Analysis of murine coronavirus surface glycoprotein functions by using monoclonal antibodies

Peroxiredoxin-3 plays a neuroprotective role in early brain injury after experimental subarachnoid hemorrhage in rats

Subarachnoid hemorrhage (SAH), a type of hemorrhagic stroke, is a neurological emergency associated with a high morbidity and mortality rate. After SAH, early brain injury (EBI) is the leading cause of poor prognosis in SAH patients. Peroxiredoxins (PRDXs) are a family of sulphhydryl-dependent peroxidases. Peroxiredoxin-3 (PRDX3) is mainly located in the mitochondria of neurons, which can remove hydrogen peroxide (H2O2); however, the effect of PRDX3 on EBI after SAH remains unclear. In this study, an endovascular perforation model was used to mimic SAH in Sprague Dawley rats in vivo. The results revealed that after SAH, PRDX3 levels decreased in the neurons. PRDX3 overexpression by neuron-specific adeno-associated viruses upregulated PRDX3 levels. Furthermore, PRDX3 overexpression improved long- and short-term behavioral outcomes and alleviated neuronal impairment in rats. Nissl staining revealed that the upregulation of PRDX3 promoted cortical neuron survival. PRDX3 overexpression decreased the H2O2 content and downregulated caspase-9 expression. In conclusion, PRDX3 participates in neuronal protection by inhibiting the neuronal mitochondria-mediated death pathway; PRDX3 may be an important target for EBI intervention after SAH.

Reduction of Cell Fusion by Deletion in the Hypervariable Region of the Spike Protein of Mouse Hepatitis Virus

Deletions in the spike gene of mouse hepatitis virus (MHV) produce several variants with diverse biological characteristics, highlighting the significance of the spike gene in viral pathogenesis. In this study, we characterized the JHM-X strain, which has a deletion in the hypervariable region (HVR) of the spike gene, compared with the cl-2 strain, which has a full spike gene. Cytopathic effects (CPEs) induced by the two strains revealed that the size of the CPE produced by cl-2 is much greater than that produced by JHM-X in delayed brain tumor (DBT) cells. Thus, this finding explains the greater fusion activity of cl-2 than JHM-X in cultured cells, and we speculate that the deletion region of the spike protein is involved in the fusion activity differences. In contrast with the fusion activity, a comparison of the virus growth kinetics revealed that the titer of JHM-X was approximately 100 times higher than that of cl-2. We found that the deletion region of the spike protein was involved in fusion activity differences, whereas cl-2 produced significantly higher luciferase activity than JHM-X upon similar expression levels of the spike protein. However, the reason behind the growth difference is still unknown. Overall, we discovered that deletion in the HVR of the spike gene could be involved in the fusion activity differences between the two strains.

A conserved immunogenic and vulnerable site on the coronavirus spike protein delineated by cross-reactive monoclonal antibodies

The coronavirus spike glycoprotein, located on the virion surface, is the key mediator of cell entry and the focus for development of protective antibodies and vaccines. Structural studies show exposed sites on the spike trimer that might be targeted by antibodies with cross-species specificity. Here we isolated two human monoclonal antibodies from immunized humanized mice that display a remarkable cross-reactivity against distinct spike proteins of betacoronaviruses including SARS-CoV, SARS-CoV-2, MERS-CoV and the endemic human coronavirus HCoV-OC43. Both cross-reactive antibodies target the stem helix in the spike S2 fusion subunit which, in the prefusion conformation of trimeric spike, forms a surface exposed membrane-proximal helical bundle. Both antibodies block MERS-CoV infection in cells and provide protection to mice from lethal MERS-CoV challenge in prophylactic and/or therapeutic models. Our work highlights an immunogenic and vulnerable site on the betacoronavirus spike protein enabling elicitation of antibodies with unusual binding breadth.

Epitope-resolved profiling of the SARS-CoV-2 antibody response identifies cross-reactivity with endemic human coronaviruses

The SARS-CoV-2 proteome shares regions of conservation with endemic human coronaviruses (CoVs), but it remains unknown to what extent these may be cross-recognized by the antibody response. Here, we study cross-reactivity using a highly multiplexed peptide assay (PepSeq) to generate an epitope-resolved view of IgG reactivity across all human CoVs in both COVID-19 convalescent and negative donors. PepSeq resolves epitopes across the SARS-CoV-2 Spike and Nucleocapsid proteins that are commonly targeted in convalescent donors, including several sites also recognized in some uninfected controls. By comparing patterns of homologous reactivity between CoVs and using targeted antibody-depletion experiments, we demonstrate that SARS-CoV-2 elicits antibodies that cross-recognize pandemic and endemic CoV antigens at two Spike S2 subunit epitopes. We further show that these cross-reactive antibodies preferentially bind endemic homologs. Our findings highlight sites at which the SARS-CoV-2 response appears to be shaped by previous CoV exposures and which have the potential to raise broadly neutralizing responses.

PepSeq enables fully in vitro, highly multiplexed peptide-based antibody assays

Epitope mapping shows preexisting antibody reactivity to SARS-CoV-2 antigens

Antibodies cross-recognize endemic and pandemic antigens in the Spike S2 subunit

Cross-reactive antibodies raised by SARS-CoV-2 preferentially bind endemic homologs

Particulate multivalent presentation of the receptor binding domain induces protective immune responses against MERS-CoV

Middle East respiratory syndrome coronavirus (MERS-CoV) is a WHO priority pathogen for which vaccines are urgently needed. Using an immune-focusing approach, we created self-assembling particles multivalently displaying critical regions of the MERS-CoV spike protein ─fusion peptide, heptad repeat 2, and receptor binding domain (RBD) ─ and tested their immunogenicity and protective capacity in rabbits. Using a "plug-and-display" SpyTag/SpyCatcher system, we coupled RBD to lumazine synthase (LS) particles producing multimeric RBD-presenting particles (RBD-LS). RBD-LS vaccination induced antibody responses of high magnitude and quality (avidity, MERS-CoV neutralizing capacity, and mucosal immunity) with cross-clade neutralization. The antibody responses were associated with blocking viral replication and upper and lower respiratory tract protection against MERS-CoV infection in rabbits. This arrayed multivalent presentation of the viral RBD using the antigen-SpyTag/LS-SpyCatcher is a promising MERS-CoV vaccine candidate and this platform may be applied for the rapid development of vaccines against other emerging viruses such as SARS-CoV-2.

Epitope-resolved profiling of the SARS-CoV-2 antibody response identifies cross-reactivity with an endemic human CoV

A high-resolution understanding of the antibody response to SARS-CoV-2 is important for the design of effective diagnostics, vaccines and therapeutics. However, SARS-CoV-2 antibody epitopes remain largely uncharacterized, and it is unknown whether and how the response may cross-react with related viruses. Here, we use a multiplexed peptide assay ('PepSeq') to generate an epitope-resolved view of reactivity across all human coronaviruses. PepSeq accurately detects SARS-CoV-2 exposure and resolves epitopes across the Spike and Nucleocapsid proteins. Two of these represent recurrent reactivities to conserved, functionally-important sites in the Spike S2 subunit, regions that we show are also targeted for the endemic coronaviruses in pre-pandemic controls. At one of these sites, we demonstrate that the SARS-CoV-2 response strongly and recurrently cross-reacts with the endemic virus hCoV-OC43. Our analyses reveal new diagnostic and therapeutic targets, including a site at which SARS-CoV-2 may recruit common pre-existing antibodies and with the potential for broadly-neutralizing responses.

