Long term immunologic consequences of experimental stroke and mucosal tolerance

Molecular Mechanisms of Neuroimmune Crosstalk in the Pathogenesis of Stroke

Stroke disrupts the homeostatic balance within the brain and is associated with a significant accumulation of necrotic cellular debris, fluid, and peripheral immune cells in the central nervous system (CNS). Additionally, cells, antigens, and other factors exit the brain into the periphery via damaged blood–brain barrier cells, glymphatic transport mechanisms, and lymphatic vessels, which dramatically influence the systemic immune response and lead to complex neuroimmune communication. As a result, the immunological response after stroke is a highly dynamic event that involves communication between multiple organ systems and cell types, with significant consequences on not only the initial stroke tissue injury but long-term recovery in the CNS. In this review, we discuss the complex immunological and physiological interactions that occur after stroke with a focus on how the peripheral immune system and CNS communicate to regulate post-stroke brain homeostasis. First, we discuss the post-stroke immune cascade across different contexts as well as homeostatic regulation within the brain. Then, we focus on the lymphatic vessels surrounding the brain and their ability to coordinate both immune response and fluid homeostasis within the brain after stroke. Finally, we discuss how therapeutic manipulation of peripheral systems may provide new mechanisms to treat stroke injury.

Stroke and immunotherapy: Potential mechanisms and its implications as immune-therapeutics

Ischemia or brain injuries are mostly associated with emergency admissions and huge mortality rates. Stroke is a fatal cerebrovascular malady and second top root of disability and death in both developing and developed countries with a projected rise of 24.9% (from 2010) by 2030. It's the most frequent cause of morbidities and systemic permanent morbidities due to its multi-organ systemic pathology. Brain edema or active immune response cause disturbed or abnormal systemic affects causing inflammatory damage leading to secondary infection and secondary immune response which leads to activation like pneumonia or urine tract infections. There are a variety of post stroke treatments available which claims their usefulness in reducing or inhibiting post stroke and recurrent stroke damage followed by heavy inflammatory actions. Stroke does change the quality of life and also ensures daily chronic rapid neurodegeneration and cognitive decline. The only approved therapies for stroke are alteplase and thrombectomy which is associated with adverse outcomes and are not a total cure for ischemic stroke. Stroke and immune response are reciprocal to the pathology and time of event and it progresses till untreated. The immune reaction during ischemia opens new doors for advanced targeted therapeutics. Nowadays stem cell therapy has shown better results in stroke-prone individuals. Few monoclonal antibodies like natalizumab have shown great impact on pre-clinical and clinical stroke trial studies. In this current review, we have explored an immunology of stroke, current therapeutic scenario and future potential targets as immunotherapeutic agents in stroke therapeutics.

The immune response of T cells and therapeutic targets related to regulating the levels of T helper cells after ischaemic stroke

Through considerable effort in research and clinical studies, the immune system has been identified as a participant in the onset and progression of brain injury after ischaemic stroke. Due to the involvement of all types of immune cells, the roles of the immune system in stroke pathology and associated effects are complicated. Past research concentrated on the functions of monocytes and neutrophils in the pathogenesis of ischaemic stroke and tried to demonstrate the mechanisms of tissue injury and protection involving these immune cells. Within the past several years, an increasing number of studies have elucidated the vital functions of T cells in the innate and adaptive immune responses in both the acute and chronic phases of ischaemic stroke. Recently, the phenotypes of T cells with proinflammatory or anti-inflammatory function have been demonstrated in detail. T cells with distinctive phenotypes can also influence cerebral inflammation through various pathways, such as regulating the immune response, interacting with brain-resident immune cells and modulating neurogenesis and angiogenesis during different phases following stroke. In view of the limited treatment options available following stroke other than tissue plasminogen activator therapy, understanding the function of immune responses, especially T cell responses, in the post-stroke recovery period can provide a new therapeutic direction. Here, we discuss the different functions and temporal evolution of T cells with different phenotypes during the acute and chronic phases of ischaemic stroke. We suggest that modulating the balance between the proinflammatory and anti-inflammatory functions of T cells with distinct phenotypes may become a potential therapeutic approach that reduces the mortality and improves the functional outcomes and prognosis of patients suffering from ischaemic stroke.

Adaptive Immunity Regulation and Cerebral Ischemia

Stroke is a disease that occurs due to a sudden interruption of the blood supply to the brain. It is a leading cause of death and disability worldwide. It is well-known that the immune system drives brain injury following an episode of ischemic stroke. The innate system and the adaptive system play distinct but synergistic roles following ischemia. The innate system can be activated by damage-associated molecular patterns (DAMPs), which are released from cells in the ischemic region. Damaged cells also release various other mediators that serve to increase inflammation and compromise the integrity of the blood–brain barrier (BBB). Within 24 h of an ischemic insult, the adaptive immune system is activated. This involves T cell and B cell-mediated inflammatory and humoral effects. These cells also stimulate the release of various interleukins and cytokines, which can modulate the inflammatory response. The adaptive immune system has been shown to contribute to a state of immunodepression following an ischemic episode, and this can increase the risk of infections. However, this phenomenon is equally important in preventing autoimmunity of the body to brain antigens that are released into the peripheral system as a result of BBB compromise. In this review, we highlight the key components of the adaptive immune system that are activated following cerebral ischemia.

