The pseudorabies virus gII gene is closely related to the gB glycoprotein gene of herpes simplex virus

Proteomic Comparison of Three Wild-Type Pseudorabies Virus Strains and the Attenuated Bartha Strain Reveals Reduced Incorporation of Several Tegument Proteins in Bartha Virions

Pseudorabies virus (PRV) is a member of the alphaherpesvirus subfamily and the causative agent of Aujeszky's disease in pigs. Driven by the large economic losses associated with PRV infection, several vaccines and vaccine programs have been developed. To this day, the attenuated Bartha strain, generated by serial passaging, represents the golden standard for PRV vaccination. However, a proteomic comparison of the Bartha virion to wild-type (WT) PRV virions is lacking. Here, we present a comprehensive mass spectrometry-based proteome comparison of the attenuated Bartha strain and three commonly used WT PRV strains: Becker, Kaplan, and NIA3. We report the detection of 40 structural and 14 presumed nonstructural proteins through a combination of data-dependent and data-independent acquisition. Interstrain comparisons revealed that packaging of the capsid and most envelope proteins is largely comparable in-between all four strains, except for the envelope protein pUL56, which is less abundant in Bartha virions. However, distinct differences were noted for several tegument proteins. Most strikingly, we noted a severely reduced incorporation of the tegument proteins IE180, VP11/12, pUS3, VP22, pUL41, pUS1, and pUL40 in Bartha virions. Moreover, and likely as a consequence, we also observed that Bartha virions are on average smaller and more icosahedral compared to WT virions. Finally, we detected at least 28 host proteins that were previously described in PRV virions and noticed considerable strain-specific differences with regard to host proteins, arguing that the potential role of packaged host proteins in PRV replication and spread should be further explored. IMPORTANCE The pseudorabies virus (PRV) vaccine strain Bartha-an attenuated strain created by serial passaging-represents an exceptional success story in alphaherpesvirus vaccination. Here, we used mass spectrometry to analyze the Bartha virion composition in comparison to three established WT PRV strains. Many viral tegument proteins that are considered nonessential for viral morphogenesis were drastically less abundant in Bartha virions compared to WT virions. Interestingly, many of the proteins that are less incorporated in Bartha participate in immune evasion strategies of alphaherpesviruses. In addition, we observed a reduced size and more icosahedral morphology of the Bartha virions compared to WT PRV. Given that the Bartha vaccine strain elicits potent immune responses, our findings here suggest that differences in protein packaging may contribute to its immunogenicity. Further exploration of these observations could aid the development of efficacious vaccines against other alphaherpesvirus vaccines such as HSV-1/2 or EHV-1.

Myricetin inhibits pseudorabies virus infection through direct inactivation and activating host antiviral defense

Myricetin, a polyhydroxyflavone compound, is one of the main ingredients of various human foods and therefore also known as dietary flavonoids. Due to the continuous emergence of resistant strains of herpesviruses, novel control measures are required. In the present study, myricetin exhibited potent antiviral activity against pseudorabies virus (PRV), a model organism of herpesvirus. The suppression rate could reach up to 96.4% at a concentration of 500 μM in cells, and the 50% inhibitory concentration (IC₅₀) was 42.69 μM. Moreover, the inhibitory activity was not attenuated by the increased amount of infective dose, and a significant reduction of intracellular PRV virions was observed by indirect immunofluorescence. A mode of action study indicated that myricetin could directly inactivate the virus in vitro, leading to inhibition of viral adsorption, penetration and replication in cells. In addition to direct killing effect, myricetin could also activate host antiviral defense through regulation of apoptosis-related gene expressions (Bcl-2, Bcl-xl, Bax), NF-κB and MAPK signaling pathways and cytokine gene expressions (IL-1α, IL-1β, IL-6, c-Jun, STAT1, c-Fos, and c-Myc). In PRV-infected mouse model, myricetin could enhance the survival rate by 40% at 5 days post infection, and viral loads in kidney, liver, lung, spleen, and brain were significantly decreased. The pathological changes caused by PRV infection were improved by myricetin treatment. The gene expressions of inflammatory factors (MCP-1, G-CSF, IL-1α, IL-1β, and IL-6) and apoptotic factors (Bcl-xl, Bcl-2, and Bax) were regulated by myricetin in PRV-infected mice. The present findings suggest that myricetin can effectively inhibit PRV infection and become a candidate for development of new anti-herpesvirus drugs.

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.

Two classes of protective antibodies against Pseudorabies virus variant glycoprotein B: Implications for vaccine design

Pseudorabies virus (PRV) belongs to the Herpesviridae family, and is an important veterinary pathogen. Highly pathogenic PRV variants have caused severe epidemics in China since 2011, causing huge economic losses. To tackle the epidemics, we identified a panel of mouse monoclonal antibodies (mAbs) against PRV glycoprotein B (gB) that effectively block PRV infection. Among these 15 mAbs, fourteen of them block PRV entry in a complement-dependent manner. The remaining one, 1H1 mAb, however can directly neutralize the virus independent of complement and displays broad-spectrum neutralizing activities. We further determined the crystal structure of PRV gB and mapped the epitopes of these antibodies on the structure. Interestingly, all the complement-dependent neutralizing antibodies bind gB at the crown region (domain IV). In contrast, the epitope of 1H1 mAb is located at the bottom of domain I, which includes the fusion loops, indicating 1H1 mAb might neutralize the virus by interfering with the membrane fusion process. Our studies demonstrate that gB contains multiple B-cell epitopes in its crown and base regions and that antibodies targeting different epitopes block virus infection through different mechanisms. These findings would provide important clues for antiviral drug design and vaccine development.

Pseudorabies virus (PRV) is an emerging veterinary pathogen that infects many domestic animals. Since 2011, highly pathogenic PRV variants have emerged in many farms in China and posed great economic burdens to the animal industry. However, the current marketed vaccines cannot provide effective protection against these emerging strains. The envelope glycoprotein B (gB) is a major viral antigen known to play a crucial role in PRV entry. In order to control PRV epidemics and treat associated diseases, we combined structural and immunological approaches to generate potential neutralizing antibodies targeting PRV gB and investigate their working mechanisms. A total of 15 monoclonal antibodies (mAbs) were identified with good neutralizing activity. Among them, fourteen mAbs target the domain IV of PRV gB and block virus entry though complement effect. In contrast, the remaining 1H1 mAb recognizes domain I of PRV gB, which can neutralize virus entry independent of complement and probably by interfering with the membrane fusion process. Our work reveals the structural details and immunogenic properties of PRV gB and may offer important guidance for developing antiviral therapeutics and vaccines against PRV infections.

