Retroviral stem cell gene therapy

Scaffold-Mediated Static Transduction of T Cells for CAR-T Cell Therapy

Chimeric antigen receptor T (CAR-T) cell therapy has produced impressive clinical responses in patients with B-cell malignancies. Critical to the success of CAR-T cell therapies is the achievement of robust gene transfer into T cells mediated by viral vectors such as gamma-retroviral vectors. However, current methodologies of retroviral gene transfer rely on spinoculation and the use of retronectin, which may limit the implementation of cost-effective CAR-T cell therapies. Herein, a low-cost, tunable, macroporous, alginate scaffold that transduces T cells with retroviral vectors under static condition is described. CAR-T cells produced by macroporous scaffold-mediated viral transduction exhibit >60% CAR expression, retain effector phenotype, expand to clinically relevant cell numbers, and eradicate CD19⁺ lymphoma in vivo. Efficient transduction is dependent on scaffold macroporosity. Taken together, the data show that macroporous alginate scaffolds serve as an attractive alternative to current transduction protocols and have high potential for clinical translation to genetically modify T cells for adoptive cellular therapy.

Gene delivery techniques for adult stem cell-based regenerative therapy

Over the past decade, stem cells have been considered to be a promising resource to cure and regenerate damaged or diseased tissues with research extending from basic studies to clinical application. Furthermore, genetically modified stem cells have the potential to reduce tumorigenic risks and achieve safe tissue formation. Recent advances in genetic modification of stem cells have rendered these cells more accessible and stable. The successful genetic modification of stem cells relies heavily on designing vector systems, either viral or nonviral vectors, which can efficiently deliver therapeutic genes to the cells with minimum toxicity. Currently, viral vectors showing high transfection efficiencies still raise safety issues, whereas safer nonviral vectors exhibit extremely poor transfection in stem cells. Here, we attempt to review and discuss the main factors raising concern in previous reports, and devise strategies to solve the issues in gene delivery systems for successful stem cell-targeting regenerative therapy.

Future of local bone regeneration - Protein versus gene therapy

The most promising attempts to achieve bone regeneration artificially are based on the application of mediators such as bone morphogenetic proteins (BMPs) directly to the deficient tissue site. BMPs, as promoters of the regenerative process, have the ability to induce de novo bone formation in various tissues, and many animal models have demonstrated their high potential for ectopic and orthotopic bone formation. However, the biological activity of the soluble factors that promote bone formation in vivo is limited by diffusion and degradation, leading to a short half-life. Local delivery remains a problem in clinical applications. Several materials, including hydroxyapatite, tricalcium phosphate, demineralised bone matrices, poly-lactic acid homo- and heterodimers, and collagen have been tested as carriers and delivery systems for these factors in a sustained and appropriate manner. Unfortunately these delivery vehicles often have limitations in terms of biodegradability, inflammatory and immunological rejection, disease transmission, and most importantly, an inability to provide a sustained, continuous release of these factors at the region of interest. In coping with these problems, new approaches have been established: genes encoding these growth factor proteins can be delivered to the target cells. In this way the transfected cells serve as local "bioreactors", as they express the exogenous genes and secrete the synthesised proteins into their vicinity. The purpose of this review is to present the different methods of gene versus growth factor delivery in tissue engineering. Our review focuses on these promising and innovative methods that are defined as regional gene therapy and provide an alternative to the direct application of growth factors. Various advantages and disadvantages of non-viral and viral vectors are discussed. This review identifies potential candidate genes and target cells, and in vivo as well as ex vivo approaches for cell transduction and transfection. In explaining the biological basis, this paper also refers to current experimental and clinical applications.

Bioreactors for hematopoietic cell culture

Hematopoietic cell culture, or ex vivo expansion of hematopoietic cells, is an enabling technology with many potential applications in bone-marrow transplantation, immunotherapy, gene therapy, and the production of blood products. Hematopoietic cultures are complex, with many different cell types of different stages of development present at any given point in time and never in steady state. Moreover, these cells interact strongly with each other and the environment through cytokines (growth factors) and adhesion molecules, as well as through their metabolism. Despite these significant challenges, cell products produced in bioreactors have shown promise in recent phase 1 clinical trials.

