Detection of latency-associated transcripts of equid herpesvirus 1 in equine leukocytes but not in trigeminal ganglia

Results from Southern hybridization and PCR amplification experiments using a randomly synthesized reverse transcription-PCR product showed that peripheral blood leukocytes from horses showing no clinical signs of disease expressed a putative latency-associated transcript antisense to and overlapping the 3' end of the equid herpesvirus 1 (EHV-1) immediate-early gene (gene 64). A PCR product derived from this transcript has > or =96% identity with the published EHV-1 sequence. In situ hybridization studies of equine bronchial lymph nodes corroborated these findings and are consistent with reactivation data (D. A. Smith, A. Hamblin, and N. Edington, unpublished data), indicating that EHV-1 latency is established predominantly in CD5+/CD8+ leukocytes.

An equine herpesvirus-1 gene 71 deletant is attenuated and elicits a protective immune response in mice

The pathogenesis of pulmonary infection and the immune response following intranasal inoculation of mice with two equine herpesvirus type 1 (EHV-1) deletion mutants have been assessed. The mutants, ED71 and ED75, have deletions in genes 71 (EUS4) and 75 (10K), respectively. Deletions were replaced by the Escherichia coli lacZ gene driven by the simian virus 40 (SV40) early promoter. It has previously been shown that the protein products of genes 71 and 75 are dispensable in vitro but that removal of gene 71 results in a defect in virus maturation and capsid envelopment which impairs the ability of mutant virus to spread via release and readsorption. This study demonstrated that the 192-kDa gene 71 product is required for full expression of virulence in mice, whereas the putative 10-kDa product of gene 75 has minimal effect. Both mutants exhibited the same tissue and cytotropism as wild-type EHV-1 and induced both humoral and cell-mediated immune responses indistinguishable from those induced by the parental strain. Irrespective of the reduced pathogenicity of the gene 71 mutant, infected mice were protected against a challenge with wild-type EHV-1. These findings highlight the potential of ED71 as a vaccine candidate.

Structural and antigenic identification of the ORF12 protein (alpha TIF) of equine herpesvirus 1

The equine herpesvirus 1 (EHV-1) homolog of the herpes simplex virus type 1 (HSV-1) tegument phosphoprotein, alpha TIF (Vmw65; VP16), was identified previously as the product of open reading frame 12 (ORF12) and shown to transactivate immediate early (IE) gene promoters. However, a specific virion protein corresponding to the ORF12 product has not been identified definitively. In the present study the ORF12 protein, designated ETIF, was identified as a 60-kDa virion component on the basis of protein fingerprint analyses in which the limited proteolysis profiles of the major 60-kDa in vitro transcription/ translation product of an ORF12 expression vector (pT7-12) were compared to those of purified virion proteins of similar size. ETIF was localized to the viral tegument in Western blot assays of EHV-1 virions and subvirion fractions using polyclonal antiserum and monoclonal antibodies generated against a glutathione-S-transferase-ETIF fusion protein. Northern and Western blot analyses of EHV-1-infected cell lysates prepared under various metabolic blocks indicated that ORF12 is expressed as a late gene, and cross reaction of polyclonal anti-GST-ETIF with a 63.5-kDa HSV-1 protein species suggested that ETIF and HSV-1 alpha TIF are antigenically related. Last, DNA band shift assays used to assess ETIF-specific complex formation indicated that ETIF participates in an infected cell protein complex with the EHV-1 IE promoter TAATGARAT motif.

Restriction endonuclease analysis of equine herpesvirus-1 isolates recovered in Ontario, 1986-1992, from aborted, stillborn, and neonatal foals

Ninety-two equine herpesvirus type 1 isolates were recovered from aborted, stillborn, or neonatal foals from Ontario, Canada, from 1986 to 1992. From this total, 32 strains were randomly chosen for further study. Four or 5 isolates from each winter were selected, each from a different premises, and characterized by restriction enzyme analysis using BamHI, KpnI, BglII, HindIII, and EcoRI. Additional isolates from 2 premises and from a zebra foal were also assessed. For the strains isolated in 1986 and 1989-1992, the DNA pattern of 18 strains was similar to that of type 1P (Kentucky D) for BamHI and KpnI. None of the 32 strains studied could be differentiated by HindIII or EcoRI. Using BglII, an inconsistent fragment pattern and distribution were observed. Of the 8 strains isolated in 1987 and 1988, 7 were assigned into the 1B prototype group. The geographic distribution of 17 type 1P and 12 1B isolates was random across southern Ontario. These findings suggest that both electropherotypes can be recovered from horses in Ontario. The patterns of the additional equine isolates from the same premises were identical. The zebra isolate was different from the prototype equine herpesvirus type 1 and type 4 patterns and from all other equine isolates.

