The role of the major tegument protein VP8 of bovine herpesvirus-1 in infection and immunity

Investigation of the safety and protective efficacy of an attenuated and marker M. bovis-BoHV-1 combined vaccine in bovines

Bovine respiratory disease (BRD) is one of the most common diseases in the cattle industry worldwide; it is caused by multiple bacterial or viral coinfections, of which Mycoplasma bovis (M. bovis) and bovine herpesvirus type 1 (BoHV-1) are the most notable pathogens. Although live vaccines have demonstrated better efficacy against BRD induced by both pathogens, there are no combined live and marker vaccines. Therefore, we developed an attenuated and marker M. bovis-BoHV-1 combined vaccine based on the M. bovis HB150 and BoHV-1 gG-/tk- strain previously constructed in our lab and evaluated in rabbits. This study aimed to further evaluate its safety and protective efficacy in cattle using different antigen ratios. After immunization, all vaccinated cattle had a normal rectal temperature and mental status without respiratory symptoms. CD4+, CD8+, and CD19+ cells significantly increased in immunized cattle and induced higher humoral and cellular immune responses, and the expression of key cytokines such as IL-4, IL-12, TNF-α, and IFN-γ can be promoted after vaccination. The 1.0 × 108 CFU of M. bovis HB150 and 1.0 × 106 TCID50 BoHV-1 gG-/tk- combined strain elicited the most antibodies while significantly increasing IgG and cellular immunity after challenge. In conclusion, the M. bovis HB150 and BoHV-1 gG-/tk- combined strain was clinically safe and protective in calves; the mix of 1.0 × 108 CFU of M. bovis HB150 and 1.0 × 106 TCID50 BoHV-1 gG-/tk- strain was most promising due to its low amount of shedding and highest humoral and cellular immune responses compared with others. This study introduces an M. bovis-BoHV-1 combined vaccine for application in the cattle industry.

The Cell-Mediated Immune Response against Bovine alphaherpesvirus 1 (BoHV-1) Infection and Vaccination

Bovine Alphaherpesvirus 1 (BoHV-1) is one of the major respiratory pathogens in cattle worldwide. Infection often leads to a compromised host immune response that contributes to the development of the polymicrobial disease known as “bovine respiratory disease”. After an initial transient phase of immunosuppression, cattle recover from the disease. This is due to the development of both innate and adaptive immune responses. With respect to adaptive immunity, both humoral and cell-mediated immunity are required to control infection. Thus, several BoHV-1 vaccines are designed to trigger both branches of the adaptive immune system. In this review, we summarize the current knowledge on cell-mediated immune responses directed against BoHV-1 infection and vaccination.

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.

Bovine Herpesvirus-1 Glycoprotein M Mediates the Translocation to the Golgi Apparatus and Packaging of VP8

VP8, the most abundant tegument protein of bovine herpesvirus-1 (BoHV-1), plays an important role in viral replication. According to our previous studies, VP8 localizes to the Golgi apparatus of BoHV-1-infected cells where it can be packaged into the virus; however, Golgi localization of VP8 does not occur outside of the context of infection. The goal of this study was to identify the viral factor(s) involved in the tropism of VP8 towards the Golgi. VP8 was found to interact with glycoprotein M (gM), and the VP8 and gM domains that are essential for this interaction were identified. VP8 and gM colocalized to the Golgi apparatus in BoHV-1-infected cells. In cells co-transfected with VP8- and gM-encoding plasmids, VP8 was also found to be localized to the Golgi, suggesting gM to be sufficient. The localization of VP8 to the Golgi was lost in cells infected with a gM deletion mutant, and the amount of VP8 incorporated into mature virus was significantly reduced. However, with the restoration of gM in a revertant virus, the localization to the Golgi and the amount of VP8 incorporated in the virions were restored. These results indicate that gM plays a critical role in VP8 subcellular localization to the Golgi and packaging into mature virions.

VP8, the Major Tegument Protein of Bovine Herpesvirus-1, Is Partially Packaged during Early Tegument Formation in a VP22-Dependent Manner

Bovine herpesvirus-1 (BoHV-1) is a major cause of rhinotracheitis and vulvovaginitis in cattle. VP8, the major tegument protein of BoHV-1, is essential for viral replication in the host. VP8 is phosphorylated by the viral kinase US3, mediating its translocation to the cytoplasm. VP8 remains nuclear when not phosphorylated. Interestingly, VP8 has a significant presence in mature BoHV-1YmVP8, in which the VP8 phosphorylation sites are mutated. This suggests that VP8 might be packaged during primary envelopment of BoHV-1. This was investigated by mass spectrometry and Western blotting, which showed VP8, as well as VP22, to be constituents of the primary enveloped virions. VP8 and VP22 were shown to interact via co-immunoprecipitation experiments, in both BoHV-1-infected and VP8-transfected cells. VP8 and VP22 also co-localised with one another and with nuclear lamin-associated protein 2 in BoHV-1-infected cells, suggesting an interaction between VP8 and VP22 in the perinuclear region. In cells infected with VP22-deleted BoHV-1 (BoHV-1ΔUL49), VP8 was absent from the primary enveloped virions, implying that VP22 might be critical for the early packaging of VP8. In conclusion, a novel VP22-dependent mechanism for packaging of VP8 was identified, which may be responsible for a significant amount of VP8 in the viral particle.

