Identification and initial characterization of the IR6 protein of equine herpesvirus 1

Major histocompatibility complex class I downregulation induced by equine herpesvirus type 1 pUL56 is through dynamin-dependent endocytosis

Equine herpesvirus type 1 (EHV-1) downregulates cell surface expression of major histocompatibility complex class I (MHC-I) in infected cells. We have previously shown that pUL56 encoded by the EHV-1 ORF1 gene regulates the process (G. Ma, S. Feineis, N. Osterrieder, and G. R. Van de Walle, J. Virol. 86:3554-3563, 2012, doi:http://dx.doi.org/10.1128/JVI.06994-11). Here, we report that cell surface MHC-I in EHV-1-infected cells is internalized and degraded in the lysosomal compartment in a pUL56-dependent fashion. pUL56-induced MHC-I endocytosis required dynamin and tyrosine kinase but was independent of clathrin and caveolin-1, the main constituents of the clathrin- and raft/caveola-mediated endocytosis pathways, respectively. Downregulation of cell surface MHC-I was significantly inhibited by the ubiquitin-activating enzyme E1 inhibitor PYR41, indicating that ubiquitination is essential for the process. Finally, we show that downregulation is not specific for MHC-I and that other molecules, including CD46 and CD63, are also removed from the cell surface in a pUL56-dependent fashion.

IMPORTANCE

We show that alphaherpesvirus induces MHC-I downregulation through endocytosis, which is mediated by pUL56. The dynamin-dependent endocytic pathway is responsible for MHC-I internalization in infected cells. Furthermore, we discovered that this endocytic process can be disrupted by the inhibiting ubiquitin-activating E1 enzyme, which is indispensable for ubiquitination. Finally, pUL56 action extends to a number of cell surface molecules that are significant for host immunity. Therefore, the protein may exert a more general immunomodulatory effect.

A potentially fatal mix of herpes in zoos

Pathogens often have a limited host range, but some can opportunistically jump to new species. Anthropogenic activities that mix reservoir species with novel, hence susceptible, species can provide opportunities for pathogens to spread beyond their normal host range. Furthermore, rapid evolution can produce new pathogens by mechanisms such as genetic recombination. Zoos unintentionally provide pathogens with a high diversity of species from different continents and habitats assembled within a confined space. Institutions alert to the problem of pathogen spread to unexpected hosts can monitor the emergence of pathogens and take preventative measures. However, asymptomatic infections can result in the causative pathogens remaining undetected in their reservoir host. Furthermore, pathogen spread to unexpected hosts may remain undiagnosed if the outcome of infection is limited, as in the case of compromised fertility, or if more severe outcomes are restricted to less charismatic species that prompt only limited investigation. We illustrate this problem here with a recombinant zebra herpesvirus infecting charismatic species including zoo polar bears over at least four years. The virus may cause fatal encephalitis and infects at least five mammalian orders, apparently without requiring direct contact with infected animals.

Properties of an equine herpesvirus 1 mutant devoid of the internal inverted repeat sequence of the genomic short region

The 150 kbp genome of equine herpesvirus-1 (EHV-1) is composed of a unique long (UL) region and a unique short (Us) segment, which is flanked by identical internal and terminal repeat (IR and TR) sequences of 12.7 kbp. We constructed an EHV-1 lacking the entire IR (vL11ΔIR) and showed that the IR is dispensable for EHV-1 replication but that the vL11ΔIR exhibits a smaller plaque size and delayed growth kinetics. Western blot analyses of cells infected with vL11ΔIR showed that the synthesis of viral proteins encoded by the immediate-early, early, and late genes was reduced at immediate-early and early times, but by late stages of replication reached wild type levels. Intranasal infection of CBA mice revealed that the vL11ΔIR was significantly attenuated as mice infected with the vL11ΔIR showed a reduced lung viral titer and greater ability to survive infection compared to mice infected with parental or revertant virus.