In vivo demethylation of a MoMuLV retroviral vector expressing the herpes simplex thymidine kinase suicide gene by 5' azacytidine

We constructed a functional MoMuLV-based bicistronic retroviral vector encoding the herpes simplex virus type I thymidine kinase gene, which induces sensitivity to the prodrug ganciclovir (gcv), and the reporter beta-galactosidase gene (MFG-tk-IRES-lacZ). The U937 histiocytic cell line was transduced with this vector, and a clone (VB71) with high-level transgene expression was selected. Severe combined immunodeficient (SCID) mice were injected with VB71 cells to evaluate the role of long terminal repeat methylation in transgene silencing in vivo and to see whether 5-azacytidine (5' aza-C) demethylating agent prevented it. We found 5' aza-C maintained gene expression at high level in vitro. In vivo, time to tumor onset was significantly longer in SCID mice receiving the VB71 cells, 5' aza-C, and gcv compared with animals treated with either 5' aza-C or gcv alone. The number of injected tumor cells influences tumor onset time and the efficacy of 5' aza-C and gcv treatment. The standard gcv treatment schedule (10 mg/kg from d + 1 until the onset of tumor) controlled tumor onset better than short-term treatment with high doses. In conclusion, the results extend our previous findings that transgene methylation in vivo may be prevented with an appropriate schedule of 5' aza-C and gcv.

Receptor-mediated targeting of gene delivery vectors: insights from molecular mechanisms for improved vehicle design

One way to deliver transgenes to cells in a selective manner is to target the delivery vehicles, or vectors, to specific cell-surface receptors as a first step toward ultimate transport of the gene to the nucleus for expression. While selective delivery, although often to undesired cell types, occurs naturally for some viral vectors and can be achieved for nonviral vehicles, current understanding and control of the delivery mechanism is inadequate for many therapeutic applications. The complicated nature of receptor-mediated transgene uptake and transport requires improved analysis to more effectively evaluate delivery vehicles. As receptor-mediated pathways for gene delivery typically involve vector binding, internalization, subcellular trafficking, vesicular escape, nuclear translocation, and unpackaging for transcription, each of these processes offer mechanisms that can be exploited to enhance targeted gene delivery via properly designed vehicles. For the purpose of this review, current targeted gene delivery vehicles are divided into three approaches: viral, synthetic, and hybrid vectors. Each approach possesses advantages as well as disadvantages at the present time for in vitro and in vivo application, and provides particular challenges to overcome in order to gain significantly improved targeted delivery properties. Quantitative experiments and mathematical modeling of the gene delivery pathway will serve to provide insight into molecular mechanisms and rate-limiting steps for effective gene expression. Information on molecular mechanisms obtained by such methodologies can then be applied to specific vectors, whether viral, synthetic, or hybrid, allowing for the creation of targeted, effective, and safe gene therapeutics.

Ex vivo expansion of transplantable human hematopoietic stem cells: where do we stand in the year 2000?

Ex vivo expansion of hematopoietic precursors, progenitors and stem cells represents the modern era of cellular therapeutics in the 21st century. For the last 10 years, increasing means for identifying and purifying hematopoietic stem cells and cytokines have facilitated and improved the development of ex vivo stem cell expansion technology. However, technology has not yet reached a stage where ex vivo-expanded hematopoietic progenitors and stem cells can be used routinely for replacement therapy. Lessons learned over the past 10 years from investigations focused at developing optimal ex vivo stem cell expansion systems have continued to a much greater understanding of stem cell biology. This knowledge has led to novel attempts at ex vivo expansion of hematopoietic precursors, progenitors, and stem cells, and should facilitate development of a new generation of cellular therapeutics. This review addresses recent progress toward development of clinically useful protocols for stem cell expansion. In addition, we discuss the results of a limited number of clinical trials that address the efficacy of such procedures. Three major areas of ex vivo stem cell expansion that impact clinical feasibility are discussed, including: (1) selection of an optimal stem cell population for expansion, (2) definition of the desired characteristics of the expanded stem cell population to be used for engraftment, and (3) development of new reagents and procedures for expansion and infusion of hematopoietic progenitors and stem cells.

