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

A Live-Attenuated Equine Influenza Vaccine Stimulates Innate Immunity in Equine Respiratory Epithelial Cell Cultures That Could Provide Protection From Equine Herpesvirus 1

Equine herpesvirus 1 (EHV-1) ubiquitously infects horses worldwide and causes respiratory disease, abortion, and equine herpesvirus myeloencephalopathy. Protection against EHV-1 disease is elusive due to establishment of latency and immune-modulatory features of the virus. These include the modulation of interferons, cytokines, chemokines, antigen presentation, and cellular immunity. Because the modulation of immunity likely occurs at the site of first infection—the respiratory epithelium, we hypothesized that the mucosal influenza vaccine Flu Avert^® I.N. (Flu Avert), which is known to stimulate strong antiviral responses, will enhance antiviral innate immunity, and that these responses would also provide protection from EHV-1 infection. To test our hypothesis, primary equine respiratory epithelial cells (ERECs) were treated with Flu Avert, and innate immunity was evaluated for 10 days following treatment. The timing of Flu Avert treatment was also evaluated for optimal effectiveness to reduce EHV-1 replication by modulating early immune responses to EHV-1. The induction of interferons, cytokine and chemokine mRNA expression, and protein secretion was evaluated by high-throughput qPCR and multiplex protein analysis. Intracellular and extracellular EHV-1 titers were determined by qPCR. Flu Avert treatment resulted in the modulation of IL-8, CCL2, and CXCL9 starting at days 5 and 6 post-treatment. Coinciding with the timing of optimal chemokine induction, our data also suggested the same timing for reduction of EHV-1 replication. In combination, our results suggest that Flu Avert may be effective at counteracting some of the immune-modulatory properties of EHV-1 at the airway epithelium and the peak for this response occurs 5–8 days post-Flu Avert treatment. Future in vivo studies are needed to investigate Flu Avert as a prophylactic in situations where EHV-1 exposure may occur.

Bovine Herpesvirus 1 Counteracts Immune Responses and Immune-Surveillance to Enhance Pathogenesis and Virus Transmission

Infection of cattle by bovine herpesvirus 1 (BoHV-1) can culminate in upper respiratory tract disorders, conjunctivitis, or genital disorders. Infection also consistently leads to transient immune-suppression. BoHV-1 is the number one infectious agent in cattle that is associated with abortions in cattle. BoHV-1, as other α-herpesvirinae subfamily members, establishes latency in sensory neurons. Stressful stimuli, mimicked by the synthetic corticosteroid dexamethasone, consistently induce reactivation from latency in latently infected calves and rabbits. Increased corticosteroid levels due to stress have a two-pronged effect on reactivation from latency by: (1) directly stimulating viral gene expression and replication, and (2) impairing antiviral immune responses, thus enhancing virus spread and transmission. BoHV-1 encodes several proteins, bICP0, bICP27, gG, UL49.5, and VP8, which interfere with key antiviral innate immune responses in the absence of other viral genes. Furthermore, the ability of BoHV-1 to infect lymphocytes and induce apoptosis, in particular CD4+ T cells, has negative impacts on immune responses during acute infection. BoHV-1 induced immune-suppression can initiate the poly-microbial disorder known as bovine respiratory disease complex, which costs the US cattle industry more than one billion dollars annually. Furthermore, interfering with antiviral responses may promote viral spread to ovaries and the developing fetus, thus enhancing reproductive issues associated with BoHV-1 infection of cows or pregnant cows. The focus of this review is to describe the known mechanisms, direct and indirect, by which BoHV-1 interferes with antiviral immune responses during the course of infection.

