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

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.

Human embryonic-derived hematopoietic repopulating cells require distinct factors to sustain in vivo repopulating function

OBJECTIVE

We have previously identified a novel circulating embryonic blood cell capable of pluripotent hematopoietic reconstitution, which may serve as a target for in utero stem cell therapy. Based on its unique biological properties and ontogenic origin, we aim to examine the ability to maintain and retrovirally transduce fetal blood (FB) reconstituting cells in ex vivo culture conditions previously optimized for pluripotent hematopoietic repopulating cells derived from later stages of human ontogeny.

METHODS

FB cells were evaluated for proliferative potential, progenitor composition, and SCID-repopulating cell (SRC) capacity before and after 3 days of serum free (SF) ex vivo culture using the previously optimized growth factor conditions of SCF, Flt-3L, IL-3, IL-6, and G-CSF (GF Mix), in comparison to cultures using GF Mix + oncostatin M (OSM), or SCF + Flt-3L. We further examined the ability to retrovirally transduce FB-SRC maintained in culture using SCF + Flt-3L alone.

RESULTS

Circulating FB-SRC could not be maintained under GF Mix conditions previously shown to sustain CB (cord blood)-SRC. Ex vivo culture with SCF + Flt-3L reduced the proliferation of primitive FB cells lacking lineage commitment markers (Lin(-)), but expanded FB progenitors and sustained FB-SRC compared to culture with GF Mix with and without OSM. Using SCF + Flt-3L, FB-SRC capable of multilineage reconstitution were successfully transduced, suggesting that SCF and Flt-3L are necessary and sufficient for the survival and transduction of human hematopoietic repopulating cells of embryonic origin.

CONCLUSION

Our study provides novel insights into the requirements of primitive FB reconstituting cells that are essential for developing in utero stem cell gene therapy protocols, and further illustrates the biological distinctiveness of FB-SRC compared to hematopoietic repopulating cells from other stages of human ontogeny.

Intrathymic and extrathymic development of human plasmacytoid dendritic cell precursors in vivo

The development of plasmacytoid dendritic cells (pDC2) from human CD34(+) stem cells in vivo was studied in RAG-2(-/-) interleukin (IL)-2Rgamma(-/-) mice that lack functional T and B cells and natural killer cells. CD34(+) cells isolated from fetal liver or thymus were labeled with 5- and 6-carboxyfluorescein diacetate succinimidyl ester (CFSE) and were injected into a human thymus grafted subcutaneously in the RAG-2(-/-) IL-2Rgamma(-/-) mice. One to 4 weeks later the CFSE label was found not only in T cells but also in CD123(+/high) CD4(+)CD45RA(+) pDC2, indicating that the CD34(+) cells can develop into pDC2 within a thymus. In addition to pDC2, CFSE-labeled dendritic cells with a mature phenotype, determined by the cell surface markers CD11c, CD83, and CD80, were found in the injected human thymus graft. pDC2 was not found in the periphery of mice carrying a human thymic graft, indicating that the intrathymic pDC2 failed to emigrate from the thymus. We also demonstrate that pDC2 can develop outside the thymus because relatively high percentages of pDC2 were found in the periphery after the intravenous injection of CD34(+)CD38(-) fetal liver cells in RAG-2(-/-) IL-2Rgamma(-/-) mice without a human thymus graft. These data indicate that the thymus and the peripheral pDC2 develop independently of each other.

Selective in vitro expansion and efficient retroviral transduction of human CD34+ CD38- haematopoietic stem cells

Ex vivo expansion of primitive human haematopoietic stem cells (HSC) is clinically relevant for stem cell transplantation and gene therapy. Here, we demonstrate the selective expansion of CD34+CD38- cells from purified CD34+ cells upon stimulation with Flt3-ligand, stem cell factor and thrombopoietin. Over a 100-fold (range 80 to 128-fold) expansion of CD34+CD38- cells was observed with bone marrow and cord blood (CB). The expanded CD34+CD38- cells remained negative for lineage-specific markers and could be induced to differentiate into granulocytes, monocytes, megakaryocytes, erythrocytes, and T and B-lymphocytes in vitro. Lineage differentiation assays with single CD34+CD38- cells showed no loss of multilineage potential of expanded cells after ex vivo culture. We also demonstrated that the increase in frequency of CD34+CD38- cells was not as a result of the downregulation of CD38 expression during the culture. Quantitative analysis showed that the number of 6 week cobblestone area forming cells (CAFCwk6), a measure of proliferating HSC, in cytokine-stimulated CD34+ cells were increased by 20-fold. Expanded CD34+CD38- cells could be transduced efficiently with retroviruses encoding the low affinity nerve growth factor receptor (LNGFR) marker gene (17% to 44%, mean 27%), resulting in long-lasting expression of retroviral-encoded genes in progeny HSC and differentiated progenitors. We conclude that the combination Flt3-ligand (FL), stem cell factor and thrombopoietin (TPO) induced strong ex vivo proliferation of CD34+CD38- cells and that the absolute number of expanded cells with stem cell activity increased substantially in this population.

