CD4-CD8 lineage commitment: an inside view

Signaling pathways in the regulation of cancer stem cells and associated targeted therapy

Cancer stem cells (CSCs) are defined as a subpopulation of malignant tumor cells with selective capacities for tumor initiation, self-renewal, metastasis, and unlimited growth into bulks, which are believed as a major cause of progressive tumor phenotypes, including recurrence, metastasis, and treatment failure. A number of signaling pathways are involved in the maintenance of stem cell properties and survival of CSCs, including well-established intrinsic pathways, such as the Notch, Wnt, and Hedgehog signaling, and extrinsic pathways, such as the vascular microenvironment and tumor-associated immune cells. There is also intricate crosstalk between these signal cascades and other oncogenic pathways. Thus, targeting pathway molecules that regulate CSCs provides a new option for the treatment of therapy-resistant or -refractory tumors. These treatments include small molecule inhibitors, monoclonal antibodies that target key signaling in CSCs, as well as CSC-directed immunotherapies that harness the immune systems to target CSCs. This review aims to provide an overview of the regulating networks and their immune interactions involved in CSC development. We also address the update on the development of CSC-directed therapeutics, with a special focus on those with application approval or under clinical evaluation.

Double-negative T cells: Setting the stage for disease control or progression

Double-negative (DN) T cells are present at relatively low frequencies in human peripheral blood, and are characterized as expressing the alpha-beta or gamma-delta T-cell receptor (TCR), but not the CD4 nor the CD8 co-receptors. Despite their low frequencies, these cells are potent producers of cytokines and, thus, are key orchestrators of immune responses. DN T cells were initially associated with induction of peripheral immunological tolerance and immunomodulatory activities related to disease prevention. However, other studies demonstrated that these cells can also display effector functions associated with pathology development. This apparent contradiction highlighted the heterogeneity of the DN T-cell population. Here, we review phenotypic and functional characteristics of DN T cells, emphasizing their role in human diseases. The need for developing biomarkers to facilitate the translation of studies from animal models to humans will also be discussed. Finally, we will examine DN T cells as promising therapeutic targets to prevent or inhibit human disease development.

Twist2 promotes CD8+ T-cell differentiation by repressing ThPOK expression

CD4/CD8 T-cell lineage differentiation is a key process in immune system development; however, a defined regulator(s) that converts the signal from T-cell receptor and co-receptor complexes into lineage differentiation remains unclear. Here, we show that Twist2 is a critical factor in CD4/CD8 thymocyte differentiation. Twist2 expression is differentially regulated by T-cell receptor signaling, leading to differentiation into the CD4 or CD8 lineage. Forced Twist2 expression perturbed CD4⁺ thymocyte differentiation while enhancing CD8⁺ thymocyte differentiation. Furthermore, Twist2 expression produced mature CD8⁺ thymocytes in B₂m^-/- mice, while its deficiency significantly impaired CD8⁺ cells in MHC class-II^-/- and TCR transgenic mice, favoring CD8 T-cell differentiation. During CD8 lineage differentiation, Twist2 interacted with Runx3 to bind to the silencer region of the ThPOK locus, thereby blocking ThPOK expression. These findings indicate that Twist2 is a part of the transcription factor network controlling CD8 lineage differentiation.

Role of the Orphan Nuclear Receptor NR4A Family in T-Cell Biology

The nuclear orphan receptors NR4A1, NR4A2, and NR4A3 are immediate early genes that are induced by various signals. They act as transcription factors and their activity is not regulated by ligand binding and are thus regulated via their expression levels. Their expression is transiently induced in T cells by triggering of the T cell receptor following antigen recognition during both thymic differentiation and peripheral T cell responses. In this review, we will discuss how NR4A family members impact different aspects of the life of a T cell from thymic differentiation to peripheral response against infections and cancer.

Targeting signalling pathways and the immune microenvironment of cancer stem cells - a clinical update

Cancer stem cells (CSCs) have important roles in tumour development, relapse and metastasis; the intrinsic self-renewal characteristics and tumorigenic properties of these cells provide them with unique capabilities to resist diverse forms of anticancer therapy, seed recurrent tumours, and disseminate to and colonize distant tissues. The findings of several studies indicate that CSCs originate from non-malignant stem or progenitor cells. Accordingly, inhibition of developmental signalling pathways that are crucial for stem and progenitor cell homeostasis and function, such as the Notch, WNT, Hedgehog and Hippo signalling cascades, continues to be pursued across multiple cancer types as a strategy for targeting the CSCs hypothesized to drive cancer progression - with some success in certain malignancies. In addition, with the renaissance of anticancer immunotherapy, a better understanding of the interplay between CSCs and the tumour immune microenvironment might be the key to unlocking a new era of oncological treatments associated with a reduced propensity for the development of resistance and with enhanced antimetastatic activity, thus ultimately resulting in improved patient outcomes. Herein, we provide an update on the progress to date in the clinical development of therapeutics targeting the Notch, WNT, Hedgehog and Hippo pathways. We also discuss the interactions between CSCs and the immune system, including the potential immunological effects of agents targeting CSC-associated developmental signalling pathways, and provide an overview of the emerging approaches to CSC-targeted immunotherapy.

