Absence of perforin expression confers axonal protection despite demyelination

Identification of crucial inflammaging related risk factors in multiple sclerosis

Background

Multiple sclerosis (MS) is an immune-mediated disease characterized by inflammatory demyelinating lesions in the central nervous system. Studies have shown that the inflammation is vital to both the onset and progression of MS, where aging plays a key role in it. However, the potential mechanisms on how aging-related inflammation (inflammaging) promotes MS have not been fully understood. Therefore, there is an urgent need to integrate the underlying mechanisms between inflammaging and MS, where meaningful prediction models are needed.

Methods

First, both aging and disease models were developed using machine learning methods, respectively. Then, an integrated inflammaging model was used to identify relative risk factors, by identifying essential "aging-inflammation-disease" triples. Finally, a series of bioinformatics analyses (including network analysis, enrichment analysis, sensitivity analysis, and pan-cancer analysis) were further used to explore the potential mechanisms between inflammaging and MS.

Results

A series of risk factors were identified, such as the protein homeostasis, cellular homeostasis, neurodevelopment and energy metabolism. The inflammaging indices were further validated in different cancer types. Therefore, various risk factors were integrated, and even both the theories of inflammaging and immunosenescence were further confirmed.

Conclusion

In conclusion, our study systematically investigated the potential relationships between inflammaging and MS through a series of computational approaches, and could present a novel thought for other aging-related diseases.

CD8+ T cells recognizing a neuron-restricted antigen injure axons in a model of multiple sclerosis

CD8+ T cells outnumber CD4+ cells in multiple sclerosis (MS) lesions associated with disease progression, but the pathogenic role and antigenic targets of these clonally expanded effectors are unknown. Based on evidence that demyelination is necessary but not sufficient for disease progression in MS, we previously hypothesized that CNS-infiltrating CD8+ T cells specific for neuronal antigens directly drive the axonal and neuronal injury that leads to cumulative neurologic disability in patients with MS. We now show that demyelination induced expression of MHC class I on neurons and axons and resulted in presentation of a neuron-specific neoantigen (synapsin promoter-driven chicken ovalbumin) to antigen-specific CD8+ T cells (anti-ovalbumin OT-I TCR-transgenic T cells). These neuroantigen-specific effectors surveilled the CNS in the absence of demyelination but were not retained. However, upon induction of demyelination via cuprizone intoxication, neuroantigen-specific CD8+ T cells proliferated, accumulated in the CNS, and damaged neoantigen-expressing neurons and axons. We further report elevated neuronal expression of MHC class I and β2-microglobulin transcripts and protein in gray matter and white matter tracts in tissue from patients with MS. These findings support a pathogenic role for autoreactive anti-axonal and anti-neuronal CD8+ T cells in MS progression.

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.

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.

Retrograde interferon-gamma signaling induces major histocompatibility class I expression in human-induced pluripotent stem cell-derived neurons

Objective

Injury-associated axon-intrinsic signals are thought to underlie pathogenesis and progression in many neuroinflammatory and neurodegenerative diseases, including multiple sclerosis (MS). Retrograde interferon gamma (IFN γ) signals are known to induce expression of major histocompatibility class I (MHC I) genes in murine axons, thereby increasing the susceptibility of these axons to attack by antigen-specific CD8⁺ T cells. We sought to determine whether the same is true in human neurons.

Methods

A novel microisolation chamber design was used to physically isolate and manipulate axons from human skin fibroblast-derived induced pluripotent stem cell (iPSC)-derived neuron-enriched neural aggregates. Fluorescent retrobeads were used to assess the fraction of neurons with projections to the distal chamber. Axons were treated with IFN γ for 72 h and expression of MHC class I and antigen presentation genes were evaluated by RT-PCR and immunofluorescence.

Results

Human iPSC-derived neural stem cells maintained as 3D aggregate cultures in the cell body chamber of polymer microisolation chambers extended dense axonal projections into the fluidically isolated distal chamber. Treatment of these axons with IFN γ resulted in upregulation of MHC class I and antigen processing genes in the neuron cell bodies. IFN γ-induced MHC class I molecules were also anterogradely transported into the distal axon.

Interpretation

These results provide conclusive evidence that human axons are competent to express MHC class I molecules, suggesting that inflammatory factors enriched in demyelinated lesions may render axons vulnerable to attack by autoreactive CD8⁺ T cells in patients with MS. Future work will be aimed at identifying pathogenic anti-axonal T cells in these patients.

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.

