Human herpesvirus-8 (HHV-8) gene expression in Kaposi's sarcoma (KS) primary lesions: an in situ hybridization study

Quantitative RNAseq analysis of Ugandan KS tumors reveals KSHV gene expression dominated by transcription from the LTd downstream latency promoter

KSHV is endemic in Uganda and the HIV epidemic has dramatically increased the incidence of Kaposi sarcoma (KS). To investigate the role of KSHV in the development of KS, we obtained KS biopsies from ART-naïve, HIV-positive individuals in Uganda and analyzed the tumors using RNAseq to globally characterize the KSHV transcriptome. Phylogenetic analysis of ORF75 sequences from 23 tumors revealed 6 distinct genetic clusters with KSHV strains exhibiting M, N or P alleles. RNA reads mapping to specific unique coding sequence (UCDS) features were quantitated using a gene feature file previously developed to globally analyze and quantitate KSHV transcription in infected endothelial cells. A pattern of high level expression was detected in the KSHV latency region that was common to all KS tumors. The clear majority of transcription was derived from the downstream latency transcript promoter P3(LTd) flanking ORF72, with little evidence of transcription from the P1(LTc) latency promoter, which is constitutive in KSHV-infected lymphomas and tissue-culture cells. RNAseq data provided evidence of alternate P3(LTd) transcript editing, splicing and termination resulting in multiple gene products, with 90% of the P3(LTd) transcripts spliced to release the intronic source of the microRNAs K1-9 and 11. The spliced transcripts encode a regulatory uORF upstream of Kaposin A with alterations in intervening repeat sequences yielding novel or deleted Kaposin B/C-like sequences. Hierarchical clustering and PCA analysis of KSHV transcripts revealed three clusters of tumors with different latent and lytic gene expression profiles. Paradoxically, tumors with a latent phenotype had high levels of total KSHV transcription, while tumors with a lytic phenotype had low levels of total KSHV transcription. Morphologically distinct KS tumors from the same individual showed similar KSHV gene expression profiles suggesting that the tumor microenvironment and host response play important roles in the activation level of KSHV within the infected tumor cells.

Kaposi’s sarcoma (KS) is among the world’s most common AIDS-associated malignancies. The Kaposi sarcoma-associated herpesvirus (KSHV) was first identified in KS tumors and is now known to be the causative agent of all forms of KS, including classical, endemic, iatrogenic and HIV-associated. KSHV is endemic to sub-Saharan Africa with high infection rates in children and adults. Compounded with the high rate of HIV and AIDS in this area, pediatric and adult KS are some of the most common malignancies with the highest fatality rates. We used RNA deep sequencing to characterize KSHV expression in a large collection of KS biopsies from HIV-infected Ugandans. Using a novel approach to quantitate expression in complex genomes like KSHV, we found that RNA from a single KSHV promoter within the latency region constituted the majority of KSHV transcripts in the KS tumors. Alternate RNA processing produced different spliced and un-spliced transcripts with different coding potentials. Differential expression of other KSHV genes was detected which segregated the tumors into three different types depending on their expression of lytic or latency genes. Quantitative analysis of KSHV expression in KS tumors provides an important basis for future studies on the role of KSHV in the development of KS.

Full-Length Isoforms of Kaposi's Sarcoma-Associated Herpesvirus Latency-Associated Nuclear Antigen Accumulate in the Cytoplasm of Cells Undergoing the Lytic Cycle of Replication

