According to research results published in Arthritis and Rheumatology, both disordered antigens and common epitopes for anticitrullinated protein antibodies and rheumatoid factor are possible unifying features for autoantibodies in rheumatoid arthritis (RA).
Using a high-density peptide array, researchers sought to evaluate autoantibodies against the citrulline-containing, homocitrulline-containing, and native peptides found in patients who were seropositive and seronegative in an effort to identify the unifying and novel features of autoantibody reactivity in RA.
Serum samples from age- and sex-matched patients with RA and controls were selected from the University of Wisconsin Rheumatology Biorepository. Investigators designed a peptide array with overlapping peptides, primarily gathered from proteins with at least 1 citrulline detected in the rheumatoid joint, including “native peptides, peptides with all arginines replaced by citrullines, and peptides with all lysines replaced by homocitrullines.” Sera from all patients were divided into 4 groups (cyclic citrullinated peptide [CCP]+rheumatoid factor [RF]+, CCP+RF-, CCP-RF+, and CCP-RF-) and were subjected to the array in order to quantify immunoglobulin M (IgM) and immunoglobulin G (IgG) binding to each peptide.
General antibody binding patterns were also evaluated in the 4 groups, first focusing on citrulline, then homocitrulline and native peptides. Patients in the CCP+RF+ group had very high, high, and moderate IgG binding to citrulline, homocitrulline, and native peptides, respectively. IgM binding to all 3 peptides was high across all groups.
Researchers noted that these results were expected and suggested that citrulline is a “major driver of autoantibody reactivity” in patients with RA who are doubly seropositive.
The investigators then evaluated whether peptides from specific proteins were preferentially targeted in RA. Among peptides bound by IgG at a level 10 times greater in RA compared with controls, every protein within the array had at least 1 peptide that met the criterion. Every peptide with >10x increased binding in RA — out of 9098 unique peptides — contained citrulline with the exception of 2 native peptides from fibrinogen.
Based on the moderate binding of native peptides in patients with RA, investigators chose to identify the native peptides bound by IgG >2x more in RA compared with controls. In total, 49 of these peptides met these criteria. Researchers noted that many of these peptides contained repeated amino acids or repeated short motifs, which are indicative of “intrinsically disordered regions of proteins.” The researchers suggest that within RA, peptides in regions of intrinsic structural disorder are targeted.
Researchers also indicated that native peptides derived from IgG heavy chain were not identified as bound by IgG 2 times more in RA compared with controls. Patients who were RF+ demonstrated modest IgM binding to all peptide types derived from the constant region of IgG1. Binding in the CCP-RF+ group was modestly greater for homocitrulline-containing peptides compared with citrulline-containing or native peptides derived from IgG1. The CCP+RF+ group demonstrated “very high” IgG binding “predominantly to citrulline-containing peptides in the constant region of IgG1,” the researchers noted.
“Using the 4 serogroups of rheumatoid arthritis and high-density peptide array, we demonstrated very strong autoantibody targeting of citrulline, particularly if adjacent to glycine or serine, as well as a moderate binding to native peptides primarily in CCP+RF+ rheumatoid arthritis,” the researchers concluded.
Disclosure: Several study authors declared affiliations with the pharmaceutical industry. Please see the original reference for a full list of authors’ disclosures.
Zheng Z, Mergaert AM, Fahmy LM, et al. Disordered antigens and epitope overlap between anti-citrullinated protein antibodies and rheumatoid factor in rheumatoid arthritis [published online August 8, 2019]. Arthritis Rheumatol. doi: 10.1002/art.41074