Pro-inflammatory functions of astrocytes correlate with viral clearance and strain-dependent protection from TMEV-induced demyelinating disease

Brain resident memory T cells rapidly expand and initiate neuroinflammatory responses following CNS viral infection

The contribution of circulating verses tissue resident memory T cells (TRMs) to clinical neuropathology is an enduring question due to a lack of mechanistic insights. The prevailing view is TRMs are protective against pathogens in the brain. However, the extent to which antigen-specific TRMs induce neuropathology upon reactivation is understudied. Using the described phenotype of TRMs, we found that brains of naïve mice harbor populations of CD69+ CD103- T cells. Notably, numbers of CD69+ CD103- TRMs rapidly increase following neurological insults of various origins. This TRM expansion precedes infiltration of virus antigen-specific CD8 T cells and is due to proliferation of T cells within the brain. We next evaluated the capacity of antigen-specific TRMs in the brain to induce significant neuroinflammation post virus clearance, including infiltration of inflammatory myeloid cells, activation of T cells in the brain, microglial activation, and significant blood brain barrier disruption. These neuroinflammatory events were induced by TRMs, as depletion of peripheral T cells or blocking T cell trafficking using FTY720 did not change the neuroinflammatory course. Depletion of all CD8 T cells, however, completely abrogated the neuroinflammatory response. Reactivation of antigen-specific TRMs in the brain also induced profound lymphopenia within the blood compartment. We have therefore determined that antigen-specific TRMs can induce significant neuroinflammation, neuropathology, and peripheral immunosuppression. The use of cognate antigen to reactivate CD8 TRMs enables us to isolate the neuropathologic effects induced by this cell type independently of other branches of immunological memory, differentiating this work from studies employing whole pathogen re-challenge. This study also demonstrates the capacity for CD8 TRMs to contribute to pathology associated with neurodegenerative disorders and long-term complications associated with viral infections. Understanding functions of brain TRMs is crucial in investigating their role in neurodegenerative disorders including MS, CNS cancers, and long-term complications associated with viral infections including COVID-19.

Biological models in multiple sclerosis

Considering the etiology of multiple sclerosis (MS) is still unknown, experimental models resembling specific aspects of this immune-mediated demyelinating human disease have been developed to increase the understanding of processes related to pathogenesis, disease evolution, evaluation of therapeutic interventions, and demyelination and remyelination mechanisms. Based on the nature of the investigation, biological models may include in vitro, in vivo, and ex vivo assessments. Even though these approaches have disclosed valuable information, every disease animal model has limitations and can only replicate specific features of MS. In vitro and ex vivo models generally do not reflect what occurs in the organism, and in vivo animal models are more likely used; nevertheless, they are able to reproduce only certain stages of the disease. In vivo MS disease animal models in mammals include: experimental autoimmune encephalomyelitis, viral encephalomyelitis, and induced demyelination. This review examines and describes the most common biological disease animal models for the study of MS, their specific characteristics and limitations.

Virus-triggered spinal cord demyelination is followed by a peripheral neuropathy resembling features of Guillain-Barré Syndrome

Theiler’s murine encephalomyelitis virus (TMEV)-induces a demyelinating disease in the spinal cord (SC) of susceptible but not in resistant (B6) mouse strains. The aim of the present study was to induce SC demyelination and a peripheral neuropathy in resistant mice by switching the infection site from cerebrum to SC. B6 mice were intraspinally inoculated with TMEV. Infected mice showed clinical signs starting at 7 days post infection (dpi). Histopathology revealed a mononuclear myelitis, centred on the injection site at 3 dpi with subsequent antero- and retrograde spread, accompanied by demyelination and axonal damage within the SC. Virus protein was detected in the SC at all time points. SC inflammation decreased until the end of the investigation period (28 dpi). Concurrent with the amelioration of SC inflammation, the emergence of a peripheral neuropathy, characterized by axonal damage, demyelination and macrophage infiltration, contributing to persistent clinical sings, was observed. Intraspinal TMEV infection of resistant mice induced inflammation, demyelination and delayed viral clearance in the spinal cord and more interestingly, subsequent, virus-triggered inflammation and degeneration within the PN associated with dramatic and progressive clinical signs. The lesions observed in the PN resemble important features of Guillain-Barré syndrome, especially of acute motor/motor-sensory axonal forms.

