Competitive repopulation of retrovirally transduced haemopoietic stem cells

The Functional Effect of Repeated Cryopreservation on Transduced CD34+ Cells from Patients with Thalassemia

Stable gene marking and effective engraftment of gene-modified CD34⁺ hematopoietic stem cells is a prerequisite for gene therapy success but may be challenged by the inevitable cryopreservation of the final product prior to extensive quality assurance testing. We investigated the β-globin gene transfer potency in fresh and cryopreserved CD34⁺ cells from mobilized patients with β-thalassemia, as well as the qualitative impact of repeated freeze/thaw cycles on the functionality of cultured and unmanipulated CD34⁺ cells in terms of engrafting capacity in a xenotransplantation model, under partial myeloablation. Cells transduced fresh or after one freeze-thaw cycle yielded similar clonogenic and gene transfer frequencies. Repeated cryopreservation cycles did not affect the transduction rates whereas either one or two freeze-thaw cycles of cultured-but not of unmanipulated-cells significantly reduced their clonogenicity. No differences in the engrafting potential of gene-corrected cells subjected to either none or up to two cryopreservation cycles, were encountered post xenotransplantation. Overall, we assessed the gene transfer efficiency, clonogenicity and engrafting capacity of cryopreserved CD34⁺ cells and the impact of repeated freeze/thaw cycles in their performance. These observations may prove essential in the design of gene therapy trials, considerably facilitating their logistics.

SLAM-enriched hematopoietic stem cells maintain long-term repopulating capacity after lentiviral transduction using an abbreviated protocol

Gene transfer to long-term repopulating hematopoietic stem cells (HSCs) using integrating viral vectors is an important goal in gene therapy. The SLAM (signaling lymphocyte activation molecule)-family receptors have recently been used for the isolation of highly enriched murine HSCs. This HSC enrichment protocol is relatively simple, and results in an HSC population with comparable repopulating capacity to c-kit(+)lin(-)Sca-1(+) (KSL) HSCs. The capacity to withstand genetic manipulation and, most importantly, to maintain long-term repopulating capacity of SLAM-enriched HSC populations has not been reported. In this study, SLAM-enriched HSCs were assessed for transduction efficiency and in vivo long-term repopulating capacity after lentiviral transduction using an abbreviated transduction protocol and KSL-enriched HSCs as a reference population. SLAM- and KSL-enriched HSCs were efficiently transduced by lentiviral vector using a simple protocol that involves minimal in vitro manipulation and no pre-stimulation. SLAM-HSCs are at least equal to KSL-HSCs with respect to efficiency of transduction and maintenance of long-term repopulating capacity. Although there was a reduction in repopulating capacity related to enrichment and culture manipulations relative to freshly isolated bone marrow (BM) cells, no detrimental effects were identified on long-term competitive capacity related to transduction, as transduced cells maintained stable levels of chimerism in competition with non-transduced cells and freshly isolated BM cells. These results support the SLAM-HSC enrichment protocol as a simple and efficient method for HSC enrichment for gene transfer studies.

In vivo selection of hematopoietic stem cells transduced at a low multiplicity-of-infection with a foamy viral MGMT(P140K) vector

OBJECTIVE

Using a clinically relevant transduction strategy, we investigated to what extent hematopoietic stem cells in lineage-negative bone marrow (Lin(neg) BM) could be genetically modified with an foamy virus (FV) vector that expresses the DNA repair protein, O(6)-methylguanine DNA methyltransferase (MGMT(P140K)) and selected in vivo with submyeloablative or myeloablative alkylator therapy.

MATERIALS AND METHODS

Lin(neg) BM was transduced at a low multiplicity-of-infection with the FV vector, MD9-P140K, which coexpresses MGMT(P140K) and the enhanced green fluorescent protein, transplanted into C57BL/6 mice, and mice treated with submyeloablative or myeloablative alkylator therapy. The BM was analyzed for the presence of in vivo selected, MD9-P140K-transduced cells at 6 months post-transplantation and subsequently transplanted into secondary recipient animals.

