Absence of neurological deficits following extensive demyelination in a class I-deficient murine model of multiple sclerosis

CXCL13 as a Biomarker: Background and Utility in Multiple Sclerosis

CXCL13 is a chemokine which is upregulated within the CNS in multiple sclerosis, Lyme neuroborreliosis, and other inflammatory diseases and is increasingly clinically useful as a biomarker. This review provides background for understanding its function in the immune system and its relationship to ectopic lymphoid follicles. Also reviewed are its utility in multiple sclerosis and Lyme neuroborreliosis and potential problems in its measurement. CXCL13 has the potential to be an exceptionally useful biomarker in a range of inflammatory diseases.

Exploring the Role of Platelets in Virus-Induced Inflammatory Demyelinating Disease and Myocarditis

Theiler's murine encephalomyelitis virus (TMEV) infection has been used as a mouse model for two virus-induced organ-specific immune-mediated diseases. TMEV-induced demyelinating disease (TMEV-IDD) in the central nervous system (CNS) is a chronic inflammatory disease with viral persistence and an animal model of multiple sclerosis (MS) in humans. TMEV infection can also cause acute myocarditis with viral replication and immune cell infiltration in the heart, leading to cardiac fibrosis. Since platelets have been reported to modulate immune responses, we aimed to determine the role of platelets in TMEV infection. In transcriptome analyses of platelets, distinct sets of immune-related genes, including major histocompatibility complex (MHC) class I, were up- or downregulated in TMEV-infected mice at different time points. We depleted platelets from TMEV-infected mice by injecting them with platelet-specific antibodies. The platelet-depleted mice had significantly fewer viral antigen-positive cells in the CNS. Platelet depletion reduced the severities of TMEV-IDD and myocarditis, although the pathology scores did not reach statistical significance. Immunologically, the platelet-depleted mice had an increase in interferon (IFN)-γ production with a higher anti-TMEV IgG2a/IgG1 ratio. Thus, platelets may play roles in TMEV infection, such as gene expression, viral clearance, and anti-viral antibody isotype responses.

Remyelination in animal models of multiple sclerosis: finding the elusive grail of regeneration

Remyelination biology and the therapeutic potential of restoring myelin sheaths to prevent neurodegeneration and disability in multiple sclerosis (MS) has made considerable gains over the past decade with many regeneration strategies undergoing tested in MS clinical trials. Animal models used to investigate oligodendroglial responses and regeneration of myelin vary considerably in the mechanism of demyelination, involvement of inflammatory cells, neurodegeneration and capacity for remyelination. The investigation of remyelination in the context of aging and an inflammatory environment are of considerable interest for the potential translation to progressive multiple sclerosis. Here we review how remyelination is assessed in mouse models of demyelination, differences and advantages of these models, therapeutic strategies that have emerged and current pro-remyelination clinical trials.

Ovalbumin-specific CD4+ and CD8+ T cells contribute to different susceptibility for Theiler's murine encephalomyelitis virus persistence

Theiler's murine encephalomyelitis virus (TMEV) is the causative agent of TMEV-induced demyelinating disease (TMEV-IDD); a well-established animal model for the chronic progressive form of human multiple sclerosis (MS). In susceptible mice with an inadequate immune response, TMEV-IDD is triggered by virus persistence and maintained by a T cell mediated immunopathology. OT-mice are bred on a TMEV-resistant C57BL/6 background and own predominantly chicken ovalbumin (OVA)-specific populations of CD8⁺ T cells (OT-I) or CD4⁺ T cells (OT-II), respectively. It is hypothesized that the lack of antigen specific T cell populations increases susceptibility for a TMEV-infection in OT-mice on a TMEV-resistant C57BL/6 background. OT-I, OT-II, and C57BL/6 control mice were infected intracerebrally with the TMEV-BeAn strain. Mice were scored weekly for clinical disease and after necropsy, histological and immunohistochemical evaluation was performed. OT-I mice started to develop progressive motor dysfunction between 7 and 21 days post infection (dpi), leading up to hind limb paresis and critical weight loss, which resulted in euthanasia for humane reasons between 14 and 35 dpi. OT-I mice displayed a high cerebral virus load, an almost complete absence of CD8⁺ T cells from the central nervous system (CNS) and a significantly diminished CD4⁺ T cell response. Contrarily, only 60% (12 of 20) of infected OT-II mice developed clinical disease characterized by mild ataxia. 25% of clinically affected OT-II mice (3 of 12) made a full recovery. 5 of 12 OT-II mice with clinical disease developed severe motor dysfunction similar to OT-I mice and were euthanized for humane reasons between 13 and 37 dpi. OT-II mice displayed only low virus-immunoreactivity, but clinical disease correlated well with severely reduced infiltration of CD8⁺ T cells and the increased presence of CD4⁺ T cells in the brains of OT-II mice. Though further studies are needed to reveal the underlying pathomechanisms following TMEV infection in OT mice, findings indicate an immunopathological process as a main contributor to clinical disease in OT-II mice, while a direct virus-associated pathology may be the main contributor to clinical disease in TMEV-infected OT-I mice.

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.

Curdlan, a Microbial β-Glucan, Has Contrasting Effects on Autoimmune and Viral Models of Multiple Sclerosis

Multiple sclerosis (MS) is an immune-mediated disease characterized by inflammatory demyelination and axonal degeneration in the central nervous system (CNS). Bacterial and fungal infections have been associated with the development of MS; microbial components that are present in several microbes could contribute to MS pathogenesis. Among such components, curdlan is a microbial 1,3-β-glucan that can stimulate dendritic cells, and enhances T helper (Th) 17 responses. We determined whether curdlan administration could affect two animal models for MS: an autoimmune model, experimental autoimmune encephalomyelitis (EAE), and a viral model, Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD). We induced relapsing-remitting EAE by sensitizing SJL/J mice with the myelin proteolipid protein (PLP)139-151 peptide and found that curdlan treatment prior to PLP sensitization converted the clinical course of EAE into hyperacute EAE, in which the mice developed a progressive motor paralysis and died within 2 weeks. Curdlan-treated EAE mice had massive infiltration of T cells and neutrophils in the CNS with higher levels of Th17 and Th1 responses, compared with the control EAE mice. On the other hand, in TMEV-IDD, we found that curdlan treatment reduced the clinical scores and axonal degeneration without changes in inflammation or viral persistence in the CNS. In summary, although curdlan administration exacerbated the autoimmune MS model by enhancing inflammatory demyelination, it suppressed the viral MS model with reduced axonal degeneration. Therefore, microbial infections may play contrasting roles in MS depending on its etiology: autoimmunity versus viral infection.

