LYVE-1–expressing Macrophages Modulate the Hyaluronan-containing Extracellular Matrix in the Mammary Stroma and Contribute to Mammary Tumor Growth

Macrophages represent a heterogeneous myeloid population with diverse functions in normal tissues and tumors. While macrophages expressing the cell surface marker lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1) have been identified in stromal regions of the normal mammary gland and in the peritumoral stroma, their functions within these regions are not well understood. Using a genetic mouse model of LYVE-1⁺ macrophage depletion, we demonstrate that loss of LYVE-1⁺ macrophages is associated with altered extracellular matrix remodeling in the normal mammary gland and reduced mammary tumor growth in vivo. In further studies focused on investigating the functions of LYVE-1⁺ macrophages in the tumor microenvironment, we demonstrate that LYVE-1 expression correlates with an increased ability of macrophages to bind, internalize, and degrade hyaluronan. Consistent with this, we show that depletion of LYVE-1⁺ macrophages correlates with increased hyaluronan accumulation in both the normal mammary gland and in mammary tumors. Analysis of single-cell RNA sequencing of macrophages isolated from these tumors reveals that depletion of LYVE-1⁺ macrophages in tumors drives a shift in the majority of the remaining macrophages toward a proinflammatory phenotype, as well as an increase in CD8⁺ T-cell infiltration. Together, these findings indicate that LYVE-1⁺ macrophages represent a tumor-promoting anti-inflammatory subset of macrophages that contributes to hyaluronan remodeling in the tumor microenvironment. Significance: We have identified a macrophage subset in mouse mammary tumors associated with tumor structural components. When this macrophage subset is absent in tumors, we report a delay in tumor growth and an increase in antitumor immune cells. Understanding the functions of distinct macrophage subsets may allow for improved therapeutic strategies for patients with breast cancer.