Structure and intracellular assembly of the transmissible gastroenteritis coronavirus

Coronaviruses have been described as pleomorphic, round particles with a helical nucleocapsid as the unique internal structure under the virion envelope. Our studies on the organization of the transmissible gastroenteritis coronavirus (TGEV) have shown that the structure of these viruses is more complex. Different electron microscopy techniques, including cryomicroscopy of vitrified viruses, revealed the existence of an internal core, most probably icosahedral, in TGEV virions. Disruption of these cores induced the release of elongated ribonucleoprotein complexes. Ultrastructural analysis of freeze-substituted TGEV-infected swine testis (ST) cells showed characteristic intracellular budding profiles as well as two types of virions. While large virions with an electron-dense internal periphery are seen at perinuclear regions, smaller viral particles exhibiting compact internal cores of poligonal contours are more abundant in areas closer to the plasma membrane of the cell. These data strongly suggest that maturation events following the budding process are responsible for the formation of the internal core shell, the new structural element that we have recently described in extracellular infectious TGEV virions.

Lactogenic immunity in transgenic mice producing recombinant antibodies neutralizing coronavirus

Protection against coronavirus infections can be provided by the oral administration of virus neutralizing antibodies. To provide lactogenic immunity, eighteen lines of transgenic mice secreting a recombinant IgG1 monoclonal antibody (rIgG1) and ten lines of transgenic mice secreting recombinant IgA monoclonal antibodies (rIgA) neutralizing transmissible gastroenteritis coronavirus (TGEV) into the milk were generated. Genes encoding the light and heavy chains of monoclonal antibody (MAb) 6A.C3 were expressed under the control of regulatory sequences derived from the mouse genomic DNA encoding the whey acidic protein (WAP) and beta-lactoglobulin (BLG), which are highly abundant milk proteins. The MAb 6A.C3 binds to a highly conserved epitope present in coronaviruses of several species. This MAb does not allow the selection of neutralization escaping virus mutants. The antibody was expressed in the milk of transgenic mice with titers of one million as determined by RIA, and neutralized TGEV infectivity by one million fold corresponding to immunoglobulin concentrations of 5 to 6 mg per ml. Matrix attachment regions (MAR) sequences were not essential for rIgG1 transgene expression, but co-microinjection of MAR and antibody genes led to a twenty to ten thousand-fold increase in the antibody titer in 50% of the rIgG1 transgenic animals generated. Co-microinjection of the genomic BLG gene with rIgA light and heavy chain genes led to the generation of transgenic mice carrying the three transgenes. The highest antibody titers were produced by transgenic mice that had integrated the antibody and BLG genes, although the number of transgenic animals generated does not allow a definitive conclusion on the enhancing effect of BLG co-integration. Antibody expression levels were transgene copy number independent and integration site dependent. The generation of transgenic animals producing virus neutralizing antibodies in the milk could be a general approach to provide protection against neonatal infections of the enteric tract.

Transgenic mice secreting coronavirus neutralizing antibodies into the milk

Ten lines of transgenic mice secreting transmissible gastroenteritis coronavirus (TGEV) neutralizing recombinant monoclonal antibodies (rMAbs) into the milk were generated. The rMAb light- and heavy-chain genes were assembled by fusing the genes encoding the variable modules of the murine MAb 6A.C3, which binds an interspecies conserved coronavirus epitope essential for virus infectivity, and a constant module from a porcine myeloma with the immunoglobulin A (IgA) isotype. The chimeric antibody led to dimer formation in the presence of J chain. The neutralization specific activity of the recombinant antibody produced in transiently or stably transformed cells was 50-fold higher than that of a monomeric rMAb with the IgG1 isotype and an identical binding site. This rMAb had titers of up to 10(4) by radioimmunoassay (RIA) and neutralized virus infectivity up to 10(4)-fold. Of 23 transgenic mice, 17 integrated both light and heavy chains, and at least 10 of them transmitted both genes to the progeny, leading to 100% of animals secreting functional TGEV neutralizing antibody during lactation. Selected mice produced milk with TGEV-specific antibody titers higher than 10(6) as determined by RIA, neutralized virus infectivity by 10(6)-fold, and produced up to 6 mg of antibody per ml. Antibody expression levels were transgene copy number independent and integration site dependent. Comicroinjection of the genomic beta-lactoglobulin gene with rMAb light- and heavy-chain genes led to the generation of transgenic mice carrying the three transgenes. The highest antibody titers were produced by transgenic mice that had integrated the antibody and beta-lactoglobulin genes, although the number of transgenic animals generated does not allow a definitive conclusion on the enhancing effect of beta-lactoglobulin cointegration. This approach may lead to the generation of transgenic animals providing lactogenic immunity to their progeny against enteric pathogens.

