Abstract 6321: Anti-BTN1A1 exhibits synergistic anti-tumor immunotherapeutic efficacy in combination with radiation therapy
Authors: Chung Ezra , Kim Young-Seung , Park Andrew , Lin Steven H. , Jung Hyunjin , Yoo Stephen S. 
Published: June 2022

Abstract
Chronic inflammation is a critical risk factor in the context of cancer development that promotes the expression of immune checkpoint proteins including PD-1, PD-L1, LAG-3, and TIM-3, which in turn suppress T-cell mediated cytotoxic tumor cell killing. Inflammation induced by treatments including chemotherapy and radiotherapy (RT) can also promote immune checkpoint upregulation and consequent tumor immune escape. However, whether the acute inflammation associated with standard chemotherapy and RT can engage distinct immune checkpoints to further support tumor persistence remains unknown. Using an in vivo screening platform in which tumors were irradiated with high-dose γ-radiation, we identified the B7-related immunoglobulin superfamily protein butyrophilin 1A1 (BTN1A1) as an acute stress-inducible immune checkpoint. Herein, we aimed to investigate how BTN1A1 contributes to post-irradiation intratumoral immunosuppression. To assess BTN1A1 induction in response to γ-radiation, human cell lines were exposed to a range of radiation doses, and cell death and BTN1A1 expression were examined by flow cytometry. Radiation promoted apoptosis and BTN1A1 expression in a dose-dependent manner, and PD-L1 was downregulated by increasing the dosage of γ-radiation. As radiation-induced cell death results in cytosolic DNA accumulation within tumors, which in turn activates the production of type I interferons (IFNs) via the cGAS/STING pathway, we examined the effect of BTN1A1 on cGAS and STING expression. BTN1A1 overexpression in human prostate adenocarcinoma PC3 cells suppressed cGAS and STING expression. IFNs can promote antigen presenting cell activation, thus priming T cell responses. We then examined the impact of BTN1A1 overexpression on type I IFN production as assessed via qRT-PCR, revealing that BTN1A1 overexpression enhanced IFN-β expression in the presence of the STING agonist cGAMP. BTN1A1 was also found to harbor a C-terminal B30.2 domain that was able to specifically interact with xanthine oxidoreductase (XOR), an oxidizing enzyme involved in purine metabolism. BTN1A1 expression induced high levels of intracellular reactive oxygen species (ROS), which have the potential to be more toxic in lymphocytes as compared to tumor cells, and BTN1A1 knockdown decreased these ROS levels. In light of these observations, we assessed the combination therapeutic efficacy of RT and anti-BTN1A1 antibodies in a syngeneic murine Lewis lung carcinoma (LLC) model system, revealing that these two treatments exhibited synergistic anti-tumor activity. FACS and IHC further confirmed that BTN1A1 was upregulated within tumors following irradiation. Together, these data offer new insights regarding the immunomodulatory role of radiation-induced BTN1A1 within tumors, providing a more robust foundation for the development of BTN1A1 as an immunologic target for cancer therapy.
Citation Format: Ezra Chung, Young-Seung Kim, Andrew Park, Steven H. Lin, Hyunjin Jung, Stephen S. Yoo. Anti-BTN1A1 exhibits synergistic anti-tumor immunotherapeutic efficacy in combination with radiation therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6321.