An overview of the role of microparticles/microvesicles in blood components: Are they clinically beneficial or harmful?

Blood cells and tissues generate heterogeneous populations of cell-derived vesicles, ranging from approximately 50 nm to 1 µm in diameter. Under normal physiological conditions and as an essential part of an energy-dependent natural process, microparticles (MPs) are continuously shed into the circulation from membranes of all viable cells such as megakaryocytes, platelets, red blood cells, white blood cells and endothelial cells. MP shedding can also be triggered by pathological activation of inflammatory processes and activation of coagulation or complement systems, or even by shear stress in the circulation. Structurally, MPs have a bilayered phospholipid structure exposing coagulant-active phosphatidylserine and expressing various membrane receptors, and they serve as cell-to-cell shuttles for bioactive molecules such as lipids, growth factors, microRNAs, and mitochondria. It was established that ex vivo processing of blood into its components, involving centrifugation, processing by various apheresis procedures, leucoreduction, pathogen reduction, and finally storage in different media and different types of blood bags, can impact MP generation and content. This is mostly due to exposure of the collected blood to anticoagulant/storage media and due to shear stresses or activation, contact with artificial surfaces, or exposure to various leucocyte-removal filters and pathogen-reduction treatments. Such artificially generated MPs, which are added to the original pool of MPs collected from the donor, may exhibit specific functional characteristics, as MPs are not an inert element of blood components. Not surprisingly, MPs' roles and functionality are therefore increasingly seen to be fully relevant to the field of transfusion medicine, and as a parameter of blood safety that must be considered in haemovigilance programmes. Continual advancements in assessment methods of MPs and storage lesions are gradually leading to a better understanding of the impacts of blood collection on MP generation, while clinical research should clarify links of MPs with transfusion reactions and certain clinical disorders. Harmonization and consensus in sampling protocols, sample handling and processing, and assessment methods are needed to achieve consensual interpretations. This review focuses on the role of MPs as an essential laboratory tool and as a most effective player in transfusion science and medicine and in health and disease.