A monoclonal natural human IgM protects axons in the absence of remyelination

BACKGROUND

Whereas demyelination underlies early neurological symptoms in multiple sclerosis (MS), axonal damage is considered critical for permanent chronic deficits. Intracerebral infection of susceptible mouse strains with Theiler's murine encephalomyelitis virus (TMEV) results in chronic induced demyelinating disease (TMEV-IDD) with progressive axonal loss and neurologic dysfunction similar to progressive forms of MS. We previously reported that treatment of chronic TMEV-IDD mice with a neurite outgrowth-promoting natural human antibody, HIgM12, improved brainstem NAA concentrations and preserved functional motor activity. In order to translate this antibody toward clinical trial, we generated a fully human recombinant form of HIgM12, rHIgM12, determined the optimal in vivo dose for functional improvement in TMEV-IDD, and evaluated the functional preservation of descending spinal cord axons by retrograde labeling.

FINDINGS

SJL/J mice at 45 to 90 days post infection (dpi) were studied. A single intraperitoneal dose of 0.25 mg/kg of rHIgM12 per mouse is sufficient to preserve motor function in TMEV-IDD. The optimal dose was 10 mg/kg. rHIgM12 treatment protected the functional transport in spinal cord axons and led to 40 % more Fluoro-Gold-labeled brainstem neurons in retrograde transport studies. This suggests that axons are not only present but also functionally competent. rHIgM12-treated mice also contained more mid-thoracic (T6) spinal cord axons than controls.

CONCLUSIONS

This study confirms that a fully human recombinant neurite outgrowth-promoting monoclonal IgM is therapeutic in a model of progressive MS using multiple reparative readouts. The minimum effective dose is similar to that of a remyelination-promoting monoclonal human IgM discovered by our group that is presently in clinical trials for MS.

Axonal degeneration in multiple sclerosis: defining therapeutic targets by identifying the causes of pathology

Current therapeutics in multiple sclerosis (MS) target the putative inflammation and immune attack on CNS myelin. Despite their effectiveness in blunting the relapse rate in MS patients, such therapeutics do not prevent MS disease progression. Importantly, specific clinical dilemma arises through inability to predict MS progression and thereby therapeutically target axonal injury during MS, limiting permanent disability. The current review identifies immune and neurobiological principles that govern the sequelae of axonal degeneration during MS disease progression. Defining the specific disease arbiters, inflammatory and autoimmune, oligodendrocyte dystrophy and degenerative myelin, we discuss a basis for a molecular mechanism in axons that may be targeted therapeutically, in spatial and temporal manner to limit axonal degeneration and thereby halt progression of MS.

Abbreviated exposure to hypoxia is sufficient to induce CNS dysmyelination, modulate spinal motor neuron composition, and impair motor development in neonatal mice

Neonatal white matter injury (nWMI) is an increasingly common cause of cerebral palsy that results predominantly from hypoxic injury to progenitor cells including those of the oligodendrocyte lineage. Existing mouse models of nWMI utilize prolonged periods of hypoxia during the neonatal period, require complex cross-fostering and exhibit poor growth and high mortality rates. Abnormal CNS myelin composition serves as the major explanation for persistent neuro-motor deficits. Here we developed a simplified model of nWMI with low mortality rates and improved growth without cross-fostering. Neonatal mice are exposed to low oxygen from postnatal day (P) 3 to P7, which roughly corresponds to the period of human brain development between gestational weeks 32 and 36. CNS hypomyelination is detectable for 2–3 weeks post injury and strongly correlates with levels of body and brain weight loss. Immediately following hypoxia treatment, cell death was evident in multiple brain regions, most notably in superficial and deep cortical layers as well as the subventricular zone progenitor compartment. PDGFαR, Nkx2.2, and Olig2 positive oligodendrocyte progenitor cell were significantly reduced until postnatal day 27. In addition to CNS dysmyelination we identified a novel pathological marker for adult hypoxic animals that strongly correlates with life-long neuro-motor deficits. Mice reared under hypoxia reveal an abnormal spinal neuron composition with increased small and medium diameter axons and decreased large diameter axons in thoracic lateral and anterior funiculi. Differences were particularly pronounced in white matter motor tracts left and right of the anterior median fissure. Our findings suggest that 4 days of exposure to hypoxia are sufficient to induce experimental nWMI in CD1 mice, thus providing a model to test new therapeutics. Pathological hallmarks of this model include early cell death, decreased OPCs and hypomyelination in early postnatal life, followed by dysmyelination, abnormal spinal neuron composition, and neuro-motor deficits in adulthood.

