CD34+ cells from mobilized peripheral blood retain fetal bone marrow repopulating capacity within the Thy-1+ subset following cell division ex vivo

Chromatin-modifying agents permit human hematopoietic stem cells to undergo multiple cell divisions while retaining their repopulating potential

Human hematopoietic stem cells (HSCs) exposed to cytokines in vitro rapidly divide and lose their characteristic functional properties presumably due to the alteration of a genetic program that determines the properties of an HSC. We have attempted to reverse the silencing of this HSC genetic program by the sequential treatment of human cord blood CD34+ cells with the chromatin-modifying agents, 5-aza-2′-deoxycytidine (5azaD) and trichostatin A (TSA). We determined that all CD34+CD90+ cells treated with 5azaD/TSA and cytokines after 9 days of incubation divide, but to a lesser degree than cells exposed to only cytokines. When CD34+CD90+ cells that have undergone extensive number of cell divisions (5-10) in the presence of cytokines alone were transplanted into immunodeficient mice, donor cell chimerism was not detectable. By contrast, 5azaD/TSA-treated cells that have undergone similar numbers of cell divisions retained their marrow repopulating potential. The expression of several genes and their products previously implicated in HSC self-renewal were up-regulated in the cells treated with 5azaD/TSA as compared to cells exposed to cytokines alone. These data indicate that HSC treated with chromatin-modifying agents are capable of undergoing repeated cell divisions in vitro while retaining their marrow-repopulating potential.

Inhibitors of histone deacetylases promote hematopoietic stem cell self-renewal

BACKGROUND

Histone deacetylases (HDAC) are associated with a variety of transcriptional repressors that control cellular differentiation and proliferation. HDAC inhibitors such as trichostatin A, trapoxin and chlamydocin could be useful tools to modulate these cellular processes. We investigated their effect on the self-renewal of hematopoietic stem cells (HSC) during ex vivo culture.

METHODS

Purified murine HSC with the phenotype c-Kit+,Thy-1.1(lo), Lin(-/lo), Sca-1+ were cultured for 4 days with IL-3, IL-6 and c-Kit ligand without or with HDAC inhibitors, after which their degree of phenotypic differentiation in culture was assessed by flow cytometric analysis. To explore whether HDAC inhibitors could have a beneficial role in human HSC transplantation, mobilized peripheral blood CD34+ cells were cultured with thrombopoietin mimetic peptide, flt3 ligand, and c-Kit ligand, without or with various HDAC inhibitors. The fluorescent dye, carboxyfluorescein-diacetate succinimidylester (CFSE), was used to track division of cell subsets, and engrafting ability was evaluated in a non-obese diabetic (NOD) -SCID xenotransplantation model.

RESULTS

Murine HSC cultured with HDAC inhibitors maintained a more primitive phenotype than control cultures. The number of human HSC expressing Thy-1 increased up to seven-fold during a 5-day culture with HDAC inhibitors compared with control cultures. Chlamydocin was the most effective of the HDAC inhibitors tested at promoting Thy-1 expression on human cells. CFSE tracking showed that the increase in Thy-1+ cells resulted from cell division. In a NOD-SCID repopulation assay, cells exposed to chlamydocin for 24 h displayed an average four-fold higher engrafting ability over control cells.

DISCUSSION

Our studies suggest that HDAC inhibitors can induce ex vivo expansion of human HSC, and may improve engraftment in hematopoietic transplant patients when cell dose is limiting.

Relationship between Thy-1 expression and cell-cycle distribution in human bone marrow hematopoietic progenitors

