Equine herpesvirus 1 mutants devoid of glycoprotein B or M are apathogenic for mice but induce protection against challenge infection

Vaccination of foals with a modified live, equid herpesvirus-1 gM deletion mutant (RacHΔgM) confers partial protection against infection

Equid herpesvirus-1 (EHV-1) causes respiratory and neurological disease and late gestation abortion in pregnant mares. Current vaccines contain either inactivated or live EHV-1, but fail to provide complete clinical or virological protection, namely prevention of nasopharyngeal shedding and cell-associated viraemia. Thus, the development of novel products, such as modified live virus (MLV) vaccines which stimulate virus-specific, humoral and cell mediated immune responses more effectively remains a priority. Two groups of weaned foals (n = 6 each group) were used in a longitudinal, prospective, experimental study to evaluate immune responses elicited by two vaccinations with a glycoprotein M (gM) deletion mutant of EHV-1 (RacHdeltagM). Following two concurrent intranasal and intramuscular inoculations six weeks apart, vaccinated (8.4 ± 0.2 months old) and control foals (6.2 ± 0.4 months) were challenge infected intranasally with EHV-1 Ab4/8 four weeks after the second vaccination and clinical signs and virological replication measured. Vaccination caused no adverse events, but did stimulate significantly higher complement fixing and virus neutralizing antibodies in serum compared with control foals at either equivalent or pre-vaccination time points. Virus-specific nasopharyngeal antibody levels and cytotoxic T lymphocyte responses were not significantly different between the groups. Following challenge infection, these immune responses were associated with a reduction in clinical signs and virological replication in the vaccinated foals, including a reduction in duration and magnitude of pyrexia, nasopharyngeal shedding and cell-associated viraemia. We conclude that the RacHΔgM MLV primed EHV-1-specific humoral immune responses in weaned foals. However, complete virological protection by vaccination against EHV-1 requires further research.

Ubiquitination and degradation of the ORF34 gene product of equine herpesvirus type 1 (EHV-1) at late times of infection

The equine herpesvirus type 1 (EHV-1) open reading frame 34 (ORF34) is predicted to encode a polypeptide of 161 amino acids. We show that an ORF34 deletion mutant exhibited a significant growth defect in equine peripheral blood mononuclear cells taken directly ex vivo during early but not late times of infection. ORF34 protein (pORF34)-specific antibodies specifically reacted with a 28-kDa early polypeptide present in the cytosol of infected cells. From 10h post infection, multiple smaller pORF34-specific protein moieties were detected indicating that expression of a late viral gene product(s) caused pORF34 degradation. Proteasome inhibitors blocked pORF34 degradation as did treatment of infected cells with a ubiquitin-activating enzyme (E1) inhibitor. Finally, kinetic studies showed that pORF34 is modified by addition of multiple copies of ubiquitin. Taken together, our findings suggest that the ubiquitin proteasome pathway is required for pORF34 degradation that may modulate protein activity in the course of infection.

Glycoproteins D of equine herpesvirus type 1 (EHV-1) and EHV-4 determine cellular tropism independently of integrins

Equine herpesvirus type 1 (EHV-1) and EHV-4 are genetically and antigenically very similar, but their pathogenic potentials are strikingly different. The differences in pathogenicity between both viruses seem to be reflected in cellular host range: EHV-1 can readily be propagated in many cell types of multiple species, while EHV-4 entry and replication appear to be restricted mainly to equine cells. The clear difference in cellular tropism may well be associated with differences in the gene products involved in virus entry and/or spread from cell to cell. Here we show that (i) most of the EHV-1 permissive cell lines became resistant to EHV-1 expressing EHV-4 glycoprotein D (gD4) and the opposite was observed for EHV-4 harboring EHV-1 gD (gD1). (ii) The absence of integrins did not inhibit entry into and replication of EHV-1 in CHO-K1 or peripheral blood mononuclear cells (PBMC). Furthermore, integrin-negative K562 cells did not acquire the ability to bind to gD1 when αVβ3 integrin was overexpressed. (iii) PBMC could be infected with similar efficiencies by both EHV-1 and EHV-4 in vitro. (iv) In contrast to results for equine fibroblasts and cells of endothelial or epithelial origin, we were unable to block entry of EHV-1 or EHV-4 into PBMC with antibodies directed against major histocompatibility complex class I (MHC-I), a result that indicates that these viruses utilize a different receptor(s) to infect PBMC. Cumulatively, we provide evidence that efficient EHV-1 and EHV-4 entry is dependent mainly on gD, which can bind to multiple cell surface receptors, and that gD has a defining role with respect to cellular host range of EHV-1 and EHV-4.

