Demonstration of three major species of pseudorabies virus glycoproteins and identification of a disulfide-linked glycoprotein complex

Preparation and Identification of a Monoclonal Antibody against the Pseudorabies Virus gE Glycoprotein through a Novel Strategy

Since 2011, pseudorabies virus (PRV) has recurred in several vaccinated pig farms in China. PRV variants with high virulence were found to be the main cause of the outbreaks. In the face of the PRV epidemic, detection of the wild strain is as important as vaccine immunization, so we hoped to achieve differential diagnosis of PRV by obtaining a monoclonal antibody (mAB) that could be used to identify the wild strain. In this study, we used a novel immunization and screening strategy to prepare an mAB and obtained mAB 1H5 against the gE glycoprotein. An immunofluorescence assay (IFA) revealed that this mAB was specific to both classic and variant strains of PRV. Subsequently, we further identified the linear epitopes of B cells recognized using the mAB. The mAB 1H5 bound at ⁶⁷RRAG⁷⁰, which is a novel epitope and is conserved in almost all PRV strains. These findings provide novel insight into the structure and function of PRV proteins, the analysis of antigenic epitope characteristics, and the establishment of antigen or antibody detection methods.

Identification of two novel epitopes targeting glycoprotein E of pseudorabies virus using monoclonal antibodies

Pseudorabies virus (PRV), the agent of pseudorabies, has raised considerable attention since 2011 due to the outbreak of emerging PRV variants in China. In the present study, we obtained two monoclonal antibodies (mAbs) known as 2E5 and 5C3 against the glycoprotein E (gE) of a PRV variant (JS-2012 strain). The two mAbs reacted with wild PRV but not the vaccine strain (gE-deleted virus). The 2E5 was located in ¹⁶¹RLRRE¹⁶⁵, which was conserved in almost of all PRV strains, while 5C3 in ¹⁴⁸EMGIGDY¹⁵⁴ was different from almost of all genotype I PRV, in which the 149th amino acid is methionine (M) instead of arginine (R). The two epitopes peptides located in the hydrophilic region and reacted with positive sera against genotype II PRV (JS-2012), which suggests they were likely dominant B-cell epitopes. Furthermore, the mutant peptide ¹⁴⁸ERGIGDY¹⁵⁴ (genotype I) did not react with the mAb 5C3 or positive sera against genotype II PRV (JS-2012). In conclusion, both mAb 2E5 and 5C3 could be used to identify wild PRV strains from vaccine strains, and mAb 5C3 and the epitope peptide of 5C3 might be used for epidemiological investigation to distinguish genotype II from genotype I PRV.

Variations in glycoprotein B contribute to immunogenic difference between PRV variant JS-2012 and Bartha-K61

A newly emerged pseudorabies virus (PRV) variant has been identified in many Bartha-K61-vaccinated pig farms. This variant has caused great economic losses to the swine industry in China since 2011. Sequence analysis demonstrated that the gB gene of the emerging PRV variant JS-2012 had multiple variations compared with the vaccine strain Bartha-K61. In the study, a specific CRISPR/Cas9 system combined with homologous recombination was used to construct two recombinant viruses, BJB (Bartha-K61+JS-2012gB) and JBJ (JS-2012-ΔgE/gI+Bartha-K61gB), by interchanging the full-length gB genes between Bartha-K61 and JS-2012-ΔgE/gI. The two recombinant viruses showed similar characteristics in growth kinetics in vitro and similar pathogenicity in mice, as compared to their parental strains. Immunization of mice with inactivated BJB or JBJ followed by challenge of JS-2012 showed that BJB could increase protective efficacy to 80%, compared to only 40% protection by the parental Bartha-K61 strain. JBJ had a decreased protective efficacy of 65%, as compared to 90% protection by its parental JS-2012-ΔgE/gI strain. Exchange of the gB gene markedly altered the immunogenicity of the recombinant PRV. These data suggest that variations in gB might play an important role in the virulence of the reemergent PRV variant in China. Our results demonstrate the importance of gB in protective immunity and suggest that the recombinant virus BJB could be a promising vaccine candidate for eradication of the PRV variant.

Glycoprotein-based enzyme-linked immunosorbent assays for serodiagnosis of infectious laryngotracheitis

For detection of infectious laryngotracheitis virus (ILTV) antibody, glycoprotein B-, C-, and D-based enzyme-linked immunosorbent assays (B-, C-, and D-ELISAs, respectively) were developed. The B- and D-ELISAs showed enhanced detection of anti-ILTV antibodies in infected chickens compared to that of the commercial ELISA. Furthermore, the D-ELISA was efficient in detecting seroconversion with vectored vaccine, using recombinant Newcastle disease virus (rNDV) expressing glycoprotein D (gD) as the vaccine vector.

A recombinant Newcastle disease virus (NDV) expressing infectious laryngotracheitis virus (ILTV) surface glycoprotein D protects against highly virulent ILTV and NDV challenges in chickens

Infectious laryngotracheitis (ILT) is a highly contagious acute respiratory disease of chickens caused by infectious laryngotracheitis virus (ILTV). Currently, modified live ILTV vaccines are used to control ILT infections. However, the live ILTV vaccines can revert to virulence after bird-to-bird passage and are capable of establishing latent infections, suggesting the need to develop safer vaccines against ILT. We have evaluated the role of three major ILTV surface glycoproteins, namely, gB, gC, and gD in protection and immunity against ILTV infection in chickens. Using reverse genetics approach, three recombinant Newcastle disease viruses (rNDVs) designated rNDV gB, rNDV gC, and rNDV gD were generated, each expressing gB, gC, and gD, respectively, of ILTV. Chickens received two immunizations with rNDVs alone (gB, gC, and gD) or in combination (gB+gC, gB+gD, gC+gD, and gB+gC+gD). Immunization with rNDV gD induced detectable levels of neutralizing antibodies with the magnitude of response greater than the rest of the experimental groups including those vaccinated with commercially available vaccines. The birds immunized with rNDV gD showed complete protection against virulent ILTV challenge. The birds immunized with rNDV gC alone or multivalent vaccines consisting of combination of rNDVs displayed partial protection with minimal disease and reduced replication of challenge virus in trachea. Immunization with rNDV gB neither reduced the severity of the disease nor the replication of challenge virus in trachea. The superior protective efficacy of rNDV gD vaccine compared to rNDV gB or rNDV gC vaccine was attributed to the higher levels of envelope incorporation and infected cell surface expression of gD than gB or gC. Our results suggest that rNDV expressing gD is a safe and effective bivalent vaccine against NDV and ILTV.