Concatemeric intermediates of equine herpesvirus type 1 DNA replication contain frequent inversions of adjacent long segments of the viral genome

In common with other alpha-herpesviruses, the genome of equine herpesvirus type-1 (EHV-1) comprises covalently linked long and short unique sequences of DNA, each flanked by inverted repeats. Equimolar amounts of two genomic isomers, generated by free inversion of the short segment, relative to the long segment, are packaged into EHV-1 virions. In contrast with herpes simplex virus (HSV), inversion of genomic long segments has not been described. In the current work, the structures of high molecular weight intermediates of EHV-1 DNA replication were studied by field inversion gel electrophoresis. It is shown that adjacent long segments of the viral genome are frequently inverted in concatemeric intermediates of EHV-1 DNA replication. Further, like HSV concatemers, high molecular weight intermediates of EHV-1 replication are flanked exclusively by the long segment of the viral genome. Hence, despite the fact that only two, rather than four, isomers of EHV-1 DNA are packaged into virions, the intermediates of EHV-1 DNA replication closely resemble those of herpes simplex virus type 1 in structure. These data have implications relating to the mechanisms involved in packaging of alpha-herpesvirus DNA.

Detection of herpes simplex DNA in semen and menstrual blood of individuals attending an infertility clinic

OBJECTIVE

To determine a possible link between herpes simplex virus 1 (HSV) and infertility.

METHOD

A specifically designed polymerase chain reaction with nested primers, was developed and used to test for HSV in 153 men and 20 women attending an infertility clinic.

RESULTS

HSV DNA was detected in 37 (24%) out of 153 semen samples and in 11 (55%) out of 20 menstrual blood samples. However, HSV DNA (0%) was not detected in the semen of 16 males with children. A significant association between the evidence for infertility and an HSV positive test was observed in men (Fisher's exact test, p = 0.024), and a stronger effect was found in females after failed in vitro fertilization (Fisher's exact test p = 0.0086).

CONCLUSION

This is the first report of the detection of herpes simplex virus DNA in semen and menstrual blood. Encouraging preliminary results justify antiviral therapy in case of a positive test.

The ICP22 protein of equine herpesvirus 1 cooperates with the IE protein to regulate viral gene expression

The equine herpesvirus 1 (EHV-1) immediate-early (IE) phosphoprotein is essential for the activation of transcription from viral early and late promoters and regulates transcription from its own promoter. The EHV-1 EICP22 protein, a homolog of ICP22 of herpes simplex virus, increased the in vitro DNA binding activity of the IE protein for sequences in the IE, early, and late promoters. The EICP22 protein affected the rate as well as the extent of the IE protein binding to promoter DNA sequences. To study the DNA binding activity of the IE protein, Trp493, Gln495, Asn496, and Lys498 of the WLQN region, which is directly involved in DNA binding, were replaced with Ser (IEW493S), Glu (IEQ495E), Ile (IEN496I), and Glu (IEK498E), respectively. Gel shift assays revealed that the glutathione S-transferase (GST)-IEQ495E(407-615) and GST-IEK498E(407-615) proteins failed to bind to the IE promoter, indicating that the Gln and Lys residues are important for the DNA binding activity. In the presence of the GST-EICP22 protein, DNA binding activity of the GST-IEQ495E(407-615) protein was restored, suggesting that the EICP22 protein cooperates with the IE protein to regulate EHV-1 gene expression. Transient-transfection assays also showed that the EICP22 protein allowed the IEQ495E mutant to be functional as a transactivator. These results are unique and may represent an important role for the EICP22 protein in EHV-1 gene regulation.