Long noncoding RNA Malat1 is not essential for T cell development and response to LCMV infection

Long noncoding RNAs (lncRNAs) are emerging as critical mediators of various biological processes in the immune system. The current data showed that the lncRNA Malat1 is highly expressed in T cell subsets, but the function of Malat1 in T cell remains unclear. In this study, we detected the T cell development and both CD8⁺ and CD4⁺ T cell response to LCMV infection using Malat1^-/- mice model. To our surprise, there were no significant defects in thymocytes at different developmental stages and the peripheral T cell pool with ablation of Malat1. During LCMV infection, Malat1^-/- mice exhibited normal effector and memory CD8⁺ T cells as well as T_FH cells differentiation. Our results indicated that Malat1 is not essential for T cell development and T cell-mediated antiviral response though it expresses at very high level in different T cell populations.

Future Path Toward TB Vaccine Development: Boosting BCG or Re-educating by a New Subunit Vaccine

Tuberculosis (TB), an infectious disease caused by Mycobacterium tuberculosis (Mtb), kills 5,000 people per day globally. Rapid development and spread of various multi drug-resistant strains of Mtb emphasize that an effective vaccine is still the most cost-effectives and efficient way of controlling and eradicating TB. Bacillus Calmette-Guerin (BCG), the only licensed TB vaccine, still remains the most widely administered human vaccine, but is inefficient in protecting from pulmonary TB in adults. The protective immunity afforded by BCG is thought to wane with time and considered to last only through adolescent years. Heterologous boosting of BCG-primed immune responses using a subunit vaccine represents a promising vaccination approach to promote strong cellular responses against Mtb. In our earlier studies, we discovered lipopeptides of ESAT-6 antigen with strong potential as a subunit vaccine candidate. Here, we have investigated that potential as a booster to BCG vaccine in both a pre-exposure preventive vaccine and a post-exposure therapeutic vaccine setting. Surprisingly, our results demonstrated that boosting BCG with subunit vaccine shortly before Mtb challenge did not improve the BCG-primed immunity, whereas the subunit vaccine boost after Mtb challenge markedly improved the quantity and quality of effector T cell responses and significantly reduced Mtb load in lungs, liver and spleen in mice. These studies suggest that ESAT-6 lipopeptide-based subunit vaccine was ineffective in overcoming the apparent immunomodulation induced by BCG vaccine in Mtb uninfected mice, but upon infection, the subunit vaccine is effective in re-educating the protective immunity against Mtb infection. These important results have significant implications in the design and investigation of effective vaccine strategies and immunotherapeutic approaches for individuals who have been pre-immunized with BCG vaccine but still get infected with Mtb.

Effect of Metal-Containing Nanocomposites on Functional Status of the Thymus in Experimental Animals

The effects of nanostructurized composites argentogalactomannan and argento-poly-1-vinyl-1,2,4-triasole on the kinetics of T cell maturation in the thymus of albino mice were studied. These nanocomposites stimulated the formation of CD3⁺CD4⁺CD8^- and CD3⁺CD4^-CD8⁺ cells from CD3^-CD4^-CD8^- cells, the effect of argentogalactomannan was more potent. The efficiency of these compounds as agents improving the defense potential deserves special studies.

Lethal and nonlethal murine malarial infections differentially affect apoptosis, proliferation, and CD8 expression on thymic T cells

Although thymic atrophy and apoptosis of the double-positive (DP) T cells have been reported in murine malaria, comparative studies investigating the effect of lethal and nonlethal Plasmodium infections on the thymus are lacking. We assessed the effects of P. yoelii lethal (17XL) and nonlethal (17XNL) infections on thymic T cells. Both strains affected the thymus. 17XL infection induced DP T-cell apoptosis and a selective decrease in surface CD8 expression on developing thymocytes. By contrast, more severe but reversible effects were observed during 17XNL infection. DP T cells underwent apoptosis, and proliferation of both DN and DP cells was affected around peak parasitemia. A transient increase in surface CD8 expression on thymic T cells was also observed. Adult thymic organ culture revealed that soluble serum factors, but not IFN-γ or TNF-α, contributed to the observed effects. Thus, lethal and nonlethal malarial infections led to multiple disparate effects on thymus. These parasite-induced thymic changes are expected to impact the naïve T-cell repertoire and the subsequent control of the immune response against the parasite. Further investigations are required to elucidate the mechanism responsible for these disparate effects, especially the reversible involution of the thymus in case of nonlethal infection.

The nuclear receptor nr4a1 controls CD8 T cell development through transcriptional suppression of runx3

The NR4A nuclear receptor family member Nr4a1 is strongly induced in thymocytes undergoing selection, and has been shown to control the development of T_reg cells; however the role of Nr4a1 in CD8⁺ T cells remains undefined. Here we report a novel role for Nr4a1 in regulating the development and frequency of CD8⁺ T cells through direct transcriptional control of Runx3. We discovered that Nr4a1 recruits the corepressor, CoREST to suppress Runx3 expression in CD8⁺ T cells. Loss of Nr4a1 results in increased Runx3 expression in thymocytes which consequently causes a 2-fold increase in the frequency and total number of intrathymic and peripheral CD8⁺ T cells. Our findings establish Nr4a1 as a novel and critical player in the regulation of CD8 T cell development through the direct suppression of Runx3.

