Abstract
The high mortality and morbidity rate of stroke is a chronic problem that plagues human society. The activation of microglia is one of the principal reasons why neuroinflammation induces cerebral dysfunction. Because of their vital functions in the regulation of neuroinflammation, microglia constitute an important target for stroke. Given that there is an innate self-preservation mechanism between neurons and microglia, the transmembrane glycoproteins on the surface of their membranes, namely CD200 and CD200R, have become a popular topic of research. Numerous studies have demonstrated that CD200-CD200R interaction, microglial activation, and poststroke neuroinflammatory damage are inextricably linked. In this review, we describe the above relationship from a new perspective. We specifically focus on neuroinflammation after stroke. The role of crosstalk of CD200-CD200R inhibitory immune ligand receptors in immune regulation will also be illustrated. Thus, we will see how poststroke injury can be influenced by the CD200-CD200R crosstalk. Finally, we will discuss the possibility of clinical application of the result of CD200-CD200R interaction to manage neuroinflammatory injury after stroke.
Introduction
Stroke with high mortality and morbidity rates have a high incidence worldwide and threatens human health and quality of life. It is a devastating disease and continues to play an important role in current research in the field of medicine. Portegies, from the Epidemiology Department of Erasmus MC University Medical Center, found that between 1990 and 2012, 74.3% of patients with their first-ever stroke died (919/1237), but only 53.8% of stroke-free participants died (2654/4928) (). Stroke is classified into two types: ischemic stroke and hemorrhagic stroke. The former accounts for over 80% of stroke cases (). In recent years, several therapeutic strategies for stroke have been used in clinical practice. Unfortunately, most patients are unable to obtain timely and proper treatment because of the narrow therapeutic time window, and only 3.4–5.2% of patients receive timely treatment during the acute phase (). During the subacute and chronic phases, it is therefore quite essential to discover other combinations of treatments to facilitate the functional rehabilitation of patients with stroke ().
Following the use of the magnetic resonance imaging (MRI) techniques at the cellular and molecular level, researchers have obtained a new understanding of the inflammatory reaction after stroke because of the ability to dynamically monitor the reaction changes occurring at spatial and temporal levels (). The immune response after stroke is a significant factor that affects the pathobiology and prognosis of acute ischemic stroke (). Microglia/macrophages are the main immune cells in the defense against brain damage (). These cells release mediators, which may have positive effects on brain repair and neurogenesis. Furthermore, superabundant proinflammatory mediators can cause secondary neuronal injury and hinder brain regeneration ().
CD200 and its receptor CD200R are transmembrane glycoproteins present on the surface of cells. Studies have shown that CD200 is expressed on the surface of a wide variety of cells such as thymocytes, B cells, T cells, tonsil follicles, kidney glomeruli, syncytiotrophoblasts, and endothelial cells (). Indirect immunoperoxidase staining showed that CD200 is primarily distributed in all neurons of the spinal cord and brain in the central nervous system (). CD200R is expressed to a lesser extent than CD200 and is mainly distributed in myeloid cells (). In the central nervous system, CD200R is mainly expressed in microglia (). CD200R1, a member of the CD200R family, binds to CD200 with a higher affinity than other CD200R family members (). Recently, we have focused our attention on the relationship between CD200 and CD200R to elucidate their immunoregulatory functions in neuroinflammation.
Neurons are thought to have a self-protection mechanism in which they express CD200 that binds to CD200R on the microglial surface, which further prevents secondary neuronal injury caused by microglia (; ; ). Although there are no studies on the specific repair mechanisms and therapeutic effects of CD200-CD200R in poststroke inflammation injury, CD200R, could still be considered as a high potential target in the study of stroke immunotherapy.
Neuroinflammation After Stroke
The innate immune system plays a role in the cerebral damage due to ischemia, hemorrhage, and other brain injuries, and inflammatory signal transduction is involved in all periods of stroke, from the early stages where it mainly causes damage to the late regenerative processes responsible for brain tissue repair (). After stroke, primary brain damage results from the death of cerebral cells. Secondary brain damage is caused by cytoplasmic substances released into the extracellular environment, which initiate a cascade of inflammatory events that can amplify cell damage (). Microglia have profound effects on neuroinflammation. Activated microglia act as a double-edged sword. These cells are normally responsible for clearing up necrotic neural cells and restoring neuronal functions. However, when overly activated after stroke, they produce a high amount of proinflammatory mediators that can destroy the blood-brain barrier and neurons and affect neurogenesis (). In this review, we have mainly focused our attention on the proinflammatory functions of microglial activation closely linked to the regulator CD200-CD200R.
