Enhanced megakaryocyte and erythroid development from normal human CD34(+) cells: consequence of enforced expression of SCL

The product of the SCL gene is a basic helix-loop-helix (bHLH) transcription factor that is essential for the development of hematopoietic stem cells in both the embryo and the adult. However, once the stem cell compartment is established, the function of SCL in subsequent differentiation and commitment events within normal hematopoietic cells remains undefined. The aim of the current study was to investigate this role using purified normal human hematopoietic CD34(+) cells. An SCL retrovirus was used to transduce CD34(+) cells isolated from human bone marrow, peripheral blood, and umbilical cord blood. Enforced expression of SCL increased by a median of twofold the number of erythroid colonies, with an increase in both colony size and the rate of hemoglobinization. Unexpectedly, enforced expression in CD34(+) cells also significantly increased the number of megakaryocyte colonies, but with no impact on the size of colonies. There was no consistent effect on the number nor size of granulocyte-macrophage (GM) colonies. The proliferative effect of enforced SCL expression on erythroid cells was attributed to a shortened cell cycle time; the self-renewal capacity of erythroid or GM progenitors was unchanged, as was survival of cells within colonies. These results demonstrate a role for SCL in determining erythroid and megakaryocyte differentiation from normal human hematopoietic CD34(+) cells.

Bone marrow failure in the Fanconi anemia group C mouse model after DNA damage

Fanconi anemia (FA) is a pleiotropic inherited disease that causes bone marrow failure in children. However, the specific involvement of FA genes in hematopoiesis and their relation to bone marrow (BM) failure is still unclear. The increased sensitivity of FA cells to DNA cross-linking agents such as mitomycin C (MMC) and diepoxybutane (DEB), including the induction of chromosomal aberrations and delay in the G2 phase of the cell cycle, have suggested a role for the FA genes in DNA repair, cell cycle regulation, and apoptosis. We previously reported the cloning of the FA group C gene (FAC) and the generation of a Fac mouse model. Surprisingly, the Fac -/- mice did not show any of the hematologic defects found in FA patients. To better understand the relationship of FA gene functions to BM failure, we have analyzed the in vivo effect of an FA-specific DNA damaging agent in Fac -/- mice. The mice were found to be highly sensitive to DNA cross-linking agents; acute exposure to MMC produced a marked BM hypoplasia and degeneration of proliferative tissues and caused death within a few days of treatment. However, sequential, nonlethal doses of MMC caused a progressive decrease in all peripheral blood parameters of Fac -/- mice. This treatment targeted specifically the BM compartment, with no effect on other proliferative tissues. The progressive pancytopenia resulted from a reduction in the number of early and committed hematopoietic progenitors. These results indicate that the FA genes are involved in the physiologic response of hematopoietic progenitor cells to DNA damage.

High level engraftment of NOD/SCID mice by primitive normal and leukemic hematopoietic cells from patients with chronic myeloid leukemia in chronic phase

We have previously shown that intravenously injected peripheral blood (PB) or bone marrow (BM) cells from newly diagnosed chronic myeloid leukemia (CML) patients can engraft the BM of sublethally irradiated severe combined immunodeficient (SCID) mice. We now report engraftment results for chronic phase CML cells in nonobese diabetic (NOD)/SCID recipients which show the superiority of this latter model. Transplantation of NOD/SCID mice with 7 to 10 x 10(7) patient PB or BM cells resulted in the continuing presence of human cells in the BM of the mice for up to 7 months, and primitive human CD34+ cells, including those detectable as colony-forming cells (CFC), as long-term culture-initiating cells, or by their coexpression of Thy-1, were found in a higher proportion of the NOD/SCID recipients analyzed, and at higher levels than were seen previously in SCID recipients. The human CFC and total human cells present in the BM of the NOD/SCID mice transplanted with CML cells also contained higher proportions of leukemic cells than were obtained in the SCID model, and NOD/SCID mice could be repopulated with transplants of enriched CD34+ cells from patients with CML. These results suggest that the NOD/SCID mouse may allow greater engraftment and amplification of both normal and leukemic (Ph+) cells sufficient for the quantitation and characterization of the normal and leukemic stem cells present in patients with CML. In addition, this model should make practical the investigation of mechanisms underlying progression of the disease and the development of more effective in vivo therapies.

