The preTCR-dependent DN3 to DP transition requires Notch signaling, is improved by CXCL12 signaling and is inhibited by IL-7 signaling

The Role of NOTCH1, GATA3, and c-MYC in T Cell Non-Hodgkin Lymphomas

Lymphomas are heterogeneous malignant tumours of white blood cells characterised by the aberrant proliferation of mature lymphoid cells or their precursors. Lymphomas are classified into main types depending on the histopathologic evidence of biopsy taken from an enlarged lymph node, progress stages, treatment strategies, and outcomes: Hodgkin and non-Hodgkin lymphoma (NHL). Moreover, lymphomas can be further divided into subtypes depending on the cell origin, and immunophenotypic and genetic aberrations. Many factors play vital roles in the progression, pathogenicity, incidence, and mortality rate of lymphomas. Among NHLs, peripheral T cell lymphomas (PTCLs) are rare lymphoid malignancies, that have various cellular morphology and genetic mutations. The clinical presentations are usually observed at the advanced stage of the disease. Many recent studies have reported that the expressions of NOTCH1, GATA3, and c-MYC are associated with poorer prognosis in PTCL and are involved in downstream activities. However, questions have been raised about the pathological relationship between these factors in PTCLs. Therefore, in this review, we investigate the role and relationship of the NOTCH1 pathway, transcriptional factor GATA3 and proto-oncogene c-MYC in normal T cell development and malignant PTCL subtypes.

The perception and response of T cells to a changing environment are based on the law of initial value

αβ T cells are critical components of the adaptive immune system and are capable of inducing sterilizing immunity after pathogen infection and eliminating transformed tumor cells. The development and function of T cells are controlled through the T cell antigen receptor, which recognizes peptides displayed on major histocompatibility complex (MHC) molecules. Here, we review how T cells generate the ability to recognize self-peptide-bound MHC molecules and use signals derived from these interactions to instruct cellular development, activation thresholds, and functional specialization in the steady state and during immune responses. We argue that the basic tenants of T cell development and function follow Weber-Fetcher's law of just noticeable differences and Wilder's law of initial value. Together, these laws argue that the ability of a system to respond and the quality of that response are scalable to the basal state of that system. Manifestation of these laws in T cells generates clone-specific activation thresholds that are based on perceivable differences between homeostasis and pathogen encounter (self versus nonself discrimination), as well as poised states for subsequent differentiation into specific effector cell lineages.

A 2020 View of Thymus Stromal Cells in T Cell Development

The thymus is an intricate primary lymphoid organ, wherein bone marrow-derived lymphoid progenitor cells are induced to develop into functionally competent T cells that express a diverse TCR repertoire, which is selected to allow for the recognition of foreign Ags while avoiding self-reactivity or autoimmunity. Thymus stromal cells, which can include all non-T lineage cells, such as thymic epithelial cells, endothelial cells, mesenchymal/fibroblast cells, dendritic cells, and B cells, provide signals that are essential for thymocyte development as well as for the homeostasis of the thymic stroma itself. In this brief review, we focus on the key roles played by thymic stromal cells during early stages of T cell development, such as promoting the homing of thymic-seeding progenitors, inducing T lineage differentiation, and supporting thymocyte survival and proliferation. We also discuss recent advances on the transcriptional regulation that govern thymic epithelial cell function as well as the cellular and molecular changes that are associated with thymic involution and regeneration.

Intrathymic Selection and Defects in the Thymic Epithelial Cell Development

Intimate interactions between thymic epithelial cells (TECs) and thymocytes (T) have been repeatedly reported as essential for performing intrathymic T-cell education. Nevertheless, it has been described that animals exhibiting defects in these interactions were capable of a proper positive and negative T-cell selection. In the current review, we first examined distinct types of TECs and their possible role in the immune surveillance. However, EphB-deficient thymi that exhibit profound thymic epithelial (TE) alterations do not exhibit important immunological defects. Eph and their ligands, the ephrins, are implicated in cell attachment/detachment and govern, therefore, TEC–T interactions. On this basis, we hypothesized that a few normal TE areas could be enough for a proper phenotypical and functional maturation of T lymphocytes. Then, we evaluated in vivo how many TECs would be necessary for supporting a normal T-cell differentiation, concluding that a significantly low number of TEC are still capable of supporting normal T lymphocyte maturation, whereas with fewer numbers, T-cell maturation is not possible.

