Kaposi's sarcoma-associated human herpesvirus-8 encodes homologues of macrophage inflammatory protein-1 and interleukin-6

Detection of human herpesvirus 8 (HHV-8) in normal prostates

BACKGROUND

Human herpesvirus 8 (HHV-8) DNA has been detected in semen and prostatic tissues in some, but not all reports. We have analyzed prostate tissues from HHV-8 seropositive men for the expression of viral proteins and determined if expression of these proteins are associated with increased inflammation.

METHODS

Paraffin sections of non-cancerous prostates from HHV-8 seropositive (n = 16) and seronegative (n = 2) men who died with AIDS were screened for expression of three viral proteins by immunohistochemistry. Levels of inflammation were determined by expression of CD68 and CD20. Cellular proliferation was determined by expression of Ki67.

RESULTS

Among the 16 HHV-8 seropositive cases, 68.9% (11/16) (95% C.I. = 0.41-0.89) were positive for HHV-8 protein expression, while the 2 seronegative patients showed no HHV-8 protein expression. There was increased inflammation among HHV-8 positive prostates.

CONCLUSIONS

These results demonstrate that HHV-8 is present in normal prostates of HIV-infected men and the expression of viral proteins is associated with increased localized inflammation.

Kaposi's sarcoma-associated herpesvirus immune modulation: an overview

Kaposi's sarcoma-associated herpesvirus (KSHV) is the most recently discovered human herpesvirus. It is the aetiological agent of Kaposi's sarcoma (KS), a tumour frequently affecting AIDS patients not receiving treatment. KSHV is also a likely cause of two lymphoproliferative diseases: multicentric Castleman's disease and primary effusion lymphoma. The study of KSHV offers exciting challenges for understanding the mechanisms of virus pathogenesis, including those involved in establishing infection and dissemination in the host. To facilitate these processes, approximately one-quarter of KSHV genes encode cellular homologues or unique proteins that have immunomodulatory roles in cytokine production, apoptosis, cell signalling and the immunological synapse. The activities of these molecules are considered in the present review and the positions of their genes are mapped from a complete KSHV genome sequence derived from a KS biopsy. The understanding gained enables the significance of different components of the immune response in protection against KSHV infection to be evaluated. It also helps to unravel the complexities of cellular and immunological pathways and offers the potential for exploiting viral immunomodulators and derivatives in disease therapy.

Remission from Kaposi's sarcoma on HAART is associated with suppression of HIV replication and is independent of protease inhibitor therapy

Highly active antiretroviral therapy (HAART) reduces the incidence and improves the prognosis of Kaposi's sarcoma (KS). This study was designed to identify factors associated with KS clinical responses in HIV-infected patients during HAART. We reviewed the files of 138 HIV-1-infected patients with KS. Epidemiologic and HIV-related clinical and biological parameters were recorded at KS diagnosis (baseline) and every 6 months thereafter. In a subset of 73 antiretroviral-naive patients, we compared the clinical outcome of KS according to the use or nonuse of protease inhibitors (PI). After 6 months of follow-up, KS remission was more frequent in patients who were naive of HAART and who were at ACTG stage S0 at baseline (P=0.03 and 0.02). Undetectable HIV viral load was strongly associated with KS remission (P⩽0.004 at all time points), while CD4 cell count was not. Among the 73 antiretroviral-naive patients at baseline, and who were studied for 24 months, KS outcome did not differ between patients who were prescribed PI-containing and PI-sparing regimens. Intercurrent multicentric Castleman's disease was associated with poor outcome after 60 months of follow-up (P⩽0.0001). Fourteen deaths occurred after a median follow-up of 37.5 months, eight of which were KS related. Suppression of HIV replication appears to be crucial to control KS. Non-PI-based regimens were equivalent to PI-based regimens as regards the clinical and virological outcome of antiretroviral-naive HIV-infected patients with KS.

The pleiotropic effects of Kaposi's sarcoma herpesvirus

Kaposi's sarcoma herpesvirus (KSHV), or human herpesvirus 8 (HHV8), is an essential factor in the pathogenesis of Kaposi's sarcoma (KS), multicentric Castleman's disease (MCD), and primary effusion lymphoma (PEL). Case reports suggest an occasional involvement in bone marrow hypoplasia and haemophagocytic syndrome, but other disease associations are unconfirmed or controversial. KSHV-associated disease is of particular importance in immunosuppressed individuals, in particular in patients with HIV infection and transplant recipients. KSHV establishes a latent infection in the majority of infected cells in KS, MCD, and PEL, but lytic replication occurs in a small fraction of infected cells. Viral proteins expressed during both the latent and the lytic phase of the viral life cycle contribute to the pathogenesis of KSHV-associated diseases.

AIDS-related malignancies: emerging challenges in the era of highly active antiretroviral therapy

Human immunodeficiency virus (HIV)-infected patients are at increased risk of developing cancer, particularly in the later stages of acquired immune deficiency syndrome (AIDS). Despite the advent of highly active anti-retroviral therapy (HAART), malignancy in this population is a leading cause of morbidity and mortality. Kaposi's sarcoma (KS) and AIDS-related non-Hodgkin's lymphoma (ARL) are the most common AIDS-defining malignancies. AIDS-related KS varies from minimal to fulminant disease. Treatment decisions for AIDS-related KS are guided largely by the presence and extent of symptomatic disease. In addition to HAART, excellent treatments exist for both localized disease (topical gel, radiotherapy, and intralesional therapy) and advanced disease (liposomal anthracyclines, paclitaxel). Novel therapies that have become available to treat AIDS-related KS include angiogenesis inhibitors and antiviral agents. ARL comprises a heterogeneous group of malignancies. With the immune restoration afforded by HAART, standard-dose chemotherapies now can be safely administered to treat ARL with curative intent. The role of analogous treatments used in HIV-negative patients, including monoclonal antibodies and autologous stem cell transplantation, requires further clarification in HIV-positive patients. HIV-infected patients also appear to be at increased risk for developing certain non-AIDS-defining cancers, such as Hodgkin's lymphoma and multiple myeloma. Although the optimal treatment of these neoplasms is at present uncertain, recent advances in chemotherapy, antiretroviral drugs, and supportive care protocols are allowing for more aggressive management of many of the AIDS-related cancers. This article provides an up-to-date review of the epidemiology, pathogenesis, clinical features, and treatment of various AIDS-related malignancies that are likely to be encountered by an oncologist practicing in the current HAART era.

Linking KSHV to human cancer

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), has been linked to several malignancies in humans. KSHV is the etiologic agent associated with the development of Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman's disease (MCD). KSHV is a double-stranded DNA virus that has been classified as a gammaherpesvirus. Here, we review the association of KSHV with human cancer, viral genes that may potentially be involved in the neoplastic process, and current therapies used to treat KS, PEL, and MCD.

Human gammaherpesvirus cytokines and chemokine receptors

Most herpesviruses of the beta and gamma subfamilies encode homologues of cytokines and chemokine receptor- related G protein-coupled receptors (GPCRs). The roles of these proteins during normal virus replication in the infected host have not been defined in most cases, but the available data and extrapolation from what is known about the properties and functions of their cellular counterparts indicate that they play primary roles in immune evasion or in activating cellular signaling cascades that enhance virus productive replication. Cytokines and chemokine receptors specified by the two human gammaherpesviruses, human herpesvirus 8 (HHV-8) and Epstein-Barr virus (EBV), are the subject of this review. HHV-8 encodes three chemokines, a homologue of interleukin-6, and a CXCR2-related chemokine receptor, while EBV encodes a distinct GPCR and a homologue of interleukin-10. While these viral cytokines and chemokine receptors no doubt contribute to virus biology, their properties indicate that they may also be involved in virus-induced neoplasia. This review discusses the properties, functions, and likely roles of HHV-8 and EBV cytokines and chemokine receptors in relation to both virus biology and virus-associated disease.

