Vaccination against lethal coronavirus-induced encephalitis with a synthetic decapeptide homologous to a domain in the predicted peplomer stalk

Animal and human RNA viruses: genetic variability and ability to overcome vaccines

RNA viruses, in general, exhibit high mutation rates; this is mainly due to the low fidelity displayed by the RNA-dependent polymerases required for their replication that lack the proofreading machinery to correct misincorporated nucleotides and produce high mutation rates. This lack of replication fidelity, together with the fact that RNA viruses can undergo spontaneous mutations, results in genetic variants displaying different viral morphogenesis, as well as variation on their surface glycoproteins that affect viral antigenicity. This diverse viral population, routinely containing a variety of mutants, is known as a viral ‘quasispecies’. The mutability of their virions allows for fast evolution of RNA viruses that develop antiviral resistance and overcome vaccines much more rapidly than DNA viruses. This also translates into the fact that pathogenic RNA viruses, that cause many diseases and deaths in humans, represent the major viral group involved in zoonotic disease transmission, and are responsible for worldwide pandemics.

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.

Identification of previously unknown antigenic epitopes on the S and N proteins of avian infectious bronchitis virus

This paper describes mapping of antigenic and host-protective epitopes of infectious bronchitis virus proteins by assessing the ability of defined peptide regions within the S1, S2 and N proteins to elicit humoral, cell-mediated and protective immune responses. Peptides corresponding to six regions in the S1 (Sp1–Sp6), one in the S2 (Sp7) and four in the N protein (Np1–Np4) were synthesized and coupled to either diphtheria toxoid (dt) or biotin (bt). Bt-peptides were used to assess if selected regions were antigenic and contained B- or T-cell epitopes and dt-peptides if regions induced an antibody response and protection against virulent challenge. All S1 and S2 peptides were antigenic, being recognised by IBV immune sera and also induced an antibody response following inoculation into chicks. Three S1-and one S2-bt peptides also induced a delayed type hypersensitivity response indicating the presence of T-cell epitopes. The S2 peptide Sp7 (amino acid position 566–584) previously identified as an immundominant region, was the most antigenic of all peptides used in this study. Two S1 (Sp4 and Sp6) and one S2 peptide (Sp7), protected kidney tissue against virulent challenge. From four N peptides located in the amino-terminal part of the N protein, only one, Np2 (amino acid position 72–86), was antigenic and also induced a delayed type hypersensitivity response. None of the N peptides induced protection against virulent challenge. The results suggest that the S1 glycoprotein carries additional antigenic regions to those previously identified and that two regions located in the S1 and one in the S2 at amino acid positions 294–316 (Sp4), 532–537 (Sp6) and 566–584 (Sp7) may have a role in protection.

A review of coronavirus infection in the central nervous system of cats and mice

Feline infectious peritonitis (FIP) is a common cause of death in cats. Management of this disease has been hampered by difficulties identifying the infection and determining the immunological status of affected cats and by high variability in the clinical, pathological, and immunological characteristics of affected cats. Neurological FIP, which is much more homogeneous than systemic effusive or noneffusive FIP, appears to be a good model for establishing the basic features of FIP immunopathogenesis. Very little information is available about the immunopathogenesis of neurologic FIP, and it is reasonable to use research from the well-characterized mouse hepatitis virus (MHV) immune-mediated encephalitis system, as a template for FIP investigation, and to contrast findings from the MHV model with those of FIP. It is expected that the immunopathogenic mechanisms will have important similarities. Such comparative research may lead to better understanding of FIP immunopathogenesis and rational prospects for management of this frustrating disease.

A review of coronavirus infection in the central nervous system of cats and mice

Feline infectious peritonitis (FIP) is a common cause of death in cats. Management of this disease has been hampered by difficulties identifying the infection and determining the immunological status of affected cats and by high variability in the clinical, pathological, and immunological characteristics of affected cats. Neurological FIP, which is much more homogeneous than systemic effusive or noneffusive FIP, appears to be a good model for establishing the basic features of FIP immunopathogenesis. Very little information is available about the immunopathogenesis of neurologic FIP, and it is reasonable to use research from the well-characterized mouse hepatitis virus (MHV) immune-mediated encephalitis system, as a template for FIP investigation, and to contrast findings from the MHV model with those of FIP. It is expected that the immunopathogenic mechanisms will have important similarities. Such comparative research may lead to better understanding of FIP immunopathogenesis and rational prospects for management of this frustrating disease.

