Increased stable retroviral gene transfer in early hematopoietic progenitors released from quiescence

Autocrine transforming growth factor-beta regulation of hematopoiesis: many outcomes that depend on the context

Transforming growth factor-beta (TGF-beta) is a pleiotropic regulator of all stages of hematopoieis. The three mammalian isoforms (TGF-beta1, 2 and 3) have distinct but overlapping effects on hematopoiesis. Depending on the differentiation stage of the target cell, the local environment and the concentration and isoform of TGF-beta, in vivo or in vitro, TGF-beta can be pro- or antiproliferative, pro- or antiapoptotic, pro- or antidifferentiative and can inhibit or increase terminally differentiated cell function. TGF-beta is a major regulator of stem cell quiescence, at least in vitro. TGF-beta can act directly or indirectly through effects on the bone marrow microenvironment. In addition, paracrine and autocrine actions of TGF-beta have overlapping but distinct regulatory effects on hematopoietic stem/progenitor cells. Since TGF-beta can act in numerous steps in the hematopoietic cascade, loss of function mutations in hematopoeitic stem cells (HSC) have different effects on hematopoiesis than transient blockade of autocrine TGF-beta1. Transient neutralization of autocrine TGF-beta in HSC has therapeutic potential. In myeloid and erythroid leukemic cells, autocrine TGF-beta1 and/or its Smad signals controls the ability of these cells to respond to various differentiation inducers, suggesting that this pathway plays a role in determining the cell fate of leukemic cells.

The role of Smad signaling in hematopoiesis

The TGF-beta family of ligands, including TGF-beta, bone morphogenetic protein (BMP) and activin, signal through Smad pathways to regulate the fate of hematopoietic progenitor and stem cells during development and postnatally. BMP regulates hematopoietic stem cell (HSC) specification during development, while TGF-beta1, 2 and 3 are not essential for the generation of HSCs. BMP4 can increase proliferation of human hematopoietic progenitors, while TGF-beta acts as a negative regulator of hematopoietic progenitor and stem cells in vitro. In contrast, TGF-beta signaling deficiency in vivo does not affect proliferation of HSCs and does not affect lineage choice either. Therefore, the outcome of Smad signaling is very context dependent in hematopoiesis and regulation of hematopoietic stem and progenitor cells is more complicated in the bone marrow microenvironment in vivo than is seen in liquid cultures ex vivo. Smad signaling regulates hematopoiesis by crosstalk with other regulatory signals and future research will define in more detail how the various pathways interact and how the knowledge obtained can be used to develop advanced cell therapies.

Cell cycle inhibitors in normal and tumor stem cells

Emerging data suggest that stem cells may be one of the key elements in normal tissue regeneration and cancer development, although they are not necessarily the same entity in both scenarios. As extensively demonstrated in the hematopoietic system, stem cell repopulation is hierarchically organized and is intrinsically limited by the intracellular cell cycle inhibitors. Their inhibitory effects appear to be highly associated with the differentiation stage in stem/progenitor pools. While this negative regulation is important for maintaining homeostasis, especially at the stem cell level under physiological cues or pathological insults, it constrains the therapeutic use of adult stem cells in vitro and restricts endogenous tissue repair after injury. On the other hand, disruption of cell cycle inhibition may contribute to the formation of the so-called 'tumor stem cells' (TSCs) that are currently hypothesized to be partially responsible for tumorigenesis and recurrence of cancer after conventional therapies. Therefore, understanding how cell cycle inhibitors control stem cells may offer new strategies not only for therapeutic manipulations of normal stem cells but also for novel therapies selectively targeting TSCs.

