The UL41-encoded virion host shutoff (vhs) protein and vhs-independent mechanisms are responsible for down-regulation of MHC class I molecules by bovine herpesvirus 1

The herpesvirus UL49.5 protein hijacks a cellular C-degron pathway to drive TAP transporter degradation

The transporter associated with antigen processing (TAP) is a key player in the major histocompatibility class I-restricted antigen presentation and an attractive target for immune evasion by viruses. Bovine herpesvirus 1 impairs TAP-dependent antigenic peptide transport through a two-pronged mechanism in which binding of the UL49.5 gene product to TAP both inhibits peptide transport and triggers its proteasomal degradation. How UL49.5 promotes TAP degradation has, so far, remained unknown. Here, we use high-content siRNA and genome-wide CRISPR-Cas9 screening to identify CLR2KLHDC3 as the E3 ligase responsible for UL49.5-triggered TAP disposal. We propose that the C terminus of UL49.5 mimics a C-end rule degron that recruits the E3 to TAP and engages the cullin-RING E3 ligase in endoplasmic reticulum-associated degradation.

Understanding the Neurotrophic Virus Mechanisms and Their Potential Effect on Systemic Lupus Erythematosus Development

Central nervous system (CNS) pathologies are a public health concern, with viral infections one of their principal causes. These viruses are known as neurotropic pathogens, characterized by their ability to infiltrate the CNS and thus interact with various cell populations, inducing several diseases. The immune response elicited by neurotropic viruses in the CNS is commanded mainly by microglia, which, together with other local cells, can secrete inflammatory cytokines to fight the infection. The most relevant neurotropic viruses are adenovirus (AdV), cytomegalovirus (CMV), enterovirus (EV), Epstein-Barr Virus (EBV), herpes simplex virus type 1 (HSV-1), and herpes simplex virus type 2 (HSV-2), lymphocytic choriomeningitis virus (LCMV), and the newly discovered SARS-CoV-2. Several studies have associated a viral infection with systemic lupus erythematosus (SLE) and neuropsychiatric lupus (NPSLE) manifestations. This article will review the knowledge about viral infections, CNS pathologies, and the immune response against them. Also, it allows us to understand the relevance of the different viral proteins in developing neuronal pathologies, SLE and NPSLE.

The herpesvirus UL49.5 protein hijacks a cellular C-degron pathway to drive TAP transporter degradation

The transporter associated with antigen processing (TAP) is a key player in the MHC class I-restricted antigen presentation and an attractive target for immune evasion by viruses. Bovine herpesvirus 1 (BoHV-1) impairs TAP-dependent antigenic peptide transport through a two-pronged mechanism in which binding of the UL49.5 gene product to TAP both inhibits peptide transport and promotes its proteasomal degradation. How UL49.5 promotes TAP degradation is unknown. Here, we use high-content siRNA and genome-wide CRISPR-Cas9 screening to identify CLR2KLHDC3 as the E3 ligase responsible for UL49.5-triggered TAP disposal in human cells. We propose that the C-terminus of UL49.5 mimics a C-end rule degron that recruits the E3 to TAP and engages the CRL2 E3 in ER-associated degradation.

The N-terminal Proline Hinge Motif Controls the Structure of Bovine Herpesvirus 1-encoded Inhibitor of the Transporter Associated with Antigen Processing Required for its Immunomodulatory Function

Due to unique features, proline residues may control protein structure and function. Here, we investigated the role of ⁵²PPQ⁵⁴ residues, indicated by the recently established experimental 3D structure of bovine herpesvirus 1-encoded UL49.5 protein as forming a characteristic proline hinge motif in its N-terminal domain. UL49.5 acts as a potent inhibitor of the transporter associated with antigen processing (TAP), which alters the antiviral immune response. Mechanisms employed by UL49.5 to affect TAP remain undetermined on a molecular level. We found that mutations in the ⁵²PPQ⁵⁴ region had a vast impact on its immunomodulatory function, increasing cell surface MHC class I expression, TAP levels, and peptide transport efficiency. This inhibitory effect was specific for UL49.5 activity towards TAP but not towards the viral glycoprotein M. To get an insight into the impact of proline hinge modifications on structure and dynamics, we performed all-atom and coarse-grained molecular dynamics studies on the native protein and PPQ mutants. The results demonstrated that the proline hinge sequence with its highly rigid conformation served as an anchor into the membrane. This anchor was responsible for the structural and dynamical behavior of the whole protein, constraining the mobility of the C-terminus, increasing the mobility of the transmembrane region, and controlling the accessibility of the C-terminal residues to the cytoplasmic environment. Those features appear crucial for TAP binding and inhibition. Our findings significantly advance the structural understanding of the UL49.5 protein and its functional regions and support the importance of proline motifs for the protein structure.

The Involvement of Histone H3 Acetylation in Bovine Herpesvirus 1 Replication in MDBK Cells

During bovine herpesvirus 1 (BoHV-1) productive infection in cell cultures, partial of intranuclear viral DNA is present in nucleosomes, and viral protein VP22 associates with histones and decreases histone H4 acetylation, indicating the involvement of histone H4 acetylation in virus replication. In this study, we demonstrated that BoHV-1 infection at the late stage (at 24 h after infection) dramatically decreased histone H3 acetylation [at residues K9 (H3K9ac) and K18 (H3K18ac)], which was supported by the pronounced depletion of histone acetyltransferases (HATs) including CBP/P300 (CREB binding protein and p300), GCN5L2 (general control of amino acid synthesis yeast homolog like 2) and PCAF (P300/CBP-associated factor). The depletion of GCN5L2 promoted by virus infection was partially mediated by ubiquitin-proteasome pathway. Interestingly, the viral replication was enhanced by HAT (histone acetyltransferase) activator CTPB [N-(4-Chloro-3-trifluoromethylphenyl)-2-ethoxy-6-pentadecylbenzamide], and vice versa, inhibited by HAT inhibitor Anacardic acid (AA), suggesting that BoHV-1 may take advantage of histone acetylation for efficient replication. Taken together, we proposed that the HAT-dependent histone H3 acetylation plays an important role in BoHV-1 replication in MDBK (Madin-Darby bovine kidney) cells.

