Rta of murine gammaherpesvirus 68 reactivates the complete lytic cycle from latency

Induction of protective immunity against murine gammaherpesvirus 68 infection in the absence of viral latency

Human gammaherpesviruses, Epstein-Barr virus, and human herpesvirus 8/Kaposi's sarcoma-associated herpesvirus are important pathogens associated with diseases, including lymphomas and other malignancies. Murine gammaherpesvirus 68 (MHV-68) is used as an experimental model system to study the host immune control of infection and explore novel vaccine strategies based on latency-deficient live viruses. We studied the properties and the potential of a recombinant MHV-68 (AC-RTA) in which the genes required for persistent infection were replaced by a constitutively expressed viral transcription activator, RTA, which dictates the virus to lytic replication. After intranasal infection of mice, replication of AC-RTA in the lung was attenuated, and no AC-RTA virus or viral DNA was detected in the isolated splenocytes, indicating a lack of latency in the spleen. Infection of the AC-RTA virus elicited both cellular immune responses and virus-specific IgG at a level comparable to that elicited by infection of the wild-type virus. Importantly, vaccination of AC-RTA was able to protect mice against subsequent challenge by the wild-type MHV-68. AC-RTA provides a vaccine strategy for preventing infection of human gammaherpesviruses. Furthermore, our results suggest that immunity to the major latent antigens is not required for protection.

Tpl2/AP-1 enhances murine gammaherpesvirus 68 lytic replication

How cellular factors regulate gammaherpesvirus lytic replication is not well understood. Here, through functional screening of a cellular kinase expression library, we identified mitogen-activated protein kinase kinase kinase 8 (MAP3K8/Tpl2) as a positive regulator of murine gammaherpesvirus 68 (MHV-68 or gammaHV-68) lytic gene expression and replication. Tpl2 enhances MHV-68 lytic replication by upregulating lytic gene expression and promoter activities of viral lytic genes, including RTA and open reading frame 57 (ORF57). By screening a cellular transcription factor library, we identified the Fos AP-1 transcription factor as a downstream factor that is both necessary and sufficient for mediating the enhancement of MHV-68 lytic replication by Tpl2. In addition, Tpl2 stimulates the promoter activities of key viral lytic genes, including RTA and ORF57, in an AP-1-dependent manner. We identified an AP-1-responsive element on the MHV-68 RTA promoter as the cis element mediating the upregulation of RTA promoter activity by Tpl2. MHV-68 lytic infection upregulates Fos expression, AP-1 activity, and RTA promoter activity in a Tpl2-dependent manner. We constructed a mutant MHV-68 virus that abolished this AP-1-responsive element. This mutant virus exhibited attenuated lytic replication kinetics, indicative of a critical role of this AP-1-responsive element during lytic replication. Moreover, Tpl2 knockdown inhibited the lytic replication of wild-type MHV-68 (MHV-68-WT) but not that of the MHV-68 mutant virus, indicating that endogenous Tpl2 promotes efficient virus lytic replication through AP-1-dependent upregulation of RTA expression. In summary, through tandem functional screens, we identified the Tpl2/AP-1 signaling transduction pathway as a positive regulator of MHV-68 lytic replication.

Open reading frame 33 of a gammaherpesvirus encodes a tegument protein essential for virion morphogenesis and egress

Tegument is a unique structure of herpesvirus, which surrounds the capsid and interacts with the envelope. Morphogenesis of gammaherpesvirus is poorly understood due to lack of efficient lytic replication for Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8, which are etiologically associated with several types of human malignancies. Murine gammaherpesvirus 68 (MHV-68) is genetically related to the human gammaherpesviruses and presents an excellent model for studying de novo lytic replication of gammaherpesviruses. MHV-68 open reading frame 33 (ORF33) is conserved among Alpha-, Beta-, and Gammaherpesvirinae subfamilies. However, the specific role of ORF33 in gammaherpesvirus replication has not yet been characterized. We describe here that ORF33 is a true late gene and encodes a tegument protein. By constructing an ORF33-null MHV-68 mutant, we demonstrated that ORF33 is not required for viral DNA replication, early and late gene expression, viral DNA packaging or capsid assembly but is required for virion morphogenesis and egress. Although the ORF33-null virus was deficient in release of infectious virions, partially tegumented capsids produced by the ORF33-null mutant accumulated in the cytoplasm, containing conserved capsid proteins, ORF52 tegument protein, but virtually no ORF45 tegument protein and the 65-kDa glycoprotein B. Finally, we found that the defect of ORF33-null MHV-68 could be rescued by providing ORF33 in trans or in an ORF33-null revertant virus. Taken together, our results indicate that ORF33 is a tegument protein required for viral lytic replication and functions in virion morphogenesis and egress.

Murine gammaherpesvirus 68 infection of IFNgamma unresponsive mice: a small animal model for gammaherpesvirus-associated B-cell lymphoproliferative disease

Gammaherpesviruses are tightly controlled by the host immune response, with gammaherpesvirus-associated malignancies prevalent in immune-suppressed individuals. Previously, infection of IFNgamma-unresponsive mice with gammaherpesvirus 68 (gammaHV68) showed that IFNgamma controlled chronic infection, limiting chronic diseases including arteritis and pulmonary fibrosis. Here, we show that gammaHV68-infected IFNgamma receptor-deficient (IFNgammaR(-/-)) mice uniformly develop angiocentric inflammatory lesions in the lung. Prolonged infection revealed a range of outcomes, from spontaneous regression to pulmonary lymphoma. By 12 months of infection, 80% of mice had lymphoid hyperplasia or pulmonary lymphoma; 45% of infected mice developed frank tumors between 5 and 12 months postinfection, with some mice showing systemic involvement. Lymphomas were composed of B lymphocytes and contained latently infected cells. Although IFNgammaR(-/-) mice control chronic gammaHV68 infection poorly, both early and late pathologies were indistinguishable between wild-type and reactivation-defective virus infection, indicating that, in contrast with other previously described gammaHV68-associated pathologies, these chronic diseases were not dependent on the reactivation of latent infection. This distinct combination of latent infection and defined host defect led to a specific and consistent lymphoproliferative disease. Significantly, this mouse model of virus-associated pulmonary B-cell lymphoma closely mimics the full spectrum of human lymphomatoid granulomatosis, an EBV-associated malignancy with no effective treatment.

Genome-wide identification of binding sites for Kaposi's sarcoma-associated herpesvirus lytic switch protein, RTA

Kaposi's sarcoma-associated herpesvirus (KSHV) replication and transcription activator (RTA) encoded by ORF50 is a lytic switch protein for viral reactivation from latency. The expression of RTA activates the expression of downstream viral genes, and is necessary for triggering the full viral lytic program. Using chromatin immunoprecipitation assay coupled with a KSHV whole-genome tiling microarray (ChIP-on-chip) approach, we identified a set of 19 RTA binding sites in the KSHV genome in a KSHV-infected cell line BCBL-1. These binding sites are located in the regions of promoters, introns, or exons of KSHV genes including ORF8, ORFK4.1, ORFK5, PAN, ORF16, ORF29, ORF45, ORF50, ORFK8, ORFK10.1, ORF59, ORFK12, ORF71/72, ORFK14/ORF74, and ORFK15, the two origins of lytic replication OriLyt-L and OriLyt-R, and the microRNA cluster. We confirmed these RTA binding sites by ChIP and quantitative real-time PCR. We further mapped the RTA binding site in the first intron of the ORFK15 gene, and determined that it is RTA-responsive. The ORFK15 RTA binding sequence TTCCAGGAA TTCCTGGAA consists of a palindromic structure of two tandem repeats, of which each itself is also an imperfect inverted repeat. Reporter assay and electrophoretic mobility shift assay confirmed the binding of the RTA protein to this sequence in vitro. Sequence alignment with other RTA binding sites identified the RTA consensus binding motif as TTCCAGGAT(N)(0-16)TTCCTGGGA. Interestingly, most of the identified RTA binding sites contain only half or part of this RTA binding motif. These results suggest the complexity of RTA binding in vivo, and the involvement of other cellular or viral transcription factors during RTA transactivation of target genes.