Development of a competitive double antibody lateral flow assay for the detection of antibodies specific to glycoprotein B of Aujeszky's disease virus in swine sera

Three lateral flow assays (LFAs) for the detection of antibodies against glycoprotein B (gB) of Aujeszky's disease virus (ADV) in swine sera: a competitive double antibody sandwich LFA without a preincubation step (CDAS-gB-LFA), a CDAS-gB-LFA with a preincubation step (pCDAS-gB-LFA), and a competitive direct gB-LFA have been developed and were compared with each other and with a gB-ELISA. The assays are based on monoclonal antibodies to immunodominant epitopes of ADV gB. The pCDAS-gB-LFA proved to be the most specific and sensitive assay to detect antibodies directed to ADV gB. The specificity and sensitivity of the pCDAS-gB-LFA with the use of an LFA reader for test line intensity measurements were 97.6 and 94.9%, respectively. The lower diagnostic sensitivity of the pCDAS-gB-LFA compared to a gB-ELISA reflects its reduced analytical sensitivity, which was shown in titration experiments with positive sera. The pCDAS-gB-LFA, using the reader-based and visual detection modes, showed good agreement in respect to specificity; however, the LFA reader detection provided a higher diagnostic and analytical sensitivity compared to visual detection. The developed pCDAS-gB-LFA is a rapid, sensitive, and specific method for the detection of antibodies to ADV gB and can be used for screening ADV-infected swine in unvaccinated herds.

Production of pseudorabies virus recombinant glycoprotein B and its use in an agar gel immunodiffusion (AGID) test for detection of antibodies with sensitivity and specificity equal to the virus neutralization assay

Pseudorabies virus (PrV) causes Aujeszky's disease (AD), which affects mainly swine, but also cattle, sheep, and wild animals, resulting in substantial economic losses due to animal mortality and lost productivity worldwide. To combat PrV, eradication programs using PrV strains lacking the gene encoding glycoprotein E (gE) are ongoing in several countries. These eradication programs have generated a currently unmet demand for affordable, easy-to-use, and sensitive tests that can detect PrV infection in pigs infected with either wild-type virus or vaccine strain (gE-deleted) virus. To meet this demand, we used the baculovirus-insect cell system to produce recombinant glycoprotein B (gB) as antigen for an immune assay. The high GC-content (70% average) of the gB gene from the Argentinian PrV CL15 strain necessitated the use of betaine as a PCR enhancer to amplify the extracellular domain. Recombinant gB was expressed at high levels and reacted strongly with sera from PrV infected pigs. We used the recombinant gB to develop an agar gel immunodiffusion (AGID) test for detection of PrV antibodies. Compared to the gold standard virus neutralization (VN) assay, the AGID sensitivity and specificity were 95% and 96.6% respectively. Thus, recombinant gB produced in the baculovirus-insect cell system is a viable source of antigen for the detection of PrV antibodies in AGID tests. Considering its relatively lower cost, simplicity of use and result interpretation, our AGID is a valuable alternative tool to the VN assay.

Entry of herpes simplex virus 1 and other alphaherpesviruses via the paired immunoglobulin-like type 2 receptor alpha

Herpes simplex virus 1 (HSV-1) enters cells either via fusion of the virion envelope and host cell plasma membrane or via endocytosis, depending on the cell type. In the study reported here, we investigated a viral entry pathway dependent on the paired immunoglobulin-like type 2 receptor alpha (PILRalpha), a recently identified entry coreceptor for HSV-1 that associates with viral envelope glycoprotein B (gB). Experiments using inhibitors of endocytic pathways and ultrastructural analyses of Chinese hamster ovary (CHO) cells transduced with PILRalpha showed that HSV-1 entry into these cells was via virus-cell fusion at the cell surface. Together with earlier observations that HSV-1 uptake into normal CHO cells and those transduced with a receptor for HSV-1 envelope gD is mediated by endocytosis, these results indicated that expression of PILRalpha produced an alternative HSV-1 entry pathway in CHO cells. We also showed that human and murine PILRalpha were able to mediate entry of pseudorabies virus, a porcine alphaherpesvirus, but not of HSV-2. These results indicated that viral entry via PILRalpha appears to be conserved but that there is a PILRalpha preference among alphaherpesviruses.

Molecular basis for resistance of herpes simplex virus type 1 mutants to the sulfated oligosaccharide inhibitor PI-88

Herpes simplex virus type 1 variants selected by virus propagation in cultured cells in the presence of the sulfated oligosaccharide PI-88 were analyzed. Many of these variants were substantially resistant to the presence of PI-88 during their initial infection of cells and/or their cell-to-cell spread. Nucleotide sequence analysis revealed that the deletion of amino acids 33-116 of gC but not lack of gC expression provided the virus with selective advantage to infect cells in the presence of PI-88. Purified gC (Delta33-116) was more resistant to PI-88 than unaltered protein in its binding to cells. Alterations that partly contributed to the virus resistance to PI-88 in its cell-to-cell spread activity were amino acid substitutions Q27R in gD and R770W in gB. These results suggest that PI-88 targets several distinct viral glycoproteins during the course of initial virus infection and cell-to-cell spread.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.

