Neuropathogenicity of mouse hepatitis virus JHM isolates differing in hemagglutinin-esterase protein expression

SDAV, the Rat Coronavirus-How Much Do We Know about It in the Light of Potential Zoonoses

Sialodacryoadenitis virus (SDAV) is known to be an etiological agent, causing infections in laboratory rats. Until now, its role has only been considered in studies on respiratory and salivary gland infections. The scant literature data, consisting mainly of papers from the last century, do not sufficiently address the topic of SDAV infections. The ongoing pandemic has demonstrated, once again, the role of the Coronaviridae family as extremely dangerous etiological agents of human zoonoses. The ability of coronaviruses to cross the species barrier and change to hosts commonly found in close proximity to humans highlights the need to characterize SDAV infections. The main host of the infection is the rat, as mentioned above. Rats inhabit large urban agglomerations, carrying a vast epidemic threat. Of the 2277 existing rodent species, 217 are reservoirs for 66 zoonotic diseases caused by viruses, bacteria, fungi, and protozoa. This review provides insight into the current state of knowledge of SDAV characteristics and its likely zoonotic potential.

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.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Molecular epidemiology of human coronavirus OC43 reveals evolution of different genotypes over time and recent emergence of a novel genotype due to natural recombination

Although human coronavirus OC43-OC43 (HCoV-OC43) is the coronavirus most commonly associated with human infections, little is known about its molecular epidemiology and evolution. We conducted a molecular epidemiology study to investigate different genotypes and potential recombination in HCoV-OC43. Twenty-nine HCoV-OC43 strains from nasopharyngeal aspirates, collected from 2004 to 2011, were subjected to RNA-dependent RNA polymerase (RdRp), spike, and nucleocapsid gene analysis. Phylogenetic analysis showed at least three distinct clusters of HCoV-OC43, although 10 unusual strains displayed incongruent phylogenetic positions between RdRp and spike genes. This suggested the presence of four HCoV-OC43 genotypes (A to D), with genotype D most likely arising from recombination. The complete genome sequencing of two genotype C and D strains and bootscan analysis showed recombination events between genotypes B and C in the generation of genotype D. Of the 29 strains, none belonged to the more ancient genotype A, 5 from 2004 belonged to genotype B, 15 from 2004 to 2006 belonged to genotype C, and 1 from 2004 and all 8 from 2008 to 2011 belonged to the recombinant genotype D. Molecular clock analysis using spike and nucleocapsid genes dated the most recent common ancestor of all genotypes to the 1950s, genotype B and C to the 1980s, genotype B to the 1990s, and genotype C to the late 1990s to early 2000s, while the recombinant genotype D strains were detected as early as 2004. This represents the first study to describe natural recombination in HCoV-OC43 and the evolution of different genotypes over time, leading to the emergence of novel genotype D, which is associated with pneumonia in our elderly population.

Binding of avian coronavirus spike proteins to host factors reflects virus tropism and pathogenicity

The binding of viruses to host cells is the first step in determining tropism and pathogenicity. While avian infectious bronchitis coronavirus (IBV) infection and avian influenza A virus (IAV) infection both depend on α2,3-linked sialic acids, the host tropism of IBV is restricted compared to that of IAV. Here we investigated whether the interaction between the viral attachment proteins and the host could explain these differences by using recombinant spike domains (S1) of IBV strains with different pathogenicities, as well as the hemagglutinin (HA) protein of IAV H5N1. Protein histochemistry showed that S1 of IBV strain M41 and HA of IAV subtype H5N1 displayed sialic acid-dependent binding to chicken respiratory tract tissue. However, while HA bound with high avidity to a broad range of α2,3-linked sialylated glycans, M41 S1 recognized only one particular α2,3-linked disialoside in a glycan array. When comparing the binding of recombinant IBV S1 proteins derived from IBV strains with known differences in tissue tropism and pathogenicity, we observed that while M41 S1 displayed binding to cilia and goblet cells of the chicken respiratory tract, S1 derived from the vaccine strain H120 or the nonvirulent Beaudette strain had reduced or no binding to chicken tissues, respectively, in agreement with the reduced abilities of these viruses to replicate in vivo. While the S1 protein derived from the nephropathogenic IBV strain B1648 also hardly displayed binding to respiratory tract cells, distinct binding to kidney cells was observed, but only after the removal of sialic acid from S1. In conclusion, our data demonstrate that the attachment patterns of the IBV S proteins correlate with the tropisms and pathogenicities of the corresponding viruses.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

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.

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.

