A mouse model for testing the pathogenicity of equine herpes virus-1 strains

Ensuring Viral Safety of Equine Immunoglobulins during Production

Equine blood plasma/serum and intermediates must be monitored for the presence of live viruses pathogenic in humans during production of equine immunoglobulins. Information concerning low-cost and simple methods for the detection of live horse viruses pathogenic and non-pathogenic to humans was gained using data of modern domestic and foreign literature. These methods are based on cultivation of these viruses on sensitive biosystems. The presented information can be used to set up blood plasma/serum control of horses at different stages of immunoglobulin production, i.e., when taking blood from horses during their quarantine period, when collecting blood from immunized horses, and before bottling the medicinal intermediate in the primary package.

Comparative Genomic Sequencing and Pathogenic Properties of Equine Herpesvirus 1 KyA and RacL11

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 Kentucky A (KyA) is attenuated in the mouse and equine, whereas wild-type pathogenic strain RacL11 induces severe inflammatory infiltration of the lung, causing infected mice to succumb. The complete DNA sequencing of the KyA genome revealed that genes UL17 (ORF17), US6 (ORF73; gI), US7 (ORF74; gE), and US8 (ORF75; 10 K) are deleted as compared to the RacL11 and Ab4 genomes. In-frame deletions in the US1 (ORF68), US4 (ORF71; gp2), and UL63 (ORF63; EICP0) genes and point mutations in 14 different open reading frames (ORFs) were detected in the KyA genome. Interestingly, UL1 (ORF1) and UL2 (ORF2) were deleted in both KyA and RacL11. Our previous studies showed that EHV-1 glycoproteins gI, gE, and full-length gp2 contribute to the pathogenesis of the RacL11 strain. The confirmation of these gene deletions in KyA suggests their contribution to the attenuation of this virus. The growth kinetics results revealed that KyA replicates to high titers in cell culture as compared to RacL11 and Ab4, indicating that the above genomic deletions and mutations in KyA do not have an inhibitory effect on KyA replication in cells of mouse, rabbit, equine, or human origin. Studies of EHV-1 pathogenesis in CBA mice showed that KyA is attenuated whereas mice infected with RacL11 succumbed by 3–6 days post-infection, which is consistent with our previous results.

Intramuscular Immunization of Mice with the Live-Attenuated Herpes Simplex Virus 1 Vaccine Strain VC2 Expressing Equine Herpesvirus 1 (EHV-1) Glycoprotein D Generates Anti-EHV-1 Immune Responses in Mice

Vaccination remains the best option to combat equine herpesvirus 1 (EHV-1) infection, and several different strategies of vaccination have been investigated and developed over the past few decades. Herein, we report that the live-attenuated herpes simplex virus 1 (HSV-1) VC2 vaccine strain, which has been shown to be unable to enter into neurons and establish latency in mice, can be utilized as a vector for the heterologous expression of EHV-1 glycoprotein D (gD) and that the intramuscular immunization of mice results in strong antiviral humoral and cellular immune responses. The VC2-EHV-1-gD recombinant virus was constructed by inserting an EHV-1 gD expression cassette under the control of the cytomegalovirus immediate early promoter into the VC2 vector in place of the HSV-1 thymidine kinase (UL23) gene. The vaccines were introduced into mice through intramuscular injection. Vaccination with both the VC2-EHV-1-gD vaccine and the commercially available vaccine Vetera EHV^XP 1/4 (Vetera; Boehringer Ingelheim Vetmedica) resulted in the production of neutralizing antibodies, the levels of which were significantly higher in comparison to those in VC2- and mock-vaccinated animals (P < 0.01 or P < 0.001). Analysis of EHV-1-reactive IgG subtypes demonstrated that vaccination with the VC2-EHV-1-gD vaccine stimulated robust IgG1 and IgG2a antibodies after three vaccinations (P < 0.001). Interestingly, Vetera-vaccinated mice produced significantly higher levels of IgM than mice in the other groups before and after challenge (P < 0.01 or P < 0.05). Vaccination with VC2-EHV-1-gD stimulated strong cellular immune responses, characterized by the upregulation of both interferon- and tumor necrosis factor-positive CD4⁺ T cells and CD8⁺ T cells. Overall, the data suggest that the HSV-1 VC2 vaccine strain may be used as a viral vector for the vaccination of horses as well as, potentially, for the vaccination of other economically important animals.IMPORTANCE A novel virus-vectored VC2-EHV-1-gD vaccine was constructed using the live-attenuated HSV-1 VC2 vaccine strain. This vaccine stimulated strong humoral and cellular immune responses in mice, suggesting that it could protect horses against EHV-1 infection.