A bimodal switch in global protein translation coupled to eIF4H relocalisation during advancing cell-cell transmission of herpes simplex virus

We used the bioorthogonal protein precursor, homopropargylglycine (HPG) and chemical ligation to fluorescent capture agents, to define spatiotemporal regulation of global translation during herpes simplex virus (HSV) cell-to-cell spread at single cell resolution. Translational activity was spatially stratified during advancing infection, with distal uninfected cells showing normal levels of translation, surrounding zones at the earliest stages of infection with profound global shutoff. These cells further surround previously infected cells with restored translation close to levels in uninfected cells, reflecting a very early biphasic switch in translational control. While this process was dependent on the virion host shutoff (vhs) function, in certain cell types we also observed temporally altered efficiency of shutoff whereby during early transmission, naïve cells initially exhibited resistance to shutoff but as infection advanced, naïve target cells succumbed to more extensive translational suppression. This may reflect spatiotemporal variation in the balance of oscillating suppression-recovery phases. Our results also strongly indicate that a single particle of HSV-2, can promote pronounced global shutoff. We also demonstrate that the vhs interacting factor, eIF4H, an RNA helicase accessory factor, switches from cytoplasmic to nuclear localisation precisely correlating with the initial shutdown of translation. However translational recovery occurs despite sustained eIF4H nuclear accumulation, indicating a qualitative change in the translational apparatus before and after suppression. Modelling simulations of high multiplicity infection reveal limitations in assessing translational activity due to sampling frequency in population studies and how analysis at the single cell level overcomes such limitations. The work reveals new insight and a revised model of translational manipulation during advancing infection which has important implications both mechanistically and with regards to the physiological role of translational control during virus propagation. The work also demonstrates the potential of bioorthogonal chemistry for single cell analysis of cellular metabolic processes during advancing infections in other virus systems.

Molecular characterization of duck enteritis virus UL41 protein

BACKGROUND

Duck enteritis virus (DEV) belongs to the subfamily Alphaherpesvirinae, and information on the DEV UL41 gene is limited.

METHODS

The DEV UL41 gene was cloned into the pET32a(+) vector and expressed in a prokaryotic expression system. Antiserum was raised against a bacterially expressed UL41-His fusion protein for further experiments. Transcription was quantified and UL41 protein expression levels were determined in DEV-infected cells at different time points by RT-qPCR and western blotting, respectively. DEV virions were purified by sucrose gradient centrifugation and analyzed by mass spectrometry to identify protein content. We confirmed the DEV UL41 gene kinetic class using a pharmacological test. IFA was used to analyze the intracellular localization of pUL41.

RESULTS

The recombinant expression plasmid, pET-32a(+)-UL41, which highly expresses a 76.0 kDa fusion protein, was constructed and expressed in E. coli BL21 (DE3) after induction with 0.2 mM IPTG at 30 °C for 10 h, generating a specific mouse anti-UL41 protein polyclonal antibody. RT-qPCR and western blot analyses revealed that the UL41 transcript number peaked at 36 hpi, and peak protein expression occurred at 48 hpi. The pharmacological test showed that UL41 was a γ2 gene. Mass spectrometry analysis showed that pUL41 was a virion component. IFA results revealed that pUL41 was localized throughout DEV-infected cells but only localized to the cytoplasm of transfected cells. DEV pUL47 translocated pUL41 to the nuclei of DEF cells; this translocation was dependent on predicted pUL47 NLS signals (40-50 aa and 768-777 aa).

CONCLUSIONS

DEV UL41 is a γ2 gene that encodes a virion structural protein, pUL41 localizes throughout DEV-infected cells but only localizes to the cytoplasm of transfected cells. pUL41 cannot autonomously localize to the nucleus, as this nuclear localization is dependent on predicted DEV pUL47 NLS signals (40-50 aa and 768-777 aa).

RNA-seq comparative analysis of Peking ducks spleen gene expression 24 h post-infected with duck plague virulent or attenuated virus

Duck plague virus (DPV), a member of alphaherpesvirus sub-family, can cause significant economic losses on duck farms in China. DPV Chinese virulent strain (CHv) is highly pathogenic and could induce massive ducks death. Attenuated DPV vaccines (CHa) have been put into service against duck plague with billions of doses in China each year. Researches on DPV have been development for many years, however, a comprehensive understanding of molecular mechanisms underlying pathogenicity of CHv strain and protection of CHa strain to ducks is still blank. In present study, we performed RNA-seq technology to analyze transcriptome profiling of duck spleens for the first time to identify differentially expressed genes (DEGs) associated with the infection of CHv and CHa at 24 h. Comparison of gene expression with mock ducks revealed 748 DEGs and 484 DEGs after CHv and CHa infection, respectively. Gene pathway analysis of DEGs highlighted valuable biological processes involved in host immune response, cell apoptosis and viral invasion. Genes expressed in those pathways were different in CHv infected duck spleens and CHa vaccinated duck spleens. The results may provide valuable information for us to explore the reasons of pathogenicity caused by CHv strain and protection activated by CHa strain.

