Dissection of progenitor compartments resolves developmental trajectories in B-lymphopoiesis

Terminal deoxynucleotidyl transferase and CD84 identify human multi-potent lymphoid progenitors

Lymphoid specification in human hematopoietic progenitors is not fully understood. To better associate lymphoid identity with protein-level cell features, we conduct a highly multiplexed single-cell proteomic screen on human bone marrow progenitors. This screen identifies terminal deoxynucleotidyl transferase (TdT), a specialized DNA polymerase intrinsic to VDJ recombination, broadly expressed within CD34+ progenitors prior to B/T cell emergence. While these TdT+ cells coincide with granulocyte-monocyte progenitor (GMP) immunophenotype, their accessible chromatin regions show enrichment for lymphoid-associated transcription factor (TF) motifs. TdT expression on GMPs is inversely related to the SLAM family member CD84. Prospective isolation of CD84lo GMPs demonstrates robust lymphoid potentials ex vivo, while still retaining significant myeloid differentiation capacity, akin to LMPPs. This multi-omic study identifies human bone marrow lymphoid-primed progenitors, further defining the lympho-myeloid axis in human hematopoiesis.

The Function of E2A in B-Cell Development

Helix-loop-helix (HLH) transcription factors (TFs) play a key role in various cellular differentiation and function through the regulation of enhancer activity. E2A, a member of the mammalian E-protein family (class I HLH protein), is well known to play an important role in hematopoiesis, especially in adaptive lymphocyte development. E2A instructs B- and T-cell lineage development through the regulation of enhancer activity for B- or T-cell signature gene expression, including Rag1 and Rag2 (Rag1/2) genes. In this chapter, we mainly focus on the function of E2A in B-cell development and on the roles of E2A in establishing the enhancer landscape through the recruitment of EP300/KAT3B, chromatin remodeling complex, mediator, cohesion, and TET proteins. Finally, we demonstrate how E2A orchestrates the assembly of the Rag1/2 gene super-enhancer (SE) formation by changing the chromatin conformation across the Rag gene locus.

Early B-Cell Factor 1: An Archetype for a Lineage-Restricted Transcription Factor Linking Development to Disease

The development of highly specialized blood cells from hematopoietic stem cells (HSCs) in the bone marrow (BM) is dependent upon a stringently orchestrated network of stage- and lineage-restricted transcription factors (TFs). Thus, the same stem cell can give rise to various types of differentiated blood cells. One of the key regulators of B-lymphocyte development is early B-cell factor 1 (EBF1). This TF belongs to a small, but evolutionary conserved, family of proteins that harbor a Zn-coordinating motif and an IPT/TIG (immunoglobulin-like, plexins, transcription factors/transcription factor immunoglobulin) domain, creating a unique DNA-binding domain (DBD). EBF proteins play critical roles in diverse developmental processes, including body segmentation in the Drosophila melanogaster embryo, and retina formation in mice. While several EBF family members are expressed in neuronal cells, adipocytes, and BM stroma cells, only B-lymphoid cells express EBF1. In the absence of EBF1, hematopoietic progenitor cells (HPCs) fail to activate the B-lineage program. This has been attributed to the ability of EBF1 to act as a pioneering factor with the ability to remodel chromatin, thereby creating a B-lymphoid-specific epigenetic landscape. Conditional inactivation of the Ebf1 gene in B-lineage cells has revealed additional functions of this protein in relation to the control of proliferation and apoptosis. This may explain why EBF1 is frequently targeted by mutations in human leukemia cases. This chapter provides an overview of the biochemical and functional properties of the EBF family proteins, with a focus on the roles of EBF1 in normal and malignant B-lymphocyte development.

PTEN regulates hematopoietic lineage plasticity via PU.1-dependent chromatin accessibility

PTEN loss in fetal liver hematopoietic stem cells (HSCs) leads to alterations in myeloid, T-, and B-lineage potentials and T-lineage acute lymphoblastic leukemia (T-ALL) development. To explore the mechanism underlying PTEN-regulated hematopoietic lineage choices, we carry out integrated assay for transposase-accessible chromatin using sequencing (ATAC-seq), single-cell RNA-seq, and in vitro culture analyses using in vivo-isolated mouse pre-leukemic HSCs and progenitors. We find that PTEN loss alters chromatin accessibility of key lineage transcription factor (TF) binding sites at the prepro-B stage, corresponding to increased myeloid and T-lineage potentials and reduced B-lineage potential. Importantly, we find that PU.1 is an essential TF downstream of PTEN and that altering PU.1 levels can reprogram the chromatin accessibility landscape and myeloid, T-, and B-lineage potentials in Ptennull prepro-B cells. Our study discovers prepro-B as the key developmental stage underlying PTEN-regulated hematopoietic lineage choices and suggests a critical role of PU.1 in modulating the epigenetic state and lineage plasticity of prepro-B progenitors.