US3 Kinase-Mediated Phosphorylation of Tegument Protein VP8 Plays a Critical Role in the Cellular Localization of VP8 and Its Effect on the Lipid Metabolism of Bovine Herpesvirus 1-Infected Cells

Bovine herpesvirus 1 (BoHV-1) infects bovine species, causing respiratory infections, genital disorders and abortions. VP8 is the most abundant tegument protein of BoHV-1 and is critical for virus replication in cattle. In this study, the cellular transport of VP8 in BoHV-1-infected cells and its ability to alter the cellular lipid metabolism were investigated. A viral kinase, US3, was found to be involved in regulating these processes. In the early stages of infection VP8 was localized in the nucleus. Subsequently, presumably after completion of its role in the nucleus, VP8 was translocated to the cytoplasm. When US3 was deleted or the essential US3 phosphorylation site of VP8 was mutated in BoHV-1, the majority of VP8 was localized in the nuclei of infected cells. This suggests that phosphorylation by US3 may be critical for cytoplasmic localization of VP8. Eventually, the cytoplasmic VP8 was accumulated in the cis-Golgi apparatus but not in the trans-Golgi network, implying that VP8 was not involved in virion transport toward and budding from the cell membrane. VP8 caused lipid droplet (LD) formation in the nuclei of transfected cells and increased cellular cholesterol levels. Lipid droplets were not found in the nuclei of BoHV-1-infected cells when VP8 was cytoplasmic in the presence of US3. However, when US3 was deleted or phosphorylation residues in VP8 were mutated, nuclear VP8 and LDs appeared in BoHV-1-infected cells. The total cholesterol level was increased in BoHV-1-infected cells but not in ΔU_L47-BoHV-1-infected cells, further supporting a role for VP8 in altering the cellular lipid metabolism during infection.IMPORTANCE Nuclear localization signals (NLSs) and nuclear export signals (NESs) are important elements directing VP8 to the desired locations in the BoHV-1-infected cell. In this study, a critical regulator that switches the nuclear and cytoplasmic localization of VP8 in BoHV-1-infected cells was identified. BoHV-1 used viral kinase US3 to regulate the cellular localization of VP8. Early during BoHV-1 infection VP8 was localized in the nucleus, where it performs various functions; once US3 was expressed, phosphorylated VP8 was cytoplasmic and ultimately accumulated in the cis-Golgi apparatus, presumably to be incorporated into virions. The Golgi localization of VP8 was only observed in virus-infected cells and not in US3-cotransfected cells, suggesting that this is mediated by other viral factors. Interestingly, VP8 was shown to cause increased cholesterol levels, which is a novel function for VP8 and a potential strategy to supply lipid for viral replication.

The bovine herpesvirus-1 major tegument protein, VP8, interacts with host HSP60 concomitant with deregulation of mitochondrial function

The U_L47 gene product, VP8, is a major tegument protein of BoHV-1. While VP8 is not essential for virus replication in cell culture, a U_L47-deleted virus exhibits a smaller tegument structure and is avirulent in cattle. To obtain pure VP8 protein for structural analysis, we expressed a N-terminally truncated version of VP8 in Eschericia coli. However, the recombinant VP8 was consistently co-purified with a tightly associated bacterial protein; this protein was identified by mass spectrometry as GroEL, which has considerable homology with mammalian heat shock protein-60 (HSP60), thus suggesting a new role for VP8 in virus-host interaction. A physical interaction of HSP60 and VP8 in both VP8-transfected and BoHV-1-infected cells was demonstrated by immunoprecipitation. Analysis of different truncated VP8 constructs revealed that amino acids 259-482 and 632-741 are involved in binding to HSP60. Full-length VP8 and VP8 219-741 (containing both interacting domains, 259-482 and 632-741) co-localized with HSP60 and mitochondria. VP8 was localized in the mitochondria from 2 to 14 h post infection in BoHV-1-infected cells. The mitochondrial membrane potential was reduced in both VP8-transfected and BoHV-1-infected cells and was further diminished by overexpression of HSP60 in the presence of VP8. In addition, VP8 expression decreased the ATP concentration during transfection, as well as BoHV-1 infection. Thus, VP8 may play a role in the deregulation of mitochondrial function through interaction with HSP60. This is consistent with the fact that BoHV-1 infection is known to promote mitochondrial dysfunction.