Taxonomic patterns in the zoonotic potential of mammalian viruses

Predicting and simplifying which pathogens may spill over from animals to humans is a major priority in infectious disease biology. Many efforts to determine which viruses are at risk of spillover use a subset of viral traits to find trait-based associations with spillover. We adapt a new method-phylofactorization-to identify not traits but lineages of viruses at risk of spilling over. Phylofactorization is used to partition the International Committee on Taxonomy of Viruses viral taxonomy based on non-human host range of viruses and whether there exists evidence the viruses have infected humans. We identify clades on a range of taxonomic levels with high or low propensities to spillover, thereby simplifying the classification of zoonotic potential of mammalian viruses. Phylofactorization by whether a virus is zoonotic yields many disjoint clades of viruses containing few to no representatives that have spilled over to humans. Phylofactorization by non-human host breadth yields several clades with significantly higher host breadth. We connect the phylogenetic factors above with life-histories of clades, revisit trait-based analyses, and illustrate how cladistic coarse-graining of zoonotic potential can refine trait-based analyses by illuminating clade-specific determinants of spillover risk.

Abortigenic but Not Neurotropic Equine Herpes Virus 1 Modulates the Interferon Antiviral Defense

Equine herpesvirus 1 (EHV1) is considered as a major pathogen of Equidae, causing symptoms from mild respiratory disease to late-term abortion and neurological disorders. Different EHV1 strains circulating in the field have been characterized to be of abortigenic or neurovirulent phenotype. Both variants replicate in a plaque-wise manner in the epithelium of the upper respiratory tract (URT), where the abortigenic strains induce more prominent viral plaques, compared to the neurovirulent strains. Considering the differences in replication at the URT, we hypothesized that abortigenic strains may show an increased ability to modulate the type I IFN secretion/signaling pathway, compared to strains that display the neurovirulent phenotype. Here, we analyze IFN levels induced by abortigenic and neurovirulent EHV1 using primary respiratory epithelial cells (EREC) and respiratory mucosa ex vivo explants. Similar levels of IFNα (~70 U/ml) were detected in explants inoculated with both types of EHV1 strains from 48 to 72 hpi. Second, EREC and mucosa explants were treated with recombinant equine IFNα (rEqIFNα) or Ruxolitinib (Rux), an IFN signaling inhibitor, prior to and during inoculation with abortigenic or neurovirulent EHV1. Replication of both EHV1 variants was suppressed by rEqIFNα. Further, addition of Rux increased replication in a concentration-dependent manner, indicating an IFN-susceptibility for both variants. However, in two out of three horses, at a physiological concentration of 100 U/ml of rEqIFNα, an increase in abortigenic EHV1 replication was observed compared to 10 U/ml of rEqIFNα, which was not observed for the neurovirulent strains. Moreover, in the presence of Rux, the plaque size of the abortigenic variants remained unaltered, whereas the typically smaller viral plaques induced by the neurovirulent variants became larger. Overall, our results demonstrate the importance of IFNα in the control of EHV1 replication in the URT for both abortigenic and neurovirulent variants. In addition, our findings support the speculation that abortigenic variants of EHV1 may have developed anti-IFN mechanisms that appear to be absent or less pronounced in neurovirulent EHV1 strains.

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.

Equid herpesvirus 1 (EHV1) infection of equine mesenchymal stem cells induces a pUL56-dependent downregulation of select cell surface markers

Equid herpesvirus 1 (EHV1) is an ubiquitous alphaherpesvirus that can cause respiratory disease, abortion and central nervous disorders. EHV1 is known to infect a variety of different cell types in vitro, but its tropism for cultured primary equine mesenchymal stem cells (MSC) has never been explored. We report that equine MSC were highly permissive for EHV1 and supported lytic replication of the virus in vitro. Interestingly, we observed that an infection of MSC with EHV1 resulted in a consistent downregulation of cell surface molecules CD29 (β1-integrin), CD105 (endoglin), major histocompatibility complex type I (MHCI) and a variable downregulation of CD172a. In contrast, expression of CD44 and CD90 remained unchanged upon wild type infection. In addition, we found that this selective EHV1-mediated downregulation of cell surface proteins was dependent on the viral protein UL56 (pUL56). So far, pUL56-dependent downregulation during EHV1 infection of equine cells has only been described for MHCI, but our present data indicate that pUL56 may have a broader function in downregulating cell surface proteins. Taken together, our results are the first to show that equine MSC are susceptible for EHV1 and that pUL56 induces downregulation of several cell surface molecules on infected cells. These findings provide a basis for future studies to evaluate the mechanisms underlying for this selective pUL56-induced downregulation and to evaluate the potential role of MSC during EHV1 pathogenesis.

