Characterization of a novel human herpesvirus 8-encoded protein, vIRF-3, that shows homology to viral and cellular interferon regulatory factors

Unraveling the Kaposi Sarcoma-Associated Herpesvirus (KSHV) Lifecycle: An Overview of Latency, Lytic Replication, and KSHV-Associated Diseases

Kaposi sarcoma-associated herpesvirus (KSHV) is an oncogenic gammaherpesvirus and the etiological agent of several diseases. These include the malignancies Kaposi sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman disease (MCD), as well as the inflammatory disorder KSHV inflammatory cytokine syndrome (KICS). The KSHV lifecycle is characterized by two phases: a default latent phase and a lytic replication cycle. During latency, the virus persists as an episome within host cells, expressing a limited subset of viral genes to evade immune surveillance while promoting cellular transformation. The lytic phase, triggered by various stimuli, results in the expression of the full viral genome, production of infectious virions, and modulation of the tumor microenvironment. Both phases of the KSHV lifecycle play crucial roles in driving viral pathogenesis, influencing oncogenesis and immune evasion. This review dives into the intricate world of the KSHV lifecycle, focusing on the molecular mechanisms that drive its latent and lytic phases, their roles in disease progression, and current therapeutic strategies.

The Interplay between KSHV Infection and DNA-Sensing Pathways

During viral infection, the innate immune system utilizes a variety of specific intracellular sensors to detect virus-derived nucleic acids and activate a series of cellular signaling cascades that produce type I IFNs and proinflammatory cytokines and chemokines. Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic double-stranded DNA virus that has been associated with a variety of human malignancies, including Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman disease. Infection with KSHV activates various DNA sensors, including cGAS, STING, IFI16, and DExD/H-box helicases. Activation of these DNA sensors induces the innate immune response to antagonize the virus. To counteract this, KSHV has developed countless strategies to evade or inhibit DNA sensing and facilitate its own infection. This review summarizes the major DNA-triggered sensing signaling pathways and details the current knowledge of DNA-sensing mechanisms involved in KSHV infection, as well as how KSHV evades antiviral signaling pathways to successfully establish latent infection and undergo lytic reactivation.

Human Gammaherpesvirus 8 Oncogenes Associated with Kaposi’s Sarcoma

Kaposi’s sarcoma-associated herpesvirus (KSHV), also known as human gammaherpesvirus 8 (HHV-8), contains oncogenes and proteins that modulate various cellular functions, including proliferation, differentiation, survival, and apoptosis, and is integral to KSHV infection and oncogenicity. In this review, we describe the most important KSHV genes [ORF 73 (LANA), ORF 72 (vCyclin), ORF 71 or ORFK13 (vFLIP), ORF 74 (vGPCR), ORF 16 (vBcl-2), ORF K2 (vIL-6), ORF K9 (vIRF 1)/ORF K10.5, ORF K10.6 (vIRF 3), ORF K1 (K1), ORF K15 (K15), and ORF 36 (vPK)] that have the potential to induce malignant phenotypic characteristics of Kaposi’s sarcoma. These oncogenes can be explored in prospective studies as future therapeutic targets of Kaposi’s sarcoma.

Kaposi Sarcoma, a Trifecta of Pathogenic Mechanisms

Kaposi's sarcoma is a rare disease with four known variants: classic, epidemic, endemic and iatrogenic (transplant-related), all caused by an oncogenic virus named Human Herpes Virus 8. The viral infection in itself, along with the oncogenic properties of HHV8 and with immune system dysfunction, forms the grounds on which Kaposi's Sarcoma may develop. Infection with HHV8 occurs through saliva via close contacts, blood, blood products, solid organ donation and, rarely, vertical transmission. Chronic inflammation and oncogenesis are promoted by a mix of viral genes that directly promote cell survival and transformation or interfere with the regular cell cycle and cell signaling (of particular note: LANA-1, v-IL6, vBCL-2, vIAP, vIRF3, vGPCR, gB, K1, K8.1, K15). The most common development sites for Kaposi's sarcoma are the skin, mucocutaneous zones, lymph nodes and visceral organs, but it can also rarely appear in the musculoskeletal system, urinary system, endocrine organs, heart or eye. Histopathologically, spindle cell proliferation with slit-like vascular spaces, plasma cell and lymphocyte infiltrate are characteristic. The clinical presentation is heterogenic depending on the variant; some patients have indolent disease and others have aggressive disease. The treatment options include highly active antiretroviral therapy, surgery, radiation therapy, chemotherapy, and immunotherapy. A literature search was carried out using the MEDLINE/PubMed, SCOPUS and Google Scholar databases with a combination of keywords with the aim to provide critical, concise, and comprehensive insights into advances in the pathogenic mechanism of Kaposi's sarcoma.

KSHV/HHV8-mediated hematologic diseases

Kaposi sarcoma (KS) herpesvirus (KSHV), also known as human herpesvirus 8, is the causal agent of KS but is also pathogenetically related to several lymphoproliferative disorders, including primary effusion lymphoma (PEL)/extracavitary (EC) PEL, KSHV-associated multicentric Castleman disease (MCD), KSHV+ diffuse large B-cell lymphoma, and germinotropic lymphoproliferative disorder. These different KSHV-associated diseases may co-occur and may have overlapping features. KSHV, similar to Epstein-Barr virus (EBV), is a lymphotropic gammaherpesvirus that is preferentially present in abnormal lymphoid proliferations occurring in immunecompromised individuals. Notably, both KSHV and EBV can infect and transform the same B cell, which is frequently seen in KSHV+ EBV+ PEL/EC-PEL. The mechanisms by which KSHV leads to lymphoproliferative disorders is thought to be related to the expression of a few transforming viral genes that can affect cellular proliferation and survival. There are critical differences between KSHV-MCD and PEL/EC-PEL, the 2 most common KSHV-associated lymphoid proliferations, including viral associations, patterns of viral gene expression, and cellular differentiation stage reflected by the phenotype and genotype of the infected abnormal B cells. Advances in treatment have improved outcomes, but mortality rates remain high. Our deepening understanding of KSHV biology, clinical features of KSHV-associated diseases, and newer clinical interventions should lead to improved and increasingly targeted therapeutic interventions.

Human cytomegalovirus blocks canonical TGFβ signaling during lytic infection to limit induction of type I interferons

Human cytomegalovirus (HCMV) microRNAs (miRNAs) significantly rewire host signaling pathways to support the viral lifecycle and regulate host cell responses. Here we show that SMAD3 expression is regulated by HCMV miR-UL22A and contributes to the IRF7-mediated induction of type I IFNs and IFN-stimulated genes (ISGs) in human fibroblasts. Addition of exogenous TGFβ interferes with the replication of a miR-UL22A mutant virus in a SMAD3-dependent manner in wild type fibroblasts, but not in cells lacking IRF7, indicating that downregulation of SMAD3 expression to limit IFN induction is important for efficient lytic replication. These findings uncover a novel interplay between SMAD3 and innate immunity during HCMV infection and highlight the role of viral miRNAs in modulating these responses.

