Multiple regions of the murine coronavirus spike glycoprotein influence neurovirulence

Reduction of Cell Fusion by Deletion in the Hypervariable Region of the Spike Protein of Mouse Hepatitis Virus

Deletions in the spike gene of mouse hepatitis virus (MHV) produce several variants with diverse biological characteristics, highlighting the significance of the spike gene in viral pathogenesis. In this study, we characterized the JHM-X strain, which has a deletion in the hypervariable region (HVR) of the spike gene, compared with the cl-2 strain, which has a full spike gene. Cytopathic effects (CPEs) induced by the two strains revealed that the size of the CPE produced by cl-2 is much greater than that produced by JHM-X in delayed brain tumor (DBT) cells. Thus, this finding explains the greater fusion activity of cl-2 than JHM-X in cultured cells, and we speculate that the deletion region of the spike protein is involved in the fusion activity differences. In contrast with the fusion activity, a comparison of the virus growth kinetics revealed that the titer of JHM-X was approximately 100 times higher than that of cl-2. We found that the deletion region of the spike protein was involved in fusion activity differences, whereas cl-2 produced significantly higher luciferase activity than JHM-X upon similar expression levels of the spike protein. However, the reason behind the growth difference is still unknown. Overall, we discovered that deletion in the HVR of the spike gene could be involved in the fusion activity differences between the two strains.

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.

Viral mouse models used to study multiple sclerosis: past and present

Multiple sclerosis (MS) is a common inflammatory demyelinating disease of the central nervous system. Although the etiology of MS is unknown, genetics and environmental factors, such as infections, play a role. Viral infections of mice have been used as model systems to study this demyelinating disease of humans. Three viruses that have long been studied in this capacity are Theiler’s murine encephalomyelitis virus, mouse hepatitis virus, and Semliki Forest virus. This review describes the viruses themselves, the infection process, the disease caused by infection and its accompanying pathology, and the model systems and their usefulness in studying MS.

Spike glycoproteins: Their significance for corona viruses and receptor binding activities for pathogenesis and viral survival

The recent outbreak of Covid-19 is posing a severe threat to public health globally. Coronaviruses (CoVs) are the largest known group of positive-sense RNA viruses surviving on an extensive number of natural hosts. CoVs are enveloped and non-segmented viruses with a size between 80 and 120 nm. CoV attachment to the surface receptor and its subsequent entrance into cells is mediated by Spike glycoprotein (S). For enhanced CoV entry and successful pathogenesis of CoV, proteolytic processing and receptor-binding act synergistically for induction of large-scale S conformational changes. The shape, size and orientation of receptor-binding domains in viral attachment proteins are well conserved among viruses of different classes that utilize the same receptor. Therefore, investigations unraveling the distribution of cellular receptors with respect to CoV entry, structural aspects of glycoproteins and related conformational changes are highly significant for understanding virus invasion and infection spread. We present the characteristic features of CoV S-Proteins, their significance for CoVs and related receptor binding activities for pathogenesis and viral survival. We are analyzing the novel role of S-protein of CoVs along with their interactive receptors for improving host immunity and decreasing infection spread. This is hoped that presented information will open new ways in tackling coronavirus, especially for the ongoing epidemic.

Effects of hypervariable regions in spike protein on pathogenicity, tropism, and serotypes of infectious bronchitis virus

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For the first time using reverse genetics to reveal the roles of HVRs in coronavirus.

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The HVRs exchange from IBV S1 subunit weakened the adsorption during IBV infection in vitro.

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The HVRs exchange in IBV S1 reduced ARV with Beaudette, but not sufficiently change serotypes.

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The recombinant IBVs provided insights into reverse genetic vaccines.

To study the roles of hypervariable regions (HVRs) in receptor-binding subunit S1 of the spike protein, we manipulated the genome of the IBV Beaudette strain using a reverse genetics system to construct seven recombinant strains by separately or simultaneously replacing the three HVRs of the Beaudette strain with the corresponding fragments from a QX-like nephropathogenic isolate ck/CH/LDL/091022 from China. We characterized the growth properties of these recombinant IBVs in Vero cells and embryonated eggs, and their pathogenicity, tropism, and serotypes in specific pathogen-free (SPF) chickens. All seven recombinant IBVs proliferated in Vero cells, but the heterogenous HVRs could reduce their capacity for adsorption during in vitro infection. The recombinant IBVs did not significantly increase the pathogenicity compared with the Beaudette strain in SPF chickens, and they still shared the same serotype as the Beaudette strain, but the antigenic relatedness values between the recombinant strain and Beaudette strain generally decreased with the increase in the number of the HVRs exchanged. The results of this study demonstrate the functions of HVRs and they may help to develop a vaccine candidate, as well as providing insights into the prevention and control of IBV.

