Abstract PO-047: Early T cell responses are enhanced by heterogenous dose delivery in a murine model of head and neck cancer (2021)

Abstract Background: Combination treatment with radiotherapy can prime the immune system due and improve responses to PD-1 based immunotherapy. However, the optimal dose and schedule of combination therapy is still unclear. Although hypofractionated radiotherapy has been shown to be superior to traditional fractionation schedules, the immunostimulatory effects of this regimen may be limited by death of radiation sensitive T-cells. Here we examine the effects of heterogenous dose delivery on early T cell responses in a murine model of head and neck cancer. Methods: Dose delivery was validated for the SARRP irradiator using high spatial resolution radiochromic film absolute dosimetry. The nozzles size was selected to deliver either a uniform dose of 12 Gy or SORT using two abutted nozzles to deliver conventional (2Gy) and hypofractionated (12 Gy) dose regions. C3H/HeJ mice bearing SCCVII/SF xenografts (n=8/group) were irradiated with either a single fraction of a uniform dose of 12 Gy using the 1 × 1 cm2 nozzle to cover the entire tumor or SORT (2-12Gy) using 0.5 × 0.5 cm2 nozzle. To assess the effect on activation of T cells present within the tumor at the time of radiation, mice were sacrificed 24 hrs after treatment and the presence of T cell subsets and cytokines was determined by real-time PCR. Statistical differences were determined using a one-way ANOVA followed by a post-hoc Tukey T-Test. Results: Radiochromic film measurements indicated highly uniform dose profiles with sharp dose fall off region for 0.5 × 0.5 cm2 nozzle, allowing butting of high and low dose fields. Quantitative PCR revealed increases in CD8 gene expression in both treatment groups over sham-irradiated mice with a slight but significant increase in 2Gy-12Gy treated mice compared to mice treated with 12Gy (7 fold vs 5 fold). No statistical differences in FoxP3 gene expression were observed between the groups. Significant increases of IFN-γ gene expression were observed in SORT treated mice compared to mice irradiated with 12Gy. Mice treated with a uniform 12Gy dose showed a 13.75-fold increase in IFN-γ over sham irradiated mice. This increase was significantly larger in 2Gy-12Gy treated mice in which a mean 67 fold-difference over sham-irradiated mice was observed. In contrast, tumors treated with 12Gy had a significantly higher level of TGF-beta gene expression than mice treated with 12Gy-2GY compared to sham irradiated mice (10 fold vs 6 fold). Conclusion: Although increases in CD8 gene expression were observed in both treatment groups, this increase was not proportional to the large increase in IFN-γ observed in mice treated with a heterogenous dose, indicating enhanced T-cell survival and/or activation. Further study is needed to determine if early immune activation observed with heterogeneous dosing correlates to a sustained immune response and improved response to immunotherapy. Citation Format: Jennifer Mourtada, Adam Raben, James Keller, Neil Hockstien, Yan Yu, Bo Lu, Firas Mourtada. Early T cell responses are enhanced by heterogenous dose delivery in a murine model of head and neck cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-047.