Type I interferons are essential in controlling neurotropic coronavirus infection irrespective of functional CD8 T cells

Cystatin F attenuates neuroinflammation and demyelination following murine coronavirus infection of the central nervous system

BACKGROUND

Cystatin F is a secreted lysosomal cysteine protease inhibitor that has been implicated in affecting the severity of demyelination and enhancing remyelination in pre-clinical models of immune-mediated demyelination. How cystatin F impacts neurologic disease severity following viral infection of the central nervous system (CNS) has not been well characterized and was the focus of this study. We used cystatin F null-mutant mice (Cst7-/-) with a well-established model of murine coronavirus-induced neurologic disease to evaluate the contributions of cystatin F in host defense, demyelination and remyelination.

METHODS

Wildtype controls and Cst7-/- mice were intracranially (i.c.) infected with a sublethal dose of the neurotropic JHM strain of mouse hepatitis virus (JHMV), with disease progression and survival monitored daily. Viral plaque assays and qPCR were used to assess viral levels in CNS. Immune cell infiltration into the CNS and immune cell activation were determined by flow cytometry and 10X genomics chromium 3' single cell RNA sequencing (scRNA-seq). Spinal cord demyelination was determined by luxol fast blue (LFB) and Hematoxylin/Eosin (H&E) staining and axonal damage assessed by immunohistochemical staining for SMI-32. Remyelination was evaluated by electron microscopy (EM) and calculation of g-ratios.

RESULTS

JHMV-infected Cst7-/- mice were able to control viral replication within the CNS, indicating that cystatin F is not essential for an effective Th1 anti-viral immune response. Infiltration of T cells into the spinal cords of JHMV-infected Cst7-/- mice was increased compared to infected controls, and this correlated with increased axonal damage and demyelination associated with impaired remyelination. Single-cell RNA-seq of CD45 + cells enriched from spinal cords of infected Cst7-/- and control mice revealed enhanced expression of transcripts encoding T cell chemoattractants, Cxcl9 and Cxcl10, combined with elevated expression of interferon-g (Ifng) and perforin (Prf1) transcripts in CD8 + T cells from Cst7-/- mice compared to controls.

CONCLUSIONS

Cystatin F is not required for immune-mediated control of JHMV replication within the CNS. However, JHMV-infected Cst7-/- mice exhibited more severe clinical disease associated with increased demyelination and impaired remyelination. The increase in disease severity was associated with elevated expression of T cell chemoattractant chemokines, concurrent with increased neuroinflammation. These findings support the idea that cystatin F influences expression of proinflammatory gene expression impacting neuroinflammation, T cell activation and/or glia cell responses ultimately impacting neuroinflammation and neurologic disease.

The Impact of Innate Components on Viral Pathogenesis in the Neurotropic Coronavirus Encephalomyelitis Mouse Model

Recognition of viruses invading the central nervous system (CNS) by pattern recognition receptors (PRRs) is crucial to elicit early innate responses that stem dissemination. These innate responses comprise both type I interferon (IFN-I)-mediated defenses as well as signals recruiting leukocytes to control the infection. Focusing on insights from the neurotropic mouse CoV model, this review discusses how early IFN-I, fibroblast, and myeloid signals can influence protective anti-viral adaptive responses. Emphasis is placed on three main areas: the importance of coordinating the distinct capacities of resident CNS cells to induce and respond to IFN-I, the effects of select IFN-stimulated genes (ISGs) on host immune responses versus viral control, and the contribution of fibroblast activation and myeloid cells in aiding the access of T cells to the parenchyma. By unraveling how the dysregulation of early innate components influences adaptive immunity and viral control, this review illustrates the combined effort of resident CNS cells to achieve viral control.

Microglia influence immune responses and restrict neurologic disease in response to central nervous system infection by a neurotropic murine coronavirus

Intracranial (i.c.) inoculation of susceptible mice with a glial-tropic strain of mouse hepatitis virus (JHMV), a murine coronavirus, results in an acute encephalomyelitis followed by viral persistence in white matter tracts accompanied by chronic neuroinflammation and demyelination. Microglia serve numerous functions including maintenance of the healthy central nervous system (CNS) and are among the first responders to injury or infection. More recently, studies have demonstrated that microglia aid in tailoring innate and adaptive immune responses following infection by neurotropic viruses including flaviviruses, herpesviruses, and picornaviruses. These findings have emphasized an important role for microglia in host defense against these viral pathogens. In addition, microglia are also critical in optimizing immune-mediated control of JHMV replication within the CNS while restricting the severity of demyelination and enhancing remyelination. This review will highlight our current understanding of the molecular and cellular mechanisms by which microglia aid in host defense, limit neurologic disease, and promote repair following CNS infection by a neurotropic murine coronavirus.

Ablation of microglia following infection of the central nervous system with a neurotropic murine coronavirus infection leads to increased demyelination and impaired remyelination

Intracranial inoculation of susceptible mice with a glial-tropic strain of mouse hepatitis virus (JHMV), a murine coronavirus, results in an acute encephalomyelitis followed by viral persistence in white matter tracts accompanied by chronic neuroinflammation and demyelination. Microglia are the resident immune cell of the central nervous system (CNS) and are considered important in regulating events associated with neuroinflammation as well as influencing both white matter damage and remyelination. To better understand mechanisms by which microglia contribute to these immune-mediated events, JHMV-infected mice with established demyelination were treated with the small molecular inhibitor of colony stimulating factor 1 receptor (CSF1R), PLX5622, to deplete microglia. Treatment with PLX5622 did not affect viral replication within the CNS yet the severity of demyelination was increased and remyelination impaired compared to control mice. Gene expression analysis revealed that targeting microglia resulted in altered expression of genes associated with immune cell activation and phagocytosis of myelin debris. These findings indicate that microglia are not critical in viral surveillance in persistently JHMV-infected mice yet restrict white matter damage and remyelination, in part, by influencing phagocytosis of myelin debris.

Innate immune evasion strategies of SARS-CoV-2

SARS-CoV-2, the virus responsible for the COVID-19 pandemic, has been associated with substantial global morbidity and mortality. Despite a tropism that is largely confined to the airways, COVID-19 is associated with multiorgan dysfunction and long-term cognitive pathologies. A major driver of this biology stems from the combined effects of virus-mediated interference with the host antiviral defences in infected cells and the sensing of pathogen-associated material by bystander cells. Such a dynamic results in delayed induction of type I and III interferons (IFN-I and IFN-III) at the site of infection, but systemic IFN-I and IFN-III priming in distal organs and barrier epithelial surfaces, respectively. In this Review, we examine the relationship between SARS-CoV-2 biology and the cellular response to infection, detailing how antagonism and dysregulation of host innate immune defences contribute to disease severity of COVID-19.

Susceptibilities of CNS Cells towards Rabies Virus Infection Is Linked to Cellular Innate Immune Responses

Rabies is caused by neurotropic rabies virus (RABV), contributing to 60,000 human deaths annually. Even though rabies leads to major public health concerns worldwide, we still do not fully understand factors determining RABV tropism and why glial cells are unable to clear RABV from the infected brain. Here, we compare susceptibilities and immune responses of CNS cell types to infection with two RABV strains, Tha and its attenuated variant Th2P-4M, mutated on phospho- (P-protein) and matrix protein (M-protein). We demonstrate that RABV replicates in human stem cell-derived neurons and astrocytes but fails to infect human iPSC-derived microglia. Additionally, we observed major differences in transcription profiles and quantification of intracellular protein levels between antiviral immune responses mediated by neurons, astrocytes (IFNB1, CCL5, CXCL10, IL1B, IL6, and LIF), and microglia (CCL5, CXCL10, ISG15, MX1, and IL6) upon Tha infection. We also show that P- and M-proteins of Tha mediate evasion of NF-κB- and JAK-STAT-controlled antiviral host responses in neuronal cell types in contrast to glial cells, potentially explaining the strong neuron-specific tropism of RABV. Further, Tha-infected astrocytes and microglia protect neurons from Tha infection via a filtrable and transferable agent. Overall, our study provides novel insights into RABV tropism, showing the interest in studying the interplay of CNS cell types during RABV infection.

Evaluating the role of chemokines and chemokine receptors involved in coronavirus infection

INTRODUCTION

Coronaviruses are a large family of positive-stranded nonsegmented RNA viruses with genomes of 26-32 kilobases in length. Human coronaviruses are commonly associated with mild respiratory illness; however, the past three decades have seen the emergence of severe acute respiratory coronavirus (SARS-CoV), middle eastern respiratory coronavirus (MERS-CoV), and SARS-CoV-2 which is the etiologic agent for COVID-19. Severe forms of COVID-19 include acute respiratory distress syndrome (ARDS) associated with cytokine release syndrome that can culminate in multiorgan failure and death. Among the proinflammatory factors associated with severe COVID-19 are the chemokines CCL2, CCL3, CXCL8, and CXCL10. Infection of susceptible mice with murine coronaviruses, such as mouse hepatitis virus (MHV), elicits a similar chemokine response profile as observed in COVID-19 patients and these in vivo models have been informative and show that targeting chemokines reduces the severity of inflammation in target organs.

