Long-term culture system for selective growth of human B-cell progenitors

An in vitro platform supports generation of human innate lymphoid cells from CD34+ hematopoietic progenitors that recapitulate ex vivo identity

Innate lymphoid cells (ILCs) are critical effectors of innate immunity and inflammation, whose development and activation pathways make for attractive therapeutic targets. However, human ILC generation has not been systematically explored, and previous in vitro investigations relied on the analysis of few markers or cytokines, which are suboptimal to assign lineage identity. Here, we developed a platform that reliably generated human ILC lineages from CD34⁺ hematopoietic progenitors derived from cord blood and bone marrow. We showed that one culture condition is insufficient to generate all ILC subsets, and instead, distinct combination of cytokines and Notch signaling are essential. The identity of natural killer (NK)/ILC1s, ILC2s, and ILC3s generated in vitro was validated by protein expression, functional assays, and both global and single-cell transcriptome analysis, recapitulating the signatures and functions of their ex vivo ILC counterparts. These data represent a resource to aid in clarifying ILC biology and differentiation.

Analysis of acute lymphoblastic leukemia drug sensitivity by changes in impedance via stromal cell adherence

The bone marrow is a frequent location of primary relapse after conventional cytotoxic drug treatment of human B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Because stromal cells have a major role in promoting chemotherapy resistance, they should be included to more realistically model in vitro drug treatment. Here we validated a novel application of the xCELLigence system as a continuous co-culture to assess long-term effects of drug treatment on BCP-ALL cells. We found that bone marrow OP9 stromal cells adhere to the electrodes but are progressively displaced by dividing patient-derived BCP-ALL cells, resulting in reduction of impedance over time. Death of BCP-ALL cells due to drug treatment results in re-adherence of the stromal cells to the electrodes, increasing impedance. Importantly, vincristine inhibited proliferation of sensitive BCP-ALL cells in a dose-dependent manner, correlating with increased impedance. This system was able to discriminate sensitivity of two relapsed Philadelphia chromosome (Ph) positive ALLs to four different targeted kinase inhibitors. Moreover, differences in sensitivity of two CRLF2-drivenBCP-ALL cell lines to ruxolitinib were also seen. These results show that impedance can be used as a novel approach to monitor drug treatment and sensitivity of primary BCP-ALL cells in the presence of protective microenvironmental cells.

On-chip recapitulation of clinical bone marrow toxicities and patient-specific pathophysiology

The inaccessibility of living bone marrow hampers the study of its pathophysiology under myelotoxic stress induced by drugs, radiation or genetic mutations. Here, we show that a vascularized human bone-marrow-on-a-chip supports the differentiation and maturation of multiple blood-cell lineages over 4 weeks while improving CD34+ cell maintenance, and that it recapitulates aspects of marrow injury, including myeloerythroid toxicity after clinically relevant exposures to chemotherapeutic drugs and ionizing radiation as well as marrow recovery after drug-induced myelosuppression. The chip comprises a fluidic channel filled with a fibrin gel in which CD34⁺ cells and bone-marrow-derived stromal cells are co-cultured, a parallel channel lined by human vascular endothelium and perfused with culture medium, and a porous membrane separating the two channels. We also show that bone-marrow chips containing cells from patients with the rare genetic disorder Shwachman–Diamond syndrome reproduced key haematopoietic defects and led to the discovery of a neutrophil-maturation abnormality. As an in vitro model of haematopoietic dysfunction, the bone-marrow-on-a-chip may serve as a human-specific alternative to animal testing for the study of bone-marrow pathophysiology.

Effects of autologous stromal cells and cytokines on differentiation of equine bone marrow-derived progenitor cells

OBJECTIVE To develop an in vitro system for differentiation of equine B cells from bone marrow hematopoietic progenitor cells on the basis of protocols for other species. SAMPLE Bone marrow aspirates aseptically obtained from 12 research horses. PROCEDURES Equine bone marrow CD34⁺ cells were sorted by use of magnetic beads and cultured in medium supplemented with cytokines (recombinant human interleukin-7, equine interleukin-7, stem cell factor, and Fms-like tyrosine kinase-3), murine OP9 stromal cell preconditioned medium, and equine fetal bone marrow mesenchymal stromal cell preconditioned medium. Cells in culture were characterized by use of flow cytometry, immunocytofluorescence microscopy, and quantitative reverse-transcriptase PCR assay. RESULTS For these culture conditions, bone marrow-derived equine CD34⁺ cells differentiated into CD19⁺IgM⁺ B cells that expressed the signature transcription factors early B-cell factor and transcription factor 3. These conditions also supported the concomitant development of autologous stromal cells, and their presence was supportive of B-cell development. CONCLUSIONS AND CLINICAL RELEVANCE Equine B cells were generated from bone marrow aspirates by use of supportive culture conditions. In vitro generation of equine autologous B cells should be of use in studies on regulation of cell differentiation and therapeutic transplantation.

Erk Negative Feedback Control Enables Pre-B Cell Transformation and Represents a Therapeutic Target in Acute Lymphoblastic Leukemia

Studying mechanisms of malignant transformation of human pre-B cells, we found that acute activation of oncogenes induced immediate cell death in the vast majority of cells. Few surviving pre-B cell clones had acquired permissiveness to oncogenic signaling by strong activation of negative feedback regulation of Erk signaling. Studying negative feedback regulation of Erk in genetic experiments at three different levels, we found that Spry2, Dusp6, and Etv5 were essential for oncogenic transformation in mouse models for pre-B acute lymphoblastic leukemia (ALL). Interestingly, a small molecule inhibitor of DUSP6 selectively induced cell death in patient-derived pre-B ALL cells and overcame conventional mechanisms of drug-resistance.

