Retroviral transduction efficiency of G-CSF+SCF-mobilized peripheral blood CD34+ cells is superior to G-CSF or G-CSF+Flt3-L-mobilized cells in nonhuman primates

Clonal tracking of haematopoietic cells: insights and clinical implications

Recent advances in high-throughput genomics have enabled the direct tracking of outputs from many cell types, greatly accelerating the study of developmental processes and tissue regeneration. The capacity for long-term self-renewal with multilineage differentiation potential characterises the cellular dynamics of a special set of developmental states that are critical for maintaining homeostasis. In haematopoiesis, the archetypal model for development, lineage-tracing experiments have elucidated the roles of haematopoietic stem cells to ongoing blood production and the importance of long-lived immune cells to immunological memory. An understanding of the biology and clonal dynamics of these cellular fates and states can provide clues to the response of haematopoiesis to ageing, the process of malignant transformation, and are key to designing more efficacious and durable clinical gene and cellular therapies.

Collection of hematopoietic CD34 stem cells in rhesus macaques using Spectra Optia

BACKGROUND

Nonhuman primates, particularly rhesus macaques, are ideal preclinical large animal models to investigate organ tolerance induction protocols using donor hematopoietic stem cells (HSCs) to induce chimerism. Their relatively small size poses some challenges for the safe and effective collection of peripheral blood HSCs through apheresis procedures. We describe our experiences using the Spectra Optia apheresis unit to successfully obtain HSCs from mobilized peripheral blood of rhesus macaques.

METHOD

Mobilization of peripheral blood HSCs was induced using granulocyte stimulating factor (G-CSF) and Mozobil. The Spectra Optia unit was used in 18 apheresis procedures in 13 animals (4.9-10 kg). Animal health was carefully monitored during and after the procedure. Changes in peripheral blood cells before, during and after procedure were determined by complete blood count and flow cytometry.

RESULTS

The automatic settings of the Spectra Optia unit were applied successfully to the procedures on the rhesus macaque. All animals tolerated the procedure well with no mortality. Mobilization of HSCs were most consistently achieved using 50 μg/kg of G-CSF for 5 days and a single dose of Mozobil on the 5th day, followed by collection of cells 3 h after Mozobil injection. The final apheresis product contained an average of 23 billion total nucleated cells with 47% granulocytes, 3,871 million total CD3 cells and 77 million CD34 cells which resulted in an average of 10 million CD34+ cells/kg of donor weight.

CONCLUSION

Apheresis of peripheral blood mobilized HSCs in rhesus macaques using Spectra Optia is a safe and effective procedure.

Gene therapy for β-thalassaemia: the continuing challenge

The β-thalassaemias are inherited anaemias that form the most common class of monogenic disorders in the world. Treatment options are limited, with allogeneic haematopoietic stem cell transplantation offering the only hope for lifelong cure. However, this option is not available for many patients as a result of either the lack of compatible donors or the increased risk of transplant-related mortality in subjects with organ damage resulting from accumulated iron. The paucity of alternative treatments for patients that fall into either of these categories has led to the development of a revolutionary treatment strategy based on gene therapy. This approach involves replacing allogeneic stem cell transplantation with the transfer of normal globin genes into patient-derived, autologous haematopoietic stem cells. This highly attractive strategy offers several advantages, including bypassing the need for allogeneic donors and the immunosuppression required to achieve engraftment of the transplanted cells and to eliminate the risk of donor-related graft-versus-host disease. This review discusses the many advances that have been made towards this endeavour as well as the hurdles that must still be overcome before gene therapy for β-thalassaemia, as well as many other gene therapy applications, can be widely applied in the clinic.

Genetic modification of human hematopoietic cells: preclinical optimization of oncoretroviral-mediated gene transfer for clinical trials

This chapter provides information about the oncoretroviral transduction of human hematopoietic stem/ progenitor cells under clinically applicable conditions. We describe in detail a short -60 h transduction protocol which consistently yields transduction efficiencies in the range of 30-50% with five different oncoretroviral vectors. We discuss a number of parameters that affect transduction efficiency, including the oncoretroviral vector characteristics, the vector stock collection, the source of CD34+ cells and transduction conditions.

Hematopoietic stem-cell behavior in nonhuman primates

Little is known about the behavior of hematopoietic stem cells (HSCs) in primates because direct observations and competitive-repopulation assays are not feasible. Therefore, we used 2 different and independent experimental strategies, the tracking of transgene expression after retroviral-mediated gene transfer (N = 11 baboons; N = 7 rhesus macaques) and quantitation of the average telomere length of granulocytes (N = 132 baboons; N = 14 macaques), together with stochastic methods, to study HSC kinetics in vivo. The average replication rate for baboon HSCs is once per 36 weeks according to gene-marking analyses and once per 23 weeks according to telomere-shortening analyses. Comparable results were derived from the macaque data. These rates are substantially slower than the average replication rates previously reported for HSCs in mice (once per 2.5 weeks) and cats (once per 8.3 weeks). Because baboons and macaques live for 25 to 45 years, much longer than mice ( approximately 2 years) and cats (12-18 years), we can compute that HSCs undergo a relatively constant number ( approximately 80-200) of lifetime replications. Thus, our data suggest that the self-renewal capacity of mammalian stem cells in vivo is defined and evolutionarily conserved.

