Characterisation and properties of ectosomes released by human polymorphonuclear neutrophils

Human neutrophils release vesicles when activated in vitro and in vivo, in local and systemic inflammation. We have suggested that the presence of these vesicles is due to ectocytosis, defined as the release of rightside-out oriented vesicles expressing a select set of membrane proteins. Herein we have characterised the vesicles released by neutrophils to be ectosomes with specific properties. They contained cytosolic F-actin indicating their outside-out orientation. They bound Annexin V, suggesting that they expose phosphatidylserine, similarly to platelet microparticles. They expressed a subset of cell surface proteins (selectins and integrins, complement regulators, HLA-1, FcgammaRIII, and CD66b, but not CD14, FcgammaRII, and CD87). There was no specificity for transmembrane or glycosyl-phosphatidylinositol-linked proteins and, unexpectedly, L-selectin, known to be cleaved from the surface of activated neutrophils, was present. Ectosomes exposed active enzymes released by neutrophils upon degranulation (matrix metalloproteinase-9, myeloperoxidase, proteinase 3, and elastase). In particular, released myeloperoxidase was able to bind back to ectosomes. The purified complement protein C1q and C1q from serum bound to ectosomes as well. Another aspect of ectosomes was that they became specifically adherent to monocytic and endothelial cells. These observations suggest that neutrophil-derived ectosomes have unique characteristics that make them candidates for playing roles in inflammation and cell signaling.