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

Equine Herpesvirus-I Infection in Horses: Recent Updates on its Pathogenicity, Vaccination, and Preventive Management Strategies

Equine herpesvirus-1 (EHV-1) is one of the most common and ubiquitous viral pathogens infecting equines, particularly horses worldwide. The EHV-1 is known to induce not only humoral but also cellular immune responses in horses. Respiratory distress, abortion in pregnant mares, neurological disorders, and neonatal foal deaths represent EHV-1 infection. Despite the limited success of inactivated, subunit, live, and DNA vaccines, over the past few decades, vaccination remains the prime preventive option to combat EHV-1 infection in horses. However, current vaccines lack the potentiality to protect the neurological form of infections in horses. There is desperate necessity to search effectual EHV-1 vaccines that may stimulate not only mucosal and systemic cellular immunity but also humoral immunity in the horses. This review highlights the state of knowledge regarding EHV-1 biology, EHV-1 pathogenesis, and disparate vaccines studied in the past to prevent EHV-1 infection. The review also underlines the best management strategies which certainly need to be adopted by veterinarians in order to avoid and prevent EHV-1 infection and outbreak in horses in the future.

TLR-5 agonist Salmonella abortus equi flagellin FliC enhances FliC-gD-based DNA vaccination against equine herpesvirus 1 infection

Equine herpesvirus 1 (EHV-1) induces serious respiratory infections, viral abortion, neurological signs, and neonatal mortality in horses. Despite the use of vaccines, EHV-1 infection also causes a high annual economic burden to the equine industry. The poor immunogenicity of and protection conferred by EHV-1 vaccines are the major factors responsible for the spread of EHV-1 infection. The present study examined the immunogenicity of a novel DNA vaccine co-expressing FliC, a flagellin protein, in Salmonella abortus equi and the gD protein of EHV-1. Mice and horses were immunized intramuscularly with the vaccine, and mice were challenged with EHV-1. Immunofluorescence and western blotting revealed that FliC and gD can be efficiently expressed in cells. This novel vaccine significantly increased gD-specific antibody and interferon gamma (IFN-γ) levels in immunized mice and horses. Compared with controls, the viral load and morbidity were markedly reduced in FliC-gD-immunized mice after they were challenged with EHV-1. Furthermore, the immunogenicity of FliC-gD in a natural host was tested. Our results indicate that vaccinated mice and horses exhibit increased humoral and improved cellular immune responses.

Immunoprotective response induced by recombinant glycoprotein D in the BALB/c respiratory mouse model of Equid alphaherpesvirus 1 infection

Equid alphaherpesvirus 1 (EHV-1) infection causes abortion, respiratory disease, perinatal deaths and neurological disorders in horses. The natural infection and available vaccines provide only partial and short-lived protection against reinfections. In the present study, we analyzed the ability of purified baculovirus-expressed glycoprotein D (gD) administered by different routes to induce protective immunity in BALB/c mice after challenge with the EHV-1 AR8 strain. Clinical signs varied among the different groups of mice immunized by parenteral routes, and, although gD induced a specific serum IgG response, it did not prevent the virus from reaching the lungs. Intranasally immunized mice showed no clinical signs, and virus isolation from lungs, histological lesions and antigen detection by immunohistochemistry were negative. In addition, by this route, gD did not stimulate the production of serum IgG and IgA. However, a specific IgA response in the respiratory tract was confirmed in intranasally immunized mice. Thus, we conclude that the mucosal immune response could reduce the initial viral attachment and prevent the virus from reaching the lungs. Our findings provide additional data to further study new immunization strategies in the natural host.

Immunization with Attenuated Equine Herpesvirus 1 Strain KyA Induces Innate Immune Responses That Protect Mice from Lethal Challenge

