Role of glycoprotein gIII of pseudorabies virus in virulence

The mutations on the envelope glycoprotein D contribute to the enhanced neurotropism of the pseudorabies virus variant

The pseudorabies virus (PRV) TJ strain, a variant of PRV, induces more severe neurological symptoms and higher mortality in piglets and mice than the PRV SC strain isolated in 1980. However, the mechanism underlying responsible for the discrepancy in virulence between these strains remains unclear. Our study investigated the differences in neurotropism between PRV TJ and PRV SC using both in vitro and in vivo models. We discovered that PRV TJ enters neural cells more efficiently than PRV SC. Furthermore, we found that PRV TJ has indistinguishable genomic DNA replication capability and axonal retrograde transport dynamics compared to the PRV SC. To gain deeper insights into the mechanisms underlying these differences, we constructed gene-interchanged chimeric virus constructs and assessed the affinity between envelope glycoprotein B, C, and D (gD) and corresponding receptors. Our findings confirmed that mutations in these envelope proteins, particularly gD, significantly contributed to the heightened attachment and penetration capabilities of PRV TJ. Our study revealed the critical importance of the gDΔR278/P279 and gDV338A in facilitating viral invasion. Furthermore, our observations indicated that mutations in envelope proteins have a more significant impact on viral invasion than on virulence in the mouse model. Our findings provide valuable insights into the roles of natural mutations on the PRV envelope glycoproteins in cell tropism, which sheds light on the relationship between cell tropism and clinical symptoms and offers clues about viral evolution.

The Attenuated Pseudorabies Virus Vaccine Strain Bartha K61: A Brief Review on the Knowledge Gathered During 60 Years of Research

Pseudorabies virus (PRV) is a member of the alphaherpesvirus subfamily of the herpesviruses and is the causative agent of Aujeszky’s disease in pigs, causing respiratory, neurological, and reproductive symptoms. Given the heavy economic losses associated with Aujeszky’s disease epidemics, great efforts were made to develop efficacious vaccines. One of the best modified live vaccines to this day is the attenuated Bartha K61 strain. The use of this vaccine in extensive vaccination programs worldwide has assisted considerably in the eradication of PRV from the domesticated pig population in numerous countries. The Bartha K61 strain was described in 1961 by Adorján Bartha in Budapest and was obtained by serial passaging in different cell cultures. Ever since, it has been intensively studied by several research groups, for example, to explore its efficacy as a vaccine strain, to molecularly and mechanistically explain its attenuation, and to use it as a retrograde neuronal tracer and as a vector vaccine. Given that the Bartha K61 vaccine strain celebrates its 60th birthday in 2021 with no sign of retirement, this review provides a short summary of the knowledge on its origin, characteristics, and use as a molecular tool and as a vaccine.

A single dose glycoprotein D-based subunit vaccine against pseudorabies virus infection

Pseudorabies Virus (PRV) is the causative agent of Pseudorabies (PR), also known as Aujeszky's Disease, one of the most important infectious diseases in swine, resulting in huge economic losses to the swine industry globally. The emergence of mutant PRV strains after 2011 resulted in a sharp decrease in the efficacy of available commercial vaccines. To develop a more effective vaccine that can prevent the spread of PRV, glycoprotein B (gB), glycoprotein C (gC) and glycoprotein D (gD) from recent PRV isolates were expressed in a baculovirus system and their protective efficacy was tested in mice and piglets. Neutralizing antibody titers (NAs) in mice vaccinated with gB, gC and gD peaked at 28 days after immunization and then slowly declined. NAs in the mice immunized with gD were remarkably higher than other groups. After a lethal challenge of 5 LD₅₀ with mutant PRV-HNLH strain, the survival rates of gB and gD were 100% and 87.5% respectively, which was significantly higher than gC group (50%). Piglets vaccinated with the gD and gB + D vaccines developed the highest NAs 7 days post immunization. No piglets in these two groups exhibited clinical symptoms, high body temperature or virus shedding following challenge with 10^6.6 TCID₅₀ with the mutant PRV-HNLH strain. Histopathology and immunohistochemistry showed remarkably reduced pathological damage and viral loads in gD and gB + D groups. Furthermore, the duration of the NAs induced by gD vaccine could maintain as long as four months after a single dose. The current study indicates that a gD-based vaccine could be developed for the efficient control of PRV.

Distribution characteristics of DNA vaccine encoded with glycoprotein C from Anatid herpesvirus 1 with chitosan and liposome as deliver carrier in ducks

Background

A eukaryotic expression plasmid encoding glycoprotein C (gC) of Anatid herpesvirus 1 (AnHV-1) (pcDNA3.1-gC) was constructed and validated. The tissue distribution of chitosan/DNA complexes, liposome/DNA complexes and pcDNA3.1-gC alone were evaluated using a quantitative real-time PCR based TaqMan™ probe following intramuscular administration in ducklings.

Results

Compared with pcDNA3.1-gC alone, liposomes universally increased the plasmid DNA copy number at the injection sites, liver, spleen, heart, brain, bursa of Fabricius, and especially in the enteron (esophagus, duodenum, rectum, and cecum). Chitosan also universally increased the plasmid DNA copy number at the injection sites, liver, spleen, heart, brain and esophagus. Compared with lipoplex-gC, higher chitosan-gC plasmid DNA copy numbers were detected at the injection sites, liver, spleen, heart, brain and esophagus. In contrast, compared with lipoplex-gC, lower copy numbers of chitosan-gC plasmid DNA were detected in the duodenum, rectum and cecum.

Conclusions

The results of this study demonstrated that chitosan and liposomes mediated rapid and extensive plasmid distribution in duck tissues, with low levels maintained from 1 d after DNA vaccination.

Identification and characterization of duck plague virus glycoprotein C gene and gene product

BACKGROUND

Viral envelope proteins have been proposed to play significant roles in the process of viral infection.

RESULTS

In this study, an envelope protein gene, gC (NCBI GenBank accession no. EU076811), was expressed and characterized from duck plague virus (DPV), a member of the family herpesviridae. The gene encodes a protein of 432 amino acids with a predicted molecular mass of 45 kDa. Sequence comparisons, multiple alignments and phylogenetic analysis showed that DPV gC has several features common to other identified herpesvirus gC, and was genetically close to the gallid herpervirus.Antibodies raised in rabbits against the pET32a-gC recombinant protein expressed in Escherichia coli BL21 (DE3) recognized a 45-KDa DPV-specific protein from infected duck embryo fibroblast (DEF) cells. Transcriptional and expression analysis, using real-time fluorescent quantitative PCR (FQ-PCR) and Western blot detection, revealed that the transcripts encoding DPV gC and the protein itself appeared late during infection of DEF cells. Immunofluorescence localization further demonstrated that the gC protein exhibited substantial cytoplasm fluorescence in DPV-infected DEF cells.

CONCLUSIONS

In this work, the DPV gC protein was successfully expressed in a prokaryotic expression system, and we presented the basic properties of the DPV gC product for the first time. These properties of the gC protein provided a prerequisite for further functional analysis of this gene.

