Brain-immune interactions and ischemic stroke: clinical implications

Increasing evidence shows that the central nervous system and the immune system interact in complex ways, and better insight into these interactions may be relevant to the treatment of patients with stroke and other forms of central nervous system injury. Atherosclerosis, autoimmune disease, and physiological stressors, such as infection or surgery, cause inflammation that contributes to vascular injury and increases the risk of stroke. In addition, the immune system actively participates in the acute pathogenesis of stroke. Thrombosis and hypoxia trigger an intravascular inflammatory cascade, which is further augmented by the innate immune response to cellular damage occurring in the parenchyma. This immune activation may cause secondary tissue injury, but it is unclear whether modulating the acute immune response to stroke can produce clinical benefits. Attempts to dampen immune activation after stroke may have adverse effects because central nervous system injury causes significant immunodepression that places patients at higher risk of infections, such as pneumonia. The activation of innate immunity after stroke sets the stage for an adaptive immune response directed against brain antigens. The pathogenic significance of adaptive immunity and its long-term effects on the postischemic brain remains unclear, but it cannot be ruled out that a persistent autoimmune response to brain antigens has deleterious and long-lasting consequences. Further research will be required to determine what role, if any, immunity has in long-term outcomes after stroke, but elucidation of potential mechanisms may open promising avenues for the development of new therapeutics to improve neurological recovery after brain injury.     

Inflammation,plasticity and real-time imaging after cerebral ischemia

With an incidence of approximately 350 in 100,000, stroke is the third leading cause of death and a major cause of disability in industrialized countries. At present, although progress has been made in understanding the molecular pathways that lead to ischemic cell death, the current clinical treatments remain poorly effective. There is mounting evidence that inflammation plays an important role in cerebral ischemia. Experimentally and clinically, brain response to ischemic injury is associated with an acute and prolonged inflammatory process characterized by the activation of resident glial cells, production of inflammatory cytokines as well as leukocyte and monocyte infiltration in the brain, events that may contribute to ischemic brain injury and affect brain recovery and plasticity. However, whether the post-ischemic inflammatory response is deleterious or beneficial to brain recovery is presently a matter of debate and controversies. Here, we summarize the current knowledge on the molecular mechanisms underlying post-ischemic neuronal plasticity and the potential role of inflammation in regenerative processes and functional recovery after stroke. Furthermore, because of the dynamic nature of the brain inflammatory response, we highlight the importance of the development of novel experimental approaches such as real-time imaging. Finally, we discuss the novel transgenic reporter mice models that have allowed us to visualize and to analyze the processes such as neuroinflammation and neuronal repair from the ischemic brains of live animals.     

Interferon regulatory factor 2 binding protein 2: a new player of the innate immune response for stroke recovery

Ischemic brain injury triggers an inflammatory response. This response is necessary to clear damaged brain tissue but can also exacerbate brain injury. Microglia are the innate immune cells of the brain that execute this critical function. In healthy brain, microglia perform a housekeeping function, pruning unused synapses between neurons. However, microglia become activated to an inflammatory phenotype upon brain injury. Interferon regulatory factors modulate microglial activation and their production of inflammatory cytokines. This review briefly discusses recent findings pertaining to these regulatory mechanisms in the context of stroke recovery.     

The acute-phase protein PTX3 is an essential mediator of glial scar formation and resolution of brain edema after ischemic injury

Acute-phase proteins (APPs) are key effectors of the immune response and are routinely used as biomarkers in cerebrovascular diseases, but their role during brain inflammation remains largely unknown. Elevated circulating levels of the acute-phase protein pentraxin-3 (PTX3) are associated with worse outcome in stroke patients. Here we show that PTX3 is expressed in neurons and glia in response to cerebral ischemia, and that the proinflammatory cytokine interleukin-1 (IL-1) is a key driver of PTX3 expression in the brain after experimental stroke. Gene deletion of PTX3 had no significant effects on acute ischemic brain injury. In contrast, the absence of PTX3 strongly compromised blood–brain barrier integrity and resolution of brain edema during recovery after ischemic injury. Compromised resolution of brain edema in PTX3-deficient mice was associated with impaired glial scar formation and alterations in scar-associated extracellular matrix production. Our results suggest that PTX3 expression induced by proinflammatory signals after ischemic brain injury is a critical effector of edema resolution and glial scar formation. This highlights the potential role for inflammatory molecules in brain recovery after injury and identifies APPs, in particular PTX3, as important targets in ischemic stroke and possibly other brain inflammatory disorders.     