Analysis of pseudorabies and herpes simplex virus recombinants simultaneously lacking the pUL17 and pUL25 components of the C-capsid specific component

Homologs of the UL17 and UL25 gene products of herpes simplex virus 1 (HSV-1) are conserved throughout the Herpesviridae and essential for virus replication. However, their exact function is still unknown. Although both proteins form a complex on DNA-containing C-capsids defects observed in the absence of either protein differ. Absence of pUL17 from HSV-1 or the related alphaherpesvirus pseudorabies virus (PrV) precludes cleavage and packaging of newly replicated viral DNA, whereas in the absence of pUL25 genomic DNA is encapsidated but nuclear egress of capsids to the cytosol is abolished. HSV-1 pUL25 partially complemented the defect in a PrV UL25 deletion mutant indicating overlapping functions. However, reciprocal complementation did not ensue, and the present study demonstrates that UL17-deleted HSV-1 or PrV mutants are also not rescued by heterologous pUL17. To analyze whether simultaneous substitution of both complex partners may allow or increase trans-complementation we generated rabbit kidney cell lines co-expressing either PrV or HSV-1 pUL17 and pUL25, and respective HSV-1 and PrV double deletion mutants. Whereas the defects of both double mutants were trans-complemented by cell lines co-expressing the homologous complex partners, productive replication was not restored by heterologous pUL17 and pUL25. Thus, the protein complexes of PrV and HSV-1 either possess distinct functions, or require interactions with other viral proteins which are impaired in a heterologous context.

Relevance of the interaction between alphaherpesvirus UL3.5 and UL48 proteins for virion maturation and neuroinvasion

The UL3.5 and UL48 genes, which are conserved in most alphaherpesvirus genomes, are important for maturation of pseudorabies virus (PrV) particles in the cytoplasm of infected cells (W. Fuchs, B. G. Klupp, H. J. Rziha, and T. C. Mettenleiter, J. Virol. 70:3517-3527, 1996; W. Fuchs, H. Granzow, B. G. Klupp, M. Kopp and T. C. Mettenleiter, J. Virol. 76:6729-6742, 2002). In bovine herpesvirus 1 (BoHV-1), the homologous gene products pUL3.5 and pUL48 have been demonstrated to interact physically (N. Lam and G. Letchworth, J. Virol. 74:2876-2884, 2000). Moreover, BoHV-1 pUL3.5 partially complemented a pUL3.5 defect in PrV (W. Fuchs, H. Granzow, and T. C. Mettenleiter, J. Virol. 71:8886-8892, 1997). By using coimmunoprecipitation and yeast two-hybrid studies, we observed a similar interaction between pUL3.5 and pUL48 of PrV, as well as a heterologous interaction between the PrV and BoHV-1 gene products. The relevant domain could be confined to the first 43 amino acids of PrV pUL3.5. Unlike its BoHV-1 homologue, PrV pUL3.5 is processed by proteolytic cleavage, and only an abundant 14-kDa fragment consisting of amino acids 1 to >or=116 could be detected by peptide mass fingerprint analysis of purified wild-type PrV particles, which also contain the pUL48 tegument component. To determine the biological relevance of the protein-protein interaction, pUL3.5-, pUL48-, and double-negative PrV mutants were analyzed in parallel. All deletion mutants were replication competent but exhibited significantly reduced plaque sizes and virus titers in cultured rabbit kidney cells compared to wild-type and rescued viruses, which correlated with a delayed neuroinvasion in intranasally infected mice. Remarkably, the defects of the double-negative mutant were similar to those of pUL48-negative virus. Electron microscopy of cells infected with either deletion mutant revealed the retention of naked nucleocapsids in the cytoplasm and the absence of mature virus particles. In summary, our studies for the first time demonstrate the relevance of the pUL3.5-pUL48 interaction for secondary envelopment of an alphaherpesvirus, give a molecular basis for the observed trans-complementation between the PrV and BHV-1 pUL3.5 homologs, yield conclusive evidence for the incorporation of a proteolytically processed pUL3.5 into PrV virions, and demonstrate the importance of both proteins for neuroinvasion and neurovirulence of PrV.

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.

Composition of pseudorabies virus particles lacking tegument protein US3, UL47, or UL49 or envelope glycoprotein E

Proteins located in the tegument layer of herpesvirus particles play important roles in the replicative cycle at both early and late times after infection. As major constituents of the virion, they execute important functions in particular during formation of progeny virions. These functions have mostly been elucidated by construction and analysis of mutant viruses deleted in single or multiple tegument protein-encoding genes (reviewed in the work of T. C. Mettenleiter, Virus Res. 106:167-180, 2004). However, since tegument proteins have been shown to be involved in numerous protein-protein interactions, the impact of single protein deletions on the composition of the virus particle is unknown, but they could impair correct interpretation of the results. To analyze how the absence of single virion constituents influences virion composition, we established a procedure to assay relative amounts of virion structural proteins in deletion mutants of the alphaherpesvirus Pseudorabies virus (PrV) in comparison to wild-type particles. The assay is based on the mass spectrometric quantitation of virion protein-derived peptides carrying stable isotope mass tags. After deletion of the US3, UL47, UL49, or glycoprotein E gene, relative amounts of a capsid protein (UL38), a capsid-associated protein (UL25), several tegument proteins (UL36 and UL47, if present), and glycoprotein H were unaffected, whereas the content of other tegument proteins (UL46, UL48, and UL49, if present) varied significantly. In the case of the UL48 gene product, a specific increase in incorporation of a smaller isoform was observed after deletion of the UL47 or UL49 gene, whereas a larger isoform remained unaffected. The cellular protein actin was enriched in virions of mutants deficient in any of the tegument proteins UL47, UL49, or US3. By two-dimensional gel electrophoresis multiple isoforms of host cell-derived heat shock protein 70 and annexins A1 and A2 were also identified as structural components of PrV virions.

Complex formation between the UL16 and UL21 tegument proteins of pseudorabies virus

The products of the UL16 and UL21 genes represent tegument proteins which are conserved throughout the mammalian herpesviruses. To identify and functionally characterize the respective proteins in the alphaherpesvirus pseudorabies virus, monospecific antisera against bacterially expressed fusion proteins were generated. In immunoblots the UL16 antiserum detected a ca. 40-kDa protein in infected cells and purified virion preparations, whereas the anti-UL21 serum recognized a protein of approximately 60 kDa. Interestingly, in immunoprecipitations using either antiserum, both proteins were coprecipitated, demonstrating the formation of a physical complex. To investigate protein function, viruses lacking either UL16, UL21, or both were constructed. Mutant viruses could be propagated on noncomplementing cells, indicating that these proteins, either alone or in combination, are not required for viral replication in cell culture. However, plaque sizes and viral titers were reduced. Electron microscopy showed only slight alterations in cytoplasmic virion morphogenesis, whereas intranuclear maturation stages were not affected. Similar results were obtained with a triple mutant simultaneously lacking the three conserved tegument proteins UL11, UL16, and UL21. In summary, our results uncover a novel interaction between conserved herpesvirus tegument proteins that increases the complexity of the intricate network of protein-protein interactions involved in herpesvirus morphogenesis.

