Murine coronavirus spike protein determines the ability of the virus to replicate in the liver and cause hepatitis

Recombinant mouse hepatitis viruses (MHV) differing only in the spike gene, containing A59, MHV-4, and MHV-2 spike genes in the background of the A59 genome, were compared for their ability to replicate in the liver and induce hepatitis in weanling C57BL/6 mice infected with 500 PFU of each virus by intrahepatic injection. Penn98-1, expressing the MHV-2 spike gene, replicated to high titer in the liver, similar to MHV-2, and induced severe hepatitis with extensive hepatocellular necrosis. S(A59)R13, expressing the A59 spike gene, replicated to a somewhat lower titer and induced moderate to severe hepatitis with zonal necrosis, similar to MHV-A59. S4R21, expressing the MHV-4 spike gene, replicated to a minimal extent and induced few if any pathological changes, similar to MHV-4. Thus, the extent of replication and the degree of hepatitis in the liver induced by these recombinant viruses were determined largely by the spike protein.

Sequence analysis of the S gene of recombinant MHV-2/A59 coronaviruses reveals three candidate mutations associated with demyelination and hepatitis

Coronaviruses, mouse hepatitis virus (MHV) strains, exhibit various degrees of neurotropism and hepatotropism following intracerebral (IC) infection of 4-week-old C57Bl/6 mice. Whereas MHV-A59 produces acute meningitis, encephalitis, hepatitis, and chronic demyelination, a closely related strain, MHV-2, produces only acute meningitis and hepatitis. We previously reported that the spike glycoprotein gene of MHV contains determinants of demyelination and hepatitis. To further investigate the site of demyelination and hepatitis determinants within the S gene, we sequenced the S gene of several nondemyelinating recombinant viruses. We found that three encephalitis-positive, demyelination-negative, hepatitis-negative recombinant viruses have an MHV-A59-derived S gene, which contains three identical point mutations (I375M, L652I, and T1087N). One or more of the sites of these mutations in the MHV-A59 genome are likely to contribute to demyelination and hepatitis.

Neurovirulence in mice of soluble receptor-resistant (srr) mutants of mouse hepatitis virus: intensive apoptosis caused by less virulent srr mutant

Three soluble receptor-resistant (srr) mutants, srr7, srr11 and srr18, derived from a highly neurotropic mouse hepatitis virus (MHV) JHMV have a single amino acid mutation in the spike (S) protein. We examined using ICR mice whether the amino acids mutated in the mutants were involved in the neurovirulence. Srr7 showed apparently reduced neurovirulence relative to the wild-type (wt) JHMV in terms of the LD50 and survival time, while the others showed slightly reduced virulence. In the brain and spinal cord, the growth of srr7 was more than 2 log10 lower than that of the wt virus. Histopathologically, no significant difference was revealed between wt and srr7-infected mice on day 2 postinoculation (p.i.), with only scant inflammation and a minimum degree of neuropathological changes. The major difference was that apoptotic cells were frequently encountered in the srr7-infected mouse brain, but not in wt-infected mice on day 2 p.i. However, there was no difference between these viruses in the potential to induce apoptosis in cultured cells. The apoptosis in the brain did not appear to result from the direct viral attack, since apoptotic cells were found in the lesion where viral antigens were barely detected. The present study suggests that the amino acids mutated in the S protein of srr mutants, especially the amino acid at position 1114 mutated in srr7, influence the neurovirulence in mice.

Multiple regions of the murine coronavirus spike glycoprotein influence neurovirulence

The spike (S) glycoprotein of mouse hepatitis virus (MHV) is a major determinant of neurovirulence. Using targeted recombination we previously demonstrated that the S gene of the highly neurovirulent MHV-4 conferred a dramatic increase in neurovirulence to the mildly neurovirulent MHV-A59. To identify the genetic determinants of neurovirulence within the MHV-4 spike, we generated isogenic recombinant viruses containing various MHV-4/MHV-A59 chimeric spike genes, and studied their phenotypes in vivo. The MHV-4/MHV-A59 chimeric spike genes consisted of either reciprocal exchanges between the S1 and S2 spike subunits, or smaller exchanges specifically in the hypervariable region (HVR) of S1. The chimeric spike gene containing recombinants all exhibited efficient replication in vitro, yet many were severely attenuated for virulence in vivo. Furthermore, these attenuated recombinants exhibited decreased titers of infectious virus in the brain relative to the parental recombinant viruses containing the full-length MHV-4 or MHV-A59 spike genes. This is the first report that compares the neurovirulence and pathogenesis of isogenic viruses with defined alterations in the MHV spike protein. From these studies, it appears that the interactions of multiple regions of the MHV spike, including the HVR, act in concert to allow for efficient infection of and virulence in the murine central nervous system.

