Monocytes/macrophages isolated from the mouse central nervous system contain infectious Theiler's murine encephalomyelitis virus (TMEV)

The Contribution of Microglia and Brain-Infiltrating Macrophages to the Pathogenesis of Neuroinflammatory and Neurodegenerative Diseases during TMEV Infection of the Central Nervous System

The infection of the central nervous system (CNS) with neurotropic viruses induces neuroinflammation and is associated with the development of neuroinflammatory and neurodegenerative diseases, including multiple sclerosis and epilepsy. The activation of the innate and adaptive immune response, including microglial, macrophages, and T and B cells, while required for efficient viral control within the CNS, is also associated with neuropathology. Under healthy conditions, resident microglia play a pivotal role in maintaining CNS homeostasis. However, during pathological events, such as CNS viral infection, microglia become reactive, and immune cells from the periphery infiltrate into the brain, disrupting CNS homeostasis and contributing to disease development. Theiler's murine encephalomyelitis virus (TMEV), a neurotropic picornavirus, is used in two distinct mouse models: TMEV-induced demyelination disease (TMEV-IDD) and TMEV-induced seizures, representing mouse models of multiple sclerosis and epilepsy, respectively. These murine models have contributed substantially to our understanding of the pathophysiology of MS and seizures/epilepsy following viral infection, serving as critical tools for identifying pharmacological targetable pathways to modulate disease development. This review aims to discuss the host-pathogen interaction during a neurotropic picornavirus infection and to shed light on our current understanding of the multifaceted roles played by microglia and macrophages in the context of these two complexes viral-induced disease.

Transcriptome analysis of human macrophages upon chikungunya virus (CHIKV) infection reveals regulation of distinct signaling and metabolic pathways during the early and late stages of infection

Macrophages are efficient reservoirs for viruses that enable the viruses to survive over a longer period of infection. Alphaviruses such as chikungunya virus (CHIKV) are known to persist in macrophages even after the acute febrile phase. The viral particles replicate in macrophages at a very low level over extended period of time and are localized in tissues that are often less accessible by treatment. Comprehensive experimental studies are thus needed to characterize the CHIKV-induced modulation of host genes in these myeloid lineage cells and in one such pursuit, we obtained global transcriptomes of a human macrophage cell line infected with CHIKV, over its early and late timepoints of infection. We analyzed the pathways, especially immune related, perturbed over these timepoints and observed several host factors to be differentially expressed in infected macrophages in a time-dependent manner. We postulate that these pathways may play crucial roles in the persistence of CHIKV in macrophages.

IL-33 promotes increased replication of Theiler's Murine Encephalomyelitis Virus in RAW264.7 macrophage cells with an IRF3-dependent response

Interleukin-33 (IL-33), which promotes M2 macrophage development, may influence the control of viruses, such as Theiler's Murine Encephalomyelitis Virus (TMEV) that infect macrophages. Because Interferon Regulatory Factor-3 (IRF3) is also critical to control of TMEV infection in macrophages, information on the relationship between IL-33 and IRF3 is important. Thus, RAW264.7 Lucia murine macrophage lineage cells with an endogenous IRF3-ISRE promoter driving secreted luciferase and IRF3KO RAW Lucia, a subline deficient in IRF3, were challenged with TMEV. After the challenge, considerable TMEV RNA detected at 18 and 24 h in RAW cells was significantly elevated in IRF3KO RAW cells. TMEV induction of ISRE-IRF3 promoter activity, IFN-β and IL-33 gene expression, and IL-6 and IL-10 protein production, which was strong in RAW cells, was less in IRF3KO RAW cells. In contrast, expression of CD206 and ARG1, classical M2 macrophage markers, was significantly elevated in IRF3KO RAW cells. Moreover, RAW and IRF3KO RAW cells produced extracellular IL-33 prior to and after infection with TMEV and antibody blockade of the IL-33 receptor, ST2, reduced CD206 and ARG1 expression, but increased IL-6 gene expression. Pre-treating both RAW and IRF3KO RAW cells with IL-33 prior to challenge significantly increased TMEV infection, but also increased IL-33, IL-10, IL-6 mRNA expression, and NO production without increasing IFN-β. Notably, IL-33 induction of IL-33, IRF3-ISRE promoter activity, and IL-10 by TMEV or poly I:C/IFN-γ was significantly dependent upon IRF3. The results show that the expression of IL-33 and the repression of M2 macrophage phenotypic markers are dependent on IRF3 and that IL-33 decreases the ability of macrophages to control infection with macrophage-tropic viruses.

Critical role of TLR activation in viral replication, persistence, and pathogenicity of Theiler's virus

Theiler's murine encephalomyelitis virus (TMEV) establishes persistent viral infections in the central nervous system and induces chronic inflammatory demyelinating disease in susceptible mice. TMEV infects dendritic cells, macrophages, B cells, and glial cells. The state of TLR activation in the host plays a critical role in initial viral replication and persistence. The further activation of TLRs enhances viral replication and persistence, leading to the pathogenicity of TMEV-induced demyelinating disease. Various cytokines are produced via TLRs, and MDA-5 signals linked with NF-κB activation following TMEV infection. In turn, these signals further amplify TMEV replication and the persistence of virus-infected cells. The signals further elevate cytokine production, promoting the development of Th17 responses and preventing cellular apoptosis, which enables viral persistence. Excessive levels of cytokines, particularly IL-6 and IL-1β, facilitate the generation of pathogenic Th17 immune responses to viral antigens and autoantigens, leading to TMEV-induced demyelinating disease. These cytokines, together with TLR2 may prematurely generate functionally deficient CD25-FoxP3+ CD4⁺ T cells, which are subsequently converted to Th17 cells. Furthermore, IL-6 and IL-17 synergistically inhibit the apoptosis of virus-infected cells and the cytolytic function of CD8+ T lymphocytes, prolonging the survival of virus-infected cells. The inhibition of apoptosis leads to the persistent activation of NF-κB and TLRs, which continuously provides an environment of excessive cytokines and consequently promotes autoimmune responses. Persistent or repeated infections of other viruses such as COVID-19 may result in similar continuous TLR activation and cytokine production, leading to autoimmune diseases.

Connecting Neuroinflammation and Neurodegeneration in Multiple Sclerosis: Are Oligodendrocyte Precursor Cells a Nexus of Disease?

