Disruption of alpha beta but not of gamma delta T cell development by overexpression of the helix-loop-helix protein Id3 in committed T cell progenitors

Mechanism study of ubiquitination in T cell development and autoimmune disease

T cells play critical role in multiple immune processes including antigen response, tumor immunity, inflammation, self-tolerance maintenance and autoimmune diseases et. Fetal liver or bone marrow-derived thymus-seeding progenitors (TSPs) settle in thymus and undergo T cell-lineage commitment, proliferation, T cell receptor (TCR) rearrangement, and thymic selections driven by microenvironment composed of thymic epithelial cells (TEC), dendritic cells (DC), macrophage and B cells, thus generating T cells with diverse TCR repertoire immunocompetent but not self-reactive. Additionally, some self-reactive thymocytes give rise to Treg with the help of TEC and DC, serving for immune tolerance. The sequential proliferation, cell fate decision, and selection during T cell development and self-tolerance establishment are tightly regulated to ensure the proper immune response without autoimmune reaction. There are remarkable progresses in understanding of the regulatory mechanisms regarding ubiquitination in T cell development and the establishment of self-tolerance in the past few years, which holds great potential for further therapeutic interventions in immune-related diseases.

Site-specific development and progressive maturation of human tissue-resident memory T cells over infancy and childhood

Infancy and childhood are critical life stages for generating immune memory to protect against pathogens; however, the timing, location, and pathways for memory development in humans remain elusive. Here, we investigated T cells in mucosal sites, lymphoid tissues, and blood from 96 pediatric donors aged 0-10 years using phenotypic, functional, and transcriptomic profiling. Our results revealed that memory T cells preferentially localized in the intestines and lungs during infancy and accumulated more rapidly in mucosal sites compared with blood and lymphoid organs, consistent with site-specific antigen exposure. Early life mucosal memory T cells exhibit distinct functional capacities and stem-like transcriptional profiles. In later childhood, they progressively adopt proinflammatory functions and tissue-resident signatures, coincident with increased T cell receptor (TCR) clonal expansion in mucosal and lymphoid sites. Together, our findings identify staged development of memory T cells targeted to tissues during the formative years, informing how we might promote and monitor immunity in children.

Speed and navigation control of thymocyte development by the fetal T-cell gene regulatory network

T-cell differentiation is a tightly regulated developmental program governed by interactions between transcription factors (TFs) and chromatin landscapes and affected by signals received from the thymic stroma. This process is marked by a series of checkpoints: T-lineage commitment, T-cell receptor (TCR)β selection, and positive and negative selection. Dynamically changing combinations of TFs drive differentiation along the T-lineage trajectory, through mechanisms that have been most extensively dissected in adult mouse T-lineage cells. However, fetal T-cell development differs from adult in ways that suggest that these TF mechanisms are not fully deterministic. The first wave of fetal T-cell differentiation occurs during a unique developmental window during thymic morphogenesis, shows more rapid kinetics of differentiation with fewer rounds of cell division, and gives rise to unique populations of innate lymphoid cells (ILCs) and invariant γδT cells that are not generated in the adult thymus. As the characteristic kinetics and progeny biases are cell-intrinsic properties of thymic progenitors, the differences could be based on distinct TF network circuitry within the progenitors themselves. Here, we review recent single-cell transcriptome data that illuminate the TF networks involved in T-cell differentiation in the fetal and adult mouse thymus.

Development of γδ T cells in the thymus - A human perspective

γδ T cells are increasingly emerging as crucial immune regulators that can take on innate and adaptive roles in the defence against pathogens. Although they arise within the thymus from the same hematopoietic precursors as conventional αβ T cells, the development of γδ T cells is less well understood. In this review, we focus on summarising the current state of knowledge about the cellular and molecular processes involved in the generation of γδ T cells in human.

Direct regulation of TCR rearrangement and expression by E proteins during early T cell development

γδ T cells are widely distributed throughout mucosal and epithelial cell-rich tissues and are an important early source of IL-17 in response to several pathogens. Like αβ T cells, γδ T cells undergo a stepwise process of development in the thymus that requires recombination of genome-encoded segments to assemble mature T cell receptor (TCR) genes. This process is tightly controlled on multiple levels to enable TCR segment assembly while preventing the genomic instability inherent in the double-stranded DNA breaks that occur during this process. Each TCR locus has unique aspects in its structure and requirements, with different types of regulation before and after the αβ/γδ T cell fate choice. It has been known that Runx and Myb are critical transcriptional regulators of TCRγ and TCRδ expression, but the roles of E proteins in TCRγ and TCRδ regulation have been less well explored. Multiple lines of evidence show that E proteins are involved in TCR expression at many different levels, including the regulation of Rag recombinase gene expression and protein stability, induction of germline V segment expression, chromatin remodeling, and restriction of the fetal and adult γδTCR repertoires. Importantly, E proteins interact directly with the cis-regulatory elements of the TCRγ and TCRδ loci, controlling the predisposition of a cell to become an αβ T cell or a γδ T cell, even before the lineage-dictating TCR signaling events. This article is categorized under: Immune System Diseases > Stem Cells and Development Immune System Diseases > Genetics/Genomics/Epigenetics.

Transcriptional dynamics and epigenetic regulation of E and ID protein encoding genes during human T cell development

T cells are generated from hematopoietic stem cells through a highly organized developmental process, in which stage-specific molecular events drive maturation towards αβ and γδ T cells. Although many of the mechanisms that control αβ- and γδ-lineage differentiation are shared between human and mouse, important differences have also been observed. Here, we studied the regulatory dynamics of the E and ID protein encoding genes during pediatric human T cell development by evaluating changes in chromatin accessibility, histone modifications and bulk and single cell gene expression. We profiled patterns of ID/E protein activity and identified up- and downstream regulators and targets, respectively. In addition, we compared transcription of E and ID protein encoding genes in human versus mouse to predict both shared and unique activities in these species, and in prenatal versus pediatric human T cell differentiation to identify regulatory changes during development. This analysis showed a putative involvement of TCF3/E2A in the development of γδ T cells. In contrast, in αβ T cell precursors a pivotal pre-TCR-driven population with high ID gene expression and low predicted E protein activity was identified. Finally, in prenatal but not postnatal thymocytes, high HEB/TCF12 levels were found to counteract high ID levels to sustain thymic development. In summary, we uncovered novel insights in the regulation of E and ID proteins on a cross-species and cross-developmental level.

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.

