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

Viral bacterial artificial chromosomes: generation, mutagenesis, and removal of mini-F sequences

Maintenance and manipulation of large DNA and RNA virus genomes had presented an obstacle for virological research. BAC vectors provided a solution to both problems as they can harbor large DNA sequences and can efficiently be modified using well-established mutagenesis techniques in Escherichia coli. Numerous DNA virus genomes of herpesvirus and pox virus were cloned into mini-F vectors. In addition, several reverse genetic systems for RNA viruses such as members of Coronaviridae and Flaviviridae could be established based on BAC constructs. Transfection into susceptible eukaryotic cells of virus DNA cloned as a BAC allows reconstitution of recombinant viruses. In this paper, we provide an overview on the strategies that can be used for the generation of virus BAC vectors and also on systems that are currently available for various virus species. Furthermore, we address common mutagenesis techniques that allow modification of BACs from single-nucleotide substitutions to deletion of viral genes or insertion of foreign sequences. Finally, we review the reconstitution of viruses from BAC vectors and the removal of the bacterial sequences from the virus genome during this process.

Back to BAC: the use of infectious clone technologies for viral mutagenesis

Bacterial artificial chromosome (BAC) vectors were first developed to facilitate the propagation and manipulation of large DNA fragments in molecular biology studies for uses such as genome sequencing projects and genetic disease models. To facilitate these studies, methodologies have been developed to introduce specific mutations that can be directly applied to the mutagenesis of infectious clones (icBAC) using BAC technologies. This has resulted in rapid identification of gene function and expression at unprecedented rates. Here we review the major developments in BAC mutagenesis in vitro. This review summarises the technologies used to construct and introduce mutations into herpesvirus icBAC. It also explores developing technologies likely to provide the next leap in understanding these important viruses.

Recombinant herpesvirus of turkeys as a vector-based vaccine against highly pathogenic H7N1 avian influenza and Marek's disease

A major challenge for poultry vaccination is the design of vaccines that protect against multiple pathogens via a single protective dose delivered through mass vaccination methods. In this investigation, we examined herpesvirus of turkeys (HVT) as a vaccine vector for delivery of haemagglutinin (HA) antigen of highly pathogenic H7N1 avian influenza virus that can act as a dual vaccine against avian influenza and Marek's disease. The HVT vector was developed using reverse genetics based on an infectious bacterial artificial chromosome (BAC) clone of HVT. The BAC carrying the HVT genome was genetically modified to express the HA gene of a highly pathogenic H7N1 virus. The resultant recombinant BAC construct containing the modified HVT sequence was transfected into chicken embryo fibroblast (CEF) cells, and HVT recombinants (rHVT-H7HA) harbouring the H7N1 HA were recovered. Analysis of cultured CEF cells infected with the rHVT-H7HA showed that HA was expressed and that the rescued rHVT-H7HA stocks were stable during several in vitro passages with no difference in growth kinetics compared with the parent HVT. Immunisation of one-day-old chicks with rHVT-H7HA induced H7-specific antibodies and protected chickens challenged with homologous H7N1 virus against virus shedding, clinical disease and death. This vaccine supports differentiation between infected and vaccinated animals (DIVA) vaccination strategies because no nucleoprotein-(NP) specific antibodies were detected in the rHVT-H7HA vaccinated birds. The rHVT-H7HA not only provided protection against a lethal challenge with highly pathogenic H7N1 virus but also against highly virulent Marek's disease virus and can be used as a DIVA vaccine.

Expression of HA of HPAI H5N1 virus at US2 gene insertion site of turkey herpesvirus induced better protection than that at US10 gene insertion site

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.

