Astrocyte-induced T cell elimination is CD95 ligand dependent

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.

The astrocyte LAMP lights a T cell TRAIL of death

In a recent issue of Nature, Sanmarco et al. reveal a novel mechanism by which astrocytes maintain an anti-inflammatory state in the central nervous system (CNS). IFNγ released by gut-licensed meningeal NK cells was found to induce TRAIL expression on astrocytes, causing effector T cell apoptosis.

Decipher the Glioblastoma Microenvironment: The First Milestone for New Groundbreaking Therapeutic Strategies

Glioblastoma (GBM) is the most common primary malignant brain tumour in adults. Despite the combination of novel therapeutical approaches, it remains a deadly malignancy with an abysmal prognosis. GBM is a polymorphic tumour from both molecular and histological points of view. It consists of different malignant cells and various stromal cells, contributing to tumour initiation, progression, and treatment response. GBM’s microenvironment is multifaceted and is made up of soluble factors, extracellular matrix components, tissue-resident cell types (e.g., neurons, astrocytes, endothelial cells, pericytes, and fibroblasts) together with resident (e.g., microglia) or recruited (e.g., bone marrow-derived macrophages) immune cells. These latter constitute the so-called immune microenvironment, accounting for a substantial GBM’s tumour volume. Despite the abundance of immune cells, an intense state of tumour immunosuppression is promoted and developed; this represents the significant challenge for cancer cells’ immune-mediated destruction. Though literature data suggest that distinct GBM’s subtypes harbour differences in their microenvironment, its role in treatment response remains obscure. However, an in-depth investigation of GBM’s microenvironment may lead to novel therapeutic opportunities to improve patients’ outcomes. This review will elucidate the GBM’s microenvironment composition, highlighting the current state of the art in immunotherapy approaches. We will focus on novel strategies of active and passive immunotherapies, including vaccination, gene therapy, checkpoint blockade, and adoptive T-cell therapies.

Astrocytes in the regulation of cerebrovascular functions

Astrocytes are the most numerous type of neuroglia in the brain and have a predominant influence on the cerebrovascular system; they control perivascular homeostasis, the integrity of the blood-brain barrier, the dialogue with the peripheral immune system, the transfer of metabolites from the blood, and blood vessel contractility in response to neuronal activity. These regulatory processes occur in a specialized interface composed of perivascular astrocyte extensions that almost completely cover the cerebral blood vessels. Scientists have only recently started to study how this interface is formed and how it influences cerebrovascular functions. Here, we review the literature on the astrocytes' role in the regulation of the cerebrovascular system. We cover the anatomy and development of the gliovascular interface, the known gliovascular functions, and molecular factors, the latter's implication in certain pathophysiological situations, and recent cutting-edge experimental tools developed to examine the astrocytes' role at the vascular interface. Finally, we highlight some open questions in this field of research.

Harnessing regulatory T cell neuroprotective activities for treatment of neurodegenerative disorders

Emerging evidence demonstrates that adaptive immunity influences the pathobiology of neurodegenerative disorders. Misfolded aggregated self-proteins can break immune tolerance leading to the induction of autoreactive effector T cells (Teffs) with associated decreases in anti-inflammatory neuroprotective regulatory T cells (Tregs). An imbalance between Teffs and Tregs leads to microglial activation, inflammation and neuronal injury. The cascade of such a disordered immunity includes the drainage of the aggregated protein antigens into cervical lymph nodes serving to amplify effector immune responses. Both preclinical and clinical studies demonstrate transformation of this altered immunity for therapeutic gain. We posit that the signs and symptoms of common neurodegenerative disorders such as Alzheimer's and Parkinson's diseases, amyotrophic lateral sclerosis, and stroke can be attenuated by boosting Treg activities.

Spinal Cord Tumor Microenvironment

Intramedullary spinal cord tumors (IMSCT) are rare entities for which there currently exist no standardized treatment paradigms. Consequently, patients usually receive treatment modalities that were established for intracerebral tumors; these approaches, however, typically result in functional impairment, recurrent tumor growth, and short overall survival. There is a distinct lack of promising research efforts in this field, which raises questions about whether spinal cord tumor microenvironment (TME) might promote the development, progression, and treatment resistance of IMSCT. In this review, we aim to examine spinal cord biology, compare spinal cord and brain microenvironments, and discuss mutual interactions between IMSCT and TME. Manipulating these pathways may provide new treatment approaches for future patient groups.

A T Cell Suppressive Circuitry Mediated by CD39 and Regulated by ShcC/Rai Is Induced in Astrocytes by Encephalitogenic T Cells

Multiple sclerosis is an autoimmune disease caused by autoreactive immune cell infiltration into the central nervous system leading to inflammation, demyelination, and neuronal loss. While myelin-reactive Th1 and Th17 are centrally implicated in multiple sclerosis pathogenesis, the local CNS microenvironment, which is shaped by both infiltrated immune cells and central nervous system resident cells, has emerged a key player in disease onset and progression. We have recently demonstrated that ShcC/Rai is as a novel astrocytic adaptor whose loss in mice protects from experimental autoimmune encephalomyelitis. Here, we have explored the mechanisms that underlie the ability of Rai^-/- astrocytes to antagonize T cell-dependent neuroinflammation. We show that Rai deficiency enhances the ability of astrocytes to upregulate the expression and activity of the ectonucleotidase CD39, which catalyzes the conversion of extracellular ATP to the immunosuppressive metabolite adenosine, through both contact-dependent and-independent mechanisms. As a result, Rai-deficient astrocytes acquire an enhanced ability to suppress T-cell proliferation, which involves suppression of T cell receptor signaling and upregulation of the inhibitory receptor CTLA-4. Additionally, Rai-deficient astrocytes preferentially polarize to the neuroprotective A2 phenotype. These results identify a new mechanism, to which Rai contributes to a major extent, by which astrocytes modulate the pathogenic potential of autoreactive T cells.