A pseudorabies virus recombinant simultaneously lacking the major tegument proteins encoded by the UL46, UL47, UL48, and UL49 genes is viable in cultured cells

The UL46, UL47, UL48, and UL49 genes, which encode major tegument proteins, are conserved in most alphaherpesvirus genomes. However, the relative importance of each of these proteins for replication of individual alphaherpesviruses appears to be different. Recently, we demonstrated that single deletions of UL47 or UL48 impair maturation and egress of pseudorabies virus (PrV) particles to different extents, whereas deletions of UL46 or UL49 have no significant effects on virus replication in cell culture (W. Fuchs, H. Granzow, B. G. Klupp, M. Kopp, and T. C. Mettenleiter, J. Virol. 76:6729-6742, 2002; M. Kopp, B. G. Klupp, H. Granzow, W. Fuchs, and T. C. Mettenleiter, J. Virol. 76:8820-8833, 2002). To test for possible functional redundancy between the four tegument proteins, a quadruple gene deletion mutant (PrV-DeltaUL46-49) was generated and characterized in vitro. Although plaque formation by this mutant was almost abolished and maximum titers were reduced more than 100-fold compared to those of parental wild-type virus, PrV-DeltaUL46-49 could be propagated and serially passaged in noncomplementing porcine and rabbit kidney cells. Electron-microscopic studies revealed that nucleocapsid formation and egress of PrV-DeltaUL46-49 from the host cell nucleus were not affected, but secondary envelopment of nucleocapsids in the cytoplasm was only rarely observed. The replication defect of PrV-DeltaUL46-49 could be fully corrected by reinsertion of the UL46-to-UL49 gene cluster. Plaque sizes and virus titers were only slightly increased after restoration of only UL47 expression, whereas repair of only UL48 resulted in a significant increase in replication capacity to the level of a UL47 deletion mutant. In conclusion, we show that none of the UL46 to UL49 tegument proteins is absolutely required for productive replication of PrV. Moreover, our data indicate that the UL47 and UL48 proteins function independently during cell-to-cell spread and virus egress.

The pseudorabies virus Us2 protein, a virion tegument component, is prenylated in infected cells

The Us2 gene is conserved among alphaherpesviruses, but its function is not known. We demonstrate here that the pseudorabies virus (PRV) Us2 protein is synthesized early after infection and localizes to cytoplasmic vesicles and to the plasma membrane, despite the lack of a recognizable signal sequence or membrane-spanning domain. Us2 protein is also packaged as part of the tegument of mature virions. The Us2 carboxy-terminal four amino acids comprise a CAAX motif, a well-characterized signal for protein prenylation. Treatment of infected cells with lovastatin, a drug that disrupts protein prenylation, changed the relative electrophoretic mobility of Us2 in sodium dodecyl sulfate-polyacrylamide gels. In addition, lovastatin treatment caused a dramatic relocalization of Us2 to cytoplasmic punctate structures associated with microtubules, which appeared to concentrate over the microtubule organizing center. When the CAAX motif was changed to GAAX and the mutant protein was synthesized from an expression plasmid, it concentrated in punctate cytoplasmic structures reminiscent of Us2 localization in infected cells treated with lovastatin. We suggest that prenylation of PRV Us2 protein is required for proper membrane association. Curiously, the Us2 protein isolated from purified virions does not appear to be prenylated. This is the first report to describe the prenylation of an alphaherpesvirus protein.

Identification of the authentic varicella-zoster virus gB (gene 31) initiating methionine overlapping the 3' end of gene 30

The varicella-zoster virus (VZV) gB sequence was re-examined in light of recent knowledge about unusually long gB signal peptides in other herpesviral gB homologs. Through mutational analysis, the discovery was made that the authentic initiating methionine for VZV gB is a codon beginning at genome nucleotide 56,819. The total length for the VZV gB primary translation product was 931 amino acids (aa) with a 71-aa signal sequence. Considering the likely signal sequence cleavage site to be located between Ser 71 and Val 72, the length of the mature VZV gB polypeptide would then be 860 amino acids prior to further internal endoproteolytic cleavage between amino acids Arg 494 and Ser 495. In this report, we also produced a full-length gB and demonstrated its association with VZV gE, suggesting a possible gE-gB interaction during gB trafficking before its cleavage in the Golgi.

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.

The interacting UL31 and UL34 gene products of pseudorabies virus are involved in egress from the host-cell nucleus and represent components of primary enveloped but not mature virions

A 2.6-kbp fragment of the pseudorabies virus (PrV) genome was sequenced and shown to contain the homologues of the highly conserved herpesvirus genes UL31 and UL32. By use of a monospecific antiserum, the UL31 gene product was identified as a nuclear protein with an apparent molecular mass of 29 kDa. For functional analysis, UL31 was deleted by mutagenesis in Escherichia coli of an infectious full-length clone of the PrV genome. The resulting virus mutants were deficient in plaque formation, and titers were reduced more than 100-fold from those of wild-type PrV. Ultrastructural analyses demonstrated that capsid maturation and DNA packaging were not affected. However, neither budding at the inner nuclear membrane nor cytoplasmic or extracellular virus particles were observed. These replication defects were similar to those of a UL34 deletion mutant (B. G. Klupp, H. Granzow, and T. C. Mettenleiter, J. Virol. 74:10063-10073, 2000) and could be completely repaired in a cell line which constitutively expresses the UL31 protein. Yeast two-hybrid studies revealed that a UL31 fusion protein specifically interacts with plasmids of a PrV genome library expressing the N-terminal part of UL34. Vice versa, UL34 selected UL31-encoding plasmids from the library. Immunofluorescence studies and immune electron microscopy demonstrated that in cells infected with wild-type PrV, both proteins accumulate at the nuclear membrane, whereas in the absence of UL34 the UL31 protein is dispersed throughout the nucleus. Like the UL34 protein, the UL31 gene product is a component of enveloped virus particles within the perinuclear space and absent from mature virions. Our findings suggest that physical interaction between these two virus proteins might be a prerequisite for primary envelopment of PrV at the inner nuclear membrane and that this envelope is removed by fusion with the outer nuclear membrane.

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.

Glycoprotein D-independent infectivity of pseudorabies virus results in an alteration of in vivo host range and correlates with mutations in glycoproteins B and H