Interleukin-1 and stroke: biomarker, harbinger of damage, and therapeutic target

Inflammation is established as a contributor to cerebrovascular disease. Risk factors for stroke include many conditions associated with chronic or acute inflammation, and inflammatory changes in the brain after cerebrovascular events contribute to outcome in experimental studies, with growing evidence from clinical research. The brain is extremely susceptible to inflammatory challenge, but resident glia, endothelial cells and neurones can all mount a pronounced inflammatory response to infection or injury. Recent discoveries highlight the importance of peripherally-derived immune cells and inflammatory molecules in various central nervous system disorders, including stroke. The inflammatory cytokine, interleukin-1 (IL-1), plays a pivotal role in both local and systemic inflammation, and is a key driver of peripheral and central immune responses to infection or injury. Inhibition of IL-1 has beneficial effects in a variety of experimental paradigms of acute brain injury and is a promising clinical target in stroke. We propose that blockade of IL-1 could be therapeutically useful in several diseases which are risk factors for stroke, and there is already considerable pre-clinical and clinical evidence that inhibition of IL-1 by IL-1 receptor antagonist may be valuable in the management of acute stroke.     

CC and CXC chemokines are pivotal mediators of cerebral injury in ischaemic stroke

The definition of ischaemic stroke has been recently updated as an acute episode of neurological dysfunction caused by focal brain, spinal cord, or retinal ischaemia in the presence of a cerebral infarction. This "tissular" definition has highlighted the importance of pathophysiological processes underlying cerebral damage. In particular, post- ischaemic inflammation in the brain and in the blood stream could influence crucial steps of the tissue injury/repair cascade. CC and CXC chemokines orchestrate the inflammatory response in atherosclerotic plaque vulnerability and cerebral infarction. These molecules exert their activities through the binding to selective transmembrane receptors. CC and CXC chemokines modulate crucial processes (such as inflammatory cell recruitment and activation, neuronal survival, neoangiogenesis). On the other hand, CXC chemokines could also modulate stem cell homing, thus favouring tissue repair. Given this evidence, both CC and CXC chemokines could represent promising therapeutic targets in primary and secondary prevention of ischaemic stroke. Only preliminary studies have been performed investigating treatments with selective chemokine agonists/antagonists. In this review, we will update evidence on the role and the potential therapeutic strategies targeting CC and CXC chemokines in the pathophysiology of ischaemic stroke.     

Is Immunomodulation a Principal Mechanism Underlying How Cell-Based Therapies Enhance Stroke Recovery?

Inflammation within the brain and in peripheral tissues contributes to brain injury following ischemic stroke. Therapeutic modulation of the inflammatory response has been actively pursued as a novel stroke treatment approach for decades, without success. In recent years, extensive studies support the high potential for cell-based therapies to become a new treatment modality for stroke and other neurological disorders. In this review, we explore different types of cellular therapies and discuss how they modulate central and peripheral inflammatory processes after stroke. Apart from identifying potential targets for cell therapy, we also discuss paracrine and immunomodulatory mechanisms of cell therapy.     

Role of inflammation and cellular stress in brain injury and central nervous system diseases

The importance of cytokines and the complement system in the propagation and maintenance of the brain inflammatory response to injury are emphasised. Much data supports the case that ischemia and trauma elicit an inflammatory response in the injured brain. This inflammatory response consists of mediators (cytokines, complement activation, chemokines and adhesion molecules) followed by cells (neutrophils early after the onset of brain injury and then a later monocyte infiltration). De novo up-regulation of pro-inflammatory cytokines, chemokines and endothelial-leukocyte adhesion molecules occurs soon following focal ischemia and trauma and at a time when the tissue injury is evolving. The significance of this brain inflammatory response and its contribution to brain injury is now better understood. In this review, we discuss the role of TNFα and IL-1β in traumatic and brain injury and associated inflammation, and the co-operative actions of the complement system, chemokines and adhesion molecules in this process. Celluar stress and cellular stress signalling is key to the neurodegenerative process in brain injury. Therefore, we also address novel approaches to target cytokines and reduce the brain inflammatory response, and thus brain injury, in stroke and neurotrauma. The mitogen-activated protein kinase (MAPK), p38, has been linked to inflammatory cytokine production and cell death following cellular stress. Stroke-induced p38 enzyme activation in the brain has been demonstrated, and treatment with p38 MAPK inhibitors can provide a significant reduction in infarct size, neurological deficits and increased inflammatory cytokines/proteins expression produced by focal stroke. p38 MAPK inhibition can also provide direct protection of cultured brain tissue to in vitro ischemia. This robust neuroprotection that can be produced by inhibition of p38 MAPK signalling emphasizes a significant opportunity for targeting MAPK pathways CNS injury/disease. Many examples of the roles of inflammation, cellular oxidative stress and MAPK signalling in Psychiatric and Neurodegenerative Diseases are also provided. As a whole, the available data suggests that inflammation, cellular stress and p38 MAPK signalling are important in nervous disease pathologies and that inhibition of cellular stress signalling should be considered for improving outcome in many CNS diseases.     

