Exosomes: endosomal-derived vesicles shipping extracellular messages
Introduction
The concept of the exosome as a non-plasma-membrane-derived vesicle emerged with the description of the process of ‘shedding’ of the transferrin receptor (TfR) during reticulocyte maturation. The release process appears to require, first, sorting into the small vesicles contained in endosomes and, second, the fusion of the limiting membrane of these endosomes with the cell surface, resulting in the release of these small vesicles into the extracellular space 1., 2., 3.. The small vesicles, which measure about 50–90 nm in diameter, are termed ‘intraluminal vesicles’ (ILVs) while they are contained within multivesicular endosomes and ‘exosomes’ when they are released into the extracellular environment.
Multivesicular endosomes, commonly called multivesicular bodies (MVBs) on the basis of their morphology [4], have well-known functions as intermediates in the degradation of proteins internalized from the cell surface or sorted from the trans Golgi network (Figure 1). Proteins destined for degradation are sorted into the ILVs of the newly forming MVB. The MVB then fuses with pre-existing lysosomes [5]. The pioneering studies in reticulocytes demonstrated that MVBs and their ILVs can also, as an alternative fate, fuse with the plasma membrane to eliminate ‘obsolete’ proteins that do not follow an intracellular degradation pathway [6].
In recent years, exosome research has been stimulated by the finding that antigen presenting cells (APCs) such as B lymphocytes and dendritic cells (DCs) secrete exosomes during exocytic fusion of multivesicular MHC class II compartments with the cell surface 7., 8.. In contrast to exosomes released by reticulocytes during their differentiation to get rid of the TfR, exosomes secreted by APCs appeared to be functionally relevant. They had the ability to stimulate T cell proliferation in vitro [7] and to induce anti tumor immune responses in vivo [8]. Since then, exosomes have been isolated from the cell culture supernatants of many other haematopoietic and non-haematopoietic cells such as T lymphocytes, mast cells, platelets, intestinal epithelial cells and several tumor cell lines 9., 10., 11., 12., 13.. In this review we highlight recent studies that have contributed to a better comprehension of the composition, biogenesis and functions of exosomes. The proposed evolutionary link between exosomes and transmissible pathogens, such as human immunodeficiency virus (HIV), is also discussed.
Section snippets
Morphological and biochemical definition of exosomes
The identification of vesicles as exosomes is based on both morphological and biochemical criteria. Given their small size, exosomes can only be visualized by electron microscopy. Exosome-secreting cells show exocytic fusion of MVBs, a process analogous to the secretion of secretory granules. The ILVs, then called exosomes, are detected in the intercellular space (Figure 2). Immunocytochemical analysis shows that proteins present in MVBs and, in particular, in the ILVs are enriched in the
The protein and lipid composition of exosomes relates to their biogenesis
The availability of highly purified exosome preparations has allowed their analysis by proteomics. The protein composition of exosomes, analyzed by SDS–PAGE and Coomassie blue staining, differs from that of total cell lysates. A compilation of the data obtained for exosomes secreted by murine DCs, human intestinal epithelial cells (IECs) in inflammatory conditions and EBV-transformed human B lymphocytes revealed common and cell-type-specific constituents 12., 16.••, 18., 19., 20.. Exosomes from
Exosome functions
In recent years, as a result of initial studies showing that exosomes display functional MHC class II and class I as well as co-stimulatory and cell adhesion molecules, a particular interest has emerged in their use as acellular vehicles for stimulation of anti-tumoral immune responses in vivo [35]. These studies have already led to clinical trials with DC-derived exosomes carrying MHC class II and class I loaded with tumor antigens. Vaccination strategies could also be envisioned using
Intercellular transfer of exosomes and regulation of exosome secretion
Together, the recent studies on the fate and functions of exosomes have lead to the hypothesis that exosomes may represent a novel method of intercellular communication 15., 46.. Nonetheless, there are as yet no experimental indications of how exosomes interact with their target cells. Different modes of interaction can be envisioned for different cell types, and these may be directly related to their functions. Exosomes could fuse with the plasma membrane or they could be endocytosed via an as
Exosomes, an easy vehicle for transmissible pathogens?
Studies on retroviruses and in particular on HIV have revealed the ability of viruses to hijack the intracellular machinery of MVB biogenesis for their budding at the cell surface; they can also exploit the machinery in endosomes where it normally acts [51]. In human macrophages, MHC class II compartments with all the hallmarks of late endocytic MVBs are the major site of HIV budding and accumulation 52.•, 53.. These findings, together with a comparative study of exosomes and HIV particles [54]
Conclusions and perspectives
Since the initial studies on reticulocyte exosomes, recent work has revealed that exosome secretion is a much more general phenomenon with a range of important regulatory functions. The discovery of prostasomes [58] and argosomes [59] has led to the suggestion that an ‘exosomal’ type of communication is not limited to the immune system, but may also function in conferring fertility (in the case of prostasomes) and participating in tissue-developmental processes (in the case of argosomes). This
Update
In addition to the proposed implication of exosomes in the regulation of the immune response, recent studies also highlight their role in pathological states. Proteomic analysis of exosomes secreted by mesothelioma cells revealed the presence of developmental endothelial locus-1 (DEL-1). DEL-1 is a strong angiogenic factor involved in the vascuolar development in the neighborhood of the tumor, suggesting a role for exosomes in the interaction between tumor cells and their environment [61]. An
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
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of special interest
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of outstanding interest
Acknowledgements
This review is dedicated to the memory of our colleague Christian Bonnerot.
We thank our colleagues and members of their laboratories for their outstanding contribution to the field: W Stoorvogel, S Amigorena, L Zitvogel, JB Le Pecq and M Vidal. We are grateful to A Pelchen-Matthews and W Stoorvogel for critical comments and helpful suggestions. GR is indebted to H Geuze for support on initial studies on B cell exosomes. Thanks to R Johnstone for her pioneering work on reticulocyte exosomes and
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