Limitations and challenges in the characterization of extracellular vesicles from stem cells and serum

Exosomes are a subpopulation of nanosized extracellular vesicles (EVs), formed by a lipid bilayer and naturally secreted by cells. They transport RNA, microRNAs, bioactive proteins, and lipids and play an important role in intercellular communication. Exosomes are a promising alternative cell-free therapy in regenerative medicine, immunotherapy, and drug delivery. The implementation of new exosomes treatments requires knowledge of its concentration, purity, and full characterization. However, comparing different studies is highly challenging due to the lack of validated methodologies for isolation and determination, as well as a lack of well-characterized exosomes reference standards. In this work, human uterine cervical mesenchymal stem cell (hUCESC) EVs have been isolated by ultracentrifugation and characterized, discussing the current limitations of characterization methods. First, total protein assays are heavily influenced by free-protein and lipid contaminations which confirm the need of employing several methods for vesicular protein determination. This purity variation seems heavily influenced by the vesicles origin as more complex mediums originate more matrix interferences. Size exclusion high performance liquid chromatography has been demonstrated as a new methodology for purity assessment of hUCESC-EVs and commercial EVs (adipose stem cells and human serum). The results found low purity in the commercial exosomes highlighting that protein and lipid purity must be included in the commercial EVs. Finally, the combination of this chromatography method with total protein assays proved that particle concentration could be estimated using vesicular protein concentration.