Antibody is required for clearance of infectious murine hepatitis virus A59 from the central nervous system, but not the liver

Neuro-axonal injury in COVID-19: the role of systemic inflammation and SARS-CoV-2 specific immune response

Background:

In coronavirus disease-2019 (COVID-19) patients, there is increasing evidence of neuronal injury by the means of elevated serum neurofilament light chain (sNfL) levels. However, the role of systemic inflammation and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–specific immune response with regard to neuronal injury has not yet been investigated.

Methods:

In a prospective cohort study, we recruited patients with mild–moderate (n = 39) and severe (n = 14) COVID-19 and measured sNfL levels, cytokine concentrations, SARS-CoV-2-specific antibodies including neutralizing antibody titers, and cell-mediated immune responses at enrollment and at 28(±7) days. We explored the association of neuro-axonal injury as by the means of sNfL measurements with disease severity, cytokine levels, and virus-specific immune responses.

Results:

sNfL levels, as an indicator for neuronal injury, were higher at enrollment and increased during follow-up in severely ill patients, whereas during mild–moderate COVID-19, sNfL levels remained unchanged. Severe COVID-19 was associated with increased concentrations of cytokines assessed [interleukin (IL)-6, IL-8, interleukin-1 beta (IL-1β), and tumor necrosis factor-alpha (TNF-α)], higher anti-spike IgG and anti-nucleocapsid IgG concentrations, and increased neutralizing antibody titers compared with mild–moderate disease. Patients with more severe disease had higher counts of defined SARS-CoV-2-specific T cells. Increases in sNfL concentrations from baseline to day 28(±7) positively correlated with anti-spike protein IgG antibody levels and with titers of neutralizing antibodies.

Conclusion:

Severe COVID-19 is associated with increased serum concentration of cytokines and subsequent neuronal injury as reflected by increased levels of sNfL. Patients with more severe disease developed higher neutralizing antibody titers and higher counts of SARS-CoV-2-specific T cells during the course of COVID-19 disease. Mounting a pronounced virus-specific humoral and cell-mediated immune response upon SARS-CoV-2 infection did not protect from neuro-axonal damage as by the means of sNfL levels.

Activation of TCA cycle restrains virus-metabolic hijacking and viral replication in mouse hepatitis virus-infected cells

Background

One of coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has caused coronavirus disease 2019 (COVID-19) pandemic and threatened worldwide. However, therapy for COVID-19 has rarely been proven to possess specific efficacy. As the virus relies on host metabolism for its survival, several studies have reported metabolic intervention by SARS-CoV-2.

Results

We investigated the coronavirus-metabolic hijacking using mouse hepatitis virus (MHV) as a surrogate for SARS-CoV-2. Based on the altered host metabolism by MHV infection, an increase of glycolysis with low mitochondrial metabolism, we tried to investigate possible therapeutic molecules which increase the TCA cycle. Endogenous metabolites and metabolic regulators were introduced to restrain viral replication by metabolic intervention. We observed that cells deprived of cellular energy nutrition with low glycolysis strongly suppress viral replication. Furthermore, viral replication was also significantly suppressed by electron transport chain inhibitors which exhaust cellular energy. Apart from glycolysis and ETC, pyruvate supplement suppressed viral replication by the TCA cycle induction. As the non-glucose metabolite, fatty acids supplement decreased viral replication via the TCA cycle. Additionally, as a highly possible therapeutic metabolite, nicotinamide riboside (NR) supplement, which activates the TCA cycle by supplying NAD+, substantially suppressed viral replication.

Conclusions

This study suggests that metabolite-mediated TCA cycle activation suppresses replication of coronavirus and suggests that NR might play a role as a novel therapeutic metabolite for coronavirus.

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.

Immune responses to human respiratory coronaviruses infection in mouse models

Human respiratory coronaviruses (HCoVs), including the recently emerged SARS-CoV-2, the causative agent of the coronavirus disease 2019 (COVID-19) pandemic, potentially cause severe lung infections and multiple organ damages, emphasizing the urgent need for antiviral therapeutics and vaccines against HCoVs. Small animal models, especially mice, are ideal tools for deciphering the pathogenesis of HCoV infections as well as virus-induced immune responses, which is critical for antiviral drug development and vaccine design. In this review, we focus on the antiviral innate immune response, antibody response and T cell response in HCoV infected mouse models, and discuss the potential implications for understanding the anti-HCoV immunity and fighting the COVID-19 pandemic.

Insights into coronavirus immunity taught by the murine coronavirus

Coronaviruses (CoVs) represent enveloped, ss RNA viruses with the ability to infect a range of vertebrates causing mainly lung, CNS, enteric, and hepatic disease. While the infection with human CoV is commonly associated with mild respiratory symptoms, the emergence of SARS‐CoV, MERS‐CoV, and SARS‐CoV‐2 highlights the potential for CoVs to cause severe respiratory and systemic disease. The devastating global health burden caused by SARS‐CoV‐2 has spawned countless studies seeking clinical correlates of disease severity and host susceptibility factors, revealing a complex network of antiviral immune circuits. The mouse hepatitis virus (MHV) is, like SARS‐CoV‐2, a beta‐CoV and is endemic in wild mice. Laboratory MHV strains have been extensively studied to reveal coronavirus virulence factors and elucidate host mechanisms of antiviral immunity. These are reviewed here with the aim to identify translational insights for SARS‐CoV‐2 learned from murine CoVs.

