Exosomes and communication between tumours and the immune system: are all exosomes equal?

Communication between cells is particularly important during tumour progression. Communication can take place through direct cell–cell interactions, but also through extracellular secretion of mediators acting at a distance. These mediators can be either soluble molecules or more complex structures called membrane vesicles, enclosing soluble factors within a lipid bilayer. A variety of extracellular membrane vesicles have been described, for instance microvesicles, ectosomes and a subtype called exosomes. The role of exosomes in tumour progression has been studied extensively in the last 10 years. In the present mini-review, we discuss our recent results, first showing the heterogeneity of the vesicles called exosomes and the probable existence of subpopulations of these exosomes, and secondly demonstrating that in vivo secretion of exosomes by some tumours can promote tumour progression, but that such a function cannot be generalized to all tumours and all exosomes.