Mechanisms of Microglial Activation
During the occurrence of acute brain injuries, microglia shift their activated states depending on two factors: the expression of “on signals” and/or the abnormity of “off signals” (). The “On” signal is principally found in pathological states and involves purines, chemokines, matrix metalloproteinase-3, and glutamate. The “Off” signal mainly appears in the healthy brain and is regulated by the release of CD22, CX3CL1, neurotrophins, and neurotransmitters from neural cells, which could combine with receptors on microglia and help microglia to function in the physiological process ().
Immediately after the occurrence of cerebral injuries, microglia become activated through signals. Damage-associated molecular patterns (DAMPs), which include modified extracellular matrix components, modified or oxidized lipid species, DNA, RNA, and cytoplasmic proteins, are released from the cell intracellular structures at the appropriate time of cell death (). DAMPs activate families of scavenger receptors and Toll-like receptors on microglia, thereby triggering stroke-induced neuroinflammation. In particular, Toll-like receptor 4 (TLR4) and TLR2 are considered to mediate the inflammatory response involved in the pathophysiological processes of cerebral ischemia-induced injury (). Mice lacking TLR4 showed less expression of mediators relevant to brain damage and inflammation, including IFN-β, COX2, inducible nitric oxide synthase (iNOS), interferon regulatory factor-1(IRF1), and matrix metalloproteinase-9 (MMP-9) (). Moreover, ATP from injured neurons, a type of DAMPs, is a contributing factor that causes microglia to function by binding to the purinergic receptors on microglia ().
Proinflammatory Functions of Microglial Activation
Once microglia are activated, they undergo major changes in their morphology, functions, and behaviors, such as migration, proliferation, and phagocytosis. The production of proinflammatory cytokines such as IL-6, IL-1, and TNF-α is one of the most crucial abilities of microglia (). Furthermore, activated microglia synthesize iNOS, which is essential to produce NO. High levels of NO damage the brain under the oxidative stress state because of the oxidation and nitro-tyrosination of useful substances (). On the one hand, the proinflammatory cytokines lead to neuronal death through direct or indirect pathways, for instance, apoptosis and necrosis regulated by the caspase family of proteins and inflammasome (; ). On the other hand, the proinflammatory cytokines could mediate the phagocytic activity of microglia. After ischemic stroke, higher levels of TNF-α expression were detected in accordance with higher phagocytic activity of microglia (). TNF-α has strong immune activity and neurotoxicity ().
Role of the CD200-CD200R Interaction in Neuroinflammation
One of the most useful endogenous immunoregulatory molecule candidates that could prevent the neuroinflammation status of the brain tissue seriously altered in cerebral injuries is the CD200/CD200R inhibitory immune ligand-receptor crosstalk. The immunomodulatory effect of CD200-CD200R is produced in the different processes of microglial functions, as shown in the following text.
Impact on Microglial Proliferation
In this segment, the proliferation of microglia probably affected by the CD200-CD200R interaction is discussed. found that in a CD200-deficient mice model with toxoplasma encephalitis, the number of microglia with the proliferation-associated antigen Ki67 + dramatically increased; this finding demonstrates that the deficiency of the CD200-CD200R interaction upregulates microglial proliferation. Furthermore, a study by showed a high amount of microglia following the exposure to rotenone in the neuron-microglia co-cultures after using an anti-CD200R blocking antibody (ACDR). observed an increase in the quantity of microglial cells with active phenotype by using a retinoic acid (RA)-induced spina bifida animal model to downregulate CD200 and upregulate CD200R. Thus, it can be speculated that the loss of the CD200-CD200R pair will stimulate microglial proliferation.
Influence of Microglial Activation
One of the most pivotal roles played by the CD200-CD200R interaction is in maintaining microglia in a resting state to inhibit the release of the proinflammatory mediators. As shown in Figure 1, once CD200R binds to CD200, its tyrosine residues are phosphorylated. Simultaneously, the downstream of tyrosine kinase (DOK)1, DOK2, and RAS p21 protein activator 1 (RasGAP) are recruited. Finally, MAPK p38, extracellular kinase (ERK), and c-Jun terminal kinase (JNK), the common signaling pathways in microglial activation, are suppressed (). Ultimately, the production of TNF-α, IL-6, and iNOS is reduced (). In one study, the authors used mice lacking CD200 to reveal that microglia exhibited a more activated phenotype and were large in numbers (). observed that the level of proinflammatory cytokines increased using a selective blocking antibody against CD200R1; this finding indicates that the destruction of CD200R1 induced microglial cells to exhibit a proinflammatory phenotype. administered a CD200R1 agonist (CD200Fc) in mice with experimental autoimmune encephalomyelitis (EAE) during the chronic phase of the disease; they found that the activation markers of microglia were decreased. To the best of our knowledge, we can predict that the immunomodulatory outcome of CD200-CD200R binding is to inhibit the activation of microglia.