A single dose of a neuron-binding human monoclonal antibody improves brainstem NAA concentrations, a biomarker for density of spinal cord axons, in a model of progressive multiple sclerosis

BACKGROUND

Intracerebral infection of susceptible mouse strains with Theiler's murine encephalomyelitis virus (TMEV) results in chronic demyelinating disease with progressive axonal loss and neurologic dysfunction similar to progressive forms of multiple sclerosis (MS). We previously showed that as the disease progresses, a marked decrease in brainstem N-acetyl aspartate (NAA; metabolite associated with neuronal integrity) concentrations, reflecting axon health, is measured. We also demonstrated stimulation of neurite outgrowth by a neuron-binding natural human antibody, IgM12. Treatment with either the serum-derived or recombinant human immunoglobulin M 12 (HIgM12) preserved functional motor activity in the TMEV model. In this study, we examined IgM-mediated changes in brainstem NAA concentrations and central nervous system (CNS) pathology.

FINDINGS

(1)H-magnetic resonance spectroscopy (MRS) showed that treatment with HIgM12 significantly increased brainstem NAA concentrations compared to controls in TMEV-infected mice. Pathologic analysis demonstrated a significant preservation of axons in the spinal cord of animals treated with HIgM12.

CONCLUSIONS

This study links drug efficacy of slowing deficits with axon preservation and NAA concentrations in the brainstem in a model of progressive MS. HIgM12-mediated changes of NAA concentrations in the brainstem are a surrogate marker of axon injury/preservation throughout the spinal cord. This study provides proof-of-concept that a neuron-reactive human IgM can be therapeutic and provides a biomarker for clinical trials.

Organ-specific protective role of NKT cells in virus-induced inflammatory demyelination and myocarditis depends on mouse strain

Theiler's murine encephalomyelitis virus (TMEV) can induce demyelination or myocarditis in susceptible mouse strains. A deficiency of NKT cells exacerbated TMEV-induced demyelinating disease (TMEV-IDD) in SJL/J and BALB/c mice. In C57BL/6 background, however, NKT-cell-deficient Jα18 KO mice remained as resistant to TMEV-IDD as wild-type mice. Echocardiography and histology showed that Jα18 KO mice developed more severe myocarditis (greater T cell infiltration and fibrosis) than wild-type mice, suggesting a protective role of NKT cells in myocarditis in C57BL/6 mice. Jα18 KO mice had higher cardiac viral RNA and anti-viral antibody titers, but had lower lymphoproliferation and IL-4 and IL-10 production.

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.

Promoting return of function in multiple sclerosis: An integrated approach

Multiple sclerosis is a disease characterized by inflammatory demyelination, axonal degeneration and progressive brain atrophy. Most of the currently available disease modifying agents proved to be very effective in managing the relapse rate, however progressive neuronal damage continues to occur and leads to progressive accumulation of irreversible disability. For this reason, any therapeutic strategy aimed at restoration of function must take into account not only immunomodulation, but also axonal protection and new myelin formation. We further highlight the importance of an holistic approach, which considers the variability of therapeutic responsiveness as the result of the interplay between genetic differences and the epigenome, which is in turn affected by gender, age and differences in life style including diet, exercise, smoking and social interaction.

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.

Immunity and inflammation in neurodegenerative diseases

Immune reactions inside the central nervous system are finely regulated, thanks to the presence of several checkpoints that have the fundamental purpose to preserve this fragile tissue form harmful events. The current knowledge on the role of neuroinflammation and neuro-immune interactions in the fields of multiple sclerosis, Alzheimer's disease and Parkinson's disease is reviewed. Moreover, a focus on the potential role of both active and passive immunotherapy is provided. Finally, we propose a common perspective, which implies that, under pathological conditions, inflammation may exert both detrimental and protective functions, depending on local factors and the timing of immune activation and shutting-off systems.

Pathoetiology of multiple sclerosis: are we barking up the wrong tree?

Despite a century of intensive investigation, the underlying cause of multiple sclerosis has eluded us. It is clear that there exists a prominent progressive degenerative phenotype together with an important autoimmune inflammatory component, and careful histopathological examination always shows, to a greater or lesser degree, concomitant degeneration/demyelination and adaptive T cell-dependent immune responses. Given this picture, it is difficult, if not impossible, to definitively say whether degeneration or autoimmunity is the initiator of the disease. In this review, I put forward the evidence for and against both models and speculate that, in contrast to the accepted view, it is equally likely that multiple sclerosis may be a degenerative disease that secondarily elicits an autoimmune response, and suggest how this might influence therapeutic approaches.

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.

