MHCII-independent CD4+ T cells protect injured CNS neurons via IL-4

The far-reaching scope of neuroinflammation after traumatic brain injury

The 'silent epidemic' of traumatic brain injury (TBI) has been placed in the spotlight as a result of clinical investigations and popular press coverage of athletes and veterans with single or repetitive head injuries. Neuroinflammation can cause acute secondary injury after TBI, and has been linked to chronic neurodegenerative diseases; however, anti-inflammatory agents have failed to improve TBI outcomes in clinical trials. In this Review, we therefore propose a new framework of targeted immunomodulation after TBI for future exploration. Our framework incorporates factors such as the time from injury, mechanism of injury, and secondary insults in considering potential treatment options. Structuring our discussion around the dynamics of the immune response to TBI - from initial triggers to chronic neuroinflammation - we consider the ability of soluble and cellular inflammatory mediators to promote repair and regeneration versus secondary injury and neurodegeneration. We summarize both animal model and human studies, with clinical data explicitly defined throughout this Review. Recent advances in neuroimmunology and TBI-responsive neuroinflammation are incorporated, including concepts of inflammasomes, mechanisms of microglial polarization, and glymphatic clearance. Moreover, we highlight findings that could offer novel therapeutic targets for translational and clinical research, assimilate evidence from other brain injury models, and identify outstanding questions in the field.

IL-13-Mediated Regulation of Learning and Memory

The role of proinflammatory cytokines in cognitive function has been investigated with both beneficial and possible detrimental effects, depending on the cytokine. More recently, the type 2 IL-4 has been demonstrated to play a role in cognition. In this study, using the Morris water maze task, we demonstrate that IL-13-deficient mice are significantly impaired in working memory as well as attenuated reference memory, both functions essential for effective complex learning. During the learning process, wild-type mice increased the number of CD4⁺ T cells in the meninges and production of IL-13, whereas neither Morris water maze-trained IL-4 nor trained IL-13-deficient mice were able to increase CD4⁺ T cells in the meninges. Mechanistically, we showed that IL-13 is able to stimulate primary astrocytes to produce brain-derived neurotrophic factor, which does foster cognitive functions. Moreover, Morris water maze-trained wild-type mice were able to increase astrocyte-produced glial fibrillary acidic protein in the hippocampus, which was impaired in Morris water maze-trained IL-4- and IL-13-deficient mice. Collectively, this study strongly suggests that the Th2 cytokines, not only IL-4 but also IL-13, are involved in cognitive functions by stimulating astrocytes from the meninges and hippocampus. These results may be important for future development of therapeutic approaches associated with neurologic disorders such as Parkinson disease-associated dementia and HIV-associated dementia among others.

Immunization with Bacillus Calmette-Guérin (BCG) alleviates neuroinflammation and cognitive deficits in APP/PS1 mice via the recruitment of inflammation-resolving monocytes to the brain

The immune system plays a crucial role in the progression of Alzheimer's disease (AD). Recently, immune-dependent cascade induced by systemic immune activation has been verified to play a beneficial role in AD mouse models. Here, we tested whether Bacillus Calmette-Guérin (BCG) immunization alters AD pathology and cognitive dysfunction in APP/PS1 AD mouse model, and with 4Aβ1-15 vaccination as positive control. It was found that BCG treatment reversed the cognitive decline to the extent observed in 4Aβ1-15 group, but did not reduce the β-amyloid (Aβ) burden in the brain. Then, we demonstrated the enhanced recruitment of inflammation-resolving monocytes across the choroid plexus and perivascular spaces to cerebral sites of plaque pathology in APP/PS1 mice immunized with BCG. Furthermore, elevated splenocyte Foxp3⁺ regulatory T cell levels in the control APP/PS1 mice were down-regulated back to the wild-type (WT) levels by BCG treatment but not 4Aβ1-15 vaccination. In addition, BCG treatment induced the production of more circulating interferon (IFN)-γ than the controls and 4Aβ1-15 vaccination. Though the similar reductions in brain levels of pro-inflammatory cytokines were observed in the BCG and 4Aβ1-15 groups compared to the controls, only BCG had the great effect in upregulating cerebral anti-inflammatory cytokine levels as well as elevating the expression of neurotrophic factors in the brain of APP/PS1 mice. Thus, it is suggested that BCG exerts a beneficial immunomodulatory effect in APP/PS1 mice through mitigation of systemic immune suppression, induction of IFN-γ response and alleviation of the neuroinflammatory response.

