Measuring herpes simplex virus thymidine kinase reporter gene expression in vitro

PEG10 Promoter-Driven Expression of Reporter Genes Enables Molecular Imaging of Lethal Prostate Cancer

The retrotransposon-derived paternally expressed gene 10 (PEG10) protein is ordinarily expressed at high levels in the placenta. Recently, it was discovered that PEG10 isoforms promote the progression of prostate cancer to a highly lethal androgen receptor (AR)-negative phenotype. The presence of PEG10 in other subtypes of prostate cancer has not been explored and a utility for PEG10 overexpression has not been developed. Here, we found that in addition to AR-null disease, PEG10 was also expressed in prostate cancer with constitutively active AR-splice variants. A molecular genetic imaging strategy for noninvasive imaging of AR-splice variant prostate cancer was developed by utilizing the cancer specificity of the PEG10 promoter to drive the expression of reporter genes. Plasmid insertion of a PEG10 promoter sequence optimized for enhanced output upstream of a reporter gene allowed detection of prostate cancer by near-infrared and positron emission tomography imaging after systemic administration of the plasmid in vivo. PEG10 expressing subcutaneous xenograft and intratibial tumor models were imaged by both modalities using this molecular genetic imaging strategy. This study demonstrates a preclinical proof-of-concept that the PEG10 promoter is a powerful and specific tool that can be utilized for noninvasive detection of aggressive prostate cancer subtypes. SIGNIFICANCE: PEG10 is expressed by prostate cancer with constitutively active AR-splice variants that can be exploited for noninvasive molecular imaging of this aggressive prostate cancer subytpe.

A Tri-fusion Reporter Mouse Reveals Tissue-Specific FGF1B Promoter Activity in vivo

Transgenic mice harboring imaging reporters take full advantage of imaging technologies in studies using living mice. Here, we established a tri-fusion multimodal reporter gene containing fragments from firefly luciferase, enhanced green fluorescent protein, and herpes simplex virus type 1 thymidine kinase and generated tri-fusion reporter Tg mice. Fibroblast growth factor type 1 (FGF1), a multifunctional mitogen to a wide range of tissues, regulates proliferation of neural stem cells of the brain, where FGF1 expression is initiated through activation of the FGF1B (F1B) promoter. The reporter mouse under the control of the human F1B promoter enables visualization in vivo where F1B activity is elevated, including tissues not only in the brain but also in the nasopharynx, skull, spine, and testes, particularly in Leydig cells. Treating Tg mice with the alkylating agent busulfan, which is known to eradicate Leydig cells and disrupt spermatogenesis in mice, eliminated the reporter signals. Restoring Leydig cells recovered reporter expression, indicating that the reporter can be used as a surrogate marker for Leydig cells. The F1B tri-fusion reporter mouse model can be utilized in longitudinal monitoring of the health status of the male reproductive system, such as in studies exploring the toxicity of chemicals to spermatogenesis.

A Fluorescent, [18F]-Positron-Emitting Agent for Imaging Prostate-Specific Membrane Antigen Allows Genetic Reporting in Adoptively Transferred, Genetically Modified Cells

Clinical trials involving genome-edited cells are growing in popularity, where CAR-T immunotherapy and CRISPR/Cas9 editing are more recognized strategies. Genetic reporters are needed to localize the molecular events inside these cells in patients. Specifically, a nonimmunogenic genetic reporter is urgently needed as current reporters are immunogenic due to derivation from nonhuman sources. Prostate-specific membrane antigen (PSMA) is potentially nonimmunogenic due to its natural, low-level expression in select tissues (self-MHC display). PSMA overexpression on human prostate adenocarcinoma is also visible with excellent contrast. We exploit these properties in a transduced, two-component, Human-Derived, Genetic, Positron-emitting, and Fluorescent (HD-GPF) reporter system. Mechanistically analogous to the luciferase and luciferin reporter, PSMA is genetically encoded into non-PSMA expressing 8505C cells and tracked with ACUPA-Cy3-BF3, a single, systemically injected small molecule that delivers positron emitting fluoride (¹⁸F) and a fluorophore (Cy3) to report on cells expressing PSMA. PSMA-lentivirus transduced tissues become visible by Cy3 fluorescence, [¹⁸F]-positron emission tomography (PET), and γ-scintillated biodistribution. HD-GPF fluorescence is visible at subcellular resolution, while a reduced PET background is achieved in vivo, due to rapid ACUPA-Cy3-BF3 renal excretion. Co-transduction with luciferase and GFP show specific advantages over popular genetic reporters in advanced murine models including, a "mosaic" model of solid-tumor intratumoral heterogeneity and a survival model for observing postsurgical recurrence. We report an advanced genetic reporter that tracks genetically modified cells in entire animals and with subcellular resolution with PET and fluorescence, respectively. This reporter system is potentially nonimmunogenic and will therefore be useful in human studies. PSMA is a biomarker of prostate adenocarcinoma and ACUPA-Cy3-BF3 potential in radical prostatectomy is demonstrated.

