Duck enteritis virus pUL47, as a late structural protein localized in the nucleus, mainly depends on residues 40 to 50 and 768 to 777 and inhibits IFN-β signalling by interacting with STAT1

The precise function of alphaherpesvirus tegument proteins and their interactions during the viral life cycle

Alphaherpesvirus is a widespread pathogen that causes diverse diseases in humans and animals and can severely damage host health. Alphaherpesvirus particles comprise a DNA core, capsid, tegument and envelope; the tegument is located between the nuclear capsid and envelope. According to biochemical and proteomic analyses of alphaherpesvirus particles, the tegument contains at least 24 viral proteins and plays an important role in the alphaherpesvirus life cycle. This article reviews the important role of tegument proteins and their interactions during the viral life cycle to provide a reference and inspiration for understanding alphaherpesvirus infection pathogenesis and identifying new antiviral strategies.

Pathogenicity studies and molecular characterization of DPV infection in ducklings

In the spring of 2023, 10 to 21-day-old chicks in a broiler duck farm in Shandong Province, China, developed swelling of the head and neck, moist eyes with mucous discharge, difficulty in walking, shrinking of the neck, and loose and disorganized coat. Anatomical observation revealed hemorrhages in the esophageal mucosa, myocardium, and liver, and severe hemorrhages in the trachea with copious inflammatory secretions. Soon after, similar symptoms appeared in a large number of ducks in the flock, which eventually led to the elimination of all the 20,000-odd newly introduced ducklings on the farm, resulting in huge economic losses. We detected duck plague virus in the tissues of liver, spleen and lungs of diseased and dead ducks, and successfully isolated the pathogenic strain, named SD423, by inoculating duck embryos and inoculating duck embryo fibroblasts. We successfully conducted animal regression experiments with the isolated strain, and the experimental animals in the 1 d of age group showed symptoms of swollen eyes and tearing, shrinking of the neck, crouching, and hemorrhage in organs such as the liver and intestines successively from the 3rd d. We sequenced the whole genome of the isolated duck plague strain, and by comparing the homology with the published duck plague virus whole sequences in Genbank, the virus strain obtained in this study had the highest homology with the Chinese virulent strain SD (MN518864.1), with nucleotide (nt) homology of about 99.90% and amino acid (aa) homology of about 99.75%, which indicated that the isolate is a virulent strain. Previously, it was reported that the natural infection of duck plague virus mainly occurs above 30 d of age, but the duck plague virus found in this study can naturally infect ducklings up to 20 d of age, and the mortality rate is as high as 100%. In this study, the pathogenicity test and whole genome sequence analysis of this isolate provided data support and theoretical basis for further research on pathogenicity and virulence-related gene analysis of duck plague virus.

Effects of US3 protein kinase activity on localization of UL31/UL34 protein and nucleocapsids egress of duck plague virus

Duck plague virus (DPV) is a pathogen causing duck plague and has caused huge economic losses in poultry industry. In our previous report, US3 gene deletion from DPV genome seriously impaired virus replication. In this study, we constructed a US3 kinase-inactive mutant (US3K213A) to further explore the function of US3 protein (pUS3) in DPV. Our results showed that the loss of pUS3 kinase activity caused lower viral titers, smaller plaque sizes and a blockage of capsids nuclear egress including primary enveloped virion (PEV) accumulation compared to the parental virus infection. It indicates that the effects of DPV pUS3 on viral propagation depended on its kinase activity. In addition, we conducted electron microscopy analysis to show the outer nuclear membrane (ONM) evaginations and the nuclear envelope (NE) deep invagination in US3K213A-infected cells. Finally, an irregular distribution of pUL31/pUL34 in the NE in △US3- and US3K213A-infected cells and an interaction of pUS3 and pUL31 were found, which suggests that pUS3 potentially targets pUL31 and regulates the localization of pUL31/pUL34 to promote nucleocapsids egress through its kinase activity.