Gene transfer in purified human hematopoietic peripheral-blood stem cells by means of electroporation without prestimulation

Gene transfer in hematopoietic stem cells (HSCs) is an important tool, exploring regulation of the hematopoietic system and understanding the development and expansion of malignant cell clones. It is also a mandatory step for gene therapy of hematopoietic disorders. Although retroviral transduction of HSCs is effective, prestimulation of cells is generally required, also inducing differentiation of HSCs. Furthermore, the risk of viral recombination and insertional mutagenesis cannot be ruled out. Potential advantages of nonviral transfection are biosafety and easy management. However, experience in nonviral methods for transfecting peripheral-blood stem cells (PBSCs) is limited. To avoid differentiation, we evaluated the efficiency of gene transfer by means of electroporation without cytokine prestimulation. Compared with prestimulated (stem-cell factor, granulocyte-colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, interleukin-3, interleukin-6, erythropoietin, and monoclonal antibody to transforming growth factor-beta), transfection of thawed nonstimulated PBSCs was equally efficient, with a median transfection rate of 3.7%, transfection efficiency of 0.8%, and survival of 19.5% (n = 5). With freshly isolated HSCs, the rate of transfected cells could be increased to a median of 27.0% (range 8.3%-31.0%), transfection efficiency of 6.9% (range 4.5%-12.6%), and survival of 43% (range 22%-64%) (n = 5). However, the percentage of transfected cells declined with time; almost no cells were detectable by day 11. One cause for the lack of long-term expression of the heterologous gene in this system was induction of apoptosis in transgenic PBSCs, shown by up-regulation of CD95 (FAS antigen).

Efficient oncoretroviral transduction of extended long-term culture-initiating cells and NOD/SCID repopulating cells: enhanced reconstitution with gene-marked cells through an ex vivo expansion approach

Recent developments of surrogate assays for human hematopoietic stem cells (HSC) have facilitated efforts at improving HSC gene transfer efficiency. Through the use of xenograft transplantation models, such as nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice, successful oncoretroviral gene transfer to transplantable hematopoietic cells has been achieved. However, because of the low frequency and/or homing efficiency of SCID repopulating cells (SRC) in bone marrow (BM), studies have primarily focused on cord blood (CB). The recently developed extended (> 60 days) long-term culture-initiating cell (ELTC-IC) assay detects an infrequent and highly quiescent candidate stem cell population in BM as well as CB of the CD34(+)CD38(-) phenotype. Although these characteristics suggest that ELTC-IC and SRC might be closely related, attempts to oncoretrovirally transduce ELTC-IC have been unsuccessful. Here, recently developed conditions (high concentrations of SCF + FL + Tpo in serum-free medium) supporting expansion of BM CD34(+)CD38(-) 12 week ELTC-IC promoted efficient oncoretroviral transduction of BM and CB ELTC-IC. Although SRC can be transduced with oncoretroviral vectors, this is frequently associated with loss of reconstituting activity, posing a problem for development of clinical HSC gene therapy. However, previous attempts at expanding transduced HSC posttransduction resulted in compromised rather than improved gene marking. Utilizing conditions promoting cell divisions and transduction of ELTC-IC we show that although 5 days of ex vivo culture is sufficient to obtain maximum gene transfer efficiency to SRC, extension of the expansion period to 12 days significantly enhances multilineage reconstitution activity of transduced SRC, supporting the feasibility of improving gene marking through ex vivo expansion.

Cell cycle entry of hematopoietic stem and progenitor cells controlled by distinct cyclin-dependent kinase inhibitors

The therapeutic promise of hematopoietic stem cells in medicine has been expanded as broader differentiation potential of the cells has gained experimental support. However, hurdles for stem cell manipulation in vitro and tissue regeneration in vivo remain because of lack of the molecular biology of the stem cells. In particular, elucidating the molecular control of cell cycle entry is necessary for rational stem cell expansion strategies. Understanding how the stem and progenitor cell populations are controlled by negative regulators of cell cycle entry may provide one basis for manipulating these cells. In this mini-review, we focus on the rationale of targeting the cyclin-dependent kinase inhibitors (CKIs) in stem cell biology. Two CKI members, p21(Cip1/Waf1) (p21) and p27kip1 (p27), have been shown to govern the pool sizes of hematopoietic stem and progenitor cells, respectively. Of note, their inhibitory roles in primitive hematopoietic cells are distinct from the action of the inhibitory cytokine, transforming growth factor-beta1 (TGF-beta1). Therefore, the distinct roles of p21, p27, and TGF-beta1 in hematopoietic cells offer attractive targets for specific manipulation of the stem or progenitor cell populations in therapeutic strategies.