Equine herpesvirus type 1 pUL56 modulates innate responses of airway epithelial cells

Recently, the product of equine herpesvirus type 1 (EHV-1) ORF1, a homolog to HSV-1 pUL56, was shown to modulate MHC-I expression and innate immunity. Here, we investigated modulation of respiratory epithelial immunity by EHV-1 pUL56 and compared responses to those of PBMCs, which are important target cells that allow cell-associated EHV-1 viremia. The salient observations are as follows: (i) EHV-1 significantly down-modulated MHC-I and MHC-II expression in equine respiratory epithelial cells (ERECs). MHC-I expression remained unaffected in PBMCs and MHC-II expression was increased. (ii) Infection with an EHV-1 ORF1 deletion mutant partially restored MHC-I and MHC-II expression and altered IFN-alpha and IL-10 mRNA expression. (iii) Deletion of EHV-1 ORF1 also significantly increased chemokine expression and chemotaxis of monocytes and neutrophils in ERECs. Collectively, these results suggest a role for EHV-1 pUL56 in modulation of antigen presentation, cytokine expression and chemotaxis at the respiratory epithelium, but not in PBMC.

Immunity to bovine herpesvirus 1: II. Adaptive immunity and vaccinology

Bovine herpesvirus 1 (BHV-1) infection is widespread and causes a variety of diseases. Although similar in many respects to the human immune response to human herpesvirus 1, the differences in the bovine virus proteins, immune system components and strategies, physiology, and lifestyle mean the bovine immune response to BHV-1 is unique. The innate immune system initially responds to infection, and primes a balanced adaptive immune response. Cell-mediated immunity, including cytotoxic T lymphocyte killing of infected cells, is critical to recovery from infection. Humoral immunity, including neutralizing antibody and antibody-dependent cell-mediated cytotoxicity, is important to prevention or control of (re-)infection. BHV-1 immune evasion strategies include suppression of major histocompatibility complex presentation of viral antigen, helper T-cell killing, and latency. Immune suppression caused by the virus potentiates secondary infections and contributes to the costly bovine respiratory disease complex. Vaccination against BHV-1 is widely practiced. The many vaccines reported include replicating and non-replicating, conventional and genetically engineered, as well as marker and non-marker preparations. Current development focuses on delivery of major BHV-1 glycoproteins to elicit a balanced, protective immune response, while excluding serologic markers and virulence or other undesirable factors. In North America, vaccines are used to prevent or reduce clinical signs, whereas in some European Union countries marker vaccines have been employed in the eradication of BHV-1 disease.

Herpes B virus, macacine herpesvirus 1, breaks simplex virus tradition via major histocompatibility complex class I expression in cells from human and macaque hosts

B virus of the family Herpesviridae is endemic to rhesus macaques but results in 80% fatality in untreated humans who are zoonotically infected. Downregulation of major histocompatibility complex (MHC) class I in order to evade CD8(+) T-cell activation is characteristic of most herpesviruses. Here we examined the cell surface presence and total protein expression of MHC class I molecules in B virus-infected human foreskin fibroblast cells and macaque kidney epithelial cells in culture, which are representative of foreign and natural host initial target cells of B virus. Our results show <20% downregulation of surface MHC class I molecules in either type of host cells infected with B virus, which is statistically insignificantly different from that observed in uninfected cells. We also examined the surface expression of MHC class Ib molecules, HLA-E and HLA-G, involved in NK cell inhibition. Our results showed significant upregulation of HLA-E and HLA-G in host cells infected with B virus relative to the amounts observed in other herpesvirus-infected cells. These results suggest that B virus-infected cell surfaces maintain normal levels of MHC class Ia molecules, a finding unique among simplex viruses. This is a unique divergence in immune evasion for B virus, which, unlike human simplex viruses, does not inhibit the transport of peptides for loading onto MHC class Ia molecules because B virus ICP47 lacks a transporter-associated protein binding domain. The fact that MHC class Ib molecules were significantly upregulated has additional implications for host-pathogen interactions.