Profile of Circulatory Metabolites in a Relapsing-remitting Animal Model of Multiple Sclerosis using Global Metabolomics

Multiple sclerosis (MS) is a chronic inflammatory and demyelinating disease of the CNS. Although, MS is well characterized in terms of the role played by immune cells, cytokines and CNS pathology, nothing is known about the metabolic alterations that occur during the disease process in circulation. Recently, metabolic aberrations have been defined in various disease processes either as contributing to the disease, as potential biomarkers, or as therapeutic targets. Thus in an attempt to define the metabolic alterations that may be associated with MS disease progression, we profiled the plasma metabolites at the chronic phase of disease utilizing relapsing remitting-experimental autoimmune encephalomyelitis (RR-EAE) model in SJL mice. At the chronic phase of the disease (day 45), untargeted global metabolomic profiling of plasma collected from EAE diseased SJL and healthy mice was performed, using a combination of high-throughput liquid-and-gas chromatography with mass spectrometry. A total of 282 metabolites were identified, with significant changes observed in 44 metabolites (32 up-regulated and 12 down-regulated), that mapped to lipid, amino acid, nucleotide and xenobiotic metabolism and distinguished EAE from healthy group (p<0.05, false discovery rate (FDR)<0.23). Mapping the differential metabolite signature to their respective biochemical pathways using the Kyoto Encyclopedia of Genes and Genomics (KEGG) database, we found six major pathways that were significantly altered (containing concerted alterations) or impacted (containing alteration in key junctions). These included bile acid biosynthesis, taurine metabolism, tryptophan and histidine metabolism, linoleic acid and D-arginine metabolism pathways. Overall, this study identified a 44 metabolite signature drawn from various metabolic pathways which correlated well with severity of the EAE disease, suggesting that these metabolic changes could be exploited as (1) biomarkers for EAE/MS progression and (2) to design new treatment paradigms where metabolic interventions could be combined with present and experimental therapeutics to achieve better treatment of MS.

Enhanced CD8 T-cell anti-viral function and clinical disease in B7-H1-deficient mice requires CD4 T cells during encephalomyelitis

Background

Anti-viral CD8 T-cell activity is enhanced and prolonged by CD4 T-cell-mediated help, but negatively regulated by inhibitory B7-H1 interactions. During viral encephalomyelitis, the absence of CD4 T cells decreases CD8 T cell activity and impedes viral control in the central nervous system (CNS). By contrast, the absence of B7-H1 enhances CD8 T-cell function and accelerates viral control, but increases morbidity. However, the relative contribution of CD4 T cells to CD8 function in the CNS, in the absence of B7-H1, remains unclear.

Methods

Wild-type (WT) and B7-H1^−/− mice were infected with a gliatropic coronavirus and CD4 T cells depleted to specifically block T helper function in the CNS. Flow cytometry and gene expression analysis of purified T-cell populations from lymph nodes and the CNS was used to directly monitor ex vivo T-cell effector function. The biological affects of altered T-cell responses were evaluated by analysis of viral control and spinal-cord pathology.

Results

Increased anti-viral activity by CD8 T cells in the CNS of B7-H1^−/− mice was lost upon depletion of CD4 T cells; however, despite concomitant loss of viral control, the clinical disease was less severe. CD4 depletion in B7-H1^−/− mice also decreased inducible nitric oxide synthase expression by microglia and macrophages, consistent with decreased microglia/macrophage activation and reduced interferon (IFN)-γ. Enhanced production of IFN-γ, interleukin (IL)-10 and IL-21 mRNA was seen in CD4 T cells from infected B7-H1^−/− compared with WT mice, suggesting that over-activated CD4 T cells primarily contribute to the increased pathology.

Conclusions

The local requirement of CD4 T-cell help for CD8 T-cell function is not overcome if B7-H1 inhibitory signals are lost. Moreover, the increased effector activity by CD8 T cells in the CNS of B7-H1^−/− mice is attributable not only to the absence of B7-H1 upregulation on major histocompatibility complex class I-presenting resident target cells, but also to enhanced local CD4 T-cell function. B7-H1-mediated restraint of CD4 T-cell activity is thus crucial to dampen both CD8 T-cell function and microglia/macrophage activation, thereby providing protection from T-cell-mediated bystander damage.

Neuroinflammation by cytotoxic T-lymphocytes impairs retrograde axonal transport in an oligodendrocyte mutant mouse

Mice overexpressing proteolipid protein (PLP) develop a leukodystrophy-like disease involving cytotoxic, CD8+ T-lymphocytes. Here we show that these cytotoxic T-lymphocytes perturb retrograde axonal transport. Using fluorogold stereotactically injected into the colliculus superior, we found that PLP overexpression in oligodendrocytes led to significantly reduced retrograde axonal transport in retina ganglion cell axons. We also observed an accumulation of mitochondria in the juxtaparanodal axonal swellings, indicative for a disturbed axonal transport. PLP overexpression in the absence of T-lymphocytes rescued retrograde axonal transport defects and abolished axonal swellings. Bone marrow transfer from wildtype mice, but not from perforin- or granzyme B-deficient mutants, into lymphocyte-deficient PLP mutant mice led again to impaired axonal transport and the formation of axonal swellings, which are predominantly located at the juxtaparanodal region. This demonstrates that the adaptive immune system, including cytotoxic T-lymphocytes which release perforin and granzyme B, are necessary to perturb axonal integrity in the PLP-transgenic disease model. Based on our observations, so far not attended molecular and cellular players belonging to the immune system should be considered to understand pathogenesis in inherited myelin disorders with progressive axonal damage.

