Engraftment of gene-modified umbilical cord blood cells in neonates with adenosine deaminase deficiency.

Haematopoietic stem cells in umbilical cord blood are an attractive target for gene therapy of inborn errors of metabolism. Three neonates with severe combined immunodeficiency were treated by retroviral-mediated transduction of the CD34+ cells from their umbilical cord blood with a normal human adenosine deaminase complementary DNA followed by autologous transplantation. The continued presence and expression of the introduced gene in leukocytes from bone marrow and peripheral blood for 18 months demonstrates that umbilical cord blood cells may be genetically modified with retroviral vectors and engrafted in neonates for gene therapy.

T lymphocytes with a normal ADA gene accumulate after transplantation of transduced autologous umbilical cord blood CD34+ cells in ADA-deficient SCID neonates.

Adenosine deaminase-deficient severe combined immunodeficiency was the first disease investigated for gene therapy because of a postulated production or survival advantage for gene-corrected T lymphocytes, which may overcome inefficient gene transfer. Four years after three newborns with this disease were given infusions of transduced autologous umbilical cord blood CD34+ cells, the frequency of gene-containing T lymphocytes has risen to 1-10%, whereas the frequencies of other hematopoietic and lymphoid cells containing the gene remain at 0.01-0.1%. Cessation of polyethylene glycol-conjugated adenosine deaminase enzyme replacement in one subject led to a decline in immune function, despite the persistence of gene-containing T lymphocytes. Thus, despite the long-term engraftment of transduced stem cells and selective accumulation of gene-containing T lymphocytes, improved gene transfer and expression will be needed to attain a therapeutic effect.

Retroviral-mediated transfer of the human glucocerebrosidase gene into cultured Gaucher bone marrow.

Gaucher disease, a lysosomal glycolipid storage disorder, results from the genetic deficiency of an acidic glucosidase, glucocerebrosidase (GC). The beneficial effects of allogeneic bone marrow transplantation (BMT) for Gaucher disease suggest that GC gene transduction and the transplantation of autologous hematopoietic stem cells (gene therapy) may similarly alleviate symptoms. We have constructed a retroviral vector, L-GC, produced by a clone of the amphotropic packaging cell line PA317, which transduces the normal human GC cDNA with high efficiency. Whole-marrow mononuclear cells and CD34-enriched cells from a 4-yr-old female with type 3 Gaucher disease were transduced by the L-GC vector and studied in long-term bone marrow culture (LTBMC). Prestimulation of marrow with IL-3 and IL-6, followed by co-cultivation with vector-producing fibroblasts, produced gene transfer into 40-45% of the hematopoietic progenitor cells. The levels of GC expression in progeny cells (primarily mature myelomonocytic) produced by the LTBMC were quantitatively analyzed by Northern blot, Western blot, and glucocerebrosidase enzyme assay. Normal levels of GC RNA, immunoreactive protein, and enzymatic activity were detected throughout the duration of culture. These studies demonstrate that retroviral vectors can efficiently transfer the GC gene into long-lived hematopoietic progenitor cells from the bone marrow of patients with Gaucher disease and express physiologically relevant levels of GC enzyme activity.

Immunodeficient mice as models of human hematopoietic stem cell engraftment.

The past year has brought forth some exciting developments in the use of murine xenotransplantation systems to study the biology and transduction of human hematopoietic stem cells. The effects of cytokines have been studied by injection into the mice or by treatment of the cell inoculum prior to injection. The importance of the cell cycle and integrin expression has been evaluated. New methods of gene therapy have been tested in xenograft models - including cell cycle manipulation and a promising new lentiviral vector system, based on HIV.

CD34: to select or not to select? That is the question.

Recent evidence suggests that expression of CD34 on the cell membrane does not always correlate with stem cell activity. In the mouse, there is a highly quiescent population of stem cells that lacks CD34 expression, but has full reconstituting capacity. The current review addresses the discovery of a similar population of dormant CD34-negative human hematopoietic stem cells. This information casts some uncertainty on the benefits of CD34+ cell isolation for stem cell transplantation, until more is known about the novel CD34-negative stem cell population. Methods designed to achieve removal of specific mature blood cell lineages might prove to be most advantageous in the future.

The AFT024 stromal cell line supports long-term ex vivo maintenance of engrafting multipotent human hematopoietic progenitors.