Differential expression and function of CD27 in chronic lymphocytic leukemia cells expressing ZAP-70

Chronic lymphocytic leukemia is a malignancy driven by abberant B cell signaling and survival. Leukemic B cells accumulate in the peripheral blood and the lymphoid organs where contact with stromal cells and T cells provide critical survival signals. Clinical severity of CLL is associated with several prognostic markers including expression of the kinase ZAP-70. ZAP-70 expression enhances signaling via the B cell antigen receptor and is associated with increased cell adhesion and migration capacity. Here we report that ZAP-70-positive CLL patients display significantly higher expression of the TNF superfamily receptor and memory marker CD27 than do ZAP-70 negative patients. CD27 expression by CLL was acutely elevated upon BCR cross-linking, or upon ectopic expression of ZAP-70. CD27 expression correlated with functional capacity to adhere to stromal cells and antibody blockade of CD27 impaired CLL binding to stroma. These results provide the first evidence for differential expression of CD27 among CLL prognostic groups, suggest a role for ZAP-70 dependent signaling in CD27 induction and implicate CD27 in cell-cell interactions with the lymphoid tissue microenvironment.

B cell biology: an overview

In this review we summarize recent insights into the development of human B cells primarily by studying immunodeficiencies. Development and differentiation of B cells can be considered as a paradigm for many other developmental processes in cell biology. However, it differs from the development of many other cell types by phases of extremely rapid cell division and by defined series of somatic recombination and mutation events required to assemble and refine the B cell antigen receptors. Both somatic DNA alteration and proliferation phases take place in defined sites but in different organs. Thus, cell migration and timely arrival at defined sites are additional features of B cell development. By comparing experimental mouse models with insights gained from studying defined genetic defects leading to primary immunodeficiencies and hypogammaglobulinemia, we address important features that are characteristic for human B cells. We also summarize recent advances made by developing improved in vitro and in vivo systems allowing the development of human B cells from hematopoietic stem cells. Combined with genetic and functional studies of immunodeficiencies, these models will contribute not only to a better understanding of disease affecting the B lymphocyte compartment, but also to designing better and safer novel B cell-targeted therapies in autoimmunity and allergy.

A feeder-free differentiation system identifies autonomously proliferating B cell precursors in human bone marrow

The peripheral B cell compartment is maintained by homeostatic proliferation and through replenishment by bone marrow precursors. Because hematopoietic stem cells cycle at a slow rate, replenishment must involve replication of precursor B cells. To study proliferation of early human B cell progenitors, we established a feeder cell-free in vitro system allowing the development of B cells from CD34(+) hematopoietic stem cells up to the stage of immature IgM(+) B cells. We found that pro-B and pre-B cells generated in vitro can proliferate autonomously and persist up to 7 wk in culture in the absence of signals induced by exogenously added cytokines. Nevertheless, addition of IL-7 enhanced pre-B cell expansion and inhibited maturation into IgM(+) B cells. The B cell precursor subsets replicating in vitro were highly similar to the bone marrow B cell precursors cycling in vivo. The autonomous proliferation of B cell precursor subsets in vitro and their long-term persistence implies that proliferation during pro-B and pre-B cell stages plays an important role in the homeostasis of the peripheral B cell compartment. Our in vitro culture can be used to study defects in B cell development or in reconstitution of the B cell pool after depletion and chemotherapy.

Adhesion of ZAP-70+ chronic lymphocytic leukemia cells to stromal cells is enhanced by cytokines and blocked by inhibitors of the PI3-kinase pathway

CLL cell survival and proliferation is enhanced through direct contact with supporting cells present in lymphoid tissues. PI3Ks are critical signal transduction enzymes controlling B cell survival and activation. PI3K inhibitors have entered clinical trials and show promising therapeutic activity; however, it is unclear whether PI3K inhibitor drugs differentially affect ZAP-70 positive versus negative CLL cells or target specific microenvironmental interactions. Here we provide evidence that CD40L+IL-4, IL-8 or IL-6 enhance adhesion to stromal cells, with IL-6 showing a selective effect on ZAP-70 positive cells. Stimulatory effects of IL-8 or IL-6 are fully reversed by PI3K inhibition, while the effects of CD40L+IL-4 are partially reversed. While CD40L+IL-4 is the only stimulation increasing CLL cell survival for all patient groups, IL-6 protects ZAP-70 positive cells from cell death induced by PI3K inhibition. Altogether, our results indicate that targeting the PI3K pathway can reverse protective CLL-microenvironment interactions in both ZAP-70 positive and negative CLL despite their differences in cytokine responsiveness.

Aging-associated changes in hematopoiesis and leukemogenesis: what's the connection?

Aging is associated with a marked increase in a number of diseases, including many types of cancer. Due to the complex and multi-factorial nature of both aging and cancer, accurate deciphering of causative links between aging and cancer remains a major challenge. It is generally accepted that initiation and progression of cancers are driven by a process of clonal evolution. In principle, this somatic evolution should follow the same Darwinian logic as evolutionary processes in populations in nature: diverse heritable types arising as a result of mutations are subjected to selection, resulting in expansion of the fittest clones. However, prevalent paradigms focus primarily on mutational aspects in linking aging and cancer. In this review, we will argue that age-related changes in selective pressures are likely to be equally important. We will focus on aging-related changes in the hematopoietic system, where age-associated alterations are relatively well studied, and discuss the impact of these changes on the development of leukemias and other malignancies.

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.

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.

In Vitro Differentiation of Human Placenta-Derived Multipotent Cells into Hepatocyte-Like Cells

Multipotent cells isolated from human term placenta (placenta-derived multipotent cells [PDMCs]) have been known to be able to differentiate into mesodermal lineage cells, including adipocytes and osteoclasts. The low infection rate and young age of placenta compared with other tissue origins of adult stem cells make theses cells attractive target for cell-based therapy. However, the differentiation potential of PDMCs toward hepatic cells has not been evaluated yet. In this study, we cultivated PDMCs with hepatic differentiation medium to evaluate the ability of these cells in differentiating toward hepatic cells. After treatment, the morphologies of differentiated PDMCs changed to polygonal epithelial cell-like. The differentiated cells not only show the hepatocyte-like morphologies but also express hepatocyte-specific markers, including albumin and cytokeratin 18. The bioactivity assays revealed that these hepatocyte-like cells could uptake lipoprotein and store glycogen. Furthermore, the addition of rifampicin increased the gene expression of CYP3A4, which is similar with the activities of human liver cells. According to our previous results, PDMCs were capable of differentiating into mesodermal and ectodermal lineage cells. Our results indicate that PDMCs can differentiate into three germ layer cells, which is similar to embryonic stem cells. In conclusion, placenta might be an easily accessible source for progenitor cells that are capable of differentiating toward hepatocyte-like cells in vitro.