AREAS COVERED

PubMed was used using keywords: Chemokines and coronaviruses; Chemokines and mouse hepatitis virus; Chemokines and COVID-19. Clinicaltrials.gov was used using keywords: COVID-19 and chemokines; COVID-19 and cytokines; COVID-19 and neutrophil.

EXPERT OPINION

Chemokines and chemokine receptors are clinically relevant therapeutic targets for reducing coronavirus-induced inflammation.

Potential differences in cleavage of the S protein and type-1 interferon together control human coronavirus infection, propagation, and neuropathology within the central nervous system

Human coronaviruses (HCoV) are respiratory pathogens which have been known since the 1960's. In December 2019, a new betacoronavirus, SARS-CoV-2, was reported and is responsible for one of the biggest pandemics of the last two centuries. Similar to the HCoV-OC43 strain, available evidence suggests SARS-CoV-2 neuroinvasion associated with potential neurological disorders. Coronavirus infection of the central nervous system (CNS) is largely controlled by a viral factor, the spike glycoprotein (S) and a host factor, innate immunity. However, the interaction between these two factors remains elusive. Proteolytic cleavage of the S protein can occur at the interface between receptor binding (S1) and fusion (S2) domains (S1/S2), as well as in a position adjacent to a fusion peptide within S2 (S2'). Herein, using HCoV-OC43 as a surrogate for SARS-CoV-2, we report that both S protein sites are involved in neurovirulence and are required for optimal CNS infection. Whereas efficient cleavage at S1/S2 is associated with decreased virulence, the potentially cleavable putative S2' site is essential for efficient viral infection. Furthermore, type 1 interferon (IFN 1)-related innate immunity also plays an important role in the control of viral spread towards the spinal cord, by preventing infection of ependymal cells. Our results underline the link between the differential S cleavage and IFN 1 in the prevention of viral spread, to control the severity of infection and pathology in both immunocompetent and immunodeficient mice. Taken together, these results point towards two potential therapeutic anti-viral targets: cleavage of the S protein in conjunction with efficient IFN 1-related innate immunity to prevent or at least reduce neuroinvasion, neural spread, and potential associated neurovirulence of human coronaviruses.ImportanceHuman coronaviruses (HCoV) are recognized respiratory pathogens. The emergence of the novel pathogenic member of this family in December 2019 (SARS-CoV-2, which causes COVID-19) poses a global health emergency. As with other coronaviruses reported previously, invasion of the human central nervous system (CNS), associated with diverse neurological disorders, was suggested for SARS-CoV-2. Herein, using the related HCoV-OC43 strain, we show that the viral spike protein constitutes a major neurovirulence factor and that type 1 interferon (IFN 1), in conjunction with cleavage of S protein by host proteases, represent important host factors that participate in the control of CNS infection.To our knowledge, this is the first demonstration of a direct link between cleavage of the S protein, innate immunity and neurovirulence. Understanding mechanisms of viral infection and spread in neuronal cells is essential to better design therapeutic strategies, and to prevent infection by human coronaviruses such as SARS-CoV-2 in human CNS especially in the vulnerable populations such as the elderly and immune-compromised individuals.

Microglia influence host defense, disease, and repair following murine coronavirus infection of the central nervous system

The present study examines functional contributions of microglia in host defense, demyelination, and remyelination following infection of susceptible mice with a neurotropic coronavirus. Treatment with PLX5622, an inhibitor of colony stimulating factor 1 receptor (CSF1R) that efficiently depletes microglia, prior to infection of the central nervous system (CNS) with the neurotropic JHM strain of mouse hepatitis virus (JHMV) resulted in increased mortality compared with control mice that correlated with impaired control of viral replication. Single cell RNA sequencing (scRNASeq) of CD45+ cells isolated from the CNS revealed that PLX5622 treatment resulted in muted CD4+ T cell activation profile that was associated with decreased expression of transcripts encoding MHC class II and CD86 in macrophages but not dendritic cells. Evaluation of spinal cord demyelination revealed a marked increase in white matter damage in PLX5622-treated mice that corresponded with elevated expression of transcripts encoding disease-associated proteins Osteopontin (Spp1), Apolipoprotein E (Apoe), and Triggering receptor expressed on myeloid cells 2 (Trem2) that were enriched within macrophages. In addition, PLX5622 treatment dampened expression of Cystatin F (Cst7), Insulin growth factor 1 (Igf1), and lipoprotein lipase (Lpl) within macrophage populations which have been implicated in promoting repair of damaged nerve tissue and this was associated with impaired remyelination. Collectively, these findings argue that microglia tailor the CNS microenvironment to enhance control of coronavirus replication as well as dampen the severity of demyelination and influence repair.

Neuronal Ablation of Alpha/Beta Interferon (IFN-α/β) Signaling Exacerbates Central Nervous System Viral Dissemination and Impairs IFN-γ Responsiveness in Microglia/Macrophages

Alpha/beta interferon (IFN-α/β) signaling through the IFN-α/β receptor (IFNAR) is essential to limit virus dissemination throughout the central nervous system (CNS) following many neurotropic virus infections. However, the distinct expression patterns of factors associated with the IFN-α/β pathway in different CNS resident cell populations implicate complex cooperative pathways in IFN-α/β induction and responsiveness. Here we show that mice devoid of IFNAR1 signaling in calcium/calmodulin-dependent protein kinase II alpha (CaMKIIα) expressing neurons (CaMKIIcre:IFNAR^fl/fl mice) infected with a mildly pathogenic neurotropic coronavirus (mouse hepatitis virus A59 strain [MHV-A59]) developed severe encephalomyelitis with hind-limb paralysis and succumbed within 7 days. Increased virus spread in CaMKIIcre:IFNAR^fl/fl mice compared to IFNAR^fl/fl mice affected neurons not only in the forebrain but also in the mid-hind brain and spinal cords but excluded the cerebellum. Infection was also increased in glia. The lack of viral control in CaMKIIcre:IFNAR^fl/fl relative to control mice coincided with sustained Cxcl1 and Ccl2 mRNAs but a decrease in mRNA levels of IFNα/β pathway genes as well as Il6, Tnf, and Il1β between days 4 and 6 postinfection (p.i.). T cell accumulation and IFN-γ production, an essential component of virus control, were not altered. However, IFN-γ responsiveness was impaired in microglia/macrophages irrespective of similar pSTAT1 nuclear translocation as in infected controls. The results reveal how perturbation of IFN-α/β signaling in neurons can worsen disease course and disrupt complex interactions between the IFN-α/β and IFN-γ pathways in achieving optimal antiviral responses.IMPORTANCE IFN-α/β induction limits CNS viral spread by establishing an antiviral state, but also promotes blood brain barrier integrity, adaptive immunity, and activation of microglia/macrophages. However, the extent to which glial or neuronal signaling contributes to these diverse IFN-α/β functions is poorly understood. Using a neurotropic mouse hepatitis virus encephalomyelitis model, this study demonstrated an essential role of IFN-α/β receptor 1 (IFNAR1) specifically in neurons to control virus spread, regulate IFN-γ signaling, and prevent acute mortality. The results support the notion that effective neuronal IFNAR1 signaling compensates for their low basal expression of genes in the IFN-α/β pathway compared to glia. The data further highlight the importance of tightly regulated communication between the IFN-α/β and IFN-γ signaling pathways to optimize antiviral IFN-γ activity.

The molecular virology of coronaviruses

Few human pathogens have been the focus of as much concentrated worldwide attention as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of COVID-19. Its emergence into the human population and ensuing pandemic came on the heels of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV), two other highly pathogenic coronavirus spillovers, which collectively have reshaped our view of a virus family previously associated primarily with the common cold. It has placed intense pressure on the collective scientific community to develop therapeutics and vaccines, whose engineering relies on a detailed understanding of coronavirus biology. Here, we present the molecular virology of coronavirus infection, including its entry into cells, its remarkably sophisticated gene expression and replication mechanisms, its extensive remodeling of the intracellular environment, and its multifaceted immune evasion strategies. We highlight aspects of the viral life cycle that may be amenable to antiviral targeting as well as key features of its biology that await discovery.

Lessons for COVID-19 Immunity from Other Coronavirus Infections

A key goal to controlling coronavirus disease 2019 (COVID-19) is developing an effective vaccine. Development of a vaccine requires knowledge of what constitutes a protective immune response and also features that might be pathogenic. Protective and pathogenic aspects of the response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are not well understood, partly because the virus has infected humans for only 6 months. However, insight into coronavirus immunity can be informed by previous studies of immune responses to non-human coronaviruses, common cold coronaviruses, and SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV). Here, we review the literature describing these responses and discuss their relevance to the SARS-CoV-2 immune response.

Lessons for COVID-19 Immunity from Other Coronavirus Infections

A key goal to controlling coronavirus disease 2019 (COVID-19) is developing an effective vaccine. Development of a vaccine requires knowledge of what constitutes a protective immune response and also features that might be pathogenic. Protective and pathogenic aspects of the response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are not well understood, partly because the virus has infected humans for only 6 months. However, insight into coronavirus immunity can be informed by previous studies of immune responses to non-human coronaviruses, common cold coronaviruses, and SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV). Here, we review the literature describing these responses and discuss their relevance to the SARS-CoV-2 immune response.