PART 2: Mouse models for multiple sclerosis research

Lower urinary tract symptoms and dysfunction (LUTS/LUTD) contribute to loss of quality of life, morbidity, and need for medical intervention in most patients with multiple sclerosis (MS). Although MS is an inflammatory neurodegenerative disease, clinical manifestations including continence control disorders have traditionally been attributed to the loss of neural signaling due to neurodegeneration. Clinical approaches to MS-LUTS/LUTD have focused on addressing symptoms in the context of urodynamic dysfunctions as pathophysiologic understandings are incomplete. The mouse model provides a useful research platform for discovery of more detailed molecular, cellular, and tissue-level knowledge of the disease and its clinical manifestations. The aim of this two-part review is to provide a state-of-the-art update on the use of the mouse model for MS research, with a focus on lower urinary tract symptoms. Part I presents a summary of current understanding of MS pathophysiology, the impact on lower urinary tract symptoms, and briefly introduces the types of mouse models available to study MS. Part II presents the common animal models that are currently available to study MS, their mechanism, relevance to MS-LUTS/LUTD and their urinary pathophysiology, advantages and disadvantages.

LY6E impairs coronavirus fusion and confers immune control of viral disease

Zoonotic coronaviruses (CoVs) are substantial threats to global health, as exemplified by the emergence of two severe acute respiratory syndrome CoVs (SARS-CoV and SARS-CoV-2) and Middle East respiratory syndrome CoV (MERS-CoV) within two decades1-3. Host immune responses to CoVs are complex and regulated in part through antiviral interferons. However, interferon-stimulated gene products that inhibit CoVs are not well characterized4. Here, we show that lymphocyte antigen 6 complex, locus E (LY6E) potently restricts infection by multiple CoVs, including SARS-CoV, SARS-CoV-2 and MERS-CoV. Mechanistic studies revealed that LY6E inhibits CoV entry into cells by interfering with spike protein-mediated membrane fusion. Importantly, mice lacking Ly6e in immune cells were highly susceptible to a murine CoV-mouse hepatitis virus. Exacerbated viral pathogenesis in Ly6e knockout mice was accompanied by loss of hepatic immune cells, higher splenic viral burden and reduction in global antiviral gene pathways. Accordingly, we found that constitutive Ly6e directly protects primary B cells from murine CoV infection. Our results show that LY6E is a critical antiviral immune effector that controls CoV infection and pathogenesis. These findings advance our understanding of immune-mediated control of CoV in vitro and in vivo-knowledge that could help inform strategies to combat infection by emerging CoVs.

Acute RNA Viral Encephalomyelitis and the Role of Antibodies in the Central Nervous System

Acute RNA viral encephalomyelitis is a serious complication of numerous virus infections. Antibodies in the cerebral spinal fluid (CSF) are correlated to better outcomes, and there is substantive evidence of antibody secreting cells (ASCs) entering the central nervous system (CNS) and contributing to resolution of infection. Here, we review the RNA viruses known to cause acute viral encephalomyelitis with mechanisms of control that require antibody or ASCs. We compile the cytokines, chemokines, and surface receptors associated with ASC recruitment to the CNS after infection and compare known antibody-mediated mechanisms as well as potential noncytolytic mechanisms for virus control. These non-canonical functions of antibodies may be employed in the CNS to protect precious non-renewable neurons. Understanding the immune-specialized zone of the CNS is essential for the development of effective treatments for acute encephalomyelitis caused by RNA viruses.

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.

Glycoproteins C and D of PRV Strain HB1201 Contribute Individually to the Escape From Bartha-K61 Vaccine-Induced Immunity

The newly emerged pseudorabies virus (PRV) novel variants can escape from the immunity induced by the classical vaccine Bartha-K61. Here we investigated the underlying mechanisms by constructing chimeric mutants between epidemic strain HB1201 and the Bartha-K61 vaccine. Our analyses focused on three viral envelope glycoproteins, namely gB, gC, and gD, as they exhibit remarkable genetic variations and are also involved in induction of protective immunity. The corresponding genes were swapped reciprocally either individually or in combination by using CRISPR/Cas9 technology and homologous recombination. The rescued chimeric viruses exhibited differential sensitivity to neutralizing antibodies in vitro, and gC was found to be the major contributor to inefficient neutralization against HB1201 by anti-Bartha-K61 serum. When tested in the 4-week-piglet model, substitution with HB1201 gC enabled Bartha-K61 to induce a protective immunity against HB1201 at a high challenge dose of 10⁷ TCID₅₀. Interestingly, despite a relatively lower cross-neutralization ability, the gD exchange also enabled Bartha-K61 to protect piglets from lethal challenge. In both cases, clinical signs and microscopic lesions were eased, and so was the viral tissue load with the exception of brain. A better protection could be achieved when both gC and gD were swapped in terms of reducing viral load in brain and virus-induced microscopic lesions. Thus, our studies not only revealed individual roles of gC and gD variations in the immune escape and also suggested a synergistic effect of both proteins on induction of protective immunity. These findings have important implications in novel vaccine development for PRV control in China.

