Granulocyte colony-stimulating factor mobilized peripheral blood stem cells enter into G1 of the cell cycle and express higher levels of amphotropic retrovirus receptor mRNA

CD34+ cells obtained from "good mobilizers" are more activated and exhibit lower ex vivo expansion efficiency than their counterparts from "poor mobilizers"

BACKGROUND

The classification of patients into "good" or "poor" mobilizers is based on CD34+ cell count in their peripheral blood (PB) after granulocyte-colony-stimulating factor (G-CSF) injection. We hypothesized that, apart from their mobilization from marrow to the blood, the response to G-CSF of CD34+ cells also includes activation of proliferation, metabolic activity, and proliferative capacity.

STUDY DESIGN AND METHODS

Mobilized PB CD34+ cells purified from samples obtained by cytapheresis of multiple myeloma or non-Hodgkin's lymphoma patients of both good (>50 CD34+ cells/microL) and poor (< or =50 CD34+ cells/microL) mobilizers were studied. The initial cell cycle state of CD34+ cells after selection and their kinetics of activation (exit from G(0) phase) during ex vivo culture were analyzed. Their proliferative capacity was estimated on the basis of ex vivo generation of total cells, CD34+ cells, and colony-forming cells (CFCs), in a standardized expansion culture. Indirect insight in metabolic activity was obtained on the basis of their survival (viability and apoptosis follow-up) during the 7-day-long conservation in hypothermia (4 degrees C) in the air or in atmosphere containing 3% O(2)/6% CO(2).

RESULTS

CD34+ cells obtained from good mobilizers were in lower proportion in the G(0) phase, their activation in a cytokine-stimulated culture was accelerated, and they exhibited a lower ex vivo expansion efficiency than those from poor mobilizers. The resistance to hypothermia of good immobilizers' CD34+ cells is impaired.

CONCLUSION

A good response to G-CSF mobilization treatment is associated with a higher degree of proliferative and metabolic activation of mobilized CD34+ cells with a decrease in their expansion capacity.

The use of experimental murine models to assess novel agents of hematopoietic stem and progenitor cell mobilization

The recent explosion in the understanding of the cellular and molecular mechanisms underlying hematopoietic stem and progenitor cell (HSPC) mobilization has facilitated development of novel therapeutic agents, targeted at improving mobilization kinetics as well as HSPC yield. With the development of new agents comes the challenge of choosing efficient and relevant preclinical studies for the testing of the HSPC mobilization efficacy of these agents. This article reviews the use of the mouse as a convenient small animal model of HSPC mobilization and transplantation, and outlines the range of murine assays that can be applied to assess novel HSPC mobilizing agents. Techniques to demonstrate murine HSPC mobilization are discussed, as well as the role of murine assays to confirm human HSPC mobilization, and techniques to investigate the biologic phenotype of HSPC mobilized by these novel agents. Technical aspects regarding mobilization regimens and control arms, and choice of experimental animals are also discussed.

The late dividing population of gamma-retroviral vector transduced human mobilized peripheral blood progenitor cells contributes most to gene-marked cell engraftment in nonobese diabetic/severe combined immunodeficient mice

We used the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse model to assess the repopulation potential of subpopulations of mobilized human CD34+ peripheral blood progenitor cells (PBPC). First, PBPC were transduced with gamma-retrovirus vector RD114-MFGS-CFP, which requires cell division for successful transduction, at 24 hours, 48 hours, and 72 hours to achieve 96% cyan fluorescent protein (CFP)-positive cells. Cells were sorted 12 hours after the last transduction into CFP-positive (divided cells) and CFP-negative populations. CFP-positive cells were transplanted postsort, whereas the CFP-negative cells were retransduced and injected at 120 hours. The CFP-negative sorted and retransduced cells contained markedly fewer vector copies and resulted in a 32-fold higher overall engraftment and in a 13-fold higher number of engrafted transgene positive cells. To assess cell proliferation as an underlying cause for the different engraftment levels, carboxyfluorescein succinimidyl ester-labeling of untransduced PBPC was performed to track the number of cell divisions. At 72 hours after initiation of culture, when 95% of all cells have divided, PBPC were sorted into nondivided and divided fractions and transplanted into NOD/SCID mice. Nondivided cells demonstrated 45-fold higher engraftment than divided cells. Late dividing PBPC in ex vivo culture retain high expression of the stem cell marker CD133, whereas rapidly proliferating cells lose CD133 in correlation to the number of cell divisions. Our studies demonstrate that late dividing progenitors transduced with gamma-retroviral vectors contribute most to NOD/SCID engraftment and transgene marking. Confining the gamma-retroviral transduction to CD133-positive cells on days 3 and 4 could greatly reduce the number of transplanted vector copies, limiting the risk of leukemia from insertional mutagenesis. Disclosure of potential conflicts of interest is found at the end of this article.