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.

In vitro differentiation and measurement of B cell progenitor activity in culture

As well as other blood cells, B lymphocytes originate from hematopoietic stem cells. However, it is not fully understood how their production is controlled. In the serum-free, stromal-cell-free B cell differentiation culture described here, early steps of the B lineage differentiation process are reproduced under defined conditions. This assay is useful for examining the direct effects of various soluble factors on B cell progenitors because it does not contain stromal cells or unknown factors. Additionally, this assay yields sufficient cloning to measure B cell progenitors from single cell cultures. Stromal cell coculture assays are also described that cover a wider category of precursors such as human B cell progenitors.

Induction of T-cell development from human cord blood hematopoietic stem cells by Delta-like 1 in vitro

The Notch signaling pathway plays a key role at several stages of T-lymphocyte differentiation. However, it remained unclear whether signals induced by the Notch ligand Delta-like 1 could support full T-cell differentiation from a defined source of human hematopoietic stem cells (HSCs) in vitro. Here, we show that human cord blood–derived HSCs cultured on Delta-like 1–expressing OP9 stromal cells undergo efficient T-cell lineage commitment and sustained T-cell differentiation. A normal stage-specific program of T-cell development was observed, including the generation of CD4 and CD8 αβ–T-cell receptor (TCR)–bearing cells. Induction of T-cell differentiation was dependent on the expression of Delta-like 1 by the OP9 cells. Stimulation of the in vitro–differentiated T cells by TCR engagement induced the expression of T-cell activation markers and costimulatory receptors. These results establish an efficient in vitro coculture system for the generation of T cells from human HSCs, providing a new avenue for the study of early T-cell differentiation and function.

A rare fraction of human hematopoietic stem cells with large telomeres

The lack of specific markers for stem cells makes the physical identification of this compartment difficult. Hematopoietic stem cells differ in their repopulating and self-renewal potential. Our study shows that multiple classes of human hematopoietic CD34+ greatly differ in telomere length. Flow-cytometry-based fluorescent in situ hybridization and confocal microscopy of CD34+ cells has revealed remarkable telomere length heterogeneity, with a hybridization pattern consistent with different classes of human hematopoietic progenitor cells. These results also point to the existence of a significant clonal heterogeneity among primitive hematopoietic cells and provide the first evidence of a rare fraction of CD34+ cells with large telomeres in humans.

Biology of human hematopoietic stem and progenitor cells present in circulation

Circulating hematopoietic stem and progenitor cells play important roles in the physiology and homeostasis of the hematopoietic system. The frequency of these cells varies throughout development, being more abundant during gestation. In the adult, the numbers of such cells are extremely low; however, they can be increased by intravenous administration of chemotherapy and/or recombinant cytokines to individuals. This mechanism--known as mobilization--involves the disruption of the interactions between primitive hematopoietic cells and microenvironment elements (stromal cells and extracellular matrix molecules), which are mediated by a group of molecules known as cell adhesion molecules. During the last two decades, circulating cells of newborns (those present in umbilical cord blood) and adults (mobilized peripheral blood) have gained relevance not only because of their biology, but also because of their clinical application. Indeed, at present the number of mobilized peripheral blood-derived hematopoietic cell transplants performed worldwide is clearly superior to the number of bone marrow transplants being done annually. On the other hand, the number of cord blood transplants has significantly increased during the last few years, and cord blood banking has expanded in a significant manner over the last decade. Circulating stem and progenitor cells are being manipulated ex vivo, both in cellular and molecular terms, and the clinical use of these manipulated cells is just beginning. Undoubtedly, hematopoietic cells present in circulation will play a key role in the development of both gene and cellular therapies for a variety of diseases.

Ageing, autoimmunity and arthritis: senescence of the B cell compartment - implications for humoral immunity

Immunosenescence is associated with a decline in both T and B lymphocyte function. Although aged individuals have normal numbers of B cells in the periphery and are capable of mounting robust humoral responses, the antibodies produced are generally of lower affinity and are less protective than those produced by young animals. Here we review multiple studies that address the mechanisms that contribute to this decline. Taken together, these studies suggest that age-associated loss of the ability to generate protective humoral immunity results in part from reduced B lymphopoiesis. As the output of new, naïve B cells declines, homeostatic pressures presumably force the filling of the peripheral B cell pool by long-lived antigen-experienced cells. Because the antibody repertoire of these cells is restricted by previous antigenic experience, they make poor quality responses to new immunologic insults.

The spatial organization of centromeric heterochromatin during normal human lymphopoiesis: evidence for ontogenically determined spatial patterns

It is believed that pericentromeric heterochromatin may play a major role in the epigenetic regulation of gene expression. We have previously shown that centromeres in human peripheral blood cells aggregate into distinct "myeloid" and "lymphoid" spatial patterns, suggesting that the three-dimensional organization of centromeric heterochromatin in interphase may be ontogenically determined during hematopoietic differentiation. To investigate this possibility, the spatial patterns of association of different centromeres were analyzed in hematopoietic progenitors and compared with those in early-B and early-T cells, mature B and T lymphocytes, and, additionally, mature granulocytes and monocytes. We show that those patterns change during lymphoid differentiation, with major spatial arrangements taking place at different stages during T and B cell differentiation. Heritable patterns of centromere association are observed, which can occur either at the level of the common lymphoid progenitor, or in early-T or early-B committed cells. A correlation of the observed patterns of centromere association with the gene content of the respective chromosomes further suggests that the variation in the composition of these heterochromatic structures may contribute to the dynamic relocation of genes in different nuclear compartments during cell differentiation, which might have functional implications for cell-stage-specific gene expression.

