Pseudorabies virus mutants lacking the essential glycoprotein gII can be complemented by glycoprotein gI of bovine herpesvirus 1

Neuropilin-1 Facilitates Pseudorabies Virus Replication and Viral Glycoprotein B Promotes Its Degradation in a Furin-Dependent Manner

Pseudorabies virus (PRV), which is extremely infectious and can infect numerous mammals, has a risk of spillover into humans. Virus-host interactions determine viral entry and spreading. Here, we showed that neuropilin-1 (NRP1) significantly potentiates PRV infection. Mechanistically, NRP1 promoted PRV attachment and entry, and enhanced cell-to-cell fusion mediated by viral glycoprotein B (gB), gD, gH, and gL. Furthermore, through in vitro coimmunoprecipitation (Co-IP) and bimolecular fluorescence complementation (BiFC) assays, NRP1 was found to physically interact with gB, gD, and gH, and these interactions were C-end Rule (CendR) motif independent, in contrast to currently known viruses. Remarkably, we illustrated that the viral protein gB promotes NRP1 degradation via a lysosome-dependent pathway. We further demonstrate that gB promotes NRP1 degradation in a furin-cleavage-dependent manner. Interestingly, in this study, we generated gB furin cleavage site (FCS)-knockout PRV (Δfurin PRV) and evaluated its pathogenesis; in vivo, we found that Δfurin PRV virulence was significantly attenuated in mice. Together, our findings demonstrated that NRP1 is an important host factor for PRV and that NRP1 may be a potential target for antiviral intervention. IMPORTANCE Recent studies have shown accelerated PRV cross-species spillover and that PRV poses a potential threat to humans. PRV infection in humans always manifests as a high fever, tonic-clonic seizures, and encephalitis. Therefore, understanding the interaction between PRV and host factors may contribute to the development of new antiviral strategies against PRV. NRP1 has been demonstrated to be a receptor for several viruses that harbor CendR, including SARS-CoV-2. However, the relationships between NRP1 and PRV are poorly understood. Here, we found that NRP1 significantly potentiated PRV infection by promoting PRV attachment and enhanced cell-to-cell fusion. For the first time, we demonstrated that gB promotes NRP1 degradation via a lysosome-dependent pathway. Last, in vivo, Δfurin PRV virulence was significantly attenuated in mice. Therefore, NRP1 is an important host factor for PRV, and NRP1 may be a potential target for antiviral drug development.

Serological Cross-Reactivity Between Bovine alphaherpesvirus 2 and Bovine alphaherpesvirus 1 in a gB-ELISA: A Case Report in Italy

In this study, we demonstrated for the first time in Italy, the serological cross-reactivity between Bovine alphaherpesvirus 2 (BoHV-2) and Bovine alphaherpesvirus 1 (BoHV-1). Five months after arriving at a performance test station in Central Italy, a 6-month-old calf, which was part of a group of 57 animals, tested positive for BoHV-1 in a commercial gB-ELISA test. It was immediately transferred to the quarantine unit and subjected to clinical observation and serological and virological investigations. During this period, the calf showed no clinical signs. The results from laboratory investigations demonstrated the presence of antibodies via competitive glycoprotein B (gB) ELISAs, indirect BoHV-1 ELISAs, and indirect BoHV-2 ELISAs. Furthermore, the plaque reduction assay provided evidence for the presence of antibodies only for BoHV-2, whereas the virus neutralization test showed negative results for both BoHV-1 and BoHV-5. These findings strongly suggest the occurrence of a serological cross-reactivity between BoHV-2 and BoHV-1. Interference of BoHV-2 antibodies in serological BoHV-1 diagnostics should be considered during routine IBR tests, especially when animals are kept in a performance test station.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

Regulation of Herpes Simplex Virus Glycoprotein-Induced Cascade of Events Governing Cell-Cell Fusion

Receptor-dependent herpes simplex virus (HSV)-induced cell-cell fusion requires glycoproteins gD, gH/gL, and gB. Our current model posits that during fusion, receptor-activated conformational changes in gD activate gH/gL, which subsequently triggers the transformation of the prefusion form of gB into a fusogenic state. To examine the role of each glycoprotein in receptor-dependent cell-cell fusion, we took advantage of our discovery that fusion by wild-type herpes simplex virus 2 (HSV-2) glycoproteins occurs twice as fast as that achieved by HSV-1 glycoproteins. By sequentially swapping each glycoprotein between the two serotypes, we established that fusion speed was governed by gH/gL, with gH being the main contributor. While the mutant forms of gB fuse at distinct rates that are dictated by their molecular structure, these restrictions can be overcome by gH/gL of HSV-2 (gH2/gL2), thereby enhancing their activity. We also found that deregulated forms of gD of HSV-1 (gD1) and gH2/gL2 can alter the fusogenic potential of gB, promoting cell fusion in the absence of a cellular receptor, and that deregulated forms of gB can drive the fusion machinery to even higher levels. Low pH enhanced fusion by affecting the structure of both gB and gH/gL mutants. Together, our data highlight the complexity of the fusion machinery, the impact of the activation state of each glycoprotein on the fusion process, and the critical role of gH/gL in regulating HSV-induced fusion. IMPORTANCE Cell-cell fusion mediated by HSV glycoproteins requires gD, gH/gL, gB, and a gD receptor. Here, we show that fusion by wild-type HSV-2 glycoproteins occurs twice as fast as that achieved by HSV-1 glycoproteins. By sequentially swapping each glycoprotein between the two serotypes, we found that the fusion process was controlled by gH/gL. Restrictions imposed on the gB structure by mutations could be overcome by gH2/gL2, enhancing the activity of the mutants. Under low-pH conditions or when using deregulated forms of gD1 and gH2/gL2, the fusogenic potential of gB could only be increased in the absence of receptor, underlining the exquisite regulation that occurs in the presence of receptor. Our data highlight the complexity of the fusion machinery, the impact of the activation state of each glycoprotein on the fusion process, and the critical role of gH/gL in regulating HSV-induced fusion.

Varicella-Zoster Virus Glycoproteins: Entry, Replication, and Pathogenesis

Varicella-zoster virus (VZV), an alphaherpesvirus that causes chicken pox (varicella) and shingles (herpes zoster), is a medically important pathogen that causes considerable morbidity and, on occasion, mortality in immunocompromised patients. Herpes zoster can afflict the elderly with a debilitating condition, postherpetic neuralgia, triggering severe, untreatable pain for months or years. The lipid envelope of VZV, similar to all herpesviruses, contains numerous glycoproteins required for replication and pathogenesis.

PURPOSE OF REVIEW

To summarize the current knowledge about VZV glycoproteins and their roles in cell entry, replication and pathogenesis.

RECENT FINDINGS

The functions for some VZV glycoproteins are known, such as gB, gH and gL in membrane fusion, cell-cell fusion regulation, and receptor binding properties. However, the molecular mechanisms that trigger or mediate VZV glycoproteins remains poorly understood.

SUMMARY

VZV glycoproteins are central to successful replication but their modus operandi during replication and pathogenesis remain elusive requiring further mechanistic based studies.

Comparative analysis of glycoprotein B (gB) of equine herpesvirus type 1 and type 4 (EHV-1 and EHV-4) in cellular tropism and cell-to-cell transmission

Glycoprotein B (gB) plays an important role in alphaherpesvirus cellular entry and acts in concert with gD and the gH/gL complex. To evaluate whether functional differences exist between gB1 and gB4, the corresponding genes were exchanged between the two viruses. The gB4-containing-EHV-1 (EHV-1_gB4) recombinant virus was analyzed for growth in culture, cell tropism, and cell entry rivaling no significant differences when compared to parental virus. We also disrupted a potential integrin-binding motif, which did not affect the function of gB in culture. In contrast, a significant reduction of plaque sizes and growth kinetics of gB1-containing-EHV-4 (EHV-4_gB1) was evident when compared to parental EHV-4 and revertant viruses. The reduction in virus growth may be attributable to the loss of functional interaction between gB and the other envelope proteins involved in virus entry, including gD and gH/gL. Alternatively, gB4 might have an additional function, required for EHV-4 replication, which is not fulfilled by gB1. In conclusion, our results show that the exchange of gB between EHV-1 and EHV-4 is possible, but results in a significant attenuation of virus growth in the case of EHV-4_gB1. The generation of stable recombinant viruses is a valuable tool to address viral entry in a comparative fashion and investigate this aspect of virus replication further.

