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

Equine herpesvirus type 1 (EHV-1) replication at the upper respiratory entry site is inhibited by neutralizing EHV-1-specific IgG1 and IgG4/7 mucosal antibodies

Equine herpesvirus type 1 (EHV-1) is a contagious respiratory pathogen that infects the mucosa of the upper respiratory tract (URT). Mucosal immune responses at the URT provide the first line of defense against EHV-1 and are crucial for orchestrating immunity. To define host-pathogen interactions, we characterized B-cell responses, antibody isotype functions, and EHV-1 replication of susceptible (non-immune) and clinically protected (immune) horses after experimental EHV-1 infection. Nasal secretion and nasal wash samples were collected and used for the isolation of DNA, RNA, and mucosal antibodies. Shedding of infectious virus, EHV-1 copy numbers, viral RNA expression, and host B-cell activation in the URT were compared based on host immune status. Mucosal EHV-1-specific antibody responses were associated with EHV-1 shedding and viral RNA transcription. Finally, mucosal immunoglobulin G (IgG) and IgA isotypes were purified and tested for neutralizing capabilities. IgG1 and IgG4/7 neutralized EHV-1, while IgG3/5, IgG6, and IgA did not. Immune horses secreted high amounts of mucosal EHV-1-specific IgG4/7 antibodies and quickly upregulated B-cell pathway genes, while EHV-1 was undetected by virus isolation and PCR. RNA transcription analysis reinforced incomplete viral replication in immune horses. In contrast, complete viral replication with high viral copy numbers and shedding of infectious viruses was characteristic for non-immune horses, together with low or absent EHV-1-specific neutralizing antibodies during viral replication. These data confirm that pre-existing mucosal IgG1 and IgG4/7 and rapid B-cell activation upon EHV-1 infection are essential for virus neutralization, regulation of viral replication, and mucosal immunity against EHV-1.IMPORTANCEEquine herpesvirus type 1 (EHV-1) causes respiratory disease, abortion storms, and neurologic outbreaks known as equine herpes myeloencephalopathy (EHM). EHV-1 is transmitted with respiratory secretions by nose-to-nose contact or via fomites. The virus initially infects the epithelium of the upper respiratory tract (URT). Host-pathogen interactions and mucosal immunity at the viral entry site provide the first line of defense against the EHV-1. Robust mucosal immunity can be essential in protecting against EHV-1 and to reduce EHM outbreaks. It has previously been shown that immune horses do not establish cell-associated viremia, the prerequisite for EHM. Here, we demonstrate how mucosal antibodies can prevent the replication of EHV-1 at the epithelium of the URT and, thereby, the progression of the virus to the peripheral blood. The findings improve the mechanistic understanding of mucosal immunity against EHV-1 and can support the development of enhanced diagnostic tools, vaccines against EHM, and the management of EHV-1 outbreaks.

Monoclonal antibody development advances immunological research in horses

Host immune analyses require specific reagents to identify cellular and soluble components of the immune system. These immune reagents are often species-specific. For horses, various immunological tools have been developed and tested by different initiatives during the past decades. This article summarizes the development of well characterized monoclonal antibodies (mAbs) for equine immune cells, immunoglobulin isotypes, cytokines, and chemokines.

Relationship between equine herpesvirus-1 viremia and abortion or equine herpesvirus myeloencephalopathy in domesticated horses: A systematic review

BACKGROUND

Equine herpes virus type 1 (EHV-1) infection in horses is associated with upper respiratory disease, neurological disease, abortions, and neonatal death.

OBJECTIVE

To determine if there is an association between the level and duration of EHV-1 viremia and either abortion or equine herpesvirus myeloencephalopathy (EHM) in domesticated horses?

METHODS

A systematic review was performed searching numerous databases to identify peer reviewed reports that evaluated viremia and EHM, or viremia and abortion published before January 19, 2021. Randomized controlled trials and observational studies were assessed for risk of bias or publication quality.

