Role of mouse hepatitis virus-A59 receptor Bgp1a expression in virus-induced pathogenesis

Expression of Bgp1a, a glycoprotein that serves as receptor for mouse hepatitis virus-A59 has been analyzed in various mouse tissues and correlated with the pathogenicity that this virus induces in the corresponding organs. Expression of Bgp1a was observed in many cells of epithelial origin, including hepatocytes and endothelial cells. It was also shown on macrophages and B lymphocytes. Bgp1a localization may easily explain infection and lysis of some cell types like hepatocytes. In contrast, other cell types that express the viral receptor are not infected after in vivo inoculation with mouse hepatitis virus-A59, which may be due to inaccessibility of the receptor to the virus during mouse infection, or to resistance to this virus in some cell types. This may account for the ability of the blood-brain barrier to prevent mouse hepatitis virus-A59 spreading into the central nervous system. In other organs, the virus may induce pathogenesis indirectly, resulting in the destruction of cells that do not express Bgp1a, like thymic lymphocytes, or else impair cell functions such as cytokine and immunoglobulin production by macrophages and B lymphocytes, respectively.

Mice lacking IL-12 develop polarized Th1 cells during viral infection

Studies in IL-12-deficient mice established the necessity for IL-12 to generate a Th1 cytokine response that is often required for elimination of intracellular pathogens. In this study, we demonstrate that mice with a targeted disruption of the IL-12p40 and/or p35 gene effectively control liver damage induced by mouse hepatitis virus (MHV) infection, similar to wild-type animals. In contrast, MHV-infected IFN-gamma receptor-deficient (IFN-gammaR[-/-]) mice showed an increased susceptibility to coronaviral hepatitis. Surprisingly, MHV-infected mice lacking IL-12 produced a polarized Th1-type cytokine response, as evidenced by high IFN-gamma and nondetectable IL-4 production by CD4+ splenocytes and normal virus-specific serum IgG2a/IgG1 ratios. The virus-induced type 1 cytokine secretion pattern was not reversed in IL-12-deficient mice by in vivo neutralization of IFN-gamma nor in IFN-gammaR(-/-) mice receiving IL-12-neutralizing Abs. In IL-12-deficient mice, Th1-type responses were also generated upon immunization with inactivated MHV. In contrast, following immunization with keyhole limpet hemocyanin, mice lacking IL-12 mounted strongly reduced specific IgG2a and increased IgE responses, indicative of a type 2-dominated cytokine pattern. These findings demonstrate that following a virus infection, IL-12 is not essential for the generation of polarized T cell type 1 cytokine expression and associated immune responses, which is in marked contrast to nonviral systems. Our data suggest that viruses may selectively induce IFN-gamma production and Th1-type immune reactions even in the absence of IL-12.

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

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

The viral spike protein is not involved in the polarized sorting of coronaviruses in epithelial cells

Coronaviruses are assembled by budding into a pre-Golgi compartment from which they are transported along the secretory pathway to leave the cell. In cultured epithelial cells, they are released in a polarized fashion; depending on the virus and cell type, they are sorted preferentially either to the apical domain or to the basolateral plasma membrane domain. In this study, we investigated the role of the coronavirus spike protein, because of its prominent position in the virion the prime sorting candidate, in the directionality of virus release. Three independent approaches were taken. (i) The inhibition of N glycosylation by tunicamycin resulted in the synthesis of spikeless virions. The absence of spikes, however, did not influence the polarity in the release of virions. Thus, murine hepatitis virus strain A59 (MHV-A59) was still secreted from the basolateral membranes of mTAL and LMR cells and from the apical sides of MDCK(MHVR) cells, whereas transmissible gastroenteritis virus (TGEV) was still released from the apical surfaces of LMR cells. (ii) Spikeless virions were also studied by using the MHV-A59 temperature-sensitive mutant Albany 18. When these virions were produced in infected LMR and MDCK(MHVR) cells at the nonpermissive temperature, they were again preferentially released from basolateral and apical membranes, respectively. (iii) We recently demonstrated that coronavirus-like particles resembling normal virions were assembled and released when the envelope proteins M and E were coexpressed in cells (H. Vennema, G.-J. Godeke, J. W. A. Rossen, W. F. Voorhout, M. C. Horzinek, D.-J. E. Opstelten, and P. J. M. Rottier, EMBO J. 15:2020-2028, 1996). The spikeless particles produced in mTAL cells by using recombinant Semliki Forest viruses to express these two genes of MHV-A59 were specifically released from basolateral membranes, i.e., with the same polarity as that of wild-type MHV-A59. Our results thus consistently demonstrate that the spike protein is not involved in the directional sorting of coronaviruses in epithelial cells. In addition, our observations with tunicamycin show that contrary to the results with some secretory proteins, the N-linked oligosaccharides present on the viral M proteins of coronaviruses such as TGEV also play no role in viral sorting. The implications of these conclusions are discussed.