Microvascular lesions and changes in cell proliferation and death, and cytokine expression in the placentas of mice experimentally infected with Equid Herpesvirus 1

This study describes the changes observed in the placentas of mice experimentally infected with an abortigenic strain of EHV-1 at mid-pregnancy and euthanized at days 3 and 4 post-infection. We analyzed microscopic vascular alterations, cell proliferation and death by immunohistochemistry, and the expression of IFN-γ, TNF-α and the IL-10 by qPCR and flow cytometry. Infected mice showed slight respiratory signs and ruffled fur during the first two days post-infection. Virus isolation and DNA detection were positive only in the lungs of the infected mice. Vascular congestion, increase in the labyrinth area, and a significant reduction in fetal capillary endothelium surface of infected placentas were found. Cell proliferation was significantly reduced in the infected placentas, whereas the apoptosis was significantly increased. IL10, TNF and IFN-γ showed different expression in the infected placentas and uteri. The effects of EHV-1 during pregnancy depend on different pathogenic mechanisms in which vascular alterations, and cell death and proliferation and local cytokine changes are compromised.

Replication kinetics of neurovirulent versus non-neurovirulent equine herpesvirus type 1 strains in equine nasal mucosal explants

Equine herpesvirus type 1 (EHV-1) is the causative agent of equine herpes myeloencephalopathy, of which outbreaks are reported with increasing frequency throughout North America and Europe. This has resulted in its classification as a potentially emerging disease by the US Department of Agriculture. Recently, it was found that a single nucleotide polymorphism (SNP) in the viral DNA polymerase gene (ORF30) at aa 752 (N-->D) is associated with the neurovirulent potential of EHV-1. In the present study, equine respiratory mucosal explants were inoculated with several Belgian isolates typed in their ORF30 as D(752) or N(752), to evaluate a possible difference in replication in the upper respiratory tract. In addition, to evaluate whether any observed differences could be attributed to the SNP associated with neurovirulence, the experiments were repeated with parental Ab4 (reference neurovirulent strain), parental NY03 (reference non-neurovirulent strain) and their N/D revertant recombinant viruses. The salient findings were that EHV-1 spreads plaquewise in the epithelium, but plaques never cross the basement membrane (BM). However, single EHV-1-infected cells could be observed below the BM at 36 h post-inoculation (p.i.) for all N(752) isolates and at 24 h p.i. for all D(752) isolates, and were identified as monocytic cells and T lymphocytes. Interestingly, the number of infected cells was two to five times higher for D(752) isolates compared with N(752) isolates at every time point analysed. Finally, this study showed that equine respiratory explants are a valuable and reproducible model to study EHV-1 neurovirulence in vitro, thereby reducing the need for horses as experimental animals.

Diverse pathogenicity of equine herpesvirus 1 (EHV-1) isolates in CBA mouse model

The pathogenicity of equine herpesvirus 1 (EHV-1) isolates of Japan were evaluated by using the CBA mouse model. CBA mice were inoculated with eight Japanese EHV-1 strains (89c1, 90c16, 90c18, 97c11, 98c12, 00c19, 01c1 and HH-1) and one British strain (Ab4p). 89c1 caused slight body weight loss and nervous signs in mice at 8 days post infection (dpi). Severe weight loss and nervous signs were observed in mice inoculated with Ab4p at 6 dpi. The other strains did not cause apparent clinical signs. Infectious viruses were recovered from the lungs of all groups at 2 dpi. Histopathological analysis revealed interstitial pneumonia in the lungs of all mice inoculated with EHV-1. Encephalitis or meningoencephalitis was observed in the brains of mice inoculated with 89c1, 90c18, 97c11, 98c12, 01c1 and Ab4p. Japanese EHV-1 strains showed low pathogenicity in CBA mice, whereas the sequential affects of infection are similar to those of the highly pathogenic strain Ab4p. These results suggest that field isolates of EHV-1 have varying degrees of pathogenicity in CBA mice.

