DNA-mediated immunization with glycoprotein D of equine herpesvirus 1 (EHV-1) in a murine model of EHV-1 respiratory infection

DNA-mediated immunization was assessed in a murine model of equine herpesvirus 1 (EHV-1) respiratory infection. A single intramuscular injection with plasmid DNA encoding EHV-1 glycoprotein D (EHV-1 gD), including its predicted C-terminal membrane anchor sequence, induced a specific antibody response detectable by 2 weeks and maintained through 23 weeks post injection. A second injection at 4 weeks markedly enhanced the antibody response and all EHV-1 gD-injected mice developed neutralizing antibodies. A lymphocyte proliferative response to whole EHV-1 was observed and a predominance of IgG2a antibodies after DNA injection was consistent with the generation of a type 1 helper T-cell (Th1) response. Following intranasal challenge with EHV-1, mice immunized with EHV-1 gD DNA were able to clear virus significantly more rapidly from lung tissue and showed reduced lung pathology, in comparison to control mice.

Infection with equine herpesvirus 1 (EHV-1) strain HVS25A in pregnant mice

The abortigenic effects of equine herpesvirus 1 (EHV-1) strain HVS25A, given intranasally, were assessed in pregnant BALB/c, C57BL/6J and Quakenbush mice at day 16 of pregnancy. All EHV-1-infected BALB/c mice showed clinical signs typical of EHV-1-induced disease, together with evidence of abortion. However, although there were fetal and neonatal deaths in some C57BL/6J and Quakenbush litters, the respiratory and systemic effects of EHV-1 infection in the dams were inconsistent. BALB/c dams were then inoculated at day 15 of pregnancy with either EHV-1 or rabbit kidney (RK) cell lysate (controls) and animals were killed at days 1-5 post-inoculation (pi), i.e., before the occurrence of abortions. EHV-1-infected mice showed a significant fall in rectal temperature between days 1 and 2 pi and lost weight during the first 4 days pi, demonstrating a significant mean difference in weight gain from the control group at days 2, 3, 4 and 5 pi. Death in utero was seen in five of 90 fetuses of EHV-1-infected mice, but in no fetuses from RK-inoculated mice. On days 1, 2, 3, 4 and 5 pi, the fetuses from EHV-1-infected dams were significantly smaller than those from RK-inoculated dams. Congestion and necrosis of the middle layer of trophoblast and chorionic necrosis were observed in the placentae from EHV-1-infected dams and assessed by a scoring system. Virus was isolated rarely from the fetuses (1/73), placentae (3/72) and uteri (1/16) of EHV-1-infected dams, and only from those killed on day 1 or 2 pi. This indicates that, as in the horse, abortion caused by EHV-1 infection in mice is not necessarily a consequence of fetal infection but may be due to fetal compromise due to vascular effects on the placenta.

An equine herpesvirus 1 mutant with a lacZ insertion between open reading frames 62 and 63 is replication competent and causes disease in the murine respiratory model

An equine herpesvirus 1 (EHV-1) mutant was constructed by inserting a lacZ expression cassette into the intergenic region upstream of gene 62 (glycoprotein L; gL) and downstream of gene 63 (a homologue of the herpes simplex virus transcriptional activator ICP0). The recombinant lacZ62/63-EHV-1 had similar growth kinetics in cell culture to those of the parental wild type (wt) virus, with indistinguishable cytopathic effects and plaque morphology. Reverse transcriptase PCR confirmed that the lacZ insertion did not interfere with transcription of gL and immunoblot analysis indicated there was no modification to late gene expression as monitored by synthesis of EHV-1 glycoproteins C and D. The parental EHV-1 isolate HVS25A used here had almost identical nucleotide sequence to that published for isolate Ab4, in a 1200 bp region surrounding the insert, but lacked a HindIII site corresponding to Ab4 position 109,048. The lacZ62/63-EHV-1 caused respiratory disease in BALB/c mice with clinical signs, histopathology and virus titres in lungs throughout days 1-5 post infection similar to those induced by wt EHV-1. X-gal staining for beta-galactosidase expression in murine lungs clearly demonstrated EHV-1 infection in cells of the bronchiolar epithelium and pulmonary parenchyma, with a peak of infection evident at day 2 post infection, when up to 50% of bronchioles demonstrated blue-staining and thus virus-infected epithelial cells. The construction of this replication competent virus carrying a reporter gene identifies a site for insertion of foreign genes and will facilitate studies on the pathogenesis of EHV-1.