RTA promoter demethylation and histone acetylation regulation of murine gammaherpesvirus 68 reactivation

Gammaherpesviruses have a common biological characteristic, latency and lytic replication. The balance between these two phases in murine gammaherpesvirus 68 (MHV-68) is controlled by the replication and transcription activator (RTA) gene. In this report, we investigated the effect of DNA demethylation and histone acetylation on MHV-68 replication. We showed that distinctive methylation patterns were associated with MHV-68 at the RTA promoter during latency or lytic replication. Treatment of MHV-68 latently-infected S11E cells with a DNA methyltransferases (DNMTs) inhibitor 5-azacytidine (5-AzaC), only weakly reactivated MHV-68, despite resulted in demethylation of the viral RTA promoter. In contrast, treatment with a histone deacetylase (HDAC) inhibitor trichostatin A (TSA) strongly reactivated MHV-68 from latency, and this was associated with significant change in histone H3 and H4 acetylation levels at the RTA promoter. We further showed that HDAC3 was recruited to the RTA promoter and inhibited RTA transcription during viral latency. However, TSA treatment caused rapid removal of HDAC3 and also induced passive demethylation at the RTA promoter. In vivo, we found that the RTA promoter was hypomethylated during lytic infection in the lung and that methylation level increased with virus latent infection in the spleen. Collectively, our data showed that histone acetylation, but not DNA demethylation, is sufficient for effective reactivation of MHV-68 from latency in S11E cells.

Two phenylalanines in the C-terminus of Epstein-Barr virus Rta protein reciprocally modulate its DNA binding and transactivation function

The Rta (R transactivator) protein plays an essential role in the Epstein-Barr viral (EBV) lytic cascade. Rta activates viral gene expression by several mechanisms including direct and indirect binding to target viral promoters, synergy with EBV ZEBRA protein, and stimulation of cellular signaling pathways. We previously found that Rta proteins with C-terminal truncations of 30 aa were markedly enhanced in their capacity to bind DNA (Chen, L.W., Chang, P.J., Delecluse, H.J., and Miller, G., (2005). Marked variation in response of consensus binding elements for the Rta protein of Epstein-Barr virus. J. Virol. 79(15), 9635-9650.). Here we show that two phenylalanines (F600 and F605) in the C-terminus of Rta play a crucial role in mediating this DNA binding inhibitory function. Amino acids 555 to 605 of Rta constitute a functional DNA binding inhibitory sequence (DBIS) that markedly decreased DNA binding when transferred to a minimal DNA binding domain of Rta (aa 1-350). Alanine substitution mutants, F600A/F605A, abolished activity of the DBIS. F600 and F605 are located in the transcriptional activation domain of Rta. Alanine substitutions, F600A/F605A, decreased transcriptional activation by Rta protein, whereas aromatic substitutions, such as F600Y/F605Y or F600W/F605W, partially restored transcriptional activation. Full-length Rta protein with F600A/F605A mutations were enhanced in DNA binding compared to wild-type, whereas Rta proteins with F600Y/F605Y or F600W/F605W substitutions were, like wild-type Rta, relatively poor DNA binders. GAL4 (1-147)/Rta (416-605) fusion proteins with F600A/F605A mutations were diminished in transcriptional activation, relative to GAL4/Rta chimeras without such mutations. The results suggest that, in the context of a larger DBIS, F600 and F605 play a role in the reciprocal regulation of DNA binding and transcriptional activation by Rta. Regulation of DNA binding by Rta is likely to be important in controlling its different modes of action.

ORF30 and ORF34 are essential for expression of late genes in murine gammaherpesvirus 68

A hallmark of productive infection by DNA viruses is the coupling of viral late gene expression to genome replication. Here we report the identification of open reading frame 30 (ORF30) and ORF34 as viral trans factors crucial for activating late gene transcription following viral DNA replication during lytic infection of murine gammaherpesvirus 68 (MHV-68). The mutant virus lacking either ORF30 or ORF34 underwent normal DNA replication but failed to express viral late gene transcripts, leading to nonproductive infection. In a reporter assay system, ORF30 and ORF34 were required for MHV-68 to activate the viral late gene promoters. Furthermore, studies using chromatin immunoprecipitation assays showed that the recruitment of RNA polymerase II to the viral late promoters during lytic infection was significantly reduced in the absence of ORF30 or ORF34. Together, the results suggest that ORF30 and ORF34 may play an important role in the assembly of the transcription initiation complex at the late gene promoters. Our discovery of the viral mutants that uncouple late gene transcription from DNA replication lays an important foundation to dissect the mechanism of this critical step of gene expression regulation.

High-resolution functional profiling of a gammaherpesvirus RTA locus in the context of the viral genome

Gammaherpesviruses Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus are associated with multiple human cancers. Our goal was to develop a quantitative, high-throughput functional profiling system to identify viral cis-elements and protein subdomains critical for virus replication in the context of the herpesvirus genome. In gamma-2 herpesviruses, the transactivating factor RTA is essential for initiation of lytic gene expression and viral reactivation. We used the RTA locus as a model to develop the functional profiling approach. The mutant murine gammaherpesvirus 68 viral library, containing 15-bp random insertions in the RTA locus, was passaged in murine fibroblast cells for multiple rounds of selection. The effect of each 15-bp insertion was characterized using fluorescent-PCR profiling. We identified 1,229 insertions in the 3,845-bp RTA locus, of which 393, 282, and 554 were critically impaired, attenuated, and tolerated, respectively, for viral growth. The functional profiling phenotypes were verified by examining several individual RTA mutant clones for transactivating function of the RTA promoter and transcomplementing function of the RTA-null virus. Thus, the profiling approach enabled us to identify several novel functional domains in the RTA locus in the context of the herpesvirus genome. Importantly, our study has demonstrated a novel system to conduct high-density functional genetic mapping. The genome-scale expansion of the genetic profiling approach will expedite the functional genomics research on herpesvirus.

Signaling through Toll-like receptors induces murine gammaherpesvirus 68 reactivation in vivo

Murine gammaherpesvirus 68 (MHV68) establishes a lifelong infection in mice and is used as a model pathogen to study the role of viral and host factors in chronic infection. The maintenance of chronic MHV68 infection, at least in some latency reservoirs, appears to be dependent on the capacity of the virus to reactivate from latency in vivo. However, the signals that lead to MHV68 reactivation in vivo are not well characterized. Toll-like receptors (TLRs), by recognizing the specific patterns of microbial components, play an essential role in the activation of innate immunity. In the present study, we investigated the capacity of TLR ligands to induce MHV68 reactivation, both in vitro and in vivo. The stimulation of latently infected B cell lines with ligands for TLRs 3, 4, 5, and 9 enhanced MHV68 reactivation; the ex vivo stimulation of latently infected primary splenocytes, recovered from infected mice, with poly(I:C), lipopolysaccharide, flagellin, or CpG DNA led to early B-cell activation, B-cell proliferation, and a significant increase in the frequency of latently infected cells reactivating the virus. In vivo TLR stimulation also induced B-cell activation and MHV68 reactivation, resulting in heightened levels of virus replication in the lungs which correlated with an increase in MHV68-specific CD8(+) T-cell responses. Importantly, TLR stimulation also led to an increase in MHV68 latency, as evidenced by an increase in viral genome-positive cells 2 weeks post-in vivo stimulation by specific TLR ligands. Thus, these data demonstrate that TLR stimulation can drive MHV68 reactivation from latency and suggests that periodic pathogen exposure may contribute to the homeostatic maintenance of chronic gammaherpesvirus infection through stimulating virus reactivation and reseeding latency reservoirs.