RESULTS

Following submyeloablative therapy, 55% of the mice expressed MGMT(P140K) in the BM. Proviral integration was observed in approximately 50% of committed BM-derived progenitors and analysis of proviral insertion sites indicated up to two integrations per transduced progenitor colony. Transduced BM cells selected with submyeloablative therapy reconstituted secondary recipient mice for up to 6 months post-transplantation. In contrast, after delivery of myeloablative therapy to primary recipient mice, only 25% survived. Hematopoietic stem cells were transduced because BM cells from the surviving animals reconstituted secondary recipients with MGMT(P140K)-positive cells for 5 to 6 months.

CONCLUSIONS

In vivo selection of MD9-P140K-transduced BM cells was more efficient following submyeloablative than myeloablative therapy. These data indicate that a critical number of transduced stem cells must be present to produce sufficient numbers of genetically modified progeny to protect against acute toxicity associated with myeloablative therapy.

A novel competitive repopulation strategy to quantitate engraftment of ex vivo manipulated murine marrow cells in submyeloablated hosts

OBJECTIVE

Standard competitive repopulation assays have proven valuable in evaluating engraftment potential in ablated hosts, permitting comparisons between various test cell populations. However, no similar method exists to compare engraftment of test cells in submyeloablated hosts, which would be helpful given the applications of reduced-intensity conditioning for hematopoietic gene-replacement therapy and other cellular therapies. Here, we developed a novel assay to quantitate engraftment of hematopoietic stem cells in submyeloablated hosts.

MATERIALS AND METHODS

Engraftment of murine marrow cells transduced with retroviral vectors using two separate protocols was compared to engraftment of fresh untreated competitor cells within low-dose radiation-conditioned hosts using a "three-way" marking system, so that test, competitor, and host cell chimerism could be reliably determined posttransplantation.

RESULTS

We demonstrate that the repopulating ability of marrow cells transduced using two distinct protocols was reduced approximately 10-fold compared to fresh competitor cells in submyeloablated hosts utilizing the novel "three-way" transplant assay.

CONCLUSIONS

Murine marrow cells transduced using a clinically applicable protocol acquire an engraftment defect in submyeloablated hosts, similar to cells transduced using a research protocol. We conclude that the submyeloablative competitive repopulation assay described here will be of benefit to comparatively assess the engraftment ability of manipulated hematopoietic stem cells using various culture protocols, such as to test the impact of modifications in transduction protocols needed to attain therapeutic levels of gene-corrected blood cells, or the effect of ex vivo expansion protocols on engraftment potential.

A microarray analysis of the emergence of embryonic definitive hematopoiesis

OBJECTIVE

Human embryonic stem (ES) cells provide a unique model for studying the development and function of human tissues and have proven utility in a number of areas. However, results from ES cell-based studies have been limited by the paucity of information available about early human hematopoietic development.

METHODS

To better understand early development of the hematopoietic lineage, we use microarray analysis to examine the temporal patterns of gene expression in embryoid bodies derived from human ES cells, focusing around the time of the emergence of definitive hematopoiesis. We use an empirical Bayes hierarchical modeling approach, called EBarrays, to classify genes into each of the possible temporal patterns of gene expression for five different time points, and correlate those patterns with the emergence of hematopoiesis.

RESULTS

We find a distinct group of genes previously identified as important in adult hematopoietic self-renewal (such as PIK3R1, ABCB1/MDR-1, RGS18, IRS1, SENP6/SUMO-1, and Wnt5A, etc.) temporally correlates with the emergence of the definitive hematopoiesis. Microarray-based results are further supported via flow cytometry and reverse transcription-polymerase chain reaction studies.

CONCLUSION

The novel genes demonstrating the same expression pattern as this group could further facilitate the understanding of the molecular mechanisms of embryonic hematopoiesis.

Towards hematopoietic stem cell-mediated protection against infection with human immunodeficiency virus

The failure of pharmacological approaches to cure infection with the human immunodeficiency virus (HIV) has renewed the interest in gene-based therapies. Among the various strategies that are currently explored, the blockade of HIV entry into susceptible T cells and macrophages promises to be the most powerful intervention. For long-term protection of both of these lineages, genetic modification of hematopoietic stem cells (HSCs) would be required. Here, we tested whether HSCs and their progeny can be modified to express therapeutic levels of M87o, a gammaretroviral vector encoding an artificial transmembrane molecule that blocks fusion-mediated uptake of HIV. In serial murine bone marrow transplantations, efficient and multilineage expression of M87o was observed for more than 1 year (range 37-75% of mononuclear cells), without signs of toxicity related to the transmembrane molecule. To allow enrichment of M87o-modified HSCs after transplant, we constructed vectors coexpressing the P140K mutant of O(6)-methylguanine-DNA-methyltransferase (MGMT-P140K). This clinically relevant selection marker mediates a survival advantage in HSCs if exposed to combinations of methylguanine-methyltransferase (MGMT) inhibitors and alkylating agents. A bicistronic vector mediated sufficient expression of both M87o and MGMT to confer a selective survival advantage in the presence of HIV and alkylating agents, respectively. These data encourage further investigations in large animal models and clinical trials.