Advances in avian immunology--prospects for disease control: a review

In order to develop novel solutions to avian disease problems, including novel vaccines and/or vaccine adjuvants, and the identification of disease resistance genes which can feed into conventional breeding programmes, it is necessary to gain a more thorough understanding of the avian immune response and how pathogens can subvert that response. Birds occupy the same habitats as mammals, have similar ranges of longevity and body mass, and face similar pathogen challenges, yet birds have a different repertoire of organs, cells, molecules and genes of the immune system compared to mammals. This review summarises the current state of knowledge of the chicken's immune response, highlighting differences in the bird compared to mammals, and discusses how the availability of the chicken genome sequence and the associated postgenomics technologies are contributing to theses studies and also to the development of novel intervention strategies againts avian and zoonotic disease.

Pathogenicity of a very virulent strain of Marek's disease herpesvirus cloned as infectious bacterial artificial chromosomes

Bacterial artificial chromosome (BAC) vectors containing the full-length genomes of several herpesviruses have been used widely as tools to enable functional studies of viral genes. Marek's disease viruses (MDVs) are highly oncogenic alphaherpesviruses that induce rapid-onset T-cell lymphomas in chickens. Oncogenic strains of MDV reconstituted from BAC clones have been used to examine the role of viral genes in inducing tumours. Past studies have demonstrated continuous increase in virulence of MDV strains. We have previously reported on the UK isolate C12/130 that showed increased virulence features including lymphoid organ atrophy and enhanced tropism for the central nervous system. Here we report the construction of the BAC clones (pC12/130) of this strain. Chickens were infected with viruses reconstituted from the pC12/130 clones along with the wild-type virus for the comparison of the pathogenic properties. Our studies show that BAC-derived viruses induced disease similar to the wild-type virus, though there were differences in the levels of pathogenicity between individual viruses. Generation of BAC clones that differ in the potential to induce cytolytic disease provide the opportunity to identify the molecular determinants of increased virulence by direct sequence analysis as well as by using reverse genetics approaches on the infectious BAC clones.

Herpesvirus BACs: past, present, and future

The herpesviridae are a large family of DNA viruses with large and complicated genomes. Genetic manipulation and the generation of recombinant viruses have been extremely difficult. However, herpesvirus bacterial artificial chromosomes (BACs) that were developed approximately 10 years ago have become useful and powerful genetic tools for generating recombinant viruses to study the biology and pathogenesis of herpesviruses. For example, BAC-directed deletion mutants are commonly used to determine the function and essentiality of viral genes. In this paper, we discuss the creation of herpesvirus BACs, functional analyses of herpesvirus mutants, and future applications for studies of herpesviruses. We describe commonly used methods to create and mutate herpesvirus BACs (such as site-directed mutagenesis and transposon mutagenesis). We also evaluate the potential future uses of viral BACs, including vaccine development and gene therapy.

Marek's disease viruses lacking either R-LORF10 or LORF4 have altered virulence in chickens

The Marek's disease virus (MDV, Gallid herpesvirus 2) genome encodes approximately 110 open reading frames (ORFs). Many of these ORFs are annotated based purely on homology to other herpesvirus genes, thus, direct experiments are needed to verify the gene products, especially the hypothetical or MDV-specific ORFs, and characterize their biological function, particularly with respect to pathogenicity in chickens. Previously, a comprehensive two-hybrid assay screen revealed nine specific chicken-MDV protein-protein interactions. In order to characterize the role of hypothetical MDV proteins R-LORF10 and LORF4, which were shown to interact with major histocompatibility complex (MHC) class II beta chain and Ii (invariant or gamma) chain, respectively, recombinant MDVs derived from virulent MDV-BAC clone rMd5-B40 were generated. Recombinant MDV rMd5DeltaR-LORF10 lacked part of the promoter and the first 17 amino acids in both copies of R-LORF10, and rMd5mLORF4 had point mutations in LORF4 that disrupted the start codon and introduced a premature stop codon without altering the amino acid sequence of overlapping ORF UL1, which encodes glycoprotein L (gL). Mutations in either R-LORF10 or LORF4 neither prevent MDV reconstitution from modified MDV-BACs nor significantly alter virus growth rate in vitro. However, MDV generated from rMd5DeltaR-LORF10 had reduced virulence compared to the parental MDV. Surprisingly, MDV with the LORF4 mutations had significantly higher overall MD incidence as measured by mortality, tumor production, and MD symptoms in infected chickens. These results indicate R-LORF10 and LORF4 encode real products, and are involved in MDV virulence although their mechanisms, especially with respect to modulation of MHC class II cell surface expression, are not clearly understood.

