The Epstein-Barr virus G-protein-coupled receptor contributes to immune evasion by targeting MHC class I molecules for degradation

Defining immune engagement thresholds for in vivo control of virus-driven lymphoproliferation

Persistent infections are subject to constant surveillance by CD8⁺ cytotoxic T cells (CTL). Their control should therefore depend on MHC class I-restricted epitope presentation. Many epitopes are described for γ-herpesviruses and form a basis for prospective immunotherapies and vaccines. However the quantitative requirements of in vivo immune control for epitope presentation and recognition remain poorly defined. We used Murid Herpesvirus-4 (MuHV-4) to determine for a latently expressed viral epitope how MHC class-I binding and CTL functional avidity impact on host colonization. Tracking MuHV-4 recombinants that differed only in epitope presentation, we found little latitude for sub-optimal MHC class I binding before immune control failed. By contrast, control remained effective across a wide range of T cell functional avidities. Thus, we could define critical engagement thresholds for the in vivo immune control of virus-driven B cell proliferation.

Chronic viral infections cause huge morbidity and mortality worldwide. γ-herpesviruses provide an example relevant to all human demographics, causing, inter alia, Hodgkin's disease, Burkitt's lymphoma, Kaposi's Sarcoma, and nasopharyngeal carcinoma. The proliferation of latently infected B cells and their control by CD8⁺ T cells are central to pathogenesis. Although many viral T cell targets have been identified in vitro, the functional impact of their engagement in vivo remains ill-defined. With the well-established Murid Herpesvirus-4 infection model, we used a range of recombinant viruses to define functional thresholds for the engagement of a latently expressed viral epitope. These data advance significantly our understanding of how the immune system must function to control γ-herpesvirus infection, with implications for vaccination and anti-cancer immunotherapy.

Novel gammaherpesvirus functions encoded by bovine herpesvirus 6 (bovine lymphotropic virus)

The genus Macavirus of the subfamily Gammaherpesvirinae includes viruses that infect lymphoid cells of domestic and wild ruminants and swine, causing asymptomatic latent infections in reservoir hosts. Here, we describe the genome of bovine herpesvirus 6 (BoHV-6), a macavirus ubiquitous in healthy cattle populations. The BoHV-6 genome exhibited architecture conserved in macaviruses, including a repetitive H-DNA region and unique 141 kbp L-DNA region predicted to encode 77 genes. BoHV-6 encoded, in variable genomic regions, a novel complement of genes relative to other characterized macaviruses, probably contributing to distinctive aspects of BoHV-6 infection biology and host range. Most notably, BoHV-6 encoded the first herpesviral protein (Bov2.b2) similar to cellular ornithine decarboxylase, an enzyme that catalyses the first and rate-limiting step in the biosynthesis of polyamines. Bov2.b2 conceivably mediates a novel mechanism by which BoHV-6 promotes cell-cycle-dependent viral replication.

Immune escape of γ-herpesviruses from adaptive immunity

Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are two γ-herpesviruses identified in humans and are strongly associated with the development of malignancies. Murine γ-herpesvirus (MHV-68) is a naturally occurring rodent pathogen, representing a unique experimental model for dissecting γ-herpesvirus infection and the immune response. These γ-herpesviruses actively antagonize the innate and adaptive antiviral responses, thereby efficiently establishing latent or persistent infections and even promoting development of malignancies. In this review, we summarize immune evasion strategies of γ-herpesviruses. These include suppression of MHC-I-restricted and MHC-II-restricted antigen presentation, impairment of dendritic cell functions, downregulation of costimulatory molecules, activation of virus-specific regulatory T cells, and induction of inhibitory cytokines. There is a focus on how both γ-herpesvirus-derived and host-derived immunomodulators interfere with adaptive antiviral immunity. Understanding immune-evasive mechanisms is essential for developing future immunotherapies against EBV-driven and KSHV-driven tumors.

Etiological factors of nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a common disease among southern Chinese. The major etiological factors proposed for NPC pathogenesis include genetic susceptibility, environment factors and EBV infection. In the high risk population, genetic susceptibility to NPC has been mapped to the HLA loci and adjacent genes in MHC region on chromosome 6p21. Consumption of preserved food including salted fish has been implicated in its etiology in earlier studies. Its contribution to pathogenesis of NPC remains to be determined. A decreasing trend of NPC incidence was observed in Hong Kong, Taiwan and Singapore in recent years which may be accounted by a change of dietary habits. A comprehensive epidemiological study will help to elucidate the relative importance of various risk factors in the pathogenesis of NPC. Despite the close association of EBV infection with NPC, the etiological role of EBV in NPC pathogenesis remains enigmatic. EBV infection in primary nasopharyngeal epithelial cells is uncommon and difficult to achieve. EBV does not transform primary nasopharyngeal epithelial cells into proliferative clones, which contrasts greatly with the well-documented ability of EBV to transform and immortalize primary B cells. Genetic alterations identified in premalignant nasopharyngeal epithelium may play crucial roles to support stable EBV infection. Subsequently, latent and lytic EBV gene products may drive clonal expansion and transformation of premalignant nasopharyngeal epithelial cells into cancer cells. Stromal inflammation in nasopharyngeal mucosa is believed to play an important role in modulating the growth and possibly drive the malignant transformation of EBV-infected nasopharyngeal epithelial cells. Furthermore, there are increasing evidences supporting a role of EBV infection to evade host immune surveillance. EBV-infected cells may have selective growth advantages in vivo by acquiring a stress-resistance phenotype. Understanding the etiological factors and pathogenesis of NPC will contribute effectively to the prevention and treatment of this disease.

