Increased Recruitment of Hematopoietic Progenitor Cells Underlies the Ex Vivo Expansion Potential of FLT3 Ligand

A Novel High-Throughput Screening Platform Reveals an Optimized Cytokine Formulation for Human Hematopoietic Progenitor Cell Expansion

The main limitations of hematopoietic cord blood (CB) transplantation, viz, low cell dosage and delayed reconstitution, can be overcome by ex vivo expansion. CB expansion under conventional culture causes rapid cell differentiation and depletion of hematopoietic stem and progenitor cells (HSPCs) responsible for engraftment. In this study, we use combinatorial cell culture technology (CombiCult^®) to identify medium formulations that promote CD133⁺ CB HSPC proliferation while maintaining their phenotypic characteristics. We employed second-generation CombiCult screens that use electrospraying technology to encapsulate CB cells in alginate beads. Our results suggest that not only the combination but also the order of addition of individual components has a profound influence on expansion of specific HSPC populations. Top protocols identified by the CombiCult screen were used to culture human CD133⁺ CB HSPCs on nanofiber scaffolds and validate the expansion of the phenotypically defined CD34⁺CD38^lo/-CD45RA^-CD90⁺CD49f⁺ population of hematopoietic stem cells and their differentiation into defined progeny.

Computational modeling of the expansion of human cord blood CD133+ hematopoietic stem/progenitor cells with different cytokine combinations

MOTIVATION

Many important problems in cell biology require dense non-linear interactions between functional modules to be considered. The importance of computer simulation in understanding cellular processes is now widely accepted, and a variety of simulation algorithms useful for studying certain subsystems have been designed. Expansion of hematopoietic stem and progenitor cells (HSC/HPC) in ex vivo culture with cytokines and small molecules is a method to increase the restricted numbers of stem cells found in umbilical cord blood (CB), while also enhancing the content of early engrafting neutrophil and platelet precursors. The efficacy of the expanded product depends on the composition of the cocktail of cytokines and small molecules used for culture. Testing the influence of a cytokine or small molecule on the expansion of HSC/HPC is a laborious and expensive process. We therefore developed a computational model based on cellular signaling interactions that predict the influence of a cytokine on the survival, duplication and differentiation of the CD133(+) HSC/HPC subset from human umbilical CB.

RESULTS

We have used results from in vitro expansion cultures with different combinations of one or more cytokines to develop an ordinary differential equation model that includes the effect of cytokines on survival, duplication and differentiation of the CD133(+) HSC/HPC. Comparing the results of in vitro and in silico experiments, we show that the model can predict the effect of a cytokine on the fold expansion and differentiation of CB CD133(+) HSC/HPC after 8-day culture on a 3D scaffold. Supplementary data are available at Bioinformatics online.

Preparation and characterization of nano-liposome-mediated FL gene in the Lovo cells

AIMS

The purpose of the present work was to formulate and evaluate cationic nano-liposomes as novel nonviral gene delivery for colon cancer treatment.

METHODS

Recombinant pEGFP-c1-Fms-like tyrosine kinase receptor 3 ligand (FL) plasmids containing human FL gene and green fluorescent protein (GFP) reporter genes were constructed. FL and GFP Gene-carrying cationic nano-liposomes were prepared based on the electrostatic adherence principle and then transfected into Lovo cells. The morphology, particle size, and zeta potential of gene-carrying cationic nano-liposomes were observed using an electron microscope. GFP expression was observed by fluorescence microscopy to assay the transfection efficiency. The cytotoxicity of FL/nano-liposomes was evaluated by the MTT method.

RESULTS

Recombinant plasmids pEGFP-c1-FL are successfully constructed using gene cloning methods and confirmed by restriction enzyme digestion and sequencing. The cationic nano-liposomes carrying pEGFP-cl-FL were observed by an electron micrograph and showed uniform spherical or elliptical shapes and many pores. The fluorescence microscopy images of gene-carrying cationic nano-liposomes showed good expression of GFP in pEGFP and pEGFP-cl-FL groups. The MTT assay of cell death indicated a significantly higher level of cell death between the FL group and the control group at 24, 48, and 96 hours after transplantation.

CONCLUSION

Cationic nano-liposomes show safe and high-performance transfection as gene carriers. Gene therapy has significant implications for colon cancer treatment in future.

Nanoliposome-mediated FL/TRAIL double-gene therapy for colon cancer: in vitro and in vivo evaluation

OBJECTIVE

To investigate the therapeutic effects of cationic nanoliposome-mediated gene therapy combined with immunotherapy for colon cancer treatment.

METHODS

Recombinant plasmids containing green and red fluorescent protein reporter genes were constructed using gene cloning methods. Gene-carrying cationic nanoliposomes were prepared based on the electrostatic adherence principle and then transfected into dendritic cells (DC), which were transplanted into colon cancer cells.

RESULTS

Recombinant plasmids containing green or red fluorescent protein reporter genes were successfully constructed by gene cloning and confirmed by restriction enzyme digestion and sequencing. Gene-carrying cationic nanoliposomes were transfected into colon cancer cells, and good gene expression was detected. A better level of apoptosis was observed in the combined group of tyrosine kinase receptor 3 ligand (FL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), while the lowest level was detected in the control group. The parameters in the FL and TRAIL groups were between the above-mentioned combined group.

CONCLUSION

Cationic nanoliposomes have the advantage of being gene carriers. The joint therapeutic effects of the two genes are superior to those of a single gene. Gene therapy combined with immunotherapy has significant implications for cancer treatment.

The role of CD40/CD40 ligand interactions in bone marrow granulopoiesis

The CD40 ligand (CD40L) and CD40 are two molecules belonging to the TNF/TNF receptor superfamily, and their role in adaptive immune system has widely been explored. However, the wide range of expression of these molecules on hematopoietic as well as nonhematopoietic cells has revealed multiple functions of the CD40/CD40L interactions on different cell types and processes such as granulopoiesis. CD40 triggering on stromal cells has been documented to enhance the expression of granulopoiesis growth factors such as granulocyte-colony-stimulating factor (G-CSF) and granulocyte/monocyte-colony-stimulating factor (GM-CSF), and upon disruption of the CD40/CD40L-signaling pathway, as in the case of X-linked hyperimmunoglobulin M (IgM) syndrome (XHIGM), it can lead to neutropenia. In chronic idiopathic neutropenia (CIN) of adults, however, under the influence of an inflammatory microenvironment, CD40L plays a role in granulocytic progenitor cell depletion, providing thus a pathogenetic cause of CIN.

Ex-vivo expansion of CFU-GM and BFU-E in unselected PBMC cultures with Flt3L is enhanced by autologous plasma

BACKGROUND

Previous ex-vivo expansion studies in our laboratory, comparing unselected and CD34(+)-selected PBMC, have shown no advantage for CD34(+) cell selection, in terms of the expansion achieved. Our goal was to develop procedures for consistent generation of large numbers of hematopoietic progenitor and post-progenitor cells from unselected PBMC.

METHODS

Unselected PBMC, collected from cancer patients undergoing apheresis prior to high-dose chemotherapy and autologous stem cell rescue, were expanded ex vivo in static cultures, without a stromal layer, in the presence of Flt3 ligand (Flt3L), a recombinant GM-CSF/IL-3 fusion protein (PIXY321), G-CSF and GM-CSF for 10 days.

RESULTS

The addition of 2% autologous plasma to this cytokine combination enhanced expansion of total cell numbers (3.2 fold versus 1.9 fold; p < 0.01), colony-forming units granulocyte-macrophage (CFU-GM) (22.0 fold versus 8.1 fold, p < 0.01) and burst-forming units erythroid (BFU-E) (17.6 fold versus 7.0 fold, 0.01 < p < 0.02). The optimal seeding density for a given specimen was inversely related to the frequency of CD34(+) cells in the sample. CFU-GM expansion with the Flt3L-containing cytokine cocktail was equivalent to that obtained with IL-3, IL-6, G-CSF and SCF, whether or not the cultures were supplemented with autologous plasma. In plasma-free cultures, BFU-E expansion was significantly higher with IL-3, IL-6, G-CSF and SCF than with Flt3L, PIXY321, G-CSF and GM-CSF. In the presence of autologous plasma, however BFU-E expansion was higher in the Flt3L-containing media. In comparison studies, autologous plasma suppressed BFU-E expansion in SCF-containing cultures. Consistent with our colony assay results, dual-parameter flow cytometric analysis of the expanded cell population revealed that supplementation with autologous plasma yielded a significant increase in the numbers of myeloid progenitors in Flt3L-containing cultures.