KSHV G protein-coupled receptor inhibits lytic gene transcription in primary-effusion lymphoma cells via p21-mediated inhibition of Cdk2

Kaposi sarcoma (KS) remains the most common AIDS-associated malignancy worldwide. In sub-Saharan Africa especially, this aggressive endothelial-cell tumor is a cause of widespread morbidity and mortality. Infection with Kaposi sarcoma-associated herpesvirus (KSHV) is now known to be an etiologic force behind KS and primary-effusion lymphoma (PEL). Over time, KSHV has pirated many human genes whose products regulate angiogenesis, inflammation, and the cell cycle. One of these, the KSHV vGPCR, is a lytic product that is a constitutively active homolog of the IL-8 receptor. Although it is considered a viral oncogene and causes KS-like lesions in mice, vGPCR expression results in cell-cycle arrest of KSHV-infected PEL cells. In the present study, we show that this arrest is mediated by p21 in a p53-independent manner; the resulting Cdk2 inhibition decreases the efficiency of chemical induction of KSHV lytic transcripts ORF 50 and 26. Importantly, Cdk2 activity is also essential for replication in other human herpesviruses. The ability of vGPCR to delay or abort KSHV replication may explain how despite being a lytic product, this potent signaling molecule has a vital role in tumor formation via its induction of various KS-associated cytokines.

A relapsing inflammatory syndrome and active human herpesvirus 8 infection

We describe an immunocompetent 61-year-old woman who was negative for human immunodeficiency virus and who had recurrent human herpesvirus 8 (HHV-8) infection associated with a relapsing systemic inflammatory syndrome characterized by fever, lymphadenopathy, splenomegaly, edema, arthrosynovitis, and rash. Kaposi's sarcoma developed 10 months after the initial clinical presentation. A correlation was documented between the recurrent clinical manifestations and the HHV-8 load in plasma and peripheral-blood mononuclear cells. Histologic examination of an enlarged lymph node heavily infected with HHV-8 revealed an atypical lymphoproliferative disorder characterized by paracortical hyperplasia and collapsed primary and secondary follicles.

Multiplex detection and quantitation of latent and lytic transcripts of human herpesvirus-8 using RNase Protection Assay

Human herpesvirus-8 (HHV-8, also called Kaposi's sarcoma-associated herpesvirus) infectious cycle can be divided into latent and lytic phases. During the latent phase viral gene expression is reduced to a minimum, while during the lytic phase, numerous genes are expressed sequentially. The development of an RNase Protection Assay (RPA) is described that allows the detection and quantitation of three latent (ORFs 71-72-73) and three lytic (ORFs 74-K4-K2) HHV-8 genes as well as two cellular housekeeping gene transcripts (glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and cyclophilin) for normalization purposes is described. The RPA was validated using 293T cells transfected with corresponding HHV-8 expression vectors and using resting and phorbol ester-butyric acid-activated BC-3 and BCBL-1 cells. The results obtained indicate that this RPA is specific, sensitive and allows for the simultaneous monitoring of HHV-8 latent and lytic genes expression. HHV-8-RPA is therefore a useful technique to monitor the status of HHV-8 infection in infected cells (latent versus lytic) by comparing and quantitating multiple viral transcripts expression from a single RNA sample.

CODEHOP-mediated PCR - a powerful technique for the identification and characterization of viral genomes

Consensus-Degenerate Hybrid Oligonucleotide Primer (CODEHOP) PCR primers derived from amino acid sequence motifs which are highly conserved between members of a protein family have proven to be highly effective in the identification and characterization of distantly related family members. Here, the use of the CODEHOP strategy to identify novel viruses and obtain sequence information for phylogenetic characterization, gene structure determination and genome analysis is reviewed. While this review describes techniques for the identification of members of the herpesvirus family of DNA viruses, the same methodology and approach is applicable to other virus families.

New developments in IL-6 dependent biology and therapy: where do we stand and what are the options?

Interleukin-6 (IL-6) is a four-helical protein which, on target cells, binds to a specific IL-6-receptor and two molecules of the promiscuous signal transducing protein gp130. Structure-function analysis defined three molecular contact sites between IL-6 and its receptor subunits. Using this information, competitive antagonistic proteins as well as hyperagonistic proteins were developed. Possible therapeutic applications of IL-6 antagonists are in IL-6 dependent haematological disorders (Castleman's disease, POEMS syndrome, multiple myeloma) and bone diseases (Paget's disease, osteoporosis). Designer IL-6 antagonists could suppress inflammatory activity in rheumatic and autoimmune diseases and could prevent secondary amyloidosis. IL-6 antagonists could also prove advantageous in myocardial infarction and unstable angina pectoris. IL-6 antagonists might slow down development of (mesangioproliferative) glomerulonephritis. On the other hand, hyperagonistic variants of IL-6 have a potential in ex vivo expansion of bone marrow stem cells and as thrombopoietic agents. They might also be developed into drugs to support liver regeneration in vivo and to treat stress-induced cardiac insufficiency.

The English strain of rat cytomegalovirus (CMV) contains a novel captured CD200 (vOX2) gene and a spliced CC chemokine upstream from the major immediate-early region: further evidence for a separate evolutionary lineage from that of rat CMV Maastricht

Sequence data for eight genes, together with time-course Northern blotting and 3′- and 5′-RACE (rapid amplification of cDNA ends) analysis for some mRNAs from a 12 kb region upstream from the major immediate-early (MIE) genes of the English isolate of rat cytomegalovirus (RCMV), are presented. The results identified important differences compared to both murine cytomegalovirus (MCMV) and the Maastricht isolate of RCMV. A striking finding is the presence of a highly conserved, rightwards-oriented homologue of the rat cellular CD200 (OX2) gene immediately to the right of the MIE region, which replaces either the leftwards-oriented AAV REP gene of RCMV (Maastricht) or the upstream spliced portions of the immediate-early 2 gene (ie2) in MCMV. From the presence of other homologues of MCMV- and RCMV-specific genes, such as the β-chemokine MCK-2, SGG1 and an Fcγ receptor gene, as reported here, the basic architecture of the MIE region (reported previously) and the level of IE2 and DNA polymerase (POL) protein conservation in phylogenetic analyses, it is clear that the English strain of RCMV is also a member of the genus Muromegalovirus, but is a β-herpesvirus species that is very distinct from both MCMV and RCMV (Maastricht). Both the lack of a CD200 homologue in the other two rodent viruses and the depth of sequence divergence of the rodent CMV IE2 and POL proteins suggest that these three viruses have evolved as separate species in the genus Muromegalovirus since very early in the host rodent lineage.

Viral macrophage-inflammatory protein-II: a viral chemokine that differentially affects adaptive mucosal immunity compared with its mammalian counterparts

Chemokines play a profound role in leukocyte trafficking and the development of adaptive immune responses. Perhaps due to their importance in host defense, viruses have adopted many of the hallmarks displayed by chemokines. In particular, viral MIP-II (vMIP-II) is a human chemokine homologue that is encoded by human herpes virus 8. vMIP-II is angiogenic, selectively chemotactic for Th2 lymphocytes, and a homologue of human I-309 and mouse TCA-3, which also differentially attracts Th2 cells. To better understand the effect of viral chemokines on mucosal immunity, we compared the affects of vMIP-II, I-309, and TCA-3 on cellular and humoral immune responses after nasal immunization with OVA. These CCR8 ligands significantly enhanced Ag-specific serum and mucosal Abs through increasing Th2 cytokine secretion by CD4+ T cells. These alterations in adaptive humoral and cellular responses were preceded (12 h after immunization) by an increase in CD4+ T and B cells in nasal tracts with decreases of these leukocyte populations in the lung. Interestingly, vMIP-II increased neutrophil infiltration in the lung and Ag-specific IL-10-secreting CD4+ T cells after immunization. Although I-309 increased the number of CD28-, CD40L-, and CD30-positive, Ag-stimulated naive CD4+ T cells, vMIP-II and TCA-3 decreased the number of CD28-, CD40L-, and CD30-positive, resting naive CD4+ T cells. Taken together, these studies suggest that CCR8 ligands direct host Th2 responses, and vMIP-II up-regulates IL-10 responses and limits costimulatory molecule expression to mitigate host immunity.