The spleen contributes to stroke-induced neurodegeneration

Stroke, a cerebrovascular injury, is the leading cause of disability and third leading cause of death in the world. Recent reports indicate that inhibiting the inflammatory response to stroke enhances neurosurvival and limits expansion of the infarction. The immune response that is initiated in the spleen has been linked to the systemic inflammatory response to stroke, contributing to neurodegeneration. Here we show that removal of the spleen significantly reduces neurodegeneration after ischemic insult. Rats splenectomized 2 weeks before permanent middle cerebral artery occlusion had a >80% decrease in infarction volume in the brain compared with those rats that were subjected to the stroke surgery alone. Splenectomy also resulted in decreased numbers of activated microglia, macrophages, and neutrophils present in the brain tissue. Our results demonstrate that the peripheral immune response as mediated by the spleen is a major contributor to the inflammation that enhances neurodegeneration after stroke.     

Reduced cerebral injury in CRH-R1 deficient mice after focal ischemia: a potential link to microglia and atrocytes that express CRH-R1.

Corticotropin releasing hormone (CRH) and its family of related peptides are involved in regulating physiologic responses to multiple stressors, including stroke. Although CRH has been implicated in the exacerbation of injury after stroke, the mechanism remains unclear. After ischemia, both excitotoxic damage and inflammation contribute to the pathology of stroke. CRH is known to potentiate excitotoxic damage in the brain and has been shown to modulate inflammatory responses in the periphery. Here the present authors examine the relative contribution of the two known CRH receptors, CRH-R1 and CRH-R2, to ischemic injury using CRH receptor knockout mice. These results implicate CRH-R1 as the primary mediator of ischemic injury in this mouse model of stroke. In addition, the authors examine a potential role for CRH in inflammatory injury after stroke by identifying functional CRH receptors on astrocytes and microglia, which are cells that are known to be involved in brain inflammation. By single cell PCR, the authors show that microglia and astrocytes express mRNA for both CRH-R1 and CRH-R2. However, CRH-R1 is the primary mediator of cAMP accumulation in response to CRH peptides in these cells. The authors suggest that astrocytes and microglia are cellular targets of CRH, which could serve as a link between CRH and inflammatory responses in ischemic injury via CRH-R1.     

Models That Matter: White Matter Stroke Models

Stroke is a devastating neurological disease with limited functional recovery. Stroke affects all cellular elements of the brain and impacts areas traditionally classified as both gray matter and white matter. In fact, stroke in subcortical white matter regions of the brain accounts for approximately 30% of all stroke subtypes, and white matter injury is a component of most classes of stroke damage. However, most basic scientific information in stroke cell death and neural repair relates principally to neuronal cell death and repair. Despite an emerging biological understanding of white matter development, adult function, and reorganization in inflammatory diseases, such as multiple sclerosis, little is known of the specific molecular and cellular events in white matter ischemia. This limitation stems in part from the difficulty in generating animal models of white matter stroke. This review will discuss recent progress in studies of animal models of white matter stroke, and the emerging principles of cell death and repair in oligodendrocytes, axons, and astrocytes in white matter ischemic injury.     