The immortalized murine stromal cell line AFT024 has been reported to maintain human hematopoietic progenitors in an undifferentiated state in vitro. In the current studies the beige/nude/xid (bnx) mouse in vivo xenograft model was used to examine the engraftment and multilineage generative potential of human hematopoietic progenitors after 2-3 weeks growth on AFT024 stroma, in comparison to primary stromal monolayers derived from post-natal human bone marrow. Eight to 12 months after transplantation of human CD34+CD38- cells from umbilical cord blood, cultured on AFT024 vs human stroma for 2-3 weeks, the murine bone marrow was harvested and analyzed for the presence of human myeloid and lymphoid cells. The mean percent engraftment of total human hematopoietic cells in the murine marrow was significantly higher after co-cultivation on AFT024 than on human stroma. Human myeloid and lymphoid lineage cells were detected in all mice. However, engraftment of myeloid lineage cells (CD33+), B lymphoid (CD19+), and T lymphoid cells (CD4+and CD8+) were significantly higher after co-cultivation of the human cells on AFT024 than on human stroma, prior to transplantation. Interestingly, the length of time in culture did not significantly affect the engraftment of the myeloid and T lymphoid lineage progenitors, but the percentage of B lymphoid lineage engraftment decreased significantly between 2 and 3 weeks of co-cultivation on both types of stroma. Cells with a primitive phenotype (CD45+/CD34-/CD38- and CD45+/CD34-/lin-) and cells with the capacity to generate secondary human CFU after recovery from the bnx bone marrow were maintained at significantly higher levels during culture on AFT024 stroma than on human stroma. The current studies demonstrate that the AFT024 murine stromal cell line supports the ex vivo survival and maintenance of human hematopoietic progenitors that are capable of long-term multilineage reconstitution for 2-3 weeks ex vivo, to levels superior to those that can be obtained using human stromal cells.

Cbl functions downstream of Src kinases in Fc gamma RI signaling in primary human macrophages.

Cbl is a cytosolic protein that is rapidly tyrosine phosphorylated in response to Fc receptor activation and binds to the adaptor proteins Grb2, CrkL, and Nck. A few reports describe Cbl interactions in primary human hematopoietic cells. We show evidence that Cbl participates in signaling initiated by Fc gammaRI receptor cross-linking in human primary macrophages, and functions downstream of Src family kinases in this pathway. Fc gammaRI stimulation in human macrophages was associated with rapid and transient tyrosine phosphorylation of the Cbl adaptor protein. Immunoprecipitated Cbl was complexed with several tyrosine phosphorylated proteins, the most prominent of which was a 38-kDa band identified as the CrkL adaptor protein. CrkL associated with tyrosine-phosphorylated Cbl and itself became tyrosine phosphorylated after Fc gammaRI cross-linking. SLP-76, a recently cloned Grb2-associated protein, was strongly tyrosine phosphorylated after Fc gammaRI stimulation and was associated with both Cbl and Grb2. Grb2 and Cbl binding to SLP-76 were inducible after Fc gammaRI stimulation of the macrophages. Nck was inducibly bound to Cbl after Fc gammaRI stimulation, whereas Grb2 was constitutively associated with it. Shc was also inducibly tyrosine phosphorylated and bound to Grb2 after Fc gammaRI stimulation of the macrophages. PP1, a specific inhibitor of Src kinases, inhibited the Fc gammaRI-induced respiratory burst, as well as the tyrosine phosphorylation of Cbl and its inducible association with CrkL. These results suggest a fundamental role for the tyrosine phosphorylation of Cbl, CrkL, SLP-76, and Shc and the association of Cbl with CrkL, SLP-76, and Nck in Fc gammaRI signaling in human macrophages. Experiments performed with PP1, the specific Src kinase inhibitor, demonstrate the first evidence that Cbl and the Cbl-Crkl interaction are downstream targets for myeloid Src kinases required for the activation of myeloid NADPH oxidase activity.

Clonal diversity of primitive human hematopoietic progenitors following retroviral marking and long-term engraftment in immune-deficient mice.

