Factors Influencing The Attenuation of Serotype 1 Marek's Disease Virus by Serial Cell Culture Passage and Evaluation of Attenuated Strains for Protection and Replication

Fully Attenuated meq and pp38 Double Gene Deletion Mutant Virus Confers Superior Immunological Protection against Highly Virulent Marek's Disease Virus Infection

Marek's disease virus (MDV) induces immunosuppression and neoplastic disease in chickens. The virus is controllable via an attenuated meq deletion mutant virus, which has the disadvantage of retaining the ability to induce lymphoid organ atrophy. To overcome this deficiency and produce more vaccine candidates, a recombinant MDV was generated from the highly virulent Md5BAC strain, in which both meq and a cytolytic replication-related gene, pp38, were deleted. Replication of the double deletion virus, Md5BAC ΔmeqΔpp38, was comparable with that of the parental virus in vitro. The double deletion virus was shown to be fully attenuated and to reduce lymphoid organ atrophy in vivo. Crucially, Md5BAC ΔmeqΔpp38 confers superior protection against highly virulent virus compared with a commercial vaccine strain, CVI988/Rispens. Transcriptomic profiling indicated that Md5BAC ΔmeqΔpp38 induced a different host immune response from CVI988/Rispens. In summary, a novel, effective, and safe vaccine candidate for prevention and control of MD caused by highly virulent MDV is reported. IMPORTANCE MDV is a highly contagious immunosuppressive and neoplastic pathogen. The virus can be controlled through vaccination via an attenuated meq deletion mutant virus that retains the ability to induce lymphoid organ atrophy. In this study, we overcame the deficiency by generating meq and pp38 double deletion mutant virus. Indeed, the successfully generated meq and pp38 double deletion mutant virus had significantly reduced replication capacity in vivo but not in vitro. It was fully attenuated and conferred superior protection efficacy against very virulent MDV challenge. In addition, the possible immunological protective mechanism of the double deletion mutant virus was shown to be different from that of the gold standard MDV vaccine, CVI988/Rispens. Overall, we successfully generated an attenuated meq deletion mutant virus and widened the range of potential vaccine candidates. Importantly, this study provides for the first time the theoretical basis of vaccination induced by fully attenuated gene-deletion mutant virus.

Attenuation of a recombinant Marek's disease virus lacking the meq oncogene and evaluation on its immune efficacy against Marek's disease virus

SC9-2 is a recombinant Marek's disease virus (MDV) strain lacking the meq oncogene. Previous study demonstrated that SC9-2 virus provides good protection against challenge with a very virulent MDV rMd5, but it induces immunosuppressive effects in specific pathogen-free (SPF) chickens. In the present study, SC9-2 was serially passaged on chicken embryo fibroblast (CEF) cell cultures. The pathogenicity and immune efficacy of SC9-2/10^th and SC9-2/40^th against rMd5 were evaluated. Animal experimental results showed that SC9-2/10^th and SC9-2/40^th showed no lethality or tumorigenicity in SPF chickens. Body weight of chickens inoculated with SC9-2/40^th were significantly higher than that of the chickens inoculated with SC9-2/10^th but lower than that of the uninoculated controls. The severity of bursa and thymus atrophy (BTA) and spleen enlargement in SC9-2/40^th-inoculated chickens were also weaker than the SC9-2/10^th-inoculated ones but stronger than the uninoculated controls. Chickens inoculated with SC9-2/40^th and SC9-2/10^th showed similar antibody levels induced by H9N2 subtype avian influenza virus/Newcastle disease virus inactivated vaccines, both of which were lower than the uninoculated controls. Replication of SC9-2/40^th was significantly lower than SC9-2/10^th in feather follicle epithelium (FFE) of infected chickens. The immune protection index of SC9-2/40^th was also lower than that of SC9-2/10^th, but the difference was not significantly, and both of which were significant higher than that of the commercial MDV vaccine CVI988/Rispens. The results of our studies demonstrated that SC9-2/40^th showed weaker severity of BTA, spleen enlargement, and body weight loss and lower replication level in FFE than SC9-2/10^th in SPF chickens. However, SC9-2/40^th was able to confer better immune protection as compared with CVI988/Rispens vaccination in SPF chickens. In conclusion, serially attenuation of SC9-2 in CEFs reduced the lymphoid organ atrophy and replication in SPF chickens, and the immune protective efficacy of attenuated viruses was still superior than CVI988/Rispens.