Coronavirus-induced demyelination occurs in the absence of inducible nitric oxide synthase

Demyelination induced by mouse hepatitis virus (MHV), strain JHM, is in large part immune mediated, but little is known about the mechanisms involved in this process. Previous results suggest that inducible nitric oxide synthase (NOS2) contributes transiently to MHV-induced demyelination. Herein, we show that equivalent amounts of demyelination were evident at day 12 after MHV infection in mice genetically deficient in NOS2 (NOS2(-/-)) and in C57BL/6 mice. Furthermore, using an established adoptive transfer model and pharmacological inhibitors of NOS2 function, we could demonstrate no effect on MHV-induced demyelination. These results indicate that NOS2 function is not required for demyelination in mice infected with MHV.

Liver-specific alpha 2 interferon gene expression results in protection from induced hepatitis

The current therapy for hepatitis B and C is based on systemic administration of recombinant human alpha interferon (r-hIFN-alpha). However, systemic delivery of r-hIFN-alpha is associated with severe side effects, but more importantly, it is effective in only a small percentage of patients. In an effort to maximize IFN-alpha antiviral efficacy, we have explored the therapeutic potential of murine IFN-alpha2 (mIFNalpha2) selectively expressed in the liver. To this end, we have developed a helper-dependent adenovirus vector (HD) containing the mIFN-alpha2 gene under the control of the liver-specific transthyretin promoter (HD-IFN). Comparison with a first-generation adenovirus carrying the same mIFN-alpha2 expression cassette indicates that at certain HD-IFN doses, induction of antiviral genes can be achieved in the absence of detectable circulating mIFN-alpha2. Challenge of injected mice with mouse hepatitis virus type 3 showed that HD-IFN provides high liver protection. Moreover, liver protection was also observed in acute nonviral liver inflammation hepatitis induced by concanavalin A at 1 month postinfection. These results hold promise for the development of a gene therapy treatment for chronic viral hepatitis based on liver-restricted expression of IFN-alpha2.

Contributions of CD8+ T cells and viral spread to demyelinating disease

Acute and chronic demyelination are hallmarks of CNS infection by the neurotropic JHM strain of mouse hepatitis virus. Although infectious virus is cleared by CD8+ T cells, both viral RNA and activated CD8+ T cells remain in the CNS during persistence potentially contributing to pathology. To dissociate immune from virus-mediated determinants initiating and maintaining demyelinating disease, mice were infected with two attenuated viral variants differing in a hypervariable region of the spike protein. Despite similar viral replication and tropism, one infection was marked by extensive demyelination and paralysis, whereas the other resulted in no clinical symptoms and minimal neuropathology. Mononuclear cells from either infected brain exhibited virus specific ex vivo cytolytic activity, which was rapidly lost during viral clearance. As revealed by class I tetramer technology the paralytic variant was superior in inducing specific CD8+ T cells during the acute disease. However, after infectious virus was cleared, twice as many virus-specific IFN-gamma-secreting CD8+ T cells were recovered from the brains of asymptomatic mice compared with mice undergoing demyelination, suggesting that IFN-gamma ameliorates rather than perpetuates JHM strain of mouse hepatitis virus-induced demyelination. The present data thus indicate that in immunocompetent mice, effector CD8+ T cells control infection without mediating either clinical disease or demyelination. In contrast, demyelination correlated with early and sustained infection of the spinal cord. Rapid viral spread, attributed to determinants within the spike protein and possibly perpetuated by suboptimal CD8+ T cell effector function, thus ultimately leads to the process of immune-mediated demyelination.

Contributions of Fas-Fas ligand interactions to the pathogenesis of mouse hepatitis virus in the central nervous system

The pathogenesis of the neurotropic strain of mouse hepatitis virus in Fas-deficient mice suggested that Fas-mediated cytotoxicity may be required during viral clearance after the loss of perforin-mediated cytotoxicity. The absence of both Fas- and perforin-mediated cytolysis resulted in an uncontrolled infection, suggesting a redundancy of cytolytic pathways to control virus replication.

