Abstract B014: Dual checkpoint blockade mitigates the immunosuppressive effects of radiotherapy amplified by experimental liver metastasis
Authors: Zang Han , Bunyan Asha , Kim Luke , Mistry Dhruvi , Bhatt Karan , Tao Shao , Lee James 
Published: January 2025

Abstract
Background: Stereotactic Radiotherapy (RT) is commonly used to manage tumor burden and progression in metastatic cancer patients, often presenting with synchronous liver metastases (LM). Our previous work demonstrated LM is associated with poor responses to checkpoint inhibitor (CPI) and induces systemic tumor-specific immune tolerance in an immunocompetent LM murine model. RT can either stimulate or suppress immune responses, but its immunomodulatory effects and whether it can complement immunotherapy in the context of LM remain unclear. We hypothesize RT in the presence of LM may have unique systemic immunomodulatory effects and aim to measure its impact on innate and adaptive antitumor immunity. Methods: Mice were injected with MC38-Luc cells both subcutaneously (SQ) and intrahepatically to generate SQ-LM mice to mimic multiple disease sites in LM patients. A commonly used dual-SQ model was generated as tumor-site control, as it reliably produces the abscopal response with RT+CPI treatment. SQ tumor of each RT groups were treated with one ablative dose of 14Gy using an image-guided small animal irradiator. Tumor growth of both irradiated and non-irradiated tumor by in vivo imaging were recorded for the following treatment group: No treatment (no-tx), RT alone, RT+aPD-1, RT+dual-CPI (aPD-1+aCTLA-4). Additionally, NRG mice were used to rule out lymphocyte effects. Finally, innate and adaptive responses in both tumor sites were analyzed by 24-color spectral cytometry. Results: SQ RT reduced the local tumor growth compared to no-tx control. Compared to dual-SQ mice, RT-targeted to SQ tumors in the SQ-LM mice had reduced local efficacy. SQ RT paradoxically accelerated growth at the non-irradiated LM site in 3 out of 9 cases, compared to no-tx. The distant LM promoting effect from RT was reproduced in NRG mice, suggesting a T/B cell-independent pro-tumor mechanism. Combining aPD-1 to SQ RT treatment did not significantly enhance response at the LM site nor yield significant abscopal effects. However, combining dual CPI with SQ RT produced significant efficacy improvements over no-tx control at both local and abscopal sites, achieving ∼40% complete response rate. Spectral flow analysis revealed distinct immune modulation as a result RT, revealing SQ RT increased CD80/86 expression on MHC II⁺ monocytes exclusively at the SQ site and decreased ICOS⁺ and Ki67⁺ on CD8⁺ T cells at the liver tumor. In contrast, triple combination therapy enhanced ICOS⁺ CD8⁺ T cells at both sites, compared to control groups. Conclusions: RT targeting to SQ tumor in the presence of LM may inadvertently foster systemic immunosuppression through non-redundant innate immune pathways, promoting growth of non-irradiated liver tumors. However, RT+aPD-1+aCTLA-4 treatment counteracted this effect, suggesting a promising approach to overcome the adverse impact of RT in the context of LM. Data supports dual CPI therapy mitigates potential negative immune impacts from RT in patients with LM, warranting further clinical investigation. Ethics Animal study IACUC approval AN202115 by UCSF
Citation Format: Han Zang, Asha Bunyan, Luke Kim, Dhruvi Mistry, Karan Bhatt, Shao Tao, James Lee. Dual checkpoint blockade mitigates the immunosuppressive effects of radiotherapy amplified by experimental liver metastasis. [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Translating Targeted Therapies in Combination with Radiotherapy; 2025 Jan 26-29; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(2_Suppl):Abstract nr B014.