The terminal repeats and latency-associated nuclear antigen of herpesvirus saimiri are essential for episomal persistence of the viral genome

Self-Repairing Herpesvirus Saimiri Deletion Variants

Herpesvirus saimiri (HVS) is discussed as a possible vector in gene therapy. In order to create a self-repairing HVS vector, the F plasmid vector moiety of the bacterial artificial chromosome (BAC) was transposed via Red recombination into the virus genes ORF22 or ORF29b, both important for virus replication. Repetitive sequences were additionally inserted, allowing the removal of the F-derived sequences from the viral DNA genome upon reconstitution in permissive epithelial cells. Moreover, these self-repair-enabled BACs were used to generate deletion variants of the transforming strain C488 in order to minimalize the virus genome. Using the en passant mutagenesis with two subsequent homologous recombination steps, the BAC was seamlessly manipulated. To ensure the replication capacity in permissive monkey cells, replication kinetics for all generated virus variants were documented. HVS variants with increased insert capacity reached the self-repair within two to three passages in permissive epithelial cells. The seamless deletion of ORFs 3/21, 12–14, 16 or 71 did not abolish replication competence. Apoptosis induction did not seem to be altered in human T cells transformed with deletion variants lacking ORF16 or ORF71. These virus variants form an important step towards creating a potential minimal virus vector for gene therapy, for example, in human T cells.

Engineering a recombinant Herpesvirus saimiri strain by co-culturing transfected and permissive cells

Recombinant herpesviruses can be used as oncolytic therapeutic agents and high packaging capacity vectors for delivering expression cassettes into the cell. Herpesvirus saimiri is a gamma-herpesvirus that normally infects squirrel monkeys but also has a unique ability to infect and immortalize human lymphocytes while allowing them to retain their mature phenotype and functional activity. Recombination of the Herpesvirus saimiri genome in permissive cells is impeded by its resistance to chemical transfection and electroporation. The aim of this study was to develop an effective method for incorporating expression cassettes into the genome of Herpesvirus saimiri without having to transfect a permissive cell culture. Transfected HEK-293T cells expressing glycoproteins of the measles virus vaccine strain were co-cultured with permissive OMK cells infected with Herpesvirus saimiri. Cell fusion and formation of syncytia stimulated recombination between the viral genome and the expression cassette; this allowed us to obtain a recombinant Herpesvirus saimiri variant without chemical transfection in permissive cells. The genetically modified virus expressed a selectable marker and retained its ability to persist in the cell in the latent state; it also caused immortalization of primary lymphoid cells. The proposed approach allows engineering recombinant Herpesvirus saimiri strains carrying a variety of expression cassettes in its genome.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

Potential of herpesvirus saimiri-based vectors to reprogram a somatic Ewing's sarcoma family tumor cell line

Herpesvirus saimiri (HVS) infects a range of human cell types with high efficiency. Upon infection, the viral genome can persist as high-copy-number, circular, nonintegrated episomes that segregate to progeny cells upon division. This allows HVS-based vectors to stably transduce a dividing cell population and provide sustained transgene expression in vitro and in vivo. Moreover, the HVS episome is able to persist and provide prolonged transgene expression during in vitro differentiation of mouse and human hemopoietic progenitor cells. Together, these properties are advantageous for induced pluripotent stem cell (iPSC) technology, whereby stem cell-like cells are generated from adult somatic cells by exogenous expression of specific reprogramming factors. Here we assess the potential of HVS-based vectors for the generation of induced pluripotent cancer stem-like cells (iPCs). We demonstrate that HVS-based exogenous delivery of Oct4, Nanog, and Lin28 can reprogram the Ewing's sarcoma family tumor cell line A673 to produce stem cell-like colonies that can grow under feeder-free stem cell culture conditions. Further analysis of the HVS-derived putative iPCs showed some degree of reprogramming into a stem cell-like state. Specifically, the putative iPCs had a number of embryonic stem cell characteristics, staining positive for alkaline phosphatase and SSEA4, in addition to expressing elevated levels of pluripotent marker genes involved in proliferation and self-renewal. However, differentiation trials suggest that although the HVS-derived putative iPCs are capable of differentiation toward the ectodermal lineage, they do not exhibit pluripotency. Therefore, they are hereby termed induced multipotent cancer cells.