In vitro and in vivo characterization of equine herpesvirus type 1 (EHV-1) mutants devoid of the viral chemokine-binding glycoprotein G (gG)

Glycoprotein G (gG) of equine herpesvirus type 1 (EHV-1), a structural component of virions and secreted from virus-infected cells, was shown to bind to a variety of different chemokines and as such might be involved in immune modulation. Little is known, however, about its role in the replication cycle and infection of EHV-1 in vivo. Here we report on the function of gG in context of virus infection in vitro and in vivo. A gG deletion mutant of pathogenic EHV-1 strain RacL11 (vL11DeltagG) was constructed and analyzed. Deletion of gG had virtually no effect on the growth properties of vL11DeltagG in cell culture when compared to parental virus or a rescuant virus vL11DeltagGR, respectively, and virus titers and plaque formation were unaffected in the absence of the glycoprotein. Similarly, in the murine model of EHV-1 infection, no significant differences in virulence between the gG deletion mutant and RacL11 or vL11DeltagGR were found at high doses of infection. However, infection of mice at lower doses revealed that the gG deletion mutant was able to replicate to higher titers in lungs of infected mice. Additionally, these mice lost significantly more weight than those infected with RacL11 and a more pronounced inflammatory response in lungs was observed. Therefore we concluded that deletion of gG in EHV-1 seems to lead to an exacerbation of respiratory disease in the mouse.

Molecular Characterization of the Equine Herpesvirus 1 Strains RacL11 and Kentucky D

The pathogenicities of RacL11 and Kentucky D strains of equine herpesvirus 1 in the hamster infection model are different from those of Ab4p and the Japanese isolates. Virus genome restriction fragment length polymorphism analysis and sequence comparison of an intergenic region, glycoproteins and tegument genes showed higher conservation but with some strain-specific differences. These results indicate that point nucleotide differences in RacL11 and Kentucky D might be responsible for their pathogenicity in rodent models.

Pathogenicity and immunogenicity of equine herpesvirus type 1 mutants defective in either gI or gE gene in murine and hamster models

To develop a live vaccine for equine herpesvirus type 1 (EHV-1), two EHV-1 mutants containing no heterogeneous DNA, DeltagI and DeltagE, were constructed with deletions in the open reading frame of either glycoprotein I (gI) or E (gE), respectively. In equine cell culture, deletion mutants formed smaller plaques than the parental and revertant viruses, but the one-step growth patterns of the deletion mutants and the parental strain were approximately the same. These results suggest that both gI and gE contribute to the ability of EHV-1 to spread directly from cell-to-cell, but that these glycoproteins are not required for viral growth in vitro. Mice and hamsters inoculated intranasally with these mutants showed no clinical signs, and continued to gain weight, whereas those inoculated with the parental virus exhibited a reduction in mean body weight. Furthermore, nervous manifestations were observed in hamsters inoculated with the parental virus. These results suggest that gI and gE have an important role in EHV-1 virulence including neurovirulence in experimental animal models. On the other hand, serum neutralizing antibodies were detected in mice immunized with DeltagI or DeltagE at two weeks after inoculation. Following challenge with the parental virus, DeltagI- or DeltagE-immunized mice were able to clear parental virus from their lungs faster than mock-immunized mice. These results suggest that the EHV-1 mutants defective in gI and in gE are attenuated but have ability to elicit immune responses in inoculated mice that contribute to virus clearance.

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

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

Equine herpesvirus type 1 modified live virus vaccines: quo vaditis?

Infections of horses with equine herpesvirus type 1 (EHV-1) have garnered new attention over the last few years. Devastating outbreaks occurring worldwide, primarily of the neurologic form of the disease, have resulted in a reassessment of the control strategies, and particularly the prophylactic measures, that are necessary to keep the infection and spread of disease in check. Most of the available EHV-1 vaccines are based on preparations of inactivated virus, which are applied monovalently for prevention of EHV-1-caused abortion in pregnant mares or as part of multivalent vaccines to prevent respiratory disease. Despite the importance of an induction of cytotoxic immune responses for protection against EHV-1-induced disease, only two modified live virus vaccine preparations, which are both based on the avirulent EHV-1 strain RacH and were developed more than 40 years ago, are commercially available. Current efforts focus on exploiting the available infectious bacterial artificial chromosome clones of various EHV-1 strains to engineer a new generation of modified live virus vaccines. Both more efficient and long-lasting anti-EHV-1 immunity and delivery of immunogens of other pathogens are attempted and within immediate reach. The improvement of modified live virus vaccines will likely be a major focus of research in the future, and will hopefully help to more completely protect horses against one of the most important and devastating viral diseases.

