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

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.

Immune response to a murine coronavirus: identification of a homing receptor-negative CD4+ T cell subset that responds to viral glycoproteins

The lymphocyte proliferative response to mouse hepatitis virus, strain JHM (MHV-JHM), a well-described cause of chronic and acute neurological infections, has been studied using vaccinia virus recombinants expressing individual MHV proteins. The surface (S) and transmembrane (M) glycoproteins were the most active proteins in causing proliferation of lymphocytes isolated from immunized adult mice, whereas lymphocytes from persistently infected mice proliferated only in response to the S protein. The cells from immunized mice which proliferated most actively in response to MHV were positive for the CD4 antigen and secreted interferon-gamma. In addition, the most responsive subset of cells did not express gp90MEL-14, the lymph node-specific homing receptor. The results identify a subpopulation of CD4+ T cells that may be an important component of the cell-mediated immune response to this virus. The data also suggest that response to the M protein is important in preventing disease progression in C57BL/6 mice since cells which recognize this protein are absent from persistently infected mice.

Maternally-derived passive immunity to enterotropic mouse hepatitis virus

Maternally-derived antibody to enterotropic mouse hepatitis virus (MHV) strain Y was transferred to pups by both intrauterine (IgG) and lactogenic (IgA and IgG) routes. Antibody present in the gastric whey of pups suckling immune dams dropped to undetectable levels by weaning age (21 days post partum). MHV-specific IgG was found in the serum of passively immune pups up to 10 weeks of age. Immune dams transferred equal levels of antibody to 3 consecutive litters of pups, without evidence of decline. Immunoblots showed that IgA and IgG in whey and serum were directed against nucleoprotein N and glycoprotein S. MHV-specific IgM was not detected in any sample.

Passively acquired challenge immunity to enterotropic coronavirus in mice

Maternally-derived passive immunity of infant mice to challenge infection with enterotropic coronavirus mouse hepatitis virus strain Y (MHV-Y) was studied. Pups born to both naive and immune dams, but nursed by naive foster dams, were susceptible to infection, while naive or immune pups nursed by immune foster dams were protected. The MHV infectious dose was identical among naive pups inoculated at 1, 2, 3, or 4 weeks of age. Pups nursing immune dams resisted infection when inoculated at 1, 2, or 3 weeks of age. Three week old pups were protected only if they were allowed access to their immune dams. Pups born to MHV immune dams 4 in consecutive litters acquired equal MHV IgG titers in serum and whey and were all protected against challenge infection. Only pups actively ingesting immune whey at the time of or within two hours after virus inoculation were effectively protected. Pups born to dams immunized by oral inoculation with live MHV acquired both MHV-specific IgA and IgG in their whey, while pups born to dams immunized with killed virus acquired only IgG. Both IgA and IgG, but not IgG alone, were required for complete protection.

Identification of the spinal cord as a major site of persistence during chronic infection with a murine coronavirus

After intranasal inoculation, mouse hepatitis virus (MHV) gains entry into the central nervous system (CNS) via the olfactory and trigeminal nerves. Under the appropriate conditions, some mice develop clinically apparent demyelinating encephalomyelitis several weeks later, with virus always present in the spinal cord. To determine the pathway by which virus reaches the cord, brains and spinal cords of infected, asymptomatic mice were analyzed by in situ hybridization. Viral RNA was always detected in the anterior part of the upper spinal cord. A similar analysis of mice with the recent onset of hindlimb weakness showed that viral RNA was detected in the same location. The results suggest that MHV is transported to the spinal cord via well-defined neuroanatomic pathways and that viral amplification with resultant clinical disease occurs from this site of persistence in the anterior spinal cord. This process of viral amplification may involve the generation of viral variants as has been described for MHV-infected rats. No major changes in viral RNA or protein could be detected when MHV isolated from mice with hindlimb paralysis was analyzed. The data suggest that the generation of viral variants is not important in the pathogenesis of the late onset of neurological disease induced by MHV in mice.