Murine gammaherpesvirus 68 LANA acts on terminal repeat DNA to mediate episome persistence

Murine gammaherpesvirus 68 (MHV68) ORF73 (mLANA) has sequence homology to Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA). LANA acts on the KSHV terminal repeat (TR) elements to mediate KSHV episome maintenance. Disruption of mLANA expression severely reduces the ability of MHV68 to establish latent infection in mice, consistent with the possibility that mLANA mediates episome persistence. Here we assess the roles of mLANA and MHV68 TR (mTR) elements in episome persistence. mTR-associated DNA persisted as an episome in latently MHV68-infected tumor cells, demonstrating that the mTR elements can serve as a cis-acting element for MHV68 episome maintenance. In some cases, both control vector and mTR-associated DNAs integrated into MHV68 episomal genomes. Therefore, we also assessed the roles of mTRs as well as mLANA in the absence of infection. DNA containing both mLANA and mTRs in cis persisted as an episome in murine A20 or MEF cells. In contrast, mTR DNA never persisted as an episome in the absence of mLANA. mLANA levels were increased when mLANA was expressed from its native promoters, and episome maintenance was more efficient with higher mLANA levels. Increased numbers of mTRs conferred more efficient episome maintenance, since DNA containing mLANA and eight mTR elements persisted more efficiently in A20 cells than did DNA with mLANA and two or four mTRs. Similar to KSHV LANA, mLANA broadly associated with mitotic chromosomes but relocalized to concentrated dots in the presence of episomes. Therefore, mLANA acts on mTR elements to mediate MHV68 episome persistence.

CCAAT/enhancer binding proteins play a role in oriLyt-dependent genome replication during MHV-68 de novo infection

Murine gammaherpesvirus 68 (MHV-68), a member of the gammaherpesvirus family, replicates robustly in permissive cell lines and is able to infect laboratory mice. MHV-68 has emerged as a model for studying the basic aspects of viral replication and host-virus interactions of its human counterparts. Herpesvirus genome replication is mediated through a cis-element in the viral genome called the origin of lytic replication (oriLyt). A family of transcription factors, CCAAT/enhancer binding proteins (C/EBPs), assists in oriLyt-mediated DNA replication during gammaherpesvirus reactivation. In this study, we examined the role of C/EBPs in gammaherpesvirus DNA replication during de novo infection, using MHV-68 as a model. We found that C/EBP α and β bind to the CCAAT boxes in the MHV-68 oriLyt core region both in vitro and in vivo, as demonstrated by electrophoretic mobility shift assay and chromatin immunoprecipitation assay. A dominant negative form of C/EBPs significantly impaired the lytic replication efficiency of MHV-68 on both the plasmid and genome levels in a replication assay, indicating that functional C/EBPs are required for maximal MHV-68 genome DNA replication. Collectively, our data demonstrate that C/EBPs interact with the oriLyt core region and play an important role in MHV-68 lytic DNA replication during de novo infection.

Mutation of herpesvirus Saimiri ORF51 glycoprotein specifically targets infectivity to hepatocellular carcinoma cell lines

Herpesvirus saimiri (HVS) is a gamma herpesvirus with several properties that make it an amenable gene therapy vector; namely its large packaging capacity, its ability to persist as a nonintegrated episome, and its ability to infect numerous human cell types. We used RecA-mediated recombination to develop an HVS vector with a mutated virion protein. The heparan sulphate-binding region of HVS ORF51 was substituted for a peptide sequence which interacts with somatostatin receptors (SSTRs), overexpressed on hepatocellular carcinoma (HCC) cells. HVS mORF51 showed reduced infectivity in non-HCC human cell lines compared to wild-type virus. Strikingly, HVS mORF51 retained its ability to infect HCC cell lines efficiently. However, neutralisation assays suggest that HVS mORF51 has no enhanced binding to SSTRs. Therefore, mutation of the ORF51 glycoprotein has specifically targeted HVS to HCC cell lines by reducing the infectivity of other cell types; however, the mechanism for this targeting is unknown.

Murine gammaherpesvirus 68 LANA is essential for virus reactivation from splenocytes but not long-term carriage of viral genome

ORF73, which encodes the latency-associated nuclear antigen (LANA), is a conserved gamma-2-herpesvirus gene. The murine gammaherpesvirus 68 (MHV68) LANA (mLANA) is critical for efficient virus replication and the establishment of latent infection following intranasal inoculation. To test whether the initial host immune response limits the capacity of mLANA-null virus to traffic to and establish latency in the spleen, we infected type I interferon receptor knockout (IFN-alpha/betaR(-/-)) mice via intranasal inoculation and observed the presence of viral genome-positive splenocytes at day 18 postinfection at approximately 10-fold-lower levels than in the genetically repaired marker rescue-infected mice. However, no mLANA-null virus reactivation from infected IFN-alpha/betaR(-/-) splenocytes was observed. To more thoroughly define a role of mLANA in MHV68 infection, we evaluated the capacity of an mLANA-null virus to establish and maintain infection apart from restriction in the lungs of immunocompetent mice. At day 18 following intraperitoneal infection of C57BL/6 mice, the mLANA-null virus was able to establish a chronic infection in the spleen albeit at a 5-fold-reduced level. However, as in IFN-alpha/betaR(-/-) mice, little or no virus reactivation could be detected from mLANA-null virus-infected splenocytes upon explant. An examination of peritoneal exudate cells (PECs) following intraperitoneal inoculation revealed nearly equivalent frequencies of PECs harboring the mLANA-null virus relative to the marker rescue virus. Furthermore, although significantly compromised, mLANA-null virus reactivation from PECs was detected upon explant. Notably, at later times postinfection, the frequency of mLANA-null genome-positive splenocytes was indistinguishable from that of marker rescue virus-infected animals. Analyses of viral genome-positive splenocytes revealed the absence of viral episomes in mLANA-null infected mice, suggesting that the viral genome is integrated or maintained in a linear state. Thus, these data provide the first evidence that a LANA homolog is directly involved in the formation and/or maintenance of an extrachromosomal viral episome in vivo, which is likely required for the reactivation of MHV68.

