Control of erythropoietin secretion by doxycycline or mifepristone in mice bearing polymer-encapsulated engineered cells

Cell microencapsulation with synthetic polymers

The encapsulation of cells into polymeric microspheres or microcapsules has permitted the transplantation of cells into human and animal subjects without the need for immunosuppressants. Cell-based therapies use donor cells to provide sustained release of a therapeutic product, such as insulin, and have shown promise in treating a variety of diseases. Immunoisolation of these cells via microencapsulation is a hotly investigated field, and the preferred material of choice has been alginate, a natural polymer derived from seaweed due to its gelling conditions. Although many natural polymers tend to gel in conditions favorable to mammalian cell encapsulation, there remain challenges such as batch to batch variability and residual components from the original source that can lead to an immune response when implanted into a recipient. Synthetic materials have the potential to avoid these issues; however, historically they have required harsh polymerization conditions that are not favorable to mammalian cells. As research into microencapsulation grows, more investigators are exploring methods to microencapsulate cells into synthetic polymers. This review describes a variety of synthetic polymers used to microencapsulate cells.

Detection of EPO gene doping in blood

Gene doping--or the abuse of gene therapy--will continue to threaten the sports world. History has shown that progress in medical research is likely to be abused in order to enhance human performance. In this review, we critically discuss the progress and the risks associated with the field of erythropoietin (EPO) gene therapy and its applicability to EPO gene doping. We present typical vector systems that are employed in ex vivo and in vivo gene therapy trials. Due to associated risks, gene doping is not a feasible alternative to conventional EPO or blood doping at this time. Nevertheless, it is well described that about half of the elite athlete population is in principle willing to risk its health to gain a competitive advantage. This includes the use of technologies that lack safety approval. Sophisticated detection approaches are a prerequisite for prevention of unapproved and uncontrolled use of gene therapy technology. In this review, we present current detection approaches for EPO gene doping, with a focus on blood-based direct and indirect approaches. Gene doping is detectable in principle, and recent DNA-based detection strategies enable long-term detection of transgenic DNA (tDNA) following in vivo gene transfer.

Control of the timing and dosage of IGF-I delivery from encapsulated cells

We report here on the development and characterization of a cell-based system for the regulated delivery of bioactive insulin-like growth factor I (IGF-I). A stable mammalian cell line, CHO-K1 Tet-IGFI, was genetically modified to have tetracycline-induced transcription of the human IGF-I gene. Cells were activated to express IGF-I in the presence of doxycycline (DOX), a tetracycline derivative, while expression was inactivated in the absence of DOX. Temporal, or on-off, release of IGF-I from cells encapsulated within Ca²⁺-alginate hydrogels was demonstrated in a pilot study over the course of 10 days in culture. Released growth factor was bioactive, exhibiting a proliferative effect comparable to recombinant purified IGF-I protein. The dosage levels and temporal control of IGF-I release from encapsulated cells meet the requirements of orthopedic wound repair, making this approach an attractive means for the controlled synthesis and delivery of growth factors in situ for wound healing.

Lack of humoral immune response to the tetracycline (Tet) activator in rats injected intracranially with Tet-off rAAV vectors

The ability to safely control transgene expression from viral vectors is a long-term goal in the gene therapy field. We have previously reported tight regulation of GFP expression in rat brain using a self-regulating tet-off rAAV vector. The immune responses against tet regulatory elements observed by other groups in nonhuman primates after intramuscular injection of tet-on encoding vectors raise concerns about the clinical value of tet-regulated vectors. However, previous studies have not examined immune responses following injection of AAV vectors into brain. Therefore, rat striatum was injected with tet-off rAAV harboring a therapeutic gene for Parkinson's disease, either hAADC or hGDNF. The expression of each gene was tightly controlled by the tet-off regulatory system. Using an ELISA developed with purified GST-tTA protein, no detectable immunogenicity against tTA was observed in sera of rats that received an intrastriatal injection of either vector. In contrast, sera from rats intradermally injected with an adenovirus containing either tTA or rtTA, as positive controls, had readily detectable antibodies. These observations suggest that tet-off rAAV vectors do not elicit an immune response when injected into rat brain and that these may offer safer vectors for Parkinson's disease than vectors with constitutive expression.

In vivo gene regulation using tetracycline-regulatable systems

Numerous preclinical studies have demonstrated the efficacy of viral gene delivery vectors, and recent clinical trials have shown promising results. However, the tight control of transgene expression is likely to be required for therapeutic applications and in some instances, for safety reasons. For this purpose, several ligand-dependent transcription regulatory systems have been developed. Among these, the tetracycline-regulatable system is by far the most frequently used and the most advanced towards gene therapy trials. This review will focus on this system and will describe the most recent progress in the regulation of transgene expression in various organs, including the muscle, the retina and the brain. Since the development of an immune response to the transactivator was observed following gene transfer in the muscle of nonhuman primate, focus will be therefore, given on the immune response to transgene products of the tetracycline inducible promoter.