In vitro identification of human pro-B cells that give rise to macrophages, natural killer cells, and T cells

In this study we report the molecular and functional characterization of very early interleukin 7 receptor alpha (IL-7Ralpha)+-CD79a+CD19- B-cell progenitors, produced by human CD34+CD19-CD10- cord blood cells grown in the presence of stromal cells and cytokines. Purified IL-7Ralpha+CD79a+CD19- cells transcribed the B-lymphoid specific genes E2A, EBF, TdT, Rag-1, had initiated DJH rearrangements, but almost lacked Pax-5 mRNA. When exposed to appropriate environmental conditions, these cells repressed B-cell genes and completely differentiated into CD14+ macrophages, CD56+ natural killer cells, and CD4high T cells. Retention of the DJH rearranged genes in both CD14+ and CD56+ cells unambiguously demonstrates that early B-cell genes, expressed prior to Pax-5, can be activated in a multipotent human progenitor cell whose final fate, including in non-B lineages, is determined by external signals.

B lymphopoiesis is active throughout human life, but there are developmental age-related changes

This study addressed several questions concerning age-related changes in human B lymphopoiesis. The relative abundance of pro-B, pre-B, immature, naive, and mature B cells among the CD19(+) lymphocyte fraction of human bone marrow was found not to change appreciably over the interval between 24 and 88 years of age. Moreover, proliferation of pro-B and large pre-B cells in adult marrow equaled that observed with fetal marrow specimens. Exceptionally low numbers of lymphocyte precursors were found in some marrow samples, and the values obtained were used to determine parameters that best reflect B lymphopoiesis. Cord blood always contained higher incidences of functional precursors than adult cells. However, sorted CD34(+) Lin(-) CD10(+) progenitors from cord blood and adult marrow had equivalent potential for differentiation in culture, and notable age-related changes were found in more primitive subsets. A recently described subset of CD34(+)CD38(-)CD7(+) cord blood cells had no exact counterpart in adult marrow. That is, all adult CD34(+)Lin(-)CD7(+)CD10(-) cells expressed CD38, displayed less CD45RA, and had little B-lineage differentiation potential. The CD7(+) fractions in either site contained progenitors for erythroid and natural killer (NK) lineages, and ones sorted from marrow expressed high levels of transcripts for the CD122 interleukin 2 (IL-2)/IL-15 receptor required by NK-lineage precursors. Dramatic changes in human B lymphopoiesis occur early in life, and more information is required to construct a probable sequence of differentiation events prior to the acquisition of CD10.

The microenvironment of AFT024 cells maintains primitive human hematopoiesis by counteracting contact mediated inhibition of proliferation

We have previously shown that maintenance of primitive human hematopoietic stem cells is poor when cultured in contact with marrow stromal feeders. However, when separated from stromal contact, human progenitors can be maintained because adhesion mediated proliferation inhibition does not occur. In this study we demonstrate how the murine fetal liver cell line, AFT024, supports primitive human hematopoiesis better in contact cultures compared to primary feeders. We evaluated if better progenitor maintenance in contact with AFT024 cells can be explained by decreased adhesion itself or decreased adhesion mediated inhibition of proliferation. We show that primitive human hematopoietic cells adhered equally well to AFT024 and primary feeders, such as M2-10B4. Further, contact with metabolically inactive AFT024 cells prevented cell cycle progression and decreased maintenance of primitive progenitors to the same extent as contact with M2-10B4 feeders. However, contact with viable AFT024 feeders did not inhibit proliferation, suggesting that AFT024-factors counteract contact mediated inhibition of proliferation. Cytokine production by M2-10B4 and AFT024 cells was similar. Large-size O-sulfated heparan sulfate glycosaminoglycans, known to be important for hematopoietic support, were found only in AFT024-matrix. We hypothesize that these factors may explain, in part, our observations. Finally, we show that more than 100% of primitive myeloid progenitors could be maintained for at least five weeks when cultured in contact with AFT024 feeders in the presence of Interleukin-3 and Macrophage Inflammatory Protein-1alpha. In conclusion, AFT024 cells produce factor(s), that counteract contact induced growth inhibition of primitive human hematopoietic progenitors, leading to expansion of these cells in contact with the microenvironment.

In vitro proliferation, expansion, and differentiation of a CD34+ cell-enriched hematopoietic cell population from human umbilical cord blood in response to recombinant cytokines

BACKGROUND

The conditions and mechanisms that control the in vitro growth of hematopoietic stem/progenitor cells (contained within the population of CD34+ cells) are still not completely understood.

METHODS

By using an immunomagnetic system, we have enriched for umbilical cord blood (UCB)-derived CD34+ cells (55% of total cells recovered vs. 0.8% of total cells prior to the enrichment procedure) and analyzed their in vitro growth (proliferation, expansion, and differentiation) in a liquid culture system in the absence or presence of different recombinant cytokine combinations.

RESULTS

When the selected cells were cultured in the absence of recombinant cytokines, no proliferation or expansion was observed. In the presence of steel factor (SF) and interleukin-6 (IL-6), total cell number was increased nearly fourfold; however, no progenitor cell expansion took place. When cultures were supplemented with SF and IL-6 together with IL-3 and erythropoietin (EPO), a rapid proliferation of the CD34+ -enriched cell population was observed with a selective stimulation of erythropoiesis. However, this stimulation was only transient, suggesting that there was a rapid exhaustion of erythroid progenitor cells within the first 10 days. Significantly higher levels of proliferation and expansion of progenitor cells were observed in the presence of SF, IL-6, GM-CSF, and G-CSF with preferential stimulation of myelopoiesis. Interestingly, such stimulation of myelopoiesis was sustained for the entire culture period (>30 days). The highest levels of proliferation and expansion were observed in the presence of all six cytokines. Under these conditions, erythropoiesis was also sustained only transiently (10 days), whereas myelopoiesis was sustained for >30 days.