Ectonucleotidase activity and immunosuppression in astrocyte-CD4 T cell bidirectional signaling

Astrocytes play a crucial role in neuroinflammation as part of the glia limitans, which regulates infiltration of the brain parenchyma by leukocytes. The signaling pathways and molecular events, which result from the interaction of activated T cells with astrocytes are poorly defined. Here we show that astrocytes promote the expression and enzymatic activity of CD39 and CD73 ectonucleotidases in recently activated CD4 cells by a contact dependent mechanism that is independent of T cell receptor interaction with class II major histocompatibility complex (MHC). Transforming growth factor-β (TGF-β) is robustly upregulated and sufficient to promote ectonucleotidases expression. T cell adhesion to astrocyte results in differentiation to an immunosuppressive phenotype defined by expression of the transcription factor Rorγt, which characterizes the CD4 T helper 17 subset. CD39 activity in T cells in turn inhibits spontaneous calcium oscillations in astrocytes that correlated with enhanced and reduced transcription of CCL2 chemokine and Sonic hedgehog (Shh), respectively. We hypothesize this TCR-independent interaction promote an immunosuppressive program in T cells to control possible brain injury by deregulated T cell activation during neuroinflammation. On the other hand, the increased secretion of CCL2 with concomitant reduction of Shh might promote leukocytes extravasation into the brain parenchyma.

Immunization with neural derived peptides plus scar removal induces a permissive microenvironment, and improves locomotor recovery after chronic spinal cord injury

Background

Immunization with neural derived peptides (INDP) as well as scar removal—separately—have shown to induce morphological and functional improvement after spinal cord injury (SCI). In the present study, we compared the effect of INDP alone versus INDP with scar removal on motor recovery, regeneration-associated and cytokine gene expression, and axonal regeneration after chronic SCI. Scar removal was conducted through a single incision with a double-bladed scalpel along the stump, and scar renewal was halted by adding α,α′-dipyridyl.

Results

During the chronic injury stage, two experiments were undertaken. The first experiment was aimed at testing the therapeutic effect of INDP combined with scar removal. Sixty days after therapeutic intervention, the expression of genes encoding for TNFα, IFNγ, IL4, TGFβ, BDNF, IGF1, and GAP43 was evaluated at the site of injury. Tyrosine hydroxylase and 5-hydroxytryptamine positive fibers were also studied. Locomotor evaluations showed a significant recovery in the group treated with scar removal + INDP. Moreover; this group presented a significant increase in IL4, TGFβ, BDNF, IGF1, and GAP43 expression, but a decrease of TNFα and IFNγ. Also, the spinal cord of animals receiving both treatments presented a significant increase of serotonergic and catecholaminergic fibers as compared to other the groups. The second experiment compared the results of the combined approach versus INDP alone. Rats receiving INDP likewise showed improved motor recovery, although on a lesser scale than those who received the combined treatment. An increase in inflammation and regeneration-associated gene expression, as well as in the percentage of serotonergic and catecholaminergic fibers was observed in INDP-treated rats to a lesser degree than those in the combined therapy group.

Conclusions

These findings suggest that INDP, both alone and in combination with scar removal, could modify the non-permissive microenvironment prevailing at the chronic phase of SCI, providing the opportunity of improving motor recovery.