Facets of Theiler's Murine Encephalomyelitis Virus-Induced Diseases: An Update

Theiler’s murine encephalomyelitis virus (TMEV), a naturally occurring, enteric pathogen of mice is a Cardiovirus of the Picornaviridae family. Low neurovirulent TMEV strains such as BeAn cause a severe demyelinating disease in susceptible SJL mice following intracerebral infection. Furthermore, TMEV infections of C57BL/6 mice cause acute polioencephalitis initiating a process of epileptogenesis that results in spontaneous recurrent epileptic seizures in approximately 50% of affected mice. Moreover, C3H mice develop cardiac lesions after an intraperitoneal high-dose application of TMEV. Consequently, TMEV-induced diseases are widely used as animal models for multiple sclerosis, epilepsy, and myocarditis. The present review summarizes morphological lesions and pathogenic mechanisms triggered by TMEV with a special focus on the development of hippocampal degeneration and seizures in C57BL/6 mice as well as demyelination in the spinal cord in SJL mice. Furthermore, a detailed description of innate and adaptive immune responses is given. TMEV studies provide novel insights into the complexity of organ- and mouse strain-specific immunopathology and help to identify factors critical for virus persistence.

Rosiglitazone as a Modulator of TLR4 and TLR3 Signaling Pathways in Rat Primary Neurons and Astrocytes

An antidiabetic drug of the thiazolidinedione class, rosiglitazone (RG) demonstrates anti-inflammatory properties in various brain pathologies. The mechanism of RG action in brain cells is not fully known. To unravel mechanisms of RG modulation of toll-like receptor (TLR) signaling pathways, we compare primary rat neuron and astrocyte cultures stimulated with the TLR4 agonist lipopolysaccharide (LPS) and the TLR3 agonist poly I:C (PIC). Both TLR agonists induced tumor necrosis factor (TNFα) release in astrocytes, but not in neurons. Neurons and astrocytes released interleukin-10 (IL-10) and prostaglandin E2 (PGE₂) in response to LPS and PIC. RG decreased TLR-stimulated TNFα release in astrocytes as well as potentiated IL-10 and PGE₂ release in both astrocytes and neurons. RG induced phosphorylation of p38 and JNK MAPK (mitogen-activated protein kinase) in neurons. The results reveal new role of RG as a modulator of resolution of neuroinflammation.

A Refined Bead-Free Method to Identify Astrocytic Exosomes in Primary Glial Cultures and Blood Plasma

Astrocytes are the most abundant glial cell type in the central nervous system (CNS) and are known to fulfill critical homeostatic functions. Dysfunction of activated astrocytes is also known to participate in the development of several neurological diseases. Astrocytes can be uniquely identified by expression of the intermediate filament protein glial acidic fibrillary protein (GFAP). Herein, we report on the development of a rigorous and sensitive methodology to identify GFAP+ exosomes in primary culture using flow cytometry. We then demonstrate that activated astrocytes release increased amounts of exosomes in response to treatment with interleukin-1β. Using this methodology, we report the identification of GFAP+ exosomes in blood and then use a mouse model of inflammatory demyelination, experimental autoimmune encephalomyelitis (EAE), to examine whether the abundance of GFAP+ exosomes in blood circulation changes during clinical illness. We find a detectable increase in the presence of GFAP+ exosomes in EAE mice when compared with non-EAE, control mice. Our data provide a novel perspective on the presence of GFAP in blood as it identifies exosomes as potential astrocyte-derived signals within blood. These data are complementary to previous clinical studies that reported elevated GFAP protein in blood samples from multiple sclerosis (MS) patients during a clinical relapse. These data also reveal the existence of a potential systemic role for astrocyte-derived exosomes in CNS conditions involving inflammation such as multiple sclerosis.

Lack of effect of Theiler's murine encephalomyelitis virus infection on system xc⁻

Changes in the expression of xCT, the specific subunit of system xc(-) or the cystine/glutamate antiporter, have been associated with several neurological disorders and system xc(-) was recently proposed as a potential target for the development of new treatment strategies for multiple sclerosis (MS). In this study we used Theiler's murine encephalomyelitis virus (TMEV) infection, both in vitro and in vivo, as a model to further evaluate the involvement of system xc(-) in MS. Protein levels of xCT, as well as activity of system xc(-) were unaffected in RAW264.7 macrophages after infection with the demyelinating DA strain of TMEV. Also, protein expression of xCT remained stable in spinal cord and brain of FVB mice 1-2 and 6 weeks after intracranial injection of the DA strain of TMEV. These results demonstrate that TMEV infection of macrophages or FVB mice has no effect on system xc(-) and as such cannot be used as a model to study the involvement of system xc(-) in MS.