Analysis of the relationship between Thy-1 expression and cell-cycle distribution of hematopoietic stem cells (HSCs) showed that freshly isolated Thy-1+ and Thy-1- subsets of the CD34highCD38-flt-3-Lin- population were predominantly in G0/G1 phase and remained essentially quiescent, whereas after 6 days of cytokine stimulation, the Thy-1+ subset of the population entered the cycling state while the Thy-1- subset again remained quiescent. Expression of Thy-1 antigen resulted in a drastic increase in the percentage of cycling cells in CD34highCD38-flt-3-Lin-Thy-1+- as well as CD34highCD38-flt-3-Lin- Thy-1(-)-cell-initiated cultures. The Thy-1+ subset of the CD34highCD38-flt-3-Lin- population exists in the freshly isolated CD34highCD38-flt-3-Lin- Thy-1+ fraction, loses Thy-1 expression during 6 days, and re-expresses Thy-1 for an additional 2 days. Cell-cycle analysis demonstrated that this unique subset contains abundant S/G2M cells. Thus, Thy-1 expression appears to be an indicator of cell-cycle phase in targeting HSC, which might serve in the cell subset best suited for gene transfer.

Peripheral blood and BM CD34+ CD38− cells show better resistance to cryopreservation than CD34+ CD38+ cells in autologous stem cell transplantation

BACKGROUND

We and others have shown a critical role for CD34+ CD38- cells in hematopoietic recovery after autologous stem cell transplantation (ASCT), in particular for platelet reconstitution. Thus a routine assessment of CD34+ CD38- cells in freezing-thawing procedures for autografting could represent an important tool for predicting poor engraftment.

METHODS

To compare the impact of cryopreservation on CD34+ CD38+ and CD34+ CD38- hematopoietic stem cell subsets, 193 autograft products collected in 84 patients with malignancies were assessed before controlled-rate cryopreservation in 10% DMSO and after thawing for autografting.

RESULTS

Cell counts after thawing were significantly different from the pre-freezing counts for total CD34+ (P<0.0001) and CD34+ CD38+ (P<0.0001) cells, but not for CD34+ CD38- cells (P=0.252). Median losses for CD34+, CD34+ CD38+ and CD34+ CD38- cells were, respectively, 11.8%, 11.4% and 0.0%. The magnitude of fresh/post-thawing percentage cell variation was significantly different when comparing between the CD34+ CD38+ and CD34+ CD38- cell subsets (P<0.001). Moreover, CD34+ CD38- cells exhibited recovery values > or =100% in 85/160 graft products, compared with 51/193 in CD34+ CD38+ cells (P<0.0001). Also, recovery values > or =90% were significantly better in the CD34+ CD38- (98/160 grafts) than in the CD34+ CD38+ subsets (89/193 grafts) (P<0.01).

DISCUSSION

In this work we have demonstrated that CD34+ cells that do not express the CD38 Ag show a significantly better resistance to cryopreservation. This could represent another example of the particular ability of less committed progenitor cells to overcome environmental injuries. Moreover, we consider routine assessment of CD34+ CD38- cells before freezing as clinically relevant, but post-thawing controls may be avoided because of their good resistance to freezing.

Ex vivo expansion of apheresis-derived peripheral blood hematopoietic progenitors

Because the administration of hematopoietic growth factors and the use of stem cell support often fails to alleviate the neutropenic phase induced by cytotoxic drugs, several investigators have attempted to expand ex vivo hematopoietic progenitors for clinical use. These attempts have clearly shown that the cultured cells are functional and can be safely administered to patients, but that the in vivo performance is disappointing and the concept as a whole is not yet clinically useful. The major reasons for these unsuccessful attempts are thought to be cumbersome cell fractionation techniques, contamination, prolonged incubation, and the use of less than ideal cytokine combinations. In response, we have developed a simple procedure for ex vivo expansion of myeloid progenitor cells. In this assay, unfractionated mononuclear cells from apheresis donors are incubated in nonpyrogenic plastic bags for 7 days in the presence of culture medium either containing fetal calf serum or human plasma, granulocyte colony-stimulating factor, and stem cell factor. We have demonstrated that under these conditions the number of colony-forming units (CFU) granulocyte-macrophage (CFU-GM) and of CFU-granulocyte-macrophage-erythroid-megakaryocyte (CFU-GEMM) increased 7- and 9-fold, respectively, by day 7 and the number of burst-forming units-erythroid (BFU-E) increased 2.7-fold by day 5 of culture. Significant increases in the numbers of cells expressing CD34+, CD34+/CD38+, CD34+/CD33+, CD34+/CD15+, and CD34+/CD90+ and significant declines in the numbers of cells expressing CD34+/CD38- and CD19 surface antigens were also observed. The relative numbers of cells expressing T-cell markers and CD56 surface antigen did not change. By using different concentrations of various hematopoietic growth factor combinations, we can increase the number of mature and immature cells of different hematopoietic lineages.