Equine herpesvirus type 4 UL56 and UL49.5 proteins downregulate cell surface major histocompatibility complex class I expression independently of each other

Major histocompatibility complex class I (MHC-I) molecules are critically important in the host defense against various pathogens through presentation of viral peptides to cytotoxic T lymphocytes (CTLs), a process resulting in the destruction of virus-infected cells. Herpesviruses interfere with CTL-mediated elimination of infected cells by various mechanisms, including inhibition of peptide transport and loading, perturbation of MHC-I trafficking, and rerouting and proteolysis of cell surface MHC-I. In this study, we show that equine herpesvirus type 4 (EHV-4) modulates MHC-I cell surface expression through two different mechanisms. First, EHV-4 can lead to a significant downregulation of MHC-I expression at the cell surface through the product of ORF1, a protein expressed with early kinetics from a gene that is homologous to herpes simplex virus 1 UL56. The EHV-4 UL56 protein reduces cell surface MHC-I as early as 4 h after infection. Second, EHV-4 can interfere with MHC-I antigen presentation, starting at 6 h after infection, by inhibition of the transporter associated with antigen processing (TAP) through its UL49.5 protein. Although pUL49.5 has no immediate effect on overall surface MHC-I levels in infected cells, it blocks the supply of antigenic peptides to the endoplasmic reticulum (ER) and transport of peptide-loaded MHC-I to the cell surface. Taken together, our results show that EHV-4 encodes at least two viral immune evasion proteins: pUL56 reduces MHC-I molecules on the cell surface at early times after infection, and pUL49.5 interferes with MHC-I antigen presentation by blocking peptide transport in the ER.

Identification and characterization of equine herpesvirus type 1 pUL56 and its role in virus-induced downregulation of major histocompatibility complex class I

Major histocompatibility complex class I (MHC-I) molecules play an important role in host immunity to infection by presenting antigenic peptides to cytotoxic T lymphocytes (CTLs), which recognize and destroy virus-infected cells. Members of the Herpesviridae have developed multiple mechanisms to avoid CTL recognition by virtue of downregulation of MHC-I on the cell surface. We report here on an immunomodulatory protein involved in this process, pUL56, which is encoded by ORF1 of equine herpesvirus type 1 (EHV-1), an alphaherpesvirus. We show that EHV-1 pUL56 is a phosphorylated early protein which is expressed as different forms and predominantly localizes to Golgi membranes. In addition, the transmembrane (TM) domain of the type II membrane protein was shown to be indispensable for correct subcellular localization and a proper function. pUL56 by itself is not functional with respect to interference with MHC-I and likely needs another unidentified viral protein(s) to perform this action. Surprisingly, pUL49.5, an inhibitor of the transporter associated with antigen processing (TAP) and encoded by EHV-1 and related viruses, appeared not to be required for pUL56-induced early MHC-I downmodulation in infected cells. In conclusion, our data identify a new immunomodulatory protein, pUL56, involved in MHC-I downregulation which is unable to perform its function outside the context of viral infection.

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.

Equus caballus major histocompatibility complex class I is an entry receptor for equine herpesvirus type 1

