Comprehensive analysis of the systemic immune landscape across breast cancer subtypes and disease stages

Background: Breast cancer is a systemic disease, yet the impact of tumor molecular subtype and disease stage on the systemic immune landscape remains poorly understood. In this study, we comprehensively analyzed the systemic immune landscape in a large cohort of breast cancer patients, encompassing all molecular subtypes and disease stages, alongside a control group of healthy donors. Materials and methods: Using multi-parameter flow cytometry, we assessed the abundance, phenotype, and activation status of diverse innate and adaptive immune cell populations across peripheral blood samples from 355 breast cancer patients and 65 healthy donors. Analyzing all blood samples immediately after collection enabled analysis of often overlooked, but highly abundant granulocyte populations, including neutrophils and eosinophils. Results: Our findings reveal that early-stage breast cancer patients exhibit increased cell counts of neutrophils, classical monocytes, and CD1c− dendritic cells (DCs) compared with healthy donors. In late-stage breast cancer patients, we observed elevated counts of neutrophils, classical monocytes, and non-classical monocytes compared with healthy donors. Additionally, reductions were observed in memory B cells, plasmablast-like cells, conventional CD4 T cells, and regulatory T cells. Notably, distinct molecular subtypes were associated with specific changes in the immune landscape, with the most significant changes observed in the triple-negative subtype. Conclusions: Our data indicate that the systemic immune landscape undergoes more profound alterations in metastatic breast cancer than non-metastatic cases, with disease stage exerting a greater influence on systemic immune composition than tumor subtype.