Reference Transcriptomes of Porcine Peripheral Immune Cells Created Through Bulk and Single-Cell RNA Sequencing

Pigs are a valuable human biomedical model and an important protein source supporting global food security. The transcriptomes of peripheral blood immune cells in pigs were defined at the bulk cell-type and single cell levels. First, eight cell types were isolated in bulk from peripheral blood mononuclear cells (PBMCs) by cell sorting, representing Myeloid, NK cells and specific populations of T and B-cells. Transcriptomes for each bulk population of cells were generated by RNA-seq with 10,974 expressed genes detected. Pairwise comparisons between cell types revealed specific expression, while enrichment analysis identified 1,885 to 3,591 significantly enriched genes across all 8 cell types. Gene Ontology analysis for the top 25% of significantly enriched genes (SEG) showed high enrichment of biological processes related to the nature of each cell type. Comparison of gene expression indicated highly significant correlations between pig cells and corresponding human PBMC bulk RNA-seq data available in Haemopedia. Second, higher resolution of distinct cell populations was obtained by single-cell RNA-sequencing (scRNA-seq) of PBMC. Seven PBMC samples were partitioned and sequenced that produced 28,810 single cell transcriptomes distributed across 36 clusters and classified into 13 general cell types including plasmacytoid dendritic cells (DC), conventional DCs, monocytes, B-cell, conventional CD4 and CD8 αβ T-cells, NK cells, and γδ T-cells. Signature gene sets from the human Haemopedia data were assessed for relative enrichment in genes expressed in pig cells and integration of pig scRNA-seq with a public human scRNA-seq dataset provided further validation for similarity between human and pig data. The sorted porcine bulk RNAseq dataset informed classification of scRNA-seq PBMC populations; specifically, an integration of the datasets showed that the pig bulk RNAseq data helped define the CD4CD8 double-positive T-cell populations in the scRNA-seq data. Overall, the data provides deep and well-validated transcriptomic data from sorted PBMC populations and the first single-cell transcriptomic data for porcine PBMCs. This resource will be invaluable for annotation of pig genes controlling immunogenetic traits as part of the porcine Functional Annotation of Animal Genomes (FAANG) project, as well as further study of, and development of new reagents for, porcine immunology.

Distinct and temporary-restricted epigenetic mechanisms regulate human αβ and γδ T cell development

The development of TCRαβ and TCRγδ T cells comprises a step-wise process in which regulatory events control differentiation and lineage outcome. To clarify these mechanisms, we employed RNA-sequencing, ATAC-sequencing and ChIPmentation on well-defined thymocyte subsets that represent the continuum of human T cell development. The chromatin accessibility dynamics show clear stage specificity and reveal that human T cell-lineage commitment is marked by GATA3- and BCL11B-dependent closing of PU.1 sites. A temporary increase in H3K27me3 without open chromatin modifications is unique for β-selection, whereas emerging γδ T cells, which originate from common precursors of β-selected cells, show large chromatin accessibility changes due to strong T cell receptor (TCR) signaling. Furthermore, we unravel distinct chromatin landscapes between CD4⁺ and CD8⁺ αβ-lineage cells that support their effector functions and reveal gene-specific mechanisms that define mature T cells. This resource provides a framework for studying gene regulatory mechanisms that drive normal and malignant human T cell development.

HEB is required for the specification of fetal IL-17-producing γδ T cells

IL-17-producing γδ T (γδT17) cells are critical components of the innate immune system. However, the gene networks that control their development are unclear. Here we show that HEB (HeLa E-box binding protein, encoded by Tcf12) is required for the generation of a newly defined subset of fetal-derived CD73⁻ γδT17 cells. HEB is required in immature CD24⁺CD73⁻ γδ T cells for the expression of Sox4, Sox13, and Rorc, and these genes are repressed by acute expression of the HEB antagonist Id3. HEB-deficiency also affects mature CD73⁺ γδ T cells, which are defective in RORγt expression and IL-17 production. Additionally, the fetal TCRγ chain repertoire is altered, and peripheral Vγ4 γδ T cells are mostly restricted to the IFNγ-producing phenotype in HEB-deficient mice. Therefore, our work identifies HEB-dependent pathways for the development of CD73⁺ and CD73⁻ γδT17 cells, and provides mechanistic evidence for control of the γδT17 gene network by HEB.

The γδ T cell pool includes abundant IL-17-producing cells that protect mucosal surfaces, but the signals that control γδ T cell specification are unclear. Here the authors identify a role for the transcription factor HEB, and antagonistic activity of Id3, in the development of these cells.

Human CD5+ Innate Lymphoid Cells Are Functionally Immature and Their Development from CD34+ Progenitor Cells Is Regulated by Id2

Innate lymphoid cells (ILCs) have emerged as a key cell type involved in surveillance and maintenance of mucosal tissues. Mouse ILCs rely on the transcriptional regulator Inhibitor of DNA-binding protein 2 (Id2) for their development. Here, we show that Id2 also drives development of human ILC because forced expression of Id2 in human thymic progenitors blocked T cell commitment, upregulated CD161 and promyelocytic leukemia zinc finger (PLZF), and maintained CD127 expression, markers that are characteristic for human ILCs. Surprisingly CD5 was also expressed on these in vitro generated ILCs. This was not an in vitro artifact because CD5 was also found on ex vivo isolated ILCs from thymus and from umbilical cord blood. CD5 was also expressed on small proportions of ILC2 and ILC3. CD5⁺ ILCs were functionally immature, but could further differentiate into mature CD5⁻ cytokine-secreting ILCs. Our data show that Id2 governs human ILC development from thymic progenitor cells toward immature CD5⁺ ILCs.

E proteins in lymphocyte development and lymphoid diseases

As members of the basic helix-loop-helix (bHLH) family of transcription factors, E proteins function in the immune system by directing and maintaining a vast transcriptional network that regulates cell survival, proliferation, differentiation, and function. Proper activity of this network is essential to the functionality of the immune system. Aberrations in E protein expression or function can cause numerous defects, ranging from impaired lymphocyte development and immunodeficiency to aberrant function, cancer, and autoimmunity. Additionally, disruption of inhibitor of DNA-binding (Id) proteins, natural inhibitors of E proteins, can induce additional defects in development and function. Although E proteins have been investigated for several decades, their study continues to yield novel and exciting insights into the workings of the immune system. The goal of this chapter is to discuss the various classical roles of E proteins in lymphocyte development and highlight new and ongoing research into how these roles, if compromised, can lead to disease.