Type II interferon signaling in the brain during a viral infection with age-dependent pathogenesis

Viral infections of the central nervous system (CNS) often cause disease in an age-dependent manner, with greater neuropathology during the fetal and neonatal periods. Transgenic CD46+ mice model these age-dependent outcomes through a measles virus infection of CNS neurons. Adult CD46+ mice control viral spread and survive the infection in an interferon gamma (IFNγ)-dependent manner, whereas neonatal CD46+ mice succumb despite similar IFNγ expression in the brain. Thus, we hypothesized that IFNγ signaling in the adult brain may be more robust, potentially due to greater basal expression of IFNγ signaling proteins. To test this hypothesis, we evaluated the expression of canonical IFNγ signaling proteins in the neonatal and adult brain, including the IFNγ receptor, Janus kinase (JAK) 1/2, and signal transducer and activator of transcription-1 (STAT1) in the absence of infection. We also analyzed the expression and activation of STAT1 and IFNγ-stimulated genes during MV infection. We found that neonatal brains have equivalent or greater JAK/STAT1 expression in the hippocampus and the cerebellum than adults. IFNγ receptor expression varied by cell type in the brain but was widely expressed on neuronal and glial cells. During MV infection, increased STAT1 expression and activation correlated with viral load in the hippocampus regardless of age, but not in the cerebellum where viral load was consistently undetectable in adults. These results suggest the neonatal brain is capable of initiating IFNγ signaling during a viral infection, but that downstream STAT1 activation is insufficient to limit viral spread.

Murine Coronavirus Infection Activates the Aryl Hydrocarbon Receptor in an Indoleamine 2,3-Dioxygenase-Independent Manner, Contributing to Cytokine Modulation and Proviral TCDD-Inducible-PARP Expression

The aryl hydrocarbon receptor (AhR) is a cytoplasmic receptor/transcription factor that modulates several cellular and immunological processes following activation by pathogen-associated stimuli, though its role during virus infection is largely unknown. Here, we show that AhR is activated in cells infected with mouse hepatitis virus (MHV), a coronavirus (CoV), and contributes to the upregulation of downstream effector TCDD-inducible poly(ADP-ribose) polymerase (TiPARP) during infection. Knockdown of TiPARP reduced viral replication and increased interferon expression, suggesting that TiPARP functions in a proviral manner during MHV infection. We also show that MHV replication induced the expression of other genes known to be downstream of AhR in macrophages and dendritic cells and in livers of infected mice. Further, we found that chemically inhibiting or activating AhR reciprocally modulated the expression levels of cytokines induced by infection, specifically, interleukin 1β (IL-1β), IL-10, and tumor necrosis factor alpha (TNF-α), consistent with a role for AhR activation in the host response to MHV infection. Furthermore, while indoleamine 2,3-dioxygenase (IDO1) drives AhR activation in other settings, MHV infection induced equal expression of downstream genes in wild-type (WT) and IDO1-/- macrophages, suggesting an alternative pathway of AhR activation. In summary, we show that coronaviruses elicit AhR activation by an IDO1-independent pathway, contributing to upregulation of downstream effectors, including the proviral factor TiPARP, and to modulation of cytokine gene expression, and we identify a previously unappreciated role for AhR signaling in CoV pathogenesis.IMPORTANCE Coronaviruses are a family of positive-sense RNA viruses with human and agricultural significance. Characterizing the mechanisms by which coronavirus infection dictates pathogenesis or counters the host immune response would provide targets for the development of therapeutics. Here, we show that the aryl hydrocarbon receptor (AhR) is activated in cells infected with a prototypic coronavirus, mouse hepatitis virus (MHV), resulting in the expression of several effector genes. AhR is important for modulation of the host immune response to MHV and plays a role in the expression of TiPARP, which we show is required for maximal viral replication. Taken together, our findings highlight a previously unidentified role for AhR in regulating coronavirus replication and the immune response to the virus.

The microbiota protects from viral-induced neurologic damage through microglia-intrinsic TLR signaling

Symbiotic microbes impact the function and development of the central nervous system (CNS); however, little is known about the contribution of the microbiota during viral-induced neurologic damage. We identify that commensals aid in host defense following infection with a neurotropic virus through enhancing microglia function. Germfree mice or animals that receive antibiotics are unable to control viral replication within the brain leading to increased paralysis. Microglia derived from germfree or antibiotic-treated animals cannot stimulate viral-specific immunity and microglia depletion leads to worsened demyelination. Oral administration of toll-like receptor (TLR) ligands to virally infected germfree mice limits neurologic damage. Homeostatic activation of microglia is dependent on intrinsic signaling through TLR4, as disruption of TLR4 within microglia, but not the entire CNS (excluding microglia), leads to increased viral-induced clinical disease. This work demonstrates that gut immune-stimulatory products can influence microglia function to prevent CNS damage following viral infection.

Innate Immune Responses and Viral-Induced Neurologic Disease

Multiple sclerosis (MS) is a disease of the central nervous system (CNS) characterized by chronic neuroinflammation, axonal damage, and demyelination. Cellular components of the adaptive immune response are viewed as important in initiating formation of demyelinating lesions in MS patients. This notion is supported by preclinical animal models, genome-wide association studies (GWAS), as well as approved disease modifying therapies (DMTs) that suppress clinical relapse and are designed to impede infiltration of activated lymphocytes into the CNS. Nonetheless, emerging evidence demonstrates that the innate immune response e.g., neutrophils can amplify white matter damage through a variety of different mechanisms. Indeed, using a model of coronavirus-induced neurologic disease, we have demonstrated that sustained neutrophil infiltration into the CNS of infected animals correlates with increased demyelination. This brief review highlights recent evidence arguing that targeting the innate immune response may offer new therapeutic avenues for treatment of demyelinating disease including MS.

Fine Tuning the Cytokine Storm by IFN and IL-10 Following Neurotropic Coronavirus Encephalomyelitis

The central nervous system (CNS) is vulnerable to several viral infections including herpes viruses, arboviruses and HIV to name a few. While a rapid and effective immune response is essential to limit viral spread and mortality, this anti-viral response needs to be tightly regulated in order to limit immune mediated tissue damage. This balance between effective virus control with limited pathology is especially important due to the highly specialized functions and limited regenerative capacity of neurons, which can be targets of direct virus cytolysis or bystander damage. CNS infection with the neurotropic strain of mouse hepatitis virus (MHV) induces an acute encephalomyelitis associated with focal areas of demyelination, which is sustained during viral persistence. Both innate and adaptive immune cells work in coordination to control virus replication. While type I interferons are essential to limit virus spread associated with early mortality, perforin, and interferon-γ promote further virus clearance in astrocytes/microglia and oligodendrocytes, respectively. Effective control of virus replication is nonetheless associated with tissue damage, characterized by demyelinating lesions. Interestingly, the anti-inflammatory cytokine IL-10 limits expansion of tissue lesions during chronic infection without affecting viral persistence. Thus, effective coordination of pro- and anti-inflammatory cytokines is essential during MHV induced encephalomyelitis in order to protect the host against viral infection at a limited cost.

Intercellular Communication Is Key for Protective IFNα/β Signaling During Viral Central Nervous System Infection

A variety of viruses can induce central nervous system (CNS) infections and neurological diseases, although the incidence is rare. Similar to peripheral infections, IFNα/β induction and signaling constitutes a first line of defense to limit virus dissemination. However, CNS-resident cells differ widely in their repertoire and magnitude of both basal and inducible components in the IFNα/β pathway. While microglia as resident myeloid cells have been implicated as prominent sentinels of CNS invading pathogens or insults, astrocytes are emerging as key responders to many neurotropic RNA virus infections. Focusing on RNA viruses, this review discusses the role of astrocytes as IFNα/β inducers and responders and touches on the role of IFNα/β receptor signaling in regulating myeloid cell activation and IFNγ responsiveness. A summary picture emerges implicating IFNα/β not only as key in establishing the classical "antiviral" state, but also orchestrating cell mobility and IFNγ-mediated effector functions.

Promoting remyelination through cell transplantation therapies in a model of viral-induced neurodegenerative disease

Multiple sclerosis (MS) is a central nervous system (CNS) disease characterized by chronic neuroinflammation, demyelination, and axonal damage. Infiltration of activated lymphocytes and myeloid cells are thought to be primarily responsible for white matter damage and axonopathy. Several United States Food and Drug Administration-approved therapies exist that impede activated lymphocytes from entering the CNS thereby limiting new lesion formation in patients with relapse-remitting forms of MS. However, a significant challenge within the field of MS research is to develop effective and sustained therapies that allow for axonal protection and remyelination. In recent years, there has been increasing evidence that some kinds of stem cells and their derivatives seem to be able to mute neuroinflammation as well as promote remyelination and axonal integrity. Intracranial infection of mice with the neurotropic JHM strain of mouse hepatitis virus (JHMV) results in immune-mediated demyelination and axonopathy, making this an excellent model to interrogate the therapeutic potential of stem cell derivatives in evoking remyelination. This review provides a succinct overview of our recent findings using intraspinal injection of mouse CNS neural progenitor cells and human neural precursors into JHMV-infected mice. JHMV-infected mice receiving these cells display extensive remyelination associated with axonal sparing. In addition, we discuss possible mechanisms associated with sustained clinical recovery. Developmental Dynamics 248:43-52, 2019. © 2018 Wiley Periodicals, Inc.