RESULTS

A total of 189 unique studies were identified, of which 34 met the inclusion criteria. Thirty studies evaluated viremia and neurologic outcomes including 4 observational studies. Eight experimental studies examined viremia and abortion, which used the Ab4 and OH03 virus strains or recombinant Ab4 derivatives. Incidence rates for both EHM and abortion in experimental studies varied among the studies as did the level of evidence. Viremia was generally detectable before the onset of either EHM or abortion. Risk of bias was generally low to moderate, sample sizes were small, and multiple studies reported negative outcome data.

CONCLUSIONS AND CLINICAL IMPORTANCE

The results of this study support that viremia is regularly present before EHM or abortion occurs. However, no inferences could be made about the relationship between the occurrence of either neurological signs or abortion and the magnitude or duration of viremia.

Impact of the host immune response on the development of equine herpesvirus myeloencephalopathy in horses

Herpesviruses establish a well-adapted balance with their host's immune system. Despite this co-evolutionary balance, infections can lead to severe disease including neurological disorders in their natural host. In horses, equine herpesvirus 1 (EHV-1) causes respiratory disease, abortions, neonatal foal death and myeloencephalopathy (EHM) in ~10 % of acute infections worldwide. Many aspects of EHM pathogenesis and protection from EHM are still poorly understood. However, it has been shown that the incidence of EHM increases to >70 % in female horses >20 years of age. In this study we used old mares as an experimental equine EHV-1 model of EHM to identify host-specific factors contributing to EHM. Following experimental infection with the neuropathogenic strain EHV-1 Ab4, old mares and yearling horses were studied for 21 days post-infection. Nasal viral shedding and cell-associated viremia were assessed by quantitative PCR. Cytokine/chemokine responses were evaluated in nasal secretions and cerebrospinal fluid (CSF) by Luminex assay and in whole blood by quantitative real-time PCR. EHV-1-specific IgG sub-isotype responses were measured by ELISA. All young horses developed respiratory disease and a bi-phasic fever post-infection, but only 1/9 horses exhibited ataxia. In contrast, respiratory disease was absent in old mares, but all old mares developed EHM that resulted in euthanasia in 6/9 old mares. Old mares also presented significantly decreased nasal viral shedding but higher viremia coinciding with a single fever peak at the onset of viremia. According to clinical disease manifestation, horses were sorted into an EHM group (nine old horses and one young horse) and a non-EHM group (eight young horses) for assessment of host immune responses. Non-EHM horses showed an early upregulation of IFN-α (nasal secretions), IRF7/IRF9, IL-1β, CXCL10 and TBET (blood) in addition to an IFN-γ upregulation during viremia (blood). In contrast, IFN-α levels in nasal secretions of EHM horses were low and peak levels of IRF7, IRF9, CXCL10 and TGF-β (blood) coincided with viremia. Moreover, EHM horses showed significantly higher IL-10 levels in nasal secretions, peripheral blood mononuclear cells and CSF and higher serum IgG3/5 antibody titres compared to non-EHM horses. These results suggest that protection from EHM depends on timely induction of type 1 IFN and upregulation cytokines and chemokines that are representative of cellular immunity. In contrast, induction of regulatory or TH-2 type immunity appeared to correlate with an increased risk for EHM. It is likely that future vaccine development for protection from EHM must target shifting this 'at-risk' immunophenotype.

Experimental challenge of horses after prime-boost immunization with a modified live equid alphaherpesvirus 1 vaccine administered by two different routes

The immune response and protective efficacy of a modified equid alphaherpesvirus 1 (EHV-1) vaccine administered by two different routes were tested in horses. Horses that received intramuscular (IM) priming and an intranasal (IN) booster with a 28-day interval (IM-IN group [n = 6]), IN priming and IM booster (IN-IM group [n = 5]), or no vaccination (control group [n = 6]) were challenged with EHV-1 strain 10-I-224 28 days after the second vaccination. Both vaccinated groups had significantly higher serum virus-neutralizing titers than the control group, with increased levels of serum IgGa, IgGb, and IgA antibodies (p < 0.05). The nasal antibody response was dominated by the IgGa and IgGb subclasses in both vaccinated groups, with no IgA antibody response. After challenge infection, three of six control horses were pyretic for 1-4 days post-inoculation (dpi), whereas none in the vaccinated groups were pyretic during this period. The number of horses that were pyretic at 5-10 dpi was 4 out of 6 for the controls, 3 out of 6 for the IM-IN group, and 2 out of 5 for the IN-IM group. Nasal virus replication in the IN-IM group (3-4 dpi) and IM-IN group (3 dpi) was significantly lower than in the control group (p < 0.05). All of the control horses showed viremia, whereas two horses in the IM-IN group and one in the IN-IM group did not. In conclusion, although IM-IN or IN-IM vaccination did not elicit a mucosal IgA response, it provided partial protection at a level similar to that of the conventional program, likely due to systemic antibodies and mucosal IgG subclass responses.

