Temperature-sensitive mutants of mouse hepatitis virus strain A59: isolation, characterization and neuropathogenic properties

A yeast-assembled, plasmid-launched reverse genetics system for the murine coronavirus MHV-A59

The Betacoronavirus murine hepatitis virus (MHV) is an important model system for studying coronavirus (CoV) molecular and cell biology. Despite this, few reagents for MHV are available through repositories such as ATCC or Addgene, potentially limiting the widespread adoption of MHV as a tractable model system. To overcome some challenges inherent in the existing MHV reverse genetics systems, we developed a plasmid-launched transformation-associated recombination (TAR) cloning-based system to assemble the MHV (strain A59; MHV-A59) genome. Following assembly in yeast, virus replication was launched by transfecting the fully assembled genome into HEK-293T cells. MHV-A59 recovered using this TAR cloning-based approach (WTTAR MHV-A59) replicated with kinetics identical to the virus recovered using a ligation- and T7-based approach (WTLIG MHV-A59). Additionally, WTTAR MHV-A59 can be detected at least 10 h post-transfection without requiring additional nucleocapsid (N) provided in trans. Lastly, we demonstrated the tractability of this TAR cloning-based system by recovering MHV-A59 expressing an 11 amino acid-containing HiBiT tag fused to the C-terminus of spike (S). While this virus, SC MHV-A59, replicated with reduced kinetics compared to WTTAR MHV-A59, the kinetics of virion production could be measured over time directly from the supernatant. This report represents the first plasmid-launched, TAR cloning-based system for MHV-A59. Furthermore, it describes a new reporter virus that could be used to study early steps during MHV-A59 entry and be used in the screening of antiviral compounds. To support future research with MHV-A59, we have made the necessary plasmids for this system available through ATCC.

The coronavirus spike protein induces endoplasmic reticulum stress and upregulation of intracellular chemokine mRNA concentrations

Murine hepatitis virus (MHV) and severe acute respiratory syndrome (SARS) coronavirus (CoV) are two of the best-studied representatives of the family Coronaviridae. During CoV infection, numerous cytokines and chemokines are induced in vitro and in vivo. Human interleukin 8 and its mouse functional counterpart, CXCL2, are early-expressed chemokines. Here we show that SARS-CoV and MHV induce endoplasmic reticulum (ER) stress and Cxcl2 mRNA transcription during infection in vitro. Expression of the viral spike protein significantly induced ER stress and Cxcl2 mRNA upregulation, while expression of the other structural genes did not. Additional experiments with UV-inactivated virus, cell-cell fusion-blocking antibodies, and an MHV mutant with a defect in spike protein maturation demonstrated that spike-host interactions in the ER are responsible for the induction of ER stress and subsequent Cxcl2 mRNA transcription. Despite significant increases in levels of Cxcl2 mRNA and functional nucleus-to-cytoplasm RNA transport, no CXCL2 protein was released into the medium from MHV-infected cells. Yet Sendai virus-infected cells showed substantial Cxcl2 mRNA induction and a simultaneous increase in levels of secreted CXCL2 protein. Our results demonstrate that expression of CoV spike proteins induces ER stress, which could subsequently trigger innate immune responses. However, at that point in infection, translation of host mRNA is already severely reduced in infected cells, preventing the synthesis of CXCL2 and ER stress proteins despite their increased mRNA concentrations.

The molecular biology of coronaviruses

Coronaviruses are large, enveloped RNA viruses of both medical and veterinary importance. Interest in this viral family has intensified in the past few years as a result of the identification of a newly emerged coronavirus as the causative agent of severe acute respiratory syndrome (SARS). At the molecular level, coronaviruses employ a variety of unusual strategies to accomplish a complex program of gene expression. Coronavirus replication entails ribosome frameshifting during genome translation, the synthesis of both genomic and multiple subgenomic RNA species, and the assembly of progeny virions by a pathway that is unique among enveloped RNA viruses. Progress in the investigation of these processes has been enhanced by the development of reverse genetic systems, an advance that was heretofore obstructed by the enormous size of the coronavirus genome. This review summarizes both classical and contemporary discoveries in the study of the molecular biology of these infectious agents, with particular emphasis on the nature and recognition of viral receptors, viral RNA synthesis, and the molecular interactions governing virion assembly.