VP8, the Major Tegument Protein of Bovine Herpesvirus 1, Interacts with Cellular STAT1 and Inhibits Interferon Beta Signaling

The UL47 gene product, VP8, is the most abundant tegument protein of bovine herpesvirus 1 (BoHV-1). Previously, we demonstrated that a UL47-deleted BoHV-1 mutant (BoHV1-ΔUL47) exhibits 100-fold-reduced virulence in vitro and is avirulent in vivo In this study, we demonstrated that VP8 expression or BoHV-1 infection inhibits interferon beta (IFN-β) signaling by using an IFN-α/β-responsive plasmid in a luciferase assay. As transducer and activator of transcription (STAT) is an essential component in the IFN-signaling pathways, the effect of VP8 on STAT was investigated. An interaction between VP8 and STAT1 was established by coimmunoprecipitation assays in both VP8-transfected and BoHV-1-infected cells. Two domains of VP8, amino acids 259 to 482 and 632 to 686, were found to be responsible for its interaction with STAT1. The expression of VP8 did not induce STAT1 ubiquitination or degradation. Moreover, VP8 did not reduce STAT1 tyrosine phosphorylation to downregulate IFN-β signaling. However, the expression of VP8 or a version of VP8 (amino acids 219 to 741) that contains the STAT1-interacting domains but not the nuclear localization signal prevented nuclear accumulation of STAT1. Inhibition of nuclear accumulation of STAT1 also occurred during BoHV-1 infection, while nuclear translocation of STAT1 was observed in BoHV1-ΔUL47-infected cells. During BoHV-1 infection, VP8 was detected in the cytoplasm at 2 h postinfection without any de novo protein synthesis, at which time STAT1 was already retained in the cytoplasm. These results suggest that viral VP8 downregulates IFN-β signaling early during infection, thus playing a role in overcoming the antiviral response of BoHV-1-infected cells.

IMPORTANCE

Since VP8 is the most abundant protein in BoHV-1 virions and thus may be released in large amounts into the host cell immediately upon infection, we proposed that it might have a function in the establishment of conditions suitable for viral replication. Indeed, while nonessential in vitro, it is critical for BoHV-1 replication in vivo In this study, we determined that VP8 plays a role in downregulation of the antiviral host response by inhibiting IFN-β signaling. VP8 interacted with and prevented nuclear accumulation of STAT1 at 2 h postinfection in the absence of de novo viral protein synthesis. Two domains of VP8, amino acids 259 to 482 and 632 to 686, were found to be responsible for this interaction. These results provide a new functional role for VP8 in BoHV-1 infection and a potential explanation for the lack of viral replication of the UL47 deletion mutant in cattle.

Phosphorylation of Bovine Herpesvirus 1 VP8 Plays a Role in Viral DNA Encapsidation and Is Essential for Its Cytoplasmic Localization and Optimal Virion Incorporation

VP8 is a major tegument protein of bovine herpesvirus 1 (BoHV-1) and is essential for viral replication in cattle. The protein undergoes phosphorylation after transcription through cellular casein kinase 2 (CK2) and a viral kinase, US3. In this study, a virus containing a mutated VP8 protein that is not phosphorylated by CK2 and US3 (BoHV-1-YmVP8) was constructed by homologous recombination in mammalian cells. When BoHV-1-YmVP8-infected cells were observed by transmission electron microscopy, blocking phosphorylation of VP8 was found to impair viral DNA encapsidation, resulting in release of incomplete viral particles to the extracellular environment. Consequently, less infectious virus was produced by the mutant virus than by wild-type (WT) virus. A comparison of mutant and WT VP8 by confocal microscopy revealed that mutant VP8 is nuclear throughout infection while WT VP8 is nuclear early during infection and is associated with the Golgi apparatus at later stages. This, together with the observation that mutant VP8 is present in virions, albeit in smaller amounts, suggests that the incorporation of VP8 may occur at two stages. The first takes place without the need for phosphorylation and before or during nuclear egress of capsids, whereas the second occurs in the Golgi apparatus and requires phosphorylation of VP8. The results indicate that phosphorylated VP8 plays a role in viral DNA encapsidation and in the secondary virion incorporation of VP8. To perform these functions, the cellular localization of VP8 is adjusted based on the phosphorylation status.

IMPORTANCE

In this study, phosphorylation of VP8 was shown to have a function in BoHV-1 replication. A virus containing a mutated VP8 protein that is not phosphorylated by CK2 and US3 (BoHV-1-YmVP8) produced smaller numbers of infectious virions than wild-type (WT) virus. The maturation and egress of WT and mutant BoHV-1 were studied, showing a process similar to that reported for other alphaherpesviruses. Interestingly, lack of phosphorylation of VP8 by CK2 and US3 resulted in reduced incorporation of viral DNA into capsids during mutant BoHV-1 infection, as well as lower numbers of extracellular virions. Furthermore, mutant VP8 remained nuclear throughout infection, in contrast to WT VP8, which is nuclear at early stages and Golgi apparatus associated late during infection. This correlates with smaller amounts of mutant VP8 in virions and suggests for the first time that VP8 may be assembled into the virions at two stages, with the latter dependent on phosphorylation.