Neem leaf glycoprotein reverses tumor-induced and age-associated thymic involution to maintain peripheral CD8+ T cell pool

Aim: As tumor causes atrophy in the thymus to target effector-T cells, this study is aimed to decipher the efficacy of neem leaf glycoprotein (NLGP) in tumor- and age-associated thymic atrophy. Materials & methods: Different thymus parameters were studied using flow cytometry, reverse transcriptase PCR and immunocyto-/histochemistry in murine melanoma and sarcoma models. Results: Longitudinal NLGP therapy in tumor hosts show tumor-reduction along with significant normalization of thymic alterations. NLGP downregulates intrathymic IL-10, which eventually promotes Notch1 to rescue blockade in CD25⁺CD44⁺c-Kit⁺DN2 to CD25⁺CD44^-c-Kit^-DN3 transition in T cell maturation and suppress Ikaros/IRF8/Pu.1 to prevent DN2-T to DC differentiation in tumor hosts. The CD5^intTCRαβ^high DP3 population was also increased to endorse CD8⁺ T cell generation. Conclusion: NLGP rescues tumor-induced altered thymic events to generate more effector T cells to restrain tumor.

Guiding T lymphopoiesis from pluripotent stem cells by defined transcription factors

Achievement of immunocompetent and therapeutic T lymphopoiesis from pluripotent stem cells (PSCs) is a central aim in T cell regenerative medicine. To date, preferentially reconstituting T lymphopoiesis in vivo from PSCs remains a practical challenge. Here we documented that synergistic and transient expression of Runx1 and Hoxa9 restricted in the time window of endothelial-to-hematopoietic transition and hematopoietic maturation stages in a PSC differentiation scheme (iR9-PSC) in vitro induced preferential generation of engraftable hematopoietic progenitors capable of homing to thymus and developing into mature T cells in primary and secondary immunodeficient recipients. Single-cell transcriptome and functional analyses illustrated the cellular trajectory of T lineage induction from PSCs, unveiling the T-lineage specification determined at as early as hemogenic endothelial cell stage and identifying the bona fide pre-thymic progenitors. The induced T cells distributed normally in central and peripheral lymphoid organs and exhibited abundant TCRαβ repertoire. The regenerative T lymphopoiesis restored immune surveillance in immunodeficient mice. Furthermore, gene-edited iR9-PSCs produced tumor-specific T cells in vivo that effectively eradicated tumor cells. This study provides insight into universal generation of functional and therapeutic T cells from the unlimited and editable PSC source.

Notch and the pre-TCR coordinate thymocyte proliferation by induction of the SCF subunits Fbxl1 and Fbxl12

Proliferation is tightly regulated during T cell development and is limited to immature CD4⁻CD8⁻ thymocytes. The major proliferative event is initiated at the ‘β-selection’ stage following successful rearrangement of Tcrβ and is triggered by and dependent on concurrent signaling by Notch and the pre-TCR; however, it is unclear how these signals cooperate to promote cell proliferation. Here we found that β-selection-associated proliferation required the combined activity of two SCF ubiquitin ligase complexes that included as substrate recognition subunits the F-box proteins Fbxl1 or Fbxl12. Both SCF complexes targeted the cyclin-dependent kinase inhibitor Cdkn1b for ubiquitinylaton and degradation. We found that Notch signals induced the transcription of Fbxl1 whereas pre-TCR signals induced the transcription of Fbxl12. Thus, concurrent Notch and pre-TCR signaling induced the expression of two genes, Fbxl1 and Fbxl12, whose products functioned identically but additively to promote degradation of Cdkn1b, cell cycle progression, and proliferation of β-selected thymocytes.

