Function of a 5'-end genomic RNA mutation that evolves during persistent mouse hepatitis virus infection in vitro

Persistently infected cultures of DBT cells were established with mouse hepatitis virus strain A59 (MHV-A59), and the evolution of the MHV leader RNA and 5' end of the genome was studied through 119 days postinfection. Sequence analysis of independent clones demonstrated an overall mutation frequency approaching 1.2 x 10(-3) to 6.7 x 10(-3). The rate of fixation of mutations was about 1.2 x 10(-5) to 7.6 x 10(-5) per nucleotide (nt) per day. In contrast to finding in bovine coronavirus, the MHV leader RNA sequences were extremely stable and did not evolve significantly during persistent infection. Rather, a 5' untranslated region (UTR) A-to-G mutation at nt 77 in the genomic RNA emerged by day 56 and accumulated until 50 to 80% of the genome-length molecules retained the mutation by 119 days postinfection. Although other 5'-end mutations were noted, only the nt 77 mutation was significantly associated with viral persistence in vitro. Mutations were also found in the 5' end of the p28 coding region, but no specific alterations accumulated in genome-length molecules through 119 days postinfection. The 5' UTR nt 77 mutation resulted in an 18-amino-acid open reading frame (ORF) upstream of the ORF 1a AUG start site. By in vitro translation assays, the small ORF was not translated into detectable product but the mutation significantly enhanced translation of the downstream p28 ORF about 2.5-fold. Variant viruses, containing either the nt 77 A-to-G mutation (V16-ATG+) or wild-type sequences at this locus (V1-ATG-), were isolated at 119 days postinfection. The variant viruses replicated more efficiently than wild-type virus and were extremely cytolytic in DBT cells, suggesting that the A-to-G mutation did not encode a nonlytic or attenuated phenotype. Consistent with the in vitro translation results, a significant increase (approximately 3.5-fold) in p28 expression was also observed with the mutant virus (V16-ATG+) in DBT cells compared with that in wild-type controls. These data indicate that MHV persistence was significantly associated with mutation and evolution in the 5'-end UTR which enhanced the translation of the ORF 1a and potentially ORF 1b polyproteins which function in virus transcription and replication.

CD8+ T-cell epitopes within the surface glycoprotein of a neurotropic coronavirus and correlation with pathogenicity

CD8+ T cells with cytotoxic activity against the surface glycoprotein (S) of mouse hepatitis virus, strain JHM, have been identified in the central nervous system (CNS) of both acutely and chronically infected C57BL/6 mice. In this report, two specific epitopes recognized by these CNS-derived cells were identified, using a panel of peptides chosen because they conformed to the allele-specific binding motif for MHC class I H-2Kb and H-2Db. The active peptides encompassed residues 510 to 518 (CSLWNGPHL, H-2Db) and 598 to 605 (RCQIFANI, H-2Kb). Both epitopes are located within the region of the S protein previously shown to be prone to deletion after passage in animals. These deleted strains are generally less neurovirulent than the wild-type virus but still are able to cause demyelination. Since C57BL/6 mice become persistently infected more commonly than many other strains of mice, these data are consistent with a role for CD8+ T-cell escape mutants in the pathogenesis of the demyelinating disease. This is the first report of CD8+ T-cell epitope localization within the S protein, the protein most strongly implicated thus far in pathogenesis in the host.

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

The hemagglutinin-esterase (HE) protein of mouse hepatitis virus (MHV) is an optional envelope protein present in only some MHV isolates. Its expression is regulated by the copy number of a UCUAA pentanucleotide sequence present in the leader sequence of the viral genomic RNA. The functional significance of this viral protein so far is not clear. In this report, we compared the neuropathogenicity of two MHV isolates, JHM(2) and JHM(3), which express different amounts of HE protein. Intracerebral inoculation of these two viruses into C57BL/6 mice showed that JHM(2), which expresses an abundant amount of HE protein, was more neurovirulent than JHM(3), which expresses very little HE. Histopathology showed that early in infection, JHM(2) infected primarily neurons, while JHM(3) infected mainly glial cells. JHM(3) eventually infected neurons and caused a delayed death relative to JHM(2)-infected mice, suggesting that the progression of JHM(3) infection in the central nervous system was slower than JHM(2). In vitro infection of JHM(3) in primary mixed glial cell cultures of astrocyte-enriched cultures yielded higher virus titers than JHM(2), mimicking the preferential growth of JHM(3) in glial cells in vivo. These findings suggest that the reduced neuropathogenicity of JHM(3) may correlate with its preferential growth in glial cells. Sequence analysis showed that the S genes of these two viruses are identical, thus ruling out the S gene as the cause of the difference in neuropathogenicity between these two viruses. We conclude that the HE protein contributes to viral neuropathogenicity by influencing either the rate of virus spread, viral cell tropism or both.

