Characterization of Md5-BAC-REV-LTR virus as Marek's disease vaccine in commercial meat-type chickens: protection and immunosuppression.

Md5-BAC-REV-LTR is a recombinant Marek's disease virus (MDV), with an insertion of the long terminal repeat (LTR) of reticuloendotheliosis virus (REV) into the genome of the highly virulent MDV strain rMd5. It has been shown that Md5-BAC-REV-LTR does not induce tumours and confers high protection against challenge with MDV in 15 × 7 chickens. The objective of the present study was to evaluate the protection and safety (in terms of oncogenicity and immunosuppression) of Md5-BAC-REV-LTR in commercial meat-type chickens bearing maternal antibodies against MDV. Our results show that sub-cutaneous administration of Md5-BAC-REV-LTR at 1 day of age conferred high protection (protection index PI = 84.2) against an early challenge (1 day) by contact exposure to shedder birds infected with the vv+ MDV 648A strain. In such stringent challenge conditions, Md5-BAC-REV-LTR was more protective than a commercial CVI988 (PI = 12.4) and similar to the experimental vaccine Md5-BACΔmeq (PI = 92.4). Furthermore, Md5-BAC-REV-LTR did not induce either tumours or immunosuppression in this study. Immunosuppression was evaluated by the relative lymphoid organ weights and also by the ability of the vaccine to induce late-MDV-induced immunosuppression associated with reactivation of the virus. This study shows that Md5-BAC-REV-LTR has the potential to be used as a MD vaccine and is highly protective against early challenge with vv+ MDV.RESEARCH HIGHLIGHTSMd5-BAC-REV-LTR is highly protective against early challenge with vv+ MDV in commercial meat-type chickens.Md5-BAC-REV-LTR does not cause early immunosuppression.Md5-BAC-REV-LTR does not cause late immunosuppression.Unlike other serotype 1 vaccines, Md5-BAC-REV-LTR is not detected in feather pulp at 7 days post vaccination.

Codon Deoptimization of UL54 in meq-Deleted Marek's Disease Vaccine Candidate Eliminates Lymphoid Atrophy but Reduces Vaccinal Protection.

Marek's disease (MD) is an oncogenic, lymphoproliferative, and highly contagious disease of chickens. Its etiologic agent is the alphaherpesvirus Marek's disease virus (MDV, Gallid alphaherpesvirus 2), and it is a chronic and ubiquitous problem for the poultry industry with significant economic impact in the United States and worldwide. We have previously demonstrated that MDV attenuated by dicodon deoptimization of the UL54 gene results in reduced gene product accumulation in vitro, with reduced viral genome copy number upon infection and reduced atrophy of bursa and thymus in vivo as well. In this report we detail our attempts to use the same attenuation strategy on a meq-deleted MDV mutant, rMd5B40ΔMeq. Unlike the wild-type rMd5B40 virus the rMd5B40ΔMeq is no longer oncogenic, but infected birds experience an unacceptable amount of bursa and thymus atrophy (BTA). We produced two meq-deleted MDV recombinants with a dicodon-deoptimized UL54 (rMd5B40ΔMeq/UL54deop1 and -deop2) and tested their tendency to cause BTA and to serve as a protective vaccine. We found that, although dicodon deoptimization of the UL54 gene results in a virus that spares the infected animal from atrophy of the bursa and thymus, the meq-deleted UL54-deoptimized recombinant is also less protective than the meq-deleted virus without UL54 deoptimization, the HVT + SB1 combination vaccine, or the Rispens (CVI988) vaccine.

Attenuation of Marek's disease virus by codon pair deoptimization of a core gene.