Chemokine CXCL10 and Coronavirus-Induced Neurologic Disease

Chemokines (chemotactic cytokines) are involved in a wide variety of biological processes. Following microbial infection, there is often robust chemokine signaling elicited from infected cells, which contributes to both innate and adaptive immune responses that control growth of the invading pathogen. Infection of the central nervous system (CNS) by the neuroadapted John Howard Mueller (JHM) strain of mouse hepatitis virus (JHMV) provides an excellent example of how chemokines aid in host defense as well as contribute to disease. Intracranial inoculation of the CNS of susceptible mice with JHMV results in an acute encephalomyelitis characterized by widespread dissemination of virus throughout the parenchyma. Virus-specific T cells are recruited to the CNS, and control viral replication through release of antiviral cytokines and cytolytic activity. Sterile immunity is not acquired, and virus will persist primarily in white matter tracts leading to chronic neuroinflammation and demyelination. Chemokines are expressed and contribute to defense as well as chronic disease by attracting targeted populations of leukocytes to the CNS. The T cell chemoattractant chemokine CXCL10 (interferon-inducible protein 10 kDa, IP-10) is prominently expressed in both stages of disease, and serves to attract activated T and B lymphocytes expressing CXC chemokine receptor 3 (CXCR3), the receptor for CXCL10. Functional studies that have blocked expression of either CXCL10 or CXCR3 illuminate the important role of this signaling pathway in host defense and neurodegeneration in a model of viral-induced neurologic disease.

Distinct Gene Profiles of Bone Marrow-Derived Macrophages and Microglia During Neurotropic Coronavirus-Induced Demyelination

Multiple Sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) characterized by demyelination and axonal loss. Demyelinating lesions are associated with infiltrating T lymphocytes, bone marrow-derived macrophages (BMDM), and activated resident microglia. Tissue damage is thought to be mediated by T cell produced cytokines and chemokines, which activate microglia and/or BMDM to both strip myelin and produce toxic factors, ultimately damaging axons and promoting disability. However, the relative contributions of BMDM and microglia to demyelinating pathology are unclear, as their identification in MS tissue is difficult due to similar morphology and indistinguishable surface markers when activated. The CD4 T cell-induced autoimmune murine model of MS, experimental autoimmune encephalitis (EAE), in which BMDM are essential for demyelination, has revealed pathogenic and repair-promoting phenotypes associated with BMDM and microglia, respectively. Using a murine model of demyelination induced by a gliatropic coronavirus, in which BMDM are redundant for demyelination, we herein characterize gene expression profiles of BMDM versus microglia associated with demyelination. While gene expression in CNS infiltrating BMDM was upregulated early following infection and subsequently sustained, microglia expressed a more dynamic gene profile with extensive mRNA upregulation coinciding with peak demyelination after viral control. This delayed microglia response comprised a highly pro-inflammatory and phagocytic profile. Furthermore, while BMDM exhibited a mixed phenotype of M1 and M2 markers, microglia repressed the vast majority of M2-markers. Overall, these data support a pro-inflammatory and pathogenic role of microglia temporally remote from viral control, whereas BMDM retained their gene expression profile independent of the changing environment. As demyelination is caused by multifactorial insults, our results highlight the plasticity of microglia in responding to distinct inflammatory settings, which may be relevant for MS pathogenesis.

Selective Packaging in Murine Coronavirus Promotes Virulence by Limiting Type I Interferon Responses

Selective packaging is a mechanism used by multiple virus families to specifically incorporate genomic RNA (gRNA) into virions and exclude other types of RNA. Lineage A betacoronaviruses incorporate a 95-bp stem-loop structure, the packaging signal (PS), into the nsp15 locus of ORF1b that is both necessary and sufficient for the packaging of RNAs. However, unlike other viral PSs, where mutations generally resulted in viral replication defects, mutation of the coronavirus (CoV) PS results in large increases in subgenomic RNA packaging with minimal effects on gRNA packaging in vitro and on viral titers. Here, we show that selective packaging is also required for viral evasion of the innate immune response and optimal pathogenicity. We engineered two distinct PS mutants in two different strains of murine hepatitis virus (MHV) that packaged increased levels of subgenomic RNAs, negative-sense genomic RNA, and even cellular RNAs. All PS mutant viruses replicated normally in vitro but caused dramatically reduced lethality and weight loss in vivo. PS mutant virus infection of bone marrow-derived macrophages resulted in increased interferon (IFN) production, indicating that the innate immune system limited the replication and/or pathogenesis of PS mutant viruses in vivo. PS mutant viruses remained attenuated in MAVS^−/− and Toll-like receptor 7-knockout (TLR7^−/−) mice, two well-known RNA sensors for CoVs, but virulence was restored in interferon alpha/beta receptor-knockout (IFNAR^−/−) mice or in MAVS^−/− mice treated with IFNAR-blocking antibodies. Together, these data indicate that coronaviruses promote virulence by utilizing selective packaging to avoid innate immune detection.

Coronaviruses (CoVs) produce many types of RNA molecules during their replication cycle, including both positive- and negative-sense genomic and subgenomic RNAs. Despite this, coronaviruses selectively package only positive-sense genomic RNA into their virions. Why CoVs selectively package their genomic RNA is not clear, as disruption of the packaging signal in MHV, which leads to loss of selective packaging, does not affect genomic RNA packaging or virus replication in cultured cells. This contrasts with other viruses, where disruption of selective packaging generally leads to altered replication. Here, we demonstrate that in the absence of selective packaging, the virulence of MHV was significantly reduced. Importantly, virulence was restored in the absence of interferon signaling, indicating that selective packaging is a mechanism used by CoVs to escape innate immune detection.

Alpha/Beta Interferon (IFN-α/β) Signaling in Astrocytes Mediates Protection against Viral Encephalomyelitis and Regulates IFN-γ-Dependent Responses

The contribution of distinct central nervous system (CNS) resident cells to protective alpha/beta interferon (IFN-α/β) function following viral infections is poorly understood. Based on numerous immune regulatory functions of astrocytes, we evaluated the contribution of astrocyte IFN-α/β signaling toward protection against the nonlethal glia- and neuronotropic mouse hepatitis virus (MHV) strain A59. Analysis of gene expression associated with IFN-α/β function, e.g., pattern recognition receptors (PRRs) and interferon-stimulated genes (ISGs), revealed lower basal mRNA levels in brain-derived astrocytes than in microglia. Although astrocytes poorly induced Ifnβ mRNA following infection, they upregulated various mRNAs in the IFN-α/β pathway to a higher extent than microglia, supporting effective IFN-α/β responsiveness. Ablation of the IFN-α/β receptor (IFNAR) in astrocytes using mGFAPcre IFNAR^fl/fl mice resulted in severe encephalomyelitis and mortality, coincident with uncontrolled virus replication. Further, virus spread was not restricted to astrocytes but also affected microglia and neurons, despite increased and sustained Ifnα/β and ISG mRNA levels within the CNS. IFN-γ, a crucial mediator for MHV control, was not impaired in infected mGFAPcre IFNAR^fl/fl mice despite reduced T cell CNS infiltration. Unexpectedly however, poor induction of IFN-γ-dependent major histocompatibility complex (MHC) class II expression on microglia supported that defective IFN-γ signaling contributes to uncontrolled virus replication. A link between sustained elevated IFN-α/β and impaired responsiveness to IFN-γ supports the novel concept that temporally limited early IFN-α/β responses are critical for effective antiviral IFN-γ function. Overall, our results imply that IFN-α/β signaling in astrocytes is not only critical in limiting early CNS viral spread but also promotes protective antiviral IFN-γ function.IMPORTANCE An antiviral state established by IFN-α/β contains initial viral spread as adaptive immunity develops. While it is apparent that the CNS lacks professional IFN-α/β producers and that resident cells have distinct abilities to elicit innate IFN-α/β responses, protective interactions between inducer and responder cells require further investigation. Infection with a glia- and neuronotropic coronavirus demonstrates that astrocytes mount a delayed but more robust response to infection than microglia, despite their lower basal mRNA levels of IFN-α/β-inducing components. Lethal, uncontrolled viral dissemination following ablation of astrocyte IFN-α/β signaling revealed the importance of IFN-α/β responses in a single cell type for protection. Sustained global IFN-α/β expression associated with uncontrolled virus did not suffice to protect neurons and further impaired responsiveness to protective IFN-γ. The results support astrocytes as critical contributors to innate immunity and the concept that limited IFN-α/β responses are critical for effective subsequent antiviral IFN-γ function.