New insight into hedgehog signaling in hematological malignancies

The hedgehog (Hh) signaling pathway is well established as being evolutionarily conserved across vertebrates, and is involved in organogenesis, hematopoiesis, embryogenesis and homeostasis of adult tissues. At a microscopic level, the Hh signaling pathway controls the proliferation, apoptosis, cell-cycle and differentiation programs of stem and progenitor cells. Increasing evidence suggests that aberrant activation of the Hh signaling pathway is related to neoplasm, including solid tumors and hematologic malignancies. Currently the Hh signaling pathway has become one of the most studied potential therapeutic targets in hematological malignancies. In this review, we focus on findings related to Hh signaling in the initiation, maintenance, progression and chemoresistance of hematological malignancies, looking forward to better targeted treatment strategies.

A silencer-proximal intronic region is required for sustained CD4 expression in postselection thymocytes

It has been proposed that differential kinetics of CD4/CD8 coreceptors regulate fate choice of selected thymocytes. Sustained signals by interaction between MHC class II and TCR/CD4 is required for commitment to the CD4 helper lineage. Although prematurely terminated MHC-TCR/CD4 interaction in transgenic mouse models results in lineage redirection, it is unclear whether CD4 expression is actively maintained by endogenous cis-elements to facilitate prolonged signaling under physiological conditions. In this article, we show that sustained CD4 expression in postselection thymocytes requires an intronic sequence containing an uncharacterized DNase I hypersensitivity (DHS) site located 3' to the silencer. Despite normal CD4 expression before selection, thymocytes lacking a 1.5-kb sequence in intron 1 including the 0.4-kb silencer and the DHS, but not the 0.4-kb silencer alone, failed to maintain CD4 expression upon positive selection and are redirected to the CD8 lineage after MHC class II-restricted selection. Furthermore, CpG dinucleotides adjacent to the DHS are hypermethylated in CD8(+) T cells. These results indicate that the 1.5-kb cis-element is required in postselection thymocytes for helper lineage commitment, presumably mediating the maintenance of CD4 expression, and suggest that inactivation of the cis-element by DNA methylation may contribute to epigenetic Cd4 silencing.

cAMP-responsive element modulator α (CREMα) trans-represses the transmembrane glycoprotein CD8 and contributes to the generation of CD3+CD4-CD8- T cells in health and disease

T cell receptor-αβ(+) CD3(+)CD4(-)CD8(-) "double-negative" T cells are expanded in the peripheral blood of patients with systemic lupus erythematosus and autoimmune lymphoproliferative syndrome. In both disorders, double-negative T cells infiltrate tissues, induce immunoglobulin production, and secrete proinflammatory cytokines. Double-negative T cells derive from CD8(+) T cells through down-regulation of CD8 surface co-receptors. However, the molecular mechanisms orchestrating this process remain unclear. Here, we demonstrate that the transcription factor cAMP-responsive element modulator α (CREMα), which is expressed at increased levels in T cells from systemic lupus erythematosus patients, contributes to transcriptional silencing of CD8A and CD8B. We provide the first evidence that CREMα trans-represses a regulatory element 5' of the CD8B gene. Therefore, CREMα represents a promising candidate in the search for biomarkers and treatment options in diseases in which double-negative T cells contribute to the pathogenesis.

Canonical and noncanonical Hedgehog/GLI signaling in hematological malignancies

The highly conserved Hedgehog/GLI signaling pathway regulates multiple aspects of embryonic development and plays a decisive role in tissue homeostasis and the hematopoietic system by controlling cell fate decisions, stem cell self-renewal, and activation. Loss of negative control of Hedgehog signaling contributes to tumor pathogenesis and progression. In the classical view of canonical Hedgehog signaling, Hedgehog ligand binding to its receptor Patched culminates in the activation of the key pathway activator Smoothened, followed by activation of the GLI transcription factors. Its essential function and druggability render Smoothened well suited to therapeutic intervention. However, recent evidence suggests a critical role of Smoothened-independent regulation of GLI activity by several other signaling pathways including the PI3K/AKT and RAS/RAF/MEK/ERK axes. In addition, the contribution of canonical Hedgehog signaling via Patched and Smoothened to normal and malignant hematopoiesis has been the subject of recent controversies. In this review, we discuss the current understanding and controversial findings of canonical and noncanonical GLI activation in hematological malignancies in light of the current therapeutic strategies targeting the Hedgehog pathway.

CD3+CD4-CD8- (double negative) T cells: saviours or villains of the immune response?