Vitamin H-regulated transgene expression in mammalian cells

Although adjustable transgene expression systems are considered essential for future therapeutic and biopharmaceutical manufacturing applications, the currently available transcription control modalities all require side-effect-prone inducers such as immunosupressants, hormones and antibiotics for fine-tuning. We have designed a novel mammalian transcription-control system, which is reversibly fine-tuned by non-toxic vitamin H (also referred to as biotin). Ligation of vitamin H, by engineered Escherichia coli biotin ligase (BirA), to a synthetic biotinylation signal fused to the tetracycline-dependent transactivator (tTA), enables heterodimerization of tTA to a streptavidin-linked transrepressor domain (KRAB), thereby abolishing tTA-mediated transactivation of specific target promoters. As heterodimerization of tTA to KRAB is ultimately conditional upon the presence of vitamin H, the system is vitamin H responsive. Transgenic Chinese hamster ovary cells, engineered for vitamin H-responsive gene expression, showed high-level, adjustable and reversible production of a human model glycoprotein in bench-scale culture systems, bioreactor-based biopharmaceutical manufacturing scenarios, and after implantation into mice. The vitamin H-responsive expression systems showed unique band pass filter-like regulation features characterized by high-level expression at low (0–2 nM biotin), maximum repression at intermediate (100–1000 nM biotin), and high-level expression at increased (>100 000 nM biotin) biotin concentrations. Sequential ON-to-OFF-to-ON, ON-to-OFF and OFF-to-ON expression profiles with graded expression transitions can all be achieved by simply increasing the level of a single inducer molecule without exchanging the culture medium. These novel expression characteristics mediated by an FDA-licensed inducer may foster advances in therapeutic cell engineering and manufacturing of difficult-to-produce protein therapeutics.

Systemic and Localized Reversible Regulation of Transgene Expression by Tetracycline with tetR-Mediated Transcription Repression Switch

BACKGROUND

We recently developed a new tetracycline-inducible gene switch employing the tetracycline operator-containing hCMV major immediate-early promoter and the tetracycline repressor, tetR, rather than the previously used tetR-mammalian cell transcription factor fusion derivatives.

MATERIALS AND METHODS

The present study demonstrates that this tetR-mediated transcription repression system can function as a powerful gene switch for On-and-Off regulation of therapeutic gene expression in ex vivo gene transfer protocols. Firstly, for achieving regulated gene expression in a localized tissue environment, R11/OEGF cells, a stable line that expresses hEGF under the control of the tetR-mediated transcription repression switch, were transplanted into porcine full-thickness wounds enclosed by wound chambers.

RESULTS

By topically applying tetracycline in wound chambers at various concentrations or at different time points post-transplantation, the levels and timing of hEGF expression in transplanted wounds could be reversibly regulated by tetracycline. Over 3000-fold induction in hEGF expression was achieved in the local wound microenvironment. Secondly, R11/OEGF cells were intramuscularly injected into NCr outbread nude mice to test the efficacy of intermittent systemic gene delivery of a soluble peptide(s).

CONCLUSIONS

Basal circulating hEGF was undetectable and induced up to at least 1,500-fold after administration of tetracycline. Furthermore, the timing and duration of hEGF expression could be finely adjusted by the presence or the absence of tetracycline in the drinking water.

Long-term doxycycline-regulated transgene expression in the retina of nonhuman primates following subretinal injection of recombinant AAV vectors

Adeno-associated viral gene therapy has shown promise for the treatment of inherited and acquired retinal disorders. In most applications, regulation of expression is a critical concern for both safety and efficacy. The purpose of our study was to evaluate the ability of the tetracycline-regulatable system to establish long-term transgene regulation in the retina of nonhuman primates. Three rAAV vectors expressing the tetracycline-dependent transactivator (rtTA) under the control of either the ubiquitous CAG promoter or the specific RPE65 promoter (AAV2/5.CAG.TetOn.epo, AAV2/4.CAG.TetOn.epo, and AAV2/4.RPE65.TetOn.epo) were generated and administered subretinally to seven macaques. We demonstrated that repeated inductions of transgene expression in the nonhuman primate retina can be achieved using a Tet-inducible system via rAAV vector administration over a long period (2.5 years). Maximum erythropoietin (EPO) secretion in the anterior chamber depends upon the rAAV serotype and the nature of the promoter driving rtTA expression. We observed that the EPO isoforms produced in the retina differ from one another based on the transduced cell type of origin within the retina and also differ from both the physiological EPO isoforms and the isoforms produced by AAV-transduced skeletal muscle.

Pharmacologic transgene control systems for gene therapy

Pharmacologic transgene-expression dosing is considered essential for future gene therapy scenarios. Genetic interventions require precise transcription or translation fine-tuning of therapeutic transgenes to enable their titration into the therapeutic window, to adapt them to daily changing dosing regimes of the patient, to integrate them seamlessly into the patient's transcriptome orchestra, and to terminate their expression after successful therapy. In recent years, decisive progress has been achieved in designing high-precision trigger-inducible mammalian transgene control modalities responsive to clinically licensed and inert heterologous molecules or to endogenous physiologic signals. Availability of a portfolio of compatible transcription control systems has enabled assembly of higher-order control circuitries providing simultaneous or independent control of several transgenes and the design of (semi-)synthetic gene networks, which emulate digital expression switches, regulatory transcription cascades, epigenetic expression imprinting, and cellular transcription memories. This review provides an overview of cutting-edge developments in transgene control systems, of the design of synthetic gene networks, and of the delivery of such systems for the prototype treatment of prominent human disease phenotypes.

Recombinant AAV-mediated gene transfer to the retina: gene therapy perspectives

Retinal degenerative diseases such as retinal macular degeneration and retinitis pigmentosa constitute a broad group of diseases that all share one critical feature, the progressive apoptotic loss of cells in the retina. There is currently no effective treatment available by which the course of these disorders can be modified, and visual dysfunction often progresses to total blindness. Gene therapy represents an attractive approach to treating retinal degeneration because the eye is easily accessible and allows local application of therapeutic vectors with reduced risk of systemic effects. Furthermore, transgene expression within the retina and effects of treatments may be monitored by a variety of noninvasive examinations. An increasing number of strategies for molecular treatment of retinal disease rely on recombinant adeno-associated virus (rAAV) as a therapeutic gene delivery vector. Before rAAV-mediated gene therapy for retinal degeneration becomes a reality, there are a number of important requirements that include: (1) evaluation of different rAAV serotypes, (2) screening of vectors in large animals in order to ensure that they mediate safe and long-term gene expression, (3) appropriate regulation of therapeutic gene expression, (4) evaluation of vectors carrying a therapeutic gene in relevant animal models, (5) identification of suitable patients, and finally (6) manufacture of clinical grade vector. All these steps towards gene therapy are still being explored. Outcomes of these studies will be discussed in the order in which they occur, from vector studies to preclinical assessment of the therapeutic potential of rAAV in animal models of retinal degeneration.

