Five-Day Spacing of Two Fractionated Ablative Radiotherapies Enhances Antitumor Immunity

This study aimed to enhance tumor control and abscopal effects by applying diverse stereotactic ablative radiation therapy (SABR) schedules.FSaII, CT-26, and 4T1 cells were used for tumor growth delay and lung metastases analysis after 1- or 5-day intervals radiation therapy (RT) with 40, 20, and 20 Gy, respectively. Immunodeficient BALB/c-nude, immunocompetent C3H, and BALB/c mouse models were used. For immune monitoring, FSaII tumors were analyzed using flow cytometry, immunofluorescence staining, and real-time quantitative reverse transcription polymerase chain reaction. The spleens were used for the ELISpot assay and flow cytometry to determine effector CD8 T cells. For abscopal effect analysis in CT-26 tumors, the volume of the nonirradiated secondary tumors was measured after primary tumors were irradiated with 1-day or 5-day intervals.Contrary to the high-dose 1-day interval RT, the 5-day interval RT significantly delayed tumor growth in immunocompetent mice, which was not observed in immunodeficient mice. In addition, the 5-day interval RT significantly reduced the number of lung metastases in FSaII and CT-26 tumors. Five-day spacing was more effective than 1-day interval in enhancing the antitumor immunity via increasing the secretion of tumor-specific IFN-γ, activating the CD8 T cells, and suppressing the monocytic myeloid-derived suppressor cells. The 5-day spacing inhibited nonirradiated secondary tumor growth more effectively than did the 1-day interval.Compared with the 1-day interval RT, the 5-day interval RT scheme demonstrated enhanced antitumor immunity of CD8 T cells associated with inhibition of myeloid-derived suppressor cells. Enhancing antitumor immunity leads to significant improvements in both primary tumor control and the abscopal effect.