Identification of functional domains of the IR2 protein of equine herpesvirus 1 required for inhibition of viral gene expression and replication

Interferon Gamma Inhibits Equine Herpesvirus 1 Replication in a Cell Line-Dependent Manner

The sole equine herpesvirus 1 (EHV-1) immediate-early protein (IEP) is essential for viral replication by transactivating viral immediate-early (IE), early (E), and late (L) genes. Here, we report that treatment of mouse MH-S, equine NBL6, and human MRC-5 cells with 20 ng/mL of IFN-γ reduced EHV-1 yield by 1122-, 631-, and 10,000-fold, respectively. However, IFN-γ reduced virus yield by only 2–4-fold in mouse MLE12, mouse L-M, and human MeWo cells compared to those of untreated cells. In luciferase assays with the promoter of the EHV-1 early regulatory EICP0 gene, IFN-γ abrogated trans-activation activity of the IEP by 96% in MH-S cells, but only by 21% in L-M cells. Similar results were obtained in assays with the early regulatory UL5 and IR4 promoter reporter plasmids. IFN-γ treatment reduced IEP protein expression by greater than 99% in MH-S cells, but only by 43% in L-M cells. The expression of IEP and UL5P suppressed by IFN-γ was restored by JAK inhibitor treatment, indicating that the inhibition of EHV-1 replication is mediated by JAK/STAT1 signaling. These results suggest that IFN-γ blocks EHV-1 replication by inhibiting the production of the IEP in a cell line-dependent manner. Affymetrix microarray analyses of IFN-γ-treated MH-S and L-M cells revealed that five antiviral ISGs (MX1, SAMHD1, IFIT2, NAMPT, TREX1, and DDX60) were upregulated 3.2–18.1-fold only in MH-S cells.

Intranasal treatment with CpG-B oligodeoxynucleotides protects CBA mice from lethal equine herpesvirus 1 challenge by an innate immune response

Equine herpesvirus 1 (EHV-1) is the causative agent of a number of equine disease manifestations, including severe disease of the central nervous system, respiratory infections, and abortion storms. Our results showed that intranasal treatment with CpG-B oligodeoxynucleotides (ODN 1826) protected CBA mice from pathogenic EHV-1 RacL11 challenge. The IFN-γ gene and seven interferon-stimulated genes (ISGs) were upregulated 39.4- to 260.3-fold at 8 h postchallenge in the lungs of RacL11-challenged mice that had been treated with CpG-B ODN. Interestingly, IFN-γ gene expression was upregulated by 26-fold upon RacL11 challenge in CpG-B ODN-treated mice lungs as compared to that of CpG-A ODN (ODN 1585)-treated mice lungs; however, the seven ISGs were upregulated by 2.4-5.0-fold, suggesting that IFN-γ is a major factor in the protection of CBA mice from the lethal challenge. Pre-treatment with IFN-γ significantly reduced EHV-1 yield in murine alveolar macrophage MH-S cells, but not in mouse lung epithelial MLE12 cells. These results suggest that CpG-B ODN may be used as a prophylactic agent in horses and provide a basis for more effective treatment of EHV-1 infection.

Development and characterization of ORF68 negative equine herpes virus type-1, Ab4p strain

Equine herpesvirus-1 (EHV-1) is an important pathogen, which infects horses worldwide with high morbidity but low mortality rates. The respiratory disorders and abortions are the most common indicators. Ab4p (an abortigenic and paralytic virus) is one of the most important and virulent strains. The development and functional characterization of the open reading frame-68 (ORF68) negative EHV-1 Ab4p mutants and an assessment of their roles in the infection at the cellular level were the main targets of the current study. Escherichia coli DH10β containing the Ab4p bacterial artificial chromosome (pAb4pBAC) and Red/ET expression vector were used to develop different ORF68 mutants. Multi-step growth kinetic experiments were conducted in order to evaluate the growth properties of the constructed mutant viruses. Growth of the Ab4pΔORF68 showed the lowest titer, compared to the Ab4pΔORF68R, Ab4pΔORF68R non-sense, and the parent Ab4p viruses without any significant difference (P > 0.05). The growth of the mutant viruses was almost similar across the cell types, but viruses growth was more efficient in FHK cells as judged by the number of the obtained virus particles. The plaque size of Ab4pΔORF68 was significantly (40%) smaller than those of Ab4p (P < 0.01), Ab4pΔORF68R, and Ab4pΔORF68R non-sense viruses which confirmed the importance of ORF68 protein in the cell-to-cell transmission of EHV-1. Subcellular localization of the green fluorescent protein (GFP) ORF68 gene fusion product showed late expression with intranuclear localization of the transfected cells while immunofluorescent antibody technique (IFAT) localized it at the nucleus and nuclear membranes of the infected cells. Hence, it could be concluded that ORF68 protein may not be essential for EHV-1 Ab4p growth but plays a crucial role in virus penetration and transmission at the cellular level. Therefore, the generated EHV-1 ORF68 negative mutant could be a prospective candidate for the development of a vaccine marker.