Human-compatible collagen matrix for prolonged and reversible systemic delivery of erythropoietin in mice from gene-modified marrow stromal cells

Bone marrow stromal cells (MSCs) can be exploited therapeutically in transgenic cell therapy approaches. Our aim was to determine if gene-modified MSCs sequestered within a clinically approved, bovine type I collagen-based viscous bulking material could serve as a retrievable implant for systemic delivery of erythropoietin (Epo). To test this hypothesis, we embedded Epo-secreting MSCs in viscous collagen (Contigen) and determined the pharmacological effect following implantation in normal mice. Primary MSCs from C57Bl/6 mice were retrovirally engineered to express murine Epo (mEpo) and 10(7) cells of a clonal population secreting 3 U of mEpo/10(6) cells/24 h were implanted subcutaneously in normal C57Bl/6 mice with and without viscous collagen. Without matrix support, Hct rose to >70% for <25 days and returned to baseline by 60 days. However, in mice implanted with viscous collagen-embedded MSCs, the Hct rose to >70% up to 203 days postimplantation (P < 0.0001). In parallel, plasma Epo concentration was significantly increased (P < 0.05) for >145 days. Moreover, surgical removal of the viscous collagen organoid 24 days after implantation led to reduction of Hct to baseline levels within 14 days. In conclusion, this investigation demonstrates that mEpo(+) MSCs embedded in a human-compatible viscous collagen matrix offers a potent, durable, and reversible approach for delivery of plasma-soluble therapeutic proteins.

Lentivectors for regulated and reversible cutaneous gene delivery

Systemic administration of therapeutic proteins has value in treating a wide variety of disorders, including erythropoietin (Epo)-responsive anemias. Recombinant proteins, however, are costly and require repeated injections, while gene delivery approaches have suffered from inefficiency and difficulties with regulation. The skin effectively delivers polypeptides to the circulation, and improved approaches would support sustainable, topically regulated protein expression after a single vector injection. Toward this goal, we generated lentivectors in which both gene delivery and persistence in skin are regulated by administration of distinct steroid ligands. Following a single injection of regulated lentivector into human skin regenerated on immunodeficient mice, topical glucocorticoid ligands regulated Epo levels and hematocrit over time. Abrogation of gene delivery was achieved by both glucocorticoid cessation and proviral excision via a 4-hydroxytamoxifen-inducible Cre recombinase. These findings establish an approach to durable, topically controlled systemic delivery of therapeutic proteins from human skin tissue.

In vivo calcium deposition on polyvinyl alcohol matrix used in hollow fiber cell macroencapsulation devices

The encapsulation of genetically modified cells represents a promising approach for the delivery of therapeutic proteins. The functionality of the device is dependent on the characteristics of the biomaterials, the procedures used in its confection and the adaptability of the encapsulated cells in the host. We report conditions leading to the development of calcifications on the polyvinyl alcohol (PVA) matrix introduced in hollow fiber devices for the encapsulation of primary human fibroblasts implanted in mice. The manufacturing procedures, batches of PVA matrix and cell lineages were assessed for their respective role in the development of the phenomenon. The results showed that the calcification is totally prevented by substituting phosphate-buffer saline with ultra-pure sterile water in the rinsing procedure of the matrix. Moreover, a positive correlation was found, when comparing two fibroblast cell lineages, between the level of lactate dehydrogenase (LDH) activity measured in the cells and the degree of calcium deposition. Higher LDH activity may decrease calcium depositions because it generates in the device a more acidic microenvironment inhibiting calcium precipitation. The present study defines optimized conditions for the encapsulation of primary human fibroblasts in order to avoid potentially detrimental calcifications and to allow long-term survival of encapsulated cells.

Survival of Encapsulated Human Primary Fibroblasts and Erythropoietin Expression Under Xenogeneic Conditions

Allogeneic cells are the most attractive source for cell transplantation, as the use of xenogeneic cells is hampered by safety concerns and the use of autologous cells involves practical difficulties. The immune rejection of allogeneic cells can be overcome by physical immunoprotection provided by polymer encapsulation. To study the variability of cell and donor sources, we compared different primary human cells as candidates for gene therapy-mediated delivery of human erythropoietin (hEpo). DARC-3.1 fibroblasts, MDX-01 fibroblasts, and ARPE-19 retinal pigment epithelial cells were encapsulated into polyethersulfone hollow fibers and implanted for 1 month in nude mice as well as in immunocompetent and FK506-immunosuppressed mice to test their in vivo resistance, with the assumption that xenogeneic conditions constitute a stringent model for human application. DARC-3.1 fibroblasts showed the best survival, prompting us to evaluate cell lineages from the same donor (DARC-3.2) or another donor (DARC-4.3 and DARC-4.4). With the exception of DARC-4.3, the remaining three lineages showed comparable survival in immunocompetent C3H and DBA/2J mice. DARC-3.1 fibroblasts were retrovirally engineered with hEpo cDNA, reaching a secretion level of 170 IU of hEpo per 10(6) cells per day. Encapsulated DARC-3.1-hEpo cells led to significantly increased hematocrits in the various hosts and under various transplantation conditions. The present study shows that encapsulated primary human DARC-3.1 fibroblasts are able to survive under xenogeneic conditions and, once engineered with hEpo cDNA, to increase the hematocrit of transplanted mice.