A central role for CD4(+) T cells and RANTES in virus-induced central nervous system inflammation and demyelination

Infection of C57BL/6 mice with mouse hepatitis virus (MHV) results in a demyelinating encephalomyelitis characterized by mononuclear cell infiltration and white matter destruction similar to the pathology of the human demyelinating disease multiple sclerosis. The contributions of CD4(+) and CD8(+) T cells in the pathogenesis of the disease were investigated. Significantly less severe inflammation and demyelination were observed in CD4(-/-) mice than in CD8(-/-) and C57BL/6 mice (P < or = 0.002 and P < or = 0.001, respectively). Immunophenotyping of central nervous system (CNS) infiltrates revealed that CD4(-/-) mice had a significant reduction in numbers of activated macrophages/microglial cells in the brain compared to the numbers in CD8(-/-) and C57BL/6 mice, indicating a role for these cells in myelin destruction. Furthermore, CD4(-/-) mice displayed lower levels of RANTES (a C-C chemokine) mRNA transcripts and protein, suggesting a role for this molecule in the pathogenesis of MHV-induced neurologic disease. Administration of RANTES antisera to MHV-infected C57BL/6 mice resulted in a significant reduction in macrophage infiltration and demyelination (P < or = 0.001) compared to those in control mice. These data indicate that CD4(+) T cells have a pivotal role in accelerating CNS inflammation and demyelination within infected mice, possibly by regulating RANTES expression, which in turn coordinates the trafficking of macrophages into the CNS, leading to myelin destruction.

Inverted immunodominance and impaired cytolytic function of CD8+ T cells during viral persistence in the central nervous system

Mice infected with the neurotropic JHM strain of mouse hepatitis virus (JHMV) clear infectious virus; nevertheless, virus persists in the CNS as noninfectious RNA, resulting in ongoing primary demyelination. Phenotypic and functional analysis of CNS infiltrating cells during acute infection revealed a potent regional CD8+ T cell response comprising up to 50% virus-specific T cells. The high prevalence of virus-specific T cells correlated with ex vivo cytolytic activity and efficient reduction in viral titers. Progressive viral clearance coincided with the loss of cytolytic activity, but retention of IFN-gamma secretion and increased expression of the early activation marker CD69, indicating differential regulation of effector function. Although the total number of infiltrating T cells declined following clearance of infectious virus, CD8+ T cells, both specific for the dominant viral epitopes and of unknown specificity, were retained within the CNS, suggesting an ongoing T cell response during persistent CNS infection involving a virus-independent component. Reversed immunodominance within the virus-specific CD8+ T cell population further indicated epitope-specific regulation, supporting ongoing T cell activation. Even in the absence of infectious virus, the CNS thus provides an environment that maintains both unspecific and Ag-specific CD8+ T cells with restricted effector function. Chronic T cell stimulation may thus play a role in preventing viral recrudescence, while increasing the risk of pathological conditions, such as demyelination.

Pathogenesis of chimeric MHV4/MHV-A59 recombinant viruses: the murine coronavirus spike protein is a major determinant of neurovirulence

The mouse hepatitis virus (MHV) spike glycoprotein, S, has been implicated as a major determinant of viral pathogenesis. In the absence of a full-length molecular clone, however, it has been difficult to address the role of individual viral genes in pathogenesis. By using targeted RNA recombination to introduce the S gene of MHV4, a highly neurovirulent strain, into the genome of MHV-A59, a mildly neurovirulent strain, we have been able to directly address the role of the S gene in neurovirulence. In cell culture, the recombinants containing the MHV4 S gene, S4R22 and S4R21, exhibited a small-plaque phenotype and replicated to low levels, similar to wild-type MHV4. Intracranial inoculation of C57BL/6 mice with S4R22 and S4R21 revealed a marked alteration in pathogenesis. Relative to wild-type control recombinant viruses (wtR13 and wtR9), containing the MHV-A59 S gene, the MHV4 S gene recombinants exhibited a dramatic increase in virulence and an increase in both viral antigen staining and inflammation in the central nervous system. There was not, however, an increase in the level of viral replication in the brain. These studies demonstrate that the MHV4 S gene alone is sufficient to confer a highly neurovirulent phenotype to a recombinant virus deriving the remainder of its genome from a mildly neurovirulent virus, MHV-A59. This definitively confirms previous findings, suggesting that the spike is a major determinant of pathogenesis.

