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

Orchestration of antiviral responses within the infected central nervous system

Many newly emerging and re-emerging viruses have neuroinvasive potential, underscoring viral encephalitis as a global research priority. Upon entry of the virus into the CNS, severe neurological life-threatening conditions may manifest that are associated with high morbidity and mortality. The currently available therapeutic arsenal against viral encephalitis is rather limited, emphasizing the need to better understand the conditions of local antiviral immunity within the infected CNS. In this review, we discuss new insights into the pathophysiology of viral encephalitis, with a focus on myeloid cells and CD8+ T cells, which critically contribute to protection against viral CNS infection. By illuminating the prerequisites of myeloid and T cell activation, discussing new discoveries regarding their transcriptional signatures, and dissecting the mechanisms of their recruitment to sites of viral replication within the CNS, we aim to further delineate the complexity of antiviral responses within the infected CNS. Moreover, we summarize the current knowledge in the field of virus infection and neurodegeneration and discuss the potential links of some neurotropic viruses with certain pathological hallmarks observed in neurodegeneration.

Contribution of microglia/macrophage to the pathogenesis of TMEV infection in the central nervous system

The infection of the central nervous system (CNS) with neurotropic viruses induces neuroinflammation and an immune response, which is associated with the development of neuroinflammatory and neurodegenerative diseases, including multiple sclerosis (MS). The activation of both innate and adaptive immune responses, involving microglia, macrophages, and T and B cells, while required for efficient viral control within the CNS, is also associated with neuropathology. Under pathological events, such as CNS viral infection, microglia/macrophage undergo a reactive response, leading to the infiltration of immune cells from the periphery into the brain, disrupting CNS homeostasis and contributing to the pathogenesis of disease. The Theiler's murine encephalomyelitis virus (TMEV)-induced demyelination disease (TMEV-IDD), which serves as a mouse model of MS. This murine model made significant contributions to our understanding of the pathophysiology of MS following subsequent to infection. Microglia/macrophages could be activated into two different states, classic activated state (M1 state) and alternative activated state (M2 state) during TMEV infection. M1 possesses the capacity to initiate inflammatory response and secretes pro-inflammatory cytokines, and M2-liked microglia/macrophages are anti-inflammatory characterized by the secretion of anti-inflammatory cytokines. This review aims to discuss the roles of microglia/macrophages M1/M2-liked polarization during TMEV infection, and explore the potential therapeutic effect of balancing M1/M2-liked polarization of microglia/macrophages on MS.

Increased expression of chondroitin sulfate proteoglycans in dentate gyrus and amygdala causes postinfectious seizures

Alterations in the extracellular matrix are common in patients with epilepsy and animal models of epilepsy, yet whether they are the cause or consequence of seizures and epilepsy development is unknown. Using Theiler's murine encephalomyelitis virus (TMEV) infection-induced model of acquired epilepsy, we found de novo expression of chondroitin sulfate proteoglycans (CSPGs), a major extracellular matrix component, in dentate gyrus (DG) and amygdala exclusively in mice with acute seizures. Preventing the synthesis of CSPGs specifically in DG and amygdala by deletion of the major CSPG aggrecan reduced seizure burden. Patch-clamp recordings from dentate granule cells revealed enhanced intrinsic and synaptic excitability in seizing mice that was significantly ameliorated by aggrecan deletion. In situ experiments suggested that dentate granule cell hyperexcitability results from negatively charged CSPGs increasing stationary cations on the membrane, thereby depolarizing neurons, increasing their intrinsic and synaptic excitability. These results show increased expression of CSPGs in the DG and amygdala as one of the causal factors for TMEV-induced acute seizures. We also show identical changes in CSPGs in pilocarpine-induced epilepsy, suggesting that enhanced CSPGs in the DG and amygdala may be a common ictogenic factor and potential therapeutic target.

Virus-Induced Epilepsy vs. Epilepsy Patients Acquiring Viral Infection: Unravelling the Complex Relationship for Precision Treatment

The intricate relationship between viruses and epilepsy involves a bidirectional interaction. Certain viruses can induce epilepsy by infecting the brain, leading to inflammation, damage, or abnormal electrical activity. Conversely, epilepsy patients may be more susceptible to viral infections due to factors, such as compromised immune systems, anticonvulsant drugs, or surgical interventions. Neuroinflammation, a common factor in both scenarios, exhibits onset, duration, intensity, and consequence variations. It can modulate epileptogenesis, increase seizure susceptibility, and impact anticonvulsant drug pharmacokinetics, immune system function, and brain physiology. Viral infections significantly impact the clinical management of epilepsy patients, necessitating a multidisciplinary approach encompassing diagnosis, prevention, and treatment of both conditions. We delved into the dual dynamics of viruses inducing epilepsy and epilepsy patients acquiring viruses, examining the unique features of each case. For virus-induced epilepsy, we specify virus types, elucidate mechanisms of epilepsy induction, emphasize neuroinflammation's impact, and analyze its effects on anticonvulsant drug pharmacokinetics. Conversely, in epilepsy patients acquiring viruses, we detail the acquired virus, its interaction with existing epilepsy, neuroinflammation effects, and changes in anticonvulsant drug pharmacokinetics. Understanding this interplay advances precision therapies for epilepsy during viral infections, providing mechanistic insights, identifying biomarkers and therapeutic targets, and supporting optimized dosing regimens. However, further studies are crucial to validate tools, discover new biomarkers and therapeutic targets, and evaluate targeted therapy safety and efficacy in diverse epilepsy and viral infection scenarios.

How to Use the Cuprizone Model to Study De- and Remyelination

Multiple sclerosis (MS) is an autoimmune and inflammatory disorder affecting the central nervous system whose cause is still largely unknown. Oligodendrocyte degeneration results in demyelination of axons, which can eventually be repaired by a mechanism called remyelination. Prevention of demyelination and the pharmacological support of remyelination are two promising strategies to ameliorate disease progression in MS patients. The cuprizone model is commonly employed to investigate oligodendrocyte degeneration mechanisms or to explore remyelination pathways. During the last decades, several different protocols have been applied, and all have their pros and cons. This article intends to offer guidance for conducting pre-clinical trials using the cuprizone model in mice, focusing on discovering new treatment approaches to prevent oligodendrocyte degeneration or enhance remyelination.

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

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

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.

