Mouse hepatitis virus-induced recurrent demyelination. A preliminary report

Coronaviruses and their relationship with multiple sclerosis: is the prevalence of multiple sclerosis going to increase after the Covid-19 pandemia?

The purpose of this review is to examine whether there is a possible (etiological/triggering) relationship between infection with various Coronaviruses, including Severe Acute Respiratory Syndrome-related Coronavirus-2 (SARS-CoV-2), the virus responsible for the Coronavirus disease-19 (Covid-19) pandemia, and Multiple Sclerosis (MS), and whether an increase of the prevalence of MS after the current Covid-19 pandemia should be expected, examining new and preexisting data. Although the exact pathogenesis of MS remains unknown, environmental agents seem to greatly influence the onset of the disease, with viruses being the most popular candidate. Existing data support this possible etiological relationship between viruses and MS, and experimental studies show that Coronaviruses can actually induce an MS-like demyelinating disease in animal models. Findings in MS patients could also be compatible with this coronaviral MS hypothesis. More importantly, current data from the Covid-19 pandemia show that SARS-CoV-2 can trigger autoimmunity and possibly induce autoimmune diseases, in the Central Nervous System as well, strengthening the viral hypothesis of MS. If we accept that Coronaviruses can induce MS, it is reasonable to expect an increase in the prevalence of MS after the Covid-19 pandemia. This knowledge is of great importance in order to protect the aging groups that are more vulnerable against autoimmune diseases and MS specifically, and to establish proper vaccination and health policies.

Neurological manifestations of coronavirus infections, before and after COVID-19: a review of animal studies

Coronavirus disease 2019 (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This virus, which was first identified in December 2019 in China, has resulted in a yet ongoing viral pandemic. Coronaviridae could potentially cause several disorders in a wide range of hosts such as birds and mammals. Although infections caused by this family of viruses are predominantly limited to the respiratory tract, Betacoronaviruses are potentially able to invade the central nervous system (CNS) as well as many other organs, thereby inducing neurological damage ranging from mild to lethal in both animals and humans. Over the past two decades, three novel CoVs, SARS-CoV-1, MERS-CoV, and SARS-CoV-2, emerging from animal reservoirs have exhibited neurotropic properties causing severe and even fatal neurological diseases. The pathobiology of these neuroinvasive viruses has yet to be fully known. Both clinical features of the previous CoV epidemics (SARS-CoV-1 and MERS-CoV) and lessons from animal models used in studying neurotropic CoVs, especially SARS and MERS, constitute beneficial tools in comprehending the exact mechanisms of virus implantation and in illustrating pathogenesis and virus dissemination pathways in the CNS. Here, we review the animal research which assessed CNS infections with previous more studied neurotropic CoVs to demonstrate how experimental studies with appliable animal models can provide scientists with a roadmap in the CNS impacts of SARS-CoV-2. Indeed, animal studies can finally help us discover the underlying mechanisms of damage to the nervous system in COVID-19 patients and find novel therapeutic agents in order to reduce mortality and morbidity associated with neurological complications of SARS-CoV-2 infection.

Viral mouse models used to study multiple sclerosis: past and present

Multiple sclerosis (MS) is a common inflammatory demyelinating disease of the central nervous system. Although the etiology of MS is unknown, genetics and environmental factors, such as infections, play a role. Viral infections of mice have been used as model systems to study this demyelinating disease of humans. Three viruses that have long been studied in this capacity are Theiler’s murine encephalomyelitis virus, mouse hepatitis virus, and Semliki Forest virus. This review describes the viruses themselves, the infection process, the disease caused by infection and its accompanying pathology, and the model systems and their usefulness in studying MS.

FTY720 (fingolimod) modulates the severity of viral-induced encephalomyelitis and demyelination

Background

FTY720 (fingolimod) is the first oral drug approved by the Food and Drug Administration for treatment of patients with the relapsing-remitting form of the human demyelinating disease multiple sclerosis. Evidence suggests that the therapeutic benefit of FTY720 occurs by preventing the egress of lymphocytes from lymph nodes thereby inhibiting the infiltration of disease-causing lymphocytes into the central nervous system (CNS). We hypothesized that FTY720 treatment would affect lymphocyte migration to the CNS and influence disease severity in a mouse model of viral-induced neurologic disease.

Methods

Mice were infected intracranially with the neurotropic JHM strain of mouse hepatitis virus. Infected animals were treated with increasing doses (1, 3 and 10 mg/kg) of FTY720 and morbidity and mortality recorded. Infiltration of inflammatory virus-specific T cells (tetramer staining) into the CNS of FTY720-treated mice was determined using flow cytometry. The effects of FTY720 treatment on virus-specific T cell proliferation, cytokine production and cytolytic activity were also determined. The severity of neuroinflammation and demyelination in FTY720-treated mice was examined by flow cytometry and histopathologically, respectively, in the spinal cords of the mice.

Results

Administration of FTY720 to JHMV-infected mice resulted in increased clinical disease severity and mortality. These results correlated with impaired ability to control viral replication (P < 0.05) within the CNS at days 7 and 14 post-infection, which was associated with diminished accumulation of virus-specific CD4+ and CD8+ T cells (P < 0.05) into the CNS. Reduced neuroinflammation in FTY720-treated mice correlated with increased retention of T lymphocytes within draining cervical lymph nodes (P < 0.05). Treatment with FTY720 did not affect virus-specific T cell proliferation, expression of IFN-γ, TNF-α or cytolytic activity. FTY720-treated mice exhibited a reduction in the severity of demyelination associated with dampened neuroinflammation.

