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

Transcriptional network dynamics in early T cell development

The rate at which cells enter the T cell pathway depends not only on the immigration of hematopoietic precursors into the strong Notch signaling environment of the thymus but also on the kinetics with which each individual precursor cell reaches T-lineage commitment once it arrives. Notch triggers a complex, multistep gene regulatory network in the cells in which the steps are stereotyped but the transition speeds between steps are variable. Progenitor-associated transcription factors delay T-lineage differentiation even while Notch-induced transcription factors within the same cells push differentiation forward. Progress depends on regulator cross-repression, on breaching chromatin barriers, and on shifting, competitive collaborations between stage-specific and stably expressed transcription factors, as reviewed here.

E Protein Transcription Factors as Suppressors of T Lymphocyte Acute Lymphoblastic Leukemia

T Lymphocyte Acute Lymphoblastic Leukemia (ALL) is an aggressive disease arising from transformation of T lymphocytes during their development. The mutation spectrum of T-ALL has revealed critical regulators of the growth and differentiation of normal and leukemic T lymphocytes. Approximately, 60% of T-ALLs show aberrant expression of the hematopoietic stem cell-associated helix-loop-helix transcription factors TAL1 and LYL1. TAL1 and LYL1 function in multiprotein complexes that regulate gene expression in T-ALL but they also antagonize the function of the E protein homodimers that are critical regulators of T cell development. Mice lacking E2A, or ectopically expressing TAL1, LYL1, or other inhibitors of E protein function in T cell progenitors, also succumb to an aggressive T-ALL-like disease highlighting that E proteins promote T cell development and suppress leukemogenesis. In this review, we discuss the role of E2A in T cell development and how alterations in E protein function underlie leukemogenesis. We focus on the role of TAL1 and LYL1 and the genes that are dysregulated in E2a^-/- T cell progenitors that contribute to human T-ALL. These studies reveal novel mechanisms of transformation and provide insights into potential therapeutic targets for intervention in this disease.

How transcription factors drive choice of the T cell fate

Recent evidence has elucidated how multipotent blood progenitors transform their identities in the thymus and undergo commitment to become T cells. Together with environmental signals, a core group of transcription factors have essential roles in this process by directly activating and repressing specific genes. Many of these transcription factors also function in later T cell development, but control different genes. Here, we review how these transcription factors work to change the activities of specific genomic loci during early intrathymic development to establish T cell lineage identity. We introduce the key regulators and highlight newly emergent insights into the rules that govern their actions. Whole-genome deep sequencing-based analysis has revealed unexpectedly rich relationships between inherited epigenetic states, transcription factor-DNA binding affinity thresholds and influences of given transcription factors on the activities of other factors in the same cells. Together, these mechanisms determine T cell identity and make the lineage choice irreversible.

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.

Transcriptional regulation of natural killer cell development and maturation

Natural killer cells are lymphocytes that respond rapidly to intracellular pathogens or cancer/stressed cells by producing pro-inflammatory cytokines or chemokines and by killing target cells through direct cytolysis. NK cells are distinct from B and T lymphocytes in that they become activated through a series of broadly expressed germ line encoded activating and inhibitory receptors or through the actions of inflammatory cytokines. They are the founding member of the innate lymphoid cell family, which mirror the functions of T lymphocytes, with NK cells being the innate counterpart to CD8 T lymphocytes. Despite the functional relationship between NK cells and CD8 T cells, the mechanisms controlling their specification, differentiation and maturation are distinct, with NK cells emerging from multipotent lymphoid progenitors in the bone marrow under the control of a unique transcriptional program. Over the past few years, substantial progress has been made in understanding the developmental pathways and the factors involved in generating mature and functional NK cells. NK cells have immense therapeutic potential and understanding how to acquire large numbers of functional cells and how to endow them with potent activity to control hematopoietic and non-hematopoietic malignancies and autoimmunity is a major clinical goal. In this review, we examine basic aspects of conventional NK cell development in mice and humans and discuss multiple transcription factors that are known to guide the development of these cells.

The influence of space and time on the establishment of B cell identity

During embryonic development multiple waves of hematopoietic progenitors with distinct lineage potential are differentially regulated in time and space. Consistent with this view, some specialized lymphocytes emerge during a limited time-window in embryogenesis and migrate to the tissues where they contribute to organogenesis and to tissue homeostasis. These cells are not constantly produced by bone marrow derived hematopoietic stem cells but are maintained in tissues and self-renew throughout life. These particular cell subsets are produced from lymphoid restricted progenitors only found in the first days of fetal liver hematopoietic activity. Growing evidence of the heterogeneity and layered organization of the hematopoietic system is leading to a common view that some lymphocyte subsets are functionally different because they follow distinct developmental programs and emerge from distinct waves of lymphoid progenitors. However, understanding the influence of developmental origin and the relative contribution of local microenvironment on the development of these specialized lymphocyte subsets needs further analysis. In this review, we discuss how different pathways followed by developing B cells during ontogeny may contribute to the diverse functions.