Theiler's Murine Encephalomyelitis Virus Replicates in Primary Neuron Cultures and Impairs Spine Density Formation

In this study, we examined infection with the highly neurovirulent GDVII, the less neurovirulent DA strains, and with a mutant DA, which lacks the L* protein (L*-1) involved in viral persistence and demyelinating disease, to analyze the direct effects of Theiler's murine encephalomyelitis virus (TMEV) replication using primary cultures of mouse brain hippocampal neurons. All viruses replicate in cultured neurons, with GDVII having the highest titers and L*-1 the lowest. Accordingly, all were positive for viral antigen staining 3 days postinfection (dpi), and DA and L*-1 were also positive after 12 dpi. NeuN + immunostaining showed an early and almost complete absence of positive cells in cultures infected with GDVII, an approximately 50% reduction in cultures infected with DA, and fewer changes in L*-1 strains at 3 dpi. Accordingly, staining with chloromethyltetramethylrosamine orange (Mitotracker OrangeTM) as a parameter for cell viability showed similar results. Moreover, at 1 dpi, the strain DA induced higher transcript levels of neuroprotective genes such as IFN-Iβ, IRF7, and IRF8. At 3 dpi, strains GDVII and DA, but not the L*-1 mutant, showed lower PKR expression. In addition, confocal analysis showed that L*-1-infected neurons exhibited a decrease in spine density. Treatment with poly (I:C), which is structurally related to dsRNA and is known to trigger IFN type I synthesis, reduced spine density even more. These results confirmed the use of mouse hippocampal neuron cultures as a model to study neuronal responses after TMEV infection, particularly in the formation of spine density.

C57BL/6J mice exposed to perfluorooctanoic acid demonstrate altered immune responses and increased seizures after Theiler's murine encephalomyelitis virus infection

Introduction

Neurological diseases can stem from environmental influences such as antecedent viral infections or exposure to potential toxicants, some of which can trigger immune responses leading to neurological symptoms. Theiler's murine encephalomyelitis virus (TMEV) is used to model human neurological conditions associated with prior viral infections, with outcomes partly attributable to improper induction and regulation of the immune response. Perfluorooctanoic acid (PFOA) can alter pathologies known to influence neurological disease such as inflammatory responses, cytokine expression, and glial activation. Co-exposure to TMEV and PFOA was used to test the hypothesis that early life exposure to the potential immunotoxicant PFOA would affect immune responses so as to render TMEV-resistant C57BL/6J (B6) mice susceptible to viral-induced neurological disease.

Methods

Neonate B6 mice were exposed to different treatments: non-injected, sham-infected with PBS, and TMEV-infected, with the drinking water of each group including either 70 ppt PFOA or filtered water. The effects of PFOA were evaluated by comparing neurological symptoms and changes in immune-related cytokine and chemokine production induced by viral infection. Immune responses of 23 cytokines and chemokines were measured before and after infection to determine the effects of PFOA exposure on immune response.

Results

Prior to infection, an imbalance between Th1, Th2, and Treg cytokines was observed in PFOA-exposed mice, suppressing IL-4 and IL-13 production. However, the balance was restored and characterized by an increase in pro-inflammatory cytokines in the non-infected group, and a decrease in IL-10 in the PFOA + TMEV group. Furthermore, the PFOA + TMEV group experienced an increase in seizure frequency and severity.

Discussion

Overall, these findings provide insight into the complex roles of immune responses in the pathogenesis of virus-associated neurological diseases influenced by co-exposures to viruses and immunotoxic compounds.

RNA-binding proteins as a common ground for neurodegeneration and inflammation in amyotrophic lateral sclerosis and multiple sclerosis

The neurodegenerative and inflammatory illnesses of amyotrophic lateral sclerosis and multiple sclerosis were once thought to be completely distinct entities that did not share any remarkable features, but new research is beginning to reveal more information about their similarities and differences. Here, we review some of the pathophysiological features of both diseases and their experimental models: RNA-binding proteins, energy balance, protein transportation, and protein degradation at the molecular level. We make a thorough analysis on TDP-43 and hnRNP A1 dysfunction, as a possible common ground in both pathologies, establishing a potential link between neurodegeneration and pathological immunity. Furthermore, we highlight the putative variations that diverge from a common ground in an atemporal course that proposes three phases for all relevant molecular events.

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

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

Infection-induced epilepsy is caused by increased expression of chondroitin sulfate proteoglycans in hippocampus and amygdala

Alterations in the extracellular matrix (ECM) are common in epilepsy, yet whether they are cause or consequence of disease is unknow. Using Theiler's virus infection model of acquired epilepsy we find de novo expression of chondroitin sulfate proteoglycans (CSPGs), a major ECM component, in dentate gyrus (DG) and amygdala exclusively in mice with seizures. Preventing synthesis of CSPGs specifically in DG and amygdala by deletion of major CSPG aggrecan reduced seizure burden. Patch-clamp recordings from dentate granule cells (DGCs) revealed enhanced intrinsic and synaptic excitability in seizing mice that was normalized by aggrecan deletion. In situ experiments suggest that DGCs hyperexcitability results from negatively charged CSPGs increasing stationary cations (K+, Ca2+) on the membrane thereby depolarizing neurons, increasing their intrinsic and synaptic excitability. We show similar changes in CSPGs in pilocarpine-induced epilepsy suggesting enhanced CSPGs in the DG and amygdala may be a common ictogenic factor and novel therapeutic potential.

Contribution of Intravital Neuroimaging to Study Animal Models of Multiple Sclerosis

Multiple sclerosis (MS) is a complex and long-lasting neurodegenerative disease of the central nervous system (CNS), characterized by the loss of myelin within the white matter and cortical fibers, axonopathy, and inflammatory responses leading to consequent sensory-motor and cognitive deficits of patients. While complete resolution of the disease is not yet a reality, partial tissue repair has been observed in patients which offers hope for therapeutic strategies. To address the molecular and cellular events of the pathomechanisms, a variety of animal models have been developed to investigate distinct aspects of MS disease. Recent advances of multiscale intravital imaging facilitated the direct in vivo analysis of MS in the animal models with perspective of clinical transfer to patients. This review gives an overview of MS animal models, focusing on the current imaging modalities at the microscopic and macroscopic levels and emphasizing the importance of multimodal approaches to improve our understanding of the disease and minimize the use of animals.