Assay of human stem cells by repopulation of NOD/SCID mice

The only conclusive method to assay stem cells is to follow their ability to repopulate conditioned recipients, making it difficult to study human stem cells. The development of systems to transplant human hematopoietic cells into immune-deficient mice lays the foundation for such an experimental repopulation assay for primitive human cells. Cell purification and gene marking studies have shown that the repopulating cells, termed severe-combined immunodeficiency (SCID) mouse-repopulating cells (SRC), are primitive and distinct from most of the progenitors that are detected using short and long-term in vitro culture assays. The SRC are exclusively CD34+CD38- and poorly infected with retrovirus vectors. These gene marking data are reminiscent of the human clinical trials establishing that the SRC assay is a good surrogate to develop improved transduction methods. Limiting dilution analysis has been used to establish a quantitative assay for SRC that can be used to precisely determine the effect of various cytokine cocktails on the proliferation and differentiation of SRC during in vitro culture.

Quantitative analysis reveals expansion of human hematopoietic repopulating cells after short-term ex vivo culture

Ex vivo culture of human hematopoietic cells is a crucial component of many therapeutic applications. Although current culture conditions have been optimized using quantitative in vitro progenitor assays, knowledge of the conditions that permit maintenance of primitive human repopulating cells is lacking. We report that primitive human cells capable of repopulating nonobese diabetic (NOD)/severe combined immunodeficiency (SCID) mice (SCID-repopulating cells; SRC) can be maintained and/or modestly increased after culture of CD34+CD38- cord blood cells in serum-free conditions. Quantitative analysis demonstrated a 4- and 10-fold increase in the number of CD34+CD38- cells and colony-forming cells, respectively, as well as a 2- to 4-fold increase in SRC after 4 d of culture. However, after 9 d of culture, all SRC were lost, despite further increases in total cells, CFC content, and CD34+ cells. These studies indicate that caution must be exercised in extending the duration of ex vivo cultures used for transplantation, and demonstrate the importance of the SRC assay in the development of culture conditions that support primitive cells.

Differential maintenance of primitive human SCID-repopulating cells, clonogenic progenitors, and long-term culture-initiating cells after incubation on human bone marrow stromal cells

Many experimental and clinical protocols are being developed that involve ex vivo culture of human hematopoietic cells on stroma or in the presence of cytokines. However, the effect of these manipulations on primitive hematopoietic cells is not known. Our severe combined immune-deficient mouse (SCID)-repopulating cell (SRC) assay detects primitive human hematopoietic cells based on their ability to repopulate the bone marrow (BM) of immune-deficient non-obese diabetic/SCID (NOD/SCID) mice. We have examined here the maintenance of SRC, colony-forming cells (CFC), and long-term culture-initiating cells (LTC-IC) during coculture of adult human BM or umbilical cord blood (CB) cells with allogeneic human stroma. Transplantation of cultured cells in equivalent doses as fresh cells resulted in lower levels of human cell engraftment after 1 and 2 weeks of culture for BM and CB, respectively. Similar results were obtained using CD34+-enriched CB cells. By limiting dilution analysis, the frequency of SRC in BM declined sixfold after 1 week of culture. In contrast to the loss of SRC as measured by reduced repopulating capacity, the transplanted inocula of cultured cells frequently contained equal or higher numbers of CFC and LTC-IC compared with the inocula of fresh cells. The differential maintenance of CFC/LTC-IC and SRC suggests that SRC are biologically distinct from the majority of these in vitro progenitors. This report demonstrates the importance of the SRC assay in the development of ex vivo conditions that will allow maintenance of primitive human hematopoietic cells with repopulating capacity.

Engraftment and development of human CD34(+)-enriched cells from umbilical cord blood in NOD/LtSz-scid/scid mice

Understanding the repopulating characteristics of human hematopoietic stem/progenitor cell fractions is crucial for predicting their performance after transplant into high-risk patients following high-dose therapy. We report that human umbilical cord blood cells, 78% to 100% of which express the hematopoietic progenitor cell surface marker CD34, can consistently engraft, develop, and proliferate in the hematopoietic tissues of sublethally irradiated NOD/LtSz-scid/scid mice. Engraftment and development of CD34+ cells is not dependent on human growth factor support. CD34+ cells home to the mouse bone marrow (BM) that becomes the primary site of human hematopoietic development containing myeloid, lymphoid, erythroid, and CD34+ progenitor populations. Myeloid, and in particular lymphoid cells possessing more mature cell surface markers, comprise the human component of mouse spleen and peripheral blood, indicating that development proceeds from primary hematopoietic sites to the periphery. Repopulation of secondary recipients with human cells by BM from primary recipients demonstrates the maintenance of substantial proliferation capacity of the input precursor population. These data suggest that the cells capable of initiating human cell engraftment (SCID-repopulating cells) are contained in the CD34+ cell fraction, and that this mouse model will be useful for assaying the developmental potential of other rare human hematopoietic cell fractions in vivo.

Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell

On the subject of acute myeloid leukemia (AML), there is little consensus about the target cell within the hematopoietic stem cell hierarchy that is susceptible to leukemic transformation, or about the mechanism that underlies the phenotypic, genotypic and clinical heterogeneity. Here we demonstrate that the cell capable of initiating human AML in non-obese diabetic mice with severe combined immunodeficiency disease (NOD/SCID mice) - termed the SCID leukemia-initiating cell, or SL-IC - possesses the differentiative and proliferative capacities and the potential for self-renewal expected of a leukemic stem cell. The SL-ICs from all subtypes of AML analyzed, regardless of the heterogeneity in maturation characteristics of the leukemic blasts, were exclusively CD34++ CD38-, similar to the cell-surface phenotype of normal SCID-repopulating cells, suggesting that normal primitive cells, rather than committed progenitor cells, are the target for leukemic transformation. The SL-ICs were able to differentiate in vivo into leukemic blasts, indicating that the leukemic clone is organized as a hierarchy.

A rapid screening procedure for the identification of high-titer retrovirus packaging clones

We have developed a viral RNA (vRNA) dot blot assay for rapid identification of high-titer retrovirus vector production by packaging cell clones. The procedure employs Trizol LS reagent to purify vRNA from packaging cell supernatants, a sensitive dot blot assay, and Phosphorlmager technology to quantify packaged viral genomes in 2 days. Experiments performed on viral supernatants of known biological titer demonstrated that the vRNA dot blot assay was extremely sensitive and that dot intensity correlated directly with viral titer. It is often necessary to analyze approximately 100 virus producing cell clones, making this method useful as a rapid screen to identify the highest virus producing clones. The vRNA dot blot assay consistently identified a subset of candidate high-titer producer cell clones. In three independent screens the supernatant with the highest biological titer was produced by one of the previously defined candidate high-titer producer clones. Our procedure greatly facilitates virus titration by: (1) rapidly eliminating the vast majority of low-titer producer cell clones; (2) accurately identifying the subset of candidate high-titer producer clones for further biological titration and assessment of the proviral genomic structure; and (3) reducing laborious tissue culture manipulations to a minimum. Furthermore, the reliance of this method on molecular detection makes it ideally suited for the isolation of high-titer clones lacking a drug selection marker.

Kinetic evidence of the regeneration of multilineage hematopoiesis from primitive cells in normal human bone marrow transplanted into immunodeficient mice

Based on initial observations of human CD34+ Thy-1+ cells and long-term culture-initiating cells (LTC-IC) in the bone marrow of some sublethally irradiated severe combined immunodeficient (SCID) mice transplanted intravenously with normal human marrow cells, and the subsequent finding that the NOD/LtSz-scid/scid (NOD/SCID) mouse supports higher levels of human cell engraftment, we undertook a series of time course experiments to examine posttransplant changes in the number, tissue distribution, cycling activity, and in vivo differentiation pattern of various human hematopoietic progenitor cell populations in this latter mouse model. These studies showed typical rapid posttransplant recovery curves for human CD34- CD19+ (B-lineage) cells, CD34+ granulopoietic, erythroid, and multilineage colony-forming cells (CFC), LTC-IC, and CD34+ Thy-1+ cells from a small initial population representing <0.1% of the original transplant. The most primitive human cell populations reached maximum values at 5 weeks posttransplant, after which they declined. More mature cell types peaked after another 5 weeks and then declined. A 2-week course of thrice weekly injections of human Steel factor, interleukin (IL)-3, granulocyte-macrophage colony-stimulating factor (GM-CSF), and erythropoietin (administered just before the mice were killed for analysis) did not alter the pace of regeneration of either primitive or mature human hematopoietic cells, or their predominantly granulopoietic and B-lymphoid pattern of differentiation, although a significant enhancing effect on the level of human cell engraftment sustained after 3 months was noted. Cycling studies showed the human CFC present at 4 to 5 weeks posttransplant to be rapidly proliferating even in mice not given human growth factors. However, by 10 weeks and thereafter, only quiescent human CFC were detected; interestingly, even in mice that were given the 2-week course of growth factor injections. These studies indicate the use of this model for future analysis of the properties and in vivo regulation of primitive human hematopoietic cells that possess in vivo repopulating ability.