Isolation of equine herpesvirus-1 lacking glycoprotein C from a dead neonatal foal in Japan

We isolated a variant equine herpesvirus-1 (EHV-1), strain 5089, from the lung of a dead neonatal foal in Japan and characterized the biological nature of the virus. The virus spread in cultured cells mainly by cell-to-cell infection, unlike wild-type EHV-1, which spreads efficiently as a cell-free virus. The virus titer in cultured supernatant and the intracellular virus titer were low compared to those of wild-type EHV-1. Heparin treatment of the virus had no effect on viral infectivity in cell culture. Glycoprotein C (gC) was not detected by Western blotting and fluorescent antibody tests in 5089 virions and 5089-infected cells, respectively. RT-PCR analysis revealed that the expression level of 5089 gC mRNA was reduced considerably compared to that of wild-type EHV-1. Sequencing analysis of the 5089 gC coding region showed a point mutation in the promoter region of the gC open reading frame. However, the mutation did not affect the promoter activity. These results suggested that the lack of gC in 5089 virions might be one of the reasons for spread of the virus by cell-to-cell infection and that gC mRNA expression might not be activated efficiently due to factors other than the mutation in the gC promoter region.

High-level expression of Marek's disease virus glycoprotein C is detrimental to virus growth in vitro

Expression levels of Marek's disease virus (MDV) glycoprotein C (gC) are significantly reduced after serial virus passage in cell culture. Reduced gC expression coincides with enhanced MDV growth in vitro and attenuation. To analyze this phenomenon in detail, a full-length infectious MDV clone was modified by Red-based and shuttle mutagenesis in Escherichia coli. Besides a gC-negative deletion mutant harboring a kanamycin resistance gene, a markerless mutant with the U(L)44 gene deleted was constructed. On the basis of this deletion mutant, the original or a modified U(L)44 gene with a mutated start codon (AUG-->ACG) was reinserted into the authentic locus. Similarly, mutants expressing authentic gC or the start codon mutation under the control of a strong constitutive promoter were generated. In vitro studies demonstrated that gC deletion mutants induced twofold-larger plaques than the parental virus did, whereas constitutive overexpression of the glycoprotein resulted in a more than twofold reduction in plaque size. In addition, plaque sizes of the gC deletion mutant were reduced when virus was grown using supernatants from cells infected with parental virus, but supernatants obtained from cells infected with the gC deletion mutant had no measurable effect on plaque size. The results indicated that (i) expression of MDV gC, albeit at low levels in a highly passaged virus, had a significant negative impact on the cell-to-cell spread capabilities of the virus, which was alleviated in its absence and exacerbated by its overexpression, and that (ii) this activity was mediated by the secreted form of MDV gC.

Novel tracing paradigms--genetically engineered herpesviruses as tools for mapping functional circuits within the CNS: present status and future prospects

The mammalian CNS is composed of an extremely complex meshwork of highly ordered interconnections among billions of neurons. To understand the diverse functions of this neuronal network we need to differentiate between functionally related and nonrelated elements. A powerful labeling method for defining intricate neural circuits is based on the utilization of neurotropic herpesviruses, including pseudorabies virus and herpes simplex virus type 1. The recent development of genetically engineered tracing viruses can open the way toward the conception of novel tract-tracing paradigms. These new-generation tracing viruses may facilitate the clarification of problems, which were inaccessible to earlier approaches. This article first presents a concise review of the general aspects of neuroanatomical tracing protocols. Subsequently, it discusses the molecular biology of alpha-herpesviruses, and the genetic manipulation and gene expression techniques that are utilized for the construction of virus-based tracers. Finally, it describes the current utilization of genetically modified herpesviruses for circuit analysis, and the future directions in their potential applications.

Cloning and expression of the complement receptor glycoprotein C from Herpesvirus simiae (herpes B virus): protection from complement-mediated cell lysis

Simian herpes B virus (SHBV) is the herpes simplex virus (HSV) homologue for the species MACACA: Unlike in its natural host, and unlike other animal herpesviruses, SHBV causes high mortality in accidentally infected humans. SHBV-infected cells, like those infected with HSV-1 and equine herpesvirus types 1 and 4, express complement C3 receptor activity. To study immunoregulatory functions involved in susceptibility/resistance against interspecies transmission, the SHBV glycoprotein C (gC(SHBV)) gene (encoding 467 aa) was isolated. Sequence analysis revealed amino acid identity with gC proteins from HSV-2 (46.9 %), HSV-1 (44.5 %) and pseudorabies virus (21.2 %). Highly conserved cysteine residues were also noted. Similar to gC(HSV-2), gC(SHBV) is less glycosylated than gC(HSV-1), resulting in a molecular mass of 65 kDa if expressed in replication-deficient vaccinia virus Ankara. Stable transfectants expressing full-length gC(SHBV) on the cell surface induced C3 receptor activity and were substantially protected from complement-mediated lysis; no protection was observed with control constructs. This suggests that expression of the gC homologues on infected cell surfaces might also contribute to the survival of infected cells in addition to decreased virion inactivation. Interestingly, soluble gC(SHBV) isolated from protein-free culture supernatants did not interfere with the binding of the alternative complement pathway activator properdin to C3b, which is similar to our findings with gC(HSV-2) and could be attributed to major differences in the amino-terminal portion of the protein with extended deletions in both gC(SHBV) and gC(HSV-2). Binding of recombinant gC(SHBV) to polysulphates was observed. This, together with the heparin-sensitivity of the gC(SHBV)-C3 interaction on the infected cell surface, suggests a role in adherence to heparan sulphate, similar to the gC proteins of other herpesviruses.

Involvement of cellular cytoskeleton components in antibody-induced internalization of viral glycoproteins in pseudorabies virus-infected monocytes

Addition of pseudorabies virus (PrV)-specific polyclonal immunoglobulins to PrV-infected monocytes induces internalization of plasma membrane-anchored viral glycoproteins and this may interfere with antibody-dependent cell lysis. We investigated the role of actin, microtubules, clathrin, and dynein, the major cellular components involved in physiological endocytosis during this virological internalization. Porcine monocytes were infected in vitro for 13 h and afterward treated with different concentrations of colchicine, cytochalasin D, latrunculin B, and amantadine-HCl, which inhibit polymerization of microtubules, actin/clathrin, actin, and clathrin, respectively. This resulted in a significant reduction of internalization compared to the nontreated control, indicating that these components are involved in the process. A double labeling was performed during the internalization process and a clear colocalization of actin, microtubules, clathrin, and dynein with the viral glycoproteins was observed at different stages during the internalization process. We conclude that these cellular components are used by PrV to generate the antibody-induced internalization of viral glycoproteins.

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.

