Japanese Kampo Medicine Juzentaihoto Enhances Antitumor Immunity in CD1d^-/- Mice Lacking NKT Cells

Although the Japanese traditional herbal medicine (Kampo), Juzentaihoto (JTT), has been reported to have antitumor effects in several tumor models, its role in tumor immunology remains controversial. In the present study, we tested whether oral administration of JTT enhances antitumor immunity in CD1d^-/- mice, in which immunosuppression was partially relieved due to the lack of NKT cells. In a subcutaneous murine syngeneic CT26 colorectal tumor model, JTT had no impact on tumor growth in wild type (WT) BALB/c mice. However, the growth rate of tumors was significantly slower in CD1d^-/- mice than in WT mice. Surprisingly, JTT significantly delayed tumor growth in such CD1d^-/- mice. In vivo depletion of CD8⁺ T cells revealed that CD8⁺ T cells are required for JTT's antitumor activity. Moreover, tumor-reactive cytotoxic T-lymphocytes were detected exclusively in JTT-treated mice with well-controlled tumors. JTT did not affect the number of tumor-infiltrating CD4⁺ regulatory T cells. On the contrary, JTT increased the degranulation marker CD107a⁺ CD8⁺ T cells and decreased Ly6G⁺ Ly6C^lo polymorphonuclear myeloid-derived suppressor cells in tumor-infiltrating lymphocytes, most probably contributing to the suppression of tumor growth in JTT-treated mice. Nonetheless, JTT had no impact on the proportion of monocytic myeloid-derived suppressor cells. In conclusion, our results indicate that in the absence of NKT cells, JTT augments antitumor immunity by CD8⁺ T cells, suggesting that this Kampo medicine is a promising anticancer adjuvant when negative immune regulation is partially relieved.