Evaluation of different protocols for gene transfer into non-obese diabetes/severe combined immunodeficiency disease mouse repopulating cells

PURPOSE

Although gene transfer with retroviral vectors has shown distinct clinical success in defined settings, efficient genetic manipulation of hematopoietic progenitor cells remains a challenge. To address this issue we have evaluated different transduction protocols and retroviral constructs in the non-obese diabetes (NOD)/severe combined immunodeficiency disease (SCID) xenograft model.

METHODS

An extended transduction protocol requiring 144 h of in vitro manipulation was compared to a more conventional protocol requiring 96 h only.

RESULT

While pretransplantation analysis of cells transduced with a retroviral vector, expressing the enhanced green fluorescent protein (EGFP) marker gene, demonstrated significantly higher overall transduction rates for the extended protocol (33.6 +/- 2.3 vs. 22.1 +/- 3.8%), EGFP expression in CD34+ cells before transplantation (4.0 +/- 0.9 vs. 11.6 +/- 2.5%), engraftment of human cells in NOD/SCID bone marrow 4 weeks after transplantation (4.5 +/- 1.7 vs. 36.5 +/- 9.4%) and EGFP expression in these cells (0 +/- 0 vs. 11.3 +/- 2.8%) were significantly impaired. When the 96 h protocol was used in combination with the spleen focus forming virus (SFFV)/murine embryonic stem cell (MESV) hybrid vector SFbeta11-EGFP, high transduction rates for CD45+ (41.0 +/- 5.3%) and CD34+ (38.5 +/- 3.7%) cells prior to transplantation, as well as efficient human cell engraftment in NOD/SCID mice 4 weeks after transplantation (32.4 +/- 3.5%), was detected. Transgene expression was observed in B-lymphoid (15.9 +/- 2.0%), myeloid (36.5 +/- 3.5%) and CD34+ cells (10.1 +/- 1.5%).

CONCLUSION

Our data show that CD34+ cells maintained in cytokines for multiple days may differentiate and loose their capacity to contribute to the haematological reconstitution of NOD/SCID mice. In addition, the SFFV/MESV hybrid vector SFbeta11-EGFP allows efficient transduction of and gene expression in haematopoietic progenitor cells.

Catalytic Activities of G1Cyclin-Dependent Kinases and Phosphorylation of Retinoblastoma Protein in Mobilized Peripheral Blood CD34+Hematopoietic Progenitor Cells

Depending on the source of cells, the cell cycle status of hematopoietic stem and progenitor cells capable of repopulating the marrow of transplant recipients is controversial. In this study, using biochemical methods, the cell cycle status of mobilized CD34+ cells was analyzed. It was demonstrated in CD34+ cell extracts that there was high catalytic activity of G(1) cyclin-dependent kinases 4 and 6 (CDK4 and CDK6) but low activity of CDK2. This was in contrast to the resting reference cells that showed only minimal or no activity of these CDKs. Since at the G0-->G1-->S transition CDK4/6 and CDK2 sequentially phosphorylate the retinoblastoma protein (pRB), its phosphorylation status was analyzed. Previously, we showed that p110RB was unphosphorylated at serine (Ser)-608 in CD34+ cells, consistent with the ability to suppress cell growth. Here, it was established that this form of pRB was phosphorylated at Ser-780, Ser-795, and Ser-807/811 in CD34+ but not in resting reference cells. This result was therefore consistent with the presence of high CDK4/6 activities in CD34+ cells. Conversely, CDK2 activity was low and the pRB residues Ser-612 and threonine (Thr)-821, which are exclusively phosphorylated by CDK2 in conjunction with either cyclin E or A, were unphosphorylated in >90% of CD34+ cells. We therefore show for the first time the exact position of mobilized CD34+ cells within the cell cycle; that is, they do not reside in G0 but in early G1 phase and did not cross the restriction point into late G1 phase.