IFN Regulatory Factor 3 in Health and Disease

Immunity to viruses requires an array of critical cellular proteins that include IFN regulatory factor 3 (IRF3). Consequently, most viruses that infect vertebrates encode proteins that interfere with IRF3 activation. This review describes the cellular pathways linked to IRF3 activation and where those pathways are targeted by human viral pathogens. Moreover, key regulatory pathways that control IRF3 are discussed. Besides viral infections, IRF3 is also involved in resistance to some bacterial infections, in anticancer immunity, and in anticancer therapies involving DNA damage agents. A recent finding shows that IRF3 is needed for T cell effector functions that are involved in anticancer immunity and also in T cell autoimmune diseases. In contrast, unregulated IRF3 activity is clearly not beneficial, considering it is implicated in certain interferonopathies, in which heightened IRF3 activity leads to IFN-β-induced disease. Therefore, IRF3 is involved largely in maintaining health but sometimes contributing to disease.

Novel Functions and Virus-Host Interactions Implicated in Pathogenesis and Replication of Human Herpesvirus 8

Human herpesvirus 8 (HHV-8) is classified as a γ2-herpesvirus and is related to Epstein-Barr virus (EBV), a γ1-herpesvirus. One important aspect of the γ-herpesviruses is their association with neoplasia, either naturally or in animal model systems. HHV-8 is associated with B-cell-derived primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD), endothelial-derived Kaposi's sarcoma (KS), and KSHV inflammatory cytokine syndrome (KICS). EBV is also associated with a number of B-cell malignancies, such as Burkitt's lymphoma, Hodgkin's lymphoma, and posttransplant lymphoproliferative disease, in addition to epithelial nasopharyngeal and gastric carcinomas. Despite the similarities between these viruses and their associated malignancies, the particular protein functions and activities involved in key aspects of virus biology and neoplastic transformation appear to be quite distinct. Indeed, HHV-8 specifies a number of proteins for which counterparts had not previously been identified in EBV, other herpesviruses, or even viruses in general, and these proteins are believed to play vital functions in virus biology and to be involved centrally in viral pathogenesis. Additionally, a set of microRNAs encoded by HHV-8 appears to modulate the expression of multiple host proteins to provide conditions conductive to virus persistence within the host and possibly contributing to HHV-8-induced neoplasia. Here, we review the molecular biology underlying these novel virus-host interactions and their potential roles in both virus biology and virus-associated disease.

Molecular Virology of KSHV in the Lymphocyte Compartment-Insights From Patient Samples and De Novo Infection Models

The incidence of Kaposi’s sarcoma-associated herpesvirus (KSHV)-associated Kaposi Sarcoma has declined precipitously in the present era of effective HIV treatment. However, KSHV-associated lymphoproliferative disorders although rare, have not seen a similar decline. Lymphoma is now a leading cause of death in people living with HIV (PLWH), indicating that the immune reconstitution provided by antiretroviral therapy is not sufficient to fully correct the lymphomagenic immune dysregulation perpetrated by HIV infection. As such, novel insights into the mechanisms of KSHV-mediated pathogenesis in the immune compartment are urgently needed in order to develop novel therapeutics aimed at prevention and treatment of KSHV-associated lymphoproliferations. In this review, we will discuss our current understanding of KSHV molecular virology in the lymphocyte compartment, concentrating on studies which explore mechanisms unique to infection in B lymphocytes.

Comparative analysis of the viral interferon regulatory factors of KSHV for their requisite for virus production and inhibition of the type I interferon pathway

Unique among human viruses, Kaposi's sarcoma-associated herpesvirus (KSHV) encodes several homologs of cellular interferon regulatory factors (vIRFs). Since KSHV expresses multiple factors that can inhibit interferon (IFN) signaling to promote virus production, it is still unclear to what extent vIRFs contribute to these specific processes during KSHV infection. To study the function of vIRFs during viral infection, we engineered 3xFLAG-tagged-vIRF and vIRF-knockout recombinant KSHV clones, which were utilized to test vIRF expression, as well as their requirement for viral replication, virus production, and inhibition of the type I IFN pathway in different models of lytic KSHV infection. Our data show that all vIRFs can be expressed as lytic viral proteins, yet were dispensable for KSHV production and inhibition of type I IFN. Nevertheless, as vIRFs were able to suppress IFN-stimulated antiviral genes, vIRFs may still promote the KSHV lytic cycle in the presence of an ongoing antiviral response.

Mechanistic Insights into Chemoresistance Mediated by Oncogenic Viruses in Lymphomas

Viral lymphomagenesis induced by infection with oncogenic viruses, such as Kaposi’s sarcoma associated herpesvirus (KSHV), Epstein–Barr virus (EBV) and human T-cell leukemia virus (HTLV-1), represents a group of aggressive malignancies with a diverse range of pathological features. Combined chemotherapy remains the standard of care for these virus-associated lymphomas; however, frequent chemoresistance is a barrier to achieving successful long-term disease-free survival. There is increasing evidence that indicates virus-associated lymphomas display more resistance to cytotoxic chemotherapeutic agents than that observed in solid tumors. Although the tumor microenvironment and genetic changes, such as key oncogene mutations, are closely related to chemoresistance, some studies demonstrate that the components of oncogenic viruses themselves play pivotal roles in the multidrug chemoresistance of lymphoma cells. In this review, we summarize recent advances in the understanding of the mechanisms through which oncogenic viruses mediate lymphoma cell chemoresistance, with a particular focus on KSHV and EBV, two major oncogenic viruses. We also discuss the current challenges to overcome these obstacles in the treatment of virus-associated lymphomas.

Pathogenesis of Human Gammaherpesviruses: Recent Advances

PURPOSE OF THIS REVIEW

Human gammaherpesviruses have complex lifecycles that drive their pathogenesis. KSHV and EBV are the etiological agents of multiple cancers worldwide. There is no FDA-approved vaccine for either KSHV or EBV. This review will describe recent progress in understanding EBV and KSHV lifecycles during infection.

RECENT FINDINGS

Determining how latency is established, particularly how non-coding RNAs influence latent and lytic infection, is a rapidly growing area of investigation into how gammaherpesviruses successfully persist in the human population. Many factors have been identified as restrictors of reactivation from latency, especially innate immune antagonism. Finally, new host proteins that play a role in lytic replication have been identified.

SUMMARY

In this review we discuss recent findings over the last 5 years on both host and viral factors that are involved in EBV and KSHV pathogenesis.

Strategies for immune evasion by human tumor viruses

Immune evasion is a hallmark of viral persistence. For the seven human tumor viruses to establish lifelong infection in their hosts, they must successfully control the host response to them. Viral inhibition of immune responses occurs at many levels. While some viruses directly target the pattern recognition receptors (PRR) of the innate immune system, they may also antagonize downstream effectors of PRR signaling cascades or activation of transcription, which would otherwise induce a type I interferon (IFN) and/or pro-inflammatory cytokine response. Secretion of IFN activates the type I interferon receptor (IFNAR) signaling pathway, which is also prone to viral inhibition. To evade the adaptive host response, viruses also target various mechanisms including antigen processing and presentation.

