Abstract
Herpesvirus of turkey (HVT) is being widely used as a vector for development of recombinant vaccines and US2 and US10 genes are often chosen as insertion sites for targeted gene expression. However, the different effects of the two genes for generation of recombinant HVT vaccines were unknown. In order to compare the effects of inserted genes in the two sites on the efficacy of the recombinant vaccines, host-protective haemagglutinin (HA) gene of the highly pathogenic avian influenza virus (HPAIV) H5N1 was inserted into either US2 or US10 gene locus of the HVT. The resulting US2 (rHVT-US2-HA) or US10 (rHVT-US10-HA) recombinant HVT viruses were used to infect chicken embryo fibroblasts. Plaques and the growth kinetics of rHVT-US2-HA-infected chicken embryo fibroblasts were similar to those of parental HVT whereas rHVT-US10-HA infected chicken embryo fibroblasts had different growth kinetics and plaque formation. The viremia levels in rHVT-US10-HA virus-infected chickens were significantly lower than those of rHVT-US2-HA group on 28 days post infection. The vaccine efficacy of the two recombinant viruses against H5N1 HPAIV and virulent Marek's disease virus was also evaluated in 1-day-old vaccinated chickens. rHVT-US2-HA-vaccinated chickens were better protected with reduced mortality than rHVT-US10-HA-vaccinated animals following HPAIV challenge. Furthermore, the overall hemaglutination inhibition antibody titers of rHVT-US2-HA-vaccinated chickens were higher than those of rHVT-US10-HA-vaccinated chickens. Protection levels against Marek's disease virus challenge following vaccination with either rHVT-US2-HA or rHVT-US10-HA, however, were similar to those of the parental HVT virus. These results, for the first time, indicate that US2 gene provides a favorable foreign gene insertion site for generation of recombinant HVT vaccines.
Highlights
Herpesvirus of turkey (HVT) is a naturally occurring, nonpathogenic alphaherpesvirus originally isolated from domestic turkeys in the late of 1960s [1]
HA DNA and another major 20 kb fragment from the BamHIdigested Recombinant HVT (rHVT)-US10-HA DNA were detected with HA probe by Southern blotting hybridization, respectively, indicating that the transfer vectors were correctly inserted in the US2 region or the US10 region
Two recombinant HVTs were generated, and preliminary experiments indicated that the respective insertions did not affect the rHVT growth in chicken embryo fibroblasts (CEFs) as previously described [10,19]
Summary
Herpesvirus of turkey (HVT) is a naturally occurring, nonpathogenic alphaherpesvirus originally isolated from domestic turkeys in the late of 1960s [1]. HVT possesses some ideal characteristics: (1) HVT is a herpesvirus that infects chickens persistently, resulting in continuous immune system stimulation that helps maintain protective antibody levels elevated, (2) HVT vaccine is available in a cell-free ‘dry’ (lyophilized) form that is convenient for long-term storage and transport [7,8] and (3) MDV genome is large enough to accommodate multiple foreign genes. Recombinant HVT (rHVT) vaccine has been proven to be one of useful viral vectors of targeted gene expression and developed for the prevention of diseases caused by infections with various fowl disease-associated viruses [7,9,10,11,12,13]
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