Tumor necrosis factor (TNF)-mediated activation of the p55 TNF receptor negatively regulates maintenance of cycling reconstituting human hematopoietic stem cells

Hematopoietic stem cell (HSC) fate decisions between self-renewal and commitment toward differentiation are tightly regulated in vivo. Recent developments in HSC culture and improvements of human HSC assays have facilitated studies of these processes in vitro. Through such studies stimulatory cytokines critically involved in HSC maintenance in vivo have been demonstrated to also promote HSC self-renewing divisions in vitro. Evidence for negative regulators of HSC self-renewal is, however, lacking. Tumor necrosis factor (TNF), if overexpressed, has been implicated to mediate bone marrow suppression. However, whether and how TNF might affect the function of HSC with a combined myeloid and lymphoid reconstitution potential has not been investigated. In the present studies in vitro conditions recently demonstrated to promote HSC self-renewing divisions in vitro were used to study the effect of TNF on human HSCs capable of reconstituting myelopoiesis and lymphopoiesis in nonobese diabetic-severe combined immunodeficient (NOD-SCID) mice. Although all cord blood and adult bone marrow CD34(+)CD38(-) cells were capable of undergoing cell divisions in the presence of TNF, cycling HSCs exposed to TNF in vitro and in vivo were severely compromised in their ability to reconstitute NOD-SCID mice and long-term cultures. The negative effect of TNF was not dependent on the Fas pathway, and a similar effect could be observed using a mutant TNF exclusively targeting the p55 TNF receptor. TNF did not appear to enhance apoptosis or affect cell-cycle distribution of cultured progenitors, but rather promoted myeloid differentiation. Thus, TNF might regulate HSC fate by promoting their differentiation rather than self-renewal.

Philadelphia-positive B-cell acute lymphoblastic leukemia is initiated in an uncommitted progenitor cell

BCR-ABL is a chimeric oncogene generated by translocation of sequences from the c-ABLgene on chromosome 9 into the BCR gene on chromosome 22. Alternative chimeric proteins, BCR-ABLp190 and BCR-ABLp210, are produced that are characteristic of chronic myelogenous leukemia (CML) and acute lymphoblastic leukemia (Ph1-ALL) respectively. In CML, it is evident that the transformation occurs at the level of pluripotent stem cells. However, Ph1-ALL has been thought to affect progenitor cells with lymphoid differentiation. Recently, it has been demonstrated that normal primitive cells, rather than committed progenitor cells, are the target for leukemic transformation in Ph1-ALL. In this review, we discuss what is known about the relationship between the specific BCR-ABLp190 oncogene, the target cell and the characteristics of the subsequent disease process it causes. We also discuss how this information may be applied to the establishment of new directions in therapy.

Isolation and characterization of canine hematopoietic progenitor cells

The purpose of this study was to purify and characterize canine hematopoietic progenitor cells for surface antigen phenotype and reconstitution ability. Canine hematopoietic progenitor cells were isolated by density gradient sedimentation, lineage depletion with monoclonal antibodies, and fluorescence-activated cell sorting (FACS) for selection of cells with low-forward and right-angle scatter that were rhodamine 123 (Rh-123)(dull). Isolated cells were characterized for expression of CD34, c-kit, and Flt-3. A canine/murine xenograft model and a mixed-chimerism assay were used to examine the in vivo proliferative potential of isolated cells. The lineage-positive (Lin(+)) cells represented 80 +/- 11% (n = 22) of the input mononuclear cells. Lineage depletion resulted in a fourfold increase in colony-forming unit granulocyte/monocyte (CFU-GM), a 2.5-fold increase in burst-forming unit-erythroid (BFU-E), and a twofold increase in the number of Rh-123(dull) cells over nonlineage-depleted bone marrow mononuclear cells (BMMCs). Lineage depletion led to a 2.7-fold enrichment of CD34 cells, a 10.4-fold enrichment of c-kit cells, and a 10.8-fold enrichment of CD34/c-kit(+1) cells over total BMMCs. Nineteen percent of lineage-negative (Lin(-)) cells were positive for Flt-3. Injection of canine cells into irradiated (400 rads) NOD/SCID mice resulted in the detection of canine CD45(+) cells with BMMCs, Lin(-) cells, or Rh-123(dull) cells. Transplantation of purified Lin(-) cells in dog leukocyte antigen-matched littermates resulted in low-level engraftment for at least 10 weeks. The development of methods for purification and characterization of canine hematopoietic progenitor cells should enhance the utilization of the canine model for a variety of experimental and therapeutic purposes.

Current status of retroviral vector mediated gene transfer into human hematopoietic stem cells

Genetic modification of hematopoietic stem cells (HSCs) has been proposed as a treatment strategy for a variety of hematologic diseases, tracking marked cells or conferring resistance to chemotherapeutic agents. Despite early enthusiasm, the results of clinical studies involving gene transfer into HSCs has not resulted in therapeutic benefits for the vast majority of treated patients. This review describes the limitations and advances that have been made in the areas of gene transfer vectors, identification of the appropriate HSCs to target for genetic modifications and the methods used to perform gene transfer.