Mouse hepatitis virus infection upregulates genes involved in innate immune responses

Neurotropic recombinant strain of Mouse Hepatitis Virus, RSA59, induces meningo-encephalitis, myelitis and demyelination following intracranial inoculation. RSA59 induced neuropathology is partially caused by activation of CNS resident microglia, as demonstrated by changes in cellular morphology and increased expression of a microglia/macrophage specific calcium ion binding factor, Iba1. Affymetrix Microarray analysis for mRNA expression data reveals expression of inflammatory mediators that are known to be released by activated microglia. Microglia-specific cell surface molecules, including CD11b, CD74, CD52 and CD68, are significantly upregulated in contrast to CD4, CD8 and CD19. Protein analysis of spinal cord extracts taken from mice 6 days post-inoculation, the time of peak inflammation, reveals robust expression of IFN-γ, IL-12 and mKC. Data suggest that activated microglia and inflammatory mediators contribute to a local CNS microenvironment that regulates viral replication and IFN-γ production during the acute phase of infection, which in turn can cause phagolysosome maturation and phagocytosis of the myelin sheath, leading to demyelination.

Identification of novel functional regions within the spike glycoprotein of MHV-A59 based on a bioinformatics approach

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Identification of functional regions within MHV-A59 spike (S) protein through analysis of sequence similarities with FcγR.

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C547 substitution abolishes the recognition of cleaved S by monoclonal antibodies.

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Substitution of residues 547 and 581–586 in S prevents the recovery of a viable virus.

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Amino acid replacements at positions 562/589 and 667/687 in S affect viral replication.

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Replacement of residue 939 in S affects the fusogenic properties of the virus.

Mouse Hepatitis Virus (MHV) is a single-stranded positive sense RNA virus with the ability to promote acute and chronic diseases in mice. The MHV spike protein (S) is a major virulence determinant which in addition to binding to cellular receptors to mediate cell entry and facilitate virus spread to adjacent cells by cell–cell fusion, also is a molecular mimic of the FcγRII receptor. This molecular mimicry of FcγRII by the MHV S protein is also exhibited by other lineage 2a betacoronaviruses, with the exception of the human coronavirus HCoV-OC43. In this work we undertook a mutational analysis to attempt to identify specific amino acid sequences within the spike glycoprotein crucial for molecular mimicry of FcγRII. Although we were unsuccessful in isolating mutant viruses which were specifically defective in that property, we identified several mutations with interesting phenotypes. Mutation of the cysteine in position 547 to alanine and alanine replacements at residues 581–586 was lethal. Replacing proline 939 with the corresponding HCoV-OC43 residue, leucine, decreased the ability MHV to induce cell–cell fusion, providing experimental support for an earlier proposal that residues 929–944 make up the fusion peptide of the MHV S protein.

Genetic diversity and selection regulates evolution of infectious bronchitis virus

Conventional and molecular epidemiologic studies have confirmed the ability of infectious bronchitis virus (IBV) to rapidly evolve and successfully circumvent extensive vaccination programs implemented since the early 1950s. IBV evolution has often been explained as variation in gene frequencies as if evolution were driven by genetic drift alone. However, the mechanisms regulating the evolution of IBV include both the generation of genetic diversity and the selection process. IBV's generation of genetic diversity has been extensively investigated and ultimately involves mutations and recombination events occurring during viral replication. The relevance of the selection process has been further understood more recently by identifying genetic and phenotypic differences between IBV populations prior to, and during, replication in the natural host. Accumulating evidence suggests that multiple environmental forces within the host, including immune responses (or lack thereof) and affinity for cell receptors, as well as physical and biochemical conditions, are responsible for the selection process. Some scientists have used or adopted the related quasispecies frame to explain IBV evolution. The quasispecies frame, while providing a distinct explanation of the dynamics of populations in which mutation is a frequent event, exhibits relevant limitations which are discussed herein. Instead, it seems that IBV populations evolving by the generation of genetic variability and selection on replicons follow the evolutionary mechanisms originally proposed by Darwin. Understanding the mechanisms underlying the evolution of IBV is of basic relevance and, without doubt, essential to appropriately control and prevent the disease.