Epizootic of low-virulence hepatotropic murine hepatitis virus in a nude mice breeding colony in Taiwan

A natural outbreak of mouse hepatitis virus infection developed in a breeding colony of nude mice in Taiwan. The outbreak was unique in that morbidity was high in both adult and suckling mice, but only sucklings died. In contrast, all suckling heterozygous (nu/+) mice survived, and no lesions were found in adult female heterozygous (nu/+) mice. Adult male nude mice had chronic, active, necrotizing hepatitis with syncytial giant cells, but no lesions were detected in other tissues. Immunohistochemistry with anti-A59 and anti-JHM serum revealed mouse hepatitis virus antigen in the liver of infected adult and suckling nude mice, although less intensively in the kidney of adult nude mice. Suckling BALB/c mice inoculated with filtrates of the liver of adult nude mice developed hepatitis similar to that in the naturally infected nude mice. Virus was isolated by inoculating cell-free liver filtrate from infected adult nude mice onto 3T3 cells. Electron microscopy of purified virus revealed 100-nm-diameter enveloped particles with characteristic petal-shaped surface projections. We conclude that the outbreak was caused by a weakly virulent, highly hepatotropic murine hepatitis virus.

Persistent infection of cultured cells with mouse hepatitis virus (MHV) results from the epigenetic expression of the MHV receptor

The A59 strain of murine coronavirus mouse hepatitis virus (MHV) can cause persistent infection of 17C1-1 cells and other murine cell lines. Persistently infected cultures released large amounts of virus (10(7) to 10(8) PFU/ml) and were resistant to superinfection with MHV but not to infection with unrelated Semliki Forest and vesicular stomatitis viruses. The culture medium from persistently infected cultures did not contain a soluble inhibitor such as interferon that protected uninfected cells from infection by MHV or vesicular stomatitis virus. The persistent infection was cured if fewer than 100 cells were transferred during subculturing, and such cured cultures were susceptible to reinfection and the reestablishment of persistent infection. Cultures of 17C1-1 cells that had been newly cloned from single cells consisted of a mixture of MHV-resistant and -susceptible cells. 17C1-1/#97 cells, which were cured by subcloning after 97 passages of a persistently infected culture over a 1-year period, contained 5 to 10% of their population as susceptible cells, while 17C1-1/#402 cells, which were cured by subcloning after 402 passages over a 3-year period, had less than 1% susceptible cells. Susceptibility to infection correlated with the expression of MHV receptor glycoprotein (MHVR [Bgp1a]). Fluorescence-activated cell sorter analysis with antibody to MHVR showed that 17C1-1/#97 cells contained a small fraction of MHVR-expressing cells. These MHVR-expressing cells were selectively eliminated within 24 h after challenge with MHV-A59, and pretreatment of 17C1-1/#97 cells with monoclonal antibody CC1, which binds to the N-terminal domain of MHVR, blocked infection. We conclude that the subpopulation of MHVR-expressing cells were infected and killed in acutely or persistently infected cultures, while the subpopulation of MHVR-nonexpressing cells survived and proliferated. The subpopulation of MHVR-negative cells produced a small proportion of progeny cells that expressed MHVR and became infected, thereby maintaining the persistent infection as a steady-state carrier culture. Thus, in 17C1-1 cell cultures, the unstable or epigenetic expression of MHVR permitted the establishment of a persistent, chronic infection.

Tumor necrosis factor expression during mouse hepatitis virus-induced demyelinating encephalomyelitis

Neutralizing anti-tumor necrosis factor alpha (TNF-alpha) antibody treatment of mice infected with the neurotropic JHMV strain of mouse hepatitis virus showed no reduction of either virus-induced encephalomyelitis or central nervous system demyelination. TNF-alpha-positive cells were present in the central nervous system during infection; however, TNF-alpha could not be colocalized with JHMV-infected cells. In vitro, TNF-alpha mRNA rapidly accumulated following JHMV infection; however, no TNF-alpha was secreted because of inhibition of translation. Both live and UV-inactivated virus inhibited TNF-alpha secretion induced by lipopolysaccharide. These data show that TNF-alpha is not secreted from infected cells and indicate that if contributes to either JHMV-induced acute encephalomyelitis nor primary demyelination.

