Major histocompatibility complex class I-restricted cytotoxic T-lymphocyte responses in horses infected with equine herpesvirus 1

Equine herpesvirus 1 elicits a strong pro-inflammatory response in the brain of mice

Equine herpesvirus type 1 (EHV-1) is an emerging pathogen that causes encephalomyelitis in horses and non-equid species. Several aspects of the immune response in the central nervous system (CNS), mainly regarding the role of inflammatory mediators during EHV-1 encephalitis, remain unknown. Moreover, understanding the mechanisms underlying extensive neuropathology induced by viruses would be helpful to establish therapeutic strategies. Therefore, we aimed to evaluate some aspects of the innate immune response during highly neurovirulent EHV-1 infection. C57BL/6 mice infected intranasally with A4/72 and A9/92 EHV-1 strains developed a fulminant neurological disease at 3 days post-inoculation with high viral titres in the brain. These mice developed severe encephalitis with infiltration of monocytes and CD8⁺ T cells to the brain. The inflammatory infiltrate followed the detection of the chemokines CCL2, CCL3, CCL4, CCL5, CXCL2, CXCL9 and CXCL-10 in the brain. Notably, the levels of CCL3, CCL4, CCL5 and CXCL9 were higher in A4/72-infected mice, which presented higher numbers of inflammatory cells within the CNS. Pro-inflammatory cytokines, such as interleukins (ILs) IL-1α, IL-1β, IL-6, IL-12β, and tumour necrosis factor (TNF), were also detected in the CNS, and Toll-like receptor (TLR) TLR2, TLR3 and TLR9 genes were also upregulated within the brain of EHV-1-infected mice. However, no expression of interferon-γ (IFN-γ) and IL-12α, which are important for controlling the replication of other herpesviruses, was detected in EHV-1-infected mice. The results show that the activated innate immune mechanisms could not prevent EHV-1 replication within the CNS, but most likely contributed to the extensive neuropathology. The mouse model of viral encephalitis proposed here will also be useful to study the mechanisms underlying extensive neuropathology.

An Equine Herpesvirus Type 1 (EHV-1) Ab4 Open Reading Frame 2 Deletion Mutant Provides Immunity and Protection from EHV-1 Infection and Disease

Equine herpesvirus type 1 (EHV-1) outbreaks continue to occur despite widely used vaccination. Therefore, development of EHV-1 vaccines providing improved immunity and protection is ongoing. Here, an open reading frame 2 deletion mutant of the neuropathogenic EHV-1 strain Ab4 (Ab4ΔORF2) was tested as a vaccine candidate. Three groups of horses (n = 8 each) were infected intranasally with Ab4ΔORF2 or the parent Ab4 virus or were kept as noninfected controls. Horses infected with Ab4ΔORF2 had reduced fever and nasal virus shedding compared to those infected with Ab4 but mounted similar adaptive immunity dominated by antibody responses. Nine months after the initial infection, all horses were challenged intranasally with Ab4. Previously noninfected horses (control/Ab4) displayed clinical signs, shed large amounts of virus, and developed cell-associated viremia. In contrast, 5/8 or 3/8 horses previously infected with Ab4ΔORF2 or Ab4, respectively, were fully protected from challenge infection as indicated by the absence of fever, clinical disease, nasal virus shedding, and viremia. All of these outcomes were significantly reduced in the remaining, partially protected 3/8 (Ab4ΔORF2/Ab4) and 5/8 (Ab4/Ab4) horses. Protected horses had EHV-1-specific IgG4/7 antibodies prior to challenge infection, and intranasal antibodies increased rapidly postchallenge. Intranasal inflammatory markers were not detectable in protected horses but quickly increased in control/Ab4 horses during the first week after infection. Overall, our data suggest that preexisting nasal IgG4/7 antibodies neutralize EHV-1, prevent viral entry, and thereby protect from disease, viral shedding, and cell-associated viremia. In conclusion, improved protection from challenge infection emphasizes further evaluation of Ab4ΔORF2 as a vaccine candidate.IMPORTANCE Nasal equine herpesvirus type 1 (EHV-1) shedding is essential for virus transmission during outbreaks. Cell-associated viremia is a prerequisite for the most severe disease outcomes, abortion and equine herpesvirus myeloencephalopathy (EHM). Thus, protection from viremia is considered essential for preventing EHM. Ab4ΔORF2 vaccination prevented EHV-1 challenge virus replication in the upper respiratory tract in fully protected horses. Consequently, these neither shed virus nor developed cell-associated viremia. Protection from virus shedding and viremia during challenge infection in combination with reduced virulence at the time of vaccination emphasizes ORF2 deletion as a promising modification for generating an improved EHV-1 vaccine. During this challenge infection, full protection was linked to preexisting local and systemic EHV-1-specific antibodies combined with rapidly increasing intranasal IgG4/7 antibodies and lack of nasal type I interferon and chemokine induction. These host immune parameters may constitute markers of protection against EHV-1 and be utilized as indicators for improved vaccine development and informed vaccination strategies.

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

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

Intranasal treatment with CpG-B oligodeoxynucleotides protects CBA mice from lethal equine herpesvirus 1 challenge by an innate immune response

Equine herpesvirus 1 (EHV-1) is the causative agent of a number of equine disease manifestations, including severe disease of the central nervous system, respiratory infections, and abortion storms. Our results showed that intranasal treatment with CpG-B oligodeoxynucleotides (ODN 1826) protected CBA mice from pathogenic EHV-1 RacL11 challenge. The IFN-γ gene and seven interferon-stimulated genes (ISGs) were upregulated 39.4- to 260.3-fold at 8 h postchallenge in the lungs of RacL11-challenged mice that had been treated with CpG-B ODN. Interestingly, IFN-γ gene expression was upregulated by 26-fold upon RacL11 challenge in CpG-B ODN-treated mice lungs as compared to that of CpG-A ODN (ODN 1585)-treated mice lungs; however, the seven ISGs were upregulated by 2.4-5.0-fold, suggesting that IFN-γ is a major factor in the protection of CBA mice from the lethal challenge. Pre-treatment with IFN-γ significantly reduced EHV-1 yield in murine alveolar macrophage MH-S cells, but not in mouse lung epithelial MLE12 cells. These results suggest that CpG-B ODN may be used as a prophylactic agent in horses and provide a basis for more effective treatment of EHV-1 infection.

Equine Herpesvirus 1 Bridles T Lymphocytes To Reach Its Target Organs

Equine herpesvirus 1 (EHV1) is an ancestral alphaherpesvirus that is related to herpes simplex virus 1 and causes respiratory, reproductive, and neurological disorders in Equidae. EHV1 is indisputably a master at exploiting leukocytes to reach its target organs, accordingly evading the host immunity. However, the role of T lymphocytes in cell-associated viremia remains poorly understood. Here we show that activated T lymphocytes efficiently become infected and support viral replication despite the presence of protective immunity. We demonstrate a restricted expression of viral proteins on the surfaces of infected T cells, which prevents immune recognition. In addition, we indicate a hampered release of progeny, which results in the accumulation of nucleocapsids in the T cell nucleus. Upon engagement with the target endothelium, late viral proteins orchestrate viral synapse formation and viral transfer to the contact cell. Our findings have significant implications for the understanding of EHV1 pathogenesis, which is essential for developing innovative therapies to prevent the devastating clinical symptoms of infection.