Conclusion

These findings indicate that FTY720 mutes effective anti-viral immune responses through impacting migration and accumulation of virus-specific T cells within the CNS during acute viral-induced encephalomyelitis. FTY720 treatment reduces the severity of neuroinflammatory-mediated demyelination by restricting the access of disease-causing lymphocytes into the CNS but is not associated with viral recrudescence in this model.

Infections and multiple sclerosis

Multiple sclerosis is a chronic inflammatory condition of unknown cause. Increasing evidence suggests that the disease develops as a result of interactions between the environment and the immune system in genetically susceptible individuals. It has long been recognized that infections may serve as environmental triggers for the disease, and a large number of pathogens have been proposed to be associated with multiple sclerosis. Here, we detail the historical basis linking infections to multiple sclerosis and review the epidemiology of the disease, which suggests a possible relationship with infectious agents. We also describe pathophysiologic studies in animals and other human demyelinating diseases that have demonstrated a variety of mechanisms by which infectious agents may induce chronic, relapsing central nervous system disease with myelin damage and relative preservation of axons, similar to multiple sclerosis. In addition, we discuss recent studies in individuals with multiple sclerosis indicating enhanced immune responses to infectious antigens, though not consistently demonstrating evidence for ongoing infection. Taken together, these studies suggest a role for infectious agents in the development of multiple sclerosis. Conclusive evidence, however, remains lacking.

Cell replacement therapies to promote remyelination in a viral model of demyelination

Persistent infection of the central nervous system (CNS) of mice with the neuroadapted JHM strain of mouse hepatitis (MHV) is characterized by ongoing demyelination mediated by inflammatory T cells and macrophages that is similar both clinically and histologically with the human demyelinating disease multiple sclerosis (MS). Although extensive demyelination occurs in mice persistently infected with MHV there is only limited remyelination. Therefore, the MHV model of demyelination is a relevant model for studying disease and evaluating therapeutic approaches to protect cells of the oligodendrocyte lineage and promote remyelination. This concept is further highlighted as the etiology of MS remains enigmatic, but viruses have long been considered as potential triggering agents in initiating and/or maintaining MS symptoms. As such, understanding mechanisms associated with promoting repair within the CNS in the context of a persistent viral infection is critical given the possible viral etiology of MS. This review focuses on recent studies using either mouse neural stem cells (NSCs) or human oligodendrocyte progenitor cells (OPCs) derived from human embryonic stem cell (hESC) to promote remyelination in mice persistently infected with MHV. In addition, the potential role for chemokines in positional migration of transplanted cells is addressed.

Induction of transcription factor Egr-1 gene expression in astrocytoma cells by Murine coronavirus infection

Mouse hepatitis virus (MHV) causes encephalitis and demyelination in the central nervous system (CNS) of susceptible rodents. Astrocytes are one of the major targets for MHV infection in the CNS, and respond to MHV infection by expressing diverse molecules that may contribute to CNS pathogenesis. Here we characterized the activation of an immediate-early transcription factor Egr-1 by MHV infection in an astrocytoma cell line. We found that the expression of Egr-1 was dramatically increased following virus infection. Using various inhibitors of mitogen-activated protein kinases, we identified that the extracellular signal-regulated kinases 1/2 were involved in the activation of Egr-1 transcription by MHV infection. Experiments with ultraviolet light-inactivated virus revealed that the induction of Egr-1 did not require virus replication and was likely mediated during cell entry. We further found that over-expression of Egr-1 suppressed the expression of BNip3, a pro-apoptotic member of the Bcl-2 family. This finding may provide an explanation for our previously observed down-regulation of BNip3 by MHV infection in astrocytoma cells (Cai, Liu, Yu, and Zhang, Virology 316:104-115, 2003). Furthermore, knockdown of Egr-1 by an siRNA inhibited MHV propagation, suggesting the biological relevance of Egr-1 induction to virus replication. In addition, the persistence/demylinating-positive strains (JHM and A59) induced Egr-1 expression, whereas the persistence/demylinating-negative strain (MHV-2) did not. These results indicate a correlation between the ability of MHVs to induce Egr-1 expression and their ability to cause demyelination in the CNS, which may suggest a potential role for the induction of Egr-1 in viral pathogenesis.