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.

Dissection of progenitor compartments resolves developmental trajectories in B-lymphopoiesis

Jensen et al. report the identification and characterization of novel lymphoid progenitor populations in the mouse bone marrow. The work resolves the complexity of the BLP/pre-pro–B/Fraction A compartments and provides a developmental trajectory for early B cell development.

To understand the developmental trajectories in early lymphocyte differentiation, we identified differentially expressed surface markers on lineage-negative lymphoid progenitors (LPs). Single-cell polymerase chain reaction experiments allowed us to link surface marker expression to that of lineage-associated transcription factors (TFs) and identify GFRA2 and BST1 as markers of early B cells. Functional analyses in vitro and in vivo as well as single-cell gene expression analyses supported that surface expression of these proteins defined distinct subpopulations that include cells from both the classical common LPs (CLPs) and Fraction A compartments. The formation of the GFRA2-expressing stages of development depended on the TF EBF1, critical both for the activation of stage-specific target genes and modulation of the epigenetic landscape. Our data show that consecutive expression of Ly6D, GFRA2, and BST1 defines a developmental trajectory linking the CLP to the CD19⁺ progenitor compartment.

The E-Id Protein Axis Specifies Adaptive Lymphoid Cell Identity and Suppresses Thymic Innate Lymphoid Cell Development

Innate and adaptive lymphoid development is orchestrated by the activities of E proteins and their antagonist Id proteins, but how these factors regulate early T cell progenitor (ETP) and innate lymphoid cell (ILC) development remains unclear. Using multiple genetic strategies, we demonstrated that E proteins E2A and HEB acted in synergy in the thymus to establish T cell identity and to suppress the aberrant development of ILCs, including ILC2s and lymphoid-tissue-inducer-like cells. E2A and HEB orchestrated T cell fate and suppressed the ILC transcription signature by activating the expression of genes associated with Notch receptors, T cell receptor (TCR) assembly, and TCR-mediated signaling. E2A and HEB acted in ETPs to establish and maintain a T-cell-lineage-specific enhancer repertoire, including regulatory elements associated with the Notch1, Rag1, and Rag2 loci. On the basis of these and previous observations, we propose that the E-Id protein axis specifies innate and adaptive lymphoid cell fate.

Asynchronous lineage priming determines commitment to T cell and B cell lineages in fetal liver

The molecular events that initiate lymphoid-lineage specification remain unidentified because the stages of differentiation during which lineage commitment occurs are difficult to characterize. We isolated fetal liver progenitor cells undergoing restriction of their differentiation potential toward the T cell-innate lymphoid cell lineage or the B cell lineage. Transcripts that defined the molecular signatures of these two subsets were sequentially upregulated in lympho-myeloid precursor cells and in common lymphoid progenitor cells, respectively, and this preceded lineage restriction; this indicates that T cell-versus-B cell commitment is not a binary fate 'decision'. The T cell-bias and B cell-bias transcriptional programs were frequently co-expressed in common lymphoid progenitor cells and were segregated in subsets biased toward T cell differentiation or B cell differentiation, after interleukin 7 (IL-7) signaling that controlled the number of progenitor cells engaging in T cell differentiation versus B cell differentiation.

The ETS1 transcription factor is required for the development and cytokine-induced expansion of ILC2

Zook et al. use a novel mouse model to demonstrate a requirement for the transcription factor ETS1 in the development and function of group 2 innate lymphoid cells.

Group 2 innate lymphoid cells (ILC2s) are a subset of ILCs that play a protective role in the response to helminth infection, but they also contribute to allergic lung inflammation. Here, we report that the deletion of the ETS1 transcription factor in lymphoid cells resulted in a loss of ILC2s in the bone marrow and lymph nodes and that ETS1 promotes the fitness of the common progenitor of all ILCs. ETS1-deficient ILC2 progenitors failed to up-regulate messenger RNA for the E protein transcription factor inhibitor ID2, a critical factor for ILCs, and these cells were unable to expand in cytokine-driven in vitro cultures. In vivo, ETS1 was required for the IL-33–induced accumulation of lung ILC2s and for the production of the T helper type 2 cytokines IL-5 and IL-13. IL-25 also failed to elicit an expansion of inflammatory ILC2s when these cells lacked ETS1. Our data reveal ETS1 as a critical regulator of ILC2 expansion and cytokine production and implicate ETS1 in the regulation of Id2 at the inception of ILC2 development.

Exploring the multifaceted nature of the common lymphoid progenitor compartment

While the common lymphoid progenitor compartment was originally thought to be a rather homogenous cell population, it has become increasingly clear that this compartment is highly heterogeneous both with regard to phenotypic and functional features. The exploration of this cellular complexity has generated novel molecular insights into regulatory events in lymphoid lineage restriction and provided support for the idea that multiple lineage restriction events occur at this developmental stage. Furthermore, the identification of multiple lineage-restricted progenitors with mixed lineage potential challenges a strictly hierarchical model for lymphoid development. Instead we propose a model based on competence windows during which cell fates are established through the action of lineage determining factors.

