Enhancement of plaque formation and cell fusion of an enteropathogenic coronavirus by trypsin treatment

Cleavage-Activation of Respiratory Viruses - Half a Century of History from Sendai Virus to SARS-CoV-2

Many viruses require the cleavage-activation of membrane fusion proteins by host proteases in the course of infection. This knowledge is based on historical studies of Sendai virus in the 1970s. From the 1970s to the 1990s, avian influenza virus and Newcastle disease virus were studied, showing a clear link between virulence and the cleavage-activation of viral membrane fusion proteins (hemagglutinin and fusion proteins) by host proteases. In these viruses, cleavage of viral membrane fusion proteins by furin is the basis for their high virulence. Subsequently, from the 2000s to the 2010s, the importance of TMPRSS2 in activating the membrane fusion proteins of various respiratory viruses, including seasonal influenza viruses, was demonstrated. In late 2019, severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) emerged and caused a pandemic. The virus continues to mutate, producing variants that have caused global pandemics. The spike protein of SARS-CoV-2 is characterized by two cleavage sites, each of which is cleaved by furin and TMPRSS2 to achieve membrane fusion. SARS-CoV-2 variants exhibit altered sensitivity to these proteases. Thus, studying the cleavage-activation of membrane fusion proteins by host proteases is critical for understanding the ongoing pandemic and developing countermeasures against it.

ACE2-Independent Bat Sarbecovirus Entry and Replication in Human and Bat Cells

Hundreds of sarbecoviruses have been found in bats, but only a fraction of them have the ability to infect cells using angiotensin-converting enzyme 2 (ACE2), the receptor for SARS-CoV and -2. To date, only ACE2-dependent sarbecoviruses have been isolated from field samples or grown in the laboratory. ACE2-independent sarbecoviruses, comprising the majority of the subgenus, have not been propagated in any type of cell culture, as the factors and conditions needed for their replication are completely unknown. Given the significant zoonotic threat posed by sarbecoviruses, cell culture models and in vitro tools are urgently needed to study the rest of this subgenus. We previously showed that the exogenous protease trypsin could facilitate cell entry of viral-like particles pseudotyped with spike protein from some of the ACE2-independent sarbecoviruses. Here, we tested if these conditions were sufficient to support bona fide viral replication using recombinant bat sarbecoviruses. In the presence of trypsin, some of the spike proteins from clade 2 viruses were capable of supporting bat sarbecovirus infection and replication in human and bat cells. Protease experiments showed a specific viral dependence on high levels of trypsin, as TMPRSS2 and furin had no effect on clade 2 virus entry. These results shed light on how sarbecoviruses transmit and coexist in their natural hosts, provide key insights for future efforts to isolate and grow these viruses from field samples, and further underscore the need for broadly protective, universal coronavirus vaccines. IMPORTANCE Our studies demonstrate that some unexplored sarbecoviruses are capable of replicating in human and bat cells in an ACE2-independent way but need a high trypsin environment. We found that trypsin is not compensated by other known proteases involved in some coronavirus entry. This work provides important information that the trypsin-dependent entry may be a widely employed mechanism for coronaviruses and will help for further understanding the biological features of the less-studied viruses.

Evolutionary dynamics of bovine coronaviruses: natural selection pattern of the spike gene implies adaptive evolution of the strains

Coronaviruses demonstrate great potential for interspecies transmission, including zoonotic outbreaks. Although bovine coronavirus (BCoV) strains are frequently circulating in cattle farms worldwide, causing both enteric and respiratory disease, little is known about their genomic evolution. We sequenced and analysed the full-length spike (S) protein gene of 33 BCoV strains from dairy and feedlot farms collected during outbreaks that occurred from 2002 to 2010 in Sweden and Denmark. Amino acid identities were >97 % for the BCoV strains analysed in this work. These strains formed a clade together with Italian BCoV strains and were highly similar to human enteric coronavirus HECV-4408/US/94. A high similarity was observed between BCoV, canine respiratory coronavirus (CRCoV) and human coronavirus OC43 (HCoV-OC43). Molecular clock analysis of the S gene sequences estimated BCoV and CRCoV diverged from a common ancestor in 1951, while the time of divergence from a common ancestor of BCoV and HCoV-OC43 was estimated to be 1899. BCoV strains showed the lowest similarity to equine coronavirus, placing the date of divergence at the end of the eighteenth century. Two strongly positive selection sites were detected along the receptor-binding subunit of the S protein gene: spanning amino acid residues 109-131 and 495-527. By contrast, the fusion subunit was observed to be under negative selection. The selection pattern along the S glycoprotein implies adaptive evolution of BCoVs, suggesting a successful mechanism for BCoV to continuously circulate among cattle and other ruminants without disappearance.

Cleavage of the SARS coronavirus spike glycoprotein by airway proteases enhances virus entry into human bronchial epithelial cells in vitro

Background

Entry of enveloped viruses into host cells requires the activation of viral envelope glycoproteins through cleavage by either intracellular or extracellular proteases. In order to gain insight into the molecular basis of protease cleavage and its impact on the efficiency of viral entry, we investigated the susceptibility of a recombinant native full-length S-protein trimer (triSpike) of the severe acute respiratory syndrome coronavirus (SARS-CoV) to cleavage by various airway proteases.

Methodology/Principal Findings

Purified triSpike proteins were readily cleaved in vitro by three different airway proteases: trypsin, plasmin and TMPRSS11a. High Performance Liquid Chromatography (HPLC) and amino acid sequencing analyses identified two arginine residues (R667 and R797) as potential protease cleavage site(s). The effect of protease-dependent enhancement of SARS-CoV infection was demonstrated with ACE2 expressing human bronchial epithelial cells 16HBE. Airway proteases regulate the infectivity of SARS-CoV in a fashion dependent on previous receptor binding. The role of arginine residues was further shown with mutant constructs (R667A, R797A or R797AR667A). Mutation of R667 or R797 did not affect the expression of S-protein but resulted in a differential efficacy of pseudotyping into SARS-CoVpp. The R667A SARS-CoVpp mutant exhibited a lack of virus entry enhancement following protease treatment.

Conclusions/Significance

These results suggest that SARS S-protein is susceptible to airway protease cleavage and, furthermore, that protease mediated enhancement of virus entry depends on specific conformation of SARS S-protein upon ACE2 binding. These data have direct implications for the cell entry mechanism of SARS-CoV along the respiratory system and, furthermore expand the possibility of identifying potential therapeutic agents against SARS-CoV.