The pathology in neurodegenerative diseases is often accompanied by inflammation. It is well-known that many cells within the central nervous system (CNS) also contribute to ongoing neuroinflammation, which can promote neurodegeneration. Multiple sclerosis (MS) is both an inflammatory and neurodegenerative disease in which there is a complex interplay between resident CNS cells to mediate myelin and axonal damage, and this communication network can vary depending on the subtype and chronicity of disease. Oligodendrocytes, the myelinating cell of the CNS, and their precursors, oligodendrocyte precursor cells (OPCs), are often thought of as the targets of autoimmune pathology during MS and in several animal models of MS; however, there is emerging evidence that OPCs actively contribute to inflammation that directly and indirectly contributes to neurodegeneration. Here we discuss several contributors to MS disease progression starting with lesion pathology and murine models amenable to studying particular aspects of disease. We then review how OPCs themselves can play an active role in promoting neuroinflammation and neurodegeneration, and how other resident CNS cells including microglia, astrocytes, and neurons can impact OPC function. Further, we outline the very complex and pleiotropic role(s) of several inflammatory cytokines and other secreted factors classically described as solely deleterious during MS and its animal models, but in fact, have many neuroprotective functions and promote a return to homeostasis, in part via modulation of OPC function. Finally, since MS affects patients from the onset of disease throughout their lifespan, we discuss the impact of aging on OPC function and CNS recovery. It is becoming clear that OPCs are not simply a bystander during MS progression and uncovering the active roles they play during different stages of disease will help uncover potential new avenues for therapeutic intervention.

Excessive Innate Immunity Steers Pathogenic Adaptive Immunity in the Development of Theiler's Virus-Induced Demyelinating Disease

Several virus-induced models were used to study the underlying mechanisms of multiple sclerosis (MS). The infection of susceptible mice with Theiler’s murine encephalomyelitis virus (TMEV) establishes persistent viral infections and induces chronic inflammatory demyelinating disease. In this review, the innate and adaptive immune responses to TMEV are discussed to better understand the pathogenic mechanisms of viral infections. Professional (dendritic cells (DCs), macrophages, and B cells) and non-professional (microglia, astrocytes, and oligodendrocytes) antigen-presenting cells (APCs) are the major cell populations permissive to viral infection and involved in cytokine production. The levels of viral loads and cytokine production in the APCs correspond to the degrees of susceptibility of the mice to the TMEV-induced demyelinating diseases. TMEV infection leads to the activation of cytokine production via TLRs and MDA-5 coupled with NF-κB activation, which is required for TMEV replication. These activation signals further amplify the cytokine production and viral loads, promote the differentiation of pathogenic Th17 responses, and prevent cellular apoptosis, enabling viral persistence. Among the many chemokines and cytokines induced after viral infection, IFN α/β plays an essential role in the downstream expression of costimulatory molecules in APCs. The excessive levels of cytokine production after viral infection facilitate the pathogenesis of TMEV-induced demyelinating disease. In particular, IL-6 and IL-1β play critical roles in the development of pathogenic Th17 responses to viral antigens and autoantigens. These cytokines, together with TLR2, may preferentially generate deficient FoxP3⁺CD25^- regulatory cells converting to Th17. These cytokines also inhibit the apoptosis of TMEV-infected cells and cytolytic function of CD8⁺ T lymphocytes (CTLs) and prolong the survival of B cells reactive to viral and self-antigens, which preferentially stimulate Th17 responses.

Viral mouse models used to study multiple sclerosis: past and present

Multiple sclerosis (MS) is a common inflammatory demyelinating disease of the central nervous system. Although the etiology of MS is unknown, genetics and environmental factors, such as infections, play a role. Viral infections of mice have been used as model systems to study this demyelinating disease of humans. Three viruses that have long been studied in this capacity are Theiler’s murine encephalomyelitis virus, mouse hepatitis virus, and Semliki Forest virus. This review describes the viruses themselves, the infection process, the disease caused by infection and its accompanying pathology, and the model systems and their usefulness in studying MS.

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.

Monocytes and Macrophages as Viral Targets and Reservoirs

Viruses manipulate cell biology to utilize monocytes/macrophages as vessels for dissemination, long-term persistence within tissues and virus replication. Viruses enter cells through endocytosis, phagocytosis, macropinocytosis or membrane fusion. These processes play important roles in the mechanisms contributing to the pathogenesis of these agents and in establishing viral genome persistence and latency. Upon viral infection, monocytes respond with an elevated expression of proinflammatory signalling molecules and antiviral responses, as is shown in the case of the influenza, Chikungunya, human herpes and Zika viruses. Human immunodeficiency virus initiates acute inflammation on site during the early stages of infection but there is a shift of M1 to M2 at the later stages of infection. Cytomegalovirus creates a balance between pro- and anti-inflammatory processes by inducing a specific phenotype within the M1/M2 continuum. Despite facilitating inflammation, infected macrophages generally display abolished apoptosis and restricted cytopathic effect, which sustains the virus production. The majority of viruses discussed in this review employ monocytes/macrophages as a repository but certain viruses use these cells for productive replication. This review focuses on viral adaptations to enter monocytes/macrophages, immune escape, reprogramming of infected cells and the response of the host cells.

Traditional Uses of Cannabinoids and New Perspectives in the Treatment of Multiple Sclerosis

Recent findings highlight the emerging role of the endocannabinoid system in the control of symptoms and disease progression in multiple sclerosis (MS). MS is a chronic, immune-mediated, demyelinating disorder of the central nervous system with no cure so far. It is widely reported in the literature that cannabinoids might be used to control MS symptoms and that they also might exert neuroprotective effects and slow down disease progression. This review aims to give an overview of the principal cannabinoids (synthetic and endogenous) used for the symptomatic amelioration of MS and their beneficial outcomes, providing new potentially possible perspectives for the treatment of this disease.

SJL bone marrow-derived macrophages do not have IRF3 mutations and are highly susceptible to Theiler's virus infection

It is well known that SJL mice are susceptible to Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease while C57BL6 (B6) and B10 mice are resistant, and H-2^s on a B10 background (B10.S) contributes modestly to susceptibility. A recent study linked two IRF3 non-conservative mutations in SJL compared to B10.S mice to resistance to TMEV infection of SJL peritoneal-derived macrophages, an observation of practical interest in light of the central role of IRF3 transcription factor in the type I interferon (IFN) response. However, we did not find these non-conservative mutations among SJL, B10.S, B6 and B10 mice in the IRF3 amino acid sequence, and show SJL bone marrow-derived macrophages infected with TMEV exhibit increased virus RNA replication and infectious virus yields as well as greater IL-6 production than C57Bl strain (including B10.S) cultures.

The influence of macrophage growth factors on Theiler's Murine Encephalomyelitis Virus (TMEV) infection and activation of macrophages

Macrophages are common targets for infection and innate immune activation by many pathogenic viruses including the neurotropic Theiler's Murine Encephalomyelitis Virus (TMEV). As both infection and innate activation of macrophages are key determinants of viral pathogenesis especially in the central nervous system (CNS), an analysis of macrophage growth factors on these events was performed. C3H mouse bone-marrow cells were differentiated in culture using either recombinant macrophage colony stimulating factor (M-CSF) or granulocyte-macrophage colony-stimulating factor (GM-CSF), inoculated with TMEV (BeAn) and analyzed at various times thereafter. Cytokine RNA and protein analysis, virus titers, and flow cytometry were performed to characterize virological parameters under these culture conditions. GM-CSF-differentiated macrophages showed higher levels of TMEV viral RNA and proinflammatory molecules compared to infected M-CSF-differentiated cells. Thus, GM-CSF increases both TMEV infection and TMEV-induced activation of macrophages compared to that seen with M-CSF. Moreover, while infectious viral particles decreased from a peak at 12h to undetectable levels at 48h post infection, TMEV viral RNA remained higher in GM-CSF- compared to M-CSF-differentiated macrophages in concert with increased proinflammatory gene expression. Analysis of a possible basis for these differences determined that glycolytic rates contributed to heightened virus replication and proinflammatory cytokine secretion in GM-CSF compared to M-CSF-differentiated macrophages. In conclusion, we provide evidence implicating a role for GM-CSF in promoting virus replication and proinflammatory cytokine expression in macrophages, indicating that GM-CSF may be a key factor for TMEV infection and the induction of chronic TMEV-induced immunopathogenesis in the CNS.