Essential function of the pseudorabies virus UL36 gene product is independent of its interaction with the UL37 protein

The large tegument protein encoded by the UL36 gene of pseudorabies virus (PrV) physically interacts with the product of the adjacent UL37 gene (B. G. Klupp, W. Fuchs, H. Granzow, R. Nixdorf, and T. C. Mettenleiter, J. Virol. 76:3065-3071, 2002). To analyze UL36 function, two PrV recombinants were generated by mutagenesis of an infectious PrV full-length clone in Escherichia coli: PrV-DeltaUL36F exhibited a deletion of virtually the complete UL36 coding region, whereas PrV-UL36BSF contained two in-frame deletions of 238 codons spanning the predicted UL37 binding domain. Coimmunoprecipitation experiments confirmed that the mutated gene product of PrV-UL36BSF did not interact with the UL37 protein. Like the previously described PrV-DeltaUL37 (B. G. Klupp, H. Granzow, and T. C. Mettenleiter, J. Virol. 75:8927-8936, 2001) but in contrast to PrV-DeltaUL36F, PrV-UL36BSF was able to replicate in rabbit kidney (RK13) cells, although maximum virus titers were reduced ca. 50-fold and plaque diameters were reduced by ca. 45% compared to wild-type PrV. PrV-DeltaUL36F was able to productively replicate after repair of the deleted gene or in a trans-complementing cell line. Electron microscopy of infected RK13 cells revealed that PrV-UL36BSF and phenotypically complemented PrV-DeltaUL36F were capable of nucleocapsid formation and egress from the nucleus by primary envelopment and deenvelopment at the nuclear membrane. However, reenvelopment of nucleocapsids in the cytoplasm was blocked. Only virus-like particles without capsids were released efficiently from cells. Interestingly, cytoplasmic nucleocapsids of PrV-UL36BSF but not of PrV-DeltaUL36F were found in large ordered structures similar to those which had previously been observed with PrV-DeltaUL37. In summary, our results demonstrate that the interaction between the UL36 and UL37 proteins is important but not strictly essential for the formation of secondary enveloped, infectious PrV particles. Furthermore, UL36 possesses an essential function during virus replication which is independent of its ability to bind the UL37 protein.

Influenza A virus replication is dependent on an antioxidant pathway that involves GSH and Bcl-2

Growing evidence indicates that viral replication is regulated by the redox state of the host cell. We demonstrate that cells of different origins display differential permissivity for influenza A virus replication, depending on their intracellular redox power as reflected by Bcl-2 expression and glutathione (GSH) content. Bcl-2 expressing cells were found to have higher intracellular levels of GSH and to produce lower amounts of virus than Bcl-2 negative cells. Two different steps in the virus life-cycle were involved in Bcl-2/GSH mediated viral inhibition: 1) expression of late viral proteins (in particular hemagglutinin and matrix); and 2) nuclear-cytoplasmic translocation of viral ribonucleoproteins (vRNPs). Buthionine-sulfoximine-induced inhibition of GSH synthesis in Bcl-2 expressing cells caused an increase in the expression of late viral proteins but did not restore vRNP export to the cytoplasm. Collectively, our findings show that both Bcl-2 expression and GSH content contribute to the host cell's ability to down-regulate influenza virus replication, although their effects are exerted at different stages of the viral life-cycle. In certain cell populations, this form of down-regulation might conceivably favor the establishment of persistent viral infection.

The attenuated pseudorabies virus strain Bartha fails to package the tegument proteins Us3 and VP22

The Bartha strain of pseudorabies virus has several recognized mutations, including a deletion in the unique short region encompassing the glycoprotein I (gI), gE, Us9, and Us2 genes and point mutations in the gC, gM, and UL21 genes. We have determined that Bartha has mutations in the serine/threonine kinase encoded by the Us3 gene relative to the wild-type Becker strain. Our analysis revealed that Becker virions contain the Us3 protein, whereas Bartha virions do not. To test whether the mutations in the Bartha Us3 protein were responsible for this observation, we constructed a recombinant Bartha strain, PRV632, which expresses the Becker Us3 protein. PRV632 failed to package Us3 into the tegument, indicating that mutations other than those in the Us3 primary amino acid sequence were responsible for the failure of Bartha to package its Us3 protein. A recombinant Becker strain, PRV634, which expresses the Bartha Us3 protein, was constructed to test whether it was capable of being packaged into virions. The Bartha Us3 protein was not incorporated into PRV634 virions efficiently, suggesting that the primary sequence of the Bartha Us3 protein affects packaging into the tegument. To determine whether the packaging of other tegument proteins was affected in the Bartha strain, we examined VP22. Whereas Becker packaged VP22 into virions, Bartha had a severe deficiency in VP22 incorporation. Analysis of VP22 expression in Bartha-infected cells revealed that Bartha VP22 had a slower mobility on sodium dodecyl sulfate-polyacrylamide gels, indicating either primary sequence differences and/or different posttranslational modifications relative to Becker VP22. Taken together, these data indicate that, while the primary sequence of the Us3 protein does affect its incorporation into the tegument, other factors are involved. Furthermore, our data suggest that one or more of the gI, gE, Us9, or Us2 genes influences the localization of the Us3 protein in infected cells, and this effect may be important for the proper incorporation of Us3 into virions.

Pseudorabies virus UL36 tegument protein physically interacts with the UL37 protein

The UL36 open reading frame encoding the tegument protein ICP1/2 represents the largest open reading frame in the genome of herpes simplex virus type 1 (HSV-1). Polypeptides homologous to the HSV-1 UL36 protein are present in all subfamilies of HERPESVIRIDAE: We sequenced the UL36 gene of the alphaherpesvirus pseudorabies virus (PrV) and prepared a monospecific polyclonal rabbit antiserum against a bacterial glutathione S-transferase (GST)-UL36 fusion protein for identification of the protein. The antiserum detected a >300-kDa protein in PrV-infected cells and in purified virions. Interestingly, in coprecipitation analyses using radiolabeled infected-cell extracts, the anti-UL36 serum reproducibly coprecipitated the UL37 tegument protein, and antiserum directed against the UL37 protein coprecipitated the UL36 protein. This physical interaction could be verified using yeast two-hybrid analysis which demonstrated that the UL37 protein interacts with a defined region within the amino-terminal part of the UL36 protein. By use of immunogold labeling, capsids which accumulate in the cytoplasm in the absence of the UL37 protein (B. G. Klupp, H. Granzow, E. Mundt, and T. C. Mettenleiter, J. Virol. 75:8927-8936, 2001) as well as wild-type intracytoplasmic and extracellular virions were decorated by the anti-UL36 antiserum, whereas perinuclear primary enveloped virions were not. We postulate that the physical interaction of the UL36 protein, which presumably constitutes the innermost layer of the tegument (Z. Zhou, D. Chen, J. Jakana, F. J. Rixon, and W. Chiu, J. Virol. 73:3210-3218, 1999), with the UL37 protein is an important early step in tegumentation during virion morphogenesis in the cytoplasm.

Restoration of function of carboxy-terminally truncated pseudorabies virus glycoprotein B by point mutations in the ectodomain

Glycoprotein B (gB) of pseudorabies virus (PrV) is essential for virus entry into target cells and direct viral cell-to-cell spread. Recently, we described a carboxy-terminally truncated derivative of PrV gB, gB-007, which was inefficiently incorporated into virions, was unable to complement infectivity, but was fully capable of restoring direct viral cell-to-cell spread of gB-negative PrV (R. Nixdorf, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 74:7137-7145, 2000). Since recombinant PrV-007, which expresses gB-007 instead of wild-type gB, was able to spread directly from cell to cell, we attempted to obtain compensatory mutations leading to restoration of the entry defect by performing serial passages in cell culture. This procedure has previously been used to successfully restore entry defects in gD- or gL-deficient PrV mutants. From an initial titer of 100 PFU per ml in the supernatant, titers increased, reaching wild-type levels of up to 10(7) PFU after ca. 20 passages. One single-plaque isolate of the passaged mutant, designated PrV-007Pass, was further characterized. PrV-007Pass gB was efficiently incorporated into the viral envelope and restored infectivity to a gB-negative PrV mutant, PrV-gB(-). Interestingly, localization of PrV-007Pass gB in the plasma membrane was similar to that of PrV-007. In contrast, wild-type gB is mainly found in intracellular vesicles. Marker rescue experiments and trans-complementation assays demonstrated the presence of compensatory mutations within the gB gene of PrV-007Pass. DNA sequencing revealed two point mutations in the gB open reading frame of PrV-007Pass, resulting in amino acid substitutions at positions 305 and 744 of gB, both of which are required for compensation of the defect in PrV-007. Our data again demonstrate the power of reversion analysis of herpesviruses and suggest that cytosolic and ectodomains play a role in incorporation of gB into virions.