Human homologues of Delta-1 and Delta-4 function as mitogenic regulators of primitive human hematopoietic cells

Delta-mediated Notch signaling controls cell fate decisions during invertebrate and murine development. However, in the human, functional roles for Delta have yet to be described. This study reports the characterization of Delta-1 and Delta-4 in the human. Human Delta-4 was found to be expressed in a wide range of adult and fetal tissues, including sites of hematopoiesis. Subsets of immature hematopoietic cells, along with stromal and endothelial cells that support hematopoiesis, were shown to express Notch and both Delta-1 and Delta-4. Soluble forms of human Delta-1 (hDelta-1) and hDelta-4 proteins were able to augment the proliferation of primitive human hematopoietic progenitors in vitro. Intravenous transplantation of treated cultures into immune-deficient mice revealed that hDelta-1 is capable of expanding pluripotent human hematopoietic repopulating cells detected in vivo. This study provides the first evidence for a role of Delta ligands as a mitogenic regulator of primitive hematopoietic cells in the human.

Human homologues of Delta-1 and Delta-4 function as mitogenic regulators of primitive human hematopoietic cells

Delta-mediated Notch signaling controls cell fate decisions during invertebrate and murine development. However, in the human, functional roles for Delta have yet to be described. This study reports the characterization of Delta-1 and Delta-4 in the human. Human Delta-4 was found to be expressed in a wide range of adult and fetal tissues, including sites of hematopoiesis. Subsets of immature hematopoietic cells, along with stromal and endothelial cells that support hematopoiesis, were shown to express Notch and both Delta-1 and Delta-4. Soluble forms of human Delta-1 (h Delta-1) and h Delta-4 proteins were able to augment the proliferation of primitive human hematopoietic progenitors in vitro. Intravenous transplantation of treated cultures into immune-deficient mice revealed that h Delta-1 is capable of expanding pluripotent human hematopoietic repopulating cells detected in vivo. This study provides the first evidence for a role of Delta ligands as a mitogenic regulator of primitive hematopoietic cells in the human. (Blood. 2001;97:1960-1967)

Current status of retroviral vector mediated gene transfer into human hematopoietic stem cells

Genetic modification of hematopoietic stem cells (HSCs) has been proposed as a treatment strategy for a variety of hematologic diseases, tracking marked cells or conferring resistance to chemotherapeutic agents. Despite early enthusiasm, the results of clinical studies involving gene transfer into HSCs have not resulted in therapeutic benefits for the vast majority of treated patients. This review describes the limitations and advances that have been made in the areas of gene transfer vectors, identification of the appropriate HSCs to target for genetic modifications and the methods used to perform gene transfer.

[Gene therapy of X-linked severe combined immunologic deficiency (SCID-X1)]

X-linked severe combined immunodeficiency (SCID-X1) is a recessive hereditary disorder in which early T and Natural Killer (NK) lymphocyte development is blocked. The genetic disorder results from mutations in the common gamma c chain that participates in several cytokine receptors including the interleukin-2 (Il-2), Il-4, Il-7, Il-9, Il-15 receptors. SCID-X1 offers a reliable model for gene therapy as it is a lethal condition that is, in many cases, curable by allogeneic bone marrow transplantation. We have shown that retrovirus-mediated transfer of the gamma c cDNA induced gamma c chain expression and restored the function of the high-affinity IL-2 receptor on SCI-X1 EBV-transformed B-cell lines. We have the designed culture conditions to study NK-cell and T-cell development of CD34+ hematopoietic progenitor cells. In the culture systems, gamma c transduced CD34+ marrow cells from two SCID-X1 patients were able to mature into CD56+ and/or CD16+ NK cells and into CD4+ TCR alpha beta+ T cells. These preclinical results set the basis for a clinical study of ex-vivo gamma c gene transfer into CD34+ cells from SCID-X1 patients.

Umbilical cord blood progeny cells that retain a CD34+ phenotype after ex vivo expansion have less engraftment potential than unexpanded CD34+ cells

BACKGROUND

Because of the limitation of cell numbers associated with cord blood harvests, there is a need to determine the efficacy of using ex vivo-expanded cord blood cells in a transplantation setting. In this study, limiting-dilution analysis was used in nonobese diabetic mice with severe combined immunodeficiency (NOD/SCID) to compare the engraftment potential of progeny cells expressing the CD34+ phenotype after expansion with that of uncultured CD34+ cells.

STUDY DESIGN AND METHODS

Cord blood CD34+ cells were cultured in Iscove's modified Dulbecco medium supplemented with 10-percent fetal calf serum (FCS) and IL-6, SCF, megakaryocyte growth and development factor, and Flt3 ligand. The resulting ex vivo-expanded products were assessed for total numbers of nucleated cells, CD34+ cells, and CFUs and long-term culture-initiating cell activity. The engraftment potentials of cultured progeny CD34+ cells and uncultured CD34+ cells were determined by using NOD/SCID mice.

RESULTS

After 14 days of culture, total nucleated cell counts increased over input values by 180 +/- 59-fold, CD34+ cell numbers by 44 +/- 13-fold, CFU activity by 23 +/- 5-fold, and long-term culture-initiating cell activity by 20 +/- 6-fold (mean +/- SD; n = 6). The frequency of SCID-repopulating cells (SRC) in mice transplanted with uncultured products was 1 per 20,000 CD34+ cells (95% CI, 1:10,000-1:38,000) and that in mice receiving ex vivo-expanded products was 1 per 418,000 progeny CD34+ cells (95% CI, 1:158,000-1:1,100,000). Taken together, these data indicated that, after 2 weeks of culture, there was a modest twofold increase in the total number of SRCs. However, the levels of human CD45 cell engraftment in NOD/SCID recipients of progeny CD34+ cells were significantly lower than those in mice receiving equivalent numbers of uncultured CD34+ cells (p<0.05).