The Genetic Engineering of Hematopoietic Stem Cells: the Rise of Lentiviral Vectors, the Conundrum of the LTR, and the Promise of Lineage-restricted Vectors

Recent studies on the integration patterns of different categories of retroviral vectors, the genotoxicity of long-terminal repeats (LTRs) and other genetic elements, the rise of lentiviral technology and the emergence of regulated vector systems providing tissue-restricted transgene expression and RNA interference, are profoundly changing the landscape of stem cell-based therapies. New developments in vector design and an increasing understanding of the mechanisms underlying insertional oncogenesis are ushering in a new phase in hematopoietic stem cell (HSC) engineering, thus bringing the hitherto exclusive reliance on LTR-driven, gamma-retroviral vectors to an end. Based on their ability to transduce non-dividing cells and their genomic stability, lentiviral vectors offer new prospects for the manipulation of HSCs. Tissue-specific vectors, as exemplified by globin vectors, not only provide therapeutic efficacy, but may also enhance safety, insofar that they restrict transgene expression in stem cells, progenitor cells and blood cells in all but the transcriptionally targeted lineage. This review provides a survey of these advances as well as several remaining challenges, focusing in particular on the importance of achieving adequate levels of protein expression from a limited number of vector copies per cell-ideally one to two.

Additive effect of endothelial progenitor cell mobilization and bone marrow mononuclear cell transplantation on angiogenesis in mouse ischemic limbs

The methods of therapeutic angiogenesis include endothelial progenitor cell (EPC) mobilization with cytokines [e.g., granulocyte colony-stimulating factor (G-CSF)] and bone marrow mononuclear cell (BMMNC) transplantation. Combined angiogenic therapies may be superior to a single angiogenic therapy for the treatment of limb ischemia. Therefore, we investigated whether the angiogenic efficacy of a combination of two angiogenic strategies is superior to either strategy alone. One day after the surgical induction of hindlimb ischemia, mice were randomized to receive either no treatment, EPC mobilization with G-CSF administration, BMMNC transplantation using a fibrin matrix, or a combination of EPC mobilization with BMMNC transplantation using a fibrin matrix. EPC mobilization with G-CSF or BMMNC transplantation using a fibrin matrix significantly increased the microvessel density compared with no treatment. Importantly, a combination of EPC mobilization with BMMNC transplantation using a fibrin matrix further increased the densities of microvessels and BrdU-positive capillaries compared to either strategy alone. Basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) expression was higher in the EPC mobilization with G-CSF or BMMNC transplantation group than in the no treatment group. The combination therapy of EPC mobilization with G-CSF and BMMNC transplantation resulted in more extensive expression of bFGF and VEGF than the single therapy of either EPC mobilization with G-CSF treatment or BMMNC transplantation. This study demonstrates that the combination therapy of BMMNC transplantation and EPC mobilization potentiates the angiogenic efficacy of either single therapy in mouse limb ischemia models.

Stem cell gene transfer: insights into integration and hematopoiesis from primate genetic marking studies

Genetic marking strategies in the nonhuman primate model have elucidated a number of principles with relevance to implementation of clinical stem cell therapies, including the lineage potential, number, and life span of hematopoietic stem and progenitor cells. The recent occurrence of leukemias likely related to insertional activation of a proto-oncogene in two patients with X-severe combined immunodeficiency (SCID) syndromes treated with CD34(+) cells transduced with retroviral vectors expressing the corrective common gamma cytokine receptor gene has refocused attention on the issue of retroviral integration. We have analyzed >1500 independent insertions from rhesus macaques transplanted with CD34(+) cells transduced with either MLV or SIV vectors. Of these, 46 rhesus macaques followed long term have not had progression to leukemia, abnormal hematopoiesis, or clonal hematopoiesis. However, the pattern of both MLV and SIV integrants in cells of these animals was found to be highly nonrandom, with a propensity for insertions of both vectors within genes: for MLV particularly near the transcription start site, and for SIV particularly in gene-dense regions.

In vivo gene transfer into adult stem cells in unconditioned mice by in situ delivery of a lentiviral vector

The potential of in vivo lentivirus-mediated bone marrow stem cell gene transfer by bone cavity injection, which could take full advantage of any source of stem cells present there, has not been previously explored. Such an approach may avoid several difficulties encountered by ex vivo hematopoietic stem cell (HSC) gene transfer. We sought to determine if efficient gene transfer could be achieved in HSC and mesenchymal stem/progenitor cells (MSC) by intrafemoral injection of a lentivirus vector in mice. Four months after injection, up to 12% GFP-expressing cells were observed in myeloid and lymphoid subpopulations. Significant transduction efficiencies were seen in Lin(-)c-kit(+)Sca1(+) HSC/progenitors and CFU with multilineage potential, which were also confirmed by duplex PCR analysis of progenitor-derived colonies. Four months after secondary BMT, we observed 8.1 to 15% vector(+) CFU in all recipients. Integration analysis by LAM-PCR demonstrated that multiple transduced clones contributed to hematopoiesis in these animals. We also showed that GFP-expressing MSC retained multilineage differentiation potential, with 2.9 to 8.8% GFP-containing CFU-fibroblasts detected in both injected and BMT recipients. Our data provide evidence that adult stem cells in bone marrow can be efficiently transduced "in situ" by in vivo vector administration without preconditioning. This approach could lead to a novel application for treatment of human diseases.