Identification of TMEM131L as a novel regulator of thymocyte proliferation in humans

In this study, we identify transmembrane protein 131-like (TMEM131L) as a novel regulator of thymocyte proliferation and demonstrate that it corresponds to a not as yet reported inhibitor of Wnt signaling. Short hairpin RNA-mediated silencing of TMEM131L in human CD34(+) hematopoietic progenitors, which were then grafted in NOD-SCID/IL-2rγ(null) mice, resulted in both thymocyte hyperproliferation and multiple pre- and post-β-selection intrathymic developmental defects. Consistent with deregulated Wnt signaling, TMEM131L-deficient thymocytes expressed Wnt target genes at abnormally high levels, and they displayed both constitutive phosphorylation of Wnt coreceptor LRP6 and β-catenin intranuclear accumulation. Using T cell factor reporter assays, we found that membrane-associated TMEM131L inhibited canonical Wnt/β-catenin signaling at the LRP6 coreceptor level. Whereas membrane-associated TMEM131L did not affect LRP6 expression under basal conditions, it triggered lysosome-dependent degradation of its active phosphorylated form following Wnt activation. Genetic mapping showed that phosphorylated LRP6 degradation did not depend on TMEM131L cytoplasmic part but rather on a conserved extracellular domain proximal to the membrane. Collectively, these data indicate that, during thymopoiesis, stage-specific surface translocation of TMEM131L may regulate immature single-positive thymocyte proliferation arrest by acting through mixed Wnt-dependent and -independent mechanisms.

Inhibitors of DNA binding proteins restrict T cell potential by repressing Notch1 expression in Flt3-negative common lymphoid progenitors

Lineage commitment is regulated during hematopoiesis, with stepwise loss of differentiation potential ultimately resulting in lineage commitment. In this study we describe a novel population of B/NK bipotent precursors among common lymphoid progenitors in the fetal liver and the bone marrow. The absence of T cell precursor potential, both in vivo and in vitro, is due to low Notch1 expression and secondary to inhibition of E2A activity by members of the inhibitor of DNA binding (Id) protein family. Our results demonstrate a new, Id protein-dependent, molecular mechanism of Notch1 repression, operative in both fetal and adult common lymphoid progenitors, where T cell potential is selectively inhibited without affecting either the B or NK programs. This study identifies Id proteins as negative regulators of T cell specification, before B and NK commitment, and provides important insights into the transcriptional networks orchestrating hematopoiesis.

On becoming a T cell, a convergence of factors kick it up a Notch along the way

The thymus is seeded by bone marrow-derived progenitors, which undergo a series of differentiation and proliferation events in order to generate functional T lymphocytes. The Notch signaling pathway, together with multiple transcription factors, act in concert to commit progenitors to a T-lineage fate, extinguishing non-T cell potential, inducing thymocyte differentiation and supporting proliferation and survival along the way to becoming a mature T cell. This review focuses on recent evidence regarding the complex interplay between the Notch pathway and other key transcription factors at specific lineage-decision points during the program of T cell development.

αβ versus γδ fate choice: counting the T-cell lineages at the branch point

Both αβ and γδ T cells develop in the thymus from a common progenitor. Historically distinguished by their T-cell receptor (TCR), these lineages are now defined on the basis of distinct molecular programs. Intriguingly, in many transgenic and knockout systems these programs are mismatched with the TCR type, leading to the development of γδ lineage cells driven by αβTCR and vice versa. These puzzling observations were recently explained by the demonstration that TCR signal strength, rather than TCR type per se, instructs lineage fate, with stronger TCR signal favoring γδ and weaker signal favoring αβ lineage fates. These studies also highlighted the ERK (extracellular signal regulated kinase)-Egr (early growth response)-Id3 (inhibitor of differentiation 3) axis as a potential molecular switch downstream of TCR that determines lineage choice. Indeed, removal of Id3 was sufficient to redirect TCRγδ transgenic cells to the αβ lineage, even in the presence of strong TCR signal. However, in TCR non-transgenic Id3 knockout mice the overall number of γδ lineage cells was increased due to an outgrowth of a Vγ1Vδ6.3 subset, suggesting that not all γδ T cells depend on this molecular switch for lineage commitment. Thus, the γδ lineage may in fact be a collection of two or more lineages not sharing a common molecular program and thus equipollent to the αβ lineage. TCR signaling is not the only factor that is required for development of αβ and γδ lineage cells; other pathways, such as signaling from Notch and CXCR4 receptors, cooperate with the TCR in this process.

Towards a molecular understanding of the differential signals regulating alphabeta/gammadelta T lineage choice

While insights into the molecular processes that specify adoption of the alphabeta and gammadelta fates are beginning to emerge, the basis for control of specification remains highly controversial. This review highlights the current models attempting to explain T lineage commitment. Recent observations support the hypothesis that the T cell receptor (TCR) provides instructive cues through differences in TCR signaling intensity and/or longevity. Accordingly, we review evidence addressing the importance of differences in signal strength/longevity, how signals differing in intensity/longevity may be generated, and finally how such signals modulate the activity of downstream effectors to promote the opposing developmental fates.

gammadelta and alphabeta T cell lineage choice: resolution by a stronger sense of being

A common bipotent thymocyte precursor gives rise to both lineages of T cells, alphabeta and gammadelta. However, the cell intrinsic and extrinsic factors that influence alphabeta- versus gammadelta-lineage bifurcation remain controversial. gammadelta T cells play a unique and vital role in host defense, from maintaining integrity at epithelial and mucosal barriers to their newly defined role as an important innate source of interleukin-17. Although a T cell receptor (TCR)-independent fate choice may take place, emerging data supports a model in which the differential signaling capacity of alphabeta and gammadeltaTCRs play an instructional role in specifying lineage fate, with strength of signal measured by the amount of ERK/MAPK pathway activation. Here we discuss how the interplay between intrinsic TCR signals and cell extrinsic signals provided by Notch and TCR ligands help to assign and support a final lineage fate decision.

