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

LY6E protects mice from pathogenic effects of murine coronavirus and SARS-CoV-2

LY6E is an antiviral protein that inhibits coronavirus entry. Its expression in immune cells allows mice to control murine coronavirus infection. However, it is not known which immune cell subsets mediate this control or whether LY6E protects mice from SARS-CoV-2. In this study, we used tissue-specific Cre recombinase expression to ablate Ly6e in distinct immune compartments or in all epiblast-derived cells, and bone marrow chimeras to target Ly6e in a subset of radioresistant cells. Mice lacking Ly6e in Lyz2 -expressing cells and radioresistant Vav1 -expressing cells were more susceptible to lethal murine coronavirus infection. Mice lacking Ly6e globally developed clinical disease when challenged with the Gamma (P.1) variant of SARS-CoV-2. By contrast, wildtype mice and mice lacking type I and type III interferon signaling had no clinical symptoms after SARS-CoV-2 infection. Transcriptomic profiling of lungs from SARS-CoV-2-infected wildtype and Ly6e knockout mice revealed a striking reduction of secretory cell-associated genes in infected knockout mice, including Muc5b , an airway mucin-encoding gene that may protect against SARS-CoV-2-inflicted respiratory disease. Collectively, our study reveals distinct cellular compartments in which Ly6e confers cell intrinsic antiviral effects, thereby conferring resistance to disease caused by murine coronavirus and SARS-CoV-2.

Neurotropic Coronavirus Infections

Neurotropic strains of the mouse hepatitis virus (MHV) cause a range of diseases in infected mice ranging from mild encephalitis with clearance of the virus followed by demyelination to rapidly fatal encephalitis. This chapter discusses the structure, life cycle, transmission, and pathology of neurotropic coronaviruses, as well as the immune response to coronavirus infection. Mice infected with neurotropic strains of MHV have provided useful systems in which to study processes of virus- and immune-mediated demyelination and virus clearance and/or persistence in the CNS, and the mechanisms of virus evasion of the immune system.

Brain Invasion by Mouse Hepatitis Virus Depends on Impairment of Tight Junctions and Beta Interferon Production in Brain Microvascular Endothelial Cells

Coronaviruses (CoVs) have shown neuroinvasive properties in humans and animals secondary to replication in peripheral organs, but the mechanism of neuroinvasion is unknown. The major aim of our work was to evaluate the ability of CoVs to enter the central nervous system (CNS) through the blood-brain barrier (BBB). Using the highly hepatotropic mouse hepatitis virus type 3 (MHV3), its attenuated variant, 51.6-MHV3, which shows low tropism for endothelial cells, and the weakly hepatotropic MHV-A59 strain from the murine coronavirus group, we investigated the virus-induced dysfunctions of BBB in vivo and in brain microvascular endothelial cells (BMECs) in vitro. We report here a MHV strain-specific ability to cross the BBB during acute infection according to their virulence for liver. Brain invasion was observed only in MHV3-infected mice and correlated with enhanced BBB permeability associated with decreased expression of zona occludens protein 1 (ZO-1), VE-cadherin, and occludin, but not claudin-5, in the brain or in cultured BMECs. BBB breakdown in MHV3 infection was not related to production of barrier-dysregulating inflammatory cytokines or chemokines by infected BMECs but rather to a downregulation of barrier protective beta interferon (IFN-β) production. Our findings highlight the importance of IFN-β production by infected BMECs in preserving BBB function and preventing access of blood-borne infectious viruses to the brain.

IMPORTANCE

Coronaviruses (CoVs) infect several mammals, including humans, and are associated with respiratory, gastrointestinal, and/or neurological diseases. There is some evidence that suggest that human respiratory CoVs may show neuroinvasive properties. Indeed, the severe acute respiratory syndrome coronavirus (SARS-CoV), causing severe acute respiratory syndrome, and the CoVs OC43 and 229E were found in the brains of SARS patients and multiple sclerosis patients, respectively. These findings suggest that hematogenously spread CoVs may gain access to the CNS at the BBB level. Herein we report for the first time that CoVs exhibit the ability to cross the BBB according to strain virulence. BBB invasion by CoVs correlates with virus-induced disruption of tight junctions on BMECs, leading to BBB dysfunction and enhanced permeability. We provide evidence that production of IFN-β by BMECs during CoV infection may prevent BBB breakdown and brain viral invasion.

