Microvesicles

Microvesicles (sometimes called, circulating microvesicles, or microparticles.) are fragments of plasma membrane ranging from 100 nm to 1000 nm shed from almost all cell types. Not to be confused with smaller intracellularly generated extracellular vesicles known as exosomes. Microvesicles play a role in intercellular communication and can transport mRNA, miRNA, and proteins between cells. Microvesicles have been implicated in the process of a remarkable anti-tumor reversal effect in cancer, tumor immune suppression, metastasis, tumor-stroma interactions and angiogenesis along with having a primary role in tissue regeneration. They originate directly from the plasma membrane of the cell and reflect the antigenic content of the cells from which they originate. They remove misfolded proteins, cytotoxic agents and metabolic waste from the cell.

Different cells can release microvesicles from the plasma membrane. Sources of microvesicles include megakaryocytes, blood platelets, monocytes, neutrophils, tumor cells and placenta.

Platelets play an important role in maintaining hemostasis: they promote thrombus growth, and thus they prevent loss of blood. Moreover, they enhance immune response, since they express the molecule CD154 (CD40L). Platelets are activated by inflammation, infection, or injury, and after their activation microvesicles containing CD154 are released from platelets. CD154 is a crucial molecule in the development of T cell-dependent humoral immune response. CD154 knockout mice are incapable of producing IgG, IgE, or IgA as a response to antigens. Microvesicles can also transfer prions and molecules CD41 and CXCR4.

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