In vivo selective expansion of gene-modified hematopoietic cells in a nonhuman primate model

Combinatorial RNA-based gene therapy for the treatment of HIV/AIDS

INTRODUCTION

HIV/AIDS continues to be a worldwide health problem and viral eradication has been an elusive goal. HIV+ patients are currently treated with combination antiretroviral therapy (cART) which is not curative. For many patients, cART is inaccessible, intolerable or unaffordable. Therefore, a new class of therapeutics for HIV is required to overcome these limitations. Cell and gene therapy for HIV has been proposed as a way to provide a functional cure for HIV in the form of a virus/infection resistant immune system.

AREAS COVERED

In this review, the authors describe the standard therapy for HIV/AIDS, its limitations, current areas of investigation and the potential of hematopoietic stem cells modified with anti-HIV RNAs as a means to affect a functional cure for HIV.

EXPERT OPINION

Cell and gene therapy for HIV/AIDS is a promising alternative to antiviral drug therapy and may provide a functional cure. In order to show clinical benefit, multiple mechanisms of inhibition of HIV entry and lifecycle are likely to be required. Among the most promising antiviral strategies is the use of transgenic RNA molecules that provide protection from HIV infection. When these molecules are delivered as gene-modified hematopoietic stem and progenitor cells, long-term repopulation of the patient's immune system with gene-modified progeny has been observed.

RNA-based gene therapy for HIV with lentiviral vector-modified CD34(+) cells in patients undergoing transplantation for AIDS-related lymphoma

AIDS patients who develop lymphoma are often treated with transplanted hematopoietic progenitor cells. As a first step in developing a hematopoietic cell-based gene therapy treatment, four patients undergoing treatment with these transplanted cells were also given gene-modified peripheral blood-derived (CD34(+)) hematopoietic progenitor cells expressing three RNA-based anti-HIV moieties (tat/rev short hairpin RNA, TAR decoy, and CCR5 ribozyme). In vitro analysis of these gene-modified cells showed no differences in their hematopoietic potential compared with nontransduced cells. In vitro estimates of successful expression of the anti-HIV moieties were initially as high as 22% but declined to approximately 1% over 4 weeks of culture. Ethical study design required that patients be transplanted with both gene-modified and unmanipulated hematopoietic progenitor cells obtained from the patient by apheresis. Transfected cells were successfully engrafted in all four infused patients by day 11, and there were no unexpected infusion-related toxicities. Persistent vector expression in multiple cell lineages was observed at low levels for up to 24 months, as was expression of the introduced small interfering RNA and ribozyme. Therefore, we have demonstrated stable vector expression in human blood cells after transplantation of autologous gene-modified hematopoietic progenitor cells. These results support the development of an RNA-based cell therapy platform for HIV.

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

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.

Novel gene switches

Controlling gene activity in space and time represents a cornerstone technology in gene and cell therapeutic applications, bioengineering, drug discovery as well as fundamental and applied research. This chapter provides a comprehensive overview of the different approaches for regulating gene activity and product protein formation at different biosynthetic levels, from genomic rearrangements over transcription and translation control to strategies for engineering inducible secretion and protein activity with a focus on the development during the past 2 years. Recent advances in designing second-generation gene switches, based on novel inducer administration routes (gas phase) as well as on the combination of heterologous switches with endogenous signals, will be complemented by an overview of the emerging field of mammalian synthetic biology, which enables the design of complex synthetic and semisynthetic gene networks. This article will conclude with an overview of how the different gene switches have been applied in gene therapy studies, bioengineering and drug discovery.

Hematopoietic stem cell transduction and amplification in large animal models

Progress in retroviral gene transfer to large animal hematopoietic stem cells (HSCs) has led to efficient, reproducible long-term marking in both canine and nonhuman primate models. Successes for HSC gene therapy have occurred in the severe combined immunodeficiency setting, in which transduced cells have a selective advantage. However, for most diseases, the therapeutic transgene does not confer a sufficient survival advantage, and increasing the percentage of gene-marked cells in vivo will be necessary to observe a therapeutic effect. In vivo amplification should expand the potential of HSC gene therapy, and progress in this area has benefited greatly from the use of large animal models where efficacy and toxicity have often not correlated with results in murine models. To date, the best results have been observed with O(6)-methylguanine-DNA methyltransferase (MGMT) selection, with which increases in gene-marked repopulating cells have been maintained long-term, likely because of the toxicity of 1,3-bis-(2-chloroethyl)-1-nitrosourea and temozolomide to quiescent HSCs. Using MGMT selection, long-term marking levels exceeding 50% can now be routinely attained with minimal toxicity. There is cause to be optimistic that HSC gene therapy with in vivo amplification will soon allow the treatment of several genetic and infectious diseases.