Mechanisms of coronavirus cell entry mediated by the viral spike protein

Coronaviruses are enveloped positive-stranded RNA viruses that replicate in the cytoplasm. To deliver their nucleocapsid into the host cell, they rely on the fusion of their envelope with the host cell membrane. The spike glycoprotein (S) mediates virus entry and is a primary determinant of cell tropism and pathogenesis. It is classified as a class I fusion protein, and is responsible for binding to the receptor on the host cell as well as mediating the fusion of host and viral membranes—A process driven by major conformational changes of the S protein. This review discusses coronavirus entry mechanisms focusing on the different triggers used by coronaviruses to initiate the conformational change of the S protein: receptor binding, low pH exposure and proteolytic activation. We also highlight commonalities between coronavirus S proteins and other class I viral fusion proteins, as well as distinctive features that confer distinct tropism, pathogenicity and host interspecies transmission characteristics to coronaviruses.

Mutations in the spike glycoprotein of human coronavirus OC43 modulate disease in BALB/c mice from encephalitis to flaccid paralysis and demyelination

The etiology of most neurodegenerative diseases of the central nervous system remains unknown and likely involves a combination of genetic susceptibility and environmental triggering factors. Given that exposure to numerous infectious pathogens occurs during childhood, and that some viral infections can lead to neurodegeneration and demyelination, it is conceivable that some viruses may act as triggering factors in neuropathogenesis. We have previously shown that the prototype OC43 strain of the common cold—associated human respiratory coronavirus has the capacity to infect human neuronal and glial cells and does persist in human brains. Moreover, it has neuroinvasive properties in susceptible BALB/c mice, where it leads to a chronic encephalitis with accompanying disabilities. Here, we show that mutations in the viral spike glycoprotein, reproducibly acquired during viral persistence in human neural cell cultures, led to a drastically modified virus-induced neuropathology in BALB/c mice, characterized by flaccid paralysis and demyelination. Even though infection by both mutated and wild-type viruses led to neuroinflammation, the modified neuropathogenesis induced by the mutated virus was associated with increased viral spread and significantly more CD4+ and CD8+ T-lymphocyte infiltration into the central nervous system, as well as significantly increased levels of the proinflammatory cytokine interleukin (IL)-6 and the chemokine CCL2 (monocyte chemoattractant protein [MCP]-1). Moreover, recombinant virus harboring the S glycoprotein mutations retained its neurotropism, productively infecting neurons. Therefore, interaction of a human respiratory coronavirus with the central nervous system may modulate virus and host factors resulting in a modified neuropathogenesis in genetically susceptible individuals.

Genetic determinants of mouse hepatitis virus strain 1 pneumovirulence

We report here investigation into the genetic basis of mouse hepatitis virus strain 1 (MHV-1) pneumovirulence. Sequencing of the 3' one-third of the MHV-1 genome demonstrated that the genetic organization of MHV-1 was similar to that of other strains of MHV. The hemagglutinin esterase (HE) protein was truncated, and reverse transcription-PCR (RT-PCR) studies confirmed previous work that suggested that the MHV-1 HE is a pseudogene. Targeted recombination was used to select chimeric viruses containing either the MHV-1 S gene or genes encoding all of the MHV-1 structural proteins, on an MHV-A59 background. Challenge studies in mice demonstrated that expression of the MHV-1 S gene within the MHV-A59 background (rA59/S(MHV-1)) increased the pneumovirulence of MHV-A59, and mice infected with this recombinant virus developed pulmonary lesions that were similar to those observed with MHV-1, although rA59/S(MHV-1) was significantly less virulent. Chimeras containing all of the MHV-1 structural genes on an MHV-A59 background were able to reproduce the severe acute respiratory syndrome (SARS)-like pathology observed with MHV-1 and reproducibly increased pneumovirulence relative to rA59/S(MHV-1), but were still much less virulent than MHV-1. These data suggest that important determinants of pneumopathogenicity are contained within the 3' one-third of the MHV-1 genome, but additional important virulence factors must be encoded in the genome upstream of the S gene. The severity of the pulmonary lesions observed correlates better with elevated levels of inflammatory cytokines than with viral replication in the lungs, suggesting that pulmonary disease has an important immunological component.