Equine herpesvirus 1 (EHV1) replicates in the respiratory epithelium and disseminates through the body via a cell-associated viremia in leukocytes, despite the presence of neutralizing antibodies. “Hijacked” leukocytes, previously identified as monocytic cells and T lymphocytes, transmit EHV1 to endothelial cells of the endometrium or central nervous system, causing reproductive (abortigenic variants) or neurological (neurological variants) disorders. In the present study, we questioned the potential route of EHV1 infection of T lymphocytes and how EHV1 misuses T lymphocytes as a vehicle to reach the endothelium of the target organs in the absence or presence of immune surveillance. Viral replication was evaluated in activated and quiescent primary T lymphocytes, and the results demonstrated increased infection of activated versus quiescent, CD4⁺ versus CD8⁺, and blood- versus lymph node-derived T cells. Moreover, primarily infected respiratory epithelial cells and circulating monocytic cells efficiently transferred virions to T lymphocytes in the presence of neutralizing antibodies. Albeit T-lymphocytes express all classes of viral proteins early in infection, the expression of viral glycoproteins on their cell surface was restricted. In addition, the release of viral progeny was hampered, resulting in the accumulation of viral nucleocapsids in the T cell nucleus. During contact of infected T lymphocytes with endothelial cells, a late viral protein(s) orchestrates T cell polarization and synapse formation, followed by anterograde dynein-mediated transport and transfer of viral progeny to the engaged cell. This represents a sophisticated but efficient immune evasion strategy to allow transfer of progeny virus from T lymphocytes to adjacent target cells. These results demonstrate that T lymphocytes are susceptible to EHV1 infection and that cell-cell contact transmits infectious virus to and from T lymphocytes.

IMPORTANCE Equine herpesvirus 1 (EHV1) is an ancestral alphaherpesvirus that is related to herpes simplex virus 1 and causes respiratory, reproductive, and neurological disorders in Equidae. EHV1 is indisputably a master at exploiting leukocytes to reach its target organs, accordingly evading the host immunity. However, the role of T lymphocytes in cell-associated viremia remains poorly understood. Here we show that activated T lymphocytes efficiently become infected and support viral replication despite the presence of protective immunity. We demonstrate a restricted expression of viral proteins on the surfaces of infected T cells, which prevents immune recognition. In addition, we indicate a hampered release of progeny, which results in the accumulation of nucleocapsids in the T cell nucleus. Upon engagement with the target endothelium, late viral proteins orchestrate viral synapse formation and viral transfer to the contact cell. Our findings have significant implications for the understanding of EHV1 pathogenesis, which is essential for developing innovative therapies to prevent the devastating clinical symptoms of infection.

Deletion of the ORF2 gene of the neuropathogenic equine herpesvirus type 1 strain Ab4 reduces virulence while maintaining strong immunogenicity

BACKGROUND

Equine herpesvirus type 1 (EHV-1) induces respiratory infection, abortion, and neurologic disease with significant impact. Virulence factors contributing to infection and immune evasion are of particular interest. A potential virulence factor of the neuropathogenic EHV-1 strain Ab4 is ORF2. This study on 24 Icelandic horses, 2 to 4 years of age, describes the infection with EHV-1 Ab4, or its deletion mutant devoid of ORF2 (Ab4ΔORF2) compared to non-infected controls (each group n = 8). The horses' clinical presentation, virus shedding, viremia, antibody and cellular immune responses were monitored over 260 days after experimental infection.

RESULTS

Infection with Ab4ΔORF2 reduced fever and minimized nasal virus shedding after infection compared to the parent virus strain Ab4, while Ab4ΔORF2 established viremia similar to Ab4. Concurrently with virus shedding, intranasal cytokine and interferon α (IFN-α) production increased in the Ab4 group, while horses infected with Ab4ΔORF2 expressed less IFN-α. The antibody response to EHV-1 was evaluated by a bead-based multiplex assay and was similar in both infected groups, Ab4 and Ab4ΔORF2. EHV-1 specific immunoglobulin (Ig) G1 was induced 8 days after infection (d8 pi) with a peak on d10-12 pi. EHV-1 specific IgG4/7 increased starting on d10 pi, and remained elevated in serum until the end of the study. The intranasal antibody response to EHV-1 was dominated by the same IgG isotypes and remained elevated in both infected groups until d130 pi. In contrast to the distinct antibody response, no induction of EHV-1 specific T-cells was detectable by flow cytometry after ex vivo re-stimulation of peripheral blood mononuclear cells (PBMC) with EHV-1 in any group. The cellular immune response was characterized by increased secretion of IFN-γ and interleukin10 in response to ex vivo re-stimulation of PBMC with EHV-1. This response was present during the time of viremia (d5-10 pi) and was similar in both infected groups, Ab4 and Ab4ΔORF2.

CONCLUSIONS

ORF2 is a virulence factor of EHV-1 Ab4 with impact on pyrexia and virus shedding from the nasal mucosa. In contrast, ORF2 does not influence viremia. The immunogenicity of the Ab4ΔORF2 and parent Ab4 viruses are identical. Graphical abstract - Deletion of ORF2 reduces virulence of EHV-1 Ab4. Graphical summary of the main findings of this study: ORF2 is a virulence factor of EHV-1 Ab4 with impact on pyrexia and virus shedding from the nasal mucosa.

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.

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.

Polarisation of equine pregnancy outcome associated with a maternal MHC class I allele: Preliminary evidence

Identification of risk factors which are associated with severe clinical signs can assist in the management of disease outbreaks and indicate future research areas. Pregnancy loss during late gestation in the mare compromises welfare, reduces fecundity and has financial implications for horse owners. This retrospective study focussed on the identification of risk factors associated with pregnancy loss among 46 Thoroughbred mares on a single British stud farm, with some but not all losses involving equid herpesvirus-1 (EHV-1) infection. In a sub-group of 30 mares, association between pregnancy loss and the presence of five common Thoroughbred horse haplotypes of the equine Major Histocompatibility Complex (MHC) was assessed. This involved development of sequence specific, reverse transcriptase polymerase chain reactions and in several mares, measurement of cytotoxic T lymphocyte activity. Of the 46 mares, 10 suffered late gestation pregnancy loss or neonatal foal death, five of which were EHV-1 positive. Maternal factors including age, parity, number of EHV-1 specific vaccinations and the number of days between final vaccination and foaling or abortion were not significantly associated with pregnancy loss. In contrast, a statistically significant association between the presence of the MHC class I B2 allele and pregnancy loss was identified, regardless of the fetus/foal's EHV-1 status (p=0.002). In conclusion, this study demonstrated a significantly positive association between pregnancy loss in Thoroughbred mares and a specific MHC class I allele in the mother. This association requires independent validation and further investigation of the mechanism by which the mare's genetic background contributes to pregnancy outcome.

Inability of FMDV replication in equine kidney epithelial cells is independent of integrin αvβ3 and αvβ6

Integrins can function as receptors for foot-and-mouth disease virus (FMDV) in epithelium. Horses are believed to be insusceptible to this disease, but the mechanism of resistance remains unclear. To detect whether FMDV can use integrin to attach to equine epithelial, we compared the utilities of αvβ3 and αvβ6 between bovine and equine kidney epithelial cells (KECs). Equine KECs showed almost equal efficiency to those of bovine. Further, the integrin αv, β3, and β6 subunits from bovine and equine were cloned and vectors were transfected into SW480 cells and COS-1 cells alone or together, and virus titers were used to determine the viral replication. In all cases, the virus reproduced successfully. Overall, FMDV can replicate in SW480 cells transfected with equine β3/β6 subunits and COS-1 cells transfected with equine αvβ3/αvβ6 integrins, but not in EKECs. These results indicated that failure of FMDV replication in EKECs was not attributed to integrin receptors.