Prevention of Immune Responses to Human Erythropoietin in Cynomologus Monkeys (Macaca fascicularis)

Genes and proteins of human origin are often administered to monkeys for research purposes, however, it can be difficult to obtain sufficient levels of the products in vivo due to immunological clearance. In this study, we showed that human erythropoietin (hEPO) induces generation of anti-hEPO antibody in cynomolgus macaques (n=2), although 92% of amino acid residues are common between the human and macaque EPO. The administered hEPO was thus eliminated from the animals. On the other hand, when an immunosuppressant, cyclosporin A (CyA), was administered (6 mg/kg) intramuscularly every other day in combination with hEPO (n=2), no anti-hEPO antibody was generated and high serum levels of hEPO were obtained during administration of hEPO, resulting in an increase in serum hemoglobin levels. No adverse effects associated with CyA were observed. Thus, CyA treatment is useful for prevention of immune responses associated with the administration of human proteins in monkeys.

Hematopoietic stem cell gene therapy with drug resistance genes: an update

Transfer of drug resistance genes into hematopoietic stem cells (HSCs) has promise for the treatment of a variety of inherited, that is, X-linked severe combined immune deficiency, adenosine deaminase deficiency, thalassemia, and acquired disorders, that is, breast cancer, lymphomas, brain tumors, and testicular cancer. Drug resistance genes are transferred into HSCs either for providing myeloprotection against chemotherapy-induced myelosuppression or for selecting HSCs that are concomitantly transduced with another gene for correction of an inherited disorder. In this review, we describe ongoing experimental approaches, observations from clinical trials, and safety concerns related to the drug resistance gene transfer.

Survival of the fittest: in vivo selection and stem cell gene therapy

Stem cell gene therapy has long been limited by low gene transfer efficiency to hematopoietic stem cells. Recent years have witnessed clinical success in select diseases such as X-linked severe combined immunodeficiency (SCID) and ADA deficiency. Arguably, the single most important factor responsible for the increased efficacy of these recent protocols is the fact that the genetic correction provided a selective in vivo survival advantage. Since, for most diseases, there will be no selective advantage of gene-corrected cells, there has been a significant effort to arm vectors with a survival advantage. Two-gene vectors can be used to introduce the therapeutic gene and a selectable marker gene. Efficient in vivo selection strategies have been demonstrated in clinically relevant large-animal models. Mutant forms of the DNA repair-enzyme methylguanine methyltransferase in particular have allowed for efficient in vivo selection and have achieved sustained marking with virtually 100% gene-modified cells in large animals, and with clinically acceptable toxicity. Translation of these strategies to the clinical setting is imminent. Here, we review how in vivo selection strategies can be used to make stem cell gene therapy applicable to the treatment of a wider scope of genetic diseases and patients.

Competitive engraftment of hematopoietic stem cells genetically modified with a truncated erythropoietin receptor

Transplantation of genetically modified hematopoietic stem cells (HSCs) has therapeutic potential for a variety of blood genetic disorders. Engraftment of HSCs, however, requires toxic myeloablative treatments, which render this approach questionable for non-life-threatening disorders. A potential alternative is the use of transgenes, which allows positive selection of HSCs in vivo. We used retroviral vectors to express a truncated derivative of the erythropoietin receptor (tEpoR) in murine and human hematopoietic cells. Murine HSCs expressing tEpoR at different levels (1500 to 13,000 receptors/cell) acquire a competitive repopulation capacity in vivo upon transplantation into fully or partially myeloablated co-isogenic mouse recipients. Long-term analysis of transplanted mice showed that expression of tEpoR at paraphysiological levels (approximately 1500 receptors/cell) has no effect on steady-state hematopoiesis and induces no further expansion of transduced cells after the engraftment period. Human cord blood-derived CD34+ stem/progenitor cells transduced with a lentiviral vector expressing tEpoR expand their clonogenic capacity in vitro, and significantly increase their marrow repopulation capacity upon xenotransplantation into sublethally irradiated NOD-SCID mice, with no alteration in their phenotype, survival, and differentiation properties. These data indicate that expression of tEpoR is an effective strategy to promote selective engraftment of genetically modified HSCs upon transplantation in both myeloablative and nonmyeloablative conditions, without the use of toxic drugs for selection.