Turning "Cold" Into "Hot" Tumors-Opportunities and Challenges for Radio-Immunotherapy Against Primary and Metastatic Brain Cancers

The development of immunotherapies has revolutionized intervention strategies for a variety of primary cancers. Despite this promising progress, treatment options for primary brain cancer and brain metastasis remain limited and still largely depend on surgical resection, radio- and/or chemotherapy. The paucity in the successful development of immunotherapies for brain cancers can in part be attributed to the traditional view of the brain as an immunologically privileged site. The presence of the blood-brain barrier and the absence of lymphatic drainage were believed to restrict the entry of blood-borne immune and inflammatory cells into the central nervous system (CNS), leading to an exclusion of the brain from systemic immune surveillance. However, recent insight from pre-clinical and clinical studies on the immune landscape of brain cancers challenged this dogma. Recruitment of blood-borne immune cells into the CNS provides unprecedented opportunities for the development of tumor microenvironment (TME)-targeted or immunotherapies against primary and metastatic cancers. Moreover, it is increasingly recognized that in addition to genotoxic effects, ionizing radiation represents a critical modulator of tumor-associated inflammation and synergizes with immunotherapies in adjuvant settings. This review summarizes current knowledge on the cellular and molecular identity of tumor-associated immune cells in primary and metastatic brain cancers and discusses underlying mechanisms by which ionizing radiation modulates the immune response. Detailed mechanistic insight into the effects of radiation on the unique immune landscape of brain cancers is essential for the development of multimodality intervention strategies in which immune-modulatory effects of radiotherapy are exploited to sensitize brain cancers to immunotherapies by converting immunologically “cold” into “hot” environments.

A Blazing Landscape: Neuroinflammation Shapes Brain Metastasis

Brain metastases are more common than primary CNS tumors and confer grave prognosis on patients, as existing treatments have very limited efficacy. The tumor microenvironment has a central role in facilitating tumorigenesis and metastasis. In recent years, there has been much progress in our understanding of the functional role of the brain metastatic microenvironment. In this review, we discuss the latest advances in brain metastasis research, with special emphasis on the role of the brain microenvironment and neuroinflammation, integrating insights from comparable findings in neuropathologies and primary CNS tumors. In addition, we overview findings on the formation of a hospitable metastatic niche and point out the major gaps in knowledge toward developing new therapeutics that will cotarget the stromal compartment in an effort to improve the treatment and prevention of brain metastases.

Immunoregulation at the gliovascular unit in the healthy brain: A focus on Connexin 43

In the brain, immune cell infiltration is normally kept at a very low level and a unique microenvironment strictly restricts immune reactions and inflammation. Even in such quiescent environment, a constant immune surveillance is at work allowing the brain to rapidly react to threats. To date, knowledge about the factors regulating the brain-immune system interrelationship in healthy conditions remains elusive. Interestingly, astrocytes, the most abundant glial cells in the brain, may participate in many aspects of this unique homeostasis, in particular due to their close interaction with the brain vascular system and expression of a specific molecular repertoire. Indeed, astrocytes maintain the blood-brain barrier (BBB) integrity, interact with immune cells, and participate in the regulation of intracerebral liquid movements. We recently showed that Connexin 43 (Cx43), a gap junction protein highly expressed by astrocytes at the BBB interface, is an immunoregulating factor. The absence of astroglial Cx43 leads to a transient endothelial activation, a continuous immune recruitment as well as the development of a specific humoral autoimmune response against the von Willebrand factor A domain-containing protein 5a, an extracellular matrix protein expressed by astrocytes. In this review, we propose to gather current knowledge on how astrocytes may influence the immune system in the healthy brain, focusing on their roles at the gliovascular interface. We will also consider pathological situations involving astrocyte-specific autoimmunities. Finally, we will discuss the specific role of astroglial Cx43 and the physiological consequences of immune regulations taking place on inflammation, cognition and behavior in the absence of Cx43.

The Role of Astrocytes in Multiple Sclerosis Progression

Multiple sclerosis (MS) is an inflammatory disorder causing central nervous system (CNS) demyelination and axonal injury. Although its etiology remains elusive, several lines of evidence support the concept that autoimmunity plays a major role in disease pathogenesis. The course of MS is highly variable; nevertheless, the majority of patients initially present a relapsing–remitting clinical course. After 10–15 years of disease, this pattern becomes progressive in up to 50% of untreated patients, during which time clinical symptoms slowly cause constant deterioration over a period of many years. In about 15% of MS patients, however, disease progression is relentless from disease onset. Published evidence supports the concept that progressive MS reflects a poorly understood mechanism of insidious axonal degeneration and neuronal loss. Recently, the type of microglial cell and of astrocyte activation and proliferation observed has suggested contribution of resident CNS cells may play a critical role in disease progression. Astrocytes could contribute to this process through several mechanisms: (a) as part of the innate immune system, (b) as a source of cytotoxic factors, (c) inhibiting remyelination and axonal regeneration by forming a glial scar, and (d) contributing to axonal mitochondrial dysfunction. Furthermore, regulatory mechanisms mediated by astrocytes can be affected by aging. Notably, astrocytes might also limit the detrimental effects of pro-inflammatory factors, while providing support and protection for oligodendrocytes and neurons. Because of the dichotomy observed in astrocytic effects, the design of therapeutic strategies targeting astrocytes becomes a challenging endeavor. Better knowledge of molecular and functional properties of astrocytes, therefore, should promote understanding of their specific role in MS pathophysiology, and consequently lead to development of novel and more successful therapeutic approaches.