The beta-galactosidase gene as a marker for hematopoietic reconstitution: individual cell analysis in a murine bone marrow transplantation model

We have used a simple, single-gene retrovirus carrying the Escherichia coli beta-galactosidase reporter gene (lacZ), termed LlacZ. This virus was found to infect immortalized myeloid and lymphoid precursor/leukemic cell lines efficiently as well as primary murine bone marrow clonogenic progenitors, without apparent modulation of growth or phenotype. Following infection of bone marrow cells, a significant proportion of progenitors--36% of lineage-negative cells with low levels of c-kit expression (lin-/c-kit(lo)) known to be enriched with pluripotent hemopoietic stem cells, and 19% of Sca1-positive cells known to be enriched with transplantable cells with lymphomyeloid-reconstituting ability--were shown to express lacZ. Use of an LlacZ-infected population of post 5-fluorouracil bone marrow cells to reconstitute lethally irradiated mice demonstrated the presence of lacZ-expressing cells in the spleen at day 12 post-transplantation with provirus detected in individual spleen colonies (CFU-S). In the long term (3-6 months following transplantation), lacZ expression was detected in hematopoietic tissues of all recipient mice. The use of two-color in situ and flow cytometry analysis combined with lineage-specific antibodies showed lacZ expression in both myeloid and lymphoid cells in spleen and bone marrow. In addition, lacZ-expressing cells were detected in secondary recipient mice injected with bone marrow cells derived from primary LlacZ recipients. Overall, these data show the efficacy of a single gene vector for stem cell transduction, the utility of beta-galactosidase as a single cell marker for stem cell transduction and reconstitution ability, and the need for protocol optimization to see high-level multilineage gene expression.

GST-pi gene-transduced hematopoietic progenitor cell transplantation overcomes the bone marrow toxicity of cyclophosphamide in mice

Autologous transplantation of bone marrow cells (BMCs) transduced with the multidrug resistance 1 (MDR1) gene or dihydrofolate reductase (DHFR) gene has already been applied in clinical chemoprotection trials. However, anticancer drugs frequently used in high-dose chemotherapy (HDC), such as alkylating agents, are not relevant to MDR1 or DHFR gene products. In this context, we have previously reported that glutathione S-transferase-pi (GST-pi) gene-transduced human CD34(+) cells showed resistance in vitro against 4-hydroperoxicyclophosphamide, an active form of cyclophosphamide (CY). In the present study, a subsequent attempt was made in a murine model to evaluate the effectiveness of transplantation of GST-pi-transduced BMCs to protect bone marrow against high-dose CY. The gene transfection was carried out retrovirally, employing a recombinant fibronectin fragment. Transfection efficiency into CFU-GM was 30%. After the transplantation, recipient mice (GST-pi mice) received three sequential courses of high-dose CY. As the chemotherapy courses advanced, both shortening of recovery period from WBC nadir and shallowing of WBC nadir were observed. In contrast to the fact that three of seven control mice died, possibly due to chemotoxicity, all seven GST-pi mice were alive after the third course, at which point the vector GST-pi gene was detected in 50% of CFU-GM derived from their BMCs and peripheral blood mononuclear cells. When BMCs obtained from these seven mice were retransplanted into secondary recipient mice, 20% of CFU-GM from BMCs showed positive signals for vector GST-pi DNA after 6 months. These data indicate that the GST-pi gene can confer resistance to bone marrow against CY by being transduced into long-term repopulating cells.