Isolation and in vitro generation of gene-manipulated human plasmacytoid and conventional dendritic cells

Our understanding of human lymphocyte development has increased significantly over the past 20 years. In particular, our insight into human T- and B-cell development has improved (1, 2). Nonetheless, there are many gaps in our understanding, particularly regarding the early stages of development of hematopoietic progenitor cells (HPCs) into downstream lineage-biased and lineage-restricted precursors and the molecular mechanisms underlying these activities. The same holds true for our knowledge of human dendritic cell (DC) development. While the amount of data on the different subsets of conventional DCs (cDCs) and plasmacytoid DCs (pDCs) rapidly increases in mice (3, 4), the developmental stages of different DC subsets in humans remain poorly defined (2). The relatively easy access to patient material and therefore human precursor cells that can be isolated from these tissues combined with the availability of in vitro and in vivo differentiation assays allows studies in the field of human hematopoietic development, including that of DCs. In addition, the opportunities to manipulate gene expression, by stable overexpression of a gene of interest or RNA interference-mediated knockdown, generate valuable information about the mechanisms underlying lineage commitment and differentiation.

Regulation of V(D)J recombination by E-protein transcription factors

Extensive study of the E-proteins E2A and HEB duringlymphocyte development has revealed various functions for these bHLH transcription factors in regulating V(D)J recombination in both B- and T-cells. The study of E-proteins in mammals began with the identification of E2A by its ability to bind immunoglobulin heavy and light chain enhancers. Subsequent analysis has identified numerous roles for E2A and HEB at the immunoglobulin and T-cell receptor loci. E-protein targets also include the rag genes and other factors critical for recombination and for regulation of the developmental windows when cells undergo recombination. E-proteins appear to be master regulators that coordinate antigen receptor gene rearrangement and expression. This chapter focuses on how E-proteins regulate V(D)J recombination by activating transcription, initiating rearrangement and driving differentiation during B- and T-cell development.

Molecular mechanisms that control mouse and human TCR-alphabeta and TCR-gammadelta T cell development

Following specification of hematopoietic precursor cells into the T cell lineage, several developmental options remain available to the immature thymocytes. The paradigm is that the outcome of the T cell receptor rearrangements and the corresponding T cell receptor signaling events will be predominant to determine the first of these choices: the alphabeta versus gammadelta T cell pathways. Here, we review the thymus-derived environmental signals, the transcriptional mediators, and other molecular mechanisms that are also involved in this decision in both the mouse and human. We discuss the differences in cellular events between the alphabeta and gammadelta developmental pathways and try to correlate these with a corresponding complexity of the molecular mechanisms that support them.

Phylogenetic and expression analysis of the basic helix-loop-helix transcription factor gene family: genomic approach to cellular differentiation

A phylogenetic analysis of seven different species (human, mouse, rat, worm, fly, yeast, and plant) utilizing all (541) basic helix-loop-helix (bHLH) genes identified, including expressed sequence tags (EST), was performed. A super-tree involving six clades and a structural categorization involving the entire coding sequence was established. A nomenclature was developed based on clade distribution to discuss the functional and ancestral relationships of all the genes. The position/location of specific genes on the phylogenetic tree in relation to known bHLH factors allows for predictions of the potential functions of uncharacterized bHLH factors, including EST's. A genomic analysis using microarrays for four different mouse cell types (i.e. Sertoli, Schwann, thymic, and muscle) was performed and considered all known bHLH family members on the microarray for comparison. Cell-specific groups of bHLH genes helped clarify those bHLH genes potentially involved in cell specific differentiation. This phylogenetic and genomic analysis of the bHLH gene family has revealed unique aspects of the evolution and functional relationships of the different genes in the bHLH gene family.

Negotiation of the T lineage fate decision by transcription-factor interplay and microenvironmental signals

Notch-Delta signaling of hematopoietic precursors sets in motion a train of events that activates expression of T lineage genes. Even so, through many cell generations the pro-T cells remain uncommitted to the T cell fate, preserving alternative potentials as divergent as monocyte or dendritic cell fates. This plasticity can be explained by the tenacious expression of stem- and progenitor-associated regulatory genes in the cells, and by the combinatorial coding of T cell identity by factors that are not intrinsically T lineage specific in their spectra of activity. T lineage developmental success depends on precise temporal and quantitative regulation of these factors and on the continuing modulating influence of Notch-Delta signals that buffer the cells against mechanisms promoting non-T outcomes. An additional mechanism, still not fully defined, is required just prior to T cell receptor-mediated selection to end plasticity and make T lineage commitment irreversible.

Forced expression of Id2 in fetal thymic T cell progenitors allows some of their progeny to adopt NK cell fate

The E proteins are indispensable for early T cell development. On the other hand, we previously demonstrated that their inhibitor Id2 is essential for NK lineage commitment from bipotent progenitors generating both T and NK cells (p-T/NK). To shed more light on the role of E proteins and Id2 in the development of early intrathymic progenitors, we performed a clonal analysis: individual fetal thymic CD4(-)CD8(-)CD44(+)CD25(-)CD122(-) (DN1CD122(-)) cells were retrovirally transduced with an Id2-internal ribosomal entry site (IRES)-green fluorescent protein (GFP) (Id2-GFP) gene or a control IRES-GFP (GFP) gene, and cultured in a modified fetal thymus organ culture able to support T and NK cell development. After the culture, both T and NK cells, T cells and no NK cells, NK cells and no T cells, or completely no cells were generated from single cells in each lobe. Hence, the seeded cells were regarded as p-T/NK, unipotent progenitors generating T cells (p-T), unipotent NK progenitors, or cells without progenitor activity, respectively. With Id2-GFP transduction, p-T disappeared and more p-T/NK emerged than with GFP transduction. This increase corresponded to the number of p-T that was counted when the vector-transduced-DN1CD122(-) cells of the same number were examined. Additionally, a fraction of GFP(-) NK cells obtained after Id2-GFP transduction underwent TCRbeta D-J rearrangement. Our data strongly suggest that forced expression of Id2 allows some progeny of p-T to adopt an NK cell fate, and that p-T retain a program for NK lineage development that can be implemented by inhibiting the function of E proteins.