Furin cleavage of the SARS coronavirus spike glycoprotein enhances cell-cell fusion but does not affect virion entry

The fusogenic potential of Class I viral envelope glycoproteins is activated by proteloytic cleavage of the precursor glycoprotein to generate the mature receptor-binding and transmembrane fusion subunits. Although the coronavirus (CoV) S glycoproteins share membership in this class of envelope glycoproteins, cleavage to generate the respective S1 and S2 subunits appears absent in a subset of CoV species, including that responsible for the severe acute respiratory syndrome (SARS). To determine whether proteolytic cleavage of the S glycoprotein might be important for the newly emerged SARS-CoV, we introduced a furin recognition site at single basic residues within the putative S1–S2 junctional region. We show that furin cleavage at the modified R667 position generates discrete S1 and S2 subunits and potentiates membrane fusion activity. This effect on the cell–cell fusion activity by the S glycoprotein is not, however, reflected in the infectivity of pseudotyped lentiviruses bearing the cleaved glycoprotein. The lack of effect of furin cleavage on virion infectivity mirrors that observed in the normally cleaved S glycoprotein of the murine coronavirus and highlights an additional level of complexity in coronavirus entry.

Circulation of genetically distinct contemporary human coronavirus OC43 strains

In this study, we report the complete genome sequence of two contemporary human coronavirus OC43 (HCoV-OC43) strains detected in 2003 and 2004, respectively. Comparative genetic analyses of the circulating strains and the prototype HCoV-OC43 strain (ATCC VR759) were performed. Remarkably, a lower than expected similarity is found between the complete genomes and more in particular between the spike genes of the BE03 and BE04 strains. This finding suggests the existence of two genetically distinct HCoV-OC43 strains, circulating in Belgium in 2003 and 2004. Spike gene sequencing of three additional 2003 and two additional 2004 HCoV-OC43 strains, and subsequent phylogenetic analysis confirm this assumption. Compared to the ATCC prototype HCoV-OC43 strain, an important amino acid substitution is present in the potential cleavage site sequence of the spike protein of all contemporary strains, restoring the N-RRXRR-C motif, associated with increased spike protein cleavability in bovine coronaviruses. We here describe specific characteristics associated with circulating HCoV-OC43 strains, and we provide substantial evidence for the genetic variability of HCoV-OC43.

Identification of a receptor-binding domain of the spike glycoprotein of human coronavirus HCoV-229E

Human coronavirus HCoV-229E uses human aminopeptidase N (hAPN) as its receptor (C. L. Yeager et al., Nature 357:420-422, 1992). To identify the receptor-binding domain of the viral spike glycoprotein (S), we expressed soluble truncated histidine-tagged S glycoproteins by using baculovirus expression vectors. Truncated S proteins purified by nickel affinity chromatography were shown to be glycosylated and to react with polyclonal anti-HCoV-229E antibodies and monoclonal antibodies to the viral S protein. A truncated protein (S(547)) that contains the N-terminal 547 amino acids bound to 3T3 mouse cells that express hAPN but not to mouse 3T3 cells transfected with empty vector. Binding of S(547) to hAPN was blocked by an anti-hAPN monoclonal antibody that inhibits binding of virus to hAPN and blocks virus infection of human cells and was also blocked by polyclonal anti-HCoV-229E antibody. S proteins that contain the N-terminal 268 or 417 amino acids did not bind to hAPN-3T3 cells. Antibody to the region from amino acid 417 to the C terminus of S blocked binding of S(547) to hAPN-3T3 cells. Thus, the data suggest that the domain of the spike protein between amino acids 417 and 547 is required for the binding of HCoV-229E to its hAPN receptor.

Antibody responses of cattle with respiratory coronavirus infections during pathogenesis of shipping fever pneumonia are lower with antigens of enteric strains than with those of a respiratory strain

The serum antibody responses of cattle with respiratory coronavirus infections during the pathogenesis of shipping fever pneumonia were analyzed with different bovine coronavirus antigens, including those from a wild-type respiratory bovine coronavirus (RBCV) strain (97TXSF-Lu 15-2) directly isolated from lung tissue from a fatally infected bovine, a wild-type enteropathogenic bovine coronavirus (EBCV) strain (Ly 138-3), and the highly cell culture-adapted, enteric prototype strain (EBCV L9-81). Infectivity-neutralizing (IN) and hemagglutinin-inhibiting (HAI) activities were tested. Sequential serum samples, collected during the onset of the respiratory coronavirus infection and at weekly intervals for 5 weeks thereafter, had significantly higher IN and HAI titers for antigens of RBCV strain 97TXSF-Lu15-2 than for the wild-type and the highly cell culture-adapted EBCV strains, with P values ranging from <0.0001 to 0.0483. The IN and HAI antibody responses against the two EBCV strains did not differ significantly, but the lowest titers were detected with EBCV strain L9-81.

The spike but not the hemagglutinin/esterase protein of bovine coronavirus is necessary and sufficient for viral infection

The spike (S) and hemagglutinin/esterase (HE) of bovine coronavirus (BCV) are the two envelope proteins that recognize the same receptor-determinant of 9-O-acetylneuraminic acid on host cells. However, the precise and relative roles of the two proteins in BCV infectivity remain elusive. To unequivocally determine their roles in viral cytopathogenicity, we developed a system in which phenotypically chimeric viruses were generated by infecting a closely related mouse hepatitis virus (MHV) in cells that stably express an individual BCV protein (S or HE). The chimeric viruses were then used to infect human rectal tumor (HRT)-18 cells that are permissive to BCV but are nonsusceptible to MHV. Using this approach, we found that the chimeric virus containing the BCV S protein on the virion surface entered and replicated in HRT-18 cells; this was specifically blocked by prior treatment of the virus with a neutralizing antibody specific to the BCV S protein, indicating that the BCV S protein is responsible for initiating chimeric virus infection. In contrast, chimeric viruses that contain biologically active and functional BCV HE protein on the surface failed to enter HRT-18 cells, indicating that the BCV HE protein alone is not sufficient for BCV infection. Taken together, these results demonstrate that the S protein but not the HE protein of BCV is necessary and sufficient for infection of the chimeric viruses in HRT-18 cells, suggesting that BCV likely uses the S protein as a primary vehicle to infect permissive cells.