Recent studies have shown that T cells are not just the latecomers in inflammation but might also play a key role in the early phase of this response. In this context, a number of T cell subsets including NKT cells, mucosal-associated invariant T cells and γ/δ T cells have been shown, together with classical innate immune cells, to contribute significantly to the development and establishment of acute and chronic inflammatory diseases. In this commentary we will focus our attention on a somewhat neglected class of T cells called CD3(+)CD4(-)CD8(-) double negative T cells and on their role in inflammation and autoimmunity. We will summarize the most recent views on their origin at the thymic and peripheral levels as well as their tissue localization in immune and non-lymphoid organs. We will then outline their potential pathogenic role in autoimmunity as well as their homeostatic role in suppressing excessive immune responses deleterious to the host. Finally, we will discuss the potential therapeutic benefits or disadvantages of targeting CD3(+)CD4(-)CD8(-) double negative T cells for the treatment of autoimmune disease. We hope that this overview will shed some light on the function of these immune cells and attract the interest of investigators aiming at the design of novel therapeutic approaches for the treatment of autoimmune and inflammatory conditions.

p300-mediated acetylation stabilizes the Th-inducing POK factor

The lineage-specifying factor Th-inducing POK (ThPOK) directs the intrathymic differentiation of CD4 T cells. Although the regulation of ThPOK at the transcription level has been extensively studied, specific posttranslational modifications regulating the activity of ThPOK have not been addressed. In this paper, we show that ThPOK is an unstable protein that is more readily degraded in CD8 T cells compared with CD4 T cells. Among the various proteins that bind ThPOK, acetyltransferase p300 specifically promotes the acetylation of ThPOK at K210, K216, and K339, outcompeting ubiquitination, thereby stabilizing the protein. In CD4 T cells, attenuation of p300-mediated acetylation promotes the degradation of ThPOK. In contrast, mutation of lysines 210, 216, and 339 to arginines stabilizes ThPOK and enhances its ability to suppress the expression of CD8 molecule and cytotoxic effectors in CD8 T cells. Our results reveal an essential role of p300-mediated acetylation in regulating the stability of ThPOK and suggest that such regulation may play a part in CD4/CD8 lineage differentiation.

Regulation of Zap70 expression during thymocyte development enables temporal separation of CD4 and CD8 repertoire selection at different signaling thresholds

To investigate the temporal regulation of the commitment of immature thymocytes to either the CD4(+) or the CD8(+) lineage in the thymus, we developed a transgenic mouse that expressed a tetracycline-inducible gene encoding the tyrosine kinase zeta chain-associated protein kinase of 70 kD (Zap70), which restored development in Zap70(-/-) thymocytes arrested at the preselection, CD4(+)CD8(+) double-positive (DP) stage. After induction of the expression of Zap70 and the production of Zap70 protein, CD4(+) single-positive (SP) cells that expressed Zbtb7b (which encodes the CD4(+) T cell-associated transcription factor ThPOK) became abundant within 30 hours, whereas CD8(+) SP cells were not detectable until day 4. We found that mature CD4(+) and CD8(+) cells arose from phenotypically distinct subsets of DP thymocytes that developed with different kinetics and contrasting sensitivities to stimulation of the T cell antigen receptor (TCR). In wild-type mice, expression of endogenous Zap70 progressively increased during maturation of the DP subsets, and the abundance of Zap70 protein determined the sensitivity of the cells to stimulation of the TCR. This temporal gradient in the amount of Zap70 protein enabled the selection of CD4(+) and CD8(+) repertoires in separate temporal windows and at different TCR signaling thresholds, thereby facilitating discrimination of distinct positive selection signals in these lineages.

A conserved CXXC motif in CD3epsilon is critical for T cell development and TCR signaling

Structural integrity of the extracellular membrane-proximal stalk region of CD3ε is required for efficient signaling by the T cell antigen receptor complex. The results in this article suggest that receptor aggregation may not be sufficient for a complete T cell receptor signal and that some type of direct allosteric signal may be involved.

Virtually all T cell development and functions depend on its antigen receptor. The T cell receptor (TCR) is a multi-protein complex, comprised of a ligand binding module and a signal transmission module. The signal transmission module includes proteins from CD3 family (CD3ε, CD3δ, CD3γ) as well as the ζ chain protein. The CD3 proteins have a short extracellular stalk connecting their Ig-like domains to their transmembrane regions. These stalks contain a highly evolutionarily conserved CXXC motif, whose function is unknown. To understand the function of these two conserved cysteines, we generated mice that lacked endogenous CD3ε but expressed a transgenic CD3ε molecule in which these cysteines were mutated to serines. Our results show that the mutated CD3ε could incorporate into the TCR complex and rescue surface TCR expression in CD3ε null mice. In the CD3ε mutant mice, all stages of T cell development and activation that are TCR-dependent were impaired, but not eliminated, including activation of mature naïve T cells with the MHCII presented superantigen, staphylococcal enterotoxin B, or with a strong TCR cross-linking antibody specific for either TCR-Cβ or CD3ε. These results argue against a simple aggregation model for TCR signaling and suggest that the stalks of the CD3 proteins may be critical in transmitting part of the activation signal directly through the membrane.

