Progress towards the construction of a transmissible gastroenteritis coronavirus self-replicating RNA using a two-layer expression system

In vitro and in vivo expression of foreign genes by transmissible gastroenteritis coronavirus-derived minigenomes

A helper-dependent expression system based on transmissible gastroenteritis coronavirus (TGEV) has been developed using a minigenome of 3.9 kb (M39). Expression of the reporter gene beta-glucuronidase (GUS) (2-8 microg per 10(6) cells) and the porcine respiratory and reproductive syndrome virus (PRRSV) ORF5 (1-2 microg per 10(6) cells) has been shown using a TGEV-derived minigenome. GUS expression levels increased about eightfold with the m.o.i. and were maintained for more than eight passages in cell culture. Nevertheless, instability of the GUS and ORF5 subgenomic mRNAs was observed from passages five and four, respectively. About a quarter of the cells in culture expressing the helper virus also produced the reporter gene as determined by studying GUS mRNA production by in situ hybridization or immunodetection to visualize the protein synthesized. Expression of GUS was detected in the lungs, but not in the gut, of swine immunized with the virus vector. Around a quarter of lung cells showing replication of the helper virus were also positive for the reporter gene. Interestingly, strong humoral immune responses to both GUS and PRRSV ORF5 were induced in swine with this virus vector. The large cloning capacity and the tissue specificity of the TGEV-derived minigenomes suggest that these virus vectors are very promising for vaccine development.

Transcription regulatory sequences and mRNA expression levels in the coronavirus transmissible gastroenteritis virus

The transcription regulatory sequences (TRSs) of the coronavirus transmissible gastroenteritis virus (TGEV) have been characterized by using a helper virus-dependent expression system based on coronavirus-derived minigenomes to study the synthesis of subgenomic mRNAs. The TRSs are located at the 5' end of TGEV genes and include a highly conserved core sequence (CS), 5'-CUAAAC-3', that is essential for mediating a 100- to 1,000-fold increase in mRNA synthesis when it is located in the appropriate context. The relevant sequences contributing to TRS activity have been studied by extending the CS 5' upstream and 3' downstream. Sequences from virus genes flanking the CS influenced transcription levels from moderate (10- to 20-fold variation) to complete mRNA synthesis silencing, as shown for a canonical CS at nucleotide (nt) 120 from the initiation codon of the S gene that did not lead to the production of the corresponding mRNA. An optimized TRS has been designed comprising 88 nt from the N gene TRS, the CS, and 3 nt 3' to the M gene CS. Further extension of the 5'-flanking nucleotides (i.e., by 176 nt) decreased subgenomic RNA levels. The expression of a reporter gene (beta-glucuronidase) by using the selected TRS led to the production of 2 to 8 microg of protein per 10(6) cells. The presence of an appropriate Kozak context led to a higher level of protein expression. Virus protein levels were shown to be dependent on transcription and translation regulation.

Complete genome sequence of transmissible gastroenteritis coronavirus PUR46-MAD clone and evolution of the purdue virus cluster

The complete sequence (28580 nt) of the PUR46-MAD clone of the Purdue cluster of transmissible gastroenteritis coronavirus (TGEV) has been determined and compared with members of this cluster and other coronaviruses. The computing distances among their S gene sequences resulted in the grouping of these coronaviruses into four clusters, one of them exclusively formed by the Purdue viruses. Three new potential sequence motifs with homology to the alpha-subunit of the polymerase-associated nucleocapsid phosphoprotein of rinderpest virus, the Bowman-Birk type of proteinase inhibitors, and the metallothionein superfamily of cysteine rich chelating proteins have been identified. Comparison of the TGEV polymerase sequence with that of other RNA viruses revealed high sequence homology with the A-E domains of the palm subdomain of nucleic acid polymerases.