DISCUSSION

Unselected PBMC from cancer patients can be effectively expanded ex vivo in Flt3L, PIXY321, G-CSF and GM-CSF, supplemented with autologous plasma, yielding high numbers of myeloid and erythroid progenitors.

IL-16 can synergize with early acting cytokines to expand ex vivo CD34+ isolated from cord blood

We previously reported that interleukin (IL)-16 can induce CD34(+) hematopoietic cells to proliferate and differentiate in vitro into phenotypically and functionally mature dendritic cells. In this study, we investigated the effects of IL-16 on the expansion of CD34(+) cells from human cord blood (CB). CD34(+) CB cells were cultured for 14 days in medium containing a basal cocktail (BC) containing stem cell factor, Flt-3 ligand, thrombopoietin, IL-6, and IL-3 with and without IL-16 as a control. Interleukin-16 added to BC significantly enhanced the expansion of CD34(+) cells (66.47 +/- 1.46-fold vs. 36.23 +/- 1.67-fold), as well as CD34(+)CD38(-) early stem cells (106.67 +/- 2.34-fold vs. 63.42 +/- 1.89-fold) and progenitor cells [colony-forming unit (CFU) -mixed -(GEMM)] and multilineage-committed progenitors [burst-forming unit (BFU-E), CFU-granulocyte, macrophage (-GM), CFU-megakaryocyte (-MK)]. Interleukin-16 also significantly increased long-term culture-initiating cells (160.8 +/- 3.45-fold vs. 83 +/- 2.89-fold with BC alone). Moreover, CD34(+) cells expanded with IL-16 maintained the capacity to differentiate into the lymphoid-B and -NK lineage. The addition of IL-16 to BC increased the migratory capacity of expanded CD34(+) cells compared to BC alone, leaving the expression of CXCR4 unaffected, and decreased the percentage of CD34(+)CD4(+) cells. We showed that IL-16 released endogenously affected the ex vivo expansion of CD34(+) cells. Overall, this study suggests that IL-16 may have a new role in promoting the expansion of hematopoietic stem cells and may represent a new tool for the expansion of CD34(+) cells for clinical applications.

Angiotensin-converting enzyme (CD143) marks hematopoietic stem cells in human embryonic, fetal, and adult hematopoietic tissues

Previous studies revealed that mAb BB9 reacts with a subset of CD34+ human BM cells with hematopoietic stem cell (HSC) characteristics. Here we map BB9 expression throughout hematopoietic development and show that the earliest definitive HSCs that arise at the ventral wall of the aorta and surrounding endothelial cells are BB9+. Thereafter, BB9 is expressed by primitive hematopoietic cells in fetal liver and in umbilical cord blood (UCB). BB9+CD34+ UCB cells transplanted into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice contribute 10-fold higher numbers of multilineage blood cells than their CD34+BB9− counterparts and contain a significantly higher incidence of SCID-repopulating cells than the unfractionated CD34+ population. Protein microsequencing of the 160-kDa band corresponding to the BB9 protein established its identity as that of somatic angiotensin-converting enzyme (ACE). Although the role of ACE on human HSCs remains to be determined, these studies designate ACE as a hitherto unrecognized marker of human HSCs throughout hematopoietic ontogeny and adulthood.

Preclinical ex vivo expansion of peripheral blood CD34+ selected cells from cancer patients mobilized with combination chemotherapy and granulocyte colony-stimulating factor

BACKGROUND AND OBJECTIVES

Ex vivo peripheral blood progenitor cell (PBPC) expansion has been proposed as a strategy to increase the number of haematopoietic progenitors available for cell transplantation. We have expanded CD34+ cells from PBPCs obtained from four patients with haematological malignancies and one patient with an Ewing's sarcoma.

MATERIALS AND METHODS

Cells were expanded in the Dideco 'Pluricell system'. After 12 days in culture, we evaluated cell phenotype, total nucleated cells, CD34+ fold increase, cell apoptosis and colony assay of expanded cells. Cell engraftment has been evaluated by transplanting two groups of irradiated non-obese diabetic/severe combined immunodeficient (NOD-SCID) mice with expanded and non-expanded cell populations.

RESULTS

Total nucleated cells and CD34+ cells increased 59.5 and 4.0 times, respectively. The expanded cells were mainly constituted of myeloid and megakaryocytic cells. A significant increase in the number of colony-forming unit-granulocyte macrophage (CFU-GM) was observed in the CFU assay. Ten mice transplanted with expanded cells showed a best overall survival (80%) compared to 10 mice transplanted with non-expanded cells (20%). Human CD45+ cells were detected by flow cytometry and polymerase chain reaction in bone marrow and spleen of transplanted animals. The relative low engraftment level obtained with the expanded cells suggests a loss of SCID repopulating cells maybe due to cell differentiation during expansion.

CONCLUSIONS

We have demonstrated the feasibility of the ex vivo expansion of mobilized PBPCs from cancer patients, evidencing a clonal expansion of CFUs and the ability of the expanded cells to engraft the bone marrow and spleen of immunosuppressed mice. The differentiation of the CD34+ stem cell compartment could be further minimized by ameliorating the expansion conditions.

Distinct roles of IL-7 and stem cell factor in the OP9-DL1 T-cell differentiation culture system

OBJECTIVE

The OP9-DL1 culture system is an in vitro model for T-cell development in which activation of the Notch pathway by Delta-like 1 promotes differentiation of mature T cells from progenitors. The roles of specific cytokines in this culture system have not been well defined, and controversy regarding the role of IL-7 has recently emerged. We examined the roles played by IL-7, Flt3 ligand, and stem cell factor (SCF) in differentiation of adult bone marrow cells in the OP9-DL1 culture system.

METHODS

Hematopoietic progenitor cells isolated from mouse bone marrow were cultured with OP9 or OP9-DL1 stromal cells and evaluated for T and B lymphocyte differentiation using immunofluorescent staining.

RESULTS

IL-7 provided both survival/proliferation and differentiation signals in a dose-dependent manner. T-cell development from the CD4/CD8 double-negative (DN) stage to the CD4/CD8 double-positive (DP) stage required IL-7 provided by the stromal cells, while differentiation from the DP to the CD8 single-positive (SP) stage required addition of exogenous IL-7. SCF favored the proliferation of DN lymphoid progenitors and inhibited differentiation to the DP stage in a dose-dependent manner. Conversely, blocking the function of SCF expressed endogenously by OP9-DL1 cells inhibited proliferation of lymphoid progenitors and accelerated T-lineage differentiation. Flt3 ligand promoted proliferation without affecting differentiation.

CONCLUSION

These results validate the OP9-DL1 model for the analysis of T-cell development from bone marrow-derived progenitor cells, and demonstrate specific roles of SCF, IL-7, and Flt3L in promoting efficient T-lineage differentiation.

Differential gene expression in human hematopoietic stem cells specified toward erythroid, megakaryocytic, and granulocytic lineage

To better understand the transcriptional program that accompanies orderly lineage-specific hematopoietic differentiation, we analyzed expression changes during the lineage-specific differentiation of human hematopoietic stem cells (HSC; CD34+/CD38-/CD33-); HSC and multipotent myeloid progenitors (MMP; CD34+/CD38-/CD33+) were isolated from the bone marrow of healthy individuals by MACS. CD34+ cells in semi-solid culture were stimulated with the cytokines erythropoietin, IL-6, and G-CSF to promote differentiation to committed erythroid, megakaryocytic, and granulocytic clones, respectively. Differential display RT-PCR analysis was performed to compare the mRNA transcripts in HSC, MMP, and the committed lineage-specific clones derived from these committed lineage-specific progenitors. Expressed sequence tags (n=256), which were differentially expressed, were identified. One hundred ninety-four were homologous to known genes, and some were associated with hematopoiesis. These known genes were classified as involved in transcription/translation, signal transduction, cell surface receptors/ligands, cell signaling, cell metabolism, cell cycle, cell apoptosis, and oncogenesis. We identified genes, which were up- or down-regulated specifically in the lineage-committed clones compared with HSC or/and MMP, suggesting that specific gene activation and repression might be necessary for specific lineage commitment and differentiation. Our data provide an extensive transcriptional profile of human hematopoiesis during in vitro, lineage-specific differentiation.

