Human cord blood CD34+Pax-5+ B-cell progenitors: single-cell analyses of their gene expression profiles

Increasing Complexity of Molecular Landscapes in Human Hematopoietic Stem and Progenitor Cells during Development and Aging

The past five decades have seen significant progress in our understanding of human hematopoiesis. This has in part been due to the unprecedented development of advanced technologies, which have allowed the identification and characterization of rare subsets of human hematopoietic stem and progenitor cells and their lineage trajectories from embryonic through to adult life. Additionally, surrogate in vitro and in vivo models, although not fully recapitulating human hematopoiesis, have spurred on these scientific advances. These approaches have heightened our knowledge of hematological disorders and diseases and have led to their improved diagnosis and therapies. Here, we review human hematopoiesis at each end of the age spectrum, during embryonic and fetal development and on aging, providing exemplars of recent progress in deciphering the increasingly complex cellular and molecular hematopoietic landscapes in health and disease. This review concludes by highlighting links between chronic inflammation and metabolic and epigenetic changes associated with aging and in the development of clonal hematopoiesis.

The Fetal-to-Adult Hematopoietic Stem Cell Transition and its Role in Childhood Hematopoietic Malignancies

As with most organ systems that undergo continuous generation and maturation during the transition from fetal to adult life, the hematopoietic and immune systems also experience dynamic changes. Such changes lead to many unique features in blood cell function and immune responses in early childhood. The blood cells and immune cells in neonates are a mixture of fetal and adult origin due to the co-existence of both fetal and adult types of hematopoietic stem cells (HSCs) and progenitor cells (HPCs). Fetal blood and immune cells gradually diminish during maturation of the infant and are almost completely replaced by adult types of cells by 3 to 4 weeks after birth in mice. Such features in early childhood are associated with unique features of hematopoietic and immune diseases, such as leukemia, at these developmental stages. Therefore, understanding the cellular and molecular mechanisms by which hematopoietic and immune changes occur throughout ontogeny will provide useful information for the study and treatment of pediatric blood and immune diseases. In this review, we summarize the most recent studies on hematopoietic initiation during early embryonic development, the expansion of both fetal and adult types of HSCs and HPCs in the fetal liver and fetal bone marrow stages, and the shift from fetal to adult hematopoiesis/immunity during neonatal/infant development. We also discuss the contributions of fetal types of HSCs/HPCs to childhood leukemias.

Transcription Factors: The Fulcrum Between Cell Development and Carcinogenesis

Higher eukaryotic development is a complex and tightly regulated process, whereby transcription factors (TFs) play a key role in controlling the gene regulatory networks. Dysregulation of these regulatory networks has also been associated with carcinogenesis. Transcription factors are key enablers of cancer stemness, which support the maintenance and function of cancer stem cells that are believed to act as seeds for cancer initiation, progression and metastasis, and treatment resistance. One key area of research is to understand how these factors interact and collaborate to define cellular fate during embryogenesis as well as during tumor development. This review focuses on understanding the role of TFs in cell development and cancer. The molecular mechanisms of cell fate decision are of key importance in efforts towards developing better protocols for directed differentiation of cells in research and medicine. We also discuss the dysregulation of TFs and their role in cancer progression and metastasis, exploring TF networks as direct or indirect targets for therapeutic intervention, as well as specific TFs' potential as biomarkers for predicting and monitoring treatment responses.

The Origin of B-cells: Human Fetal B Cell Development and Implications for the Pathogenesis of Childhood Acute Lymphoblastic Leukemia

