Abstract:

【Objective】Duck enteritis virus (DEV) taxonomically belongs to family Herpesviridae and infects ducks, geese, and swans, which results in high mortality and decreased egg production in domestic and wild waterfowl. Several DEV whole genomic sequences were published, which contained 158-162 kb. Compared with DEV vaccine strain, the virulent DEV strain for vaccine evaluation had a 3513-bp insertion, resulting to UL56 frameshift mutation. At present, there are few papers about DEV gene function and pathogenic mechanism. To study the effect of the 3513-bp insertion on DEV characterization, a recombinant DEV with the 3513-bp deletion was constructed.【Method】The extracted DEV genomic DNA was used as template to amplify the UL56-u and UL56-d of the upstream and downstream ends of UL56 gene, respectively. The two homologous arm fragments were cloned into pMD18T-Simple vector, and the recombinant plasmid pT-UL56ud was obtained. The recombinant plasmid pT-UL56ud was cut by MluI enzyme, and after electrophoretic recovery and dephosphorylation, the recombinant plasmid pT-UL56-RFP was obtained by inserting RFP expression box into pT-UL56ud. After DEV was inoculated with duck embryo fibroblast (DEF) (MOI=0.1) for 1 to 2 h, the purified plasmid pT-UL56-RFP was transfected according to Lipofectamine 2000 instructions, and then purified by plaque. A 3513-bp deletion mutant, DEVΔ3513-RFP, was generated by targeted homologous recombination, in which red fluorescent protein (RFP) as a reporter replaced the 3 513 bp. The RFP was then removed to generate DEV△3513. A rescue mutant, DEVΔ3513(R), was constructed by reinserting the 3 513 bp into the genome of DEVΔ3513-RFP. The recombinant virus and its parent virus were inoculated in DEF (MOI =0.01), the virus titer was measured and the growth curve was plotted. The recombinant virus and its parent virus were diluted properly and inoculated into monolayers DEF, covered with M199 agarose and cultured 5-7 days. Twenty plaques were selected randomly and the area of plaques was assayed. Six-week old susceptible ducks were inoculated with 10 ^5.0TCID₅₀ of the 3513-bp deletion mutant, rescue mutant and its parental virus, respectively. After 10 days, all the surviving ducks were euthanized. The liver tissues were taken from all the animals and fixed to 4% poly Formaldehyde Solution; the pathological sections were prepared by routine procedures and stained with HE. The recombinant virus and the Duck Plague Live Vaccine (CVCC AV1222) were injected with 10 ^3.0TCID₅₀ in muscles for 6 weeks of age susceptible ducks to be tested for immunogenicity. After 14 days, all vaccinated ducks were challenged with 10 ^3.0MLD of lethal DEV (CVCC AV1221).【Result】The recombinant viruses, DEVΔ3513 and DEVΔ3513(R), and their parental virus possessed similar growth kinetics, and their titer peaked at 72 hours with the peak titer 10 ^6.2-6.5TCID₅₀/0.1mL. Their average plaques sizes were not significantly different; DEVΔ3513 was avirulent in 6-week ducks; the ducks vaccinated with 10 ³TCID₅₀ were protected against subsequent challenge with lethal DEV.【Conclusion】We successfully constructed a DEV mutant with 3 513 bp deletion, and firstly confirmed that the deletion of the 3 513 bp had no effect on virus replication in cells and immunogenicity in ducks. Moreover, the 3 513 bp was associated with DEV virulence.

Key words: duck enteritis virus, downstream 3513 bp of UL56, recombinant virus, property