The ultimate goal of human gene therapy in treating hematopoietic disorders is to insert a functional copy of the affected gene into self-renewing stem cells. The engineered pluripotent cells should then provide all lineages of corrected blood cells for the lifetime of the recipient. It is therefore important to develop methods of tracking and studying the progeny of individual human hematopoietic stem cells. Using the technique of single-colony inverse polymerase chain reaction (PCR), we assessed the clonal diversity of marked colony-forming cells that had developed from transduced human hematopoietic progenitors in a long-term xenograft system. The LN retroviral vector, which carries the neo gene, was used to individually mark human CD34+ progenitors. The marked cells were then transplanted into immune-deficient mice for periods of up to 1 year to assess their survival and retention of clonogenic capacity. Following long-term engraftment, bone marrow cells recovered from each mouse were plated in human-specific colony-forming unit (CFU) assay with and without the drug G418, which selects for cells expressing the neo gene. Three weeks later, well-isolated colonies that had grown in G418 were plucked, and PCR for the neo gene was performed to confirm the presence of the vector. Inverse PCR was then performed on neo+ colonies to analyze the integration site of the LN provirus in human DNA. The clonal diversity of G418-resistant (G418R) human CFU recovered from 18 long-term engrafted beige/nude/xid (bnx) mice was assessed. From one to six human hematopoietic precursors had generated all marked colony-forming progenitors (3-39) recovered from the marrow of each animal. To assess the extent of in vitro self-renewal divisions, marrow samples from 22 sets of experiments, with 2-4 mice transplanted in each set, were studied using the single-colony inverse PCR technique. Proviral integrants at identical sites were found in only two mice transplanted with cells transduced in the same flask. The presence of identical integration sites in human progenitors recovered from two mice demonstrated that a long-lived, marked cell had self-renewed in vitro before transplantation and that both daughter cells had retained the capacity to home to the bnx bone marrow and survive for 10 months. Our in vivo xenograft model and the inverse PCR technique have allowed us to identify, trace, and quantitate the clonogenic progeny of primitive human hematopoietic cells for up to 1 year after retroviral-mediated transduction.

Retroviral vector-mediated gene transfer into primitive human hematopoietic progenitor cells: effects of mast cell growth factor (MGF) combined with other cytokines.

Retroviral vector-mediated gene transfer into human hematopoietic stem cells may permit gene therapy of numerous genetic diseases. Stimulation of marrow with hematopoietic growth factors (HGFs) has been shown to increase the level of retroviral transduction. We have examined the effects of recombinant human mast cell growth factor (MGF), alone and in combination with other HGFs, on the efficiency of gene transfer into human hematopoietic progenitor cells. MGF acts in concert with interleukin 3 (IL-3) and interleukin 6 (IL-6) to increase the percentage of CD34+ progenitors transduced with a retroviral vector expressing the neo gene. The most potent combination of growth factors that we examined, interleukin 1 (IL-1)/IL-3/IL-6/MGF, resulted in the conferral of G418 resistance to 45% of progenitors and long-term culture-initiating cells. Extending the time of cocultivation of the marrow cells with the vector-producing cells did not further increase gene transfer frequency, suggesting that the amount of available vector is not limiting. To analyze the effects of the HGF on gene transfer into more primitive hematopoietic progenitors, CD34+ cells were isolated from marrow samples that were purged of committed progenitor cells by treatment with 4-hydroperoxycyclophosphamide (4-HC). Preculturing the CD34+ 4-HC-treated cells with the combination of four HGF (IL-1/IL-3/IL-6/MGF) permitted transduction of 20%-28% of the progenitors that formed colonies after 30 days in culture. These results demonstrate that MGF in combination with other HGFs enhances gene transduction of human hematopoietic progenitor cells.

Comparison of the effects of growth factors on retroviral vector-mediated gene transfer and the proliferative status of human hematopoietic progenitor cells.

We have examined the ability of the recombinant hematopoietic growth factors (HGF) interleukin-3 (IL-3), IL-6, and granulocyte-macrophage colony-stimulating factor (GM-CSF) to increase retroviral vector-mediated gene transfer into human hematopoietic progenitor cells (HPC). The efficiency of neo gene transfer by the N2 vector into human HPC was enhanced by preculture with either GM-CSF or IL-3 (but not IL-6) and with each combination of the three factors. The combination of IL-3 plus IL-6 consistently produced significantly higher levels of G418-resistant colonies (50-60%) than any of the other combinations of HGF tested. Following preculture with HGF and transduction by N2, marrow was maintained in long-term bone marrow culture (LTBMC) for 2 months. The levels of G418-resistant HPC remained stable, and no apparent depletion of total HPC content resulted from the prior exposure to highly stimulatory doses of factors. The proliferative status of the HPC, following exposure to the HGF, was measured as the percentage of HPC that were inhibited from forming colonies by exposure to the S-phase-specific drug, hydroxyurea. The ability of the different HGF to increase the rate of gene transfer by N2 correlated significantly with the extent to which they stimulated HPC proliferation. These results suggest that the mechanism by which HGF increase rates of gene transfer into HPC is by stimulating cell proliferation. Techniques that produce high rates of gene transfer into long-lived human HPC will facilitate studies to quantitate expression of exogenous genes in hematopoietic cells and may be applicable to clinical gene therapy.