Interference with virus and bacteria replication by the tissue specific expression of antibodies and interfering molecules

Historically, protection against virus infections has relied on the use of vaccines, but the induction of an immune response requires several days and in certain situations, like in newborn animals that may be infected at birth and die in a few days, there is not sufficient time to elicit a protective immune response. Immediate protection in new born could be provided either by vectors that express virus-interfering molecules in a tissue specific form, or by the production of animals expressing resistance to virus replication. The mucosal surface is the largest body surface susceptible to virus infection that can serve for virus entry. Then, it is of high interest to develop strategies to prevent infections of these areas. Virus growth can be interfered intracellularly, extracellularly or both. The antibodies neutralize virus intra- and extracellularly and their molecular biology is well known. In addition, antibodies efficiently neutralize viruses in the mucosal areas. The autonomy of antibody molecules in virus neutralization makes them functional in cells different from those that produce the antibodies and in the extracellular medium. These properties have identified antibodies as very useful molecules to be expressed by vectors or in transgenic animals to provide resistance to virus infection. A similar role could be played by antimicrobial peptides in the case of bacteria. Intracellular interference with virus growth (intracellular immunity) can be mediated by molecules of very different nature: (i) full length or single chain antibodies; (ii) mutant viral proteins that strongly interfere with the replication of the wild type virus (dominant-negative mutants); (iii) antisense RNA and ribozyme sequences; and (iv) the product of antiviral genes such as the Mx proteins. All these molecules inhibiting virus replication may be used to obtain transgenic animals with resistance to viral infection built in their genomes. We have developed two strategies to target into mucosal areas either antibodies to provide immediate protection, or antigens to elicit immune responses in the enteric or respiratory surfaces in order to prevent virus infection. One strategy is based on the development of expression vectors using coronavirus derived defective RNA minigenomes, and the other relies on the development of transgenic animals providing virus neutralizing antibodies in the milk during lactation. Two types of expression vectors are being engineered based on transmissible gastroenteritis coronavirus (TGEV) defective minigenomes. The first one is a helper virus dependent expression system and the second is based on self-replicating RNAs including the information required to encode the TGEV replicase. The minigenomes expressing the heterologous gene have been improved by using a two-step amplification system based on cytomegalovirus (CMV) and viral promoters. Expression levels around 5 micrograms per 10(6) cells were obtained. The engineered minigenomes will be useful to understand the mechanism of coronavirus replication and for the tissue specific expression of antigen, antibody or virus interfering molecules. To protect from viral infections of the enteric tract, transgenic animals secreting virus neutralizing recombinant antibodies in the milk during lactation have been developed. Neutralizing antibodies with isotypes IgG1 or IgA were produced in the milk with titers of 10(6) in RIA that reduced virus infectivity by one million-fold. The recombinant antibodies recognized a conserved epitope apparently essential for virus replication. Antibody expression levels were transgene transgene copy number independent and were related to the transgene integration site. This strategy may be of general use since it could be applied to protect newborn animals against infections of the enteric tract by viruses or bacteria for which a protective MAb has been identified. Alternatively, the same strategy could be used to target the expression of antibio

Replication and packaging of transmissible gastroenteritis coronavirus-derived synthetic minigenomes

The sequences involved in the replication and packaging of transmissible gastroenteritis virus (TGEV) RNA have been studied. The structure of a TGEV defective interfering RNA of 9.7 kb (DI-C) was described previously (A. Mendez, C. Smerdou, A. Izeta, F. Gebauer, and L. Enjuanes, Virology 217: 495-507, 1996), and a cDNA with the information to encode DI-C RNA was cloned under the control of the T7 promoter. The molecularly cloned DI-C RNA was replicated in trans upon transfection of helper virus-infected cells and inhibited 20-fold the replication of the parental genome. A collection of 14 DI-C RNA deletion mutants (TGEV minigenomes) was synthetically generated and tested for their ability to be replicated and packaged. The smallest minigenome (M33) that was replicated by the helper virus and efficiently packaged was 3.3 kb. A minigenome of 2.1 kb (M21) was also replicated, but it was packaged with much lower efficiency than the M33 minigenome, suggesting that it had lost either the sequences containing the main packaging signal or the required secondary structure in the packaging signal due to alteration of the flanking sequences. The low packaging efficiency of the M21 minigenome was not due to minimum size restrictions. The sequences essential for minigenome replication by the helper virus were reduced to 1,348 nt and 492 nt at the 5' and 3' ends, respectively. The TGEV-derived RNA minigenomes were successfully expressed following a two-step amplification system that couples pol II-driven transcription in the nucleus to replication supported by helper virus in the cytoplasm, without any obvious splicing. This system and the use of the reporter gene beta-glucuronidase (GUS) allowed minigenome detection at passage zero, making it possible to distinguish replication efficiency from packaging capability. The synthetic minigenomes have been used to design a helper-dependent expression system that produces around 1.0 microgram/10(6) cells of GUS.