Transforming growth factor beta 1 mediates cell-cycle arrest of primitive hematopoietic cells independent of p21(Cip1/Waf1) or p27(Kip1)

The regulation of stem cell proliferation is a poorly understood process balancing rapid, massive blood cell production in times of stress with maintenance of a multipotent stem cell pool over decades of life. Transforming growth factor beta 1 (TGF-beta 1) has pleiotropic effects on hematopoietic cells, including the inhibition of primitive cell proliferation. It was recently demonstrated that the cyclin-dependent kinase inhibitors, p21(Cip1/Waf1) (p21) and p27(Kip1) (p27), can inhibit the proliferation of hematopoietic stem cells and progenitor cells, respectively. The relation of TGF-beta 1 stimulation to p21 and p27 was examined using a fine-mapping approach to gene expression in individual cells. Abundant TGF-beta 1 expression and p21 expression were documented in quiescent, cytokine-resistant hematopoietic stem cells and in terminally differentiated mature blood cells, but not in proliferating progenitor cell populations. TGF-beta 1 receptor (T beta R II) was expressed ubiquitously without apparent modulation. Cell- cycle-synchronized 32D cells exposed to TGF-beta 1 demonstrated a marked antiproliferative effect of TGF-beta 1, yet neither the level of p21 mRNA nor the protein level of either p21 or p27 was altered. To corroborate these observations in primary cells, bone marrow mononuclear cells derived from mice engineered to be deficient in p21 or p27 were assessed. Progenitor and primitive cell function was inhibited by TGF-beta 1 equivalently in -/- and +/+ littermate controls. These data indicate that TGF-beta 1 exerts its inhibition on cell cycling independent of p21 and p27 in hematopoietic cells. TGF-beta 1 and p21 or p27 participate in independent pathways of stem cell regulation, suggesting that targeting each may provide complementary strategies for enhancing stem or progenitor cell expansion and gene transduction.

Transforming growth factor beta, pleiotropic regulator of hematopoietic stem cells: potential physiological and clinical relevance

Transforming growth factor beta (TGF-beta) is a pleiotropic regulator of all stages of hematopoieis. Depending on the differentiation stage of the target cell, the local environment, and the concentration of TGF-beta, TGF-beta can be proproliferative or antiproliferative, proapoptotic or antiapoptotic, and/or prodifferentiative or antidifferentiative. TGF-beta is the major regulator of stem cell quiescence and can act directly or indirectly through effects on the marrow microenvironment. In addition, paracrine and autocrine actions of TGF-beta have overlapping but distinct regulatory effects on hematopoietic stem/progenitor cells. Neutralization of autocrine TGF-beta has therapeutic potential.

The high proliferative potential-quiescent (HPP-Q) cell assay allows an optimized evaluation of gene transfer efficiency into primitive hematopoietic stem/progenitor cells

Various protocols have been described to optimize gene transfer into hematopoietic cells. However, most of these methods do not specify whether they are associated with an improved transduction of the more primitive stem/progenitor cells, the best candidates for long-term engraftment. The majority of these primitive cells remains in quiescence because of the negative control of TGF-beta1, effective on these cells at low concentrations (10 pg/ml). In this study, CD34- cells were activated by a 10 h pretreatment with anti-TGF-beta1 followed by four successive retroviral supernatant incubations of 6 h each. After 12 h (two incubations), a significant increase in TGF-beta1 mRNA in CD34+ cells was observed. We wondered whether neo-synthesized autocrine TGF-beta1 could induce reversion to quiescence of the more primitive CD34+ cells transduced after one cell cycle. This would prevent their subsequent detection in a classic clonal assay. Using the HPP-Q assay comparing a rapid mixed colony assay with or without anti-TGF-beta1, we indeed observed, that in clonal growth conditions the more primitive transduced cells were activated and detectable only with anti-TGF-beta1. Therefore, this assay represents not only a rapid means to detect quiescent multipotent stem/progenitor cells but also a necessary step for the detection of the more primitive transduced cells which have returned to quiescence after retroviral induction of TGF-beta1 secretion.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