LY6E impairs coronavirus fusion and confers immune control of viral disease

Zoonotic coronaviruses (CoVs) are significant threats to global health, as exemplified by the recent emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ¹ . Host immune responses to CoV are complex and regulated in part through antiviral interferons. However, the interferon-stimulated gene products that inhibit CoV are not well characterized ² . Here, we show that interferon-inducible lymphocyte antigen 6 complex, locus E (LY6E) potently restricts cellular infection by multiple CoVs, including SARS-CoV, SARS-CoV-2, and Middle East respiratory syndrome coronavirus (MERS-CoV). Mechanistic studies revealed that LY6E inhibits CoV entry into cells by interfering with spike protein-mediated membrane fusion. Importantly, mice lacking Ly6e in hematopoietic cells were highly susceptible to murine CoV infection. Exacerbated viral pathogenesis in Ly6e knockout mice was accompanied by loss of hepatic and splenic immune cells and reduction in global antiviral gene pathways. Accordingly, we found that Ly6e directly protects primary B cells and dendritic cells from murine CoV infection. Our results demonstrate that LY6E is a critical antiviral immune effector that controls CoV infection and pathogenesis. These findings advance our understanding of immune-mediated control of CoV in vitro and in vivo , knowledge that could help inform strategies to combat infection by emerging CoV.

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.

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.

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.

Neurotropic Coronavirus Infections

Neurotropic strains of the mouse hepatitis virus (MHV) cause a range of diseases in infected mice ranging from mild encephalitis with clearance of the virus followed by demyelination to rapidly fatal encephalitis. This chapter discusses the structure, life cycle, transmission, and pathology of neurotropic coronaviruses, as well as the immune response to coronavirus infection. Mice infected with neurotropic strains of MHV have provided useful systems in which to study processes of virus- and immune-mediated demyelination and virus clearance and/or persistence in the CNS, and the mechanisms of virus evasion of the immune system.

TLR3 mediated innate immune response in mice brain following infection with Chikungunya virus

Chikungunya virus (CHIKV) has received global attention due to the series of large-scale outbreaks in different parts of the world. Many unusual clinical severities including neurological complications and death were reported in recent outbreaks. The mechanism underlying the host immune response to CHIKV in the brain is poorly characterized. In this study, the neuropathogenesis of CHIKV with E1:A226V mutation was elucidated in 1 week old BALB/c mice. The virus was found to replicate in mice brain with peak titer of 10(4) on 6th day post infection. Immunohistochemical analysis revealed preferential virus localization in neuronal cells of cerebellum. The expression profiling of TLR, antiviral genes and cytokines in mice brain revealed significant up regulation of TLR3, TRAF-6, TICAM-1, MCP-1, CXCL-10, IL-6, IL-4, ISG-15, MX-2, IFN-β, OAS-3 genes that ultimately resulted in virus clearance from brain by day 9-10 suggesting activation of innate immune pathway. Further the effect of poly I: C (Polyinosinic: Polycytidylic acid), a TLR-3 agonist and potent IFN inducer on CHIKV neuropathogenesis was studied. Pretreatment of mice with Poly I: C caused reduction of CHIKV titer in brain and offered 100% protection of animals. The protection was mediated by an increased induction of TLR3, IFN-β and antiviral genes in mice brain. Our result demonstrates that pre immune stimulation of animals by Poly I: C is effective inhibitor of CHIKV replication and might be a promising prevention agent against this virus.

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.

Astrocyte-derived CXCL10 drives accumulation of antibody-secreting cells in the central nervous system during viral encephalomyelitis

Microbial infections of the central nervous system (CNS) are often associated with local accumulation of antibody (Ab)-secreting cells (ASC). By providing a source of Ab at the site of infection, CNS-localized ASC play a critical role in acute viral control and in preventing viral recrudescence. Following coronavirus-induced encephalomyelitis, the CNS accumulation of ASC is chemokine (C-X-C motif) receptor 3 (CXCR3) dependent. This study demonstrates that CNS-expressed CXCR3 ligand CXCL10 is the critical chemokine regulating ASC accumulation. Impaired ASC recruitment in CXCL10(-/-) but not CXCL9(-/-) mice was consistent with reduced CNS IgG and κ-light chain mRNA and virus-specific Ab. Moreover, the few ASC recruited to the CNS in CXCL10(-/-) mice were confined to the vasculature, distinct from the parenchymal localization in wild-type and CXCL9(-/-) mice. However, neither CXCL9 nor CXCL10 deficiency diminished neutralizing serum Ab, supporting a direct role for CXCL10 in ASC migration. T cell accumulation, localization, and effector functions were also not affected in either CXCL9(-/-) or CXCL10(-/-) mice, consistent with similar control of infectious virus. There was also no evidence for dysregulation of chemokines or cytokines involved in ASC regulation. The distinct roles of CXCL9 and CXCL10 in ASC accumulation rather coincided with their differential localization. While CXCL10 was predominantly expressed by astrocytes, CXCL9 expression was confined to the vasculature/perivascular spaces. These results suggest that CXCL10 is critical for two phases: recruitment of ASC to the CNS vasculature and ASC entry into the CNS parenchyma.

Clearance of virus infection from the CNS

Viruses that cause encephalomyelitis infect neurons and recovery from infection requires noncytolytic clearance of virus from the nervous system to avoid damaging these irreplaceable cells. Several murine model systems of virus infection have been used to identify clearance mechanisms. Quantitative analysis of Sindbis virus clearance over 6 months shows three phases: day 5-7, clearance of infectious virus, but continued presence of viral RNA; day 8-60, decreasing levels of viral RNA; day 60-180, maintenance of viral RNA at low levels. Antiviral antibody and interferon-γ have major roles in clearance with a likely role for IgM as well as IgG antibody. Long-term residence of virus-specific immune cells in the nervous system is necessary to prevent virus reactivation.