The common equine class I molecule Eqca-1*00101 (ELA-A3.1) is characterized by narrow peptide binding and T cell epitope repertoires

Here we describe a detailed quantitative peptide-binding motif for the common equine leukocyte antigen (ELA) class I allele Eqca-1*00101, present in roughly 25 % of Thoroughbred horses. We determined a preliminary binding motif by sequencing endogenously bound ligands. Subsequently, a positional scanning combinatorial library (PSCL) was used to further characterize binding specificity and derive a quantitative motif involving aspartic acid in position 2 and hydrophobic residues at the C-terminus. Using this motif, we selected and tested 9- and 10-mer peptides derived from the equine herpesvirus type 1 (EHV-1) proteome for their capacity to bind Eqca-1*00101. PSCL predictions were very efficient, with an receiver operating characteristic (ROC) curve performance of 0.877, and 87 peptides derived from 40 different EHV-1 proteins were identified with affinities of 500 nM or higher. Quantitative analysis revealed that Eqca-1*00101 has a narrow peptide-binding repertoire, in comparison to those of most human, non-human primate, and mouse class I alleles. Peripheral blood mononuclear cells from six EHV-1-infected, or vaccinated but uninfected, Eqca-1*00101-positive horses were used in IFN-γ enzyme-linked immunospot (ELISPOT) assays. When we screened the 87 Eqca-1*00101-binding peptides for T cell reactivity, only one Eqca-1*00101 epitope, derived from the intermediate-early protein ICP4, was identified. Thus, despite its common occurrence in several horse breeds, Eqca-1*00101 is associated with a narrow binding repertoire and a similarly narrow T cell response to an important equine viral pathogen. Intriguingly, these features are shared with other human and macaque major histocompatibility complex (MHC) molecules with a similar specificity for D in position 2 or 3 in their main anchor motif.

The equine immune responses to infectious and allergic disease: a model for humans?

The modern horse, Equus caballus has historically made important contributions to the field of immunology, dating back to Emil von Behring's description of curative antibodies in equine serum over a century ago. While the horse continues to play an important role in human serotherapy, the mouse has replaced the horse as the predominant experimental animal in immunology research. Nevertheless, continuing efforts have led to an improved understanding of the equine immune response in a variety of infectious and non-infectious diseases. Based on this information, we can begin to identify specific situations where the horse may provide a unique immunological model for certain human diseases.

Immediate-early protein of equid herpesvirus type 1 as a target for cytotoxic T-lymphocytes in the Thoroughbred horse

Cytotoxic T-lymphocytes (CTLs) are associated with protective immunity against disease caused by equid herpesvirus type 1 (EHV-1). However, the EHV-1 target proteins for CTLs are poorly defined. This limits the development of vaccine candidates designed to stimulate strong CTL immunity. Here, classical CTL assays using lymphocytes from horses of three defined MHC class I types that experienced natural infection with EHV-1 and a modified vaccinia virus construct containing an EHV-1 gene encoding the immediate-early (IE) protein are reported. Horses homozygous for the equine leukocyte antigen (ELA)-A2 haplotype, but not the ELA-A5 haplotype, produced MHC-restricted CTL responses against the IE protein. Previously, horses homozygous for the ELA-A3 haplotype also mounted CTL responses against the IE protein. Both haplotypes are common in major horse breeds, including the Thoroughbred. Thus, the IE protein is an attractive candidate molecule for future studies of T-cell immunity to EHV-1 in the horse.

Serological responses and clinical outcome after vaccination of mares and foals with equine herpesvirus type 1 and 4 (EHV-1 and EHV-4) vaccines

Equine herpesvirus type 1 and type 4 (EHV-1 and EHV-4) cause infections of horses worldwide. While both EHV-1 and EHV-4 cause respiratory disease, abortion and myeloencephalopathy are observed after infection with EHV-1 in the vast majority of cases. Disease control is achieved by hygiene measures that include immunization with either inactivated or modified live virus (MLV) vaccine preparations. We here compared the efficacy of commercially available vaccines, an EHV-1/EHV-4 inactivated combination and an MLV vaccine, with respect to induction of humoral responses and protection of clinical disease (abortion) in pregnant mares and foals on a large stud with a total of approximately 3500 horses. The MLV vaccine was administered twice during pregnancy (months 5 and 8 of gestation) to 383 mares (49.4%), while the inactivated vaccine was administered three times (months 5, 7, and 9) to 392 mares (50.6%). From the vaccinated mares, 192 (MLV) and 150 (inactivated) were randomly selected for serological analyses. There was no significant difference between the groups with respect to magnitude or duration of the humoral responses as assessed by serum neutralization assays (median range from 1:42 to 1:130) and probing for EHV-1-specific IgG isotypes, although neutralizing responses were higher in animals vaccinated with the MLV preparation at all time points sampled. The total number of abortions in the study population was 55/775 (7.1%), 9 of which were attributed to EHV-1. Seven of the abortions were in the inactivated and two in the MLV vaccine group (p=0.16). When foals of vaccinated mares were followed up, a dramatic drop of serum neutralizing titers (median below 1:8) was observed in all groups, indicating that the half-life of maternally derived antibody is less than 4 weeks.

Immunological correlates of vaccination and infection for equine herpesvirus 1

Equine herpesvirus 1 (EHV-1) induces a variety of disease manifestations, including respiratory disease, abortions, and myeloencephalopathy. Several vaccines are commercially available but could not previously be distinguished by serologic testing from infection with EHV-1 (or the closely related EHV-4). Currently available vaccines are not reliably protective against the severe manifestations of the disease, including fatal myeloencephalopathy. We determined immunological parameters that can differentiate vaccinated from previously infected animals by comparing humoral and cellular EHV-1-specific responses in clinically healthy horses 10 months after vaccination. Forty-seven horses with known histories of vaccination and infection were studied, including a group of horses that survived a severe neurological outbreak 5 years prior to vaccination. Results of serum virus neutralization (SN), serum IgG isotyping, and cytokine profiling of lymphocyte subsets were compared. IgG4/7 levels strongly correlated with virus neutralization (P < 0.0001). IgG1/3 and SN values distinguished vaccinated/outbreak-exposed (vacc/outbreak) horses from vaccinated horses (P < 0.05). EHV-1-specific gamma interferon (IFN-γ)-producing CD4(+) (but not CD8(+)) T-cell numbers were also increased in vacc/outbreak horses, which distinguished them from vaccinated horses (P < 0.01). IFN-α secretion was similar between all groups and independent of previous exposure or vaccination. Our data suggest that IgG isotype responses to EHV-1 are more diverse under field conditions than is revealed by experimental studies and that the current modified-live virus (MLV) vaccine induces a more restricted IgG isotype response than does natural exposure to EHV-1. Since these parameters can be assessed in a high-throughput manner, they may prove useful in screening future vaccine candidates and assessing levels of protection.

