Culture system for extensive production of CD19+IgM+ cells by human cord blood CD34+ progenitors

In Vitro Human Haematopoietic Stem Cell Expansion and Differentiation

The haematopoietic system plays an essential role in our health and survival. It is comprised of a range of mature blood and immune cell types, including oxygen-carrying erythrocytes, platelet-producing megakaryocytes and infection-fighting myeloid and lymphoid cells. Self-renewing multipotent haematopoietic stem cells (HSCs) and a range of intermediate haematopoietic progenitor cell types differentiate into these mature cell types to continuously support haematopoietic system homeostasis throughout life. This process of haematopoiesis is tightly regulated in vivo and primarily takes place in the bone marrow. Over the years, a range of in vitro culture systems have been developed, either to expand haematopoietic stem and progenitor cells or to differentiate them into the various haematopoietic lineages, based on the use of recombinant cytokines, co-culture systems and/or small molecules. These approaches provide important tractable models to study human haematopoiesis in vitro. Additionally, haematopoietic cell culture systems are being developed and clinical tested as a source of cell products for transplantation and transfusion medicine. This review discusses the in vitro culture protocols for human HSC expansion and differentiation, and summarises the key factors involved in these biological processes.

Regeneration of immunocompetent B lymphopoiesis from pluripotent stem cells guided by transcription factors

Regeneration of functional B lymphopoiesis from pluripotent stem cells (PSCs) is challenging, and reliable methods have not been developed. Here, we unveiled the guiding role of three essential factors, Lhx2, Hoxa9, and Runx1, the simultaneous expression of which preferentially drives B lineage fate commitment and in vivo B lymphopoiesis using PSCs as a cell source. In the presence of Lhx2, Hoxa9, and Runx1 expression, PSC-derived induced hematopoietic progenitors (iHPCs) immediately gave rise to pro/pre-B cells in recipient bone marrow, which were able to further differentiate into entire B cell lineages, including innate B-1a, B-1b, and marginal zone B cells, as well as adaptive follicular B cells. In particular, the regenerative B cells produced adaptive humoral immune responses, sustained antigen-specific antibody production, and formed immune memory in response to antigen challenges. The regenerative B cells showed natural B cell development patterns of immunoglobulin chain switching and hypermutation via cross-talk with host T follicular helper cells, which eventually formed T cell-dependent humoral responses. This study exhibits de novo evidence that B lymphopoiesis can be regenerated from PSCs via an HSC-independent approach, which provides insights into treating B cell-related deficiencies using PSCs as an unlimited cell resource.

Toward a better definition of hematopoietic progenitors suitable for B cell differentiation

The success of inducing human pluripotent stem cells (hIPSC) offers new opportunities for cell-based therapy. Since B cells exert roles as effector and as regulator of immune responses in different clinical settings, we were interested in generating B cells from hIPSC. We differentiated human embryonic stem cells (hESC) and hIPSC into B cells onto OP9 and MS-5 stromal cells successively. We overcame issues in generating CD34+CD43+ hematopoietic progenitors with appropriate cytokine conditions and emphasized the difficulties to generate proper hematopoietic progenitors. We highlight CD31intCD45int phenotype as a possible marker of hematopoietic progenitors suitable for B cell differentiation. Defining precisely proper lymphoid progenitors will improve the study of their lineage commitment and the signals needed during the in vitro process.

What is the future for cord blood stem cells?

Stem and progenitor cells are present in cord blood at a high frequency making these cells a major target population for experimental and clinical studies. Over the past decade there has been considerable developments in cord blood research and transplantation but despite the rapid progress many problems remain. The initial hope that cord blood would be an alternative source of haemopoietic cells for transplantation has been tempered by the fact that there are insufficient cells in most cord blood collections to engraft an adult of average weight. In attempts to increase the cell number, a plethora of techniques for ex-vivo expansion have been developed.These techniques have also proved useful for gene therapy. As cord blood cells possess unique properties this allows them to be utilised as suitable vehicles for gene therapy and long-term engraftment of transduced cells has been achieved. Current work examining the nature of the stem cells present in this haematological source indicates that cord blood contains not only haemopoietic stem cells but also primitive non-haemopoietic cells with high proliferative and developmental potential. As attention focuses on stem cell biology and the controversies surrounding the potential use of embryonic stem cells in treatment of disease, the properties of stem cells from other sources including cord blood are being re-appraised. The purpose of this article is to review some of the current areas of work and highlight biological problems associated with the use of cord blood cells.

IL-7R expression and IL-7 signaling confer a distinct phenotype on developing human B-lineage cells

IL-7 is an important cytokine for lymphocyte differentiation. Similar to what occurs in vivo, human CD19⁺ cells developing in human/murine xenogeneic cultures show differential expression of the IL-7 receptor α (IL-7Rα) chain (CD127). We now describe the relationship between CD127 expression/signaling and Ig gene rearrangement. In the present study, < 10% of CD19⁺CD127⁺ and CD19⁺CD127⁻ populations had complete VDJ(H) rearrangements. IGH locus conformation measurements by 3D FISH revealed that CD127⁺ and CD127⁻ cells were less contracted than pediatric BM pro-B cells that actively rearrange the IGH locus. Complete IGH rearrangements in CD127⁺ and CD127⁻ cells had smaller CDR3 lengths and fewer N-nucleotide insertions than pediatric BM B-lineage cells. Despite the paucity of VDJ(H) rearrangements, microarray analysis indicated that CD127⁺ cells resembled large pre-B cells, which is consistent with their low level of Ig light-chain rearrangements. Unexpectedly, CD127⁻ cells showed extensive Ig light-chain rearrangements in the absence of IGH rearrangements and resembled small pre-B cells. Neutralization of IL-7 in xenogeneic cultures led to an increase in Ig light-chain rearrangements in CD127⁺ cells, but no change in complete IGH rearrangements. We conclude that IL-7-mediated suppression of premature Ig light-chain rearrangement is the most definitive function yet described for IL-7 in human B-cell development.