What Kaplan-Meier survival curves don't tell us about CNS disease

Central nervous system consequences of viral infections are rare, but when they do occur, they are often serious and clinically challenging to manage. Our awareness of the perils of neuroinvasion by viruses is growing: the recently appreciated impact of Ebola and Zika virus infections on CNS integrity, decreases in vaccination coverage for potentially neurotropic viruses such as measles, and increased neurovirulence of some influenza strains collectively highlight the need for a better understanding of the viral-neural interaction. Defining these interactions and how they result in neuropathogenesis is paramount for the development of better clinical strategies, especially given the limited treatment options that are available due to the unique physiology of the brain that limits migration of blood-borne molecules into the CNS parenchyma. In this perspective, we discuss some unique aspects of neuronal viral infections and immune-mediated control that impact the pathogenic outcomes of these infections. Further, we draw attention to an often overlooked aspect of neuropathogenesis research: that lack of overt disease, which is often equated with survival post-infection, likely only scratches the surface of the myriad ways by which neurotropic infections can impair CNS function.

Virus-induced inflammasome activation is suppressed by prostaglandin D2/DP1 signaling

Prostaglandin D2 (PGD₂), an eicosanoid with both pro- and anti-inflammatory properties, is the most abundantly expressed prostaglandin in the brain. Here we show that PGD₂ signaling through the D-prostanoid receptor 1 (DP1) receptor is necessary for optimal microglia/macrophage activation and IFN expression after infection with a neurotropic coronavirus. Genome-wide expression analyses indicated that PGD₂/DP1 signaling is required for up-regulation of a putative inflammasome inhibitor, PYDC3, in CD11b⁺ cells in the CNS of infected mice. Our results also demonstrated that, in addition to PGD₂/DP1 signaling, type 1 IFN (IFN-I) signaling is required for PYDC3 expression. In the absence of Pydc3 up-regulation, IL-1β expression and, subsequently, mortality were increased in infected DP1^-/- mice. Notably, survival was enhanced by IL1 receptor blockade, indicating that the effects of the absence of DP1 signaling on clinical outcomes were mediated, at least in part, by inflammasomes. Using bone marrow-derived macrophages in vitro, we confirmed that PYDC3 expression is dependent upon DP1 signaling and that IFN priming is critical for PYDC3 up-regulation. In addition, Pydc3 silencing or overexpression augmented or diminished IL-1β secretion, respectively. Furthermore, DP1 signaling in human macrophages also resulted in the up-regulation of a putative functional analog, POP3, suggesting that PGD₂ similarly modulates inflammasomes in human cells. These findings demonstrate a previously undescribed role for prostaglandin signaling in preventing excessive inflammasome activation and, together with previously published results, suggest that eicosanoids and inflammasomes are reciprocally regulated.

Keeping it in check: chronic viral infection and antiviral immunity in the brain

It is becoming clear that the manner by which the immune response resolves or contains infection by a pathogen varies according to the tissue that is affected. Unlike many peripheral cell types, CNS neurons are generally non-renewable. Thus, the cytolytic and inflammatory strategies that are effective in controlling infections in the periphery could be damaging if deployed in the CNS. Perhaps for this reason, the immune response to some CNS viral infections favours maintenance of neuronal integrity and non-neurolytic viral control. This modified immune response - when combined with the unique anatomy and physiology of the CNS - provides an ideal environment for the maintenance of viral genomes, including those of RNA viruses. Therefore, it is possible that such viruses can reactivate long after initial viral exposure, contributing to CNS disease.

Interferon gamma modulation of disease manifestation and the local antibody response to alphavirus encephalomyelitis

Infection of mice with Sindbis virus (SINV) produces encephalomyelitis and provides a model for examination of the central nervous system (CNS) immune response to alphavirus infection. Clearance of infectious virus is accomplished through a cooperative effort between SINV-specific antibody and IFN-γ, but the regulatory interactions are poorly understood. To determine the effects of IFN-γ on clinical disease and the antiviral immune response, C57BL/6 mice lacking IFN-γ (Ifng-/-) or IFN-γ receptor (Ifngr1-/-) were studied in comparison to WT mice. Maximum production of Ifng mRNA and IFN-γ protein in the CNS of WT and Ifngr1-/- mice occurred 5-7 days after infection, with higher levels of IFN-γ in Ifngr1-/- mice. Onset of clinical disease was earlier in mice with impaired IFN-γ signalling, although Ifngr1-/- mice recovered more rapidly. Ifng-/- and Ifngr1-/- mice maintained body weight better than WT mice, associated with better food intake and lower brain levels of inflammatory cytokines. Clearance of infectious virus from the spinal cords was slower, and CNS, but not serum, levels of SINV-specific IgM, IgG2a and IgG2b were lower in Ifngr1-/- and Ifng-/- mice compared to WT mice. Decreased CNS antiviral antibody was associated with lower expression of mRNAs for B-cell attracting chemokines CXCL9, CXCL10 and CXCL13 and fewer B cells in the CNS. Therefore, IFN-γ signalling increases levels of CNS pro-inflammatory cytokines, leading to clinical disease, but synergistically clears virus with SINV-specific antibody at least in part by increasing chemokine production important for infiltration of antibody-secreting B cells into the CNS.

Toll-like receptor-2 exacerbates murine acute viral hepatitis

Viral replication in the liver is generally detected by cellular endosomal Toll‐like receptors (TLRs) and cytosolic helicase sensors that trigger antiviral inflammatory responses. Recent evidence suggests that surface TLR2 may also contribute to viral detection through recognition of viral coat proteins but its role in the outcome of acute viral infection remains elusive. In this study, we examined in vivo the role of TLR2 in acute infections induced by the highly hepatotrophic mouse hepatitis virus (MHV) type 3 and weakly hepatotrophic MHV‐A59 serotype. To address this, C57BL/6 (wild‐type; WT) and TLR2 knockout (KO) groups of mice were intraperitoneally infected with MHV3 or MHV‐A59. MHV3 infection provoked a fulminant hepatitis in WT mice, characterized by early mortality and high alanine and aspartate transaminase levels, histopathological lesions and viral replication whereas infection of TLR2 KO mice was markedly less severe. MHV‐A59 provoked a comparable mild and subclinical hepatitis in WT and TLR2 KO mice. MHV3‐induced fulminant hepatitis in WT mice correlated with higher hepatic expression of interferon‐β, interleukin‐6, tumour necrosis factor‐α, CXCL1, CCL2, CXCL10 and alarmin (interleukin‐33) than in MHV‐A59‐infected WT mice and in MHV3‐infected TLR2 KO mice. Intrahepatic recruited neutrophils, natural killer cells, natural killer T cells or macrophages rapidly decreased in MHV3‐infected WT mice whereas they were sustained in MHV‐A59‐infected WT mice and MHV3‐infected TLR2 KO. MHV3 in vitro infection of macrophagic cells induced rapid and higher viral replication and/or interleukin‐6 induction in comparison to MHV‐A59, and depended on viral activation of TLR2 and p38 mitogen‐activated protein kinase. Taken together, these results support a new aggravating inflammatory role for TLR2 in MHV3‐induced acute fulminant hepatitis.

Activation of RNase L by Murine Coronavirus in Myeloid Cells Is Dependent on Basal Oas Gene Expression and Independent of Virus-Induced Interferon

The oligoadenylate synthetase (OAS)-RNase L pathway is a potent interferon (IFN)-induced antiviral activity. Upon sensing double-stranded RNA, OAS produces 2',5'-oligoadenylates (2-5A), which activate RNase L. Murine coronavirus (mouse hepatitis virus [MHV]) nonstructural protein 2 (ns2) is a 2',5'-phosphodiesterase (PDE) that cleaves 2-5A, thereby antagonizing RNase L activation. PDE activity is required for robust replication in myeloid cells, as a mutant of MHV (ns2(H126R)) encoding an inactive PDE fails to antagonize RNase L activation and replicates poorly in bone marrow-derived macrophages (BMM), while ns2(H126R) replicates to high titer in several types of nonmyeloid cells, as well as in IFN receptor-deficient (Ifnar1(-/-)) BMM. We reported previously that myeloid cells express significantly higher basal levels of OAS transcripts than nonmyeloid cells. Here, we investigated the contributions of Oas gene expression, basal IFN signaling, and virus-induced IFN to RNase L activation. Infection with ns2(H126R) activated RNase L in Ifih1(-/-) BMM to a similar extent as in wild-type (WT) BMM, despite the lack of IFN induction in the absence of MDA5 expression. However, ns2(H126R) failed to induce RNase L activation in BMM treated with IFNAR1-blocking antibody, as well as in Ifnar1(-/-) BMM, both expressing low basal levels of Oas genes. Thus, activation of RNase L does not require virus-induced IFN but rather correlates with adequate levels of basal Oas gene expression, maintained by basal IFN signaling. Finally, overexpression of RNase L is not sufficient to compensate for inadequate basal OAS levels.

IMPORTANCE

The oligoadenylate synthetase (OAS)-RNase L pathway is a potent antiviral activity. Activation of RNase L during murine coronavirus (mouse hepatitis virus [MHV]) infection of myeloid cells correlates with high basal Oas gene expression and is independent of virus-induced interferon secretion. Thus, our data suggest that cells with high basal Oas gene expression levels can activate RNase L and thereby inhibit virus replication early in infection upon exposure to viral double-stranded RNA (dsRNA) before the induction of interferon and prior to transcription of interferon-stimulated antiviral genes. These findings challenge the notion that activation of the OAS-RNase L pathway requires virus to induce type I IFN, which in turn upregulates OAS gene expression, as well as to provide dsRNA to activate OAS. Our data further suggest that myeloid cells may serve as sentinels to restrict viral replication, thus protecting other cell types from infection.

