Herpesvirus saimiri-based vector biodistribution using noninvasive optical imaging

The use of high-frequency ultrasound imaging and biofluorescence for in vivo evaluation of gene therapy vectors

BACKGROUND

Non-invasive imaging of the biodistribution of novel therapeutics including gene therapy vectors in animal models is essential.

METHODS

This study assessed the utility of high-frequency ultrasound (HF-US) combined with biofluoresence imaging (BFI) to determine the longitudinal impact of a Herpesvirus saimiri amplicon on human colorectal cancer xenograft growth.

RESULTS

HF-US imaging of xenografts resulted in an accurate and informative xenograft volume in a longitudinal study. The volumes correlated better with final ex vivo volume than mechanical callipers (R2 = 0.7993, p = 0.0002 vs. R2 = 0.7867, p = 0.0014). HF-US showed that the amplicon caused lobe formation. BFI demonstrated retention and expression of the amplicon in the xenografts and quantitation of the fluorescence levels also correlated with tumour volumes.

CONCLUSIONS

The use of multi-modal imaging provided useful and enhanced insights into the behaviour of gene therapy vectors in vivo in real-time. These relatively inexpensive technologies are easy to incorporate into pre-clinical studies.

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.

Production of recombinant Herpesvirus saimiri-based vectors

Herpesvirus saimiri (HVS) is capable of infecting a range of human cell types with high efficiency. The viral genome persists as high-copy-number, circular, nonintegrated episomes that segregate to progeny upon cell division. This allows HVS-based vectors to transduce stably 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 (BAC) cassette into the HVS genome simplifies the incorporation of large amounts of heterologous DNA for gene delivery. These properties offer characteristics similar to that of an artificial chromosome combined with an efficient delivery system. To insert and express a heterologous gene in an HVS-based vector, a recombinant virus must be constructed, as described in this protocol. An HVS-BAC is used to simplify and enhance the production of recombinant viruses. This requires a two-step process to insert the heterologous expression cassette first into the pHVS-Shuttle and then into the HVS-BAC.

Assessment of infectivity using a Herpesvirus saimiri (HVS) recombinant that expresses HVS-GFP

Herpesvirus saimiri (HVS) is capable of infecting a wide range of human cell types with high efficiency. The viral genome persists as high-copy-number, circular, nonintegrated episomes that segregate to progeny on cell division. This allows HVS-based vectors to transduce stably 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 (BAC) cassette into the HVS genome simplifies the incorporation of large amounts of heterologous DNA for gene delivery. These properties offer characteristics similar to that of an artificial chromosome combined with an efficient delivery system. This protocol describes the use of an HVS recombinant virus expressing green fluorescent protein (GFP) (HVS-GFP) to assess the infectivity of a specific cell line.

Gardella gel analysis to detect Herpesvirus saimiri episomal DNA

Herpesvirus saimiri (HVS) is capable of infecting a range of human cell types with high efficiency. The viral genome persists as high-copy-number, circular, nonintegrated episomes that segregate to progeny on cell division. This allows HVS-based vectors to transduce stably 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 (BAC) cassette into the HVS genome simplifies the incorporation of large amounts of heterologous DNA for gene delivery. These properties offer characteristics similar to that of an artificial chromosome combined with an efficient delivery system. In this protocol, Gardella gel analysis is performed to determine whether the HVS genome is maintained in a nonintegrated episomal form. This method allows the identification of chromosomal/integrated DNA and episomal and linear forms of viral DNA, which run at the top, middle, and bottom of the gel, respectively. To obtain the required sensitivity to detect HVS episomes within host tissues, a modified polymerase chain reaction (PCR)-based Gardella gel can also be utilized.

