Abstract
Local transplantation of human CD34+ hematopoietic stem cells has been shown to promote neovascularization in pre-clinical studies in models of myocardial and limb ischemia. In early phase clinical trials, transplantation of CD34+ cells has been associated with reduced angina, improved exercise time and reduced amputation rates. Several studies have suggested that paracrine effects by these pro-angiogenic cells mediate the effects induced by cell transplantation. We hypothesized that CD34+ cells secrete exosomes (Exo), which mediate at least a part of the therapeutic function of the cells. Methods and Results: We isolated Exo from the conditioned media of adult human peripheral blood (PB) CD34+ cells. The angiogenic and therapeutic potency of CD34+ Exo was compared with the intact CD34+ cells and also with PB mononuclear cell (MNC) Exo. Exo from both CD34+ cells and MNC are 50–90nm in size, have cup shaped morphology, and carry known Exo-marker proteins such as CD63, TSG101 and Annexin V as shown by electron microscopy, Western blot and flow cytometry. Compared to CD34+ cells or MNC Exo, CD34+ Exo significantly induces in vitro angiogenic activities such as viability, proliferation and tube formation of HUVECs on matrigel- in a dose dependent manner. In vivo, CD34+ Exo stimulated significant neovascularization in mouse corneal angiogenesis assay (14±4 mm v MNC Exo, 4±1 mm, p<0.01) and incorporation of endothelial (CD31+) cells in mouse matrigel-plug assay (6±1.7% v CD34+ cells, 2±0.8%, p<0.01). Finally, in a mouse model of hind limb ischemia (HLI), CD34+ Exo significantly improved perfusion (ratio: 1.01±0.04 v 0.57±0.1, P<0.05), increased capillary density (1.8±0.3/HPF v 0.9±0.1/HPF, p<0.001) and prevented ischemic leg amputation (16% v 100%), as compared with MNC Exo. Conclusions: These data demonstrate that CD34+ Exo induce angiogenic activity and ischemic tissue repair in the absence of CD34+ cells, and suggest that Exo represent important mediators of the therapeutic effects associated with CD34+ cell therapy. We speculate that Exo derived from CD34+ cells may represent a significant component of the paracrine effect of progenitor-cell transplantation for therapeutic angiogenesis.
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