Background and Aim: Activation of the complement cascade and oxidative stress in tubular epithelial and peritubular endothelial cells are hallmarks of delayed kidney graft function (DGF) due to ischemia-reperfusion injury (IRI). Endothelial progenitor cells (EPCs) are bone marrow-derived precursors known to reverse IRI by paracrine mechanisms including the release of extracellular vesicles (EV), small particles playing a role in intercellular communication through the transfer of specific mRNA and microRNA. The aim of this study was to evaluate the regenerative role of EPC-derived EV in kidney IRI through the horizontal transfer of RNA involved in the inhibition of complement activation and of oxidative stress. 
Methods: EPC were isolated from peripheral blood of healthy volouteers and EV obtained by supernatant ultracentrifugation were characterized for size, protein and RNA content. We evaluated the effects of EV in a rat model of kidney IRI and in vitro in human tubular epithelial and endothelial cells cultured in hypoxia in vitro.
Results: EPC-derived EV sized 60-130 nm and carried different subsets of mRNAs and microRNAs able to modulate cell proliferation, angiogenesis and apoptosis (eNOS, Akt, Bcl-XL, miR-126, miR-296). By RT-PCR, we also found within EV mRNAs coding for the complement inhibitors factor H, DAF (CD55), CD59 and for the anti-oxidant transription factgor nrf2. In experimental kidney IRI, EPC EV localized within peritubular capillaries and tubular cells exerting morphologic and functional protection from by reducing tubular cell apoptosis/senescence, endothelial-to-mesenchymal transition (EndoMT) and leukocyte infiltration. Moreover, EV administration reduced C5b9 deposition and enhanced the expression of factor H, DAF, CD59 and nrf2 in the ischemic kidney. In vitro, EV reduced hypoxia-induced apoptosis/senescence of tubular epithelial (caspase activation, Klotho expression) and endothelial cells (EndoMT) by up-regulating the expression of factor H, DAF, CD59 and nrf2, thus confirming the in vivo data. The role of specific mRNA transfer to hypoxic renal cells was confirmed by experiments using RNase-treated EV, EV released from EPC engineered to knock-down the complement inhibitors or nrf2  by specific siRNA, or EV produced by EPC transfected with siRNA Dicer, the intracellular enzyme essential for microRNA maturation. 
Conclusions: EPC-derived EV protect the kidney from IRI by delivering pro-angiogenic, anti-apoptotic, complement inhibitors and anti-oxidant microRNAs and mRNAs coding for factor H, DAF, CD59 and nrf2 to injured tubular epithelial and endothelial cells. These results suggest the potential use of EPC-derived EVs as therapeutic tool to avoid or at least to limit IRI-associated DGF in kidney transplantation without the potential adverse effects of whole stem cell therapy including maldifferentiation and tumorigenesis.