Stromal cell-free conditions favorable for human B lymphopoiesis in culture

Progress has been slow in defining molecular requirements for human B lymphopoiesis in part because of differences from experimental animals and also because of the lack of culture conditions that efficiently support the process. We recently found that human CD10+ lymphocytes were produced when CD34+ hematopoietic stem and progenitor cells were cultured in contact with human mesenchymal stem cells (hMSC). Further investigation revealed that it occurred even when progenitors were separated from hMSC by membrane filters. Experiments with neutralizing antibodies suggested that important heat labile factors produced by hMSC are unlikely to be IL-7, TSLP, CXCL12 or hemokinin-1. Further manipulation of culture conditions revealed that optimal lymphopoiesis required careful selection of fetal calf serum lots, maintenance of high cell densities, as well as recombinant cytokines (SCF, FL and G-CSF). G-CSF was particularly important when adult bone marrow rather than umbilical cord blood derived CD34+ cells were used to initiate the cultures. These improved methods should facilitate identification of molecules that can be used to speed regeneration of the humoral immune system following chemotherapy and might suggest ways to inhibit growth of B lineage malignancies.

Reciprocal t(9;22) ABL/BCR fusion proteins: leukemogenic potential and effects on B cell commitment

Background

t(9;22) is a balanced translocation, and the chromosome 22 breakpoints (Philadelphia chromosome – Ph⁺) determine formation of different fusion genes that are associated with either Ph⁺ acute lymphatic leukemia (Ph⁺ ALL) or chronic myeloid leukemia (CML). The “minor” breakpoint in Ph⁺ ALL encodes p185^BCR/ABL from der22 and p96^ABL/BCR from der9. The “major” breakpoint in CML encodes p210^BCR/ABL and p40^ABL/BCR. Herein, we investigated the leukemogenic potential of the der9-associated p96^ABL/BCR and p40^ABL/BCR fusion proteins and their roles in the lineage commitment of hematopoietic stem cells in comparison to BCR/ABL.

Methodology

All t(9;22) derived proteins were retrovirally expressed in murine hematopoietic stem cells (SL cells) and human umbilical cord blood cells (UCBC). Stem cell potential was determined by replating efficiency, colony forming - spleen and competitive repopulating assays. The leukemic potential of the ABL/BCR fusion proteins was assessed by in a transduction/transplantation model. Effects on the lineage commitment and differentiation were investigated by culturing the cells under conditions driving either myeloid or lymphoid commitment. Expression of key factors of the B-cell differentiation and components of the preB-cell receptor were determined by qRT-PCR.

Principal Findings

Both p96^ABL/BCR and p40^ABL/BCR increased proliferation of early progenitors and the short term stem cell capacity of SL-cells and exhibited own leukemogenic potential. Interestingly, BCR/ABL gave origin exclusively to a myeloid phenotype independently from the culture conditions whereas p96^ABL/BCR and to a minor extent p40^ABL/BCR forced the B-cell commitment of SL-cells and UCBC.

Conclusions/Significance

Our here presented data establish the reciprocal ABL/BCR fusion proteins as second oncogenes encoded by the t(9;22) in addition to BCR/ABL and suggest that ABL/BCR contribute to the determination of the leukemic phenotype through their influence on the lineage commitment.

IL-7 Dependence in human B lymphopoiesis increases during progression of ontogeny from cord blood to bone marrow

IL-7 is critical for B cell production in adult mice; however, its role in human B lymphopoiesis is controversial. One challenge was the inability to differentiate human cord blood (CB) or adult bone marrow (BM) hematopoietic stem cells (HSCs) without murine stroma. Here, we examine the role of IL-7 in human B cell development using a novel, human-only model based on coculturing human HSCs on primary human BM stroma. In this model, IL-7 increases human B cell production by >60-fold from both CB and adult BM HSCs. IL-7-induced increases are dose-dependent and specific to CD19(+) cells. STAT5 phosphorylation and expression of the Ki-67 proliferation Ag indicate that IL-7 acts directly on CD19(+) cells to increase proliferation at the CD34(+) and CD34(-) pro-B cell stages. Without IL-7, HSCs in CB, but not BM, give rise to a small but consistent population of CD19(lo) B lineage cells that express EBF (early B cell factor) and PAX-5 and respond to subsequent IL-7 stimulation. Flt3 ligand, but not thymic stromal-derived lymhopoietin (TSLP), was required for the IL-7-independent production of human B lineage cells. As compared with CB, adult BM shows a reduction of in vitro generative capacity that is progressively more profound in developmentally sequential populations, resulting in an approximately 50-fold reduction in IL-7-dependent B lineage generative capacity. These data provide evidence that IL-7 is essential for human B cell production from adult BM and that IL-7-induced expansion of the pro-B compartment is increasingly critical for human B cell production during the progression of ontogeny.

