Abstract B008: Understanding myeloid and lymphoid cell heterogeneity following radiotherapy and its impact on metastasis development in breast cancer
Authors: Perini Alessandra , Brucoli Marti , Acton Sophie , Ombrato Luigi 
Published: January 2025

Abstract
Breast cancer (BC) is the most frequent malignancy in women worldwide and the gold standard for its treatment currently entails resection of the primary tumor, followed by radiotherapy (RT), chemotherapy and/or immunotherapy. Specifically, adjuvant RT directed at the breast has been successfully employed to improve the risk of local recurrences, but the main obstacle to full remission of most BC patients remains the development of metastasis. RT directed at the primary tumor (pRT) acts via a combination of direct cytotoxicity and immunomodulatory effects on the local tumor microenvironment (TME), whereas its impact at distal sites and on the metastatic TME is still poorly understood. Thus, we hypothesized that pRT-driven systemic effects could change the nature and function of immune cell populations at distal sites, affecting survival and growth of disseminated tumor cells and ultimately influencing the development of metastasis. Analysis of scRNAseq data from murine lungs at the pre-metastatic stage showed that pRT towards a tumor-bearing breast induces alterations within the lung immune landscape, particularly suggesting a dual role of neutrophils and alterations in their interactions with the adaptive immune system. Moreover, we demonstrated that in a mouse model of metastasis, experimentally induced via intravenous (IV) injection of 4T1 cells, pRT directed to a tumor-bearing breast induces a significant increase in lung metastasis, accompanied by numerical and functional differences in lymphoid and myeloid cells which collectively suggest the establishment of a local immunosuppressive microenvironment. Interestingly, the metastatic increase observed in irradiated mice resembled the phenotype of mice IV injected with 4T1 cells in absence of a primary tumor, thus when there is no pre-activation of the immune system prior to the IV injection. This result suggested that pRT could alter systemic anti-tumor immunity in our model, resulting in metastatic growth. Therefore, current experiments are focusing on elucidating the specific immune populations involved in this effect by selectively depleting cell types of interest, such as neutrophils and Treg cells. Moreover, we are further investigating the mechanisms influencing DTCs’ outgrowth via scRNAseq of lungs bearing niche-labelling metastasis, which allow to identify changes occurring specifically in the metastatic niche. Finally, we are confirming the effects observed in our experimentally-induced metastatic model also in a clinically relevant mouse model, comprising both the spontaneous metastatic spread of 4T1 cells and the resection of the primary tumor prior to RT, resulting in a therapeutic protocol more similar to current clinical practice. Our data show a pro-metastatic effect of RT, possibly driven by alterations in the activation of the immune system. Results from this project could better inform on RT’s influence on immune cell behavior in the metastatic TME, allowing to identify relevant targets for novel immunotherapy approaches against metastatic disease.
Citation Format: Alessandra Perini, Marti Brucoli, Sophie Acton, Luigi Ombrato. Understanding myeloid and lymphoid cell heterogeneity following radiotherapy and its impact on metastasis development in breast cancer. [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 B008.