A novel full-length isoform of murine pregnancy-specific glycoprotein 16 (psg16) is expressed in the brain but does not mediate murine coronavirus (MHV) entry

The mouse pregnancy-specific glycoprotein 16 (PSG16) has been reported to be an alternative receptor for mouse hepatitis virus (MHV), some strains of which cause encephalitis in mice lacking the canonical receptor CEACAM1a. The known isoforms of PSG16 are N-terminally truncated relative to other PSG family proteins and are expressed in neurons as well as in the placenta. We have cloned a novel full-length isoform of psg16 that is also expressed in the brain, placenta, and retina but, like the truncated form, lacks MHV receptor activity when expressed on 293T cells, suggesting that PSG16 does not mediate CEACAM1a-independent spread of MHV.

Role of the systemic immune system in brain metastasis

Metastatic disease in the central nervous system (CNS) is a cause of increasing mortality amongst cancer patients. As with other types of cancer, cells of the systemic immune system play a range of important roles in the development of metastatic lesions in the CNS, both repressing and promoting tumour growth. Recent advances in immunotherapy have changed the emphasis in cancer treatment away from conventional chemotherapy and radiotherapy for certain tumour types. Despite this, our understanding of systemic immune system involvement in CNS metastases remains poor. The blood-brain barrier prevents the majority of diagnostic and therapeutic agents from crossing into the brain parenchyma until the late stages of metastatic disease. Thus, the development of immunotherapy for CNS pathologies is particularly desirable. This review draws together our current understanding in the relationships between CNS metastases and circulating systemic immune cells. We discuss the roles that circulating systemic immune cells may play in the homing of metastatic cells to the perivascular space, and the pro-metastatic and antagonistic roles that infiltrating systemic immune cells may play at sites of metastasis. This article is part of a Special Issue entitled 'Neuroinflammation in neurodegeneration and neurodysfunction'.

Murine coronavirus receptors are differentially expressed in the central nervous system and play virus strain-dependent roles in neuronal spread

Coronavirus infection of the murine central nervous system (CNS) provides a model for studies of viral encephalitis and demyelinating disease. Mouse hepatitis virus (MHV) neurotropism varies by strain: MHV-A59 causes mild encephalomyelitis and demyelination, while the highly neurovirulent strain JHM.SD (MHV-4) causes fatal encephalitis with extensive neuronal spread of virus. In addition, while neurons are the predominant CNS cell type infected in vivo, the canonical receptor for MHV, the carcinoembryonic antigen family member CEACAM1a, has been demonstrated only on endothelial cells and microglia. In order to investigate whether CEACAM1a is also expressed in other cell types, ceacam1a mRNA expression was quantified in murine tissues and primary cells. As expected, among CNS cell types, microglia expressed the highest levels of ceacam1a, but lower levels were also detected in oligodendrocytes, astrocytes, and neurons. Given the low levels of neuronal expression of ceacam1a, primary neurons from wild-type and ceacam1a knockout mice were inoculated with MHV to determine the extent to which CEACAM1a-independent infection might contribute to CNS infection. While both A59 and JHM.SD infected small numbers of ceacam1a knockout neurons, only JHM.SD spread efficiently to adjacent cells in the absence of CEACAM1a. Quantification of mRNA for the ceacam1a-related genes ceacam2 and psg16 (bCEA), which encode proposed alternative MHV receptors, revealed low ceacam2 expression in microglia and oligodendrocytes and psg16 expression exclusively in neurons; however, only CEACAM2 mediated infection in human 293T cells. Therefore, neither CEACAM2 nor PSG16 is likely to be an MHV receptor on neurons, and the mechanism for CEACAM1a-independent neuronal spread of JHM.SD remains unknown.