In this study, Equus caballus major histocompatibility complex class I (MHC-I) was identified as a cellular entry receptor for the alphaherpesvirus equine herpesvirus type 1 (EHV-1). This novel EHV-1 receptor was discovered using a cDNA library from equine macrophages. cDNAs from this EHV-1-susceptible cell type were inserted into EHV-1-resistant B78H1 murine melanoma cells, these cells were infected with an EHV-1 lacZ reporter virus, and cells that supported virus infection were identified by X-Gal (5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside) staining. Positive cells were subjected to several rounds of purification to obtain homogeneous cell populations that were shown to be uniformly infected with EHV-1. cDNAs from these cell populations were amplified by PCR and then sequenced. The sequence data revealed that the EHV-1-susceptible cells had acquired an E. caballus MHC-I cDNA. Cell surface expression of this receptor was verified by confocal immunofluorescence microscopy. The MHC-I cDNA was cloned into a mammalian expression vector, and stable B78H1 cell lines were generated that express this receptor. These cell lines were susceptible to EHV-1 infection while the parental B78H1 cells remained resistant to infection. In addition, EHV-1 infection of the B78H1 MHC-I-expressing cell lines was inhibited in a dose-dependent manner by an anti-MHC-I antibody.

Regulation of Innate Immune Responses by Bovine Herpesvirus 1 and Infected Cell Protein 0 (bICP0)

Bovine herpesvirus 1 (BoHV-1) infected cell protein 0 (bICP0) is an important transcriptional regulatory protein that stimulates productive infection. In transient transfection assays, bICP0 also inhibits interferon dependent transcription. bICP0 can induce degradation of interferon stimulatory factor 3 (IRF3), a cellular transcription factor that is crucial for activating beta interferon (IFN-β) promoter activity. Recent studies also concluded that interactions between bICP0 and IRF7 inhibit trans-activation of IFN-β promoter activity. The C3HC4 zinc RING (really important new gene) finger located near the amino terminus of bICP0 is important for all known functions of bICP0. A recombinant virus that contains a single amino acid change in a well conserved cysteine residue of the C3HC4 zinc RING finger of bICP0 grows poorly in cultured cells, and does not reactivate from latency in cattle confirming that the C3HC4 zinc RING finger is crucial for viral growth and pathogenesis. A bICP0 deletion mutant does not induce plaques in permissive cells, but induces autophagy in a cell type dependent manner. In summary, the ability of bICP0 to stimulate productive infection, and repress IFN dependent transcription plays a crucial role in the BoHV-1 infection cycle.

Biology of bovine herpesvirus 5

Bovine herpesvirus 5 (BoHV-5) is an alphaherpesvirus responsible for meningoencephalitis in young cattle and is closely antigenically and genetically related to bovine herpesvirus 1 (BoHV-1). Both viruses have common aspects in their pathogenesis: (1) they infect epithelial cells at the portal of entry and (2) they establish a latent infection in the sensory nerve ganglia, i.e., the trigeminal ganglia. However, they have different neuroinvasion and neurovirulence capacities. Only in rare cases can BoHV-1 reach the brain of infected cattle. BoHV-5 infection induces different degrees of severity of neurological disease depending on both viral and host factors. Although a case of BoHV-5 associated disease in Europe and some outbreaks in USA and Australia have been reported, the current geographical distribution of BoHV-5 infection is mainly restricted to South America, especially Brazil and Argentina. This review focuses on the genomic characteristics, pathobiology and epidemiology of BoHV-5, in order to provide information on the possible basis of alphaherpesvirus neuropathogenesis.

Feline herpesvirus-1 down-regulates MHC class I expression in an homologous cell system

Cytotoxic T lymphocytes (CTLs) are an essential component of the immune defense against many virus infections. CTLs recognize viral peptides in the context of the major histocompatibility complex (MHC) class I molecules on the surface of infected cells. Many viruses have evolved mechanisms to interfere with MHC class I expression as a means of evading the host immune response. In the present research we have studied the effect of in vitro Feline Herpesvirus 1 (FeHV-1) infection on MHC class I expression. The results of this study demonstrate that FeHV-1 down regulates surface expression of MHC class I molecules on infected cells, presumably to evade cytotoxic T-cell recognition and, perhaps, attenuate induction of immunity. Sensitivity to UV irradiation and insensitivity to a viral DNA synthesis inhibitor, like phosphonacetic acid, revealed that immediate early or early viral gene(s) are responsible. Use of the protein translation inhibitor cycloheximide confirmed that an early gene is primarily responsible.