Antiviral activities of phosphonoformate sodium to pseudorabies herpesvirus infection in vitro

CONTEXT

Phosphonoformate sodium (PFS) has been used as an anti-herpesvirus drug; nevertheless, studies of the use of PFS for treatment of pseudorabies herpesvirus (PrV) infection in the veterinary setting have not been widely reported.

OBJECTIVE

The present study aimed to analyze the inhibitory effect of PFS on cell infection and apoptosis induced by PrV.

MATERIALS AND METHODS

The infectivity of PrV was determined by plaque assays when PFS was applied to the virus, to the virus-infected cells, and to the cells prior to infection. PCR amplifying DNA polymerase, gE, gG, and gD genes of PrV was performed. PrV-induced cell apoptosis was analyzed by immunofluorescence and flow cytometry.

RESULTS

PFS inhibits cell infection by PrV. Addition of the drug decreased the number of apoptotic cells. Amplification of DNA polymerase and other viral structural genes detected in this study by PCR was reduced, because there were fewer viral DNA copies being made in the presence of the drug. The drug has an inhibitory effect on cell apoptosis induced by PrV.

DISCUSSION AND CONCLUSION

PFS has inhibitory effects on cell infection by PrV, which may be used as an anti-PrV agent or combined with other anti-PrV agents. PrV-induced cell apoptotic cells and viral DNA copies decreased in the presence of the PFS.

Pseudorabies virus infection alters neuronal activity and connectivity in vitro

Alpha-herpesviruses, including human herpes simplex virus 1 & 2, varicella zoster virus and the swine pseudorabies virus (PRV), infect the peripheral nervous system of their hosts. Symptoms of infection often include itching, numbness, or pain indicative of altered neurological function. To determine if there is an in vitro electrophysiological correlate to these characteristic in vivo symptoms, we infected cultured rat sympathetic neurons with well-characterized strains of PRV known to produce virulent or attenuated symptoms in animals. Whole-cell patch clamp recordings were made at various times after infection. By 8 hours of infection with virulent PRV, action potential (AP) firing rates increased substantially and were accompanied by hyperpolarized resting membrane potentials and spikelet-like events. Coincident with the increase in AP firing rate, adjacent neurons exhibited coupled firing events, first with AP-spikelets and later with near identical resting membrane potentials and AP firing. Small fusion pores between adjacent cell bodies formed early after infection as demonstrated by transfer of the low molecular weight dye, Lucifer Yellow. Later, larger pores formed as demonstrated by transfer of high molecular weight Texas red-dextran conjugates between infected cells. Further evidence for viral-induced fusion pores was obtained by infecting neurons with a viral mutant defective for glycoprotein B, a component of the viral membrane fusion complex. These infected neurons were essentially identical to mock infected neurons: no increased AP firing, no spikelet-like events, and no electrical or dye transfer. Infection with PRV Bartha, an attenuated circuit-tracing strain delayed, but did not eliminate the increased neuronal activity and coupling events. We suggest that formation of fusion pores between infected neurons results in electrical coupling and elevated firing rates, and that these processes may contribute to the altered neural function seen in PRV-infected animals.

Human cytomegalovirus glycoprotein B is required for virus entry and cell-to-cell spread but not for virion attachment, assembly, or egress

Glycoprotein B (gB) homologs are conserved throughout the family Herpesviridae and appear to serve essential, universal functions, as well as specific functions unique to a particular herpesvirus. Genetic analysis is a powerful tool to analyze protein function, and while it has been possible to generate virus mutants, complementation of essential virus knockouts has been problematic. Human cytomegalovirus (HCMV) gB (UL55) plays an essential role in the replication cycle of the virus. To define the function(s) of gB in HCMV infection, the BAC system was used to generate a recombinant virus in which the UL55 gene was replaced with galK (pAD/CreDeltaUL55). UL55 deletions in the viral genome have been made before, demonstrating that UL55 is an essential gene. However, without being able to successfully complement the genetic defect, a phenotypic analysis of the mutant virus was impossible. We generated fibroblasts expressing HCMV gB that complement pAD/CreDeltaUL55 and produce infectious virions lacking the UL55 gene but containing wild-type gB on the virion surface (DeltaUL55-gB HCMV). This is the first successful complementation of an HCMV mutant with a glycoprotein deleted. To characterize DeltaUL55 infection in the absence of gB, noncomplementing cells were infected with DeltaUL55-gB virus. All stages of gene expression were detected, and significant amounts of DNase-resistant viral DNA genomes, representing whole intact virions, were released into the infected cell supernatant. Gradient purification of these virions revealed they lacked gB but contained other viral structural proteins. The gB-null virions were able to attach to the cell surface similarly to wild-type gB-containing virions but were defective in virus entry and cell-to-cell spread. Glycoprotein B-null virions do, however, contain infectious DNA, as IE gene expression can be detected in fibroblasts following treatment of attached gB-null virions with a membrane fusion agent, polyethylene glycol. Taken together, our results indicate that gB is required for virus entry and cell-to-cell spread of the virus. However, HCMV gB is not absolutely required for virus attachment or assembly and egress from infected cells.

Effects of simultaneous deletion of pUL11 and glycoprotein M on virion maturation of herpes simplex virus type 1

The conserved membrane-associated tegument protein pUL11 and envelope glycoprotein M (gM) are involved in secondary envelopment of herpesvirus nucleocapsids in the cytoplasm. Although deletion of either gene had only moderate effects on replication of the related alphaherpesviruses herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PrV) in cell culture, simultaneous deletion of both genes resulted in a severe impairment in virion morphogenesis of PrV coinciding with the formation of huge inclusions in the cytoplasm containing nucleocapsids embedded in tegument (M. Kopp, H. Granzow, W. Fuchs, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 78:3024-3034, 2004). To test whether a similar phenotype occurs in HSV-1, a gM and pUL11 double deletion mutant was generated based on a newly established bacterial artificial chromosome clone of HSV-1 strain KOS. Since gM-negative HSV-1 has not been thoroughly investigated ultrastructurally and different phenotypes have been ascribed to pUL11-negative HSV-1, single gene deletion mutants were also constructed and analyzed. On monkey kidney (Vero) cells, deletion of either pUL11 or gM resulted in ca.-fivefold-reduced titers and 40- to 50%-reduced plaque diameters compared to those of wild-type HSV-1 KOS, while on rabbit kidney (RK13) cells the defects were more pronounced, resulting in ca.-50-fold titer and 70% plaque size reduction for either mutant. Electron microscopy revealed that in the absence of either pUL11 or gM virion formation in the cytoplasm was inhibited, whereas nuclear stages were not visibly affected, which is in line with the phenotypes of corresponding PrV mutants. Simultaneous deletion of pUL11 and gM led to additive growth defects and, in RK13 cells, to the formation of large intracytoplasmic inclusions of capsids and tegument material, comparable to those in PrV-DeltaUL11/gM-infected RK13 cells. The defects of HSV-1DeltaUL11 and HSV-1DeltaUL11/gM could be partially corrected in trans by pUL11 of PrV. Thus, our data indicate that PrV and HSV-1 pUL11 and gM exhibit similar functions in cytoplasmic steps of virion assembly.

Neuronal pathways of viral invasion in mice after intranasal inoculation of pseudorabies virus PrV-9112C2 expressing bovine herpesvirus 1 glycoprotein B

In contrast to wild-type Pseudorabies virus (PrV), which infects the central nervous system mainly via fibres of the trigeminal and autonomous nerves, the PrV mutant PrV-9112C2, deleted in glycoprotein B but expressing its bovine herpesvirus 1 (BHV-1) homologue, was shown to infect the swine central nervous system (CNS) via the olfactory route. In this study application of PrV-9112C2 into the nose of mice resulted in CNS infection as described for wild-type PrV. These findings indicate that gB((BHV-1))-dependent changes in PrV's capability to infect swine olfactory sensory neurons (OSNs) are not prominent in mice and give evidence for viral entry receptors present in swine but not mice OSNs.

