Carbon ion irradiation enhances the antitumor efficacy of dual immune checkpoint blockade therapy both for local and distant sites in murine osteosarcoma

// Yutaka Takahashi 1 , Tomohiro Yasui 2 , Kazumasa Minami 1 , Keisuke Tamari 1 , Kazuhiko Hayashi 3 , Keisuke Otani 1 , Yuji Seo 1 , Fumiaki Isohashi 1 , Masahiko Koizumi 2 and Kazuhiko Ogawa 1 1 Department of Radiation Oncology, Osaka University Graduate School of Medicine, Osaka, Japan 2 Department of Medical Physics and Engineering, Osaka University Graduate School of Medicine, Suita, Osaka, Japan 3 Hospital of the National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Chiba, Japan Correspondence to: Yutaka Takahashi, email: ytakahashi@radonc.med.osaka-u.ac.jp Keywords: carbon ion beam; radiation; immune checkpoint; osteosarcoma; abscopal effect Received: July 18, 2018     Accepted: December 20, 2018     Published: January 18, 2019 ABSTRACT Carbon ion radiotherapy has been utilized even for X-ray resistant tumors. However, control of distant metastasis remains a major challenge in carbon ion irradiation. We investigated whether carbon ion irradiation combined with dual immune checkpoint blockade therapy (anti-PD-L1 and anti-CTLA-4 antibodies [P1C4]) provides anti-tumor efficacy for both local and distant sites. A mouse osteosarcoma cell line (LM8) was inoculated into both hind legs of C3H mice assigned to four groups: no treatment (NoTX), P1C4, 5.3 Gy of carbon ion irradiation to one leg (Cion), and combination (Comb) groups. In the Comb group, tumor growth delay was observed not only in the irradiated tumors but also in the unirradiated tumors. Notably, a complete response of unirradiated tumors was observed in 64% of mice in the Comb group, while only 20% of mice in the P1C4 group showed a complete response. Significant activation of immune cells was observed in the Comb group, with an increase in CD8+/GzmB+ tumor-infiltrating lymphocytes (TILs) in the irradiated tumor, and of CD8+/GzmB+ and CD4+ TILs in the unirradiated tumor, respectively. Depletion of CD8 abolished the tumor growth delay in unirradiated tumors in mice treated by Cion and P1C4. Overall survival was significantly prolonged in the Comb group. HMGB-1 release from irradiated tumors was significantly increased after Cion both in vitro and in vivo . These data suggest that carbon ion therapy enhances P1C4 efficacy against osteosarcoma in both the primary tumor and distant metastases mediated by immune activation.