Targeting Glycoproteins as a therapeutic strategy for diabetes mellitus and its complications

OBJECTIVES

Glycoproteins are organic compounds formed from proteins and carbohydrates, which are found in many parts of the living systems including the cell membranes. Furthermore, impaired metabolism of glycoprotein components plays the main role in the pathogenesis of diabetes mellitus. The aim of this study is to investigate the influence of glycoprotein levels in the treatment of diabetes mellitus.

METHODS

All relevant papers in the English language were compiled by searching electronic databases, including Scopus, PubMed and Cochrane library. The keywords of glycoprotein, diabetes mellitus, glycan, glycosylation, and inhibitor were searched until January 2019.

RESULTS

Glycoproteins are pivotal elements in the regulation of cell proliferation, growth, maturation and signaling pathways. Moreover, they are involved in drug binding, drug transportation, efflux of chemicals and stability of therapeutic proteins. These functions, structure, composition, linkages, biosynthesis, significance and biological effects are discussed as related to their use as a therapeutic strategy for the treatment of diabetes mellitus and its complications.

CONCLUSIONS

The findings revealed several chemical and natural compounds have significant beneficial effects on glycoprotein metabolism. The comprehension of glycoprotein structure and functions are very essential and inevitable to enhance the knowledge of glycoengineering for glycoprotein-based therapeutics as may be required for the treatment of diabetes mellitus and its associated complications. Graphical abstract.

Tectonic conformational changes of a coronavirus spike glycoprotein promote membrane fusion

The tremendous pandemic potential of coronaviruses was demonstrated twice in the past few decades by two global outbreaks of deadly pneumonia. The coronavirus spike (S) glycoprotein initiates infection by promoting fusion of the viral and cellular membranes through conformational changes that remain largely uncharacterized. Here we report the cryoEM structure of a coronavirus S glycoprotein in the postfusion state, showing large-scale secondary, tertiary, and quaternary rearrangements compared with the prefusion trimer and rationalizing the free-energy landscape of this conformational machine. We also biochemically characterized the molecular events associated with refolding of the metastable prefusion S glycoprotein to the postfusion conformation using limited proteolysis, mass spectrometry, and single-particle EM. The observed similarity between postfusion coronavirus S and paramyxovirus F structures demonstrates that a conserved refolding trajectory mediates entry of these viruses and supports the evolutionary relatedness of their fusion subunits. Finally, our data provide a structural framework for understanding the mode of neutralization of antibodies targeting the fusion machinery and for engineering next-generation subunit vaccines or inhibitors against this medically important virus family.

Two-step conformational changes in a coronavirus envelope glycoprotein mediated by receptor binding and proteolysis

The coronaviruses mouse hepatitis virus type 2 (MHV-2) and severe acute respiratory syndrome coronavirus (SARS-CoV) utilize proteases to enter host cells. Upon receptor binding, the spike (S) proteins of both viruses are activated for membrane fusion by proteases, such as trypsin, present in the environment, facilitating virus entry from the cell surface. In contrast, in the absence of extracellular proteases, these viruses can enter cells via an endosomal pathway and utilize endosomal cathepsins for S protein activation. We demonstrate that the MHV-2 S protein uses multistep conformational changes for membrane fusion. After interaction with a soluble form of the MHV receptor (CEACAM1a), the metastable form of S protein is converted to a stable trimer, as revealed by mildly denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Liposome-binding assays indicate that the receptor-bound virions are associated with the target membrane through hydrophobic interactions. The exposure of receptor-bound S protein to trypsin or cathepsin L (CPL) induces the formation of six-helix bundles (6HB), the final conformation. This trypsin- or CPL-mediated conversion to 6HB can be blocked by a heptad repeat peptide known to block the formation of 6HB. Although trypsin treatment enabled receptor-bound MHV-2 to enter from the cell surface, CPL failed to do so. Interestingly, consecutive treatment with CPL and then chlorpromazine enabled a portion of the virus to enter from cell surface. These results suggest that trypsin suffices for the induction of membrane fusion of receptor-primed S protein, but an additional unidentified cellular factor is required to trigger membrane fusion by CPL.

Organ-specific attenuation of murine hepatitis virus strain A59 by replacement of catalytic residues in the putative viral cyclic phosphodiesterase ns2

The Murine hepatitis virus (MHV) strain A59 ns2 protein is a 30-kDa nonstructural protein that is expressed from a subgenomic mRNA in the cytoplasm of virus-infected cells. Its homologs are also encoded in other closely related group 2a coronaviruses and more distantly related toroviruses. Together, these proteins comprise a subset of a large superfamily of 2H phosphoesterase proteins that are distinguished by a pair of conserved His-x-Thr/Ser motifs encompassing catalytically important residues. We have used a vaccinia virus-based reverse genetic system to produce recombinant viruses encoding ns2 proteins with single-amino-acid substitutions in, or adjacent to, these conserved motifs, namely, inf-ns2 H46A, inf-ns2 S48A, inf-ns2-S120A, and inf-ns2-H126R. All of the mutant viruses replicate in mouse 17 clone 1 fibroblast cells and mouse embryonic cells to the same extent as the parental wild-type recombinant virus, inf-MHV-A59. However, compared to inf-MHV-A59, the inf-ns2 H46A and inf-ns2-H126R mutants are highly attenuated for replication in mouse liver following intrahepatic inoculation. Interestingly, none of the mutant viruses were attenuated for replication in mouse brain following intracranial inoculation. These results show that the ns2 protein of MHV-A59 has an important role in virus pathogenicity and that a substitution of the histidine residues of the MHV-A59 ns2 His-x-Thr/Ser motifs is critical for virus virulence in the liver but not in the brain. This novel phenotype suggests a strategy to investigate the function of the MHV-A59 ns2 protein involving the search for organ-specific proteins or RNAs that react differentially to wild-type and mutant ns2 proteins.

Sequence changes of infectious bronchitis virus isolates in the 3' 7.3 kb of the genome after attenuating passage in embryonated eggs

Attenuated infectious bronchitis virus (IBV) vaccines are available but the relationship between sequence and virulence is not clear. In this study, the sequences of the 3' 7.3 kb of the genome, amplified using reverse transcription-polymerase chain reaction before and after attenuation, were compared to study the relationship between virulence and the sequences of three IBV strains. After attenuation, two to six amino acid substitutions were found in the spike 1 subunit, and two or three amino acid substitutions were found in the spike 2 subunit. None or one amino acid substitution was found in the small membrane protein, and one or three amino acid substitutions were found in the membrane protein. However, no amino acid substitution was found in the nucleocapsid (N) protein, indicating that the N protein might not be related to this attenuation. The 3' untranslated region after the N gene of one strain was partially deleted after attenuation, and might be correlated with virulence. This study is the first demonstration for IBV comparing sequence changes in the 3' 7.3 kb genome after attenuation. The aforementioned information on amino acid changes might be useful in future virulence studies.

