Isolation of a Marek's disease virus (MDV) recombinant containing the lacZ gene of Escherichia coli stably inserted within the MDV US2 gene

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.

Duck virus enteritis (duck plague) - a comprehensive update

Duck virus enteritis (DVE), also called duck plague, is one of the major contagious and fatal diseases of ducks, geese and swan. It is caused by duck enteritis virus (DEV)/Anatid herpesvirus-1 of the genus Mardivirus, family Herpesviridae, and subfamily Alpha-herpesvirinae. Of note, DVE has worldwide distribution, wherein migratory waterfowl plays a crucial role in its transmission within and between continents. Furthermore, horizontal and/ or vertical transmission plays a significant role in disease spread through oral-fecal discharges. Either of sexes from varying age groups of ducks is vulnerable to DVE. The disease is characterized by sudden death, vascular damage and subsequent internal hemorrhage, lesions in lymphoid organs, digestive mucosal eruptions, severe diarrhea and degenerative lesions in parenchymatous organs. Huge economic losses are connected with acute nature of the disease, increased morbidity and mortality (5%-100%), condemnations of carcasses, decreased egg production and hatchability. Although clinical manifestations and histopathology can provide preliminary diagnosis, the confirmatory diagnosis involves virus isolation and detection using serological and molecular tests. For prophylaxis, both live-attenuated and killed vaccines are being used in broiler and breeder ducks above 2 weeks of age. Since DEV is capable of becoming latent as well as shed intermittently, recombinant subunit and DNA vaccines either alone or in combination (polyvalent) are being targeted for its benign prevention. This review describes DEV, epidemiology, transmission, the disease (DVE), pathogenesis, and advances in diagnosis, vaccination and antiviral agents/therapies along with appropriate prevention and control strategies.

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.

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.

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.

Analysis of viral and cellular factors influencing herpesvirus-induced nuclear envelope breakdown

Herpesvirus nucleocapsids are translocated from their assembly site in the nucleus to the cytosol by acquisition of a primary envelope at the inner nuclear membrane which subsequently fuses with the outer nuclear membrane. This transport through the nuclear envelope requires homologs of the conserved herpesviral pUL31 and pUL34 proteins which form the nuclear egress complex (NEC). In its absence, 1,000-fold less virus progeny is produced. We isolated a UL34-negative mutant of the alphaherpesvirus pseudorabies virus (PrV), PrV-ΔUL34Pass, which regained replication competence after serial passages in cell culture by inducing nuclear envelope breakdown (NEBD) (B. G. Klupp, H. Granzow, and T. C. Mettenleiter, J. Virol. 85:8285-8292, 2011). To test whether this phenotype is unique, passaging experiments were repeated with a UL31 deletion mutant. After 60 passages, the resulting PrV-ΔUL31Pass replicated similarly to wild-type PrV. Ultrastructural analyses confirmed escape from the nucleus via NEBD, indicating an inherent genetic disposition in herpesviruses. To identify the mutated viral genes responsible for this phenotype, the genome of PrV-ΔUL34Pass was sequenced and compared to the genomes of parental PrV-Ka and PrV-ΔUL34. Targeted sequencing of PrV-ΔUL31Pass disclosed congruent mutations comprising genes encoding tegument proteins (pUL49, pUL46, pUL21, pUS2), envelope proteins (gI, pUS9), and protease pUL26. To investigate involvement of cellular pathways, different inhibitors of cellular kinases were tested. While induction of apoptosis or inhibition of caspases had no specific effect on the passaged mutants, roscovitine, a cyclin-dependent kinase inhibitor, and U0126, an inhibitor of MEK1/2, specifically impaired replication of the passaged mutants, indicating involvement of mitosis-related processes in herpesvirus-induced NEBD.

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.

Protection of chickens, with or without maternal antibodies, against IBDV infection by a recombinant IBDV-VP2 protein

The use of avian herpesviruses (Marek's disease virus, MDV) as vectors to express the capsid protein of infectious bursal disease virus (IBDV) was well established, and its protection against IBDV challenge has been evaluated previously. However, there is little data about rMDV1 expressing the VP2 protein of IBDV protecting SPF and commercial chickens against virulent IBDV (vIBDV) challenge. In this study, we constructed a stable rMDV1 expressing the VP2 protein of IBDV by inserting the coding sequence within the US10 gene of MDVl by homologous recombination and designated this as rMDVl-US10L, and evaluated effectiveness of the recombinant VP2 protein with SPF chickens and commercial chickens with maternal antibodies in vIBDV challenge. The results can be summarized as follows: (1) We constructed a rMDV1 expressing IBDV-VP2 under the control of the MDV1 glycoprotein B (gB) promoter [rMDV1-US10L]. (2) rMDV-VP2 protein was readily expressed and induced 53% protection against a vIBDV challenge in SPF chickens with 10(3)PFU/chicken, whereas 10(4)PFU induced 73% protection. (3) Vaccination of commercial chickens having maternal antibodies to rMDV1-VP2 induced 87% protection in vIBDV challenge, which was similar to results using the live vaccine, BJ87 IBDV strain, in commercial chickens. These results demonstrate that the VP2 antigen expressed in the MDV vector was an effective and stable vaccine in correlation with the vaccine efficacy against lethal IBDV challenge, and can provide a better protective effect that is likely to persist for the life of the chickens.

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.