Equine herpesvirus 1 (EHV-1) is a major pathogen affecting equines worldwide. The virus causes respiratory disease, abortion, and, in some cases, neurological disease. EHV-1 strain KyA is attenuated in the mouse and equine, whereas wild-type strain RacL11 induces severe inflammation of the lung, causing infected mice to succumb at 4 to 6 days postinfection. Our previous results showed that KyA immunization protected CBA mice from pathogenic RacL11 challenge at 2 and 4 weeks postimmunization and that KyA infection elicited protective humoral and cell-mediated immune responses. To investigate the protective mechanisms of innate immune responses to KyA, KyA-immunized mice were challenged with RacL11 at various times postvaccination. KyA immunization protected mice from RacL11 challenge at 1 to 7 days postimmunization. Immunized mice lost less than 10% of their body weight and rapidly regained weight. Virus titers in the lungs of KyA-immunized mice were 1,000-fold lower at 2 days post-RacL11 challenge than virus titers in the lungs of nonimmunized mice, indicating accelerated virus clearance. Affymetrix microarray analysis revealed that gamma interferon (IFN-γ) and 16 antiviral interferon-stimulated genes (ISGs) were upregulated 3.1- to 48.2-fold at 8 h postchallenge in the lungs of RacL11-challenged mice that had been immunized with KyA. Murine IFN-γ inhibited EHV-1 infection of murine alveolar macrophages and protected mice against lethal EHV-1 challenge, suggesting that IFN-γ expression is important in mediating the protection elicited by KyA immunization. These results suggest that EHV-1 KyA may be used as a live attenuated EHV-1 vaccine as well as a prophylactic agent in horses.

IMPORTANCE

Viral infection of cells initiates a signal cascade of events that ultimately attempts to limit viral replication and prevent infection through the expression of host antiviral proteins. In this study, we show that EHV-1 KyA immunization effectively protected CBA mice from pathogenic RacL11 challenge at 1 to 7 days postvaccination and increased the expression of IFN-γ and 16 antiviral interferon-stimulated genes (ISGs). The administration of IFN-γ blocked EHV-1 replication in murine alveolar macrophages and mouse lungs and protected mice from lethal challenge. To our knowledge, this is the first report of an attenuated EHV-1 vaccine that protects the animal at 1 to 7 days postimmunization by innate immune responses. Our findings suggested that IFN-γ serves as a novel prophylactic agent and may offer new strategies for the development of anti-EHV-1 agents in the equine.

Antibody and cellular immune responses following DNA vaccination and EHV-1 infection of ponies

Equine herpesvirus-1 (EHV-1) is the cause of serious disease with high economic impact on the horse industry, as outbreaks of EHV-1 disease occur every year despite the frequent use of vaccines. Cytotoxic T-lymphocytes (CTLs) are important for protection from primary and reactivating latent EHV-1 infection. DNA vaccination is a powerful technique for stimulating CTLs, and the aim of this study was to assess antibody and cellular immune responses and protection resulting from DNA vaccination of ponies with combinations of EHV-1 genes. Fifteen ponies were divided into three groups of five ponies each. Two vaccination groups were DNA vaccinated on four different occasions with combinations of plasmids encoding the gB, gC, and gD glycoproteins or plasmids encoding the immediate early (IE) and early proteins (UL5) of EHV-1, using the PowderJect XR research device. Total dose of DNA/plasmid/vaccination were 25 microg. A third group comprised unvaccinated control ponies. All ponies were challenge infected with EHV-1 6 weeks after the last vaccination, and protection from clinical disease, viral shedding, and viremia was determined. Virus neutralizing antibodies and isotype specific antibody responses against whole EHV-1 did not increase in either vaccination group in response to vaccination. However, glycoprotein gene vaccinated ponies showed gD and gC specific antibody responses. Vaccination did not affect EHV-1 specific lymphoproliferative or CTL responses. Following challenge infection with EHV-1, ponies in all three groups showed clinical signs of disease. EHV-1 specific CTLs, proliferative responses, and antibody responses increased significantly in all three groups following challenge infection. In summary, particle-mediated EHV-1 DNA vaccination induced limited immune responses and protection. Future vaccination strategies must focus on generating stronger CTL responses.