Effect of the pseudorabies virus US3 protein on nuclear membrane localization of the UL34 protein and virus egress from the nucleus

The alphaherpesvirus UL34 protein is necessary for the primary envelopment of intranuclear capsids at the inner leaflet of the nuclear membrane. In herpes simplex virus type 1, the UL34 protein is exclusively phosphorylated by the protein kinase encoded by the non-essential US3 gene. To investigate the effect of the pseudorabies virus (PrV) US3 product on the intracellular localization of the UL34 protein and on virus morphogenesis, PrV US3 deletion mutants were isolated and characterized. Immunofluorescence analyses demonstrated that in the absence of the US3 protein, the localization of the UL34 polypeptide to the nuclear membrane was not as pronounced as that seen with US3, although immunoelectron microscopy indicated the presence of the UL34 protein in both leaflets of the nuclear membrane. Ultrastructurally, an accumulation of enveloped virions in the perinuclear space in large invaginations of the inner nuclear membrane was observed, which were shown by immunoelectron microscopy to contain the UL34 protein, but not glycoproteins gB or gC. Thus, the US3 protein appears to be involved in the de-envelopment of perinuclear virions by fusion with the outer leaflet of the nuclear membrane. Surprisingly, no difference in the phosphorylation of the PrV UL34 protein was observed in the presence or absence of the US3 kinase. Therefore, the observed effects of the PrV US3 protein on the intracellular localization of the UL34 protein and on virus morphogenesis are probably not due to the phosphorylation of the UL34 protein by the US3 kinase.

Role of the cytoplasmic tails of pseudorabies virus glycoproteins B, E and M in intracellular localization and virion incorporation

The cytoplasmic domains of several herpesviral glycoproteins encompass potential intracellular sorting signals. To analyse the function of the cytoplasmic domains of different pseudorabies virus (PrV) glycoproteins, hybrid proteins were constructed consisting of the extracellular and transmembrane domains of envelope glycoprotein D (gD) fused to the cytoplasmic tails of gB, gE or gM (designated gDB, gDE and gDM), all of which contain putative endocytosis motifs. gD is a type I membrane protein required for binding to and entry into target cells. Localization of hybrid proteins compared to full-length gB, gE and gM as well as carboxy-terminally truncated variants of gD was studied by confocal laser scanning microscopy. The function of gD hybrids was assayed by trans-complementation of a gD-negative PrV mutant. The carboxy-terminal domains of gB and gM directed a predominantly intracellular localization of gDB and gDM, while full-length gD and a tail-less gD mutant (gDc) were preferentially expressed on the cell surface. In contrast gDE, and a gDB lacking the putative gB endocytosis signal (gDB Delta 29), were predominantly located in the plasma membrane. Despite the different intracellular localization, all tested proteins were able to complement infectivity of a PrV gD(-) mutant. Cells which stably express full-length gD and plasma-membrane-associated gD hybrids exhibit a significant resistance to PrV infection, while cells expressing predominantly intracellularly located forms do not. This suggests that the assumed sequestration of receptors by gD, which is supposed to be responsible for the interference phenomenon, occurs at the cell surface.

Effects of truncation of the carboxy terminus of pseudorabies virus glycoprotein B on infectivity

Glycoproteins homologous to the type I membrane glycoprotein B (gB) of herpes simplex virus 1 (HSV-1) are the most highly conserved glycoproteins within the family Herpesviridae and are present in members of each herpesvirus subfamily. In the alphaherpesvirus pseudorabies virus (PrV), gB is required for entry into target cells and for direct viral cell-to-cell spread. These processes, though related, appear to be distinct, and thus it was interesting to analyze whether they require different functions of gB. To this end, we established cell lines stably expressing different carboxy-terminally truncated versions of PrV gB by deleting either (i) one predicted intracytoplasmic alpha-helical domain encompassing putative YQRL and dileucine internalization signals, (ii) two predicted intracytoplasmic alpha-helical domains, (iii) the complete intracytoplasmic domain, or (iv) the intracytoplasmic domain and the transmembrane anchor region. Confocal laser scanning microscopy showed that gB derivatives lacking at least the last 29 amino acids (aa) localize close to the plasma membrane, while the full-length protein accumulates in intracellular aggregations. Trans-complementation studies with a gB-deleted PrV (PrV-gB(-)) demonstrated that the 29-aa truncated form lacking the putative internalization signals and the C-terminal alpha-helical domain (gB-008) was efficiently incorporated into PrV-gB(-) virions and efficiently complemented infectivity and cell-to-cell spread. Moreover, gB-008 exhibited an enhanced fusogenic activity. In contrast, gB proteins lacking both alpha-helical domains (gB-007), the complete intracytoplasmic domain, or the intracytoplasmic domain and transmembrane anchor were only inefficiently or not at all incorporated into PrV-gB(-) virions and did not complement infectivity. However, gB-007 was able to mediate cell-to-cell spread of PrV-gB(-). Similar phenotypes were observed when virus recombinants expressing gB-008 or gB-007, respectively, instead of wild-type gB were isolated and analyzed. Thus, our data show that internalization of gB is not required for gB incorporation into virions nor for its function in either entry or cell-to-cell spread. Moreover, they indicate different requirements for gB in these membrane fusion processes.

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.

The porcine humoral immune response against pseudorabies virus specifically targets attachment sites on glycoprotein gC

High titers of virus-neutralizing antibodies directed against glycoprotein gC of Pseudorabies virus (PRV) (Suid herpesvirus 1) are generally observed in the serum of immunized pigs. A known function of the glycoprotein gC is to mediate attachment of PRV to target cells through distinct viral heparin-binding domains (HBDs). Therefore, it was suggested that the virus-neutralizing activity of anti-PRV sera is directed against HBDs on gC. To address this issue, sera with high virus-neutralizing activity against gC were used to characterize the anti-gC response. Epitope mapping demonstrated that amino acids of HBDs are part of an antigenic antibody binding domain which is located in the N-terminal part of gC. Binding of antibodies to this antigenic domain of gC was further shown to interfere with the viral attachment. Therefore, these results show that the viral HBDs are accessible targets for the humoral anti-PRV response even after tolerance induction against self-proteins, which utilize similar HBDs to promote host protein-protein interactions. The findings indicate that the host's immune system can specifically block the attachment function of PRV gC. Since HBDs promote the attachment of a number of herpesviruses, the design of future antiherpesvirus vaccines should aim to induce a humoral immune response that prevents HBD-mediated viral attachment.

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.