Gene therapy for the hemoglobin disorders

The hemoglobin disorders of beta-thalassemia and sickle cell disease together constitute the most prevalent group of human monogenic diseases. Although curative allogeneic stem cell transplantation therapy and palliative therapies have been developed for these disorders, the majority of patients still suffer significant morbidity and early mortality. The development of therapeutic approaches based on genetic manipulation of autologous stem cells therefore remains an attractive alternative. In the past 4 years, significant advances have been made toward this goal using lentiviral vectors to obtain high-level expression of complex globin gene cassettes. Therapeutic correction in murine models of both beta-thalassemia and sickle cell anemia has been achieved using this approach. These advances, coupled with progress in the ability to achieve in vivo selection of genetically modified cells, can now be evaluated in the well-developed nonhuman primate autologous transplant model. The goal in these studies is to provide preclinical safety and efficacy data prior to human clinical trials in order to maximize the likelihood of success in the context of an acceptable risk to benefit ratio. Here we review progress in each of these areas.

Safe And Efficient Collection of Cytokine-Mobilized Peripheral Blood Cells From Cynomolgus Monkeys (Macaca fascicularis) with Human Newborn-Equivalent Body Weights

Hematopoietic stem cells in bone marrow can be mobilized into peripheral blood by cytokine administration. Cytokine-mobilized peripheral blood stem cells are of great use in clinical applications. We previously established a modified procedure for the collection of cytokine-mobilized peripheral blood cells from rhesus monkeys (Macaca mulata) using a commercially available apparatus originally developed for human subjects. In this study, we examined the efficacy and safety of this method with even smaller macaques, cynomolgus monkeys (Macaca fascicularis), which are equivalent to human newborns in body weight (mean = 3.3 kg). Using the manufacturer's unmodified protocol (n=6), one monkey died of cardiac failure and three developed severe anemia. In contrast, using our modified procedure (n=6), no such complication was observed in any animal. In addition, the harvested nuclear cell, mononuclear cell and CD34(+) cell counts were significantly higher with the modified method. The modified method should allow safe and efficient collection of cytokine-mobilized peripheral blood cells from non-human primates as small as human newborns in a non-invasive manner.

[Development and application of gene therapy technologies]

The success of hematopoietic stem cell gene therapy for X-linked severe combined immunodeficiency (X-SCID) was a major breakthrough in the field of gene therapy. However, two patients treated with this gene therapy developed leukemia at a later time, and retroviral vector-mediated gene transfer was considered to trigger leukemogenesis; i.e. insertional mutagenesis caused activation of LMO 2 gene, which was one step toward leukemia development. To cope with this serious problem, basic studies are required to improve the safety of retroviral vectors and to develop the method for site-specific integration of transgenes. In addition, we have to develop technologies such as selective amplifier genes (SAGs), the system for selective expansion of transduced cells, in order to obtain therapeutic efficacy of hematopoietic stem cell gene therapy in many other disorders. Moreover, clinical applications of AAV vector are promising from the standpoint of safety issue, because this vector is derived from non-pathogenic virus. AAV vector is appropriate for gene transfer into neurons, muscles, and hepatocytes. For example, gene therapy for Parkinson's disease is investigated using AAV vectors. Genetic manipulation is also one of the indispensable technologies in the field of regeneration medicine, and further promotion of basic research is important.

