Myeloid-Derived Lymphatic Endothelial Cell Progenitors Significantly Contribute to Lymphatic Metastasis in Clinical Breast Cancer

Lymphatic metastasis is a high impact prognostic factor for mortality of breast cancer (BC) patients, and directly depends on tumor-associated lymphatic vessels. We previously reported that lipopolysaccharide-induced inflammatory lymphangiogenesis is strongly promoted by myeloid-derived lymphatic endothelial cell progenitors (M-LECP) derived from the bone marrow (BM). As BC recruit massive numbers of pro-vascular myeloid cells, we hypothesized that M-LECP, within this recruited population, are specifically programmed to promote tumor lymphatics that increase lymph node metastasis. In support of this hypothesis, high levels of M-LECP were found in peripheral blood and tumor tissues of BC patients. Moreover, the density of M-LECP and lymphatic vessels positive for myeloid marker proteins strongly correlated with patient node status. It was also established that tumor M-LECP co-express lymphatic-specific, stem/progenitor and M2-type macrophage markers that indicate their BM hematopoietic-myeloid origin and distinguish them from mature lymphatic endothelial cells, tumor-infiltrating lymphoid cells, and tissue-resident macrophages. Using four orthotopic BC models, we show that mouse M-LECP are similarly recruited to tumors and integrate into pre-existing lymphatics. Finally, we demonstrate that adoptive transfer of in vitro differentiated M-LECP, but not naive or non-differentiated BM cells, significantly increased metastatic burden in ipsilateral lymph nodes. These data support a causative role of BC-induced lymphatic progenitors in tumor lymphangiogenesis and suggest molecular targets for their inhibition.