Regulatory T cell is critical for interleukin-33-mediated neuroprotection against stroke

Interleukin-33 (IL-33) is known to activate the regulatory T lymphocytes (Tregs), which are negatively correlated with brain damage after ischemic stroke. In this study, we aimed to investigate the role of Tregs in IL-33-mediated neuroprotection and elucidate the underlying mechanisms. In vivo, male C57BL/6 N mice were subjected to 60 min of transient middle cerebral artery occlusion (tMCAO), followed by daily administration of vehicle or IL-33 immediately after injury. Tregs were depleted by intraperitoneal administration of anti-CD25 antibody (anti-CD25Ab). Behavioral changes, brain edema, neuronal injury, Treg percentages, and cytokine expression levels were investigated in each group. In vitro experiments, primary mouse neuronal cells were subjected to oxygen-glucose deprivation (OGD) for 3 h. Vehicle- or drug-conditioned Tregs were applied to the neurons at the time of induction of hypoxia. Neuronal apoptosis and cytokine expression were measured in each group. The results indicate that intraperitoneal administration of anti-CD25Ab reduced CD4 + CD25 + Foxp3+ Tregs, increased infarct volume, enhanced stroke-induced cell death, and decreased sensorimotor functions. Notably, IL-33 increased CD4 + CD25 + Foxp3+ Tregs in the spleen and brain. However, blockading ST2 attenuated these effects of IL-33. The supernatant of the IL-33-treated Treg culture reduced neuronal apoptosis and elevated the production of the Treg cytokines IL-10, IL-35, and transforming growth factor-β (TGF-β). Anti-CD25Ab abrogated the neuroprotective effect of IL-33. Mechanistically, the neuroprotective effects of IL-33 were associated with reduction in apoptosis-related proteins and production of Tregs related cytokines. Overall, these findings showed that IL-33 afforded neuroprotection against ischemic brain injury by enhancing ST2-dependent regulatory T-cell expansion and activation via a mechanism involving anti-apoptosis proteins and cytokines, representing a promising immune modulatory target for the treatment of stroke.

Protective and Regenerative Roles of T Cells in Central Nervous System Disorders

Pathogenic mechanisms of T cells in several central nervous system (CNS) disorders are well-established. However, more recent studies have uncovered compelling beneficial roles of T cells in neurological diseases, ranging from tissue protection to regeneration. These divergent functions arise due to the diversity of T cell subsets, particularly CD4⁺ T cells. Here, we review the beneficial impact of T cell subsets in a range of neuroinflammatory and neurodegenerative diseases including multiple sclerosis, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, stroke, and CNS trauma. Both T cell-secreted mediators and direct cell contact-dependent mechanisms deliver neuroprotective, neuroregenerative and immunomodulatory signals in these settings. Understanding the molecular details of these beneficial T cell mechanisms will provide novel targets for therapeutic exploitation that can be applied to a range of neurological disorders.

Immunomodulatory Therapeutic Strategies in Stroke

The role of immunity in all stages of stroke is increasingly being recognized, from the pathogenesis of risk factors to tissue repair, leading to the investigation of a range of immunomodulatory therapies. In the acute phase of stroke, proposed therapies include drugs targeting pro-inflammatory cytokines, matrix metalloproteinases, and leukocyte infiltration, with a key objective to reduce initial brain cell toxicity. Systemically, the early stages of stroke are also characterized by stroke-induced immunosuppression, where downregulation of host defences predisposes patients to infection. Therefore, strategies to modulate innate immunity post-stroke have garnered greater attention. A complementary objective is to reduce longer-term sequelae by focusing on adaptive immunity. Following stroke onset, the integrity of the blood–brain barrier is compromised, exposing central nervous system (CNS) antigens to systemic adaptive immune recognition, potentially inducing autoimmunity. Some pre-clinical efforts have been made to tolerize the immune system to CNS antigens pre-stroke. Separately, immune cell populations that exhibit a regulatory phenotype (T- and B- regulatory cells) have been shown to ameliorate post-stroke inflammation and contribute to tissue repair. Cell-based therapies, established in oncology and transplantation, could become a strategy to treat the acute and chronic stages of stroke. Furthermore, a role for the gut microbiota in ischaemic injury has received attention. Finally, the immune system may play a role in remote ischaemic preconditioning-mediated neuroprotection against stroke. The development of stroke therapies involving organs distant to the infarct site, therefore, should not be overlooked. This review will discuss the immune mechanisms of various therapeutic strategies, surveying published data and discussing more theoretical mechanisms of action that have yet to be exploited.

