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

Impact of viral telomeric repeat sequences on herpesvirus vector vaccine integration and persistence

Marek's disease virus (MDV) vaccines were the first vaccines that protected against cancer. The avirulent turkey herpesvirus (HVT) was widely employed and protected billions of chickens from a deadly MDV infection. It is also among the most common vaccine vectors providing protection against a plethora of pathogens. HVT establishes latency in T-cells, allowing the vaccine virus to persist in the host for life. Intriguingly, the HVT genome contains telomeric repeat arrays (TMRs) at both ends; however, their role in the HVT life cycle remains elusive. We have previously shown that similar TMRs in the MDV genome facilitate its integration into host telomeres, which ensures efficient maintenance of the virus genome during latency and tumorigenesis. In this study, we investigated the role of the TMRs in HVT genome integration, latency, and reactivation in vitro and in vivo. Additionally, we examined HVT infection of feather follicles. We generated an HVT mutant lacking both TMRs (vΔTMR) that efficiently replicated in cell culture. We could demonstrate that wild type HVT integrates at the ends of chromosomes containing the telomeres in T-cells, while integration was severely impaired in the absence of the TMRs. To assess the role of TMRs in vivo, we infected one-day-old chickens with HVT or vΔTMR. vΔTMR loads were significantly reduced in the blood and hardly any virus was transported to the feather follicle epithelium where the virus is commonly shed. Strikingly, latency in the spleen and reactivation of the virus were severely impaired in the absence of the TMRs, indicating that the TMRs are crucial for the establishment of latency and reactivation of HVT. Our findings revealed that the TMRs facilitate integration of the HVT genome into host chromosomes, which ensures efficient persistence in the host, reactivation, and transport of the virus to the skin.

Transgenerational epigenetic inheritance and immunity in chickens that vary in Marek's disease resistance

Marek's disease virus (MDV), a naturally oncogenic, highly contagious alpha herpesvirus, induces a T cell lymphoma in chickens that causes severe economic loss. Marek's disease (MD) outcome in an individual is attributed to genetic and environmental factors. Further investigation of the host-virus interaction mechanisms that impact MD resistance is needed to achieve greater MD control. This study analyzed genome-wide DNA methylation patterns in 2 highly inbred parental lines 63 and 72 and 5 recombinant congenic strains (RCS) C, L, M, N, and X strains from those parents. Lines 63 and 72, are MD resistant and susceptible, respectively, whereas the RCS have different combinations of 87.5% Line 63 and 12.5% Line 72. Our DNA methylation cluster showed a strong association with MD incidence. Differentially methylated regions (DMRs) between the parental lines and the 5 RCS were captured. MD-resistant and MD-susceptible markers of DNA methylation were identified as transgenerational epigenetic inheritable. In addition, the growth of v-src DNA tumors and antibody response against sheep red blood cells differed among the 2 parental lines and the RCS. Overall, our results provide very solid evidence that DNA methylation patterns are transgenerational epigenetic inheritance (TEI) in chickens and also play a vital role in MD tumorigenesis and other immune responses; the specific methylated regions may be important modulators of general immunity.

The genome evolution of Marek's disease viruses in chickens and turkeys in China

The virus that causes Marek's disease (MD) is globally ubiquitous in chickens, continuously evolving, and poses a significant threat to the poultry industry. Although vaccines are extensively used, MD still occurs frequently and the virus has evolved increased virulence in China. Here, we report an outbreak of MD in vaccinated chickens and unvaccinated turkeys in a backyard farm in Guangdong province, China, in 2018. Phylogenetic analysis revealed two lineages of MDVs at this farm, with one lineage, containing isolates from two turkeys and five chickens, clustering with virulent Chinese strains and displays a relatively high genetic divergence from the vaccine strains. These new isolates appear to have broken through vaccine immunity, yielding this outbreak of MD in chickens and turkeys. The second lineage included four chicken isolates that clustered with the CVI988 and 814 vaccine strains. The large diversity of MDVs in this single outbreak reveals a complex circulation of MDVs in China. Poor breeding conditions and the weak application of disease prevention and control measures make backyard farms a hotbed for the evolution of viruses that cause infectious diseases. This is especially important in MDV as the MD vaccines do not provide sterilizing immunity, which allows the replication and shedding of virulent field viruses by vaccinated individuals and supporting the continuous evolution of MDVs. Hence, constant monitoring of the evolution of MDVs is necessary to understand the evolution of these field viruses and potential expansions of their host range.

Poultry Vaccine Technology Platforms

The different technology platforms used to make poultry vaccines are reviewed. Vaccines based on classical technologies are either live attenuated or inactivated vaccines. Genetic engineering is applied to design by deletion, mutation, insertion, or chimerization, genetically modified target microorganisms that are used either as live or inactivated vaccines. Other vaccine platforms are based on one or a few genes of the target pathogen agent coding for proteins that can induce a protective immune response ("protective genes"). These genes can be expressed in vitro to produce subunit vaccines. Alternatively, vectors carrying these genes in their genome or nucleic acid-based vaccines will induce protection by in vivo expression of these genes in the vaccinated host. Properties of these different types of vaccines, including advantages and limitations, are reviewed, focusing mainly on vaccines targeting viral diseases and on technologies that succeeded in market authorization.

Recombinant Turkey Herpesvirus Expressing H9N2 HA Gene at the HVT005/006 Site Induces Better Protection Than That at the HVT029/031 Site

Turkey herpesvirus (HVT) is widely used as an effective recombinant vaccine vector for expressing protective antigens of multiple avian pathogens from different loci of the HVT genome. These include the HVT029/031 (UL22-23) locus for the insertion of IBDV VP2 and the recently identified HVT005/006 locus as a novel site for expressing heterologous proteins. In order to compare the efficacy of recombinant vaccines with the HA gene at different sites, the growth curves and the HA expression levels of HVT-005/006-hCMV-HA, HVT-005/006-MLV-HA, and HVT-029/031-MLV-HA were first examined in vitro. While the growth kinetics of three recombinant viruses were not significantly different from those of parent HVT, higher expression of the HA gene was achieved from the HVT005/006 site than that from the HVT029/031 site. The efficacy of the three recombinant viruses against avian influenza H9N2 virus was also evaluated using one-day-old SPF chickens. Chickens immunized with HVT-005/006-MLV-HA or HVT-005/006-hCMV-HA displayed reduced virus shedding compared to HVT-029/031-MLV-HA vaccinated chickens. Moreover, the overall hemagglutination inhibition (HI) antibody titers of HVT-005/006-HA-vaccinated chickens were higher than that of HVT-029/031-HA-vaccinated chickens. However, HVT-005/006-MLV-HA and HVT-005/006-hCMV-HA did not result in a significant difference in the level of HA expression in vitro and provided the same protective efficacy (100%) at 5 days after challenge. In the current study, the results suggested that recombinant HVT005/006 vaccines caused better expression of HA than recombinant HVT029/031 vaccine, and that HVT-005/006-MLV-HA or HVT-005/006-hCMV-HA could be a candidate vaccine for the protection of chickens against H9N2 influenza.