Infectivity-neutralizing and hemagglutinin-inhibiting antibody responses to respiratory coronavirus infections of cattle in pathogenesis of shipping fever pneumonia

Respiratory bovine coronaviruses (RBCV) emerged as an infectious agent most frequently isolated from respiratory tract samples of cattle with acute respiratory tract diseases. Infectivity-neutralizing (IN) and hemagglutinin-inhibiting (HAI) antibodies induced by RBCV infections were monitored in sequential serum samples collected from cattle during a naturally evolving and experimentally monitored epizootic of shipping fever pneumonia (SFP). Cattle nasally shedding RBCV at the beginning of the epizootic started with low levels of serum IN and HAI antibodies. An increase in serum IN antibody after day 7 led to reduction of virus shedding in nasal secretions by the majority of the cattle between days 7 and 14. A substantial rise in the serum HAI antibody was observed during the initial phase among the sick but not the clinically normal cattle which were infected with RBCV. The RBCV isolation-positive cattle that developed fatal SFP had minimal serum IN and HAI antibodies during the course of disease development. Cattle that remained negative in RBCV isolation tests entered this epizootic with high levels of serum IN and HAI antibodies, which dramatically increased during the next two weeks. Protection against SFP was apparently associated with significantly higher levels of serum IN antibodies at the beginning of the epizootic. The RBCV-neutralizing activity is associated with serum immunoglobulin G (IgG), particularly the IgG2 subclass, while RBCV-specific HAI antibody is related to both serum IgG and IgM fractions.

A single amino acid change within antigenic domain II of the spike protein of bovine coronavirus confers resistance to virus neutralization

The spike glycoprotein is a major neutralizing antigen of bovine coronavirus (BCV). Conformational neutralizing epitopes of group A and group B monoclonal antibodies (MAbs) have previously been mapped to two domains at amino acids 351 to 403 (domain I) and amino acids 517 to 621 (domain II). To further map antigenic sites, neutralization escape mutants of BCV were selected with a group A MAb which has both in vitro and in vivo virus-neutralizing ability. The escape mutants were demonstrated to be neutralization resistant to the selecting group A MAb and remained sensitive to neutralization by a group B MAb. In radioimmunoprecipitation assays, the spike proteins of neutralization escape mutants were shown to have lost their reactivities with the selecting group A MAb. Sequence analysis of the spike protein genes of the escape mutants identified a single nucleotide substitution of C to T at position 1583, resulting in the change of alanine to valine at amino acid position 528 (A528V). The mutation occurs in domain II and in a location which corresponds to the hypervariable region of the spike protein of the coronavirus mouse hepatitis virus. Experimental introduction of the A528V mutation into the wild-type spike protein resulted in the loss of MAb binding of the mutant protein, confirming that the single point mutation was responsible for the escape of BCV from immunological selective pressure.

Isolation of respiratory bovine coronavirus, other cytocidal viruses, and Pasteurella spp from cattle involved in two natural outbreaks of shipping fever

OBJECTIVE

To identify cytocidal viruses and Pasteurella spp that could be isolated from cattle involved in 2 natural outbreaks of shipping fever.

ANIMALS

105 and 120 castrated male 4- to 8-month-old feedlot cattle involved in 1997 and 1998 outbreaks, respectively.

PROCEDURES

Nasal swab specimens and blood samples were collected, and cattle were vaccinated on arrival at an order-buyer barn from 4 local auction houses. Four days later, they were transported to a feedlot, and additional nasal swab specimens and blood samples were collected. Nasal swab specimens were submitted for virus isolation and bacterial culture; blood samples were submitted for measurement of respiratory bovine coronavirus (RBCV) hemagglutinin inhibition titers.

RESULTS

93 of 105 cattle and 106 of 120 cattle developed signs of respiratory tract disease during 1997 and 1998, respectively, and RBCV was isolated from 81 and 89 sick cattle, respectively, while at the order-buyer's barn or the day after arrival at the feedlot. During the 1997 outbreak, bovine herpesvirus 1 was isolated from 2 cattle at the order-buyer's barn and from 5 cattle 7 and 14 days after arrival at the feedlot, and parainfluenza virus 3 was isolated from 4 cattle 14 days after arrival at the feedlot. During the 1998 outbreak, bovine herpesvirus 1 was isolated from 2 cattle at the order-buyer's barn and on arrival at the feedlot and from 5 cattle 7 and 14 days after arrival at the feedlot, and parainfluenza virus 3 was isolated from 1 animal the day of, and from 18 cattle 7 and 14 days after, arrival at the feedlot. Pasteurella spp was cultured from 4 and 6 cattle at the order-buyer's barn and from 92 and 72 cattle on arrival at the feedlot during the 1997 and 1998 outbreaks, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that RBCV may play a causative role in outbreaks of shipping fever in cattle. More than 80% of the sick cattle shed RBCV at the beginning of 2 outbreaks when the Pasteurella spp infection rate was low.

Adaptation of human enteric coronavirus to growth in cell lines

BACKGROUND

The existence of human enteric coronavirus (HEC) has been debated since its first description in stool by electron microscopy (EM) in 1975. Needed to resolve the issue is its cultivation in readily available cell lines.

OBJECTIVES

To grow HEC in cell lines. To describe its characteristics and to differentiate it from other human and animal coronaviruses.

STUDY DESIGN

Originally grown in human fetal intestinal organ culture, HEC was passed in J774 cells (a mouse macrophage cell line) and C6/36 cells (a mosquito cell line). Its cytopathic effect (CPE) and pattern of immunofluorescence were described. Its appearance was ascertained by negative staining and transmission EM. Its structural proteins were delineated by polyacrylamide gel electrophoresis (PAGE) and Western blotting (WB). The antigenic character of the virus was determined by immunofluorescence and WB. Agglutination with mouse erythrocytes was performed.

RESULTS

In J774 cells, HEC induced the formation of giant cells and small syncytia. Immunofluorescence in both J774 and C6/36 cells was limited to the cytoplasm. Studies with transmission EM revealed the virus to have the typical appearance of other coronaviruses, to be 80-120 nm in diameter, and to bud into cysternae of the endoplasmic reticulum. By PAGE and WB, its major protein has an average molecular weight (MW) of 41 kilodaltons (kDa). Two other proteins had MWs of 190 and 24 kDa. By immunofluorescence and WB, HEC is antigenically distinct from human coronaviruses 0C43 and 229E and mouse hepatitis virus (A59 strain). Preparations of HEC did not agglutinate mouse erythrocytes.