Infection of peripheral blood mononuclear cells with neuropathogenic equine herpesvirus type-1 strain Ab4 reveals intact interferon-α induction and induces suppression of anti-inflammatory interleukin-10 responses in comparison to other viral strains

The recent increase in incidence, morbidity, and mortality of neurological disease induced by equine herpesvirus type 1 (EHV-1) has suggested a change of virulence of the virus. The exact mechanisms by which EHV-1 induces neurologic disease are not known. Environmental, viral, and host risk factors might contribute to neurological manifestation. Here, we investigated innate interferon-α (IFN-α), interleukin-10 (IL-10) and IL-4 responses after infection of equine peripheral blood mononuclear cells (PBMC) with EHV-1 using an available cytokine multiplex assay. Three viral strains representing an older isolate (RacL11), a recent abortigenic (NY03) and a neuropathogenic isolate (Ab4) were compared to identify differences in cytokine induction that might explain the increased pathogenicity of Ab4. Cytokine concentrations were also compared between foals, mares after birth, pregnant and non-pregnant mares to investigate whether immune responses to EHV-1 infection are influenced by age or pregnancy status. PBMC from all groups secreted high concentrations of anti-viral IFN-α in response to EHV-1. A reduced response was observed in foals compared to non-pregnant mares. EHV-1 infection induced moderate IL-10 and overall low IL-4 secretion. Ab4 infection resulted in a significant reduction of IL-10 responses in adult horses. IL-10 and IL-4 responses were lower in foals than in most mare groups. These data suggested that EHV-1 induces robust IFN-α secretion without major differences between viral strains. However, anti-inflammatory IL-10 production was significantly reduced after infection with neuropathogenic Ab4. The ability of this EHV-1 isolate to down-regulate IL-10 production might contribute to increased local inflammation and a higher risk for neurological manifestation of the disease after infection with Ab4.

Evaluation of immune responses following infection of ponies with an EHV-1 ORF1/2 deletion mutant

Equine herpesvirus-1 (EHV-1) infection remains a significant problem despite the widespread use of vaccines. The inability to generate a protective immune response to EHV-1 vaccination or infection is thought to be due to immunomodulatory properties of the virus, and the ORF1 and ORF2 gene products have been hypothesized as potential candidates with immunoregulatory properties. A pony infection study was performed to define immune responses to EHV-1, and to determine if an EHV-1 ORF1/2 deletion mutant (ΔORF1/2) would have different disease and immunoregulatory effects compared to wild type EHV-1 (WT). Infection with either virus led to cytokine responses that coincided with the course of clinical disease, particularly the biphasic pyrexia, which correlates with respiratory disease and viremia, respectively. Similarly, both viruses caused suppression of proliferative T-cell responses on day 7 post infection (pi). The ΔORF1/ORF2 virus caused significantly shorter primary pyrexia and significantly reduced nasal shedding, and an attenuated decrease in PBMC IL-8 as well as increased Tbet responses compared to WT-infected ponies. In conclusion, our findings are (i) that infection of ponies with EHV-1 leads to modulation of immune responses, which are correlated with disease pathogenesis, and (ii) that the ORF1/2 genes are of importance for disease outcome and modulation of cytokine responses.

Interferon-gamma, interleukin-4 and interleukin-10 production by T helper cells reveals intact Th1 and regulatory TR1 cell activation and a delay of the Th2 cell response in equine neonates and foals

Cytokines produced by T helper (Th) cells are important in orchestrating the immune response during health and disease. Recent reports indicated that cytokine mRNA expression in foals is often quantitatively lower than that of adult horses suggesting that foal T cells are not fully mature. Here, peripheral blood mononuclear cells from foals and adult horses were stimulated with phorbol 12-myristate 13-acetate and analyzed for intracellular interferon-gamma (IFN-γ), interleukin-4 (IL-4) and IL-10 production, representing the Th1, Th2 and regulatory T_R1 cell phenotypes respectively, by flow cytometry. In agreement with previous reports, all three cytokines were quantitatively reduced in foals compared to adults. However, the balance between Th1 and Th2 cytokines (IFN-γ/IL-4 ratio) showed a clear Th1-biased response in foals by 6 and 12 weeks of life, while similar IFN-γ/IL-10 ratios were found in foals and adult horses. By day 5 after birth, intracellular IFN-γ production by foal CD4⁺ and CD8⁺ T cells resembled that in adults. Overall, IL-4 production was low in foals. IL-4⁺ cells peaked at day 5 of age when IL-4 was mainly produced by IgE⁺ cells. Relative percentages of IL-4⁺ Th2 cells were significantly lower in foals at all time points. The data suggested that equine neonates and young foals have an impaired Th2 response, that the immune response of foals is Th1 biased, that IFN-γ production by Th and cytotoxic T cells is qualitatively similar to adult horses, and regulatory IL-10 production by T cells is developmentally mature in foals during the first three months of life.

Vaccination of ponies with the IE gene of EHV-1 in a recombinant modified live vaccinia vector protects against clinical and virological disease

The control of EHV-1 infection by cytotoxic T-cell responses (CTL) via a reduction in cell associated viremia remains an important goal in horses. Unfortunately, current vaccines are inefficient at inducing these responses. We have identified the immediate early (IE) gene of EHV-1 as a potent stimulator of virus-specific CTL responses in ponies expressing a specific MHC class I serological haplotype (A3/B2). This study was designed to determine if vaccination of A3/B2 MHC I positive ponies with the IE gene could induce protection and immune responses associated with cell mediated immunity. Ponies expressing the MHC-I A3/B2 haplotype (A3/B2 vaccinates) and ponies with a different MHC I haplotype (either non-A3 vaccinates or A3-non-B2 vaccinates) were vaccinated with a recombinant modified vaccinia Ankara (rMVA) vector expressing the IE gene on 3 occasions and vaccinates and unvaccinated controls were challenge infected 8 weeks after the last vaccination. Interferon gamma (IFN-gamma) mRNA and antibody titers were determined throughout the study and clinical signs, nasal virus shedding and viremia were determined following challenge infection. Vaccination of A3/B2 vaccinates conferred significant clinical protection and a significant reduction in EHV-1 viremia. IFN-gamma mRNA increased significantly following vaccination in the A3/B2 vaccinates. Antibody titers remained low until after challenge infection, indicating that no accidental field acquired or recrudescent EHV-1 infection had occurred. In summary, this is an important study showing that vaccination of ponies with the EHV-1 IE protein provides not only reduction in clinical disease but also reduction of cell associated viremia, which is a prerequisite for the prevention of abortion and neurological disease.

New assays to measure equine influenza virus-specific Type 1 immunity in horses

Equine influenza virus (EIV) is a leading cause of respiratory disease in horses. Equine influenza infection induces a long-term immunity to re-infection. Recent strategies of vaccination aim to mimic this immunity by stimulating both antibody and cellular immune responses. Cell-mediated immunity (CMI) to influenza is well defined in man, but little has been done to characterise the responses in the horse. Additionally, the development of reliable assays for the measurement of equine CMI has lagged behind serological methods and vaccine development. In this study, two methods of measuring EIV-specific T lymphocyte responses have been developed. An EIV 'bulk' cytotoxic T lymphocytes (CTL) assay using equine dermal fibroblasts as target cells has been adapted from a method used in the 1980s. This method was also complemented with a new EIV-specific IFNgamma synthesis assay. When compared with the measurement of EIV-specific IFNgamma synthesis previously described, this method required the amplification of EIV-specific lymphocytes by culture and was sensitive enough to detect stimulation of EIV-specific T lymphocytes induced by experimental infection with EIV or vaccination with recombinant canarypox viruses coding for EIV-HA molecules. This study provides the tools to characterise the stimulation of CMI by the new generation of vaccines against equine influenza.