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.

Detection and characterization of bovine torovirus from the respiratory tract in Japanese cattle

Bovine torovirus (BToV), a member of the Coronaviridae family, is a causative agent of diarrhea in cattle, but it may also possess tropism for the respiratory tract. However, no surveys concerning with the relation between respiratory symptoms and the detection of BToV have been conducted in wide range. Among 311 nasal samples, BToV gene products were detected in seven samples (rBToV-1 to -7) derived only from calves with respiratory symptoms, suggesting that BToV may be a predisposing factor and/or causative agent for bovine respiratory disease. Regarding the degree of similarity between the spike and hemagglutinin-esterase coding regions, the rBToVs showed over 90.8% similarity with one another and 73.5–99.0% similarity with fecal tract-derived BToVs. rBToV-1, -2, and -3 were identical despite their being collected during different seasons; in comparison, rBToV-4 and -5 were distinct despite the fact that they were collected from the same herd, suggesting the existence of diversity among domestic rBToVs. One animal with a BToV-positive nasal sample also shed the virus in its feces, suggesting dual tropisms for BToV.

Epidemiological analysis of bovine torovirus in Japan

Bovine torovirus (BToV), a member of the Coronaviridae family, is an established gastrointestinal infectious agent in cattle. No epidemiological research on BToV has been reported from Japan. In this study, we performed a survey to detect BToV in Japan in 2004 and 2005 using 231 fecal samples (167 from diarrheic cattle and 64 from asymptomatic cattle) that were analyzed by nested reverse transcription (RT) PCR using primers located in the consensus sequences of the reported BToV nucleocapsid (N), membrane (M), and spike (S) genes. BToV N, M, and S genes were detected in 6.5% (15/231), 6.1% (14/231), and 5.6% (13/231) of samples by nested-RT-PCR, respectively. In conclusion, detectability was improved compared to the results of the first round of RT-PCR. BToV was detected at a significantly higher rate in diarrheic samples than in asymptomatic samples (14/167 diarrheic samples [8.4%] and 1/64 asymptomatic samples [1.6%]), suggesting that BToV may act as a risk factor for diarrhea in Japanese cattle. The nucleotide sequence of M fragments from the BToV isolates including the newly identified Japanese isolates showed more than 97% identity. A similar degree of homology was observed in the N gene fragment among BToV isolates with the exception of BRV-1 and BRV-2. Domestic samples were classified into three clusters by phylogenetic analysis of the S gene fragment, which were considerably correlated with the geographic origin of the samples. BToV positive areas did not adjoin each other but were spread across a wide range, suggesting that BToV exists conventionally in Japan and is geographically differentiated. We also developed an RFLP method to distinguish these clusters using two restriction enzymes, HaeIII and AccI. This method should be useful for comparing newly acquired BToV-positive samples with the reported BToVs.

Murine hepatitis virus strain 1 produces a clinically relevant model of severe acute respiratory syndrome in A/J mice

Severe acute respiratory syndrome (SARS) is a life-threatening infectious disease which has been difficult to study and treat because of the lack of a readily available animal model. Intranasal infection of A/J mice with the coronavirus murine hepatitis virus strain 1 (MHV-1) produced pulmonary pathological features of SARS. All MHV-1-infected A/J mice developed progressive interstitial pneumonitis, including dense macrophage infiltrates, giant cells, and hyaline membranes, resulting in death of all animals. In contrast, other mouse strains developed only mild transitory disease. Infected A/J mice had significantly higher cytokine levels, particularly macrophage chemoattractant protein 1 (MCP-1/CCL-2), gamma interferon, and tumor necrosis factor alpha. Furthermore, FGL2/fibroleukin mRNA transcripts and protein and fibrin deposits were markedly increased in the lungs of infected A/J mice. These animals developed a less robust type I interferon response to MHV-1 infection than resistant C57BL/6J mice, and treatment with recombinant beta interferon improved survival. This study describes a potentially useful small animal model of human SARS, defines its pathogenesis, and suggests treatment strategies.

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.