Stable marker gene transfer into human bone marrow stromal cells and their progenitors using novel herpesvirus saimiri-based vectors

We have evaluated the ability of new herpesvirus saimiri (HVS)-based vectors to deliver a marker gene green fluorescent protein (GFP) into human bone marrow (BM) stromal cells and their progenitors. Stromal cells expanded from adherent layers of long-term BM cultures (LTC) were susceptible to HVS-based infection in a dose-dependent manner, and the efficiency of 94.8 +/- 2.0% was achieved using single exposure with HVS/EGFP vector at multiplicity of infection (moi) of approximately 50. Colony-forming unit-fibroblast (CFU-F) assay established the ability of HVS-based vectors to infect progenitors for bone marrow stroma fibroblasts and transfer the marker gene over multiple cell divisions in the absence of selective pressure. HVS was not toxic for stromal cells and progenitors and no viral replication was detected upon growth of modified stroma. On the basis these data, we believe that HVS-based constructs can offer a new opportunity for selective gene delivery into bone marrow stromal cells and progenitors. The ability of HVS to infect nondividing cells can be considered advantageous in the development of both ex vivo and in vivo strategies.

Inactivation of a GFP retrovirus occurs at multiple levels in long-term repopulating stem cells and their differentiated progeny

Hematopoietic stem cell gene therapy holds promise for the treatment of many hematologic disorders. One major variable that has limited the overall success of gene therapy to date is the lack of sustained gene expression from viral vectors in transduced stem cell populations. To understand the basis for reduced gene expression at a single-cell level, we have used a murine retroviral vector, MFG, that expresses the green fluorescent protein (GFP) to transduce purified populations of long-term self-renewing hematopoietic stem cells (LT-HSC) isolated using the fluorescence-activated cell sorter. Limiting dilution reconstitution of lethally irradiated recipient mice with 100% transduced, GFP+ LT-HSC showed that silencing of gene expression occurred rapidly in most integration events at the LT-HSC level, irrespective of the initial levels of GFP expression. When inactivation occurred at the LT-HSC level, there was no GFP expression in any hematopoietic lineage clonally derived from silenced LT-HSC. Inactivation downstream of LT-HSC that stably expressed GFPin long-term reconstituted animals was restricted primarily to lymphoid cells. These observations suggest at least 2 distinct mechanisms of silencing retrovirally expressed genes in hematopoietic cells.

Inactivation of a GFP retrovirus occurs at multiple levels in long-term repopulating stem cells and their differentiated progeny

Hematopoietic stem cell gene therapy holds promise for the treatment of many hematologic disorders. One major variable that has limited the overall success of gene therapy to date is the lack of sustained gene expression from viral vectors in transduced stem cell populations. To understand the basis for reduced gene expression at a single-cell level, we have used a murine retroviral vector, MFG, that expresses the green fluorescent protein (GFP) to transduce purified populations of long-term self-renewing hematopoietic stem cells (LT-HSC) isolated using the fluorescence-activated cell sorter. Limiting dilution reconstitution of lethally irradiated recipient mice with 100% transduced, GFP+ LT-HSC showed that silencing of gene expression occurred rapidly in most integration events at the LT-HSC level, irrespective of the initial levels of GFP expression. When inactivation occurred at the LT-HSC level, there was no GFP expression in any hematopoietic lineage clonally derived from silenced LT-HSC. Inactivation downstream of LT-HSC that stably expressed GFPin long-term reconstituted animals was restricted primarily to lymphoid cells. These observations suggest at least 2 distinct mechanisms of silencing retrovirally expressed genes in hematopoietic cells.