Characterization of protection against coronavirus infection by noninternal image antiidiotypic antibody

Previously, we have reported protective vaccination of mice against a coronavirus infection using rabbit polyclonal noninternal image Ab2gamma anti-idiotypic (anti-Id) antibody specific for a virus-neutralizing and protective monoclonal antibody (mAb) 7-10A against the viral surface S glycoprotein. To characterize further the mechanisms involved in the induction of protective immunity by this noninternal image anti-Id, plasma and splenocytes from Ab2gamma-immunized BALB/c mice were passively transferred to naive BALB/c mice, followed by viral challenge. A reproducible significant delay in mortality observed in mice to which plasma was passively transferred, together with the presence of specific in vitro neutralizing antiviral Ab3 identified the humoral immune response as the major element responsible for protection. The activation of specific and cross-reactive T lymphocytes by both virus and anti-Id in immunized mice and the absence of adoptive transfer of protection by splenocytes suggested the participation of T helper activity in the induction of protective virus-neutralizing Ab3. To obtain more defined monoclonal reagents for a better understanding of anti-Id-induced protection, mAb2 were generated against the same mAb1 7-10A and characterized. We report the successful generation of mAb2 of the gamma type. However, unlike the polyclonal Ab2gamma, they were not capable of inducing a protective immune response.

Protective immunity against murine hepatitis virus (MHV) induced by intranasal or subcutaneous administration of hybrids of tobacco mosaic virus that carries an MHV epitope

Hybrids of tobacco mosaic virus (TMV) were constructed with the use of fusion to the coat protein peptides of 10 or 15 amino acids, containing the 5B19 epitope from the spike protein of murine hepatitis virus (MHV) and giving rise to TMV-5B19 and TMV-5B19L, respectively. The TMV hybrids were propagated in tobacco plants, and the virus particles were purified. Immunogold labeling, with the use of the monoclonal MAb5B19 antibody, showed specific decoration of hybrid TMV particles, confirming the expression and display of the MHV epitope on the surface of the TMV. Mice were immunized with purified hybrid viruses after several regimens of immunization. Mice that received TMV-5B19L intranasally developed serum IgG and IgA specific for the 5B19 epitope and for the TMV coat protein. Hybrid TMV-5B19, administered by subcutaneous injections, elicited high titers of serum IgG that was specific for the 5B19 epitope and for coat protein, but IgA that was specific against 5B19 was not observed. Mice that were immunized with hybrid virus by subcutaneous or intranasal routes of administration survived challenge with a lethal dose (10 x LD50) of MHV strain JHM, whereas mice administered wild-type TMV died 10 d post challenge. Furthermore, there was a positive correlation between the dose of administered immunogen and protection against MHV infection. These studies show that TMV can be an effective vaccine delivery vehicle for parenteral and mucosal immunization and for protection from challenge with viral infection.

Persistent infection of human oligodendrocytic and neuroglial cell lines by human coronavirus 229E

Human coronaviruses (HuCV) cause common colds. Previous reports suggest that these infectious agents may be neurotropic in humans, as they are for some mammals. With the long-term aim of providing experimental evidence for the neurotropism of HuCV and the establishment of persistent infections in the nervous system, we have evaluated the susceptibility of various human neural cell lines to acute and persistent infection by HuCV-229E. Viral antigen, infectious virus progeny and viral RNA were monitored during both acute and persistent infections. The astrocytoma cell lines U-87 MG, U-373 MG, and GL-15, as well as neuroblastoma SK-N-SH, neuroglioma H4, and oligodendrocytic MO3.13 cell lines, were all susceptible to an acute infection by HuCV-229E. The CHME-5 immortalized fetal microglial cell line was not susceptible to infection by this virus. The MO3.13 and H4 cell lines also sustained a persistent viral infection, as monitored by detection of viral antigen and infectious virus progeny. Sequencing of the S1 gene from viral RNA after approximately 130 days of infection showed two point mutations, suggesting amino acid changes during persistent infection of MO3.13 cells but none for H4 cells. Thus, persistent in vitro infection did not generate important changes in the S1 portion of the viral spike protein, which was shown for murine coronaviruses to bear hypervariable domains and to interact with cellular receptor. These results are consistent with the potential persistence of HuCV-229E in cells of the human nervous system, such as oligodendrocytes and possibly neurons, and the virus's apparent genomic stability.