Varicellovirus UL 49.5 proteins differentially affect the function of the transporter associated with antigen processing, TAP

Cytotoxic T-lymphocytes play an important role in the protection against viral infections, which they detect through the recognition of virus-derived peptides, presented in the context of MHC class I molecules at the surface of the infected cell. The transporter associated with antigen processing (TAP) plays an essential role in MHC class I–restricted antigen presentation, as TAP imports peptides into the ER, where peptide loading of MHC class I molecules takes place. In this study, the UL49.5 proteins of the varicelloviruses bovine herpesvirus 1 (BHV-1), pseudorabies virus (PRV), and equine herpesvirus 1 and 4 (EHV-1 and EHV-4) are characterized as members of a novel class of viral immune evasion proteins. These UL49.5 proteins interfere with MHC class I antigen presentation by blocking the supply of antigenic peptides through inhibition of TAP. BHV-1, PRV, and EHV-1 recombinant viruses lacking UL49.5 no longer interfere with peptide transport. Combined with the observation that the individually expressed UL49.5 proteins block TAP as well, these data indicate that UL49.5 is the viral factor that is both necessary and sufficient to abolish TAP function during productive infection by these viruses. The mechanisms through which the UL49.5 proteins of BHV-1, PRV, EHV-1, and EHV-4 block TAP exhibit surprising diversity. BHV-1 UL49.5 targets TAP for proteasomal degradation, whereas EHV-1 and EHV-4 UL49.5 interfere with the binding of ATP to TAP. In contrast, TAP stability and ATP recruitment are not affected by PRV UL49.5, although it has the capacity to arrest the peptide transporter in a translocation-incompetent state, a property shared with the BHV-1 and EHV-1 UL49.5. Taken together, these results classify the UL49.5 gene products of BHV-1, PRV, EHV-1, and EHV-4 as members of a novel family of viral immune evasion proteins, inhibiting TAP through a variety of mechanisms.

Herpesviruses have the conspicuous property that they persist for life in the infected host. This is also the case for varicelloviruses, a large subfamily of herpesviruses with representatives in humans (varicella zoster virus or VZV), cattle (bovine herpesvirus 1 or BHV-1), pigs (pseudorabies virus or PRV), and horses (equine herpesvirus or EHV type 1 and 4), among many others. Cytotoxic T-lymphocytes play an important role in the protection against viral infections, which they detect through the recognition of virus-derived peptides, presented in the context of MHC class I molecules at the surface of the infected cell. The transporter associated with antigen processing (TAP) plays an essential role in this process, as TAP imports peptides into the compartment where peptide loading of the MHC class I molecules takes place. In this study, we show that the UL49.5 proteins of BHV-1, PRV, EHV-1, and EHV-4 all block the supply of peptides through the inhibition of TAP, but that the mechanisms employed by these proteins to inhibit TAP function exhibit surprising diversity. VZV UL49.5, on the other hand, binds to TAP, but does not interfere with peptide transport. Our study classifies the UL49.5 proteins of BHV-1, PRV, EHV-1, and EHV-4 as members of a novel family of viral immune evasion proteins, inhibiting TAP through a variety of mechanisms.

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.

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.

Absence of viral envelope proteins in equine herpesvirus 1-infected blood mononuclear cells during cell-associated viremia

In vitro studies demonstrated that most equine herpesvirus 1 (EHV-1)-infected peripheral blood mononuclear cells (PBMC) do not expose viral envelope proteins on their surface. This protects them against antibody-dependent lysis. We examined whether viral envelope proteins are also undetectable on infected PBMC during cell-associated viremia. Further, surface expression of major histocompatibility complex (MHC)-I was examined, since MHC-I assists in making infected cells recognizable for cytotoxic T-lymphocytes (CTL). Four ponies, previously exposed to EHV, and two ponies that had no contact with EHV before, were inoculated with EHV-1. PBMC were collected at different time points up to 28 days post inoculation. Ninety-eight percent of the infected PBMC did not show viral envelope proteins on their surface. Moreover, infected PBMC without surface expression only produced immediate early and, at least, one early protein, ICP22, but not late envelope proteins gB and gM. This indicates that surface expression of viral envelope proteins is absent, simply because the PBMC are in an early phase of infection. The percentage of infected PBMC showing surface expression of MHC-I was similar as observed in non-infected PBMC from the same ponies (80-100%). Therefore, inefficient recognition of EHV-1-infected PBMC by CTLs does not arise from absent surface expression of MHC-I.

