Comparison of pseudorabies virus China reference strain with emerging variants reveals independent virus evolution within specific geographic regions

Establishment of pseudorabies virus latency and reactivation model in mice dorsal root ganglia culture

Pseudorabies virus (PRV) belongs to the alpha herpesvirus family and is responsible for Aujeszky's disease in pigs. Similar to other alpha herpesviruses, PRV establishes a lifelong latent infection in trigeminal ganglion. These latently infected pigs serve as a reservoir for recurrent infections when reactivation is triggered, making the eradication of PRV a challenging task. However, the molecular mechanism underlying PRV latency and reactivation in neurons is still poorly understood due to limitations in the in vitro model. To establish a pseudorabies virus latency and reactivation model in primary neuron cultures, we isolated dorsal root ganglion (DRG) from newborn Kunming mice using a method named epineurium-pulling for DRG collection (EPDC) and cultured primary neurons in vitro. A dual-colour recombinant PRV BAC mRuby-VP16 was constructed and 0.5 multiplicity of infection (MOI) was found as an appropriate dose in the presence of aciclovir to establish latency. Reactivation was induced using UV-inactivated herpesviruses or a series of chemical inhibitors. Interestingly, we found that not only UV-PRV, but also UV-HSV-1 and UV-BHoV-5 were able to induce rapid PRV reactivation. The efficiency of reactivation for LY294002, forskolin, etoposide, dexamethasone, and acetylcholine was found to be dependent on their concentration. In conclusion, we developed a valuable model of PRV latency and reactivation, which provides a basis for future mechanism research.

Recombinant Pseudorabies Virus Usage in Vaccine Development against Swine Infectious Disease

Pseudorabies virus (PRV) is the pathogen of pseudorabies (PR), which belongs to the alpha herpesvirus subfamily with a double stranded DNA genome encoding approximately 70 proteins. PRV has many non-essential regions for replication, has a strong capacity to accommodate foreign genes, and more areas for genetic modification. PRV is an ideal vaccine vector, and multivalent live virus-vectored vaccines can be developed using the gene-deleted PRV. The immune system continues to be stimulated by the gene-deleted PRVs and maintain a long immunity lasting more than 4 months. Here, we provide a brief overview of the biology of PRV, recombinant PRV construction methodology, the technology platform for efficiently constructing recombinant PRV, and the applications of recombinant PRV in vaccine development. This review summarizes the latest information on PRV usage in vaccine development against swine infectious diseases, and it offers novel perspectives for advancing preventive medicine through vaccinology.

Free ISG15 inhibits Pseudorabies virus infection by positively regulating type I IFN signaling

Interferon-stimulated gene 15 (ISG15) is strongly upregulated during viral infections and exerts pro-viral or antiviral actions. While many viruses combat host antiviral defenses by limiting ISG expression, PRV infection notably increases expression of ISG15. However, studies on the viral strategies to regulate ISG15-mediated antiviral responses are limited. Here, we demonstrate that PRV-induced free ISG15 and conjugated proteins accumulation require viral gene expression. Conjugation inhibition assays showed that ISG15 imposes its antiviral effects via unconjugated (free) ISG15 and restricts the viral release. Knockout of ISG15 in PK15 cells interferes with IFN-β production by blocking IRF3 activation and promotes PRV replication. Mechanistically, ISG15 facilitates IFNα-mediated antiviral activity against PRV by accelerating the activation and nuclear translocation of STAT1 and STAT2. Furthermore, ISG15 facilitated STAT1/STAT2/IRF9 (ISGF3) formation and ISGF3-induced IFN-stimulated response elements (ISRE) activity for efficient gene transcription by directly interacting with STAT2. Significantly, ISG15 knockout mice displayed enhanced susceptibility to PRV, as evidenced by increased mortality and viral loads, as well as more severe pathology caused by excessive production of the inflammatory cytokines. Our studies establish the importance of free ISG15 in IFNα-induced antiviral immunity and in the control of viral infections.

Serological Investigation and Genetic Characteristics of Pseudorabies Virus between 2019 and 2021 in Henan Province of China

In late 2011, severe pseudorabies (PR) outbreaks occurred among swine herds vaccinated with the Bartha-K61 vaccine in many provinces of China, causing enormous economic losses for the pork industry. To understand the epidemic profile and genetic characteristics of the pseudorabies virus (PRV), a total of 35,796 serum samples were collected from 1090 pig farms of different breeding scales between 2019 and 2021 in the Henan province where swine had been immunized with the Bartha-K61 vaccine, and PRV glycoprotein E (gE)-specific antibodies were detected using an enzyme-linked immunosorbent assay (ELISA). The results reveal that the overall positive rate for PRV gE antibodies was 20.33% (7276/35,796), which decreased from 25.00% (2596/10,385) in 2019 to 16.69% (2222/13,315) in 2021, demonstrating that PR still existed widely in pig herds in the Henan province but displayed a decreasing trend. Further analysis suggested that the PRV-seropositive rate may be associated with farm size, farm category, quarter, region and the cross-regional transportation of livestock. Moreover, the gE gene complete sequences of 18 PRV isolates were obtained, and they shared a high identity (97.1–100.0%) with reference strains at the nucleotide level. Interestingly, the phylogenetic analysis based on the gE complete sequences found that there were both classical strains and variant strains in pig herds. The deduced amino acid sequence analysis of the gE gene showed that there were unique amino acids in the classical strains, the variant strains and genotype Ⅱ strains. This study provides epidemiological data that could be useful in the prevention of pseudorabies in Henan, China, and this finding contributed to our understanding of the epidemiology and evolution of PRV.