Pathogenesis of murine coronavirus in the central nervous system

Murine coronavirus (mouse hepatitis virus, MHV) is a collection of strains that induce disease in several organ systems of mice. Infection with neurotropic strains JHM and A59 causes acute encephalitis, and in survivors, chronic demyelination, the latter of which serves as an animal model for multiple sclerosis. The MHV receptor is a carcinoembryonic antigen-related cell adhesion molecule, CEACAM1a; paradoxically, CEACAM1a is poorly expressed in the central nervous system (CNS), leading to speculation of an additional receptor. Comparison of highly neurovirulent JHM isolates with less virulent variants and the weakly neurovirulent A59 strain, combined with the use of reverse genetics, has allowed mapping of pathogenic properties to individual viral genes. The spike protein, responsible for viral entry, is a major determinant of tropism and virulence. Other viral proteins, both structural and nonstructural, also contribute to pathogenesis in the CNS. Studies of host responses to MHV indicate that both innate and adaptive responses are crucial to antiviral defense. Type I interferon is essential to prevent very early mortality after infection. CD8 T cells, with the help of CD4 T cells, are crucial for viral clearance during acute disease and persist in the CNS during chronic disease. B cells are necessary to prevent reactivation of virus in the CNS following clearance of acute infection. Despite advances in understanding of coronavirus pathogenesis, questions remain regarding the mechanisms of viral entry and spread in cell types expressing low levels of receptor, as well as the unique interplay between virus and the host immune system during acute and chronic disease.

Cytopathy of an infiltrating monocyte lineage during the early phase of infection with murinecoronavirus in the brain

Viral spread during the early stages after infection was compared between a highly neurovirulent mouse hepatitis virus (MHV), JHMV cl-2 strain (cl-2), and its low-virulent mutant, soluble-receptor-resistant (srr)7. The infection of cells with srr7 (soluble-receptor-resistant mutant 7) is dependent on a known MHV receptor (MHVR), carcinoembryonic cell adhesion molecule 1a, whereas cl-2 shows MHVR-independent infection. Initial viral antigens were detected between 12 and 24 h post-inoculation (p.i) in the infiltrating cells that appeared in the subarachnoidal space of mouse brains infected with viruses. There were no significant differences in the intensity or spread of viral antigens in the inflammatory cells between the two viruses. However, 48 h after infection with cl-2, viral antigen-positive cells in the grey matter with the shape of neurons, which do not express MHVR, were detected, while srr7 infection was observed primarily in the white matter. Some of the viral antigen-positive inflammatory cells found in the subarachnoidal space during the early phase of infection reacted with anti-F4/80 or anti-CD11b monoclonal antibodies. Syncytial giant cells (SGCs) expressing viral and CD11b antigens were also detected among these inflammatory cells. These antigen-positive cells appeared in the subarachnoidal space prior to viral antigen spread into the brain parenchyma, indicating that viral encephalitis starts with the infection of infiltrating monocytes which express MHVR. Furthermore, the observation indicates that viral infection has cytopathic effects on the monocyte lineage, which plays a critical role in innate immunity, leading to the rapid spread of viruses during the early stage of infection.

Fine specificity of autoantibodies induced by mouse hepatitis virus A59

We have shown that mice infected with mouse hepatitis virus A59 (MHV-A59) develop autoantibodies (autoAb) to liver and kidney fumarylacetoacetate hydrolase (FAH). The autoAb recognized conformational as well as linear antigenic determinants in the enzyme, and the autoimmune response was not entirely restricted to molecular mimicry and/or epitope spreading. Since the N- and C-terminal portions of the enzyme were the most reactive with autoAb, the fine specificity of these Ab was investigated. Immobilized 15-mer linear peptides (overlapping by 14 amino acids) spanning the N-terminal FAH sequence 1-49 were recognized by Ab from MHV-infected mice. The pattern of reactivity indicated the existence of two major epitope cores (i.e., sequences 9-23 and 30-44), and sequence comparison permitted the identification of two minimal epitopes, DSDFPIQ (amino acids 9-15) and IGDQILD (amino acids 36-42). Mutational analysis of sequences 9-23 and 30-44 indicated that residues 9-12 (DSDF) from the first major N-terminal epitope, and residue 36 (I) from the second, were the key amino acids energetically important for Ab contact. Interestingly, those residues were inside the two minimal epitopes previously predicted. The C-terminal portion of the enzyme (sequence 390-419) presented only one major epitope, located between residues 390 and 409. In this case, the minimal epitope had nine amino acids, CQGDGYRVG, corresponding to the FAH sequence 396-404 that outlines a loop specific for the enzyme. Data indicated that neither the FAH minimal epitopes nor the key residues important for binding to Ab from MHV-infected mice have their counterparts in the viral proteins. However, location of the energetically important residues in the tertiary structure of the enzyme originates a virtual conformational epitope. Such hypothetical B-cell epitopes could be present in any viral protein, originating a cross-reaction leading to the autoimmune response induced by MHV.

