Microparticles: a critical component in the nexus between inflammation, immunity, and thrombosis

Plasma membrane remodeling characterized by phosphatidylserine exposure and consecutive microparticle (MP) shedding is an ubiquitous process enabling the clearance of senescent cells and the maintenance of tissue homeostasis. MPs are released as fragments from the budding plasma membrane of virtually all eukaryotic cell types undergoing stimulation or apoptosis and may be considered a broad primitive response to stress. MP release is dependent on cytoskeleton degradation pathways involving caspases, requires a sustained increase in intracellular calcium triggering K+ and Cl- efflux and is possibly tuned by mitochondria permeability changes. Because they convey a broad spectrum of bioactive molecules, circulating MPs may serve as shuttles promoting cellular cross talk in various pathological settings such as inflammation or immunity-induced thrombotic disorders. If the drastic shedding of procoagulant MPs appears clearly noxious in thrombotic disorders or in some models of inflammation-induced coagulopathy, this does not necessarily endorse their invariably harmful nature. In the vessel, endothelial cytoprotection reported in the early regulation of inflammation-induced coagulopathy is emblematic of the beneficial effects provided by MPs. In addition, MPs would prove beneficial in the prevention of blood leakage. Because of their multiple properties that are characteristic of a private response of the parental cell, MPs could act as cytoprotective and anti-inflammatory agents through the delivery of activated protein C or annexin 1 and could contribute to the limitation of vascular hyporeactivity. Owing to their ability to cargo bioactive signals, MPs could be viewed as an integrated communication network enabling the coordination of complex cellular responses in biological fluids and the maintenance of the homeostasis equation. A better understanding of the molecular mechanisms involved in MP shedding would pave the way of a new pharmacological approach aiming at the control of MP-driven cellular responses.