Interaction Effects of Season of Birth and Cytokine Genes on Schizotypal Traits in the General Population

Literature suggests that the effect of winter birth on vulnerability to schizophrenia might be mediated by increased expression of proinflammatory cytokines due to prenatal infection and its inadequate regulation by anti-inflammatory factors. As the response of the immune system depends on genotype, this study assessed the interaction effects of cytokine genes and season of birth (SOB) on schizotypy measured with the Schizotypal Personality Questionnaire (SPQ-74). We searched for associations of IL1B rs16944, IL4 rs2243250, and IL-1RN VNTR polymorphisms, SOB, and their interactions with the SPQ-74 total score in a sample of 278 healthy individuals. A significant effect of the IL4 X SOB interaction was found, p = 0.007 and η² = 0.028. We confirmed this effect using an extended sample of 373 individuals. Homozygotes CC born in winter showed the highest SPQ total score and differed significantly from winter-born T allele carriers, p = 0.049. This difference was demonstrated for cognitive-perceptual and disorganized but not interpersonal dimensions. The findings are consistent with the hypothesis that the cytokine genes by SOB interaction can influence variability of schizotypal traits in the general population. The IL4 T allele appeared to have a protective effect against the development of positive and disorganized schizotypal traits in winter-born individuals.

Emerging Roles for the Immune System in Traumatic Brain Injury

Traumatic brain injury (TBI) affects an ever-growing population of all ages with long-term consequences on health and cognition. Many of the issues that TBI patients face are thought to be mediated by the immune system. Primary brain damage that occurs at the time of injury can be exacerbated and prolonged for months or even years by chronic inflammatory processes, which can ultimately lead to secondary cell death, neurodegeneration, and long-lasting neurological impairment. Researchers have turned to rodent models of TBI in order to understand how inflammatory cells and immunological signaling regulate the post-injury response and recovery mechanisms. In addition, the development of numerous methods to manipulate genes involved in inflammation has recently expanded the possibilities of investigating the immune response in TBI models. As results from these studies accumulate, scientists have started to link cells and signaling pathways to pro- and anti-inflammatory processes that may contribute beneficial or detrimental effects to the injured brain. Moreover, emerging data suggest that targeting aspects of the immune response may offer promising strategies to treat TBI. This review will cover insights gained from studies that approach TBI research from an immunological perspective and will summarize our current understanding of the involvement of specific immune cell types and cytokines in TBI pathogenesis.

Characterization of meningeal type 2 innate lymphocytes and their response to CNS injury

The meningeal space is occupied by a diverse repertoire of immune cells. Central nervous system (CNS) injury elicits a rapid immune response that affects neuronal survival and recovery, but the role of meningeal inflammation remains poorly understood. Here, we describe type 2 innate lymphocytes (ILC2s) as a novel cell type resident in the healthy meninges that are activated after CNS injury. ILC2s are present throughout the naive mouse meninges, though are concentrated around the dural sinuses, and have a unique transcriptional profile. After spinal cord injury (SCI), meningeal ILC2s are activated in an IL-33–dependent manner, producing type 2 cytokines. Using RNAseq, we characterized the gene programs that underlie the ILC2 activation state. Finally, addition of wild-type lung-derived ILC2s into the meningeal space of IL-33R^−/− animals partially improves recovery after SCI. These data characterize ILC2s as a novel meningeal cell type that responds to SCI and could lead to new therapeutic insights for neuroinflammatory conditions.

A Three-Day Consecutive Fingolimod Administration Improves Neurological Functions and Modulates Multiple Immune Responses of CCI Mice