Astrocyte roles in traumatic brain injury

Astrocytes sense changes in neural activity and extracellular space composition. In response, they exert homeostatic mechanisms critical for maintaining neural circuit function, such as buffering neurotransmitters, modulating extracellular osmolarity and calibrating neurovascular coupling. In addition to upholding normal brain activities, astrocytes respond to diverse forms of brain injury with heterogeneous and progressive changes of gene expression, morphology, proliferative capacity and function that are collectively referred to as reactive astrogliosis. Traumatic brain injury (TBI) sets in motion complex events in which noxious mechanical forces cause tissue damage and disrupt central nervous system (CNS) homeostasis, which in turn trigger diverse multi-cellular responses that evolve over time and can lead either to neural repair or secondary cellular injury. In response to TBI, astrocytes in different cellular microenvironments tune their reactivity to varying degrees of axonal injury, vascular disruption, ischemia and inflammation. Here we review different forms of TBI-induced astrocyte reactivity and the functional consequences of these responses for TBI pathobiology. Evidence regarding astrocyte contribution to post-traumatic tissue repair and synaptic remodeling is examined, and the potential for targeting specific aspects of astrogliosis to ameliorate TBI sequelae is considered.

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.

Vitamin D Actions on CD4(+) T Cells in Autoimmune Disease

This review summarizes and integrates research on vitamin D and CD4⁺ T-lymphocyte biology to develop new mechanistic insights into the molecular etiology of autoimmune disease. A deep understanding of molecular mechanisms relevant to gene–environment interactions is needed to deliver etiology-based autoimmune disease prevention and treatment strategies. Evidence linking sunlight, vitamin D, and the risk of multiple sclerosis and type 1 diabetes is summarized to develop the thesis that vitamin D is the environmental factor that most strongly influences autoimmune disease development. Evidence for CD4⁺ T-cell involvement in autoimmune disease pathogenesis and for paracrine calcitriol signaling to CD4⁺ T lymphocytes is summarized to support the thesis that calcitriol is sunlight’s main protective signal transducer in autoimmune disease risk. Animal modeling and human mechanistic data are summarized to support the view that vitamin D probably influences thymic negative selection, effector Th1 and Th17 pathogenesis and responsiveness to extrinsic cell death signals, FoxP3⁺CD4⁺ T-regulatory cell and CD4⁺ T-regulatory cell type 1 (Tr1) cell functions, and a Th1–Tr1 switch. The proposed Th1–Tr1 switch appears to bridge two stable, self-reinforcing immune states, pro- and anti-inflammatory, each with a characteristic gene regulatory network. The bi-stable switch would enable T cells to integrate signals from pathogens, hormones, cell–cell interactions, and soluble mediators and respond in a biologically appropriate manner. Finally, unanswered questions and potentially informative future research directions are highlighted to speed delivery of etiology-based strategies to reduce autoimmune disease.

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.

Galectin-9 protein is up-regulated in astrocytes by tumor necrosis factor and promotes encephalitogenic T-cell apoptosis

Demyelination and axonal damage in multiple sclerosis (MS) are thought to be a consequence of inflammatory processes that are perpetuated by activated glia and infiltrating leukocytes. Galectin-9 is a β-galactoside binding lectin capable of modulating immune responses and appears to be up-regulated in MS. However, its role in the pathogenesis of MS has yet to be determined. Here, we report that proinflammatory cytokines induce galectin-9 (Gal-9) expression in primary astrocytes and the mechanism by which TNF up-regulates Gal-9. Astrocytes did not express Gal-9 under basal conditions nor did IL-6, IL-10, or IL-13 trigger Gal-9 expression. In contrast, IL-1β, IFN-γ, and particularly TNF up-regulated Gal-9 in astrocytes. TNF-induced Gal-9 expression was dependent on TNF receptor 1 (TNFR1) as TNF failed to induce Gal-9 in TNFR1(-/-) astrocytes. Blockade of the JNK MAP kinase pathway with the JNK inhibitor SP600125 abrogated TNF-induced Gal-9, whereas p38 and MEK inhibitors had minimal effects. Furthermore, specific knockdown of c-Jun via siRNA in astrocytes before TNF treatment greatly suppressed Gal-9 transcription, suggesting that TNF induces astroglial Gal-9 through the TNF/TNFR1/JNK/cJun signaling pathway. Finally, utilizing astrocytes from Lgals9 mutant (Gal-9(-/-)) mice as well as a myelin basic protein-specific Tim-3(+) encephalitogenic T-cell clone (LCN-8), we found that conditioned medium from TNF-stimulated Gal-9(+/+) but not Gal-9(-/-) astrocytes increased the percentage of apoptotic encephalitogenic T-cells. Together, our results suggest that Gal-9 is induced in astrocytes by TNF via the JNK/c-Jun pathway and that astrocyte-derived Gal-9 may function as an immunoregulatory protein in response to ongoing neuroinflammation.