Stage specific gene expression of serpins and their cognate proteases during myeloid differentiation

Proteases and their serpin inhibitors are abundantly expressed in haemopoietic and peripheral blood cells. There is, however, relatively little information about the role played by serpins in the control of protease activity within these cells and in the pericellular region. The observation that mutations in the neutrophil elastase gene, which cause cyclic and severe congenital neutropenia, are associated with protease maldistribution gives some clue as to the potential importance of inhibitor proteins. To begin to address the role of protease/inhibitor balance in blood cells we used reverse transcription polymerase chain reaction to examine protease and serpin gene expression in mature peripheral blood cells, differentiating haemopoietic progenitors, leukaemic blasts and haemopoietic cell lines. The results demonstrate stage-specific expression of proteases together with widespread expression of intra- and extra-cellular serpins. The elastase inhibitors monocyte neutrophil elastase inhibitor (MNEI) and antitrypsin (AT) showed overlapping expression. MNEI is predominantly expressed in early haemopoietic progenitors while antitrypsin is mainly expressed in more mature myeloid precursors, peripheral blood granulocytes and mononuclear cells. Our results give an overall picture of serpin and protease gene expression and draws attention to the potential importance of elastase regulators at all stages of myelopoiesis.

Ex vivo expansion of umbilical cord blood CD34+ cells in a closed system: a multicentric study

BACKGROUND AND OBJECTIVES

The Dideco 'Pluricell System' is a CE-marked medical device allowing haematopoietic stem cell (HSC) expansion. It comprises a kit of cGMP cytokines and reagents, a closed-cell expansion chamber and a cell-washing set. We tested the system in a multicentric study by expanding CD34(+) cells from eight fresh umbilical cord blood (UCB) samples.

MATERIALS AND METHODS

During culture, the mean nucleated cell (NC) count, the mean CD34(+) cell count, fold expansion, viability and apoptosis were measured. Clonogenic assays and immunophenotypical characterization were performed on days 0, 7 and 12. On the expanded cellular product, in three cases cell genotyping, endotoxin level and mycoplasma detection (by polymerase chain reaction) were performed.

RESULTS

The mean CD34(+) cell expansion on days 7 and 12 was sevenfold and 12-fold respectively and the mean NC expansion was 69-fold and 180-fold. The mean NC viability on day 12 was 96.9% (94.4-99.1). After 12 days, granulocyte-macrophage colony-forming units (GM-CFU) showed a 20-fold increase: a slight increase in CD34(+) cell apoptosis was observed during culture. In all of three cases neither chromosomal alterations nor mycoplasma contamination was detected. No significant endotoxin levels were detected after expansion.

CONCLUSIONS

The device allows the ex vivo expansion of NC and CD34(+) cells in a closed system. The expanded cellular product is a mixture of progenitors (CD34(+) cells) and differentiated (mainly myeloid and megakaryocytic) cells. To reduce cell apoptosis, more frequent cell feeding during culture should be tested.

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.

The Expansion of Megakaryocyte Progenitors from CD34+-Enriched Mobilized Peripheral Blood Stem Cells Is Inhibited by Flt3-L

This study aimed to determine the optimal growth factor combination for expansion of megakaryocyte (Mk) progenitors with clonogenic potential from CD34+-enriched mobilized peripheral blood stem cells (PBSC). Mobilized PBSC were monocyte depleted and CD34+ enriched, then cultured with various combinations of interleukin-3 (IL-3), IL-6, IL-11, Flt3 ligand (Flt3-L), stem cell factor (SCF), granulocyte-macrophage colonystimulating factor (GM-CSF), and erythropoietin (EPO), using a 2(7-3) IV fractional factorial design. Expansion of Mk committed progenitors (CD41+) and primitive precursors (CD61+ CD34+) was determined using FACS and colony-forming assays. Amplification of Mk progenitor production was attributed to IL-3 (p < 0.002), SCF (p < 0.001), and GM-CSF (p < 0.05). Flt3-L inhibited the production of total CD61+ cells (p < 0.05), CD61+CD34+ cells (p < 0.03), and total CD41a+ cells (p < 0.01). Addition of Flt3-L to the optimum growth factor combination of megakaryocyte growth and development factor (MGDF), SCF, IL-3, and GM-CSF caused the greatest increase in total nucleated cells but reduced Mk progenitor expansion. There was also a 20% reduction in Mk+ colonies from cells expanded in the presence of Flt3-L. Factorial analysis identified the optimal combination of growth factors required to expand Mk precursors with clonogenic potential. The addition of Flt3-L to the optimal combination of MGDF, SCF, IL-3, and GM-CSF reduced both the fold expansion of Mk progenitors and Mk colony numbers.

Hematopoietic stem cell gene therapy with drug resistance genes: an update

Transfer of drug resistance genes into hematopoietic stem cells (HSCs) has promise for the treatment of a variety of inherited, that is, X-linked severe combined immune deficiency, adenosine deaminase deficiency, thalassemia, and acquired disorders, that is, breast cancer, lymphomas, brain tumors, and testicular cancer. Drug resistance genes are transferred into HSCs either for providing myeloprotection against chemotherapy-induced myelosuppression or for selecting HSCs that are concomitantly transduced with another gene for correction of an inherited disorder. In this review, we describe ongoing experimental approaches, observations from clinical trials, and safety concerns related to the drug resistance gene transfer.

Neural Cell Adhesion Molecule Contributes to Hemopoiesis-Supporting Capacity of Stromal Cell Lines

To clarify mechanisms underlying cell-to-cell interactions between hemopoietic stem cells (HSCs) and stromal cells, we established a stromal cell line (FMS/PA6-P) from day-16 fetal bone marrow (BM) adherent cells using an anti-PA6 monoclonal antibody (mAb) specific for BM stromal cells. Importantly, this FMS/PA6-P cell line, showing homogenous fibroblastic morphology, is absent from hematolymphoid and endothelial lineage markers and maintains a high level of expression of PA6 molecule, recognized by the anti-PA6 mAb, for approximately 20 passages. Further, the cell line expressing a high level of PA6 molecule has a better hemopoiesis-supporting capacity in vitro than other stromal cell lines such as PA6 and MS-5. In fact, the PA6 molecule is closely related to the hemopoiesis-supporting capacity of the stromal cells because the proliferation of HSCs was suppressed to a great extent by the anti-PA6 mAb. Affinity chromatography and mass peptide fingerprinting revealed that the protein reacting with the anti-PA6 mAb is neural cell adhesion molecule (NCAM). The frequencies of long-term cobblestone area-forming cells and long-term culture-initiating cells were significantly suppressed by repression of NCAM in the FMS/PA6-P cells using NCAM small interfering RNA. Our findings clearly indicate that NCAM functions on the maintenance of HSCs.

Characterization of mesenchymal stem cells isolated from mouse fetal bone marrow

Mesenchymal stem cells (MSCs) are defined as cells that can differentiate into multiple mesenchymal lineage cells. MSCs have some features (surface molecules and cytokine production, etc.) common to so-called traditional bone marrow (BM) stromal cells, which have the capacity to support hemopoiesis. In the present study, we isolated murine MSCs (mMSCs) from the fetal BM using an anti-PA6 monoclonal antibody (mAb) that is specific for bone marrow stromal cells. The mMSCs, called FMS/PA6-P cells, are adherent, fibroblastic, and extensively expanded and have the ability to differentiate not only into osteoblasts and adipocytes but also into vascular endothelial cells. The FMS/PA6-P cells produce a broad spectrum of cytokines and growth factors closely related to hemopoiesis and show good hemopoiesis-supporting capacity both in vivo and in vitro, suggesting that they are a component of the hemopoietic stem cell niche in vivo. Interestingly, although the FMS/PA6-P cells express a high level of the PA6 molecule, which is reactive with anti-PA6 mAb, they gradually lose their ability to express this molecule during the course of differentiation into osteoblasts and adipocytes, indicating that the PA6 molecule might serve as a novel marker of mMSCs.