The T cells of the immune system have surface receptors that detect unique features (called antigens) of foreign invaders such as viruses, bacteria and toxins. An encounter between an antigen and the T cell receptor sets off a chain of events that activates the T cell to proliferate and thus call to action the various arms of the immune response that ultimately eliminate the invader. A set of proteins, called CD3, associates with the T cell receptor, spanning the cell membrane. Their function is to deliver a signal to the inside of T cell that its receptor has encountered antigen on the outside of the cell. Two general ideas have been proposed to explain how the CD3 proteins accomplish this: That the engagement of the T cell receptor outside the cell directly causes a change in conformation in the intracellular portion of the associated CD3 proteins that is recognized by the intracellular signaling machinery; and that engagement of the T cell receptor causes clustering of multiple receptor and CD3 proteins such that interactions among the cytoplasmic portions of the many CD3 proteins now attract other proteins to start the chain of intercellular signaling. These two ideas are not mutually exclusive. We show here that mutations in a highly conserved extracellular portion of one of the CD3 proteins can impair the transmission of the activation signal without preventing receptor clustering. These results suggest that direct transmission of a conformational change across the membrane may constitute part of the CD3-mediated activation signal.

TCR down-regulation controls T cell homeostasis

TCR and cytokine receptor signaling play key roles in the complex homeostatic mechanisms that maintain a relative stable number of T cells throughout life. Despite the homeostatic mechanisms, a slow decline in naive T cells is typically observed with age. The CD3gamma di-leucine-based motif controls TCR down-regulation and plays a central role in fine-tuning TCR expression and signaling in T cells. In this study, we show that the age-associated decline of naive T cells is strongly accelerated in CD3gammaLLAA knock-in mice homozygous for a double leucine to alanine mutation in the CD3gamma di-leucine-based motif, whereas the number of memory T cells is unaffected by the mutation. This results in premature T cell population senescence with a severe dominance of memory T cells and very few naive T cells in middle-aged to old CD3gamma mutant mice. The reduced number of naive T cells in CD3gamma mutant mice was caused by the combination of reduced thymic output, decreased T cell apoptosis, and increased transition of naive T cells to memory T cells. Experiments with bone marrow chimeric mice confirmed that the CD3gammaLLAA mutation exerted a T cell intrinsic effect on T cell homeostasis that resulted in an increased transition of CD3gammaLLAA naive T cells to memory T cells and a survival advantage of CD3gammaLLAA T cells compared with wild-type T cells. The experimental observations were further supported by mathematical modeling of T cell homeostasis. Our study thus identifies an important role of CD3gamma-mediated TCR down-regulation in T cell homeostasis.

Gene expression profiling in mice with enforced Gata3 expression reveals putative targets of Gata3 in double positive thymocytes

The zinc-finger transcription factors Gata3 and ThPOK have both been implicated in positive selection of double positive (DP) thymocytes towards the CD4 lineage. As in the absence of Gata3, expression of ThPOK is lacking, Gata3 may directly regulate ThPOK expression. As ThPOK failed to promote CD4(+) lineage differentiation of Gata3-deficient cells, ThPOK cannot be the only Gata3 target gene essential for the induction of the CD4(+) lineage program. Therefore, it is conceivable that Gata3 is essential for selected DP T cells to reach the developmental stage at which ThPOK expression is induced. Here, we show that Gata3 overexpression does not affect ThPOK expression levels in DP or CD4(+) thymocytes, providing evidence that Gata3 does not directly regulate ThPOK. To identify additional target genes that clarify Gata3 function at the DP thymocyte stage, we performed gene expression profiling assays in wild-type mice and transgenice mice with enforced expression of Gata3, in the presence or absence of the MHC class II-restricted DO11.10 TCR. We found that Gata3 expression in DP cells undergoing positive selection was associated with downregulation of the V(D)J-recombination machinery genes Rag1, Rag2 and TdT. Moreover, Gata3 overexpression was associated with downregulation of many signaling molecules and the induction of modulators of TCR signaling, including Ctla-4 and thrombospondin 2. Together with our previous finding that Gata3 reduces expression of CD5, a negative regulator of TCR signaling, and upregulates TCR expression, these findings indicate that Gata3 in DP cells mainly functions to (i) terminate TCRalpha gene rearrangement, and (ii) regulate TCR signal intensity or duration in cells undergoing positive selection towards the CD4 lineage.

Themis is a member of a new metazoan gene family and is required for the completion of thymocyte positive selection

T cell antigen receptor (TCR) signaling in CD4(+)CD8(+) double-positive thymocytes determines cell survival and lineage commitment, but the genetic and molecular basis of this process is poorly defined. To address this issue, we used ethylnitrosourea mutagenesis to identify a previously unknown T lineage-specific gene, Themis, which is critical for the completion of positive selection. Themis contains a tandem repeat of a unique globular domain (called 'CABIT' here) that includes a cysteine motif that defines a family of five uncharacterized vertebrate proteins with orthologs in most animal species. Themis-deficient thymocytes showed no substantial impairment in early TCR signaling but did show altered expression of genes involved in the cell cycle and survival before and during positive selection. Our data suggest a unique function for Themis in sustaining positive selection.