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus of Gallus gallus, the domesticated chicken. Control strategies rely upon vaccination with live attenuated viruses of antigenically similar avian herpesviruses or attenuated strains of MDV. Recent studies in other viruses have shown that recoding certain viral genes to employ synonymous but rarely-used codon pairs resulted in viral attenuation. We deoptimized two MDV proteins, UL54/ICP27 and UL49/VP22, and demonstrate that the more severely deoptimized variant of UL54 accumulates significantly less gene product in vitro. Using these UL54 deoptimized mutants, we further demonstrate that animals infected with the UL54-recoded recombinant virus exhibited decreased viral genome copy number in lymphocytes, reduced lymphoid atrophy and reduced tumor incidence. This study demonstrates that codon pair deoptimization of a single viral gene can produce attenuated strains of MDV. This approach may be useful as a rational way of making novel live attenuated virus vaccines for MDV.

Evaluation of the Protection Efficacy of a Serotype 1 Marek's Disease Virus-Vectored Bivalent Vaccine Against Infectious Laryngotracheitis and Marek's Disease.

Laryngotracheitis (LT) is a highly contagious respiratory disease of chickens that produces significant economic losses to the poultry industry. Traditionally, LT has been controlled by administration of modified live vaccines. In recent years, the use of recombinant DNA-derived vaccines using turkey herpesvirus (HVT) and fowlpox virus has expanded, as they protect not only against the vector used but also against LT. However, HVT-based vaccines confer limited protection against challenge, with emergent very virulent plus Marek's disease virus (vv+MDV). Serotype 1 vaccines have been proven to be the most efficient against vv+MDV. In particular, deletion of oncogene MEQ from the oncogenic vvMDV strain Md5 (BACδMEQ) resulted in a very efficient vaccine against vv+MDV. In this work, we have developed two recombinant vaccines against MD and LT by using BACδMEQ as a vector that carries either the LT virus (LTV) gene glycoprotein B (gB; BACΔMEQ-gB) or LTV gene glycoprotein J (gJ; BACδMEQ-gJ). We have evaluated the protection that these recombinant vaccines confer against MD and LT challenge when administered alone or in combination. Our results demonstrated that both bivalent vaccines (BACΔMEQ-gB and BACδMEQ-gJ) replicated in chickens and were safe to use in commercial meat-type chickens bearing maternal antibodies against MDV. BACΔMEQ-gB protected as well as a commercial recombinant (r)HVT-LT vaccine against challenge with LTV. However, BACδMEQ-gJ did not protect adequately against LT challenge or increase protection conferred by BACΔMEQ-gB when administered in combination. On the other hand, both BACΔMEQ-gB and BACδMEQ-gJ, administered alone or in combination, protected better against an early challenge with vv+MDV strain 648A than commercial strains of rHVT-LT or CVI988. Our results open a new avenue in the development of recombinant vaccines by using serotype 1 MDV as vectors.

Detection and differentiation of CVI988 (Rispens vaccine) from other serotype 1 Marek's disease viruses.

The serotype 1 Marek's disease virus (MDV) is the causative agent for Marek's disease (MD), a lymphoproliferative disease of chickens of great concern to the poultry industry. CVI988 (Rispens vaccine), an attenuated serotype 1 MDV, is currently the most efficacious commercially available vaccine for preventing MD. However, it is difficult to detect and differentiate CVI988 when other serotype 1 MDVs are present. To facilitate the detection of CVI988, we developed two sets of primers for a mismatch amplification mutation assay (MAMA) PCR that targeted the single nulceotide polymorphism associated with the H19 epitope of the phosphorylated protein 38 gene. The PCR was very specific. One primer set (oncogenic primers) amplified DNA from 15 different serotype 1 MDVs except CVI988. The other primer set (CVI988 primers) amplified DNA from CVI988 but not from any of the other 15 serotype 1 MDVs. A real-time PCR assay was developed using MAMA primers, and specificity and sensitivity was evaluated in vitro and in vivo. Mixtures of plasmids (CVI988 plasmid and oncogenic plasmid) at various concentrations were used to evaluate the sensitivity/specificity of MAMA primers in vitro. Both primer setswere able to amplify as little as one copy of their respective plasmid. Oncogenic primers were highly specific and only amplified CVI988 plasmid when the concentration of oncogenic plasmid was very low (1 X 10(1)) and CVI988 plasmid was very high (1 X 10(6)). Specificity of CVI988 primers was not as high because they could amplify oncogenic plasmids when the concentration of CVI988 plasmid was 1 x 10(3) and the concentration of oncogenic 1 x 10(2). Validation of MAMA primers in in vivo samples demonstrated that oncogenic primers can be used for both early diagnosis of MD in feather pulp (FP) samples collected at 3 wk of age and confirmation of MD diagnosis in tumors. CVI988 primers could be used to monitor CVI988 vaccination in samples with a low load of oncogenic MDV DNA (latently infected samples or negative) but not in samples with a high load of oncogenic MDV DNA (tumors). Our results suggest that monitoring CVI988 vaccination in FP samples collected at 1 wk of age ensures the specificity of the CVI988 primers.