The Biology of Persistent Infection: Inflammation and Demyelination following Murine Coronavirus Infection of the Central Nervous System

Multiple Sclerosis (MS) is an immune-mediated demyelinating disease of humans. Although causes of MS are enigmatic, underlying elements contributing to disease development include both genetic and environmental factors. Recent epidemiological evidence has pointed to viral infection as a trigger to initiating white matter damage in humans. Mouse hepatitis virus (MHV) is a positive strand RNA virus that, following intracranial infection of susceptible mice, induces an acute encephalomyelitis that later resolves into a chronic fulminating demyelinating disease. Immune cell infiltration into the central nervous system is critical both to quell viral replication and instigate demyelination. Recent efforts by our laboratory and others have focused upon strategies capable of enhancing remyelination in response to viral-induced demyelination, both by dampening chronic inflammation and by surgical engraftment of remyelination - competent neural precursor cells.

The autoimmune response induced by mouse hepatitis virus A59 is expanded by a hepatotoxic agent

Mouse hepatitis virus strain A59 (MHV-A59) triggers various pathologies in several mouse strains, including hypergammaglobulinaemia, hepatitis and thymus involution. We reported previously the presence of autoantibodies (autoAb) to liver and kidney fumarylacetoacetate hydrolase (FAH) in sera from mice infected with MHV-A59.

Long-term MHV-infected mice represented a good model of non-pathogenic autoimmune response since the animals were apparently healthy in spite of the presence of autoAb. The aim of this work was to see whether a severe liver injury, which releases endogenous adjuvants, i.e. danger signals, could elicit a broader spectrum of autoAb and perhaps signs of autoimmune hepatitis. Carbon tetrachloride (CCl₄) was injected into mice 30 days after MHV infection, and serum was assayed for autoAb and total IgG 20 days later. The association of MHV infection with the toxic effects of CCl₄ resulted in hypergammaglobulinaemia and the production of autoAb to various liver and kidney proteins. Histological examination of liver samples showed tissue damages but without significant differences between the animals submitted to MHV + CCl₄ and controls, which were either infected by MHV without CCl₄, or poisoned by CCl₄ in the absence of MHV infection. Those results show that liver injury after viral infection may lead to the spreading of the immune response and to an increase of serum IgG, suggesting that the procedure used herein could simulate the onset of autoimmune hepatitis.

Mouse hepatitis virus infection of the CNS: a model for defense, disease, and repair

Viral infection of the central nervous system (CNS) results in varied outcomes ranging from encephalitis, paralytic poliomyelitis or other serious consequences. One of the principal factors that directs the outcome of infection is the localized innate immune response, which is proceeded by the adaptive immune response against the invading viral pathogen. The role of the immune system is to contain and control the spread of virus within the CNS, and paradoxically, this response may also be pathological. Studies with a neurotropic murine coronavirus, mouse hepatitis virus (MHV) have provided important insights into how the immune system combats neuroinvasive viruses, and have identified molecular and cellular mechanisms contributing to chronic disease in persistently infected mice.

The spike glycoprotein of murine coronavirus MHV-JHM mediates receptor-independent infection and spread in the central nervous systems of Ceacam1a-/- Mice