Persistent gammaherpesvirus replication and dynamic interaction with the host in vivo

Gammaherpesviruses establish life-long persistency inside the host and cause various diseases during their persistent infection. However, the systemic interaction between the virus and host in vivo has not been studied in individual hosts continuously, although such information can be crucial to control the persistent infection of the gammaherpesviruses. For the noninvasive and continuous monitoring of the interaction between gammaherpesvirus and the host, a recombinant murine gammaherpesvirus 68 (MHV-68, a gammaherpesvirus 68) was constructed to express a firefly luciferase gene driven by the viral M3 promoter (M3FL). Real-time monitoring of M3FL infection revealed novel sites of viral replication, such as salivary glands, as well as acute replication in the nose and the lung and progression to the spleen. Continuous monitoring of M3FL infection in individual mice demonstrated the various kinetics of transition to different organs and local clearance, rather than systemically synchronized clearance. Moreover, in vivo spontaneous reactivation of M3FL from latency was detected after the initial clearance of acute infection and can be induced upon treatment with either a proteasome inhibitor Velcade or an immunosuppressant cyclosporine A. Taken together, our results demonstrate that the in vivo replication and reactivation of gammaherpesvirus are dynamically controlled by the locally defined interaction between the virus and the host immune system and that bioluminescence imaging can be successfully used for the real-time monitoring of this dynamic interaction of MHV-68 with its host in vivo.

A replication-defective gammaherpesvirus efficiently establishes long-term latency in macrophages but not in B cells in vivo

Murine gammaherpesvirus 68 (gammaHV68 or MHV68) is genetically related to the human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV), providing a useful system for in vivo studies of the virus-host relationship. To begin to address fundamental questions about the mechanisms of the establishment of gammaherpesvirus latency, we previously generated a replication-defective gammaHV68 lacking the expression of the single-stranded DNA binding protein encoded by orf6. In work presented here, we demonstrate that this mutant virus established a long-term infection in vivo that was molecularly identical to wild-type virus latency. Thus, despite the absence of an acute phase of lytic replication, the mutant virus established a chronic infection in which the viral genome (i) was maintained as an episome and (ii) expressed latency-associated, but not lytic replication-associated, genes. Macrophages purified from mice infected with the replication-defective virus harbored viral genome at a frequency that was nearly identical to that of wild-type gammaHV68; however, the frequency of B cells harboring viral genome was greatly reduced in the absence of lytic replication. Thus, this replication-defective gammaherpesvirus efficiently established in vivo infection in macrophages that was molecularly indistinguishable from wild-type virus latency. These data point to a critical role for lytic replication or reactivation in the establishment or maintenance of latent infection in B cells.

The ORF49 protein of murine gammaherpesvirus 68 cooperates with RTA in regulating virus replication

Our functional mapping study of murine gammaherpesvirus 68 (MHV-68, or gammaHV-68) revealed that a mutant harboring a transposon at the ORF49 locus (ORF49(null)) evidenced a highly attenuated in vitro growth. ORF49 resides adjacent to and in an opposite direction from RTA, the primary switch of the gammaherpesvirus life cycle. A FLAG-tagged ORF49 protein was able to transcomplement ORF49(null), and a revertant of ORF49(null) restored its attenuated growth to a level comparable to that of the wild type. The FLAG-tagged ORF49 protein promoted the ability of RTA to activate downstream target promoters and enhanced virus replication from the ORF50(null) virus in the presence of RTA. Furthermore, ORF49 enhanced wild-type virus replication by increasing the RTA transcript levels. Our data indicate that ORF49 may perform an important function in MHV-68 replication in cooperation with RTA.

Murine gammaherpesvirus 68 ORF52 encodes a tegument protein required for virion morphogenesis in the cytoplasm

The tegument, a semiordered matrix of proteins overlying the nucleocapsid and underlying the virion envelope, in viruses in the gamma subfamily of Herpesviridae is poorly understood. Murine gammaherpesvirus 68 (MHV-68) is a robust model for studying gammaherpesvirus virion structure, assembly, and composition, as MHV-68 efficiently completes the lytic phase and productively infects cultured cells. We have found that MHV-68 ORF52 encodes an abundant tegument protein conserved among gammaherpesviruses. Detergent sensitivity experiments revealed that the MHV-68 ORF52 protein is more tightly bound to the virion nucleocapsid than the ORF45 tegument protein but could be dissociated from particles that retained the ORF65 small capsomer protein. ORF52, tagged with enhanced green fluorescent protein or FLAG epitope, localized to the cytoplasm. A recombinant MHV-68 bacterial artificial chromosome mutant with a nonsense mutation incorporated into ORF52 exhibited viral DNA replication, expression of late lytic genes, and capsid assembly and packaging at levels near those of the wild type. However, the MHV-68 ORF52-null virus was deficient in the assembly and release of infectious virion particles. Instead, partially tegumented capsids produced by the ORF52-null mutant accumulated in the cytoplasm, containing conserved capsid proteins, the ORF64 and ORF67 tegument proteins, but virtually no ORF45 tegument protein. Thus, ORF52 is essential for the tegumentation and egress of infectious MHV-68 particles in the cytoplasm, suggesting an important conserved function in gammaherpesvirus virion morphogenesis.

Systematic identification of cellular signals reactivating Kaposi sarcoma-associated herpesvirus

The herpesvirus life cycle has two distinct phases: latency and lytic replication. The balance between these two phases is critical for viral pathogenesis. It is believed that cellular signals regulate the switch from latency to lytic replication. To systematically evaluate the cellular signals regulating this reactivation process in Kaposi sarcoma-associated herpesvirus, the effects of 26,000 full-length cDNA expression constructs on viral reactivation were individually assessed in primary effusion lymphoma-derived cells that harbor the latent virus. A group of diverse cellular signaling proteins were identified and validated in their effect of inducing viral lytic gene expression from the latent viral genome. The results suggest that multiple cellular signaling pathways can reactivate the virus in a genetically homogeneous cell population. Further analysis revealed that the Raf/MEK/ERK/Ets-1 pathway mediates Ras-induced reactivation. The same pathway also mediates spontaneous reactivation, which sets the first example to our knowledge of a specific cellular pathway being studied in the spontaneous reactivation process. Our study provides a functional genomic approach to systematically identify the cellular signals regulating the herpesvirus life cycle, thus facilitating better understanding of a fundamental issue in virology and identifying novel therapeutic targets.

Control of Rta expression critically determines transcription of viral and cellular genes following gammaherpesvirus infection

The replication and transcriptional activator (Rta), encoded by ORF50 of gammaherpesviruses, initiates the lytic cycle of gene expression; therefore understanding the impact of Rta on viral and cellular gene expression is key to elucidating the transcriptional events governing productive infection and reactivation from latency. To this end, the impact of altering Rta transcription on viral and cellular gene expression was studied in the context of a whole virus infection. Recombinant murine gammaherpesvirus (MHV)-68 engineered to overexpress Rta greatly accelerated expression of specific lytic cycle ORFs, but repressed transcription of the major latency gene, ORF73. Increased expression of Rta accelerated the dysregulation in transcription of specific cellular genes when compared with cells infected with wild-type and revertant viruses. A subset of cellular genes was dysregulated only in cells infected with Rta-overexpressing virus, and never in those infected with non-overexpressing viruses. These data highlight the critical role of Rta abundance in governing viral and cellular gene transcription, and demonstrate the importance of understanding how the relative expression of ORF50 during the virus life cycle impacts on these processes.