Effect of the innate immune response on development of Theiler's murine encephalomyelitis virus-induced demyelinating disease

Theiler's murine encephalomyelitis virus (TMEV) infection of susceptible mice leads to the development of demyelinating disease in the central nervous system (CNS) associated with an inflammatory immune response. The demyelinating disease in mice has similarities to multiple sclerosis in humans and is used as an experimental model for the human disease. The innate immune response initiates the immune response to TMEV through innate immune receptors on cells that recognize components of the virus and activate intracellular signaling that leads to the expression of innate immune cytokines, chemokines, and effector molecules. The innate immune response directly affects the development of the adaptive immune response, especially the T cell response, which mediates viral clearance. However, infection of Swiss Jim Laboratory (SJL) mice with TMEV leads to a persistent virus infection of the microglia/macrophage in the CNS which contributes to the development of demyelinating disease. Susceptibility to demyelinating disease has been linked to the T cell response against the virus. However, the current studies will examine the role of the innate immune response to TMEV and the affect it has on the adaptive immune response and development of demyelinating disease following TMEV infection. The innate immune cytokines, chemokines, and effector molecules as well as the innate immune cells, both CNS resident and infiltrating peripheral cells, all contribute to the innate immune response following TMEV and may affect susceptibility to demyelinating disease.

Protocol to isolate a large amount of functional oligodendrocyte precursor cells from the cerebral cortex of adult mice and humans

During development, oligodendrocytes are generated from oligodendrocyte precursor cells (OPCs), a cell type that is a significant proportion of the total cells (3-8%) in the adult central nervous system (CNS) of both rodents and humans. Adult OPCs are responsible for the spontaneous remyelination that occurs in demyelinating diseases like Multiple Sclerosis (MS) and they constitute an interesting source of cells for regenerative therapy in such conditions. However, there is little data regarding the neurobiology of adult OPCs isolated from mice since an efficient method to isolate them has yet to be established. We have designed a protocol to obtain viable adult OPCs from the cerebral cortex of different mouse strains and we have compared its efficiency with other well-known methods. In addition, we show that this protocol is also useful to isolate functional OPCs from human brain biopsies. Using this method we can isolate primary cortical OPCs in sufficient quantities so as to be able to study their survival, maturation and function, and to facilitate an evaluation of their utility in myelin repair.

Astrocyte regulation of CNS inflammation and remyelination

Astrocytes regulate fundamentally important functions to maintain central nervous system (CNS) homeostasis. Altered astrocytic function is now recognized as a primary contributing factor to an increasing number of neurological diseases. In this review, we provide an overview of our rapidly developing understanding of the basal and inflammatory functions of astrocytes as mediators of CNS responsiveness to inflammation and injury. Specifically, we elaborate on ways that astrocytes actively participate in the pathogenesis of demyelinating diseases of the CNS through their immunomodulatory roles as CNS antigen presenting cells, modulators of blood brain barrier function and as a source of chemokines and cytokines. We also outline how changes in the extracellular matrix can modulate astrocytes phenotypically, resulting in dysregulation of astrocytic responses during inflammatory injury. We also relate recent studies describing newly identified roles for astrocytes in leukodystrophies. Finally, we describe recent advances in how adapting this increasing breadth of knowledge on astrocytes has fostered new ways of thinking about human diseases, which offer potential to modulate astrocytic heterogeneity and plasticity towards therapeutic gain. In summary, recent studies have provided improved insight in a wide variety of neuroinflammatory and demyelinating diseases, and future research on astrocyte pathophysiology is expected to provide new perspectives on these diseases, for which new treatment modalities are increasingly necessary.