Transcription factor networks link B-lymphocyte development and malignant transformation in leukemia

Rapid advances in genomics have opened unprecedented possibilities to explore the mutational landscapes in malignant diseases, such as B-cell acute lymphoblastic leukemia (B-ALL). This disease is manifested as a severe defect in the production of normal blood cells due to the uncontrolled expansion of transformed B-lymphocyte progenitors in the bone marrow. Even though classical genetics identified translocations of transcription factor-coding genes in B-ALL, the extent of the targeting of regulatory networks in malignant transformation was not evident until the emergence of large-scale genomic analyses. There is now evidence that many B-ALL cases present with mutations in genes that encode transcription factors with critical roles in normal B-lymphocyte development. These include PAX5, IKZF1, EBF1, and TCF3, all of which are targeted by translocations or, more commonly, partial inactivation in cases of B-ALL. Even though there is support for the notion that germline polymorphisms in the PAX5 and IKZF1 genes predispose for B-ALL, the majority of leukemias present with somatic mutations in transcription factor-encoding genes. These genetic aberrations are often found in combination with mutations in genes that encode components of the pre-B-cell receptor or the IL-7/TSLP signaling pathways, all of which are important for early B-cell development. This review provides an overview of our current understanding of the molecular interplay that occurs between transcription factors and signaling events during normal and malignant B-lymphocyte development.

Mitochondrial DNA Mutations as Natural Barcodes for Lineage Tracing of Murine Tumor Models

Murine models are indispensable tools for functional genomic studies and preclinical testing of novel therapeutic approaches. Mitochondrial single-cell assay for transposase-accessible chromatin using sequencing (mtscATAC-seq) enables the dissection of cellular heterogeneity and clonal dynamics by capturing chromatin accessibility, copy-number variations (CNV), and mitochondrial DNA (mtDNA) mutations, yet its applicability to murine studies remains unexplored. By leveraging mtscATAC-seq in novel chronic lymphocytic leukemia and Richter syndrome mouse models, we report the detection of mtDNA mutations, particularly in highly proliferative murine cells, alongside CNV and chromatin state changes indicative of clonal evolution upon secondary transplant. This study thus demonstrates the feasibility and utility of multi-modal single-cell and natural barcoding approaches to characterize murine cancer models.

SIGNIFICANCE

mtDNA mutations can serve as natural barcodes to enable lineage tracing in murine cancer models, which can be used to provide new insights into disease biology and to identify therapeutic vulnerabilities.

EBF1 is continuously required for stabilizing local chromatin accessibility in pro-B cells

The establishment of de novo chromatin accessibility in lymphoid progenitors requires the "pioneering" function of transcription factor (TF) early B cell factor 1 (EBF1), which binds to naïve chromatin and induces accessibility by recruiting the BRG1 chromatin remodeler subunit. However, it remains unclear whether the function of EBF1 is continuously required for stabilizing local chromatin accessibility. To this end, we replaced EBF1 by EBF1-FKBP^F36V in pro-B cells, allowing the rapid degradation by adding the degradation TAG13 (dTAG13) dimerizer. EBF1 degradation results in a loss of genome-wide EBF1 occupancy and EBF1-targeted BRG1 binding. Chromatin accessibility was rapidly diminished at EBF1-binding sites with a preference for sites whose occupancy requires the pioneering activity of the C-terminal domain of EBF1. Diminished chromatin accessibility correlated with altered gene expression. Thus, continuous activity of EBF1 is required for the stable maintenance of the transcriptional and epigenetic state of pro-B cells.