Identification of an Rta responsive promoter involved in driving gammaHV68 v-cyclin expression during virus replication

Among the distinguishing characteristics of members of the gamma-2 herpesvirus family is the expression of a mammalian D-type cyclin homolog, termed v-cyclin. Murine gammaherpesvirus 68 (gammaHV68) is a gamma2-herpesvirus that can infect inbred and outbred strains of mice, providing a genetic system for the study of gammaherpesvirus pathogenesis. Disruption of the v-cyclin gene of gammaHV68 results in a virus that establishes latency in infected mice to wild-type levels, but is severely attenuated for virus reactivation [van Dyk, L.F., Virgin IV, H.W., Speck, S.H., 2000. J. Virol. 74:7451-7461]. Transcriptional regulation of the gammaHV68 v-cyclin has not been defined. We report here the initial characterization of the v-cyclin transcript expressed in permissive murine fibroblasts. Based on 5' mapping of the v-cyclin transcript, we identified a promoter that is involved in driving v-cyclin expression during virus replication. In addition, we determined that the promoter is responsive to the major viral lytic transactivator, Rta, encoded by orf 50. Using reporter plasmids we have analyzed both basal and Rta-induced v-cyclin promoter activity, initially identifying two regions of the v-cyclin promoter important for both basal and Rta-induced activity. Notably, only one of these regions could be shown to confer Rta responsiveness on a reporter construct containing the hsp70 TATA box. The importance of this region in regulating v-cyclin expression during virus replication was confirmed by introducing these mutations into the context of the viral genome and assessing v-cyclin expression following infection of permissive murine fibroblasts in tissue culture. In addition, we show that mutations that severely cripple Rta-induction of v-cyclin expression did not adversely impact virus reactivation from splenocytes recovered from latently infected mice, indicating that alternatively regulated v-cyclin gene expression is required for virus reactivation.

Gamma interferon blocks gammaherpesvirus reactivation from latency

Establishment of latent infection and reactivation from latency are critical aspects of herpesvirus infection and pathogenesis. Interfering with either of these steps in the herpesvirus life cycle may offer a novel strategy for controlling herpesvirus infection and associated disease pathogenesis. Prior studies show that mice deficient in gamma interferon (IFN-gamma) or the IFN-gamma receptor have elevated numbers of cells reactivating from murine gammaherpesvirus 68 (gammaHV68) latency, produce infectious virus after the establishment of latency, and develop large-vessel vasculitis. Here, we demonstrate that IFN-gamma is a powerful inhibitor of reactivation of gammaHV68 from latency in tissue culture. In vivo, IFN-gamma controls viral gene expression during latency. Importantly, depletion of IFN-gamma in latently infected mice results in an increased frequency of cells reactivating virus. This demonstrates that IFN-gamma is important for immune surveillance that limits reactivation of gammaHV68 from latency.

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.

The Rta/Orf50 transactivator proteins of the gamma-herpesviridae

The replication and transcription activator protein, Rta, is encoded by Orf50 in Kaposi's sarcoma-associated herpesvirus (KSHV) and other known gammaherpesviruses including Epstein-Barr virus (EBV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS), and murine herpesvirus 68 (MHV-68). Each Rta/Orf50 homologue of each gammaherpesvirus plays a pivotal role in the initiation of viral lytic gene expression and lytic reactivation from latency. Here we discuss the Rta/Orf50 of KSHV in comparison to the Rta/Orf50s of other gammaherpesviruses in an effort to identify structural motifs, mechanisms of action, and modulating host factors.

ORF18 is a transfactor that is essential for late gene transcription of a gammaherpesvirus

Lytic replication of the tumor-associated human gammaherpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus has important implications in pathogenesis and tumorigenesis. Herpesvirus lytic genes have been temporally classified as exhibiting immediate-early (IE), early, and late expression kinetics. Though the regulation of IE and early gene expression has been studied extensively, very little is known regarding the regulation of late gene expression. Late genes, which primarily encode virion structural proteins, require viral DNA replication for their expression. We have identified a murine gammaherpesvirus 68 (MHV-68) early lytic gene, ORF18, essential for viral replication. ORF18 is conserved in both beta- and gammaherpesviruses. By generating an MHV-68 ORF18-null virus, we characterized the stage of the virus lytic cascade that requires the function of ORF18. Gene expression profiling and quantitation of viral DNA synthesis of the ORF18-null virus revealed that the expression of early genes and viral DNA replication were not affected; however, the transcription of late genes was abolished. Hence, we have identified a gammaherpesvirus-encoded factor essential for the expression of late genes independently of viral DNA synthesis.

The porcine lymphotropic herpesvirus 1 encodes functional regulators of gene expression

The porcine lymphotropic herpesviruses (PLHV) are discussed as possible risk factors in xenotransplantation because of the high prevalence of PLHV-1, PLHV-2 and PLHV-3 in pig populations world-wide and the fact that PLHV-1 has been found to be associated with porcine post-transplant lymphoproliferative disease. To provide structural and functional knowledge on the PLHV immediate-early (IE) transactivator genes, the central regions of the PLHV genomes were characterized by genome walking, sequence and splicing analysis. Three spliced genes were identified (ORF50, ORFA6/BZLF1(h), ORF57) encoding putative IE transactivators, homologous to (i) ORF50 and BRLF1/Rta, (ii) K8/K-bZIP and BZLF1/Zta and (iii) ORF57 and BMLF1 of HHV-8 and EBV, respectively. Expressed as myc-tag or HA-tag fusion proteins, they were located to the cellular nucleus. In reporter gene assays, several PLHV-promoters were mainly activated by PLHV-1 ORF50, to a lower level by PLHV-1 ORFA6/BZLF1(h) and not by PLHV-1 ORF57. However, the ORF57-encoded protein acted synergistically on ORF50-mediated activation.

A gamma-herpesvirus deficient in replication establishes chronic infection in vivo and is impervious to restriction by adaptive immune cells

Chronic gamma-herpesvirus infection is a dynamic process involving latent infection, reactivation from latency, and low level persistent replication. The gamma-herpesviruses maintain latent infection in restricted subsets of hematopoietic cells as a result of an intricate balance between host factors that suppress infection and viral factors that facilitate evasion of the immune response. Immune effectors limit reactivation and subsequent replication events, and the adaptive immune response ultimately restricts infection to a level compatible with life-long infection. However, it has not been possible to determine whether the immune system constrains chronic infection by directly targeting latently infected cells in vivo due to the complex nature of chronic infection. To begin to address this issue, we generated a murine gamma-herpesvirus 68 (gammaHV68) deficient in its ability to replicate or undergo reactivation from latency via a mutation in the single-stranded DNA binding protein encoded by ORF6. Even in the absence of lytic replication, this virus established long-term infection in peritoneal cells of wild-type mice at levels identical to that of wild-type gammaHV68, and generated an immune response that was sufficient to protect against secondary challenge with wild-type gammaHV68. Nevertheless, the number of latently infected cells was not significantly altered in mice deficient in T cells or both T cells and B cells, demonstrating that the adaptive immune system is incapable of altering infection with a virus lacking the capacity for lytic replication and reactivation from latency. Thus, these data support the conclusion that latency is immunologically silent.

Characterization of Kaposi's sarcoma-associated herpesvirus (KSHV) K1 promoter activation by Rta

The K1 gene of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a 46-kDa transmembrane glycoprotein that possesses transforming properties, initiates signaling pathways in B cells, and prevents apoptosis. Here, we demonstrate a mechanism for activation of the K1 promoter by the Rta transactivator. Electrophoretic mobility shift assay (EMSA) analysis of the K1 promoter demonstrated that purified Rta protein bound to the K1 promoter at three locations, independent of other DNA-binding factors. Transcriptional assays revealed that only two of these Rta DNA-binding sites are functionally significant, and that they could impart Rta responsiveness to a heterologous E4 TATA box promoter. In addition, TATA-binding protein (TBP) bound to a TATA box element located 25 bp upstream of the K1 transcription start site and was also shown to associate with Rta by coimmunoprecipitation analysis. Rta transactivation may therefore be mediated in part through recruitment of TBP to target promoters.