The latency-associated nuclear antigen (LANA) of the Kaposi's sarcoma-associated herpesvirus (KSHV) performs a variety of functions to establish and maintain KSHV latency. During latency, LANA localizes to discrete punctate spots in the nucleus, where it tethers viral episomes to cellular chromatin and interacts with nuclear components to regulate cellular and viral gene expression. Using highly sensitive tyramide signal amplification, we determined that LANA localizes to the cytoplasm in different cell types undergoing the lytic cycle of replication after de novo primary infection and after spontaneous, tetradecanoyl phorbol acetate-, or open reading frame 50 (ORF50)/replication transactivator (RTA)-induced activation. We confirmed the presence of cytoplasmic LANA in a subset of cells in lytically active multicentric Castleman disease lesions. The induction of cellular migration by scratch-wounding confluent cell cultures, culturing under subconfluent conditions, or induction of cell differentiation in primary cultures upregulated the number of cells permissive for primary lytic KSHV infection. The induction of lytic replication was characterized by high-level expression of cytoplasmic LANA and nuclear ORF59, a marker of lytic replication. Subcellular fractionation studies revealed the presence of multiple isoforms of LANA in the cytoplasm of ORF50/RTA-activated Vero cells undergoing primary infection. Mass spectrometry analysis demonstrated that cytoplasmic LANA isoforms were full length, containing the N-terminal nuclear localization signal. These results suggest that trafficking of LANA to different subcellular locations is a regulated phenomenon, which allows LANA to interact with cellular components in different compartments during both the latent and the replicative stages of the KSHV life cycle.IMPORTANCE Kaposi's sarcoma-associated herpesvirus (KSHV) causes AIDS-related malignancies, including lymphomas and Kaposi's sarcoma. KSHV establishes lifelong infections using its latency-associated nuclear antigen (LANA). During latency, LANA localizes to the nucleus, where it connects viral and cellular DNA complexes and regulates gene expression, allowing the virus to maintain long-term infections. Our research shows that intact LANA traffics to the cytoplasm of cells undergoing permissive lytic infections and latently infected cells in which the virus is induced to replicate. This suggests that LANA plays important roles in the cytoplasm and nuclear compartments of the cell during different stages of the KSHV life cycle. Determining cytoplasmic function and mechanism for regulation of the nuclear localization of LANA will enhance our understanding of the biology of this virus, leading to therapeutic approaches to eliminate infection and block its pathological effects.

Activating transcription factor 4 (ATF4) is upregulated by human herpesvirus 8 infection, increases virus replication and promotes proangiogenic properties

Human herpesvirus 8 (HHV-8) triggers proangiogenic behaviour in endothelial cells by inducing monocyte chemoattractant protein 1 (MCP-1) through activation of Nuclear Factor κB (NF-κB). However, NF-κB inhibition still results in partial MCP-1 induction and consequent angiogenesis, suggesting the involvement of another transcriptional pathway. We analysed activating transcription factor 4 (ATF4), since it is central in the cellular response to stress and is involved in angiogenesis. The results show that HHV-8 upregulates ATF4 expression, which in turn promotes HHV-8 infection, and induces MCP-1 production and proangiogenic properties in endothelial cells. By contrast, ATF4 silencing decreases virus replication and inhibits virus-induced MCP-1 production and induction of tube-like structures. Therefore, ATF4 plays a role in HHV-8 replication and associated virus-induced angiogenesis. The elucidation of molecular pathways involved in this process will result in a better understanding of the virus-induced angiogenic process and might help in designing novel therapies to reduce tumour growth.

Integrated microarray and multiplex cytokine analyses of Kaposi's Sarcoma Associated Herpesvirus viral FLICE Inhibitory Protein K13 affected genes and cytokines in human blood vascular endothelial cells

Background

Kaposi's sarcoma (KS) associated herpesvirus (KSHV) is the etiological agent of KS, a neoplasm characterized by proliferating spindle cells, extensive neoangiogenesis and a prominent inflammatory infiltrate. Infection of blood vascular endothelial cells with KSHV in vitro results in their spindle cell transformation, which is accompanied by increased expression of inflammatory chemokines and cytokines, and acquisition of lymphatic endothelial markers. Mimicking the effect of viral infection, ectopic expression of KSHV-encoded latent protein vFLIP K13 is sufficient to induce spindle transformation of vascular endothelial cells. However, the effect of K13 expression on global gene expression and induction of lymphatic endothelial markers in vascular endothelial cells has not been studied.

Methods

We used gene array analysis to determine change in global gene expression induced by K13 in human vascular endothelial cells (HUVECs). Results of microarray analysis were validated by quantitative RT-PCR, immunoblotting and a multiplex cytokine array.

