To study how soluble citrullinated telopeptides of type I and II collagens inhibit the binding of autoantibodies to the respective antigens immobilized onto solid phase, and to use modifications of previous methods to re-analyse rheumatoid arthritis (RA) and control serum samples.
Autoantibody binding was inhibited with normal or citrullinated carboxytelopeptides using enzyme-linked immunosorbent assay (ELISA) methods. Serum samples were available from 120 patients with RA and 81 controls.
Autoantibodies that bind normal C-telopeptides were not inhibited with soluble normal or citrullinated telopeptides. However, the antibodies that bind only citrullinated telopeptides could be inhibited with corresponding citrullinated telopeptides. Thus, it is not necessary to study the binding of autoantibodies to normal collagens if the specificity of their binding is assessed by immunological inhibition. For type I telopeptide, there are two arginines, the latter of which, when citrullinated, is important for binding. For type II telopeptide, there is one arginine, which is important when citrullinated. The other amino acids, e.g. the last alanine, have only a slight effect on binding. These improved methods differentiate better between RA patients, who have specific citrullinated autoantibodies, and controls than previous ELISA methods.
Based on inhibition assay, it is possible to measure the autoantibodies binding specifically to citrullinated telopeptides of type I and II collagens. When only one assay is involved, variance is decreased and the overall performance is easier than before.