Will the real multiple sclerosis please stand up?

Multiple sclerosis (MS) is considered to be an autoimmune, inflammatory disease of the CNS. In most patients, the disease follows a relapsing-remitting course and is characterized by dynamic inflammatory demyelinating lesions in the CNS. Although on the surface MS may appear consistent with a primary autoimmune disease, questions have been raised as to whether inflammation and/or autoimmunity are really at the root of the disease, and it has been proposed that MS might in fact be a degenerative disorder. We argue that MS may be an 'immunological convolution' between an underlying primary degenerative disorder and the host's aberrant immune response. To better understand this disease, we might need to consider non-inflammatory primary progressive MS as the 'real' MS, with inflammatory forms reflecting secondary, albeit very important, reactions.

Morphological and functional characterization of leech circulating blood cells: role in immunity and neural repair

Unlike most invertebrates, annelids possess a closed vascular system distinct from the coelomic liquid. The morphology and the function of leech blood cells are reported here. We have demonstrated the presence of a unique cell type which participates in various immune processes. In contrast to the mammalian spinal cord, the leech CNS is able to regenerate and restore function after injury. The close contact of the blood with the nerve cord also led us to explore the participation of blood in neural repair. Our data evidenced that, in addition to exerting peripheral immune functions, leech blood optimizes CNS neural repair through the release of neurotrophic substances. Circulating blood cells also appeared able to infiltrate the injured CNS where, in conjunction with microglia, they limit the formation of a scar. In mammals, CNS injury leads to the generation of a glial scar that blocks the mechanism of regeneration by preventing axonal regrowth. The results presented here constitute the first description of neuroimmune functions of invertebrate blood cells. Understanding the basic function of the peripheral circulating cells and their interactions with lesioned CNS in the leech would allow us to acquire insights into the complexity of the neuroimmune response of the injured mammalian brain.

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.

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.

Granule-derived granzyme B mediates the vulnerability of human neurons to T cell-induced neurotoxicity

Multiple sclerosis (MS) is considered an autoimmune disease of the CNS and is characterized by inflammatory cells infiltrating the CNS and inducing demyelination, axonal loss, and neuronal death. Recent evidence strongly suggests that axonal and neuronal degeneration underlie the progression of permanent disability in MS. In this study, we report that human neurons are selectively susceptible to the serine-protease granzyme B (GrB) isolated from cytotoxic T cell granules. In vitro, purified human GrB induced neuronal death to the same extent as the whole activated T cell population. On the contrary, activated T cells isolated from GrB knockout mice failed to induce neuronal injury. We found that following internalization through various parts of neurons, GrB accumulated in the neuronal soma. Within the cell body, GrB diffused out of endosomes possibly through a perforin-independent mechanism and induced subsequent activation of caspases and cleavage of α-tubulin. Inhibition of caspase-3, a well-known substrate for GrB, significantly reduced GrB-mediated neurotoxicity. We demonstrated that treatment of neurons with mannose-6-phosphate prevented GrB entry and inhibited GrB-mediated neuronal death, suggesting mannose-6-phosphate receptor-dependent endocytosis. Together, our data unveil a novel mechanism by which GrB induces selective neuronal injury and suggest potential new targets for the treatment of inflammatory-mediated neurodegeneration in diseases such as MS.

Experimental in vivo and in vitro models of multiple sclerosis: EAE and beyond

Although the primary cause of multiple sclerosis (MS) is unknown, the widely accepted view is that aberrant (auto)immune responses possibly arising following infection(s) are responsible for the destructive inflammatory demyelination and neurodegeneration in the central nervous system (CNS). This notion, and the limited access of human brain tissue early in the course of MS, has led to the development of autoimmune, viral and toxin-induced demyelination animal models as well as the development of human CNS cell and organotypic brain slice cultures in an attempt to understand events in MS. The autoimmune models, collectively known as experimental autoimmune encephalomyelitis (EAE), and viral models have shaped ideas of how environmental factors may trigger inflammation, demyelination and neurodegeneration in the CNS. Understandably, these models have also heavily influenced the development of therapies targeting the inflammatory aspect of MS. Demyelination and remyelination in the absence of overt inflammation are better studied in toxin-induced demyelination models using cuprizone and lysolecithin. The paradigm shift of MS as an autoimmune disease of myelin to a neurodegenerative disease has required more appropriate models reflecting the axonal and neuronal damage. Thus, secondary progressive EAE and spastic models have been crucial to develop neuroprotective approaches. In this review the current in vivo and in vitro experimental models to examine pathological mechanisms involved in inflammation, demyelination and neuronal degeneration, as well as remyelination and repair in MS are discussed. Since this knowledge is the basis for the development of new therapeutic approaches for MS, we particularly address whether the currently available models truly reflect the human disease, and discuss perspectives to further optimise and develop more suitable experimental models to study MS.