Results

K13 affected the expression of several genes whose expression is known to be modulated by KSHV infection, including genes involved in immune and inflammatory responses, anti-apoptosis, stress response, and angiogenesis. The NF-κB pathway was the major signaling pathway affected by K13 expression, and genetic and pharmacological inhibitors of this pathway effectively blocked K13-induced transcriptional activation of the promoter of CXCL10, one of the chemokines whose expression was highly upregulated by K13. However, K13, failed to induce expression of lymphatic markers in blood vascular endothelial cells.

Conclusion

While K13 may account for change in the expression of a majority of genes observed following KSHV infection, it is not sufficient for inducing lymphatic reprogramming of blood vascular endothelial cells.

Functional characterization of the M-type K15-encoded membrane protein of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 is the causative agent of Kaposi's sarcoma, primary effusion lymphoma and the plasma-cell variant of multicentric Castleman's disease. Its alternatively spliced K15 gene encodes several membrane proteins with varying numbers of transmembrane domains. Two highly diverged alleles of the K15 gene, termed predominant (P) and minor (M), exist and share only 33 % amino acid identity with one another, but retain conserved putative src homology (SH) 2- and SH3-binding motifs. K15-M is thought to have entered the KSHV genome as the result of recombination with a related gamma(2)-herpesvirus. The more common K15-P allele has been shown to activate the mitogen-activated protein kinases Erk2 and JNK1 and the nuclear factor kappaB (NF-kappaB) pathway. To explore possible functional differences between K15-P and K15-M that might have influenced their spread in the KSHV population, here, the ability of the M form of K15 to activate these pathways was investigated. Similarly to K15-P, K15-M induces the activation of the Erk2 and JNK1 kinases, the NF-kappaB transcription factor and the expression of a similar range of cellular inflammatory genes, as assessed by gene-expression microarray studies and reporter assays. In epithelial cells, the activation of most K15-M target genes is impaired by mutagenesis of Y(490) in its SH2-binding motif Y(490)EEV, although this motif appears less important in endothelial cells. Therefore, K15-M and K15-P can trigger similar intracellular signalling pathways, despite their extensive sequence divergence.

Molecular biology of KSHV in relation to AIDS-associated oncogenesis

KSHV has been established as the causative agent of KS, PEL, and MCD, malignancies occurring more frequently in AIDS patients. The aggressive nature of KSHV in the context of HIV infection suggests that interactions between the two viruses enhance pathogenesis. KSHV latent infection and lytic reactivation are characterized by distinct gene expression profiles, and both latency and lytic reactivation seem to be required for malignant progression. As a sophisticated oncogenic virus, KSHV has evolved to possess a formidable repertoire of potent mechanisms that enable it to target and manipulate host cell pathways, leading to increased cell proliferation, increased cell survival, dysregulated angiogenesis, evasion of immunity, and malignant progression in the immunocompromised host. Worldwide, approximately 40.3 million people are currently living with HIV infection. Of these, a significant number are coinfected with KSHV. The complex interplay between the two viruses dramatically elevates the risk for development of KSHV-induced malignancies, KS, PEL, and MCD. Although HAART significantly reduces HIV viral load, the entire T-cell repertoire and immune function may not be completely restored. In fact, clinically significant immune deficiency is not necessary for the induction of KSHV-related malignancy. Because of variables such as lack of access to therapy noncompliance with prescribed treatment, failure to respond to treatment and the development of drug-resistant strains of HIV, KSHV-induced malignancies will continue to present as major health concerns.