Protein Composition of the Bovine Herpesvirus 1.1 Virion

Bovine herpesvirus (BoHV) type 1 is an important agricultural pathogen that infects cattle and other ruminants worldwide. Acute infection of the oro-respiratory tract leads to immune suppression and allows commensal bacteria to infect an otherwise healthy lower respiratory tract. This condition is known as the Bovine Respiratory Disease (BRD). BoHV-1 latently infects the host for life and periodical stress events re-initiate BRD, translating into high morbidity and large economic losses. To gain a better understanding of the biology of BoHV-1 and the disease it causes, we elucidated the protein composition of extracellular virions using liquid chromatography-mass spectrometry analysis. We detected 33 viral proteins, including the expected proteins of the nucleocapsid and envelope as well as other regulatory proteins present in the viral tegument. In addition to viral proteins, we have also identified packaged proteins of host origin. This constitutes the first proteomic characterization of the BoHV virion.

Seroprevalence of respiratory viral pathogens of indigenous calves in Western Kenya

Most studies of infectious diseases in East African cattle have concentrated on gastro-intestinal parasites and vector-borne diseases. As a result, relatively little is known about viral diseases, except for those that are clinically symptomatic or which affect international trade such as foot and mouth disease, bluetongue and epizootic haemorrhagic disease. Here, we investigate the seroprevalence, distribution and relationship between the viruses involved in respiratory disease, infectious bovine rhinotracheitis virus (IBR), bovine parainfluenza virus Type 3 (PIV3) and bovine viral diarrhoea virus (BVDV) in East African Shorthorn Zebu calves. These viruses contribute to the bovine respiratory disease complex (BRD) which is responsible for major economic losses in cattle from intensive farming systems as a result of pneumonia. We found that calves experience similar risks of infection for IBR, PIV3, and BVDV with a seroprevalence of 20.9%, 20.1% and 19.8% respectively. We confirm that positive associations exist between IBR, PIV3 and BVDV; being seropositive for any one of these three viruses means that an individual is more likely to be seropositive for the other two viruses than expected by chance.

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.

Cytomegalovirus immune evasion by perturbation of endosomal trafficking

Cytomegaloviruses (CMVs), members of the herpesvirus family, have evolved a variety of mechanisms to evade the immune response to survive in infected hosts and to establish latent infection. They effectively hide infected cells from the effector mechanisms of adaptive immunity by eliminating cellular proteins (major histocompatibility Class I and Class II molecules) from the cell surface that display viral antigens to CD8 and CD4 T lymphocytes. CMVs also successfully escape recognition and elimination of infected cells by natural killer (NK) cells, effector cells of innate immunity, either by mimicking NK cell inhibitory ligands or by downregulating NK cell-activating ligands. To accomplish these immunoevasion functions, CMVs encode several proteins that function in the biosynthetic pathway by inhibiting the assembly and trafficking of cellular proteins that participate in immune recognition and thereby, block their appearance at the cell surface. However, elimination of these proteins from the cell surface can also be achieved by perturbation of their endosomal route and subsequent relocation from the cell surface into intracellular compartments. Namely, the physiological route of every cellular protein, including immune recognition molecules, is characterized by specific features that determine its residence time at the cell surface. In this review, we summarize the current understanding of endocytic trafficking of immune recognition molecules and perturbations of the endosomal system during infection with CMVs and other members of the herpesvirus family that contribute to their immune evasion mechanisms.

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.

Cell-mediated immune responses induced by BHV-1: rational vaccine design

Bovine herpesvirus-1 (BHV-1) is one of the major respiratory pathogens in cattle worldwide. Although antibodies have been correlated with protection and recovery from BHV-1 infection, the cell-mediated immune response is also a critical defense mechanism because cell-to-cell spread occurs before hematogenous spread. Furthermore, induction of robust T-cell memory is critical for the long-term duration of immunity. Among current commercial vaccines, the attenuated conventional vaccines induce a balanced immune response and long-term memory but may result in viral shedding. By contrast, inactivated vaccines primarily elicit a humoral immune response and relative short-term memory. These vaccines do not allow differentiation of vaccinated from infected cattle. Recent efforts are focusing on the development of vaccines that induce a balanced immune response and long-term memory, as well as having differentiation markers. This includes well-defined genetically engineered gene-deleted, subunit and vectored vaccines.

Immune regulation and evasion of Mammalian host cell immunity during viral infection

The mammalian host immune system has wide array of defence mechanisms against viral infections. Depending on host immunity and the extent of viral persistence, either the host immune cells might clear/restrict the viral load and disease progression or the virus might evade host immunity by down regulating host immune effector response(s). Viral antigen processing and presentation in the host cells through major histocompatibility complex (MHC) elicit subsequent anti-viral effector T cell response(s). However, modulation of such response(s) might generate one of the important viral immune evasion strategies. Viral peptides are mostly generated by proteolytic cleavage in the cytosol of the infected host cells. CD8(+) T lymphocytes play critical role in the detection of viral infection by recognizing these peptides displayed at the plasma membrane by MHC-I molecules. The present review summarises the current knowledge on the regulation of mammalian host innate and adaptive immune components, which are operative in defence mechanisms against viral infections and the variety of strategies that viruses have evolved to escape host cell immunity. The understanding of viral immune evasion strategies is important for designing anti-viral immunotherapies.