Metabolic signaling directs the reciprocal lineage decisions of αβ and γδ T cells

The interaction between extrinsic factors and intrinsic signal strength governs thymocyte development, but the mechanisms linking them remain elusive. We report that mechanistic target of rapamycin complex 1 (mTORC1) couples microenvironmental cues with metabolic programs to orchestrate the reciprocal development of two fundamentally distinct T cell lineages, the αβ and γδ T cells. Developing thymocytes dynamically engage metabolic programs including glycolysis and oxidative phosphorylation, as well as mTORC1 signaling. Loss of RAPTOR-mediated mTORC1 activity impairs the development of αβ T cells but promotes γδ T cell generation, associated with disrupted metabolic remodeling of oxidative and glycolytic metabolism. Mechanistically, we identify mTORC1-dependent control of reactive oxygen species production as a key metabolic signal in mediating αβ and γδ T cell development, and perturbation of redox homeostasis impinges upon thymocyte fate decisions and mTORC1-associated phenotypes. Furthermore, single-cell RNA sequencing and genetic dissection reveal that mTORC1 links developmental signals from T cell receptors and NOTCH to coordinate metabolic activity and signal strength. Our results establish mTORC1-driven metabolic signaling as a decisive factor for reciprocal αβ and γδ T cell development and provide insight into metabolic control of cell signaling and fate decisions.

The transcription factor Duxbl mediates elimination of pre-T cells that fail β-selection

During β-selection, T cells without productive TCRβ rearrangements are eliminated. Klein et al. show that the transcription factor Duxbl regulates this process by inducing apoptosis through activation of the Oas/RNaseL pathway. Successful TCRβ rearrangement rescues cells by pre-TCR–mediated Duxbl suppression.

T cell development is critically dependent on successful rearrangement of antigen-receptor chains. At the β-selection checkpoint, only cells with a functional rearrangement continue in development. However, how nonselected T cells proceed in their dead-end fate is not clear. We identified low CD27 expression to mark pre-T cells that have failed to rearrange their β-chain. Expression profiling and single-cell transcriptome clustering identified a developmental trajectory through β-selection and revealed specific expression of the transcription factor Duxbl at a stage of high recombination activity before β-selection. Conditional transgenic expression of Duxbl resulted in a developmental block at the DN3-to-DN4 transition due to reduced proliferation and enhanced apoptosis, whereas RNA silencing of Duxbl led to a decrease in apoptosis. Transcriptome analysis linked Duxbl to elevated expression of the apoptosis-inducing Oas/RNaseL pathway. RNaseL deficiency or sustained Bcl2 expression led to a partial rescue of cells in Duxbl transgenic mice. These findings identify Duxbl as a regulator of β-selection by inducing apoptosis in cells with a nonfunctional rearrangement.

A new role for Notch in the control of polarity and asymmetric cell division of developing T cells

A fundamental question in biology is how single cells can reliably produce progeny of different cell types. Notch signalling frequently facilitates fate determination. Asymmetric cell division (ACD) often controls segregation of Notch signalling by imposing unequal inheritance of regulators of Notch. Here, we assessed the functional relationship between Notch and ACD in mouse T cell development. To attain immunological specificity, developing T cells must pass through a pivotal stage termed β-selection, which involves Notch signalling and ACD. We assessed functional interactions between Notch1 and ACD during β-selection using direct presentation of Notch ligands, DL1 and DL4, and pharmacological inhibition of Notch signalling. Contrary to prevailing models, we demonstrate that Notch controls the distribution of Notch1 itself and cell fate determinants, α-Adaptin and Numb. Further, Notch and CXCR4 signalling cooperated to drive polarity during division. Thus, Notch signalling directly orchestrates ACD, and Notch1 is differentially inherited by sibling cells.