Immunization with Attenuated Equine Herpesvirus 1 Strain KyA Induces Innate Immune Responses That Protect Mice from Lethal Challenge

Equine herpesvirus 1 (EHV-1) is a major pathogen affecting equines worldwide. The virus causes respiratory disease, abortion, and, in some cases, neurological disease. EHV-1 strain KyA is attenuated in the mouse and equine, whereas wild-type strain RacL11 induces severe inflammation of the lung, causing infected mice to succumb at 4 to 6 days postinfection. Our previous results showed that KyA immunization protected CBA mice from pathogenic RacL11 challenge at 2 and 4 weeks postimmunization and that KyA infection elicited protective humoral and cell-mediated immune responses. To investigate the protective mechanisms of innate immune responses to KyA, KyA-immunized mice were challenged with RacL11 at various times postvaccination. KyA immunization protected mice from RacL11 challenge at 1 to 7 days postimmunization. Immunized mice lost less than 10% of their body weight and rapidly regained weight. Virus titers in the lungs of KyA-immunized mice were 1,000-fold lower at 2 days post-RacL11 challenge than virus titers in the lungs of nonimmunized mice, indicating accelerated virus clearance. Affymetrix microarray analysis revealed that gamma interferon (IFN-γ) and 16 antiviral interferon-stimulated genes (ISGs) were upregulated 3.1- to 48.2-fold at 8 h postchallenge in the lungs of RacL11-challenged mice that had been immunized with KyA. Murine IFN-γ inhibited EHV-1 infection of murine alveolar macrophages and protected mice against lethal EHV-1 challenge, suggesting that IFN-γ expression is important in mediating the protection elicited by KyA immunization. These results suggest that EHV-1 KyA may be used as a live attenuated EHV-1 vaccine as well as a prophylactic agent in horses.

IMPORTANCE

Viral infection of cells initiates a signal cascade of events that ultimately attempts to limit viral replication and prevent infection through the expression of host antiviral proteins. In this study, we show that EHV-1 KyA immunization effectively protected CBA mice from pathogenic RacL11 challenge at 1 to 7 days postvaccination and increased the expression of IFN-γ and 16 antiviral interferon-stimulated genes (ISGs). The administration of IFN-γ blocked EHV-1 replication in murine alveolar macrophages and mouse lungs and protected mice from lethal challenge. To our knowledge, this is the first report of an attenuated EHV-1 vaccine that protects the animal at 1 to 7 days postimmunization by innate immune responses. Our findings suggested that IFN-γ serves as a novel prophylactic agent and may offer new strategies for the development of anti-EHV-1 agents in the equine.

Functional Characterization of the Serine-Rich Tract of Varicella-Zoster Virus IE62