Effective treatment of respiratory alphaherpesvirus infection using RNA interference

BACKGROUND

Equine herpesvirus type 1 (EHV-1), a member of the Alphaherpesvirinae, is spread via nasal secretions and causes respiratory disease, neurological disorders and abortions. The virus is a significant equine pathogen, but current EHV-1 vaccines are only partially protective and effective metaphylactic and therapeutic agents are not available. Small interfering RNAs (siRNA's), delivered intranasally, could prove a valuable alternative for infection control. siRNA's against two essential EHV-1 genes, encoding the viral helicase (Ori) and glycoprotein B, were evaluated for their potential to decrease EHV-1 infection in a mouse model. METHODOLOGY/PRINCIPAL FNDINGS: siRNA therapy in vitro significantly reduced virus production and plaque size. Viral titers were reduced 80-fold with 37.5 pmol of a single siRNA or with as little as 6.25 pmol of each siRNA when used in combination. siRNA therapy in vivo significantly reduced viral replication and clinical signs. Intranasal treatment did not require a transport vehicle and proved effective when given up to 12 h before or after infection.

CONCLUSIONS/SIGNIFICANCE

siRNA treatment has potential for both prevention and early treatment of EHV-1 infections.

An Argentine equine herpesvirus strain with special restriction patterns protect mice challenged with a pathogenic strain

Equine herpesvirus 1 (EHV-1) was first isolated in Argentina in 1979. This strain SPv has special restriction patterns, but a previous study demonstrated that SPv did not modify its growth in cell culture. In addition, it showed low virulence in the mouse respiratory model consistently with results found in female BALB/C at different state of gestation. This study evaluates in a mouse respiratory model, if primary infection with SPv strain protects animals from subsequent challenge with a pathogenic strain. Body weight loss was not observed in mice intranasally inoculated with SPv strain and challenged with HH1 Japanese strain. The SPv primary infection does not completely prevent clinical presentation by HH1 infection but the SPv inoculated animals recovered more quickly, with less intense and less persistent histological lesions. The challenge infection caused a rapid and prolonged increase in anti-EHV-1 antibodies in the mice previously infected with SPv, along with a more rapid reduction of viral titres in lungs. In this work it was demonstrated that this EHV-1 strain constitute a good immunogen. These results show that this SPv strain could be considered to produce an EHV-1 vaccine.

Use of polarised equine endothelial cell cultures and an in vitro thrombosis model for potential characterisation of EHV-1 strain variation

Equine herpesvirus-1 (EHV-1) is responsible for respiratory disease and abortion in pregnant mares. Some high virulence isolates of EHV-1 also cause neurological disease. The pathogenesis of both abortion and neurological disease relates in part, to thrombus formation occurring in the pregnant uterus and central nervous system. The differences in disease outcome may relate to differing abilities of high and low virulence EHV-1 isolates to cause cell-associated viraemia, infect endothelial cells and cause thrombosis at sites distant from the respiratory tract. This study attempted to identify in vitro assays, which could be used to characterise the interaction between these isolates, equine endothelial cells and clotting factors. No significant difference was found between the growth kinetics of high and low virulence isolates of EHV-1 in polarised endothelial cells. For both isolates, virus was released preferentially from the apical surface of the polarised cells. The functional effects of viral infection on endothelial cells, with reference to virally-induced thrombosis were then investigated. Endothelial cells were grown on microcarrier beads, infected with EHV-1 and assayed for procoagulant activity. No significant difference in clotting time was observed between mock and EHV-1 infected endothelial cells in microcarrier cultures. Thus the degree of thrombosis may reflect a more complex interaction between endothelial cells, circulating leucocytes and other factors in the microenvironment.

Argentine strain of equine herpesvirus 1 isolated from an aborted foetus shows low virulence in mouse respiratory and abortion models

The equine herpesvirus 1 (EHV-1) was isolated in Argentina from an aborted equine foetus in 1979. This virus (SPv) has special restriction patterns (RP) in comparison with other Argentine isolates. In addition, SPv could be distinguished on the basis of its pathogenicity in baby mice inoculated intracerebrally. We studied the growth properties of the SPv in cell culture and its effects in a mouse respiratory and abortion model. We observed that SPv did not modify its capacity to grow in cell culture with respect to reference HH1 strain. Nevertheless, we found significant differences between the titres of the two strains at 8-14 h post-infection (PI). In this work we demonstrated that SPv showed low virulence in female at different stages of gestation, consistently, with results found in the mouse respiratory model. We considered that this low virulence of SPv could be related to its RP because the RP of HH1 strain are similar to those of the HVS25A strain and both showed effect on pregnant mice. More specific studies about genomic alterations to the SPv are necessary for identifying, more clearly, if the intra-strain variations have relation with the low virulence in the mouse respiratory and abortion model.