DN2 Thymocytes Activate a Specific Robust DNA Damage Response to Ionizing Radiation-Induced DNA Double-Strand Breaks

For successful bone marrow transplantation (BMT), a preconditioning regime involving chemo and radiotherapy is used that results in DNA damage to both hematopoietic and stromal elements. Following radiation exposure, it is well recognized that a single wave of host-derived thymocytes reconstitutes the irradiated thymus, with donor-derived thymocytes appearing about 7 days post BMT. Our previous studies have demonstrated that, in the presence of donor hematopoietic cells lacking T lineage potential, these host-derived thymocytes are able to generate a polyclonal cohort of functionally mature peripheral T cells numerically comprising ~25% of the peripheral T cell pool of euthymic mice. Importantly, we demonstrated that radioresistant CD44⁺ CD25⁺ CD117+ DN2 progenitors were responsible for this thymic auto-reconstitution. Until recently, the mechanisms underlying the radioresistance of DN2 progenitors were unknown. Herein, we have used the in vitro “Plastic Thymus” culture system to perform a detailed investigation of the mechanisms responsible for the high radioresistance of DN2 cells compared with radiosensitive hematopoietic stem cells. Our results indicate that several aspects of DN2 biology, such as (i) rapid DNA damage response (DDR) activation in response to ionizing radiation-induced DNA damage, (ii) efficient repair of DNA double-strand breaks, and (iii) induction of a protective G₁/S checkpoint contribute to promoting DN2 cell survival post-irradiation. We have previously shown that hypoxia increases the radioresistance of bone marrow stromal cells in vitro, at least in part by enhancing their DNA double-strand break (DNA DSB) repair capacity. Since the thymus is also a hypoxic environment, we investigated the potential effects of hypoxia on the DDR of DN2 thymocytes. Finally, we demonstrate for the first time that de novo DN2 thymocytes are able to rapidly repair DNA DSBs following thymic irradiation in vivo.

Regulation of Hematopoietic Cell Development and Function Through Phosphoinositides

One of the most paramount receptor-induced signal transduction mechanisms in hematopoietic cells is production of the lipid second messenger phosphatidylinositol(3,4,5)trisphosphate (PIP₃) by class I phosphoinositide 3 kinases (PI3K). Defective PIP₃ signaling impairs almost every aspect of hematopoiesis, including T cell development and function. Limiting PIP₃ signaling is particularly important, because excessive PIP₃ function in lymphocytes can transform them and cause blood cancers. Here, we review the key functions of PIP₃ and related phosphoinositides in hematopoietic cells, with a special focus on those mechanisms dampening PIP₃ production, turnover, or function. Recent studies have shown that beyond “canonical” turnover by the PIP₃ phosphatases and tumor suppressors phosphatase and tensin homolog (PTEN) and SH2 domain-containing inositol-5-phosphatase-1 (SHIP-1/2), PIP₃ function in hematopoietic cells can also be dampened through antagonism with the soluble PIP₃ analogs inositol(1,3,4,5)tetrakisphosphate (IP₄) and inositol-heptakisphosphate (IP₇). Other evidence suggests that IP₄ can promote PIP₃ function in thymocytes. Moreover, IP₄ or the kinases producing it limit store-operated Ca²⁺ entry through Orai channels in B cells, T cells, and neutrophils to control cell survival and function. We discuss current models for how soluble inositol phosphates can have such diverse functions and can govern as distinct processes as hematopoietic stem cell homeostasis, neutrophil macrophage and NK cell function, and development and function of B cells and T cells. Finally, we will review the pathological consequences of dysregulated IP₄ activity in immune cells and highlight contributions of impaired inositol phosphate functions in disorders such as Kawasaki disease, common variable immunodeficiency, or blood cancer.