CONCLUSIONS

This study indicates that significant proliferation and expansion of hematopoietic progenitors can be achieved in vitro when culturing a cell population in which CD34+ cells comprise only >50% of the total cells. Our results also suggest that myeloid progenitors (those responding to GM-CSF and G-CSF) possess higher expansion potentials in vitro than their erythroid counterparts. The methods described here for the enrichment and culture of CD34+ cells may be relevant in the development of protocols for the ex vivo proliferation and expansion of hematopoietic progenitors for transplantation.

Forced expression of the Ikaros 6 isoform in human placental blood CD34(+) cells impairs their ability to differentiate toward the B-lymphoid lineage

Studies in mice suggest that the Ikaros (Ik) gene encodes several isoforms and is a critical regulator of hematolymphoid differentiation. Little is known on the role of Ikaros in human stem cell differentiation. Herein, the biological consequences of the forced expression of Ikaros 6 (Ik6) in human placental blood CD34(+) progenitors are evaluated. Ik6 is one of the isoforms produced from the Ikaros premessenger RNA by alternative splicing and is thought to behave as a dominant negative isoform of the gene product because it lacks the DNA binding domain present in transcriptionally active isoforms. The results demonstrate that human cord blood CD34(+) cells that express high levels of Ik6 as a result of retrovirally mediated gene transfer have a reduced capacity to produce lymphoid B cells in 2 independent assays: (1) in vitro reinitiation of human hematopoiesis during coculture with the MS-5 murine stromal cell line and (2) xenotransplantation in nonobese diabetic-severe combined immunodeficient mice. These results suggest that Ikaros plays an important role in stem cell commitment in humans and that the balance between the different isoforms is a key element of this regulatory system; they support the hypothesis that posttranscriptional events can participate in the control of human hematopoietic differentiation.

Differential effects of Notch ligands Delta-1 and Jagged-1 in human lymphoid differentiation

Notch signaling is known to differentially affect the development of lymphoid B and T cell lineages, but it remains unclear whether such effects are specifically dependent on distinct Notch ligands. Using a cell coculture assay we observed that the Notch ligand Delta-1 completely inhibits the differentiation of human hematopoietic progenitors into the B cell lineage while promoting the emergence of cells with a phenotype of T cell/natural killer (NK) precursors. In contrast, Jagged-1 did not disturb either B or T cell/NK development. Furthermore, cells cultured in the presence of either Delta-1 or Jagged-1 can acquire a phenotype of NK cells, and Delta-1, but not Jagged-1, permits the emergence of a de novo cell population coexpressing CD4 and CD8. Our results thus indicate that distinct Notch ligands can mediate differential effects of Notch signaling and provide a useful system to further address cell-fate decision processes in lymphopoiesis.

Inhibitory effect of interleukin 3 on early development of human B-lymphopoiesis

Interleukin 3 (IL-3) is known as a stimulator of proliferation and differentiation of non-lymphoid cells, but information about the activity of IL-3 in lymphopoiesis is limited. In the present study, we examined the effect of IL-3 on human B-lymphopoiesis using the co-culture system on a murine stromal cell line, MS-5, with stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF). Although a large number of CD45+CD19+ B cells were generated in the co-culture of cord blood CD34+ cells, the addition of IL-3 to the co-culture suppressed the B-cell generation in a dose-dependent manner. In the co-culture, CD34+IL-3 receptor alpha-chain (IL-3Ralpha)+ cells, but not IL-3Ralpha- cells, produced B cells, and IL-3 suppressed the B-cell generation from CD34+IL-3Ralpha+ cells, suggesting that IL-3 exerts an inhibitory effect through the binding to IL-3Ralpha on CD34+ cells. Because the delayed addition of IL-3 and the short-term exposure to IL-3 showed that IL-3 acted as an inhibitor at an early stage of B-cell development from CD34+ cells, before the generation of CD19+ B cells, the effect of IL-3 on the early development of B cells from immature CD34+CD38- cells was examined. Individual colonies, produced from clone-sorted CD34+CD38- cells by SCF, thrombopoietin and a complex of IL-6/soluble IL-6R with or without IL-3, were divided into two fractions and analysed for B-cell potential using co-culture on MS-5 with SCF and G-CSF, and haematopoietic potential using methylcellulose clonal culture. Although some colonies cultured without IL-3 showed both B-cell and haematopoietic potential, all the colonies cultured with IL-3 showed only haematopoietic potential. These results indicate that IL-3 has an inhibitory effect on the B-cell generation from human lymphohaematopoietic progenitor cells.

Human natural killer cells with polyclonal lectin and immunoglobulinlike receptors develop from single hematopoietic stem cells with preferential expression of NKG2A and KIR2DL2/L3/S2

The stage of progenitor maturation and factors that determine the fate and clonal acquisition of human natural killer (NK) cell receptors during development are unknown. To study human NK cell receptor ontogeny, umbilical cord blood CD34(+)/Lin(-)/CD38(-) cells were cultured with a murine fetal liver line (AFT024) and defined cytokines. In the absence of lymphocyte-stimulating cytokines or when contact with AFT024 was prohibited, NK cell progeny were killer immunoglobulinlike receptor (KIR) and CD94 lectin receptor negative. In contrast, efficient NK cell differentiation and receptor acquisition was dependent on direct contact of progenitors with AFT024 and the addition of interleukin-15 (IL-15) or IL-2 but not IL-7. To address the question of whether receptor acquisition was determined at the stem cell level, single CD34(+)/Lin(-)/CD38(-) progenitors were studied. More than 400 single cell progeny were analyzed from cultures containing IL-15 or IL-2 and NK cells were always polyclonal, suggesting that receptor fate is determined beyond an uncommitted progenitor and that receptor-negative NK cells acquire class I-recognizing receptors after lineage commitment. KIR2DL2/L3/S2 was expressed more than KIR2DL1/S1 or KIR3DL1, and NKG2A was the dominant CD94 receptor, independent of whether the stem cell source contained the respective major histocompatibility complex class I ligand, suggesting a nonrandom sequence of receptor acquisition. The conclusion is that NK receptor fate is determined after NK cell commitment, does not require stromal presentation of human class I alleles, and is clonally stable after expression but dynamic because new receptors are acquired over time. (Blood. 2001;98:705-713)