Neuronal Presentation of Antigen and Its Possible Role in Parkinson's Disease

Patients with Parkinson's disease (PD) and other synucleinopathies often exhibit autoimmune features, including CD4+ and some CD8+ T lymphocytes that recognize epitopes derived from alpha-synuclein. While neurons have long been considered to not present antigens, recent data indicate that they can be induced to do so, particularly in response to interferons and other forms of stress. Here, we review literature on neuronal antigen presentation and its potential role in PD. Although direct evidence for CD8+ T cell-mediated neuronal death is lacking in PD, neuronal antigen presentation appears central to the pathology of Rasmussen's encephalitis, a pediatric neurological disorder driven by cytotoxic T cell infiltration and neuroinflammation. Emerging data suggest that T cells enter the brain in PD and other synucleinopathies, where the majority of neuromelanin-containing substantia nigra and locus coeruleus neurons express MHC Class I molecules. In cell culture, CD8+ T cell recognition of antigen:MHC Class I complexes on neuronal membranes leads to cytotoxic responses and neuronal cell death. Recent animal models suggest the possibility of T cell autoreactivity to mitochondrial antigens in PD. It remains unclear if neuronal antigen presentation plays a role in PD or other neurodegenerative disorders, and efforts are underway to better elucidate the potential impact of autoimmune responses on neurodegeneration.

Double-edged effects of tamoxifen-in-oil-gavage on an infectious murine model for multiple sclerosis

Tamoxifen gavage is a commonly used method to induce genetic modifications in cre-loxP systems. As a selective estrogen receptor modulator (SERM), the compound is known to have immunomodulatory and neuroprotective properties in non-infectious central nervous system (CNS) disorders. It can even cause complete prevention of lesion development as seen in experimental autoimmune encephalitis (EAE). The effect on infectious brain disorders is scarcely investigated. In this study, susceptible SJL mice were infected intracerebrally with Theiler's murine encephalomyelitis virus (TMEV) and treated three times with a tamoxifen-in-oil-gavage (TOG), resembling an application scheme for genetically modified mice, starting at 0, 18, or 38 days post infection (dpi). All mice developed 'TMEV-induced demyelinating disease' (TMEV-IDD) resulting in inflammation, axonal loss, and demyelination of the spinal cord. TOG had a positive effect on the numbers of oligodendrocytes and oligodendrocyte progenitor cells, irrespective of the time point of application, whereas late application (starting 38 dpi) was associated with increased demyelination of the spinal cord white matter 85 dpi. Furthermore, TOG had differential effects on the CD4+ and CD8+ T cell infiltration into the CNS, especially a long lasting increase of CD8+ cells was detected in the inflamed spinal cord, depending of the time point of TOG application. Number of TMEV-positive cells, astrogliosis, astrocyte phenotype, apoptosis, clinical score, and motor function were not measurably affected. These data indicate that tamoxifen gavage has a double-edged effect on TMEV-IDD with the promotion of oligodendrocyte differentiation and proliferation, but also increased demyelination, depending on the time point of application. The data of this study suggest that tamoxifen has also partially protective functions in infectious CNS disease. These effects should be considered in experimental studies using the cre-loxP system, especially in models investigating neuropathologies.

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.

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.

Clinical implications of myelin regeneration in the central nervous system

INTRODUCTION

Amongst strategies to repair the brain, myelin repair offers genuine cause for optimism. Myelin, which sheaths most axons in the central nervous system (CNS), is vital for normal neurological function, as demonstrated by the functional deficits that accrue when it is absent in a range of debilitating myelin diseases. Following demyelination, post-mortem and imaging studies have shown that extensive regeneration of myelin is possible in the human brain. Over recent decades preclinical research has given us a strong understanding of the biology of myelin regeneration, opening up several exciting therapeutic opportunities that are on the cusp of clinical translation. Areas covered: This review discusses diseases that compromise the function of myelin, the endogenous capacity of the CNS to regenerate myelin, and why this sometimes fails. We then outline the extensive progress that has been made towards therapies that promote the regeneration of myelin. Expert commentary: Finally, a commentary on the first examples of these therapies to reach human patients and the evidence base that supports them, giving our opinion on where attention should be focused going forward is provided.

Cytotoxic CD8+ T cell ablation enhances the capacity of regulatory T cells to delay viral elimination in Theiler's murine encephalomyelitis

Theiler's murine encephalomyelitis (TME) of susceptible mouse strains is a commonly used infectious animal model for multiple sclerosis. The study aim was to test the hypothesis whether cytotoxic T cell responses account for the limited impact of regulatory T cells on antiviral immunity in TME virus-induced demyelinating disease (TMEV-IDD) resistant C57BL/6 mice. TME virus-infected C57BL/6 mice were treated with (i) interleukin-2/-anti-interleukin-2-antibody-complexes to expand regulatory T cells ("Treg-expansion"), (ii) anti-CD8-antibodies to deplete cytotoxic T cells ("CD8-depletion") or (iii) with a combination of Treg-expansion and CD8-depletion ("combined treatment") prior to infection. Results showed that "combined treatment", but neither sole "Treg-expansion" nor "CD8-depletion," leads to sustained hippocampal infection and virus spread to the spinal cord in C57BL/6 mice. Prolonged infection reduces myelin basic protein expression in the spinal cord together with increased accumulation of β-amyloid precursor protein in axons, characteristic of myelin loss and axonal damage, respectively. Chronic spinal cord infection upon "combined treatment" was also associated with increased T and B cell recruitment, accumulation of CD107b⁺ microglia/macrophages and enhanced mRNA expression of interleukin (IL)-1α, IL-10 and tumor necrosis factor α. In conclusion, data revealed that the suppressive capacity of Treg on viral elimination is efficiently boosted by CD8-depletion, which renders C57BL/6 mice susceptible to develop chronic neuroinfection and TMEV-IDD.