Characterization of pseudorabies virus glycoprotein B expressed by canine herpesvirus

A recombinant canine herpesvirus (CHV) which expressed glycoprotein B (gB) of pseudorabies virus (PrV) was constructed. The antigenicity of the PrV gB expressed by the recombinant CHV is similar to that of the native PrV. The expressed PrV gB was shown to be transported to the surface of infected cells as judged by an indirected immunofluorescence test. Antibodies raised in mice immunized with the recombinant CHV neutralized the infectivity of PrV in vitro. It is known that the authentic PrV gB exists as a glycoprotein complex, which consists of gBa, gBb and gBc. In MDCK cells, PrV gB expressed by the recombinant CHV was processed like authentic PrV gB, suggesting that the cleavage mechanism of PrV gB depends on a functional cleavage domain from PrV gB gene and protease from infected cells.

Induction of antibodies in mice by a recombinant baculovirus expressing pseudorabies virus glycoprotein B in mammalian cells

The glycoprotein gB of pseudorabies virus (PrV) was expressed in various mammalian cells by a recombinant baculovirus carrying the PrV gB gene under the control of the CAG promoter. When the recombinant baculovirus was inoculated into the stable porcine kidney cell line CPK, expression of PrV gB was detected by immunofluorescent antibody analysis and a 155 kDa of protein, which has the same molecular mass as the native PrV gB, was detected by Western blotting. High levels of expression of PrV gB were observed in BHK-21, HmLu-1 and SK-H cell lines. Furthermore, anti-PrV gB-specific antibodies against PrV gB protein were detected by the enzyme-linked immunosorbent assay in mice inoculated the recombinant baculovirus. The recombinant baculovirus containing the PrV glycoprotein gB gene under the CAG promoter could be a candidate for a pseudorabies vaccine.

Glycoprotein gL-independent infectivity of pseudorabies virus is mediated by a gD-gH fusion protein

Envelope glycoproteins gH and gL, which form a complex, are conserved throughout the family Herpesviridae. The gH-gL complex is essential for the fusion between the virion envelope and the cellular cytoplasmic membrane during penetration and is also required for direct viral cell-to-cell spread from infected to adjacent noninfected cells. It has been proposed for several herpesviruses that gL is required for proper folding, intracellular transport, and virion localization of gH. In pseudorabies virus (PrV), glycoprotein gL is necessary for infectivity but is dispensable for virion localization of gH. A virus mutant lacking gL, PrV-DeltagLbeta, is defective in entry into target cells, and direct cell-to-cell spread is drastically reduced, resulting in only single or small foci of infected cells (B. G. Klupp, W. Fuchs, E. Weiland, and T. C. Mettenleiter, J. Virol. 71:7687-7695, 1997). We used this limited cell-to-cell spreading ability of PrV-DeltagLbeta for serial passaging of cells infected with transcomplemented virus by coseeding with noninfected cells. After repeated passaging, plaque formation was restored and infectivity in the supernatant was observed. One single-plaque isolate, designated PrV-DeltagLPass, was further characterized. To identify the mutation leading to this gL-independent infectious phenotype, Southern and Western blot analyses, radioimmunoprecipitations, and DNA sequencing were performed. The results showed that rearrangement of a genomic region comprising part of the gH gene into a duplicated copy of part of the unique short region resulted in a fusion fragment predicted to encode a protein consisting of the N-terminal 271 amino acids of gD fused to the C-terminal 590 residues of gH. Western blotting and radioimmunoprecipitation with gD- and gH-specific antibodies verified the presence of a gDH fusion protein. To prove that this fusion protein mediates infectivity of PrV-DeltagLPass, cotransfection of PrV-DeltagLbeta DNA with the cloned fusion fragment was performed, and a cell line, Nde-67, carrying the fusion gene was established. After cotransfection, infectious gL-negative PrV was recovered, and propagation of PrV-DeltagLbeta on Nde-67 cells produced infectious virions. Thus, a gDH fusion polypeptide can compensate for function of the essential gL in entry and cell-to-cell spread of PrV.

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.

Vaccine-induced, pseudorabies virus-specific, extrathymic CD4+CD8+ memory T-helper cells in swine

Pseudorabies virus (PRV; suid herpesvirus 1) infection causes heavy economic losses in the pig industry. Therefore, vaccination with live attenuated viruses is practiced in many countries. This vaccination was demonstrated to induce extrathymic virus-specific memory CD4+CD8+ T lymphocytes. Due to their major histocompatibility complex (MHC) class II-restricted proliferation, it is generally believed that these T lymphocytes function as memory T-helper cells. To directly prove this hypothesis, 15-amino-acid, overlapping peptides of the viral glycoprotein gC were used for screening in proliferation assays with peripheral blood mononuclear cells of vaccinated d/d haplotype inbred pigs. In these experiments, two naturally processed T-cell epitopes (T1 and T2) which are MHC class II restricted were identified. It was shown that extrathymic CD4+CD8+ T cells are the T-lymphocyte subpopulation that responds to epitope T2. In addition, we were able to show that cytokine secretion can be induced in these T cells through recall with inactivated PRV and demonstrated that activated PRV-primed CD4+CD8+ T cells are able to induce PRV-specific immunoglobulin synthesis by PRV-primed, resting B cells. Taken together, these results demonstrate that the glycoprotein gC takes part in the priming of humoral anti-PRV memory responses. The experiments identified the first T-cell epitopes so far known to induce the generation of virus-specific CD4+CD8+ memory T lymphocytes and showed that CD4+CD8+ T cells are memory T-helper cells. Therefore, this study describes the generation of virus-specific CD4+CD8+ T cells, which is observed during vaccination, as a part of the potent humoral anti-PRV memory response induced by the vaccine.

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.

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.

Glutathione inhibits HIV replication by acting at late stages of the virus life cycle

We investigated the effect of glutathione on the replication of human immunodeficiency virus (HIV) in chronically infected macrophages, a known reservoir of the virus in the body. We found that exogenous GSH strongly suppresses the production of p24gag protein as well as the virus infectivity. This is related to a dramatic decrease in both budding and release of virus particles from chronically infected cells (either macrophages or lymphocytes), together with a selective decrease in the expression of gp120, the major envelope glycoprotein, rich in intrachain disulfide bonds and thus potentially sensitive to the effect of a reducing agent such as GSH. Overall data suggest that GSH can interfere with late stages of virus replication. This would be in agreement with data obtained in cells exposed to herpesvirus type 1 (a DNA virus) or to Sendai (an RNA virus), showing that the suppression of virus replication by GSH is related to the selective inhibition of envelope glycoproteins. These results suggest a potential role of GSH in combination with other antivirals in the treatment of virus-related diseases.

Glycoprotein Bb, the N-terminal subunit of bovine herpesvirus 1 gB, can bind to heparan sulfate on the surfaces of Madin-Darby bovine kidney cells

The present study confirms our previous findings made by using heparin affinity chromatography that bovine herpesvirus 1 gB can bind to heparin-like structures. In order to locate the functional domain for heparin binding, we expressed the extracellular portion of gB (gBt) and the large subunit of gB (gBb) in Madin Darby bovine kidney (MDBK) cells under the control of the bovine heat shock protein 70A gene promoter. The recombinant gBt and gBb were both efficiently secreted from the transfected cells. They were shown to have structural and antigenic properties similar to those of authentic gB. Like authentic gB, both gBt and gBb were able to bind heparin-Sepharose as well as heparan sulfates on MDBK cells. Thus, we suggest that at least one heparin-binding domain is localized in gBb, the N-terminal portion of gB, which agrees with the presence of clusters of prolines and basic residues, thought to be essential for heparin binding.