Two types of virus-related particles are found during transmissible gastroenteritis virus morphogenesis

The intracellular assembly of the transmissible gastroenteritis coronavirus (TGEV) was studied in infected swine testis (ST) cells at different postinfection times by using ultrathin sections of conventionally embedded infected cells, freeze-substitution, and methods for detecting viral proteins and RNA at the electron microscopy level. This ultrastructural analysis was focused on the identification of the different viral components that assemble in infected cells, in particular the spherical, potentially icosahedral internal core, a new structural element of the extracellular infectious coronavirus recently characterized by our group. Typical budding profiles and two types of virion-related particles were detected in TGEV-infected cells. While large virions with an electron-dense internal periphery and a clear central area are abundant at perinuclear regions, smaller viral particles, with the characteristic morphology of extracellular virions (exhibiting compact internal cores with polygonal contours) accumulate inside secretory vesicles that reach the plasma membrane. The two types of virions coexist in the Golgi complex of infected ST cells. In nocodazole-treated infected cells, the two types of virions coexist in altered Golgi stacks, while the large secretory vesicles filled with virions found in normal infections are not detected in this case. Treatment of infected cells with the Golgi complex-disrupting agent brefeldin A induced the accumulation of large virions in the cisternae that form by fusion of different membranous compartments. These data, together with the distribution of both types of virions in different cellular compartments, strongly suggest that the large virions are the precursors of the small viral particles and that their transport through a functional Golgi complex is necessary for viral maturation.

Interference of coronavirus infection by expression of immunoglobulin G (IgG) or IgA virus-neutralizing antibodies

Immunoglobulin gene fragments encoding the variable modules of the heavy and light chains of a transmissible gastroenteritis coronavirus (TGEV)-neutralizing monoclonal antibody (MAb) have been cloned and sequenced. The selected MAb recognizes a highly conserved viral epitope and does not lead to the selection of neutralization escape mutants. The sequences of MAb 6A.C3 kappa and gamma 1 modules were identified as subgroup V and subgroup IIIC, respectively. The chimeric immunoglobulin genes encoding the variable modules from the murine MAb and constant modules of human gamma 1 and kappa chains were constructed by reverse transcriptase PCR. Chimeric immunoglobulins were stably or transiently expressed in murine myelomas or COS cells, respectively. The secreted recombinant antibodies had radioimmunoassay titers (i.e., the highest dilution giving a threefold increase over the background) higher than 10(3) and reduced the infectious virus more than 10(4)-fold. Recombinant dimeric immunoglobulin A (IgA) showed a 50-fold enhanced neutralization of TGEV relative to a recombinant monomeric IgG1 which contained the identical antigen binding site. Stably transformed epithelial cell lines which expressed either recombinant IgG or IgA TGEV-neutralizing antibodies reduced virus production by > 10(5)-fold after infection with homologous virus, although a residual level of virus production (< 10(2) PFU/ml) remained in less than 0.1% of the cells. This low-level persistent infection was shown not to be due to the selection of neutralization escape mutants. The implications of these findings for somatic gene therapy with recombinant antibodies are discussed.

Two amino acid changes at the N-terminus of transmissible gastroenteritis coronavirus spike protein result in the loss of enteric tropism