CD8+ T cell responses to lytic EBV infection: late antigen specificities as subdominant components of the total response

EBV elicits primary CD8(+) T cell responses that, by T cell cloning from infectious mononucleosis (IM) patients, appear skewed toward immediate early (IE) and some early (E) lytic cycle proteins, with late (L) proteins rarely targeted. However, L Ag-specific responses have been detected regularly in polyclonal T cell cultures from long-term virus carriers. To resolve this apparent difference between responses to primary and persistent infection, 13 long-term carriers were screened in ex vivo IFN-γ ELISPOT assays using peptides spanning the two IE, six representative E, and seven representative L proteins. This revealed memory CD8 responses to 44 new lytic cycle epitopes that straddle all three protein classes but, in terms of both frequency and size, maintain the IE > E > L hierarchy of immunodominance. Having identified the HLA restriction of 10 (including 7 L) new epitopes using memory CD8(+) T cell clones, we looked in HLA-matched IM patients and found such reactivities but typically at low levels, explaining why they had gone undetected in the original IM clonal screens. Wherever tested, all CD8(+) T cell clones against these novel lytic cycle epitopes recognized lytically infected cells naturally expressing their target Ag. Surprisingly, however, clones against the most frequently recognized L Ag, the BNRF1 tegument protein, also recognized latently infected, growth-transformed cells. We infer that BNRF1 is also a latent Ag that could be targeted in T cell therapy of EBV-driven B-lymphoproliferative disease.

PML nuclear bodies and SATB1 are associated with HLA class I expression in EBV+ Hodgkin lymphoma

Tumor cells of classical Hodgkin lymphoma (cHL) are characterized by a general loss of B cell phenotype, whereas antigen presenting properties are commonly retained. HLA class I is expressed in most EBV+ cHL cases, with an even enhanced expression in a proportion of the cases. Promyelocytic leukemia protein (PML) and special AT-rich region binding protein 1 (SATB1) are two global chromatin organizing proteins that have been shown to regulate HLA class I expression in Jurkat cells. We analyzed HLA class I, number of PML nuclear bodies (NBs) and SATB1 expression in tumor cells of 54 EBV+ cHL cases and used 27 EBV- cHL cases as controls. There was a significant difference in presence of HLA class I staining between EBV+ and EBV- cases (p<0.0001). We observed normal HLA class I expression in 35% of the EBV+ and in 19% of the EBV- cases. A stronger than normal HLA class I expression was observed in approximately 40% of EBV+ cHL and not in EBV- cHL cases. 36 EBV+ cHL cases contained less than 10 PML-NBs per tumor cell, whereas 16 cases contained more than 10 PML-NBs. The number of PML-NBs was positively correlated to the level of HLA class I expression (p<0.01). The percentage of SATB1 positive cells varied between 0% to 100% in tumor cells and was inversely correlated with the level of HLA class I expression, but only between normal and strong expression (p<0.05). Multivariable analysis indicated that the number of PML-NBs and the percentage of SATB1+ tumor cells are independent factors affecting HLA class I expression in EBV+ cHL. In conclusion, both PML and SATB1 are correlated to HLA class I expression levels in EBV+ cHL.

Autocrine CCL3 and CCL4 induced by the oncoprotein LMP1 promote Epstein-Barr virus-triggered B cell proliferation

Epstein-Barr virus (EBV) alters the regulation and expression of a variety of cytokines in its host cells to modulate host immune surveillance and facilitate viral persistence. Using cytokine antibody arrays, we found that, in addition to the cytokines reported previously, two chemotactic cytokines, CCL3 and CCL4, were induced in EBV-infected B cells and were expressed at high levels in all EBV-immortalized lymphoblastoid cell lines (LCLs). Furthermore, EBV latent membrane protein 1 (LMP1)-mediated Jun N-terminal protein kinase activation was responsible for upregulation of CCL3 and CCL4. Inhibition of CCL3 and CCL4 in LCLs using a short hairpin RNA approach or by neutralizing antibodies suppressed cell proliferation and caused apoptosis, indicating that autocrine CCL3 and CCL4 are required for LCL survival and growth. Importantly, significant amounts of CCL3 were detected in EBV-positive plasma from immunocompromised patients, suggesting that EBV modulates this chemokine in vivo. This study reveals the regulatory mechanism and a novel function of CCL3 and CCL4 in EBV-infected B cells. CCL3 might be useful as a therapeutic target in EBV-associated lymphoproliferative diseases and malignancies.

Evasion of adaptive and innate immune response mechanisms by γ-herpesviruses

γ-Herpesviral immune evasion mechanisms are optimized to support the acute, lytic and the longterm, latent phase of infection. During acute infection, specific immune modulatory proteins limit, but also exploit, the antiviral activities of cell intrinsic innate immune responses as well as those of innate and adaptive immune cells. During latent infection, a restricted gene expression program limits immune targeting and cis-acting mechanisms to reduce the antigen presentation as well as antigenicity of latency-associated proteins. Here, we will review recent progress in our understanding of γ-herpesviral immune evasion strategies.

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.

Immune regulation of human herpesviruses and its implications for human transplantation

Human herpesviruses including cytomegalovirus, Epstein-Barr virus, HHV6, HHV7, HHV8, Herpes simplex virus (HSV)-1 and HSV-2 and varicella zoster virus (VZV) have developed an intricate relationship with the human immune system. This is characterized by the interplay between viral immune evasion mechanisms that promote the establishment of a lifelong persistent infection and the induction of a broad humoral and cellular immune response, which prevents the establishment of viral disease. Understanding the immune parameters that control herpesvirus infection, and the strategies the viruses use to evade immune recognition, has been critical in understanding why immunological dysfunction in transplant patients can lead to disease, and in the development of immunological strategies to prevent and control herpesvirus associated diseases.

EBV BILF1 evolved to downregulate cell surface display of a wide range of HLA class I molecules through their cytoplasmic tail

Coevolution of herpesviruses and their hosts has driven the development of both host antiviral mechanisms to detect and eliminate infected cells and viral ploys to escape immune surveillance. Among the immune-evasion strategies used by the lymphocryptovirus (γ(1)-herpesvirus) EBV is the downregulation of surface HLA class I expression by the virally encoded G protein-coupled receptor BILF1, thereby impeding presentation of viral Ags and cytotoxic T cell recognition of the infected cell. In this study, we show EBV BILF1 to be expressed early in the viral lytic cycle. BILF1 targets a broad range of HLA class I molecules, including multiple HLA-A and -B types and HLA-E. In contrast, HLA-C was only marginally affected. We advance the mechanistic understanding of the process by showing that the cytoplasmic C-terminal tail of EBV BILF1 is required for reducing surface HLA class I expression. Susceptibility to BILF1-mediated downregulation, in turn, is conferred by specific residues in the intracellular tail of the HLA class I H chain. Finally, we explore the evolution of BILF1 within the lymphocryptovirus genus. Although the homolog of BILF1 encoded by the lymphocryptovirus infecting Old World rhesus primates shares the ability of EBV to downregulate cell surface HLA class I expression, this function is not possessed by New World marmoset lymphocryptovirus BILF1. Therefore, this study furthers our knowledge of the evolution of immunoevasive functions by the lymphocryptovirus genus of herpesviruses.

