Spatial organization of the tumor-immune microenvironment in ER-positive breast cancer with treatment remodeling and associated clinical responses

The tumor microenvironment influences treatment response in ER-positive breast cancer, but what distinguishes responders from non-responders and how it changes during treatment is poorly understood. Here, we profiled ER-positive breast tumors treated with neoadjuvant chemotherapy with or without bevacizumab with bulk proteomics pre-, on-, and post-treatment. Addition of bevacizumab to neoadjuvant chemotherapy amplified the chemotherapy-associated effects on proteomic signaling. A subset of the tumors was profiled with spatial single-cell proteomics pre- and on-treatment, in which cell phenotypes, spatial location, and activation states were determined. Cellular colocalization metrics revealed a shift in the immune contexture from suppressive to supportive during treatment with bevacizumab and chemotherapy. Finally, we identified spatially distinct immune microenvironments in responders and non-responders, where colocalization of M1-like macrophages with epithelial cells and CD44 high PD-1 high CD8 + T-cells was of importance in responders. Our findings highlight the potential of spatial single-cell analysis to support future patient stratification in ER-positive breast cancer for guiding treatment decisions.