Extremely strong infiltration of WT1-specific CTLs into mouse tumor by the combination vaccine with WT1-specific CTL and helper peptides

In immunotherapy by cancer antigen-derived peptide vaccine, vaccination of cytotoxic T lymphocyte (CTL) peptide alone is common, while it remains unclear whether the addition of helper peptide vaccine to the CTL peptide vaccine is of great advantage for the enhancement of tumor immunity. In the present study, combination vaccine of Wilms’ tumor gene 1(WT1) protein-derived CTL and helper peptides induced the strong infiltration of WT1-specific CD8⁺ T cells into mouse tumor at frequencies of 8.8%, resulting in the formation of multiple microscopic necrotic lesions in the tumor, whereas the frequencies of WT1-specific CD8⁺ T cell infiltration into the tumor in the vaccination of the CTL peptide alone were only 0.32%. The majority of the infiltrated WT1-specific CD8⁺ T cells was effector phenotype T cells, but importantly, WT1-specific CD8⁺CD44⁺CD62L⁺CD103⁺ resident memory T cells, which could differentiate into a lot of effector phenotype T cells, existed in the tumor of mice vaccinated with the both WT1 peptides. Furthermore, T-cell receptor repertoire analysis showed the oligoclonality of these tumor infiltrating WT1 tetramer⁺ CD8⁺ T cells, and 3 clones occupied about half of them. These results indicated that WT1-specific CD4⁺ T cells played an essential role not only in the priming and activation of WT1-specific CD8⁺ T cells, but also in trafficking and infiltration of the CD8⁺ T cells into tumors. These results should provide us with the concept that in the clinical setting, combination vaccine of WT1-specific CTL and helper peptides would be more advantageous than the CTL peptide vaccine alone.