Construction and characterization of an equine herpesvirus 1 glycoprotein C negative mutant

An equine herpesvirus 1 (EHV-1) strain RacL 11 mutant was constructed that carries the Escherichia coli LacZ gene instead of the open reading frame encoding glycoprotein C (gC). The engineered virus mutant (L11(delta)gC) lacked codons 46-440 of the 1404 bp gene. On rabbit kidney cell line Rk13 and equine dermal cell line Edmin337, the L11(delta)gC virus grew to titers which were reduced by approximately 5- to 10-fold compared with wild-type RacL11 virus or a repaired virus (R-L11(delta)gC). However, when L11(delta)gC growth properties were analyzed on primary equine cells a decrease of viral titers was observed such that extracellular L11(delta)gC titers were reduced by 48- to 210-fold compared with those of wild-type or repaired virus. Heparin sensitive and heparin resistant attachment was assessed by binding studies using radiolabeled virion preparations. These studies revealed that EHV-1 gC is important for heparin sensitive attachment to the target cell. Similar results were obtained when cellular glycosaminoglycan (GAG) synthesis was inhibited by chlorate treatment or when cells defective in GAG synthesis were used. L11(delta)gC also exhibited significantly delayed penetration kinetics on Rk13 and primary equine cells. Infection of mice with L11(delta)gC did not cause EHV-1-related disease, whereas mice infected with either RacL11 or R-L11(delta)gC exhibited massive bodyweight losses, high virus titers in the lungs, and viremia. Taken together, EHV-1 gC was shown to play important roles in the early steps of infection and in release of virions, especially in primary equine cells, and contributes to EHV-1 virulence.

The renal afferent pathways in the rat: a pseudorabies virus study

Retrograde tract tracing studies have indicated that dorsal root ganglion cells from T8 to L2 innervate the rat's left kidney. Electrophysiology studies have indicated that putative second-order sympathetic afferents are found in the dorsal horn at spinal segments T10 to L1 in laminae V-VII. Here, the spread of pseudorabies virus through renal sensory pathways was examined following 2-5 days post-infection (PI) and the virus was located immunocytochemically using a rabbit polyclonal antibody. Two days PI, dorsal root ganglion neurons (first-order sympathetic afferents) were infected with PRV. An average of 1.2, 0.8, 2.1 and 4.4% of the infected dorsal root ganglion neurons were contralateral to the injected kidney at spinal segments T10, T11, T12 and T13, respectively. Four days PI, infected neurons were detected within laminae I and II of the dorsal horn of the caudal thoracic and upper lumbar spinal cord segments. The labeling patterns in the spinal cord are consistent with previous work indicating the location of renal sympathetic sensory pathways. The nodose ganglia were labeled starting 4 days PI, suggesting the involvement of parasympathetic sensory pathways. Five days PI, infected neurons were found in the nucleus tractus solitarius. In the present study, it was unclear whether the infected neurons in the nucleus tractus solitarius are part of sympathetic or parasympathetic afferent pathways or represent a convergence of sensory information. Renal denervation prevented the spread of the virus into the dorsal root ganglia and spinal cord. Sectioning the dorsal roots from T10-L3 blocked viral spread into the spinal cord dorsal horn, but did not prevent infection of neurons in dorsal root ganglion nor did it prevent infection of putative preganglionic neurons in the intermediolateral cell column. The present results indicated that renal afferent pathways can be identified after pseudorabies virus infection of the kidney. Our results suggest that renal afferents travel in sympathetic and parasympathetic nerves and that this information may converge at the NTS.

A chicken embryo eye model for the analysis of alphaherpesvirus neuronal spread and virulence

We describe use of developing chicken embryos as a model to study neuronal spread and virulence of pseudorabies virus (PRV). At embryonic day 12, beta-galactosidase-expressing PRV strains were injected into the vitreous humor of one eye, and virus replication and spread from the eye to the brain were measured by beta-galactosidase activity and the recovery of infectious virus from tissues. The wild-type PRV strain, Becker, replicated in the eye and then spread to the brain, causing extensive pathology characterized by edema, hemorrhage, and necrosis that localized to virally infected tissue. The attenuated vaccine strain, Bartha, replicated in the eye and spread throughout specific regions of the brain, producing little to no overt pathology. Becker mutants lacking membrane proteins gE or gI replicated in the eye and were able to spread to the brain efficiently. The pathology associated with replication of these mutants in the brain was intermediate to that induced by Becker or Bartha. Mixed infection of a gE deletion mutant and a gI deletion mutant restored the pathogenic phenotype to wild-type levels. These data indicate that the replication of virus in embryonic brain tissue is not sufficient to induce the characteristic pathological response and that the gE and gI gene products actively affect pathological responses in the developing chicken brain.

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

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

Antiviral activity of an extract from leaves of the tropical plant Acanthospermum hispidum

Incubation of the alphaherpesviruses pseudorabiesvirus (PRV) and bovine herpesvirus 1 during infection of cell cultures with an extract prepared from the leaves of Acanthospermum hispidum impaired productive replication of these viruses in a concentration-dependent manner whereas propagation of classical swine fever virus, foot-and-mouth disease virus and vaccinia virus was not affected. The 50% inhibitory concentration for cell growth (IC50) was 107 +/- 5 microliters/ml, and the concentration reducing PRV yield by 1 log10 (90% effective concentration, EC90) was 8 +/- 3 microliters/ml. The selectivity index calculated as the IC50/EC90 ration was 13 +/- 4. Delineation of the mechanism of the antiviral activity demonstrated inhibition of alphaherpesvirus attachment to and, to a lesser extent, penetration into the cells. In contrast, viral gene expression was not inhibited by the extract when added after entry of virions into the target cells. Reduced antiviral activity of A.h. against PRV deletion mutants lacking glycoprotein C (gC) or glycoproteins gC, gE, gG and gI altogether indicated that gC alone and/or viral attachment complexes of which gC is a component constitute the target structures for A. hispidum.

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

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

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.

Central command neurons of the sympathetic nervous system: basis of the fight-or-flight response

During stress, the activity of the sympathetic nervous system is changed in a global fashion, leading to an increase in cardiovascular function and a release of adrenal catecholamines. This response is thought to be regulated by a common set of brain neurons that provide a dual input to the sympathetic preganglionic neurons regulating cardiac and adrenal medullary functions. By using a double-virus transneuronal labeling technique, the existence of such a set of central autonomic neurons in the hypothalamus and brainstem was demonstrated. These neurons innervate both of the sympathetic outflow systems and likely function in circumstances where parallel sympathetic processing occurs, such as in the fight-or-flight response.

Pseudorabies virus mutants as transneuronal markers

The transneuronal labeling properties of three genetically engineered forms of the Bartha strain of pseudorabies virus (PRV) were studied in the ocular sympathetic pathway of rats. Bartha PRV mutants in which expression of the viral glycoprotein gI (homologous to gE of herpes simplex virus type 1, HSV-1) was restored (Bartha gI+) or which express a wildtype form of glycoprotein gIII (homologous to gC of HSV-1 and referred here as Bartha gIIIKa) were analyzed. In addition, a Bartha PRV mutant (Bartha beta-gal) containing the lacZ gene encoding E. coli beta-galactosidase inserted into the gX gene (homologous to gG of HSV-1) was also studied. These were compared to the parental strain--Bartha PRV. The pattern of transneuronal labeling in the intermediolateral cell column was studied 4 days after 5 microliters of different concentrations of viral stocks were injected into the anterior chamber of the eye. The optimal infectious dose required to produce the maximal number of cases with specific transneuronal labeling of sympathetic preganglionic neurons was determined and these were as follows: Bartha PRV = 10(7.5) pfu/ml, Bartha beta-galactosidase = 10(6.5) pfu/ml, Bartha gIIIKa = 10(5) pfu/ml, Bartha gI+ = 10(4) pfu/ml. An inverse relationship between specificity and infectivity rate was observed. Bartha beta-gal produced the greatest number of cases with specific labeling (76%); Bartha gI+ produced the lowest level (10%) and thus, this virus is not useful for transneuronal labeling studies. Bartha gIIIKa labeled more sympathetic preganglionic neurons (second-order neurons) than Bartha beta-gal or Bartha PRV. Bartha gIIIKa and Bartha beta-gal viruses labeled more interneurons (third-order) than the standard Bartha PRV.(ABSTRACT TRUNCATED AT 250 WORDS)