Differential Expression Patterns of Irf3 and Irf7 in Pediatric Lymphoid Disorders

Interferon regulatory factors (IRFs) are multifunctional transcriptional factors. To define the role of IRFs in lymphoid disorders, we determined the expression patterns of IRF3 and IRF7 by immunohistochemistry in 5 normal lymph nodes, 12 reactive hyperplastic lymph nodes, and 27 pediatric lymphomas. IRF3 was prominently expressed in the nuclei of the histiocytes, and was expressed very weakly in the cytoplasm of most of the lymphocytes of the normal lymph nodes. However, IRF7 was expressed strongly in the nuclei of over 50% of the lymphocytes throughout the normal lymph nodes, but the histiocytes and fibroblasts were spared. In the reactive hyperplastic lymph nodes, the number of IRF3- and IRF7-positive cells in the nuclei was elevated. In the lymphomas, the number of IRF3-positive cells in the nucleus appeared to have decreased, and the cells were scattered throughout the lymphoma tissue in no specific pattern. However, in most cases the number of IRF7-positive cells was elevated.

These results suggested that IRF3 was activated principally in the histiocytes and T cells under inflammatory conditions, but IRF3 activation was attenuated in cases of lymphoma. However, the number of IRF7-positive cells was found to be elevated in the reactive hyperplastic lymph nodes and pediatric lymphoma.

Signal Transduction Pathways Associated with KSHV-Related Tumors

Signal transduction pathways play a key role in the regulation of cell growth, cell differentiation, cell survival, apoptosis, and immune responses. Bacterial and viral pathogens utilize the cell signal pathways by encoding their own proteins or noncoding RNAs to serve their survival and replication in infected cells. Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is classified as a rhadinovirus in the γ-herpesvirus subfamily and was the eighth human herpesvirus to be discovered from Kaposi's sarcoma specimens. KSHV is closely associated with an endothelial cell malignancy, Kaposi's sarcoma, and B-cell malignancies, primary effusion lymphoma, and multicentric Castleman's disease. Recent studies have revealed that KSHV manipulates the cellular signaling pathways to achieve persistent infection, viral replication, cell proliferation, anti-apoptosis, and evasion of immune surveillance in infected cells. This chapter summarizes recent developments in our understanding of the molecular mechanisms used by KSHV to interact with the cell signaling machinery.

Current status and future prospects for human papillomavirus vaccines

Cervical cancer is the fourth most frequent cancer among women worldwide. Human papillomaviruses (HPVs) cause almost all cervical cancers in low-income countries. Three prophylactic HPV virus-like particle-based vaccines have been licensed to date, and they have all shown high efficacy and reliable safety profiles. However, isolated safety issues have resulted in a reluctance to use these vaccinations. In addition, the high prices of the vaccinations have caused the inequitable distribution of the vaccine: the prices are unaffordable for low-income countries. Meanwhile, great effort has been put into the development of therapeutic HPV vaccines, including protein/peptide-, live vector-, DNA- and cell-based vaccines. These new vaccines have considerable therapeutic potential but limited practical use. The development of immune checkpoint inhibitors and personalized immunotherapy remain challenges for future study. In this article, the current status of the licensed vaccines, therapeutic HPV vaccines and biosimilars, and new platforms for HPV vaccines, are reviewed, and safety issues related to the licensed vaccines are discussed. In addition, the prospects for HPV vaccines are considered.

Rhadinoviral interferon regulatory factor homologues

Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8 (HHV8) is a gammaherpesvirus and the etiological agent of Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman disease. The KSHV genome contains genes for a unique group of proteins with homology to cellular interferon regulatory factors, termed viral interferon regulatory factors (vIRFs). This review will give an overview over the oncogenic, antiapoptotic and immunomodulatory characteristics of KSHV and related vIRFs.

Primary lymphocyte infection models for KSHV and its putative tumorigenesis mechanisms in B cell lymphomas

Kaposi's sarcoma-associated herpesvirus (KSHV) is the latest addition to the human herpesvirus family. Unlike alpha- and beta-herpesvirus subfamily members, gamma-herpesviruses, including Epstein-Barr virus (EBV) and KSHV, cause various tumors in humans. KSHV primarily infects endothelial and B cells in vivo, and is associated with at least three malignancies: Kaposi's sarcoma and two B cell lymphomas, respectively. Although KSHV readily infects endothelial cells in vitro and thus its pathogenic mechanisms have been extensively studied, B cells had been refractory to KSHV infection. As such, functions of KSHV genes have mostly been elucidated in endothelial cells in the context of viral infection but not in B cells. Whether KSHV oncogenes, defined in endothelial cells, play the same roles in the tumorigenesis of B cells remains an open question. Only recently, through a few ground-breaking studies, B cell infection models have been established. In this review, those models will be compared and contrasted and putative mechanisms of KSHV-induced B cell transformation will be discussed.

A Rhesus Rhadinovirus Viral Interferon (IFN) Regulatory Factor Is Virion Associated and Inhibits the Early IFN Antiviral Response

The interferon (IFN) response is the earliest host immune response dedicated to combating viral infection. As such, viruses have evolved strategies to subvert this potent antiviral response. Two closely related gammaherpesviruses, Kaposi's sarcoma-associated herpesvirus (KSHV) and rhesus macaque rhadinovirus (RRV), are unique in that they express viral homologues to cellular interferon regulatory factors (IRFs), termed viral IRFs (vIRFs). Cellular IRFs are a family of transcription factors that are particularly important for the transcription of type I IFNs. Here, we demonstrate a strategy employed by RRV to ensure rapid inhibition of virus-induced type I IFN induction. We found that RRV vIRF R6, when expressed ectopically, interacts with a transcriptional coactivator, CREB-binding protein (CBP), in the nucleus. As a result, phosphorylated IRF3, an important transcriptional regulator in beta interferon (IFN-β) transcription, fails to effectively bind to the IFN-β promoter, thus inhibiting the activation of IFN-β genes. In addition, we found R6 within RRV virion particles via immunoelectron microscopy and, furthermore, that virion-associated R6 is capable of inhibiting the type I IFN response by preventing efficient binding of IRF3/CBP complexes to the IFN-β promoter in the context of infection. The work shown here is the first example of a vIRF being associated with either the KSHV or RRV virion. The presence of this immunomodulatory protein in the RRV virion provides the virus with an immediate mechanism to evade the host IFN response, thus enabling the virus to effectively establish an infection within the host.

IMPORTANCE

Kaposi's sarcoma-associated herpesvirus (KSHV) and the closely related rhesus macaque rhadinovirus (RRV) are the only viruses known to encode viral homologues to cellular interferon regulatory factors (IRFs), known as vIRFs. In KSHV, these proteins have been shown to play major roles in a variety of cellular processes and are particularly important in the evasion of the host type I interferon (IFN) response. In this study, we delineate the immunomodulatory mechanism of an RRV vIRF and its ability to assist the virus in rapid immune evasion by being prepackaged within the virion, thus providing evidence, for the first time, of a virion-associated vIRF. This work further contributes to our understanding of the mechanisms behind immunomodulation by the RRV vIRFs during infection.

