Herpesvirus saimiri pathogenicity enhanced by thymidine kinase of herpes simplex virus

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.

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.

Herpesvirus saimiri infection of rhesus macaques: a model for acute rhadinovirus-induced t-cell transformation and oncogenesis

Herpesvirus saimiri (HVS) causes acute lymphoma and leukemia upon experimental infection of various monkey species. HVS strain C488 is also capable of transforming human T-lymphocytes to stable growth in culture. The most susceptible species for oncogenesis are New World primates, in particular the cottontop tamarin (Saguinus oedipus). However, Old World monkeys such as macaques are the most used animal model for the close-to-human situation. The limited data on HVS infection in Old World monkeys prompted us to investigate susceptibility to infection and disease induction by HVS in macaques. After having established that rhesus macaques can be infected productively, and that rhesus T-cells can be transformed in vivo by HVS, we observed induction of lymphoma in all inoculated animals. Pre-existing humoral immunity in part of the rhesus colony capable of blocking HVS infection could be overcome by preselecting rhesus macaques for lack of this immunity of unknown origin. HVS infection of rhesus macaques as compared to that of New World monkeys has the advantages that disease progression is more prolonged, and larger blood volumes can be collected, which allows more extended analyses. Also, rhesus monkeys are the best immunologically and immunogenetically characterized primate species next to humans. This model could be useful for the evaluation of candidate tumor vaccines and to test novel approaches for cancer immunotherapy. In addition, HVS infection of macaques could eventually be useful as a surrogate model to address certain questions in rhadinovirus-induced human cancer such as effusion lymphoma or Kaposi's sarcoma.

Transformation by herpesviruses: focus on T cells

Acute T-lymphoproliferative syndromes are caused by herpesvirus saimiri (HVS) and ateles in neotropical primates; by alcelaphine herpesvirus-1 and ovine herpesvirus-2 strains in domestic cattle and other ungulates; and by the α-herpesvirus of Marek's disease in chickens. T-cell lymphoproliferation caused by these herpesviruses has short incubation periods and a rapid course when compared with retroviral disease. The B-lymphotropic Epstein–Barr virus (EBV) is also associated with some human T-cell malignancies. Analogous to EBV in B cells, HVS isolates of the subgroup C are uniquely capable of transforming human and Old World primate T lymphocytes to continuous growth in cell culture and can provide useful tools for T-cell immunology or gene transfer. Signal transduction pathways stimulated by the viral oncoproteins seem to converge at related cellular effector proteins, in total providing a proproliferative signal. However, the viral oncoproteins most likely evolved to evade immune recognition and to support persistent infection in the natural host, where these viruses are frequently apathogenic.

Efficient infection and persistence of a herpesvirus saimiri-based gene delivery vector into human tumor xenografts and multicellular spheroid cultures

Herpesvirus saimiri (HVS) has the ability to infect a variety of human cell lines and establish a persistent infection by virtue of episomal maintenance. Moreover, the viral episome provides sustained expression of a heterologous transgene. HVS-based vectors can also persist for a long term in tumor xenografts generated from HVS-infected human carcinoma cell lines. The viral episome remains latent within the xenograft allowing long-term transgene expression. These properties, in addition to its ability to incorporate large amounts of heterologous DNA, make HVS an attractive potential gene delivery vector. Here we report on the further evaluation of such HVS-based vectors. We demonstrate for the first time that HVS can efficiently infect solid tumor xenografts derived from a variety of human carcinoma cells via direct intratumoral injections. Furthermore, HVS can efficiently infect spheroid cultures, a three-dimensional cell culture system that closely resembles a tumor. Upon infection of both the tumor xenografts and spheroid cultures, HVS-based vectors can establish a persistent episomal infection within the tumor xenograft allowing expression of a heterologous transgene. These results suggest that HVS-based vectors may be suitable for cancer gene therapy applications.

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.

Transcriptional Activation of Gammaherpesviral Oncogene Promoters by the Hepatitis B Viral X Protein (HBx)

The latent membrane protein-1 (LMP1) of Epstein-Barr Virus (EBV), saimiri transformation protein (STP) of Herpesvirus saimiri (HVS), and K1 protein of Kaposi's sarcoma-associated herpesvirus (KSHV) are potent gammaherpesvirus oncogenes. To study the possible effects of double viral infection, we investigated the effects of oncogenic early proteins of DNA viruses E1A and E1B (adenovirus-5), E6 and E7 (human papillomavirus-16), HBx (hepatitis B virus), Tag (SV40), and gammaherpesviral oncogene during co-infection in human B-lymphoma (Ramos) and human T-cell leukemia (Jurkat) cell lines. HBx transactivated the promoters of LMP1, STP, and K1 the most, by about six-, three-, and twofold, respectively. Analyses of site-directed mutation and the heterologous promoter system showed that HBx activated the promoter activity of these genes via the NF-kappaB site. These results suggest that HBV (HBx) infection of cells previously infected by gammaherpesviruses transactivates their oncogenes, resulting in possible virus-related disease pathogenesis.

Murine gammaherpesvirus 68 lacking thymidine kinase shows severe attenuation of lytic cycle replication in vivo but still establishes latency

The lytic cycle functions of gammaherpesviruses have received relatively little attention to date, at least in part due to the lack of a convenient experimental model. The murine gammaherpesvirus 68 (MHV-68) now provides such a model and allows the roles of individual lytic cycle gammaherpesvirus proteins to be evaluated in vivo. We have used MHV-68 to determine the contribution of a gammaherpesvirus thymidine kinase (TK) to viral lytic replication and latency establishment. MHV-68 mutants with a disrupted TK gene grew normally in vitro but showed a severe attenuation of replication in the lungs after intranasal inoculation, with lytic titers at least 1,000-fold lower than those of wild-type and revertant viruses. Nevertheless, the establishment of latency by the TK-deficient mutants, while delayed, was not prevented by their lytic replication deficit. The viral TK clearly plays a crucial role in the capacity of MHV-68 to replicate efficiently in its natural host but does not seem to be essential to establish a persistent infection. The potential of TK-deficient mutants as gammaherpesvirus vaccines is discussed.

The herpesvirus saimiri ORF 73 regulatory region provides long-term transgene expression in human carcinoma cell lines

Herpesvirus saimiri (HVS) is capable of establishing a persistent infection in a variety of human carcinoma cell lines, by virtue of episomal maintenance. Moreover, the viral episome provides expression of a transgene in both in vitro and in vivo environments. At present, HVS vectors utilize heterologous promoters such as the IE hCMV promoter. However, this promoter maybe unsuitable for long-term expression in vivo, as promoter silencing has been observed in this and other herpesvirus-based vector systems. Ideal regulatory regions would be functional when the herpesvirus genome is maintained as a latent episome. We have previously shown that gene expression in an HVS-persistently-infected human carcinoma cell line is limited to an adjacent set of genes encoding ORFs 71-73. These genes are transcribed as a polycistronic mRNA species from a common regulatory region upstream of the ORF 73 gene. In this report, we assess the potential of the ORF 73 regulatory region to provide heterologous gene expression in a wide variety of human cancer cell lines. We demonstrate, utilizing transient transfection assays, that the ORF 73 regulatory region can provide transgene expression in a variety of human carcinoma cell lines, although levels of transgene expression are not as high as achieved under the control of heterologous promoters such as the IE hCMV promoter. Furthermore, incorporation of the minimal ORF 73 regulatory region in a recombinant HVS-based vector provides sustained expression of the green fluorescent protein in both in vitro and in vivo environments. These results suggest that the ORF 73 regulatory region may be suitable for use in HVS-based cancer gene therapy applications.