Characterization of the interaction between astrocytes and encephalitogenic lymphocytes during the development of experimental autoimmune encephalitomyelitis (EAE) in mice

Neuroinflammation, memory, and depression: new approaches to hippocampal neurogenesis

As one of most common and severe mental disorders, major depressive disorder (MDD) significantly increases the risks of premature death and other medical conditions for patients. Neuroinflammation is the abnormal immune response in the brain, and its correlation with MDD is receiving increasing attention. Neuroinflammation has been reported to be involved in MDD through distinct neurobiological mechanisms, among which the dysregulation of neurogenesis in the dentate gyrus (DG) of the hippocampus (HPC) is receiving increasing attention. The DG of the hippocampus is one of two niches for neurogenesis in the adult mammalian brain, and neurotrophic factors are fundamental regulators of this neurogenesis process. The reported cell types involved in mediating neuroinflammation include microglia, astrocytes, oligodendrocytes, meningeal leukocytes, and peripheral immune cells which selectively penetrate the blood-brain barrier and infiltrate into inflammatory regions. This review summarizes the functions of the hippocampus affected by neuroinflammation during MDD progression and the corresponding influences on the memory of MDD patients and model animals.

Neuroinflammation: An astrocyte perspective

Astrocytes are abundant glial cells in the central nervous system (CNS) that play active roles in health and disease. Recent technologies have uncovered the functional heterogeneity of astrocytes and their extensive interactions with other cell types in the CNS. In this Review, we highlight the intricate interactions between astrocytes, other CNS-resident cells, and CNS-infiltrating cells as well as their potential therapeutic value in the context of inflammation and neurodegeneration.

iAstrocytes do not restrain T cell proliferation in vitro

The cross-talk between T cells and astrocytes occurring under physiological and, even more, neuroinflammatory conditions may profoundly impact the generation of adaptive immune responses in the nervous tissue. In this study, we used a standardized in vitro co-culture assay to investigate the immunomodulatory properties of astrocytes differing for age, sex, and species. Mouse neonatal astrocytes enhanced T cell vitality but suppressed T lymphocyte proliferation in response to mitogenic stimuli or myelin antigens, regardless of the Th1, Th2 or Th17 T cell phenotype. Studies comparing glia cells from adult and neonatal animals showed that adult astrocytes were more efficient in inhibiting T lymphocyte activation than neonatal astrocytes, regardless of their sex. Differently from primary cultures, mouse and human astrocytes derived from reprogrammed fibroblasts did not interfere with T cell proliferation. Overall, we describe a standardized astrocyte-T cell interaction in vitro assay and demonstrate that primary astrocytes and iAstrocytes may differ in modulating T cell function.

Phospholipase B Is Critical for Cryptococcus neoformans Survival in the Central Nervous System

Cryptococcus neoformans (Cn) is an opportunistic, encapsulated, yeast-like fungus that causes severe meningoencephalitis, especially in countries with high HIV prevalence. In addition to its well-known polysaccharide capsule, Cn has other virulence factors such as phospholipases, a heterogeneous group of enzymes that hydrolyze ester linkages in glycerophospholipids. Phospholipase B (PLB1) has been demonstrated to play a key role in Cn pathogenicity. In this study, we used a PLB1 mutant (plb1) and its reconstituted strain (Rec1) to assess the importance of this enzyme on Cn brain infection in vivo and in vitro. Mice infected with the plb1 strain survive significantly longer, have lower peripheral and central nervous system (CNS) fungal loads, and have fewer and smaller cryptococcomas or biofilm-like brain lesions compared to H99- and Rec1-infected animals. PLB1 causes extensive brain tissue damage and changes microglia morphology during cryptococcal disease, observations which can have important implications in patients with altered mental status or dementia as these manifestations are related to poorer survival outcomes. plb1 cryptococci are significantly more phagocytosed and killed by NR-9460 microglia-like cells. plb1 cells have altered capsular polysaccharide biophysical properties which impair their ability to stimulate glial cell responses or morphological changes. Here, we provide significant evidence demonstrating that Cn PLB1 is an important virulence factor for fungal colonization of and survival in the CNS as well as in the progression of cryptococcal meningoencephalitis. These findings may potentially help fill in a gap of knowledge in our understanding of cerebral cryptococcosis and provide novel research avenues in Cn pathogenesis. IMPORTANCE Cryptococcal meningoencephalitis (CME) is a serious disease caused by infection by the neurotropic fungal pathogen Cryptococcus neoformans. Due to the increasing number of cases in HIV-infected individuals, as well as the limited therapies available, investigation into potential targets for new therapeutics has become critical. Phospholipase B is an enzyme synthesized by Cn that confers virulence to the fungus through capsular enlargement, immunomodulation, and intracellular replication. In this study, we examined the properties of PLB1 by comparing infection of a Cn PLB1 mutant strain with both the wild-type and a PLB1-reconstituted strain. We show that PLB1 augments the survival and proliferation of the fungus in the CNS and strengthens virulence by modulating the immune response and enhancing specific biophysical properties of the fungus. PLB1 expression causes brain tissue damage and impacts glial cell functions, which may be responsible for the dementia observed in patients which may persist even after resolving from CME. The implications of PLB1 inhibition reveal its involvement in Cn infection and suggest that it may be a possible molecular target in the development of antifungal therapies. The results of this study support additional investigation into the mechanism of PLB1 to further understand the intricacies of cerebral Cn infection.