Equine herpesvirus 1 glycoprotein D expressed in E. coli provides partial protection against equine herpesvirus infection in mice and elicits virus-neutralizing antibodies in the horse

The envelope glycoprotein D of EHV-1 (EHV-1 gD) is essential for virus infectivity and entry of virus into cells and is a potent inducer of virus-neutralizing antibody. In this study, truncated EHV-1 gD (gDt) was expressed with a C-terminal hexahistidine tag in E. coli using a pET vector. Western blot analysis using an anti-gD monoclonal antibody demonstrated the presence of gDt bands at 37.5, 36, 29.5 and 28 kDa. The immunogenicity and protective efficacy of partially purified gDt was compared with gD expressed in insect cells by a recombinant baculovirus (Bac gD) using a BALB/c mouse model of EHV-1 respiratory infection. The proteins were also compared in a prime-boost protocol following an initial inoculation with gD DNA. gDt elicited similar levels of gD-specific antibody and neutralizing antibody compared with Bac gD and also provided a similar level of protection against EHV-1 challenge in mice. Inoculation of horses with gDt elicited EHV-1 gD-specific antibodies including virus-neutralizing antibody, suggesting that despite the lack of glycosylation, E. coli may be a useful vehicle for large scale production of EHV-1 gD for vaccine studies.

Inoculation of mares and very young foals with EHV-1 glycoproteins D and B reduces virus shedding following respiratory challenge with EHV-1

We have previously demonstrated that intramuscular inoculation of EHV-1 glycoprotein D (gD) and glycoprotein B (gB) produced by a recombinant baculovirus and formulated with the adjuvant Iscomatrix elicited virus-neutralizing antibody and gD- and gB-specific ELISA antibody in adult horses. In this study, 14 mares and their very young foals were inoculated with a combination of baculovirus-expressed EHV-1 gD and EHV-1 gB (EHV-1 gDBr) and challenged with a respiratory strain of EHV-1. Following experimental challenge, inoculated mares and foals shed virus in nasal secretions on significantly fewer occasions compared to uninoculated mares and foals. Uninoculated foals born from inoculated mares were no more protected against experimental challenge than uninoculated foals born from uninoculated mares. The results suggest that it is indeed possible to induce partial protection in very young foals through vaccination, and while the inoculation did not prevent infection, it did reduce the frequency of viral shedding with the potential to thereby reduce the risk and prevalence of infection in a herd situation.

Purification of phocid herpesvirus type 1 glycoproteins B and D and pilot studies of immunogenicity in mice

In the process of developing a subunit vaccine against phocid herpesvirus type 1, we have cloned and expressed the glycoproteins B and D (gB and gD) of phicid herpesvirus type 1, using an eukaryotic baculovirus expression system. To establish the proof of concept, candidate iscom vaccines based on these affinity-purified proteins either alone or in combination, were tested for their immunogenicity in BALB/C mice. Mice immunised with a combination of gB and gD developed higher antibody and proliferative T cell responses against PhHV-1 than those immunised with gB or gD alone. The corresponding antibody and T cell proliferative responses were higher against PhHV-1 than against FHV. These data favour further testing of these candidate vaccines based on gB and gD in the FHV-cat model.

Equine herpesviruses 1 (EHV-1) and 4 (EHV-4) – epidemiology, disease and immunoprophylaxis: A brief review

This review concentrates on the epidemiology, latency and pathogenesis of, and the approaches taken to control infection of horses by equine herpesvirus types 1 (EHV-1) and 4 (EHV-4). Although both viruses may cause febrile rhinopneumonitis, EHV-1 is the main cause of abortions, paresis and neonatal foal deaths. The lesion central to these three conditions is necrotising vasculitis and thrombosis resulting from lytic infection of endothelial cells lining blood capillaries. The initiation of infection in these lesions is likely to be by reactivated EHV-1 from latently infected leukocytes. However, host factors responsible for reactivation remain poorly understood. While vaccine development against these important viruses of equines involving classical and modern approaches has been ongoing for over five decades, progress, compared to other alpha herpesviruses of veterinary importance affecting cattle and pigs, has been slow. However recent data with a live temperature sensitive EHV-1 vaccine show promise.