Complex formation by glycoproteins M and N of human cytomegalovirus: structural and functional aspects

The genomes of herpesviruses contain a number of genes which are conserved throughout the family of Herpesviridae, indicating that the proteins may serve important functions in the replication of these viruses. Among these are several envelope glycoproteins, including glycoprotein M (gM) and gN, which form a complex that is covalently linked via disulfide bonds in some herpesviruses. However, deletion of gM and/or gN from most alphaherpesviruses has limited effects on replication of the respective viruses in vitro. In contrast, insertional inactivation of the gM gene of the betaherpesvirus human cytomegalovirus (HCMV) results in a replication-incompetent virus. We have started to analyze the structural and functional aspects of the interaction between gM and gN of HCMV. We show that large parts of gM are dispensable for the formation of a gM/gN complex that is transported to distal parts of the cellular secretory pathway. In addition, we demonstrate that the disulfide bond is between the cysteine at position 44 in gM and cysteine 90 in gN. However, disulfide linkage is not a prerequisite for modification and transport of the gM/gN complex. Moreover, mutant viruses that lack a disulfide bridge between gM and gN replicate with efficiencies similar to that of wild-type viruses.

Potential of equine herpesvirus 1 as a vector for immunization

Key problems using viral vectors for vaccination and gene therapy are antivector immunity, low transduction efficiencies, acute toxicity, and limited capacity to package foreign genetic information. It could be demonstrated that animal and human cells were efficiently transduced with equine herpesvirus 1 (EHV-1) reconstituted from viral DNA maintained and manipulated in Escherichia coli. Between 13 and 23% of primary human CD3+, CD4+, CD8+, CD11b+, and CD19+ cells and more than 70% of CD4+ MT4 cells or various human tumor cell lines (MeWo, Huh7, HeLa, 293T, or H1299) could be transduced with one infectious unit of EHV-1 per cell. After intranasal instillation of EHV-1 into mice, efficient transgene expression in lungs was detectable. Successful immunization using EHV-1 was shown after delivery of the human immunodeficiency virus type 1 Pr55gag precursor by the induction of a Gag-specific CD8+ immune response in mice. Because EHV-1 was not neutralized by human sera containing high titers of antibodies directed against human herpesviruses 1 to 5, it is concluded that this animal herpesvirus has enormous potential as a vaccine vector, because it is able to efficiently transduce a variety of animal and human cells, has high DNA packaging capacity, and can conveniently be maintained and manipulated in prokaryotic cells.

Equine herpesvirus type 1 (EHV-1) glycoprotein K is required for efficient cell-to-cell spread and virus egress

The function of the equine herpesvirus type 1 (EHV-1) glycoprotein K (gK) homologue was investigated. Deletion of 88% of the UL53-homologous open reading frame in EHV-1 strain RacH resulted in a severe growth defect of the gK-negative virus (HDeltagK) as reflected by a significant decrease in the production of infectious virus progeny on RK13 cells. The HDeltagK virus induced only minute plaques, was unable to form syncytia, and its penetration efficiency into RK13 cells was reduced by approximately 40%. To further analyze gK function and intracellular trafficking, gK of strain RacH was replaced by a C-terminally truncated gK-green fluorescent protein fusion protein (gK-GFP). The generated recombinant virus was shown to replicate well on non-complementing cells, and virus penetration and syncytium formation were comparable to parental RacH. A reduction in plaque size and slightly decreased intra- and extracellular virus titers, however, were observed. The gK-GFP fusion protein was expressed with early-late kinetics, and multiple forms of the protein exhibiting M(r)s between 50,000 and 85,000 were detected by Western blot analysis. The various gK-GFP forms were shown to be N-glycosylated, associated with membranes of the Golgi apparatus, and were incorporated into extracellular virions. Complete processing of gK-GFP was only observed within the context of viral infection. From the results, we concluded that EHV-1 gK is required for efficient virus growth in vitro and that the carboxy-terminal amino acids are not required for its function, because the gK-GFP fusion protein was able to complement for EHV-1 growth in the absence of authentic gK.