Construction and characterization of Marek's disease viruses having green fluorescent protein expression tied directly or indirectly to phosphoprotein 38 expression

Marek's disease (MD) is caused by Marek's disease virus (MDV), a highly cell-associated alphaherpesvirus. MD is primarily characterized by lymphocyte infiltration of the nerves and the development of lymphomas in visceral organs, muscle, and skin. MDV encodes two phosphoproteins, pp24 and pp38, that are highly expressed during lytic infection. These proteins were initially identified in MDV-induced tumors but are now known to be linked primarily to MDV lytic infection. Despite the recent characterization of a pp38 deletion mutant MDV, the functions of these phosphoproteins remain unknown. The goal of this work was to construct recombinant MDVs having direct fusions of a marker gene, the green fluorescent protein (GFP), to pp38 in order to study the expression patterns and localization of this protein during stages of MDV infection. We report the construction of two recombinant viruses, one having the enhanced green fluorescent protein (eGFP) fused in-frame to the pp38 open reading frame (ORF) (RB1Bpp38/eGFP) and the other having soluble-modified GFP (smGFP) downstream but out-of-frame with pp38 (RB1Bpp38/smGFP). During construction of RB1Bpp38/eGFP, an ORF located downstream of pp38 (LORF12) was partially deleted. In RB1Bpp38/smGFP, however, LORF12 and its immediate 5' upstream sequence was left intact. This report describes the construction, cell culture, and in vivo characterization of RB1Bpp38/eGFP and RB1Bpp38/smGFP. Structural analysis showed that the virus stocks of RB1Bpp38/eGFP and RB1Bpp38/smGFP had incorporated the GFP cassette and were free of contaminating parent virus (RB1B). Moreover, RB1Bpp38/eGFP and RB1Bpp38/smGFP contained two and three and four and five copies of the 132-bp repeats, respectively. Expression analysis showed that the transcription of genes in RB1Bpp38/eGFP-and RB1Bpp38/smGFP-infected chicken embryo fibroblasts (CEFs) were similar to RB1B-infected CEFs, with the notable exception of deletion of a LORF12-specific transcript in RB1Bpp38/ eGFP-infected cells. In CEFs, RB1Bpp38/eGFP and RB1Bpp38/smGFP showed comparable one-step growth kinetics to parental virus (RB1B). RB1Bpp38/eGFP and RB1Bpp38/smGFP, however, showed quite distinct growth characteristics in vivo. Two independent clones of RB1Bpp38/eGFP were highly attenuated, whereas RB1Bpp38/smGFP exhibited pathogenesis similar to parent virus and retained oncogenicity. Our results suggest that the RB1Bpp38/eGFP phenotype could be due to an interference with an in vivo-specific pp38 function via GFP direct fusion, to the deletion of LORF12, or to a targeting of the immune response to eGFP. Because deletion of pp38 was recently found not to fully attenuate very virulent MDV strain MD-5, it is possible that deletion of LORF12 may be at least partially responsible for the attenuation of RB1Bpp38/eGFP. The construction of these viruses and the establishment of cell lines from RB1Bpp38/smGFP provide useful tools for the study of MDV lyric infection in cell culture and in vivo, in studies of the reactivation of MDV from latency, and in the functional analysis of LORF12.

Comparative analysis of Marek’s disease virus (MDV) glycoprotein-, lytic antigen pp38- and transformation antigen Meq-encoding genes: association of meq mutations with MDVs of high virulence

Marek's disease (MD) is a highly contagious lymphoproliferative and demyelinating disorder of chickens. MD is caused by Marek's disease virus (MDV), a cell-associated, acute-transforming alphaherpesvirus. For three decades, losses to the poultry industry due to MD have been greatly limited through the use of live vaccines. MDV vaccine strains are comprised of antigenically related, apathogenic MDVs originally isolated from chickens (MDV-2), turkeys (herpesvirus of turkeys, HVT) or attenuated-oncogenic strains of MDV-1 (CVI-988). Since the inception of high-density poultry production and MD vaccination, there have been two discernible increases in the virulence of MDV field strains. Our objectives were to determine if common mutations in the major glycoprotein genes, a major lytic antigen phosphoprotein 38 (pp38) or a major latency/transformation antigen Meq (Marek's EcoRI-Q-encoded protein) were associated with enhanced MDV virulence. To address this, we cloned and sequenced the major surface glycoprotein genes (gB, gC, gD, gE, gH, gI, and gL) of five MDV strains that were representative of the virulent (v), very virulent (vv) and very virulent plus (vv+) pathotypes of MDV. We found no consistent mutations in these genes that correlated strictly with virulence level. The glycoprotein genes most similar among MDV-1, MDV-2 and HVT (gB and gC, approximately 81 and 75%, respectively) were among the most conserved across pathotype. We found mutations mapping to the putative signal cleavage site in the gL genes in four out of eleven vv+MDVs, but this mutation was also identified in one vvMDV (643P) indicating that it did not correlate with enhanced virulence. In further analysis of an additional 12 MDV strains, we found no gross polymorphism in any of the glycoprotein genes. Likewise, by PCR and RFLP analysis, we found no polymorphism at the locus encoding the pp38 gene, an early lytic-phase gene associated with MDV replication. In contrast, we found distinct mutations in the latency and transformation-associated Marek's EcoRI-Q-encoded protein, Meq. In examination of the DNA and deduced amino acid sequence of meq genes from 26 MDV strains (9 m/vMDV, 5 vvMDV and 12 vv+MDVs), we found distinct polymorphism and point mutations that appeared to correlate with virulence. Although a complex trait like MDV virulence is likely to be multigenic, these data describe the first sets of mutations that appear to correlate with MDV virulence. Our conclusion is that since Meq is expressed primarily in the latent/transforming phase of MDV infection, and is not encoded by MDV-2 or HVT vaccine viruses, the evolution of MDV virulence may be due to selection on MDV-host cell interactions during latency and may not be mediated by the immune selection against virus lytic antigens such as the surface glycoproteins.