Viral Macrophage Inflammatory Protein-II and Fractalkine (CX3CL1) Chimeras Identify Molecular Determinants of Affinity, Efficacy, and Selectivity at CX3CR1

Fractalkine (FKN/CX3CL1) is a cell surface-expressed chemokine involved in many aspects of leukocyte trafficking and activation. The various structural domains of FKN play distinct roles in its ability to bind and activate its receptor, CX3CR1. A human herpesvirus 8-encoded chemokine, termed viral macrophage inflammatory protein (vMIP)-II, is structurally similar to FKN; vMIP-II is a nonselective chemokine receptor antagonist (binding multiple chemokine receptors, including CX3CR1). The goal of this study was to identify FKN determinants of selectivity for its receptor and to further refine domains important in affinity and efficacy at CX3CR1. Chimeric and insertional mutagenesis was used to generate mutants of both vMIP-II and FKN, and the expressed proteins were evaluated for chemokine receptor binding affinities and efficacy at CX3CR1. Modification of the intervening amino acids between the first two conserved cysteine residues of FKN or vMIP-II indicated a role of the X3 bulge of FKN in affinity and selectivity for CX3CR1. Substitution of the vMIP-II N terminus with that of FKN created an agonist that was just as potent and efficacious as FKN for binding and stimulating CX3CR1, whereas replacement of the FKN N terminus with the cognate domain of vMIP-II disrupted the ability of FKN to bind CX3CR1. Furthermore, the entire N terminus of FKN was necessary for the high-affinity and full agonist properties of FKN at CX3CR1. These results refine the pharmacophore for chemokine binding to and activation of CX3CR1 and demonstrate the usefulness of modified virally encoded chemokines as templates for the development of selective chemokine receptor antagonists.

N-linked glycosylation is required for optimal function of Kaposi's sarcoma herpesvirus-encoded, but not cellular, interleukin 6

Kaposi's sarcoma-associated herpesvirus interleukin-6 (vIL-6) is a structural and functional homologue of the human cytokine IL-6 (hIL-6). hIL-6 and vIL-6 exhibit similar biological functions and both act via the gp130 receptor subunit to activate the Janus tyrosine kinase (JAK)1 and signal transducer and activator of transcription (STAT)1/3 pathway. Here we show that vIL-6 is N-linked glycosylated at N78 and N89 and demonstrate that N-linked glycosylation at site N89 of vIL-6 markedly enhances binding to gp130, signaling through the JAK1-STAT1/3 pathway and functions in a cytokine-dependent cell proliferation bioassay. Although hIL-6 is also N-glycosylated at N73 and multiply O-glycosylated, neither N-linked nor O-linked glycosylation is necessary for IL-6 receptor alpha-dependent binding to gp130 or signaling through JAK1-STAT1/3. As distinct from vIL-6, unglycosylated hIL-6 is as potent as glycosylated hIL-6 in stimulating B cell proliferation. These findings highlight distinct functional roles of N-linked glycosylation in viral and cellular IL-6.

RBP-J (CSL) is essential for activation of the K14/vGPCR promoter of Kaposi's sarcoma-associated herpesvirus by the lytic switch protein RTA

The Kaposi's sarcoma-associated herpesvirus (KSHV) gene product virally encoded G protein-coupled receptor (vGPCR) is a homolog of cellular GPCRs and has been proposed to play important roles in KSHV-induced angiogenesis. The most abundant vGPCR-containing transcripts are K14/vGPCR bicistronic RNAs that are strongly induced during lytic reactivation. Here we show that the promoter governing this transcript is strongly responsive to activation by the viral lytic switch protein RTA. By deletion mapping and scanning mutation analyses, we have identified three putative RTA response elements (A, B, and C) in this promoter. However, none of these sites appear to directly bind RTA in electrophoretic mobility shift assays (EMSA). Site C corresponds to a canonical binding site for RBP-J, a sequence-specific transcriptional repressor that is normally the target of Notch signaling. RBP-J can bind RTA and recruit it to its cognate recognition site; when this happens, the activation function of RTA can relieve RBP-J-mediated repression and upregulate expression of the targeted gene. EMSA studies reveal that both sites A and C can bind to RBP-J; sequence inspection reveals that site A is a novel functional variant of known RBP-J recognition sites. (Site B corresponds to an as-yet-unknown host DNA-binding protein.) The importance of sites A and C in vivo is underscored by the observation that K14/vGPCR promoter function is dramatically inhibited in cells genetically deficient in RBP-J. The regulation of K14/vGPCR transcripts by RBP-J raises the possibility that other modulators of Notch signaling might be able to induce expression of this RNA outside the context of lytic KSHV replication.

Alpha-chemokine-mediated signal transduction in human Kaposi's sarcoma spindle cells

The role of chemokines and their receptors in HIV biology and Kaposi's sarcoma (KS) pathogenesis has recently gained considerable attention. It has been shown that KS-associated human herpes virus type 8 (KSHV/HHV-8) encodes functional homologues of certain chemokines and chemokine receptors. This suggests that chemokines may contribute to the growth and spread of KS seen in AIDS. We found the expression of CXCR4 in primary KS tissue by using in situ hybridization (ISH). Recently, alpha-chemokine receptors CXCR1 and CXCR2 have also been shown to be expressed by KS tissues. We further characterized the expression of these chemokines as well as the signaling events induced upon binding to their respective cognate ligands in the KS 38 spindle cell line. These cells express authentic characteristics of primary KS spindle cells and provide a useful in vitro model for these studies. We observed using RT-PCR that KS 38 cells express mRNA for the alpha-chemokine receptors CXCR1, CXCR2, and CXCR4. We also confirmed the cell surface protein expression by FACS analysis. Characterization of signaling pathways revealed that the alpha-chemokines, IL-8 and stromal cell-derived factor 1alpha (SDF1alpha/CXCL12), activated members of the mitogen-activated protein (MAP) kinase family, including Erk kinase, c-Jun amino terminal kinase (JNK)/stress-activated protein kinase (SAPK) and the p38 MAP kinase. Furthermore, using DNA protein-binding experiments, we have shown that IL-8 increased AP-1 and NF Kappa B activity in these cells. IL-8 also enhanced the chemotaxis of KS cells. These results reveal that chemokine-induced signaling pathways may mediate cell growth, transcriptional activation and cell migration in KS.

Chemokines and Kaposi’s sarcoma

Chemokines participate in many biological processes in homeostasis and disease. Recently, they have been implicated in cancer, more specifically in tumor angiogenesis and metastasis. Here we review evidence supporting a role for chemokines in the pathogenesis of Kaposi's sarcoma and discuss a possible role for these molecules in angioproliferation and immune evasion.