The TEL-AML1 leukemia fusion gene dysregulates the TGF-beta pathway in early B lineage progenitor cells

Chromosome translocation to generate the TEL-AML1 (also known as ETV6-RUNX1) chimeric fusion gene is a frequent and early or initiating event in childhood acute lymphoblastic leukemia (ALL). Our starting hypothesis was that the TEL-AML1 protein generates and maintains preleukemic clones and that conversion to overt disease requires secondary genetic changes, possibly in the context of abnormal immune responses. Here, we show that a murine B cell progenitor cell line expressing inducible TEL-AML1 proliferates at a slower rate than parent cells but is more resistant to further inhibition of proliferation by TGF-beta. This facilitates the competitive expansion of TEL-AML1-expressing cells in the presence of TGF-beta. Further analysis indicated that TEL-AML1 binds to a principal TGF-beta signaling target, Smad3, and compromises its ability to activate target promoters. In mice expressing a TEL-AML1 transgene, early, pre-pro-B cells were increased in number and also showed reduced sensitivity to TGF-beta-mediated inhibition of proliferation. Moreover, expression of TEL-AML1 in human cord blood progenitor cells led to the expansion of a candidate preleukemic stem cell population that had an early B lineage phenotype (CD34+CD38-CD19+) and a marked growth advantage in the presence of TGF-beta. Collectively, these data suggest a plausible mechanism by which dysregulated immune responses to infection might promote the malignant evolution of TEL-AML1-expressing preleukemic clones.

Engineering human hematopoietic stem/progenitor cells to produce a broadly neutralizing anti-HIV antibody after in vitro maturation to human B lymphocytes

Broadly neutralizing anti-HIV antibodies are rare and have proved hard to elicit with any immunogen. We have tested in vitro the notion that such antibodies or other antiviral proteins could be made by lentivirus-mediated gene transfer into human hematopoietic stem/progenitor cells (HSPCs), followed by differentiation of the transduced cells into B cells, the most potent antibody-producing cells. To do this, we have developed a highly efficient system for in vitro maturation of secreting B lymphocytes and plasma cells from CD34(+) HSPCs. It is a 3-stage, in vitro culture system that supports normal human B-lineage development from HSPCs to antibody-secreting plasmablasts (approximately 36%) and plasma cells (approximately 20%). By transducing human cord blood CD34(+) cells with lentiviral vectors encoding a secretory monoclonal anti-HIV antibody, b12 (IgG(1)), we were able to program human B cells to produce in vitro up to 1.5 microg/mL of this broadly neutralizing antibody. Our results suggest that an HIV vaccine might be delivered by autologous transplantation of in vitro-programmed HSPCs, which would develop into antibody-secreting B cells in vivo and provide a continuous supply of anti-HIV neutralizing antibodies.

Interleukin-7 contributes to human pro–B-cell development in a mouse stromal cell–dependent culture system

OBJECTIVE

The role of interleukin (IL)-7 in human B lymphopoiesis is still controversial. We used an in vitro culture system to verify involvement of IL-7 in development of human pro-B cells from hematopoietic stem cells.

MATERIALS AND METHODS

Human CD34(+) bone marrow cells were cultured for 4 weeks on MS-5 mouse stromal cells to induce pro-B cells. Expression of IL-7 receptor alpha or other B-cell differentiation marker genes on cultured human CD34(+)bone marrow cells was investigated by reverse transcription polymerase chain reaction (RT-PCR). Colony assay of human CD34(+) bone marrow cells was also performed to determine the effect of IL-7 on colony-forming ability. Neutralizing antibody or reagent that eliminates the effect of IL-7 was added to the culture system, and the number of pro-B cells induced was estimated by flow cytometry.

RESULTS

RT-PCR analysis revealed mRNA expression of IL-7 receptor alpha as well as B-cell differentiation marker genes in not only CD19(+) pro-B cells but also CD19(-) CD33(-) cells induced from CD34(+) bone marrow cells after cultivation for 4 weeks on MS-5 cells. Addition of anti-mouse IL-7 antibody, anti-human IL-7 receptor alpha antibody, or JAK3 kinase inhibitor reduced the number of pro-B cells induced, demonstrating that elimination of IL-7 reduces pro-B-cell development. Addition of anti-mouse IL-7 antibody emphasized the colony-forming ability of burst-forming unit erythroid cells.

CONCLUSIONS

IL-7 produced by MS-5 cells is required for human pro-B-cell development from CD34(+)bone marrow cells in our culture system, and IL-7 appears to play a certain role in early human B lymphopoiesis.

Identification and comparative analysis of Pax5 C-terminal isoforms expressed in human cord blood-derived B cell progenitors

We identified three Pax5 isoforms due to alternative splicing of the C-terminal exons of its gene in cord blood (CB)-derived B cell progenitors cultivated on the murine bone marrow stromal (HESS-5) cells. Apart from wild type (wt), one isoform skips exon 9 without subsequent frameshift (del9), while the other has a frameshift insert between exons 8 and 9, resulting in novel C-terminal sequences (ins8'). Quantitative reverse transcription-polymerase chain reaction analysis revealed that wt mRNA could be detected in CB CD34(+) cells, but that del9 and ins8' isoforms only appeared after 1 or 3 weeks of co-culture, respectively. Expression of each isoform mRNA was markedly upregulated during B cell differentiation in vitro, and wild type continued to be the most abundant isoform. In a luciferase reporter assay using a synthetic CD19 enhancer, del9 isoform revealed slightly lower activity and ins8' isoform showed much lower activity, compared with Pax5-wt. Furthermore, retroviral expression of each Pax5 isoform in CB CD34(+) cells induced aberrant CD19 expression in a fraction of immature myeloid cells after 1 week of culture, although del9 and ins8' isoforms showed much less potent activity than Pax5-wt. These results suggest that Pax5-wt is quantitatively and qualitatively dominant over other C-terminal isoforms during human B cell differentiation.