Id2 and Id3 inhibit development of CD34(+) stem cells into predendritic cell (pre-DC)2 but not into pre-DC1. Evidence for a lymphoid origin of pre-DC2

We found previously that Id3, which inhibits transcriptional activities of many basic helix-loop-helix transcription factors, blocked T and B cell development but stimulated natural killer (NK) cell development. Here we report that ectopic expression of Id3 and another Id protein, Id2, strongly inhibited the development of primitive CD34(+)CD38(-) progenitor cells into CD123(high) dendritic cell (DC)2 precursors. In contrast, development of CD34(+)CD38(-) cells into CD4(+)CD14(+) DC1 precursors and mature DC1 was not affected by ectopic Id2 or Id3 expression. These observations support the notion of a common origin of DC2 precursors, T and B cells. As Id proteins did not block development of NK cells, a model presents itself in which these proteins drive common lymphoid precursors to develop into NK cells by inhibiting their options to develop into T cells, B cells, and pre-DC2.

Engagement of the human pre-B cell receptor generates a lipid raft-dependent calcium signaling complex

Pre-B cell receptor (pre-BCR) expression is critical for B lineage development. The signaling events initiated by the pre-BCR, however, remain poorly defined. We demonstrate that lipid rafts are the major functional compartment for human pre-B cell activation. A fraction of pre-BCR was constitutively raft associated, and receptor engagement enhanced this association. These events promoted Lyn activation and Igbeta phosphorylation and led to the generation of a raft-associated signaling module composed of tyrosine phosphorylated Lyn, Syk, BLNK, PI3K, Btk, VAV, and PLCgamma2. Formation of this module was essential for pre-BCR calcium signaling. Together, these observations directly link the previously identified genetic requirement for the components of this module in B lineage development with theirfunctional role(s) in human preBCR signaling.

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.

Genetic Modification of Human B-Cell Development: B-Cell Development Is Inhibited by the Dominant Negative Helix Loop Helix Factor Id3

Transgenic and gene targeted mice have contributed greatly to our understanding of the mechanisms underlying B-cell development. We describe here a model system that allows us to apply molecular genetic techniques to the analysis of human B-cell development. We constructed a retroviral vector with a multiple cloning site connected to a gene encoding green fluorescent protein by an internal ribosomal entry site. Human CD34+CD38− fetal liver cells, cultured overnight in a combination of stem cell factor and interleukin-7 (IL-7), could be transduced with 30% efficiency. We ligated the gene encoding the dominant negative helix loop helix (HLH) factor Id3 that inhibits many enhancing basic HLH transcription factors into this vector. CD34+CD38− FL cells were transduced with Id3-IRES-GFP and cultured with the murine stromal cell line S17. In addition, we cultured the transduced cells in a reaggregate culture system with an SV-transformed human fibroblast cell line (SV19). It was observed that overexpression of Id3 inhibited development of B cells in both culture systems. B-cell development was arrested at a stage before expression of the IL-7R. The development of CD34+CD38− cells into CD14+ myeloid cells in the S17 system was not inhibited by overexpression of Id3. Moreover, Id3+ cells, although inhibited in their B-cell development, were still able to develop into natural killer (NK) cells when cultured in a combination of Flt-3L, IL-7, and IL-15. These findings confirm the essential role of bHLH factors in B-cell development and demonstrate the feasibility of retrovirus-mediated gene transfer as a tool to genetically modify human B-cell development.

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.

Follicular dendritic cells mediated maintenance of primary lymphocyte cultures for long-term analysis of a functional in vitro immune system

Primary lymphocyte cultures are important for analysis of cellular and molecular events occurring during immune responses. However, the lymphoid cells (especially B cells) typically only survive for a few days in vitro which limits studies. Establishment of long-term primary lymphocyte cultures where a functioning humoral immune responses can be maintained and regulated is still a challenge. Follicular dendritic cells (FDC) are immune accessory cells that reside in the follicles of secondary lymphoid organs and are known to protect lymphocytes from apoptosis. We hypothesized that addition of FDC to primary lymphocyte cultures may help maintain humoral immune responses in vitro as they do in vivo. To test the hypothesis, freshly isolated lymphocytes were cultured with or without FDC. The B cells in cultures were labeled using B220 and apoptotic cells were labeled using the TUNEL assay. Antibody production was monitored in supernatant fluids using ELISA. The results showed that FDC reduced apoptosis and helped sustain primary lymphocyte cultures and antibody production was maintained throughout the entire period (e.g., 8 weeks). This FDC dependent system should be useful for analysis of cellular and molecular events over extended periods in vitro.

Characterization and retroviral transduction of an early human lymphomyeloid precursor assayed in nonswitched long-term culture on murine stroma

In the hierarchy of human hematopoietic progenitors, long-term culture-initiating cells (LTC-IC) and extended LTC-IC belong to the earliest cell populations that can be assayed in vitro. We report the identification of a multipotential lymphomyeloid progenitor detected in a nonswitch culture system. We observed the emergence of CD33+ myeloid and CD19+ B-lymphoid cells following plating of lineage-depleted (Lin-) CD34 -enriched or purified CD34+ CD38- cord blood cells on MS-5 stroma in the absence of exogenous cytokines. Both CD19+ CD20- pro-B and CD19+ CD20+ pre-B lymphocytes coexist with myeloid cells in long-term culture. A limiting dilution approach was used to show that a single CD34+ CD38- cell can generate lymphomyeloid progeny in conventional (5-week) and extended (10-week) cultures. Most of the clones in long-term culture or extended long-term culture contained not only lymphoid and myeloid cells, but also myeloid clonogenic progenitors. A high proportion of CD34+ CD38- cells gave rise to lymphomyeloid clones after 5 and 10 weeks of culturing (up to 48% and 16%, respectively), which distinguishes the assay reported here from those using switch culture conditions. We performed retroviral gene transfer experiments involving 1-3 days of exposure of Lin CD34+ -enriched cells to virus encoding enhanced green fluorescent protein. Monitoring of gene transfer efficiency into LTC-IC by enhanced green fluorescent protein fluorescence showed that it is possible to achieve marking of lymphomyeloid LTC-IC, albeit to a lesser extent than myeloid-restricted LTC-IC.