Viral induced demyelination

Viral induced demyelination, in both humans and rodent models, has provided unique insights into the cell biology of oligodendroglia, their complex cell-cell interactions and mechanisms of myelin destruction. They illustrate mechanisms of viral persistence, including latent infections in which no infectious virus is readily evident, virus reactivation and viral-induced tissue damage. These studies have also provided excellent paradigms to study the interactions between the immune system and the central nervous system (CNS). Although of interest in their own right, an understanding of the diverse mechanisms used by viruses to induce demyelination may shed light into the etiology and pathogenesis of the common demyelinating disorder multiple sclerosis (MS). This notion is supported by the persistent view that a viral infection acquired during adolescence might initiate MS after a long period of quiescence. Demyelination in both humans and rodents can be initiated by infection with a diverse group of enveloped and non-enveloped RNA and DNA viruses (Table 1). The mechanisms that ultimately result in the loss of CNS myelin appear to be equally diverse as the etiological agents capable of causing diseases which result in demyelination. Although demyelination can be a secondary result of axonal loss, in many examples of viral induced demyelination, myelin loss is primary and associated with axonal sparing. This suggests that demyelination induced by viral infections can result from: 1) a direct viral infection of oligodendroglia resulting in cell death with degeneration of myelin and its subsequent removal; 2) a persistent viral infection, in the presence or absence of infectious virus, resulting in the loss of normal cellular homeostasis and subsequent oligodendroglial death; 3) a vigorous virus-specific inflammatory response wherein the virus replicates in a cell type other than oligodendroglia, but cytokines and other immune mediators directly damage the oligodendroglia or the myelin sheath; or 4) infection initiates activation of an immune response specific for either oligodendroglia or myelin components. Virus-induced inflammation may be associated with the processing of myelin or oligodendroglial components and their presentation to the host's own T cell compartment. Alternatively, antigenic epitopes derived from the viral proteins may exhibit sufficient homology to host components that the immune response to the virus activates autoreactive T cells, i.e. molecular mimicry. Although it is not clear that each of these potential mechanisms participates in the pathogenesis of human demyelinating disease, analysis of the diverse demyelinating viral infections of both humans and rodents provides examples of many of these potential mechanisms.

Histopathology in Coronavirus-Induced Demyelination

The experimental model system of coronavirus mouse hepatitis virus (MHV) induced demyelination in 4–6 week old C57Bl/6 or Balb/c mice exhibits a biphasic disease and two distinct forms of virus-induced demyelination. During the acute phase of the disease MHV infection causes acute encephalitis, and some strains of virus cause also hepatitis. Infection with the JHM strain of MHV causes severe panencephalitis, whereas MHV-A59 causes mild to moderate encephalitis involving specific limbic and limbic related areas of the brain and brain stem. The target cells are neurons and glia including oligodendrocytes. Demyelination during the acute stage is due to cytolytic infection of oligodendrocytes. After two weeks, the disease process enters a chronic stage of immune-mediated demyelination, in the presence of high levels of anti-viral antibodies and persistent low levels viral RNA in glial cells, without detectable levels of infectious virus or viral antigens.

Virus demyelination

A number of viruses can initiate central nervous system (CNS) diseases that include demyelination as a major feature of neuropathology. In humans, the most prominent demyelinating diseases are progressive multifocal leukoencephalopathy, caused by JC papovirus destruction of oligodendrocytes, and subacute sclerosing panencephalitis, an invariably fatal childhood disease caused by persistent measles virus. The most common neurological disease of young adults in the developed world, multiple sclerosis, is also characterized by lesions of inflammatory demyelination; however, the etiology of this disease remains an enigma. A viral etiology is possible, because most demyelinating diseases of known etiology in both man and animals are viral. Understanding of the pathogenesis of virus-induced demyelination derives for the most part from the study of animal models. Studies with neurotropic strains of mouse hepatitis virus, Theiler's virus, and Semliki Forest virus have been at the forefront of this research. These models demonstrate how viruses enter the brain, spread, persist, and interact with immune responses. Common features are an ability to infect and persist in glial cells, generation of predominantly CD8(+) responses, which control and clear the early phase of virus replication but which fail to eradicate the infection, and lesions of inflammatory demyelination. In most cases demyelination is to a limited extent the result of direct virus destruction of oligodendrocytes, but for the most part is the consequence of immune and inflammatory responses. These models illustrate the roles of age and genetic susceptibility and establish the concept that persistent CNS infection can lead to the generation of CNS autoimmune responses.

Murine hepatitis virus--a model for virus-induced CNS demyelination

Most murine hepatitis virus (MHV) strains, as their name suggests, infect the liver. However, several murine strains are tropic for the central nervous system (CNS) and cause encephalitis with subsequent CNS demyelination. The CNS demyelination shares pathological similarities with human CNS demyelinating diseases such as multiple sclerosis (MS). These viruses are, therefore, used to study the role of the immune system in viral clearance from the CNS, in CNS demyelination, and in remyelination. Nevertheless, it is still unclear exactly how MHV induces demyelination and to what extent the immune system plays a role in this pathology. Here we review this field in the context of the immune response to MHV in the liver and the CNS focusing on studies that have been published in the past 5 years.

Mouse hepatitis virus strain JHM infects a human hepatocellular carcinoma cell line

Mouse hepatitis virus (MHV) strain JHM is a coronavirus that causes encephalitis and demyelination in susceptible rodents. The known receptors for MHV are all members of the carcinoembryonic antigen family. Although human forms of the MHV receptor can function as MHV receptors in some assays, no human cell line has been identified that can support wild-type MHV infection. Here we describe the infection of a human hepatocellular carcinoma cell line, HuH-7, with MHV. HuH-7 cells were susceptible to strains JHM-DL and JHM-DS, yielding virus titers nearly identical to those seen in mouse DBT cells. In contrast, HuH-7 cells were only marginally susceptible or completely resistant to infection by other MHV strains, including A59. JHM produced a strong cytopathic effect in HuH-7 cells with the formation of round plaques. Studies of various recombinant viruses between JHM and A59 strains suggested that the ability of JHM to infect HuH-7 cells was determined by multiple viral genetic elements. Blocking the viral spike (S) protein with a neutralizing antibody or a soluble form of the MHV receptor inhibited infection of HuH-7 cells, suggesting that infection is mediated through the S protein. Transfection with the prototype mouse receptor, biliary glycoprotein, rendered HuH-7 cells susceptible to infection by other MHV strains as well, suggesting that JHM uses a receptor distinct from the classical MHV receptor to infect HuH-7 cells. Possible implications for human disease are discussed.