Functional and genetic analysis of coronavirus replicase-transcriptase proteins

The coronavirus replicase-transcriptase complex is an assembly of viral and cellular proteins that mediate the synthesis of genome and subgenome-sized mRNAs in the virus-infected cell. Here, we report a genetic and functional analysis of 19 temperature-sensitive (ts) mutants of Murine hepatitis virus MHV-A59 that are unable to synthesize viral RNA when the infection is initiated and maintained at the non-permissive temperature. Both classical and biochemical complementation analysis leads us to predict that the majority of MHV-A59 ORF1a replicase gene products (non-structural proteins nsp1-nsp11) form a single complementation group (cistron1) while the replicase gene products encoded in ORF1b (non-structural proteins nsp12-nsp16) are able to function in trans and comprise at least three, and possibly five, further complementation groups (cistrons II-VI). Also, we have identified mutations in the non-structural proteins nsp 4, nsp5, nsp10, nsp12, nsp14, and nsp16 that are responsible for the ts phenotype of eight MHV-A59 mutants, which allows us to conclude that these proteins are essential for the assembly of a functional replicase-transcriptase complex. Finally, our analysis of viral RNA synthesis in ts mutant virus-infected cells allows us to discriminate three phenotypes with regard to the inability of specific mutants to synthesize viral RNA at the non-permissive temperature. Mutant LA ts6 appeared to be defective in continuing negative-strand synthesis, mutant Alb ts16 appeared to form negative strands but these were not utilized for positive-strand RNA synthesis, and mutant Alb ts22 was defective in the elongation of both positive- and negative-strand RNA. On the basis of these results, we propose a model that describes a pathway for viral RNA synthesis in MHV-A59-infected cells. Further biochemical analysis of these mutants should allow us to identify intermediates in this pathway and elucidate the precise function(s) of the viral replicase proteins involved.

Viral and cellular proteins involved in coronavirus replication

As the largest RNA virus, coronavirus replication employs complex mechanisms and involves various viral and cellular proteins. The first open reading frame of the coronavirus genome encodes a large polyprotein, which is processed into a number of viral proteins required for viral replication directly or indirectly. These proteins include the RNA-dependent RNA polymerase (RdRp), RNA helicase, proteases, metal-binding proteins, and a number of other proteins of unknown function. Genetic studies suggest that most of these proteins are involved in viral RNA replication. In addition to viral proteins, several cellular proteins, such as heterogeneous nuclear ribonucleoprotein (hnRNP) A1, polypyrimidine-tract-binding (PTB) protein, poly(A)-binding protein (PABP), and mitochondrial aconitase (m-aconitase), have been identified to interact with the critical cis-acting elements of coronavirus replication. Like many other RNA viruses, coronavirus may subvert these cellular proteins from cellular RNA processing or translation machineries to play a role in viral replication.

Localization of mouse hepatitis virus nonstructural proteins and RNA synthesis indicates a role for late endosomes in viral replication

The aim of the present study was to define the site of replication of the coronavirus mouse hepatitis virus (MHV). Antibodies directed against several proteins derived from the gene 1 polyprotein, including the 3C-like protease (3CLpro), the putative polymerase (POL), helicase, and a recently described protein (p22) derived from the C terminus of the open reading frame 1a protein (CT1a), were used to probe MHV-infected cells by indirect immunofluorescence (IF) and electron microscopy (EM). At early times of infection, all of these proteins showed a distinct punctate labeling by IF. Antibodies to the nucleocapsid protein also displayed a punctate labeling that largely colocalized with the replicase proteins. When infected cells were metabolically labeled with 5-bromouridine 5'-triphosphate (BrUTP), the site of viral RNA synthesis was shown by IF to colocalize with CT1a and the 3CLpro. As shown by EM, CT1a localized to LAMP-1 positive late endosomes/lysosomes while POL accumulated predominantly in multilayered structures with the appearance of endocytic carrier vesicles. These latter structures were also labeled to some extent with both anti-CT1a and LAMP-1 antibodies and could be filled with fluid phase endocytic tracers. When EM was used to determine sites of BrUTP incorporation into viral RNA at early times of infection, the viral RNA localized to late endosomal membranes as well. These results demonstrate that MHV replication occurs on late endosomal membranes and that several nonstructural proteins derived from the gene 1 polyprotein may participate in the formation and function of the viral replication complexes.

Genetic complementation among three panels of mouse hepatitis virus gene 1 mutants

Temperature-sensitive (ts) mutant viruses have been useful for the study of replication processes in many viral systems. To determine how our panel of MHV-JHM-derived RNA- ts mutants (Robb et al., 1979) is genetically related to other panels of MHV RNA- ts mutants, we tested our mutants for complementation with representatives from two different sets of MHV-A59 ts mutants (Koolen et al., 1983; Schaad et al., 1990). These three ts mutant panels together comprise eight genetically distinct complementation groups. Considerable genetic similarity was observed among the three mutant panels. Only three complementation classes are unique to their particular mutant panel, and genetically equivalent mutants were not observed within the other two mutant panels. There are two overlapping complementation groups between the mutant sets derived from MHV-A59 and four overlapping complementation classes between the MHV-JHM panel and the MHV-A59 panels. Two complementation groups had representative mutants in all three mutant panels. One of these latter complementation classes demonstrated nonreciprocal complementation patterns consistent with intragenic complementation.