CONCLUSION

Umbilical cord blood progeny cells retaining a CD34+ phenotype after ex vivo expansion have less engraftment potential than do unexpanded CD34+ cells.

Sonic hedgehog induces the proliferation of primitive human hematopoietic cells via BMP regulation

A pool of stem cells that arise from the mesoderm during embryogenesis initiates hematopoiesis. However, factors that regulate the expansion of blood stem cells are poorly understood. We show here that cytokine-induced proliferation of primitive human hematopoietic cells could be inhibited with antibodies to hedgehog (Hh). Conversely, Sonic hedgehog (Shh) treatment induced the expansion of pluripotent human hematopoietic repopulating cells detected in immunodeficient mice. Noggin, a specific inhibitor of bone morphogenetic protein 4 (BMP-4), was capable of inhibiting Shh-induced proliferation in a similar manner to anti-Hh; however, anti-Hh had no effect on BMP-4-induced proliferation. Our study shows that Shh functions as a regulator of primitive hematopoietic cells via mechanisms that are dependent on downstream BMP signals.

The human hematopoietic stem cell compartment is heterogeneous for CXCR4 expression

The chemokine stromal derived factor-1alpha (SDF-1alpha) has been implicated recently in the chemotaxis of primitive human hematopoietic cells, suggesting that pluripotent human stem cells express the SDF-1alpha receptor, CXCR4. By using flow cytometry and confocal microscopy, we have identified and isolated primitive subsets of human CXCR4(+) and CXCR4(-) cells. Distinctions in the progenitor content and response to SDF-1alpha in vitro indicate that CXCR4(+) and CXCR4(-) cells represent discrete populations of primitive blood cells. The i.v. transplantation of these subfractions into immune-deficient mice established that both possess comparable engraftment capacity in vivo. Human myeloid, lymphoid, and primitive CD34(+) CXCR4(+) cells were present in chimeric animals transplanted with either subset, indicating that CXCR4(+) and CXCR4(-) stem cells have equivalent proliferative and differentiative abilities. Our study indicates that the human stem cell compartment is heterogeneous for CXCR4 expression, suggesting that the relationship between CXCR4 expression and stem cell repopulating function is not obligatory.

The human immunodeficiency virus type-1 central DNA flap is a crucial determinant for lentiviral vector nuclear import and gene transduction of human hematopoietic stem cells

Gene transfer in human hematopoietic stem cells (HSCs) has great potential for both gene therapy and the understanding of hematopoiesis. As HSCs have extensive proliferative capacities, stable gene transfer should include genomic integration of the transgene. Lentiviral vectors are now preferred to oncoretroviral vectors especially because they integrate in nondividing cells such as HSCs, thereby avoiding the use of prolonged cytokine stimulation. Human immunodeficiency virus type-1 (HIV-1) has evolved a complex reverse transcription strategy including a central strand displacement event controlled in cis by the central polypurine tract (cPPT) and the central termination sequence (CTS). This creates, at the center of HIV-1 linear DNA molecules, a 99-nucleotide-long plus-strand overlap, the DNA flap, which acts as a cis-determinant of HIV-1 genome nuclear import. The reinsertion of the DNA flap sequence in an HIV-derived lentiviral vector promotes a striking increase of gene transduction efficiency in human CD34+ hematopoietic cells, and the complementation of the nuclear import defect present in the parental vector accounts for this result. In a short ex vivo protocol, the flap-containing vector allows efficient transduction of the whole hierarchy of human HSCs including both slow-dividing or nondividing HSCs that have multiple lymphoid and myeloid potentials and primitive cells with long-term engraftment ability in nonobese diabetic/severe combined immunodeficiency mice (NOD/SCID).

The human immunodeficiency virus type-1 central DNA flap is a crucial determinant for lentiviral vector nuclear import and gene transduction of human hematopoietic stem cells

Gene transfer in human hematopoietic stem cells (HSCs) has great potential for both gene therapy and the understanding of hematopoiesis. As HSCs have extensive proliferative capacities, stable gene transfer should include genomic integration of the transgene. Lentiviral vectors are now preferred to oncoretroviral vectors especially because they integrate in nondividing cells such as HSCs, thereby avoiding the use of prolonged cytokine stimulation. Human immunodeficiency virus type-1 (HIV-1) has evolved a complex reverse transcription strategy including a central strand displacement event controlled in cis by the central polypurine tract (cPPT) and the central termination sequence (CTS). This creates, at the center of HIV-1 linear DNA molecules, a 99-nucleotide-long plus-strand overlap, the DNA flap, which acts as a cis-determinant of HIV-1 genome nuclear import. The reinsertion of the DNA flap sequence in an HIV-derived lentiviral vector promotes a striking increase of gene transduction efficiency in human CD34+ hematopoietic cells, and the complementation of the nuclear import defect present in the parental vector accounts for this result. In a short ex vivo protocol, the flap-containing vector allows efficient transduction of the whole hierarchy of human HSCs including both slow-dividing or nondividing HSCs that have multiple lymphoid and myeloid potentials and primitive cells with long-term engraftment ability in nonobese diabetic/severe combined immunodeficiency mice (NOD/SCID).