Complete, long-lasting protection against lethal infectious bursal disease virus challenge by a single vaccination with an avian herpesvirus vector expressing VP2 antigens

Marek's disease herpesvirus is a vaccine vector of great promise for chickens; however, complete protection against foreign infectious diseases has not been achieved. In this study, two herpesvirus of turkey recombinants (rHVTs) expressing large amounts of infectious bursal disease virus (IBDV) VP2 antigen under the control of a human cytomegalovirus (CMV) promoter or CMV/beta-actin chimera promoter (Pec promoter) (rHVT-cmvVP2 and rHVT-pecVP2) were constructed. rHVT-pecVP2, which expressed the VP2 antigen approximately four times more than did rHVT-cmvVP2 in vitro, induced complete protection against a lethal IBDV challenge in chickens, whereas rHVT-cmvVP2 induced 58% protection. All of the chickens vaccinated with rHVT-pecVP2 had a protective level of antibodies to the VP2 antigen at the time of challenge, whereas only 42 and 67% of chickens vaccinated with rHVT-cmvVP2 or the conventional live IBDV vaccine, respectively, had the antibodies. The antibody level of chickens vaccinated with rHVT-pecVP2 increased for 16 weeks, and the peak antibody level persisted throughout the experiment. The serum antibody titer at 30 weeks of age was about 20 or 65 times higher than that of chickens vaccinated with rHVT-cmvVP2 or the conventional live vaccine, respectively. rHVT-pecVP2, isolated consistently for 30 weeks from the vaccinated chickens, expressed the VP2 antigen after cultivation, and neither nucleotide mutations nor deletion in the VP2 gene was found. These results demonstrate that the amount of VP2 antigen expressed in the HVT vector was correlated with the vaccine efficacy against lethal IBDV challenge, and complete protective immunity that is likely to persist for the life of the chickens was induced.

The products of the UL10 (gM) and the UL49.5 genes of Marek's disease virus serotype 1 are essential for virus growth in cultured cells

The role of the products of the UL10 and the UL49.5 homologous genes of Marek's disease virus serotype 1 (MDV-1) in virus replication was investigated. Deletion of either open reading frame in an infectious bacterial artificial chromosome clone (BAC20) of MDV-1 resulted in progeny viruses that were unable to spread from cell to cell. After transfection of UL10- or UL49.5-negative BAC20 DNA into chicken or quail cells, only single infected cells were observed by indirect immunofluorescence analysis. In contrast, plaque formation was restored when mutant BAC DNAs were co-transfected with the corresponding expression plasmid encoding either the UL10-encoded gM or the UL49.5 gene product. These data demonstrate that gM and its putative complex partner, the UL49.5 homologous protein, are essential for MDV-1 growth in cultured cells. Thus, MDV-1 represents the first example of a member of the family Herpesviridae for which the highly conserved membrane proteins are indispensable for cell-to-cell spread.

Reconstitution of Marek's disease virus serotype 1 (MDV-1) from DNA cloned as a bacterial artificial chromosome and characterization of a glycoprotein B-negative MDV-1 mutant

The complete genome of Marek's disease virus serotype 1 (MDV-1) strain 584Ap80C was cloned in Escherichia coli as a bacterial artificial chromosome (BAC). BAC vector sequences were introduced into the U(S)2 locus of the MDV-1 genome by homologous recombination. Viral DNA containing the BAC vector was used to transform Escherichia coli strain DH10B, and several colonies harboring the complete MDV-1 genome as an F plasmid (MDV-1 BACs) were identified. DNA from various MDV-1 BACs was transfected into chicken embryo fibroblasts, and from 3 days after transfection, infectious MDV-1 was obtained. Growth of MDV-1 recovered from BACs was indistinguishable from that of the parental virus, as assessed by plaque formation and determination of growth curves. In one of the MDV-1 BAC clones, sequences encoding glycoprotein B (gB) were deleted by one-step mutagenesis using a linear DNA fragment amplified by PCR. Mutant MDV-1 recovered after transfection of BAC DNA that harbored a 2.0-kbp deletion of the 2.6-kbp gB gene were able to grow and induce MDV-1-specific plaques only on cells providing MDV-1 gB in trans. The gB-negative virus reported here represents the first MDV-1 mutant with a deletion of an essential gene and demonstrates the power and usefulness of BACs to analyze genes and gene products in slowly growing and strictly cell-associated herpesviruses.

Dual-viral vector approach induced strong and long-lasting protective immunity against very virulent infectious bursal disease virus

To induce strong protective immunity against very virulent infectious bursal disease virus (vvIBDV) in chickens, two viral vector systems, Marek's disease and Fowlpox viruses expressing the vvIBDV host-protective antigen VP2 (rMDV, rFPV), were used. Most of chickens vaccinated with the rFPV or rMDV alone, or vaccinated simultaneously with both at their hatch (rMDV-rFPV(1d)), were protected against developing clinical signs and mortality; however, only zero to 14% of the chickens were protected against gross lesions. In contrast, gross lesions were protected in 67% of chickens vaccinated primarily with the rMDV followed by boosting with the rFPV 2 weeks later (rMDV-rFPV(14d)). Protection against the severe histopathological lesions of rFPV, rMDV, rMDV-rFPV(1d), and rMDV-rFPV(14d) vaccine groups were 33, 42, 53, and 73%, respectively. Geometric mean antibody titers to VP2 of chickens vaccinated with the rFPV, rMDV, rMDV-rFPV(1d), and rMDV-rFPV(14d) before the challenge were 110, 202, 254, and 611, respectively. Persistent infection of the rMDV in chickens after the booster vaccination with rFPV was suggested by detection of the rMDV genes from peripheral blood lymphocyte DNA at 28 weeks of age. These results indicate that the dual-viral vector approach is useful for quickly and safely inducing strong and long-lasting protective immunity against vvIBDV in chickens.