Serum antibody responses to equine herpesvirus 1 glycoprotein D in horses, pregnant mares and young foals

The envelope glycoprotein D of equine herpesvirus 1 (EHV-1 gD) has been shown in laboratory animal models to elicit protective immune responses against EHV-1 challenge, and hence is a potential vaccine antigen. Here we report that intramuscular inoculation of EHV-1 gD produced by a recombinant baculovirus and formulated with the adjuvant Iscomatrix elicited virus-neutralizing antibody and gD-specific ELISA antibody in the serum of over 90% of adult mixed breed horses. The virus-neutralizing antibody responses to EHV-1 gD were similar to those observed after inoculation with a commercially available killed EHV-1/4 whole virus vaccine. Intramuscular inoculation of EHV-1 gD DNA encoded in a mammalian expression vector was less effective in inducing antibody responses when administered as the sole immunogen, but inoculation with EHV-1 gD DNA followed by recombinant EHV-1 gD induced increased gD ELISA and virus-neutralizing antibody titres in six out of seven horses. However, these titres were not higher than those induced by either EHV-1 gD or the whole virus vaccine. Isotype analysis revealed elevated gD-specific equine IgGa and IgGb relative to IgGc, IgG(T) and IgA in horses inoculated with EHV-1 gD or with the whole virus vaccine. Following inoculation of pregnant mares with EHV-1 gD, their foals had significantly higher levels of colostrally derived anti-gD antibody than foals out of uninoculated mares. The EHV-1 gD preparation did not induce a significant mean antibody response in neonatal foals following inoculation at 12 h post-partum and at 30 days of age, irrespective of the antibody status of the mare. The ability of EHV-1 gD to evoke comparable neutralizing antibody responses in horses to those of a whole virus vaccine confirms EHV-1 gD as a promising candidate for inclusion in subunit vaccines against EHV-1.

Cytokine profiles and long-term virus-specific antibodies following immunization of CBA mice with equine herpesvirus 1 and viral glycoprotein D

Equine herpesvirus 1 (EHV-1)-specific antibody-secreting cells (ASC) isolated from the lung and spleen of mice at 12 months after immunization with attenuated EHV-1 KyA, heat-killed KyA, or recombinant viral glycoprotein D (rgD) assessed by ELISPOT showed a three- to fivefold increase in three immunoglobulin isotypes at 3 days post-challenge with pathogenic EHV-1 RacL11 as compared to control mice. ELISPOT assays demonstrated a high frequency of cells secreting proinflammatory tumor necrosis factor-alpha (TNF-alpha), interferon gamma (IFN-gamma), and interleukin 4 (IL-4) in the lungs in response to infection with KyA or RacL11 or immunization with rgD. Cytokine production elicited by EHV-1 KyA or RacL11 infection revealed similar frequencies of EHV-1-specific IFN-gamma and IL-4 spot forming cells in the mediastinal lymph nodes and spleen. However, KyA induced significantly greater amounts of IFN-gamma producing cells in the lungs than did RacL11. Intranasal immunization with KyA or rgD induced long-term immunity that provided protection against pathogenic EHV-1 challenge infection at 12 months post-immunization. Overall, the data indicate that immunization with infectious KyA or rgD induces significant levels of cytokines, virus-specific ASC in the lungs and spleen, and long-term virus specific B-cell responses.

A protective effect of epidermal powder immunization in a mouse model of equine herpesvirus-1 infection

To evaluate the protective effect of epidermal powder immunization (EPI) against equine herpesvirus-1 (EHV-1) infection, we prepared a powder vaccine in which formalin-inactivated virions were embedded in water-soluble, sugar-based particles. A PowderJect device was used to immunize mice with the powder vaccine via their abdominal skin. We found that twice-immunized mice were protected against challenge with the wild-type virus. This protective effect was equivalent to or better than that observed in mice immunized with other types of vaccines, including a gene gun-mediated DNA vaccine containing the glycoprotein D (gD) gene or conventional inactivated virus vaccines introduced via intramuscular or intranasal injections. These findings indicate that the powder vaccine is a promising approach for the immunological control of EHV-1 infection, either alone or as a part of prime-boost vaccination strategies.

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.

Identification of equine herpesvirus-1 antigens recognized by cytotoxic T lymphocytes

