Engraftment and Development of Human CD34+-Enriched Cells From Umbilical Cord Blood in NOD/LtSz-scid/scid Mice

High Efficiency Gene Transfer to Human Hematopoietic SCID-Repopulating Cells Under Serum-Free Conditions

Stable gene transfer to human pluripotent hematopoietic stem cells (PHSCs) is an attractive strategy for the curative treatment of many genetic hematologic disorders. In clinical trials, the levels of gene transfer to this cell population have generally been low, reflecting deficiencies in both the vector systems and transduction conditions. In this study, we have used a pseudotyped murine retroviral vector to transduce human CD34+ cells purified from bone marrow (BM) and umbilical cord blood (CB) under optimized conditions. After transduction, 71% to 97% of the hematopoietic cells were found to express a low-affinity nerve growth factor receptor (LNGFR) marker gene. Six weeks after transplantation into immunodeficient NOD/LtSz-scid/scid (NOD/SCID) mice, LNGFR expression was detected in 6% to 57% of CD45+ cells in eight of nine engrafted animals. Moreover, proviral DNA was detected in 8.3% to 45% of secondary colonies derived from BM cells of engrafted NOD/SCID mice. Our data show consistent transduction of SCID-repopulating cells (SRCs) and suggest that the efficiency of gene transfer to human hematopoietic repopulating cells can be improved using existing retroviral vector systems and carefully optimized transduction conditions.

© 1998 by The American Society of Hematology.

Biology of CD34+CD38- cells in lymphohematopoiesis

Since the discovery of the CD34 stem/progenitor cell antigen, considerable progress has been made in further purifying human lymphohematopoietic stem cells (HSC). These studies have identified a number of antigens which can be targeted to subfractionate the CD34+ cell population. In particular, several lines of evidence suggest that the rare CD38(-)subpopulation of CD34+ cells may be enriched in HSC. This review briefly summarizes relevant knowledge concerning the CD38 molecule and the results of in vitro and in vivo studies of CD34+38(-)cells. Possible clinical uses for purified CD34+38(-)cells are outlined.

Nonobese Diabetic/Severe Combined Immunodeficiency (NOD/SCID) Mouse as a Model System to Study the Engraftment and Mobilization of Human Peripheral Blood Stem Cells

Mobilized CD34+ cells from human peripheral blood (PB) are increasingly used for hematopoietic stem-cell transplantation. However, the mechanisms involved in the mobilization of human hematopoietic stem and progenitor cells are largely unknown. To study the mobilization of human progenitor cells in an experimental animal model in response to different treatment regimens, we injected intravenously a total of 92 immunodeficient nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice with various numbers of granulocyte colony-stimulating factor (G-CSF) –mobilized CD34+ PB cells (ranging from 2 to 50 × 106cells per animal). Engraftment of human cells was detectable for up to 6.5 months after transplantation and, depending on the number of cells injected, reached as high as 96% in the bone marrow (BM), displaying an organ-specific maturation pattern of T- and B-lymphoid and myeloid cells. Among the different mobilization regimens tested, human clonogenic cells could be mobilized from the BM into the PB (P= .019) with a high or low dose of human G-CSF, alone or in combination with human stem-cell factor (SCF), with an average increase of 4.6-fold over control. Therefore, xenotransplantation of human cells in NOD/SCID mice will provide a basis to further study the mechanisms of mobilization and the biology of the mobilized primitive human hematopoietic cell.

Nonobese Diabetic/Severe Combined Immunodeficiency (NOD/SCID) Mouse as a Model System to Study the Engraftment and Mobilization of Human Peripheral Blood Stem Cells