An Epstein-Barr virus encoded inhibitor of Colony Stimulating Factor-1 signaling is an important determinant for acute and persistent EBV infection

Acute Epstein-Barr virus (EBV) infection is the most common cause of Infectious Mononucleosis. Nearly all adult humans harbor life-long, persistent EBV infection which can lead to development of cancers including Hodgkin Lymphoma, Burkitt Lymphoma, nasopharyngeal carcinoma, gastric carcinoma, and lymphomas in immunosuppressed patients. BARF1 is an EBV replication-associated, secreted protein that blocks Colony Stimulating Factor 1 (CSF-1) signaling, an innate immunity pathway not targeted by any other virus species. To evaluate effects of BARF1 in acute and persistent infection, we mutated the BARF1 homologue in the EBV-related herpesvirus, or lymphocryptovirus (LCV), naturally infecting rhesus macaques to create a recombinant rhLCV incapable of blocking CSF-1 (ΔrhBARF1). Rhesus macaques orally challenged with ΔrhBARF1 had decreased viral load indicating that CSF-1 is important for acute virus infection. Surprisingly, ΔrhBARF1 was also associated with dramatically lower virus setpoints during persistent infection. Normal acute viral load and normal viral setpoints during persistent rhLCV infection could be restored by Simian/Human Immunodeficiency Virus-induced immunosuppression prior to oral inoculation with ΔrhBARF1 or infection of immunocompetent animals with a recombinant rhLCV where the rhBARF1 was repaired. These results indicate that BARF1 blockade of CSF-1 signaling is an important immune evasion strategy for efficient acute EBV infection and a significant determinant for virus setpoint during persistent EBV infection.

Epstein-Barr virus (EBV) is a herpesvirus that persistently infects nearly all humans by adulthood. Acute and persistent phases of EBV infection are associated with a variety of human diseases, including infectious mononucleosis and cancer. To investigate how EBV interacts with the host to successfully establish acute and persistent infection, we combined the power of the rhesus macaque animal model for EBV infection with genetic engineering of the EBV-related herpesvirus, or lymphocryptovirus (LCV), that naturally infects rhesus macaques. We created a recombinant rhLCV carrying a mutated EBV BARF1 homologue, a replication-associated viral protein that is secreted and blocks Colony Stimulating Factor-1 (CSF-1) signaling, a cytokine important for innate immunity. Oral inoculation of rhesus macaques showed that the virus' ability to block CSF-1 was important for achieving the normally high viral loads during acute infection, and surprisingly, was also needed to establish normal levels of virus infection, or viral setpoint, during persistent infection. These studies show that virus-mediated interruption of innate immunity is critical for both acute and persistent phases of EBV infection. Understanding how EBV successfully infects humans and how the natural history of EBV infection can be disrupted will aid in development of vaccines to prevent EBV-associated diseases.

Hiding lipid presentation: viral interference with CD1d-restricted invariant natural killer T (iNKT) cell activation

The immune system plays a major role in protecting the host against viral infection. Rapid initial protection is conveyed by innate immune cells, while adaptive immunity (including T lymphocytes) requires several days to develop, yet provides high specificity and long-lasting memory. Invariant natural killer T (iNKT) cells are an unusual subset of T lymphocytes, expressing a semi-invariant T cell receptor together with markers of the innate NK cell lineage. Activated iNKT cells can exert direct cytolysis and can rapidly release a variety of immune-polarizing cytokines, thereby regulating the ensuing adaptive immune response. iNKT cells recognize lipids in the context of the antigen-presenting molecule CD1d. Intriguingly, CD1d-restricted iNKT cells appear to play a critical role in anti-viral defense: increased susceptibility to disseminated viral infections is observed both in patients with iNKT cell deficiency as well as in CD1d- and iNKT cell-deficient mice. Moreover, viruses have recently been found to use sophisticated strategies to withstand iNKT cell-mediated elimination. This review focuses on CD1d-restricted lipid presentation and the strategies viruses deploy to subvert this pathway.

Molecular mechanisms deployed by virally encoded G protein-coupled receptors in human diseases

G protein-coupled receptors (GPCRs) represent the largest family of cell surface molecules involved in signal transduction. Surprisingly, open reading frames for multiple GPCRs were hijacked in the process of coevolution between Herpesviridae family viruses and their human and mammalian hosts. Virally encoded GPCRs (vGPCRs) evolved as parts of viral genomes, and this evolution allowed the power of host GPCR signaling circuitries to be harnessed in order to ensure viral replicative success. Phylogenetically, vGPCRs are distantly related to human chemokine receptors, although they feature several unique characteristics. Here, we describe the molecular mechanisms underlying vGPCR-mediated viral pathogenesis. These mechanisms include constitutive activity, aberrant coupling to human G proteins and β-arrestins, binding and activation by human chemokines, and dimerization with other GPCRs expressed in infected cells. The likely structural basis for these molecular events is described for the two closest viral homologs of human GPCRs. This information may aid in the development of novel targeted therapeutic strategies against viral diseases.