Long-term expression and transfer of arylsulfatase A into brain of arylsulfatase A-deficient mice transplanted with bone marrow expressing the arylsulfatase A cDNA from a retroviral vector

A deficiency of arylsulfatase A (ASA) results in the lysosomal lipid storage disease metachromatic leukodystrophy. The disease mainly affects the central nervous system causing a progressive demyelination. A therapeutic effect will depend on the delivery of the deficient enzyme to the central nervous system. We have transplanted ASA-deficient mice with bone marrow transduced with a retroviral vector expressing the human ASA cDNA. All transplanted animals initially showed high serum levels of human ASA. In 50% of the recipients high ASA serum levels were sustained for 12 months after transplantation. In the remaining mice, serum levels decreased rapidly to low or undetectable levels. ASA activity and immunoreactivity was detectable in all organs of animals with continuous levels of ASA in serum. Most notably, substantial amounts of ASA activity were transferred into the brain, reaching up to 33% of the normal tissue level. In contrast to peripheral organs, the amount of enzyme delivered to the brain did not correlate with ASA serum levels as an indicator of overexpression. This reveals that enzyme transfer to the brain is not due to endocytosis of serum ASA by endothelial cells, but rather to bone marrow-derived cells migrated into the brain. Gene Therapy (2000) 7, 1250-1257.

Retrovirus-mediated gene transfer into T cells: 95% transduction efficiency without further in vitro selection

This study was designed to retrovirally transduce T cells by a protocol that would be simple, short, cost effective, applicable for clinical use, and efficient enough to avoid further selection of transduced T cells. Because retrovirally mediated infection is depending on the cell cycle, we first optimized the conditions for activating T cells in the presence of immobilized CD3 monoclonal antibodies and recombinant interleukin 2. Cell cycle analysis indicated that CD8+ and total T cells reach a maximum of cycling within 4 days whereas CD4+ T cells attain their maximum of cycling only by day 6. Taking into account these data, CD4+, CD8+, and total T cells were preactivated for 5 and 3 days, respectively, and then infected for 24 hr with supernatant containing retrovirus pseudotyped with gibbon-ape leukemia virus envelope, using a cell centrifugation protocol. Results show that approximately 95% of CD4+, CD8+, and total T cells can be transduced, this transduction efficiency being significantly higher than that obtained with amphotropic retrovirus vectors. Furthermore, under permanent growth stimulation, transduced T cells can be expanded approximately 1,000-fold in 4 weeks of culture with maintenance of transgene expression. However, Immunoscope analysis revealed alterations of T cell repertoire diversity after 2-3 weeks in culture that was not due to retroviral transduction per se. Overall, these data provide evidence that T cells can be transduced at levels that may alleviate the need for both further selection of transduced cells and in vitro expansion, thereby preserving the repertoire diversity of the transduced T cells to be reinfused.

Stem cell therapy and gene transfer for regeneration

The committed stem and progenitor cells have been recently isolated from various adult tissues, including hematopoietic stem cell, neural stem cell, mesenchymal stem cell and endothelial progenitor cell. These adult stem cells have several advantages as compared with embryonic stem cells as their practical therapeutic application for tissue regeneration. In this review, we discuss the promising gene therapy application of adult stem and progenitor cells in terms of modifying stem cell potency, altering organ property, accelerating regeneration and forming expressional organization.

Improved retroviral transduction of hematopoietic progenitors by combining methods to enhance virus-cell interaction

One of the factors required for successful retroviral transduction is contact between viral particles and target cells. We hypothesized that combining agents that improve virus-target cell interaction via different mechanisms will increase transduction efficiency. We examined the transduction efficiency of leukemic K562 cells, primary normal and chronic myelogenous leukemia CD34+ cells with the amphotropic retroviral vector, G1Na, packaged in PA317 by enumerating G418-resistant colonies in semisolid media. We evaluated the ability of the recombinant fibronectin fragment, CH296, cationic lipids, or a transwell flow-through system, alone or in combination to improve retroviral transduction. Transduction of K562 cells improved 1.5 to two-fold with lipids or CH296, while their combination improved transduction 2.5-fold. Transduction of K562 cells in the transwell flow-through system improved transduction three-fold. Transduction of normal (NL) CD34+ CFC improved 10-fold with lipids and 20-fold with CH296. Lipid and CH296 had synergistic effects. The transwell flow-through system improved transduction of normal CD34+ CFC 30-fold. Finally, similar to what was seen for K562 cells, transduction of CML CFC improved two- to three-fold with either CH296 or lipids, whereas the combination had synergistic effects. We conclude that any physical means that enhances contact between viral particles and target cells improves transduction. Two such methods that have different action mechanisms have additive or synergistic effects on transduction.