Pseudorabies Virus: From Pathogenesis to Prevention Strategies

Pseudorabies (PR), also called Aujeszky’s disease (AD), is a highly infectious viral disease which is caused by pseudorabies virus (PRV). It has been nearly 200 years since the first PR case occurred. Currently, the virus can infect human beings and various mammals, including pigs, sheep, dogs, rabbits, rodents, cattle and cats, and among them, pigs are the only natural host of PRV infection. PRV is characterized by reproductive failure in pregnant sows, nervous disorders in newborn piglets, and respiratory distress in growing pigs, resulting in serious economic losses to the pig industry worldwide. Due to the extensive application of the attenuated vaccine containing the Bartha-K61 strain, PR was well controlled. With the variation of PRV strain, PR re-emerged and rapidly spread in some countries, especially China. Although researchers have been committed to the design of diagnostic methods and the development of vaccines in recent years, PR is still an important infectious disease and is widely prevalent in the global pig industry. In this review, we introduce the structural composition and life cycle of PRV virions and then discuss the latest findings on PRV pathogenesis, following the molecular characteristic of PRV and the summary of existing diagnosis methods. Subsequently, we also focus on the latest clinical progress in the prevention and control of PRV infection via the development of vaccines, traditional herbal medicines and novel small RNAs. Lastly, we provide an outlook on PRV eradication.

Phospho-proteomics identifies a critical role of ATF2 in pseudorabies virus replication

Pseudorabies virus (PRV), an etiological agent of pseudorabies in livestock, has negatively affected the porcine industry all over the world. Epithelial cells are reported as the first site of PRV infection. However, the role of host proteins and its related signaling pathways in PRV replication is largely unclear. In this study, we performed a quantitative phosphoproteomics screening on PRV-infected porcine kidney (PK-15) epithelial cells. Totally 5723 phosphopeptides, corresponding to 2180 proteins, were obtained, and the phosphorylated states of 810 proteins were significantly different in PRV-infected cells compared with mock-infected cells (P ​< ​0.05). GO and KEGG analysis revealed that these differentially expressed phosphorylated proteins were predominantly related to RNA transport and MAPK signaling pathways. Further functional studies of NF-κB, transcription activator factor-2 (ATF2), MAX and SOS genes in MAPK signaling pathway were analyzed using RNA interference (RNAi) knockdown. It showed that only ATF2-knockdown reduces both PRV titer and viral genome copy number. JNK pathway inhibition and CRISPR/Cas9 gene knockout showed that ATF2 was required for the effective replication of PRV, especially during the biogenesis of viral genome DNA. Subsequently, by overexpression of the ATF2 gene and point mutation of the amino acid positions 69/71 of ATF2, it was further demonstrated that the phosphorylation of ATF2 promoted PRV replication. These findings suggest that ATF2 may provide potential therapeutic target for inhibiting PRV infection.

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Phosphoproteomic profiling of PRV-infected PK-15 ​cells with iTRAQ-quantification.

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JNK pathway regulates ATF2 phosphorylation and PRV replication.

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Phosphorylation of ATF2 promotes PRV replication.

The Epidemiological Analysis of Pseudorabies Virus and Pathogenicity of the Variant Strain in Shandong Province

Pseudorabies (PR) is a disease that is seriously endangering the pig industry in China. To understand the current prevalence of pseudorabies virus (PRV) in Shandong Province, China, 19,292 serum samples were collected from 16 locations in Shandong from 2018 to 2020. The gE antibody was detected by enzyme-linked immunosorbent assay. Ninety-seven suspected cases of PRV infection were collected from sick pigs vaccinated with Bartha-K61 to isolate PRV. The results showed that the average positive rate of the PRV gE antibody decreased from 38.20% in 2018 to 18.12% in 2020, but there was a high positive rate in sows. The isolation rate of PRV was 13.40% (13/97), and four strains were purified through plaque assay (named PRV-SD1, PRV-SD2, PRV-SD3, and PRV-SD4). The homology and genetic evolution of four PRV strains based on gE, gC, gI, and TK genes were analyzed and showed that these four strains shared more than 99.0% nucleotide homology with the variant PRV XJ5 strain, and they clustered in the same sub-branch with the domestic variant PRV strains, including JS-2012 and XJ5. Furthermore, the pathogenicity of the isolated variant strain was assessed by intranasal infection of 16-week-old pigs with 1 mL PRV-SD1 strain. The results of the animal experiment demonstrated that the PRV-SD1–infected pigs exhibited obvious clinical symptoms as early as 2 days post inoculation (dpi), and all infected pigs died within 1 week. The severe hyperemia of meninges and swelling of lungs and tonsils were observed. Histopathology analysis showed the obvious lymphocytes necrosis of tonsils, interstitial pneumonia, and viral encephalitis. Many positive staining cells were observed in tonsils and brains through immunohistochemistry staining assay. Viral shedding in oropharyngeal and rectal swabs were detected at 2 dpi, reached a peak at 3 dpi, and then gradually decreased. The detection of viral loads in the tissues showed that tonsils had the highest virus titer, further proving it may be the target organ of variant PRV infection. In conclusion, variant PRV strains were still highly prevalent in Shandong Province, and they had a strong pathogenicity in pigs.