Mimicry of cytokine pathways by human herpesviruses

ABSTRACT 

Viruses have evolved to subvert host cell pathways to enable their replication and persistence. In particular, virus-encoded gene products target the host's immune system to evade elimination by antiviral immune defenses. Cytokines are soluble, secreted proteins, which regulate many aspects of immune responses, by providing signals through cell surface receptors on target cells. Cytokine pathways are therefore attractive targets for modulation by viruses during their replication cycle. This review deals with modulation of cytokine pathways by the human herpesvirus, a family of viruses that are capable of life-long persistence in the host and cause severe disease particularly in immunocompromised individuals.

Molecular biology of human herpesvirus 8: novel functions and virus-host interactions implicated in viral pathogenesis and replication

Human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma-associated herpesvirus (KSHV), is the second identified human gammaherpesvirus. Like its relative Epstein-Barr virus, HHV-8 is linked to B-cell tumors, specifically primary effusion lymphoma and multicentric Castleman's disease, in addition to endothelial-derived KS. HHV-8 is unusual in its possession of a plethora of "accessory" genes and encoded proteins in addition to the core, conserved herpesvirus and gammaherpesvirus genes that are necessary for basic biological functions of these viruses. The HHV-8 accessory proteins specify not only activities deducible from their cellular protein homologies but also novel, unsuspected activities that have revealed new mechanisms of virus-host interaction that serve virus replication or latency and may contribute to the development and progression of virus-associated neoplasia. These proteins include viral interleukin-6 (vIL-6), viral chemokines (vCCLs), viral G protein-coupled receptor (vGPCR), viral interferon regulatory factors (vIRFs), and viral antiapoptotic proteins homologous to FLICE (FADD-like IL-1β converting enzyme)-inhibitory protein (FLIP) and survivin. Other HHV-8 proteins, such as signaling membrane receptors encoded by open reading frames K1 and K15, also interact with host mechanisms in unique ways and have been implicated in viral pathogenesis. Additionally, a set of micro-RNAs encoded by HHV-8 appear to modulate expression of multiple host proteins to provide conditions conducive to virus persistence within the host and could also contribute to HHV-8-induced neoplasia. Here, we review the molecular biology underlying these novel virus-host interactions and their potential roles in both virus biology and virus-associated disease.

p53 tumor suppressor protein stability and transcriptional activity are targeted by Kaposi's sarcoma-associated herpesvirus-encoded viral interferon regulatory factor 3

Viruses have developed numerous strategies to counteract the host cell defense. Kaposi's sarcoma-associated herpesvirus (KSHV) is a DNA tumor virus linked to the development of Kaposi's sarcoma, Castleman's disease, and primary effusion lymphoma (PEL). The virus-encoded viral interferon regulatory factor 3 (vIRF-3) gene is a latent gene which is involved in the regulation of apoptosis, cell cycle, antiviral immunity, and tumorigenesis. vIRF-3 was shown to interact with p53 and inhibit p53-mediated apoptosis. However, the molecular mechanism underlying this phenomenon has not been established. Here, we show that vIRF-3 associates with the DNA-binding domain of p53, inhibits p53 phosphorylation on serine residues S15 and S20, and antagonizes p53 oligomerization and the DNA-binding affinity. Furthermore, vIRF-3 destabilizes p53 protein by increasing the levels of p53 polyubiquitination and targeting p53 for proteasome-mediated degradation. Consequently, vIRF-3 attenuates p53-mediated transcription of the growth-regulatory p21 gene. These effects of vIRF-3 are of biological relevance since the knockdown of vIRF-3 expression in KSHV-positive BC-3 cells, derived from PEL, leads to an increase in p53 phosphorylation, enhancement of p53 stability, and activation of p21 gene transcription. Collectively, these data suggest that KSHV evolved an efficient mechanism to downregulate p53 function and thus facilitate uncontrolled cell proliferation and tumor growth.

IRF-4-mediated CIITA transcription is blocked by KSHV encoded LANA to inhibit MHC II presentation

Peptides presentation to T cells by MHC class II molecules is of importance in initiation of immune response to a pathogen. The level of MHC II expression directly influences T lymphocyte activation and is often targeted by various viruses. Kaposi's sarcoma-associated herpesvirus (KSHV) encoded LANA is known to evade MHC class I peptide processing, however, the effect of LANA on MHC class II remains unclear. Here, we report that LANA down-regulates MHC II expression and presentation by inhibiting the transcription of MHC II transactivator (CIITA) promoter pIII and pIV in a dose-dependent manner. Strikingly, although LANA knockdown efficiently disrupts the inhibition of CIITA transcripts from its pIII and pIV promoter region, the expression of HLA-DQβ but no other MHC II molecules was significantly restored. Moreover, we revealed that the presentation of HLA-DQβ enhanced by LANA knockdown did not help LANA-specific CD4+ T cell recognition of PEL cells, and the inhibition of CIITA by LANA is independent of IL-4 or IFN-γ signaling but dependent on the direct interaction of LANA with IRF-4 (an activator of both the pIII and pIV CIITA promoters). This interaction dramatically blocked the DNA-binding ability of IRF-4 on both pIII and pIV promoters. Thus, our data implies that LANA can evade MHC II presentation and suppress CIITA transcription to provide a unique strategy of KSHV escape from immune surveillance by cytotoxic T cells.

Role of IRF4 in IFN-stimulated gene induction and maintenance of Kaposi sarcoma-associated herpesvirus latency in primary effusion lymphoma cells

IFN regulatory factor (IRF) 4 is a hematopoietic cell-specific transcription factor that regulates the maturation and differentiation of immune cells. Using an inducible expression system, we found that IRF4 directly induced a specific subset of IFN-stimulated genes (ISGs) in a type I IFN-independent manner in both epithelial and B cell lines. Moreover, Kaposi sarcoma-associated herpesvirus (KSHV)-encoded viral FLICE inhibitory protein (vFLIP) enhances IRF4-mediated gene induction. Coexpression of IRF4 with vFLIP significantly increased ISG60 (IFIT3) and Cig5 (RSAD2) transcription that was dependent on the ability of vFLIP to activate NF-κB. A vFLIP mutant (A57L) defective in NF-κB activation failed to enhance IRF4-mediated ISG induction. Thus, we provide a physiologically relevant mechanism by which viral protein-mediated NF-κB activation modulates specific ISG induction by IRF4. In contrast, IRF4 also acted as a negative regulator of KSHV replication and transcription activator expression after induction of KSHV lytic reactivation in KSHV-positive primary effusion lymphoma cells. Taken together, these results suggest a dual role for IRF4 in regulating ISG induction and KSHV lytic reactivation in primary effusion lymphoma cells.

Distinct roles of Kaposi's sarcoma-associated herpesvirus-encoded viral interferon regulatory factors in inflammatory response and cancer

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent associated with Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman disease (MCD). Similar to other herpesviruses, KSHV has two life cycles, latency and lytic replication. In latency, the KSHV genome persists as a circular episome in the nucleus of the host cell and only a few viral genes are expressed. In this review, we focus on oncogenic, antiapoptotic, and immunomodulating properties of KSHV-encoded homologues of cellular interferon regulatory factors (IRFs)--viral IRF1 (vIRF1) to vIRF4--and their possible role in the KSHV-mediated antiviral response, apoptosis, and oncogenicity.