Drugs for increasing oxygen and their potential use in doping: a review

Blood oxygenation is a fundamental factor in optimising muscular activity. Enhancement of oxygen delivery to tissues is associated with a substantial improvement in athletic performance, particularly in endurance sports. Progress in medical research has led to the identification of new chemicals for the treatment of severe anaemia. Effective and promising molecules have been created and sometimes used for doping purposes. The aim of this review is to present methods, and drugs, known to be (or that might be) used by athletes to increase oxygen transport in an attempt to improve endurance capacity. These methods and drugs include: (i) blood transfusion; (ii) endogenous stimulation of red blood cell production at altitude, or using hypoxic rooms, erythropoietins (EPOs), EPO gene therapy or EPO mimetics; (iii) allosteric effectors of haemoglobin; and (iv) blood substitutes such as modified haemoglobin solutions and perfluorochemicals. Often, new chemicals are used before safety tests have been completed and athletes are taking great health risks. Such new chemicals have also created the need for new instrumental strategies in doping control laboratories, but not all of these chemicals are detectable. Further progress in analytical research is necessary.

Sustained tetracycline-regulated transgene expression in vivo in rat retinal ganglion cells using a single type 2 adeno-associated viral vector

BACKGROUND

Viral vector delivery of neurotrophic-expressing transgenes in the retina may retard or prevent the onset of blindness associated with photoreceptor degeneration. A key safety issue is to achieve regulated expression of these genes in the retina. The purpose of our study was to evaluate whether a single recombinant AAV-2 (rAAV) encoding for a tetracycline (Tet)-regulated destabilized reporter gene could provide quantitative profiles of gene regulation targeted to the rat neuroretina.

METHODS

A rAAV vector carrying a destabilized green fluorescent protein (dgfp) under a tet-regulatable promoter and the tetracycline-repressed transactivator (tTA) was generated (rAAVtetoff.dgfp) and administered intravitreally in nine Wistar rats. Retinas were monitored for 6 months using noninvasive fluorescence imaging and the animals were subjected to two cycles of doxycycline (Dox), a tetracycline analog. Eyes were ultimately examined by histology.

RESULTS

Intravitreal injection of rAAVtetoff.dgfp resulted in effective transduction of ganglion cells. Following full expression of the transgene in the absence of Dox, 95% of the GFP signal was shut down 48 h post Dox administration and the signal was undetectable 7 days later. Initial levels of GFP expression were restored 21 days after Dox administration ceased. This pattern of expression was repeated twice over a period of 6 months.

CONCLUSIONS

This report demonstrates that rAAVtetoff.dgfp intravitreally injected rats displayed tight and sustained long-term regulation of the reporter gene in ganglion cells. These findings may have important implications regarding rAAV-mediated gene therapy using neuroprotective approaches for retinitis pigmentosa and glaucoma.

Optimization of human erythropoietin secretion from MLV-infected human primary fibroblasts used for encapsulated cell therapy

BACKGROUND

The transplantation of encapsulated cells genetically engineered to secrete human erythropoietin (hEpo) represents an alternative to repeated injections of the recombinant hormone for the treatment of Epo-responsive anemia. In the present study, the ability of primary human foreskin fibroblasts to secrete high levels of hEpo and the importance of cis-acting elements and infection conditions on transgene expression level were assessed.

METHODS

The transduction efficiency was first evaluated with beta-galactosidase (LacZ)-encoding retroviral vectors derived from the murine leukemia retrovirus (MLV) pseudotyped either with an amphotropic envelope or with the G glycoprotein of vesicular stomatitis virus (VSV-G). Human fibroblasts were then infected with an amphotropic hEpo-expressing retroviral vector, which was modified by insertion of a post-transcriptional regulatory element from the woodchuck hepatitis virus (WPRE) and a Kozak consensus sequence (KZ). Human Epo production was further optimized by increasing the multiplicity of infection and by selecting high producer cells. The survival and the transgene expression of these fibroblasts were finally evaluated in vivo. The cells were encapsulated into microporous hollow fibers and subcutaneously implanted in nude mice.

RESULTS

A secretion level of approximately 5 IU hEpo/10(6) cells/day was obtained with the basal vector. A 7.5-fold increase in transgene expression was observed with the insertion of WPRE and KZ elements. Finally, according to the optimization of infection conditions, we obtained a 40-fold increase in hEpo secretion, reaching approximately 200 IU hEpo/10(6) cells/day. The in vivo experiments showed an increase in the hematocrit during the first 2 weeks and elevated levels exceeding 60% were maintained over a 6-week period.

CONCLUSIONS

These results indicate that primary human fibroblasts represent a promising source for encapsulated cell therapy.

Injection of genetically engineered fibroblasts corrects regenerated human epidermolysis bullosa skin tissue

Current therapeutic strategies for genetic skin disorders rely on the complex process of grafting genetically engineered tissue to recipient wound beds. Because fibroblasts synthesize and secrete extracellular matrix, we explored their utility in recessive dystrophic epidermolysis bullosa (RDEB), a blistering disease due to defective extracellular type VII collagen. Intradermal injection of RDEB fibroblasts overexpressing type VII collagen into intact RDEB skin stably restored correctly localized type VII collagen expression in vivo and normalized hallmark RDEB disease features, including subepidermal blistering and anchoring fibril defects. This article was published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Hollow fiber membrane diffusive permeability regulates encapsulated cell line biomass, proliferation, and small molecule release

Using histological and HPLC methods, we examined the influence of hollow fiber membrane transport properties on encapsulated PC12 cell biomass, proliferation and the release of dopamine over 4 weeks in culture. Our data indicated that encapsulated cell biomass, the number of proliferating cells, and the quantity of dopamine released increased as a function of increasing hollow fiber encapsulation membrane diffusive permeability. Overall the percentage of viable cells and the biomass architecture, however, was not significantly affected by differences in membrane transport. When compared to membrane sieving properties, membrane diffusive transport and membrane hydraulic permeability were better indicators of biomass size, proliferating cell number, and dopamine release from encapsulated cells. Studies examining the sustained release of DA from membranes of differing permeability suggest that membrane diffusive permeability can be used to regulate the quantity of small molecules released per unit time at steady state, and should be considered when dosing is an important determinant of implant efficacy.

