Activation of cellular and heterologous promoters by the human herpesvirus 8 replication and transcription activator

Cell Cultures for Virology: Usability, Advantages, and Prospects

Virus detection in natural and clinical samples is a complicated problem in research and diagnostics. There are different approaches for virus isolation and identification, including PCR, CRISPR/Cas technology, NGS, immunoassays, and cell-based assays. Following the development of genetic engineering methods, approaches that utilize cell cultures have become useful and informative. Molecular biology methods allow increases in the sensitivity and specificity of cell cultures for certain viruses and can be used to generate reporter cell lines. These cell lines express specific reporter proteins (e.g., GFP, luciferase, and CAT) in response to virus infection that can be detected in a laboratory setting. The development of genome editing and synthetic biology methods has given rise to new perspectives regarding the design of virus reporter systems in cell cultures. This review is aimed at describing both virology methods in general and examples of the development of cell-based methods that exist today.

IL-10 promoter transactivation by the viral K-RTA protein involves the host-cell transcription factors, specificity proteins 1 and 3

Kaposi's sarcoma-associated herpesvirus (KSHV)/human herpesvirus-8 (HHV-8) causes a persistent infection, presenting latent and lytic replication phases during its life cycle. KSHV-related diseases are associated with deregulated expression of inflammatory cytokines, including IL-6 and IL-10, but the mechanisms underlying this dysregulation are unclear. Herein, we report a molecular mechanism for KSHV-induced IL-10 gene expression. KSHV replication and transcription activator (K-RTA) is a molecular switch for the initiation of expression of viral lytic genes, and we describe, for the first time, that K-RTA significantly activates the promoter of the human IL-10 gene. Of note, mutations involving a basic region of K-RTA reduced the association of K-RTA with the IL-10 promoter. Moreover, the host-cell transcription factors, specificity proteins (SP) 1 and 3, play a pivotal cooperative role in K-RTA-mediated transactivation of the IL-10 promoter. K-RTA can interact with SP1 and SP3 directly in vitro, and electrophoresis mobility shift assays (EMSAs) revealed co-operative interaction involving K-RTA, SP1, and SP3 in binding to the IL-10 promoter. As DNase I footprinting assays indicated that K-RTA did not affect SP3 binding to the IL-10 promoter, SP3 can function to recruit K-RTA to the IL-10 promoter. These findings indicate that K-RTA can directly contribute to IL-10 up-regulation via a functional interplay with the cellular transcription factors SP1 and SP3.

Identification of Novel Kaposi's Sarcoma-Associated Herpesvirus Orf50 Transcripts: Discovery of New RTA Isoforms with Variable Transactivation Potential

Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus that has been associated with primary effusion lymphoma and multicentric Castleman's disease, as well as its namesake Kaposi's sarcoma. As a gammaherpesvirus, KSHV is able to acutely replicate, enter latency, and reactivate from this latent state. A key protein involved in both acute replication and reactivation from latency is the replication and transcriptional activator (RTA) encoded by the gene Orf50 RTA is a known transactivator of multiple viral genes, allowing it to control the switch between latency and virus replication. We report here the identification of six alternatively spliced Orf50 transcripts that are generated from four distinct promoters. These newly identified promoters are shown to be transcriptionally active in 293T (embryonic kidney), Vero (African-green monkey kidney epithelial), 3T12 (mouse fibroblast), and RAW 264.7 (mouse macrophage) cell lines. Notably, the newly identified Orf50 transcripts are predicted to encode four different isoforms of the RTA which differ by 6 to 10 residues at the amino terminus of the protein. We show the global viral transactivation potential of all four RTA isoforms and demonstrate that all isoforms can transcriptionally activate an array of KSHV promoters to various levels. The pattern of transcriptional activation appears to support a transcriptional interference model within the Orf50 region, where silencing of previously expressed isoforms by transcription initiation from upstream Orf50 promoters has the potential to modulate the pattern of viral gene activation.

IMPORTANCE

Gammaherpesviruses are associated with the development of lymphomas and lymphoproliferative diseases, as well as several other types of cancer. The human gammaherpesvirus, Kaposi's sarcoma-associated herpesvirus (KSHV), is tightly associated with the development of Kaposi's sarcoma and multicentric Castleman's disease, as well as a rare form of B cell lymphoma (primary effusion lymphoma) primarily observed in HIV-infected individuals. RTA is an essential viral gene product involved in the initiation of gammaherpesvirus replication and is conserved among all known gammaherpesviruses. We show here for KSHV that transcription of the gene encoding RTA is complex and leads to the expression of several isoforms of RTA with distinct functions. This observed complexity in KSHV RTA expression and function likely plays a critical role in the regulation of downstream viral and cellular gene expression, leading to the efficient production of mature virions.