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.

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.

γδ T cells and multiple sclerosis: Friends, foes, or both?

Multiple sclerosis (MS) is a debilitating CNS disease characterized by demyelination and neuro-axonal loss. Though the exact etiology is still unknown, accumulated evidence points to the immune system being involved in the MS disease-process. Both ill-fated adaptive and innate immune responses can potentially contribute to the etiopathogenesis. We have been interested in deciphering how innate immunity might be involved; in particular, the role of γδ T cells. In this review, we discuss the current understanding about γδ T cells and describe the evidence implicating them in myelin injury, neurotoxicity, and immunoregulation in the development of MS.

Enhanced antiviral T cell function in the absence of B7-H1 is insufficient to prevent persistence but exacerbates axonal bystander damage during viral encephalomyelitis

The T cell inhibitory ligand B7-H1 hinders T cell-mediated virus control, but also ameliorates clinical disease during autoimmune and virus-induced CNS disease. In mice infected with gliatropic demyelinating coronavirus, B7-H1 expression on oligodendroglia delays virus control, but also dampens clinical disease. To define the mechanisms by which B7-H1 alters pathogenic outcome, virus-infected B7-H1-deficient (B7-H1(-/-)) mice were analyzed for altered peripheral and CNS immune responses. B7-H1 deficiency did not affect peripheral T or B cell activation or alter the magnitude or composition of CNS-infiltrating cells. However, higher levels of IFN-γ mRNA in CNS-infiltrating virus-specific CD8 T cells as well as CD4 T cells contributed to elevated IFN-γ protein in the B7-H1(-/-) CNS. Increased effector function at the single-cell level was also evident by elevated granzyme B expression specifically in virus-specific CNS CD8 T cells. Although enhanced T cell activity accelerated virus control, 50% of mice succumbed to infection. Despite enhanced clinical recovery, surviving B7-H1(-/-) mice still harbored persisting viral mRNA, albeit at reduced levels compared with wild-type mice. B7-H1(-/-) mice exhibited extensive loss of axonal integrity, although demyelination, a hallmark of virus-induced tissue damage, was not increased. The results suggest that B7-H1 hinders viral control in B7-H1 expressing glia cells, but does not mediate resistance to CD8 T cell-mediated cytolysis. These data are the first, to our knowledge, to demonstrate that B7-H1-mediated protection from viral-induced immune pathology associated with encephalomyelitis resides in limiting T cell-mediated axonal bystander damage rather than direct elimination of infected myelinating cells.

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.

Ectopic T-cell specificity and absence of perforin and granzyme B alleviate neural damage in oligodendrocyte mutant mice

In transgenic mice overexpressing the major myelin protein of the central nervous system, proteolipid protein, CD8+ T-lymphocytes mediate the primarily genetically caused myelin and axon damage. In the present study, we investigated the cellular and molecular mechanisms underlying this immune-related neural injury. At first, we investigated whether T-cell receptors (TCRs) are involved in these processes. For this purpose, we transferred bone marrow from mutants carrying TCRs with an ectopic specificity to ovalbumin into myelin mutant mice that also lacked normal intrinsic T-cells. T-lymphocytes with ovalbumin-specific TCRs entered the mutant central nervous system to a similar extent as T-lymphocytes from wild-type mice. However, as revealed by histology, electron microscopy and axon- and myelin-related immunocytochemistry, these T-cells did not cause neural damage in the myelin mutants, reflecting the need for specific antigen recognition by cytotoxic CD8+ T-cells. By chimerization with bone marrow from perforin- or granzyme B (Gzmb)-deficient mice, we demonstrated that absence of these cytotoxic molecules resulted in reduced neural damage in myelin mutant mice. Our study strongly suggests that pathogenetically relevant immune reactions in proteolipid protein-overexpressing mice are TCR-dependent and mediated by the classical components of CD8+ T-cell cytotoxicity, perforin, and Gzmb. These findings have high relevance with regard to our understanding of the pathogenesis of disorders primarily caused by genetically mediated oligodendropathy.