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.

Spontaneous and induced remyelination in multiple sclerosis and the Theiler's virus model of central nervous system demyelination

Remyelination in the central nervous system, originally thought to occur rarely, if ever, is now an established phenomena in multiple sclerosis patients. However, the extent of myelin repair is incomplete and limited. Experimental models of central nervous system demyelination provide an opportunity to study the cellular and molecular events involved in remyelination. These models may provide some clue to why remyelination in multiple sclerosis is incomplete as well as suggest potential methods to stimulate central nervous system repair. In this review we examine the morphological aspects of central nervous system remyelination and discuss both spontaneous and induced remyelination in multiple sclerosis and experimental models of central nervous system demyelination. We give special emphasis to the Theiler's virus model of central nervous system demyelination and its usefulness to identify therapeutic agents to promote remyelination. The role of immunoglobulins in promoting remyelination in both the Theiler's model system and in multiple sclerosis is discussed. Finally, we examine the potential physiological role of demyelination and remyelination and its relationship with clinical manifestations of central nervous system disease.

Persistence of viral RNA in the central nervous system of mice inoculated with MHV-4

In order to study the role that viral persistence may play in chronic central nervous system (CNS) disease induced by murine coronaviruses, we have used the reverse transcriptase-polymerase chain reaction (RT-PCR) to study viral RNA in the brains of mice after intracerebral inoculation of JHM virus (JHMV or MHV-4). Quantitative RT-PCR showed that JHMV RNA decreased from approximately 2 ng/ug total brain RNA at day 6 post-inoculation (PI) to 0.1 pg/ug total brain RNA at 360 days PI. Double-stranded viral RNA could be detected up to day 20 PI. By the selective use of upstream or downstream primers during the RT step, it was possible to measure negative sense and positive sense JHMV RNA respectively, and we found that there was a marked rise in the ratio of positive to negative sense JHMV RNA after day 13 PI. Analysis of amplified products by dideoxy DNA sequencing showed that the characteristic mutation of our input virus (at position 3340 of gene 3) is maintained to at least day 42 PI. Taken together, these results favor a model of JHMV persistence in vivo in which viral RNA is present as double stranded forms initially and predominantly as single stranded, positive sense forms at late timepoints. Further analysis of this model in quantitative terms may contribute to our understanding of the biological significance of coronavirus persistence in the CNS.

Coronavirus JHM OMP1 pathogenesis in owl monkey CNS and coronavirus infection of owl monkey CNS via peripheral routes

Two separate studies are described in this report. First, 5 Owl monkeys were inoculated intracerebrally (IC) with coronavirus JHM OMP1; this virus isolate was cultured from the brain of an animal inoculated with uncloned MHV JHM. Two of the animals became neurological impaired and were sacrificed; these animals had developed severe encephalomyelitis as previously described. Two of the remaining 3 healthy animals were inoculated IC again at 90 days post-inoculation (DPI) and all 3 were sacrificed approximately 5 months after the first virus inoculation. Despite the lack of detectable infectious virus, viral RNA and antigen, all 3 animals had significant white matter inflammation and areas of demyelination in the spinal cord. In the second study 4 Owl monkeys were inoculated intranasally (IN) and ocularly and 4 inoculated intravenously (i.v.) with JHM OMP1. The animals were sacrificed between 16 and 215 DPI with 2 IN and 2 i.v. animals receiving a second i.v. inoculum at 152 DPI. Viral RNA and/or antigen was detected in the brains of all animals and the distribution corresponded to areas of inflammation and edema. One of the animals that received the second inoculum developed neurological impairment and subsequent analysis of tissues showed viral antigen in both brain and spinal cord. Viral products were predominantly found in blood vessels suggesting hematogenous spread with entry into the central nervous system (CNS) through endothelium.

Coronavirus infects and causes demyelination in primate central nervous system

Two species of primates, Owl and African green monkeys, were inoculated intracerebrally with either the neurotropic mouse hepatitis virus JHM or the putative multiple sclerosis brain coronavirus isolate SD. These viruses caused an acute to subacute panencephalitis and/or demyelination in the infected animals. The course of pathogenesis and sites of detected viral RNA and antigen was dependent both on animal species and virus strain but the results clearly showed that these viruses replicated and disseminated in the central nervous system (CNS) of these primates. This study suggests that human CNS may be susceptible to coronavirus infection.

Coronavirus induced primary demyelination: indications for the involvement of a humoral immune response

Coronavirus MHV-JHM infection of rodents can result in demyelinating encephalomyelitis. We analysed histological changes induced by coronavirus MHV-JHM infection in Lewis rats. Besides an acute disease (AE), chronic panencephalitis (CPE) and subacute demyelinating encephalomyelitis (SDE) were induced. These disease types were differentiated by the incubation period, the localization of lesions, the type of tissue damage and distribution of virus antigen. In AE and CPE, virus antigen was detected in neurons, astrocytes and oligodendrocytes, whereas in SDE neurons lacked virus antigen. Viral nucleocapsid protein (N) was present in the cytoplasm and the spike protein (S) was displayed on the surface of infected neural cells. However, expression of S protein relative to N protein was severely impaired in SDE lesions. Quantitative analysis of infiltrating inflammatory cells revealed that the number of macrophages and T cells were similar in lesions of AE, CPE and SDE. In contrast to that, SDE lesions contained a significantly higher number of IgG + B cells and plasma cells. In addition active demyelinating SDE lesions displayed an enhanced IgG content and deposits of complement C9. These results indicate that virus induced primary demyelination could be a consequence of antibody mediated cytotoxicity. Furthermore, a reduction in the number of cells producing spike protein in the chronic forms of the disease indicates down-regulation of this protein, possibly mediated by anti-S antibodies.