Toward gene therapy for Gaucher disease.

We are studying the transfer and expression by retroviral vectors of the human glucocerebrosidase (GC) gene into bone marrow cells as a model of gene therapy for genetic diseases of hematopoietic cells. A simple retroviral vector (G2) was developed that contains a normal human GC cDNA under the control of the Moloney murine leukemia virus long-terminal repeat (LTR) enhancer/promoter. Murine bone marrow was transduced with the G2 vector and maintained in long-term bone marrow culture (LTBMC). Expression of the human GC gene in the transduced murine LTBMC cells exceeded the level of endogenous murine GC mRNA. Murine bone marrow cells were also transduced with G2 and transplanted into irradiated syngeneic recipients. High levels of GC gene transfer and expression were seen in day-12 CFU-S foci, and to a lesser extent in the hematopoietic organs 4 months after gene transfer/bone marrow transplant (BMT). Human bone marrow, from a patient with Gaucher disease, was also used in studies of GC gene transduction. Gene transfer into 35-40% of the Gaucher hematopoietic progenitor cells was achieved, following prestimulation of the marrow with recombinant hematopoietic growth factors. Equal rates of gene transfer were obtained using either total marrow mononuclear cells or progenitor cells enriched 100-fold by immunomagnetic bead separation. GC gene transduction corrected the enzymatic deficiency of the Gaucher marrow. Our results demonstrate the potential utility of retroviral vector-mediated gene transfer for gene therapy of Gaucher disease. Current efforts are aimed at achieving more consistent in vivo GC expression in the murine BMT model and demonstrating transduction of pluripotent human hematopoietic stem cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Retroviral transfer of the glucocerebrosidase gene into CD34+ cells from patients with Gaucher disease: in vivo detection of transduced cells without myeloablation.

Retroviral gene transfer of the glucocerebrosidase gene to hematopoietic progenitor and stem cells has shown promising results in animal models and corrected the enzyme deficiency in cells from Gaucher patients in vitro. Therefore, a clinical protocol was initiated to explore the safety and feasibility of retroviral transduction of peripheral blood (PB) or bone marrow (BM) CD34+ cells with the G1Gc vector. This vector uses the viral LTR promoter to express the human glucocerebrosidase cDNA. Three adult patients have been entered with follow-up of 6-15 months. Target cells were G-CSF-mobilized and CD34-enriched PB cells or CD34-enriched steady state BM cells, and were transduced ex vivo for 72 hr. Patient 1 had PB cells transduced in the presence of autologous stromal marrow cells. Patient 2 had PB cells transduced in the presence of autologous stroma, IL-3, IL-6, and SCF. Patient 3 had BM cells transduced in the presence of autologous stroma, IL-3, IL-6, and SCF. At the end of transduction, the cells were collected and infused immediately without any preparative treatment of the patients. The transduction efficiency of the CD34+ cells at the end of transduction was approximately 1, 10, and 1 for patients 1, 2, and 3, respectively, as estimated by semiquantitative PCR on bulk samples and PCR analysis of individual hematopoietic colonies. Gene marking in vivo was demonstrated in patients 2 and 3. Patient 2 had vector-positive PB granulocytes and mononuclear bone marrow cells at 1 month postinfusion and positive PB mononuclear cells at 2 and 3 months postinfusion. Patient 3 had a positive BM sample at 1 month postinfusion but was negative thereafter. These results indicate that gene-marked cells can engraft and persist for at least 3 months postinfusion, even without myeloablation. However, the level of corrected cells (<0.02%) is too low to result in any clinical benefit, and glucocerebrosidase enzyme activity did not increase in any patient following infusion of transduced cells. Modifications of vector systems and transduction conditions, along with partial myeloablation to allow higher levels of engraftment, may be necessary to achieve beneficial levels of correction in patients with Gaucher disease.