Functional diversity of chemokines and chemokine receptors in response to viral infection of the central nervous system

Encounters with neurotropic viruses result in varied outcomes ranging from encephalitis, paralytic poliomyelitis or other serious consequences to relatively benign infection. One of the principal factors that control the outcome of infection is the localized tissue response and subsequent immune response directed against the invading toxic agent. It is the role of the immune system to contain and control the spread of virus infection in the central nervous system (CNS), and paradoxically, this response may also be pathologic. Chemokines are potent proinflammatory molecules whose expression within virally infected tissues is often associated with protection and/or pathology which correlates with migration and accumulation of immune cells. Indeed, studies with a neurotropic murine coronavirus, mouse hepatitis virus (MHV), have provided important insight into the functional roles of chemokines and chemokine receptors in participating in various aspects of host defense as well as disease development within the CNS. This chapter will highlight recent discoveries that have provided insight into the diverse biologic roles of chemokines and their receptors in coordinating immune responses following viral infection of the CNS.

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.

Substitutions of conserved amino acids in the receptor-binding domain of the spike glycoprotein affect utilization of murine CEACAM1a by the murine coronavirus MHV-A59

The host range of the murine coronavirus (MHV) is limited to susceptible mice and murine cell lines by interactions of the spike glycoprotein (S) with its receptor, mCEACAM1a. We identified five residues in S (S33, L79, T82, Y162 and K183) that are conserved in the receptor-binding domain of MHV strains, but not in related coronaviruses. We used targeted RNA recombination to generate isogenic viruses that differ from MHV-A59 by amino acid substitutions in S. Viruses with S33R and K183R substitutions had wild type growth, while L79A/T82A viruses formed small plaques. Viruses with S33G, L79M/T82M or K183G substitutions could only be recovered from cells that over-expressed a mutant mCEACAM1a. Viruses with Y162H or Y162Q substitutions were never recovered, while Y162A viruses formed minute plaques. However, viruses with Y162F substitutions had wild type growth, suggesting that Y162 may comprise part of a hydrophobic domain that contacts the MHV-binding site of mCEACAM1a.

Single-amino-acid substitutions in open reading frame (ORF) 1b-nsp14 and ORF 2a proteins of the coronavirus mouse hepatitis virus are attenuating in mice

A reverse genetic system was recently established for the coronavirus mouse hepatitis virus strain A59 (MHV-A59), in which cDNA fragments of the RNA genome are assembled in vitro into a full-length genome cDNA, followed by electroporation of in vitro-transcribed genome RNA into cells with recovery of viable virus. The "in vitro-assembled" wild-type MHV-A59 virus (icMHV-A59) demonstrated replication identical to laboratory strains of MHV-A59 in tissue culture; however, icMHV-A59 was avirulent following intracranial inoculation of C57BL/6 mice. Sequencing of the cloned genome cDNA fragments identified two single-nucleotide mutations in cloned genome fragment F, encoding a Tyr6398His substitution in open reading frame (ORF) 1b p59-nsp14 and a Leu94Pro substitution in the ORF 2a 30-kDa protein. The mutations were repaired individually and together in recombinant viruses, all of which demonstrated wild-type replication in tissue culture. Following intracranial inoculation of mice, the viruses encoding Tyr6398His/Leu94Pro substitutions and the Tyr6398His substitution alone demonstrated log10 50% lethal dose (LD50) values too great to be measured. The Leu94Pro mutant virus had reduced but measurable log10 LD5), and the "corrected" Tyr6398/Leu94 virus had a log10 LD50 identical to wild-type MHV-A59. The experiments have defined residues in ORF 1b and ORF 2a that attenuate virus replication and virulence in mice but do not affect in vitro replication. The results suggest that these proteins serve roles in pathogenesis or virus survival in vivo distinct from functions in virus replication. The study also demonstrates the usefulness of the reverse genetic system to confirm the role of residues or proteins in coronavirus replication and pathogenesis.

Molecular Determinants of Coronavirus Mhv- Induced Demyelination

Mouse hepatitis virus (MHV) is a member of the coronavirus family of the nidovirales order. MHV is an enveloped virus with single-stranded, positive genomic RNA of about 31kb. Infection of susceptible strains of mice with the MHV-JHM and A59 strains results in acute encephalomyelitis and chronic demyelinating disease with features similar to the human demyelination disease multiple sclerosis (MS). Because the mechanism of demyelination in MS is not completely understood, various experimental models, including MHV infection in mice, have been used to study the pathogenesis of inflammatory autoimmune demyelination. The spike (S) glycoprotein of MHV has been implicated as the most critical genomic determinant of MHV pathogenesis and demyelination. However, other genes and proteins are likely to contribute to MHV pathogenesis as well.

