JHM infections in rats as a model for acute and subacute demyelinating disease

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

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

Induction of protective immunity against coronavirus-induced encephalomyelitis: evidence for an important role of CD8+ T cells in vivo

Coronavirus MHV-JHM infections of rats provide useful models to study the pathogenesis of virus-induced central nervous system disease. To analyze the role of the immune response against defined MHV-JHM antigens, we tested the protective efficacy of vaccinia virus (VV) recombinants expressing either the nucleocapsid (N) or the spike (S) protein. A strong protection was mediated in animals by immunization with recombinant VV encoding a wild-type S protein (VV-Swildtype), whereas VV recombinant expressing a mutant S354CR protein (VV-S354CR) had no protective effect. Recombinant VV encoding N protein (VV-N) induces a humoral and a CD4+ T cell response, but did not prevent acute disease regardless of the immunization protocol. In these experiments, challenge with an otherwise lethal dose of MHV-JHM was performed prior to the induction of virus-neutralizing antibodies and studies with the anti-CD8+ monoclonal antibody. MRC OX8 showed that elimination of the CD8+ subset of T cells abrogates the protective effect. This result indicates that CD8+ T cells primed by recombinant VV expressing wild-type S protein are a primary mechanism of immunological defense against MHV-JHM infection in rats.

Neurovirulence of six different murine coronavirus JHMV variants for rats

Six variant viruses of the JHMV strain of murine coronavirus with large (cl-2, CNSV, DL and DS) or small (sp-4 and JHM-X) S proteins were compared in terms of their relative neurovirulence in weanling Lewis rats. Inoculation of various doses of the variants revealed that the cl-2 and CNSV were highly virulent and DL and DS were low-virulent, while sp-4 and JHM-X were avirulent. Pathological examination of rats infected with variants cl-2, DL and sp-4 showed that the cl-2 and DL induced severe and mild acute encephalomyelitis, respectively, while no lesions were observed in the central nervous system of rats infected with sp-4. Virus growth and distribution of antigen in rat brains correlated strongly with neurovirulence. These results suggest that S protein plays a role in neurovirulence in rats. In addition, these variant viruses were shown to be useful tools for further analysis of JHMV neurovirulence in animals as well as in cultured cells.

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

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

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

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

Increased levels of myelin basic protein transcripts in virus-induced demyelination

In multiple sclerosis, a demyelinating disease of young adults, there is a paucity of myelin repair in the central nervous system (CNS) which is necessary for the restoration of fast saltatory conduction in axons. Consequently, this relapsing disease often causes marked disability. In similar diseases of small rodents, however, remyelination can be quite extensive, as in the demyelinating disease caused by the A59 strain of mouse hepatitis virus (MHV-A59), a coronavirus of mice. To investigate when and where oligodendrocytes are first triggered to repair CNS myelin in such disease, we have used a complementary DNA probe specific for one major myelin protein gene, myelin basic protein (MBP), which hybridizes with the four forms of MBP messenger RNA in rodents. Using Northern blot and in situ hybridization techniques, we previously found that MBP mRNA is first detected at about 5 days after birth, peaks at 18 days and progressively decreases to 25% of the peak levels in the adult. We now report that in spinal cord sections of adult animals with active demyelination and inflammatory cells, in situ hybridization reveals a dramatic increase in probe binding to MBP-specific mRNA at 2-3 weeks after virus inoculation and before remyelination can be detected by morphological methods. This increase of MBP-specific mRNA is found at the edge of the demyelinating area and extends into surrounding areas of normal-appearing white matter. Thus, in situ hybridization with myelin-specific probes appears to be a useful method for detecting the timing, intensity and location of myelin protein gene reactivation preceding remyelination. This method could be used to elucidate whether such a reactivation occurs in multiple sclerosis brain tissue. Our results suggest that in mice, glial cells react to a demyelinating process with widespread MBP mRNA synthesis which may be triggered by a diffusible factor released in the demyelinated areas.

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

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

Influence of maternal immunity on the outcome of murine coronavirus JHM infection in suckling mice

Adult C3H mice are resistant to intraperitoneal infection with murine coronavirus JHM, whereas suckling offspring of non-immune females are susceptible. Resistance can be conferred on suckling C3H mice by postnatal transmission of maternal immunity, if transfer precedes infection. Suckling mice succumb to infection even when they receive maternal antibodies within 1 day after infection. Prenatal transmission alone without subsequent postnatal transmission of maternal immunity is not sufficient to provide resistance. Persistence of virus without clinical consequences was observed when the supply of breast milk anti-JHMV antibodies was terminated 5 days before infection. Immune reactions restricted by histocompatibility antigens do not play a crucial role in bestowing resistance. As neutralizing anti-JHM serum antibody titers of adult mice only rise sharply 5 to 7 days after infection, these results indicate that infection of adults can be arrested by immunological means but that, in addition, the rate of virus dissemination must be limited by other non-immunological mechanisms.