Comparison of three retroviral vector systems for transduction of nonobese diabetic/severe combined immunodeficiency mice repopulating human CD34+ cord blood cells

The use of recombinant vectors based on wild-type viruses that are absent in humans and are not associated with any disease in their natural animal hosts or in accidentally infected humans would add an additional level of safety for human somatic gene therapy approaches. These criteria are fulfilled by foamy viruses (FVs), a family of complex retroviruses whose members are widely found among mammals and are apathogenic in all hosts. Here, we show by comparison of identically designed vector constructs that recombinant retroviral vectors based on FVs were as efficient as lentiviral vectors in transducing nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice repopulating human CD34(+) cord blood (CB) cells. The FV vector was able to achieve gene transfer levels up to 84% of engrafted human cells in a short overnight transduction protocol. In contrast, without prestimulation of the target cells, a human immunodeficiency virus type 1 (HIV-1)-based lentiviral vector pseudotyped with gibbon ape leukemia virus envelope (GALV Env) was nearly as inefficient as murine leukemia virus (MLV)-based oncoretroviral vectors in transducing NOD/SCID repopulating cells. The same HIV vector pseudotyped with the vesicular stomatitis virus glycoprotein G (VSV-G) achieved high marking efficiency. Clonality analysis of bone marrow samples showed oligoclonal hematopoiesis with single to multiple insertions per cell, both for FV and HIV vectors. These data demonstrate that vectors based on FVs warrant further investigation and development for medical use.

Third-generation, self-inactivating gp91(phox) lentivector corrects the oxidase defect in NOD/SCID mouse-repopulating peripheral blood-mobilized CD34+ cells from patients with X-linked chronic granulomatous disease

HIV-1-derived lentivectors are promising for gene transfer into hematopoietic stem cells but require preclinical in vivo evaluation relevant to specific human diseases. Nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice accept human hematopoietic stem cell grafts, providing a unique opportunity for in vivo evaluation of therapies targeting human hematopoietic diseases. We demonstrate for the first time that hematopoietic stem cells from patients with X-linked chronic granulomatous disease (X-CGD) give rise to X-CGD-phenotype neutrophils in the NOD/SCID model that can be corrected using VSV-G-pseudotyped, 3rd-generation, self-inactivating (SIN) lentivector encoding gp91(phox). We transduced X-CGD patient-mobilized CD34(+) peripheral blood stem cells (CD34(+)PBSCs) with lentivector-gp91(phox) or amphotropic oncoretrovirus MFGS-gp91(phox) and evaluated correction ex vivo and in vivo in NOD/SCID mice. Only lentivector transduced CD34(+)PBSCs under ex vivo conditions nonpermissive for cell division, but both vectors performed best under conditions permissive for proliferation (multiple growth factors). Under the latter conditions, lentivector and MFGS achieved significant ex vivo correction of X-CGD CD34(+)PBSCs (18% and 54% of cells expressing gp91(phox), associated with 53% and 163% of normal superoxide production, respectively). However, lentivector, but not MFGS, achieved significant correction of human X-CGD neutrophils arising in vivo in NOD/SCID mice that underwent transplantation (20% and 2.4%, respectively). Thus, 3rd-generation SIN lentivector-gp91(phox) performs well as assessed in human X-CGD neutrophils differentiating in vivo, and our studies suggest that the NOD/SCID model is generally applicable for in vivo study of therapies evaluated in human blood cells expressing a specific disease phenotype.