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.

The astrocyte is a target cell in mice persistently infected with mouse hepatitis virus, strain JHM

Mouse hepatitis virus, strain JHM (MHV-JHM), causes a late onset, clinically apparent, demyelinating encephalomyelitis in 40% of suckling C57BL/6 mice born to immunized dams. Suckling mice born to unimmunized dams rapidly succumb to an acute encephalomyelitis. MHV-JHM can be isolated from the brains and spinal cords of maternal antibody-protected mice when the late onset disease becomes clinically apparent, showing that the virus must be present in these mice when they are still asymptomatic. To determine which cells of the central nervous system (CNS) were potential reservoirs for the virus during the asymptomatic period, tissue sections were assayed simultaneously by immunoperoxidase and immunofluorescence staining for the presence of viral antigen and for glial fibrillary acidic protein (GFAP), a marker for astrocytes. The results indicate that 20% (range 0-52%) of the MHV-JHM infected cells in asymptomatic mice were astrocytes. In mice symptomatic with late onset hindlimb paralysis, a higher percentage of infected cells were astrocytes. These results indicate that astrocytes are a target cell in both symptomatic and asymptomatic mice persistently infected with MHV-JHM, and suggest that the astrocyte is a potential cellular reservoir for MHV-JHM in asymptomatic mice.

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.

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.

Effects of regional spinal X-irradiation on demyelinating disease caused by Theiler's virus, mouse hepatitis virus or experimental allergic encephalomyelitis

The effects of X-irradiation on the course of chronic demyelinating disease were examined in mice with experimental allergic encephalitis (EAE), mouse hepatitis virus (MHV) or Theiler's virus (DAV) infection. One month after the induction of EAE or 2-16 months after inoculation of DAV, exposure of the cervical spinal cord to 20 Gy X-rays caused local exacerbation of disease activity but spinal irradiation did not affect MHV-induced demyelination. In EAE, there was a significant increase in the number of inflammatory cells in the irradiated part of the cord. Mice infected with DAV showed locally increased demyelination and axonal degeneration but no change in the titer of infectious virus within the cord. Thus in DAV infection, as in EAE, the exacerbation of disease seemed to be due to vascular or immunological factors rather than viral reactivation.

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.

In vivo and in vitro models of demyelinating diseases. XV. Differentiation influences the regulation of coronavirus infection in primary explants of mouse CNS

Mouse oligodendrocytes and astrocytes, in primary cerebral explant cultures, were infected with JHMV and MHV3 coronaviruses. Contrary to previous findings with neural cells from the rat (S. Beushausen and S. Dales, 1985, Virology 141, 89-101), these agents show no discrimination in the tropism and have the ability to replicate in either type of murine glial cell. Effects of the differentiation inducer dbcAMP on levels of the myelinspecific enzyme 2':3'-cyclic nucleotide-3'-phosphohydrolase (CNPase) activity and virus replication were determined. In the mouse system there was a gradual, continuous elevation of CNPase beyond 30 days whereas in comparable rat cell cultures maximum CNPase enhancement is elicited within 21 days (F. A. McMorris, 1983, J. Neurochem. 41, 506-515). After dbcAMP treatment replication of both coronaviruses was profoundly suppressed in murine oligodendrocytes, consistent with our findings on JHMV replication in treated rat oligodendrocytes. By contrast the replication of JHMV and MHV3 in dbcAMP-treated murine astrocytes was influenced only marginally. These findings provide further support for the hypothesis that susceptibility of rodents to CNS infection by coronaviruses is determined, in part, by the age-related maturation process of oligodendrocytes.

High-frequency RNA recombination of murine coronaviruses

The RNA genome of coronaviruses consists of a single species of nonsegmented RNA. In this communication, we demonstrate that the RNA genomes of different strains of murine coronaviruses recombine during mixed infection at a very high frequency. Susceptible cells were coinfected with a temperature-sensitive mutant of one strain of mouse hepatitis virus (MHV) and a wild-type virus of a different strain. Of 21 randomly isolated viruses released from the coinfected cells at the nonpermissive temperature, 2 were recombinants which differed in the site of recombination. After three serial passages of the original virus pool derived from the mixed infection, the majority of the progeny viruses were recombinants. These recombinant viruses represented at least five different recombination sites between the two parental MHV strains. Such a high-frequency recombination between nonsegmented RNA genomes of MHV suggests that segmented RNA intermediates might be generated during MHV replication. We propose that the RNA replication of MHV proceeds in a discontinuous and nonprocessive manner, thus generating free segmented RNA intermediates, which could be used in RNA recombination via a copy-choice mechanism.