KSHV Rta Promoter Specification and Viral Reactivation

Viruses are obligate intracellular pathogens whose biological success depends upon replication and packaging of viral genomes, and transmission of progeny viruses to new hosts. The biological success of herpesviruses is enhanced by their ability to reproduce their genomes without producing progeny viruses or killing the host cells, a process called latency. Latency permits a herpesvirus to remain undetected in its animal host for decades while maintaining the potential to reactivate, or switch, to a productive life cycle when host conditions are conducive to generating viral progeny. Direct interactions between many host and viral molecules are implicated in controlling herpesviral reactivation, suggesting complex biological networks that control the decision. One viral protein that is necessary and sufficient to switch latent Kaposi’s sarcoma-associated herpesvirus (KSHV) into the lytic infection cycle is called K-Rta. K-Rta is a transcriptional activator that specifies promoters by binding DNA directly and interacting with cellular proteins. Among these cellular proteins, binding of K-Rta to RBP-Jk is essential for viral reactivation. In contrast to the canonical model for Notch signaling, RBP-Jk is not uniformly and constitutively bound to the latent KSHV genome, but rather is recruited to DNA by interactions with K-Rta. Stimulation of RBP-Jk DNA binding requires high affinity binding of Rta to repetitive and palindromic “CANT DNA repeats” in promoters, and formation of ternary complexes with RBP-Jk. However, while K-Rta expression is necessary for initiating KSHV reactivation, K-Rta’s role as the switch is inefficient. Many factors modulate K-Rta’s function, suggesting that KSHV reactivation can be significantly regulated post-Rta expression and challenging the notion that herpesviral reactivation is bistable. This review analyzes rapidly evolving research on KSHV K-Rta to consider the role of K-Rta promoter specification in regulating the progression of KSHV reactivation.

Deep sequencing reveals direct targets of gammaherpesvirus-induced mRNA decay and suggests that multiple mechanisms govern cellular transcript escape

One characteristic of lytic infection with gammaherpesviruses, including Kaposi's sarcoma-associated herpesvirus (KSHV), Epstein-Barr virus (EBV) and murine herpesvirus 68 (MHV68), is the dramatic suppression of cellular gene expression in a process known as host shutoff. The alkaline exonuclease proteins (KSHV SOX, MHV-68 muSOX and EBV BGLF5) have been shown to induce shutoff by destabilizing cellular mRNAs. Here we extend previous analyses of cellular mRNA abundance during lytic infection to characterize the effects of SOX and muSOX, in the absence of other viral genes, utilizing deep sequencing technology (RNA-seq). Consistent with previous observations during lytic infection, the majority of transcripts are downregulated in cells expressing either SOX or muSOX, with muSOX acting as a more potent shutoff factor than SOX. Moreover, most cellular messages fall into the same expression class in both SOX- and muSOX-expressing cells, indicating that both factors target similar pools of mRNAs. More abundant mRNAs are more efficiently downregulated, suggesting a concentration effect in transcript targeting. However, even among highly expressed genes there are mRNAs that escape host shutoff. Further characterization of select escapees reveals multiple mechanisms by which cellular genes can evade downregulation. While some mRNAs are directly refractory to SOX, the steady state levels of others remain unchanged, presumably as a consequence of downstream effects on mRNA biogenesis. Collectively, these studies lay the framework for dissecting the mechanisms underlying the susceptibility of mRNA to destruction during lytic gammaherpesvirus infection.

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

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

RNA editing of the human herpesvirus 8 kaposin transcript eliminates its transforming activity and is induced during lytic replication

Human herpesvirus 8 is the etiologic agent associated with Kaposi's sarcoma and primary effusion lymphoma (PEL). The K12 RNA, which produces as many as three variants of the kaposin protein, as well as a microRNA, is the most abundant transcript expressed in latent Kaposi's sarcoma-associated herpesvirus infection, and yet it is also induced during lytic replication. The portion of the transcript that includes the microRNA and the kaposin A sequence has been shown to have tumorigenic potential. Genome coordinate 117990, which is within this transcript, has been found to be heterogeneous, primarily in RNAs but also among viral DNA sequences. This sequence heterogeneity affects an amino acid in kaposins A and C and the microRNA. The functional effects of this sequence heterogeneity have not been studied, and its origin has not been definitively settled; both RNA editing and heterogeneity at the level of the viral genome have been proposed. Here, we show that transcripts containing A at position 117990 are tumorigenic, while those with G at this position are not. Using a highly sensitive quantitative assay, we observed that, in PEL cells under conditions where more than 60% of cDNAs derived from K12 RNA transcripts have G at coordinate 117990, there is no detectable G in the viral DNA sequence at this position, only A. This result is consistent with RNA editing by one of the host RNA adenosine deaminases (ADARs). Indeed, we observed that purified human ADAR1 efficiently edits K12 RNA in vitro. Remarkably, the amount of editing correlated with the replicative state of the virus; editing levels were nearly 10-fold higher in cells treated to induce lytic viral replication. These results suggest that RNA editing controls the function of one segment of the kaposin transcript, such that it has transforming activity during latent replication and possibly another, as-yet-undetermined, function during lytic replication.