MDA5 Is Critical to Host Defense during Infection with Murine Coronavirus

Infection with the murine coronavirus mouse hepatitis virus (MHV) activates the pattern recognition receptors melanoma differentiation-associated gene 5 (MDA5) and Toll-like receptor 7 (TLR7) to induce transcription of type I interferon. Type I interferon is crucial for control of viral replication and spread in the natural host, but the specific contributions of MDA5 signaling to this pathway as well as to pathogenesis and subsequent immune responses are largely unknown. In this study, we use MHV infection of the liver as a model to demonstrate that MDA5 signaling is critically important for controlling MHV-induced pathology and regulation of the immune response. Mice deficient in MDA5 expression (MDA5(-/-) mice) experienced more severe disease following MHV infection, with reduced survival, increased spread of virus to additional sites of infection, and more extensive liver damage than did wild-type mice. Although type I interferon transcription decreased in MDA5(-/-) mice, the interferon-stimulated gene response remained intact. Cytokine production by innate and adaptive immune cells was largely intact in MDA5(-/-) mice, but perforin induction by natural killer cells and levels of interferon gamma, interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF-α) in serum were elevated. These data suggest that MDA5 signaling reduces the severity of MHV-induced disease, at least in part by reducing the intensity of the proinflammatory cytokine response.

IMPORTANCE

Multicellular organisms employ a wide range of sensors to detect viruses and other pathogens. One such sensor, MDA5, detects viral RNA and triggers induction of type I interferons, chemical messengers that induce inflammation and help regulate the immune responses. In this study, we sought to determine the role of MDA5 during infection with mouse hepatitis virus, a murine coronavirus used to model viral hepatitis as well as other human diseases. We found that mice lacking the MDA5 sensor were more susceptible to infection than were mice with MDA5 and experienced decreased survival. Viral replication in the liver was similar in mice with and without MDA5, but liver damage was increased in MDA5(-/-) mice, suggesting that the immune response is causing the damage. Production of several proinflammatory cytokines was elevated in MDA5(-/-) mice, suggesting that MDA5 may be responsible for keeping pathological inflammatory responses in check.

The ns12.9 Accessory Protein of Human Coronavirus OC43 Is a Viroporin Involved in Virion Morphogenesis and Pathogenesis

An accessory gene between the S and E gene loci is contained in all coronaviruses (CoVs), and its function has been studied in some coronaviruses. This gene locus in human coronavirus OC43 (HCoV-OC43) encodes the ns12.9 accessory protein; however, its function during viral infection remains unknown. Here, we engineered a recombinant mutant virus lacking the ns12.9 protein (HCoV-OC43-Δns12.9) to characterize the contributions of ns12.9 in HCoV-OC43 replication. The ns12.9 accessory protein is a transmembrane protein and forms ion channels in both Xenopus oocytes and yeast through homo-oligomerization, suggesting that ns12.9 is a newly recognized viroporin. HCoV-OC43-Δns12.9 presented at least 10-fold reduction of viral titer in vitro and in vivo. Intriguingly, exogenous ns12.9 and heterologous viroporins with ion channel activity could compensate for the production of HCoV-OC43-Δns12.9, indicating that the ion channel activity of ns12.9 plays a significant role in the production of infectious virions. Systematic dissection of single-cycle replication revealed that ns12.9 protein had no measurable effect on virus entry, subgenomic mRNA (sgmRNA) synthesis, and protein expression. Further characterization revealed that HCoV-OC43-Δns12.9 was less efficient in virion morphogenesis than recombinant wild-type virus (HCoV-OC43-WT). Moreover, reduced viral replication, inflammatory response, and virulence in HCoV-OC43-Δns12.9-infected mice were observed compared to the levels for HCoV-OC43-WT-infected mice. Taken together, our results demonstrated that the ns12.9 accessory protein functions as a viroporin and is involved in virion morphogenesis and the pathogenesis of HCoV-OC43 infection.

IMPORTANCE

HCoV-OC43 was isolated in the 1960s and is a major agent of the common cold. The functions of HCoV-OC43 structural proteins have been well studied, but few studies have focused on its accessory proteins. In the present study, we demonstrated that the ns12.9 protein is a newly recognized viroporin, and the ns12.9 gene knockout virus (HCoV-OC43-Δns12.9) presents a growth defect in vitro and in vivo. We identified the important functions of the ns12.9 viroporin in virion morphogenesis during HCoV-OC43 infection. Furthermore, mice infected with HCoV-OC43-Δns12.9 exhibited reduced inflammation and virulence accompanied by a lower titer in the brain than that of wild-type-infected mice, suggesting the ns12.9 viroporin influences virus pathogenesis. Therefore, our findings revealed that the ns12.9 viroporin facilitates virion morphogenesis to enhance viral production, and these results provided a deeper understanding of HCoV-OC43 pathogenesis.

Myd88 Initiates Early Innate Immune Responses and Promotes CD4 T Cells during Coronavirus Encephalomyelitis

Myd88 signaling is critical to the control of numerous central nervous system (CNS) infections by promoting both innate and adaptive immune responses. Nevertheless, the extent to which Myd88 regulates type I interferon (IFN) versus proinflammatory factors and T cell function, as well as the anatomical site of action, varies extensively with the pathogen. CNS infection by neurotropic coronavirus with replication confined to the brain and spinal cord induces protective IFN-α/β via Myd88-independent activation of melanoma differentiation-associated gene 5 (MDA5). However, a contribution of Myd88-dependent signals to CNS pathogenesis has not been assessed. Infected Myd88(-/-) mice failed to control virus, exhibited enhanced clinical disease coincident with increased demyelination, and succumbed to infection within 3 weeks. The induction of IFN-α/β, as well as of proinflammatory cytokines and chemokines, was impaired early during infection. However, defects in both IFN-α/β and select proinflammatory factors were rapidly overcome prior to T cell recruitment. Myd88 deficiency also specifically blunted myeloid and CD4 T cell recruitment into the CNS without affecting CD8 T cells. Moreover, CD4 T cells but not CD8 T cells were impaired in IFN-γ production. Ineffective virus control indeed correlated most prominently with reduced antiviral IFN-γ in the CNS of Myd88(-/-) mice. The results demonstrate a crucial role for Myd88 both in early induction of innate immune responses during coronavirus-induced encephalomyelitis and in specifically promoting protective CD4 T cell activation. In the absence of these responses, functional CD8 T cells are insufficient to control viral spread within the CNS, resulting in severe demyelination.

IMPORTANCE

During central nervous system (CNS) infections, signaling through the adaptor protein Myd88 promotes both innate and adaptive immune responses. The extent to which Myd88 regulates antiviral type I IFN, proinflammatory factors, adaptive immunity, and pathology is pathogen dependent. These results reveal that Myd88 protects from lethal neurotropic coronavirus-induced encephalomyelitis by accelerating but not enhancing the induction of IFN-α/β, as well as by promoting peripheral activation and CNS accumulation of virus-specific CD4 T cells secreting IFN-γ. By controlling both early innate immune responses and CD4 T cell-mediated antiviral IFN-γ, Myd88 signaling limits the initial viral dissemination and is vital for T cell-mediated control of viral loads. Uncontrolled viral replication in the absence of Myd88 leads to severe demyelination and pathology despite overall reduced inflammatory responses. These data support a vital role of Myd88 signaling in protective antimicrobial functions in the CNS by promoting proinflammatory mediators and T cell-mediated IFN-γ production.

Promoting remyelination: utilizing a viral model of demyelination to assess cell-based therapies

Multiple sclerosis (MS) is a chronic inflammatory disease of the CNS. While a broad range of therapeutics effectively reduce the incidence of focal white matter inflammation and plaque formation for patients with relapse-remitting forms of MS, a challenge within the field is to develop therapies that allow for axonal protection and remyelination. In the last decade, growing interest has focused on utilizing neural precursor cells (NPCs) to promote remyelination. To understand how NPCs function in chronic demyelinating environments, several excellent pre-clinical mouse models have been developed. One well accepted model is infection of susceptible mice with neurotropic variants of mouse hepatitis virus (MHV) that undergo chronic demyelination exhibiting clinical and histopathologic similarities to MS patients. Combined with the possibility that an environmental agent such as a virus could trigger MS, the MHV model of demyelination presents a relevant mouse model to assess the therapeutic potential of NPCs transplanted into an environment in which inflammatory-mediated demyelination is established.

Distinct CD4 T-cell effects on primary versus recall CD8 T-cell responses during viral encephalomyelitis

CD4 T-cell help is not a universal requirement for effective primary CD8 T cells but is essential to generate memory CD8 T cells capable of recall responses. This study examined how CD4 T cells affect primary and secondary anti-viral CD8 T-cell responses within the central nervous system (CNS) during encephalomyelitis induced by sublethal gliatropic coronavirus. CD4 T-cell depletion before infection did not impair peripheral expansion, interferon-γ production, CNS recruitment or initial CNS effector capacity of virus-specific CD8 T cells ex vivo. Nevertheless, impaired virus control in the absence of CD4 T cells was associated with gradually diminished CNS CD8 T-cell interferon-γ production. Furthermore, within the CD8 T-cell population short-lived effector cells were increased and memory precursor effector cells were significantly decreased, consistent with higher T-cell turnover. Transfer of memory CD8 T cells to reduce viral load in CD4-depleted mice reverted the recipient CNS CD8 T-cell phenotype to that in wild-type control mice. However, memory CD8 T cells primed without CD4 T cells and transferred into infected CD4-sufficient recipients expanded less efficiently and were not sustained in the CNS, contrasting with their helped counterparts. These data suggest that CD4 T cells are dispensable for initial expansion, CNS recruitment and differentiation of primary resident memory CD8 T cells as long as the duration of antigen exposure is limited. By contrast, CD4 T cells are essential to prolong primary CD8 T-cell function in the CNS and imprint memory CD8 T cells for recall responses.

