In vitro self-renewal division of hematopoietic stem cells

Influence of lineage-specific cytokines on commitment and asymmetric cell division of haematopoietic progenitor cells

We examined the influence of cytokines on erythroid- and myeloid-lineage development of AC133+ cells during primary and secondary cultures. Cells cultured for 14 d in liquid medium containing erythropoietin (EPO) were amplified 831-fold with 98.2% erythroid cells. A similar culture exposed to granulocyte colony-stimulating factor (G-CSF) grew 1350-fold with 97.4% myeloid cells. To assess whether the cells with EPO inducement could respond at this point to G-CSF signal, or vice versa, the EPO-stimulated population was re-grown with G-CSF, constituting 95.2% myeloid, of 5075-fold, cells after 14 d of re-culture. Conversely, reculture of the G-CSF-stimulated population with EPO resulted in a 4083-fold growth with 81.4% erythroid cells. Semisolid culture containing EPO orG-CSF showed that some individual colonies had self- renewal potential after 14 d culture and could be induced todevelop into a different lineage. Analysis of primitive markers, CD34 and Notch1, or lineage markers, EPO-R and CD13, by single-cell reverse transcription polymerase chain reaction showed that individual colonies of 2-16 cells contained at least one CD34-positive cell with expression ofNotch1 and co-expression of EPO-R and CD13 appeared on either CD34-positive or CD34-negative cells. In situ hybridization with the same cell surface markers in cell populations confirmed the asymmetric cell division and co-expression from single cell data. The study provides a useful model for the analysis of multipotential progenitor development, and indicates that progenitor cells co-express genes from different lineage pathways before commitment and that cytokines influence lineage commitment.

Development of allogeneic hematopoietic stem cell transplantation (HSCT)

The field of BMT has entered a new phase based on insights into basic molecular and cellular biology. The mechanism of cure mediated by allogeneic HSCT is now believed to be at least partially mediated by cellular attack against tumor-associated antigens. Better understanding of these cellular targets and the means of targeting them specifically will allow a more precise application of cellular therapy than is currently possible. This in turn may make it possible to design therapies that are increasingly selective without the collateral toxicities of GVHD, infection, and direct organ damage that currently are the limiting features of allogeneic HSCT as practiced until recently. However, it remains a challenge to demonstrate that disease-control using reduced intensity preparative regimens is at least comparable to that achieved by traditional myeloablative HSCT. This will require carefully controlled studies in each the major diseases that have shown sensitivity to the immune-mediated effects of HSCT. The chapters below will highlight progress towards the goals of elucidating the role of NST.

Functional analysis of initial cell divisions defines the subsequent fate of individual human CD34(+)CD38(-) cells

OBJECTIVE

We assessed the relationship of individual cell divisional behavior with the functional fate of stem cell candidates at the single cell level.

MATERIALS AND METHODS

Individual CD34(+)CD38(-) cells derived from cord blood (88-352 cells in each of 25 experiments) were cultured in early-acting conditioned medium (EACM) or late-acting proliferation medium (LAPM). The initial cell divisions were microscopically monitored every 12 to 24 hours and then assessed for primitive function in the myeloid lymphoid-initiating cell assay and committed function in the colony-forming cell (CFC) assay.

RESULTS

Despite a higher proliferative capacity in LAPM, significantly more quiescent cells (11.1 +/- 1.7%) were found in LAPM than in EACM cultures (1.1 +/- 0.4%; p < 0.001). No differences were observed in the initially plated CD34(+)Cd38(-) cells that produced asymmetrically dividing progeny. The majority of cells with subsequent ML-IC function divided in EACM but were found among quiescent cells in LAPM conditions. All cycling cells with subsequent ML-IC capacity initially remained quiescent for at least 96 hours. All ML-IC had been recruited exclusively (100%) from either cytokine nonresponsive (quiescent) or slow and asymmetrically dividing cells (1-2 divisions). In contrast, the majority of CFCs entered the cell cycle immediately after plating, have divided more than two times, and only 20.2 +/- 5.5% of the cycled CFC divided asymmetrically.

CONCLUSIONS

Asymmetric divisional behavior of CD34(+)CD38(-)cells cannot be influenced by culture conditions. Primitive ML-IC can be distinguished from committed CFC by initial quiescence or asymmetric divisions. Committed CFC cycle rapidly and symmetrically.

JAK2, complemented by a second signal from c-kit or flt-3, triggers extensive self-renewal of primary multipotential hemopoietic cells

Defining signals that can support the self-renewal of multipotential hemopoietic progenitor cells (MHPCs) is pertinent to understanding leukemogenesis and may be relevant to developing stem cell-based therapies. Here we define a set of signals, JAK2 plus either c-kit or flt-3, which together can support extensive MHPC self-renewal. Phenotypically and functionally distinct populations of MHPCs were obtained, depending on which receptor tyrosine kinase, c-kit or flt-3, was activated. Self-renewal was abrogated in the absence of STAT5a/b, and in the presence of inhibitors targeting either the mitogen-activated protein kinase or phosphatidylinositol 3' kinase pathways. These findings suggest that a simple two-component signal can drive MHPC self-renewal.