Ability of MEQ-deleted MDV vaccine candidates to adversely affect lymphoid organs and chicken weight gain.

CVI988 (Rispens) is currently the most effective vaccine used to protect against Marek's disease, a lymphoproliferative disease of chickens. A MEQ-deleted Marek's disease virus strain has shown promise as a vaccine candidate; however, unpublished results from vaccine safety trials suggest that this candidate vaccine induces unwanted lymphoid atrophy. The current study evaluated lymphoid atrophy at multiple time points between 2- and 8-wk postinoculation and attempted to correlate results with virus replication in the thymus. Results confirm reports that MEQ-deleted virus strains are able to cause thymus and bursa atrophy, which is most severe at 2-wk postinoculation. The MEQ-deleted virus strains induced lower body weights and relative thymus and bursa weights compared to uninoculated and Rispens-vaccinated chickens at multiple time points between 2- and 8-wk postinoculation. Both MEQ-deleted virus strains produced high levels of in vivo virus replication in the thymus at rates significantly greater than in Rispens-vaccinated chickens and were comparable to levels of RM1 virus, a MDV previously shown to induce severe thymus and bursa atrophy. Virus replication was highly correlated with relative thymus weights at each time point. Understanding this delicate balance between inducing maximum disease protection and preventing immunodepressive effects is critical for the development of future Marek's disease vaccines.

Insertion of reticuloendotheliosis virus long terminal repeat into a bacterial artificial chromosome clone of a very virulent Marek's disease virus alters its pathogenicity.

Co-cultivation of the JM/102W strain of Marek's disease virus (MDV) with reticuloendotheliosis virus (REV) resulted in the generation of a recombinant MDV containing the REV long terminal repeat (LTR) named the RM1 strain of MDV, a strain that was highly attenuated for oncogenicity but induced severe bursal and thymic atrophy. We hypothesize that the phenotypic changes were solely due to the LTR insertion. Furthermore, we hypothesize that insertion of REV LTR into an analogous location in a different MDV would result in a similar phenotypic change. To test these hypotheses, we inserted the REV LTR into a bacterial artificial chromosome (BAC) clone of a very virulent strain of MDV, Md5, and designated the virus rMd5-RM1-LTR. The rMd5-RM1-LTR virus and the rMd5 virus were passaged in duck embryo fibroblast cells for up to 40 passages before pathogenicity studies. Susceptible chickens were inoculated intra-abdominally at hatch with the viruses rMd5-RM1-LTR, rMd5 BAC parental virus, wild-type strain Md5, or strain RM1 of MDV. The rMd5-RM1-LTR virus was attenuated at cell culture passage 40, whereas the rMd5 BAC without RM1 LTR retained its pathogenicity at cell culture passage 40. Using polymerase chain analysis, the RM1 LTR insert was detected in MDV isolated from buffy coat cells collected from chickens inoculated with rMd5-RM1-LTR, but only at 1 week post inoculation. The data suggest that the presence of the RM1 LTR insert within MDV genome for 1 week post inoculation with virus at hatch is sufficient to cause a reduction in pathogenicity of strain Md5 of MDV.

Competition between two virulent Marek's disease virus strains in vivo.

