A deletion in the glycoprotein L (gL) gene of U.S. Marek's disease virus (MDV) field strains is insufficient to confer increased pathogenicity to the bacterial artificial chromosome (BAC)-based strain, RB-1B

Genetic characterization of a Marek's disease virus strain isolated in Japan

Background

Marek’s disease virus (MDV) causes malignant lymphomas in chickens (Marek’s disease, MD). MD is currently controlled by vaccination; however, MDV strains have a tendency to develop increased virulence. Distinct diversity and point mutations are present in the Meq proteins, the oncoproteins of MDV, suggesting that changes in protein function induced by amino acid substitutions might affect MDV virulence. We previously reported that recent MDV isolates in Japan display distinct mutations in Meq proteins from those observed in traditional MDV isolates in Japan, but similar to those in MDV strains isolated from other countries.

Methods

To further investigate the genetic characteristics in Japanese field strains, we sequenced the whole genome of an MDV strain that was successfully isolated from a chicken with MD in Japan. A phylogenetic analysis of the meq gene was also performed.

Results

Phylogenetic analysis revealed that the Meq proteins in most of the Japanese isolates were similar to those of Chinese and European strains, and the genomic sequence of the Japanese strain was classified into the Eurasian cluster. Comparison of coding region sequences among the Japanese strain and MDV strains from other countries revealed that the genetic characteristics of the Japanese strain were similar to those of Chinese and European strains.

Conclusions

The MDV strains distributed in Asian and European countries including Japan seem to be genetically closer to each other than to MDV strains from North America. These findings indicate that the genetic diversities of MDV strains that emerged may have been dependent on the different vaccination-based control approaches.

Newly detected mutations in the Meq oncogene and molecular pathotyping of very virulent Marek's disease herpesvirus in Tunisia

Marek's disease (MD) is a contagious avian viral disease that is responsible for large economic losses to farmers. The disease is caused by Marek's disease virus (species Gallid alphaherpesvirus 2), which causes neurological lesions, immune suppression, and tumor proliferation of lymphoid cells that invade a large number of organs and tissues. Despite widespread vaccination, Marek's disease virus (MDV), has shown a continuous increase in its virulence and has acquired the ability to overcome immune responses induced by vaccines. In the present study, the oncogenic serotype MDV-1 was detected by real-time PCR in DNA samples extracted from organs developing tumor infiltrations. Identification of the pathotype based on a 132-bp tandem repeat and sequencing and phylogenetic analysis of the Meq gene and its encoded protein allowed classification of the isolated viruses as "very virulent", with two new and unique mutations in the Meq gene resulting in amino acid substitutions. Sequencing of pp38, vIl-8, UL1 and UL44 genes did not reveal any new mutations that were characteristic of the Tunisian isolates or correlated with virulence. These results raised concerns about the ability of HVT and CVI988 vaccines, which are currently used in Tunisia and other countries, to protect chickens against highly virulent virus strains.

Transcriptional Analyses of Innate and Acquired Immune Patterning Elicited by Marek's Disease Virus Vaccine Strains: Turkey Herpesvirus (HVT), Marek's Disease Virus 2 (strain SB1), and Bivalent Vaccines (HVT/SB1 and HVT-LT/SB1)

