The dual promise of extracellular vesicles in lung diseases: towards reliable biomarkers and drug delivery vectors.

Extracellular vesicles (EVs) are small, membrane-bound particles released by cells into the extracellular space. Once considered as cellular waste disposal organelles, EVs are now recognised as functional cellular components because they carry a variety of cargo biomolecules, including nucleic acids, proteins and lipids. EVs are constitutively released under homeostatic conditions, but their numbers increase and their cargo composition is altered under nonhomeostatic conditions, such as exposure to environmental pollutants, viral infections or disease states. Therefore, EVs are being actively explored as noninvasive biomarkers for many diseases, including lung diseases. In addition, EVs are key mediators of intercellular communication through the transfer of their cargo biomolecules from EV-releasing donor cells to recipient (target) cells through membrane fusion, endocytosis or receptor-ligand interactions. This intercellular communication between the cells positions EVs as novel drug delivery vectors because of their low immunogenicity, high biocompatibility and unique cargo compositions. While EV-based drugs have not yet been approved by regulatory authorities, numerous clinical trials are evaluating their use either as therapeutics or as delivery systems. In this review, we discuss EVs, with particular emphasis on recent advances in identifying reliable and sensitive biomarkers for lung diseases, and on their emerging role as targeted drug delivery systems.