Investigation into an engraftment defect induced by culturing primitive hematopoietic cells with cytokines

BACKGROUND

Strategies for transplanting primitive hematopoietic progenitor (PHP) cells are under development that require in vitro manipulation of cells for several hours to several days prior to transplantation. This applies to gene-therapy protocols involving transduction with adenoviral or lentiviral vectors (typically 1 day of ex vivo culture) or retroviral vectors (up to 3 days of culture).

METHODS

Human mobilized peripheral blood (MPB) CD34(+) cells were cultured with the cytokines thrombopoietin mimetic peptide (mTPO), flt3 ligand (FL), and c-kit ligand (KL). Equal numbers of CD34(+) cells, either uncultured or cultured for various time periods up to 5 days, were tested for engraftment in sublethally irradiated 8-10 week-old NOD/SCID mice. Cells were also compared for expression and function of several key surface molecules.

RESULTS

At a limiting dose of 1 million cells, mice receiving uncultured cells had a mean of 20% CD45(+) (human) cells in their BM 6 weeks post-transplantation, versus 3% for mice receiving 3-5 day cultured cells. Analysis of 10 surface molecules, CD11a, CD18, CD29, CD49d, CD49e, CXCR-4, CD62L, CD31, CD43, and CD44 over a 5-day culture period showed that their expression levels were either maintained or up-regulated on CD34(+) cells and the primitive Thy-1(+) subset. Similar percentages of uncultured and 3-day cultured MPB CD34(+) cells bound to plates coated with vascular cell adhesion molecule-1 (VCAM-1) under both static and physiological flow conditions, and chemotaxis of cultured cells towards stromal-derived factor-1 (SDF-1) was not impaired, suggesting that VLA-4 and CXCR-4 were functional on cultured cells. CD34(+) Thy-1(+) MPB cells cultured with cytokines expressed increasing levels of Fas receptor beginning at 20 h in culture, with peak expression levels after 3 days (mean Day 0 expression, 39%; mean Day 3 expression, 86%), without increased apoptosis. Including inhibitors of caspases in the media of cells cultured for 24-48 h significantly improved their engraftment in a SCID-hu bone-engraftment model.

DISCUSSION

Increased susceptibility to apoptosis upon in vivo injection may contribute to impaired engraftment of in vitro manipulated cells. Inhibitors of apoptosis may increase their engrafting capacity in clinical settings.

Human hematopoietic lineage commitment

The ultimate goal of developmental immunology is to understand the normal processes that give rise to the immune system in order to diagnose and develop effective treatments for diseases that occur as a consequence of immune system defects. Central to achieving this goal is understanding the complex interplay between microenvironmental signals and transcription factors that direct human hematopoietic differentiation and lineage commitment. The ability to isolate highly purified populations of human hematopoietic cells at critical points in differentiation make it possible to answer very specific questions about the hematopoietic process and lineage restriction. This review describes the use of surface immunophenotypes to identify human hematopoietic cells at particular points in differentiation or with particular patterns of lineage restriction. Culture models are discussed in the context of the ability to detect, characterize and determine the lineage potential of human hematopoietic stem cells and progenitors. Variations in hematopoeises that correspond to ontogeny will be examined. Potential roles for the HOX and Ikaros proteins in human lineage commitment will be considered. Also included will be discussion of a number of factors that provide challenges to experimental design, to experimental interpretation, and to the development of a comprehensive model of human hematopoiesis.