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.

Granulomatous peritonitis and pleuritis in interferon-gamma gene knockout mice naturally infected with mouse hepatitis virus

OBJECTIVE

To investigate a disease outbreak in a colony of laboratory mice with targeted disruption of the gene for interferon-gamma. FORMAT: A case report based on necropsy, histopathology, serology and immunohistochemistry.

RESULTS

Affected mice exhibited depression and variable ascites. Necropsy revealed a granulomatous peritonitis and pleuritis with extensive adhesions although parenchymal lesions were minimal. Serum samples had high concentrations of antibody to mouse hepatitis virus and immunohistochemical examination revealed the presence of mouse hepatitis virus antigen in granuloma macrophages. Sero-logical testing for other infectious agents and bacterial culture were negative and wild type mice kept in the same facility remained healthy. Despite the association between the disease and mouse hepatitis virus infection, the precise role played by mouse hepatitis virus was not determined. While the disease is superficially similar to feline infectious peritonitis (another coronavirus-induced serositis), differences exist between the histopathological findings in these two conditions.

CONCLUSION

This unusual disease process illustrates how new diagnostic challenges can arise in novel mouse genotypes created through molecular genetics. Furthermore, the association between the disease and mouse hepatitis virus illustrates the importance of maintaining laboratory animals under specific-pathogen free conditions.

Depletion of blood-borne macrophages does not reduce demyelination in mice infected with a neurotropic coronavirus

Mice infected with the neurotropic coronavirus mouse hepatitis virus strain JHM (MHV-JHM) develop a chronic demyelinating disease with symptoms of hindlimb paralysis. Histological examination of the brains and spinal cords of these animals reveals the presence of large numbers of activated macrophages/microglia. In two other experimental models of demyelination, experimental allergic encephalomyelitis and Theiler's murine encephalomyelitis virus-induced demyelination, depletion of hematogenous macrophages abrogates the demyelinating process. In both of these diseases, early events in the demyelinating process are inhibited by macrophage depletion. From these studies, it was not possible to determine whether infiltrating macrophages were required for late steps in the process, such as myelin removal. In this study, we show that when macrophages are depleted with either unmodified or mannosylated liposomes encapsulating dichloromethylene diphosphate, the amount of demyelination detected in MHV-infected mice is not affected. At a time when these cells were completely depleted from the liver, approximately equivalent numbers of macrophages were present in the spinal cords of control and drug-treated animals. These results suggest that blood-borne macrophages are not required for MHV-induced demyelination and also suggest that other cells, such as perivascular macrophages or microglia, perform the function of these cells in the presence of drug.

Selection of CD8+ T cells with highly focused specificity during viral persistence in the central nervous system

The relationships between T cell populations during primary viral infection and persistence are poorly understood. Mice infected with the neurotropic JHMV strain of mouse hepatitis virus mount potent regional CTL responses that effectively reduce infectious virus; nevertheless, viral RNA persists in the central nervous system (CNS). To evaluate whether persistence influences Ag-specific CD8+ T cells, functional TCR diversity was studied in spleen and CNS-derived CTL populations based on differential recognition of variant peptides for the dominant nucleocapsid epitope. Increased specificity of peripheral CTL from persistently infected mice for the index epitope compared with immunized mice suggested T cell selection during persistence. This was confirmed with CD8+ T cell clones derived from the CNS of either acutely (CTLac) or persistently (CTLper) infected mice. Whereas CTLac clones recognized a broad diversity of amino acid substitutions, CTLper clones exhibited exquisite specificity for the wild-type sequence. Highly focused specificity was CD8 independent but correlated with longer complementarity-determining regions 3 characteristic of CTLper clonotypes despite limited TCR alpha/beta-chain heterogeneity. Direct ex vivo analysis of CNS-derived mononuclear cells by IFN-gamma enzyme-linked immunospot assay confirmed the selection of T cells with narrow Ag specificity during persistence at the population level. These data suggest that broadly reactive CTL during primary infection are capable of controlling potentially emerging mutations. By contrast, the predominance of CD8+ T cells with dramatically focused specificity during persistence at the site of infection and in the periphery supports selective pressure driven by persisting Ag.