Brainstem 1H nuclear magnetic resonance (NMR) spectroscopy: marker of demyelination and repair in spinal cord

Measuring in vivo spinal cord injury and repair remains elusive. Using magnetic resonance spectroscopy (MRS) we examined brainstem N-acetyl-aspartate (NAA) as a surrogate for spinal cord injury in two mouse strains with different reparative phenotypes following virus-induced demyelination. Swiss Jim Lambert (SJL) and Friend Virus B (FVB) mice progressively demyelinate with axonal loss. FVB mice demyelinate similarly but eventually remyelinate coincident with functional recovery. Brainstem NAA levels drop in both but recover in FVB mice. Chronically infected SJL mice lost 30.5% of spinal cord axons compared to FVB mice (7.3%). In remyelination-enhancing or axon-preserving clinical trials, brainstem MRS may be a viable endpoint to represent overall spinal cord dysfunction.

Axonal and neuronal pathology in multiple sclerosis: what have we learnt from animal models

Axonal and neuronal injury and loss are of critical importance for permanent clinical disability in multiple sclerosis patients. Axonal injury occurs already early during the disease and accumulates with disease progression. It is not restricted to focal demyelinated lesions in the white matter, but also affects the normal appearing white matter and the grey matter. Experimental studies show that many different immunological mechanisms may lead to axonal and neuronal injury, including antigen-specific destruction by specific T-cells and auto-antibodies as well as injury induced by products of activated macrophages and microglia. They all appear to be relevant for multiple sclerosis pathogensis in different patients and at different stages of the disease. However, in MS lesions a major mechanism of axonal and neuronal damage appears to be related to the action of reactive oxygen and nitrogen species, which may induce neuronal injury through impairment of mitochondrial function and subsequent energy failure.

Modeling multiple sclerosis in laboratory animals

Inflammatory demyelinating disease of the central nervous system is one of the most frequent causes of neurological disability in young adults. While in situ analysis and in vitro models do shed some light onto the processes of tissue damage and cellular interactions, the development of neuroinflammation and demyelination is a far too complex process to be adequately modeled by simple test tube systems. Thus, animal models using primarily genetically modified mice have been proven to be of paramount importance. In this chapter, we discuss recent advances in modeling brain diseases focusing on murine models and report on new tools to study the pathogenesis of complex diseases such as multiple sclerosis.

RNase L mediated protection from virus induced demyelination

IFN-α/β plays a critical role in limiting viral spread, restricting viral tropism and protecting mice from neurotropic coronavirus infection. However, the IFN-α/β dependent mechanisms underlying innate anti-viral functions within the CNS are poorly understood. The role of RNase L in viral encephalomyelitis was explored based on its functions in inhibiting translation, inducing apoptosis, and propagating the IFN-α/β pathway through RNA degradation intermediates. Infection of RNase L deficient (RL^−/−) mice with a sub-lethal, demyelinating mouse hepatitis virus variant revealed that the majority of mice succumbed to infection by day 12 p.i. However, RNase L deficiency did not affect overall control of infectious virus, or diminish IFN-α/β expression in the CNS. Furthermore, increased morbidity and mortality could not be attributed to altered proinflammatory signals or composition of cells infiltrating the CNS. The unique phenotype of infected RL^−/− mice was rather manifested in earlier onset and increased severity of demyelination and axonal damage in brain stem and spinal cord without evidence for enhanced neuronal infection. Increased tissue damage coincided with sustained brain stem infection, foci of microglia infection in grey matter, and increased apoptotic cells. These data demonstrate a novel protective role for RNase L in viral induced CNS encephalomyelitis, which is not reflected in overall viral control or propagation of IFN-α/β mediated signals. Protective function is rather associated with cell type specific and regional restriction of viral replication in grey matter and ameliorated neurodegeneration and demyelination.

Initial spread of viruses is controlled by type I interferon induced antiviral molecules. Early intervention with viral replication is especially critical in central nervous system infections to reduce loss of non-renewable cells and mitigate immune pathology. One of the best characterized anti-viral mechanisms is mediated by ribonuclease L (RNase L). RNase L exerts activity at multiple levels, including degradation of viral and host RNA, induction of apoptosis, and propagation of the IFN-α/β pathway. Recent studies suggest that RNase L antiviral activity is dependent on the virus, as well as the cell type and tissue infected. This study demonstrates that RNase L protects mice infected with a sub-lethal, demyelinating neurotropic coronavirus by ameliorating encephalitis and morbidity, albeit without affecting control of infectious virus or IFN-α/β expression. RNase L specifically protected the brain stem from sustained infection and prevented spread of virus to microglia/macrophages located in spinal cord grey matter. The subtle regional alteration in tropism in the absence of RNase L coincided with increased apoptotic cells and earlier onset as well as increased severity of axonal damage and demyelination. The results demonstrate how subtle regional alterations in viral tropism within the CNS may severely affect the balance between neuroprotection and neurotoxicity mediated by microglia/macrophages.

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.