Previous studies have demonstrated the presence of multiple strains of Marek's disease virus simultaneously circulating within poultry flocks, leading to the assumption that individual birds are repeatedly exposed to a variety of virus strains in their lifetime. Virus competition within individual birds may be an important factor that influences the outcome of co-infection under field conditions, including the potential outcome of emergence or evolution of more virulent strains. A series of experiments was designed to evaluate virus competition within chickens following simultaneous challenge with two virulent serotype 1 Marek's disease virus strains, using either pathogenically similar (rMd5 and rMd5/pp38CVI) or dissimilar (JM/102W and rMd5/pp38CVI) virus pairs. Bursa of Fabricius, feather follicle epithelium, spleen, and tumour samples were collected at multiple time points to determine the frequency and distribution of each virus present using pyrosequencing, immunohistochemistry and virus isolation. In the similar pair, rMd5 appeared to have a competitive advantage over rMd5/pp38CVI, which in turn had a competitive advantage over the less virulent JM/102W in the dissimilar virus pair. Dominance of one strain over the other was not absolute for either virus pair, as the subordinate virus was rarely eliminated. Interestingly, competition between two viruses with either pair rarely ended in a draw. Further work is needed to identify factors that influence virus-specific dominance to better understand what characteristics favour emergence of one strain in chicken populations at the expense of other strains.

Gene expression profiling in rMd5- and rMd5deltameq-infected chickens.

Marek's disease (MD) is a lymphoproliferative disorder of domestic chickens caused by a highly contagious and oncogenic alpha-herpesvirus, Marek's disease virus (MDV). MD is characterized by bursal-thymic atrophy and rapid onset of T-cell lymphomas that infiltrate lymphoid tissues, visceral organs, and peripheral nerves with severe clinical signs that include transient paralysis, anemia, weight loss, and neurologic disorders. Using overlapping cosmids- and BAC-cloned MDV, it has been shown that MDV-encoded vIL-8, pp38, vTR, vLIP, RLORF4, and meq are among the many essential genes that play critical roles in viral pathogenesis. Of all the genes investigated so far, only meq has been shown to be consistently expressed in all MDV-derived tumors and lymphoblastoid cell lines. Meq is a basic leucine-zipper protein that shares homology with the jun/fos family of transcriptional factors. There are two copies of meq gene within the MDV genome that are only present in the serotype-1 strains. It has been shown conclusively that deletion of meq results in loss of transformation of T cells in chickens, with no effect on the early cytolytic phase of infection in lymphoid organs, which is essential for induction of innate and adaptive immunity. The goal of this study was to investigate 1) the effect of the meq oncogene on the expression pattern of select chicken immune and nonimmune-related genes, and 2) its potential role in MDV-induced apoptosis. We used real-time reverse transcriptase-polymerase chain reaction to evaluate the expression profiling of a panel of chicken genes in rMd5- and rMd5deltameq-infected chickens at 5, 14, 21, and 35 days postinfection (dpi). Although the transcriptional activities of several immune-related genes, including IL-6, IL-10, cMGF, GM-CSF, iNOS, IFNbeta, and INFgamma, were higher in rMd5deltameq-infected chickens at 5 dpi when compared to the rMd5-infected birds, the differences in expression levels of the tested genes between the two viral constructs were not significant. In addition, a reduction in the transcriptional activity of Bdcl2 in recombinant fowlpox virus (rFPV)+meq-infected chicken embryonic fibroblasts suggested that meq alone did not impede FPV-induced apoptosis. The likely suppressive nature and anti-inflammatory function of the meq oncogene and its possible role in virus-induced cell death is discussed.

Artificially inserting a reticuloendotheliosis virus long terminal repeat into a bacterial artificial chromosome clone of Marek's disease virus (MDV) alters expression of nearby MDV genes.