Role of spleen macrophages in innate and acquired immune responses against mouse hepatitis virus strain A59

Owing to their scavenging and phagocytic functions, spleen macrophages are regarded to be important in the induction and maintenance of both innate and acquired immune defence mechanisms. In this study, we investigated the role of spleen macrophages in immunity against mouse hepatitis virus strain A59 (MHV-A59). Previous studies showed that spleen and liver macrophages are the first target cells for infection by MHV-A59 in vivo, suggesting that they could be involved in the induction of immune responses against MHV-A59. We used a macrophage depletion technique to deplete macrophages in vivo and studied the induction of virus-specific antibody and cytotoxic T-cell (CTL) responses and non-immune resistance against MHV-A59 in normal and macrophage-depleted mice. Virus titres in spleen and liver increased rapidly in macrophage-depleted mice, resulting in death of mice within 4 days after infection. Elimination of macrophages before immunization with MHV-A59 resulted in increased virus-specific humoral and T-cell proliferative responses. However, virus-specific CTL responses were not altered in macrophage-depleted mice. Our results show that spleen macrophages are of major importance as scavenger cells during MHV-A59 infection and are involved in clearance of virus from the host. In addition, macrophages may be involved in the regulation of acquired immune responses. In the absence of macrophages, increased virus-specific T-cell and antibody responses are detectable, suggesting that macrophages suppress MHV-A59-specific T- and B-cell responses and that other cells serve as antigen-presenting cells.

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.

Characterization of two temperature-sensitive mutants of coronavirus mouse hepatitis virus strain A59 with maturation defects in the spike protein

Two temperature-sensitive (ts) mutants of mouse hepatitis virus strain A59, ts43 and ts379, have been described previously to be ts in infectivity but unaffected in RNA synthesis (M. J. M. Koolen, A. D. M. E. Osterhaus, G. van Steenis, M. C. Horzinek, and B. A. M. van der Zeijst, Virology 125:393-402, 1983). We present a detailed analysis of the protein synthesis of the mutant viruses at the permissive (31 degrees C) and nonpermissive (39.5 degrees C) temperatures. It was found that synthesis of the nucleocapsid protein N and the membrane protein M of both viruses was insensitive to temperature. However, the surface protein S of both viruses was retained in the endoplasmic reticulum at the nonpermissive temperature. This was shown first by analysis of endoglycosidase H-treated and immunoprecipitated labeled S proteins. The mature Golgi form of S was not present at the nonpermissive temperature for the ts viruses, in contrast to wild-type (wt) virus. Second, gradient purification of immunoprecipitated S after pulse-chase labeling showed that only wt virus S was oligomerized. We conclude that the lack of oligomerization causes the retention of the ts S proteins in the endoplasmic reticulum. As a result, ts virus particles that were devoid of S were produced at the nonpermissive temperature. This result could be confirmed by biochemical analysis of purified virus particles and by electron microscopy.

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.

Differential activation of mouse hepatitis virus-specific CD4+ cytotoxic T cells is defined by peptide length

In this study we have characterized the core epitope recognized by the MHV-A59-specific CD4+ cytotoxic T lymphocyte (CTL) clones HS1 and B6.1, derived from BALB/c and C57/BL6 mice, respectively. These CD4+ clones respond to the promiscuous peptide fragment S-329-343 of the glycoprotein S of MHV-A59. The results indicate that the core peptides of both clones overlap but are not identical. The core region of the HS1 clone is an 8-mer, and comprises the amino acid residues S-332-339, whereas the minimal epitope for clone B6.1 is a 9-mer and comprises the amino acid residues S-334-342. The peptide fragment S-329-343 activates all T-cell effector functions, including proliferation, cytokine secretion and cytolysis. However, in the present study we show that T-cell activation is not an all-or-none phenomenon, in which T-cell stimulation leads to activation of all T-cell effector functions. It appears that changes in the length of a peptide ligand can differentially activate the cytolytic machinery from proliferation and cytokine secretion. Furthermore, the results indicate that, in our case, modulation of the flanking residues of the core epitopes did not convert the cytokine profile of polarized T-helper type-1 (Th1) clones into a Th2-type pattern.

Activation of virus-specific major histocompatibility complex class II-restricted CD8+ cytotoxic T cells in CD4-deficient mice

Acute enteritic or respiratory disease is a consequence of coronavirus infection in man and rodents. Mouse hepatitis virus, stain A59 (MHV-A59) causes acute hepatitis in mice and rats and induces a response of major histocompatibility complex (MHC) class II-restricted CD4+ cytotoxic T cells, protecting mice against acute infection. In the present study we show that MHV-A59 infection of mice that lack a functional CD4 gene activates effector cells of the CD8+ phenotype. These cytotoxic T cells lyse virus-infected target cells in a MHC class II-restricted fashion. The results indicate that CD8+ T cells have the potential to utilize MHC class II as restriction element, illustrating that the immune system can effectively deal with evading microorganisms, such as viruses which down-regulate MHC class I.