Human B-lymphopoiesis is a dynamic life-long process that starts in utero by around six post-conception weeks. A detailed understanding of human fetal B-lymphopoiesis and how it changes in postnatal life is vital for building a complete picture of normal B-lymphoid development through ontogeny, and its relevance in disease. B-cell acute lymphoblastic leukemia (B-ALL) is one of the most common cancers in children, with many of the leukemia-initiating events originating in utero. It is likely that the biology of B-ALL, including leukemia initiation, maintenance and progression depends on the developmental stage and type of B-lymphoid cell in which it originates. This is particularly important for early life leukemias, where specific characteristics of fetal B-cells might be key to determining how the disease behaves, including response to treatment. These cellular, molecular and/or epigenetic features are likely to change with age in a cell intrinsic and/or microenvironment directed manner. Most of our understanding of fetal B-lymphopoiesis has been based on murine data, but many recent studies have focussed on characterizing human fetal B-cell development, including functional and molecular assays at a single cell level. In this mini-review we will give a short overview of the recent advances in the understanding of human fetal B-lymphopoiesis, including its relevance to infant/childhood leukemia, and highlight future questions in the field.

MLL-rearranged infant leukaemia: A 'thorn in the side' of a remarkable success story

Advances in treatment of childhood leukaemia has led to vastly improved survival rates, however some subtypes such as those characterised by MLL gene rearrangement (MLL-r), especially in infants, continue to have high relapse rates and poor survival. Natural history and molecular studies indicate that infant acute lymphoblastic leukaemia (ALL) originates in utero, is distinct from childhood ALL, and most cases are caused by MLL-r resulting in an oncogenic MLL fusion protein. Unlike childhood ALL, only a very small number of additional mutations are present in infant ALL, indicating that MLL-r alone may be sufficient to give rise to this rapid onset, aggressive leukaemia in an appropriate fetal cell context. Despite modifications in treatment approaches, the outcome of MLL-r infant ALL has remained dismal and a clear understanding of the underlying biology of the disease is required in order to develop appropriate disease models and more effective therapeutic strategies.

Transcription Factors That Govern Development and Disease: An Achilles Heel in Cancer

Development requires the careful orchestration of several biological events in order to create any structure and, eventually, to build an entire organism. On the other hand, the fate transformation of terminally differentiated cells is a consequence of erroneous development, and ultimately leads to cancer. In this review, we elaborate how development and cancer share several biological processes, including molecular controls. Transcription factors (TF) are at the helm of both these processes, among many others, and are evolutionarily conserved, ranging from yeast to humans. Here, we discuss four families of TFs that play a pivotal role and have been studied extensively in both embryonic development and cancer-high mobility group box (HMG), GATA, paired box (PAX) and basic helix-loop-helix (bHLH) in the context of their role in development, cancer, and their conservation across several species. Finally, we review TFs as possible therapeutic targets for cancer and reflect on the importance of natural resistance against cancer in certain organisms, yielding knowledge regarding TF function and cancer biology.

Discovery of a CD10-negative B-progenitor in human fetal life identifies unique ontogeny-related developmental programs

Human lymphopoiesis is a dynamic lifelong process that starts in utero 6 weeks postconception. Although fetal B-lymphopoiesis remains poorly defined, it is key to understanding leukemia initiation in early life. Here, we provide a comprehensive analysis of the human fetal B-cell developmental hierarchy. We report the presence in fetal tissues of 2 distinct CD19+ B-progenitors, an adult-type CD10+ve ProB-progenitor and a new CD10-ve PreProB-progenitor, and describe their molecular and functional characteristics. PreProB-progenitors and ProB-progenitors appear early in the first trimester in embryonic liver, followed by a sustained second wave of B-progenitor development in fetal bone marrow (BM), where together they form >40% of the total hematopoietic stem cell/progenitor pool. Almost one-third of fetal B-progenitors are CD10-ve PreProB-progenitors, whereas, by contrast, PreProB-progenitors are almost undetectable (0.53% ± 0.24%) in adult BM. Single-cell transcriptomics and functional assays place fetal PreProB-progenitors upstream of ProB-progenitors, identifying them as the first B-lymphoid-restricted progenitor in human fetal life. Although fetal BM PreProB-progenitors and ProB-progenitors both give rise solely to B-lineage cells, they are transcriptionally distinct. As with their fetal counterparts, adult BM PreProB-progenitors give rise only to B-lineage cells in vitro and express the expected B-lineage gene expression program. However, fetal PreProB-progenitors display a distinct, ontogeny-related gene expression pattern that is not seen in adult PreProB-progenitors, and they share transcriptomic signatures with CD10-ve B-progenitor infant acute lymphoblastic leukemia blast cells. These data identify PreProB-progenitors as the earliest B-lymphoid-restricted progenitor in human fetal life and suggest that this fetal-restricted committed B-progenitor might provide a permissive cellular context for prenatal B-progenitor leukemia initiation.