EBF1 primes B-lymphoid enhancers and limits the myeloid bias in murine multipotent progenitors

Hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs) generate all cells of the blood system. Despite their multipotency, MPPs display poorly understood lineage bias. Here, we examine whether lineage-specifying transcription factors, such as the B-lineage determinant EBF1, regulate lineage preference in early progenitors. We detect low-level EBF1 expression in myeloid-biased MPP3 and lymphoid-biased MPP4 cells, coinciding with expression of the myeloid determinant C/EBPα. Hematopoietic deletion of Ebf1 results in enhanced myelopoiesis and reduced HSC repopulation capacity. Ebf1-deficient MPP3 and MPP4 cells exhibit an augmented myeloid differentiation potential and a transcriptome with an enriched C/EBPα signature. Correspondingly, EBF1 binds the Cebpa enhancer, and the deficiency and overexpression of Ebf1 in MPP3 and MPP4 cells lead to an up- and downregulation of Cebpa expression, respectively. In addition, EBF1 primes the chromatin of B-lymphoid enhancers specifically in MPP3 cells. Thus, our study implicates EBF1 in regulating myeloid/lymphoid fate bias in MPPs by constraining C/EBPα-driven myelopoiesis and priming the B-lymphoid fate.

FOXO Dictates Initiation of B Cell Development and Myeloid Restriction in Common Lymphoid Progenitors

The development of B cells relies on an intricate network of transcription factors critical for developmental progression and lineage commitment. In the B cell developmental trajectory, a temporal switch from predominant Foxo3 to Foxo1 expression occurs at the CLP stage. Utilizing VAV-iCre mediated conditional deletion, we found that the loss of FOXO3 impaired B cell development from LMPP down to B cell precursors, while the loss of FOXO1 impaired B cell commitment and resulted in a complete developmental block at the CD25 negative proB cell stage. Strikingly, the combined loss of FOXO1 and FOXO3 resulted in the failure to restrict the myeloid potential of CLPs and the complete loss of the B cell lineage. This is underpinned by the failure to enforce the early B-lineage gene regulatory circuitry upon a predominantly pre-established open chromatin landscape. Altogether, this demonstrates that FOXO3 and FOXO1 cooperatively govern early lineage restriction and initiation of B-lineage commitment in CLPs.

Helix-Loop-Helix Proteins in Adaptive Immune Development

The E/ID protein axis is instrumental for defining the developmental progression and functions of hematopoietic cells. The E proteins are dimeric transcription factors that activate gene expression programs and coordinate changes in chromatin organization. Id proteins are antagonists of E protein activity. Relative levels of E/Id proteins are modulated throughout hematopoietic development to enable the progression of hematopoietic stem cells into multiple adaptive and innate immune lineages including natural killer cells, B cells and T cells. In early progenitors, the E proteins promote commitment to the T and B cell lineages by orchestrating lineage specific programs of gene expression and regulating VDJ recombination of antigen receptor loci. In mature B cells, the E/Id protein axis functions to promote class switch recombination and somatic hypermutation. E protein activity further regulates differentiation into distinct CD4+ and CD8+ T cells subsets and instructs mature T cell immune responses. In this review, we discuss how the E/Id proteins define the adaptive immune system lineages, focusing on their role in directing developmental gene programs.

Mutant U2AF1-induced alternative splicing of H2afy (macroH2A1) regulates B-lymphopoiesis in mice

Somatic mutations in spliceosome genes are found in ∼50% of patients with myelodysplastic syndromes (MDS), a myeloid malignancy associated with low blood counts. Expression of the mutant splicing factor U2AF1(S34F) alters hematopoiesis and mRNA splicing in mice. Our understanding of the functionally relevant alternatively spliced target genes that cause hematopoietic phenotypes in vivo remains incomplete. Here, we demonstrate that reduced expression of H2afy1.1, an alternatively spliced isoform of the histone H2A variant gene H2afy, is responsible for reduced B cells in U2AF1(S34F) mice. Deletion of H2afy or expression of U2AF1(S34F) reduces expression of Ebf1 (early B cell factor 1), a key transcription factor for B cell development, and mechanistically, H2AFY is enriched at the EBF1 promoter. Induced expression of H2AFY1.1 in U2AF1(S34F) cells rescues reduced EBF1 expression and B cells numbers in vivo. Collectively, our data implicate alternative splicing of H2AFY as a contributor to lymphopenia induced by U2AF1(S34F) in mice and MDS.