The immediate early 62 protein (IE62) of varicella-zoster virus (VZV), a major viral trans-activator, initiates the virus life cycle and is a key component of pathogenesis. The IE62 possesses several domains essential for trans-activation, including an acidic trans-activation domain (TAD), a serine-rich tract (SRT), and binding domains for USF, TFIIB, and TATA box binding protein (TBP). Transient-transfection assays showed that the VZV IE62 lacking the SRT trans-activated the early VZV ORF61 promoter at only 16% of the level of the full-length IE62. When the SRT of IE62 was replaced with the SRT of equine herpesvirus 1 (EHV-1) IEP, its trans-activation activity was completely restored. Herpes simplex virus 1 (HSV-1) ICP4 that lacks a TAD very weakly (1.5-fold) trans-activated the ORF61 promoter. An IE62 TAD-ICP4 chimeric protein exhibited trans-activation ability (10.2-fold), indicating that the IE62 TAD functions with the SRT of HSV-1 ICP4 to trans-activate viral promoters. When the serine and acidic residues of the SRT were replaced with Ala, Leu, and Gly, trans-activation activities of the modified IE62 proteins IE62-SRTΔSe and IE62-SRTΔAc were reduced to 46% and 29% of wild-type activity, respectively. Bimolecular complementation assays showed that the TAD of IE62, EHV-1 IEP, and HSV-1 VP16 interacted with Mediator 25 in human melanoma MeWo cells. The SRT of IE62 interacted with the nucleolar-ribosomal protein EAP, which resulted in the formation of globular structures within the nucleus. These results suggest that the SRT plays an important role in VZV viral gene expression and replication.

IMPORTANCE

The immediate early 62 protein (IE62) of varicella-zoster virus (VZV) is a major viral trans-activator and is essential for viral growth. Our data show that the serine-rich tract (SRT) of VZV IE62, which is well conserved within the alphaherpesviruses, is needed for trans-activation mediated by the acidic trans-activation domain (TAD). The TADs of IE62, EHV-1 IEP, and HSV-1 VP16 interacted with cellular Mediator 25 in bimolecular complementation assays. The interaction of the IE62 SRT with nucleolar-ribosomal protein EAP resulted in the formation of globular structures within the nucleus. Understanding the mechanisms by which the TAD and SRT of IE62 contribute to the function of this essential regulatory protein is important in understanding the gene program of this human pathogen.

Full trans-activation mediated by the immediate-early protein of equine herpesvirus 1 requires a consensus TATA box, but not its cognate binding sequence

The immediate-early protein (IEP) of equine herpesvirus 1 (EHV-1) has extensive homology to the IEP of alphaherpesviruses and possesses domains essential for trans-activation, including an acidic trans-activation domain (TAD) and binding domains for DNA, TFIIB, and TBP. Our data showed that the IEP directly interacted with transcription factor TFIIA, which is known to stabilize the binding of TBP and TFIID to the TATA box of core promoters. When the TATA box of the EICP0 promoter was mutated to a nonfunctional TATA box, IEP-mediated trans-activation was reduced from 22-fold to 7-fold. The IEP trans-activated the viral promoters in a TATA motif-dependent manner. Our previous data showed that the IEP is able to repress its own promoter when the IEP-binding sequence (IEBS) is located within 26-bp from the TATA box. When the IEBS was located at 100 bp upstream of the TATA box, IEP-mediated trans-activation was very similar to that of the minimal IE(nt -89 to +73) promoter lacking the IEBS. As the distance from the IEBS to the TATA box decreased, IEP-mediated trans-activation progressively decreased, indicating that the IEBS located within 100 bp from the TATA box sequence functions as a distance-dependent repressive element. These results indicated that IEP-mediated full trans-activation requires a consensus TATA box of core promoters, but not its binding to the cognate sequence (IEBS).

Similar regulation of two distinct UL24 promoters by regulatory proteins of equine herpesvirus type 1 (EHV-1)

To characterise the pattern of the transcriptional regulation of equine herpesvirus type 1 (EHV-1) UL24 by regulatory proteins, we identified two distinct promoter regions and two transcription initiation (Tci) sites located upstream of the UL24 open reading frame (ORF). The ORF proximal promoter exhibited higher cis-activity than that of the distal one. Contrary to the former, the latter performed its function dependent on an initiator (INR) due to its lack of a TATA box. Our results showed that the EHV-1 regulatory proteins EICP0, EICP22 and ETIF trans-activated the two promoters, whereas IEP and IR2P displayed negative regulation. In summary, the regulatory proteins exhibited similar regulatory patterns for the two distinct promoters of EHV-1 UL24.