B cells going viral in the CNS: Dynamics, complexities, and functions of B cells responding to viral encephalitis

A diverse number of DNA and RNA viruses have the potential to invade the central nervous system (CNS), causing inflammation and injury to cells that have a limited capacity for repair and regeneration. While rare, viral encephalitis in humans is often fatal and survivors commonly suffer from permanent neurological sequelae including seizures. Established treatment options are extremely limited, predominantly relying on vaccines, antivirals, or supportive care. Many viral CNS infections are characterized by the presence of antiviral antibodies in the cerebral spinal fluid (CSF), indicating local maintenance of protective antibody secreting cells. However, the mechanisms maintaining these humoral responses are poorly characterized. Furthermore, while both viral and autoimmune encephalitis are associated with the recruitment of diverse B cell subsets to the CNS, their protective and pathogenic roles aside from antibody production are just beginning to be understood. This review will focus on the relevance of B cell responses to viral CNS infections, with an emphasis on the importance of intrathecal immunity and the potential contribution to autoimmunity. Specifically, it will summarize the newest data characterizing B cell activation, differentiation, migration, and localization in clinical samples as well as experimental models of acute and persistent viral encephalitis.

Viral Clearance and Neuroinflammation in Acute TMEV Infection Vary by Host Genetic Background

A wide range of viruses cause neurological manifestations in their hosts. Infection by neurotropic viruses as well as the resulting immune response can irreversibly disrupt the complex structural and functional architecture of the brain, depending in part on host genetic background. The interaction between host genetic background, neurological response to viral infection, and subsequent clinical manifestations remains poorly understood. In the present study, we used the genetically diverse Collaborative Cross (CC) mouse resource to better understand how differences in genetic background drive clinical signs and neuropathological manifestations of acute Theiler's murine encephalomyelitis virus (TMEV) infection. For the first time, we characterized variations of TMEV viral tropism and load based on host genetic background, and correlated viral load with microglial/macrophage activation. For five CC strains (CC002, CC023, CC027, CC057, and CC078) infected with TMEV, we compared clinical signs, lesion distribution, microglial/macrophage response, expression, and distribution of TMEV mRNA, and identified genetic loci relevant to the early acute (4 days post-infection [dpi]) and late acute (14 dpi) timepoints. We examined brain pathology to determine possible causes of strain-specific differences in clinical signs, and found that fields CA1 and CA2 of the hippocampal formation were especially targeted by TMEV across all strains. Using Iba-1 immunolabeling, we identified and characterized strain- and timepoint-specific variation in microglial/macrophage reactivity in the hippocampal formation. Because viral clearance can influence disease outcome, we used RNA in situ hybridization to quantify viral load and TMEV mRNA distribution at both timepoints. TMEV mRNA expression was broadly distributed in the hippocampal formation at 4 dpi in all strains but varied between radiating and clustered distribution depending on the CC strain. We found a positive correlation between microglial/macrophage reactivity and TMEV mRNA expression at 4 dpi. At 14 dpi, we observed a dramatic reduction in TMEV mRNA expression, and localization to the medial portion of field CA1 and field CA2. To better understand how host genetic background can influence pathological outcomes, we identified quantitative trait loci associated with frequency of lesions in a particular brain region and with microglial/macrophage reactivity. These QTL were located near several loci of interest: lysosomal trafficking regulator (Lyst) and nidogen 1 (Nid1), and transmembrane protein 106 B (Tmem106b). Together, these results provide a novel understanding about the influences of genetic variation on the acute neuropathological and immunopathological environment and viral load, which collectively lead to variable disease outcomes. Our findings reveal possible avenues for future investigation which may lead to more effective intervention strategies and treatment regimens.

Serum Cytokines Predict Neurological Damage in Genetically Diverse Mouse Models

Viral infections contribute to neurological and immunological dysfunction driven by complex genetic networks. Theiler's murine encephalomyelitis virus (TMEV) causes neurological dysfunction in mice and can model human outcomes to viral infections. Here, we used genetically distinct mice from five Collaborative Cross mouse strains and C57BL/6J to demonstrate how TMEV-induced immune responses in serum may predict neurological outcomes in acute infection. To test the hypothesis that serum cytokine levels can provide biomarkers for phenotypic outcomes of acute disease, we compared cytokine levels at pre-injection, 4 days post-injection (d.p.i.), and 14 d.p.i. Each strain produced unique baseline cytokine levels and had distinct immune responses to the injection procedure itself. Thus, we eliminated the baseline responses to the injection procedure itself and identified cytokines and chemokines induced specifically by TMEV infection. Then, we identified strain-specific longitudinal cytokine profiles in serum during acute disease. Using stepwise regression analysis, we identified serum immune markers predictive for TMEV-induced neurological phenotypes of the acute phase, e.g., IL-9 for limb paralysis; and TNF-α, IL-1β, and MIP-1β for limb weakness. These findings indicate how temporal differences in immune responses are influenced by host genetic background and demonstrate the potential of serum biomarkers to track the neurological effects of viral infection.

Molecular Mechanisms in the Genesis of Seizures and Epilepsy Associated With Viral Infection

Seizures are a common presenting symptom during viral infections of the central nervous system (CNS) and can occur during the initial phase of infection ("early" or acute symptomatic seizures), after recovery ("late" or spontaneous seizures, indicating the development of acquired epilepsy), or both. The development of acute and delayed seizures may have shared as well as unique pathogenic mechanisms and prognostic implications. Based on an extensive review of the literature, we present an overview of viruses that are associated with early and late seizures in humans. We then describe potential pathophysiologic mechanisms underlying ictogenesis and epileptogenesis, including routes of neuroinvasion, viral control and clearance, systemic inflammation, alterations of the blood-brain barrier, neuroinflammation, and inflammation-induced molecular reorganization of synapses and neural circuits. We provide clinical and animal model findings to highlight commonalities and differences in these processes across various neurotropic or neuropathogenic viruses, including herpesviruses, SARS-CoV-2, flaviviruses, and picornaviruses. In addition, we extensively review the literature regarding Theiler's murine encephalomyelitis virus (TMEV). This picornavirus, although not pathogenic for humans, is possibly the best-characterized model for understanding the molecular mechanisms that drive seizures, epilepsy, and hippocampal damage during viral infection. An enhanced understanding of these mechanisms derived from the TMEV model may lead to novel therapeutic interventions that interfere with ictogenesis and epileptogenesis, even within non-infectious contexts.