Isoform-specific potentiation of stem and progenitor cell engraftment by AML1/RUNX1

BACKGROUND

AML1/RUNX1 is the most frequently mutated gene in leukaemia and is central to the normal biology of hematopoietic stem and progenitor cells. However, the role of different AML1 isoforms within these primitive compartments is unclear. Here we investigate whether altering relative expression of AML1 isoforms impacts the balance between cell self-renewal and differentiation in vitro and in vivo.

METHODS AND FINDINGS

The human AML1a isoform encodes a truncated molecule with DNA-binding but no transactivation capacity. We used a retrovirus-based approach to transduce AML1a into primitive haematopoietic cells isolated from the mouse. We observed that enforced AML1a expression increased the competitive engraftment potential of murine long-term reconstituting stem cells with the proportion of AML1a-expressing cells increasing over time in both primary and secondary recipients. Furthermore, AML1a expression dramatically increased primitive and committed progenitor activity in engrafted animals as assessed by long-term culture, cobblestone formation, and colony assays. In contrast, expression of the full-length isoform AML1b abrogated engraftment potential. In vitro, AML1b promoted differentiation while AML1a promoted proliferation of progenitors capable of short-term lymphomyeloid engraftment. Consistent with these findings, the relative abundance of AML1a was highest in the primitive stem/progenitor compartment of human cord blood, and forced expression of AML1a in these cells enhanced maintenance of primitive potential both in vitro and in vivo.

CONCLUSIONS

These data demonstrate that the "a" isoform of AML1 has the capacity to potentiate stem and progenitor cell engraftment, both of which are required for successful clinical transplantation. This activity is consistent with its expression pattern in both normal and leukaemic cells. Manipulating the balance of AML1 isoform expression may offer novel therapeutic strategies, exploitable in the contexts of leukaemia and also in cord blood transplantation in adults, in whom stem and progenitor cell numbers are often limiting.

Establishment and characterization of a human telomerase catalytic subunit-transduced fetal bone marrow-derived osteoblastic cell line

The fate of human hematopoietic stem cells (HSCs)/progenitor cells (HPCs) is influenced by bone marrow (BM) stromal cells. To investigate the role of stromal cells in the hematopoietic support, we have transduced human fetal BM stromal cells (FBMSCs) with a human telomerase catalytic subunit (hTERT). One of the resultant cell lines was identified as osteoblasts, because it contained mineral deposits and constitutively expressed osteogenic genes osteocalcin, osteopontin, collagen type I, osteoblast marker alkaline phosphatase, but not marrow stromal cell marker STRO-1 and CD105. The hTERT-transduced fetal BM-derived osteoblastic cells (FBMOB-hTERT) can actively maintain the capacity of self-renewal and multipotency of HSCs/HPCs at least partly through transcriptional up-regulation of hematopoietic growth factors such as stem cell growth factors (SCFs) and Wnt-5A during interaction with HSCs/HPCs. The enhanced transcription of SCFs and Wnt-5A appears to be mediated by CD29 signaling. Moreover, the FBMOB-hTERT cells seem superior to primary FBMSCs in supporting hematopoiesis, because they are more potent than primary FBMSCs in supporting the ex vivo expansion and long-term culture initiating cells activity of HSCs. The FBMOB-hTERT cell line has been maintained in vitro more than 125 population doublings without tumorigenicity. The results indicate that the FBMOB-hTERT is useful for the study of molecular mechanisms by which osteoblasts support hematopoiesis.

Involvement of insulin-like growth factor-I and insulin-like growth factor binding proteins in pro-B-cell development

OBJECTIVE

Insulin-like growth factor (IGF)-binding proteins (IGFBPs) are a family of proteins thought to modulate IGF function. By employing an in vitro culture system of human hematopoietic stem cells cocultured with murine bone marrow stromal cells, we examined the effects of IGF-I and IGFBPs on early B-cell development.

MATERIALS AND METHODS

Human CD34(+) bone marrow cells were cocultured with murine stromal MS-5 cells for 4 weeks, and pro-B-cell number was analyzed by flow cytometry. After administration of reagents that are supposed to modulate IGF-I or IGFBP function to the culture, the effect on pro-B-cell development was examined.

RESULTS

After cultivation for 4 weeks, effective induction of pro-B-cell proliferation was observed. Experiments using several distinct factors, all of which neutralize IGF-I function, revealed that impairment of IGF-I function results in a significant reduction in pro-B-cell development from CD34(+) cells. In addition, when the effect of recombinant proteins of IGFBPs and antibodies against IGFBPs were tested, IGFBP-3 was found to inhibit pro-B-cell development, while IGFBP-6 was required for pro-B-cell development.

CONCLUSIONS

IGF-I is essential for development of bone marrow CD34(+) cells into pro-B cells. Moreover, IGFBPs are likely involved in regulation of pro-B-cell development.