Pattern of disease after murine hepatitis virus strain 3 infection correlates with macrophage activation and not viral replication

Murine hepatitis virus strain (MHV-3) produces a strain-dependent pattern of disease which has been used as a model for fulminant viral hepatitis. This study was undertaken to examine whether there was a correlation between macrophage activation and susceptibility or resistance to MHV-3 infection. Peritoneal macrophages were isolated from resistant A/J and susceptible BALB/cJ mice and, following stimulation with MHV-3 or lipopolysaccharide (LPS), analyzed for transcription of mRNA and production of interleukin-1 (IL-1), tumor necrosis factor alpha (TNF-alpha), transforming growth factor beta (TGF-beta), mouse fibrinogen-like protein (musfiblp), tissue factor (TF), leukotriene B4, and prostaglandin E2 (PGE2). Macrophages from BALB/cJ mice produced greater amounts of IL-1, TNF-alpha, TGF-beta, leukotriene B4, and musfiblp following MHV-3 infection than macrophages from resistant A/J mice, whereas in response to LPS, equivalent amounts of IL-1, TNF-alpha, TGF-beta, and TF were produced by macrophages from both strains of mice. Levels of mRNA of IL-1, TNF-alpha, and musfiblp were greater and more persistent in BALB/cJ than in A/J macrophages, whereas the levels and kinetics of IL-1, TNF-alpha, and TF mRNA following LPS stimulation were identical in macrophages from both strains of mice. Levels of production of PGE2 by MHV-3-stimulated macrophages from resistant and susceptible mice were equivalent; however, the time course for induction of PGE2, differed, but the total quantity of PGE2 produced was insufficient to inhibit induction of musfiblp, a procoagulant known to correlate with development of fulminant hepatic necrosis in susceptible mice. These results demonstrate marked differences in production of inflammatory mediators to MHV-3 infection in macrophages from resistant A/J and susceptible BALB/cJ mice, which may explain the marked hepatic necrosis and fibrin deposition and account for the lethality of MHV-3 in susceptible mice.

Localization of neurovirulence determinant for rats on the S1 subunit of murine coronavirus JHMV

A cloned virus of murine coronavirus JHMV, cl-2, was shown to be highly neurovirulent for rats in comparison with other JHMV variants. We have isolated cl-2-derived variant viruses resistant to neutralization by monoclonal antibodies (MAbs) specific for the spike (S) protein of cl-2. The variants MM6 and MM13, selected by the MAbs specific for the JHMV S protein, were revealed to have a point mutation located within the N-terminal 100 amino acids (aa) of the S1 protein. The variants MM56, MM85, and MM78, selected by MAbs specific for the larger S protein of JHMV, were shown to have a deletion composed of about 150 aa located in the middle of the S1 subunit (MM56 and MM85) or one amino acid deletion, aspartic acid at number 543 from the N-terminus of the S1 (MM78). These five MAb-resistant variants were not different from cl-2 in growth pattern on cultured DBT cells. MM6 and MM13 were shown to be highly neurovirulent for 4-week-old Lewis rats, growing to high titers in the brain and causing as high an incidence of neurological disease and death as the parental cl-2. In contrast, MM56 and MM85 were nonneurovirulent for rats. They did not cause any central nervous system disorders nor did they multiply in the rat brains. MM78 showed intermediate neurovirulence as well as intermediate growth potential in the rat brain. However, there was no apparent difference in neurovirulence between the parental and the MAb-resistant variants for mice; all of these viruses showed high neurovirulence for mice. These results suggest that the domain composed of about 150 aa in the middle of the S1 is critical for high neurovirulence of JHMV for rats. Furthermore, it is suggested that the neurovirulence of cl-2 for mice is controlled by a different viral factor.