Analysis of the contributions of the equine herpesvirus 1 glycoprotein gB homolog to virus entry and direct cell-to-cell spread

Experiments to analyze the functions of the equine herpesvirus 1 (EHV-1) glycoprotein gB were performed. Cell lines which stably expressed either the full-length EHV-1 gB or only the extracellular portion of gB (amino acids 1 to 844) were constructed and were termed TCgBf and TCgB delta, respectively. Using the cell line TCgBf, a gB-negative viral mutant, L11delta gB, was generated by replacing a 2.1-kb BglII-NruI fragment in the EHV-1 strain RacL11 gB with the Escherichia coli LacZ gene. EHV-1 strain RacL11, the modified live vaccine strain RacH, and L11delta gB were used for functional studies. It was shown that: (i) EHV-1 gB is essential for virus growth in vitro since gB-negative L11delta gB exhibited titers of <10 PFU/ml when grown and titrated on noncomplementing cells. (ii) The cell line expressing truncated gB (TCgB delta) did not complement for the growth of L11delta gB, but the RacH virus grew to titers comparable to those of RacL11 in all cell lines tested. Since RacH had amino acids 944-980 of gB replaced by 7 missense amino acids as determined by nucleotide sequence analysis, the extreme carboxyterminus but not a domain between amino acid residues 845 and 943, probably the transmembrane domain, of EHV-1 gB is dispensable for virus growth in cultured cells. (iii) Single infected cells but no plaque formation were observed after infection of noncomplementing cells with L11delta gB, demonstrating the requirement of EHV-1 gB for direct cell-to-cell spread of infection. (iv) The attachment of gB-negative L11delta gB virions to target cells was similar to both phenotypically complemented L11delta gB and parent RacL11 virus. (v) L11delta gB viral titers could be enhanced by using the fusogen polyethylene glycol (PEG). The increase of L11delta gB titers by PEG treatment, however, was considerably lower compared to gB-negative pseudorabies virus, suggesting that EHV-1 gB might not be as stringently required for virus penetration as are its homologs in other Alphaherpesvirinae.

The equine herpesvirus 1 IR6 protein influences virus growth at elevated temperature and is a major determinant of virulence

The diploid IR6 gene (ORF 67) of equine herpesvirus type 1 (EHV-1) is absent in the modified live EHV-1 vaccine strain RacH and is present in a mutated form in the avirulent EHV-1 strains RacM24 and RacM36, such that the IR6 protein fails to form the typical rod-like structures observed for wild-type EHV-1 RacL11. To assess the role of the IR6 protein in EHV-1 replication and virulence, two recombinant RacH viruses, HIR6-1 and HIR6-2, that harbor a single copy of the wild-type IR6 gene were engineered and characterized. It was shown that: (i) HIR6-1 or HIR6-2 virus encoded for an IR6 protein that was capable of forming the rod-like structures typical of cells infected with the wild-type virulent virus strain RacL11. (ii) Whereas the avirulent EHV-1 strains RacH and RacM36 are temperature-sensitive (in that virus replication at 40 degrees versus that at 37 degrees was reduced by as much as 7,500-fold), the HIR6-1 and HIR6-2 viruses, like RacL11 virus, were capable of significant replication at the elevated temperature. (iii) Electron microscopic analyses revealed that cells infected with the HIR6-1 or HIR6-2 virus, like those infected with virulent RacL11 virus, produced and released comparable numbers of virus particles at both 37 and 40 degrees. In cells infected with the RacH virus at 40 degrees, however, release of extracellular particles was inhibited by greater than 90% and relatively few of the particles were enveloped. (iv) Infections of BALB/cA mice revealed that both the HIR6-1 and HIR6-2 viruses, unlike the parent RacH virus, were as virulent as the wild-type RacL11 strain as judged by the criteria of body weight loss, development of clinical signs of EHV-1 infection, virus titers in the lung, and ability to cause viremia. These findings and those of our recent studies indicate that the IR6 protein is a major determinant of EHV-1 virulence and that the IR6 protein may play a role in virus maturation and/or egress.