Production and characterization of anti-peptide monoclonal antibodies with specificity for staphylococcal enterotoxins A and B

A synthetic peptide containing selected epitopes from staphylococcal enterotoxin A (SEA) and enterotoxin B (SEB) was used to produce monoclonal antibodies (Mabs) to respective enterotoxins in a single fusion procedure. The peptide inhibited the reaction of polyclonal anti-SEA or anti-SEB antisera with their homologous enterotoxin, thus showing that the chosen epitopes are part of the antibody-inducing enterotoxin sequences. Two Mabs, Mab-A and Mab-B, reacted with both the peptide and with either SEA or SEB. Used in a double antibody sandwich ELISA, the Mabs were able to quantitate the native SEA or SEB toxins at nanogram levels.

Production and immunogenicity of multiple antigenic peptide (MAP) constructs derived from the S1 glycoprotein of infectious bronchitis virus (IBV)

Synthetic peptides were prepared as multiple antigenic peptide (MAP) constructs to the S1 glycoprotein of infectious bronchitis virus (IBV). The MAP system has been used in the production of anti-peptide and anti-protein antibodies. It has an advantage over linking peptides to a highly immunogenic carrier molecule because antibodies are not produced to the MAP core matrix of lysine residues. Two 25-residue peptides were synthesized to the Arkansas serotype and two were synthesized to the Massachusetts serotype of IBV. The peptide sequences correspond to amino acid residues 64 to 88 and to residues 117 to 141 for each of the IBV serotypes. A MAP construct for each peptide was prepared by linking 4 copies of a peptide to the immunogenetically inert core matrix of lysine residues. The MAP constructs were used to immunize specific pathogen free chickens. Anti-peptide ELISA titers and the dot immunobinding assay against the homologous peptide were positive for all of the sera tested whereas the anti-whole virus ELISA titers and virus neutralization titers were negative for all of the sera tested. Hyperimmune sera against whole virus did not cross react with synthetic peptides made to the heterologous virus suggesting a possible role for the MAP constructs in a serotype specific dot blot or ELISA test for IBV.

Serological recognition of feline infectious peritonitis virus spike gene regions expressed as synthetic peptides and E. coli fusion protein

Cats exposed to feline infectious peritonitis virus (FIPV) or feline enteric coronavirus (FECV) cannot be differentiated by serological analysis. Three synthetic peptides and an E. coli recombinant fusion protein generated from FIPV 79-1146 spike gene sequence were produced. Coronavirus positive cat sera reacted to peptide aa 950-990 but were non-reactive to aa137-151 and aa 150-180 peptides as demonstrated by ELISA. Amino acid sequence 97-222 expressed as a galk fusion protein in E. coli was tested against coronavirus positive cat sera by western blot analysis. Only sera from cats exposed to the FIPV type-II strains DF-2 or 79-1146 that were exhibiting signs of FIP recognized the fusion protein. Sera from FECV exposed cats did not recognize the 97-222 fusion protein in western blot analysis.

The S1 glycoprotein but not the N or M proteins of avian infectious bronchitis virus induces protection in vaccinated chickens

The S1, N and M proteins, obtained from the nephropathogenic N1/62 strain of infectious bronchitis virus (IBV) by immunoaffinity purification with monoclonal antibodies, were used for immunization of chickens. For all three antigens multiple immunizations were necessary for induction of an antibody response. Protection of chickens vaccinated with the S1 glycoprotein against virulent challenge was demonstrated by the complete absence of virus in tracheas and kidneys of vaccinated chickens. Following four immunizations with the S1 glycoprotein 71% and 86% of chickens were protected at the level of tracheas and kidneys, respectively. Three immunizations with the S1 glycoprotein protected 70% and 10% of chickens at the level of kidney and trachea, respectively. Neither the N nor the M antigen induced protection to a virulent challenge with the nephropathogenic N1/62 strain of IBV after four immunizations. Virus neutralizing, haemagglutination inhibiting and ELISA antibodies were detected in chickens immunized with the S1 glycoprotein and inactivated N1/62 virus, however there was no correlation between the presence of any of these antibodies and protection.