Inhibition of NF-kappaB activation in vivo impairs establishment of gammaherpesvirus latency

A critical determinant in chronic gammaherpesvirus infections is the ability of these viruses to establish latency in a lymphocyte reservoir. The nuclear factor (NF)-κB family of transcription factors represent key players in B-cell biology and are targeted by gammaherpesviruses to promote host cell survival, proliferation, and transformation. However, the role of NF-κB signaling in the establishment of latency in vivo has not been addressed. Here we report the generation and in vivo characterization of a recombinant murine gammaherpesvirus 68 (γHV68) that expresses a constitutively active form of the NF-κB inhibitor, IκBαM. Inhibition of NF-κB signaling upon infection with γHV68-IκBαM did not affect lytic replication in cell culture or in the lung following intranasal inoculation. However, there was a substantial decrease in the frequency of latently infected lymphocytes in the lung (90% reduction) and spleens (97% reduction) 16 d post intranasal inoculation. Importantly, the defect in establishment of latency in lung B cells could not be overcome by increasing the dose of virus 100-fold. The observed decrease in establishment of viral latency correlated with a loss of activated, CD69^hi B cells in both the lungs and spleen at day 16 postinfection, which was not apparent by 6 wk postinfection. Constitutive expression of Bcl-2 in B cells did not rescue the defect in the establishment of latency observed with γHV68-IκBαM, indicating that NF-κB–mediated functions apart from Bcl-2–mediated B-cell survival are critical for the efficient establishment of gammaherpesvirus latency in vivo. In contrast to the results obtained following intranasal inoculation, infection of mice with γHV68-IκBαM by the intraperitoneal route had only a modest impact on splenic latency, suggesting that route of inoculation may alter requirements for establishment of virus latency in B cells. Finally, analyses of the pathogenesis of γHV68-IκBαM provides evidence that NF-κB signaling plays an important role during multiple stages of γHV68 infection in vivo and, as such, represents a key host regulatory pathway that is likely manipulated by the virus to establish latency in B cells.

A central aspect of chronic infection of a host by herpesviruses is the ability of these viruses to establish a quiescent infection (latent infection) in some cell type(s) in which there is only intermittent production of progeny virus (virus reactivation). The establishment of a latent infection in the antibody producing cells of the host immune system (B lymphocytes) is critical for life-long persistence of gammaherpesviruses, as well as the development of virus-associated lymphoproliferative diseases (e.g., B-cell lymphomas). Nuclear factor (NF)-κB transcription factors are a family of cellular proteins that play an important role regulating gene expression in B cells, and it has been shown that gammaherpesviruses have evolved multiple strategies for manipulating NF-κB activity. However, to date there has been no reported examination of the role of NF-κB in the establishment of chronic gammaherpesvirus infection in vivo. Murine gammaherpesvirus 68 (γHV68) infects rodents and shares genetic and biologic properties with the human gammaherpesviruses, Epstein-Barr virus and Kaposi sarcoma–associated herpesvirus. To selectively block the function of NF-κB in infected cells, we engineered a transgenic virus that expresses a repressor of NF-κB activation (IκBαM). Notably, this recombinant virus was defective in the establishment of latency in B cells in the lungs and spleen following intranasal inoculation. We also observed that the decrease in B-cell infection could not be rescued by forced expression of the cellular Bcl-2 protein, which is normally upregulated by NF-κB and serves to protect B cells from some forms of cell death. Thus, we conclude that NF-κB is an important host factor for the successful establishment of a chronic infection by gammaherpesviruses, and likely requires functions of NF-κB apart from its role in B-cell survival.

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.