Production of human B cells from CD34+CD38- T- B- progenitors in organ culture by sequential cytokine stimulation

We investigated sequential cytokine addition on human hematopoietic stem cell (HSC) differentiation in murine fetal liver (FL), fetal spleen (FS) and bone marrow (BM) organ cultures (OC). Tissues were colonized with unpurified or FACS sorted CD34+CD38-CD10-CD19-CD3-CD8-CD4-(T- B-) cells from human cord blood (HUCB). CD19+ cell production and kinetics differed in each tissue. Fetal liver organ cultures (FLOC) inoculated with CD34+CD38-T-B- cells produced fewer CD19+ cells than fetal liver organ culture (FLOC) cultured with unpurified HUCB. CD19+ cell production was restored in the CD34+CD38-T-B- organ cultures by treating with SCF, LIF and IL-6 followed by IL-7 and removing all cytokines for the last 3 days of culture (a six-fold increase). FLOC also produced CD34+CD38-T-B- cells and monocyte-lineage CD33+CD14- cells, both of which increased after cytokine treatment. Re-colonization of secondary FLOC with CD34+CD38-T-B- cells generated in primary FLOC produced additional B-cells, monocytes and CD34+CD38- cells suggesting that the primary cells retained HSC activity. Expansion and differentiation of HSCs depended on the microenvironment of the recipient tissue as well as addition of cytokines in the appropriate order.

Murine and Human IL-7 Activate STAT5 and Induce Proliferation of Normal Human Pro-B Cells

The role of IL-7 in lymphoid development and T cell homeostasis has been extensively documented. However, the role of IL-7 in human B cell development remains unclear. We used a xenogeneic human cord blood stem cell/murine stromal cell culture to study the development of CD19+ B-lineage cells expressing the IL-7R. CD34+ cord blood stem cells were cultured on the MS-5 murine stromal cell line supplemented with human G-CSF and stem cell factor. Following an initial expansion of myeloid/monocytoid cells within the initial 2 wk, CD19+/pre-BCR- pro-B cells emerged, of which 25-50% expressed the IL-7R. FACS-purified CD19+/IL-7R+ cells were larger and, when replated on MS-5, underwent a dose-dependent proliferative response to exogenous human IL-7 (0.01-10.0 ng/ml). Furthermore, STAT5 phosphorylation was induced by the same concentrations of human IL-7. CD19+/IL-7R- cells were smaller and did not proliferate on MS-5 after stimulation with IL-7. In a search for cytokines that promote human B cell development in the cord blood stem cell/MS-5 culture, we made the unexpected finding that murine IL-7 plays a role. Murine IL-7 was detected in MS-5 supernatants by ELISA, recombinant murine IL-7 induced STAT5 phosphorylation in CD19+/IL-7R+ pro-B cells and human B-lineage acute lymphoblastic leukemias, and neutralizing anti-murine IL-7 inhibited development of CD19+ cells in the cord blood stem cell/MS-5 culture. Our results support a model wherein IL-7 transduces a replicative signal to normal human B-lineage cells that is complemented by additional stromal cell-derived signals essential for normal human B cell development.

Cell-cell contact and anatomical compatibility in stromal cell-mediated HSC support during development

Hematopoietic stem cells (HSCs) are able to generate the wide variety of blood cells found in the adult and are maintained in the bone marrow (BM) stromal microenvironment. In the aorta-gonads-mesonephros (AGM), which autonomously generates the first HSCs, the stromal microenvironment is largely uncharacterized. We have previously made an extensive panel of stromal clones from AGM subregions and have found that clones from the urogenital ridges (UG) provide the most potent support for adult BM HSCs. However, it is unknown to what extent the stroma from this developmentally and anatomically distinct microenvironment can support HSCs from other regions of the embryo, such as yolk sac. Moreover, it is unknown whether cell-cell contact is necessary in this microenvironment. Here, we show that the HSCs from the embryonic aorta are the most potently supported HSCs in UG stromal clone co-cultures and that contact is required for the maintenance and expansion of embryo-derived HSCs.

Enhancement of stress-induced apoptosis in B-lineage cells by caspase-9 inhibitor

We have established human B-lineage (BLIN) acute lymphoblastic leukemia cell lines that retain a dependency on fibroblast monolayers for survival and proliferation. Eight hours following removal from adherent cell contact BLIN cells undergo a decrease in mitochondrial transmembrane potential and an increase in annexin V binding. Unexpectedly, the caspase-9 inhibitor (C9i) benzyloxycarbonyl-Leu-Glu-His-Asp-fluoromethylketone enhanced the appearance of apoptotic cells within 8 hours following removal of BLIN cells from fibroblast monolayers. C9i enhancement of apoptosis was dose dependent and did not occur with irreversible inhibitors of caspases-2, -3, -6, and -8. C9i also enhanced apoptosis in cord blood-derived CD19+ B-lineage cells (but not myeloid cells) removed from murine stromal cells. Longer exposure (&gt; 18 hours) to C9i culminated in apoptosis in a panel of B-lineage acute lymphoblastic leukemia (ALL) cell lines in the presence or absence of fibroblast monolayers, as well as in 2 proliferating leukemic cell lines (RAMOS and CEM). BLIN-4L cells made deficient in caspase-9 by RNA interference exhibited no resistance to apoptotic signals and actually showed increased apoptotic sensitivity to staurosporine. These collective results suggest that a 4-amino acid caspase inhibitor of caspase-9 can promote apoptosis and that at least some types of apoptotic pathways in B-lineage ALL do not require caspase-9.