Recombinant vaccinia viruses which express MHV-JHM proteins: protective immune response and the influence of vaccination on coronavirus-induced encephalomyelitis

Vaccinia-virus (VV) recombinants encoding either the nucleocapsid (N) or the spike (S) protein of MHV-JHM were constructed to study the role of the immune response against defined coronavirus antigens. For the S-protein, a fusogenic (Sfus+) or non fusogenic variant (Sfus-) of the gene was inserted into the VV genome. A strong protection against acute encephalomyelitis (AE) was mediated in Lewis rats which were immunized by VV-Sfus+ and challenged with an otherwise lethal dose of MHV-JHM before the induction of S-specific IgG antibodies. By contrast, a VV recombinant encoding a variant non fusogenic S-protein or the N-protein was not capable conferring protection. In addition, we demonstrated that MHV-JHM S-specific IgG antibodies elicited before MHV-JHM challenge modulated the disease process, changing it from an acute disease to subacute demyelinating encephalomyelitis (SDE).

Structural proteins of avian infectious bronchitis virus: role in immunity and protection

The antigenicity of the S1, M and N proteins of avian infectious bronchitis virus was compared following immunization of chickens with live and inactivated virus. The N protein was immunodominant antigen inducing cross-reactive antibodies in high titres whereas the S1 glycoprotein induced serotype-specific and cross-reactive antibodies. The M glycoprotein elicited antibodies in low titres and of limited cross-reactivity. Immunization of chickens with the purified N and M proteins did not induce protection against virulent challenge whereas immunization with the S1 glycoprotein prevented replication of nephropathogenic IBV in kidneys but not in tracheas of immunized chickens.

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

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

Identification of B-cell epitopes on the S4 subunit of pertussis toxin

The main purpose of the present study was to identify B-cell epitopes on the S4 subunit of pertussis toxin (PT) by the synthetic peptide approach. Two strategies were followed: (i) screening of two series of overlapping peptides (12- and 25-residue peptides) covering the entire S4 sequence by a panel of murine monoclonal anti-PT antibodies and various polyclonal anti-PT antisera in an enzyme-linked immunosorbent assay (ELISA), and (ii) analysis of the S4 amino acid sequence by a predictive algorithm followed by synthesis and immunization of mice with the predicted peptides coupled to diphtheria toxoid. The anti-peptide conjugate antisera were tested in an ELISA for cross-reactivity with native PT, B oligomer, and S4. Screening of the free peptides in an ELISA by the PT antisera indicated the presence of six B-cell epitope-containing domains covered by residues 18 to 32, 33 to 46, 39 to 52, 51 to 65, 71 to 84, and 91 to 106. None of the peptides, however, were recognized by the monoclonal anti-PT antibodies in an ELISA. Immunization with six computer-predicted peptides (B1 to B6) and three potential T-cell epitopes (T1 to T3) gave rise to very high antibody responses towards the homologous conjugates. With the exception of the anti-T1/diphtheria toxoid antisera, all anti-peptide conjugate antisera cross-reacted with PT in an ELISA at different levels. None of these anti-peptide conjugate antisera, however, showed any PT-neutralizing effect as measured by the Chinese hamster ovary cell assay and the leukocytosis-promoting activity test. The results of the present study suggest that discontinuous epitopes are predominant in the S4 subunit of native PT.

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.

Structural analysis of the conformational domains involved in neutralization of bovine coronavirus using deletion mutants of the spike glycoprotein S1 subunit expressed by recombinant baculoviruses

Two conformation-dependent neutralizing epitopes, A and B, have been mapped to the S1 subunit of the S spike glycoprotein of bovine coronavirus (BCV). In order to characterize the structure of these antigenic sites, we constructed a series of cDNA clones encoding deleted or truncated S1 derivatives and expressed the modified genes in insect cells using recombinant baculoviruses. Monoclonal antibodies directed against epitopes A and B recognized only the mutant S1 polypeptides containing amino acids 324-720, as demonstrated by immunoprecipitation and Western blot analysis in the absence of beta-mercaptoethanol. In addition, two domains within this region were identified and only mutants containing both domains were immunoreactive, indicating that both were critical in the formation of the antigenic determinants. One domain was localized between residues 324 and 403 and the other at residues 517-720. Deletion of either domain inhibited extracellular secretion of the mutant proteins whereas mutants containing both or none of the domains were secreted efficiently. This observation suggests a vital function of the native conformation of the S1 protein in both antigenic structure and intracellular transport. Antigenic determinants A and B were not distinguished, but these determinants appeared to require both domains for epitope formation. Our results suggest that the antigenic determinants formed by two domains are likely associated with the probable polymorphic region of the BCV S1 subunit.