Mutagenesis of the repeat regions of herpesviruses cloned as bacterial artificial chromosomes

Cloning of infectious and pathogenic herpesvirus genomes in a bacterial artificial chromosome (BAC) vector greatly facilitates genetic manipulation of their genomes. BAC-based mutagenesis strategies of viruses can advance our understanding of the viral gene functions and determinants of pathogenicity, and can ultimately help to develop molecularly defined improved vaccines against virus diseases. Unlike the virus stocks, where continuous passage in tissue culture can lead to phenotypic alterations such as loss of virulence or immunogenicity, viral genomes can be stably maintained with high fidelity as BAC clones in bacteria. Thanks to the "RecA" or the inducible phage "lambda Red" homologous recombination systems and a variety of positive and negative selection strategies, viral genomes cloned as BAC can be efficiently manipulated in E. coli. All the manipulations, including DNA fragment deletion or insertion, point mutations, or even multiple modifications in repeat regions can be carried out accurately in E. coli, and the mutated DNA can be used directly to reconstitute mutant viruses in transfected host cells. Furthermore, using self-excision strategies, the non-viral bacterial replicon sequence can be excised automatically during virus reconstitution, thus generating recombinant viruses virtually identical to the wild-type parent viruses. Here, we describe the various technologies of manipulating the infectious BAC clones of a group E herpesvirus as an example through a combination of different approaches.

Functional evaluation of the role of reticuloendotheliosis virus long terminal repeat (LTR) integrated into the genome of a field strain of Marek's disease virus

MDV-GX0101 is a field strain of Marek's disease virus with a naturally occurring insertion of the reticuloendotheliosis virus (REV) LTR fragment. In order to study the biological properties of REV-LTR insertion in the MDV genome, we constructed a full-length infectious BAC clone of MDV-GX0101 strain and deleted the LTR sequences by BAC mutagenesis. The pathogenic properties of the LTR-deleted virus were evaluated in infected SPF birds. The study demonstrated that the LTR-deleted virus had a stronger inhibitory effect on the growth rates of the infected birds and induced stronger immunosuppressive effects. Surprisingly, however, the ability for horizontal transmission of the LTR-deleted virus appeared to be significantly weaker than its parental LTR-intact virus. Even though the precise molecular mechanisms are still not clear, the results of our studies demonstrate that the retention of the REV-LTR in the MDV genome decreases its pathogenic effects but increases its potential for horizontal transmission.

Homodimerization of the Meq viral oncoprotein is necessary for induction of T-cell lymphoma by Marek's disease virus

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus that induces fatal rapid-onset T-cell lymphomas in chickens, its natural host. The MDV-encoded nuclear oncoprotein Meq is essential for lymphomagenesis and acts as a regulator of transcription. Meq has structural features, including a basic domain adjacent to a leucine zipper motif (B-ZIP), that suggest it is related to the Jun/Fos family of transcription factors. Via the leucine zipper, Meq can form homodimers or heterodimerize with c-Jun. Meq/Meq homodimers are associated with transrepression, and Meq/Jun heterodimers can transactivate target genes carrying an AP-1-like binding site. In order to determine the role of the leucine zipper and of Meq dimerization in T lymphomagenesis, specific point mutations were engineered into the highly oncogenic RB-1B strain of MDV to produce virus completely lacking a functional Meq leucine zipper (RB-1B Meq(BZIP/BZIP)) or virus encoding Meq that cannot homodimerize but can still bind to c-Jun and an AP-1-like site on DNA (RB-1B Meq(Hom/Hom)). Both of these mutant viruses were capable of replication in cultured chicken embryo fibroblasts. However both mutations resulted in a complete loss of oncogenicity, since no lymphomas were produced up to 90 days postinfection in experimentally infected chicks. We conclude that the leucine zipper is necessary for the oncogenic activity of Meq and/or the efficient establishment of long-term MDV latency in T cells. Moreover, it appears that the ability to form homodimers is an absolute requirement and the ability to bind c-Jun alone is insufficient for the T-cell lymphomagenesis associated with virulent MDV.