Localization of virus and antibody response in mice infected persistently with MHV-JHM

Suckling mice infected intranasally with MHV-JHM and nursed by immunized dams develop a late onset demyelinating encephalomyelitis. Analysis by in situ hybridization revealed that MHV-JHM entered the central nervous system (CNS) via the olfactory and trigeminal nerves and spread over the next two weeks to the spinal cord, prior to amplification at this site. Serial measurements of neutralizing antibody titers showed that the late onset disease developed in some mice at levels of antibody which protected mice from the fatal, acute encephalitis, supporting the notion that cell-mediated and not humoral immunity is important in protecting mice from MHV-JHM persistence.

T-cell-mediated clearance of mouse hepatitis virus strain JHM from the central nervous system

Clearance of the neurotropic JHM strain of mouse hepatitis virus from the central nervous system was examined by the transfer of spleen cells from immunized donors. A T cell with the surface phenotype of Thy1.2+ CD4+ CD8- asialo-GM1+ Mac-1- was found to be necessary for viral clearance. The surface phenotype and adherence to nylon wool suggest that these cells are activated helper-inducer T cells. Adoptive transfer to congenic histocompatibility strains demonstrated the necessity for compatibility at the D locus of the major histocompatibility complex. The expression of the CD4 surface marker and the requirement for major histocompatibility complex class I were further studied by the transfer of cells to recipients treated with anti-CD4 or anti-CD8 monoclonal antibodies. Treatment of recipients with either the anti-CD8 or the anti-CD4 antibodies inhibited virus clearance from the central nervous system. This suggests that the CD4+ cell acts as a helper and that virus is cleared from the central nervous system. This suggests that the CD4+ cell acts as a helper and that virus is cleared from the central nervous system by CD8+ cells that recognize viral antigen in the context of the H-2Db gene product.

Spread of a neurotropic murine coronavirus into the CNS via the trigeminal and olfactory nerves

The route of entry into the central nervous system (CNS) of most neurtropic viruses has not been established. The coronavirus, mouse hepatitis virus strain JHM (MHV-JHM), causes acute encephalomyelitis and acute and chronic demyelinating diseases and is an important model system for virus-induced neurological disease. Suckling C57BL/6 mice infected intranasally with MHV-JHM develop either the acute encephalomyelitis or a late onset, symptomatic demyelinating encephalomyelitis, depending on whether they are nursed by unimmunized or immunized dams. Analysis by in situ hybridization was used to determine the route of entry of MHV-JHM into the CNS in these mice. At early times, viral RNA was detected only in the trigeminal and olfactory nerves and in their immediate connections in all mice. A few days later, MHV-JHM RNA was found throughout the brain in mice dying of the acute encephalomyelitis, but remained confined to the entry sites in mice which did not develop acute disease. These results suggest that MHV-JHM enters the CNS via an interneuronal route in all mice, but that the presence of maternal antibody prevents the dissemination of virus via extracellular fluid. In addition, MHV-JHM may establish low-level persistence in the trigeminal or olfactory nerve or in one of its connections in mice that do not develop acute encephalomyelitis.

Regional localization of virus in the central nervous system of mice persistently infected with murine coronavirus JHM

Suckling C57BL/6 mice infected with mouse hepatitis virus strain JHM (MHV-JHM) develop either a fatal acute encephalomyelitis or a late onset demyelinating disease, depending on whether they are nursed by unimmunized or immunized dams. To determine the localization of virus-specific RNA, serial sections of brains from infected and uninfected mice were annealed with a 35S-labeled antisense RNA probe and analyzed by film autoradiography. In the mice with acute encephalomyelitis, viral RNA was present in the mesencephalon, hypothalamus, hippocampus, basal ganglia, subcortical white matter, and thalamus. Viral RNA was detected in the spinal cords of all mice with the late onset, demyelinating encephalomyelitis, but was distributed into three different patterns in the brains of these mice, even though all had the same clinical disease. In the first group, viral RNA was detected only in the brainstem. In the second group, viral RNA was detected in the brainstem, thalamus, and cerebral grey matter. This distribution was consistent with viral spread along well-defined tracts connecting these parts of the brain. In the third group, viral RNA could be detected both in the brainstem and in several white matter tracts within close physical proximity to the optic chiasm. This distribution was consistent with viral spread by an extracellular route from one white matter tract to other tracts which were physically close, but which were not part of the same pathways. These results suggest that MHV-JHM spreads through the central nervous system both along well-defined neuronal pathways and by spread from contiguous structures, but also suggest that viral replicates preferentially in a limited number of areas of the brain. The technique of in situ hybridization with film autoradiography should be generally useful for analyzing macroscopic movements of virus within infected organs.