Predominance of MHC class II-restricted CD4+ cytotoxic T cells against mouse hepatitis virus A59

Coronavirus-induced acute hepatitis is a complex event and the role of different components of the immune system with regard to defined viral proteins and the course of the infection is not yet clear. We have analysed the cytotoxic T-lymphocyte (CTL) response in mouse hepatitis virus (MHV-A59) infection. Surprisingly, we detected only a very clear virus-specific major histocompatibility complex (MHC) class II-restricted cytotoxicity in mice infected with MHV-A59. We found no evidence of activation of the classical CD8+ MHC class I-restricted CTL. The virus-specific CD4+ CTL derived from two different mouse strains having different MHC haplotypes recognized the same immunodominant epitope. This epitope, comprising the amino acid residues 329-343 of the viral S-glycoprotein, was recognized both at the polyclonal level and by virus-specific CTL clones. Transfer studies using a MHV-A59-specific CD4+ CTL clone showed significant protection against a lethal challenge with MHV-A59, implicating that these CD4+ CTL play a pivotal role in the protection against MHV-A59 infections.

Map locations of mouse hepatitis virus temperature-sensitive mutants: confirmation of variable rates of recombination

Using standard genetic recombination techniques, studies in our laboratory suggest that recombination rates are very high and vary in different portions of the mouse hepatitis virus (MHV) genome. To determine the actual recombination frequencies in the MHV genome and localize the nucleotide boundaries of individual viral genes, we have sequenced temperature-sensitive and revertant viruses to identify the location of specific mutant alleles. Complementation group F RNA+ ts mutants (LA7, NC6, and NC16) each contained a unique mutation which was tightly linked to the ts phenotype and resulted in a conservative or nonconservative amino acid change in the MHV S glycoprotein gene. In agreement with previous recombination mapping studies, the mutation in LA7 and NC6 mapped within the S1 domain while NC16 mapped within the S2 domain. To determine the map coordinates of the MHV polymerase genes, several RNA- mutants and their revertants belonging to complementation groups C (NC3 and LA9) and E (LA18 and NC4) were also sequenced. Mutations were identified in each virus that were tightly linked to the ts phenotype and resulted in either a conservative or nonconservative amino acid change. The group C allele spanned the ORF 1a/ORF 1b junction, while the group E mutants mapped at the C terminus of ORF 1b about 20 to 22 kb from the 5' end of the genome. Mutation rates, calculated from the reversion frequencies of plaque-purified ts viruses requiring a single nucleotide alteration for reversion, approached 1.32 (+/- 0.89) x 10(-4) substitutions per nucleotide site per round of template copying. Detailed recombination mapping studies across known distances between these different ts alleles has confirmed that homologous recombination rates approached 25% and varied within different portions of the MHV genome.

Beta-endorphin protects mice from neurological disease induced by the murine coronavirus MHV-JHM

The neurotropic murine coronavirus, MHV-JHM (JHMV) causes encephalitis and paralytic-demyelinating disease in susceptible strains of mice and rats, serving as a model for human demyelinating diseases such as multiple sclerosis. In this communication, we report that a single intracerebral administration of the naturally occurring neuropeptide, beta-endorphin, reduced the incidence of JHMV-induced paralytic-demyelinating disease 40-50% in C57Bl/6 mice. Protection from disease was accompanied by significantly reduced virus replication in the brain as early as 3 days post-infection and did not occur in irradiated, or immunoincompetent mice. The data suggest that beta-endorphin engages immune mechanisms of host resistance to JHMV infection to protect the mice from disease.

Homologous RNA recombination allows efficient introduction of site-specific mutations into the genome of coronavirus MHV-A59 via synthetic co-replicating RNAs

We describe a novel strategy to site-specifically mutagenize the genome of an RNA virus by exploiting homologous RNA recombination between synthetic defective interfering (DI) RNA and the viral RNA. The construction of a full-length cDNA clone, pMIDI, of a DI RNA of coronavirus MHV strain A59 was reported previously (R.G. Van der Most, P.J. Bredenbeek, and W.J.M. Spaan (1991). J. Virol. 65, 3219-3226). RNA transcribed from this construct, is replicated efficiently in MHV-infected cells. Marker mutations introduced in MIDI RNA were replaced by the wild-type residues during replication. More importantly, however, these genetic markers were introduced into viral genome: even in the absence of positive selection MHV recombinants could be isolated. This finding provides new prospects for the study of coronavirus replication using recombinant DNA techniques. As a first application, we describe the rescue of the temperature sensitive mutant MHV Albany-4 using DI-directed mutagenesis. Possibilities and limitations of this strategy are discussed.

