Quality of Repopulation in Nonobese Diabetic Severe Combined Immunodeficient Mice Engrafted With Expanded Cord Blood CD34+ Cells

Glucose metabolism, hyperosmotic stress, and reprogramming of somatic cells

The availability of glucose and oxygen are important regulatory elements that help directing stem cell fate. In the undifferentiated state, stem cells, and their artificially reprogrammed equivalent-induced pluripotent stem cells (iPS) are characterized by limited oxidative capacity and active anaerobic glycolysis. Recent studies have shown that pluripotency-a characteristic of staminality-is associated with a poorly developed mitochondrial patrimony, while differentiation is accompanied by an activation of mitochondrial biogenesis. Besides being an important energy source in hypoxia, high glucose level results in hyperosmotic stress. The identification of specific metabolic pathways and biophysical factors that regulate stem cell fate, including high glucose in the extracellular medium, may therefore facilitate reprogramming efficiency and control the differentiation and fate of iPS cells, which are increasingly being explored as therapeutic tools. In this article, we review recent knowledge of the role of glucose metabolism and high glucose level as major anaerobic energy source, and a determinant of osmolarity as possible tools for reprogramming therapies in clinical applications. As in the diabetic setting hyperglycemia negatively affect the stem/progenitor cell fate and likely somatic reprogramming, we also discuss the in vivo potential transferability of the available in vitro findings.

Serial transplantations in nonobese diabetic/severe combined immunodeficiency mice of transduced human CD34+ cord blood cells: efficient oncoretroviral gene transfer and ex vivo expansion under serum-free conditions

Stable oncoretroviral gene transfer into hematopoietic stem cells (HSCs) provides permanent genetic disease correction. It is crucial to transplant enough transduced HSCs to compete with and replace the defective host hemopoiesis. To increase the number of transduced cells, the role of ex vivo expansion was investigated. For a possible clinical application, all experiments were carried out in serum-free media. A low-affinity nerve growth factor receptor (LNGFR) pseudotyped murine retroviral vector was used to transduce cord blood CD34(+) cells, which were then expanded ex vivo. These cells engrafted up to three generations of serially transplanted nonobese diabetic/severe combined immunodeficiency mice: 54.26% +/- 5.59%, 19.05% +/- 2.01%, and 6.15% +/- 5.16% CD45(+) cells from primary, secondary, and tertiary recipient bone marrow, respectively, were LNGFR(+). Repopulation in secondary and tertiary recipients indicates stability of transgene expression and long-term self-renewal potential of transduced HSCs, suggesting that retroviral gene transfer into HSCs, followed by ex vivo expansion, could facilitate long-term engraftment of genetically modified HSCs.

Impaired bone marrow homing of cytokine-activated CD34+ cells in the NOD/SCID model

The reduced engraftment potential of hematopoietic stem/progenitor cells (HSPCs) after exposure to cytokines may be related to the impaired homing ability of actively cycling cells. We tested this hypothesis by quantifying the short-term homing of human adult CD34+ cells in nonobese diabetic/severe combined immunodeficient (NOD/SCID) animals. We show that the loss of engraftment ability of cytokine-activated CD34+ cells is associated with a reduction in homing of colony-forming cells (CFCs) to bone marrow (BM) at 24 hours after transplantation (from median 2.8% [range, 1.9%-6.1%] to 0.3% [0.0%-0.7%]; n = 3; P < .01), coincident with an increase in CFC accumulation in the lungs (P < .01). Impaired BM homing of cytokine-activated cells was not restored by using sorted cells in G0G1 or by inducing cell cycle arrest at the G1/S border. Blocking Fas ligation in vivo did not increase the BM homing of cultured cells. Finally, we tested cytokine combinations or culture conditions previously reported to restore the engraftment of cultured cells but did not find that any of these was able to reverse the changes in homing behavior of cytokine-exposed cells. We suggest that these changes in homing and, as a consequence, engraftment result from the increased migratory capacity of infused activated cells, leading to the loss of selectivity of the homing process.

Distinct hematopoietic stem/progenitor cell populations are responsible for repopulating NOD/SCID mice compared with nonhuman primates

The nonobese diabetic/severe combined immune-deficient (NOD/SCID) mouse xenotransplantation assay is the most commonly used surrogate assay for the study of human candidate stem cells. In contrast to large animal and human studies, however, it is limited by the short life span of the recipients, the limited proliferative demand placed on the transplanted cells, and the inability to support differentiation into all hematopoietic lineages. In the present study, we directly compared hematopoietic repopulation in NOD/SCID mice with autologous reconstitution in the baboon, a well-established preclinical large animal model for stem cell transplantation. Baboon CD34-enriched marrow cells were retrovirally marked and infused into the irradiated baboon and the NOD/SCID mice. Although the percentage of gene-marked cells was high and remained stable in NOD/SCID mice up to 12 weeks and in those that underwent secondary transplantation, we observed a considerable decline and overall a significantly (10-fold) lower percentage of gene-marked cells in the baboons. In addition, clonal integration site analysis revealed common proviral vector integrants in NOD/SCID repopulating cells and in the baboon at 6 weeks but not at 6 months after transplantation. These results suggest that distinct hematopoietic stem/progenitor cells are responsible for hematopoietic reconstitution in NOD/SCID mice compared with nonhuman primates.