The nsp3 macrodomain promotes virulence in mice with coronavirus-induced encephalitis

All coronaviruses encode a macrodomain containing ADP-ribose-1"-phosphatase (ADRP) activity within the N terminus of nonstructural protein 3 (nsp3). Previous work showed that mouse hepatitis virus strain A59 (MHV-A59) with a mutated catalytic site (N1348A) replicated similarly to wild-type virus but was unable to cause acute hepatitis in mice. To determine whether this attenuated phenotype is applicable to multiple disease models, we mutated the catalytic residue in the JHM strain of MHV (JHMV), which causes acute and chronic encephalomyelitis, using a newly developed bacterial artificial chromosome (BAC)-based MHV reverse genetics system. Infection of mice with the macrodomain catalytic point mutant virus (N1347A) resulted in reductions in lethality, weight loss, viral titers, proinflammatory cytokine and chemokine expression, and immune cell infiltration in the brain compared to mice infected with wild-type virus. Specifically, macrophages were most affected, with approximately 2.5-fold fewer macrophages at day 5 postinfection in N1347A-infected brains. Tumor necrosis factor (TNF) and interferon (IFN) signaling were not required for effective host control of mutant virus as all N1347A virus-infected mice survived the infection. However, the adaptive immune system was required for protection since N1347A virus was able to cause lethal encephalitis in RAG1(-/-) (recombination activation gene 1 knockout) mice although disease onset was modestly delayed. Overall, these results indicate that the BAC-based MHV reverse genetics system will be useful for studies of JHMV and expand upon previous studies, showing that the macrodomain is critical for the ability of coronaviruses to evade the immune system and promote viral pathogenesis.

IMPORTANCE

Coronaviruses are an important cause of human and veterinary diseases worldwide. Viral processes that are conserved across a family are likely to be good targets for the development of antiviral therapeutics and vaccines. The macrodomain is a ubiquitous structural domain and is also conserved among all coronaviruses. The coronavirus macrodomain has ADP-ribose-1"-phosphatase activity; however, its function during infection remains unclear as does the reason that coronaviruses have maintained this enzymatic activity throughout evolution. For MHV, this domain has now been shown to promote multiple types of disease, including hepatitis and encephalitis. These data indicate that this domain is vital for the virus to replicate and cause disease. Understanding the mechanism used by this enzyme to promote viral pathogenesis will open up novel avenues for therapies and may give further insight into the role of macrodomain proteins in the host cell since these proteins are found in all living organisms.

Coronavirus infection, ER stress, apoptosis and innate immunity

The replication of coronavirus, a family of important animal and human pathogens, is closely associated with the cellular membrane compartments, especially the endoplasmic reticulum (ER). Coronavirus infection of cultured cells was previously shown to cause ER stress and induce the unfolded protein response (UPR), a process that aims to restore the ER homeostasis by global translation shutdown and increasing the ER folding capacity. However, under prolonged ER stress, UPR can also induce apoptotic cell death. Accumulating evidence from recent studies has shown that induction of ER stress and UPR may constitute a major aspect of coronavirus–host interaction. Activation of the three branches of UPR modulates a wide variety of signaling pathways, such as mitogen-activated protein (MAP) kinase activation, autophagy, apoptosis, and innate immune response. ER stress and UPR activation may therefore contribute significantly to the viral replication and pathogenesis during coronavirus infection. In this review, we summarize the current knowledge on coronavirus-induced ER stress and UPR activation, with emphasis on their cross-talking to apoptotic signaling.

PKR mediated regulation of inflammation and IL-10 during viral encephalomyelitis

Double-stranded RNA-dependent protein kinase (PKR) regulates antiviral activity, immune responses, apoptosis and neurotoxicity. Gliatropic coronavirus infection induced PKR activation in infected as well uninfected cells within the central nervous system (CNS). However, PKR deficiency only modestly increased viral replication and did not affect IFN-α/β or IL-1β expression. Despite reduced Il-6, Ccl5, and Cxcl10 mRNA, protein levels remained unaltered. Furthermore, PKR deficiency selectively reduced IL-10 production in CD4, but not CD8 T cells, without affecting CNS pathology. The results demonstrate the ability of PKR to balance neuroinflammation by selectively modulating key cytokines and chemokines in CNS resident and CD4 T cells.

Neutrophils are needed for an effective immune response against pulmonary rat coronavirus infection, but also contribute to pathology

Polymorphonuclear neutrophils (PMN) infiltrate the respiratory tract early after viral infection and can contribute to both host defence and pathology. Coronaviruses are important causes of respiratory tract infections, ranging from mild to severe depending on the viral strain. This study evaluated the role of PMN during a non-fatal pulmonary coronavirus infection in the natural host. Rat coronavirus (RCoV) causes respiratory disease in adult rats, characterized by an early PMN response, viral replication and inflammatory lesions in the lungs, mild weight loss and effective resolution of infection. To determine their role during RCoV infection, PMN were depleted and the effects on disease progression, viral replication, inflammatory response and lung pathology were analysed. Compared with RCoV infection in control animals, PMN-depleted rats had worsened disease with weight loss, clinical signs, mortality and prolonged pulmonary viral replication. PMN-depleted animals had fewer macrophages and lymphocytes in the respiratory tract, corresponding to lower chemokine levels. Combined with in vitro experiments showing that PMN express cytokines and chemokines in response to RCoV-infected alveolar epithelial cells, these findings support a role for PMN in eliciting an inflammatory response to RCoV infection. Despite their critical role in the protection from severe disease, the presence of PMN was correlated with haemorrhagic lesions, epithelial barrier permeability and cellular inflammation in the lungs. This study demonstrated that while PMN are required for an effective antiviral response, they also contribute to lung pathology during RCoV infection.

Ifit2 deficiency results in uncontrolled neurotropic coronavirus replication and enhanced encephalitis via impaired alpha/beta interferon induction in macrophages

Type I interferons (IFN-α/β) limit viral dissemination prior to the emergence of adaptive immune responses through the concerted action of interferon-stimulated genes (ISGs). Although IFN-α/β induction by coronaviruses is modest, it effectively limits viral spread within the central nervous system (CNS) and protects against mortality. The protective roles of specific ISGs against the mouse hepatitis virus (MHV) members of the coronaviruses are largely unknown. This study demonstrates a protective role of the ISG Ifit2 in encephalitis induced by the dual hepato- and neurotropic MHV-A59. Contrasting the mild encephalitis and 100% survival of MHV-A59-infected wild-type (wt) mice, nearly 60% of infected Ifit2(-/-) mice exhibited severe encephalitis and succumbed between 6 and 8 days postinfection. Increased clinical disease in Ifit2(-/-) mice coincided with higher viral loads and enhanced viral spread throughout the CNS parenchyma. Ifit2(-/-) mice also expressed significantly reduced IFN-α/β and downstream ISG mRNAs Ifit1, Isg15, and Pkr, while expression of proinflammatory cytokines and chemokines was only modestly affected in the CNS. Impaired IFN-α/β induction in the absence of Ifit2 was confirmed by ex vivo mRNA analysis of microglia and macrophages, the prominent cell types producing IFN-α/β following MHV CNS infection. Furthermore, both IFN-α/β mRNA and protein production were significantly reduced in MHV-infected Ifit2(-/-) relative to wt bone marrow-derived macrophages. Collectively, the data implicate Ifit2 as a positive regulator of IFN-α/β expression, rather than direct antiviral mediator, during MHV-induced encephalitis.

Confounding roles for type I interferons during bacterial and viral pathogenesis

Although type I interferons (IFN-I) were initially defined as potent antiviral agents, they can also cause decreased host resistance to some bacterial and viral infections. The many antiviral functions of the IFN-I include direct suppression of viral replication and activation of the immune response against viruses. In addition to their antiviral effects, IFN-I are also protective against several extracellular bacterial infections, in part, by promoting the induction of TNF-α and nitric oxide. In contrast, there is a negative effect of IFN-I on host resistance during chronic infection with lymphocytic choriomeningitis virus (LCMV) and acute infections with intracellular bacteria. In the case of LCMV, chronic IFN-I signaling induces adaptive immune system suppression. Blockade of IFN-I signaling removes the suppression and allows CD4 T-cell- and IFN-γ-mediated resolution of the infection. During acute intracellular bacterial infection, IFN-I suppress innate immunity by at least two defined mechanisms. During Francisella infection, IFN-I prevent IL-17 upregulation on γδ T cells and neutrophil recruitment. Following Listeria infection, IFN-I promote the cell death of macrophages and lymphocytes, which leads to innate immune suppression. These divergent findings for the role of IFN-I on pathogen control emphasize the complexity of the interferons system and force more mechanistic evaluation of its role in pathogenesis. This review evaluates IFN-I during infection with an emphasis on work carried out IFN-I-receptor-deficient mice.