The MHV-JHM strain of the murine coronavirus mouse hepatitis virus is much more neurovirulent than the MHV-A59 strain, although both strains use murine CEACAM1a (mCEACAM1a) as the receptor to infect murine cells. We previously showed that Ceacam1a(-/-) mice are completely resistant to MHV-A59 infection (E. Hemmila et al., J. Virol. 78:10156-10165, 2004). In vitro, MHV-JHM, but not MHV-A59, can spread from infected murine cells to cells that lack mCEACAM1a, a phenomenon called receptor-independent spread. To determine whether MHV-JHM could infect and spread in the brain independent of mCEACAM1a, we inoculated Ceacam1a(-/-) mice. Although Ceacam1a(-/-) mice were completely resistant to i.c. inoculation with 10(6) PFU of recombinant wild-type MHV-A59 (RA59) virus, these mice were killed by recombinant MHV-JHM (RJHM) and a chimeric virus containing the spike of MHV-JHM in the MHV-A59 genome (SJHM/RA59). Immunohistochemistry showed that RJHM and SJHM/RA59 infected all neural cell types and induced severe microgliosis in both Ceacam1a(-/-) and wild-type mice. For RJHM, the 50% lethal dose (LD(50)) is <10(1.3) in wild-type mice and 10(3.1) in Ceacam1a(-/-) mice. For SJHM/RA59, the LD(50) is <10(1.3) in wild-type mice and 10(3.6) in Ceacam1a(-/-) mice. This study shows that infection and spread of MHV-JHM in the brain are dependent upon the viral spike glycoprotein. RJHM can initiate infection in the brains of Ceacam1a(-/-) mice, but expression of mCEACAM1a increases susceptibility to infection. The spread of infection in the brain is mCEACAM1a independent. Thus, the ability of the MHV-JHM spike to mediate mCEACAM1a-independent spread in the brain is likely an important factor in the severe neurovirulence of MHV-JHM in wild-type mice.

The peptide specificities of the autoantibodies elicited by mouse hepatitis virus A59

Synthetic decapeptides (N = 206) covering the entire sequence of mouse liver fumarylacetoacetate hydrolase (FAH) were used to analyze the specificities of the autoantibodies (autoAb) elicited towards this enzyme in mice infected with mouse hepatitis virus (MHV). These autoAb bound mainly to N- and C-terminal FAH peptides, the most reactive sequences being 1–50 and 390–420, respectively. Surprisingly, although FAH sequence 1–50 shares a high degree of homology with various MHV proteins, the C-terminal portion does not. Moreover, whereas the autoAb reacted with homologous peptides surrounding residues 70, 160 and 360, non-similar sequences around residues 130, 210, 240, 250, and 300 were also recognized, indicating that autoAb were not restricted to epitopes with sequence homologies. There was also a lack of correlation between the amount of anti-MHV or anti-FAH antibodies produced and the reactivity towards the peptides. Moreover, the spectrum of peptides recognized by the autoAb of a given mouse did not change significantly with time, which suggests that the MHV-elicited autoimmune response does not induce an epitope recognition spreading. Finally, anti-FAH Ab produced after immunization with rat liver FAH recognized essentially the same mouse FAH regions than autoAb from MHV-infected mice. Results indicated that the induction of the autoAb is not only related to molecular or structural mimicry, but rather supports the Danger model, in which any aggression, in this case the MHV infection, is susceptible to trigger the production of autoAb.

Replicative homeostasis II: influence of polymerase fidelity on RNA virus quasispecies biology: implications for immune recognition, viral autoimmunity and other "virus receptor" diseases

Much of the worlds' population is in active or imminent danger from established infectious pathogens, while sporadic and pandemic infections by these and emerging agents threaten everyone. RNA polymerases (RNA_pol) generate enormous genetic and consequent antigenic heterogeneity permitting both viruses and cellular pathogens to evade host defences. Thus, RNA_pol causes more morbidity and premature mortality than any other molecule. The extraordinary genetic heterogeneity defining viral quasispecies results from RNA_pol infidelity causing rapid cumulative genomic RNA mutation a process that, if uncontrolled, would cause catastrophic loss of sequence integrity and inexorable quasispecies extinction. Selective replication and replicative homeostasis, an epicyclical regulatory mechanism dynamically linking RNApol fidelity and processivity with quasispecies phenotypic diversity, modulating polymerase fidelity and, hence, controlling quasispecies behaviour, prevents this happening and also mediates immune escape. Perhaps more importantly, ineluctable generation of broad phenotypic diversity after viral RNA is translated to protein quasispecies suggests a mechanism of disease that specifically targets, and functionally disrupts, the host cell surface molecules – including hormone, lipid, cell signalling or neurotransmitter receptors – that viruses co-opt for cell entry. This mechanism – "Viral Receptor Disease (VRD)" – may explain so-called "viral autoimmunity", some classical autoimmune disorders and other diseases, including type II diabetes mellitus, and some forms of obesity. Viral receptor disease is a unifying hypothesis that may also explain some diseases with well-established, but multi-factorial and apparently unrelated aetiologies – like coronary artery and other vascular diseases – in addition to diseases like schizophrenia that are poorly understood and lack plausible, coherent, pathogenic explanations.