Ex vivo treatment of proliferating human cord blood stem cells with stroma-derived factor-1 enhances their ability to engraft NOD/SCID mice

Ex vivo proliferation of hematopoietic stem cells (HSCs) is important for cellular and gene therapy but is limited by the observation that HSCs do not engraft as they transit S/G(2)/M. Recently identified candidate inhibitors of human HSC cycling are transforming growth factor-beta(1) (TGF-beta(1)) and stroma-derived factor-1 (SDF-1). To determine the ability of these factors to alter the transplantability of human HSCs proliferating in vitro, lin(-) cord blood cells were first cultured for 96 hours in serum-free medium containing Flt3 ligand, Steel factor, interleukin-3, interleukin-6, and granulocyte colony-stimulating factor. These cells were then transferred to medium containing Steel factor and thrombopoietin with or without SDF-1 and/or TGF-beta(1) for 48 hours. Exposure to SDF-1 but not TGF-beta(1) significantly increased (> 2-fold) the recovery of HSCs able to repopulate nonobese diabetic/severe combined immunodeficiency mice. These results suggest new strategies for improving the engraftment activity of HSCs stimulated to proliferate ex vivo.

Early hematopoietic reconstitution after clinical stem cell transplantation: evidence for stochastic stem cell behavior and limited acceleration in telomere loss

Our inability to purify hematopoietic stem cells (HSCs) precludes direct study of many aspects of their behavior in the clinical hematopoietic stem cell transplantation (HSCT) setting. We indirectly assessed stem/progenitor cell behavior in the first year after HSCT by examining changes in neutrophil telomere length, X-inactivation ratios, and cycling of marrow progenitors in 25 fully engrafted allogeneic HSCT recipients. Donors were sampled once and recipients at engraftment and 2 to 6 months and 12 months after HSCT. Telomere length was measured by an in-gel hybridization technique, X-inactivation ratios were measured by the human androgen receptor assay, and cell cycle status was determined by flow cytometric analysis of pyronin Y- and Hoechst 33342-stained CD34(+)CD90(+) and CD34(+)CD90(-) marrow cells. Compared with their donors, recipients' telomeres were shortened at engraftment (-424 base pairs [bp]; P <.0001), 6 months (-495 bp; P =.0001) after HSCT, and 12 months after HSCT (-565 bp; P <.0001). There was no consistent pattern of change in telomere length from 1 to 12 months after HSCT; marked, seemingly random, fluctuations were common. In 11 of 11 informative recipients, donor X-inactivation ratios were faithfully reproduced and maintained. The proportion of CD34(+)CD90(+) progenitors in S/G(2)/M was 4.3% in donors, 15.7% at 2 to 6 months (P <.0001) after HSCT, and 11.5% at 12 months after HSCT (P <.0001, versus donors; P =.04, versus 2-6 months). Cycling of CD34(+) CD90(-) progenitors was largely unchanged. We infer that (1) HSCT-induced accelerated telomere loss is temporary and unlikely to promote graft failure or clonal hematopoietic disorders and (2) the striking fluctuations in telomere length and variation in pattern of telomere loss reflect stochastic determination of HSC fate after HSCT.

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.

Dissociation between stem cell phenotype and NOD/SCID repopulating activity in human peripheral blood CD34(+) cells after ex vivo expansion

OBJECTIVE

The relationship between phenotype and function in ex vivo-cultured human hematopoietic stem cells (HSC) remains poorly understood. We investigated the effects of a short-term serum-free culture on the relationship between stem cell phenotype, cell division history, and function in human CD34(+) cells.

METHODS

G-CSF-mobilized peripheral CD34(+) cells were cultured for 4 days with stem cell factor, flt-3 ligand, and thrombopoietin. The phenotype (CD34, CD38, HLA-DR, c-kit), cell division history, colony-forming cell (CFC), long-term culture-initiating cell (LTC-IC), and NOD/SCID repopulating activities were evaluated at Day 0 and 4.

RESULTS

We observed a loss of CD38, HLA-DR, and c-kit surface expression resulting in a drastic increase in CD34(+)CD38(-), CD34(+)HLA-DR(-), and CD34(+)c-kit(-/low) cells at Day 4. In contrast, the frequency of Thy-1(+) cells was maintained. We observed a 1.3-fold expansion of CFC, a 4.8-fold increase in LTC-IC, and an overall maintenance of the NOD/SCID repopulating cell activity. CD34(+)CD38(-) and CD34(+)HLA-DR(-) cells detected at Day 4 displayed the most active pattern of division (4 to 5 divisions) whereas 60% of CD34(+)Thy-1(+) cells divided 0 to 2 times during the same period. At Day 4, the NOD/SCID repopulating activity was associated with Thy-1(+) cells with no more than 2 divisions.

