Abstract SY37-02: Adapting radiotherapy to cancer immunotherapy

Abstract The fact that the immune system contributes to the response to radiotherapy (RT) was first demonstrated preclinically in 1979, in a syngeneic model of fibrosarcoma, where mice subjected to different degrees of immunosuppression (6 Gy total body irradiation or T cell deprivation) required gradually higher doses of focal radiotherapy to control their cancer compared to immunocompetent controls: interestingly once mice were Tcell deprived, despite achieving local control of the tumor with higher radiation doses, the rate of metastasis drastically increased (Stone H. et al., JNCI 1979). We and other groups originally hypothesized that radiation could be an ideal partner to immunotherapy by enhancing cancer immunogenicity. In response to DNA damage, cytosolic DNA released from the nucleus and mitochondria activates sensors cGAS/STING, leading to release of IFNβ, that recruits and activates BAFT3+ dendritic cells, for cross-presentation and cross-priming of CD8+ T cells (Deng L. et al., Immunity, 2014; Vanpouille-Box C. et al., Nature Communications. 2017; Yamazaki T. et al, Nat Immunology, 2020). Radiation has multiple other immunogenic effects, including increasing the trafficking of activated CD8+ T cells by releasing CXCL16, a chemokine that binds to CXCR6 (Matsumura et al., J Immunol, 2008). Recent evidence has demonstrated how as part of DNA damage response to radiation, mutated cancer genes are expressed, availing neoantigens to the patient’s immune system (Formenti S.C. et al., Nature Medicine, 2018; Lhullier C. et al., JCI 2021). By recruiting both the innate and adaptive immune response, RT can convert the irradiated tumor into an in situ vaccine, providing the host with a tumor specific immune response that both contributes to the response of the irradiated tumor and potentially, unirradiated metastasis, a rare clinical occurrence initially described by Mole et al. as the abscopal effect (Mole R.H., Br J Radiol. 1953). Our team has originally linked the immunogenicity of RT with the abscopal effect (Demaria S. et al., IJROBP, 2005). Abscopal effects of RT are extremely rare because of the multiple immunosuppressive networks that are already in place once established tumors become clinically apparent (Vesely M.D. et al., Annu Rev Immunol, 2011). The availability of agents targeting key mediators of such immunosuppression has enabled their combination with RT, with promising preclinical and clinical results. In general, trials have demonstrated tolerability of combining RT with immunotherapy and have followed two main paradigms to assess efficacy: 1) response is compared in eligible patients randomly assigned to an immunotherapy agent alone or in combination with focal RT and 2) response is compared in eligible patients randomly assigned to a focal RT regimen or to the same regimen in combination with immunotherapy. Success in either case has been demonstrated: for instance, operable NSCLC patients randomly assigned to 3 cycles of preoperative durvalumab achieved a significantly lower rate of pathological response compared than those assigned to durvalumab and immunogenic radiotherapy (Altorki N. et al., Lancet Oncology, 2021). An example of the second paradigm comes from a multicentric prospective randomized trial where the addition of immunotherapy to stereotactic ablative radiotherapy (SABR) in NSCLC resulted in superior local, systemic control and survival compared to SABR alone (Chang J.Y. et al., Lancet 2023). Importantly, evidence has gradually emerged on how to best harness radiotherapy to enhance cancer immunogenicity. To this end, standard radiation treatments have been modified to select specific treatment fields when combined with immunotherapy, and specific radiation dose and fractionation have been chosen. For instance, preclinical evidence discourages the inclusion of draining nodal stations in the field of radiotherapy when RT is used in combination with immunotherapy strategies (Saddawi-Konefka R. et al., Nature Communications 2022). Similarly, preclinical and clinical evidence supports the use of focal hypo-fractionated regimens, with 3-5 fractions of 8-12 Gy (Vanpouille-Box C. et al., Nature Communications 2017; Luke J.J. et al., Clin Cancer Res, 2020), that prevent the induction of the exonuclease Trex, and minimally impact the viability of circulating immune cells (Chen D. et al., Radiother Oncol, 2020). In addition, the choice of optimal sequencing of administration of radiation and immunotherapy has been chosen to impact the effect (Wei J. et al., Sci. Immunol. 2021). Since focal radiotherapy also elicits immunosuppressive effects, overcoming them is as important (Galluzzi L. et al., Nat Reviews Clin Onc, 2023). The availability of new immunotherapies has enabled the exploration of approaches that target different immunological networks. Macrophage biology plays a central role in shaping responses to immunological interventions, including focal radiotherapy. Although radiotherapy is a powerful enhancer of immunogenicity, it also causes significant tissue damage. This damage activates evolutionarily conserved wound-healing mechanisms in macrophages, promoting immune-tolerant phenotypes that ultimately diminish the immunogenic potential of radiotherapy. To overcome this challenge, our group focuses on macrophage reprogramming strategies that reverse immunosuppressive mechanisms induced by radiotherapy (Ben-Chetrit N, et al., Nat Biotechno., 2023). Inspired by a reprogramming CRISPR screen in tumor-associated macrophages (TAMs), we utilize key reprogramming targets to convert immunosuppressive TAMs into immunostimulatory macrophages. This innovative approach has led to the regression of established tumors (as a stand-alone intervention) and the promise of inducing the elusive abscopal effects on distant, non-irradiated tumors when combined with radiotherapy. The strategy of combining RT with immunotherapy is evolving to selectively target the dynamics of RT damage and inflammatory response both to the tumor and normal tissue. It requires acknowledging the temporal evolution of host’s immune response while adapting standard radiotherapy to best interact with the immune system. To this end, the field is evolving to explore novel radiation strategies including incorporating brachytherapy to generate “hot spots’ within the tumor (Jagodinsky C.J. et al., Science Translat Med, 2024), testing different dose rates for RT delivery, and the use of low dose treatment to the gut to modify the microbiome to enhance results of PD-L1blockade in metastatic patients (Chen J et al., Cancer Cell, 2025). More preclinical research as well as clinical trials that incorporate testing these many variables are warranted, to achieve synergy of radiation and immunotherapy. Citation Format: Silvia C. Formenti. Adapting radiotherapy to cancer immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr SY37-02.