IRF-1 acts as a positive regulator in the transcription of grass carp (Ctenopharyngodon idella) IFN gene

Interferon regulatory factors (IRFs) are well-known to be crucial for modulating the innate immune responses to viral infections. In the present study, the IRF-1 gene of grass carp (Ctenopharyngodon idella) (termed CiIRF-1) was cloned and characterized. The complete genomic sequence of CiIRF-1 was 3150 bp in length and comprised 9 exons and 8 introns. The CiIRF-1 promoter sequence was 558 bp in length. The largest open reading frame (ORF) of the full CiIRF-1 cDNA sequence was 870 bp, and encoded a polypeptide of 289 amino acids. The putative CiIRF-1 was characterized by a conserved N-terminal DBD (113 aa), and included a signature of five conserved tryptophan residues. Phylogenetic relationship analysis revealed that CiIRF-1 was highly homologous to the counterparts of other teleosts and mammalians. CiIRF-1 was expressed at a low constitutive level but was significantly up-regulated following stimulation with either Poly I:C or recombinant grass carp (C. idella) IFN (rCiIFN) in all 6 tested tissues, especially in spleen and gill. The recombinant CiIRF-1 was expressed in BL21 Escherichia coli, and the expressed protein was purified by affinity chromatography with the Ni-NTA His-Bind Resin. Three different fragments of promoter sequences from grass carp type I IFN (CiIFN) gene (GU139255) were amplified. These fragments included the proximal region (CiIFNP2), the distal region (CiIFNP6), and the full length of CiIFN promoter sequences (CiIFNP7). Gel mobility shift assays were employed to analyze the interaction between CiIRF-1 and CiIFN promoter sequences. The results revealed that CiIRF-1 could bind to CiIFN promoter with high affinity in vitro. Subsequently, the recombinant plasmid of pGL3-CiIFNPs and pcDNA3.1-CiIRF-1 were constructed and transiently co-transfected into C. idella kidney (CIK) cells. The impact of CiIRF-1 on CiIFN promoter sequences were measured by luciferase assays. These results demonstrated that CiIRF-1 acts as a positive regulator in the transcription of grass carp IFN gene.

Viral genome maintenance and latent replication of human gammaherpesviruses

During gammaherpesvirus latency, only a few genes are expressed and required for maintenance of viral latency over a long period. While the expressed latent viral proteins play functional roles in viral latent DNA replication, they do not have replication-associated enzymatic activity such as polymerase or helicase activity. Viral genomes are detected in a similar copy number per infected cell, suggesting that the viral genome is replicated and segregated using host replication machinery. Kaposi’s sarcoma-associated herpesvirus and EBV have trans-acting elements required for viral genome maintenance during latency; LANA1 and EBNA1, respectively. The proteins recruit host replication-associated proteins at their latent origins, leading to initiation of viral replication and segregation with host chromosomes once per cell cycle. In addition, viral latent origins (cis-elements) provide trans-element-binding sites as well as a sufficient space for recruitment of cellular factors. In this review, we describe the molecular mechanisms required for replication of the viral genome during latency, including interactions with cellular factors and the interplay between viral trans- and cis-elements.

Regulation of T helper cell differentiation by interferon regulatory factor family members

Interferon regulatory factors (IRFs) consist of a family of transcription factors with diverse functions in the transcriptional regulation of cellular responses in health and diseases. IRFs commonly contain a DNA-binding domain in the N-terminus, with most members also containing a C-terminal IRF-associated domain that mediates protein-protein interactions. Ten IRFs and several virus-encoded IRF homologs have been identified in mammals so far. In response to endogenous and microbial stimuli during an immune response, IRFs are activated, and selectively and cooperatively modulate the expression of key cytokine and transcription factors involved in T helper cell differentiation in T cells and/or antigen-presenting cells. This review focuses on recent advances in the understanding of IRF-mediated transcriptional regulation in T helper cell differentiation and discusses the implications on the development of cellular and humoral immune responses and the pathogenesis of immune disorders.

Viral interactions with microtubules: orchestrators of host cell biology?

Viral interaction with the microtubule (MT) cytoskeleton is critical to infection by many viruses. Most data regarding virus–MT interaction indicate key roles in the subcellular transport of virions/viral genomic material to sites of replication, assembly and egress. However, the MT cytoskeleton orchestrates diverse processes in addition to subcellular cargo transport, including regulation of signaling pathways, cell survival and mitosis, suggesting that viruses, expert manipulators of the host cell, may use the virus–MT interface to control multiple aspects of cell biology. Several lines of evidence support this idea, indicating that specific viral proteins can modify MT dynamics and/or structure and regulate processes such as apoptosis and innate immune signaling through MT-dependent mechanisms. Here, the authors review general aspects of virus–MT interactions, with emphasis on viral mechanisms that modify MT dynamics and functions to affect processes beyond virion transport. The emerging importance of discrete viral protein–MT interactions in pathogenic processes indicates that these interfaces may represent new targets for future therapeutics and vaccine development.

The viral interferon regulatory factors of kaposi's sarcoma-associated herpesvirus differ in their inhibition of interferon activation mediated by toll-like receptor 3

Kaposi's sarcoma-associated herpesvirus (KSHV) infection is correlated with three human malignancies and can establish lifelong latent infection in multiple cell types within its human host. In order to establish and maintain infection, KSHV utilizes multiple mechanisms to evade the host immune response. One such mechanism is the expression of a family of genes with homology to cellular interferon (IFN) regulatory factors (IRFs), known as viral IRFs (vIRFs). We demonstrate here that KSHV vIRF1, -2, and -3 have a differential ability to block type I interferon signaling mediated by Toll-like receptor 3 (TLR3), a receptor we have previously shown to be activated upon KSHV infection. vIRF1, -2, and -3 inhibited TLR3-driven activation of IFN transcription reporters. However, only vIRF1 and vIRF2 inhibited increases in both IFN-β message and protein levels following TLR3 activation. The expression of vIRF1 and vIRF2 also allowed for increased replication of a virus known to activate TLR3 signaling. Furthermore, vIRF1 and vIRF2 may block TLR3-mediated signaling via different mechanisms. Altogether, this report indicates that vIRFs are able to block IFN mediated by TLRs but that each vIRF has a unique function and mechanism for blocking antiviral IFN responses.