Cytokines, Transcription Factors, and the Initiation of T-Cell Development

Multipotent blood progenitor cells migrate into the thymus and initiate the T-cell differentiation program. T-cell progenitor cells gradually acquire T-cell characteristics while shedding their multipotentiality for alternative fates. This process is supported by extracellular signaling molecules, including Notch ligands and cytokines, provided by the thymic microenvironment. T-cell development is associated with dynamic change of gene regulatory networks of transcription factors, which interact with these environmental signals. Together with Notch or pre-T-cell-receptor (TCR) signaling, cytokines always control proliferation, survival, and differentiation of early T cells, but little is known regarding their cross talk with transcription factors. However, recent results suggest ways that cytokines expressed in distinct intrathymic niches can specifically modulate key transcription factors. This review discusses how stage-specific roles of cytokines and transcription factors can jointly guide development of early T cells.

Non-canonical antagonism of PI3K by the kinase Itpkb delays thymocyte β-selection and renders it Notch-dependent

β-selection is the most pivotal event determining αβ T cell fate. Here, surface-expression of a pre-T cell receptor (pre-TCR) induces thymocyte metabolic activation, proliferation, survival and differentiation. Besides the pre-TCR, β-selection also requires co-stimulatory signals from Notch receptors - key cell fate determinants in eukaryotes. Here, we show that this Notch-dependence is established through antagonistic signaling by the pre-TCR/Notch effector, phosphoinositide 3-kinase (PI3K), and by inositol-trisphosphate 3-kinase B (Itpkb). Canonically, PI3K is counteracted by the lipid-phosphatases Pten and Inpp5d/SHIP-1. In contrast, Itpkb dampens pre-TCR induced PI3K/Akt signaling by producing IP₄, a soluble antagonist of the Akt-activating PI3K-product PIP₃. Itpkb^-/- thymocytes are pre-TCR hyperresponsive, hyperactivate Akt, downstream mTOR and metabolism, undergo an accelerated β-selection and can develop to CD4⁺CD8⁺ cells without Notch. This is reversed by inhibition of Akt, mTOR or glucose metabolism. Thus, non-canonical PI3K-antagonism by Itpkb restricts pre-TCR induced metabolic activation to enforce coincidence-detection of pre-TCR expression and Notch-engagement.

DOI:http://dx.doi.org/10.7554/eLife.10786.001

T cells defend our body against cancer and infectious agents such as viruses. However, they can also cause rheumatoid arthritis and other autoimmune diseases by attacking healthy tissue. T cells recognize target cells via receptor proteins on their surface. To maximize the variety of infections and cancers our immune system can recognize, we generate millions of T cells with different T cell receptors every day.

To ensure T cells work correctly, T cell receptors are tested at various checkpoints. The first checkpoint involves a process called beta (β) selection, during which T cells produce their first T cell receptor – the so-called pre-T cell receptor. This receptor causes T cells to divide and mature, and sets their future identity or “fate”. To complete β-selection, T cells must also receive signals from another surface receptor – one that belongs to the Notch family, which determines cell fate in many different tissues.

The Notch receptor and the pre-T cell receptor both activate an enzyme called PI3K – a key mediator of β-selection. But the pre-T cell receptor also activates another enzyme called Itpkb that is required for T cell development. Westernberg, Conche et al. have now investigated how these different proteins and signaling processes work and interact during β-selection, using mice that lack several immune genes, including the gene that produces Itpkb.

The results of the experiments show that during β-selection, Itpkb limits the ability of PI3K to activate some of its key target proteins. This “dampened” PI3K signaling ensures that both the pre-T cell receptor and the Notch receptor must be activated to trigger T cell maturation. Without Itpkb, β-selection can occur in the absence of Notch signaling.

As Notch signaling is important for determining the fate of many different cell types, Westernberg, Conche et al.’s findings raise the possibility that Itpkb might also regulate cell fate determination in other tissues. Moreover, Itpkb may suppress tumor development, because excessive PI3K signaling drives many cancers.