Receptor-independent infection of murine coronavirus: analysis by spinoculation

A highly neurovirulent murine coronavirus JHMV (wild-type [wt] JHMV) is known to spread from cells infected via the murine coronavirus mouse hepatitis virus receptor (MHVR) to cells without MHVR (MHVR-independent infection), whereas a mutant virus isolated from wt JHMV, srr7, spread only in an MHVR-dependent fashion. These observations were obtained by the overlay of JHMV-infected cells onto receptor-negative cells that are otherwise resistant to wt JHMV infection. MHVR-independent infection is hypothetically thought to be attributed to a naturally occurring fusion activation of the wt JHMV S protein, which did not occur in the case of srr7. Attachment of S protein on cells without MHVR during the S-protein activation process seems to be a key condition. Thus, in the present study, we tried to see whether wt JHMV virions that are attached on MHVR-negative cells are able to infect those cells. In order to make virions attach to the cell surface without MHVR, we have used spinoculation, namely, the centrifugation of cells together with inoculated virus at 3,000 rpm for 2 h. This procedure forces viruses to attach to the cell surface, as revealed by quantitative estimation of attached virions by real-time PCR and also facilitated wt JHMV infection to MHVR-negative cells, but failed to do so for srr7. Virions of both wt and srr7 attached on MHVR-negative cells by spinoculation were facilitated for infection in the presence of a soluble form of MHVR that induces conformational changes of both wt and srr7. It was further revealed that wt JHMV S1, but not srr7, was released from the cell surface when S protein was expressed on cells. These observations support the hypothesis that attachment of the virion to MHVR-negative cells is a critical step and that a unique feature of wt JHMV S1 to be released from S2 in a naturally occurring event is involved in an MHVR-independent infection.

Monoclonal antibodies targeting the HR2 domain and the region immediately upstream of the HR2 of the S protein neutralize in vitro infection of severe acute respiratory syndrome coronavirus

We have previously shown that an Escherichia coli-expressed, denatured spike (S) protein fragment of the severe acute respiratory coronavirus, containing residues 1029 to 1192 which include the heptad repeat 2 (HR2) domain, was able to induce neutralizing polyclonal antibodies (C. T. Keng, A. Zhang, S. Shen, K. M. Lip, B. C. Fielding, T. H. Tan, C. F. Chou, C. B. Loh, S. Wang, J. Fu, X. Yang, S. G. Lim, W. Hong, and Y. J. Tan, J. Virol. 79:3289-3296, 2005). In this study, monoclonal antibodies (MAbs) were raised against this fragment to identify the linear neutralizing epitopes in the functional domain and to investigate the mechanisms involved in neutralization. Eighteen hybridomas secreting the S protein-specific MAbs were obtained. Binding sites of these MAbs were mapped to four linear epitopes. Two of them were located within the HR2 region and two immediately upstream of the HR2 domain. MAbs targeting these epitopes showed in vitro neutralizing activities and were able to inhibit cell-cell membrane fusion. These results provide evidence of novel neutralizing epitopes that are located in the HR2 domain and the spacer region immediately upstream of the HR2 of the S protein.

N-terminal domain of the murine coronavirus receptor CEACAM1 is responsible for fusogenic activation and conformational changes of the spike protein

The mouse hepatitis virus (MHV) receptor (MHVR), CEACAM1, has two different functions for MHV entry into cells: binding to MHV spike protein (S protein) and activation of the S protein to execute virus-cell membrane fusion, the latter of which is accompanied by conformational changes of the S protein. The MHVR comprising the N-terminal and fourth domains [R1(1,4)] displays these two activities, and the N domain is thought to be critical for binding to MHV. In this study, we have addressed whether or not the N domain alone is sufficient for these activities. We examined three types of soluble form MHVR (soMHVR), one consisting of the N domain alone [soR1(1)], one with the N and second domains [soR1(1,2)], and one [soR1(1,4)] expressed by recombinant baculoviruses. We assessed the abilities of these three types of soMHVR to bind to MHV, activate fusogenicity, and induce conformational changes of the S protein. All three types of soMHVR similarly bound to MHV, as examined by a solid-phase binding assay and neutralized MHV infectivity. They also activated S protein fusogenicity and induced its conformational changes with similar levels of efficiency. However, R1(1) expressed on the BHK cell surface failed to serve as a receptor in spite of a sufficient level of expression. The inability of expressed R1(1) to work as a receptor was due to the inaccessibility of virions to R1(1); however, these were accessible using the MHVR-specific monoclonal antibody CC1. These results collectively indicated that the N domain retains all biological activities necessary for receptor function.

Receptor-induced conformational changes of murine coronavirus spike protein

Although murine coronavirus mouse hepatitis virus (MHV) enters cells by virus-cell membrane fusion triggered by its spike (S) protein, it is not well known how the S protein participates in fusion events. We reported that the soluble form of MHV receptor (soMHVR) transformed a nonfusogenic S protein into a fusogenic one (F. Taguchi and S. Matsuyama, J. Virol. 76:950-958, 2002). In the present study, we demonstrate that soMHVR induces the conformational changes of the S protein, as shown by the proteinase digestion test. A cl-2 mutant, srr7, of the MHV JHM virus (JHMV) was digested with proteinase K after treatment with soMHVR, and the resultant S protein was analyzed by Western blotting using monoclonal antibody (MAb) 10G, specific for the membrane-anchored S2 subunit. A 58-kDa fragment, encompassing the two heptad repeats in S2, was detected when srr7 was digested after soMHVR treatment, while no band was seen when the virus was untreated. The appearance of the proteinase-resistant fragment was dependent on the temperature and time of srr7 incubation with soMHVR and also on the concentration of soMHVR. Coimmunoprecipitation indicated that the direct binding of soMHVR to srr7 S protein induced these conformational changes; this was also suggested by the inhibition of the changes following pretreatment of soMHVR with anti-MHVR MAb CC1. soMHVR induced conformational changes of the S proteins of wild-type (wt) JHMV cl-2, as well as revertants from srr7, srr7A and srr7B; however, a major proportion of these S proteins were resistant to proteinase K even without soMHVR treatment. The implications of this proteinase-resistant fraction are discussed. This is the first report on receptor-induced conformational changes of the membrane-anchored fragment of the coronavirus S protein.