Infection of cells by herpesviruses is initiated by the interaction of viral envelope glycoproteins with cellular receptors. In the alphaherpesvirus pseudorabies virus (PrV), the causative agent of Aujeszky's disease in pigs, the essential glycoprotein D (gD) mediates secondary attachment of virions to target cells by binding to newly identified cellular receptors (R. J. Geraghty, C. Krummenacher, G. H. Cohen, R. J. Eisenberg, and P. G. Spear, Science 280:1618-1620, 1998). However, in the presence of compensatory mutations, infection can also occur in the absence of gD, as evidenced by the isolation in cell culture of an infectious gD-negative PrV mutant (PrV-gD(-) Pass) (J. Schmidt, B. G. Klupp, A. Karger, and T. C. Mettenleiter, J. Virol. 71:17-24, 1997). PrV-gD(-) Pass is replication competent with an only moderate reduction in specific infectivity but appears to bind to receptors different from those recognized by wild-type PrV (A. Karger, J. Schmidt, and T. C. Mettenleiter, J. Virol. 72:7341-7348, 1998). To analyze whether this alteration in receptor usage in vitro influences infection in vivo, the model host mouse and the natural host pig were intranasally infected with PrV-gD(-) Pass and were compared to animals infected by wild-type PrV. For mice, a comparable progress of disease was observed, and all animals infected with mutant virus died, although they exhibited a slight delay in the onset of symptoms and, correspondingly, a longer time to death. In contrast, whereas wild-type PrV-infected pigs showed clinical signs and histological and histopathological findings typical of PrV infection, no signs of disease were observed after infection with PrV-gD(-) Pass. Moreover, in these animals, virus-infected cells were not detectable by immunohistochemical staining of different organ samples and no virus could be isolated from nasal swabs. Mutations in glycoproteins B and H were found to correlate with, and probably contribute to, gD-independent infectivity. In conclusion, although PrV-gD(-) Pass is virulent in mice, it is apparently unable to infect the natural host, the pig. This altered host range in vivo correlates with a difference of receptor usage in vitro and demonstrates for the first time the importance of gD receptors in alphaherpesvirus infection of an animal host.

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.

Herpes simplex virus types 1 and 2 differ in their interaction with heparan sulfate

Cell surface heparan sulfate (HS) serves as an initial receptor for many different viruses, including herpes simplex virus types 1 and 2 (HSV-1 and 2, respectively). Glycoproteins C and B (gC and gB) are the major components of the viral envelope that mediate binding to HS. In this study, purified gB and gC homologous proteins as well as purified HSV-1 and HSV-2 virions were compared for the ability to bind isolated HS receptor molecules. HSV-1 gC and HSV-2 gC bound comparable amounts of HS. Similarly, HSV-1 gB and its HSV-2 counterpart showed no difference in the HS-binding capabilities. Despite the similar HS-binding potentials of gB and gC homologs, HSV-1 virions bound more HS than HSV-2 particles. Purified gC and gB proteins differed with respect to sensitivity of their interaction with HS to increased concentrations of sodium chloride in the order gB-2 > gB-1 > gC-1 > gC-2. The corresponding pattern for binding of whole HSV virions to cells in the presence of increased ionic strength of the medium was HSV-2 gC-neg1 > HSV-1 gC(-)39 > HSV-1 KOS 321 > HSV-2 333. These results relate the HS-binding activities of individual glycoproteins with the cell-binding abilities of whole virus particles. In addition, these data suggest a greater contribution of electrostatic forces for binding of gB proteins and gC-negative mutants compared with binding of gC homologs and wild-type HSV strains. Binding of wild-type HSV-2 virions was the least sensitive to increased ionic strength of the medium, suggesting that the less extensive binding of HS molecules by HSV-2 than by HSV-1 can be compensated for by a relatively weak contribution of electrostatic forces to the binding. Furthermore, gB and gC homologs exhibited different patterns of sensitivity of binding to cells to inhibition with selectively N-, 2-O-, and 6-O-desulfated heparin compounds. The O-sulfate groups of heparin were found to be more important for interaction with gB-1 than gB-2. These results indicate that HSV-1 and HSV-2 differ in their interaction with HS.

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.

Herpes simplex virus 1 (HSV-1) strain HSZP glycoprotein B gene: comparison of mutations among strains differing in virulence

The nonpathogenic HSZP strain of HSV-1 induces large polykaryocytes due to a syn3 mutation (His for Arg at residue 858) in the C-terminal endodomain of glycoprotein B (gB) (40). We determined the nucleotide (nt) sequence of the UL27 gene specifying the gB polypeptide of HSZP (gBHSZP) and found 3 mutations in its ectodomain at aminoacids (aa) 59, 79 and 108. The ANGpath virus, which also has a syn3 mutation in the C-terminal endodomain of gB (Val for Ala at residue 855) is pathogenic for adult mice (39), but can be made nonpathogenic by replacing the gBANGpath gene by the corresponding gBKOS sequence (21). The gBANGpath had three ectodomain mutations (at aa 62, 77 and 285), while gBKOS had at least four ectomain mutations (aa 59, 79, 313, and 553). Two mutations (aa 59 and 79) in the latter, located in the variable antigenic site IV/D1 were common for gBKOS and gBHSZP. These together with the gBANGpath mutations at aa 62 and 77 create a cluster of 4 mutations in diverse region of the N-terminal part of gB (between aa 59-79), in which the gBs of pathogenic ANGpath and 17 viruses differ from the gBs of nonpathogenic HSZP and KOS viruses. The lower pathogenicity of KOS as related to gBKOS, is furthermore associated with the change of Ser to Thr at aa 313 (locus III/D2). The possibility is discussed that mutations in both above mentioned antigenic loci could result in higher immunogenicity of the corresponding antigenic epitopes, which, in turn, would contribute to the decreased virulence of HSZP and KOS viruses.

Directional transneuronal infection by pseudorabies virus is dependent on an acidic internalization motif in the Us9 cytoplasmic tail

The Us9 gene is conserved among most alphaherpesviruses. In pseudorabies virus (PRV), the Us9 protein is a 98-amino-acid, type II membrane protein found in the virion envelope. It localizes to the trans-Golgi network (TGN) region in infected and transfected cells and is maintained in this compartment by endocytosis from the plasma membrane. Viruses with Us9 deleted have no observable defects in tissue culture yet have reduced virulence and restricted spread to retinorecipient neurons in the rodent brain. In this report, we demonstrate that Us9-promoted transneuronal spread in vivo is dependent on a conserved acidic motif previously shown to be essential for the maintenance of Us9 in the TGN region and recycling from the plasma membrane. Mutant viruses with the acidic motif deleted have an anterograde spread defect indistinguishable from that of Us9 null viruses. Transneuronal spread, however, is not dependent on a dileucine endocytosis motif in the Us9 cytoplasmic tail. Through alanine scanning mutagenesis of the acidic motif, we have identified two conserved tyrosine residues that are essential for Us9-mediated spread as well as two serine residues, comprising putative consensus casein kinase II sites, that modulate the rate of PRV transneuronal spread in vivo.