A human stromal-based serum-free culture system supports the ex vivo expansion/maintenance of bone marrow and cord blood hematopoietic stem/progenitor cells

OBJECTIVE

We investigated the role of human stromal layers (hu-ST) on the ex vivo expansion/maintenance of human hematopoietic stem/progenitor cells (HSC) from adult bone marrow (BM) and umbilical cord blood (CB).

MATERIALS AND METHODS

BM and CB CD34(+)-enriched cells were cultured in serum-free medium supplemented with SCF, bFGF, LIF, and Flt-3, in the presence or absence of stroma, and analyzed for proliferation, phenotype, and clonogenic potential.

RESULTS

Significant expansion of BM and CB CD34(+) and CD34(+)CD38(-) cells were achieved in the presence of hu-ST. The differentiative potential of both BM and CB CD34(+)-enriched cells cocultured with hu-ST was primarily shifted toward the myeloid lineage, while maintaining/expanding a CD7(+) population. Clonogenic analysis of the expanded cells showed increases in progenitors of the myeloid lineage, including colony-forming unit-granulocyte, macrophage (CFU-GM) and colony-forming unit-granulocyte, erythroid, macrophage, megakaryocyte (CFU-Mix) for both BM (stroma and stroma-free conditions) and CB cells in the presence of stroma.

CONCLUSIONS

These results indicate that adult hu-ST in the presence of appropriate cytokines can be used to efficiently expand/maintain myeloid and lymphoid cell populations from human BM and CB HSC.

Evaluation of ex vivo expansion and engraftment in NOD-SCID mice of umbilical cord blood CD34+ cells using the DIDECO "Pluricell System"

The Dideco "Pluricell System" is a commercially available closed device composed of an expansion chamber and a kit of certified reagents that allow haematopoietic stem cell expansion. We have expanded seven umbilical cord blood (UCB) samples following the manufacturer's instructions; two groups of irradiated NOD-SCID mice have been transplanted with expanded and nonexpanded cells from the same UCB, and bone marrow was analysed for the presence of human cells. Average UCB volume was 61.6+/-8.8 ml; mean nucleated cell content was 1090.5+/-189.9 x 10(6). Percentage and number of CD34+ cells were 0.37+/-0.13% and 3.9+/-1.2 x 10(6). After separation, CD34+ cell purity was 82+/-11%. Mean number of inoculated cells was 760 000; mean NC and CD34+ fold expansion at 12 days was 230.4+/-91.5 and 21.0+/-11.9. Both groups of mice showed successful engraftment: the percentage of human cells was higher in the group receiving expanded cells (3.4+/-2.01%) compared to the group receiving nonexpanded cells (1.5+/-0.66%) (P<0.00018, Mann-Whitney test). The cell population obtained after 12 days expansion consisted mainly of myeloid and megakaryocytic progenitors. The CD34+ antigen reached the maximum expression level at day 12 (7.5+/-2.0%). Analysis of lineage-markers for human myelomonocytic, megakaryocytic, B, T, CD34 and erythroid cells, gave evidence that all the lineages were represented in the marrow of transplanted mice.

Converging pathways in leukemogenesis and stem cell self-renewal

Studies over the last 40 years have led to an understanding of the hierarchical organization of the hematopoietic system and the role of the pluripotential hematopoietic stem cell. Earlier recognition of the importance of bone marrow hematopoietic microenvironments has evolved into the recognition of specific niches that regulate stem cell pool size, proliferative status, mobilization, and differentiation. The discovery of the role of multiple hematopoietic growth factors and their receptors in the orchestration of stem cell self-renewal and differentiation has been followed by recognition of the importance of the Notch and Wnt pathways. The homeobox family of transcription factors serve as master regulators of development and are increasingly found to be critical regulators of hematopoiesis. In parallel with this understanding of normal hematopoiesis has come a recognition that stem cell dysregulation at various levels is involved in leukemogenesis. Furthermore, the progression from chronic leukemia or myelodysplasia to acute leukemia involves accumulation of at least two mutational events that lead to enhancement of stem cell proliferation, or acquisition of stem cell behavior by a progenitor cell, coupled with maturation inhibition. Translocations resulting in development of oncogenic fusion genes are found in AML and the transforming potential of two of these, AML1-ETO and NUP98-HOXA9, will be discussed. Secondary, constitutively activating mutations of the Flt3 and c-kit receptors and of K- and N-ras are found with high frequency in AML, and the transforming potential of mutated FLT3 and the role of STAT5A activation in human stem cell transformation will be reviewed.

Transcriptional profiling of human hematopoiesis during in vitro lineage-specific differentiation

To better understand the transcriptional program that a ccompanies orderly lineage-specific hematopoietic differentiation, we performed serial oligonucleotide microarray analysis of human normal CD34+ bone marrow cells during lineage-specific differentiation. CD34+ bone marrow cells isolated from healthy individuals were selectively stimulated in vitro with the cytokines erythropoietin (EPO), thrombopoietin (TPO), granulocyte colony-stimulating factor (G-CSF), and granulocyte macrophage colony-stimulating factor (GM-CSF). Cells from each of the lineages were harvested after 4, 7, and 11 days of culture for expression profiling. Gene expression was analyzed by oligonucleotide microarrays (HG-U133A; Affymetrix, Santa Clara, CA). Experiments were done in triplicates. We identified 258 genes that are consistently upregulated or downregulated during the course of lineage-specific differentiation within each specific lineage (horizontal change). In addition, we identified 52 genes that contributed to a specific expression profile, yielding a genetic signature specific for successive stages of differentiation within each of the three lineages. Analysis of horizontal changes selected 21 continuously upregulated genes for EPO-induced differentiation (including GTPase activator proteins RAP1GA1 and ARHGAP8, which regulate small Rho GTPases), 21 for G-CSF-induced/GM-CSF-induced differentiation, and 91 for TPO-induced differentiation (including DLK1, of which the role in normal hematopoiesis is not defined). During the lineage-specific differentiation, 58 (erythropoiesis), 30 (granulopoiesis), and 37 (thrombopoiesis) genes were significantly downregulated, respectively. The expression of selected genes was confirmed by real-time polymerase chain reaction. Our data encompass the first extensive transcriptional profile of human hematopoiesis during in vitro lineage-specific differentiation.

Enhanced effect of vascular endothelial growth factor, thrombopoietin peptide agonist, SCF, and Flt3-L on LTC-IC and reporter gene transduction from umbilical cord blood CD34+ cells

BACKGROUND

Hemangioblastic precursors have been identified that give rise to both endothelial cells and HPCs, suggesting that common growth factor requirements may exist.

STUDY DESIGN AND METHODS

The effect of vascular endothelial growth factor (VEGF) in combination with thrombopoietin peptide agonist (TPOA), Flt-3 L (F), and SCF (S) on long-term culture-initiating cell (LTC-IC), CFU, differentiation, and transduction of cord blood (CB) CD34+ were evaluated up to 4 weeks in culture.

RESULTS

At Week 4, in cultures containing T/F/S and VEGF, the LTC-IC increased 1000-fold (from 37 +/- 8 to 37,012 +/- 14,329) with a frequency of 3.4 in 10,000 cells. In the T/F/S cultures without VEGF, the LTC-IC increased 675-fold (to 25,086 +/- 12,102) with a frequency of one LTC-IC in 10,000 cells. The addition of VEGF significantly increased (p < 0.05) the LTC-IC per 10,000 CB CD34+ cells. Transduction with reporter gene enhanced green fluorescent protein (EGFP), resulted in an increase in EGFP+ CFU at Week 1 and EGFP + LTC-IC at Weeks 1 and 4. The cells maintained their multilineage expression when cultured in conditions for erythroid, myeloid, or megakaryocytic differentiation. Peak percentage EGFP coexpression of GlyA and CD11b was 51 +/- 6 percent and 63 +/- 15 percent, respectively, at Week 2, while CD41a was 34 +/- 17 percent at Week 4.

CONCLUSION

T/F/S and VEGF have an enhanced effect on total LTC-IC output and frequency but do not appear to significantly alter the differentiation or transducibility characteristics of CB HPCs in vitro.