Strategies to expand transduced hematopoietic stem cells in vivo

Data in mice suggest that in vivo selection strategies will expand the numbers of transduced hematopoietic stem cells (HSC) to levels sufficient for clinical therapies, and it is argued that comparable strategies will benefit larger animals and humans. To test this assumption, we performed virtual gene therapy in mouse and cat, species in which the in vivo kinetics of HSC are defined. In the simulated experiments, 10% of HSC and 50% of short-term repopulating cells were transduced with a gene allowing a conditional replication or apoptosis advantage. After transplantation, differentiation proceeded stochastically and contributions of transduced cells were tracked for 2 years. Fifty independent transplantations were simulated per species for each analysis. When transduced HSC had a 2-fold increased chance of replication (self-renewal) extending for 4, 10, or 20 weeks after transplantation, or a 5-fold replication advantage extending for 4 weeks, results in mice were far better than in cat, a larger animal, with slower baseline HSC cell cycle kinetics. Similarly, when transduced HSC had a 2-, 4-, or 10-fold decreased chance of apoptosis, extending for 20 or more weeks after transplantation, the murine studies were poor predictors of feline results. Simulation may allow one to optimize and/or understand the limitations of a gene therapy strategy.

Approaches for trigger-inducible viral transgene regulation in gene-based tissue engineering

Recent advances in mammalian transgene expression dosing have resulted in a portfolio of mutually compatible systems that can adjust therapeutic transgene levels in response to antibiotics, hormone analogues, quorum-sensing messengers and secondary metabolites. The molecular merger of trigger-inducible expression technology with the latest generation of virus-derived transduction systems has enabled unmatched clinical interventions to shape desired therapeutic cell and tissue phenotypes for the treatment of complex human diseases.

Genetic manipulation of hematopoietic stem cells

Over the past two decades, the ability to transfer genes into hematopoietic stem cells (HSCs) has provided new insights into the behavior of individual stem cells and offered a novel approach for the treatment of various inherited or acquired disorders. At present, gene transfer into HSCs has been achieved mainly using modified retroviruses. While retrovirus-based vectors could efficiently transduce murine HSCs, extrapolation of these methods to large mammals and human clinical trials resulted in very low numbers of gene-marked engrafted cells. In addition, in vitro progenitor assays used to optimize gene transfer procedures were found to poorly predict the outcome of stem cell gene transfer. The focus rapidly turned to the development of superior and more relevant preclinical assays in human stem cell gene transfer research. Xenogeneic transplant models and large animal transplantation system have been invaluable. The development of better assays for evaluating human gene therapy protocols and a better understanding of stem cell and vector biology has culminated over the past decade in multiple strategies to improve gene transfer efficiency into HSCs. Improved gene transfer vectors, optimization of cytokine combination, and incorporation of a recombinant fragment of fibronectin during transduction are examples of novel successful additions to the early gene transfer protocols that have contributed to the first unequivocal clinical benefits resulting from genetic manipulation of HSC.

High-Level in Vivo Gene Marking after Gene-Modified Autologous Hematopoietic Stem Cell Transplantation without Marrow Conditioning in Nonhuman Primates

The successful engraftment of genetically modified hematopoietic stem cells (HSCs) without toxic conditioning is a desired goal for HSC gene therapy. To this end, we have examined the combination of intrabone marrow transplantation (iBMT) and in vivo expansion by a selective amplifier gene (SAG) in a nonhuman primate model. The SAG is a chimeric gene consisting of the erythropoietin (EPO) receptor gene (as a molecular switch) and c-Mpl gene (as a signal generator). Cynomolgus CD34+ cells were retrovirally transduced with or without SAG and returned into the femur and humerus following irrigation with saline without prior conditioning. After iBMT without SAG, 2-30% of colony-forming cells were gene marked over 1 year. The marking levels in the peripheral blood, however, remained low (<0.1%). These results indicate that transplanted cells can engraft without conditioning after iBMT, but in vivo expansion is limited. On the other hand, after iBMT with SAG, the peripheral marking levels increased more than 20-fold (up to 8-9%) in response to EPO even at 1 year posttransplant. The increase was EPO-dependent, multilineage, polyclonal, and repeatable. Our results suggest that the combination of iBMT and SAG allows efficient in vivo gene transduction without marrow conditioning.