Development of an effective polyvalent vaccine against both Marek's and Newcastle diseases based on recombinant Marek's disease virus type 1 in commercial chickens with maternal antibodies

An earlier report (M. Sakaguchi et al., Vaccine 16:472-479, 1998) showed that recombinant Marek's disease virus type 1 (rMDV1) expressing the fusion (F) protein of Newcastle disease virus (NDV-F) under the control of the simian virus 40 late promoter [rMDV1-US10L(F)] protected specific pathogen-free chickens from NDV challenge, but not commercial chickens with maternal antibodies against NDV and MDV1. In the present study, we constructed an improved polyvalent vaccine based on MDV1 against MDV and NDV in commercial chickens with maternal antibodies. The study can be summarized as follows. (i) We constructed rMDV1 expressing NDV-F under the control of the MDV1 glycoprotein B (gB) promoter [rMDV1-US10P(F)]. (ii) Much less NDV-F protein was expressed in cells infected with rMDV1-US10P(F) than in those infected with rMDV1-US10L(F). (iii) The antibody response against NDV-F and MDV1 antigens of commercial chickens vaccinated with rMDV1-US10P(F) was much stronger and faster than with rMDV1-US10L(F), and a high level of antibody against NDV-F persisted for over 80 weeks postvaccination. (iv) rMDV1-US10P(F) was readily reisolated from the vaccinated chickens, and the recovered viruses were found to express NDV-F. (v) Vaccination of commercial chickens having maternal antibodies to rMDV1-US10P(F) completely protected them from NDV challenge. (vi) rMDV1-US10P(F) offered the same degree of protection against very virulent MDV1 as the parental MDV1 and commercial vaccines. These results indicate that rMDV1-US10P(F) is an effective and stable polyvalent vaccine against both Marek's and Newcastle diseases even in the presence of maternal antibodies.

The equine herpesvirus 1 Us2 homolog encodes a nonessential membrane-associated virion component

Experiments were conducted to analyze the equine herpesvirus 1 (EHV-1) gene 68 product which is encoded by the EHV-1 Us2 homolog. An antiserum directed against the amino-terminal 206 amino acids of the EHV-1 Us2 protein specifically detected a protein with an Mr of 34,000 in cells infected with EHV-1 strain RacL11. EHV-1 strain Ab4 encodes a 44,000-Mr Us2 protein, whereas vaccine strain RacH, a high-passage derivative of RacL11, encodes a 31,000-Mr Us2 polypeptide. Irrespective of its size, the Us2 protein was incorporated into virions. The EHV-1 Us2 protein localized to membrane and nuclear fractions of RacL11-infected cells and to the envelope fraction of purified virions. To monitor intracellular trafficking of the protein, the green fluorescent protein (GFP) was fused to the carboxy terminus of the EHV-1 Us2 protein or to a truncated Us2 protein lacking a stretch of 16 hydrophobic amino acids at the extreme amino terminus. Both fusion proteins were detected at the plasma membrane and accumulated in the vicinity of nuclei of transfected cells. However, trafficking of either GFP fusion protein through the secretory pathway could not be demonstrated, and the EHV-1 Us2 protein lacked detectable N- and O-linked carbohydrates. Consistent with the presence of the Us2 protein in the viral envelope and plasma membrane of infected cells, a Us2-negative RacL11 mutant (L11DeltaUs2) exhibited delayed penetration kinetics and produced smaller plaques compared with either wild-type RacL11 or a Us2-repaired virus. After infection of BALB/c mice with L11DeltaUs2, reduced pathogenicity compared with the parental RacL11 virus and the repaired virus was observed. It is concluded that the EHV-1 Us2 protein modulates virus entry and cell-to-cell spread and appears to support sustained EHV-1 replication in vivo.

Recombinant Marek's disease virus (MDV)-derived lymphoblastoid cell lines: regulation of a marker gene within the context of the MDV genome

Marek's disease is a herpesvirus (Marek's disease virus [MDV])-induced pathology of chickens characterized by paralysis and the rapid appearance of T-cell lymphomas. Lymphoblastoid cell lines (LBCLs) derived from MDV-induced tumors have served as models of MDV latency and transformation. We have recently reported the construction of mutant MDVs having a deletion (M. S. Parcells et al., J. Virol. 69:7888-7898, 1995) and an insertion (A. S. Anderson et al., J. Virol. 72:2548-2553, 1998) within the unique short region of the virus genome. These mutant MDVs retained oncogenicity, and LBCLs have been established from the mutant-induced tumors. We report the characterization of these cell lines with respect to (i) virus structure within and reactivated from the cell lines, (ii) surface antigen expression, (iii) kinetics of MDV and marker gene induction, (iv) localization and colocalization of induced MDV antigens and beta-galactosidase (beta-Gal), and (v) methylation status of the region of lacZ insertion in recombinant- and non-recombinant-derived cell lines. Our results indicate that (i) recombinant-derived cell lines contain no parental virus, (ii) the established cell lines are predominantly CD4(+) CD8(-), (iii) the percentage of Lac-expressing cells is low (1 to 3%) but increases dramatically upon 5'-iododeoxyuridine (IUdR) treatment, (iv) lacZ expression is induced with the same kinetics as several MDV lytic-phase genes (pp38, US1, gB, gI, and US10), and (v) the regulation of lacZ expression is not mediated by methylation. Furthermore, the MDV-encoded oncoprotein, Meq, could be detected in cells expressing beta-Gal and various lytic antigens but did not appear to be induced by IUdR treatment. Our results indicate that regulation of the lacZ marker gene can serve as sensitive measure of virus lytic-phase induction and the reactivation from latency.

