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Isolation and labeling of exosomes
B16-F10 melanoma exosomes were isolated for use in experiments according to previously established methods (15). Briefly, B16-F10 melanoma cell cultures were grown to 70% confluence in three 300 cm2 flask. Culture media was removed and cells washed in PBS. Cells were cultured for 48 hours in the presence of conditioned media. Conditioned culture media was prepared by subjecting normal culture media to overnight ultracentrifugation at 110,000 × g to remove bovine exosomes (19). B16 melanoma exosomes were collected from 48-hour culture in conditioned media through standard differential centrifugation steps using a 70 Ti rotor (19). Culture media was spun and supernatants collected from 300 × g for 10 minutes, 2,000 × g for 10 minutes, to remove residual cells and debris, 10,000 × g for 30 minutes to remove microparticles (20) and 100,000 × g for 2 hours in the presence or absence of 1.0 μmol/L DiI or DiR. Exosome pellets were washed 3 times in PBS, pooled, and reisolated in PBS at 100,000 × g for 2 hours. Exosome pellets were resuspended in 1 mL of PBS, protein content measured via BCA absorbance (Thermo Fisher Scientific Inc.) and stored at −80°C until use. Between the 10,000 and 100,000 × g centrifugation steps, exosomes were sized using dynamic light scattering (DLS) as previously reported (15) and the electrokinetic potential (zeta potential) of purified exosomes in PBS was measured using a Zeta Plus Zeta Potential Analyzer (Brookhaven Instruments Corp.). Previously, fluorescent exosome localization technique (FELT) was used to confirm the use of differential centrifugation and DLS in obtaining a purified population of B16-F10 melanoma exosomes from conditioned media (15). Using FELT, B16-F10 melanoma exosomes applied to continuous sucrose gradients (2.0–0.25 mol/L sucrose, 20 mmol/L HEPES/NaOH, pH 7.4), were found to have a density of (1.10–1.21 g/mL; ref. 15).
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