Permissive roles of cytokines interleukin-7 and Flt3 ligand in mouse B-cell lineage commitment

The role of GATA2 in adult hematopoiesis and cell fate determination

The correct maintenance and differentiation of hematopoietic stem cells (HSC) in bone marrow is vital for the maintenance and operation of the human blood system. GATA2 plays a critical role in the maintenance of HSCs and the specification of HSCs into the different hematopoietic lineages, highlighted by the various defects observed in patients with heterozygous mutations in GATA2, resulting in cytopenias, bone marrow failure and increased chance of myeloid malignancy, termed GATA2 deficiency syndrome. Despite this, the mechanisms underlying GATA2 deficiency syndrome remain to be elucidated. The detailed description of how GATA2 regulates HSC maintenance and blood lineage determination is crucial to unravel the pathogenesis of GATA2 deficiency syndrome. In this review, we summarize current advances in elucidating the role of GATA2 in hematopoietic cell fate determination and discuss the challenges of modeling GATA2 deficiency syndrome.

Human and mouse early B cell development: So similar but so different

Early B cell development in the bone marrow ensures the replenishment of the peripheral B cell pool. Immature B cells continuously develop from hematopoietic stem cells, in a process guided by an intricate network of transcription factors as well as chemokine and cytokine signals. Humans and mice possess somewhat similar regulatory mechanisms of B lymphopoiesis. The continuous discovery of monogenetic defects that impact early B cell development in humans substantiates the similarities and differences with B cell development in mice. These differences become relevant when targeted therapeutic approaches are used in patients; therefore, predicting potential immunological adverse events is crucial. In this review, we have provided a phenotypical classification of human and murine early progenitors and B cell stages, based on surface and intracellular protein expression. Further, we have critically compared the role of key transcription factors (Ikaros, E2A, EBF1, PAX5, and Aiolos) and chemo- or cytokine signals (FLT3, c-kit, IL-7R, and CXCR4) during homeostatic and aberrant B lymphopoiesis in both humans and mice.

Flt3 Signaling in B Lymphocyte Development and Humoral Immunity

The Class III receptor tyrosine kinase Flt3 and its ligand, the Flt3-ligand (FL), play an integral role in regulating the proliferation, differentiation, and survival of multipotent hematopoietic and lymphoid progenitors from which B cell precursors derive in bone marrow (BM). More recently, essential roles for Flt3 signaling in the regulation of peripheral B cell development and affinity maturation have come to light. Experimental findings derived from a multitude of mouse models have reinforced the importance of molecular and cellular regulation of Flt3 and FL in lymphohematopoiesis and adaptive immunity. Here, we provide a comprehensive review of the current state of the knowledge regarding molecular and cellular regulation of Flt3/FL and the roles of Flt3 signaling in hematopoietic stem cell (HSC) activation, lymphoid development, BM B lymphopoiesis, and peripheral B cell development. Cumulatively, the literature has reinforced the importance of Flt3 signaling in B cell development and function. However, it has also identified gaps in the knowledge regarding Flt3-dependent developmental-stage specific gene regulatory circuits essential for steady-state B lymphopoiesis that will be the focus of future studies.

Dntt expression reveals developmental hierarchy and lineage specification of hematopoietic progenitors

Intrinsic and extrinsic cues determine developmental trajectories of hematopoietic stem cells (HSCs) towards erythroid, myeloid and lymphoid lineages. Using two newly generated transgenic mice that report and trace the expression of terminal deoxynucleotidyl transferase (TdT), transient induction of TdT was detected on a newly identified multipotent progenitor (MPP) subset that lacked self-renewal capacity but maintained multilineage differentiation potential. TdT induction on MPPs reflected a transcriptionally dynamic but uncommitted stage, characterized by low expression of lineage-associated genes. Single-cell CITE-seq indicated that multipotency in the TdT⁺ MPPs is associated with expression of the endothelial cell adhesion molecule ESAM. Stable and progressive upregulation of TdT defined the lymphoid developmental trajectory. Collectively, we here identify a new multipotent progenitor within the MPP4 compartment. Specification and commitment are defined by downregulation of ESAM which marks the progressive loss of alternative fates along all lineages.

