Evaluation of Tumor Immune Microenvironment Changes in Response to Different Radiotherapy Regimens in a CT26 Tumor Model

Background: Radiotherapy (RT) is vital in cancer treatment, inducing tumor cell death and anti-tumor immunity. However, it can also enhance immunosuppressive factors like CD47 and PD-L1. While RT- immunotherapy combinations show promise, optimal strategies remain unclear. This study examines the impact of different RT regimens on the tumor immune microenvironment (TME) to guide more effective treatment approaches. Methods: We treated CT26 tumor-bearing mice with three distinct RT regimens under the same biological equivalent dose (BED). Result: We observed that the frequency of both tumor-infiltrating CD4+ and CD8+ cells were increased after ablative and hypo RT, although single high dose exposure in an ablative scheme led to a greater extent of CD8+ cells infiltration and increased the expression of IFNγ in those cells. While conventional RT enhanced the recruitment of myeloid-derived suppressor cells (MDSCs) into tumors, the Ablative and hypo schemes induced regulatory T cells (Tregs) enrichment. The two hypofractionated regimens enhanced the expression of CD47 as well as the PD-1/PD-L1 axis, although PD-1 and its ligand (PD-L1) expression were temporarily induced by conventional RT on tumor-infiltrating lymphocytes and tumor cells, respectively. Conclusion:The results suggest targeting the recruitment of MDSC and Treg are, respectively, crucial for therapeutic efficacy of conventional and hypofractionated RT regimens. Furthermore, anti-PD-1/PD-L1 and anti-CD47 treatments are necessary to improve the anti-tumor response from the hypofractionated schemes.