High-level transgene expression in primary human T lymphocytes and adult bone marrow CD34+ cells via electroporation-mediated gene delivery

The design of effective gene delivery systems for gene transfer in primary human blood cells is important both for fundamental hematopoiesis research and for cancer gene therapy strategies. Here, we evaluated electroporation as a nonviral means for transfection of activated human T lymphocytes and adult bone marrow (BM) CD34+ cells. We describe optimal culture and electroporation parameters for efficient gene delivery in prestimulated T lymphocytes (16.3 +/-1.3%), as well as 2-day cultured adult BM CD34+ cells (29.6+/-4.6%). PHA-stimulated T cells were most receptive for transfection after 48h of in vitro culture, while T cells stimulated by CD3 cross-linking and interleukin (IL)-2 achieved maximum transfection levels after 72 h of prestimulation. Kinetic analysis of EGFP expression revealed that activated T lymphocytes maintained transgene expression at high levels for a prolonged period. In addition, fresh unstimulated BM CD34+ cells were consistently transfected (5.2+/-0.4%) with minimal cytotoxicity (<5%), even without preliminary CD34+ cell purification. Both T cells and CD34+ cells retained their phenotype and functional capacity after electroporation. These results demonstrate that electroporation is a suitable nonviral transfection technique that may serve applications in gene therapy protocols using T lymphocytes or CD34+ cells.

Release from quiescence of primitive human hematopoietic stem/progenitor cells by blocking their cell-surface TGF-beta type II receptor in a short-term in vitro assay

Genetic alterations of the signaling cascade of transforming growth factor-beta (TGF-beta) are often associated with neoplastic transformation of primitive cells. This demonstrates the key role for this pleiotropic factor in the control of quiescence and cell proliferation in vivo. In the high proliferative potential-quiescent cell (HPP-Q) in vitro assay, the use of TGF-beta1 blocking antibodies (anti-TGF-beta1) allows the detection within two to three weeks of primitive hematopoietic cells called HPP-Q, which otherwise would not grow. However, the possibility of triggering cell proliferation by blocking the cell-surface TGF-beta receptors has not been investigated until now. We have tested here the efficiency of a blocking antibody against TGF-betaRII (anti-TGF-betaRII) on CD34(+)CD38(-) hematopoietic cells, a subpopulation enriched in primitive stem/progenitor cells, and compared its effect with that of anti-TGF-beta1. About twice as many HPP colony-forming cells were detected in the presence of anti-TGF-beta1 or anti-TGF-betaRII, compared to the control (p < 0.02). Moreover, anti-TGF-betaRII was as efficient as anti-TGF-beta1 for activating multipotent HPP-granulocyte erythroid macrophage megakaryocyte and HPP-Mix, bipotent HPP-granulocyte-macrophage (GM) and unipotent HPP-G, HPP-M and HPP-BFU-E. We therefore propose the use of anti-TGF-betaRII to release primitive cells from quiescence in the HPP-Q assay. This strategy could be extended to nonhematopoietic tissues, as TGF-beta1 may be a pleiotropic regulator of somatic stem cell quiescence.

p21(cip1) mRNA is controlled by endogenous transforming growth factor-beta1 in quiescent human hematopoietic stem/progenitor cells