Identification of candidate gammaherpesvirus 68 genes required for virus replication by signature-tagged transposon mutagenesis

Current methods for determining the role of a given gene product in the gammaherpesvirus 68 (gammaHV68) life cycle require generation of a specific mutation by either homologous recombination in mammalian cells or bacterial artificial chromosome-mediated mutagenesis in Escherichia coli. The mutant virus is then compared to wild-type virus, and the role of the gene in the viral life cycle is deduced from its phenotype. This process is both time-consuming and labor intensive. Here we present the use of random, transposon-mediated signature-tagged mutagenesis for the identification of candidate viral genes involved in virus replication. Pools of viral mutants, each containing a random insertion of a transposon, were generated with a transposon donor library in which each transposon contains a unique sequence identifier. These pools were transfected into mammalian cells, and the ability of each mutant to replicate was assessed by comparing the presence of virus in the output pool to that present in the input pool of viral genomes. With this approach we could rapidly screen up to 96 individual mutants simultaneously. The location of the transposon insertion was determined by sequencing individual clones with a common primer specific for the transposon end. Here we present the characterization of 53 distinct viral mutants that correspond to insertions in 29 open reading frames within the gammaHV68 genome. To confirm the results of the signature-tagged mutagenesis screen, we quantitated the ability of each mutant to replicate compared to wild-type gammaHV68. From these analyses we identified 16 gammaHV68 open reading frames that, when disrupted by transposon insertions, score as essential for virus replication, and six other open reading frames whose disruption led to significant attenuation of virus replication. In addition, transposon insertion in five other gammaHV68 open reading frames did not affect virus replication. Notably, all but one of the candidate essential replication genes identified in this screen have been shown to be essential for the replication of at least one other herpesvirus.

Pre-infection frequencies of equine herpesvirus-1 specific, cytotoxic T lymphocytes correlate with protection against abortion following experimental infection of pregnant mares

In general, vaccines containing inactivated equine herpesvirus-1 (EHV-1) fail to prevent abortion in pregnant mares following infection with a virulent strain of EHV-1. We have tested the hypothesis that resistance to EHV-1-induced abortion in pregnant mares is associated with high frequencies of EHV-1 specific, major histocompatibility complex (MHC) class I-restricted, cytotoxic T lymphocytes (CTL) in the circulation. To test this theory, three groups of pregnant mares were assembled with varying backgrounds of infection or vaccination in an attempt to mimic the immune status of the general population. Group 1 mares (n=9) were untreated controls selected at random. Group 2 mares (n=5) were vaccinated three times intramuscularly with inactivated EHV-1. Group 3 mares (n=3) had been infected with EHV-1 on four previous occasions. The frequency of CTL in blood leucocytes was measured by limiting dilution analysis at three time points; at the beginning of pregnancy (approximately 28 weeks before infection) in the Group 2 and Group 3 mares (4-7 weeks of gestation) (Group 1 was unavailable for sampling) and then 2 weeks before (30-40 weeks of gestation) and 3 weeks after experimental infection in all the mares. Serum samples were collected to monitor complement fixing (CF) antibody titres. Mares in all three groups were infected experimentally with EHV-1 strain Ab4/8 by the intranasal route after which they were monitored clinically to determine the outcome of pregnancy and samples were collected to determine the duration of nasopharyngeal shedding and cell-associated viraemia. The untreated control mares showed low pre-infection CTL. After experimental infection, they all seroconverted, aborted and demonstrated expected clinical and virological signs. Some vaccinated mares (3/5) had elevated titres of CF antibody prior to their first vaccination. All the vaccinated mares seroconverted after vaccination and exhibited higher CTL frequencies than controls before infection. Four of the five foaled normally. The multiply infected mares had low CF antibody titres prior to infection and showed neither seroconversion nor clinical or virological signs after infection. All multiply infected mares exhibited high frequencies of CTL before infection and they all foaled normally. The CTL frequencies observed differed significantly from the expected frequencies in the control and multiply infected groups at 2 weeks pre-infection (P=0.034) and between the foaling and aborting mares at 2 weeks pre-infection (P=0.005) and 3 weeks post-infection (P=0.015). The results show a positive correlation between the number of virus-specific CTL in the peripheral blood of pregnant mares and their protection against abortion induced by EHV-1 infection. Therefore, as indicated by this study, rational approaches to the development of new vaccines for EHV-1 should stimulate cytotoxic immune responses and develop virus-specific CTL as pre-requisites for protection against abortion.