Establishment of an astrocyte-like cell line from the brain of tilapia (Oreochromis niloticus) for virus pathogenesis and a vitro model of the blood-brain barrier

A new cell line was established from the brain of a cultured fish, tilapia (Oreochromis niloticus), designated as TA-02 (Tilapia Astrocyte clone 02 cell line). The TA-02 cells are grown for 300 days in an L-15 medium supplemented with 10% fetal bovine serum (FBS). This cell line showed excellent proliferative capacity and expressed various neuroglial cell markers, including SOX2, SOX10, Hes1, Notch1, Occludin, E-cadherin, and GFAP. In addition, TA-02 cells were susceptible to Tilapia Lake Virus (TiLV) as demonstrated by the presence of a severe cytopathic effect (CPE), virus particle in a transmission electron microscope (TEM), and PCR positive signal. Bacterial cytotoxicity studies showed that Streptococcus agalactiae was toxic to TA-02 cells. When co-culture with trans-well, TA-02 exhibited prominent barrier properties, manifested by tight intercellular junctions and increased trans-endothelial electrical resistance (TEER). In addition, the barrier is effective against Escherichia coli (non-meningitis pathogenic bacteria). In contrast, S. agalactiae (meningitis pathogenic bacteria) can pass through the membrane comprising the cells in the trans-well insert. The newly established TA-02 cell line provided a valuable tool for virus pathogenesis and a vitro model of the fish blood-brain barrier.

CC chemokine receptor 5 antagonist alleviates inflammation by regulating IFN-γ/IL-10 and STAT4/Smad3 signaling in a mouse model of autoimmune encephalomyelitis

Multiple sclerosis (MS) is an immunopathological disease that causes demyelination and recurrent episodes of T cell-mediated immune attack in the central nervous system. Experimental autoimmune encephalomyelitis (EAE) is a well-established mouse model of MS. The roles of T cells in MS/EAE have been well investigated, but little is known about the role of CCR5⁺ cells. In the present study, we investigated whether treatment with DAPTA, a selective CCR5 antagonist, could modulate the progression of EAE in the SJL/J mice. EAE mice were treated with DAPTA (0.01 mg/kg) intraperitoneally daily from day 14 to day 42, and the clinical scores were evaluated. We further investigated the effects of DAPTA on IFN-γ-, TGF-β-, IL-10-, IL-17A-, IL-22-, T-bet, STAT4-, RORγT-, AhR-, Smad3-, and Foxp3-expressing CCR5⁺ spleen cells using flow cytometry analysis. We further explored the effects of DAPTA on mRNA/protein expression of IFN-γ, IL-10, IL-17A, IL-22, TGF-β, T-bet, STAT4, RORγT, AhR, Foxp3, and NF-H in the brain tissue. The severity of clinical scores decreased in DAPTA-treated EAE mice as compared to that in the EAE control mice. Moreover, the percentage of CCR5⁺IFN-γ⁺, CCR5⁺T-bet⁺, CCR5⁺STAT4⁺, CCR5⁺IL-17A⁺, CCR5⁺RORγt⁺, CCR5⁺IL-22⁺, and CCR5⁺AhR⁺ cells decreased while CCR5⁺TGF-β⁺, CCR5⁺IL-10⁺, CCR5⁺Smad3⁺, and CCR5⁺Foxp3⁺ increased in DAPTA-treated EAE mice. Furthermore, DAPTA treatment significantly mitigated the EAE-induced expression of T-bet, STAT4, IL-17A, RORγT, IL-22, and AhR but upregulated Foxp3, IL-10, and NF-H expression in the brain tissue. Taken together, our data demonstrated that DAPTA could ameliorate EAE progression through the downregulation of the inflammation-related cytokines and transcription factors signaling, which may be useful for the clinical therapy of MS.