A mutant of pseudorabies virus with deletion of glycoprotein gIII gene prepared from a Japanese isolate: it fails to agglutinate mouse erythrocytes

A mutant with deletion of the glycoprotein gIII gene was produced from a Japanese isolate of pseudorabies virus (PrV) and characterized. Viral titers of the mutant propagated in PK-15 cells were always lower than those of the parental virus. The parental virus agglutinated BALB/c mouse erythrocytes, whereas the deletion mutant showed no hemagglutinating activity. Pigs inoculated with the parental virus produced not only neutralizing but also hemagglutination-inhibiting antibodies. On the other hand, the mutant induced high titers of neutralizing antibody comparable to the parental virus but no hemagglutination-inhibiting antibody in inoculated pigs, suggesting that glycoprotein gIII is an essential component for hemagglutination of PrV. Finally, no evidence that the deletion mutant lost virulence for mice was obtained.

gp13 (EHV-gC): a complement receptor induced by equine herpesviruses

Equine herpesviruses type 1 (EHV-1) and type 4 (EHV-4) induce a complement receptor protein on the surface of infected cells capable of binding to the third component of complement (C3). The protein mediating the binding to the C3 component of complement was identified as glycoprotein 13 (gp13, EHV-gC), as expression of the cloned viral gene under the control of a CMV promoter induced C3 binding activity at the transfected cell surface. Comparable to glycoprotein C (gC) from herpes simplex virus type 1 (HSV-1-gC), glycoprotein III from pseudorabiesvirus (gIII, PRV-gC) and bovine herpesvirus-1 (gIII, BHV-1-gC), gp13 derived from EHV-infected cell lysates bound to C3 fixed to solid phase, showing preferential binding to the appropriate host complement component. Similar to wild-type isolates, a highly attenuated vaccine EHV-1 strain also displayed complement receptor activity despite apparent differences of the gp13 gene in restriction enzyme digest pattern and reactivity with monoclonal antibodies. In addition, other structural proteins were altered in the vaccine strain as compared to wild-type strains, which might contribute to its attenuated phenotype. In contrast to the situation observed with HSV-1-gC, the interaction of gp13 (EHV-gC) with horse complement was not inhibited by polyanionic substances like heparin or dextran sulfate. These results suggest structural differences in the particular binding mechanism of the respective viral envelope proteins.

Cell surface proteoglycans are not essential for infection by pseudorabies virus

Cell surface proteoglycans, in particular those carrying heparan sulfate glycosaminoglycans, play a major role in primary attachment of herpesviruses to target cells. In pseudorabies virus (PrV), glycoprotein gC has been shown to represent the major heparan sulfate-binding virion envelope protein (T. C. Mettenleiter, L. Zsak, F. Zuckermann, N. Sugg, H. Kern, and T. Ben-Porat, J. Virol. 64:278-286, 1990). Since PrV gC is nonessential for viral infectivity in vitro and in vivo, either the interaction between virion envelope and cellular heparan sulfate is not necessary to mediate infection or other virion envelope proteins can substitute as heparan sulfate-binding components in the absence of gC. To answer these questions, we analyzed the infectivity of isogenic gC+ and gC- PrV on mouse L-cell derivatives with defects in glycosaminoglycan biosynthesis, using a rapid and sensitive fluorescence-based beta-galactosidase assay and single-cell counting in a fluorescence-activated cell sorter. Our data show that (i) in the virion, glycoprotein gC represents the only proteoglycan-binding envelope protein, and (ii) cellular proteoglycans are not essential for infectivity of PrV. Attachment studies using radiolabeled virions lacking either gC or the essential gD confirmed these results and demonstrated that PrV gD mainly contributes to binding of Pr virions to cell surface components other than proteoglycans. These data demonstrate the presence of a proteoglycan-independent mode of attachment for Pr virions leading to infectious entry into target cells.

The Marek's disease virus (MDV) unique short region: alphaherpesvirus-homologous, fowlpox virus-homologous, and MDV-specific genes

Despite its previous classification as a gammaherpesvirus, primarily due to its lymphotropism, Marek's disease virus (MDV), an oncogenic avian herpesvirus, is phylogenetically more related to the "neurotropic" alphaherpesviruses, characterized by its prototype, herpes simplex virus (HSV) (Buckmaster et al., 1988, J. Gen. Virol. 69, 2033-2042). In this report we present the DNA sequence of an 11,286-bp DNA segment encompassing the entire 11,160-bp-long Us region of the oncogenic avian herpesvirus, Marek's disease virus. Eleven open reading frames (ORFs) likely to code for proteins were identified; of these, 7 represent homologs exclusive to alphaherpesvirus S component genes. These include MDV counterparts of HSV US1 (ICP22), US2, US3 (a serine-threonine protein kinase), US6, US7, and US8 (HSV glycoproteins gD, gI, and gE, respectively), and US10. Three additional ORFs were identified with no apparent relation to any sequences currently present in the SwissProt or GenBank/EMBL databases, while a fourth was found to exhibit significant homology to an uncharacterized fowlpox virus (FPV) ORF. Having precisely identified the IRs-U(s) and U(s)-TRs junctions, we have corrected and clarified their previously reported locations. By characterizing genes encoding three new alphaherpesvirus-related homologs (US1, US8, and US10), completing the sequence for a fourth (US7), and identifying 2 new MDV-specific ORFs (SORF1 and SORF3) and a fowlpox homolog (SORF2), our sequence analysis of the "virulent" GA strain of MDV (vMDV) extends upon that of a 5255-bp segment located in the U(s) region of the "very virulent" RB1B strain of MDV (vvMDV) (Ross et al., 1991, J. Gen. Virol. 72, 939-947; 949-954). These two sequences were found to exhibit 99% identity at both nucleotide and predicted amino acid levels. Combined with the fact that MDV U(s) sequences failed to show statistically significant CpG deficiencies, our analysis is consistent with MDV bearing a closer phylogenetic relation to alphaherpesviruses than to gammaherpesviruses. Because alphaherpesvirus-specific U(s) region genes are primarily nonessential for virus replication, they are thought to be important biological property determinants. Thus, our sequence provides a foundation for further MDV studies aimed at resolving the apparent discrepancy between MDV's genetic and biologic properties.