The genetic organization and transcriptional analysis of the short unique region in the genome of nononcogenic Marek's disease virus serotype 2

Studies on the Marek's disease virus (MDV) serotype 2 (MDV2) genome may be important for understanding the naturally nononcogenic nature of the virus. To determine the complete DNA sequence of MDV2 unique short (Us) region, genomic BamHI fragments F, M1 and R were sequenced. The MDV2 Us region is 12109 bp long and contains 12 potential open reading frames (ORFs) likely to encode for proteins. Seven of them exhibit homologies to herpes simplex virus type 1 (HSV-1) US1 (ICP22), US2, US3 (protein kinase), US6 (gD), US7 (gI), US8 (gE) and US10 genes. These ORFs are conserved in a similar arrangement with those of HSV-1, except for US10 which is transposed in the Us regions of all three MDV serotypes. The predicted amino acid sequence of MDV2 ORF6 is homologous to SORF3 of the other serotypes of MDV serotype 1 (MDV1) and herpesvirus of turkeys (HVT) and to infectious laryngotracheitis virus SR1. In addition, four ORFs, which have been identified around the Us and inverted repeat junction regions, have no apparent relation to any other known herpesvirus genes. The identified ORFs in the MDV2 Us region were more colinear with their previously reported locations of MDV1 than with those of HVT and other alphaherpesviruses. Ten of the 12 ORFs in the MDV2 Us region were expressed and transcribed with 3'-coterminal transcripts and/or a unique transcript in the virus-infected cells. Compared to other MDV serotypes, the MDV2 Us-encoded proteins showed 46-70% and 33-59% identities with equivalent of MDV1 and HVT at the amino acid level, respectively. Our present data will be useful to understand the different pathogenicity among serotypes of MDV and to allow precise manipulation of the genes for a possible use in genetically engineered vaccines.

The glycoprotein D (US6) homolog is not essential for oncogenicity or horizontal transmission of Marek's disease virus

RB1BUS6lacgpt, a Marek's disease virus (MDV) mutant having a disrupted glycoprotein D (gD) homolog gene, established infection and induced tumors in chickens exposed to it by inoculation or by contact. Lymphoblastoid cell lines derived from RB1BUS6lacgpt-induced tumors harbored only the mutant virus. These results provide strong evidence that an intact gD homolog gene is not essential for oncogenicity or horizontal transmission of MDV.

Genomic organization of the canine herpesvirus US region

Canine herpesvirus (CHV) is an alpha-herpesvirus of limited pathogenicity in healthy adult dogs and infectivity of the virus appears to be largely limited to cells of canine origin. CHV's low virulence and species specificity make it an attractive candidate for a recombinant vaccine vector to protect dogs against a variety of pathogens. As part of the analysis of the CHV genome, the authors determined the complete nucleotide sequence of the CHV US region as well as portions of the flanking inverted repeats. Seven full open reading frames (ORFs) encoding proteins larger than 100 amino acids were identified within, or partially within the CHV US: cUS2, cUS3, cUS4, cUS6, cUS7, cUS8 and cUS9; which are homologs of the herpes simplex virus type-1 US2; protein kinase; gG, gD, gI, gE; and US9 genes, respectively. An eighth ORF was identified in the inverted repeat region, cIR6, a homolog of the equine herpesvirus type-1 IR6 gene. The authors identified and mapped most of the major transcripts for the predicted CHV US ORFs by Northern analysis.

Asymmetric deletion of the junction between the short unique region and the inverted repeat does not affect viral growth in culture and vaccine-induced immunity against Marek's disease

To construct an effective recombinant Marek's disease virus type 1 (MDV1), we localized a stable insertion site for expression of the Escherichia coli lacZ gene near or within the short inverted repeats of MDV1 strain K554 DNA. A stable recombinant MDV1 was obtained by deleting the junction region between the short unique sequence (Us) and the internal short inverted repeat (IRs). The recombinant MDV1 replicated in cultured cells as well as the parental viral DNA. Antibodies against both MDV1 antigen and beta-galactosidase encoded by the lacZ gene were detected in the sera of chickens immunized with the virus, and persisted for at least 16 weeks. Moreover, the recombinant virus conferred protection upon chickens against a challenge with virulent MDV1. These results demonstrated that the Us-IRs junction region is an effective site for the insertion of foreign genes from which to construct a polyvalent live vaccine for poultry. Analysis of the Us-IRs junction region which was deleted from the parental MDV1 indicated that there is a tandem direct repeat of a 220-bp exists within the short internal and terminal inverted repeats of avirulent MDV1 K554 strain DNA. The 220-bp sequence was well conserved among DNAs from various strains. The number of the repeat units may differ between the IRs and TRs or among various MDV1 strain DNAs.

Retention of oncogenicity by a Marek's disease virus mutant lacking six unique short region genes

We previously reported the construction of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique short (US) region of the viral genome (J.L. Cantello, A.S. Anderson, A. Francesconi, and R.W. Morgan, J. Virol. 65:1584-1588, 1991; M.S. Parcells, A.S. Anderson, and R.W. Morgan, Virus Genes 9:5-13, 1994; M.S. Parcells, A.S. Anderson, and R.W. Morgan, J. Virol. 68:8239-8253, 1994). These strains were constructed by using a high-passage-level serotype 1 MDV strain which grew well in chicken embryo fibroblasts. Despite the growth of the parent and mutant viruses in cell culture, in vivo studies were limited by poor growth of these strains in chickens. One of the mutants studied lacked 4.5 kbp of US region DNA and contained the lacZ gene of Escherichia coli inserted at the site of the deletion. The deletion removed MDV homologs to the US1, US2, and US10 genes of herpes simplex virus type 1 as well as three MDV-specific open reading frames. We now report the construction of a mutant MDV containing a similar deletion in the US region of the highly oncogenic RB1B strain. This mutant, RB1B delta 4.5lac, had a growth impairment in established chicken embryo fibroblasts similar to that described previously for MDVs lacking a functional US1 gene. In chickens, RB1B delta 4.5lac showed decreased early cytolytic infection, mortality, tumor incidence, and horizontal transmission. Several lymphoblastoid cell lines were established from RB1B delta 4.5lac-induced tumors, and virus reactivated from these cell lines was LacZ+. These results indicate that the deleted genes are nonessential for the transformation of chicken T cells or for the establishment and maintenance of latency. On the basis of the growth impairment observed for RB1B delta 4.5lac in cell culture and in vivo, we conclude that deletion of these genes affects the lytic replication of MDV. This is the first MDV mutant constructed in the RB1B oncogenic strain, and the methodology described herein provides for the direct examination of MDV-encoded determinants of oncogenicity.