Coronavirus reverse genetics by targeted RNA recombination

Targeted RNA recombination was the first reverse genetics system devised for coronaviruses at a time when it was not clear whether the construction of full-length infectious cDNA clones would become possible. In its current state targeted RNA recombination offers a versatile and powerful method for the site-directed mutagenesis of the downstream third of the coronavirus genome, which encodes all the viral structural proteins. The development of this system is described, with an emphasis on recent improvements, and multiple applications of this technique to the study of coronavirus molecular biology and pathogenesis are reviewed. Additionally, the relative strengths and limitations of targeted RNA recombination and infectious cDNA systems are contrasted.

Diversity of Coronavirus Spikes: Relationship to Pathogen Entry and Dissemination

Coronaviruses are widespread in the environment, infecting humans, domesticated and wild mammals, and birds. Infections cause a variety of diseases including bronchitis, gastroenteritis, hepatitis, and encephalitis, with symptoms ranging from being nearly undetectable to rapidly fatal. A combination of interacting variables determine the pattern and severity of coronavirus-induced disease, including the infecting virus strain, its transmission strategy, and the age and immune status of the infected host. Coronavirus pathogenesis is best understood by discerning how each of these variables dictates clinical outcomes. This chapter focuses on variabilities amongst the spike (S) proteins of infecting virus strains. Diversity of coronavirus surface proteins likely contributes to epidemic disease, an important and timely topic given the recent emergence of the human SARS coronavirus.

Recombinant infectious bronchitis coronavirus Beaudette with the spike protein gene of the pathogenic M41 strain remains attenuated but induces protective immunity

We have replaced the ectodomain of the spike (S) protein of the Beaudette strain (Beau-R; apathogenic for Gallus domesticus chickens) of avian infectious bronchitis coronavirus (IBV) with that from the pathogenic M41 strain to produce recombinant IBV BeauR-M41(S). We have previously shown that this changed the tropism of the virus in vitro (R. Casais, B. Dove, D. Cavanagh, and P. Britton, J. Virol. 77:9084-9089, 2003). Herein we have assessed the pathogenicity and immunogenicity of BeauR-M41(S). There were no consistent differences in pathogenicity between the recombinant BeauR-M41(S) and its apathogenic parent Beau-R (based on snicking, nasal discharge, wheezing, watery eyes, rales, and ciliostasis in trachea), and both replicated poorly in trachea and nose compared to M41; the S protein from the pathogenic M41 had not altered the apathogenic nature of Beau-R. Both Beau-R and BeauR-M41(S) induced protection against challenge with M41 as assessed by absence of recovery of challenge virus and nasal exudate. With regard to snicking and ciliostasis, BeauR-M41(S) induced greater protection (seven out of nine chicks [77%]; assessed by ciliostasis) than Beau-R (one out of nine; 11%) but less than M41 (100%). The greater protection induced by BeauR-M41(S) against M41 may be related to the ectodomain of the spike protein of Beau-R differing from that of M41 by 4.1%; a small number of epitopes on the S protein may play a disproportionate role in the induction of immunity. The results are promising for the prospects of S-gene exchange for IBV vaccine development.

A combination of mutations in the S1 part of the spike glycoprotein gene of coronavirus MHV-A59 abolishes demyelination

The A59 strain of coronavirus, mouse hepatitis virus (MHV), produces acute hepatitis, meningoencephalitis, and chronic demyelination. The authors have previously shown that the spike (S) glycoprotein gene of MHV contains determinants of virulence, hepatitis, and demyelination. They then identified viruses containing mutations in the S gene that exhibit alterations in viral pathogenesis. In the present study, the authors produced new recombinant viruses with each one of these S gene mutations by site-directed mutagenesis and targeted recombination and studied the effect of each individual mutation on the pathogenesis of the virus. They identified a combination of mutations in the S1 gene (I375M and L652I) that abolishes demyelination. Individual mutation and other combinations of mutations in the S gene only interfere with virulence and hepatitis and only reduce demyelination (I375M), but do not abolish demyelination completely. Thus, demyelination determinants exist within genomic regions on both sides of the hypervariable region, downstream from the receptor-binding domain in the S1 part of the MHV spike glycoprotein gene. The structure and precise function of these regions awaits further investigation.