The EICP27 protein of equine herpesvirus 1 is recruited to viral promoters by its interaction with the immediate-early protein

The equine herpesvirus 1 (EHV-1) EICP27 protein cooperates with either the immediate-early (IE) or the EICP0 protein to synergistically trans-activate viral promoters. GST-pulldown and co-immunoprecipitation assays revealed that the EICP27 protein's cooperation with the IE or the EICP0 protein involves its physical interaction with these viral proteins. In the case of the IE-EICP27 protein interaction, IE residues 424 to 826 and EICP27 residues 41 to 206 harbor the interactive domains. Electrophoretic mobility shift assays (EMSA) suggested that the EICP27 protein is not a sequence-specific DNA-binding protein as it fails to directly bind to the IE promoter, the early EICP27, EICP0, and TK promoters, or the late gD and IR5 promoters. However, EMSA studies also showed that the interaction of the IE and EICP27 proteins results in the recruitment of the EICP27 protein to representative early promoters. These results support our hypothesis that the EICP27 protein participates in the trans-activation of EHV-1 promoters, and suggest its presence within RNA polymerase II preinitiation complexes that assemble at viral promoters.

Interaction of the equine herpesvirus 1 EICP0 protein with the immediate-early (IE) protein, TFIIB, and TBP may mediate the antagonism between the IE and EICP0 proteins

The equine herpesvirus 1 (EHV-1) immediate-early (IE) and EICP0 proteins are potent trans-activators of EHV-1 promoters; however, in transient-transfection assays, the IE protein inhibits the trans-activation function of the EICP0 protein. Assays with IE mutant proteins revealed that its DNA-binding domain, TFIIB-binding domain, and nuclear localization signal may be important for the antagonism between the IE and EICP0 proteins. In vitro interaction assays with the purified IE and EICP0 proteins indicated that these proteins interact directly. At late times postinfection, the IE and EICP0 proteins colocalized in the nuclei of infected equine cells. Transient-transfection assays showed that the EICP0 protein trans-activated EHV-1 promoters harboring only a minimal promoter region (TATA box and cap site), suggesting that the EICP0 protein trans-activates EHV-1 promoters by interactions with general transcription factor(s). In vitro interaction assays revealed that the EICP0 protein interacted directly with the basal transcription factors TFIIB and TBP and that the EICP0 protein (amino acids [aa] 143 to 278) mediated the interaction with aa 125 to 174 of TFIIB. Our unpublished data showed that the IE protein interacts with the same domain (aa 125 to 174) of TFIIB and with TBP. Taken together, these results suggested that interaction of the EICP0 protein with the IE protein, TFIIB, and TBP may mediate the antagonism between the IE and EICP0 proteins.

The equine herpesvirus 1 UL34 gene product is involved in an early step in virus egress and can be efficiently replaced by a UL34-GFP fusion protein

The structure and function of the equine herpesvirus type 1 (EHV-1) UL34 homologous protein were characterized. A UL34 protein-specific antiserum reacted with an M(r)28,000 protein that could not be detected in purified extracellular virions. Confocal laser scanning microscopy demonstrated that UL34 reactivity mainly concentrated at the nuclear rim, which changed into a punctuate and filamentous pattern at late times after infection. These changes in UL34 distribution were especially prominent when analyzing the distribution of a GFP-UL34 fusion protein. A UL34-negative EHV-1 was generated by mutagenesis of a recently established BAC clone of EHV-1 strain RacH (pRacH). Release of extracellular infectious virus was severely impaired after infection of Rk13 cells with HDelta34. Electron microscopy revealed a virtual absence of virus particles in the cytoplasm of infected cells, whereas nucleocapsid formation and maturation within the nucleus appeared unaffected. A UL34-GFP fusion protein with GFP linked to the C-terminus of UL34 was able to complement for the UL34 deletion in trans, while a GFP-UL34-fusion protein with GFP linked to the N-terminus of UL34 was able to only partially restore virus growth. It was concluded that the EHV-1 UL34 product is essential for an early step in virus egress, i.e., release of capsids from infected-cell nuclei.

The equine herpesvirus 1 immediate-early protein interacts with EAP, a nucleolar-ribosomal protein