Multiple sclerosis: experimental models and reality

One of the most frequent statements, provided in different variations in the introduction of experimental studies on multiple sclerosis (MS), is that "Multiple sclerosis is a demyelinating autoimmune disease and experimental autoimmune encephalomyelitis (EAE) is a suitable model to study its pathogenesis". However, so far, no single experimental model covers the entire spectrum of the clinical, pathological, or immunological features of the disease. Many different models are available, which proved to be highly useful for studying different aspects of inflammation, demyelination, remyelination, and neurodegeneration in the central nervous system. However, the relevance of results from such models for MS pathogenesis has to be critically validated. Current EAE models are mainly based on inflammation, induced by auto-reactive CD4+ T-cells, and these models reflect important aspects of MS. However, pathological data and results from clinical trials in MS indicate that CD8+ T-cells and B-lymphocytes may play an important role in propagating inflammation and tissue damage in established MS. Viral models may reflect key features of MS-like inflammatory demyelination, but are difficult to use due to their very complex pathogenesis, involving direct virus-induced and immune-mediated mechanisms. Furthermore, evidence for a role of viruses in MS pathogenesis is indirect and limited, and an MS-specific virus infection has not been identified so far. Toxic models are highly useful to unravel mechanisms of de- and remyelination, but do not reflect other important aspects of MS pathology and pathogenesis. For all these reasons, it is important to select the right experimental model to answer specific questions in MS research.

Major histocompatibility complex class I molecules protect motor neurons from astrocyte-induced toxicity in amyotrophic lateral sclerosis

Astrocytes isolated from individuals with amyotrophic lateral sclerosis (ALS) are toxic to motor neurons (MNs) and play a non-cell autonomous role in disease pathogenesis. The mechanisms underlying the susceptibility of MNs to cell death remain unclear. Here we report that astrocytes derived from either mice bearing mutations in genes associated with ALS or human subjects with ALS reduce the expression of major histocompatibility complex class I (MHCI) molecules on MNs; reduced MHCI expression makes these MNs susceptible to astrocyte-induced cell death. Increasing MHCI expression on MNs increases survival and motor performance in a mouse model of ALS and protects MNs against astrocyte toxicity. Overexpression of a single MHCI molecule, HLA-F, protects human MNs from ALS astrocyte-mediated toxicity, whereas knockdown of its receptor, the killer cell immunoglobulin-like receptor KIR3DL2, on human astrocytes results in enhanced MN death. Thus, our data indicate that, in ALS, loss of MHCI expression on MNs renders them more vulnerable to astrocyte-mediated toxicity.

Th17-biased RORγt transgenic mice become susceptible to a viral model for multiple sclerosis

In a viral model for multiple sclerosis (MS), Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD), both immune-mediated tissue damage (immunopathology) and virus persistence have been shown to cause pathology. T helper (Th) 17 cells are a Th cell subset, whose differentiation requires the transcription factor retinoic acid-related orphan receptor (ROR) γt, secrete pro-inflammatory cytokines, including IL-17, and can antagonize Th1 cells. Although Th17 cells have been shown to play a pathogenic role in immune-mediated diseases or a protective role in bacterial and fungal infections, their role in viral infections is unclear. Using newly established Th17-biased RORγt Tg mice, we tested whether Th17 cells could play a pathogenic or protective role in TMEV-IDD by contributing to immunopathology and/or by modulating anti-viral Th1 immune responses. While TMEV-infected wild-type littermate C57BL/6 mice are resistant to TMEV-IDD, RORγt Tg mice developed inflammatory demyelinating lesions with virus persistence in the spinal cord. TMEV-infected RORγt Tg mice had higher levels of IL-17, lower levels of interferon-γ, and fewer CD8(+) T cells, without alteration in overall levels of anti-viral lymphoproliferative and antibody responses, compared with TMEV-infected wild-type mice. This suggests that a Th17-biased "gain-of-function" mutation could increase susceptibility to virus-mediated demyelinating diseases.

Axonal loss in multiple sclerosis: causes and mechanisms

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system and the leading cause of non-traumatic neurologic disability in young adults in the United States and Europe. The disease course is variable and starts with reversible episodes of neurologic disability which transforms into continuous and irreversible neurologic decline. It is well established that loss of axons and neurons is the major cause of the progressive neurologic decline that most MS patients endure. Current hypotheses support primary inflammatory demyelination as the underlying cause of axonal loss during earlier stages in MS. The transition to progressive disease course is thought to occur when a threshold of neuronal and axonal loss is reached and the compensatory capacity of the central nervous system is surpassed. Available immunomodulatory therapies are of little benefit to MS after entering this irreversible phase of the disease. Elucidation of mechanisms that are responsible for axonal loss is therefore essential for the development of therapies directed to stop neurologic decline in MS patients. The current chapter reviews existing data on mechanisms of axonal pathology in MS.

