Abstract 5427: Different immune responses in the radiotherapy-sensitive and resistant mouse model

Abstract Background and purpose: Different mouse models have distinct responses to radiotherapy. The purpose of this study was to investigate the immune effects of radiotherapy on tumor models with distinct sensitivities, including LL2 (mouse Lewis lung cancer) and CT26WT (mouse colon cancer) syngeneic models. Methods and materials: LL2 and CT26WT cell lines were irradiated with different X-ray doses for 24 h in vitro. Cell viability was determined with a CellTiter Glo assay. RNA and protein expression levels of Sting and IFNb1 were determined by real-time quantitative PCR and western blotting. The corresponding tumor models in C57BL/6 and BALB/c mice were established in vivo. Tumor growth of each irradiated model was monitored for 28 days post implantation. Mice were euthanized on day 5 from the beginning of irradiation. Expression levels of immune-related factors Sting and IFNb1 in tumor tissues were determined by real-time quantitative PCR and western blotting. PDL1 expression on tumor cell surfaces and PD1 expression on lymphocyte surfaces andon tumor-infiltrating lymphocytes (TILs) such as dendritic cells (DCs) and T cells were measured by flow cytometry. CD8a cell infiltration in tumor tissue was examined using immunohistochemistry. Results: In vivo, LL2 and CT26WT cell lines were irradiated with distinct strategies, comprising 0 Gy, 8 Gy, 20 Gy,30 Gy, and 8Gy × 3. After 24 h irradiation, no significant decrease of cell viability was observed in any group. No obvious expression level changes of Sting protein in LL2 were observed; however, increased expression of IFNb1 was observed with an increase in irradiation dose to 8 Gy × 3 (P<0.05). However, significant decreases of Sting and IFNb1 expression were observed in both the 20Gy single-dose and 8Gy × 3 irradiation groups in the CT26WT cell line. Cell viability in both the LL2 and CT26WT cell lines was significantly decreased in the 8 Gy × 3 group after 72 h irradiation (P<0.05). No anti-tumor activity of 8 Gy× 3 was found in the LL2 syngeneic model; however, there was a marked effect in the CT26WT model in vivo. In the LL2 model, after 5 days’ irradiation, there was a significant increase in the infiltration of DCs, Th, and Teff cells, and in the expression of PD1 and PDL1 in tumor tissue (P<0.05); however, these effects were not observed in the CT26WT model. Immune responses in tumor tissue were distinct in the two syngeneic mouse models with different radiosensitivities. Conclusion: In the LL2 model, which was not sensitive to radiotherapy, the immune-responsive effect was enhanced after X-ray treatment in the tumor microenvironment; however, no such effect was found in the radiotherapy-sensitive CT26WT tumor model. Our data demonstrate that radiotherapy combined with immunotherapy can greatly improve anti-tumor activity in LL2 radiotherapy-insensitive tumor models. These results also provide a possible basis for effective treatment in other radiotherapy-insensitive tumor models. Citation Format: Buhai Wang, Yichun Zeng, Qiuxian Li, Juan Gu. Different immune responses in the radiotherapy-sensitive and resistant mouse model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5427.