First isolation and molecular characterization of pseudorabies virus detected in Turkey

BACKGROUND

Pigs are the main host species for the pseudorabies virus. It causes fatal encephalitis in many species, including humans. This article aims to report the first clinical case of pseudorabies as well as isolation and molecular characterization of the virus from a hunting dog in Bursa province, Turkey.

METHODS AND RESULTS

The dog shows clinical signs including pruritus and neurological signs such as stumbling and inability to stand up compatible with pseudorabies. The virus isolates were obtained from the supernatant of fresh tissue samples from the cerebellum, cornu ammonis, spleen, salivary gland, conjunctival swab, serum, and PBMC samples. The glycoprotein C region is targeted for viral DNA amplification. Pseudorabies virus genome detected both in fresh tissues and supernatants of third passage on Vero cells. The number of PCR positive samples was dramatically increased after cell culture inoculations. Genome sequencing of strain Bursa-10303, which was isolated from a non-endemic area, identified it to belong to clade A.

CONCLUSIONS

This study confirms the possible presence of pseudorabies infection in the wildlife reservoirs in Turkey. Future studies may clarify the importance of the infection in Turkey region, where there is no prevalent pig production.

Evasion of I Interferon-Mediated Innate Immunity by Pseudorabies Virus

Type I interferon (IFN-I) mediated innate immunity serves as the first line of host defense against viral infection, ranging from IFN-I production upon viral detection, IFN-I triggered signaling pathway that induces antiviral gene transcription the antiviral effects of IFN-I induced gene products. During coevolution, herpesviruses have developed multiple countermeasures to inhibit the various steps involved to evade the IFN response. This mini-review focuses on the strategies used by the alphaherpesvirus Pseudorabies virus (PRV) to antagonize IFN-I mediated innate immunity, with a particular emphasis on the mechanisms inhibiting IFN-I induced gene transcription through the JAK-STAT pathway. The knowledge obtained from PRV enriches the current understanding of the alphaherpesviral immune evasion mechanisms and provides insight into the vaccine development for PRV control.

Isolation and Phylogenetic Analysis of Reemerging Pseudorabies Virus Within Pig Populations in Central China During 2012 to 2019

To understand the biological characteristics of the reemerging pseudorabies virus (PRV) strains, a total of 392 tissue samples were collected from diseased pigs during reemerging PR outbreaks between 2012 and 2019 on farms in central China where swine had been immunized with Bartha-K61 and 51 (13. 01%) were positive for the gE gene by PCR. Sixteen PRV strains were isolated and caused clinical symptoms and death in mice. Subsequently, gE, gC, gB, and gD complete genes were amplified from the 16 PRV isolates and sequenced. Phylogenetic analysis based on these four gene sequences shows that the 16 PRV isolates were more closely related to the Chinese PRV variants (after 2012) but genetically differed from early Chinese PRV isolates (before 2012). Sequence analysis reveals that PRV isolates exhibited amino acid insertions, substitutions, or deletions compared with early Chinese PRV isolates and European–American PRV strains. In addition, this is the first report that eight isolates (8/16) in this study harbor a unique amino acid substitution at position 280 (F to L) of the gC protein, and six isolates have an amino acid substitution at position 338 (A to V) of the gD protein compared with the Chinese PRV variants. The emulsion containing inactivated PRV NY isolate could provide complete protection against the NY isolate. This study might enrich our understanding of the evolution of reemerging PRV strains as well as pave the way for finding a model virus to develop a novel vaccine based on reemerging PRV strains.

Whole-Genome Sequence Analysis of Pseudorabies Virus Clinical Isolates from Pigs in China between 2012 and 2017 in China

Pseudorabies virus (PRV) is an economically significant swine infectious agent. A PRV outbreak took place in China in 2011 with novel virulent variants. Although the association of viral genomic variability with pathogenicity is not fully confirmed, the knowledge concerning PRV genomic diversity and evolution is still limited. Here, we sequenced 54 genomes of novel PRV variants isolated in China from 2012 to 2017. Phylogenetic analysis revealed that China strains and US/Europe strains were classified into two separate genotypes. PRV strains isolated from 2012 to 2017 in China are highly related to each other and genetically close to classic China strains such as Ea, Fa, and SC. RDP analysis revealed 23 recombination events within novel PRV variants, indicating that recombination contributes significantly to the viral evolution. The selection pressure analysis indicated that most ORFs were under evolutionary constraint, and 19 amino acid residue sites in 15 ORFs were identified under positive selection. Additionally, 37 unique mutations were identified in 19 ORFs, which distinguish the novel variants from classic strains. Overall, our study suggested that novel PRV variants might evolve from classical PRV strains through point mutation and recombination mechanisms.