Regulation and function of phosphorylation on VP8, the major tegument protein of bovine herpesvirus 1

The major tegument protein of bovine herpesvirus 1 (BoHV-1), VP8, is essential for virus replication in cattle. VP8 is phosphorylated in vitro by casein kinase 2 (CK2) and BoHV-1 unique short protein 3 (US3). In this study, VP8 was found to be phosphorylated in both transfected and infected cells but was detected as a nonphosphorylated form in mature virions. This suggests that phosphorylation of VP8 is strictly controlled during different stages of the viral life cycle. The regulation and function of VP8 phosphorylation by US3 and CK2 were further analyzed. An in vitro kinase assay, site-directed mutagenesis, and liquid chromatography-mass spectrometry were used to identify the active sites for US3 and CK2. The two kinases phosphorylate VP8 at different sites, resulting in distinct phosphopeptide patterns. S(16) is a primary phosphoreceptor for US3, and it subsequently triggers phosphorylation at S(32). CK2 has multiple active sites, among which T(107) appears to be the preferred residue. Additionally, CK2 consensus motifs in the N terminus of VP8 are essential for phosphorylation. Based on these results, a nonphosphorylated VP8 mutant was constructed and used for further studies. In transfected cells phosphorylation was not required for nuclear localization of VP8. Phosphorylated VP8 appeared to recruit promyelocytic leukemia (PML) protein and to remodel the distribution of PML in the nucleus; however, PML protein did not show an association with nonphosphorylated VP8. This suggests that VP8 plays a role in resisting PML-related host antiviral defenses by redistributing PML protein and that this function depends on the phosphorylation of VP8.

IMPORTANCE

The progression of VP8 phosphorylation over time and its function in BoHV-1 replication have not been characterized. This study demonstrates that activation of S(16) initiates further phosphorylation at S(32) by US3. Additionally, VP8 is phosphorylated by CK2 at several residues, with T(107) having the highest level of phosphorylation. Evidence for a difference in the phosphorylation status of VP8 in host cells and mature virus is presented for the first time. Phosphorylation was found to be a critical modification, which enables VP8 to attract and to redistribute PML protein in the nucleus. This might promote viral replication through interference with a PML-mediated antiviral defense. This study provides new insights into the regulation of VP8 phosphorylation and suggests a novel, phosphorylation-dependent function for VP8 in the life cycle of BoHV-1, which is important in view of the fact that VP8 is essential for virus replication in vivo.

Bovine herpesvirus-1 VP8 interacts with DNA damage binding protein-1 (DDB1) and is monoubiquitinated during infection

VP8 is the most abundant tegument protein of bovine herpesvirus-1 (BHV-1). In the present study DNA damage binding protein 1 (DDB1) was identified as interacting partner of VP8. MALDI-TOF mass spectroscopy analysis of proteins co-immunoprecipitated with VP8 identified DDB1 as a protein interacting with VP8. The interaction between VP8 and DDB1 was confirmed based on co-immunoprecipitation and co-localization in both VP8-transfected and BHV-1 infected cells. DDB1 was distributed both in the nucleus and the cytoplasm with some nuclear speckles prior to BHV-1 infection, became perinuclear by 4h and was predominantly nuclear at 5h post infection, where it co-localized with VP8. In contrast, in cells infected with a U(L)47 deletion mutant DDB1 remained cytoplasmic throughout the course of infection. This suggests that VP8 mediates nuclear re-localization of DDB1. Finally, VP8 was shown to be monoubiquitinated both in VP8-transfected and BHV-1-infected cells. These data suggest that BHV-1 VP8 interacts with DDB1-CUL4 E3 ubiquitin ligase, which correlates to monoubiquitination of this viral protein.

A UL47 gene deletion mutant of bovine herpesvirus type 1 exhibits impaired growth in cell culture and lack of virulence in cattle

Tegument protein VP8 encoded by the U(L)47 gene of bovine herpesvirus type 1 (BHV-1) is the most abundant constituent of mature virions. In the present report, we describe the characterization of U(L)47 gene-deleted BHV-1 in cultured cells and its natural host. The U(L)47 deletion mutant exhibited reduced plaque size and more than 100-fold decrease in intracellular and extracellular viral titers in cultured cells. Ultrastructural observations of infected cells showed normal maturation of BHV-1 virions in the absence of VP8. There was no evidence for a change in immediate-early gene activator function of VP16 in the U(L)47 deletion mutant virus-infected cells, since bovine ICP4 mRNA and protein levels were similar to those in the wild-type and revertant virus-infected cells throughout the course of infection. Whereas VP16, glycoprotein C (gC), gB, and VP5 were expressed to wild-type levels in the U(L)47 deletion mutant-infected cells, the gD and VP22 protein levels were significantly reduced. The reduction in gD protein was associated with increased turnover of the protein. Furthermore, some of the analyzed early and late proteins were expressed with earlier kinetics in the absence of VP8. Extracellular virions of the U(L)47 deletion mutant contained reduced amounts of gD, gB, gC, and VP22 but similar amounts of VP16 compared to those of wild-type or revertant virus particles. In addition, the U(L)47 gene product was indispensable for BHV-1 replication in vivo, since no clinical manifestations or viral shedding were detected in the U(L)47 deletion mutant-infected calves, and the virus failed to induce significant levels of humoral and cellular immunity.