MHC class I cross-presentation by dendritic cells counteracts viral immune evasion

DCs very potently activate CD8(+) T cells specific for viral peptides bound to MHC class I molecules. However, many viruses have evolved immune evasion mechanisms, which inactivate infected DCs and might reduce priming of T cells. Then MHC class I cross-presentation of exogenous viral Ag by non-infected DCs may become crucial to assure CD8(+) T cell responses. Although many vital functions of infected DCs are inhibited in vitro by many different viruses, the contributions of cross-presentation to T cell immunity when confronted with viral immune inactivation in vivo has not been demonstrated up to now, and remains controversial. Here we show that priming of Herpes Simplex Virus (HSV)-, but not murine cytomegalovirus (mCMV)-specific CD8(+) T cells was severely reduced in mice with a DC-specific cross-presentation deficiency. In contrast, while CD8(+) T cell responses to mutant HSV, which lacks crucial inhibitory genes, also depended on CD8α(+) DCs, they were independent of cross-presentation. Therefore HSV-specific CTL-responses entirely depend on the CD8α(+) DC subset, which present via direct or cross-presentation mechanisms depending on the immune evasion equipment of virus. Our data establish the contribution of cross-presentation to counteract viral immune evasion mechanisms in some, but not all viruses.

The "Bridge" in the Epstein-Barr virus alkaline exonuclease protein BGLF5 contributes to shutoff activity during productive infection

Replication of the human herpesvirus Epstein-Barr virus drastically impairs cellular protein synthesis. This shutoff phenotype results from mRNA degradation upon expression of the early lytic-phase protein BGLF5. Interestingly, BGLF5 is the viral DNase, or alkaline exonuclease, homologues of which are present throughout the herpesvirus family. During productive infection, this DNase is essential for processing and packaging of the viral genome. In contrast to this widely conserved DNase activity, shutoff is only mediated by the alkaline exonucleases of the subfamily of gammaherpesviruses. Here, we show that BGLF5 can degrade mRNAs of both cellular and viral origin, irrespective of polyadenylation. Furthermore, shutoff by BGLF5 induces nuclear relocalization of the cytosolic poly(A) binding protein. Guided by the recently resolved BGLF5 structure, mutants were generated and analyzed for functional consequences on DNase and shutoff activities. On the one hand, a point mutation destroying DNase activity also blocks RNase function, implying that both activities share a catalytic site. On the other hand, other mutations are more selective, having a more pronounced effect on either DNA degradation or shutoff. The latter results are indicative of an oligonucleotide-binding site that is partially shared by DNA and RNA. For this, the flexible "bridge" that crosses the active-site canyon of BGLF5 appears to contribute to the interaction with RNA substrates. These findings extend our understanding of the molecular basis for the shutoff function of BGLF5 that is conserved in gammaherpesviruses but not in alpha- and betaherpesviruses.

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.

Bovine herpesvirus type 1 (BHV-1) mutant lacking U(L)49.5 luminal domain residues 30-32 and cytoplasmic tail residues 80-96 induces more rapid onset of virus neutralizing antibody and cellular immune responses in calves than the wild-type strain Cooper

Bovine herpesvirus type 1 (BHV-1) envelope protein U(L)49.5 inhibits transporter associated with antigen processing (TAP) and down-regulates cell-surface expression of major histocompatibility complex (MHC) class I molecules to promote immune evasion. Earlier, we have constructed a BHV-1U(L)49.5Δ30-32 CT-null virus and determined that in the infected cells, TAP inhibition and MHC-I down regulation properties of the virus are abolished. In this study, we compared the pathogenicity and immune responses in calves infected with BHV-1U(L)49.5Δ30-32 CT-null and BHV-1 wt viruses. Following primary infection, both BHV-1 wt and BHV-1U(L)49.5Δ30-32 CT-null virus replicated in the nasal epithelium with very similar yields. BHV-1 antigen-specific CD8+ T cell proliferation as well as CD8+ T cell cytotoxicity in calves infected with the BHV-1U(L)49.5Δ30-32 CT-null virus peaked by 7 dpi (P<0.05) which is 7 days earlier than that of BHV-1 wt-infected calves. Further, virus neutralizing antibody (VN Ab) titers and IFN-γ producing peripheral blood mononuclear cells (PBMCs) in the U(L)49.5 mutant virus-infected calves, also peaked 7 days (IFN-γ; P<0.05) and 14 days (VN Ab; P<0.05) earlier, respectively. Therefore, relative to wt in the BHV-1U(L)49.5 mutant virus-infected calves, primary neutralizing antibody and cellular immune responses were induced significantly more rapidly.

Bovine herpesvirus type 1 (BHV-1) UL49.5 luminal domain residues 30 to 32 are critical for MHC-I down-regulation in virus-infected cells

Bovine herpesvirus type 1 (BHV-1) U_L49.5 inhibits transporter associated with antigen processing (TAP) and down-regulates cell-surface expression of major histocompatibility complex (MHC) class I molecules to promote immune evasion. We have constructed a BHV-1 U_L49.5 cytoplasmic tail (CT) null and several U_L49.5 luminal domain mutants in the backbone of wild-type BHV-1 or BHV-1 U_L49.5 CT- null viruses and determined their relative TAP mediated peptide transport inhibition and MHC-1 down-regulation properties compared with BHV-1 wt. Based on our results, the U_L49.5 luminal domain residues 30–32 and U_L49.5 CT residues, together, promote efficient TAP inhibition and MHC-I down-regulation functions. In vitro, BHV-1 U_L49.5 Δ30–32 CT-null virus growth property was similar to that of BHV-1 wt and like the wt U_L49.5, the mutant U_L49.5 was incorporated in the virion envelope and it formed a complex with gM in the infected cells.