EBF1 and PAX5 control pro-B cell expansion via opposing regulation of the Myc gene

Genes encoding B lineage-restricted transcription factors are frequently mutated in B-lymphoid leukemias, suggesting a close link between normal and malignant B-cell development. One of these transcription factors is early B-cell factor 1 (EBF1), a protein of critical importance for lineage specification and survival of B-lymphoid progenitors. Here, we report that impaired EBF1 function in mouse B-cell progenitors results in reduced expression of Myc. Ectopic expression of MYC partially rescued B-cell expansion in the absence of EBF1 both in vivo and in vitro. Using chromosome conformation analysis in combination with ATAC-sequencing, chromatin immunoprecipitation-sequencing, and reporter gene assays, six EBF1-responsive enhancer elements were identified within the Myc locus. CRISPR-Cas9-mediated targeting of EBF1-binding sites identified one element of key importance for Myc expression and pro-B cell expansion. These data provide evidence that Myc is a direct target of EBF1. Furthermore, chromatin immunoprecipitation-sequencing analysis revealed that several regulatory elements in the Myc locus are targets of PAX5. However, ectopic expression of PAX5 in EBF1-deficient cells inhibits the cell cycle and reduces Myc expression, suggesting that EBF1 and PAX5 act in an opposing manner to regulate Myc levels. This hypothesis is further substantiated by the finding that Pax5 inactivation reduces requirements for EBF1 in pro-B-cell expansion. The binding of EBF1 and PAX5 to regulatory elements in the human MYC gene in a B-cell acute lymphoblastic leukemia cell line indicates that the EBF1:PAX5:MYC regulatory loop is conserved and may control both normal and malignant B-cell development.

B Lymphocyte Specification Is Preceded by Extensive Epigenetic Priming in Multipotent Progenitors

B lymphocyte development is dependent on the interplay between the chromatin landscape and lineage-specific transcription factors. It has been suggested that B lineage commitment is associated with major changes in the nuclear chromatin environment, proposing a critical role for lineage-specific transcription factors in the formation of the epigenetic landscape. In this report, we have used chromosome conformation capture in combination with assay for transposase-accessible chromatin sequencing analysis to enable highly efficient annotation of both proximal and distal transcriptional control elements to genes activated in B lineage specification in mice. A large majority of these genes were annotated to at least one regulatory element with an accessible chromatin configuration in multipotent progenitors. Furthermore, the majority of binding sites for the key regulators of B lineage specification, EBF1 and PAX5, occurred in already accessible regions. EBF1 did, however, cause a dynamic change in assay for transposase-accessible chromatin accessibility and was critical for an increase in distal promoter-enhancer interactions. Our data unravel an extensive epigenetic priming at regulatory elements annotated to lineage-restricted genes and provide insight into the interplay between the epigenetic landscape and transcription factors in cell specification.

C/EBPα induces Ebf1 gene expression in common lymphoid progenitors

C/EBPα is required for formation of granulocyte-monocyte progenitors (GMP) and also participates in B lymphopoiesis. The common lymphoid progenitor (CLP) and preproB populations but not proB cells express Cebpa, and pan-hematopoietic deletion of the +37 kb Cebpa enhancer using Mx1-Cre leads not only to reduced GMP but also to 2-fold reduced marrow preproB and >15-fold reduced proB and preB cells. We now show that IL7Rα-Cre-mediated deletion of the +37 kb Cebpa enhancer, which occurs in 89% of Ly6D⁺ and 65% of upstream Ly6D^- CLP, leads to a 2-fold reduction of both preproB and proB cells, and a 3-fold reduction in preB cells, with no impact on GMP numbers. These data support a direct role for C/EBPα during B lineage development, with reduced enhancer deletion in Ly6D^- CLP mediated by IL7Rα-Cre diminishing the effect on B lymphopoiesis compared to that seen with Mx1-Cre. Amongst mRNAs encoding key transcriptional regulators that initiate B lymphoid specification (PU.1, E2A, IKAROS, EBF1, FOXO1, and BACH2), only Ebf1 levels are altered in CLP upon Mx1-Cre-mediated Cebpa enhancer deletion, with Ebf1 reduced ~40-fold in Flt3⁺Sca-1^intc-kit^intIL7Rα⁺ CLP. In addition, Cebpa and Ebf1 RNAs were 4- and 14-fold higher in hCD4⁺ versus hCD4^- CLP from Cebpa-hCD4 transgenic mice. Histone modification ChIP-Seq data for CLP indicate the presence of active, intronic Ebf1 enhancers located 270 and 280 kb upstream of the transcription start sites. We identified a cis element in this region that strongly binds C/EBPα using the electrophoretic mobility shift assay. Mutation of this C/EBPα-binding site in an Ebf1 enhancer-TK-luciferase reporter leads to a 4-fold reduction in C/EBPα-mediated trans-activation. These findings support a model of B lymphopoiesis in which induction of Ebf1 by C/EBPα in a subset of CLP contributes to initiation of B lymphopoiesis.