Alpha-herpesvirus glycoprotein D interaction with sensory neurons triggers formation of varicosities that serve as virus exit sites

α-Herpesviruses constitute closely related neurotropic viruses, including herpes simplex virus in man and pseudorabies virus (PRV) in pigs. Peripheral sensory neurons, such as trigeminal ganglion (TG) neurons, are predominant target cells for virus spread and lifelong latent infections. We report that in vitro infection of swine TG neurons with the homologous swine α-herpesvirus PRV results in the appearance of numerous synaptophysin-positive synaptic boutons (varicosities) along the axons. Nonneuronal cells that were juxtaposed to these varicosities became preferentially infected with PRV, suggesting that varicosities serve as axonal exit sites for the virus. Viral envelope glycoprotein D (gD) was found to be necessary and sufficient for the induction of varicosities. Inhibition of Cdc42 Rho GTPase and p38 mitogen-activated protein kinase signaling pathways strongly suppressed gD-induced varicosity formation. These data represent a novel aspect of the cell biology of α-herpesvirus infections of sensory neurons, demonstrating that virus attachment/entry is associated with signaling events and neuronal changes that may prepare efficient egress of progeny virus.

Assembly of pseudorabies virus genome-based transfer vehicle carrying major antigen sites of S gene of transmissible gastroenteritis virus: potential perspective for developing live vector vaccines

Two severe porcine infectious diseases, pseudorabies (PR) and transmissible gastroenteritis (TGE) caused by pseudorabies virus (PRV) and transmissible gastroenteritis virus (TGEV) respectively often result in serious economic loss in animal husbandry worldwide. Vaccination is the important prevention means against both infections. To achieve a PRV genome-based virus live vector, aiming at further TGEV/PRV bivalent vaccine development, a recombinant plasmid pUG was constructed via inserting partial PK and full-length gG genes of PRV strain Bartha K-61 amplified into pUC119 vector. In parallel, another recombinant pHS was generated by introducing a fragment designated S1 encoding the major antigen sites of S gene from TGEV strain TH-98 into a prokaryotic expression vector pP_ROEX HTc. The SV40 polyA sequence was then inserted into the downstream of S1 fragment of pHS. The continuous region containing S1fragment, SV40 polyA and four single restriction enzyme sites digested from pHS was subcloned into the downstream of gG promoter of pUG. In addition, a LacZ reporter gene was introduced into the universal transfer vector named pUGS-LacZ. Subsequently, a PRV genome-based virus live vector was generated via homologous recombination. The functionally effective vector was purified and partially characterized. Moreover, the potential advantages of this system are discussed.

Identification of functional domains in herpes simplex virus 2 glycoprotein B

Glycoprotein B (gB) is one of four membrane proteins that are essential for the entry of herpes simplex viruses (HSV) into cells, and coexpression of the same combination of proteins in transfected cells results in cell fusion. The latter effect is reminiscent of the ability of virus infection to cause cell fusion, particularly since the degree of fusion is greatly increased by syncytial mutations in gB. Despite intensive efforts with the gB homologs of HSV and some other herpesviruses, information about functionally important regions in the 700-amino-acid ectodomain of this protein is very limited at present. This is largely due to the misfolding of the majority of the mutants examined. It was shown previously that the percentage of correctly folded mutants could be increased by targeting only predicted loop regions (i.e., not alpha-helix or beta-strand), and by using this approach new functional domains in HSV-2 gB have now been identified.

Engineering glycoprotein B of bovine herpesvirus 1 to function as transporter for secreted proteins: a new protein expression approach

Glycoprotein B (gB) of bovine herpesvirus 1 (BHV-1) is essential for BHV-1 replication and is required for membrane fusion processes leading to virus penetration into the target cell and direct spreading of BHV-1 from infected to adjacent noninfected cells. Like many of the herpesvirus gB homologs, BHV-1 gB is proteolytically processed by furin, an endoproteinase localized in the trans-Golgi network. Cleavage by furin is a common mechanism for the activation of a number of viral fusion (F) proteins. Among these, the F proteins of both human and bovine respiratory syncytial virus (RSV) have the so-far unique feature that cleavage of the respective F protein precursors occurs at two furin recognition sites, resulting in the release of a 27-amino-acid intervening peptide which is secreted into the extracellular space. We showed recently that the intervening peptide of bovine RSV can be replaced by bovine interleukins which are secreted into the medium of cells infected with the respective bovine RSV recombinants (P. Konig, K. Giesow, K. Schuldt, U. J. Buchholz, and G. M. Keil, J. Gen. Virol. 85:1815-1824, 2004). To elucidate whether the approach to transport heterologous proteins as furin-excisable polypeptides functions in principle also in glycoproteins which are cleaved by furin only once, we inserted a second furin cleavage site into BHV-1 gB and integrated a 16-amino-acid peptide sequence, the 246-amino-acid green fluorescent protein (GFP), or the 167 amino acids for mature bovine alpha interferon (boIFN-alpha) as an intervening polypeptide. The resulting gB variants rescued gB-negative BHV-1 mutants, the resulting BHV-1 recombinants were fully infectious, and infected cells secreted biologically active GFP and boIFN-alpha, respectively. In contrast to the gB2Fu and gB2FuGFP precursor molecules, which were efficiently cleaved at both furin sites, the majority of pgB2FuIFN-alpha was not cleaved at the site between the amino-terminal (NH2) subunit and boIFN-alpha, whereas cleavage at the newly introduced site was normal. This resulted in virus particles that also contain the NH2-subunit/boIFN-alpha fusion protein within their envelopes. Our results demonstrate that BHV-1 gB can be used as a transporter for peptides and proteins which could be important for development of novel vaccines. In addition, the general principle might be useful for other applications, e.g., in gene therapy and also in nonviral systems.

Pathogenesis of neurotropic herpesviruses: role of viral glycoproteins in neuroinvasion and transneuronal spread

Neuroinvasion by herpesviruses requires entry into nerve endings in the periphery, transport to the cell body, replication in the cell body, axonal transport to the synapse and transneuronal viral spread. Entry occurs after receptor binding by fusion of virion envelope and cellular plasma membrane followed by microtubuli-assisted transport of capsids to the nuclear pore. By transneuronal spread, the virus gains access to synaptically linked neuronal circuits. A common set of herpesvirus glycoproteins is involved in entry and direct viral cell-cell spread. However, both processes can be distinguished by involvement of additional viral components. Interestingly, transneuronal spread appears to be functionally linked to intracytoplasmic formation of mature virions. This review will focus on the importance of herpesvirus envelope glycoproteins for infection of neurons and transneuronal spread, and their influence on viral pathogenesis.

Transmission of pseudorabies virus from immune-masked blood monocytes to endothelial cells

Pseudorabies virus (PRV) may cause abortion, even in the presence of vaccination-induced immunity. Blood monocytes are essential to transport the virus in these immune animals, including transport to the pregnant uterus. Infected monocytes express viral proteins on their cell surface. Specific antibodies recognize these proteins and should activate antibody-dependent cell lysis. Previous work showed that addition of PRV-specific polyclonal antibodies to PRV-infected monocytes induced internalization of viral cell surface proteins, protecting the cells from efficient antibody-dependent lysis in vitro (immune-masked monocytes). As a first step to reach the pregnant uterus, PRV has to cross the endothelial cell barrier of the maternal blood vessels. The current aim was to investigate in vitro whether immune-masked PRV-infected monocytes can transmit PRV in the presence of virus-neutralizing antibodies via adhesion and fusion of these monocytes with endothelial cells. Porcine blood monocytes, infected with a lacZ-carrying PRV strain, were incubated with PRV-specific antibodies to induce internalization. Then, cells were co-cultivated with endothelial cells for different periods of time. Only PRV-infected monocytes with internalized viral cell surface proteins adhered efficiently to endothelial cells. LacZ transmission to endothelial cells, as a measure for monocyte-endothelial cell fusion, could be detected after co-cultivation from 30 min onwards. Virus transmission was confirmed by the appearance of plaques. Adhesion of immune-masked PRV-infected monocytes to endothelial cells was mediated by cellular adhesion complex CD11b-CD18 and subsequent fusion was mediated by the virus. In conclusion, immune-masked PRV-infected monocytes can adhere and subsequently transmit virus to endothelial cells in the presence of PRV-neutralizing antibodies.

Different functional domains in the cytoplasmic tail of glycoprotein B are involved in Epstein-Barr virus-induced membrane fusion

A virus-free cell fusion assay relying on the transient transfection of Epstein-Barr virus (EBV) glycoproteins into cells provides an efficient and quantitative assay for characterizing the viral requirements necessary for fusion of the viral envelope with the B cell membrane. Extensive cellular fusion occurred when Daudi cells were layered onto Chinese hamster ovary K1 cells transiently expressing EBV glycoproteins gp42, gH, gL, and gB. This is the first direct evidence that gB is involved in the process of EBV entry. Moreover, mutational analysis of gB indicates that the cytoplasmic tail contains two distinct domains that function differentially in the process of fusion. The region from amino acids 802 to 816 is necessary for productive membrane fusion, while amino acids 817 to 841 comprise a domain that negatively regulates membrane fusion.