Histone acetyltransferase CBP is vital to demarcate conventional and innate CD8+ T-cell development

Defining the chromatin modifications and transcriptional mechanisms that direct the development of different T-cell lineages is a major challenge in immunology. The transcriptional coactivators CREB binding protein (CBP) and the closely related p300, which comprise the KAT3 family of histone/protein lysine acetyltransferases, interact with over 50 T-lymphocyte-essential transcriptional regulators. We show here that CBP, but not p300, modulates the thymic development of conventional adaptive T cells versus those having unconventional innate functions. Conditional inactivation of CBP in the thymus yielded CD8 single-positive (SP) thymocytes with an effector-, memory-, or innate-like T-cell phenotype. In this regard, CD8 SP thymocytes in CBP mutant mice were phenotypically similar to those reported for Itk and Rlk protein tyrosine kinase mutants, including the increased expression of the T-cell master regulatory transcription factor eomesodermin (Eomes) and the interleukin-2 and -15 receptor beta chain (CD122) and an enhanced ability to rapidly produce gamma interferon. CBP was required for the expression of the Itk-dependent genes Egr2, Egr3, and Il2, suggesting that CBP helps mediate Itk-responsive transcription. CBP therefore defines a nuclear component of the signaling pathways that demarcate the development of innate and adaptive naïve CD8(+) T cells in the thymus.

Targeting CD4 coreceptor expression to postselection thymocytes reveals that CD4/CD8 lineage choice is neither error-prone nor stochastic

The mechanism by which CD4/CD8 lineage choice is coordinated with TCR specificity during positive selection remains an unresolved problem in immunology. The stochastic/selection model proposes that CD4/CD8 lineage choice in TCR-signaled CD4(+)CD8(+) thymocytes occurs randomly and therefore is highly error-prone. This perspective is strongly supported by "coreceptor rescue" experiments in which transgenic CD4 coreceptors were ectopically expressed on thymocytes throughout their development and caused significant numbers of cells bearing MHC-II-specific TCR to differentiate into mature, CD8 lineage T cells. However, it is not known if forced coreceptor expression actually rescued positively selected thymocytes making an incorrect lineage choice or if it influenced developing thymocytes into making an incorrect lineage choice. We have now reassessed coreceptor rescue and the concept that lineage choice is highly error-prone with a novel CD4 transgene (referred to as E8(I)-CD4) that targets expression of transgenic CD4 coreceptors specifically to thymocytes that have already undergone positive selection and adopted a CD8 lineage fate. Unlike previous CD4 transgenes, the E8(I)-CD4 transgene has no effect on early thymocyte development and cannot itself influence CD4/CD8 lineage choice. We report that the E8(I)-CD4 transgene did in fact induce expression of functional CD4 coreceptor proteins on newly arising CD8 lineage thymocytes precisely at the point in thymic development that transgenic CD4 coreceptors would putatively rescue MHC-II-specific thymocytes that incorrectly adopted the CD8 lineage. However, the E8(I)-CD4 transgene did not reveal any MHC-II-selected thymocytes that adopted the CD8 lineage fate. These results demonstrate that CD4/CD8 lineage choice is neither error-prone nor stochastic.

Lineage fate and intense debate: myths, models and mechanisms of CD4- versus CD8-lineage choice

Following successful gene rearrangement at alphabeta T-cell receptor (TCR) loci, developing thymocytes express both CD4 and CD8 co-receptors and undergo a life-or-death selection event, which is known as positive selection, to identify cells that express TCRs with potentially useful ligand specificities. Positively selected thymocytes must then differentiate into either CD4(+) helper T cells or CD8(+) cytotoxic T cells, a crucial decision known as CD4/CD8-lineage choice. In this Review, we summarize recent advances in our understanding of the cellular and molecular events involved in lineage-fate decision and discuss them in the context of the major models of CD4/CD8-lineage choice.

Coronin-1A links cytoskeleton dynamics to TCR alpha beta-induced cell signaling

Actin polymerization plays a critical role in activated T lymphocytes both in regulating T cell receptor (TCR)-induced immunological synapse (IS) formation and signaling. Using gene targeting, we demonstrate that the hematopoietic specific, actin- and Arp2/3 complex-binding protein coronin-1A contributes to both processes. Coronin-1A-deficient mice specifically showed alterations in terminal development and the survival of αβT cells, together with defects in cell activation and cytokine production following TCR triggering. The mutant T cells further displayed excessive accumulation yet reduced dynamics of F-actin and the WASP-Arp2/3 machinery at the IS, correlating with extended cell-cell contact. Cell signaling was also affected with the basal activation of the stress kinases sAPK/JNK1/2; and deficits in TCR-induced Ca²⁺ influx and phosphorylation and degradation of the inhibitor of NF-κB (IκB). Coronin-1A therefore links cytoskeleton plasticity with the functioning of discrete TCR signaling components. This function may be required to adjust TCR responses to selecting ligands accounting in part for the homeostasis defect that impacts αβT cells in coronin-1A deficient mice, with the exclusion of other lympho/hematopoietic lineages.