Stable in vivo expression of glucose-6-phosphate dehydrogenase (G6PD) and rescue of G6PD deficiency in stem cells by gene transfer

Many mutations of the housekeeping gene encoding glucose-6-phosphate dehydrogenase (G6PD) cause G6PD deficiency in humans. Some underlie severe forms of chronic nonspherocytic hemolytic anemia (CNSHA) for which there is no definitive treatment. By using retroviral vectors pseudotyped with the vesicular stomatitis virus G glycoprotein that harbor the human G6PD (hG6PD) complementary DNA, stable and lifelong expression of hG6PD was obtained in all the hematopoietic tissues of 16 primary bone marrow transplant (BMT) recipient mice and 14 secondary BMT recipients. These findings demonstrate the integration of a functional gene in totipotent stem cells. The average total G6PD in peripheral blood cells of these transplanted mice, measured as enzyme activity, was twice that of untransplanted control mice. This allowed the inference that the amount of G6PD produced by the transduced gene must be therapeutically effective. With the same vectors both the cloning efficiency and the ability to form embryoid bodies were restored in embryonic stem cells, in which the G6PD gene had been inactivated by targeted homologous recombination, thus effectively rescuing their defective phenotype. Finally, expression of normal human G6PD in hG6PD-deficient primary hematopoietic cells and in human hematopoietic cells engrafted in nonobese diabetic/severe combined immunodeficient mice was obtained. This approach could cure severe CNSHA caused by G6PD deficiency.

Stable in vivo expression of glucose-6-phosphate dehydrogenase (G6PD) and rescue of G6PD deficiency in stem cells by gene transfer

Many mutations of the housekeeping gene encoding glucose-6-phosphate dehydrogenase (G6PD) cause G6PD deficiency in humans. Some underlie severe forms of chronic nonspherocytic hemolytic anemia (CNSHA) for which there is no definitive treatment. By using retroviral vectors pseudotyped with the vesicular stomatitis virus G glycoprotein that harbor the human G6PD (hG6PD) complementary DNA, stable and lifelong expression of hG6PD was obtained in all the hematopoietic tissues of 16 primary bone marrow transplant (BMT) recipient mice and 14 secondary BMT recipients. These findings demonstrate the integration of a functional gene in totipotent stem cells. The average total G6PD in peripheral blood cells of these transplanted mice, measured as enzyme activity, was twice that of untransplanted control mice. This allowed the inference that the amount of G6PD produced by the transduced gene must be therapeutically effective. With the same vectors both the cloning efficiency and the ability to form embryoid bodies were restored in embryonic stem cells, in which the G6PD gene had been inactivated by targeted homologous recombination, thus effectively rescuing their defective phenotype. Finally, expression of normal human G6PD in hG6PD-deficient primary hematopoietic cells and in human hematopoietic cells engrafted in nonobese diabetic/severe combined immunodeficient mice was obtained. This approach could cure severe CNSHA caused by G6PD deficiency.

The notch ligand jagged-1 represents a novel growth factor of human hematopoietic stem cells

The Notch ligand, Jagged-1, plays an essential role in tissue formation during embryonic development of primitive organisms. However, little is known regarding the role of Jagged-1 in the regulation of tissue-specific stem cells or its function in humans. Here, we show that uncommitted human hematopoietic cells and cells that comprise the putative blood stem cell microenvironment express Jagged-1 and the Notch receptors. Addition of a soluble form of human Jagged-1 to cultures of purified primitive human blood cells had modest effects in augmenting cytokine-induced proliferation of progenitors. However, intravenous transplantation of cultured cells into immunodeficient mice revealed that human (h)Jagged-1 induces the survival and expansion of human stem cells capable of pluripotent repopulating capacity. Our findings demonstrate that hJagged-1 represents a novel growth factor of human stem cells, thereby providing an opportunity for the clinical utility of Notch ligands in the expansion of primitive cells capable of hematopoietic reconstitution.

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.

Interleukin-3 supports expansion of long-term multilineage repopulating activity after multiple stem cell divisions in vitro

Although long-term repopulating hematopoietic stem cells (HSC) can self-renew and expand extensively in vivo, most efforts at expanding HSC in vitro have proved unsuccessful and have frequently resulted in compromised rather than improved HSC grafts. This has triggered the search for the optimal combination of cytokines for HSC expansion. Through such studies, c-kit ligand (KL), flt3 ligand (FL), thrombopoietin, and IL-11 have emerged as likely positive regulators of HSC self-renewal. In contrast, numerous studies have implicated a unique and potent negative regulatory role of IL-3, suggesting perhaps distinct regulation of HSC fate by different cytokines. However, the interpretations of these findings are complicated by the fact that different cytokines might target distinct subpopulations within the HSC compartment and by the lack of evidence for HSC undergoing self-renewal. Here, in the presence of KL+FL+megakaryocyte growth and development factor (MGDF), which recruits virtually all Lin−Sca-1+kit+ bone marrow cells into proliferation and promotes their self-renewal under serum-free conditions, IL-3 and IL-11 revealed an indistinguishable ability to further enhance proliferation. Surprisingly, and similar to IL-11, IL-3 supported KL+FL+MGDF-induced expansion of multilineage, long-term reconstituting activity in primary and secondary recipients. Furthermore, high-resolution cell division tracking demonstrated that all HSC underwent a minimum of 5 cell divisions, suggesting that long-term repopulating HSC are not compromised by IL-3 stimulation after multiple cell divisions. In striking contrast, the ex vivo expansion of murine HSC in fetal calf serum-containing medium resulted in extensive loss of reconstituting activity, an effect further facilitated by the presence of IL-3.