Optimization of in vitro growth conditions for enterotropic murine coronavirus strains

Enterotropic mouse hepatitis virus (MHV) strains have been difficult to grow in cell culture. In an attempt to develop an efficient in vitro cultivation system for enterotropic MHV strains (MHV-RI and MHV-Y), 8 murine cell lines were inoculated with MHV-RI- or MHV-Y-infected infant mouse intestinal homogenates and screened for the production of cytopathic effects. MHV-RI and MHV-Y consistently produced cytopathic effects only in J774A.1 cells. Both strains produced titers of > 10(6) TCID50/ml in subsequent passages in J774.1 cells. MHV strains-1, -3, -A59, -JHM, -S and -DVIM also produced high-titer viral stocks in J774A.1 cells. Therefore J774A.1 cells are the first cells found that support the replication of these 8 enterotropic and respiratory MHV strains. After passage in J774A.1 cells, MHV-RI and MHV-Y could infect previously non-susceptible cell lines (17Cl-1, CMT-93, N18 and NCTC 1469), though cytopathic effects were often negligible with MHV-RI. MHV-Y, but not MHV-RI, grew in L2(Percy) cells. Using L2(Percy) cells, an agarose overlay and Giemsa staining, MHV-Y could be quantified by plaque assay. Infant mouse bioassay, plaque assays and cell culture infections were compared for their sensitivity in detecting MHV-Y in infected intestinal homogenates and cell supernatants.

Mouse hepatitis virus A59-induced demyelination can occur in the absence of CD8+ T cells

Mouse hepatitis virus causes a chronic demyelinating disease in C57BL/6 mice. While early studies suggested demyelination is due to direct cytolytic effects of virus on oligodendrocytes, there is increasing evidence for the involvement of the immune system in the mechanism of demyelination. In this study we have asked whether demyelination can occur in the absence of functional MHC class I expression and CD8+ T cells. We infected transgenic mice lacking expression of beta 2 microglobulin (beta 2 M -/- mice) with MHV-A59. In beta 2M-/- mice, virus was much more lethal than in either of the parental strains used to produce the mice; furthermore, while clearance from the CNS did occur in beta 2M-/- mice, it was slower than in C57BL/6 mice. This is consistent with the importance of CD8+ cells in viral clearance. Because of the increased sensitivity of the beta 2M-/- mice to infection, only low levels of virus could be used to evaluate chronic disease. Even at these low levels, demyelination did occur in some animals. To compare infection in beta 2M-/- and C57BL/6 mice we used a higher dose of an attenuated variant of MHV-A59, C12. The attenuated variant induced less demyelination in C57BL/6 mice compared to wild type A59, but the levels observed were not significantly different from those seen in beta 2M-/- mice. Thus, MHV-induced demyelination can occur in some animals in the absence of MHC class I and CD8+ cells.

Interaction of mouse hepatitis virus (MHV) spike glycoprotein with receptor glycoprotein MHVR is required for infection with an MHV strain that expresses the hemagglutinin-esterase glycoprotein

In addition to the spike (S) glycoprotein that binds to carcinoembryonic antigen-related receptors on the host cell membrane, some strains of mouse coronavirus (mouse hepatitis virus [MHV]) express a hemagglutinin esterase (HE) glycoprotein with hemagglutinating and acetylesterase activity. Virions of strains that do not express HE, such as MHV-A59, can infect mouse fibroblasts in vitro, showing that the HE glycoprotein is not required for infection of these cells. The present work was done to study whether interaction of the HE glycoprotein with carbohydrate moieties could lead to virus entry and infection in the absence of interaction of the S glycoprotein with its receptor glycoprotein, MHVR. The DVIM strain of MHV expresses large amounts of HE glycoprotein, as shown by hemadsorption, acetylesterase activity, and immunoreactivity with antibodies directed against the HE glycoprotein of bovine coronavirus. A monoclonal anti-MHVR antibody, MAb-CC1, blocks binding of virus S glycoprotein to MHVR and blocks infection of MHV strains that do not express HE. MAb-CC1 also prevented MHV-DVIM infection of mouse DBT cells and primary mouse glial cell cultures. Although MDCK-I cells express O-acetylated sialic acid residues on their plasma membranes, these canine cells were resistant to infection with MHV-A59 and MHV-DVIM. Transfection of MDCK-I cells with MHVR cDNA made them susceptible to infection with MHV-A59 and MHV-DVIM. Thus, the HE glycoprotein of an MHV strain did not lead to infection of cultured murine neural cells or of nonmurine cells that express the carbohydrate ligand of the HE glycoprotein. Therefore, interaction of the spike glycoprotein of MHV with its carcinoembryonic antigen-related receptor glycoprotein is required for infectivity of MHV strains whether or not they express the HE glycoprotein.