Cellular reservoirs for coronavirus infection of the brain in beta2-microglobulin knockout mice

Mouse hepatitis virus (MHV) A59 infection which causes acute encephalitis, hepatitis, and chronic demyelination, is one of the experimental models for multiple sclerosis. Previous studies showed that lethal infection of beta2-microglobulin 'knockout' (beta2M(-/-)) mice required 500-fold less virus and viral clearance was delayed as compared to infection of immunocompetent C57Bl/6 (B6) mice. To investigate the mechanism of the increased susceptibility of beta2M(-/-) mice to MHV-A59, we studied organ pathology and the distribution of viral antigen and RNA during acute and chronic infection. A59-infected beta2M(-/-) mice were more susceptible to acute encephalitis and hepatitis, but did not have increased susceptibility to demyelination. Viral antigen and RNA distribution in the brain was increased in microglia, lymphocytes, and small vessel endothelial cells while the distribution in neurons and glia was similar in beta2M(-/-) mice and B6 mice. Acute hepatitis and thymus cortical hypoplasia in beta2M(-/-) mice were delayed in onset but pathologic changes in these organs were similar to those in B6 mice. The low rate of demyelination in beta2M(-/-) mice was consistent with the low dose of the virus given. A less neurotropic virus MHV-2, caused increased parenchymal inflammation in beta2M(-/-) mice, but without demyelination. Thus, CD8+ cells were important for viral clearance from endothelial cells, microglia and inflammatory cells, but not from neuronal and glial cells. In addition, CD8+ cells played a role in preventing the spread of encephalitis.

Mice lacking expression of secondary lymphoid organ chemokine have defects in lymphocyte homing and dendritic cell localization

Secondary lymphoid organ chemokine (SLC) is expressed in high endothelial venules and in T cell zones of spleen and lymph nodes (LNs) and strongly attracts naive T cells. In mice homozygous for the paucity of lymph node T cell (plt) mutation, naive T cells fail to home to LNs or the lymphoid regions of spleen. Here we demonstrate that expression of SLC is undetectable in plt mice. In addition to the defect in T cell homing, we demonstrate that dendritic cells (DCs) fail to accumulate in spleen and LN T cell zones of plt mice. DC migration to LNs after contact sensitization is also substantially reduced. The physiologic significance of these abnormalities in plt mice is indicated by a markedly increased sensitivity to infection with murine hepatitis virus. The plt mutation maps to the SLC locus; however, the sequence of SLC introns and exons in plt mice is normal. These findings suggest that the abnormalities in plt mice are due to a genetic defect in the expression of SLC and that SLC mediates the entry of naive T cells and antigen-stimulated DCs into the T cell zones of secondary lymphoid organs.

IFN-gamma is required for viral clearance from central nervous system oligodendroglia

Infection of the central nervous system (CNS) by the JHM strain of mouse hepatitis virus (JHMV) is a rodent model of the human demyelinating disease multiple sclerosis. The inability of effective host immune responses to eliminate virus from the CNS results in a chronic infection associated with ongoing recurrent demyelination. JHMV infects a variety of CNS cell types during the acute phase of infection including ependymal cells, astrocytes, microglia, oligodendroglia, and rarely in neurons. Replication within the majority of CNS cell types is controlled by perforin-dependent virus-specific CTL. However, inhibition of viral replication in oligodendroglia occurs via a perforin-independent mechanism(s). The potential role for IFN-gamma as mediator controlling JHMV replication in oligodendroglia was examined in mice deficient in IFN-gamma secretion (IFN-gamma0/0 mice). IFN-gamma0/0 mice exhibited increased clinical symptoms and mortality associated with persistent virus, demonstrating an inability to control replication. Neither antiviral Ab nor CTL responses were diminished in the absence of IFN-gamma, although increased IgG1 was detected in IFN-gamma0/0 mice. Increased virus Ag in the absence of IFN-gamma localized almost exclusively to oligodendroglia and was associated with increased CD8+ T cells localized within white matter. These data suggest that although perforin-dependent CTL control virus replication within astrocytes and microglia, which constitute the majority of infected CNS cells, IFN-gamma is critical for control of viral replication in oligodendroglia. Therefore, different mechanisms are used by the host defenses to control virus replication within the CNS, dependent upon the phenotype of the targets of virus replication.