Evidence for variable rates of recombination in the MHV genome

Mouse hepatitis virus has been shown to undergo RNA recombination at high frequency during mixed infection. Temperature-sensitive mutants were isolated using 5-fluorouracil and 5-azacytidine as mutagen. Six RNA+ mutants that reside within a single complementation group mapping within the S glycoprotein gene of MHV-A59 were isolated which did not cause syncytium at the restrictive temperature. Using standard genetic techniques, a recombination map was established that indicated that these mutants mapped into two distinct domains designated F1 and F2. These genetic domains may correspond to mutations mapping within the S1 and S2 glycoproteins, respectively, and suggest that both the S1 and S2 domains are important in eliciting the fusogenic activity of the S glycoprotein gene. In addition, assuming that most distal ts alleles map roughly 4.0 kb apart, a recombination frequency of 1% per 575-676 bp was predicted through the S glycoprotein gene. Interestingly, this represents a threefold increase in the recombination frequency as compared to rates predicted through the polymerase region. The increase in the recombination rate was probably not due to recombination events resulting in large deletions or insertions (greater than 50 bp), but rather was probably due to a combination of homologous and nonhomologous recombination. A variety of explanations could account for the increased rates of recombination in the S gene.

The V5A13.1 envelope glycoprotein deletion mutant of mouse hepatitis virus type-4 is neuroattenuated by its reduced rate of spread in the central nervous system

Following intracerebral inoculation of adult Balb/c Byj mice, the MHV-4 strain of mouse hepatitis virus (MHV) had an LD50 of less than 0.1 PFU, whereas its monoclonal antibody resistant variant V5A13.1 had an LD50 of 10(4.2) PFU. To determine the basis for this difference in neurovirulence we have studied the acute central nervous system (CNS) infection of these two viruses by in situ hybridization. Both viruses infected the same, specific neuroanatomical areas, predominantly neurons, and spread via the cerebrospinal fluid, along neuronal pathways and between adjacent cells. The neuronal nuclei infected and the spread of virus within the brain are described. The main difference between the parental and variant viruses was the rate at which the infection spread. MHV-4 spread rapidly, destroying large numbers of neurons and the animals died within 4 days of infection. The variant virus spread to the same areas of the brain but at a slower rate. This difference in the rate of virus spread was also apparent from the brain virus titers. The slower rate of spread of the variant virus appears to allow intervention by the immune response. Consistent with this, the variant virus spread slowly in athymic nu/nu mice, but in the absence of an intact immune response, infection and destruction of neurons eventually reached the same extent as that of the parental virus and the mice died within 6 days of infection. We conclude that the V5A13.1 variant of MHV-4 is neuroattenuated by its slower rate of spread in the CNS.

Genetics of mouse hepatitis virus transcription: identification of cistrons which may function in positive and negative strand RNA synthesis

A panel of 26 temperature-sensitive mutants of MHV-A59 were selected by mutagenesis with either 5-fluorouracil or 5-azacytidine. Complementation analysis revealed the presence of one RNA+ and five RNA- complementation groups. None of the RNA- complementation groups transcribed detectable levels of positive- or negative-stranded RNA at the restrictive temperature. Temperature shift experiments after the onset of mRNA synthesis revealed at least two classes of RNA- mutants. RNA- complementation groups A, B, D, and E were blocked in the ability to release infectious virus and transcribe mRNA and genome, while group C mutants continued to release infectious virus and transcribe both mRNA and genome. Temperature shift experiments at different times postinfection suggest that the group C mutants encode a function required early in viral transcription which affects the overall rate of positive strand synthesis. Analysis of steady state levels of negative strand RNA after the shift indicate that the group C mutants were probably blocked in the ability to synthesize additional minus strand RNA under conditions in which the group E mutants continued low levels of minus strand synthesis. These data suggest that at least four cistrons may be required for positive strand synthesis while the group C cistron functions during minus strand synthesis.

Establishing a genetic recombination map for murine coronavirus strain A59 complementation groups

MHV-A59 temperature-sensitive mutants, representing one RNA+ and five RNA- complementation groups, were isolated and characterized by genetic recombination techniques. Maximum recombination frequencies occurred under multiplicities of infection greater than 10 each in which 99.99% of the cells were co-infected. Recombination frequencies between different ts mutants increased steadily during infection and peaked late in the virus growth cycle. These data suggest that recombination is a late event in the virus replication cycle. Recombination frequencies were also found to range from 63 to 20,000 times higher than the sum of the spontaneous reversion frequencies of each ts mutant used in the cross. Utilizing standard genetic recombination techniques, the five RNA- complementation groups of MHV-A59 were arranged into an additive, linear, genetic map located at the 5' end of the genome in the 23-kb polymerase region. These data indicate that at least five distinct functions are encoded in the MHV polymerase region which function in virus transcription. Moreover, using well-characterized ts mutants the recombination frequency for the entire 32-kb MHV genome was found to approach 25% or more. This is the highest recombination frequency described for a nonsegmented, linear, plus-polarity RNA virus.