Theiler's murine encephalomyelitis virus infection of SJL/J and C57BL/6J mice: Models for multiple sclerosis and epilepsy

Mouse models are great tools to study the mechanisms of disease development. Theiler's murine encephalomyelitis virus is used in two distinct viral infection mouse models to study the human diseases multiple sclerosis (MS) and epilepsy. Intracerebral (i.c.) infection of the SJL/J mouse strain results in persistent viral infection of the central nervous system and a MS-like disease, while i.c. infection of the C57BL/6J mouse strain results in acute seizures and epilepsy. Our understanding of how the immune system contributes to the development of two disparate diseases caused by the same virus is presented.

The role of α4 integrin in Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease: an infectious animal model for multiple sclerosis (MS)

Natalizumab, which is an antibody against α4 integrin, has been used for the treatment of multiple sclerosis (MS). In the present study, we investigated both the role of α4 integrin and the therapeutic effect of HCA3551, a newly synthesized orally active small-molecule α4 integrin antagonist, in the development of TMEV-induced demyelinating disease (TMEV-IDD). The mRNA levels of α4 integrins were significantly up-regulated in the CNS of mice with TMEV-IDD as compared with naïve mice (*p<0.05). HCA3551 treatment in the effector phase significantly suppressed both the clinical and histological development of TMEV-IDD. The number of infiltrating mononuclear inflammatory cells in the CNS was significantly decreased in the mice treated with HCA3551 (**p<0.01). The labeling indices for CD68 antigen and the absolute cell numbers of TNF-α-producing CD4⁺ T cells and IFN-γ-producing CD8⁺ T cells were significantly decreased in the CNS of mice treated with HCA3551 (*p<0.05). HCA3551 treatment in the effector phase might inhibit the binding of α4 integrin to VCAM-1, thereby decreasing the number of MNCs in the CNS.

Close Encounters of the First Kind: Innate Sensors and Multiple Sclerosis

Although autoimmune diseases by definition imply adaptive immune system pathologies, growing evidence points to the relevance of innate receptors in modulating the initiation and progression of the autoreactive response. Multiple sclerosis (MS) is a chronic autoimmune disease characterised by central nervous system (CNS) demyelination, inflammation and axonal damage, in which the role of several pathogens such as herpes viruses have long been described as potential triggers. Encounters of these pathogens with altered innate receptors in susceptible individuals might drive pathological autoreactivity and inflammation, overcoming tolerance and causing subsequent CNS damage. In particular, functional and genetic studies reveal that Toll-like receptor (TLR) 2 and the Nod-like receptor (NLR) P3 could be involved in MS pathogenesis, whereas TLR3, the triggering receptor expressed on myeloid cells (TREM)-2 and the C-type lectin receptors (CLRs) MBL and MASP-3 would have a putative protective role. A better understanding of these interactions will provide important insights into the aetiopathogenesis of MS and could help design potential targets for novel therapies.

Depletion of Olig2 in oligodendrocyte progenitor cells infected by Theiler's murine encephalomyelitis virus

Theiler's murine encephalomyelitis virus (TMEV) infects the central nervous system of mice and causes a demyelinating disease that is a model for multiple sclerosis. During the chronic phase of the disease, TMEV persists in oligodendrocytes and macrophages. Lack of remyelination has been attributed to insufficient proliferation and differentiation of oligodendrocyte progenitor cells (OPCs), but the molecular mechanisms remain unknown. Here, we employed pluripotent stem cell technologies to generate pure populations of mouse OPCs to study the temporal and molecular effects of TMEV infection. Global transcriptome analysis of RNA sequencing data revealed that TMEV infection of OPCs caused significant up-regulation of 1926 genes, whereas 1853 genes were significantly down-regulated compared to uninfected cells. Pathway analysis revealed that TMEV disrupted many genes required for OPC growth and maturation. Down-regulation of Olig2, a transcription factor necessary for OPC proliferation, was confirmed by real-time PCR, immunofluorescence microscopy, and western blot analysis. Depletion of Olig2 was not found to be specific to viral strain and did not require expression of the leader (L) protein, which is a multifunctional protein important for persistence, modulation of gene expression, and cell death. These data suggest that direct infection of OPCs by TMEV may inhibit remyelination during the chronic phase of TMEV-induced demyelinating disease.

Self-reactive CD4(+) T cells activated during viral-induced demyelination do not prevent clinical recovery

Background

Microbial infections have been implicated in initiating and enhancing severity of autoimmune diseases including the demyelinating disease multiple sclerosis (MS). Nevertheless, the incidence of both acute and persisting viral infections without evidence of autoimmune sequelae suggests that this process is well controlled. The conditions promoting or stemming self-reactive (SR) T cells following viral-induced tissue damage thus need to be better defined. Using a non-fatal viral mouse model of encephalomyelitis associated with demyelination and disability, yet ultimate clinical improvement, this study set out to monitor uptake and presentation of endogenous myelin antigens, as well as induction and fate of SR T cells.

Methods

Activation and central nervous system (CNS) recruitment of myelin-specific CD4 T cells was analyzed by flow cytometry during encephalomyelitis induced by a glia tropic murine coronavirus. Potential antigen-presenting cells (APC) ingesting myelin were characterized by flow cytometry and their ability to activate SR T cells tested by co-culture with carboxyfluorescein succinimidyl ester (CFSE)-labeled myelin-specific CD4 T cells. Endogenous SR T cell kinetics was analyzed within both cervical lymph nodes and CNS by Enzyme-Linked ImmunoSpot (ELISPOT) following viral infection.

Results

The data demonstrate the presence of APC capable of activating SR T cells in both draining lymph nodes and the CNS temporally correlating with overt demyelination. While both the CNS-infiltrating myeloid population and microglia ingested myelin, only CNS-infiltrating APC were capable of presenting endogenous myelin antigen to SR T cells ex vivo. Finally, SR T cell activation from the endogenous T cell repertoire was most notable when infectious virus was controlled and paralleled myelin damage. Although SR T cell accumulation peaked in the persistently infected CNS during maximal demyelination, they were not preferentially retained. Their gradual decline, despite ongoing demyelination, suggested minimal re-stimulation and pathogenic function in vivo consistent with the lack of autoimmune symptoms.

Conclusions

The results demonstrate the potential for CNS tissue destruction to induce and recruit SR T cells to the injury site and support a host suppressive mechanism limiting development of autoimmunity.