Construction of a Novel Infectious Clone of Recombinant Herpesvirus of Turkey Fc-126 Expressing VP2 of IBDV

The increased virulence of infectious bursal disease virus (IBDV) is a threat to the chicken industry. The construction of novel herpesvirus of turkey-vectored (HVT) vaccines expressing VP2 of virulent IBDV may be a promising vaccine candidate for controlling this serious disease in chickens. We generated a novel infectious clone of HVT Fc-126 by inserting mini-F sequences in lieu of the glycoprotein C (gC) gene. Based on this bacterial artificial chromosome (BAC), a VP2 expression cassette containing the pMCMV IE promoter and a VP2 sequence from the virulent IBDV NJ09 strain was inserted into the noncoding area between the UL55 and UL56 genes to generate the HVT vector VP2 recombinant, named HVT-VP2-09. The recovered vectored mutant HVT-VP2-09 exhibited higher titers (p = 0.0202 at 36 h) or similar growth kinetics to the parental virus HVT Fc-126 (p = 0.1181 at 48 h and p = 0.1296 at 64 h). The high reactivation ability and strong expression of VP2 by HVT-VP2-09 in chicken embryo fibroblasts (CEFs) were confirmed by indirect immunofluorescence (IFA) and Western blotting. The AGP antibodies against IBDV were detected beginning at 3 weeks post-inoculation (P.I.) of HVT-VP2-09 in 1-day-old SPF chickens. Seven of ten chickens immunized with HVT-VP2-09 were protected post-challenge (P.C.) with the virulent IBDV NJ09 strain. In contrast, all chickens in the challenge control group showed typical IBD lesions in bursals, and eight of ten died P.C. In this study, we demonstrated that (i) a novel HVT BAC with the whole genome of the Fc-126 strain was obtained with the insertion of mini-F sequences in lieu of the gC gene; (ii) HVT-VP2-09 harboring the VP2 expression cassette from virulent IBDV exhibited in vitro growth properties similar to those of the parental HVT virus in CEF cells; and (iii) HVT-VP2-09 can provide efficient protection against the IBDV NJ09 strain.

Rapid, Sensitive, and Species-Specific Detection of Conventional and Recombinant Herpesvirus of Turkeys Vaccines Using Loop-Mediated Isothermal Amplification Coupled With a Lateral Flow Device Readout

Marek's disease, an economically important disease of chickens caused by virulent serotype 1 strains of the Mardivirus Marek's disease virus (MDV-1), is effectively controlled in the field by live attenuated vaccine viruses including herpesvirus of turkeys (HVT)—both conventional HVT (strain FC126) and, in recent years, recombinant HVT viruses carrying foreign genes from other avian viruses to protect against both Marek's disease and other avian viral diseases. Testing to monitor and confirm successful vaccination is important, but any such test must differentiate HVT from MDV-1 and MDV-2, as vaccination does not prevent infection with these serotypes. End-point and real-time PCR tests are widely used to detect and differentiate HVT, MDV-1 and MDV-2 but require expensive specialist laboratory equipment and trained operators. Here, we developed and validated two tube-based loop-mediated isothermal amplification tests coupled with detection by lateral flow device readout (LAMP-LFD): an HVT-specific test to detect both conventional and recombinant HVT strains, and a second test using novel LAMP primers to specifically detect the Vaxxitek® recombinant HVT. Specificity was confirmed using DNA extracted from virus-infected cultured cells, and limit of detection was determined using plasmid DNA carrying either the HVT or Vaxxitek® genome. The LAMP-LFD tests accurately detected all HVT vaccines, or Vaxxitek® only, in crude DNA as well as purified DNA extracted from field samples of organs, feathers, or poultry house dust that were confirmed positive for HVT by real-time PCR. These LAMP-LFD tests have potential for specific, rapid, simple, and inexpensive detection of HVT vaccines in the field.

Identification of a Novel Insertion Site HVT-005/006 for the Generation of Recombinant Turkey Herpesvirus Vector

Turkey herpesvirus (HVT) has been widely used as a successful live virus vaccine against Marek's disease (MD) in chickens for more than five decades. Increasingly, HVT is also used as a highly effective recombinant vaccine vector against multiple avian pathogens. Conventional recombination, or recombineering, techniques that involve the cloning of viral genomes and, more recently, gene editing methods have been used for the generation of recombinant HVT-based vaccines. In this study, we used NHEJ-dependent CRISPR/Cas9-based approaches to insert the mCherry cassette for the screening of the HVT genome and identifying new potential sites for the insertion of foreign genes. A novel intergenic site HVT-005/006 in the unique long (UL) region of the HVT genome was identified, and mCherry was found to be stably expressed when inserted at this site. To confirm whether this site was suitable for the insertion of other exogenous genes, haemagglutinin (HA) of the H9N2 virus was inserted into this site, and a recombinant HVT-005/006-HA was rescued. The recombinant HVT-HA can grow well and express HA protein stably, which demonstrated that HVT-005/006 is a promising site for the insertion of foreign genes.

A Recombinant Turkey Herpesvirus Expressing the F Protein of Newcastle Disease Virus Genotype XII Generated by NHEJ-CRISPR/Cas9 and Cre-LoxP Systems Confers Protection against Genotype XII Challenge in Chickens

In this study, we developed a new recombinant virus rHVT-F using a Turkey herpesvirus (HVT) vector, expressing the fusion (F) protein of the genotype XII Newcastle disease virus (NDV) circulating in Peru. We evaluated the viral shedding and efficacy against the NDV genotype XII challenge in specific pathogen-free (SPF) chickens. The F protein expression cassette was inserted in the unique long (UL) UL45–UL46 intergenic locus of the HVT genome by utilizing a clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 gene-editing technology via a non-homologous end joining (NHEJ) repair pathway. The rHVT-F virus, which expressed the F protein stably in vitro and in vivo, showed similar growth kinetics to the wild-type HVT (wtHVT) virus. The F protein expression of the rHVT-F virus was detected by an indirect immunofluorescence assay (IFA), Western blotting, and a flow cytometry assay. The presence of an NDV-specific IgY antibody was detected in serum samples by an enzyme-linked immunosorbent assay (ELISA) in SPF chickens vaccinated with the rHVT-F virus. In the challenge experiment, the rHVT-F vaccine fully protects a high, and significantly reduced, virus shedding in oral at 5 days post-challenge (dpc). In conclusion, this new rHVT-F vaccine candidate is capable of fully protecting SPF chickens against the genotype XII challenge.

Review of Poultry Recombinant Vector Vaccines

The control of poultry diseases has relied heavily on the use of many live and inactivated vaccines. However, over the last 30 yr, recombinant DNA technology has been used to generate many novel poultry vaccines. Fowlpox virus and turkey herpesvirus are the two main vectors currently used to construct recombinant vaccines for poultry. With the use of these two vectors, more than 15 recombinant viral vector vaccines against Newcastle disease, infectious laryngotracheitis, infectious bursal disease, avian influenza, and Mycoplasma gallisepticum have been developed and are commercially available. This review focuses on current knowledge about the safety and efficacy of recombinant viral vectored vaccines and the mechanisms by which they facilitate the control of multiple diseases. Additionally, the development of new recombinant vaccines with novel vectors will be briefly discussed.