Viral RNase Involvement in Strategies of Infection

The overwhelming majority of RNase activity is engaged in catabolic processes. Viruses have no metabolism of their own, but rely completely on host cellular energy and substrate provision to support the biochemical processes necessary for virus replication. It is therefore obvious that RNA hydrolysis does not represent an obligate step in the viral life cycle that would have to be governed by viral proteins. Accordingly, RNases are found only rarely in the viral proteomes and serve special functions. In this chapter, several virus-specific RNases will be described and their role in the viral life cycle discussed. The text will concentrate on RNases of members of the nidoviruses, herpesviruses, pestiviruses, and several viruses with segmented negative-strand RNA genome including influenza virus. These enzymes are involved in specific steps of viral gene expression, viral genome replication, shutoff of host cellular gene expression, and interference with the host’s immune response to virus infection.

Viral evasion of T cell immunity: ancient mechanisms offering new applications

Upon infecting a host, viruses are confronted by a coordinated and multi-faceted immune response. Indeed, evolutionary combat between virus and host has contributed signally to the host's development of a formidable innate and adaptive immune defense arsenal, and to the virus' acquisition of effective means to evade it. Cytotoxic T lymphocytes play a key role in the elimination of virus-infected cells, which they detect through recognition of virus-derived peptides displayed at the cell surface in the context of MHC class I molecules. This highly sensitive recognition system is a prime target for immune evasion strategies deployed by many viruses, particularly large DNA viruses such as herpesviruses and poxviruses. Elucidation of the mode of action of the immune evasion proteins encoded by these viruses has not only provided new insights into viral pathogenesis, but has also led to the discovery of hitherto unknown cell biological and immunological phenomena. Moreover, viral immune evasion proteins constitute extremely useful tools to block defined stages of the MHC class I presentation pathway, not only for research purposes, but also for clinical applications.

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.

Role of the UL41 protein of pseudorabies virus in host shutoff, pathogenesis and induction of TNF-α expression

The vhs (virion host shutoff) is highly conserved in alphaherpesvirus, including pseudorabies virus (PRV). In an attempt to explore the function of vhs of PRV, we constructed and characterized a mutant virus (Δ41). In the absence of vhs activity, Δ41 mutant is highly attenuated in mice model and the lethality is correlated with the virus dissemination in neural tissues. As with herpes simplex virus type 1 (HSV-1), the prototype virus of alphaherpesvirus, the pronounced decrease in cellular protein synthesis triggered by wild type PRV was largely restored in cells infected with Δ41 virus. Furthermore, tumor necrosis factor-α (TNF-α) protein expression was elevated significantly in spleen of mice infected with vhs mutant virus. Since TNF-α has been indicated to be an important cytokine in the innate immune response against various infections, our results implicate vhs may contribute to the protection against PRV lethality via the action of TNF-α. Overall, we confirm the shutoff function of vhs protein in PRV, and demonstrate the role that vhs protein plays in virulence, and regulation of cytokine production.

A UL47 gene deletion mutant of bovine herpesvirus type 1 exhibits impaired growth in cell culture and lack of virulence in cattle

Tegument protein VP8 encoded by the U(L)47 gene of bovine herpesvirus type 1 (BHV-1) is the most abundant constituent of mature virions. In the present report, we describe the characterization of U(L)47 gene-deleted BHV-1 in cultured cells and its natural host. The U(L)47 deletion mutant exhibited reduced plaque size and more than 100-fold decrease in intracellular and extracellular viral titers in cultured cells. Ultrastructural observations of infected cells showed normal maturation of BHV-1 virions in the absence of VP8. There was no evidence for a change in immediate-early gene activator function of VP16 in the U(L)47 deletion mutant virus-infected cells, since bovine ICP4 mRNA and protein levels were similar to those in the wild-type and revertant virus-infected cells throughout the course of infection. Whereas VP16, glycoprotein C (gC), gB, and VP5 were expressed to wild-type levels in the U(L)47 deletion mutant-infected cells, the gD and VP22 protein levels were significantly reduced. The reduction in gD protein was associated with increased turnover of the protein. Furthermore, some of the analyzed early and late proteins were expressed with earlier kinetics in the absence of VP8. Extracellular virions of the U(L)47 deletion mutant contained reduced amounts of gD, gB, gC, and VP22 but similar amounts of VP16 compared to those of wild-type or revertant virus particles. In addition, the U(L)47 gene product was indispensable for BHV-1 replication in vivo, since no clinical manifestations or viral shedding were detected in the U(L)47 deletion mutant-infected calves, and the virus failed to induce significant levels of humoral and cellular immunity.

Intracellular trafficking of VP22 in bovine herpesvirus-1 infected cells

The intracellular trafficking of different VP22-enhanced yellow fluorescent protein (EYFP) fusion proteins expressed by bovine herpesvirus-1 (BHV-1) recombinants was examined by live-cell imaging. Our results demonstrate that (i) the fusion of EYFP to the C terminus of VP22 does not alter the trafficking of the protein in infected cells, (ii) VP22 expressed during BHV-1 infection translocates to the nucleus through three different pathways, namely early mitosis-dependent nuclear translocation, late massive nuclear translocation that follows a prolonged cytoplasmic stage of the protein in non-mitotic cells, and accumulation of a small subset of VP22 in discrete dot-like nuclear domains during its early cytoplasmic stage, (iii) the addition of the SV40 large-T-antigen nuclear localization signal (NLS) to VP22-EYFP abrogates its early cytoplasmic stage, and (iv) the VP22 (131)PRPR(134) NLS is not required for the late massive nuclear translocation of the protein, but this motif is essential for the targeting of VP22 to discrete dot-like nuclear domains during the early cytoplasmic stage. These results show that the amount of VP22 in the nucleus is precisely regulated at different stages of BHV-1 infection and suggest that the early pathways of VP22 nuclear accumulation may be more relevant to the infection process as the late massive nuclear influx starts when most of the viral progeny has already emerged from the cell.

