Th Balance–Related Host Genetic Background Affects the Therapeutic Effects of Combining Carbon-Ion Radiation Therapy With Dendritic Cell Immunotherapy

The goal of this study is to clarify the underlying mechanisms of metastasis suppression by carbon-ion radiotherapy combined with immature dendritic cell immunotherapy (CiDC), which was shown previously to suppress pulmonary metastasis in an NR-S1–bearing C3H/He mouse model. Mouse carcinoma cell lines (LLC, LM8, Colon-26, and Colon-26MGS) were grafted into the right hind paw of syngeneic mice (C57BL/6J, C3H/He, and BALB/c). Seven days later, the tumors on the mice were locally irradiated with carbon ions (290 MeV/n, 6 cm spread-out Bragg peak, 1 or 2 Gy). At 1.5 days after irradiation, bone marrow–derived immature dendritic cells (iDCs) were administrated intravenously into a subset of the mice. The number of lung metastases was evaluated within 3 weeks after irradiation. In vitro–cultured cancer cells were irradiated with carbon ions (290 MeV/n, mono-energy, LET approximately 70-80 keV/µm), and then cocultured with iDCs for 3 days to determine the DC maturation. CiDC effectively repressed distant lung metastases in cancer cell (LLC and LM8)–bearing C57BL/6J and C3H/He mouse models. However, Colon-26– and Colon-26MGS–bearing BALB/c models did not show enhancement of metastasis suppression by combination treatment. This result was evaluated further by comparing LM8-bearing C3H/He and LLC-bearing C57BL/6J models with a Colon-26–bearing BALB/c model. In vitro coculture assays demonstrated that all irradiated cell lines were able to activate C3H/He- or C57BL/6J-derived iDCs into mature DCs, but not BALB/c-derived iDCs. The genetic background of the host could have a strong effect on the potency of combination therapy. Future animal and clinical testing should evaluate host genetic factors when evaluating treatment efficacy.