Cloning of Gallid herpesvirus 3 (Marek's disease virus serotype-2) genome as infectious bacterial artificial chromosomes for analysis of viral gene functions

Marek's disease virus serotype 2 (Gallid herpesvirus 3) is a non-pathogenic alphaherpesvirus belonging to the Mardivirus genus, used widely in live vaccines against Marek's disease. Although the complete genome sequence of the MDV-2 strain HPRS-24 has been published, very little is known about the gene functions. As a first step for carrying out functional genomic analysis of MDV-2, the full-length genome of the MDV-2 vaccine strain SB-1 was cloned as an infectious bacterial artificial chromosome (BAC) clone pSB-1. Virus reconstituted from the pSB-1 clone showed morphological and growth characteristics in cell culture very similar to the parent virus. Generation of SB-1 constructs deleted in glycoprotein E and viruses expressing Citrine-UL35 fusion protein by the application of different BAC mutagenesis techniques demonstrated the amenability of the pSB-1 clone for reverse genetics approaches to identify molecular determinants associated with different biological features of this virus. The generation of replication-competent infectious clones of SB-1, together with those of CVI988 and herpesvirus of turkey strains described previously, completes the portfolio of generating infectious BAC clones of the MD vaccine strains belonging to all the three serotypes, paving the way for the application of reverse genetics for functional analysis of immunogenic determinants of these vaccines as well as for developing novel recombinant vectors.

Construction of an infectious Marek's disease virus bacterial artificial chromosome and characterization of protection induced in chickens

Marek's disease virus (MDV) is a highly oncogenic alphaherpesvirus that induces rapid-onset T-cell lymphoma in poultry. The complete genome of the avirulent vaccine strain MDV-814 was cloned as an infectious bacterial artificial chromosome (BAC) using an 8.8-kb fragment containing the self-designed selective marker guanosine phosphoriboxyl transferase. The recombinant virus MDV-814-BAC was generated by co-transfection of a BAC transfer vector and MDV-814 total DNA, and was purified by eight rounds of selective passaging. The infectivity of the BAC DNA clone was validated by MDV reconstitution from chicken embryo fibroblasts transfected with MDV-BAC DNA, which was extracted from electroporated Escherichia coli DH10B cells. In vitro, the BAC-derived virus had similar biological characteristics and growth kinetics as the wild-type parental and recombinant viruses, and chickens immunized with BAC derivatives by various delivery mechanisms acquired protection against virulent MDV challenge. Construction of this MDV-BAC may aid the development of recombinant vaccines-containing multiple antigens.

Targeting Marek's disease virus by RNA interference delivered from a herpesvirus vaccine

Live attenuated herpesvirus vaccines such as herpesvirus of turkey (HVT) have been used since 1970 for the control of Marek's disease (MD), a highly infectious lymphoproliferative disease of poultry. Despite the success of these vaccines in reducing losses from the disease, Marek's disease virus (MDV) strains have shown a continuing increase in virulence, presumably due to the inability of the current vaccines in preventing MDV replication. The highly specific and effective nature of RNA interference (RNAi) makes this technology particularly attractive for new antiviral strategies. In order to exploit the power of RNAi-mediated suppression of MDV replication in vivo delivered through existing vaccines, we engineered recombinant HVT expressing short hairpin RNA (shRNA) against MDV genes gB and UL29. The levels of protection induced by the RNAi-expressing HVT against virulent virus challenge were similar to the parent pHVT3 virus. However, chickens vaccinated with recombinant HVT expressing shRNA showed moderate reduction of challenge virus replication in blood and feather samples. Delivery of RNAi-based gene silencing through live attenuated vaccines for reducing replication of pathogenic viruses is a novel approach for the control of infectious diseases.