CONCLUSION

We conclude that HEC is a human coronavirus that is antigenically unrelated to 0C43 and 229E viruses. Growth of HEC in readily available cell lines should aid in elucidating its role as a pathogen in human diarrheal illnesses.

Cell culture-grown putative bovine respiratory torovirus identified as a coronavirus

A putative bovine respiratory torovirus (BRTV) was propagated in bovine fetal diploid lung and human colonic tumour cells, and fringed pleomorphic particles were detected in the culture supernatants by electron microscopy. Antisera directed against a bovine (Breda strain) and equine (Berne strain) torovirus failed to react with BRTV-infected cells in immunofluorescence assays and did not neutralise BRTV. No toroviral RNA was found in the supernatants of infected cells by means of a reverse transcriptase-polymerase chain reaction with torovirus-specific primers. On the other hand, bovine coronavirus-specific antisera and monoclonal antibodies did neutralise the cytopathic effects, and coronaviral antigen was detected in the cultures by immunofluorescence. Furthermore, bovine coronavirus RNA was detected in the supernatants of BRTV-infected cells after nucleic acid amplification. It is concluded that the cytopathic BRTV isolate is a coronavirus.

Sensitive plaque assay and propagation of Chuzan (Kasba) virus, a Palyam serogroup orbivirus, in BHK-21 cells

Various factors influencing plaque formation of Chuzan virus in BHK-21 cell monolayers were studied and a practical method for plaque assay was developed. On addition of trypsin (5 micrograms/ml) and/or diethylaminoethyl (DEAE)-dextran (50 micrograms/ml) to the virus diluent as the virus adsorption medium and agar overlay medium, the number of plaques increased. When 100 micrograms/ ml DEAE-dextran was added to the diluent and overlay medium, plaques were produced in about 10-fold higher numbers than without trypsin and DEAE-dextran. Based on these results, a practical plaque assay method for Chuzan virus was established. Using this method, one-step growth of Chuzan virus was performed at an input multiplicity of 25 plaque-forming units (PFU) per cell. Cytopathic effects were first observed at 7.5 h post-inoculation (p.i.), and were complete at 12 h p.i. The titre of cell-associated virus, after gradual decline during the first 3 h of incubation, showed a rise within 4.5 h p.i. and a rise to a plateau of 10(6.3)PFU/0.2 ml at 12 h p.i. By indirect immunofluorescence, virus-specific antigen was detected in the cytoplasm of the cells at 4.5 h p.i., and all the cells fluoresced at 6 h p.i. Haemagglutination activity was first detected in infected whole cultures at 7.5 h p.i. reaching a plateau of 1:64 at 15 h p.i. Plaque formation and haemagglutination by the virus were specifically inhibited by antisera against the original and the plaque-cloned virus.

Infectious subvirion particles of reovirus type 3 Dearing exhibit a loss in infectivity and contain a cleaved sigma 1 protein

Mammalian reoviruses exhibit differences in the capacity to grow in intestinal tissue: reovirus type 1 Lang (T1L), but not type 3 Dearing (T3D), can be recovered in high titer from intestinal tissue of newborn mice after oral inoculation. We investigated whether in vitro protease treatment of virions of T1L and T3D, using conditions to generate infectious subvirion particles (ISVPs) as occurs in the intestinal lumen of mice (D. K. Bodkin, M. L. Nibert, and B. N. Fields, J. Virol. 63:4676-4681, 1989), affects viral infectivity. Chymotrypsin treatment of T1L was associated with a 2-fold increase in viral infectivity, whereas identical treatment of T3D resulted in a 10-fold decrease in infectivity. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, we found that loss of T3D infectivity was correlated with cleavage of its sigma 1 protein. We used reassortant viruses to identify viral determinants of infectivity loss and sigma 1 cleavage and found that both phenotypes segregate with the sigma 1-encoding S1 gene. Comparable results were obtained when trypsin treatment of virions of T1L and T3D was used. In experiments to determine the fate of sigma 1 fragments following cleavage, the capacity of anti-sigma 1 monoclonal antibody G5 to neutralize infectivity of T3D ISVPs was significantly decreased in comparison with its capacity to neutralize infectivity of virions, suggesting that a sigma 1 domain bound by G5 is lost from viral particles after proteolytic digestion. In contrast to the decrease in infectivity, chymotrypsin treatment of T3D virions leading to generation of ISVPs resulted in a 10-fold increase in their capacity to produce hemagglutination, indicating that a domain of sigma 1 important for binding to sialic acid remains associated with viral particles after sigma 1 cleavage. Neuraminidase treatment of L cells substantially decreased the yield of T3D ISVPs in comparison with the yield of virions, indicating that a sigma 1 domain important for binding sialic acid also can mediate attachment of T3D ISVPs to L cells and lead to productive infection. These results suggest that cleavage of T3D sigma 1 protein following oral inoculation of newborn mice is at least partly responsible for the decreased growth of T3D in the intestine and provide additional evidence that T3D sigma 1 contains more than a single receptor-binding domain.

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)

Comparative sequence analysis of a polymorphic region of the spike glycoprotein S1 subunit of enteric bovine coronavirus isolates

Complementary oligonucleotide primers which flank a 1146-nucleotide gene fragment (S1B: nt 1185 to 2333) encompassing a polymorphic region (nt 1368 to 1776) of the S1 subunit of bovine coronavirus spike glycoprotein were used for enzymatic amplification by PCR. We chose four clinical isolates, recovered from cases of epidemic diarrhea in neonatal calves in Québec dairy herds between 1987-1990, to specifically amplify and analyze their sequences in the selected genomic area. Nucleotide sequence analysis of the four clinical isolates indicated that their S1B gene fragments were highly conserved. We also compared the S1B gene sequences of the Québec BCV isolates to the published corresponding sequences from BCV-L9 [37], BCV-MEB [1], and BCV-F15 [3] reference strains. A high degree of similarity was demonstrated for all viruses, no deletions or insertions were observed, and the only variations that were identified consisted of nucleotide substitutions. The differing nucleotides and amino acids (aa) were not distributed randomly over the entire sequence but rather were clustered in the polymorphic region. Of these, four sporadic aa changes were located in antigenic domain II (aa residues 517 to 720) of S1. This correlates with varied antigenicity observed among the BCV Québec isolates when reacting with MAbs directed against the S glycoprotein of the Mebus strain. The other mutations seem to be fixed in all Québec isolates.