The latent human herpesvirus-6A genome specifically integrates in telomeres of human chromosomes in vivo and in vitro

Previous research has suggested that human herpesvirus-6 (HHV-6) may integrate into host cell chromosomes and be vertically transmitted in the germ line, but the evidence--primarily fluorescence in situ hybridization (FISH)--is indirect. We sought, first, to definitively test these two hypotheses. Peripheral blood mononuclear cells (PBMCs) were isolated from families in which several members, including at least one parent and child, had unusually high copy numbers of HHV-6 DNA per milliliter of blood. FISH confirmed that HHV-6 DNA colocalized with telomeric regions of one allele on chromosomes 17p13.3, 18q23, and 22q13.3, and that the integration site was identical among members of the same family. Integration of the HHV-6 genome into TTAGGG telomere repeats was confirmed by additional methods and sequencing of the integration site. Partial sequencing of the viral genome identified the same integrated HHV-6A strain within members of families, confirming vertical transmission of the viral genome. We next asked whether HHV-6A infection of naïve cell lines could lead to integration. Following infection of naïve Jjhan and HEK-293 cell lines by HHV-6, the virus integrated into telomeres. Reactivation of integrated HHV-6A virus from individuals' PBMCs as well as cell lines was successfully accomplished by compounds known to induce latent herpesvirus replication. Finally, no circular episomal forms were detected even by PCR. Taken together, the data suggest that HHV-6 is unique among human herpesviruses: it specifically and efficiently integrates into telomeres of chromosomes during latency rather than forming episomes, and the integrated viral genome is capable of producing virions.

Immune evasion by gammaherpesvirus genome maintenance proteins

Viruses that establish lifelong latent infections must ensure that the viral genome is maintained within the latently infected cell throughout the life of the host, yet at the same time must also be capable of avoiding elimination by the immune surveillance system. Gammaherpesviruses, which include the human viruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus, establish latent infections in lymphocytes. Infection of this dynamic host-cell population requires that the viruses have appropriate strategies for enabling the viral genome to persist while these cells go through rounds of mitosis, but at the same time must avoid detection by host CD8(+) cytotoxic T lymphocytes (CTLs). The majority of gammaherpesviruses studied have been found to encode a specific protein that is critical for maintenance of the viral genome within latently infected cells. This protein is termed the genome maintenance protein (GMP). Due to its vital role in long-term latency, this offers the immune system a crucial target for detection and elimination of virus-infected cells. GMPs from different gammaherpesviruses have evolved related strategies that allow the protein to be present within latently infected cells, but to remain effectively hidden from circulating CD8(+) CTLs. In this review, I will summarize the role of the GMPs and highlight the available data describing the immune-evasion properties of these proteins.

Chromatin organization of gammaherpesvirus latent genomes

The gammaherpesviruses are a subclass of the herpesvirus family that establish stable latent infections in proliferating lymphoid and epithelial cells. The latent genomes are maintained as multicopy chromatinized episomes that replicate in synchrony with the cellular genome. Importantly, most of the episomes do not integrate into the host chromosome. Therefore, it is essential that the viral "minichromosome" establish a chromatin structure that is suitable for gene expression, DNA replication, and chromosome segregation. Evidence suggests that chromatin organization is important for each of these functions and plays a regulatory role in the establishment and maintenance of latent infection. Here, we review recent studies on the chromatin organization of the human gammaherpesviruses, Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV). We discuss the potential role of viral origins of DNA replication and viral encoded origin-binding proteins like EBNA1 and LANA in establishment of viral chromosome organization during latent infection. We also discuss the roles of host cell factors, like CTCF and cohesins, that contribute to higher-order chromosome structures that may be important for stable gene expression programs during latent infection in proliferating cells.

Identification and functional characterization of the left origin of lytic replication of murine gammaherpesvirus 68

Murine gammaherpesvirus 68 (MHV-68) replicates robustly in cell culture, providing a model for studying viral genome replication during de novo infection of tumor-associated herpesviruses. We have previously identified a 1.25-kb origin of lytic replication (oriLyt) for MHV-68. To further investigate the molecular mechanism of viral genome replication, we first fine-mapped essential cis-elements from this oriLyt fragment using a transposon-mediated high-density mutagenesis method. The result provided information for us to identify a second oriLyt located towards the left end of MHV-68 genome using a de novo infection-replication assay. We further characterized this left oriLyt by scanning deletion analysis and site-directed mutations, and showed that several CCAAT motifs are essential for oriLyt function, whereas an AT-rich region enhances replication. However, GC-rich repeats are not important cis-element. Moreover, we identified a cellular transcription factor, NF-Y, which binds to CCAAT boxes in EMSA and associates with oriLyt in ChIP assay. Using a dominant negative expression plasmid, we demonstrated that NF-Y plays an important role in mediating MHV-68 genome replication during de novo infection.