The current status of gene therapy in autologous transplantation

Autologous hematopoietic cells have been used as targets of gene transfer, with applications in inherited disorders, cell therapy, and acquired immunodeficiency. The types of cells include hematopoietic progenitor cells, lymphocytes, and mesenchymal stem cells. The inherited disorders thus far approached in clinical trials include severe combined immunodeficiency, common variable gamma-chain immunodeficiency, chronic granulomatous disease, and Gaucher disease. Preclinical studies are vigorously under way in thalassemia, sickle cell anemia, Wiskott-Aldrich syndrome and Fanconi anemia. Clinical trials of immunological therapy with gene-modified lymphocytes are under study in the treatment of malignancies. Clinical trials using anti-viral strategies for HIV infection in combination with autologous transplantation have begun, with additional approaches being developed. Gene therapy vectors are being developed to eliminate tumor cells contaminating autologous stem cell products. However, the risk of insertional mutagenesis and the potential for development of leukemia was highlighted by the first gene therapy trials in inherited immunodeficiency syndromes that achieved a therapeutic effect. Despite the slow progress of the field to date, there is extraordinary promise for gene therapy in the future.

Competitive equality of donor cells expressing a disparate MHC antigen following stem cell-enriched bone marrow transplantation

INTRODUCTION

Bone marrow cells expressing foreign MHC antigens survive poorly after transplantation. Stable mixed hematopoietic chimerism requires reconstitution with a relatively large number of foreign bone marrow cells and intensive depletion of host cells. In addition, when foreign MHC-transduced autologous bone marrow cells are transplanted, prolonged hematopoietic transgene expression requires extensive host conditioning. The competitive disadvantage associated with engraftment of donor cells expressing foreign MHC antigens is thought to result from a defect in engraftment secondary to donor-host incompatibility or immunologic resistance by the host.

METHODS

We used a limiting-dilution competitive repopulation assay with cells from HLA-A2.1 transgenic mice to determine whether and to what extent foreign MHC antigen expression impairs engraftment in C57BL/6 hosts. Transplants were performed with Hoechst 33342 fluorescence-sorted side population (SP) cells, a subset of bone marrow enriched for stem cells. RESULTS.: Transplantation with 250 stem cell-enriched HLA-A2.1-transgenic side population cells successfully competed with nearly 5000 host C57BL/6 side population cells to produce stable long-term mixed chimerism. There was a direct relationship between the number of transplanted donor HLA-A2-expressing cells and the percentage of HLA-A2-expressing cells in the peripheral blood of reconstituted C57BL/6 mice (r2=0.1799, P=0.031). This correlation was maintained in secondary transplant recipients.

CONCLUSIONS

HLA-A2-expressing hematopoietic cells do not have an engraftment defect when transplanted into C57BL/6 hosts and immunologic resistance did not limit chimerism following lethal irradiation. These results may have relevance to understanding long-term gene expression after hematopoietic stem cell based gene therapy.

Long-term transgene expression and survival of transgene-expressing grafts following lentivirus transduction of bone marrow side population cells

BACKGROUND

Successful transduction of hematopoietic stem cells is essential if gene therapy is to be used clinically to induce immunologic tolerance.

METHODS

Hoechst 33342 staining was used to isolate a population of bone marrow cells enriched for stem cells, termed side population (SP) cells. Murine bone marrow SP cells were transduced with HLA-A2.1-expressing VSV-G-pseudotyped lentivirus or retrovirus vectors under identical conditions.

