Abstract W P222: Activated Microglia Contribute to Neurovascular Integrity and Limit Brain Injury After Acute Neonatal Focal Stroke

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It has been recently shown that microglial cells, which for a long time were considered purely injurious in the context of cerebral ischemia, can also exert beneficial effects following stroke in both adults and neonates1,2. Lack of tools to reliably distinguish resident microglia from infiltrated peripheral monocytes has been a major obstacle on the way to understand the relative role of these subpopulations of cells of the monocyte lineage in the pathophysiology of stroke. We subjected postnatal day 10 (P10) transgenic Cx3cr1GFP/-CCr2RFP/- mice, in which resident microglia (Cx3cr1GFP) and infiltrating monocytes (CCr2RFP) can be distinctively identified, to a transient 3 hour middle cerebral artery occlusion MCAO, a model that we recently developed3. Microglial cells were left unperturbed or were selectively depleted before MCAO by intracortical injection of clodronate-encapsulated liposomes. Depletion of microglia exacerbated injury and significantly increased infarct volume (75.9% Vs. 56.3%, p<0.01). Furthermore, compared to mice with unperturbed microglia, depletion of microglia significantly increased the number of hemorrhages in injured regions, adversely affected vascular density and decreased the number of both adherent and infiltrated monocytes. The extent of RFP+ monocyte adhesion to vessels and infiltration in the brain parenchyma was highly variable among individual mice and did not correlate with brain infarct, whereas a significant correlation between the overall extent of microglial activation (measured by morphological transformation) and the number of infiltrated monocytes was observed. The deleterious effect of microglial depletion on vascular integrity and function and on brain injury indicates that activated microglia act as a buffering component that limits vascular degeneration and injury severity after neonatal stroke. Our data also suggest a direct and dynamic relationship between microglial activation and monocyte recruitment into acutely reperfused neonatal brain. Support: NS55915 (ZV), NS76726 (ZV), NS080015 (ZS, KA), AHA POST10980003 (DFL). 1. Faustino J et al. J Neurosci. 2011. 2. Lalancette-Hebert M et al. J Neurosci. 2007. 3. Woo MS et al. Annals of Neurology. 2012.