Identification of new B cell epitopes in the sera of rheumatoid arthritis patients using a random nanopeptide phage library.

A random nanopeptide phage library was used to screen a pool of immunoglobulin fractions obtained from rheumatoid arthritis (RA) patients. After three rounds of panning, random individual phages were selected by their capacity to react with individual sera from RA patients. By sequencing the inserts corresponding to the peptides displayed on the surface of the phages, we found that phages displaying particular peptides were overrepresented in the selected libraries. The peptides displayed by these phages were: pep1 = Ala-Asp-Gly-Gly-Ala-Gln-Gly-Thr-Ala; pep2 = Pro-Gly-Pro-Ser-Arg-Ala-His-Phe-Leu; pep3 = Leu-Ser-Ser-Arg-Glu-Pro-Gln-Ala-Arg; pep4 = Arg-Leu-Thr-Arg-Glu-Leu-Tyr-Ala-Gln and pep5 = Tyr-Thr-Gln-Lys-His-Gln-Ala. The percentage of sera positive for pep1 was higher in RA patients as compared to the normal adults (p < 0.0004) and the reacting antibody was mainly of IgG isotype. The specificity of binding to the phage displaying pep1 was confirmed by competition experiments using both isolated phages and a synthetic peptide. Interestingly, a mutated phage displaying only Ala-Asp-Gln-Gly-Thr-Ala had no significant reactivity with the sera, indicating that the amino acids (Gly-Gly-Ala) of pep1 are the vital for the binding. Taken together this study demonstrates that it is possible to select specific ligands from a random phage library using sera from RA patients. In addition, this approach could be useful for identifying peptide antigens that might be part of causitive agents in autoimmune diseases.