Ionizing radiation (IR) has variable effects on the solid tumor immune microenvironment. While many report additive or synergistic effects between IR and immune activating therapies, most studies utilize single fractions or hypofractionated high-dose radiation of 8 Gy or greater. For head and neck squamous cell carcinoma (HNSCC), 30 or more low-dose fractions of 1.8-2Gy are utilized as standard-of-care for the up-front treatment of early stage or advanced disease. Preliminary experiments from our laboratory have demonstrated that 10 fractions of 2Gy IR decreased the rate of rejection of T-cell inflamed syngeneic carcinomas in wild-type B6 mice when combined with cyclic dinucleotide (CDN) compared to CDN treatment alone (10% rejection rate with low-dose fractionated IR plus CDN vs. 50% with CDN alone). Immune suppression following IR in mice is mediated by at least galectin-1. Further, HNSCC patients treated with standard fractionated low-dose IR develop durable neutropenia. Thus, we hypothesized that low-dose fractionated IR is immunosuppressive and does not result in additive or synergistic effects when combined with cyclic dinucleotide. To test this hypothesis, we performed a time-course analysis of tumor immune infiltration and tumor-draining lymph node antigen-specific T-lymphocyte activation in three syngeneic models of carcinoma with defined tumor rejection antigens (MOC1-p15E, B16-OVA and MC38-CEA) following exposure to two different IR regimens of 8Gy x 2 fractions or 2Gy x 10 fractions. Analysis of effector immune infiltration and antigen-specific activation pre-IR and at 5, 10, and 20 days post-IR revealed consistent trends of profound immunosuppression with the low-dose fractionated IR regimen (2Gy x 10) but preserved or enhanced immune activation with the high-dose hypofractionated regimen (8Gy x 2). Galectin-1 levels will be measured from whole tumor lysates. We plan to perform nano-string RNA array analysis on irradiated tissues to serve as correlative immune analysis and also to validate this technology against well-defined immune read-outs for potential use in the clinical trial setting. We also plan to combine both IR regimens with intra-lesional CDN (20 μg x 3 injections) to determine if either IR regimen results in additive or synergistic tumor control. This work has profound implications for future pre-clinical studies and clinical trials combining IR and immune activating therapies, such as CDN or checkpoint inhibitors.

Citation Format: Megan V. Morisada, Ellen C. Moore, Jay A. Friedman, James W. Hodge, James B. Mitchell, Clint T. Allen. High-dose versus low-dose fractionated ionizing radiation to enhance antigen-specific antitumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1696. doi:10.1158/1538-7445.AM2017-1696