Two 7-mer cyclic peptides, bearing the antigenic motif recognized by the anti-CD20 mAb Rituximab and differing because of motif-surrounding amino acids, inhibit the binding of Rituximab to raft-associated CD20 and Rituximab-induced membrane ceramide on human lymphoid DAUDI cells. These peptides displayed different immunogenic profiles, in that antibodies recognizing CD20 were induced in two and five out of five BALB/c mice immunized with Rp-15-C and Rp13-C respectively. Analysis of immunogenic motif, performed by panning a 7-mer phage display peptide library with purified anti-peptide IgGs, showed that the motif defined by anti-Rp15-C mostly included amino acids surrounding the Rituximab-specific antigenic motif "ANPS", whereas that defined by anti-Rp13-C was "NPS". These data showed that the motif-surrounding amino acids can markedly influence the specificity of antibodies, even when elicited with a short 7-mer peptide. Since anti-peptides antibodies analyzed are IgG, their specificity is likely to reflect how peptides are processed at the T cell level and suggests that, within a short peptide, the motif defined by T cells during the recognition phase may be different from that recognized by these cells upon their stimulation. Our findings can explain the failure of most peptide-based immunotherapy in cancer and autoimmune diseases and suggest strategies to implement the specificity of peptides-induced antibodies against the target antige