Efficacy of vaccination against equine herpesvirus type 1 (EHV-1) infection: systematic review and meta-analysis of randomized controlled challenge trials

BACKGROUND

Equid herpesvirus type 1 (EHV-1) infection can cause a range of disease syndromes of variable severity that can result in a lethal outcome and restriction of horse movements, especially in the case of outbreaks involving neurological disease. Vaccination is one of the tools used to control the infection. It is widely known that vaccination is not completely effective in ensuring protection against disease caused by this virus. In fact, the real efficacy of vaccination against EHV-1 related disease has not been measured and no systematic reviews exist on this topic.

OBJECTIVES

To perform a systematic review and meta-analysis on the efficacy of commercial or candidate vaccines against EHV-1 in randomised controlled trials (RCT) all of which involved experimental challenge of the test subjects.

STUDY DESIGN

Systematic review and meta-analysis.

METHODS

RCTs were searched using the search algorithm (((equid herpesvirus* OR equine herpesvirus* OR EHV-1)) AND vaccin*) AND (trial OR experimental OR challenge) on PubMed, Science Citation Index Expanded, Scopus, and CAB Abstracts. Where appropriate, meta-analysis was performed using RevMan 5.4.

RESULTS

Eight studies were selected and were analysed for their respective characteristics and possible shortcomings. The results of RCTs revealed that there was a general improvement in the clinical and virological outcomes of EHV-1 infection following vaccination, but that the effects were very slight. The reduced beneficial effect is probably amplified by the paucity of detailed data reported in the studies, that did not allow for the comparison of parameters in many of the cases analysed.

MAIN LIMITATIONS

The remarkable heterogeneity and the poor quality of reporting of the selected studies.

CONCLUSIONS

Meta-analysis has shown that EHV-1 vaccination generally results in a slight improvement in clinical and virological outcomes, although not to a significant extent. The cumulative results have probably been affected by the lack of information on some parameters not systematically reported in the studies. An improvement in the standard of reporting and better standardisation of the data collected would likely have improved the quality of each study and enabled more effective comparison of the studies with each other.

Contribution of the immune response to the pathogenesis of equine herpesvirus-1 (EHV-1): Are there immune correlates that predict increased risk or protection from EHV-1 myeloencephalopathy?

Equine herpesvirus-1 (EHV-1) myeloencephalopathy (EHM) is a devastating consequence of EHV-1 infection that has significant economic consequences. However, clinical EHM is relatively rare and occurs in only approximately 10% of infected horses. While there is a positive correlation between the duration and magnitude of viremia and incidence of EHM, it is likely that a combination of host and viral factors determine whether EHM occurs. The identification of these factors is of high interest for the equine community and has been the topic of much research for vaccine development and to predict which horses might be most at risk for developing EHM. The aim of this review is to highlight host immunity contributions to EHM pathogenesis at different sites of EHV-1 infection to shed light on the different aspects and interdependence of the response to EHV-1 in the time course of infection.