Dimethyl fumarate suppresses Theiler's murine encephalomyelitis virus-induced demyelinating disease by modifying the Nrf2-Keap1 pathway

Dimethyl fumarate (DMF) is a modifier of the nuclear factor (erythroid-derived 2)-2 (Nrf2)-kelch-like ECH-associated protein 1 (Keap1) pathway. DMF treatment in the effector phase significantly suppressed the development of Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) both clinically and histologically. DMF treatment leads to an enhanced Nrf2 antioxidant response in TMEV-IDD mice. DMF treatment in the effector phase significantly suppressed the level of IL-17A mRNA. DMF is known to inhibit differentiation of T helper 17 (Th17) cells via suppressing NF-κB. Taken together, our data suggest that DMF treatment in the effector phase may suppress TMEV-IDD not only via enhancing the antioxidant response but also via suppressing IL-17A.

Role of the Programmed Death-1 (PD-1) pathway in regulation of Theiler's murine encephalomyelitis virus-induced demyelinating disease

Programmed death-1 (PD-1) belongs to the CD28 family of co-stimulatory and co-inhibitory molecules and regulates adaptive immunity. This molecule induces the development of regulatory T cells, T cell tolerance, or apoptosis. We examined the role of PD-1 pathway in Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD) mice. Up-regulation of PD-1 and PD-1 ligand-1 (PD-L1) mRNA expression in bone marrow-derived dendritic cells were induced by TMEV infection in vitro. Furthermore, PD-1 and PD-L1 mRNA expression was increased in the spinal cords of the TMEV-infected mice in vivo. Treatment with a blocking monoclonal antibody (mAb) against PD-1, especially during the effector phase, resulted in significant deterioration of the TMEV-IDD both clinically and histologically. Flow cytometric analysis revealed a dramatically increase of CD4(+) T cells producing Th1 cytokines such as IFN-γ and TNF-α in the spinal cord of anti-PD-1 mAb-treated mice. These results indicate that the PD-1 pathway plays a pivotal regulatory role in the development of TMEV-IDD.

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.

Modeling the acute and chronic phases of Theiler murine encephalomyelitis virus infection

Theiler murine encephalomyelitis virus (TMEV) infection of a mouse's central nervous system is biphasic: first the virus infects motor neurons (acute phase), and this is followed by a chronic phase in which the virus infects glial cells (primarily microglia and macrophages [M]) of the spinal cord white matter, leading to inflammation and demyelination. As such, TMEV-induced demyelinating disease in mice provides a highly relevant experimental animal model for multiple sclerosis. Mathematical models have proven valuable in understanding the in vivo dynamics of persistent virus infections, such as HIV-1, hepatitis B virus, and hepatitis C virus infections. However, viral dynamic modeling has not been used for understanding TMEV infection. We constructed the first mathematical model of TMEV-host kinetics during acute and early chronic infections in mice and fit measured viral kinetic data with the model. The data fitting allowed us to estimate several unknown parameters, including the following: the rate of infection of neurons, 0.5 × 10(-8) to 5.6 × 10(-8) day(-1); the percent reduction of the infection rate due to the presence of virus-specific antibodies, which reaches 98.5 to 99.9% after day 15 postinfection (p.i.); the half-life of infected neurons, 0.1 to 1.2 days; and a cytokine-enhanced macrophage source rate of 25 to 350 M/day into the spinal cord starting at 10.9 to 12.9 days p.i. The model presented here is a first step toward building a comprehensive model for TMEV-induced demyelinating disease. Moreover, the model can serve as an important tool in understanding TMEV infectious mechanisms and may prove useful in evaluating antivirals and/or therapeutic modalities to prevent or inhibit demyelination.

Inflammatory monocytes and the pathogenesis of viral encephalitis

Monocytes are a heterogeneous population of bone marrow-derived cells that are recruited to sites of infection and inflammation in many models of human diseases, including those of the central nervous system (CNS). Ly6C^hi/CCR2^hi inflammatory monocytes have been identified as the circulating precursors of brain macrophages, dendritic cells and arguably microglia in experimental autoimmune encephalomyelitis; Alzheimer’s disease; stroke; and more recently in CNS infection caused by Herpes simplex virus, murine hepatitis virus, Theiler’s murine encephalomyelitis virus, Japanese encephalitis virus and West Nile virus. The precise differentiation pathways and functions of inflammatory monocyte-derived populations in the inflamed CNS remains a contentious issue, especially in regard to the existence of monocyte-derived microglia. Furthermore, the contributions of monocyte-derived subsets to viral clearance and immunopathology are not well-defined. Thus, understanding the pathways through which inflammatory monocytes migrate to the brain and their functional capacity within the CNS is critical to inform future therapeutic strategies. This review discusses some of the key aspects of inflammatory monocyte trafficking to the brain and addresses the role of these cells in viral encephalitis.

Chronic social stress impairs virus specific adaptive immunity during acute Theiler's virus infection

Prior exposure to social disruption (SDR) stress exacerbates Theiler's murine encephalomyelitis virus (TMEV) infection, a model of multiple sclerosis. Here we examined the impact of SDR on T cell responses to TMEV infection in SJL mice. SDR impaired viral clearance and exacerbated acute disease. Moreover, TMEV infection alone increased CD4 and CD8 mRNA expression in brain and spleen while SDR impaired this response. SDR decreased both CD4(+) and CD8(+) virus-specific T cells in CNS, but not spleen. These findings suggest that SDR-induced suppression of virus-specific T cell responses contributes to impairments in viral clearance and exacerbation of acute disease.

Therapeutic effects of anti-Delta1 mAb on Theiler's murine encephalomyelitis virus-induced demyelinating disease

We examined the role of Notch ligand Delta-like 1 (Delta1) in the development of Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD). Blocking of Delta1 by anti-Delta1 monoclonal antibody (mAb) in the effector phase significantly suppressed the disease development of TMEV-IDD both clinically and histologically. The number of infiltrating inflammatory mononuclear cells in the spinal cords was also decreased in mice treated with anti-Delta1 mAb at the effector phase. Flow cytometric analysis of cytokine staining revealed that IFN-γ- or IL-4-producing CD4(+) splenocytes were significantly decreased in mice treated with anti-Delta1 mAb in the spleens, whereas IL-10-producing CD4(+) splenocytes were increased. Furthermore, IFN-γ-, TNF-α-, IL-4-, or IL-10-producing CD4(+) cells were decreased in spinal cords, and IL-17-producing CD4(+) cells were increased. These data suggest that Delta1 may play important roles in the development of TMEV-IDD and that antibodies to Delta1 could be used as a novel therapeutic treatment of demyelinating diseases such as human multiple sclerosis.