Astrocytes and Inflammatory T Helper Cells: A Dangerous Liaison in Multiple Sclerosis

Multiple Sclerosis (MS) is a neurodegenerative autoimmune disorder of the central nervous system (CNS) characterized by the recruitment of self-reactive T lymphocytes, mainly inflammatory T helper (Th) cell subsets. Once recruited within the CNS, inflammatory Th cells produce several inflammatory cytokines and chemokines that activate resident glial cells, thus contributing to the breakdown of blood-brain barrier (BBB), demyelination and axonal loss. Astrocytes are recognized as key players of MS immunopathology, which respond to Th cell-defining cytokines by acquiring a reactive phenotype that amplify neuroinflammation into the CNS and contribute to MS progression. In this review, we summarize current knowledge of the astrocytic changes and behaviour in both MS and experimental autoimmune encephalomyelitis (EAE), and the contribution of pathogenic Th1, Th17 and Th1-like Th17 cell subsets, and CD8⁺ T cells to the morphological and functional modifications occurring in astrocytes and their pathological outcomes.

Functional immune cell-astrocyte interactions

Astrocytes are abundant glial cells in the central nervous system (CNS) that control multiple aspects of health and disease. Through their interactions with components of the blood–brain barrier (BBB), astrocytes not only regulate BBB function, they also sense molecules produced by peripheral immune cells, including cytokines. Here, we review the interactions between immune cells and astrocytes and their roles in health and neurological diseases, with a special focus on multiple sclerosis (MS). We highlight known pathways that participate in astrocyte crosstalk with microglia, NK cells, T cells, and other cell types; their contribution to the pathogenesis of neurological diseases; and their potential value as therapeutic targets.

Cryptococcus neoformans-astrocyte interactions: effect on fungal blood brain barrier disruption, brain invasion, and meningitis progression

Cryptococcus neoformans is an opportunistic, neurotropic, and encapsulated fungus that causes life-threatening cryptococcal meningitis (CM), especially in regions of the world where AIDS is endemic. The polysaccharide capsule of C. neoformans is the fungus major virulent factor, being copiously released during infection and causing immunosuppressive defects in the host. Although the capsular material is commonly associated with reactive astrocytes in fatal CM, little is known about the molecular and cellular interactions among astroglia and C. neoformans. As astrocytes also make up the neurovascular unit at the blood-brain barrier (BBB), which C. neoformans must transverse to colonize the central nervous system and cause CM; these cells may play a significant regulatory role in the prevention and progression of infection. For example, astrocytes are implicated in neurological disease including the regulation of cerebral intracranial pressure, immune function, and water homeostasis. Hence, in this review, we provide a general overview of astroglia biology and discuss the current knowledge on C. neoformans-astrocyte interactions including their involvement in the development of CM. This "gliocentric view" of cerebral cryptococcosis suggests that therapeutic interventions particularly targeting at preserving the neuroprotective function of astrocytes may be used in preventing and managing C. neoformans BBB transmigration, brain invasion, colonization, and meningitis.

