Rapid Recovery of Classical Swine Fever Virus Directly from Cloned cDNA

Generation of a DNA-launched classical swine fever virus infectious clone packaged in bacterial artificial chromosome

Reverse genetics system offers powerful tool for the research of RNA viruses. The infectious clones of classical swine fever virus (CSFV) were commonly constructed either in high- or low-copy number plasmids and transcribed to infectious RNA using phage RNA-polymerases. Herein, the full-length genome of CSFV Shimen strain, flanked by cytomegalovirus immediate-early (CMV) promoter (a eukaryotic RNA polymerase II promoter) sequence at the 5'-end and the hepatitis delta virus ribozyme along with the bovine growth hormone termination and polyadenylation signal sequences at the 3'-end, was packaged in bacterial artificial chromosome vector to establish a CSFV infectious clone pBAC-smCSFV. This infectious cDNA clone maintained stability after passaged 20 times in bacteria. Transfection of PK15 cells with this cDNA clone facilitated recovery of infectious progeny virus which was identical to parent virus as characterized by RT-qPCR, western blotting, indirect immunofluorescence assay, one-step growth kinetics analysis and nucleotide sequencing. Based on this CSFV infectious cDNA clone, the mCherry was inserted between viral Npro and C protein to develop reporter virus CSFV-mCherry. The mCherry was stably expressed after CSFV-mCherry was passaged 10 times in PK15 cells. Taken together, this present study develops a concise and efficient CSFV infectious cDNA clone and a reporter virus CSFV-mCherry. To the best of our knowledge, this is the first combination of CMV promoter and BAC system in construction of CSFV reverse genetics system. The CSFV infectious cDNA clone and the reporter virus will be useful in the study of CSFV virus biology, virulence determinants, molecular pathogenesis, vaccine development and virus-host interaction.

eEF1A Interacts with the NS5A Protein and Inhibits the Growth of Classical Swine Fever Virus

The NS5A protein of classical swine fever virus (CSFV) is involved in the RNA synthesis and viral replication. However, the NS5A-interacting cellular proteins engaged in the CSFV replication are poorly defined. Using yeast two-hybrid screen, the eukaryotic elongation factor 1A (eEF1A) was identified to be an NS5A-binding partner. The NS5A-eEF1A interaction was confirmed by coimmunoprecipitation, glutathione S-transferase (GST) pulldown and laser confocal microscopy assays. The domain I of eEF1A was shown to be critical for the NS5A-eEF1A interaction. Overexpression of eEF1A suppressed the CSFV growth markedly, and conversely, knockdown of eEF1A enhanced the CSFV replication significantly. Furthermore, eEF1A, as well as NS5A, was found to reduce the translation efficiency of the internal ribosome entry site (IRES) of CSFV in a dose-dependent manner, as demonstrated by luciferase reporter assay. Streptavidin pulldown assay revealed that eEF1A could bind to the CSFV IRES. Collectively, our results suggest that eEF1A interacts with NS5A and negatively regulates the growth of CSFV.

Efficient and stable rescue of classical swine fever virus from cloned cDNA using an RNA polymerase II system

Conventional reverse genetics for classical swine fever virus (CSFV) is based on the transfection of permissive cells with either in vitro or intracellularly synthesized RNA transcripts from a viral genomic cDNA clone. These strategies are complicated, inefficient and time-consuming. This study is aimed to develop an improved reverse genetics method for the direct, rapid and efficient recovery of CSFV from cloned cDNA. The cDNA clone pBRCISM was constructed, which harbors the full-length genomic sequence from the CSFV Shimen strain flanked by the cytomegalovirus promoter (an RNA polymerase II promoter), a chimeric intron, and hammerhead ribozyme sequences at the 5'-end and the hepatitis delta virus ribozyme and SV40 polyadenylation signal sequences at the 3'-end. Infectious progeny virus was rescued from PK-15 cells directly transfected with pBRCISM, and its morphology, one-step growth characteristics and pathogenicity were indistinguishable from the parent virus and virus rescued from classical reverse genetics. The reverse genetics based on RNA polymerase II yielded a 120-fold increase in the titer of nascent virus in 12-h less time than a reverse genetics method based on in vitro transcription. The full-length cDNA clone remained stable and infectious after 20 passages in bacterial cells, in contrast to the instability of the full-length clone without the intron after 9 passages. The improved reverse genetics method developed in the present study is efficient, stable, convenient and cost-effective and will be valuable for the rapid recovery of CSFV mutants.