Stem Cell Imaging: Tools to Improve Cell Delivery and Viability

Stem cell therapy (SCT) has shown very promising preclinical results in a variety of regenerative medicine applications. Nevertheless, the complete utility of this technology remains unrealized. Imaging is a potent tool used in multiple stages of SCT and this review describes the role that imaging plays in cell harvest, cell purification, and cell implantation, as well as a discussion of how imaging can be used to assess outcome in SCT. We close with some perspective on potential growth in the field.

Design of a functional cyclic HSV1-TK reporter and its application to PET imaging of apoptosis

Positron emission tomography (PET) is a sensitive and noninvasive imaging method that is widely used to explore molecular events in living subjects. PET can precisely and quantitatively evaluate cellular apoptosis, which has a crucial role in various physiological and pathological processes. In this protocol, we describe the design and use of an engineered cyclic herpes simplex virus 1-thymidine kinase (HSV1-TK) PET reporter whose kinase activity is specifically switched on by apoptosis. The expression of cyclic TK (cTK) in healthy cells leads to inactive product, whereas the activation of apoptosis through the caspase-3 pathway cleaves cTK, thus restoring its activity and enabling PET imaging. In addition to detailing the design and construction of the cTK plasmid in this protocol, we include assays for evaluating the function and specificity of the cTK reporter in apoptotic cells, such as assays for measuring the cell uptake of PET tracer in apoptotic cells, correlating doxorubicin (Dox)-induced cell apoptosis to cTK function recovery, and in vivo PET imaging of cancer cell apoptosis, and we also include corresponding data acquisition methods. The time to build the entire cTK reporter is ∼2-3 weeks. The selection of a stable cancer cell line takes ∼4-6 weeks. The time to implement assays regarding cTK function in apoptotic cells and the in vivo imaging varies depending on the experiment. The cyclization strategy described in this protocol can also be adapted to create other reporter systems for broad biomedical applications.

Molecular imaging in tracking tumor-specific cytotoxic T lymphocytes (CTLs)

Despite the remarkable progress of adoptive T cell therapy in cancer treatment, there remains an urgent need for the noninvasive tracking of the transfused T cells in patients to determine their biodistribution, viability, and functionality. With emerging molecular imaging technologies and cell-labeling methods, noninvasive in vivo cell tracking is experiencing impressive progress toward revealing the mechanisms and functions of these cells in real time in preclinical and clinical studies. Such cell tracking methods have an important role in developing effective T cell therapeutic strategies and steering decision-making process in clinical trials. On the other hand, they could provide crucial information to accelerate the regulatory approval process on the T cell therapy. In this review, we revisit the advances in tracking the tumor-specific CTLs, highlighting the latest development in human studies and the key challenges.

Evaluation of prostate-specific membrane antigen as an imaging reporter

Genetic reporters provide a noninvasive method to monitor and evaluate a population of cells. The ideal properties of a gene reporter-probe system include biocompatibility, lack of immunogenicity, low background expression or signal, and high sensitivity of detection. The prostate-specific membrane antigen (PSMA) is an attractive candidate for a genetic reporter as it is a human transmembrane protein with a selective expression pattern, and there are several PSMA imaging agents available for clinical and preclinical applications. We evaluated the use of PSMA as a genetic imaging reporter by comparison to 2 clinically established reporters, the mutant herpes simplex virus type I thymidine kinase and the human sodium-iodide symporter.

METHODS

Adenoviruses expressing each reporter were constructed and validated in vitro for expression and function. To compare PSMA with existing imaging reporters, a bilateral Matrigel suspension model was established with nude mice bearing cells equally infected with each reporter or control adenovirus. Dynamic PET was performed, and time-activity curves were generated for each reporter-probe pair.

RESULTS

A comparison of peak target-to-background ratios revealed that PSMA offered the highest ratio relative to the control Matrigel suspension as well as muscle. Further, as proof of concept, PSMA was applied as an imaging reporter to monitor adenoviral liver transduction with both nuclear and optical imaging probes.

