Virus expanded regulatory T cells control disease severity in the Theiler's virus mouse model of MS

The Plasticity of Immune Cell Response Complicates Dissecting the Underlying Pathology of Multiple Sclerosis

Multiple sclerosis (MS) is a neurodegenerative autoimmune disease characterized by the destruction of the myelin sheath of the neuronal axon in the central nervous system. Many risk factors, including environmental, epigenetic, genetic, and lifestyle factors, are responsible for the development of MS. It has long been thought that only adaptive immune cells, especially autoreactive T cells, are responsible for the pathophysiology; however, recent evidence has indicated that innate immune cells are also highly involved in disease initiation and progression. Here, we compile the available data regarding the role immune cells play in MS, drawn from both human and animal research. While T and B lymphocytes, chiefly enhance MS pathology, regulatory T cells (Tregs) may serve a more protective role, as can B cells, depending on context and location. Cells chiefly involved in innate immunity, including macrophages, microglia, astrocytes, dendritic cells, natural killer (NK) cells, eosinophils, and mast cells, play varied roles. In addition, there is evidence regarding the involvement of innate-like immune cells, such as γδ T cells, NKT cells, MAIT cells, and innate-like B cells as crucial contributors to MS pathophysiology. It is unclear which of these cell subsets are involved in the onset or progression of disease or in protective mechanisms due to their plastic nature, which can change their properties and functions depending on microenvironmental exposure and the response of neural networks in damage control. This highlights the need for a multipronged approach, combining stringently designed clinical data with carefully controlled in vitro and in vivo research findings, to identify the underlying mechanisms so that more effective therapeutics can be developed.

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.

Caspase-8 has dual roles in regulatory T cell homeostasis balancing immunity to infection and collateral inflammatory damage

Targeting the potent immunosuppressive properties of FOXP3⁺ regulatory T cells (T_regs) has substantial therapeutic potential for treating autoimmune and inflammatory diseases. Yet, the molecular mechanisms controlling T_reg homeostasis, particularly during inflammation, remain unclear. We report that caspase-8 is a central regulator of T_reg homeostasis in a context-specific manner that is decisive during immune responses. In mouse genetic models, targeting caspase-8 in T_regs led to accumulation of effector T_regs resistant to apoptotic cell death. Conversely, inflammation induced the MLKL-dependent necroptosis of caspase-8-deficient lymphoid and tissue T_regs, which enhanced immunity to a variety of chronic infections to promote clearance of viral or parasitic pathogens. However, improved immunity came at the risk of lethal inflammation in overwhelming infections. Caspase-8 inhibition using a clinical-stage compound revealed that human T_regs have heightened sensitivity to necroptosis compared with conventional T cells. These findings reveal a fundamental mechanism in T_regs that could be targeted to manipulate the balance between immune tolerance versus response for therapeutic benefit.

C-type lectin receptor DCIR contributes to hippocampal injury in acute neurotropic virus infection

Neurotropic viruses target the brain and contribute to neurologic diseases. C-type lectin receptors (CLRs) are pattern recognition receptors that recognize carbohydrate structures on endogenous molecules and pathogens. The myeloid CLR dendritic cell immunoreceptor (DCIR) is expressed by antigen presenting cells and mediates inhibitory intracellular signalling. To investigate the effect of DCIR on neurotropic virus infection, mice were infected experimentally with Theiler’s murine encephalomyelitis virus (TMEV). Brain tissue of TMEV-infected C57BL/6 mice and DCIR^−/− mice were analysed by histology, immunohistochemistry and RT-qPCR, and spleen tissue by flow cytometry. To determine the impact of DCIR deficiency on T cell responses upon TMEV infection in vitro, antigen presentation assays were utilised. Genetic DCIR ablation in C57BL/6 mice was associated with an ameliorated hippocampal integrity together with reduced cerebral cytokine responses and reduced TMEV loads in the brain. Additionally, absence of DCIR favoured increased peripheral cytotoxic CD8⁺ T cell responses following TMEV infection. Co-culture experiments revealed that DCIR deficiency enhances the activation of antigen-specific CD8⁺ T cells by virus-exposed dendritic cells (DCs), indicated by increased release of interleukin-2 and interferon-γ. Results suggest that DCIR deficiency has a supportive influence on antiviral immune mechanisms, facilitating virus control in the brain and ameliorates neuropathology during acute neurotropic virus infection.

Genetic and immunological contributors to virus-induced paralysis

Infection by a single virus can evoke diverse immune responses, resulting in different neurological outcomes, depending on the host's genetic background. To study heterogenous viral response, we use Theiler's Murine Encephalomyelitis Virus (TMEV) to model virally induced neurological phenotypes and immune responses in Collaborative Cross (CC) mice. The CC resource consists of genetically distinct and reproducible mouse lines, thus providing a population model with genetic heterogeneity similar to humans. We examined different CC strains for the effect of chronic stage TMEV-induced immune responses on neurological outcomes throughout 90 days post infection (dpi), with a particular focus on limb paralysis, by measuring serum levels of 23 different cytokines and chemokines. Each CC strain demonstrated a unique set of immune responses, regardless of presence or absence of TMEV RNA. Using stepwise regression, significant associations were identified between IL-1α, RANTES, and paralysis frequency scores. To better understand these interactions, we evaluated multiple aspects of the different CC genetic backgrounds, including haplotypes of genomic regions previously linked with TMEV pathogenesis and viral clearance or persistence, individual cytokine levels, and TMEV-relevant gene expression. These results demonstrate how loci previously associated with TMEV outcomes provide incomplete information regarding TMEV-induced paralysis in the CC strains. Overall, these findings provide insight into the complex roles of immune response in the pathogenesis of virus-associated neurological diseases influenced by host genetic background.

Efficacy and safety of current medications for treating severe and non-severe COVID-19 patients: an updated network meta-analysis of randomized placebo-controlled trials

BACKGROUND

Many recent studies have investigated the role of drug interventions for coronavirus disease 2019 (COVID-19) infection. However, an important question has been raised about how to select the effective and secure medications for COVID-19 patients. The aim of this analysis was to assess the efficacy and safety of the various medications available for severe and non-severe COVID-19 patients based on randomized placebo-controlled trials (RPCTs).

METHODS

We did an updated network meta-analysis. We searched the databases from inception until July 31, 2021, with no language restrictions. We included RPCTs comparing 49 medications and placebo in the treatment of severe and non-severe patients (aged 18 years or older) with COVID-19 infection. We extracted data on the trial and patient characteristics, and the following primary outcomes: all-cause mortality, the ratios of virological cure, and treatment-emergent adverse events. Odds ratio (OR) and their 95% confidence interval (CI) were used as effect estimates.