Receptor-independent spread of a highly neurotropic murine coronavirus JHMV strain from initially infected microglial cells in mixed neural cultures

Although neurovirulent mouse hepatitis virus (MHV) strain JHMV multiplies in a variety of brain cells, expression of its receptor carcinoembryonic antigen cell adhesion molecule 1 (CEACAM 1) (MHVR) is restricted only in microglia. The present study was undertaken to clarify the mechanism of an extensive JHMV infection in the brain by using neural cells isolated from mouse brain. In contrast to wild-type (wt) JHMV, a soluble-receptor-resistant mutant (srr7) infects and spreads solely in an MHVR-dependent fashion (F. Taguchi and S. Matsuyama, J. Virol. 76:950-958, 2002). In mixed neural cell cultures, srr7 infected a limited number of cells and infection did not spread, although wt JHMV induced syncytia in most of the cells. srr7-infected cells were positive for GS-lectin, a microglia marker. Fluorescence-activated cell sorter analysis showed that about 80% of the brain cells stained with anti-MHVR antibody (CC1) were also positive for GS-lectin. Pretreatment of those cells with CC1 prevented virus attachment to the cell surface and also blocked virus infection. These results show that microglia express functional MHVR that mediates JHMV infection. As expected, in microglial cell-enriched cultures, both srr7and wt JHMV produced syncytia in a majority of cells. Treatment with CC1 of mixed neural cell cultures and microglia cultures previously infected with wt virus failed to block the spread of infection, indicating that wt infection spreads in an MHVR-independent fashion. Thus, the present study indicates that microglial cells are the major population of the initial target for MHV infection and that the wt spreads from initially infected microglia to a variety of cells in an MHVR-independent fashion.

Sequence similarity and structural homologies are involved in the autoimmune response elicited by mouse hepatitis virus A59

The features of autoantibodies (autoAb) to liver fumarylacetoacetate hydrolase (FAH) elicited in mice infected with mouse hepatitis virus (MHV) were studied by ELISA and western-blot competition assays. All sera tested contained Ab to cryptic FAH epitopes according with results from western-blot tests, whereas ELISA data indicated that some of these same sera did recognize native epitopes of the autoantigen (autoAg). Such differences were detected in individual sera from various mouse strains, and were ascribed to the fact that proteins insolubilized on solid supports expose a variety of conformational and cryptic antigenic determinants.

On the other hand, whereas results from both experimental protocols showed that anti-MHV Ab did not cross-react with the soluble autoAg, the opposite situation did not show analogous results. Thus, binding of autoAb to insolubilized FAH could be inhibited by MHV depending on the mouse serum or the experimental protocol used.

Additionally, a set of synthetic homologous peptides from mouse FAH and various viral proteins was employed to analyze the Ab repertoire of MHV-infected mice. Results indicated that two homologous peptides were recognized by most Ab: the N-terminal sequences (1–10) from FAH and the nucleocapside, both sharing 50% of identity, and sequence 2317–2326 of the RNA polymerase, a peptide showing 30% of identity with FAH 11–20.

Results indicated that MHV-infection triggers at least three distinct Ab populations: anti-MHV, anti-FAH and cross-reacting Ab. This cross-reaction implies either sequential or conformational epitopes from both the viral proteins and the autoAg and may differ between individuals.

Expression of the mouse hepatitis virus receptor by central nervous system microglia

Detection of the mouse hepatitis virus receptor within the central nervous system (CNS) has been elusive. Receptor expression on microglia was reduced during acute infection and restored following immune-mediated virus control. Receptor down regulation was independent of neutrophils, NK cells, gamma interferon, or perforin. Infection of mice devoid of distinct inflammatory cells revealed CD4(+) T cells as the major cell type influencing receptor expression by microglia. In addition to demonstrating receptor expression on CNS resident cells, these data suggest that transient receptor down regulation on microglia aids in establishing persistence in the CNS by assisting virus infection of other glial cell types.