T helper cell- and CD40-dependent germline IgM prevents chronic virus-induced demyelinating disease

Generation of antiviral IgM is usually considered as a marker of a short-lived initial antibody response that is replaced by hypermutated and more-efficient IgG. However, once viruses have established a particular niche for their persistence (e.g., within the CNS), the immune system has to specifically mobilize a broad range of antimicrobial effectors to contain the pathogen in the long term. Infection of the CNS with the mouse hepatitis virus (MHV) provides a unique model situation in which the extent of inflammatory CNS disease is determined by the balance between antiviral immune control, viral replication, and immune-mediated damage. We show here that whereas antibody- or B cell-deficient mice failed to contain MHV CNS infection and developed progressive demyelinating disease, germline IgM produced in activation-induced cytidine deaminase-deficient mice (aicda(-/-)) provided long-term protection against the chronic multiple sclerosis-like disease. Furthermore, we found that appropriate B-cell activation within the CNS-draining lymph node and subsequent CXCR3-mediated migration of antiviral IgM-secreting cells to the infected CNS was dependent on CD40-mediated interaction of B cells with T helper cells. These data indicate that the CD40-mediated collaboration of T and B cells is critical to secure neuroprotective IgM responses during viral CNS infection.

Murine coronavirus neuropathogenesis: determinants of virulence

Murine coronavirus, mouse hepatitis virus (MHV), causes various diseases depending on the strain and route of inoculation. Both the JHM and A59 strains, when inoculated intracranially or intranasally, are neurovirulent. Comparison of the highly virulent JHM isolate, JHM.SD, with less virulent JHM isolates and with A59 has been used to determine the mechanisms and genes responsible for high neuropathogenicity of MHV. The focus of this review is on the contributions of viral spread, replication, and innate and adaptive immunity to MHV neuropathogenesis. JHM.SD spreads more quickly among neurons than less neurovirulent MHVs, and is able to spread in the absence of the canonical MHV receptor, CEACAM1a. The observation that JHM.SD infects more cells and expresses more antigen, but produces less infectious virus per cell than A59, implies that efficient replication is not always a correlate of high neurovirulence. This is likely due to the unstable nature of the JHM.SD spike protein (S). JHM.SD induces a generally protective innate immune response; however, the strong neutrophil response may be more pathogenic than protective. In addition, JHM.SD induces only a minimal T-cell response, whereas the strong T-cell response and the concomitant interferon-γ (IFN-γ) induced by the less neurovirulent A59 is protective. Differences in the S and nucleocapsid (N) proteins between A59 and JHM.SD contribute to JHM.SD neuropathogenicity. The hemmagglutinin-esterase (HE) protein may enhance neuropathogenicity of some MHV isolates, but is unlikely a major contributor to the high neuroviruence of JHM.SD. Further data suggest that neither the internal (I) protein nor nonstructural proteins ns4, and ns2 are significant contributors to neurovirulence.

Murine coronavirus neuropathogenesis: determinants of virulence

Murine coronavirus, mouse hepatitis virus (MHV), causes various diseases depending on the strain and route of inoculation. Both the JHM and A59 strains, when inoculated intracranially or intranasally, are neurovirulent. Comparison of the highly virulent JHM isolate, JHM.SD, with less virulent JHM isolates and with A59 has been used to determine the mechanisms and genes responsible for high neuropathogenicity of MHV. The focus of this review is on the contributions of viral spread, replication, and innate and adaptive immunity to MHV neuropathogenesis. JHM.SD spreads more quickly among neurons than less neurovirulent MHVs, and is able to spread in the absence of the canonical MHV receptor, CEACAM1a. The observation that JHM.SD infects more cells and expresses more antigen, but produces less infectious virus per cell than A59, implies that efficient replication is not always a correlate of high neurovirulence. This is likely due to the unstable nature of the JHM.SD spike protein (S). JHM.SD induces a generally protective innate immune response; however, the strong neutrophil response may be more pathogenic than protective. In addition, JHM.SD induces only a minimal T-cell response, whereas the strong T-cell response and the concomitant interferon-γ (IFN-γ) induced by the less neurovirulent A59 is protective. Differences in the S and nucleocapsid (N) proteins between A59 and JHM.SD contribute to JHM.SD neuropathogenicity. The hemmagglutinin-esterase (HE) protein may enhance neuropathogenicity of some MHV isolates, but is unlikely a major contributor to the high neuroviruence of JHM.SD. Further data suggest that neither the internal (I) protein nor nonstructural proteins ns4, and ns2 are significant contributors to neurovirulence.

Impaired immune responses following spinal cord injury lead to reduced ability to control viral infection

Spinal cord injuries disrupt central autonomic pathways that regulate immune function, and increasing evidence suggests that this may cause deficiencies in immune responses in people with spinal cord injuries. Here we analyze the consequences of spinal cord injury (SCI) on immune responses following experimental viral infection of mice. Female C57BL/6 mice received complete crush injuries at either thoracic level 3 (T3) or 9 (T9), and 1 week post-injury, injured mice and un-injured controls were infected with different dosages of mouse hepatitis virus (MHV, a positive-strand RNA virus). Following MHV infection, T3- and T9-injured mice exhibited increased mortality in comparison to un-injured and laminectomy controls. Infection at all dosages resulted in significantly higher viral titer in both T3- and T9-injured mice compared to un-injured controls. Investigation of anti-viral immune responses revealed impairment of cellular infiltration and effector functions in mice with SCI. Specifically, cell-mediated responses were diminished in T3-injured mice, as seen by reduction in virus-specific CD4(+) T lymphocyte proliferation and IFN-γ production and decreased numbers of activated antigen presenting cells compared to infected un-injured mice. Collectively, these data indicate that the inability to control viral replication following SCI is not level dependent and that increased susceptibility to infection is due to suppression of both innate and adaptive immune responses.