The role of astrocytes in CNS tumors: pre-clinical models and novel imaging approaches

Brain metastasis is a significant clinical problem, yet the mechanisms governing tumor cell extravasation across the blood-brain barrier (BBB) and CNS colonization are unclear. Astrocytes are increasingly implicated in the pathogenesis of brain metastasis but in vitro work suggests both tumoricidal and tumor-promoting roles for astrocyte-derived molecules. Also, the involvement of astrogliosis in primary brain tumor progression is under much investigation. However, translation of in vitro findings into in vivo and clinical settings has not been realized. Increasingly sophisticated resources, such as transgenic models and imaging technologies aimed at astrocyte-specific markers, will enable better characterization of astrocyte function in CNS tumors. Techniques such as bioluminescence and in vivo fluorescent cell labeling have potential for understanding the real-time responses of astrocytes to tumor burden. Transgenic models targeting signaling pathways involved in the astrocytic response also hold great promise, allowing translation of in vitro mechanistic findings into pre-clinical models. The challenging nature of in vivo CNS work has slowed progress in this area. Nonetheless, there has been a surge of interest in generating pre-clinical models, yielding insights into cell extravasation across the BBB, as well as immune cell recruitment to the parenchyma. While the function of astrocytes in the tumor microenvironment is still unknown, the relationship between astrogliosis and tumor growth is evident. Here, we review the role of astrogliosis in both primary and secondary brain tumors and outline the potential for the use of novel imaging modalities in research and clinical settings. These imaging approaches have the potential to enhance our understanding of the local host response to tumor progression in the brain, as well as providing new, more sensitive diagnostic imaging methods.

Blood-brain barrier alterations in the cerebral cortex in experimental autoimmune encephalomyelitis

The pathophysiology of cerebral cortical lesions in multiple sclerosis (MS) is not understood. We investigated cerebral cortex microvessels during immune-mediated demyelination in the MS model chronic murine experimental autoimmune encephalomyelitis (EAE) by immunolocalization of the endothelial cell tight junction (TJ) integral proteins claudin-5 and occludin, a structural protein of caveolae, caveolin-1, and the blood-brain barrier-specific endothelial transporter, Glut 1. In EAE-affected mice, there were areas of extensive subpial demyelination and well-demarcated lesions that extended to deeper cortical layers. Activation of microglia and absence of perivascular inflammatory infiltrates were common in these areas. Microvascular endothelial cells showed increased expression of caveolin-1 and a coincident loss of both claudin-5 and occludin normal junctional staining patterns. At a very early disease stage, claudin-5 molecules tended to cluster and form vacuoles that were also Glut 1 positive; the initially preserved occludin pattern became diffusely cytoplasmic at more advanced stages. Possible internalization of claudin-5 on TJ dismantling was suggested by its coexpression with the autophagosomal marker MAP1LC3A. Loss of TJ integrity was confirmed by fluorescein isothiocyanate-dextran experiments that showed leakage of the tracer into the perivascular neuropil. These observations indicate that, in the cerebral cortex of EAE-affected mice, there is a microvascular disease that differentially targets claudin-5 and occludin during ongoing demyelination despite only minimal inflammation.

Astrocytic Fas ligand expression is required to induce T-cell apoptosis and recovery from experimental autoimmune encephalomyelitis

In T-cell-mediated autoimmune diseases of the CNS, apoptosis of Fas(+) T cells by FasL contributes to resolution of disease. However, the apoptosis-inducing cell population still remains to be identified. To address the role of astrocytic FasL in the regulation of T-cell apoptosis in experimental autoimmune encephalomyelitis, we immunized C57BL/6 glial fibrillary acid protein (GFAP)-Cre FasL(fl/fl) mice selectively lacking FasL in astrocytes with MOG(35-55) peptide. GFAP-Cre FasL(fl/fl) mice were unable to resolve EAE and suffered from persisting demyelination and paralysis, while FasL(fl/fl) control mice recovered. In contrast to FasL(fl/fl) mice, GFAP-Cre FasL(fl/fl) mice failed to induce apoptosis of Fas(+) activated CD4(+) T cells and to increase numbers of Foxp3(+) Treg cells beyond day 15 post immunization, the time point of maximal clinical disease in control mice. The persistence of activated and GM-CSF-producing CD4(+) T cells in GFAP-Cre FasL(fl/fl) mice also resulted in an increased IL-17, IFN-γ, TNF, and GM-CSF mRNA expression in the CNS. In vitro, FasL(+) but not FasL(-) astrocytes induced caspase-3 expression and apoptosis of activated T cells. In conclusion, FasL expression of astrocytes plays an important role in the control and elimination of autoimmune T cells from the CNS, thereby determining recovery from EAE.

The innate immune system in demyelinating disease

Demyelinating diseases such as multiple sclerosis are chronic inflammatory autoimmune diseases with a heterogeneous clinical presentation and course. Both the adaptive and the innate immune systems have been suggested to contribute to their pathogenesis and recovery. In this review, we discuss the role of the innate immune system in mediating demyelinating diseases. In particular, we provide an overview of the anti-inflammatory or pro-inflammatory functions of dendritic cells, mast cells, natural killer (NK) cells, NK-T cells, γδ T cells, microglial cells, and astrocytes. We emphasize the interaction of astroctyes with the immune system and how this interaction relates to the demyelinating pathologies. Given the pivotal role of the innate immune system, it is possible that targeting these cells may provide an effective therapeutic approach for demyelinating diseases.

Tumor microenvironment in the brain

In addition to malignant cancer cells, tumors contain a variety of different stromal cells that constitute the tumor microenvironment. Some of these cell types provide crucial support for tumor growth, while others have been suggested to actually inhibit tumor progression. The composition of tumor microenvironment varies depending on the tumor site. The brain in particular consists of numerous specialized cell types such as microglia, astrocytes, and brain endothelial cells. In addition to these brain-resident cells, primary and metastatic brain tumors have also been shown to be infiltrated by different populations of bone marrow-derived cells. The role of different cell types that constitute tumor microenvironment in the progression of brain malignancies is only poorly understood. Tumor microenvironment has been shown to be a promising therapeutic target and diagnostic marker in extracranial malignancies. A better understanding of tumor microenvironment in the brain would therefore be expected to contribute to the development of improved therapies for brain tumors that are urgently required due to a poor availability of treatments for these malignancies. This review summarizes some of the known interactions between brain tumors and different stromal cells, and also discusses potential therapeutic approaches within this context.