Virus-induced central positional nystagmus in mice

Geotropic direction-changing nystagmus in lateral body positions was observed in 4-week-old BALB/c mice after intracerebral injection with a temperature-sensitive mutant of mouse hepatitis virus. The positional nystagmus was detected already 2 days after infection and it lasted half a year at least. The nystagmic responses of the semicircular canals were also evaluated before and after infection. They were unaltered during the disease, which was clinically manifested by general weakness, ataxia and tremor. Histopathological examination 2 weeks after infection revealed demyelination in various parts of the CNS.

Viruses and demyelinating disease of the central nervous system

In MS, there are many mechanisms by which viruses can produce demyelinating diseases in humans and experimental demyelinating infections in animals.

Analysis of genomic and intracellular viral RNAs of small plaque mutants of mouse hepatitis virus, JHM strain

The genomic RNA and intracellular RNA of mouse hepatitis virus, strain JHM (MHV-JHM) and two plaque mutants (1a and 2c), which have been isolated from a persistently infected culture (JHM-CC), have been analyzed by T1-resistant oligonucleotide finger-printing. The genomic RNA of the virus population (JHM-CC virus) released from different passage levels of the same persistent infection has also been analyzed. The analysis shows the locations within the genomic and intracellular RNAs of more than 45 T1-resistant oligonucleotides and confirm earlier studies (J. L. Leibowitz, K. C. Wilhelmsen, and C. W. Bond (1981), Virology 114, 39-51), showing that the six subgenomic RNAs of MHV-JHM form a 3' coterminal nested set which extends for different lengths in a 5' direction. The analysis also identifies in each subgenomic RNA those large T1 oligonucleotides derived from noncontiguous regions of the genome during mRNA synthesis. Two important conclusions can be reached from analysis of the mutant viruses. First, the virus population released from the persistent infection represents a fairly constant mixture of viruses, and the fluctuating emergence of variants as predominant species in the culture does not occur. Second, the data indicate that for particular intracellular RNAs of mutant viruses the sequence rearrangements occurring during subgenomic mRNA synthesis are different from those in the corresponding intracellular RNA of wild-type virus. The result may indicate a potential flexibility in the leader/body fusion process that has not been previously recognized.

Relapsing subacute demyelinating encephalomyelitis in rats during the course of coronavirus JHM infection

Temperature-sensitive mutants of the murine coronavirus JHM induced a subacute demyelinating encephalomyelitis (SDE) in young rats. Neurological symptoms were associated with marked lesions of primary demyelination in the white matter of the central nervous system (CNS), and developing after an incubation time of several weeks to months. Many rats survived this infection and recovered completely from this CNS disease. Among 43 survivors of SDE, 9 rats developed a relapse 27-153 days after onset of the first attack. Neuropathological examination of these animals revealed areas of fresh demyelination together with old remyelinated lesions. Viral antigens were detectable in the neighbourhood of fresh lesions and in some cases infectious virus was re-isolated from rats revealing low antibody titers to JHM virus. These results demonstrate that mutants of JHM virus can induce a relapsing demyelinating disease process, associated with a persistent infection, which possesses some similarities to chronic experimental allergic encephalomyelitis.

Virological and immunological aspects of coronavirus induced subacute demyelinating encephalomyelitis in rats

Infection of rats with the murine coronavirus JHM led to acute or subacute encephalitis. Viral and host factors greatly influenced the outcome of the infection. A number of temperature-sensitive (ts) mutants was obtained which differed widely in their capacity to induce lesions of the central nervous system (CNS) in rats. Under defined conditions a subacute demyelinating encephalomyelitis ( SDE ) with pronounced clinical signs was observed 14-160 days post infection (p.i.). A number of rats, which showed a remission of SDE later developed a relapse of the disease accompanied by neurological symptoms. Neuropathological examination of such animals revealed lesions of active demyelination and extended remyelinated areas. The presence of viral antigen or infectious virus in the CNS of these rats demonstrated that they were persistently infected. Further investigations indicated that this virus infection triggers a cell mediated immune response against basic myelin protein which may contribute to the development of subacute to chronic encephalomyelitides .

Biological and macromolecular properties of murine cells persistently infected with MHV-JHM

A persistently-infected neuroblastoma culture [Neuro-2A( JHMV )] was established with the murine hepatitis virus JHM [MHV-JHM]. After 100 days of passage, the endogenous virus [Neuro-2A( JHMV ) end] released by this culture was unable to induce the syncytia typical of MHV-JHM and the endogenous virus was not temperature-sensitive. The Neuro-2A( JHMV ) culture was cured of virus production by passage under neutralizing antibody [Neuro-2A( JHMV )Ab]. The Neuro-2A( JHMV ) and the Neuro-2A ( JHMV ) Ab cultures were as susceptible to heterologous infection with mengovirus and vesicular stomatitis virus as the uninfected Neuro-2A culture. However, the Neuro-2A ( JHMV ) and Neuro-2A( JHMV ) Ab cultures were partially resistant to homologous superinfection by MHV-JHM and the closely related MHV-A59. Virus related to MHV-JHM was rescued from the antibody-cured cells by cell fusion. The synthesis of MHV-JHM specific antigens by Neuro-2A( JHMV ) cells, Neuro-2A( JHMV ) Ab cells and 17 Cl-1 cells infected by Neuro-2A( JHMV ) end was studied by SDS-PAGE. The genomic RNAs of MHV-JHM and Neuro-2A( JHMV ) end were compared by oligonucleotide mapping. The results of the protein and RNA studies indicated that the genome of Neuro-2A( JHMV ) end was substantially modified from the genome of MHV-JHM, but the modifications did not significantly alter the molecular size of the viral-specific proteins.