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.

PAX Genes in Cancer; Friends or Foes?

PAX genes have been shown to be critically required for the development of specific tissues and organs during embryogenesis. In addition, PAX genes are expressed in a handful of adult tissues where they are thought to play important roles, usually different from those in embryogenesis. A common theme in adult tissues is a requirement for PAX gene expression in adult stem cell maintenance or tissue regeneration. The connections between adult stem cell PAX gene expression and cancer are intriguing, and the literature is replete with examples of PAX gene expression in either situation. Here we systematically review the literature and present an overview of postnatal PAX gene expression in normal and cancerous tissue. We discuss the potential link between PAX gene expression in adult tissue and cancer. In addition, we discuss whether persistent PAX gene expression in cancer is favorable or unfavorable.

The density of CD10 corresponds to commitment and progression in the human B lymphoid lineage

BACKGROUND

Requirements for human B lymphopoiesis are still poorly understood, and that has hampered investigation of differentiation events. For example, there are few cell surface antigens that can be used as milestones of lineage progression. The CD10 ectoenzyme is one such marker and has been used to define CLP, but we found substantial tissue specific variations in CD10 levels, and there was no information about how that corresponded to differentiation options.

METHODOLOGY/PRINCIPAL FINDINGS

The aim of the present study was to use recently developed culture methods to assess the nature and differentiation potential of progenitors sorted according to CD10 density from umbilical cord blood (CB), adult bone marrow (BM) or G-CSF mobilized peripheral blood (PB). Many CD34(+) cells in BM express high levels of CD10, while low or low/negative CD10 densities were found on CD34(+) cells in CB or G-CSF mobilized PB, respectively. The relative abundance of CD10(Lo) versus CD10(Hi) cells only accounts for some CB versus BM differences. Almost all of the CD34(+) CD10(Hi) cells expressed CD19 and lymphocyte transcription factors and corresponded to loss of myeloid potential. A high degree of immunoglobulin D(H)-J(H) gene rearrangements was characteristic only of the CD10(Hi) subset. In contrast, the CD34(+) CD10(Lo) progenitors efficiently produced plasmacytoid and conventional dendritic cells as well as myeloid cells. These findings suggest a positive correlation between CD10 density and degree of differentiation. Although freshly isolated CD34(+) CD10(Hi) cells were in cycle, those from CB or BM expanded poorly in culture, suggesting regulators of populations remain to be discovered.

CONCLUSIONS/SIGNIFICANCE

Steps in human B lymphopoiesis have not been sufficiently studied, and we now show that increased CD10 expression corresponds to differentiation potential and stage. CD34(+) CD10(Hi) progenitors are obviously in the B lineage but may have progressed beyond the point where they can be expanded in culture.

Ordering human CD34+CD10-CD19+ pre/pro-B-cell and CD19- common lymphoid progenitor stages in two pro-B-cell development pathways

Studies here respond to two long-standing questions: Are human "pre/pro-B" CD34(+)CD10(-)CD19(+) and "common lymphoid progenitor (CLP)/early-B" CD34(+)CD10(+)CD19(-) alternate precursors to "pro-B" CD34(+)CD19(+)CD10(+) cells, and do the pro-B cells that arise from these progenitors belong to the same or distinct B-cell development pathways? Using flow cytometry, gene expression profiling, and Ig V(H)-D-J(H) sequencing, we monitor the initial 10 generations of development of sorted cord blood CD34(high)Lineage(-) pluripotential progenitors growing in bone marrow S17 stroma cocultures. We show that (i) multipotent progenitors (CD34(+)CD45RA(+)CD10(-)CD19(-)) directly generate an initial wave of Pax5(+)TdT(-) "unilineage" pre/pro-B cells and a later wave of "multilineage" CLP/early-B cells and (ii) the cells generated in these successive stages act as precursors for distinct pro-B cells through two independent layered pathways. Studies by others have tracked the origin of B-lineage leukemias in elderly mice to the mouse B-1a pre/pro-B lineage, which lacks the TdT activity that diversifies the V(H)-D-J(H) Ig heavy chain joints found in the early-B or B-2 lineage. Here, we show a similar divergence in human B-cell development pathways between the Pax5(+)TdT(-) pre/pro-B differentiation pathway that gives rise to infant B-lineage leukemias and the early-B pathway.