Control of early Theiler's murine encephalomyelitis virus replication in macrophages by interleukin-6 occurs in conjunction with STAT1 activation and nitric oxide production

During Theiler's murine encephalomyelitis virus (TMEV) infection of macrophages, it is thought that high interleukin-6 (IL-6) levels contribute to the demyelinating disease found in chronically infected SJL/J mice but absent in B10.S mice capable of clearing the infection. Therefore, IL-6 expression was measured in TMEV-susceptible SJL/J and TMEV-resistant B10.S macrophages during their infection with TMEV DA strain or responses to lipopolysaccharide (LPS) or poly(I · C). Unexpectedly, IL-6 production was greater in B10.S macrophages than SJL/J macrophages during the first 24 h after stimulation with TMEV, LPS, or poly(I · C). Further experiments showed that in B10.S, SJL/J, and RAW264.7 macrophage cells, IL-6 expression was dependent on extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) and enhanced by exogenous IL-12. In SJL/J and RAW264.7 macrophages, exogenous IL-6 resulted in decreased TMEV replication, earlier activation of STAT1 and STAT3, production of nitric oxide, and earlier upregulation of several antiviral genes downstream of STAT1. However, neither inhibition of IL-6-induced nitric oxide nor knockdown of STAT1 diminished the early antiviral effect of exogenous IL-6. In addition, neutralization of endogenous IL-6 from SJL/J macrophages with Fab antibodies did not exacerbate early TMEV infection. Therefore, endogenous IL-6 expression after TMEV infection is dependent on ERK MAPK, enhanced by IL-12, but too slow to decrease viral replication during early infection. In contrast, exogenous IL-6 enhances macrophage control of TMEV infection through preemptive antiviral nitric oxide production and antiviral STAT1 activation. These results indicate that immediate-early production of IL-6 could protect macrophages from TMEV infection.

Periventricular demyelination and axonal pathology is associated with subependymal virus spread in a murine model for multiple sclerosis

OBJECTIVES

Theiler's murine encephalomyelitis virus (TMEV) infection of mice is a widely used animal model for demyelinating disorders, such as multiple sclerosis (MS). The aim of the present study was to identify topographical differences of TMEV spread and demyelination in the brain of experimentally infected susceptible SJL/J mice and resistant C57BL/6 mice.

METHODS

Demyelination was confirmed by Luxol fast blue and cresyl violet staining and axonal damage by neurofilament-specific and β-amyloid precursor protein-specific immunohistochemistry. Viral dissemination within the central nervous system (CNS) was quantified by immunohistochemistry and in situ hybridization. Further, the phenotype of infected cells was determined by confocal laser scanning microscopy.

RESULTS

An early transient infection of periventricular cells followed by demyelination and axonopathies around the fourth ventricle in SJL/J mice was noticed. Periventricular and brain stem demyelination was associated with a predominant infection of microglia/macrophages and oligodendrocytes.

CONCLUSIONS

Summarized, the demonstration of ependymal infection and subjacent spread into the brain parenchyma as well as regional virus clearance despite ongoing demyelination and axonal damage in other CNS compartments allows new insights into TME pathogenesis. This novel aspect of TMEV CNS interaction will enhance the understanding of region-specific susceptibilities to injury and regenerative capacities of the brain in this MS model.

Mouse models of multiple sclerosis: experimental autoimmune encephalomyelitis and Theiler's virus-induced demyelinating disease

Experimental autoimmune encephalomyelitis (EAE) and Theiler's Murine Encephalitis Virus-Induced Demyelinating Disease (TMEV-IDD) are two clinically relevant murine models of multiple sclerosis (MS). Like MS, both are characterized by mononuclear cell infiltration into the CNS and demyelination. EAE is induced by either the administration of myelin protein or peptide in adjuvant or by the adoptive transfer of encephalitogenic T cell blasts into naïve recipients. The relative merits of each of these protocols are compared. Depending on the type of question being asked, different mouse strains and peptides are used. Different disease courses are observed with different strains and different peptides in active EAE. These variations are also addressed. Additionally, issues relevant to clinical grading of EAE in mice are discussed. In addition to EAE induction, useful references for other disease indicators such as DTH, in vitro proliferation, and immunohistochemistry are provided. TMEV-IDD is a useful model for understanding the possible viral etiology of MS. This section provides detailed information on the preparation of viral stocks and subsequent intracerebral infection of mice. Additionally, virus plaque assay and clinical disease assessment are discussed. Recently, recombinant TMEV strains have been created for the study of molecular mimicry which incorporate various 30 amino acid myelin epitopes within the leader region of TMEV.

Cytokine/chemokine profile in J774 macrophage cells persistently infected with DA strain of Theiler's murine encephalomyelitis virus (TMEV)

Theiler's murine encephalomyelitis virus (TMEV) is a picornavirus and persists in the spinal cords of mice, followed by inflammatory demyelinating disease. Viral persistence is a key determinant for the TMEV-induced demyelination. Macrophages are thought to serve as the site of TMEV persistence during the chronic demyelinating phase. We previously demonstrated that two nonstructural proteins of TMEV, L and L(*), were important for virus growth in J774.1 macrophage cells. However, the key factors of macrophage cells related to virus persistence and demyelination remain poorly understood. The inflammatory response is heavily dependent on cytokine and chemokine production by cell of both the immune system and the central nervous system (CNS). In this study, we established the macrophage cells persistently infected with DA strain, and then analyzed the cytokine expression pattern in those cells. The present results are the first to demonstrate the up-regulation of B-lymphocyte chemoattractant (BLC) and granulocyte colony-stimulating factor (G-CSF) in the macrophage cells persistently infected with DA strain. Furthermore, up-regulation of interleukin (IL)-10 and down-regulation of interferon (IFN)-alpha 4, IFN-beta, and IFN-gamma were shown in those cells. The data suggest that these cytokines/chemokines may contribute to the virus persistence and the acceleration of TMEV-induced demyelination.

Microbial induction of vascular pathology in the CNS

The central nervous system (CNS) is a finely tuned organ that participates in nearly every aspect of our day-to-day function. Neurons lie at the core of this functional unit and maintain an active dialogue with one another as well as their fellow CNS residents (e.g. astrocytes, oligodendrocytes, microglia). Because of this complex dialogue, it is essential that the CNS milieu be tightly regulated in order to permit uninterrupted and efficient neural chemistry. This is accomplished in part by anatomical barriers that segregate vascular components from the cerebral spinal fluid (CSF) and brain parenchyma. These barriers impede entry of noxious materials and enable the CNS to maintain requisite protein and ionic balances for constant electrochemical signaling. Under homeostatic conditions, the CNS is protected by the presence of specialized endothelium/epithelium, the blood brain barrier (BBB), and the blood-CSF barrier. However, following CNS infection these protective barriers can be comprised, sometimes resulting in severe neurological complications triggered by an imbalance or blockage of neural chemistry. In some instances, these disruptions are severe enough to be fatal. This review focuses on a selection of microbes (both viruses and parasites) that compromise vascular barriers and induce neurological complications upon gaining access to the CNS. Emphasis is placed on CNS diseases that result from a pathogenic interplay between host immune defenses and the invading microbe.