Ross River virus--induced demyelination: II. Ultrastructural studies

Focal central nervous system demyelination is a prominent feature of Ross River virus encephalitis in mice. The present ultrastructural study shows that oligodendrocytes are a primary site of viral replication. The earliest myelin disruption occurs in association with an inflammatory infiltrate composed primarily of polymorphonuclear leukocytes, which are later replaced by macrophages. Viral particles are found in oligodendrocytes, selected neuronal populations, macrophages, and polymorphonuclear leukocytes through the end of the first week of infection as macrophages remove myelin from normal-appearing axons. Between the second and third weeks of infection, axons within foci of demyelination partially remyelinate with central myelin. Schwann cells are not found within regions of central remyelination. Cyclophosphamide treatment does not prevent or delay demyelination or remyelination. Results of this and previous studies strongly suggest that Ross River virus--induced demyelination is not immune mediated but rather the direct result of viral infection of oligodendrocytes.

Coronavirus JHM-induced demyelinating encephalomyelitis in rats: influence of immunity on the course of disease

Disease processes of the central nervous system (CNS) accompanied by demyelination may be the result of a viral infection or the consequence of an immunopathological reaction directed against myelin. In acute viral infections, the infection of oligodendroglial cells, leading to cell destruction, may be the main mechanism for inducing this neuropathological lesion. In the case of a persistent virus infection in oligodendroglia cells, however, it is conceivable that functional impairment of oligodendroglia cells, or the induction of an immune reaction to the agent that may cross-react with brain antigens, could eventually cause demyelination. Therefore, pathogenic studies on subacute or chronic demyelinating encephalomyelitides in association with viral infections may provide information on the mechanisms involved in demyelination. In rats, depending on the biological property of the virus material used, the genetic background and immune response of the host, a subacute or late demyelinating encephalomyelitis can be induced, accompanied by primary demyelination. This provides a model for analysis of the virus and host factors interacting in the pathogenesis of these diseases.

Characterization of small plaque mutants of mouse hepatitis virus, JHM strain

Two small plaque mutants designated as 1a and 2c were isolated from DBT cells persistently infected with the JHM strain of mouse hepatitis virus. Unlike the wild type JHM, these two mutant viruses grew more slowly with no prominent cell fusion. The buoyant densities of the mutants were slightly lower and 2c was revealed to have fewer peplomers than JHM by electron microscopy. The purified JHM contained five polypeptides with molecular weights (M.W.) of 260,000, 105,000 (GP105), 65,000, 60,000 (P60), and 23,000 (GP23). In addition to two polypeptides, P60 and GP23, which were common to JHM and the mutants, 1a was found to contain three other specific polypeptides with M.W. of 180,000 (GP180), 110,000, and 95,000 (GP95), while 2c had GP180, GP105, GP95, and one with a M.W. of 175,000. All of these polypeptides were shown to be glycosylated except for P60. After bromelain treatment, all these viruses lost the peplomers and contained P60 and another new 18,000 dalton polypeptide.

Cell tropism and expression of mouse hepatitis viruses (MHV) in mouse spinal cord cultures

Mouse hepatitis viruses (MHV) are coronaviruses which cause various infections in mice affecting lung, intestine, liver, and other organs as well as the central nervous system. The replication of three different MHV strains was studied in mouse dissociated spinal cord cultures containing differentiated neurons and nonneuronal cells (NN) (including astrocytes). Cell tropism and maturation of each virus strain was analyzed by immunolabeling methods using antisera to the virion or to purified membrane glycoproteins (E₁ and E₂) and by electron microscopy (EM). Wt-JHM, which causes acute encephalitis in mice, produces acute cytopathic changes in both neurons and NN cells. In neurons, virions mature in smooth ER cisternae closely associated to the Golgi apparatus. As judged by EM, fewer virions are produced by neurons than NN cells and neurons do not fuse or stain for E₂ as do NN cells. NN cells contain large inclusions made of nucleocapsid strands. A temperature-sensitive mutant of JHM, Ts8-JHM, which causes demyelination in mice, infects NN cells but not neurons. Infected NN cells synthesize E₁ and E₂, and contain large inclusions but few mature virions, even at permissive temperatures. These inclusions appear granular and rarely contain nucleocapsid strands in contrast to wt-JHM infection. NN cells infected with this mutant also display numerous membrane whorls. The hepatotropic strain A59 lacks tropism for neurons and primarily infects NN cells, thus resembling ts8-JHM. Infected NN cells become loaded with intracytoplasmic virions which are secreted from the cells. E₁ can only be detected in the perinuclear area of these cells while E₂ rapidly spreads throughout the cytoplasm. The cytoplasm of A59 infected NN cells frequently contains large tubular structures often in the lumen of the RER. In conclusion, in primary CNS cultures consisting of neurons and NN cells: (1) wt-JHM replicates in both neurons and NN cells but has different effects on these cells; (2) Ts8-JHM exhibits no productive infection of neurons, and in NN cells appears to be defective in assembly and to stimulate membrane synthesis; (3) A59 also shows tropism restricted to NN cells which produce many viruses and display differential distribution of the two virion glycoproteins. Thus, in the absence of the immune system, the MHV strains assayed exhibit differences in viral tropism, cytopathic changes, and viral assembly in CNS cells, and these differences may account for the different disease patterns.