Targeting mitotic chromosomes: a conserved mechanism to ensure viral genome persistence

Viruses that maintain their genomes as extrachromosomal circular DNA molecules and establish infection in actively dividing cells must ensure retention of their genomes within the nuclear envelope in order to prevent genome loss. The loss of nuclear membrane integrity during mitosis dictates that paired host cell chromosomes are captured and organized by the mitotic spindle apparatus before segregation to daughter cells. This prevents inaccurate chromosomal segregation and loss of genetic material. A similar mechanism may also exist for the nuclear retention of extrachromosomal viral genomes or episomes during mitosis, particularly for genomes maintained at a low copy number in latent infections. It has been heavily debated whether such a mechanism exists and to what extent this mechanism is conserved among diverse viruses. Research over the last two decades has provided a wealth of information regarding the mechanisms by which specific tumour viruses evade mitotic and DNA damage checkpoints. Here, we discuss the similarities and differences in how specific viruses tether episomal genomes to host cell chromosomes during mitosis to ensure long-term persistence.

Mapping the minimal regions within the ORF73 protein required for herpesvirus saimiri episomal persistence

Herpesvirus saimiri (HVS) establishes a persistent infection in which the viral genome persists as a circular non-integrated episome. ORF73 tethers HVS episomes to host mitotic chromosomes, allowing episomal persistence via an interaction with the chromosome-associated protein, MeCP2. Here we demonstrate that ORF73 also interacts with the linker histone H1 via its C terminus, suggesting it associates with multiple chromosome-associated proteins. In addition, we show that the C terminus is also required for the ability of ORF73 to bind the terminal repeat region of the HVS genome. These results suggest that the ORF73 C terminus contains all the necessary elements required for HVS episomal persistence. Using a range of ORF73 C terminus deletions to rescue the episomal maintenance properties of a HVSDelta73 recombinant virus, we show that a C terminus region comprising residues 285-407 is sufficient to maintain the HVS episome in a dividing cell population.

Malignant catarrhal fever induced by alcelaphine herpesvirus 1 is associated with proliferation of CD8+ T cells supporting a latent infection

Alcelaphine herpesvirus 1 (AlHV-1), carried by wildebeest asymptomatically, causes malignant catarrhal fever (WD-MCF) when cross-species transmitted to a variety of susceptible species of the Artiodactyla order. Experimentally, WD-MCF can be induced in rabbits. The lesions observed are very similar to those described in natural host species. Here, we used the rabbit model and in vivo 5-Bromo-2'-Deoxyuridine (BrdU) incorporation to study WD-MCF pathogenesis. The results obtained can be summarized as follows. (i) AlHV-1 infection induces CD8(+) T cell proliferation detectable as early as 15 days post-inoculation. (ii) While the viral load in peripheral blood mononuclear cells remains below the detection level during most of the incubation period, it increases drastically few days before death. At that time, at least 10% of CD8(+ )cells carry the viral genome; while CD11b(+), IgM(+) and CD4(+) cells do not. (iii) RT-PCR analyses of mononuclear cells isolated from the spleen and the popliteal lymph node of infected rabbits revealed no expression of ORF25 and ORF9, low or no expression of ORF50, and high or no expression of ORF73. Based on these data, we propose a new model for the pathogenesis of WD-MCF. This model relies on proliferation of infected CD8(+) cells supporting a predominantly latent infection.

A comprehensive library of mutations of Epstein Barr virus

A mutant library of 249 mutants with mutations that span the entire Epstein-Barr virus (EBV) genome was generated by transposition with EZ : : TN <KAN-2> and insertion with an apramycin resistance gene by a PCR-targeting method. This study also demonstrates the feasibility of generating deletions and site-specific mutations in the BRLF1 promoter on the EBV genome to determine the regions in the promoter that are crucial to transcription. Analysing BZLF1 and BRLF1 mutants by microarray analysis revealed that these two genes regulate the transcription of EBV lytic genes differently. A BZLF1 mutation affects global expression of EBV lytic genes; almost no lytic gene is expressed by the mutant after lytic induction. However, although a BRLF1 mutant still transcribes most lytic genes, the expression of these lytic genes is inefficient. Furthermore, this study shows that the proximal Zta-response element in the BRLF1 promoter is crucial to BRLF1 transcription from the EBV genome, despite the fact that another work demonstrated that this site was unimportant in transient transfection analysis. Furthermore, mutants with a mutation in BDLF1 and BORF1 cannot assemble viral capsids. Results of this study demonstrate the usefulness of a comprehensive mutant library in genetic analyses of EBV.