FLT3 ligand causes autocrine signaling in acute myeloid leukemia cells

The FLT3 receptor tyrosine kinase is highly expressed in most acute leukemias and frequently mutated in acute myeloid leukemia (AML). The mutated form of the receptor is constitutively activated and known to play an important role in AML, but the activation state of the overexpressed wild-type (wt) receptor is, at present, unknown. In this study, we examined the activation state of the wild-type receptor in AML. We found that the wild-type receptor was constitutively phosphorylated/activated in 8 of 12 primary AML samples and 4 of 13 leukemia cell lines. To explain why wtFLT3 is often activated, we investigated the expression of its ligand, FL, by these same cells. Coexpression of FL with FLT3 was a universal finding in both primary AML samples and leukemic-derived cell lines. To further prove that autocrine signaling was accounting for the activation, we showed that conditioned media but not fresh media was able to activate FLT3. In addition, an antibody that blocks binding of ligand to the receptor blocks FLT3 activation. Finally, depletion of FL from conditioned media is able to block the activation of FLT3. Taken together, these findings represent strong evidence that wtFLT3 is often constitutively activated in AML and thus, like its mutated form, might contribute to the altered signaling that characterizes leukemogenesis.

Adhesion to E-selectin promotes growth inhibition and apoptosis of human and murine hematopoietic progenitor cells independent of PSGL-1

Although both P- and E-selectin are constitutively expressed on bone marrow endothelial cells, their role in the regulation of hematopoiesis has only recently been investigated. We have previously shown that P-selectin glycoprotein ligand-l (PSGL-1/CD162) is expressed by primitive human bone marrow CD34+ cells, mediates their adhesion to P-selectin, and, more importantly, inhibits their proliferation. We now demonstrate that adhesion to E-selectin inhibits the proliferation of human CD34+ cells isolated either from human umbilical cord blood, adult mobilized blood, or steady-state bone marrow. Furthermore, a subpopulation, which does not contain the most primitive hematopoietic progenitor cells, undergoes apoptosis following E-selectin-mediated adhesion. The same phenomenon was observed in cells isolated from mouse bone marrow. Using lineage-negative Sca-1+ c-KIT+ bone marrow cells from PSGL-1(-/-) and wild-type mice, we establish that PSGL-1 is not the ligand involved in E-selectin-mediated growth inhibition and apoptosis. Moreover, stable transfection of the human myeloid cell line K562 (which does not express PSGL-1) with alpha(1,3) fucosyltransferase VII alone was sufficient to recapitulate the E-selectin-mediated growth inhibition and apoptosis observed in hematopoietic progenitor cells. These data demonstrate that an E-selectin ligand(s) other than PSGL-1 transduces growth inhibitory and proapoptotic signals and requires posttranslational fucosylation to be functional.

Allergen exposure-induced differences in CD34+ cell phenotype: relationship to eosinophilopoietic responses in different compartments

We hypothesized that the allergen-induced increased number of airway eosinophils results from increased recruitment of eosinophils from bone marrow (BM) and local development of CD34(+) cells into eosinophils. We also assumed that the phenotype of airway eosinophils depends on whether these cells have differentiated within BM or airway. C57BL/6 mice were sensitized and subsequently exposed to ovalbumin (OVA) on 5 consecutive days. Newly produced cells were labeled with a thymidine analog. Clonogenic activity and interleukin 5 (IL-5) release from bronchoalveolar lavage fluid (BALf) CD34(+) cells were evaluated by using cell-culture techniques. Allergen exposure induces increase in CD135(+) primitive myeloid progenitors within the BM CD34(+) cell population, without significant changes in total number of CD34(+) cells or newly produced CD34(+) cells. CD34(+)/IL-5R alpha(+) cells in the first stage of cell differentiation were found only in BM, arguing that early commitment of CD34(+) cells into the eosinophil lineage is restricted to the BM compartment. Allergen exposure induces a shift in differentiation of BM, blood, and BALf eosinophillineage-committed CD34(+) cells toward mature eosinophils and recruitment of these cells via blood into airway. We further demonstrate in vitro that ability to multiply persists in BALf CD34(+) cells but not CD34(-) cells, likely via autocrine IL-5 release and IL-5-induced up-regulation of IL-5R alpha.

Hematopoietic stem cells and hematopoiesis

BACKGROUND

The highly orchestrated process of blood cell development and homeostasis is termed "hematopoiesis." Understanding the biology of hematopoietic stem cells as well as hematopoiesis is important to developing improved treatments for hematologic malignancies, congenital disorders, chemotherapy-related cytopenias, and blood and marrow transplants.

METHODS

The author reviews the current state of the art regarding hematopoietic stem cells and hematopoiesis.

RESULTS

Several new concepts, including stem cell plasticity, suggest the possibility that stem cells may have the ability to differentiate into other tissues in addition to blood cells.

CONCLUSIONS

While much is known about hematopoietic stem cells and hematopoiesis, much remains to be clarified about the environmental and genetic processes that govern the growth and development of the blood system. In addition, careful studies remain to be conducted to determine whether hematopoietic stem cells can differentiate into extra-hematopoietic tissues.

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.

Clinical-scale production of granulocyte progenitor and post-progenitor cells using daniplestim, leridistim, Progenipoietin, Promegapoietin and autologous plasma

BACKGROUND

Supplementation of PBPC autografts with ex vivo expanded PBMC may significantly reduce or eliminate the period of neutropenia associated with high-dose chemotherapy.

METHODS

Unmanipulated growth-factor mobilized PBMC were expanded in media containing daniplestim, leridistim, Promegapoietin, and Progenipoietin (DLPP) and 2% autologous plasma at 4 x 10(5) PBMC/mL, first in 25 cm(2) T-flasks, with sampling on Days 7, 10, 13 and 15, and then in 1264 cm(2) Nunclon Cell Factories, with sampling on Days 7 and 13.

RESULTS

In T25-flasks, maximal CFU-GM expansion ([38.2 +/- 9.5]-fold) occurred on Day 10, whereas maximal total cell expansion ([6.7 +/- 1.1]-fold) occurred on Day 15. Production of CD15(+)CD11b(-) and CD15(+)CD11b(+) granulocytic post-progenitors (3.0 +/- 0.4 x 10(6) and 3.7 +/- 0.9 x 10(6), respectively) was also maximal at Day 15. Compared with the previously studied combination of Flt3L, PIXY321, G-CSF, GM-CSF and Epo, the DLPP cocktail performed similarly, with the exception of yielding larger GM colonies at Day 10 and fewer granulocyte post-progenitors on Day 15. In Cell Factories, CFU-GM were expanded (31.6 +/- 14.5)-fold, while total nonadherent cells were expanded (2.6 +/- 0.5)-fold. The two stack Cell Factory cultures seeded with 1.0 x 10(8) unselected PBMC produced approximately 3.3 x 10(6) CFU-GM and 1.3 x 10(8) myeloid post-progenitors.

DISCUSSION

Whereas expansion of cell numbers, CFU-GM and granulocytic post-progenitors in Cell Factories mirrored that achieved in T25-flasks, future preclinical studies with the DLPP cytokine combination may be performed in small volumes, with subsequent translation to the larger volume Cell Factories. Sufficient expansion can be achieved using the DLPP cytokine combination in the Cell Factories to provide the numbers of progenitors required for clinical trials.

Ex vivo expansion marginally amplifies repopulating cells from baboon peripheral blood mobilized CD34+ cells

The ability of ex vivo expansion to increase the long-term repopulating capacity of a graft is still unknown. One problem is the most reliable way to quantify transplantable cells. We addressed this point in a baboon model based on autologous transplantation of serial limiting doses of non-manipulated or ex vivo-expanded mobilized CD34+ cells and determined the threshold doses of non-manipulated and expanded cells which supported long-term multilineage engraftment. In the expansion group, CD34+ cells were cultured for 6 d with a combination of early acting cytokines (Flt3-ligand, stem cell factor, thrombopoietin and interleukin 3). Grafted cells were characterized by their surface antigens and biological properties [semisolid assays, long-term culture-initiating cells (LTC-IC) and non-obese diabetic severe combined immunodeficient reconstituting cells (SRC)]. Animals were followed for at least 12 months post transplantation. The expansion protocol yielded 12.3-fold, 16.9-fold, 3.7-fold, 3.5-fold and 2.2-fold increases in CD34+ cells, granulocyte-macrophage colony-forming units (CFU-GM), megakaryocyte CFU (CFU-MK), LTC-IC and SRC respectively. It induced a modest increase in the long term reconstitutive ability of the graft; the threshold value for long-term engraftment was 0.5 x 10(6)/kg CD34+ cells in the control group and 0.3 x 10(6)/kg CD34+ cells in the expansion group, although one animal in this latter group remained hypoplastic. Frequencies of SRC had a high predictive value of long-term engraftment (r > 0.80). The main advantage of the protocol was the acceleration of granulocyte recovery, achieved at the different doses tested. In conclusion, these experiments suggest that this ex vivo expansion protocol marginally amplifies long-term reconstituting cells.