The equine herpesvirus 1 UL11 gene product localizes to the trans-golgi network and is involved in cell-to-cell spread

Experiments were conducted to identify and characterize the equine herpesvirus type 1 (EHV-1) UL11 homologous protein. At early-late times after EHV-1 infection of Rk13 cells several proteins at an M(r) of 8000 to 12,000 were detected using a UL11 protein-specific antiserum. Particularly, an M(r) of 11,000 protein was found abundantly in purified virions and could be assigned to the tegument fraction. As demonstrated by confocal laser scanning microscopy, UL11 reactivity localized predominantly to the trans-Golgi network of infected cells, but was also noted at the plasma membrane, specifically of transfected cells. Deletion of UL11 sequences in EHV-1 vaccine strain RacH (Hdelta11) and in the virulent isolate RacL22 (Ldelta11) resulted in viruses that were able to replicate on noncomplementing cells. It was shown in one-step growth kinetics on Rk13 cells that the reduction of intracellular and of extracellular virus titers caused by the absence of UL11 expression in either virus was somewhat variable, but approximately 10- to 20-fold. In contrast, a marked influence on the plaque phenotype was noted, as mean maximal diameters of plaques were reduced to 23.2% (RacL22) or 34.7% (RacH) of parental virus plaques and as an effect on the ability of RacH to cause syncytia upon infection was noted. It was therefore concluded that the EHV-1 UL11 product is not essential for virus replication in Rk13 cells but is involved in cell-to-cell spread.

Mutagenesis of a bovine herpesvirus type 1 genome cloned as an infectious bacterial artificial chromosome: analysis of glycoprotein E and G double deletion mutants

The genome of bovine herpesvirus type 1 Schönböken was cloned as a bacterial artificial chromosome (BAC) by inserting mini F plasmid sequences into the glycoprotein (g) E gene. The resulting BAC clone, pBHV-1DeltagE, was transfected into bovine kidney cells and viable gE-negative BHV-1 (BHV-1DeltagE) was recovered. By RecE/T mutagenesis in Escherichia coli, the gG open reading frame was deleted from pBHV-1DeltagE. From the mutated BAC, double negative BHV-1DeltagE-gG was reconstituted and its growth properties were compared to those of rescuant viruses in which the gE gene was restored (BHV-1rev, BHV-1DeltagG). The mutant viruses did not exhibit markedly lowered virus titres. Plaque sizes of BHV-1DeltagE, BHV-1DeltagE-gG and BHV-1DeltagG, however, were reduced by 19 to 55 % compared to parental strain Schönböken or BHV-1rev. Our results suggested that gE and gG function independently from each other in cell-to-cell spread, because an additive effect on plaque formation was observed in the gE/gG double deletion mutant.

Equine herpesvirus type 1 devoid of gM and gp2 is severely impaired in virus egress but not direct cell-to-cell spread

Experiments were conducted to analyze the effects of a simultaneous deletion of glycoprotein M (gM) and glycoprotein 2 (gp2) of equine herpesvirus type 1 (EHV-1). EHV-1 strain RacH was cloned as a bacterial artificial chromosome (pRacH) by homologous recombination of a mini F plasmid into the unique short region of the genome, thereby deleting gene 71 encoding gp2. Upon transfection of the pRacH DNA into rabbit kidney RK13 cells, virus plaques were visible from day 1 after transfection. The mutant RacH virus (H Delta gp2) reconstituted from pRacH lacked gene 71 and did not express gp2 as assayed by indirect immunofluorescence analysis using gp2-specific monoclonal antibodies. The H Delta gp2 virus exhibited 10-fold reduced extracellular titers and an approximately 10% reduction in mean plaque diameters when compared to parental or gp2-revertant virus. The gM open reading frame was deleted from pRacH by recE/T mediated mutagenesis in Escherichia coli. The gM-gp2 double negative virus mutant (H Delta gp2gM) did not express either of the deleted glycoproteins as demonstrated by indirect immunofluorescence analysis. The H Delta gp2gM virus exhibited a 200-fold reduction of end-point extracellular titers when compared to parental RacH virus, which could not be compensated for by growth of the mutant virus on gM-expressing cells. After restoration of the gM open reading frame, however, growth of the mutant virus was comparable to the H Delta gp2 virus. Plaque diameters of the gM-gp2 double-negative mutant were reduced by only 16% when compared to that of parental RacH virus. From the results it was concluded that the simultaneous absence of gM and gp2 had an additive effect on egress but not secondary envelopment or cell-to-cell spread of EHV-1.