Emerging Role of Immunity in Cerebral Small Vessel Disease

Cerebral small vessel disease (CSVD) is one of the main causes of vascular dementia in older individuals. Apart from risk containment, efforts to prevent or treat CSVD are ineffective due to the unknown pathogenesis of the disease. CSVD, a subtype of stroke, is characterized by recurrent strokes and neurodegeneration. Blood-brain barrier (BBB) impairment, chronic inflammatory responses, and leukocyte infiltration are classical pathological features of CSVD. Understanding how BBB disruption instigates inflammatory and degenerative processes may be informative for CSVD therapy. Antigens derived from the brain are found in the peripheral blood of lacunar stroke patients, and antibodies and sensitized T cells against brain antigens are also detected in patients with leukoaraiosis. These findings suggest that antigen-specific immune responses could occur in CSVD. This review describes the neurovascular unit features of CSVD, the immune responses to specific neuronal and glial processes that may be involved in a distinct mechanism of CSVD, and the current evidence of the association between mechanisms of inflammation and interventions in CSVD. We suggest that autoimmune activity should be assessed in future studies; this knowledge would benefit the development of effective therapeutic interventions in CSVD.

The role of carbon monoxide and heme oxygenase in the prevention of sickle cell disease vaso-occlusive crises

Sickle Cell Disease (SCD) is a painful, lifelong hemoglobinopathy inherited as a missense point mutation in the hemoglobin (Hb) beta-globin gene. This disease has significant impact on quality of life and mortality, thus a substantial medical need exists to reduce the vaso-occlusive crises which underlie the pathophysiology of the disease. The concept that a gaseous molecule may exert biological function has been well known for over one hundred years. Carbon monoxide (CO), although studied in SCD for over 50 years, has recently emerged as a powerful cytoprotective biological response modifier capable of regulating a host of physiologic and therapeutic processes that, at low concentrations, exerts key physiological functions in various models of tissue inflammation and injury. CO is physiologically generated by the metabolism of heme by the heme oxygenase enzymes and is measurable in blood. A substantial amount of preclinical and clinical data with CO have been generated, which provide compelling support for CO as a potential therapeutic in a number of pathological conditions. Data underlying the therapeutic mechanisms of CO, including in SCD, have been generated by a plethora of in vitro and preclinical studies including multiple SCD mouse models. These data show CO to have key signaling impacts on a host of metallo-enzymes as well as key modulating genes that in sum, result in significant anti-inflammatory, anti-oxidant and anti-apoptotic effects as well as vasodilation and anti-adhesion of cells to the endothelium resulting in preservation of vascular flow. CO may also have a role as an anti-polymerization HbS agent. In addition, considerable scientific data in the non-SCD literature provide evidence for a beneficial impact of CO on cerebrovascular complications, suggesting that in SCD, CO could potentially limit these highly problematic neurologic outcomes. Research is needed and hopefully forthcoming, to carefully elucidate the safety and benefits of this potential therapy across the age spectrum of patients impacted by the host of pathophysiological complications of this devastating disease.

A Crucial Role of CXCL14 for Promoting Regulatory T Cells Activation in Stroke

Inflammatory processes have a detrimental role in the pathophysiology of ischemic stroke. However, little is known about the endogenous anti-inflammatory mechanisms in ischemic brain. Here, we identify CXCL14 as a critical mediator of these mechanisms. CXCL14 levels were upregulated in the ischemic brains of humans and rodents. Moreover, hypoxia inducible factor-1α (HIF-1α) drives hypoxia- or cerebral ischemia (CI)-dependent CXCL14 expression via directly binding to the CXCL14 promoter. Depletion of CXCL14 inhibited the accumulation of immature dendritic cells (iDC) or regulatory T cells (Treg) and increased the infarct volume, whereas the supplementation of CXCL14 had the opposite effects. CXCL14 promoted the adhesion, migration, and homing of circulating CD11c⁺ iDC to the ischemic tissue via the upregulation of the cellular prion protein (PrP^C), PECAM-1, and MMPs. The accumulation of Treg in ischemic areas of the brain was mediated through a cooperative effect of CXCL14 and iDC-secreted IL-2-induced Treg differentiation. Interestingly, CXCL14 largely promoted IL-2-induced Treg differentiation. These findings indicate that CXCL14 is a critical immunomodulator involved in the stroke-induced inflammatory reaction. Passive CXCL14 supplementation provides a tractable path for clinical translation in the improvement of stroke-induced neuroinflammation.

Stroke induces specific alteration of T memory compartment controlling auto-reactive CNS antigen-specific T cell responses

Whether and when auto-reactivity after stroke occurs is still a matter of debate. By using overlapping 15mer peptide pools consisting of myelin basic protein (MBP) and myelin oligodendrocyte glycoprotein (MOG) we show increased frequencies of immunodominant MOG- and MBP T cell responses in acute ischemic stroke which were associated with reduced frequencies of naïve T cells as well as CD8+ TEMRA cells. Auto-reactive CNS antigen-specific T cells responses as well as alterations of T cell subpopulations normalized in long-term follow up after stroke. Our findings suggest that stroke-induced immunodepression might function as an adaptive mechanism in order to inhibit harmful and long-lasting CNS antigen-specific immune responses.