Excessive inflammation after traumatic brain injury (TBI) is a major cause of secondary TBI. Though several inflammatory biomarkers have been postulated as the risk factors of TBI, there has not been any comprehensive description of them. Fingolimod, a new kind of immunomodulatory agent which can diminish various kinds of inflammatory responses, has shown additional therapeutic effects in the treatment of intracranial cerebral hematoma (ICH), ischemia, spinal cord injury (SCI), and many other CNS disorders. However, its therapeutic application has not been confirmed in TBI. Thus, we hypothesized that a 3-day consecutive fingolimod administration could broadly modulate the inflammatory reactions and improve the outcomes of TBI. The TBI models of C57/BL6 mice were established with the controlled cortical impact injury (CCI) system. A 3-day consecutive fingolimod therapy (given at 1, 24, and 48 h post injury) was performed at a dose of 1 mg/kg. The flow cytometry, immunoflourence, cytokine array, and ELISA were all applied to evaluate the immune cells and inflammatory markers in the injured brains. Immunohistochemical staining with anti-APP antibody was performed to assess the axonal damage. The neurological functions of these TBI models were assessed by mNSS/Rota-rod and Morris water maze (MWM). The brain water content and integrity of the blood-brain barrier (BBB) were also observed. On the 3rd day after TBI, the accumulation of inflammatory cytokines and chemokines reached the peak and administration of fingolimod reduced as many as 20 kinds of cytokines and chemokines. Fingolimod decreased infiltrated T lymphocytes and NK cells but increased the percentage of regulatory T (Treg) cells, and the concentration of IL-10 on the 3rd day after TBI. Fingolimod also notably attenuated the general activated microglia but augmented the M2/M1 ratio accompanied by decreased axonal damage. The neurological functions were improved after the fingolimod treatment accompanied with alleviation of the brain edema and BBB damage. This study suggests that the 3-day consecutive fingolimod administration extensively modulates multiple immuno-inflammatory responses and improves the neurological deficits after TBI, and therefore, it may be a new approach to the treatment of secondary TBI.

Emerging Roles for the Immune System in Traumatic Brain Injury

Traumatic brain injury (TBI) affects an ever-growing population of all ages with long-term consequences on health and cognition. Many of the issues that TBI patients face are thought to be mediated by the immune system. Primary brain damage that occurs at the time of injury can be exacerbated and prolonged for months or even years by chronic inflammatory processes, which can ultimately lead to secondary cell death, neurodegeneration, and long-lasting neurological impairment. Researchers have turned to rodent models of TBI in order to understand how inflammatory cells and immunological signaling regulate the post-injury response and recovery mechanisms. In addition, the development of numerous methods to manipulate genes involved in inflammation has recently expanded the possibilities of investigating the immune response in TBI models. As results from these studies accumulate, scientists have started to link cells and signaling pathways to pro- and anti-inflammatory processes that may contribute beneficial or detrimental effects to the injured brain. Moreover, emerging data suggest that targeting aspects of the immune response may offer promising strategies to treat TBI. This review will cover insights gained from studies that approach TBI research from an immunological perspective and will summarize our current understanding of the involvement of specific immune cell types and cytokines in TBI pathogenesis.

Molecular and Cellular Responses to Interleukin-4 Treatment in a Rat Model of Transient Ischemia

Within hours after stroke, potentially cytotoxic pro-inflammatory mediators are elevated within the brain; thus, one potential therapeutic strategy is to reduce them and skew the brain toward an anti-inflammatory state. Because interleukin-4 (IL-4) treatment induces an anti-inflammatory, "alternative-activation" state in microglia and macrophages in vitro, we tested the hypothesis that early supplementation of the brain with IL-4 can shift it toward an anti-inflammatory state and reduce damage after transient focal ischemia. Adult male rat striata were injected with endothelin-1, with or without co-injection of IL-4. Inflammation, glial responses and damage to neurons and white matter were quantified from 1 to 7 days later. At 1 day, IL-4 treatment increased striatal expression of several anti-inflammatory markers (ARG1, CCL22, CD163, PPARγ), increased phagocytic (Iba1-positive, CD68-positive) microglia/macrophages, and increased VEGF-A-positive infiltrating neutrophils in the infarcts. At 7 days, there was evidence of sustained, propagating responses. IL-4 increased CD206, CD200R1, IL-4Rα, STAT6, PPARγ, CD11b, and TLR2 expression and increased microglia/macrophages in the infarct and astrogliosis outside the infarct. Neurodegeneration and myelin damage were not reduced, however. The sustained immune and glial responses when resolution and repair processes have begun warrant further studies of IL-4 treatment regimens and long-term outcomes.

