Non-immune stromal reprogramming shapes breast cancer immunotherapy resistance: insights from mouse models

Background: PD-1/PD-L1 blockade benefits only a subset of breast cancer patients.While tumor and immune cells have been extensively studied, the role of non-immunestroma such as endothelial cells (ECs) and cancer-associated fibroblasts (CAFs) intherapeutic resistance is less understood. Methods: We performed single-cell RNA sequencing of CD45⁻ cells from threesyngeneic breast cancer models with divergent ICB responses (4T1: resistant, EMT6:partial responder, EO771: responder). Stromal composition, functional programs (GOBiological Process), and tumor-stroma paracrine communication (CellChat) wereanalyzed. Findings were compared with published human breast and lung cancerdatasets (Cell Res 30, 745-762 (2020)). Results: Distinct stromal states were linked to ICB outcomes. 4T1 were enriched forfibroblast- and extracellular matrix-dominant stroma with immune-silent endothelium,while EMT6 tumors showed endothelial expansion and interferon-active CAFs. EO771tumors displayed pericyte-stabilized, quiescent vasculature and immune-stimulatoryfibroblast programs. Pathway-level analysis highlighted conserved modes of stromalcommunication and identified fibroblast-associated signatures that correlated withdisease stage in human cohorts. Pathway-level analysis highlighted conserved modesof stromal communication and identified fibroblast-associated signatures that correlatedwith disease stage in human cohorts. Conclusions: Stromal reprogramming contributes to ICB resistance. Cross-speciesanalysis highlights endothelial and fibroblast states as potential biomarkers andtherapeutic entry points to enhance immunotherapy efficacy.