Untargeted LC-MS based metabolomic profiling of iPAMs to investigate lipid metabolic pathways alternations induced by different Pseudorabies virus strains

Owing to viral recombination, interspecies transmission, and evolution, variant pseudorabies virus (PRV) strains exhibit different biological characteristics and pathogenicity. To improve the understanding of common and specific metabolic changes that occur upon infection by different PRV strains, we herein describe the comprehensive analysis of metabolites of PRV vaccine strain (Bartha K61), classical strain (EA) and variant strain (HNX) infection in immortalized porcine alveolar macrophage cells. Compared with uninfected cells, cells infected with Bartha K61, EA and HNX had 246, 225, and 272 differing metabolites, respectively. In the three types of PRV-strain-infected cells, lipids and lipid-like molecules accounted for over 50 % of the altered metabolites. As these viruses are enveloped, viral replication, assembly and release occur on cellular membranes primed through the manipulation of the host metabolism. We analyzed the potential relationship between virus replication and the virus-mediated host metabolism. Our study resulted in the first reconstruction of the major lipid metabolic pathways involved in PRV infection, including those of glycerophospholipids, sphingolipids, glycerolipids, and fatty acyls. In addition, the metabolic perturbations caused by different PRV strain infections are consistent across many species, however, our results also revealed many specific metabolic alterations during HNX infection, such as the enrichment of phosphatidylinositol and 15R-PGE2 methyl ester 15-acetate, and the diminishment of phosphatidylethanolamine, phosphatidic acid, and ceramides. These strain-specific altered metabolites may be linked to the unique biological characteristics and pathogenicity of the HNX strain.

Better immune efficacy triggered by the inactivated gI/gE-deleted pseudorabies virus with the additional insertion of gC gene in mice and weaned pigs

A new variant of pseudorabies virus (PRV) with high pathogenicity has been prevalent in many swineherds vaccinated with Bartha-K61 in China since 2011. Several gene-deleted vaccine candidates have been developed based on new emerging PRV variants. PRV-AH, a new emerging PRV strain from Anhui Province, was isolated in our laboratory in 2013. In the present study, rPRV-AH-gI^-/gE^- and rPRV-AH-gI^-/gE^-/gC⁺ were generated based on PRV-AH by homologous recombination. The growth kinetics of rPRV-AH-gI^-/gE^- and rPRV-AH-gI^-/gE^-/gC⁺ were similar to their parental strains. Compared with the commercial inactivated vaccine of Ea strain, the immune efficacy of the inactivated vaccine based on recombinant viruses was evaluated in mice and weaned pigs. The result showed that the level of neutralizing antibody in mice immunized with rPRV-AH-gI^-/gE^-/gC⁺ was higher compared with those immunized with rPRV-AH-gI^-/gE- at a dose of 10⁶ TCID₅₀ at 8 weeks post initial immunization (p < 0.0001). Among the groups immunized at a dose of 10⁵ TCID₅₀, the rPRV-AH-gI^-/gE^- group showed a survival rate of 37.5 %, while the rPRV-AH-gI^-/gE^-/gC⁺ group showed a protection rate of 87.5 % against the PRV-AH challenge. Besides, the rPRV-AH-gI^-/gE^- and rPRV-AH-gI^-/gE^-/gC⁺ group immunized at a dose of 10⁶ TCID₅₀ showed a survival rate of 100 %. Interestingly, compared with the commercial vaccine group, the group of 10⁵ TCID₅₀ rPRV-AH-gI^-/gE^-/gC⁺ showed a lower level of neutralizing antibodies (p < 0.0001) but the same protection rate in mice. Moreover, in the pig experiment, the level of neutralizing antibodies in the group vaccinated with inactivated rPRV-AH-gI^-/gE^-/gC⁺ was higher than any other groups at 8 weeks post initial immunization (p < 0.05). More importantly, the milder symptoms and pathological lesions occurred in pigs vaccinated with rPRV-AH-gI^-/gE^-/gC⁺ after challenge with 10⁶ TCID₅₀ PRV-AH, revealing that additional insertion of gC gene could enhance the protective efficacy in PRV gI/gE-deleted vaccine in pigs. Collectively, these above-mentioned findings suggested that the inactivated vaccine of rPRV-AH-gI^-/gE^-/gC⁺ had a better immune efficacy, which could be regarded as a promising inactivated vaccine candidate for PRV control.

Pseudorabies virus infection in hunting dogs in Oita, Japan: Report from a prefecture free from Aujeszky's disease in domestic pigs

We isolated two pseudorabies virus (PRV) isolates (designated OT-1 and OT-2) from two hunting dogs exhibiting neurological manifestations after eating the flesh of wild boar hunted in Oita prefecture, Kyushu Island, Japan. The isolates corresponded to a previously reported PRV (MY-1 strain) isolated from a hunting dog in neighboring Miyazaki prefecture, and it clustered into genotype II based on the glycoprotein C sequence. Our results suggest that this common PRV strain may have been maintained in wild boars on Kyushu Island even though domestic pigs in this area have attained an Aujeszky's disease-free status.

Implication of the Identification of an Earlier Pseudorabies Virus (PRV) Strain HLJ-2013 to the Evolution of Chinese PRVs

Pseudorabies viruses (PRVs) pose a great threat to the pig industry of many countries around the world. Human infections with PRV have also been reported occasionally in China. Therefore, understanding the epidemiology and evolution of PRVs is of great importance for disease control in the pig populations and humans as well. In this study, we isolated a PRV designated HLJ-2013 from PRV-positive samples that had been collected in Heilongjiang, China, in 2013. The full genome sequence of the virus was determined to be ∼143 kbp in length using high-throughput sequencing. The genomic sequence identities between this isolate and 21 other previous PRV isolates ranged from 92.4% (with Bartha) to 97.3% (with SC). Phylogenetic analysis based on the full-length genome sequences revealed that PRV HLJ-2013 clustered together with all the Chinese strains in one group belonging to Genotype II, but this virus occurred phylogenetically earlier than all the other Chinese PRV strains. Phylogenetic trees based on both protein-coding genes and non-coding regions revealed that HLJ-2013 probably obtained its genome sequences from three origins: a yet unknown parent virus, the European viruses, and the same ancestor of all Chinese PRVs. Recombination analysis showed that HLJ-2013-like virus possibly donated the main framework of the genome of the Chinese PRVs. HLJ-2013 exhibited cytopathic and growth characteristics similar to that of the Chinese PRV strains SC and HeN1, but its pathogenicity in mice was higher than that of SC and lower than that of HeN1. The identification of HLJ-2013 takes us one step closer to understanding the origin of PRVs in China and provides new knowledge about the evolution of PRVs worldwide.