Viral models of multiple sclerosis: neurodegeneration and demyelination in mice infected with Theiler's virus

Multiple sclerosis (MS) is a complex inflammatory disease of unknown etiology that affects the central nervous system (CNS) white matter, and for which no effective cure exists. Indeed, whether the primary event in MS pathology affects myelin or axons of the CNS remains unclear. Animal models are necessary to identify the immunopathological mechanisms involved in MS and to develop novel therapeutic and reparative approaches. Specifically, viral models of chronic demyelination and axonal damage have been used to study the contribution of viruses in human MS, and they have led to important breakthroughs in our understanding of MS pathology. The Theiler's murine encephalomyelitis virus (TMEV) model is one of the most commonly used MS models, although other viral models are also used, including neurotropic strains of mouse hepatitis virus (MHV) that induce chronic inflammatory demyelination with similar histological features to those observed in MS. This review will discuss the immunopathological mechanisms involved in TMEV-induced demyelinating disease (TMEV-IDD). The TMEV model reproduces a chronic progressive disease due to the persistence of the virus for the entire lifespan in susceptible mice. The evolution and significance of the axonal damage and neuroinflammation, the importance of epitope spread from viral to myelin epitopes, the presence of abortive remyelination and the existence of a brain pathology in addition to the classical spinal cord demyelination, are some of the findings that will be discussed in the context of this TMEV-IDD model. Despite their limitations, viral models remain an important tool to study the etiology of MS, and to understand the clinical and pathological variability associated with this disease.

Hippocampal protection in mice with an attenuated inflammatory monocyte response to acute CNS picornavirus infection

Neuronal injury during acute viral infection of the brain is associated with the development of persistent cognitive deficits and seizures in humans. In C57BL/6 mice acutely infected with the Theiler's murine encephalomyelitis virus, hippocampal CA1 neurons are injured by a rapid innate immune response, resulting in profound memory deficits. In contrast, infected SJL and B6xSJL F1 hybrid mice exhibit essentially complete hippocampal and memory preservation. Analysis of brain-infiltrating leukocytes revealed that SJL mice mount a sharply attenuated inflammatory monocyte response as compared to B6 mice. Bone marrow transplantation experiments isolated the attenuation to the SJL immune system. Adoptive transfer of B6 inflammatory monocytes into acutely infected B6xSJL hosts converted these mice to a hippocampal damage phenotype and induced a cognitive deficit marked by failure to recognize a novel object. These findings show that inflammatory monocytes are the critical cellular mediator of hippocampal injury during acute picornavirus infection of the brain.

Brain-specific angiogenesis inhibitor-1 expression in astrocytes and neurons: implications for its dual function as an apoptotic engulfment receptor

Brain-specific angiogenesis inhibitor-1 (BAI1) is a transmembrane protein highly expressed in normal brain that has been ascribed two apparently distinct functions: inhibition of angiogenesis and recognition and engulfment of apoptotic cells by phagocytes. A previous localization study reported BAI1 expression only in neurons. Because a phagocytic function of BAI1 could be important for neuroglial antigen processing and presentation, we performed immunolocalization studies in adult mouse brain and cultured neural cells, using a pair of antibodies directed against N- and C-terminal epitopes. BAI1 immunoreactivity is enriched in gray matter structures and largely excluded from myelinated axon tracts. Neuronal BAI1 expression was readily detectable in the cerebellar molecular layer as well as in primary hippocampal cultures. In some brain regions, especially olfactory bulb glomeruli, BAI1 was expressed by GFAP-positive astrocytes. Cultured cortical astrocytes show small (∼0.4μm(2)) BAI1 immunoreactive membrane puncta as well as prominent focal adhesion localization in a subset of cells. In mixed neuronal-glial cultures, BAI1-expressing astrocytes frequently contained engulfed apoptotic debris. Cultured astrocytes engulfed apoptotic targets, and BAI1 showed accumulation within the phagocytic cup. We hypothesize that glial BAI1 may subserve an engulfment function in adult brain regions such as olfactory bulb with ongoing apoptotic turnover, whereas neuronal-derived BAI1 may serve primarily as an anti-angiogenic factor in the mature neuropil.

Japanese encephalitis virus induces matrix metalloproteinase-9 in rat brain astrocytes via NF-κB signalling dependent on MAPKs and reactive oxygen species

BACKGROUND AND PURPOSE

Japanese encephalitis virus (JEV) is a member of the family Flaviviridae and JEV infection is a major cause of acute encephalopathy in children, which destroys cells in the CNS, including astrocytes and neurons. However, the detailed mechanisms underlying the inflammatory action of JEV are largely unclear.