Targeted recombination within the spike gene of murine coronavirus mouse hepatitis virus-A59: Q159 is a determinant of hepatotropism

Previous studies of a group of mutants of the murine coronavirus mouse hepatitis virus (MHV)-A59, isolated from persistently infected glial cells, have shown a strong correlation between a Q159L amino acid substitution in the S1 subunit of the spike gene and a loss in the ability to induce hepatitis and demyelination. To determine if Q159L alone is sufficient to cause these altered pathogenic properties, targeted RNA recombination was used to introduce a Q159L amino acid substitution into the spike gene of MHV-A59. Recombination was carried out between the genome of a temperature-sensitive mutant of MHV-A59 (Alb4) and RNA transcribed from a plasmid (pFV1) containing the spike gene as well as downstream regions, through the 3' end, of the MHV-A59 genome. We have selected and characterized two recombinant viruses containing Q159L. These recombinant viruses (159R36 and 159R40) replicate in the brains of C57BL/6 mice and induce encephalitis to a similar extent as wild-type MHV-A59. However, they exhibit a markedly reduced ability to replicate in the liver or produce hepatitis compared to wild-type MHV-A59. These viruses also exhibit reduced virulence and reduced demyelination. A recombinant virus containing the wild-type MHV-A59 spike gene, wtR10, behaved essentially like wild-type MHV-A59. This is the first report of the isolation of recombinant viruses containing a site-directed mutation, encoding an amino acid substitution, within the spike gene of any coronavirus. This technology will allow us to begin to map the molecular determinants of pathogenesis within the spike glycoprotein.

Murine coronavirus-induced subacute fatal peritonitis in C57BL/6 mice deficient in gamma interferon

Gamma interferon-deficient (IFN-gamma-/-) mice with a C57BL/6 background were infected intraperitoneally with mouse hepatitis virus strain JHM (JHMV). In contrast to IFN-gamma-+/- and IFN-gamma+/+ mice, JHMV persisted in IFN-gamma-/- mice and induced death during the subacute phase of the infection. Unexpectedly, infected IFN-gamma-/- mice showed severe peritonitis accompanying the accumulation of a viscous fluid in the abdominal and thoracic cavities in the subacute phase. Destructive changes of hepatocytes were not observed. Administration of recombinant IFN-gamma protracted the survival time of IFN-gamma-/- mice after JHMV infection. These results demonstrate that IFN-gamma plays a critical role in viral clearance in JHMV infection. They also show that a resultant persistent JHMV infection induces another form of disease in IFN-gamma-/- mice, which bears a resemblance to feline infectious peritonitis in cats.

Selection in persistently infected murine cells of an MHV-A59 variant with extended host range

Murine coronavirus MHV-A59 normally infects only murine cells in vitro and causes transmissible infection only in mice. In the 17 C1 1 line of murine cells, the receptor for MHV-A59 is MHVR, a biliary glycoprotein in the carcinoembryonic antigen (CEA) family of glycoproteins. We found that virus released from the 600th passage of 17 C1 1 cells persistently infected with MHV-A59 (MHV/pi600) replicated in hamster (BHK-21) cells. The virus was passaged and plaque-purified in BHK-21 cells, yielding the MHV/BHK strain. Because murine cells persistently infected with MHV-A59 express a markedly reduced level of MHVR (Sawicki, et al., 1995), we tested whether virus with altered receptor interactions was selected in the persistently infected culture. Infection of 17 C1 1 cells by MHV-A59 can be blocked by treating the cells with anti-MHVR MAb-CC1, while infection by MHV/BHK was only partially blocked by MAb-CC1. MHV/BHK virus was also more resistant than wild-type MHV-A59 to neutralization by purified, recombinant, soluble MHVR glycoprotein (sMHVR). Cells in the persistently infected culture may also express reduced levels of and have altered interactions with some of the Bgp-related glycoproteins that can serve as alternative receptors for MHV-A59. Unlike the parental MHV-A59 which only infects murine cells, MHV/BHK virus was able to infect cell lines derived from mice, hamsters, rats, cats, cows, monkeys and humans. However, MHV/BHK was not able to infect all mammalian species, because a pig (ST) cell line and a dog cell line (MDCK I) were not susceptible to infection. MHV/pi600 and MHV/BHK replicated in murine cells more slowly than MHV-A59 and formed smaller plaques. Thus, in the persistently infected murine cells which expressed a markedly reduced level of MHVR, virus variants were selected that have altered interactions with MHVR and an extended host range. In vivo, in mice infected with coronavirus, virus variants with altered receptor recognition and extended host range might be selected in tissues that have low levels of receptors. Depending upon the tissue in which such a virus variant was selected, it might be shed from the infected animal or eaten by a predator, thus presenting a possible means for initiating the transition of a variant virus into a new host as a model for an emerging virus disease.