Morphological analysis of mouse hepatitis virus A59-induced pathology with regard to viral receptor expression

Mouse hepatitis virus, strain A59 (MHV-A59), is a coronavirus that triggers in susceptible mice a wide variety of pathologies, including hepatitis, thymus involution, B lymphocyte polyclonal activation and, after intra-cerebral inoculation, transient demyelination. One receptor that mediates entry of the virus into target cells has been identified: it is a glycoprotein of the carcinoembryonic antigen family, called Bgp1a. The availability of antibodies recognizing this molecule permits the analysis of its cellular expression and of the relationship between receptor expression and pathology induced by the virus. Bgp1a is found on epithelial and endothelial cells as well as on B lymphocytes and macrophages. In the liver, Bgp1a expression correlates well with infection of hepatocytes and endothelial cells, leading to the development of hepatitis. However, other cells expressing this molecule, such as central nervous system endothelial cells, are not infected by the virus. This observation may explain how the blood-brain barrier prevents dissemination of MHV-A59 from the general circulation into the brain. Thymic atrophy results from apoptosis of immature double-positive T lymphocytes which might be caused indirectly by infection of a small proportion of thymus epithelial cells that express Bgp1a rather than by infection of T cells that do not express the receptor. Finally, polyclonal activation of B lymphocytes, leading to increased secretion of antibodies of the IgG2a isotype, involves a cascade of events, including cytokine secretion, that may result from the interaction of MHV-A59 with B cells and macrophages that express Bgp1a. Therefore, after viral infection, cellular expression of Bgp1a may have different results: cell lysis; alteration of cellular functions that may lead to indirect death of other cell types, or resistance to infection.

Altered pathogenesis of a mutant of the murine coronavirus MHV-A59 is associated with a Q159L amino acid substitution in the spike protein

C12, an attenuated, fusion delayed, very weakly hepatotropic mutant of mouse hepatitis virus strain A59 (MHV-A59( has been further characterized. We have previously shown that C12 has two amino acid substitutions relative to wild type virus in the spike protein, Q159L (within a region of S1 shown to bind to viral receptor in an in vitro assay) and H716D (in the proteolytic cleavage recognition site). We have sequenced the rest of the 31-kb genome of C-12 and compared it to wild type virus. Only three additional amino acids substitutions were found, all encoded within the replicase gene. Analysis of C12 in vivo in C57Bl/6 mice has shown that despite the fact that this virus replicates in the brain to titers at least as high as wild type and causes acute encephalitis similar to wild-type, this virus causes a minimal level of demyelination and only at very high levels of virus inoculation. Thus acute encephalitis is not sufficient for the induction of demyelination by MHV-A59. Analysis of mutants isolated at earlier times from the same persistently infected glial cell culture as C12, as well as mutants isolated from a second independent culture of persistently infected glial cells, suggests that both the weakly demyelinating and the weakly hepatotropic phenotypes of C12 are associated with the Q159L amino acid substitution.