Seroprevalence of equine herpesvirus 1 in thoroughbred foals before and after weaning

OBJECTIVE

To investigate the seroprevalence of equine herpesvirus 1 in foals around weaning and after weaning on two large Thoroughbred farms using a type-specific enzyme-linked immunosorbent assay to determine exposure to infection.

DESIGN

A longitudinal population study in groups of Thoroughbred weanling foals.

STUDY POPULATION

Two hundred weanling Thoroughbred foals from a population of about 380 foals were enrolled on two adjacent stud farms in the Hunter Valley of New South Wales. Foals on both farms were weaned from February to May 1995 into randomly selected groups of 10 to 15 foals. Farms were selected because of their willingness to cooperate in the survey and because their detailed records of foals and their movements. They were representative of well-managed large Thoroughbred stud farms in New South Wales. Both studs had upper respiratory tract disease among weanling foals around weaning each year although the seroprevalence of viral respiratory disease on either farm was not known before the study.

PROCEDURE

Serum was collected from foals within each group at fortnightly intervals from 9th February until 1st June 1995, and at a single follow-up period in August 1995. Each sample was tested in triplicate using an antibody-detection ELISA which is type-specific for EHV-1 and EHV-4 antibodies.

RESULTS AND CONCLUSIONS

There was serological evidence of EHV-1 infection both before and after weaning. The prevalence of EHV-1 antibody in the sample population increased during the study and individual cases of EHV-1 infection were identified. The increase was caused both by the seroconversion of foals within the groups and by the recruitment into the study of foals with pre-existing EHV-1 antibody. Evidence of EHV-1 infection in Thoroughbred foals after weaning has not been reported previously in Australia and this has implications for vaccination regimens.

Primary and challenge infection of mice with equine herpesvirus 1, strain HSV25A

Clinical signs, haematology, lymphocyte subset analysis, viral clearance, lung histopathology and humoral and cell-mediated (CMI) immune responses were monitored throughout the acute and convalescent phases of infection in groups of BALB/c mice infected intranasally with equine herpesvirus 1 (EHV-1), strain HSV25A. Primary infection caused a leucocytosis due to a neutrophilia during days 1 and 2 post-infection (pi) and a B lymphocytosis at day 1 pi. Serum ELISA antibodies were detected by 7 days pi and neutralising antibodies by 2 weeks pi. Mice infected with EHV-1 were not protected against disease when challenged with EHV-1 12 weeks later. However, viral clearance from lungs was significantly faster and the antibody response was markedly enhanced within the first few days of challenge infection. A CMI response was detected by 5 days after primary infection, but the level of responsiveness was not increased by challenge infection, although the lungs of challenged mice had markedly increased numbers of mononuclear cells around blood vessels and bronchioles. Specific antibodies to glycoprotein (g) B were detected by 2 weeks pi, 4 weeks earlier than the detection of antibodies to gC and 10 weeks before those to gD. The primary response was relatively short-lived with neither ELISA antibody nor lymphocyte proliferation was evident by 6 months pi.

Protective effects of equine herpesvirus-1 (EHV-1) glycoprotein B in a murine model of EHV-1-induced abortion

In an attempt to determine if pregnant mice could be protected from abortion subsequent to challenge with equine herpesvirus-1 (EHV-1) in the mouse model of EHV-1 disease, female BALB/c mice were inoculated with baculovirus-expressed EHV-1 glycoprotein B (bac-gB), wild-type baculovirus (bac-wt), rabbit kidney (RK-13) or baby hamster kidney (BHK-21) cells. Using an ELISA, antibodies against EHV-1 were detected in the serum of mice following two injections of bac-gB and were enhanced by a third injection, after which low levels of neutralising antibody were also detected. After mating, mice in the bac-gB, bac-wt and RK-immunised groups were infected intranasally with 10(7) pfu of EHV-1 on day 16 of pregnancy. All challenged mice experienced body weight loss post-infection (pi). However, postnatally, the gB-immunised group demonstrated body weight gain which was not seen in the other groups. There were no maternal deaths in the gB-immunised group but 1/6 bac-wt-immunised and 3/6 RK-immunised mice died post-challenge. Litter survival rate was significantly higher (p < 0.001) for the gB-immunised dams (54%) than that of either the bac-wt-(9%) or RK-immunised (0%) dams and the mean body weight of young from the surviving bac-wt-immunised litter was significantly (p = 0.021) lower than either the gB-immunised group or the BHK-immunised unchallenged group at 10 days of age. The virus was not isolated from any foetus from a gB-immunised dam. However, the virus was detected in 9% of foetuses from bac-wt-immunised and 21% of foetuses from RK-immunised dams.