In vitro characterisation of high and low virulence isolates of equine herpesvirus-1 and -4

Basic in vitro characteristics of high and low virulence isolates of equine herpesviruses-1 and -4 were investigated with particular reference made to the Ab4 and V592 isolates of EHV-1 as both have distinct endotheliotropism and clinical outcomes in pony challenge studies. Additionally, some EHV-4 isolates that showed variations in clinical outcome were included in some experiments. The aim of the study was to identify an in vitro characteristic that would differentiate strains of known virulence. Such a system could then be applied to vaccine and virulence studies as an effective screening tool. Viral growth kinetics in a variety of cell culture systems, plaque size, ability to replicate in fetal endothelium in organ culture, and sensitivity to acyclovir were compared. No reliable marker system that differentiated between higher and lower virulence isolates of EHV-1 and EHV-4 was identified.

Neurological disease associated with EHV-1-infection in a riding school: clinical and virological characteristics

An outbreak of neurological disease caused by EHV-1 infection is described with emphasis on diagnosis and prognosis for recumbent horses. In April 1995, an outbreak of the neurological form of Equine herpesvirus type 1 (EHV-1) occurred in a well-managed riding school with 41 horses: 34 horses showed a temperature spike and 20 some degree of neurological signs, of which 10 were nursed intensively in the indoor arena of the riding school for 3 to 20 days, 8 having to be maintained in slings for 2-18 days, while 9 needed bladder catheterisation b.i.d. for 2-16 days. Within the first 3 days, one horse was subjected to euthanasia and another horse died. Postmortem examination revealed a mild vasculitis with perivascular mononuclear cuffing and axonal degeneration in the central nervous system. Clinical diagnosis was confirmed by serology and virology: 28 horses seroconverted in one or more tests during the outbreak, whereas 12 had already high CF and SN titres in the first sample, suggestive of recent infection. Virus was isolated from nasal swabs of 4 horses, and identified as EHV-1 with type-specific monoclonal antibodies. Restriction enzyme analysis revealed that the EHV-1 strains from this outbreak belonged to genome type EHV-1.IP. The electropherotypes were identical to those from another, epidemiologically unrelated, outbreak of neurological disease 2 months earlier. The timing of the temperature spikes and seroconversions indicated that the infection was probably introduced by a horse purchased 3 weeks before neurological signs occurred. At follow-up one year later, the 10 horses that showed mild neurological signs had recovered completely. Of the 8 horses that survived intensive care, 3 had returned to around their former performance level (2 of which had been in slings), while the other 5 had become pasture-sound. At follow-up 4 years later, all pasture-sound horses had been subjected to euthanasia because of persistent mild ataxia and incontinence. In conclusion, the prognosis for recumbent horses due to EHV-1 infection is grave. For virological diagnosis, extensive and strategic sampling of febrile in-contact horses is required, and the EHV-1-specific glycoprotein G (gG) ELISA is a valuable tool for specific serological diagnosis of EHV-1 infection causing neurological disease.

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.

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 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.

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.

The production of a truncated form of baculovirus expressed EHV-1 glycoprotein C and its role in protection of C3H (H-2Kk) mice against virus challenge

A truncated form of the equine herpesvirus 1 (EHV-1) glycoprotein C (gC) gene was expressed in baculovirus. The gC signal sequence was substituted with the honeybee melittin signal sequence and the transmembrane region was replaced with a histidine tag. The recombinant virus produced high levels of gC in both the cells and supernatants of infected cells. The protein was present by 24 h and maximal secretion occurred at 96 h post-infection. The recombinant protein was antigenically authentic as shown by its reaction with each of a panel of individual monoclonal antibodies specific for the five distinct antigenic sites on EHV-1 gC. Recombinant gC was purified from the supernatant of infected cells by immuno-affinity chromatography and used to immunize C3H (H-2Kk haplotype) mice. This incurred a gC specific antibody response against both the recombinant protein and EHV-1 gC. 'Pepscan' analysis showed that the gC specific antibodies in serum from these mice reacted with the same epitopes on gC as those recognized by antibodies in convalescent equine sera (i.e. antibodies were specific to antigenic sites one and five). A third previously unrecognized antibody binding site at the carboxyl terminus was also detected (Antibody binding domain I). A T-cell proliferative response against EHV-1 was detected in splenocyte populations taken from vaccinated mice. Further, the recovery of virus from the lungs and turbinates following challenge of mice with EHV-1 was significantly reduced. These findings indicate that baculovirus expressed gC may contribute significantly to a subunit vaccine preparation aimed at protecting horses from EHV-1 infection.

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.