The S2 subunit of the spike glycoprotein of bovine coronavirus mediates membrane fusion in insect cells

The hemagglutinin/esterase (HE), spike precursor (S) and the S1 and S2 subunits of the spike precursor protein of bovine coronavirus were expressed in Spodoptera frugiperda (Sf9) cells, and the cell-fusing activity of each recombinant glycoprotein was examined. Extensive syncytia formation was observed in cells infected with the S2 recombinant but not with the HE or S1 recombinant baculoviruses. Fusion of Sf9 cells expressing the intact S protein precursor was evident after trypsin treatment. These results demonstrate that proteolytic cleavage of the S spike precursor is required for fusion induction and that the fusion is mediated by the S2 subunit. These observations may reflect the biological role of the S2 subunit in fusion-penetration during bovine coronavirus infection.

Immunogenic peptide comprising a mouse hepatitis virus A59 B-cell epitope and an influenza virus T-cell epitope protects against lethal infection

The coronavirus spike protein S is responsible for important biological activities including virus neutralization by antibody, cell attachment, and cell fusion. Recently, we have elucidated the amino acid sequence of an S determinant common in murine coronaviruses (W. Luytjes, D. Geerts, W. Posthumus, R. Meloen, and W. Spaan, J. Virol. 63:1408-1412, 1989). A monoclonal antibody directed to this determinant (MAb 5B19.2) protected mice against acute fatal infection. In this study, BALB/c mice were immunized with a synthetic peptide of 13 amino acids corresponding to the binding site of MAb 5B19.2, which was either extended with an amino acid sequence of influenza virus hemagglutinin or conjugated to keyhole limpet hemocyanin. Both immunogens induced S-specific antibodies in mice, but only the hemagglutinin-peptide construct protected them against lethal challenge. In contrast to mouse hepatitis virus type 4 (MHV-4), MHV-A59 was not neutralized in vitro by MAb 5B19.2. Neither MHV-A59 nor MHV-4 was neutralized in vitro by antibodies comprising by the synthetic peptides. Our results demonstrated that antibodies elicited with a synthetic peptide comprising a B-cell epitope and a T-helper cell determinant can protect mice against an acute fetal mouse hepatitis virus infection.

Analysis of the S spike (peplomer) glycoprotein of bovine coronavirus synthesized in insect cells

The bovine coronavirus (BCV) spike glycoprotein precursor (S, formerly termed peplomer) and its two subunit polypeptides (S1 and S2) were individually expressed in Spodoptera frugiperda (Sf9) insect cells. Each recombinant baculovirus expressed both glycosylated (S, 170K; S1, 95K; S2, 80K) and unglycosylated (S0, 140K; S10, 75K; and S20, 65K) forms of BCV spike polypeptides in Sf9 cells. The mature 95K S1 polypeptide was secreted whereas the S and S2 polypeptides remained cell-associated. The S precursor was partially cleaved in Sf9 cells, and the resulting S1 was also released into the medium. Neutralizing monoclonal antibodies representing two antigenic domains bound to recombinant S and S1 but not the S2 polypeptides, indicating that two major epitopes for BCV neutralization are located on the S1 subunit.

Deduced sequence of the bovine coronavirus spike protein and identification of the internal proteolytic cleavage site

The sequence of the spike (also called peplomer or E2) protein gene of the Mebus strain of bovine coronavirus (BCV) was obtained from cDNA clones of genomic RNA. The gene sequence predicts a 150,825 mol wt apoprotein of 1363 amino acids having an N-terminal hydrophobic signal sequence of 17 amino acids, 19 potential N-linked glycosylation sites, a hydrophobic anchor sequence of approximately 17 amino acids near the C terminus, and a hydrophilic cysteine-rich C terminus of 35 amino acids. An internal Lys-Arg-Arg-Ser-Arg-Arg sequence predicts a protease cleavage site between amino acids 768 and 769 that would separate the S apoprotein into S1 and S2 segments of 85690 and 65153 mol wt, respectively. Amino terminal amino acid sequencing of the virion-derived gp 100 spike subunit confirmed the location of the predicted cleavage site, and established that gp 120 and gp 100 are the glycosylated virion forms of the S1 and S2 subunits, respectively. Sequence comparisons between BCV and the antigenically related mouse hepatitis coronavirus revealed more sequence divergence in the putative knob region of the spike protein (S1) than in the stem region (S2).