CONCLUSIONS

Our results show that the relationship between stem cell phenotype and function is dramatically altered in cultured CD34(+) cells. Thy-1 expression and cell division history appear to be superior to CD38, HLA-DR, and c-kit, or to homing molecules (CXCR4, VLA-4) as predictors of the repopulating activity of cultured peripheral CD34(+) cells.

Stromal cells retard the differentiation of CD34(+)CD38(low/neg) human primitive progenitors exposed to cytokines independent of their mitotic history

Stem cell proliferation induced by potent cytokines usually leads to a loss of primitive potential through differentiation. In this study, the ability of cytokines and murine MS5 stromal cells to independently regulate the proliferation and long-term culture-initiating cell (LTC-IC) activity of primitive CD34(+)CD38(low/neg) human bone marrow cells was evaluated. To compare populations with identical proliferation histories, cells were labeled with carboxy fluorescein diacetate succinimidyl ester, and LTC-IC activity was assessed 4 days later in cells that had accomplished the same number of divisions with or without MS5 cells. MS5 cells counteracted dramatically the loss of LTC-IC activity observed in the presence of cytokines alone. Thus, in the presence of MS5 cells, means of 1233 (n = 5) and 355 (n = 9) LTC-IC-derived colony-forming cells (CFCs) were generated by 1000 cells that performed 3 and 4 divisions respectively, whereas 311 (n = 5) and 64 (n = 5) CFCs were generated by 1000 cells cultured without MS5 cells. Interestingly, MS5 cells had no detectable effect on the LTC-IC activity of cells that divided only twice in 4 days-1606 CFCs (n = 6) and 1993 (n = 6) CFCs, respectively, without and with MS5 cells-and a 48 additional hours of coculture were necessary to unmask changes in the LTC-IC activity mediated by stromal cells. These results indicate that cytokines and stroma-derived signals can regulate independently the proliferation and differentiation of primitive cells and that these stroma-derived extracellular factors act directly on their target cells.

Hematopoietic stem cells can be CD34+ or CD34-

Until recently, it was thought that the most primitive HSC have a fixed phenotype within a hierarchical differentiation system, and that changes in engraftment and renewal potential occur in a stepwise fashion linked with differentiation. In this review, we summarize the data from several different species and different animal models of hematopoietic stem cell function. Taking into account all of the published data, it becomes clear that the hematopoietic stem cell compartment contains more than one phenotypically identifiable population capable of self-renewal and long term pluripotent engraftment. It is clear that some stem cells express CD34, and others do not. The exact phenotypic progression between these cells needs to be further defined, because different in vivo and ex vivo manipulations may shift the stem cells from one phenotype to another, and this can complicate interpretation of experimental transplant data.

Human hematopoietic stem cells stimulated to proliferate in vitro lose engraftment potential during their S/G2/M transit and do not reenter G0

An understanding of mechanisms regulating hematopoietic stem cell engraftment is of pivotal importance to the clinical use of cultured and genetically modified transplants. Human cord blood (CB) cells with lymphomyeloid repopulating activity in NOD/SCID mice were recently shown to undergo multiple self-renewal divisions within 6 days in serum-free cultures containing Flt3-ligand, Steel factor, interleukin 3 (IL-3), IL-6, and granulocyte colony-stimulating factor. The present study shows that, on the fifth day, the transplantable stem cell activity is restricted to the G1fraction, even though both colony-forming cells (CFCs) and long-term culture-initiating cells (LTC-ICs) in the same cultures are approximately equally distributed between G0/G1and S/G2/M. Interestingly, the G0 cells defined by their low levels of Hoechst 33342 and Pyronin Y staining, and reduced Ki67 and cyclin D expression (representing 21% of the cultured CB population) include some mature erythroid CFCs but very few primitive CFCs, LTC-ICs, or repopulating cells. Although these findings suggest a cell cycle–associated change in in vivo stem cell homing, the cultured G0/G1 and S/G2/M CD34+ CB cells exhibited no differences in levels of expression of VLA-4, VLA-5, or CXCR-4. Moreover, further incubation of these cells for 1 day in the presence of a concentration of transforming growth factor β1 that increased the G0/G1 fraction did not enhance detection of repopulating cells. The demonstration of a cell cycle–associated mechanism that selectively silences the transplantability of proliferating human hematopoietic stem cells poses both challenges and opportunities for the future improvement of ex vivo–manipulated grafts.