RESULTS

From 3,869 publications, we included 61 articles related to 73 RPCTs (57 in non-severe COVID-19 patients and 16 in severe COVID-19 patients), comprising 20,680 patients. The mean sample size was 160 (interquartile range 96-393) in this study. The median duration of follow-up drugs intervention was 28 days (interquartile range 21-30). For increase in virological cure, we only found that proxalutamide (OR 9.16, 95% CI 3.15-18.30), ivermectin (OR 6.33, 95% CI 1.22-32.86), and low dosage bamlanivimab (OR 5.29, 95% CI 1.12-24.99) seemed to be associated with non-severe COVID-19 patients when compared with placebo, in which proxalutamide seemed to be better than low dosage bamlanivimab (OR 5.69, 95% CI 2.43-17.65). For decrease in all-cause mortality, we found that proxalutamide (OR 0.13, 95% CI 0.09-0.19), imatinib (OR 0.49, 95% CI 0.25-0.96), and baricitinib (OR 0.58, 95% CI 0.42-0.82) seemed to be associated with non-severe COVID-19 patients; however, we only found that immunoglobulin gamma (OR 0.27, 95% CI 0.08-0.89) was related to severe COVID-19 patients when compared with placebo. For change in treatment-emergent adverse events, we only found that sotrovimab (OR 0.21, 95% CI 0.13-0.34) was associated with non-severe COVID-19 patients; however, we did not find any medications that presented a statistical difference when compared with placebo among severe COVID-19 patients.

CONCLUSION

We conclude that marked variations exist in the efficacy and safety of medications between severe and non-severe patients with COVID-19. It seems that monoclonal antibodies (e.g., low dosage bamlanivimab, baricitinib, imatinib, and sotrovimab) are a better choice for treating severe or non-severe COVID-19 patients. Clinical decisions to use preferentially medications should carefully consider the risk-benefit profile based on efficacy and safety of all active interventions in patients with COVID-19 at different levels of infection.

The CSF1R-Microglia Axis Has Protective Host-Specific Roles During Neurotropic Picornavirus Infection

Viral encephalitis is a major cause of morbidity and mortality, but the manifestation of disease varies greatly between individuals even in response to the same virus. Microglia are professional antigen presenting cells that reside in the central nervous system (CNS) parenchyma that are poised to respond to viral insults. However, the role of microglia in initiating and coordinating the antiviral response is not completely understood. Utilizing Theiler's murine encephalomyelitis virus (TMEV), a neurotropic picornavirus, and PLX5622, a small molecule inhibitor of colony-stimulating factor 1 receptor (CSF1R) signaling that can deplete microglia in the CNS; we investigated the role of the CSF1R-microglia axis in neurotropic picornavirus infection of C57BL/6J and SJL/J mice. These mouse strains differ in their ability to clear TMEV and exhibit different neurological disease in response to TMEV infection. CSF1R antagonism in C57BL/6J mice, which normally clear TMEV in the CNS, led to acute fatal encephalitis. In contrast, CSF1R antagonism in SJL/J mice, which normally develop a chronic CNS TMEV infection, did not result in acute encephalitis, but exacerbated TMEV-induced demyelination. Immunologically, inhibition of CSF1R in C57BL/6J mice reduced major histocompatibility complex II expression in microglia, decreased the proportion of regulatory T cells in the CNS, and upregulated proinflammatory pathways in CNS T cells. Acute CSF1R inhibition in SJL/J mice had no effect on microglial MHC-II expression and upregulated anti-inflammatory pathways in CNS T cells, however chronic CSF1R inhibition resulted in broad immunosuppression. Our results demonstrate strain-specific effects of the CSF1R-microglia axis in the context of neurotropic viral infection as well as inherent differences in microglial antigen presentation and subsequent T cell crosstalk that contribute to susceptibility to neurotropic picornavirus infection.

Transcriptome analysis following neurotropic virus infection reveals faulty innate immunity and delayed antigen presentation in mice susceptible to virus-induced demyelination

Viral infections of the central nervous system cause acute or delayed neuropathology and clinical consequences ranging from asymptomatic courses to chronic, debilitating diseases. The outcome of viral encephalitis is partially determined by genetically programed immune response patterns of the host. Experimental infection of mice with Theiler's murine encephalomyelitis virus (TMEV) causes diverse neurologic diseases, including TMEV‐induced demyelinating disease (TMEV‐IDD), depending on the used mouse strain. The aim of the present study was to compare initial transcriptomic changes occurring in the brain of TMEV‐infected SJL (TMEV‐IDD susceptible) and C57BL/6 (TMEV‐IDD resistant) mice. Animals were infected with TMEV and sacrificed 4, 7, or 14 days post infection. RNA was isolated from brain tissue and analyzed by whole‐transcriptome sequencing. Selected differences were confirmed on a protein level by immunohistochemistry. In mock‐infected SJL and C57BL/6 mice, >200 differentially expressed genes (DEGs) were detected. Following TMEV‐infection, the number of DEGs increased to >700. Infected C57BL/6 mice showed a higher expression of transcripts related to antigen presentation via major histocompatibility complex (MHC) I, innate antiviral immune responses and cytotoxicity, compared with infected SJL animals. Expression of many of those genes was weaker or delayed in SJL mice, associated with a failure of viral clearance in this mouse strain. SJL mice showed prolonged elevation of MHC II and chemotactic genes compared with C57BL/6 mice, which presumably facilitates the induction of chronic demyelinating disease. In addition, elevated expression of several genes associated with immunomodulatory or –suppressive functions was observed in SJL mice. The exploratory study confirms previous observations in the model and provides an extensive list of new immunologic parameters potentially contributing to different outcomes of viral encephalitis in two mouse strains.

CARD9 Deficiency Increases Hippocampal Injury Following Acute Neurotropic Picornavirus Infection but Does Not Affect Pathogen Elimination

Neurotropic viruses target the brain and contribute to neurologic diseases. Caspase recruitment domain containing family member 9 (CARD9) controls protective immunity in a variety of infectious disorders. To investigate the effect of CARD9 in neurotropic virus infection, CARD9^−/− and corresponding C57BL/6 wild-type control mice were infected with Theiler’s murine encephalomyelitis virus (TMEV). Brain tissue was analyzed by histology, immunohistochemistry and molecular analyses, and spleens by flow cytometry. To determine the impact of CARD9 deficiency on T cell responses in vitro, antigen presentation assays were utilized. Genetic ablation of CARD9 enhanced early pro-inflammatory cytokine responses and accelerated infiltration of T and B cells in the brain, together with a transient increase in TMEV-infected cells in the hippocampus. CARD9^−/− mice showed an increased loss of neuronal nuclear protein⁺ mature neurons and doublecortin⁺ neuronal precursor cells and an increase in β-amyloid precursor protein⁺ damaged axons in the hippocampus. No effect of CARD9 deficiency was found on the initiation of CD8⁺ T cell responses by flow cytometry and co-culture experiments using virus-exposed dendritic cells or microglia-enriched glial cell mixtures, respectively. The present study indicates that CARD9 is dispensable for the initiation of early antiviral responses and TMEV elimination but may contribute to the modulation of neuroinflammation, thereby reducing hippocampal injury following neurotropic virus infection.

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

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

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

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

Rapid Expansion of Virus-Specific CD4+ T Cell Types in the CNS of Susceptible Mice Infected with Theiler's Virus

The infection of susceptible mice with Theiler's murine encephalomyelitis virus (TMEV) induces a T cell-mediated demyelinating disease. This system has been studied as a relevant infection model for multiple sclerosis (MS). Therefore, defining the type of T cell responses and their functions is critically important for understanding the relevant pathogenic mechanisms. In this study, we adoptively transferred naive VP2-specific TCR-Tg CD4⁺ T cells into syngeneic susceptible SJL mice and monitored the development of the disease and the activation and proliferation of CD4⁺ T cells during the early stages of viral infection. The preexisting VP2-specific naive CD4⁺ T cells promoted the pathogenesis of the disease in a dose-dependent manner. The transferred VP2-specific CD4⁺ T cells proliferated rapidly in the CNS starting at 2-3 dpi. High levels of FoxP3⁺CD4⁺ T cells were found in the CNS early in viral infection (3 dpi) and persisted throughout the infection. Activated VP2-specific FoxP3⁺CD4⁺ T cells inhibited the production of IFN-γ, but not IL-17, via the same VP2-specific CD4⁺ T cells without interfering in proliferation. Thus, the early presence of regulatory T cells in the CNS with viral infection may favor the induction of pathogenic Th17 cells over protective Th1 cells in susceptible mice, thereby establishing the pathogenesis of virus-induced demyelinating disease.

