Combined treatment with TLR7/8 agonists and carbon ion radiation induces local and abscopal immune responses in gastrointestinal tumor models.

Abstract

292 Background: Radiotherapy is a mainstay of local tumor treatment for rectal and pancreatic carcinoma. Radiation has local and abscopal immunological effects, and may beneficially be combined with immune therapies. Photon radiotherapy can favorably be combined with a TLR7/8 agonist to induce strong immune responses in gastrointestinal tumors in mice. Here we investigated carbon ion particle radiation, which has superior physical radiation dose precision and a higher relative anti-tumor effect strength (relative biological effect, RBE, compared with photons), because it was unclear if carbon ions can also favorably be combined with immunotherapies such as TLR agonists. Methods: Therefore we investigated the effects of 3M-011, a TLR7/8 agonist (that was systemically given), and carbon ions in vivo in gastrointestinal tumors in mice in subcutaneous and orthotopic mouse models of colorectal and pancreatic cancer. In vitro, cytotoxicity assays were applied to identify the immune cell populations involved in the therapy-induced immune reaction. The tumor specific T cell immune reaction was demonstrated by ELISPOT assays. Results: We found that the carbon ions and the TLR7/8 agonist 3M-011 induced strong immune responses in s.c. and orthotopic colorectal and pancreatic cancer models in mice. The TLR agonist dramatically enhanced the carbon ion effects in both local and distant antitumor activity. Depletion experiments showed that NK and CD8 T cells mediated the cytotoxic effects of the TLR agonist and in vivo depletion of CD11c+cells using a transgenic mouse carrying the diphteria toxin receptor showed that DCs are the necessary cell population inducing the detected antitumor immunity for monotherapies and combination treatment. Conclusions: Our findings suggest that TLR7/8 agonists are potent adjuvants not only to photon radiotherapy but also to particle radiation with carbon ions, and that the combination induced local and relevant systemic immune responses. In conclusion our preclinical proof of concept data supports the notion that TLR7/8 agonists may be favorably combined with particle radiation to treat GI tumors including pancreatic and rectal cancer.