Marek's disease herpesvirus vaccines integrate into chicken host chromosomes yet lack a virus-host phenotype associated with oncogenic transformation

Marek's disease (MD) is a lymphotropic and oncogenic disease of chickens that can lead to death in susceptible and unvaccinated host birds. The causative pathogen, MD virus (MDV), a highly oncogenic alphaherpesvirus, integrates into host genome near the telomeres. MD occurrence is controlled across the globe by biosecurity, selective breeding for enhanced MD genetic resistance, and widespread vaccination of flocks using attenuated serotype 1 MDV or other serotypes. Despite over 40 years of usage, the specific mechanism(s) of MD vaccine-related immunity and anti-tumor effects are not known. Here we investigated the cytogenetic interactions of commonly used MD vaccine strains of all three serotypes (HVT, SB-1, and Rispens) with the host to determine if all were equally capable of host genome integration. We also studied the dynamic profiles of chromosomal association and integration of the three vaccine strains, a first for MD vaccine research. Our cytogenetic data provide evidence that all three MD vaccine strains tested integrate in the chicken host genome as early as 1 day after vaccination similar to oncogenic strains. However, a specific, transformation-associated virus-host phenotype observed for oncogenic viruses is not established. Our results collectively provide an updated model of MD vaccine-host genome interaction and an improved understanding of the possible mechanisms of vaccinal immunity. Physical integration of the oncogenic MDV genome into host chromosomes along with cessation of viral replication appears to have joint signification in MDV's ability to induce oncogenic transformation. Whereas for MD vaccine serotypes, a sustained viral replication stage and lack of the chromosome-integrated only stage were shared traits during early infection.

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

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

Dynamic equilibrium of Marek's disease genomes during in vitro serial passage

Attenuation of Gallid herpesvirus-2 (GaHV-2), the causative agent of Marek's disease, can occur through serial passage of a virulent field isolate in avian embryo fibroblasts. In order to gain a better understanding of the genes involved in attenuation and associate observed changes in phenotype with specific genetic variations, the genomic DNA sequence of a single GaHV-2 virulent strain (648A) was determined at defined passage intervals. Biological characterization of these "interval-isolates" in chickens previously indicated that the ability to induce transient paralysis was lost by passages 40 and the ability to induce persistent neurological disease was lost after passage 80, coincident with the loss of neoplastic lesion formation. Deep sequencing of the interval-isolates allowed for a detailed cataloguing of the mutations that exist within a single passage population and the frequency with which a given mutation occurs across passages. Gross genetic alterations were identified in both novel and well-characterized genes and cis-acting regions involved in replication and cleavage/packaging. Deletions in genes encoding the virulence factors vLipase, vIL8, and RLORF4, as well as a deletion in the promoter of ICP4, appeared between passages 61 and 101. Three mutations in the virus-encoded telomerase which predominated in late passages were also identified. Overall, the frequency of mutations fluctuated greatly during serial passage and few genetic changes were absolute. This indicates that serial passage of GaHV-2 results in the generation of a collection of genomes with limited sequence heterogeneity.

Cell culture attenuation eliminates rMd5ΔMeq-induced bursal and thymic atrophy and renders the mutant virus as an effective and safe vaccine against Marek's disease

Marek's disease virus (MDV) encodes a basic leucine zipper oncoprotein, Meq, which structurally resembles jun/fos family of transcriptional activators. It has been clearly demonstrated that deletion of Meq results in loss of transformation and oncogenic capacity of MDV. The rMd5ΔMeq virus provided superior protection than CVI988/Rispens vaccine in 15×7 chickens when challenged with a very virulent plus (vv+) strain of MDV, 648A. The rMd5ΔMeq construct was also shown to be an effective vaccine in commercial chickens that were challenged under field conditions by exposure to seeder chicken inoculated with MDV strain 686, a vv+ and arguably the most pathogenic strain of MDV. Although deletion of Meq gene renders the virus non-oncogenic, it still induces lymphoid organ atrophy like that of the parental rMd5, in highly susceptible MDV maternal antibody negative (MAb-) chickens. We have generated 50 cell culture passages of attenuated rMd5ΔMeq viruses and found no significant lymphoid organ atrophy beginning at 40(th) passage onward when compared with the normal control chickens. The protective ability of these attenuated Meq null viruses against challenge with vv+ MDV strain 686 is similar to the original virus at 19(th) passage in maternal antibody negative chickens. The data indicate that attenuation of these Meq null viruses has no influence on their protective efficacy, but eliminated lymphoid organ atrophy and rendered them safe to use even in MAb- chickens, a characteristic that should facilitate commercialization and licensing by vaccine manufacturers.