Efficient mobilization and recruitment of marrow-derived endothelial and hematopoietic stem cells by adenoviral vectors expressing angiogenic factors

Adult bone marrow (BM) is a rich reservoir for endothelial and hematopoietic stem and progenitor cells that contribute to revascularization of injured and tumor tissue. Physiological stress results in the release of specific chemo-cytokines that promote mobilization of stem cells to the circulation and direct their incorporation into the target tissues. In order to dissect the mechanism and identify the cellular mediators that regulate stem cell recruitment, we have developed an in vivo murine model, in which the plasma levels of chemokines are elevated by introducing adenoviral vectors (Advectors) expressing such chemokines. Among the known stem cell-active chemokines, the angiogenic factor VEGF through interaction with its receptors, VEGFR2 and VEGFR1 expressed on endothelial and hematopoietic stem cells, promotes mobilization and recruitment of these cells into the neo-angiogenic sites, thereby accelerating the revascularization process. Based on these studies, it has become apparent that mobilization of stem cells is a dynamic process and requires sequential release of chemocytokines, expression of adhesion molecules and activation of proteases that facilitate egress of cells from the BM to the circulation. Chemokine-activation of metalloproteinases is essential for the release of bio-active cytokines, thereby enhancing stem cell mobilization potential. Advectors are ideal for delivery of chemocytokines since they allow for long-term robust expression facilitating in vivo proliferation and mobilization of large numbers of an otherwise rare population of stem cells. VEGF-mobilized endothelial and hematopoietic stem cells provide for an enriched source of adult pluripotent cells that can be used for revascularization, tissue regeneration or gene therapy.

Mechanisms of muscle stem cell expansion with cytokines

Stem cell expansion and proliferation are important for cell transplantation and stem cell-mediated applications. While we have demonstrated that muscle stem cells can be obtained from adult skeletal muscle tissue, these cells represent only a small percentage of the muscle-derived cells and require in vitro expansion for successful stem cell-mediated therapies. In this study, we have examined the potential of several cytokines to stimulate stem cell growth by combining a non-exponential mathematical model with a unique cell culture system. The growth kinetics of two populations of muscle stem cells were characterized in culture medium supplemented with epidermal growth factor (EGF), fibroblast growth factor-2 (FGF-2), insulin-like growth factor-1 (IGF-1), FLT-3 ligand, hepatocyte growth factor, or stem cell factor (SCF). The division time (DT) and fraction of mitotically active cells were investigated as key parameters to further understand the mechanism of the expansion of the stem cell populations. Our results show that expansion of the freshly isolated, muscle-derived stem cells (MDSC) occurred by recruiting cells into the cell cycle in the presence of EGF, IGF-1, and SCF. However, expansion of the cultured stem cell clone, MC13, is attributed to a reduction of the length of the cell cycle in the presence of FGF-2, EGF, IGF-1, and SCF. Both MDSC and MC13 growth were inhibited in the presence of FLT-3 ligand by increasing the length of the cell cycle. Our results suggest that EGF, IGF-1, FGF-2, and SCF are important cytokines for stimulating the proliferation of MDSC. In addition, this study illustrates that expansion of stem cells occurs through different mechanisms, which consequently demonstrates the importance of monitoring several parameters of cell growth, such as DT and dividing fraction, following stimulation with growth factors.

Flk-2 is a marker in hematopoietic stem cell differentiation: a simple method to isolate long-term stem cells

Clonogenic multipotent mouse hematopoietic stem cells (HSCs) and progenitor cells are contained within the c-kit(+) (K) lineage(-/lo) (L) Sca-1(+) (S) population of hematopoietic cells; long-term (LT) and short-term (ST) HSCs are Thy-1.1(lo). c-kit is a member of the receptor tyrosine kinase family, a class of receptors that are important in the proliferation and differentiation of hematopoietic cells. To establish whether the Flk-2/Flt3 receptor tyrosine kinase was expressed on the most primitive LT-HSCs, we sorted highly purified multipotent stem and progenitor cells on the basis of Flk-2 surface expression and used them in competitive reconstitution assays. Low numbers of Flk-2(-) HSCs gave rise to long-term multilineage reconstitution in the majority of recipients, whereas the transfer of Flk-2(+) multipotent cells resulted in mostly short-term multilineage reconstitution. The KLS subset of adult mouse bone marrow was analyzed for Flk-2 and Thy-1.1 expression. Three phenotypically and functionally distinct populations were isolated: Thy(lo) Flk-2(-) (LT-HSCs), Thy(lo) Flk-2(+) (ST-HSCs), and Thy(-) Flk-2(+) multipotent progenitors. The loss of Thy-1.1 and gain of Flk-2 expression marks the loss of self-renewal in HSC maturation. The addition of Flk-2 antibody to the lineage mix allows direct isolation of LT-HSC from adult bone marrow as c-kit(+) lin(-) Sca-1(+) Flk-2(-) from many strains of mice. Fetal liver HSCs are contained within Flk-2(-) and Flk-2(+) KTLS cells.

The FLT3 tyrosine kinase receptor inhibits neural stem/progenitor cell proliferation and collaborates with NGF to promote neuronal survival

The FLT3 receptor tyrosine kinase (FLT3) was originally identified on hematopoietic stem cells (HSCs) and its ligand (FL) induces HSC proliferation. As stem cells originating from various tissues are more similar than once thought, the goal of this study was to determine whether neural stem cells express FLT3 and proliferate in response to FL. In fact, a subset of neural stem/progenitor cells does express FLT3, but contrary to our expectations, FL inhibited EGF and FGF-2 stimulated proliferation. Since FLT3 is expressed weakly by proliferative neuroepithelia but strongly by subsets of neurons in the CNS and PNS, we tested its ability to support neuronal survival. FL synergized with NGF to promote the survival of cultured DRG neurons, although it lacked any neurotrophic activity alone. We conclude that FL serves as an adjunct trophic factor in the nervous system, which differs from its role in the hematopoietic system.

Controlling culture dynamics for the expansion of hematopoietic stem cells

The ex vivo expansion of hematopoietic stem cells (HSCs) is the subject of intense commercial and academic interest due to the potential of HSCs to be a renewable source of material for cellular therapeutics. Unfortunately, because methodologies have not yet been developed to grow clinically relevant numbers of HSCs (or their derivatives) consistently, the potential of this technology is limited. Manipulation of the in vitro culture microenvironment, primarily through cytokine supplementation, has been the predominant approach in studies attempting to expand primary human HSC numbers in vitro. While promising results have been obtained, it is becoming clear that novel methods must be developed before cellular therapies using these stem cells can become routine. Ideally, bioprocesses must be designed to target specifically the growth of stem cell populations while incorporating positive and negative feedback from potentially dynamic mature and maturing cell populations. The product of these culture systems should consist of not only HSCs, but also of cells that allow the engraftment of HSCs and, ideally, cells responsible for the immediate or accelerated functional support of patients. Development of such "designer transplants" will require combining optimal culture conditions capable of amplifying HSC numbers with novel approaches for finely controlling the number, functional capabilities, and characteristics of potentially therapeutic cells in these very complex cell culture systems.

Progenipoietin-1: a multifunctional agonist of the granulocyte colony-stimulating factor receptor and fetal liver tyrosine kinase-3 is a potent mobilizer of hematopoietic stem cells

OBJECTIVE

Progenipoietin-1 is an agonist of both the granulocyte colony-stimulating factor and fetal liver tyrosine kinase-3 receptors capable of inducing the proliferation of multiple hematopoietic cell lineages. The potential of progenipoietin-1 to mobilize transplantable hematopoietic stem cells into the peripheral blood was evaluated.