Beneficial and Detrimental Effects of Regulatory T Cells in Neurotropic Virus Infections

Neurotropic viruses infect the central nervous system (CNS) and cause acute or chronic neurologic disabilities. Regulatory T cells (Treg) play a critical role for immune homeostasis, but may inhibit pathogen-specific immunity in infectious disorders. The present review summarizes the current knowledge about Treg in human CNS infections and their animal models. Besides dampening pathogen-induced immunopathology, Treg have the ability to facilitate protective responses by supporting effector T cell trafficking to the infection site and the development of resident memory T cells. Moreover, Treg can reduce virus replication by inducing apoptosis of infected macrophages and attenuate neurotoxic astrogliosis and pro-inflammatory microglial responses. By contrast, detrimental effects of Treg are caused by suppression of antiviral immunity, allowing for virus persistence and latency. Opposing disease outcomes following Treg manipulation in different models might be attributed to differences in technique and timing of intervention, infection route, genetic background, and the host’s age. In addition, mouse models of virus-induced demyelination revealed that Treg are able to reduce autoimmunity and immune-mediated CNS damage in a disease phase-dependent manner. Understanding the unique properties of Treg and their complex interplay with effector cells represents a prerequisite for the development of new therapeutic approaches in neurotropic virus infections.

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.

Microglia have a protective role in viral encephalitis-induced seizure development and hippocampal damage

In the central nervous system (CNS), innate immune surveillance is mainly coordinated by microglia. These CNS resident myeloid cells are assumed to help orchestrate the immune response against infections of the brain. However, their specific role in this process and their interactions with CNS infiltrating immune cells, such as blood-borne monocytes and T cells are only incompletely understood. The recent development of PLX5622, a specific inhibitor of colony-stimulating factor 1 receptor that depletes microglia, allows studying the role of microglia in conditions of brain injury such as viral encephalitis, the most common form of brain infection. Here we used this inhibitor in a model of viral infection-induced epilepsy, in which C57BL/6 mice are infected by a picornavirus (Theiler's murine encephalomyelitis virus) and display seizures and hippocampal damage. Our results show that microglia are required early after infection to limit virus distribution and persistence, most likely by modulating T cell activation. Microglia depletion accelerated the occurrence of seizures, exacerbated hippocampal damage, and led to neurodegeneration in the spinal cord, which is normally not observed in this mouse strain. This study enhances our understanding of the role of microglia in viral encephalitis and adds to the concept of microglia-T cell crosstalk.

Signaling pathways and therapeutic perspectives related to environmental factors associated with multiple sclerosis

Multiple sclerosis (MS) is an immune-mediated demyelinating disorder of unknown etiology. Both genetic-susceptibility and environment exposures, including vitamin D deficiency, Epstein-Barr viral and Herpesvirus (HHV-6) infections are strongly implicated in the activation of T cells and MS-pathogenesis. Despite precise knowledge of how these factors could be operating alone or in combination to facilitate and aggravate the disease progression, it is clear that prolonged induction of inflammatory molecules and recruitment of other immune cells by the activated T cells results in demyelination and axonal damage. It is imperative to understand the risk factors associated with MS progression and how these factors contribute to disease pathology. Understanding of the underlying mechanisms of what factors triggers activation of T cells to attack myelin antigen are important to strategize therapeutics and therapies against MS. Current review provides a detailed literature to understand the role of both pathogenic and non-pathogenic factors on the impact of MS.

Intact interleukin-10 receptor signaling protects from hippocampal damage elicited by experimental neurotropic virus infection of SJL mice

Theiler’s murine encephalomyelitis virus (TMEV) infection represents an experimental mouse model to study hippocampal damage induced by neurotropic viruses. IL-10 is a pleiotropic cytokine with profound anti-inflammatory properties, which critically controls immune homeostasis. In order to analyze IL-10R signaling following virus-induced polioencephalitis, SJL mice were intracerebrally infected with TMEV. RNA-based next generation sequencing revealed an up-regulation of Il10, Il10rα and further genes involved in IL-10 downstream signaling, including Jak1, Socs3 and Stat3 in the brain upon infection. Subsequent antibody-mediated blockade of IL-10R signaling led to enhanced hippocampal damage with neuronal loss and increased recruitment of CD3⁺ T cells, CD45R⁺ B cells and an up-regulation of Il1α mRNA. Increased expression of Tgfβ and Foxp3 as well as accumulation of Foxp3⁺ regulatory T cells and arginase-1⁺ macrophages/microglia was detected in the hippocampus, representing a potential compensatory mechanism following disturbed IL-10R signaling. Additionally, an increased peripheral Chi3l3 expression was found in spleens of infected mice, which may embody reactive regulatory mechanisms for prevention of excessive immunopathology. The present study highlights the importance of IL-10R signaling for immune regulation and its neuroprotective properties in the context of an acute neurotropic virus infection.

T-bet, but not Gata3, overexpression is detrimental in a neurotropic viral infection

Intracerebral Theiler's murine encephalomyelitis virus (TMEV) infection in mice induces inflammatory demyelination in the central nervous system. Although C57BL/6 mice normally resistant to TMEV infection with viral clearance, we have previously demonstrated that RORγt-transgenic (tg) C57BL/6 mice, which have Th17-biased responses due to RORγt overexpression in T cells, became susceptible to TMEV infection with viral persistence. Here, using T-bet-tg C57BL/6 mice and Gata3-tg C57BL/6 mice, we demonstrated that overexpression of T-bet, but not Gata3, in T cells was detrimental in TMEV infection. Unexpectedly, T-bet-tg mice died 2 to 3 weeks after infection due to failure of viral clearance. Here, TMEV infection induced splenic T cell depletion, which was associated with lower anti-viral antibody and T cell responses. In contrast, Gata3-tg mice remained resistant, while Gata3-tg mice had lower IFN-γ and higher IL-4 production with increased anti-viral IgG1 responses. Thus, our data identify how overexpression of T-bet and Gata3 in T cells alters anti-viral immunity and confers susceptibility to TMEV infection.

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.