Pathogenesis of murine coronavirus in the central nervous system

Murine coronavirus (mouse hepatitis virus, MHV) is a collection of strains that induce disease in several organ systems of mice. Infection with neurotropic strains JHM and A59 causes acute encephalitis, and in survivors, chronic demyelination, the latter of which serves as an animal model for multiple sclerosis. The MHV receptor is a carcinoembryonic antigen-related cell adhesion molecule, CEACAM1a; paradoxically, CEACAM1a is poorly expressed in the central nervous system (CNS), leading to speculation of an additional receptor. Comparison of highly neurovirulent JHM isolates with less virulent variants and the weakly neurovirulent A59 strain, combined with the use of reverse genetics, has allowed mapping of pathogenic properties to individual viral genes. The spike protein, responsible for viral entry, is a major determinant of tropism and virulence. Other viral proteins, both structural and nonstructural, also contribute to pathogenesis in the CNS. Studies of host responses to MHV indicate that both innate and adaptive responses are crucial to antiviral defense. Type I interferon is essential to prevent very early mortality after infection. CD8 T cells, with the help of CD4 T cells, are crucial for viral clearance during acute disease and persist in the CNS during chronic disease. B cells are necessary to prevent reactivation of virus in the CNS following clearance of acute infection. Despite advances in understanding of coronavirus pathogenesis, questions remain regarding the mechanisms of viral entry and spread in cell types expressing low levels of receptor, as well as the unique interplay between virus and the host immune system during acute and chronic disease.

Type I interferon response in the central nervous system

This review is dedicated to the influence of type I IFNs (also called IFN-alpha/beta) in the central nervous system (CNS). Studies in mice with type I IFN receptor or IFN-beta gene deficiency have highlighted the importance of the type I IFN system against CNS viral infections and non-viral autoimmune disorders. Direct antiviral effects of type I IFNs appear to be crucial in limiting early spread of a number of viruses in CNS tissues. Type I IFNs have also proved to be beneficial in autoimmune disorders like multiple sclerosis or experimental autoimmune encephalitis, probably through immunomodulatory effects. Increasing efforts are done to characterize IFN expression and response in the CNS: to identify type I IFN producing cells, to decipher pathways leading to type I IFN expression in those cells, and to identify responding cells. However, reversible and irreversible damages consecutive to chronic exposure of the CNS to type I IFNs underline the importance of a tightly regulated type I IFN homeostasis in this organ.

Both spike and background genes contribute to murine coronavirus neurovirulence

Various strains of mouse hepatitis virus (MHV) exhibit different pathogenic phenotypes. Infection with the A59 strain of MHV induces both encephalitis and hepatitis, while the highly neurovirulent JHM strain induces a fatal encephalitis with little, if any, hepatitis. The pathogenic phenotype for each strain is determined by the genetic composition of the viral genome, as well as the host immune response. Using isogenic recombinant viruses with A59 background genes differing only in the spike gene, we have previously shown that high neurovirulence is associated with the JHM spike protein, the protein responsible for attachment to the host cell receptor (J. J. Phillips, M. M. Chua, G. F. Rall, and S. R. Weiss, Virology 301:109-120, 2002). Using another set of isogenic recombinant viruses with JHM background genes expressing either the JHM or A59 spike, we have further investigated the roles of viral genes in pathogenesis. Here, we demonstrate that the high neurovirulence of JHM is associated with accelerated spread through the brain and a heightened innate immune response that is characterized by high numbers of infiltrating neutrophils and macrophages, suggesting an immunopathogenic component to neurovirulence. While expression of the JHM spike is sufficient to confer a neurovirulent phenotype, as well as increased macrophage infiltration, background genes contribute to virulence as well, at least in part, by dictating the extent of the T-cell immune response.

CpG oligodeoxynucleotides protect newborn mice from a lethal challenge with the neurotropic Tacaribe arenavirus

The innate immune system is key to limiting the early spread of most pathogens and directing the development of Ag-specific immunity. Recently, a number of synthetic molecules that activate the innate immune system by stimulating TLRs have been identified. Among them, synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs (CpG ODNs) were shown to activate TLR9-bearing B cells, macrophages, and dendritic cells to induce a strong proinflammatory milieu and a type 1-biased immune response that protects mice from a variety of parasitic, bacterial, and viral infections. Although the protective effect of CpG ODN in adult mice was well established, its effectiveness in neonates, which have lower numbers of dendritic, B, and T cells and tend to favor Th2 responses, was unclear. This study uses the New World arenavirus Tacaribe, a neurotropic pathogen that is lethal in newborn mice, to explore the effectiveness of TLR-mediated innate immune responses. Neonatal BALB/c mice treated with CpG ODN at the time of infection had reduced viral load (p < 0.01) and increased survival (52%, p < 0.001 i.p.; 36%, p < 0.05 intranasally). Protection was achieved in mice treated no later than 3 days postchallenge and appears to be mediated by an increase in Ag-specific Abs (IgG and IgM) and to require inducible NO synthase expression and NO production. To our knowledge, this is the first study assessing the mechanisms by which CpG ODN can protect mice from a neurotropic viral infection.