IL-6 signal transduction and its physiological roles: the signal orchestration model

Interleukin (IL)-6 is a pleiotropic cytokine that not only affects the immune system, but also acts in other biological systems and many physiological events in various organs. In a target cell, IL-6 can simultaneously generate functionally distinct or sometimes contradictory signals through its receptor complex, IL-6Ralpha and gp130. One good illustration is derived from the in vitro observations that IL-6 promotes the growth arrest and differentiation of M1 cells through gp130-mediated STAT3 activation, whereas the Y759/SHP-2-mediated cascade by gp130 stimulation has growth-enhancing effects. The final physiological output can be thought of as a consequence of the orchestration of the diverse signaling pathways generated by a given ligand. This concept, the signal orchestration model, may explain how IL-6 can elicit proinflammatory or anti-inflammatory effects, depending on the in vivo environmental circumstances. Elucidation of the molecular mechanisms underlying this issue is a challenging subject for future research. Intriguingly, recent in vivo studies indicated that the SHP-2-binding site- and YXXQ-mediated pathways through gp130 are not mutually exclusive but affect each other: a mutation at the SHP-2-binding site prolongs STAT3 activation, and a loss of STAT activation by gp130 truncation leads to sustained SHP-2/ERK MAPK phosphorylation. Although IL-6/gp130 signaling is a promising target for drug discovery for many human diseases, the interdependence of each signaling pathway may be an obstacle to the development of a nonpeptide orally active small molecule to inhibit one of these IL-6 signaling cascades, because it would disturb the signal orchestration. In mice, a consequence of the imbalanced signals causes unexpected results such as gastrointestinal disorders, autoimmune diseases, and/or chronic inflammatory proliferative diseases. However, lessons learned from IL-6 KO mice indicate that IL-6 is not essential for vital biological processes, but a significant impact on disease progression in many experimental models for human disorders. Thus, IL-6/gp130 signaling will become a more attractive therapeutic target for human inflammatory diseases when a better understanding of IL-6 signaling, including the identification of the conductor for gp130 signal transduction, is achieved.

Molecular mechanisms for viral mimicry of a human cytokine: activation of gp130 by HHV-8 interleukin-6

Kaposi's sarcoma-associated herpesvirus (KSHV, or HHV-8) encodes a pathogenic viral homologue of human interleukin-6 (IL-6). In contrast to human IL-6 (hIL-6), viral IL-6 (vIL-6) binds directly to, and activates, the shared human cytokine signaling receptor gp130 without the requirement for pre-complexation to a specific alpha-receptor. Here, we dissect the biochemical and functional basis of vIL-6 mimicry of hIL-6. We find that, in addition to the "alpha-receptor-independent" tetrameric vIL-6/gp130 complex, the viral cytokine can engage the human alpha-receptor (IL-6Ralpha) to form a hexameric vIL-6/IL-6Ralpha/gp130 complex with enhanced signaling potency. In contrast to the assembly sequence of the hIL-6 hexamer, the preformed vIL-6/gp130 tetramer can be decorated with IL-6Ralpha, post facto, in a "vIL-6-dependent" fashion. A detailed comparison of the viral and human cytokine/gp130 interfaces indicates that vIL-6 has evolved a unique molecular strategy to interact with gp130, as revealed by an almost entirely divergent structural makeup of its receptor binding sites. Viral IL-6 appears to utilize an elegant combination of both convergent, and unexpectedly divergent, molecular strategies to oligomerize gp130 and activate similar downstream signaling cascades as its human counterpart.

The KSHV G protein-coupled receptor signals via multiple pathways to induce transcription factor activation in primary effusion lymphoma cells

Kaposi's sarcoma-associated virus (KSHV) is the causative agent of Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL). The KSHV G protein-couple receptor (vGPCR) is a homologue of the human IL-8 receptor that signals constitutively, activates mitogen- and stress-activated kinases, and induces transcription via multiple transcription factors including AP-1 and NFkappaB. Furthermore, vGPCR causes cellular transformation in vitro and leads to KS-like tumors in transgenic mouse models. vGPCR has therefore become an exciting potential therapeutic target for KSHV-mediated disease, but its signaling properties need to be better understood in the context of KSHV-infected hematopoietic cells. We recently described a PEL cell line that expresses vGPCR via an inducible promoter and have shown that vGPCR has broad capabilities of affecting cellular and viral transcription patterns in this highly relevant cell type. To elucidate the predominant signaling pathways used by vGPCR in PEL cells, we have used reporter gene assays to measure vGPCR activity in the presence of various pharmacologic enzyme inhibitors and plasmid constructs. We show that vGPCR-induced activation of AP-1 and CREB is mediated cooperatively by a Gq-ERK-1/2 and a Gi-PI3K-Src axis. Furthermore, unlike in other cell types, NFkappaB activation by vGPCR seems not to be substantially mediated by Gi or PI3K/Akt in PEL cells.

A macrophage inflammatory protein homolog encoded by guinea pig cytomegalovirus signals via CC chemokine receptor 1

Cytomegaloviruses encode homologs of cellular immune effector proteins, including chemokines (CKs) and CK receptor-like G protein-coupled receptors (GPCRs). Sequence of the guinea pig cytomegalovirus (GPCMV) genome identified an open reading frame (ORF) which predicted a 101 amino acid (aa) protein with homology to the macrophage inflammatory protein (MIP) subfamily of CC (beta) CKs, designated GPCMV-MIP. To assess functionality of this CK, recombinant GPCMV-MIP was expressed in HEK293 cells and assayed for its ability to bind to and functionally interact with a variety of GPCRs. Specific signaling was observed with the hCCR1 receptor, which could be blocked with hMIP -1alpha in competition experiments. Migration assays revealed that GPCMV-MIP was able to induce chemotaxis in hCCR1-L1.2 cells. Antisera raised against a GST-MIP fusion protein immunoprecipitated species of approximately 12 and 10 kDa from GPCMV-inoculated tissue culture lysates, and convalescent antiserum from GPCMV-infected animals was immunoreactive with GST-MIP by ELISA assay. These results represent the first substantive in vitro characterization of a functional CC CK encoded by a cytomegalovirus.

Chromatin remodeling of the Kaposi's sarcoma-associated herpesvirus ORF50 promoter correlates with reactivation from latency

The switch from latent to lytic infection of Kaposi's sarcoma-associated herpesvirus is initiated by the immediate early transcriptional activator protein Rta/open reading frame 50 (ORF50). We examined the transcriptional regulation of the ORF50 core promoter in response to lytic cycle stimulation. We show that the ORF50 promoter is highly responsive to sodium butyrate (NaB) and trichostatin A (TSA), two chemicals known to inhibit histone deacetylases. The NaB and TSA responsive element was mapped to a 70-bp minimal promoter containing an essential GC box that binds Sp1/Sp3 in vitro and in vivo. Micrococcal nuclease mapping studies revealed that a nucleosome is positioned over the transcriptional initiation and the Sp1/3 binding sites. Stimulation with NaB or TSA increased histone acetylation and restriction enzyme accessibility of the ORF50 promoter transcription initiation site. Chromatin immunoprecipitation assay was used to demonstrate that the ORF50 promoter is associated with several different histone deacetylase proteins (including HDAC1, 5, and 7) in latently infected cells. NaB treatment led to the rapid association of Ini1/Snf5, a component of the Swi/Snf family of chromatin remodeling proteins, with the ORF50 promoter. Ectopic expression of the CREB-binding protein (CBP) histone acetyltransferase (HAT) stimulated plasmid-based ORF50 transcription in a HAT-dependent manner, suggesting that CBP recruitment to the ORF50 promoter can be an initiating event for transcription and viral reactivation. Together, these results suggest that remodeling of a stably positioned nucleosome at the transcriptional initiation site of ORF50 is a regulatory step in the transition from latent to lytic infection.