Deletion of gene 52 encoding glycoprotein M of equine herpesvirus type 1 strain RacH results in increased immunogenicity

The immunogenicity of equine herpesvirus type 1 (EHV-1) strain RacH was compared to a RacH virus in which gene 52 encoding glycoprotein M (gM) was interrupted by insertion of LacZ (HDeltagM-Ins) and a RacH with 75% of gene 52 was deleted and replaced by LacZ (HDeltagM-HS). HDeltagM-Ins failed to produce full-length gM, but the carboxy-terminal portion was still expressed. No gM expression was detected in HDeltagM-HS-infected cells. Mice were immunised once with 1x10(3) to 1x10(5) plaque-forming units (PFU) of RacH or mutant viruses and challenged with virulent RacL11 virus 29 days later. A dose-dependence of protection was observed in RacH-immunised mice, and following immunisation with 1x10(4) or 1x10(3) PFU body weight losses and increased virus titres in lungs were observed after challenge infection. HDeltagM-HS-immunised mice were completely protected even after immunisation with 1x10(3) PFU. Mice immunised with 1x10(3) PFU of HDeltagM-Ins but not the higher doses showed signs of disease after challenge infection.

Clinical and virological evaluation of the efficacy of an inactivated EHV1 and EHV4 whole virus vaccine (Duvaxyn EHV1,4). Vaccination/challenge experiments in foals and pregnant mares

Pregnant mares and young foals were vaccinated with Duvaxyn EHV1,4, an inactivated and adjuvanted vaccine containing both the EHV-1 and 4 antigens. SN and CF antibody titres were induced two weeks after first vaccination. Antibody levels were boosted after second vaccination, however they never reached the levels induced after virus challenge. Young foals were challenged with virulent EHV-1 and EHV-4 field viruses. Pregnant mares were challenged with the highly abortigenic EHV-1 strain Ab4. Vaccinated animals showed a clear reduction in clinical signs and virus excretion compared to unvaccinated control animals. Log transformed antibody levels could be correlated to duration of virus excretion. The incidence of EHV-1 induced abortions was drastically reduced in vaccinated mares. Therefore, although vaccinated animals are not fully protected against disease, Duvaxyn EHV1,4 clearly reduces clinical symptoms, the duration of virus shedding and the quantity of virus shed. It can be concluded that vaccination of foals and pregnant mares with Duvaxyn EHV1,4 significantly reduces the risk of abortions and outbreaks of respiratory disease caused by circulating field viruses.

The equine herpesvirus 1 UL45 homolog encodes a glycosylated type II transmembrane protein and is involved in virus egress

Experiments to analyze the product of the equine herpesvirus type 1 (EHV-1) UL45 homolog were conducted. Using an antiserum generated against the carboxylterminal 114 amino acids of the EHV-1 UL45 protein, proteins of M(r) 32,000, 40,000, and 43,000 were detected specifically in EHV-1-infected cells. Neither form of the protein was located in purified virions of EHV-1 wild-type strain RacL22 or the modified live vaccine strain RacH, but UL45 was demonstrated to be expressed as a late (gamma-2) protein. Fractionation of infected cells and deglycosylation experiments demonstrated that the EHV-1 UL45 protein represents a type II membrane glycoprotein. Deletion of the UL45 gene in RacL22 and RacH (LDelta45 and HDelta45) showed that UL45 is nonessential for EHV-1 growth in vitro, but that deletion reduced the viruses' replication efficiency. A marked reduction of virus release was observed although no significant influence was noticed either on plaque size or on the syncytial phenotype of the EHV-1 strain RacH.

Equine herpesvirus 1 (EHV-1) glycoprotein M: effect of deletions of transmembrane domains

Equine herpesvirus 1 (EHV-1) recombinants that carry either a deletion of glycoprotein M (gM) or express mutant forms of gM were constructed. The recombinants were derived from strain Kentucky A (KyA), which also lacks genes encoding gE and gI. Plaques on RK13 cells induced by the gM-negative KyA were reduced in size by 80%, but plaque sizes were restored to wild-type levels on gM-expressing cells. Electron microscopic studies revealed a massive defect in virus release after the deletion of gM in the gE- and gI-negative KyA, which was caused by a block in secondary envelopment of virions at Golgi vesicles. Recombinant KyA expressing mutant gM with deletions of predicted transmembrane domains was generated and characterized. It was shown that mutant gM was expressed and formed dimeric and oligomeric structures. However, subcellular localization of mutant gM proteins differed from that of wild-type gM. Mutant glycoproteins were not transported to the Golgi network and consequently were not incorporated into the envelope of extracellular virions. Also, a small plaque phenotype of mutant viruses that was indistinguishable from that of the gM-negative KyA was observed. Plaque sizes of mutant viruses were restored to wild-type levels by plating onto RK13 cells constitutively expressing full-length EHV-1 gM, indicating that mutant proteins did not exert a transdominant negative effect on wild-type gM.