The Requirement of Glycoprotein C for Interindividual Spread Is Functionally Conserved within the Alphaherpesvirus Genus (Mardivirus), but Not the Host (Gallid)

Marek’s disease (MD) in chickens is caused by Gallid alphaherpesvirus 2, better known as MD herpesvirus (MDV). Current vaccines do not block interindividual spread from chicken-to-chicken, therefore, understanding MDV interindividual spread provides important information for the development of potential therapies to protect against MD, while also providing a natural host to study herpesvirus dissemination. It has long been thought that glycoprotein C (gC) of alphaherpesviruses evolved with their host based on their ability to bind and inhibit complement in a species-selective manner. Here, we tested the functional importance of gC during interindividual spread and host specificity using the natural model system of MDV in chickens through classical compensation experiments. By exchanging MDV gC with another chicken alphaherpesvirus (Gallid alphaherpesvirus 1 or infectious laryngotracheitis virus; ILTV) gC, we determined that ILTV gC could not compensate for MDV gC during interindividual spread. In contrast, exchanging turkey herpesvirus (Meleagrid alphaherpesvirus 1 or HVT) gC could compensate for chicken MDV gC. Both ILTV and MDV are Gallid alphaherpesviruses; however, ILTV is a member of the Iltovirus genus, while MDV is classified as a Mardivirus along with HVT. These results suggest that gC is functionally conserved based on the virus genera (Mardivirus vs. Iltovirus) and not the host (Gallid vs. Meleagrid).

Methods for the Manipulation of Herpesvirus Genome and the Application to Marek's Disease Virus Research

Herpesviruses are a group of double-strand DNA viruses that infect a wide range of hosts, including humans and animals. In the past decades, numerous methods have been developed to manipulate herpesviruses genomes, from the introduction of random mutations to specific genome editing. The development of genome manipulation methods has largely advanced the study of viral genes function, contributing not only to the understanding of herpesvirus biology and pathogenesis, but also the generation of novel vaccines and therapies to control and treat diseases. In this review, we summarize the major methods of herpesvirus genome manipulation with emphasis in their application to Marek’s disease virus research.

Novel Insights into the Roles of Bcl-2 Homolog Nr-13 (vNr-13) Encoded by Herpesvirus of Turkeys in the Virus Replication Cycle, Mitochondrial Networks, and Apoptosis Inhibition

The Bcl-2 (B cell lymphoma 2)-related protein Nr-13 plays a major role in the regulation of cell death in developing avian B cells. With over 65% sequence similarity to the chicken Nr-13, herpesvirus of turkeys (HVT) vNr-13, encoded by the HVT079 and HVT096 genes, is the first known alphaherpesvirus-encoded Bcl-2 homolog. HVT-infected cells were reported to be relatively more resistant to serum starvation, suggested that vNr-13 could be involved in protecting the cells. Here, we describe CRISPR/Cas9-based editing of exon 1 of the HVT079 and HVT096 genes from the HVT genome to generate the mutant HVT-ΔvNr-13 to gain insights into its functional roles. Overall, wild-type HVT and HVT-ΔvNr-13 showed similar growth kinetics; however, at early time points, HVT-ΔvNr-13 showed 1.3- to 1.7-fold-lower growth of cell-associated virus and 3- to 6.2-fold-lower growth of cell-free virus. In transfected cells, HVT vNr-13 showed a mainly diffuse cytoplasmic distribution with faint nuclear staining. Further, vNr-13 localized to the mitochondria and endoplasmic reticulum (ER) and disrupted mitochondrial network morphology in the transfected cells. In the wild-type HVT-infected cells, vNr-13 expression appeared to be directly involved in the disruption of the mitochondrial network, as the mitochondrial network morphology was substantially restored in the HVT-ΔvNr-13-infected cells. IncuCyte S3 real-time apoptosis monitoring demonstrated that vNr-13 is unequivocally involved in the apoptosis inhibition, and it is associated with an increase of PFU, especially under serum-free conditions in the later stages of the viral replication cycle. Furthermore, HVT blocks apoptosis in infected cells but activates apoptosis in noninfected bystander cells.IMPORTANCE B cell lymphoma 2 (Bcl-2) family proteins play important roles in regulating apoptosis during homeostasis, tissue development, and infectious diseases. Several viruses encode homologs of cellular Bcl-2-proteins (vBcl-2) to inhibit apoptosis, which enable them to replicate and persist in the infected cells and to evade/modulate the immune response of the host. Herpesvirus of turkeys (HVT) is a nonpathogenic alphaherpesvirus of turkeys and chickens that is widely used as a live vaccine against Marek's disease and as recombinant vaccine viral vectors for protecting against multiple avian diseases. Identical copies of the HVT genes HVT079 and HVT096 encode the Bcl-2 homolog vNr-13. While previous studies have identified the potential ability of vNr-13 in inhibiting apoptosis induced by serum deprivation, there have been no detailed investigations on the functions of vNr-13. Using CRISPR/Cas9-based ablation of the vNr-13 gene, we demonstrated the roles of HVT vNr-13 in early stages of the viral replication cycle, mitochondrial morphology disruption, and apoptosis inhibition in later stages of viral replication.

Vaccinal efficacy of molecularly cloned Gallid alphaherpesvirus 3 strain 301B/1 against very virulent Marek's disease virus challenge

Marek's disease virus (MDV), a causative agent of Marek's disease, has evolved its virulence partly because the current control strategies fail to provide sterilizing immunity. Gallid alphaherpesvirus 3 (GaHV-3) and turkey herpesvirus have been developed as bivalent vaccines to improve upon the level of protection elicited by single formulations. Since the in vitro passage of vaccines can result in attenuation, a GaHV-3 strain 301B/1 was cloned as a bacterial artificial chromosome (BAC) by inserting the mini-F replicon into the virus genome. A fully infectious virus, v301B-BAC, was reconstituted from the 301B/1 BAC clone and had similar growth kinetics comparable to that of the parental 301B/1 virus with strong reactivity against anti-301B/1 chicken sera. Protective efficacies of v301B-BAC, parental 301B/1, and SB-1 vaccine were evaluated against a very virulent MDV Md5 challenge. Clinical signs were significantly lower in the v301B-BAC vaccinated groups (24-25 %), parental 301B/1 (29 %) compare to that of non-vaccinated control (100%) and the removal of BAC sequences from v301B-BAC genome further reduced this to 17 %. The protective indices of v301B-BACs (75-76 %) were comparable with those of both the 301B/1 and the SB-1 vaccine (71%). Removal of the mini-F replicon resulted in a reconstituted virus with a protective index of 83 %. The shedding of challenge virus was notably lower in the v301B-BAC, and v301B-delBAC vaccinated groups. Overall, the protective efficacy of the 301B-BAC-derived vaccine virus against a very virulent MDV challenge was comparable to that of the parental 301B/1 virus as well as the SB-1 vaccine virus.