The function of the spike protein of mouse hepatitis virus strain A59 can be studied on virus-like particles: cleavage is not required for infectivity

The spike protein (S) of the murine coronavirus mouse hepatitis virus strain A59 (MHV-A59) induces both virus-to-cell fusion during infection and syncytium formation. Thus far, only syncytium formation could be studied after transient expression of S. We have recently described a system in which viral infectivity is mimicked by using virus-like particles (VLPs) and reporter defective-interfering (DI) RNAs (E. C. W. Bos, W. Luytjes, H. Van der Meulen, H. K. Koerten, and W. J. M. Spaan, Virology 218:52-60, 1996). Production of VLPs of MHV-A59 was shown to be dependent on the expression of M and E. We now show in several ways that the infectivity of VLPs is dependent on S. Infectivity was lost when spikeless VLPs were produced. Infectivity was blocked upon treatment of the VLPs with MHV-A59-neutralizing anti-S monoclonal antibody (MAb) A2.3 but not with nonneutralizing anti-S MAb A1.4. When the target cells were incubated with antireceptor MAb CC1, which blocks MHV-A59 infection, VLPs did not infect the target cells. Thus, S-mediated VLP infectivity resembles MHV-A59 infectivity. The system can be used to identify domains in S that are essential for infectivity. As a first application, we investigated the requirements of cleavage of S for the infectivity of MHV-A59. We inserted three mutant S proteins that were previously shown to be uncleaved (E. C. W. Bos, L. Heijnen, W. Luytjes, and W. J. M. Spaan, Virology 214:453-463, 1995) into the VLPs. Here we show that cleavage of the spike protein of MHV-A59 is not required for infectivity.

Role of virus receptor-bearing endothelial cells of the blood-brain barrier in preventing the spread of mouse hepatitis virus-A59 into the central nervous system

BALB/c mice develop a neurologic demyelinating disease after inoculation of mouse hepatitis virus (MHV), strain A59, by the intracranial, but not by the intraperitoneal route. To determine the mechanisms that prevent virus spreading through the blood-brain barrier, we analyzed expression of MHVR, a glycoprotein that serves as receptor for mouse hepatitis virus on endothelial cells of cerebral blood vessels. Our results indicated that MHVR was strongly expressed on the endoluminal pole of these cells. In addition, a direct virus binding assay showed that mouse hepatitis virus was able to bind endothelial cells via this receptor. Despite this expression of a functional viral receptor, in normal mice infected with mouse hepatitis virus by the contra-peritoneal route, no in vivo viral replication could be detected in endothelial cells from the brain, contrasting with the equivalent cells from the liver. However, shortly after i.v. administration of sodium dodecylsulfate detergent to the mice, virus infection of some cerebral endothelial cells was detected in a few mice. As a consequence of detergent treatment, virus infection was able to cross the blood-brain barrier. These results suggest that the protective role of the blood-brain barrier against spreading of mouse hepatitis virus A59 into the central nervous system is determined by a specific restriction of viral entry into the endothelial cells of cerebral origin.

The murine coronavirus mouse hepatitis virus strain A59 from persistently infected murine cells exhibits an extended host range

In murine 17 Cl 1 cells persistently infected with murine coronavirus mouse hepatitis virus strain A59 (MHV-A59), expression of the virus receptor glycoprotein MHVR was markedly reduced (S. G. Sawicki, J. H. Lu, and K. V. Holmes, J. Virol. 69:5535-5543, 1995). Virus isolated from passage 600 of the persistently infected cells made smaller plaques on 17 Cl 1 cells than did MHV-A59. Unlike the parental MHV-A59, this variant virus also infected the BHK-21 (BHK) line of hamster cells. Virus plaque purified on BHK cells (MHV/BHK) grew more slowly in murine cells than did MHV-A59, and the rate of viral RNA synthesis was lower and the development of the viral nucleocapsid (N) protein was slower than those of MHV-A59. MHV/BHK was 100-fold more resistant to neutralization with the purified soluble recombinant MHV receptor glycoprotein (sMHVR) than was MHV-A59. Pretreatment of 17 Cl 1 cells with anti-MHVR monoclonal antibody CC1 protected the cells from infection with MHV-A59 but only partially protected them from infection with MHV/BHK. Thus, although MHV/BHK could still utilize MHVR as a receptor, its interactions with the receptor were significantly different from those of MHV-A59. To determine whether a hemagglutinin esterase (HE) glycoprotein that could bind the virions to 9-O-acetylated neuraminic acid moieties on the cell surface was expressed by MHV/BHK, an in situ esterase assay was used. No expression of HE activity was detected in 17 Cl 1 cells infected with MHV/BHK, suggesting that this virus, like MHV-A59, bound to cell membranes via its S glycoprotein. MHV/BHK was able to infect cell lines from many mammalian species, including murine (17 Cl 1), hamster (BHK), feline (Fcwf), bovine (MDBK), rat (RIE), monkey (Vero), and human (L132 and HeLa) cell lines. MHV/BHK could not infect dog kidney (MDCK I) or swine testis (ST) cell lines. Thus, in persistently infected murine cell lines that express very low levels of virus receptor MHVR and which also have and may express alternative virus receptors of lesser efficiency, there is a strong selective advantage for virus with altered interactions with receptor (D. S. Chen, M. Asanaka, F. S. Chen, J. E. Shively, and M. M. C. Lai, J. Virol. 71:1688-1691, 1997; D. S. Chen, M. Asanaka, K. Yokomori, F.-I. Wang, S. B. Hwang, H.-P. Li, and M. M. C. Lai, Proc. Natl. Acad. Sci. USA 92:12095-12099, 1995; P. Nedellec, G. S. Dveksler, E. Daniels, C. Turbide, B. Chow, A. A. Basile, K. V. Holmes, and N. Beauchemin, J. Virol. 68:4525-4537, 1994). Possibly, in coronavirus-infected animals, replication of the virus in tissues that express low levels of receptor might also select viruses with altered receptor recognition and extended host range.

