Abstract
BackgroundThe brain’s initial innate response to stroke is primarily mediated by microglia, the resident macrophage of the CNS. However, as early as 4 h after stroke, the blood–brain barrier is compromised and monocyte infiltration occurs. The lack of discriminating markers between these two myeloid populations has led many studies to generate conclusions based on the grouping of these two populations. A growing body of evidence now supports the distinct roles played by microglia and monocytes in many disease models.MethodsUsing a flow cytometry approach, combined with ex-vivo functional assays, we were able to distinguish microglia from monocytes using the relative expression of CD45 and assess the function of each cell type following stroke over the course of 7 days.ResultsWe found that at 72 h after a 90-min middle cerebral artery occlusion (MCAO), microglia populations decrease whereas monocytes significantly increase in the stroke brain compared to sham. After stroke, BRDU incorporation into monocytes in the bone marrow increased. After recruitment to the ischemic brain, these monocytes accounted for nearly all BRDU-positive macrophages. Inflammatory activity peaked at 72 h. Microglia produced relatively higher reactive oxygen species and TNF, whereas monocytes were the predominant IL-1β producer. Although microglia showed enhanced phagocytic activity after stroke, monocytes had significantly higher phagocytic capacity at 72 h. Interestingly, we found a positive correlation between TNF expression levels and phagocytic activity of microglia after stroke.ConclusionsIn summary, the resident microglia population is vulnerable to the effects of severe ischemia, show compromised cell cycle progression, and adopt a largely pro-inflammatory phenotype after stroke. Infiltrating monocytes are primarily involved with early debris clearance of dying cells. These findings suggest that the early wave of infiltrating monocytes may be beneficial to stroke repair and future therapies aimed at mitigating microglia cell death may prove more effective than attempting to elicit targeted anti-inflammatory responses from damaged cells.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-015-0329-1) contains supplementary material, which is available to authorized users.
Highlights
Inflammation is a key component of stroke-induced injury and elevated levels of inflammatory markers are associated with poor outcome in stroke patients [1,2,3,4]
We found a dramatic increase in monocyte (CD45hiCD11b+Ly6C+Ly6G−) counts in the stroke hemisphere compared to sham brain (Fig. 1d)
Following repeated BRDU injections starting at 12 h, microglia showed little BRDU incorporation by 72 h, whereas ~90 % of monocytes in the ischemic brain were BRDU-positive (Fig. 2c, d)
Summary
Inflammation is a key component of stroke-induced injury and elevated levels of inflammatory markers are associated with poor outcome in stroke patients [1,2,3,4]. In the protected confines of the blood–brain barrier, microglia maintain healthy brain function by clearing debris, pruning synapses, and producing growth/repair factors [6] These cells stand poised to respond to injuries in the CNS such as ischemic stroke. CNS-resident microglia are the first responders to ischemia; these cells likely serve a unique role in injury repair relative to monocytes, which have a finite lifespan and are recruited in higher numbers during the postreperfusion phase from the periphery. These differences have been made evident in recent studies that utilize transgenic bone marrow chimeras to distinguish between local and circulating myeloid populations [12]. A growing body of evidence supports the distinct roles played by microglia and monocytes in many disease models
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