Abstract Book

S177

ESTRO 37

SP-0334 From local to abscopal: preclinical evidence that radiation dose and fractionation determine its synergy with immunotherapy S. Demaria 1 1 Weill Cornell Medical College, Department of Pathology, New York, USA Abstract text Most cancer cells exposed to ionizing radiation do not undergo immediate death even when the damage is lethal but remain within the tumour mass for some time during which they generate molecular signals that modify the cross-talk of the tumour with the host immune system. Central to this process is the activation of the DNA damage repair (DDR) response, which is often dysregulated in neoplastic cells, and associated with DNA displacement to the cytosol. The presence of dsDNA in the cytosol is sensed by the cyclic GMP-AMP synthase, cGAS, which catalyzes the formation of the second messenger cGAMP and activates STING, leading to the production of type I interferon (IFN-I). IFN-I is a critical effector of cell-mediated anti-viral and anti-tumour immunity and is primarily responsible for the recruitment and activation of BATF3-dendritic cells (DCs), and downstream activation of anti-tumour CD8 T cells. Accumulation of cytosolic IFN-stimulatory dsDNA is regulated by the single radiation dose size, with an optimal window ranging between 4-12 Gy in most human and mouse carcinoma cells tested. Above these doses, upregulation of the DNA exonuclease TREX1 results in clearance of cytosolic dsDNA, abrogating radiation immunogenicity. Fractionation, i.e., repeated (three times) daily delivery of radiation therapy at doses within this window, amplifies the IFN-I pathway activation in the carcinoma cells, an effect that requires induction of IFNRA. Furthermore, the synergy of RT with immune checkpoint blockers (ICBs) anti-CTLA-4 or anti-PD-1 and the induction of abscopal effects (i.e., immune-mediated rejection of non-irradiated synchronous tumours) are completely dependent on the ability of radiotherapy to induce cancer cell-intrinsic IFN-I (1). Tumor-derived dsDNA has also been shown to be critical for cGAS/STING- mediated production of IFN-I by DCs infiltrating immunogenic tumors (2). We have recently found that activation of DCs by tumor-derived dsDNA is modulated by RT and regulated by TREX1. Exosomes derived from irradiated cancer cells deliver the IFN-stimulatory dsDNA cargo to the cytosol of DCs, leading to their STING- mediated activation and production of IFN-I (Diamond et al., in revision). The exosome dsDNA cargo is under the control of TREX1 expressed in the parent cancer cells, indicating that the dose and fractionation used to irradiate the tumor determines immune activation in the tumor microenvironment by multiple mechanisms. These findings have critical implications for the use of radiotherapy to increase the response to ICBs, a combination currently being tested in many clinical trials. Supported by NIH 1R01CA201246 and 1R01CA198533, Breast Cancer Research Foundation, and The Chemotherapy Foundation. References 1. Vanpouille-Box, C., A. Alard, M. J. Aryankalayil, Y. Sarfraz, J. M. Diamond, R. J. Schneider, G. Inghirami, C. N. Coleman, S. C. Formenti, and S. Demaria. 2017. DNA exonuclease Trex1 regulates radiotherapy-induced tumour immunogenicity. Nat Commun 8: 15618. 2. Woo, S. R., M. B. Fuertes, L. Corrales, S. Spranger, M. J. Furdyna, M. Y. Leung, R. Duggan, Y. Wang, G. N. Barber, K. A. Fitzgerald, M. L. Alegre, and T. F. Gajewski. 2014. STING-dependent cytosolic DNA sensing mediates innate immune recognition of immunogenic tumors. Immunity 41: 830-842.

position before treatment. In fact, some RTTs are dedicated to patient positioning and verification, and I was able to watch them work, therefore, becoming more familiar with the role of an RTT in IGRT treatments. A new approach, based on on-line magnetic resonance imaging, is being developed at the UMC with a new hybrid MRI-guided linear accelerator (MRI-LINAC) and I had the great opportunity to approach to this new amazing project. Thanks to ESTRO mobility grant, I was also able to broaden my experience in some clinical procedures developed for on-line IGRT with MR images. Over my insightful two-week visit, I acquired practical skills in IGRT as applied to the techniques and protocols used at UMC. I also developed new capabilities to better manage this important and delicate task and to improve my ability to identify the appropriate techniques to increase both the efficiency and the efficacy of radiotherapy cancer treatments. This mobility grant provided an important opportunity to exchange knowledge and methods of treatment and enabled me to develop practical skills with other RTTs. It was really a fruitful experience, I want to thank ESTRO for the opportunity and last but not least the UMC Radiotherapy Department for the support and hospitality that I received, which I am forever grateful. SP-0333 Overview of the molecular interactions between radiotherapy and the immune system and immune checkpoint inhibitors A. Melcher 1 1 The Institute of Cancer Research, Division of Radiotherapy and Imaging, London, United Kingdom Abstract text Radiotherapy has the potential to induce immunogenic tumour cell death, which can activate an anti-cancer immune response. Therefore, in the context of the current expansion of clinical indications for immuno- oncology (IO) drugs, radiotherapy is an ideal treatment modality to test as part of combination strategies to enhance immunotherapy, which in general has a limited response rate. However, our understanding of the factors that determine whether radiotherapy is immune stimulatory or immune suppressive, including dose, fractionation and scheduling, is very limited. This talk will first outline, in general terms, our current knowledge of interactions between cancers and the immune system, highlighting some of the key molecules and pathways which have been identified as effective targets for immune therapy to date. It will then address how radiotherapy can impact on this cancer/immune interface, including i) the effects of radiotherapy on non- malignant cell components of the tumour microenvironment and ii) how a radiotherapy-induced DNA damage response can support anti-tumour immune priming. Finally, we will cover how radiotherapy might be most effectively and rationally combined with immune checkpoint inhibitors (ICI), as these are currently the most effective class of IO agents. Although there are multiple current trials combining radiotherapy with ICI, these protocols are not generally embedded within the pre-clinical data which might inform optimally effective combinations. Moreover, it is essential to maximise translational research within radiotherapy/IO combination studies, to enhance our understanding of the biology which sculpts the immune response against a tumour during therapy, an essential requirement if we are to fully exploit the potential of immunoradiotherapy. Symposium: Immuno-radiotherapy: the critical influence of radiation dose, scheduling and dose per fraction