Efficient transduction and engraftment of G-CSF-mobilized peripheral blood CD34+ cells in nonhuman primates using GALV-pseudotyped gammaretroviral vectors

The optimal stem cell source for stem cell gene therapy has yet to be determined. Most large-animal studies have utilized peripheral blood or marrow-derived cells collected after administration of granulocyte colony-stimulating factor (G-SCF) and stem cell factor (SCF); however, SCF is unavailable for clinical use in the United States and the European Union. A recent study in a competitive repopulation assay in the rhesus macaque showed very inefficient marking of G-CSF-mobilized (G/only) peripheral blood (G-PBSC) CD34(+) cells relative to G-CSF and SCF-mobilized cells using vectors with an amphotropic pseudotype. Because G-PBSC would be the preferred target cell population for most clinical stem cell gene therapy applications, we asked whether we could achieve efficient transduction and engraftment of G-PBSC using Phoenix-GALV-pseudotyped vectors. We transplanted three baboons with G/only mobilized CD34(+) cells transduced with GALV-pseudotyped retroviral vectors. We observed high-level, persistent engraftment of gene-modified G-PBSC in all animals with gene marking levels in granulocytes up to 60%. We analyzed amphotropic (PIT2) and GALV (PIT1) receptor expression in G/only cells and found preferential expression of PIT1 after G/only, which may explain the inferior results with amphotropic pseudotypes. These findings demonstrate that high stem cell gene transfer levels can be achieved using G-CSF-mobilized PBSC with Phoenix-GALV-pseudotyped vectors.

Combined Sustained Delivery of Basic Fibroblast Growth Factor and Administration of Granulocyte Colony-Stimulating Factor:Synergistic Effect on Angiogenesis in Mouse Ischemic Limbs

PURPOSE

To investigate whether the efficacy of a single angiogenic therapy (sustained delivery of basic fibroblast growth factor [bFGF] or administration of granulocyte colony-stimulating factor [G-CSF]) can be enhanced further by combining the therapies.

METHODS

One day after surgical induction of hind-limb ischemia, groups of 6 mice were randomized to receive either no treatment, sustained delivery (SD) of bFGF, endothelial progenitor cell (EPC) mobilization with G-CSF administration, or a combination of bFGF SD + G-CSF administration.

RESULTS

G-CSF administration increased significantly (p < 0.05) the number of EPC lineages (CD34 + /AC133 + cells) in both peripheral blood and bone marrow compared to no G-CSF administration. The bFGF SD and G-CSF administration individually increased the capillary and arteriole densities significantly versus no treatment (capillary density: 659 +/- 48/mm2 and 385 +/- 59/mm2, respectively, versus 280 +/- 28/mm2; p < 0.05; arteriole density: 34 +/- 9/mm2 and 41 +/- 6/mm2, respectively, versus 15 +/- 2/mm2; p<0.05). Importantly, bFGF SD + G-CSF further increased the capillary and arteriole densities compared to either strategy alone (capillary density: 786 +/- 40/mm2 versus 659 +/- 48/mm2 and 385 +/- 59/mm2, respectively, p < 0.05; arteriole density: 55 +/- 10/mm2 versus 34 +/- 9/mm2 and 41 +/- 6/mm2, respectively, p < 0.05).

CONCLUSION

This study demonstrates that the combined therapy of sustained bFGF delivery and G-CSF administration potentiates the angiogenic efficacy of either single therapy in mouse hind-limb ischemia models.

Busulfan pharmacokinetics, toxicity, and low-dose conditioning for autologous transplantation of genetically modified hematopoietic stem cells in the rhesus macaque model

OBJECTIVE

Gene transfer to hematopoietic stem cells has recently been demonstrated to benefit a small number of patients in whom a selective advantage is conferred upon genetically modified cells; however, in disorders where no such selective advantage is conferred, conditioning appears necessary to allow adequate engraftment. To decrease the toxicity profile, we sought to develop nonmyeloablative conditioning regimens and in this work, explored the use of intravenous busulfan in a large animal model.

METHODS

Busulfan pharmacokinetics and toxicity were monitored in young rhesus macaques at two dosing levels (4 and 6 mg/kg). These doses were then employed to condition two animals at each dose level prior to autologous transplantation of genetically modified cells using our standard methods.

RESULTS

Busulfan pharmacokinetic (PK) data showed the area under the curve (AUC), drug half-life, and drug clearance were consistent within each dose group and similar to those reported in children. Single doses of busulfan were well tolerated and produced dose-dependent myelosuppression, most notably in the neutrophil and platelet counts. Although marking levels reached over 1% early in one animal, the long-term marking was low but detectable at 0.01 to 0.001%.