A gammaherpesvirus 68 gene 50 null mutant establishes long-term latency in the lung but fails to vaccinate against a wild-type virus challenge

The gammaherpesvirus immediate-early genes are critical regulators of virus replication and reactivation from latency. Rta, encoded by gene 50, serves as the major transactivator of the lytic program and is highly conserved among all the gammaherpesviruses, including Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and murine gammaherpesvirus 68 (gammaHV68). Introduction of a translation stop codon in gammaHV68 gene 50 (gene 50.stop gammaHV68) demonstrated that Rta is essential for virus replication in vitro. To investigate the role that virus replication plays in the establishment and maintenance of latency, we infected mice with gene 50.stop gammaHV68. Notably, the gene 50.stop virus established a long-term infection in lung B cells following intranasal infection of mice but was unable to establish latency in the spleen. This complete block in the establishment of latency in the spleen was also seen when lytic virus production was inhibited by treating mice infected with wild-type virus with the antiviral drug cidofovir, implicating virus replication and not an independent function of Rta in the establishment of splenic latency. Furthermore, we showed that gene 50.stop gammaHV68 was unable to prime the immune system and was unable to protect against a challenge with wild-type gammaHV68, despite its ability to chronically infect lung B cells. These data indicate gammaherpesviruses that are unable to undergo lytic replication in vivo may not be viable vaccine candidates despite the detection of cells harboring viral genome at late times postinfection.

Immune mechanisms in murine gammaherpesvirus-68 infection

The murine gamma-herpesvirus-68 (MHV-68) is a relative of the Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) that infects mice. All these gamma-herpesviruses are subject to immune control, but limit the impact of this control through immune evasion. Molecular evasion mechanisms have been described in abundance. However, we can only speculate what EBV and KSHV immune evasion contributes to the viral lifecycle. With MHV-68, we can analyze in vivo the contribution of immunological and virological gene expression to pathogenesis. While the physiology of infection seems quite well conserved between these viruses, the pathologies associated with immune suppression are obviously very different. MHV-68 is therefore more suited to uncovering the basic biology of gamma-herpesvirus infection than to testing disease interventions. Nevertheless, it may make some useful predictions about effective strategies of vaccination and infection control. This review aims to outline our current state of knowledge and to highlight some limitations of the MHV-68 model as it stands, in the hope of stimulating constructive progress.

The Herpesvirus saimiri replication and transcription activator acts synergistically with CCAAT enhancer binding protein alpha to activate the DNA polymerase promoter

The open reading frame (ORF) 50 gene product, also known as the replication and transcription activator (Rta), is an immediate-early gene which is well conserved among all gamma-2 herpesviruses and plays a pivotal role in regulating the latent-lytic switch. Herpesvirus saimiri (HVS) ORF 50a functions as a sequence-specific transactivator capable of activating delayed-early (DE) gene expression via binding directly to an ORF 50 response element (RE) within the respective promoter. Analysis of the ORF 50 REs have identified two distinct types within HVS gene promoters. The first comprises a consensus sequence motif, CCN(9)GG, the second an AT-rich sequence. Here we demonstrate that ORF 50a is capable of transactivating the DE ORF 9 promoter which encodes the DNA polymerase. Deletion analysis of the ORF 9 promoter mapped the ORF 50 RE to a 95-bp region situated 126 bp upstream of the initiation codon. Gel retardation analysis further mapped the RE to a 28-bp fragment, which was able to confer ORF 50 responsiveness on an enhancerless simian virus 40 minimal promoter. Furthermore, sequence analysis identified multiple CCAAT enhancer binding protein alpha (C/EBPalpha) binding sites within the ORF 9 promoter and specifically two within the close vicinity of the AT-rich ORF 50 RE. Analysis demonstrated that the HVS ORF 50a and C/EBPalpha proteins associate with the ORF 9 promoter in vivo, interact directly, and synergistically activate the ORF 9 promoter by binding to adjacent binding motifs. Overall, these data suggest a cooperative interaction between HVS ORF 50a and C/EBPalpha proteins to activate the DNA polymerase promoter during early stages of the lytic replication cycle.

Marked variation in response of consensus binding elements for the Rta protein of Epstein-Barr virus

The R transactivator (Rta) protein activates Epstein-Barr virus (EBV) lytic-cycle genes by several distinct mechanisms that include direct binding to viral promoters, synergy with BamHI Z EBV replication activator (ZEBRA), and activation of cellular signaling pathways. In the direct and synergistic mechanisms of action, Rta binds to specific DNA sequences that are present in the promoters of responsive genes. It has been difficult to demonstrate the capacity of Rta expressed in mammalian cells to bind DNA in vitro in order to study the relative affinities of Rta binding elements. We discovered that a short C-terminal region of Rta inhibits the ability of Rta to bind DNA in vitro. C-terminally truncated versions of Rta bind DNA efficiently and thus facilitate a comparison of consensus Rta binding elements (CRBEs) found in promoters of five Rta-responsive genes: BMLF1, BHLF1, BMRF1, BaRF1, and BLRF2. All CRBEs in the promoters of the five genes conform to the proposed recognition sequence GNCCN9GGNG, where N is any nucleotide and N9 represents a sequence of nine nucleotides. Nonetheless, CRBEs varied markedly in their abilities to bind Rta in electrophoretic mobility shift assays. Not all CRBEs bound or responded to Rta. Binding affinities of the CRBEs and the capacity to be activated by Rta in reporter assays were strongly correlated. The CRBEs from the BMLF1 and BHLF1 promoters conferred the greatest response. The response of the BMRF1, BaRF1, and BLRF2 CRBEs was less robust. By creation of chimeras, inversions, and point mutations, differences in binding affinities and transcriptional activation levels could be attributed to N9 sequence variation. The length of N9 was also critical for a maximal response. In Raji and BZLF1-knockout cells, the mRNAs of the five Rta-responsive lytic-cycle genes differed dramatically in kinetics of expression, abundance, and synergistic responses to ZEBRA and Rta. Affinities of Rta response elements for Rta are likely to play an important role in temporal regulation and the level of lytic-cycle EBV gene expression.

Murine herpesvirus pathogenesis: a model for the analysis of molecular mechanisms of human gamma herpesvirus infections

Murine herpes virus (MHV), a natural pathogen originally isolated from free-living rodents, constitutes the most amenable animal model for human gamma herpesviruses. Based on DNA sequence homology, this virus was classified as Murid Herpesvirus 4 to subfamily Gammaherpesvirinae. Pilot studies in our laboratory, using mice inoculated by the intranasal route, showed that MHV infects macrophages, B lymphocytes, lung alveolar as well as endothelial cells. From the lungs the virus spreads via the bloodstream to spleen and bone marrow and via the lymphatics to the mediastinal lymph nodes. Similarly to other gamma herpesviruses, MHV established life-long latency maintained in host B lymphocytes and macrophages. An IM-like syndrome (per analogy to EBV) may develop during acute MHV infection, in which the atypical T/CD8+ lymphocytes eliminate viral DNA carrying B cells expressing the M2 latency associated protein. During latency, the MHV LANA (a KSHV LANA homologue) maintains the latent viral genome, assuring its copying and partition to new carrier cells in the course of division of the maternal cell. The nonproductive latency is turned onto virus replication by means of Rta protein. The chronic lymphoproliferative syndrome of unclear pathogenesis, which occurs in a certain part of latent MHV carriers, is related to the expression of gamma herpesvirus common latency-associated genes such as v-cyclin and/or to that of a virus-specific (M11/bcl-2) gene. This review attempts to summarize our knowledge concerning the function of MHV genes (either gamma herpesvirus common or MHV specific) related to immune evasion, latency and lymphoproliferation when highlighting the unsolved problems and/or controversial opinions.

