Introduction: Lymph node (LN) metastasis, a key prognostic factor for poor survival of breast cancer (BC) patients, directly depends on lymphangiogenesis, i.e., formation of new lymphatic vessels. Using orthotopic BC models, we previously showed that lymphangiogenesis is strongly promoted by tumor-mobilized myeloid-derived lymphatic endothelial cell (LEC) progenitors (M-LECP) and that M-LECP density correlates with lymphatic metastasis. The goal of this study was to examine the density and a prometastatic role of M-LECP in clinical human BC.

Methods: Circulating CD14+ cells from BC patients (N=25) and healthy donors (N=24) were analyzed for expression of lymphatic-specific markers. Specimens from BC patients (N=104) along with normal mammary tissues (N=6) were co-stained with antibodies against myeloid and lymphatic markers including CD68, CD14, CD18 as well as LYVE1, podoplanin, and VEGFR-3. Sections were also double stained to identify LYVE1+ cells expressing lymphoid, M2-type macrophage, immunosuppressive, and stem cell markers. Densities of tumor CD68+ macrophages, LYVE1+ vessels, and CD68+/LYVE1+ double-stained M-LECP were quantified and correlated with node status and other parameters. Prometastatic potential of experimentally produced mouse M-LECP was determined by injecting in vitro differentiated myeloid-lymphatic cells into mice bearing syngeneic EMT6 breast tumors.

Results: 92% of BC patients had monocytes expressing at least one lymphatic marker compared to 2% of healthy donors. More than 70% of breast tumors contained M-LECP with density ranging from 2 to 50 cells per high-power field. Tumor-recruited M-LECP expressed specific markers of myeloid but not lymphoid lineages as well as M2-type, immunosuppressive and stem cell markers. Tumors with high density (>20 cells/field) of M-LECP were significantly more likely to belong to aggressive subgroups (HER2+ and TNBC) and present with lymph node metastasis (P-value <0.02).

Conclusion: These data show for the first time that human breast tumors recruit significant amounts of immature myeloid cells with immunosuppressive and lymphatic-promoting properties. Such cells are largely absent in normal breast tissues. Statistical analyses indicate that M-LECP can significantly contribute to formation of new lymphatic vessels and promote metastasis.

Citation Format: Lisa Volk-Draper, Radhika Patel, Nihit Bhattarai, Jie Yang, Sophia Ran. Myeloid-lymphatic endothelial progenitors significantly contribute to lymph node metastasis in clinical breast cancer [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr A79.