To study the molecular basis of TGEV tropism, a collection of recombinants between the PUR46-MAD strain of transmissible gastroenteritis coronavirus (TGEV) infecting the enteric and respiratory tracts and the PTV strain, which only infects the respiratory tract, was generated. The recombinant isolation frequency was about 10(-9) recombinants per nucleotide and was 3.7-fold higher at the 5'-end of the S gene than in other areas of the genome. Thirty recombinants were plaque purified and characterized phenotypically and genetically. All recombinant viruses had a single crossover and had inherited the 5'- and 3'-halves of their genome from the enteric and respiratory parents, respectively. Recombinant viruses were classified into three groups, named 1 to 3, according to the location of the crossover. Group 1 recombinants had the crossover in the S gene, while in Groups 2 and 3 the crossovers were located in ORF1b and ORF1a, respectively. The tropism of the recombinants was studied. Recombinants of Group 1 had enteric and respiratory tropism, while Group 2 recombinants infected the respiratory, but not the enteric, tract. Viruses of both groups differed by two nucleotide changes at positions 214 and 655. Both changes may be in principle responsible for the loss of enteric tropism but only the change in nucleotide 655 was specifically found in the respiratory isolates and most likely this single nucleotide change, which leads to a substitution in amino acid 219 of the S protein, was responsible for the loss of enteric tropism in the closely related PUR46 isolates. The available data indicate that in order to infect enteric tract cells with TGEV, two different domains of the S protein, mapping between amino acids 522 and 744 and around amino acid 219, respectively, are involved. The first domain binds to porcine aminopeptidase N, the cellular receptor for TGEV. In the other domain maps a second factor of undefined nature but which may be the binding site for a coreceptor essential for the enteric tropism of TGEV.

Cooperation between transmissible gastroenteritis coronavirus (TGEV) structural proteins in the in vitro induction of virus-specific antibodies

Following infection of haplotype defined NIH-miniswine with virulent transmissible gastroenteritis coronavirus (TGEV), isolated mesenteric lymph node CD4+ T-cells mounted a specific proliferative response against infectious or inactivated purified virus in secondary in vitro stimulation. A specific, dose-dependent response to the three major recombinant viral proteins: spike (S), membrane (M), and nucleoprotein (N), purified by affinity chromatography, was characterized. Induction of in vitro antibody synthesis was analyzed. The purified recombinant viral proteins induced the in vitro synthesis of neutralizing TGEV-specific antibodies when porcine TGEV-immune cells were stimulated with each of the combinations made with two of the major structural proteins: S + N, S + M, and to a minor extent with M + N, but not by the individual proteins. S-protein was dissociated from purified virus using NP-40 detergent and then micellar S-protein oligomers (S-rosettes) were formed by removing the detergent. These occurred preferentially by the association of more than 10 S-protein trimmers. These S-rosettes in collaboration with either N or M-proteins elicited TGEV-specific antibodies with titers up to 84 and 60%, respectively, of those induced by the whole virus. N-protein could be partially substituted by a 15-mer peptide that represents a T helper epitope previously identified in N-protein (Antón et al. (1995)). These results indicate that the induction of high levels of TGEV-specific antibodies requires stimulation by at least two viral proteins, and that optimum responses are induced by a combination of S-rosettes and the nucleoprotein.

Transmissible gastroenteritis coronavirus, but not the related porcine respiratory coronavirus, has a sialic acid (N-glycolylneuraminic acid) binding activity

The hemagglutinating activity of transmissible gastroenteritis virus (TGEV), an enteric porcine coronavirus, was analyzed and found to be dependent on the presence of alpha-2,3-linked sialic acid on the erythrocyte surface. N-Glycolylneuraminic acid was recognized more efficiently by TGEV than was N-acetylneuraminic acid. For an efficient hemagglutination reaction the virions had to be treated with sialidase. This result suggests that the sialic acid binding site is blocked by virus-associated competitive inhibitors. Porcine respiratory coronavirus (PRCV), which is serologically related to TGEV but not enteropathogenic, was found to be unable to agglutinate erythrocytes. Incubation with sialidase did not induce a hemagglutinating activity of PRCV, indicating that the lack of this activity is an intrinsic property of the virus and not due to the presence of competitive inhibitors. Only monoclonal antibodies to an antigenic site that is absent from the S protein of PRCV were able to prevent TGEV from agglutinating erythrocytes. The epitope recognized by these antibodies is located within a stretch of 224 amino acids that is missing in the S protein of PRCV. Our results indicate that the sialic acid binding activity is also located in that portion of the S protein. The presence of a hemagglutinating activity in TGEV and its absence in PRCV open the possibility that the sialic acid binding activity contributes to the enterotropism of TGEV.