Transforming growth factor-beta1 (TGF-beta1) has been described as an efficient growth inhibitor that maintains the CD34(+) hematopoietic progenitor cells in quiescence. The concept of high proliferative potential-quiescent cells or HPP-Q cells has been introduced as a working model to study the effect of TGF-beta1 in maintaining the reversible quiescence of the more primitive hematopoietic stem cell compartment. HPP-Q cells are primitive quiescent stem/progenitor cells on which TGF-beta1 has downmodulated the cytokine receptors. These cells can be released from quiescence by neutralization of autocrine or endogenous TGF-beta1 with a TGF-beta1 blocking antibody or a TGF-beta1 antisense oligonucleotide. In nonhematopoietic systems, TGF-beta1 cooperates with the cyclin-dependent kinase inhibitor, p21(cip1), to induce cell cycle arrest. We therefore analyzed whether endogenous TGF-beta1 controls the expression of the p21(cip1) in the CD34(+) undifferentiated cells using a sensitive in situ hybridization method. We observed that addition of anti-TGF-beta1 is followed by a rapid decrease in the level of p21(cip1) mRNA whereas TGF-beta1 enhances p21(cip1) mRNA expression concurrently with an inhibitory effect on progenitor cell proliferation. These results suggest the involvement of p21(cip1) in the cell cycle control of early human hematopoietic quiescent stem/progenitors and not only in the differentiation of more mature myeloid cells as previously described. The modulation of p21(cip1) observed in response to TGF-beta1 allows us to further precise the working model of high proliferative potential-quiescent cells.

TGF-beta1 regulation of dendritic cells

Dendritic cells (DCs) represent antigen-presenting cell (APC) populations in lymphoid and nonlymphoid organs which are considered to play key roles in the initiation of antigen-specific T-cell proliferation. According to current knowledge, the net outcome of T-cell immune responses seems to be significantly influenced by the activation stage of antigen-presenting DCs. Several studies have shown that transforming growth factor-beta 1 (TGF-beta1) inhibits in vitro activation and maturation of DCs. TGF-beta1 inhibits upregulation of critical T-cell costimulatory molecules on the surface of DCs and reduces the antigen-presenting capacity of DCs. Thus, in addition to direct inhibitory effects of TGF-beta1 on effector T lymphocytes, inhibitory effects of TGF-beta1 at the level of APCs may critically contribute to previously characterized immunosuppressive effects of TGF-beta1. In contrast to these negative regulatory effects of TGF-beta1 on function and maturation of lymphoid tissue type DCs, certain subpopulations of immature DCs in nonlymphoid tissues are positively regulated by TGF-beta1 signaling. In particular, epithelial-associated DC populations seem to critically require TGF-beta1 stimulation for development and function. Recent studies established that TGF-beta1 stimulation is absolutely required for the development of epithelial Langerhans cells (LCs) in vitro and in vivo. Furthermore, TGF-beta1 seems to enhance antigen processing and costimulatory functions of epithelial LCs.

Increased gene transfer into human cord blood cells by centrifugation-enhanced transduction in fibronectin fragment-coated tubes

We investigated whether transduction of human cord blood progenitor cells can be increased by spinoculation in fibronectin fragment CH-296 (FN)-coated tubes. Bicistronic vectors PA317/LgEIN, containing the enhanced green fluorescent protein (EGFP) and neomycin phosphotransferase (neo) genes, and PG13/LgDIN, containing the dihydrofolate reductase and neo genes, were used to transduce CD34-enriched human cord blood cells. Transduction by spinoculation in FN-coated tubes (spin/FN+) was compared with spinoculation in noncoated tubes (spin/FN-) and transduction in plates coated with FN (plate/FN+). Antibody to TGF-beta was added to spin/FN+ to evaluate its impact on transduction. Using producer cell line PA317/LgEIN for transduction of CD34+ cord blood cells, FACS analysis for expression of EGFP revealed mean transduction of 30.6+/-4.3, 9.1+/-1.6, and 21.1+/-6.5% of CD34+ cells in the spin/FN+, spin/FN-, and plate/FN+ arms, respectively. Transduction of CD+CD38low cells was also higher in the spin/FN+ arm as compared with transduction in the spin/FN- arm. These results were corroborated by colony-forming assays. Antibody to TGF-beta did not further increase transduction. Using a different producer cell line, PG13/pLgDIN, a higher number of G418-resistant CFU-GM was observed in the spin/FN+ as compared with the plate/FN+ and spin/FN-arms. NOD/SCID mice were transplanted with transduced, CD34-enriched human cord blood cells, and persistence of transduced human cells was analyzed in the mice marrows after 6-8 weeks: 32.8, 6.0, and 23.9% human G418-resistant CFU-GM colonies were observed in the spin/FN+, spin/FN-, and plate/FN+ arms, respectively. These results suggest that spinoculation in FN-coated tubes increases transduction of early human cord blood progenitor cells as compared with spinoculation in noncoated tubes.