METHODS

Cohorts of donor mice were treated with either progenipoietin-1, fetal liver tyrosine kinase-3 ligand, granulocyte colony-stimulating factor, or a vehicle control. Hematopoietic progenitor/stem-cell activity in donor blood was assayed by radioprotection, multilineage reconstitution, secondary transplantation, and competitive repopulation.

RESULTS

Only 1 microL of peripheral blood from progenipoietin-1-treated donors was required to protect 80% of lethally irradiated mice, while in contrast 1 microL of peripheral blood from granulocyte colony-stimulating factor-treated donors failed to protect any recipients. The radioprotected recipients of progenipoietin-1-treated donor cells showed donor-derived (Ly5.2) multilineage hematopoietic reconstitution for up to 6 months. Serial transplantation studies using bone marrow from radioprotected, chimeric recipients demonstrated long-term donor-derived hematopoiesis, indicating the successful transplantation of multipotent hematopoietic stem cells. The engraftment potential of progenipoietin-1 donor-derived cells was directly compared with donors treated with granulocyte colony-stimulating factor or fetal liver tyrosine kinase-3 ligand alone or in combination. Both spleen colony-forming activity and competitive repopulating activity was highest in the blood from progenipoietin-1-treated donors.

CONCLUSIONS

These studies demonstrate that progenipoietin-1 is a potent mobilizer of transplantable hematopoietic stem cells and indicate that this dual-receptor agonist has greater biologic activity than its constituent molecules.

Monoclonal antibody BB9 raised against bone marrow stromal cells identifies a cell-surface glycoprotein expressed by primitive human hemopoietic progenitors

OBJECTIVE

The identification of cell-surface antigens whose expression is limited to primitive hematopoietic progenitor cells (HPC) is of major value in the identification, isolation, and characterization of candidate stem cells in human hemopoietic tissues. Based on the observation that bone marrow stromal cells and primitive HPC share several cell-surface antigens, we sought to generate monoclonal antibodies to HPC by immunization with cultured human stromal cells.

METHODS

BALB/c mouse were immunized with human bone marrow (BM)-derived stromal cells. Splenocytes isolated from immunized mice were fused with the NS-1 murine myeloma cell line and resulting hybridomas selected in HAT medium, then screened for reactivity against stromal cells, peripheral blood (PB), and BM cells.

RESULTS

A monoclonal antibody (MAb), BB9, was identified based on its binding to stromal cells, a minor subpopulation of mononuclear cells in adult human BM, and corresponding lack of reactivity with leukocytes in PB. BB9 bound to a minor subpopulation of BM CD34(+) cells characterized by high-level CD34 antigen and Thy-1 expression, low-absent expression of CD38, low retention of Rhodamine 123, and quiescent cycle status as evidenced by lack of labeling with Ki67. CD34(+)BB9(+) cells, in contrast to CD34(+)BB9(-) cells, demonstrated a capacity to sustain hematopoiesis in pre-CFU culture stimulated by the combination of IL-3, IL-6, G-CSF, and SCF. BB9 also demonstrated binding to CD34(+) cells from mobilized PB.

CONCLUSION

Collectively, these data therefore demonstrate that MAb BB9 identifies an antigen, which is selectively expressed by hierarchically primitive human HPC and also by stromal cells.

Optimization of culture conditions to enhance transfection of human CD34+ cells by electroporation

The ability to culture CD34+ stem cells, while maintaining their pluripotency, is essential for manipulations such as gene transfection for therapeutic trials. Human peripheral blood (PB) CD34+ cells (> or = 90% purity) were cultured for up to 4 days in serum-free culture medium supplemented with thrombopoietin (TPO), stem cell factor (SCF), Flt-3 ligand (Flt-3L), with or without PIXY321 (IL-3/GM-CSF fusion protein) and human serum. The CD34 mean fluorescence intensity (MFI) and cell cycle status were evaluated daily using flow cytometry and hypotonic propidium iodide. Prior to culture (day 0), 97.0 +/- 0.9%, 1.9 +/- 0.3% and 1.0 +/- 0.6% of the selected CD34+ cells were in G0-G1, S-phase, or G2-M, respectively. After 2-4 days in culture with TPO/SCF/Flt-3L, there was an increase in the percent of cells in S-phase to 26.4 +/- 0.1% without significant loss of CD34 MFI. The addition of PIXY321 increased.the percentage of CD34+ cells in S-phase to 36.3 +/- 4.0%, but the CD34 MFI and numbers of CFU (colony-forming units) were significantly decreased at day 3 when cultured with PIXY321 or various recombinant cytokine combinations that included IL-3 and IL-6. There is an increase from day 0 to day 4 in the percentages of CD34+ with CD38-, HLA-DR-, and c-kit(low), but not Thy-1+ cells. Electroporation with EGFP reporter gene showed that 1-2 days of pre-stimulation in X-VIVO 10 supplemented with TPO/SCF/Flt-3L was necessary and sufficient for efficient transfection. Flow cytometry analysis demonstrated that 22% of the viable cells are CD34+/EGFP+ 48 h post electroporation. The introduced reporter gene appears to be stable as determined by EGFP+/LTC-IC (long-term colony-initiating cells), at 30-40 positive colonies (16 +/- 7%) per 1 x 10(5) electroporated CD34+ cells.

Major histocompatibility complex restriction between hematopoietic stem cells and stromal cells in vitro

We have previously found that a significant number of hematopoietic progenitors accumulate in engrafted bones with the same major histocompatibility complex (MHC) as the transplanted bone marrow cells. In the present study, to further clarify the MHC restriction between hematopoietic stem cells (HSC) and microenvironment, we carried out cobblestone colony formation assays by culturing HSCs with MHC-matched or -mismatched stromal cell monolayers. The formation of cobblestone colonies under MHC-mismatched stromal cells significantly decreased in comparison with MHC-matched stromal cells. However, the decrease in cobblestone colony formation under MHC-mismatched stromal cells was not significant when using MHC class I-deficient HSC or stromal cells. Taken together with the results using B10 congenic strains, it is suggested that the MHC preference is restricted by MHC class Ia molecules. Treatment with monoclonal antibodies (mAbs) against MHC class Ia molecules of stromal cell phenotypes significantly enhanced the cobblestone colony formation, whereas treatment with mAbs against HSC phenotypes significantly inhibited it. The expression of cytokines to promote hematopoiesis was enhanced by the mAbs against stromal cell phenotypes. The enhancement of cytokine expression was also observed when stromal cells and HSCs were MHC-matched. These results suggest that signaling via the MHC molecules augments stromal cell activity and elicits the MHC restriction.

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.

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.

Disparate regulation of human fetal erythropoiesis by the microenvironments of the liver and bone marrow

The liver and the bone marrow (BM) are the major organs that support hematopoiesis in the human fetus. Although both tissues contain the spectrum of hematopoietic cells, erythropoiesis dominates the liver. Previous studies suggested that a unique responsiveness of fetal burst-forming units erythroid (BFU-E) to erythropoietin (EPO) obviates the need for cytokines with burst-promoting activity (BPA) in fetal erythropoiesis. This potential regulatory mechanism whereby fetal erythropoiesis is enhanced was further investigated. Fluorescence-activated cell sorting was used to isolate liver and BM progenitors based on their levels of CD34 and CD38 expression. The most mature population of CD34+ lineage (Lin-) cells was also the most prevalent of the three subpopulations and contained BFU-E responsive to EPO alone under serum-deprived conditions. Kit ligand (KL) also strongly synergized with EPO in stimulating the growth of these BFU-E. An intermediate subset of CD34++CD38+Lin- cells contained erythroid progenitors responsive to EPO alone, but also displayed synergism between EPO and KL, granulocyte-macrophage colony-stimulating factor (GM-CSF), or interleukin (IL)-3, demonstrating that erythroid progenitors that respond to cytokines with BPA do exist in fetal tissues as in the adult BM. Candidate stem cells (CD34++CD38-Lin- cells) did not respond to EPO. Synergisms among KL, GM-CSF, and IL-3, and to a lesser extent granulocyte colony-stimulating factor (G-CSF) and FLK-2/FLT-3 ligand (FL), supported the growth of primitive multipotent progenitors that became responsive to EPO. These data define the limits of EPO activity in fetal erythropoiesis to cells that express CD38 and demonstrate the potential for various cytokine interactions to be involved in regulating fetal erythropoiesis. Furthermore, a comparison of the responses of liver and BM erythroid progenitors revealed similarity in their responses to cytokines but a difference in the frequency of BFU-E among the three subpopulations examined. A higher frequency of BFU-E among the intermediate and late progenitor subsets in the liver indicates that regulatory factors acting on stem cells and their immediate progeny are partially responsible for the high content of erythropoiesis in the liver. These data implicate a critical role for the microenvironments of the liver and BM in regulating the disparate levels of erythropoiesis in these tissues.