Murine gammaherpesvirus 68 open reading frame 45 plays an essential role during the immediate-early phase of viral replication

Murine gammaherpesvirus 68 (MHV-68) has been developed as a model for the human gammaherpesviruses Epstein-Barr virus and human herpesvirus 8/Kaposi's sarcoma-associated herpesvirus (HHV-8/KSHV), which are associated with several types of human diseases. Open reading frame 45 (ORF45) is conserved among the members of the Gammaherpesvirinae subfamily and has been suggested to be a virion tegument protein. The repression of ORF45 expression by small interfering RNAs inhibits MHV-68 viral replication. However, the gene product of MHV-68 ORF45 and its function have not yet been well characterized. In this report, we show that MHV-68 ORF45 is a phosphorylated nuclear protein. We constructed an ORF45-null MHV-68 mutant virus (45STOP) by the insertion of translation termination codons into the portion of the gene encoding the N terminus of ORF45. We demonstrated that the ORF45 protein is essential for viral gene expression immediately after the viral genome enters the nucleus. These defects in viral replication were rescued by providing ORF45 in trans or in an ORF45-null revertant (45STOP.R) virus. Using a transcomplementation assay, we showed that the function of ORF45 in viral replication is conserved with that of its KSHV homologue. Finally, we found that the C-terminal 23 amino acids that are highly conserved among the Gammaherpesvirinae subfamily are critical for the function of ORF45 in viral replication.

Ex vivo stimulation of B cells latently infected with gammaherpesvirus 68 triggers reactivation from latency

Murine gammaherpesvirus 68 (gammaHV68) infection of mice results in the establishment of a chronic infection, which is largely maintained through latent infection of B lymphocytes. Acute virus replication is almost entirely cleared by 2 weeks postinfection. Spontaneous reactivation of gammaHV68 from latently infected splenocytes upon ex vivo culture can readily be detected at the early stages of infection (e.g., day 16). However, by 6 weeks postinfection, very little spontaneous reactivation is detected upon explant into tissue culture. Here we report that stimulation of latently infected splenic B cells harvested at late times postinfection with cross-linking surface immunoglobulin (Ig), in conjunction with anti-CD40 antibody treatment, triggers virus reactivation. As expected, this treatment resulted in B-cell activation, as assessed by upregulation of CD69 on B cells, and ultimately B-cell proliferation. Since anti-Ig/anti-CD40 stimulation resulted in splenic B-cell proliferation, we assessed whether this reactivation stimulus could overcome the previously characterized defect in virus reactivation of a v-cyclin null gammaHV68 mutant. This analysis demonstrated that anti-Ig/anti-CD40 stimulation could drive reactivation of the v-cyclin null mutant virus in latently infected splenocytes, but not to the levels observed with wild-type gammaHV68. Thus, there appears to be a role for the v-cyclin in B cells following anti-Ig/anti-CD40 stimulation independent of the induction of the cell cycle. Finally, to assess signals that are not mediated through the B-cell receptor, we demonstrate that addition of lipopolysaccharide to explanted splenocyte cultures also enhanced virus reactivation. These studies complement and extend previous analyses of Epstein-Barr virus and Kaposi's sarcoma-associated virus reactivation from latently infected cell lines by investigating reactivation of gammaHV68 from latently infected primary B cells recovered from infected hosts.

Murine gammaherpesvirus 68 Rta-dependent activation of the gene 57 promoter

The Rta homolog encoded by murine gammaherpesvirus 68 (gammaHV68) gene 50 is essential for virus replication and is capable of driving virus reactivation from the S11 latently infected B lymphoma cell line. Here we characterize Rta activation of gammaHV68 gene 57, which is abundantly transcribed during the early phase of virus replication. Infection of murine fibroblasts with an Rta null virus demonstrated that transcription of gene 57 is dependent on Rta expression. Analysis of the gene 57 promoter identified 2 distinct regions that are Rta responsive, either in the context of the gene 57 promoter or when cloned upstream of a heterologous promoter. Sequence analysis of these regions revealed homology to known Rta-responsive cis-elements in the closely related Kaposi's sarcoma-associated viral (KSHV) genome. In addition, two candidate binding sites for the cellular transcription factor RBP-Jkappa/CBF1 were also identified in one of the Rta-responsive regions, which may play a role in mediating Rta transactivation similar to that observed in some KSHV Rta-responsive genes. Overall, analysis of the gammaHV68 gene 57 promoter suggests that mechanisms of Rta activation are conserved among gamma2-herpesviruses.

Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 RTA reactivates murine gammaherpesvirus 68 from latency

Murine gammaherpesvirus 68 (MHV-68), Kaposi's sarcoma-associated herpesvirus (HHV-8), and Epstein-Barr virus (EBV) are all members of the gammaherpesvirus family, characterized by their ability to establish latency in lymphocytes. The RTA protein, conserved in all gammaherpesviruses, is known to play a critical role in reactivation from latency. Here we report that HHV-8 RTA, not EBV RTA, was able to induce MHV-68 lytic viral proteins and DNA replication and processing and produce viable MHV-68 virions from latently infected cells at levels similar to those for MHV-68 RTA. HHV-8 RTA was also able to activate two MHV-68 lytic promoters, whereas EBV RTA was not. In order to define the domains of RTA responsible for their functional differences in viral promoter activation and initiation of the MHV-68 lytic cycle, chimeric RTA proteins were constructed by exchanging the N-terminal and C-terminal domains of the RTA proteins. Our data suggest that the species specificity of MHV-68 RTA resides in the N-terminal DNA binding domain.

Inhibiting primary effusion lymphoma by lentiviral vectors encoding short hairpin RNA

We use lentiviral-delivered RNA interference (RNAi) to inhibit the growth of a model of primary effusion lymphoma (PEL) in vitro and in vivo. RNAi is a phenomenon allowing the sequence-specific targeting and silencing of exogenous and endogenous gene expression and is being applied to inhibit viral replication both in vitro and in vivo. We show that silencing of genes believed to be essential for the Kaposi sarcoma-associated herpesvirus (KSHV) latent life cycle (the oncogenic cluster) has a varied effect in PEL cell lines cultured in vitro, however, concomitant silencing of the viral cyclin (vcyclin) and viral FLICE (Fas-associating protein with death domain-like interleukin-1β-converting enzyme) inhibitory protein (vFLIP) caused efficient apoptosis in all PEL lines tested. We demonstrate that in a murine model of PEL, lentiviral-mediated RNA interference both inhibits development of ascites and can act as a treatment for established ascites. We also show that the administered lentiviral vectors are essentially limited to the peritoneal cavity, which has advantages for safety and dosage in a therapeutic setting. This shows the use of lentiviral-mediated RNA interference in vivo as a potential therapeutic against a virally driven human cancer.

The herpesvirus saimiri Rta gene autostimulates via binding to a non-consensus response element

Herpesvirus saimiri ORF 50a protein expression is sufficient to reactivate the entire lytic-replication cycle. ORF 50a functions as a sequence-specific transactivator that is capable of activating delayed-early gene expression via direct binding to an ORF 50 response element (RE) within the respective promoter. Here, it is shown that ORF 50a is capable of transactivating its own promoter. Deletion analysis of the ORF 50a promoter showed that the ORF 50-responsive element is contained within an 80 bp fragment, situated 293–373 bp from the transcription initiation site. Gel-retardation analysis further mapped the RE to a 34 bp fragment that was able to confer ORF 50 responsiveness to an enhancerless SV40 minimal promoter. Sequence analysis showed that this RE has no direct similarity to previously identified ORF 50 REs. Therefore, it is concluded that ORF 50a is capable of stimulating its own promoter via a novel RE.