Suppression of antigen-specific T cell responses by the Kaposi's sarcoma-associated herpesvirus viral OX2 protein and its cellular orthologue, CD200

Regulating appropriate activation of the immune response in the healthy host despite continual immune surveillance dictates that immune responses must be either self-limiting and therefore negatively regulated following their activation or prevented from developing inappropriately. In the case of antigen-specific T cells, their response is attenuated by several mechanisms, including ligation of CTLA-4 and PD-1. Through the study of the viral OX2 (vOX2) immunoregulator encoded by Kaposi's sarcoma-associated herpesvirus (KSHV), we have identified a T cell-attenuating role both for this protein and for CD200, a cellular orthologue of the viral vOX2 protein. In vitro, antigen-presenting cells (APC) expressing either native vOX2 or CD200 suppressed two functions of cognate antigen-specific T cell clones: gamma interferon (IFN-γ) production and mobilization of CD107a, a cytolytic granule component and measure of target cell killing ability. Mechanistically, vOX2 and CD200 expression on APC suppressed the phosphorylation of ERK1/2 mitogen-activated protein kinase in responding T cells. These data provide the first evidence for a role of both KSHV vOX2 and cellular CD200 in the negative regulation of antigen-specific T cell responses. They suggest that KSHV has evolved to harness the host CD200-based mechanism of attenuation of T cell responses to facilitate virus persistence and dissemination within the infected individual. Moreover, our studies define a new paradigm in immune modulation by viruses: the provision of a negative costimulatory signal to T cells by a virus-encoded orthologue of CD200.

Immune evasion strategies of the human gamma-herpesviruses: implications for viral tumorigenesis

Two human gamma-herpesviruses, Epstein-Barr virus and Kaposi's sarcoma associated herpesvirus/human herpesvirus 8 display oncogenic potential, causing benign and malignant lymphoproliferative disorders in genetically susceptible or immunosuppressed individuals. As a family of viruses that establish persistent life-long infections, herpesviruses have evolved strategies to limit innate antiviral responses and evade host immune surveillance. Herpesviruses have developed mechanisms to disrupt antigen presentation, pirate the production of immune regulating cytokines, and inhibit pro-apoptotic signaling pathways. Although these strategies are designed to facilitate the long-term persistence of herpesviruses, in certain circumstances they can contribute to viral-driven carcinogenesis.

Epstein-Barr virus evades CD4+ T cell responses in lytic cycle through BZLF1-mediated downregulation of CD74 and the cooperation of vBcl-2

Evasion of immune T cell responses is crucial for viruses to establish persistence in the infected host. Immune evasion mechanisms of Epstein-Barr virus (EBV) in the context of MHC-I antigen presentation have been well studied. In contrast, viral interference with MHC-II antigen presentation is less well understood, not only for EBV but also for other persistent viruses. Here we show that the EBV encoded BZLF1 can interfere with recognition by immune CD4⁺ effector T cells. This impaired T cell recognition occurred in the absence of a reduction in the expression of surface MHC-II, but correlated with a marked downregulation of surface CD74 on the target cells. Furthermore, impaired CD4⁺ T cell recognition was also observed with target cells where CD74 expression was downregulated by shRNA-mediated inhibition. BZLF1 downregulated surface CD74 via a post-transcriptional mechanism distinct from its previously reported effect on the CIITA promoter. In addition to being a chaperone for MHC-II αβ dimers, CD74 also functions as a surface receptor for macrophage Migration Inhibitory Factor and enhances cell survival through transcriptional upregulation of Bcl-2 family members. The immune-evasion function of BZLF1 therefore comes at a cost of induced toxicity. However, during EBV lytic cycle induced by BZLF1 expression, this toxicity can be overcome by expression of the vBcl-2, BHRF1, at an early stage of lytic infection. We conclude that by inhibiting apoptosis, the vBcl-2 not only maintains cell viability to allow sufficient time for synthesis and accumulation of infectious virus progeny, but also enables BZLF1 to effect its immune evasion function.

Epstein-Barr virus (EBV) is a herpesvirus and an important human pathogen that can cause diseases ranging from non-malignant proliferative disease to fully malignant cancers of lymphocytes and epithelial cells. The persistence of EBV in healthy individuals relies on the balance between host immune responses and viral immune evasion. As CD4⁺ immune T cell responses include both helper and cytotoxic functions, viral mechanisms for interfering with MHC class II antigen presentation to CD4⁺ T cells have the potential to greatly influence the outcome of viral infections. Our work on Epstein-Barr virus provides a new paradigm for viral immune evasion of MHC-II presented antigen by targeting CD74. CD74 is a dual function protein; it serves as a surviving receptor as well as a chaperone for MHC-II antigen presentation. Therefore, downregulation of CD74 as a T cell evasion strategy comes at the cost of potentially inducing cell death. However, EBV also encodes a vBcl-2 to attenuate the toxicity associated with reduced CD74, thus enabling the immune-impairment function to be effected. We expect that future studies will identify other viruses utilizing a similar strategy to evade CD4⁺ immune T cell responses.

Persistent infection drives the development of CD8+ T cells specific for late lytic infection antigens in lymphocryptovirus-infected macaques and Epstein-Barr virus-infected humans

We examined the CD8(+) T cell repertoire against lytic infection antigens in rhesus macaques persistently infected with the Epstein-Barr virus (EBV)-related lymphocryptovirus (rhLCV). CD8(+) T cells specific for late (L) antigens were detected at rates comparable to those for early antigens and were associated with increasing duration of infection. L antigen-specific CD8(+) T cells were also readily detected in adult, EBV-positive humans. Thus, viral major histocompatibility complex class I (MHCI) immune evasion genes expressed during lytic LCV infection do not prevent L-specific CD8(+) T cell development over time during persistent infection.

Cytotoxic CD4+ T cell responses to EBV contrast with CD8 responses in breadth of lytic cycle antigen choice and in lytic cycle recognition

EBV, a B lymphotropic herpesvirus, encodes two immediate early (IE)-, >30 early (E)-, and >30 late (L)-phase proteins during its replication (lytic) cycle. Despite this, lytic Ag-induced CD8 responses are strongly skewed toward IE and a few E proteins only, all expressed before HLA I presentation is blocked in lytically infected cells. For comparison, we examined CD4(+) T cell responses to eight IE, E, or L proteins, screening 14 virus-immune donors to overlapping peptide pools in IFN-γ ELISPOT assays, and established CD4(+) T cell clones against 12 defined epitopes for target-recognition assays. We found that the lytic Ag-specific CD4(+) T cell response differs radically from its CD8 counterpart in that it is widely distributed across IE, E, and L Ag targets, often with multiple reactivities detectable per donor and with IE, E, or L epitope responses being numerically dominant, and that all CD4(+) T cell clones, whether IE, E, or L epitope-specific, show strong recognition of EBV-transformed B cell lines, despite the lines containing only a small fraction of lytically infected cells. Efficient recognition occurs because lytic Ags are released into the culture and are acquired and processed by neighboring latently infected cells. These findings suggested that lytic Ag-specific CD4 responses are driven by a different route of Ag display than drives CD8 responses and that such CD4 effectors could be therapeutically useful against EBV-driven lymphoproliferative disease lesions, which contain similarly small fractions of EBV-transformed cells entering the lytic cycle.