Characterization of Marek's disease virus insertion and deletion mutants that lack US1 (ICP22 homolog), US10, and/or US2 and neighboring short-component open reading frames

We report the characterization of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique short (US) region of the viral genome. A deletion mutant (GA delta 4.8lac) lacks 4.8 kbp of US region DNA, the deleted segment having been replaced by the lacZ gene of Escherichia coli. This deletion results in the loss of the MDV-encoded US1, US10, and US2 homologs of herpes simplex virus type 1, as well as three putative MDV-specific genes, Sorf1, Sorf2, and Sorf3. Two mutants containing lacZ insertions in the US1 and US10 genes have been constructed, and we have previously reported a US2lac insertion mutant (J. L. Cantello, A. S. Anderson, A. Francesconi, and R. W. Morgan, J. Virol. 65:1584-1588, 1991). The isolation of these mutants indicates that the relevant genes are not required for growth of MDV in chicken embryo fibroblasts. The mutants had early growth kinetics indistinguishable from those of their parent viruses; however, 5 to 7 days after being plated, the US1 insertion mutant (US1lac) and the GA delta 4.8lac deletion mutant showed a 5- to 10-fold decrease in virus growth. This decrease in virus accumulation correlated with a 30 to 50% decrease in plaquing efficiency when these viruses were plated onto established versus fresh chicken embryo fibroblast monolayers compared with a 10 to 15% decrease seen for the parent viruses and for the US10lac or US2lac insertion mutants. Finally, GA delta 4.8lac could be reisolated from chickens, indicating that the deleted genes are not required for the infection of chickens following intra-abdominal inoculation of an attenuated serotype 1 MDV.

Glycoprotein E of pseudorabies virus and homologous proteins in other alphaherpesvirinae

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

Pseudorabies virus infectivity for swine skin characterized in vitro

The infectivity of pseudorabies virus (PrV) was demonstrated in a cell substrate derived from swine skin explant cultures designated primary porcine skin cells (c/cSLA PPSC). c/cSLA PPSC infected with either wild type or TK- PrV strain Kaplan (Ka) developed typical cytopathologic changes (CPE) as early as 4 h post inoculation (p.i.). The CPE caused by PrV on c/cSLA PPSC was specifically neutralized by covalescent swine sera. Synthesis of late viral proteins was demonstrated in PrV-infected c/cSLA PPSC by indirect fluorescent antibody staining using monoclonal antibodies (mAbs) specific for PrV gIII. PrV induced protein synthesis was further confirmed by specific immunoprecipitation of 35S-methionine labeled viral polypeptides from PrV-infected c/cSLA PPSC with PrV convalescent swine serum, PrV immune mouse serum or mAb to PrV gIII. Moreover, the virus progeny derived from c/cSLA PPSC was shown to be infectious for MDBK cells and this infection was specifically neutralized by PrV convalescent swine serum. The capacity c/cSLA PPSC to support a complete growth cycle of PrV and the relative ease of deriving these cells from pigs can be applied in an autologous fashion in studies of cellular immunity where the MHC needs to be matched.

Glycoprotein B is a specific determinant of herpes simplex virus type 1 neuroinvasiveness

Herpes simplex virus type 1 strains ANG and KOS lack neuroinvasiveness when inoculated on the footpads of mice, and because the strains are able to complement each other, the genes associated with this phenotype differ. In this study, we used marker rescue techniques to show that at least two genes cloned from ANG are required to restore neuroinvasiveness to KOS. One of the two fragments required is the 6.3-kb BamHI-A/EcoRI-D fragment (0.15 to 0.19 map units). The second has been identified as the sequence encoding glycoprotein B (gB) (UL27). Analysis of ANG and KOS DNA sequences in the relevant region of the gB gene revealed two nucleotide differences which result in amino acid differences in the gB protein. One appears to be unique to the strain of KOS used in our laboratory. The second, at codon 523 of the mature gB protein, encodes a valine in KOS and an alanine in ANG. Recombinant KOS viruses which contained ANG sequences in this region were constructed, and two independently selected recombinants demonstrated increased neuroinvasiveness in mice. From these results, we conclude that gB significantly influences neuroinvasiveness. Mechanisms by which this might occur are discussed.

The amino-terminal one-third of pseudorabies virus glycoprotein gIII contains a functional attachment domain, but this domain is not required for the efficient penetration of Vero cells

We have examined the attachment and penetration phenotypes of several glycoprotein gIII mutants of pseudorabies virus (PRV) and have identified the first one-third of gIII as a region that mediates efficient virus attachment to PK15 and Vero cells. This portion of gIII, amino acids 25 through 157 of the wild-type sequence, appeared to support attachment by binding to heparinlike molecules on cell surfaces. Virions containing the first one-third of gIII were sensitive to heparin competition and showed greatly reduced infectivity on cells treated with heparinase. PRV virions lacking the first one-third of the mature glycoprotein exhibited only residual binding to cells if challenged by vigorous washing with phosphate-buffered saline at 2 h postinfection at 4 degrees C. This residual binding was resistant to heparin competition, and strains lacking the first one-third of gIII were able to infect cells treated with heparinase as effectively as untreated cells. When we determined the penetration phenotypes for each strain, we found that gIII-mediated virus attachment was necessary for timely penetration of PK15 cells but remarkably was not required for efficient virus penetration of Vero cells. Moreover, wild-type PRV was actually prohibited from rapid penetration of Vero cells by a gIII-heparan sulfate interaction. Our results indicate that initial virus binding to heparan sulfate via glycoprotein gIII is not required for efficient PRV infection of all cell types and may in fact be detrimental in some instances.

The virulence-determining genomic BamHI fragment 4 of pseudorabies virus contains genes corresponding to the UL15 (partial), UL18, UL19, UL20, and UL21 genes of herpes simplex virus and a putative origin of replication

The genomic BamHI-fragment 4 of pseudorabies virus (PrV) has previously been shown to encode functions necessary for expression of PrV neurovirulence (B. Lomniczi, S. Watanabe, T. Ben-Porat, and A. S. Kaplan, 1984, J. Virol. 52, 198-205). To identify proteins that might be involved in the neurotropism of PrV we sequenced the complete 9382-bp fragment BamHI-4, the longest contiguous sequence determined in the UL region of PrV so far, and analyzed its coding capacity. In an arrangement similar to that found in herpes simplex virus type 1 we identified complete open reading frames encoding proteins with strong homology to the UL18 (50% homology), UL19 (60% homology), UL20 (33% homology), and UL21 (36% homology) polypeptides and the 3'-part of a gene homologous to UL15 (67% homology) of HSV-1. In addition, a consensus sequence for an alphaherpesviral origin of replication was found at the left terminus of the fragment.

Role of different genes in the virulence and pathogenesis of Aujeszky's disease virus

In this study the role of different genes located in the unique short region of the genome of Aujeszky's disease virus was examined. Inactivation of the genes encoding the protein kinase (PK), gp63, and gI reduced virulence of the virus for pigs, in contrast to inactivation of the genes encoding the 28 kDa protein, and gX. There was no correlation between virulence and virus multiplication in vitro or in the oropharynx in vivo. The morphogenesis of the PK mutant was altered. The gI mutant replicated to normal titres in the oropharynx and could be recovered from the trigeminal ganglia but not from other parts of the central nervous system, suggesting that gI facilitates the spread of the virus from neuron to neuron. All mutants induced neutralizing antibody and complete or partial protection against a challenge infection. PK and gp63 were required for the induction of complete protection, although these proteins are reportedly not targets for neutralizing antibody or cytotoxic T cells.