Blocking stroke-induced immunodeficiency increases CNS antigen-specific autoreactivity but does not worsen functional outcome after experimental stroke

Stroke-induced immunodepression (SIDS) is an essential cause of poststroke infections. Pharmacological inhibition of SIDS appears promising in preventing life-threatening infections in stroke patients. However, SIDS might represent an adaptive mechanism preventing autoreactive immune responses after stroke. To address this, we used myelin oligodendrocyte glycoprotein (MOG) T-cell receptor transgenic (2D2) mice where >80% of peripheral CD4(+) T cells express a functional receptor for MOG. We investigated in a murine model of middle cerebral artery occlusion the effect of blocking SIDS by inhibiting body's main stress axes, the sympathetic nervous system (SNS) with propranolol and the hypothalamic-pituitary-adrenal axis (HPA) with mifepristone. Blockade of both stress axes robustly reduced infarct volumes, decreased infection rate, and increased long-term survival of 2D2 and C57BL/6J wild-type mice. Despite these protective effects, blockade of SIDS increased CNS antigen-specific Type1 T helper cell (Th1) responses in the brains of 2D2 mice 14 d after middle cerebral artery occlusion. One month after experimental stroke, 2D2 mice developed signs of polyradiculitis, which were diminished by SIDS blockade. Adoptive transfer of CD4(+) T cells, isolated from 2D2 mice, into lymphocyte-deficient Rag-1KO mice did not reveal differences between SIDS blockade and vehicle treatment in functional long-term outcome after stroke. In conclusion, inhibiting SIDS by pharmacological blockade of body's stress axes increases autoreactive CNS antigen-specific T-cell responses in the brain but does not worsen functional long-term outcome after experimental stroke, even in a mouse model where CNS antigen-specific autoreactive T-cell responses are boosted.

Promiscuity of autoimmune responses to MBP after stroke

In this study we examined Th1 and Th17 immune responses to rat myelin basic protein (MBP), bovine MBP, human MBP, MBP 68-86, MBP 63-81 and ovalbumin in Lewis rats to determine which MBP antigen is recognized following ischemic brain injury. Responses were compared to animals immunized to rat MBP. Data show that immune responses following immunization with rat MBP are promiscuous with cross reaction to MBP from other species. After stroke, few animals develop Th1 or Th17 responses to MBP, but when those responses occur, especially Th1 responses to rat MBP in the brain, they are predictive of worse stroke outcome.

The ischemic environment drives microglia and macrophage function

Cells of myeloid origin, such as microglia and macrophages, act at the crossroads of several inflammatory mechanisms during pathophysiology. Besides pro-inflammatory activity (M1 polarization), myeloid cells acquire protective functions (M2) and participate in the neuroprotective innate mechanisms after brain injury. Experimental research is making considerable efforts to understand the rules that regulate the balance between toxic and protective brain innate immunity. Environmental changes affect microglia/macrophage functions. Hypoxia can affect myeloid cell distribution, activity, and phenotype. With their intrinsic differences, microglia and macrophages respond differently to hypoxia, the former depending on ATP to activate and the latter switching to anaerobic metabolism and adapting to hypoxia. Myeloid cell functions include homeostasis control, damage-sensing activity, chemotaxis, and phagocytosis, all distinctive features of these cells. Specific markers and morphologies enable to recognize each functional state. To ensure homeostasis and activate when needed, microglia/macrophage physiology is finely tuned. Microglia are controlled by several neuron-derived components, including contact-dependent inhibitory signals and soluble molecules. Changes in this control can cause chronic activation or priming with specific functional consequences. Strategies, such as stem cell treatment, may enhance microglia protective polarization. This review presents data from the literature that has greatly advanced our understanding of myeloid cell action in brain injury. We discuss the selective responses of microglia and macrophages to hypoxia after stroke and review relevant markers with the aim of defining the different subpopulations of myeloid cells that are recruited to the injured site. We also cover the functional consequences of chronically active microglia and review pivotal works on microglia regulation that offer new therapeutic possibilities for acute brain injury.

Viral immune surveillance: Toward a TH17/TH9 gate to the central nervous system

Viral cellular immune surveillance is a dynamic and fluid system that is driven by finely regulated cellular processes including cytokines and other factors locally in the microenvironment and systemically throughout the body. It is questionable as to what extent the central nervous system (CNS) is an immune-privileged organ protected by the blood-brain barrier (BBB). Recent evidence suggests converging pathways through which viral infection, and its associated immune surveillance processes, may alter the integrity of the blood-brain barrier, and lead to inflammation, swelling of the brain parenchyma and associated neurological syndromes. Here, we expand upon the recent "gateway theory", by which viral infection and other immune activation states may disrupt the specialized tight junctions of the BBB endothelium making it permeable to immune cells and factors. The model we outline here builds upon the proposition that this process may actually be initiated by cytokines of the IL-17 family, and recognizing the intimate balance between TH17 and TH9 cytokine profiles systemically. We argue that immune surveillance events, in response to viruses such as the Human Immunodeficiency Virus (HIV), cause a TH17/TH9 induced gateway through blood brain barrier, and thus lead to characteristic neuroimmune pathology. It is possible and even probable that the novel TH17/TH9 induced gateway, which we describe here, opens as a consequence of any state of immune activation and sustained chronic inflammation, whether associated with viral infection or any other cause of peripheral or central neuroinflammation. This view could lead to new, timely and critical patient-centered therapies for patients with neuroimmune pathologies across a variety of etiologies.