Experimental Rhodococcus equi and equine infectious anemia virus DNA vaccination in adult and neonatal horses: effect of IL-12, dose, and route

Improving the ability of DNA-based vaccines to induce potent Type1/Th1 responses against intracellular pathogens in large outbred species is essential. Rhodoccocus equi and equine infectious anemia virus (EIAV) are two naturally occurring equine pathogens that also serve as important large animal models of neonatal immunity and lentiviral immune control. Neonates present a unique challenge for immunization due to their diminished immunologic capabilities and apparent Th2 bias. In an effort to augment R. equi- and EIAV-specific Th1 responses induced by DNA vaccination, we hypothesized that a dual promoter plasmid encoding recombinant equine IL-12 (rEqIL-12) would function as a molecular adjuvant. In adult horses, DNA vaccines induced R. equi- and EIAV-specific antibody and lymphoproliferative responses, and EIAV-specific CTL and tetramer-positive CD8+ T lymphocytes. These responses were not enhanced by the rEqIL-12 plasmid. In neonatal foals, DNA immunization induced EIAV-specific antibody and lymphoproliferative responses, but not CTL. The R. equi vapA vaccine was poorly immunogenic in foals even when co-administered with the IL-12 plasmid. It was concluded that DNA immunization was capable of inducing Th1 responses in horses; dose and route were significant variables, but rEqIL-12 was not an effective molecular adjuvant. Additional work is needed to optimize DNA vaccine-induced Th1 responses in horses, especially in neonates.

Cloning and large-scale expansion of epitope-specific equine cytotoxic T lymphocytes using an anti-equine CD3 monoclonal antibody and human recombinant IL-2

Cytotoxic T lymphocytes are involved in controlling intracellular pathogens in many species, including horses. Particularly, CTL are critical for the control of equine infectious anemia virus (EIAV), a lentivirus that infects horses world-wide. In humans and animal models, CTL clones are valuable for evaluating the fine specificity of epitope recognition, and for adoptive immunotherapy against infectious and neoplastic diseases. Cloned CTL would be equally useful for similar studies in the horse. Here we present the first analysis of a method to generate equine CTL clones. Peripheral blood mononuclear cells were obtained from an EIAV-infected horse and stimulated with the EIAV Rev-QW11 peptide. Sorted CD8+ T cells were cloned by limiting dilution, and expanded without further antigen addition using irradiated PBMC, anti-equine CD3, and human recombinant IL-2. Clones could be frozen and thawed without detrimental effects, and could be subsequently expanded to numbers exceeding 2 x 10(9)cells. Flow cytometry of expanded clones confirmed the CD3+/CD8+ phenotype, and chromium release assays confirmed CTL activity. Finally, sequencing TCR beta chain genes confirmed clonality. Our results provide a reliable means to generate large numbers of epitope-specific equine CTL clones that are suitable for use in downstream applications, including functional assays and adoptive transfer studies.

Frequency and phenotype of EHV-1 specific, IFN-gamma synthesising lymphocytes in ponies: the effects of age, pregnancy and infection

Equine herpesvirus-1 (EHV-1) infects horses, causing acute respiratory disease, neurological signs, and is also a leading cause of abortion. Protection from EHV-1 infection and disease depends on both humoral (virus neutralising antibody) and cellular (mainly cytotoxic T lymphocytes, CTL) immune responses. CTL activity after EHV-1 infection has been extensively investigated and is closely associated with an alternative measure of cell mediated immunity (CMI), interferon-gamma (IFN-gamma) synthesis. This study investigates EHV-1-specific IFN-gamma synthesising cells in potentially immunocompromised horses; foals, pregnant mares and aged animals, after field or experimental infection with EHV-1. In foals and pregnant mares, the kinetics after experimental infection were similar and the phenotype of IFN-gamma+ synthesising cells after EHV-1 stimulation was mainly CD8alpha+. In contrast, in samples collected from primed healthy ponies exposed to EHV-1 several months previously or in old ponies (28 years old), the majority of EHV-1-specific IFN-gamma+ lymphocytes expressed a CD5+, CD8alpha- phenotype. This study highlights the complexity of the relationship between EHV-1, a common pathogen in horses, and the virus-specific cellular immune response as measured using IFN-gamma synthesis.

Detection and quantification of equine herpesvirus-1 viremia and nasal shedding by real-time polymerase chain reaction

Equine herpesvirus-1 (EHV-1) infection is common in young horses throughout the world, resulting in respiratory disease, epidemic abortion, sporadic myelitis, or latent infections. To improve on conventional diagnostic tests for EHV-1, a real-time polymerase chain reaction (PCR) technique was developed, using primers and probes specific for the EHV-1 gB gene. Amplification efficiencies of 100% +/- 5% were obtained for DNA isolated from a plasmid, infected peripheral blood mononuclear cells (PBMCs), and nasal secretions from infected ponies. The dynamic range of the assay was 8 log10 dilutions, and the lower limit of detection was 6 DNA copies. Fifteen ponies, seronegative for EHV-1, were experimentally infected with EHV-1, and nasal samples were used to quantify shedding of virus by both virus isolation and real-time PCR analysis. Virus isolation identified nasal shedding of EHV-1 in 12/15 ponies on a total of 25 days; real-time PCR detected viral shedding in 15/15 ponies on 75 days. Viremia was quantified using PBMC DNA, subsequent to challenge infection in 3 additional ponies. Viremia was identified in 1/3 ponies on a single day by virus isolation; real-time PCR detected viremia in 3/3 ponies on 17 days. When real-time PCR was used to analyze PBMC DNA from 11 latently infected ponies (documented by nested PCR), EHV-1 was not detected. We conclude that real-time PCR is a sensitive and quantitative test for EHV-1 nasal shedding and viremia and provides a valuable tool for EHV-1 surveillance, diagnosis of clinical disease, and investigation of vaccine efficacy.

A molecular approach to the identification of cytotoxic T-lymphocyte epitopes within equine herpesvirus 1

Equine herpesvirus 1 (EHV-1) causes respiratory and neurological disease and abortion in horses. Animals with high frequencies of cytotoxic T lymphocytes (CTL) show reduced severity of respiratory disease and frequency of abortion, probably by CTL-mediated control of cell-associated viraemia. This study aimed to identify CTL epitopes restricted by selected major histocompatibility complex (MHC) class I alleles expressed in the equine leukocyte antigen (ELA) A3 haplotype. Effector CTL were induced from EHV-1-primed ponies and thoroughbreds with characterized MHC class I haplotypes and screened against P815 target cells transfected with selected EHV-1 genes and MHC class I genes. Targets that expressed EHV-1 gene 64 and the MHC B2 gene were lysed by effector CTL in a genetically restricted manner. There was no T-cell recognition of targets expressing either the MHC B2 gene and EHV-1 genes 2, 12, 14, 16, 35, 63 or 69, or the MHC C1 gene and EHV-1 genes 12, 14, 16 or 64. A vaccinia virus vector encoding gene 64 (NYVAC-64) was also investigated. Using lymphocytes from ELA-A3 horses, the recombinant NYVAC-64 virus induced effector CTL that lysed EHV-1-infected target cells; the recombinant virus also supplied a functional peptide that was expressed by target cells and recognized in an MHC-restricted fashion by CTL induced with EHV-1. This construct may therefore be used to determine the antigenicity of EHV-1 gene 64 for other MHC haplotypes. These techniques are broadly applicable to the identification of additional CTL target proteins and their presenting MHC alleles, not only for EHV-1, but for other equine viruses.