Epstein-Barr virus that lacks glycoprotein gN is impaired in assembly and infection

The Epstein-Barr virus (EBV) glycoproteins N and M (gN and gM) are encoded by the BLRF1 and BBRF3 genes. To examine the function of the EBV gN-gM complex, recombinant virus was constructed in which the BLRF1 gene was interrupted with a neomycin resistance cassette. Recombinant virus lacked not only gN but also detectable gM. A significant proportion of the recombinant virus capsids remained associated with condensed chromatin in the nucleus of virus-producing cells, and cytoplasmic vesicles containing enveloped virus were scarce. Virus egress was impaired, and sedimentation analysis revealed that the majority of the virus that was released lacked a complete envelope. The small amount of virus that could bind to cells was also impaired in infectivity at a step following fusion. These data are consistent with the hypothesis that the predicted 78-amino-acid cytoplasmic tail of gM, which is highly charged and rich in prolines, interacts with the virion tegument. It is proposed that this interaction is important both for association of capsids with cell membrane to assemble and release enveloped particles and for dissociation of the capsid from the membrane of the newly infected cell on its way to the cell nucleus. The phenotype of EBV lacking the gN-gM complex is more striking than that of most alphaherpesviruses lacking the same complex but resembles in many respects the phenotype of pseudorabies virus lacking glycoproteins gM, gE, and gI. Since EBV does not encode homologs for gE and gI, this suggests that functions that may have some redundancy in alphaherpesviruses have been concentrated in fewer proteins in EBV.

Immunization of BALB/c mice with DNA encoding equine herpesvirus 1 (EHV-1) glycoprotein D affords partial protection in a model of EHV-1-induced abortion

DNA-mediated immunization was assessed in a murine model of equine herpesvirus 1 (EHV-1) abortion. Whilst there are differences between the model and natural infection in the horse, literature suggests that EHV-1 infection of pregnant mice can be used to assess the potential ability of vaccine candidates to protect against abortion. Female BALB/c mice were inoculated twice, 4 weeks apart, with an expression vector encoding EHV-1 glycoprotein D (gD DNA). They were mated 15 days after the second inoculation, challenged at day 15 of pregnancy and killed 3 days later. The gD DNA-inoculated mice had fewer foetuses which were damaged or had died in utero (6% in gD DNA, 21% vector DNA and 28% in nil inoculated groups challenged with EHV-1), a reduction in the stunting effect of EHV-1 infection on foetuses (gD DNA: 0.40g+/-0.06, vector DNA: 0.34g+/-0.10), reduced placental and herpesvirus-specific lung histopathology and a lower titre of virus (TCID(50)+/-SEM/lung) in maternal lung than control groups (gD DNA 4.7+/-0.3, vector 5.3+/-0.2, nil 5.6+/-0.2). Maternal antibody to EHV-1 gD was demonstrated in pups born to a dam inoculated 123 days earlier with gD DNA. Although protection from abortion was incomplete, immunization of mice with gD DNA demonstrated encouragingly the potential of this vaccine strategy.

Role of the cytoplasmic tail of pseudorabies virus glycoprotein E in virion formation

Glycoproteins M (gM), E (gE), and I (gI) of pseudorabies virus (PrV) are required for efficient formation of mature virions. The simultaneous absence of gM and the gE/gI complex results in severe deficiencies in virion morphogenesis and cell-to-cell spread, leading to drastically decreased virus titers and a small-plaque phenotype (A. Brack, J. Dijkstra, H. Granzow, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 73:5364-5372, 1999). Serial passaging in noncomplementing cells of a virus mutant unable to express gM, gE, and gI resulted in a reversion of the small-plaque phenotype and restoration of infectious virus formation to the level of a gM(-) mutant. Genetic analyses showed that reversion of the phenotype was accompanied by a genomic rearrangement which led to the fusion of a portion of the gE gene encoding the cytoplasmic domain to the 3' end of the glycoprotein D gene, resulting in expression of a chimeric gD-gE protein. Since this indicated that the intracytoplasmic domain of gE was responsible for the observed phenotypic alterations, the UL10 (gM) gene was deleted in a PrV mutant, PrV-107, which specifically lacked the cytoplasmic tail of gE. Regarding one-step growth, plaque size, and virion formation as observed under the electron microscope, the mutant lacking gM and the gE cytoplasmic tail proved to be very similar to the gE/I/M triple mutant. Thus, our data indicate that it is the cytoplasmic tail of gE which is responsible for the observed phenotypic effects in conjunction with deletion of gM. We hypothesize that the cytoplasmic domain of gE specifically interacts with components of the capsid and/or tegument, leading to efficient secondary envelopment of intracytoplasmic capsids.