Chickens can durably clear herpesvirus vaccine infection in feathers while still carrying vaccine-induced antibodies

Marek’s disease (MD) is a major disease of chickens induced by Marek’s disease virus (MDV) associated to lethal lymphomas. Current MD vaccines protect against lymphomas, but fail to prevent infection and shedding. The control of MDV shedding is crucial in order to eradicate this highly contagious virus. Like pathogenic MDV, MD vaccines infect the feather follicles of the skin before being shed into the environment. MD vaccines constitute excellent models to study virus interaction with feathers, the unique excretion source of these viruses. Herein we studied the viral persistence in feathers of a MD vaccine, the recombinant turkey herpesvirus (rHVT-ND). We report that most of the birds showed a persistent HVT infection of feathers over 41 weeks with moderate viral loads. Interestingly, 20% of the birds were identified as low HVT producers, among which six birds cleared the infection. Indeed, after week 14–26, these birds named controllers had undetectable HVT DNA in their feathers through week 41. All vaccinated birds developed antibodies to NDV, which lasted until week 41 in 95% of the birds, including the controllers. No correlation was found between HVT loads in feathers and NDV antibody titers over time. Interestingly, no HVT DNA was detected in the spleens of four controllers. This is the first description of chickens that durably cleared MD vaccine infection of feathers suggesting that control of Mardivirus shedding is achievable by the host.

Generation of A Triple Insert Live Avian Herpesvirus Vectored Vaccine Using CRISPR/Cas9-Based Gene Editing

Herpesvirus of turkeys (HVT), used originally as a vaccine against Marek’s disease (MD), has recently been shown to be a highly effective viral vector for generation of recombinant vaccines that deliver protective antigens of other avian pathogens. Until the recent launch of commercial HVT-vectored dual insert vaccines, most of the HVT-vectored vaccines in the market carry a single foreign gene and are usually developed with slow and less efficient conventional recombination methods. There is immense value in developing multivalent HVT-vectored vaccines capable of inducing simultaneous protection against multiple avian pathogens, particularly to overcome the interference between individual recombinant HVT vaccines. Here we demonstrate the use of a previously developed CRISPR/Cas9 gene editing protocol for the insertion of ILTV gD-gI and the H9N2 AIV hemagglutinin expression cassettes into the distinct locations of the recombinant HVT-IBDV VP2 viral genome, to generate the triple insert HVT-VP2-gDgI-HA recombinant vaccine. The insertion, protein expression, and stability of each insert were then evaluated by PCR, immunostaining and Western blot analyses. The successful generation of the first triple insert recombinant HVT vaccine with the potential for the simultaneous protection against three major avian viral diseases in addition to MD is a major innovation in vaccination-based control of major poultry diseases.

Application of HDR-CRISPR/Cas9 and Erythrocyte Binding for Rapid Generation of Recombinant Turkey Herpesvirus-Vectored Avian Influenza Virus Vaccines

Avian influenza viruses (AIVs) are highly contagious and have caused huge economical loss to the poultry industry. AIV vaccines remain one of the most effective methods of controlling this disease. Turkey herpesvirus (HVT) is a commonly used live attenuated vaccine against Marek’s disease; it has also been used as a viral vector for recombinant AIV vaccine development. The clustered regularly interspaced palindromic repeats (CRISPR)/Cas9 system is a gene editing tool which, in vaccinology, has facilitated the development of recombinant DNA viral-vectored vaccines. Here, we utilize homology-directed repair (HDR) for the generation of a HVT–H7N9 HA bivalent vaccine; a H7N9 HA expression cassette was inserted into the intergenic region between UL45 and UL46 of HVT. To optimize the selection efficiency of our bivalent vaccine, we combined CRISPR/Cas9 with erythrocyte binding to rapidly generate recombinant HVT–H7HA candidate vaccines.

A proofreading-impaired herpesvirus generates populations with quasispecies-like structure

RNA virus populations are composed of highly diverse individuals that form a cloud of related sequences commonly referred to as a 'quasispecies'^1-3. This diversity arises as a consequence of low-fidelity genome replication^4,5. By contrast, DNA virus populations contain more uniform individuals with similar fitness⁶. Genome diversity is often correlated with increased fitness in RNA viruses, while DNA viruses are thought to require more faithful genome replication. During DNA replication, erroneously incorporated bases are removed by a 3'-5' exonuclease, a highly conserved enzymatic function of replicative DNA but not RNA polymerases. This proofreading process enhances replication fidelity and ensures the genome integrity of DNA organisms, including large DNA viruses⁷. Here, we show that a herpesvirus can tolerate impaired exonucleolytic proofreading, resulting in DNA virus populations, which, as in RNA viruses⁸, are composed of highly diverse genotypes of variable individual fitness. This indicates that herpesvirus mutant diversity may compensate for individual fitness loss. Notably, in vivo infection with diverse virus populations results in a marked increase in virulence compared to genetically homogenous parental virus. While we cannot exclude that the increase in virulence is caused by selection of and/or interactions between individual genotypes, our findings are consistent with quasispecies dynamics. Our results contrast with traditional views of DNA virus replication and evolution, and indicate that a substantial increase in population diversity can lead to higher virulence.

Utilization of herpesviridae as recombinant viral vectors in vaccine development against animal pathogens

Throughout the past few decades, numerous viral species have been generated as vaccine vectors. Every viral vector has its own distinct characteristics. For example, the family herpesviridae encompasses several viruses that have medical and veterinary importance. Attenuated herpesviruses are developed as vectors to convey heterologous immunogens targeting several serious and crucial pathogens. Some of these vectors have already been licensed for use in the veterinary field. One of their prominent features is their capability to accommodate large amount of foreign DNA, and to stimulate both cell-mediated and humoral immune responses. A better understanding of vector-host interaction builds up a robust foundation for the future development of herpesviruses-based vectors. At the time, many molecular tools are applied to enable the generation of herpesvirus-based recombinant vaccine vectors such as BAC technology, homologous and two-step en passant mutagenesis, codon optimization, and the CRISPR/Cas9 system. This review article highlights the most important techniques applied in constructing recombinant herpesviruses vectors, advantages and disadvantages of each recombinant herpesvirus vector, and the most recent research regarding their use to control major animal diseases.

Application of CRISPR/Cas9 Gene Editing System on MDV-1 Genome for the Study of Gene Function

Marek’s disease virus (MDV) is a member of alphaherpesviruses associated with Marek’s disease, a highly contagious neoplastic disease in chickens. Complete sequencing of the viral genome and recombineering techniques using infectious bacterial artificial chromosome (BAC) clones of Marek’s disease virus genome have identified major genes that are associated with pathogenicity. Recent advances in CRISPR/Cas9-based gene editing have given opportunities for precise editing of the viral genome for identifying pathogenic determinants. Here we describe the application of CRISPR/Cas9 gene editing approaches to delete the Meq and pp38 genes from the CVI988 vaccine strain of MDV. This powerful technology will speed up the MDV gene function studies significantly, leading to a better understanding of the molecular mechanisms of MDV pathogenesis.