Animal Models of Epilepsy: A Phenotype-oriented Review

Epilepsy is a serious neurological disorder characterized by abnormal, recurrent, and synchronous discharges in the brain. Long-term recurrent seizure attacks can cause serious damage to brain function, which is usually observed in patients with temporal lobe epilepsy. Controlling seizure attacks is vital for the treatment and prognosis of epilepsy. Animal models, such as the kindling model, which was the most widely used model in the past, allow the understanding of the potential epileptogenic mechanisms and selection of antiepileptic drugs. In recent years, various animal models of epilepsy have been established to mimic different seizure types, without clear merits and demerits. Accordingly, this review provides a summary of the views mentioned above, aiming to provide a reference for animal model selection.

Microglia and monocytes in inflammatory CNS disease: integrating phenotype and function

In neurological diseases, the actions of microglia, the resident myeloid cells of the CNS parenchyma, may diverge from, or intersect with, those of recruited monocytes to drive immune-mediated pathology. However, defining the precise roles of each cell type has historically been impeded by the lack of discriminating markers and experimental systems capable of accurately identifying them. Our ability to distinguish microglia from monocytes in neuroinflammation has advanced with single-cell technologies, new markers and drugs that identify and deplete them, respectively. Nevertheless, the focus of individual studies on particular cell types, diseases or experimental approaches has limited our ability to connect phenotype and function more widely and across diverse CNS pathologies. Here, we critically review, tabulate and integrate the disease-specific functions and immune profiles of microglia and monocytes to provide a comprehensive atlas of myeloid responses in viral encephalitis, demyelination, neurodegeneration and ischemic injury. In emphasizing the differential roles of microglia and monocytes in the severe neuroinflammatory disease of viral encephalitis, we connect inflammatory pathways common to equally incapacitating diseases with less severe inflammation. We examine these findings in the context of human studies and highlight the benefits and inherent limitations of animal models that may impede or facilitate clinical translation. This enables us to highlight common and contrasting, non-redundant and often opposing roles of microglia and monocytes in disease that could be targeted therapeutically.

Inflammatory monocytes and microglia play independent roles in inflammatory ictogenesis

BACKGROUND

The pathogenic contribution of neuroinflammation to ictogenesis and epilepsy may provide a therapeutic target for reduction of seizure burden in patients that are currently underserved by traditional anti-seizure medications. The Theiler's murine encephalomyelitis virus (TMEV) model has provided important insights into the role of inflammation in ictogenesis, but questions remain regarding the relative contribution of microglia and inflammatory monocytes in this model.

METHODS

Female C57BL/6 mice were inoculated by intracranial injection of 2 × 105, 5 × 104, 1.25 × 104, or 3.125 × 103 plaque-forming units (PFU) of the Daniel's strain of TMEV at 4-6 weeks of age. Infiltration of inflammatory monocytes, microglial activation, and cytokine production were measured at 24 h post-infection (hpi). Viral load, hippocampal injury, cognitive performance, and seizure burden were assessed at several timepoints.

RESULTS

The intensity of inflammatory infiltration and the extent of hippocampal injury induced during TMEV encephalitis scaled with the amount of infectious virus in the initial inoculum. Cognitive performance was preserved in mice inoculated with 1.25 × 104 PFU TMEV relative to 2 × 105 PFU TMEV, but peak viral load at 72 hpi was equivalent between the inocula. CCL2 production in the brain was attenuated by 90% and TNFα and IL6 production was absent in mice inoculated with 1.25 × 104 PFU TMEV. Acute infiltration of inflammatory monocytes was attenuated by more than 80% in mice inoculated with 1.25 × 104 PFU TMEV relative to 2 × 105 PFU TMEV but microglial activation was equivalent between groups. Seizure burden was attenuated and the threshold to kainic acid-induced seizures was higher in mice inoculated with 1.25 × 104 PFU TMEV but low-level behavioral seizures persisted and the EEG exhibited reduced but detectable abnormalities.

CONCLUSIONS

The size of the inflammatory monocyte response induced by TMEV scales with the amount of infectious virus in the initial inoculum, despite the development of equivalent peak infectious viral load. In contrast, the microglial response does not scale with the inoculum, as microglial hyper-ramification and increased Iba-1 expression were evident in mice inoculated with either 1.25 × 104 or 2 × 105 PFU TMEV. Inoculation conditions that drive inflammatory monocyte infiltration resulted in robust behavioral seizures and EEG abnormalities, but the low inoculum condition, associated with only microglial activation, drove a more subtle seizure and EEG phenotype.

Inflammation Unleashed in Viral-Induced Epileptogenesis

Viral infection of the central nervous system increasingly places people at risk of developing life-threatening and treatment-resistant acute and chronic seizures (epilepsy). The emergence of new human viruses due to ongoing social, political, and ecological changes places people at risk more than ever before. The development of new preventative or curative strategies is critical to address this burden. However, our understanding of the complex relationship between viruses and the brain has been hindered by the lack of animal models that survive the initial infection and are amenable for long-term mechanistic, behavioral, and pharmacological studies in the process of viral-induced epileptogenesis. In this review, we focus on the Theiler’s murine encephalomyelitis virus (TMEV) mouse model of viral infection–induced epilepsy. The TMEV model has a number of important advantages to address the quintessential processes underlying the development of epilepsy following a viral infection, as well as fuel new therapeutic development. In this review, we highlight the contributions of the TMEV model to our current understanding of the relationship between viral infection, inflammation, and seizures.

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.

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.

Host genetic diversity drives variable central nervous system lesion distribution in chronic phase of Theiler's Murine Encephalomyelitis Virus (TMEV) infection

Host genetic background is a significant driver of the variability in neurological responses to viral infection. Here, we leverage the genetically diverse Collaborative Cross (CC) mouse resource to better understand how chronic infection by Theiler's Murine Encephalomyelitis Virus (TMEV) elicits diverse clinical and morphologic changes in the central nervous system (CNS). We characterized the TMEV-induced clinical phenotype responses, and associated lesion distributions in the CNS, in six CC mouse strains over a 90 day infection period. We observed varying degrees of motor impairment in these strains, as measured by delayed righting reflex, paresis, paralysis, seizures, limb clasping, ruffling, and encephalitis phenotypes. All strains developed neuroparenchymal necrosis and mineralization in the brain, primarily localized to the hippocampal regions. Two of the six strains presented with axonal degeneration with myelin loss of the nerve roots in the lumbar spinal cord. Moreover, we statistically correlated lesion distribution with overall frequencies of clinical phenotypes and phenotype progression to better understand how and where TMEV targets the CNS, based on genetic background. Specifically, we assessed lesion distribution in relation to the clinical progression of these phenotypes from early to late TMEV disease, finding significant relationships between progression and lesion distribution. Finally, we identified quantitative trait loci associated with frequency of lesions in a particular brain region, revealing several loci of interest for future study: lysosomal trafficking regulator (Lyst) and nidogen 1 (Nid1). Together, these results indicate that the genetic background influences the type and severity of clinical phenotypes, phenotypic resilience to TMEV, and the lesion distribution across strains.