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.

Epstein-Barr virus evasion of CD8(+) and CD4(+) T cell immunity via concerted actions of multiple gene products

Upon primary infection, EBV establishes a latent infection in B cells, characterized by maintenance of the viral genome in the absence of viral replication. The Epstein-Barr Nuclear Antigen 1 (EBNA1) plays a crucial role in maintenance of the viral DNA episome and is consistently expressed in all EBV-associated malignancies. Compared to other EBV latent gene products, EBNA1 is poorly recognized by CD8(+) T lymphocytes. Recent studies are discussed that shed new light on the mechanisms that underlie this unusual lack of CD8(+) T cell activation. Whereas the latent phase is characterized by the expression of a limited subset of viral gene products, the full repertoire of over 80 EBV lytic gene products is expressed during the replicative phase. Despite this abundance of potential T cell antigens, which indeed give rise to a strong response of CD4(+) and CD8(+) T lymphocytes, the virus can replicate successfully. Evidence is accumulating that this paradoxical situation is the result of actions of multiple viral gene products, inhibiting discrete stages of the MHC class I and class II antigen presentation pathways. Immediately after initiation of the lytic cycle, BNLF2a prevents peptide-loading of MHC class I molecules through inhibition of the Transporter associated with Antigen Processing, TAP. This will reduce presentation of viral antigens by the large ER-resident pool of MHC class I molecules. Synthesis of new MHC class I molecules is blocked by BGLF5. Viral-IL10 causes a reduction in mRNA levels of TAP1 and bli/LMP2, a subunit of the immunoproteasome. MHC class I molecules present at the cell surface are downregulated by BILF1. Also the antigen presenting capacity of MHC class II molecules is severely compromised by multiple EBV lytic gene products, including gp42/gH/gL, BGLF5, and vIL-10. In this review, we discuss how concerted actions of these EBV lytic proteins result in highly effective interference with CD8(+) and CD4(+) T cell surveillance, thereby providing the virus with a window for undisturbed generation of viral progeny.

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.

Immune evasion by pathogens of bovine respiratory disease complex

Bovine respiratory tract disease is a multi-factorial disease complex involving several viruses and bacteria. Viruses that play prominent roles in causing the bovine respiratory disease complex include bovine herpesvirus-1, bovine respiratory syncytial virus, bovine viral diarrhea virus and parinfluenza-3 virus. Bacteria that play prominent roles in this disease complex are Mannheimia haemolytica and Mycoplasma bovis. Other bacteria that infect the bovine respiratory tract of cattle are Histophilus (Haemophilus) somni and Pasteurella multocida. Frequently, severe respiratory tract disease in cattle is associated with concurrent infections of these pathogens. Like other pathogens, the viral and bacterial pathogens of this disease complex have co-evolved with their hosts over millions of years. As much as the hosts have diversified and fine-tuned the components of their immune system, the pathogens have also evolved diverse and sophisticated strategies to evade the host immune responses. These pathogens have developed intricate mechanisms to thwart both the innate and adaptive arms of the immune responses of their hosts. This review presents an overview of the strategies by which the pathogens suppress host immune responses, as well as the strategies by which the pathogens modify themselves or their locations in the host to evade host immune responses. These immune evasion strategies likely contribute to the failure of currently-available vaccines to provide complete protection to cattle against these pathogens.

Transcriptomic analysis of the dialogue between Pseudorabies virus and porcine epithelial cells during infection

Background

Transcriptomic approaches are relevant for studying virus-host cell dialogues to better understand the physiopathology of infection and the immune response at the cellular level. Pseudorabies virus (PrV), a porcine Alphaherpesvirus, is a good model for such studies in pig. Since PrV displays a strong tropism for mucous epithelial cells, we developed a kinetics study of PrV infection in the porcine PK15 epithelial cell line. To identify as completely as possible, viral and cellular genes regulated during infection, we simultaneously analyzed PrV and cellular transcriptome modifications using two microarrays i.e. a laboratory-made combined SLA/PrV microarray, consisting of probes for all PrV genes and for porcine genes contained in the Swine Leukocyte Antigen (SLA) complex, and the porcine generic Qiagen-NRSP8 oligonucleotide microarray. We confirmed the differential expression of a selected set of genes by qRT-PCR and flow cytometry.

Results

An increase in the number of differentially expressed cellular genes and PrV genes especially from 4 h post-infection (pi) was observed concomitantly with the onset of viral progeny while no early global cellular shutoff was recorded. Many cellular genes were down-regulated from 4 h pi and their number increased until 12 h pi. UL41 transcripts encoding the virion host shutoff protein were first detected as differentially expressed at 8 h pi. The viral gene UL49.5 encoding a TAP inhibitor protein was differentially expressed as soon as 2 h pi, indicating that viral evasion via TAP inhibition may start earlier than the cellular gene shutoff. We found that many biological processes are altered during PrV infection. Indeed, several genes involved in the SLA class I antigenic presentation pathway (SLA-Ia, TAP1, TAP2, PSMB8 and PSMB9), were down-regulated, thus contributing to viral immune escape from this pathway and other genes involved in apoptosis, nucleic acid metabolism, cytoskeleton signaling as well as interferon-mediated antiviral response were also modulated during PrV infection.