Cytoplasmic domain of herpes simplex virus gE causes accumulation in the trans-Golgi network, a site of virus envelopment and sorting of virions to cell junctions

Alphaherpesviruses express a heterodimeric glycoprotein, gE/gI, that facilitates cell-to-cell spread between epithelial cells and neurons. Herpes simplex virus (HSV) gE/gI accumulates at junctions formed between polarized epithelial cells at late times of infection. However, at earlier times after HSV infection, or when gE/gI is expressed using virus vectors, the glycoprotein localizes to the trans-Golgi network (TGN). The cytoplasmic (CT) domains of gE and gI contain numerous TGN and endosomal sorting motifs and are essential for epithelial cell-to-cell spread. Here, we swapped the CT domains of HSV gE and gI onto another HSV glycoprotein, gD. When the gD-gI(CT) chimeric protein was expressed using a replication-defective adenovirus (Ad) vector, the protein was found on both the apical and basolateral surfaces of epithelial cells, as was gD. By contrast, the gD-gE(CT) chimeric protein, gE/gI, and gE, when expressed by using Ad vectors, localized exclusively to the TGN. However, gD-gE(CT), gE/gI, and TGN46, a cellular TGN protein, became redistributed largely to lateral surfaces and cell junctions during intermediate to late stages of HSV infection. Strikingly, gE and TGN46 remained sequestered in the TGN when cells were infected with a gI(-)HSV mutant. The redistribution of gE/gI to lateral cell surfaces did not involve widespread HSV inhibition of endocytosis because the transferrin receptor and gE were both internalized from the cell surface. Thus, gE/gI accumulates in the TGN in early phases of HSV infection then moves to lateral surfaces, to cell junctions, at late stages of infection, coincident with the redistribution of a TGN marker. These results are related to recent observations that gE/gI participates in the envelopment of nucleocapsids into cytoplasmic vesicles (A. R. Brack, B. G. Klupp, H. Granzow, R. Tirabassi, L. W. Enquist, and T. C. Mettenleiter, J. Virol. 74:4004-4016, 2000) and that gE/gI can sort nascent virions from cytoplasmic vesicles specifically to the lateral surfaces of epithelial cells (D. C. Johnson, M. Webb, T. W. Wisner, and C. Brunetti, J. Virol. 75:821-833, 2000). Therefore, gE/gI localizes to the TGN, through interactions between the CT domain of gE and cellular sorting machinery, and then participates in envelopment of cytosolic nucleocapsids there. Nascent virions are then sorted from the TGN to cell junctions.

Primary envelopment of pseudorabies virus at the nuclear membrane requires the UL34 gene product

Primary envelopment of several herpesviruses has been shown to occur by budding of intranuclear capsids through the inner nuclear membrane. By subsequent fusion of the primary envelope with the outer nuclear membrane, capsids are released into the cytoplasm and gain their final envelope by budding into vesicles in the trans-Golgi area. We show here that the product of the UL34 gene of pseudorabies virus, an alphaherpesvirus of swine, is localized in transfected and infected cells in the nuclear membrane. It is also detected in the envelope of virions in the perinuclear space but is undetectable in intracytoplasmic and extracellular enveloped virus particles. Conversely, the tegument protein UL49 is present in mature virus particles and absent from perinuclear virions. In the absence of the UL34 protein, acquisition of the primary envelope is blocked and neither virus particles in the perinuclear space nor intracytoplasmic capsids or virions are observed. However, light particles which label with the anti-UL49 serum are formed in the cytoplasm. We conclude that the UL34 protein is required for primary envelopment, that the primary envelope is biochemically different from the final envelope in that it contains the UL34 protein, and that perinuclear virions lack the tegument protein UL49, which is present in mature virions. Thus, we provide additional evidence for a two-step envelopment process in herpesviruses.

A highly specific and sensitive sandwich blocking ELISA based on baculovirus expressed pseudorabies virus glycoprotein B

A direct sandwich blocking enzyme-linked immunosorbent assay (BacgB ELISA) based on the reaction between a monoclonal antibody (MAb) and a recombinant glycoprotein B (gB) of pseudorabies virus (PRV) was developed. This protein was obtained in large quantities from insect cells infected with a PRV gB recombinant baculovirus. Expression of the gB was confirmed by immunoperoxidase monolayer assay (IPMA) with gB specific MAbs. The specificity and sensitivity of the developed BacgB ELISA were evaluated and compared with two commercially available tests by using sets of sera of known PRV infection or vaccination history. For validation, 347 serum samples have been tested. The BacgB ELISA had a high sensitivity and specificity, which were comparable with those of the two commercial tests. In addition, the BacgB ELISA allows detecting anti-gB antibodies in pig serum as early as 7 days following infection. Also maternal antibodies in uninfected pig sera were detected. We conclude that the BacgB ELISA is a useful tool for the detection of as well vaccinated as infected pigs (including derivatives from gE negative vaccine strains), with the added advantage that it uses an antigen that can be produced safely and in large quantities.

Pseudorabies virus expressing bovine herpesvirus 1 glycoprotein B exhibits altered neurotropism and increased neurovirulence

Herpesvirus glycoproteins play dominant roles in the initiation of infection of target cells in culture and thus may also influence viral tropism in vivo. Whereas the relative contribution of several nonessential glycoproteins to neurovirulence and neurotropism of Pseudorabies virus (PrV), an alphaherpesvirus which causes Aujeszky's disease in pigs, has recently been uncovered in studies using viral deletion mutants, the importance of essential glycoproteins is more difficult to assess. We isolated an infectious PrV mutant, PrV-9112C2, which lacks the gene encoding the essential PrV glycoprotein B (gB) but stably carries in its genome and expresses the homologous gene of bovine herpesvirus 1 (BHV-1) (A. Kopp and T. C. Mettenleiter, J. Virol. 66:2754-2762, 1992). Apart from exhibiting a slight delay in penetration kinetics, PrV-9112C2 was similar in its growth characteristics in cell culture to wild-type PrV. To analyze the effect of the exchange of these homologous glycoproteins in PrV's natural host, swine, 4-week-old piglets were intranasally infected with 10(6) PFU of either wild-type PrV strain Kaplan (PrV-Ka), PrV-9112C2, or PrV-9112C2R, in which the PrV gB gene was reinserted instead of the BHV-1 gB gene. Animals infected with PrV-Ka and PrV-9112C2R showed a similar course of disease, i.e., high fever, marked respiratory symptoms but minimal neurological disorders, and excretion of high amounts of virus. All animals survived the infection. In contrast, animals infected with PrV-9112C2 showed no respiratory symptoms and developed only mild fever. However, on day 5 after infection, all piglets developed severe central nervous system (CNS) symptoms leading to death within 48 to 72 h. Detailed histological analyses showed that PrV-9112C2R infected all regions of the nasal mucosa and subsequently spread to the CNS preferentially by the trigeminal route. In contrast, PrV-9112C2 primarily infected the olfactory epithelium and spread via the olfactory route. In the CNS, more viral antigen and significantly more pronounced histological changes resulting in more severe encephalitis were found after PrV-9112C2 infection. Thus, our results demonstrate that replacement of PrV gB by the homologous BHV-1 glycoprotein resulted in a dramatic increase in neurovirulence combined with an alteration in the route of neuroinvasion, indicating that the essential gB is involved in determining neurotropism and neurovirulence of PrV.

Glycoproteins gM and gN of pseudorabies virus are dispensable for viral penetration and propagation in the nervous systems of adult mice

Glycoproteins gM and gN are conserved throughout the herpesviruses but are dispensable for viral replication in cell cultures. To assay for a function of these proteins in infection of an animal, deletion mutants of pseudorabies virus lacking gM or gN and corresponding revertants were analyzed for the ability to penetrate and propagate in the nervous systems of adult mice after intranasal inoculation. We demonstrate that neither of the two glycoproteins is required for infection of the nervous systems of mice by pseudorabies virus.