Lack of an immune response against the tetracycline-dependent transactivator correlates with long-term doxycycline-regulated transgene expression in nonhuman primates after intramuscular injection of recombinant adeno-associated virus

We previously documented persistent regulation of erythropoietin (Epo) secretion in mice after a single intramuscular (i.m.) injection of a recombinant adeno-associated virus (rAAV) vector harboring both the tetracycline-dependent transactivator (rtTA) and the Epo cDNA (D. Bohl, A. Salvetti, P. Moullier, and J. M. Heard, Blood 92:1512-1517, 1998). Using the same vector harboring the cynomolgus macaque Epo cDNA instead, the present study evaluated the ability of the tetracycline-regulatable (tetR) system to establish long-term transgene regulation in nonhuman primates. The vector was administered i.m., after which 5-day induction pulses were performed monthly for up to 13 months by using doxycycline (DOX), a tetracycline analog. We show that initial inductions were successful in all individuals and that there was a tight regulation and a rapid deinduction pattern upon DOX withdrawal. For one macaque, regulation of Epo secretion was maintained during the entire experimental period; for the five remaining macaques, secreted Epo became indistinguishable from endogenous Epo upon repeated DOX inductions. We investigated the mechanism involved and showed that, except in the animal in which secretion persisted, delayed humoral and cellular immune responses were directed against the rtTA transactivator protein associated with the reduction of vector DNA in transduced muscles. This study provides some evidence that, when the immune system is not mobilized against the rtTA transactivator, the tetR-regulatable system is able to support long-term transgene regulation in the context of an rAAV in nonhuman primates. In addition, our results suggest potential improvements for vector design.

Long-term reversal of chronic anemia using a hypoxia-regulated erythropoietin gene therapy

Anemia is a common clinical problem, and there is much interest in its role in promoting left ventricular hypertrophy through increasing cardiac workload. Normally, red blood cell production is adjusted through the regulation of erythropoietin (Epo) production by the kidney. One important cause of anemia is relative deficiency of Epo, which occurs in most types of renal disease. Clinically, this can be corrected by supplementation with recombinant Epo. Here we describe an oxygen-regulated gene therapy approach to treating homozygous erythropoietin-SV40 T antigen (Epo-TAg(h)) mice with relative erythropoietin deficiency. We used vectors in which murine Epo expression was directed by an Oxford Biomedica hypoxia response element (OBHRE) or a constitutive cytomegalovirus (CMV) promoter. Both corrected anemia, but CMV-Epo-treated mice acquired fatal polycythemia. In contrast, OBHRE-Epo corrected the hematocrit level in anemic mice to a normal physiologic level that stabilized without resulting in polycythemia. Importantly, the OBHRE-Epo vector had no significant effect on the hematocrit of control mice. Homozygous Epo-TAg(h) mice display cardiac hypertrophy, a common adaptive response in patients with chronic anemia. In the OBHRE-Epo-treated Epo-TAg(h) mice, we observed a significant reversal of cardiac hypertrophy. We conclude that the OBHRE promoter gives rise to physiologically regulated Epo secretion such that the hematocrit level is corrected to healthy in anemic Epo-TAg(h) mice. This establishes that a hypoxia regulatory mechanism similar to the natural mechanism can be achieved, and it makes EPO gene therapy more attractive and safer in clinical settings. We envisage that this control system will allow regulated delivery of therapeutic gene products in other ischemic settings.

Macrolide-based transgene control in mammalian cells and mice

Heterologous mammalian gene regulation systems for adjustable expression of multiple transgenes are necessary for advanced human gene therapy and tissue engineering, and for sophisticated in vivo gene-function analyses, drug discovery, and biopharmaceutical manufacturing. The antibiotic-dependent interaction between the repressor (E) and operator (ETR) derived from an Escherichia coli erythromycin-resistance regulon was used to design repressible (E(OFF)) and inducible (E(ON)) mammalian gene regulation systems (E.REX) responsive to clinically licensed macrolide antibiotics (erythromycin, clarithromycin, and roxithromycin). The E(OFF) system consists of a chimeric erythromycin-dependent transactivator (ET), constructed by fusing the prokaryotic repressor E to a eukaryotic transactivation domain that binds and activates transcription from ETR-containing synthetic eukaryotic promoters (P(ETR)). Addition of macrolide antibiotic results in repression of transgene expression. The E(ON) system is based on E binding to artificial ETR-derived operators cloned adjacent to constitutive promoters, resulting in repression of transgene expression. In the presence of macrolides, gene expression is induced. Control of transgene expression in primary cells, cell lines, and microencapsulated human cells transplanted into mice was demonstrated using the E.REX (E(OFF) and E(ON)) systems. The macrolide-responsive E.REX technology was functionally compatible with the streptogramin (PIP-regulated and tetracycline (TET-regulated expression systems, and therefore may be combined for multiregulated multigene therapeutic interventions in mammalian cells and tissues.