Mutagenesis of the herpesvirus saimiri terminal repeat region reveals important elements for virus production

Deletion of the terminal repeats (TR) from herpesvirus saimiri (HVS) renders it unable to produce infectious virus or generate plaques. However, a TR-deleted HVS bacterial artificial chromosome can form replication compartments. Complementation of this mutant shows that one copy of the TR, plus the right junction of the genome with the TR, is sufficient for efficient plaque formation and generation of infectious virus. Within the TR unit, the region around the cleavage site of the genome appears both necessary and sufficient for virus production. Analysis of episomes from productive cells indicates a propensity to amplify TR numbers during the lytic cycle.

Herpesvirus saimiri episomal persistence is maintained via interaction between open reading frame 73 and the cellular chromosome-associated protein MeCP2

Herpesvirus saimiri (HVS) is the prototype gamma-2 herpesvirus, which naturally infects the squirrel monkey Saimiri sciureus, causing an asymptomatic but persistent infection. The latent phase of gamma-2 herpesviruses is characterized by their ability to persist in a dividing cell population while expressing a limited subset of latency-associated genes. In HVS only three genes, open reading frame 71 (ORF71), ORF72, and ORF73, are expressed from a polycistronic mRNA. ORF73 has been shown to be the only gene essential for HVS episomal maintenance and can therefore be functionally compared to the human gammaherpesvirus latency-associated proteins, EBNA-1 and Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA). HVS ORF73 is the positional homologue of KSHV LANA and, although it shares limited sequence homology, has significant structural and functional similarities. Investigation of KSHV LANA has demonstrated that it is able to mediate KSHV episomal persistence by tethering the KSHV episome to host mitotic chromosomes via interactions with cellular chromosome-associated proteins. These include associations with core and linker histones, several bromodomain proteins, and the chromosome-associated proteins methyl CpG binding protein 2 (MeCP2) and DEK. Here we show that HVS ORF73 associates with MeCP2 via a 72-amino-acid domain within the ORF73 C terminus. Furthermore, we have assessed the functional significance of this interaction, using a variety of techniques including small hairpin RNA knockdown, and show that association between ORF73 and MeCP2 is essential for HVS chromosomal attachment and episomal persistence.

Acetylation of the latency-associated nuclear antigen regulates repression of Kaposi's sarcoma-associated herpesvirus lytic transcription

Reactivation of the Kaposi's sarcoma-associated herpesvirus (KSHV) lytic cycle can be initiated by transcription activation of the ORF50 immediate early gene (Rta). We show that ORF50 transcription is actively repressed by the KSHV latency-associated nuclear antigen (LANA) during latency. Depletion of LANA by small interfering RNA derepressed ORF50 transcription in the latently infected BCBL1 pleural effusion lymphoma-derived cell line. In contrast, overexpression of LANA suppressed ORF50 mRNA levels in BCBL1 cells. ORF50 transcription was significantly elevated during primary infection with recombinant virus lacking LANA, further indicating that LANA plays a role in lytic gene silencing during the establishment of latency. Chromatin immunoprecipitation assays indicated that LANA interacts with the ORF50 promoter region in latently infected cells. Histone deacetylase inhibitors, including sodium butyrate (NaB) and trichostatin A, caused the rapid dissociation of LANA from the ORF50 promoter. NaB treatment of latently infected BCBL1 cells disrupted a stable interaction between LANA and the cellular proteins Sp1 and histone H2B. We also found immunological and radiochemical evidence that LANA is subject to lysine acetylation after NaB treatment. These findings support the role of LANA as a transcriptional repressor of lytic reactivation and provide evidence that lysine acetylation regulates LANA interactions with chromatin, Sp1, and ORF50 promoter DNA.

Open reading frame 73 is required for herpesvirus saimiri A11-S4 episomal persistence

Herpesvirus saimiri (HVS) establishes a latent infection in which the viral genome persists as a non-integrated episome. Analysis has shown that only open reading frames (ORFs) 71-73 are transcribed in an in vitro model of HVS latency. ORF73 also colocalizes with HVS genomic DNA on host mitotic chromosomes and maintains the stability of HVS terminal-repeat-containing plasmids. However, it is not known whether ORF73 is the only HVS-encoded protein required for episomal maintenance. In this study, the elements required for episomal maintenance in the context of a full-length HVS genome were examined by mutational analysis. A recombinant virus, HVS-BAC delta71-73, lacking the latency-associated genes was unable to persist in a dividing cell population. However, retrofitting an ORF73 expression cassette into the recombinant virus rescued episomal maintenance. This indicates that ORF73 is the key trans-acting factor for episomal persistence and efficient establishment of a latent infection.