Primitive human hematopoietic cells are enriched in cord blood compared with adult bone marrow or mobilized peripheral blood as measured by the quantitative in vivo SCID-repopulating cell assay

We have previously reported the development of in vivo functional assays for primitive human hematopoietic cells based on their ability to repopulate the bone marrow (BM) of severe combined immunodeficient (SCID) and nonobese diabetic/SCID (NOD/SCID) mice following intravenous transplantation. Accumulated data from gene marking and cell purification experiments indicate that the engrafting cells (defined as SCID-repopulating cells or SRC) are biologically distinct from and more primitive than most cells that can be assayed in vitro. Here we demonstrate through limiting dilution analysis that the NOD/SCID xenotransplant model provides a quantitative assay for SRC. Using this assay, the frequency of SRC in cord blood (CB) was found to be 1 in 9.3 x 10(5) cells. This was significantly higher than the frequency of 1 SRC in 3.0 x 10(6) adult BM cells or 1 in 6.0 x 10(6) mobilized peripheral blood (PB) cells from normal donors. Mice transplanted with limiting numbers of SRC were engrafted with both lymphoid and multilineage myeloid human cells. This functional assay is currently the only available method for quantitative analysis of human hematopoietic cells with repopulating capacity. Both CB and mobilized PB are increasingly being used as alternative sources of hematopoietic stem cells in allogeneic transplantation. Thus, the findings reported here will have important clinical as well as biologic implications.

Purification of primitive human hematopoietic cells capable of repopulating immune-deficient mice

The purification of primitive human hematopoietic stem cells has been impaired by the absence of repopulation assays. By using a stringent two-step strategy involving depletion of lineage-positive cells followed by fluorescence-activated cell sorting, we have purified a cell population that is highly enriched for cells capable of multilineage repopulation in nonobese diabetic/severe combined immunodeficient (NOD/SCID) recipients. These SCID-repopulating cells (SRCs) were exclusively found in a cell fraction that expressed high levels of CD34 and no CD38. Through limiting dilution analysis using Poisson statistics, we calculated a frequency of 1 SRC in 617 CD34(+) CD38(-) cells. The highly purified SRC were capable of extensive proliferation in NOD/SCID mice. Mice transplanted with 1 SRC (at limiting cell doses) were able to produce approximately 400, 000 progeny 6 weeks after the transplant. Detailed flow cytometric analysis of the marrow of highly engrafted mice demonstrated both lymphoid and myeloid differentiation, as well as the retention of a significant fraction of CD34(+) CD38(-) cells. These highly purified fractions should be useful for identification of the cellular and molecular mechanisms that regulate primitive human hematopoietic cells. Moreover, the ability to detect and purify primitive cells provides a means to develop conditions for maintaining and/or expanding these cells during in vitro culture.

Identification of primitive human hematopoietic cells capable of repopulating NOD/SCID mouse bone marrow: implications for gene therapy

The development of stem-cell gene therapy is hindered by the absence of repopulation assays for primitive human hematopoietic cells. Current methods of gene transfer rely on in vitro colony-forming cell (CFC) and long-term culture-initiating cell (LTC-IC) assays, as well as inference from other mammalian species. We have identified a novel human hematopoietic cell, the SCID-repopulating cell (SRC), a cell more primitive than most LTC-ICs and CFCs. The SRC, exclusively present in the CD4+CD8- fraction, is capable of multilineage repopulation of the bone marrow of nonobese diabetic mice with severe combined immunodeficiency disease (NOD/SCID mice). SRCs were rarely transduced with retroviruses, distinguishing them from most CFCs and LTC-ICs. This observation is consistent with the low level of gene marking seen in human gene therapy trials. An SRC assay may aid in the characterization of hematopoiesis, as well as the improvement of transduction methods.

