Impact of neoadjuvant chemotherapy on local breast microbiota and macrophages in ER-positive breast cancer with links to treatment response

Gut microbiota can modulate the tumor microenvironment and influence therapeutic outcomes in breast cancer. We hypothesized that similar interactions occur between the breast microbiota and tumor microenvironment. This exploratory study evaluated whether in situ microbial and macrophage densities differ between good and poor neoadjuvant chemotherapy responders and compared microbial density and composition post treatment with surgery-only controls. In 20 postmenopausal estrogen receptor-positive breast cancer patients, bacterial density was quantified by fluorescence in situ hybridization and macrophages density by multiplexed immunofluorescence in pre and post treatment tissue. Bacterial composition was analyzed by 16S rRNA sequencing of post-treatment resection specimens. Nine surgery-only patients served as controls. Neoadjuvant chemotherapy significantly reduced CD68 macrophage density, but not CD163 macrophages or bacterial density. Macrophages and bacterial densities were not correlated, and bacterial densities did not differ between good and poor responders. Higher pretreatment CD68 and CD163 macrophage densities predicted good response. Post treatment resections showed reduced bacterial density and alpha-diversity compared to surgery-only controls. These findings suggest that neoadjuvant chemotherapy affects local breast microbiota and macrophages, with macrophages as promising biomarkers to predict treatment response. The interaction between breast microbiota and local immunity deserves more attention in future research.