Human T-cell leukemia virus type 1 Tax protein down-regulates pre-T-cell receptor alpha gene transcription in human immature thymocytes

The human pre-T-cell receptor alpha (TCRalpha; pTalpha) gene encodes a polypeptide which associates with the TCRbeta chain and CD3 molecules to form the pre-TCR complex. The surface expression of the pre-TCR is pTalpha dependent, and signaling through this complex triggers an early alphabeta T-cell developmental checkpoint inside the thymus, known as beta-selection. E2A transcription factors, which are involved at multiple stages of T-cell development, regulate the transcription of the pTalpha gene. Here we show that the regulatory protein Tax of the human T-cell leukemia virus type 1 (HTLV-1) efficiently suppresses the E47-mediated activation of the pTalpha promoter. Furthermore, we report that in Tax lentivirally transduced human MOLT-4 T cells, which constitutively express the pTalpha gene, the amount of pTalpha transcripts decreases. Such a decrease is not observed in MOLT-4 cells transduced by a vector encoding the Tax mutant K88A, which is unable to interact with p300. These data underline that Tax inhibits pTalpha transcription by recruiting this coactivator. Finally, we show that the expression of Tax in human immature thymocytes results in a decrease of pTalpha gene transcription but does not modify the level of E47 transcripts. These observations indicate that Tax, by silencing E proteins, down-regulates pTalpha gene transcription during early thymocyte development. They further provide evidence that Tax can interfere with an important checkpoint during T-cell differentiation in the thymus.

Recent insights into the signals that control alphabeta/gammadelta-lineage fate

During thymopoiesis, two major types of mature T cells are generated that can be distinguished by the clonotypic subunits contained within their T-cell receptor (TCR) complexes: alphabeta T cells and gammadelta T cells. Although there is no consensus as to the exact developmental stage where alphabeta and gammadelta T-cell lineages diverge, gammadelta T cells and precursors to the alphabeta T-cell lineage (bearing the pre-TCR) are thought to be derived from a common CD4- CD8- double-negative precursor. The role of the TCR in alphabeta/gammadelta lineage commitment has been controversial, in particular whether different TCR isotypes intrinsically favor adoption of the corresponding lineage. Recent evidence supports a signal strength model of lineage commitment, whereby stronger signals promote gammadelta development and weaker signals promote adoption of the alphabeta fate, irrespective of the TCR isotype from which the signals originate. Moreover, differences in the amplitude of activation of the extracellular signal-regulated kinase- mitogen-activated protein kinase-early growth response pathway appear to play a critical role. These findings will be placed in context of previous analyses in an effort to more precisely define the signals that control T-lineage fate during thymocyte development.

Progression of regulatory gene expression states in fetal and adult pro-T-cell development

Precursors entering the T-cell developmental pathway traverse a progression of states characterized by distinctive patterns of gene expression. Of particular interest are regulatory genes, which ultimately control the dwell time of cells in each state and establish the mechanisms that propel them forward to subsequent states. Under particular genetic and developmental circumstances, the transitions between these states occur with different timing, and environmental feedbacks may shift the steady-state accumulations of cells in each state. The fetal transit through pro-T-cell stages is faster than in the adult and subject to somewhat different genetic requirements. To explore causes of such variation, this review presents previously unpublished data on differentiation gene activation in pro-T cells of pre-T-cell receptor-deficient mutant mice and a quantitative comparison of the profiles of transcription factor gene expression in pro-T-cell subsets of fetal and adult wildtype mice. Against a background of consistent gene expression, several regulatory genes show marked differences between fetal and adult expression profiles, including those encoding two basic helix-loop-helix antagonist Id factors, the Ets family factor SpiB and the Notch target gene Deltex1. The results also reveal global differences in regulatory alterations triggered by the first T-cell receptor-dependent selection events in fetal and adult thymopoiesis.

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.

Human alpha beta and gamma delta thymocyte development: TCR gene rearrangements, intracellular TCR beta expression, and gamma delta developmental potential--differences between men and mice

To evaluate the role of the TCR in the alphabeta/gammadelta lineage choice during human thymocyte development, molecular analyses of the TCRbeta locus in gammadelta cells and the TCRgamma and delta loci in alphabeta cells were undertaken. TCRbeta variable gene segments remained largely in germline configuration in gammadelta cells, indicating that commitment to the gammadelta lineage occurred before complete TCRbeta rearrangements in most cases. The few TCRbeta rearrangements detected were primarily out-of-frame, suggesting that productive TCRbeta rearrangements diverted cells away from the gammadelta lineage. In contrast, in alphabeta cells, the TCRgamma locus was almost completely rearranged with a random productivity profile; the TCRdelta locus contained primarily nonproductive rearrangements. Productive gamma rearrangements were, however, depleted compared with preselected cells. Productive TCRgamma and delta rearrangements rarely occurred in the same cell, suggesting that alphabeta cells developed from cells unable to produce a functional gammadelta TCR. Intracellular TCRbeta expression correlated with the up-regulation of CD4 and concomitant down-regulation of CD34, and plateaued at the early double positive stage. Surprisingly, however, some early double positive thymocytes retained gammadelta potential in culture. We present a model for human thymopoiesis which includes gammadelta development as a default pathway, an instructional role for the TCR in the alphabeta/gammadelta lineage choice, and a prolonged developmental window for beta selection and gammadelta lineage commitment. Aspects that differ from the mouse are the status of TCR gene rearrangements at the nonexpressed loci, the timing of beta selection, and maintenance of gammadelta potential through the early double positive stage of development.

Developmental and molecular characterization of emerging beta- and gammadelta-selected pre-T cells in the adult mouse thymus

The first checkpoint in T cell development, beta selection, has remained incompletely characterized for lack of specific surface markers. We show that CD27 is upregulated in DN3 thymocytes initiating beta selection, concomitant with intracellular TCR-beta expression. Clonal analysis determined that CD27high DN3 cells generate CD4+CD8+ progeny with more than 90% efficiency, faster and more efficiently than the CD27low majority. CD27 upregulation also occurs in gammadelta-selected DN3 thymocytes in TCR-beta-/- mice and in IL2-GFP transgenic reporter mice where GFP marks the earliest emerging TCR-gammadelta cells from DN3 thymocytes. With CD27 to distinguish pre- and postselection DN3 cells, a detailed gene expression analysis defined regulatory changes associated with checkpoint arrest, with beta selection, and with gammadelta selection. gammadelta selection induces higher CD5, Egr, and Runx3 expression as compared to beta selection, but it triggers less proliferation. Our results also reveal differences in Notch/Delta dependence at the earliest stages of divergence between developing alphabeta and gammadelta T-lineage cells.