Transgenic expression of the 3D polymerase inhibits Theiler's virus infection and demyelination

The RNA-dependent RNA polymerase 3D(pol) is required for the elongation of positive- and negative-stranded picornavirus RNA. During the course of investigating the effect of the transgenic expression of viral genes on the host immune response, we evaluated the viral load present in the host after infection. To our surprise, we found that 3D transgenic expression in genetically susceptible FVB mice led to substantially lower viral loads after infection with Theiler's murine encephalomyelitis virus (TMEV). As a result, spinal cord damage caused by chronic viral infection in the central nervous system was reduced in FVB mice that expressed 3D. This led to the preservation of large-diameter axons and motor function in these mice. The 3D transgene also lowered early viral loads when expressed in FVB-D(b) mice resistant to persistent TMEV infection. The protective effect of 3D transgenic expression was not altered in FVB-Rag(-/-).3D mice that are deficient in T and B cells, thus ruling out a mechanism by which the overexpression of 3D enhanced the adaptive immune clearance of the virus. Understanding how endogenously overexpressed 3D polymerase inhibits viral replication may lead to new strategies for targeting therapies to all picornaviruses.

Functional characterization of mouse spinal cord infiltrating CD8+ lymphocytes

Understanding the immunopathogenesis of neuroimmunological diseases of the CNS requires a robust method for isolating and characterizing the immune effector cells that infiltrate the spinal cord in animal models. We have developed a simple and rapid isolation method that produces high yields of spinal cord infiltrating leukocytes from a single demyelinated spinal cord and which maintains high surface expression of key immunophenotyping antigens. Using this method and the Theiler's virus model of chronic demyelination, we report the presence of spinal cord infiltrating acute effector CD8(+) lymphocytes that are CD45(hi)CD44(lo)CD62L(-) and a population of spinal cord infiltrating target effector memory CD8(+) lymphocytes that are CD45(hi)CD44(hi)CD62L(-). These cells respond robustly to ex vivo stimulation by producing interferon gamma but do not exhibit specificity for Theiler's virus in a cytotoxicity assay. We conclude that target-derived lymphocytes in a mouse model of chronic spinal cord demyelination may have unique functional specificities.

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.

Mucopolysaccharidosis IIIB: oxidative damage and cytotoxic cell involvement in the neuronal pathogenesis

Sanfilippo B syndrome (Mucopolysaccharidosis IIIB, MPS IIIB) is a lysosomal storage disease due to mutations in the gene encoding alpha-N-acetylglucosaminidase and is characterized by a severe neurological disorder. Although several studies have been reported for the murine model of the disease, the molecular basis and the sequence of events leading to neurodegeneration remain to be clarified. We previously suggested the possible involvement of the reactive oxygen species in the disease pathogenesis. In the present paper we extended the analysis of oxidative stress by evaluating the production of superoxide ions throughout the CNS and by evaluating the effect of the stress on the cellular macromolecules. These approaches applied to one-month-old, three-month-old and six-month-old mice revealed that oxidative stress is present in the affected cerebrum and cerebellum tissues from one month from birth, and that it results primarily in protein oxidation, both in the cerebrum and cerebellum, with lipid peroxidation, and especially DNA oxidation, appearing milder and restricted essentially to the cerebellum. We also identified additional genes possibly associated with the neuropathology of MPS IIIB disease. Real time RT-PCR analysis revealed an altered expression of the Sod1, Ret, Bmp4, Tgfb, Gzmb and Prf1 genes. Since Gzmb and Prf1 are proteins secreted by NK/cytotoxic T-cells, these data suggest the involvement of cytotoxic cells in the neuronal pathogenesis. Extending our previous study, findings reported in the present paper show that oxidative stress and all the analyzed stress-related pathological changes occur very early in the disease course, most likely before one month of age.

The multiple sclerosis degradome: enzymatic cascades in development and progression of central nervous system inflammatory disease

An array of studies implicate different classes of protease and their endogenous inhibitors in multiple sclerosis (MS) pathogenesis based on expression patterns in MS lesions, sera, and/or cerebrospinal fluid (CSF). Growing evidence exists regarding their mechanistic roles in inflammatory and neurodegenerative aspects of this disease. Proteolytic events participate in demyelination, axon injury, apoptosis, and development of the inflammatory response including immune cell activation and extravasation, cytokine and chemokine activation/inactivation, complement activation, and epitope spreading. The potential significance of proteolytic activity to MS therefore relates not only to their potential use as important biomarkers of disease activity, but additionally as prospective therapeutic targets. Experimental data indicate that understanding the net physiological consequence of altered protease levels in MS development and progression necessitates understanding protease activity in the context of substrates, endogenous inhibitors, and proteolytic cascade interactions, which together make up the MS degradome. This review will focus on evidence regarding the potential physiologic role of those protease families already identified as markers of disease activity in MS; that is, the metallo-, serine, and cysteine proteases.