Genetically modified CD34+ cells as cellular vehicles for gene delivery into areas of angiogenesis in a rhesus model

To develop a cellular vehicle able to reach systemically disseminated areas of angiogenesis, we sought to exploit the natural tropism of circulating endothelial progenitor cells (EPCs). Primate CD34+ EPCs were genetically modified with high efficiency and minimal toxicity using a non-replicative herpes virus vector. These EPCs localized in a skin autograft model of angiogenesis in rhesus monkeys, and sustained the expression of a reporter gene for several weeks while circulating in the blood. In animals infused with autologous CD34+ EPCs transduced with a thymidine kinase-encoding herpes virus, skin autografts and subcutaneous Matrigel pellets impregnated with vascular growth factors underwent necrosis or accelerated regression after administration of ganciclovir. Importantly, the whole intervention was perfectly well tolerated. The accessibility, easy manipulation, lack of immunogenicity of the autologous CD34+ cell vehicles, and tropism for areas of angiogenesis render autologous CD34+ circulating endothelial progenitors as ideal candidates for exploration of their use as cellular vehicles when systemic gene delivery to those areas is required. Gene Therapy (2000) 7, 43-52.

In vivo methotrexate selection of murine hemopoietic cells transduced with a retroviral vector for Gaucher disease

The studies described were performed to investigate whether in vivo selection of retrovirus-transduced hemopoietic cells is feasible starting from a low percentage of transduced hemopoietic stem cells (PHSCs). The vector used is an amphotropic bicistronic retroviral vector carrying a cDNA for human lysosomal glucocerebrosidase (hGC) for treatment of Gaucher disease and a methotrexate (MTX) resistant mutant cDNA encoding human dihydrofolate reductase (DHFR). We tested the effect of MTX selection in mice that were either myeloablated or not before infusion of transduced cells. In addition, we determined whether repeated administration of transduced bone marrow cells has an additional effect on the percentage of hGC expressing cells. The results obtained have shown that, in myeloablated mice transplanted once with transduced bone marrow and treated twice weekly with 10 mg/kg of MTX for a total of 6 months, a two- to three-fold increased numbers of hGC expressing cells could be detected in both peripheral blood and bone marrow as compared with non-MTX treated mice. In mice transplanted with transduced bone marrow once every 2 weeks for a total of four times, percentages of hGC expressing cells were not significantly increased as compared with mice transplanted once. In non-ablated mice neither MTX selection nor multiple infusions of transduced bone marrow resulted in detection of hGC expressing cells 6 months after transplantation, indicating that the success of in vivo selection using MTX is highly dependent on the ratio of transduced hemopoietic stem cells transplanted versus residing and untransduced stem cells.

Biological barriers to cellular delivery of lipid-based DNA carriers

Although lipid-based DNA delivery systems are being assessed in gene therapy clinical trials, many investigators in this field are concerned about the inefficiency of lipid-based gene transfer technology, a criticism directed at all formulations used to enhance transfer of plasmid expression vectors. It is important to recognize that many approaches have been taken to improve transfection efficiency, however because of the complex nature of the formulation technology being developed, it has been extremely difficult to define specific carrier attributes that enhance transfection. We believe that these optimization processes are flawed for two reasons. First, a very defined change in formulation components affects the physical and chemical characteristics of the carrier in many ways. As a consequence, it has not been possible to define structure/activity relationships. Second, the primary endpoint used to assess plasmid delivery has been transgene expression, an activity that is under the control of cellular processes that have nothing to do with delivery. Gene expression following administration of a plasmid expression vector involves a number of critical steps: (i) DNA protection, (ii) binding to a specific cell population, (iii) DNA transfer across the cell membrane, (iv) release of DNA into the cytoplasm, (v) transport through the cell and across the nuclear membrane as well as (vi) transcription and translation of the gene. The objective of this review is to describe lipid-based DNA carrier systems and the attributes believed to be important in regulating the transfection activity of these formulations. Although membrane destabilization activity of the lipid-based carriers plays an important role, we suggest here that a critical element required for efficient transfection is dissociation of lipids bound to the plasmid expression vector following internalization.