Expression and function of the equine herpesvirus 1 virion-associated host shutoff homolog

The ability of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2, respectively) to repress host cell protein synthesis early in infection has been studied extensively and found to involve the activities of the UL41 gene product, the virion-associated host shutoff (vhs) protein. To date, UL41 homologs have been identified in the genomes of three other alphaherpesviruses: equine herpesvirus 1 (EHV-1), varicella-zoster virus, and pseudorabies virus, but very little is known about the putative products of these homologous genes. Our earlier observations that no rapid early host protein shutoff occurred in EHV-1-infected cells led us to test EHV-1 vhs activity more thoroughly and to examine the expression and function of the EHV-1 UL41 homolog, ORF19. In the present study, the effects of EHV-1 and HSV-1 infections on cellular protein synthesis and mRNA degradation were compared at various multiplicities of infection in several cell types under an actinomycin D block. No virion-associated inhibition of cellular protein synthesis or vhs-induced cellular mRNA degradation was detected in cells infected with any of three EHV-1 strains (Ab4, KyA, and KyD) at multiplicities of infection at which HSV-1 strain F exhibited maximal vhs activity. However, further analyses revealed that (i) the EHV-1 vhs homolog gene, ORF19, was transcribed and translated into a 58-kDa protein in infected cells; (ii) the ORF19 protein was packaged into viral particles in amounts detectable in Western blots (immunoblots) with monoclonal antibodies; (iii) in cotransfection vhs activity assays, transiently-expressed ORF19 protein had intrinsic vhs activity comparable to that of wild-type HSV-1 vhs; and (iv) this intrinsic vhs activity was ablated by in vitro site-directed mutations in which either the functionally inactive HSV-1 vhs1 UL41 mutation (Thr at position 214 replaced by Ile [Thr-214-->Ile]) was recreated within ORF19 or two conserved residues within the putative poly(A) binding region of the ORF19 sequence were altered (Tyr-190, 192-->Phe). From these results we conclude that EHV-1's low vhs activity in infected cells is not a reflection of the ORF19 protein's intrinsic vhs activity but may be due instead to the amount of ORF19 protein associated with viral particles or to modulation of ORF19 protein's intrinsic activity by another viral component(s).

Direct detection of equine herpesvirus DNA in tissues of aborted equine fetuses

Restriction endonuclease analysis of equine herpesviruses 1 (EHV-1) and 4 has been investigated using cultured cells infected with these viruses. The DNA cleavage patterns of these viruses were observed in the intracellular DNA after digestion with Eco RI and electrophoresis. This procedure was applied to the diagnosis of equine herpesvirus infection in aborted equine fetuses. The characteristic Eco RI restriction pattern of EHV-1 DNA was directly detectable in the emulsion of lungs collected from aborted equine fetuses.

Application of polymerase chain reaction (PCR) for diagnosis of equine herpes virus-1 (EHV-1)

Fifty aborted foetus samples were diagnosed for the presence of equine herpes virus-1 (EHV-1) by polymerase chain reaction (PCR) technique. Specific primer pair for amplification of a particular segment of EHV-1 DNA in gc region having 3 Hae III restriction endonuclease sites was used. A 409 base pair segment obtained as PCR amplification product in 9 samples was digested with Hae III to confirm the presence of EHV-1 as the infectious agent in aborted tissues. It was observed that PCR technique was more sensitive, specific and rapid than the conventional virological diagnostic methods.

The highly O-glycosylated glycoprotein gp2 of equine herpesvirus 1 is encoded by gene 71

There have been conflicting reports regarding the gene assignment of the high-molecular-mass envelope glycoprotein gp2 (gp300) of equine herpesvirus 1. Here, we provide an unequivocal demonstration that gp2 is encoded by gene 71. gp2 that was purified with a defining monoclonal antibody was cleaved internally to yield a 42-kDa protein encoded by gene 71. Amino acid composition data and N-terminal sequence analysis of a tryptic peptide identified gp2 as the product of equine herpesvirus 1 gene 71 with the SWISS-PROT database. Analysis of gp2's monosaccharide composition and the 42-kDa subunit showed that the high level of O glycosylation occurs on the serine/threonine-rich region upstream of the cleavage site.