Whole-genome transcription profiling of rhesus monkey rhadinovirus

Rhesus monkey rhadinovirus (RRV) and Kaposi's sarcoma-associated herpesvirus (KSHV; also called human herpesvirus 8) belong to the gamma-2 grouping of herpesviruses. RRV and KSHV share a high degree of sequence similarity, and their genomes are organized in a similar fashion. RRV serves as an excellent animal model system to study the gamma herpesvirus life cycle both in vitro and in vivo. We have developed a high-sensitivity, high-throughput, high-specificity real-time quantitative reverse transcriptase-based PCR assay for RRV and have used this assay to profile transcription from the whole RRV genome during de novo productive infection of rhesus fibroblasts. Using this assay, we demonstrate that the genome-wide transcription profile for RRV closely parallels the genome-wide transcription profile for KSHV.

T-cell transformation and oncogenesis by gamma2-herpesviruses

gamma2-Herpesviruses, also termed rhadinoviruses, have long been known as animal pathogens causing lymphoproliferative diseases such as malignant catarrhal fever in cattle or T-cell lymphoma in certain Neotropical primates. The rhadinovirus prototype is Herpesvirus saimiri (HVS), a T-lymphotropic agent of squirrel monkeys (Saimiri sciureus); Herpesvirus ateles (HVA) is closely related to HVS. The first human rhadinovirus, human herpesvirus type 8 (HHV-8), was discovered a decade ago in Kaposi's sarcoma (KS) biopsies. It was found to be strongly associated with all forms of KS, as well as with multicentric Castleman's disease and primary effusion lymphoma (PEL). Since DNA of this virus is regularly found in all KS forms, and specifically in the spindle cells of KS, it was also termed KS-associated herpesvirus (KSHV). Several simian rhadinoviruses related to KSHV have been discovered in various Old World primates, though they seem only loosely associated with pathogenicity or tumor induction. In contrast, HVS and HVA cause T-cell lymphoma in numerous non-natural primate hosts; HVS strains of the subgroup C are capable of transforming human and simian T-lymphocytes to continuous growth in cell culture and can provide useful tools for T-cell immunology or gene transfer. Here, we describe their natural history, genome structure, biology, and pathogenesis in T-cell transformation and oncogenesis.

Cell transformation by Herpesvirus saimiri

Herpesvirus saimiri (Saimiriine herpesvirus-2), a gamma2-herpesvirus (rhadinovirus) of non-human primates, causes T-lymphoproliferative diseases in susceptible organisms and transforms human and non-human T lymphocytes to continuous growth in vitro in the absence of stimulation. T cells transformed by H. saimiri retain many characteristics of intact T lymphocytes, such as the sensitivity to interleukin-2 and the ability to recognize the corresponding antigens. As a result, H. saimiri is widely used in immunobiology for immortalization of various difficult-to-obtain and/or -to-maintain T cells in order to obtain useful experimental models. In particular, H. saimiri-transformed human T cells are highly susceptible to infection with HIV-1 and -2. This makes them a convenient tool for propagation of poorly replicating strains of HIV, including primary clinical isolates. Therefore, the mechanisms mediating transformation of T cells by H. saimiri are of considerable interest. A single transformation-associated protein, StpA or StpB, mediates cell transformation by H. saimiri strains of group A or B, respectively. Strains of group C, which exhibit the highest oncogenic potential, have two proteins involved in transformation-StpC and Tip. Both proteins have been shown to dramatically affect signal transduction pathways leading to the activation of crucial transcription factors. This review is focused on the biological effects and molecular mechanisms of action of proteins involved in H. saimiri-dependent transformation.

Development of herpesvirus-based episomally maintained gene delivery vectors

Successful gene therapy aims to deliver and express therapeutic genes to cure or slow the progression of disease. However, a major obstacle in the application of gene therapy has been the development of the vectors used to deliver heterologous DNA to the cell or tissue of choice. A number of viral- and non-viral-based vector systems have undergone clinical trials with varying success. However, at present, no vector system possesses the full complement of properties that are generally believed necessary in an ideal gene delivery system. Therefore, alongside attempts to improve current gene delivery vectors, the identification and evaluation of new viral vectors is crucial for the long-term success of gene therapy. Herpesviruses are large DNA viruses which possess a number of advantages as gene delivery vectors. These relate to an ability to package large DNA insertions and establish lifelong latent infections in which the genomic material exists as a stable episome. This review aims to highlight the potential of herpesvirus vectors, in particular an alternative vector system based on herpesvirus saimiri (HVS). HVS is capable of infecting a range of human cell lines with high efficiencies, and the viral genome persists as high copy number, circular, non-integrated episomes which segregate to progeny following cell division. This allows the virus-based vector to stably transduce a dividing cell population and provide sustained transgene expression for an extended period of time both in vitro and in vivo. Moreover, the insertion of a bacterial artificial chromosome cassette into the HVS genome simplifies the incorporation of large amounts of heterologous DNA for gene delivery. These properties offer characteristics similar to an artificial chromosome combined with an efficient delivery system and merit its continual development as a possible gene delivery vector for the future.