Retroviral transduction of IL2RG into CD34(+) cells from X-linked severe combined immunodeficiency patients permits human T- and B-cell development in sheep chimeras

X-linked severe combined immunodeficiency (XSCID) is caused by mutations of the common gamma chain of cytokine receptors, gamma(c). Because bone marrow transplantation (BMT) for XSCID does not provide complete immune reconstitution for many patients and because of the natural selective advantage conferred on lymphoid progenitors by the expression of normal gamma(c), XSCID is a good candidate disease for therapeutic retroviral gene transfer to hematopoietic stem cells. We studied XSCID patients who have persistent defects in B-cell and/or combined B- and T-cell function despite having received T cell-depleted haploidentical BMT. We compared transduction of autologous B-cell lines and granulocyte colony-stimulating factor-mobilized peripheral CD34(+) cells from these patients using an MFGS retrovirus vector containing the gamma(c) gene IL2RG pseudotyped with amphotropic, gibbon ape leukemia virus, or RD114 envelopes. Transduced B-cell lines and peripheral CD34(+) cells demonstrated provirus integration and new cell-surface gamma(c) expression. The chimeric sheep model was exploited to test development of XSCID CD34(+) cells into mature myeloid and lymphoid lineages. Transduced and untransduced XSCID CD34(+) cells injected into developing sheep fetuses gave rise to myeloid cells. However, only transduced gamma progenitors from XSCID patients developed into T and B cells. These results suggest that gene transfer to autologous peripheral CD34(+) cells using MFGS-gc retrovirus may benefit XSCID patients with persistent T- and B-cell deficits despite prior BMT.

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.

Prospects for gene therapy using haemopoietic stem cells

Gene therapy has thus far promised much and delivered little. Much of this has been due to deficiencies in the reagents and methodologies employed in early clinical trials. Recent technological advances in vectors and haemopoietic stem cell manipulation, coupled with improved pre-clinical assays of gene transfer and expression in re-populating stem cells give cause for greater optimism. Here we review these advances and indicate areas requiring further development before clinical gene therapy in the haemopoietic system becomes a widely applicable treatment modality.

Human CD34+ cell preparations contain over 100-fold greater NOD/SCID mouse engrafting capacity than do CD34- cell preparations

OBJECTIVE

The CD34 cell surface marker is used widely for stem/progenitor cell isolation. Since several recent studies reported that CD34(-) cells also have in vivo engrafting capacity, we quantitatively compared the engraftment potential of CD34(+) vs CD34(-) cell preparations from normal human placental/umbilical cord blood (CB), bone marrow (BM), and mobilized peripheral blood (PBSC) specimens, using the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse model.

METHODS

CD34(+) and CD34(-) cell preparations were purified by four different approaches in 14 individual experiments involving 293 transplanted NOD/SCID mice. In most experiments, CD34(+) cells were depleted twice (CD34(=)) in order to obtain efficient depletion of CD34(+) cells from the CD34(-) cell preparations.

RESULTS

Dose-dependent levels of human hematopoietic cells were observed after transplantation of CD34(+) cell preparations. To rigorously assess the complementary CD34(-) cell preparations, cell doses 10- to 1000-fold higher than the minimum dose of the CD34(+) cell preparations necessary for engraftment were transplanted. Nevertheless, of 125 NOD/SCID mice transplanted with CD34(-) cell preparations purified from the same starting cells, only six mice had detectable human hematopoiesis, by flow cytometric or PCR assay.

CONCLUSIONS

CD34(-) cells provide only a minor contribution to hematopoietic engraftment in this in vivo model system, as compared to CD34(+) cells from the same samples of noncultured human cells. Hematopoiesis derived from actual CD34(-) cells is difficult to distinguish from that due to CD34(+) cells potentially contaminating the preparations.

Differences in cell cycle kinetics of candidate engrafting cells in human bone marrow and mobilized peripheral blood

Patients undergoing hematopoietic stem cell transplantation (HSCT) with mobilized peripheral blood (MPB) engraft quicker than those receiving bone marrow (BM). Our objective was to determine whether candidate engrafting cells--primitive hematopoietic progenitors (PHPs)--from MPB and BM exhibit different responses to cytokines that could explain this observation. We compared the cell cycle kinetics and ex vivo expansion of PHP-enriched cells obtained from MPB (n = 12) and BM (n = 10) by fluorescence-activated sorting of CD90+, AC133+ or CD38(dull) subsets of pre-selected CD34(+) cells. Cell cycle status, before and after 40 hours of serum-free culture with a cytokine cocktail, was assessed by multiparameter flow cytometry following incubation with Hoechst 33342 and pyronin Y. We found that 0.2% +/- 0.3% of MPB CD34(+)CD90(+) cells were in S/G(2)/M phases at hour 0, compared with 5% +/- 2.5% of those from BM (p = 0.0001), and 86.3% +/- 9.7% were in G(0), compared with 65.3% +/- 10% of those in BM (p = 0.0001). After 40 hours of culture, CD34(+)CD90(+) cells from MPB were more mitotically active than those from BM, with 29% +/- 4.9% in S/G(2)/M and 20% +/- 11.4% in G(0), compared to 19% +/- 6.5% (p = 0.001) and 39.2% +/- 22% (p = 0.027) of cells from BM. There was greater expansion of both total CD34(+) cells and the CD90(+) subset from MPB samples (p = 0.001 and 0.0001, respectively). Results from PHPs defined on the basis of AC133 expression correlated well with results obtained in CD90(+) subsets (r(2) = 0.81; p = 0.014).MPB PHPs appear to be primed for a greater acceleration in mitotic activity upon cytokine exposure. This qualitative difference may contribute to the earlier engraftment seen after HSCT using MPB grafts.