Deletion of Virus-specific T-cells Enhances Remyelination in a Model of Multiple Sclerosis

We used transgenic expression of capsid antigens to Theiler's murine encephalomyelitis virus (TMEV) to study how the immune response to VP1 and VP2 influences spinal cord demyelination, remyelination and axonal loss during the acute and chronic phases of infection. Expression from birth of capsid antigen under the ubiquitin promoter resulted in tolerance to the antigen and absence of an immune response to the respective capsid antigen following virus infection. The transgenic mice were crossed to B10.Q mice normally susceptible to demyelination but which, when compared to FVB mice of the same H2 ^q haplotype, show poor remyelination. The major finding in this study was that VP1+ and VP2+ animals featured more remyelination at all three chronic time points (90, 180 and 270 dpi) than transgene-negative controls. Interestingly, at 270 dpi, remyelination in VP1+ mice tended to be higher and more complete than that in VP2+ mice. Compared with transgene- negative controls, VP1+ and VP2+ animals showed similar demyelination in but less only late in the disease (270 dpi). The number of mid-thoracic axons at the last time point correlated with the levels of remyelination. The increase in number of axons in VP1+ mice with remyelination was driven by counts in medium- and large-caliber axons. This study supports the hypothesis that expression of viral capsid proteins as self and subsequent genetic deletion of capsid-specific T cells influences the extent of spinal cord remyelination following Theiler's virus-induced demyelination. We propose that VP1- and, to a lesser extent, VP2-specific CD8⁺ T cells limit and/or prevent the naturally occurring process of remyelination. This finding may have relevance to human multiple sclerosis, as targeted removal of CD8⁺ T cells specific for a yet-to-be-discovered causative peptide may enhance remyelination and prevent axonal loss in patients.

Axons are injured by antigen-specific CD8(+) T cells through a MHC class I- and granzyme B-dependent mechanism

Axon injury is a central determinant of irreversible neurological deficit and disease progression in patients with multiple sclerosis (MS). CD8(+) lymphocytes (CTLs) within inflammatory demyelinated MS lesions correlate with acute axon injury and neurological deficits. The mechanisms of these correlations are unknown. We interrogated CTL-mediated axon injury using the transgenic OT-I antigen-specific CTL model system in conjunction with a chambered cortical neuron culture platform that permitted the isolated manipulation of axons independent of neuron cell bodies and glia. Interferon gamma upregulated, through a dose dependent mechanism, the axonal expression of functional major histocompatibility complex class I (MHC I) molecules competent to present immunologically-relevant antigens derived from endogenously expressed proteins. Antigen-specific CTLs formed cytotoxic immune synapses with and directly injured axons expressing antigen-loaded MHC I molecules. CTL-mediated axon injury was mechanistically dependent upon axonal MHC I antigen presentation, T cell receptor specificity and axoplasmic granzyme B activity. Despite extensive distal CTL-mediated axon injury, acute neuron cell body apoptosis was not observed. These findings present a novel model of immune-mediated axon injury and offer anti-axonal CTLs and granzyme B as targets for the therapeutic protection of axons and prevention of neurological deficits in MS patients.

CD8(+) T cells in multiple sclerosis

INTRODUCTION

CD8(+) T cells were originally considered to exert a suppressive role in demyelinating disease because of bias toward the CD4(+) T cell-mediated experimental autoimmune encephalomyelitis, the most common multiple sclerosis (MS) model. However, recent studies of human MS lesion samples and cerebrospinal fluid (CSF) provided compelling evidence about the pathogenic role of CD8(+) T cells. In this article, we discuss the theoretical roles of different CD8(+) T-cell subsets in MS.

AREAS COVERED

A revised focus from CD4(+) to CD8(+) T cell-mediated demyelinating disease is summarized. Clonal expansion of CD8(+) T cells in MS lesions and in vitro evidence that CD8(+) T cells injure every central nervous system (CNS) cell type and transect axons are discussed. The role of CD8(+) T cells in two animal models of MS and of regulatory, interleukin (IL)-17-secreting CD8(+) T cells is reviewed. Lastly, an overview about the pathogenic and/or beneficial role of various CD8(+) T-cell subsets is offered.

EXPERT OPINION

Growing evidence supports the pathogenic role of CD8(+) T cells. Clonally expanded CD8(+) T cells within MS lesions may damage the nervous system. Revealing the specific antigen is critical to design novel efficient treatments with minimal adverse effects. Increasing evidence exists for the role of regulatory, IL-17-secreting CD8(+) T cells in MS.

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.

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.

Animal models of multiple sclerosis: the good, the bad and the bottom line

Multiple sclerosis is a debilitating inflammatory demyelinating disease of the CNS. In this perspective, the author examines the strengths and weaknesses of the numerous animal models that are currently being used to analyze the pathogenesis of this disease with an eye toward the precise pathological aspect that each model recapitulates.

Multiple sclerosis (MS) is a spontaneous, acquired, inflammatory demyelinating disease of the human CNS. Because it involves a complex interaction between two of the most intricate biological systems, immune system and CNS, animal modeling has been critical for addressing MS pathogenesis. MS models were originally developed serendipitously more than 75 years ago. Immune-mediated, toxic, viral and genetic models of demyelination are now used to understand the manifold aspects of MS. MS treatments evolved in part from animal model research, and further progress is envisaged in large part because these systems have been continually refined and their use focused on questions whose relevance was established by studying the human disease.

Kallikrein 6 regulates early CNS demyelination in a viral model of multiple sclerosis

Kallikrein 6 (Klk6) is a secreted serine protease that is elevated in active multiple sclerosis lesions and patient sera. To further evaluate the involvement of Klk6 in chronic progressive demyelinating disease, we determined its expression in the brain and spinal cord of SJL mice infected with Theiler's murine encephalomyelitis virus (TMEV) and assessed the effects of Klk6-neutralizing antibodies on disease progression. Klk6 RNA expression was elevated in the brain and spinal cord by 7 days postinfection (dpi). Thereafter, Klk6 expression persisted primarily in the spinal cord reaching a peak of fivefold over controls at mid-chronic stages (60 dpi-120 dpi). Significant elevations in Klk6 RNA were also induced in splenocytes stimulated with viral capsid proteins in vitro and in activated human acute monocytic leukemia cells. Klk6-neutralizing antibodies reduced TMEV-driven brain and spinal cord pathology and delayed-type hypersensitivity (DTH) responses when examined at early chronic time points (40 dpi). Reductions in spinal cord pathology included a decrease in activated monocytes/microglia and reductions in the loss of myelin basic protein (MBP). By 180 dpi, pathology scores no longer differed between groups. These findings point to regulatory activities for Klk6 in the development and progression of central nervous system (CNS) inflammation and demyelination that can be effectively targeted through the early chronic stages with neutralizing antibody.