RESULTS

After transduction without prestimulating cytokines, which minimizes cell cycling and helps maintain stem cell pluripotency, the HLA-A2.1 gene was found in the DNA of 56% of CFU-GM colonies derived from lentivirus-transduced SP cells, but in only 4% of colonies derived from retrovirus-transduced SP cells. Lentivirus and retrovirus transduction including cytokine prestimulation produced the same degree of integration as that following lentivirus-transduction of non-prestimulated cells. Transplantation of 5,000 lentivirus-transduced SP cells into lethally irradiated mice resulted in long-term expression of the HLA-A2.1 transgene in peripheral blood progeny of bone marrow SP cells and prolonged skin graft survival across this class I MHC barrier until the time of animal sacrifice.

CONCLUSIONS

Recombinant lentivirus, but not retrovirus vectors, effectively transduced SP cells that were not prestimulated with cytokines and lentivirus-transduced SP cells successfully repopulated lethally irradiated C57BL/6 mice, animals where there is no selective advantage to repopulation with transduced cells. Transplantation of a relatively small number of transduced SP cells led to prolonged transgene mRNA expression and antigen-specific survival of grafts expressing the foreign MHC transgene.

Modulating erythrocyte chimerism in a mouse model of pyruvate kinase deficiency

In vivo selection may provide a means to increase the relative number of cells of donor origin in recipients with hemopoietic chimerism. We have tested whether in vivo selection using chemical inducers of dimerization (CIDs) can direct the expansion of transduced normal donor erythrocytes in recipients with chimerism using a mouse model of pyruvate kinase deficiency. Marrow cells from normal CBA/N mice were transduced with a vector (F36Vmpl(GFP)) that promotes cell growth in the presence of CIDs. Transduced cells were then transplanted into minimally conditioned, pyruvate kinase-deficient recipients (CBA-Pk-1(slc)/Pk-1(slc)) to establish stable chimerism. CID administration resulted in expansion of normal donor erythrocytes and improvement of the anemia. The preferential expansion of normal erythrocytes also resulted in a decrease in erythropoietin levels, reducing the drive for production of pyruvate kinase-deficient red blood cells. CID-mediated expansion of genetically modified erythrocytes could prove a useful adjunct to transplantation methods that achieve erythroid chimerism.

Side effects of retroviral gene transfer into hematopoietic stem cells

Recent conceptual and technical improvements have resulted in clinically meaningful levels of gene transfer into repopulating hematopoietic stem cells. At the same time, evidence is accumulating that gene therapy may induce several kinds of unexpected side effects, based on preclinical and clinical data. To assess the therapeutic potential of genetic interventions in hematopoietic cells, it will be important to derive a classification of side effects, to obtain insights into their underlying mechanisms, and to use rigorous statistical approaches in comparing data. We here review side effects related to target cell manipulation; vector production; transgene insertion and expression; selection procedures for transgenic cells; and immune surveillance. We also address some inherent differences between hematopoiesis in the most commonly used animal model, the laboratory mouse, and in humans. It is our intention to emphasize the need for a critical and hypothesis-driven analysis of "transgene toxicology," in order to improve safety, efficiency, and prognosis for the yet small but expanding group of patients that could benefit from gene therapy.

Cytokine and chemokine networks influencing stem cell proliferation, differentiation, and marrow homing

The hematopoietic stem cell (HSC) is an attractive target for gene therapy of genetic diseases of the immune and hematopoietic system, and for drug-resistance strategies in which genes conferring resistance to a variety of chemotherapeutic agents can be transduced. Stem cells are relatively easy to obtain; e.g., by marrow aspiration or G-CSF mobilization into the peripheral blood, and can be enriched e.g., by the use of anti-CD34 + monoclonal antibody. For conventional retroviral transduction, normally quiescent HSC must be activated into the cell cycle by priming with appropriate cytokines, and it has been critical to identify cytokine combinations that preserve the self-renewal capacity of long-term repopulating HSC. It has become apparent that strategies designed to optimize HSC cycling and proviral integration can compromise the capacity of transduced HSC to compete in vivo against endogenous HSC or HSC that have not been activated into cell cycle. Lentiviral vectors can integrate genes into non-cycling cells but there is an increased efficiency of transduction if Go HSC are activated into G1-phase of the cell cycle. This reduced efficiency of long-term engraftment of ex vivo cultured HSC may be due to impaired self-renewal capacity or reduced marrow homing efficiency. The latter may be attributed to down modulation of chemokine receptors necessary for chemotactic homing to the marrow. Alternatively, or in addition, there may be down modulation of (1) HSC adhesion molecules necessary for endothelial adhesion and egress from the circulation: (2) metalloproteinases secreted by HSC that facilitate their migration through extracellular matrix and promote release of critical soluble regulatory factors in the marrow microenvironment. A more controversial view is that cell death pathways, for example those involving FasR (CD95) may be activated in cycling HSC, resulting in their selective destruction upon transplantation and localization to sites rich in Fas ligand such as the liver.