DOI:http://dx.doi.org/10.7554/eLife.10786.002

Developmental gene networks: a triathlon on the course to T cell identity

Cells acquire their ultimate identities by activating combinations of transcription factors that initiate and sustain expression of the appropriate cell type-specific genes. T cell development depends on the progression of progenitor cells through three major phases, each of which is associated with distinct transcription factor ensembles that control the recruitment of these cells to the thymus, their proliferation, lineage commitment and responsiveness to T cell receptor signals, all before the allocation of cells to particular effector programmes. All three phases are essential for proper T cell development, as are the mechanisms that determine the boundaries between each phase. Cells that fail to shut off one set of regulators before the next gene network phase is activated are predisposed to leukaemic transformation.

An epigenetic profile of early T-cell development from multipotent progenitors to committed T-cell descendants

Cellular differentiation of the T-cell branch of the immune system begins with the HSC, which undergoes a series of stages characterized by progressive restriction in multipotency and acquisition of specific lineage identity At the molecular level, the restriction of cell potential, commitment, and differentiation to a specific lineage is achieved through the coordinated control of gene expression and epigenetic mechanisms. Here, we analyzed and compared the gene expression profiles and the genome-wide histone modification marks H3K4me3 (H3 lysine 4 trimethylation) and H3K27me3 (H3 lysine 27 trimethylation) in (i) in vitro propagated HSCs, (ii) in vitro generated and propagated pro-T cells derived from these stem cells, and (iii) double-positive thymocytes derived from these pro-T cells after injection into Rag-deficient mice. The combined analyses of the different datasets in this unique experimental system highlighted the importance of both transcriptional and epigenetic repression in shaping the early phases of T-cell development.

IL-7R-mediated signaling in T-cell acute lymphoblastic leukemia

Interleukin-7 (IL-7), a cytokine produced in the bone marrow, thymus and other organs, is mandatory for normal human T-cell development and peripheral homeostasis. Different studies, including phase I clinical trials, have indicated the potential therapeutic value of recombinant IL-7 in the context of anti-cancer immunotherapy and as a booster of immune reconstitution. However, the two main pathways activated by IL-7, JAK/STAT5 and PI3K/Akt/mTOR, have both been implicated in cancer and there is considerable evidence that IL-7 and its receptor (IL-7R), formed by IL-7Rα (encoded by IL7R) and γc, may partake in T-cell acute lymphoblastic leukemia (T-ALL) development. In this context, the most compelling data comes from recent studies demonstrating that around 10% of T-ALL patients display IL7R gain-of-function mutations leading, in most cases, to disulfide bond-dependent homodimerization of two mutant receptors and consequent constitutive activation of downstream signaling, with ensuing cell transformation in vitro and tumorigenic ability in vivo. Here, we review the data on the involvement of IL-7 and IL-7R in T-ALL, further discussing the peculiarities of IL-7R-mediated signaling in human leukemia T-cells that may be of therapeutic value, namely regarding the potential use of PI3K and mTOR pharmacological inhibitors.

Interleukin-7 supports survival of T-cell receptor-β-expressing CD4(-) CD8(-) double-negative thymocytes

Among the milestones that occur during T-cell development in the thymus is the expression of T-cell receptor-β (TCR-β) and the formation of the pre-TCR complex. Signals emanating from the pre-TCR trigger survival, proliferation and differentiation of T-cell precursors. Although the pre-TCR is essential for these cell outcomes, other receptors, such as Notch and CXCR4, also contribute. Whether interleukin-7 (IL-7) participates in promoting the survival or proliferation of pre-TCR-expressing cells is controversial. We used in vitro and in vivo models of T-cell development to examine the function of IL-7 in TCR-β-expressing thymocytes. Culturing TCR-β-expressing CD4(-) CD8(-) double-negative thymocytes in an in vitro model of T-cell development revealed that IL-7 reduced the frequency of CD4(+) CD8(+) double-positive thymocytes at the time of harvest. The mechanism for this change in the percentage of double-positive cells was that IL-7 promoted the survival of thymocytes that had not yet differentiated. By preserving the double-negative population, IL-7 reduced the frequency of double-positive thymocytes. Interleukin-7 was not required for proliferation in the in vitro system. To follow this observation, we examined mice lacking CD127 (IL-7Rα). In addition to the known effect of CD127 deficiency on T-cell development before TCR-β expression, CD127 deficiency also impaired the development of TCR-β-expressing double-negative thymocytes. Specifically, we found that Bcl-2 expression and cell cycle progression were reduced in TCR-β-expressing double-negative thymocytes in mice lacking CD127. We conclude that IL-7 continues to function after TCR-β is expressed by promoting the survival of TCR-β-expressing double-negative thymocytes.