Role of pseudorabies virus Us9, a type II membrane protein, in infection of tissue culture cells and the rat nervous system

The protein product of the pseudorabies virus (PRV) Us9 gene is a phosphorylated, type II membrane protein that is inserted into virion envelopes and accumulates in the trans-Golgi network. It is among a linked group of three envelope protein genes in the unique short region of the PRV genome which are absent from the attenuated Bartha strain. We found that two different Us9 null mutants exhibited no obvious phenotype after infection of PK15 cells in culture. Unlike those of gE and gI null mutants, the plaque size of Us9 null mutants on Madin-Darby bovine kidney cells was indistinguishable from that of wild-type virus. However, both of the Us9 null mutants exhibited a defect in anterograde spread in the visual and cortical circuitry of the rat. The visual system defect was characterized by restricted infection of a functionally distinct subset of visual projections involved in the temporal organization of behavior, whereas decreased anterograde spread of virus to the cortical projection targets was characteristic of animals receiving direct injections of virus into the cortex. Spread of virus through retrograde pathways in the brain was not compromised by a Us9 deletion. The virulence of the Us9 null mutants, as measured by time to death and appearance of symptoms of infection, also was reduced after their injection into the eye, but not after cortical injection. Through sequence analysis, construction of revertants, measurement of gE and gI protein synthesis in the Us9 null mutants, and mixed-infection studies of rats, we conclude that the restricted-spread phenotype after infection of the rat nervous system reflects the loss of Us9 and is not an indirect effect of the Us9 mutations on expression of glycoproteins gE and gI. Therefore, at least three viral envelope proteins, Us9, gE, and gI, function together to promote efficient anterograde transneuronal infection by PRV in the rat central nervous system.

The UL25 protein of pseudorabies virus associates with capsids and localizes to the nucleus and to microtubules

The UL25 gene of pseudorabies virus (PrV) can encode a protein of about 57 kDa which is well conserved among herpesviruses. The UL25 protein of herpes simplex virus type 1 is a capsid constituent involved in virus penetration and capsid maturation. To identify and characterize the UL25 gene product of PrV, polyclonal mouse anti-UL25 antibodies were raised to a bacterially expressed fusion protein. In immunoblotting and immunoprecipitation assays of PrV-infected cell lysates, these anti-UL25 antisera specifically recognized a protein of the expected size with late expression kinetics. This 57-kDa product was also present in purified virions and was found to be associated with all types of capsids. Synthesis of a protein migrating at the same size point was directed from the eukaryotic expression plasmid pCG-UL25. To determine the subcellular localization of UL25, immunofluorescence studies with anti-UL25 antisera were performed on Nonidet P-40-extracted COS-7 cells infected with PrV or transfected with pCG-UL25. In PrV-infected cells, newly synthesized UL25 is directed mainly to distinct nuclear compartments, whereas UL25 expressed in the absence of other viral proteins is distributed more uniformly in the nucleus and colocalizes also with microtubules. To study the fate of UL25 at very early stages of infection, immunofluorescence experiments were performed on invading PrV particles in the presence or absence of drugs that specifically depolymerize components of the cytoskeleton. We found that the incoming nucleocapsids colocalize with microtubules during their transport to the nucleus and that UL25 remains associated with nucleocapsids during this transport.

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.

Studies of genetic relationships between bovine, caprine, cervine, and rangiferine alphaherpesviruses and improved molecular methods for virus detection and identification

The glycoprotein B (gB) and D (gD) genes from five ruminant alphaherpesviruses, bovine herpesvirus 1 (BHV-1), bovine herpesvirus 5 (BHV-5), caprine herpesvirus 1 (CapHV-1), cervine herpesvirus 1, and rangiferine herpesvirus 1, were partially sequenced. The nucleotide sequence alignments revealed a highly conserved gB gene, with homologies ranging between 87.2 and 99.6%, and a more variable gD gene, with homologies ranging between 71.3 and 98.9%. The phylogenetic analysis of the gB and gD nucleotide and deduced amino acid sequences revealed that BHV-5 is the most closely related virus to the BHV-1 subtype 1 and BHV-1 subtype 2 cluster and that CapHV-1 is the most distantly related virus. The phylogenetic data showed a close relationship of all the studied viruses with suid herpesvirus 1. On the basis of sequence data for the gB gene, a nested PCR combined with restriction enzyme analysis (REA) of the PCR products was developed for the simultaneous detection and identification of the viruses that were studied. Nested primers from highly conserved sequence stretches were selected in order to amplify a region of 294 bp in all five viruses, and a subsequent REA of the PCR products allowed specific identification. A mimic molecule that served as an internal standard of the amplification efficiency was constructed. The practical diagnostic applicability of the assay was evaluated with clinical samples consisting of semen and organ specimens from experimentally infected animals.

Mutation of the YXXL endocytosis motif in the cytoplasmic tail of pseudorabies virus gE

The role of alphaherpesvirus membrane protein internalization during the course of viral infection remains a matter of speculation. To determine the role of internalization of the pseudorabies virus (PRV) gE and gI proteins, we constructed viral mutants encoding specific mutations in the cytoplasmic tail of the gE gene that inhibited internalization of the gE-gI complex. We used these mutants to assess the role of gE-gI endocytosis in incorporation of the proteins into the viral envelope and in gE-mediated spread or gE-promoted virulence. In addition, we report that another viral mutant, PRV 25, which encodes a gE protein defective in endocytosis, contains an additional, previously uncharacterized mutation in the gE gene. We compared PRV 25 to another viral mutant, PRV 107, that does not express the cytoplasmic tail of the gE protein. The gE protein encoded by PRV 107 is also defective in endocytosis. We conclude that efficient endocytosis of gE is not required for gE incorporation into virions, gE-mediated virulence, or spread of virus in the rat central nervous system. However, we do correlate the defect in endocytosis to a small-plaque phenotype in cultured cells.