ORC, MCM, and histone hyperacetylation at the Kaposi's sarcoma-associated herpesvirus latent replication origin

The viral genome of Kaposi's sarcoma-associated herpesvirus (KSHV) persists as an extrachromosomal plasmid in latently infected cells. The KSHV latency-associated nuclear antigen (LANA) stimulates plasmid maintenance and DNA replication by binding to an approximately 150-bp region within the viral terminal repeats (TR). We have used chromatin immunoprecipitation assays to demonstrate that LANA binds specifically to the replication origin sequence within the KSHV TR in latently infected cells. The latent replication origin within the TR was also bound by LANA-associated proteins CBP, double-bromodomain-containing protein 2 (BRD2), and the origin recognition complex 2 protein (ORC2) and was enriched in hyperacetylated histones H3 and H4 relative to other regions of the latent genome. Cell cycle analysis indicated that the minichromosome maintenance complex protein, MCM3, bound TR in late-G(1)/S-arrested cells, which coincided with the loss of histone H3 K4 methylation. Micrococcal nuclease studies revealed that TRs are embedded in a highly ordered nucleosome array that becomes disorganized in late G(1)/S phase. ORC binding to TR was LANA dependent when reconstituted in transfected plasmids. DNA affinity purification confirmed that LANA, CBP, BRD2, and ORC2 bound TR specifically and identified the histone acetyltransferase HBO1 (histone acetyltransferase binding to ORC1) as a potential TR binding protein. Disruption of ORC2, MCM5, and HBO1 expression by small interfering RNA reduced LANA-dependent DNA replication of TR-containing plasmids. These findings are the first demonstration that cellular replication and origin licensing factors are required for KSHV latent cycle replication. These results also suggest that the KSHV latent origin of replication is a unique chromatin environment containing histone H3 hyperacetylation within heterochromatic tandem repeats.

Vaccine potential of a murine gammaherpesvirus-68 mutant deficient for ORF73

A murine gammaherpesvirus (MHV-68) containing a deletion of the putative plasmid maintenance protein ORF73 exhibits a severe latency deficit. In this study the ability of an ORF73 deletion mutant (Delta73) to confer in vivo protection against subsequent challenge with wild-type virus has been examined. Vaccination studies have shown that Delta73 vaccination reduced latent infection of wild-type challenge virus to a level below the limit of detection. These results indicate that a live-attenuated gammaherpesvirus that cannot persist is an effective vaccine.

ORF73 of murine herpesvirus-68 is critical for the establishment and maintenance of latency

In vitro studies have established that the latency-associated nuclear antigen encoded by human Kaposi's sarcoma-associated herpesvirus and the related ORF73 gene product of herpesvirus saimiri interact with virus origins of replication to facilitate maintenance of episomal DNA. Such a function implies a critical role for ORF73 in the establishment and maintenance of latency in vivo. To determine the role of ORF73 in virus pathogenesis, the ORF73 gene product encoded by murine herpesvirus-68 (MHV-68) was disrupted by making an ORF73 deletion mutant, Delta73, and an independent ORF73 frameshift mutant, FS73. The effect of the mutations introduced in ORF73 on MHV-68 pathogenesis was analysed in vivo using a well-characterized murine model system. These studies have revealed that ORF73 is not required for efficient lytic replication either in vitro or in vivo. In contrast, a severe latency deficit is observed in splenocytes of animals infected with an ORF73 mutant, as assessed by infectious centre reactivation assay or by in situ hybridization detection of latent virus. Assessment of viral genome-positive cells in sorted splenocyte populations confirmed the absence of ORF73 mutant virus from splenic latency reservoirs, including germinal centre B cells. These data indicate a crucial role for ORF73 in the establishment of latency and for virus persistence in the host.

ORF73 of herpesvirus Saimiri strain C488 tethers the viral genome to metaphase chromosomes and binds to cis-acting DNA sequences in the terminal repeats

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV), a human oncogenic gamma-2-herpesvirus, transforms human endothelial cells and establishes latent infection at a low efficiency in vitro. During latent infection, only a limited number of genes are expressed, and the circularized viral genome is maintained as a multicopy episome. Latency-associated nuclear antigen (LANA), exclusively expressed during latency, has been shown to have a multifunctional role in KS pathogenesis. LANA tethers the viral episome to the host chromosome, thus ensuring efficient persistence of the viral genome during successive rounds of cell division. Besides episome maintenance, LANA modulates the expression of genes of various cellular and viral pathways, including those of retinoblastoma protein and p53. Herpesvirus saimiri (HVS), another gamma-2-herpesvirus, primarily infects New World primates. Orf73, encoding the nuclear antigen of HVS, is the positional homolog of the LANA gene, and the ORF73 protein has some sequence homology to KSHV LANA. However, the function of ORF73 of HVS has not been thoroughly investigated. In this report, we show that HVS ORF73 may be important for episome persistence and colocalizes with the HVS genomic DNA on metaphase chromosomes. Furthermore, HVS terminal repeats (TRs) contain a cis-acting sequence similar to that in KSHV TRs, suggesting that the LANA binding sequence is conserved between these two viruses. This cis-acting element is sufficient to bind HVS ORF73 from strains C488 and A11, and plasmids containing the HVS C488 TR element are maintained and replicate in HVS C488 ORF73-expressing cells.