Equine herpesvirus-1 strain KyA, a candidate vaccine strain, reduces viral titers in mice challenged with a pathogenic strain, RacL

The equine herpesvirus type-1 (EHV-1) strain Kentucky A (KyA) has a long history of repeated passage either in vivo in the Syrian hamster or in vitro in mouse L-M fibroblast tissue culture. This repeated passage in cells other than those of the natural host has caused genomic alterations of the KyA chromosome resulting in deletion of several genes or portions of open reading frames (ORFs). This report presents in vivo data from a mouse model of EHV-1 infection demonstrating the attenuated nature of EHV-1 strain KyA and that intranasal infection with KyA protects animals from subsequent challenge with a pathogenic strain, RacL, by reducing RacL viral titers in the lungs of the challenged animals. Mice infected with KyA exhibit no clinical manifestations of EHV-1 disease and do not experience the wasting that occurs with RacL infection. KyA-infected mice clear virus from the lung by day 5 post-infection (p.i.), whereas RacL infected mice have substantial virus titers (5 x 10(5) pfu/lung) at this time point. Intranasal infection with KyA followed by a challenge with RacL 4 weeks post-KyA infection resulted in a significant (P = 0.0079) reduction in the lung titers of the RacL virus. RacL was identified as the virus present in the lungs of the challenged mice by a PCR assay employing primers to amplify the EUS4 gene which differs in size by 1.2 kilobase pairs (kbp) in the two strains. Importantly, the protection afforded by KyA is long lasting in that challenge with RacL 15 months after KyA infection, results in reduced virus titers and viral clearance by day 5 post-challenge. These results support the further consideration of EHV-1 KyA as a live virus vaccine.

The equine herpesvirus 1 IR6 protein is nonessential for virus growth in vitro and modified by serial virus passage in cell culture

The IR6 protein of different plaque isolates from three passages of the equine herpesvirus 1 strain Rac was investigated. Southern blot and DNA sequence analyses revealed that plaque isolates from the 12th passage (RacL11 and RacL22) retained both copies of the IR6 gene, whereas two different genotypes were observed by the 185th passage: RacM24 still harbored both copies of the IR6 gene, whereas RacM36 deleted one of the two copies. In the 256th passage (RacH), both copies of the IR6 gene were absent. As compared to the wild-type IR6 protein, both the RacM24 and RacM36 IR6 protein displayed amino acid exchanges at positions 34, 42, 110, and 134 of the 272 amino acid polypeptide. It is shown that (i) the IR6 protein is nonessential for virus growth in vitro. (ii) In RacL11-infected equine and rodent cells, the typical rod-like appearance of the IR6 protein could be detected from 6 hr p.i., whereas in RacM24- and RacM36-infected cells formation of these structures was not observed. (iii) The RacL11 IR6 gene product was present in both the nuclei and cytoplasmic fraction of infected cells. In contrast, the IR6 protein of both RacM24 and RacM36 colocalized with cytoplasmic membrane vesicles. (iv) The RacL11 and RacL22 IR6 protein is present in viral nucleocapsids, whereas that of RacM24 and RacM36 is not incorporated into virions. (v) The RacL11 IR6 gene product aggregated to disulfide-linked oligomers, whereas the RacM24 and RacM36 IR6 protein showed only marginal oligomerization. (vi) In COS7 cells transfected with constructs expressing either the full-length RacL11-IR6 protein or a truncated form lacking the 81 carboxyterminal amino acids, the formation of rod-like structures was observed, indicating that another viral protein is not necessary for aggregation of the IR6 protein. In contrast, the IR6 protein expressed from constructs derived from either RacM24 or RacM36 failed to form these structures. (vii) Analyses of chimeric RacL11-RacM24 IR6 proteins suggest a crucial role for amino acid Leu-134 in the ability of the IR6 protein to form the rod-like structures.