Development and optimization of plaque assays for rat coronaviruses

Plaque assays under Sephadex or agarose overlays are described for rat coronaviruses (RCVs) grown in L2 mouse fibroblasts. A plaque assay using Sephadex was simple; however, viable plaques could not be collected for propagation, and fixation was necessary before evaluation. Plaque formation under agarose was optimized using diethylaminoethyl-dextran (DEAE-D) in the pre-treatment and absorption media and trypsin added to the absorption media and agarose overlay. The use of DEAE-D alone, trypsin alone or trypsin combined with DEAE-D significantly increased plaque numbers and visibility. Plaque numbers were highest when pre-treatment media contained DEAE-D, absorption media contained DEAE-D and trypsin, and the agarose overlay contained trypsin. The assay was useful for plaque isolation and quantification of sialodacryoadenitis virus (SDA), Parker's rat coronavirus (PRCV) and other coronavirus isolates from rats and its specificity was demonstrated by plaque-reduction neutralization testing. These methods will facilitate production of cloned virus stocks for study of RCV biology and virus quantification for in vitro and in vivo studies of RCVs.

Physical and chemical methods for enhancing rapid detection of viruses and other agents

Viral replication events can be enhanced by physical, chemical, or heat treatment of cells. The centrifugation of cells can stimulate them to proliferate, reduce their generation times, and activate gene expression. Human endothelial cells can be activated to release cyclo-oxygenase metabolites after rocking for 5 min, and mechanical stress can stimulate endothelial cells to proliferate. Centrifugation of virus-infected cultures can increase cytopathic effects (CPE), enhance the number of infected cells, increase viral yields, and reduce viral detection times and may increase viral isolation rates. The rolling of virus-infected cells also has an effect similar to that of centrifugation. The continuous rolling of virus-infected cultures at < or = 2.0 rpm can enhance enterovirus, rhinovirus, reovirus, rotavirus, paramyxovirus, herpesvirus, and vaccinia virus CPE or yields or both. For some viruses, the continuous rolling of infected cell cultures at 96 rpm (1.9 x g) is superior to rolling at 2.0 rpm for viral replication or CPE production. In addition to centrifugation and rolling, the treatment of cells with chemicals or heat can also enhance viral yields or CPE. For example, the treatment of virus-infected cells with dimethyl sulfoxide can enhance viral transformation, increase plaque numbers and plaque size, increase the number of cells producing antigens, and increase viral yields. The infectivity of fowl plague virus is increased by 80-fold when 4% dimethyl sulfoxide is added to culture medium immediately after infection. The heat shocking of virus-infected cells also has been shown to have a stimulatory effect on the replication events of cytomegalovirus, Epstein-Barr virus, and human immunodeficiency virus. The effects of motion, chemicals, or heat treatments on viral replication are not well understood. These treatments apparently activate cells to make them more permissive to viral infection and viral replication. Perhaps heat shock proteins or stress proteins are a common factor for this enhancement phenomenon. The utility of these treatments alone or in combination with other methods for enhancing viral isolation and replication in a diagnostic setting needs further investigation.

Bovine coronavirus

This review aims to summarize current data describing the characteristics of bovine coronavirus (BCV) and the three clinical syndromes with which this virus is associated. The first half of this paper consists of a general description of the virus, commencing with a brief outline of the methods used for in vitro growth. The structure of the virus is then described in more detail, with particular reference to the structure and functions of the four major viral proteins. This is followed by an outline of the unique replication strategy adopted by coronaviruses. The second half of this review discusses the clinical significance of the virus, beginning with a detailed account of BCV-induced neonatal calf diarrhoea, the clinical syndrome with which this virus is most commonly associated. The clinical and epidemiological importance of BCV respiratory tract infection is then discussed, and finally the evidence supporting the aetiological role of BCV in outbreaks of winter dysentery in adult cattle is examined.

Virus infection of polarized epithelial cells

This chapter focuses on the interaction of viruses with epithelial cells. The role of specific pathways of virus entry and release in the pathogenesis of viral infection is examined together with the mechanisms utilized by viruses to circumvent the epithelial barrier. Polarized epithelial cells in culture, which can be grown on permeable supports, provide excellent systems for investigating the events in virus entry and release at the cellular level, and much information is being obtained using such systems. Much remains to be learned about the precise routes by which many viruses traverse the epithelial barrier to initiate their natural infection processes, although important information has been obtained in some systems. Another area of great interest for future investigation is the process of virus entry and release from other polarized cell types, including neuronal cells.

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.

Comparison of hemagglutinating, receptor-destroying, and acetylesterase activities of avirulent and virulent bovine coronavirus strains

Hemagglutinating and acetylesterase functions as well as the 124 kDa glycoprotein were present in the highly cell-culture adapted, avirulent bovine coronavirus strain BCV-L9, in the Norden vaccine strain derived from it, and in 5 wild-type, virulent strains that multiplied in HRT-18 cells but were restricted in several types of cultured bovine cells. The BCV-L9 and the wild-type strain BCV-LY-138 agglutinated chicken and mouse erythrocytes. The acetylesterase facilitated break-down of the BCV-erythrocyte complex with chicken but only to a minimal extent with mouse erythrocytes in the receptor-destroying enzyme test. Purified preparations of the vaccine and the wild-type strains agglutinated chicken erythrocytes at low titers and mouse erythrocytes at 128 to 256 times higher titers whereas receptor destroying enzyme activity was detectable only with chicken erythrocytes. When wild-type strains were propagated in HRT cells at low passage levels, they produced 5 x 10(5) to 4.5 x 10(6) plaque forming units per 50 microliters which agglutinated erythrocytes from mice but not from chickens. Diisopropylfluoro-phosphate moderately increased the hemagglutination titers, but completely inhibited the receptor destroying enzyme of purified virus of all strains. It had virtually no influence on the plaque-forming infectivity of the different BCV strains. The acetylesterase of strain BCV-L9 reacting in the receptor-destroying enzyme test was stable for 3 h at 37 and 42 degrees C. It was inactivated within 30 min at 56 degrees C while the hemagglutinin function of this strain was stable for 3 h at 37, 42, and 56 degrees C, but it was inactivated at 65 degrees C within 1 h.

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.