Donor chimerism and stem cell function in a murine congenic transplantation model after low-dose radiation conditioning: effects of a retroviral-mediated gene transfer protocol and implications for gene therapy

OBJECTIVE

We investigated low-dose radiation conditioning for the transplantation of retrovirus-transduced cells in a C57Bl6/J murine model.

MATERIALS AND METHODS

The effect of low-dose radiation on stem cell function was investigated using a competitive repopulation assay. Stem cell function of marrow cells that underwent a retroviral-mediated gene transfer (RMGT) protocol was examined by this assay, and donor chimerism of these cells when transplanted into 160-cGy conditioned syngeneic hosts was compared to fresh marrow.

RESULTS

Irradiation with 300 or 160 cGy substantially decreased stem cell function as measured by competitive repopulation. Animals conditioned with 160 cGy and transplanted with 20 x 10(6) fresh marrow cells permitted donor cell engraftment of 53.6% +/- 11.4% 6 months after transplant compared to 100% donor cell engraftment after 1100 cGy irradiation. Lymphoid and myeloid engraftment did not significantly differ from total engraftment in submyeloablated hosts. When transplanted into lethally irradiated hosts, the competitive repopulating activity of marrow treated with a single dose of 5-fluorouracil followed by ex vivo culture according to a standard RMGT protocol was equal to 5-fluorouracil-only treated marrow. However, cells treated with 5-fluorouracil or 5-fluorouracil plus ex vivo culture for RMGT repopulated less well than fresh marrow cells in 160 cGy conditioned hosts.

CONCLUSIONS

Low-dose irradiation decreases host stem cell function, allowing engraftment of both fresh and RMGT protocol-treated marrow, although the engraftment of 5-fluorouracil-treated cells was reduced at least two-fold, and 5-fluorouracil plus RMGT protocol-treated cells at least three-fold, compared to fresh marrow. Modification of current RMGT protocols may be important for optimizing engraftment under these conditions.

Myeloablation enhances engraftment of transduced murine hematopoietic cells, but does not influence long-term expression of the transgene

To investigate to what extent myeloablation, graft size, and ex vivo manipulation influence the engraftment and long-term survival of transduced murine hematopoietic cells, groups of C57BL/6J (CD45.2) mice receiving total body irradiation (TBI) (1-9 Gy) or no irradiation were transplanted with either transduced bone marrow (BM) cells, at two cell doses, or with fresh BM cells from B6/SJL (CD45.1) congenic mice. Short (40 days) and long-term (5 months) engraftment and transgene expression were measured by FACS analysis. No donor cells were detected in the hematopoietic tissues of non-myeloablated mice, whereas in the irradiated animals, levels of engraftment correlated well with the dose of TBI administered. Similar percentages of transgene-expressing cells were found in the grafted hematopoietic cells of all groups of mice, regardless of the dose of TBI administered or the level of engraftment achieved. This suggests that the engrafted animals could become tolerant to the transgene product (enhanced green fluorescent protein, EGFP). Our results indicate that TBI facilitates the engraftment of manipulated hematopoietic cells in a dose-dependent manner, that mice engrafted with EGFP(+) hematopoietic cells probably acquire tolerance to EGFP, and that increasing the graft size and reducing the ex vivo manipulation required for retroviral gene transfer of hematopoietic cells also enhances their engrafting potential.