Identification of novel circulating human embryonic blood stem cells

Using murine models, primitive hematopoietic cells capable of repopulation have been shown to reside in various anatomic locations, including the aortic gonad mesonephros, fetal liver, and bone marrow. These sites are thought to be seeded by stem cells migrating through fetal circulation and would serve as ideal targets for in utero cellular therapy. In humans, however, it is unknown whether similar stem cells exist. Here, we identify circulating hematopoeitic cells present during human in utero development that are capable of multilineage repopulation in immunodeficient NOD/SCID (nonobese diabetic/severe combined immunodeficient) mice. Using limiting dilution analysis, the frequency of these fetal stem cells was found to be 1 in 3.2 × 105, illustrating a 3- and 22-fold enrichment compared with full-term human cord blood and circulating adult mobilized–peripheral blood, respectively. Comparison of in vivo differentiation and proliferative capacity demonstrated that circulating fetal stem cells are intrinsically distinct from hematopoietic stem cells found later in human development and those derived from the fetal liver or fetal bone marrow compartment at equivalent gestation. Taken together, these studies demonstrate the existence of unique circulating stem cells in early human embryonic development that provide a novel and previously unexplored source of pluripotent stem cell targets for cellular and gene-based fetal therapies.

Identification of novel circulating human embryonic blood stem cells

Using murine models, primitive hematopoietic cells capable of repopulation have been shown to reside in various anatomic locations, including the aortic gonad mesonephros, fetal liver, and bone marrow. These sites are thought to be seeded by stem cells migrating through fetal circulation and would serve as ideal targets for in utero cellular therapy. In humans, however, it is unknown whether similar stem cells exist. Here, we identify circulating hematopoeitic cells present during human in utero development that are capable of multilineage repopulation in immunodeficient NOD/SCID (nonobese diabetic/severe combined immunodeficient) mice. Using limiting dilution analysis, the frequency of these fetal stem cells was found to be 1 in 3.2 × 105, illustrating a 3- and 22-fold enrichment compared with full-term human cord blood and circulating adult mobilized–peripheral blood, respectively. Comparison of in vivo differentiation and proliferative capacity demonstrated that circulating fetal stem cells are intrinsically distinct from hematopoietic stem cells found later in human development and those derived from the fetal liver or fetal bone marrow compartment at equivalent gestation. Taken together, these studies demonstrate the existence of unique circulating stem cells in early human embryonic development that provide a novel and previously unexplored source of pluripotent stem cell targets for cellular and gene-based fetal therapies.

Interleukin-3 supports expansion of long-term multilineage repopulating activity after multiple stem cell divisions in vitro

Although long-term repopulating hematopoietic stem cells (HSC) can self-renew and expand extensively in vivo, most efforts at expanding HSC in vitro have proved unsuccessful and have frequently resulted in compromised rather than improved HSC grafts. This has triggered the search for the optimal combination of cytokines for HSC expansion. Through such studies, c-kit ligand (KL), flt3 ligand (FL), thrombopoietin, and IL-11 have emerged as likely positive regulators of HSC self-renewal. In contrast, numerous studies have implicated a unique and potent negative regulatory role of IL-3, suggesting perhaps distinct regulation of HSC fate by different cytokines. However, the interpretations of these findings are complicated by the fact that different cytokines might target distinct subpopulations within the HSC compartment and by the lack of evidence for HSC undergoing self-renewal. Here, in the presence of KL+FL+megakaryocyte growth and development factor (MGDF), which recruits virtually all Lin−Sca-1+kit+ bone marrow cells into proliferation and promotes their self-renewal under serum-free conditions, IL-3 and IL-11 revealed an indistinguishable ability to further enhance proliferation. Surprisingly, and similar to IL-11, IL-3 supported KL+FL+MGDF-induced expansion of multilineage, long-term reconstituting activity in primary and secondary recipients. Furthermore, high-resolution cell division tracking demonstrated that all HSC underwent a minimum of 5 cell divisions, suggesting that long-term repopulating HSC are not compromised by IL-3 stimulation after multiple cell divisions. In striking contrast, the ex vivo expansion of murine HSC in fetal calf serum-containing medium resulted in extensive loss of reconstituting activity, an effect further facilitated by the presence of IL-3.

Cellular and molecular aspects of human CD34+ CD38- precursors: analysis of a primitive hematopoietic population

Hematopoiesis is a complex, highly regulated process in which a small number of primitive stem cells produce all the mature blood and immune cells required by an animal throughout its life. In this review, we summarize current understanding of human CD34+ CD38- cells, a small subclass of hematopoietic cells, which is enriched for primitive precursors, including stem cells. This review emphasizes functional, molecular and immunophenotypic characteristics of the cells and includes some of the factors which are believed to regulate the survival, growth and differentiation of this important class of cells.