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.

Animal Models of Multiple Sclerosis

Animal models with high translational validity are essential tools in understanding disease pathogenesis and in the development of therapeutic strategies. Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system characterized by progressive neurological deficits and socioeconomic burden. Experimental autoimmune encephalomyelitis (EAE) is the most extensively utilized animal model of MS, with well-characterized rodent and non-human primate variants. The EAE model is typically induced by either active immunization with myelin-derived proteins or peptides in adjuvant or by passive transfer of activated myelin-specific CD4⁺ T lymphocytes. To date, the EAE model has been an essential tool in the development of at least seven U.S. Food and Drug Administration (FDA)-approved immunomodulatory drugs for the treatment of MS, including glatiramer acetate, fingolimod, and natalizumab. However, the translational validity of the EAE model is frequently compromised due to poor study design, inconsistent clinical scoring endpoints, and inappropriate statistical calculations. No single animal model accurately reflects the complexity of human MS pathogenesis. Beyond EAE, multiple additional animal models are described, including Theiler's murine encephalomyelitis virus and cuprizone-induced demyelination, which facilitate the study of pathogen-induced CNS autoimmunity and remyelination, respectively. This overview summarizes several of the most frequently used animal models of MS and highlights key factors that significantly influence the experimental outcome and affect translational validity. © 2021 Wiley Periodicals LLC.

Neuroinflammation: A Signature or a Cause of Epilepsy?

Epilepsy can be both a primary pathology and a secondary effect of many neurological conditions. Many papers show that neuroinflammation is a product of epilepsy, and that in pathological conditions characterized by neuroinflammation, there is a higher probability to develop epilepsy. However, the bidirectional mechanism of the reciprocal interaction between epilepsy and neuroinflammation remains to be fully understood. Here, we attempt to explore and discuss the relationship between epilepsy and inflammation in some paradigmatic neurological and systemic disorders associated with epilepsy. In particular, we have chosen one representative form of epilepsy for each one of its actual known etiologies. A better understanding of the mechanistic link between neuroinflammation and epilepsy would be important to improve subject-based therapies, both for prophylaxis and for the treatment of epilepsy.

Remyelination therapies for multiple sclerosis: optimizing translation from animal models into clinical trials

Introduction: Multiple sclerosis (MS) is the most common inflammatory disease of the central nervous system (CNS). Demyelination, the main pathology in MS, contributes to clinical symptoms and long-term neurological deficits if left untreated. Remyelination, the natural repair of damaged myelin by cells of the oligodendrocyte lineage, occurs in MS, but eventually fails in most patients as they age. Encouraging timely remyelination can restore axon conduction and minimize deficits.Areas covered: We discuss and correlate human MS pathology with animal models, propose methods to deplete resident oligodendrocyte progenitor cells (OPCs) to determine whether mature oligodendrocytes support remyelination, and review remyelinating agents, mechanisms of action, and available clinical trial data.Expert opinion: The heterogeneity of human MS may limit successful translation of many candidate remyelinating agents; some patients lack the biological targets necessary to leverage current approaches. Development of therapeutics for remyelination has concentrated almost exclusively on mobilization of innate OPCs. However, mature oligodendrocytes appear an important contributor to remyelination in humans. Limiting the contribution of OPC mediated repair in models of MS would allow the evaluation of remyelination-promoting agents on mature oligodendrocytes. Among remyelinating reagents reviewed, only rHIgM22 targets both OPCs and mature oligodendrocytes.

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.

The role of picornavirus infection in epileptogenesis

Picornaviridae are a family of small positive-strand RNA viruses, and transmitted via the respiratory or fecal-oral route. The neurotropic picornaviruses can induce acute or late recurrent seizures following central nervous system infection, by infecting the peripheral nerve, crossing the blood-brain barrier and migrating in the Trojan-horse method. Theiler’s murine encephalomyelitis virus (TMEV), as a member of Picornaviridae family, can cause encephalitis, leading to chronic spontaneous seizures. TMEV-infected C57BL/6 mice have been used as an animal model for exploring the mechanism of epileptogenesis and assessing new antiepileptic drugs. Astrogliosis, neuronal death and microglial recruitment have been detected in the hippocampus following the picornaviruse-induced encephalitis. The macrophages, monocytes, neutrophils, as well as IL-6 and TNF-α released by them, play an important role in the epileptogenesis. In this review, we summarize the clinical characteristics of picornavirus infection, and the immunopathology involved in the TMEV-induced epilepsy.

Myelin-specific T cells in animals with Japanese macaque encephalomyelitis

Objective

To determine whether animals with Japanese macaque encephalomyelitis (JME), a spontaneous demyelinating disease similar to multiple sclerosis (MS), harbor myelin‐specific T cells in their central nervous system (CNS) and periphery.

Methods

Mononuclear cells (MNCs) from CNS lesions, cervical lymph nodes (LNs) and peripheral blood of Japanese macaques (JMs) with JME, and cervical LN and blood MNCs from healthy controls or animals with non‐JME conditions were analyzed for the presence of myelin‐specific T cells and changes in interleukin 17 (IL‐17) and interferon gamma (IFNγ) expression.

Results

Demyelinating JME lesions contained CD4⁺ T cells and CD8⁺ T cells specific to myelin oligodendrocyte glycoprotein (MOG), myelin basic protein (MBP), and/or proteolipid protein (PLP). CD8⁺ T‐cell responses were absent in JME peripheral blood, and in age‐ and sex‐matched controls. However, CD4⁺ Th1 and Th17 responses were detected in JME peripheral blood versus controls. Cervical LN MNCs from eight of nine JME animals had CD3⁺ T cells specific for MOG, MBP, and PLP that were not detected in controls. Mapping myelin epitopes revealed a heterogeneity in responses among JME animals. Comparison of myelin antigen sequences with those of JM rhadinovirus (JMRV), which is found in JME lesions, identified six viral open reading frames (ORFs) with similarities to myelin antigen sequences. Overlapping peptides to these JMRV ORFs did not induce IFNγ responses.