Immunology meets neuroscience--opportunities for immune intervention in neurodegenerative diseases

Neuroinflammatory changes are characteristic of many, if not all, neurodegenerative diseases but the extent to which the immune system is involved in the pathogenesis of these diseases is unclear. The findings of several studies during the past decade has established that there is a well-developed communication between the central nervous system (CNS) and the peripheral immune system, but also has revealed that the immune system in the CNS is much more sophisticated that previously acknowledged. In this mini-review, we discuss two major neurodegenerative disorders, Alzheimer's disease (AD) and multiple sclerosis (MS), and consider whether the therapies most likely to succeed are those that are identified by studying the marriage of neuroscience and immunology.

Sensing the microenvironment of the central nervous system: immune cells in the central nervous system and their pharmacological manipulation

Immune responses are highly regulated in all organs and severely restricted in certain tissues within the central nervous system (CNS). This phenomenon, called 'immune privilege', has been linked to the existence of multiple anatomical and physiological protective mechanisms. The finely balanced anti-inflammatory microenvironment within the CNS contributes to the immune privilege status of this tissue. The regulation of this compartment changes under pathological conditions when pro-inflammatory mediators might dominate. The past few years brought a wealth of novel information fostering our understanding of how CNS resident cells regulate the functions of immune cells, particularly helper T lymphocytes (Ths) and dendritic cells (DCs). These two cell types play a crucial role in the initiation and maintenance of neuroinflammatory diseases. The change from anti-inflammatory to pro-inflammatory microenvironment in the inflamed CNS affects Th and DC accumulation and function in the nervous tissue. A new era of DC-targeted therapies has begun, with the possibility of designing novel immunomodulatory therapies to intervene with neuroinflammation in a wide range of neurological diseases.

Structural reorganization of the dentate gyrus following entorhinal denervation: species differences between rat and mouse

Deafferentation of the dentate gyrus by unilateral entorhinal cortex lesion or unilateral perforant pathway transection is a classical model to study the response of the central nervous system (CNS) to denervation. This model has been extensively characterized in the rat to clarify mechanisms underlying denervation-induced gliosis, transneuronal degeneration of denervated neurons, and collateral sprouting of surviving axons. As a result, candidate molecules have been identified which could regulate these changes, but a causal link between these molecules and the postlesional changes has not yet been demonstrated. To this end, mutant mice are currently studied by many groups. A tacit assumption is that data from the rat can be generalized to the mouse, and fundamental species differences in hippocampal architecture and the fiber systems involved in sprouting are often ignored. In this review, we will (1) provide an overview of some of the basics and technical aspects of the entorhinal denervation model, (2) identify anatomical species differences between rats and mice and will point out their relevance for the axonal reorganization process, (3) describe glial and local inflammatory changes, (4) consider transneuronal changes of denervated dentate neurons and the potential role of reactive glia in this context, and (5) summarize the differences in the reorganization of the dentate gyrus between the two species. Finally, we will discuss the use of the entorhinal denervation model in mutant mice.

The type 1 interleukin 1 receptor is not required for the death of murine hippocampal dentate granule cells and microglia activation

Alterations in inflammatory process, neuronal death, and glia response have been observed under manipulation of interleukin-1 (IL-1) and subsequent signaling through the type 1 IL-1 receptor (IL-1R1). To investigate the influence of IL-1R1 activation in the pathophysiology of a chemical-induced injury to the murine hippocampus, we examined the level and pattern of neuronal death and neuroinflammation in male weanling mice exposed to trimethyltin hydroxide (2.0 mg TMT/kg, i.p.). Dentate granule cell death occurred at 6 h post-TMT as detected by active caspase 3 immunostaining and presence of lectin positive microglia. The severity of neuronal death and microglia response increased by 12-24 h with elevations in mRNA levels for TNFalpha and IL-1alpha. In IL-1R1 null (IL-1R1-/-) mice, the pattern and severity of neuronal death at 24 or 72 h post-TMT was similar as compared to wildtype (WT) mice. In both groups, mRNA levels for TNFalpha and MIP-1alpha were elevated, no significant change was seen in either IL-1alpha or IL-1beta, and the early activation of microglia, including their ability to progress to a phagocytic phenotype, was maintained. Compared to WT mice, IL-1R1-/- mice displayed a limited glial fibrillary acidic protein (GFAP) astrocytic response, as well as a preferential induction in mRNA levels of Fas signaling components. Cumulatively, these results indicate that IL-1R1 activation is not necessary for TMT-induced death of dentate granule neurons or local activation of microglia; however, IL-1R1 signaling is involved in mediating the structural response of astrocytes to injury and may regulate apoptotic mechanisms via Fas signaling components.

Drainage of tears: impact on the ocular surface and lacrimal system

The human efferent tear ducts are part of the lacrimal system. Because little knowledge exists concerning the physiology of the nasolacrimal system, and hence its patho- physiology, the nasolacrimal system has received almost no consideration as a possible factor in dry eye. The human nasolacrimal ducts consist of the upper and the lower lacrimal canaliculus, the lacrimal sac, and the nasolacrimal duct. As a draining and secretory system, the efferent tear ducts play a role in tear transport and nonspecific immune defense. Moreover, components of tear fluid are absorbed in the nasolacrimal passage and are transported into a surrounding vascular system. This system is similar to a cavernous body that is subject to autonomic control and regulates tear outflow. Tear duct-associated lymphoid tissue (TALT) is present in the efferent tear ducts, displaying the cytomorphological and immunophenotypic features of mucosa-associated lymphoid tissue (MALT). Under normal conditions, tear fluid components are constantly absorbed into the blood vessels of the surrounding cavernous body. These vessels are connected to the blood vessels of the outer eye and could act as a feedback signal for tear fluid production, which ceases if these tear components are not absorbed. In this way, dry eye could be initiated. Defective stimulation of TALT could result in abnormal immune deviation at the ocular surface, leading to an autoimmunological response that causes dry eye pathology.