Kaposi sarcoma herpesvirus-encoded interferon regulator factors

The Kaposi sarcoma herpesvirus (KSHV) encodes multiple proteins that disrupt host antiviral responses, including four viral proteins that have homology to the interferon regulatory factor (IRF) family of transcription factors. At least three of the KSHV vIRFs (vIRFs 1-3) alter responses to cellular IRFs and to interferons (IFNs), whereas functional changes resulting from the fourth vIRF (vIRF-4) have not been reported. The vIRFs also affect other important regulatory proteins in the cell, including responses to transforming growth factor beta (TGF-beta) and the tumor suppressor protein p53. This review examines the expression of the vIRFs during the life cycle of KSHV and the functional consequences of their expression.

Targeting human herpesvirus-8 for treatment of Kaposi??s sarcoma and primary effusion lymphoma

PURPOSE OF REVIEW

Human herpesvirus-8, also called the Kaposi's sarcoma herpesvirus, is present in all cases of Kaposi's sarcoma and primary effusion lymphoma and in some cases of multicentric Castleman's disease. This review discusses mechanisms by which human herpesvirus-8 contributes to tumorigenesis and how this knowledge can be used to target the virus for the treatment of these tumors.

RECENT FINDINGS

Most primary effusion lymphomas and Kaposi's sarcoma tumor cells are latently infected with human herpesvirus-8 and hence resistant to antiherpesvirus drugs that are dependent on lytic replication. In contrast, many of the cells infected with human herpesvirus-8 in multicentric Castleman's disease support lytic replication, so that clinical improvement frequently occurs in response to treatment with antiherpesvirus drugs. The resistance of latently-infected tumor cells to antiherpesvirus drugs can be overcome by inducing human herpesvirus-8 to reenter the lytic cascade in the presence of antiherpesvirus drugs. This leads to apoptosis of virally infected cells without increasing production of infectious virus. Alternatively, the replication and maintenance of the human herpesvirus-8 episome during latency can be disrupted by glycyrrhizic acid or hydroxyurea so that the virus no longer contributes to tumorigenesis. Both the innate and acquired immune systems can also be augmented to help prevent or treat human herpesvirus-8-associated tumors.

SUMMARY

Novel strategies targeting human herpesvirus-8, which is present in all cases of Kaposi's sarcoma and primary effusion lymphoma, provide opportunities for selectively killing tumor cells.

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.

Higher prevalence of human herpesvirus 8 DNA sequence and specific IgG antibodies in patients with pemphigus in China

BACKGROUND

Environmental factors, including virus infection, may play a role in the onset and/or development of pemphigus. However, it is controversial whether human herpesvirus (HHV)-8 is involved in pathogenesis of pemphigus.

OBJECTIVE

The possible association of pemphigus with HHV-8 was investigated.

METHODS

A total of 36 lesional skin and 13 peripheral blood mononuclear cell specimens from 58 patients with pemphigus, and 18 normal skin and 230 peripheral blood mononuclear cell specimens from healthy individuals, were tested for HHV-8 DNA sequence by a nested polymerase chain reaction assay. In all, 29 sera from the patients and 109 sera from healthy individuals were tested for HHV-8-specific IgG antibodies by enzyme-linked immunosorbent assays using HHV-8-specific oligopeptides as antigens.

RESULTS

Prevalence of both HHV-8 DNA sequence (36.1% and 30.8% in lesional skin and in peripheral blood mononuclear cells, respectively) and HHV-8-specific IgG antibodies (34.5%) for patients with pemphigus was statistically higher than that of control subjects (<8% in both assays). There was no significant difference in HHV-8 prevalence among different types of pemphigus.

CONCLUSION

HHV-8 infection might be a contributing factor in the development of pemphigus.

The small GTPase Rac1 links the Kaposi sarcoma–associated herpesvirus vGPCR to cytokine secretion and paracrine neoplasia

Kaposi sarcoma (KS) is a multifocal angioproliferative neoplasm strictly dependent on angiogenic growth factors and cytokines and invariably associated with infection by the Kaposi sarcoma–associated herpesvirus (KSHV or HHV8). A G protein–coupled receptor encoded by KSHV (vGPCR) is able to initiate KS-like tumors when targeted to the vascular endothelium of mice. Analogous to human KS, vGPCR sarcomagenesis involves the paracrine secretion of angiogenic growth factors and proinflammatory molecules from vGPCR-expressing cells. Here we demonstrate that vGPCR up-regulates expression and secretion of critical KS cytokines by stimulating key transcription factors, including nuclear factor–κB (NF-κB), activator protein-1 (AP-1), and nuclear factor of activated T cells (NFAT), through the activation of the small G protein Rac1. Inhibition of Rac1 blocked vGPCR-induced transcription and secretion of KS cytokines, including interleukin-6 (IL-6), IL-8, and growth-regulated oncogene α (GROα), in vitro and reduced vGPCR tumorigenesis in vivo. Moreover, endothelial-specific infection with the constitutively active Rac1QL induced vascular lesions in mice that were remarkably similar to early vGPCR experimental lesions. These results identify Rac1 as a key mediator of vGPCR paracrine neoplasia, suggesting that this small G protein and its downstream effectors may represent suitable therapeutic targets for the treatment of KS.

