Background: The presence of lymph node metastasis in patients with solid tumors is associated with tumor aggressiveness, poorer prognosis and the recommendation for systemic therapy. However, whether tumor cells exit the lymph node and contribute to distant metastases remains controversial. Evidence shows that treating disease in the lymph node improves survival in some patients. In this study, we address the controversy of whether tumor cells colonizing the axillary lymph nodes could disseminate to distant sites.
Materials and Methods: We used syngeneic murine cell lines representing breast (4T1), melanoma (B16F10), head and neck (SCCVII) cancer that spontaneously metastasize to the lymph node. We engineered these cells to express Dendra2, a photoconvertable protein, to determine if metastatic tumor cells from the lymph node are able to seed distant organs like the lung. Using high-resolution multiphoton microscopy and a chronic lymph node window we monitored the behavior of tumor cells in the lymph node and their interaction with blood vessels.
Results: By photoconverting cancer cells only in lymph nodes, we show that spontaneous lymph node metastasis from breast cancer and melanoma mouse models can leave the lymph node and enter the systemic blood circulation. Our data show that the circulating tumor cells originating from the lymph node are viable and are able to proliferate in vitro. Using a combination of intravital microscopy and immunofluorescence staining, our data show that 7.4%±1.8% of isolated tumor cells in the lymph node are intraluminal and an additional 16.3%±1.9% were within 5μm of a blood vessel compared to an expected 8.8%±0.9% if they were randomly distributed. Further, we identified lung micrometastases that originated from the lymph node in both mouse models. Analysis of human metastatic lymph node samples from head and neck cancer patients revealed perivascular association of isolated tumor cells as well as the presence of cancer cells inside blood vessels similar to our observations in the mouse models.
Conclusions: Together, our data show for the first time that in spontaneous breast and melanoma mouse models, tumor cells in the lymph node can invade blood vessels, exit the lymph node and colonize distant organs like the lung. These studies exemplify the importance of treating metastatic disease in the lymph node, which could potentially lead to distant metastasis.