(E)-5-(2'-bromovinyl)-2'-deoxyuridine inhibition of macropodid herpesvirus 1 in vitro

Herpesviruses have caused the death of kangaroos and wallabies in European and North American zoos. Eight antiherpetic purine or pyrmidine nucleoside compounds were tested in plaque reduction neutralization tests for in vitro inhibition of macropodid herpesvirus 1, a virus that has been associated with illness in captive macropods in Australia. The virus was most susceptible to inhibition by (E)-5-(2'-bromovinyl)-2'-deoxyuridine (BVDU) and 5'-iodo-2'-deoxycytidine. Because BVDU effectively inhibits macropodid herpesvirus 2 in vitro, it may be the drug of choice for experimental therapy in herpesvirus infections in captive macropodids.

Immune responses and protective efficacy of recombinant baculovirus-expressed glycoproteins of equine herpesvirus 1 (EHV-1) gB, gC and gD alone or in combinations in BALB/c mice

Baculovirus-expressed glycoproteins of EHV-1 gB, gC and gD alone or in combination evoked antibody responses and protected vaccinated mice against a challenge with EHV-1. gB, gD, gB + gC, gB + gD and gC + gD elicited very high levels of ELISA antibodies while gC and gC + gD elicited high levels of virus neutralising antibodies. Western blotting demonstrated that the antibodies produced were not only specific for the baculovirus-expressed glycoproteins gB, gC and gD, but also highly specific for each EHV-1 glycoprotein. Vaccination of mice with gB or gD prevented clinical signs of infection in mice challenged with EHV-1 and all vaccinated groups of mice except controls showed a rapid clearance of virus from the lungs and a reduction in lesions characteristic of herpesviruses in the lungs post-challenge. Notably, the lungs of mice vaccinated with gB, gD or gB + gD and challenged with EHV-1 showed prominent peribronchiolar and perivascular aggregations of mononuclear cells, predominantly lymphocytes. Immunocytochemical staining of these sections showed large numbers of T cells, suggesting an active role for these cells at the site of virus replication post-challenge.

Study of the protective immunity of co-expressed glycoprotein H and L of equine herpesvirus-1 in a murine intranasal infection model

Equine herpesvirus-1 (EHV-1) glycoproteins H, and L (gH and gL) expressed individually or co-expressed by recombinant baculoviruses were used to immunise BALB/c mice prior to intranasal challenge in a murine model of respiratory infection. Only the co-expressed material (EHV-1 gH/gL) induced neutralising antibody (low levels). The same immunogen also produced the strongest cellular responses. Immunisation with gH/gL and, to a lesser extent, with gH alone was associated with a reduction of virus load in nasal turbinates and olfactory bulbs after challenge infection. Viraemia, detected by polymerase chain reaction, was also reduced. No such protective effects were observed for gL alone. Adoptive transfer of lymphocytes from gH/gL-immunised mice to näive mice subsequently challenged with EHV-1 indicated that both CD4+ and CD8+ cells had a role in protective immunity. Although clearance of EHV-1 from respiratory tissue was not as effective as previously found for glycoproteins D or C, these experiments provide evidence that the co-expression of EHV-1 gL with gH generates a conformational neutralising epitope which is not present in either molecule alone, and suggests that gH/gL antigen may have a better potential as a component of an EHV-1 vaccine than gH alone.

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.

Expression and characterization of equine herpesvirus 1 glycoprotein D in mammalian cell lines

Equine herpesvirus 1 glycoprotein D (EHV-1 gD) expressed constitutively in mammalian cell lines had similar electrophoretic mobility to gD produced in EHV-1 infected cells but lacked a possibly complexed higher molecular weight form seen in the latter. Recombinant gD was N-terminally cleaved at the same site as gD in EHV-1 infected cells and expression was associated with enhanced levels of cell-cell fusion, indicating a role for EHV-1 gD in cell-to-cell transmission of virus.