Mouse hepatitis virus and host determinants of vertical transmission and maternally-derived passive immunity in mice

Transmission of mouse hepatitis virus (MHV) in utero following oronasal inoculation of pregnant mice was found to depend upon MHV strain and host genotype. Virulent, polytropic MHV-JHM was recovered from multiple maternal tissues, including liver and uterus, as well as placenta and fetus in susceptible BALB/cByJ mice. Fetuses were infected during all 3 trimesters of pregnancy. Low virulence, polytropic MHV-S infected fetuses in a low percentage of susceptible BALB/cByJ dams. Infection of resistant CD-1 mice with MHV-JHM was limited, with no fetal infection. Enterotropic MHV-Y was largely restricted to intestine of BALB/cByJ and CD-1 dams, with minimal dissemination and no fetal infection. Maternally-derived MHV IgG antibody was detectable in pup sera through 4 weeks of age. Antibody titers were generally lower in second litters of the same dam. Cross-fostering experiments showed that antibody was transferred via colostrum and not in utero, and that pups were capable of absorption through 2 weeks of age. Pups nursing immune dams were protected against MHV challenge at 1 and 2 weeks of age, compared to pups nursing naive dams. Immunity to MHV challenge was cross-protective against both antigenically homotypic and heterotypic strains of MHV.

Late onset, symptomatic, demyelinating encephalomyelitis in mice infected with MHV-JHM in the presence of maternal antibody

The presence of maternal antibodies protected suckling C57BL/6 mice from the clinical manifestations of the acute encephalomyelitis caused by mouse hepatitis virus, strain JHM (MHV-JHM), a coronavirus, even though histological evidence of encephalomyelitis was found at early times after inoculation. 100% of infected suckling mice developed a fatal disease in the absence of maternal antibody. By 14 days after inoculation, the brains of all antibody-protected mice examined were nearly normal on histological examination. At 3-8 weeks post-inoculation, approximately 40% of the antibody-protected mice developed a neurological disease characterized by hindlimb paralysis and wasting. Evidence of inflammation and demyelination was apparent in the spinal cord and brainstem. The mice that remained asymptomatic at this time showed few signs of inflammation and none developed clinical disease over the following 9 months. Viral antigen could be detected in most of the mice examined at all times after inoculation, whether symptomatic or not, and was particularly evident in the animals with hindlimb paralysis. MHV-JHM could be consistently cultured from the mice with hindlimb paralysis. These results show that maternal immune factors can completely protect susceptible mice from the acute, fatal, clinical encephalomyelitis caused by MHV-JHM, but cannot prevent the establishment of a latent state and subsequent development of virus-induced, clinically evident, demyelinating disease. This model will be useful for studying the virus and host factors important for the development of MHV-JHM latency and subsequent virus-induced demyelination.

Maternal antibody-modulated MHV-JHM infection in C57BL/6 and BALB/c mice

1. Maternal antibody protected C57BL/6 and BALB/c suckling mice from the acute, fatal encephalomyelitis caused by MHV-JHM. 2. 40% of the C57BL/6 mice and 25% of the BALB/c mice which were protected by maternal antibody developed neurological disease days to weeks later. Although the clinical syndromes developed by the two different strains were different, in both cases the mice developed a demyelinating encephalomyelitis with fewer inflammatory changes present in the grey matter. 3. Presence or absence of neutralizing antibody in the sera of maternal antibody-protected C57BL/6 mice did not correlate with the development of clinically evident neurological disease. 4. Infectious virus could only be isolated from C57BL/6 mice with neurological disease, although viral antigen could be detected in most mice whether symptomatic or not. 5. This model should be useful for the study of the viral and immune factors important in MHV-induced viral demyelination.