Vulnerability of rat and mouse brain cells to murine hepatitis virus (JHM-strain): studies in vivo and in vitro

The pathogenicity and cell tropism of mouse hepatitis virus (MHV-JHM-strain) in the developing mouse (Balb/c) and rat (Wistar and Lewis) brain were analysed. Intracranial infection of Balb/c mice at postnatal day 5 induced a lethal encephalitis in all animals. Of Wistar rats infected at day 2 or 5 after birth, 30 to 70%, respectively, survived. The distribution of viral antigen was studied in frozen brain sections of animals that died after infection; astrocytes were found to be the major virus-infected cell type throughout the central nervous system. More than 75% of the surviving rat pups developed paralysis, but viral antigen was detected in only few brain cells and not in astrocytes. The cell tropism of MHV-JHM was examined further in virus-infected glial cell cultures derived from brains of rats or mice. In the glial cultures derived from Wistar rats, only oligodendrocytes were infected, whereas in cultures derived from mouse or Lewis rat brain viral antigen was detected in both astrocytes and oligodendrocytes. Infection of astrocytes led to the formation of syncytia and degradation of the cytoskeleton. Infected rat oligodendrocytes gradually disappeared from the cultures because of cell death. These phenomena indicate that, besides an indirect autoimmune response triggered by infected astrocytes, direct virus-induced injury to astrocytes or to oligodendrocytes can have a dominant role in the neuropathogenicity of mouse hepatitis virus. The present results underscore the importance of species and developmental stage of experimental animals in the neurotropism and pathogenicity of MHV-JHM.

Temperature-sensitive mutants of mouse hepatitis virus type 3 (MHV-3): isolation, biochemical and genetic characterization

Mouse hepatitis virus 3 (MHV-3) is highly hepatotropic in sensitive mice. Temperature-sensitive mutants (ts mutants) induced by N-methyl-N'-nitrosoguanidine and 5-fluorouracil were isolated. Twelve mutants which were able to induce the formation of syncytia at 33 degrees C but not at the restrictive temperature (39.5 degrees C) were selected for detailed study. No viral RNA synthesis was detected after infection at the restrictive temperature with six of the mutants (RNA-) whereas six others were RNA+, although they displayed RNA synthesis which was generally reduced. No differences have been detected in the size of the genome or the viral-intracellular RNA species found in wild type virus or ts mutant infected cells at permissive temperature. The pattern of virus-induced proteins analyzed after immunoprecipitation by SDS-PAGE was similar in wild type virus and RNA+ mutant infected cells at 39.5 degrees C. Complementation experiments between ts mutants enabled us to distinguish five groups. Three of the groups contained RNA- mutants and two of them RNA+. Plaques made by mutants in one group displayed characteristic features that distinguished them from the wild type.

Olfactory neural pathway in mouse hepatitis virus nasoencephalitis

The mechanism of brain infection with mouse hepatitis virus-JHM was studied in BALB/cByJ mice following intranasal inoculation, and found to be a consequence of direct viral spread along olfactory nerves into olfactory bulbs of the brain. Infection was followed sequentially from nose to brain, using microscopy, immunohistochemistry and virus quantification. Lesions, antigen and virus were observed in the olfactory bulb and anterior brain as early as 2 days and posterior brain by 4 days after inoculation. Viral antigen extended through nasal mucosa into submucosa, then coursed along the olfactory nerve perineurium and fibers, through the cribriform plate into the olfactory bulbs. On days 4 and 7, viral antigen was found in the antero-ventral brain, along ventral meninges, olfactory tracts and anterior ramifications of the lateral ventricles. Virus was cleared from nose by 10 days and anterior brain by 20 days, but persisted in posterior brain for 20 days after inoculation. Mice also developed disseminated infection, with viremia and hepatitis. Infection of brain did not correlate with presence of viremia. In contrast to intranasally inoculated mice, orally-inoculated mice did not develop encephalitis, despite evidence of disseminated infection.

Response of genetically susceptible and resistant mice to intranasal inoculation with mouse hepatitis virus JHM

Mouse hepatitis virus (MHV)-JHM infection was studied in genetically susceptible (BALB/cByJ) and resistant (SJL/J) mice following intranasal inoculation at 1, 3, 6 or 12 wk of age. Markers of infection included histology, immunohistochemistry, virus quantification and virus serology. All BALB mice developed severe disseminated disease with high mortality due to encephalitis and hepatitis. Peak MHV titers appeared in brain, liver, spleen and intestine on days 3 or 5. Age at inoculation did not influence virus titers in brain, spleen or intestine, but virus titers in liver were inversely proportional to age at inoculation. In 6-wk-old BALB mice, virus was cleared from spleen, intestine and liver by day 30 and from brain by day 60. In intestine, MHV was localized to lymphoid tissue, without fecal excretion. SJL mice of all ages developed remarkably milder disease with low mortality occurring only among mice inoculated at 1 wk of age. SJL mice inoculated at 1 wk had disseminated infection at day 3, but lesions and antigen were cleared from most organs by day 5. Mice inoculated at 3 and 6 wk of age had minimal or no involvement of peripheral organs, and mice inoculated at 12 wk of age had infections restricted to the nose. At day 5, MHV titers in brain, liver, spleen and intestine were significantly lower or undetectable in SJL mice of all ages compared to age-matched BALB mice. In 6-wk-old mice, MHV was cleared from all organs by day 10. Serum antibody titers to MHV were many-fold higher in BALB mice, compared to SJL mice, which mounted only a modest response.