Deleting UL49.5 in duck plague virus causes attachment, entry and spread defects

Duck plague is a disease with high morbidity and mortality rates, and it causes great losses for the duck breeding industry. Duck plague virus (DPV) is the causative agent of duck plague, and DPV UL49.5 protein (pUL49.5) is homologue of glycoprotein N (gN), which is conserved in herpesviruses. UL49.5 homologues are known to be involved in processes such as immune escape, virus assembly, viral fusion, transporter associated with antigen processing (TAP) inhibition and degradation, and maturation and incorporation of glycoprotein M. However, few studies have focused on the role of gN in the early stage of virus infection cells. In this study, we determined that DPV pUL49.5 was distributed in the cytoplasm and colocalized with the endoplasmic reticulum (ER). Moreover, we found that DPV pUL49.5 was a virion component and nonglycosylated protein. To better explore its function, BAC-DPV-ΔUL49.5 was constructed, and its attachment was only approximately 25 % of the revertant virus. Additionally, the penetration ability of BAC-DPV-ΔUL49.5 has only reached 73 % of the revertant virus. The plaque sizes produced by the UL49.5-deleted virus were approximately 58 % smaller than those produced by the revertant virus. Deleting UL49.5 mainly resulted in attachment and cell-to-cell-spread defects. Taken together, these findings suggest important roles for DPV pUL49.5 in viral attachment, penetration and spread.

Characterization of a Unique Novel LORF3 Protein of Duck Plague Virus and Its Potential Pathogenesis

Duck plague virus (DPV) is a high-morbidity fowl alphaherpesvirus that causes septicemic lesions in various organs. Most DPV genes are conserved among herpesviruses, while a few are specific to fowl herpesviruses, including the LORF3 gene, for which there is currently no literature describing its biological properties and functions. This study first addressed whether the LORF3 protein is expressed by making specific polyclonal antibodies. We could demonstrate that DPV LORF3 is an early gene and encodes a protein involved in virion assembly, mainly localized in the nucleus of DPV-infected DEF cells. To investigate the role of this novel LORF3 protein in DPV pathogenesis, we generated a recombinant virus that lacks expression of the LORF3 protein. Our data revealed that the LORF3 protein is not essential for viral replication but contributes to DPV replication in vitro and in vivo and promotes duck plague disease morbidity and mortality. Interestingly, deletion of the LORF3 protein abolished thymus atrophy in DPV-vaccinated ducks. In conclusion, this study revealed the expression of avian herpesviruses-specific genes and unraveled the role of the early protein LORF3 in the pathogenesis of DPV. IMPORTANCE DPV is a highly lethal alphaherpesvirus that causes duck plague in birds of the order Anseriformes. The virus has caused huge economic losses to the poultry industry due to high morbidity and mortality and the cost of vaccination. DPV encodes 78 open reading frames (ORFs), and these genes are involved in various processes of the viral life cycle. Functional characterization of DPV genes is important for understanding the complex viral life cycle and DPV pathogenesis. Here, we identified a novel protein encoded by LORF3, and our data suggest that the LORF3 protein is involved in the occurrence and development of duck plague.

Duck plague virus US3 protein kinase phosphorylates UL47 and regulates the subcellular localization of UL47

Duck plague virus (DPV) belongs to the alphaherpesvirinae and causes high morbidity and mortality in waterfowl. UL47 is a large abundant structural protein in DPV, which means that UL47 protein plays an important role in virus replication. US3 protein, as a viral protein kinase in alphaherpesviruses, has been reported to be critical for DPV virion assembly. In this study, we over-expressed UL47 and US3 proteins and found that DPV UL47 protein was a phosphorylated substrate of US3 protein, which interacted and co-localized with US3 protein in the cytoplasm. US3-regulated phosphorylation of UL47 was important for the cytoplasmic localization of UL47 because non-phosphorylated UL47 was localized in the nucleus. The six sites of UL47 at Thr29, Ser30, Ser42, Thr47, Ser161, and Thr775 were identified as the phosphorylation targets of US3 protein. In vivo, UL47 phosphorylation was also detected but not in ΔUS3-infected cells. US3 protein promoted the cytoplasmic localization of UL47 at the late stage of infection, and the lack of US3 protein caused a delay in UL47 translocation to the cytoplasm. These results enhance our understanding of the functions of US3 during DPV infection and provide some references for DPV assembly.