Major tegument protein VP8 of bovine herpesvirus 1 is phosphorylated by viral US3 and cellular CK2 protein kinases

The UL47 gene product, VP8, is one of the major tegument proteins of bovine herpesvirus 1 (BoHV-1) and is subject to phosphorylation. Analysis of protein bands co-immunoprecipitated with VP8 from BoHV-1-infected cells by mass spectroscopy suggested that VP8 interacts with two protein kinases: cellular CK2 and viral US3. CK2 is a highly conserved cellular protein, expressed ubiquitously and known to phosphorylate numerous proteins. The US3 gene product is one of the viral kinases produced by BoHV-1 during infection. Interactions of CK2 and US3 with VP8 were confirmed outside the context of infection when FLAG–VP8 was expressed alone or co-expressed with US3–haemagglutinin tag in Cos-7 cells. Furthermore, VP8 and US3 were found to co-localize in the nucleus during viral infection. To explore the significance of these interactions, an in vitro kinase assay was performed, which demonstrated that VP8 is heavily phosphorylated by CK2. In the presence of the highly specific CK2 kinase inhibitor 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT), phosphorylation of VP8 was significantly reduced. Phosphorylation of VP8 was also inhibited by the presence of kenpaullone, a less specific CK2 inhibitor, but not by protein kinase CK1 or protein kinase C inhibitors. When VP8 and US3 were both included in the kinase assay in the presence of DMAT, phosphorylation of VP8 was again observed. Autophosphorylation of US3 was also detected and was not inhibited by DMAT. Based on these results, it is proposed that VP8 interacts with cellular CK2 and viral US3 in BoHV-1-infected cells, and is in turn subject to kinase activities associated with both of these proteins.

Intracellular trafficking of VP22 in bovine herpesvirus-1 infected cells

The intracellular trafficking of different VP22-enhanced yellow fluorescent protein (EYFP) fusion proteins expressed by bovine herpesvirus-1 (BHV-1) recombinants was examined by live-cell imaging. Our results demonstrate that (i) the fusion of EYFP to the C terminus of VP22 does not alter the trafficking of the protein in infected cells, (ii) VP22 expressed during BHV-1 infection translocates to the nucleus through three different pathways, namely early mitosis-dependent nuclear translocation, late massive nuclear translocation that follows a prolonged cytoplasmic stage of the protein in non-mitotic cells, and accumulation of a small subset of VP22 in discrete dot-like nuclear domains during its early cytoplasmic stage, (iii) the addition of the SV40 large-T-antigen nuclear localization signal (NLS) to VP22-EYFP abrogates its early cytoplasmic stage, and (iv) the VP22 (131)PRPR(134) NLS is not required for the late massive nuclear translocation of the protein, but this motif is essential for the targeting of VP22 to discrete dot-like nuclear domains during the early cytoplasmic stage. These results show that the amount of VP22 in the nucleus is precisely regulated at different stages of BHV-1 infection and suggest that the early pathways of VP22 nuclear accumulation may be more relevant to the infection process as the late massive nuclear influx starts when most of the viral progeny has already emerged from the cell.

Characterization of a CRM1-dependent nuclear export signal in the C terminus of herpes simplex virus type 1 tegument protein UL47

The herpes simplex virus type 1 tegument protein known as VP13/14, or hUL47, localizes to the nucleus and binds RNA. Using fluorescence loss in photobleaching analysis, we show that hUL47 undergoes nucleocytoplasmic shuttling during infection. We identify the hUL47 nuclear export signal (NES) as a C-terminal 10-residue hydrophobic peptide and measure its efficiency relative to that of the classical human immunodeficiency virus type 1 Rev NES. Finally, we show that the hUL47 NES is sensitive to the inhibitor of CRM1-mediated nuclear export leptomycin B. Hence, hUL47 joins a growing list of virus-encoded RNA-binding proteins that use CRM1 to exit the nucleus.

Phylogeny and recombination history of gallid herpesvirus 2 (Marek's disease virus) genomes

Phylogenetic analyses based on concatenated amino acid sequences from orthologous loci from eight genomes of alpha herpesviruses infecting birds provided strong support for the following hypotheses: (1) gallid HV3 is a sister taxon to gallid HV2 but gallid HV1 is not closely related to the other two chicken herpesviruses; (2) meleagrid HV1 is closer to both gallid HV2 and gallid HV3 than is gallid HV1; (3) within gallid HV2, the virulent GA genome forms an outgroup to both the avirulent CVI988 genome and the highly virulent Md5 and Md11 genomes. Analysis of the pattern of synonymous nucleotide substitution between orthologous genes shared by four complete genomes of gallid HV2 showed strong evidence of past events of homologous recombination that homogenized certain loci between genomes. Eight of these loci represented cases of loci homogenized between the CVI988, on the one hand, and the Md5 and Md11 genomes, on the other hand. Two others represented loci where the GA genome was homogenized with those of Md5 and Md11. The two loci (UL49.5 and RLORF12) that were homogenized among the virulent genomes GA, Md5, and Md11 are candidates for contributing to viral virulence.

Bovine herpesvirus 1 infection and infectious bovine rhinotracheitis

Bovine herpesvirus 1 (BoHV-1), classified as an alphaherpesvirus, is a major pathogen of cattle. Primary infection is accompanied by various clinical manifestations such as infectious bovine rhinotracheitis, abortion, infectious pustular vulvovaginitis, and systemic infection in neonates. When animals survive, a life-long latent infection is established in nervous sensory ganglia. Several reactivation stimuli can lead to viral re-excretion, which is responsible for the maintenance of BoHV-1 within a cattle herd. This paper focuses on an updated pathogenesis based on a molecular characterization of BoHV-1 and the description of the virus cycle. Special emphasis is accorded to the impact of the latency and reactivation cycle on the epidemiology and the control of BoHV-1. Several European countries have initiated BoHV-1 eradication schemes because of the significant losses incurred by disease and trading restrictions. The vaccines used against BoHV-1 are described in this context where the differentiation of infected from vaccinated animals is of critical importance to achieve BoHV-1 eradication.