Recombinant single chain Fv antibodies specific for glycoprotein D of equid herpesvirus 1

Single chain Fv (scFv) molecules generated by phage-display technology represent a new and efficient tool in the research and diagnostics of infectious diseases. The recombinant glycoprotein D of Equid herpesvirus 1 was successfully expressed in E. coli cells. The protein was produced predominantly in soluble fraction and was then purified on a nickel-agarose column. The scFv antibodies against the glycoprotein were selected and several clones of glycoprotein D-specific scFv-antibodies were identified; t of them was expressed as a soluble scFv molecule, purified by immobilized metal-affinity chromatography and used as reagent in an immunofluorescence test.

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

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

The equine immune response to equine herpesvirus-1: The virus and its vaccines

Equine herpesvirus-1 (EHV-1) is an alphaherpesvirus which infects horses, causing respiratory and neurological disease and abortion in pregnant mares. Latency is established in trigeminal ganglia and lymphocytes. Immunity to EHV-1 lasts between 3 and 6 months. Current vaccines, many of which contain inactivated virus, have reduced the incidence of abortion storms in pregnant mares but individual animals, which may be of high commercial value, remain susceptible to infection. The development of effective vaccines which stimulate both humoral and cellular immune responses remains a priority. Utilising data generated following experimental and field infections of the target species, this review describes the immunopathogenesis of EHV-1 and the interaction between the horse's immune system and this virus, both in vivo and in vitro, and identifies immune responses, highlighting those which have been associated with protective immunity. It then goes on to recount a brief history of vaccination, outlines factors likely to influence the outcome of vaccine administration and describes the immune response stimulated by a selection of commercial and experimental vaccines. Finally, based on the available data, a rational strategy designed to stimulate protective immune responses by vaccination is outlined.

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.

Report of the equine herpesvirus-1 Havermeyer Workshop, San Gimignano, Tuscany, June 2004

Amongst the infectious diseases that threaten equine health, herpesviral infections remain a world wide cause of serious morbidity and mortality. Equine herpesvirus-1 infection is the most important pathogen, causing an array of disorders including epidemic respiratory disease abortion, neonatal foal death, myeloencephalopathy and chorioretinopathy. Despite intense scientific investigation, extensive use of vaccination, and established codes of practice for control of disease outbreaks, infection and disease remain common. While equine herpesvirus-1 infection remains a daunting challenge for immunoprophylaxis, many critical advances in equine immunology have resulted in studies of this virus, particularly related to MHC-restricted cytotoxicity in the horse. A workshop was convened in San Gimignano, Tuscany, Italy in June 2004, to bring together clinical and basic researchers in the field of equine herpesvirus-1 study to discuss the latest advances and future prospects for improving our understanding of these diseases, and equine immunity to herpesviral infection. This report highlights the new information that was the focus of this workshop, and is intended to summarize this material and identify the critical questions in the field.

Detection of equine herpesvirus-specific effector and memory cytotoxic immunity in the equine upper respiratory tract

Immunological protection of horses from equine herpesvirus-1 (EHV-1) infection and disease depends on the cooperation of virus-specific humoral and cellular immune responses. EHV-specific mucosal immunity may be an important component of such immune responses. This study demonstrates the induction of anti-EHV cytotoxic cellular immune responses in various mucosal and systemic lymphoid tissues associated with the upper respiratory tract (URT) of the horse. Four young horses (1-2 years of age) were inoculated intranasally with the Army 183 strain of EHV-1 and euthanized 1 week later. One untreated foal served as a non-infected control. Mucosa-associated tonsillar tissues, draining lymph nodes and PBMC were harvested. Virus-specific memory and effector cytolytic activity were individually assessed using 4 h chromium release assays, with and without in vitro restimulation with EHV-1, respectively. EHV-specific cytotoxic activity was detected ex vivo in several URT-associated mucosal lymphoid tissues of horses, particularly within the lining of the nasopharynx, a principal site of EHV-1 replication. This activity was also detected in the circulation of some horses 1 week post-challenge. Virus-specific memory cytotoxic activity was elevated in the circulation, and detectable in the draining lymph nodes of all horses following challenge infection.

Lymphocyte proliferation responses induced to broadly reactive Th peptides did not protect against equine infectious anemia virus challenge

The effect of immunization with five lipopeptides, three containing T-helper (Th) epitopes and two with both Th and cytotoxic T-lymphocyte (CTL) epitopes, on equine infectious anemia virus (EIAV) challenge was evaluated. Peripheral blood mononuclear cells from EIAV lipopeptide-immunized horses had significant proliferative responses to Th peptides compared with those preimmunization, and the responses were attributed to significant responses to peptides Gag from positions 221 to 245 (Gag 221-245), Gag 250-269, and Pol 326-347; however, there were no consistent CTL responses. The significant proliferative responses in the EIAV lipopeptide-immunized horses allowed testing of the hypothesis that Th responses to immunization would enhance Th and CTL responses following EIAV challenge and lessen the viral load and the severity of clinical disease. The EIAV lipopeptide-immunized group did have a significant increase in proliferative responses to Th peptides 1 week after virus challenge, whereas the control group did not. Two weeks after challenge, a significant CTL response to virus-infected cell targets occurred in the EIAV lipopeptide-immunized group compared to that in the control group. These Th and CTL responses did not significantly alter either the number of viral RNA copies/ml or disease severity. Thus, lipopeptide-induced proliferative responses and enhanced Th and CTL responses early after virus challenge were unable to control challenge virus load and clinical disease.

Equine interferon gamma synthesis in lymphocytes after in vivo infection and in vitro stimulation with EHV-1

Equine cytotoxic T lymphocyte (CTL) responses to equine herpesvirus-1 (EHV-1) are well characterised but little is known about the cytokine response after infection or vaccination. EHV-1 is common in horses and infects lymphocytes in vivo. This virus was used as a model to measure the synthesis of interferon gamma (IFN-gamma) by equine peripheral blood mononuclear cells (PBMC) after in vivo infection and/or in vitro stimulation with EHV-1. Both flow cytometry and ELISPOT assays were used to quantify equine IFN-gamma using a mouse anti-bovine IFN-gamma monoclonal antibody (clone CC302; shown to cross-react with recombinant equine IFN-gamma) and a rabbit anti-canine IFN-gamma polyclonal antibody. The percentage of PBMC synthesising IFN-gamma after in vitro stimulation with EHV-1 increased with age. In yearlings infected experimentally with EHV-1, PBMC showed two peaks of IFN-gamma synthesis, 11 and 56 days after infection. The IFN-gamma synthesis was principally associated with CD8(+) cells. The patterns of IFN-gamma synthesis detected by intracellular IFN-gamma staining or ELISPOT were compared with CTL data and shown to be similar. These methods were also applied successfully to frozen samples of PBMC. Measurement of equine IFN-gamma using these simple techniques can now be applied to future studies on protective cellular immune responses following virus infection and/or vaccination of horses.

Characterisation and quantification of equine interferon gamma

Interferon-gamma (IFN-gamma) is a key cytokine in cell-mediated immunity. To measure IFN-gamma production of equine lymphocytes (eqIFN-gamma), we developed a quantitative ELISA. Monoclonal antibodies (mAb) were produced against bacterially derived eqIFN-gamma. The mAbs recognised recombinant and lymphocyte-derived eqIFN-gamma in ELISA, Western blotting, as well as flow cytometric and microscopic analysis. In contrast to bacterially derived material, mammalian and insect cell-derived eqIFN-gamma was biologically active but could be neutralised by one of the monoclonal antibodies. Unexpectedly, glycosylation seemed to be required for antiviral activity of eqIFN-gamma.