RNA duplex unwinding activity of alfalfa mosaic virus RNA-dependent RNA polymerase

An RNA-dependent RNA polymerase (RdRp) purified from alfalfa mosaic virus-infected tobacco is capable of synthesizing in vitro full-size RNAs of minus and plus polarities. However, the enzyme is not able to perform a complete replication cycle in vitro. The products were found to be completely base-paired to their templates. The enzyme was able to use double-stranded RNA as a template for RNA synthesis if it could initiate from a single-stranded promoter. The inability (of most) of our enzyme preparations to create a single-stranded initiation site could explain why they could not perform a complete replication cycle in vitro. This is the first report on duplex RNA unwinding activities by a plant viral RdRp.

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.

Evidence for antiviral activity of glutathione: in vitro inhibition of herpes simplex virus type 1 replication

The role of glutathione (GSH) in the in vitro infection and replication of human herpes simplex virus type 1 (HSV-1) was investigated. Intracellular endogenous GSH levels dramatically decreased in the first 24 h after virus adsorption, starting immediately after virus challenge. The addition of exogenous GSH was not only able to restore its intracellular levels almost up to those found in uninfected cells, but also to inhibit > 99% the replication of HSV-1. This inhibition was concentration-dependent, not related to toxic effects on host cells and also maintained if the exogenous GSH was added as late as 24 h after virus challenge, i.e. when virus infection was fully established. Electron microscopic examination of HSV-1-infected cells showed that GSH dramatically reduced the number of extracellular and intracytoplasmic virus particles, whereas some complete nucleocapsids were still detected within the nuclei of GSH-treated cells. Consistent with this observation, immunoblot analysis showed that the expression of HSV-1-glycoprotein B, crucial for the release and the infectivity of virus particles, was significantly decreased. Data suggest that exogenous GSH inhibits the replication of HSV-1 by interfering with very late stages of the virus life cycle, without affecting cellular metabolism.

Characterization of pseudorabies virus glycoprotein gII expressed by recombinant baculovirus

The gene encoding the complete glycoprotein gII (homologue of gB of herpes simplex virus) of pseudorabies virus (PrV) was inserted into a baculovirus transfer vector, and a recombinant virus expressing gII was isolated. Three gII-related recombinant baculovirus-expressed peptides of 100, 60, and 45 to 50 kDa were detected with a polyclonal antibody against gII; these correspond to the authentic subunits gIIa and its cleavage products gIIb and gIIc, respectively. These proteins were subjected to N-terminal sequencing, and the results showed that the protease cleavage sites were identical to those of authentic gII. The expressed gII was shown to be transported to the surface of infected cells as judged by an indirect immunofluorescence test. Antibodies raised in mice immunized with the recombinant gII neutralized the infection of PrV in vitro. Mice inoculated with the recombinant gII were completely protected from lethal challenge with PrV.

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.

A novel intergenic site for integration and expression of foreign genes in the genome of pseudorabies virus

Restriction enzyme analysis of DNA of a number of Pseudorabies virus (PRV) single plaque isolates revealed in several cases the existence of a unique EcoRI cleavage site, which has not been observed in PRV DNA before. This EcoRI site was mapped to the right end of the unique long region of the PRV genome, in BamHI-fragment 6. Sequence analysis of this region demonstrated the presence of an 11 bp tandem repeat in variable copy numbers in different PRV strains, suggesting the creation of the EcoRI recognition site by a recombinational event. The occurrence of variable reiterations and Northern blot analysis indicated an intergenic region. We therefore, used this site for integration and expression of heterologous DNA (the multiple cloning site of phage M13 and the E. coli lacZ gene). Viable PRV recombinants could be obtained which showed no detectable differences in virus growth in vitro compared to wild-type PRV. The novel insertion site can be used for the construction of PRV recombinants expressing foreign genes without apparent impairment of PRV genes.

Specific pseudorabies virus infection of the rat visual system requires both gI and gp63 glycoproteins

Transneuronal transport of pseudorabies virus (PRV) from the retina to visual centers that mediate visual discrimination and reflexes requires specific genes in the unique short region of the PRV genome. In contrast, these same viral genes are not required to infect retinorecipient areas of the brain involved in circadian rhythm regulation. In this report, we demonstrate that viral mutants carrying defined deletions of the genes encoding glycoprotein gI or gp63, or both, result in the same dramatic transport defect. Efficient export of either gI or gp63 from the endoplasmic reticulum to the Golgi apparatus in a fibroblast cell line requires the presence of both proteins. We also show that gI and gp63 physically interact, as demonstrated by pulse-chase and sucrose gradient sedimentation experiments. Complex formation is rapid compared with homodimerization of PRV glycoprotein gII. We suggest that gI and gp63 function in concert to affect neurotropism in the rat visual circuitry and that a heterodimer is likely to be the unit of function.

Efficacy of various non-oily adjuvants in immunization against the Aujeszky's disease (pseudorabies) virus

Standard oil and various non-oily adjuvants were compared for use in immunization against the Aujeszky's disease (pseudorabies) virus, both in mice and swine, and using either inactivated virions or purified glycoproteins as antigen. Mineral oil, sodium alginate, aluminium hydroxide, and saponin were assayed in mice as adjuvants for inactivated virions, saponin being the most efficient. The addition of Mab anti-CD3 did not improve either immune response or protection achieved in mice using viral particles with oil or sodium alginate. When purified glycoproteins were used as antigens, the use of ISCOM greatly enhanced specific T-cell responses and protection of mice. The incorporation of Mab anti-CD3 into ISCOM conferred 100% protection of mice. Surprisingly, when an ISCOM containing glycoproteins was assayed in swine in a single-dose trial, no improvement on the protection conferred by the oily adjuvant was observed.

Comparison of the protective efficacy of Aujeszky's disease (pseudorabies) virus glycoproteins obtained from different sources

The immunogenic properties of a series of glycoprotein preparations are compared using inactivated conventional vaccines as reference. Serological response and protective efficacy of vaccination of mice and pigs are evaluated for glycoprotein immunogens obtained from various sources. BHK-21 cell cultures were infected with Aujeszky's disease virus and used as antigenic source. Glycoproteins were obtained from (i) the whole culture (ii) the cell sediment and (iii) the clarified supernatant. Both in pigs and in mice, protection was greater with glycoproteins purified from infected-cell membranes than with viral mature particle glycoproteins. The specific profiles of humoral responses were basically identical regardless of the source of glycoprotein. Bartha strain, one of the gI- strains most commonly used as an attenuated vaccine, was also used as a glycoprotein source. Immunogens obtained from this strain were protective in challenge trials with the virulent E-974 strain of the Aujeszky's disease virus. Glycoproteins did not induce detectable delayed type hypersensitivity in mice but conferred greater protection than particulate antigens (which, conversely, did induce a detectable delayed type hypersensitivity reaction). Until the recent proposal of the potency criterion delta 7, no objective method was available to evaluate the degree of protection conferred by Aujeszky's disease vaccines. In this study, we thus used the protection index, a quantitative parameter designed to evaluate potency of vaccines against Aujeszky's disease virus.