ThPOK acts late in specification of the helper T cell lineage and suppresses Runx-mediated commitment to the cytotoxic T cell lineage

The transcription factor ThPOK has been shown to be required and sufficient for CD4⁺CD8⁻ thymocyte generation, yet the mechanism through which ThPOK orchestrates CD4 helper T cell lineage differentiation remains unclear. Here we utilized reporter mice to track expression of transcription factors in developing thymocytes. Distal promoter-driven Runx3 (Runx3d) expression was restricted to MHC class I-selected thymocytes. In ThPOK-deficient mice, Runx3d expression was de-repressed in MHCII-selected thymocytes, contributing to their redirection to the CD8 T cell lineage. In the absence of both ThPOK and Runx, redirection was prevented and cells potentially belonging to the CD4 lineage, presumably specified independently of ThPOK, were generated. Our results suggest that MHCII-selected thymocytes are directed towards the CD4 lineage independently of ThPOK, but require ThPOK to prevent Runx-dependent differentiation towards the CD8 lineage.

Cooperation of Gata3, c-Myc and Notch in malignant transformation of double positive thymocytes

Gata transcription factors are critical regulators of proliferation and differentiation implicated in various human cancers, but specific genes activated by Gata proteins remain to be identified. We previously reported that enforced expression of Gata3 during T cell development in CD2-Gata3 transgenic mice induced CD4(+)CD8(+) double-positive (DP) T cell lymphoma. Here, we show that the presence of the DO11.10 T-cell receptor transgene, which directs DP cells towards the CD4 lineage, resulted in enhanced lymphoma development and a dramatic increase in thymocyte cell size in CD2-Gata3 transgenic mice. CD2-Gata3 DP cells expressed high levels of the proto-oncogene c-Myc but the Notch1 signaling pathway, which is known to induce c-Myc, was not activated. Gene expression profiling showed that in CD2-Gata3 lymphoma cells transcription of c-Myc and its target genes was further increased. A substantial fraction of CD2-Gata3 lymphomas had trisomy of chromosome 15, leading to an increased c-Myc gene dose. Interestingly, most lymphomas showed high expression of the Notch targets Deltex1 and Hes1, often due to activating Notch1 PEST domain mutations. Therefore, we conclude that enforced Gata3 expression converts DP thymocytes into a pre-malignant state, characterized by high c-Myc expression, whereby subsequent induction of Notch1 signaling cooperates to establish malignant transformation. The finding that Gata3 regulates c-Myc expression levels, in a direct or indirect fashion, may explain the parallel phenotypes of mice with overexpression or deficiency of either of the two transcription factors.

Sonic hedgehog signalling in T-cell development and activation

The production of mature functional T cells in the thymus requires signals from the thymic epithelium. Here, we review recent experiments showing that one way in which the epithelium controls the production of mature T cells is by the secretion of sonic hedgehog (SHH). We consider the increasing evidence that SHH-induced signalling is not only important for the differentiation and proliferation of early thymocyte progenitors, but also for modulating T-cell receptor signalling during repertoire selection, with implications for positive selection, CD4 versus CD8 lineage commitment, and clonal deletion of autoreactive cells. We also review the influence of hedgehog signalling in peripheral T-cell activation.

c-Myb regulates lineage choice in developing thymocytes via its target gene Gata3

During T-cell development, thymocytes with intermediate avidity for antigen-MHC complexes are positively selected and then differentiate into functional cytotoxic and helper T cells. This process is controlled by signalling from the T-cell receptor (TCR). Here, we show that the c-Myb transcription factor is a critical downstream regulator of positive selection, promoting the development of helper T cells and blocking the development of cytotoxic T cells. A gain-of-function c-Myb transgene stops development of cytotoxic T cells, instead causing accumulation of a precursor population. Conversely, loss of c-Myb in selecting cells results in significantly fewer helper T cells. In c-Myb-null thymocytes, Gata3, a critical inducer of T-helper cell fate, is not upregulated in response to T-cell receptor signaling, following selection. We show that Gata3 is a direct target of c-Myb, and propose that c-Myb is an important regulator of Gata3, required for transduction of the T-cell receptor signal for subsequent helper cell lineage differentiation.

GATA3 controls the expression of CD5 and the T cell receptor during CD4 T cell lineage development

The transcription factor GATA3 is essential at multiple stages of T cell development, including the earliest double-negative stages, beta-selection and CD4 single-positive thymocytes. Here, we show that in CD2-GATA3 transgenic mice, with enforced GATA3 expression driven by the CD2 promoter, thymocytes have reduced levels of CD5, which is a negative regulator of TCR signaling participating in TCR repertoire fine-tuning. Reduction of CD5 expression was most prominent in CD4(+)CD8(+) double-positive (DP) cells and was associated with increased levels of the transcription factor E2A. Conversely, GATA3-deficient DP thymocytes showed consistently higher CD5 levels and defective TCR up-regulation during their development towards the CD4(lo)CD8(lo) subpopulation. CD2-GATA3 transgenic mice carrying the MHC class II-restricted TCR DO11.10 also manifested decreased CD5 levels. As in these TCR-transgenic mice reduced CD5 expression cannot result from an effect of GATA3 on repertoire selection, we conclude that enforced GATA3 interferes with the developmentally regulated increase of CD5 levels. Enforced GATA3 expression in DO11.10 transgenic mice was also accompanied by enhanced TCR expression during CD4 positive selection. Because GATA3 is induced by TCR signaling in DP thymocytes, our findings indicate that GATA3 establishes a positive feedback loop that increases TCR surface expression in developing CD4 lineage cells.