CONCLUSIONS

We conclude that low-dose intravenous bolus infusion of busulfan is well tolerated, has dose-dependent effects on peripheral blood counts, and allows long-term engraftment of genetically modified cells, but at levels too low for most clinical disorders.

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.

In vitro culture during retroviral transduction improves thymic repopulation and output after total body irradiation and autologous peripheral blood progenitor cell transplantation in rhesus macaques

Immunodeficiency after peripheral blood progenitor cell (PBPC) transplantation may be influenced by graft composition, underlying disease, and/or pre-treatment. These factors are difficult to study independently in humans. Ex vivo culture and genetic manipulation of PBPC grafts may also affect immune reconstitution, with relevance to gene therapy applications. We directly compared the effects of three clinically relevant autologous graft compositions on immune reconstitution after myeloblative total body irradiation in rhesus macaques, the first time these studies have been performed in a large animal model with direct clinical relevance. Animals received CD34(+) cell dose-matched grafts of either peripheral blood mononuclear cells, purified CD34(+) PBPCs, or purified CD34(+) PBPCs expanded in vitro and retrovirally transduced. We evaluated the reconstitution of T, B, natural killer, dendritic cells, and monocytes in blood and lymph nodes for up to 1 year post-transplantation. Animals receiving selected-transduced CD34(+) cells had the fastest recovery of T-cell numbers, along with the highest T-cell-receptor gene rearrangement excision circles levels, the fewest proliferating Ki-67(+) T-cells in the blood, and the best-preserved thymic architecture. Selected-transduced CD34(+) cells may therefore repopulate the thymus more efficiently and promote a higher output of naïve T-cells. These results have implications for the design of gene therapy trials, as well as for the use of expanded PBPCs for improved T-cell immune reconstitution after transplantation.

AMD3100 mobilizes hematopoietic stem cells with long-term repopulating capacity in nonhuman primates

AMD3100, a bicyclam antagonist of the chemokine receptor CXCR4, has been shown to induce rapid mobilization of CD34(+) hematopoietic cells in mice, dogs, and humans, offering an alternative to G-CSF mobilization of peripheral-blood hematopoietic stem cells. In this study, AMD3100-mobilized CD34(+) cells were phenotypically analyzed, marked with Neo(R)-containing retroviral vectors, and subsequently transplanted into myeloablated rhesus macaques. We show engraftment of transduced AMD3100-mobilized CD34(+) cells with Neo(R) gene marked myeloid and lymphoid cells up to 32 months after transplantation, demonstrating the ability of AMD3100 to mobilize true long-term repopulating hematopoietic stem cells. More AMD3100-mobilized CD34(+) cells are in the G(1) phase of the cell cycle and more cells express CXCR4 and VLA-4 compared with G-CSF-mobilized CD34(+) cells. In vivo gene marking levels obtained with AMD3100-mobilized CD34(+) cells were better than those obtained using CD34(+) cells mobilized with G-CSF alone. Overall, these results indicate that AMD3100 mobilizes a population of hematopoietic stem cells with intrinsic characteristics different from those of hematopoietic stem cells mobilized with G-CSF, suggesting fundamental differences in the mechanism of AMD3100-mediated and G-CSF-mediated hematopoietic stem cell mobilization. Thus, AMD3100-mobilized CD34(+) cells represent an alternative source of hematopoietic stem cells for clinical stem cell transplantation and genetic manipulation with integrating retroviral vectors.

Large animal models for stem and progenitor cell analysis

Extrapolation of an understanding regarding hematopoiesis and, in particular, hematopoietic stem cells (HSCs) from rodent models or in vitro human cell models to applications in humans has proven very difficult. This is not surprising, given the differences between rodent and human hematopoietic physiology and the lack of true in vitro assays for HSCs. Therefore, translational preclinical development of genetic and cellular therapies is dependent on the utilization of practical and well-defined large animal models. This chapter will introduce the most commonly used model species, including macaques, baboons, dogs, cats, and sheep, and explain the particular advantages and limitations of each. Specific protocols for the support of macaques through ablative cell and gene therapy procedures will be included to introduce investigators to the types of resources and support required to maintain a large animal facility dedicated to high-intensity experimentation, and also to introduce investigators to the types of procedures that are possible.

Hematopoietic stem cell gene therapy: dead or alive?

Despite some reports of toxicity in recent clinical trials, many scientists believe that the use of gene therapy in the treatment of congenital genetic defects and acquired disorders has too much potential to abandon. Hematopoietic stem cells (HSCs) have been primary targets for gene therapy owing to their capacity for differentiation and self-renewal, whereby multiple cell lineages can potentially be corrected for the lifetime of an individual. These efforts represent a long-term investment towards broadening physicians' treatment options for patients whose diseases, in particular certain immunodeficiencies, are fatal and where no other therapy is available. We review the recent progress and clinical triumphs as well as the reported toxicity related to insertional mutagenesis. We also discuss the current risk-to-benefit estimates and future strategies to reduce the risks and allow full realization of clinical potential. Scientists are continually revising protocols: going both from "bench to bedside" and, as strikingly demonstrated by HSC gene therapy, from "bedside to bench."