Quaternary structure of coronavirus spikes in complex with carcinoembryonic antigen-related cell adhesion molecule cellular receptors

Oligomeric spike (S) glycoproteins extend from coronavirus membranes. These integral membrane proteins assemble within the endoplasmic reticulum of infected cells and are subsequently endoproteolyzed in the Golgi, generating noncovalently associated S1 and S2 fragments. Once on the surface of infected cells and virions, peripheral S1 fragments bind carcinoembryonic antigen-related cell adhesion molecule (CEACAM) receptors, and this triggers membrane fusion reactions mediated by integral membrane S2 fragments. We focused on the quaternary structure of S and its interaction with CEACAMs. We discovered that soluble S1 fragments were dimers and that CEACAM binding was entirely dependent on this quaternary structure. However, two differentially tagged CEACAMs could not co-precipitate with the S dimers, suggesting that binding sites were closely juxtaposed in the dimer (steric hindrance) or that a single CEACAM generated global conformational changes that precluded additional interactions (negative cooperativity). CEACAM binding did indeed alter S1 conformations, generating alternative disulfide linkages that were revealed on SDS gels. CEACAM binding also induced separation of S1 and S2. Differentially tagged S2 fragments that were free of S1 dimers were not co-precipitated, suggesting that S1 harbored the primary oligomerization determinants. We discuss the distinctions between the S.CEACAM interaction and other virus-receptor complexes involved in receptor-triggered entry.

Communication between S1N330 and a region in S2 of murine coronavirus spike protein is important for virus entry into cells expressing CEACAM1b receptor

The soluble receptor-resistant (srr) mutants, srr7 and srr11, isolated from a murine coronavirus, mouse hepatitis virus (MHV) JHMV, have an amino acid mutation at positions 1114 (Leu to Phe) and 65 (Leu to His), respectively, in the spike (S) protein. These mutants failed to efficiently infect BHK cells expressing CEACAM1b (BHK-R2), due to their low entry into this cell line, although they infected cells expressing CEACAM1a (BHK-R1) in a manner similar to that of wild-type (wt) JHMV cl-2 (Matsuyama and Taguchi, Virology 273, 80-89, 2000). Following the repeated passage of these mutants through BHK-R2 cells, viruses were no longer isolated from srr11-infected cells, while two distinct mutants, srr7A and srr7B, were obtained from srr7-infected cells. Srr7A and srr7B grew 2 log10 higher than srr7 and induced fusion in BHK-R2 cells, being similar to wt virus. In addition to the amino acid change at position 1114 that stemmed from parental srr7, srr7A and srr7B had mutations around position 280, corresponding to the third region of the S1N330 receptor-binding site (S1N330-III) common to all MHV strains examined thus far. Srr7A and srr7B S proteins showed high fusogenicity in both BHK-R1 and BHK-R2 cells, like the wt virus, while srr7Aa and srr7Ba S proteins, which had mutations in S1N330-III but not at amino acid 1114, exhibited profoundly reduced fusion activity in these cell lines. These findings suggest that communication between S1N330-III and the amino acid at position 1114 is important for efficient fusion activity in BHK-R2 cells. S1N330-III is a possible region in the S1 involved in viral entry into cells.

Comparison of genomic and predicted amino acid sequences of respiratory and enteric bovine coronaviruses isolated from the same animal with fatal shipping pneumonia

The complete genome sequences are reported here of two field isolates of bovine coronavirus (BCoV), which were isolated from respiratory and intestinal samples of the same animal experiencing fatal pneumonia during a bovine shipping fever epizootic. Both genomes contained 31028 nucleotides and included 13 open reading frames (ORFs) flanked by 5'- and 3'-untranslated regions (UTRs). ORF1a and ORF1b encode replicative polyproteins pp1a and pp1ab, respectively, that contain all of the putative functional domains documented previously for the closest relative, mouse hepatitis virus. The genomes of the BCoV isolates differed in 107 positions, scattered throughout the genome except the 5'-UTR. Differences in 25 positions were non-synonymous and were located in all proteins except pp1b. Six replicase mutations were identified within or immediately downstream of the predicted largest pp1a-derived protein, p195/p210. Single amino acid changes within p195/p210 as well as within the S glycoprotein might contribute to the different phenotypes of the BCoV isolates.

Virus-neutralizing monoclonal antibody expressed in milk of transgenic mice provides full protection against virus-induced encephalitis

Neutralizing antibodies represent a major host defense mechanism against viral infections. In mammals, passive immunity is provided by neutralizing antibodies passed to the offspring via the placenta or the milk as immunoglobulin G and secreted immunoglobulin A. With the long-term goal of producing virus-resistant livestock, we have generated mice carrying transgenes that encode the light and heavy chains of an antibody that is able to neutralize the neurotropic JHM strain of murine hepatitis virus (MHV-JHM). MHV-JHM causes acute encephalitis and acute and chronic demyelination in susceptible strains of mice and rats. Transgene expression was targeted to the lactating mammary gland by using the ovine beta-lactoglobulin promoter. Milk from these transgenic mice contained up to 0.7 mg of recombinant antibody/ml. In vitro analysis of milk derived from different transgenic lines revealed a linear correlation between antibody expression and virus-neutralizing activity, indicating that the recombinant antibody is the major determinant of MHV-JHM neutralization in murine milk. Offspring of transgenic and control mice were challenged with a lethal dose of MHV-JHM. Litters suckling nontransgenic dams succumbed to fatal encephalitis, whereas litters suckling transgenic dams were fully protected against challenge, irrespective of whether they were transgenic. This demonstrates that a single neutralizing antibody expressed in the milk of transgenic mice is sufficient to completely protect suckling offspring against MHV-JHM-induced encephalitis.

Characterization of cell-surface determinants important for baculovirus infection

Baculovirus gp64 envelope glycoprotein is a major component of the envelope of the budded virus and is involved in virus entry into the host cells by endocytosis. To investigate the cell-surface molecules important for infection of baculovirus into mammalian cells, we constructed a recombinant baculovirus, Ac64-CAluc, which has gp64 and luciferase genes under the polyhedrin and the CAG promoter, respectively. For controls, we constructed recombinant viruses possessing vesicular stomatitis virus (VSV) G protein, mouse hepatitis virus (MHV) S protein, or green fluorescent protein (GFP) gene under the polyhedrin promoter and the luciferase gene under the CAG promoter (AcVSVG-CAluc, AcMHVS-CAluc, and AcGFP-CAluc). Treatment of HepG2 cells with phospholipase C markedly reduced the reporter gene expression by Ac64-CAluc or AcVSVG-CAluc in a dose-dependent manner, whereas AcMHVS-CAluc was shown to be resistant to the treatment. Inhibition with purified lipids and susceptibility to the mutant CHO hamster cell lines deficient in phospholipids synthesis suggest that the interaction of gp64 and phospholipids on the cell surface might play an important role in baculovirus infection into mammalian cells.