Long-term doxycycline-regulated secretion of erythropoietin by encapsulated myoblasts

We developed an ex vivo gene therapy approach for the regulated delivery of therapeutic proteins based on the implantation of encapsulated, genetically engineered C(2)C(12) myoblasts. We investigated doxycycline-based regulation of gene expression to modulate the secretion of erythropoietin (EPO) from encapsulated myoblasts in a mouse model. An autoregulatory tet-off system provided high induction levels with low basal expression in the noninduced state. Stable C(2)C(12) clones constitutively secreted between 25 and 50 IU mouse EPO/10(6)cells/24 hours in the on-state. The clone C15, selected for its in vivo survival characteristics, displayed a desirable secretion profile when encapsulated. Devices released 5 IU EPO per capsule in the on-state, with EPO levels being undetectable upon the addition of doxycycline (dox). Capsules subcutaneously implanted in DBA/2J mice demonstrated a tightly regulated secretion of EPO through up to four on-off cycles during a period lasting 40 weeks. Hematocrits could be modulated between basal levels (40-50%) and elevated levels (70-90%) through the presence or absence of dox in the drinking water. Hematocrit returned to normal levels, paralleling the kinetics observed following capsule explantation, 6 to 8 weeks following dox administration to polycythemic mice. The results of this study suggest that encapsulation and implantation of a tet-off regulated C(2)C(12) cell clone represents a safe method for the controlled long-term delivery of proteins in vivo.

The tet-off system is more effective than the tet-on system for regulating transgene expression in a single adenovirus vector

BACKGROUND

Control of transgene expression in mammalian cells is desirable for gene therapy and the study of gene function in basic research. This study evaluates the functionality of single adenovirus (Ad) vectors containing a tetracycline-controllable expression system (tet-off or tet-on system).

METHODS

An Ad-mediated binary transgene expression system was generated containing a tet-off or a tet-on system, which introduced the gene of interest with a tetracycline-regulatable promoter and the tetracycline-responsive transcriptional activator gene into the E1 and E3 deletion regions, respectively. The functionality of the Ad-mediated tet-off and tet-on systems was compared under various conditions in vitro and in vivo.

RESULTS

The Ad vector containing the tet-off system provided negative control of gene expression ranging from 20- to 500-fold, depending on the cell type and condition. In contrast, the Ad vector containing the tet-on system increased gene expression by only 2- to 28-fold and required about two-log orders higher concentration of an inducer (doxycycline) to switch on the gene expression, compared with the Ad vector containing the tet-off system.

CONCLUSION

Ad vectors containing the tet-off system are a better choice for regulated gene expression than Ad vectors containing the tet-on system. Single Ad vectors containing the tetracycline-controllable expression system will greatly facilitate in vitro and in vivo analyses of gene function and may be useful for gene therapy.

Establishment of an artificial β-cell line expressing insulin under the control of doxycycline

AIM: Artificial β-cell lines may offer an abundant source of cells for the treatment of type I diabetes, but insulin secretion in β-cells is tightly regulated in physiological conditions. The Tet-On system is a “gene switch” system, which can induce gene expression by administration of tetracycline (Tet) derivatives such as doxcycline (Dox). Using this system, we established 293 cells to an artificial cell line secreting insulin in response to stimulation by Dox.

METHODS: The mutated proinsulin cDNA was obtained from plasmid pcDNA3.1/C-mINS by the polymerase chain reaction (PCR), and was inserted downstream from the promoter on the expression vector pTRE2, to construct a recombined expression vector pTRE2mINS. The promoter on pTRE2 consists of the tetracycline-response element and the CMV minimal promoter and is thus activated by the reverse tetracycline-controlled transactivator (rtTA) when Dox is administrated. pTRE2mINS and plasmid pTK-Hyg encoding hygromycin were co-transfected in the tet293 cells, which express rtTA stably. Following hygromycin screening, the survived cells expressing insulin were selected and enriched. Dox was used to control the expression of insulin in these cells. At the levels of mRNA and protein, the regulating effect of Dox in culture medium on the expression of proinsulin gene was estimated respectively with Northern blot, RT-PCR, and radioimmunoassay.

RESULTS: From the 28 hygromycin-resistant cell strains, we selected one cell strain (tet293/Ins6) secreting insulin not only automatically, but in response to stimulation by Dox. The amount on insulin secretion was dependent on the Dox dose (0, 10, 100, 200, 400, 800 and 1000 μg•L⁻¹), the level of insulin secreted by the cells treated with Dox (1000 μg·L^-1) was 241.0 pU·d^-1× cell^-1, which was 25-fold that of 9.7 pU·d^-1× cell^-1 without Dox treatment. Northern blot analyses and RT-PCR further confirmed that the transcription of insulin gene had already been up-regulated after exposing tet293/Ins6 cells to Dox for 15 min, and was also induced in a dose-dependent manner. However, the concentration of insulin in the media did not increase significantly until 5 h following the addition of Dox.

CONCLUSION: Human proinsulin gene was transfected successfully and expressed efficiently in 293 cells, and the expression was modulated by tetracycline and its derivatives, improving the accuracy, safety, and reliability of gene therapy, suggesting that conditional establishment of artificial β-cells may be a useful approach to develop cellular therapy for diabetes mellitus.

Dual-regulated expression technology: a new era in the adjustment of heterologous gene expression in mammalian cells

BACKGROUND

On the basis of the compatible streptogramin- and tetracycline-responsive expression systems, a series of dual-regulated expression systems have been established for use in sophisticated biopharmaceutical manufacturing, advanced gene therapy, and tissue engineering.

METHODS

Dual-regulated expression concepts enable streptogramin- and tetracycline-responsive control of two different (sets of) transgenes (multi-regulated multigene metabolic engineering), dual-autoregulated expression configurations for one-step chromosomal integration of two antibiotic-adjustable expression units, and artificial regulatory cascades for multi-level regulation of transgenes and optimized integration of molecular interventions into mammalian regulatory networks.