MHVR-independent cell-cell spread of mouse hepatitis virus infection requires neutral pH fusion

Receptor-specificity is a key determinant of viral tropism. In this report, however, we have demonstrated that cell-associated spread of MHV can bypass the requirement for binding to primary receptors and thereby spread to cells that are resistant to MHV infection. Anti-receptor antibody CC1, which blocks infection by MHV virions, failed to prevent cell-associated spread of MHV to receptor-negative BHK cells or receptor-positive DBT cells. Cell-associated MHV may be utilizing an alternative, low-affinity receptor that is inadequate for functional interaction with MHV virions. Theoretically, dissemination of MHV infection through a receptor-independent, cell-associated mechanism in vivo provides the potential for broader host and tissue range, and for spread of infection despite the presence neutralizing antibodies. Receptor-independent, cell-associated spread of MHV requires neutral pH fusion capability. The low pH-dependent MHV variant OBLV60, which utilizes an endocytic route of entry, does not spread through a receptor-independent mechanism. Additionally, antiviral antibodies that block MHV spike-mediated fusion inhibited cell-associated spread of infection.

Evolution and persistence mechanisms of mouse hepatitis virus

We established and characterized persistently-infected DBT cells with mouse hepatitis virus to study the molecular mechanisms of MHV persistence and evolution in vitro. Following infection, viral mRNA and RF RNA were coordinately reduced by about 70% as compared to acute infection suggesting that the reduction in mRNA synthesis was due to reduced levels of transcriptionally active full length and subgenomic length negative-stranded RNAs. Although the rates of mRNA synthesis were also reduced, the relative percent molar ratio of the mRNAs and RF RNAs were similar to those detected during acute infection. In contrast to the finding during BCV persistence, analysis of the MHV leader RNA indicated that the leader RNA and leader/body junction sequences were extremely stable. These data suggested that polymorphism and mutations resulting in intraleader ORFs was not required for MHV persistence. Conversely MHV persistence was significantly associated with a A to G mutation at nt 77 in the 5' end untranslated region (UTR) of the genomic RNA.

Multiple receptor-dependent steps determine the species specificity of HCV-229E infection

Human coronavirus (HCV)-229E causes disease only in humans and grows in human cells and in cells of other species that express recombinant human aminopeptidase N (hAPN), the receptor for HCV-229E. We compared the species specificity of HCV-229E infection with the species specificity of virus binding using immunofluorescence, assay of virus yields, fluorescence activated cell sorting and a monoclonal antibody directed against hAPN that blocks infection. We found that HCV-229E binds to intestinal brush border membranes (BBM) and to membranes of cell lines from cats, dogs, pigs, and humans, however the virus only infects two of these species. HCV-229E will not bind to BBM or to membranes from cell lines derived from hamster or mice. Animal coronaviruses related to HCV-229E, including FIPV, CCV, and TGEV bind to cell membranes from cats, dogs, cows, pigs and humans (but not mice), while each virus infects cells from only a subset of these species. Infectious genomic HCV-229E RNA, can infect cells of all of these species. These data suggest that the species-specificity of infection for this serogroup of coronaviruses is determined at the levels of virus binding and penetration. Since binding of viral spike glycoprotein to cellular receptors is not the only limiting factor, we suggest that one or more steps associated with virus penetration may determine the species specificity of infection with the HCV-229E serogroup of coronaviruses.

Characterization of the S protein of enterotropic murine coronavirus strain-Y

The pathogenesis of enterotropic murine coronavirus strain MHV-Y differs extensively from that of prototypic respiratory strains of murine coronaviruses. The S protein of MHV-Y was characterized as a first step towards identifying viral determinants of enterotropism. Immunoblots of MHV-Y virions using anti-S protein specific antiserum revealed that the MHV-Y S protein was inefficiently cleaved. The MHV-Y S gene was cloned and sequenced. It encodes a protein predicted to be 1361 amino acids long. The presence of several amino acids changes within and surrounding the predicted cleavage site of the MHV-Y S protein may contribute to its inefficient cleavage.