CONCLUSION

These preliminary studies support further development of PSMA as a noninvasive genetic reporter.

Synthesis of a probe for monitoring HSV1-tk reporter gene expression using chemical exchange saturation transfer MRI

In experiments involving transgenic animals or animals treated with transgenic cells, it is important to have a method to monitor the expression of the relevant genes longitudinally and noninvasively. An MRI-based reporter gene enables monitoring of gene expression in the deep tissues of living subjects. This information can be co-registered with detailed high-resolution anatomical and functional information. We describe here the synthesis of the reporter probe, 5-methyl-5,6-dihydrothymidine (5-MDHT), which can be used for imaging of the herpes simplex virus type 1 thymidine kinase (HSV1-tk) reporter gene expression in rodents by MRI. The protocol also includes data acquisition and data processing routines customized for chemical exchange saturation transfer (CEST) contrast mechanisms. The dihydropyrimidine 5-MDHT is synthesized through a catalytic hydrogenation of the 5,6-double bond of thymidine to yield 5,6-dihydrothymidine, which is methylated on the C-5 position of the resulting saturated pyrimidine ring. The synthesis of 5-MDHT can be completed within 5 d, and the compound is stable for more than 1 year.

A transgenic tri-modality reporter mouse

Transgenic mouse with a stably integrated reporter gene(s) can be a valuable resource for obtaining uniformly labeled stem cells, tissues, and organs for various applications. We have generated a transgenic mouse model that ubiquitously expresses a tri-fusion reporter gene (fluc2-tdTomato-ttk) driven by a constitutive chicken β-actin promoter. This “Tri-Modality Reporter Mouse” system allows one to isolate most cells from this donor mouse and image them for bioluminescent (fluc2), fluorescent (tdTomato), and positron emission tomography (PET) (ttk) modalities. Transgenic colonies with different levels of tri-fusion reporter gene expression showed a linear correlation between all three-reporter proteins (R²=0.89 for TdTomato vs Fluc, R²=0.94 for Fluc vs TTK, R²=0.89 for TdTomato vs TTK) in vitro from tissue lysates and in vivo by optical and PET imaging. Mesenchymal stem cells (MSCs) isolated from this transgenics showed high level of reporter gene expression, which linearly correlated with the cell numbers (R²=0.99 for bioluminescence imaging (BLI)). Both BLI (R²=0.93) and micro-PET (R²=0.94) imaging of the subcutaneous implants of Tri-Modality Reporter Mouse derived MSCs in nude mice showed linear correlation with the cell numbers and across different imaging modalities (R²=0.97). Serial imaging of MSCs transplanted to mice with acute myocardial infarction (MI) by intramyocardial injection exhibited significantly higher signals in MI heart at days 2, 3, 4, and 7 (p<0.01). MSCs transplanted to the ischemic hindlimb of nude mice showed significantly higher BLI and PET signals in the first 2 weeks that dropped by 4^th week due to poor cell survival. However, laser Doppler perfusion imaging revealed that blood circulation in the ischemic limb was significantly improved in the MSCs transplantation group compared with the control group. In summary, this mouse can be used as a source of donor cells and organs in various research areas such as stem cell research, tissue engineering research, and organ transplantation.

Synthesis and biological evaluation of a new acyclic pyrimidine derivative as a probe for imaging herpes simplex virus type 1 thymidine kinase gene expression

With the idea of finding a more selective radiotracer for imaging herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene expression by means of positron emission tomography (PET), a novel [¹⁸F]fluorine radiolabeled pyrimidine with 4-hydroxy-3-(hydroxymethyl)butyl side chain at N-1 (HHB-5-[¹⁸F]FEP) was prepared and evaluated as a potential PET probe. Unlabeled reference compound, HHB-5-FEP, was synthesized via a five-step reaction sequence starting from 5-(2-acetoxyethyl)-4-methoxypyrimidin-2-one. The radiosynthesis of HHB-[¹⁸F]-FEP was accomplished by nucleophilic radiofluorination of a tosylate precursor using [¹⁸F]fluoride-cryptate complex in 45% ± 4 (n = 4) radiochemical yields and high purity (>99%). The biological evaluation indicated the feasibility of using HHB-5-[¹⁸F]FEP as a PET radiotracer for monitoring HSV1-tk expression in vivo.