Recent advances in viral vectors in veterinary vaccinology

Viral vectored vaccines, particularly using vectors such as adenovirus, herpesvirus and poxviruses, are used widely in veterinary medicine, where this technology has been adopted much more quickly than in human medicine. There are now a large number of programmes to develop viral vector vaccine platforms for humans and very similar or identical vectors are being developed for veterinary medicine. The shared experiences of developing these new vaccine platforms across the two disciplines is accelerating progress, a striking example of the value of a 'One Health' approach. In particular, there is growing use of adenoviruses, either replicating or replication-incompetent, to create new vaccines for use in livestock or companion animals. Live replicating avian herpesvirus vectors are increasingly used as vaccines against poultry diseases.

A simple and rapid approach to develop recombinant avian herpesvirus vectored vaccines using CRISPR/Cas9 system

Herpesvirus of turkeys (HVT) has been successfully used as live vaccine against Marek's disease (MD) worldwide for more than 40 years either alone or in combination with other serotypes. HVT is also widely used as a vector platform for generation of recombinant vaccines against a number of avian diseases such as infectious bursal disease (IBD), Newcastle disease (ND) and avian influenza (AI) using conventional recombination methods or recombineering tools on cloned viral genomes. In the present study, we describe the application of CRISPR/Cas9-based genome editing as a rapid and efficient method of generating HVT recombinants expressing VP2 protein of IBDV. This approach offers an efficient method to introduce other viral antigens into the HVT genome for rapid development of recombinant vaccines.

Impairment of infectious laryngotracheitis virus replication by deletion of the UL[-1] gene

Infectious laryngotracheitis virus (ILTV) encodes several unique genes, including a pair of unique nuclear proteins UL0 and UL[-1] that are expressed during replication in cell culture. Although the UL0 gene has been shown to be dispensable for replication, the role of UL[-1] has not been elucidated. In this study a deletion mutant of ILTV lacking the UL[-1] gene was constructed using homologous recombination. The coding sequences of the gene were replaced with the gene for enhanced green fluorescent protein and the cytomegalovirus major immediate early promoter element. The progeny virus carrying the reporter gene was readily identified using fluorescent microscopy, but was unable to propagate in the permissive cells in the absence of wild type ILTV. Even after plaque purification and fluorescent associated cell sorting the recombinant virus deficient in UL[-1] gene could not be successfully isolated. Our findings suggest that the UL[-1] gene has an important role in ILTV replication.

Recombinant Marek's disease virus type 1 provides full protection against very virulent Marek's and infectious bursal disease viruses in chickens

Marek’s disease virus (MDV) is a preferred vector in the construction of recombinant vaccines. However, bivalent vaccine based on MDV that confers full protection against both very virulent Marek’s and infectious bursal disease virus (IBDV) infections in chickens has not been produced. Here we developed a system utilizing overlapping fosmid DNAs transfection that rescues an MDV type 1 (MDV1) vaccine strain. Using this system, we inserted the IBDV VP2 gene at MDV1 genome sites UL41, US10 and US2. The VP2 protein was stably expressed in the recombinant MDV-infected cells and self-assembled into IBDV subviral particles. Insertion of the VP2 gene did not affect the replication phenotype of MDV in cell cultures, nor did it increase the virulence of the MDV vaccine strain in chickens. After challenge with very virulent IBDV, r814US2VP2 conferred full protection, whereas r814UL41VP2 and r814US10VP2 provided partial or no protection. All the three recombinant vaccines provided full protection against very virulent MDV challenge in chickens. These results demonstrated that r814US2VP2 could be used as a promising bivalent vaccine against both Marek’s and infectious bursal diseases in chickens.

Recombinant Marek's Disease Virus as a Vector-Based Vaccine against Avian Leukosis Virus Subgroup J in Chicken

Avian leukosis virus subgroup J (ALV-J) is an immunosuppressive virus that causes considerable economic losses to the chicken industry in China. However, there is currently no effective vaccine to prevent ALV-J infection. In order to reduce the losses caused by ALV-J, we constructed two effective ALV-J vaccines by inserting the ALV-J (strain JL093-1) env or gag+env genes into the US2 gene of the Marek’s disease herpesviruses (MDV) by transfection of overlapping fosmid DNAs, creating two recombinant MDVs, rMDV/ALV-gag+env and rMDV/ALV-env. Analysis of cultured chicken embryo fibroblasts infected with the rMDVs revealed that Env and Gag were successfully expressed and that there was no difference in growth kinetics in cells infected with rMDVs compared with that of cells infected with the parent MDV. Chickens vaccinated with either rMDV revealed that positive serum antibodies were induced. Both rMDVs also effectively reduced the rate of positive viremia in chicken flocks challenged with ALV-J. The protective effect provided by rMDV/ALV-env inoculation was slightly stronger than that provided by rMDV/ALV-gag+env. This represents the first study where a potential rMDV vaccine, expressing ALV-J antigenic genes, has been shown to be effective in the prevention of ALV-J. Our study also opens new avenues for the control of MDV and ALV-J co-infection.

Deletion of the BAC sequences from recombinant meq-null Marek's disease (MD) virus increases immunosuppression while maintaining protective efficacy against MD

Marek's disease virus (MDV) GX0101 is a field strain of MDV with a naturally occurring insertion of the reticuloendotheliosis virus (REV) long terminal repeat (LTR) fragment. Both copies of the meq gene were knocked out in the GX0101 bacterial artificial chromosome (BAC) clone to construct the recombinant virus SC9-1, resulting in a complete lack of pathogenicity and providing better protection against MD than CVI988/Rispens. In the present study, the BAC sequences in SC9-1 were removed using a cre-loxP system, and the virus termed SC9-2. SC9-2 showed a significant increase in replication in vitro and in vivo. There was a significant decrease in chicken weight, immune organ index, and antibody levels compared with those of SC9-1-inoculated chickens. The immune protection index of SC9-2 was similar to that of SC9-1, and the difference was not significant. The results of our studies demonstrate that the SC9-2 virus provides protection in specific pathogen free (SPF) chickens when challenged with a very virulent MDV rMd5, but it induces immunosuppressive effects in SPF chickens.

Virus and host genomic, molecular, and cellular interactions during Marek's disease pathogenesis and oncogenesis

Marek's Disease Virus (MDV) is a chicken alphaherpesvirus that causes paralysis, chronic wasting, blindness, and fatal lymphoma development in infected, susceptible host birds. This disease and its protective vaccines are highly relevant research targets, given their enormous impact within the poultry industry. Further, Marek's disease (MD) serves as a valuable model for the investigation of oncogenic viruses and herpesvirus patterns of viral latency and persistence--as pertinent to human health as to poultry health. The objectives of this article are to review MDV interactions with its host from a variety of genomic, molecular, and cellular perspectives. In particular, we focus on cytogenetic studies, which precisely assess the physical status of the MDV genome in the context of the chicken host genome. Combined, the cytogenetic and genomic research indicates that MDV-host genome interactions, specifically integration of the virus into the host telomeres, is a key feature of the virus life cycle, contributing to the viral achievement of latency, transformation, and reactivation of lytic replication. We present a model that outlines the variety of virus-host interactions, at the multiple levels, and with regard to the disease states.