Stem and progenitor cell therapies: insights from non-human primate models

Genetic marking strategies in the non-human primate model have elucidated a number of principles relevant to implementation of clinical stem cell therapies, including the lineage potential, number and lifespan of hematopoietic stem and progenitor cells, and differences in the functional properties of marrow cells mobilized into the peripheral blood utilizing different regimens.

Recurrent retroviral vector integration at the Mds1/Evi1 locus in nonhuman primate hematopoietic cells

Recent reports linking insertional activation of LMO2 following gene therapy for X-linked severe combined immunodeficiency (X-SCID) have led to a re-evaluation of risks following gene therapy with retroviral vectors. In our analysis of 702 integration sites in rhesus macaques that underwent transplantation up to 7 years earlier with autologous CD34+ cells transduced with amphotropic murine leukemia virus (MLV)-derived retroviral vectors containing marker genes, we detected insertion into one locus, the Mds1/Evi1 region, a total of 14 times in 9 animals. Mds1/Evi1 integrations were observed stably long term, primarily in myeloid cells. We hypothesize that this over-representation likely results from an impact on the self-renewal and engraftment potential of CD34+ progenitor cells via insertional mutagenesis at this specific locus. There is no evidence of ongoing in vivo clonal expansion of the Mds1/Evi1 populations, and all animals are hematologically normal without evidence for leukemia. Characterization of integration sites in this relevant preclinical model provides critical information for gene therapy risk assessment as well as identification of genes controlling hematopoiesis.

Improved gene transfer and normalized enzyme levels in primitive hematopoietic progenitors from patients with mucopolysaccharidosis type I using a bioreactor

BACKGROUND

One of the major barriers to the clinical application of hematopoietic stem cell (HSC) gene therapy has been relatively low gene transfer efficiency. Other inadequacies of current transduction protocols are related to their multi-step procedures, e.g., using tissue-culture flasks, roller bottles or gas-permeable bags for clinical application.

METHODS

In comparison with a conventional bag transduction protocol, a 'closed' hollow-fiber bioreactor system (HBS) was exploited to culture and transduce human peripheral blood CD34(+) progenitor cells (PBPC(MPS)) from patients with mucopolysaccharidosis type I (MPS I) using an amphotropic retroviral vector based on a murine Moloney leukemia virus LN prototype. Both short-term colony-forming cell (CFC) and long-term culture initiating cell (LTCIC) assays were employed to determine transduction frequency and transgene expression in committed progenitor cells and primitive progenitors with multi-lineage potentials.

RESULTS

A novel ultrafiltration-transduction method was established to culture and transduce enzyme-deficient PBPC(MPS) over a 5-day period without loss in viability and CD34 identity (n = 5). Significantly higher transduction efficiencies were achieved in primary CFC that derived from the HBS (5.8-14.2%) in comparison with those from gas-permeable bags (undetectable to 1.7%; p < 0.01). Up to 15-fold higher-than-normal enzyme activity was found in selected PBPC(MPS)-LP1CD transductants. Moreover, higher gene transfer (4.4-fold) and expression in very primitive progenitors were observed in products from the HBS compared with bag experiments as indicated by CFC derived from primitive LTCIC. Remarkably, with relatively modest gene transfer levels in LTCIC from HBS experiments, the expression of the IDUA transgene corrected the enzyme-deficiency in 5-week long-term cultures (LTC).

CONCLUSIONS

MPS I progenitor cells achieved normalized enzyme levels in LTC after transduction in a HBS system. These studies demonstrate the advantages of a bioreactor-transduction system for viral-mediated stem cell gene transfer.

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.

Distinct genomic integration of MLV and SIV vectors in primate hematopoietic stem and progenitor cells

Murine leukemia virus (MLV)-derived vectors are widely used for hematopoietic stem cell (HSC) gene transfer, but lentiviral vectors such as the simian immunodeficiency virus (SIV) may allow higher efficiency transfer and better expression. Recent studies in cell lines have challenged the notion that retroviruses and retroviral vectors integrate randomly into their host genome. Medical applications using these vectors are aimed at HSCs, and thus large-scale comprehensive analysis of MLV and SIV integration in long-term repopulating HSCs is crucial to help develop improved integrating vectors. We studied integration sites in HSCs of rhesus monkeys that had been transplanted 6 mo to 6 y prior with MLV- or SIV-transduced CD34⁺ cells. Unique MLV (491) and SIV (501) insertions were compared to a set of in silico-generated random integration sites. While MLV integrants were located predominantly around transcription start sites, SIV integrants strongly favored transcription units and gene-dense regions of the genome. These integration patterns suggest different mechanisms for integration as well as distinct safety implications for MLV versus SIV vectors.