Interpretations

JME possesses an immune‐mediated component that involves both CD4⁺ and CD8⁺ T cells specific for myelin antigens. JME may shed new light on inflammatory demyelinating disease pathogenesis linked to gamma‐herpesvirus infection.

The role of herpesvirus 6A and 6B in multiple sclerosis and epilepsy

Human herpesvirus 6A (HHV‐6A) and 6B (HHV‐6B) are two closely related viruses that can infect cells of the central nervous system (CNS). The similarities between these viruses have made it difficult to separate them on serological level. The broad term HHV‐6 remains when referring to studies where the two species were not distinguished, and as such, the seroprevalence is over 90% in the adult population. HHV‐6B has been detected in up to 100% of infants with the primary infection roseola infantum, but less is known about the primary infection of HHV‐6A. Both viruses are neurotropic and have capacity to establish lifelong latency in cells of the central nervous system, with potential to reactivate and cause complications later in life. HHV‐6A infection has been associated with an increased risk of multiple sclerosis (MS), whereas HHV‐6B is indicated to be involved in pathogenesis of epilepsy. These two associations show how neurological diseases might be caused by viral infections, but as suggested here, through completely different molecular mechanisms, in an autoimmune disease, such as MS, by triggering an overreaction of the immune system and in epilepsy by hampering internal cellular functions when the immune system fails to eliminate the virus. Understanding the viral mechanisms of primary infection and reactivation and their spectrum of associated symptoms will aid our ability to diagnose, treat and prevent these severe and chronic diseases. This review explores the role of HHV‐6A and HHV‐6B specifically in MS and epilepsy, the evidence to date and the future directions of this field.

Conditional Silencing of H-2Db Class I Molecule Expression Modulates the Protective and Pathogenic Kinetics of Virus-Antigen-Specific CD8 T Cell Responses during Theiler's Virus Infection

Theiler's murine encephalomyelitis virus (TMEV) infection of the CNS is cleared in C57BL/6 mice by a CD8 T cell response restricted by the MHC class I molecule H-2D^b The identity and function of the APC(s) involved in the priming of this T cell response is (are) poorly defined. To address this gap in knowledge, we developed an H-2D^b LoxP-transgenic mouse system using otherwise MHC class I-deficient C57BL/6 mice, thereby conditionally ablating MHC class I-restricted Ag presentation in targeted APC subpopulations. We observed that CD11c⁺ APCs are critical for early priming of CD8 T cells against the immunodominant TMEV peptide VP2_121-130 Loss of H-2D^b on CD11c⁺ APCs mitigates the CD8 T cell response, preventing early viral clearance and immunopathology associated with CD8 T cell activity in the CNS. In contrast, animals with H-2D^b-deficient LysM⁺ APCs retained early priming of D^b:VP2_121-130 epitope-specific CD8 T cells, although a modest reduction in immune cell entry into the CNS was observed. This work establishes a model enabling the critical dissection of H-2D^b-restricted Ag presentation to CD8 T cells, revealing cell-specific and temporal features involved in the generation of CD8 T cell responses. Employing this novel system, we establish CD11c⁺ cells as pivotal to the establishment of acute antiviral CD8 T cell responses against the TMEV immunodominant epitope VP2_121-130, with functional implications both for T cell-mediated viral control and immunopathology.

Neuroimmune interaction in seizures and epilepsy: focusing on monocyte infiltration

Epilepsy is a major neurological condition that affects millions of people globally. While a number of interventions have been developed to mitigate this condition, a significant number of patients are refractory to these treatments. Consequently, other avenues of research are needed. One such avenue is modulation of the immune system response to this condition, which has mostly focused on microglia, the resident immune cells of the central nervous system (CNS). However, other immune cells can impact neurological conditions, principally blood-borne monocytes that can infiltrate into brain parenchyma after seizures. As such, this review will first discuss how monocytes can be recruited to the CNS and how they can be distinguished from there immunological cousins, microglia. Then, we will explore what is known about the role monocytes have within seizure pathogenesis and epilepsy. Considering how little is known about monocyte function in seizure- and epilepsy-related pathologies, further studies are warranted that investigate infiltrated blood-borne monocytes as a potential therapeutic target for epilepsy treatment.

Mechanisms of Pathogen Invasion into the Central Nervous System

CNS infections continue to rise in incidence in conjunction with increases in immunocompromised populations or conditions that contribute to the emergence of pathogens, such as global travel, climate change, and human encroachment on animal territories. The severity and complexity of these diseases is impacted by the diversity of etiologic agents and their routes of neuroinvasion. In this review, we present historical, clinical, and molecular concepts regarding the mechanisms of pathogen invasion of the CNS. We also discuss the structural components of CNS compartments that influence pathogen entry and recent discoveries of the pathways exploited by pathogens to facilitate CNS infections. Advances in our understanding of the CNS invasion mechanisms of different neurotropic pathogens may enable the development of strategies to control their entry and deliver drugs to mitigate established infections.

Viral infections and multiple sclerosis

The etiology and pathogenesis of MS is likely to involve multiple factors interacting with each other, and the role of infectious and viral agents is still under debate, however a consistent amount of studies suggests that some viruses are associated with the disease. The strongest documentation has come from the detection of viral nucleic acid or antigen or of an anti-viral antibody response in MS patients. A further step for the study of the mechanism viruses might be involved in can be made using in vitro and in vivo models. While in vitro models, based on glial and neural cell lines from various sources are widely used, in vivo animal models present challenges. Indeed neurotropic animal viruses are currently used to study demyelination in well-established models, but animal models of demyelination by human virus infection have only recently been developed, using animal gammaherpesviruses closely related to Epstein Barr virus (EBV), or using marmosets expressing the specific viral receptor for Human Herpesvirus 6 (HHV-6). The present review will illustrate the main potential mechanisms of MS pathogenesis possibly associated with viral infections and viruses currently used to study demyelination in animal models. Then the viruses most strongly linked with MS will be discussed, in the perspective that more than one virus might have a role, with varying degrees of interaction, contributing to MS heterogeneity.