Mouse susceptibility to mouse hepatitis virus infection is linked to viral receptor genotype

We have reported that the receptor for mouse hepatitis virus (MHV) expressed in MHV-susceptible BALB/c mice (MHVR1) has 10 to 30 times the virus-binding activity of the MHV receptor expressed in MHV-resistant SJL mice (MHVR2) (N. Ohtsuka, Y. K. Yamada, and F. Taguchi, J. Gen. Virol. 77:1683-1992, 1996). This fact indicates the possibility that the difference in MHV susceptibility between BALB/c and SJL mice is determined by the virus-binding activity of the receptor. To test this possibility, we have examined MHV susceptibility in mice with the homozygous MHVR1 gene (R1/R1 genotype), mice with the MHVR1 and MHVR2 genes (R1/R2 genotype), and mice with the homozygous MHVR2 gene (R2/R2 genotype) produced by cross and backcross mating between BALB/c and SJL mice. All 63 F2 and backcrossed mice with the MHVR1 gene (R1/R1 and R1/R2) were susceptible to MHV infection, and all 57 with the homozygous MHVR2 gene (R2/R2) were resistant. We have also examined the MHV receptor genotypes of several mouse strains that were reported to be susceptible to MHV infection. All of those mice had the MHVR1 gene. These results suggest the possibility that the viral receptor determines the susceptibility of the whole animal to MHV infection.

Expression of interferon-gamma by a coronavirus defective-interfering RNA vector and its effect on viral replication, spread, and pathogenicity

A defective-interfering (DI) RNA of the murine coronavirus mouse hepatitis virus (MHV) was developed as a vector for expressing interferon-gamma (IFN-gamma). The murine IFN-gamma gene was cloned into the DI vector under the control of an MHV transcriptional promoter and transfected into MHV-infected cells. IFN-gamma was secreted into culture medium as early as 6 hr posttransfection and reached a peak level (up to 180 U/ml) at 12 hr posttransfection. The DI-expressed IFN-gamma (DE-IFN-gamma) exhibited an antiviral activity comparable to that of recombinant IFN-gamma and was blocked by a neutralizing monoclonal antibody against IFN-gamma. Treatment of macrophages with DE-IFN-gamma selectively induced the expression of the cellular inducible nitric oxide synthase and the IFN-gamma-inducing factor (IGIF) but did not affect the amounts of the MHV receptor mRNA. Antiviral activity was detected only when cells were pretreated with IFN-gamma for 24 hr prior to infection; no inhibition of virus replication was detected when cells were treated with IFN-gamma during or after infection. Furthermore, addition of IFN-gamma together with MHV did not prevent infection, but appeared to prevent subsequent viral spread. MHV variants with different degrees of neurovirulence in mice had correspondingly different levels of sensitivities to IFN-gamma treatment in vitro, with the most virulent strain being most resistant to IFN-gamma treatment. Infection of susceptible mice with DE-IFN-gamma-containing virus caused significantly milder disease, accompanied by more pronounced mononuclear cell infiltrates into the CNS and less virus replication, than that caused by virus containing a control DI vector. This study thus demonstrates the feasibility and usefulness of this MHV DI vector for expressing cytokines and may provide a model for studying the role of cytokines in MHV pathogenesis.

