Type-I interferon signalling through IFNAR1 plays a deleterious role in the outcome after stroke
Introduction
The immune system and the central nervous system (CNS) have long been considered separate systems with minimal interaction. However, the immune molecular mechanisms and pathways that exist in the periphery are also present in the CNS. These same pathways in the CNS become activated in the event of a cerebral ischemia/reperfusion injury or stroke, resulting in neuroinflammation, which further exacerbates primary brain damage. It is becoming increasingly clear that the regulation of inflammation after stroke is multifaceted and comprises vascular effects, distinct cellular responses, apoptosis, and chemotaxis. There are many cell types that are affected including neurons, glial cells, microglia, endothelial cells and they all respond to the resultant neuroinflammation in different ways.
Hallmark features of neuroinflammation including activated microglia, astrogliosis, and increased cytokine and chemokine levels (Downes and Crack, 2010, Downes et al., 2013) have all been reported in animal models and postmortem human brain samples of stroke. With the development of highly specific molecular, biochemical, and immunohistochemical techniques, the presence of numerous inflammatory mediators in and around ischaemic brain tissue has also been documented. In brief, the presence of inflammatory cytokines, chemokines and adhesion molecules has triggered intense research on strategies for blocking their action. However, it is likely that some inflammatory cell–derived mediators will be critical in the proper repair and recovery of neuronal networks and in enhancing plasticity and reformatting of circuits necessary for taking over tasks for which the lost brain tissue was responsible. This is evidenced by a recent study using a brain slice model showing microglia playing a neuroprotective role in acute brain injury (Neumann et al., 2006). The timing, as well as the specific cytokine or factor to be targeted are important variables in determining whether inflammation helps or hinders improved outcomes after stroke.
In the periphery type-I interferons (IFNs) are key regulators in the production of cytokines (de Weerd et al., 2007, Kawai and Akira, 2006) however their role in the generation of neuroinflammation is not well described. In view of the potential proinflammatory effects of type I IFNs, we investigated the impact of this family of cytokines in the acute neural injury model of murine stroke. Since we have had considerable interest in the factors that regulate the pathogenesis of stroke for some years, and IFNs have been implied in brain pathology, we investigated the impact of type I interferon receptor (IFNAR1) deficient mice in the MCAO model of ischaemic brain injury. Our data presented in this study proposes that signalling through IFNAR1 is a previously unrecognised factor that is critical to neural injury after stroke.
Section snippets
Animals
All animal experiments complied with the regulatory standards of, and were approved by, the University of Melbourne, Medicine Dentistry and Health Sciences Animal Ethics Committee. IFNAR1-/- and IFNAR2-/- mice were on a C57Bl6 background, male, 8–10 weeks of age and backcrossed to 15 generations and were previously generated in the laboratory of Prof Hertzog (de Weerd et al., 2013, Hwang et al., 1995, Owczarek et al., 1997). All surgery was performed under isoflurane anesthesia, and all efforts
Type-I interferon expression is elevated after MCAO
In view of the proinflammatory nature of the environment in the brain after stroke we examined type-I interferon responses in wild-type mice in the MCAO model using quantitative PCR. The expression of IFNα was induced at 8 h post-arterial occlusion and several interferon-regulated genes (IRGs); CCL2, CCL3, CXCL1, SAA3, SOCs3 and IRF7 also induced 2–8 h after stroke (Fig. 1). Since neither IFNβ, nor the newly identified IFNε, nor IFNγ was detectable at these times (data not shown), it suggests
Discussion
Over the past 20 years, researchers examining brain tissue at various time intervals after stroke have observed the presence of inflammatory cells, neutrophils and monocytes at the site of injury, as well as the activation of endogenous glia and microglia (Le Thuc et al., 2015). These observations have led to the hypothesis that this resultant neuroinflammation plays a role in the progression and exacerbation of the neural injury that results from stroke. Here we describe that blocking
Conflict of interest statement
The authors report that there are no conflicts of interest concerning this paper.
Author contributions
MZ, CED, CHYW, KMB, PLGA, JG, RA carried out experiments, PJH contributed critical reagents and contributed to experimental design, JMT and PJC designed experiments, analysed data and wrote the manuscript.
Acknowledgements
This study was supported by grants from the National Health and Medical Research Council (NHMRC) of Australia. PJC was supported by an Australian Research Council (ARC) Future Fellowship. PJH is a Senior Principle Research Fellow of the NHMRC.
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