PD-L1 (B7-H1) regulation in zones of axonal degeneration

Fibre tract injury evokes recruitment of antigen-presenting- and T cells, but does not cause autoimmune demyelination. This implies that immune tolerance to myelin is actively maintained or readily re-established. Using entorhinal cortex lesion (ECL) to induce axonal degeneration in the hippocampus of adult mice, we studied the induction of B7-H1 (PD-L1) in zones of axonal degeneration. This member of the B7-family has been shown to be expressed on parenchymal cells of various organs, where it strongly down-modulates the activity of T cells. Real-time reverse transcriptase (RT)-PCR revealed low mRNA levels in brain compared to lung and spleen under normal conditions. After ECL, a twofold increase could be observed. Immunocytochemistry revealed astrocytes as source of B7-H1, while immune positive microglia were not detected. Thus, axonal degeneration induces astrocytes to express B7-H1, a potent inhibitor of effector T cells.

Mediators of apoptosis Fas and FasL predict disability progression in multiple sclerosis over a period of 10 years

TNF-alpha, IL-12p35, IL-12p40, IL-4, IL-10, TGF-beta1, CCR3, CXCR3, CCR5, Fas and FasL mRNA levels in PBMC of 25 multiple sclerosis (MS) patients were quantified at baseline by real-time PCR according to a post-hoc study design. The baseline values of the different markers were analysed with respect to their correlation with the increase in disability over a period of 10 years. High levels of Fas mRNA were associated with a favourable disease course in relapsing-remitting (RR) MS (R2 = 0.74, P = 0.0001, n = 13), as measured by the Expanded Disability Status Scale (EDSS); high levels of FasL mRNA were associated with relatively mild disease progression (R2 = 0.86, P = 0.0001, n =12) in secondary progressive (SP) MS. These findings suggest that Fas-mediated apoptosis plays a major role in the mechanism underlying long-term disease progression in MS.:

CNS immune privilege: hiding in plain sight

Central nervous system (CNS) immune privilege is an experimentally defined phenomenon. Tissues that are rapidly rejected by the immune system when grafted in sites, such as the skin, show prolonged survival when grafted into the CNS. Initially, CNS immune privilege was construed as CNS isolation from the immune system by the blood-brain barrier (BBB), the lack of draining lymphatics, and the apparent immunoincompetence of microglia, the resident CNS macrophage. CNS autoimmunity and neurodegeneration were presumed automatic consequences of immune cell encounter with CNS antigens. Recent data have dramatically altered this viewpoint by revealing that the CNS is neither isolated nor passive in its interactions with the immune system. Peripheral immune cells can cross the intact BBB, CNS neurons and glia actively regulate macrophage and lymphocyte responses, and microglia are immunocompetent but differ from other macrophage/dendritic cells in their ability to direct neuroprotective lymphocyte responses. This newer view of CNS immune privilege is opening the door for therapies designed to harness autoreactive lymphocyte responses and also implies (i) that CNS autoimmune diseases (i.e. multiple sclerosis) may result as much from neuronal and/or glial dysfunction as from immune system dysfunctions and (ii) that the severe neuronal and glial dysfunction associated with neurodegenerative disorders (i.e. Alzheimer's disease) likely alters CNS-specific regulation of lymphocyte responses affecting the utility of immune-based therapies (i.e. vaccines).

Tolerogenic effect of fiber tract injury: reduced EAE severity following entorhinal cortex lesion

Despite transient, myelin-directed adaptive immune responses in regions of fiber tract degeneration, none of the current models of fiber tract injuries evokes disseminated demyelination, implying effective mechanisms maintaining or re-establishing immune tolerance. In fact, we have recently detected CD95L upregulation accompanied by apoptosis of leukocytes in zones of axonal degeneration induced by entorhinal cortex lesion (ECL), a model of layer-specific axonal degeneration. Moreover, infiltrating monocytes readily transformed into ramified microglia exhibiting a phenotype of immature (CD86+/CD80-) antigen-presenting cells. We now report the appearance of the axonal antigen neurofilament-light along with increased T cell apoptosis and enhanced expression of the pro-apoptotic gene Bad in cervical lymph nodes after ECL. In order to test the functional significance of such local and systemic depletory/regulatory mechanisms on subsequent immunity to central nervous system antigens, experimental autoimmune encephalomyelitis was induced by proteolipid protein immunization 30 days after ECL. In three independent experiments, we found significantly diminished disease scores and infiltrates in lesioned compared to sham-operated SJL mice. This is consistent with a previous meta-statistical analysis (Goodin et al. in Neurology 52:1737-1745, 1999) rejecting the O-hypothesis that brain trauma causes or exacerbates multiple sclerosis. Conversely, brain injuries may involve long-term tolerogenic effects towards brain antigens.