Characterization of early stages of human B cell development by gene expression profiling

We have characterized several stages of normal human B cell development in adult bone marrow by gene expression profiling of hemopoietic stem cells, early B (E-B), pro-B, pre-B, and immature B cells, using RNA amplification and Lymphochip cDNA microarrays (n = 6). Hierarchical clustering of 758 differentially expressed genes clearly separated the five populations. We used gene sets to investigate the functional assignment of the differentially expressed genes. Genes involved in VDJ recombination as well as B lineage-associated transcription factors (TCF3 (E2A), EBF, BCL11A, and PAX5) were turned on in E-B cells, before acquisition of CD19. Several transcription factors with unknown roles in B lymphoid cells demonstrated interesting expression patterns, including ZCCHC7 and ZHX2. Compared with hemopoietic stem cells and pro-B cells, E-B cells had increased expression of 18 genes, and these included IGJ, IL1RAP, BCL2, and CD62L. In addition, E-B cells expressed T/NK lineage and myeloid-associated genes including CD2, NOTCH1, CD99, PECAM1, TNFSF13B, and MPO. Expression of key genes was confirmed at the protein level by FACS analysis. Several of these Ags were heterogeneously expressed, providing a basis for further subdivision of E-B cells. Altogether, these results provide new information regarding expression of genes in early stages of human B cell development.

Expression and production of aberrant PAX5 with deletion of exon 8 in B-lineage acute lymphoblastic leukaemia of children

Summary We investigated PAX5 expression in childhood B-lineage acute lymphoblastic leukaemia (ALL). Seven of 21 children with B-lineage ALL had multiple PAX5 variants, while 14 children and healthy controls showed full-length (FL) and one variant PAX5. By Western blotting, healthy controls displayed Pax5-FL, while one short Pax5, derived from the deletion of exon 8 (Pax5-DeltaE8) was produced in 90% of ALL samples, as well as in ALL cell lines. PAX5-DeltaE8 lacked more than 50% of the transactivation domain, indicating that aberrant Pax5 production might lead to the arrest of B-cell differentiation, contributing to the pathogenesis of B-lineage ALL.

Molecular genetic studies of gene identification for osteoporosis: a 2004 update

This review summarizes comprehensively the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of December 2004. It is intended to constitute a sequential update of our previously published review covering the available data up to the end of 2002. Evidence from candidate gene association studies and genome-wide linkage studies in humans, as well as quantitative trait locus mapping animal models are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. An important extension of this update is incorporation of functional genomic studies (including DNA microarrays and proteomics) on osteogenesis and osteoporosis, in light of the rapid advances and the promising prospects of the field. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

Development of human lymphoid cells

The lymphocytes, T, B, and NK cells, and a proportion of dendritic cells (DCs) have a common developmental origin. Lymphocytes develop from hematopoietic stem cells via common lymphocyte and various lineage-restricted precursors. This review discusses the current knowledge of human lymphocyte development and the phenotypes and functions of the rare intermediate populations that together form the pathways of development into T, B, and NK cells and DCs. Clearly, development of hematopoietic cells is supported by cytokines. The studies of patients with genetic deficiencies in cytokine receptors that are discussed here have illuminated the importance of cytokines in lymphoid development. Lineage decisions are under control of transcription factors, and studies performed in the past decade have provided insight into transcriptional control of human lymphoid development, the results of which are summarized and discussed in this review.