Neurovirulence for rats of the JHMV variants escaped from neutralization with the S1-specific monoclonal antibodies

We have studied the neurovirulence for rats of the MAb-resistant variants isolated from a highly neurovirulent JHMV, cl-2. The variants, MM6 and MM13, with point mutation located within the N terminal 100 amino acids (aa) of the S1 protein showed no alteration in neurovirulence in comparison with cl-2, showing high neurovirulence. The variants, MM65 and MM85, with a deletion composed of about 150 aa located in the middle of the S1 subunit were revealed to be non-neurovirulent. A variant MM78 with one aa deletion, asparagic acid at number 543 from the N terminus of the S1, was shown to be low-virulence. The neurovirulence of these viruses paralleled with the viral growth potential in the rat brain. However, all of these variants as well as parental cl-2 showed high neurovirulence for mice. These results suggest that the domain composed of about 150 aa in the middle of the S1 is critical for high-neurovirulence of JHMV for rats.

Hepatitis mutants of mouse hepatitis virus strain A59

MHV-A59 causes acute meningoencephalitis and hepatitis in susceptible mice, and a persistent productive, but nonlytic, infection of cultured glial cells. We have shown previously that viruses isolated from persistently infected glial cell cultures have a fusion-defective phenotype and were impaired in their abilities to cause hepatitis compared to wild-type MHV-A59. Two mutants chosen for detailed study, B11 and C12, display two distinct hepatitis phenotypes. The ability of B11 to replicate in the liver was dependent on infectious dose and route of inoculation, while C12 consistently displayed decreased liver titers regardless of dose and route of inoculation. Sequence analysis of wild-type, mutant and revertant S proteins indicates that 1) a mutation in the N terminal subunit of S, resulting in a glutamine to leucine amino acid substitution (Q159L), may affect ability to cause hepatitis and 2) a cleavage site mutation (H716D) which determines fusogenicity is not responsible for the altered hepatitis phenotype. Sequence analysis indicated that hepatitis-producing revertants did not revert at mutation Q159L, although it is possible that a mutation in the heptad repeat domain of S2 may compensate for the mutation in S1. Since B11, C12 and a nonattenuated fusion mutant (B12) have identical S protein sequences, there must be additional mutations outside of S which influence both virulence and ability to replicate in the liver.

Spread of MHV-JHM from nasal cavity to white matter of spinal cord. Transneuronal movement and involvement of astrocytes

C57B1/6 mice infected intranasally with mouse hepatitis virus, strain JHM (MHV-JHM) develop hindlimb paralysis with histological evidence of demyelination several weeks after inoculation. Virus must spread from the site of inoculation, the nasal cavity, to the site of disease, the white matter of the spinal cord. It has been shown previously that after intranasal inoculation, virus enters the brain via the olfactory nerve and spreads to infect many of its neuroanatomic connections within the central nervous system (CNS). In this report, it is shown that virus infecting the spinal cord is first detected in the gray matter, with spread occurring to the white matter soon thereafter. Astrocytes are heavily infected during the process of spread from the gray to the white matter of the spinal cord. Since astrocytes are in intimate contact with neuronal synapses and are themselves connected via gap junctions, these results suggest that astrocytes may be a conduit for the spread of virus in these mice. Astrocytes provide factors for the proliferation and survival of oligodendrocytes, and widespread infection of these cells might contribute to the demyelinating process eventually observed in these mice. Additionally, since virus first appears at specific locations in the spinal cord, it should be possible to determine the source of the virus infecting the cord. While the results are not definitive, the data are most consistent with virus spreading from the ventral reticular formation to the gray matter of the cervical spinal cord.

Genomic regions associated with neurotropism identified in MHV by RNA-RNA recombination