Cytokine responses to EHV-1 infection in immune and non-immune ponies

Protecting equids against equine herpesvirus-1 (EHV-1) infection remains an elusive goal. Repeated infection with EHV-1 leads to protective immunity against clinical respiratory disease, and a study was conducted to measure the regulatory cytokine response (IFN-gamma and IL-4) in repeatedly infected immune ponies compared to non-immune ponies. Two groups of four ponies were established. Group 1 ponies had previously been infected on two occasions, and most recently 7 months before this study. Group 2 ponies had no history no vaccination or challenge infection prior to this study. Both groups were subjected to an intranasal challenge infection with EHV-1, and blood samples were collected pre-infection, and at 7 and 21 days post-infection for preparation of PBMCs. At each time point, the in vitro responses of PBMCs to stimulation with EHV-1 were measured, including IFN-gamma and IL-4 mRNA production, and lymphoproliferation. Group 1 ponies showed no signs of clinical disease or viral shedding after challenge infection. Group 2 ponies experienced a biphasic pyrexia, mucopurulent nasal discharge, and nasal shedding of virus after infection. Group 1 ponies had an immune response characterized both before and subsequent to challenge infection by an IFN-gamma response to EHV-1 in the absence of an IL-4 response, and demonstrated increased EHV-1-specific lymphoproliferation post-infection. Group 2 ponies had limited cytokine or lymphoproliferative responses to EHV-1 pre-challenge, and demonstrated increases in both IFN-gamma and IL-4 responses post-challenge, but without any lymphoproliferative response. Protective immunity to EHV-1 infection was therefore characterized by a polarized IFN-gamma dependent immunoregulatory cytokine response.

Immune escape of equine herpesvirus 1 and other herpesviruses of veterinary importance

Equine herpesvirus (EHV)-1 is a pathogen of horses, well known for its ability to induce abortion and nervous system disorders. Clinical signs may occur despite the presence of a virus-specific immune response in the horse. The current review will summarize the research, on how, EHV-1-infected cells can hide from recognition by the immune system. Research findings on immune evasion of EHV-1 will be compared with those of other herpesviruses of veterinary importance.

Molecular mechanisms of HLA class I antigen abnormalities following viral infection and transformation

In humans as in other animal species, CD8+ cytotoxic T lymphocytes (CTLs) play an important if not the major role in controlling virus-infected and malignant cell growth. The interactions between CD8+ T cells and target cells are mediated by human leukocyte antigen (HLA) class I antigens loaded with viral and tumor antigen-derived peptides along with costimulatory receptor/ligand stimuli. Thus, to escape from CD8+ T-cell recognition and destruction, viruses and tumor cells have developed strategies to inhibit the expression and/or function of HLA class I antigens. In contrast, cells with downregulated MHC class I surface expression can be recognized by NK cells, although NK cell-mediated lysis could be abrogated by the expression of inhibiting NK cell receptors. This review discusses the molecular mechanisms utilized by viruses to inhibit the formation, transport and/or expression of HLA class I antigen/peptide complexes on the cell surface. The knowledge about viral interference with MHC class I antigen presentation is not only crucial to understand the pathogenesis of viral diseases, but contributes also to the design of novel strategies to counteract the escape mechanisms utilized by viruses. These investigations may eventually lead to the development of effective immunotherapies to control viral infections and virus-associated malignant diseases.