Conclusion

Our results show that the gene expression of both PrV and porcine cells can be analyzed simultaneously with microarrays, providing a chronology of PrV gene transcription, which has never been described before, and a global picture of transcription with a direct temporal link between viral and host gene expression.

The DNase of gammaherpesviruses impairs recognition by virus-specific CD8+ T cells through an additional host shutoff function

The DNase/alkaline exonuclease (AE) genes are well conserved in all herpesvirus families, but recent studies have shown that the AE proteins of gammaherpesviruses such as Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) exhibit an additional function which shuts down host protein synthesis. One correlate of this additional shutoff function is that levels of cell surface HLA molecules are downregulated, raising the possibility that shutoff/AE genes of gammaherpesviruses might contribute to viral immune evasion. In this study, we show that both BGLF5 (EBV) and SOX (KSHV) shutoff/AE proteins do indeed impair the ability of virus-specific CD8+ T-cell clones to recognize endogenous antigen via HLA class I. Random mutagenesis of the BGLF5 gene enabled us to genetically separate the shutoff and AE functions and to demonstrate that the shutoff function was the critical factor determining whether BGLF5 mutants can impair T-cell recognition. These data provide further evidence that EBV has multiple mechanisms to modulate HLA class I-restricted T-cell responses, thus enabling the virus to replicate and persist in the immune-competent host.

Downregulation of class I major histocompatibility complex surface expression by varicella-zoster virus involves open reading frame 66 protein kinase-dependent and -independent mechanisms

We show here that the varicella-zoster virus (VZV) open reading frame 66 (ORF66) protein kinase is one mechanism employed to reduce class I major histocompatibility complex (MHC-I) surface expression in VZV-infected cells. Cells expressing enhanced green fluorescent protein-tagged functional and inactivated ORF66 (GFP-66 and GFP-66kd) from replication-defective adenovirus vectors revealed that ORF66 reduced MHC-I surface levels in a manner dependent on kinase activity. Cells infected with recombinant VZV expressing GFP-66 exhibited a significantly greater reduction in MHC-I surface expression than that observed in cells infected with VZV disrupted in GFP-66 expression. MHC-I maturation was delayed in its transport from the endoplasmic reticulum through the Golgi in both adenovirus-transduced cells expressing only GFP-66 and in VZV-infected cells expressing high levels of GFP-66, and this was predominantly kinase dependent. MHC-I levels were reduced in VZV-infected cells, and analyses of intracellular MHC-I revealed accumulation of folded MHC-I in the Golgi region, irrespective of ORF66 expression. Thus, the ORF66 kinase is important for VZV-mediated MHC-I downregulation, but additional mechanisms also may be involved. Analyses of the VZV ORF9a protein, the ortholog of the bovine herpesvirus 1 transporter associated with antigen processing inhibitor UL49.5 revealed no effects on MHC-I. These results establish a new role for viral protein kinases in immune evasion and suggest that VZV utilizes unique mechanisms to inhibit antigen presentation.

Host shutoff during productive Epstein-Barr virus infection is mediated by BGLF5 and may contribute to immune evasion

Relatively little is known about immune evasion during the productive phase of infection by the gamma(1)-herpesvirus Epstein-Barr virus (EBV). The use of a unique system to isolate cells in lytic cycle allowed us to identify a host shutoff function operating in productively EBV-infected B cells. This impairment of protein synthesis results from mRNA degradation induced upon expression of the early lytic-cycle gene product BGLF5. Recently, a gamma(2)-herpesvirus, Kaposi sarcoma herpesvirus, has also been shown to encode a host shutoff function, indicating that host shutoff appears to be a general feature of gamma-herpesviruses. One of the consequences of host shutoff is a block in the synthesis of HLA class I and II molecules, reflected by reduced levels of these antigen-presenting complexes at the surface of cells in EBV lytic cycle. This effect could lead to escape from T cell recognition and elimination of EBV-producing cells, thereby allowing generation of viral progeny in the face of memory T cell responses.

Bovine herpesvirus 1 infection and infectious bovine rhinotracheitis

Bovine herpesvirus 1 (BoHV-1), classified as an alphaherpesvirus, is a major pathogen of cattle. Primary infection is accompanied by various clinical manifestations such as infectious bovine rhinotracheitis, abortion, infectious pustular vulvovaginitis, and systemic infection in neonates. When animals survive, a life-long latent infection is established in nervous sensory ganglia. Several reactivation stimuli can lead to viral re-excretion, which is responsible for the maintenance of BoHV-1 within a cattle herd. This paper focuses on an updated pathogenesis based on a molecular characterization of BoHV-1 and the description of the virus cycle. Special emphasis is accorded to the impact of the latency and reactivation cycle on the epidemiology and the control of BoHV-1. Several European countries have initiated BoHV-1 eradication schemes because of the significant losses incurred by disease and trading restrictions. The vaccines used against BoHV-1 are described in this context where the differentiation of infected from vaccinated animals is of critical importance to achieve BoHV-1 eradication.