IFN-beta inhibits T cell activation capacity of central nervous system APCs

We have previously investigated the physiological effects of IFN-beta on chronic CNS inflammation and shown that IFN-beta(-/-) mice develop a more severe experimental autoimmune encephalomyelitis than their IFN-beta(+/-) littermates. This result was shown to be associated with a higher activation state of the glial cells and a higher T cell cytokine production in the CNS. Because this state suggested a down-regulatory effect of IFN-beta on CNS-specific APCs, these results were investigated further. We report that IFN-beta pretreatment of astrocytes and microglia (glial cells) indeed down-modulate their capacity to activate autoreactive Th1 cells. First, we investigated the intrinsic ability of glial cells as APCs and report that glial cells prevent autoreactive Th1 cells expansion while maintaining Ag-specific T cell effector functions. However, when the glial cells are treated with IFN-beta before coculture with T cells, the effector functions of T cells are impaired as IFN-gamma, TNF-alpha, and NO productions are decreased. Induction of the T cell activation marker, CD25 is also reduced. This suppression of T cell response is cell-cell dependent, but it is not dependent on a decrease in glial expression of MHC class II or costimulatory molecules. We propose that IFN-beta might exert its beneficial effects mainly by reducing the Ag-presenting capacity of CNS-specific APCs, which in turn inhibits the effector functions of encephalitogenic T cells. This affect is of importance because activation of encephalitogenic T cells within the CNS is a prerequisite for the development of a chronic progressive CNS inflammation.

Loss of Fas ligand-function improves survival in G93A-transgenic ALS mice

ALS is a devastating neurodegenerative disorder for which no effective treatment exists. The precise molecular mechanisms underlying the selective degeneration of motor neurons are still unknown. A motor neuron specific apoptotic pathway involving Fas and NO has been discovered. Motor neurons from ALS-mice have an increased sensitivity to Fas-induced cell death via this pathway. In this study we therefore crossed G93A-SOD1 overexpressing ALS mice with Fas ligand (FasL) mutant (gld) mice to investigate whether the reduced Fas signaling could have beneficial effects on motor neuron death. G93A-SOD1 mutant mice with a homozygous FasL mutant showed a modest but statistically significant extension of survival, and reduced loss of motor neurons. These results indicate that motor neuron apoptosis triggered by Fas is relevant in ALS pathogenesis.

Inhibition of transcription factor NF-κB in the central nervous system ameliorates autoimmune encephalomyelitis in mice

Activation of transcription factor NF-kappaB in the central nervous system (CNS) has been linked to autoimmune demyelinating disease; however, it remains unclear whether its function is protective or pathogenic. Here we show that CNS-restricted ablation of 'upstream' NF-kappaB activators NEMO or IKK2 but not IKK1 ameliorated disease pathology in a mouse model of multiple sclerosis, suggesting that 'canonical' NF-kappaB activation in cells of the CNS has a mainly pathogenic function in autoimmune demyelinating disease. NF-kappaB inhibition prevented the expression of proinflammatory cytokines, chemokines and the adhesion molecule VCAM-1 from CNS-resident cells. Thus, NF-kappaB-dependent gene expression in non-microglial cells of the CNS provides a permissive proinflammatory milieu that is critical for CNS inflammation and tissue damage in autoimmune demyelinating disease.

Efficient presentation of myelin oligodendrocyte glycoprotein peptides but not protein by astrocytes from HLA-DR2 and HLA-DR4 transgenic mice

The role of astrocytes in the pathogenesis of multiple sclerosis (MS) is not well understood. Astrocytes may modulate the activity of pathogenic T cells by presenting myelin antigens in combination with pro- or anti-inflammatory signals. Astrocytes have been shown to present myelin basic protein (MBP) and proteolipid protein (PLP) to T cells, but it has remained unresolved whether astrocytes present myelin oligodendrocyte glycoprotein (MOG), which has been implicated as an important autoantigen in MS. Here, we asked whether astrocytes presented MOG to T cells. To closer model presentation of human MOG by astrocytes in MS patients, we generated astrocytes from transgenic mice expressing the MS-associated MHC class II alleles HLA-DR2 (DRB1*1501) and HLA-DR4 (DRB1*0401). The results show that IFN-gamma-activated HLA-DR2 and HLA-DR4 expressing astrocytes efficiently presented immunodominant and subdominant MOG peptides to T cells. The hierarchy of the presented MOG epitopes was comparable to that of professional APCs, including dendritic cells and microglia. Importantly, astrocytes were poor at processing and presenting native MOG protein. Furthermore, astrocytes induced a mixed Th1/Th2 cytokine response in MOG-specific T cells, whereas dendritic cells induced a predominantly Th1 cell response. Collectively, the results suggest that astrocytes may modulate anti-MOG T cell responses in the CNS.

Indolamine 2,3-dioxygenase is expressed in the CNS and down-regulates autoimmune inflammation

The tryptophan (trp)-catabolizing enzyme indolamine 2,3-dioxygenase (IDO) is induced by the T helper 1 (Th 1) cytokine IFN-gamma during infections in various tissues including the brain. Recent studies demonstrated an immune modulatory function of this enzyme, since IDO-mediated depletion of trp hinders T cell proliferation, while its inhibition by 1-methyl-tryptophan (1-Mt) induces breakdown of immune tolerance in the placenta, leading to rejection of allogeneic concepti. Here, we tested IDO expression and function during experimental autoimmune encephalomyelitis (EAE) actively induced in adult SJL mice by immunization with PLP139-151. IDO activity (determined by HPLC analysis of the kynurenine/tryptophan ratio) was increased in the spleen during the preclinical phase, and within the brain and spinal cord at the onset of symptoms. Immunocytochemistry revealed macrophages/activated microglia expressing IDO during EAE and in vitro experiments confirmed IDO induction in microglia upon IFN-gamma treatment with synergistic effects of TNF-alpha. Inhibition of IDO by systemic administration of 1-Mt at clinical onset significantly exacerbated disease scores. From these data, it is tempting to speculate that IFN-gamma from encephalitogenic Th 1 cells induces local IDO expression, thereby initiating a negative feedback loop which may underlie the self-limitation of autoimmune inflammation during EAE and multiple sclerosis.