The transmissible gastroenteritis coronavirus contains a spherical core shell consisting of M and N proteins

Coronaviruses are enveloped RNA viruses involved in a variety of pathologies that affect animals and humans. Existing structural models of these viruses propose a helical nucleocapsid under the virion envelope as the unique internal structure. In the present work, we have analyzed the structure of the transmissible gastroenteritis coronavirus. The definition of its organization supports a new structural model for coronaviruses, since a spherical, probably icosahedral, internal core has been characterized. Disruption of these cores induces the release of N-protein-containing helical nucleocapsids. Immunogold mapping and protein analysis of purified cores showed that they consist of M and N proteins, M being the main core shell component. This surprising finding, together with the fact that M protein molecules are also located in the virion envelope, indicates that a reconsideration of the assembly and maturation of coronaviruses, as well as a study of potential M-protein subclasses, is needed.

Tropism of human adenovirus type 5-based vectors in swine and their ability to protect against transmissible gastroenteritis coronavirus

The infection of epithelia] swine testicle and intestinal porcine epithelial (IPEC-1) cell lines by adenovirus type 5 (Ad5) has been studied in vitro by using an Ad5-luciferase recombinant containing the firefly luciferase gene as a reporter. Porcine cell lines supported Ad5 replication, showing virus titers, kinetics of virus production, and luciferase expression levels similar to those obtained in human 293 cells, which constitutively express the 5'-end 11% of the Ad5 genome. The tropism of Ad5-based vectors in swine and its ability to induce an efficient immune response against heterologous antigens expressed by foreign genes inserted in these vectors has been determined. Ad5 vectors replicate and express heterologous antigens in porcine lungs and mediastinal and mesenteric lymph nodes. Significant levels of heterologous antigen expression were also demonstrated in the small intestine (jejunum and ileum), but Ad5 replication in this organ was very poor, suggesting that Ad vectors undergo an abortive replication in the porcine small intestine. The tissues infected by Ad5 were dependent on the inoculation route. The oronasal route appeared to be best for inoculation of bronchus-associated lymphoid tissue infection, while the intraperitoneal route was best for gut-associated lymphoid tissue infection. Epithelial cells of bronchioles, macrophages, type II pneumocytes, and follicular dendritic cells were identified as targets for Ad5, while epithelial cells of the intestine were not infected by Ad5. Viruses with a deletion from 79.5 to 84.8 map units in the E3 region, with or without heterologous inserted genes, replicated to lower levels in porcine tissues than did wild-type Ad5. It was also shown that an Ad5 recombinant expressing the four antigenic sites (A, B, C, and D) of transmissible gastroenteritis coronavirus (TGEV) spike protein induced in swine immune responses which neutralized TGEV infectivity. In addition, porcine serum from Ad-TGEV-immune animals provide passive protection when mixed with fully virulent TGEV and orally administered to highly susceptible newborn piglets. These results taken together indicate that swine may be a good animal model for human Ad5 lung infection to aid in the evaluation of candidate adenovirus vaccines and that Ad5 may be suitable as a recombinant viral vaccine or for other applications in swine.

Molecular characterization of transmissible gastroenteritis coronavirus defective interfering genomes: packaging and heterogeneity

Three transmissible gastroenteritis virus (TGEV) defective RNAs were selected by serial undiluted passage of the PUR46 strain in ST cells. These RNAs of 22, 10.6, and 9.7 kb (DI-A, DI-B, and DI-C, respectively) were detected at passage 30, remained stable upon further passage in cell culture, and significantly interfered with helper mRNA synthesis. RNA analysis from purified virions showed that the three defective RNAs were efficiently packaged. Virions of different densities containing either full-length or defective RNAs were sorted in sucrose gradients, indicating that defective and full-length genomes were independently encapsidated. DI-B and DI-C RNAs were amplified by the reverse transcription-polymerase chain reaction, cloned, and sequenced. DI-B and DI-C genomes are formed by three and four discontinuous regions of the wild-type genome, respectively. DI-C contains 2144 nucleotides (nt) from the 5'-end of the genome, two fragments of 4540 and 2531 nt mostly from gene 1b, and 493 nt from the 3' end of the genome. DI-B and DI-C RNAs include sequences with the pseudoknot motif and encoding the polymerase, metal ion binding, and helicase motifs. DI-B RNA has a structure closely related to DI-C RNA with two main differences: it maintains the entire ORF 1b and shows heterogeneity in the size of the 3' end deletion. This heterogeneity maps at the beginning of the S gene, where other natural TGEV recombination events have been observed, suggesting that either a process of template switching occurs with high frequency at this point or that the derived genomes have a selective advantage.

