Post-translational modifications of the major linear epitope 169-190aa of Ro60 kDa autoantigen alter the autoantibody binding.

Ro60 kDa is a member of the Ro/LaRNP ribonucleoprotein complex and its major linear B cell epitope, corresponding to the region 169-190aa, has been found to be the initial target of the autoimmune response in patients with systemic lupus erythematosus. This sequence contains one serine and two arginine amino acid residues, which can potentially be modified post-translationally by phosphorylation or citrullination, respectively. The aim of this study was to develop an immunoassay for anti-Ro60 kDa epitope antibody detection and to investigate the changes in the antigenicity of the Ro60 kDa epitope when it is post-translationally modified, by either citrullination or phosphorylation. Peptide analogues corresponding to the unmodified form of the epitope, its phosphorylated form, and a form with both arginine residues citrullinated were synthesized. The peptide coating conditions were investigated and it was found that the use of highly hydrophilic surfaces increase the efficiency of the coating, as well as the sensitivity of the method for anti-peptide antibody detection. All peptides were tested by the optimized enzyme-linked immunosorbent assay (ELISA) against 119 sera from patients with primary Sjögren's syndrome, systemic lupus erythematosus and rheumatoid arthritis with anti-Ro/SSA reactivity, 20 sera from patients with systemic diseases without anti-Ro/SSA immune reactivity, as well as against 65 sera from normal individuals. A large proportion of the tested sera reacted against all three peptide analogues, although with a preference for the unmodified form of the epitope. In conclusion, post-translational modifications of the major Ro60 kDa B cell epitope can alter the autoantibody binding.