Prostaglandin E2 produced following infection with Theiler's virus promotes the pathogenesis of demyelinating disease

Infection of various cells with Theiler’s murine encephalomyelitis virus (TMEV) activates the TLR- and melanoma differentiation-associated gene 5 (MDA5)-dependent pathways, resulting in the production of IL-1β via the activation of caspase-1 upon assembly of the node-like receptor protein 3 (NLRP3) inflammasome. The role of IL-1β in the pathogenesis of TMEV-induced demyelinating disease was previously investigated. However, the signaling effects of prostaglandin E2 (PGE₂) downstream of the NLRP3 inflammasome on the immune responses to viral determinants and the pathogenesis of demyelinating disease are unknown. In this study, we investigated the levels of intermediate molecules leading to PGE₂ signaling and the effects of blocking PGE₂ signaling on the immune response to TMEV infection, viral persistence and the development of demyelinating disease. We demonstrate here that TMEV infection activates the NLRP3 inflammasome and PGE₂ signaling much more vigorously in dendritic cells (DCs) and CD11b⁺ cells from susceptible SJL mice than in cells from resistant B6 mice. Inhibition of virus-induced PGE₂ signaling using AH23848 resulted in decreased pathogenesis of demyelinating disease and viral loads in the central nervous system (CNS). In addition, AH23848 treatment caused the elevation of protective early IFN-γ-producing CD4⁺ and CD8⁺ T cell responses. Because the levels of IFN-β were lower in AH23848-treated mice but the level of IL-6 was similar, over-production of pathogenic IFN-β was modulated and the generation of IFN-γ-producing T cell responses was enhanced by the inhibition of PGE₂ signaling. These results strongly suggest that excessive activation of the NLRP3 inflammasome and downstream PGE₂ signaling contribute to the pathogenesis of TMEV-induced demyelinating disease.

Gut dysbiosis and neuroimmune responses to brain infection with Theiler's murine encephalomyelitis virus

Recent studies have begun to point out the contribution of microbiota to multiple sclerosis (MS) pathogenesis. Theiler's murine encephalomyelitis virus induced demyelinating disease (TMEV-IDD) is a model of progressive MS. Here, we first analyze the effect of intracerebral infection with TMEV on commensal microbiota and secondly, whether the early microbiota depletion influences the immune responses to TMEV on the acute phase (14 dpi) and its impact on the chronic phase (85 dpi). The intracranial inoculation of TMEV was associated with a moderate dysbiosis. The oral administration of antibiotics (ABX) of broad spectrum modified neuroimmune responses to TMEV dampening brain CD4⁺ and CD8⁺ T infiltration during the acute phase. The expression of cytokines, chemokines and VP2 capsid protein was enhanced and accompanied by clusters of activated microglia disseminated throughout the brain. Furthermore, ABX treated mice displayed lower levels of CD4⁺ and CD8⁺T cells in cervical and mesenteric lymph nodes. Increased mortality to TMEV was observed after ABX cessation at day 28pi. On the chronic phase, mice that survived after ABX withdrawal and recovered microbiota diversity showed subtle changes in brain cell infiltrates, microglia and gene expression of cytokines. Accordingly, the surviving mice of the group ABX-TMEV displayed similar disease severity than TMEV mice.

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

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

Viral-induced suppression of self-reactive T cells: Lessons from neurotropic coronavirus-induced demyelination

Genetic and environmental factors, i.e. infections, have been proposed to contribute to disease induction and relapsing events in multiple sclerosis (MS), an autoimmune demyelinating disease of the central nervous system (CNS). While research has mainly focused on virus associated autoimmune activation, less is known about prevention of autoimmunity, especially following resolving infections associated with CNS tissue damage. This review discusses novel insights on control of self-reactive (SR) T cells activated during neurotropic coronavirus-induced demyelination. A new concept is introduced that SR T cells can be dampened by distinct regulatory mechanisms in the periphery and the CNS, thereby preventing autoimmune disease.

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Virus-induced demyelination activates myelin specific T cells.

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Virus-induced regulatory mechanisms limit pathogenic self-reactive R CD4 T cells.

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Self-reactive CD4 T cells are controlled by distinct mechanisms in the CLN and CNS.

Roles of regulatory T cells and IL-10 in virus-induced demyelination

Neurotropic viruses are important causes of morbidity and mortality in human populations. Some of these viruses preferentially infect oligodendrocytes in the white matter, causing either direct lysis of infected cells, or more commonly myelin damage as a consequence of the host immune response to the virus. Virus-induced demyelination has similarities to the human disease multiple sclerosis. To study this disease process in experimental animals, mice are infected, most commonly, with neurotropic strains of mouse hepatitis virus, a coronavirus or Theiler's murine encephalomyelitis, a picornavirus. While the diseases caused by these two viruses differ in some aspects, in both cases demyelination is a major consequence of the infection. As in autoimmune disease, therapeutic interventions that diminish an overactive immune response would be useful. However, unlike autoimmune disease, complete suppression would result in unchecked virus replication, generally leading to more severe disease. Here we discuss two approaches that dampen but do not fully suppress the host immune response. Regulatory T cells, especially those that are specific for antigens recognized by pathogenic T cells, and IL-10 are two anti-inflammatory/pro-resolution factors that demonstrate efficacy in experimental models of virus-induced demyelination and may be useful in patients infected with viruses that cause demyelination.

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MHV and TMEV are common causes of virus-induced demyelination in mice

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Virus-induced demyelination is often host cell-mediated.

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Virus-specific Treg dampen pathogenic T cells responding to cognate epitope

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IL-10 expressed by Tregs and Tr1 cells dampens pathogenic T cell responses

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Tregs and IL-10 suppress pathogenic T cells without broad immunosuppression

Viral Infection of the Central Nervous System Exacerbates Interleukin-10 Receptor Deficiency-Mediated Colitis in SJL Mice

Theiler´s murine encephalomyelitis virus (TMEV)-infection is a widely used animal model for studying demyelinating disorders, including multiple sclerosis (MS). The immunosuppressive cytokine Interleukin (IL)-10 counteracts hyperactive immune responses and critically controls immune homeostasis in infectious and autoimmune disorders. In order to investigate the effect of signaling via Interleukin-10 receptor (IL-10R) in infectious neurological diseases, TMEV-infected SJL mice were treated with IL-10R blocking antibody (Ab) in the acute and chronic phase of the disease. The findings demonstrate that (i) Ab-mediated IL-10 neutralization leads to progressive colitis with a reduction in Foxp3+ regulatory T cells and increased numbers of CD8+CD44+ memory T cells as well as activated CD4+CD69+ and CD8+CD69+ T cells in uninfected mice. (ii) Concurrent acute TMEV-infection worsened enteric disease-mediated by IL-10R neutralization. Virus-triggered effects were associated with an enhanced activation of CD4+ T helper cells and CD8+ cytotoxic T lymphocytes and augmented cytokine expression. By contrast, (iii) IL-10R neutralization during chronic TMEV-infection was not associated with enhanced peripheral immunopathology but an increased CD3+ T cell influx in the spinal cord. IL-10R neutralization causes a breakdown in peripheral immune tolerance in genetically predisposed mice, which leads to immune-mediated colitis, resembling inflammatory bowel disease. Hyperactive immune state following IL-10R blockade is enhanced by central nervous system-restricted viral infection in a disease phase-dependent manner.

Role of CD4+ T Cells in the Pathophysiology of Multiple Sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system. Although the precise etiology of MS remains unclear, CD4⁺ T cells have been proposed to play not only effector but also regulatory roles in MS. CD4⁺ T cells can be divided into four subsets: pro-inflammatory helper T (Th) 1 and Th17 cells, anti-inflammatory Th2 cells and regulatory T cells (Tregs). The roles of CD4⁺ T cells in MS have been clarified by either “loss-of-function” or “gain-of-function” methods, which have been carried out mainly in autoimmune and viral models of MS: experimental autoimmune encephalomyelitis and Theiler's murine encephalomyelitis virus infection, respectively. Observations in MS patients were consistent with the mechanisms found in the MS models, that is, increased pro-inflammatory Th1 and Th17 activity is associated with disease exacerbation, while anti-inflammatory Th2 cells and Tregs appear to play a protective role.