Bugs and Brain: How Infection Makes You Feel Blue

In this issue of Immunity, Prinz and colleagues (2016) describe an unexpected mechanism underlying the role of type I interferon in the initiation of cognitive impairment and sickness behavior during viral infection through induction of chemokine CXCL10 in central nervous system epithelial and endothelial cells.

Inflammatory neuroprotection following traumatic brain injury

Traumatic brain injury (TBI) elicits an inflammatory response in the central nervous system (CNS) that involves both resident and peripheral immune cells. Neuroinflammation can persist for years following a single TBI and may contribute to neurodegeneration. However, administration of anti-inflammatory drugs shortly after injury was not effective in the treatment of TBI patients. Some components of the neuroinflammatory response seem to play a beneficial role in the acute phase of TBI. Indeed, following CNS injury, early inflammation can set the stage for proper tissue regeneration and recovery, which can, perhaps, explain why general immunosuppression in TBI patients is disadvantageous. Here, we discuss some positive attributes of neuroinflammation and propose that inflammation be therapeutically guided in TBI patients rather than globally suppressed.

Treatment of surgical brain injury by immune tolerance induced by intrathymic and hepatic portal vein injection of brain antigens

Surgical brain injury (SBI) defines complications induced by intracranial surgery, such as cerebral edema and other secondary injuries. In our study, intrathymic and hepatic portal vein injection of allogeneic myelin basic protein (MBP) or autogeneic brain cell suspensions were administered to a standard SBI model. Serum pro-inflammatory IL-2, anti-inflammatory IL-4 concentrations and the CD4⁺T/CD8⁺T ratio were measured at 1, 3, 7, 14 and 21 d after surgery to verify the establishment of immune tolerance. Furthermore, we confirmed neuroprotective effects by evaluating neurological scores at 1, 3, 7, 14 and 21 d after SBI. Anti-Fas ligand (FasL) immunohistochemistry and TUNEL assays of brain sections were tested at 21 d after surgery. Intrathymic injections of MBP or autogeneic brain cell suspensions functioned by both suppressing secondary inflammatory reactions and improving prognoses, whereas hepatic portal vein injections of autogeneic brain cell suspensions exerted a better effect than MBP. Intrathymic and hepatic portal vein injections of MBP had equal effects on reducing secondary inflammation and improving prognoses. Otherwise, hepatic portal vein injections of autogeneic brain cell suspensions had better outcomes than intrathymic injections of autogeneic brain cell suspensions. Moreover, the benefit of injecting antigens into the thymus was outweighed by hepatic portal vein injections.

Multifaceted interactions between adaptive immunity and the central nervous system

Neuroimmunologists seek to understand the interactions between the central nervous system (CNS) and the immune system, both under homeostatic conditions and in diseases. Unanswered questions include those relating to the diversity and specificity of the meningeal T cell repertoire; the routes taken by immune cells that patrol the meninges under healthy conditions and invade the parenchyma during pathology; the opposing effects (beneficial or detrimental) of these cells on CNS function; the role of immune cells after CNS injury; and the evolutionary link between the two systems, resulting in their tight interaction and interdependence. This Review summarizes the current standing of and challenging questions related to interactions between adaptive immunity and the CNS and considers the possible directions in which these aspects of neuroimmunology will be heading over the next decade.

Interleukin-25 is detrimental for recovery after spinal cord injury in mice

Background

The cytokine, interleukin (IL)-25, is thought to be critically involved in inducing a type 2 immune response which may contribute to regeneration after central nervous system (CNS) trauma. We investigated whether applying recombinant IL-25, locally or systemically, in a mouse model of spinal cord injury (SCI) improves functional and histological recovery.

Findings

Repeated systemic administration of IL-25 did not influence functional recovery following SCI. In contrast, a single local administration of IL-25 significantly worsened locomotor outcome, which was evident from a decreased Basso mouse scale (BMS) score compared with phosphate-buffered saline (PBS)-treated controls. This was accompanied by a significant increase in lesion size, demyelination, and T helper cell infiltration.

