Radiation therapy enhanced therapeutic efficacy of anti-PD1 against gastric cancer.

Radiation therapy is an important method in tumor treatment with distinct responses. This study aimed to investigate the immune effects of radiation therapy on the syngeneic gastric tumor model. Mouse forestomach carcinoma (MFC) cells were irradiated with different X-ray doses. Cell proliferation was determined by clonogenic assay. Gene and protein expression were determined by real-time quantitative PCR and western blot, respectively. The tumor model was established by subcutaneously injecting tumor cells in 615-(H-2 K) mice. Levels of immune-related factors in tumor tissues were determined by immunohistochemistry and flow cytometry. 5 Gy × 3 (three subfractions with 4 h interval) treatment significantly inhibited cell proliferation. Protein expression of stimulator of interferon genes (Sting) and gene expression of IFNB1, TNFα as well as CXCL-9 significantly increased in MFC cells after irradiation. In the MFC mouse model, no obvious tumor regression was observed after irradiation treatment. Further studies showed Sting protein expression, infiltration of dendritic cells and T cells, and significantly increased PD-1/PD-L1 expression in tumor tissues. Moreover, the irradiation treatment activated T cells and enhanced the therapeutic effects of anti-PD1 antibody against MFC tumor. Our data demonstrated that although the MFC tumor was not sensitive to radiation therapy, the tumor microenvironment could be primed after irradiation. Radiation therapy combined with immunotherapy can greatly improve anti-tumor activities in radiation therapy-insensitive tumor models.