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

Background

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

Methods

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

Results

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

Conclusions

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

The immune response in the CNS in Theiler's virus induced demyelinating disease switches from an early adaptive response to a chronic innate-like response

Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) is an important model of the progressive disability caused by irreversible CNS tissue injury, and provides an example of how a CNS pathogen can cause inflammation, demyelination, and neuronal damage. We were interested in which molecules, especially inflammatory mediators, might be upregulated in the CNS throughout TMEV-IDD. We quantitated by a real-time RT-PCR multi-gene system the expression of a pathway-focused panel of genes at 30 and 165 days post infection, characterizing both the early inflammatory and the late neurodegenerative stages of TMEV-IDD. Also, we measured 32 cytokines/chemokines by multiplex Luminex analysis in CSF specimens from early and late TMEV-IDD as well as sham-treated mice. Results indicate that, in the later stage of TMEV-IDD, activation of the innate immune response is most prominent: TLRs, type I IFN response genes, and innate immunity-associated cytokines were highly expressed in late TMEV-IDD compared to sham (p ≤ 0.0001) and early TMEV-IDD (p < 0.05). Conversely, several molecular mediators of adaptive immune response were highly expressed in early TMEV-IDD (all p ≤ 0.001). Protein detection in the CSF was broadly concordant with mRNA abundance of the corresponding gene measured by real-time RT-PCR in the spinal cord, since several cytokines/chemokines were increased in the CSF of TMEV-IDD mice. Results show a clear shift from adaptive to innate immunity from early to late TMEV-IDD, indicating that adaptive and innate immune pathways are likely involved in the development and progression of the disease to different extents. CSF provides an optimal source of biomarkers of CNS neuroinflammation.

Deficient Natural Killer Dendritic Cell Responses Underlay the Induction of Theiler's Virus-Induced Autoimmunity

The initiating events in autoimmune disease remain to be completely understood, but it is thought that genetic predisposition synergizes with “environmental” factors, including viral infection, leading to disease. One elegant animal model used to study the pathogenesis of multiple sclerosis that perfectly blends genetics and environmental components in the context of virus-induced autoimmunity is Theiler’s murine encephalitis virus-induced demyelinating disease (TMEV-IDD). TMEV-infected disease-susceptible SJL/J mice develop a persistent central nervous system (CNS) infection and later develop autoimmune demyelination, while disease-resistant C57BL/6 (B6) mice rapidly clear the infection and develop no autoimmune pathology. Mice of the (B6 × SJL/J)F₁ cross between these two mouse strains are classified as intermediately susceptible. We employed this model to investigate if rapid virus clearance in B6 versus SJL/J mice was perhaps related to differences in the innate immune response in the CNS of the two strains in the first few days following intracerebral virus inoculation. Here we show that SJL/J mice lack, in addition to NK cells, a novel innate immune subset known as natural killer dendritic cells (NKDCs), which express phenotypic markers (CD11c^int NK1.1⁺) and functional activity of both NK cells and DCs. These NKDCs are activated in the periphery and migrate into the infected CNS in a very late antigen 4 (VLA-4)-dependent fashion. Most significantly, NKDCs are critical for CNS clearance of TMEV, as transfer of NKDCs purified from B6 mice into TMEV-IDD-susceptible (B6 × SJL/J)F₁ mice promotes viral clearance. Together the findings of this work show for the first time a link between NKDCs, viral infection, and CNS autoimmunity.

Viral infection is an important cofactor, along with genetic susceptibility, in the initiation of a variety of organ-specific autoimmune diseases. Thus, in-depth understanding of how virus infections trigger autoimmunity may lead to novel ways to prevent or treat these diseases. Theiler’s murine encephalitis virus-induced demyelinating disease (TMEV-IDD) serves as an important model for the human T cell-mediated autoimmune demyelinating disease multiple sclerosis. Induction of TMEV-IDD is genetically controlled as SJL/J mice develop persistent central nervous system (CNS) infection leading to chronic autoimmune demyelination, while C57BL/6 mice rapidly clear virus and are disease resistant. We determined that, as opposed to resistant B6 mice, disease-susceptible SJL/J mice lacked a unique innate immune population, the natural killer dendritic cell (NKDC), which was shown to play a critical role in early CNS virus clearance via its ability to both present virus antigen to T cells and to lyse target cells.

Interleukin-10 is a critical regulator of white matter lesion containment following viral induced demyelination

Neurotropic coronavirus induces an acute encephalomyelitis accompanied by focal areas of demyelination distributed randomly along the spinal column. The initial areas of demyelination increase only slightly after the control of infection. These circumscribed focal lesions are characterized by axonal sparing, myelin ingestion by macrophage/microglia, and glial scars associated with hypertrophic astrocytes, which proliferate at the lesion border. Accelerated virus control in mice lacking the anti‐inflammatory cytokine IL‐10 was associated with limited initial demyelination, but low viral mRNA persistence similar to WT mice and declining antiviral cellular immunity. Nevertheless, lesions exhibited sustained expansion providing a model of dysregulated white matter injury temporally remote from the acute CNS insult. Expanding lesions in the absence of IL‐10 are characterized by sustained microglial activation and partial loss of macrophage/microglia exhibiting an acquired deactivation phenotype. Furthermore, IL‐10 deficiency impaired astrocyte organization into mesh like structures at the lesion borders, but did not prevent astrocyte hypertrophy. The formation of discrete foci of demyelination in IL‐10 sufficient mice correlated with IL‐10 receptor expression exclusively on astrocytes in areas of demyelination suggesting a critical role for IL‐10 signaling to astrocytes in limiting expansion of initial areas of white matter damage. GLIA 2015;63:2106–2120

Distinct Immune Responses in Resistant and Susceptible Strains of Mice during Neurovirulent Alphavirus Encephalomyelitis

Susceptibility to alphavirus encephalomyelitis is dependent on a variety of factors, including the genetic background of the host. Neuroadapted Sindbis virus (NSV) causes uniformly fatal disease in adult C57BL/6 (B6) mice, but adult BALB/c (Bc) mice recover from infection. In B6 mice, fatal encephalomyelitis is immune mediated rather than a direct result of virus infection. To identify the immunological determinants of host susceptibility to fatal NSV-induced encephalomyelitis, we compared virus titers and immune responses in adult B6 and Bc mice infected intranasally with NSV. B6 mice had higher levels of virus replication, higher levels of type I interferon (IFN), and slower virus clearance than did Bc mice. B6 mice had more neuronal apoptosis, more severe neurologic disease, and higher mortality than Bc mice. B6 mice had more infiltration of inflammatory cells and higher levels of IL1b, IL-6, TNFa, Csf2, and CCL2 mRNAs and interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-α), IFN-γ, and C-C motif ligand 2 (CCL2) protein in brains than Bc mice. However, Bc mice had more brain antibody at day 7 and a higher percentage of CD4(+) T cells. CD4(+) T cells in the brains of Bc mice included fewer Th17 cells and more regulatory T cells (Tregs) producing IL-10 than B6 mice, accompanied by higher levels of Il2 and Cxcl10 mRNAs. In the absence of IL-10, resistant Bc mice became susceptible to fatal encephalomyelitis after NSV infection. These studies demonstrate the importance of the immune response and its regulation in determining host survival during alphavirus encephalomyelitis.