Mobilized CD34+ cells from human peripheral blood (PB) are increasingly used for hematopoietic stem-cell transplantation. However, the mechanisms involved in the mobilization of human hematopoietic stem and progenitor cells are largely unknown. To study the mobilization of human progenitor cells in an experimental animal model in response to different treatment regimens, we injected intravenously a total of 92 immunodeficient nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice with various numbers of granulocyte colony-stimulating factor (G-CSF) –mobilized CD34+ PB cells (ranging from 2 to 50 × 106cells per animal). Engraftment of human cells was detectable for up to 6.5 months after transplantation and, depending on the number of cells injected, reached as high as 96% in the bone marrow (BM), displaying an organ-specific maturation pattern of T- and B-lymphoid and myeloid cells. Among the different mobilization regimens tested, human clonogenic cells could be mobilized from the BM into the PB (P= .019) with a high or low dose of human G-CSF, alone or in combination with human stem-cell factor (SCF), with an average increase of 4.6-fold over control. Therefore, xenotransplantation of human cells in NOD/SCID mice will provide a basis to further study the mechanisms of mobilization and the biology of the mobilized primitive human hematopoietic cell.

Treatment of Non-Obese Diabetic (NOD)/Severe-Combined Immunodeficient Mice (SCID) With flt3 Ligand and Interleukin-7 Impairs the B-Lineage Commitment of Repopulating Cells After Transplantation of Human Hematopoietic Cells

Until recently, the identification of cellular factors that govern the developmental program of human stem cells has been difficult due to the absence of repopulation assays that detect human stem cells. The transplantation of human bone marrow (BM) or cord blood (CB) into non-obese diabetic (NOD)/severe-combined immunodeficient (SCID) mice has enabled identification of primitive human cells capable of multilineage repopulation of NOD/SCID mice (termed the SCID–repopulating cell [SRC]). Here, we examined the effect of long-term in vivo treatment with various combinations of human cytokines on the developmental program of SRC. Detailed flow cytometric analysis of engrafted mice indicated that the vast majority of the human graft of untreated mice was comprised of B lymphocytes at various stages of development as well as myeloid and primitive cells; T cells were not reproducibly detected. Many studies, including murine in vitro and in vivo data and human in vitro experiments, have suggested that flt3 ligand (FL) and/or Interleukin-7 (IL-7) promotes T- and B-cell development. Unexpectedly, we found that treatment of engrafted mice with the FL/IL-7 combination did not induce human T- or B-cell development, but instead markedly reduced B-cell development with a concomitant shift in the lineage distribution towards the myeloid lineage. Effects on lineage distribution were similar in engrafted mice transplanted with highly purified cells indicating that the action of the cytokines was not via cotransplanted mature cells from CB or BM cells. These data show that the lineage development of the human graft in NOD/SCID mice can be modulated by administration of human cytokines providing a valuable tool to evaluate the in vivo action of human cytokines on human repopulating cells.

© 1998 by The American Society of Hematology.

Treatment of Non-Obese Diabetic (NOD)/Severe-Combined Immunodeficient Mice (SCID) With flt3 Ligand and Interleukin-7 Impairs the B-Lineage Commitment of Repopulating Cells After Transplantation of Human Hematopoietic Cells

Until recently, the identification of cellular factors that govern the developmental program of human stem cells has been difficult due to the absence of repopulation assays that detect human stem cells. The transplantation of human bone marrow (BM) or cord blood (CB) into non-obese diabetic (NOD)/severe-combined immunodeficient (SCID) mice has enabled identification of primitive human cells capable of multilineage repopulation of NOD/SCID mice (termed the SCID–repopulating cell [SRC]). Here, we examined the effect of long-term in vivo treatment with various combinations of human cytokines on the developmental program of SRC. Detailed flow cytometric analysis of engrafted mice indicated that the vast majority of the human graft of untreated mice was comprised of B lymphocytes at various stages of development as well as myeloid and primitive cells; T cells were not reproducibly detected. Many studies, including murine in vitro and in vivo data and human in vitro experiments, have suggested that flt3 ligand (FL) and/or Interleukin-7 (IL-7) promotes T- and B-cell development. Unexpectedly, we found that treatment of engrafted mice with the FL/IL-7 combination did not induce human T- or B-cell development, but instead markedly reduced B-cell development with a concomitant shift in the lineage distribution towards the myeloid lineage. Effects on lineage distribution were similar in engrafted mice transplanted with highly purified cells indicating that the action of the cytokines was not via cotransplanted mature cells from CB or BM cells. These data show that the lineage development of the human graft in NOD/SCID mice can be modulated by administration of human cytokines providing a valuable tool to evaluate the in vivo action of human cytokines on human repopulating cells.