Ephrin-B-dependent thymic epithelial cell-thymocyte interactions are necessary for correct T cell differentiation and thymus histology organization: relevance for thymic cortex development

Previous analysis on the thymus of erythropoietin-producing hepatocyte kinases (Eph) B knockout mice and chimeras revealed that Eph-Eph receptor-interacting proteins (ephrins) are expressed both on T cells and thymic epithelial cells (TECs) and play a role in defining the thymus microenvironments. In the current study, we have used the Cre-LoxP system to selectively delete ephrin-B1 and/or ephrin-B2 in either thymocytes (EfnB1(thy/thy), EfnB2(thy/thy), and EfnB1(thy/thy)EfnB2(thy/thy) mice) or TECs (EfnB1(tec/tec), EfnB2(tec/tec), and EfnB1(tec/tec)EfnB2(tec/tec) mice) and determine the relevance of these Eph ligands in T cell differentiation and thymus histology. Our results indicate that ephrin-B1 and ephrin-B2 expressed on thymocytes play an autonomous role in T cell development and, expressed on TECs, their nonautonomous roles are partially overlapping. The effects of the lack of ephrin-B1 and/or ephrin-B2 on either thymocytes or TECs are more severe and specific on thymic epithelium, contribute to the cell intermingling necessary for thymus organization, and affect cortical TEC subpopulation phenotype and location. Moreover, ephrin-B1 and ephrin-B2 seem to be involved in the temporal appearance of distinct cortical TECs subsets defined by different Ly51 levels of expression on the ontogeny.

β-Selection-induced proliferation is required for αβ T cell differentiation

Proliferation and differentiation are tightly coordinated to produce an appropriate number of differentiated cells and often exhibit an antagonistic relationship. Developing T cells, which arise in the thymus from a minute number of bone-marrow-derived progenitors, undergo a major expansion upon pre-T cell receptor (TCR) expression. The burst of proliferation coincides with differentiation toward the αβ T cell lineage-but the two processes were previously thought to be independent from one another, although both were driven by signaling from pre-TCR and Notch receptors. Here we report that proliferation at this step was not only absolutely required for differentiation but also that its ectopic activation was sufficient to substantially rescue differentiation in the absence of Notch signaling. Consistently, pharmacological inhibition of the cell cycle machinery also blocked differentiation in vivo. Thus the proliferation step is strictly required prior to differentiation of immature thymocytes.

Expression of Notch receptors and ligands on immature and mature T cells

Notch plays multiple roles in T cell development in the thymus and T cell differentiation in the periphery. In order to systematically examine the role of Notch in T cell biology, we determined the cell surface expression of all Notch receptors and ligands on various populations of T cells by using a panel of specific monoclonal antibodies we recently established. Notch1 and Notch3 were upregulated at double-negative (DN) 2-DN4 stages of immature thymocytes, then downregulated on mature single-positive thymocytes and peripheral T cells, but were rapidly upregulated again upon activation. Notch2 was consistently expressed on T cells while Notch4 was not. Jagged1 and Jagged2 were expressed at double-positive stage of immature T cells. Jagged2 was also inducible on mature T cells upon activation. In contrast, no Delta-like (Dll) 1 or Dll4 expression was observed on T cells. These comprehensive profiling of the expression of Notch receptors and ligands would be informative to fully understand the role of individual Notch receptors and ligands in T cell development and differentiation.