Animal xenograft models for evaluation of gene transfer into human hematopoietic stem cells.

Animal xenograft models of gene therapy have become increasingly popular to study the effects of various transduction strategies on human hematopoietic stem cells (HSC). Xenograft models provide an in vivo setting in which to monitor the duration and effects of vector expression in the progeny of the transduced stem and progenitor cells. Also, the ability of HSC to home to the bone marrow and differentiate into multilineage progeny following ex vivo manipulation can only be tested in a transplantation system. The current review will discuss the murine xenograft models that have been used recently to determine optimized methods for gene transfer into normal human hematopoietic stem cells.

Expression of human glucocerebrosidase following retroviral vector-mediated transduction of murine hematopoietic stem cells.

The human glucocerebrosidase (GC) gene has been expressed in the progeny of murine hematopoietic stem cells following transduction of marrow with a retroviral vector (G2) containing the human GC cDNA. Murine marrow was transduced via co-cultivation following prestimulation in the presence or absence of recombinant IL-3 and IL-6. A high rate of gene transfer and expression (95%) was demonstrated in primary day 12 CFU-S foci following bone marrow transplantation (BMT) of G2-transduced marrow into lethally irradiated syngeneic recipient mice. Immunoreactive human GC protein was also documented in the CFU-S foci. Primary recipient mice were examined 4-6 months following BMT. A higher rate of gene transfer (87%) was seen in hematopoietic organs of recipients of prestimulated donor marrow compared with organs from initially unstimulated marrow (25%). A high rate of expression of human GC was also documented in the prestimulated organs (50%) when compared with the unstimulated group (25%). Secondary BMT was performed using marrow from the long-lived primary recipients. The human GC gene was present in 88% of secondary day 12 CFU-S foci examined in the prestimulated group versus 23% in the unstimulated group. Expression of the human GC gene was documented in secondary day 12 CFU-S foci, providing strong evidence of initial hematopoietic stem cell transduction.

The number and generative capacity of human B lymphocyte progenitors, measured in vitro and in vivo, is higher in umbilical cord blood than in adult or pediatric bone marrow.

The lack of human B lymphocyte development in beige/nude/XID (bnx) mice is in sharp contrast to the robust development observed in another immune deficient strain, the NOD/SCID mouse. The ability to generate human B lymphocytes in the NOD/SCID, but not bnx mouse has been hypothesized to be caused by differences in the microenvironments or systemic cytokine concentrations. In the current studies we report that the differences in development can be primarily attributed to the source of the progenitors transplanted into the mice. The prior studies in bnx mice used cultured pediatric or adult bone marrow (BM) as the source of the CD34+ cells, whereas the NOD/SCID studies have predominantly used fresh or cultured umbilical cord blood (UCB). We have analyzed BM and UCB for the number of human CD34+/CD38- cells capable of in vitro B lymphocyte development, and have found a lower frequency of B lymphocyte generation in BM. The individual B lymphocyte clones that developed from bone marrow produced 100-fold fewer cells than the UCB-derived clones. In agreement with the in vitro studies, human B lymphocytes developed in bnx mice from both CD34+ and CD34+/CD38- cells isolated from human umbilical cord blood, but not from equivalent numbers of CD34+ and CD34+/CD38- progenitors from bone marrow. Therefore, the lower generative capacity, and frequency of B lymphocyte precursors in human marrow may be responsible for the previous results that showed a lack of B lymphocyte development in bnx mice.

Clinical infection control in gene therapy: a multidisciplinary conference.

Gene therapy is being studied for the treatment of a variety of acquired and inherited disorders. Retroviruses, adenoviruses, poxviruses, adeno-associated viruses, herpesviruses, and others are being engineered to transfer genes into humans. Treatment protocols using recombinant viruses are being introduced into clinical settings. Infection control professionals will be involved in reviewing the safety of these agents in their clinics and hospitals. To date, only a limited number of articles have been written on infection control in gene therapy, and no widely available recommendations exist from federal or private organizations to guide infection control professionals. The goals of the conference were to provide a forum where gene therapy experts could share their perspectives and experience with infection control in gene therapy and to provide an opportunity for newcomers to the field to learn about issues specific to infection control in gene therapy. Recommendations for infection control in gene therapy were proposed.

Multidrug resistance gene expression in pediatric primitive neuroectodermal tumors of the central nervous system.