Marek's disease (MD) is a complex pathology of chickens caused by MD virus (MDV) 1 and is observed as paralysis, immune suppression, neurologic signs, and the rapid formation of T-cell lymphomas. The incidence of MD in commercial broilers is largely controlled via vaccination, either in ovo or at hatch with live attenuated vaccines, i.e., turkey herpesvirus (HVT) or a bivalent combination of HVT with the MDV 2 strain (SB1). To further extend the protection conferred by bivalent HVT/SB-1, recombinant HVTs encoding transgenes of other avian viruses have similarly been used for in ovo administration. Despite decades of use, the specific mechanisms associated with vaccine-induced protection remain obscure. Additionally, the mechanistic basis for vaccine synergism conferred by bivalent HVT/SB-1, compared with HVT or SB-1 administered alone, is largely unknown. In the present study, we report on temporal changes in innate and acquired immune-patterning gene expression by using ex vivo splenocyte infection and in ovo vaccination models. We report that in the ex vivo splenocyte infection model, by 72 hr postinfection, vaccines induced IFN and IFN-stimulated gene expression, with lesser proinflammatory cytokine induction. For several genes (TLR3, IFN-γ, OASL, Mx1, NOS2A, and IL-1β), the effects on gene expression were additive for HVT, SB1, and HVT/SB1 infection. We observed similar patterns of induction in in ovo-vaccinated commercial broiler embryos and chicks with HVT/SB-1 or recombinant HVT-based bivalent combination (HVT-LT/SB-1). Furthermore, HVT/SB-1 or HVT-LT/SB-1 in ovo vaccination appeared to hasten immune maturation, with expression patterns suggesting accelerated migration of T and natural killer cells into the spleen. Finally, HVT/SB-1 vaccination resulted in a coordinated induction of IL-12p40 and downregulation of suppressors of cytokine signaling 1 and 3, indicative of classical macrophage 1 and T-helper 1 patterning.

Induction of the unfolded protein response (UPR) during Marek's disease virus (MDV) infection

Marek's disease (MD) is a pathology of chickens associated with paralysis, immune suppression, and the rapid formation of T-cell lymphomas. MD is caused by the herpesvirus, Marek's disease virus (MDV). We examined endoplasmic reticulum (ER) stress and the activation of unfolded protein response (UPR) pathways during MDV infection of cells in culture and lymphocytes in vivo. MDV strains activate the UPR as measured by increased mRNA expression of GRP78/BiP with concomitant XBP1 splicing and induction of its target gene, EDEM1. Cell culture replication of virulent, but not vaccine MDVs, activated the UPR at late in infection. Pathotype-associated UPR activation was induced to a greater level by a vv + MDV. Discrete UPR activation was observed during MDV in vivo infection, with the level of UPR modulation being affected by the MDV oncoprotein Meq. Finally, ATF6 was found to be activated in vv + MDV-induced primary lymphomas, suggesting a possible role in tumor progression.

DNA from Dust: Comparative Genomics of Large DNA Viruses in Field Surveillance Samples

The intensification of the poultry industry over the last 60 years facilitated the evolution of increased virulence and vaccine breaks in Marek's disease virus (MDV-1). Full-genome sequences are essential for understanding why and how this evolution occurred, but what is known about genome-wide variation in MDV comes from laboratory culture. To rectify this, we developed methods for obtaining high-quality genome sequences directly from field samples without the need for sequence-based enrichment strategies prior to sequencing. We applied this to the first characterization of MDV-1 genomes from the field, without prior culture. These viruses were collected from vaccinated hosts that acquired naturally circulating field strains of MDV-1, in the absence of a disease outbreak. This reflects the current issue afflicting the poultry industry, where virulent field strains continue to circulate despite vaccination and can remain undetected due to the lack of overt disease symptoms. We found that viral genomes from adjacent field sites had high levels of overall DNA identity, and despite strong evidence of purifying selection, had coding variations in proteins associated with virulence and manipulation of host immunity. Our methods empower ecological field surveillance, make it possible to determine the basis of viral virulence and vaccine breaks, and can be used to obtain full genomes from clinical samples of other large DNA viruses, known and unknown. IMPORTANCE Despite both clinical and laboratory data that show increased virulence in field isolates of MDV-1 over the last half century, we do not yet understand the genetic basis of its pathogenicity. Our knowledge of genome-wide variation between strains of this virus comes exclusively from isolates that have been cultured in the laboratory. MDV-1 isolates tend to lose virulence during repeated cycles of replication in the laboratory, raising concerns about the ability of cultured isolates to accurately reflect virus in the field. The ability to directly sequence and compare field isolates of this virus is critical to understanding the genetic basis of rising virulence in the wild. Our approaches remove the prior requirement for cell culture and allow direct measurement of viral genomic variation within and between hosts, over time, and during adaptation to changing conditions.