Hsp90 is involved in pseudorabies virus virion assembly via stabilizing major capsid protein VP5

Many viruses utilize molecular chaperone heat shock protein 90 (Hsp90) for protein folding and stabilization, however, the role of Hsp90 in herpesvirus lifecycle is obscure. Here, we provide evidence that Hsp90 participates in pseudorabies virus (PRV) replication. Viral growth kinetics assays show that Hsp90 inhibitor geldanamycin (GA) abrogates PRV replication at the post-penetration step. Transmission electron microscopy demonstrates that dysfunction of Hsp90 diminishes the quantity of PRV nucleocapsids. Overexpression and knockdown of Hsp90 suggest that de novo Hsp90 is involved in PRV replication. Mechanismly, dysfunction of Hsp90 inhibits PRV major capsid protein VP5 expression. Co-immunoprecipitation and indirect immunofluorescence assays indicate that Hsp90 interacts with VP5. Interestingly, Hsp70, a collaborator of Hsp90, also interacts with VP5, but doesn't affect PRV growth. Finally, inhibition of Hsp90 results in PRV VP5 degradation in a proteasome-dependent manner. Collectively, our data suggest that Hsp90 contributes to PRV virion assembly and replication via stabilization of VP5.

Pseudorabies virus glycoprotein gE suppresses interferon-β production via CREB-binding protein degradation

Cyclic GMP-AMP synthase (cGAS) is a main sensor used to detect microbial DNA in the cytoplasm, which subsequently induces the production of interferon (IFN) via the cGAS/STING/IRF3 signaling pathway, leading to an antiviral response. However, some viruses have evolved multiple strategies to escape this process. Pseudorabies virus (PRV) is a double-stranded DNA virus belonging to the Alphaherpesvirinae subfamily, which can cause serious damage to the porcine industry. Many herpesvirus components have been reported to counteract IFN production, whereas little is known of PRV. In the present study, we found that PRV glycoprotein E (gE) was involved in counteracting cGAS/STING-mediated IFN production. Ectopic expression of gE decreased cGAS/STING-mediated IFN-β promoter activity and the level of mRNA expression. Moreover, gE targeted at or downstream of IRF3 was found to inhibit IFN-β production. However, gE did not affect the phosphorylation, dimerization and nuclear translocation of IRF3. Furthermore, gE is located on the nuclear membrane and could subsequently degrade CREB-binding protein (CBP). MG132, a proteasome inhibitor, decreased CBP degradation and restored the IFN-β production induced by gE. Finally, gE-deleted PRV induced a higher level of IFN-β production and reduced CBP degradation compared to wild-type PRV. Together, these results demonstrate that PRV gE can inhibit cGAS/STING-mediated IFN-β production by degrading CBP to interrupt the enhanced assembly of IRF3 and CBP.

Feedback inhibition of bovine herpesvirus 5 replication by dual-copy bhv5-miR-B10-3p

Bovine herpesvirus 5 (BoHV-5) is a pathogen of cattle responsible for fatal meningoencephalitis. Like alpha herpesvirus subfamily members, BoHV-5 also encodes microRNA in lytic infections of epithelial cells. BoHV-5-miR-B10 was the most abundant miRNA detected in a high-throughput sequencing study. Here, we evaluated the kinetics of miR-B10 expression after BoHV-5 productive infection by stem-loop real-time quantitative PCR. miR-B10 candidate target sites in the virus were predicted, and BoHV-5 UL39 was confirmed as a target gene by dual-luciferase assay with the design of an miR-B10 tough decoy (TuD). The UL39 gene encoding ribonucleotide reductase (RR) large subunit plays an important role in the early stage of BoHV-5 lytic infection. As BoHV-5-miR-B10 is located in internal and terminal repeat regions, we generated a TuD gene-integrated BoHV-5 strain, which effectively down-regulated miR-B10-3p. Strikingly, the suppression of miR-B10-3p significantly improved BoHV-5 replication. Taking these findings together, our study established an efficient method to deliver and express TuD RNA for viral miRNA suppression, and demonstrated that virus-encoded miRNA suppresses viral-genome biogenesis with a feedback mode, which might serve as a brake for viral replication. Herpesviruses infect humans and a variety of animals. Almost all herpesviruses can encode miRNAs, but the functions of these miRNAs remain to be elucidated. Most herpesvirus-encoded miRNA harbours dual copies, which is difficult to be deleted by current genetic modulation. Here, we developed an efficient method to deliver and express TuD RNA to efficiently suppress viral miRNA with multiple copies. Using this method, we demonstrated for the first time that viral miRNA feedback regulates viral replication by suppressing the expression of RR.

