Although it has long since been realised that eukaryotic cells release complex structures, termed membrane vesicles, into their extracellular environment, only in the past decade has it been realised that these entities are not merely junk or debris indicative of cell death, but that they may be micro- or nano-maps of their cell of origin and may be of both physiological and pathological relevance.

Microvesicles and exosomes (ME) are often defined and sub-grouped first-and-foremost on size and proposed origin (exosomes ~30nm-100nm, endosomal origin; microvesicles >100-1000nm, from the cell membrane). However, there is no reason to believe that microvesicles of 30-100nm cannot also exist and the relevance of size per se is unknown.

Furthermore, once the vesicles have been released, their origin cannot be identified as unique markers for the different vesicle types have not yet been identified. Vesicles for analysis are typically isolated from medium conditioned by cultured cells, serum, saliva and other body fluids, based on size; through a range of methods, based on the Researcher’s choice but not standardised between laboratories.

Similarly, basic quantification and characterisation of the resulting ME populations are varied and non-standardised (again, see D.2). Collaborative efforts to review this area and reach a consensus on guidelines will, undoubtedly, lead to more robust output from European research and help fast-track exploitation of its clinical relevance through academic, industry, clinical collaborations. 

Considering origin and biogenesis of ME, exosomes are believed to be generated in endosomal compartments termed Multivesicular Bodies (MVBs) that pack and store molecules in membrane-bound structures. These MVBs may then fuse with the cell membrane, releasing their contents of small vesicles extracellularly. Since 1987, these small vesicles are termed exosomes. In contrast, socalled microvesicles are typically described as derived from the cell membrane. Frequently the term extracellular vesicles or cell-derived vesicles are used to include both microvesicles and exosomes; although some authors sub-divide their description as exosomes and “other vesicles”. However, the origin of isolated vesicles cannot be studied retrospectively, as markers specific for different vesicle types are unknown. So, in this regard, there is no consensus on the relative importance of separate mechanisms of biogenesis. Adding to the complexity with regards to terminology, ME from different origins are sometimes randomly assigned other titles (e.g. those isolated from blood are sometimes termed “microparticles”; from seminal fluid, “prostasomes”; from cancer/tumour cells, “oncosomes”,etc.).

Notwithstanding the necessity for networking and a consensus on terminology and methodology, it is important to highlight the importance that has been attributed to MEs to date. Major milestones include the observation, in 1996, that these vesicles play a role in intercellular communication. In brief, EBV-transformed B-lymphocytes were found to secrete exosomes that bore MHC II bound to antigenic peptides, essential for the adaptive immune response. In 1998, dendritic cells were also reported to secrete exosomes that stimulate T cells, thus initiating adaptive immune response and ultimately anti-tumour activity in mice. Arising from this, a limited number of exosomes-based cancer vaccines are being explored in clinical trials. 

Global analysis of ME contents, possible so far, in dicates that they contain both molecules common to ME from different origins and others specific to a given cell of origin. ME from normal cells shows that typically the proteins included are from the cell membrane, endocytic pathway, and cytoplasm, but rarely from the ER, mitochondria, nucleus and Golgi apparatus. However, MEs from abnormal (e.g. cancerous) may also originate from other organelles e.g. “leaky” mitochondria. In addition to proteins, ME also contain mRNAs and non-coding RNAs. In fact, further support for a role of exosomes in cellular communication was the observation, in 2007, that mRNAs secreted within exosomes from a “donor” cells can be translated by “recipient/target” cells, at least in vitro.

Relevance of exosomes in vivo has also recently (2011) been shown, where exosomes administered into mice prepared niches for attaching tumours cells/metastasis. Another seminal example (2011) involved pay-loading dendritic cell exosomes with Beta-secretase 1(BAEC1)-specific siRNA, administering i.v. and then observing knock-down, in the brain, of BAEC1 i.e. an important protein in Alzheimer’s. Additionally, exosomes have been successfully used as delivery systems for chemical drugs (e.g. curcumin) that show more efficacies when delivered in exosomes than in their soluble form. 

Overall the importance of studying and understanding ME is evident by their implication in cell communication and mis-communication; in antigen-presentation; tumour immune suppression; tumour-stromal interactions; metastasis; angiogenesis; drug-resistance; stem cell differentiation; neurodegeneration; reproduction; and other processes and that the contents of vesicles in body fluids provide valuable information on the state of the tissue of origin. Yet we don’t yet know the breadth and scope of their relevance in either physiological or pathological circumstances.

We do know, however, that -depending on their source and target cells- these vesicles incorporate and transfer selected mRNA, non-coding RNAs, and proteins. These, thus, have potential as biomarkers for disease which can be harvested from body fluids overcoming the requirement for biopsies (ie non- or minimally-invasive) as well as "self" delivery systems which could be harnessed for therapeutic purposes. This Action is innovative as it brings together experience and young researchers from academic, clinical and industry settings all who share a common goal of progressing the field of European ME research in a logical and co-ordinated manner. This new integrative interdisciplinary approach for the critical concerted assessment of terminology/nomenclature, methodology, experimental design and data interpretation will advance from current state-of-play regarding the basic understanding to the diagnostic and therapeutic potential of ME. This action will also improve creativity and entrepreneurial mindset among ME researchers. The collaboration with industrial partners will allow us to explore the exploitability of European-based research with respect to biomarker and drug generation.