© 1998 by The American Society of Hematology.

A newly discovered class of human hematopoietic cells with SCID-repopulating activity

The detection of primitive hematopoietic cells based on repopulation of immune-deficient mice is a powerful tool to characterize the human stem-cell compartment. Here, we identify a newly discovered human repopulating cell, distinct from previously identified repopulating cells, that initiates multilineage hematopoiesis in NOD/SCID mice. We call such cells CD34neg-SCID repopulating cells, or CD34neg-SRC. CD34neg-SRC are restricted to a Lin-CD34-CD38- population without detectable surface markers for multiple lineages and CD38 or those previously associated with stem cells (HLA-DR, Thy-1 and CD34). In contrast to CD34+ subfractions, Lin-CD34-CD38- cells have low clonogenicity in short-and long-term in vitro assays. The number of CD34neg-SRC increased in short-term suspension cultures in conditions that did not maintain SRC derived from CD34+ populations, providing independent biological evidence of their distinctiveness. The identification of this newly discovered cell demonstrates complexity of the organization of the human stem-cell compartment and has important implications for clinical applications involving stem-cell transplantation.

Biology of human umbilical cord blood-derived hematopoietic stem/progenitor cells

Reported in 1989, studies by Broxmeyer, Gluckman, and colleagues demonstrated that umbilical cord blood (UCB) is a rich source of hematopoietic stem/progenitor cells (HSPC) and that UCB could be used in clinical settings for hematopoietic cell transplantation. Since then, a great interest has been generated on the biological characterization of these cells. Over the last nine years, several groups have focused on the study of UCB HSPC, addressing different aspects, such as the frequency of these cells in UCB, the identification of different HSPC subsets based on their immunophenotype, their ability to respond to hematopoietic cytokines, the factors that control their proliferation and expansion potentials, and their capacity to reconstitute hematopoiesis in animal models. Most of these studies have shown that significant functional differences exist between HSPC from UCB and adult bone marrow (i.e., the former possess higher proliferation and expansion potential than the latter). It is also noteworthy that genetic manipulation of UCB HSPC has been achieved by several groups and that genetically modified UCB cells have already been used in the clinic. In spite of the significant advances in the characterization of these cells, we are still in the process of trying to fully understand their biology, both at the cellular and the molecular levels. In the present article, we describe and discuss what is currently known about the biology of UCB HSPC.

SCID mouse models of human stem cell engraftment

The discovery of the severe combined immunodeficiency (scid) mouse mutation has provided a tool for establishment of small animal models as hosts for the in vivo analysis of normal and malignant human pluripotent hemopoietic stem cells. Intravenous injection of irradiated scid mice with human bone marrow, cord blood, or G-CSF cytokine-mobilized peripheral blood mononuclear cells, all rich in human hemopoietic stem cell activity, results in the engraftment of a human hemopoietic system in the murine recipient. This model has been used to identify a pluripotent stem cell, termed "scid-repopulating cell" (SRC) that is more primitive than any of the hemopoietic stem cell populations identified using the currently available in vitro methodology. In this review, we describe the development and use of this model system, termed Hu-SRC-SCID, and summarize the discoveries that have resulted from the investigation of human stem cells in this model. Finally, we detail the recent extension of the original Hu-SRC-SCID model system based on the C.B-17-scid mouse as the murine host to the Hu-SRC-NOD-SCID model based on the NOD-scid mouse as the host. The engraftment of human stem cells in the Hu-SRC-NOD-SCID model is enhanced over that observed in the Hu-SRC-SCID model and results in exceptionally high levels of human hemopoietic cells in the murine recipient. Future directions to further improve the Hu-SRC-NOD-SCID model system and the potential utility of this model in the preclinical and diagnostic arenas of hematology and oncology are discussed.