Researchers reported that co-cultivating the JM/102W strain of Marek's disease virus (MDV) with reticuloendotheliosis virus (REV) resulted in an REV long terminal repeat (LTR) being inserted into the internal repeat short (IRS) region of JM/102W. When the resulting recombinant virus was serially passed in cell culture, the initial LTR was duplicated and a second LTR spontaneously appeared in the terminal repeat short (TRS) region of the MDV genome. The virus, designated RM1, was significantly attenuated but still induced severe bursal and thymic atrophy (Isfort et al. PNAS 89:991-995). To determine whether the altered phenotype was due solely to the LTR, we cloned the LTR from the RM1 IRS region and inserted it into the IRS region of a very virulent bacterial artificial clone (BAC) of the Md5 strain of MDV, which we designated rMd5-RM1-LTR. During blind passage in duck embryo fibroblast cultures, the initial LTR in the rMd5-RM1-LTR was also duplicated, with LTRs appearing in both IRS and TRS regions of the MDV genome. The inserted LTR sequences and transcripts associated with the MDV open reading frames MDV085, MDV086, SORF2, US1, and US10 were molecularly characterized. The parental Md5 BAC contains a family of transcripts of 3, 2, and 1 kb that all terminate at the end of the US10 gene. The rMd5-RM1-LTR and RM1 viruses both express an additional 4 kb transcript that originates in the LTR and also terminates after US10. Collectively, the data suggest that our engineered rMd5-RM1-LTR virus very closely resembles the RM1 virus in its structure and transcription patterns.

Virulent Marek's disease virus generated from infectious bacterial artificial chromosome clones with complete DNA sequence and the implication of viral genetic homogeneity in pathogenesis.

Genetic homogeneity of a test population is essential to precisely associate a viral genome sequence and its phenotype at the nucleotide level. However, homogeneity is not easy to achieve for Marek's disease virus (MDV) due to its strictly cell-associated replication. To address this problem, two virulent infectious bacterial artificial chromosome (BAC) clones of MDV were generated from an MDV genome previously cloned as five overlapping cosmids. The Md5SN5BAC clone has the BAC vector inserted between the 3' ends of UL3 and UL4, such that no known ORFs should be disrupted. The BAC vector is flanked by loxP sites, so that it can be deleted from the viral genome by transfecting Md5SN5BAC into a newly developed chicken cell line that constitutively expresses Cre recombinase. The Md5B40BAC clone has the BAC vector replacing a portion of US2, a location similar to that used by other groups to construct MDV-BAC clones. Although both BACs were capable of producing infectious virulent MDV when inoculated into susceptible chickens, Md5B40BAC-derived viruses showed somewhat better replication in vivo and higher virulence. Removal of the BAC vector in Md5SN5BAC-derived viruses had no influence on virulence. Interestingly, when genetically homogeneous virulent MDV generated from Md5B40BAC was mixed with avirulent virus, the overall virulence of the mixed population was noticeably compromised, which emphasizes the importance of MDV population complexity in pathogenesis.

The effect of the time interval between exposures on the susceptibility of chickens to superinfection with Marek's disease virus.

Marek's disease virus (MDV) is ubiquitous within commercial poultry flocks because current vaccines do not prevent MDV infection or transmission. In order for newly-evolved MDV strains to become established within a flock, it seems inevitable that any new strain would need to infect and replicate in chickens previously infected with resident MDV strains. This phenomenon is difficult to detect and there is no clear evidence that it is even possible. Four experiments were performed to demonstrate superinfection and evaluate the effect of time between challenges on the effect of superinfection with the use of two pairs of fully virulent MDV strains that could be discriminated by novel technology: 1) JM/102W and rMd5//38CVI, and 2) rMd5 and rMd5//38CVI. Feather follicle epithelium (FFE), spleen, and tumor samples were collected at single or multiple time points from the same bird to determine the frequency and distribution of each virus present following superinfection, with the use of pyrosequencing and immunohistochemistry. Superinfection was observed in 82 of 149 (55%) FFE samples following short-interval challenge (24 hr) compared to only 6 of 121 (5%) samples following long-interval challenge (13 days), indicating a strong influence of challenge interval. In cases where the first inoculated virus was weak or delayed, the second inoculated virus was detected in 42 of 95 (44%) birds. In tumors from dually challenged birds, the second virus was again present much more often following short-interval challenge (68%) compared to long-interval challenge (11%). Virus mixtures in tumors were less common compared to those in FFE samples. Vaccination with turkey herpesvirus had no significant effect on the virus frequency for either virus pair or challenge time interval, suggesting these conclusions may be applicable to vaccinated chickens in the field. These studies demonstrated superinfection for the first time with two fully virulent MDV strains and suggest that short-interval challenge exposure and/or weak initial exposures may be important factors leading to superinfection--a prerequisite for the establishment of a second virus strain in the population. This model system should be useful to elucidate this important phenomenon further.