Mutations in the conserved carboxy-terminal hydrophobic region of glycoprotein gB affect infectivity of herpes simplex virus

Glycoprotein gB is the most highly conserved glycoprotein in the herpesvirus family and plays a critical role in virus entry and fusion. Glycoprotein gB of herpes simplex virus type 1 contains a hydrophobic stretch of 69 aa near the carboxy terminus that is essential for its biological activity. To determine the role(s) of specific amino acids in the carboxy-terminal hydrophobic region, a number of amino acids were mutagenized that are highly conserved in this region within the gB homologues of the family HERPESVIRIDAE: Three conserved residues in the membrane anchor domain, namely A786, A790 and A791, as well as amino acids G743, G746, G766, G770 and P774, that are non-variant in Herpesviridae, were mutagenized. The ability of the mutant proteins to rescue the infectivity of the gB-null virus, K082, in trans was measured by a complementation assay. All of the mutant proteins formed dimers and were incorporated in virion particles produced in the complementation assay. Mutants G746N, G766N, F770S and P774L showed negligible complementation of K082, whereas mutant G743R showed a reduced activity. Virion particles containing these four mutant glycoproteins also showed a markedly reduced rate of entry compared to the wild-type. The results suggest that non-variant residues in the carboxy-terminal hydrophobic region of the gB protein may be important in virus infectivity.

BHV-1: new molecular approaches to control a common and widespread infection

BACKGROUND

Herpesviruses are widespread viruses, causing severe infections in both humans and animals. Eradication of herpesviruses is extremely difficult because of their ability to establish latent and life-long infections. However, latency is only one tool that has evolved in herpesviruses to successfully infect their hosts; such viruses display a wide (and still incompletely known) panoply of genes and proteins that are able to counteract immune responses of their hosts. Envelope glycoproteins and cytokine inhibitors are two examples of such weapons. All of these factors make it difficult to develop diagnostics and vaccines, unless they are based on molecular techniques.

MATERIALS AND METHODS

Animal herpesviruses, because of their striking similarity to human ones, are suitable models to study the molecular biology of herpesviruses and develop strategies aimed at designing neurotropic live vectors for gene therapy as well as engineered attenuated vaccines.

RESULTS

BHV-1 is a neurotropic herpesvirus causing infectious rhinotracheitis (IBR) in cattle. It is a major plague in zootechnics and commercial trade, because of its ability to spread through asymptomatic carrier animals, frozen semen, and embryos. Such portals of infections are also important for human herpesviruses, which mainly cause systemic, eye, and genital tract infections, leading even to the development of cancer.

CONCLUSIONS

This review covers both the genetics and molecular biology of BHV-1 and its related herpesviruses. Epidemiology and diagnostic approaches to herpesvirus infections are presented. The role of herpesviruses in gene therapy and a broad introduction to classic and engineered vaccines against herpesviruses are also provided. http://link.springer-ny. com/link/service/journals/00020/bibs/5n5p261.html

Human herpesvirus-8 glycoprotein B interacts with Epstein-Barr virus (EBV) glycoprotein 110 but fails to complement the infectivity of EBV mutants

To characterize human herpesvirus 8 (HHV-8) gB, the open reading frame was PCR amplified from the HHV-8-infected cell line BCBL-1 and cloned into an expression vector. To facilitate detection of expressed HHV-8 gB, the cytoplasmic tail of the glycoprotein was tagged with the influenza hemagglutinin (HA) epitope. Expression of tagged HHV-8 gB (gB-HA), as well as the untagged form, was readily detected in CHO-K1 cells and several lymphoblastoid cell lines (LCLs). HHV-8 gB-HA was sensitive to endoglycosidase H treatment, and immunofluorescence revealed that HHV-8 gB-HA was detectable in the perinuclear region of CHO-K1 cells. These observations suggest that HHV-8 gB is not processed in the Golgi and localizes to the endoplasmic reticulum or nuclear membrane. Because both HHV-8 and EBV are gamma-herpesviruses, the ability of HHV-8 gB to interact with and functionally complement EBV gp110 was examined. HHV-8 gB-HA and EBV gp110 co-immunoprecipitated, indicating formation of hetero-oligomers. However, HHV-8 gB-HA and HHV-8 gB failed to restore the infectivity of gp110-negative EBV mutants. These findings indicate that although HHV-8 gB and EBV gp110 have similar patterns of intracellular localization and can interact, there is not sufficient functional homology to allow efficient complementation.

Receptor-binding properties of a soluble form of human cytomegalovirus glycoprotein B

The human cytomegalovirus (HCMV) glycoprotein B (gB) (also known as gpUL55) homolog is an important mediator of virus entry and cell-to-cell dissemination of infection. To examine the potential ligand-binding properties of gB, a soluble form of gB (gB-S) was radiolabeled, purified, and tested in cell-binding experiments. Binding of gB-S to human fibroblast cells was found to occur in a dose-dependent, saturable, and specific manner. Scatchard analysis demonstrated a biphasic plot with the following estimated dissociation constants (Kd): Kd1, 4.96 x 10(-6) M; Kd2, 3.07 x 10(-7) M. Cell surface heparan sulfate proteoglycans (HSPGs) were determined to serve as one class of receptors able to facilitate gB-S binding. Both HSPG-deficient Chinese hamster ovary (CHO) cells and fibroblast cells with enzymatically removed HSPGs had 40% reductions in gB-S binding, whereas removal of chondroitin sulfate had no effect. However, a significant proportion of gB-S was able to associate with the cell surface in the absence of HSPGs via an undefined nonheparin component. Binding affinity analysis of gB-S binding to wild-type CHO-K1 cells demonstrated biphasic binding kinetics (Kd1, 9.85 x 10(-6) M; Kd2, 4.03 x 10(-8) M), whereas gB-S binding to HSPG-deficient CHO-677 cells exhibited single-component binding kinetics (Kd, 7.46 x 10(-6) M). Together, these data suggest that gB-S associates with two classes of cellular receptors. The interaction of gB with its receptors is physiologically relevant, as evidenced by an inhibitory effect on HCMV entry when cells were pretreated with purified gB-S. This inhibition was determined to be manifested at the level of virus attachment. We conclude that gB is a ligand for HCMV that mediates an interaction with a cellular receptor(s) during HCMV infection.

Pseudorabies virus glycoprotein gK is a virion structural component involved in virus release but is not required for entry

The pseudorabies virus (PrV) gene homologous to herpes simplex virus type 1 (HSV-1) UL53, which encodes HSV-1 glycoprotein K (gK), has recently been sequenced (J. Baumeister, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 69:5560-5567, 1995). To identify the corresponding protein, a rabbit antiserum was raised against a 40-kDa glutathione S-transferase-gK fusion protein expressed in Escherichia coli. In Western blot analysis, this serum detected a 32-kDa polypeptide in PrV-infected cell lysates as well as a 36-kDa protein in purified virion preparations, demonstrating that PrV gK is a structural component of virions. After treatment of purified virions with endoglycosidase H, a 34-kDa protein was detected, while after incubation with N-glycosidase F, a 32-kDa protein was specifically recognized. This finding indicates that virion gK is modified by N-linked glycans of complex as well as high-mannose type. For functional analysis, the UL53 open reading frame was interrupted after codon 164 by insertion of a gG-lacZ expression cassette into the wild-type PrV genome (PrV-gKbeta) or by insertion of the bovine herpesvirus 1 gB gene into a PrV gB- genome (PrV-gK(gB)). Infectious mutant virus progeny was obtained only on complementing gK-expressing cells, suggesting that gK has an important function in the replication cycle. After infection of Vero cells with either gK mutant, only single infected cells or small foci of infected cells were visible. In addition, virus yield was reduced approximately 30-fold, and penetration kinetics showed a delay in entry which could be compensated for by phenotypic gK complementation. Interestingly, the plating efficiency of PrV-gKbeta was similar to that of wild-type PrV on complementing and noncomplementing cells, pointing to an essential function of gK in virus egress but not entry. Ultrastructurally, virus assembly and morphogenesis of PrV gK mutants in noncomplementing cells were similar to wild-type virus. However, late in infection, numerous nucleocapsids were found directly underneath the plasma membrane in stages typical for the entry process, a phenomenon not observed after wild-type virus infection and also not visible after infection of gK-complementing cells. Thus, we postulate that presence of gK is important to inhibit immediate reinfection.