Coronavirus infection of the central nervous system: host-virus stand-off

Several viruses infect the mammalian central nervous system (CNS), some with devastating consequences, others resulting in chronic or persistent infections associated with little or no overt pathology. Coronavirus infection of the murine CNS illustrates the contributions of both the innate immune response and specific host effector mechanisms that control virus replication in distinct CNS cell types. Despite T-cell-mediated control of acute virus infection, host regulatory mechanisms, probably designed to protect CNS integrity, contribute to the failure to eliminate virus. Distinct from cytolytic effector mechanisms expressed during acute infection, non-lytic humoral immunity prevails in suppressing infectious virus during persistence.

Coronavirus pathogenesis and the emerging pathogen severe acute respiratory syndrome coronavirus

Coronaviruses are a family of enveloped, single-stranded, positive-strand RNA viruses classified within the Nidovirales order. This coronavirus family consists of pathogens of many animal species and of humans, including the recently isolated severe acute respiratory syndrome coronavirus (SARS-CoV). This review is divided into two main parts; the first concerns the animal coronaviruses and their pathogenesis, with an emphasis on the functions of individual viral genes, and the second discusses the newly described human emerging pathogen, SARS-CoV. The coronavirus part covers (i) a description of a group of coronaviruses and the diseases they cause, including the prototype coronavirus, murine hepatitis virus, which is one of the recognized animal models for multiple sclerosis, as well as viruses of veterinary importance that infect the pig, chicken, and cat and a summary of the human viruses; (ii) a short summary of the replication cycle of coronaviruses in cell culture; (iii) the development and application of reverse genetics systems; and (iv) the roles of individual coronavirus proteins in replication and pathogenesis. The SARS-CoV part covers the pathogenesis of SARS, the developing animal models for infection, and the progress in vaccine development and antiviral therapies. The data gathered on the animal coronaviruses continue to be helpful in understanding SARS-CoV.

Contributions of the viral genetic background and a single amino acid substitution in an immunodominant CD8+ T-cell epitope to murine coronavirus neurovirulence

The immunodominant CD8+ T-cell epitope of a highly neurovirulent strain of mouse hepatitis virus (MHV), JHM, is thought to be essential for protection against virus persistence within the central nervous system. To test whether abrogation of this H-2Db-restricted epitope, located within the spike glycoprotein at residues S510 to 518 (S510), resulted in delayed virus clearance and/or virus persistence we selected isogenic recombinants which express either the wild-type JHM spike protein (RJHM) or spike containing the N514S mutation (RJHM(N514S)), which abrogates the response to S510. In contrast to observations in suckling mice in which viruses encoding inactivating mutations within the S510 epitope (epitope escape mutants) were associated with persistent virus and increased neurovirulence (Pewe et al., J Virol. 72:5912-5918, 1998), RJHM(N514S) was not more virulent than the parental, RJHM, in 4-week-old C57BL/6 (H-2b) mice after intracranial injection. Recombinant viruses expressing the JHM spike, wild type or encoding the N514S substitution, were also selected in which background genes were derived from the neuroattenuated A59 strain of MHV. Whereas recombinants expressing the wild-type JHM spike (SJHM/RA59) were highly neurovirulent, A59 recombinants containing the N514S mutation (SJHM(N514S)/RA59) were attenuated, replicated less efficiently, and exhibited reduced virus spread in the brain at 5 days postinfection (peak of infectious virus titers in the central nervous system) compared to parental virus encoding wild-type spike. Virulence assays in BALB/c mice (H-2d), which do not recognize the S510 epitope, revealed that attenuation of the epitope escape mutants was not due to the loss of a pathogenic immune response directed against the S510 epitope. Thus, an intact immunodominant S510 epitope is not essential for virus clearance from the CNS, the S510 inactivating mutation results in decreased virulence in weanling mice but not in suckling mice, suggesting that specific host conditions are required for epitope escape mutants to display increased virulence, and the N514S mutation causes increased attenuation in the context of A59 background genes, demonstrating that genes other than that for the spike are also important in determining neurovirulence.

The Role of T Cells in Corona-Virus-Induced Demyelination

Mice infected with neurotropic strains of coronavirus develop acute encephalomyelitis and eliminate infectious virus. However, control of acute infection is incomplete resulting in persistence of viral RNA in the central nervous system (CNS) associated with ongoing primary demyelination. A high prevalence of virus specific CD8 and CD4 T cells within the CNS correlates with ex vivo cytolytic activity and IFN-γ secretion, which are both required for virus reduction during the acute infection. Although most infected cell types are susceptible to perforin mediated clearance, IFN-γ is required for controlling infection of oligodendrocytes. Furthermore, by enhancing class I expression and inducing class II expression within resident CNS cells IFN-γ optimizes T cell receptor dependent functions. In addition to its direct anti viral activity, these multifactorial effects make IFN-γ more essential than perforin for viral control. CD4 T cells enhance CD8 T cell expansion, survival and effectiveness. Although both CD8 and CD4 T cells are retained within the CNS during persistence, they cannot control viral recrudescence in the absence of humoral immunity. Demyelination can be mediated by either CD8 or CD4 T cells; however, although a variety of effector molecules have been excluded, a dominant common denominator remains elusive. Thus concerted efforts to control infection coincide with a variety of potential mechanisms causing chronic demyelinating disease.