Structure and function of latency-associated nuclear antigen

Latency-associated nuclear antigen (LANA) encoded by open reading frame 73 (ORF73) is the major latent protein expressed in all forms of KSHV-associated malignancies. LANA is a large (222-234 kDa) nuclear protein that interacts with various cellular as well as viral proteins. LANA has been classified as an oncogenic protein as it dysregulates various cellular pathways including tumor suppressor pathways associated with pRb and p53 and can transform primary rat embryo fibroblasts in cooperation with the cellular oncogene Hras. It associates with GSK-3beta, an important modulator of Wnt signaling pathway leading to the accumulation of cytoplasmic beta-catenin, which upregulates Tcf/Lef regulated genes after entering into the nucleus. LANA also blocks the expression of RTA, the reactivation transcriptional activator, which is critical for the latency to lytic switch, and thus helps in maintaining viral latency. LANA tethers the viral episomal DNA to the host chromosomes by directly binding to its cognate binding sequence within the TR region of the genome through its C terminus and to the nucleosomes through the N terminus of the molecule. Tethering to the host chromosomes helps in efficient partitioning of the viral episomes in the dividing cells. Disruptions of LANA expression led to reduction in the episomal copies of the viral DNA, supporting its role in persistence of the viral DNA. The functions known so far suggest that LANA is a key player in KSHV-mediated pathogenesis.

Kaposi's sarcoma-associated herpesvirus infection and Kaposi's sarcoma in Brazil

Kaposi's sarcoma (KS) became a critical health issue with the emergence of acquired immunodeficiency syndrome (AIDS) in the 1980s. Four clinical-epidemiological forms of KS have been described: classical KS, endemic KS, iatrogenic KS, and AIDS-associated KS. In 1994, Kaposi's sarcoma-associated herpesvirus (KSHV) or human herpesvirus type 8 was identified by Chang and colleagues, and has been detected worldwide at frequencies ranging from 80 to 100%. The aim of the present study was to evaluate the frequency of KSHV infection in KS lesions from HIV-positive and HIV-negative patients in Brazil, as well as to review the current knowledge about KS transmission and detection. For these purposes, DNA from 51 cases of KS was assessed by PCR: 20 (39.2%) cases of classical KS, 29 (56.9%) of AIDS-associated KS and 2 (3.9%) of iatrogenic KS. Most patients were males (7.5:1, M/F), and mean age was 47.9 years (SD = +/- 18.7 years). As expected, HIV-positive KS patients were younger than patients with classical KS. On the other hand, patients with AIDS-associated KS have early lesions (patch and plaque) compared to classical KS patients (predominantly nodular lesions). This is assumed to be the result of the early diagnose of KS in the HIV-positive setting. KSHV infection was detected by PCR in almost all cases (48/51; 94.1%), irrespectively of the clinical-epidemiological form of KS. These results show that KSHV is associated with all forms of KS in Brazilian patients, a fact that supports the role of this virus in KS pathogenesis.

Human herpesvirus 8 enhances human immunodeficiency virus replication in acutely infected cells and induces reactivation in latently infected cells

Human herpesvirus 8 (HHV-8) is etiologically associated with Kaposi sarcoma (KS), the most common AIDS-associated malignancy. Previous results indicate that the HHV-8 viral transactivator ORF50 interacts synergistically with Tat protein in the transactivation of human immunodeficiency virus (HIV) long terminal repeat (LTR), leading to increased cell susceptibility to HIV infection. Here, we analyze the effect of HHV-8 infection on HIV replication in monocyte-macrophage and endothelial cells, as potential targets of coinfection. Primary or transformed monocytic and endothelial cells were infected with a cell-free HHV-8 inoculum and subsequently infected with lymphotropic or monocytotropic strains of HIV. The results show that HHV-8 coinfection markedly increases HIV replication in both cell types. HHV-8 infection induces also HIV reactivation in chronically infected cell lines and in peripheral blood mononuclear cells (PBMCs) from patients with asymptomatic HIV, suggesting the possibility that similar interactions might take place also in vivo. Furthermore, coinfection is not an essential condition, since contiguity of differently infected cells is sufficient for HIV reactivation. The results suggest that HHV-8 might be a cofactor for HIV progression and that HHV-8-infected endothelial cells might play a relevant role in transendothelial HIV spread.