Expansion of genetically corrected neutrophils in chronic granulomatous disease mice by cotransferring a therapeutic gene and a selective amplifier gene

Hematopoietic stem cell gene therapy has not provided clinical success in disorders such as chronic granulomatous disease (CGD), where genetically corrected cells do not show a selective advantage in vivo. To facilitate selective expansion of transduced cells, we have developed a fusion receptor system that confers drug-induced proliferation. Here, a 'selective amplifier gene (SAG)' encodes a chimeric receptor (GcRER) that generates a mitotic signal in response to estrogen. We evaluated the in vivo efficacy of SAG-mediated cell expansion in a mouse disease model of X-linked CGD (X-CGD) that is deficient in the NADPH oxidase gp91phox subunit. Bone marrow cells from X-CGD mice were transduced with a bicistronic retrovirus encoding GcRER and gp91phox, and transplanted to lethally irradiated X-CGD recipients. Estrogen was administered to a cohort of the transplants, and neutrophil superoxide production was monitored. A significant increase in oxidase-positive cells was observed in the estrogen-treated mice, and repeated estrogen administration maintained the elevation of transduced cells for 20 weeks. In addition, oxidase-positive neutrophils were increased in the X-CGD transplants given the first estrogen even at 9 months post-transplantation. These results showed that the SAG system would enhance the therapeutic effects by boosting genetically modified, functionally corrected cells in vivo.

Transient in vivo selection of transduced peripheral blood cells using antifolate drug selection in rhesus macaques that received transplants with hematopoietic stem cells expressing dihydrofolate reductase vectors

One of the main obstacles for effective human gene therapy for hematopoietic disorders remains the achievement of an adequate number of genetically corrected blood cells. One approach to this goal is to incorporate drug resistance genes into vectors to enable in vivo selection of hematopoietic stem cells (HSCs). Although a number of drug resistance vectors enable HSC selection in murine systems, little is known about these systems in large animal models. To address this issue, we transplanted cells transduced with dihydrofolate resistance vectors into 6 rhesus macaques and studied whether selection of vector-expressing cells occurred following drug treatment with trimetrexate and nitrobenzylmercaptopurineriboside-phosphate. In some of the 10 administered drug treatment courses, substantial increases in the levels of transduced peripheral blood cells were noted; however, numbers returned to baseline levels within 17 days. Attempts to induce stem cell cycling with stem cell factor and granulocyte-colony stimulating factor prior to drug treatment did not lead to sustained enrichment for transduced cells. These data highlight an important species-specific difference between murine and nonhuman primate models for assessing in vivo HSC selection strategies and emphasize the importance of using drugs capable of inducing selective pressure at the level of HSCs.

Current developments in the design of onco-retrovirus and lentivirus vector systems for hematopoietic cell gene therapy

Over the past dozen years, the majority of clinical gene therapy trials for inherited genetic diseases and cancer therapy have been performed using murine onco-retrovirus as the gene delivery vector. The earliest systems used were relatively inefficient in both the rates of transduction and expression of the transgene. Formidable obstacles inherent in the cell biology and/or the immunology of the target cell systems limited the efficacy of gene therapy for many target diseases. Development of novel retrovirus gene transfer systems that are in progress have begun to overcome these obstacles. Evidence of this progress is the recent successful functional correction of the immune T and B lymphocyte deficiency in patients with X-linked severe combined immunodeficiency (X-SCID) and adenosine deaminase (ADA)-deficient SCID following onco-retrovirus vector ex vivo transduction of autologous marrow stem cells [Science 296 (2002) 2410; Science 288 (2000) 669; N. Engl. J. Med. 346 (2002) 1185]. These achievements of prolonged clinical benefit from gene therapy were tempered by the finding of insertional mutageneses in two of the treated X-SCID patients [N. Engl. J. Med. 348 (2003) 255].