Arginine transcriptional response does not require inositol phosphate synthesis

Inositol phosphates are key signaling molecules affecting a large variety of cellular processes. Inositol-polyphosphate multikinase (IPMK) is a central component of the inositol phosphate biosynthetic routes, playing essential roles during development. IPMK phosphorylates inositol 1,4,5-trisphosphate to inositol tetrakisphosphate and subsequently to inositol pentakisphosphate and has also been described to function as a lipid kinase. Recently, a catalytically inactive mammalian IPMK was reported to be involved in nutrient signaling by way of mammalian target of rapamycin and AMP-activated protein kinase. In yeast, the IPMK homologue, Arg82, is the sole inositol-trisphosphate kinase. Arg82 has been extensively studied as part of the transcriptional complex regulating nitrogen sensing, in particular arginine metabolism. Whether this role requires Arg82 catalytic activity has long been a matter of contention. In this study, we developed a novel method for the real time study of promoter strength in vivo and used it to demonstrate that catalytically inactive Arg82 fully restored the arginine-dependent transcriptional response. We also showed that expression in yeast of catalytically active, but structurally very different, mammalian or plant IPMK homologue failed to restore arginine regulation. Our work indicates that inositol phosphates do not regulate arginine-dependent gene expression.

Imaging beyond the diagnosis: image-guided enzyme/prodrug cancer therapy

The ideal therapy would target cancer cells while sparing normal tissue. However, in most conventional chemotherapies normal cells are damaged together with cancer cells resulting in the unfortunate side effects. The principle underlying enzyme/prodrug therapy is that a prodrug-activating enzyme is delivered or expressed in tumor tissue following which a non-toxic prodrug is administered systemically. Non-invasive imaging modalities can fill an important niche in guiding prodrug administration when the enzyme concentration is detected to be high in the tumor tissue but low in the normal tissue. Therefore, high therapeutic efficacy with minimized toxic effect can be anticipated. This review introduces the latest developments of molecular imaging in enzyme/prodrug cancer therapies. We focus on the application of imaging modalities including magnetic resonance imaging, position emission tomography and optical imaging in monitoring the enzyme delivery/expression, guiding the prodrug administration and evaluating the real-time therapeutic response in vivo.

In vivo trafficking and immunostimulatory potential of an intranasally-administered primary dendritic cell-based vaccine

BACKGROUND

Coccidioidomycosis or Valley fever is caused by a highly virulent fungal pathogen: Coccidioides posadasii or immitis. Vaccine development against Coccidioides is of contemporary interest because a large number of relapses and clinical failures are reported with antifungal agents. An efficient Th1 response engenders protection. Thus, we have focused on developing a dendritic cell (DC)-based vaccine for coccidioidomycosis. In this study, we investigated the immunostimulatory characteristics of an intranasal primary DC-vaccine in BALB/c mouse strain that is most susceptible to coccidioidomycosis. The DCs were transfected nonvirally with Coccidioides-Ag2/PRA-cDNA. Expression of DC-markers, Ag2/PRA and cytokines were studied by flow cytometry, dot-immunoblotting and cytometric bead array methods, respectively. The T cell activation was studied by assessing the upregulation of activation markers in a DC-T cell co-culture assay. For trafficking, the DCs were co-transfected with a plasmid DNA encoding HSV1 thymidine kinase (TK) and administered intranasally into syngeneic mice. The trafficking and homing of TK-expressing DCs were monitored with positron emission tomography (PET) using 18F-FIAU probe. Based on the PET-probe accumulation in vaccinated mice, selected tissues were studied for antigen-specific response and T cell phenotypes using ELISPOT and flow cytometry, respectively.

RESULTS

We found that the primary DCs transfected with Coccidioides-Ag2/PRA-cDNA were of immature immunophenotype, expressed Ag2/PRA and activated naïve T cells. In PET images and subsequent biodistribution, intranasally-administered DCs were found to migrate in blood, lung and thymus; lymphocytes showed generation of T effector memory cell population (T(EM)) and IFN-γ release.

CONCLUSIONS

In conclusion, our results demonstrate that the intranasally-administered primary DC vaccine is capable of inducing Ag2/PRA-specific T cell response. Unique approaches utilized in our study represent an attractive and novel means of producing and evaluating an autologous DC-based vaccine.

Molecular imaging as a tool for translating breast cancer science

The ability to measure biochemical and molecular processes underlies progress in breast cancer biology and treatment. These assays have traditionally been performed by analysis of cell culture or tissue samples. More recently, functional and molecular imaging has allowed the in vivo assay of biochemistry and molecular biology, which is highly complementary to tissue-based assays. This review briefly describes different imaging modalities used in molecular imaging and then reviews applications of molecular imaging to breast cancer, with a focus on translational work. It includes sections describing work in functional and physiological tumor imaging, imaging gene product expression, imaging the tumor microenvironment, reporter gene imaging, and cell labeling. Work in both animal models and human is discussed with an eye towards studies that have relevance to breast cancer treatment in patients.

