Ectosomes include small-sized EVs (such as small ectosomes and arrestin domain-containing protein 1-mediated microvesicles), medium-sized microvesicles and the larger-sized apoptotic bodies. Of note, some ectosomes may also carry endosomal cargo components. By contrast, ectosomes are generated by plasma membrane budding and blebbing. Amphisomes are formed by the fusion of autophagosomes and MVBs. Exosomes are small EVs of endosomal origin released by the exocytosis of multivesicular bodies (MVBs) and amphisomes. Two main types of EV are distinguished based on their biogenesis, known as exosomes and ectosomes. It is also increasingly recognized that EV biogenesis can intersect with viral egress 13, secretory autophagy, the cellular senescence-associated secretory phenotype and the DNA damage response 14.Įxtracellular vesicles (EVs) are heterogeneous, phospholipid membrane-enclosed structures. Of note, EVs also include vesicles generated by different cell death mechanisms (such as apoptosis, necroptosis or pyroptosis). The heterogeneity of EVs is a consequence of the variety of types and functional states of the releasing cells as well as of the different biogenetic routes. Medium-sized EVs, with an approximate diameter of 200–800 nm, are present in smaller numbers than small EVs, and large EVs (diameter ≥1 μm such as migrasomes, exophers, apoptotic bodies, large oncosomes and en bloc-released MVB-like small EV clusters 12) are the least abundant population of EVs (Table 1). EVs that are present in the greatest numbers in biological fluids are small EVs with an approximate diameter of 50–150 nm. However, definitive molecular markers of the different biogenetic routes are not yet available, and operational terms have been suggested to distinguish EV types based on their biophysical or biochemical properties 2. The other basic route of EV biogenesis is the release of plasma membrane-derived EVs (known as ectosomes). Recent data suggest the involvement of additional endomembranes (such as endoplasmic reticulum 10 and nuclear envelope 11) in the biogenesis of exosomes. Exosomes are of endosomal origin, released upon the fusion of the limiting membrane of multivesicular bodies (MVBs) or amphisomes with the plasma membrane 7, 8, 9. Based on their biogenesis, we distinguish two basic types of EV (Fig. Since their initial description, a previously unexpected biophysical, biochemical and functional heterogeneity of EVs has been discovered 2, 6, 7. The broad term of bacterial extracellular vesicles is increasingly used to refer to all EVs released by bacteria 5. For example, the release of outer membrane vesicles by Gram-negative bacteria and the more recently described discharge of cytoplasmic membrane vesicles by Gram-positive bacteria and archaea demonstrate that EV production is characteristic of all three domains of life (archaea, bacteria and eukaryota) 4. EVs are released by all cellular organisms. The designation ‘extracellular vesicles’ was suggested in 2011 as a collective term for lipid bilayer-enclosed, cell-derived particles 3. However, after several decades of sporadic observations of extracellular, membrane-enclosed structures, the early 2000s brought a renewed research focus on these EVs, leading to an exponential development of the field in the past two decades 1, 2. For a long time, this connotation discouraged scientists from investigating extracellular particles in depth, thus obscuring the discovery of both EVs and non-EV nanoparticles in this compartment. The commonly used word ‘debris’ is a nonspecific collective designation of all undefined extracellular particles and its negative tone suggests that all such particles represent cellular waste. The history of research into extracellular vesicles (EVs) is an example of how a single term can delay the development of an entire scientific field. It ends with a focus on the relevance of EVs to immunotherapy and vaccination, drawing attention to ongoing or recently completed clinical trials that aim to harness the therapeutic potential of EVs. It also highlights key progress related to deciphering the roles of EVs in antimicrobial defence and in allergic, autoimmune and antitumour immune responses. This Review summarizes the roles of EVs in basic processes of innate and adaptive immunity, including inflammation, antigen presentation, and the development and activation of B cells and T cells. With this progress in mind, an updated comprehensive overview of the roles of EVs in the immune system is timely. The recent recognition that EVs have the potential to function as biomarkers or as therapeutic tools has attracted even greater attention to their study. The twenty-first century has witnessed major developments in the field of extracellular vesicle (EV) research, including significant steps towards defining standard criteria for the separation and detection of EVs.
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