Construction and immunogenicity of a gE/gI/TK-deleted PRV based on porcine pseudorabies virus variant

Pseudorabies (PR) caused by re-emerging pseudorabies virus (PRV) variant has outbroken among PRV vaccine-immunized swine herds on many Chinese pig farms, with severe socioeconomic consequences since late 2011. Here, a gE/gI/TK-deleted recombinant virus (rPRV NY-gE^-/gI^-/TK^-) was constructed based on PRV NY strain from 2012 through homologous DNA recombination and gene-editing technology termed clustered regularly interspaced palindromic repeats (CRISPR)/associated (Cas9) system. The rPRV NY-gE^-/gI^-/TK^- strain showed similar growth kinetics to the parental PRV NY strain in vitro, and was safe for mice. Sixty mice were injected subcutaneously (s.c.) twice with 10^6.0 TCID₅₀ of rPRV NY-gE^-/gI^-/TK^- and DMEM, respectively, with two-week interval. The levels of PRV gB antibodies and neutralizing antibodies against PRV NY in mice immunized with rPRV NY-gE^-/gI^-/TK^- were higher than those in the DMEM control group. The number of T lymphocyte subclasses CD3⁺, CD4⁺ and CD8⁺ in rPRV NY-gE^-/gI^-/TK^--immunized mice was higher than that in DMEM-injected mice. After challenge with 10^6.0 TCID₅₀ PRV NY at 42 dpi, all rPRV NY-gE^-/gI^-/TK^--immunized mice survived without exhibiting any pathological lesions in different tissues and intranuclear eosinophilic inclusions of the brain, and the viral genomic copy numbers in various organs of mice were obviously lower than DMEM group. These results showed the rPRV NY-gE^-/gI^-/TK^- could be a promising next-generation vaccine to control now epidemic PR in China.

G2-quadruplex in the 3'UTR of IE180 regulates Pseudorabies virus replication by enhancing gene expression

RNA secondary structure elements in the mRNA 3'-untranslated regions (3'UTR) play important roles in post-transcriptional regulation. RNA structure elements in the viral RNA provide valuable model for studying diverse regulation mechanisms. Herpesvirus genomes are double-stranded DNA with GC-rich sequences, which can be transcribed into abundant GC-rich RNAs. It is valuable to explore the structures and function of those GC-rich RNAs. We identified a G₂-quadruplex-forming sequence named PQS18-1 in the 3'UTR of the unique immediate early gene of Pseudorabies virus (PRV), an important member of Alphaherpesvirinae subfamily. The RNA PQS18-1 was folded into parallel G-quadruplex structure, enhancing gene expression. Both non-G-quadruplex mutant and G₃-quadruplex mutant in the 3'UTR showed lower gene expression level than the wildtype G₂-quadruplex. TMPyP4 destroyed PQS18-1 G₂-quadruplex and suppressed gene expression, accordingly reducing PRV replication by one titre in the PK15 cells at 24 h post infection. Our findings indicated that the RNA G₂-quadruplex in 3'UTR was essential for high expression of IE180 gene, and it could be a specific post-transcription regulation element in response to small molecules or other macromolecules. This study discovers a novel RNA G₂-quadruplex in the 3'UTR of an immediate early gene of alphaherpesvirus and provides a new nucleic acid target for anti-virus drug design.

Highly Efficient Base Editing in Viral Genome Based on Bacterial Artificial Chromosome Using a Cas9-Cytidine Deaminase Fused Protein

Viruses evolve rapidly and continuously threaten animal health and economy, posing a great demand for rapid and efficient genome editing technologies to study virulence mechanism and develop effective vaccine. We present a highly efficient viral genome manipulation method using CRISPR-guided cytidine deaminase. We cloned pseudorabies virus genome into bacterial artificial chromosome, and used CRISPR-guided cytidine deaminase to directly convert cytidine (C) to uridine (U) to induce premature stop mutagenesis in viral genes. The editing efficiencies were 100%. Comprehensive bioinformatic analysis revealed that a large number of editable sites exist in pseudorabies virus (PRV) genomes. Notably, in our study viral genome exists as a plasmid in E. coli, suggesting that this method is virus species-independent. This application of base-editing provided an alternative approach to generate mutant virus and might accelerate study on virulence and vaccine development.

Genome Characteristics and Evolution of Pseudorabies Virus Strains in Eastern China from 2017 to 2019

Since late 2011, outbreaks of pseudorabies virus (PRV) have occurred in southern China causing major economic losses to the pig industry. We previously reported that variant PRV forms and recombination in China could be the source of continued epidemics. Here, we analyzed samples from intensive pig farms in eastern China between 2017 and 2019, and sequenced the main glycoproteins (gB, gC, gD, and gE) to study the evolution characteristics of PRV. Based on the gC gene, we found that PRV variants belong to clade 2 and detected a founder effect during by the PRV epidemic. In addition, we detected inter- and intra-clade recombination; in particular, inter-clade recombination in the gB genes of strains FJ-ZXF and FJ-W2, which were recombinant with clade 1 strains. We also found specific amino-acid changes and positively selected sites, possibly associated with functional changes. This analysis of the emergence of PRV in China illustrates the need for continuous monitoring and the development of vaccines against specific variants of PRV.