Noninvasive detection of therapeutic cytolytic T cells with 18F-FHBG PET in a patient with glioma

BACKGROUND

A 57-year-old man had been diagnosed with grade IV glioblastoma multiforme and was enrolled in a trial of adoptive cellular immunotherapy. The trial involved infusion of ex vivo expanded autologous cytolytic CD8+ T cells (CTLs), genetically engineered to express the interleukin 13 zetakine gene (which encodes a receptor protein that targets these T cells to tumor cells) and the herpes simplex virus 1 thymidine kinase (HSV1 tk) suicide gene, and PET imaging reporter gene.

INVESTIGATIONS

MRI, whole-body and brain PET scan with (18)F-radiolabelled 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine ((18)F-FHBG) to detect CTLs that express HSV1 tk, and safety monitoring after injection of (18)F-FHBG.

DIAGNOSIS

MRI detected grade III-IV glioblastoma multiforme plus two tumors recurrences that developed after resection of the initial tumor.

MANAGEMENT

Surgical resection of primary glioblastoma tumor, enrollment in CTL therapy trial, reresection of glioma recurrences, infusion of approximately 1 x 10(9) CTLs into the site of tumor reresection, and (18)F-FHBG PET scan to detect infused CTLs.

Applications of lentiviral vectors in noninvasive molecular imaging

Noninvasive imaging of molecular-genetic and cellular processes is an effective way to determine the location(s), magnitude, and time variation of action of gene products used for many therapeutic strategies. Lentiviral vectors provide effective means for the delivery, integration, and expression of transgenes in cultured mammalian cells as well as in vivo. Therefore, the combination of lentiviral vector-mediated therapeutic and imaging-targeted reporter gene delivery to various target organs holds promise for the future treatment of diseases. In this chapter, we provide protocols for developing lentiviral vectors that can be utilized for noninvasive monitoring/imaging of reporter gene expression. We have described the procedures to perform cellular assays and animal imaging based on positron emission tomography (PET), optical bioluminescence, and fluorescence reporter genes. The protocols described here are standardized for mouse models, which can also be adapted for other small animal models (e.g., rats).

PET imaging of herpes simplex virus type 1 thymidine kinase (HSV1-tk) or mutant HSV1-sr39tk reporter gene expression in mice and humans using [18F]FHBG

The herpes simplex virus type 1 thymidine kinase (HSV1-tk) positron emission tomography (PET) reporter gene (PRG) or its mutant HSV1-sr39tk are used to investigate intracellular molecular events in cultured cells and to image intracellular molecular events and cell trafficking in living subjects. The expression of these PRGs can be imaged using 18F- or 124I-radiolabeled acycloguanosine or pyrimidine analog PET reporter probes (PRPs). This protocol describes the procedures for imaging HSV1-tk or HSV1-sr39tk PRG expression in living subjects with the acycloguanosine analog 9-4-[18F]fluoro-3-(hydroxymethyl)butyl]guanine ([18F]FHBG). [18F]FHBG is a high-affinity substrate for the HSV1-sr39TK enzyme with relatively low affinity for mammalian TK enzymes, resulting in improved detection sensitivity. Furthermore, [18F]FHBG is approved by the US Food and Drug Administration as an investigational new imaging agent and has been shown to detect HSV1-tk transgene expression in the liver tumors of patients. MicroPET imaging of each small animal can be completed in approximately 1.5 h, and each patient imaging session takes approximately 3 h.

Studying the biodistribution of positron emission tomography reporter probes in mice

Positron emission tomography (PET) reporter probes (PRPs) are used to detect PET reporter gene (PRG) expression in living subjects. This article details protocols for analyzing the biodistribution of a PRP used to detect herpes simplex virus 1 thymidine kinase (HSV1-tk) or mutant HSV1-sr39tk PRG expression. However, the methods described are generalizable to other beta- or gamma/positron-emitting probes. Accumulation of PRPs in animal tissues can be determined by counting PRP activity of isolated tissues, whereas digital whole-body autoradiography (DWBA) provides high-resolution images of PRP biodistribution in 5- to 45-microm tissue slices of killed research animals at a single time point. Biodistribution assay results may be obtained in less than a week after beginning the assay, and DWBA image acquisitions can take up to 3 months depending on the probe's radioisotope.