EXPERIMENTAL APPROACH

The effect of JEV on the expression of matrix metalloproteinase (MMP)-9 was determined by gelatin zymography, Western blot analysis, real-time PCR and promoter assay. The involvement of the NADPH oxidase and reactive oxygen species (ROS), MAPKs, and the transcription factor NF-κB in these responses was investigated by using selective pharmacological inhibitors and transfection with appropriate siRNAs.

KEY RESULTS

JEV induced the expression of the pro-form of MMP-9 in rat brain astrocytes (RBA-1 cells). In RBA-1 cells, JEV induced MMP-9 expression and promoter activity, which was inhibited by pretreatment with inhibitors of NADPH oxidase (diphenylene iodonium chloride or apocynin), MAPKs (U0126, SB203580 or SP600125) and a ROS scavenger (N-acetylcysteine), or transfection with siRNAs of p47(phox) , ERK1, JNK2 and p38. In addition, JEV-induced MMP-9 expression was reduced by pretreatment with an inhibitor of NF-κB (helenalin) or transfection with p65 siRNA. Moreover, JEV-stimulated NF-κB activation was inhibited by pretreatment with the inhibitors of NADPH oxidase and MAPKs.

CONCLUSIONS AND IMPLICATIONS

MMP-9 expression induced by JEV infection of RBA-1 cells was mediated through the generation of ROS and activation of p42/p44 MAPK, p38 MAPK and JNK1/2, leading to NF-κB activation.

The role of antigen presenting cells in multiple sclerosis

Multiple sclerosis (MS) is a debilitating T cell mediated autoimmune disease of the central nervous system (CNS). Animal models of MS, such as experimental autoimmune encephalomyelitis (EAE) and Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) have given light to cellular mechanisms involved in the initiation and progression of this organ-specific autoimmune disease. Within the CNS, antigen presenting cells (APC) such as microglia and astrocytes participate as first line defenders against infections or inflammation. However, during chronic inflammation they can participate in perpetuating the self-destructive environment by secretion of inflammatory factors and/or presentation of myelin epitopes to autoreactive T cells. Dendritic cells (DC) are also participants in the presentation of antigen to T cells, even within the CNS. While the APCs alone are not solely responsible for mediating the destruction to the myelin sheath, they are critical players in perpetuating the inflammatory milieu. This review will highlight relevant studies which have provided insight to the roles played by microglia, DCs and astrocytes in the context of CNS autoimmunity.

Development of postinfection epilepsy after Theiler's virus infection of C57BL/6 mice

Viral infection of the central nervous system can lead to long-term neurologic defects, including increased risk for the development of epilepsy. We describe the development of the first mouse model of viral-induced epilepsy after intracerebral infection with Theiler's murine encephalomyelitis virus. Mice were monitored with long-term video-electroencephalogram at multiple time points after infection. Most mice exhibited short-term symptomatic seizures within 3 to 7 days of infection. This was followed by a distinct latent period in which no seizures were observed. Prolonged video-electroencephalogram recordings at 2, 4, and 7 months after the initial infection revealed that a significant proportion of the mice developed profound, spontaneous epileptic seizures. Neuropathologic examination revealed hippocampal sclerosis in animals with epilepsy. Theiler's murine encephalomyelitis virus-infected C57BL/6 mice represent a novel "hit-and-run" model to investigate mechanisms underlying viral-induced short-term symptomatic seizures, epileptogenesis, and epilepsy. Importantly, this model will also be useful to investigate novel therapies for the treatment and prevention of epilepsy.

EV71 induces COX-2 expression via c-Src/PDGFR/PI3K/Akt/p42/p44 MAPK/AP-1 and NF-kappaB in rat brain astrocytes