Neurological consequences of systemic inflammation in the premature neonate

Despite substantial progress in neonatal care over the past two decades leading to improved survival of extremely premature infants, extreme prematurity continues to be associated with long term neurodevel-opmental impairments. Cerebral white matter injury is the predominant form of insult in preterm brain leading to adverse neurological consequences. Such brain injury pattern and unfavorable neurologic se-quelae is commonly encountered in premature infants exposed to systemic inflammatory states such as clinical or culture proven sepsis with or without evidence of meningitis, prolonged mechanical ventilation, bronchopulmonary dysplasia, necrotizing enterocolitis and chorioamnionitis. Underlying mechanisms may include cytokine mediated processes without direct entry of pathogens into the brain, developmental differences in immune response and complex neurovascular barrier system that play a critical role in regu-lating the cerebral response to various systemic inflammatory insults in premature infants. Understanding of these pathologic mechanisms and clinical correlates of such injury based on serum biomarkers or brain imaging findings on magnetic resonance imaging will pave way for future research and translational thera-peutic opportunities for the developing brain.     

Microglial activation in stroke: Therapeutic targets

Microglial activation is an early response to brain ischemia and many other Stressors. Microglia continuously monitor and respond to changes in brain homeostasis and to specific signaling molecules expressed or released by neighboring cells. These signaling molecules, including ATP, glutamate, cytokines, prostaglandins, zinc, reactive oxygen species, and HSP60, may induce microglial proliferation and migration to the sites of injury. They also induce a nonspecific innate immune response that may exacerbate acute ischemic injury. This innate immune response includes release of reactive oxygen species, cytokines, and proteases. Microglial activation requires hours to days to fully develop, and thus presents a target for therapeutic intervention with a much longer window of opportunity than acute neuroprotection. Effective agents are now available for blocking both microglial receptor activation and the microglia effector responses that drive the inflammatory response after stroke. Effective agents are also available for targeting the signal transduction mechanisms linking these events. However, the innate immune response can have beneficial as well deleterious effects on outcome after stoke, and a challenge will be to find ways to selectively suppress the deleterious effects of microglial activation after stroke without compromising neurovascular repair and remodeling.     

The vascular effects of infection in Pediatric Stroke (VIPS) Study

Understanding the vascular injury pathway is crucial to developing rational strategies for secondary stroke prevention in children. The multicenter Vascular Effects of Infection in Pediatric Stroke (VIPS) cohort study will test the hypotheses that (1) infection can lead to childhood arterial ischemic stroke by causing vascular injury and (2) resultant arteriopathy and inflammatory markers predict recurrent stroke. The authors are prospectively enrolling 480 children (aged 1 month through 18 years) with arterial ischemic stroke and collecting extensive infectious histories, blood and serum samples (and cerebrospinal fluid, when clinically obtained), and standardized brain and cerebrovascular imaging studies. Laboratory assays include serologies (acute and convalescent) and molecular assays for herpesviruses and levels of inflammatory markers. Participants are followed prospectively for recurrent ischemic events (minimum of 1 year). The analyses will measure association between markers of infection and cerebral arteriopathy and will assess whether cerebral arteriopathy and inflammatory markers predict recurrent stroke.     

缺血性卒中后的神经炎性反应

神经炎性反应在缺血性卒中后的病理损伤中起重要作用。越来越多的证据表明,神经炎性反应是一把"双刃剑",在加重急性期卒中脑损伤的同时,亦可促进卒中后的神经修复。本文阐述了缺血性卒中后神经炎性反应的关键因素,如炎性细胞、炎性介质和黏附分子的变化,探讨了其可能的神经损伤及神经保护作用;同时,对缺血性卒中后神经炎性反应相关研究的进展及前景进行了综述。     

Inflammation and Stroke: An Overview

The immune response to acute cerebral ischemia is a major factor in stroke pathobiology and outcome. While the immune response starts locally in occluded and hypoperfused vessels and the ischemic brain parenchyma, inflammatory mediators generated in situ propagate through the organism as a whole. This “spillover” leads to a systemic inflammatory response first, followed by immunosuppression aimed at dampening the potentially harmful proinflammatory milieu. In this overview we will outline the inflammatory cascade from its starting point in the vasculature of the ischemic brain to the systemic immune response elicited by brain ischemia. Potential immunomodulatory therapeutic approaches, including preconditioning and immune cell therapy will also be discussed.     