IMPORTANCE

Mosquito-borne alphavirus infections are an important cause of encephalomyelitis in humans. The severity of disease is dependent both on the strain of the virus and on the age and genetic background of the host. A neurovirulent strain of Sindbis virus causes immune-mediated fatal encephalomyelitis in adult C57BL/6 mice but not in BALB/c mice. To determine the host-dependent immunological mechanisms underlying the differences in susceptibility between these two strains of mice, we compared their immune responses to infection. Resistance to fatal disease in BALB/c mice was associated with better antibody responses, more-rapid virus clearance, fewer Th17 cells, and more-potent regulatory T cell responses than occurred in susceptible C57BL/6 mice. In the absence of interleukin-10, a component of the regulatory immune response, resistant mice became susceptible to lethal disease. This study demonstrates the importance of the immune response and its regulation for host survival during alphavirus encephalomyelitis.

Limited role of regulatory T cells during acute Theiler virus-induced encephalitis in resistant C57BL/6 mice

Background

Theiler’s murine encephalomyelitis virus (TMEV) infection represents a commonly used infectious animal model to study various aspects of the pathogenesis of multiple sclerosis (MS). In susceptible SJL mice, dominant activity of Foxp3⁺ CD4⁺ regulatory T cells (Tregs) in the CNS partly contributes to viral persistence and progressive demyelination. On the other hand, resistant C57BL/6 mice rapidly clear the virus by mounting a strong antiviral immune response. However, very little is known about the role of Tregs in regulating antiviral responses during acute encephalitis in resistant mouse strains.

Methods

In this study, we used DEREG mice that express the diphtheria toxin (DT) receptor under control of the foxp3 locus to selectively deplete Foxp3⁺ Tregs by injection of DT prior to infection and studied the effect of Treg depletion on the course of acute Theiler’s murine encephalomyelitis (TME).

Results

As expected, DEREG mice that are on a C57BL/6 background were resistant to TMEV infection and cleared the virus within days of infection, regardless of the presence or absence of Tregs. Nevertheless, in the absence of Tregs we observed priming of stronger effector T cell responses in the periphery, which subsequently resulted in a transient increase in the frequency of IFNγ-producing T cells in the brain at an early stage of infection. Histological and flow cytometric analysis revealed that this transiently increased frequency of brain-infiltrating IFNγ-producing T cells in Treg-depleted mice neither led to an augmented antiviral response nor enhanced inflammation-mediated tissue damage. Intriguingly, Treg depletion did not change the expression of IL-10 in the infected brain, which might play a role for dampening the inflammatory damage caused by the increased number of effector T cells.

Conclusion

We therefore propose that unlike susceptible mice strains, interfering with the Treg compartment of resistant mice only has negligible effects on virus-induced pathologies in the CNS. Furthermore, in the absence of Tregs, local anti-inflammatory mechanisms might limit the extent of damage caused by strong anti-viral response in the CNS.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0180-9) contains supplementary material, which is available to authorized users.

Protective and detrimental roles for regulatory T cells in a viral model for multiple sclerosis

Multiple sclerosis (MS) has been proposed to be an immune-mediated disease in the central nervous system (CNS) that can be triggered by virus infections. In Theiler's murine encephalomyelitis virus (TMEV) infection, during the first week (acute stage), mice develop polioencephalomyelitis. After 3 weeks (chronic stage), mice develop immune-mediated demyelination with virus persistence, which has been used as a viral model for MS. Regulatory T cells (Tregs) can suppress inflammation, and have been suggested to be protective in immune-mediated diseases, including MS. However, in virus-induced inflammatory demyelination, although Tregs can suppress inflammation, preventing immune-mediated pathology, Tregs may also suppress antiviral immune responses, leading to more active viral replication and/or persistence. To determine the role and potential translational usage of Tregs in MS, we treated TMEV-infected mice with ex vivo generated induced Tregs (iTregs) on day 0 (early) or during the chronic stage (therapeutic). Early treatment worsened clinical signs during acute disease. The exacerbation of acute disease was associated with increased virus titers and decreased immune cell recruitment in the CNS. Therapeutic iTreg treatment reduced inflammatory demyelination during chronic disease. Immunologically, iTreg treatment increased interleukin-10 production from B cells, CD4(+) T cells and dendritic cells, which may contribute to the decreased CNS inflammation.

IL-27 limits central nervous system viral clearance by promoting IL-10 and enhances demyelination

IL-27 is a pleiotropic member of the IL-6 and IL-12 cytokine family composed of the IL-27p28 and the EBV-induced gene 3. IL-27 and its receptor mRNA are both upregulated in the CNS during acute encephalomyelitis induced by the JHM strain of mouse hepatitis virus (JHMV) and sustained during viral persistence. Contributions of IL-27 to viral pathogenesis were evaluated by infection of IL-27Rα-chain-deficient (IL-27Rα(-/-)) mice. The absence of IL-27 signaling accelerated virus control within the CNS associated with increased IFN-γ secreting virus-specific CD4+ and CD8+ T cells. Abrogation of IL-27 signaling did not affect virus-specific CD8+ T cell-mediated IL-10 production or cytolytic activity or Foxp3+ regulatory T cell populations. However, IL-10 production by virus-specific CD4+ T cells was reduced significantly. Despite increased T cell-mediated antiviral function in IL-27Rα(-/-) mice, the virus persisted in the CNS at similar levels as in wild-type mice. Nevertheless, IL-27Rα(-/-) mice exhibited decreased clinical disease during persistence, coincident with less severe demyelination, the hallmark tissue damage associated with JHMV infection. Overall, these data demonstrate that in contrast to viral infections at other sites, IL-27 does not play a proinflammatory role during JHMV-induced encephalomyelitis. Rather, it limits CNS inflammation and impairs control of CNS virus replication via induction of IL-10 in virus-specific CD4+ T cells. Furthermore, in contrast to its protective role in limiting CNS autoimmunity and preventing immunopathology, these data define a detrimental role of IL-27 in promoting demyelination by delaying viral control.

Role of regulatory T cells during virus infection

The host response to viruses includes multiple cell types that have regulatory function. Most information focuses on CD4(+) regulatory T cells that express the transcription factor Foxp3(+) (Tregs), which are the topic of this review. We explain how viruses through specific and non-specific means can trigger the response of thymus-derived natural Tregs as well as induce Tregs. The latter derive under appropriate stimulation conditions either from uncommitted precursors or from differentiated cells that convert to become Tregs. We describe instances where Tregs appear to limit the efficacy of antiviral protective immunity and other, perhaps more common, immune-mediated inflammatory conditions, where the Tregs function to limit the extent of tissue damage that occurs during a virus infection. We discuss the controversial roles that Tregs may play in the pathogenesis of human immunodeficiency and hepatitis C virus infections. The issue of plasticity is discussed, as this may result in Tregs losing their protective function when present in inflammatory environments. Finally, we mention approaches used to manipulate Treg numbers and function and assess their current value and likely future success to manage the outcome of virus infection, especially those that are responsible for chronic tissue damage.