Gene therapy-based treatment for HIV-positive patients with malignancies

Gene therapy for the treatment of HIV has long been a goal of many investigators. The majority of trials have involved the use of lymphocytes transduced with vectors promoting resistance to HIV infection or replication. Unfortunately, the results have been less than encouraging with low-level marking and, more importantly, clearance of these lymphocytes from the circulation. Conversely, gene-modified hematopoietic stem cells appear able to introduce foreign transgenes while avoiding immunologic clearance. Furthermore, the use of less toxic conditioning regimens for allogeneic transplantation provides an attractive approach to conferring HIV resistance while allowing treatment of HIV-related disorders such as malignancies. This combination of nonmyeloablative allogeneic transplantation using gene-modified hematopoietic stem cell theoretically overcomes the high transplant mortality associated with traditional conditioning regimens in patients with HIV as well as providing a self-renewing source of HIV-resistant cells. To assess the safety and feasibility of such an approach, a clinical protocol was initiated in those patients infected with HIV with a hematologic malignancy meeting the standard indications for allogeneic transplantation and provided here is an update to the previously published original report. Only patient 1 received genetically modified cells. Both patients tolerated the procedure with no effect on viral load and improved CD4 counts, and patient 1 remains in complete remission from acute myelogenous leukemia 3 years post transplant. Patient 2 also achieved clinical remission from chemorefractory Hodgkin's disease but died of relapsed disease 12 months after transplantation. Vector-transduced cells remain detectable at low levels more than 3 years post-transplantation, suggesting the potential for gene therapy as a reasonable goal for the treatment of HIV.

Gene therapy of hematopoietic stem cells: strategies for improvement

Gene therapy of hematopoietic stem cells (HSC) is limited by low frequency of the target cells, their quiescent nature, poor engraftment of treated HSC, and lack of a selective growth advantage of genetically modified cells. Lentiviral vectors combined with positive selection strategies using conditional cell-growth switches should allow for improvement.

Transduction of long-term-engrafting human hematopoietic stem cells by retroviral vectors

Gene therapy using retroviral vectors to transfer functional exogenous genes into hematopoietic stem cells (HSCs) promises to provide a permanent cure for a wide array of both hematopoietic and nonhematopoietic disorders by virtue of the fact that retroviral vectors permanently integrate into the host cell genome and HSCs are able to self-renew and give rise to differentiated progeny throughout the life span of the patient. However, for transduction and genomic integration to occur, the target cells must undergo cell division and express the appropriate retroviral receptor, requirements that have thus far hindered attempts at inserting exogenous genes into human HSCs in vitro. In the present studies, we used the fetal sheep xenograft model of human hematopoiesis to evaluate whether human long-term engrafting HSCs could be transduced in vivo, within a fetal microenvironment. We transplanted adult human bone marrow-derived CD34(+)Lin(-) cells into preimmune fetal sheep recipients and subsequently (19 days later) administered clinical-grade murine retroviral vector supernatants to these fetal hematopoietic chimeras. Our results demonstrate that this approach successfully transduced adult human HSCs within all seven sheep that survived the procedure, and that these transduced HSCs had the ability to serially engraft primary, secondary, and tertiary fetal sheep recipients. Transgene expression persisted throughout the serial transplantation. The successful in vivo transduction of long-term engrafting human HSCs with the existing generation of murine retroviral vectors has significant implications for developing new approaches to pre- and postnatal gene therapy.

Nonmyeloablative conditioning followed by transplantation of genetically modified HLA-matched peripheral blood progenitor cells for hematologic malignancies in patients with acquired immunodeficiency syndrome

To assess the safety and efficacy of nonmyeloablative allogeneic transplantation in patients with HIV infection, a clinical protocol was initiated in patients with refractory hematologic malignancies and concomitant HIV infection. The results from the first 2 patients are reported. The indications for transplantation were treatment-related acute myelogenous leukemia and primary refractory Hodgkin disease in patients 1 and 2, respectively. Only patient 1 received genetically modified cells. Both patients tolerated the procedure well with minimal toxicity, and complete remissions were achieved in both patients, but patient 2 died of relapsed Hodgkin disease 12 months after transplantation. Patient 1 continues in complete remission with undetectable HIV levels and rising CD4 counts, and with both the therapeutic and control gene transfer vectors remaining detectable at low levels more than 2 years after transplantation. These results suggest that nonmyeloablative allogeneic transplantation in the context of highly active antiretroviral therapy is feasible in patients with treatment-sensitive HIV infection.