Inhibition of infection and replication of human herpesvirus 8 in microvascular endothelial cells by alpha interferon and phosphonoformic acid

Infection of endothelial cells with human herpesvirus 8 (HHV-8) is an essential event in the development of Kaposi's sarcoma. When primary microvascular endothelial cells (MECs) were infected with HHV-8 at a low multiplicity of infection, considerable latent replication of HHV-8 occurred, leading to a time-dependent increase in the percentage of virus-infected cells that was accompanied by cellular spindling and growth to a high density with loss of contact inhibition. Only a low percentage of MECs supported lytic replication of HHV-8 and produced infectious virus. Phosphonoformic acid blocked production of infectious virus but did not inhibit the rapid expansion of latently infected MECs. Pretreatment of MECs with alpha interferon (IFN-alpha) prior to infection effectively reduced HHV-8 viral gene expression, latent replication, and production of infectious virus. High levels of the double-stranded RNA activated protein kinase (PKR) were expressed in HHV-8-infected cells, and incubation with IFN-alpha increased PKR expression more in virus-infected cells than in uninfected cells. MECs that were immortalized with simian virus 40 large-T antigen differed from nonimmortalized MECs in their response to infection with HHV-8 and demonstrated that cells with elevated levels of expression of antiviral transcripts expressed viral transcripts at reduced levels. These studies demonstrate that MECs respond to HHV-8 with enhanced expression of cellular antiviral genes and that augmentation of innate antiviral defenses with IFN-alpha is a more effective strategy than inhibition of viral lytic replication to protect MECs from infection with HHV-8 and to restrict proliferation of virus-infected MECs.

Use of the red fluorescent protein as a marker of Kaposi's sarcoma-associated herpesvirus lytic gene expression

A hallmark of all herpesvirus is the ability to exist in either a latent, or lytic, state of replication, enabling the lifelong infection of its host. Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) can efficiently establish a latent infection in a variety of cell types in vitro, making it a valuable model for the study of latency and reactivation. To facilitate the identification of KSHV lytic replication, and allow subsequent experiments with live cells, a recombinant virus, rKSHV.219, was constructed using JSC-1 cells that expresses the red fluorescent protein (RFP) from the KSHV lytic PAN promoter, the green fluorescent protein (GFP) from the EF-1alpha promoter, and with the gene for puromycin resistance as a selectable marker. rKSHV.219 from JSC-1 cells was used to infect Vero cells for purification of the recombinant virus. Vero cells were also used for the production of rKSHV.219 at levels of 10(5)-10(6) infectious units (IU) of virus per milliliter using a combination of KSHV/RTA expressed from a baculovirus vector, BacK50, and butyrate. Virus produced from Vero cells was used to infect human fibroblasts (HF), 293, DU145, T24, HaCaT, and HEp-2 cells, and in all cells except 293 cells, only a latent infection was established with GFP expression, but no RFP expression. In 293 cells, 10-15% of cells showed lytic gene expression. Both primary and immortalized microvascular endothelial cells (MVEC) were also infected with rKSHV.219, and reduced spontaneous lytic replication was found in immortalized cells. In all cells used in this study, rKSHV.219 efficiently established latent infections from which the virus could be reactivated to productive lytic replication. This work also demonstrated strong synergy between KSHV/RTA and butyrate for the activation of KSHV lytic replication and the production of infectious virus.

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.

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.

Retrospective analysis of HHV-8 viremia and cellular viral load in HIV-seropositive patients receiving interleukin 2 in combination with antiretroviral therapy

The combination of interleukin 2 (IL-2) and antiretroviral therapy (ART) represents an emerging strategy in the treatment of patients infected with HIV. Aside from its immunomodulatory role, however, IL-2 may induce replication of human herpesvirus 8 (HHV-8)/Kaposi sarcoma (KS)–associated herpesvirus. We retrospectively evaluated HHV-8 plasma viremia and cellular load, as well as anti–HHV-8 antibody titers, in sequential samples from 84 patients receiving ART alone or in combination with IL-2. At baseline, HHV-8 plasma viremia was present only in 2 HHV-8–seropositive patients in whom KS subsequently developed during or immediately after termination of IL-2 therapy. The level of viremia increased during follow-up and peaked at the time of the clinical manifestation of KS. Moreover, transient peaks of HHV-8 viremia were temporally associated with administration of IL-2. HHV-8 plasma viremia was never detected in the other 47 patients receiving IL-2 nor in 35 controls treated only with ART. Thus, IL-2 therapy seems safe in most patients infected with both HIV and HHV-8, except for those with detectable HHV-8 viremia, who may not be eligible for IL-2 treatment.