A dietary restriction influences the progression but not the initiation of MSG-Induced nonalcoholic steatohepatitis

The metabolic syndrome is a major worldwide health care issue and a dominant risk factor for cardiovascular disease. The liver manifestations of this syndrome include nonalcoholic fatty liver disease (NAFLD) and its progressive variant nonalcoholic steatohepatitis (NASH). Although significant research has been performed, the basic pathogenesis of NAFLD/NASH remains controversial and effective treatments are still unavailable. We have previously reported on a murine model of NASH induced by the neonatal injection of monosodium glutamate (MSG), which includes the clinical manifestations of central obesity, diabetes, hyperlipidemia, and ultimately liver inflammation, fibrosis, and cancer. Although MSG is considered a safe food additive, its administration to pregnant rats increases the voracity and growth hormone levels in the offspring. To further understand the biology of this model, we have investigated the influence of the calorie intake on these clinical manifestations by feeding animals a restrictive diet. MSG-treated animals fed a restrictive diet continue to manifest obesity and early stage NASH but have improvements in serum lipid profiles. At 12 months of age, mice had manifestations of obesity, whether animals were fed a restricted or control diet, but animals fed a restrictive diet had a reduction in the progression of NASH. In conclusion, MSG appears to be a critical factor in the initiation of obesity, whereas calorie intake may modulate the progression of disease.

Chronic follicular bronchiolitis requires antigen-specific regulatory T cell control to prevent fatal disease progression

To study regulatory T (Treg) cell control of chronic autoimmunity in a lymphoreplete host, we created and characterized a new model of autoimmune lung inflammation that targets the medium and small airways. We generated transgenic mice that express a chimeric membrane protein consisting of hen egg lysozyme and a hemoglobin epitope tag under the control of the Clara cell secretory protein promoter, which largely limited transgene expression to the respiratory bronchioles. When Clara cell secretory protein-membrane hen egg lysozyme/hemoglobin transgenic mice were crossed to N3.L2 TCR transgenic mice that recognize the hemoglobin epitope, the bigenic progeny developed dense, pseudo-follicular lymphocytic peribronchiolar infiltrates that resembled the histological pattern of follicular bronchiolitis. Aggregates of activated IFN-γ- and IL-17A-secreting CD4(+) T cells as well as B cells surrounded the airways. Lung pathology was similar in Ifng(-/-) and Il17a(-/-) mice, indicating that either cytokine is sufficient to establish chronic disease. A large number of Ag-specific Treg cells accumulated in the lesions, and Treg cell depletion in the affected mice led to an interstitial spread of the disease that ultimately proved fatal. Thus, Treg cells act to restrain autoimmune responses, resulting in an organized and controlled chronic pathological process rather than a progressive disease.

Dynamic changes of Foxp3(+) regulatory T cells in spleen and brain of canine distemper virus-infected dogs

Canine distemper virus (CDV) infection causes immunosuppression and demyelinating leukoencephalitis in dogs. In viral diseases, an ambiguous function of regulatory T cells (Treg), with both beneficial effects by reducing immunopathology and detrimental effects by inhibiting antiviral immunity, has been described. However, the role of Treg in the pathogenesis of canine distemper remains unknown. In order to determine the effect of CDV upon immune homeostasis, the amount of Foxp3(+) Treg in spleen and brain of naturally infected dogs has been determined by immunohistochemistry. In addition, splenic cytokine expression has been quantified by reverse transcriptase polymerase chain reaction. Splenic depletion of Foxp3(+) Treg was associated with an increased mRNA-expression of tumor necrosis factor and decreased transcription of interleukin-2 in the acute disease phase, indicative of disturbed immunological counter regulation in peripheral lymphoid organs. In the brain, a lack of Foxp3(+) Treg in predemyelinating and early demyelinating lesions and significantly increased infiltrations of Foxp3(+) Treg in chronic demyelinating lesions were observed. In conclusion, disturbed peripheral and CNS immune regulation associated with a reduction of Treg represents a potential prerequisite for excessive neuroinflammation and early lesion development in canine distemper leukoencephalitis.

Regulatory T cells inhibit Th1 cell-mediated bile duct injury in murine biliary atresia

BACKGROUND & AIMS

Biliary atresia (BA) is a pediatric inflammatory disease of the biliary system which leads to cirrhosis and the need for liver transplantation. One theory regarding etiology is that bile duct injury is due to virus-induced autoreactive T cell-mediated inflammation. Regulatory T cell (Treg) abnormalities in BA could result in unchecked bystander inflammation and autoimmunity targeting bile ducts. The aim of this study was to determine if Tregs are dysfunctional in the rotavirus-induced mouse model of BA (murine BA).

METHODS

Murine BA resulted from infection of BALB/c neonates with Rhesus rotavirus (RRV).

RESULTS

Liver Tregs from BA mice were decreased in number, activation marker expression, and suppressive function. Adoptive transfer studies revealed that RRV-infected mice that received Tregs had significantly increased survival (84%) compared to controls (12.5%). In addition, ablation of Tregs in older mice, followed by RRV infection, resulted in increased bile duct injury.

CONCLUSIONS

These studies demonstrate that dysregulation of Tregs is present in murine BA and that diminished Treg function may be implicated in the pathogenesis of human BA.

Role of CD25(+) CD4(+) T cells in acute and persistent coronavirus infection of the central nervous system

The influence of CD25(+)CD4(+) regulatory T cells (Treg) on acute and chronic viral infection of the central nervous system (CNS) was examined using a glial tropic murine coronavirus. Treg in the CNS were highest during initial T cell mediated virus control, decreased and then remained relatively stable during persistence. Anti-CD25 treatment did not affect CNS recruitment of inflammatory cells. Viral control was initially delayed; however, neither the kinetics of viral control nor viral persistence were affected. By contrast, the absence of Treg during the acute phase resulted in increased demyelination during viral persistence. These data suggest that CNS inflammation, progression of viral control and viral persistence are relatively independent of CD25(+)CD4(+) Treg. However, their absence during acute infection alters the ability of the host to limit tissue damage.

Targeting the B7 family of co-stimulatory molecules: successes and challenges

As more patient data is cross-referenced with animal models of disease, the primary focus on T(h)1 autoreactive effector cell function in autoimmune diseases, such as rheumatoid arthritis and multiple sclerosis, has shifted towards the role of T(h)17 autoreactive effector cells and the ability of regulatory T cells (T(reg)) to modulate the pro-inflammatory autoimmune response. Therefore, the currently favored hypothesis is that a delicate balance between T(h)1/17 effector cells and T(reg) cell function is critical in the regulation of inflammatory autoimmune disease. An intensive area of research with regard to the T(h)1/17:T(reg) cell balance is the utilization of blockade and/or ligation of various co-stimulatory or co-inhibitory molecules, respectively, during ongoing disease to skew the immune response toward a more tolerogenic/regulatory state. Currently, FDA-approved therapies for multiple sclerosis patients are all aimed at the suppression of immune cell function. The other favored method of treatment is a modulation or deletion of autoreactive immune cells via short-term blockade of activating co-stimulatory receptors via treatment with fusion proteins such as CTLA4-Ig and CTLA4-FasL. Based on the initial success of CTLA4-Ig, there are additional fusion proteins that are currently under development. Examples of the more recently identified B7/CD28 family members are PD-L1, PD-L2, inducible co-stimulatory molecule-ligand (ICOS-L), B7-H3, and B7-H4, all of which may emerge as potential fusion protein therapeutics, each with unique, yet often overlapping functions. The expression of both stimulatory and inhibitory B7 molecules seems to play an essential role in modulating immune cell function through a variety of mechanisms, which is supported by findings that suggest each B7 molecule has developed its own indispensable niche in the immune system. As more data are generated, the diagnostic and therapeutic potential of the above B7 family-member-derived fusion proteins becomes ever more apparent. Besides defining the biology of these B7/CD28 family members in vivo, additional difficulty in the development of these therapies lies in maintaining the normal immune functions of recognition and reaction to non-self-antigens following viral or bacterial infection in the patient. Further complicating the clinical translation of these therapies, the mechanism of action identified for a particular reagent may depend upon the method of immune-cell activation and the subset of immune cells targeted in the study.