Cell fate determination from stem cells

In the adult, tissue-specific stem cells are thought to be responsible for the replacement of differentiated cells within continuously regenerating tissues, such as the liver, skin, and blood system. In this review, we will consider the factors that influence stem cell fate, taking as a primary example the cell fate determination of hematopoietic stem cells.

Long-term support of hematopoiesis by a single stem cell clone in patients with paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematopoietic stem cell disorder characterized by clonal blood cells that are deficient in glycosylphosphatidylinositol-anchored proteins because of somatic mutations of the PIG-A gene. Many patients with PNH have more than one PNH clone, but it is unclear whether a single PNH clone remains dominant or minor clones eventually become dominant. Furthermore, it is unknown how many hematopoietic stem cells (HSCs) sustain hematopoiesis and how long a single HSC can support hematopoiesis in humans. To understand dynamics of HSCs, we reanalyzed the PIG-A gene mutations in 9 patients 6 to 10 years after the previous analyses. The proportion of affected peripheral blood polymorphonuclear cells (PMNs) in each patient was highly variable; it increased in 2 (from 50% and 65% to 98% and 97%, respectively), was stable in 4 (changed less than 20%), and diminished in 3 (94%, 99%, and 98% to 33%, 57%, and 43%, respectively) patients. The complexity of these results reflects the high variability of the clinical course of PNH. In all patients, the previously predominant clone was still present and dominant. Therefore, one stem cell clone can sustain hematopoiesis for 6 to 10 years in patients with PNH. Two patients whose affected PMNs decreased because of a decline of the predominant PNH clone and who have been followed up for 24 and 31 years now have an aplastic condition, suggesting that aplasia is a terminal feature of PNH.

HOXB4-induced expansion of adult hematopoietic stem cells ex vivo

Hox transcription factors have emerged as important regulators of primitive hematopoietic cell proliferation and differentiation. In particular, HOXB4 appears to be a strong positive regulator of hematopoietic stem cell (HSC) self-renewal. Here we demonstrate the potency of HOXB4 to enable high-level ex vivo HSC expansion. Cultures of nontransduced or GFP-transduced murine bone marrow cells experienced large HSC losses over 10-14 days. In sharp contrast, cultures of HOXB4-transduced cells achieved rapid, extensive, and highly polyclonal HSC expansions, resulting in over 1000-fold higher levels relative to controls and a 40-fold net HSC increase. Importantly, these HSCs retained full lympho-myeloid repopulating potential and enhanced in vivo regenerative potential, demonstrating the feasibility of achieving significant ex vivo expansion of HSCs without functional impairment.

Enhanced hematopoiesis by hematopoietic progenitor cells lacking intracellular adaptor protein, Lnk

Hematopoietic stem cells (HSCs) give rise to variety of hematopoietic cells via pluripotential progenitors and lineage-committed progenitors and are responsible for blood production throughout adult life. Amplification of HSCs or progenitors represents a potentially powerful approach to the treatment of various blood disorders and to applying gene therapy by bone marrow transplantation. Lnk is an adaptor protein regulating the production of B cells. Here we show that Lnk is also expressed in hematopoietic progenitors in bone marrow, and that in the absence of Lnk, the number and the hematopoietic ability of progenitors are significantly increased. Augmented growth signals through c-Kit partly contributed to the enhanced hematopoiesis by lnk-/- cells. Lnk was phosphorylated by and associated with c-Kit, and selectively inhibited c-Kit-mediated proliferation by attenuating phosphorylation of Gab2 and activation of mitogen-activated protein kinase cascade. These observations indicate that Lnk plays critical roles in the expansion and function of early hematopoietic progenitors, and provide useful clues for the amplification of hematopoietic progenitor cells.

Differential gene expression profiling of adult murine hematopoietic stem cells

Hematopoietic stem cells (HSCs) have self-renewal capacity and multilineage developmental potentials. The molecular mechanisms that control the self-renewal of HSCs are still largely unknown. Here, a systematic approach using bioinformatics and array hybridization techniques to analyze gene expression profiles in HSCs is described. To enrich mRNAs predominantly expressed in uncommitted cell lineages, 54 000 cDNA clones generated from a highly enriched population of HSCs and a mixed population of stem and early multipotent progenitor (MPP) cells were arrayed on nylon membranes (macroarray or high-density array), and subtracted with cDNA probes derived from mature lineage cells including spleen, thymus, and bone marrow. Five thousand cDNA clones with very low hybridization signals were selected for sequencing and further analysis using microarrays on glass slides. Two populations of cells, HSCs and MPP cells, were compared for differential gene expression using microarray analysis. HSCs have the ability to self-renew, while MPP cells have lost the capacity for self-renewal. A large number of genes that were differentially expressed by enriched populations of HSCs and MPP cells were identified. These included transcription factors, signaling molecules, and previously unknown genes.

