The immunogenicity of a novel cytotoxic T lymphocyte epitope from tumor antigen PL2L60 could be enhanced by 4-chlorophenylalanine substitution at position 1.

PIWIL2, a member of PIWI/AGO family, is expressed in germline stem cells and precancerous stem cells, but not in adult somatic cells. PIWIL2 plays an important role in tumor development. It is considered as a cancer–testis antigen (CT80). It has been reported that the spliced fragment of PIWIL2, PL2L60, was widely expressed in cancer cell lines. In this study, HLA-A2-restricted epitopes from PL2L60 were predicted by online tools. To improve the activity of the native epitope, a candidate peptide P281 with potent binding affinity was chosen to investigate the modification strategy. A series of aromatic amino acids were introduced to substitute the first residue of P281. Then, we tested the binding affinity and stability of the peptide analogs and their ability to elicit specific immune responses both in vitro and in vivo. Our results indicated that the cytotoxic T lymphocytes (CTLs) induced by [4-Cl-Phe1]P281 could elicit more potent activities than that of P281 and other analogs. The CTLs induced by this analog could lyze target cells in HLA-A2-restricted and antigen-specific manners. [4-Cl-Phe1]P281 also showed the best resistance against degradation in human serum. In conclusion, the introduction of the unnatural amino acid, 4-Cl-Phe, into the first position could enhance the activity of the native epitope to induce cytotoxic T lymphocytes. It might be a good strategy to modify other promising native epitopes. The novel epitopes identified in this study could be used as novel candidates to the immunotherapy of HLA-A2 positive patients with tumors expressing PL2L60.