Development of herpesvirus-based episomally maintained gene delivery vectors

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.

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.

Herpesvirus saimiri-based vector biodistribution using noninvasive optical imaging

Herpesvirus saimiri (HVS) is capable of infecting a range of human cell types with high efficiency and the viral genome persists as high copy number, circular, nonintegrated episomes which segregate to progeny upon cell division. This allows the HVS-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. Here we assess the dissemination of HVS-based vectors in vivo following intravenous and intraperitoneal administration. Bioluminescence imaging of an HVS-based vector expressing luciferase demonstrates that the virus can infect and establish a persistent latent infection in a variety of mouse tissues. Moreover, the long-term in vivo maintenance of the HVS genome as a nonintegrated circular episome provided sustained expression of luciferase over a 10-week period. A particularly high level of transgene expression in the liver and the ability of HVS to infect and persist in hepatic stellate cells suggest that HVS-based vectors may have potential for the treatment of inherited and acquired liver diseases.

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.

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.