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.

Evidence for involvement of equine herpesvirus 1 glycoprotein B in cell-cell fusion

Monoclonal antibodies specific for equine herpesvirus 1 (EHV-1) glycoproteins (gB, gD, gp2 and a cleaved translation product of gene 71) were tested for ability to inhibit cell-cell fusion as measured by syncytium formation in EHV-1 infected cell cultures. Syncytium formation was inhibited by a complement-dependent neutralising antibody (7B10) which recognised the large subunit of EHV-1 gB. This indicated that EHV-1 gB, in common with gB homologues of herpes simplex virus and other herpesviruses, plays a role in the cell-cell fusion process.

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.

Characteristics of equine herpesvirus 1 glycoproteins expressed in insect cells

A series of recombinant baculoviruses containing genes for glycoproteins C, D, H and L of equine herpesvirus 1 (EHV-1) have been constructed, and the EHV-1 products characterised by gel electrophoresis and immunoblotting. The EHV-1 glycoproteins expressed in insect cells were similar but not identical in apparent sizes to those expressed in EHV-1 infected mammalian cells. Each of the EHV-1 products was recognised by convalescent equine sera, indicating that they were all targets for an equine immune response. Mice immunised with baculovirus-expressed EHV-1 gD and gC acquired an enhanced ability to clear challenge EHV-1 from respiratory tissues, in association with both neutralising antibody and cell mediated immune responses.

Immunization with glycoprotein C of equine herpesvirus-1 is associated with accelerated virus clearance in a murine model

The glycoprotein C (gC) gene of equine herpesvirus-1 (EHV-1) was expressed in insect cells by a recombinant baculovirus as several products with apparent molecular weights of 66 kDa-80 kDa. The baculovirus EHV-1 gC products were recognised by monoclonal antibody and by EHV-1 convalescent equine sera, indicating conservation of antigenic determinants and confirming this glycoprotein as a target for the equine immune system. Mice immunized with recombinant EHV-1 gC showed accelerated clearance of EHV-1 from respiratory tissues following intranasal challenge. Virus clearance was accompanied by virus specific antibodies and by cell mediated immune responses measured by a delayed type hypersensitivity reaction and lymphocyte stimulation by killed EHV-1 as antigen.

Expression and characterisation of equine herpesvirus 1 glycoprotein H using a recombinant baculovirus

A recombinant baculovirus capable of expressing the glycoprotein H (gH) gene of equine herpesvirus 1 (EHV-1) was constructed. EHV-1 gH gene products in recombinant baculovirus infected insect cells were identified as 105 kDa and 110 kDa species compared with a 115 kDa product detected in EHV-1 infected mammalian cells. The extent of N-glycosylation of EHV-1 gH in both insect and mammalian cells was indicated by a shift in apparent molecular weights after PNGase F treatment to 90 kDa and 95 kDa forms, which compared with the predicted value of 90 kDa for the unglycosylated polypeptide. The recombinant EHV-1 gH was recognised by equine sera demonstrating that EHV-1 gH is a target for the immune system of the natural host. However, while the recombinant EHV-1 gH product from infected insect cells was immunogenic in mice, it did not induce a neutralizing antibody response against EHV-1.

Characterization of immune responses to baculovirus-expressed equine herpesvirus type 1 glycoproteins D and H in a murine model

A murine intranasal infection model for equine herpesvirus type 1 (EHV-1) was used to evaluate immune responses following immunization with insect cells infected by baculoviruses that express EHV-1 glycoproteins. Baculovirus recombinant glycoprotein D (gD) and gH both induced serum antibodies to EHV-1 when measured by ELISA. The gD recombinant also produced a neutralizing antibody response. Protective immunity, determined by accelerated clearance of virus from the target organs in the respiratory tract, was demonstrated in mice immunized with a baculovirus recombinant expressing gD. In addition to the serological response, evidence is presented which shows that cell-mediated responses also play an important role in protection. Both recombinants induced delayed-type hypersensitivity and lymphoproliferation to EHV-1 antigen. The protective effects of T cells were confirmed by adoptive transfer of spleen cells from baculovirus gD-immunized donors to recipients that were challenged with live EHV-1. Depletion of either CD4- or CD8-bearing cells from the gD-immunized donors reduced the ability of the recipients to clear virus from the target organs, although depletion of CD4 cells had a more marked effect.