Efficient expression of foreign genes in human CD34(+) hematopoietic precursor cells using electroporation

Introduction of foreign genes into human CD34(+) hematopoietic precursor cells offers a means to correct inborn errors or to protect human stem cells from chemotherapeutic damage. Electroporation is a non-chemical, nonviral, highly reproducible means to introduce foreign genes into mammalian cells that has been used primarily for rapidly dividing cells. CD34(+) cells isolated from mobilized peripheral blood of patients were cultured for 48 h in serum-free culture medium supplemented with Flt-3 ligand, stem cell factor and thrombopoietin. Cell cycle analysis showed an increase in % S-phase from 2% on day 0 to 28% on day 2 without significant loss of mean fluorescence intensity (MFI). Optimal electroporation conditions for CD34(+) cells were 550 V/cm, 38 ms, 30 microg DNA/500 microl at cell densities between 0.2 x 10(6) and 10 x 10(6) cells/ml resulting in transient EGFP gene expression in 21% (+/- 1%) of CD34(+) precursor cells, as determined by flow cytometry 48 h after electroporation. The more primitive cells were also found to be EGFP(+) as determined by subset analysis using Thy1, CD38, AC133 and c-kit conjugated monoclonal antibodies. Methylcellulose assays on electroporated CD34(+) cells yielded 20% (+/- 7%) EGFP(+) colonies (CFU-GM, BFU-E and CFU-mix) and 22% (+/- 5%) EGFP(+) long-term colony-initiating cells (LTC-IC). The reporter gene was found to be integrated into the LTC-IC genomic DNA as determined by inverse PCR and DNA sequencing. These results suggest that electroporation has the potential to effectively and stably deliver exogenous genes into human hematopoietic precursor cells.

Optimization of retroviral gene transfer protocol to maintain the lymphoid potential of progenitor cells

We have attempted to improve retrovirus-mediated gene transfer efficacy into hematopoietic progenitor cells (HPCs) without causing them to lose their lymphoid potential. Highly purified CD34(+) cells on CH-296 fibronectin fragments have been transduced with three different cytokine combinations. Murine CD2 was used as a marker gene. Transgene expression was assayed by FACS analysis shortly after transduction of CD34(+) cells and after long-term culture (LTC) extended by differentiation of various lymphoid lineages: NK cells, B cells, and dendritic cells. Compared with the historical cytokine mix, i.e., SCF (stem cell factor) + IL-3 (interleukin 3) + IL-6, the combination SCF + FL (Flt-3 ligand) + M-GDF (megakaryocyte growth and differentiation factor) + IL-3 significantly improved the total number of viable cells and CD34(+) cells after transduction and the long term-cultured progenitors after 6 weeks. In addition, the combination of SCF + FL + M-GDF + IL-3 maintained more efficiently the lymphoid potential of the progeny of transduced long term-cultured CD34(+) cells, as attested by the significantly higher number of CD56(+), CD19(+), and CD1a(+) cells recovered when FL and M-GDF were added to SCF + IL-3. Thus, even though additional improvements may still be needed in transduction of HPCs, these conditions were adopted for a clinical trial of gene therapy for X-linked severe combined immunodeficiency.

Murine M2-10B4 and SL/SL cell lines differentially affect the balance between CD34+ cell expansion and maturation

The ability of bone marrow stroma to modulate hematopoietic progenitor cell expansion is of considerable interest for gene transfer strategies and transplantation of limited stem cell numbers. We compared the capacity of 2 murine stromal cell lines to affect the balance between maturation and proliferation of human CD34+ cells in short-term expansion cultures. In 7-day serum-free cultures, cytokine-induced amplification of granulocyte-macrophage colony-forming cells (CFC-GM), erythroid burst-forming units (BFU-E), and total cells was significantly increased by the presence of genetically engineered Sl/Sl and M2-10B4 stromal cells in a 1:1 ratio (Sl/M2 cells) compared with stroma-free cultures (P < .05). Sl/M2 cultures generated 21-fold more mature CD15+ cells than stroma-free cultures, without further amplifying the number of CD34+ cells. The addition of serum led to a further increase of CFC-GM, total cells, and CD15+ cells, whereas BFU-E were no longer maintained. Pure Sl/Sl stromal layers were likewise superior to stroma-free cultures in expansion of CD34+ cells and total cells when serum was present. However, the differentiation of CD34+ cells was less pronounced in Sl/Sl cultures compared with Sl/M2 layers, as demonstrated by a lower content of CD15+ cells. Neutralization experiments revealed differential contributions of Flt3 ligand and thrombopoietin to the support of total cell and CFC expansion by Sl/M2 and Sl/Sl stromal feeders.

FLT3 ligand preserves the uncommitted CD34+CD38- progenitor cells during cytokine prestimulation for retroviral transduction

Before stem cell gene therapy can be considered for clinical applications, problems regarding cytokine prestimulation remain to be solved. In this study, a retroviral vector carrying the genes for the enhanced version of green fluorescent protein (EGFP) and neomycin resistance (neo(r)) was used for transduction of CD34+ cells. The effect of cytokine prestimulation on transduction efficiency and the population of uncommitted CD34+CD38- cells was determined. CD34+ cells harvested from umbilical cord blood were kept in suspension cultures and stimulated with combinations of the cytokines stem cell factor (SCF), FLT3 ligand, interleukin-3 (IL-3), IL-6, and IL-7 prior to transduction. Expression of the two genes was assessed by flow cytometry and determination of neomycin-resistant colonies in a selective colony-forming unit (CFU) assay, respectively. The neomycin resistance gene was expressed in a higher percentage of cells than the EGFP gene, but there seemed to be a positive correlation between expression of the two genes. The effect of cytokine prestimulation was therefore monitored using EGFP as marker for transduction. When SCF was compared to SCF in combination with more potent cytokines, highest transduction efficiency was found with SCF and IL-3 and IL-6 (5.05% +/- 0.80 versus 2.66% +/- 0.53 with SCF alone, p = 0.04). However, prestimulation with SCF in combination with IL-3 and IL-6 also reduced the percentage of CD34+ cells (p = 0.02). Then, prestimulation with SCF and FLT3 ligand was compared. Significant difference in transduction efficiency was not found. Interestingly, FLT3 ligand seemed to preserve the population of CD34+CD38- cells compared to SCF (16.56% +/- 2.02 versus 9.39% +/- 2.35, p = 0.03). In conclusion, prestimulation with potent cytokine combinations increased the transduction efficiency, but reduced the fraction of CD34+ cells. Importantly, the use of FLT3 ligand seemed to preserve the population of uncommitted cells.

Ex vivo expansion of transplantable human hematopoietic stem cells: where do we stand in the year 2000?

Ex vivo expansion of hematopoietic precursors, progenitors and stem cells represents the modern era of cellular therapeutics in the 21st century. For the last 10 years, increasing means for identifying and purifying hematopoietic stem cells and cytokines have facilitated and improved the development of ex vivo stem cell expansion technology. However, technology has not yet reached a stage where ex vivo-expanded hematopoietic progenitors and stem cells can be used routinely for replacement therapy. Lessons learned over the past 10 years from investigations focused at developing optimal ex vivo stem cell expansion systems have continued to a much greater understanding of stem cell biology. This knowledge has led to novel attempts at ex vivo expansion of hematopoietic precursors, progenitors, and stem cells, and should facilitate development of a new generation of cellular therapeutics. This review addresses recent progress toward development of clinically useful protocols for stem cell expansion. In addition, we discuss the results of a limited number of clinical trials that address the efficacy of such procedures. Three major areas of ex vivo stem cell expansion that impact clinical feasibility are discussed, including: (1) selection of an optimal stem cell population for expansion, (2) definition of the desired characteristics of the expanded stem cell population to be used for engraftment, and (3) development of new reagents and procedures for expansion and infusion of hematopoietic progenitors and stem cells.