Astrocyte Crosstalk in CNS Inflammation

Astrocytes control multiple processes in the nervous system in health and disease. It is now clear that specific astrocyte subsets or activation states are associated with specific genomic programs and functions. The advent of novel genomic technologies has enabled rapid progress in the characterization of astrocyte heterogeneity and its control by astrocyte interactions with other cells in the central nervous system (CNS). In this review, we provide an overview of the multifaceted roles of astrocytes in the context of CNS inflammation, highlighting recent discoveries on astrocyte subsets and their regulation. We explore mechanisms of crosstalk between astrocytes and other cells in the CNS in the context of neuroinflammation and neurodegeneration and discuss how these interactions shape pathological outcomes.

Exocytosis of large-diameter lysosomes mediates interferon γ-induced relocation of MHC class II molecules toward the surface of astrocytes

Astrocytes are the key homeostatic cells in the central nervous system; initiation of reactive astrogliosis contributes to neuroinflammation. Pro-inflammatory cytokine interferon γ (IFNγ) induces the expression of the major histocompatibility complex class II (MHCII) molecules, involved in antigen presentation in reactive astrocytes. The pathway for MHCII delivery to the astrocyte plasma membrane, where MHCII present antigens, is unknown. Rat astrocytes in culture and in organotypic slices were exposed to IFNγ to induce reactive astrogliosis. Astrocytes were probed with optophysiologic tools to investigate subcellular localization of immunolabeled MHCII, and with electrophysiology to characterize interactions of single vesicles with the plasmalemma. In culture and in organotypic slices, IFNγ augmented the astrocytic expression of MHCII, which prominently co-localized with lysosomal marker LAMP1-EGFP, modestly co-localized with Rab7, and did not co-localize with endosomal markers Rab4A, EEA1, and TPC1. MHCII lysosomal localization was corroborated by treatment with the lysosomolytic agent glycyl-L-phenylalanine-β-naphthylamide, which reduced the number of MHCII-positive vesicles. The surface presence of MHCII was revealed by immunolabeling of live non-permeabilized cells. In IFNγ-treated astrocytes, an increased fraction of large-diameter exocytotic vesicles (lysosome-like vesicles) with prolonged fusion pore dwell time and larger pore conductance was recorded, whereas the rate of endocytosis was decreased. Stimulation with ATP, which triggers cytosolic calcium signaling, increased the frequency of exocytotic events, whereas the frequency of full endocytosis was further reduced. In IFNγ-treated astrocytes, MHCII-linked antigen surface presentation is mediated by increased lysosomal exocytosis, whereas surface retention of antigens is prolonged by concomitant inhibition of endocytosis.

Epigenetic insights into multiple sclerosis disease progression

Multiple sclerosis (MS), a chronic inflammatory demyelinating and neurodegenerative disease of the central nervous system, is today a leading cause of unpredictable lifelong disability in young adults. The treatment of patients in progressive stages remains highly challenging, alluding to our limited understanding of the underlying pathological processes. In this review, we provide insights into the mechanisms underpinning MS progression from a perspective of epigenetics, that refers to stable and mitotically heritable, yet reversible, changes in the genome activity and gene expression. We first recapitulate findings from epigenetic studies examining the brain tissue of progressive MS patients, which support a contribution of DNA and histone modifications in impaired oligodendrocyte differentiation, defective myelination/remyelination and sustained neuro-axonal vulnerability. We next explore possibilities for identifying factors affecting progression using easily accessible tissues such as blood by comparing epigenetic signatures in peripheral immune cells and brain tissue. Despite minor overlap at individual methylation sites, nearly 30% of altered genes reported in peripheral immune cells of progressive MS patients were found in brain tissue, jointly converging on alterations of neuronal functions. We further speculate about the mechanisms underlying shared epigenetic patterns between blood and brain, which likely imply the influence of internal (genetic control) and/or external (e.g. smoking and ageing) factors imprinting a common signature in both compartments. Overall, we propose that epigenetics might shed light on clinically relevant mechanisms involved in disease progression and open new avenues for the treatment of progressive MS patients in the future.