Cdk1 inhibition induces mutually inhibitory apoptosis and reactivation of Kaposi's sarcoma-associated herpesvirus

Primary effusion lymphoma (PEL) cells are predominantly infected by the latent form of Kaposi's sarcoma-associated herpesvirus (KSHV), with virus reactivation occurring in a small percentage of cells. Latency enables KSHV to persist in the host cell and promotes tumorigenesis through viral gene expression, thus presenting a major barrier to the elimination of KSHV and the treatment of PEL. Therefore, it is important to identify cellular genes that are essential for PEL cell survival or the maintenance of KSHV latency. Here we report that cyclin-dependent kinase 1 (Cdk1) inhibition can induce both apoptosis and KSHV reactivation in a population of PEL cells. Caspases, but not p53, are required for PEL cell apoptosis induced by Cdk1 inhibition. p38 kinase is activated by Cdk1 inhibition and mediates KSHV reactivation. Interestingly, upon Cdk1 inhibition, KSHV is reactivated predominantly in the nonapoptotic subpopulation of PEL cells. We provide evidence that this is due to mutual inhibition between apoptosis and KSHV reactivation. In addition, we found that KSHV reactivation activates protein kinase B (AKT/PKB), which promotes cell survival and facilitates KSHV reactivation. Our study thus establishes a key role for Cdk1 in PEL cell survival and the maintenance of KSHV latency and reveals a multifaceted relationship between KSHV reactivation and PEL cell apoptosis.

Kaposi sarcoma-associated herpesvirus vIRF-3 protein binds to F-box of Skp2 protein and acts as a regulator of c-Myc protein function and stability

The Kaposi sarcoma-associated herpesvirus (KSHV) has been linked to Kaposi sarcoma, body cavity-based lymphoma, and Castleman disease. vIRF-3 is a KSHV latent gene that is critical for proliferation of KSHV-positive lymphoid cells. Furthermore, vIRF-3 contributes to KSHV-associated pathogenesis by stimulating c-Myc transcription activity. Here we show that vIRF-3 can associate with Skp2, a key component of the SCF(skp2) ubiquitin ligase complex. Skp2 is a transcriptional co-factor for c-Myc that was shown to regulate the stability of c-Myc protein as well as c-Myc-dependent transcription. In this study, we show that vIRF-3 binds to the F-box of Skp2 and recruits it to c-Myc-regulated promoters to activate c-Myc-dependent transcription. Additionally, cells overexpressing vIRF-3 exhibit higher levels of c-Myc ubiquitylation, suggesting that ubiquitylation is necessary for c-Myc-mediated transcription. Moreover, vIRF-3 can stabilize the c-Myc protein by increasing its half-life. Collectively, these results indicate that vIRF-3 can effectively manipulate c-Myc stability and function and thus contribute to c-Myc-induced KSHV-associated lymphomagenesis.

Viral interferon regulatory factors decrease the induction of type I and type II interferon during rhesus macaque rhadinovirus infection

Kaposi's sarcoma-associated herpesvirus and rhesus macaque rhadinovirus (RRV), two closely related gammaherpesviruses, are unique in their expression of viral homologs of cellular interferon regulatory factors (IRFs), termed viral IRFs (vIRFs). To assess the role of vIRFs during de novo infection, we have utilized the bacterial artificial chromosome clone of wild-type RRV(17577) (WT(BAC) RRV) to generate a recombinant virus with all 8 of the vIRFs deleted (vIRF-ko RRV). The infection of primary rhesus fibroblasts and peripheral blood mononuclear cells (PBMCs) with vIRF-ko RRV resulted in earlier and increased induction of type I interferon (IFN) (IFN-α/β) and type II IFN (IFN-γ). Additionally, plasmacytoid dendritic cells maintained higher levels of IFN-α production in PBMC cultures infected with vIRF-ko RRV than in cultures infected with WT(BAC) RRV. Moreover, the nuclear accumulation of phosphorylated IRF-3, which is necessary for the induction of type I IFN, was also inhibited following WT(BAC) RRV infection. These findings demonstrate that during de novo RRV infection, vIRFs are inhibiting the induction of IFN at the transcriptional level, and one potential mechanism for this is the disruption of the activation and localization of IRF-3.

Kaposi's Sarcoma-Associated Herpesvirus Genome Replication, Partitioning, and Maintenance in Latency

Kaposi's sarcoma-associated herpesvirus (KSHV) is thought to be an oncogenic member of the γ-herpesvirus subfamily. The virus usually establishes latency upon infection as a default infection pattern. The viral genome replicates according to the host cell cycle by recruiting the host cellular replication machinery. Among the latently expressing viral factors, LANA plays pivotal roles in viral genome replication, partitioning, and maintenance. LANA binds with two LANA-binding sites (LBS1/2) within a terminal repeat (TR) sequence and is indispensable for viral genome replication in latency. The nuclear matrix region seems to be important as a replication site, since LANA as well as cellular replication factors accumulate there and recruit the viral replication origin in latency (ori-P) by its binding activity to LBS. KSHV ori-P consists of LBS followed by a 32-bp GC-rich segment (32GC). Although it has been reported that LANA recruits cellular pre-replication complexes (pre-RC) such as origin recognition complexes (ORCs) to the ori-P through its interaction with ORCs, this mechanism does not account completely for the requirement of the 32GC. On the other hand, there are few reports about the partitioning and maintenance of the viral genome. LANA interacts with many kinds of chromosomal proteins, including Brd2/RING3, core histones, such as H2A/H2B and histone H1, and so on. The detailed molecular mechanisms by which LANA enables KSHV genome partitioning and maintenance still remain obscure. By integrating the findings reported thus far on KSHV genome replication, partitioning, and maintenance in latency, we will summarize what we know now, discuss what questions remain to be answered, and determine what needs to be done next to understand the mechanisms underlying viral replication, partitioning, and maintenance strategy.

Viral interferon regulatory factors are critical for delay of the host immune response against rhesus macaque rhadinovirus infection

Kaposi's sarcoma-associated herpesvirus (KSHV) and the closely related gamma-2 herpesvirus rhesus macaque (RM) rhadinovirus (RRV) are the only known viruses to encode viral homologues of the cellular interferon (IFN) regulatory factors (IRFs). Recent characterization of a viral IRF (vIRF) deletion clone of RRV (vIRF-knockout RRV [vIRF-ko RRV]) demonstrated that vIRFs inhibit induction of type I and type II IFNs during RRV infection of peripheral blood mononuclear cells. Because the IFN response is a key component to a host's antiviral defenses, this study has investigated the role of vIRFs in viral replication and the development of the immune response during in vivo infection in RMs, the natural host of RRV. Experimental infection of RMs with vIRF-ko RRV resulted in decreased viral loads and diminished B cell hyperplasia, a characteristic pathology during acute RRV infection that often develops into more severe lymphoproliferative disorders in immune-compromised animals, similar to pathologies in KSHV-infected individuals. Moreover, in vivo infection with vIRF-ko RRV resulted in earlier and sustained production of proinflammatory cytokines and earlier induction of an anti-RRV T cell response compared to wild-type RRV infection. These findings reveal the broad impact that vIRFs have on pathogenesis and the immune response in vivo and are the first to validate the importance of vIRFs during de novo infection in the host.

Kaposi's sarcoma-associated herpesvirus viral interferon regulatory factor-2 inhibits type 1 interferon signalling by targeting interferon-stimulated gene factor-3

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes four viral interferon regulatory factors (vIRF-1-4). We investigated the mechanism and consequences of vIRF-2-mediated inhibition of interferon-response element signalling following type I interferon (IFN) induction. Western blot and electrophoretic mobility-shift assays identified the interferon-stimulated gene factor-3 (ISGF-3) components STAT1 and IRF-9 as the proximal targets of vIRF-2 activity. The biological significance of vIRF-2 inhibition of ISGF-3 was demonstrated by vIRF-2-mediated rescue of the replication of the IFN-sensitive virus encephalomyocarditis virus. This study provides both a mechanism and evidence for KSHV vIRF-2-mediated suppression of the consequences of type 1 IFN-induced signalling.

