Release of membrane vesicles, a process conserved in both prokaryotes and eukaryotes, represents an evolutionary link, and suggests essential functions of a dynamic extracellular vesicular compartment (including exosomes, microparticles or microvesicles and apoptotic bodies). Compelling evidence supports the significance of this compartment in a broad range of physiological and pathological processes. However, classification of membrane vesicles, protocols of their isolation and detection, molecular details of vesicular release, clearance and biological functions are still under intense investigation. Here, we give a comprehensive overview of extracellular vesicles. After discussing the technical pitfalls and potential artifacts of the rapidly emerging field, we compare results from meta-analyses of published proteomic studies on membrane vesicles. We also summarize clinical implications of membrane vesicles. Lessons from this compartment challenge current paradigms concerning the mechanisms of intercellular communication and immune regulation. Furthermore, its clinical implementation may open new perspectives in translational medicine both in diagnostics and therapy.

Numerous diseases, recently reported to associate with elevated microvesicle/ microparticle (MP) counts, have also long been known to be characterized by accelerated immune complex (IC) formation. The goal of this study was to investigate the potential overlap between parameters of protein complexes (eg, ICs or avidinbiotin complexes) and MPs, which might perturb detection and/or isolation of MPs. In this work, after comprehensive characterization of MPs by electron microscopy, atomic force microscopy, dynamic lightscattering analysis, and flow cytometry, for the first time, we drive attention to the fact that protein complexes, especially insoluble ICs, overlap in biophysical properties (size, light scattering, and sedimentation) with MPs. This, in turn, affects MP quantification by flow cytometry and purification by differential centrifugation, especially in diseases in which IC formation is common, including not only autoimmune diseases, but also hematologic disorders, infections, and cancer. These data may necessitate reevaluation of certain published data on patient-derived MPs and contribute to correct the clinical laboratory assessment of the presence and biologic functions of MPs in health and disease. (Blood. 2011;117(4):e39-e48) IntroductionMembrane vesicles are small subcellular structures surrounded by a phospholipid bilayer. Their release by various cell types is enhanced during activation and apoptosis. 1 They represent heterogeneous structures and can be classified into several groups depending on their size, antigenic features, and mechanism of cellular release. 1 The two best characterized categories include exosomes and microvesicles/microparticles (MPs). Both populations are characterized by the exposure of phosphatidylserine, which allows annexin-V (AX) to bind to these-lipid surfaces. 1 Exosomes are composed of small, 50-to 100-nm-sized structures released on exocytosis of multivesicular bodies. 1 The diameter of MPs, formed by membrane blebbing, is described to be 100 to 1000 nm 2 . However, precise definitions of MPs are still lacking. 1,2 MPs are found in various biologic fluids, including blood plasma, 3 urine, 4 and synovial fluid (SF). 5,6 Numerous flow cytometry (FC) studies using blood plasma have shown correlation of MP counts with human cardiovascular 7 and autoimmune diseases, 8 hematologic disorders, 9 and cancer. 10 Of particular interest, autoimmune diseases were reported to be characterized by elevated levels of MPs. 11 The assessment of exosomes and MPs is complicated by the presence of further known categories of membrane bound subcellular structures, such as apoptotic vesicles, exosome-like vesicles, membrane particles, and ectosomes. 1 There is a substantial size overlap among the aforementioned vesicle categories, and the size distribution of a given vesicle preparation may also be affected by the method used for their isolation. 3,12,13 Recently, attempts have been made to standardize isolation and detection protocols for membrane vesicles. 3,14 Up until now, no systemat...

Microvesicles (MVs) are membrane-covered cell fragments released by most cell types during apoptosis or activation. They are increasingly considered to play a pivotal role in information transfer between cells. Their presence and role have been proven in several physiological and pathological processes, such as immune modulation in inflammation and pregnancy, or blood coagulation and cancer. MVs represent a newly recognized system of intercellular communications. They not only may serve as prognostic markers in different diseases, but could also hold the potential to be new therapeutic targets or drug delivery systems. The present overview aims to highlight some aspects of this new means of cellular communication: "microvesicular communication".