Gene transfer approaches to the lysosomal storage disorders

The work summarized in this paper used animal and cell culture models systems to develop gene therapy approaches for the lysosomal storage disorders. The results have provided the scientific basis for a clinical trial of gene transfer to hematopoietic stem cells (HSC) in Gaucher disease which is now in progress. The clinical experiment is providing evidence of HSC transduction, competitive engraftment of genetically corrected HSC, expression of the GC transgene, and the suggestion of a clinical response. In this paper we will review the progress made in Gaucher disease and include how gene transfer might be studied in other lysosomal storage disorders.

Human cord blood CD34+CD38- cell transduction via lentivirus-based gene transfer vectors

The efficient transfer and sustained expression of a transgene in human hematopoietic cells with in vivo repopulating potential would provide a significant advancement in the development of protocols for the treatment of hematopoietic diseases. Recent advances in the ability to purify and culture hematopoietic cells with the CD34+CD38- phenotype and with in vivo repopulating potential from human umbilical cord blood provide a direct means of testing the ability of transfer vectors to transduce these cells. Here we demonstrate the efficient transduction and expression of enhanced green fluorescent protein (EGFP) in human umbilical cord-derived CD34+CD38- cells, without prestimulation, using a lentivirus-based gene transfer system. Transduced CD34+CD38- cells cultured in serum-free medium supplemented with SCF, Flt-3, IL-3, and IL-6 maintained their surface phenotype for 5 days and expressed readily detectable levels of the transgene. The average transduction efficiency of the CD34+CD38- cells was 59 +/- 7% as determined by flow cytometry. Erythroid and myeloid colonies derived from transduced CD34+CD38- cells were EGFP positive at a high frequency (66 +/- 9%). In contrast, a murine leukemia virus-based vector transduced the CD34+CD38- cells at a low frequency (<4%). These results demonstrate the utility of lentiviral-based gene transfer vectors in the transduction of primitive human hematopoietic CD34+CD38- cells.

Development of a modified selective amplifier gene for hematopoietic stem cell gene therapy

We have proposed a novel concept, ie selective expansion of transduced cells, to overcome the low efficiency of gene transfer into hematopoietic stem cells. Previously, a fusion gene encoding a chimeric receptor (DeltaGCRER) between the mouse granulocyte colony-stimulating factor receptor (G-CSFR) and the hormone-binding domain of rat estrogen receptor was constructed as a 'selective amplifier gene'. Although the chimeric gene conferred estrogen-inducible proliferation on the transduced Ba/F3 cells, it also mediated differentiation of the retrovirally transduced 32D cells upon estrogen treatment. Since only a growth signal is required for our purpose, we further modified the DeltaGCRER gene to attenuate its differentiation signal. Based on the observation that tyrosine-703 in wild-type G-CSFR plays a pivotal role in transmitting the differentiation signal, phenylalanine was substituted for this residue in DeltaGCRER. When the resultant selective amplifier gene (DeltaY703F-GCRER gene) was expressed in 32D cells, sustained growth was supported by estrogen, while differentiation was suppressed. These cells ceased to grow upon estrogen withdrawal and differentiated with G-CSF treatment. The present findings suggested that DeltaY703F-GCRER may have desirable properties as a selective amplifier for hematopoietic stem cell expansion and gene therapy.