Alterations in the equine herpesvirus type-1 (EHV-1) strain RacH during attenuation

The equine herpesvirus type-1 modified live-vaccine strain RacH (256th passage on porcine embryonic kidney cells) was investigated by restriction-enzyme analysis and compared to representative plaque isolates of the 12th passage (RacL11, RacL22) and 185th passage (RacM24, RacM36). The restriction patterns of all Rac plaque isolates differed compared with reference strain Ab4. The left UL terminus was shortened by 0.1 kbp and a missing BamHI site led to the fusion of the f and t fragments. In some Rac derivatives, losses of restriction sites without deletions were observed: 1. One BamHI site located in the ribosyl reductase gene was missing in RacH, RacM24, RacM36, and RacL22; and 2. An SalI site mapping to the gp14 (gB) gene was absent in RacM24, RacM36 and RacH. An identical deletion of 0.85 kbp in size was found in both copies of the inverted repeat (IR) regions of RacH. The deletion was present only in the terminal IR of the medium-passage derivative RacM36. By contrast, in the genomes of the apathogenic RacM24, as well as the pathogenic plaque isolates RacL11 and RacL22, no deletions in the IRs were detectable. Nucleotide-sequence and Northern-blot analyses revealed that the deletions led to the elimination of one or both copies of the gene 67 (IR6) open-reading frame in RacM36 and RacH and affected the gene 68 (EUS1) in RacH.

The role of the gene 71 product in the life cycle of equine herpesvirus 1

Equine herpesvirus type 1 (EHV-1) gene 71 encodes a heavily O-glycosylated 192 kDa protein with no identified herpesvirus homologue. Isolation of a deletion mutant in gene 71 (ED71) demonstrated that its protein product is not essential in vitro. To investigate the role of the gene 71 protein in the virus life cycle, ED71 has been characterized in vitro in terms of cellular adsorption, penetration, egress and transmission compared to wild-type and revertant virus. ED71 virions adsorbed to cells less efficiently than wild-type and revertant virus with a consequential effect on virus penetration; virus egress was significantly impaired and the timing of release was also delayed. The percentage of both full and empty capsids accumulating in the nuclei of ED71-infected cells was significantly higher than in wild-type virus-infected cells but the most notable differences were the low number of particles and the low ratio of enveloped to unenveloped capsids in the cytoplasm. The primary mode of transmission of the mutant virus is by direct cell-to-cell spread and the fact that a neutralizing antiserum did not reduce ED71 plaque size, supported the conclusion that deletion of gene 71 impairs the ability of virus to spread via release and readsorption to uninfected cells. Thus, deletion of EHV-1 gene 71 results in a defect in virus maturation and capsid envelopment. Progeny virus is consequently impaired in adsorption/penetration presumably due to the particles lacking the glycoprotein spikes predicted to be encoded by this gene and hence spreads by direct cell-to-cell contact.

Sequence and expression of a bovine herpesvirus-1 gene homologous to the glycoprotein K-encoding gene of herpes simplex virus-1

In the bovine herpes virus-1 (BHV-1) genome, a gene equivalent to the glycoprotein k (gK)-encoding gene of other herpesviruses was identified and sequenced. The primary translation product is predicted to comprise 338 amino acids (aa) and to exhibit a molecular mass of 37.5 kDa. It possesses characteristics typical for membrane glycoproteins including a potential cleavable signal sequence, three transmembrane domains and two potential N-linked glycosylation sites. Comparison to the gK proteins of the other herpesviruses revealed aa sequence homologies of 46, 44, 53, 43, and 46% with the gK counterparts of herpes simplex viruses-1 and 2 (HSV-1 and 2), equine herpesvirus-1 (EHV-1), Marek's disease virus (MDV) and varicella zoster virus (VZV), respectively. A 30-kDa primary translation product was identified following in vitro translation of in vitro transcribed mRNA. When canine microsomal membranes were added to the translation reaction, a 38-kDa glycosylated protein was detected. Treatment with endoglycosidase F or H (endo or H) removed the glycosyl groups and reduced the apparent molecular mass of the 38-kDa glycoprotein.