Enterovirus 71 (EV71) induces the expression of cyclooxgenase (COX)-2 served as a major neurotoxic factor in CNS injury. However, the mechanisms underlying EV71-initiated intracellular signaling pathways leading to COX-2 expression remain unknown. Therefore, we investigated the mechanisms underlying EV71-induced COX-2 expression and prostaglandin E(2) (PGE(2)) production in rat brain astrocytes (RBA)-1, determined by Western blotting, RT-PCR, and promoter assay. Here, we reported that EV71-induced COX-2 expression and PGE(2) production were attenuated by pretreatment with the inhibitors of c-Src (PP1), PDGFR (AG1296), PI3K (Wortmannin), MEK1/2 (PD98059), NF-kappaB (helenalin), and AP-1 (Tanshinone) and transfection with shRNA or siRNA of c-Src, PDGFR, p85, c-Jun, c-Fos, ERK1, or ERK2. We further observed that EV71-induced activation of Akt and p42/p44 MAPK were mediated via c-Src and PDGFR. Pretreatment with PP1 attenuated EV71-stimulated phosphorylation of Src, PDGFR, Akt, and p42/p44 MAPK. Inhibition of PI3K by Wortmannin attenuated EV71-induced Akt and p42/p44 MAPK phosphorylation, but had no effect on PDGFR phosphorylation, suggesting that PDGFR is an upstream and p42/p44 MAPK is a downstream component of PI3K/Akt in these responses. EV71-stimulated NF-kappaB translocation from the cytoplasm to the nucleus, IkappaBalpha degradation and NF-kappaB promoter activity were attenuated by pretreatment with helenalin, but not AG1296, Wortmannin, and PD98059. EV71-induced c-Jun mRNA expression was attenuated by pretreatment with PD98059, AG1296, or Wortmannin. These results demonstrate that in RBA-1 cells, EV71-induced COX-2 expression associated with PGE(2) production is mediated through activation of c-Src/PDGFR/PI3K/Akt/p42/p44 MAPK to initiate the expression of AP-1.

Astrocytes modulate the chemokine network in a pathogen-specific manner

Immune responses in the central nervous system (CNS) are carefully regulated. Despite the absence of most immune processes and a substantive blood brain barrier, potent immune responses form during infection and autoimmunity. Astrocytes are innate immune sentinels that ensheath parenchymal blood vessels and sit at the gateway to the CNS parenchyma. Viral and bacterial infections trigger the influx of distinct leukocyte subsets. We show that astrocytes alone are sufficient for distinguishing between these two main types of infection and triggers release of relevant chemokines that relate to the microbe recognised. Bacterial-associated molecules induced the preferential expression of CCL2, CXCL1, CCL20 and CCL3 whilst a virus-associated dsRNA analogue preferentially up-regulated CXCL10 and CCL5. Thus, astrocytes can respond to infection in a distinct and appropriate manner suggesting they have the capacity to attract appropriate sets of leukocytes into the brain parenchyma. Astrocytes themselves are unable to respond to these chemokines since they were devoid of most chemokine receptors but expressed CXCR4, CXCR7 and CXCR6 at rest. Stimulation with TGF-beta specifically up-regulated CXCR6 expression and may explain how TGF-beta/CXCL16-expressing gliomas are so effective at attracting astroglial cells.

Disruption of astrocyte STAT3 signaling decreases mitochondrial function and increases oxidative stress in vitro

BACKGROUND

Astrocytes exert a wide variety of functions in health and disease and respond to a wide range of signaling pathways, including members of the Janus-kinase signal transducers and activators of transcription (Jak-STAT) family. We have recently shown that STAT3 is an important regulator of astrocyte reactivity after spinal cord injury in vivo[1].

METHODOLOGY/PRINCIPAL FINDINGS

Here, we used both a conditional gene deletion strategy that targets the deletion of STAT3 selectively to astrocytes (STAT3-CKO), and a pharmacological inhibitor of JAK-2, AG490, in cultured astrocytes in vitro, to investigate potential functions and molecules influenced by STAT3 signaling in relation to mitochondrial function and oxidative stress. Our findings show that the absence of STAT3 signaling in astrocytes leads to (i) increased production of superoxide anion and other reactive oxygen species and decreased level of glutathione, (ii) decreased mitochondrial membrane potential and decreased ATP production, and (iii) decreased rate of cell proliferation. Many of the differences observed in STAT3-CKO astrocytes were distinctly altered by exposure to rotenone, suggesting a role for complex I of the mitochondrial electron transport chain. Gene expression microarray studies identified numerous changes in STAT3-CKO cells that may have contributed to the identified deficits in cell function.

CONCLUSIONS/SIGNIFICANCE

Taken together, these STAT3-dependent alterations in cell function and gene expression have relevance to both reactive gliosis and to the support and protection of surrounding cells in neural tissue.