Intravenous administration of human umbilical cord blood cells in a mouse model of amyotrophic lateral sclerosis: distribution, migration, and differentiation

Amyotrophic lateral sclerosis (ALS), a multifactorial disease characterized by diffuse motor neuron degeneration, has proven to be a difficult target for stem cell therapy. The primary aim of this study was to determine the long-term effects of intravenous mononuclear human umbilical cord blood cells on disease progression in a well-defined mouse model of ALS. In addition, we rigorously examined the distribution of transplanted cells inside and outside the central nervous system (CNS), migration of transplanted cells to degenerating areas in the brain and spinal cord, and their immunophenotype. Human umbilical cord blood (hUCB) cells (10(6)) were delivered intravenously into presymptomatic G93A mice. The major findings in our study were that cord blood transfusion into the systemic circulation of G93A mice delayed disease progression at least 2-3 weeks and increased lifespan of diseased mice. In addition, transplanted cells survived 10-12 weeks after infusion while they entered regions of motor neuron degeneration in the brain and spinal cord. There, the cells migrated into the parenchyma of the brain and spinal cord and expressed neural markers [Nestin, III Beta-Tubulin (TuJ1), and glial fibrillary acidic protein (GFAP)]. Infused cord blood cells were also widely distributed in peripheral organs, mainly the spleen. Transplanted cells also were recovered in the peripheral circulation, possibly providing an additional cell supply. Our results indicate that cord blood may have therapeutic potential in this noninvasive cell-based treatment of ALS by providing cell replacement and protection of motor neurons. Replacement of damaged neurons by progeny of cord blood stem cells is probably not the only mechanism by which hUCB exert their effect, since low numbers of cells expressed neural antigens. Most likely, cord blood efficacy is partially due to neuroprotection by modulation of the autoimmune process.

Bone marrow stromal cells prepared using AB serum and bFGF for hematopoietic stem cells expansion

BACKGROUND

An ex vivo culture system was previously established for stem cell expansion using human marrow stromal cells and serum-free medium. However, the stromal cells were prepared using long-term culture medium containing horse serum and FCS, which may transmit infectious diseases of xenogeneic origin. In this study, therefore, a method was established to prepare stromal cells using an AB serum-based medium. In the case that serum from a transplant recipient or PBPC donor is available, additional infectious diseases would not be transmitted.

STUDY DESIGN AND METHODS

Cord blood CD34+ cells were cultured with thrombopoietin, stem cell factor, and flt3/flk2 ligand on a monolayer of human marrow primary stromal cells prepared using long-term culture medium or AB serum-based medium. After 2 weeks, clonogenic progenitor activity and SCID mouse-reconstituting cell activity were assayed. mRNA expression of cytokines and Notch ligand by stromal cells was also examined.

RESULTS

There were no remarkable differences in expansion-supporting activity and mRNA expression between stromal cells established by the two methods.

CONCLUSION

An ex vivo expansion system completely based on AB serum has been established.

B-cell precursors differentiated from cord blood CD34+ cells are more immature than those derived from granulocyte colony-stimulating factor-mobilized peripheral blood CD34+ cells

Umbilical cord blood (CB) has been widely used instead of bone marrow (BM) and peripheral blood (PB) for stem cell transplantation (SCT). However, problems of sustained immunodeficiency after CB transplantation remain to be resolved. To elucidate the mechanism of immunodeficiency, we compared the characteristics of B cells differentiated in vitro from CD34+ cells of CB with those of PB. Purified CD34+ cells from CB and PB were cultured on murine stroma cell-line MS-5 with stem cell factor and granulocyte colony-stimulating factor for 6 weeks. The B-cell precursors (pre-B cells) that differentiated in this culture system, were analysed as to their immunoglobulin heavy chain (IgH) variable region gene repertoire and the expression of B-cell differentiation-related genes. CD10+ CD19+ pre-B cells were differentiated from both PB and CB. Although the usages of IgH gene segments in pre-B cells differentiated from CB and PB were similar, the N region was significantly shorter in CB-derived than PB-derived cells. Productive rearrangements were significantly fewer in cells of CB than PB in the third week. Among a number of B-cell differentiation-related genes, the terminal deoxynucleotidyl transferase (TdT) gene was not expressed in CB-derived cells during the culture. These results indicated that immature features of pre-B cells from CB, such as lack of TdT expression, and a short N region and few productive rearrangements in the IgH gene, might cause the delay in mature B-cell production.

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

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

Serum-free coculture system for ex vivo expansion of human cord blood primitive progenitors and SCID mouse-reconstituting cells using human bone marrow primary stromal cells

OBJECTIVE

In an attempt to maintain and expand human stem cells, many investigators have used xenogeneic, especially murine, stromal cells and fetal calf serum. Because of the possible transmission of infectious diseases, however, the safety of the delivery of grafts expanded in culture using xenogeneic cells and serum has been debated. Using primary human marrow stromal cells, we established a novel serum-free culture system to expand human primitive progenitors and transplantable stem cells.

MATERIALS AND METHODS

Cord blood CD34(+) cells were cultured on a monolayer of human primary marrow stromal cells in the presence of thrombopoietin (TPO), flt3/flk2 ligand (FL), and/or stem cell factor (SCF) under serum-free conditions. After 2 or 4 weeks of culture, cells were examined for clonogenic progenitors and severe combined immunodeficient disorder (SCID) mouse-reconstituting cells (SRC).

RESULTS

In the presence of TPO, FL, and SCF, marrow stromal cells supported more than a 100- and 1,000-fold expansion of CD34(+) cells and colony-forming units in culture after 2 and 4 weeks of incubation, respectively. In addition, cobblestone area-forming cells were expanded more than 18- and 60-fold after 2 and 4 weeks of culture, respectively. Furthermore, SRC assay demonstrated augmented engraftment by cultured cells.