Glycoprotein gp50-negative pseudorabies virus: a novel approach toward a nonspreading live herpesvirus vaccine

Essential herpesvirus glycoproteins are involved in membrane fusion processes during infection, e.g., viral penetration and direct cell-to-cell transmission. We previously showed that the gD-homologous glycoprotein gp50 of pseudorabies virus (PrV) is essential for virus entry into target cells but proved to be dispensable for direct viral cell-to-cell spread in cell culture (I. Rauh and T. C. Mettenleiter, J. Virol. 65:5348-5456, 1991). For gp50-negative (gp50-) viruses, after phenotypic complementation necessary for primary infection, the only means of viral spread is by way of direct cell-to-cell transmission. In contrast, virus mutants lacking the essential gB-homologous glycoprotein gII after phenotypic complementation are only able to infect primary target cells and are blocked in further viral spread. To analyze how these in vitro phenotypes translate into virus replication in the animal, mice were infected intranasally with gp50- or gII- PrV mutants after prior phenotypic complementation by propagation on cell lines providing the essential glycoprotein in trans. Our results show that whereas the gII- mutants did not cause disease or any symptoms, gp50- mutants derived from two different PrV strains were fully virulent, with animals exhibiting severe symptoms ultimately leading to death. However, free infectious virus could not be recovered from either gp50- or gII- PrV-infected animals. We conclude that direct cell-to-cell transmission as the only means of viral spread of the gp50- mutants is sufficient for a full virulent phenotype in mice. After infection of pigs with phenotypically complemented gp50- PrV, only mild symptoms were observed, whereas the gII- mutant was totally avirulent. In both cases, shedding of infectious virus did not occur, in contrast to results with animals infected by gX- PrV that showed severe signs of disease and extensive virus shedding. After challenge infection with the highly virulent NIA-3 strain, the previously gII- PrV-infected animals exhibited severe symptoms, whereas the gp50- PrV-infected pigs showed a significant level of protection. In conclusion, vaccination with a PrV mutant lacking glycoprotein gp50, which is unable to spread between animals because of a lack of formation of free infectious virions, can confer on pigs protection against challenge infection. These results provide the basis for the development of new, nonspreading live herpesvirus vaccines based on gp50- PrV mutants.

Pseudorabies virus protein homologous to herpes simplex virus type 1 ICP18.5 is necessary for capsid maturation

In pseudorabies virus (PrV), an open reading frame that partially overlaps the gene for the essential glycoprotein gII has been shown to encode a protein homologous to the ICP18.5 polypeptide of herpes simplex virus type 1 (N. Pederson and L. Enquist, Nucleic Acids Res. 17:3597, 1989). To study the function of this protein during the viral replicative cycle, a PrV mutant which carries a beta-galactosidase expression cassette interrupting the ICP18.5(PrV) gene was constructed. This mutant could be propagated only on cell lines that were able to provide ICP18.5(PrV) in trans after transformation with a corresponding genomic PrV DNA fragment. Detailed analysis showed that inactivation of the ICP18.5(PrV) gene did not impair infection of noncomplementing cells, nor did it impair early or late gene expression, as shown by immunoprecipitation of glycoproteins gII, gIII, and gp50. Surface localization of glycoproteins as demonstrated by fluorescence-activated cell sorting analyses was also not affected. Southern blot hybridizations, however, showed that cleavage of replicative concatemeric viral DNA did not occur in noncomplementing cells infected by the ICP18.5 mutant PrV. In addition, electron microscopic analysis revealed an accumulation of empty capsids in the nucleus of mutant-infected noncomplementing cells. We conclude that the ICP18.5(PrV) protein is necessary for viral replication and plays an essential role in the process of mature capsid formation.

Structural, functional, and immunological characterization of bovine herpesvirus-1 glycoprotein gl expressed by recombinant baculovirus

The major glycoprotein complex gl of bovine herpesvirus-1 was expressed at high levels (36 micrograms per 1 x 10(6) cells) in insect cells using a recombinant baculovirus. The recombinant gl had an apparent molecular weight of 116 kDa and was partially cleaved to yield 63-kDa (glb) and 52-kDa (glc) subunits. This processing step was significantly less efficient in insect cells than the analogous step in mammalian cells, even though the cleavage sites of authentic and recombinant gl were shown to be identical. The oligosaccharide linkages were mostly endoglycosidase-H-sensitive, in contrast to those of authentic gl, which has mostly endoglycosidase-H-resistant linkages and an apparent molecular weight of 130/74/55 kDa. Despite the reduced cleavage and altered glycosylation, the recombinant glycoprotein was transported and expressed on the surface of infected insect cells. These surface molecules were biologically active as demonstrated by their ability to induce cell-cell fusion. Fusion was inhibited by three monoclonal antibodies specific for antigenic domains I and IV on gl. Domain I maps to the extracellular region of the carboxy terminal fragment glc and domain IV to the very amino terminus of the glb fragment, indicating that domains mapping in two distinct regions of gl function in cell fusion. Monoclonal antibodies specific for eight different epitopes recognized recombinant gl, indicating that the antigenic characteristics of the recombinant and authentic glycoproteins are similar. In addition, the recombinant gl was as immunogenic as the authentic gl, resulting in the induction of gl-specific antibodies in cattle.

Antigens involved in vaccination of swine against Aujeszky's disease (pseudorabies) virus

The polypeptide and glycopolypeptide composition of a local virulent Aujeszky's disease virus (suid herpesvirus 1, SHV-1) strain (E-974) was determined in order to characterize the individual SHV-1 antigens inducing the serological responses in immunized and non-immunized animals. A commercially available inactivated vaccine of known efficacy and three experimental immunogen preparations (whole inactivated SHV-1 particles, lectin-purified glycoproteins from SHV-1 culture, and a combination of both) were used for immunization. Sera of two-month old immunized and non-immunized animals were analyzed by ELISA, seroneutralization and Western immunoblotting prior to and following challenge with E-974. Sera of 7- to 30-day-old piglets littered by immunized and non-immunized sows were likewise analyzed by immunoblotting. The following variables were determined: the total level of anti-SHV-1 antibodies, the level of neutralizing antibodies, the IgG responses to individual SHV-1 antigens, and the clinical parameters and degree of protection of the animals. The whole-particle experimental immunogen conferred greatest protection, but correlation between antibody levels and the degree of protection was imperfect. Serological responses seemed to be directed against certain structural polypeptides and viral envelope glycoproteins. The glycoprotein immunogen caused a selective response to bands which closely resemble the glycopolypeptides gII and gIII. A 71 kDa component of uncertain location within the viral structure appeared to be one of the main antigens involved in porcine serological response to SHV-1 and colostral protection of piglets.