An adenovirus recombinant expressing the spike glycoprotein of porcine respiratory coronavirus is immunogenic in swine

The full-length spike (S) gene of porcine respiratory coronavirus (PRCV) was inserted into the genome of human adenovirus type 5 downstream of the early transcription region 3 promoter. The recombinant virus replicated in cultures of the swine testicle ST cell line and directed the synthesis of S antigen with a maximum yield of approximately 26 micrograms per 10(6) cells. The antigen was cell-associated except in the late phase of the infection, when a small amount (3.5 micrograms per 10(6) cells) was released. The cell-associated antigen consisted of polypeptides of molecular mass 160 kDa and 175 kDa, comigrating with the authentic precursor S' and the mature S protein of PRCV, respectively. The extracellular recombinant antigen corresponded to the 175 kDa mature protein. Some recombinant S protein was exposed on the cell surface and was recognized by neutralization-mediating anti-S monoclonal antibodies. Piglets, inoculated oronasally with the recombinant adenovirus vector developed PRCV-neutralizing serum antibodies and were partially protected against PRCV challenge, demonstrating the potential of live adenovirus as vaccine vector.

Induction of antibodies protecting against transmissible gastroenteritis coronavirus (TGEV) by recombinant adenovirus expressing TGEV spike protein

Ten recombinant adenoviruses expressing either fragments of 1135, 1587, or 3329 nt or the full-length spike gene of transmissible gastroenteritis coronavirus (TGEV) have been constructed. These recombinants produce S polypeptides with apparent molecular masses of 68, 86, 135, and 200 kDa, respectively. Expression of the recombinant antigen driven by Ad5 promoters was inhibited by the insertion of an exogenous SV-40 promoter. Most of the recombinant antigens remain intracytoplasmic in infected cells. All the recombinant-directed expression products contain functional antigenic sites C and B (Gebauer et al., 1991, Virology 183, 225-238). The recombinant antigen of 135 kDa and that of 200 kDa, which represents the whole spike protein, also contain antigenic sites D and A, which have previously been shown to be the major inducers of TGEV-neutralizing antibodies. Interestingly, here we show that recombinant S protein fragments expressing only sites C and B also induced TGEV-neutralizing antibodies. The chimeric Ad5-TGEV recombinants elicited lactogenic immunity in hamsters, including the production of TGEV-neutralizing antibodies. The antisera induced in swine by the Ad5 recombinants expressing the amino-terminal 26% of the spike protein (containing sites C and B) or the full-length spike protein, when mixed with a lethal dose of virus prior to administration to susceptible piglets, delayed or completely prevented the induction of symptoms of disease, respectively.

A transmissible gastroenteritis coronavirus nucleoprotein epitope elicits T helper cells that collaborate in the in vitro antibody synthesis to the three major structural viral proteins

Four strong T cell epitopes have been identified studying the blastogenic response of lymphocytes from haplotype-defined transmissible gastroenteritis virus (TGEV) immune miniswine to sixty-one 15-mer synthetic peptides. Three of these epitopes are located on the nucleoprotein (N46, amino acids 46 to 60; N272, amino acids 272 to 286; and N321, amino acids 321 to 335), and one on the membrane protein (M196, amino acids 196 to 210). N321 peptide induced the highest T cell response and was recognized by immune miniswine lymphocytes with haplotypes dd, aa, and cc. T lymphocytes from peptide N321-immune miniswine reconstituted the in vitro synthesis of TGEV-specific antibodies by complementing CD4- TGEV-immune cells. This response was directed at least against the three major structural proteins. The synthesized antibodies specific for S protein preferentially recognized discontinuous epitopes and neutralized TGEV infectivity. These results show that peptide N321 defines a functional T helper epitope eliciting T cells capable of collaborating with B cells specific for different proteins of TGEV.

Membrane protein molecules of transmissible gastroenteritis coronavirus also expose the carboxy-terminal region on the external surface of the virion

The binding domains of four monoclonal antibodies (MAbs) specific for the M protein of the PUR46-MAD strain of transmissible gastroenteritis coronavirus (TGEV) have been located in the 46 carboxy-terminal amino acids of the protein by studying the binding of MAbs to recombinant M protein fragments. Immunoelectron microscopy using these MAbs demonstrated that in a significant proportion of the M protein molecules, the carboxy terminus is exposed on the external surface both in purified viruses and in nascent TGEV virions that recently exited infected swine testis cells. The same MAbs specifically neutralized the infectivity of the PUR46-MAD strain, indicating that the C-terminal domain of M protein is exposed on infectious viruses. This topology of TGEV M protein probably coexists with the structure currently described for the M protein of coronaviruses, which consists of an exposed amino terminus and an intravirion carboxy-terminal domain. The presence of a detectable number of M protein molecules with their carboxy termini exposed on the surface of the virion has relevance for viral function, since it has been shown that the carboxy terminus of M protein is immunodominant and that antibodies specific for this domain both neutralize TGEV and mediate the complement-dependent lysis of TGEV-infected cells.