The Role of Humoral Immunity in Mouse Hepatitis Virus Induced Demyelination

Pathogenesis induced by mouse hepatitis virus (MHV) infection of rodents is characterized by acute viral encephalomyelitis and demyelination which progresses to a persistent CNS infection associated with ongoing myelin loss, pathologically similar to multiple sclerosis (MS). Although humoral immunity appears redundant for the control of acute virus replication, it is vital in maintaining virus at levels detectable only by RNA analysis. T cell mediated control of acute infection cannot be sustained in antibody (Ab) deficient mice, resulting in virus reactivation. The protective role of Ab during persistence is strongly supported by detection of Ab in the cerebrospinal fluid of MHV infected rodents and maintenance of virus specific Ab secreting cells (ASC) in the CNS long after virus clearance. Ab mediated neutralization constitutes the major mechanism of protection, although fusion inhibition also plays a minor role. Delayed accumulation of ASC, concomitant with a decline in T cell function, assures control of residual virus while minimizing T cell mediated pathology. Although there is little evidence for a detrimental role of Ab in demyelination, an association between Ab mediated protection and remyelination is unclear.

Coronavirus replication and pathogenesis: Implications for the recent outbreak of severe acute respiratory syndrome (SARS), and the challenge for vaccine development

A novel coronavirus has been recently identified as the causative agent of the severe acute respiratory syndrome (SARS) outbreak that has accounted for more than 8000 infected people worldwide. This review will discuss current knowledge on coronavirus replication, pathogenesis, evolution, and vaccine strategies, as well as the most recent findings on SARS coronavirus.

Effects of an epitope-specific CD8+ T-cell response on murine coronavirus central nervous system disease: protection from virus replication and antigen spread and selection of epitope escape mutants

Both CD4(+) and CD8(+) T cells are required for clearance of the murine coronavirus mouse hepatitis virus (MHV) during acute infection. We investigated the effects of an epitope-specific CD8(+) T-cell response on acute infection of MHV, strain A59, in the murine CNS. Mice with CD8(+) T cells specific for gp33-41 (an H-2D(b)-restricted CD8(+) T-cell epitope derived from lymphocytic choriomeningitis glycoprotein) were infected with a recombinant MHV-A59, also expressing gp33-41, as a fusion protein with enhanced green fluorescent protein (EGFP). By 5 days postinfection, these mice showed significantly (approximately 20-fold) lower titers of infectious virus in the brain compared to control mice. Furthermore mice with gp33-41-specific CD8(+) cells exhibited much reduced levels of viral antigen in the brain as measured by immunohistochemistry using an antibody directed against viral nucleocapsid. More than 90% of the viruses recovered from brain lysates of such protected mice, at 5 days postinfection, had lost the ability to express EGFP and had deletions in their genomes encompassing EGFP and gp33-41. In addition, genomes of viruses from about half the plaques that retained the EGFP gene had mutations within the gp33-41 epitope. On the other hand, gp33-41-specific cells failed to protect perforin-deficient mice from infection by the recombinant MHV expressing gp33, indicating that perforin-mediated mechanisms were needed. Virus recovered from perforin-deficient mice did not exhibit loss of EGFP expression and the gp33-41 epitope. These observations suggest that the cytotoxic T-cell response to gp33-41 exerts a strong immune pressure that quickly selects epitope escape mutants to gp33-41.

A critical role for induced IgM in the protection against West Nile virus infection

In humans, the elderly and immunocompromised are at greatest risk for disseminated West Nile virus (WNV) infection, yet the immunologic basis for this remains unclear. We demonstrated previously that B cells and IgG contributed to the defense against disseminated WNV infection (Diamond, M.S., B. Shrestha, A. Marri, D. Mahan, and M. Engle. 2003. J. Virol. 77:2578–2586). In this paper, we addressed the function of IgM in controlling WNV infection. C57BL/6J mice (sIgM^−/−) that were deficient in the production of secreted IgM but capable of expressing surface IgM and secreting other immunoglobulin isotypes were vulnerable to lethal infection, even after inoculation with low doses of WNV. Within 96 h, markedly higher levels of infectious virus were detected in the serum of sIgM^−/− mice compared with wild-type mice. The enhanced viremia correlated with higher WNV burdens in the central nervous system, and was also associated with a blunted anti-WNV IgG response. Passive transfer of polyclonal anti-WNV IgM or IgG protected sIgM^−/− mice against mortality, although administration of comparable amounts of a nonneutralizing monoclonal anti-WNV IgM provided no protection. In a prospective analysis, a low titer of anti-WNV IgM antibodies at day 4 uniformly predicted mortality in wild-type mice. Thus, the induction of a specific, neutralizing IgM response early in the course of WNV infection limits viremia and dissemination into the central nervous system, and protects against lethal infection.

Antibody-mediated protection against cytotoxic T-cell escape in coronavirus-induced demyelination

C57BL/6 (B6) mice infected with mouse hepatitis virus (MHV) strain JHM develop a clinically evident, demyelinating encephalomyelitis. Infectious virus can be isolated from the spinal cords of these mice and is invariably mutated in the immunodominant CD8 T-cell epitope recognized in this strain. We showed previously that these persistently infected mice did not mount a measurable serum anti-MHV neutralizing antibody response. Here we show that cytotoxic T-lymphocyte (CTL) escape was not detected in MHV-infected BALB/b mice (H-2(b) haplotype), even though the same CD8 T-cell epitopes were recognized as in B6 mice. BALB/b mice had 25-fold more MHV-specific antibody-secreting cells in the central nervous system, the site of infection, than B6 mice, suggesting that local production of anti-MHV antibody contributed to this absence of CTL escape. Additionally, administration of anti-MHV neutralizing antibody to infected B6 mice suppressed the development of CTL escape mutants. These findings indicate a key role for the anti-MHV antibody response in suppressing virus replication, thereby minimizing the emergence and competitive advantage of CTL escape mutants.