Messenger RNA turnover and its regulation in herpesviral infection

The ability to regulate cellular gene expression is a key aspect of the lifecycles of a diverse array of viruses. In fact, viral infection often results in a global shutoff of host cellular gene expression; such inhibition serves not only to ensure maximal viral gene expression without competition from the host for essential machinery and substrates but also aids in evasion of immune responses detrimental to successful viral replication and dissemination. Within the herpesvirus family, host shutoff is a prominent feature of both the alpha- and gamma-herpesviruses. Intriguingly, while both classes of herpesviruses block cellular gene expression by inducing decay of messenger RNAs, the viral factors responsible for this phenotype as well as the mechanisms by which it is achieved are quite distinct. However, data suggest that the host shutoff functions of alpha- and gamma-herpesviruses are likely achieved both through the activity of virally encoded nucleases as well as via modulation of cellular RNA degradation pathways. This review highlights the processes governing normal cellular messenger RNA decay and then details the mechanisms by which herpesviruses promote accelerated RNA turnover. Parallels between the viral and cellular degradation systems as well as the known interactions between viral host shutoff factors and the cellular RNA turnover machinery are highlighted.

Bovine herpesvirus 1 UL49.5 protein inhibits the transporter associated with antigen processing despite complex formation with glycoprotein M

Bovine herpesvirus 1 (BHV-1) interferes with peptide translocation by the transporter associated with antigen processing (TAP). Recently, the UL49.5 gene product of BHV-1 was identified as the protein responsible for the observed inhibition of TAP. In BHV-1-infected cells and virions, the UL49.5 protein forms a complex with glycoprotein M (gM). Hence, it was investigated whether UL49.5 can combine the interactions with gM and the TAP complex. In cell lines constitutively expressing both UL49.5 and gM, UL49.5 appears to be required for functional processing of gM. Immunofluorescence-confocal laser scanning microscopy demonstrated that both proteins are interdependent for their redistribution from the endoplasmic reticulum to the trans-Golgi network. Remarkably, expression of cloned gM results in the abrogation of the UL49.5-mediated inhibition of TAP and prevents the degradation of the transporter. However, in BHV-1-infected cells, differences in UL49.5 and gM expression kinetics were seen to create a window of opportunity at the early stages of infection, during which time the UL49.5 protein can act on TAP without gM interference. Moreover, in later periods, non-gM-associated UL49.5 can be detected in addition to the UL49.5/gM complex. Thus, it has been deduced that different functions of UL49.5, editing of gM processing and inhibition of TAP, can be combined during BHV-1 infection.

Herpes simplex virus evades natural killer T cell recognition by suppressing CD1d recycling

Natural killer T cells, which are stimulated by lipids presented by CD1d molecules, are crucial in antiviral host defense. How viruses evade natural killer T cell recognition remains unclear. Here we show that infection with herpes simplex virus type 1 (HSV-1) reduced CD1d surface expression on antigen-presenting cells. HSV-1 did not inhibit CD1d protein synthesis or enhance constitutive CD1d endocytosis. Instead, HSV-1 prevented the reappearance of endocytosed CD1d on the cell surface by redistributing endocytosed CD1d to the lysosome limiting membrane. HSV-1 might also inhibit the transport of newly synthesized CD1d to the cell surface. Such inhibition of CD1d surface expression impaired antigen-presenting cell-mediated stimulation of natural killer T cells, supporting the idea that this mechanism may be an important HSV-1 immune evasion strategy.

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.

Modulation of the antigen transport machinery TAP by friends and enemies

The transporter associated with antigen processing (TAP) is a key factor of the major histocompatibility complex (MHC) class I antigen presentation pathway. This ABC transporter translocates peptides derived mainly from proteasomal degradation from the cytosol into the ER lumen for loading onto MHC class I molecules. Manifold mechanisms have evolved to regulate TAP activity. During infection, TAP expression is upregulated by interferon-gamma. Furthermore, the assembly and stability of the transport complex is promoted by various auxiliary factors. However, tumors and viruses have developed sophisticated strategies to escape the immune surveillance by suppressing TAP function. The activity of TAP can be impaired on the transcriptional or translational level, by enhanced degradation or by inhibition of peptide translocation. In this review, we briefly summarize existing data concerning the regulation of the TAP complex.

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.

Role of the VP16-binding domain of vhs in viral growth, host shutoff activity, and pathogenesis

The virion host shutoff (vhs) protein of herpes simplex virus type 1 causes the degradation of host and viral mRNA immediately upon infection of permissive cells. vhs can interact with VP16 through a 20-amino-acid binding domain, and viruses containing a deletion of this VP16-binding domain of vhs (Delta20) and a corresponding marker rescue (Delta20R) were constructed and characterized. Transient-transfection assays showed that this domain was dispensable for vhs activity. The Delta20 recombinant virus, however, was unable to induce mRNA degradation in the presence of actinomycin D, while degradation induced by Delta20R was equivalent to that for wild-type virus. Delta20, Delta20R, and KOS caused comparable RNA degradation in the absence of actinomycin D. Western blot analysis of infected cells indicated that comparable levels of vhs were expressed by Delta20, Delta20R, and KOS, and there was only a modest reduction of vhs packaging in Delta20. Immunoprecipitation of protein from cells infected with Delta20 and Delta20R showed equivalent coprecipitation of vhs and VP16. Pathogenesis studies with Delta20 showed a significant decrease in replication in the corneas, trigeminal ganglia, and brains, as well as a significant reduction in clinical disease and lethality, but no significant difference in the establishment of, or reactivation from, latency compared to results with KOS and Delta20R. These results suggest that the previously described VP16-binding domain is not required for vhs packaging or for binding to VP16. It is required, however, for RNA degradation activity of tegument-derived vhs and wild-type replication and virulence in mice.

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.