Resistance to infection with mouse hepatitis virus (MHV) in the cell clones derived from persistently infected DBT cells with the JHM strain of MHV

PiD-10 and piD-11 cells that have been established from persistently infected DBT cells with the JHM strain of MHV (JHMV) were resistant to infection with JHMV. There was no significant difference in the amount of adsorbed virus among piD-10, piD-11 and DBT cells. When an expression of mRNA of the MHV receptor in piD-10 and piD-11 cells was analyzed by the RT-PCR method, no significant difference was observed in the intensities of the amplified products among piD-10, piD-11 and DBT cells. Treatment of virus-adsorbed cells with PEG, which induces fusion of the cellular membrane with the viral envelop, causes entry of virus particles into cells. There was no significant difference in the yields of virus between PEG-treated and PEG-untreated cells. The titers of infectious virus internalized into piD-10 and piD-11 cells were the same as those in DBT cells. When piD-10 and piD-11 cells were fused with PEG and infected with JHMV, the yields of infectious virion particles from the fused cells between piD-10 and piD-11 cells were significantly lower than those from the fused cells between DBT and piD-10 or piD-11 cells. The present study showed that resistance of piD-10 and piD-11 cells to JHMV infection is not due to an inhibition of JHMV entry into the cells.

CD4+ and CD8+ T cells are not major effectors of mouse hepatitis virus A59-induced demyelinating disease

We examined murine hepatitis virus strain A59 (MHV-A59)-induced demyelinating disease in C57BL/6 mice which had previously been thymectomized at 25 days of age. Demyelination was observed in 51-96% of spinal cord quadrants examined 30 or 60 days post infection (dpi), indicating that neither an intact thymus nor thymic infection is a prerequisite to demyelination. Depletion of CD4+ or CD8+ T cells at 5, 7 or 10 dpi did not influence the extent of demyelination indicating that neither T cell subset is a major effector of demyelination. However, these findings do not exclude the possibility that T cells are involved in initiating demyelinating disease very early in infection.

Enterotropic mouse hepatitis virus

Mouse hepatitis virus [MHV], the coronavirus of the mouse, is the most common viral pathogen in contemporary laboratory mouse colonies throughout the world. It is highly contagious with variable clinical manifestations. The majority of infections are subclinical, but can still significantly influence biological responses, thus interfering with research, mainly in the field of immunology. MHV has been intensively studied from a number of research perspectives and has become the prototype for studying the molecular biology of coronaviruses. MHV contains a single-stranded, positive-sense RNA genome ranging from 27 to 31 kb, which is divided into seven genes. Virions consist of four to five structural proteins. There are many MHV strains that vary in virulence, organotropism and cell tropism, and are constantly evolving by naturally occurring mutation and recombination. Based on pathogenesis studies MHV strains are usually grouped according to their primary tissue tropism into two biotypes: polytropic and enterotropic. Enterotropic strains of MHV replicate in the intestinal mucosa and only rarely spread to other tissues. No morphological structure of the virion has as yet been identified that is responsible for enterotropism. The course of an MHV infection is dependent on the virus strain and host factors. Generally, MHV causes an acute, self-limiting infection which is inapparent in adult mice. Neonates are highly susceptible to disease and show high mortality. In an enzootically infected colony, however, they are protected by maternally derived passive immunity. Detection of MHV infections depends on serological screening of colonies. MHV is controlled by culling and rederivation of the affected colony using hysterectomy or embryo transfer or by elimination through cessation of breeding.