The spike protein of murine coronavirus regulates viral genome transport from the cell surface to the endoplasmic reticulum during infection

We observed that the nonfusogenic mouse hepatitis virus (MHV) strain MHV-2 reached a titer of approximately 2 log10 higher than that of the fusogenic strain A59 in astrocytoma DBT cells. To determine whether the spike protein is responsible for the difference, a recombinant virus, Penn-98-1, that contains the A59 genome with a spike from MHV-2 was used to infect DBT cells. Results showed that Penn-98-1 behaved like MHV-2, thus establishing a role for the spike protein in viral growth. The inverse correlation between viral fusogenicity and growth was further established in four different cell types and with a fusogenic mutant, the S757R mutant, derived from isogenic Penn-98-1. While both A59 and Penn-98-1 entered cells at similar levels, viral RNA and protein syntheses were significantly delayed for A59. Interestingly, when the genomic RNAs were delivered directly into the cells via transfection, the levels of gene expression for these viruses were similar. Furthermore, cell fractionation experiments revealed that significantly more genomic RNAs for the nonfusogenic MHVs were detected in the endoplasmic reticulum (ER) within the first 2 h after infection than for the fusogenic MHVs. Pretreatment of Penn-98-1 with trypsin reversed its properties in syncytium formation, virus production, and genome transport to the ER. These findings identified a novel role for the spike protein in regulating the uncoating and delivery of the viral genome to the ER after internalization.

Sialic acids as receptor determinants for coronaviruses

Among coronaviruses, several members are able to interact with sialic acids. For bovine coronavirus (BCoV) and related viruses, binding to cell surface components containing N-acetyl-9- O-acetylneuraminic acid is essential for initiation of an infection. These viruses resemble influenza C viruses because they share not only the receptor determinant, but also the presence of an acetylesterase that releases the 9- O-acetyl group from sialic acid and thus abolishes the ability of the respective sialoglycoconjugate to function as a receptor for BCoV. As in the case of influenza viruses, the receptor-destroying enzyme of BCoV is believed to facilitate the spread of virus infection by removing receptor determinants from the surface of infected cells and by preventing the formation of virus aggregates. Another coronavirus, porcine transmissible gastroenteritis virus (TGEV) preferentially recognizes N-glycolylneuraminic acid. TGEV does not contain a receptor-destroying enzyme and does not depend on the sialic acid binding activity for infection of cultured cells. However, binding to sialic acids is required for the enteropathogenicity of TGEV. Interaction with sialoglycoconjugates may help the virus to pass through the sialic acid-rich mucus layer that covers the viral target cells in the epithelium of the small intestine. We discuss that the BCoV group of viruses may have evolved from a TGEV-like ancestor by acquiring an acetylesterase gene through heterologous recombination.

Luxury at a cost? Recombinant mouse hepatitis viruses expressing the accessory hemagglutinin esterase protein display reduced fitness in vitro

Group 2 coronaviruses encode an accessory envelope glycoprotein species, the hemagglutinin esterase (HE), which possesses sialate-O-acetylesterase activity and which, presumably, promotes virus spread and entry in vivo by facilitating reversible virion attachment to O-acetylated sialic acids. While HE may provide a strong selective advantage during natural infection, many laboratory strains of mouse hepatitis virus (MHV) fail to produce the protein. Apparently, their HE genes were inactivated during cell culture adaptation. For this report, we have studied the molecular basis of this phenomenon. By using targeted RNA recombination, we generated isogenic recombinant MHVs which differ exclusively in their expression of HE and produce either the wild-type protein (HE+), an enzymatically inactive HE protein (HE0), or no HE at all. HE expression or the lack thereof did not lead to gross differences in in vitro growth properties. Yet the expression of HE was rapidly lost during serial cell culture passaging. Competition experiments with mixed infections revealed that this was not due to the enzymatic activity: MHVs expressing HE+ or HE0 propagated with equal efficiencies. During the propagation of recombinant MHV-HE+, two types of spontaneous mutants accumulated. One produced an anchorless HE, while the other had a Gly-to-Trp substitution at the predicted C-terminal residue of the HE signal peptide. Neither mutant incorporated HE into virion particles, suggesting that wild-type HE reduces the in vitro propagation efficiency, either at the assembly stage or at a postassembly level. Our findings demonstrate that the expression of "luxury" proteins may come at a fitness penalty. Apparently, under natural conditions the costs of maintaining HE are outweighed by the benefits.