Characterization of reticuloendotheliosis virus isolates obtained from broiler breeders, turkeys, and prairie chickens located in various geographical regions in the United States.

Nine reticuloendotheliosis virus (REV) isolates obtained from broiler breeders, turkeys, and prairie chickens located in three different geographical regions in the USA, and three isolates obtained from known contaminated live-virus vaccines were characterized using polymerase chain reaction (PCR) and indirect immunofluorescence (IFA) assays. All isolates were propagated in chicken embryo fibroblasts obtained from a specific pathogen free breeder flock. PCR analysis of all 12 isolates resulted in the amplification of the 291-bp REV long-terminal repeat region (LTR); none of the isolates exhibited a different pattern or shift from the expected PCR product of REV LTR. The subtype of the REV isolates was determined by IFA using REV-specific monoclonal antibodies, 11B118.22, 11C237.8, and 11D182. Results from sub-typing indicated that all nine isolates from broiler breeders, turkeys, and prairie chickens belonged to subtype 3, and are antigenically related to the chick syncytial virus (CSV) strain of REV, the prototype of subtype 3 REV. In contrast, the three isolates from contaminated vaccines were classified as subtype 2, and were antigenically related to spleen necrosis virus (SNV) strain of REV, the prototype of subtype 2 REV. Three isolates representing REV isolated from broiler breeders, turkeys, and prairie chickens were cloned and further evaluated by DNA sequence analysis of the envelope gene. Results from DNA sequence analysis confirmed those from sub-typing and indicated that the three REV isolates representing those from broiler breeders, turkeys, and prairie chickens are closely related to CSV of REV, with an amino acid homology of 98% or greater as compared with SNV with an amino acid homology of 95% or less. Data from this study clearly indicate that subtype 3 is the most common subtype of REV circulating in three different avian species, namely broiler breeders, turkeys and prairie chickens, located in three different geographical regions in the United States.

A MEQ-deleted Marek's disease virus cloned as a bacterial artificial chromosome is a highly efficacious vaccine.

The Marek's disease virus (MDV) induces T-cell tumors in susceptible chickens. Of the 80 to 100 known MDV genes, only the MDV MEQ gene was shown to have transforming properties. Further evidence that MEQ is probably the principal oncogene in MDV came when researchers used overlapping cosmid clones of MDV and demonstrated that deleting MEQ resulted in a highly protective Marek's disease (MD) vaccine. We deleted both copies of MEQ from a bacterial artificial chromosome clone (BAC) of MDV. The virus, BACdelMEQ, was completely attenuated and did not appear to have any adverse effect on chicken body weight in MDV maternal-antibody-positive chickens, as measured at 8 wk of age. In two protection studies, BACdelMEQ efficiently protected susceptible chickens from a challenge by MDV strain 686, one of the most virulent MDV strains. In both protection studies, the BACdelMEQ protected chickens significantly better than the commercial MD vaccine, CVI988/Rispens. Only the protein-coding sequences of MEQ were deleted and all upstream and downstream regulatory sequences were left intact. Thus, BACdelMEQ has the potential to be a superior MD vaccine as well as a vector to deliver various foreign genes to poultry.

Deletion of the Marek's disease virus UL41 gene (vhs) has no measurable effect on latency or pathogenesis.