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.

SARS: Lessons Learned from Other Coronaviruses

The identification of a new coronavirus as the etiological agent of severe acute respiratory syndrome (SARS) has evoked much new interest in the molecular biology and pathogenesis of coronaviruses. This review summarizes present knowledge on coronavirus molecular biology and pathogenesis with particular emphasis on mouse hepatitis virus (MHV). MHV, a member of coronavirus group 2, is a natural pathogen of the mouse; MHV infection of the mouse is considered one of the best models for the study of demyelinating disease, such as multiple sclerosis, in humans. As a result of the SARS epidemic, coronaviruses can now be considered as emerging pathogens. Future research on SARS needs to be based on all the knowledge that coronavirologists have generated over more than 30 years of research.

Amino acid substitutions within the heptad repeat domain 1 of murine coronavirus spike protein restrict viral antigen spread in the central nervous system

Targeted recombination was carried out to select mouse hepatitis viruses (MHVs) in a defined genetic background, containing an MHV-JHM spike gene encoding either three heptad repeat 1 (HR1) substitutions (Q1067H, Q1094H, and L1114R) or L1114R alone. The recombinant virus, which expresses spike with the three substitutions, was nonfusogenic at neutral pH. Its replication was significantly inhibited by lysosomotropic agents, and it was highly neuroattenuated in vivo. In contrast, the recombinant expressing spike with L1114R alone mediated cell-to-cell fusion at neutral pH and replicated efficiently despite the presence of lysosomotropic agents; however, it still caused only subclinical morbidity and no mortality in animals. Thus, both recombinant viruses were highly attenuated and expressed viral antigen which was restricted to the olfactory bulbs and was markedly absent from other regions of the brains at 5 days postinfection. These data demonstrate that amino acid substitutions, in particular L1114R, within HR1 of the JHM spike reduced the ability of MHV to spread in the central nervous system. Furthermore, the requirements for low pH for fusion and viral entry are not prerequisites for the highly attenuated phenotype.

Conformational changes in the spike glycoprotein of murine coronavirus are induced at 37 degrees C either by soluble murine CEACAM1 receptors or by pH 8

The spike glycoprotein (S) of the murine coronavirus mouse hepatitis virus (MHV) binds to viral murine CEACAM receptor glycoproteins and causes membrane fusion. On virions, the 180-kDa S glycoprotein of the MHV-A59 strain can be cleaved by trypsin to form the 90-kDa N-terminal receptor-binding subunit (S1) and the 90-kDa membrane-anchored fusion subunit (S2). Incubation of virions with purified, soluble CEACAM1a receptor proteins at 37 degrees C and pH 6.5 neutralizes virus infectivity (B. D. Zelus, D. R. Wessner, R. K. Williams, M. N. Pensiero, F. T. Phibbs, M. deSouza, G. S. Dveksler, and K. V. Holmes, J. Virol. 72:7237-7244, 1998). We used liposome flotation and protease sensitivity assays to investigate the mechanism of receptor-induced, temperature-dependent virus neutralization. After incubation with soluble receptor at 37 degrees C and pH 6.5, virions became hydrophobic and bound to liposomes. Receptor binding induced a profound, apparently irreversible conformational change in S on the viral envelope that allowed S2, but not S1, to be degraded by trypsin at 4 degrees C. Various murine CEACAM proteins triggered conformational changes in S on recombinant MHV strains expressing S glycoproteins of MHV-A59 or MHV-4 (MHV-JHM) with the same specificities as seen for virus neutralization and virus-receptor activities. Increased hydrophobicity of virions and conformational change in S2 of MHV-A59 could also be induced by incubating virions at pH 8 and 37 degrees C, without soluble receptor. Surprisingly, the S protein of recombinant MHV-A59 virions with a mutation, H716D, that precluded cleavage between S1 and S2 could also be triggered to undergo a conformational change at 37 degrees C by soluble receptor at neutral pH or by pH 8 alone. A novel 120-kDa subunit was formed following incubation of the receptor-triggered S(A59)H716D virions with trypsin at 4 degrees C. The data show that unlike class 1 fusion glycoproteins of other enveloped viruses, the murine coronavirus S protein can be triggered to a membrane-binding conformation at 37 degrees C either by soluble receptor at neutral pH or by alkaline pH alone, without requiring previous activation by cleavage between S1 and S2.