Molecular pathogenesis of virus infections

Although a very wide range of viral diseases exists in vertebrates, certain generalizations can be made regarding pathogenetic pathways on the molecular level. The presentation will focus on interactions of virions and their components with target cells. Using coronaviruses as examples the changes in virulence have been traced back to single mutational events; recombination, however, is likely to be an alternative mechanism by which virus-host interactions (e.g. the cell-, organ- or animal species-spectrum) can dramatically change. Receptor molecules are essential for the early interactions during infection and some of these have been identified. Events in the target cell and the host organism are discussed, and wherever possible, aspects of virus evolution and cooperation between infectious agents are highlighted.

Fatty acid acylation of viral proteins in murine hepatitis virus-infected cells. Brief report

The fatty acid acylation of the cell-associated virus-specific proteins of mouse hepatitis virus (A 59-strain) was studied. 3H-palmitate label was associated with E2, one of the two virion glycoproteins and its intracellular precursor gp 150. A 110 K protein, the unglycosylated apoprotein of gp 150, accumulated by tunicamycin treatment, also incorporated radiolabeled palmitic acid. The addition of fatty acid to the MHV-A 59 E 2 protein is therefore not dependent on glycosylation.

Restricted replication of mouse hepatitis virus A59 in primary mouse brain astrocytes correlates with reduced pathogenicity

Temperature-sensitive (ts) mutants of mouse hepatitis virus A59 (MHV-A59) are drastically attenuated in their pathogenic properties. Intracerebral inoculation of mice with 10(5) PFU of mutant ts342 results in prolonged infection of the central nervous system, whereas 100 PFU of wild-type virus are lethal (M. J. M. Koolen, A. D. M. E. Osterhaus, G. van Steenis, M. C. Horzinek, and B. A. M. van der Zeijst, Virology 125:393-402, 1983). In the Sac(-) cell line ts342 grows as well at 37 degrees C (the body temperature of mice) as at 31 degrees C (the permissive temperature). There is, however, a difference in primary cultures of mouse brain astrocytes. After infection with ts342, astrocytes produced low levels of infectious virus (5.2 +/- 3.7%) compared with virus yields after infection with wild-type virus. The fraction of wild-type virus- and ts342-infected cells was similar. Electron microscopy showed in wild-type virus-infected cells abundant virions in smooth vesicles usually closely associated with a well-developed Golgi apparatus. In mutant-infected cells no mature ts342 virus particles were found. There was no difference between ts342 and wild-type virus regarding the intracellular virus-specific RNAs. In ts342-infected cells the viral glycoproteins E2 and E1 were not detectable or were barely detectable. Either the mRNAs for the glycoproteins are not translated or the proteins are rapidly broken down. Revertants of ts342 were isolated. They grew as well as wild-type virus in astrocytes, indicating that they apparently produced sufficient amounts of E2 and E1, the ts defect itself rather than a second site mutation is responsible for the defect in replication, and the ts defect acts in unison with host-cell factors. The revertants also regained the lethal properties of wild-type virus.

Mouse hepatitis virus nasoencephalopathy is dependent upon virus strain and host genotype

Mouse hepatitis virus (MHV) S induced typical MHV spongiform lesions in brainstem 28 days following intranasal inoculation of adult A/J, BALB/cByJ, CBA/J, C 3 H/HeJ and C 3 H/RV, but not SJL mice. In all but SJL mice, brain lesions occurred at or near the infectious dose level, based on seroconversion by the indirect immunofluorescence assay. During the acute phase of infection (day 5), lesions were limited to the nose and brain in most genotypes. Exceptions were BALB mice, which had mild hepatitis and SJL mice, which had lesions restricted to the nose. No mortality occurred in any genotype. Following intranasal inoculation of adult mice, MHV-1, -3, -A 59, -JHM and -S all caused brain lesions at 28 days after inoculation. MHV-1 and -3 caused lesions that were usually restricted to the anterior olfactory tracts, while MHV-A 59, -S and -JHM also caused more generalized and pronounced lesions involving the midbrain and pons. These studies suggest that avirulent MHV-S given intranasally to most mouse genotypes is a good model for induction of brain infection in the absence of mortality. They also confirm observations made by others in which MHV-JHM, -S and -A 59 are relatively more neurotropic than other MHV strains, such as MHV-1 and -3.