The left border of the genomic inversion of pseudorabies virus contains genes homologous to the UL46 and UL47 genes of herpes simplex virus type 1, but no UL45 gene

The genome of pseudorabies virus (PrV) is collinear with the herpes simplex virus type 1 (HSV1) genome, except for an inversion in the unique long region, the right extremity of which resides within the BamHI fragment 9 and the left within the BamHI fragment 1. We previously sequenced the right border of the inversion which is situated next to the UL44-gC gene and found that it encodes the UL24, UL25, UL26 and UL26.5 gene counterparts of HSV1. We have now sequenced 5317 base pairs of the BamHI fragment 1, upstream of the UL27-gB gene. We found two open reading frames homologous to UL46 and UL47 of HSV1 yet UL45 was absent and replaced by a set of strictly repeated sequences. PrV UL46 and UL47 are transcribed into two 3' co-terminal messenger RNAs with early and late kinetics, respectively. Comparison of the PrV UL46 and UL47 protein sequences with their counterparts from alphaherpesviruses indicated a strong similarity. The genome is rearranged in this region with respect to HSV1 and the inversion must have taken place, on the left side, within the UL46-UL27 intergenic region. Thus, the inversion should include genes UL27 to UL44.

Molecular mechanisms of neurotropic herpesvirus invasion and spread in the CNS

Pseudorabies virus (PRV) is a herpesvirus in the subfamily alphaherpesvirinae (the alpha herpesviruses). After primary infection at mucosal surfaces, PRV infects the peripheral nervous system in its natural host (swine) with occasional invasion of the central nervous system. When other hosts (including cows and rodents) are infected, the infection almost always gives rise to fatal disease in the CNS as a result of infection of peripheral neurons and subsequent spread to the brain. Part of the ability to cause fatal CNS disease can be attributed to a viral glycoprotein called gE. Viruses lacking gE are thought to be less virulent because they do not spread efficiently from cell to cell. Based on a set of gE mutations we have constructed, we suggest that these two phenotypes of cell-cell spread and virulence reflect separate functions of the gE protein. In this report, we show that viruses carrying these new gE mutations have marked reduction in virulence, yet spread efficiently in defined neural circuits in the rat brain. As such, they offer new insight and opportunities for understanding of viral disease and host response to injury, as well as in the construction of viral tracers of neuronal connections.

Role of envelope protein gE endocytosis in the pseudorabies virus life cycle

Several groups have reported that certain herpesvirus envelope proteins do not remain on the surface of cells that express them but rather are internalized by endocytosis in a recycling process. The biological function of membrane protein endocytosis in the virus life cycle remains a matter of speculation and debate. In this report, we demonstrate that some, but not all, membrane proteins encoded by the alphaherpesvirus pseudorabies virus (PRV) are internalized after reaching the plasma membrane. Glycoproteins gE and gB are internalized from the plasma membrane of cells, while gI and gC are not internalized efficiently. We show for gE that the cytoplasmic domain of the protein is required for endocytosis. While the gI protein is incapable of endocytosis on its own, it can be internalized when complexed with gE. We demonstrate that endocytosis of the gE-gI complex and gB occurs early after infection of tissue culture cells but that this process stops completely after 6 h of infection, a time that correlates with significant shutoff of host protein synthesis. We also show that gE protein internalized at 4 h postinfection is not present in virions formed at a later time. We discuss the differences in PRV gE and gI endocytosis compared to that of the varicella-zoster virus homologs and the possible roles of glycoprotein endocytosis in the virus life cycle.

The Us9 gene product of pseudorabies virus, an alphaherpesvirus, is a phosphorylated, tail-anchored type II membrane protein

The Us9 gene is highly conserved among the alphaherpesviruses sequenced to date, yet its function remains unknown. In this report, we demonstrate that the pseudorabies virus (PRV) Us9 protein is present in infected cell lysates as several phosphorylated polypeptides ranging from 17 to 20 kDa. Synthesis is first detected at 6 h postinfection and is sensitive to the DNA synthesis inhibitor phosphonoacetic acid. Unlike the herpes simplex virus type 1 Us9 homolog, which was reported to be associated with nucleocapsids in the nuclei of infected cells (M. C. Frame, D. J. McGeoch, F. J. Rixon, A. C. Orr, and H. S. Marsden, Virology 150:321-332, 1986), PRV Us9 localizes to the secretory pathway (predominately to the Golgi apparatus) and not to the nucleus. By fusing the enhanced green fluorescent protein (EGFP) reporter molecule to the carboxy terminus of Us9, we demonstrated that Us9 not only is capable of targeting a Us9-EGFP fusion protein to the Golgi compartment but also is able to direct efficient incorporation of such chimeric molecules into infectious viral particles. Moreover, through protease digestion experiments with Us9-EGFP-containing viral particles, we demonstrated that the Us9 protein is inserted into the viral envelope as a type II, tail-anchored membrane protein.

Glycoprotein B of Aujeszky's disease virus: topographical epitope mapping and epitope-specific antibody response

A panel of 26 monoclonal antibodies (mAbs) against glycoprotein B (gB) of Aujeszky's disease (pseudorabies) virus (ADV), a glycoprotein complex consisting of three glycoproteins, gBa, gBb, and gBc, was produced by two research groups and was used for the topographical epitope mapping of gB. An epitope map was constructed in which the identified epitopes of gB were situated in 14 topologically distinct antigenic domains; ten antigenic domains represented by 22 mAbs were localized on gBc, while four antigenic domains represented by four mAbs resided on gBb of the gB complex. All the epitopes located on gBc appeared to be conformation-dependent, whereas all the epitopes on gBb were conformation-independent. The identified epitopes of gB were conserved among laboratory, vaccine and field ADV strains. Conformation-dependent epitopes were shown to contribute largely to the overall antibody response to gB in naturally infected swine and immunized mice. Moreover, it was found that most of the infected animals responded relatively weakly to the identified conformation-independent epitopes of gB, while a group of immunodominant epitopes that induced a strong antibody response was represented exclusively by conformation-dependent epitopes from different antigenic domains. The results clearly demonstrated that conformation-dependent epitopes of gBc play a crucial role in inducing the humoral immune response to gB of ADV during the natural infection of swine and immunization of mice. The application of mAbs of our panel as research and diagnostic tools is discussed.

Characterization of pseudorabies virus mutants expressing carboxy-terminal truncations of gE: evidence for envelope incorporation, virulence, and neurotropism domains

Glycoprotein E (gE) gene of pseudorabies virus (PRV) is conserved among diverse alphaherpesviruses and therefore is predicted to be important for virus survival. gE contributes to viral spread from cell to cell in a variety of hosts and is responsible, in part, for increased virulence or pathogenesis of the virus. Virulence and spread mediated by gE are thought to be highly correlated. We initiated this study to explore the hypothesis that these two phenotypes might reflect separate functions of the gE protein. We did so by focusing on the role of the gE carboxy terminus in neuronal spread. Viruses harboring nonsense mutations affecting the expression of the gE cytoplasmic domain had several notable phenotypes. First, the truncated gE proteins expressed from these mutants are not found in virion envelopes. Second, the mutants retain the ability to spread to all retinorecipient regions of the rodent brain after retinal infection of rats. Third, the mutants have the reduced virulence phenotype of a gE deletion mutant in rats. Finally, the mutants have distinct plaque-size phenotypes on MDBK cells but not PK15 cells. Based on these observations, we suggest that gE-mediated virulence and spread may reflect separate functions that are not mediated by gE on virus particles.