The equine herpesvirus 1 (EHV-1) immediate-early (IE) phosphoprotein is essential for the activation of transcription from viral early and late promoters and regulates transcription from its own promoter. The IE protein of 1487 amino acids contains a serine-rich tract (SRT) between residues 181 and 220. Deletion of the SRT decreased transactivation activity of the IE protein. Previous results from investigation of the ICP4 protein, the IE homolog of herpes simplex virus 1 (HSV-1), revealed that a domain containing a serine-rich tract interacts with EAP (Epstein-Barr virus-encoded small nuclear RNA-associated protein), a 15-kDa nucleolar-ribosomal protein (R. Leopardi, and B. Roizman, Proc. Natl. Acad. Sci. USA 93, 4572-4576, 1996). DNA binding assays revealed that (i) glutathione S-transferase (GST)-EAP disrupted the binding of HSV-1 ICP4 to its cognate DNA in a dose-dependent manner, (ii) GST-EAP interacted with the EHV-1 IE protein, but did not disrupt its binding to its cognate site in viral DNA. GST-pulldown assays indicated that the SRT of the IE protein is required for physical interaction with EAP. The IE protein and EAP colocalized in the cytoplasm of the infected equine ETCC cells at late times of the infection cycle. This latter finding may be important in EHV-1 gene regulation since late viral gene expression is greatly influenced by the EICP0 trans-activator protein whose function is antagonized by the IE protein.

The equine herpesvirus 1 Us2 homolog encodes a nonessential membrane-associated virion component

Experiments were conducted to analyze the equine herpesvirus 1 (EHV-1) gene 68 product which is encoded by the EHV-1 Us2 homolog. An antiserum directed against the amino-terminal 206 amino acids of the EHV-1 Us2 protein specifically detected a protein with an Mr of 34,000 in cells infected with EHV-1 strain RacL11. EHV-1 strain Ab4 encodes a 44,000-Mr Us2 protein, whereas vaccine strain RacH, a high-passage derivative of RacL11, encodes a 31,000-Mr Us2 polypeptide. Irrespective of its size, the Us2 protein was incorporated into virions. The EHV-1 Us2 protein localized to membrane and nuclear fractions of RacL11-infected cells and to the envelope fraction of purified virions. To monitor intracellular trafficking of the protein, the green fluorescent protein (GFP) was fused to the carboxy terminus of the EHV-1 Us2 protein or to a truncated Us2 protein lacking a stretch of 16 hydrophobic amino acids at the extreme amino terminus. Both fusion proteins were detected at the plasma membrane and accumulated in the vicinity of nuclei of transfected cells. However, trafficking of either GFP fusion protein through the secretory pathway could not be demonstrated, and the EHV-1 Us2 protein lacked detectable N- and O-linked carbohydrates. Consistent with the presence of the Us2 protein in the viral envelope and plasma membrane of infected cells, a Us2-negative RacL11 mutant (L11DeltaUs2) exhibited delayed penetration kinetics and produced smaller plaques compared with either wild-type RacL11 or a Us2-repaired virus. After infection of BALB/c mice with L11DeltaUs2, reduced pathogenicity compared with the parental RacL11 virus and the repaired virus was observed. It is concluded that the EHV-1 Us2 protein modulates virus entry and cell-to-cell spread and appears to support sustained EHV-1 replication in vivo.

The equine herpesvirus 1 IR6 protein that colocalizes with nuclear lamins is involved in nucleocapsid egress and migrates from cell to cell independently of virus infection

The equine herpesvirus 1 (EHV-1) IR6 protein forms typical rod-like structures in infected cells, influences virus growth at elevated temperatures, and determines the virulence of EHV-1 Rac strains (Osterrieder et al., Virology 226:243-251, 1996). Experiments to further elucidate the functions and properties of the IR6 protein were conducted. It was shown that the IR6 protein of wild-type RacL11 virus colocalizes with nuclear lamins very late in infection as demonstrated by confocal laser scan microscopy and coimmunoprecipitation experiments. In contrast, the mutated IR6 protein encoded by the RacM24 strain did not colocalize with the lamin proteins at any time postinfection (p.i.). Electron microscopical examinations of ultrathin sections were performed on cells infected at 37 and 40 degreesC, the latter being a temperature at which the IR6-negative RacH virus and the RacM24 virus are greatly impaired in virus replication. These analyses revealed that nucleocapsid formation is efficient at 40 degreesC irrespective of the virus strain. However, whereas cytoplasmic virus particles were readily observed at 16 h p.i. in cells infected with the wild-type EHV-1 RacL11 or an IR6-recombinant RacH virus (HIR6-1) at 40 degreesC, virtually no capsid translocation to the cytoplasm was obvious in RacH- or RacM24-infected cells at the elevated temperature, demonstrating that the IR6 protein is involved in nucleocapsid egress. Transient transfection assays using RacL11 or RacM24 IR6 plasmid DNA and COS7 or Rk13 cells, infection studies using a gB-negative RacL11 mutant (L11DeltagB) which is deficient in direct cell-to-cell spread, and studies using lysates of IR6-transfected cells demonstrated that the wild-type IR6 protein is transported from cell to cell in the absence of virus infection and can enter cells by a yet unknown mechanism.