Analysis of the sialic acid-binding activity of the transmissible gastroenteritis virus

Porcine transmissible gastroenteritis virus (TGEV) has been shown to agglutinate erythrocytes using alpha 2,3-linked sialic acid on the cell surface as binding site. The hemagglutinating activity requires the pretreatment of virus with neuraminidase. We obtained evidence that TGEV recognizes not only N-acetylneuraminic acid but also N-glycoloylneuraminic acid, a sialic acid present on many porcine cells.

Structure and encapsidation of transmissible gastroenteritis coronavirus (TGEV) defective interfering genomes

Serial undiluted passages were performed with the PUR46 strain of TGEV in swine testis (ST) cells. Total cellular RNA was analyzed at different passages after orthophosphate metabolic labeling. Three new defective RNA species of 24, 10.5, and 9.5 kb (DI-A, DI-B, and DI-C respectively) were detected at passage 30, which were highly stable and significantly interfered with helper mRNA synthesis in subsequent passages. By Northern hybridization DIs A, B, and C were detected in purified virions at amounts similar to those of helper RNA. Standard and defective TGEV virions could be sorted in sucrose gradients, indicating that defective and full-length genomes are independently packaged. cDNA synthesis of DI-B and DI-C RNAs was performed by the reverse transcription-polymerase chain reaction (RT-PCR) to give four fragments in each case. Cloning and sequencing of the DI-C PCR products showed that the smallest DI particle comprises 9.5 kb and has 4 discontinuous regions of the genome. It contains 2.1 kb from the 5'-end of the genome, about 7 kb from gene 1b, the first 24 nucleotides of the S gene, 12 nucleotides of ORF 7, and the 0.4 kb of the UTR at the 3'-end.

Molecular bases of tropism in the PUR46 cluster of transmissible gastroenteritis coronaviruses

Transmissible gastroenteritis coronavirus (TGEV) infects both, the enteric and the respiratory tract of swine. S protein, that is recognized by the cellular receptor, has been proposed that plays an essential role in controlling the dominant tropism. The genetic relationship of S gene from different enteric strains and non-enteropathogenic porcine respiratory coronaviruses (PRCVs) was determined. A correlation between tropism and the genetic structure of the S gene was established. PRCVs, derived from enteric isolates have a large deletion at the N-terminus of the S protein. Interestingly, two respiratory isolates, attenuated Purdue type virus (PTV-ATT) and Toyama (TOY56) have a full-length S gene. PTV-ATT has two specific amino acid differences with the S protein of the enteric viruses. One is located around position 219, within the deleted area, suggesting that alterations around this amino acid may result in the loss of enteric tropism. To study the role of different genes in tropism, a cluster of viruses closely related to PUR46 strain was analyzed. All of them have been originated by accumulating point mutations from a common, virulent isolate which infected the enteric tract. During their evolution these viruses have lost, virulence first, and then, enteric tropism. Sequencing analysis proved that enteric tropism could be lost without changes in ORFs 3a, 3b, 4, 6, and 7, and in 3'-end untranslated regions (3'-UTR). To study the role of the S protein in tropism recombinants were obtained between an enteric and a respiratory virus of this cluster. Analysis of the recombinants supported the hypothesis on the role in tropism of S protein domain around position 219.