Deletion of beta-2-microglobulin ameliorates spinal cord lesion load and promotes recovery of brainstem NAA levels in a murine model of multiple sclerosis

We used genetic deletion of β2-microglobulin to study the influence of CD8(+) T cells on spinal cord demyelination, remyelination, axonal loss and brainstem N-acetyl aspartate levels during the acute and chronic phases of Theiler's murine encephalomyelitis virus (TMEV) infection. We used β2m(-/-) and β2m(+/+) B10.Q mice (of H-2(q) background) normally susceptible to TMEV-induced demyelination. Over the disease course, β2m(+/+) mice had increasing levels of demyelination and minimal late-onset remyelination. In contrast, β2m(-/-) mice had steady levels of demyelination from 45-390 dpi and remyelination was extensive and more complete. Early in the disease, brainstem NAA levels drop in both strains, but accordingly with remyelination and axonal preservation, NAA recover in β2m(-/-) mice despite equivalent brainstem pathology. At 270 dpi, β2m(+/+) mice had significantly fewer spinal cord axons than β2m(-/-) mice (up to 28% less). In addition, β2m(+/+) mice lost axons of all calibers, whereas β2m(-/-) mice had a modest loss of only medium- and large-caliber axons. This study further supports the hypothesis that CD8(+) T cells are involved in demyelination, and axonal loss following Theiler's virus-induced demyelination.

Cuprizone inhibits demyelinating leukomyelitis by reducing immune responses without virus exacerbation in an infectious model of multiple sclerosis

Multiple sclerosis is one of the most common demyelinating central nervous system diseases in young adults. Theiler's murine encephalomyelitis (TME) is a widely used virus-induced murine model for human myelin disorders. Immunosuppressive approaches generally reduce antiviral immunity and therefore increase virus dissemination with clinical worsening. In the present study, the progressive course of TME was significantly delayed due to a five-week cuprizone feeding period. Cuprizone was able to minimize demyelinating leukomyelitis without virus exacerbation. This phenomenon is supposed to be a consequence of selective inhibition of detrimental inflammatory responses with maintained protective immunity against the virus.

A single dose of neuron-binding human monoclonal antibody improves spontaneous activity in a murine model of demyelination

Our laboratory demonstrated that a natural human serum antibody, sHIgM12, binds to neurons in vitro and promotes neurite outgrowth. We generated a recombinant form, rHIgM12, with identical properties. Intracerebral infection with Theiler's Murine Encephalomyelitis Virus (TMEV) of susceptible mouse strains results in chronic demyelinating disease with progressive axonal loss and neurologic dysfunction similar to progressive forms of multiple sclerosis. To study the effects of rHIgM12 on the motor function of TMEV-infected mice, we monitored spontaneous nocturnal activity over many weeks. Nocturnal behavior is a sensitive measure of rodent neurologic function because maximal activity changes are expected to occur during the normally active night time monitoring period. Mice were placed in activity boxes eight days prior to treatment to collect baseline spontaneous activity. After treatment, activity in each group was continuously recorded over 8 weeks. We chose a long 8-week monitoring period for two reasons: (1) we previously demonstrated that IgM induced remyelination is present by 5 weeks post treatment, and (2) TMEV-induced demyelinating disease in this strain progresses very slowly. Due to the long observation periods and large data sets, differences among treatment groups may be difficult to appreciate studying the original unfiltered recordings. To clearly delineate changes in the highly fluctuating original data we applied three different methods: (1) binning, (2) application of Gaussian low-pass filters (GF) and (3) polynomial fitting. Using each of the three methods we showed that compared to control IgM and saline, early treatment with rHIgM12 induced improvement in both horizontal and vertical motor function, whereas later treatment improved only horizontal activity. rHIgM12 did not alter activity of normal, uninfected mice. This study supports the hypothesis that treatment with a neuron-binding IgM not only protects neurons in vitro, but also influences functional motor improvement.

The relevance of animal models in multiple sclerosis research

Multiple Sclerosis (MS) is a complex disease with an unknown etiology and no effective cure, despite decades of extensive research that led to the development of several partially effective treatments. Researchers have only limited access to early and immunologically active MS tissue samples, and the modification of experimental circumstances is much more restricted in human studies compared to studies in animal models. For these reasons, animal models are needed to clarify the underlying immune-pathological mechanisms and test novel therapeutic and reparative approaches. It is not possible for a single mouse model to capture and adequately incorporate all clinical, radiological, pathological and genetic features of MS. The three most commonly studied major categories of animal models of MS include: (1) the purely autoimmune experimental autoimmune/allergic encephalomyelitis (EAE); (2) the virally induced chronic demyelinating disease models, with the main model of Theiler's Murine Encephalomyelitis Virus (TMEV) infection and (3) toxin-induced models of demyelination, including the cuprizone model and focal demyelination induced by lyso-phosphatidyl choline (lyso-lecithine). EAE has been enormously helpful over the past several decades in our overall understanding of CNS inflammation, immune surveillance and immune-mediated tissue injury. Furthermore, EAE has directly led to the development of three approved medications for treatment in multiple sclerosis, glatiramer acetate, mitoxantrone and natalizumab. On the other hand, numerous therapeutical approaches that showed promising results in EAE turned out to be either ineffective or in some cases harmful in MS. The TMEV model features a chronic-progressive disease course that lasts for the entire lifespan in susceptible mice. Several features of MS, including the role and significance of axonal injury and repair, the partial independence of disability from demyelination, epitope spread from viral to myelin epitopes, the significance of remyelination has all been demonstrated in this model. TMEV based MS models also feature several MRI findings of the human disease. Toxin-induced demyelination models has been mainly used to study focal demyelination and remyelination. None of the three main animal models described in this review can be considered superior; rather, they are best viewed as complementary to one another. Despite their limitations, the rational utilization and application of these models to address specific research questions will remain one of the most useful tools in studies of human demyelinating diseases.