Epstein-Barr virus Zta-induced immunomodulators from nasopharyngeal carcinoma cells upregulate interleukin-10 production from monocytes

During lytic infection with Epstein-Barr virus (EBV), several viral lytic proteins function to evade immune recognition or to actively suppress immune cells. An EBV lytic transactivator, Zta, induces an immunosuppressive cytokine interleukin 10 (IL-10) in B cells, but whether it regulates IL-10 in the context of epithelial cells is unclear. In this study, we tested nasopharyngeal carcinoma (NPC) cell lines and found that Zta did not induce IL-10 in these epithelial cells. Interestingly, the conditioned medium of Zta-expressing NPC cells enhanced IL-10 production from monocytes. We further revealed that the IL-10-inducing effect involved at least two immunomodulators that were upregulated by Zta and secreted from NPC cells: granulocyte-macrophage colony-stimulating factor (GM-CSF) and prostaglandin E(2) (PGE(2)). Zta was recruited to and activated the GM-CSF promoter, thus upregulating GM-CSF expression. Zta also activated the promoter of cyclooxygenase-2 (COX-2), and Zta-induced COX-2 increased downstream PGE(2) production. Cotreatment with GM-CSF and PGE(2) synergistically induced IL-10 production from monocytes. The IL-10-inducing effect of the Zta-conditioned medium was reduced when GM-CSF or the COX-2/PGE(2) pathway was blocked. The conditioned medium of NPC cells with EBV lytic infection showed a similar increase of GM-CSF and PGE(2) levels as well as the IL-10-inducing effect on monocytes, and knockdown of Zta abolished all the effects. Therefore, through Zta-induced immunomodulators, EBV lytic infection in NPC cells can direct bystander monocytes to produce IL-10, which may be a novel way of EBV to promote local immunosuppression.

Immune defence against EBV and EBV-associated disease

Epstein-Barr virus (EBV), a B-lymphotropic herpesvirus widespread in the human population and normally contained as an asymptomatic infection by T cell surveillance, nevertheless causes infectious mononucleosis and is strongly linked to several types of human cancer. Here we describe new findings on the range of cellular immune responses induced by EBV infection, on viral strategies to evade those responses and on the links between HLA gene loci and EBV-induced disease. The success of adoptive T cell therapy for EBV-driven post-transplant lymphoproliferative disease is stimulating efforts to target other EBV-associated tumours by immunotherapeutic means, and has reawakened interest in the ultimate intervention strategy, a prophylactic EBV vaccine.

Exploiting human herpesvirus immune evasion for therapeutic gain: potential and pitfalls

Herpesviruses stand out for their capacity to establish lifelong infections of immunocompetent hosts, generally without causing overt symptoms. Herpesviruses are equipped with sophisticated immune evasion strategies, allowing these viruses to persist for life despite the presence of a strong antiviral immune response. Although viral evasion tactics appear to target virtually any stage of the innate and adaptive host immune response, detailed knowledge is now available on the molecular mechanisms underlying herpesvirus obstruction of MHC class I-restricted antigen presentation to T cells. This opens the way for clinical application. Here, we review and discuss recent efforts to exploit human herpesvirus MHC class I evasion strategies for the rational design of novel strategies for vaccine development, cancer treatment, transplant protection and gene therapy.

The Epstein-Barr virus-encoded BILF1 protein modulates immune recognition of endogenously processed antigen by targeting major histocompatibility complex class I molecules trafficking on both the exocytic and endocytic pathways

Despite triggering strong immune responses, Epstein-Barr virus (EBV) has colonized more than 90% of the adult human population. Successful persistence of EBV depends on the establishment of a balance between host immune responses and viral immune evasion. Here we have extended our studies on the EBV-encoded BILF1 protein, which was recently identified as an immunoevasin that functions by enhancing degradation of major histocompatibility complex class I (MHC-I) antigens via lysosomes. We now demonstrate that disruption of the EKT signaling motif of BILF1 by a K122A mutation impairs the ability of BILF1 to enhance endocytosis of surface MHC-I molecules, while subsequent lysosomal degradation was impaired by deletion of the 21-residue C-terminal tail of BILF1. Furthermore, we identified another mechanism of BILF1 immunomodulation: it targets newly synthesized MHC-I/peptide complexes en route to the cell surface. Importantly, although the diversion of MHC-I on the exocytic pathway caused a relatively modest reduction in cell surface MHC-I, presentation of endogenously processed target peptides to immune CD8(+) effector T cells was reduced by around 65%. The immune-modulating functions of BILF1 in the context of the whole virus were confirmed in cells lytically infected with a recombinant EBV in which BILF1 was deleted. This study therefore extends our initial observations on BILF1 to show that this immunoevasin can target MHC-I antigen presentation via both the exocytic and endocytic trafficking pathways. The results also emphasize the merits of including functional T cell recognition assays to gain a more complete picture of immunoevasin effects on the antigen presentation pathway.

G protein-coupled receptors: novel targets for drug discovery in cancer

G protein-coupled receptors (GPCRs) belong to a superfamily of cell surface signalling proteins that have a pivotal role in many physiological functions and in multiple diseases, including the development of cancer and cancer metastasis. Current drugs that target GPCRs - many of which have excellent therapeutic benefits - are directed towards only a few GPCR members. Therefore, huge efforts are currently underway to develop new GPCR-based drugs, particularly for cancer. We review recent findings that present unexpected opportunities to interfere with major tumorigenic signals by manipulating GPCR-mediated pathways. We also discuss current data regarding novel GPCR targets that may provide promising opportunities for drug discovery in cancer prevention and treatment.

