SPP1+ macrophages in HR+ breast cancer are associated with tumour-infiltrating lymphocytes.

Breast cancer, categorised into hormone receptor-positive (HR+), HER2-positive (HER2+), and triple-negative (TNBC) subtypes, exhibits varied outcomes based on the number of tumour-infiltrating lymphocytes (TILs). Increased TIL levels are associated with better outcomes in HER2+ and TNBC, while HR+ individuals with high TIL levels show shorter survival but greater neoadjuvant chemotherapy response. To explore the divergent roles of TIL levels across various subtypes and their effect on immune cell composition, we employed single-cell RNA sequencing on 31 patients with breast cancer. HR+ breast cancer with high TIL levels (TIL-high) revealed increased SPP1+ macrophages, increased SPP1 expression in other monocytes/macrophages (mono/macro) subgroups, and enriched pathways associated with extracellular matrix (ECM) remodelling in mono/macro. Moreover, cell–cell interaction analyses revealed enhanced SPP1, MIF, and FN1 signalling in the interaction between SPP1+ macrophages and T-cells in TIL-high HR+ breast cancer. Spatial transcriptomics data highlighted the close proximity of SPP1+ macrophages, CD8+ T-cells, and CD4+ T-cells in TIL-high HR+ breast cancer. Our findings unveil the novel influence of SPP1+ macrophages on T-cells in TIL-high HR+ breast cancer, potentially explaining the poor prognosis and offering insights for targeted interventions.