Understanding the molecular basis of disease is crucial to improving the design and construction of herpesviral vectors for veterinary vaccines

Viral infections are associated with production losses in many animal production industries. Important examples of this are Marek's disease (MD) and bovine respiratory disease (BRD) which are significant issues in the chicken and cattle industries, respectively. Viruses play key roles in MD and BRD development and consequently have also been utilised in vaccination strategies to control these diseases. Despite the widespread availability and use of vaccines to control these diseases both are still major issues for their respective industries. Here the dual role of members of viruses from the family Herpesviridae in causation and control of MD and BRD will be discussed. The technologies that may lead to the development of improved vaccines to provide more sustainable control of MD and BRD will also be identified.

Identification of non-essential loci within the Meleagrid herpesvirus 1 genome

Background

Meleagrid herpesvirus 1 (MeHV-1) infectious bacterial artificial chromosomes (iBACs) are ideal vectors for the development of recombinant vaccines for the poultry industry. However, the full potential of iBACS as vectors can only be realised after thorough genetic characterisation, including identification of those genetic locations that are non-essential for virus replication. Generally, transposition has proven to be a highly effective strategy for rapid and efficient mutagenesis of iBAC clones. The current study describes the characterisation of 34 MeHV-1 mutants containing transposon insertions within the pMeHV1-C18 iBAC genome.

Methods

Tn5 and MuA transposition methods were used to generate a library of 76 MeHV-1 insertion mutants. The capacity of each mutant to facilitate the recovery of infectious MeHV-1 was determined by the transfection of clone DNA into chicken embryo fibroblasts.

Results

Attempts to recover infectious virus from the modified clones identified 14 genetic locations that were essential for MeHV-1 replication in cell culture. Infectious MeHV-1 was recovered from the remaining 14 intragenic insertion mutants and six intergenic insertion mutants, suggesting that the respective insertion locations are non-essential for MeHV-1 replication in cell culture.

Conclusions

The essential and non-essential designations for those MeHV-1 genes characterised in this study were generally in agreement with previous reports for other herpesviruses homologues. However, the requirement for the mardivirus-specific genes LORF4A and LORF5 are reported for the first time. These findings will help direct future work on the development of recombinant poultry vaccines using MeHV-1 as a vector by identifying potential transgene insertion sites within the viral genome.

Electronic supplementary material

The online version of this article (doi10.1186/s12985-015-0362-9) contains supplementary material, which is available to authorized users.

Differential expression of Marek's disease virus (MDV) late proteins during in vitro and in situ replication: role for pUL47 in regulation of the MDV UL46-UL49 gene locus

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus that replicates in a highly cell-associated manner in vitro. Production of infectious cell-free virus only occurs in feather follicle epithelial (FFE) cells of infected chicken skins. Previously, we described differential expression for a core alphaherpesvirus protein, pUL47 that was found to be abundantly expressed in FFE cells of infected chickens, while barely detectable during in vitro propagation. Here, we further examined the dynamics of expression of four tegument proteins within the UL46-49 locus during in vitro and in situ replication. All four proteins examined were expressed abundantly in situ, whereas both pUL47 and pUL48 expression were barely detectable in vitro. Replacement of the putative UL47 and UL48 promoters with the minimal cytomegalovirus promoter enhanced mRNA and protein expression in vitro. Interestingly, enhanced expression of pUL47 resulted in increased UL46, UL48, and UL49 transcripts that resulted in increased pUL46 and pUL48 expression.

Construction of Recombinant HVT Expressing PmpD, and Immunological Evaluation against Chlamydia psittaci and Marek's Disease Virus

Chlamydia psittaci (C. psittaci) is an obligate intracellular zoonotic pathogen that can be transmitted to humans from birds. No efficacious commercial vaccine is available for clearing chlamydial infection due to lack of potential vaccine candidates and effective delivery vehicles. Herpesvirus of turkeys (HVT) is an efficacious commercially available vaccine against Marek’s Disease virus (MDV). In this study, a recombinant HVT-delivered vaccine against C. psittaci and Marek’s disease was developed and examined. The 5'-terminus of pmpD gene (pmpD-N) encoding the N-terminal fragment of polymorphic membrane protein D of C. psittaci was inserted into a nonessential region of HVT genome using reverse genetics based on an infectious bacterial artificial chromosome (BAC) clone of HVT. The recombinant virus (rHVT-pmpD-N) was recovered from primary chicken embryo fibroblast (CEF) cells by transfection of modified HVT BAC DNA containing the pmpD-N gene. The rHVT-pmpD-N construct was confirmed to express PmpD-N by immunoblot and immunofluorescence. The rHVT-pmpD-N was stable during 20 passages in vitro. The growth kinetics of rHVT-pmpD-N was comparable to that of parental HVT in vitro and in vivo. One-day-old SPF chickens inoculated subcutaneously with rHVT-pmpD-N displayed increased PmpD-specific antibody levels and a vigorous PmpD-specific lymphocyte proliferation response using HVT vector or CEF cells as control. Furthermore, the percentage of CD4+ cells was significantly elevated in rHVT-pmpD-N-immunized birds as compared to the parental HVT. All chickens vaccinated with rHVT-pmpD-N or parental HVT were protected completely against challenge with a very virulent strain of Marek’s Disease virus (MDV) RB-1B. Post challenge with C. psittaci CB7 strain, a significant decrease in respiratory distress, lesions and Chlamydia load was found in the rHVT-pmpD-N-vaccinated group compared to the parental HVT. In conclusion, our study suggests that the rHVT-pmpD-N live vaccine may be viable as a candidate dual vaccine that provides protection against both very virulent MDV and C. psittaci.

Construction of a triple gene-deleted Chinese Pseudorabies virus variant and its efficacy study as a vaccine candidate on suckling piglets

New-emerging variants of Pseudorabies virus (PRV) compromise the protection provided by current vaccines and cause the death of all ages of vaccinated pigs since 2011. New vaccines based on current circulating PRV strain are needed to control the spread of disease since the variants are antigenically different from classical strains of virus. In this study, a TK/gE/gI triple gene-deleted PRV derived from current circulating field isolate was generated by using bacterial artificial chromosome techniques, and the rescued virus showed similar growth properties in vitro to its parent strain but reduced plaque size. To evaluate it as vaccine candidate, 9 day-old pigs were vaccinated and challenged with a virulent PRV variant. The results showed that vaccination can generate high level of protective gB-specific antibodies after vaccination and provide complete protection to the viral challenge. By contrast, the unvaccinated piglets all died within 6 days after viral challenge. Therefore, the TK/gE/gI triple gene-deleted PRV could be a promising vaccine candidate to control the wide spreading of PR variants in China.