Quantitation of virus-specific classes of antibodies following immunization of mice with attenuated equine herpesvirus 1 and viral glycoprotein D

The antibody responses of CBA/J mice infected intranasally (i.n.) with either the attenuated KyA strain or the pathogenic RacL11 strain of equine herpesvirus 1 (EHV-1) or immunized with recombinant glycoprotein D (rgD) were investigated using the ELISPOT assay to measure EHV-1-specific antibody-secreting cells (ASC) in the regional lymphoid tissue of the respiratory tract. IgG, IgA, and IgM ASC specific for EHV-1 were detected in the mediastinal lymph nodes (MLN) and lungs 2 weeks after i.n. infection with EHV-1 strain KyA or RacL11, or immunization with heat-killed KyA or rgD. EHV-1-specific ASC were present in the MLN and lungs at 4 and 8 weeks, but declined in frequency by fivefold in the lung at 8 weeks. However, i.n. immunized (2 x 10(6) pfu KyA or 50 microgram rgD/mouse) mice infected at 8 weeks with pathogenic EHV-1 RacL11 resisted challenge and showed eight- and tenfold increases in MLN ASC and lung ASC, respectively, by 3 days after challenge. In contrast to the intranasal route of immunization, intraperitoneal immunization yielded ASC frequencies in the MLN and lungs that were only slightly above those of nonimmunized control mice. These data indicate that immunization with infectious or heat-killed EHV-1 KyA, or rgD, induces significant levels of virus-specific ASC both in the MLN and lungs, a specific memory B-cell response, and long-term protective immunity. The finding that the numbers of ASC induced by the pathogenic strain versus the attenuated strain of EHV-1, which were virtually identical, indicated that the ability to generate a B-cell response is independent of and does not contribute to EHV-1 virulence.

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

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

Comparison of the pathogenesis of acute equine herpesvirus 1 (EHV-1) infection in the horse and the mouse model: a review

The mouse models of the respiratory and abortion forms of equine herpesvirus 1 (EHV-1) infection have been used to investigate the vaccine potential of various EHV-1 immunogens, the effect of antiviral agents on EHV-1 infection and the pathogenicity of EHV-1 strain variants and deletion or insertional mutants. This review examines the similarities and differences in the pathogenesis of primary EHV-1 infection in the natural host, the horse, and in the mouse by comparing tissue tropism, clinical signs of infection, the effects of EHV-1 on pregnancy, haematological changes following infection, viral clearance, histopathology and latency. The evidence suggests that the mouse model provides a valid method for investigation of virological and histopathological aspects of EHV-1-induced disease in the horse. However, the extent to which useful and valid comparisons and extrapolations can be made of immunological parameters from mouse to horse is yet to be determined.

Characterization of the cytolytic T-lymphocyte response to a candidate vaccine strain of equine herpesvirus 1 in CBA mice

The cytolytic T-lymphocyte (CTL) response to respiratory infection with equine herpesvirus 1 (EHV-1) in CBA (H-2(k)) mice was investigated. Intranasal (i.n.) inoculation of mice with the attenuated EHV-1 strain KyA resulted in the generation of a primary virus-specific CTL response in the draining mediastinal lymph nodes 5 days following infection. EHV-1-specific CTL could be restimulated from the spleen up to 26 weeks after the resolution of infection, indicating that a long-lived memory CTL population was generated. Depletion of CD8+ T cells by treatment with antibody and complement prior to assay eliminated CTL activity from both primary and memory populations, indicating that cytolytic activity in this model was mediated by class I major histocompatibility complex-restricted, CD8+ T cells. A single i.n. inoculation with KyA induced protective immunity against infection with the pathogenic EHV-1 strain, RacL11. The adoptive transfer of splenocytes from KyA-immune donors into sublethally irradiated recipients resulted in a greater than 250-fold reduction in RacL11 in the lung. The elimination of both CD4+ and CD8+ T cells from the transferred cells abrogated clearance of RacL11, while the selective depletion of either subpopulation alone had little effect. These results suggested that both lymphocyte subpopulations contribute to viral clearance, with either subpopulation alone being sufficient.