Transplantation of Human Umbilical Cord Blood Cells in Macrophage-Depleted SCID Mice: Evidence for Accessory Cell Involvement in Expansion of Immature CD34+CD38−Cells

In vivo expansion and multilineage outgrowth of human immature hematopoietic cell subsets from umbilical cord blood (UCB) were studied by transplantation into hereditary immunodeficient (SCID) mice. The mice were preconditioned with Cl2MDP-liposomes to deplete macrophages and 3.5 Gy total body irradiation (TBI). As measured by immunophenotyping, this procedure resulted in high levels of human CD45+ cells in SCID mouse bone marrow (BM) 5 weeks after transplantation, similar to the levels of human cells observed in NOD/SCID mice preconditioned with TBI. Grafts containing approximately 107 unfractionated cells, approximately 105purified CD34+ cells, or 5 × 103 purified CD34+CD38− cells yielded equivalent numbers of human CD45+ cells in the SCID mouse BM, which contained human CD34+ cells, monocytes, granulocytes, erythroid cells, and B lymphocytes at different stages of maturation. Low numbers of human GpA+ erythroid cells and CD41+ platelets were observed in the peripheral blood of engrafted mice. CD34+CD38+ cells (5 × 104/mouse) failed to engraft, whereas CD34− cells (107/mouse) displayed only low levels of chimerism, mainly due to mature T lymphocytes. Transplantation of graded numbers of UCB cells resulted in a proportional increase of the percentages of CD45+ and CD34+ cells produced in SCID mouse BM. In contrast, the number of immature, CD34+CD38− cells produced in vivo showed a second-order relation to CD34+graft size, and mice engrafted with purified CD34+CD38− grafts produced 10-fold fewer CD34+ cells without detectable CD34+CD38− cells than mice transplanted with equivalent numbers of unfractionated or purified CD34+ cells. These results indicate that SCID repopulating CD34+CD38− cells require CD34+CD38+ accessory cell support for survival and expansion of immature cells, but not for production of mature multilineage progeny in SCID mouse BM. These accessory cells are present in the purified, nonrepopulating CD34+CD38+ subset as was directly proven by the ability of this fraction to restore the maintenance and expansion of immature CD34+CD38− cells in vivo when cotransplanted with purified CD34+CD38−grafts. The possibility to distinguish between maintenance and outgrowth of immature repopulating cells in SCID mice will facilitate further studies on the regulatory functions of accessory cells, growth factors, and other stimuli. Such information will be essential to design efficient stem cell expansion procedures for clinical use.

Transplantation of Human Umbilical Cord Blood Cells in Macrophage-Depleted SCID Mice: Evidence for Accessory Cell Involvement in Expansion of Immature CD34+CD38−Cells

In vivo expansion and multilineage outgrowth of human immature hematopoietic cell subsets from umbilical cord blood (UCB) were studied by transplantation into hereditary immunodeficient (SCID) mice. The mice were preconditioned with Cl2MDP-liposomes to deplete macrophages and 3.5 Gy total body irradiation (TBI). As measured by immunophenotyping, this procedure resulted in high levels of human CD45+ cells in SCID mouse bone marrow (BM) 5 weeks after transplantation, similar to the levels of human cells observed in NOD/SCID mice preconditioned with TBI. Grafts containing approximately 107 unfractionated cells, approximately 105purified CD34+ cells, or 5 × 103 purified CD34+CD38− cells yielded equivalent numbers of human CD45+ cells in the SCID mouse BM, which contained human CD34+ cells, monocytes, granulocytes, erythroid cells, and B lymphocytes at different stages of maturation. Low numbers of human GpA+ erythroid cells and CD41+ platelets were observed in the peripheral blood of engrafted mice. CD34+CD38+ cells (5 × 104/mouse) failed to engraft, whereas CD34− cells (107/mouse) displayed only low levels of chimerism, mainly due to mature T lymphocytes. Transplantation of graded numbers of UCB cells resulted in a proportional increase of the percentages of CD45+ and CD34+ cells produced in SCID mouse BM. In contrast, the number of immature, CD34+CD38− cells produced in vivo showed a second-order relation to CD34+graft size, and mice engrafted with purified CD34+CD38− grafts produced 10-fold fewer CD34+ cells without detectable CD34+CD38− cells than mice transplanted with equivalent numbers of unfractionated or purified CD34+ cells. These results indicate that SCID repopulating CD34+CD38− cells require CD34+CD38+ accessory cell support for survival and expansion of immature cells, but not for production of mature multilineage progeny in SCID mouse BM. These accessory cells are present in the purified, nonrepopulating CD34+CD38+ subset as was directly proven by the ability of this fraction to restore the maintenance and expansion of immature CD34+CD38− cells in vivo when cotransplanted with purified CD34+CD38−grafts. The possibility to distinguish between maintenance and outgrowth of immature repopulating cells in SCID mice will facilitate further studies on the regulatory functions of accessory cells, growth factors, and other stimuli. Such information will be essential to design efficient stem cell expansion procedures for clinical use.