The Coronaviridae now comprises two genera, coronavirus and torovirus: report of the Coronaviridae Study Group

At the April 1992, mid-term meeting of the International Committee on Taxonomy of Viruses (ICTV) a proposal from the Coronaviridae Study Group (CSG) to include the torovirus genus in the Coronaviridae was accepted. Following another proposal, the arterivirus genus was removed from the Togaviridae but not assigned to another family. The arteriviruses have some features in common with the Coronaviridae but also have major differences. After much debate, culminating in September 1992, it was decided that the CSG would not recommend inclusion of arterivirus in the Coronaviridae. It was agreed that (a) the nomenclature used for coronavirus genes, mRNAs and polypeptides (Cavanagh et al., 1990) should be used for toroviruses, (b) that the small (about 100 amino acids) membrane-associated protein, which is distinct from the integral membrane glycoprotein M, associated with virions of infectious bronchitis (Liu & Inglis, 1991) and transmissible gastroenteritis (Godet et al., 1992) coronaviruses would be referred to by the acronym sM (lower case 's') and (c) that 'pol' (polymerase) should be used as a working term for gene 1, which comprises open reading frames (ORFs) 1a and 1b in both genera of the Coronaviridae.

N-acetylneuraminic acid plays a critical role for the haemagglutinating activity of avian infectious bronchitis virus and porcine transmissible gastroenteritis virus

Porcine transmissible gastroenteritis virus (TGEV) was found to resemble avian infectious bronchitis virus (IBV) in its interaction with erythrocytes. Inactivation of the receptors on erythrocytes by neuraminidase treatment and restoration of receptors by reattaching N-acetylneuraminic acid (Neu5Ac) to cell surface components indicated that alpha 2,3-linked Neu5Ac serves as a receptor determinant for TGEV as has been reported recently for IBV. Similar to IBV, the haemagglutinating activity of TGEV is evident only after pretreatment of virus with neuraminidase indicating that inhibitors on the virion surface have to be inactivated in order to induce the HA-activity of these viruses. A model is presented to explain why the HA-activity of untreated virus is masked and how neuraminidase treatment results in the unmasking of this activity.

Evolution and tropism of transmissible gastroenteritis coronavirus

Transmissible gastroenteritis coronavirus (TGEV) is an enteropathogenic coronavirus isolated for the first time in 1946. Nonenteropathogenic porcine respiratory coronaviruses (PRCVs) have been derived from TGEV. The genetic relationship among six European PRCVs and five coronaviruses of the TGEV antigenic cluster has been determined based on their RNA sequences. The S proteins of six European PRCVs have an identical deletion of 224 amino acids starting at position 21. The deleted area includes the antigenic sites C and B of TGEV S glycoprotein. Interestingly, two viruses (NEB72 and TOY56) with respiratory tropism have the S protein with a similar size to the enteric viruses. NEB72 and TOY56 viruses have 2 and 15 specific amino acid differences with the enteric viruses, respectively. Four of the residues changed are located within the deletion present in the PRCVs and may influence the enteric tropism of TGEV in vivo. A receptor binding site (RBS) used by the virus to infect ST and other cell types might be located between sites A and D of the S glycoprotein, since monoclonal antibodies (MAbs) specific for these sites inhibit the binding of the virus to ST cells. An evolutionary tree relating 13 enteric and respiratory isolates has been proposed. According to this tree, a main virus lineage evolved from a recent progenitor which was circulating around 1941. From this, secondary lineages originated PUR46, NEB72, TOY56, MIL65, BRI70, and the PRCVs, in this order. Least squares estimation of the origin of TGEV-related coronaviruses showed a significant constancy in the mutation fixation rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Epitope specificity of protective lactogenic immunity against swine transmissible gastroenteritis virus

The epitope specificity of the protective immune response against swine transmissible gastroenteritis (TGE) has been investigated by using circulating and secretory antibodies. This study was carried out with sows vaccinated with TGEV or the antigenically related porcine respiratory coronavirus (PRCV). TGEV vaccination of sows resulted in greater lactogenic protection of suckling piglets against TGEV challenge and a higher secretory immune response than PRCV vaccination did. These differences in the immune response were conditioned by the route of antigen presentation as a result of the different tropism of each virus. Epitopes on S protein, and in particular those contained in its antigenic site. A, were more immunogenic than epitopes on N and M proteins in both groups of vaccinated sows, as determined by a competitive radioimmunoassay. Minor differences in antibody response against the previously defined antigenic subsites Aa, Ab, and Ac were also detected, with subsite Ab being the most antigenic in both TGEV- and PRCV-immune sows. These findings suggest that antigenic site A on S protein, involved in virus neutralization, is the immunodominant site in pregnant sows that confer lactogenic protection. They also validate, in experiments with secretory antibodies, the antigenic maps made with murine monoclonal antibodies. Therefore, this antigenic site should be considered for vaccine or diagnostic development.