Tehran University of Medical Sciences Immunology and Genetics Journal 2645-4831 8 0 2025 02 24 100 110 Masoumeh Alimohammadi Department of Immunology, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran Haniyeh Ghaffari Nazari Department of Immunology, School of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran Reza Alimohammadi Department of Immunology, School of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran Mohsen Bakhshandeh Department of Radiology Technology, Allied Medical Faculty, Shahid Beheshti University of Medical Sciences, Tehran, Iran Seyed Amir Jalali Department of Immunology, School of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran Nima Rezaei Department of Immunology, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran; Research Center for Immunodeficiencies (RCID), Children's Medical Center Hospital, Tehran University of Medical Sciences, Tehran, Iran; Network of Immunity in Infection, Malignancy and Autoimmunity (NIIMA), Universal Scientific Education and Research Network (USERN), Tehran, Iran 2025 02 24 2025 02 24 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. https://igj.tums.ac.ir/index.php/igj/article/view/195 https://igj.tums.ac.ir/index.php/igj/article/download/195/172