Circulating monocytic cells infiltrate layers of anterograde axonal degeneration where they transform into microglia

In this study, we demonstrate the infiltration of blood-derived monocytic cells and their morphologic transformation into microglia in zones of acute, anterograde (Wallerian) axonal degeneration induced by entorhinal cortex lesion (ECL). ECL was performed in mice which had received green fluorescent protein (GFP)-transduced bone marrow grafts allowing identification of blood-derived elements within the brain. While in the unlesioned hemisphere GFP+ cells were restricted to perivascular and leptomeningeal sites, many round fluorescent cells appeared in hippocampal zones of axonal degeneration at 24 h post lesion (hpl). Within 72 hpl, these GFP+ cells acquired ramified, microglia-like morphologies, which persisted for at least 7 days post ECL. Differentiation of GFP+ cells into glial fibrillary acidic protein (GFAP)+ astrocytes was never observed. To exclude that this recruitment is an artifact of irradiation or bone marrow transplantation, the fluorescent cell tracker 6-carboxylfluorescein diacetate (CFDA) was injected into spleens of normal mice 1 day before ECL. Again, fluorescent cells appeared at the lesion site and along the layers of axonal degeneration at 48 hpl and CFDA+/MAC-1+, cells exhibited amoeboid and ramified morphologies. Thus, blood-derived cells infiltrate not only the site of mechanical lesion, but also the layers of anterograde axonal degeneration, where they readily transform into microglia-like elements. A role for infiltrating leukocytes in facilitating or modulating postlesional plasticity, e.g., by phagocytosis of growth-inhibiting myelin should now be considered. Moreover, monocytic cells may serve as vehicles to transport therapeutic substances such as neurotrophic factors or caspase inhibitors to zones of axonal degeneration.

Astrocyte-induced regulatory T cells mitigate CNS autoimmunity

Although astrocytes presumably participate in maintaining the immune privilege of the central nervous system (CNS), the mechanisms behind their immunoregulatory properties are still largely undefined. In this study, we describe the development of regulatory T cells upon contact with astrocytes. Rat T cells pre-incubated with astrocytes completely lost the ability to proliferate in response to mitogenic stimuli. The cells were blocked in G0/G1 phase of the cell cycle, expressed less IL-2R, and produced significantly lower amounts of interferon-gamma (IFN-gamma), but not interleukin-2 (IL-2), IL-10, or tumor necrosis factor (TNF). These anergic cells completely prevented mitogen-induced growth of normal T lymphocytes, as well as CNS antigen-driven proliferation of autoreactive T cells. The suppressive activity resided in both CD4+ and CD8+ T-cell compartments. Heat-sensitive soluble T-cell factors, not including transforming growth factor-beta (TGF-beta) or IL-10, were solely responsible for the observed suppression, as well as for the transfer of suppressive activity to normal T cells. The administration of astrocyte-induced regulatory T cells markedly alleviated CNS inflammation and clinical symptoms of CNS autoimmunity in rats with experimental allergic encephalomyelitis. Finally, the cells with suppressive properties were readily generated from human lymphocytes after contact with astrocytes. Taken together, these data indicate that astrocyte-induced regulatory T cells might represent an important mechanism for self-limitation of excessive inflammation in the brain.

Defined inflammatory states in astrocyte cultures: correlation with susceptibility towards CD95-driven apoptosis

A complete cytokine mix (CCM) or its individual components tumour necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma) were used to switch resting murine astrocytes to reactive states. The transformation process was characterized by differential up-regulation of interleukin-6 (IL-6), cyclooxygenase-2 (COX-2) and inducible nitric oxide synthetase (iNOS) mRNA and protein and a subsequent release of prostaglandin E2, nitric oxide (NO) and IL-6. Both CD95L and anti-CD95 antibodies triggered caspase activation followed by apoptotic death in fully pro-inflammatory astrocytes, whereas resting cells were totally resistant. Two other death-inducing ligands, TNF and TNF-related apoptosis-inducing ligand (TRAIL) did not induce apoptosis in reactive astrocytes. The switch in astrocyte sensitivity was accompanied by up-regulation of caspase-8 and CD95 as well as the capacity to recruit Fas-associated death domain (FADD) to the activated death receptor complex. Neither CD95-mediated death, nor other inflammatory parameters were affected by inhibition of iNOS or COX, respectively. Accordingly, IFN-gamma was absolutely essential for up-regulation of iNOS, but not for the switch in apoptosis sensitivity. In contrast, p38 kinase activity was identified as an important controller of both the inflammatory reaction and apoptosis both in astrocytes stimulated with CCM and in glia exposed to TNF and IL-1 only.

HIV-1 and IL-1 beta regulate Fas ligand expression in human astrocytes through the NF-kappa B pathway

Reactive astrogliosis is a prominent pathological feature of HIV-1-associated dementia (HAD). We hypothesized that in HAD, astrocytes activated with proinflammatory stimuli such as IL-1beta express Fas ligand (FasL), a death protein. IL-1beta and HIV-1-activated astrocytes expressed FasL mRNA and protein. Luciferase reporter constructs showed that IL-1beta and HIV-1 upregulated FasL promoter activity (p<0.001). The NF-kappaB pathway was involved as shown by inhibition with SN50 and dominant negative IkappaBalpha mutants. Brain extracts from HAD patients had significantly elevated FasL levels compared to HIV-seropositive (p<0.001) and seronegative individuals (p<0.01). We propose that astrocyte expression of FasL may participate in neuronal injury in HAD.