Survival of human leukaemic B-cell precursors is supported by stromal cells and cytokines: association with the expression of bcl-2 protein

We searched for cytokines with the potential to support the survival of human B-cell precursor acute lymphoblastic leukaemia (pre-B ALL) cells. 47 patients with pre-B ALL were classified into four stages: stage I, CD19+CD10-CD20-; stage II, CD19+CD10+CD20-; stage III, CD19+CD10+CD20+cytoplasmic mu-heavy chain (cmu)-; stage IV, CD19+CD10+CD20+cmu. Interleukin (IL)-3 receptor alpha chain (IL-3Ralpha) was expressed in all stages, whereas the expressions of IL-7Ralpha and IL-2Ralpha were pronounced in stages IV and II, respectively. Neither IL-3, IL-7 nor IL-2 supported the survival of pre-B ALL cells. When pre-B ALL cells were layered on stromal, MS-10, cells, viability of the pre-B ALL cells increased. Addition of IL-3 to culture containing MS-10 cells enhanced the survival of pre-B ALL cells in all cases, whereas addition of IL-7 augmented the survival of pre-B ALL cells of some cases of stage III and all cases of stage IV. The survival of pre-B ALL cells was also supported by the conditioned media of MS-10 cells. Stromal-cell-derived factor 1 (SDF-1) supported the survival of pre-B ALL cells. Effects of the conditioned media of MS-10 cells were abrogated by an anti-SDF-1 neutralizing antibody. The extent of survival of pre-B ALL cells supported by stromal cells and IL-3 and IL-7, correlated with the expression level of bcl-2 protein. The effects of stromal cells may be in part related to SDF-1.

Single Adult Human CD34+/Lin−/CD38− Progenitors Give Rise to Natural Killer Cells, B-Lineage Cells, Dendritic Cells, and Myeloid Cells

Marrow stromal cultures support adult CD34+/Lin−/HLA-DR− or CD34+/Lin−/CD38− cell differentiation into natural killer (NK) or myeloid cells, but unlike committed lymphoid progenitors (CD34+/Lin−/CD45RA+/CD10+), no B cells are generated. We tested whether different microenvironments could establish a developmental link between the NK and B-cell lineages. Progenitors were cultured in limiting dilutions with interleukin-7 (IL-7), flt3 ligand (FL), c-kit ligand (KL), IL-3, IL-2, and AFT024, a murine fetal liver line, which supports culture of transplantable murine stem cells. NK cells, CD10+/CD19+ B-lineage cells and dendritic cells (DC) developed from the same starting population and IL-7, FL, and KL were required in this process. Single cell deposition of 3,872 CD34+/Lin−/CD38− cells onto AFT024 with IL-7, FL, KL, IL-2, and IL-3 showed that a one time addition of IL-3 at culture initiation was essential for multilineage differentiation from single cells. Single and double lineage progeny were frequently detected, but more importantly, 2% of single cells could give rise to at least three lineages (NK cells, B-lineage cells, and DC or myeloid cells) providing direct evidence that NK and B-lineage differentiation derive from a common lymphomyeloid hematopoietic progenitor under the same conditions. This study provides new insights into the role of the microenvironment niche, which governs the earliest events in lymphoid development.

Single Adult Human CD34+/Lin−/CD38− Progenitors Give Rise to Natural Killer Cells, B-Lineage Cells, Dendritic Cells, and Myeloid Cells

Marrow stromal cultures support adult CD34+/Lin−/HLA-DR− or CD34+/Lin−/CD38− cell differentiation into natural killer (NK) or myeloid cells, but unlike committed lymphoid progenitors (CD34+/Lin−/CD45RA+/CD10+), no B cells are generated. We tested whether different microenvironments could establish a developmental link between the NK and B-cell lineages. Progenitors were cultured in limiting dilutions with interleukin-7 (IL-7), flt3 ligand (FL), c-kit ligand (KL), IL-3, IL-2, and AFT024, a murine fetal liver line, which supports culture of transplantable murine stem cells. NK cells, CD10+/CD19+ B-lineage cells and dendritic cells (DC) developed from the same starting population and IL-7, FL, and KL were required in this process. Single cell deposition of 3,872 CD34+/Lin−/CD38− cells onto AFT024 with IL-7, FL, KL, IL-2, and IL-3 showed that a one time addition of IL-3 at culture initiation was essential for multilineage differentiation from single cells. Single and double lineage progeny were frequently detected, but more importantly, 2% of single cells could give rise to at least three lineages (NK cells, B-lineage cells, and DC or myeloid cells) providing direct evidence that NK and B-lineage differentiation derive from a common lymphomyeloid hematopoietic progenitor under the same conditions. This study provides new insights into the role of the microenvironment niche, which governs the earliest events in lymphoid development.

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.

Early stages in the development of human T, natural killer and thymic dendritic cells

T-cell development is initiated when CD34+ pluripotent stem cells or their immediate progeny leave the bone marrow to migrate to the thymus. Upon arrival in the thymus the stem cell progeny is not yet committed to the T-cell lineage as it has the capability to develop into T, natural killer (NK) and dendritic cells (DC). Primitive hematopoietic progenitor cells in the human thymus express CD34 and lack CD1a. When these progenitor cells develop into T cells they traverse a number of checkpoints. One early checkpoint is the induction of T-cell commitment, which correlates with appearance of CD1a and involves the loss of capacity to develop into NK cells and DC and the initiation of T-cell receptor (TCR) gene rearrangements. Basic helix-loop-helix transcription factors play a role in induction of T-cell commitment. CD1a+CD34+ cells develop into CD4+CD8 alpha+ beta+ cells by upregulating first CD4, followed by CD8 alpha and then CD8 beta. Selection for productive TCR beta gene rearrangements (beta selection) likely occurs in the CD4+CD8 alpha+ beta- and CD4+CD8 alpha+ beta+ populations. Although the T and NK-cell lineages are closely related to each other, NK cells can develop independently of the thymus. The fetal thymus is most likely one site of NK-cell development.