Protective effect of glycoprotein gC-rich antigen against pseudorabies virus

A trial vaccine containing pseudorabies virus (PRV) glycoprotein gC as the main component showed excellent protection against virulent virus infection in pigs. Glycoprotein gC-rich antigen was prepared by heparin affinity chromatography from PRV-infected cell lysates. The preparations were mixed with mineral oil adjuvant as a water-in-oil emulsion. Six-week-old pigs were immunized twice at two-week intervals with trial vaccines containing 128,000, 12,800 and 1,280 HA units per dose of gC antigen. They were then challenged with a virulent PRV at day 7 after the final immunization. Neutralizing (NT) antibodies were produced with increase of antibody titers after challenge. Pigs immunized with 128,000 HA units per dose of gC survived and showed no virus shedding during the 2-week experimental period after the challenge. The role of cell-mediated immunity was examined using BALB/c mice, and induction of gC-specific cytotoxic T lymphocytes (CTLs) was detected by 51Cr release assay. From these results with mice, it is inferred that cell-mediated immunity, especially CTL, may play an important role in the effectiveness of our trial vaccine in addition to humoral immunity.

Genital infection and transmission of pseudorabies virus in feral swine in Florida, USA

Seventeen feral swine (FS) naturally infected with pseudorabies virus (PRV) and treated with dexamethasone (4 mg/kg body wt) on five consecutive days shed virus primarily from the genital tract and less frequently from the upper respiratory tract. The FS isolates were identified as PRV by virus neutralization with specific polyclonal antiserum and by direct immunofluorescence. Restriction endonuclease analysis with BamHI showed that representative samples from a total of 62 isolates were identical to each other, but differed in at least 5 DNA bands from the PRV Shope reference strain profile. DNA purified from FS isolates propagated in Vero cells or DNA extracted directly from genital swabs were amplified in the polymerase chain reaction using primers specific for the gpII (gB) gene of PRV. This amplification yielded a product of the expected size (200 bp), which specifically hybridized to a digoxigenin-labelled 30-mer probe complementary to an area within the region defined by the primers. In a transmission experiment, PRV was recovered from the vagina at 1 and 6 weeks after uninfected feral gilts were mixed with infected feral boars. PRV was not isolated from the upper respiratory tract of either gilts or boars. At eight weeks, 4 of the 5 gilts had developed low titer neutralizing antibodies to PRV. Our results indicate that PRV in FS is transmitted through sexual contact.

Pseudorabies virus recombinants expressing functional virulence determinants gE and gI from bovine herpesvirus 1.1

In the Alphaherpesvirinae subfamily, the gE and gI genes are conserved and encode membrane glycoproteins required for efficient pathogenesis (virulence). The molecular mechanism(s) responsible is not well understood, but the existence of similar phenotypes of gE and gI mutations in diverse Alphaherpesvirinae implies conservation of function(s). In this report, we describe construction of pseudorabies virus (PRV) recombinants that efficiently express the bovine herpesvirus 1 (BHV-1) membrane proteins gI and gE at the PRV gG locus. Each BHV-1 gene was cloned in a PRV mutant lacking both the PRV gI and gE coding sequences. All recombinant viruses expressed the BHV-1 proteins at levels similar to or greater than that observed after infection with parental BHV-1, and there were no observable differences in processing or ability to form gE-gI oligomers. The important observation resulting from this report is that the BHV-1 gE and gI proteins functioned together to complement the virulence defect of PRV lacking its own gE and gI genes in a rodent model, despite being derived from a highly restricted host range virus with a different pathogenic profile.

Detection of pseudorabies virus genomic sequences in apparently uninfected 'single reactor' pigs

With a pseudorabies virus (PrV) gB ELISA, performed on 480,000 pigs on 8,900 Swedish farms, approximately 1,300 cases were observed with only one single animal reacting positively. These animals were termed 'single reactors' (SR). In order to find explanations for this peculiar phenomenon, the presence of PrV was investigated in organs of immunosuppressed and non-immunosuppressed SR animals. The virus was not detected by immunohistochemistry, virus isolation or co-cultivation. An in situ DNA hybridization test detected PrV gC gene sequences in the olfactory bulb of one sow. A nested polymerase chain reaction (PCR) assay revealed gB, gE and gD gene sequences of PrV in the tissues of trigeminal ganglia, olfactory bulb, tonsils and brain. The nucleotide sequences of the amplicons revealed 98 to 100% homology with the corresponding sequences of PrV. The large latency transcript (LLT) was not detected in the organs of the SR pigs. Transmission of the SR phenomenon to animals in contact or to the next generation was not observed. Considering the present observations and the facts that (i) PrV vaccination is not applied in Sweden; (ii) the SR animals occur not only in the South, but also in Northern Scandinavia, which has no history of PrV infection and (iii) viral reactivation was not observed under natural conditions or after experimental immunosuppression, it is concluded that the SR phenomenon should hardly be considered as a typical PrV latency. The present findings show that certain herpesviral genomic sequences exist in apparently uninfected individuals.