Genomic organization of the canine herpesvirus US region

Canine herpesvirus (CHV) is an alpha-herpesvirus of limited pathogenicity in healthy adult dogs and infectivity of the virus appears to be largely limited to cells of canine origin. CHV's low virulence and species specificity make it an attractive candidate for a recombinant vaccine vector to protect dogs against a variety of pathogens. As part of the analysis of the CHV genome, the authors determined the complete nucleotide sequence of the CHV US region as well as portions of the flanking inverted repeats. Seven full open reading frames (ORFs) encoding proteins larger than 100 amino acids were identified within, or partially within the CHV US: cUS2, cUS3, cUS4, cUS6, cUS7, cUS8 and cUS9; which are homologs of the herpes simplex virus type-1 US2; protein kinase; gG, gD, gI, gE; and US9 genes, respectively. An eighth ORF was identified in the inverted repeat region, cIR6, a homolog of the equine herpesvirus type-1 IR6 gene. The authors identified and mapped most of the major transcripts for the predicted CHV US ORFs by Northern analysis.

Equine herpesvirus-1 strain KyA, a candidate vaccine strain, reduces viral titers in mice challenged with a pathogenic strain, RacL

The equine herpesvirus type-1 (EHV-1) strain Kentucky A (KyA) has a long history of repeated passage either in vivo in the Syrian hamster or in vitro in mouse L-M fibroblast tissue culture. This repeated passage in cells other than those of the natural host has caused genomic alterations of the KyA chromosome resulting in deletion of several genes or portions of open reading frames (ORFs). This report presents in vivo data from a mouse model of EHV-1 infection demonstrating the attenuated nature of EHV-1 strain KyA and that intranasal infection with KyA protects animals from subsequent challenge with a pathogenic strain, RacL, by reducing RacL viral titers in the lungs of the challenged animals. Mice infected with KyA exhibit no clinical manifestations of EHV-1 disease and do not experience the wasting that occurs with RacL infection. KyA-infected mice clear virus from the lung by day 5 post-infection (p.i.), whereas RacL infected mice have substantial virus titers (5 x 10(5) pfu/lung) at this time point. Intranasal infection with KyA followed by a challenge with RacL 4 weeks post-KyA infection resulted in a significant (P = 0.0079) reduction in the lung titers of the RacL virus. RacL was identified as the virus present in the lungs of the challenged mice by a PCR assay employing primers to amplify the EUS4 gene which differs in size by 1.2 kilobase pairs (kbp) in the two strains. Importantly, the protection afforded by KyA is long lasting in that challenge with RacL 15 months after KyA infection, results in reduced virus titers and viral clearance by day 5 post-challenge. These results support the further consideration of EHV-1 KyA as a live virus vaccine.

Alterations in the equine herpesvirus type-1 (EHV-1) strain RacH during attenuation

The equine herpesvirus type-1 modified live-vaccine strain RacH (256th passage on porcine embryonic kidney cells) was investigated by restriction-enzyme analysis and compared to representative plaque isolates of the 12th passage (RacL11, RacL22) and 185th passage (RacM24, RacM36). The restriction patterns of all Rac plaque isolates differed compared with reference strain Ab4. The left UL terminus was shortened by 0.1 kbp and a missing BamHI site led to the fusion of the f and t fragments. In some Rac derivatives, losses of restriction sites without deletions were observed: 1. One BamHI site located in the ribosyl reductase gene was missing in RacH, RacM24, RacM36, and RacL22; and 2. An SalI site mapping to the gp14 (gB) gene was absent in RacM24, RacM36 and RacH. An identical deletion of 0.85 kbp in size was found in both copies of the inverted repeat (IR) regions of RacH. The deletion was present only in the terminal IR of the medium-passage derivative RacM36. By contrast, in the genomes of the apathogenic RacM24, as well as the pathogenic plaque isolates RacL11 and RacL22, no deletions in the IRs were detectable. Nucleotide-sequence and Northern-blot analyses revealed that the deletions led to the elimination of one or both copies of the gene 67 (IR6) open-reading frame in RacM36 and RacH and affected the gene 68 (EUS1) in RacH.