| Summary: | BACKGROUND: Herpesvirus of turkey (HVT) as a vector to express the haemagglutinin (HA) of avian influenza virus (AIV) H5 was developed and its protection against lethal Marek's disease virus (MDV) and highly pathogenic AIV (HPAIV) challenges was evaluated previously. It is well-known that avirulemt MDV type 1 vaccines are more effective than HVT in prevention of lethal MDV infection. To further increase protective efficacy against HPAIV and lethal MDV, a recombinant MDV type 1 strain 814 was developed to express HA gene of HPAIV H5N1. METHODOLOGY/PRINCIPAL FINDINGS: A recombinant MDV-1 strain 814 expressing HA gene of HPAIV H5N1 virus A/goose/Guangdong/3/96 at the US2 site (rMDV-HA) was developed under the control of a human CMV immediate-early promoter. The HA expression in the rMDV-HA was tested by immunofluorescence and Western blot analyses, and in vitro and in vivo growth properties of rMDV-HA were also analyzed. Furthermore, we evaluated and compared the protective immunity of rMDV-HA and previously constructed rHVT-HA against HPAIV and lethal MDV. Vaccination of chickens with rMDV-HA induced 80% protection against HPAIV, which was better than the protection rate by rHVT-HA (66.7%). In the animal study with MDV challenge, chickens immunized with rMDV-HA were completely protected against virulent MDV strain J-1 whereas rHVT-HA only induced 80% protection with the same challenge dose. CONCLUSIONS/SIGNIFICANCE: The rMDV-HA vaccine was more effective than rHVT-HA vaccine for protection against lethal MDV and HPAIV challenges. Therefore, avirulent MDV type 1 vaccine is a better vector than HVT for development of a recombinant live virus vaccine against virulent MDV and HPAIV in poultry.
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