The DNA architectural protein HMGB1 facilitates RTA-mediated viral gene expression in gamma-2 herpesviruses

Replication and transcription activator (RTA), an immediate-early gene product of gamma-2 herpesviruses including Kaposi's sarcoma-associated herpesvirus (KSHV) and murine gamma herpesvirus 68 (MHV-68), plays a critical role in controlling the viral life cycle. RTA acts as a strong transcription activator for several downstream genes of KSHV and MHV-68 through direct DNA binding, as well as via indirect mechanisms. HMGB1 (also called HMG-1) protein is a highly conserved nonhistone chromatin protein with the ability to bind and bend DNA. HMGB1 protein promoted RTA binding to different RTA target sites in vitro, with greater enhancement to low-affinity sites than to high-affinity sites. Box A or box B and homologues of HMGB1 also enhanced RTA binding to DNA. Transient transfection of HMGB1 stimulated RTA transactivation of RTA-responsive promoters from KSHV and MHV-68. Furthermore, MHV-68 viral gene expression, as well as viral replication, was significantly reduced in HMGB1-deficient cells than in the wild type. This abated viral gene expression was partially restored by HMGB1 transfection into HMGB1(-/-) cells. These results suggest an important function of the DNA architectural protein, HMGB1, in RTA-mediated gene expression, as well as viral replication in gamma-2 herpesviruses.

The M2 gene product of murine gammaherpesvirus 68 is required for efficient colonization of splenic follicles but is not necessary for expansion of latently infected germinal centre B cells

Infection of mice with murine gammaherpesvirus 68 is characterized by a marked transient expansion of latently infected splenic germinal centre (GC) B cells, which is followed by lower levels of persistent infection in GC and memory B cells. Virus transcription within GC B cells is restricted to a number of latency-associated open reading frames, including M2. This gene encodes a structurally unique protein of unknown function, which has been shown to be essential for the transient peak of virus latency during the establishment of latent infection in the spleen. This study shows that upon infection of mice with M2-defective viruses, at 14 days post-infection during the establishment of latency in the spleen, there was a reduction in the number of latently infected follicles when compared with wild-type virus. However, the mean number of latently infected cells within each follicle was equivalent between wild-type and M2-defective viruses. Late in infection, disruption of M2 resulted in sustained and abnormally high levels of virus persistence in splenic GC B cells but not memory B cells. These data indicate that during the establishment of latency in the spleen, the M2 gene product is required for efficient colonization of splenic follicles but is dispensable for the expansion of latently infected GC B cells and that M2 might be a critical modulator of B-cell function.

Generation of a latency-deficient gammaherpesvirus that is protective against secondary infection

Kaposi's sarcoma-associated herpesvirus and murine gammaherpesvirus-68 (MHV-68) establish latent infections and are associated with various types of malignancies. They are members of the gamma-2 herpesvirus subfamily and encode a replication and transcriptional activator, RTA, which is necessary and sufficient to disrupt latency and initiate the viral lytic cycle in vitro. We have constructed a recombinant MHV-68 virus that overexpresses RTA. This virus has faster replication kinetics in vitro and in vivo, is deficient in establishing latency, exhibits a reduction in the development of a mononucleosis-like disease in mice, and can protect mice against challenge by wild-type MHV-68. The present study, by using MHV-68 as an in vivo model system, demonstrated that RTA plays a critical role in the control of viral latency and suggests that latency is a determinant of viral pathogenesis in vivo.

Identification of cis sequences required for lytic DNA replication and packaging of murine gammaherpesvirus 68

Human gammaherpesviruses are associated with lymphomas and other malignancies. Murine gammaherpesvirus 68 (MHV-68) infection of mice has emerged as a model for understanding gammaherpesvirus pathogenesis in vivo. In contrast to human gammaherpesviruses, MHV-68 replicates in permissive cell lines in a robust manner, presenting an efficient model to study the basic mechanisms for DNA replication and recombination processes. In addition, MHV-68 also infects a broad range of cells of different tissue types and from different host species, and the viral genome persists as an episome in infected cells. These features make MHV-68 an attractive system on which to build gene delivery vectors. We have therefore undertaken a study to identify the cis elements required for MHV-68 genome replication and packaging. Here we report that an 8.4-kb MHV-68 genomic fragment between ORF66 and ORF73 conferred on the plasmid the ability to replicate; replication required the presence of either de novo viral infection or viral reactivation from latency. We further mapped the origin of lytic replication (oriLyt) to a 1.25-kb region. Moreover, we demonstrated that the terminal repeat of the viral genome is sufficient for packaging of the replicated oriLyt plasmid into mature viral particles. Functional identification of the MHV-68 oriLyt and packaging signal has laid a foundation for investigating the mechanisms controlling gammaherpesvirus DNA replication during the viral lytic phase and will also serve as a base on which to design gene delivery vectors.

PIAS1 selectively inhibits interferon-inducible genes and is important in innate immunity

Interferon (IFN) activates the signal transducer and activator of transcription (STAT) pathway to regulate immune responses. The protein inhibitor of activated STAT (PIAS) family has been suggested to negatively regulate STAT signaling. To understand the physiological function of PIAS1, we generated Pias1(-/-) mice. Using PIAS1-deficient cells, we show that PIAS1 selectively regulates a subset of IFN-gamma- or IFN-beta-inducible genes by interfering with the recruitment of STAT1 to the gene promoter. The antiviral activity of IFN-gamma or IFN-beta was consistently enhanced by Pias1 disruption. Pias1(-/-) mice showed increased protection against pathogenic infection. Our data indicate that PIAS1 is a physiologically important negative regulator of STAT1 and suggest that PIAS1 is critical for the IFN-gamma- or IFN-beta-mediated innate immune responses.

Murine gammaherpesvirus 68 open reading frame 31 is required for viral replication

Murine gammaherpesvirus 68 (MHV-68) is genetically related to the human gammaherpesviruses, Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8) and Epstein-Barr virus (EBV). It has been proposed as a model for gammaherpesvirus infection and pathogenesis. Open reading frame 31 (ORF31) is conserved among the Beta- and Gammaherpesvirinae subfamily, and there is no known mammalian homologue of this protein. The function of MHV-68 ORF31 and its viral homologues has not yet been determined. We described here a primary characterization of this protein and its requirement for lytic replication. The native MHV-68 ORF31 was detected at peak levels by 24 h postinfection, and the FLAG-tagged and green fluorescent protein fusion ORF31 were localized in the cytoplasm and nucleus in a diffuse pattern. Two independent experimental approaches were then utilized to demonstrate that ORF31 was required for lytic replication. First, small interfering RNA generated against ORF31 expression blocked protein expression and virus production in transfected cells. Then, two-independent bacterial artificial chromosome-derived ORF31-null MHV-68 mutants (31STOP) were generated and found to be defective in virus production in fibroblast cells. This defect can be rescued in trans by MHV-68 ORF31 and importantly by its KSHV homologue. A repair virus of 31STOP was also generated by homologous recombination in fibroblast cells. Finally, we showed that the defect in ORF31 blocked late lytic protein expression. Our results demonstrate that MHV-68 ORF31 is required for viral lytic replication, and its function is conserved in its KSHV homologue.

Comparison of the Rta/Orf50 transactivator proteins of gamma-2-herpesviruses

The viral immediate-early transactivator Rta/Orf50 is necessary and sufficient to initiate Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 (KSHV/HHV-8) reactivation from latently infected cells. Since Rta/Orf50 is conserved among all known gamma-2-herpesviruses, we investigated whether the murine gamma-68-herpesvirus (MHV-68) and rhesus monkey rhadinovirus (RRV) homologs can functionally substitute for KSHV Rta/Orf50. (i) Our comparison of 12 KSHV promoters showed that most responded to all three Rta/Orf50proteins, but three promoters (vGPCR, K8, and gB) responded only to the KSHV Rta/Orf50 transactivator. Overall, the activation of KSHV promoters was higher with KSHV Rta than with the RRV and MHV-68 Rta. (ii) Only the primate Rta/Orf50 homologs were able to interfere with human p53-depedent transcriptional activation. (iii) Transcriptional profiling showed that the KSHV Rta/Orf50 was more efficient than it's homologs in inducing KSHV lytic transcription from the latent state. These results suggest that the core functionality of Rta/Orf50 is conserved and independent of its host, but the human protein has evolved additional, human-specific capabilities.