Equine herpesvirus-1 (EHV-1) causes serious disease in horses throughout the world, despite the frequent use of vaccines. CTLs are thought to be critical for protection from primary and reactivating latent EHV-1 infections. However, the antigen-specificity of EHV-1-specific CTLs is unknown. The aim of this study was to identify EHV-1 genes that encode proteins containing CTL epitopes and to determine their MHC I (or ELA-A in the horse) restriction. Equine dendritic cells, transfected with a series of EHV-1 genes, were used to stimulate autologous CTL precursor populations derived from previously infected horses. Cytotoxicity was subsequently measured against EHV-1-infected PWM lymphoblast targets. Dendritic cells were infected with EHV-1 (positive control) or transfected with plasmids encoding the gB, gC, gD, gE, gH, gI, gL, immediate-early (IE) or early protein of EHV-1 using the PowderJect XR-1 research device. Dendritic cells transfected with the IE gene induced CTL responses in four of six ponies. All four of these ponies shared a common ELA-A3.1 haplotype. Dendritic cells transfected with gC, gD, gI and gL glycoproteins induced CTLs in individual ponies. The cytotoxic activity was ELA-A-restricted, as heterologous targets from ELA-A mismatched ponies were not killed and an MHC I blocking antibody reduced EHV-1-specific killing. This is the first identification of an EHV-1 protein containing ELA-A-restricted CTL epitopes. This assay can now be used to study CTL specificity for EHV-1 proteins in horses with a broad range of ELA-A haplotypes, with the goal of developing a multi-epitope EHV-1 vaccine.

Equine herpesvirus 1 glycoprotein D expressed in Pichia pastoris is hyperglycosylated and elicits a protective immune response in the mouse model of EHV-1 disease

Equine herpesvirus 1 glycoprotein D (EHV-1 gD) has been shown in mouse models and in the natural host to have potential as a subunit vaccine, using various expression systems that included Escherichia coli, baculovirus and plasmid DNA. With the aim of producing secreted recombinant protein, we have cloned and expressed EHV-1 gD, lacking its native signal sequence and C-terminal transmembrane region, into the methylotrophic yeast Pichia pastoris. The truncated glycoprotein D (gD) gene was placed under the control of the methanol inducible alcohol oxidase 1 promoter and directed for secretion with the Saccharomyces cerevisiae alpha-factor prepro secretion signal. SDS-PAGE and Western blot analysis of culture supernatant fluid 24 h after induction revealed gD-specific protein products between 40 and 200 kDa. After treatment with PNGase F and Endo H, three predominant bands of 34, 45 and 48 kDa were detected, confirming high mannose N-linked glycosylation of Pichia-expressed gD (Pic-gD). N-terminal sequence analysis of PNGase F-treated affinity-purified protein showed that the native signal cleavage site of gD was being recognised by P. pastoris and the 34 kDa band could be explained by internal proteolytic cleavage effected by a putative Kex2-like protease. Pic-gD, when used in a DNA prime/protein boost inoculation schedule, induced high EHV-1 ELISA and virus neutralizing antibodies and provided protection from challenge infection in BALB/c mice.

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.

Solid matrix-antibody-antigen complexes incorporating equine herpesvirus 1 glycoproteins C and D elicit anti-viral immune responses in BALB/c (H-2K(d)) and C3H (H-2K(k)) mice

Glycoproteins C and D (gC and gD) derived from equine herpesvirus 1 (EHV-1)-infected cells were incorporated into individual solid matrix-antibody-antigen (SMAA) complexes and administered to BALB/c (H-2K(d)) and C3H (H-2K(k)) mice. Antibodies against each of the glycoproteins were produced that neutralised virus infectivity and mediated the lysis of EHV-1-infected target cells in the presence of complement. Immunoglobulin (Ig)G2b was the predominant antibody isotype produced in BALB/c mice against gC, while equal amounts of IgG2a/2b were found in the serum of C3H mice (indicative of a T-helper(1) response). Glycoprotein D immunisation elicited predominantly an IgG1 response in BALB/c mice (indicative of a T-helper(2) response) and an IgG2a/2b response in C3H mice. EHV-1-specific local and systemic T-cell proliferative responses were detected in vitro following administration of SMAA complexes. Suppression of the local T-cell response was seen following virus challenge of mice immunised with SMAA gC. SMAA gD provided some protection against intranasal EHV-1 challenge. These data show that the SMAA system is an effective way of presenting subviral components to the immune system and further emphasises the importance of including glycoprotein D as a component of a subunit EHV-1 vaccine.

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.

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.

