Interferon regulatory factor 4/5 signaling impacts on microglial activation after ischemic stroke in mice

Abstract

Microglial activation is a key element in initiating and perpetuating inflammatory responses to stroke. Interferon regulatory factor 5 (IRF5) and IRF4 signaling have been found critical in mediating macrophage pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes respectively in peripheral inflammation. We hypothesize that the IRF5/4 regulatory axis also mediates microglial activation after stroke. C57BL6 mice of 8-12 weeks were subject to a 90-minute middle cerebral artery occlusion and the brains evaluated at 24h, 3d, 10d and 30d after reperfusion. Flow cytometry was utilized to examine microglial activation and cytokine expression. RT-PCR was performed for mRNA levels of IRF5/4 in sorted microglia. Microglial expression of IRF5/4 was examined by immunohistochemistry, and brain cytokine levels were determined by ELISA. Our results revealed that the IRF5 mRNA level in sorted microglia increased at 3d of stroke; whereas IRF4 mRNA level exhibited bi-phasic increases, with a transient rise at 24h and a peak at 10d. The same pattern was seen in IRF5/4 protein colocalization with Iba-1⁺ cells by IHC. Intracellular levels of TNFα and IL-1β in microglia peaked at 3d of stroke, and IL-4⁺IL-10⁺ double positive microglia significantly increased at day 10. Brain levels of these cytokines were consistent with microglial cytokine changes. Worse behavior test results were seen at 3d vs. 10d of stroke. We conclude that microglia phenotypes are dynamic to ischemic stroke and IRF5/4 signaling may regulate microglial M1/M2 activation and impact on stroke outcomes.

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