Assessing the anterior visual pathway in optic neuritis: recent experimental and clinical aspects

PURPOSE OF REVIEW

Multiple sclerosis (MS) and related autoimmune disorders of the central nervous system such as neuromyelitis optica spectrum disorders (NMOSD) are characterized by chronic disability resulting from autoimmune neuroinflammation, with demyelination, astrocyte damage, impaired axonal transmission and neuroaxonal loss. Novel therapeutics stopping or reversing the progression of disability are still urgently warranted. This review addresses research on optic neuritis in preclinical experimental models and their translation to clinical trials.

RECENT FINDINGS

Optic neuritis can be used as paradigm for an MS relapse which can serve to evaluate the efficacy of novel therapeutics in clinical trials with a reasonable duration and cohort size. The advantage is the linear structure of the visual pathway allowing the assessment of visual function and retinal structure as highly sensitive outcome parameters. Experimental autoimmune encephalomyelitis is an inducible, inflammatory and demyelinating central nervous system disease extensively used as animal model of MS. Optic neuritis is part of the clinicopathological manifestations in a number of experimental autoimmune encephalomyelitis models. These have gained increasing interest for studies evaluating neuroprotective and/or remyelinating substances as longitudinal, visual and retinal readouts have become available.

SUMMARY

Translation of preclinical experiments, evaluating neuroprotective or remyelinating therapeutics to clinical studies is challenging. In-vivo readouts like optical coherence tomography, offers the possibility to transfer experimental study designs to clinical optic neuritis trials.

Protective effects of pharmacological therapies in animal models of multiple sclerosis: a review of studies 2014-2019

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system. The disability caused by inflammatory demyelination clinically dominates the early stages of relapsing-remitting MS and is reversible. Once there is considerable loss of axons, MS patients enter a secondary progressive stage. Disease-modifying drugs currently in use for MS suppress the immune system and reduce relapse rates but are not effective in the progressive stage. Various animal models of MS (mostly mouse and rat) have been established and proved useful in studying the disease process and response to therapy. The experimental autoimmune encephalomyelitis animal studies reviewed here showed that a chronic progressive disease can be induced by immunization with appropriate amounts of myelin oligodendrocyte glycoprotein together with mycobacterium tuberculosis and pertussis toxin in Freund's adjuvant. The clinical manifestations of autoimmune encephalomyelitis disease were prevented or reduced by treatment with certain pharmacological agents given prior to, at, or after peak disease, and the agents had protective effects as shown by inhibiting demyelination and damage to neurons, axons and oligodendrocytes. In the cuprizone-induced toxicity animal studies, the pharmacological agents tested were able to promote remyelination and increase the number of oligodendrocytes when administered therapeutically or prophylactically. A monoclonal IgM antibody protected axons in the spinal cord and preserved motor function in animals inoculated with Theiler's murine encephalomyelitis virus. In all these studies the pharmacological agents were administered singly. A combination therapy may be more effective, especially using agents that target neuroinflammation and neurodegeneration, as they may exert synergistic actions.

Biological models in multiple sclerosis

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

Differential transcriptional profiles identify microglial- and macrophage-specific gene markers expressed during virus-induced neuroinflammation

Background

In the healthy central nervous system (CNS), microglia are found in a homeostatic state and peripheral macrophages are absent from the brain. Microglia play key roles in maintaining CNS homeostasis and acting as first responders to infection and inflammation, and peripheral macrophages infiltrate the CNS during neuroinflammation. Due to their distinct origins and functions, discrimination between these cell populations is essential to the comprehension of neuroinflammatory disorders. Studies comparing the gene profiles of microglia and peripheral macrophages, or macrophages in vitro-derived from bone marrow, under non-infectious conditions of the CNS, have revealed valuable microglial-specific genes. However, studies comparing gene profiles between CNS-infiltrating macrophages and microglia, when both are isolated from the CNS during viral-induced neuroinflammation, are lacking.

Methods

We isolated, via flow cytometry, microglia and infiltrating macrophages from the brains of Theiler’s murine encephalomyelitis virus-infected C57BL/6 J mice and used RNA-Seq, followed by validation with qPCR, to examine the differential transcriptional profiles of these cells. We utilized primary literature defining subcellular localization to determine whether or not particular proteins extracted from the transcriptional profiles were expressed at the cell surface. The surface expression and cellular specificity of triggering receptor expressed on myeloid cells 1 (TREM-1) protein were examined via flow cytometry. We also examined the immune response gene profile within the transcriptional profiles of these isolated microglia and infiltrating macrophages.

Results

We have identified and validated new microglial- and macrophage-specific genes, encoding cell surface proteins, expressed at the peak of neuroinflammation. TREM-1 protein was confirmed to be expressed by infiltrating macrophages, not microglia, at the peak of neuroinflammation. We also identified both unique and redundant immune functions, through examination of the immune response gene profiles, of microglia and infiltrating macrophages during neurotropic viral infection.

Conclusions

The differential expression of cell surface-specific genes during neuroinflammation can potentially be used to discriminate between microglia and macrophages as well as provide a resource that can be further utilized to target and manipulate specific cell responses during neuroinflammation.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1545-x) contains supplementary material, which is available to authorized users.

Microglial cell depletion is fatal with low level picornavirus infection of the central nervous system

Microglia are the only resident myeloid cell in the central nervous system (CNS) parenchyma, but the role of microglia in the context of neurotropic viral infection is poorly understood. Using different amounts of Theiler's murine encephalomyelitis virus (TMEV) in a preclinical model of epilepsy and PLX5622, a colony stimulating factor-1 receptor inhibitor that selectively depletes microglia in the CNS, we report that microglia-depleted, TMEV-infected mice develop seizures, manifest paralysis, and uniformly succumb to fatal encephalitis regardless of viral amount. CNS demyelination correlates with viral amount; however, viral amount does not correlate with axon damage and TMEV antigen in the CNS.

