Chapter 2. Color-coded fluorescent mouse models of cancer cell interactions with blood vessels and lymphatics

Several new strategies now exist for imaging cancer cell interactions with both blood vessels and lymphatics in living animals. Tumors labeled with fluorescent proteins allow the nonluminous capillaries and larger blood vessels to be clearly visualized against the bright tumor fluorescence via either intravital or whole-body imaging. Signal attenuation by overlying tissue can be markedly reduced by opening a reversible skin flap in the light path, increasing detection sensitivity. With this increase in observable depth of tissue, many previously obscured small tumor vessels can be imaged. In addition, dual-color fluorescence imaging, effected by using red fluorescent protein (RFP)-expressing tumors growing in green fluorescent protein (GFP)-expressing transgenic mice, can show with great clarity tumor-stroma interactions, including the developing tumor vasculature. The GFP-expressing host vasculature, both mature and nascent, can be distinguished from the RFP-expressing tumor itself in this model. Transgenic mice with GFP gene expression driven by the nestin promoter offer another way to image the developing tumor vasculature. In this model system, only nascent blood vessels express GFP, allowing newly developing blood vessels to be imaged against a background of RFP-expressing tumor cells. Finally, dual-color imaging technology can facilitate the imaging of cancer cell interactions with lymphatics. Delivery of FITC-dextran or fluorescent antibodies specific for lymphatic endothelium to the lymphatics around an RFP-expressing tumor allows imaging of tumor cell shedding into the lymphatic system. This imaging technology has the potential to visualize each step of tumor progress.