The EHV-1 UL4 protein that tempers viral gene expression interacts with cellular transcription factors

The UL4 gene is conserved within the genome of defective interfering particles of equine herpesvirus type 1 (EHV-1) that mediate persistent infection. Here, we show that the UL4 protein inhibits EHV-1 reporter gene expression by decreasing the level of transcribed mRNA. The UL4 protein did not bind any gene class of EHV-1 promoters in electromobility or chromatin immunoprecipitation assays, but directly interacted with the TATA box-binding protein (TBP) and the carboxy-terminal domain of RNA polymerase II both in vitro (GST-pulldown assays) and in infected cells (coimmunoprecipitation analyses). Microarray analyses of the expression of the 78 EHV-1 genes revealed that viral late genes important for virion assembly displayed enhanced expression in cells infected with UL4-null virus as compared to wild-type or UL4-restored EHV-1. Quantitative PCR analyses showed that viral DNA replication was not retarded in cells infected with the UL4-null virus as compared to wild-type EHV-1.

The early UL31 gene of equine herpesvirus 1 encodes a single-stranded DNA-binding protein that has a nuclear localization signal sequence at the C-terminus

The amino acid sequence of the UL31 protein (UL31P) of equine herpesvirus 1 (EHV-1) has homology to that of the ICP8 of herpes simplex virus type 1 (HSV-1). Here we show that the UL31 gene is synergistically trans-activated by the IEP and the UL5P (EICP27). Detection of the UL31 RNA transcript and the UL31P in EHV-1-infected cells at 6h post-infection (hpi) as well as metabolic inhibition assays indicated that UL31 is an early gene. The UL31P preferentially bound to single-stranded DNA over double-stranded DNA in gel shift assays. Subcellular localization of the green fluorescent protein (GFP)-UL31 fusion proteins revealed that the C-terminal 32 amino acid residues of the UL31P are responsible for the nuclear localization. These findings may contribute to defining the role of the UL31P single-stranded DNA-binding protein in EHV-1 DNA replication.

Development of a bacterial artificial chromosome (BAC) recombineering procedure using galK-untranslated region (UTR) for the mutation of diploid genes

Bacterial artificial chromosome (BAC) recombineering using galK selection allows DNA cloned in Escherichia coli to be modified without introducing an unwanted selectable marker at the modification site. Genomes of some herpesviruses have a pair of inverted repeat sequences that makes it very difficult to introduce mutations into diploid (duplicate) genes using the galK selection method. To mutate diploid genes, we developed a galK-UTR BAC recombineering procedure that blocks one copy of the target diploid gene by insertion of a galK untranslated region (UTR), which enables the simple mutation of the other copy. The blocked copy can then be replaced with an UTR-specific primer pair. The IR2 gene of equine herpesvirus 1 (EHV-1) maps within both the internal (IR) and terminal repeat (TR) of the genomic short region and is expressed at low levels because its promoter is TATA-less. Both IR2 promoters in EHV-1 BAC were replaced with a mutant IR2 promoter containing three Sp1-binding motifs and a consensus TATA box by galK-UTR BAC recombineering. The expression level of the IR2 protein controlled by the modified promoter increased approximately 4-fold as compared to that of wild-type EHV-1. The galK-UTR method will provide a useful tool in studies of herpesviruses.

Characterization of cis-acting elements required for autorepression of the equine herpesvirus 1 IE gene

The immediate-early protein (IEP), the major regulatory protein encoded by the IE gene of equine herpesvirus 1 (EHV-1), plays a crucial role as both transcription activator and repressor during a productive lytic infection. To investigate the mechanism by which the EHV-1 IEP inhibits its own promoter, IE promoter-luciferase reporter plasmids containing wild-type and mutant IEP-binding site (IEBS) were constructed and used for luciferase reporter assays. The IEP inhibited transcription from its own promoter in the presence of a consensus IEBS (5'-ATCGT-3') located near the transcription initiation site but did not inhibit when the consensus sequence was deleted. To determine whether the distance between the TATA box and the IEBS affects transcriptional repression, the IEBS was displaced from the original site by the insertion of synthetic DNA sequences. Luciferase reporter assays revealed that the IEP is able to repress its own promoter when the IEBS is located within 26-bp from the TATA box. We also found that the proper orientation and position of the IEBS were required for the repression by the IEP. Interestingly, the level of repression was significantly reduced when a consensus TATA sequence was deleted from the promoter region, indicating that the IEP efficiently inhibits its own promoter in a TATA box-dependent manner. Taken together, these results suggest that the EHV-1 IEP delicately modulates autoregulation of its gene through the consensus IEBS that is near the transcription initiation site and the TATA box.