Evasion and subversion of interferon-mediated antiviral immunity by Kaposi's sarcoma-associated herpesvirus: an overview

Viral invasion of a host cell triggers immune responses with both innate and adaptive components. The innate immune response involving the induction of type I interferons (alpha and beta interferons [IFN-α and -β]) constitutes the first line of antiviral defenses. The type I IFNs signal the transcription of a group of antiviral effector proteins, the IFN-stimulated genes (ISGs), which target distinct viral components and distinct stages of the viral life cycle, aiming to eliminate invading viruses. In the case of Kaposi's sarcoma-associated herpesvirus (KSHV), the etiological agent of Kaposi's sarcoma (KS), a sudden upsurge of type I IFN-mediated innate antiviral signals is seen immediately following both primary de novo infection and viral lytic reactivation from latency. Potent subversion of these responses thus becomes mandatory for the successful establishment of a primary infection following viral entry as well as for efficient viral assembly and egress. This review gives a concise overview of the induction of the type I IFN signaling pathways in response to viral infection and provides a comprehensive understanding of the antagonizing effects exerted by KSHV on type I IFN pathways wielded at various stages of the viral life cycle. Information garnered from this review should result in a better understanding of KSHV biology essential for the development of immunotherapeutic strategies targeted toward KSHV-associated malignancies.

Innate antiviral response: role in HIV-1 infection

As an early response to infection, cells induce a profile of the early inflammatory proteins including antiviral cytokines and chemokines. Two families of transcriptional factors play a major role in the transcriptional activation of the early inflammatory genes: The well-characterized family of NFkB factors and the family of interferon regulatory factors (IRF). The IRFs play a critical role in the induction of type I interferon (IFN) and chemokine genes, as well as genes mediating antiviral, antibacterial, and inflammatory responses. Type I IFNs represent critical components of innate antiviral immunity. These proteins not only exert direct antiviral effects, but also induce maturation of dendritic cells (DC), and enhance functions of NK, T and B cells, and macrophages. This review will summarize the current knowledge of the mechanisms leading to the innate antiviral response with a focus on its role in the regulation of HIV-1 infection and pathogenicity. We would like this review to be both historical and a future perspective.

The viral interferon regulatory factors of KSHV: immunosuppressors or oncogenes?

Kaposi’s sarcoma-associated herpesvirus (KSHV) is a large double-stranded DNA gammaherpesvirus, and the etiological agent for three human malignancies: Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease. To establish and maintain infection, KSHV has evolved unique mechanisms to evade the host immune response. Cellular interferon regulatory factors (IRFs) are a critical part of the host anti-viral immune response. KSHV encodes four homologs of IRFs, vIRF1–4, which inhibit the activity of their cellular counterparts. vIRF1, 2, and 3 have been shown to interact directly with cellular IRFs. Additionally, the vIRFs have other functions such as modulation of Myc, p53, Notch, transforming growth factor-β, and NF-κB signaling. These activities of vIRFs may contribute to KSHV tumorigenesis. KSHV vIRF1 and vIRF3 have been implicated as oncogenes, making the understanding of KSHV vIRF function vital to understanding KSHV pathogenesis.

IRF7: activation, regulation, modification and function

Interferon regulatory factor 7 (IRF7) was originally identified in the context of Epstein-Barr virus (EBV) infection, and has since emerged as the crucial regulator of type I interferons (IFNs) against pathogenic infections, which activate IRF7 by triggering signaling cascades from pathogen recognition receptors (PRRs) that recognize pathogenic nucleic acids. Moreover, IRF7 is a multifunctional transcription factor, underscored by the fact that it is associated with EBV latency, in which IRF7 is induced as well as activated by the EBV principal oncoprotein latent membrane protein-1 (LMP1). Aberrant production of type I IFNs is associated with many types of diseases such as cancers and autoimmune disorders. Thus, tight regulation of IRF7 expression and activity is imperative in dictating appropriate type I IFN production for normal IFN-mediated physiological functions. Posttranslational modifications have important roles in regulation of IRF7 activity, exemplified by phosphorylation, which is indicative of its activation. Furthermore, mounting evidence has shed light on the importance of regulatory ubiquitination in activation of IRF7. Albeit these exciting findings have been made in the past decade since its discovery, many questions related to IRF7 remain to be addressed.

Malignancy after renal transplantation: the role of immunosuppression

Outcomes of kidney transplantation, in terms of graft and patient survival, have improved over the past few decades, partly as a result of the introduction of new immunosuppressive drugs. Many immunosuppressive agents are associated with an increased risk of cardiovascular events and an increased risk of cancer, however, which can compromise patient survival. Cancer is more common among solid-organ transplant recipients than it is in the general population or in patients on dialysis. In fact, malignancy is the third most common cause of death in renal transplant recipients. Immunosuppressive treatments used in renal transplant recipients can cause malignancy by supporting oncogenesis caused by certain viruses or by impairing immune surveillance thereby enabling faster tumor growth. In this Review, we describe the epidemiological and clinical characteristics of common tumor types occurring after kidney transplantation, and the etiopathogenetic factors that lead to their appearance, with a particular focus on the relationship between immunosuppressive treatment and malignancy. Immunosuppressive drugs associated with an increased risk of malignancy after transplantation are also discussed, as are immunosuppressive drugs that seem to have antioncogenic properties.

Recognition of herpesviruses by the innate immune system

Advances in innate immunity over the past decade have revealed distinct classes of pattern recognition receptors (PRRs) that detect pathogens at the cell surface and in intracellular compartments. This has shed light on how herpesviruses, which are large disease-causing DNA viruses that replicate in the nucleus, are initially recognized during cellular infection. Surprisingly, this involves multiple PRRs both on the cell surface and within endosomes and the cytosol. In this article we describe recent advances in our understanding of innate detection of herpesviruses, how this innate detection translates into anti-herpesvirus host defence, and how the viruses seek to evade this innate detection to establish persistent infections.