Evaluation of the effect of cryopreservation on ex vivo expansion of hematopoietic progenitors from cord blood

In previous studies, we identified a cytokine cocktail including thrombopoietin, Flt-3 ligand, interleukin (IL)-6 and IL-11 in serum-free medium, suitable to induce significant and sustained ex vivo expansion of primitive hematopoietic stem cells (HSCs) from cord blood (CB) for up to 10 weeks. The aim of the present study was to evaluate the effects of cryopreservation on ex vivo expansion of HSCs and their committed progenitors. CD34+ cells were purified from CB units, each of which was processed in part as such and in part as cryopreserved and thawed, then expanded for 5 weeks in serum-free medium with the cytokine cocktail described above. We determined the number of nucleated cells (NC), CD34+, CD34+/38(-)/33(-), CD34+/61+, CD61+ cells and the clonogenic potential. After 2 weeks the median fold expansion of NC, CD34+ and CD34+/38(-)/33(-) cells was around two log both with fresh and cryopreserved CB and the expansion continued similarly until week 5. Our data suggest that this serum free protocol induces similar ex vivo expansion of HSCs and their committed progenitors from both fresh and cryopreserved CB. Our findings can be useful in view of clinical applications, since CB used for transplantation is stored in the cryopreserved state.

Variables to predict engraftment of umbilical cord blood into immunodeficient mice: usefulness of the non-obese diabetic--severe combined immunodeficient assay

Umbilical cord blood is an alternative stem cell source for patients without matched family donors. In this study, we examined several parameters that have not been studied in detail -- radiation dose, cell dose, age of mice, and maternal and neonatal characteristics of the cord blood donor -- that affect engraftment of cord blood in non-obese diabetic-severe combined immunodeficient (NOD--scid) mice. Engraftment, measured using flow cytometry analyses of human CD45(+) cells, was highest in 400 cGy-treated mice. Successful engraftment was demonstrated up to 6 months, with a mean engraftment of 31% (range 0--67%) of human cells in recipient bone marrow. Engraftment was skewed to B lymphocytes. The radiation dose of 350 cGy resulted in superior survival of the murine recipients compared with 400 cGy (P = 0.03). The sex of the NOD--scid recipients had a significant effect on survival (female superior to male, P = 0.01), but not on engraftment. There were high levels of variability among different cord units and among animals injected with the same cord unit. This variability may limit the clinical usefulness of the NOD--scid mice as hosts for the quantification of human stem cells.

Experimental culture conditions are critical for ex vivo expansion of hematopoietic cells

The ex vivo expansion of hematopoietic stem cells (HSC) for clinical use is now recognized to be a feasible and very promising approach for hematotherapy. Expansion of specific HSC subsets is required for different clinical applications, for example, to increase the number of mature cells, to produce specific cells for adoptive therapy, or to increase the number of primitive stem cells available for engraftment. Although hematopoietic growth factors can play an important role in this setting, in this review we emphasize that other variables affect the outcome of stem and progenitor cell expansion. These variables include the serum supplement, the purity of CD34(+) cells, the initial cell concentration, and the duration of culture. It is also essential to define standard culture conditions for normal stem cells and to limit or prevent expansion of residual tumor cells. In clinical applications, determination of the hematopoietic value of the expanded population is mandatory. Thus, we have to demonstrate the expansion of primitive hematopoietic progenitor and stem cells, with maintenance of their hematopoietic potential as assessed by in vitro or in vivo assays. We draw attention to the challenges in the clinical application of ex vivo expansion. These include the establishment of well-defined experimental conditions and the determination of the hematopoietic value of the expanded grafts, whatever the graft source: bone marrow, mobilized peripheral blood, or cord blood. Future studies hopefully will optimize these procedures and allow not only expansion but engineering of defined cellular functions as HSCs grow under defined conditions.

Long-term expansion and maintenance of cord blood haematopoietic stem cells using thrombopoietin, Flt3-ligand, interleukin (IL)-6 and IL-11 in a serum-free and stroma-free culture system

Although cord blood (CB) compares favourably with other haematopoietic stem cell (HSCs) sources, its use in large patients is limited by the low number of cells available. Ex vivo expansion of CB HSCs has been used to overcome this limitation. In this study, we investigated the effect of different cytokine cocktails, including interleukin (IL)-6, IL-11, Flt3-ligand (FL) and thrombopoietin (TPO) combined with serum or serum-free medium on the ex vivo expansion of CD34+ cells from CB. Initial experiments showed that expansion could be slightly improved using serum, but we chose to use serum-free medium in the subsequent investigations to apply good medical practice (GMP) conditions suitable for clinical use. The highest expansion of CD34+ cells was obtained with a cocktail containing FL + TPO + IL-6 + IL-11. The median (range) fold expansions of CD34+ cells at 5 and 10 weeks with serum-free medium were 235.6 (131.3-340) and 5205.6 (4736.6-5674.7) respectively. The absence of IL-11 was associated with a similar fold expansion after 5 weeks (median 215.6, range 149.8-281.5), but after 10 weeks expansion was slightly lower (median 1314.7, range 645-1984.4). Our data support the possibility of maintaining long-term expansion of CB HSCs in a simple stroma- and serum-free system.

