KS03.5.A DISSECTING THE CELLULAR ARCHITECTURE OF BREAST CANCER BRAIN METASTASES REVEALS PROGNOSTICALLY DISTINCT IMMUNE LANDSCAPES

Abstract BACKGROUND Breast cancer brain metastases are associated with a poor prognosis, and there is a high demand for improved personalized treatment strategies. It is still poorly understood which breast cancer brain metastases patients benefit from immune checkpoint blockade. MATERIAL AND METHODS Here, we multimodally characterized the immune niche of breast cancer brain metastases at single-cell resolution in a clinically annotated patient cohort (n = 156), combining multiplexed imaging and cell type quantification by tissue cytometry, single-nuclei RNA-sequencing, flow cytometry, spatial transcriptomics, and functional studies in patient-derived organoids. RESULTS Overall, infiltration of breast cancer brain metastases by CD8 T cells was strongly heterogeneous between patients, and high numbers were prognostically favorable. In-depth characterization of cell types and integrative analyses including clinical data revealed distinct immune landscapes across patients, two of which independently predicted prolonged overall survival: i) Breast cancer brain metastases containing T and B cells accumulating together with characteristic cell communities in stromal tertiary lymphoid structures, and ii) breast cancer brain metastases characterized by a high content of tissue resident-like CD8 memory T cells (CD8 TRM-like cells) spatially enriched within tumor islands. CD8 TRM-like cells exhibited high levels of activation, checkpoint, and functional markers and showed tumor cell killing capacity ex vivo, suggesting them to be critical players of tumor immune control. CONCLUSION Together, our work provides novel insights into anti-tumor immunity in breast cancer brain metastases and reveals prognostic immune biomarkers with potential translational relevance.