Expression of hemagglutinin esterase protein from recombinant mouse hepatitis virus enhances neurovirulence

Murine hepatitis virus (MHV) infection provides a model system for the study of hepatitis, acute encephalitis, and chronic demyelinating disease. The spike glycoprotein, S, which mediates receptor binding and membrane fusion, plays a critical role in MHV pathogenesis. However, viral proteins other than S also contribute to pathogenicity. The JHM strain of MHV is highly neurovirulent and expresses a second spike glycoprotein, the hemagglutinin esterase (HE), which is not produced by MHV-A59, a hepatotropic but only mildly neurovirulent strain. To investigate a possible role for HE in MHV-induced neurovirulence, isogenic recombinant MHV-A59 viruses were generated that produced either (i) the wild-type protein, (ii) an enzymatically inactive HE protein, or (iii) no HE at all (A. Lissenberg, M. M. Vrolijk, A. L. W. van Vliet, M. A. Langereis, J. D. F. de Groot-Mijnes, P. J. M. Rottier, and R. J. de Groot, J. Virol. 79:15054-15063, 2005 [accompanying paper]). A second, mirror set of recombinant viruses was constructed in which, in addition, the MHV-A59 S gene had been replaced with that from MHV-JHM. The expression of HE in combination with A59 S did not affect the tropism, pathogenicity, or spread of the virus in vivo. However, in combination with JHM S, the expression of HE, regardless of whether it retained esterase activity or not, resulted in increased viral spread within the central nervous system and in increased neurovirulence. Our findings suggest that the properties of S receptor utilization and/or fusogenicity mainly determine organ and host cell tropism but that HE enhances the efficiency of infection and promotes viral dissemination, at least in some tissues, presumably by serving as a second receptor-binding protein.

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.

Molecular interactions in the assembly of coronaviruses

This chapter describes the interactions between the different structural components of the viruses and discusses their relevance for the process of virion formation. Two key factors determine the efficiency of the assembly process: intracellular transport and molecular interactions. Many viruses have evolved elaborate strategies to ensure the swift and accurate delivery of the virion components to the cellular compartment(s) where they must meet and form (sub) structures. Assembly of viruses starts in the nucleus by the encapsidation of viral DNA, using cytoplasmically synthesized capsid proteins; nucleocapsids then migrate to the cytosol, by budding at the inner nuclear membrane followed by deenvelopment, to pick up the tegument proteins.

Inhibition, escape, and attenuated growth of severe acute respiratory syndrome coronavirus treated with antisense morpholino oligomers

The recently emerged severe acute respiratory syndrome coronavirus (SARS-CoV) is a potent pathogen of humans and is capable of rapid global spread. Peptide-conjugated antisense morpholino oligomers (P-PMO) were designed to bind by base pairing to specific sequences in the SARS-CoV (Tor2 strain) genome. The P-PMO were tested for their capacity to inhibit production of infectious virus as well as to probe the function of conserved viral RNA motifs and secondary structures. Several virus-targeted P-PMO and a random-sequence control P-PMO showed low inhibitory activity against SARS coronavirus. Certain other virus-targeted P-PMO reduced virus-induced cytopathology and cell-to-cell spread as a consequence of decreasing viral amplification. Active P-PMO were effective when administered at any time prior to peak viral synthesis and exerted sustained antiviral effects while present in culture medium. P-PMO showed low nonspecific inhibitory activity against translation of nontargeted RNA or growth of the arenavirus lymphocytic choriomeningitis virus. Two P-PMO targeting the viral transcription-regulatory sequence (TRS) region in the 5' untranslated region were the most effective inhibitors tested. After several viral passages in the presence of a TRS-targeted P-PMO, partially drug-resistant SARS-CoV mutants arose which contained three contiguous base point mutations at the binding site of a TRS-targeted P-PMO. Those partially resistant viruses grew more slowly and formed smaller plaques than wild-type SARS-CoV. These results suggest PMO compounds have powerful therapeutic and investigative potential toward coronavirus infection.

Characterization and complete genome sequence of a novel coronavirus, coronavirus HKU1, from patients with pneumonia

