Exosomes are nanovesicles that transmit membrane proteins and information from cell to cell. In particular, they trigger potent immune responses

What are exosomes?

Exosomes are membrane vesicles ~50-100 nm in diameter, secreted by a wide range of animal cell types. They originate from late endosomes called MultiVesicular Bodies (MVB) that contain intraluminal nanovesicles. Upon fusion with the plasma membrane, intraluminal nano-vesicles are released in the extracellular environment and become exosomes. Exosomes contain membrane proteins, cytosolic proteins and small RNAs (miRNA). Exosomes can likely fuse back with plasma membranes, introducing new proteins and RNA in new cells. Increasing bodies of evidences have shown that exosomes are intercellular shuttle vehicles of materials and of information that can reprogram targeted cells; in particular, they are able to pass through the Blood Brain Barrier.

Leveraging exosome immuno-stimulatory potential

There are strong evidences that exosomes are potent modulators of immune information in inhibitory or enhancing ways. In the presence of corresponding immunogens, activated dendritic cells produce exosomes that transfer immune information to T cells. For example, exosomes can trigger cellular and/or humoral immune responses allowing either tumor regression or antibody protection against pathogens.

The molecular mechanisms of exosome secretion and of protein sorting in exosomes are still not fully understood. This could be because several kinds of MVBs may exist, differing in their origins and/or by their purpose. It has been reported that vesicle budding in MVBs could be triggered by sphingolipid ceramide, but several results revealed that other factors are at work in vesicle budding and exosome secretion. Proteins sorted in intraluminal vesicles could originate either from plasma membrane or directly from the Golgi apparatus. The proteins sorted with intraluminal vesicles could be either sorted for degradation by fusion of MVB with lysosome or secreted with exosomes by fusion of MVB with plasma membrane. Thus, several pathways may occur simultaneously in cells depending most likely on specific motifs present in each sorted protein.