CONCLUSION

This ex vivo expansion system should prove valuable in clinical settings in which stromal cells are available from recipients or stem cell donors.

Stromal cell-dependent ex vivo expansion of human cord blood progenitors and augmentation of transplantable stem cell activity

In vitro maintenance and expansion of human hematopoietic stem cells is crucial for many clinical applications. Thrombopoietin (TPO) and flt3/flk2 ligand (FL) have been suggested to support the proliferation of primitive hematopoietic progenitors and the expansion of transplantable stem cells in culture. In this study, we examined the synergistic effects of the murine stromal cell line MS-5 and a combination of the two cytokines, TPO and FL, on the ex vivo expansion of human cord blood primitive progenitors and transplantable stem cells. A monolayer of MS-5 cells with TPO/FL synergistically supported a more than 600-fold expansion of human cord blood CD34+ cells and CD34+CD38- cells in 2 weeks of culture. Colony-forming unit in culture (CFU-C) and 5-week and 8-week cobblestone area-forming cells (CAFC) were also expanded approximately 300-, 4- and 13-fold, respectively. When MS-5 cells were physically separated from progenitors by a Transwell filter, the synergy was reduced to a quarter of the control, suggesting that direct cell-cell contact between MS-5 cells and progenitors is required for maximum expansion. The severe-combined immunodeficient (scid) mouse-reconstituting cell (SRC) assay demonstrated the slight augmentation of transplantable stem cell activity in culture. These results indicated that MS-5 cells provide a milieu that stimulates the proliferation of primitive progenitors including transplantable stem cells.

Fates of human B-cell precursors

Development of mammalian B-lineage cells is characterized by progression through a series of checkpoints defined primarily by rearrangement and expression of immunoglobulin genes. Progression through these checkpoints is also influenced by stromal cells in the microenvironment of the primary tissues wherein B-cell development occurs, ie, fetal liver and bone marrow and adult bone marrow. This review focuses on the developmental biology of human bone marrow B-lineage cells, including perturbations that contribute to the origin and evolution of B-lineage acute lymphoblastic leukemia and primary immunodeficiency diseases characterized by agammaglobulinemia. Recently described in vitro and in vivo models that support development and expansion of human B-lineage cells through multiple checkpoints provide new tools for identifying the bone marrow stromal cell–derived molecules necessary for survival and proliferation. Mutations in genes encoding subunits of the pre-B cell receptor and molecules involved in pre-B cell receptor signaling culminate in X-linked and non–X-linked agammaglobulinemia. A cardinal feature of these immunodeficiencies is an apparent apoptotic sensitivity of B-lineage cells at the pro-B to pre-B transition. On the other end of the spectrum is the apoptotic resistance that accompanies the development of B-lineage acute lymphoblastic leukemia, potentially a reflection of genetic abnormalities that subvert normal apoptotic programs. The triad of laboratory models that mimic the bone marrow microenvironment, immunodeficiency diseases with specific defects in B-cell development, and B-lineage acute lymphoblastic leukemia can now be integrated to deepen our understanding of human B-cell development.

Fates of human B-cell precursors

Development of mammalian B-lineage cells is characterized by progression through a series of checkpoints defined primarily by rearrangement and expression of immunoglobulin genes. Progression through these checkpoints is also influenced by stromal cells in the microenvironment of the primary tissues wherein B-cell development occurs, ie, fetal liver and bone marrow and adult bone marrow. This review focuses on the developmental biology of human bone marrow B-lineage cells, including perturbations that contribute to the origin and evolution of B-lineage acute lymphoblastic leukemia and primary immunodeficiency diseases characterized by agammaglobulinemia. Recently described in vitro and in vivo models that support development and expansion of human B-lineage cells through multiple checkpoints provide new tools for identifying the bone marrow stromal cell–derived molecules necessary for survival and proliferation. Mutations in genes encoding subunits of the pre-B cell receptor and molecules involved in pre-B cell receptor signaling culminate in X-linked and non–X-linked agammaglobulinemia. A cardinal feature of these immunodeficiencies is an apparent apoptotic sensitivity of B-lineage cells at the pro-B to pre-B transition. On the other end of the spectrum is the apoptotic resistance that accompanies the development of B-lineage acute lymphoblastic leukemia, potentially a reflection of genetic abnormalities that subvert normal apoptotic programs. The triad of laboratory models that mimic the bone marrow microenvironment, immunodeficiency diseases with specific defects in B-cell development, and B-lineage acute lymphoblastic leukemia can now be integrated to deepen our understanding of human B-cell development.

Stromal cell-independent differentiation of human cord blood CD34+CD38- lymphohematopoietic progenitors toward B cell lineage

To study the cytokine regulation of early stages of human B-lymphopoiesis, we developed a stroma-free two-step culture system. Single human cord blood CD34+CD38- cells were individually cultured by micromanipulation with interleukin (IL)-3, stem cell factor (SCF), fIt3 ligand (FL), IL-6 and granulocyte colony-stimulating factor (G-CSF). About 10% of the cells formed primary colonies, which were individually tested for myeloid and B-lymphoid potentials by reculturing aliquots of the primary colony cells into secondary myeloid and B-lymphoid cultures. One third of the primary colonies proved capable of differentiation into CD19+IgM+ cells, as well as into myeloid lineage cells. RT-PCR analyses revealed that some cells in the primary culture had already matured to express B cell-specific transcripts. Thus, the combination of IL-3, SCF, FL, IL-6 and G-CSF supported the differentiation of CD34+CD38- lymphohematopoietic progenitors toward B cell lineage in addition to myeloid lineages. Screening of cytokines to identify the minimum requirement of cytokines in the primary culture revealed that IL-3 and SCF were essential and that the addition of FL, and to a lesser extent IL-6 or G-CSF, to the combination of IL3 and SCF remarkably enhanced the primary colony formation and the generation of CD19+ cells in the secondary B-lymphoid culture.