Rapid, single-step differentiation of equid herpesviruses 1 and 4 from clinical material using the polymerase chain reaction and virus-specific primers

Sets of primers were designed which enabled specific amplification of homologous regions of the glycoprotein C and gene 76 genetic loci of equine herpesviruses 1 and 4 (EHV-1 and EHV-4). The resultant virus-specific polymerase chain reaction (PCR) products arising from each loci could be discriminated easily on the basis of size on an agarose gel, allowing rapid differentiation of the two equine herpesviruses. Specificity of the amplifications were confirmed by Southern hybridization and restriction endonuclease digestion. The PCR test was applied to nasal swab samples from weanling foals and to archival aborted fetal tissue samples and the results compared to those obtained by virus isolation. A strong correlation was found between this PCR assay and virus isolation methods of EHV-1 and EHV-4 detection and discrimination.

Epidemiological investigation of equid herpesvirus-4 (EHV-4) excretion assessed by nasal swabs taken from thoroughbred foals

Equid herpesvirus-4 (EHV-4) was detected in nasal swabs taken from foals using a PCR based test and this information used to study the epidemiology of EHV-4 disease on three Australian Thoroughbred stud farms in NSW in 1992. There was a very high level of agreement (kappa value of 0.84) between the PCR results and virus isolation using cell culture techniques. There was a strong seasonal distribution of EHV-4 shedding. Twenty-five of 26 positive samples were collected in January and March with the remaining positive sample collected in February. Foals with clinical signs of upper respiratory tract infection per se were no more likely to be shedders of EHV-4 (odds ratio [OR] 1.4, 95% confidence limits [CL] 0.5-3.8). However, EHV-4 was more likely to be isolated from foals exhibiting copious serous or mucopurulent nasal discharge than those with no clinical signs (OR 4.6, 95% CL 1.1-19.0 and OR 2.5, 95% CL 0.8-8.0, respectively). The month of the year was more important than weaning or age as a risk factor for excretion of EHV-4. Male foals and those with a history of respiratory disease that had required veterinary treatment were more likely to shed EHV-4.

Expression of equine herpesvirus 1 glycoprotein D by using a recombinant baculovirus

Glycoprotein D (gD) of equine herpesvirus 1 (EHV-1) was expressed at the surface of insect cells infected by a recombinant baculovirus. EHV-1 gD was detected as multiple forms (56, 52, and 48 kDa) from 18 to 96 h postinfection. Laboratory animals inoculated with the recombinant EHV-1 gD developed neutralizing antibody responses against both EHV-1 and EHV-4.

Herpesvirus ICP18.5 and DNA-binding protein genes are conserved in equine herpesvirus-1

The genome of equine herpesvirus-1 (EHV-1) contained three open reading frames (ORFs) in a 3.9 kbp BamHI-SmaI fragment at 0.38-0.41 map units in the long unique region. The most 5' ORF encoded the carboxy terminus of a protein with 45-55 percent amino acid homology to the DNA-binding proteins (ICP8-DBP) of four other alpha-herpesviruses. The middle ORF translated to a polypeptide of 775 residues with 43-55% homology to the ICP18.5 proteins. The most 3' ORF encoded the EHV-1 glycoprotein B (gB) gene. Three mRNAs of 4.3, 4.4-4.8, and 3.5-3.9 kb (corresponding to the three sequenced ORFs) were all transcribed from the same strand. The gene order of this group was conserved in all herpesviruses examined.

Characterization of the glycoprotein D gene products of equine herpesvirus 1 using a prokaryotic cell expression vector

The gene encoding equine herpesvirus 1 (equine abortion virus; EHV-1) glycoprotein D was engineered into the prokaryotic vector pEX, and expressed as a beta-galactosidase fusion product, which was recognized by pooled equine sera and anti-EHV-1 rabbit sera. Antibodies raised against the EHV-1 gD fusion product identified strong bands in infected cells at 66 and 68 K and at 138 K in purified virus, thus characterizing the several forms of this major envelope glycoprotein which is an important candidate for inclusion in subunit vaccines.