LYVE-1-expressing macrophages modulate the extracellular matrix in the mammary gland and contribute to mammary tumor growth

Macrophages represent a heterogeneous myeloid population with diverse functions in normal tissues and tumors. While LYVE-1 macrophages have been identified in stromal regions of the normal mammary gland and mammary tumor, their functions in these regions remain unknown. Using a genetic LYVE-1+ macrophage deletion model, we demonstrate that loss of LYVE-1+ macrophages leads to accumulation of the extracellular matrix glycosaminoglycan hyaluronan. Consistent with this finding, we demonstrate that LYVE-1 expression correlates with an increased ability of macrophages to bind, internalize, and degrade hyaluronan. Furthermore, we demonstrate that deletion of LYVE-1+ macrophages results in increased hyaluronan accumulation in mammary tumors, which correlates with reduced tumor growth. Finally, using scRNA-seq, we demonstrate that deletion of LYVE-1 macrophages in tumors results in a shift in the remaining macrophages towards a pro-inflammatory phenotype. Together, these findings demonstrate that LYVE-1+ macrophages represent an anti-inflammatory macrophage population that contributes to tissue remodeling in the tumor microenvironment. Overall design: Two Lyve1CreCsf1rfl/fl female mice and two Csf1rfl/fl female mice were orthotopically injected with EO771 tumor cells. Tumors were grown to 1.5cm3 and CD45+ leukocytes were flow sorted for single cell RNA sequencing