Antiviral Activity of Germacrone against Pseudorabies Virus in Vitro

Pseudorabies virus (PRV), a member of the Herpesviridae, is the causative agent of an acute infectious disease in a variety of animals. The emergence of a novel variant strain brought huge economic losses to the pig industry since classical vaccine strains were not completely effective against variant strains. Therefore, the development of new anti-pseudorabies virus drugs and vaccines is of great significance for the treatment and prevention of pseudorabies. In this study, we found that germacrone, one of the major components of the essential oils extracted from Rhizoma Curcuma, was able to effectively inhibit PRV replication in a dose-dependent manner in vitro. Germacrone showed antiviral activity against PRV in the early phase of the viral replication cycle. Moreover, we found that germacrone does not directly kill the virus, nor does it affect the expression of the PRV receptor protein nectin-1, nectin-2, and CD155. Our results suggest germacrone could be used as an efficient microbicide or immunomodulatory agent in the control of the emerging variant PRV.

Isolation and molecular characterization of a variant of Chinese gC-genotype II pseudorabies virus from a hunting dog infected by biting a wild boar in Japan and its pathogenicity in a mouse model

We isolated a variant of Chinese pseudorabies virus from a hunting dog with symptoms similar to Aujeszky's disease and designated the isolate MY-1 strain. The dog developed symptoms 6 days after hunting and biting a wild boar and died the day after onset. The Bam HI restriction profile of MY-1 DNA was different from those of the Japanese reference strain Yamagata-S81 and two vaccine strains, Bartha and Begonia, and resembled Bam HI-RFLP (restriction fragment length polymorphism) type IV. Complete nucleotide sequences were determined, and phylogenetic analyses revealed that MY-1 belonged to the same cluster of old Chinese strains and variant strains isolated recently in China, but most of the open reading frames of MY-1 were located on a different branch from those of these Chinese strains. Based on a gC phylogenetic analysis, MY-1 belonged to gC-genotype II composed of those Chinese strains. In mice, the 50% lethal dose (LD₅₀) of MY-1 (10^3.0 TCID₅₀) was almost the same as those of Yamagata-S81 and Bartha. The LD₅₀ value of Begonia was 10^≥4.5 TCID₅₀. The mean survival periods of mice after infection with 10⁴ TCID₅₀ of MY-1, Yamagata-S81 and Bartha were 3.9 days, 2.3 days, and 8.0 days, respectively. The results suggested that the variant of Chinese PRV with slightly weaker pathogenicity than that of wild virulent viruses might be maintained in wild boars in Japan. Furthermore, we would like to propose that old Chinese strains, recent Chinese variant strains, and MY-1 should be grouped as an Asian type PRV.

The Carboxyl Terminus of Tegument Protein pUL21 Contributes to Pseudorabies Virus Neuroinvasion

Following its entry into cells, pseudorabies virus (PRV) utilizes microtubules to deliver its nucleocapsid to the nucleus. Previous studies have shown that PRV VP1/2 is an effector of dynein-mediated capsid transport. However, the mechanism of PRV for recruiting microtubule motor proteins for successful neuroinvasion and neurovirulence is not well understood. Here, we provide evidence that PRV pUL21 is an inner tegument protein. We tested its interaction with the cytoplasmic light chains using a bimolecular fluorescence complementation (BiFC) assay and observed that PRV pUL21 interacts with Roadblock-1. This interaction was confirmed by coimmunoprecipitation (co-IP) assays. We also determined the efficiency of retrograde and anterograde axonal transport of PRV strains in explanted neurons using a microfluidic chamber system and investigated pUL21's contribution to PRV neuroinvasion in vivo Further data showed that the carboxyl terminus of pUL21 is essential for its interaction with Roadblock-1, and this domain contributes to PRV retrograde axonal transport in vitro and in vivo Our findings suggest that the carboxyl terminus of pUL21 contributes to PRV neuroinvasion.IMPORTANCE Herpesviruses are a group of DNA viruses that infect both humans and animals. Alphaherpesviruses are distinguished by their ability to establish latent infection in peripheral neurons. After entering neurons, the herpesvirus capsid interacts with cellular motor proteins and undergoes retrograde transport on axon microtubules. This elaborate process is vital to the herpesvirus lifecycle, but the underlying mechanism remains poorly understood. Here, we determined that pUL21 is an inner tegument protein of pseudorabies virus (PRV) and that it interacts with the cytoplasmic dynein light chain Roadblock-1. We also observed that pUL21 promotes retrograde transport of PRV in neuronal cells. Furthermore, our findings confirm that pUL21 contributes to PRV neuroinvasion in vivo Importantly, the carboxyl terminus of pUL21 is responsible for interaction with Roadblock-1, and this domain contributes to PRV neuroinvasion. This study offers fresh insights into alphaherpesvirus neuroinvasion and the interaction between virus and host during PRV infection.

Intensive Distribution of G₂-Quaduplexes in the Pseudorabies Virus Genome and Their Sensitivity to Cations and G-Quadruplex Ligands

Guanine-rich sequences in the genomes of herpesviruses can fold into G-quadruplexes. Compared with the widely-studied G₃-quadruplexes, the dynamic G₂-quadruplexes are more sensitive to the cell microenvironment, but they attract less attention. Pseudorabies virus (PRV) is the model species for the study of the latency and reactivation of herpesvirus in the nervous system. A total of 1722 G₂-PQSs and 205 G₃-PQSs without overlap were identified in the PRV genome. Twelve G₂-PQSs from the CDS region exhibited high conservation in the genomes of the Varicellovirus genus. Eleven G₂-PQSs were 100% conserved in the repeated region of the annotated PRV genomes. There were 212 non-redundant G₂-PQSs in the 3′ UTR and 19 non-redundant G₂-PQSs in the 5′ UTR, which would mediate gene expression in the post-transcription and translation processes. The majority of examined G₂-PQSs formed parallel structures and exhibited different sensitivities to cations and small molecules in vitro. Two G₂-PQSs, respectively, from 3′ UTR of UL5 (encoding helicase motif) and UL9 (encoding sequence-specific ori-binding protein) exhibited diverse regulatory activities with/without specific ligands in vivo. The G-quadruplex ligand, NMM, exhibited a potential for reducing the virulence of the PRV Ea strain. The systematic analysis of the distribution of G₂-PQSs in the PRV genomes could guide further studies of the G-quadruplexes’ functions in the life cycle of herpesviruses.

