Modulation of Coreceptor Transcription during Positive Selection Dictates Lineage Fate Independently of TCR/Coreceptor Specificity

How autoreactive thymocytes differentiate into regulatory versus effector CD4+ T cells after avoiding clonal deletion

Thymocytes bearing autoreactive T cell receptors (TCRs) are agonist-signaled by TCR/co-stimulatory molecules to either undergo clonal deletion or to differentiate into specialized regulatory T (T_reg) or effector T (T_eff) CD4⁺ cells. How these different fates are achieved during development remains poorly understood. We now document that deletion and differentiation are agonist-signaled at different times during thymic selection and that T_reg and T_eff cells both arise after clonal deletion as alternative lineage fates of agonist-signaled CD4⁺CD25⁺ precursors. Disruption of agonist signaling induces CD4⁺CD25⁺ precursors to initiate Foxp3 expression and become T_reg cells, whereas persistent agonist signaling induces CD4⁺CD25⁺ precursors to become IL-2⁺ T_eff cells. Notably, we discovered that transforming growth factor-β induces Foxp3 expression and promotes T_reg cell development by disrupting weaker agonist signals and that Foxp3 expression is not induced by IL-2 except under non-physiological in vivo conditions. Thus, TCR signaling disruption versus persistence is a general mechanism of lineage fate determination in the thymus that directs development of agonist-signaled autoreactive thymocytes.

Reversal of the T cell immune system reveals the molecular basis for T cell lineage fate determination in the thymus

T cell specificity and function are linked during development, as MHC-II-specific TCR signals generate CD4 helper T cells and MHC-I-specific TCR signals generate CD8 cytotoxic T cells, but the basis remains uncertain. We now report that switching coreceptor proteins encoded by Cd4 and Cd8 gene loci functionally reverses the T cell immune system, generating CD4 cytotoxic and CD8 helper T cells. Such functional reversal reveals that coreceptor proteins promote the helper-lineage fate when encoded by Cd4, but promote the cytotoxic-lineage fate when encoded in Cd8—regardless of the coreceptor proteins each locus encodes. Thus, T cell lineage fate is determined by cis-regulatory elements in coreceptor gene loci and is not determined by the coreceptor proteins they encode, invalidating coreceptor signal strength as the basis of lineage fate determination. Moreover, we consider that evolution selected the particular coreceptor proteins that Cd4 and Cd8 gene loci encode to avoid generating functionally reversed T cells because they fail to promote protective immunity against environmental pathogens.

To determine how T cell lineage fates are determined in the thymus, Singer and colleagues generated ‘FlipFlop’ mice with a functionally reversed T cell immune system that distinguishes TCR signal strength versus TCR signal duration.

Constitutive CD8 expression drives innate CD8+ T-cell differentiation via induction of iNKT2 cells

Temporal down-regulation of the CD8 co-receptor after receiving positive-selection signals has been proposed to serve as an important determinant to segregate helper versus cytotoxic lineages by generating differences in the duration of TCR signaling between MHC-I and MHC-II selected thymocytes. By contrast, little is known about whether CD8 also modulates TCR signaling engaged by the non-classical MHC-I-like molecule, CD1d, during development of invariant natural killer T (iNKT) cells. Here, we show that constitutive transgenic CD8 expression resulted in enhanced differentiation of innate memory-like CD8⁺ thymocytes in both a cell-intrinsic and cell-extrinsic manner, the latter being accomplished by an increase in the IL-4-producing iNKT2 subset. Skewed iNKT2 differentiation requires cysteine residues in the intracellular domain of CD8α that are essential for transmitting cellular signaling. Collectively, these findings shed a new light on the relevance of CD8 down-regulation in shaping the balance of iNKT-cell subsets by modulating TCR signaling.

CD4 Helper and CD8 Cytotoxic T Cell Differentiation

A fundamental question in developmental immunology is how bipotential thymocyte precursors generate both CD4⁺ helper and CD8⁺ cytotoxic T cell lineages. The MHC specificity of αβ T cell receptors (TCRs) on precursors is closely correlated with cell fate-determining processes, prompting studies to characterize how variations in TCR signaling are linked with genetic programs establishing lineage-specific gene expression signatures, such as exclusive CD4 or CD8 expression. The key transcription factors ThPOK and Runx3 have been identified as mediating development of helper and cytotoxic T cell lineages, respectively. Together with increasing knowledge of epigenetic regulators, these findings have advanced our understanding of the transcription factor network regulating the CD4/CD8 dichotomy. It has also become apparent that CD4⁺ T cells retain developmental plasticity, allowing them to acquire cytotoxic activity in the periphery. Despite such advances, further studies are necessary to identify the molecular links between TCR signaling and the nuclear machinery regulating expression of ThPOK and Runx3.