Human hematopoietic stem cells stimulated to proliferate in vitro lose engraftment potential during their S/G2/M transit and do not reenter G0

An understanding of mechanisms regulating hematopoietic stem cell engraftment is of pivotal importance to the clinical use of cultured and genetically modified transplants. Human cord blood (CB) cells with lymphomyeloid repopulating activity in NOD/SCID mice were recently shown to undergo multiple self-renewal divisions within 6 days in serum-free cultures containing Flt3-ligand, Steel factor, interleukin 3 (IL-3), IL-6, and granulocyte colony-stimulating factor. The present study shows that, on the fifth day, the transplantable stem cell activity is restricted to the G1fraction, even though both colony-forming cells (CFCs) and long-term culture-initiating cells (LTC-ICs) in the same cultures are approximately equally distributed between G0/G1and S/G2/M. Interestingly, the G0 cells defined by their low levels of Hoechst 33342 and Pyronin Y staining, and reduced Ki67 and cyclin D expression (representing 21% of the cultured CB population) include some mature erythroid CFCs but very few primitive CFCs, LTC-ICs, or repopulating cells. Although these findings suggest a cell cycle–associated change in in vivo stem cell homing, the cultured G0/G1 and S/G2/M CD34+ CB cells exhibited no differences in levels of expression of VLA-4, VLA-5, or CXCR-4. Moreover, further incubation of these cells for 1 day in the presence of a concentration of transforming growth factor β1 that increased the G0/G1 fraction did not enhance detection of repopulating cells. The demonstration of a cell cycle–associated mechanism that selectively silences the transplantability of proliferating human hematopoietic stem cells poses both challenges and opportunities for the future improvement of ex vivo–manipulated grafts.

Primitive human HPCs are better maintained and expanded in vitro at 1 percent oxygen than at 20 percent

BACKGROUND

The liquid culture of murine bone marrow cells at 1-percent oxygen maintains the balance between primative progenitor cell renewal and clonogenic progenitor expansion better than that at 20-percent oxygen. These results are of potential interest for the ex vivo expansion of human progenitor cells, as low O(2) tension could preserve the engraftment potential of cultured apheresis products.

STUDY DESIGN AND METHODS

G-CSF-mobilized blood cells collected by apheresis, now the main source of progenitor cells for autologous transplantation, were cultured at 1-percent and 20-percent O(2) for 7 days in serum-free liquid cultures in the presence of IL-3 and SCF (5 ng/mL). The growth of the clonogenic progenitors (CFU-GM, BFU-E, CFU-Mix) and of the more primitive human HPCs that are capable of generating clongenic progenitors in secondary liquid culture, as well as the proliferation and differentiation of total and CD34+ cells, was analyzed.

RESULTS

The expansion of CD34+ cells and of clonogenic progenitors was significantly lower in liquid cultures at 1-percent O(2) than at 20-percent O(2). On the contrary, the primitive human HPCs were better maintained and expanded at 1-percent O(2), although the number of CD34+ cells remaining quiescent was lower. After 7 days of liquid culture at 1-percent or 20-percent O(2) the percentage of CD34+ cells was similar. However, the CD34+ cells that divided more than four times (PKH2 staining) were more numerous in liquid cultures incubated at 1-percent O(2).

CONCLUSION

When cultured at 1-percent O(2) for 7 days in presence of IL-3 and SCF, the CD34+ cells present in apheresis components underwent more cell divisions and better maintained their primitive progenitor cell potential. As suggested by previous results in mice, our data on human cells emphasize the potential interest of cultures at low O(2) tension (1%) for cell therapy protocols aimed at expanding primitive HPCs in autografts.