A critical role for virus-specific CD8(+) CTLs in protection from Theiler's virus-induced demyelination in disease-susceptible SJL mice

Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD) is a relevant mouse model of multiple sclerosis. Infection of susceptible SJL/J mice leads to life-long CNS virus persistence and development of a chronic T cell-mediated autoimmune demyelinating disease triggered via epitope spreading to endogenous myelin epitopes. Potent CNS-infiltrating CD8(+) T cell responses to TMEV epitopes have previously been shown to be induced in both disease-susceptible SJL/J and resistant C57BL/6 mice, in which the virus is rapidly cleared. Specific tolerization of SJL CD8(+) T cells specific for the immunodominant TMEV VP3(159)(-)(166) epitope has no effect on viral load or development of clinical TMEV-IDD, but adoptive transfer of activated CD8(+) VP3(159)(-)(166)-specific T cell blasts shortly after TMEV infection to boost the early anti-viral response leads to clearance of CNS virus and protection from subsequent TMEV-IDD. These studies have important implications for vaccine strategies and treatment of chronic infections in humans.

Central neuroinvasion and demyelination by inflammatory macrophages after peripheral virus infection is controlled by SHP-1

SHP-1 is a protein tyrosine phosphatase that negatively regulates cytokine signaling and inflammatory gene expression. Mice genetically lacking SHP-1 (me/me) display severe inflammatory demyelinating disease following intracranial inoculation with the BeAn strain of Theiler's murine encephalomyelitis virus (TMEV) compared to infected wild-type mice. Furthermore, SHP-1-deficient mice show a profound and predominant infiltration of blood-derived macrophages into the CNS following intracerebral injection of TMEV, and these macrophages are concentrated in areas of demyelination in brain and spinal cord. In the present study we investigated the role of SHP-1 in controlling CNS inflammatory demyelination following a peripheral instead of an intracerebral inoculation of TMEV. Surprisingly, we found that while wild-type mice were entirely refractory to intraperitoneal (IP) infection by TMEV, in agreement with previous studies, all SHP-1-deficient mice displayed profound macrophage neuroinvasion and macrophage-mediated inflammatory demyelination. Moreover, SHP-1 deficiency led to increased expression of inflammatory molecules in macrophages, serum, and CNS following IP infection with TMEV. Importantly, pharmacological depletion of peripheral macrophages significantly decreased both paralysis and CNS viral loads in SHP-1-deficient mice. In addition, peripheral MCP-1 neutralization attenuated disease severity, decreased macrophage infiltration into the CNS, and decreased monocyte numbers in the blood of SHP-1-deficient mice, implicating MCP-1 as an important mediator of monocyte migration between multiple tissues. These results demonstrate that peripheral TMEV infection results in a unique evolution of macrophage-mediated demyelination in SHP-1-deficient mice, implicating SHP-1 in the control of neuroinvasion of inflammatory macrophages and neurotropic viruses into the CNS.

Effects of anti-CD70 mAb on Theiler's murine encephalomyelitis virus-induced demyelinating disease

Ligation of CD27, a member of the tumor necrosis factor (TNF) receptor family, by its ligand CD70 is thought to be important in T cell activation, expansion and survival, B cell activation, and NK cell activation. We examined the role of CD70 in Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) mice. Blocking of CD70 in effector phase by anti-CD70 monoclonal antibody (mAb) suppressed the development of TMEV-IDD. The number of IFN-gamma- or TNF-alpha-producing cells in the spleen and mRNA levels of IFN-gamma and TNF-alpha in spinal cord were decreased in mice treated with anti-CD70 mAb at the effector phase. In contrast, treatment with anti-CD70 mAb in induction phase failed to reduce these responses, compared to nonspecific IgG-treated control mice. These data suggest that CD70 is critically involved in the pathogenesis of TMEV-IDD and that antibodies against CD70 could be a novel therapeutic approach in the clinical treatment of demyelinating diseases such as human multiple sclerosis.

Restraint stress modulates virus specific adaptive immunity during acute Theiler's virus infection

Multiple sclerosis (MS) is a devastating CNS disease of unknown origin. Multiple factors including genetic background, infection, and psychological stress affect the onset or progression of MS. Theiler's murine encephalomyelitis virus (TMEV) infection is an animal model of MS in which aberrant immunity leads to viral persistence and subsequently results in demyelination that resembles MS. Here, we examined how stress during acute TMEV infection altered virus-specific cell mediated responses. Using immunodominant viral peptides specific for either CD4(+) or CD8(+) T cells, we found that stress reduced IFN-gamma producing virus-specific CD4(+) and CD8(+) T cells in the spleen and CD8(+) T cells CNS. Cytokine production by cells isolated from the CNS or spleens following stimulation with virus or viral peptides, indicated that stress decreased both type 1 and type 2 responses. Glucocorticoids were implicated in the decreased T cell function as the effects of stress were partially reversed by concurrent RU486 administration but mimicked by dexamethasone. As T cells mediate viral clearance in this model, our data support the hypothesis that stress-induced immunosuppression may provide a mechanism for enhanced viral persistence within the CNS.

Modulation of macrophage infiltration and inflammatory activity by the phosphatase SHP-1 in virus-induced demyelinating disease

The protein tyrosine phosphatase SHP-1 is a crucial negative regulator of cytokine signaling and inflammatory gene expression, both in the immune system and in the central nervous system (CNS). Mice genetically lacking SHP-1 (me/me) display severe inflammatory demyelinating disease following inoculation with the Theiler's murine encephalomyelitis virus (TMEV) compared to infected wild-type mice. Therefore, it became essential to investigate the mechanisms of TMEV-induced inflammation in the CNS of SHP-1-deficient mice. Herein, we show that the expression of several genes relevant to inflammatory demyelination in the CNS of infected me/me mice is elevated compared to that in wild-type mice. Furthermore, SHP-1 deficiency led to an abundant and exclusive increase in the infiltration of high-level-CD45-expressing (CD45(hi)) CD11b(+) Ly-6C(hi) macrophages into the CNS of me/me mice, in concert with the development of paralysis. Histological analyses of spinal cords revealed the localization of these macrophages to extensive inflammatory demyelinating lesions in infected SHP-1-deficient mice. Sorted populations of CNS-infiltrating macrophages from infected me/me mice showed increased amounts of viral RNA and an enhanced inflammatory profile compared to wild-type macrophages. Importantly, the application of clodronate liposomes effectively depleted splenic and CNS-infiltrating macrophages and significantly delayed the onset of TMEV-induced paralysis. Furthermore, macrophage depletion resulted in lower viral loads and lower levels of inflammatory gene expression and demyelination in the spinal cords of me/me mice. Finally, me/me macrophages were more responsive than wild-type macrophages to chemoattractive stimuli secreted by me/me glial cells, indicating a mechanism for the increased numbers of infiltrating macrophages seen in the CNS of me/me mice. Taken together, these findings demonstrate that infiltrating macrophages in SHP-1-deficient mice play a crucial role in promoting viral replication by providing abundant viral targets and contribute to increased proinflammatory gene expression relevant to the effector mechanisms of macrophage-mediated demyelination.

