Association of tumor microenvironment landscape with neoadjuvant therapy resistance in hormone receptor–positive breast cancer.

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Background: Hormone receptor (HR)–positive breast cancer shows limited sensitivity to neoadjuvant therapy, with pCR rates below 10%—significantly lower than the rates in HER2-positive and triple-negative breast cancer. We investigated the immune characteristics of the tumor microenvironment associated with different pCR outcomes in HR-positive breast cancer. Methods: Pretreatment core biopsy samples were obtained from 36 patients with HR-positive breast cancer who received eight cycles of neoadjuvant EC–T chemotherapy and were stratified based on postoperative pCR status. Single-cell RNA sequencing was performed on samples from 8 patients, while imaging mass cytometry (IMC) was conducted on samples from 28 patients to characterize the spatial immune landscape. In addition, functional assays were performed to further investigate cellular interactions within the tumor immune microenvironment. Results: All patients had clinical stage II–III, HER2-negative, ER/PR-positive breast cancer. Imaging mass cytometry identified 24 distinct cellular clusters and 10 tumor communities within the HR-positive tumor microenvironment. Integrated single-cell RNA sequencing and IMC analyses defined five tumor cell subpopulations. A tumor community dominated by E-cadherin–low tumor cells was significantly enriched in the non-pCR group and was characterized by surrounding fibroblasts that spatially separated tumor cells from immune infiltration. Multicolor immunofluorescence further demonstrated a significant positive correlation between tumor E-cadherin expression and pCR rates following neoadjuvant therapy in HR-positive breast cancer. Mechanistically, E-cadherin–low tumor cells exhibited increased secretion of NAMPT, which interacted specifically with fibroblasts, promoted collagen barrier formation, and contributed to an immune-desert tumor microenvironment. Conclusions: Single-cell and spatial analyses reveal tumor microenvironment–driven mechanisms of neoadjuvant therapy resistance in HR-positive breast cancer, providing a basis for early response prediction and precision therapy. Research Sponsor: "Pioneer" and "Leading Goose" R&D Program of Zhejiang (2026C02A1111); National Natural Science Foundation of China (82273449).