Liver kinase B1 depletion from astrocytes worsens disease in a mouse model of multiple sclerosis

Liver kinase B1 (LKB1) is a ubiquitously expressed kinase involved in the regulation of cell metabolism, growth, and inflammatory activation. We previously reported that a single nucleotide polymorphism in the gene encoding LKB1 is a risk factor for multiple sclerosis (MS). Since astrocyte activation and metabolic function have important roles in regulating neuroinflammation and neuropathology, we examined the serine/threonine kinase LKB1 in astrocytes in a chronic experimental autoimmune encephalomyelitis mouse model of MS. To reduce LKB1, a heterozygous astrocyte-selective conditional knockout (het-cKO) model was used. While disease incidence was similar, disease severity was worsened in het-cKO mice. RNAseq analysis identified Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways enriched in het-cKO mice relating to mitochondrial function, confirmed by alterations in mitochondrial complex proteins and reductions in mRNAs related to astrocyte metabolism. Enriched pathways included major histocompatibility class II genes, confirmed by increases in MHCII protein in spinal cord and cerebellum of het-cKO mice. We observed increased numbers of CD4+ Th17 cells and increased neuronal damage in spinal cords of het-cKO mice, associated with reduced expression of choline acetyltransferase, accumulation of immunoglobulin-γ, and reduced expression of factors involved in motor neuron survival. In vitro, LKB1-deficient astrocytes showed reduced metabolic function and increased inflammatory activation. These data suggest that metabolic dysfunction in astrocytes, in this case due to LKB1 deficiency, can exacerbate demyelinating disease by loss of metabolic support and increase in the inflammatory environment.

Astrocytic Interleukin-15 Reduces Pathology of Neuromyelitis Optica in Mice

Astrocyte loss induced by neuromyelitis optica (NMO)-IgG and complement-dependent cytotoxicity (CDC) is the hallmark of NMO pathology. The survival of astrocytes is thought to reflect astrocyte exposure to environmental factors in the CNS and the response of astrocytes to these factors. However, still unclear are how astrocytes respond to NMO-IgG and CDC, and what CNS environmental factors may impact the survival of astrocytes. In a murine model of NMO induced by intracerebral injection of NMO-IgG and human complement, we found dramatic upregulation of IL-15 in astrocytes. To study the role of astrocytic IL-15 in NMO, we generated a transgenic mouse line with targeted expression of IL-15 in astrocytes (IL-15^tg), in which the expression of IL-15 is controlled by a glial fibrillary acidic protein promoter. We showed that astrocyte-targeted expression of IL-15 attenuates astrocyte injury and the loss of aquaporin-4 in the brain. Reduced blood–brain barrier leakage and immune cell infiltration are also found in the lesion of IL-15^tg mice subjected to NMO induction. IL-15^tg astrocytes are less susceptible to NMO-IgG-mediated CDC than their wild-type counterparts. The enhanced resistance of IL-15^tg astrocytes to cytotoxicity and cell death involves NF-κB signaling pathway. Our findings suggest that IL-15 reduces astrocyte loss and NMO pathology.

Effectors of Th1 and Th17 cells act on astrocytes and augment their neuroinflammatory properties

Background

Autoreactive Th1 and Th17 cells are believed to mediate the pathology of multiple sclerosis in the central nervous system (CNS). Their interaction with microglia and astrocytes in the CNS is crucial for the regulation of the neuroinflammation. Previously, we have shown that only Th1 but not Th17 effectors activate microglia. However, it is not clear which cells are targets of Th17 effectors in the CNS.

Methods

To understand the effects driven by Th17 cells in the CNS, we induced experimental autoimmune encephalomyelitis in wild-type mice and CD4⁺ T cell-specific integrin α4-deficient mice where trafficking of Th1 cells into the CNS was affected. We compared microglial and astrocyte response in the brain and spinal cord of these mice. We further treated astrocytes with supernatants from highly pure Th1 and Th17 cultures and assessed the messenger RNA expression of neurotrophic factors, cytokines and chemokines, using real-time PCR. Data obtained was analyzed using the Kruskal-Wallis test.

Results

We observed in α4-deficient mice weak microglial activation but comparable astrogliosis to that of wild-type mice in the regions of the brain populated with Th17 infiltrates, suggesting that Th17 cells target astrocytes and not microglia. In vitro, in response to supernatants from Th1 and Th17 cultures, astrocytes showed altered expression of neurotrophic factors, pro-inflammatory cytokines and chemokines. Furthermore, increased expression of chemokines in Th1- and Th17-treated astrocytes enhanced recruitment of microglia and transendothelial migration of Th17 cells in vitro.