The Marek's disease virus (MDV) unique short region: alphaherpesvirus-homologous, fowlpox virus-homologous, and MDV-specific genes

Despite its previous classification as a gammaherpesvirus, primarily due to its lymphotropism, Marek's disease virus (MDV), an oncogenic avian herpesvirus, is phylogenetically more related to the "neurotropic" alphaherpesviruses, characterized by its prototype, herpes simplex virus (HSV) (Buckmaster et al., 1988, J. Gen. Virol. 69, 2033-2042). In this report we present the DNA sequence of an 11,286-bp DNA segment encompassing the entire 11,160-bp-long Us region of the oncogenic avian herpesvirus, Marek's disease virus. Eleven open reading frames (ORFs) likely to code for proteins were identified; of these, 7 represent homologs exclusive to alphaherpesvirus S component genes. These include MDV counterparts of HSV US1 (ICP22), US2, US3 (a serine-threonine protein kinase), US6, US7, and US8 (HSV glycoproteins gD, gI, and gE, respectively), and US10. Three additional ORFs were identified with no apparent relation to any sequences currently present in the SwissProt or GenBank/EMBL databases, while a fourth was found to exhibit significant homology to an uncharacterized fowlpox virus (FPV) ORF. Having precisely identified the IRs-U(s) and U(s)-TRs junctions, we have corrected and clarified their previously reported locations. By characterizing genes encoding three new alphaherpesvirus-related homologs (US1, US8, and US10), completing the sequence for a fourth (US7), and identifying 2 new MDV-specific ORFs (SORF1 and SORF3) and a fowlpox homolog (SORF2), our sequence analysis of the "virulent" GA strain of MDV (vMDV) extends upon that of a 5255-bp segment located in the U(s) region of the "very virulent" RB1B strain of MDV (vvMDV) (Ross et al., 1991, J. Gen. Virol. 72, 939-947; 949-954). These two sequences were found to exhibit 99% identity at both nucleotide and predicted amino acid levels. Combined with the fact that MDV U(s) sequences failed to show statistically significant CpG deficiencies, our analysis is consistent with MDV bearing a closer phylogenetic relation to alphaherpesviruses than to gammaherpesviruses. Because alphaherpesvirus-specific U(s) region genes are primarily nonessential for virus replication, they are thought to be important biological property determinants. Thus, our sequence provides a foundation for further MDV studies aimed at resolving the apparent discrepancy between MDV's genetic and biologic properties.

Nucleotide sequence analysis of an infectious laryngotracheitis virus gene corresponding to the US3 of HSV-1 and a unique gene encoding a 67 kDa protein

The DNA sequence of 4005 nucleotides from the Kpnl O and part of Kpnl K fragments in the short unique region of infectious laryngotracheitis virus (ILTV) was determined. The sequence contained two complete and one partial open reading frames (ORFs). The partial ORF was open at the 5' end of the sequence and represented the NH2-terminal 118 amino acids (aa) of a polypeptide. Its partial predicted protein product exhibited significant homology to the US2 gene product of HSV-1 (herpes simplex virus type 1) and it homologs in other herpesviruses. ORF 2 is 471 aa long and could encode a protein of 53.8 kDa which shared aa homology with the protein kinases encoded by HSV-1 US3 and its gene homologs. Analysis of the ORF 2 aa sequence revealed domains characteristic of protein-serine/threonine (S/T) kinases of cellular and viral origin. The ORF 3 encoded a predicted protein of 601 aa (M(r) 67.5 kDa) which exhibited limited homology (18% overall identity) with the UL47 protein (major tegument protein) of HSV-1. Northern (RNA) blot hybridization and metabolic inhibitors were used to characterize the ILTV protein kinase and the 67K mRNAs. The data revealed that protein kinase is a gamma-1 gene encoding a 1.6 mRNa, while the 67K ORF is a gamma-2 gene encoding a 2 kb mRNA.

Characterization of Marek's disease virus insertion and deletion mutants that lack US1 (ICP22 homolog), US10, and/or US2 and neighboring short-component open reading frames

We report the characterization of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique short (US) region of the viral genome. A deletion mutant (GA delta 4.8lac) lacks 4.8 kbp of US region DNA, the deleted segment having been replaced by the lacZ gene of Escherichia coli. This deletion results in the loss of the MDV-encoded US1, US10, and US2 homologs of herpes simplex virus type 1, as well as three putative MDV-specific genes, Sorf1, Sorf2, and Sorf3. Two mutants containing lacZ insertions in the US1 and US10 genes have been constructed, and we have previously reported a US2lac insertion mutant (J. L. Cantello, A. S. Anderson, A. Francesconi, and R. W. Morgan, J. Virol. 65:1584-1588, 1991). The isolation of these mutants indicates that the relevant genes are not required for growth of MDV in chicken embryo fibroblasts. The mutants had early growth kinetics indistinguishable from those of their parent viruses; however, 5 to 7 days after being plated, the US1 insertion mutant (US1lac) and the GA delta 4.8lac deletion mutant showed a 5- to 10-fold decrease in virus growth. This decrease in virus accumulation correlated with a 30 to 50% decrease in plaquing efficiency when these viruses were plated onto established versus fresh chicken embryo fibroblast monolayers compared with a 10 to 15% decrease seen for the parent viruses and for the US10lac or US2lac insertion mutants. Finally, GA delta 4.8lac could be reisolated from chickens, indicating that the deleted genes are not required for the infection of chickens following intra-abdominal inoculation of an attenuated serotype 1 MDV.

