The presence of RNA splicing signals in the cDNA construct of the E2 gene of classical swine fever virus affected its expression

Single-Round Infectious Particle Production by DNA-Launched Infectious Clones of Bungowannah Pestivirus

Reverse genetics systems are powerful tools for functional studies of viral genes or for vaccine development. Here, we established DNA-launched reverse genetics for the pestivirus Bungowannah virus (BuPV), where cDNA flanked by a hammerhead ribozyme sequence at the 5′ end and the hepatitis delta ribozyme at the 3′ end was placed under the control of the CMV RNA polymerase II promoter. Infectious recombinant BuPV could be rescued from pBuPV-DNA-transfected SK-6 cells and it had very similar growth characteristics to BuPV generated by conventional RNA-based reverse genetics and wild type BuPV. Subsequently, DNA-based E^RNS deleted BuPV split genomes (pBuPV∆E^RNS/E^RNS)—co-expressing the E^RNS protein from a separate synthetic CAG promoter—were constructed and characterized in vitro. Overall, DNA-launched BuPV genomes enable a rapid and cost-effective generation of recombinant BuPV and virus mutants, however, the protein expression efficiency of the DNA-launched systems after transfection is very low and needs further optimization in the future to allow the use e.g., as vaccine platform.

Flavivirus reverse genetic systems, construction techniques and applications: a historical perspective

The study of flaviviruses, which cause some of the most important emerging tropical and sub-tropical human arbovirus diseases, has greatly benefited from the use of reverse genetic systems since its first development for yellow fever virus in 1989. Reverse genetics technology has completely revolutionized the study of these viruses, making it possible to manipulate their genomes and evaluate the direct effects of these changes on their biology and pathogenesis. The most commonly used reverse genetics system is the infectious clone technology. Whilst flavivirus infectious clones provide a powerful tool, their construction as full-length cDNA molecules in bacterial vectors can be problematic, laborious and time consuming, because they are often unstable, contain unwanted induced substitutions and may be toxic for bacteria due to viral protein expression. The incredible technological advances that have been made during the past 30years, such as the use of PCR or new sequencing methods, have allowed the development of new approaches to improve preexisting systems or elaborate new strategies that overcome these problems. This review summarizes the evolution and major technical breakthroughs in the development of flavivirus reverse genetics technologies and their application to the further understanding and control of these viruses and their diseases.

Evaluation of efficacy of mammalian and baculovirus expressed E2 subunit vaccine candidates to bovine viral diarrhoea virus

Bovine viral diarrhoea virus (BVDV) is a worldwide pathogen of cattle causing a wide spectrum of clinical disease. The major envelope glycoprotein of BVDV, E2, induces the production of neutralising antibodies. In this study we compared the protection afforded to cattle after BVDV challenge by two separate E2 vaccine candidates produced by different heterologous protein expression systems. E2 antigen was expressed using the baculovirus expression system (brE2) and a mammalian cell expression system (mrE2). In the first vaccination study the quantity of recombinant protein expressed by the two systems differed. Vaccination of cattle with a higher dose of brE2 or low dose mrE2 gave comparable protection from viral challenge. Immunised animals showed no pyrexia and reduced leucopaenia which contrasted to the unvaccinated controls. In addition virus shedding from the nasal mucosa was decreased in the vaccinated groups and strong humoral responses were evident post-challenge. However, the efficacy of the brE2 vaccine was greatly diminished when a reduced dose was tested, indicating the importance of assessing the type of expression system used in antigen production.

Nonstructural proteins NS2-3 and NS4A of classical swine fever virus: essential features for infectious particle formation

The nonstructural protein NS2-3 of pestiviruses undergoes tightly regulated processing. For bovine viral diarrhea virus it was shown that uncleaved NS2-3 is required for infectious particle formation while cleaved NS3 is essential for genome replication. To further investigate the functions of NS2-3 and NS4A in the pestivirus life cycle, we established T7 RNA polymerase-dependent trans-complementation for p7-NS2-3-4A of classical swine fever virus (CSFV). Expression of NS2-3 and NS4A in trans restored the production of infectious particles from genomes lacking NS2-3 expression. Co-expression of cleaved NS4A was essential. None of the enzymatic activities harbored by NS2-3 were required for infectious particle formation. Importantly, expression of uncleavable NS2-3 together with NS4A rescued infectious particles from a genome lacking NS2, demonstrating that cleaved NS2 per se has no additional essential function. These data indicate that NS2-3 and NS3, each in association with NS4A, have independent functions in the CSFV life cycle.