Human stem cell assays in immune-deficient mice

Much of our understanding of the organization of the cells that comprise the hematopoietic system and the cellular and molecular mechanisms that regulate their development is derived from mouse models. However, knowledge of the human hematopoietic system and identification of human stem cells have, until recently, been hampered by the absence of in vivo assays that measure their repopulation capacity. The development of methods to transplant normal and leukemic human hematopoietic cells into immune-deficient SCID mice provides the foundation for human stem cell assays. This review will focus on recent evidence that normal and leukemic human stem cells can be assayed in these systems.

Normal and leukemic human stem cells assayed in SCID mice

Understanding the processes that regulate the developmental program of normal stem cells and those that initiate proliferative diseases such as leukemia remains one of the major challenges in biology. Progress to address these major questions in the human hematopoietic system have been hampered, until recently, by the lack of in-vivo assays for normal and leukemic stem cells. The recent development of methods to transplant normal and leukemic human hematopoietic cells into immune-deficient mice provides an important approach to identify, characterize and purify stem cells. This review will focus on the development of assays for normal and leukemic human stem cells and on the new insights these assays are beginning to provide on the organization of the human stem cell hierarchy and mechanisms of leukemogenesis.

Inactivation of Fac in mice produces inducible chromosomal instability and reduced fertility reminiscent of Fanconi anaemia

Fanconi anaemia (FA) is an autosomal recessive disease characterized by bone marrow failure, variable congenital malformations and predisposition to malignancies. Cells derived from FA patients show elevated levels of chromosomal breakage and an increased sensitivity to bifunctional alkylating agents such as mitomycin C (MMC) and diepoxybutane (DEB). Five complementation groups have been identified by somatic cell methods, and we have cloned the gene defective in group C (FAC)(7). To understand the in vivo role of this gene, we have disrupted murine Fac and generated mice homozygous for the targeted allele. The -/- mice did not exhibit developmental abnormalities nor haematologic defects up to 9 months of age. However, their spleen cells had dramatically increased numbers of chromosomal aberrations in response to MMC and DEB. Homozygous male and female mice also had compromised gametogenesis, leading to markedly impaired fertility, a characteristic of FA patients. Thus, inactivation of Fac replicates some of the features of the human disease.

Normal and leukemic SCID-repopulating cells (SRC) coexist in the bone marrow and peripheral blood from CML patients in chronic phase, whereas leukemic SRC are detected in blast crisis

Progress in understanding the abnormal regulation of hematopoiesis in chronic myelogenous leukemia (CML) would be facilitated if neoplastic cells, at all stages of the disease, could be studied in an animal model. In this report, we show that irradiated severe combined immunodeficient (SCID) mice can be transplanted with both normal (Philadelphia chromosome [Ph]-negative) and neoplastic (Ph+) cells from CML patients with either chronic or blast phase disease. Mice transplanted with peripheral blood (PB) or bone marrow (BM) cells from 9 of 12 chronic phase CML patients were well engrafted with human cells including multilineage colony-forming progenitors and CD34+ cells for at least 90 days posttransplantation. Repeated posttransplant injections of cytokines did not enhance the number of engrafted human cells. Interestingly, approximately 70% of the human progenitors found in the engrafted SCID BM were Ph-, suggesting that the growth of primitive normal cells is favored in this in vivo transplant model. A similar number of normal cells were found in mice transplanted with either PB or BM cells, suggesting that elevated numbers of primitive normal cells are present in CML PB. When cells from patients with CML in either myeloid or lymphoid blast crisis were transplanted into SCID mice, the BM of these mice was more rapidly repopulated and to a higher level than that observed with transplants of chronic phase cells. Moreover, all human colony-forming progenitors present in the BM of mice transplanted with blast crisis cells were Ph+, and the majority of cells showed the same morphological features of the blast crisis cells originally transplanted. These experiments provide a starting point for the creation of an animal model of CML and establish the feasibility of using this model for the future characterization of transplantable CML stem cells during disease progression.

