CXCL12-CXCR4 signaling mediates the formation of a macrophageal niche for mammary gland stem cells

Niche-associated signals, essential for stem cell maintenance, are spatially confined and exert their influence locally among adjacent cells. Here, we demonstrate that CXCR4+ macrophages are enriched in the mammary ducts and enhance MaSC activity in basal cells in response to the CXCL12 secreted by luminal cells. Conditional knockout of CXCR4 in macrophages or CXCL12 in luminal cells results in similar phenotypes, including impaired branching morphogenesis, decreased stem cell functionality in basal cells, and diminished ductal association of macrophages. CXCL12 stimulation of macrophages triggers an AKT-mediated stabilization of β-catenin, increasing the expression of pro-migratory genes and multiple Wnt ligands. This process enhances the infiltration of macrophages into intraepithelial regions and their ability to support MaSC functions. Our findings elucidate a crucial role of the CXCL12-CXCR4 axis in facilitating a complex interaction among ductal macrophages and two mammary epithelial cell lineages in establishing a supportive environment for MaSCs during mammary gland development.