Image: The ABI 7500 real-time PCR system (Photo courtesy of Thermo Fisher Scientific).
Rheumatoid arthritis (RA) is an autoimmune disorder that occurs when the immune system mistakenly attacks the body's tissues. Unlike the wear-and-tear damage of osteoarthritis, rheumatoid arthritis affects the lining of the joints, causing painful swelling that can eventually result in bone erosion and joint deformity.
Most RA patients are positive for anti-citrullinated protein antibodies (ACPA), and these antibodies are highly specific for RA diagnosis. ACPA recognizes various citrullinated proteins, such as fibrinogen, vimentin and glucose- 6-phosphate isomerase. Citrullinated proteins are proteins that have the amino acid arginine converted into the citrulline, which is not one of the 20 standard amino acids encoded by DNA in the genetic code.
Scientists at the University of Tsukuba (Tsukuba, Japan) collected serum samples from 60 Japanese patients with RA (mean age 52.2 years, range 20–73 years, 80% females) and from 30 healthy subjects (HS); (mean age 49.0 years, range 34–65 years, 80% females). Serum samples were also collected from 17 patients with RA before and 24 weeks after treatment with biologic drugs. A murine model was used to test the methodologies.
The investigators used a variety of techniques to explore the expression and commonality of citrullinated proteins in peptide glucose-6-phosphate isomerase-induced arthritis (pGIA) and patients with RA, and went one step further to investigate its correlation with RA disease activity. These included the measurement of anti-citrullinated peptide (CCP) antibodies in pGIA using the Immunoscan CCPlus test kit; measurement of anti-inter-alpha-trypsin inhibitor heavy chain 4 (ITIH4) antibodies with an ELISA test in patients with RA.
Real-time quantitative polymerase chain reaction (RT-qPCR) analysis carried out using an ABI 7500 analyzer. Sera from pGIA mice, control mice, patients with RA, and HS were separated by 2D-PAGE, and gels were stained. The resultant peptides were analyzed with the nanoACQUITY ultrahigh-performance liquid chromatography (UPLC) system.
The scientists found that citrullinated ITIH4 was highly specific to patients with RA, compared with patients with other autoimmune and arthritic diseases or in healthy subjects, indicating a potential role for citrullinated ITIH4 in RA pathogenesis. Notably, its levels were decreased in correlation with the reduction of disease activity score after effective treatment in patients with RA. Moreover, antibody response to citrullinated epitope in ITIH4 was specifically observed in patients with RA.
Isao Matsumoto. MD, PhD, a clinical immunologist and corresponding author of the study, said,” We examined serum citrullinated proteins from pGIA by western blotting, and the sequence was identified by mass spectrometry. With the same methods, serum citrullinated proteins were analyzed in patients with RA, primary Sjögren's syndrome, systemic lupus erythematosus, and osteoarthritis as well as in healthy subjects. Our results suggest that citrullinated ITIH4 might be a novel biomarker to distinguish RA from other rheumatic diseases and for assessing disease activity in patients with RA.” The study was published on April 10, 2018, in the journal Arthritis Research & Therapy.
University of Tsukuba