ABBREVIATIONS

BBB - blood brain barrier, BDV - Borna disease virus, CARD - caspase activation and recruitment domains, CD - clusters of differentiation, CNS - central nervous system, DAMP - damage-associated molecular patterns, DENV - Dengue virus, EBOV - Ebola virus, ESCRT - endosomal sorting complex required for transport-I, HepC - Hepatitis C virus, HIV - human immunodeficiency virus, IFN - interferon, ILn - interleukin-n, IRF-n - interferon regulatory factor-n, MAVS - mitochondrial antiviral-signaling, MBGV - Marburg virus, M-CSF - macrophage colony-stimulating factor, MCP-1 - monocyte chemotactic protein 1 (aka CCL2), MHC - major histocompatibility complex, MIP-α β - macrophage inflammatory protein-1 α β (aka CCL3 & CCL4), MIF - macrophage migration inhibitory factor, NVE - Nipah virus encephalitis, NK - natural killer cell, NLR - NLR, NOD - like receptor, NOD - nucleotide oligomerization domain, PAMP - pathogen-associated molecular patterns, PtdIns - phosphoinositides, PV - Poliovirus, RIG-I - retinoic acid-inducible gene I, RIP - Receptor-interacting protein (RIP) kinase, RLR - RIG-I-like receptor, sICAM1 - soluble intracellular adhesion molecule 1, STAT-3 - signal tranducer and activator of transcription-3, sVCAM1 - soluble vascular cell adhesion molecule 1, TANK - TRAF family member-associated NF- . B activator, TBK1 - TANK-binding kinase 1, TLR - Toll-like receptor, TNF - tumor necrosis factor, TNFR - TNF receptor, TNFRSF21 - tumor necrosis factor receptor superfamily member 21, TRADD TNFR-SF1A - associated via death domain, TRAF TNFR - associated factor, Tregs - regulatory T cellsubpopulation (CD4/8+CD25+FoxP3+), VHF - viral hemorrhagic fever.

Antigen-specific immune reactions to ischemic stroke

Brain proteins are detected in the cerebrospinal fluid (CSF) and blood of stroke patients and their concentration is related to the extent of brain damage. Antibodies against brain antigens develop after stroke, suggesting a humoral immune response to the brain injury. Furthermore, induced immune tolerance is beneficial in animal models of cerebral ischemia. The presence of circulating T cells sensitized against brain antigens, and antigen presenting cells (APCs) carrying brain antigens in draining lymphoid tissue of stroke patients support the notion that stroke might induce antigen-specific immune responses. After stroke, brain proteins that are normally hidden from the periphery, inflammatory mediators, and danger signals can exit the brain through several efflux routes. They can reach the blood after leaking out of the damaged blood-brain barrier (BBB) or following the drainage of interstitial fluid to the dural venous sinus, or reach the cervical lymph nodes through the nasal lymphatics following CSF drainage along the arachnoid sheaths of nerves across the nasal submucosa. The route and mode of access of brain antigens to lymphoid tissue could influence the type of response. Central and peripheral tolerance prevents autoimmunity, but the actual mechanisms of tolerance to brain antigens released into the periphery in the presence of inflammation, danger signals, and APCs, are not fully characterized. Stroke does not systematically trigger autoimmunity, but under certain circumstances, such as pronounced systemic inflammation or infection, autoreactive T cells could escape the tolerance controls. Further investigation is needed to elucidate whether antigen-specific immune events could underlie neurological complications impairing recovery from stroke.

Variation in behavioral deficits and patterns of recovery after stroke among different rat strains

Pre-clinical models of stroke therapeutics depend upon the ability to detect differences in infarct volume as well as in the short- and long-term outcomes of treated animals. Little attention has been paid to interstrain differences in these outcomes and the importance of defining the most appropriate behavioral tests. In this study, we evaluate long-term outcome from stroke in three different rat strains. Lewis, Wistar, and Sprague Dawley (SD) rats were subjected to 2-h middle cerebral artery occlusion and survived for up to 49 days. Behavioral tests were performed weekly. There was continuous assessment of rotational/circling activity in the home cage by use of an automated software program. A separate group of animals was sacrificed at 24 h to determine infarct volume. Infarct volume was similar in all three strains. Mortality was significantly higher in SD rats (P < 0.001). Rotational/circling activity at 24 h was correlated with cortical infarct volume in Wistar and SD rats (ρ = 0.67, P = 0.04 and ρ = 0.72, P = 0.01, respectively). Wistar and SD rats displayed more rotational/circling activity following stroke than Lewis rats, but Lewis rats evidenced more impairment on complex motor tasks like the rotarod. Further, computer automated analysis of rotational activity was more sensitive than subjective assessment, with SD rats showing a preference for clockwise rotations to 49 days after stroke despite normalization of the neurological score after 21 days. There are significant interstrain differences in survival and in the patterns of neurological impairment and recovery after stroke. These differences must be taken into account in pre-clinical studies, but may also be capitalized upon to understand genetic contributions to injury. Finally, computerized assessment of behavior is more sensitive than subjective assessment for detecting behavioral changes.

