Herpesvirus of turkey reconstituted from bacterial artificial chromosome clones induces protection against Marek's disease.

Herpesvirus of turkey (HVT) is an alphaherpesvirus that is widely used as a live vaccine against Marek's disease because of its antigenic relationship with Marek's disease virus (MDV). In spite of a similar genome structure, HVT has several unique genes, the functions of which are not completely understood. As a first step in carrying out detailed analysis of the functions of the HVT genes, a full-length infectious bacterial artificial chromosome (BAC) clone of HVT was constructed. DNA from two independent BAC clones, upon transfection into chicken embryo fibroblasts, produced plaques similar to those produced by the wild-type virus. Viruses derived from the BAC clones were stable during in vitro passage, but showed differences in in vitro growth kinetics compared with the wild-type virus. Using a one-step mutagenesis protocol to delete the essential glycoprotein B gene from the HVT genome, followed by construction of the revertant virus, BAC clones of HVT were shown to be amenable to standard mutagenesis techniques. In spite of the difference in in vitro growth, viruses from both clones induced 100 % protection against infection by the virulent MDV strain RB-1B, indicating that the BAC-derived viruses could be used as vaccines with efficacies similar to that of the parental virus. The construction of HVT BAC is a major step in understanding the functions of HVT genes by exploiting the power of BAC technology. Furthermore, the availability of the BAC clones enables use of HVT as a vector for expressing foreign genes.