Presence of primitive lymphoid progenitors with NK or B potential in ex vivo expanded bone marrow cell cultures

OBJECTIVE

In previous work, we showed that CD34+ bone marrow cells can be successfully expanded along the myeloid pathway in stroma- and serum-free conditions in the presence of SCF+IL-3+IL-6+Flt3-l+G-CSF+MGDF. Due to the lack of phenotypically detectable lymphoid cells, it was necessary to address the question of the lymphoid potential of the expanded populations under these conditions.

MATERIALS AND METHODS

The present report describes a long-term culture system that supports human B- and NK-cell differentiation from the day 14 fraction without further selection of the more primitive cells. In NK proliferation assays, the cells were maintained over stroma cells in the presence of IL-2 for 4-5 weeks. NK initiating cells (NK-IC) were determined by a limiting dilution assay. In B-cell cultures, the expanded cells were maintained over MS5 in the presence of Flt3-l for 4-8 weeks.

RESULTS

NK cells rose from 0.2%+/-0.04% at culture initiation to 71%+/-6% at week 5. These cells displayed cytolytic activity. NK-IC evaluation showed a mean 18-fold expansion in the day 14 expanded fraction as compared to the initial day 0 fraction. Similarly, CD19+ cells rose from 0.1% at culture initiation to 30%+/-1% at week 6. Cells produced under these B-LTC conditions were CD34-CD19+CD10+. We also demonstrated that the CD34+/Lin- sorted cells from the day 14 fraction gave rise to NK and B cells.

CONCLUSION

This culture system permits the revelation of a population that, although poorly represented in terms of phenotypically detectable cells, nevertheless retains high levels of lymphoid NK and B potential after 14 days expansion. Such data suggest the persistence, or expansion, of lymphoid progenitors and, hence, the multipotentiality of the expanded progenitor/stem cells.

Comparative studies of different stromal cell microenvironments in support of human B-cell development

This study compared human murine stromal cells for their capacity to support human hematopoietic stem cell (HSC) development into the B lineage. FACS sorted human fetal bone marrow (BM) HSC (CD34+CD19- or CD34+/CD10-/CD19-/CD45RA) were cultured on human fetal BM stromal cells, human skin fibroblasts, or murine S17 stromal cells and analyzed by flow cytometry or reverse transcriptase polymerase chain reaction. CD34+CD19- HSC on human BM stromal cells or fibroblasts differentiated into B-lineage cells with a continuum in density of surface CD19 expression, and some cells expressing micro/kappa or micro/lambda B-cell receptors. In contrast, CD19+ cells from S17 cultures had two- to fourfold higher levels of CD19, but no cells expressing B-cell receptors. The number and percentage of CD19+ cells was high, intermediate, or low in the human BM, human fibroblast, or murine S17 stromal cell cultures, respectively. Reverse transcriptase polymerase chain reaction analysis showed that TdT, CD19, and DHQ52-J(H) rearrangements were expressed at comparable levels when CD34+/CD19- HSC were plated on human or murine stromal cells. In contrast, CD34+/CD10-/CD19-/CD45RA HSC plated on human or murine stromal cells expressed CD19 in both cultures, but TdT was only expressed in human stromal cell cultures. We conclude that human BM stromal cell, human skin fibroblasts, and murine S17 stromal cell cultures can provide complementary and comparative tools for identification of stromal cell ligands with potentially unique functions in regulating human B-cell development.

The hematopoietic stroma

All blood cells are derived from a small common pool of totipotent cells, called hematopoietic stem cells. The process is strictly regulated by the hematopoietic microenvironment, which includes stromal cells, extracellular matrix molecules and soluble regulatory factors. Several experimental in vitro assays have been developed for the study of hematopoietic differentiation, and have provided valuable information on the stroma, which includes, among other cell types, macrophages, fibroblasts, adipocytes, and endothelial cells. The composition, ontogeny, and function in physiological as well as pathological conditions of stroma are discussed.

A clonal culture assay for human cord blood lymphohematopoietic progenitors

We describe a two-step clonal culture assay system for human lymphohematopoietic progenitors present in umbilical cord blood which are capable of differentiation along both myeloid and B-lymphoid lineages. Human cord blood CD34+ cells were plated in methylcellulose in the presence of stem cell factor (SCF), granulocyte-colony stimulating factor (G-CSF), interleukin (IL)-7, and the murine stroma cell line, MS-5. The growing primary colonies were individually examined for their potentials to differentiate along both myeloid and B-lymphoid lineages by reculturing aliquots of the primary colonies in methylcellulose culture containing IL-3, G-CSF and erythropoietin (Epo), and on a monolayer of MS-5 in the presence of SCF and G-CSF. Approximately 10-15% of the primary colonies generated various combinations of myeloid cells and CD19+ sIgM+ cells. Subsequent studies using micromanipulated single CD34+ cells unequivocally demonstrated the clonal origin of the lymphohematopoietic progenitors. This culture system should prove valuable for elucidation of the mechanisms regulating early stages of human lymphohematopoiesis.