Getting the message direct manipulation of host mRNA accumulation during gammaherpesvirus lytic infection

The Gammaherpesvirinae subfamily of herpesviruses comprises lymphotropic viruses, including the oncogenic human pathogens Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. During lytic infection, gammaherpesviruses manipulate host gene expression to optimize the cellular environment for viral replication and to evade the immune response. Additionally, although a lytically infected cell will itself be killed in the process of viral replication, lytic infection can contribute to pathogenesis by inducing the secretion of paracrine factors with functions in cell survival and proliferation, and angiogenesis. The mechanisms by which these viruses manipulate host gene expression are varied and target the accumulation of cellular mRNAs and their translation, signaling pathways, and protein stability. Here, we discuss how gammaherpesviral proteins directly influence host mRNA biogenesis and stability, either selectively or globally, in order to fine-tune the cellular environment to the advantage of the virus. Appreciation of the mechanisms by which these viruses interface with and adapt normal cellular processes continues to inform our understanding of gammaherpesviral biology and the regulation of mRNA accumulation and turnover in our own cells.

Herpesvirus-encoded GPCRs rewire cellular signaling

Viral G-protein-coupled receptors (vGPCRs) are chemokine receptor homologues encoded by the Herpes- and Capripoxviridae. They are thought to have been hijacked from the host genome during the course of evolution. These vGPCRs play different roles in the viral lifecycle and associated pathologies. Three members of the Herpesviridae, Kaposi sarcoma-associated herpesvirus (KSHV), Epstein-Barr virus (EBV) and human cytomegalovirus (HCMV) are capable of setting up persistent latent infections in humans. Two of the herpesviruses, KSHV and EBV, are associated with cancer, while HCMV may have an oncomodulary effect. The vGPCRs may contribute to the escape of immune surveillance and (constitutively) activate signaling pathways linked to proliferation and inflammation. Some vGPCRs induce activation of autocrine and paracrine signaling, resulting in secretion of growth factors and/or cytokines. As a result, vGPCRs effectively rewire cellular signaling networks. Delineating the cellular signaling networks modulated by these vGPCRs will be crucial for treatment of virus-associated pathologies.

Innate immune modulation in EBV infection

Epstein-Barr Virus (EBV) belongs to the gammaherpesvirus family, members of which are oncogenic. Compared with other closely related herpesviruses, EBV has developed much more elaborate and sophisticated strategies for subverting host immune system, which may account for its high prevalence in immune competent hosts. Thus, study of EBV-specific immune dysregulation is important for understanding EBV latency and oncogenesis, and will identify potential molecular targets for immunotherapeutic interventions. Here I summarize the recent findings of individual EBV products in regulating host immune responses, with emphasis on the innate immune modulation.

EBV lytic-phase protein BGLF5 contributes to TLR9 downregulation during productive infection

Viruses use a wide range of strategies to modulate the host immune response. The human gammaherpesvirus EBV, causative agent of infectious mononucleosis and several malignant tumors, encodes proteins that subvert immune responses, notably those mediated by T cells. Less is known about EBV interference with innate immunity, more specifically at the level of TLR-mediated pathogen recognition. The viral dsDNA sensor TLR9 is expressed on B cells, a natural target of EBV infection. Here, we show that EBV particles trigger innate immune signaling pathways through TLR9. Furthermore, using an in vitro system for productive EBV infection, it has now been possible to compare the expression of TLRs by EBV(-) and EBV(+) human B cells during the latent and lytic phases of infection. Several TLRs were found to be differentially expressed either in latently EBV-infected cells or after induction of the lytic cycle. In particular, TLR9 expression was profoundly decreased at both the RNA and protein levels during productive EBV infection. We identified the EBV lytic-phase protein BGLF5 as a protein that contributes to downregulating TLR9 levels through RNA degradation. Reducing the levels of a pattern-recognition receptor capable of sensing the presence of EBV provides a mechanism by which the virus could obstruct host innate antiviral responses.

Epstein-Barr virus genetics: talking about the BAC generation

Genetic mutant organisms pervade all areas of Biology. Early on, herpesviruses (HV) were found to be amenable to genetic analysis using homologous recombination techniques in eukaryotic cells. More recently, HV genomes cloned onto a bacterial artificial chromosome (BAC) have become available. HV BACs can be easily modified in E.coli and reintroduced in eukaryotic cells to produce infectious viruses. Mutants derived from HV BACs have been used both to understand the functions of all types of genetic elements present on the virus genome, but also to generate mutants with potentially medically relevant properties such as preventative vaccines. Here we retrace the development of the BAC technology applied to the Epstein-Barr virus (EBV) and review the strategies available for the construction of mutants. We expand on the appropriate controls required for proper use of the EBV BACs, and on the technical hurdles researchers face in working with these recombinants. We then discuss how further technological developments might successfully overcome these difficulties. Finally, we catalog the EBV BAC mutants that are currently available and illustrate their contributions to the field using a few representative examples.

Prospects of a novel vaccination strategy for human gamma-herpesviruses

Due to the oncogenic potential associated with persistent infection of human gamma-herpesviruses, including Epstein-Barr virus (EBV or HHV-4) and Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8), vaccine development has focused on subunit vaccines. However, the results using an animal model of mouse infection with a related rodent virus, murine gamma-herpesvirus 68 (MHV-68, γHV-68, or MuHV-4), have shown that the only effective vaccination strategy is based on live attenuated viruses, including viruses engineered to be incapable of establishing persistence. Vaccination with a virus lacking persistence would eliminate many potential complications. Progress in understanding persistent infections of EBV and KSHV raises the possibility of engineering a live attenuated virus without persistence. Therefore, we should keep the option open for developing a live EBV or KSHV vaccine.