Glycoproteins M and N of pseudorabies virus form a disulfide-linked complex

Genes homologous to the herpes simplex virus UL49.5 open reading frame are conserved throughout the Herpesviridae. In the alphaherpesvirus pseudorabies virus (PrV), the UL49.5 product is an O-glycosylated structural protein of the viral envelope, glycoprotein N (gN) (A. Jöns, H. Granzow, R. Kuchling, and T. C. Mettenleiter, J. Virol. 70:1237-1241, 1996). For functional characterization of gN, a gN-negative PrV mutant, PrV-gNbeta, and the corresponding rescuant, PrV-gNbetaR, were constructed, gN-negative PrV was able to productively replicate on noncomplementing cells, and one-step growth in cell culture was only slightly reduced compared to that of wild-type PrV. However, penetration was significantly delayed. In indirect immunofluorescence assays with rabbit serum directed against baculovirus-expressed gN, specific staining of wild-type PrV-infected cells occurred only after permeabilization of cells, whereas live cells failed to react with the antiserum. This indicates the lack of surface accessibility of gN in the plasma membrane of a PrV-infected cell. Western blot analyses and radioimmunoprecipitation experiments under reducing and nonreducing conditions led to the discovery of a heteromeric complex composed of gM and gN. The complex was stable in the absence of 2-mercaptoethanol but dissociated after the addition of the reducing agent, indicating that the partners are linked by disulfide bonds. Finally, gN was absent from gM-negative PrV virions, whereas gM was readily detected in virions in the absence of gN. Thus, gM appears to be required for virion localization of gN.

Expression, purification, and characterization of a soluble form of human cytomegalovirus glycoprotein B

The human cytomegalovirus glycoprotein B gene (gB; gpUL55) was truncated at amino acid 692 and recombined into Autographa californica nuclear polyhedrosis virus (baculovirus). Infection of insect cells with the recombinant baculovirus resulted in high-level expression and secretion of the truncated gB protein (gB-S) into the culture medium. Purification of gB-S by monoclonal antibody affinity chromatography yielded a protein of ca. 200 kDa. Characterization of the 200-kDa purification product indicated that the recombinant gB protein retained many structural and functional features of the viral gB. Comparison of electrophoretic migration patterns in reduced versus nonreduced protein samples and immune blotting analysis with antibodies specific for the amino or carboxy-terminus of gB demonstrated that the recombinant protein was composed of disulfide linked 69 kDa amino terminal and 35-kDa carboxy-terminal fragments. In addition, recognition of the 200-kDa gB-S by a conformational-dependent, oligomer-specific monoclonal antibody suggested that gB-S was properly folded and dimeric. Like the viral gB, gB-S had heparin binding ability. One heparin binding site was found to reside within the 35-kDa carboxy-terminal fragment (aa 492-692). Heparin binding was abolished when gB-S was denatured. These data suggest that gB contains a novel heparin binding motif that is at least partially conformational dependent.

Functional complementation of UL3.5-negative pseudorabies virus by the bovine herpesvirus 1 UL3.5 homolog

The UL3.5 gene is positionally conserved but highly variable in size and sequence in different members of the Alphaherpesvirinae and is absent from herpes simplex virus genomes. We have shown previously that the pseudorabies virus (PrV) UL3.5 gene encodes a nonstructural protein which is required for secondary envelopment of intracytoplasmic virus particles in the trans-Golgi region. In the absence of UL3.5 protein, naked nucleocapsids accumulate in the cytoplasm, release of infectious virions is drastically reduced, and plaque formation in cell culture is inhibited (W. Fuchs, B. G. Klupp, H. Granzow, H.-J. Rziha, and T. C. Mettenleiter, J. Virol. 70:3517-3527, 1996). To assay functional complementation by a heterologous herpesviral UL3.5 protein, the UL3.5 gene of bovine herpesvirus 1 (BHV-1) was inserted at two different sites within the genome of UL3.5-negative PrV. In cells infected with the PrV recombinants the BHV-1 UL3.5 gene product was identified as a 17-kDa protein which was identical in size to the UL3.5 protein detected in BHV-1-infected cells. Expression of BHV-1 UL3.5 compensated for the lack of PrV UL3.5, resulting in a ca. 1,000-fold increase in virus titer and restoration of plaque formation in cell culture. Also, the intracellular block in viral egress was resolved by the BHV-1 UL3.5 gene. We conclude that the UL3.5 proteins of PrV and BHV-1 are functionally related and are involved in a common step in the egress of alphaherpesviruses.

Antibody-induced and cytoskeleton-mediated redistribution and shedding of viral glycoproteins, expressed on pseudorabies virus-infected cells

Fluorescein isothiocyanate-labeled porcine pseudorabies virus (PrV) polyclonal antibodies were added to PrV-infected swine kidney cells in vitro at 37 degrees C. In approximately 47% of the infected cells, the addition induced passive patching and subsequent energy- and microtubule-dependent capping of all viral envelope glycoproteins, expressed on the plasma membranes of the infected cells. Further contraction and extrusion of the capped viral glycoproteins occurred in approximately 30% of the capped cells 2 h after the addition of antibodies and was accompanied by a concentration of F-actin beneath the caps. At that time, about 18% of the extruded caps were shed spontaneously into the surrounding medium. Mechanical force released 85% of the extruded caps, leaving viable cells with no microscopically detectable levels of viral glycoproteins on their plasma membranes. Experiments with PrV deletion mutants showed that viral glycoproteins gE and gI are important in triggering viral glycoprotein redistribution. Since the PrV gE-gI complex exhibits Fc receptor activity which facilitates capping, the importance of gE and gI may be partially explained by antibody bipolar bridging.

Failure to complement infectivity of EBV and HSV-1 glycoprotein B (gB) deletion mutants with gBs from different human herpesvirus subfamilies

Glycoprotein B (gB) is conserved among the herpesvirus family which infects a broad range of species. To investigate the functional homology of human alpha-herpesviruses, beta-herpesviruses, and gamma-herpesviruses gB proteins, complementation studies were performed with gB genes from each subfamily member using EBV gp110 (EBV gB homologue) and HSV-1 gB null mutants. Neither the alpha-herpesvirus HSV-1 gB gene nor the beta-herpesvirus HCMV gB gene were able to complement the gp110 null mutant. Conversely, neither the beta-herpesvirus HCMV gB or the gamma-herpesvirus EBV gp110 gene were able to complement HSV-1 gB null mutants. To further investigate functional domains of EBV gp110 and HSV-1 gB, gB-gp110 chimeric proteins were constructed. Surprisingly, none of the chimeric proteins were able to complement either HSV-1 gB null mutants or EBV gp110 null mutants. These results demonstrate that there is not sufficient functional homology between the different gBs to allow complementation in other subfamily members of the herpesvirus family.

Bovine herpesvirus 1 glycoprotein B does not productively interact with cell surface heparan sulfate in a pseudorabies virion background

Attachment to cell surface heparan sulfate proteoglycans is the first step in infection by several alphaherpesviruses. This interaction is primarily mediated by virion glycoprotein C (gC). In herpes simplex virus, in the absence of the nonessential gC, heparan sulfate binding is effected by glycoprotein B. In contrast, gC-negative pseudorabies virus (PrV) infects target cells via a heparan sulfate-independent mechanism, indicating that PrV virion gB does not productively interact with heparan sulfate. To assay whether a heterologous alphaherpesvirus gB protein will confer productive heparan sulfate binding on gC-negative PrV, gC was deleted from an infectious PrV recombinant, PrV-9112C2, which expresses bovine herpesvirus 1 (BHV-1) gB instead of PrV gB. Our data show that gC-negative PrV-BHV-1 gB recombinant 9112C2-delta gCbeta was not inhibited in infection by soluble heparin, in contrast to the gC-positive parental strain. Similar results were obtained when wild-type BHV-1 was compared with a gC-negative BHV-1 mutant. Moreover, infection of cells proficient or deficient in heparan sulfate biosynthesis occurred with equal efficiency by PrV-9112C2-delta gCbeta, whereas heparan sulfate-positive cells showed an approximately fivefold higher plating efficiency than heparan sulfate-negative cells with the parental gC-positive virus. In summary, our data show that in a PrV gC-negative virion background, BHV-1 gB is not able to mediate infection by productive interaction with heparan sulfate, and they indicate the same lack of heparin interaction for BHV-1 gB in gC-negative BHV-1.