Further observations on coronavirus infection of primate CNS

Previously, we demonstrated that intracerebral (IC) inoculation of a murine coronavirus, MHV-JHM, into two species of primates can result in acute encephalomyelitis (Murray et al., 1992a). Infectious virus isolated from acutely infected animals, designated JHM-OMp1, was inoculated IC into a second group of monkeys. In this report we describe observations on the acutely infected animals and those surviving the acute infection were sacrificed at later times post-infection. Results from dual in situ hybridization/immunohistochemistry screening of tissues show that astrocytes are target cells in white matter lesions during acute infection. In animals sacrificed 150 days post-infection, areas of demyelinated gliotic lesions, prominent in the spinal cord, were seen throughout the neuraxis. No virus products were detected in these late-infection lesions.

Murine coronavirus infection: a paradigm for virus-induced demyelinating disease

A variety of neurological diseases in humans, including multiple sclerosis (MS), have been postulated to have a viral etiology. The use of animal models provides insights into potential mechanism(s) involved in the disease process. The murine coronavirus-induced demyelinating disease in rodents is one such model for demyelinating disease in humans.

Pathogenesis of mouse hepatitis virus-induced demyelination

Infection of rodents with neurotropic mouse hepatitis virus (MHV) may result in lethal encephalitis or paralytic demyelinating disease resembling the human disease multiple sclerosis. The outcome of MHV infection is dependent on a number of variables, including the passage history of the viral isolate, dose and route of inoculation, and the age and immune status of the host. Alterations in surface glycoproteins, especially the spike protein, can profoundly influence pathogenesis. Innate resistance to MHV infection may be related to the expression of cellular receptors or to immunological factors. The immune system plays a major role in MHV pathogenesis, affecting encephalitis, viral clearance, and demyelination. Antiviral antibodies, CD4+ T lymphocytes, or CD8+ T lymphocytes may protect infected animals from lethal encephalitis, but both CD4+ and CD8+ T lymphocytes are required for effective viral clearance. Demyelination in MHV-infected animals has been attributed to the cytolytic effects of viral infection on myelin-producing oligodendrocytes, but more recent evidence supports an immunopathological mechanism for demyelination. Immunopathological models for demyelination include autoimmunity, direct immune cytotoxicity, and indirect 'bystander' damage. Although evidence exists supporting all of these models, the authors favor the bystander demyelination model. Much remains to be revealed about the processes leading to demyelination in MHV-infected mice, and information gained from these investigations may aid in the study of demyelinating disease in humans.

Chronic infection of Balb/c mice with murine herpesvirus 72 is associated with neoplasm development

One hundred Balb/c mice were infected with murine herpesvirus strain 72 (MHV-72) and observed for 2.5 years for neoplasm development and virus presence in tumour as well as non-tumour tissues. Out of 13 neoplasm-bearing mice the virus was recovered from solid tumours (one lymphoma, two non-differentiated lymphoblastomas and two fibrosarcomas) of five mice and from the spleen of one mouse with lymphatic leukemia. The virus persisted frequently also in various organs of the neoplasm-bearing mice.

Spike glycoprotein-mediated fusion in biliary glycoprotein-independent cell-associated spread of mouse hepatitis virus infection

The mouse hepatitis virus (MHV) spike glycoprotein mediates attachment of the virus to the MHV receptor, the murine biliary glycoprotein (BGP) carcinoembryonic antigen. Monoclonal antibody CC1 directed against BGP specifically inhibited infection of DBT, Sac-, GT1-7, and OBL21 cells by wild-type MHV-4 and the neuron-adapted variant OBLV60. Binding to this receptor was necessary to establish infection by cell-free MHV; however, the presence of BGP was not required for infection by cell-associated virus. Cell-associated infectious induced syncytium formation on Vero and BHK cells, which lack murine BGP; this activity was not inhibited by monoclonal antibody CC1. Antibody CC1 also did not prevent syncytium formation on DBT cells, which bear BGP. In infectious center assays, the MHV-4 variant OBLV60, which exhibits acid-dependent fusion, spread to cells lacking BGP only when exposed to acidic media. Therefore, spike-mediated fusion was required for BGP-independent spread of MHV infection. Furthermore, BGP-independent, cell-associated spread of MHV-4 was prevented by monoclonal antibodies 5A13.5 and 5B19.2 directed against the spike glycoprotein, but not by other neutralizing and nonneutralizing anti-spike antibodies. Expression of spike glycoprotein by recombinant vaccinia virus resulted in fusion of BGP-negative cells; monoclonal antibodies 5A13.5 and 5B19.2 strongly inhibited spike-mediated fusion in this assay.

