High Systemic VEGF-C Levels Lead to a Specific Neutrophil Accumulation in the Lung and Oromote Metastasis in a Syngeneic Rat Breast Cancer Model

VEGF-C (Vascular endothelial growth factor-C) and its receptor VEGFR-3 are key regulators of lymphangiogenesis. Tumors that express VEGF-C can induce lymphangiogenesis peritumorally. Enhanced peritumoral lymphatic vessel density induced by VEGF-C is thought to increase the probability that invasive tumor cells enter the vasculature, disseminate and form metastases. Nevertheless, we have found high levels of VEGF-C in blood from breast cancer patients compared to healthy volunteers, and here provide evidence that tumor-derived VEGF‑C may not only act locally but also systemically to promote metastasis. By intravenously injecting recombinant VEGF-C or adeno-associated viruses that induce VEGF-C expression (AAV-VEGF-C), we artificially increased the circulating levels of VEGF-C in healthy experimental animals to mimic the situation in breast cancer patients. This led to an accumulation of CD11b⁺ myeloid cells in the lungs, but not liver or spleen. These CD11b⁺ cells virtually all co-expressed Gr-1 and Ly6G, identifying them as neutrophils. Accordingly, enhanced circulating VEGF-C levels resulted in increased numbers of neutrophils in the blood, with a concomitant reduction in the number of circulating lymphocytes. In the lungs, VEGF-C-induced myeloid cell clusters localized to the periphery of terminal bronchioles, sites associated with pre-metastatic niche formation and lung metastasis. Pre-conditioning of experimental animals with daily injections of recombinant VEGF-C for five days prior to injection of poorly metastatic breast cancer cells strongly stimulated lung metastasis formation. Furthermore, in a highly metastatic VEGF-C-secreting breast cancer model, neutrophilia in the circulation was observed, and clusters of CD11b⁺ myeloid cells accumulated in the pre-metastatic lung. Together these data suggest that increased systemic VEGF-C induces neutrophil accumulation in the lung, thereby contributing to the formation of metastatic niches and the development of lung metastases.