Generation and characterization of an Npro-disrupted marker bovine viral diarrhea virus derived from a BAC cDNA

Attenuated porcine-derived type 2 bovine viral diarrhea virus as vector stably expressing viral gene

Infectious bovine viral diarrhea virus (BVDV) cDNA clones have been used for the expression of classical swine fever virus (CSFV) genes for immune prevention and control. However, can it be used for the expression of an allogenetic fragment? To answer this question, a BVDV chimeric virus expressing the spike (S) antigen fragment of porcine epidemic diarrhea virus (PEDV) was constructed. Antigen S_499-602 was inserted into pig-derived BVDV-2 infectious cDNA clone pASH28 in tandem by overlapping PCR, located between the seventh and eighth amino acids at the N-terminus of the capsid (C) protein of BVDV. Indirect immunofluorescence assay confirmed that the chimeric virus vASH-dS312 containing double S_499-602 sequences was successfully assembled, which could react with the monoclonal antibody (MAb) against BVDV E2 and PEDV S proteins. Further western blot analysis confirmed that the exogenous S_499-602 double protein could be stably expressed. Next, the chimeric virus vASH-dS312 was administered to BALB/C mice either orally or by intramuscular injection. The immunized mice were healthy and showed no signs of toxicity. IgG against BVDV and PEDV antibodies could be detected in the mice administered vASH-dS312 by intramuscular injection, which had neutralization activity against BVDV and PEDV. Thus, this study reported a new insertion site in the BVDV infectious cDNA clone that could successfully express an allogenetic antigen.

Insertion and stable expression of Gaussia luciferase gene by the genome of bovine viral diarrhea virus

As a tool to address selected issues of virus biology, we constructed a recombinant cDNA clone of bovine viral diarrhea virus (BVDV) expressing Gaussia luciferase (Gluc) reporter gene. A full-length genomic cDNA clone of a non-cytopathic BVDV isolate was assembled by recombination in yeast Saccharomyces cerevisiae. The Gluc gene was inserted between the N(pro) and Core protein coding regions by recombination. The cDNA transcribed in vitro was infectious upon transfection of MDBK cells, resulting in reporter gene expression and productive virus replication. The rescued viruses were stable for 15 passages in cell culture, maintaining the replication kinetics, focus size and morphology similar to those of the parental virus. Expression and correct processing of the reporter protein were also maintained, as demonstrated by Gluc activity. These results demonstrate that genes up to 555 bp are simply assembled by a single step in yeast recombination and are stably expressed by this cDNA clone.

N pro of Bungowannah virus exhibits the same antagonistic function in the IFN induction pathway than that of other classical pestiviruses

Bungowannah virus is the most divergent atypical pestivirus that had been detected up to now, and does not fit into any of the four approved species: Bovine viral diarrhea virus type 1 (BVDV-1) and type 2 (BVDV-2), Classical swine fever virus (CSFV) and Border disease virus (BDV). However, the presence of N(pro) and E(rns) coding regions, which are unique to pestiviruses, provides clear evidence of a pestivirus. Nevertheless, the amino acid identity of Bungowannah virus N(pro) and BVDV-1 N(pro) (strain CP7) is only 51.5%. By using a BVDV-1 backbone, a novel chimeric construct was generated, in which the genomic region encoding the non-structural protein N(pro) was replaced by that of Bungowannah virus (CP7_N(pro)-Bungo). In vitro studies of CP7_N(pro)-Bungo revealed autonomous replication with the same efficacy as the BVDV backbone CP7 and infectious high-titer virus could be collected. In order to compare the ability of interferon (IFN) suppression, two reporter gene assays, specific for type-I IFN, were carried out. In virus-infected cells, no significant difference in blocking of IFN expression between the parental virus CP7, Bungowannah virus and the chimeric construct CP7_N(pro)-Bungo could be detected. In contrast, an N(pro) deletion mutant showed an impaired replication in bovine cells and a marked type-I IFN response. Taken together, our findings reveal the compatibility of non-structural protein N(pro) of atypical Bungowannah virus with a BVDV type 1 backbone and its characteristic feature as an inhibitor of type-I IFN induction with an inhibitor-activity comparable to other pestiviruses.

Development of a reporter bovine viral diarrhea virus and initial evaluation of its application for high throughput antiviral drug screening

Bovine viral diarrhea virus (BVDV) causes lethal mucosal disease of cattle and leads to severe economic loss of cattle production and reproduction worldwide. Over the past decades, vaccination was not very successful in providing prevention of BVDV infection. This reality demands that anti-BVDV drugs should be used as an alternative treatment strategy. In this study, a BAC cDNA of noncytopathic BVDV strain SD-1 is constructed to contain an enhanced green fluorescence protein (eGFP) gene between viral NS3 and NS4A coding sequences. The recombinant reporter virus is generated subsequently by transfection of MDBK cells with the transcripts produced in vitro. The rescued reporter virus is stable in MDBK cells and the eGFP protein is expressed and processed properly. Of most importance, the reporter virus shows a growth property similar to the SD-1 parent and the fluorescent signal intensity increases in parallel to the reporter virus RNA and protein replication. In addition, two known anti-BVDV drug G418 (viral assembly/release inhibitor) and ribavirin (viral RNA replication inhibitor) are identified as hits in a high-throughput format, suggesting that this system is capable of identifying BVDV inhibitors that target different steps in viral life cycle. The cell-based system developed provides a useful and versatile tool which should facilitate the identification of BVDV inhibitors on a large scale.