The holy grail of epilepsy prevention: Preclinical approaches to antiepileptogenic treatments

A variety of acute brain insults can induce epileptogenesis, a complex process that results in acquired epilepsy. Despite advances in understanding mechanisms of epileptogenesis, there is currently no approved treatment that prevents the development or progression of epilepsy in patients at risk. The current concept of epileptogenesis assumes a window of opportunity following acute brain insults that allows intervention with preventive treatment. Recent results suggest that injury-induced epileptogenesis can be a much more rapid process than previously thought, suggesting that the 'therapeutic window' may only be open for a brief period, as in stroke therapy. However, experimental data also suggest a second, possibly delayed process ("secondary epileptogenesis") that influences the progression and refractoriness of the epileptic state over time, allowing interfering with this process even after onset of epilepsy. In this review, both methodological issues in preclinical drug development and novel targets for antiepileptogenesis will be discussed. Several promising drugs that either prevent epilepsy (antiepileptogenesis) or slow epilepsy progression and alleviate cognitive or behavioral comorbidities of epilepsy (disease modification) have been described in recent years, using diverse animal models of acquired epilepsy. Promising agents include TrkB inhibitors, losartan, statins, isoflurane, anti-inflammatory and anti-oxidative drugs, the SV2A modulator levetiracetam, and epigenetic interventions. Research on translational target validity and on prognostic biomarkers that can be used to stratify patients (or experimental animals) at high risk of developing epilepsy will hopefully soon lead to proof-of-concept clinical trials with the most promising drugs, which will be essential to make prevention of epilepsy a reality. This article is part of the special issue entitled 'New Epilepsy Therapies for the 21st Century - From Antiseizure Drugs to Prevention, Modification and Cure of Epilepsy'.

Music-Enhanced Analgesia and Antiseizure Activities in Animal Models of Pain and Epilepsy: Toward Preclinical Studies Supporting Development of Digital Therapeutics and Their Combinations With Pharmaceutical Drugs

Digital therapeutics (software as a medical device) and mobile health (mHealth) technologies offer a means to deliver behavioral, psychosocial, disease self-management and music-based interventions to improve therapy outcomes for chronic diseases, including pain and epilepsy. To explore new translational opportunities in developing digital therapeutics for neurological disorders, and their integration with pharmacotherapies, we examined analgesic and antiseizure effects of specific musical compositions in mouse models of pain and epilepsy. The music playlist was created based on the modular progression of Mozart compositions for which reduction of seizures and epileptiform discharges were previously reported in people with epilepsy. Our results indicated that music-treated mice exhibited significant analgesia and reduction of paw edema in the carrageenan model of inflammatory pain. Among analgesic drugs tested (ibuprofen, cannabidiol (CBD), levetiracetam, and the galanin analog NAX 5055), music intervention significantly decreased paw withdrawal latency difference in ibuprofen-treated mice and reduced paw edema in combination with CBD or NAX 5055. To the best of our knowledge, this is the first animal study on music-enhanced antinociceptive activity of analgesic drugs. In the plantar incision model of surgical pain, music-pretreated mice had significant reduction of mechanical allodynia. In the corneal kindling model of epilepsy, the cumulative seizure burden following kindling acquisition was lower in animals exposed to music. The music-treated group also exhibited significantly improved survival, warranting further research on music interventions for preventing Sudden Unexpected Death in Epilepsy (SUDEP). We propose a working model of how musical elements such as rhythm, sequences, phrases and punctuation found in K.448 and K.545 may exert responses via parasympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. Based on our findings, we discuss: (1) how enriched environment (EE) can serve as a preclinical surrogate for testing combinations of non-pharmacological modalities and drugs for the treatment of pain and other chronic diseases, and (2) a new paradigm for preclinical and clinical development of therapies leading to drug-device combination products for neurological disorders, depression and cancer. In summary, our present results encourage translational research on integrating non-pharmacological and pharmacological interventions for pain and epilepsy using digital therapeutics.

TLR signals license CD8 T cells to destroy oligodendrocytes expressing an antigen shared with a Listeria pathogen

Increasing evidence suggests a role of CD8 T cells in autoimmune demyelinating CNS disease, which, however, is still controversially discussed. Mice, which express ovalbumin (OVA) as cytosolic self-antigen in oligodendrocytes (ODC-OVA mice), respond to CNS infection induced by OVA-expressing attenuated Listeria with CD8 T cell-mediated inflammatory demyelination. This model is suitable to decipher the contribution of CD8 T cells and the pathogen in autoimmune CNS disease. Here, we show that both antigen and pathogen are required in the CNS for disease induction, though not in a physically linked fashion. Intracerebral challenge with combined toll like receptor (TLR) TLR2 and TLR9 as well as TLR7 and TLR9 agonists substituted for the bacterial stimulus, but not with individual TLR agonists (TLR2, TLR3,TLR5,TLR7, TLR9). Furthermore, MyD88 inactivation rendered ODC-OVA mice resistant to disease induction. Collectively, CD8 T cell-mediated destruction of oligodendrocytes is activated if (i) an antigen shared with an infectious agent is provided in the CNS microenvironment and (ii) innate immune signals inform the CNS microenvironment that pathogen removal warrants an immune attack by CD8 T cells, even at the expense of locally restricted demyelination.

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.

Automated quantification of EEG spikes and spike clusters as a new read out in Theiler's virus mouse model of encephalitis-induced epilepsy

Intracerebral infection of C57BL/6 mice with Theiler's murine encephalomyelitis virus (TMEV) replicates many features of viral encephalitis-induced epilepsy in humans, including neuroinflammation, early (insult-associated) and late (spontaneous) seizures, neurodegeneration in the hippocampus, and cognitive and behavioral alterations. Thus, this model may be ideally suited to study mechanisms involved in encephalitis-induced epilepsy as potential targets for epilepsy prevention. However, spontaneous recurrent seizures (SRS) occur too infrequently to be useful as a biomarker of epilepsy, e.g., for drug studies. This prompted us to evaluate whether epileptiform spikes or spike clusters in the cortical electroencephalogram (EEG) may be a useful surrogate of epilepsy in this model. For this purpose, we developed an algorithm that allows efficient and large-scale EEG analysis of early and late seizures, spikes, and spike clusters in the EEG. While 77% of the infected mice exhibited early seizures, late seizures were only observed in 33% of the animals. The clinical characteristics of early and late seizures did not differ except that late generalized convulsive (stage 5) seizures were significantly longer than early stage 5 seizures. Furthermore, the frequency of SRS was much lower than the frequency of early seizures. Continuous (24/7) video-EEG monitoring over several months following infection indicated that the latent period to onset of SRS was 61 (range 16-91) days. Spike and spike clusters were significantly more frequent in infected mice with late seizures than in infected mice without seizures or in mock-infected sham controls. Based on the results of this study, increases in EEG spikes and spike clusters in groups of infected mice may be used as a new readout for studies on antiepileptogenic or disease-modifying drug effects in this model, because the significant increase in average spike counts in mice with late seizures obviously indicates a proepileptogenic alteration.