Proteolytic cleavage of the E2 glycoprotein of murine coronavirus: host-dependent differences in proteolytic cleavage and cell fusion

Cell fusion induced by infection with mouse hepatitis virus strain A59 (MHV-A59) varied markedly in extent and time course in four different murine cell lines. When inoculated at a multiplicity of 3 to 5 PFU per cell, the Sac-, L2, and DBT cell lines began to fuse by 7 h, were fused into confluent syncytia by 9 to 12 h, and peeled from the substrate by 10 to 14 h. These virulent virus-cell interactions were in striking contrast to the moderate interaction of MHV-A59 with the 17 Cl 1 cell line, in which only small syncytia were observed 18 h postinoculation, and greater than 50% of the cells remained unfused by 24 h. The yield of infectious virus produced by 17 Cl 1 cells was 10-fold higher than the yields from the other three cell lines. The processing of the nucleocapsid protein, the membrane glycoprotein E1, and the peplomeric glycoprotein E2 were found to differ significantly in the four cell lines. Since the E2 glycoprotein is responsible for virus-induced cell fusion, we attempted to correlate differences in cellular processing of E2 with differences in fusion of infected cells. The predominant intracellular form of E2 in all cell lines was the 180K species. Pulse-chase experiments showed that a small portion of the 17 Cl 1 cell-associated 180K E2 was cleaved by 1 h after synthesis to yield 90K E2, shown in the preceding paper to consist of two different glycoproteins called 90A and 90B (L. S. Sturman, C. S. Ricard, and K. V. Holmes, J. Virol. 56:904-911, 1985). This cleavage occurred shortly before the release of virions from cells, as shown by pulse-chase experiments. After budding at intracellular membranes, virions released into the medium by the four cell lines contained different ratios of 180K to 90K E2. Virions from Sac- cells, which contained 100% 90K E2, fused L2 cells rapidly without requiring virus replication, whereas virions from 17 Cl 1 cells, which had 50% 90K E2, required trypsin activation to induce rapid fusion (Sturman et al., J. Virol. 56:904-911, 1985). The addition of protease inhibitors to the medium markedly delayed L2 cell fusion induced by MHV infection. The extent of coronavirus-induced cell fusion does not depend solely upon the percent cleavage of the E2 glycoprotein by cellular proteases, since extensive fusion was induced by infection of L2 and DBT cells but not 17 Cl 1 cells, although all three cell lines cleaved E2 to the same extent.(ABSTRACT TRUNCATED AT 400 WORDS)

Virus-induced central positional nystagmus in mice

Geotropic direction-changing nystagmus in lateral body positions was observed in 4-week-old BALB/c mice after intracerebral injection with a temperature-sensitive mutant of mouse hepatitis virus. The positional nystagmus was detected already 2 days after infection and it lasted half a year at least. The nystagmic responses of the semicircular canals were also evaluated before and after infection. They were unaltered during the disease, which was clinically manifested by general weakness, ataxia and tremor. Histopathological examination 2 weeks after infection revealed demyelination in various parts of the CNS.

Mouse hepatitis virus strain--related patterns of tissue tropism in suckling mice

The pattern of tissue tropism for several prototype and uncharacterized strains of mouse hepatitis virus (MHV) was studied by intranasal inoculation of each virus strain into groups of neonatal Swiss mice under otherwise identical conditions. Mice were killed at intervals up to 18 days after inoculation, and their tissues were examined for the presence of MHV antigen by indirect immunofluorescence. Two patterns of infection were apparent. Prototype MHV strains 1, 3, A59, JHM, S and uncharacterized MHV strains Tettnang and wt-1 produced a respiratory pattern, in which nose and lung were consistently involved with dissemination to other organs in a vascular distribution. Pulmonary vascular endothelium and alveolar septal cells, but not airway epithelium, were infected. An enteric pattern was observed with MHV-Y and wt-2 in which MHV antigen was largely restricted to the nose and bowel, with limited dissemination to other abdominal organs but not lung. Intestinal lesions in these mice were severe compared to those manifesting the respiratory pattern of infection. These results indicate that, like coronaviruses of other species, different strains of MHV possess different primary and secondary organotropisms following a natural route of inoculation in a susceptible host.

The molecular biology of coronaviruses

Coronaviruses have recently emerged as an important group of animal and human pathogens that share a distinctive replicative cycle. Some of the unique characteristics in the replication of coronaviruses include generation of a 3' coterminal-nested set of five or six subgenomic mRNAs, each of which appears to direct the synthesis of one protein. Two virus-specific RNA polymerase activities have been identified. Many of the distinctive features of coronavirus infection and coronavirus-induced diseases may result from the properties of the two coronavirus glycoproteins. The intracellular budding site, which may be important in the establishment and maintenance of persistent infections, appears to be due to the restricted intracytoplasmic migration of the E1 glycoprotein, which acts as a matrix-like transmembrane glycoprotein. E1 also exhibits distinctive behavior by self-aggregating on heating at 100°C in sodium dodecyl sulfate (SDS) and by its interaction with RNA in the viral nucleocapsid. The E1 of mouse hepatitis virus (MHV) is an O -linked glycoprotein, unlike most other viral glycoproteins. Thus, the coronavirus system may be a useful model for the study of synthesis, glycosylation, and transport of O -linked cellular glycoproteins.