Role of CTL mutants in demyelination induced by mouse hepatitis virus, strain JHM

Mouse hepatitis virus, strain JHM (MHV-JHM) is a well described cause of demyelination. C57B1/6 (B6) mice infected at the suckling stage in the presence of protective antibodies remain asymptomatic initially but later develop clinical disease (hindlimb paralysis). Infectious virus can be isolated from these mice. Recently, two MHV-specific target epitopes for cytotoxic CD8 T cells have been identified in B6 mice. Our results show that in all mice with hindlimb paralysis, mutations can be detected in the RNA encoding the immunodominant of the two epitopes. These mutations result in a loss of recognition by MHV-specific cytotoxic T cells. These changes are not detected, for the most part, in mice that remain asymptomatic nor in mice with acute encephalitis. These results suggest that the development of CTL escape mutants is necessary for hindlimb paralysis to develop in this model.

Targeted recombination between MHV-2 and MHV-A59 to study neurotropic determinants of MHV

MHV-A59 produces acute encephalitis, acute hepatitis and chronic demyelination in infected mice. MHV-2 produces only hepatitis and mild meningitis but without encephalitis or demyelination. We have previously studied a set of recombinant viruses between these two strains. The common denominator of viruses that produced encephalitis was a membrane (M) gene derived from MHV-A59. Thus to study the potential contribution of the M gene to acute encephalitis, chimeric viruses were produced in which the M gene of MHV-A59 was substituted with the M gene of MHV-2 by targeted recombination. A control virus was produced in which the M gene of A59 was recombined back into an A59 background. Viruses were then analyzed for their biologic properties and compared with the phenotypes of MHV-A59 and MHV-2 by histopathology and plaque assays for viral titers in organs following intracerebral (IC) inoculation. All three chimeric viruses had a phenotype similar to MHV-A59. Thus, the replacement of the M gene of MHV-A59 with that of MHV-2 is insufficient to produce a phenotype that lacks encephalitis similar to MHV-2.

Coronavirus infection and demyelination. Sequence conservation of the S-gene during persistent infection of Lewis-rats

Coronaviruses display a large phenotypic variability, which may be an important factor for diversification and selection. Previous studies have demonstrated that the S-protein is an essential determinant of virulence and pathogenicity. Therefore we studied the S-gene as an indicator molecule for selection processes employing two different MHV-JHM variants. First, Lewis-rats were infected with MHV-JHM-Pi, a variant that causes demyelinating disease after several weeks p.i. It was not possible to isolate infectious MHV-JHM-Pi from such rats, although viral proteins were expressed. The S-gene was rescued directly from brain tissue employing RT-PCR technology. The amplicons were sequenced in bulk or at the level of single clones. We detected no evidence for an increase of S-gene mutants during the length of time. Only few mutations were found at the clonal level. The changes were distributed throughout the analysed S-gene fragments without a predilection in their location. The frequency of mutation remained low within a range of 0.03 to 0.5 mutations per thousand nucleotides. As a second approach, we sequenced the S-genes of viruses isolated from brain tissue infected with MHV-JHM-ts43. Infection of adult Lewis rats with that mutant resulted several weeks to months p.i. in demyelinating encephalomyelitis. The S-gene of this virus contains an insertion of 423 bp in the S1 region, which is identical to a polymorphic region described for MHV-4. In contrast to JHM-Pi, infectious MHV-JHM-ts43 was readily to isolate from brain tissue. The S-gene sequences of virus isolated 45-106 days p.i. from diseased rats were identical with that of the input virus. These results show, that during a persistent infection of Lewis-rats the S-gene was highly conserved.