Chronic progressive multiple sclerosis - pathogenesis of neurodegeneration and therapeutic strategies

Multiple sclerosis (MS) is an inflammatory, autoimmune, demyelinating disease of the central nervous system (CNS) that usually starts as a relapsing-remitting disease. In most patients the disease evolves into a chronic progressive phase characterized by continuous accumulation of neurological deficits. While treatment of relapsing-remitting MS (RRMS) has improved dramatically over the last decade, the therapeutic options for chronic progressive MS, both primary and secondary, are still limited. In order to find new pharmacological targets for the treatment of chronic progressive MS, the mechanisms of the underlying neurodegenerative process that becomes apparent as the disease progresses need to be elucidated. New animal models with prominent and widespread progressive degenerative components of MS have to be established to study both inflammatory and non-inflammatory mechanisms of neurodegeneration. Here, we discuss disease mechanisms and treatment strategies for chronic progressive MS.

Neuroprotective role of fibroblast growth factor-2 in experimental autoimmune encephalomyelitis

The role of fibroblast growth factor-2 (FGF-2) in multiple sclerosis and its animal model experimental autoimmune encephalomyelitis is discussed. This study is the first to use FGF-2(-/-) mice to further address the involvement of FGF-2 in the disease process. We demonstrate that immunization with myelin oligodendrocyte glycoprotein peptide 35-55 induces more severe experimental autoimmune encephalomyelitis in FGF-2(-/-) mice compared with FGF-2(+/+) mice. The antigen-specific cytokine response to myelin oligodendrocyte glycoprotein peptide and the degree of central nervous system inflammation was similar in both groups. However, FGF-2(-/-) mice displayed increased infiltration of CD8(+) T cells and macrophages/microglia. In addition, nerve fibre degeneration and axonal loss were augmented, whereas the extent of remyelination in central nervous system lesions was reduced. FGF-2 has been associated with the induction of demyelination and the inhibition of myelin production by oligodendrocytes. Our study supports the opposing notion that FGF-2 can also assert a neuroprotective function. This may be particularly appealing when it comes to targeting the neurodegenerative aspect of multiple sclerosis.

Transgenic expression of viral capsid proteins predisposes to axonal injury in a murine model of multiple sclerosis

We used transgenic expression of capsid antigens to Theiler's murine encephalomyelitis virus (TMEV) to study the influence of VP1, VP2 or VP2(121-130) to either protection or pathogenesis to chronic spinal cord demyelination, axonal loss and functional deficits during the acute and chronic phases of infection. We used both mice that are normally susceptible (FVB) and mice normally resistant (FVB.D(b) ) to demyelination. Transgenic expression of VP2(121-130) epitope in resistant FVB.D(b) mice caused spinal cord pathology and virus persistence because the VP2(121-130) epitope is the dominant peptide recognized by D(b) , which is critical for virus clearance. In contrast, all three FVB TMEV transgenic mice showed more demyelination, inflammation and axonal loss as compared with wild-type FVB mice, even though virus load was not increased. Motor function measured by rotarod showed weak correlation with total number of midthoracic axons, but a strong correlation with large-caliber axons (>10µm(2) ). This study supports the hypothesis that expression of viral capsid proteins as self influences the extent of axonal pathology following Theiler's virus-induced demyelination. The findings provide insight into the role of axonal injury in the development of functional deficits that may have relevance to human demyelinating disease.

Rapid formation of extended processes and engagement of Theiler's virus-infected neurons by CNS-infiltrating CD8 T cells

A fundamental question in neuroimmunology is the extent to which CD8 T cells actively engage virus-infected neurons. In the Theiler's murine encephalomyelitis virus (TMEV) model of multiple sclerosis, an effective central nervous system (CNS)-infiltrating antiviral CD8 T cell response offers protection from this demyelinating disease. However, the specific CNS cell types engaged by these protective CD8 T cells in TMEV-resistant strains remains unknown. We used confocal microscopy to visualize the morphology, migration, and specific cellular interactions between adoptively transferred CD8 T cells and specific CNS cell types. Adoptively transferred GFP+ CD8+ splenocytes migrated to the brain and became 93% specific for the immunodominant virus epitope D(b):VP2(121-130). These CD8 T cells also polarized T cell receptor, CD8 protein, and granzyme B toward target neurons. Furthermore, we observed CD8 T cells forming cytoplasmic processes up to 45 μm in length. Using live tissue imaging, we determined that these T cell-extended processes (TCEPs) could be rapidly formed and were associated with migratory behavior through CNS tissues. These studies provide evidence that antiviral CD8 T cells have the capacity to engage virus-infected neurons in vivo and are the first to document and measure the rapid formation of TCEPs on these brain-infiltrating lymphocytes using live tissue imaging.

CD8+ T cells cause disability and axon loss in a mouse model of multiple sclerosis

Background

The objective of this study was to test the hypothesis that CD8⁺ T cells directly mediate motor disability and axon injury in the demyelinated central nervous system. We have previously observed that genetic deletion of the CD8⁺ T cell effector molecule perforin leads to preservation of motor function and preservation of spinal axons in chronically demyelinated mice.

Methodology/Principal Findings

To determine if CD8⁺ T cells are necessary and sufficient to directly injure demyelinated axons, we adoptively transferred purified perforin-competent CD8⁺ spinal cord-infiltrating T cells into profoundly demyelinated but functionally preserved perforin-deficient host mice. Transfer of CD8⁺ spinal cord-infiltrating T cells rapidly and irreversibly impaired motor function, disrupted spinal cord motor conduction, and reduced the number of medium- and large-caliber spinal axons. Likewise, immunodepletion of CD8⁺ T cells from chronically demyelinated wildtype mice preserved motor function and limited axon loss without altering other disease parameters.

Conclusions/Significance

In multiple sclerosis patients, CD8⁺ T cells outnumber CD4⁺ T cells in active lesions and the number of CD8⁺ T cells correlates with the extent of ongoing axon injury and functional disability. Our findings suggest that CD8⁺ T cells may directly injure demyelinated axons and are therefore a viable therapeutic target to protect axons and motor function in patients with multiple sclerosis.