A primate model of gene transfer into hematopoietic stem cells demonstrated MLV integration around transcription start sites whereas SIV integrated into gene-dense regions, indicating distinct safety implications for each

Competitive Repopulation Assay of Two Gene-Marked Cord Blood Units in NOD/SCID/γcnull Mice

In multiunit cord blood transplantation, hematopoietic stem cells from each unrelated cord blood (UCB) unit competitively reconstitute the hematopoietic system in a recipient. To evaluate the fate of the progeny of each UCB unit and to determine the effects of graft-versus-graft reaction, we established a novel competitive repopulation assay using NOD/SCID/gammac(null) mice in which human T lymphocytes develop from CD34+ cells. CD34+ cells from each UCB unit were labeled with recombinant lentivirus vectors carrying genes encoding either enhanced green fluorescent protein (EGFP) or enhanced yellow fluorescent protein (EYFP). Hematopoietic chimerism composed of both EGFP+ and EYFP+ cells was stably maintained up to 6 months after transplantation with purified CD34+ cells; the ratio of EGFP+ to EYFP+ cells in peripheral blood and bone marrow posttransplantation was equivalent to the ratio of these cells at transplantation. However, when mononuclear cells from two UCB units were cotransplanted with CD34+ cells, engraftment was highly competitive, with cells from only one or the other of the two UCB units surviving. Further subfractionations of mononuclear cells indicate that the skewed chimerism that is often observed in clinical multiunit cord blood transplantation may be mediated by the cooperation of both CD4+ and CD8+ T cells. The assay established here will be a useful tool for analyzing hematopoietic reconstitution in clinical multiunit cord blood transplantation.

Comparison of Retroviral Transduction Efficiency in CD34+Cells Derived from Bone Marrow versus G-CSF-Mobilized or G-CSF Plus Stem Cell Factor-Mobilized Peripheral Blood in Nonhuman Primates

Hematopoietic stem cells (HSCs) are ideal targets for genetic manipulation in the treatment of several congenital and acquired disorders affecting the hematopoietic compartment. Although G-CSF-mobilized peripheral blood CD34(+) cells are the favored source of hematopoietic stem cells in clinical transplantation, this source of stem cells does not provide meaningful engraftment levels of genetically modified cells compared with G-CSF + stem cell factor (SCF)-mobilized cells in nonhuman primates. Furthermore, the use of G-CSF mobilization can have disastrous consequences in patients with sickle cell disease, a long-held target disorder for HSC-based gene therapy approaches. We therefore conducted a study to compare the levels of genetically modified cells attainable after retroviral transduction of CD34(+) cells collected from a bone marrow (BM) harvest with CD34(+) cells collected from a leukapheresis product after mobilization with G-CSF (n = 3) or G-CSF in combination with SCF (n = 3) in the rhesus macaque autologous transplantation model. Transductions were performed using retroviral vector supernatant on fibronectin-coated plates for 96 hours in the presence of stimulatory cytokines. BM was equal to or better than G-CSF-mobilized peripheral blood as a source of HSCs for retroviral transduction. Although the highest marking observed was derived from G-SCF + SCF-mobilized peripheral blood in two animals, marking in the third originated only from the BM fraction. These results demonstrate that steady-state BM is at least equivalent to G-CSF-mobilized peripheral blood as a source of HSCs for retroviral gene transfer and the only currently available source for patients with sickle cell disease.

Synergistic Effect of Bone Marrow Mobilization and Vascular Endothelial Growth Factor-2 Gene Therapy in Myocardial Ischemia

Background— We performed a series of investigations to test the hypothesis that combining angiogenic gene therapy and cytokine (CK)-induced endothelial progenitor cell mobilization would be superior to either strategy alone for treatment of chronic myocardial ischemia.

Methods and Results— A swine model of chronic myocardial ischemia and a murine model of acute myocardial infarction were used in this study. In both models, animals were randomly assigned to 1 of 4 treatment groups: Combo group, intramyocardial vascular endothelial growth factor (VEGF)-2 gene transfer plus subcutaneous injection of CKs; VEGF-2, VEGF-2 gene transfer plus saline subcutaneously injected; CK, empty vector transfer plus CKs; and control, empty vector plus subcutaneous saline. Acute myocardial infarction was also induced in wild-type mice 4 weeks after bone marrow transplantation from enhanced green fluorescent protein transgenic mice to permit observation of bone marrow–derived cells in the myocardium after acute myocardial infarction. In chronic myocardial ischemia, combination therapy resulted in superior improvement in all indexes of perfusion and function compared with all other treatment groups. In the bone marrow transplant mice, double immunofluorescent staining revealed that the combination of CK-induced mobilization and local VEGF-2 gene transfer resulted in a significant increase in the number of bone marrow–derived cells incorporating into the neovasculature, indicating that recruitment and/or retention of bone marrow–derived progenitors was enhanced by mobilization and that local VEGF-2 gene transfer can provide signals for recruitment or incorporation of circulating progenitor cells.

Conclusions— Mobilization of endothelial progenitor cells with cytokines potentiates VEGF-2 gene therapy for myocardial ischemia and enhances bone marrow cell incorporation into ischemic myocardium.