Infectious delivery of alphaherpesvirus bacterial artificial chromosomes

Bacterial artificial chromosomes (BACs) can accommodate and stably propagate the genomes of large DNA viruses in E. coli. As DNA virus genomes are often per se infectious upon transfection into mammalian cells, their cloning in BACs and easy modification by homologous recombination in bacteria has become an important strategy to investigate the functions of individual virus genes. This chapter describes a strategy to clone the genomes of viruses of the Alphaherpesvirinae subfamily within the family of the Herpesviridae, which is a group of large DNA viruses that can establish both lytic and latent infections in most animal species including humans. The cloning strategy includes the following steps: (1) Construction of a transfer plasmid that contains the BAC backbone with selection and screening markers, and targeting sequences which support homologous recombination between the transfer plasmid and the alphaherpesvirus genome. (2) Introduction of the transfer plasmid sequences into the alphaherpesvirus genome via homologous recombination in mammalian cells. (3) Isolation of recombinant virus genomes containing the BAC backbone sequences from infected mammalian cells and electroporation into E. coli. (4) Preparation of infectious BAC DNA from bacterial cultures and transfection into mammalian cells. (5) Isolation and characterization of progeny virus.

Transcriptional activity comparison of different sites in recombinant Marek's disease virus for the expression of the H9N2 avian influenza virus hemagglutinin gene

Over the last two decades, much attention has been paid to MDV-vectored recombinant vaccines. Many factors have influenced their protective efficacy, and insertion site has been among the main influential factors for the expression of foreign genes in recombinant Marek's disease virus (rMDV). To compare the transcriptional activity of different sites of rMDV, an H9N2 avian influenza virus hemagglutinin gene (AIV-H9N2-HA) expression cassette that used the bi-directional promoter of serotype 1 MDV (MDV1) in the 1.8kb RNA transcript direction (p1.8kb) as a promoter was inserted into 4 different regions of MDV using the bacterial artificial chromosome (BAC) vector and FLP/FRT recombination technique. The insertion regions included 3 of its own sites (US2, US10 and one of Meq genes) in the MDV genome and a foreign site (gpt gene) in the BAC vector. Quantitative PCR and enzyme-linked immunosorbent assay (ELISA) were used to analyze and compare the H9N2-HA expression levels of these different rMDVs both at the mRNA level and at the protein level. The results indicated that among the four tested insertion regions, the HA expression cassette in the US2 region demonstrated the highest activity, followed by that in the Meq region, which was almost equal to that of US10. Further, the expression cassette had the lowest activity in the foreign region gpt gene. The above data could be useful for choosing proper recombinant insertion regions in the construction of rMDV to express different foreign genes, and it is a prerequisite for developing effective MDV-vectored recombinant vaccines.

Construction of recombinant Marek's disease virus (rMDV) co-expressing AIV-H9N2-NA and NDV-F genes under control of MDV's own bi-directional promoter

To qualitatively analyze and evaluate a bi-directional promoter transcriptional function in both transient and transgenic systems, several different plasmids were constructed and recombinant MDV type 1 strain GX0101 was developed to co-express a Neuraminidase (NA) gene from Avian Influenza Virus H9N2 strain and a Fusion (F) gene from the Newcastle disease virus (NDV). The two foreign genes, NDV-F gene and AIV-NA gene, were inserted in the plasmid driven in each direction by the bi-directional promoter. To test whether the expression of pp38/pp24 heterodimers are the required activators for the expression of the foreign genes, the recombinant plasmid pPpp38-NA/1.8kb-F containing expression cassette for the two foreign genes was co-transfected with a pp38/pp24 expression plasmid, pBud-pp38-pp24, in chicken embryo fibroblast (CEF) cells. Alternatively, plasmid pPpp38-NA/1.8kb-F was transfected in GX0101-infected CEFs where the viral endogenous pp38/pp24 were expressed via virus infection. The expression of both foreign genes was activated by pp38/pp24 dimers either via virus infection, or co-expression. The CEFs transfected with pPpp38-NA/1.8kb-F alone had no expression. We chose to insert the expression cassette of Ppp38-NA/1.8kb-F in the non-essential region of GX0101ΔMeq US2 gene, and formed a new rMDV named MZC13NA/F through homologous recombination. Indirect fluorescence antibody (IFA) test, ELISA and Western blot analyses indicated that F and NA genes were expressed simultaneously under control of the bi-directional promoter, but in opposite directions. The data also indicated the activity of the promoter in the 1.8-kb mRNA transcript direction was higher than that in the direction for the pp38 gene. The expression of pp38/pp24 dimers either via co-tranfection of the pBud-pp38-pp24 plasmid, or by GX0101 virus infection were critical to activate the bi-directional promoter for expression of two foreign genes in both directions. Therefore, the confirmed function of the bi-directional promoter provides better feasibilities to insert multiple foreign genes in MDV genome based vectors.

Recombinant duck enteritis virus expressing the HA gene from goose H5 subtype avian influenza virus

The duck enteritis virus (DEV) may be a promising candidate viral vector for an aquatic poultry vaccination that can protect against multiple pathogens because it has a very large genome and a narrow host range. Recently, we described two DEV recombinants that contained deletions of the viral US2 or gIgE genes. The hemagglutinin (HA) gene of an H5N1-type highly pathogenic avian influenza virus (HPAIV) of goose origin was inserted into the deletion sites to construct two rDEVs expressing the AIV HA antigen. The resulting rDEV-ΔgIgE-HA or rDEV-ΔUS2-HA recombinant DEV viruses were used to infect duck embryo fibroblasts. Reverse transcription PCR, immunofluorescence and western blot analysis results indicated that rDEV-ΔgIgE-HA and rDEV-ΔUS2-HA were successfully expressed in duck embryo fibroblasts (DEFs). To investigate whether the HA gene could be stably maintained in the recombinant viruses, the viruses were passaged in DEFs 18 times. The HA gene in both recombinants could be detected by PCR amplification. The immunized four-week-old ducks induced specific antibodies against DEV and AIV HA and were protected against challenge infections with DEV AV1221 viruses.

Application of a new bivalent Marek's disease vaccine does not interfere with infectious bronchitis or Newcastle disease vaccinations and proves efficacious

A possible interference after Marek's disease (MD) vaccination using an experimental bivalent vaccine, consisting of a redesigned CVI-988/Rispens-type MDV-1 strain and herpesvirus of turkeys, with vaccination against infectious bronchitis (IB) virus (IBV) or Newcastle disease (ND) virus (NDV) was examined. Day-old specific-pathogen-free chicks were concomitantly vaccinated with the bivalent MD vaccine (either intramuscularly or subcutaneously) and with commercially available vaccines against ND or IB. Afterward chickens were challenged with either lethal MD virus (MDV) or NDV strains or with a pathogenic IBV strain. After challenge, neither mortality nor notable clinical signs of MD, ND, or IB were observed in the vaccinated birds. The experimental bivalent MDV vaccine proved efficacious against lethal MDV challenge and did not affect the efficacy of the NDV or IBV vaccines. In conclusion, no signs of interference or adverse effects were detected. Thus, the vaccines can be administered concomitantly on chickens' first day of life.