Ivermectin inhibits DNA polymerase UL42 of pseudorabies virus entrance into the nucleus and proliferation of the virus in vitro and vivo

Pseudorabies virus (PRV) is an important viral pathogen of pigs that causes huge losses in pig herds worldwide. Ivermectin is a specific inhibitor of importin-α/β-dependent nuclear transport and shows antiviral potential against several RNA viruses by blocking the nuclear localization of viral proteins. Since the replication of DNA viruses is in the nucleus, ivermectin may be functional against DNA virus infections if the DNA polymerase or other important viral proteins enter the nucleus via the importin-α/β-mediated pathway. Here, we determined whether ivermectin suppresses PRV replication in hamster kidney BHK-21 cells and investigated the effect of ivermectin on the subcellular localization of the PRV UL42 protein, the accessory subunit of PRV DNA polymerase. Also, an in vivo anti-PRV assay was conducted in mice. Our data demonstrate that ivermectin treatment inhibits PRV infection in cells in a dose-dependent manner. Treatment of PRV-infected cells with ivermectin significantly suppressed viral DNA synthesis and progeny virus production. Ivermectin disrupted the nuclear localization of UL42 by targeting the nuclear localization signal of the protein in transfected cells. Ivermectin treatment increased the survival rates of mice infected with PRV and relieved infection as indicated by lower clinical scores and fewer gross lesions in the brain. Together, our results suggest that ivermectin may be a therapeutic or preventative agent against PRV infection.

Establishment of an Efficient and Flexible Genetic Manipulation Platform Based on a Fosmid Library for Rapid Generation of Recombinant Pseudorabies Virus

Conventional genetic engineering of pseudorabies virus (PRV) is essentially based on homologous recombination or bacterial artificial chromosome. However, these techniques require multiple plaque purification, which is labor-intensive and time-consuming. The aim of the present study was to develop an efficient, direct, and flexible genetic manipulation platform for PRV. To this end, the PRV genomic DNA was extracted from purified PRV virions and sheared into approximately 30–45-kb DNA fragments. After end-blunting and phosphorylation, the DNA fragments were separated by pulsed-field gel electrophoresis, the recovered DNA fragments were inserted into the cloning-ready fosmids. The fosmids were then transformed into Escherichia coli and selected clones were end-sequenced for full-length genome assembly. Overlapping fosmid combinations that cover the complete genome of PRV were directly transfected into Vero cells and PRV was rescued. The morphology and one-step growth curve of the rescued virus were indistinguishable from those of the parent virus. Based on this system, a recombinant PRV expressing enhanced green fluorescent protein fused with the VP26 gene was generated within 2 weeks, and this recombinant virus can be used to observe the capsid transport in axons. The new genetic manipulation platform developed in the present study is an efficient, flexible, and stable method for the study of the PRV life cycle and development of novel vaccines.

Phylogenetic and recombination analysis of the herpesvirus genus varicellovirus

BACKGROUND

The varicelloviruses comprise a genus within the alphaherpesvirus subfamily, and infect both humans and other mammals. Recently, next-generation sequencing has been used to generate genomic sequences of several members of the Varicellovirus genus. Here, currently available varicellovirus genomic sequences were used for phylogenetic, recombination, and genetic distance analysis.

RESULTS

A phylogenetic network including genomic sequences of individual species, was generated and suggested a potential restriction between the ungulate and non-ungulate viruses. Intraspecies genetic distances were higher in the ungulate viruses (pseudorabies virus (SuHV-1) 1.65%, bovine herpes virus type 1 (BHV-1) 0.81%, equine herpes virus type 1 (EHV-1) 0.79%, equine herpes virus type 4 (EHV-4) 0.16%) than non-ungulate viruses (feline herpes virus type 1 (FHV-1) 0.0089%, canine herpes virus type 1 (CHV-1) 0.005%, varicella-zoster virus (VZV) 0.136%). The G + C content of the ungulate viruses was also higher (SuHV-1 73.6%, BHV-1 72.6%, EHV-1 56.6%, EHV-4 50.5%) compared to the non-ungulate viruses (FHV-1 45.8%, CHV-1 31.6%, VZV 45.8%), which suggests a possible link between G + C content and intraspecies genetic diversity. Varicellovirus clade nomenclature is variable across different species, and we propose a standardization based on genomic genetic distance. A recent study reported no recombination between sequenced FHV-1 strains, however in the present study, both splitstree, bootscan, and PHI analysis indicated recombination. We also found that the recently sequenced Brazilian CHV-1 strain BTU-1 may contain a genetic signal in the UL50 gene from an unknown varicellovirus.

CONCLUSION

Together, the data contribute to a greater understanding of varicellovirus genomics, and we also suggest a new clade nomenclature scheme based on genetic distances.