Impaired entry of soluble receptor-resistant mutants of mouse hepatitis virus into cells expressing MHVR2 receptor

Mouse hepatitis virus (MHV) JHMV and its soluble receptor-resistant (srr) mutants, srr7, srr11, and srr18, grew and induced syncytia equally well in BHK-R1 cells expressing the MHVR1 receptor derived from MHV-susceptible BALB/c mice. In contrast, srr growth and syncytia formations were drastically reduced relative to wild-type (wt) virus in BHK-R2 cells expressing the MHVR2 receptor from MHV-resistant SJL mice. Infections by these srr mutants in BHK-R2 cells were 0.7 to 1.5 log10 less efficient than those of wt virus. BHK cells expressing both MHVR1 and MHVR2 supported srr replication to the same extent as did BHK-R1 cells, suggesting that inefficient infection by srr mutants in BHK-R2 cells resulted from the absence of the effective receptor MHVR1. Virus-receptor binding tests failed to demonstrate a difference between the abilities of wt and srr18 to bind MHVR2. The binding of srr7 and srr11 to both MHVR1 and MHVR2 was revealed lower by two- to fourfold relative to the wt binding. The fusion activity of srr S proteins as examined by the expression with recombinant vaccinia virus was apparently lower than that of the wt S protein in BHK-R2 cells, while there was not such a remarkable difference in BHK-R1 cells. This suggests that the most likely reason for inefficient infection by mutants in BHK-R2 is impaired virus entry into cells. These observations suggest that inefficient infections in BHK-R2 cells by srr mutants occur in the absence of a functional receptor MHVR1, which plays an important role in srr entry into cells.

A 12-amino acid stretch in the hypervariable region of the spike protein S1 subunit is critical for cell fusion activity of mouse hepatitis virus

The spike (S) glycoprotein of mouse hepatitis virus (MHV) plays a major role in the viral pathogenesis. It is often processed into the N-terminal S1 and the C-terminal S2 subunits that were evidently important for binding to cell receptor and inducing cell-cell fusion, respectively. As a consequence of cell-cell fusion, most of the naturally occurring infections of MHV are associated with syncytia formation. So far, only MHV-2 was identified to be fusion-negative. In this study, the S gene of MHV-2 was molecularly cloned, and the nucleotide sequence was determined. The MHV-2 S protein lacks a 12-amino acid stretch in the S1 hypervariable region from amino acid residue 446 to 457 when compared with the fusion-positive strain MHV-JHM. In addition, there are three amino acid substitutions in the S2 subunit, Tyr-1144 to Asp, Glu-1165 to Asp, and Arg-1209 to Lys. The cloned MHV-2 S protein exhibited the fusion-negative property in DBT cells as the intrinsic viral protein. Furthermore, similar to the fusion-positive MHV-JHM strain, proteolytic cleavage activity was detected both in DBT cells infected with the fusion-negative MHV-2 and in the transfected cells that expressed the cloned MHV-2 S protein. Domain swapping experiments demonstrated that the 12-amino acid stretch missing in the MHV-2 S1 subunit, but not the proteolytic cleavage site, was critical for the cell-fusion activity of MHV.

Isolation and characterization of murine coronavirus mutants resistant to neutralization by soluble receptors

Murine coronavirus mutants resistant to neutralization with soluble receptors were isolated to study the receptor-binding site on the S proteins since such mutants were expected to have mutations in an important site for receptor-binding. We have isolated five soluble receptor-resistant (srr) mutants which had mutations of a single amino acid at 3 different positions in S protein. Srr mutant 11 with an amino acid change at position 65 (Leu to His) in the S1 subunit showed an extremely reduced binding by virus overlay protein blot assay. However srr mutants with a mutation at 1114 (Leu to Phe) (srr mutants 3, 4 and 7) or 1163 (Cys to Phe) (srr mutant 18) in the S2 subunit had receptor-binding activity similar to that of wild type cl-2. These results suggest that an amino acid at position 62 located in a conserved region among MHV strains is in particular important for receptor binding. We also discuss why srr mutants with a mutation in S2 showed high resistance to neutralization by soluble receptor, irrespective of their binding to MHV receptors.

Isolation and recombinant expression of an MHV-JHM neutralising monoclonal antibody

The monoclonal antibody A1 (mab A1) efficiently neutralises the infection of susceptible cells by the murine hepatitis virus MHV-JHM in vitro and in vivo (Wege et al., 1984). The variable regions of mab A1 were amplified from mRNA of the respective hybridoma cell line by RT-PCR and integrated into different eukaryotic expression vectors. The biological function of the recombinant antibody constructs was verified by virus neutralisation assays. Whereas a complete recombinant antibody (mab A1rec.) expressed in transfected murine myeloma cells inhibited the MHV-JHM infection as well as the parental antibody, a single-chain Fv derived from mab A1 did not show any neutralising activity.

Identification of spike protein residues of murine coronavirus responsible for receptor-binding activity by use of soluble receptor-resistant mutants

We previously demonstrated by site-directed mutagenesis analysis that the amino acid residues at positions 62 and 214 to 216 in the N-terminal region of mouse hepatitis virus (MHV) spike (S) protein are important for receptor-binding activity (H. Suzuki and F. Taguchi, J. Virol. 70:2632-2636, 1996). To further identify the residues responsible for the activity, we isolated the mutant viruses that were not neutralized with the soluble form of MHV receptor proteins, since such mutants were expected to have mutations in amino acids responsible for receptor-binding activity. Five soluble-receptor-resistant (srr) mutants isolated had mutations in a single amino acid at three different positions: one was at position 65 (Leu to His) (srr11) in the S1 subunit and three were at position 1114 (Leu to Phe) (srr3, srr4, and srr7) and one was at position 1163 (Cys to Phe) (srr18) in the S2 subunit. The receptor-binding activity examined by a virus overlay protein blot assay and by a coimmunoprecipitation assay showed that srr11 S protein had extremely reduced binding activity, while the srr7 and srr18 proteins had binding activity similar to that of wild-type cl-2 protein. However, when cell surface receptors were used for the binding assay, all srr mutants showed activity similar to that of the wild type or only slightly reduced activity. These results, together with our previous observations, suggest that amino acids located at positions 62 to 65 of S1, a region conserved among the MHV strains examined, are important for receptor-binding activity. We also discuss the mechanism by which srr mutants with a mutation in S2 showed high resistance to neutralization by a soluble receptor, despite their sufficient level of binding to soluble receptors.

Expression of a recombinant monoclonal antibody from a bicistronic mRNA

The variable regions of the murine monoclonal antibody A1, which effectively neutralizes the infection of susceptible cells by the murine hepatitis virus strain JHM, were cloned, sequenced, and expressed in mammalian cells as a functional recombinant antibody. To accomplish the concurrent synthesis of both antibody chains, the light- and heavy-chain-coding regions were inserted into a bicistronic expression cassette based on the encephalomyocarditis virus internal ribosomal entry site. The strategy of combining both coding regions in one bicistronic mRNA allows for the rapid isolation of cell clones expressing high levels of recombinant antibody.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.

The S2 subunit of the murine coronavirus spike protein is not involved in receptor binding

The receptor-binding capacity of the S2 subunit of the murine coronavirus S protein was examined by testing the inhibition of virus-receptor binding. Sp-4 virus and S1N(330), which consists of the N-terminal 330 amino acids of the S1 protein, both of which exhibited receptor-binding capacity, were able to prevent the binding of cl-2 virus to the receptor, while the mutant protein S1N(330)-159, which failed to bind to the receptor protein, and S2TM-, which lacks the transmembrane and cytoplasmic domains normally existing in the S2, were unable to prevent the binding of cl-2. By using cultured DBT cells, it was revealed that the infection of cells by cl-2 virus was significantly inhibited by S1N(330) but not by S2TM-. These results indicate that the S2 protein is not involved in the receptor binding of murine coronaviruses.