Conclusion

Our results demonstrate the delicate interaction between T cell subsets and glial cells and how they communicate to mediate their effects. Effectors of Th1 act on both microglia and astrocytes whereas Th17 effectors preferentially target astrocytes to promote neuroinflammation.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-0978-3) contains supplementary material, which is available to authorized users.

HIV-1 increases TLR responses in human primary astrocytes

Astrocytes are the major glial cell within the central nervous system and have a number of important physiological properties related to brain homeostasis. They provide trophic support to neurons and are immune cells with key roles during states-of-inflammation. The potential for production of proinflammatory cytokines and its consequences has been studied in the context of HIV-1 infection of normal human astrocytes (NHA). NHA express TLR3, TLR4, and TLR5. TLR3 ligation induced the strongest proinflammatory polarizing response, characterized by generation of high levels of TNF-α, IL-6, and IL-8. HIV-1 increased the transient production of key inflammatory mediators, and exposure to LPS of HIV-1-infected cells increased significantly the cytokine secretion. We confirmed that it is necessary viral gene expression from the moment of pretreatment with antiretrovirals inhibited totally HIV-1-induced TLR response. The higher response to LPS from HIV-1-infected cells did not correlate with TLR4 or MyD88 increased expression. LPS responsiveness of infected cells parallels MHC class II expression, but not CD14. HIV-1-infected NHA present increased sensitivity to the proinflammatory effects of LPS. If this phenomenon occurs in vivo, it will contribute to the immunopathogenesis of this disease and may ultimately offer novel targets for immunomodulatory therapy.

Production of IL-27 in multiple sclerosis lesions by astrocytes and myeloid cells: Modulation of local immune responses

The mechanisms whereby human glial cells modulate local immune responses are not fully understood. Interleukin-27 (IL-27), a pleiotropic cytokine, has been shown to dampen the severity of experimental autoimmune encephalomyelitis, but it is still unresolved whether IL-27 plays a role in the human disease multiple sclerosis (MS). IL-27 contribution to local modulation of immune responses in the brain of MS patients was investigated. The expression of IL-27 subunits (EBI3 and p28) and its cognate receptor IL-27R (the gp130 and TCCR chains) was elevated within post-mortem MS brain lesions compared with normal control brains. Moreover, astrocytes (GFAP(+) cells) as well as microglia and macrophages (Iba1(+) cells) were important sources of IL-27. Brain-infiltrating CD4 and CD8 T lymphocytes expressed the IL-27R specific chain (TCCR) implying that these cells could respond to local IL-27 sources. In primary cultures of human astrocytes inflammatory cytokines increased IL-27 production, whereas myeloid cell inflammatory M1 polarization and inflammatory cytokines enhanced IL-27 expression in microglia and macrophages. Astrocytes in postmortem tissues and in vitro expressed IL-27R. Moreover, IL-27 triggered the phosphorylation of the transcription regulator STAT1, but not STAT3 in human astrocytes; indeed IL-27 up-regulated MHC class I expression on astrocytes in a STAT1-dependent manner. These findings demonstrated that IL-27 and its receptor were elevated in MS lesions and that local IL-27 can modulate immune properties of astrocytes and infiltrating immune cells. Thus, therapeutic strategies targeting IL-27 may influence not only peripheral but also local inflammatory responses within the brain of MS patients.

Control of autoimmune CNS inflammation by astrocytes

Multiple sclerosis is a neurologic disease caused by immune cell infiltration into the central nervous system, resulting in gray and white matter inflammation, progressive demyelination, and neuronal loss. Astrocytes, the most abundant cell population in the central nervous system (CNS), have been considered inert scaffold or housekeeping cells for many years. However, recently, it has become clear that this cell population actively modulates the immune response in the CNS at multiple levels. While being exposed to a plethora of cytokines during ongoing autoimmune inflammation, astrocytes modulate local CNS inflammation by secreting cytokines and chemokines, among other factors. This review article gives an overview of the most recent understanding about cytokine networks operational in astrocytes during autoimmune neuroinflammation and highlights potential targets for immunomodulatory therapies for multiple sclerosis.