Efficient infection by a recombinant Kaposi's sarcoma-associated herpesvirus cloned in a bacterial artificial chromosome: application for genetic analysis

Kaposi's sarcoma-associated herpesvirus (KSHV) is etiologically associated with Kaposi's sarcoma and several other malignancies. The lack of an efficient infection system has impeded the understanding of KSHV-related pathogenesis. A genetic approach was used to isolate infectious KSHV. Recombinant bacteria artificial chromosome (BAC) KSHV containing hygromycin resistance and green fluorescent protein (GFP) markers was generated by homologous recombination in KSHV-infected BCBL-1 cells. Recombinant KSHV genomes from cell clones that were resistant to hygromycin, expressed GFP, and produced infectious virions after induction with tetradecanoyl phorbol acetate (TPA) were rescued in Escherichia coli and reconstituted in 293 cells. Several 293 cell lines resulting from infection with recombinant virions induced from a full-length recombinant KSHV genome, named BAC36, were obtained. BAC36 virions established stable latent infection in 293 cells, harboring 1 to 2 copies of viral genome per cell and expressing viral latent proteins, with approximately 0.5% of cells undergoing spontaneous lytic replication, which is reminiscent of KSHV infection in Kaposi's sarcoma tumors. TPA treatment induced BAC36-infected 293 cell lines into productive lytic replication, expressing lytic proteins and producing virions that efficiently infected normal 293 cells with a approximately 50% primary infection rate. BAC36 virions were also infectious to HeLa and E6E7-immortalized human endothelial cells. Since BAC36 can be efficiently shuttled between bacteria and mammalian cells, it is useful for KSHV genetic analysis. The feasibility of the system was illustrated through the generation of a KSHV mutant with the vIRF gene deleted. This cellular model is useful for the investigation of KSHV infection and pathogenesis.

Angiogenic effects of extracellular human immunodeficiency virus type 1 Tat protein and its role in the pathogenesis of AIDS-associated Kaposi's sarcoma

The Tat protein of human immunodeficiency virus (HIV) type 1 is a transactivator of viral gene expression that is required for virus replication and spread. Moreover, Tat is released by acutely HIV-infected cells via a leaderless secretory pathway and in a biologically active form that exerts effects on both HIV-infected and uninfected cells from different organs and systems. This review focuses on the activities of extracellular Tat protein on endothelial cells, on angiogenesis, and on the pathogenesis of AIDS-associated angioproliferative diseases such as Kaposi's sarcoma. In particular, we discuss results from different groups indicating that Tat mimics the proangiogenic activities of extracellular matrix molecules and that it enhances the effects of angiogenic factors.

The molecular pathology of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the eighth and most recently identified human herpesvirus (HHV-8). KSHV was discovered in 1994 by Chang et al. who used representational difference analysis to search for DNA sequences present in AIDS-associated KS but not in adjacent normal skin [1]. The virus has since been shown to be specifically associated with all forms of this disease and has fulfilled all of Hill's criteria for causation (reviewed in ). KSHV is also found in all cases of primary effusion lymphoma and in a plasmablastic variant of multicentric Castleman's disease. Over the last few years a wealth of data has been gained on the role of KSHV genes during infection. This review is an attempt to assemble this information into a more complete picture of how KSHV may cause disease.