Characterization of a human coronavirus (strain 229E) 3C-like proteinase activity

The RNA polymerase gene of human coronavirus (HCV) 229E encodes a large polyprotein that contains domains with motifs characteristic of both papain-like cysteine proteinases and proteinases with homology to the 3C proteinase of picornaviruses. In this study, we have, first, expressed the putative HCV 229E 3C-like proteinase domain as part of a beta-galactosidase fusion protein in Escherichia coli and have shown that the expressed protein has proteolytic activity. The substitution of one amino acid within the predicted proteinase domain (His-3006-->Asp-3006) abolishes, or at least significantly reduces, this activity. Amino-terminal sequence analysis of a purified, 34-kDa cleavage product shows that the bacterial fusion protein is cleaved at the dipeptide Gln-2965-Ala-2966, which is the predicted amino-terminal end of the putative 3C-like proteinase domain. Second, we have confirmed the proteolytic activity of a bacterially expressed polypeptide with the amino acid sequence of the predicted HCV 229E 3C-like proteinase by trans cleavage of an in vitro translated polypeptide encoded within open reading frame 1b of the RNA polymerase gene. Finally, using fusion protein-specific antiserum, we have identified a 34-kDa, 3C-like proteinase polypeptide in HCV 229E-infected MRC-5 cells. This polypeptide can be detected as early as 3 to 5 h postinfection but is present in the infected cell in very low amounts. These data contribute to the characterization of the 3C-like proteinase activity of HCV 229E.

Molecular mimicry between Fc receptor and S peplomer protein of mouse hepatitis virus, bovine corona virus, and transmissible gastroenteritis virus

We have previously demonstrated molecular mimicry between the S peplomer protein of mouse hepatitis virus (MHV) and Fc gamma R (Fc gamma R). A monoclonal antibody (MAb) to mouse Fc gamma R (2.4G2 anti-Fc gamma R MAb), purified rabbit immunoglobulin, but not their F(ab')2 fragments, as well as mouse and rat IgG, immunoprecipitated (1) recombinant S peplomer protein expressed by a vaccinia virus recombinant in human, rabbit, and mouse cells, and (2) natural S peplomer protein from cells infected with several strains of MHV and MHV escaped mutants. We report here results of studies documenting molecular mimicry between Fc gamma R and S peplomer protein of viruses representing three distinct antigenic subgroups of the Coronaviridae. We have shown a molecular mimicry between the S peplomer protein of bovine corona virus (BCV) and Fc gamma R. The 2.4G2 anti-Fc gamma R MAb, rabbit IgG, but not its F(ab')2 fragments, as well as homologous bovine serum, free of anti-BCV antibodies, immunoprecipitated S peplomer protein of BCV (Mebus strain). In contrast, we did not find molecular mimicry between S peplomer protein of human corona virus (HCV-OC43) and Fc gamma R. Although the OC43 virus belongs to the same antigenic group as MHV and BCV, MAb specific for human Fc gamma RI or Fc gamma RII and purified human IgG1, IgG2, and IgG3 myeloma proteins did not immunoprecipitate the S peplomer protein from HCV-OC43-infected RD cells. In addition, we did demonstrate molecular mimicry between the S peplomer protein of porcine transmissible gastroenteritis virus (TGEV) and Fc gamma R. TGEV belongs to the second antigenic subgroup of coronaviridae.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of neutralizing epitopes and the receptor-binding site within the amino-terminal 330 amino acids of the murine coronavirus spike protein

To localize the epitopes recognized by monoclonal antibodies (MAbs) specific for the S1 subunit of the murine coronavirus JHMV spike protein, we have expressed S1 proteins with different deletions from the C terminus of S1. S1utt is composed of the entire 769-amino-acid (aa) S1 protein; S1NM, S1N, S1n(330), and S1n(220) are deletion mutants with 594, 453, 330, and 220 aa from the N terminus of the S1 protein. The expressed S1 deletion mutant proteins were examined for reactivities to a panel of MAbs. All MAbs classified in groups A and B, those reactive to most mouse hepatitis virus (MHV) strains and those specific for isolate JHMV, respectively, recognized S1N(330) and the larger S1 deletion mutants but failed to react with S1N(220). MAbs in group C, specific for the larger S protein of JHMV, reacted only with the S1utt protein without any deletion. These results indicated that the domain composed of the N-terminal 330 aa comprised the cluster of conformational epitopes recognized by MAbs in groups A and B. It was also shown that the epitopes of MAbs in group C were not restricted to the region missing in the smaller S protein. These results together with the fact that all MAbs in group B retained high neutralizing activity suggested the possibility that the N-terminal 330 aa are responsible for binding to the MHV-specific receptors. In investigate this possibility, we expressed the receptor protein and examined the binding of each S1 deletion mutant to the receptor. It was demonstrated that the S1N(330) protein as well as other S1 deletion mutants larger than S1N(330) bound to the receptor. These results indicated that a domain composed of 330 aa at the N terminus of the S1 protein is responsible for binding to the MHV-specific receptor.

Proteolytic cleavage of the murine coronavirus surface glycoprotein is not required for its fusion activity

The surface glycoprotein (S) of the murine hepatitis coronavirus MHV normally undergoes proteolytic cleavage during transport to the cell surface. To determine whether the cleavage of the MHV-JHM S glycoprotein is required to activate its ability to fuse cellular membranes, the protease recognition sequence in a cDNA copy of the S gene was altered from Arg-Arg-Ala-Arg-Arg into Ser-Val-Ser-Gly-Gly by site directed mutagenesis. The mutated and wild type S genes were expressed by means of recombinant vaccinia viruses and it could be shown that the mutated S protein was not cleaved when it was expressed in mouse DBT cells, in contrast to the wild type S protein. Nevertheless, the non-cleaved S protein induced extensive syncytium formation in mouse DBT cells. These results clearly indicate that the non-cleaved form of the MHV S protein is able to mediate cell membrane fusion. Thus, proteolytic cleavage is not an absolute requirement for its fusion function.

Characterization of a monoclonal antibody resistant variant of MHV

A monoclonal antibody resistant (MAR) variant of MHV was isolated after infection of hybridoma cells secreting the neutralizing and fusion-inhibiting monoclonal antibody, mAb 11F. The isolated variant was able to mediate syncytia formation even in the presence of high concentrations of mAb 11F. The S gene of the variant was cloned and sequenced. There were three nucleotide exchanges in comparison to the wild-type S gene, resulting in two amino acid alterations. However, both amino acid substitutions (at positions 255 and 1116) were located outside the binding site of mAb 11F.