Protection of mice from lethal coronavirus MHV-A59 infection by monoclonal affinity-purified spike glycoprotein

Numerous studies have provided indirect evidence that the spike glycoprotein of coronaviruses (E2 or S) bears determinants for pathogenesis and the induction of protective immunity. In order to directly evaluate its immunogenicity, the E2 glycoprotein of the murine hepatitis virus, strain A59, was purified by immunoaffinity chromatography. High titers of neutralizing and fusion inhibiting antibodies were induced in mice vaccinated with purified E2/S in Freund's adjuvant, which were protected from an intracerebral challenge with 10 LD50 of MHV-A59. This study provides a direct demonstration of the importance of the coronavirus spike glycoprotein in the induction of a protective immune response.

Protection from lethal coronavirus infection by affinity-purified spike glycoprotein of murine hepatitis virus, strain A59

Murine hepatitis viruses provide excellent animal models for the study of virus-induced diseases of the central nervous system and gastrointestinal tract. Several studies have indirectly provided evidence that the spike glycoprotein (S) of these coronaviruses bears determinants for pathogenesis and the induction of protective immunity. In order to directly evaluate the immunogenicity of this protein, it was purified by affinity chromatography with an in vitro neutralizing and in vivo protective monoclonal antibody which immunoprecipitated the 180-kDa spike glycoprotein of the neurotropic A59 strain of murine hepatitis virus (MHV-A59). Mice immunized twice with approximately 1 micrograms of purified S in Freund's adjuvant developed high titers of neutralizing and fusion inhibiting antibodies, even though the protein was at least partially denaturated after elution from the affinity column. Moreover, these mice were protected from lethal encephalitis when challenged intracerebrally with 10 LD50 of MHV-A59. This study provides a direct demonstration of the importance of the coronavirus spike glycoprotein in the induction of a protective immune response.

Stably expressed FIPV peplomer protein induces cell fusion and elicits neutralizing antibodies in mice

We have established bovine papilloma virus (BPV)-transformed mouse C127 cell lines that synthesize the peplomer protein of the feline infectious peritonitis virus (FIPV) strain 79-1146. For this purpose, a new cassette expression vector pHSL, which carries the Drosophila HSp70 promotor and the polyadenylation signal of the Moloney murine leukemia virus long terminal repeat, was constructed. Cocultivation of the BPV-transformed cell lines with FIPV-permissive feline fcwf-D cells resulted in polykaryocyte formation. Since it depended on the presence of fcwf-D cells, binding of E2 to the cell receptor may be required for membrane fusion. E2 was synthesized as a core-glycosylated protein of 180K which was only slowly transported from the endoplasmic reticulum to the medial Golgi: of the E2-molecules labeled during a 1-hr pulse about half was still completely sensitive to endoglycosidase H after a 2-hr chase, while the remaining E2 had been chased into multiple, partially endoglycosidase H-resistant forms. Immunofluorescence studies also indicated that most E2 was retained intracellularly. Mice immunized with whole lysates of the transformed cells produced FIPV-neutralizing antibodies as shown by plaque reduction.

Growth of a murine coronavirus in a microcarrier cell culture system

The growth of the murine coronavirus MHV-A59 on murine DBT cells adapted to dextran-made Cytodex 1 microcarriers was studied in comparison with cells grown on plastic dishes. With a microcarrier concentration of 5 g/l in spinner flasks, a density of 3 x 10(6) cells/ml was reached in 7 days. Under these conditions, cells supported virus growth to the same extent as when they were grown on the plastic substratum. This was shown by a similar development of virus-induced syncytia, the release of an equivalent number of infectious progeny virions per cell, similar recoveries observed after concentration and purification and an identical appearance of the purified virus under the electron microscope. On the other hand, the technical convenience of microcarriers and the ease of scale-up emphasize their potential for the growth of coronaviruses.