At the crossroads: diverse roles of early thymocyte transcriptional regulators

Transcriptional regulation of T-cell development involves successive interactions between complexes of transcriptional regulators and their binding sites within the regulatory regions of each gene. The regulatory modules that control expression of T-lineage genes frequently include binding sites for a core set of regulators that set the T-cell-specific background for signal-dependent control, including GATA-3, Notch/CSL, c-myb, TCF-1, Ikaros, HEB/E2A, Ets, and Runx factors. Additional regulators in early thymocytes include PU.1, Id-2, SCL, Spi-B, Erg, Gfi-1, and Gli. Many of these factors are involved in simultaneous regulation of non-T-lineage genes, T-lineage genes, and genes involved in cell cycle control, apoptosis, or survival. Potential and known interactions between early thymic transcription factors such as GATA-3, SCL, PU.1, Erg, and Spi-B are explored. Regulatory modules involved in the expression of several critical T-lineage genes are described, and models are presented for shifting occupancy of the DNA-binding sites in the regulatory modules of pre-Talpha, T-cell receptor beta (TCRbeta), recombinase activating genes 1 and 2 (Rag-1/2), and CD4 during T-cell development. Finally, evidence is presented that c-kit, Erg, Hes-1, and HEBAlt are expressed differently in Rag-2(-/-) thymocytes versus normal early thymocytes, which provide insight into potential regulatory interactions that occur during normal T-cell development.

Molecular genetics of T cell development

T cell development is guided by a complex set of transcription factors that act recursively, in different combinations, at each of the developmental choice points from T-lineage specification to peripheral T cell specialization. This review describes the modes of action of the major T-lineage-defining transcription factors and the signal pathways that activate them during intrathymic differentiation from pluripotent precursors. Roles of Notch and its effector RBPSuh (CSL), GATA-3, E2A/HEB and Id proteins, c-Myb, TCF-1, and members of the Runx, Ets, and Ikaros families are critical. Less known transcription factors that are newly recognized as being required for T cell development at particular checkpoints are also described. The transcriptional regulation of T cell development is contrasted with that of B cell development, in terms of their different degrees of overlap with the stem-cell program and the different roles of key transcription factors in gene regulatory networks leading to lineage commitment.

Differential regulation of granulopoiesis by the basic helix-loop-helix transcriptional inhibitors Id1 and Id2

Inhibitor of DNA binding (Id) proteins function as inhibitors of members of the basic helix-loop-helix family of transcription factors and have been demonstrated to play an important role in regulating lymphopoiesis. However, the role of these proteins in regulation of myelopoiesis is currently unclear. In this study, we have investigated the role of Id1 and Id2 in the regulation of granulopoiesis. Id1 expression was initially up-regulated during early granulopoiesis, which was then followed by a decrease in expression during final maturation. In contrast, Id2 expression was up-regulated in terminally differentiated granulocytes. In order to determine whether Id expression plays a critical role in regulating granulopoiesis, Id1 and Id2 were ectopically expressed in CD34(+) cells by retroviral transduction. Our experiments demonstrate that constitutive expression of Id1 inhibits eosinophil development, whereas in contrast neutrophil differentiation was modestly enhanced. Constitutive Id2 expression accelerates final maturation of both eosinophils and neutrophils, whereas inhibition of Id2 expression blocks differentiation of both lineages. Transplantation of beta2-microglobulin(-/-) nonobese diabetic severe combined immunodeficient (NOD/SCID) mice with CD34(+) cells ectopically expressing Id1 resulted in enhanced neutrophil development, whereas ectopic expression of Id2 induced both eosinophil and neutrophil development. These data demonstrate that both Id1 and Id2 play a critical, although differential role in granulopoiesis.

SCL: from the origin of hematopoiesis to stem cells and leukemia

In the hematopoietic system, lineage commitment and differentiation is controlled by the combinatorial action of transcription factors from diverse families. SCL is a basic helix-loop-helix transcription factor that is an essential regulator at several levels in the hematopoietic hierarchy and whose inappropriate regulation frequently contributes to the development of pediatric T-cell acute lymphoblastic leukemia. This review discusses advances that have shed important light on the functions played by SCL during normal hematopoiesis and leukemogenesis and have revealed an unexpected robustness of hematopoietic stem cell function. Molecular studies have unraveled a mechanism through which gene expression is tightly controlled, as SCL functions within multifactorial complexes that exhibit an all-or-none switch-like behavior in transcription activation, arguing for a quantal process that depends on the concurrent occupation of target loci by all members of the complex. Finally, variations in composition of SCL-containing complexes may ensure flexibility and specificity in the regulation of lineage-specific programs of gene expression, thus providing the molecular basis through which SCL exerts its essential functions at several branch points of the hematopoietic hierarchy.

Retroviral transduction of acute myeloid leukaemia-derived dendritic cells with OX40 ligand augments their antigen presenting activity

Recent studies have shown that human myeloid leukaemia cells can differentiate into dendritic cell (DC)-like cells (leukaemia-DCs) when cultured with a combination of cytokines. In the present study, we examined whether the transduction of leukaemia-DCs with OX40 ligand (OX40L), a member of the tumour necrosis factor (TNF) family, resulted in augmentation of their antigen presenting activity. Bicistronic retroviral vectors expressing both human OX40L and enhanced green fluorescent protein (EGFP) or EGFP alone were generated and used for transduction. Fresh leukaemic cells from five patients with acute myeloid leukaemia (AML) were isolated and retrovirally transduced with OX40L during the culture with a combination of cytokines from stem cell factor, fms-like tyrosine kinase (Flt)-3 ligand, granulocyte-macrophage colony stimulating factor (GM-CSF), interleukin-4 (IL-4) and TNF-alpha. After 7 d, the majority of cells showed DC-like morphology, and expressed higher levels of CD80, CD86 and HLA-DR than fresh leukaemic cells. The transduction efficiency was 8.5-27.2%. Leukaemia-DCs transduced with OX40L elicited higher proliferative response of allogeneic CD4+ T cells than fresh leukaemic cells, non-transduced, or mock-transduced leukaemia-DCs. Co-culture of allogeneic CD4+ T cells with OX40L-transduced leukaemia-DCs was superior in the generation of interferon (IFN)-gamma producing CD4+ T cells and in production of IFN-gamma. Furthermore, OX40L-transduced leukaemia-DCs could elicit significant proliferative response of human leucocyte antigen-matched T cells from the donor in allogeneic stem cell transplantation. These results indicate that retroviral transduction of leukaemia-DCs with OX40L augments their antigen presenting cell activity and thus renders them more suitable for tumour vaccines or ex vivo stimulation of leukaemia-specific T cells.

