Characterization and biological role of extracellular vesicles

Extracellular vesicles (EV) form a heterogeneous population of mostly spherical membrane structures released by almost all cells, including tumour cells, both in vivo and in vitro. Their size varies from 30 nm to 1 mu m, and size is one of the main criteria of the selection of two categories of EV: small (30-100 nm), more homogeneous exosomes and larger fragments (0.1-1 mu m) called membrane microvesicles or ectosomes. The presence of EV has already been detected in many human body fluids: blood, urine, saliva, semen and amniotic fluid. Formation of EV is tightly controlled, and their function and biochemical composition depend on the cell type they originate from. EV are the vehicles of bioactive molecules, such as proteins, mRNA and microRNA, and may play an important role in intercellular communication and modulation of e.g. immune system cell activity. In addition, on the surface of tumour-derived microvesicles (TMV), called oncosomes, several markers specific for cancer cells were identified, which indicates a role of TMV in tumour growth and cancer development. On the other hand, TMV may be an important source of tumour-associated antigens (TAA) which can be potentially useful as biomarkers with prognostic value, as well as in development of new forms of targeted immunotherapy of cancer.