Latest Insights into Unique Open Reading Frames Encoded by Unique Long (UL) and Short (US) Regions of Marek's Disease Virus

Marek's disease virus-1 unique gene LORF1 is involved in viral replication and MDV-1/Md5-induced atrophy of the bursa of Fabricius

Marek's disease virus (MDV), an alphaherpesvirus, causes severe immunosuppression and T cell lymphomas in chickens, known as Marek's disease (MD), an economically important poultry disease primarily controlled by vaccination. Importantly, it also serves as a comparative model for studying herpesvirus-induced tumor formation in humans. MDV encodes more than 100 genes, most of which have unknown functions. MDV LORF1 is unique to serotype I MDV (MDV-1), lacking homologs in other herpesviruses, and has not been explored yet. To this end, an infectious bacterial artificial chromosome (BAC) harboring the complete genome of the MDV-1 very virulent strain Md5 was generated, and the rescued rMd5 maintained biological properties similar to the parental virus both in vitro and in vivo. Subsequently, rMd5ΔLORF1, a recombinant Md5 virus deficient in pLORF1 expression, was generated by a frameshift mutation in the LORF1 gene. Chickens infected with rMd5ΔLORF1 exhibited a lower mortality rate and delayed bursal atrophy than those infected with the parental rMd5 and the revertant virus (rMd5-reLORF1). Consistently, viral loads of rMd5ΔLORF1 were obviously lower than those of rMd5 or rMd5-reLORF1 in the bursa, but not in the spleen. Importantly, we found that pLORF1 deficiency impairs viral replication in bursal B cells. Furthermore, we showed that pLORF1 associated with the cellular membrane, interacted with MDV structural proteins, and exhibited punctate colocalization with tegument or capsid proteins in the cytoplasm. Taken together, this study demonstrates for the first time that the MDV-1 unique gene LORF1 is involved in MDV-induced bursal atrophy but not in tumor formation.

MDV-encoded protein kinase US3 phosphorylates WTAP to inhibit transcriptomic m6A modification and cellular protein translation

Marek's disease virus (MDV)-encoded US3 is a highly conserved serine/threonine protein kinase in alpha-herpesviruses. In other alpha-herpesviruses, such as pseudorabies virus (PRV), US3 phosphorylates the N6-methyladenosine (m6A) methyltransferase Wilms tumor 1-associated protein (WTAP), inhibiting m6A modification. However, the role and mechanism of US3-mediated WTAP phosphorylation during MDV infection remain undefined. Our study revealed that MDV infection in vitro does not alter WTAP expression, while significant changes in WTAP expression occur during the MDV life cycle in vivo. We demonstrated that MDV-encoded US3 interacts with and co-localizes with WTAP in the nucleus. Further analysis showed that US3 binds to WTAP's C-terminal domain and phosphorylates WTAP at S273, S305, S314, and S375. Notably, the interaction between US3 and WTAP does not affect WTAP stability but inhibits transcriptomic m6A modification and cellular protein translation. Therefore, these findings enhance our understanding of the molecular mechanisms underlying MDV infection.

Duck enteritis virus LORF4 gene is a late gene and nonessential for virus replication in vitro

Duck enteritis virus (DEV) is an avian alpha-herpesvirus that primarily causes an acute and highly contagious infectious disease of ducks. The LORF4 gene is one of the specific genes of DEV, with limited reports on its biological characteristics and functions. This study investigates the basic biological properties of LORF4 protein (pLORF4). The results show that DEV LORF4 is a late gene mainly localized in the cytoplasm of DEV-infected DEF. To explore the role of pLORF4 in the DEV replication life cycle, a recombinant virus lacking pLORF4 expression was constructed. The results showed that pLORF4 is not essential for virus replication and does not affect virus adsorption, assembly and release, it plays a positive role in virus invasion and DNA replication. In summary, this study provides a foundation for further research on the function of the LORF4 gene.

LORF9 of Marek's disease virus is involved in the early cytolytic replication of B lymphocytes and can act as a target for gene deletion vaccine development

IMPORTANCE

Marek's disease virus (MDV) is a highly infectious and oncogenic virus that can induce severe T cell lymphomas in chickens. MDV encodes more than 100 genes, most of which have unknown functions. This work indicated that the LORF9 gene is necessary for MDV early cytolytic replication in B lymphocytes. In addition, we have found that the LORF9 deletion mutant has a comparative immunological protective effect with CVI988/Rispens vaccine strain against very virulent MDV challenge. This is a significant discovery that LORF9 can be exploited as a possible target for the development of an MDV gene deletion vaccine.

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.

Complete genome analysis reveals evolutionary history and temporal dynamics of Marek’s disease virus

Marek’s disease has caused enormous losses in poultry production worldwide. However, the evolutionary process and molecular mechanisms underlying Marek’s disease virus (MDV) remain largely unknown. Using complete genomic sequences spanning an unprecedented diversity of MDVs, we explored the evolutionary history and major patterns in viruses sampled from 1964 to 2018. We found that the evolution of MDV strains had obvious geographical features, with the Eurasian and North American strains having independent evolutionary paths, especially for Asian strains. The evolution of MDVs generally followed a clock-like structure with a relatively high evolutionary rate. Asian strains had evolved at a faster rate than European strains, with most genetic mutations occurring in Asian strains. Our results showed that all recombination events occurred in the UL and US subregions. We found direct evidence of a closer correlation between Eurasian strains, related to a series of reorganization events represented by the European strain ATE2539. We also discovered that the vaccine strains had recombined with the wild virulent strains. Base substitution and recombination were found to be the two main mechanisms of MDV evolution. Our study offers novel insights into the evolution of MDVs that could facilitate predicting the spread of infections, and hence their control.

Genetic characterization of Marek's disease virus in chickens in Thailand reveals a high genetic diversity of circulating strains

Marek's disease (MD) is a highly contagious lymphoproliferative disease of chickens caused by Gallid alphaherpesvirus 2, commonly known as serotype 1 Marek's disease virus (MDV-1). Despite widespread vaccination, MD-related cases have been frequently observed worldwide, including in Thailand. However, no information is available on the genetic characteristics of MDV-1 field strains circulating in chickens in Thailand. This study investigated the geographic distribution and genetic characteristics of MDV-1 field strains circulating in chickens in Thailand between 2013 and 2021 by analysing the Meq and pp38 genes. Out of a total of the 286 clinical samples obtained from 70 chicken farms located in major chicken raising areas of Thailand, 138 samples (48.25%) from 46 chicken farms (65.71%) tested positive for MDV-1 field strains. Results demonstrated that MDV-1 field strains were extensively distributed in major chicken raising areas. Phylogenetic analyses based on the Meq gene revealed that four clusters of MDV-1 circulated in chickens in Thailand between 2013 and 2021. Among these clusters, cluster 1 was the predominant cluster circulating in chickens in Thailand. Additionally, our findings based on molecular characteristics of Meq and pp38 gene/protein suggested that most of the Thai MDV-1 field strains were potentially highly virulent. In conclusion, our data demonstrated the circulation of different clusters of MDV-1 with virulence characteristics in chickens in Thailand, indicating high genetic diversity of MDV-1 in Thailand. This study highlights the importance of more effective vaccine development and routine MDV-1 surveillance for early detection and control of highly virulent MDV-1.