Protection of mice and swine from pseudorabies virus conferred by vaccinia virus-based recombinants

Glycoproteins gp50, gII, and gIII of pseudorabies virus (PRV) were expressed either individually or in combination by vaccinia virus recombinants. In vitro analysis by immunoprecipitation and immunofluorescence demonstrated the expression of a gII protein of approximately 120 kDa that was proteolytically processed to the gIIb (67- to 74-kDa) and gIIc (58-kDa) mature protein species similar to those observed in PRV-infected cells. Additionally, the proper expression of the 90-kDa gIII and 50-kDa gp50 was observed. All three of these PRV-derived glycoproteins were detectable on the surface of vaccinia virus-PRV recombinant-infected cells. In vivo, mice were protected against a virulent PRV challenge after immunization with the PRV glycoprotein-expressing vaccinia virus recombinants. The coexpression of gII and gIII by a single vaccinia virus recombinant resulted in a significantly reduced vaccination dose required to protect mice against PRV challenge. Inoculation of piglets with the various vaccinia virus-PRV glycoprotein recombinants also resulted in protection against virulent PRV challenge as measured by weight gain. The simultaneous expression of gII and gp50 in swine resulted in a significantly enhanced level of protection as evaluated by weight evolution following challenge with live PRV.

Stable rescue of a glycoprotein gII deletion mutant of pseudorabies virus by glycoprotein gI of bovine herpesvirus 1

Glycoproteins homologous to glycoprotein B (gB) of herpes simplex virus constitute the most highly conserved group of herpesvirus glycoproteins. This strong conservation of amino acid sequences might be indicative of a common functional role. Indeed, gB homologs have been implicated in the processes of viral entry and virus-mediated cell-cell fusion. Recently, we showed that pseudorabies virus (PrV) lacking the essential gB-homologous glycoprotein gII could be propagated on a cell line expressing the gB homolog of bovine herpesvirus 1, gI(BHV-1), leading to a phenotypic complementation of the gII defect (I. Rauh, F. Weiland, F. Fehler, G. Keil, and T.C. Mettenleiter, J. Virol. 65:621-631, 1991). However, this pseudotypic virus could still replicate only on complementing cell lines, thereby limiting experimental approaches to analyze the effects of the gB exchange in detail. We describe here the construction and isolation of a PrV recombinant, 9112C2, that lacks gII(PrV) but instead stably carries and expresses the gene encoding gI(BHV-1). The recombinant is able to replicate on noncomplementing cells with growth kinetics and final titers similar to those of its gII-positive wild-type PrV parent. Neutralization tests and immunoprecipitation analyses demonstrated incorporation of gI(BHV-1) into 9112C2 virions with concomitant absence of gII(PrV). Analysis of in vitro host ranges of wild-type PrV, BHV-1, and recombinant 9112C2 showed that in cells of pig, rabbit, canine, monkey, or human origin, the plating efficiency of 9112C2 was similar to that of its PrV parent. Exchange of gII(PrV) for gI(BHV-1) in recombinant 9112C2 or by phenotypic complementation of gII- PrV propagated on gI(BHV-1)-expressing cell lines resulted in penetration kinetics intermediate between those of wild-type PrV and BHV-1. In conclusion, we report the first isolation of a viral recombinant in which a lethal glycoprotein mutation has been rescued by a homologous glycoprotein of a different herpesvirus. Our data show that in gII- PrV, gI(BHV-1) in vitro fully complements the lethal defect associated with lack of gII(PrV). These results conclusively demonstrate that gI(BHV-1) in a PrV background can execute all essential functions normally provided by gII(PrV). They also indicate that the origin of gB-homologous glycoproteins influences the penetration kinetics of herpesviruses.

Measurement of isotype-specific antibody responses to Aujeszky's disease virus in sera and mucosal secretions of pigs

Enzyme-linked immunosorbent assays (ELISAs) for the detection of porcine IgM, IgA, IgG1 and IgG2 antibodies directed against Aujeszky's disease virus (ADV) are described. ADV-specific IgA and IgM were detected in an antibody capture assay, and ADV-specific IgG1 and IgG2 were detected in an indirect double antibody sandwich assay. A selected set of samples was tested in the four ELISAs and in a 24 h virus neutralization assay. Comparison of the results showed that the ELISAs were isotype-specific, sensitive, and reproducible. Samples with ADV antibody of one isotype showed that ADV-specific IgG1, IgG2 and IgM were able to neutralize the virus in vitro. In vitro neutralization of virus can be enhanced by complement. ADV-specific IgA neutralized virus only weakly. ADV-infected cells activated complement in the absence of antibody. Specific IgG2 and IgM enhanced complement activation. Analysis of the time course of antibody responses after infection or vaccination revealed that the isotype-specific ELISAs are suitable to study the humoral antibody response of pigs to the virus in mucosal secretions. Wild-type virus (strain NIA-3) and an attenuated vaccine strain (Bartha) administered intranasally induced mucosal IgM and IgA responses to the virus. In contrast, a killed vaccine (Nobivac) administered intramuscularly induced only weak mucosal IgM responses. The attenuated vaccine strain primed for a mucosal IgA memory response evoked upon challenge infection with wild-type virus.

Pseudorabies virus glycoproteins gII and gp50 are essential for virus penetration

Pseudorabies virus (PrV) glycoproteins gII and gp50 are major constituents of the viral envelope and targets of neutralizing monoclonal antibodies. Both are homologs of essential glycoproteins found in herpes simplex virus, gB (gII) and gD (gp50). We recently isolated a gII-negative PrV deletion mutant on complementing cell lines and established the essential character of gII for PrV replication (I. Rauh, F. Weiland, F. Fehler, G. Keil, and T.C. Mettenleiter, J. Virol. 65: 621-631, 1991). In this report, we describe the isolation of a gp50-negative PrV mutant after constructing cell lines that constitutively express gp50 and phenotypically complement the gp50 defect. Analysis of the gp50- mutant proved that gp50 is essential for PrV replication. Further studies showed that both gII and gp50 are required for viral penetration into target cells. The penetration defect in the gII and gp50 deletion mutants could be overcome by experimental polyethylene glycol-induced membrane fusion. Surprisingly, whereas gII proved to be essential for both penetration and cell-cell spread of the virus, gp50 was required only for penetration and appeared dispensable for direct cell-cell spread.

Monoclonal antibodies to double-stranded RNA as probes of RNA structure in crude nucleic acid extracts

We describe four monoclonal antibodies (MAB) which specifically recognize double-stranded RNA (dsRNA) together with their use in new methods for detecting and characterizing dsRNA in unfractionated nucleic acid extracts. The specificity of the antibodies was analyzed using a panel of 27 different synthetic and naturally occurring nucleic acids. All four antibodies reacted in a highly specific manner with long dsRNA helices, irrespective of their sequence; no binding to single-stranded RNA homopolymers or to DNA or RNA-DNA hybrids was observed. The apparent affinity of the antibodies to short (less than or equal to 11 bp) RNA helices was very low in all test systems used: only background levels of binding were obtained on single-stranded RNA species which contain double-helical secondary structures (e.g. rRNA, tRNA, viroid RNA). A sandwich ELISA and a dsRNA-immunoblotting procedure have been established which allow detection and characterization of dsRNA by MAB even in the presence of a large excess of other nucleic acids. In combination with temperature-gradient gelelectrophoresis (TGGE) not only the molecular weights but also the highly characteristic Tm-values of conformational transitions of individual dsRNA species could be determined by immunoblotting. An example of the general use of these methods for the detection of plant virus infections is demonstrated with groundnut rosette virus (GRV) dsRNAs. We were able to estimate the dsRNA content of infected leaves, identify the dsRNA species present in crude extracts and to determine the Tm- values of GRV dsRNA-3.