Nucleocytoplasmic shuttling of bovine herpesvirus 1 UL47 protein in infected cells

Previous studies with transfected cells have shown that the herpes simplex virus type 1 (HSV-1) and bovine herpesvirus 1 (BHV-1) UL47 proteins shuttle between the nucleus and the cytoplasm. HSV-1 UL47 has also been shown to bind RNA. Here we examine the BHV-1 UL47 protein in infected cells using a green fluorescent protein-UL47-expressing virus. We show that UL47 is detected in the nucleus early in infection. We use fluorescence loss in photobleaching to show that nuclear UL47 undergoes rapid nucleocytoplasmic shuttling. Furthermore, we demonstrate that actinomycin D inhibits the reaccumulation of UL47 in the nuclei of infected cells. These results suggest that UL47 exhibits behavior similar to that of previously characterized RNA-transporting proteins.

Characterization of nuclear localization and export signals of the major tegument protein VP8 of bovine herpesvirus-1

Bovine herpesvirus-1 (BHV-1) VP8 is found in the nucleus immediately after infection. Transient expression of VP8 fused to yellow fluorescent protein (YFP) in COS-7 cells confirmed the nuclear localization of VP8 in the absence of other viral proteins. VP8 has four putative nuclear localization signals (NLS). Deletion of pat4 ((51)RRPR(54)) or pat7 ((48)PRVRRPR(54)) NLS2 abrogated nuclear accumulation, whereas deletion of (48)PRV(50) did not, so pat4 NLS2 is critical for nuclear localization of VP8. Furthermore, NLS1 ((11)RRPRR(15)), pat4 NLS2, and pat7 NLS2 were all capable of transporting the majority of YFP to the nucleus. Finally, a 12-amino-acid peptide with the sequence RRPRRPRVRRPR directed all of YFP into the nucleus, suggesting that reiteration of the RRPR motif makes the nuclear localization more efficient. Heterokaryon assays demonstrated that VP8 is also capable of shuttling between the nucleus and cytoplasm of the cell. Deletion mutant analysis revealed that this property is attributed to a leucine-rich nuclear export sequence (NES) consisting of amino acids (485)LSAYLTLFVAL(495). This leucine-rich NES caused transport of YFP to the cytoplasm. These results demonstrate that VP8 shuttles between the nucleus and cytoplasm.

Identification and characterization of the pseudorabies virus tegument proteins UL46 and UL47: role for UL47 in virion morphogenesis in the cytoplasm

Proteins encoded by the UL46 and UL47 genes of herpes simplex virus type 1 (HSV-1) constitute major components of the viral tegument. However, their functions have so far not been elucidated in detail. By use of monospecific antisera directed against bacterially expressed glutathione-S-transferase fusion proteins, the homologous UL46 and UL47 proteins of the alphaherpesvirus pseudorabies virus (PrV) were identified in virus-infected cells and in virions. The PrV UL46 gene product of 693 amino acids (aa) exhibits an apparent molecular mass of 95 kDa, whereas the UL47 product of 750 aa was identified as a 97-kDa protein. Both are present in purified virions, correlating with their role as tegument proteins. Immunofluorescence analysis by confocal laser scan microscopy showed that late in infection the UL46 product is detectable in the cytoplasm, whereas the UL47 product was observed to be diffuse in the cytoplasm and speckled in the nucleus. Virus mutants lacking either the UL46 or the UL47 gene or both were isolated on noncomplementing cells, demonstrating that these genes either singly or in combination are not required for productive viral replication. However, plaque sizes were decreased. Interestingly, in one-step growth analysis, UL47 deletion mutants exhibited an approximately 10-fold decrease in final titers, whereas the UL46 deletion mutant was not affected. This finding correlated with ultrastructural observations which showed unimpaired virion morphogenesis in the absence of the UL46 protein, whereas in the absence of the UL47 protein intracytoplasmic aggregates of partially tegumented capsids were observed. In summary, we identified the PrV UL46 and UL47 proteins and show that the UL47 protein plays an important role in virion assembly in the cytoplasm.

The significance of the Golgi complex in envelopment of bovine herpesvirus 1 (BHV-1) as revealed by cryobased electron microscopy