Lack of virulence of the murine fibroblast adapted strain, Kentucky A (KyA), of equine herpesvirus type 1 (EHV-1) in young horses

The virulence of the cell culture adapted KyA strain of equine herpesvirus type 1 (EHV-1), which lacks at least six genes by deletions in its genome, was assessed by intranasal inoculation of six young horses that were serologically negative for EHV-1. No horses showed clinical signs, and a neutralizing antibody response against EHV-1 was detected in two horses which had antibodies against EHV-4 prior to the inoculation. A challenge experiment using a highly virulent strain of EHV-1 conducted 4 weeks later against 4 of the 6 horses inoculated intranasally with the KyA strain and 2 control horses revealed that (i) the KyA inoculated horses were protected from manifestation of clinical signs detected in both control horses, with the exception of pyrexia, (ii) duration of virus isolation from the KyA inoculated horses after the challenge was remarkably shortened as compared to that from control horses; (iii) thus, animals inoculated with the KyA and challenged with pathogenic EHV-1 showed a reduction in the time of virus shedding and viremia; (iv) two horses which exhibited no antibody responses after the KyA inoculation showed antibody responses after the challenge significantly higher than those of control horses. The results reveal that the KyA strain has no virulence but still possesses immunogenicity for horses, suggesting that some of the genes deleted from the KyA strain might have importance in the expression of EHV-1 virulence.

The expression of the proteins of equine herpesvirus 1 which share homology with herpes simplex virus 1 glycoproteins H and L

Several expression systems were used in studies aimed at characterizing the equine herpesvirus 1 (EHV-1) glycoprotein H and L homologues of HSV-1 (EHV-1 gH and gL) and the products were compared to the authentic proteins synthesized in virus infected cells. Using an in vitro transcription/translation system two gH species were detected (an unprocessed 89 kDa and a processed 116 kDa product). Three low molecular weight proteins were found in the case of gL (21.8 kDa, 22.9 kDa and 26.9 kDa) and these showed a slight reduction in mobility on the addition of microsomal membranes to the reactions. A gL fusion protein was produced in pGEX-2T, expression being confirmed by Western blotting using a gL-specific antiserum raised against a peptide incorporating the 13 carboxyl terminal amino acids of the protein. A gH specific peptide antiserum precipitated both gH and two smaller proteins from EHV-1 infected cells thought to be two forms of gL. Insect cells infected with gH or gL baculovirus recombinants were used to vaccinate C3H (H-2k) mice. Some protection against EHV-1 infection was conferred to the gH inoculated mice. The results will enable further studies on the importance of the gH and gL interaction in the pathogenesis of EHV-1 to be evaluated and their potential in contributing to a subunit vaccine to be assessed.

N-terminal sequence analysis of equine herpesvirus 1 glycoproteins D and B and evidence for internal cleavage of the gene 71 product

Signal cleavage sites of equine herpesvirus 1 (EHV-1) glycoproteins D and B (gD and gB) and an endoproteolytic cleavage site of EHV-1 gB were determined by N-terminal amino acid sequencing and compared with known cleavage sites of homologues in other herpesvirus. Signal cleavage of EHV-1 gD occurred between Arg35 and Ala36 in a region of basic amino acids resembling the endoproteolytic cleavage sites of viral glycoproteins, nine amino acids downstream of the predicted site, while EHV-1 gB was cleaved as predicted between Ala85 and Val86. Endoproteolytic cleavage of EHV-1 gB occurred between Arg548 and Ala549, 28 amino acids downstream of the cleavage site predicted from conserved sequences of other herpesvirus gB homologous. One interpretation of these data is that EHV-1 gB is cleaved internally at both sites, a possibility which was supported by the apparent molecular masses of the unglycosylated gB subunits produced in the presence of tunicamycin. This double cleavage would release a stretch of amino acids which is not present in sequenced gB molecules of other herpesviruses. Experiments with glycosylation inhibitors indicated that cleavage of EHV-1 gB can occur in the absence of glycosylation. N-terminal sequencing also determined that a 42 kDa EHV-1 glycoprotein was a product of internal cleavage of the protein encoded by gene 71. Staggered endoproteolytic cleavage after adjacent arginine residues 506 and 507 separates the 42 kDa C-terminal subunit containing all the cysteine residues from the serine and threonine rich N-terminal region.