IL-21 restricts virus-driven Treg cell expansion in chronic LCMV infection

Foxp3⁺ regulatory T (Treg) cells are essential for the maintenance of immune homeostasis and tolerance. During viral infections, Treg cells can limit the immunopathology resulting from excessive inflammation, yet potentially inhibit effective antiviral T cell responses and promote virus persistence. We report here that the fast-replicating LCMV strain Docile triggers a massive expansion of the Treg population that directly correlates with the size of the virus inoculum and its tendency to establish a chronic, persistent infection. This Treg cell proliferation was greatly enhanced in IL-21R^−/− mice and depletion of Treg cells partially rescued defective CD8⁺ T cell cytokine responses and improved viral clearance in some but not all organs. Notably, IL-21 inhibited Treg cell expansion in a cell intrinsic manner. Moreover, experimental augmentation of Treg cells driven by injection of IL-2/anti-IL-2 immune complexes drastically impaired the functionality of the antiviral T cell response and impeded virus clearance. As a consequence, mice became highly susceptible to chronic infection following exposure to low virus doses. These findings reveal virus-driven Treg cell proliferation as potential evasion strategy that facilitates T cell exhaustion and virus persistence. Furthermore, they suggest that besides its primary function as a direct survival signal for antiviral CD8⁺ T cells during chronic infections, IL-21 may also indirectly promote CD8⁺ T cell poly-functionality by restricting the suppressive activity of infection-induced Treg cells.

T cell exhaustion represents a state of T cell dysfunction associated with clinically relevant diseases, such as persistent viral infections or cancer. Although the molecular signature of exhausted T cells has been characterized in detail at the functional and transcriptional level, the immunological mechanisms that lead to T cell exhaustion during chronic infections remain poorly understood. Our present study reports two major findings that illustrate a pathway that contributes to T cell exhaustion during viral infection, and indicate its modulation by both, the pathogen and the host. First, we show that a persistence-inducing virus triggers the massive proliferation of Foxp3⁺ regulatory T (Treg) cells and demonstrate the potential of Treg cells to promote T cell exhaustion and chronic infection. Second, we identify IL-21 as a crucial host factor that antagonizes this virus-driven expansion of the Treg population in a cell intrinsic manner independent of IL-2. Thus, in addition to its known pre-dominant direct positive effects on antiviral T cells, IL-21 can also alleviate the suppressive activity of Treg cells. Together, these results suggest enhanced Treg cell responses as a mechanism of immune evasion that could be therapeutically targeted with IL-21.

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.

Autoimmunity in 2011

The mechanisms leading to the onset and perpetuation of systemic and tissue-specific autoimmune diseases are complex, and numerous hypotheses have been proposed or confirmed over the past 12 months. It is particularly of note that the number of articles published during 2011 in the major immunology and autoimmunity journals increased by 3 % compared to the previous year. The present article is dedicated to a brief review of the reported data and, albeit not comprehensive of all articles, is aimed at identifying common and future themes. First, clinical researchers were particularly dedicated to defining refractory forms of diseases and to discuss the use and switch of therapeutic monoclonal antibodies in everyday practice. Second, following the plethora of genome-wide association studies reported in most multifactorial diseases, it became clear that genomics cannot fully explain the individual susceptibility and additional environmental or epigenetic factors are necessary. Both these components were widely investigated, both in organ-specific (i.e., type 1 diabetes) and systemic (i.e., systemic lupus erythematosus) diseases. Third, a large number of 2011 works published in the autoimmunity area are dedicated to dissect pathogenetic mechanisms of tolerance breakdown in general or in specific conditions. While our understanding of T regulatory and Th17 cells has significantly increased in 2011, it is of note that most of the proposed lines of evidence identify potential targets for future treatments and should not be overlooked.

Immune heterogeneity in neuroinflammation: dendritic cells in the brain

Dendritic cells (DC) are critical to an integrated immune response and serve as the key link between the innate and adaptive arms of the immune system. Under steady state conditions, brain DC’s act as sentinels, continually sampling their local environment. They share this function with macrophages derived from the same basic hemopoietic (bone marrow-derived) precursor and with parenchymal microglia that arise from a unique non-hemopoietic origin. While multiple cells may serve as antigen presenting cells (APCs), dendritic cells present both foreign and self-proteins to naïve T cells that, in turn, carry out effector functions that serve to protect or destroy. The resulting activation of the adaptive response is a critical step to resolution of injury or infection and is key to survival. In this review we will explore the critical roles that DCs play in the brain’s response to neuroinflammatory disease with emphasis on how the brain’s microenvironment impacts these actions.

IL-1 signal affects both protection and pathogenesis of virus-induced chronic CNS demyelinating disease

Background

Theiler’s virus infection induces chronic demyelinating disease in mice and has been investigated as an infectious model for multiple sclerosis (MS). IL-1 plays an important role in the pathogenesis of both the autoimmune disease model (EAE) and this viral model for MS. However, IL-1 is known to play an important protective role against certain viral infections. Therefore, it is unclear whether IL-1-mediated signaling plays a protective or pathogenic role in the development of TMEV-induced demyelinating disease.

Methods

Female C57BL/6 mice and B6.129S7-Il1r1^tm1Imx/J mice (IL-1R KO) were infected with Theiler’s murine encephalomyelitis virus (1 x 10⁶ PFU). Differences in the development of demyelinating disease and changes in the histopathology were compared. Viral persistence, cytokine production, and immune responses in the CNS of infected mice were analyzed using quantitative PCR, ELISA, and flow cytometry.

Results

Administration of IL-1β, thereby rending resistant B6 mice susceptible to TMEV-induced demyelinating disease, induced a high level of Th17 response. Interestingly, infection of TMEV into IL-1R-deficient resistant C57BL/6 (B6) mice also induced TMEV-induced demyelinating disease. High viral persistence was found in the late stage of viral infection in IL-1R-deficient mice, although there were few differences in the initial anti-viral immune responses and viral persistent levels between the WT B6 and IL-1R-deficiecent mice. The initial type I IFN responses and the expression of PDL-1 and Tim-3 were higher in the CNS of TMEV-infected IL-1R-deficient mice, leading to deficiencies in T cell function that permit viral persistence.

Conclusions

These results suggest that the presence of high IL-1 level exerts the pathogenic role by elevating pathogenic Th17 responses, whereas the lack of IL-1 signals promotes viral persistence in the spinal cord due to insufficient T cell activation by elevating the production of inhibitory cytokines and regulatory molecules. Therefore, the balance of IL-1 signaling appears to be extremely important for the protection from TMEV-induced demyelinating disease, and either too much or too little signaling promotes the development of disease.