Pediatric primitive neuroectodermal tumor (PNET) is a malignancy of the central nervous system currently treated with surgery, radiation therapy, and chemotherapy. Despite aggressive management, tumors recur in almost one-half of all patients. Drug resistance of tumor cells may, in part, explain the poor outcome. Resistance to chemotherapeutic agents may be related to expression of the multidrug resistance gene (MDR1) and its protein product, P-glycoprotein. The role of MDR1 in 16 instances of PNET was investigated using Western blot analysis to detect the expression of P-glycoprotein, messenger ribonucleic acid (mRNA), polymerase chain reaction to detect MDR1 mRNA expression, and Southern blot analysis to assess gene amplification. Analysis of proteins extracted from 15 tumors revealed that two of the 15 patients expressed detectable levels of P-glycoprotein. Polymerase chain reaction of ribonucleic acid from 12 PNET's revealed that six of the 12 patients (four of 10 de novo tumors and both recurrent tumors) expressed MDR1 mRNA. Southern blot analysis of deoxyribonucleic acid from 16 PNET's revealed no evidence of MDR1 amplification in any tumor. This is the first report of MDR1 expression in pediatric brain tumors. These data suggest a possible role for MDR1 in de novo and acquired drug resistance in PNET's.

Use of the bnx/hu xenograft model of human hematopoiesis to optimize methods for retroviral-mediated stem cell transduction (Review).

The potentiality of primitive human hematopoietic cells can be profoundly affected by in vitro culture. Due to the growing number of protocols proposed for stem cell gene therapy and ex vivo expansion, it is crucial to define methods to preserve the generative capacity of human stem cells in culture while promoting self-renewal divisions. Stem cell division, homing, and subsequent lineage development can only be studied definitively by marking of pluripotent cells, followed by tracking and clonal analysis of the progeny in a long-term transplantation system. We have developed a bnx/hu xenograft model, in which transduced human hematopoietic cells can be individually tracked into different lineages over the course of one year post-transplantation. The tracking is accomplished by single cell cloning of individual T lymphoid and myeloid progenitors recovered from the marrow of the mice, and clonal integration analysis by the sensitive technique of single-colony inverse PCR. All cells derived from a stem cell transduced by a retroviral vector will carry the unique restriction fragment length polymorphism (RFLP) created by the random integration event. We have used the bnx/hu xenograft system coupled with single-colony inverse PCR to determine that human stem cells require stromal support, fibronectin support with cytokines, or the presence of Flt3 ligand during a 72-h ex vivo culture to maintain the ability to sustain long-term multilineage hematopoiesis.

Inclusion of IL-3 during retrovirally-mediated transduction on stromal support does not increase the extent of gene transfer into long-term engrafting human hematopoietic progenitors.

Interleukin 3 (IL-3) supports the survival of multilineage hematopoietic progenitors, and increases the extent of retrovirally mediated gene transfer into colony-forming cells. However, effects from the supraphysiological levels used in ex-vivo expansion and gene-therapy protocols on subsequent differentiation of human progenitors have not been well defined. In the current studies, the extents of retrovirally mediated transduction and lineage development from CD34+ cells cultured ex vivo in the presence or absence of IL-3 were compared. All transductions were performed in the presence of an irradiated stromal support layer, IL-6 and SCF, with and without inclusion of 10 ng/ml IL-3. Following transduction, colony-forming (CFU) assays were done, and the remaining cells were transplanted into cohorts of sibling beige/nude/xid (bnx) mice. Marrow from the mice was harvested 9-10 months post transplantation. The average extent of human CD45+ cell engraftment in the bone marrow and the human hematopoietic lineages recovered from the mice in the +IL-3 and -IL-3 groups did not vary significantly. No deleterious effects on the extent of engraftment, lineage generation, or survival of clonogenic progenitors was observed with inclusion of IL-3 in the transductions performed on stromal support. The percentage of G418-resistant human progenitors recovered from mice was equivalent. The extent of marking by the neo gene in the marrow of the mice was equal in both groups, and inverse PCR revealed that primitive cells transduced in the absence of IL-3 had generated progeny with slightly better clonal diversity than progenitors transduced in the presence of IL-3. These data show that, while transduction of colony-forming progenitors may not always be apparent, primitive human hematopoietic cells can be transduced to significant levels in the absence of IL-3.