Marek's disease is an economically important disease of poultry and is caused by an alphaherpesvirus, Marek's disease virus (MDV). The predicted protein product of the MDV UL41 open reading frame has significant protein sequence identity with the virion host shutoff (vhs) protein gene in other alphaherpesviruses. To determine whether the MDV UL41 gene functions as a vhs protein, we utilized a transient co-transfection assay and demonstrated that the MDV UL41 protein was as active in degrading RNA as the vhs protein of herpes simplex virus type 1. To evaluate whether the MDV UL41 gene was involved in pathogenesis, we deleted the MDV UL41 gene. The UL41 deletion mutant replicated in cell culture as well as the parental MDV. The deletion mutant was inoculated into susceptible day-old chicks. The pattern and degree of tumor lesions and neurovirulence produced by the deletion mutant was same as the pattern of lesions induced by the parental virus. The only observable difference between the inoculation of MDV and the MDV deletion mutant was that the early in vivo cytolytic infection with the deletion mutant was of a longer duration than in the non-mutant MDV.

Recombinant Marek's disease virus (MDV) lacking the Meq oncogene confers protection against challenge with a very virulent plus strain of MDV.

Marek's disease virus (MDV) encodes a basic leucine-zipper protein, Meq, that shares homology with the Jun/Fos family of transcriptional factors. Conclusive evidence that Meq is an oncogene of MDV came from recent studies of a Meq-null virus, rMd5 Delta Meq. This virus replicated well in vitro, but was non-oncogenic in vivo. Further characterization of this virus in vivo indicated that the meq gene is dispensable for cytolytic infection since it replicated well in the lymphoid organs and feather follicular epithelium. Since rMd5 Delta Meq virus was apathogenic for chickens, we set out to investigate whether this virus could be a good candidate vaccine. Vaccine efficacy experiments conducted in Avian Disease and Oncology Laboratory (ADOL) 15I(5)x 7(1) chickens vaccinated with rMd5 Delta Meq virus or an ADOL preparation of CVI988/Rispens indicated that the Meq-null virus provided protection superior to CVI988/Rispens, the most efficacious vaccine presently available, following challenge with a very virulent (rMd5) and a very virulent plus (648A) MDV strains.

Transcriptional profiling of Marek's disease virus genes during cytolytic and latent infection.

Marek's disease (MD), a lymphoproliferative disease of chicken is caused by a highly cell-associated alpha-herpesvirus, Marek's disease virus (MDV). MDV replicates in chicken lymphocytes and establishes a latent infection within CD4(+) T cells. The expression analysis of limited viral transcripts have revealed differences in gene expression pattern during cytolytic and latent phases of MDV infection. In this study, we conducted a global gene expression profiling of MDV using oligonucleotide-based Affymetrix GeneChip Chicken Genome Arrays. These arrays contain probe for more than 32,000 chicken transcripts and most of the known MDV genes and open reading frames. Two-week-old MD-susceptible chickens were inoculated with an oncogenic strain of MDV, and spleen samples were collected 5 and 15 days post inoculation (cytolytic and latent infection, respectively) for RNA isolation and microarray analysis. Array results displayed a significant differential pattern of viral transcriptome between the two phases of MDV infection. The expression levels of more than 78 MDV genes were increased during the cytolytic infection when compared to latent infection (2-11-fold increase). A 23-KD nuclear protein, meq oncoprotein, and R-LORF5 were among the few viral genes that were expressed during both phases of infection. In addition, there were at least 11 known and hypothetical genes that had no significant transcriptional activities during either stages of infection. These chicken genome arrays have considerable promise, as a valuable tool in understanding the molecular mechanism regulating MDV cytolytic and latent infection, and providing insights into the chicken gene expression pattern and associated biological pathways in response to different phases of viral pathogenesis.

Quantitative evaluation of viral fitness due to a single nucleotide polymorphism in the Marek's disease virus UL41 gene via an in vitro competition assay.