Soft Statistical Mechanics for Biology

The multilevel organization of nature is self-evident: proteins do interact among them to give rise to an organized metabolism and the same hierarchical organization is in action for gene expression, tissue and organ architectures, and ecological systems.The still more common approach to such state of affairs is to think that causally relevant events originate from the lower level in the form of perturbations, that climb up the hierarchy reaching the ultimate layer of macroscopic behavior (e.g., causing a specific disease). Such rigid bottom-up causative model is unable to offer realistic models of many biological phenomena.Complex network approach allows to uncover the nature of multilevel organization, but in order to operationally define the organization principles of biological systems, we need to go further and complement network approach with sensible measures of order and organization. These measures, while keeping their original physical meaning, must not impose theoretical premises not verifiable in biological frameworks. We will show here how relatively simple and largely hypothesis-free multidimensional statistics tools can satisfactorily meet these criteria.

Fibroblasts as confederates of the immune system

Fibroblastic stromal cells are as diverse, in origin and function, as the niches they fashion in the mammalian body. This cellular variety impacts the spectrum of responses elicited by the immune system. Fibroblast influence on the immune system keeps evolving our perspective on fibroblast roles and functions beyond just a passive structural part of organs. This review discusses the foundations of fibroblastic stromal-immune crosstalk, under the scope of stromal heterogeneity as a basis for tissue-specific tutoring of the immune system. Focusing on the skin as a relevant immunological organ, we detail the complex interactions between distinct fibroblast populations and immune cells that occur during homeostasis, injury repair, scarring, and disease. We further review the relevance of fibroblastic stromal cell heterogeneity and how this heterogeneity is central to regulate the immune system from its inception during embryonic development into adulthood.

Single-cell analyses reveal the clonal and molecular aetiology of Flt3L-induced emergency dendritic cell development

Regulation of haematopoietic stem and progenitor cell (HSPC) fate is crucial during homeostasis and under stress conditions. Here we examine the aetiology of the Flt3 ligand (Flt3L)-mediated increase of type 1 conventional dendritic cells (cDC1s). Using cellular barcoding we demonstrate this occurs through selective clonal expansion of HSPCs that are primed to produce cDC1s and not through activation of cDC1 fate by other HSPCs. In particular, multi/oligo-potent clones selectively amplify their cDC1 output, without compromising the production of other lineages, via a process we term tuning. We then develop Divi-Seq to simultaneously profile the division history, surface phenotype and transcriptome of individual HSPCs. We discover that Flt3L-responsive HSPCs maintain a proliferative 'early progenitor'-like state, leading to the selective expansion of multiple transitional cDC1-primed progenitor stages that are marked by Irf8 expression. These findings define the mechanistic action of Flt3L through clonal tuning, which has important implications for other models of 'emergency' haematopoiesis.

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.

New Molecular Insights into Immune Cell Development

During development innate lymphoid cells and specialized lymphocyte subsets colonize peripheral tissues, where they contribute to organogenesis and later constitute the first line of protection while maintaining tissue homeostasis. A few of these subsets are produced only during embryonic development and remain in the tissues throughout life. They are generated through a unique developmental program initiated in lympho-myeloid-primed progenitors, which lose myeloid and B cell potential. They either differentiate into innate lymphoid cells or migrate to the thymus to give rise to embryonic T cell receptor-invariant T cells. At later developmental stages, adaptive T lymphocytes are derived from lympho-myeloid progenitors that colonize the thymus, while lymphoid progenitors become specialized in the production of B cells. This sequence of events highlights the requirement for stratification in the establishment of immune functions that determine efficient seeding of peripheral tissues by a limited number of cells.