Ex vivo bioluminescence detection of alcelaphine herpesvirus 1 infection during malignant catarrhal fever

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 reproduced in rabbits. WD-MCF is described as a combination of lymphoproliferation and degenerative lesions in virtually all organs and is caused by unknown mechanisms. Recently, we demonstrated that WD-MCF is associated with the proliferation of CD8(+) cells supporting a latent type of infection in lymphoid tissues. Here, we investigated the macroscopic distribution of AlHV-1 infection using ex vivo bioluminescence imaging in rabbit to determine whether it correlates with the distribution of lesions in lymphoid and nonlymphoid organs. To reach that goal, a recombinant AlHV-1 strain was produced by insertion of a luciferase expression cassette (luc) in an intergenic region. In vitro, the reconstituted AlHV-1 luc(+) strain replicated comparably to the parental strain, and luciferase activity was detected by bioluminescence imaging. In vivo, rabbits infected with the AlHV-1 luc(+) strain developed WD-MCF comparably to rabbits infected with the parental wild-type strain, with hyperthermia and increases of both CD8(+) T cell frequencies and viral genomic charge over time in peripheral blood mononuclear cells and in lymph nodes at time of euthanasia. Bioluminescent imaging revealed that AlHV-1 infection could be detected ex vivo in lymphoid organs but also in lung, liver, and kidney during WD-MCF, demonstrating that AlHV-1 infection is prevalent in tissue lesions. Finally, we show that the infiltrating mononuclear leukocytes in nonlymphoid organs are mainly CD8(+) T cells and that latency is predominant during WD-MCF.

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.

Chitosan-based systems for molecular imaging

Molecular imaging enables the non-invasive assessment of biological and biochemical processes in living subjects. Such technologies therefore have the potential to enhance our understanding of disease and drug activity during preclinical and clinical drug development. Molecular imaging allows a repetitive and non-invasive study of the same living subject using identical or alternative biological imaging assays at different time points, thus harnessing the statistical power of longitudinal studies, and reducing the number of animals required and cost. Chitosan is a hydrophilic and non-antigenic biopolymer and has a low toxicity toward mammalian cells. Hence, it has great potential as a biomaterial because of its excellent biocompatibility. Conjugated to additional materials, chitosan composites result in a new class of biomaterials that possess mechanical, physicochemical and functional properties, which have potential for use in advanced biomedical imaging applications. The present review will discuss the strengths, limitations and challenges of molecular imaging as well as applications of chitosan nanoparticles in the field of molecular imaging.

Non-invasive Imaging in Gene Therapy

Several methods are available for non-invasive imaging of gene delivery and transgene expression, including magnetic resonance imaging (MRI), single photon emission tomography (SPECT)/positron emission tomography (PET), and fluorescence and bioluminescence imaging. However, these imaging modalities differ greatly in terms of their sensitivity, cost, and ability to measure the signal. Whereas MRI can produce a resolution of approximately 50 mum, optical imaging achieves only 3-5 mm but outperforms MRI in terms of the cost of the imaging device. Similarly, SPECT and PET give a resolution of only 1-2 mm but provide for relatively easy quantitation of the signal and need only nanograms of probe, compared with the microgram or milligram levels required for MRI and optical imaging. To develop safer and more efficient gene delivery vectors, it is essential to perform rigorous in vivo experiments, to image particle biodistribution and transduction patterns, and to quantify the transgene expression profile. Differences between modalities have a significant effect on the resultant imaging resolution for gene therapy. This review describes the methodologies in use and highlights recent key approaches using the latest imaging modalities in gene therapy. Future trends in gene therapy imaging are also discussed.

Nanoparticles for bioimaging

The emergence of synthesis strategies for the fabrication of nanosized contrast agents is anticipated to lead to advancements in understanding biological processes at the molecular level in addition to progress in the development of diagnostic tools and innovative therapies. Imaging agents such as fluorescent dye-doped silica nanoparticles, quantum dots and gold nanoparticles have overcome many of the limitations of conventional contrast agents (organic dyes) such as poor photostability, low quantum yield, insufficient in vitro and in vivo stability, etc. Such particulates are now being developed for absorbance and emission in the near infrared region, which is expected to allow for real time and deep tissue imaging via optical routes. Other efforts to facilitate deep tissue imaging with pre-existing technologies have lead to the development of multimodal nanoparticles which are both optical and MRI active. The main focus of this article is to provide an overview of properties and design of contrast agents such as dye-doped silica nanoparticles, quantum dots and gold nanoparticles for non-invasive bioimaging.