Conclusions

These data show for the first time that IL-25 is either ineffective when applied systemically or detrimental to spinal cord recovery when applied locally. Our findings question the potential neuroprotective role of IL-25 following CNS trauma.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0566-y) contains supplementary material, which is available to authorized users.

Immune Surveillance of the CNS following Infection and Injury

The central nervous system (CNS) contains a sophisticated neural network that must be constantly surveyed in order to detect and mitigate a diverse array of challenges. The innate and adaptive immune systems actively participate in this surveillance, which is critical for the maintenance of CNS homeostasis and can facilitate the resolution of infections, degeneration, and tissue damage. Infections and sterile injuries represent two common challenges imposed on the CNS that require a prompt immune response. While the inducers of these two challenges differ in origin, the resultant responses orchestrated by the CNS share some overlapping features. Here, we review how the CNS immunologically discriminates between pathogens and sterile injuries, mobilizes an immune reaction, and, ultimately, regulates local and peripherally-derived immune cells to provide a supportive milieu for tissue repair.

Cannabinoids to treat spinal cord injury

Spinal cord injury (SCI) is a devastating condition for which there is no standard treatment beyond rehabilitation strategies. In this review, we discuss the current knowledge on the use of cannabinoids to treat this condition. The endocannabinoid system is expressed in the intact spinal cord, and it is dramatically upregulated after lesion. Endogenous activation of this system counteracts secondary damage following SCI, and treatments with endocannabinoids or synthetic cannabinoid receptor agonists promote a better functional outcome in experimental models. The use of cannabinoids in SCI is a new research field and many questions remain open. Here, we discuss caveats and suggest some future directions that may help to understand the role of cannabinoids in SCI and how to take advantage of this system to regain functions after spinal cord damage.

Regulation of IL-4 Expression in Immunity and Diseases

IL-4 was first identified as a T cell-derived growth factor for B cells. Studies over the past several decades have markedly expanded our understanding of its cellular sources and function. In addition to T cells, IL-4 is produced by innate lymphocytes, such as NTK cells, and myeloid cells, such as basophils and mast cells. It is a signature cytokine of type 2 immune response but also has a nonimmune function. Its expression is tightly regulated at several levels, including signaling pathways, transcription factors, epigenetic modifications, microRNA, and long noncoding RNA. This chapter will review in detail the molecular mechanism regulating the cell type-specific expression of IL-4 in physiological and pathological type 2 immune responses.

Arginase 1+ microglia reduce Aβ plaque deposition during IL-1β-dependent neuroinflammation

Background

Neuroinflammation has long been considered a driver of Alzheimer’s disease progression. However, experiments developed to explore the interaction between neuroinflammation and Alzheimer’s disease (AD) pathology showed a surprising reduction in amyloid beta (Aβ) plaque deposition. We sought to understand this unexpected outcome by examining microglia phenotypes during chronic neuroinflammation.

Methods

Using an adeno-associated virus vector carrying hIL-1β cDNA, inflammation was induced in one hippocampus of 8-month-old amyloid precursor protein (APP)/PS1 mice for 4 weeks, while the other hemisphere received control injections. Bone marrow chimeras and staining analysis were used to identify the origins and types of immune cells present during sustained inflammation. Arginase 1 (Arg1) and inducible nitric oxide synthase (iNOS) immunoreactivity were used as markers of alternatively activated and classically activated cells, respectively, and changes in cellular uptake of Aβ by Arg1+ or iNOS+ microglia was demonstrated by confocal microscopy. To determine if an anti-inflammatory phenotype was present during neuroinflammation, RNA was extracted on flow-sorted microglia and rt-PCR was performed. Interleukin-4 injection was used to induce alternatively activated cells, whereas a minipump and intrahippocampal cannula was used to deliver an interleukin (IL)-4Rα antibody to block the induction of Arg1+ cells in the setting of sustained IL-1β expression.