Stimulation of mouse bone marrow cells with kit ligand, FLT3 ligand, and thrombopoietin leads to efficient retrovirus-mediated gene transfer to stem cells, whereas interleukin 3 and interleukin 11 reduce transduction of short- and long-term repopulating cells

The effects of cytokine stimulation during retroviral transduction on in vivo reconstitution of mouse hematopoietic stem cells was tested in a murine competitive repopulation assay with alpha-thalassemia as a marker to distinguish donor and recipient red blood cells (RBCs) and the enhanced green fluorescent protein (EGFP) as a marker for gene transfer. After transplantation, EGFP was detected in up to 90% of circulating RBCs, platelets, and leukocytes, and in primitive progenitors in bone marrow (BM), spleen, and thymus of individual transplanted mice for observation periods of more than 6 months. Large quantitative differences in reconstitution were observed after transplantation with graded numbers (1000-30, 000) of EGFP(+) cells preconditioned with various combinations of Kit ligand (KL), FLT-3 ligand (FL), thrombopoietin (TPO), interleukin 3 (IL-3), and IL-11. Relative to nonmanipulated BM cells, repopulation of EGFP(+) cells was maintained by KL/FL/TPO stimulation, but approximately 30-fold reduced after KL/FL/TPO/IL-3, or KL/FL/IL-3/IL-11. These differences were not caused by changes in the ability of immature hematopoietic cells to home to the BM, which was only moderately reduced. In conclusion, these quantitative transplantation studies of mice demonstrate the importance of optimal ex vivo cytokine stimulation for gene transfer to stem cells with retention of their in vivo hematopoietic potential, and also emphasize that overall in vitro transduction frequency does not predict gene transfer to repopulating stem cells.

Addition of the human interferon beta scaffold attachment region to retroviral vector backbones increases the level of in vivo transgene expression among progeny of engrafted human hematopoietic stem cells

Absence of durable high-level expression of transgenes from Moloney murine leukemia (Mo-MuLV) retroviral vectors has been a hurdle in bringing effective gene therapy to the clinic. In this study we have analyzed transgene expression among the progeny of mobilized hematopoietic stem cells (HSCs), comparing Mo-MuLV and mouse stem cell virus (MSCV) vectors, with or without addition of a scaffold attachment region (SAR) from the human interferon beta gene. Retroviral (RV) vector supernatant quality was assessed by comparing NGFR transgene expression by HEL cells, and transgene delivery and expression by CD34(+) cells 72 hr after transduction, using real-time PCR and FACS analysis. This is the first description of the effect of SAR within both Mo-MuLV and MSCV vector backbones on long-term RV transgene expression among in vivo HSC progeny in HSC repopulation assays (SCID-hu bone and NOD/SCID). After transduction of mobilized CD34(+) cells with MSCV-SAR vector, transgene expression was observed among a mean of 10% of donor HSC progeny in the SCID-hu bone (range, 0.6-43%). The predominant effect of SAR was to increase the mean fluorescence intensity (MFI) of transgene expression among HSC progeny in both in vivo bone repopulation models (three- to fourfold), and after long-term stromal cultures (twofold).

Expression of hematopoietic growth factor receptors on early hematopoietic precursors: detection and regulation

Since the original isolation of colony-stimulating factors from human serum, conditioned medium of murine or human cell lines, or freshly isolated human mononuclear cells, a revolutionary explosion of ideas has occurred in our understanding of molecular controls of the hematopoietic stem cell self-renewal and differentiation. With the availability of techniques of molecular cloning in the early 1 980s, the first hematopoietically activated cytokines led to molecular clones expressed in bacteria, yeast, or mammalian cellular systems. There then followed a development of techniques leading to the molecular cloning and expression of many hematopoietic growth factors and their receptors, as well as the primary, secondary, and tertiary molecules in signal transduction into activation of specific genes for differentiation or self-renewal. The clinical use of these factors in the diagnosis, treatment, and incorporation into new cell therapies for a variety of diseases is a subject of current interest.