Characterization of a Marek's disease virus mutant containing a lacZ insertion in the US6 (gD) homologue gene

We report the construction of a Marek's disease virus (MDV) mutant containing the lacZ gene of Escherichia coli inserted into a homologue of the US6 (glycoprotein D, gD) gene of herpes simplex virus. The mutant was constructed using the high-passage GAatt85 MDV strain as the parent virus, since that strain grows readily in chicken embryo fibroblasts using culture conditions conducive to mutant virus construction. The lacZ insertion site was positioned one third of the way into the US6 (gD) open reading frame. Insertion of the lacZ gene disrupted a major 6.2 kb transcript that initiated approximately 2.5 kb upstream of the gD homologue gene in the vicinity of the US3 homologue and sorf4 genes, and extended into the US7 (gI) homologue gene. The mutant virus (US6lac) and the parent virus had similar growth kinetics in cell culture at 37 degrees C and 41 degrees C. Furthermore, the US6lac mutant could be reisolated from the spleens and peripheral blood of infected chickens with a frequency comparable to that of the parent virus. Our results indicate that the gene encoding the gD homologue is nonessential for growth in cell culture or for infection of chickens following intra-abdominal inoculation with an attenuated serotype-1 MDV.

Construction of recombinant Marek's disease virus type 1 (MDV1) expressing the Escherichia coli lacZ gene as a possible live vaccine vector: the US10 gene of MDV1 as a stable insertion site

This paper describes the construction of a recombinant Marek's disease virus serotype 1 (MDV1) in which the Escherichia coli lacZ gene was inserted into the open reading frame homologous to the US10 gene of herpes simplex virus 1 (HSV1). The recombinant virus replicated as well in cell culture as the parental MDV1 K-554 strain. Chickens immunized with the virus were protected against challenge with virulent MDV1, and produced a high level of antibodies against beta-galactosidase as well as against MDV1 antigens. The antibody titres persisted for at least 16 weeks. These results demonstrate that the US10 gene of MDV1 is an effective site for the insertion of foreign genes from which to construct a polyvalent live vaccine for poultry.

Expression of a novel Marek's disease virus type 1 (MDV1)-specific protein p40 in insect cells by a baculovirus vector and in chick embryo fibroblasts infected with MDV1

We found that the open reading frame of 1053 nucleotides within the short unique sequence of Marek's disease virus (MDV) type 1 (MDV1) expresses a 40 kDa protein, using a baculovirus vector in insect cells. Mouse antisera against the 40 kDa protein reacted with a 40 kDa protein in MDV1-infected cells. A cross-reactive polypeptide of 37 kDa was also detected in cells infected with herpesvirus of turkey.

Alterations in DNA sequence and RNA transcription of the Bam HI-H fragment accompany attenuation of oncogenic Marek's disease herpesvirus

The nucleotide sequences of the BamHI-H fragment of the HPRS16 strain of Marek's disease virus (MDV) and of its attenuated derivative HPRS16/att have been determined. The results show that in addition to the tandem expansion of a 132 bp sequence from two copies in HPRS16 virus to eight copies in HPRS16/att, nucleotide substitutions, deletions, and insertions were also noted. Several potential open reading frames (ORFs) were identified. One of these (ORF 13) encoded a deduced protein, mol wt 32 kD, which is likely to be the serotype-1 specific phosphoprotein expressed in tumours (1) and mapped to an EcoRI fragment within BamHI-H (2). Our results suggest that this ORF is unlikely to be the B antigen (3). ORF 4, which had some similarity to CD4 and immunoglobulin M heavy chain, was encoded by a transcript that originated within the first copy of the 132 bp repeat. ORF 21, which mapped entirely within UL, encoded a deduced protein at least 322 amino acids long that had some similarity to varicella zoster virus (VZV) alpha trans-inducing protein. None of these ORFs was altered significantly by attenuation, except ORF 4 and another small ORF (ORF 3), 5' of the 132 bp repeats, which would probably fail to be transcribed because of truncation of an RNA transcript.

Avian herpesvirus as a live viral vector for the expression of heterologous antigens

Control of Marek's disease in the poultry industry has been successfully achieved for several decades by large-scale vaccination of day-old chickens with live herpesvirus of turkeys (HVT) strains. Several features of this virus including lack of pathogenicity and long-term immune protection due to a persistent viraemic infection made us decide to use HVT as a live viral vector for the expression of foreign antigens. Potential sites for the integration of foreign DNA in the unique short region of the HVT genome were identified by the insertion of a beta-galactosidase expression cassette. Vaccination trials with recombinant virus strains indicated that the marker gene was expressed and stably maintained during animal passage. Based on an insertion site mapping in one of the open reading frames of the unique short region, a general recombination vector was designed for the integration of foreign genes into HVT. Recombinant virus-directed expression of individual antigens from Newcastle disease virus was driven by a strong promoter element derived from the lung terminal repeat sequence of Rous sarcoma virus.