Repetitive hypoxic preconditioning induces an immunosuppressed B cell phenotype during endogenous protection from stroke

Background

Repetitive hypoxic preconditioning (RHP) creates an anti-inflammatory phenotype that protects from stroke-induced injury for months after a 2-week treatment. The mechanisms underlying long-term tolerance are unknown, though one exposure to hypoxia significantly increased peripheral B cell representation. For this study, we sought to determine if RHP specifically recruited B cells into the protected ischemic hemisphere, and whether RHP could phenotypically alter B cells prior to stroke onset.

Methods

Adult, male SW/ND4 mice received RHP (nine exposures over 2 weeks; 8 to 11 % O₂; 2 to 4 hours) or identical exposures to 21 % O₂ as control. Two weeks following RHP, a 60-minute transient middle cerebral artery occlusion was induced. Standard techniques quantified CXCL13 mRNA and protein expression. Two days after stroke, leukocytes were isolated from brain tissue (70:30 discontinuous Percoll gradient) and profiled on a BD-FACS Aria flow cytometer. In a separate cohort without stroke, sorted splenic CD19⁺ B cells were isolated 2 weeks after RHP and analyzed on an Illumina MouseWG-6 V2 Bead Chip. Final gene pathways were determined using Ingenuity Pathway Analysis. Student’s t-test or one-way analysis of variance determined significance (P < 0.05).

Results

CXCL13, a B cell-specific chemokine, was upregulated in post-stroke cortical vessels of both groups. In the ischemic hemisphere, RHP increased B cell representation by attenuating the diapedesis of monocyte, macrophage, neutrophil and T cells, to quantities indistinguishable from the uninjured, contralateral hemisphere. Pre-stroke splenic B cells isolated from RHP-treated mice had >1,900 genes differentially expressed by microarray analysis. Genes related to B-T cell interactions, including antigen presentation, B cell differentiation and antibody production, were profoundly downregulated. Maturation and activation were arrested in a cohort of B cells from pre-stroke RHP-treated mice while regulatory B cells, a subset implicated in neurovascular protection from stroke, were upregulated.

Conclusions

Collectively, our data characterize an endogenous neuroprotective phenotype that utilizes adaptive immune mechanisms pre-stroke to protect the brain from injury post-stroke. Future studies to validate the role of B cells in minimizing injury and promoting central nervous system recovery, and to determine whether B cells mediate an adaptive immunity to systemic hypoxia that protects from subsequent stroke, are needed.

Immunological consequences of ischemic stroke

The treatment of ischemic stroke is one of the great challenges in modern neurology. The localization and the size of the infarct determine the long-term disability of stroke survivors. Recent observations have revealed that stroke also alters the function of the immune system and vice versa: At the site of the infarct, a local inflammatory response develops that enhances brain lesion development. In experimental stroke, proof-of-concept studies confirm that inhibition of this immune response reduces lesion volume and improves outcome. In the peripheral blood of stroke patients, though, lymphocytopenia and monocyte dysfunction develop. These changes reflect a clinically relevant impairment of bacterial defense mechanisms because they are associated with an enhanced risk to acquire post-stroke infections. Stress hormones have been identified as important mediators of stroke-induced immune suppression. The pharmacological inhibition of beta adrenergic receptors, but not the inhibition of steroids, is effective in reducing infection and improving clinical outcome in experimental stroke; catecholamine release therefore appears causally related to stroke-induced immune suppression. Strong evidence supports the hypothesis that these immune alterations impact the clinical course of stroke patients. Thus, the development of new therapeutic strategies targeted to alter the immunological consequences of stroke appears promising. However, to date, the beneficial effects seen in experimental stroke have not been successfully translated into a clinical trial. This brief review summarizes the current understanding of the immunological consequences of ischemic stroke. Finally, we propose a concept that links the peripheral immune suppression with the development of local inflammation.

Leukocytosis in patients with neurologic deterioration after acute ischemic stroke is associated with poor outcomes

BACKGROUND

Neurologic deterioration (ND) after acute ischemic stroke (AIS) has been shown to result in poor outcomes. ND is thought to arise from penumbral excitotoxic cell death caused in part by leukocytic infiltration. Elevated admission peripheral leukocyte levels are associated with poor outcomes in stroke patients who suffer ND, but little is known about the dynamic changes that occur in leukocyte counts around the time of ND. We sought to determine if peripheral leukocyte levels in the days surrounding ND are correlated with poor outcomes.

METHODS

Patients with AIS who presented to our center within 48 hours of symptom onset between July 2008 and June 2010 were retrospectively identified by chart review and screened for ND (defined as an increase in National Institutes of Health Stroke Scale score ≥ 2 within a 24-hour period). Patients were excluded for steroid use during hospitalization or in the month before admission and infection within the 48 hours before or after ND. Demographics, daily leukocyte counts, and poor functional outcome (modified Rankin Scale score 3-6) were investigated.

RESULTS

Ninety-six of the 292 (33%) patients screened had ND. The mean age was 69.5 years; 62.5% were male and 65.6% were black. Patients with a poor functional outcome had significantly higher leukocyte and neutrophil levels 1 day before ND (P = .048 and P = .026, respectively), and on the day of ND (P = .013 and P = .007, respectively), compared to patients with good functional outcome.