A novel latent membrane 2 transcript expressed in Epstein-Barr virus-positive NK- and T-cell lymphoproliferative disease encodes a target for cellular immunotherapy

Therapeutic targeting of virus-encoded proteins using cellular immunotherapy has proved successful for Epstein-Barr virus (EBV)-associated posttransplant lymphoproliferative disease. However, the more limited repertoire and immunogenicity of EBV-encoded proteins in other malignancies such as Hodgkin lymphoma and extranodal natural killer (NK)/T lymphoma has been more challenging to target. The immunosubdominant latent membrane protein 2 (LMP2) is considered the optimal target in such Latency II tumors, although data relating to its expression in T/NK malignancies are limited. In addressing the validity of LMP2 as an immunotherapeutic target we found that LMP2-specific effector CD8(+) T cells recognized and killed EBV-positive NK- and T-cell tumor lines, despite an apparent absence of LMP2A protein and barely detectable levels of LMP2 transcripts from the conventional LMP2A and LMP2B promoters. We resolved this paradox by identifying in these lines a novel LMP2 mRNA, initiated from within the EBV terminal repeats and containing downstream, epitope-encoding exons. This same mRNA was also highly expressed in primary (extra-nodal) NK/T lymphoma tissue, with virtually undetectable levels of conventional LMP2A/B transcripts. Expression of this novel transcript in T/NK-cell lymphoproliferative diseases validates LMP2 as an attractive target for cellular immunotherapy and implicates this truncated LMP2 protein in NK- and T-cell lymphomagenesis. This study is registered at clinicaltrials.gov as NCT00062868.

Epstein-Barr virus lytic cycle activation alters proteasome subunit expression in Burkitt's lymphoma cells

We have shown that Epstein-Barr virus (EBV) lytic cycle activation in Burkitt's lymphoma (BL) cells down-regulates chymotrypsin- and caspase-like activities of the proteasome. The aim of the present study was to evaluate whether EBV activation might also affect proteasome subunit composition. Our results indicate that, independently of the latency program established in the host cells, induction of the EBV lytic cycle reduces the expression of the proteasomal components β5, β1 and β2i, whereas it increases that of β2, β1i, PA28α and PA28β. The modulation of the composition and enzymatic activities of the proteolytic complex are indicative of a less efficient generation of viral immunoepitopes.

The Epstein-Barr virus-encoded G protein-coupled receptor BILF1 hetero-oligomerizes with human CXCR4, scavenges Gαi proteins, and constitutively impairs CXCR4 functioning

Cells express distinct G protein-coupled receptor (GPCR) subtypes on their surface, allowing them to react to a corresponding variety of extracellular stimuli. Cross-regulation between different ligand-GPCR pairs is essential to generate appropriate physiological responses. GPCRs can physically affect each other's functioning by forming heteromeric complexes, whereas cross-regulation between activated GPCRs also occurs through integration of shared intracellular signaling networks. Human herpesviruses utilize virally encoded GPCRs to hijack cellular signaling networks for their own benefit. Previously, we demonstrated that the Epstein-Barr virus-encoded GPCR BILF1 forms heterodimeric complexes with human chemokine receptors. Using a combination of bimolecular complementation and bioluminescence resonance energy transfer approaches, we now show the formation of hetero-oligomeric complexes between this viral GPCR and human CXCR4. BILF1 impaired CXCL12 binding to CXCR4 and, consequently, also CXCL12-induced signaling. In contrast, the G protein uncoupled mutant BILF1-K(3.50)A affected CXCL12-induced CXCR4 signaling to a much lesser extent, indicating that BILF1-mediated CXCR4 inhibition is a consequence of its constitutive activity. Co-expression of Gα(i1) with BILF1 and CXCR4 restored CXCL12-induced signaling. Likewise, BILF1 formed heteromers with the human histamine H(4) receptor (H(4)R). BILF1 inhibited histamine-induced Gα(i)-mediated signaling by H(4)R, however, without affecting histamine binding to this receptor. These data indicate that functional cross-regulation of Gα(i)-coupled GPCRs by BILF1 is at the level of G proteins, even though these GPCRs are assembled in hetero-oligomeric complexes.

Cell transformation mediated by the Epstein-Barr virus G protein-coupled receptor BILF1 is dependent on constitutive signaling

Epstein-Barr virus (EBV) open reading frame BILF1 encodes a seven trans-membrane (TM) G protein-coupled receptor that signals with high constitutive activity through Galpha(i) (Beisser et al., 2005; Paulsen et al., 2005). In this paper, the transforming potential of BILF1 is investigated in vitro in a foci formation assay using retrovirally transduced NIH3T3 cells, as well as in vivo by using nude mice. BILF1 revealed a substantial transforming potential that was dependent on constitutive signaling, as a signaling-deficient mutant completely lost its ability to transform cells in vitro, and an intermediately active triple-mutated receptor possessed an intermediate transforming potential. Furthermore, BILF1 expression induced vascular endothelial growth factor secretion in a constitutively active manner. In nude mice, BILF1 promoted tumor formation in 90% of cases, ORF74 (from Kaposi's sarcoma-associated herpes virus) in 100% of cases, whereas the signaling-deficient receptor resulted in tumor establishment in 40% of cases. These data suggest that BILF1, when expressed during EBV infection, could indeed be involved in the pathogenesis of EBV-associated diseases and malignancies. Furthermore, the correlation between receptor activity and the ability to mediate cell transformation in vitro and tumor formation in vivo supports the idea that inverse agonists for BILF1 could inhibit cell transformation and be relevant therapeutic candidates.

The RING-CH ligase K5 antagonizes restriction of KSHV and HIV-1 particle release by mediating ubiquitin-dependent endosomal degradation of tetherin

Tetherin (CD317/BST2) is an interferon-induced membrane protein that inhibits the release of diverse enveloped viral particles. Several mammalian viruses have evolved countermeasures that inactivate tetherin, with the prototype being the HIV-1 Vpu protein. Here we show that the human herpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV) is sensitive to tetherin restriction and its activity is counteracted by the KSHV encoded RING-CH E3 ubiquitin ligase K5. Tetherin expression in KSHV-infected cells inhibits viral particle release, as does depletion of K5 protein using RNA interference. K5 induces a species-specific downregulation of human tetherin from the cell surface followed by its endosomal degradation. We show that K5 targets a single lysine (K18) in the cytoplasmic tail of tetherin for ubiquitination, leading to relocalization of tetherin to CD63-positive endosomal compartments. Tetherin degradation is dependent on ESCRT-mediated endosomal sorting, but does not require a tyrosine-based sorting signal in the tetherin cytoplasmic tail. Importantly, we also show that the ability of K5 to substitute for Vpu in HIV-1 release is entirely dependent on K18 and the RING-CH domain of K5. By contrast, while Vpu induces ubiquitination of tetherin cytoplasmic tail lysine residues, mutation of these positions has no effect on its antagonism of tetherin function, and residual tetherin is associated with the trans-Golgi network (TGN) in Vpu-expressing cells. Taken together our results demonstrate that K5 is a mechanistically distinct viral countermeasure to tetherin-mediated restriction, and that herpesvirus particle release is sensitive to this mode of antiviral inhibition.