Pathogenicity of neutralization escape mutants of mouse hepatitis virus: correlation with T- and B-cell depletions

Viral pathogenicity is a result of an imbalance between viral replication and the host's immune defences. When the virus is lymphotropic, understanding the pathogenic process of the viral disease becomes complicated because virus/lymphocyte interactions can alter the cell's integrity and subsequently induce immunodeficiency. The immune system plays an important role in the outcome of acute disease induced by the mouse hepatitis virus type 3 (MHV3). The use of attenuated escape mutants provides a tool to study the role of viral properties involved in its pathogenicity. We selected MHV3 mutants by virtue of their resistance to neutralization by monoclonal antibodies (mAb), in order to study their pathogenic properties. We reported that two MHV3 escape mutants were attenuated in their pathogenic properties according to inoculation site and with regard to survival time and ability to deplete T- and B-cell subpopulations in the spleen, thymus and bone marrow of susceptible Balb/c mice. The highly attenuated CL12 mutant could not induce depletion in T or B cells following intraperitoneal (i.p.) or intranasal (i.n.) inoculations, at three days postinfection. The less attenuated 51.6 mutant, however, maintained the ability to deplete T and B cells following i.p. inoculation, as described with the pathogenic MHV3. In contrast, no depletion of T cells following i.n. inoculation was induced with this mutant, although B lineage cells decreased. The use of such mutants enabled us to examine the role of each compartment of the immune system, since the highly attenuated CL12 mutant induced no immunodeficiency, as defined by immune cell depletion, whereas the less attenuated 51.6 mutant maintained its ability to decrease only the B-cell compartment after i.n. inoculation. Results are discussed with regard to the virus/lymphocyte interactions during the pathogenic process.

A review of feline infectious peritonitis virus: molecular biology, immunopathogenesis, clinical aspects, and vaccination

Feline infectious peritonitis (FIP) has been an elusive and frustrating problem for veterinary practitioners and cat breeders for many years. Over the last several years, reports have begun to elucidate aspects of the molecular biology of the causal virus (FIPV). These papers complement a rapidly growing base of knowledge concerning the molecular organization and replication of coronaviruses in general. The fascinating immunopathogenesis of FIPV infection and the virus' interaction with macrophages has also been the subject of several recent papers. It is now clear that FIPV may be of interest to scientists other than veterinary virologists since its pathogenesis may provide a useful model system for other viruses whose infectivity is enhanced in the presence of virus-specific antibody. With these advances and the recent release of the first commercially-available FIPV vaccine, it is appropriate to review what is known about the organization and replication of coronaviruses and the pathogenesis of FIPV infection.

Identification of an immunodominant linear neutralization domain on the S2 portion of the murine coronavirus spike glycoprotein and evidence that it forms part of complex tridimensional structure

Numerous studies have demonstrated that the spike glycoprotein of coronaviruses bears major determinants of pathogenesis. To elucidate the antigenic structure of the protein, a panel of monoclonal antibodies was studied by competitive ELISA, and their reactivities were assayed against fragments of the murine coronavirus murine hepatitis virus strain A59 S gene expressed in prokaryotic vectors. An immunodominant linear domain was localized within the predicted stalk, S2, of the peplomer. It is recognized by several neutralizing antibodies. Other domains were also identified near the proteolytic cleavage site, in the predicted globular head, S1, and in another part of the stalk. Furthermore, competition results suggest that the immunodominant functional domain forms part of a complex three-dimensional structure. Surprisingly, some antibodies which have no antiviral biological activities were shown to bind the immunodominant neutralization domain.

Fusion formation by the uncleaved spike protein of murine coronavirus JHMV variant cl-2

The fusogenic properties of the uncleaved spike (S) protein of murine coronavirus JHMV variant cl-2 were studied by expressing the S protein with a deleted putative cleavage site. The amino acid sequence of the putative cleavage site, Arg-Arg-Ala-Arg-Arg, was replaced by Arg-Thr-Ala-Leu-Glu by in vitro mutagenesis of the cl-2 S protein cDNA. Recombinant vaccinia viruses containing the cl-2 S cDNA [RVV t(+)] or the mutated cDNA [RVV t(-)] were constructed and monitored for fusion formation and cleavage of the expressed S proteins. When cultured DBT cells were infected with RVV t(+) at a multiplicity of infection of 0.5, fusion formation was first observed at 10 to 12 h postinoculation and spread throughout the whole culture by 20 to 24 h postinoculation. In cells infected with RVV t(-) under the same conditions, fusion formation appeared by 12 to 14 h. This result represented a 2- to 4-h delay in the onset of fusion, compared with its appearance in cells expressing the wild-type S protein. By 25 to 30 h, most of the cells infected by RVV t(-) had fused. By immunoprecipitation and Western blotting (immunoblotting), the 170-kDa S protein was detected in DBT cells expressing the wild-type S protein and the mutated S protein. However, interestingly, the cleavage products of the S protein, S1 and S2, were not detected in RVV t(-)-infected cells, producing the mutated S protein, even though fusion was clearly visible. Both products were, of course, detected in RVV t(+)-infected DBT cells, producing the wild-type S protein. The same results concerning the fusion formation and cleavage properties of the S proteins were reproduced by the transiently expressed S proteins. These results suggest that the cleavage event in the S protein of murine coronavirus JHMV is not a prerequisite for fusion formation but that it does facilitate fusion formation.

Evidence for variable rates of recombination in the MHV genome

Mouse hepatitis virus has been shown to undergo RNA recombination at high frequency during mixed infection. Temperature-sensitive mutants were isolated using 5-fluorouracil and 5-azacytidine as mutagen. Six RNA+ mutants that reside within a single complementation group mapping within the S glycoprotein gene of MHV-A59 were isolated which did not cause syncytium at the restrictive temperature. Using standard genetic techniques, a recombination map was established that indicated that these mutants mapped into two distinct domains designated F1 and F2. These genetic domains may correspond to mutations mapping within the S1 and S2 glycoproteins, respectively, and suggest that both the S1 and S2 domains are important in eliciting the fusogenic activity of the S glycoprotein gene. In addition, assuming that most distal ts alleles map roughly 4.0 kb apart, a recombination frequency of 1% per 575-676 bp was predicted through the S glycoprotein gene. Interestingly, this represents a threefold increase in the recombination frequency as compared to rates predicted through the polymerase region. The increase in the recombination rate was probably not due to recombination events resulting in large deletions or insertions (greater than 50 bp), but rather was probably due to a combination of homologous and nonhomologous recombination. A variety of explanations could account for the increased rates of recombination in the S gene.