Equine herpesvirus-1 infection induces IFN-γ production by equine T lymphocyte subsets

A commercial bovine IFN-gamma-specific monoclonal antibody was used to measure antigen-specific IFN-gamma production by equine lymphocytes. Paired PBMC samples were collected from six ponies prior to and 10 days after challenge infection with equine herpesvirus-1 (EHV-1). Each sample was stimulated in vitro with EHV-1, virus-free medium, or PMA and ionomycin, and labelled with monoclonal antibodies specific for various equine lymphocyte subset markers. Following fixation, intracellular IFN-gamma was detected using a FITC-conjugated bovine IFN-gamma-specific monoclonal antibody. In vitro restimulation of PBMC with EHV-1 induced IFN-gamma production by a significantly higher percentage of total (CD5(+)) T lymphocytes, and CD4(+) and CD8(+) T lymphocyte subsets among post-EHV-1 infection PBMC samples compared to pre-infection samples. This response was associated with an increase in virus-specific CTL activity, a critical immune effector for the control of EHV-1 infection and disease. No significant increase in IFN-gamma production by B lymphocytes was observed. These data demonstrate that EHV-1 challenge infection of ponies results in increased production of IFN-gamma by virus-specific T lymphocytes, and that this response can be quantitated using flow cytometry.

The immune response to equine arteritis virus: potential lessons for other arteriviruses

The members of the family Arteriviridae, genus Arterivirus, include equine arteritis virus (EAV), porcine reproductive and respiratory syndrome virus (PRRSV), lactate dehydrogenase-elevating virus (LDV) of mice, and simian hemorrhagic fever virus (SHFV). PRRSV is the newest member of the family (first isolated in North America and Europe in the early 1990s), whereas the other three viruses were recognized earlier (EAV in 1953, LDV in 1960, and SHFV in 1964). Although arterivirus infections are strictly species-specific, the causative agents share many biological and molecular properties, including their virion morphology, replication strategy, unique properties of their structural proteins, and their ability to establish distinctive persistent infections in their natural hosts. The arteriviruses are each antigenically distinct and cause different disease syndromes in their natural hosts. Similarly, the mechanism(s) responsible for the prolonged and/or persistent infections that characterize infections with each arterivirus in their natural hosts are remarkably different. The objective of this review is to compare and contrast the immune response to EAV with that to the other three arteriviruses, and emphasize the potential relevance of apparent similarities and differences in the neutralization characteristics of each virus.

Peptide transport activity of the transporter associated with antigen processing (TAP) is inhibited by an early protein of equine herpesvirus-1

Equine herpesvirus-1 (EHV-1) downregulates surface expression of major histocompatibility complex (MHC) class I molecules on infected cells. The objective of this study was to investigate whether EHV-1 interferes with peptide translocation by the transporter associated with antigen processing (TAP) and to identify the proteins responsible. Using an in vitro transport assay, we showed that EHV-1 inhibited transport of peptides by TAP as early as 2 h post-infection (p.i). Complete shutdown of peptide transport was observed by 8 h p.i. Furthermore, pulse–chase experiments revealed that maturation of class I molecules in the endoplasmic reticulum (ER) was delayed in EHV-1-infected cells, which may be due to reduced availability of peptides in the ER as a result of TAP inhibition. Metabolic inhibition studies indicated that an early protein(s) of EHV-1 is responsible for this effect.

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.

Rhodococcus equi secreted antigens are immunogenic and stimulate a type 1 recall response in the lungs of horses immune to R. equi infection

Rhodococcus equi is an opportunistic pathogen in immunocompromised humans and an important primary pathogen in young horses. Although R. equi infection can produce life-threatening pyogranulomatous pneumonia, most foals develop a protective immune response that lasts throughout life. The antigen targets of this protective response are currently unknown; however, Mycobacterium tuberculosis is a closely related intracellular pathogen and provides a model system. Based on previous studies of M. tuberculosis protective antigens released into culture filtrate supernatant (CFS), a bacterial growth system was developed for obtaining R. equi CFS antigens. Potential immunogens for prevention of equine rhodococcal pneumonia were identified by using immunoblots. The 48-h CFS contained five virulence-associated protein bands that migrated between 12 and 24 kDa and were recognized by sera from R. equi-infected foals and immune adult horses. Notably, the CFS contained the previously characterized proteins VapC, VapD, and VapE, which are encoded by genes on the R. equi virulence plasmid. R. equi CFS was also examined for the ability to stimulate a type 1-like memory response in immune horses. Three adult horses were challenged with virulent R. equi, and cells from the bronchoalveolar lavage fluid were recovered before and 1 week after challenge. In vitro stimulation of pulmonary T-lymphocytes with R. equi CFS resulted in significant proliferation and a significant increase in gamma interferon mRNA expression 1 week after challenge. These results were consistent with a memory effector response in immune adult horses and provide evidence that R. equi CFS proteins are antigen targets in the immunoprotective response against R. equi infection.

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.

Selecting peptides to optimize Th1 responses to an equine lentivirus using HLA-DR binding motifs and defined HIV-1 Th peptides

Three moderately to broadly recognized equine infectious anemia virus (EIAV) peptides that contained helper T-lymphocyte (Th) 1 epitopes were previously identified. Although lipopeptide immunization was only weakly immunostimulatory in a preliminary study, as measured by T-lymphocyte proliferation responses, it was of interest to define additional broadly recognized Th1 epitopes to include in future immunization trials. Using broadly cross-reactive and conserved Th epitopes known in the related human immunodeficiency virus-1 (HIV-1) and binding motifs defined in human leukocyte antigen DR molecules as guides, this work identified three new peptides containing Th1 epitopes recognized by 60-75% of EIAV infected horses. The observed similarity across species of major histocompatibility complex (MHC) class II binding motifs and the conservation of Th peptides between related viruses should allow easier targeting of Th epitope regions in less well characterized pathogens and/or in species whose MHC class II molecules are poorly defined.

Detection of equine arteritis virus (EAV)-specific cytotoxic CD8+ T lymphocyte precursors from EAV-infected ponies

Equine arteritis virus (EAV) causes a systemic infection in equids with variable outcome, ranging from subclinical infections to severe disease, and also has the capacity to induce abortion in pregnant mares and persistent infections in stallions. The serum virus-neutralizing antibody response that invariably develops in the infected animal lasts for many months or years and is believed to play an important role in virus clearance. However, very little is known about cellular immunity against EAV because of a lack of methods for evaluating these immune responses. In the present study, we describe methods for detecting cytotoxic T lymphocyte (CTL) precursors in the peripheral blood of EAV-convalescent ponies using a 51Cr release cytolysis assay. Primary equine dermal cells, used as CTL targets, were shown to express MHC I but not MHC II and to retain 51Cr efficiently and support EAV replication. Peripheral blood mononuclear cells (PBMC) collected from EAV-convalescent ponies that had been incubated with or without live EAV were used as effectors. EAV-induced PBMC cultures showed evidence of expansion and activation of lymphoblasts, with an increase in the CD8+/CD4+ ratio in comparison with mock-induced PBMC. The cytotoxicity induced by EAV-stimulated PBMC was virus specific, showed genetic restriction, was mediated by CD8+ T lymphocytes and could be detected for periods of 4 months to more than 1 year post-infection. These findings and methods will hopefully contribute to an understanding of virus–host interactions in horses, in particular the mechanisms of virus clearance occurring during EAV infection.