Tyrosine residues of the erythropoietin receptor are dispensable for erythroid differentiation of human CD34+ progenitors

To study the role of the cytoplasmic domain and particularly the tyrosine residues of the erythropoietin receptor (EpoR) in erythroid differentiation of human primary stem cells, we infected cord blood-derived CD34+ cells with retroviruses encoding chimeric receptors containing the extracellular domain of the prolactin receptor (PRLR) and the cytoplasmic domain of either the normal EpoR or a truncated EpoR devoid of tyrosine residues. Erythroid differentiation of the infected progenitors could thus be studied after stimulation by PRL. The complete PRLR was used to assess its ability to substitute for EpoR in erythroid differentiation. Typical erythroid day-14 colonies were observed from CD34+ cells grown in PRL when infected with any of the three viral constructs. These results demonstrate that: (i) the activation of the virally transduced PRLR leads to erythroid colony formation showing that erythroid terminal differentiation can be induced by a non-erythroid receptor in human progenitors; (ii) a chimeric receptor PRLR/EpoR is able to transduce a signal leading to terminal erythroid differentiation of human CD34+ cells; (iii) in contrast to results previously reported in murine models, tyrosine residues of the EpoR are not required for growth and terminal differentiation of human erythroid progenitors.

Lipofectamine and related cationic lipids strongly improve adenoviral infection efficiency of primitive human hematopoietic cells

Adenoviral vectors have the potential to infect a large number of cell types including quiescent cells. Their use in hematopoietic cells is limited by the episomal form of their DNA, leading to transgene loss in the progeny cells. However, the use of this vector may be interesting for short-term in vitro modifications of primitive human hematopoietic cells. Therefore, we have investigated the ability of adenovirus to transduce cord blood CD34+ cells. Several promoters were tested using the lacZ reporter gene. The PGK and CMV promoters induced transgene expression in 18-25% of the cells, whereas the HTLV-I and especially the RSV promoter were almost inactive. To improve infection efficiency, adenovirus was complexed with cationic lipids. Lipofectamine, Cellfectin, and RPR120535b, but not Lipofectin, Lipofectace, or DOTAP, markedly improved transgene expression in CD34+ cells (from 19 to 35%). Lipofectamine strongly enhanced infection efficiency of the poorly infectable primitive CD34+CD38low cells (from 11 to 28%) whereas the more mature CD34+CD38+ cells were only slightly affected (from 24 to 31%). Lipofectamine tripled the infection of CFU-GMs and LTC-ICs derived from the CD34+CD38low cell fraction (from 4 to 12% and from 5 to 16%, respectively) and doubled that of BFU-Es (from 13 to 26%). We conclude that cationic lipids can markedly increase the efficiency of adenovirus-mediated gene transfer into primitive hematopoietic cells.