Nucleocapsids of herpesviruses originate in the nucleus of host cells and bud through the inner nuclear membrane acquiring tegument and envelope. The release of the enveloped virus particle from the perinuclear space is unknown. Cryobased electron microscopic imaging revealed enveloped virus particles within cisterns associated with the perinuclear space, a pre-Golgi compartment connecting Golgi cisterns to the perinuclear space, and enveloped virus particles in Golgi cisterns where they are packaged into transport vacuoles by membrane fission. To our knowledge, our images show for the first time the connectivity from the perinuclear space to Golgi cisterns. The data strongly indicate an intracisternal transport of enveloped virus particles from the budding site to the packaging site. Budding starts by condensation at the inner membrane. Condensation involving the viral envelope and peripheral tegument was persistent in virus particles within perinuclear space and associated cisterns. Virus particles within Golgi cisterns and transport vacuoles originating by Golgi membrane fission, however, lacked condensation. Instead, spikes were clearly evident. The phenomenon of condensation is considered likely to be responsible for preventing fusion of the viral envelope with cisternal membranes and/or for driving virions from the perinuclear space to Golgi cisterns. Glycoprotein K is discussed to likely play a role in the intracisternal transportation of virions. In addition to the pathway including intracisternal transport and packaging, there were clear indications for the well-known pathway involving wrapping of cytoplasmic nucleocapsids by Golgi membranes. The origin of the cytoplasmic nucleocapsids, however, remains obscure. Lack of evidence for release of nucleocapsids at the outer nuclear membrane suggests that the process is very rapid, or that nucleocapsids pass the nucleocytoplasmic barrier via an alternative route.

Distinctions between bovine herpesvirus 1 and herpes simplex virus type 1 VP22 tegument protein subcellular associations

The alphaherpesvirus tegument protein VP22 has been characterized with multiple traits including microtubule reorganization, nuclear localization, and nonclassical intercellular trafficking. However, all these data were derived from studies using herpes simplex virus type 1 (HSV-1) and may not apply to VP22 homologs of other alphaherpesviruses. We compared subcellular attributes of HSV-1 VP22 (HVP22) with bovine herpesvirus 1 (BHV-1) VP22 (BVP22) using green fluorescent protein (GFP)-fused VP22 expression vectors. Fluorescence microscopy of cell lines transfected with these constructs revealed differences as well as similarities between the two VP22 homologs. Compared to that of HVP22, the BVP22 microtubule interaction was much less pronounced. The VP22 nuclear interaction varied, with a marbled or halo appearance for BVP22 and a speckled or nucleolus-bound appearance for HVP22. Both VP22 homologs associated with chromatin at various stages of mitosis and could traffic from expressing cells to the nuclei of nonexpressing cells. However, distinct qualitative differences in microtubule, nuclear, and chromatin association as well as trafficking were observed. The differences in VP22 homolog characteristics revealed in this study will help define VP22 function within HSV-1 and BHV-1 infection.

A role for bovine herpesvirus 1 (BHV-1) glycoprotein E (gE) tyrosine phosphorylation in replication of BHV-1 wild-type virus but not BHV-1 gE deletion mutant virus

Bovine herpesvirus 1 (BHV-1), an alphaherpesvirus, is a major pathogen that causes respiratory and reproductive infections. We observed tyrosine phosphorylation of a 95-kDa viral protein and dephosphorylation of 55- and 103-kDa cellular proteins during the course of BHV-1 infection. We demonstrated BHV-1 glycoprotein E (gE) to be the tyrosine phosphorylated viral protein by immunoprecipitation. Inhibition of phosphorylation of BHV-1 gE by tyrosine kinase inhibitors genistein and tyrphostin AG1478 substantially lowered the viral titer in Madin-Darby bovine kidney cells. The decrease in viral titer was directly proportional to the decrease in phosphorylation of the BHV-1 gE. Interestingly, these kinase inhibitors did not inhibit the replication of the BHV-1 gE deletion mutant virion (BHV-1gEDelta3.1). Our findings suggest that the wild-type BHV-1, with a functional gE protein, uses a different pathway of signaling events than the BHV-1 gE deletion mutant in replication. Our results indicate that the tyrosine phosphorylation of the cytoplasmic tail of BHV-1 gE is an important post-translational modification of the functional protein. An application of this study may be the use of tyrosine kinase inhibitors in controlling the BHV-1 infection.

Molecular virology of ruminant herpesviruses

Molecular virology has served to establish bovine herpesvirus 1 (BHV-1) as the prototype member of ruminant herpesviruses. Based on the genomic sequence of the virus, we aim to identify and characterize virus-specified components, to explain their concerted action, and to predict how the chain of events during the lytic and latent phases of the viral life cycle may be interrupted. The nucleotide sequence of the BHV-1 genome (136 kb) has just been completed by international cooperation (July 1995; except for a small gap in UL36). It comprises 67 unique genes and 2 genes, both duplicated, in the inverted repeats. In general, these genes exhibit strong homology at the amino acid sequence level to those of other alphaherpesviruses (HSV-1, VZV, EHV-1) and are arranged in similar order. A few genes are peculiar to only one or two herpesviruses, e.g. in BHV-1 the circ, UL0.5, UL3.5 and US1.5 genes. Not long ago, the repertoire of BHV-1 proteins under study was restricted to the three major glycoproteins (gB, gC, and gD) and thymidine kinase. The repertoire is now growing rapidly and includes 7 additional glycoproteins (gE, gI, gH, gL, gG, gK and gM), a number of enzymes (e.g. ribonucleotide reductase, DNA Polymerase, dUTPase), and a group of regulatory proteins (BICPO, 4, 22, and 27, alpha TIF). Investigations into the functions of these proteins and comparison with their counterparts in other herpesviruses should reveal which are useful targets for diagnosis, prevention or antiviral treatment. Recombinant viruses containing deletions or replacements of individual genes are being created, aiming at vaccine development and insights into pathogenesis, notably latency, neurotropism, and interference with host functions. Molecular analysis of other ruminant herpesviruses is much less advanced. Over a dozen virus species have been described; most share basic properties with BHV-1 and may be classified as alphaherpesviruses. The gammaherpesviruses are represented by the proposed agent of malignant catarrhal fever, alcelaphine herpesvirus 1, and by bovine herpesvirus 4, whose partial sequences exhibit similarity to herpesvirus saimiri.