Generation of a candidate live marker vaccine for equine arteritis virus by deletion of the major virus neutralization domain

Equine arteritis virus (EAV) is an enveloped plus-strand RNA virus of the family Arteriviridae (order Nidovirales) that causes respiratory and reproductive disease in equids. Protective, virus-neutralizing antibodies (VNAb) elicited by infection are directed predominantly against an immunodominant region in the membrane-proximal domain of the viral envelope glycoprotein G(L), allowing recently the establishment of a sensitive peptide enzyme-linked immunosorbent assay (ELISA) based on this particular domain (J. Nugent et al., J. Virol. Methods 90:167-183, 2000). By using an infectious cDNA we have now generated, in the controlled background of a nonvirulent virus, a mutant EAV from which this immunodominant domain was deleted. This virus, EAV-G(L)Delta, replicated to normal titers in culture cells, although at a slower rate than wild-type EAV, and caused an asymptomatic infection in ponies. The antibodies induced neutralized the mutant virus efficiently in vitro but reacted poorly to wild-type EAV strains. Nevertheless, when inoculated subsequently with virulent EAV, the immunized animals, in contrast to nonvaccinated controls, were fully protected against disease; replication of the challenge virus occurred briefly at low though detectable levels. The levels of protection achieved suggest that an immune effector mechanism other than VNAb plays an important role in protection against infection. As expected, infection with EAV-G(L)Delta did not induce a measurable response in our G(L)-peptide ELISA while the challenge infection of the animals clearly did. EAV-G(L)Delta or similar mutants are therefore attractive marker vaccine candidates, enabling serological discrimination between vaccinated and wild-type virus-infected animals.

Down-regulation of MHC class I expression by equine herpesvirus-1

There is good evidence that cytotoxic T lymphocytes play an important role in the clearance of equine herpesvirus-1 (EHV1) in horses. We have demonstrated that, in common with other alphaherpesviruses, EHV1 infection can lead to dramatic down-regulation of MHC class I expression at the cell surface, a common strategy for pathogen evasion of the host immune response. This down-regulation is specific for MHC class I and does not reflect a general shut-off of host-cell protein synthesis. The use of monoclonal antibodies that recognize different MHC class I epitopes has demonstrated that the effect may be allele- or locus-specific. Use of the viral DNA synthesis inhibitor phosphonoacetic acid, which prevents late viral gene expression, showed that the effect is mediated by an immediate-early or early viral gene, and use of the protein translation inhibitor cycloheximide confirmed that an early gene is primarily responsible. The data indicate that EHV1 infection results in enhanced endocytosis of MHC class I from the cell surface; the only other herpesvirus reported to use this mechanism is human herpesvirus-8. Elucidation of the precise mechanisms used by EHV1 in this process and identification of the genes responsible may lead to improved vaccination strategies.

Identification of broadly recognized, T helper 1 lymphocyte epitopes in an equine lentivirus

Equine infectious anaemia virus (EIAV) is a horse lentivirus causing lifelong, persistent infection. During acute infection, CD8(+) cytotoxic T lymphocytes (CTL) are probably involved in terminating plasma viraemia. However, only a few EIAV CTL epitopes, restricted to fewer horse major histocompatibility complex (MHC) class I alleles, are known. As interferon-gamma (IFN-gamma)-secreting CD4(+), T helper 1 (Th1) lymphocytes promote CTL activity and help maintain memory CTL, identifying broadly recognized EIAV Th1 epitopes would contribute significantly to vaccine strategies seeking to promote strong CTL responses among horses with varying class I haplotypes. To this end, peripheral blood mononuclear cells (PBMC) from 10 MHC disparate, EIAV-infected horses were tested in T-lymphocyte proliferation assays for recognition of peptides from the Gag p26 capsid region and a portion of Pol. Both regions are highly conserved among EIAV isolates, and this Pol region is 51-63% homologous to other lentiviral Pol proteins. Seven of 10 horses recognized peptide Gag 221-245, and peptides Gag 242-261 and Pol 323-344 were recognized by five and four horses, respectively. Furthermore, the Gag peptides were recognized by two additional horses after resolving their initial plasma viraemia, indicating that these two peptides can be immunodominant early in infection. Gag peptide-responsive PBMC produced only IFN-gamma, indicating a Th1 response, while Pol 323-344-responsive PBMC produced IFN-gamma both with and without interleukin-4. PBMC from uninfected horses failed to either proliferate or secrete cytokines in response to peptide stimulation. Finally, CD4(+) T lymphocytes were required for proliferation responses, as shown by assays using CD4- versus CD8-depleted PBMC.

The mucosal humoral immune response of the horse to infective challenge and vaccination with equine herpesvirus-1 antigens

Equine herpesvirus-1 (EHV-1) remains a frequent cause of upper respiratory tract infection and abortion in horses worldwide. However, little is known about the local antibody response elicited in the upper airways of horses following exposure to EHV-1. This study analysed the mucosal humoral immune response of weanling foals following experimental infection with virulent EHV-1, or vaccination with either of 2 commercial vaccines. Twenty weanlings were assigned to 5 groups and were inoculated with, or vaccinated against, EHV-1 following different regimens. Finally, all weanlings were simultaneously challenged intranasally with virulent EHV-1 Army 183 (A183). Nasal wash and serum samples were collected at regular intervals until 13 weeks after final challenge. Nasal washes were assayed for EHV-1-specific equine IgGa, IgGb, IgG(T), IgA, IgM and total virus-specific antibody using an indirect, quantitative ELISA. Total serum antibody responses were also monitored, and clinical signs of EHV-disease were recorded for each individual. Virus-specific IgA dominated the mucosal antibody response elicited in weanlings inoculated with A183, being detectable at up to 3.1 microg/mg total IgA 13 weeks after challenge. Neither inactivated EHV-1 administered i.m., nor attenuated EHV-1 administered intranasally induced detectable mucosal antibodies. EHV-1-specific mucosal antibodies impeded EHV-1 plaque formation in vitro. Such virus-neutralising antibody probably contributes to a reduction of shedding of EHV-1 from the respiratory tract of virus-infected horses.

Equine herpesvirus 1 (EHV-1) glycoprotein D DNA inoculation in horses with pre-existing EHV-1/EHV-4 antibody

We have shown previously that equine herpesvirus 1 (EHV-1) glycoprotein D (gD) DNA elicited protective immune responses against EHV-1 challenge in murine respiratory and abortion models of EHV-1 disease. In this study, 20 horses, all with pre-existing antibody to EHV-4 and two with pre-existing antibody to EHV-1, were inoculated intramuscularly with three doses each of 50, 200 or 500microg EHV-1 gD DNA or with 500microg vector DNA. In 8 of 15 horses, inoculation with EHV-1 gD DNA led to elevated gD-specific antibody and nine horses exhibited increased virus neutralising (VN) antibody titres compared to those present when first inoculated. A lack of increase in gC-specific antibody during the 66 weeks of the experiment showed that the increase in gD-specific antibodies was not due to a natural infection with either EHV-1 or EHV-4. The increase in EHV-1 gD-specific antibodies was predominantly an IgGa and IgGb antibody response, similar to the isotype profile reported following natural EHV-1 infection.