Reactivation and role of HHV-8 in Kaposi's sarcoma initiation

Kaposi's sarcoma (KS) is an angioproliferative disease occurring in several clinical-epidemio-logic forms but all associated with infection by the human herpesvirus-8 (HHV-8). At least in early stages, KS is a reactive disease associated with a state of immune dysregulation characterized by CD8+ T-cell activation and production of Th1-type inflammatory cytokines (IC) that precedes lesion development. In fact, evidence indicates that IC can trigger lesion formation by inducing the activation of endothelial cells that leads to adhesion and tissue extravasation of lymphomonocytes, spindle cell formation, and angiogenesis, and HHV-8 reactivation that, in turn, leads to virus spread to all circulating cell types and virus dissemination into tissues. Due to virus escape mechanisms and deficient immune responses toward HHV-8, virus reactivation and spread are not controlled by the immune system but induce immune responses that may paradoxically exacerbate the reactive process. The virus is recruited into "activated" tissue sites where it finds an optimal environment for growth. In fact, viral load is very low in early lesions, whereas almost all spindle cells are infected in late-stage lesions. Although early KS is a reactive process of polyclonal nature that can regress, in time and in the presence of immunodeficiency, it can progress to a true sarcoma. This is likely due to the long-lasting expression of HHV-8 latency genes in spindle cells associated with the deregulated expression of oncogenes and oncosuppressor genes and, for AIDS-KS, with the effects of the HIV-1 Tat protein.

Human herpesvirus-8 and Kaposi's sarcoma: relationship with the multistep concept of tumorigenesis

Kaposi's sarcoma (KS) develops through discrete inflammatory-angiogenic stages of polyclonal nature (early-stage lesions) to monomorphic nodules of spindle-shaped cells that can be clonal (late-stage lesions) and resemble true sarcomas. Molecular and epidemiological studies indicate that development of KS is tightly associated with infection by the human herpesvirus-8 (HHV-8). However, only individuals with specific conditions of immunodysregulation develop KS. In these individuals the systemic and tissue increase of Th-1-type cytokines (IC) reactivate HHV-8 infection, leading to increased viral load, antibody titers, and an expanded cell tropism that precedes the clinical appearance of KS. Recruitment of the virus into tissues by infected monocytes and other cell types is facilitated by the endothelial cell activation due to IC. In clinical lesions, HHV-8 infection increases with lesion stage and in late-stage lesions most of the spindle cells are latently infected, whereas only few lyrically infected cells are present, suggesting that latent genes may have a role in the transformation of the early inflammatory-hyperplastic lesion into a real sarcoma. The development of tumors, however, is regulated through a multistep process based on the acquisition by cells of several different capabilities leading to malignant growth. Here we review the available data on the expression of HHV-8-encoded genes in primary KS lesions and, in view of their biological activity, analyze their potential function in different steps of tumorigenesis. By this pragmatic approach interesting insights into potential key functions of HHV-8-encoded genes are found and steps of potential cooperativity with other viral factors (HIV-1-Tat) in the pathogenesis of KS are identified.

The emerging role of the human herpesvirus 8 (HHV8) in human neoplasia

The human herpesvirus 8 (HHV8) was initially described and characterised in Kaposi's sarcoma tissue. The virus was found in the lesion of most cases of Kaposi's sarcoma. Whilst there is a large body of evidence to implicate its role in the pathogenesis of Kaposi's sarcoma, it has recently been found that the virus may also be important in a number of other human neoplasias. This review will examine the molecular pathology of HHV8 in the pathogenesis of Kaposi's sarcoma and summarise the current evidence and postulated mechanisms in its role in other human neoplasias.

Transcription pattern of human herpesvirus 8 open reading frame K3 in primary effusion lymphoma and Kaposi's sarcoma