L-selectin defines a bone marrow analog to the thymic early T-lineage progenitor

The recent description of an early T-lineage progenitor (ETP) population in adult mouse thymus implies the presence of a bone marrow predecessor that has not yet been identified. Here we describe a Lin(Neg) Sca-1(Pos) c-kit(Hi) Thy-1.1(Neg) L-selectin(Pos) adult mouse bone marrow population that resembles the thymic ETP in both antigen expression phenotype and posttransplantation lineage potential. These cells produce wavelike kinetics of thymic seeding and reconstitute the irradiated thymus with kinetics comparable to a thymocyte graft after intravenous transplantation. Transient B-lineage reconstitution is also observed, but little myeloid potential can be detected in transplant experiments. A second subset of progenitors is L-selectin(Neg) and is highly enriched for rapid and persistent T- and B-lineage potential, as well as some myeloid potential. L-selectin (CD62L) is therefore an effective marker for separating lymphoid progenitors from myeloid progenitors and hematopoietic stem cells in mouse bone marrow.

Sustained Notch1 signaling instructs the earliest human intrathymic precursors to adopt a gammadelta T-cell fate in fetal thymus organ culture

Notch1 activity is essential for the specification of T-lineage fate in hematopoietic progenitors. Once the T-cell lineage is specified, T-cell precursors in the thymus must choose between alphabeta and gammadelta lineages. However, the impact of Notch1 signaling on intrathymic pro-T cells has not been addressed directly. To approach this issue, we used retroviral vectors to express constitutively active Notch1 in human thymocyte progenitors positioned at successive developmental stages, and we followed their differentiation in fetal thymus organ culture (FTOC). Here we show that sustained Notch1 signaling impairs progression to the double-positive (DP) stage and efficiently diverts the earliest thymic progenitors from the main alphabeta T-cell pathway toward development of gammadelta T cells. The impact of Notch1 signaling on skewed gammadelta production decreases progressively along intrathymic maturation and is restricted to precursor stages upstream of the pre-T-cell receptor checkpoint. Close to and beyond that point, Notch1 is not further able to instruct gammadelta cell fate, but promotes an abnormal expansion of alphabeta-committed thymocytes. These results stress the stage-specific impact of Notch1 signaling in intrathymic differentiation and suggest that regulation of Notch1 activity at defined developmental windows is essential to control alphabeta versus gammadelta T-cell development and to avoid deregulated expansion of alphabeta-lineage cells.

Gene expression signatures in MLL-rearranged T-lineage and B-precursor acute leukemias: dominance of HOX dysregulation

Rearrangements of the MLL locus, located on human chromosome 11q23, are frequent in both infant and therapy-related leukemias. Gene expression analysis of MLL-rearranged B-precursor acute lymphoblastic leukemias (MLL B-ALLs) has identified these cases as a unique subtype of leukemia, characterized by the expression of genes associated with both lymphoid and myeloid hematopoietic lineages. Here we show that MLL fusions also generate a distinct genetic subtype of T-lineage ALL (MLL T-ALL), in which leukemic cells are characterized by an early arrest in thymocyte differentiation, with suggestive evidence of commitment to the gammadelta lineage. Interestingly, multiple genes linked to cell proliferation (eg, PCNA, MYC, CDK2, and POLA) were down-regulated in MLL-fusion samples, relative to those transformed by other T-ALL oncogenes (P <.000 001, Fisher exact test). Overall, MLL T-ALL cases consistently demonstrated increased levels of expression of a subset of major HOX genes--HOXA9, HOXA10, and HOXC6--and the MEIS1 HOX coregulator (P <.008, one-sided Wilcoxon test), a pattern of gene expression that was reiterated in MLL B-ALLs. However, expression of myeloid lineage genes, previously reported in MLL B-ALLs, was not identified in T-lineage cases with this abnormality, suggesting that myeloid gene dysregulation is dispensable in leukemic transformation mediated by MLL fusion proteins. Our findings implicate dysregulation of HOX gene family members as a dominant mechanism of leukemic transformation induced by chimeric MLL oncogenes.

Regulation of pT alpha gene expression by a dosage of E2A, HEB, and SCL

The expression of the pT alpha gene is required for effective selection, proliferation, and survival of beta T-cell receptor (beta TCR)-expressing immature thymocytes. Here, we have identified two phylogenetically conserved E-boxes within the pT alpha enhancer sequence that are required for optimal enhancer activity and for its stage-specific activity in immature T cells. We have shown that the transcription factors E2A and HEB associate with high affinity to these E-boxes. Moreover, we have identified pT alpha as a direct target of E2A-HEB heterodimers in immature thymocytes because they specifically occupy the enhancer in vivo. In these cells, pT alpha mRNA levels are determined by the presence of one or two functional E2A or HEB alleles. Furthermore, E2A/HEB transcriptional activity is repressed by heterodimerization with SCL, a transcription factor that is turned off in differentiating thymocytes exactly at a stage when pT alpha is up-regulated. Taken together, our observations suggest that the dosage of E2A, HEB, and SCL determines pT alpha gene expression in immature T cells.

Regulation of early lymphocyte development by E2A family proteins

Lymphocytes develop from hematopoietic stem cells through a series of highly regulated differentiation events in the bone marrow and thymus. A number of transcription factors are known to collaborate in controlling the timing and specificity of gene expression required for these developmental processes to occur. The basic helix-loop-helix (bHLH) proteins encoded by the E2A gene have been shown to play particularly important roles in the initiation and progression of lymphocyte differentiation. Gene targeting experiments in mice have demonstrated a requirement for E2A proteins at the onset of B lymphocyte development. More recent studies have broadened our view on the function of E2A proteins at multiple stages of lymphopoiesis and in the regulation of lymphoid-specific gene expression. Here we review the mammalian E2A proteins and the accumulated evidence demonstrating central roles for E2A throughout early B and T lymphocyte development. We also speculate on the direction of future research on the mechanisms underlying the lineage and stage-specific functions of E2A in lymphopoiesis.

Developmental origin of pre-DC2

It is generally accepted that dendritic cells can be generated from either myeloid or lymphoid derived progenitors. Ample information has been collected on the development and nature of myeloid DC type 1 (DC1). In contrast, our current understanding on the origin and function of the lymphoid derived DC type 2 (DC2) is still limited but is increasing rapidly. Here we will summarize recent findings on the developmental origin of the precursor of DC2 (pre-DC2). The presence of pre-DC2 has been revealed in bone marrow, fetal liver, and cord blood, where they develop from hematopoietic stem cells (HSC) most likely via an intermediate pro-DC2 stage. Both in human and mouse, development of pre-DC2 depends on the cytokine FLT3-ligand (FLT3-L). In addition, transcription factors such as Spi-B and members of the basic helix-loop helix (bHLH) family have been shown to be involved in the proper differentiation of HSC into pre-DC2. The human thymus contains a population of cells that closely resembles the peripheral pre-DC2, including interferon (INF)-a production after viral stimulation. Some phenotypic differences have been observed however. Furthermore, we have shown that the thymic microenvironment is able to support development of pre-DC2 from HSC in vivo. A thymus independent pathway of pre-DC2 development exists as well, although at present it is not clear where these extrathymic pre-DC2 are generated. In regard of the absence of a phenotypic defined pro-DC2 population in the thymus, we speculate that development of thymic pre-DC2 may differ from peripheral pre-DC2. The challenge of the near future will be to determine the role of pre-DC2 during thymic T cell development.