Despite extensive laboratory investigations in patients with respiratory tract infections, no microbiological cause can be identified in a significant proportion of patients. In the past 3 years, several novel respiratory viruses, including human metapneumovirus, severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV), and human coronavirus NL63, were discovered. Here we report the discovery of another novel coronavirus, coronavirus HKU1 (CoV-HKU1), from a 71-year-old man with pneumonia who had just returned from Shenzhen, China. Quantitative reverse transcription-PCR showed that the amount of CoV-HKU1 RNA was 8.5 to 9.6 x 10(6) copies per ml in his nasopharyngeal aspirates (NPAs) during the first week of the illness and dropped progressively to undetectable levels in subsequent weeks. He developed increasing serum levels of specific antibodies against the recombinant nucleocapsid protein of CoV-HKU1, with immunoglobulin M (IgM) titers of 1:20, 1:40, and 1:80 and IgG titers of <1:1,000, 1:2,000, and 1:8,000 in the first, second and fourth weeks of the illness, respectively. Isolation of the virus by using various cell lines, mixed neuron-glia culture, and intracerebral inoculation of suckling mice was unsuccessful. The complete genome sequence of CoV-HKU1 is a 29,926-nucleotide, polyadenylated RNA, with G+C content of 32%, the lowest among all known coronaviruses with available genome sequence. Phylogenetic analysis reveals that CoV-HKU1 is a new group 2 coronavirus. Screening of 400 NPAs, negative for SARS-CoV, from patients with respiratory illness during the SARS period identified the presence of CoV-HKU1 RNA in an additional specimen, with a viral load of 1.13 x 10(6) copies per ml, from a 35-year-old woman with pneumonia. Our data support the existence of a novel group 2 coronavirus associated with pneumonia in humans.

Gene therapy in a murine model for clinical application to multiple sclerosis

Female SJL/J mice, suffering from experimental autoimmune encephalomyelitis (EAE), were injected with 1 x 10(7) cells from a syngeneic fibroblast line transduced with a retroviral vector designed to encode proteolipid protein (101-157) targeted for secretion. A striking abrogation of both clinical and histological signs of disease resulted. The treatment was efficacious when given after the first or the third relapses, protected naive mice from challenge with spinal cord homogenate, and was dose dependent. This strategy was devised to provide a systemic, antigen-specific signal to pathogenic T cells in the absence of costimulation and, hence, render them anergic. Cytokine analyses of brain and spinal cord lymphocytes demonstrate that the treatment induces an antiinflammatory Th2 profile, indicating that this antigen-specific therapy acts by a cytokine-induced pathway. This study was designed for translation to the clinic. We envision using allogeneic transduced fibroblasts, encapsulated in a chamber, to deliver the antigen-specific signal. This will enable us to use one therapeutic cell line for all patients and to remove the device should the therapy exacerbate disease.

Mouse hepatitis virus strain JHM infects a human hepatocellular carcinoma cell line

Mouse hepatitis virus (MHV) strain JHM is a coronavirus that causes encephalitis and demyelination in susceptible rodents. The known receptors for MHV are all members of the carcinoembryonic antigen family. Although human forms of the MHV receptor can function as MHV receptors in some assays, no human cell line has been identified that can support wild-type MHV infection. Here we describe the infection of a human hepatocellular carcinoma cell line, HuH-7, with MHV. HuH-7 cells were susceptible to strains JHM-DL and JHM-DS, yielding virus titers nearly identical to those seen in mouse DBT cells. In contrast, HuH-7 cells were only marginally susceptible or completely resistant to infection by other MHV strains, including A59. JHM produced a strong cytopathic effect in HuH-7 cells with the formation of round plaques. Studies of various recombinant viruses between JHM and A59 strains suggested that the ability of JHM to infect HuH-7 cells was determined by multiple viral genetic elements. Blocking the viral spike (S) protein with a neutralizing antibody or a soluble form of the MHV receptor inhibited infection of HuH-7 cells, suggesting that infection is mediated through the S protein. Transfection with the prototype mouse receptor, biliary glycoprotein, rendered HuH-7 cells susceptible to infection by other MHV strains as well, suggesting that JHM uses a receptor distinct from the classical MHV receptor to infect HuH-7 cells. Possible implications for human disease are discussed.

Induction of apoptosis in murine coronavirus-infected cultured cells and demonstration of E protein as an apoptosis inducer

We demonstrated that infection of 17Cl-1 cells with the murine coronavirus mouse hepatitis virus (MHV) induced caspase-dependent apoptosis. MHV-infected DBT cells did not show apoptotic changes, indicating that apoptosis was not a universal mechanism of cell death in MHV-infected cells. Expression of MHV structural proteins by recombinant vaccinia viruses showed that expression of MHV E protein induced apoptosis in DBT cells, whereas expression of other MHV structural proteins, including S protein, M protein, N protein, and hemagglutinin-esterase protein, failed to induce apoptosis. MHV E protein-mediated apoptosis was suppressed by a high level of Bcl-2 oncogene expression. Our data showed that MHV E protein is a multifunctional protein; in addition to its known function in coronavirus envelope formation, it also induces apoptosis.