A secreted and LIF-mediated stromal cell–derived activity that promotes ex vivo expansion of human hematopoietic stem cells

The development of culture systems that facilitate ex vivo maintenance and expansion of transplantable hematopoietic stem cells (HSCs) is vital to stem cell research. Establishment of such culture systems will have significant impact on ex vivo manipulation and expansion of transplantable stem cells in clinical applications such as gene therapy, tumor cell purging, and stem cell transplantation. We have recently developed a stromal-based culture system that facilitates ex vivo expansion of transplantable human HSCs. In this stromal-based culture system, 2 major contributors to the ex vivo stem cell expansion are the addition of leukemia inhibitory factor (LIF) and the AC6.21 stromal cells. Because the action of LIF is indirect and mediated by stromal cells, we hypothesized that LIF binds to the LIF receptor on AC6.21 stromal cells, leading to up-regulated production of stem cell expansion promoting factor (SCEPF) and/or down-regulated production of stem cell expansion inhibitory factor (SCEIF). Here we demonstrate a secreted SCEPF activity in the conditioned media of LIF-treated AC6.21 stromal cell cultures (SCM-LIF). The magnitude of ex vivo stem cell expansion depends on the concentration of the secreted SCEPF activity in the SCM-LIF. Furthermore, we have ruled out the contribution of 6 known early-acting cytokines, including interleukin-3, interleukin-6, granulocyte macrophage colony-stimulating factor, stem cell factor, flt3 ligand, and thrombopoietin, to this SCEPF activity. Although further studies are required to characterize this secreted SCEPF activity and to determine whether this secreted SCEPF activity is mediated by a single factor or by multiple growth factors, our results demonstrate that stromal cells are not required for this secreted SCEPF activity to facilitate ex vivo stem cell expansion.

CD164--a novel sialomucin on CD34+ cells

Hematopoiesis in adult bone marrow is a tightly regulated process involving interactions between cytokine and adhesion receptors on hematopoietic progenitor cells and their cognate ligands in the immediate microenvironment. These interactions control hematopoietic stem cell self-renewal, quiescence, commitment and migration. Recently, sialomucins have assumed some importance in hematopoiesis, with six of these receptors, CD34, PSGL-1, CD43, PCLP, CD45RA and CD164, having been identified on primitive hematopoietic precursor cells and/or their associated stromal/endothelial elements. This article reviews the cloning, expression and function of the recently identified sialomucin, CD164, which is highly expressed by primitive hematopoietic progenitor cells. The CD164 receptor is implicated in mediating or regulating hematopoietic precursor cell adhesion to stroma, and may serve as a potent negative regulator of hematopoietic progenitor cell proliferation.

Optimization of retroviral gene transduction of mobilized primitive hematopoietic progenitors by using thrombopoietin, Flt3, and Kit ligands and RetroNectin culture

We have investigated the ability of several cytokine combinations to improve retrovirus-mediated transduction of human primitive hematopoietic progenitors (PHPs) from mobilized peripheral blood (MPB). Retroviral infection of CD34+ cells was performed by culture on fibronectin fragment CH-296 (RetroNectin, RN), using the truncated human nerve growth factor receptor (NGFR) as the transgene reporter. Transgene expression among progeny of PHPs was assayed by FACS analysis after long-term stromal culture (LTC). Transgene delivery to PHPs was assessed by PCR of individual stromal culture-derived methylcellulose colonies (LTC-CFCs). Compared with interleukin 3 (IL-3), IL-6, and leukemia inhibitory factor (LIF), the combination of thrombopoietin (TPO), Flt3 ligand (FL), and Kit ligand (KL) effected a 73-fold increase in NGFR expression among CD34+ cells (to 14%) and a 14-fold increase in NGFR expression among total cells (to 10%) after LTC. In addition, a 2.4-fold increase in neo gene marking of LTC-CFCs was observed. A preclinical study comparing the effect of high-speed centrifugation ("spinoculation") or culture on RN during exposure to retroviral particles in teflon cell culture bags showed no difference in the efficiency of transduction of PHPs between these two methods.