Gene transfer: a review of methods and applications

Gene transfer is a potentially powerful tool for the treatment of a wide variety of diseases. The transfer of these genes is achieved by utilizing a variety of vectors, including retroviral, adenoviral, adeno-associated virus (AAV) and a number of non-viral mechanisms. Numerous studies have successfully demonstrated transduction of genes into target cells with a variety of vectors, and have provided 'proof-in-principle' that gene transfer can result in prolonged in vivo expression of transduced genes, albeit at low quantities. Furthermore, gene marking studies in acute myeloblastic leukemia (AML), chronic myeloid leukemia (CML) and neuroblastoma have elegantly demonstrated that gene-marked tumor cells contribute to relapse following autologous transplantation. However none of the studies examining the therapeutic benefit of gene therapy has definitively demonstrated a clinically meaningful benefit. Nonetheless, the results of studies involving gene transfer for severe combined immunodeficiency (SCID), chronic granulomatous disease (CGD), melanoma and lung cancer highlight the potential benefit of this strategy. This review will discuss mechanisms of achieving gene transfer into target cells. It will examine some of the pre-clinical and clinical results to date and will discuss some of the potential uses of gene transfer for therapeutic purposes.

Reduction in levels of the cyclin-dependent kinase inhibitor p27(kip-1) coupled with transforming growth factor beta neutralization induces cell-cycle entry and increases retroviral transduction of primitive human hematopoietic cells

Successful gene therapy depends on stable transduction of hematopoietic stem cells. Target cells must cycle to allow integration of Moloney-based retroviral vectors, yet hematopoietic stem cells are quiescent. Cells can be held in quiescence by intracellular cyclin-dependent kinase inhibitors. The cyclin-dependent kinase inhibitor p15(INK4B) blocks association of cyclin-dependent kinase (CDK)4/cyclin D and p27(kip-1) blocks activity of CDK2/cyclin A and CDK2/cyclin E, complexes that are mandatory for cell-cycle progression. Antibody neutralization of beta transforming growth factor (TGFbeta) in serum-free medium decreased levels of p15(INK4B) and increased colony formation and retroviral-mediated transduction of primary human CD34(+) cells. Although TGFbeta neutralization increased colony formation from more primitive, noncycling hematopoietic progenitors, no increase in M-phase-dependent, retroviral-mediated transduction was observed. Transduction of the primitive cells was augmented by culture in the presence of antisense oligonucleotides to p27(kip-1) coupled with TGFbeta-neutralizing antibodies. The transduced cells engrafted immune-deficient mice with no alteration in human hematopoietic lineage development. We conclude that neutralization of TGFbeta, plus reduction in levels of the cyclin-dependent kinase inhibitor p27, allows transduction of primitive and quiescent hematopoietic progenitor populations.

Efficient transduction of hemopoietic CD34+ progenitors of human origin using an original retroviral vector derived from Fr-MuLV-FB29: in vitro assessment

A novel retroviral vector has been designed based on a Friend-murine leukemia virus (Fr-MuLV) FB29 strain. The latter has been selected according to characteristics of pathogenicity in mice where it induces a disease of the haemopoietic system affecting all lineages. Higher infectivity has also been demonstrated as compared to other strains. In accordance with these findings, the amphotropic producer clone used in this study carrying along the neomycine resistance gene (FOCH-Neo), harbors viral titers over 10(7) cfu/ml. To investigate the potential of genetically engineering hematopoietic precursors, CD34+ progenitors were selected from cord blood, bone marrow, and peripheral blood mobilized stem cells (patients + solid tumors) and transduced with FOCH-Neo. High transduction rates were achieved using virus supernatant and minimal doses of hematopoietic growth factors during pretransduction and transduction steps. A polymerase chain reaction (PCR) assay investigating the presence of both neomycin-encoding and viral vector sequences tested positive in 45-90% of granulocyte-macrophage colony-forming units (CFU-GM) generating cells (bone marrow and peripheral blood derived cells) following transduction. An average of 35% colonies showed resistance to G418. Such levels of transduction proved reproducible using only supernatants harboring over 10(7) cfu/ml. In those experiments where long-term in vitro cultures could be maintained over 5 weeks (all cord blood and 5 among 23 PBSC), efficient transduction of long-term culture initiating cell (LTC-IC) hematopoietic progenitors was demonstrated on the basis of both resistance to G418 and virus integration. In the latter case, the PCR assay tested positive in as much as 35-60% of late unselected CFU-colonies. This novel retroviral vector harbors interesting features toward genetic modification of hematopoietic progenitors.

