline - metabolism; Cytokines; Cytokines - biosynthesis; Cytokines - metabolism; Cytometry; Diagnosis; Disease; Enzyme-Linked Immunosorbent Assay; Epitopes; Epitopes - chemistry; Epitopes - immunology; Flow Cytometry; Health aspects; Humans; Identification and classification; Immunogenicity; Joint diseases; Lymphocytes; Lymphocytes T; Medicine; Medicine and Health Sciences; Mice; Orthopedics; Pathogenesis; Patients; Peptides; Peripheral blood mononuclear cells; Physiological aspects; Proteins; Proteoglycans; Proteoglycans - chemistry; Proteoglycans - toxicity; Research and Analysis Methods; Rheumatoid arthritis; Risk factors; Spleen - cytology; Spleen - metabolism; Splenocytes; T cell receptors; T cells; T-Lymphocytes - cytology; T-Lymphocytes - immunology
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0160284

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. 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. 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. 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.