CONCLUSIONS

Leukocytosis at the time of ND correlates with poor functional outcomes and may represent a marker of greater cerebral damage through increased parenchymal inflammation.

Regulatory T cell in stroke: a new paradigm for immune regulation

Stroke is a common, debilitating trauma that has an incompletely elucidated pathophysiology and lacks an effective therapy. FoxP3⁺CD25⁺CD4⁺ regulatory T cells (Tregs) suppress a variety of normal physiological and pathological immune responses via several pathways, such as inhibitory cytokine secretion, direct cytolysis induction, and antigen-presenting cell functional modulation. FoxP3⁺CD25⁺CD4⁺ Tregs are involved in a variety of central nervous system diseases and injuries, including axonal injury, neurodegenerative diseases, and stroke. Specifically, FoxP3⁺CD25⁺CD4⁺ Tregs exert neuroprotective effects in acute experimental stroke models. These beneficial effects, however, are difficult to elucidate. In this review, we summarized evidence of FoxP3⁺CD25⁺CD4⁺ Tregs as potentially important immunomodulators in stroke pathogenesis and highlight further investigations for possible immunotherapeutic strategies by modulating the quantity and/or functional effects of FoxP3⁺CD25⁺CD4⁺ Tregs in stroke patients.

The immunologic profile of adoptively transferred lymphocytes influences stroke outcome of recipients

Animals that have myelin basic protein (MBP) specific lymphocytes with a Th1(+) phenotype have worse stroke outcome than those that do not. Whether these MBP specific cells contribute to worsened outcome or are merely a consequence of worse outcome is unclear. In these experiments, lymphocytes were obtained from donor animals one month after stroke and transferred to naïve recipient animals at the time of cerebral ischemia. The MBP specific phenotype of donor cells was determined prior to transfer. Animals that received either MBP specific Th1(+) or Th17(+) cells experienced worse neurological outcome, and the degree of impairment correlated with the robustness of MBP specific Th1(+) and Th17(+) responses. These data demonstrate that the immunologic phenotype of antigen specific lymphocytes influences stroke outcome.

Post-stroke immunodeficiency: effects of sensitization and tolerization to brain antigens

Acute onset of cerebrovascular diseases seems to be related to a number of immunological alternations. After the initial pro-inflammatory response to brain ischemia accompanied by systemic inflammatory response syndrome, stroke interferes with function of the innate and the adaptive immune cells, resulting in systemic immunosuppression. Although post-stroke immunodeficiency could predispose patients to life-threatening infections, it could potentially protect brain via reducing autoimmune reaction to the brain antigens. In this paper, we review current knowledge on the immunological alterations after brain ischemia, particularly effects of infection for stimulation of autoimmune response against brain antigens.

Regulatory T cells accumulate and proliferate in the ischemic hemisphere for up to 30 days after MCAO

Local and peripheral immune responses are activated after ischemic stroke. In our present study, we investigated the temporal distribution, location, induction, and function of regulatory T cells (Tregs) and the possible involvement of microglia, macrophages, and dendritic cells after middle cerebral artery occlusion (MCAO). C57BL/6J and Foxp3(EGFP) transgenic mice were subjected to 30 minutes MCAO. On days 7, 14, and 30 after MCAO, Tregs and antigen presenting cells were analyzed using fluorescence activated cell sorting multicolor staining and immunohistochemistry. A strong accumulation of Tregs was observed on days 14 and 30 in the ischemic hemisphere accompanied by the elevated presence and activation of microglia. Dendritic cells and macrophages were found on each analyzed day. About 60% of Foxp3(+) Tregs in ischemic hemispheres were positive for the proliferation marker Ki-67 on days 7 and 14 after MCAO. The transfer of naive CD4(+) cells depleted of Foxp3(+) Tregs into RAG1(-/-) mice 1 day before MCAO did not lead to a de novo generation of Tregs 14 days after surgery. After depletion of CD25(+) Tregs, no changes regarding neurologic outcome were detected. The sustained presence of Tregs in the brain after MCAO indicates a long-lasting immunological alteration and involvement of brain cells in immunoregulatory mechanisms.

Activation of immune responses to brain antigens after stroke

Infection is common after stroke and is independently associated with a worse outcome. The predisposition to infection following stroke is in part related to a sympathetically mediated suppression of the peripheral immune response. The teleological explanation for this immune dysfunction is that it serves to prevent autoimmune responses to brain antigens. We believe that the systemic immune response in patients who develop infection, however, thwarts this seemingly protective response and predisposes to central nervous system autoimmunity. These autoimmune responses may mediate, at least in part, the worse outcome associated with post-stroke infection.

The immunology of acute stroke

Recent clinical and experimental studies have highlighted a complex role for the immune system in the pathophysiological changes that occur after acute stroke. Sensors of the innate immune system such as Toll-like receptors, or effectors such as the lectin pathway of complement activation and innate immune cells, are activated by brain ischaemia and tissue damage, leading to amplification of the inflammatory cascade. Activation of the adaptive arm of the immune system, mediated by lymphocyte populations including T and B cells, regulatory T cells, and γδT cells, in response to stroke can lead to deleterious antigen-specific autoreactive responses but can also have cytoprotective effects. Increased incidence of infections is observed after acute stroke, and might result from activation of long-distance feedback loops between the CNS and peripheral immune organs, which are thought to play a part in stroke-induced immunodepression. Ongoing clinical trials are investigating whether the preventive use of antibiotics improves functional outcome after stroke. This Review discusses the multifaceted role of the immune system in the pathophysiology of acute stroke.