Preincubation with endothelial cell monolayers increases gene transfer efficiency into human bone marrow CD34(+)CD38(-) progenitor cells

Retroviral gene transfer studies targeting bone marrow CD34(+)CD38(-) stem cells have been disappointing because of the rarity of these cells, their G(0) cell cycle status, and their low or absent expression of surface retroviral receptors. In this study, we examined whether preincubation of bone marrow CD34(+)CD38(-) stem cells with a hematopoietically supportive porcine microvascular endothelial cell line (PMVECs) could impact the cell cycle status and expression of retroviral receptors in pluripotent CD34+CD38- cells and the efficiency of gene transfer into these primitive target cells. PMVEC coculture supplemented with GM-CSF + IL-3 + IL-6 + SCF + Flt-3 ligand induced >93% of the CD34(+)CD38(-) population to enter the G(1) or G(2)/S/M phase while increasing this population from 1.4% on day 0 to 6.5% of the total population by day 5. Liquid cultures supplemented with the identical cytokines induced 73% of the CD34(+)CD38(-) population into cell cycle but did not maintain cells with the CD34(+)CD38(-) phenotype over time. We found no significant increase in the levels of AmphoR or GaLVR mRNA in PMVEC-expanded CD34(+)CD38(-) cells after coculture. Despite this, the efficiency of gene transfer using either amphotropic vector (PA317) or GaLV vector (PG13) was significantly greater in PMVEC-expanded CD34(+)CD38(-) cells (11.4 +/- 5.6 and 10.9 +/- 5.2%, respectively) than in either steady state bone marrow CD34(+)CD38(-) cells (0.6 +/- 1.7 and 0.2 +/- 0.6%, respectively; p < 0.01 and p < 0.01) or liquid culture-expanded CD34(+)CD38(-) cells (1.4 +/- 3.5 and 0.0%, respectively; p < 0.01 and p < 0.01). Since PMVEC coculture induces a high level of cell cycling in human bone marrow CD34(+)CD38(-) cells and expands hematopoietic cells capable of in vivo repopulation, this system offers potential advantages for application in clinical gene therapy protocols.

CD44v10 in hematopoiesis and stem cell mobilization

Transplantation of hematopoietic progenitor cells provides in many instances of malignant tumors an ultimate chance of curative therapy, whereby the transfer of peripheral blood stem cells (PBSC) may even be advantageous as compared to bone marrow cells. Yet, the transfer of PBSC requires mobilization of stem cells into the periphery, which is mostly achieved via hematopoietic growth factors like G-CSF. Although G-CSF has been found to efficiently mobilize stem cells in most instances, some patients do not or insufficiently respond to G-CSF treatment In addition, G-CSF treatment may by accompanied by maturation of the most primitive progenitors and this may have an impact on stem cell homing and recovery of hemopoiesis. Therefore, additional approaches for stem cell mobilization have been searched for, in particular mobilization via a blockade of an adhesion molecule expressed by CD34-positive cells, like VLA-4 (CD49d) and the hematopoietic isoform of CD44 (CD44s). We recently described that in the mouse one of the CD44 variant isoforms, CD44v10, is expressed on a subpopulation of bone marrow cells, whereas a CD44v10 receptor-globulin only binds to stromal elements. These features appeared promising for anti-CD44v10 as a means of stem cell mobilization. Indeed, treatment with anti-CD44v10 revealed promising results concerning the recovery of multilineage colony forming units in the spleen and the peripheral blood. We here summarize features of expression and function of CD44 in hematopoiesis an provide further evidence for anti-CD44v10 as a means to mobilize hematopoietic progenitor cells.