Theiler's virus infection chronically alters seizure susceptibility

PURPOSE

Central nervous system infections greatly increase the risk for the development of seizures and epilepsy (recurrent unprovoked seizures). We have previously shown that Theiler's murine encephalomyelitis virus (Theiler's virus or TMEV) infection causes acute symptomatic seizures in C57BL/6 (B6) mice. The objective of the present study was threefold: (1) to assess pathologic changes associated with acute TMEV infection and infection-induced seizures, (2) to determine whether Theiler's virus infection and associated acute seizures lead to chronically altered seizure susceptibility, and (3) to determine whether genetic background influences seizure susceptibility following Theiler's virus infection.

METHODS

Immunohistochemical techniques were used to assess Theiler's virus antigen localization in the brain and associated neuronal cell death. A battery of electroconvulsive threshold (ECT) tests and corneal kindling studies were conducted to assess whether there were chronic alterations in seizure susceptibility and kindling development. Studies were conducted in both B6 and SJL/J mice to assess strain-dependent effects.

RESULTS

Histopathologic analyses indicate that TMEV has specific tropism for limbic structures and causes widespread cell death in these regions. Results from ECT studies demonstrate that B6 mice that displayed acute symptomatic seizures have significantly reduced seizure thresholds and kindle faster than either control mice or infected mice without acute seizures. Furthermore, these effects were mouse-strain dependent, since SJL/J mice displayed a different seizure threshold spectrum.

DISCUSSION

These findings indicate that Theiler's virus infection leads to chronically altered seizure susceptibility in mice. It is important to note that Theiler's virus infection of B6 mice represents a novel model to study postinfection hyperexcitability.

Neuropathogenesis of Theiler's murine encephalomyelitis virus infection, an animal model for multiple sclerosis

Theiler's murine encephalomyelitis virus (TMEV) infection of mice is an experimental model for multiple sclerosis (MS). TMEV induces a biphasic disease in susceptible mouse strains. During the acute phase, 1 week after infection, TMEV causes polioencephalomyelitis characterized by infection and apoptosis of neurons in the gray matter of the brain. During the chronic phase, about 1 month after infection, virus infects glial cells and macrophages, and induces inflammatory demyelination with oligodendrocyte apoptosis and axonal degeneration in the white matter of the spinal cord. Although antibody, CD4(+), and CD8(+) T cell responses against TMEV capsid proteins play important roles in neuropathogenesis, infectious virus with persistence is necessary to induce demyelination; in general, adoptive transfer of antibody or T cells alone did not induce central nervous system (CNS) disease. The TMEV model can be useful for testing new therapeutic strategies specifically as a viral model for MS. Therapies targeting adhesion molecules, axonal degeneration, and immunosuppression can be beneficial for pure autoimmune CNS demyelinating diseases, such as experimental autoimmune encephalomyelitis, but could be detrimental in virus-induced demyelinating diseases, such as progressive multifocal leukoencephalopathy.

Fulminant lymphocytic choriomeningitis virus-induced inflammation of the CNS involves a cytokine-chemokine-cytokine-chemokine cascade

Intracerebral inoculation of immunocompetent mice with lymphocytic choriomeningitis virus (LCMV) normally results in fatal CD8+ T cell mediated meningoencephalitis. However, in CXCL10-deficient mice, the virus-induced CD8+ T cell accumulation in the neural parenchyma is impaired, and only 30-50% of the mice succumb to the infection. Similar results are obtained in mice deficient in the matching chemokine receptor, CXCR3. Together, these findings point to a key role for CXCL10 in regulating the severity of the LCMV-induced inflammatory process. For this reason, we now address the mechanisms regulating the expression of CXCL10 in the CNS of LCMV-infected mice. Using mice deficient in type I IFN receptor, type II IFN receptor, or type II IFN, as well as bone marrow chimeras expressing CXCL10 only in resident cells or only in bone marrow-derived cells, we analyzed the up-stream regulation as well as the cellular source of CXCL10. We found that expression of CXCL10 initially depends on signaling through the type I IFN receptor, while late expression and up-regulation requires type II IFN produced by the recruited CD8+ T cells. Throughout the infection, the producers of CXCL10 are exclusively resident cells of the CNS, and astrocytes are the dominant expressors in the neural parenchyma, not microglial cells or recruited bone marrow-derived cell types. These results are consistent with a model suggesting a bidirectional interplay between resident cells of the CNS and the recruited virus-specific T cells with astrocytes as active participants in the local antiviral host response.