Exosome separation and enrichment technologies and their applications in disease diagnosis and treatment

Exosomes are nanoscale vesicles wrapped in lipid bilayers that are secreted by cells and carry a variety of proteins, lipids, nucleic acids, and metabolites. Exosomes are widely present in various bodily fluids and mediate intercellular communication. They participate in a variety of physiological and pathological processes, including immune regulation, angiogenesis, tumorigenesis, and metastasis, and have significant clinical diagnosis and treatment potential. Exosomes are source-rich, structurally stable, and reflect the states of their parental cells. Therefore, they are expected to serve as novel diagnostic markers for various diseases. In addition, stem-cell-derived exosomes show therapeutic potential and have the advantages of low immunogenicity, high safety and easy storage, and exhibit therapeutic potential for neurodegenerative disorder, cardiovascular disease, and cancer. Furthermore, exosomes are highly biocompatible, have natural homing properties, and are capable of easily penetrating biological barriers, making them excellent drug-delivery carriers. Isolation and enrichment of exosomes is a prerequisite for downstream analysis and application. High-purity, high-yield, and high-throughput exosome-isolation methods are expected to be used in clinical diagnosis and treatment applications. Based on the physicochemical properties of exosomes, including density, size, charge, and surface composition, exosome-isolation methods are mainly divided into density-based (e.g., differential ultracentrifugation, density-gradient ultracentrifugation), size-based (e.g., ultrafiltration, size-exclusion chromatography, field-flow fractionation), polymer-precipitation (e.g., polyethylene-glycol-based precipitation), and chemical affinity (e.g., antibody-based, aptamer-based, and surface-lipid-based lipid probes) methods. Currently, basic research into exosomes and their clinical applications face a number of challenges. Firstly, the complexity and heterogeneity of exosomes and the lack of standardized isolation methods has led to highly variable research results that hinder comparing and reproducing results between different laboratories and clinical settings. Current isolation methods are generally hindered by insufficient purity, low yield, low throughput, and difficulties separating specific subpopulations, which seriously restrict the development of the exosome field. Secondly, exosome-isolation methods that are easy to use in the clinic, have few technical requirements, and are highly efficient and inexpensive are lacking. Commonly used classical methods, such as ultracentrifugation, are time-consuming, labor-intensive, require large sample volumes, and are inappropriate for clinical settings. Methods such as immunoaffinity can be used to isolate exosomes from precious trace samples in clinical practice; however, high costs, low recoveries, and high operating requirements are shortcomings that restrict sample analysis in the clinic. In addition, robust large-scale methods for preparing exosomes are lacking. There is an urgent need to develop repeatable and scalable methods for preparing batches of high-quality exosomes owing to the rapid development of exosomes for the treatment of clinical diseases. Generally, exosome research progress is expected to greatly improve our understanding of the biological functions and components of exosomes, which will help transform the exosome research into effective diagnostic and therapeutic strategies and lead to new precision-medicine and personalized-treatment applications. This article summarizes the latest progress in exosome-isolation and -enrichment technologies and introduces the application of exosomes as disease diagnostic markers, therapeutic agents, and drug delivery carriers. Finally, the future developmental trends in exosome isolation and enrichment technologies for disease diagnosis and treatment are discussed.