RESULTS

This report describes the construction and testing of a family of dual-regulated expression vectors which are compatible with the pTRIDENT vector construction kit, and, in some cases, adapted for retroviral expression technology enabling straightforward transduction of difficult-to-transfect cell lines such as primary cells and stem cells.

CONCLUSIONS

Dual-regulated expression technology will probably become of prime interest for a variety of therapeutic applications, including biopharmaceutical manufacturing, gene therapy, and tissue engineering.

Sustainable systemic delivery via a single injection of lentivirus into human skin tissue

The skin offers a tissue site accessible for delivery of gene-based therapeutics. To develop the capability for sustained systemic polypeptide delivery via cutaneous gene transfer, we generated and injected pseudotyped HIV-1 lentiviral vectors intradermally at a range of doses into human skin grafted on immune-deficient mice. Unlike Moloney murine leukemia virus (MLV)-based retrovectors, which failed to achieve detectable cutaneous gene transfer by this approach, lentivectors effectively targeted all major cell types within human skin tissue, including fibroblasts, endothelial cells, keratinocytes, and macrophages. After a single injection, lentivectors encoding human erythropoietin (EPO) produced dose-dependent increases in serum human EPO levels and hematocrit that increased rapidly within one month and remained stable subsequently. Delivered gene expression was confined locally at the injection site. Excision of engineered skin led to rapid and complete loss of human EPO in the bloodstream, confirming that systemic EPO delivery was entirely due to lentiviral targeting of cells within skin rather than via spread of the injected vector to visceral tissues. These findings indicate that the skin can sustain dosed systemic delivery of therapeutic polypeptides via direct lentivector injection and thus provide an accessible and reversible approach for gene-based delivery to the bloodstream.

Exogenously regulated stem cell-mediated gene therapy for bone regeneration

Regulated expression of transgene production and function is of great importance for gene therapy. Such regulation can potentially be used to monitor and control complex biological processes. We report here a regulated stem cell-based system for controlling bone regeneration, utilizing genetically engineered mesenchymal stem cells (MSCs) harboring a tetracycline-regulated expression vector encoding the osteogenic growth factor human BMP-2. We show that doxycycline (a tetracycline analogue) is able to control hBMP-2 expression and thus control MSC osteogenic differentiation both in vitro and in vivo. Following in vivo transplantation of genetically engineered MSCs, doxycycline administration controlled both bone formation and bone regeneration. Moreover, our findings showed increased angiogenesis accompanied by bone formation whenever genetically engineered MSCs were induced to express hBMP-2 in vivo. Thus, our results demonstrate that regulated gene expression in mesenchymal stem cells can be used as a means to control bone healing.

Delivery of FGF-2 but not VEGF by encapsulated genetically engineered myoblasts improves survival and vascularization in a model of acute skin flap ischemia

Stimulating angiogenesis by gene transfer approaches offers the hope of treating tissue ischemia which is untreatable by currently practiced techniques of vessel grafting and bypass surgery. Vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (FGF-2) are potent angiogenic molecules, making them ideal candidates for novel gene transfer protocols designed to promote new blood vessel growth. In this study, an ex vivo gene therapy approach utilizing cell encapsulation was employed to deliver VEGF and FGF-2 in a continuous and localized manner. C(2)C(12) myoblasts were genetically engineered to secrete VEGF(121), VEGF(165) and FGF-2. These cell lines were encapsulated in hollow microporous polymer membranes for transplantation in vivo. Therapeutic efficacy was evaluated in a model of acute skin flap ischemia. Capsules were positioned under the distal, ischemic region of the flap. Control flaps showed 50% necrosis at 1 week. Capsules releasing either form of VEGF had no effect on flap survival, but induced a modest increase in distal vascular supply. Delivery of FGF-2 significantly improved flap survival, reducing necrosis to 34.2% (P < 0.001). Flap vascularization was significantly increased by FGF-2 (P < 0.01), with numerous vessels, many of which had a large lumen diameter, growing in the proximity of the implanted capsules. These results demonstrate that FGF-2, delivered from encapsulated cells, is more efficacious than either VEGF(121) or VEGF(165) in treating acute skin ischemia and improving skin flap survival. Furthermore, these data attest to the applicability of cell encapsulation for the delivery of angiogenic factors for the treatment and prevention of tissue ischemia.

Transcriptional control: an essential component of cancer gene therapy strategies?

The therapeutic index of cancer gene therapy approaches will, at least in part, be dictated by the spatial and temporal control of expression of the therapeutic transgenes. Strategies which allow precise control of gene transcription are likely to play a crucial role in the future pre-clinical and clinical development of gene therapy. In this review, we discuss these issues as they relate to tissue and tumor specific promoters. In addition, the exciting opportunities offered by the development of regulated gene expression systems using small molecules, radiation and heat are reviewed. It is realistic to expect that the future offers the prospect of amalgamating elements of a number of these different systems in a co-ordinated gene delivery approach with the potential to increase the efficacy and reduce the toxicity of treatment.

Fibroblasts and dermal gene therapy: a minireview

This review highlights our current understanding of the biology of, survival of, and transgene expression by genetically modified fibroblasts (GMFb) carrying stably integrated transgenes in vivo. Experimental data demonstrate that three elements will enhance expression by and survival of GMFb in vivo: a matrix scaffolding to take the place of the existing dermis, the presence of elements of the extracellular matrix in the construct used to move GMFb to the in vivo setting, and the utilization of immortalized fibroblasts to carry the transgenes. Although moving GMFb to an in vivo setting is an invasive procedure, there are a number of clinical settings where GMFb appear to be the suitable cell for gene therapy.