Characterization of a spontaneously transformed chicken mononuclear cell line

We describe the characterization of a spontaneously transformed chicken monocytic cell line that developed as a single colony of cells in a heterophil culture that was inadvertently left in the incubator over a period of 25 days. These cells, hitherto named HTC, grow efficiently at both 37 or 41 degrees C in culture medium containing either 5% FBS or 2% chicken serum. The HTC cells are acid phosphatase positive, show expressions of both class I and class II major histocompatibility complex (MHC), CD44, K1, and K55 cell surface antigens, and engulf latex beads, produce nitrite and interleukin-6 on stimulation with bacterial lipopolysaccharide (LPS). Treatment with phorbol myristate acetate (PMA) induces respiratory burst in HTC cells and the secretion of matrix metalloproteinase (MMP) into culture medium. Using gene-specific primers and reverse transcriptase-polymerase chain reaction (RT-PCR), the presence of mRNA trancripts for interferon-gamma (IFN-gamma), interleukin-1 (IL-1), interleukin-6 (IL-6), nitric oxide synthase (NOS), matrix metalloproteinase-2 (MMP-2), and transforming growth factor-beta (TGF-beta) were detected. Lipopolysaccharide (LPS) treatment of HTC cells modulated IL-1, IL-6, IFN-gamma, NOS mRNA levels as detected by RT-PCR analyses. Using different avian tumor virus gene-specific primers and PCR, the HTC cells were positive for the presence of avian leukosis virus (ALV) and Marek's disease virus (MDV) but negative for reticuloendothelial virus (REV), chicken infectious anemia virus (CIAV), and herpes virus of turkeys (HVT). The production of ALV antigens by HTC cells was further confirmed using p27 gag protein ELISA. Collectively, these results show that the HTC cells belong to myeloid/macrophage lineage and were likely transformed by ALV and MDV but retain many interesting and useful biological activities.

Human herpesvirus-8-encoded signalling ligands and receptors

Analysis of the genome of human herpesvirus 8 (HHV-8) led to the discovery of several novel genes, unique among the characterized gammaherpesviruses. These include cytokines (interleukin-6 and chemokine homologues), two putative signal-transducing transmembrane proteins encoded by genes K1 and K15 at the genome termini, and an OX-2 (CD200) receptor homologue that had not previously been identified in a gammaherpesvirus. HHV-8 also specifies a diverged version of the gammaherpesvirus-conserved G protein-coupled chemokine receptor (vGCR) and a latently expressed protein unique to HHV-8 specified by open reading frame (ORF) K12. These cytokine and receptor homologues mediate signal transduction or modulate the activities of other endogenous cytokines and receptors to enhance viral productive replication, regulate latent-lytic switching, evade host attack, or mediate cell survival. The viral signalling ligands and receptors are also potential contributors to virus-associated diseases, Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease, and so represent potentially important targets for therapeutic and antiviral drugs. Understanding these proteins' modes of action and functions in viral biology and disease is therefore of considerable importance, and the subject of this review.

The role of Kaposi's sarcoma-associated herpesvirus/human herpesvirus-8 regulator of transcription activation (RTA) in control of gene expression

The mechanisms that control the replication state, latency versus lytic, of human herpesviruses have been under intense investigations. Here we summarize some of the recent findings that help define such mechanisms for Kaposi's sarcoma-associated herpesvirus/human herpesvirus type 8 (KSHV/HHV-8). For HHV-8, the viral regulator of transcription activation (RTA) is a key mediator of the switch from latency to lytic gene expression in infected cells. RTA is necessary and sufficient to drive HHV-8 lytic replication and the production of viral progeny. The RTA is an immediate-early gene product, it is the initial activator of expression of a multitude of viral and cellular genes that have been implicated in the replication of HHV-8 and pathogenesis of KS. Interactions of RTA with a number of viral promoters, and with a number of transcription factors or transcriptional co-activators are highlighted. Modulation of transactivation, through alternate RTA-protein, or RTA-promoter interactions, is hypothesized to participate in the selective tissue tropism and differential pathogenesis observed in KS.

Lytic but not latent infection by Kaposi's sarcoma-associated herpesvirus requires host CSL protein, the mediator of Notch signaling

Infection by Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is a key factor in the development of KS. Both latent and lytic KSHV infection is observed in KS tumor cells, and both genetic programs contribute importantly to KS pathogenesis. The viral replication and transcription activator (RTA) protein is a transcription factor that controls the switch from latency to lytic replication. We have previously shown that RTA can activate the expression of several lytic viral genes in transfected cells by interaction with recombination signal sequence-binding protein-J kappa (RBP-J kappa, also called CSL), which in uninfected cells is a transcriptional repressor that is the target of the Notch-signaling pathway. The recognition that many KSHV lytic genes, including RTA itself, contain RBP-J kappa-binding sites raised the possibility that RBP-J kappa-mediated repression may be central to the establishment of latency. Here, we have tested this hypothesis by examining KSHV infection of RBP-J kappa-null murine fibroblasts. Our results show that KSHV latency is efficiently induced in such cells; however, the reactivation of lytic gene expression, viral DNA replication, and the release of progeny viruses are dramatically inhibited in the absence of RBP-J kappa. RBP-J kappa-mediated repression is therefore not essential for establishment of latent infection, but the RTA-mediated redirection of RBP-J kappa activity from repression to activation is critical for lytic viral replication.

Molecular genetics of Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8) epidemiology and pathogenesis

Kaposi's sarcoma had been recognized as unique human cancer for a century before it manifested as an AIDS-defining illness with a suspected infectious etiology. The discovery of Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8, in 1994 by using representational difference analysis, a subtractive method previously employed for cloning differences in human genomic DNA, was a fitting harbinger for the powerful bioinformatic approaches since employed to understand its pathogenesis in KS. Indeed, the discovery of KSHV was rapidly followed by publication of its complete sequence, which revealed that the virus had coopted a wide armamentarium of human genes; in the short time since then, the functions of many of these viral gene variants in cell growth control, signaling apoptosis, angiogenesis, and immunomodulation have been characterized. This critical literature review explores the pathogenic potential of these genes within the framework of current knowledge of the basic herpesvirology of KSHV, including the relationships between viral genotypic variation and the four clinicoepidemiologic forms of Kaposi's sarcoma, current viral detection methods and their utility, primary infection by KSHV, tissue culture and animal models of latent- and lytic-cycle gene expression and pathogenesis, and viral reactivation from latency. Recent advances in models of de novo endothelial infection, microarray analyses of the host response to infection, receptor identification, and cloning of full-length, infectious KSHV genomic DNA promise to reveal key molecular mechanisms of the candidate pathogeneic genes when expressed in the context of viral infection.

Global changes in Kaposi's sarcoma-associated virus gene expression patterns following expression of a tetracycline-inducible Rta transactivator

An important step in the herpesvirus life cycle is the switch from latency to lytic reactivation. In order to study the life cycle of Kaposi's sarcoma-associated herpesvirus (KSHV), we developed a gene expression system in KSHV-infected primary effusion lymphoma cells. This system uses Flp-mediated efficient recombination and tetracycline-inducible expression. The Rta transcriptional activator, which acts as a molecular switch for lytic reactivation of KSHV, was efficiently integrated downstream of the Flp recombination target site, and its expression was tightly controlled by tetracycline. Like stimulation with tetradecanoyl phorbol acetate (TPA), the ectopic expression of Rta efficiently induced a complete cycle of viral replication, including a well-ordered program of KSHV gene expression and production of infectious viral progeny. A striking feature of Rta-mediated lytic gene expression was that Rta induced KSHV gene expression in a more powerful and efficient manner than TPA stimulation, indicating that Rta plays a central, leading role in KSHV lytic gene expression. Thus, our streamlined gene expression system provides a novel means not only to study the effects of viral gene products on overall KSHV gene expression and replication, but also to understand the natural viral reactivation process.