The N-terminal domain of the murine coronavirus spike glycoprotein determines the CEACAM1 receptor specificity of the virus strain

Using isogenic recombinant murine coronaviruses expressing wild-type murine hepatitis virus strain 4 (MHV-4) or MHV-A59 spike glycoproteins or chimeric MHV-4/MHV-A59 spike glycoproteins, we have demonstrated the biological functionality of the N-terminus of the spike, encompassing the receptor binding domain (RBD). We have used two assays, one an in vitro liposome binding assay and the other a tissue culture replication assay. The liposome binding assay shows that interaction of the receptor with spikes on virions at 37 degrees C causes a conformational change that makes the virions hydrophobic so that they bind to liposomes (B. D. Zelus, J. H. Schickli, D. M. Blau, S. R. Weiss, and K. V. Holmes, J. Virol. 77: 830-840, 2003). Recombinant viruses with spikes containing the RBD of either MHV-A59 or MHV-4 readily associated with liposomes at 37 degrees C in the presence of soluble mCEACAM1(a), except for S(4)R, which expresses the entire wild-type MHV-4 spike and associated only inefficiently with liposomes following incubation with soluble mCEACAM1(a). In contrast, soluble mCEACAM1(b) allowed viruses with the MHV-A59 RBD to associate with liposomes more efficiently than did viruses with the MHV-4 RBD. In the second assay, which requires virus entry and replication, all recombinant viruses replicated efficiently in BHK cells expressing mCEACAM1(a). In BHK cells expressing mCEACAM1(b), only viruses expressing chimeric spikes with the MHV-A59 RBD could replicate, while replication of viruses expressing chimeric spikes with the MHV-4 RBD was undetectable. Despite having the MHV-4 RBD, S(4)R replicated in BHK cells expressing mCEACAM1(b); this is most probably due to spread via CEACAM1 receptor-independent cell-to-cell fusion, an activity displayed only by S(4)R among the recombinant viruses studied here. These data suggest that the RBD domain and the rest of the spike must coevolve to optimize function in viral entry and spread.

Murine coronavirus spike glycoprotein mediates degree of viral spread, inflammation, and virus-induced immunopathology in the central nervous system

The mouse hepatitis virus (MHV) spike glycoprotein is a major determinant of neurovirulence. We investigated how alterations in spike affect neurovirulence using two isogenic recombinant viruses differing exclusively in spike. S(4)R, containing the MHV-4 spike gene, is dramatically more neurovirulent than S(A59)R, containing the MHV-A59 spike gene (J. J. Phillips, M. M. Chua, E. Lavi, and S. R. Weiss, 1999, J. Virol. 73, 7752-7760). We examined the contribution of differences in cellular tropism, viral spread, and the immune response to infection to the differential neurovirulence of S(4)R and S(A59)R. MHV-4 spike-mediated neurovirulence was associated with extensive viral spread in the brain in both neurons and astrocytes. Infection of primary hippocampal neuron cultures demonstrated that S(4)R spread more rapidly than S(A59)R and suggested that spread may occur between cells in close physical contact. In addition, S(4)R infection induced a massive influx of lymphocytes into the brain, a higher percentage of CD8(+) T cells, and a higher frequency of MHV-specific CD8(+) T cells relative S(A59)R infection. Despite this robust and viral-specific immune response to S(4)R infection, infection of RAG1-/- mice suggested that immune-mediated pathology also contributes to the high neurovirulence of S(4)R.

The virulence of mouse hepatitis virus strain A59 is not dependent on efficient spike protein cleavage and cell-to-cell fusion

The cleavage and fusion properties of recombinant murine hepatitis viruses (MHV) were examined to assess the role of the cleavage signal in determining the extent of S protein cleavage, and the correlation between cleavage and induction of cell-to-cell fusion. Targeted recombination was used to introduce amino acid substitutions into the cleavage signal of the fusion glycoprotein (spike or S protein) of MHV strain A59. The recombinants were then used to address the question of the importance of S protein cleavage and viral-mediated cell-to-cell fusion on pathogenicity. Our data indicate that cleavage of spike is not solely determined by the amino acid sequence at the cleavage site, but may also depend on sequences removed from the cleavage site. In addition, efficient cell-to-cell fusion is not necessary for virulence.

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.