The Reliability and Specificity of c-kit for the Diagnosis of Acute Myeloid Leukemias and Undifferentiated Leukemias

We document findings on c-kit (CD117) expression in 1,937 pediatric and adult de novo acute leukemia cases, diagnosed in five single European centers. All cases were well characterized as to the morphologic, cytochemical, and immunologic features, according to the European Group for the Immunological Classification of Leukemias (EGIL). The cases included 1,103 acute myeloid leukemia (AML), 819 acute lymphoblastic leukemia (ALL), 11 biphenotypic acute leukemia (BAL), and 4 undifferentiated (AUL). c-kit was expressed in 741 (67%) AML cases, regardless of the French-American-British (FAB) subtype, one third of BAL, all four AUL, but only in 34 (4%) of ALL cases. The minority of c-kit+ ALL cases were classified as: T-cell lineage (two thirds), mainly pro-T–ALL or T-I, and B lineage (one third); cells from 62% of these ALL cases coexpressed other myeloid markers (CD13, CD33, or both). There were no differences in the frequency of c-kit+ AML or ALL cases according to age being similar in the adult and pediatric groups. Our findings demonstrate that c-kit is a reliable and specific marker to detect leukemia cells committed to the myeloid lineage, and therefore should be included in a routine basis for the diagnosis of acute leukemias to demonstrate myeloid commitment of the blasts. c-kit expression should score higher, at least one point, in the system currently applied to the diagnosis of BAL, as its myeloid specificity is greater than CD13 and CD33. Findings in ALL and AUL suggest that c-kit identifies a subgroup of cases, which may correspond to leukemias either arising from early prothymocytes and/or early hematopoietic cells, both able to differentiate to the lymphoid and myeloid pathways.

The Reliability and Specificity of c-kit for the Diagnosis of Acute Myeloid Leukemias and Undifferentiated Leukemias

We document findings on c-kit (CD117) expression in 1,937 pediatric and adult de novo acute leukemia cases, diagnosed in five single European centers. All cases were well characterized as to the morphologic, cytochemical, and immunologic features, according to the European Group for the Immunological Classification of Leukemias (EGIL). The cases included 1,103 acute myeloid leukemia (AML), 819 acute lymphoblastic leukemia (ALL), 11 biphenotypic acute leukemia (BAL), and 4 undifferentiated (AUL). c-kit was expressed in 741 (67%) AML cases, regardless of the French-American-British (FAB) subtype, one third of BAL, all four AUL, but only in 34 (4%) of ALL cases. The minority of c-kit+ ALL cases were classified as: T-cell lineage (two thirds), mainly pro-T–ALL or T-I, and B lineage (one third); cells from 62% of these ALL cases coexpressed other myeloid markers (CD13, CD33, or both). There were no differences in the frequency of c-kit+ AML or ALL cases according to age being similar in the adult and pediatric groups. Our findings demonstrate that c-kit is a reliable and specific marker to detect leukemia cells committed to the myeloid lineage, and therefore should be included in a routine basis for the diagnosis of acute leukemias to demonstrate myeloid commitment of the blasts. c-kit expression should score higher, at least one point, in the system currently applied to the diagnosis of BAL, as its myeloid specificity is greater than CD13 and CD33. Findings in ALL and AUL suggest that c-kit identifies a subgroup of cases, which may correspond to leukemias either arising from early prothymocytes and/or early hematopoietic cells, both able to differentiate to the lymphoid and myeloid pathways.

Biology of human umbilical cord blood-derived hematopoietic stem/progenitor cells

Reported in 1989, studies by Broxmeyer, Gluckman, and colleagues demonstrated that umbilical cord blood (UCB) is a rich source of hematopoietic stem/progenitor cells (HSPC) and that UCB could be used in clinical settings for hematopoietic cell transplantation. Since then, a great interest has been generated on the biological characterization of these cells. Over the last nine years, several groups have focused on the study of UCB HSPC, addressing different aspects, such as the frequency of these cells in UCB, the identification of different HSPC subsets based on their immunophenotype, their ability to respond to hematopoietic cytokines, the factors that control their proliferation and expansion potentials, and their capacity to reconstitute hematopoiesis in animal models. Most of these studies have shown that significant functional differences exist between HSPC from UCB and adult bone marrow (i.e., the former possess higher proliferation and expansion potential than the latter). It is also noteworthy that genetic manipulation of UCB HSPC has been achieved by several groups and that genetically modified UCB cells have already been used in the clinic. In spite of the significant advances in the characterization of these cells, we are still in the process of trying to fully understand their biology, both at the cellular and the molecular levels. In the present article, we describe and discuss what is currently known about the biology of UCB HSPC.

In Vitro Identification of Single CD34+CD38− Cells With Both Lymphoid and Myeloid Potential

Human hematopoietic stem cells are pluripotent, ie, capable of producing both lymphoid and myeloid progeny, and are therefore used for transplantation and gene therapy. An in vitro culture system was developed to study the multi-lineage developmental potential of a candidate human hematopoietic stem cell population, CD34+CD38− cells. CD34+CD38− cells cocultivated on the murine stromal line S17 generated predominantly CD19+ B-cell progenitors. Transfer of cells from S17 stroma to myeloid-specific conditions (“switch culture”) showed that a fraction of the immunophenotypically uncommitted CD19− cells generated on S17 stroma had myeloid potential (defined by expression of CD33 and generation of colony-forming unit-cells). Using the switch culture system, single CD34+CD38− cells were assessed for their lymphoid and myeloid potential. Nineteen of 50 (38%) clones generated from single CD34+CD38− cells possessed both B-lymphoid and myeloid potential. 94.7% of the CD34+CD38− cells with lympho-myeloid potential were late-proliferating (clonal appearance after 30 days), demonstrating that pluripotentiality is detected significantly more often in quiescent progenitors than in cytokine-responsive cells (P = .00002). The S17/switch culture system permits the in vitro assessment of the pluripotentiality of single human hematopoietic cells.