(a) Measurement of the total protein per million cells in exosomes isolated from human and mouse melanoma cells in culture. Error bars represent s.e.m. (b) Confocal microscopic analysis of B16-F10 exosome tissue distribution (green) 5 min (lung, left panel) or 24 h (lung and BM, right panels) after tail vein injection. Scale bar, 50 μm. (c) Analysis of lung endothelial permeability following fluorescently-labeled dextran perfusion (red) 24 h after tail vein injection of B16-F10 exosomes, conditioned medium or control particles. Scale bar, 50 μm. (d) Analysis of primary tumor growth (left panel) after subcutaneous flank injection of B16-F10mCherry cells in WT mice treated with B16-F10 exosomes for 3 weeks (n = 6 mice per group; error bars represent s.e.m.; * P < 0.05 by ANOVA). Red arrows indicate exosome injections. The black arrow denotes the timepoint (day 19) at which lung micrometastatic lesions were analyzed. Lung micrometastases (mCherry+, middle panels, scale bar, 50 μm) were quantified by immunofluorescence (right panel). (e) Analysis of primary tumor growth after subcutaneous flank injection of B16-F10-luciferase cells in mice pre-treated with 5μg of B16-F10 and B16-F1 exosomes three times a week for 28 d. (f) Metastatic burden was quantified by luciferin photon flux at 21 d post-tumor injection (left panel). Scale bar, 200 μm. Quantification of total photon flux in lungs and bones (right panel), n = 10 mice per group; error bars represent s.e.m.; *P < 0.05 by ANOVA.