Induction of the expression of SCF in mouse by lethal irradiation

To clarify what kinds of cytokines are actually contributing to proliferation of hemopoietic stem cells in vivo after lethal irradiation, we have investigated the expression of some cytokines by RT-PCR method. Above all, expression of the SCF was increased significantly in the bone marrow cells soon after lethal irradiation in both the Sca-1 (+) bone marrow cells injected and non-injected mice. The day 6 serum from the lethally irradiated mice could support the proliferation of the Sca-1 (+) bone marrow cells, even though the serum from normal mice could not. The quantification analyses have revealed the increase of the amounts of IL-6 and flt3-ligand in their serum, but not significant increase of the amount of SCF. Precise PCR analysis has revealed that the cell surface associated form of SCF was significantly induced in the bone marrow after lethal irradiation. These data indicate that the cell surface form of SCF mainly promotes the proliferation of hemopoietic stem cells with some soluble cytokines under sever lack of hemopoietic stem cells in vivo caused by lethal irradiation and also suggest the importance of direct cell-to-cell interaction on proliferation of hematopoietic stem cells in vivo.

Cocultivation of umbilical cord blood cells with endothelial cells leads to extensive amplification of competent CD34+CD38- cells

OBJECTIVE

In this report, methods to expand the number of human cord blood hematopoietic stem cells were explored.

MATERIALS AND METHODS

CD34+ cord blood cells were expanded in the presence of various cytokine combinations in either a stroma-free cell culture system or a preformed porcine microvascular endothelial cell layer. After 7 to 21 days, stem cell number and function were monitored. In addition, the replicative history of stem cells was tracked using the fluorescent dye, PKH26.

RESULTS

With the addition of various cytokine combinations, total cellular expansion was equivalent for both culture systems, although the endothelial cell-based system contained statistically greater numbers of CD34+ cells. By day 21, the endothelial-based system receiving the FLT3L, SCF, IL-6, and GM-CSF cytokine combination contained five-fold greater numbers of CD34+ than the stroma cell-free culture cell system. Endothelial-based cultures receiving these four cytokines plus megakaryocyte growth and development factor produced a 640-fold expansion of CD34+CD38- cells as compared to a four-fold expansion in the stroma-free system. The number of progenitor cells generated was similar with both systems, yet the greatest degree of expansion of cobblestone area-forming cells was observed in the endothelial based cultures (11-fold vs four-fold). Virtually all CD34+ and CD34+CD38+ cells expanded in the presence of endothelial cells had undergone self replication by day 10, yet stromal cell-free cultures contained a significant number (4.8%) of quiescent cells. Identical numbers of re-isolated cord blood CD34+ cells expanded in both systems exhibited a similar ability to engraft and generate cells belonging to multiple hematopoietic lineages in human fetal bones implanted in immunodeficient mice.

CONCLUSIONS

These results suggest that the use of preformed endothelial cell monolayers might permit the ex vivo generation of sufficient numbers of cord stem cells to serve as successful grafts for adult transplant recipients.

A defined window for efficient gene marking of severe combined immunodeficient-repopulating cells using a gibbon ape leukemia virus-pseudotyped retroviral vector

We have investigated the minimal time required for efficient transduction of human hematopoietic repopulating cells using a surrogate nonobese diabetic (NOD)/severe combined immunodeficient (SCID) xenoengraftment assay. Cord blood CD34+ cells were transduced to high levels over 24-48 hr in the presence of Flt-3 ligand, stem cell factor, interleukin 3, and interleukin 6. Under these conditions, high levels of NOD/SCID repopulating activity were preserved, but the levels of gene marking in engrafting cell populations measured by expression of a reporter transgene were low. Extension of the transduction period by 24 hr (total culture period, 72 hr) under the same cytokine conditions resulted in high levels of gene marking, but on closer analysis expression was limited predominantly to the myeloid population. Efficient transduction of both lymphoid and myeloid lineages could be achieved only if the transduction protocol was extended by a further 24 hr (total culture period, 96 hr), suggesting that myeloid lineage-committed precursors are capable of repopulation, and that over shorter time periods transduction is largely restricted to this population. This adds to the emerging evidence of heterogeneity within the SRC compartment, and has important implications for the interpretation of this assay in stem cell transplantation and gene transfer studies.