Infection of mice with a neurotropic strain of MHV (MHV-A59), a non-neurotropic strain of MHV (MHV-2), and a set of recombinant viruses (kindly provided by Dr. Michael Lai) were used to map genetic determinants of viral neurotropism and demyelination. Following intracerebral (IC) inoculation of 4-week old C57B1/6 mice, 1LD50 of MHV-A59 produced acute meningoencephalitis and hepatitis, and subsequently chronic CNS demyelinating disease. IC inoculation of 1LD50 of MHV-2 produced acute hepatitis without CNS disease. Recombinants ML-3, ML-11, ML-7, ML-8, ML-9 and ML-10 produced acute encephalitis similar to MHV-A59. According to previous oligonucleotide fingerprinting analysis the only common denominator of the neurotropic recombinant viruses was an M gene derived from MHV-A59. Sequencing of PCR-amplified viral S and M genes confirmed that the M genes of neurotropic viruses are derived from A59 while the S genes of neurotropic viruses are either derived from MHV-2 or from A59. In tissue culture, ML-11, ML-3 and MHV-2 are fusion negative, while A59, ML-7, ML-8 and ML-10 are fusion positive. Thus, neurotropism in MHVs is not linked to fusion or the S gene. Moreover, the M gene may be a significant determinant of neurotropism and acute encephalitis.

Treatment of resistant A/J mice with methylprednisolone (MP) results in loss of resistance to murine hepatitis strain 3 (MHV-3) and induction of macrophage procoagulant activity (PCA)

BALB/cJ mice die of fulminant hepatitis within 7 days of exposure to murine hepatitis virus strain 3 (MHV-3) whereas A/J mice are fully resistant to the lethal effects of MHV-3 infection. Previous studies have implicated macrophage activation with production of a unique macrophage prothrombinase (PCA) and lymphocyte cytokine secretion in the pathogenesis of MHV-3 susceptibility and have demonstrated that immunosuppression induces susceptibility in resistant mice. This study was undertaken to determine whether macrophages, derived from resistant A/J mice and treated in vitro with methylprednisolone sodium succinate (MP), elaborated PCA following MHV-3 exposure and whether therapy with MP altered resistance of A/J mice to MHV-3 infection in vivo. Macrophages, incubated with MP in vitro, expressed dose dependent increases in PCA following infection with MHV-3. No induction of PCA occurred in macrophages treated with MHV-3 or MP alone. Analysis of mRNA transcripts for mouse fibrinogen like protein (musfiblp), the MHV-3 specific prothrombinase, in macrophages which were incubated with MP prior to exposure to MHV-3 demonstrated significantly increased mRNA levels as compared to macrophages not incubated with MP prior to MHV-3 exposure. In vivo, A/J mice treated for 3 days with 500 mg/kg/day of MP prior to infection with MHV-3 demonstrated extensive hepatocyte necrosis and fibrin deposition in hepatic sinusoids on histological examination of liver tissue, elevated serum transaminases and 100% mortality within 10 days of infection. These results therefore provide further support for the role of increased PCA in the pathogenesis of MHV-3 related liver necrosis.

Coronaviruses in polarized epithelial cells

Coronaviruses have a marked tropism for epithelial cells. In this paper the interactions of the porcine transmissible gastroenteritis virus (TGEV) and mouse hepatitis virus (MHV-A59) with epithelial cells are compared. Porcine (LLC-PK1) and murine (mTAL) epithelial cells were grown on permeable supports. By inoculation from the apical or basolateral side both TGEV and MHV-A59 were found to enter the polarized cells only through the apical membrane. The release of newly synthesized TGEV from LLC-PK1 cells occurred preferentially from the apical plasma membrane domain, as evidenced by the accumulation of viral proteins and infectivity in the apical culture fluid. In contrast, MHV was released preferentially from the basolateral membrane of mTAL cells. The apical release of TGEV and the basolateral release of MHV may explain the in vivo establishment of a local and systemic infection, respectively.

T cell immunodeficiency involved in pathogenicity of attenuated MHV3 mutants

Viral pathogenicity results when there is an imbalance between viral replication and the host's immune defenses. The immune system plays and important role in the outcome of an acute disease induced by the mouse hepatitis virus type 3 (MHV3). Of use in the study of the role of viral properties involved in its pathogenicity is the attenuated escape mutants. We reported that two MHV3 escape mutants were attenuated in their ability to deplete T cell subpopulations in the spleen in BALB/c mice according to inoculation route and time postinfection. The highly attenuated CL12 mutant cannot induce depletion in T cells following intraperitoneal (i.p.) or intranasal (i.n.) inoculations, at three days postinfection (p.i.). The less attenuated 51.6 mutant, however, maintained the ability to deplete T cells following i.p. inoculation, as described for the pathogenic MHV3. In contrast, no depletion of T cells following i.n. inoculation was induced with this mutant. The use of such mutants enables us to dissect the role of each compartment of the immune system.