Marking and gene expression by a lentivirus vector in transplanted human and nonhuman primate CD34(+) cells

Recently, gene delivery vectors based on human immunodeficiency virus (HIV) have been developed as an alternative mode of gene delivery. These vectors have a number of advantages, particularly in regard to the ability to infect cells which are not actively dividing. However, the use of vectors based on human immunodeficiency virus raises a number of issues, not the least of which is safety; therefore, further characterization of marking and gene expression in different hematopoietic lineages in primate animal model systems is desirable. We use two animal model systems for gene therapy to test the efficiency of transduction and marking, as well as the safety of these vectors. The first utilizes the rhesus animal model for cytokine-mobilized autologous peripheral blood CD34(+) cell transplantation. The second uses the SCID-human (SCID-hu) thymus/liver chimeric graft animal model useful specifically for human T-lymphoid progenitor cell reconstitution. In the rhesus macaques, detectable levels of vector were observed in granulocytes, lymphocytes, monocytes, and, in one animal with the highest levels of marking, erythrocytes and platelets. In transplanted SCID-hu mice, we directly compared marking and gene expression of the lentivirus vector and a murine leukemia virus-derived vector in thymocytes. Marking was observed at comparable levels, but the lentivirus vector bearing an internal cytomegalovirus promoter expressed less efficiently than did the murine retroviral vector expressed from its own long terminal repeats. In assays for infectious HIV type 1 (HIV-1), no replication-competent HIV-1 was detected in either animal model system. Thus, these results indicate that while lentivirus vectors have no apparent deleterious effects and may have advantages over murine retroviral vectors, further study of the requirements for optimal use are warranted.

Expansion of human cord blood CD34+CD38−cells in ex vivo culture during retroviral transduction without a corresponding increase in SCID repopulating cell (SRC) frequency: dissociation of SRC phenotype and function

Current procedures for the genetic manipulation of hematopoietic stem cells are relatively inefficient due, in part, to a poor understanding of the conditions for ex vivo maintenance or expansion of stem cells. We report improvements in the retroviral transduction of human stem cells based on the SCID-repopulating cell (SRC) assay and analysis of Lin− CD34+CD38−cells as a surrogate measure of stem cell function. Based on our earlier study of the conditions required for ex vivo expansion of Lin−CD34+ CD38− cells and SRC, CD34+–enriched lineage–depleted umbilical cord blood cells were cultured for 2 to 6 days on fibronectin fragment in MGIN (MSCV-EGFP-Neo) retroviral supernatant (containing 1.5% fetal bovine serum) and IL-6, SCF, Flt-3 ligand, and G-CSF. Both CD34+CD38− cells (20.8%) and CFC (26.3%) were efficiently marked. When the bone marrow of engrafted NOD/SCID mice was examined, 75% (12/16) contained multilineage (myeloid and B lymphoid) EGFP+ human cells composing as much as 59% of the graft. Half of these mice received a limiting dose of SRC, suggesting that the marked cells were derived from a single transduced SRC. Surprisingly, these culture conditions produced a large expansion (166-fold) of cells with the CD34+CD38− phenotype (n = 20). However, there was no increase in SRC numbers, indicating dissociation between the CD34+CD38− phenotype and SRC function. The underlying mechanism involved apparent downregulation of CD38 expression within a population of cultured CD34+CD38+ cells that no longer contained any SRC function. These results suggest that the relationship between stem cell function and cell surface phenotype may not be reliable for cultured cells. (Blood. 2000;95:102-110)

Expansion of human cord blood CD34+CD38−cells in ex vivo culture during retroviral transduction without a corresponding increase in SCID repopulating cell (SRC) frequency: dissociation of SRC phenotype and function

Current procedures for the genetic manipulation of hematopoietic stem cells are relatively inefficient due, in part, to a poor understanding of the conditions for ex vivo maintenance or expansion of stem cells. We report improvements in the retroviral transduction of human stem cells based on the SCID-repopulating cell (SRC) assay and analysis of Lin− CD34+CD38−cells as a surrogate measure of stem cell function. Based on our earlier study of the conditions required for ex vivo expansion of Lin−CD34+ CD38− cells and SRC, CD34+–enriched lineage–depleted umbilical cord blood cells were cultured for 2 to 6 days on fibronectin fragment in MGIN (MSCV-EGFP-Neo) retroviral supernatant (containing 1.5% fetal bovine serum) and IL-6, SCF, Flt-3 ligand, and G-CSF. Both CD34+CD38− cells (20.8%) and CFC (26.3%) were efficiently marked. When the bone marrow of engrafted NOD/SCID mice was examined, 75% (12/16) contained multilineage (myeloid and B lymphoid) EGFP+ human cells composing as much as 59% of the graft. Half of these mice received a limiting dose of SRC, suggesting that the marked cells were derived from a single transduced SRC. Surprisingly, these culture conditions produced a large expansion (166-fold) of cells with the CD34+CD38− phenotype (n = 20). However, there was no increase in SRC numbers, indicating dissociation between the CD34+CD38− phenotype and SRC function. The underlying mechanism involved apparent downregulation of CD38 expression within a population of cultured CD34+CD38+ cells that no longer contained any SRC function. These results suggest that the relationship between stem cell function and cell surface phenotype may not be reliable for cultured cells. (Blood. 2000;95:102-110)

Distinct Requirements for Optimal Growth and In Vitro Expansion of Human CD34+CD38− Bone Marrow Long-Term Culture-Initiating Cells (LTC-IC), Extended LTC-IC, and Murine In Vivo Long-Term Reconstituting Stem Cells