Interleukin-6 promotes cervical tumor growth by VEGF-dependent angiogenesis via a STAT3 pathway

Interleukin-6 (IL-6) has received particular attention in the pathogenesis of cervical cancer, although the underlying mechanism remains elusive. This study revealed that IL-6 promotes in vivo tumor growth of human cervical cancer C33A cells, but does not substantially alter their in vitro growth kinetics. The in vivo angiogenic assays showed that IL-6 increases angiogenic activity in human cervical cancer cells, an effect that is specifically associated with upregulation of vascular endothelial growth factor (VEGF). Also, using anti-VEGF antibody to block VEGF function significantly inhibited IL-6-mediated angiogenesis and tumor growth in nude mice, strongly supporting the critical role of VEGF in the IL-6-mediated cervical tumorigenesis. Accordingly, the signaling pathway downstream of IL-6/IL-6R responsible for the regulation of VEGF was investigated. Notably, pharmacological inhibition of PI3-K or MAPK failed to inhibit IL-6-mediated transcriptional upregulation of VEGF. Meanwhile, blocking STAT3 pathway with dominant-negative mutant STAT3D effectively abolished IL-6-induced VEGF mRNA. In transient transfections, a luciferase reporter construct containing the full-length 1.5-kb VEGF promoter or a 1.2-kb fragment lacking the known hypoxic-response element also exhibited the same degree of response to IL-6. Additionally, transient transfection of STAT3D downregulated the 1.2-kb VEGF promoter luciferase reporter stimulated by IL-6. Based on the above phenomenon combined with the concomitant increased tumor expression of IL-6 and VEGF in cervical cancer tissues, we conclude that IL-6 may promote cervical tumorigenesis by activating VEGF-mediated angiogenesis via a STAT3 pathway.

Kaposi's sarcoma: aetiopathogenesis, histology and clinical features

Kaposi's sarcoma (KS) represents today one of the most common skin cancers in transplanted Mediterranean subjects and, since the epidemic of human immunodeficiency virus/acquired immune deficiency syndrome, in young unmarried single men. The disease has been associated with the recent identified human herpesvirus (HHV)-8 or KS herpesvirus and its incidence in the general population shows a north to south gradient that parallels the HHV-8 increasing prevalence from Nordic countries to sub-Saharan regions. The identification of the aetiopathogenetic mechanisms (viral agents and immunodeficiency) involved in the pathogenesis of KS, are relevant for identifying susceptible subjects (HHV-8 seropositive subjects), monitoring the immune levels in iatrogenic immune suppressed patients, and developing new therapeutic approaches based on antiviral and immune modulators.

LEARNING OBJECTIVE

This article should enable the reader: (i) to learn about the clinical and molecular aspects of KS in order to have a multidisciplinary approach to a tumour that shows unique features; (ii) to consider the role of viral agents and immunity; and (iii) to recognize properties of an opportunistic neoplasm. The identification of the HHV-8 role in KS pathogenesis should establish a relevant tool in the clinical management of KS patients.

Detection of human herpesvirus-8 in peripheral blood mononuclear cells from adult Japanese patients with multicentric Castleman's disease

Human herpesvirus-8 (HHV-8) encodes viral homologues of cellular genes, including viral interleukin 6 (vIL-6), which induces endogenous human IL-6 (hIL-6) secretion. Unregulated overproduction of hIL-6 in lymph nodes (LN) is thought to be responsible for the systemic manifestations of multicentric Castleman's disease (MCD). In the present study, we assessed the presence of HHV-8 and HHV-8-encoded viral homologues in LN and peripheral blood mononuclear cells (PBMC) from adult Japanese patients with MCD. HHV-8 DNA was amplified by nested polymerase chain reaction (PCR) and was detected in LN from 13 out of 16 MCD patients (81%). HHV-8 DNA was also detected in PBMC from six out of seven patients (86%) whose LN were positive for HHV-8 DNA. Because mRNA could not be successfully extracted from LN sections that were either formalin-fixed or embedded in paraffin, we examined the expression of mRNA for HHV-8-encoded viral homologues, such as vIL-6, vBCL-2, vCyclin-D and viral G-protein-coupled receptor (vGPCR) by nested reverse transcription (RT)-PCR in PBMC from 10 MCD patients. However, mRNA of these HHV-8-encoded viral homologues was not detected in any patients tested. Although our results do not indicate a role for HHV-8-encoded viral homologues in the pathogenesis of MCD, they do suggest that HHV-8 infection may be associated with MCD in adult Japanese patients.

Kaposi's sarcoma-like tumors in a human herpesvirus 8 ORF74 transgenic mouse

The product of human herpesvirus 8 (HHV-8) open reading frame 74 (ORF74) is related structurally and functionally to cellular chemokine receptors. ORF74 activates several cellular signaling pathways in the absence of added ligands, and NIH 3T3 cells expressing ORF74 are tumorigenic in nude mice. We have generated a line of transgenic (Tg) mice with ORF74 driven by the simian virus 40 early promoter. A minority (approximately 30%) of the Tg mice, including the founder, developed tumors resembling Kaposi's sarcoma (KS) lesions, which occurred most typically on the tail or legs. The tumors were highly vascularized, had a spindle cell component, expressed VEGF-C mRNA, and contained a majority of CD31(+) cells. CD31 and VEGF-C are typically expressed in KS. Tumors generally (but not always) occurred at single sites and most were relatively indolent, although several mice developed large visceral tumors. ORF74 was expressed in a minority of cells in the Tg tumors and in a few other tissues of mice with tumors; mice without tumors did not express detectable ORF74 in any tissues tested. Cell lines established from tumors expressed ORF74 in a majority of cells, expressed VEGF-C mRNA, and were tumorigenic in nude mice. The resultant tumors grew rapidly, metastasized, and continued to express ORF74. Cell lines established from these secondary tumors also expressed ORF74 and were tumorigenic. These data strongly suggest that ORF74 plays a role in the pathology of KS and confirm and extend previous findings on the tumorigenic potential of ORF74.

MIP-1alpha and MIP-1beta differentially mediate mucosal and systemic adaptive immunity

Macrophage inflammatory protein-1alpha (MIP-1alpha) and MIP-1beta are distinct but highly homologous CC chemokines produced by a variety of host cells in response to various external stimuli and share affinity for CCR5. To better elucidate the role of these CC chemokines in adaptive immunity, we have characterized the affects of MIP-1alpha and MIP-1beta on cellular and humoral immune responses. MIP-1alpha stimulated strong antigen (Ag)-specific serum immunoglobulin G (IgG) and IgM responses, while MIP-1beta promoted lower IgG and IgM but higher serum IgA and IgE antibody (Ab) responses. MIP-1alpha elevated Ag-specific IgG1 and IgG2b followed by IgG2a and IgG3 subclass responses, while MIP-1beta only stimulated IgG1 and IgG2b subclasses. Correspondingly, MIP-1beta produced higher titers of Ag-specific mucosal secretory IgA Ab levels when compared with MIP-1alpha. Splenic T cells from MIP-1alpha- or MIP-1beta-treated mice displayed higher Ag-specific Th1 (interferon-gamma [IFN-gamma]) as well as selective Th2 (interleukin-5 [IL-5] and IL-6) cytokine responses than did T cells from control groups. Interestingly, mucosally derived T cells from MIP-1beta-treated mice displayed higher levels of IL-4 and IL-6 compared with MIP-1alpha-treated mice. However, MIP-1alpha effectively enhanced Ag-specific cell-mediated immune responses. In correlation with their selective effects on humoral and cellular immune responses, these chemokines also differentially attract CD4(+) versus CD8(+) T cells and modulate CD40, CD80, and CD86 expressed by B220(+) cells as well as CD28, 4-1BB, and gp39 expression by CD4(+) and CD8(+) T cells in a dose-dependent fashion. Taken together, these studies suggest that these CC chemokines differentially enhance mucosal and serum humoral as well as cellular immune responses.