Functional analysis of the transmembrane anchor region of bovine herpesvirus 1 glycoprotein gB

In herpesviruses, homologues of glycoprotein B (gB) are essential membrane proteins which are involved in fusion. However, there is no clear evidence regarding the location of the fusogenic domain on gB. By using bovine herpesvirus 1 (BHV-1) as a model, we studied the relationship between the structure and the fusogenic activity of gB. This was achieved by expressing genes of different gB derivatives containing specific truncations at the end of segments 2 or 3 of the transmembrane region in Madin-Darby bovine kidney cells under the control of the bovine heat-shock protein hsp70A gene promoter. All expressed gB products were structurally similar to authentic gB. One truncated form of gB, gBt, which contains residues 1-763, was efficiently secreted. However, gBtM (residues 1-807), which includes the first two segments at the carboxyl terminus, showed unstable retention on the cell surface, whereas gBtMA (residues 1 829), which contains all three membrane-spanning segments, was mostly intracellularly retained with some unstable surface anchorage. Another truncated gB, gBtDAF, which has gB residues 1-763 (gBt) and a human decay-accelerating factor (DAF) carboxyl tail, was also expressed. The DAF fragment provided a signal for the addition of a glycosyl phosphatidylinositol-based membrane anchor, which could target the gBt chimeric protein on the cell membrane. Immunofluorescence staining and pulse-chase kinetic studies support the theory that gBtM, gBtMA, and gBtDAF are retained on nuclear and cellular membranes via different segments of the transmembrane region or the DAF fragment, respectively. For the cells expressing gBt or gBtM, no cell fusion was observed, whereas cells expressing gBtMA clearly showed fusion. However, in gBtDAF cells, the overexpression and cellular accumulation of recombinant gB products did not cause fusion either, which supports our contention that the fusion phenomenon in gBtMA cells is caused by the fusogenic activity of the expressed gBtMA. With the help of sequence analysis, our results indicate that segment 2 of the transmembrane anchor region might be a fusogenic domain, whereas the real anchor is segment 3.

Adaptability in herpesviruses: glycoprotein D-independent infectivity of pseudorabies virus

Initial contact between herpesviruses and host cells is mediated by virion envelope glycoproteins which bind to cellular receptors. In several alphaherpesviruses, the nonessential glycoprotein gC has been found to interact with cell surface proteoglycans, whereas the essential glycoprotein gD is involved in stable secondary attachment. In addition, gD is necessary for penetration, which involves fusion between virion envelope and cellular cytoplasmic membrane. As opposed to other alphaherpesvirus gD homologs, pseudorabies virus (PrV) gD is not required for direct viral cell-to-cell spread. Therefore, gD- PrV can be passaged in noncomplementing cells by cocultivating infected and noninfected cells. Whereas infectivity was found to be strictly cell associated in early passages, repeated passaging resulted in the appearance of infectivity in the supernatant, finally reaching titers as high as 10(7) PFU/ml (PrV gD- Pass). Filtration experiments indicated that this infectivity was not due to the presence of infected cells, and the absence of gD was verified by Southern and Western blotting and by virus neutralization. Infection of bovine kidney cells constitutively expressing PrV gD interfered with the infectivity of wild-type PrV but did not inhibit that of PrV gD- Pass. Similar results were obtained after passaging of a second PrV mutant, PrV-376, which in addition to gD also lacks gG, gI, and gE. Penetration assays demonstrated that PrV gD- Pass entered cells much more slowly than wild-type PrV. In summary, our data demonstrate the existence of a gD-independent mode of initiation of infection in PrV and indicate that the essential function(s) that gD performs in wild-type PrV infection can be compensated for after passaging. Therefore, regarding the requirement for gD, PrV seems to be intermediate between herpes simplex virus type 1, in which gD is necessary for penetration and cell-to-cell spread, and varicella-zoster virus (VZV), which lacks a gD gene. Our data show that the relevance of an essential protein can change under selective pressure and thus demonstrate a way in which VZV could have evolved from a PrV-like ancestor.

From essential to beneficial: glycoprotein D loses importance for replication of bovine herpesvirus 1 in cell culture

Glycoprotein D (gD) of bovine herpesvirus 1 (BHV-1) has been shown to be an essential component of virions involved in virus entry. gD expression in infected cells is also required for direct cell-to-cell spread. Therefore, BHV-1 gD functions are identical in these aspects to those of herpes simplex virus 1 (HSV-1) gD. In contrast, the gD homolog of pseudorabies virus (PrV), although essential for penetration, is not necessary for direct cell-to-cell spread. Cocultivation of cells infected with phenotypically gD-complemented gD- mutant BHV-1/80-221 with noncomplementing cells resulted in the isolation of the cell-to-cell-spreading gD-negative mutant ctcs+BHV-1/80-221, which was present in the gD-null BIV-1 stocks. ctcs+BHV-1/80-221 could be propagated only by mixing infected with uninfected cells, and virions released into the culture medium were noninfectious. Marker rescue experiments revealed that a single point mutation in the first position of codon 450 of the glycoprotein H open reading frame, resulting in a glycine-to-tryptophan exchange, enabled complementation of the gD function for cell-to-cell spread. After about 40 continuous passages of ctcs+BHV-1/80-221-infected cells with noninfected cells, the plaque morphology in the cultures started to change from roundish to comet shaped. Cells from such plaques produced infectious gD- virus, named gD-infBHV-1, which entered cells much more slowly than wild-type BHV-1. In contrast, integration of the gD gene into the genomes of gD-infBHV-1 and ctcs+BHV-1/80-221 resulted in recombinants with accelerated penetration in comparison to wild-type virions. In summary, our results demonstrate that under selective conditions, the function of BHV-1 gD for direct cell-to-cell spread and entry into cells can be compensated for by mutations in other viral (glyco)proteins, leading to the hypothesis that gD is involved in formation of penetration-mediating complexes in the viral envelope of which gH is a component. Together with results for PrV, varicella-zoster virus, which lacks a gD homolog, and Marek's disease virus, whose gD homolog is not essential for infectivity, our data may open new insights into the evolution of alphaherpesviruses.

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.

Identification and characterization of pseudorabies virus glycoprotein gM as a nonessential virion component

Sequence analysis within BamHI fragment 3 of the pseudorabies virus (PrV) genome revealed an open reading frame homologous to the UL10 gene of herpes simplex virus. A rabbit antiserum directed against a synthetic oligopeptide representing the carboxy-terminal 18 amino acids of the predicted UL10 product recognized a major 45-kDa protein in lysates of purified Pr virions. In addition, a second protein of 90 kDa which could represent a dimeric form was observed. Enzymatic deglycosylation showed that the PrV UL10 protein is N glycosylated. Therefore, it was designated PrV gM according to its homolog in herpes simplex virus. A PrV mutant lacking ca. 60% of UL10 coding sequences was able to productively replicate on noncomplementing cells, demonstrating that PrV gM is not required for viral replication in cell culture. However, infectivity of the mutant virus was reduced and penetration was delayed, indicating a modulatory role of PrV gM in the initiation of infection.

Identification and characterization of the pseudorabies virus UL3.5 protein, which is involved in virus egress

Alphaherpesvirus genomes exhibit a generally collinear gene arrangement, and most of their genes are conserved among the different members of the subfamily. Among the exceptions is the UL3.5 gene of pseudorabies virus (PrV) for which positional homologs have been detected in the genomes of varicella-zoster virus, equine herpesvirus 1, and bovine herpesvirus 1 but not in the genomes of herpes simplex virus types 1 and 2. To identify and characterize the predicted 224 amino acid UL3.5 protein of PrV, a rabbit antiserum was prepared against a UL3.5 fusion protein expressed in Escherichia coli. In Western blot (immunoblot) analyses the antiserum detected a 30-kDa protein in the cytoplasm of PrV infected cells which was absent from purified virions. For functional analysis, UL3.5-expressing cell lines were established and virus mutants were isolated after the rescue of defective, glycoprotein B-negative PrV by insertion of the complementing glycoprotein B-encoding gene of bovine herpesvirus 1 at two sites within the UL3.5 locus. A PrV mutant carrying the insertion at codon 159 and expressing a truncated UL3.5 protein was still capable of efficient productive replication in noncomplementing cells. In contrast, a PrV mutant carrying the insertion at codon 10 of the UL3.5 gene did not express detectable UL3.5 protein and exhibited a dramatic growth deficiency on non-complementing cells with regard to plaque formation and one-step replication. Electron microscopical studies showed an accumulation of unenveloped capsids in the vicinity of the Golgi apparatus. This defect could be compensated by propagation on complementing UL3.5-expressing cell lines. Our results thus demonstrate that the PrV UL3.5 gene encodes a nonstructural protein which plays an important role in virus replication, presumably during virus egress. The functionally relevant domains appear to be located within the N-terminal part of the UL3.5 protein which also comprises the region exhibiting the highest level of homology between the predicted UL3.5 homologous proteins of other alphaherpesviruses.

