Exosome: from internal vesicle of the multivesicular body to intercellularsignaling device

Exosomes are small membrane vesicles that are secreted by a multitude of ce ll types as a consequence of fusion of multivesicular late endosomes/lysoso mes with the plasma membrane. Depending on their origin, exosomes can play roles in different physiological processes. Maturing reticulocytes external ize obsolete membrane proteins such as the transferrin receptor by means of exosomes, whereas activated platelets release exosomes whose function is n ot yet known. Exosomes are also secreted by cytotoxic T cells, and these mi ght ensure specific and efficient targeting of cytolytic substances to targ et cells. Antigen presenting cells, such as B lymphocytes and dendritic cel ls, secrete MHC class-I- and class-II-carrying exosomes that stimulate T ce ll proliferation in vitro, In addition, dendritic-cell-derived exosomes, wh en used as a cell-free vaccine, can eradicate established murine tumors, Al though the precise physiological target(s) and functions of exosomes remain largely to be resolved, follicular dendritic cells (accessory cells in the germinal centers of secondary lymphoid organs) have recently been shown to bind B-lymphocyte-derived exosomes at their cell surface, which supports t he notion that exosomes play an immunoregulatory role. Finally, since exoso mes are derived from multivesicular bodies, their molecular composition mig ht provide clues to the mechanism of protein and lipid sorting in endosomes .