Kaposi's sarcoma associated herpesvirus G protein-coupled receptor immortalizes human endothelial cells by activation of the VEGF receptor-2/ KDR

The G protein-coupled receptor oncogene (vGPCR) of the Kaposi's sarcoma (KS) associated herpesvirus (KSHV), an oncovirus implicated in angioproliferative neoplasms, induces angiogenesis by VEGF secretion. Accordingly, we found that expression of vGPCR in human umbilical vein endothelial cells (HUVEC) leads to immortalization with constitutive VEGF receptor-2/ KDR expression and activation. vGPCR immortalization was associated with anti-senescence mediated by alternative lengthening of telomeres and an anti-apoptotic response mediated by vGPCR constitutive signaling and KDR autocrine signaling leading to activation of the PI3K/AKT pathway. In the presence of the KS growth factor VEGF, this mechanism can sustain suppression of signaling by the immortalizing gene. We conclude that vGPCR can cause an oncogenic immortalizing event and recapitulate aspects of the KS angiogenic phenotype in human endothelial cells, pointing to this gene as a pathogenic determinant of KSHV.

Kaposi's sarcoma-associated herpesvirus immunoevasion and tumorigenesis: two sides of the same coin?

Kaposi's sarcoma-associated herpesvirus (KSHV) [or human herpesvirus 8 (HHV-8)] is the most frequent cause of malignancy among AIDS patients. KSHV and related herpesviruses have extensively pirated cellular cDNAs from the host genome, providing a unique opportunity to examine the range of viral mechanisms for controlling cell proliferation. Many of the viral regulatory homologs encode proteins that directly inhibit host adaptive and innate immunity. Other viral proteins target retinoblastoma protein and p53 control of tumor suppressor pathways, which also play key effector roles in intracellular immune responses. The immune evasion strategies employed by KSHV, by targeting tumor suppressor pathways activated during immune system signaling, may lead to inadvertent cell proliferation and tumorigenesis in susceptible hosts.

Gamma herpesviruses: pathogenesis of infection and cell signaling

Altered cell signaling is the molecular basis for cell proliferation occurring in association with several gamma herpesvirus infections. Three gamma herpesviruses, namely EBV/HHV-4, KSHV/HHV-8 and the MHV-68 (and/or MHV-72) and their unusual cell-pirated gene products are discussed in this respect. The EBV, KSHV as well as the MHV DNA may persist lifelong in an episomal form in the host carrier cells (mainly in lymphocytes but also in macrophages, in non-hornifying squamous epithelium and/or in blood vessel endothelial cells). Under conditions of extremely limited transcription, the EBV-infected cells express EBNA1 (EB nuclear antigen 1), the KSHV infected cells express LANA1 (latent nuclear antigen 1), while the MHV DNA carrier cells express the latency-associated protein M2. With the full set of latency-associated proteins expressed, EBV carrier cells synthesize additional EBNAs and at least one LMP (latent membrane protein 1). The latent KSHV carrier cells, in addition to LANA1, may express a viral cyclin, a viral Fas-DD-like ICE inhibitor protein (vFLIP) and a virus-specific transformation protein called kaposin (K12). In MHV latency with a wide expression of latency-associated proteins, the carrier cells express a LANA analogue (ORF73), the M3 protein, the K3/IE (immediate early) proteins and M11/bcl-2 homologue proteins. During the period of limited gene expression, the latency-associated proteins serve mainly for the maintenance of the latent episomal DNA (a typical example is EBNA1). In contrast, during latency with a broader spectrum gene expression, the virus-encoded products activate transcription of otherwise silenced cellular genes, which leads to the synthesis of enzymes capable of promoting not only viral but also cellular DNA replication. Thus, the latency-associated proteins block apoptosis and drive host cells towards division and immortalization. Proliferation of hemopoetic cells, which had become gamma herpesvirus DNA carriers, can be initiated and strongly enhanced in the presence of inflammatory cytokines and by virus-encoded analogues of interleukins, chemokines and IFN regulator proteins. At early stages of tumor formation, many proliferating hemopoetic and/or endothelium cells, which had became transcriptionally active under the influence of chemokines and cytokines, may not yet be infected. In contrast, at later stages of oncogenesis, the virus-encoded proteins, inducing false signaling and activating the proliferation pathways, bring the previously infected cells into full transformation burst.

Split genes and their expression in Kaposi's sarcoma-associated herpesvirus

A split or interrupted gene is defined as a gene consisting of introns and exons. Removal (splicing) of the intron(s) from a primary transcript (pre-mRNA) is essential for creating a mRNA. Initial assignment of a potential protein coding region in the KSHV genome was based on the initiation codon context and predicted protein size larger than 100 amino acids, but the gene discontinuity was disregarded. Experimental investigation of the assigned ORFs has demonstrated that there are up to 25 split genes, more than one fourth of the total KSHV genes described in the KSHV genome. This includes the genes involved in all phases (latent, immediate early, early and late) of KSHV infection. The complexity of a split gene expression depends upon the availability of a proximal promoter and polyadenylation (pA) signal. Sharing a single promoter or a single pA signal by two or three genes is not uncommon in the expression of KSHV split genes and the resulting transcripts are usually polycistronic. Among those of KSHV split genes, 15 genes express a bicistronic or tricistronic RNA and 10 genes express a monocistronic RNA. Alternative RNA splicing could happen in a particular pre-mRNA due to intron or exon inclusion or skipping or the presence of an alternative 5' splice site or 3' splice site. This may, respectively, result in at least 8 species of K8 and 14 species of K15 transcripts. This appears to be related to cell differentiation and stages of the virus infection, presumably involving viral cis elements and trans splicing factors.

Immune evasion strategies of Kaposi's sarcoma-associated herpesvirus

To establish lifelong infection in the presence of an active host immune system, herpesviruses have acquired an impressive array of immune modulatory mechanisms that contribute to their success as long-term parasites. Kaposi's sarcoma-associated herpesvirus (KSHV) is the most recently discovered human tumor virus and is associated with the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. KSHV has acquired a battery of genes to assist in viral survival against the host immune response. These viral gene products target a variety of host immune surveillance mechanisms, including the cytokine-mediated immune response, apoptosis, natural killer (NK) cell killing and T cell-mediated responses. This review summarizes our understanding of the role of these viral proteins in the escape from host immune surveillance, which ultimately contributes to lifelong infection and pathogenesis of KSHV.

Suppression of tetradecanoyl phorbol acetate-induced lytic reactivation of Kaposi's sarcoma-associated herpesvirus by K1 signal transduction

The K1 protein of Kaposi's sarcoma-associated herpesvirus (KSHV) contains an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic region and elicits cellular signal transduction through this motif. To investigate the role of K1 signal transduction in KSHV replication, we expressed full-length K1 and CD8-K1 chimeras in BCBL1 cells. Unlike its strong signaling activity in uninfected B lymphocytes, K1 did not induce intracellular calcium mobilization or NF-AT activation at detectable levels in KSHV-infected BCBL1 cells. Instead, K1 signaling dramatically suppressed KSHV lytic reactivation induced by tetradecanoyl phorbol acetate (TPA) stimulation, but not by ORF50 ectopic expression. Mutational analysis showed that the cytoplasmic ITAM sequence of K1 was required for this suppression. Viral microarray and immunoblot analyses demonstrated that K1 signaling suppressed the TPA-mediated increase in the expression of a large subset of viral lytic genes in KSHV-infected BCBL1 cells. Furthermore, electrophoretic mobility shift assays demonstrated that TPA-induced activation of AP-1, NF-kappaB, and Oct-1 activities was severely diminished in BCBL1 cells expressing the K1 cytoplasmic domain. The reduced activities of these transcription factors may confer the observed reduction in viral lytic gene expression. These results demonstrate that K1-mediated signal transduction in KSHV-infected cells is profoundly different from that in KSHV-negative cells. Furthermore, K1 signal transduction efficiently suppresses TPA-mediated viral reactivation in an ITAM-dependent manner, and this suppression may contribute to the establishment and/or maintenance of KSHV latency in vivo.