A prime-boost immunization strategy with DNA and recombinant baculovirus-expressed protein enhances protective immunogenicity of glycoprotein D of equine herpesvirus 1 in naïve and infection-primed mice

The immunogenicity and protective efficacy afforded by intramuscular inoculation of plasmid DNA encoding equine herpesvirus 1 (EHV-1) glycoprotein D (gD) followed by EHV-1 gD expressed by a recombinant baculovirus was assessed in a murine model of EHV-1 respiratory infection. Compared with mice inoculated with DNA or protein only, mice inoculated with the combination of gD DNA and protein had enhanced ELISA and neutralizing antibody titres to EHV-1 and had accelerated clearance of virus from lungs following challenge infection. The enhanced protective effects of this consecutive immunization were also evident in mice which had a previous infection with EHV-1 and had pre-existing antibodies. The T-helper 1 (Th1) type of immune response induced by EHV-1 gD DNA was maintained after the protein boost, despite the gD protein alone appearing to direct a Th2 response.

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.

Potential of DNA-mediated vaccination for equine herpesvirus 1

The potential of DNA-mediated immunisation to protect against equine herpesvirus 1 (EHV-1) disease was assessed in a murine model of EHV-1 respiratory infection. Intramuscular injection with DNA encoding the EHV-1 envelope glycoprotein D (gD) in a mammalian expression vector induced a specific antibody response detectable by two weeks and maintained through 23 weeks post injection. Immune responses were proportional to the dose of DNA and a second injection markedly enhanced the antibody response. EHV-1 gD DNA-injected mice developed neutralising antibodies, and a predominance of IgG2a antibodies after the DNA injection was consistent with the generation of a type 1 helper T-cell (Th1) response. Following intranasal challenge with EHV-1, mice immunised with 50 microg of EHV-1 gD DNA were able to clear virus more rapidly from lung tissue and showed reduced lung pathology in comparison with control mice. The data indicate that DNA-mediated immunisation may be a useful strategy for vaccination against EHV-1.

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.

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.

Protective immunity against equine herpesvirus type-1 (EHV-1) infection in mice induced by recombinant EHV-1 gD

The ability of recombinant preparations of equine herpesvirus type 1 (EHV-1) glycoprotein D (gD) to elicit specific antibody and T lymphocyte responses in the BALB/c mouse model of respiratory infection was investigated. Recombinant gD (rgD) expressed as a glutathione-S-transferase (GST) fusion protein in Escherichia coli elicited both high titer neutralizing antibody (nAb) and CD4 T cell proliferative responses following subcutaneous or intranasal immunization, but elicited only a weak antibody response after intraperitoneal immunization. Protection against respiratory tract infection with pathogenic EHV-1 RacL11 was observed in mice immunized subcutaneously with GST-gD. Furthermore, the degree of protection correlated to the titer of nAb and the T cell response observed. Finally, GST-gD was more effective in protecting against respiratory RacL11 infection if delivered intranasally. These results confirm that gD plays an important role in eliciting the protective immune response against EHV-1 infection, and indicate that subunit vaccines containing preparations of gD may be very effective if delivered directly to the upper respiratory tract.

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.

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.

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

Equine herpesvirus 1 (EHV-1) mutants devoid of the open reading frames (ORFs) of either glycoprotein (g) B or M were constructed and tested for their immunogenic potential in a murine model of EHV-1 infection. The mutant viruses were engineered using the virulent EHV-1 strain RacL11 or the modified live vaccine strain RacH by inserting the Escherichia coli LacZ gene into the viral ORFs. RacL11-infected mice showed signs typical of an EHV-1 infection, whereas mice infected with the EHV-1 gB- or gM-negative mutants or with RacH did not develop disease. No difference in the pathogenic potential of RacL11 gB- and gM-negative viruses was observed after application of either phenotypically completed or negative viruses. However, revertant RacL11 viruses in which the gB or gM gene had been restored caused EHV-1-related symptoms that were indistinguishable from those induced by RacL11. Mice that had been immunized with phenotypically negative gB- and gM-deficient EHV-1 were challenged with the RacL11 virus 25 days after immunization. Mock-immunized mice developed EHV-1 disease and high virus loads in their lungs were observed. In contrast, mice developed not exhibit EHV-1-caused disease. It was concluded (i) that deletion of either gB or gM abolished the virulence of strain RacL11 and (ii) that immunization with gB- or gM-negative EHV-1 elicited a protective immunity that was reflected by both virus-neutralizing antibodies and EHV-1-specific T-cells in spleens of immunized mice.