Genetic manipulation of primate embryonic and hematopoietic stem cells with simian lentivirus vectors

During the past several years, many articles have described how human embryonic stem (ES) cells and adult hematopoietic stem cells (HSCs) can differentiate into cardiac muscle, blood vessels, and various other types of cells. The articles raised the expectation that these stem cells may become useful for the treatment of a variety of diseases, including cardiovascular diseases. Genetic manipulation of ES cells and HSCs would be important for such future applications of the cells. Until now, retroviral vectors have been used primarily for stable expression of transgenes in murine ES cells and HSCs. Because murine models may not predict reliably the biology of ES cells and HSCs in humans, we have utilized primate ES cells and HSCs as targets of gene transfer. We have shown that primate ES cells and HSCs can be transduced efficiently with lentiviral vectors derived from the simian immunodeficiency virus, and that the high transgene expression persists without transcriptional silencing. This highly efficient gene transfer method allows for safe and faithful gene delivery to primate ES cells and HSCs to test potential research and therapeutic applications.

New selective amplifier genes containing c-Mpl for hematopoietic cell expansion

We previously developed "selective amplifier genes (SAGs)" which confer a growth advantage to transduced cells. The SAG is a chimeric gene encoding the G-CSF receptor (GCR) and the estrogen or tamoxifen (Tm) receptor and is able to expand transduced hematopoietic cells by treatment with estrogen or Tm. In the current study, we examined the in vitro efficacy of modified SAGs containing the thrombopoietin (TPO) receptor (c-Mpl) gene instead of GCR as a more potent signal generator. In addition, we constructed various mutant Mpl-type SAGs to abolish the responsiveness to endogenous TPO while retaining Tm-dependency. When Ba/F3 cells were retrovirally transduced with the Mpl-type SAGs, the cells showed Tm- and TPO-dependent growth even without IL-3. The Mpl-type SAGs induced more potent proliferation of Ba/F3 and cynomolgus CD34(+) cells than the GCR-type SAG. One mutant Mpl-type SAG (Delta GCRMplTmR) successfully lost the responsiveness to TPO without affecting the Tm-dependence.

In vivo expansion of transduced murine hematopoietic cells with a selective amplifier gene

BACKGROUND

Hematopoietic stem-cell-directed gene transfer has achieved limited success in transducing clinically relevant levels of target cells. The expansion of gene-modified cells is one way to circumvent the problem of inefficient transduction with current vectors. To this end, we have developed 'selective amplifier genes' (SAGs) that encode chimeric proteins that are a fusion of granulocyte colony-stimulating factor receptor and the steroid-binding domain. Prototype SAGs conferred estrogen-responsive growth on murine hematopoietic progenitors.

METHODS

We constructed a retroviral vector coexpressing an SAG for 4-hydroxytamoxifen (Tm)-specific proliferation and the enhanced green fluorescent protein (EGFP). Murine bone marrow cells were transduced with this vector and transplanted into myeloablated mice. Subsequently, recipients were challenged with Tm, and EGFP(+) cells were enumerated.

RESULTS

The challenge induced a significant increase in EGFP(+) leukocytes (21 +/- 4% to 27 +/- 5%), while EGFP(+) cells decreased in untreated animals (21 +/- 5% to 10 +/- 3%). Three months later, bone marrow cells were transplanted from the unchallenged mice to secondary hosts. Again the administration of Tm resulted in an increase of EGFP(+) cells (16 +/- 4% to 35 +/- 3%), contrasting to a decrease in controls (22 +/- 4% to 12 +/- 4%), and the difference was significant for more than 3 months. A detailed study of lineage showed a preferential expansion of EGFP(+) cells in granulocytes and monocytes following Tm administration.

CONCLUSIONS

Long-term repopulating cells were transduced with the SAG, and the transduced granulocyte/monocyte precursors were most likely to be expandable in vivo upon Tm stimulation.

Selective expansion of transduced cells for hematopoietic stem cell gene therapy

Although gene transfer into hematopoietic stem cells holds a considerable therapeutic potential, clinical trials targeting this cell compartment have achieved limited success. Poor transduction efficiency with gene transfer vectors used in human studies has hindered delivering therapeutic genes to clinically relevant numbers of target cells. One way to overcome the low-efficiency problem is by selecting or expanding the number of genetically modified cells to a suprathreshold level to achieve clinical efficacy. This approach may be further classified into 2 categories: one is to transfer a drug resistance gene and eliminate unmodified cells with cytotoxic drugs, and the other is to confer a direct growth advantage on target cells. This review aims at an overview of recent advances involving these strategies, with some details of "selective amplifier genes," a novel system that we have developed for specific expansion of genetically modified hematopoietic cells.