Self-renewal, differentiation or death: regulation and manipulation of hematopoietic stem cell fate

Hematopoietic stem cells (HSCs) are the rare cells from which all hematopoietic cells are derived. The absence of HSCs is not compatible with life because many essential cells, such as myeloid and erythroid cells, are short lived. The hematopoietic system is the first essential organ system that fails following cytotoxic treatments. It is the vulnerability of HSCs that prevents regeneration following treatment and thus long-term survival. Because HSCs have the capacity to regenerate a functional hematopoietic system, the manipulation of these cells in vitro holds many promises for gene-therapeutic and other applications; however, these are severely curtailed by current difficulties in maintaining and expanding HSCs in culture. This review focuses on recent approaches towards understanding how the HSC compartment is regulated in vivo and discusses how this knowledge might be applied to manipulating HSC numbers.

Transduction of human CD34+ cells that mediate long-term engraftment of NOD/SCID mice by HIV vectors

Efficient gene transfer into human hematopoietic stem cells (HSCs) is an important goal in the study of the hematopoietic system as well as for gene therapy of hematopoietic disorders. A lentiviral vector based on the human immunodeficiency virus (HIV) was able to transduce human CD34+ cells capable of stable, long-term reconstitution of nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. High-efficiency transduction occurred in the absence of cytokine stimulation and resulted in transgene expression in multiple lineages of human hematopoietic cells for up to 22 weeks after transplantation.

Retroviral transduction of enriched hematopoietic stem cells allows lifelong Bcl-2 expression in multiple lineages but does not perturb hematopoiesis

Transduction of hematopoietic stem cells with a novel retrovirus has allowed long-term expression of human Bcl-2 in multiple hematopoietic lineages. Thy-1.2lo Sca-1+ H-2Khi stem cells enriched from the bone marrow of 5-fluorouracil-treated (Ly5-2) mice were infected with the bcl-2 retrovirus and injected into (Ly5-1) irradiated recipients. Analysis at 5 months indicated that reconstitution of hematopoiesis occurred predominantly from donor-derived (Ly5-2+) stem cells and that, in half the mice (18 of 35), most blood cells derived from virally transduced stem cells. The level of Bcl-2 expression achieved with the retroviral vector approached that of a well-characterized transgenic vector and could be sustained for life in several blood cell lineages. In the 25 mice assessed at 10 months, human Bcl-2 was readily detectable in 62+/-22% of Ly5-2+ peripheral blood leukocytes. More detailed analysis of a cohort killed between 14 and 20 months established that human Bcl-2 protein could be detected in B and T lymphocytes, granulocytes, macrophages, and some immature erythroid cells. Furthermore, hematopoietic stem cells from the bone marrow of these mice maintained Bcl-2 expression in hematopoietic tissues of secondary recipients for at least another 19 months. These data provide clear evidence for efficient infection of primitive hematopoietic stem cells and for maintenance of proviral expression for over 2.5 years, the lifespan of mice. The level of exogenous Bcl-2 was sufficient to enhance survival of B and T lymphoid cells, granulocytes, and myeloid colony-forming cells cultured under suboptimal conditions, but hematopoiesis in the mice was not notably perturbed.

Vector development: a major obstacle in human gene therapy

Gene therapy has been proposed for a wide variety of human conditions including monogenic disorders, such as the haemoglobinopathies and immunodeficiency syndromes, cancer and many other diseases. Prerequisites for the success of this approach include the ability to deliver the therapeutic gene intact to the target cell, persistent levels of transgene expression sufficient to correct the disease phenotype, lack of unwanted side-effects associated with vector exposure or gene transfer and relative simplicity allowing the widespread use of this methodology. Although substantial progress has been made in animal models since the inception of genetic therapy in the early 1980s, significant obstacles remain for human therapy, most notably in the area of vector development. The first generation of gene therapy vectors has failed to overcome many of the biological hurdles cited above necessitating the development of alternate means of gene delivery and expression.