An efficient method for precise gene substitution in the AcMNPV genome by homologous recombination in E. coli

The Host Specificities of Baculovirus per os Infectivity Factors

Baculoviruses are insect-specific pathogens with a generally narrow host ranges. Successful primary infection is initiated by the proper interaction of at least 8 conserved per os infectivity factors (PIFs) with the host's midgut cells, a process that remains largely a mystery. In this study, we investigated the host specificities of the four core components of the PIF complex, P74, PIF1, PIF2 and PIF3 by using Helicoverpa armigera nucleopolyhedrovirus (HearNPV) backbone. The four pifs of HearNPV were replaced by their counterparts from a group I Autographa californica multiple nucleopolyhedrovirus (AcMNPV) or a group II Spodoptera litura nucleopolyhedrovirus (SpltNPV). Transfection and infection assays showed that all the recombinant viruses were able to produce infectious budded viruses (BVs) and were lethal to H. armigera larvae via intrahaemocoelic injection. However, feeding experiments using very high concentration of occlusion bodies demonstrated that all the recombinant viruses completely lost oral infectivity except SpltNPV pif3 substituted pif3-null HearNPV (vHaBacΔpif3-Sppif3-ph). Furthermore, bioassay result showed that the median lethal concentration (LC50) value of vHaBacΔpif3-Sppif3-ph was 23-fold higher than that of the control virus vHaBacΔpif3-Hapif3-ph, indicating that SpltNPV pif3 can only partially substitute the function of HearNPV pif3. These results suggested that most of PIFs tested have strict host specificities, which may account, at least in part, for the limited host ranges of baculoviruses.

A bacmid approach to the genetic manipulation of granuloviruses

A Cydia pomonella granulovirus (CpGV) bacmid has been constructed, which allows rapid and efficient production of recombinant baculoviruses in Escherichia coli. An 8.6kbp bacterial DNA cassette derived from the AcMNPV Bac-to-Bac system was ligated into a unique PacI restriction site within an intergenic region flanking the DNA ligase gene of the CpGV genome. The CpGV bacmids produced in E. coli were transfected into a CpGV-permissive C. pomonella cell line and the transfected cells fed to larvae to amplify the virus. The enhanced green fluorescent protein (EGFP) gene under the constitutive Drosophila heat-shock promoter was transposed into the mini-attTn7 transposition site, using a modified pFASTBAC donor plasmid, to generate a recombinant CpGV bacmid which caused infected larvae to glow under UV light. Targeted homologous recombination was also achieved in a recombinant proficient E. coli strain (BJ5183). A chloramphenicol acetyl transferase (CAT) gene replaced the cathepsin (v-cath) gene in the bacmid to produce a v-cath-deletion mutant. This is the first published report of a granulovirus bacmid, which will allow easy manipulation of the CpGV genome, enabling future studies on granulovirus genes and biology.

Generation of a recombinant Oka varicella vaccine expressing mumps virus hemagglutinin-neuraminidase protein as a polyvalent live vaccine

We constructed a recombinant varicella-zoster virus (VZV) Oka vaccine strain (vOka) that contained the mumps virus (MuV) hemagglutinin-neuraminidase (HN) gene, inserted into the site of the ORF 13 gene by using the bacterial artificial chromosome (BAC) system in Escherichia coli. Insertion of the HN gene into the VZV genome was confirmed by PCR and Southern blot. The infectious virus reconstituted from the vOka-HN genome (rvOka-HN) had a growth curve similar to the original recombinant vOka without the HN gene. The mumps virus HN protein expressed in rvOka-HN infected cells was expressed diffusely in the cytoplasm, and modification of the protein was similar to that seen in MuV-infected cells. Electron microscopic examination of infected cells revealed that HN was expressed on the plasma membrane of the cells but not in the viral envelope, suggesting that the tropism of rvOka-HN would be unchanged from that of the original vOka strain. Immunization of guinea pigs with rvOka-HN-induced VZV- and HN-specific antibodies. Interestingly, the induced antibodies had a strong neutralizing activity against virus-cell infections of both MuV and VZV. Therefore, the novel varicella vaccine expressing MuV HN protein is suitable as a polyvalent live attenuated vaccine against VZV and MuV infections.

Characterization of AcMNPV with a deletion of me53 gene

The Autographa Californica multiple nucleopolyhedrovirus (AcMNPV) me53 gene, which was previously reported as one of the major early-transcribed genes, was deleted through homologous recombination from an AcMNPV genome propagated as a bacmid DNA in E. coli, generating a me53 gene knockout bacmid. Green fluorescent protein (GFP) expression analysis and supernatant passage assay revealed that the me53 knockout bacmid was unable to replicate in cell culture, while me53 repair bacmid, which was generated by reinsertion of the me53 gene into the mutant at a different locus (the gentamicin locus) with ET-recombination technique, resumed viral replication ability at wild-type levels, indicating that the defective phenotype of the me53 knockout virus was directly due to the deletion of the me53-ORF. Subsequent electron microscopy revealed that the me53 knockout bacmid failed to form nucleocapsid in the nuclei of the transfected cells, though viral infection seemed to be initiated. Meanwhile, real-time PCR analysis based on SYBR Green fluorescence indicated abolishment of the viral DNA replication by me53 gene inactivation. Thus, it is demonstrated for the first time that me53 knockout blocked viral DNA replication, nucleocapsid formation, and consequent BV and ODV production.