SJL/J resistance to mouse hepatitis virus-JHM-induced neurologic disease can be partially overcome by viral variants of S and host immunosuppression

The basis of the resistance of SJL/J mice to various strains of mouse hepatitis virus (MHV) has been the subject of some debate, especially as it relates to the number and nature of the determinants involved. Our previous work demonstrated that resistance by primary SJL/J glial cultures may involve events subsequent to viral gene expression, possibly due to a defect in cell-to-cell spread of the infection. Since S, the virion's major spike glycoprotein, is known to facilitate the spread of infection due to its syncytiogenic properties, we decided to investigate the role of this viral structural protein in resistance by primary SJL/J glial cells. Variants possessing deletions within the S coding region were able to grow in SJL/J glial cells 10-100 times easier and fuse five-times more efficiently than wt virus. Induction of neurologic disease in SJL/J mice following intracranial inoculation with either wt JHMV or the S deletion variant, AT11f cord, was age-dependent, occurring only in animals inoculated under 4 weeks of age. Resistance in older animals to wt and variant viruses could be abrogated by immunosuppression with cyclosporin A. However, both disease incidence and viral brain titers were higher in animals receiving the JHM variant AT11f cord virus, suggesting that SJL/J resistance to neurologic disease may manifest itself through interactions between inefficient cell-to-cell spread of the infection and protective aspects of the immune response.

Sequence analysis of the spike protein gene of murine coronavirus variants: study of genetic sites affecting neuropathogenicity

Mouse hepatitis virus (MHV), a coronavirus, causes encephalitis and demyelination in susceptible rodents. Previous investigations have shown that the MHV spike (S) protein is a critical determinant of viral tropism and pathogenicity in mice and rats. To understand the molecular basis of MHV neuropathogenesis, we studied the spike protein gene sequences of several neutralization-resistant variants of the JHM strain of MHV, which were selected with monoclonal antibodies (MAbs) specific for the S protein. We found that variant 2.2-V-1, which was selected with MAb J.2.2 and primarily caused demyelination, had a single point mutation at nucleotide (NT) 3340, as compared to the parental JHM virus, which predominantly caused encephalitis. This site was in the S2 subunit of the S protein. In contrast, variant 7.2-V-1, which was selected with MAb J.7.2 and primarily caused encephalitis, had two point mutations at NT 1766 and 1950, which were in the S1 subunit. Finally, the double mutant 2.2/7.2-V-2, which was selected with both MAbs J.2.2 and J.7.2, and was attenuated with respect to both virulence and the ability to cause demyelination, had a deletion spanning from NT 1523 to 1624 in the S1 and a point mutation at NT 3340 in the S2. We conclude that at least two regions of the S protein contribute to neuropathogenicity of MHV. We have also isolated a partial revertant of 2.2-V-1, which was partially resistant to MAb J.2.2 but retained the same neuropathogenicity as the variant 2.2-V-1. This revertant retained the mutation at NT 3340, but had a second-site mutation at NT 1994, further confirming that NT 3340 contributed to the pathogenic phenotype of MHV. By comparing these results with MHV variants isolated in other laboratories, which had mutations in other sites on the S gene and yet retained the demyelinating ability, we suggest that the ability of JHM viruses to induce demyelination is determined by the interaction of multiple sites on the S gene, rather than the characteristics of a single, unique site. Our study also revealed the possible presence of microheterogeneity of S gene sequence, particularly in the S1 region, in these viruses. The sequence microheterogeneity may also contribute to the differences in their biological properties.

Identification of epitopes associated with different biological activities on the glycoprotein of vesicular stomatitis virus by use of monoclonal antibodies

Thirteen monoclonal antibodies (MAbs) to the glycoprotein (G) of vesicular stomatitis virus (VSV) serotype Indiana were prepared and examined for their effects on various biological activities of VSV, including in vitro infection, hemagglutination, adsorption to cells, and mediation of cell fusion. Competitive binding assays with these MAbs revealed the presence of at least seven distinct antigenic determinants (epitopes) on the G protein. In some cases, overlappings among epitopes to various degrees were observed as partial inhibition or binding enhancement. The MAbs to all the epitopes but one (epitopes 1-6) reacted with the denatured G protein in a Western immunoblot analysis. Four of the epitopes (epitopes 2, 4, 5, and 7) were involved in neutralization and two (epitopes 1 and 2) in hemagglutination inhibition. None of the MAbs inhibited the adsorption of radiolabeled VSV to BHK-21 cells; the MAbs to epitope 2 slightly enhanced the virus adsorption. All neutralization epitopes except epitope 2 (epitopes 4, 5, and 7) were associated with inhibition of VSV-mediated cell fusion. These results show a direct spatial relationship between the epitopes recognized by the MAbs and functional sites on G protein and further insights into the structure and function of G protein.

MHV S peplomer protein expressed by a recombinant vaccinia virus vector exhibits IgG Fc-receptor activity

We have previously shown that cells infected with mouse hepatitis virus (MHV) bind rabbit, mouse, and rat IgG by the Fc portion of the IgG molecule. This Fc-binding activity appeared to be mediated by the MHV S protein. S protein could also be precipitated from MHV-infected cells by a monoclonal antibody directed against the murine Fc gamma receptor (Fc gamma R). To prove definitively that the S protein mediates Fc-binding activity, we have expressed the MHV S protein utilizing recombinant vaccinia viruses. The anti-Fc gamma R monoclonal antibody, 2.4G2, precipitated recombinant S protein in cells of murine, human, and rabbit origin. Since the anti-Fc receptor monoclonal antibody does not react with human and rabbit Fc receptors these results demonstrate that the epitope recognized by this antibody is carried on the MHV S protein and is not murine in origin. Examination of various MHV isolates and escape mutants failed to identify the precise sequences in S responsible for the molecular mimicry of the murine Fc gamma R. These data are consistent with the hypothesis that a previously identified region of similarity between the S protein and the Fc gamma R mediates this activity. The Fc binding activity of S was expressed on the cell surface, since MHV-JHM-infected cells, but not uninfected cells, formed rosettes with anti-sheep red blood cell (SRBC) antibody-coated SRBC. The anti-Fc gamma R monoclonal antibody neutralized MHV-JHM and inhibited syncytium formation induced by the MHV S protein.

Carbohydrate-induced conformational changes strongly modulate the antigenicity of coronavirus TGEV glycoproteins S and M

The carbohydrate composition and the immunoreactivity of the S and M glycoproteins of the coronavirus TGEV were studied at different stages of their maturation. The biosynthesis of S and M was analyzed in the presence of tunicamycin and monensin. The effect of treatment with endoglycosidases H and F and glycopeptidase F on the precursors and mature forms of S and M were also examined. Species 175K and 29K were characterized as high mannose forms of S and M, respectively, and species 220K and 30-36K as complex type glycosylated forms of these two proteins. M was present mainly as a 29K species in mature virions whereas the 175K form of S was not detected, thus implying that the two proteins undergo Golgi modifications at a far different efficiency. Anti-S and -M monoclonal antibodies were examined for their reactivity towards polypeptide species either treated with endo H or produced in the presence of tunicamycin. It was found that (i) among the four major antigenic sites previously defined (Delmas et al., 1986), only site C (amino acids 363 to 371) was notably expressed by the unglycosylated S polypeptide 155K, whereas the three other sites were dependent upon core-glycosylation, (ii) three of the four anti-M mAbs tested did not recognize the unglycosylated M polypeptide 26K. These data led us to conclude that co-translational, but not terminal glycosylation is an essential requirement for both acquisition and maintenance of the antigenicity of TGEV glycoproteins.