Using a defective-interfering RNA system to express the HE protein of mouse hepatitis virus for studying viral pathogenesis

We have developed a defective-interfering (DI) RNA of mouse hepatitis virus (MHV) as a vector for expressing a variety of cellular and viral genes including the chloramphenicol acetyltransferase (CAT), hemagglutinin' esterase (HE), and gamma interferon. Here, we used the HE-expressing DI RNA for examining the role of HE protein in viral pathogenesis. The pseudorecombinant virus containing an expressed HE protein was generated by infecting cells with MHV-A59, which does not express, HE, and transfecting the in vitro-transcribed DI RNA containing the HE gene. These pseudorecombinant viruses (DE-HE A59) were then inoculated intracerebrally into mice. Viruses recovered from cells infected with A59 and transfected with DI RNA expressing the CAT gene (DE-CAT A59) were used as a control. At various time points after inoculation, mice were observed for clinical symptoms. Tissues (brains and livers) were obtained for determining the replication of DI RNA by RT-PCR, virus replication by plaque assay, antigen expression by immunohistochemistry, and pathological changes. Results showed that all mice infected with DE-CAT A59 succumbed to infection by 9 days postinfection (d p.i). These data are identical to the pathogenesis of the parental A59 virus, demonstrating that inclusion of the DI RNA did not by itself alter pathogenesis. In contrast, only 40% of mice infected with DE-HE A59 succumbed to infection. The subgenomic mRNAs transcribed from the DI vector were detected at 1 and 2 d p.i. but not at subsequent time points, indicating that the genes in the DI vector were expressed only at an early stage of viral infection. No significant difference in virus replication in the brains was detected between these two groups of mice, suggesting that virus replication in brains was not affected by the expression of the HE. Histopathological examination showed only a small increase in the extent of inflammatory cell infiltration and reduced viral antigen in the mice infected with DE-HE A59. There was no difference in virus replication in the livers at 2 and 4 d p.i., but a 3 log10 reduction was detected in the livers of mice infected with DE-HE A59 at 6 d p.i. Histological examination showed a significant reduction in viral antigen, inflammation and necrosis in mice infected with DE-HE A59. These results indicate that the expression of HE from the DI vector altered the viral pathogenesis. This study thus demonstrates the usefulness of this system in studying the role of viral or cellular genes expressed locally at the sites of viral infection in viral pathogenesis.

Expression of hemagglutinin/esterase by a mouse hepatitis virus coronavirus defective-interfering RNA alters viral pathogenesis

A defective-interfering (DI) RNA of mouse hepatitis virus (MHV) was developed as a vector for expressing MHV hemagglutinin/esterase (HE) protein. The virus containing an expressed HE protein (A59-DE-HE) was generated by infecting cells with MHV-A59, which does not express HE, and transfecting the in vitro-transcribed DI RNA containing the HE gene. A similar virus (A59-DE-CAT) expressing the chloramphenicol acetyltransferase (CAT) was used as a control. These viruses were inoculated intracerebrally into mice, and the role of the HE protein in viral pathogenesis was evaluated. Results showed that all mice infected with parental A59 or A59-DE-CAT succumbed to infection by 9 days postinfection (p.i.), demonstrating that inclusion of the DI did not by itself alter pathogenesis. In contrast, 60% of mice infected with A59-DE-HE survived infection. HE- or CAT-specific subgenomic mRNAs were detected in the brains at days 1 and 2 p.i. but not later, indicating that the genes in the DI vector were expressed only in the early stage of viral infection. No significant difference in virus titer or viral antigen expression in brains was observed between A59-DE-HE- and A59-DE-CAT-infected mice, suggesting that virus replication in brain was not affected by the expression of HE. However, at day 3 p.i. there was a slight increase in the extent of inflammatory cell infiltration in the brains of the A59-DE-HE-infected mice. Surprisingly, virus titers in the livers of A59-DE-HE-infected mice were 3 log10 lower than that of the A59-DE-CAT-infected mice at day 6 p.i. Also, substantially less necrosis and viral antigen were detected in the livers of the A59-DE-HE-infected mice. This may account for the reduced mortality of these mice. The possible contribution of the host immune system to this difference in pathogenesis was analyzed by comparing the expression of four cytokines. Results showed that both tumor necrosis factor-alpha and interleukin-6 mRNAs increased in the brains of the A59-DE-HE-infected mice at day 2 p.i., whereas interferon-gamma and interleukin-1 alpha mRNAs were similar between A59-DE-HE- and A59-DE-CAT-infected mice. These data suggest that the transient expression of HE protein enhances an early innate immune response, possibly contributing to the eventual clearance of virus from the liver. This study indicates the feasibility of the DI expression system for studying roles of viral proteins during MHV infection.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.