Marek's disease, a T cell lymphoma, is an economically important disease of poultry caused by the Marek's disease virus (MDV), a highly cell-associated alphaherpesvirus. A greater understanding of viral gene function and the contribution of sequence variation to virulence should facilitate efforts to control Marek's disease in chickens. To characterize a naturally occurring single nucleotide polymorphism (SNP; AY510475:g.108,206C>T) in the MDV UL41 gene that results in a missense mutation (AAS01683:p.Arg377Cys), bacterial artificial chromosome (BAC)-derived MDVs that differed only in the UL41 SNP were evaluated using a head-to-head competition assay in vitro. Monitoring the frequency of each SNP by pyrosequencing during virus passage determined the ratio of each viral genome in a single monolayer, which is a very sensitive method to monitor viral fitness. MDV with the UL41*Cys allele showed enhanced fitness in vitro. To evaluate the mechanism of altered viral fitness caused by this SNP, the virion-associated host shutoff (vhs) activity of both UL41 alleles was determined. The UL41*Cys allele had no vhs activity, which suggests that enhanced fitness in vitro for MDV with inactive vhs was due to reduced degradation of viral transcripts. The in vitro competition assay should be applicable to other MDV genes and mutations.

Creation of diversity in the animal virus world by inter-species and intra-species recombinations: lessons learned from poultry viruses.

The biological diversity within viruses is one of the largest found in all other forms of nature. Many mechanisms contribute to virus diversity and include incorporating genetic material from the host, recombination between viruses belonging to the same or to a different family, and even recombination between viruses normally infecting different hosts. In particular, avian viruses can utilize all three of these mechanisms to generate new viruses. It is well documented that recombinations can occur between Marek's disease virus (MDV), an oncogenic herpesvirus, fowlpox virus (FPV), and various avian retroviruses. In addition, chicken infectious anemia virus (CIAV), a circovirus, was created by several inter-family recombination events, which occurred between plant and animal viruses. The circovirus represents the ancestral creation of a recombination between a plant DNA virus (nanovirus) and a mammalian RNA virus (calicivirus), through a transition of RNA to DNA made by an endogenous mammalian retrovirus. The present review will discuss the current knowledge on recombination events that have occurred between avian herpesviruses and retroviruses following dual infections in vitro and in vivo. In addition, we will discuss recombinations between fowlpox viruses and the avian retrovirus reticuloendotheliosis (REV). Finally, the review will address the creation of CIAV and how it evolved from recombinations between a plant virus and an animal virus.

Development of a polymerase chain reaction to differentiate avian leukosis virus (ALV) subgroups: detection of an ALV contaminant in commercial Marek's disease vaccines.

Avian leukosis viruses (ALVs) are common in many poultry flocks and can be detected using an enzyme-linked immunosorbent assay or any other test designed to identify p27, the group-specific antigen located in gag. However, endogenous retroviruses expressing p27 are often present and can be confused with exogenous ALVs. A more specific and informative assay involves targeting the variable envelope glycoprotein gene (gp85) that is the basis for dividing ALVs into their different subgroups. We designed polymerase chain reaction (PCR) primers that would specifically detect and amplify viruses from each of the six ALV subgroups: A, B, C, D, E, and J. Subgroup B and D envelopes are related, and our B-specific primers also amplified subgroup D viruses. We also designed a set of common primers to amplify any ALV subgroup virus. To demonstrate the usefulness of these primers, we obtained from the Center for Veterinary Biologics in Iowa culture supernatant from chicken embryo fibroblasts infected with an ALV that was found to be a contaminant in two commercial Marek's disease vaccines. Using our PCR primers, we demonstrate that the contaminant was a subgroup A ALV. We cloned and sequenced a portion of the envelope gene and confirmed that the ALV was a subgroup A virus. Unlike typical subgroup A viruses, the contaminant ALV grew very slowly in cell culture. We also cloned and sequenced a portion of the long terminal repeat (LTR) from the contaminant virus. The LTR was found to be similar to those LTRs found in endogenous ALVs (subgroup E) and very dissimilar to LTRs normally found in subgroup A viruses. The E-like LTR probably explains why the contaminant grew so poorly in cell culture.