Abstract P003: Harnessing macrophage plasticity and radiotherapy for breast cancer immunotherapy
Authors: Sotelo Jesus , Niu Xiang , Vinagolu Rajasekhar K. , Jiao Maria S. , Neville Sanjana E. , Landau Dan Avi , Fermonti Silvia C. , Ben Chetrit Nir 
Published: January 2025

Abstract
Immunotherapies are a transformative force in clinical oncology, delivering prolonged disease-free survival in refractory cancers. However, in the setting of breast malignancies, clinical responses to immune checkpoint inhibitors (ICI) are both low and ephemeral, particularly in the hormone receptor-positive (HR+) subtype. Effective responses are predicated on abundant neoantigens, the activity of immune effector cells within the tumor microenvironment (TME), and the establishment of immunological memory. These prerequisites pose a significant efficacy barrier as BC patients carry low mutational burdens (neoantigens), and T-cells are profoundly hindered by the presence of immunosuppressive tumor-associated macrophages (TAMs). Radiotherapy (RT) holds a significant untapped potential to overcome the lack of neoantigens by triggering immunogenic cell death and recruiting adaptive immunity. However, the unavoidable tissue damage and clearance of dead cells by adjacent macrophages further enhance their wound-healing (immunosuppressive) phenotype, dampening the immunogenic potential of RT. Macrophages regulate immune responses through remarkably plastic activation states, including immune-stimulatory (M1) and immune-suppressive (M2). In breast tumors, macrophages undergo a well-documented but poorly understood “M2-education” process that facilitates immune evasion, utilizing a myriad of immune modulators (hormones, cytokines, and enzymes). Collectively, these dependencies highlight a tremendous therapeutic potential for combining RT with strategies that recover the M1 phenotype of TAMs, creating immune-supportive TME. Despite the potential, modulating TAM activities has proven exceptionally challenging, primarily due to the pleiotropic nature of their suppressive functions and the lack of scalable TME models to enable high-throughput target discovery. To overcome these limitations, we developed a scalable platform that recreates the breast TME at scale and phenotypic fidelity. This model contains the major BC constituents, including macrophages, tumor cells, and their supporting stroma, while preserving the functions, transcriptional programs, cell-cell interactions, and typical spatial organization. To identify reprogramming targets in TAMs, we performed a CRISPR screen utilizing Arg1 (a hallmark M2 marker) as a surrogate marker. Our screen exposed targets that convert immunosuppressive (M2) TAMs into immunostimulatory (M1) macrophages in established tumors. In vivo data show that TAM reprogramming is remarkably effective, promoting tumor regression and abscopal effect in HR+ and ovarian cancers. Reprogrammed TAMs enhanced responses to ICI and radiotherapy, a path we plan to optimize for future clinical development as “first-in-human” trials. Interventions that re-potentiate innate and adaptive immunities can benefit cancer patients, particularly those with limited therapeutic options. Our findings pioneer an actionable roadmap to unlock the full therapeutic potential of RT and immunotherapies in refractory human cancers.
Citation Format: Jesus Sotelo, Xiang Niu, Rajasekhar K. Vinagolu, Maria S. Jiao, Sanjana E. Neville, Dan Avi Landau, Silvia C. Fermonti, Nir Ben Chetrit. Harnessing macrophage plasticity and radiotherapy for breast cancer immunotherapy. [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 P003