Viral infection and allergy - What equine immune responses can tell us about disease severity and protection

Horses have many naturally occurring diseases that mimic similar conditions in humans. The ability to conduct environmentally controlled experiments and induced disease studies in a genetically diverse host makes the horse a valuable intermediate model between mouse studies and human clinical trials. This review highlights important similarities in the immune landscape between horses and humans using current research on two equine diseases as examples. First, equine herpesvirus type 1 (EHV-1) infection initiates a series of innate inflammatory signals at its mucosal entry site in the upper respiratory tract. These inflammatory markers are highly synchronized and predictable between individuals during viral respiratory infection and ultimately lead to adaptive immune induction and protection. The timing of early inflammatory signals, followed by specific adaptive immune markers correlating with immunity and protection, allow accurate outbreak tracking and also provide a foundation for understanding the importance of local mucosal immunity during other viral respiratory infections. Second, rare peripheral blood immune cells that promote allergic inflammation can be analyzed during Culicoides hypersensitivity, a naturally occurring type I IgE-mediated allergic disease of horses. Rare immune cells, such as IgE-binding monocytes or basophils, can be studied repeatedly in the horse model to unravel their larger mechanistic role in inflammation during allergic and other inflammatory diseases. We conclude with a survey of all other common equine inflammatory conditions. Together, this review serves as a reference and rationale for the horse as a non-rodent model for immunological research.

Equine Herpesvirus Type 1 Modulates Cytokine and Chemokine Profiles of Mononuclear Cells for Efficient Dissemination to Target Organs

Equine herpesvirus type 1 (EHV-1) causes encephalomyelopathy and abortion, for which cell-associated viremia and subsequent virus transfer to and replication in endothelial cells (EC) are responsible and prerequisites. Viral and cellular molecules responsible for efficient cell-to-cell spread of EHV-1 between peripheral blood mononuclear cells (PBMC) and EC remain unclear. We have generated EHV-1 mutants lacking ORF1, ORF2, and ORF17 genes, either individually or in combination. Mutant viruses were analyzed for their replication properties in cultured equine dermal cells, PBMC infection efficiency, virus-induced changes in the PBMC proteome, and cytokine and chemokine expression profiles. ORF1, ORF2, and ORF17 are not essential for virus replication, but ORF17 deletion resulted in a significant reduction in plaque size. Deletion of ORF2 and ORF17 gene significantly reduced cell-to-cell virus transfer from virus-infected PBMC to EC. EHV-1 infection of PBMC resulted in upregulation of several pathways such as Ras signaling, oxidative phosphorylation, platelet activation and leukocyte transendothelial migration. In contrast, chemokine signaling, RNA degradation and apoptotic pathways were downregulated. Deletion of ORF1, ORF2 and ORF17 modulated chemokine signaling and MAPK pathways in infected PBMC, which may explain the impairment of virus spread between PBMC and EC. The proteomic results were further confirmed by chemokine assays, which showed that virus infection dramatically reduced the cytokine/chemokine release in infected PBMC. This study uncovers cellular proteins and pathways influenced by EHV-1 after PBMC infection and provide an important resource for EHV-1 pathogenesis. EHV-1-immunomodulatory genes could be potential targets for the development of live attenuated vaccines or therapeutics against virus infection.

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 IgG4/7 antibody responses protect horses against equid herpesvirus-1 (EHV-1) infection including nasal virus shedding and cell-associated viremia

Equid herpesvirus-1 (EHV-1) outbreaks continue despite widely used vaccination. We demonstrated previously that an ORF1/ORF71 gene deletion mutant of the EHV-1 strain Ab4 (Ab4ΔORF1/71) is less virulent than its parent Ab4 virus. Here, we describe the Ab4 challenge infection evaluating protection induced by the Ab4ΔORF1/71 vaccine candidate. Susceptible control horses developed respiratory disease, fever, nasal shedding, and viremia. Full protection after challenge infection was observed in 5/5 previously Ab4 infected horses and 3/5 Ab4ΔORF1/71 horses. Two Ab4ΔORF1/71 horses developed short-lasting viremia and/or virus shedding. Protective immunity in the respiratory tract was characterized by pre-existing EHV-1-specific IgG4/7 antibodies, the absence of IFN-α secretion and rapidly increasing IgG4/7 upon challenge infection. Pre-existing systemic EHV-1-specific IgG4/7 highly correlated with protection. T-cell immunity was overall low. In conclusion, protective immunity against EHV-1 infection including prevention of viremia was associated with robust systemic and intranasal IgG4/7 antibodies suggesting immediate virus neutralization at the local site.