Peptide mimics of a carbohydrate-associated epitope expressed by cancer cells: Identification of vaccine candidates

Abstract

Cancer-associated antigen 215 (CA215) is an immunoglobulin molecule expressed by numerous tumor types. Membrane-bound and soluble CA215 have been detected in the majority of cancer cells and rarely identified in normal tissues. In addition, CA215C is a carbohydrate-associated epitope in the variable region of CA215, which is specifically recognized by the monoclonal antibody, RP215. However, CA215C is not a suitable vaccine candidate as it is a thymus-independent antigen. In the present study, RP215 was used as a target to screen short peptide mimics of CA215C from a phage display peptide library. Following three rounds of screening, 30 positive phage clones that specifically bound to RP215 were identified and sequenced. The result of amino-acid sequence analysis revealed five conserved sequence groups for seventeen of the positive phage clones. The sequences of phage clones 2, 13 and 42 were selected for peptide synthesis and binding analysis. The synthetic peptides R2 and R42 specifically bound RP215. Antisera from mice immunized with R2-BSA or R42-BSA bound purified CA215C and innate CA215C expressed on human hepatic and rectal carcinoma tissues, as demonstrated by immunohistochemistry. Furthermore, R2-BSA and R42-BSA antisera inhibited RP215 binding to cancer tissues. These results revealed that R2-BSA and R42-BSA antisera had similar characteristics to RP215 and that the synthetic peptides R2 and R42 may mimic the CA215C epitope. R2 and R42 peptides may therefore have potential for development into a tumor vaccine.