Short duration of elevated vIRF-1 expression during lytic replication of human herpesvirus 8 limits its ability to block antiviral responses induced by alpha interferon in BCBL-1 cells

Human herpesvirus 8 (HHV-8) encodes multiple proteins that disrupt the host antiviral response, including viral interferon (IFN) regulatory factor 1 (vIRF-1). The product of the vIRF-1 gene blocks responses to IFN when overexpressed by transfection, but the functional consequence of vIRF-1 that is expressed during infection with HHV-8 is not known. These studies demonstrate that BCBL-1 cells that were latently infected with HHV-8 expressed low levels of vIRF-1 that were associated with PML bodies, whereas much higher levels of vIRF-1 were transiently expressed during the lytic phase of HHV-8 replication. The low levels of vIRF-1 that were associated with PML bodies were insufficient to block alpha IFN (IFN-alpha)-induced alterations in gene expression, whereas cells that expressed high levels of vIRF-1 were resistant to some changes induced by IFN-alpha, including the expression of the double-stranded-RNA-activated protein kinase. High levels of vIRF-1 were expressed for only a short period during the lytic cascade, so many cells with HHV-8 in the lytic phase responded to IFN-alpha with increased expression of antiviral genes and enhanced apoptosis. Furthermore, the production of infectious virus was severely compromised when IFN-alpha was present early during the lytic cascade. These studies indicate that the transient expression of high levels of vIRF-1 is inadequate to subvert many of the antiviral effects of IFN-alpha so that IFN-alpha can effectively induce apoptosis and block production of infectious virus when present early in the lytic cascade of HHV-8.

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.

NF-kappaB inhibits gammaherpesvirus lytic replication

Nasopharyngeal carcinoma, Kaposi's sarcoma, and B-cell lymphomas are human malignancies associated with gammaherpesvirus infections. Members of this virus family are characterized by their ability to establish latent infections in lymphocytes. The latent viral genome expresses very few gene products. The infected cells are therefore poorly recognized by the host immune system, allowing the virus to persist for long periods of time. We sought to identify the cell-specific factors that allow these viruses to redirect their life cycle from productive replication to latency. We find that the cellular transcription factor NF-kappaB can regulate this process. Epithelial cells and fibroblasts support active (lytic) gammaherpesvirus replication and have low NF-kappaB activity. However, overexpression of NF-kappaB in these cells inhibits the replication of the gammaherpesvirus murine herpesvirus 68 (MHV68). In addition, overexpression of NF-kappaB inhibits the activation of lytic promoters from MHV68 and human gammaherpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV). In lymphocytes latently infected with KSHV or EBV, the level of NF-kappaB activity is high, and treatment of these cells with an NF-kappaB inhibitor leads to lytic protein synthesis consistent with virus reactivation. These results suggest that high levels of NF-kappaB can inhibit gammaherpesvirus lytic replication and may therefore contribute to the establishment and maintenance of viral latency in lymphocytes. They also suggest that NF-kappaB may be a novel target for the disruption of virus latency and therefore the treatment of gammaherpesvirus-related malignancies.

Characterization of entry mechanisms of human herpesvirus 8 by using an Rta-dependent reporter cell line

To analyze the mechanisms of entry of human herpesvirus 8 (HHV-8), we established a reporter cell line T1H6 that contains the lacZ gene under the control of the polyadenylated nuclear RNA promoter, known to be strongly activated by a viral transactivator, Rta. We found that infection with cell-free virus, as well as cocultivation with HHV-8-positive primary effusion lymphoma cell lines, activated the lacZ gene of T1H6 in a sensitive and dose-dependent manner. Addition of Polybrene and centrifugation enhanced, but polysulfonate compounds inhibited, the HHV-8 infectivity. RGD-motif-containing polypeptides and integrins did not decrease the infectivity, suggesting the presence of an additional cellular receptor other than the reported one. The entry was dependent on pH acidification but not on the clathrin pathway. Although conditioned media obtained from human immunodeficiency virus (HIV)-infected cells did not have any effect on the early steps of HHV-8 infection, intracellular expression of a proviral HIV type 1, but not of Tat alone, increased the HHV-8-dependent reporter activation slightly, suggesting a potential of HIV-mediated enhancement of an early step of HHV-8 infection.