Control of central nervous system viral persistence by neutralizing antibody

Replication of the neurotropic JHM strain of mouse hepatitis virus within the central nervous system is controlled by cellular immunity. However, following initial clearance, virus reactivates in the absence of humoral immunity. Viral recrudescence is prevented by the transfer of antiviral antibody (Ab). To characterize the specificity and biological functions of Ab critical for maintaining viral persistence, monoclonal Abs specific for the viral spike, matrix, and nucleocapsid proteins were transferred into infected B-cell-deficient mice following initial virus clearance. Neutralizing immunoglobulin G (IgG) but not IgA anti-spike Ab suppressed virus recrudescence, reduced viral antigen in most cell types except oligodendroglia, and was associated with reduced demyelination. Nonneutralizing monoclonal Abs specific for the spike, matrix, and nucleocapsid proteins did not prevent recrudescence, demonstrating that neutralization is critical for maintaining JHM mouse hepatitis virus persistence within the central nervous system. Ab-mediated protection was not associated with alterations in virus-specific T-cell function or inflammation. Furthermore, neutralizing Ab delayed but did not prevent virus recrudescence. These data indicate that following acute viral clearance cellular immunity is ineffective in controlling virus recrudescence and suggest that the continued presence of neutralizing Ab is the essential effector in maintaining viral persistence within the central nervous system.

B cells and antibody play critical roles in the immediate defense of disseminated infection by West Nile encephalitis virus

West Nile virus (WNV) causes severe central nervous system (CNS) infection primarily in humans who are immunocompromised or elderly. In this study, we addressed the mechanism by which the immune system limits dissemination of WNV infection by infecting wild-type and immunodeficient inbred C57BL/6J mice with a low-passage WNV isolate from the recent epidemic in New York state. Wild-type mice replicated virus extraneuronally in the draining lymph nodes and spleen during the first 4 days of infection. Subsequently, virus spread to the spinal cord and the brain at virtually the same time. Congenic mice that were genetically deficient in B cells and antibody (microMT mice) developed increased CNS viral burdens and were vulnerable to lethal infection at low doses of virus. Notably, an approximately 500-fold difference in serum viral load was detected in micro MT mice as early as 4 days after infection, a point in the infection when low levels of neutralizing immunoglobulin M antibody were detected in wild-type mice. Passive transfer of heat-inactivated serum from infected and immune wild-type mice protected micro MT mice against morbidity and mortality. We conclude that antibodies and B cells play a critical early role in the defense against disseminated infection by WNV.

Neither B cells nor T cells are required for CNS demyelination in mice persistently infected with MHV-A59

Murine hepatitis virus A59 infection of the central nervous system (CNS) results in CNS demyelination in susceptible strains of mice. In infected B-cell-deficient mice, demyelination not only occurred but was also more severe than in parental C57BL/6 animals. This increase may be due to the persistence of virus in the CNS in the absence of B cells. In mice lacking antibody receptors or complement pathway activity, virus did not persist yet demyelination was similar to parental mice. In infected RAG1(-/-) mice, moderately sized, typical demyelinating lesions were identified. Therefore, demyelination can occur in the absence of B and T cells.

Murine hepatitis virus--a model for virus-induced CNS demyelination

Most murine hepatitis virus (MHV) strains, as their name suggests, infect the liver. However, several murine strains are tropic for the central nervous system (CNS) and cause encephalitis with subsequent CNS demyelination. The CNS demyelination shares pathological similarities with human CNS demyelinating diseases such as multiple sclerosis (MS). These viruses are, therefore, used to study the role of the immune system in viral clearance from the CNS, in CNS demyelination, and in remyelination. Nevertheless, it is still unclear exactly how MHV induces demyelination and to what extent the immune system plays a role in this pathology. Here we review this field in the context of the immune response to MHV in the liver and the CNS focusing on studies that have been published in the past 5 years.

Enhanced green fluorescent protein expression may be used to monitor murine coronavirus spread in vitro and in the mouse central nervous system

Targeted recombination was used to select mouse hepatitis virus isolates with stable and efficient expression of the gene encoding the enhanced green fluorescent protein (EGFP). The EGFP gene was inserted into the murine coronavirus genome in place of the nonessential gene 4. These viruses expressed the EGFP gene from an mRNA of slightly slower electrophoretic mobility than mRNA 4. EGFP protein was detected on a Western blot of infected cell lysates and EGFP activity (fluorescence) was visualized by microscopy in infected cells and in viral plaques. Expression of EGFP remained stable through at least six passages in tissue culture and during acute infection in the mouse central nervous system. These viruses replicated with similar kinetics and to similar final extents as wild-type virus both in tissue culture and in the mouse central nervous system (CNS). They caused encephalitis and demyelination in animals as wild-type virus; however, they were somewhat attenuated in virulence. Isogenic EGFP-expressing viruses that differ only in the spike gene and express either the spike gene of the highly neurovirulent MHV-4 strain or the more weakly neurovirulent MHV-A59 strain were compared; the difference in virulence and patterns of spread of viral antigen reflected the differences between parental viruses expressing each of these spike genes. Thus, EGFP-expressing viruses will be useful in the studies of murine coronavirus pathogenesis in mice.