Replication ability of three highly protective Marek's disease vaccines: implications in lymphoid organ atrophy and protection

The present work is a chronological study of the pathogenesis of three attenuated serotype 1 Marek's disease (MD) virus strains (RM1, CVI988 and 648A80) that provide high protection against MD but have been attenuated by different procedures and induce different degrees of lymphoid organ atrophy. All studied strains replicated in the lymphoid organs (bursa,x thymus and spleen) and a peak of replication was detected at 6 days post inoculation (d.p.i.). Differences, however, were observed among vaccine strains. RM1 strain replicates more in all lymphoid organs compared with CVI988 and 648A80 strains. In addition, replication of RM1 in the thymus did not decrease after 6 d.p.i. but continued at high levels at 14 d.p.i. and until the thymus was completely destroyed. Lung infection occurred very early after infection with all of the three vaccines and the level of replication was similar to that found in the lymphoid organs. Infected cells were very large and appeared scattered in the lung parenchyma and in the parabronchial lining. The study of the target cells for the early infection in cell suspensions of blood and spleen showed that both non-adherent cell populations (enriched in lymphoid cells) and adherent cells (enriched in monocytes/macrophages) supported MD virus infection. Infection in adherent cells was especially high at very early stages of the infection (3 to 6 d.p.i.). Atrophy of lymphoid organs is a major drawback in the production of highly protective vaccines against MD. A better understanding of the mechanisms associated with lymphoid organ atrophy will aid in overcoming this problem.

Accumulation of attenuating mutations in varying proportions within a high passage very virulent plus strain of Gallid herpesvirus type 2

Marek's disease (MD) is controlled through mass vaccination. Although these vaccines reduce or delay tumor formation they fail to induce sterilizing immunity and prevent virus shedding. Relatively little is known about the genetic changes that lead to attenuation. It has been established that serial passage of virulent strains in avian cell lines results in the generation of attenuated progeny at some undefined passage level. A detailed cataloging of the mutations needed for attenuation will be important for advancing our understanding of MD biology and should facilitate the development of better vaccines. Using deep sequencing, the complete nucleotide sequence of the very virulent plus (vv+) strain 648A representing the 101 st passage was determined. Pathotyping studies have indicated that 648Ap101 is indeed attenuated. Comparative sequence analysis with the 648A strain at passage 11 has identified numerous gross genetic changes and subtle single nucleotide polymorphisms scattered throughout the genome. Relative to the strain (648Ap11), deletions were identified in MD-specific genes located in the repeat long (R(L)), unique long (U(L)) and repeat short (R(S)) regions. A deletion in the R(L) region, present in 33% of the p101 sequences, mapped to the genes encoding viral interleukin 8 (vIL8), RLORF4 and RLORF5. An R(S) deletion was mapped to the Meq oncoprotein binding site within the ICP4 promoter and was present in 97.8% of the p101 sequences. A short deletion in the U(L) region mapped to the 3' terminus of the gene encoding vLipase and was present in 54% of the p101 sequences. The cis-acting sites involved in DNA replication and packaging also contained deletions in varying proportions (64% and 100%, respectively). Three mutations, present in 100% of p101 sequences, were identified in the overlapping genes encoding the Arg-rich protein (MDV002/079) and the virus-encoding telomerase (vTR). Varying proportions of 12 single nucleotide polymorphisms (SNPs) were identified within 11 open reading frames (ORFs) and 3 noncoding regions. This comparative sequencing study has provided a wealth of information regarding genetic changes which have occurred during the attenuation process and has indicated that serial passage results in the generation of mixed populations.