Evaluation of the sensitivity and specificity of two diagnostic tests for antibodies to pseudorabies virus glycoprotein X

The diagnostic performance of 2 enzyme-linked immunosorbent assays (gX-T, gX-H) for antibodies to pseudorabies virus (PRV) glycoprotein X (gX) were evaluated using 311 serum samples from a nonvaccinated quarantined herd. When the standardized virus neutralization (VN) test, which uses the Shope strain (VN Shope), was used as the comparative diagnostic standard, the gX-T test had a 7% false-negative rate and a 52% false-positive rate, and the gX-H test had a 19% false-negative rate and a 19% false-positive rate. When the VN test with a Bartha recombinant strain (VN Bartha gIIIKa) was used as the diagnostic standard, the gX-T test had a 9% false-negative rate and a 26% false-positive rate, and the gX-H test had a 24% false-negative rate and a 11% false-positive rate. Thus, the gX-T test was more sensitive and the gX-H test was more specific. Additional diagnostic tests on 79 serum samples from a noninfected herd did not produce false positives for the gX-H test, but there was an 8% false-positive rate for the gX-T test. Previous studies from our laboratory have demonstrated that VN Bartha gIIIKa has higher sensitivity than VN Shope, without losing specificity, and thus is a better comparative diagnostic standard. When adding a suspect range to the gX-T test, using the same criteria as the suspect range for the gX-H test, the false-positive rate of the gX-T test was reduced to 5% when evaluated versus VN Bartha gIIIKa in the infected herd and to 1% for the PRV-negative herd.(ABSTRACT TRUNCATED AT 250 WORDS)

Pseudorabies virus envelope glycoprotein gI influences both neurotropism and virulence during infection of the rat visual system

We previously demonstrated that intraocular injections of virulent and attenuated strains of pseudorabies virus (PRV) produce transneuronal infection of functionally distinct central visual circuits in the rat. The virulent Becker strain of PRV induces two temporally separated waves of infection that ultimately target all known retinorecipient neurons; the attenuated Bartha strain only infects a functionally distinct subset of these neurons. In this study, we demonstrate that deletion of a single viral gene encoding glycoprotein gI is sufficient to reproduce both the novel pattern of infectivity and the reduced neurovirulence of the Bartha strain of PRV. Glycoprotein gIII, a major viral membrane protein required for efficient adsorption of virus in cell culture, has no obvious role in determining the pattern of neuronal infectivity, but appears to function with gI to influence neurovirulence. These data suggest that neuroinvasiveness and virulence are the products of an interaction of viral envelope glycoproteins with as yet unidentified cellular receptors.

Glycoprotein gI of pseudorabies virus promotes cell fusion and virus spread via direct cell-to-cell transmission

Mutants of pseudorabies virus defective in either glycoprotein gI or gIII are only slightly less virulent for mice and chickens than is wild-type virus, while mutants defective in both gI and gIII are avirulent. To clarify the reason for the lack of virulence of the gI- gIII- mutants, we have analyzed in some detail the interactions of these mutants with their hosts. The results obtained showed that the gI glycoprotein is an accessory protein that promotes cell fusion. This conclusion is based on the findings that in some cell types, syncytium formation is significantly reduced in mutants deficient in gI. Furthermore, despite efficient replication, gI- mutants form significantly smaller plaques on some cell types. Finally, while wild-type and gI- virus are neutralized similarly by antisera, the size of the plaques formed by gI- mutants, but not by wild-type virus, is reduced by the presence of neutralizing antibodies in the overlay. Passive immunization of mice with neutralizing antipseudorabies virus sera is also considerably more effective in protecting them against challenge with gI- mutants than in protecting them against challenge with wild-type virus. These results show that gI- mutants are deficient in their ability to form syncytia and to spread directly by cell-to-cell transmission and that these mutants spread mainly by adsorption of released virus to uninfected cells. Wild-type virus and gIII- mutants, however, spread mainly via direct cell-to-cell transmission both in vivo and in vitro. We postulate that the lack of virulence of the gIII- gI- virus is attributable to its inability to spread by either mode, the defect in gIII affecting virus spread by adsorption of released virus and the defect in gI affecting cell-to-cell spread. Although a gI- gIII- mutant replicates as well as a gIII- mutant, it will be amplified much less well. Our results with in vitro systems show that this is indeed the case.

Sensitivity of the standardized pseudorabies virus neutralization test varies with the test strain used

The effect of altering the strain of the test virus used in the standardized pseudorabies virus neutralization (VN) test on the sensitivity of the assay was evaluated. Comparative VN tests were performed using 4 different strains: the avirulent Bartha parental, the avirulent recombinant Bartha gIIIKa, the moderately virulent Shope (currently used for the VN test at the National Veterinary Services Laboratory, Ames, IA), and the highly virulent P2208 (Funkhauser). A radioimmunoassay and a Western immunoblotting technique were employed to verify the presence of anti-pseudorabies virus (PrV) antibodies in sera. Statistical analysis indicated that replacement of the Shope strain by the Bartha gIIIKa or the P2208 strain resulted in VN titers that were 4.23- and 2.00-fold higher, respectively. Despite these differences, specificity with regard to PrV diagnosis was unaltered. This apparent enhancement of the sensitivity of the PrV VN test would be beneficial for the serologic identification of PrV-infected animals during an eradication effort.

The gIII glycoprotein of pseudorabies virus is involved in two distinct steps of virus attachment

The entry of herpesviruses into cells involves two distinct stages: attachment or adsorption to the cell surface followed by internalization. The virus envelope glycoproteins have been implicated in both stages. Pseudorabies virus attaches to cells by an early interaction that involves the viral glycoprotein gIII and a cellular heparinlike substance. We examined the role of gIII in the attachment process by analysis of a set of viruses carrying defined gIII mutations. The initial attachment of gIII mutants with an internal deletion of 134 amino acids (PrV2) to MDBK cells was indistinguishable from that of wild-type virus. The adsorption of these mutants was, however, much more sensitive than that of wild-type virus to competing heparin. Furthermore, while attachment of wild-type virus to MDBK cells led to a rapid loss of sensitivity to heparin, this was not the case with PrV2, which could be displaced from the cell surface by heparin after it had attached to the cells. We conclude that glycoprotein gIII is involved in two distinct steps of virus attachment and that the second of these steps but not the first is defective in PrV2.

The nucleotide sequence of the equine herpesvirus 4 gC gene homologue

The genomic position of an equine herpesvirus 4 (EHV-4) gene homologue of the herpes simplex virus 1 (HSV-1) gC gene was determined by Southern analysis and DNA sequencing. The gene lies within a 2-kbp Bg/II-EcoRI fragment mapping between 0.15 and 0.17 within the long unique component of the EHV-4 genome and is transcribed from right to left. Putative promoter elements were identified upstream of the 1455-bp open reading frame which encodes a 485-amino-acid protein of unglycosylated molecular weight 52,513. Computer-assisted analysis of the primary sequence predicts the protein possesses a domain structure characteristic of a type 1 integral membrane glycoprotein. Four domains were distinguished--(i) an N-terminal signal sequence, (ii) a large extracellular domain containing 11 putative N-linked glycosylation sites, (iii) a hydrophobic transmembrane domain, and (iv) a C-terminal charged domain. Comparison of the predicted amino acid sequence to that of other herpesvirus glycoproteins indicated identities of between 22 and 29% with HSV-1 gC, HSV-2 gC, VZV gpV, PRV gIII, BHV-1 gIII, and MDV A antigen and of 79% with EHV-1 gp13. A gene with no apparent homologue in HSV-1 or VZV maps immediately downstream of the EHV-4 gC gene homologue.