Comparison of bovine coronavirus (BCV) antigens: monoclonal antibodies to the spike glycoprotein distinguish between vaccine and wild-type strains

Monoclonal antibodies (MAbs) against two major structural proteins of the cell-adapted Mebus strain of bovine coronavirus (BCV-L9) were produced and characterized. Seven MAbs reacted with the peplomeric glycoprotein, gp 100/S, while three MAbs reacted with the nucleoprotein p53/N in Western blot analysis of BCV polypeptides. MAbs to gp 100/S reacted with discontinuous epitopes of gp 100/S in Westerns under mild but not under standard denaturing conditions. In contrast, MAbs to p53/N reacted in both types of Westerns, and those epitopes were thus continuous. MAbs to p53/N failed to neutralize BCV infectivity, while 4 MAbs to gp 100/S neutralized BCV effectively. Cross reactivity of MAbs to gp 100/S specified by five virulent wild-type strains and two high passage, cell-culture-adapted strains in mildly denaturing Westerns and neutralization assays indicated that two epitopes were conserved in all seven strains, while two epitopes of the avirulent strains were not detected in the wild-type strains. Non-neutralizing MAbs of gp 100/S reacted with all seven strains in Westerns with the exception of one MAb that was specific for the highly cell-adapted strain BCV-L9.

Comparison of the nucleotide and deduced amino acid sequences of the S genes specified by virulent and avirulent strains of bovine coronaviruses

The entire nucleotide sequences of the spike glycoprotein (S) genes of the highly virulent bovine coronavirus (BCV) strain BCV-LY138, the avirulent BCV-L9 and related Norden Vaccine (BCV-Vaccine) strains were determined using the polymerase chain reaction (PCR) to amplify cDNAs obtained by reverse transcription of viral RNA, and to produce single strand cDNAs for DNA sequencing. The S gene sequences of these viral strains were compared with those of recently published strains BCV-Mebus, BCV-Quebec, and BCV-F15. An open reading frame of 4092 nucleotides, encoding a protein of 1363 amino acid residues, was found in all six strains. Frameshifts and insertions or deletions were not observed except for the BCV-F15. The S gene sequences were more than 98% conserved overall inspite of different origins of the six viruses. There were 45 to 56 nt differences between the virulent and avirulent groups while there were 6 to 14 nt differences among four avirulent strains. Comparison of the deduced amino acid sequences indicated that the S proteins had typical properties of membrane glycoproteins. Nineteen N-linked glycosylation sites were predicted in five strains, and 18 of them were conserved in the avirulent strain BCV-L9. The sequence KRRSRR at the predicted proteolytic cleavage site was identified in five strains while the sequence KRRSVR was found in BCV-F15. Substitutions of few amino acids in the putative fusogenic domains and two prolines at 507 and 567 in the antigenic domains may cause altered immunogenic and other functional properties of the S proteins specified by the virulent and avirulent BCV strains. Nine amino acid substitutions between the virulent and avirulent groups may correlate with BCV virulence.

Susceptibility of rodent cell lines to rat coronaviruses and differential enhancement by trypsin or DEAE-dextran

Cell lines of rodent origin were tested for susceptibility to infection with rat coronavirus (RCV), including sialodacryoadenitis virus (SDAV) and Parker's rat coronavirus (PRCV). LBC rat mammary adenocarcinoma cells were susceptible only if the cells were treated with diethylaminoethyl-dextran (DEAE-D). A recent report that RCVs grow well in L2 mouse fibroblast cells was confirmed and expanded. RCV infection of L2 cells was substantially enhanced by treatment of cells with trypsin but not by treatment with DEAE-D. Primary isolation of SDAV from experimentally infected rats was accomplished using trypsin-treated L2 cells. One of 13 additional cell lines tested (rat urinary bladder epithelium, RBL-02) supported growth of RCVs, and growth was slightly enhanced by DEAE-D, but not by trypsin. These refinements of in vitro growth conditions for RCVs should facilitate further studies of their basic biology and improve options for primary isolation.

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.

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.

Bovine coronavirus antigen in the host cell plasmalemma

Expression of bovine coronavirus (BCV) antigen in the plasmalemma of epithelioid human rectal tumor (HRT-18) and fibroblastic bovine fetal spleen (BFS) cell lines was traced by immunofluorescence and immunoelectron microscopy facilitated by colloidal gold. Cytoplasmic fluorescence was first observed at 12 hr postinfection (h.p.i) in infected HRT-18 cultures. This fluorescence coincided with the appearance of cell surface antigen reacting with colloidal gold-labeled antibodies to BCV antigens. At 24 h.p.i the amount of viral antigens at the surface of HRT-18 had increased, although cytoplasmic fluorescence remained constant. Infected BFS cells but not HRT-18 cells formed polykaryons when incubated in the presence of trypsin. One viral antigen in the plasma membrane of BFS cells was thus identified as the S glycoprotein with a fusion domain. In contrast to HRT-18 cells, the overall amount of BCV antigens at the surface of BFS cells remained constant after the onset of fusion. Analysis of the labeling characteristics established that the gold-marked-sites represented de novo expression of BCV antigen in the plasma membrane of infected cells.

Scanning electron microscopic characterization of bovine coronavirus plaques in HRT cells

The ecology of cytopathic expression of bovine coronavirus (BCV) in HRT-18 cells was analyzed within virus-induced plaques by scanning electron microscopy. Virus replication was cytocidal for many HRT-18 cells, a function enhanced in the presence of trypsin. A monolayer of cells remained that imparted a characteristic turbidity to the plaque. These structurally normal, lysis-resistant cells did not stain with fluorescent antibodies specific for BCV antigens, failed to adsorb virus particles or mouse erythrocytes in contrast to the susceptible cells. The survival of cells in the plaque interior reflects a non-productively infected population with evidence of viral persistence.

Cell culture propagation of a coronavirus isolated from cows with winter dysentery

Fecal filtrates from cows with winter dysentery were inoculated into gnotobiotic and conventional calves, and a coronavirus was isolated from calf feces. Cytopathic effects were observed on human rectal tumor cells but not bovine cell cultures. The winter dysentery isolates morphologically and antigenically resembled the Mebus strain of bovine coronavirus.

Intraluminal proteolytic activation plays an important role in replication of type 1 reovirus in the intestines of neonatal mice

Oral inoculation of suckling mice with reovirus serotype 1 (strain Lang) results in the conversion of intact virions to intermediate subviral particles (ISVPs) in the intestinal lumen. Digestion of virus in vitro with chymotrypsin or trypsin reveals two distinct forms of ISVPs, while the predominant species of ISVPs found in the small intestinal lumen appears to be identical to the chymotrypsin product. The in vivo conversion of virions to ISVPs was blocked by pretreatment of mice with protease inhibitors, resulting in inefficient replication of reovirus in intestinal tissue. The early inhibition of viral replication in suckling mice pretreated with protease inhibitors was not observed when suckling mice were inoculated with ISVPs generated by in vitro digestion with either chymotrypsin or trypsin. However, replication was decreased during secondary rounds of replication in mice receiving repeated doses of protease inhibitors, suggesting that luminal proteolytic digestion is important in rendering progeny virions infectious in the gut.