Enhanced virulence mediated by the murine coronavirus, mouse hepatitis virus strain JHM, is associated with a glycine at residue 310 of the spike glycoprotein

The coronavirus, mouse hepatitis virus strain JHM, causes acute and chronic neurological diseases in rodents. Here we demonstrate that two closely related virus variants, both of which cause acute encephalitis in susceptible strains of mice, cause markedly different diseases if mice are protected with a suboptimal amount of an anti-JHM neutralizing antibody. One strain, JHM.SD, caused acute encephalitis, while infection with JHM.IA resulted in no acute disease. Using recombinant virus technology, we found that the differences between the two viruses mapped to the spike (S) glycoprotein and that the two S proteins differed at four amino acids. By engineering viruses that differed by only one amino acid, we identified a serine-to-glycine change at position 310 of the S protein (S310G) that recapitulated the more neurovirulent phenotype. The increased neurovirulence mediated by the virus encoding glycine at position S310 was not associated with a different tropism within the central nervous system (CNS) but was associated with increased lateral spread in the CNS, leading to significantly higher brain viral titers. In vitro studies revealed that S310G was associated with decreased S1-S2 stability and with enhanced ability to mediate infection of cells lacking the primary receptor for JHM ("receptor-independent spread"). These enhanced fusogenic properties of viruses encoding a glycine at position 310 of the S protein may contribute to spread within the CNS, a tissue in which expression of conventional JHM receptors is low.

In vivo expression of major histocompatibility complex molecules on oligodendrocytes and neurons during viral infection

Demyelination in multiple sclerosis and in animal models is associated with infiltrating CD8+ and CD4+ T cells. Although oligodendrocytes and axons are damaged in these diseases, the roles T cells play in the demyelination process are not completely understood. Antigen-specific CD8+ T cell lysis of target cells is dependent on interactions between the T cell receptor and major histocompatibility complex (MHC) class I-peptide complexes on the target cell. In the normal central nervous system, expression of MHC molecules is very low but often increases during inflammation. We set out to precisely define which central nervous system cells express MHC molecules in vivo during infection with a strain of murine hepatitis virus that causes a chronic, inflammatory demyelinating disease. Using double immunofluorescence labeling, we show that during acute infection with murine hepatitis virus, MHC class I is expressed in vivo by oligodendrocytes, neurons, microglia, and endothelia, and MHC class II is expressed only by microglia. These data indicate that oligodendrocytes and neurons have the potential to present antigen to T cells and thus be damaged by direct antigen-specific interactions with CD8+ T lymphocytes.

Identification of two liver proteins recognized by autoantibodies elicited in mice infected with mouse hepatitis virus A59

Western blot experiments showed that sera from mice infected with the mouse hepatitis virus strain A59 (MHV-A59) contained autoantibodies (autoAb) that bound to a 40-kDa protein present in liver and kidney extracts. No reaction was observed with extracts of the heart, muscles, spleen, brain and lung. The Ab cross-reacted with a 40-kDa protein from human, rat and sheep liver, but not with liver extracts from the silver side fish (Odontesthes bonariensis). No correlation was found between the development of the hypergammaglobulinemia that followed the viral infection and the occurrence of the autoAb. Reactive immunoglobulins pertained to the IgG1, IgG2a and IgG2b subclasses, recognized cryptic epitopes and were detected from 10 days up to 8 weeks after MHV-infection. The 40-kDa protein was purified from mouse liver extracts by ion-exchange chromatography, gel filtration and SDS-PAGE. Because the N-terminal was blocked, we digested the protein in-gel with trypsin and sequenced various peptides. Results indicated a 100% homology of sequence between the protein recognized by the autoAb and liver fumarylacetoacetate hydrolase (FAH), the enzyme that mediates the last step of tyrosine catabolism. Additionally, a second protein recognized by the autoAb was detected during FAH purification steps and was identified as liver alcohol dehydrogenase.