Differential virus replication, cytokine production, and antigen-presenting function by microglia from susceptible and resistant mice infected with Theiler's virus

Infection with Theiler's murine encephalomyelitis virus (TMEV) in the central nervous system (CNS) causes an immune system-mediated demyelinating disease similar to human multiple sclerosis in susceptible but not resistant strains of mice. To understand the underlying mechanisms of differential susceptibility, we analyzed viral replication, cytokine production, and costimulatory molecule expression levels in microglia and macrophages in the CNS of virus-infected resistant C57BL/6 (B6) and susceptible SJL/J (SJL) mice. Our results indicated that message levels of TMEV, tumor necrosis factor alpha, beta interferon, and interleukin-6 were consistently higher in microglia from virus-infected SJL mice than in those from B6 mice. However, the levels of costimulatory molecule expression, as well as the ability to stimulate allogeneic T cells, were significantly lower in TMEV-infected SJL mice than in B6 mice. In addition, microglia from uninfected naïve mice displayed differential viral replication, T-cell stimulation, and cytokine production, similar to those of microglia from infected mice. These results strongly suggest that different levels of intrinsic susceptibility to TMEV infection, cytokine production, and T-cell activation ability by microglia contribute to the levels of viral persistence and antiviral T-cell responses in the CNS, which are critical for the differential susceptibility to TMEV-induced demyelinating disease between SJL and B6 mice.

Expression of L* protein of Theiler's murine encephalomyelitis virus in the chronic phase of infection

The DA strain and other members of the TO subgroup of Theiler's murine encephalomyelitis virus synthesize the L* protein from an alternative initiation codon. L* is considered to play a key role in viral persistence and demyelination in susceptible strains of mice, although this hypothesis is still controversial. By using a mutant virus that expresses FLAG epitope-tagged L*, it was demonstrated previously that L* is expressed exclusively in neurons in vivo in the acute phase of infection in the central nervous system (CNS). However, in the mutant virus, the C-H-C-C zinc-binding motif in the leader protein (L) was disrupted by the insertion of the FLAG epitope, resulting in clearance of the virus from the CNS. Therefore, a further two mutant viruses were newly generated, expressing FLAG epitope-tagged L* in which the C-H-C-C zinc-binding motif within L is spared. Both mutant viruses caused persistence and demyelination successfully in spinal cords and enabled us to identify L* immunohistochemically in the demyelinating lesions.

CCR2 regulates development of Theiler's murine encephalomyelitis virus-induced demyelinating disease

Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease, a murine model for multiple sclerosis, involves recruitment of T cells and macrophages to the CNS after infection. We hypothesized that CCR2, the only known receptor for CCL2, would be required for TMEV-induced demyelinating disease development because of its role in macrophage recruitment. TMEV-infected SJL CCR2 knockout (KO) mice showed decreased long-term clinical disease severity and less demyelination compared with controls. Flow cytometric data indicated that macrophages (CD45(high) CD11b(+) ) in the CNS of TMEV-infected CCR2 KO mice were decreased compared with control mice throughout disease. CD4(+) and CD8(+) T cell percentages in the CNS of TMEV-infected control and CCR2 KO mice were similar over the course of disease. There were no apparent differences between CCR2 KO and control peripheral immune responses. The frequency of interferon-gamma-producing T cells in response to proteolipid protein 139-151 in the CNS was also similar during the autoimmunity stage of TMEV-induced demyelinating disease. These data suggest that CCR2 is important for development of clinical disease by regulating macrophage accumulation after TMEV infection.

Differential usage of carbohydrate co-receptors influences cellular tropism of Theiler's murine encephalomyelitis virus infection of the central nervous system

Theiler's murine encephalomyelitis viruses (TMEV) are ubiquitous pathogens of mice, producing either rapidly fatal encephalitis (high-neurovirulence strains) or persistent central nervous system infection and inflammatory demyelination (low-neurovirulence strains). Although a protein entry receptor has not yet been identified, carbohydrate co-receptors that effect docking and concentration of the virus on the cell surface are known for both TMEV neurovirulence groups. Low-neurovirulence TMEV use alpha2,3-linked N-acetylneuramic acid (sialic acid) on an N-linked glycoprotein, whereas high-neurovirulence TMEV use the proteoglycan heparan sulfate (HS) as a co-receptor. While the binding of low-neurovirulence TMEV to sialic acid can be inhibited completely, only a third of the binding of high-neurovirulence TMEV to HS is inhibitable, suggesting that high-neurovirulence strains use another co-receptor or bind directly to the putative protein entry receptor. Four amino acids on the surface (VP2 puff B) of low-neurovirulence strains make contact with sialic acid through non-covalent hydrogen bonds. Since these virus residues are conserved in all TMEV strains, the capsid conformation of this region is probably responsible for sialic acid binding. A persistence determinant that maps within the virus coat using recombinant TMEV is also conformational in nature. Low-neurovirulence virus variants that do not bind to sialic acid fail to persist in the central nervous system of mice, indicating a role for sialic acid binding in TMEV persistence. Analysis of high-neurovirulence variants that do not bind HS demonstrates that HS co-receptor usage influences neuronal tropism in brain, whereas, the HS co-receptor use is not required for the infection of spinal cord anterior horn cells associated with poliomyelitis.

Anti-CCL2 treatment inhibits Theiler's murine encephalomyelitis virus-induced demyelinating disease

Theiler's murine encephalomyelitis virus induces a demyelinating disease (TMEV-IDD) of the central nervous system (CNS) in susceptible mouse strains with accompanying histopathology characterized by mononuclear cell infiltrates. In susceptible strains of mice such as SJL, virus establishes a persistent infection in macrophages, induces a CNS infiltration by macrophages, T cells, and B cells, which results in chronic-progressive paralysis. In the present report the authors have investigated the functional role of CCL2 (monocyte chemotactic protein-1) in the induction and progression of demyelinating disease. Treatment of infected mice at day 0, 14, or 28 with anti-CCL2 resulted in a significant decrease in the clinical disease progression. Further analysis of anti-CCL2-treated mice revealed decreased CNS inflammation and mononuclear cell infiltration with an accompanying change in inflammatory cytokine responses. There was an overall decrease in the absolute numbers of CNS-infiltrating CD4+ T cells, macrophages, and B cells. Finally, anti-CCL2 treatment resulted in decreased viral load in the CNS. These data directly demonstrate a role for CCL2 in the pathogenesis of TMEV-IDD.

The immunodominant CD8+ T cell epitope region of Theiler's virus in resistant C57BL/6 mice is critical for anti-viral immune responses, viral persistence, and binding to the host cells

Theiler's virus infection induces an immune-mediated demyelinating disease, providing a relevant animal model of human multiple sclerosis. VP2(121-130)-specific CD8+ T cells in resistant H-2b mice account for the majority of CNS-infiltrating CD8+ T cells. To further study the role of the CD8(+) T cells, we generated a panel of mutant viruses substituted with L, G, or T at the anchor residue (M130) of the VP2(121-130) epitope. M130L virus (M130L-V) with a substitution of M with L displayed similar properties as wild-type virus (WT-V). However, M130G-V and M130T-V could not establish a persistent infection in the CNS. The level of both virus-specific CD8+ and CD4+ T cell responses is significantly reduced in mice infected with these variant viruses. While all mutant and wild-type viruses replicate comparably in BHK cells, replication of M130G-V and M130T-V in macrophages was significantly lower compared to those infected with WT-V and M130L-V. Interestingly, these mutant viruses deficient in replication in primary mouse cells showed drastically reduced binding ability to the cells. These results suggest that the anchor residue of the predominant CD8+ T cell epitope of TMEV in resistant mice is critical for the virus to infect target cells and this deficiency may result in poor viral persistence leading to correspondingly low T cell responses in the periphery and CNS. Thus, selection of the cellular binding region of the virus as the predominant epitope for CD8+ T cells in resistant mice may provide a distinct advantage in controlling viral persistence by preventing escape mutations.