Macrophage antiviral activity: extrinsic versus intrinsic activity

Peritoneal exudate cells from strains of mice both resistant and susceptible to challenge with mouse hepatitis virus strain JHM were examined for extrinsic and intrinsic antiviral activity. Thioglycolate-elicited and resident peritoneal cells from uninfected mice were able to suppress viral growth in a permissive cell. The active cell in both populations is an adherent, radiation-resistant, Thy-1.2 antigen- and Ia antigen-negative cell. The suppression of virus replication was not related to nonspecific cellular cytotoxicity directed against the permissive host cell, and no interferon was detected. The expression of extrinsic antiviral activity was not related to the ability of the host to resist mouse hepatitis virus infection by virtue of either age or genetic background. The expression of intrinsic antiviral activity, on the other hand, correlated with the ability of the host to resist virus challenge, indicating a characteristic distinction between these two in vitro mechanisms of macrophage-mediated antiviral activity with regard to host resistance to viral infection. Further, the ability of a macrophage to support viral replication itself was independent of the ability of the macrophage to suppress virus growth in another cell.

Experimental models of virus-induced demyelination of the central nervous system

One of the arguments in favor of a viral pathogenesis for multiple sclerosis is the existence of several experimental and natural animal models of virus-induced primary demyelination. This review deals comprehensively with such models. Well-known examples of demyelinating viral infections in their natural host are JHM, Theiler, visna, and canine distemper encephalomyelitides. Recent reports of experimental murine infections with pathogens such as vesicular stomatitis, Chandipura, herpes simplex, Venezuelan equine encephalomyelitis, and Semliki Forest viruses are also discussed. The thrust of the review is to include viral models suspected of producing primary demyelination on an immunopathological basis.

Virus persistence and recurring demyelination produced by a temperature-sensitive mutant of MHV-4

Mouse hepatitis virus type 4 (MHV-4, the JHM strain), a positive-strand RNA virus of the coronavirus family, is well documented as an inducer of acute and chronic demyelination in mice, as well as subacute demyelination in rats, due to a cytolytic infection of oligodendrocytes. However, experiments to explore the role of virus and host factors in the production of chronic or recurrent demyelinating disease have been limited because MHV-4 usually produces demyelination in conditions that frequently induce a fatal necrotizing encephalomyelitis. To circumvent this problem, we had made and selected mutant viruses that caused both a high incidence of demyelination and a low incidence of encephalitis-induced mortality. One such mutant, designated ts8, consistently caused acute demyelinating disease in over 90% of intracerebrally or intranasally (natural route of infection) inoculated, 4-5 week-old mice from several susceptible strains within 6-10 days. In addition, ts8 typically did not cause fatal necrotizing encephalitis, showing a low mortality (less than 5%). This reflected a unique tropism of ts8 for oligodendrocytes, but a limited one for neuronal cells. We now report that ts8 is also useful for inducing persistent infection of the mouse central nervous system (CNS). The histopathological correlate of this infection is chronic recurrent demyelination, and virus can be demonstrated ultrastructurally in intact oligodendrocytes, in the vicinity of demyelinated areas.

Coronavirus 229E susceptibility in man-mouse hybrids is located on human chromosome 15

Human coronavirus 229E, n enveloped, RNA-containing virus, causes respiratory illness in man and is serologically related to murine coronavirus JHM, which causes acute and chronic demyelination in rodents. 229E displays a species-specific host range restriction whose genetic basis was studied in human-mouse hybrids. 229E replicated in human WI-38 cells but not in three mouse cell lines tested (RAG, LM/TK-, and A9). Human coronavirus sensitivity (HCVS) was expressed as a dominant phenotype in hybrids, indicating that mouse cells do not actively suppress 229E replication. HCVS segregated concordantly with the human chromosome 15 enzyme markers mannose phosphate isomerase (MPI) and the muscle form of pyruvate kinase (PKM2), and analysis of hybrids containing an X/15 translocation [t(X;15)(p11;q11)] localized HCVS to the q11 leads to qter region of chromosome 15. HCVS might code for a specific surface receptor, allowing 229E to be absorbed to and received within the host cell.

Ependymitis, leukoencephalitis, hydrocephalus, and thrombotic vasculitis following chronic infection by mouse hepatitis virus 3 (MHV 3)

Mouse hepatitis virus 3 (MHV 3) is either avirulent (resistant mice), hepatotropic (susceptible mice), or neurotropic (semisusceptible mice), depending on the strain of mice infected. In semisusceptible mice, infection led first to a transient meningitis, ependymitis, and leukoencephalitis, followed by a permanent communicating hydrocephalus and, later on, to a chronic thrombotic vasculitis affecting meningeal and parenchymal vessels at the brain stem level. Small foci of ischemic necrosis related to vascular occlusions were seen in the dorsal brain stem. Cyclophosphamide treatment of semisusceptible mice significantly reduced the meningeal infiltrates but did not prevent the development of hydrocephalus and other neuropathologic changes. Identical lesions occurred in fully susceptible mice infected with a low dose of virus, but no neurologic disorder could be induced in genetically resistant mice even following immunosuppression or intracranial inoculation. The leukoencephalitis differed from the demyelinating lesions observed with MHV 4. Vascular lesions were of particular interest. More attention should be given to the possibility of virus induced chronic cerebral vasculitis in man.