A clustering of RNA recombination sites adjacent to a hypervariable region of the peplomer gene of murine coronavirus

Coronaviruses undergo RNA recombination at a very high frequency. To understand the mechanism of recombination in murine coronavirus, we have performed RNA sequencing of viral genomic RNA to determine the precise sites of recombination in a series of recombinants which have crossovers within the gene encoding the peplomer protein. We found that all of the recombination sites are clustered within a region of 278 nucleotides in the 5'-half of the gene. This region in which all of the crossovers occurred represents a small fraction of the distance between the two selection markers used for the isolation of these recombinant viruses. This result suggests that this region may be a preferred site for RNA recombination. The crossover sites are located within and immediately adjacent to a hypervariable area of the gene. This area has undergone deletions of varying sizes in several virus strains which have been passaged either in vivo or in vitro. These results suggest that a similar RNA structure may be involved in the occurrence of both recombination and deletion events.

Biosynthesis, structure, and biological activities of envelope protein gp65 of murine coronavirus

We have previously shown that gp65 (E3) is a virion structural protein which varies widely in quantity among different strains of mouse hepatitis virus (MHV). In this study, the biosynthetic pathway and possible biological activities of this protein were examined. The glycosylation of gp65 in virus-infected cells was inhibited by tunicamycin but not by monensin, suggesting that it contains an N-glycosidic linkage. Glycosylation is cotranslational and appears to be complete before the glycoprotein reaches the Golgi complex. Pulse-chase experiments showed that this protein decreased in size after 30 min of chase, suggesting that the carbohydrate chains of gp65 undergo trimming during its transport across the Golgi. This interpretation is supported by the endoglycosidase treatment of gp65, which showed that the peptide backbone of gp65 did not decrease in size after pulse-chase periods. This maturation pathway is distinct from that of the E1 or E2 glycoproteins. Partial endoglycosidase treatment indicated that gp65 contains 9 to 10 carbohydrate side chains; thus, almost all of the potential glycosylation sites of gp65 were glycosylated. In vitro translation studies coupled with protease digestion suggest that gp65 is an integral membrane protein. The presence of gp65 in the virion is correlated with the presence of an acetylesterase activity. No hemagglutinin activity was detected.

Identification and location of the structural glycoproteins of a tissue culture-adapted turkey enteric coronavirus

The Minnesota strain of turkey enteric coronavirus (TCV) was grown on a human rectal tumor (HRT-18) cell line in the presence of radiolabeled amino acids and glucosamine to analyse virion structural proteins. In addition to the 52,000 unglycosylated nucleocapsid protein, three major glycoprotein species were found to be associated with the viral envelope. A predominant glycosylated protein with a molecular weight of 22-24,000 represented the transmembrane matrix protein. Larger glycoproteins with apparent molecular weights of 180-200,000 (gp 200), 120-125,000 (gp 120) and 95-100,000 (gp 100) were associated to the characteristic large bulbous projections (peplomers) located at the surface of the virion. The gp 100 and gp 120 species apparently arose from a proteolytic cleavage of gp 200, as suggested by digestion studies with trypsin and chymotrypsin. An additional large glycoprotein with mol. wt. of 140,000 (gp 140), that behaved as a disulfide-linked dimer of a 66,000 molecule, was found to be associated to granular projections located near the base of the large peplomers. Digestion studies with trypsin, bromelain and pronase demonstrated that gp 140 was related to the hemagglutinating activity of the virus. An inner membranous sac or tongue-shaped structure could be visualized in the interior of the viral particles following treatment with pronase. In contrast, trypsin or chymotrypsin treatments resulted in evaginations ("budding") on the virus surface. Progeny viral particles produced in TCV-infected cell cultures in the presence of tunicamycin lacked both types of surface projections, as demonstrated by electron microscopy and electrophoresis. The matrix protein also appeared to be reduced to its unglycosylated form, concomitant with a considerable loss of its antigenicity. Thus, with respect to its morphological and biochemical characteristics, TCV resembles viruses belonging to the group of mammalian hemagglutinating coronaviruses, but differs in that both types of envelope glycoproteins are N-glycosylated as in case of the avian infectious bronchitis virus.

Propagation of the virus of porcine epidemic diarrhea in cell culture

Porcine epidemic diarrhea virus (PEDV) was adapted to serial propagation in Vero cell cultures by adding trypsin to the medium. PEDV-infected cells showed a distinct cytoplasmic fluorescence when examined by a fluorescent-antibody-staining technique. Cytopathic effects, such as vacuolation, formation of syncytia, and fusion of cells, were detected even at passage 1 of the PEDV in Vero cells. Once adapted, the virus induced numerous syncytia containing over 100 nuclei. From virus passage 5 on, all cells forming the monolayer were fused and totally destroyed within 24 h after inoculation. Cell culture-grown PEDV had typical coronavirus morphology when viewed by electron microscopy. Attempts to propagate PEDV in several primary and secondary fetal porcine cell cultures in the presence or absence of trypsin were unsuccessful.

In vivo and in vitro models of demyelinating disease: efficiency of virus spread and formation of infectious centers among glial cells is genetically determined by the murine host

Resistance or susceptibility of various mouse strains to central nervous system disease caused by different strains of coronavirus is well known. Data from the present study draw attention to an additional, genetically determined mechanism controlling CV infections. The resistance to A59 and JHM virus (JHMV) associated with SJL mice was maintained in explanted glial cultures which, by contrast, fully supported a productive infection by the serorelated mouse hepatitis virus type 3. A comparative analysis of the infectious process in glial cell explants from SJL and CD.1 mice helped to define the stage at which restriction is manifested. Cultures of oligodendrocytes and astrocytes from these strains of mice were challenged with JHMV or mouse hepatitis virus type 3, and cell-virus interactions were monitored, including adsorption, uptake of inoculum, transcription, and cell-to-cell dissemination. The sequence of early events from adsorption to genome activation occurred with about equal efficiency with both viruses and genetically different cells, indicating that SJL resistance is not due to any deficiency in specific receptors or penetration of the inoculum or general expression of viral functions. However, intercellular spread of the infection was restricted in SJL glial cells owing to an as yet undefined component. Since cells from (SJL x CD.1)F1 mice were fully susceptible to JHMV, resistance to virus spread must be due to a deficiency in some factor, perhaps a proteolytic activity necessary for dissemination.