Activation of bovine herpesvirus 4 lytic replication in a non-permissive cell line by overexpression of BoHV-4 immediate early (IE) 2 gene

Bovine herpesvirus 4 (BoHV-4) is a gammaherpesvirus with no clear disease association, it establishes persistent infections in its natural host, the bovine, and in an experimental host, the rabbit. BoHV-4 immediate early 2 (IE2) RNA is the less abundant, spliced, 1.8 kb RNA. The predicted amino acid sequence, of the IE2 protein, reveals that it could encode a 61 kDa protein with amino acid sequence homology to the Epstein-Barr virus (EBV) transactivator R protein and its homologues including, herpesvirus saimiri (HVS), equine herpesvirus 2 (EHV-2), murine herpesvirus 68 and Kaposi's sarcoma-associated herpesvirus (KSHV). We examined recently the interaction of BoHV-4 with a human rhabdomyosarcoma cell line, RD-4, and found that although some infectious viruses can be produced, no cytopathic effect (CPE) was observed [J. Gen. Virol. 81 (2000) 1807]. Because IE2 could play a critical role in BoHV-4 productive infection and its overexpression in RD-4 cells could switch the non-permissive RD-4 status to a permissive one. RD-4 cells expressing stably BoHV-4 IE2 gene were generated. BoHV-4 IE2 induced an increased production of infectious viral particles sufficient to obtain an apparent cytopathic effect. It is concluded that BoHV-4 IE2 is a key factor in determining the outcome of BoHV-4 infection.

Protection against wild-type murine gammaherpesvirus-68 latency by a latency-deficient mutant

A murine gammaherpesvirus-68 (MHV-68) mutant with deregulated transcription of its ORF50 transactivator was severely impaired in latency establishment. The deregulated virus showed reduced immunogenicity, probably reflecting a lower antigen load. However, it still elicited effective immunity to a subsequent wild-type (WT) virus challenge. Infection was not completely prevented, but was very substantially reduced in extent and the long-term level of WT viral DNA in lungs and spleens remained low. Thus latency-deficient MHV-68 illustrates a possible general approach to creating attenuated gammaherpesvirus vaccines that can protect against pathogenic WT infections.

Forced lytic replication impairs host colonization by a latency-deficient mutant of murine gammaherpesvirus-68

A regulated switch between latent and lytic gene expression is common to all known herpesviruses. However, the effects on host colonization of altering this switch are largely unknown. We deregulated the transcription of the gene encoding the major lytic transactivator of murine gammaherpesvirus-68, ORF50, by inserting a new and powerful promoter element in its 5' untranslated region. In vitro, the mutant virus (M50) transcribed ORF50 at a high level and showed more rapid lytic spread in permissive fibroblast cultures, but in vivo, the M50 virus showed a severe deficit in latency establishment, with no sign of the infectious mononucleosis-like illness normally associated with wild-type infection. Although a low level of M50 viral DNA was detectable by PCR in spleens, replication-competent virus could not be recovered beyond 10 days post-infection. The M50 virus was also attenuated in immunocompromised mice. Thus a gammaherpesvirus unable to shut off lytic cycle gene expression showed severely restricted host colonization.

An internal ribosome entry site directs translation of the murine gammaherpesvirus 68 MK3 open reading frame

The gammaherpesviruses characteristically drive the proliferation of latently infected lymphocytes. The murine gammaherpesvirus 68 (MHV-68) MK3 protein contributes to this process in vivo by evading CD8(+)-T-cell recognition during latency, as well as during lytic infection. We analyzed some of the molecular mechanisms that control MK3 expression. No dedicated MK3 mRNA was detected. Instead, the MK3 open reading frame (ORF) was transcribed as part of a bicistronic mRNA, downstream of a previously unidentified ORF, 13M. The 13M/MK3 promoter appeared to extend approximately 1 kb 5' of the transcription start site and included elements both dependent on and independent of the ORF50 lytic transactivator. MK3 was translated from the bicistronic transcript by virtue of an internal ribosome entry site (IRES) element. RNA structure mapping identified two stem-loops between 13M and MK3 that were sufficient for IRES activity in a bicistronic reporter plasmid and a third stem-loop just within the MK3 coding sequence, with a subtler, perhaps regulatory role. Overall, translation of the MHV-68 MK3 bore a striking resemblance to that of the Kaposi's sarcoma-associated herpesvirus vFLIP, suggesting that IRES elements are a common theme of latent gammaherpesvirus immune evasion in proliferating cells.

COX-2 induction during murine gammaherpesvirus 68 infection leads to enhancement of viral gene expression

The murine gammaherpesvirus 68 (MHV-68 or gammaHV-68) model provides many advantages for studying virus-host interactions involved in gammaherpesvirus replication, including the role of cellular responses to infection. We examined the effects of cellular cyclooxygenase-2 (COX-2) and its by-product prostaglandin E(2) (PGE(2)) on MHV-68 gene expression and protein production following de novo infection of cultured cells. Western blot analyses revealed an induction of COX-2 protein in MHV-68-infected cells but not in cells infected with UV-irradiated MHV-68. Luciferase reporter assays demonstrated activation of the COX-2 promoter during MHV-68 replication. Two nonsteroidal anti-inflammatory drugs, a COX-2-specific inhibitor (NS-398) and a COX-1-COX-2 inhibitor (indomethacin), substantially reduced MHV-68 protein production in infected cells. Inhibition of viral protein expression and virion production by NS-398 was reversed in the presence of exogenous PGE(2). Global gene expression analysis using an MHV-68 DNA array showed that PGE(2) increased production of multiple viral gene products, and NS-398 inhibited production of many of the same genes. These studies suggest that COX-2 activity and PGE(2) production may play significant roles during MHV-68 de novo infection.

Poly(ADP-ribose) polymerase 1 and Ste20-like kinase hKFC act as transcriptional repressors for gamma-2 herpesvirus lytic replication

The replication and transcription activator (RTA) of gamma-2 herpesvirus is sufficient to drive the entire virus lytic cycle. Hence, the control of RTA activity should play an important role in the maintenance of viral latency. Here, we demonstrate that cellular poly(ADP-ribose) polymerase 1 (PARP-1) and Ste20-like kinase hKFC interact with the serine/threonine-rich region of gamma-2 herpesvirus RTA and that these interactions efficiently transfer poly(ADP-ribose) and phosphate units to RTA. Consequently, these modifications strongly repressed RTA-mediated transcriptional activation by inhibiting its recruitment onto the promoters of virus lytic genes. Conversely, the genetic ablation of PARP-1 and hKFC interaction or the knockout of the PARP-1 gene and activity considerably enhanced gamma-2 herpesvirus lytic replication. Thus, this is the first demonstration that cellular PARP-1 and hKFC act as molecular sensors to regulate RTA activity and thereby, herpesvirus latency.

The genome of herpesvirus saimiri C488 which is capable of transforming human T cells

Herpesvirus saimiri (HVS), the rhadinovirus prototype, is apathogenic in the persistently infected natural host, the squirrel monkey, but causes acute T cell leukemia in other New World primate species. In contrast to subgroups A and B, only strains of HVS subgroup C such as C488 are capable of transforming primary human T cells to stable antigen-independent growth in culture. Here, we report the complete 155-kb genome sequence of the transformation-competent HVS strain C488. The A+T-rich unique L-DNA of 113,027 bp encodes at least 77 open reading frames and 5 URNAs. In addition to the viral oncogenes stp and tip, only a few genes including the transactivator orf50 and the glycoprotein orf51 are highly divergent. In a series of new primary HVS isolates, the subgroup-specific divergence of the orf50/orf51 alleles was studied. In these new isolates, the orf50/orf51 alleles of the respective subgroup segregate with the stp and/or tip oncogene alleles, which are essential for transformation.