All-trans retinoic acid facilitates oncoretrovirus-mediated transduction of hematopoietic repopulating stem cells

A major limiting factor in achieving high levels of gene transfer into hematopoietic stem cells is the ability to retain significant repopulating activity of the stem cells during the ex vivo exposure to oncoretroviral vectors. Recently, we reported that pharmacological levels (1 microM) of all-trans retinoic acid (ATRA) enhanced the maintenance of in vivo repopulating hematopoietic stem cells during liquid suspension culture. Therefore, we investigated the use of ATRA to improve transduction of hematopoietic repopulating cells. Hematopoietic precursors cultured and transduced with a GFP-containing oncoretroviral vector with or without ATRA were transplanted immediately post-transduction (day 3 post-culture initiation) or following extended culture without further transduction (day 7 post-culture initiation). Mice transplanted with 3-day ATRA-treated cells had four-fold more donor cells than the untreated cells. In contrast, there were more GFP-expressing donor cells in recipients of cells cultured without ATRA (31.31 +/- 8.47% no ATRA vs. 16.52 +/- 9.35% ATRA). After 7 days of culture, however, the repopulating ability of the hematopoietic precursors was the same for both treatment groups, but the ATRA-treated cells had significantly more green fluorescence protein (GFP)-expressing donor cells (5.57 +/- O.53% no ATRA vs. 13.67 +/- 2.14% ATRA). Secondary recipients of marrow from recipients of the 3 day cultured cells had similar donor cell levels, but the percentage of GFP-expressing cells within the donor cell population was higher in the recipients of ATRA-treated cells (3.25 +/- 0.70% no ATRA vs. 7.97 +/- 2.71% ATRA). Our data show that the addition of ATRA to cultures of hematopoietic precursors resulted in increased gene transfer into murine hematopoietic repopulating cells. These data suggest that ATRA may be useful in clinical gene therapy protocols using oncoretroviral vectors.

Impact of cell growth morphology on retroviral transduction: effect of contact inhibition

Retrovirus-mediated gene transfer is one of the most commonly used methods to deliver, integrate, and express the gene of interest because the retrovirus can insert the desired gene into the chromosome of the target cells with high stability. However, to deliver the gene successfully, the retrovirus requires active division to integrate reversely transcribed DNA into the chromosome of target cells. In this study, we focused on the effect of cell-cell contact inhibition on the efficiency of retroviral transduction with two anchorage-dependent cell lines: NIH 3T3 and 293 cells. These two cell lines have very different cell morphologies and growth patterns on surfaces. Human embryonic kidney epithelial 293 cells tend to stick together after dividing, while NIH 3T3 cells migrate to occupy available surface and spread. Experimental data indicate that the abatement of the transduction rate of 293 cells was initiated in the early stage of the culture, whereas effect of contact inhibition of NIH 3T3 cells on the transduction rate became dominating at the end of the culture period. Experimental results were also quantitatively illustrated by plotting normalized multiplicity of infection (MOI) versus normalized cell density. According to the outcomes, cell inoculation density plays an important role in optimizing the retroviral transduction rate. The optimal time of retroviral transduction should be confined to the accelerating growth phase for 293 cells and at the exponential growth phase for NIH 3T3 cells. The implication drawn from this study is that contact inhibition effect on retroviral transduction should be taken into account for large-scale gene transfer systems such as the microcarrier bioreactor.

In vivo competitive studies between normal and common gamma chain-defective bone marrow cells: implications for gene therapy

Corrective gene transfer into hematopoietic stem cells (HSCs) is being investigated as therapy for X-linked severe combined immunodeficiency (XSCID) and it is hoped that selective advantage of gene-corrected HSCs will help in achieving full immune reconstitution after treatment. Lines of evidence from the results of allogeneic bone marrow transplantation in patients with XSCID support this hypothesis that, however, has not been rigorously tested in an experimental system. We studied the competition kinetics between normal and XSCID bone marrow (BM) cells using a murine bone marrow transplantation (BMT) model. For easy chimerism determination, we used genetic marking with retrovirus-mediated expression of the enhanced green fluorescent protein (EGFP). We found that XSCID BM cells were able to compete with normal BM cells for engraftment of myeloid lineages in a dose-dependent manner, whereas we observed selective repopulation of T, B, and NK cells deriving from normal BM cells. This was true despite the evidence of competitive engraftment of XSCID lineage marker-negative/c-Kit-positive (Lin-/c-Kit+) cells in the bone marrow of treated animals. From these results we extrapolate that genetic correction of XSCID HSCs will result in selective advantage of gene-corrected lymphoid lineages with consequent restoration of lymphocyte populations and high probability of clinical benefit.