Maternal antibodies protect immunoglobulin deficient neonatal mice from mouse hepatitis virus (MHV)-associated wasting syndrome

PROBLEM

Neonatal mice nursed by dams lacking immunoglobulins (Igs) may often suffer from lethal runting if raised under conventional conditions. The present study was performed in order to clarify a) the cause of the wasting syndrome and b) the protective role of antigen-specific milk antibodies.

METHOD

Ig-deficient mouse embryos in a conventional environment were embryo-transferred to specified pathogen free (SPF) dams. Neonatal growth, mortality, and health status of mice from both environments was recorded. Suspected presence of mouse hepatitis virus (MHV) was tested by RT-PCR. Protective effects on neonatal mortality of milk containing different titers of anti-MHV antibodies were investigated in cross-fostering experiments.

RESULTS

The SPF colony of Ig-deficient mice exhibited no breeding problems, whereas Ig-deficient neonates in the conventional environment suffered from lethal wasting syndrome. Serological screening of the mice kept in the two environments revealed that mice in the conventional room had high titers of antibodies against mouse hepatitis virus. Presence of MHV in runting neonates was confirmed by pathological examinations and RT-nested-PCR using MHV genome specific primers. Milk containing high titers of anti-MHV antibodies, when provided for 8 days or more, completely prevented Ig-deficient neonates from developing wasting syndrome in the conventional environment.

CONCLUSION

These findings show that the neonatal wasting syndrome is associated with the presence of MHV and that neonates are efficiently protected by MHV-specific antibodies in the milk.

Molecular anatomy of mouse hepatitis virus persistence: coevolution of increased host cell resistance and virus virulence

Persistent infection of murine astrocytoma (DBT) cells with mouse hepatitis virus (MHV) has been established. From this in vitro virus-host system, persistence is mediated at the level of cellular MHV receptor (MHVR) expression and increased virus virulence. MHV persistence selects for resistant host cell populations which abate virus replication. Reductions in MHVR expression were significantly associated with increased host resistance, and transfection of MHVR into resistant host cells completely restored the capacity of cells to support efficient replication of MHV strain A59. The emergence of resistant host cells coselected for variant viruses that had increased avidity for MHVR and also recognized different receptors for entry into resistant cells. These data illustrate that MHV persistence in vitro provides a model to identify critical sites of virus-host interaction at the cellular level which are altered during the evolution of host cell resistance to viral infection and the coevolution of virus virulence.

Hepatitogenicity of three plaque purified variants of hepatotropic mouse hepatitis virus, MHV-2 in athymic nude mice

Hepatitogenicity of 3 plaque purified variants of hepatotropic mouse hepatitis virus, MHV-2 were examined in athymic BALB/c-nu/nu mice up to 9 weeks post infection (9WPI). All of the MHV-2S- and MHV-2M-infected mice died with severe acute hepatitis in 3WPI. On the other hand, MHV-2L-infected mice did not die until 9WPI and showed signs of slow-developing chronic hepatitis with persistent infection under low serum virus neutralizing antibody titers. This suggests that MHV-2L-infected athymic nude mice may be useful as a new model of chronic viral hepatitis.

Role of CD4+ and CD8+ T cells in mouse hepatitis virus infection in mice

Viral growth and histopathological changes in the liver after intraperitoneal infection with mouse hepatitis virus, strain JHM were compared among normal C57BL/6 mice, those depleted of CD4+ T cells, CD8+ T cells and both T cell subsets. Viral growth in mice depleted of CD4+ T cells increased slightly, but pathological changes resembled those in normal mice. In contrast, the hepatitis was exacerbated in mice depleted of CD8+ T cells and those depleted of both T cell subsets. These results suggest that CD8+ T cells play a key role although both T cell subsets are involved in protection against mouse hepatitis virus infection in mice.

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.