Human herpesvirus 8 (HHV-8) is found in immunoblastic B cells of patients with multicentric Castleman's disease (MCD) and, predominantly in a latent form, in primary effusion lymphoma (PEL) cells and Kaposi's sarcoma (KS) spindle cells. Recent studies have shown that upon reactivation, HHV-8 expresses factors that downregulate major histocompatibility class I proteins and coactivation molecules and that may enable productively infected cells to escape cytotoxic T lymphocytes and natural killer cell responses. One of these viral factors is encoded by open reading frame (ORF) K3. Here we show that in PEL cells, ORF K3 is expressed through viral transcripts that are induced very early upon virus reactivation, including bicistronic RNA molecules containing coding sequences from viral ORFs K3 and 70. Specifically, we found that a bicistronic transcript was expressed in the absence of de novo protein synthesis, thereby identifying a novel HHV-8 immediate-early gene product. Several features of the RNA molecules encoding the K3 product, including multiple transcriptional start sites, multiple donor splicing sites, and potential alternative ATG usage, suggest that there exists a finely tuned modulation of ORF K3 expression. By contrast, ORF K3 transcripts are not detected in the majority of cells present in KS lesions that are latently infected by the virus, suggesting that there are other, as-yet-unknown mechanisms of immune evasion for infected KS spindle cells. Nevertheless, because HHV-8 viremia precedes the development of KS lesions and is associated with the recrudescence of MCD symptoms, the prompt expression of ORF K3 in productively infected circulating cells may be important for virus pathogenesis. Thus, molecules targeting host or viral factors that activate ORF K3 expression or inactivate the biological functions of the K3 product should be exploited for the prevention or treatment of HHV-8-associated diseases in at-risk individuals.

Biology of Kaposi's sarcoma

Kaposi's sarcoma (KS) is an angioproliferative disease occurring in several different clinical-epidemiological forms that, however, share the same histological traits and are all associated with infection by the human herpesvirus 8 (HHV8). KS initiates in a context of immune dysregulation characterised by CD8+ T cell activation and the production of Th1-type cytokines that induce a generalised activation of endothelial cells leading to adhesion and tissue extravasation of lympho-monocytes, spindle cell formation and angiogenesis. These phenomena are triggered or enhanced by infection with HHV8 that, in turn, is reactivated by the same cytokines. Productively-infected circulating cells are recruited into 'activated' tissue sites where HHV8 finds an optimal environment for establishing a persistent, latent infection of KS spindle cells (KSC). HHV8 dissemination is favoured by virus escape mechanisms and immune dysregulation, and leads to immune responses that are not effective against the virus but, paradoxically, exacerbates the reactive process. Although early KS is a reactive process of polyclonal nature that can regress, in time it can progress in to a true sarcoma. The progression of KS appears to be due to the deregulated expression of oncogenes and oncosuppressor genes, to the long-lasting expression of the HHV8 latency genes and, for AIDS-KS, is promoted by the proliferative and angiogenic effects of the HIV-1 Tat protein.

Induction of HHV-8 lytic cycle replication by inflammatory cytokines produced by HIV-1-infected T cells

Human herpesvirus 8 (HHV-8) is a gamma2-herpesvirus consistently identified in Kaposi's sarcoma (KS), primary effusion lymphoma, and multicentric Castleman's disease. Although HHV-8 infection appears to be necessary, it may not be sufficient for development of KS without the involvement of other cofactors. One potentially important cofactor is HIV-1. HIV-1-infected cells produce HIV-1-related proteins and cytokines, both of which have been shown to promote growth of KS cells in vitro. Though HIV-1 is not absolutely necessary for KS development, KS is the most frequent neoplasm in AIDS patients, and AIDS-KS is recognized as a particularly aggressive form of the disease. To determine whether HIV-1 could participate in the pathogenesis of KS by modulating HHV-8 replication (rather than by inducing immunodeficiency), HIV-1-infected T cells were cocultured with the HHV-8-infected cell line, BCBL-1. The results demonstrate soluble factors produced by or in response to HIV-1-infected T cells induced HHV-8 replication, as determined by production of lytic phase mRNA transcripts, viral proteins, and detection of progeny virions. By focusing on cytokines produced in the coculture system, several cytokines known to be important in growth and proliferation of KS cells in vitro, particularly Oncostatin M, hepatocyte growth factor/scatter factor, and interferon-gamma, were found to induce HHV-8 lytic replication when added individually to BCBL-1 cells. These results suggest specific cytokines can play an important role in the initiation and progression of KS through reactivation of HHV-8. Thus, HIV-1 may participate more directly than previously recognized in KS by promoting HHV-8 replication and, hence, increasing local HHV-8 viral load.