U6/miR-211 expression ratio as a purity parameter for HEK293 cell-derived exosomes

Small extracellular vesicles (sEVs) including exosomes are produced by all cell types and can be isolated from biological fluids and cell culture supernatants. The separation of exosomes with high purity from protein-rich media remains challenging. Besides contaminating proteins, small microvesicles (MVs) and apoptotic bodies are usually co-isolated with exosomes. The optimization of exosome separation and purification depends on reliable methods for the determination of the purity of the preparation, but no standard measurement has been defined so far. We tried to advance purity assessment. sEVs were isolated from HEK293 cell culture supernatants by various combinations of centrifugation, precipitation and size exclusion chromatography. sEVs with a diameter within the size range of 30-150 nm, typical for exosomes, were obtained with all tested isolation methods as shown by electron microscopy. To estimate the levels of protein contamination, flow cytometric analysis of the obtained vesicles was used. Based on the controlled preferential loading and enrichment of miR-211 into exosomes, a novel approach for the estimation of the fraction of HEK293 derived exosomes as opposed to MVs and apoptotic bodies in sEV mixtures was developed. This novel approach represents a simple qRT-PCR-based approach to improve the precise characterization of sEV isolates that is necessary for the usage of exosomes as carriers for therapeutic nucleic acids. Compared to the precipitation and size exclusion chromatography, the differential ultracentrifugation turned out to give sEVs with fairly intact shape and the highest purity according to the novel qRT-PCR-based approach, as well as to other established methods for purification assessment.