Decision‐making by the immune response

Decisions by uncommitted cells to differentiate down one lineage pathway or another is fundamental to developmental biology. In the immune system, lymphocyte precursors commit to T- or B-cell lineages and T-cell precursors to CD4 or CD8 independently of foreign antigen. T and B cells must also decide whether or not to respond to antigen and when a response is initiated, what sort of response to make such as the type of antibody, CD4 or CD8, and CD4 Th1 or Th2. The two basic mechanisms for these decision-making processes are selection and instruction. Selection depends on prior stochastic production of precommitted cells, which are then selected to respond by an appropriate signal; for example, CD8 and CD4 responses selected by peptide presented in association with major histocompatibility complex class I or II. In contrast, instruction occurs when an uncommitted precursor embarks upon a differentiation pathway in response to a particular set of signals; for example, Th1 and Th2 lineage commitment. In this paper, the signals that determine Th1 and Th2 differentiation are examined with a mathematical model and shown to act as a bistable switch permitting either Tbet or Gata3 to be expressed in an individual cell but not both. The model is used to show how the Tbet Gata3 network within an individual cell interacts with cytokine signals between cells and suggests how Th1 and Th2 lineage commitment can become irreversible. These considerations provide an example of how mathematical models can be used to gain a better understanding of lymphocyte differentiation in an immune response.

Impairment of organ-specific T cell negative selection by diabetes susceptibility genes: genomic analysis by mRNA profiling

BACKGROUND

T cells in the thymus undergo opposing positive and negative selection processes so that the only T cells entering circulation are those bearing a T cell receptor (TCR) with a low affinity for self. The mechanism differentiating negative from positive selection is poorly understood, despite the fact that inherited defects in negative selection underlie organ-specific autoimmune disease in AIRE-deficient people and the non-obese diabetic (NOD) mouse strain

RESULTS

Here we use homogeneous populations of T cells undergoing either positive or negative selection in vivo together with genome-wide transcription profiling on microarrays to identify the gene expression differences underlying negative selection to an Aire-dependent organ-specific antigen, including the upregulation of a genomic cluster in the cytogenetic band 2F. Analysis of defective negative selection in the autoimmune-prone NOD strain demonstrates a global impairment in the induction of the negative selection response gene set, but little difference in positive selection response genes. Combining expression differences with genetic linkage data, we identify differentially expressed candidate genes, including Bim, Bnip3, Smox, Pdrg1, Id1, Pdcd1, Ly6c, Pdia3, Trim30 and Trim12.

CONCLUSION

The data provide a molecular map of the negative selection response in vivo and, by analysis of deviations from this pathway in the autoimmune susceptible NOD strain, suggest that susceptibility arises from small expression differences in genes acting at multiple points in the pathway between the TCR and cell death.

Activation of the Hedgehog signaling pathway in T-lineage cells inhibits TCR repertoire selection in the thymus and peripheral T-cell activation

TCR signal strength is involved in many cell fate decisions in the T-cell lineage. Here, we show that transcriptional events induced by Hedgehog (Hh) signaling reduced TCR signal strength in mice. Activation of Hh signaling in thymocytes in vivo by expression of a transgenic transcriptional-activator form of Gli2 (Gli2DeltaN(2)) changed the outcome of TCR ligation at many stages of thymocyte development, allowing self-reactive cells to escape clonal deletion; reducing transgenic TCR-mediated positive selection; reducing the ratio of CD4/CD8 single-positive (SP) cells; and reducing cell surface CD5 expression. In contrast, in the Shh(-/-) thymus the ratio of CD4/CD8 cells and both positive and negative selection of a transgenic TCR were increased, demonstrating that Shh does indeed influence TCR repertoire selection and the transition from double-positive (DP) to SP cell in a physiological situation. In peripheral T cells, Gli2DeltaN(2) expression attenuated T-cell activation and proliferation, by a mechanism upstream of ERK phosphorylation.

The Tec family tyrosine kinases Itk and Rlk regulate the development of conventional CD8+ T cells

The Tec family tyrosine kinases, Itk and Rlk, are expressed in thymocytes and peripheral T cells and regulate thresholds of T cell receptor signaling. Yet little is known about the specific role of Itk- and Rlk-dependent signals in CD8(+) T cell maturation. We show here that Itk(-/-) and Rlk(-/-)Itk(-/-) mice were nearly devoid of conventional CD8(+) T cells and, instead, contained a large population of CD8(+) T cells that bear striking similarity to lineages of innate lymphocytes. Itk(-/-) and Rlk(-/-)Itk(-/-) CD8(+) thymocytes and T cells were CD44(hi), CD122(+), and NK1.1(+); were able to produce interferon-gamma directly ex vivo; and were dependent on interleukin-15. Itk(-/-) and Rlk(-/-)Itk(-/-) CD8(+) thymocytes expressed abundant transcripts for the T box transcription factor, eomesodermin, correlating with their phenotype and function. These data indicate a critical role for Itk and Rlk in conventional CD8(+) T cell development in the thymus.