Construction of bovine herpesvirus-1 (BHV-1) recombinants which express pseudorabies virus (PRV) glycoproteins gB, gC, gD, and gE

We have improved the method for constructing recombinants of bovine herpesvirus type-1 (BHV-1). Using this method, we constructed three recombinants in which the pseudorabies virus (PRV) thymidine kinase (tk) gene was inserted at three different sites in the unique short region of BHV-1. These three sites are located in the open reading frame of gE, gG and gI genes. Previously, two sites (tk and gC) had been used to insert foreign DNA fragments to BHV-1 genome. Therefore we now have 5 sites in BHV-1 where DNA can be inserted. The gB, gC, gD, gE and gI genes of PRV were successfully inserted at the tk or the gC gene of BHV-1 genome and Western blot analyses confirmed that the recombinants express PRV gB, gC, gD and gE. Anti-PRV gB and gC antibodies as well as anti-PRV polyclonal serum neutralized BHV-1 recombinants which express PRV gB and gC. The latter was neutralized more strongly. However, anti-gD monoclonal antibody and anti-PRV polyclonal serum failed to neutralize gD-expressing recombinants. This suggests that PRV gC and some gB are integrated into the viral envelope of the recombinants, but very little gD is present in the viral envelope.

Role of glycoprotein gD in the adhesion of pseudorabies virus infected cells and subsequent cell-associated virus spread

Pseudorabies virus (PrV) infected cells in suspension are able to adhere to a monolayer of uninfected cells by means of PrV glycoproteins expressed at the outer cell membrane, with gB and gC playing a major role as ligands and a heparinlike substance as receptor. In order to investigate the role of gD in this process and subsequent transmission of infectivity to contact cells, experiments with a gD deletion mutant, heparin and a monoclonal antibody (Mab) against gD were performed. The first indication that gD is active during cell adhesion was found by the observation that the binding of gD- PrV infected cells was five times weaker than that of wild type (WT) PrV infected cells. Further evidence was given by the use of a Mab against gD. Preincubation of WT PrV infected cells with this Mab led to a reduction of the percentage adhering cells from 69% to 49%. The same Mab inhibited the heparin independent and heparin resistant binding of WT PrV infected cells indicating that gD is important during both processes. Furthermore, it was demonstrated in a plaque assay that, after contact with a monolayer, gD- PrV infected cells in suspension were able to induce plaques with an efficiency of 1%. In conclusion, we can state that beside the interaction of the ligands gB and gC with a heparinlike receptor also the interaction of gD with a receptor which differs from a heparinlike substance mediates the binding of WT PrV infected cells to uninfected cells and that gD is not essential for the subsequent cell-to-cell spread of the virus.

Induction of protective antibody responses against pseudorabies virus by intranasal vaccination with glycoprotein B in mice

Intranasal vaccination of mice with glycoprotein B (gB) of pseudorabies virus (PRV) induced specific IgA and IgG antibody responses in the secretion of the respiratory tract, resulting in protection of the animals against intranasal challenge with a lethal dose of virulent PRV. The immune response was enhanced by the use of cholera toxin B subunit as an adjuvant. The present results indicate that local vaccination with gB is a promising strategy to confer protective immunity on animals against PRV infection by inducing secretory antibodies on their mucosal surfaces where the primary replication of the virus occurs.

Glycoprotein E of pseudorabies virus and homologous proteins in other alphaherpesvirinae

This paper reviews biological properties of glycoprotein E (gE) of pseudorabies virus (Aujeszky's disease virus) and homologous proteins in other alphaherpesvirinae. It focuses on the gene encoding gE, conserved regions in the gE protein and its homologs, the complex of gE and gI, biological functions of gE in vitro and in vivo, the role of gE in latency and the role of gE in the induction of humoral and cellular immune responses. Special emphasis is placed on the use of gE as a marker protein in the control and eradication of pseudorabies virus.

Identification and characterization of a novel structural glycoprotein in pseudorabies virus, gL

Herpesvirus envelope glycoproteins play important roles in the interaction between virions and target cells. In the alphaherpesvirus pseudorabies virus (PrV), seven glycoproteins that all constitute homologs of glycoproteins found in herpes simplex virus type 1 (HSV-1) have been characterized, including a homolog of HSV-1 glycoprotein H (gH). Since HSV-1 gH is found associated with another essential glycoprotein, gL, we analyzed whether PrV also encodes a gL homolog. DNA sequence analysis of a corresponding part of the UL region adjacent to the internal inverted repeat in PrV strains Kaplan and Becker revealed the presence of two open reading frames (ORF). Deduced proteins exhibited homology to uracil-DNA glycosylase encoded by HSV-1 ORF UL2 (54% identity) and gL encoded by HSV-1 ORF UL1 (24% identity), respectively. To identify the PrV UL1 protein, rabbit antisera were prepared against two synthetic oligopeptides that were predicted by computer analysis to encompass antigenic epitopes. Sera against both peptides reacted in Western blots of purified virions with a 20-kDa protein. The specificity of the reaction was demonstrated by peptide competition. Since the PrV UL1 sequence did not reveal the presence of a consensus N-linked glycosylation site, concanavalin A affinity chromatography and enzymatic deglycosylation of virion glycoproteins were used to ascertain that the PrV UL1 product is O glycosylated. Therefore, we designated this protein PrV gL. Analysis of mutant PrV virions lacking gH showed that concomitantly with the absence of gH, gL was also missing in purified virions. In summary, we identified and characterized a novel structural PrV glycoprotein, gL, which represents the eighth PrV glycoprotein described. In addition, we show that virion location of PrV gL is dependent on the presence of PrV gH.

Proteolytic cleavage of bovine herpesvirus 1 (BHV-1) glycoprotein gB is not necessary for its function in BHV-1 or pseudorabies virus

Glycoprotein B homologs represent the most highly conserved group of herpesvirus glycoproteins. They exist in oligomeric forms based on a dimeric structure. Despite the high degree of sequence and structural conservation, differences in posttranslational processing are observed. Whereas gB of herpes simplex virus is not proteolytically processed after oligomerization, most other gB homologs are cleaved by a cellular protease into subunits that remain linked via disulfide bonds. Proteolytic cleavage is common for activation of viral fusion proteins, and it has been shown that herpesvirus gB homologs are essential for membrane fusion events during infection, e.g., virus penetration and direct viral cell-to-cell spread. To analyze the importance of proteolytic cleavage for the function of gB homologs, we isolated a mutant bovine herpesvirus 1 (BHV-1) expressing a BHV-1 gB that is no longer proteolytically processed because of a deletion of the proteolytic cleavage site and analyzed its phenotype in cell culture. We showed previously that BHV-1 gB can functionally substitute for the homologous glycoprotein in pseudorabies virus (PrV), based on the isolation of a PrV gB-negative PrV recombinant that expresses BHV-1 gB (A. Kopp and T. C. Mettenleiter, J. Virol, 66:2754-2762, 1992). Therefore, we also isolated a mutant PrV lacking PrV gB but expressing a noncleavable BHV-1 gB. Our results show that cleavage of BHV-1 gB is not essential for its function in either a BHV-1 or a PrV background. Compared with the PrV recombinant expressing cleavable BHV-1 gB, deletion of the cleavage site in the recombinant PrV did not detectably alter the viral phenotype, as analyzed by plaque assays, one-step growth kinetics, and penetration kinetics. In the BHV-1 mutant, the uncleaved BHV-1 gB was functionally equivalent to the wild-type protein with regard to penetration and showed only slightly delayed one-step growth kinetics compared with parental wild-type BHV-1. However, the resulting plaques were significantly smaller, indicating a role for proteolytic cleavage of BHV-1 gB in cell-to-cell spread of BHV-1.

Glycoprotein gB (gII) of pseudorabies virus can functionally substitute for glycoprotein gB in herpes simplex virus type 1

Glycoproteins homologous to gB of herpes simplex virus (HSV) constitute the most highly conserved family of herpesvirus glycoproteins. All gB homologs analyzed so far have been shown to play essential roles in penetration and direct viral cell-to-cell spread. In studies aimed at assessing whether the high sequence homology is also indicative of functional homology, we analyzed the ability of the gB-homologous glycoprotein (former designation gII) of pseudorabies virus (PrV) to complement a gB- HSV type 1 (HSV-1) mutant and vice versa. The results show that a PrV gB-expressing cell line phenotypically complemented the lethal defect in gB- HSV-1 whereas reciprocal complementation of a gB- PrV mutant by HSV-1 gB was not observed.