Herpesviruses and immunity: the art of evasion

Herpesviruses have evolved several effective strategies to counter the host immune response. Chief among these is inhibition of the host MHC class I antigen processing and presentation pathway, thereby reducing the presentation of virus-derived epitopes on the surface of the infected cell. This review summarizes the mechanisms used by herpesviruses to achieve this goal, including shut-down of MHC class I molecule synthesis, blockage of proteasome-mediated peptide generation and prevention of TAP-mediated peptide transport. Furthermore, herpesvirus proteins can retain MHC class I molecules in the endoplasmic reticulum, or direct their retrograde translocation from the endoplasmic reticulum or endocytosis from the plasma membrane, with subsequent degradation. The resulting down-regulation of cell surface MHC class I peptide complexes thwarts the ability of cytotoxic T lymphocytes to recognize and eliminate virus-infected cells. The subversion of the natural killer cell response by herpesvirus proteins and microRNAs is also discussed.

Immune responses to Epstein-Barr virus: molecular interactions in the virus evasion of CD8+ T cell immunity

Persistent viruses have mechanisms for modulating the host immune responses that are essential for achieving a lifelong virus–host balance while minimizing the viral pathogenicity. Here we review some of the immune-modulating mechanisms evolved by the ubiquitous but potentially oncogenic Epstein–Barr virus, with particular emphasis on the molecular mechanisms of genes interfering with HLA class I antigen presentation.

Herpesvirus-encoded G protein-coupled receptors as modulators of cellular function

Human herpesviruses (HHVs) are widespread pathogens involved in proliferative diseases, inflammatory conditions, and cardiovascular diseases. During evolution, homologs of human chemokine receptors were integrated into the HHV genomes. In addition to binding endogenous chemokines, these viral G protein-coupled receptors (vGPCRs) have acquired the ability to signal in a constitutive manner. Ligand-induced and ligand-independent and autocrine and paracrine signaling properties of vGPCRs modify the functions of the expressing cells and lead to transformation and escape from immune surveillance. Furthermore, cross-talk or heterodimerization with endogenous chemokine receptors represent other ways for vGPCRs to modify intracellular signaling and cellular functions. As such, these viral receptors seem to play a prominent role during viral pathogenesis and life cycle and thus represent innovative antiviral therapies.

Upregulation of interleukin 7 receptor alpha and programmed death 1 marks an epitope-specific CD8+ T-cell response that disappears following primary Epstein-Barr virus infection

In immunocompetent individuals, the stability of the herpesvirus-host balance limits opportunities to study the disappearance of a virus-specific CD8(+) T-cell response. However, we noticed that in HLA-A 0201-positive infectious mononucleosis (IM) patients undergoing primary Epstein-Barr virus (EBV) infection, the initial CD8 response targets three EBV lytic antigen-derived epitopes, YVLDHLIVV (YVL), GLCTLVAML (GLC), and TLDYKPLSV (TLD), but only the YVL and GLC reactivities persist long-term; the TLD response disappears within 10 to 27 months. While present, TLD-specific cells remained largely indistinguishable from YVL and GLC reactivities in many phenotypic and functional respects but showed unique temporal changes in two markers of T-cell fate, interleukin 7 receptor alpha (IL-7Ralpha; CD127) and programmed death 1 (PD-1). Thus, following the antigen-driven downregulation of IL-7Ralpha seen on all populations in acute IM, in every case, the TLD-specific population recovered expression unusually quickly post-IM. As well, in four of six patients studied, TLD-specific cells showed very strong PD-1 upregulation in the last blood sample obtained before the cells' disappearance. Our data suggest that the disappearance of this individual epitope reactivity from an otherwise stable EBV-specific response (i) reflects a selective loss of cognate antigen restimulation (rather than of IL-7-dependent signals) and (ii) is immediately preceded, and perhaps mediated, by PD-1 upregulation to unprecedented levels.

Stage-specific inhibition of MHC class I presentation by the Epstein-Barr virus BNLF2a protein during virus lytic cycle

The gamma-herpesvirus Epstein-Barr virus (EBV) persists for life in infected individuals despite the presence of a strong immune response. During the lytic cycle of EBV many viral proteins are expressed, potentially allowing virally infected cells to be recognized and eliminated by CD8+ T cells. We have recently identified an immune evasion protein encoded by EBV, BNLF2a, which is expressed in early phase lytic replication and inhibits peptide- and ATP-binding functions of the transporter associated with antigen processing. Ectopic expression of BNLF2a causes decreased surface MHC class I expression and inhibits the presentation of indicator antigens to CD8+ T cells. Here we sought to examine the influence of BNLF2a when expressed naturally during EBV lytic replication. We generated a BNLF2a-deleted recombinant EBV (DeltaBNLF2a) and compared the ability of DeltaBNLF2a and wild-type EBV-transformed B cell lines to be recognized by CD8+ T cell clones specific for EBV-encoded immediate early, early and late lytic antigens. Epitopes derived from immediate early and early expressed proteins were better recognized when presented by DeltaBNLF2a transformed cells compared to wild-type virus transformants. However, recognition of late antigens by CD8+ T cells remained equally poor when presented by both wild-type and DeltaBNLF2a cell targets. Analysis of BNLF2a and target protein expression kinetics showed that although BNLF2a is expressed during early phase replication, it is expressed at a time when there is an upregulation of immediate early proteins and initiation of early protein synthesis. Interestingly, BNLF2a protein expression was found to be lost by late lytic cycle yet DeltaBNLF2a-transformed cells in late stage replication downregulated surface MHC class I to a similar extent as wild-type EBV-transformed cells. These data show that BNLF2a-mediated expression is stage-specific, affecting presentation of immediate early and early proteins, and that other evasion mechanisms operate later in the lytic cycle.