A gamma-herpesvirus immune evasion gene allows tumor cells in vivo to escape attack by cytotoxic T cells specific for a tumor epitope

DNA vaccines induce CTL attack on target tumor epitopes, but tumor elimination in vivo also requires sufficient effector CTL to enter the site, guided by inflammatory chemokines. Many herpesviruses contain genes for chemokine and chemokine receptor-like proteins to protect infected cells from immune attack. To assess if this evasion strategy could protect tumor cells, we used a model where CTL specific for a single epitope were the only effectors. Following DNA vaccination, CTL eliminated tumor cells from a subcutaneous site. However, introducing a viral gene encoding a secreted broad-spectrum chemokine-binding protein (M3) into tumor cells completely blocked CTL attack. Transduced tumor cells also protected neighboring non-transduced tumor. These findings confirm the importance of chemokines for migration of CTL to a non-lymphoid site. They may have relevance for escape of human virus-associated malignancies, and raise the question of whether analogous molecules might contribute to the failure of CTL to eliminate tumors.

Viral IL-6-induced cell proliferation and immune evasion of interferon activity

Lymphoma cells infected with Kaposi's sarcoma-associated herpesvirus are autocrine dependent on virus-derived interleukin-6 (IL-6), but not on cellular IL-6. During viral infection, host cells induce the antiviral factor interferon (IFN) to up-regulate p21, initiate cell cycle arrest, and inhibit virus replication. Viral IL-6, however, blocks IFN signaling. A viral transcriptional program exists in which only the viral IL-6 gene is directly activated by IFN-alpha, allowing the virus to modify its cellular environment by sensing and responding to levels of intracellular IFN signaling. The human cytokine cannot mimic this effect because IFN-alpha down-regulates the IL-6 receptor, gp80. Viral IL-6 bypasses the gp80 regulatory checkpoint by binding directly to the gp130 transducer molecule, resulting in tumor cell autocrine dependence on the viral cytokine for proliferation and survival.

The role of transsignalling via the agonistic soluble IL-6 receptor in human diseases

The activation of cells that do not express the membrane bound interleukin-6 6 receptor (IL-6R) by IL-6 and the soluble IL-6 receptor (sIL-6R) is termed transsignalling. Transsignalling may be an pathogenetic factor in human diseases as diverse as multiple myeloma (MM), Castleman's disease, prostate carcinoma, Crohn's disease, systemic sclerosis, Still's disease, osteoporosis and cardiovascular diseases. IL-6 and sIL-6R may directly or indirectly enhance their own production on endothelial or bone marrow stromal cells. Positive feedback autocrine loops thus created in affected organs may either cause or maintain disease progression. In autoimmune or vasculitic disease, the ability of the IL-6/sIL-6R complex to inhibit apoptosis of autoreactive T-cells may be central to the development of tissue specific autoimmunity. The anti-apoptotic effect of the IL-6/sIL-6R complex may be involved in tumour genesis and resistance to chemotherapy. Only in rare cases, where counterregulation has failed, there is a notable systemic effect of IL-6/sIL-6R. Appropriate animal models are necessary to establish the pathogenetic role of the IL-6/sIL-6R complex. A specific treatment option for diseases influenced by the sIL-6R could be based on gp130-Fc, a soluble gp130 (sgp130) linked to the Fc-fragment of IgG1. gp130-Fc has shown efficacy in vivo in animal models of Crohn's disease.

Human interleukin-6 induces human herpesvirus-8 replication in a body cavity-based lymphoma cell line

Human herpesvirus-8 (HHV-8) is etiologically associated with Kaposi's sarcoma (KS), body cavity-based lymphoma (BCBL), and multicentric Castleman's disease (MCD). These HHV-8-associated diseases arise predominantly in acquired immunodeficiency syndrome (AIDS) patients. Human interleukin-6 (huIL-6) elevated in the serum of AIDS patients is suggested to stimulate the growth of KS and BCBL and to augment the symptoms of MCD. To determine whether huIL-6 stimulates HHV-8 replication directly, expression of the HHV-8 ORF-50 immediate-early gene (transcription activator) and ORF-26 late lytic gene (a capsid protein) was assessed in a BCBL-1 cell line stimulated by huIL-6 by means of real-time reverse transcription-polymerase chain reaction. huIL-6 induced both ORF-50 and ORF-26 expression, and the maximal ORF-50 expression appeared earlier than that of ORF-26. The data indicate that huIL-6 reactivates HHV-8 in BCBL-1 cells through inducing ORF-50. We also confirmed the previously reported activities of HHV-8-encoded huIL-6 homologue (viral interleukin-6 [vIL-6]) on human immunodeficiency virus (HIV) replication in U1 cell line and huIL-6 production by MT-4 T cells, and utilizing monoclonal antibodies to the huIL-6 receptor components, we elucidated that gp130 is the signaling molecule necessary for these vIL-6 activities. These data suggest the possible existence of interaction between HIV and HHV-8 via IL-6, and that the blockade of IL-6 signal by anti-IL-6R antibody or anti-gp130 antibody can constitute a strategy to treat HIV/HHV-8 dually infected patients.

Development of real-time NASBA assays with molecular beacon detection to quantify mRNA coding for HHV-8 lytic and latent genes

BACKGROUND

Human herpesvirus-8 (HHV-8) is linked to the pathogenesis of Kaposi's sarcoma (KS), and the HHV-8 DNA load in peripheral blood mononuclear cells (PBMC) is associated with the clinical stage of KS. To examine the expression of HHV-8 in PBMC, four HHV-8 mRNA specific NASBA assays were developed

METHODS

We have developed four quantitative nucleic acid sequence-based amplification assays (NASBA-QT) specifically to detect mRNA coding for ORF 73 (latency-associated nuclear antigen, LANA), vGCR (a membrane receptor), vBcl-2 (a viral inhibitor of apoptosis) and vIL-6 (a viral growth factor). The NASBA technique amplifies nucleic acids without thermocycling and mRNA can be amplified in a dsDNA background. A molecular beacon is used during amplification to enable real-time detection of the product. The assays were tested on PBMC samples of two AIDS-KS patients from the Amsterdam Cohort.

RESULTS

For all four assays, the limit of detection (LOD) of 50 molecules and the limit of quantification (LOQ) of 100 molecules were determined using in vitro transcribed RNA. The linear dynamic range was 50 to 10(7) molecules of HHV-8 mRNA. We found HHV-8 mRNA expression in 9 out of the 10 tested samples.

CONCLUSION

These real-time NASBA assays with beacon detection provide tools for further study of HHV-8 expression in patient material.

Rta of the human herpesvirus 8/Kaposi sarcoma-associated herpesvirus up-regulates human interleukin-6 gene expression

Human herpesvirus 8 (HHV-8)/Kaposi sarcoma-associated herpesvirus (KSHV) is linked to a number of malignancies thought to be driven by cytokines, including interleukin-6 (IL-6). Rta, a transcriptional activator encoded by HHV-8/KSHV, activates the viral lytic cycle leading to the expression of several viral genes implicated in viral pathogenesis. However, the effect of HHV-8/KSHV Rta on cellular genes has not been reported. We present evidence that the human IL-6 (hIL-6) gene is up-regulated by Rta. Rta potently activated (up to 164-fold) the hIL-6 promoter in a dose-dependent manner in a transient transfection reporter system. Rta also induced expression of the endogenous hIL-6 gene, as shown by enzyme-linked immunosorbent assays. Activation of the hIL-6 gene by HHV-8/KSHV supports the role of hIL-6 in the development of these malignancies.