Injection of the sciatic nerve with TMEV: a new model for peripheral nerve demyelination

Demyelination of the human peripheral nervous system (PNS) can be caused by diverse mechanisms including viral infection. Despite association of several viruses with the development of peripheral demyelination, animal models of the condition have been limited to disease that is either autoimmune or genetic in origin. We describe here a model of PNS demyelination based on direct injection of sciatic nerves of mice with the cardiovirus, Theiler's murine encephalomyelitis virus (TMEV). Sciatic nerves of FVB mice develop inflammatory cell infiltration following TMEV injection. Schwann cells and macrophages are infected with TMEV. Viral replication is observed initially in the sciatic nerves and subsequently the spinal cord. Sciatic nerves are demyelinated by day 5 post-inoculation (p.i.). Injecting sciatic nerves of scid mice resulted in increased levels of virus recovered from the sciatic nerve and spinal cord relative to FVB mice. Demyelination also occurred in scid mice and by 12 days p.i., hindlimbs were paralyzed. This new model of virus-induced peripheral demyelination may be used to dissect processes involved in protection of the PNS from viral insult and to study the early phases of lesion development.

Restraint stress facilitates systemic dissemination of Theiler's virus and alters its pathogenecity

Theiler's murine encephalomyelitis virus (TMEV), a Picornavirus used as a viral model for multiple sclerosis (MS), causes an acute encephalomyelitis and chronic demyelination. The failure to clear the virus, which can result from stress, is a prerequisite for development of the later disease. Similarly, stressful life events have been associated with the development of MS. In the present study, a restraint stress (RS) model was used to investigate the effect of stress on the systemic dissemination of TMEV during the early stage of disease. Experimental data demonstrated that repeated RS remarkably facilitated the spread of virus from the CNS to such systemic organs as the spleen, lymph nodes, thymus, lungs and heart and compromised the ability of viral clearance within those tissues. RS also altered the pathogenecity of TMEV, enabling it to become cardiotropic, resulting in higher myocardial infectivity. These results demonstrate the profound impact that RS has upon both the tissue and organ dissemination of the virus, and the organ tropism of TMEV. An additional finding associated with stress was hepatic necrosis in the restrained animals, regardless of whether or not they were infected.

Leader (L) and L* proteins of Theiler's murine encephalomyelitis virus (TMEV) and their regulation of the virus' biological activities

Theiler's murine encephalomyelitis virus (TMEV) is divided into two subgroups on the basis of their different biological activities. GDVII subgroup strains produce fatal poliomyelitis in mice without virus persistence or demyelination. In contrast, TO subgroup strains induce demyelinating disease with virus persistence in the spinal cords of weanling mice. Two proteins, whose open reading frames are located in the N-terminus of the polyprotein, recently have been reported to be important for TMEV biological activities. One is leader (L) protein and is processed from the most N-terminus of the polyprotein; its function is still unknown. Although the homology of capsid proteins between DA (a representative strain of TO subgroup) and GDVII strains is over 94% at the amino acid level, that of L shows only 85%. Therefore, L is thought to be a key protein for the subgroup-specific biological activities of TMEV. Various studies have demonstrated that L plays important roles in the escape of virus from host immune defenses in the early stage of infection. The second protein is a 17–18 kDa protein, L*, which is synthesized out-of-frame with the polyprotein. Only TO subgroup strains produce L* since GDVII subgroup strains have an ACG rather than AUG at the initiation site and therefore do not synthesize L*. 'Loss and gain of function' experiments demonstrate that L* is essential for virus growth in macrophages, a target cell for TMEV persistence. L* also has been demonstrated to be necessary for TMEV persistence and demyelination. Further analysis of L and L* will help elucidate the pathomechanism(s) of TMEV-induced demyelinating disease.

Induction of chemokine and cytokine genes in astrocytes following infection with Theiler's murine encephalomyelitis virus is mediated by the Toll-like receptor 3

Theiler's murine encephalomyelitis virus (TMEV) infection in the central nervous system (CNS) induces a demyelinating disease similar to human multiple sclerosis. TMEV infection results in activation of various chemokine and cytokine genes that are important in the initiation of an inflammatory response. We have previously shown that the production of these chemokines and cytokines in astrocytes is induced via the NF-kappaB pathway following TMEV and Coxsackie virus infection. In this study, we investigated whether the NF-kappaB-dependent inflammatory responses after TMEV infection is triggered through TLR3 and/or TLR7. The activation of NF-kappaB or IRF/ISRE, as well as the production of both MCP-1/CCL2 and IL-8/CXCL8, was observed in only TLR3-transfected HEK 293 cells, but not in TLR7-tranfected cells. The potential involvement of TLR3 in mouse embryonic fibroblasts and primary astrocytes was further investigated following transfection with wildtype or dominant negative form of TLRs and MyD88, as well as astrocytes from TLR3- and MyD88-deficient mice. Similarly, the activation of transcription factors and chemokine genes is induced in these mouse cells through primarily TLR3 signaling pathway, but not TLR7 or other MyD88-mediated pathways following TMEV infection. However, the TLR3-mediated cellular activation does not appear to affect the level of viral replication in astrocytes. These results strongly suggest that TLR3-signaling by TMEV alone is sufficient to induce the initial inflammatory cytokine responses that could be very important for the outcome of virus-induced encephalitis and/or demyelinating diseases, such as multiple sclerosis.

CNS cell populations are protected from virus-induced pathology by distinct arms of the immune system

The basis for the distinct patterns of brain pathology in individuals experiencing virus-induced encephalitis may be related to either the tropism of the virus or the host's response to virus infection of the central nervous system (CNS). In these studies we used Theiler's murine encephalomyelitis virus (TMEV) and a series of mice deficient in various immune system components (alpha/beta T cells, antibody, Class I MHC, and Class II MHC) to examine the hypothesis that discrete populations of CNS cells are protected differentially from virus infection by distinct arms of the immune response. Here we demonstrate that the Class I-mediated immune response provided more protection from areas of the brain (brainstem, corpus callosum and cerebellum) with abundant white matter as there was significantly more disease in these areas in beta2m -/- (Class I-deficient) mice as compared to A beta(0) (Class II-deficient) mice. In contrast, the striatum, with an abundance of neurons, was protected from virus-induced pathology primarily by antibody. In addition, we determined that antibody and alpha/beta T cells provided protection from severe deficits and death during the acute phase of the disease. The data presented here support the hypothesis that distinct immune system components function to protect discrete areas of the CNS from virus-induced pathology.