FIGURE 1
Role of CD200-CD200R in Stroke
Stroke leads to the death of neural cells. In this condition, the CD200-CD200R has ceased to function. Although understanding the interaction between microglial cells and neurons is a significant challenge, it has a high therapeutic potential to restore damage caused by microglia in neuroinflammation after stroke. For the past few years, various studies have been conducted on CD200-CD200R interaction in stroke in animal models, and satisfactory results have been achieved.
Effects on Microglial Activation by Mechanisms
Correlations With Stem Cell Treatment in Stroke
Cell therapy represents a potential breakthrough in the treatment of stroke. Preclinical studies have revealed that cell therapy is effective in the improvement of sensorimotor functions and facilitation of behavioral recovery in animal models of stroke (
In a previous study, the authors used an in vitro human allogeneic co-culture model to reveal the interplay between neural stem/progenitor cells (NPCs) and a microglia population. They showed that the proportion of NPCs expressing CD200 and microglia expressing CD200R in the co-culture were higher than those in the mono-culture, leading to the enhanced possibility of the CD200 ligand-receptor binding to ameliorate the detrimental neuroinflammation mediated by microglia (
CD200-CD200R in Other Neuronal Injuries
The EAE model is used to investigate the relationship between CD200-CD200R and multiple sclerosis (MS). In the experiment, the phenomenon of reduction in CD200 and an increase in CD200R1 was observed in the EAE model. More specifically, CD200 expression showed an apparent reduction before the appearance of clinical symptoms in EAE. This perhaps indicates that changes in CD200 expression might occur in the early phase of MS, which may be the reason for the downregulated control of macrophage/microglial activation, thus contributing to the inflammatory response and the development of pathological processes. In comparison, a succedent increase in CD200R1 expression followed behind that seemingly portended compensatory reaction to re-build control of the inflammation (
The CD200-CD200R pair also acts as a contributing factor to neurodegenerative diseases such as PD and Alzheimer’s disease (AD). After preinjection of a CD200R blocking antibody to block the CD200-CD200R inhibition signal in rats, a sub-lethal dose of 6-hydroxydopamine, which only caused slight death of dopaminergic neurons in the substantia nigra, resulted in apparent PD symptoms in rats. Histopathological examination showed the death of dopaminergic neurons and activation of microglia cells in the substantia nigra (
Conclusion and Future Prospects
Recently, CD200 and CD200R have been increasingly investigated, and the immune regulation induced by the CD200-CD200R axis is valuable. As we already know, the toxic damage caused to neurons by overactivated microglia far outweighs its benefits (
In accordance with the evidence reviewed earlier, stem cells are optimal materials for the cell therapy of patients with stroke. Many animal trials have been conducted, in which the anti-inflammatory effects of stem cells are hot topics (
In conclusion, compared to traditional treatment regimens, the use of the CD200-CD200R pair as target sites to alleviate poststroke inflammatory injury is certainly one of the best options (
Statements
Author contributions
HS conceived the review. JL participated in drafting the manuscript. All authors wrote the manuscript.
Funding
This work was supported by the National Natural Science Foundation of China (81701243), the Pearl River S&T Nova Program of Guangzhou (201710010047), and the Presidential Foundation of Zhujiang Hospital of Southern Medical University (No. yzjj2018rc03).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
stroke, microglia, neuroinflammation, CD200-CD200R interaction, clinical potential
Citation
Zhao X, Li J and Sun H (2019) CD200-CD200R Interaction: An Important Regulator After Stroke. Front. Neurosci. 13:840. doi: 10.3389/fnins.2019.00840
Received
29 April 2019
Accepted
26 July 2019
Published
07 August 2019
Volume
13 - 2019
Edited by
Mohammad Badruzzaman Khan, Medical College of Georgia, Augusta University, United States
Reviewed by
Rajaneesh K. Gupta, University of Pittsburgh, United States; Koteswara Rao Nalamolu, University of Illinois College of Medicine at Peoria, United States
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© 2019 Zhao, Li and Sun.
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*Correspondence: Haitao Sun, 2009sht@smu.edu.cn; msunhaitao1988@126.com
This article was submitted to Neurodegeneration, a section of the journal Frontiers in Neuroscience
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