Antigenic mimicry by linear peptides of the conformational epitope recognized by the anti-CD20 monoclonal antibody (mAb) Rituximab

The only reported mimotope able to target CD20 antigen in the context of an active immunotherapy is a 40-mer polypeptide, corresponding to the exposed domain of the molecule (from amino acid 142 to182). Due to its length, however, it fails to efficiently induce a CD20-specific response. Therefore, we have bio-panned a 12-mer phage display peptide library with the anti-CD20 mAb Rituximab and isolated 14 phage clones specifically reacting with Rituximab in ELISA and Western blot. Peptides expressed by 3 positive clones, were synthesized and purified (> than 70%) for immunological assay. Peptides R-p1L, R-p5L and R-p10L (R-p) bound Rituximab. Their binding was dose-dependent and specific, since they did not react with the isotype-matched mAb Infliximab. R-p1L, R-p5L and R-p10L recognize the same idiotope on Rituximab, since they cross-block to each other in their binding to the mAb. It is likely that the idiotope recognized by these peptides on Rituximab is within its antigen-combining site, since all of them, though to a different extent, specifically and dose-dependently inhibited the binding of the mAb to CD20+ human lymphoid Raji cells. Of interest, the motif amino acids resulting from the alignment of the peptide sequences had no homology with any portion of the exposed CD20 domain. Because CD20 is considered a target for passive immunotherapy of autoimmune diseases, these peptides may be useful reagents to induce an anti-CD20 response and eventually be utilized in the active immunotherapy of autoimmune diseases.