Hhex regulates murine lymphoid progenitor survival independently of Stat5 and Cdkn2a

The transcription factor Hhex (hematopoietically expressed homeobox gene) is critical for development of multiple lymphoid lineages beyond the common lymphoid progenitor. In addition, Hhex regulates hematopoietic stem cell (HSC) self-renewal, emergency hematopoiesis, and acute myeloid leukemia initiation and maintenance. Hhex mediates its effects on HSCs and acute myeloid leukemia stem cells via repression of the Cdkn2a tumor suppressor locus. However, we report here that loss of Cdkn2a does not rescue the failure of lymphoid development caused by loss of Hhex. As loss of Hhex causes apoptosis of lymphoid progenitors associated with impaired Bcl2 expression and defective Stat5b signaling, we tested the effects of rescuing these pathways using transgenic mice. Expression of the anti-apoptotic factor Bcl2, but not activated Stat5, rescued the development of T-, B-, and NK-cell lineages in the absence of Hhex. These results indicate that Bcl2 expression, but not Stat5b signaling or loss of Cdkn2a, can overcome the lymphoid deficiencies caused by the absence of Hhex, suggesting that the primary role of this transcription factor is to promote survival of lymphoid progenitors during early lymphoid development.

Modeling the Hematopoietic Landscape

Some time ago, we proposed a continuum-like view of the lineages open to hematopoietic stem cells (HSCs); each HSC self-renews or chooses from the spectrum of all end-cell options and can then "merely" differentiate. Having selected a cell lineage, an individual HSC may still "step sideways" to an alternative, albeit closely related, fate: HSC and their progeny therefore remain versatile. The hematopoietic cytokines erythropoietin, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, granulocyte/macrophage colony-stimulating factor and ligand for the fms-like tyrosine kinase 3 instruct cell lineage. Sub-populations of HSCs express each of the cytokine receptors that are positively auto-regulated upon cytokine binding. Many years ago, Waddington proposed that the epigenetic landscape played an important role in cell lineage choice. This landscape is dynamic and unstable especially regarding DNA methylation patterns across genomic DNA. This may underlie the receptor diversity of HSC and their decision-making.

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.

Accumulation of Multipotent Hematopoietic Progenitors in Peripheral Lymphoid Organs of Mice Over-expressing Interleukin-7 and Flt3-Ligand

Interleukin-7 (IL-7) and Flt3-ligand (FL) are two cytokines important for the generation of B cells, as manifested by the impaired B cell development in mice deficient for either cytokine or their respective receptors and by the complete block in B cell differentiation in the absence of both cytokines. IL-7 is an important survival and proliferation factor for B cell progenitors, whereas FL acts on several early developmental stages, prior to B cell commitment. We have generated mice constitutively over-expressing both IL-7 and FL. These double transgenic mice develop splenomegaly and lymphadenopathy characterized by tremendously enlarged lymph nodes even in young animals. Lymphoid, myeloid and dendritic cell numbers are increased compared to mice over-expressing either of the two cytokines alone and the effect on their expansion is synergistic, rather than additive. B cell progenitors, early progenitors with myeloid and lymphoid potential (EPLM), common lymphoid progenitors (CLP) and lineage⁻, Sca1⁺, kit⁺ (LSK) cells are all increased not only in the bone marrow but also in peripheral blood, spleen and even lymph nodes. When transplanted into irradiated wild-type mice, lymph node cells show long-term multilineage reconstitution, further confirming the presence of functional hematopoietic progenitors therein. Our double transgenic mouse model shows that sustained and combined over-expression of IL-7 and FL leads to a massive expansion of most bone marrow hematopoietic progenitors and to their associated presence in peripheral lymphoid organs where they reside and potentially differentiate further, thus leading to the synergistic increase in mature lymphoid and myeloid cell numbers. The present study provides further in vivo evidence for the concerted action of IL-7 and FL on lymphopoiesis and suggests that extramedullary niches, including those in lymph nodes, can support the survival and maintenance of hematopoietic progenitors that under physiological conditions develop exclusively in the bone marrow.

The Making of Hematopoiesis: Developmental Ancestry and Environmental Nurture

Evidence from studies of the behaviour of stem and progenitor cells and of the influence of cytokines on their fate determination, has recently led to a revised view of the process by which hematopoietic stem cells and their progeny give rise to the many different types of blood and immune cells. The new scenario abandons the classical view of a rigidly demarcated lineage tree and replaces it with a much more continuum-like view of the spectrum of fate options open to hematopoietic stem cells and their progeny. This is in contrast to previous lineage diagrams, which envisaged stem cells progressing stepwise through a series of fairly-precisely described intermediate progenitors in order to close down alternative developmental options. Instead, stem and progenitor cells retain some capacity to step sideways and adopt alternative, closely related, fates, even after they have “made a lineage choice.” The stem and progenitor cells are more inherently versatile than previously thought and perhaps sensitive to lineage guidance by environmental cues. Here we examine the evidence that supports these views and reconsider the meaning of cell lineages in the context of a continuum model of stem cell fate determination and environmental modulation.