Modulation of interferon regulatory factor 5 activities by the Kaposi sarcoma-associated herpesvirus-encoded viral interferon regulatory factor 3 contributes to immune evasion and lytic induction

Multiple Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded proteins with potential roles in KSHV-associated neoplasms have been identified. KSHV encodes 4 genes with homology to transcription factors of the interferon (IFN) regulatory factor (IRF) family. Viral IRF3 (vIRF3) is expressed in latently KSHV-infected primary effusion lymphoma (PEL) cells and was recently shown to be essential for the survival of PEL cells. The focus of this study was to determine the mechanism(s) of vIRF3 oncogenic activity contributing to KSHV-associated lymphoma. We report that vIRF3 interacts with the amino-terminal DNA binding domain of human IRF5, leading to a complex manipulation of IRF5 function. vIRF3 associated with both exogenous and endogenous IRF5, thereby inhibiting IRF5-mediated IFN promoter activation and the synthesis of biologically active type I IFNs by blocking its binding to endogenous IFNA promoters. The function of this interaction was not limited to the IFN system as IRF5-mediated cell growth regulation was significantly altered by overexpression of vIRF3 in B cells. vIRF3 prevented IRF5-mediated growth inhibition and G2/M cell cycle arrest. Important, IRF5 was upregulated by the protein kinase C agonist 12-O-tetradecanoyl-phorbol-13-acetate in BCBL1 PEL cells and interaction with vIRF3 was observed at the endogenous p21 promoter in response to 12-O-tetradecanoyl-phorbol-13-acetate, suggesting that these 2 proteins cooperate in the regulation of lytic cycle-induced G1 arrest, which is an important early step for the reactivation of KSHV. In conclusion, cellular IRF5 and vIRF3 interact, leading to the functional modulation of IRF5-mediated type I IFN expression and cell cycle regulation. These findings support an important role for vIRF3 in immune evasion and cell proliferation that likely contribute to the survival of PEL cells.

Covalent modification by SUMO is required for efficient disruption of PML oncogenic domains by Kaposi's sarcoma-associated herpesvirus latent protein LANA2

The multifunctional Kaposi's sarcoma-associated herpesvirus (KSHV) latent protein latency-associated nuclear antigen 2 (LANA2) has a critical role in KSHV-induced B-cell malignancies. LANA2 increases the level of small ubiquitin-like modifier (SUMO)2-ubiquitin-modified PML and induces the disruption of PML oncogenic domains (PODs) by a process that requires a non-covalent SUMO interaction domain (SIM) in LANA2. We now demonstrate that LANA2 is covalently conjugated to SUMO1 and SUMO2 both in vitro and in latently KSHV-infected B-cells. We show that a LANA2 SIM mutant exhibits a slightly altered sumoylation pattern, which suggests that non-covalent SUMO interactions represent a mechanism for determining SUMO substrate recognition and modification. In addition, several lysine residues were mapped as SUMO conjugation sites. A sumoylation-deficient mutant shows impaired ability to induce disruption of PODs, which suggests that either directly bound or covalently conjugated SUMO moieties may act as a bridge for interaction between LANA2 and other SUMO-modified or SUMO-interacting proteins required for disruption of PODs.

Antagonism of host antiviral responses by Kaposi's sarcoma-associated herpesvirus tegument protein ORF45

Virus infection of a cell generally evokes an immune response by the host to defeat the intruder in its effort. Many viruses have developed an array of strategies to evade or antagonize host antiviral responses. Kaposi's sarcoma-associated herpesvirus (KSHV) is demonstrated in this report to be able to prevent activation of host antiviral defense mechanisms upon infection. Cells infected with wild-type KSHV were permissive for superinfection with vesicular stomatitis virus (VSV), suggesting that KSHV virions fail to induce host antiviral responses. We previously showed that ORF45, a KSHV immediate-early protein as well as a tegument protein of virions, interacts with IRF-7 and inhibits virus-mediated type I interferon induction by blocking IRF-7 phosphorylation and nuclear translocation (Zhu et al., Proc. Natl. Acad. Sci. USA. 99:5573-5578, 2002). Here, using an ORF45-null recombinant virus, we demonstrate a profound role of ORF45 in inhibiting host antiviral responses. Infection of cells with an ORF45-null mutant recombinant KSHV (BAC-stop45) triggered an immune response that resisted VSV super-infection, concomitantly associated with appreciable increases in transcription of type I IFN and downstream anti-viral effector genes. Gain-of-function analysis showed that ectopic expression of ORF45 in human fibroblast cells by a lentivirus vector decreased the antiviral responses of the cells. shRNA-mediated silencing of IRF-7, that predominantly regulates both the early and late phase induction of type I IFNs, clearly indicated its critical contribution to the innate antiviral responses generated against incoming KSHV particles. Thus ORF45 through its targeting of the crucial IRF-7 regulated type I IFN antiviral responses significantly contributes to the KSHV survival immediately following a primary infection allowing for progression onto subsequent stages in its life-cycle.

Kaposi's sarcoma-associated herpesvirus and innate immunity

Kaposi's sarcoma-associated herpesvirus (KSHV) is the most recently discovered human herpesvirus, first isolated and identified from a Kaposi's sarcoma lesion in 1994. It is the etiological agent of Kaposi's sarcoma, a vascular lesion that is the predominant cancer among AIDS patients. KSHV is also the primary etiological agent of two B-cell lymphomas, primary effusion lymphoma and multicentric Castleman's disease. KSHV can exist in either a lytic phase, in which the viral DNA is actively replicated and virions are assembled, or in a latent phase, in which the viral genome is tethered to the host chromosome via protein-protein interactions. The lytic cycle generally occurs following primary infection, and within 72-96 h in most cell types, the virus enters the latent state. Reactivation from latency also leads to the intiation of the lytic cycle, which is necessary for virus propagation and survival in the host. Several KSHV proteins have been implicated in modulation of the host immune response to viral infection. This article summarizes recent discoveries involving the innate immune response to KSHV infection.

Molecular biology of Kaposi's sarcoma-associated herpesvirus and related oncogenesis

Kaposi's Sarcoma-associated Herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is the most recently identified human tumor virus,and is associated with the pathogenesis of Kaposi's sarcoma and two lymphoproliferative disorders known to occur frequently in AIDS patients-primary effusion lymphoma and multicentric Castleman disease. In the 15 years since its discovery, intense studies have demonstrated an etiologic role for KSHV in the development of these malignancies. Here, we review the recent advances linked to understanding KSHV latent and lytic life cycle and the molecular mechanisms of KSHV-mediated oncogenesis in terms of transformation, cell signaling, cell growth and survival, angiogenesis, immune invasion and response to microenvironmental stress, and highlight the potential therapeutic targets for blocking KSHV tumorigenesis.

Kaposi's sarcoma-associated herpesvirus protein LANA2 disrupts PML oncogenic domains and inhibits PML-mediated transcriptional repression of the survivin gene

Infection by herpesviruses causes a dramatic disturbance of PML oncogenic domains (PODs) that has been suggested to be essential for viral lytic replication. Several proteins from Kaposi's sarcoma-associated herpesvirus (KSHV) have been tested as putative POD-disrupting factors with negative results. Here, we show that LANA2, a viral protein that is absolutely required for the viability and proliferation of KSHV-infected primary effusion lymphoma (PEL) cells, increases the levels of SUMO2-ubiquitin-modified PML and induces the disruption of PODs by a proteasome-mediated mechanism. In addition, we demonstrate that this disruption is largely dependent on both the integrity of a SUMO interaction motif in LANA2 and the lysine 160 from PML. Moreover, silencing of LANA2 expression in PEL cells by RNA interference led to an increase in the PML levels. Finally, we demonstrate that LANA2 relieves PML-mediated transcriptional repression of survivin, a protein that directly contributes to malignant progression of PEL. This represents the first example of inactivation of these important antiviral structures by KSHV.