Exosomes are 50-90 nm vesicles secreted by a wide range of mammalian cell types.^[1] First discovered in maturing mammalian reticulocytes, they were shown to be a mechanism for selective removal of many plasma membrane proteins.^[2] These proteins are lost or reduced in amount, without concomitant degradation, during the maturation to the erythrocyte. Although the exosomal protein composition varies with the cell of origin, most exosomes contain the soluble protein Hsc 70. Certain immune cells, such as dendritic cells and B cells, secrete exosomes that many scientists believe play a functional role in mediating adaptive immune responses to pathogens and tumors.^[3]

An exosome is created intracellularly when a segment of the cell membrane spontaneously invaginates and is endocytosed.^[1] The internalized segment is broken into smaller vesicles that are subsequently expelled from the cell. The latter stage occurs when the late endosome, containing many small vesicles, fuses with the cell membrane, triggering the release of the vesicles from the cell. The vesicles (once released are called exosomes) consist of a lipid raft embedded with ligands common to the original cell membrane.

Exosomes from red cells contain the transferrin receptor which is absent in mature erythrocytes. Dendritic cell-derived exosomes express MHC I, MHC II, and costimulatory molecules. Exosomes derived from dendritic cells have proven to be able to induce and enhance antigen-specific T cell responses in vivo. In addition, the first exosome-based cancer vaccination platforms are being explored in early clinical trials.^[4]. Exosomes can also contain both functional mRNA and microRNA, which can be shuttled from one cell to another, affecting the recipient cell’s protein production. This RNA is called “exosomal shuttle RNA” (esRNA; Valadi et al., Nat Cell Biol 2007 Jun;9(6):654-9).