Establishment of a Reproducible Model of Chronic-Phase Chronic Myeloid Leukemia in NOD/SCID Mice Using Blood-Derived Mononuclear or CD34+ Cells

An animal model of chronic myeloid leukemia (CML) will help characterize leukemic and normal stem cells and also help evaluate experimental therapies in this disease. We have established a model of CML in the NOD/SCID mouse. Infusion of ≥4 × 107chronic-phase CML peripheral blood cells results in engraftment levels of ≥1% in the bone marrow (BM) of 84% of mice. Engraftment of the spleen was seen in 60% of mice with BM engraftment. Intraperitoneal injection of recombinant stem cell factor produced a higher level of leukemic engraftment without increasing Philadelphia-negative engraftment. Granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor did not increase the level of leukemic or residual normal engraftment. Assessment of differential engraftment of normal and leukemic cells by fluorescence in situ hybridization analysis with bcr and abl probes showed that a median of 35% (range, 5% to 91%) of engrafted cells present in the murine BM were leukemic. BM engraftment was multilineage with myeloid, B-cell, and T-cell engraftment, whereas T cells were the predominant cell type in the spleen. BM morphology showed evidence of eosinophilia and increased megakaryocytes. We also assessed the ability of selected CD34+ CML blood cells to engraft NOD/SCID mice and showed engraftment with cell doses of 7 to 10 × 106 cells. CD34− cells failed to engraft at cell doses of 1.2 to 5 × 107. CD34+ cells produced myeloid and B-cell engraftment with high levels of CD34+ cells detected. Thus, normal and leukemic stem cells are present in CD34+ blood cells from CML patients at diagnosis and lead to development of the typical features of CML in murine BM. This model is suitable to evaluate therapy in CML.

Establishment of a Reproducible Model of Chronic-Phase Chronic Myeloid Leukemia in NOD/SCID Mice Using Blood-Derived Mononuclear or CD34+ Cells

An animal model of chronic myeloid leukemia (CML) will help characterize leukemic and normal stem cells and also help evaluate experimental therapies in this disease. We have established a model of CML in the NOD/SCID mouse. Infusion of ≥4 × 107chronic-phase CML peripheral blood cells results in engraftment levels of ≥1% in the bone marrow (BM) of 84% of mice. Engraftment of the spleen was seen in 60% of mice with BM engraftment. Intraperitoneal injection of recombinant stem cell factor produced a higher level of leukemic engraftment without increasing Philadelphia-negative engraftment. Granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor did not increase the level of leukemic or residual normal engraftment. Assessment of differential engraftment of normal and leukemic cells by fluorescence in situ hybridization analysis with bcr and abl probes showed that a median of 35% (range, 5% to 91%) of engrafted cells present in the murine BM were leukemic. BM engraftment was multilineage with myeloid, B-cell, and T-cell engraftment, whereas T cells were the predominant cell type in the spleen. BM morphology showed evidence of eosinophilia and increased megakaryocytes. We also assessed the ability of selected CD34+ CML blood cells to engraft NOD/SCID mice and showed engraftment with cell doses of 7 to 10 × 106 cells. CD34− cells failed to engraft at cell doses of 1.2 to 5 × 107. CD34+ cells produced myeloid and B-cell engraftment with high levels of CD34+ cells detected. Thus, normal and leukemic stem cells are present in CD34+ blood cells from CML patients at diagnosis and lead to development of the typical features of CML in murine BM. This model is suitable to evaluate therapy in CML.