An alternative -1/+2 open reading frame exists within viral N(pro)(1-19) region of bovine viral diarrhea virus SD-1

We previously reported the engineering of an N(pro)-disrupted bovine viral diarrhea virus (BVDV), BSD1-N(pro)/eGFP2A (Fan and Bird, 2008a). Here, we report that BSD1-N(pro)/eGFP2A survives a single nucleotide missing in its C-terminal eGFP region. By using our established reverse genetics system for BVDV, we confirm that the viral mutant is rescued through a -1/+2 ORF initiated in the N(pro)(1-19)/eGFP region of the mutant viral genome. We furthermore uncover that this event occurs in the N(pro)(1-19) region of BVDV strain SD-1. The rescued viral mutant showed dramatic reductions in levels of both viral RNA and viral protein in host cells. Although the mutant is similar to the native strain in viral kinetics, the peak yield of the mutant is decreased dramatically. These findings reveal the existence of an alternative -1/+2 ORF in the N(pro)(1-19) region during the replication of BVDV and open a new avenue to understand the life cycle and pathogenesis of pestiviruses.

Characterization of essential domains and plasticity of the classical Swine Fever virus Core protein

Pestiviruses are pathogens of cloven-hoofed animals, belonging to the Flaviviridae. The pestiviral particle consists of a lipid membrane containing the three envelope glycoproteins Erns, E1, and E2 and a nucleocapsid of unknown symmetry, which is composed of the Core protein and the viral positive-sense RNA genome. The positively charged pestiviral Core protein consists of 86 to 89 amino acids. To analyze the organization of essential domains, N- and C-terminal truncations, as well as internal deletions, were introduced into the Core coding sequence in the context of an infectious cDNA clone of classical swine fever virus strain Alfort. Amino acids 179 to 180, 194 to 198, and 208 to 212 proved to be of special importance for the generation of progeny virus. The results of transcomplementation of a series of C-terminally truncated Core molecules indicate the importance of Ala255 at the C terminus. The plasticity of Core protein was examined by the construction of concatemeric arrays of Core coding regions and the insertion of up to three yellow fluorescent protein (YFP) genes between two Core genes. Even a Core fusion protein with more than 10-fold-increased molecular mass was integrated into the viral particle and supported the production of infectious progeny virus. The unexpected plasticity of Core protein brings into question the formation of a regular icosahedric particle and supports the idea of a histone-like protein-RNA interaction. All viruses with a duplicated Core gene were unstable and reverted to the wild-type sequence. Interestingly, a nonviable YFP-Core construct was rescued by a mutation within the C-terminal domain of the nonstructural protein NS3.

Regeneration and characterization of a recombinant bovine viral diarrhea virus and determination of its efficacy to cross the bovine placenta

The capacity of different bovine viral diarrhea virus (BVDV) strains to cause transplacental infection is variable. BVDV strain SD-1 was isolated from a persistently infected heifer. Its genome represents the only reported nucleotide sequence of a noncytopathic viral isolate determined without cell culture passage in the laboratory. Thus, SD-1 might possess biological advantages over other NCP BVDV strains to be used as a model virus for investigation of viral transplacental transmission. To evaluate if a molecularly generated BVDV SD-1 is capable of crossing the bovine placenta efficiently, a full-length cDNA clone of SD-1 was constructed using RT-PCR amplification and standard molecular techniques. In vitro transcripts synthesized from the cDNA template directed the generation of infectious virus in MDBK cells with a transfection efficiency as high as 4.7 x 10(5) FFU/mug RNA. The recovered virus termed ASD1 harbored five silent point mutations engineered as genetic markers and was similar to wild type (wt) SD-1 in viral growth kinetics. As evaluated in the pregnant heifers, ASD1 was capable of crossing the bovine placenta efficiently, suggesting that NCP BVDV SD-1 is a suitable viral backbone for investigation of the role of viral genetic element(s) in viral transplacental transmission by allowing for evaluation of newly created viral mutants.

Bovine viral diarrhea virus is a suitable viral vector for stable expression of heterologous gene when inserted in between N(pro) and C genes

Bovine viral diarrhea virus (BVDV) is a group of small enveloped viruses with a single-stranded, positive-oriented RNA genome of approximately 12.3 kb. BVDV genome directs the production of a viral polyprotein that is subsequently cleaved to release the mature viral proteins. To explore the potential of using BVDV as viral vector for stable expression of heterologous genes, eGFP2A was inserted in between N(pro) and C genes of a noncytopathic type-I BVDV strain SD1. eGFP2A was designed with eGFP protein in frame fused to the N terminus of the foot-and-mouth disease virus 2A protease. This strategy promised not only the correct processing of both viral N(pro) and C protein but also releasing of the chimeric protein from the nascent viral polyprotein. The recombinant reporter virus was successfully rescued in MDBK cells. In vitro study showed that eGFP2A protein, as expected, was expressed and processed properly from the nascent viral polyprotein. The reporter virus was similar to wt SD1 in viral RNA replication and protein expression and comparable to wt SD1 in growth kinetics except that this virus had a peak virus titer approximately 0.5 log(10) lower and a maximum yield about 4h later than wt SD1. In summary, these results indicated that BVDV is a suitable viral vector for stable expression of heterologous genes when inserted in between N(pro) and C genes.