Direct comparison of steady-state marrow, primed marrow, and mobilized peripheral blood for transduction of hematopoietic stem cells in dogs

The optimal stem cell source for stem cell gene therapy has not been defined. Most gene transfer studies have used peripheral blood or marrow repopulating cells collected after administration of granulocyte colony-stimulating factor and stem cell factor (G-CSF/SCF). For clinical applications, however, growth factor administration may not be feasible. Thus, in the current study we used a competitive repopulation assay in the dog to directly compare transduction efficiency of steady-state marrow, G-CSF/SCF-primed marrow, and G-CSF/SCF-mobilized peripheral blood. Cells from all three sources were transduced, cryopreserved, and thawed together before infusion into myeloablated dogs. Gene marking in hematopoietic repopulating cells was assessed by polymerase chain reaction. While primed marrow resulted in the highest long-term marking levels, steady-state marrow was transduced at least as efficiently as mobilized peripheral blood in all three dogs. These results suggest that steady-state marrow may be an appropriate source for genetic modification of hematopoietic stem cells.

Effect of chronic cytokine therapy on clonal dynamics in nonhuman primates

Hematopoietic cytokines such as filgrastim are used extensively to stimulate granulocyte production or to mobilize hematopoietic progenitors into the circulation; however, their effect on more primitive hematopoietic progenitor and stem cells in vivo is unknown, particularly in large animals or humans. In particular, there is concern that chronic therapy with cytokines could result in stem cell exhaustion or clonal dominance; however, direct assessment of the dynamics of individual stem and progenitor cell clones in vivo has not been previously reported. A number of models can be proposed regarding the mechanisms by which the marrow responds to cytokine stimulation, including recruitment of previously quiescent clones, stimulation of proliferation of already active clones, or prevention of apoptosis of more mature progenitors from all clones. Using retroviral marking and comprehensive insertion site tracking of individual stem and progenitor cell clones in 2 rhesus macaques, we analyzed the effect of chronic administration of granulocyte colony-stimulating factor (G-CSF), or a combination of G-CSF plus stem cell factor (SCF). The overall number of contributing clones remained constant, and the relative output from each clone did not change significantly during or following cytokine treatments. These results suggest that individual transduced stem or progenitor cells can contribute to hematopoiesis for prolonged periods, with no evidence for an effect of G-CSF or G-CSF/SCF on the number, the lifespan, or the relative activity of individual stem or progenitor cell clones. These relevant large animal studies are reassuring regarding clinical applications of cytokines and provide new insights into their mechanisms of action.

Efficient lentiviral gene transfer to canine repopulating cells using an overnight transduction protocol

The use of lentiviral vectors for the transduction of hematopoietic stem cells has evoked much interest owing to their ability to stably integrate into the genome of nondividing cells. However, published large animal studies have reported highly variable gene transfer rates of typically less than 1%. Here we report the use of lentiviral vectors for the transduction of canine CD34+ hematopoietic repopulating cells using a very short, 18-hour transduction protocol. We compared lentiviral transduction of hematopoietic repopulating cells from either stem cell factor (SCF)– and granulocyte-colony stimulating factor (G-CSF)–primed marrow or mobilized peripheral blood in a competitive repopulation assay in 3 dogs. All dogs engrafted rapidly within 9 days. Transgene expression was detected in all lineages (B cells, T cells, granulocytes, and red blood cells as well as platelets) indicating multilineage engraftment of transduced cells, with overall long-term marking levels of up to 12%. Gene transfer levels in mobilized peripheral blood cells were slightly higher than in primed marrow cells. In conclusion, we show efficient lentiviral transduction of canine repopulating cells using an overnight transduction protocol. These results have important implications for the design of stem cell gene therapy protocols, especially for those diseases in which the maintenance of stem cells in culture is a major limitation.

The impact of low-dose busulfan on clonal dynamics in nonhuman primates

An understanding of the number and contribution of individual pluripotent hematopoietic stem cells (HSCs) to the formation of blood lineages has important clinical implications for gene therapy and stem cell transplantation. We have been able to efficiently mark rhesus macaque long-term repopulating stem and progenitor cells with retroviral vectors, and track their in vivo contributions to hematopoiesis using the linear amplification mediated-polymerase chain reaction (LAM-PCR) technique of insertion site analysis. We assessed the impact of busulfan on contributions of individual retrovirally marked clones to hematopoiesis. There were 2 macaques that received transplants of retrovirally transduced CD34(+) cells 2 years previously that were then treated with 4 mg/kg busulfan. Despite only transient and mild suppression of peripheral blood counts, the numbers of individual stem/progenitor clones contributing to granulocyte production decreased dramatically, by 80% in the first monkey and by 60% in the second monkey. A similar impact was seen on clones contributing to T cells. The clone numbers recovered gradually back toward baseline by 5 months following busulfan in the first monkey and by 3 months in the second monkey, and have remained stable for more than one year in both animals. Tracking of individual clones with insertion-site-specific primers suggested that clones contributing to hematopoiesis prior to busulfan accounted for the majority of this recovery, but that some previously undetected clones began to contribute during this recovery phase. These results indicate that even low-dose busulfan significantly affects stem and progenitor cell dynamics. The clonal diversity of hematopoiesis was significantly decreased after even a single, clinically well-tolerated dose of busulfan, with slow but almost complete recovery over the next several months, suggesting that true long-term repopulating stem cells were not permanently deleted. However, the prolonged period of suppression of many clones suggests that transplanted HSCs may have a marked competitive advantage if they can engraft and proliferate during this time period, and supports the use of this agent in nonmyeloablative regimens