The Intracellular Antigen Transport Machinery TAP in Adaptive Immunity and Virus Escape Mechanisms

The transporter associated with antigen processing (TAP) is a crucial element of the adaptive immune system, which translocates proteasomal degradation products into the endoplasmic reticulum, for transfer of these peptides on major histocompatibility complex (MHC) I molecules within a macromolecular peptide-loading complex. After loading and intracellular transport to the cell surface, these peptide/MHC complexes are monitored by cytotoxic T-lymphocytes. This review summarizes the structural organization and function of the ABC transporter TAP. Furthermore, we discuss human diseases and viral evasion strategies associated with TAP function.

Viral interference with MHC class I antigen presentation pathway: the battle continues

CD8+ cytotoxic T lymphocytes (CTLs) play a critical role in the defense against viral infections. In general, CD8+ CTLs recognize antigenic peptides in the context of the major histocompatibility complex (MHC) class I molecule. The MHC class I molecules are expressed on almost all the nucleated cells in the body. The trimolecular complex consisting of the class I heavy chain, beta2-microglobulin and the peptide are generated by the MHC class I antigen presentation pathway. This pathway is designed to sample the intracellular milieu and present the information to the CTLs trafficking the area. This rigorous sampling of intracellular environment enables the CTLs to quickly identify and eliminate the cells that synthesize non-self proteins as a result of a viral infection. Many viruses, including several viruses of veterinary importance, have evolved astounding strategies to interfere with the MHC class I antigen presentation pathway, as a means of evading the CTL response of the host. This review focuses on the diverse mechanisms of viral evasion of the MHC class I antigen presentation pathway with particular emphasis on viruses of veterinary importance.

Viral evasion of the MHC class I antigen-processing machinery

In their adaptation to the immune system in vertebrates, viruses have been forced to evolve elaborate strategies for evading the host's immune response. To ensure life-long persistence in the host, herpes viruses, adenoviruses and retroviruses have exploited multiple cellular pathways for their purpose, including the class I antigen-processing machinery. Attractive and prominent targets for viral attacks are the proteasome complex, the transporter associated with antigen processing, and MHC class I molecules. This review briefly outlines the different mechanisms of viral interference with the antigen-presentation pathway.

Varicelloviruses avoid T cell recognition by UL49.5-mediated inactivation of the transporter associated with antigen processing

Detection and elimination of virus-infected cells by cytotoxic T lymphocytes depends on recognition of virus-derived peptides presented by MHC class I molecules. A critical step in this process is the translocation of peptides from the cytoplasm into the endoplasmic reticulum by the transporter associated with antigen processing (TAP). Here, we identified the bovine herpesvirus 1-encoded UL49.5 protein as a potent inhibitor of TAP. The expression of UL49.5 results in down-regulation of MHC class I molecules at the cell surface and inhibits detection and lysis of the cells by cytotoxic T lymphocytes. UL49.5 homologs encoded by two other varicelloviruses, pseudorabies-virus and equine herpesvirus 1, also block TAP. Homologs of UL49.5 are widely present in herpesviruses, acting as interaction partners for glycoprotein M, but in several varicelloviruses UL49.5 has uniquely evolved additional functions that mediate its participation in TAP inhibition. Inactivation of TAP by UL49.5 involves two events: inhibition of peptide transport through a conformational arrest of the transporter and degradation of TAP by proteasomes. UL49.5 is degraded along with TAP via a reaction that requires the cytoplasmic tail of UL49.5. Thus, UL49.5 represents a unique immune evasion protein that inactivates TAP through a unique two-tiered process.

TAP genes and immunity

The transporter associated with antigen processing (TAP) is a member of the ATP-binding cassette transporter family that specializes in delivering cytosolic peptides to class I molecules in the endoplasmic reticulum. The TAP is a major target of genetic alteration in tumours and disruption by viral inhibitors. In some species, TAP genes have co-evolved with MHC class I molecules to deliver peptides that are customised for particular alleles. In humans, MHC class I polymorphism determines the level of tapasin-mediated association with TAP and subsequent peptide optimisation within the peptide-loading complex (PLC). MHC class I molecules that still load peptides without complexing to the TAP might be more resistant to viral interference of the PLC and less sensitive to competition for TAP by other class I allotypes.