Abstract WP243: Csf1r Antagonism Abolishes Ischemic Preconditioning Mediated Protection In White Matter

Background: Ischemic preconditioning (IPC) is a robust protective phenomenon whereby brief ischemic exposure confers protection against a subsequent prolonged ischemic challenge. IPC has been studied primarily in gray matter predominant models, however, stroke significantly impacts white matter (WM). We have previously reported development of a WM IPC model in the mouse optic nerve (MON), a fully myelinated CNS WM tract. We identified innate immune signaling pathways as required for axonal protection, however the cell type(s) responsible for IPC in WM are unknown. Here we characterize the effects of microglial depletion on IPC-induced protection against OGD-mediated injury to: (i) axonal compound action potential (CAP) recovery, (ii) axonal structural integrity, (iii) oligodendrocyte viability and (iv) nodes of Ranvier (NoR) in MON. Methods: Following microglial depletion by pharmacologic treatment with colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622, MONs were exposed to transient ischemia in vivo, acutely isolated, and subjected to oxygen-glucose deprivation (OGD) ex vivo to simulate a severe ischemic injury. Functional and structural axonal recovery was assessed by electrophysiology with recording of CAP and immunofluorescent/confocal microscopy followed by quantitative stereology. Results: Microglial depletion eliminated IPC-mediated protection of axonal function but intriguingly had no effect on recovery after acute ischemic injury alone (i.e. in the absence of IPC). Microglial depletion abrogated IPC-mediated protective effects on both axonal integrity and the survival of mature (APC+) oligodendrocytes after exposure to OGD. IPC-mediated protection was determined to be independent of retinal injury. Effects on NoR remain under investigation. Conclusions: Based on these findings, we conclude that preconditioned, but not naïve, microglia are critical in the endogenous IPC-induced protective response against ischemic injury that occurs in WM. Thus, preconditioned microglia are a critical cellular target for future therapeutics designed to enhance WM recovery from acute ischemic injury (stroke). Furthermore, our data suggest that IPC-mediated protection in WM is anatomically intrinsic to WM.