Neurturin regulates the lung-resident macrophage inflammatory response to viral infection

The neurotrophic factor RET is induced in lung macrophages by viral triggers and activation via its ligand neurturin regulates matrix proteins and cytokines that shape the inflammatory response.

Lung-resident macrophages are crucial to the maintenance of health and in the defence against lower respiratory tract infections. Macrophages adapt to local environmental cues that drive their appropriate function; however, this is often dysregulated in many inflammatory lung pathologies. In mucosal tissues, neuro-immune interactions enable quick and efficient inflammatory responses to pathogenic threats. Although a number of factors that influence the antimicrobial response of lung macrophages are known, the role of neuronal factors is less well understood. Here, we show an intricate circuit involving the neurotrophic factor, neurturin (NRTN) on human lung macrophages that dampens pro-inflammatory cytokine release and modulates the type of matrix metalloproteinases produced in response to viral stimuli. This circuit involves type 1 interferon–induced up-regulation of RET that when combined with the glial cell line-derived neurotrophic factor (GDNF) receptor α2 (GFRα2) allows binding to epithelial-derived NRTN. Our research highlights a non-neuronal immunomodulatory role for NRTN and a novel process leading to a specific antimicrobial immune response by human lung-resident macrophages.

Cell Fate Decisions: The Role of Transcription Factors in Early B-cell Development and Leukemia

B-cells are an integral part of the adaptive immune system and regulate innate immunity. Derived from hematopoietic stem cells they mature through a series of cell fate decisions. Complex transcriptional circuits form and dissipate dynamically during these lineage restrictions. Genomic aberrations of involved transcription factors underlie various B-cell disorders. Acquired somatic aberrations are associated with cancer, whereas germline variations predispose to both malignant and non-malignant diseases. We review the opposing role of transcription factors during B-cell development in health and disease. We focus on early B-cell leukemia and discuss novel causative gene-environment cooperations and their implications for precision medicine.

ARID1a Associates with Lymphoid-Restricted Transcription Factors and Has an Essential Role in T Cell Development

Maturation of lymphoid cells is controlled by the action of stage and lineage-restricted transcription factors working in concert with the general transcription and chromatin remodeling machinery to regulate gene expression. To better understand this functional interplay, we used Biotin Identification in human embryonic kidney cells to identify proximity interaction partners for GATA3, TCF7 (TCF1), SPI1, HLF, IKZF1, PAX5, ID1, and ID2. The proximity interaction partners shared among the lineage-restricted transcription factors included ARID1a, a BRG1-associated factor complex component. CUT&RUN analysis revealed that ARID1a shared binding with TCF7 and GATA3 at a substantial number of putative regulatory elements in mouse T cell progenitors. In support of an important function for ARID1a in lymphocyte development, deletion of Arid1a in early lymphoid progenitors in mice resulted in a pronounced developmental arrest in early T cell development with a reduction of CD4⁺CD8⁺ cells and a 20-fold reduction in thymic cellularity. Exploring gene expression patterns in DN3 cells from Wt and Arid1a-deficient mice suggested that the developmental block resided in the DN3a to DN3b transition, indicating a deficiency in β-selection. Our work highlights the critical importance of functional interactions between stage and lineage-restricted factors and the basic transcription machinery during lymphocyte differentiation.

PAX5 is part of a functional transcription factor network targeted in lymphoid leukemia

One of the most frequently mutated proteins in human B-lineage leukemia is the transcription factor PAX5. These mutations often result in partial rather than complete loss of function of the transcription factor. While the functional dose of PAX5 has a clear connection to human malignancy, there is limited evidence for that heterozygote loss of PAX5 have a dramatic effect on the development and function of B-cell progenitors. One possible explanation comes from the finding that PAX5 mutated B-ALL often display complex karyotypes and additional mutations. Thus, PAX5 might be one component of a larger transcription factor network targeted in B-ALL. To investigate the functional network associated with PAX5 we used BioID technology to isolate proteins associated with this transcription factor in the living cell. This identified 239 proteins out of which several could be found mutated in human B-ALL. Most prominently we identified the commonly mutated IKZF1 and RUNX1, involved in the formation of ETV6-AML1 fusion protein, among the interaction partners. ChIP- as well as PLAC-seq analysis supported the idea that these factors share a multitude of target genes in human B-ALL cells. Gene expression analysis of mouse models and primary human leukemia suggested that reduced function of PAX5 increased the ability of an oncogenic form of IKZF1 or ETV6-AML to modulate gene expression. Our data reveals that PAX5 belong to a regulatory network frequently targeted by multiple mutations in B-ALL shedding light on the molecular interplay in leukemia cells.