Cell culture propagation of bovine coronavirus

Although most field strains of bovine coronavirus (BCV) grow poorly in cell culture and fail to produce cytopathic effects (CPE) until after blind passage, primary calf kidney (PCK) and Vero cells have permitted primary isolation of virus. Cell culture-adapted strains of BCV replicate in PCK, bovine embryonic lung, bovine fetal thyroid, bovine fetal brain, bovine skin cells, ovine fetal kidney cells, and the cell lines pig kidney K3 and 15, Vero, human embryonic lung fibroblasts, HRT-18, MDBK and BEK-1, with trypsin useful for enhancing replication. Organ culture as well as suckling mouse, rat, and hamster brains also support the growth of cell culture-adapted BCV strains. Viral growth is most commonly detected by CPE, immunofluorescence, hemagglutination, and hemadsorption assays or electron microscopy of supernatants from infected cells. In this report, the optimal conditions for the growth and plaque assay of the NCDV strain of BCV in MDBK cells are described.

Cultivation techniques for animal coronaviruses: Emphasis on feline infectious peritonitis virus, canine coronavirus, transmissible gastroenteritis virus, and porcine hemagglutinating encephalomyelitis virus

Techniques are described for the growth and characterization of some mammalian coronaviruses. Because of the fastidious nature of their growth requirements, most will replicate only in cells derived from the natural host or a closely related species. Fetal cat cells are used to grow FIPV, and porcine cells are used to grow TGEV and HEV. However, CCV will replicate in both feline and canine cells. Although all four of these viruses prefer to replicate in a cell in the stationary phase of growth, FIPV is able to replicate in an actively growing cell. Each virus causes a cytopathic effect in monolayer cell cultures under agar or media 18 to 72 h postinfection. Primary isolation of each virus from field specimens is difficult, although most can usually be isolated after 1 to 3 blind passages in the cell culture.

Structural proteins of bovine coronavirus strain L 9: effects of the host cell and trypsin treatment

The polypeptide profile of the cell-adapted strain of bovine coronavirus (Mebus BCV-L 9) is remarkably affected by the host cell and trypsin. We compared the structural proteins of virus purified from different cell lines and found cell-dependent differences in the virus structure. BCV was purified from four clones of human rectal tumour cells (HRT-18): 3F3, D2, 3E3, and 4B3. The structural profiles of BCV propagated in clones 3E3 and 3F3 were identical, consisting of proteins with molecular weights of 185, 160, 140, 125, 110, 100, 52, 46, 37, 31-34, and 26-28 kilodaltons (kd). BCV purified from clone D2 lacked the 100 kd species, and clone 4B3 yielded virus lacking the 46 kd protein. We compared the structures of BCV propagated in HRT-18 cells [BCV(HRT-18)] and virus raised in bovine fetal spleen cells [BCV(D2 BFS)]. The concentration of the 185 kd protein was higher in BCV (D2BFS), and it also contained a 200 kd species. Protein profiles of in vitro trypsin treated and untreated BCV(HRT-18) differed only under reducing conditions, suggesting that trypsin cleavage sites are located within disulfide-linked regions of affected proteins. Propagation of BCV in D2 BFS cells in the presence of trypsin resulted in cleavage of the 185 kd protein and a concomitant increase of the 100 kd protein. Activation of the fusion function probably depends on this cleavage process because fusion of BCV-infected D2 BFS cells is trypsin dependent.

Analysis of cell fusion induced by bovine coronavirus infection

Polykaryon formation in bovine fetal spleen (BFS) cells infected with bovine coronavirus L9 occurred only in media supplemented with trypsin. A single 1 to 2 h trypsin treatment 10 h and later after infection induced formation of polykaryons. Trypsin treatment at pH 7.5 and 8.0 induced polykaryons while treatments at lower or higher pH levels did not. Cell fusion activity was partially suppressed by the presence of antibody.

The role of protease-dependent cell membrane fusion in persistent and lytic infections of murine hepatitis virus

We have defined three categories of cultured cell lines on the basis of their permissiveness (susceptibility to initial infection) to mouse hepatitis virus (MHV). Fully permissive L-2 cells gave rise to 100-1000-fold higher numbers of infectious centers than did semi-permissive LM, LM-K or C-1300 cells, whereas non-permissive Vero or C-6 cells were refractory to MHV infection. On an infected cell basis, there was no deficiency on the part of semi-permissive cell lines to replicate total viral RNA, viral polypeptides or progeny virions. Two of the semi-permissive cell lines (LM and LM-K) supported persistent MHV infection, while a third (C-1300) succumbed to lytic infection. LM and LM-K cells, but not C-1300 cells showed resistance to MHV-induced membrane fusion, even when placed in contact with fusion-active MHV-infected L-2 cells. The ability of a given cell to undergo fusion did not correlate with membrane lipid characteristics (unsaturated fatty acid and sterol content) which contribute to membrane "fluidity". In order to more closely study the parameters of MHV-induced cell fusion, membranes were prepared from MHV-infected L-2 cells and monitored for their fusogenic potential with permissive L-2 cells, semi-permissive LM cells and non-permissive vero cells. Fusion was only observed with the permissive L-2 cells, and only when exogenous protease (trypsin or chymotrypsin) was added. When the membranes were prepared from 35S-methionine-labeled MHV-infected L-2 cells and subjected to protease treatment, the radiolabeled 180,000 dalton form of the E2-glycoprotein underwent proteolytic cleavage to yield a major product of approximately 90,000 daltons. Both trypsin and chymotrypsin were effective in this proteolytic cleavage and in activating membrane fusion. In a normally permissive, fusogenic infection of MHV in L-2 cells, the protease inhibitors TPCK and ZPCK, but not TLCK, were found to inhibit cell fusion. In MHV-infected L-2 cells, E2 was found almost exclusively as the 180,000 dalton form but turned over rapidly as shown by pulse-chase studies. TPCK and ZPCK but not TLCK inhibited turnover. The results suggest that L-2 cells contain a protease which cleaves at aromatic amino acids such as phenylalanine, and that this protease cleaves the 180,000 dalton form of the E2 to peptide fragments, one or more of which may activate cell fusion.