Isolation of a viable herpesvirus (pseudorabies virus) mutant specifically lacking all four known nonessential glycoproteins

Recently we described the isolation and characterization of a pseudorabies virus (PrV) mutant lacking the nonessential glycoproteins gI, gp63, and gIII. Using insertional mutagenesis with a functional gX-beta-galactosidase fusion gene we describe here the isolation of a PrV mutant specifically lacking all four known nonessential glycoproteins, gI, gp63, gIII, and gX. The quadruple mutant did not show any significant alterations in the vitro growth characteristics compared to its triple mutant parent. These results prove that PrV nonessential glycoproteins are dispensable for viral replication in cell culture altogether.

Linker insertion mutagenesis of herpesviruses: inactivation of single genes within the Us region of pseudorabies virus

We describe a technique for the systematic inactivation of nonessential genes within the genome of a herpesvirus without the requirement for phenotypic selection. This technique is based on the insertion of an oligonucleotide containing translational stop codons at a random site within a large cloned viral DNA fragment. Mutant virus is then reconstituted by cotransfection with overlapping viral clones, together comprising the entire viral genome, as described previously (M. van Zijl, W. Quint, J. Briaire, T. de Rover, A. Gielkens, and A. Berns, J. Virol. 62:2191-2195, 1988). This technique was used to construct, in a single experiment, a set of 13 viable pseudorabies virus strains with oligonucleotide insertions within all known genes of the Us region except for the gp50 gene, which proved essential for virus growth in cell culture. The growth rate in porcine kidney cells of mutants of all nonessential Us genes was similar to that of the parental virus, with the exception of a mutant of the recently identified protein kinase gene.

Marker vaccines, virus protein-specific antibody assays and the control of Aujeszky's disease

Vaccination of pigs is widely practised to control Aujeszky's disease (AD). Molecular biological research revealed that several conventionally attenuated virus vaccines harbour deletions in their genomes. The deleted genes are nonessential for virus replication and can be involved in the expression of virulence. These findings have prompted several groups to construct well-characterized deletion mutants of AD virus that do not express either glycoprotein gI, gX or gIII. These mutants have also been rendered thymidine kinase negative. Although data on vaccine efficacy and safety have been published, widely varying test conditions have made it impossible to identify the most efficacious deletion mutant vaccine(s). Vaccination enhances the amount of virus required for infection and reduces, but does not prevent, the shedding of virulent virus and the establishment of latency in pigs infected with virulent AD virus. Therefore, while a vaccination programme will reduce the circulation of virus in the field, it will not eliminate AD virus from pig populations. To eradicate AD, the ability to differentiate infected from vaccinated pigs is crucial. The use of marker vaccines enables us to identify infected pigs in vaccinated populations by detecting antibodies against the protein whose gene is deleted from vaccine strains. The antibody response to gI appears to persist for more than 2 years, and all of about 300 field strains tested so far express gI. The use of vaccines lacking gI in combination with an enzyme linked immunosorbent assay to detect antibodies to gI and culling of gI-seropositive pigs, may help to eradicate AD in countries where vaccination is widely practised.

Pseudorabies virus glycoprotein gIII is a major target antigen for murine and swine virus-specific cytotoxic T lymphocytes

Pseudorabies virus (PrV) is the etiological agent of Aujeszky's disease, a disease that causes heavy economic losses in the swine industry. A rational approach to the generation of an effective vaccine against this virus requires an understanding of the immune response induced by it and of the role of the various viral antigens in inducing such a response. We have constructed mutants of PrV [strain PrV (Ka)] that differ from each other only in expression of the viral nonessential glycoproteins gI, gp63, gX, and gIII (i.e., are otherwise isogenic). These mutants were used to ascertain the importance of each of the nonessential glycoproteins in eliciting a PrV-specific cytotoxic T-lymphocyte (CTL) response in mice and pigs. Immunization of DBA/2 mice and pigs with a thymidine kinase-deficient (TK-) mutant of PrV elicits the formation of cytotoxic cells that specifically lyse syngeneic infected target cells. These PrV-specific cytolytic cells have the phenotype of major histocompatibility complex class I antigen-restricted CTLs. The relative number of CTLs specific for glycoproteins gI, gp63, gX, and gIII induced in mice vaccinated with a TK- mutant of PrV was ascertained by comparing their levels of cytotoxicity against syngeneic cells infected with either wild-type virus or gI-/gp63-, gX-, or gIII- virus deletion mutants. The PrV-specific CLTs were significantly less effective in lysing gIII(-)-infected targets than in lysing gI-/gp63-, gX-, or wild-type-infected targets. The in vitro secondary CTL response of lymphocytes obtained from either mice or pigs 6 or more weeks after immunization with a TK- mutant of PrV was also tested. Lymphocytes obtained from these animals were cultured with different glycoprotein-deficient mutants of PrV, and their cytolytic activities against wild-type-infected targets were ascertained. The importance of each of the nonessential viral glycoproteins in eliciting CTLs was assessed from the effectiveness of each of the virus mutants to stimulate the secondary anti-PrV CTL response. Cultures of both murine or swine lymphocytes that had been stimulated with gIII- virus contained only approximately half as many lytic units as did those stimulated with either wild-type virus, a gX- virus mutant, or a gI-/gp63- virus mutant. Thus, a large proportion of the PrV-specific CTLs that are induced by immunization with PrV of both mice and pigs are directed against gIII. Furthermore, glycoproteins gI, gp63, and gX play at most a minor role in the CTL response of these animals to PrV.

Interaction of glycoprotein gIII with a cellular heparinlike substance mediates adsorption of pseudorabies virus

Glycoprotein gIII is one of the major envelope glycoproteins of pseudorabies virus (PrV) (Suid herpesvirus 1). Although it is dispensable for viral growth, it has been shown to play a prominent role in the attachment of the virus to target cells, since gIII- deletion mutants are severely impaired in adsorption (C. Schreurs, T. C. Mettenleiter, F. Zuckermann, N. Sugg, and T. Ben-Porat, J. Virol. 62:2251-2257, 1988). We show here that during the process of adsorption of PrV, the viral glycoprotein gIII interacts with a cellular heparinlike receptor. This conclusion is based on the following findings. (i) Heparin inhibits plaque formation of PrV by preventing the adsorption of wild-type virions to target cells. However, heparin does not interfere with the plaque formation of PrV mutants that lack glycoprotein gIII. (ii) Wild-type virions readily adsorb to matrix-bound heparin, whereas gIII- mutants do not. (iii) Pretreatment of cells with heparinase reduces considerably the ability of wild-type PrV to adsorb to these cells and to form plaques but does not negatively affect gIII- mutants. (iv) Glycoprotein gIII binds to heparin and appears to do so in conjunction with glycoprotein gII. Although heparin significantly reduces the adsorption of wild-type virus to all cell types tested, quantitative differences in the degree of inhibition of virus adsorption by heparin to different cell types were observed. Different cell types also retain their abilities to adsorb wild-type PrV to a different extent after treatment with heparinase and differ somewhat in their relative abilities to adsorb gIII- mutants. Our results show that while the primary pathway of adsorption of wild-type PrV to cells occurs via the interaction of viral glycoprotein gIII with a cellular heparinlike receptor, an alternative mode of adsorption, which is not dependent on either component, exists. Furthermore, the relative abilities of different cell types to adsorb PrV by the gIII-dependent or the alternative mode vary to some extent.