Interaction of astrocytes and T cells in physiological and pathological conditions

The central nervous system (CNS) has long been recognized as a site of 'immune privilege' because of the existence of the blood brain barrier (BBB) which presumably isolates CNS from the peripheral immunosurveillance. Different from the peripheral organs, CNS is unique in response to all forms of CNS injury and disease which is mainly mediated by resident microglia and astrocyte. There is increasing evidence that immune cells are not only involved in neuroinflammation process but also the maintenance of CNS homeostasis. T cells, an important immune cell population, are involved in the pathogenesis of some neurological diseases by inducing either innate or adaptive immune responses. Astrocytes, which are the most abundant cell type in the CNS, maintain the integrity of BBB and actively participate in the initiation and progression of neurological diseases. Surprisingly, how astrocytes and T cells interact and the consequences of their interaction are not clear. In this review we briefly summarized T cells diversity and astrocyte function. Then, we examined the evidence for the astrocytes and T cells interaction under physiological and pathological conditions including ischemic stroke, multiple sclerosis, viral infection, and Alzheimer's disease. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

Toll-like receptor signaling directly increases functional IL-17RA expression in neuroglial cells

IL-17, the hallmark cytokine of Th17 cells, plays a pivotal role in the pathogenesis of autoimmune diseases, including encephalomyelitis. In the central nervous system, neuroglial cells are the main residents that express IL-17R and respond to IL-17 by producing chemokines/cytokines and boosting local inflammation. Factors that influence the IL-17R expression in neuroglial cells can also exert their impacts on the outbreak, progression and outcome of encephalomyelitis. Here, we reported that Toll-like receptor signaling has its bias for promoting the IL-17RA, but not the IL-17RC, expression in mouse neuroglial cells in a T cell infiltration independent manner. Elevated IL-17R functionally responded to IL-17 by secreting more chemokines and accelerating CD4 cell migration. First, real-time PCR confirmed that the expression of Il-17ra, but not Il-17rc, was significantly increased in the brain and spinal cord of EAE-induced mice. This effect was elicited by something in complete Freund's adjuvant (CFA), because markedly increased IL-17R was detected in mice immunized with CFA only, even though no evidence of EAE was found. Furthermore, in Rag1(-/-) mice, it was confirmed that CFA could augment the IL-17RA expression in the CNS in the absence of T cell infiltration. In vivo immunization with TLR ligands and in vitro treatment of purified neuroglial cells demonstrated that TLR ligands directly and effectively evoke the IL-17RA expression in the CNS and in cultured astrocytes, microglia and oligodendrocytes. LPS was the most effective inducer of the IL-17RA expression in astrocytes, and polyIC was superior to LPS for microglia and oligodendrocytes. Activated CD4 cells can also promote the secretion of chemokines by LPS pre-treated astrocytes, and hence accelerate the migration of CD4 cells, which was blocked by the neutralization of IL-17RA on the surface of the astrocyte. Taken together, we concluded that TLR signaling can directly stimulate the expression of IL-17RA, but not IL-17RC, in neuroglial cells, which functionally respond to IL-17A by secreting chemokines, accelerating CD4 cell migration, and contributing to the pathogenesis of encephalomyelitis.

Immune privilege as an intrinsic CNS property: astrocytes protect the CNS against T-cell-mediated neuroinflammation

Astrocytes have many functions in the central nervous system (CNS). They support differentiation and homeostasis of neurons and influence synaptic activity. They are responsible for formation of the blood-brain barrier (BBB) and make up the glia limitans. Here, we review their contribution to neuroimmune interactions and in particular to those induced by the invasion of activated T cells. We discuss the mechanisms by which astrocytes regulate pro- and anti-inflammatory aspects of T-cell responses within the CNS. Depending on the microenvironment, they may become potent antigen-presenting cells for T cells and they may contribute to inflammatory processes. They are also able to abrogate or reprogram T-cell responses by inducing apoptosis or secreting inhibitory mediators. We consider apparently contradictory functions of astrocytes in health and disease, particularly in their interaction with lymphocytes, which may either aggravate or suppress neuroinflammation.