Lymphoid Gene Upregulation on Circulating Progenitors Participates in Their T-Lineage Commitment

Extrathymic T cell precursors can be detected in many tissues and represent an immediately competent population for rapid T cell reconstitution in the event of immunodeficiencies. Blood T cell progenitors have been detected, but their source in the bone marrow (BM) remains unclear. Prospective purification of BM-resident and circulating progenitors, together with RT-PCR single-cell analysis, was used to evaluate and compare multipotent progenitors (MPPs) and common lymphoid progenitors (CLPs). Molecular analysis of circulating progenitors in comparison with BM-resident progenitors revealed that CCR9(+) progenitors are more abundant in the blood than CCR7(+) progenitors. Second, although Flt3(-) CLPs are less common in the BM, they are abundant in the blood and have reduced Cd25(+)-expressing cells and downregulated c-Kit and IL-7Rα intensities. Third, in contrast, stage 3 MPP (MPP3) cells, the unique circulating MPP subset, have upregulated Il7r, Gata3, and Notch1 in comparison with BM-resident counterparts. Evaluation of the populations' respective abilities to generate splenic T cell precursors (Lin(-)Thy1.2(+)CD25(+)IL7Rα(+)) after grafting recipient nude mice revealed that MPP3 cells were the most effective subset (relative to CLPs). Although several lymphoid genes are expressed by MPP3 cells and Flt3(-) CLPs, the latter only give rise to B cells in the spleen, and Notch1 expression level is not modulated in the blood, as for MPP3 cells. We conclude that CLPs have reached the point where they cannot be a Notch1 target, a limiting condition on the path to T cell engagement.

ID'ing innate and innate-like lymphoid cells

The immune system can be divided into innate and adaptive components that differ in their rate and mode of cellular activation, with innate immune cells being the first responders to invading pathogens. Recent advances in the identification and characterization of innate lymphoid cells have revealed reiterative developmental programs that result in cells with effector fates that parallel those of adaptive lymphoid cells and are tailored to effectively eliminate a broad spectrum of pathogenic challenges. However, activation of these cells can also be associated with pathologies such as autoimmune disease. One major distinction between innate and adaptive immune system cells is the constitutive expression of ID proteins in the former and inducible expression in the latter. ID proteins function as antagonists of the E protein transcription factors that play critical roles in lymphoid specification as well as B- and T-lymphocyte development. In this review, we examine the transcriptional mechanisms controlling the development of innate lymphocytes, including natural killer cells and the recently identified innate lymphoid cells (ILC1, ILC2, and ILC3), and innate-like lymphocytes, including natural killer T cells, with an emphasis on the known requirements for the ID proteins.

Transcriptional control of early T and B cell developmental choices

T and B cells share a common somatic gene rearrangement mechanism for assembling the genes that code for their antigen receptors; they also have developmental pathways with many parallels. Shared usage of basic helix-loop-helix E proteins as transcriptional drivers underlies these common features. However, the transcription factor networks in which these E proteins are embedded are different both in membership and in architecture for T and B cell gene regulatory programs. These differences permit lineage commitment decisions to be made in different hierarchical orders. Furthermore, in contrast to B cell gene networks, the T cell gene network architecture for effector differentiation is sufficiently modular so that E protein inputs can be removed. Complete T cell-like effector differentiation can proceed without T cell receptor rearrangement or selection when E proteins are neutralized, yielding natural killer and other innate lymphoid cells.

RAG-1 and Ly6D independently reflect progression in the B lymphoid lineage

Common lymphoid progenitors (CLPs) are thought to represent major intermediates in the transition of hematopoietic stem cells (HSCs) to B lineage lymphocytes. However, it has been obvious for some time that CLPs are heterogeneous, and there has been controversy concerning their differentiation potential. We have now resolved four Flt3⁺ CLP subsets that are relatively homogenous and capable of forming B cells. Differentiation potential and gene expression patterns suggest Flt3⁺ CLPs lacking both Ly6D and RAG-1 are the least differentiated. In addition to B cells, they generate natural killer (NK) and dendritic cells (DCs). At the other extreme is a subset of the recently described Flt3⁺ Ly6D⁺ CLPs that have a history of RAG-1 expression and are B lineage restricted. These relatively abundant and potent CLPs were depleted within 48 hours of acute in vivo estrogen elevation, suggesting they descend from hormone regulated progenitors. This contrasts with the hormone insensitivity of other CLP subsets that include NK lineage progenitors. This progenitor heterogeneity and differentiation complexity may add flexibility in response to environmental changes. Expression of RAG-1 and display of Ly6D are both milestone events, but they are neither synchronized nor dependent on each other.