Generation and precise modification of a herpesvirus saimiri bacterial artificial chromosome demonstrates that the terminal repeats are required for both virus production and episomal persistence

Herpesvirus saimiri (HVS) is the prototype gamma-2 herpesvirus, and shares considerable homology with the human gammaherpesviruses Kaposi's sarcoma-associated herpesvirus and Epstein–Barr virus. The generation of herpesvirus mutants is a key facet in the study of virus biology. The use of F-factor-based bacterial artificial chromosomes (BACs) to clone and modify the genomes of herpesviruses has enhanced the variety, precision and simplicity of mutant production. Here we describe the cloning of the genome of HVS non-transforming strain A11-S4 into a BAC. The cloning of the BAC elements disrupts open reading frame (ORF) 15 but the HVS-BAC can still replicate at levels similar to wild-type virus, and can persistently infect fibroblasts. The HVS-BAC was modified by RecA-mediated recombination initially to substitute reporter genes and also to delete the terminal repeats (TR). After deletion of the TR, the HVS-BAC fails to enter a productive virus lytic cycle, and cannot establish a persistent episomal infection when transfected into fibroblast cell lines. This shows that while ORF 15 is dispensable for virus function in vitro, the TR is required for both virus latency and lytic virus production. In addition, the HVS-BAC promises to be a valuable tool that can be used for the routine and precise production and analysis of viral mutants to further explore gammaherpesvirus biology.

The gammaherpesvirus 68 latency-associated nuclear antigen homolog is critical for the establishment of splenic latency

Open reading frame 73 (ORF 73) is conserved among the gamma-2-herpesviruses (rhadinoviruses) and, in Kaposi's sarcoma-associated herpesvirus (KSHV) and herpesvirus saimiri (HVS), has been shown to encode a latency-associated nuclear antigen (LANA). The KSHV and HVS LANAs have also been shown to be required for maintenance of the viral genome as an episome during latency. LANA binds both the viral latency-associated origin of replication and the host cell chromosome, thereby ensuring efficient partitioning of viral genomes to daughter cells during mitosis of a latently infected cell. In gammaherpesvirus 68 (gammaHV68), the role of the LANA homolog in viral infection has not been analyzed. Here we report the construction of a gammaHV68 mutant containing a translation termination codon in the LANA ORF (73.STOP). The 73.STOP mutant virus replicated normally in vitro, in both proliferating and quiescent murine fibroblasts. In addition, there was no difference between wild-type (WT) and 73.STOP virus in the kinetics of induction of lethality in mice lacking B and T cells (Rag 1(-/-)) infected with 1000 PFU of virus. However, compared to WT virus, the 73.STOP mutant exhibited delayed kinetics of replication in the lungs of immunocompetent C57BL/6 mice. In addition, the 73.STOP mutant exhibited a severe defect in the establishment of latency in the spleen of C57BL/6 mice. Increasing the inoculum of 73.STOP virus partially overcame the acute replication defected observed in the lungs at day 4 postinfection but did not ameliorate the severe defect in the establishment of splenic latency. Thus, consistent with its proposed role in replication of the latent viral episome, LANA appears to be a critical determinant in the establishment of gammaHV68 latency in the spleen post-intranasal infection.

The latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus interacts preferentially with the terminal repeats of the genome in vivo and this complex is sufficient for episomal DNA replication

The genome of Kaposi's sarcoma-associated herpesvirus (KSHV) persists in latently infected cells as a circular episome. The latency-associated nuclear antigen (LANA) has been shown to tether viral DNA fragments to chromosomes and is proposed to maintain the KSHV genome. In order to identify the in vivo-binding sites for LANA on the whole KSHV genome and to analyse the function of this protein-DNA interaction, different in vivo systems have been developed. Chromatin immunoprecipitation experiments using three different cell lines latently infected with KSHV demonstrated that LANA binds preferentially and directly to the terminal repeats (TRs) but not to other regions of the viral chromosome in vivo. In contrast, in vitro LANA-DNA binding was much less specific. To identify autonomously replicating sequences within the KSHV genome, BCBL-1 cells were transfected with cosmids representing the entire genome. Cosmid Z2, consisting of the right end of the unique region and TRs, persisted as an episome in short-term assays. Long term, stable episome replication was observed with constructs derived from Z2 containing TRs only. LANA expression constructs containing a variable number of TRs replicated stably as episomes in uninfected cells. A 424 bp subfragment of the 801 bp TR could mediate episome replication. These studies show that LANA is a trans-acting protein that binds preferentially to TRs in vivo and these two elements are sufficient for episome replication. These results also suggest that the LANA expression plasmids reported here could be utilized as episomal vectors in a manner similar to Epstein-Barr virus-based vectors.