The use of modern high throughput DNA-sequencing has dramatically increased our ability to identify genetic alterations associated with cancer. However, while the mutations per se are rather easily identified, our understanding of how these mutations impact cellular functions and drive malignant transformation is more limited. We have explored the function of the transcription factor PAX5, commonly mutated in human B-lymphocyte leukemia, to identify a regulatory network of transcription factors often targeted in human disease. Hence, we propose that malignant conversion of B-lymphocyte progenitors involves multiple targeting of a central transcription factor network aggravating the impact of the individual mutations. These data increase our understanding for how individual mutations collaborate to drive the formation of B—lineage leukemia.

Signalling circuits that direct early B-cell development

In mammals, the B-cell lineage arises from pluripotent progenitors in the bone marrow. During their development, B-cells undergo lineage specification and commitment, followed by expansion and selection. These processes are mediated by regulated changes in gene expression programmes, rearrangements of immunoglobulin (Ig) genes, and well-timed rounds of proliferation and apoptosis. Many of these processes are initiated by environmental factors including cytokines, chemokines, and cell-cell contacts. Developing B-cells process these environmental cues into stage-specific functions via signalling pathways including the PI3K, MAPK, or JAK-STAT pathway. The cytokines FLT3-Ligand and c-Kit-Ligand are important for the early expansion of the B-cell precursors at different developmental stages and conditions. Interleukin 7 is essential for commitment to the B-cell lineage and for orchestrating the Ig recombination machinery. After rearrangement of the immunoglobulin heavy chain, proliferation and apoptosis, and thus selection, are mediated by the clonal pre-B-cell receptor, and, following light chain rearrangement, by the B-cell receptor.

GDNF family receptor alpha 2 promotes neuroblastoma cell proliferation by interacting with PTEN

Neuroblastoma is a childhood tumor, and high-stage neuroblastoma has a poor prognosis. The regulatory mechanisms for neuroblastoma progression are poorly understood. In present study, we found that GDNF family receptor alpha 2 (GFRA2) was upregulated in neuroblastoma cells and tissues, and its overexpression promoted neuroblastoma cell proliferation, as revealed using colony formation, soft agar growth, and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays Tumor suppressor phosphatase and tensin homolog (PTEN) is an inhibitor of the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/AKT serine/threonine kinase (AKT) pathway that interacts with GFRA2. A luciferase activity assay showed GFRA2 inhibits the transcriptional activity of the forkhead box O (FOXO) family proteins, which suggested that GFRA2 activated the PI3K/AKT pathway. Inhibition of the PI3K/AKT pathway in GFRA2 overexpressing cells decreased cell proliferation, confirming that GFRA2 promoted neuroblastoma cell proliferation by activating the PI3K/AKT pathway. In summary, cell proliferation via the GFRA2-PTEN-PI3K/AKT axis may represent new target to develop treatments for neuroblastoma.

Molecular Regulation of Differentiation in Early B-Lymphocyte Development

B-lymphocyte differentiation is one of the best understood developmental pathways in the hematopoietic system. Our understanding of the developmental trajectories linking the multipotent hematopoietic stem cell to the mature functional B-lymphocyte is extensive as a result of efforts to identify and prospectively isolate progenitors at defined maturation stages. The identification of defined progenitor compartments has been instrumental for the resolution of the molecular features that defines given developmental stages as well as for our understanding of the mechanisms that drive the progressive maturation process. Over the last years it has become increasingly clear that the regulatory networks that control normal B-cell differentiation are targeted by mutations in human B-lineage malignancies. This generates a most interesting link between development and disease that can be explored to improve diagnosis and treatment protocols in lymphoid malignancies. The aim of this review is to provide an overview of our current understanding of molecular regulation in normal and malignant B-cell development.