Mesenchymal Stem Cell (MSC)-Derived Extracellular Vesicles Protect from Neonatal Stroke by Interacting with Microglial Cells.

Mesenchymal stem cell (MSC)-based therapies are beneficial in models of perinatal stroke and hypoxia-ischemia. Mounting evidence suggests that in adult injury models, including stroke, MSC-derived small extracellular vesicles (MSC-sEV) contribute to the neuroprotective and regenerative effects of MSCs. Herein, we examined if MSC-sEV protect neonatal brain from stroke and if this effect is mediated via communication with microglia. MSC-sEV derived from bone marrow MSCs were characterized by size distribution (NanoSight™) and identity (protein markers). Studies in microglial cells isolated from the injured or contralateral cortex of postnatal day 9 (P9) mice subjected to a 3-h middle cerebral artery occlusion (tMCAO) and cultured (in vitro) revealed that uptake of fluorescently labeled MSC-sEV was significantly greater by microglia from the injured cortex vs. contralateral cortex. The cell-type-specific spatiotemporal distribution of MSC-sEV was also determined in vivo after tMCAO at P9. MSC-sEV administered at reperfusion, either by intracerebroventricular (ICV) or by intranasal (IN) routes, accumulated in the hemisphere ipsilateral to the occlusion, with differing spatial distribution 2 h, 18 h, and 72 h regardless of the administration route. By 72 h, MSC-sEV in the IN group was predominantly observed in Iba1+ cells with retracted processes and in GLUT1+ blood vessels in ischemic-reperfused regions. MSC-sEV presence in Iba1+ cells was sustained. MSC-sEV administration also significantly reduced injury volume 72 h after tMCAO in part via modulatory effects on microglial cells. Together, these data establish feasibility for MSC-sEV delivery to injured neonatal brain via a clinically relevant IN route, which affords protection during sub-acute injury phase.