Determination of the sequence of the complete open reading frame and the 5'NTR of the Paderborn isolate of classical swine fever virus

The classical swine fever (CSF) epidemic in the Netherlands in 1997-1998 lasted 14 months, during which 429 infected and 1300 at risk herds were culled, at an estimated economical cost of 2 billion US dollars. Despite the overwhelming scale of the epizootic, the CSF virus (CSFV) strain causing the outbreak has remained largely uncharacterized. The Dutch epizootic is epidemiologically linked to a small CSF outbreak in 1997, in Paderborn in Germany. E2 and partial 5' NTR sequencing has shown that the index Paderborn isolate, and several Dutch isolates taken during the 1997-1998 epizootic, are virtually identical, confirming that the Paderborn isolate triggered the Dutch outbreak, and furthermore showing that this single isolate was stable throughout the whole Dutch outbreak (the above reviewed in [C. Terpstra, A. J. de Smit, Veterinary Microbiol. 77 (2000) 3-15]). We determined the nucleotide sequence of the 5' NTR (by 5' RACE) and the complete open reading frame of the Paderborn isolate (GenBank AY072924). Our sequence was identical to previously published partial 5'NTR and E2 sequences for the index Paderborn 1997 and Dutch 1997 (Venhorst) isolates, confirming the identity of the virus we sequenced. Phylogenetic analysis based on the complete open reading frame showed that Paderborn is genetically very different from common European laboratory reference strains. Neutralization studies showed that Paderborn is also antigenically very different from common laboratory strains such as Alfort 187. Paderborn is the only recent European CSFV field isolate for which a complete sequence is available, and given Paderborns genetic and antigenic uniqueness, the Paderborn sequence may have practical use for diagnostic and vaccine antigen development.

A new strategy in design of +RNA virus infectious clones enabling their stable propagation in E. coli

Infectious clone methodology is a valuable tool of modern experimental virology. However, its use is often constrained by the instability of infectious clone constructs during propagation in E. coli. To circumvent this problem, we have devised a strategy that could be suitable for design of +RNA virus molecular clones in general. An infectious clone is assembled as "infectious DNA," and expression of problem regions present in the viral cDNA is prevented during propagation in E. coli by insertion of short introns. To demonstrate the feasibility of this approach, a highly unstable Japanese encephalitis flavivirus infectious clone has been successfully converted into a remarkably stable infectious DNA construct with the specific infectivity of 10(6) pfu/microg in cell culture. The proposed strategy may be useful in the design of self-amplifying gene therapy vectors and development of new immunization methodologies, and could facilitate creation of molecular repositories of existing viral vaccines.

Expression of bovine viral diarrhoea virus glycoprotein E2 by bovine herpesvirus-1 from a synthetic ORF and incorporation of E2 into recombinant virions

Expression cassettes containing the codons for the pestivirus E (rns) signal peptide (Sig) followed by a chemically synthesized ORF that encoded the bovine viral diarrhoea virus (BVDV) strain C86 glycoprotein E2, a class I membrane glycoprotein, were constructed with and without a chimeric intron sequence immediately upstream of the translation start codon, and incorporated into the genome of bovine herpesvirus-1 (BHV-1). The resulting recombinants, BHV- 1/SigE2(syn) and BHV-1/SigE2(syn)-intron, expressed comparable quantities of glycoprotein E2, and Northern blot hybridizations indicated that the presence of the intron did not increase significantly the steady-state levels of transcripts encompassing the SigE2(syn) ORF. In BHV-1/SigE2(syn)- infected cells, the 54 kDa E2 glycoprotein formed a dimer with an apparent molecular mass of 94 kDa, which was further modified to a 101 kDa form found in the envelope of recombinant virus particles. Penetration kinetics and single-step growth curves indicated that the incorporation of the BVDV E2 glycoprotein in the BHV-1 envelope, which apparently did not require BHV-1-specific signals, interfered with entry into target cells and egress of progeny virions. These results demonstrate that a pestivirus glycoprotein can be expressed efficiently by BHV-1 and incorporated into the viral envelope. BHV-1 thus represents a promising tool for the development of efficacious live and inactivated BHV-1-based vector vaccines.