Rational modulation of immune mechanisms synergizes the anti-tumor effects of targeted radiation therapy in pre-clinical models

Immunotherapy has revolutionized cancer treatment, offering new hope for many patients. However, while some individuals show remarkable responses, the overall success rate remains limited. This has spurred interest in combination therapies, particularly with established treatments like radiation therapy (RT), to improve outcomes. RT is a cornerstone of cancer therapy and known to influence the immune landscape, yet a systematic characterization of its effects on tumor-infiltrating leukocytes (TILs) and a rationale-based therapy is still lacking. In this study, we employed a diverse set of pre-clinical syngeneic murine tumor models with varying immune profiles to investigate the immunological impact of tumor targeted RT. We observed that immunologically ‘hot’ tumors showed stronger tumor growth inhibition (TGI) after RT compared to ‘cold’ tumors. Additionally, RT induced both pro- and anti-inflammatory shifts within the tumor immune microenvironment. Importantly, RT led to an intra-tumoral increase in proliferating CD8 + T cells, while the population of proliferating macrophages was notably reduced. To identify immune-modulatory pathways that shape the response to RT across different tumor immune contexts, we tested RT in HPK1 (Hematopoietic Progenitor Kinase 1) and STING (Stimulator of Interferon Genes) deficient mice. These experiments revealed that STING deficiency compromises TGI in tumors with a high baseline population of myeloid cells expressing an interferon response signature. Moreover, we identified a synergistic effect on survival in tumor-bearing mice when combining HPK1 deficiency with RT. Thus, RT promotes expansion of cytotoxic T cells while limiting macrophage proliferation, with therapeutic outcomes strongly influenced by STING and HPK1 pathways. Collectively, these results highlight the complex interplay between RT, tumor immune microenvironment and response to therapy, offering potential avenues for novel therapeutic combinations.