Molecular studies of the acute infection, latency and reactivation of equine herpesvirus-1 (EHV-1) in the mouse model

The murine intranasal (i.n.) infection model was used to study the molecular distribution of equine herpesvirus-1 (EHV-1) during acute infection, latency and following a reactivation stimulus. After inoculation, infectious virus was detected in lungs, nasal turbinates, brains and olfactory bulbs during the acute phase. A nested PCR (nPCR) readily detected virus in these tissues and, in addition, virus was detected in spleens and (in the second round of nPCR) in peripheral blood mononuclear cells (PBMC). A digoxigenin-labelled in situ hybridization probe detected EHV-1 DNA in bronchiolar and vascular endothelium in the lungs and in and around germinal centres in the spleens. One month later, although infectious virus was absent from all tissues, the trigeminal ganglia, olfactory bulb and PBMC remained positive for virus DNA although this was detected only on the second round of nPCR. Furthermore, in situ hybridization, using either DNA or RNA probes, suggested that little or no transcription of virus occurred in neural tissues during the 'latent phase'. Following a reactivation stimulus, infectious virus was not isolated from any tissues, however, EHV-1 DNA was detected on the first round of nPCR in olfactory bulb, trigeminal ganglia and PBMC. This suggested a quantitative increase in EHV-1 DNA occurred following reactivation stimulus. The significance of these results is discussed in relation to the molecular state of EHV-1 in different tissues at various stages of infection and the validity of the murine model for studying latency and reactivation of EHV-1 in the horses.

Expression of an equine herpesvirus 1 ICP22/ICP27 hybrid protein encoded by defective interfering particles associated with persistent infection

Defective interfering (DI) particles of equine herpesvirus type 1 (EHV-1) are capable of mediating persistent infection (S. A. Dauenhauer, R. A. Robinson, and D. J. O'Callaghan, J. Gen. Virol. 60:1-14, 1982; R. A. Robinson, R. B. Vance, and D. J. O'Callaghan, J. Virol. 36:204-219, 1980). Sequence analysis of cloned DI particle DNA revealed that portions of two regulatory genes, ICP22 (IR4) and ICP27 (UL3), are linked in frame to form a unique hybrid open reading frame (ORF). This hybrid ORF, designated as the IR4/UL3 gene, encodes the amino-terminal 196 amino acids of the IR4 protein (ICP22 homolog) and the carboxy-terminal 68 amino acids of the UL3 protein (ICP27 homolog). Portions of DNA sequences encoding these two regulatory proteins, separated by more than 115 kbp in the standard virus genome, were linked presumably by a homologous recombination event between two identical 8-bp sequences. Reverse transcriptase-PCR and S1 nuclease analyses revealed that this unique ORF is transcribed by utilizing the transcription initiation site of ICP22 and the polyadenylation signal of ICP27 in DI particle-enriched infection. Immunoprecipitation and Western blot (immunoblot) analyses with antisera to the ICP22 and ICP27 proteins demonstrated that a 31-kDa hybrid protein was synthesized in the DI particle-enriched infection but not in standard virus infection. This 31-kDa hybrid protein was expressed at the same time as the ICP22 protein in DI particle-enriched infection and migrated at the same location on polyacrylamide gel electrophoresis as the protein expressed from a cloned IR4/UL3 expression vector. These observations suggested that the unique IR4/UL3 hybrid gene is expressed from the DI particle genome and may play a role in DI particle-mediated persistent infection.

The detection of latency-associated transcripts of equine herpesvirus 1 in ganglionic neurons

Neural tissues from specific pathogen-free ponies that had been experimentally infected with equine herpesvirus 1 (EHV-1) were analysed by in situ hybridization. Digoxigenin-labelled EHV-1 BamHI fragments spanning almost the entire EHV-1 genome were hybridized to RNA in tissue sections from latently infected trigeminal ganglia. The BamHI E fragment detected EHV-1 RNA antisense to gene 63 (HSV-1 homologue ICP0) in a small number of neurons. Sixteen other BamHI fragments gave negative results in 20 sections tested with each fragment. Latency associated transcripts (LATs) were localized to the neuronal nuclei. EHV-1 nucleotide sequence data in the region reveals the presence of a putative EHV-1 LAT promoter that shares a similar motifs with the HSV-1 LAT promoter, including the LAT promoter-binding factor, and may have a role in EHV-1 LAT expression.