Contribution of CD8 T lymphocytes to the immuno-pathogenesis of multiple sclerosis and its animal models

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) characterized by multi-focal demyelination, axonal loss, and immune cell infiltration. Numerous immune mediators are detected within MS lesions, including CD4(+) and CD8(+) T lymphocytes suggesting that they participate in the related pathogenesis. Although CD4(+) T lymphocytes are traditionally considered the main actors in MS immunopathology, multiple lines of evidence suggest that CD8(+) T lymphocytes are also implicated in the pathogenesis. In this review, we outline the recent literature pertaining to the potential roles of CD8(+) T lymphocytes both in MS and its animal models. The CD8(+) T lymphocytes detected in MS lesions demonstrate characteristics of activated and clonally expanded cells supporting the notion that these cells actively contribute to the observed injury. Moreover, several experimental in vivo models mediated by CD8(+) T lymphocytes recapitulate important features of the human disease. Whether the CD8(+) T cells can induce or aggravate tissue destruction in the CNS needs to be fully explored. Strengthening our understanding of the pathogenic potential of CD8(+) T cells in MS should provide promising new avenues for the treatment of this disabling inflammatory disease.

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.

Classical major histocompatibility complex class I molecules in motoneurons: new actors at the neuromuscular junction

Major histocompatibility complex (MHC) class I molecules have fundamental functions in the immune system. Recent studies have suggested that these molecules may also have non-immune functions in the nervous system, in particular related to synaptic function and plasticity. Because adult motoneurons express mRNAs for MHC class I molecules, we have examined their subcellular expression pattern in vivo and their role for the synaptic connectivity of these neurons. We observed immunoreactivity for classical MHC class I (Ia) protein in motoneuron somata, but the predominant expression was found in axons and presynaptically at neuromuscular junctions (NMJs). Peripheral nerve lesion induced a strong increase of motoneuron MHC class Ia (H2-K(b)/D(b)) mRNA, indicating a role for MHC class Ia molecules during regeneration. Accordingly, there was an accumulation of MHC class Ia proteins at the cut ends and in growth cones of motor axons after lesion. In K(b-/-)D(b-/-) mice (lacking MHC class Ia molecules), the time course for recovery of grip ability in reinnervated muscles was significantly delayed. Muscles from K(b-/-)D(b-/-) mice displayed an increased density and a disturbed distribution of NMJs and fewer terminal Schwann cells/NMJ compared with wild-type mice. A population of Schwann cells in sciatic nerves expressed the paired Ig receptor B, which binds to MHC class I molecules. These results provide the first evidence that neuronal MHC class Ia molecules are present in motor axons, that they are important for organization of NMJs and motor recovery after nerve lesion, and that their actions may be mediated via Schwann cells.

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.

Demyelinated axons and motor function are protected by genetic deletion of perforin in a mouse model of multiple sclerosis

Axon injury is a major determinant of the loss of neurological function in patients with multiple sclerosis. It is unclear, however, whether damage to axons is an obligatory consequence of demyelination or whether it is an independent process that occurs in the permissive environment of demyelinated lesions. Previous investigations into the role of CD8 T cells and perforin in the Theiler murine encephalomyelitis virus model of multiple sclerosis have used mouse strains resistant to Theiler murine encephalomyelitis virus infection. To test the role of CD8 T cells in axon injury, we established a perforin-deficient mouse model on the H-2 major histocompatibility complex background thereby removing confounding factors related to viral biology in this Theiler murine encephalomyelitis virus-susceptible strain. This permitted direct comparison of clinical and pathological parameters between perforin-competent and perforin-deficient mice. The extent of demyelination was indistinguishable between perforin-competent and perforin-deficient H-2 mice, but chronically infected perforin-deficient mice exhibited preservation of motor function and spinal axons despite the presence of spinal cord demyelination. Thus, demyelination is necessary but insufficient for axon injury in this model; the absence of perforin protects axons without impacting demyelination. These results suggest that perforin is a key mediator of axon injury and lend additional support to the hypothesis that CD8 T cells are primarily responsible for axon damage in multiple sclerosis.

Inflammation on the mind: visualizing immunity in the central nervous system

The central nervous system (CNS) is a remarkably complex structure that utilizes electrochemical signaling to coordinate activities throughout the entire body. Because the nervous system contains nonreplicative cells, it is postulated that, through evolutionary pressures, this compartment has acquired specialized mechanisms to limit damage. One potential source of damage comes from our immune system, which has the capacity to survey the CNS and periphery for the presence of foreign material. The immune system is equipped with numerous effector mechanisms and can greatly alter the homeostasis and function of the CNS. Degeneration, autoimmunity, and pathogen infection can all result in acute, and sometimes chronic, inflammation within the CNS. Understanding the specialized functionality of innate and adaptive immune cells within the CNS is critical to the design of more efficacious treatments to mitigate CNS inflammatory conditions. Much of our knowledge of CNS-immune interactions stems from seminal studies that have used static and dynamic imaging approaches to visualize inflammatory cells responding to different CNS conditions. This review will focus on how imaging techniques have elevated our understanding of CNS inflammation as well as the exciting prospects that lie ahead as we begin to pursue investigation of the inflamed CNS in real time.

Theiler's virus infection induces a predominant pathogenic CD4+ T cell response to RNA polymerase in susceptible SJL/J mice

Theiler's murine encephalomyelitis virus (TMEV)-induced immune-mediated demyelinating disease in susceptible mouse strains has been extensively investigated as a relevant model for human multiple sclerosis. Previous investigations of antiviral T-cell responses focus on immune responses to viral capsid proteins, while virtually nothing is reported on immune responses to nonstructural proteins. In this study, we have identified noncapsid regions recognized by CD4(+) T cells from TMEV-infected mice using an overlapping peptide library. Interestingly, a greater number of CD4(+) T cells recognizing an epitope (3D(21-36)) of the 3D viral RNA polymerase, in contrast to capsid epitopes, were detected in the CNS of TMEV-infected SJL mice, whereas only a minor population of CD4(+) T cells from infected C57BL/6 mice recognized this region. The effects of preimmunization and tolerization with these epitopes on the development of demyelinating disease indicated that capsid-specific CD4(+) T cells are protective during the early stages of viral infection, whereas 3D(21-36)-specific CD4(+) T cells exacerbate disease development. Therefore, protective versus pathogenic CD4(+) T-cell responses directed to TMEV appear to be epitope dependent, and the differences in CD4(+) T-cell responses to these epitopes between susceptible and resistant mice may play an important role in the resistance or susceptibility to virally induced demyelinating disease.