Release of pseudorabies virus from infected cells is controlled by several viral functions and is modulated by cellular components

The role of the nonessential glycoproteins gI, gp63, and gIII in the release of pseudorabies virus from different cell lines was investigated. We show that these glycoproteins may have a beneficial or deleterious effect on virus release depending on the type of cell in which the virus is grown. Inactivation of the genes encoding either gI, gp63, or gIII has no detectable effect on virus release from rabbit kidney cells. Inactivation of gI or gp63 strongly promotes virus release from chicken embryo fibroblasts, whereas inactivation of gIII reduces virus release from these cells. A defect in both gI and gIII or in both gp63 and gIII diminishes virus release from rabbit kidney cells but improves release from chicken embryo fibroblasts. We demonstrate that all three nonessential glycoproteins contribute to one specific aspect of viral growth, namely, virus release, and that they affect virus release in conjunction with each other. Furthermore, our results show that the manifestation of the role of each of these viral functions in virus growth may differ in different cell types, i.e., that release is affected by these viral functions in conjunction with some unknown cellular function.

Nucleotide sequence of bovine herpesvirus type 1 glycoprotein gIII, a structural model for gIII as a new member of the immunoglobulin superfamily, and implications for the homologous glycoproteins of other herpesviruses

The gene encoding bovine herpesvirus type 1 (BHV-1) glycoprotein gIII was mapped, cloned, and sequenced. The gene is situated between map units 0.122 and 0.135 and encodes a predicted protein of 521 amino acids. The identity of the sequenced gene has been verified previously by expression of immunologically authentic recombinant BHV-1 gIII (Fitzpatrick et al. (1988) J. Virol. 62, 4239-4248). Comparison of the BHV-1 gIII amino acid sequence with the homologous glycoproteins of other alphaherpesviruses revealed significant homology in the carboxy-terminal half of the molecules, including six invariant cysteine residues. A 96 amino acid domain with significant homology to class II major histocompatibility complex (MHC) antigen constant domains was identified in the conserved carboxy-terminal half of BHV-1 gIII. This domain is flanked by two other similarly sized domains which may be related to other immunoglobulin (Ig) superfamily domains. These homologies support a model for the structure of BHV-1 gIII as a new member of the Ig superfamily. Elements of the model may be applicable to the homologous glycoproteins of other herpesviruses and relevant to the immunobiology of herpesvirus infections.

Identification of pseudorabies virus-exposed swine with a gI glycoprotein enzyme-linked immunosorbent assay

A monoclonal antibody specific for the gI glycoprotein of virulent pseudorabies virus was produced and used to affinity purify gI glycoprotein. The purified gI was used in an enzyme-linked immunosorbent assay (ELISA) that identified and differentiated field virus-exposed animals from animals vaccinated with gI-deleted virus. The gI ELISA was evaluated by comparing it with the virus neutralization test and with a standard ELISA which does not distinguish between vaccinated and naturally infected animals. Pigs vaccinated with a gI-deleted vaccine were seropositive by the virus neutralization or standard ELISA but were seronegative in the gI ELISA. Nonvaccinated and vaccinated animals were detected as seropositive in the gI ELISA only after exposure to gI-containing field virus. Exposed animals were detected as early as day 7 and for as long as 141 days after field virus exposure. As little as 10(2.7) PFU of field virus was sufficient to seroconvert negative animals in the gI ELISA. Pseudorabies virus-seronegative animals which received multiple doses of gI-deleted vaccine remained seronegative in the gI ELISA. The use of this test to monitor swine for pseudorabies virus infection would offer significant benefits towards eradication of the disease.

Replacement of the pseudorabies virus glycoprotein gIII gene with its postulated homolog, the glycoprotein gC gene of herpes simplex virus type 1

gIII, the major envelope glycoprotein of pseudorabies virus (PRV), shares approximately 20% amino acid similarity with glycoprotein gC of herpes simplex virus type 1 (HSV-1) and HSV-2. We describe here our first experiments on the potential conservation of function between these two genes and gene products. We constructed PRV recombinants in which the gIII gene and regulatory sequences have been replaced with the entire HSV-1 gC gene and its regulatory sequences. The gC promoter functions in the PRV genome, and authentic HSV-1 gC protein is produced, albeit at a low level, in infected cells. The gC protein is present at the cell surface but cannot be detected in the PRV envelope.

Glycoprotein gIII deletion mutants of pseudorabies virus are impaired in virus entry

gIII is one of the major structural glycoproteins of pseudorabies virus (PrV). Though nonessential for replication in cell culture, it plays a prominent role in virus adsorption and virus release from infected cells. In this study the effect of inactivation of gIII on virus uptake into cells was investigated using isogenic PrV glycoprotein mutants. Kinetic analyses demonstrated that deletion of the gIII gene severely impaired entry of PrV into cells, whereas inactivation of the genes coding for nonessential glycoproteins gI, gp63, or gX had no influence on the rate of virus penetration. Loss of gIII is therefore associated with a reduced rate of virus penetration indicating a gIII-independent way of virus entry which is significantly slower than the gIII-dependent penetration process.

Early interactions of pseudorabies virus with host cells: functions of glycoprotein gIII

Adsorption of mutants of pseudorabies virus (PrV) lacking glycoprotein gIII is slower and less efficient than is that of wild-type virus (C. Schreurs, T. C. Mettenleiter, F. Zuckermann, N. Snugg, and T. Ben-Porat, J. Virol. 62:2251-2257, 1988). To ascertain the functions of gIII in the early interactions of PrV with its host cells, we compared the effect on wild-type virus and gIII- mutants of antibodies specific for various PrV proteins. Although adsorption of wild-type virus was inhibited by polyvalent antisera against PrV as well as by sera against gIII and gp50 (but not sera against gII), adsorption of the gIII- mutants was not inhibited by any of these antisera. These results suggest that, in contrast to adsorption of wild-type PrV, the initial interactions of the gIII- mutants with their host cells are not mediated by specific viral proteins. Furthermore, competition experiments showed that wild-type Prv and the gIII- mutants do not compete for attachment to the same cellular components. These findings show that the initial attachment of PrV to its host cells can occur by a least two different modes--one mediated by glycoprotein gIII and the other unspecific. gIII- mutants not only did not adsorb as readily to cells as did wild-type virus but also did not penetrate cells as rapidly as did wild-type virus after having adsorbed. Antibodies against gIII did not inhibit the penetration of adsorbed virus (wild type or gIII-), whereas antibodies against gII and gp50 did. It is unlikely, therefore, that gIII functions directly in virus penetration. Our results support the premises that efficient adsorption of PrV to host cell components is mediated either directly or indirectly by gIII (or a complex of viral proteins for which the presence of gIII is functionally essential) and that this pathway of adsorption promotes the interactions of other viral membrane proteins with the appropriate cellular proteins, leading to the rapid penetration of the virus into the cells. The slower penetration of the gIII- mutants than of wild-type PrV appears to be related to the slower and less efficient alternative mode of adsorption of PrV that occurs in the absence of glycoprotein gIII.