A recombinant bcl-x s adenovirus selectively induces apoptosis in cancer cells but not in normal bone marrow cells

Many cancers overexpress a member of the bcl-2 family of inhibitors of apoptosis. To determine the role of these proteins in maintaining cancer cell viability, an adenovirus vector that expresses bcl-xs, a functional inhibitor of these proteins, was constructed. Even in the absence of an exogenous apoptotic signal such as x-irradiation, this virus specifically and efficiently kills carcinoma cells arising from multiple organs including breast, colon, stomach, and neuroblasts. In contrast, normal hematopoietic progenitor cells and primitive cells capable of repopulating severe combined immunodeficient mice were refractory to killing by the bcl-xs adenovirus. These results suggest that Bcl-2 family members are required for survival of cancer cells derived from solid tissues. The bcl-xs adenovirus vector may prove useful in killing cancer cells contaminating the bone marrow of patients undergoing autologous bone marrow transplantation.

Engraftment of immune-deficient mice with primitive hematopoietic cells from beta-thalassemia and sickle cell anemia patients: implications for evaluating human gene therapy protocols

Permanent correction of genetic deficiencies of the hematopoietic system requires gene transfer into stem cells and long-term lineage specific expression after autologous transplantation. However, progress to develop gene therapy protocols has been hampered by the absence of in vivo assays that detect genetically deficient human hematopoietic stem cells and their diseased differentiated progeny. The establishment of systems to transplant human cells into immune-deficient SCID mice provides such an assay. We report that primitive bone marrow cells from beta-thalassemia major and sickle cell anemia patients engraft immune-deficient mice, giving rise to high levels of human erythroid and myeloid cells in response to treatment with human cytokines. The bone marrow of transplanted mice contained the entire erythroid lineage from BFU-E to mature erythrocytes expressing human gamma, beta or beta s-globin. Moreover, human erythroid cells from mice transplanted with sickle cell anemia bone marrow showed characteristic sickling under reducing conditions in an in vitro assay. This model provides a powerful in vivo system that can be used to evaluate the efficiency of globin gene transfer into primitive human hematopoietic cells, lineage-specific expression in mature erythrocytes, and ultimately correction of the cellular defect found in the erythroid lineage.

Rescue of T cell-specific V(D)J recombination in SCID mice by DNA-damaging agents

Assembly of antigen receptor V (variable), D (diversity), and J (joining) gene segments requires lymphocyte-specific genes and ubiquitous DNA repair activities. Severe combined immunodeficient (SCID) mice are defective in general double-strand (ds) DNA break repair and V(D)J coding joint formation, resulting in arrested lymphocyte development. A single treatment of newborn SCID mice with DNA-damaging agents restored functional, diverse, T cell receptor beta chain coding joints, as well as development and expansion of thymocytes expressing both CD4 and CD8 coreceptors, but did not promote B cell development. Thymic lymphoma developed in all mice treated with DNA-damaging agents, suggesting an interrelation between V(D)J recombination, dsDNA break repair, and lymphomagenesis.

Immature human cord blood progenitors engraft and proliferate to high levels in severe combined immunodeficient mice

Unseparated or Ficoll-Hypaque (Pharmacia, Piscataway, NJ)--fractionated human cord blood cells were transplanted into sublethally irradiated severe combined immunodeficient (SCID) mice. High levels of multilineage engraftment, including myeloid and lymphoid lineages, were obtained with 80% of the donor samples as assessed by DNA analysis, fluorescence-activated cell sorting (FACS), and morphology. In contrast to previous and concurrent studies with adult human bone marrow (BM), treatment with human cytokines was not required to establish high-level human cell engraftment, suggesting that neonatal cells either respond differently to the murine microenvironment or they provide their own cytokines in a paracrine fashion. Committed and multipotential myelo-erythroid progenitors were detected using in vitro colony assays and FACS analysis of the murine BM showed the presence of immature CD34+ cells. In addition, human hematopoiesis was maintained for at least 14 weeks providing further evidence that immature hematopoietic precursors had engrafted the murine BM. This in vivo model for human cord blood-derived hematopoiesis will be useful to gain new insights into the biology of neonatal hematopoietic cells and to evaluate their role in gene therapy. There is growing evidence that there are ontogeny-related changes in immature human hematopoietic cells, and therefore, the animal models we have developed for adult and neonatal human hematopoiesis provide useful tools to evaluate these changes in vivo.