Disulfide bonds of herpes simplex virus type 2 glycoprotein gB

Glycoprotein B (gB) is the most highly conserved envelope glycoprotein of herpesviruses. The gB protein is required for virus infectivity and cell penetration. Recombinant forms of gB being used for the development of subunit vaccines are able to induce virus-neutralizing antibodies and protective efficacy in animal models. To gain structural information about the protein, we have determined the location of the disulfide bonds of a 696-amino-acid residue truncated, recombinant form of herpes simplex virus type 2 glycoprotein gB (HSV gB2t) produced by expression in Chinese hamster ovary cells. The purified protein, which contains virtually the entire extracellular domain of herpes simplex virus type 2 gB, was digested with trypsin under nonreducing conditions, and peptides were isolated by reversed-phase high-performance liquid chromatography (HPLC). The peptides were characterized by using mass spectrometry and amino acid sequence analysis. The conditions of cleavage (4 M urea, pH 7) induced partial carbamylation of the N termini of the peptides, and each disulfide peptide was found with two or three different HPLC retention times (peptides with and without carbamylation of either one or both N termini). The 10 cysteines of the molecule were found to be involved in disulfide bridges. These bonds were located between Cys-89 (C1) and Cys-548 (C8), Cys-106 (C2) and Cys-504 (C7), Cys-180 (C3) and Cys-244 (C4), Cys-337 (C5) and Cys-385 (C6), and Cys-571 (C9) and Cys-608 (C10). These disulfide bonds are anticipated to be similar in the corresponding gBs from other herpesviruses because the 10 cysteines listed above are always conserved in the corresponding protein sequences.

The BamHI fragment 9 of pseudorabies virus contains genes homologous to the UL24, UL25, UL26, and UL 26.5 genes of herpes simplex virus type 1

The genomes of pseudorabies virus (PrV) and of herpes simplex virus type 1 (HSV1) are colinear, excepting an inversion in the unique long region, of which one extremity resides within the BamHI fragment 9. This fragment (4088 bp) encodes the counterparts of HSV1 UL24, UL25, UL26 and UL26.5 that are transcribed into four 3'-coterminal mRNAs. Multiple alignments of UL24, UL25 and UL26 protein homologs from alpha-, beta- and gamma-herpesviruses were performed. The PrV UL24 protein is shorter than its counterparts, missing the non-conserved COOH-terminal region. The region which is common to all viruses contains a basic NH2-terminus and a hydrophobic COOH-end, suggesting that UL24 may function as a matrix protein. The UL25 proteins are well conserved, particularly among the alpha-herpesviruses. All the domains involved in the proteolytic activity of theUL26 protein are highly conserved, as well as the two cleavage sites. Thus, its function and processing may be similar in PrV as in other herpesviruses. Due to the fact that in PrV the UL26 and UL44 genes are adjacent and their ends are conserved, the right border of the inversion must lie within their intergenic region.

Glycoprotein D-negative pseudorabies virus can spread transneuronally via direct neuron-to-neuron transmission in its natural host, the pig, but not after additional inactivation of gE or gI

Envelope glycoprotein D (gD) is essential for entry of pseudorabies virus (PRV) into cells but is not required for the subsequent steps in virus replication. Phenotypically complemented gD mutants can infect cells and can spread, both in vitro and in mice, by direct cell-to-cell transmission. Progeny virions released by infected cells are noninfectious because they lack gD. The aim of this study was to determine the role of gD in the neuropathogenicity of PRV in its natural host, the pig. We investigated whether gD-negative PRV can spread transneuronally via synaptically linked neurons of the olfactory and trigeminal routes. High doses of a phenotypically complemented gD mutant and gD mutants that are unable to express either gI or gI plus gE were inoculated intranasally in 3- to 5-week-old pigs. Compared with the wild-type virus, the virulence of the gD mutant was reduced. However, pigs inoculated with the gD mutant still developed fever and respiratory signs. Additional inactivation of either gI or gI plus gE further decreased virulence for pigs. Immunohistochemical examination of infected pigs showed that a PRV gD mutant could replicate and spread transneuronally into the central nervous system (CNS). Compared with the wild-type virus, the gD mutant had infected fewer neurons of the CNS on day 2. Nevertheless, on day 3, the gD-negative PRV had infected more neurons and viral antigens were present in second- and third-order neurons in the olfactory bulb, brain stem, and medulla oblongata. In contrast, gD mutants which are unable to express either gI or gI plus gE infected a limited number of first-order neurons in the olfactory epithelium and in the trigeminal ganglion and did not spread transneuronally or infect the CNS. Thus, transsynaptic spread of PRV in pigs can occur independently of gD. Possible mechanisms of transsynaptic transport of PRV are discussed.

Glycoprotein B (gB) of pseudorabies virus interacts specifically with the glycosaminoglycan heparin

We have previously shown that the pseudorabies virus (PrV) glycoproteins gB and gC (former PrV-gII and PrV-gIII) exhibit heparin-binding properties. While PrV-gC functions as the major adsorption protein, the biological role of the heparin-binding properties of PrV-gB are not understood. We used a gC-deleted PrV-mutant, PrV (dlg92/dltk), to analyse the heparin-binding properties of PrV-gB and the biological role of the PrV-gB-protein in adsorption. PrV-gB was the only glycoprotein of this vaccine strain binding to immobilised heparin in in vitro assays. Presence of the gC-protein was not necessary for the interaction of gB with heparin. Soluble heparin also interfered with adsorption of this mutant virus to a similar extent as it blocked adsorption of wild-type PrV (Ka), but it had only a minor inhibitory effect on infectivity of the mutant strain. These results show that PrV-gB interacts specifically with immobilized heparin and heparin-like structures on the cell surface, but this interaction is not required for a productive infection.

The receptor-binding domain of pseudorabies virus glycoprotein gC is composed of multiple discrete units that are functionally redundant

Many herpesviruses attach to cells in a two-step process, using the glycoprotein gC family of homologs to bind the primary receptor, heparan sulfate (HS) proteoglycan, and glycoprotein gD homologs to bind an unknown secondary receptor. We have previously shown by deletion analysis that the amino-terminal one-third of gC from pseudorabies virus (PRV), a swine herpesvirus, includes at least the principal HS receptor-binding domain. This portion of PRV gC contains three discrete clusters of basic residues that exactly or nearly match proposed consensus sequences for heparin-binding domains (HBDs); four additional potential HBDs lie in the distal two-thirds of the glycoprotein. We now specifically implicate each of the three amino-terminal HBDs in virus attachment. Mutational analysis demonstrated that any one of the three HBDs could mediate efficient virus infectivity; HS-dependent PRV attachment to cells was eliminated only after all three amino-terminal HBDs were altered. Furthermore, the binding dysfunction was due to a disruption of the specific HBDs and not to total charge loss. Thus, unlike previously described viral receptor-binding domains, the PRV gC receptor-binding domain is composed of multiple, discrete units that can function independently of one another. These units may function redundantly either to increase binding affinity or perhaps to effectively increase the virus's host range.

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.

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.