Development of alphabeta T cells in the human thymus

The thymus is the main producer of alphabeta T cells and is, therefore, crucial for a normal immune system. The intrathymic developmental pathway of human alphabeta T cells has now been delineated. The production of new T cells by the thymus decreases with age, and the thymus was thought to be redundant in adults once the peripheral T-cell pool has been formed early in life. However, recent work has shown that the thymus can function even at an advanced age. Research into the production of T cells in clinical settings that are associated with loss of T cells in the periphery has sparked renewed interest in the function of the human thymus.

Lineage plasticity and commitment in T-cell development

The earliest stages of intrathymic T-cell development include not only the acquisition of T-cell characteristics but also programmed loss of potentials for B, natural killer, and dendritic cell development. Evidence from genetics and cell-transfer studies suggests an order and some components of the mechanisms involved in loss of these options, but some of the interpretations conflict. The conflicts can be resolved by a view that postulates overlapping windows of developmental opportunity and individual mechanisms regulating progression along each pathway. This view is consistent with molecular evidence for the expression patterns of positive regulators of non-T developmental pathways, SCL, PU.1 and Id2, in early thymocytes. To some extent, overexpression of such regulators redirects thymocyte development in vitro. Specific commitment functions may normally terminate this developmental plasticity. Both PU.1 overexpression and stimulation of ectopically expressed growth factor receptors can perturb T- and myeloid/dendritic-cell divergence, but only in permissive stages. A cell-line system that approximates DN3-stage thymocytes reveals that PU.1 can alter specification even in a homogeneous population. However, the response of the population to PU.1 is sharply discontinuous. These studies show a critical role for regulatory context in restricting plasticity, which is probably maintained by interacting transcription factor networks.

Disruption of pre-TCR expression accelerates lymphomagenesis in E2A-deficient mice

The helix-loop-helix proteins E47 and E12, which are encoded by the E2A gene, regulate several stages of T cell development. In addition, mice deficient for E2A are highly susceptible to thymic lymphoma. Here we report that the development of lymphoma in E2A-deficient mice did not require pre- and recombinase-activating gene expression. Rather, we found that, whereas illegitimate DNA rearrangement did not play a major role in the development of these lymphomas, defects that prevented pre-T cell antigen receptor expression tended to accelerate lymphomagenesis in E2A-deficient mice. These data and previous observations also provide insight into the role of Notch in lymphoma development. Specifically, we propose that Notch activation indirectly modulates E2A activity through induction of pre-Talpha expression, ultimately leading to the development of lymphoma.

E protein function in lymphocyte development

Lymphocytes arise from hematopoietic stem cells through the coordinated action of transcription factors. The E proteins (E12, E47, HEB and E2-2) have emerged as key regulators of both B and T lymphocyte differentiation. This review summarizes the current data and examines the various functions of E proteins and their antagonists, Id2 and Id3, throughout lymphoid maturation. Beyond an established role in B and T lineage commitment, E proteins continue to be essential at subsequent stages of development. E protein activity regulates the expression of surrogate and antigen receptor genes, promotes Ig and TCR rearrangements, and coordinates cell survival and proliferation with developmental progression in response to TCR signaling. Finally, this review also discusses the role of E47 as a tumor suppressor.

Id2 negatively regulates B cell differentiation in the spleen

Early stages of B cell development occur in the bone marrow, resulting in formation of immature B cells. These immature cells migrate to the spleen where they differentiate into mature (B2 or marginal zone (MZ)) cells. This final maturation step is crucial for B cells to become responsive to Ags and to participate in the immune response. Id2 is a helix-loop-helix protein that lacks a DNA-binding region; and therefore, inhibits basic helix-loop-helix functions in a dominant negative manner. In this study, we show that Id2 expression is down-regulated during differentiation of immature B cells into mature B2 and MZ B cells. The high levels of Id2 expressed in the immature B cells result in inhibition of E2A binding activity to an E2 box site. Moreover, mice lacking Id2 show an elevation in the proportion of mature B2 cells in the spleen, while the MZ population in these mice is almost absent. Thus, Id2 acts as a regulator of the differentiation of immature B cells occurring in the spleen, it negatively controls differentiation into mature B2 cells while allowing the commitment to MZ B cells. In the absence of Id2 control, the unregulated differentiation is directed toward the mature B2 population.

Intrathymic and extrathymic development of human plasmacytoid dendritic cell precursors in vivo

The development of plasmacytoid dendritic cells (pDC2) from human CD34(+) stem cells in vivo was studied in RAG-2(-/-) interleukin (IL)-2Rgamma(-/-) mice that lack functional T and B cells and natural killer cells. CD34(+) cells isolated from fetal liver or thymus were labeled with 5- and 6-carboxyfluorescein diacetate succinimidyl ester (CFSE) and were injected into a human thymus grafted subcutaneously in the RAG-2(-/-) IL-2Rgamma(-/-) mice. One to 4 weeks later the CFSE label was found not only in T cells but also in CD123(+/high) CD4(+)CD45RA(+) pDC2, indicating that the CD34(+) cells can develop into pDC2 within a thymus. In addition to pDC2, CFSE-labeled dendritic cells with a mature phenotype, determined by the cell surface markers CD11c, CD83, and CD80, were found in the injected human thymus graft. pDC2 was not found in the periphery of mice carrying a human thymic graft, indicating that the intrathymic pDC2 failed to emigrate from the thymus. We also demonstrate that pDC2 can develop outside the thymus because relatively high percentages of pDC2 were found in the periphery after the intravenous injection of CD34(+)CD38(-) fetal liver cells in RAG-2(-/-) IL-2Rgamma(-/-) mice without a human thymus graft. These data indicate that the thymus and the peripheral pDC2 develop independently of each other.