Selection and use of chemotherapy with hematopoietic growth factors for mobilization of peripheral blood progenitor cells

Peripheral blood progenitor cells (PBPCs) have become the preferred means of stem cell support for high-dose chemotherapy in recent years. The biology of PBPC mobilization is complex and may be influenced by several variables. Signals from both stromal and hemopoietic cells may induce downregulation of adhesion molecules and upregulate the expression of metalloproteinases. Cytokines alone can mobilize PBPCs but a synergistic effect has been shown when they are used in conjunction with chemotherapy. Disease-specific mobilization strategies appear to have the advantage of less toxicity, greater stem cell yield, and enhanced antitumor activity. Studies have demonstrated that the number of peripheral blood CD34+ cells can be used as a predictor for the timing of apheresis and for estimating PBPC yield. Similarly the CD34+ cell dose is the strongest predictor of hematologic recovery after PBPC transplant. Age, prior radiotherapy, marrow involvement, and prior chemotherapy (especially with alkylating agents) are important factors influencing the yield of stem cells.

Gene Therapy 2000

This article reviews 1) the use of gene transfer methods to genetically manipulate hematopoietic stem cell targets, 2) recent advances in technology that are addressing problems that have prevented widespread successful translation of gene transfer approaches for the cure of disease, and 3) recent regulatory issues related to human gene therapy trials.

In Section I, Dr. Nienhuis describes the use of alternative viral envelopes and vector systems to improve efficiency of transduction of hematopoietic stem cells. Major limitations of stem cell transduction are related to low levels of viral receptors on the stem cells of large animal species and the low frequency of cycling stem cells in the bone marrow. Attempts to circumvent these limitations by exploiting non-oncoretroviral vectors and pseudotyping of Moloney vectors with alternative envelopes are discussed.

In Section II, Dr. Hawley addresses new strategies to improve the expression of transgenes in cells derived from long-term reconstituting hematopoietic stem cells. Transgene silencing in transduced hematopoietic stem cells remains an obstacle to gene therapy for some gene sequences. New generations of retroviral backbones designed to both improve expression and reduce silencing in primary cells are explored.

In Section III, Drs. Smith and Cornetta update regulatory issues related to human gene therapy trials. Increased scrutiny of human trials has led to changes in requirements and shifts in emphasis of existing regulations, which apply to human gene therapy trials. The current Food and Drug Administration's structure and regulations and the roles of the Recombinant DNA Advisory Committee of the NIH and other sponsors and partners in gene therapy trials are reviewed.

Gene Therapy 2000

This article reviews 1) the use of gene transfer methods to genetically manipulate hematopoietic stem cell targets, 2) recent advances in technology that are addressing problems that have prevented widespread successful translation of gene transfer approaches for the cure of disease, and 3) recent regulatory issues related to human gene therapy trials.

In Section I, Dr. Nienhuis describes the use of alternative viral envelopes and vector systems to improve efficiency of transduction of hematopoietic stem cells. Major limitations of stem cell transduction are related to low levels of viral receptors on the stem cells of large animal species and the low frequency of cycling stem cells in the bone marrow. Attempts to circumvent these limitations by exploiting non-oncoretroviral vectors and pseudotyping of Moloney vectors with alternative envelopes are discussed.

In Section II, Dr. Hawley addresses new strategies to improve the expression of transgenes in cells derived from long-term reconstituting hematopoietic stem cells. Transgene silencing in transduced hematopoietic stem cells remains an obstacle to gene therapy for some gene sequences. New generations of retroviral backbones designed to both improve expression and reduce silencing in primary cells are explored.

In Section III, Drs. Smith and Cornetta update regulatory issues related to human gene therapy trials. Increased scrutiny of human trials has led to changes in requirements and shifts in emphasis of existing regulations, which apply to human gene therapy trials. The current Food and Drug Administration's structure and regulations and the roles of the Recombinant DNA Advisory Committee of the NIH and other sponsors and partners in gene therapy trials are reviewed.