Abstract PR06: The effect of low-dose fractionated versus high-dose hypofractionated radiotherapy on antitumor immunity

Fractionated, low-dose (1.8-2 Gy) ionizing radiation (IR) is an integral component of head and neck cancer management. The mechanism of tumor control has traditionally been attributed to direct cytolytic effects on tumor cells. Substantial evidence supports enhanced antitumor immune responses when IR is given in large, single or hypofractionated doses. The effect of low-dose fractionated IR on antitumor immunity is less clear. We hypothesized that high-dose (8 Gy) IR would induce antigen release and priming of antigen-specific T-lymphocytes and sensitize tumor cells to killing by effector T-lymphocytes (CTLs) to a greater degree than low-dose (2 Gy) IR. To test this hypothesis, we stably transduced murine oral cancer 1 (MOC1) cells with full-length ovalbumin as a model antigen (MOC1ova). Upon exposure of MOC1ova cells to 8 Gy IR in vitro, splenocytes endocytosed and cross-presented SIINFEKL on H2-Kb and induced proliferation of CFSE-labeled, SIINFEKL-specific OT-1 T-lymphocytes to a greater degree than when exposed to MOC1ova cells irradiated to 0 or 2 Gy. Antigen cross-presentation was dependent upon type I interferon. In in vivo correlative experiments, MOC1ova tumors irradiated to 8 Gy induced expansion of adoptively transferred OT-1 T-lymphocytes located within the spleen, draining lymph node, and tumor to a greater degree than tumors exposed to 0 or 2 Gy. To assess if higher doses of IR enhance CTL killing of target cells, a real-time impedance-based cytotoxicity assay was utilized. Exposure of target cells to 8 Gy IR dramatically enhanced OT-1 CTL killing of MOC1ova cells to a greater degree than cells exposed to 0 or 2 Gy in a process independent of antigen-presentation. We next hypothesized that 10 days of 2Gy/day (low-dose fractionated) IR would be immunosuppressive in vivo compared to 2 fractions of 8 Gy (high-dose, hypofractionated) IR. We performed a time course analysis of effector and immunosuppressive cell peripheral accumulation and tumor infiltration as well as antigen-specific draining lymph node T-lymphocyte responses comparing these two IR regimens. Cumulatively, results demonstrated significantly greater correlative and functional T-lymphocyte immune responses with high-dose hypofractionated IR compared to daily low-dose IR. Analysis of myeloid derived suppressor cell and regulatory CD4+ T-lymphocyte changes displayed mixed alterations between regimens. Taken together, these results suggest that higher-dose, hypofractionated IR results in enhanced immune activation and less immunosuppression compared to daily fractionated low-dose IR. These results have significant implications in the rational design of combination therapies utilizing IR and immunotherapy. This abstract is also being presented as Poster 32. Citation Format: Megan V. Morisada, Ellen C. Moore, Jay Friedman, Clint T. Allen. The effect of low-dose fractionated versus high-dose hypofractionated radiotherapy on antitumor immunity [abstract]. In: Proceedings of the AACR-AHNS Head and Neck Cancer Conference: Optimizing Survival and Quality of Life through Basic, Clinical, and Translational Research; April 23-25, 2017; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(23_Suppl):Abstract nr PR06.