Abstract 1101: Combination therapy to overcome radiotherapy induced immunosuppression and improve tumor control

Background: The relationship between radiation therapy, immunosuppressive myeloid populations and adenosine is not well characterized. Here we test the hypothesis that radiation-induced adenosine signaling promotes tumor growth and causes suppression of T cell function and that therapeutic targeting may improve response to radiotherapy. Methods: Orthotopic 4T1 cells were implanted in Balb/c mice and tumors were irradiated using small animal radiation research platform (SARRP, Xstrahl; 220 kV) with 3 fractions of 8 Gy and/or treated with adenosine receptor 2A and 2B (A2AR/A2BRi) inhibitor, CD73 inhibitor (CD73i), and/or checkpoint inhibitor, aPD-1. Tumor response was assessed by tumor volume and the immune response characterized by flow cytometry. Results: Intratumoral immune response was evaluated 72 hours post-irradiation. Radiotherapy induced an expansion of myeloid cells particularly monocytic cells (13.5% vs 7% of CD45+ cells) with a corresponding contraction of lymphocytes. Monocytes from irradiated tumors had increased expression of most of the key members of the adenosine signaling pathway including enpp1 (2.5% vs 0.6%), CD73 (10.3% vs 4.8%), CD39 (10.7% vs 4.4%), A2aR (2.8% vs 0.6%) and A2bR (2.4% vs 0.3%), compared to untreated controls. In the absence of tumor irradiation, the combination of A2AR/A2BRi & CD73i with αPD-1 showed no improvement in tumor control compared to A2AR/A2BRi & CD73i alone or untreated controls. However, in the setting of tumor irradiation, we measured a significant reduction in 4T1 tumor volume when aPD-1 was added to A2AR/A2BRi & CD73i, compared to A2AR/A2BRi & CD73i alone (156mm^3 vs 250mm^3). The addition of αPD-1 to A2AR/A2BRi & CD73i and tumor irradiation resulted in decrease in monocytic cells (9.4% vs 15% of CD45+ cells) and expression of CD39 (7.4% vs 11%), CD73 (7.5% vs 11%), A2aR (2.2% vs 7%) and A2bR (2.7% vs 9%) in monocytic compartment, compared to radiation and A2AR/A2BRi & CD73i. Further, there was a corresponding expansion of Ki67+ proliferating (52% vs 36%) and IFN-γ producing CD8+ T cells (3.3% vs 2%) after treatment with radiation, A2AR/A2BRi & CD73i, and αPD-1, suggesting improved T cell effector function. Conclusion: Radiation-induced adenosine signaling facilitates immunosuppressive changes in the TME. In the immunotherapy insensitive 4T1 mouse model, adenosine blockade induces checkpoint inhibition sensitivity through alleviation of immunosuppression, thus improving tumor control. Citation Format: Shruti Bansal, Matthew Chaimowitz, Julia Ann, Patrick McCann, Jason Piersaint, Claudia Aiello, Namita Sen, Julia Kogan, Catherine Spina. Combination therapy to overcome radiotherapy induced immunosuppression and improve tumor control [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1101.