Intravascular location of breast cancer cells after spontaneous metastasis to the lung

In this study, we examined the hypothesis that early pulmonary metastases form within the vasculature. We introduced primary tumors in immunocompromised mice by subcutaneous injection of murine breast carcinoma cells (4T1) expressing green fluorescent protein. Isolated ventilated and perfused lungs from these mice were examined at various times after tumor formation by fluorescent microscopy. The vasculature was visualized by counterstaining with 1,1-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI)-acetylated low-density lipoprotein. These experiments showed that metastatic cells derived by spontaneous metastases were intravascular, and that early colony formation was intravascular. The location of the tumor cells was confirmed by deconvolution analysis. This work extends our previous study(1) that sarcoma cells injected intravenously form intravascular colonies to spontaneous metastasis and to a carcinoma model system. Many of the tumor cells seen were single implying that tumor cells may travel as single cells. These results support a model for pulmonary metastasis in mice in which 1) tumor cells can attach to lung endothelium soon after arrival; 2) surviving tumor cells proliferate intravascularly in this model; and 3) extravasation of the tumor occurs when intravascular micrometastatic foci outgrow the vessels they are in.