Gene contents in a 31-kb segment at the left genome end of bovine herpesvirus-1

We report the nucleotide sequence of a 31-kb segment at the left genome end of bovine herpesvirus-1 (BHV-1) and show that it comprises 19 different open reading frames (ORFs), including seven which have been described previously (circ, dUTPase, UL49.5, alpha TIF, VP8, glycoprotein C, and ribonucleotide reductase small subunit). The new sequence resulted in a correction at the C-terminus of glycoprotein C. All 19 ORFs exhibited strong amino acid sequence homology to the gene products of other alphaherpesviruses. The BHV-1 ORFs were arranged colinearly with the prototype sequence of herpes simplex virus 1 (HSV-1) in the range of the UL54 to UL37 genes. No BHV-1 homologs of the HSV-1 UL56, UL55, and UL45 genes were identified. The BHV-1 circ gene was the only gene without a HSV-1 counterpart. The additional ORFs 1 and 2 found at the left genome end of equine herpesvirus-1 (EHV-1) were absent in BHV-1. Among the newly sequenced BHV-1 ORFs are homologs of ICP27 (UL54), glycoprotein K (UL53), helicase-primase (UL52), DNA polymerase accessory protein (UL42), ribonucleotide reductase large subunit (UL39), and several virion proteins (UL49, UL46, UL43, UL41, UL38, UL37), most of which are strongly conserved in all herpesviruses. The possible functions of the proteins encoded within the sequenced region are assessed and features found are discussed.

The bovine herpesvirus type 1 major tegument protein VP8 expressed in recombinant vaccinia virus does not induce significant immunity in mice

We previously reported the characterization of the gene encoding the bovine herpesvirus type 1 (BHV-1) major tegument protein VP8. With the aim of defining the immunological properties of this protein, we constructed a recombinant vaccinia virus (VV-VP8) in which expression of the VP8 gene was regulated by the P7.5 early/late promoter. Since the sequence of the VP8 gene contained a TTTTTNT motif known to serve as a transcription termination signal of vaccinia virus genes of the early class, a second recombinant (VV-VP8-Mut) in which this signal was modified by site-directed mutagenesis was created. Characterization of the recombinant viruses revealed that truncated VP8 mRNA and protein (69 kDa) were synthesized in VV-VP8 infected cells, whereas cells infected with VV-VP8-Mut produced a protein which was undistinguishable from that of the BHV-1 encoded protein (92-94 kDa). Immunization of BALB/c mice (H-2d) with VV-VP8-Mut induced a low VP8-specific antibody response whereas no specific response was induced in VV-VP8 inoculated mice. The low humoral response elicited was similar in C57BL/6 (H-2b) and C3H (H-2k) mice. Furthermore, immunization of mice with VV-VP8-Mut did not induce a BHV-1-specific lymphoproliferation in the three mice strains examined. Our results contrast with a recent study showing that immunization of calves with purified VP8 stimulated both T cell proliferation and antibody production.

The dominant phosphoprotein pp65 (UL83) of human cytomegalovirus is dispensable for growth in cell culture

The phosphoprotein pp65 (ppUL83) of human cytomegalovirus (HCMV) is abundantly synthesized during lytic infection in cultured fibroblasts. As a major constituent of extracellular particles, it gains entry to infected cells immediately after adsorption and subsequently translocates to the cell nucleus. This efficient transport is mediated by unique nuclear localization signals. To study the function of pp65, a viral deletion mutant was constructed by replacing the pp65 gene with the bacterial neomycin phosphotransferase gene, driven by the simian virus 40 early promoter. The resulting virus, RVAd65, could be grown and selected on human fibroblasts without complementation. The deletion of the pp65 gene in RVAd65 was verified by using Southern blot and PCR analyses. The lack of expression from the gene was investigated by immunoblotting with pp65-specific monoclonal antibodies. Single-cycle growth analyses showed that RVAd65 grew to levels of infectivity comparable to those of the wild-type virus. Therefore, pp65 is nonessential for the growth of HCMV in human fibroblasts. Electron microscopy revealed no differences in the processes of virion morphogenesis, although the maturation appeared to be delayed. However, the kinetics of expression of the immediate-early genes UL122 and UL123, the early gene UL44, and the late gene UL32 were the same in RVAd65-infected cells as in wild-type virus-infected cells in immunoblot analyses. In vitro phosphorylation assays showed that some of the virion proteins were labelled to a markedly reduced extent by virion-associated kinases in RVAd65 compared with wild-type virus. We therefore conclude that although deletion of the pp65 gene does not abolish replication of HCMV, a recombinant virus lacking pp65 displays phenotypic alterations compared with wild-type virus during growth in cultured fibroblasts.