Lentiviral vector-mediated transduction of murine CD34(-) hematopoietic stem cells

OBJECTIVE

Efficient gene transfer into murine hematopoietic stem cells (HSCs) provides a powerful tool for exploring hematopoietic stem cell biology. In this study, we evaluated the efficiency of lentiviral vector-mediated gene transfer into murine CD34(-/low)c-Kit(+)Sca-1(+)Lin(-) (CD34(-) KSL) cells that are highly enriched for HSCs.

MATERIALS AND METHODS

FACS-sorted CD34(-) KSL cells were transduced with the vesicular stomatitis virus G glycoprotein-pseudotyped HIV-1-based lentiviral vector containing the green fluorescent protein (GFP) gene under the control of the cytomegalovirus promoter, and then 50 transduced cells were transplanted into lethally irradiated mice. Transduction efficiency was assessed by FACS analysis for GFP expression in peripheral blood (PB) cells. FACS-sorted GFP(+) KSL bone marrow (BM) cells from primary recipients were used for secondary transplantation, and GFP expression in PB cells of reconstituted mice was analyzed by FACS.

RESULTS

GFP expression was detected in PB cells of all primary recipients (n = 10) at an average of 40% (range 26-58%) when the lentiviral vector containing the woodchuck hepatitis virus posttranscriptional regulatory element was used. GFP(+) cells were found in multilineage cells in PB, BM, spleen, and thymus for at least 8 months posttransplantation. In secondary recipients, donor-derived GFP(+) KSL BM cells could reconstitute and GFP expression was detected in both myeloid and lymphoid cells in PB.

CONCLUSION

Our results indicate that lentiviral vectors can efficiently transduce highly enriched murine HSCs and sustain long-term expression of the transgene in the multilineage differentiated progeny in reconstituted mice.

Self-renewal of multipotent long-term repopulating hematopoietic stem cells is negatively regulated by Fas and tumor necrosis factor receptor activation

Multipotent self-renewing hematopoietic stem cells (HSCs) are responsible for reconstitution of all blood cell lineages. Whereas growth stimulatory cytokines have been demonstrated to promote HSC self-renewal, the potential role of negative regulators remains elusive. Receptors for tumor necrosis factor (TNF) and Fas ligand have been implicated as regulators of steady-state hematopoiesis, and if overexpressed mediate bone marrow failure. However, it has been proposed that hematopoietic progenitors rather than stem cells might be targeted by Fas activation. Here, murine Lin(-)Sca1(+)c-kit(+) stem cells revealed little or no constitutive expression of Fas and failed to respond to an agonistic anti-Fas antibody. However, if induced to undergo self-renewal in the presence of TNF-alpha, the entire short and long-term repopulating HSC pool acquired Fas expression at high levels and concomitant activation of Fas suppressed in vitro growth of Lin(-)Sca1(+)c-kit(+) cells cultured at the single cell level. Moreover, Lin(-)Sca1(+)c-kit(+) stem cells undergoing self-renewal divisions in vitro were severely and irreversibly compromised in their short- and long-term multilineage reconstituting ability if activated by TNF-alpha or through Fas, providing the first evidence for negative regulators of HSC self-renewal.

Quantitative assessment of the stem cell self-renewal capacity

Little is known about the manner in which hematopoietic stem cells (HSCs) self-renew. To address this issue, we used a serum-free single-cell culture, followed by transplantation of cultured cells into lethally irradiated mice. CD34-negative or low, c-Kit-positive, Sca-1-positive, lineage marker-negative (CD34-KSL) cells are highly enriched for murine bone marrow HSCs. Successful long-term reconstitution with a single CD34-KSL cell enabled us to study in vitro self-renewal of HSC at clonal level. Using this clonal cell transplantation system, we examined the effect of various cytokines on CD34-KSL cells. Among the cytokines examined, stem cell factor (SCF) and thrombopoietin (TPO) were minimum cytokines to induce cell division of CD34-KSL cells most efficiently. Similarly, multilineage repopulating activity was detected in the cells derived from a significant portion of single cells after culture in the presence of TPO and SCF. However, SCF + IL-3, SCF + IL-6, or SCF + IL-11 + FL appeared to be less effective for self-renewal of HSCs. The activity of HSCs as indicated by repopulation unit (RU) remaining after culture with SCF and TPO was not so different from that of freshly isolated HSCs. However, there was a substantial loss of HSC number in these cultured cells. Taken together, this study provides definitive proof that one HSC can generate at least one HSC in vitro.