High-level gene transfer to cord blood progenitors using gibbon ape leukemia virus pseudotype retroviral vectors and an improved clinically applicable protocol

The best methods for transducing hematopoietic progenitor cells usually involve either direct co-cultivation with virus-producing cells or human stromal supportive cells. However, these methods cannot be safely or easily applied to clinical use. Therefore, we aimed at improving retrovirus-mediated gene transfer into hematopoietic progenitors derived from cord blood CD34+ cells using viral supernatant to levels achieved at least with direct co-cultivation and under conditions that are suitable for clinical applications. In a first set of experiments, CD34+ cells were infected with supernatant containing amphotropic retroviral particles carrying the nls-lacZ reporter gene and the effects of centrifugation, cell adhesion to fibronectin, and Polybrene on the transduction of both clonogenic progenitors (CFC) and long-term culture initiating cells (LTC-IC) were studied. Transduction efficiency was evaluated on the percentage and total number of progenitors expressing the beta-galactosidase activity. Results show that a 48-hr infection of CD34+ cells with viral supernatant combining centrifugation at 1000 x g for 3 hr followed by adhesion to fibronectin allows transduction levels for both CFC and LTC-IC to be reached that are as good as using direct co-cultivation. In a second set of experiments, CD34+ cells were infected using this optimized protocol with pseudotyped retroviral particles carrying the gibbon ape leukemia virus (GALV) envelope protein. Under these conditions, between 50 and 100% of CFC and LTC-IC were transduced. Thus, we have developed a protocol capable of highly transducing cord blood progenitors under conditions suitable for a therapeutical use.

Gene transfer into human haematopoietic stem cells

This review of gene transfer to the human haematopoietic system (1) describes the different vectors used to transduce genes into stem cells, emphasizing retroviruses that have already shown their efficiency and innocuousness; (2) analyses which human cells should be targeted to ensure long-lasting engraftment; (3) indicates the different means of infecting these targets ex vivo, underscoring the role of cytokines and stromal cells; (4) recollects the methods used to evaluate transduction efficiency; and (5) gathers the results of clinical trials recently performed using human stem cells. The major conclusions are that good practice can ensure safe gene delivery to human beings and that long-lasting, multilineal precursors can be transduced using retroviral vectors of marker genes or genes of therapeutic interest. However, transduction rates appear to remain relatively low, which should stimulate ongoing research on both vector design and means of ex vivo gene transfer.

Retroviral stem cell gene therapy

Long-term in vivo gene transfer studies in mice have shown that recombinant murine retroviruses are able to infect murine hemopoietic stem cells with high efficiency. Taken together the results indicated that the proviral structure was present at high frequency in circulating hemopoietic cells resulting in significant expression levels. Because of the success of these murine studies, it was believed that gene therapy would soon be applicable to treat a wide variety of congenital or acquired human diseases associated with the hemopoietic system. However, results from gene transfer studies in nonhuman primates and first human clinical trails have indicated that murine retrovirus infection of primate hemopoietic stem cells is inefficient. Although there are essential differences between the murine and primate gene therapy studies with respect to the recombinant viruses and transduction protocols used, these differences cannot solely account for the differences observed in infection efficiency. Therefore, in recent years effort has been spent on the identification of factors limiting retroviral transduction of primate hemopoietic stem cells. Increasing knowledge concerning hemopoiesis and retroviral infection has helped in identifying a number of limiting factors. Novel transduction strategies and tools have been generated which attempt to circumvent these limiting factors. These factors as well as the strategies that showed increased retroviral infection of primate hemopoietic stem cells will be discussed.