Recently, primitive human bone marrow (BM) progenitors supporting hematopoiesis in extended (>60 days) long-term BM cultures were identified. Such extended long-term culture-initiating cells (ELTC-IC) are of the CD34+CD38− phenotype, are quiescent, and are difficult to recruit into proliferation, implicating ELTC-IC as the most primitive human progenitor cells detectable in vitro. However, it remains to be established whether ELTC-IC can proliferate and potentially expand in response to early acting cytokines. Here, CD34+CD38− BM ELTC-IC (12-week) were efficiently recruited into proliferation and expanded in vitro in response to early acting cytokines, but conditions for expansion of ELTC-IC activity were distinct from those of traditional (5-week) LTC-IC and murine long-term repopulating cells. Whereas c-kit ligand (KL), interleukin-3 (IL-3), and IL-6 promoted proliferation and maintenance or expansion of murine long-term reconstituting activity and human LTC-IC, they dramatically depleted ELTC-IC activity. In contrast, KL, flt3 ligand (FL), and megakaryocyte growth and development factor (MGDF) (and KL + FL + IL-3) expanded murine long-term reconstituting activity as well as human LTC-IC and ELTC-IC. Expansion of LTC-IC was most optimal after 7 days of culture, whereas optimal expansion of ELTC-IC activity required 12 days, most likely reflecting the delayed recruitment of quiescent CD34+CD38− progenitors. The need for high concentrations of KL, FL, and MGDF (250 ng/mL each) and serum-free conditions was more critical for expansion of ELTC-IC than of LTC-IC. The distinct requirements for expansion of ELTC-IC activity when compared with traditional LTC-IC suggest that the ELTC-IC could prove more reliable as a predictor for true human stem cell activity after in vitro stem cell manipulation.

Distinct Requirements for Optimal Growth and In Vitro Expansion of Human CD34+CD38− Bone Marrow Long-Term Culture-Initiating Cells (LTC-IC), Extended LTC-IC, and Murine In Vivo Long-Term Reconstituting Stem Cells

Recently, primitive human bone marrow (BM) progenitors supporting hematopoiesis in extended (&gt;60 days) long-term BM cultures were identified. Such extended long-term culture-initiating cells (ELTC-IC) are of the CD34+CD38− phenotype, are quiescent, and are difficult to recruit into proliferation, implicating ELTC-IC as the most primitive human progenitor cells detectable in vitro. However, it remains to be established whether ELTC-IC can proliferate and potentially expand in response to early acting cytokines. Here, CD34+CD38− BM ELTC-IC (12-week) were efficiently recruited into proliferation and expanded in vitro in response to early acting cytokines, but conditions for expansion of ELTC-IC activity were distinct from those of traditional (5-week) LTC-IC and murine long-term repopulating cells. Whereas c-kit ligand (KL), interleukin-3 (IL-3), and IL-6 promoted proliferation and maintenance or expansion of murine long-term reconstituting activity and human LTC-IC, they dramatically depleted ELTC-IC activity. In contrast, KL, flt3 ligand (FL), and megakaryocyte growth and development factor (MGDF) (and KL + FL + IL-3) expanded murine long-term reconstituting activity as well as human LTC-IC and ELTC-IC. Expansion of LTC-IC was most optimal after 7 days of culture, whereas optimal expansion of ELTC-IC activity required 12 days, most likely reflecting the delayed recruitment of quiescent CD34+CD38− progenitors. The need for high concentrations of KL, FL, and MGDF (250 ng/mL each) and serum-free conditions was more critical for expansion of ELTC-IC than of LTC-IC. The distinct requirements for expansion of ELTC-IC activity when compared with traditional LTC-IC suggest that the ELTC-IC could prove more reliable as a predictor for true human stem cell activity after in vitro stem cell manipulation.

Genetic Modification of Human B-Cell Development: B-Cell Development Is Inhibited by the Dominant Negative Helix Loop Helix Factor Id3

Transgenic and gene targeted mice have contributed greatly to our understanding of the mechanisms underlying B-cell development. We describe here a model system that allows us to apply molecular genetic techniques to the analysis of human B-cell development. We constructed a retroviral vector with a multiple cloning site connected to a gene encoding green fluorescent protein by an internal ribosomal entry site. Human CD34+CD38− fetal liver cells, cultured overnight in a combination of stem cell factor and interleukin-7 (IL-7), could be transduced with 30% efficiency. We ligated the gene encoding the dominant negative helix loop helix (HLH) factor Id3 that inhibits many enhancing basic HLH transcription factors into this vector. CD34+CD38− FL cells were transduced with Id3-IRES-GFP and cultured with the murine stromal cell line S17. In addition, we cultured the transduced cells in a reaggregate culture system with an SV-transformed human fibroblast cell line (SV19). It was observed that overexpression of Id3 inhibited development of B cells in both culture systems. B-cell development was arrested at a stage before expression of the IL-7R. The development of CD34+CD38− cells into CD14+ myeloid cells in the S17 system was not inhibited by overexpression of Id3. Moreover, Id3+ cells, although inhibited in their B-cell development, were still able to develop into natural killer (NK) cells when cultured in a combination of Flt-3L, IL-7, and IL-15. These findings confirm the essential role of bHLH factors in B-cell development and demonstrate the feasibility of retrovirus-mediated gene transfer as a tool to genetically modify human B-cell development.

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.