CD8+ T cells and neuronal damage: direct and collateral mechanisms of cytotoxicity and impaired electrical excitability

Cytotoxic CD8(+) T cells are increasingly recognized as key players in various inflammatory and degenerative central nervous system (CNS) disorders. CD8(+) T cells are believed to actively contribute to neural damage in these CNS conditions. Conceptually, one can separate two possible ways that CD8(+) T cells harm neuronal function or integrity: CD8(+) T cells either directly target neurons and their neurites in an antigen- or contact-dependent fashion, or exert their action via "collateral" mechanisms of neuronal damage that might follow destruction of the myelin sheath or glial cells in both the CNS gray and white matter. After introducing clinical examples, in which the putative relevance CD8(+) T cells has been demonstrated, we summarize knowledge on the sequence of initiation and execution of CD8(+) T-cell responses in the CNS. This includes the initial antigen cross-presentation and priming of naive CD8(+) T cells, followed by the invasion, migration, and target-cell recognition of CD8(+) effector T cells in the CNS parenchyma. Moreover, we discuss mechanisms of impaired electrical signaling and cell death of neurons as direct and collateral targets of CD8(+) T cells in the CNS.

Remyelination-promoting human IgMs: developing a therapeutic reagent for demyelinating disease

Promoting remyelination following injury to the central nervous system (CNS) promises to be an effective neuroprotective strategy to limit the loss of surviving axons and prevent disability. Studies confirm that multiple sclerosis (MS) and spinal cord injury lesions contain myelinating cells and their progenitors. Recruiting these endogenous cells to remyelinate may be of therapeutic value. This review addresses the use of antibodies reactive to CNS antigens to promote remyelination. Antibody-induced remyelination in a virus-mediated model of chronic spinal cord injury was initially observed in response to treatment with CNS reactive antisera. Monoclonal mouse and human IgMs, which bind to the surface of oligodendrocytes and myelin, were later identified that were functionally equivalent to antisera. A recombinant form of a human remyelination-promoting IgM (rHIgM22) targets areas of CNS injury and promotes maximal remyelination within 5 weeks after a single low dose (25 microg/kg). The IgM isoform of this reparative antibody is required for in vivo function. We hypothesize that the IgM clusters membrane domains and associated signaling molecules on the surface of target cells. Current therapies for MS are designed to modulate inflammation. In contrast, remyelination promoting IgMs are the first potential therapeutic molecules designed to induce tissue repair by acting within the CNS at sites of damage on the cells responsible for myelin synthesis.

Immunological aspects of axon injury in multiple sclerosis

The role of immune-mediated axonal injury in the induction of nonremitting functional deficits associated with multiple sclerosis is an area of active research that promises to substantially alter our understanding of the pathogenesis of this disease and modify or change our therapeutic focus. This review summarizes the current state of research regarding changes in axonal function during demyelination, provides evidence of axonal dysmorphia and degeneration associated with demyelination, and identifies the cellular and molecular effectors of immune-mediated axonal injury. Finally, a unifying hypothesis that links neuronal stress associated with demyelination-induced axonal dysfunction to immune recognition and immunopathology is provided in an effort to shape future experimentation.

Targeting inflammatory demyelinating lesions to sites of Wallerian degeneration

In Theiler's murine encephalomyelitis virus (TMEV) infection, an animal model for multiple sclerosis (MS), axonal injury precedes inflammatory demyelinating lesions, and the distribution of axonal damage present during the early phase of infection corresponds to regions where subsequent demyelination occurs during the chronic phase. We hypothesized that axonal damage recruits inflammatory cells to sites of Wallerian degeneration, leading to demyelination. Three weeks after TMEV infection, axonal degeneration was induced in the posterior funiculus of mice by injecting the toxic lectin Ricinus communis agglutinin (RCA) I into the sciatic nerve. Neuropathology was examined 1 week after lectin injection. Control mice, infected with TMEV but receiving no RCA I, had inflammatory demyelinating lesions in the anterior/lateral funiculi. Other control mice that received RCA I alone did not develop inflammatory lesions. In contrast, RCA I injection into TMEV-infected mice induced lesions in the posterior funiculus in addition to the anterior/lateral funiculi. We found no differences in lymphoproliferative responses or antibody titers against TMEV among the groups. This suggests that axonal degeneration contributes to the recruitment of inflammatory cells into the central nervous system by altering the local microenvironment. In this scenario, lesions develop from the axon (inside) to the myelin (outside) (Inside-Out model).

Effectors of demyelination and remyelination in the CNS: implications for multiple sclerosis

Most of the research on multiple sclerosis (MS) has focused on the early events that trigger demyelination and subsequent remyelination. Less attention has been given to the factors that directly mediate the demyelination that is the hallmark of the disease. Effector cells or molecules are those factors directly responsible for mediating the damage in the disease. Similarly, there are effector molecules that are critical for remyelination in the central nervous system (CNS). By understanding those effector molecules in demyelination and remyelination that directly influence the pathologic process, we should be able to generate specific therapies with the greatest potential for benefiting MS patients. This review focuses on effector cells and molecules that are critical for demyelination and remyelination in MS but also in experimental models of the disease including experimental autoimmune encephalomyelitis (EAE), virus-induced models of demyelination (Theiler's virus, murine hepatitis virus), and toxic models of demyelination (lysolecithin, ethidium bromide, and cuprizone). These are models in which the effector molecules for demyelination and remyelination have been most precisely evaluated.