Proteomic analysis of dendritic cell-derived exosomes: A secreted subcellular compartment distinct from apoptotic vesicles

Dendritic cells constitutively secrete a population of small (50-90 nm diam eter) Ag-presenting vesicles called exosomes. When sensitized with tumor an tigenic peptides, dendritic cells produce exosomes, which stimulate anti-tu mor immune responses and the rejection of established tumors in mice. Using a systematic proteomic approach, we establish the first extensive protein map of a particular exosome population; 21 new exosomal proteins were thus identified. Most proteins present in exosomes are related to endocytic comp artments. New exosomal residents include cytosolic proteins most likely inv olved in exosome biogenesis and function, mainly cytoskeleton-related (cofi lin, profilin I, and elongation factor 1 alpha) and intracellular membrane transport and signaling factors (such as several annexins, rab 7 and 11, ra p1B, and syntenin). Importantly, we also identified a novel category of exo somal proteins related to apoptosis: thioredoxin peroxidase II, Alix, 14-3- 3, and galectin-3. These findings led us to analyze possible structural rel ationships between exosomes and microvesicles released by apoptotic cells. We show that although they both represent secreted populations of membrane vesicles relevant to immune responses, exosomes and apoptotic vesicles are biochemically and morphologically distinct. Therefore, in addition to cytok ines, dendritic cells produce a specific population of membrane vesicles, e xosomes, with unique molecular composition and strong immunostimulating pro perties.