Streptogramin-based gene regulation systems for mammalian cells

Here we describe repressible (PipOFF) as well as inducible (PipON) systems for regulated gene expression in mammalian cells, based on the repressor Pip (pristinamycin-induced protein), which is encoded by the streptogramin resistance operon of Streptomyces coelicolor. Expression of genes placed under control of these systems was responsive to clinically approved antibiotics belonging to the streptogramin group (pristinamycin, virginiamycin, and Synercid). The versatility of these systems was demonstrated by streptogramin-regulated expression of mouse erythropoietin (EPO), human placental secreted alkaline phosphatase (SEAP), or green fluorescent protein (GFP) in diverse cell lines (BHK, CHO, HeLa, and mouse myoblasts). Analysis of isogenic constructs in CHO cells demonstrated the PipOFF system gave lower background and higher induction ratios than the widely used tetracycline-repressible (TetOFF) expression systems. The streptogramin-based expression technology was functionally compatible with the TetOFF system, thus enabling the selective use of different antibiotics to independently control two different gene activities in the same cell.

Delivering Erythropoietin through Genetically Engineered Cells

Abstract. Erythropoietin (Epo) is a glycoprotein hormone produced by genetic engineering. Many pathologic conditions could benefit from its administration, such as chronic renal failure or hemoglobinopathies. Epo secretion from genetically modified tissued could be proposed to patients only if the protocol is low cost and low risk. For that purpose, retroviral vectors and adeno-associated vectors expressing the Epo cDNA were developed. Gene transfer was performed into skeletal muscles. To avoid polycythemia, a tetracycline-regulated system was used to control the levels of protein secretion in vivo. β-thalassemias are among diseases that could benefit from an Epo gene transfer. β-thalassemias are attributable to deficient synthesis of β-globin and accumulation of unpaired α-chains. Stimulation of fetal globin synthesis is one strategy to correct the globin chain imbalance. There is evidence that Epo could play this role. In a mouse model of β-thalassemia, an adeno-associated vector expressing the Epo cDNA was injected intramuscularly. Epo was secreted continuously during at least 1 yr. Erythropoiesis was improved in those mice by increasing the synthesis of fetal hemoglobin.

Dose response to a single intramuscular injection of recombinant adeno-associated virus-erythropoietin in monkeys

BACKGROUND

Anemia is a significant problem in many disease states. Erythropoietin (Epo) has been used in the treatment of anemia associated with numerous chronic diseases. This study investigates the dose-response profiles of a single intramuscular (im) injection of a recombinant adeno-associated virus vector (rAAV) containing the Epo gene with the goal of achieving a sustained elevation of hematocrit (Hct).

METHODS

Cynomolgus (cm) monkeys were given single injections of different doses of rAAV-cm-Epo. The biological effect of Epo gene expression was monitored by determining the Hct levels and circulating hormone levels by ELISA. Antibody to the rAAV capsid protein was also measured over the 41-week period of the experiment.

RESULTS

Epo expression was noted only when 2 x 10(11) or more particles were injected. Epo was noted to be increased as soon as 1 week postinjection and was maximum in 6 to 8 weeks. This level of expression remained constant for nearly 20 weeks. Animals given the highest dose of rAAV developed a higher Hct over the first 8 weeks postinjection than those given an intermediate dose. However, the maximum levels of hemoglobin were the same. There was a weak correlation between amount of rAAV injected and capsid antibody response.

CONCLUSIONS

AAV vectors are able to transduce skeletal muscle and are capable of achieving sustained expression and systemic delivery of a therapeutic protein following a single im administration. Dose responses to rAAV-Epo are achievable, although a threshold inoculum of virus is necessary to produce an effect and the therapeutic window is narrow.

In-vivo delivery of therapeutic proteins by genetically-modified cells: comparison of organoids and human serum albumin alginate-coated beads

We have designed a self-assembling multimeric soluble CD4 molecule by inserting the C-terminal fragment of the alpha chain of human C4-binding protein (C4bp alpha) at the C-terminal end of human soluble CD4 genes. This CD4-C4bp alpha fusion protein (sMulti-CD4) and two other reference molecules, a fusion protein of human serum albumin (HSA) and the first two domains of CD4 (HSA-CD4) and monomeric soluble CD4 (sMono-CD4), were delivered in vivo by genetically modified 293 cells. These cells were implanted in mice as organoids and also encapsulated in HSA alginate-coated beads. sMulti-CD4 showed an apparent molecular weight of about 300-350 kDa, in accordance with a possible heptamer formula. sMulti-CD4 produced either in cell culture or in vivo in mice appeared to be a better invitro inhibitor of HIV infection than sMono-CD4. Plasma levels of sMulti-CD4, HSA-CD4, and sMono-CD4 reached approximately 2,300, 2,700, and 170 ng/mL, respectively, 13 weeks after in-vivo organoid implantation, which had formed tumours at that time. This suggests that the plasma half-life of sMulti-CD4 is much longer than that of sMono-CD4. The 293 xenogeneic cells encapsulated in HSA alginate-coated beads remained alive and kept secreting sMono-CD4 or HSA-CD4 continuously at significant levels for 18 weeks in nude mice, without tumour formation. When implanted in immunocompetent Balb/c mice, they were rejected two to three weeks after implantation. In contrast, encapsulated BL4 hybridoma cells remained alive and kept secreting BL4 anti-CD4 mAb for at least four weeks in Balb/c mice. These results suggest the clinical potential of the C4bp-multimerizing system, which could improve both the biological activity and the poor in-vivo pharmacokinetic performance of a monomeric functional protein like soluble CD4. These data also show that a systemic delivery of therapeutic proteins, including immunoglobulins, can be obtained by the in-vivo implantation of engineered allogeneic cells encapsulated in HSA alginate-coated beads.