Autoimmune responses to brain following stroke

This review provides a synthesis of the work done by our laboratory that demonstrates the presence of cellular immune responses directed towards brain antigens in animals following experimental stroke as well as in patients following ischemic stroke. These responses include both antigen-specific TH1(+) responses, which are associated with worse stroke outcome, and antigen-specific TREG responses, which are associated with better stroke outcome. The likelihood of developing a detrimental TH1(+) response to brain antigens is increased by administration of a systemic inflammatory stimulus in experimental stroke and by systemic infection in patients with stroke. We propose that the microenvironment within the lymph nodes and brain is altered by systemic inflammation and allows for bystander activation of lymphocytes and the development of autoimmune responses to brain antigens following cerebral ischemic injury.

The immunology of stroke: from mechanisms to translation

Immunity and inflammation are key elements of the pathobiology of stroke, a devastating illness second only to cardiac ischemia as a cause of death worldwide. While the immune system participates in the brain damage produced by ischemia, the damaged brain, in turn, exerts a powerful immunosuppressive effect that promotes fatal intercurrent infections and threatens the survival of stroke patients. Inflammatory signaling is instrumental in all stages of the ischemic cascade, from the early damaging events triggered by arterial occlusion, to the late regenerative processes underlying post-ischemic tissue repair. Recent developments have revealed that stroke, like multiple sclerosis, engages both innate and adaptive immunity. But, unlike multiple sclerosis, adaptive immunity triggered by newly exposed brain antigens does not have an impact on the acute phase of the damage. Nevertheless, modulation of adaptive immunity exerts a remarkable protective effect on the ischemic brain and offers the prospect of new stroke therapies. However, immunomodulation is not devoid of deleterious side effects, and gaining a better understanding of the reciprocal interaction between the immune system and the ischemic brain is essential to harness the full therapeutic potential of the immunology of stroke.

Role of dihydrotestosterone in post-stroke peripheral immunosuppression after cerebral ischemia

Stroke is a sexually dimorphic disease with male gender considered a disadvantage in terms of risk and disease outcome. In intact males, stroke induces peripheral immunosuppression, characterized by decreased splenocyte numbers and proliferation and altered percentages of viable T, B, and CD11b+ cells. To investigate whether the potent androgen and known immunomodulator, dihydrotestosterone (DHT), exacerbates post-stroke immunosuppression in castrated male mice after focal stroke, we evaluated the effect of middle cerebral artery occlusion (MCAO) on peripheral and central nervous system (CNS) immune responses in castrated mice with or without controlled levels of DHT. MCAO reduced spleen cell numbers in both groups, but altered T cell and B cell percentages in remaining splenocytes and concomitantly increased the percentage of CD11b+ blood cells solely in DHT-replaced animals at 24 h. Furthermore, DHT-replacement reduced splenocyte proliferation which was accompanied by an increased percentage of immunosuppressive regulatory T cells relative to castrates 96 h post-MCAO. In brain, the percentages of immune cell populations in the ischemic hemisphere relative to the non-ischemic hemisphere were similar between castrated and DHT-replaced mice after MCAO. These data suggest DHT modulates peripheral immunosuppression after MCAO but with relatively little effect on early immune response of the recovering CNS.

α-MSH: a potential neuroprotective and immunomodulatory agent for the treatment of stroke

Alpha-melanocyte-stimulating hormone (MSH) is a neuropeptide with profound immunomodulatory properties; we evaluated the effects of α-MSH on stroke outcome and its ability to modulate the postischemic immune response. In Lewis rats subjected to 3 hours of middle cerebral artery occlusion (MCAO), plasma concentrations of α-MSH rapidly decreased and returned to baseline over the course of days. Exogenous administration of α-MSH (100 or 500 μg/kg) improved 24 hour outcome in animals subjected to 2 hours MCAO; α-MSH 500 μg/kg also decreased infarct volume at this time point. Both doses of α-MSH were ineffective in improving outcome or decreasing infarct volume in animals subjected to 3 hours MCAO. The splenocyte response to phytohemagglutin in animals treated with α-MSH was attenuated at 24 hours after MCAO. At 1 month after MCAO, treatment with α-MSH 500 μg/kg at the time of stoke was associated with a decrease in TH1 response to myelin basic protein (MBP) in animals subjected to 2 hours MCAO, although treatment was not associated with improved outcome at this time point. Given the early benefits of α-MSH treatment and its effect on immunologic outcome, further studies to evaluate the utility of α-MSH for the treatment of cerebral ischemia are warranted.

Modulation of the postischemic immune response to improve stroke outcome

Recent advances in understanding how the poststroke immune response may contribute to ischemic brain injury are discussed in this article. In particular, the potential of modulating the postischemic immune response to improve stroke outcome is explored.