Results

We observed a robust upregulation of centrally derived Arg1+ microglia present only in the inflamed hemisphere. Furthermore, in the inflamed hemisphere, greater numbers of Arg1+ microglia contained Aβ when compared to iNOS+ microglia. RNA isolated from flow-sorted microglia from the inflamed hemisphere demonstrated elevation of mRNA species consistent with alternative activation as well as neuroprotective genes such as BDNF and IGF1. To explore if Arg1+ microglia mediated plaque reduction, we induced Arg1+ microglia with IL-4 and observed significant plaque clearance. Moreover, when we reduced Arg1+ microglia induction in the context of neuroinflammation using an anti-IL-4Rα antibody delivered via intrahippocampal cannula, we observed a clear correlation between numbers of Arg1+ microglia and plaque reduction.

Conclusions

Together, these findings suggest that Arg1+ microglia are involved in Aβ plaque reduction during sustained, IL-1β-dependent neuroinflammation, opening up possible new avenues for immunomodulatory therapy of AD.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0411-8) contains supplementary material, which is available to authorized users.

Revisiting the Mechanisms of CNS Immune Privilege

Whereas the study of the interactions between the immune system and the central nervous system (CNS) has often focused on pathological conditions, the importance of neuroimmune communication in CNS homeostasis and function has become clear over that last two decades. Here we discuss the progression of our understanding of the interaction between the peripheral immune system and the CNS. We examine the notion of immune privilege of the CNS in light of both earlier findings and recent studies revealing a functional meningeal lymphatic system that drains cerebrospinal fluid (CSF) to the deep cervical lymph nodes, and consider the implications of a revised perspective on the immune privilege of the CNS on the etiology and pathology of different neurological disorders.

Commentary: IL-4 and IL-13 receptors and signaling

Interleukin (IL)-4 and IL-13 were discovered approximately 30years ago and were immediately linked to allergy and atopic diseases. Since then, new roles for IL-4 and IL-13 and their receptors in normal gestation, fetal development and neurological function and in the pathogenesis of cancer and fibrosis have been appreciated. Studying IL-4/-13 and their receptors has revealed important clues about cytokine biology and led to the development of numerous experimental therapeutics. Here we aim to highlight new discoveries and consolidate concepts in the field of IL-4 and IL-13 structure, receptor regulation, signaling and experimental therapeutics.

Dealing with Danger in the CNS: The Response of the Immune System to Injury

Fighting pathogens and maintaining tissue homeostasis are prerequisites for survival. Both of these functions are upheld by the immune system, though the latter is often overlooked in the context of the CNS. The mere presence of immune cells in the CNS was long considered a hallmark of pathology, but this view has been recently challenged by studies demonstrating that immunological signaling can confer pivotal neuroprotective effects on the injured CNS. In this review, we describe the temporal sequence of immunological events that follow CNS injury. Beginning with immediate changes at the injury site, including death of neural cells and release of damage-associated molecular patterns (DAMPs), and progressing through innate and adaptive immune responses, we describe the cascade of inflammatory mediators and the implications of their post-injury effects. We conclude by proposing a revised interpretation of immune privilege in the brain, which takes beneficial neuro-immune communications into account.

Role reversal: infiltrating T cells protect the brain

Inflammatory conditions intensify and then resolve, often sparing and recovering some of the injured tissue. While the ebb and flow of inflammation can be followed in many tissues, there is not a great deal of information on how inflammation regresses in the brain. In this issue of the JCI, Walsh, Hendrix, and colleagues illuminate a cellular mechanism whereby T cells that infiltrate the brain after nerve crush or contusion actually protect neurons from injury. These infiltrating T cells produce IL-4 and do so independently of a classic adaptive T cell immune response. The T cells respond to mediators produced by damaged neurons, without the classic three-way interaction among antigen, the major histocompatibility complex, and the T cell receptor. After brain injury, these protective T cells produce IL-4, which attenuates damage via IL-4 receptors on neurons.