LSECtin on tumor-associated macrophages enhances breast cancer stemness via interaction with its receptor BTN3A3.

Macrophages have been suggested to contribute to constructing a cancer stem cell (CSC) niche. However, whether and how macrophages regulate the activity of CSCs through juxtacrine signaling are poorly understood. Here we report LSECtin, a transmembrane protein highly expressed on tumor-associated macrophages (TAMs), enhances stemness of breast cancer cells (BCCs). We identified BTN3A3, a B7 family member with previously unknown functions as the receptor for LSECtin on BCCs responsible for stemness-promoting effect of LSECtin. In mice bearing human tumor xenografts, either macrophage-specific ablation of LSECtin or silencing of BTN3A3 in BCCs decreased CSC frequency and tumor growth. Admixture of LSECtin-positive macrophages increased the tumorigenic activity of BCCs dependent on BTN3A3. Disruption of the LSECtin-BTN3A3 axis with BTN3A3-Fc or anti-BTN3A3 mAb has a therapeutic effect on breast cancer. These findings define a juxtacrine signaling mechanism by which TAMs promote cancer stemness. Targeting this axis in the CSC niche may provide potential therapies to breast cancer.