High level expression of equine herpesvirus 1 glycoproteins D and H and their role in protection against virus challenge in the C3H (H-2Kk) murine model

N and C-terminal truncated forms of equine herpesvirus 1 (EHV 1) glycoproteins gD and gH were expressed in baculovirus resulting in the production of secreted recombinant proteins. A carboxy-terminal histidine tag was included on each of the genes for protein isolation by nickel affinity chromatography. Recombinant gD was recognized by three gD specific monoclonal antibodies, 20C4, 5H6 and F3132. F3132 is a conformationally dependent monoclonal antibody with virus neutralizing activity. Expression of gH was confirmed by reacting the protein with the gH peptide specific antiserum R319. The truncated gD gene was also expressed as a beta-galactosidase fusion protein which was purified from E. coli by nickel affinity chromatography. C3H mice were inoculated with purified recombinant gD or gH or insect cells which had been infected with recombinant baculoviruses. Mice were subsequently challenged with EHV 1. Purified recombinant baculovirus gD provided the most protection and produced high levels of virus neutralizing antibodies. The gD fusion protein was less effective at protecting mice and insect cells infected with either of the recombinant baculoviruses or purified recombinant gH were poor at conferring protection. The results emphasize the importance of using purified proteins in vaccine formulations and of including EHV 1 gD as a component of a subunit vaccine.

Baculovirus-expressed nonstructural protein NS2 of bluetongue virus induces a cytotoxic T-cell response in mice which affords partial protection

Virus-specific cytotoxic T lymphocytes were generated in two strains of mice (BALB/c and CBA/Ca) against baculovirus recombinant proteins (minor and nonstructural) derived from bluetongue virus serotype 10. Immunization of mice with recombinant baculovirus insect cell extracts expressing the nonstructural protein NS2 (Bac-NS2) conferred partial protection against infection with vaccinia virus expressing the NS2 protein. This protective immunity was mediated by CD8+ cells. In contrast, no protection was observed when mice were immunized with similarly expressed Bac-NS1 or -NS3 or the virion minor structural proteins (Bac-VP1, -VP4, or -VP6). Furthermore, the in vitro cytotoxicity activity of T cells derived from immunized animals did not correlate to the protective efficacy of baculovirus recombinant proteins. The implications of this work with regard to the design of noninfectious subunit vaccines are discussed.

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.

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.

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 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.

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.

Comparison of efficacies of famciclovir and valaciclovir against herpes simplex virus type 1 in a murine immunosuppression model

A mouse model of herpes simplex virus type 1 infection in an immunocompromised host was established by using cyclosporin-A to impair T-cell function. Following inoculation of herpes simplex virus type 1 into the skin of the ear pinna, cyclosporin-A prolonged virus replication in the skin and neural tissues compared with that in immunocompetent mice. This model was used to investigate the activity of famciclovir (FCV) and valaciclovir (VACV), which are oral products of the antiherpesvirus agents penciclovir and acyclovir, respectively. Both prodrugs gave similar blood profiles of the antiherpesvirus agents in normal and cyclosporin-treated mice. The compounds were administered by the oral route at 50 mg/kg per dose twice daily for 5 days. Both compounds were very effective at clearing infectious virus from the tissues despite the immunosuppression; FCV-treated animals cleared virus from the ear pinna more rapidly than VACV-treated animals. The areas under the concentration-time curve (AUC) for virus replication with time were reduced to 50 and 30% of control values for ear pinna and brain stem, respectively, with VACV therapy and to < 5% in both tissues by FCV. When treatment was continued to day 10, the reductions in AUC for ear and brain stem, respectively, were to 33 and 26% of control values with VACV and to < 3 and < 5% with FCV. However, on cessation of the antiviral treatment, there was a reproducible recurrence of infectious virus in the tissues obtained from VACV-treated mice. The recurrence of infectious virus was also evident after 10 days of treatment with VACV. In mice which had received FCV for 10 or 5 days, these was no resumption of virus replication in the ear pinna or brain stem. When dosing was reduced to once per day, both compounds were less effective at controlling the infection. Nevertheless, no recurrence of infectious virus was observed on cessation of FCV therapy.

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.