Expression of bcr-abl abrogates factor-dependent growth of human hematopoietic M07E cells by an autocrine mechanism

The introduction of a retrovirus vector expressing p210bcr-abl (P210) into the human factor-dependent cell line M07E resulted in the rapid outgrowth of factor-independent cells. Early after infection, four factor-independent clones were isolated and analyzed in greater detail along with mass populations obtained from separate infections. High levels of P210 tyrosine kinase activity were measured in the factor-independent cells. The mass populations and three of the four clones remained responsive to exogenous growth factors. Concentrated conditioned media isolated from the factor-independent populations and from all clones contained biologically active granulocyte-macrophage colony-stimulating factor (GM-CSF); interleukin-3 (IL-3) was detected at low levels in the mass population and in two of the clones. Neutralizing antibodies to IL-3, GM-CSF, and mast cell growth factor inhibited proliferation of the factor responsive clones by 60% to 90%. These results indicate that the growth autonomy of the P210-expressing M07E cells was acquired via an autocrine mechanism. In addition to factor-independent growth, P210-expressing M07E cells readily acquired a more mature megakaryocytic phenotype compared with control M07E cells. These data provide experimental evidence that expression of P210 tyrosine kinase in human hematopoietic cells induced growth factor secretion resulting in a pleiotropic effect on growth factor dependence and differentiation.

A cell initiating human acute myeloid leukaemia after transplantation into SCID mice

Most human acute myeloid leukaemia (AML) cells have limited proliferative capacity, suggesting that the leukaemic clone may be maintained by a rare population of stem cells. This putative leukaemic stem cell has not been characterized because the available in vitro assays can only detect progenitors with limited proliferative and replating potential. We have now identified an AML-initiating cell by transplantation into severe combined immune-deficient (SCID) mice. These cells homed to the bone marrow and proliferated extensively in response to in vivo cytokine treatment, resulting in a pattern of dissemination and leukaemic cell morphology similar to that seen in the original patients. Limiting dilution analysis showed that the frequency of these leukaemia-initiating cells in the peripheral blood of AML patients was one engraftment unit in 250,000 cells. We fractionated AML cells on the basis of cell-surface-marker expression and found that the leukaemia-initiating cells that could engraft SCID mice to produce large numbers of colony-forming progenitors were CD34+ CD38-; however, the CD34+ CD38+ and CD34- fractions contained no cells with these properties. This in vivo model replicates many aspects of human AML and defines a new leukaemia-initiating cell which is less mature than colony-forming cells.

Gene transfer into hematopoietic stem cells: long-term maintenance of in vitro activated progenitors without marrow ablation

Adoptive transfer of genetically modified somatic cells will play an increasingly important role in the management of a wide spectrum of human diseases. Among the most appealing somatic cells as potential gene transfer vehicles are hematopoietic cells, because of their wide distribution and their well-characterized capacities for proliferation, differentiation, and self-renewal. Genes can be readily transferred into short-lived and lineage-restricted hematopoietic cells, but there remains a need to develop reliable methods for gene transfer into hematopoietic stem cells in large animals. In this work, we used a gene transfer approach in which hematopoietic cells in long-term marrow cultures were exposed to the replication-defective retrovirus N2, bearing the reporter gene neo, on multiple occasions during 21 days of culture. Genetically marked cultured autologous cells were infused into 18 canine recipients in the absence of marrow-ablative conditioning. neo was detected by Southern blotting and/or the polymerase chain reaction in the marrow, blood, marrow-derived granulocyte/macrophage and erythroid progenitors, and cultured T cells in dogs after infusion. In most dogs, the proportion of long-term marrow culture cells contributing to hematopoiesis rose during the first 3 months after infusion and peaked within the first 6. The maximal levels attained were between 10% and 30% G418-resistant (neo-positive) granulocyte/macrophage progenitors. At 12 months, five dogs maintained greater than 10% G418-resistant progenitors, and for two of them this level exceeded 20%. Two dogs had greater than 5% G418-resistant hematopoietic progenitors at 24 months after infusion. Our data suggest that very primitive hematopoietic progenitors are maintained in long-term marrow cultures, where they can be triggered into entering the cell cycle. In vivo, these activated cells likely continue normal programs of proliferation, differentiation, and self-renewal. Their progeny can be maintained at clinically relevant levels for up to 2 years without the requirement that endogenous hematopoiesis be suppressed through chemo- or radiotherapy prior to adoptive transfer. Long-term marrow culture cells may thus be ideal targets for gene therapy involving adoptive transfer of transduced hematopoietic cells.