Review: The long‐term consequences of microglial activation following acute traumatic brain injury

The brain is vulnerable to a number of acute insults, with traumatic brain injury being among the commonest. Neuroinflammation is a common response to acute injury and microglial activation is a key component of the inflammatory response. In the acute and subacute phase it is likely that this response is protective and forms an important part of the normal tissue reaction. However, there is considerable literature demonstrating an association between acute traumatic brain injury to the brain and subsequent cognitive decline. This article will review the epidemiological literature relating to both single and repetitive head injury. It will focus on the neuropathological features associated with long‐term complications of a single blunt force head injury, repetitive head injury and blast head injury, with particular reference to chronic traumatic encephalopathy, including dementia pugilistica. Neuroinflammation has been postulated as a key mechanism linking acute traumatic brain injury with subsequent neurodegenerative disease, and this review will consider the response to injury in the acute phase and how this may be detrimental in the longer term, and discuss potential genetic factors which may influence this cellular response. Finally, this article will consider future directions for research and potential future therapies.     

Interferon-gamma but not TNF alpha promotes neuronal differentiation and neurite outgrowth of murine adult neural stem cells

Neural trauma, such as traumatic brain injury or stroke, results in a vigorous inflammatory response at and near the site of injury, with cytokine production by endogenous glial cells and invading immune cells. Little is known of the effect that these cytokines have on neural stem cell function. Here we examine the effects of two inflammatory cytokines, interferon-gamma (IFN gamma) and tumour necrosis factor-alpha (TNFalpha), on adult neural stem cells. Neural stem cells grown in the presence of either cytokine failed to generate neurospheres. Cytotoxicity assays showed that TNF alpha but not IFN gamma was toxic to the neural stem cells under proliferative conditions. Under differentiating conditions, neither cytokine was toxic; however, IFN gamma enhanced neuronal differentiation, rapidly increasing beta III-tubulin positive cell numbers 3-4 fold and inhibiting astrocyte generation. Furthermore, neurite outgrowth and the number of neurites per neuron was enhanced in cells differentiated in the presence of IFN gamma. Therefore, both inflammatory cytokines examined have substantial, but different effects on neural stem cell function and suggests that regulation of the inflammatory environment following brain injury may influence the ability of neural stem cells to repair the damage.     

The flavonoid fisetin attenuates postischemic immune cell infiltration, activation and infarct size after transient cerebral middle artery occlusion in mice

The development of the brain tissue damage in ischemic stroke is composed of an immediate component followed by an inflammatory response with secondary tissue damage after reperfusion. Fisetin, a flavonoid, has multiple biological effects, including neuroprotective and antiinflammatory properties. We analyzed the effects of fisetin on infarct size and the inflammatory response in a mouse model of stroke, temporary middle cerebral artery occlusion, and on the activation of immune cells, murine primary and N9 microglial and Raw264.7 macrophage cells and human macrophages, in an in vitro model of inflammatory immune cell activation by lipopolysaccharide (LPS). Fisetin not only protected brain tissue against ischemic reperfusion injury when given before ischemia but also when applied 3 hours after ischemia. Fisetin also prominently inhibited the infiltration of macrophages and dendritic cells into the ischemic hemisphere and suppressed the intracerebral immune cell activation as measured by intracellular tumor necrosis factor α (TNFα) production. Fisetin also inhibited LPS-induced TNFα production and neurotoxicity of macrophages and microglia in vitro by suppressing nuclear factor κB activation and JNK/Jun phosphorylation. Our findings strongly suggest that the fisetin-mediated inhibition of the inflammatory response after stroke is part of the mechanism through which fisetin is neuroprotective in cerebral ischemia.     

Astrocytes: Integrative Regulators of Neuroinflammation in Stroke and Other Neurological Diseases

Astrocytes regulate neuroinflammatory responses after stroke and in other neurological diseases. Although not all astrocytic responses reduce inflammation, their predominant function is to protect the brain by driving the system back to homeostasis after injury. They receive multidimensional signals within the central nervous system and between the brain and the systemic circulation. Processing this information allows astrocytes to regulate synapse formation and maintenance, cerebral blood flow, and blood–brain barrier integrity. Similarly, in response to stroke and other central nervous system disorders, astrocytes detect and integrate signals of neuronal damage and inflammation to regulate the neuroinflammatory response. Two direct regulatory mechanisms in the astrocyte arsenal are the ability to form both physical and molecular barriers that seal the injury site and localize the neuroinflammatory response. Astrocytes also indirectly regulate the inflammatory response by affecting neuronal health during the acute injury and axonal regrowth. This ability to regulate the location and degree of neuroinflammation after injury, combined with the long time course of neuroinflammation, makes astrocytic signaling pathways promising targets for therapies.