Identification and transcriptional mapping of genes encoded at the IR/Us junction of equine herpesvirus type 1

Two open reading frames (ORFs) encoded at the inverted repeat unique short (Us) junction of the Short (S) region of the equine herpesvirus type 1 genome were identified by DNA sequencing of a 2876 base pair (bp) genomic segment, and transcripts encoding these ORFs were characterized by Northern blot, S1 nuclease, and primer extension analyses. These studies also established the size of each inverted repeat to be 12,768 nucleotides (nts). The IR6 ORF (816 bp), mapping at nts 12,317-11,502 of the S region, is the last gene completely encoded within each inverted repeat and encodes a predicted 30.1-kDa protein of 272 amino acids, which does not exhibit homology to other alphaherpesvirus proteins. IR6 is expressed as an early transcript of 1.2 kb which is detected initially at 1.5 hr p.i. and up to 12 hr p.i. The transcription initiation and termination sites of IR6 were mapped by primer extension and S1 nuclease analyses to nts 12,465 and 11,408, respectively. The first ORF encoded within the Us segment (909 bp; EUS1), mapping at nts 13,397-12,489, encodes a predicted 33.5-kDa protein of 303 amino acids that exhibits 29% identity to the US2 protein of herpes simplex virus 1. EUS1 is expressed as a 2.3-kb mRNA of the gamma-1 class, as its synthesis begins prior to viral DNA replication at 4 hr p.i. but is retarded by phosphonoacetic acid, an inhibitor of viral DNA replication. The Tci and Tct sites of EUS1 were mapped by S1 nuclease analyses to nts 13,637 and 11,408, respectively. Interestingly, this termination site is also utilized by three late mRNAs of 5.8, 3.8, and 1.7 kb which originate within the Us and overlap the IR6 mRNA encoded in the terminal inverted repeat (TR) of the prototype genomic isomer. EUS1 is 3' coterminal with IR6 in the inverted repeat, whereas, the 5.8, 3.8, and 1.7 kb transcripts are 3' coterminal with IR6 of the TR.

Plasmid-associated effects on test gene expression and Marek's disease virus plaque formation during recombination trials

Marek's disease virus (MDV), a herpesvirus of avian origin, is being examined for suitability as a vector for expressing foreign genes. We observed that plasmids encoding the LacZ gene of E. coli under the control of either the herpes simplex virus alpha 4 immediate-early promoter or the cytomegalovirus major immediate-early promoter inhibited MDV plaque formation. Plaque numbers were decreased by one-third, and transient expression of the beta-galactosidase reporter gene was increased by up to 6-fold, when the plasmids were linearized. Sequences associated with the heterologous promoter were identified as being responsible for inhibiting MDV replication.

Sequence determination and genetic content of an 8.9-kb restriction fragment in the short unique region and the internal inverted repeat of Marek's disease virus type 1 DNA

The DNA sequence (8.9 kb) covering about 70% of the short unique region (Us) and part of the short inverted repeat of the Marek's disease virus type 1 GA strain was determined. Computer analysis of the sequence showed the presence of nine potential open reading frames (ORFs), consisting of more than 300 nucleotides in the Us region. Of these ORFs, four were found to be homologous to US10 (minor virion protein), US3 (protein kinase), US2, and US6 (gD) in the Us region of alpha-herpesvirus herpes simplex virus type 1. The protein kinase homologue is especially well conserved in alpha-herpesviruses. No counterpart of the nine MDV1 ORFs was found in the beta-herpes virus human cytomegalovirus and gamma-herpesvirus Epstein-Barr virus, suggesting that MDV1 is more similar to the alpha-herpesviruses. The junction of the Us region and the short inverted repeat was also determined by comparison between the sequences of the DNA fragments, including the terminal and internal repeats. Northern blot analysis showed that the Us region within the 8.9 kb sequence was transcriptionally active in MDV1-infected cells.

Complete nucleotide sequence of the Marek's disease virus ICP4 gene

The Marek's disease virus (MDV) gene encoding a homologue to the ICP4 protein of herpes simplex virus has been mapped to BamHl fragment A based on the physical map of the MDV genome (Fukuchi et al., 1984). The gene lies completely within the inverted repeat flanking the unique short region of the genome. The complete nucleotide sequence of the MDV ICP4 gene has been determined. The coding region is 4245 nucleotides long and has an overall G+C content of 52%. The MDV ICP4 protein is predicted to have a structure similar to that of ICP4-like proteins of other herpesviruses in that it has five distinct regions, the second and fourth of which are highly conserved. In addition, the protein contains the characteristic run of serine residues located toward its amino terminus. The MDV ICP4 gene is expressed in MDV-infected chicken embryo fibroblasts.

Identification of a novel transcription factor, ACF, in cultured avian fibroblast cells that interacts with a Marek's disease virus late gene promoter

Interactions between factors in duck and chick embryo fibroblast (DEF and CEF, respectively) nuclear extracts and the Marek's disease virus (MDV) gp57-65 gene promoter were investigated. Results of in vitro transcription and gel mobility-shift assays indicated that multiple cellular factors interact with 5'-flanking sequences of the MDV gp57-65 gene. One sequence-specific DNA binding activity (termed ACF for avian cell factor(s)) was identified by interaction of DEF and CEF nuclear extract proteins with a particular site (nucleotides -193 to -177) in the MDV gp57-65 gene promoter. Binding of ACF to its apparent recognition sequence, contained within the 17-bp oligonucleotide 5'-CTAGTTTACTTGTTTGT-3' (ACF-12), was highly sequence-specific. Radiolabeled ACF-12 oligonucleotide bound significant ACF protein in the presence of a 400-fold molar excess of unlabeled nonspecific competitor DNA. A similar amount of specific competitor completely abolished ACF binding to probe DNA. Deletion of the ACF binding site from MDV gp57-65 gene promoters linked to a chloramphenicol acetyltransferase (CAT) reporter gene reduced expression of CAT activity by twofold relative to that seen with a gp57-65 promoter-CAT construct containing an intact ACF binding site. Transfection inhibition assays using double-stranded ACF binding site competitors reduced steady-state levels of gp57-65 mRNA in MDV infected cells by over twofold relative to those in control infected cells. Introduction of a similar amount of nonspecific double-stranded oligonucleotide had no adverse effect on gp57-65 mRNA levels. These data suggest that ACF is important for efficient expression of gp57-65.