Deletion of Marek's disease virus large subunit of ribonucleotide reductase impairs virus growth in vitro and in vivo

Marek's disease virus (MDV), a highly cell-associated lymphotropic alphaherpesvirus, is the causative agent of a neoplastic disease in domestic chickens called Marek's disease (MD). In the unique long (UL) region of the MDV genome, open reading frames UL39 and UL40 encode the large and small subunits of the ribonucleotide reductase (RR) enzyme, named RR1 and RR2, respectively. MDV RR is distinguishable from that present in chicken and duck cells by monoclonal antibody T81. Using recombinant DNA technology we have generated a mutant MDV (Md5deltaRR1) in which RR1 was deleted. PCR amplification of the RR gene in Md5deltaRR1-infected duck embryo fibroblasts (DEF) confirmed the deletion of the 2.4 kb RR1 gene with a resultant amplicon of a 640-bp fragment. Restriction enzyme digests with SalI confirmed a UL39 deletion and the absence of gross rearrangement. The biologic characteristics of Md5deltaRR1 virus were studied in vitro and in vivo. The Md5deltaRR1 replicated in DEF, but significantly slower than parental Md5-BAC, suggesting that RR is important but not essential for replication in fibroblasts. In vivo studies, however, showed that the RR1 deletion virus was impaired for its ability to replicate in chickens. Inoculation of specific-pathogen-free (SPF) chickens with Md5deltaRR1 showed the mutant virus is nonpathogenic and does not induce MD in birds. A revertant virus, Md5deltaRR1/R, was generated with the restored phenotype of the parental Md5-BAC in vivo, indicating that RR is essential for replication of the virus in chickens. Protection studies in SPF chickens indicated that the Md5deltaRR1 virus is not a candidate vaccine against MD.

Cloning of a very virulent plus, 686 strain of Marek's disease virus as a bacterial artificial chromosome

Bacterial artificial chromosome (BAC) vectors were first developed to facilitate propagation and manipulation of large DNA fragments. This technology was later used to clone full-length genomes of large DNA viruses to study viral gene function. Marek's disease virus (MDV) is a highly oncogenic herpesvirus that causes rapid induction of T-cell lymphomas in chickens. Based on the virus's ability to cause disease in vaccinated chickens, MDV strains are classified into pathotypes, with the most virulent strains belonging to the very virulent plus (vv+) pathotype. Here we report the construction of BAC clones of 686 (686-BAC), a vv+ strain of MDV. Transfection of DNA isolated from two independent clones into duck embryo fibroblasts resulted in recovery of infectious virus. Pathogenesis studies showed that the BAC-derived 686 viruses were more virulent than Md5, a vv strain of MDV. With the use of a two-step red-mediated mutagenesis process, both copies of viral interleukin 8 (vIL-8) were deleted from the MDV genome, showing that 686-BACs were amenable to mutagenesis techniques. The generation of BAC clones from a vv+ strain of MDV is a significant step toward understanding molecular basis of MDV pathogenesis.

The Meleagrid herpesvirus 1 genome is partially resistant to transposition

The propagation of herpesvirus genomes as infectious bacterial artificial chromosomes (iBAC) has enabled the application of highly efficient strategies to investigate gene function across the genome. One of these strategies, transposition, has been used successfully on a number of herpesvirus iBACs to generate libraries of gene disruption mutants. Gene deletion studies aimed at determining the dispensable gene repertoire of the Meleagrid herpesvirus 1 (MeHV-1) genome to enhance the utility of this virus as a vaccine vector have been conducted in this report. A MeHV-1 iBAC was used in combination with the Tn5 and MuA transposition systems in an attempt to generate MeHV-1 gene interruption libraries. However, these studies demonstrated that Tn5 transposition events into the MeHV-1 genome occurred at unexpectedly low frequencies. Furthermore, characterization of genomic locations of the rare Tn5 transposon insertion events indicated a nonrandom distribution within the viral genome, with seven of the 24 insertions occurring within the gene encoding infected cell protein 4. Although insertion events with the MuA system occurred at higher frequency compared with the Tn5 system, fewer insertion events were generated than has previously been reported with this system. The characterization and distribution of these MeHV-1 iBAC transposed mutants is discussed at both the nucleotide and genomic level, and the properties of the MeHV-1 genome that could influence transposition frequency are discussed.

Expression of chicken parvovirus VP2 in chicken embryo fibroblasts requires codon optimization for production of naked DNA and vectored meleagrid herpesvirus type 1 vaccines

Meleagrid herpesvirus type 1 (MeHV-1) is an ideal vector for the expression of antigens from pathogenic avian organisms in order to generate vaccines. Chicken parvovirus (ChPV) is a widespread infectious virus that causes serious disease in chickens. It is one of the etiological agents largely suspected in causing Runting Stunting Syndrome (RSS) in chickens. Initial attempts to express the wild-type gene encoding the capsid protein VP2 of ChPV by insertion into the thymidine kinase gene of MeHV-1 were unsuccessful. However, transient expression of a codon-optimized synthetic VP2 gene cloned into the bicistronic vector pIRES2-Ds-Red2, could be demonstrated by immunocytochemical staining of transfected chicken embryo fibroblasts (CEFs). Red fluorescence could also be detected in these transfected cells since the red fluorescent protein gene is downstream from the internal ribosome entry site (IRES). Strikingly, fluorescence could not be demonstrated in cells transiently transfected with the bicistronic vector containing the wild-type or non-codon-optimized VP2 gene. Immunocytochemical staining of these cells also failed to demonstrate expression of wild-type VP2, indicating that the lack of expression was at the RNA level and the VP2 protein was not toxic to CEFs. Chickens vaccinated with a DNA vaccine consisting of the bicistronic vector containing the codon-optimized VP2 elicited a humoral immune response as measured by a VP2-specific ELISA. This VP2 codon-optimized bicistronic cassette was rescued into the MeHV-1 genome generating a vectored vaccine against ChPV disease.

Avirulent Marek's disease virus type 1 strain 814 vectored vaccine expressing avian influenza (AI) virus H5 haemagglutinin induced better protection than turkey herpesvirus vectored AI vaccine

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.

Progress toward the development of polyvalent vaccination strategies against multiple viral infections in chickens using herpesvirus of turkeys as vector

Vaccination is the most cost effective strategy for the control and prevention of the plethora of viral diseases affecting poultry production. The major challenge for poultry vaccination is the design of vaccines that will protect against multiple pathogens via a single protective dose, delivered by mass vaccination. The Marek disease virus and the highly pathogenic avian influenza virus cause severe disease outbreaks in chickens. Vaccination with live herpesvirus of turkeys protects chickens from Marek disease and inactivated influenza viruses are used as antigens to protect chickens against influenza virus infections. We developed herpesvirus of turkeys (HVT) as a vaccine vector that can act as a dual vaccine against avian influenza and Marek 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 haemagglutinin (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. Immunization 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. The rHVT-H7HA vaccine also induced strong and long-lasting antibody titers against H7HA in chickens that were vaccinated in ovo 3 d before hatching. This vaccine supports differentiation between infected and vaccinated animals (DIVA), because no influenza virus nucleoprotein-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 disease virus and can be used as a DIVA vaccine.