Podoplanin-Expressing Macrophages (PoEMs) Promote Lymphangiogenesis and Lymphoinvasion in Breast Cancer

Tumor-associated macrophages (TAMs) constitute the most plentiful immune infiltrate in multiple tumor types and play critical roles at each stage of cancer progression. In breast malignancies, high TAM infiltration is associated with active lymphangiogenesis, lymph node (LN) involvement and distant organ metastasis. Nonetheless, the crosstalk between macrophages and tumor lymphatic vessels has not been extensively studied up to date, and specific TAM populations promoting lymphatic dissemination remain unidentified. This work describes for the first time a subset of TAMs characterized by the expression of Podoplanin (PDPN) - a glycoprotein implicated in regulating cell motility and adhesion. We hereby demonstrate that PDPN is involved in the migration and attachment of this TAM subset to lymphatic endothelial cells (LECs). On a molecular level, PDPN promotes b1 integrin activation in macrophages, mediating their binding to lymphatic-derived Galectin 8 (GAL8). When proximal to lymphatic vessels, Podoplanin-expressing macrophages (PoEMs) support lymphoinvasion through the release of extracellular matrix (ECM) components and matrix-degrading enzymes, as well as through their direct interactions with cancer cells. PoEM-induced ECM remodeling favors the formation of new lymphatic vessels which constitute conduits for cancer cell dissemination. Macrophage-specific Pdpngene knockout, antibody-mediated integrin b1 blockade or pharmacological GAL8 inhibition impair the adhesion of TAMs to lymphatic endothelium. In our cancer models, such defective perilymphatic TAM recruitment restrains lymphangiogenesis and reduces lymphatic metastasis. Furthermore, the association of PoEMs with tumor lymphatic vessels in breast cancer patients correlates with incidences of lymph node and distant organ metastasis. Consequently, PDPN, PoEMs, and GAL8 stem as potential prognostic biomarkers which could trigger the development of new therapeutics targeting metastasis in breast cancer.status: publishe