Extracellular vesicles (EVs) are nanoscale lipid bilayer vesicles that are secreted by almost all cell types into the extracellular environment. EVs have a wide range of biological functions, including involvement in homeostasis, cancer progression, and development. EVs can be classified into different types based on their size, biogenesis, and cargo. There are three main types of EVs – they are exosomes (diameter < 200 nm), microvesicles (100–1000 nm), and apoptotic bodies (diameter > 1000 nm).

Exosomes have become interesting because of their unique ability to communicate between cells and transfer important molecular cargo, such as proteins, RNA, and lipids. They are involved in various processes and have gained tremendous attention in recent years for their potential applications as biomarkers, drug delivery systems, and therapeutic agents. As biomarkers, exosomes can be isolated from bodily fluids and their contents can be analyzed to provide information about the physiological or pathological state of the originating cells. Exosomes have the ability to encapsulate and deliver therapeutic molecules, they are regarded as a promising alternative to traditional drug delivery systems. Exosomes also have the potential to be used as therapeutic agents, as they can be engineered to target specific cells and deliver therapeutic cargo to treat various diseases such as to promote tissue repair, modulate immune responses, and even inhibit cancer cell growth.  

 Exosomes are extremely tiny particles and specific methods are required to isolate them. Ultracentrifugation and ultrafiltration are the most frequently utilized methods to isolate exosomes. Alternative methods such as immunoaffinity capture, precipitation, and microfluidic have also been developed. However, many of these methods fail to provide pure and standardized preparation of exosomes.

          Although exosomes can be isolated from several sources, exosomes derived from mesenchymal stem cells (MSCs) tend to provide the greatest therapeutics and regenerative potential. Exosomes generated from MSCs exhibit biological properties similar to MSCs, inheriting the traits facilitating tissue regeneration by encapsulating and delivering active biomolecules to the affected cells and tissues. Many studies have also reported that exosomes alone are responsible for the therapeutic effects of MSCs. Therefore, MSCs-derived exosomes are regarded as an excellent cell-free therapeutic approach for treating many diseases such as cardiovascular, neurological, and renal diseases.

Notably, the biological characteristics of MSCs from various sources may vary. For instance, human adipose-derived MSC exosomes have the ability to repair and regenerate tissue as well as control angiogenesis, inflammation, transplant rejection, skin healing, and the immune system, whereas exosomes derived from human umbilical cord-derived MSC are more effective at promoting ossification, formation of new blood vessels, and nerve regeneration. On the other hand, bone marrow-derived MSC exosomes can prevent the growth of multiple myeloma cells and glioblastomas.

          Although exosomes hold promise in therapeutic applications, there are some challenges that need to be overcome in order for them to be used in clinical trials. Isolating and purifying exosomes from the source material is one of the biggest challenges, as exosomes are small and heterogeneous in size, and other extracellular vesicles and non-vesicular particles can contaminate the preparation. Another challenge is the lack of standardization in the characterization and quantification of exosomes. Exosomes are generally categorized based on their biological and physical characteristics. However, the variability and size fluctuation makes it difficult to isolate high-quality, standardized exosomes as they lack exosome-specific characteristics. The low yield of exosomes also presents another concern making it insufficient for clinical applications. Another challenge is the packaging of contents in these exosomes, as the mechanism of loading and its consistency is currently not well understood. Exosomes have also been shown to have a natural tropism for certain cell types, but the ability to engineer them for targeted delivery is still a developing area of research.  In conclusion, there is a need for standardized protocols and methods to overcome these issues in order to use exosomes in clinical therapy.

References

1. Doyle LM, Wang MZ. Overview of Extracellular Vesicles, Their Origin, Composition, Purpose, and Methods for Exosome Isolation and Analysis. Cells. 2019; 8(7):727. https://doi.org/10.3390/cells8070727
2. Tang Y, Zhou Y, Li HJ. Advances in mesenchymal stem cell exosomes: a review. Stem Cell Res Ther. 2021; 12(1): 71. doi:10.1186/s13287-021-02138-7
3. Nikfarjam S, Rezaie J, Zolbanin NM, Jafari R. Mesenchymal stem cell derived-exosomes: a modern approach in translational medicine. J Transl Med. 2020; 18(1): 449. doi:10.1186/s12967-020-02622-3
4. Hussen BM, Faraj GSH, Rasul MF, et al. Strategies to overcome the main challenges of the use of exosomes as drug carrier for cancer therapy. Cancer Cell Int. 2022; 22(1): 323. doi:10.1186/s12935-022-02743-3
5. Cheng Y, Zeng Q, Han Q, Xia W. Effect of pH, temperature and freezing-thawing on quantity changes and cellular uptake of exosomes. Protein Cell. 2019; 10(4): 295-299. doi:10.1007/s13238-018-0529-4