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.

Single-cell RNA sequencing reveals developmental heterogeneity among early lymphoid progenitors

Single-cell RNA sequencing is a powerful technology for assessing heterogeneity within defined cell populations. Here, we describe the heterogeneity of a B220⁺CD117^intCD19^-NK1.1^- uncommitted hematopoietic progenitor having combined lymphoid and myeloid potential. Phenotypic and functional assays revealed four subpopulations within the progenitor with distinct lineage developmental potentials. Among them, the Ly6D⁺SiglecH^-CD11c^- fraction was lymphoid-restricted exhibiting strong B-cell potential, whereas the Ly6D^-SiglecH^-CD11c^- fraction showed mixed lympho-myeloid potential. Single-cell RNA sequencing of these subsets revealed that the latter population comprised a mixture of cells with distinct lymphoid and myeloid transcriptional signatures and identified a subgroup as the potential precursor of Ly6D⁺SiglecH^-CD11c^- Subsequent functional assays confirmed that B220⁺CD117^intCD19^-NK1.1^- single cells are, with rare exceptions, not bipotent for lymphoid and myeloid lineages. A B-cell priming gradient was observed within the Ly6D⁺SiglecH^-CD11c^- subset and we propose a herein newly identified subgroup as the direct precursor of the first B-cell committed stage. Therefore, the apparent multipotency of B220⁺CD117^intCD19^-NK1.1^- progenitors results from underlying heterogeneity at the single-cell level and highlights the validity of single-cell transcriptomics for resolving cellular heterogeneity and developmental relationships among hematopoietic progenitors.

The Cytokine Flt3-Ligand in Normal and Malignant Hematopoiesis

The cytokine Fms-like tyrosine kinase 3 ligand (FL) is an important regulator of hematopoiesis. Its receptor, Flt3, is expressed on myeloid, lymphoid and dendritic cell progenitors and is considered an important growth and differentiation factor for several hematopoietic lineages. Activating mutations of Flt3 are frequently found in acute myeloid leukemia (AML) patients and associated with a poor clinical prognosis. In the present review we provide an overview of our current knowledge on the role of FL in the generation of blood cell lineages. We examine recent studies on Flt3 expression by hematopoietic stem cells and its potential instructive action at early stages of hematopoiesis. In addition, we review current findings on the role of mutated FLT3 in leukemia and the development of FLT3 inhibitors for therapeutic use to treat AML. The importance of mouse models in elucidating the role of Flt3-ligand in normal and malignant hematopoiesis is discussed.

Selective Expression of Flt3 within the Mouse Hematopoietic Stem Cell Compartment

The fms-like tyrosine kinase 3 (Flt3) is a cell surface receptor that is expressed by various hematopoietic progenitor cells (HPC) and Flt3-activating mutations are commonly present in acute myeloid and lymphoid leukemias. These findings underscore the importance of Flt3 to steady-state and malignant hematopoiesis. In this study, the expression of Flt3 protein and Flt3 mRNA by single cells within the hematopoietic stem cell (HSC) and HPC bone marrow compartments of C57/BL6 mice was investigated using flow cytometry and the quantitative reverse transcription polymerase chain reaction. Flt3 was heterogeneously expressed by almost all of the populations studied, including long-term reconstituting HSC and short-term reconstituting HSC. The erythropoietin receptor (EpoR) and macrophage colony-stimulating factor receptor (M-CSFR) were also found to be heterogeneously expressed within the multipotent cell compartments. Co-expression of the mRNAs encoding Flt3 and EpoR rarely occurred within these compartments. Expression of both Flt3 and M-CSFR protein at the surface of single cells was more commonly observed. These results emphasize the heterogeneous nature of HSC and HPC and the new sub-populations identified are important to understanding the origin and heterogeneity of the acute myeloid leukemias.