Duck plague virus UL41 protein inhibits RIG-I/MDA5-mediated duck IFN-β production via mRNA degradation activity

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are cytosolic pattern recognition receptors that initiate innate antiviral immunity. Recent reports found that duck RLRs significantly restrict duck plague virus (DPV) infection. However, the molecular mechanism by which DPV evades immune responses is unknown. In this study, we first found that the DPV UL41 protein inhibited duck interferon-β (IFN-β) production mediated by RIG-I and melanoma differentiation-associated gene 5 (MDA5) by broadly downregulating the mRNA levels of important adaptor molecules, such as RIG-I, MDA5, mitochondrial antiviral signalling protein (MAVS), stimulator of interferon gene (STING), TANK-binding kinase 1 (TBK1), and interferon regulatory factor (IRF) 7. The conserved sites of the UL41 protein, E229, D231, and D232, were responsible for this activity. Furthermore, the DPV CHv-BAC-ΔUL41 mutant virus induced more duck IFN-β and IFN-stimulated genes (Mx, OASL) production in duck embryo fibroblasts (DEFs) than DPV CHv-BAC parent virus. Our findings provide insights into the molecular mechanism underlying DPV immune evasion.

The intracellular domain of duck plague virus glycoprotein E affects UL11 protein incorporation into viral particles

Studies have shown that proteins in the tegument (located between the viral capsid and envelope layer) play critical roles in the assembly and budding of herpesviruses. The UL11 protein of herpesviruses is important in the process of virus particle cell entry, release, assembly and secondary envelopment. Herpesvirus glycoprotein E (gE) is involved in syncytia formation, transmission between cells and nerve invasion. In herpes simplex virus, UL11 has been shown to interact with gE. However, little is known about the relationship of duck plague virus (DPV) pUL11 and gE. In this study, we constructed DPV cytoplasmic domain (CT)-gE, and extracellular domain (ET)-gE deletion mutants, pCMV-gE, CT-gE, and ET-gE and UL11 recombinant plasmids. We found that pUL11 can interact and colocalize with gE, CT-gE and ET-gE. Together, these results highlight an important role for UL11 in the function of gE, and may also have important implications for the role of pUL11 and gE.

Development of a Colloidal Gold Immunochromatographic Assay for Duck Enteritis Virus Detection Using Monoclonal Antibodies

Duck viral enteritis is a highly contagious and fatal disease of commercial waterfowl flocks. The disease occurs sporadically or epizootically in mainland China due to insufficient vaccinations. Early and rapid diagnosis is important for preventive intervention and the control of epizootic events in clinical settings. In this study, we generated two monoclonal antibodies (MAbs) that specifically recognized the duck enteritis virus (DEV) envelope glycoprotein B and tegument protein UL47, respectively. Using these MAbs, a colloidal gold-based immunochromatographic assay (ICA) was developed for the efficient detection of DEV antigens within 15 min. Our results showed that the detection limit of the developed ICA strip was 2.52 × 10³ TCID₅₀/mL for the virus infected cell culture suspension with no cross-reactivity with other pathogenic viruses commonly encountered in commercially raised waterfowl. Using samples from experimentally infected ducks, we demonstrated that the ICA detected the virus in cloacal swab samples on day three post-infection, demonstrating an 80% concordance with the PCR. For tissue homogenates from ducks succumbing to infection, the detection sensitivity was 100%. The efficient and specific detection by this ICA test provides a valuable, convenient, easy to use and rapid diagnostic tool for DVE under both laboratory and field conditions.

DPV UL41 gene encoding protein induces host shutoff activity and affects viral replication

The virion host shutoff (VHS) protein, encoded by the UL41 gene of herpes simplex virus (HSV), specifically degrades mRNA and induces host shutoff. VHS and its homologs are highly conserved in the Alphaherpesvirinae subfamily. However, the role of the duck plague virus (DPV) UL41 gene is unclear. In this study, we found that the DPV UL41 gene-encoded protein (pUL41) degrades RNA polymerase (pol) II-transcribed translatable RNA and induces protein synthesis shutoff. DPV pUL41 was dispensable for viral replication, but the UL41-deleted mutant virus exhibited a significant viral growth defect and plaque size reduction in Duck embryo fibroblast (DEF) cells. Furthermore, DPV pUL41 regulated viral mRNA accumulation to affect viral DNA replication, release and cell-to-cell spread.