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.

Cell-type-specific activation of the oligoadenylate synthetase-RNase L pathway by a murine coronavirus

Previous studies have demonstrated that the murine coronavirus mouse hepatitis virus (MHV) nonstructural protein 2 (ns2) is a 2',5'-phosphodiesterase that inhibits activation of the interferon-induced oligoadenylate synthetase (OAS)-RNase L pathway. Enzymatically active ns2 is required for efficient MHV replication in macrophages, as well as for the induction of hepatitis in C57BL/6 mice. In contrast, following intranasal or intracranial inoculation, efficient replication of MHV in the brain is not dependent on an enzymatically active ns2. The replication of wild-type MHV strain A59 (A59) and a mutant with an inactive phosphodiesterase (ns2-H126R) was assessed in primary hepatocytes and primary central nervous system (CNS) cell types-neurons, astrocytes, and oligodendrocytes. A59 and ns2-H126R replicated with similar kinetics in all cell types tested, except macrophages and microglia. RNase L activity, as assessed by rRNA cleavage, was induced by ns2-H126R, but not by A59, and only in macrophages and microglia. Activation of RNase L correlated with the induction of type I interferon and the consequent high levels of OAS mRNA induced in these cell types. Pretreatment of nonmyeloid cells with interferon restricted A59 and ns2-H126R to the same extent and failed to activate RNase L following infection, despite induction of OAS expression. However, rRNA degradation was induced by treatment of astrocytes or oligodendrocytes with poly(I·C). Thus, RNase L activation during MHV infection is cell type specific and correlates with relatively high levels of expression of OAS genes, which are necessary but not sufficient for induction of an effective RNase L antiviral response.

Antiviral type I and type III interferon responses in the central nervous system

The central nervous system (CNS) harbors highly differentiated cells, such as neurons that are essential to coordinate the functions of complex organisms. This organ is partly protected by the blood-brain barrier (BBB) from toxic substances and pathogens carried in the bloodstream. Yet, neurotropic viruses can reach the CNS either by crossing the BBB after viremia, or by exploiting motile infected cells as Trojan horses, or by using axonal transport. Type I and type III interferons (IFNs) are cytokines that are critical to control early steps of viral infections. Deficiencies in the IFN pathway have been associated with fatal viral encephalitis both in humans and mice. Therefore, the IFN system provides an essential protection of the CNS against viral infections. Yet, basal activity of the IFN system appears to be low within the CNS, likely owing to the toxicity of IFN to this organ. Moreover, after viral infection, neurons and oligodendrocytes were reported to be relatively poor IFN producers and appear to keep some susceptibility to neurotropic viruses, even in the presence of IFN. This review addresses some trends and recent developments concerning the role of type I and type III IFNs in: i) preventing neuroinvasion and infection of CNS cells; ii) the identity of IFN-producing cells in the CNS; iii) the antiviral activity of ISGs; and iv) the activity of viral proteins of neurotropic viruses that target the IFN pathway.

Differential innate immune response programs in neuronal subtypes determine susceptibility to infection in the brain by positive-stranded RNA viruses

Different types of neurons are differentially susceptible to West Nile virus (WNV) infection. Michael Diamond and colleagues now show that cerebellar granule cell neurons (GCN) have a higher basal level of expression of type I interferon–inducible genes than cortical neurons, making GCN more resistant to infection by a variety of positive-stranded RNA viruses, including WNV.

Although susceptibility of neurons in the brain to microbial infection is a major determinant of clinical outcome, little is known about the molecular factors governing this vulnerability. Here we show that two types of neurons from distinct brain regions showed differential permissivity to replication of several positive-stranded RNA viruses. Granule cell neurons of the cerebellum and cortical neurons from the cerebral cortex have unique innate immune programs that confer differential susceptibility to viral infection ex vivo and in vivo. By transducing cortical neurons with genes that were expressed more highly in granule cell neurons, we identified three interferon-stimulated genes (ISGs; Ifi27, Irg1 and Rsad2 (also known as Viperin)) that mediated the antiviral effects against different neurotropic viruses. Moreover, we found that the epigenetic state and microRNA (miRNA)-mediated regulation of ISGs correlates with enhanced antiviral response in granule cell neurons. Thus, neurons from evolutionarily distinct brain regions have unique innate immune signatures, which probably contribute to their relative permissiveness to infection.

Neuromyelitis optica-like pathology is dependent on type I interferon response

Neuromyelitis optica is an antibody-mediated autoimmune inflammatory disease of the central nervous system. Reports have suggested that interferon beta which is beneficial for multiple sclerosis, exacerbates neuromyelitis optica. Our aim was to determine whether type I interferon plays a role in the formation of neuromyelitis optica lesions. Immunoglobulin G from a neuromyelitis optica patient was injected intracerebrally with human complement to type I interferon receptor deficient and wildtype mice. Loss of aquaporin-4 and glial fibrillary acidic protein was reduced in type I interferon receptor deficient mice brain. Our findings suggest that type I interferon signaling contributes to neuromyelitis optica pathogenesis.

Intrathecal humoral immunity to encephalitic RNA viruses

The nervous system is the target for acute encephalitic viral infections, as well as a reservoir for persisting viruses. Intrathecal antibody (Ab) synthesis is well documented in humans afflicted by infections associated with neurological complications, as well as the demyelinating disease, multiple sclerosis. This review focuses on the origin, recruitment, maintenance, and biological relevance of Ab-secreting cells (ASC) found in the central nervous system (CNS) following experimental neurotropic RNA virus infections. We will summarize evidence for a highly dynamic, evolving humoral response characterized by temporal alterations in B cell subsets, proliferation, and differentiation. Overall local Ab plays a beneficial role via complement-independent control of virus replication, although cross or self-reactive Ab to CNS antigens may contribute to immune-mediated pathogenesis during some infections. Importantly, protective Ab exert anti-viral activity not only by direct neutralization, but also by binding to cell surface-expressed viral glycoproteins. Ab engagement of viral glycoproteins blocks budding and mediates intracellular signaling leading to restored homeostatic and innate functions. The sustained Ab production by local ASC, as well as chemokines and cytokines associated with ASC recruitment and retention, are highlighted as critical components of immune control.

The chemokine receptor CXCR2 and coronavirus-induced neurologic disease

Inoculation with the neurotropic JHM strain of mouse hepatitis virus (MHV) into the central nervous system (CNS) of susceptible strains of mice results in an acute encephalomyelitis in which virus preferentially replicates within glial cells while excluding neurons. Control of viral replication during acute disease is mediated by infiltrating virus-specific T cells via cytokine secretion and cytolytic activity, however sterile immunity is not achieved and virus persists resulting in chronic neuroinflammation associated with demyelination. CXCR2 is a chemokine receptor that upon binding to specific ligands promotes host defense through recruitment of myeloid cells to the CNS as well as protecting oligodendroglia from cytokine-mediated death in response to MHV infection. These findings highlight growing evidence of the diverse and important role of CXCR2 in regulating neuroinflammatory diseases.

Production of IL-8, IL-17, IFN-gamma and IP-10 in human astrocytes correlates with alphavirus attenuation

Venezuelan equine encephalitis virus (VEEV) is an important, naturally emerging zoonotic pathogen. Recent outbreaks in Venezuela and Colombia in 1995 indicate that VEEV still poses a serious public health threat. Astrocytes may be target cells in human and mouse infection and they play an important role in repair through gliosis. In this study, we report that virulent VEEV efficiently infects cultured normal human astrocytes, three different murine astrocyte cell lines and astrocytes in the mouse brain. The attenuation of virus replication positively correlates with the increased levels of production of IL-8, IL-17, IFN-gamma and IP-10. In addition, VEEV infection induces release of basic fibroblast growth factor and production of potent chemokines such as RANTES and MIP-1-beta from cultured human astrocytes. This growth factor and cytokine profile modeled by astrocytes in vitro may contribute to both neuroprotection and repair and may play a role in leukocyte recruitment in vivo.

Type I interferon signaling protects mice from lethal henipavirus infection

Hendra virus (HeV) and Nipah virus (NiV) are closely related, recently emerged paramyxoviruses that form Henipavirus genus and are capable of causing considerable morbidity and mortality in a number of mammalian species, including humans. However, in contrast to many other species and despite expression of functional virus entry receptors, mice are resistant to henipavirus infection. We report here the susceptibility of mice deleted for the type I interferon receptor (IFNAR-KO) to both HeV and NiV. Intraperitoneally infected mice developed fatal encephalitis, with pathology and immunohistochemical features similar to what was found in humans. Viral RNA was found in the majority of analyzed organs, and sublethally infected animals developed virus-specific neutralizing antibodies. Altogether, these results reveal IFNAR-KO mice as a new small animal model to study HeV and NiV pathogenesis, prophylaxis, and treatment and suggest the critical role of type I interferon signaling in the control of henipavirus infection.