Long-Term Clinical and Molecular Follow-up of Large Animals Receiving Retrovirally Transduced Stem and Progenitor Cells: No Progression to Clonal Hematopoiesis or Leukemia

There has been significant progress toward clinically relevant levels of retroviral gene transfer into hematopoietic stem cells (HSC), and the therapeutic potential of HSC-based gene transfer has been convincingly demonstrated in children with severe combined immunodeficiency syndrome (SCID). However, the subsequent development of leukemia in two children with X-linked SCID who were apparently cured after transplantation of retrovirally corrected CD34+ cells has raised concerns regarding the safety of gene therapy approaches utilizing integrating vectors. Nonhuman primates and dogs represent the best available models for gene transfer safety and efficacy and are particularly valuable for evaluation of long-term effects. We have followed 42 rhesus macaques, 23 baboons, and 17 dogs with significant levels of gene transfer for a median of 3.5 years (range 1-7) after infusion of CD34+ cells transduced with retroviral vectors expressing marker or drug-resistance genes. None developed abnormal hematopoiesis or leukemia. Integration site analysis confirmed stable, polyclonal retrovirally marked hematopoiesis, without progression toward mono- or oligoclonality over time. These results suggest that retroviral integrations using replication-incompetent vectors, at copy numbers achieved using standard protocols, are unlikely to result in leukemogenesis and that patient- or transgene-specific factors most likely contributed to the occurrence of leukemia in the X-SCID gene therapy trial.

Efficient gene transfer into rhesus repopulating hematopoietic stem cells using a simian immunodeficiency virus-based lentiviral vector system

High-titer, HIV-1-based lentiviral vector particles were found to transduce cytokine-mobilized rhesus macaque CD34(+) cells and clonogenic progenitors very poorly (< 1%), reflecting the postentry restriction in rhesus cells to HIV infection. To overcome this barrier, we developed a simian immunodeficiency virus (SIV)-based vector system. A single exposure to a low concentration of amphotropic pseudotyped SIV vector particles encoding the green fluorescent protein (GFP) resulted in gene transfer into 68% +/- 1% of rhesus bulk CD34(+) cells and 75% +/- 1% of clonogenic progenitors. Polymerase chain reaction (PCR) analysis of DNA from individual hematopoietic colonies confirmed these relative transduction efficiencies. To evaluate SIV vector-mediated stem cell gene transfer in vivo, 3 rhesus macaques underwent transplantation with transduced, autologous cytokine-mobilized peripheral blood CD34(+) cells following myeloablative conditioning. Hematopoietic reconstitution was rapid, and an average of 18% +/- 8% and 15% +/- 7% GFP-positive granulocytes and monocytes, respectively, were observed 4 to 6 months after transplantation, consistent with the average vector copy number of 0.19 +/- 0.05 in peripheral blood leukocytes as determined by real-time PCR. Vector insertion site analysis demonstrated polyclonal reconstitution with vector-containing cells. SIV vectors appear promising for evaluating gene therapy approaches in nonhuman primate models.

Direct comparison of RD114-pseudotyped versus amphotropic-pseudotyped retroviral vectors for transduction of rhesus macaque long-term repopulating cells

Recently, RD114 (feline endogenous retrovirus envelope protein)-pseudotyped retroviral particles have been shown to transduce human NOD/SCID repopulating cells efficiently. In this study, we compared directly transduction of repopulating cells with RD114-pseudotyped vector to that with standard amphotropic vector in the rhesus macaque model. G-CSF/SCF-mobilized CD34(+) rhesus peripheral blood cells were cultured in the presence of SCF, Flt-3 ligand, and MGDF on Retronectin-coated flasks. To assess directly the ability of the two pseudotypes to transduce primitive cells, both vectors were added simultaneously to the target cells every 24 h, for a total of four exposures in 96 h. The cells were reinfused after the animals received 1000 cGy total body irradiation. At the end of transduction, gene marking efficiency of CFU was higher with amphotropic LNL6 vector (mean 88.4%) vs RD114-G1Na vector (mean 18.5%). After long-term engraftment in three animals, total neo gene marking levels were 4-5% in PBMNCs and 1.5-4% in granulocytes. The RD114-G1Na marking levels were consistently higher in granulocytes than in mononuclear cells, while amphotropic LNL6 marking levels were higher in PBMNCs than in granulocytes. The differential gene marking patterns suggest that RD114 and amphotropic vectors may target distinct progenitor or stem cell populations. There was no clear advantage for RD114-pseudotyped vectors in this predictive preclinical model in terms of overall long-term marking levels; however, optimization of transduction conditions by increasing m.o.i. or inducing the receptor could potentially improve results with this novel vector system.