Requirement for intron structures in activating the Cd8a locus

During differentiation of CD4⁺CD8⁺ double-positive (DP) thymocytes into the CD4^-CD8⁺ single-positive (CD8SP) thymocytes committed to the cytotoxic T cell lineage, Cd8a transcription is temporally terminated after positive selection and is subsequently reinitiated, a process known as coreceptor reversal. Despite the identification of a transcriptional enhancer in the Cd8a gene that directs reporter transgene expression specifically in CD8SP thymocytes, the molecular mechanisms controlling reactivation of the Cd8a gene are not fully understood. Here, we show that, after positive selection, hCD2 reporter expression from the Cd8a locus, which was generated by insertion of hCD2 cDNA into the first exon of the Cd8a gene, requires the incorporation of intron sequences into the hCD2 transcript. The presence of polyadenylation signals after hCD2 cDNA inhibited hCD2 expression in mature CD8⁺ T cells, whereas hCD2 expression in DP thymocytes recapitulated the Cd8a expression. Incorporation of the endogenous short intron structure and heterologous intron structure of the Cd4 locus restored hCD2 expression in mature CD8⁺ T cells in a variegated manner. Interestingly, stage-specific DNA demethylation was impaired in Cd8a reporter alleles that failed to express hCD2 in CD8⁺ T cells, and intron sequences lacking RNA splicing signals still restored hCD2 expression. These observations indicate that "intron-mediated enhancement" is involved in a stage-specific reactivation of the Cd8a locus harboring hCD2 cDNA. However, the Cd8a gene was transcribed in mature CD8⁺ T cells, albeit at a lower level, from a mutant Cd8a locus lacking intron structures, suggesting that protein-coding sequences in transcripts affect sensitivity to intron-mediated enhancement.

Views on helper/cytotoxic lineage choice from a bottom-up approach

There has been speculation as to how bi-potent CD4(+)  CD8(+) double-positive precursor thymocytes choose their distinct developmental fate, becoming either CD4(+) helper or CD8(+) cytotoxic T cells. Based on the clear correlation of αβT cell receptor (TCR) specificity to major histocompatibility complex (MHC) classes with this lineage choice, various studies have attempted to resolve this question by examining the cellular signaling events initiated by TCR engagements, a strategy referred to as a 'top-down' approach. On the other hand, based on the other correlation of CD4/CD8 co-receptor expression with its selected fate, other studies have addressed this question by gradually unraveling the sequential mechanisms that control the phenotypic outcome of this fate decision, a method known as the 'bottom-up' approach. Bridging these two approaches will contribute to a more comprehensive understanding of how TCR signals are coupled with developmental programs in the nucleus. Advances made during the last two decades seemed to make these two approaches more closely linked. For instance, identification of two transcription factors, ThPOK and Runx3, which play central roles in the development of helper and cytotoxic lineages, respectively, provided significant insights into the transcriptional network that controls a CD4/CD8 lineage choice. This review summarizes achievements made using the 'bottom-up' approach, followed by a perspective on future pathways toward coupling TCR signaling with nuclear programs.

Expression patterns of Ikaros family members during positive selection and lineage commitment of human thymocytes

The Ikaros family of transcription factors is essential for normal T-cell development, but their expression pattern in human thymocytes remains poorly defined. Our goal is to determine how protein levels of Ikaros, Helios and Aiolos change as human thymocytes progress through the positive selection and lineage commitment stages. To accomplish this goal, we used multi-parameter flow cytometry to define the populations in which positive selection and lineage commitment are most likely to occur. After human thymocytes express CD3 and receive positive selection signals, the cells down-regulate expression of CD4 to become transitional single-positive (TSP) CD8⁺ thymocytes. At this stage, there was a transient increase in the Ikaros, Helios and Aiolos protein levels. After the TSP CD8⁺ developmental stage, some thymocytes re-express CD4 and become CD3^hi double-positive thymocytes before down-regulating CD8 to become mature single-positive CD4⁺ thymocytes. Except for regulatory T cells, Helios protein levels declined and Aiolos protein levels transiently increased during CD4⁺ T-cell maturation. For thymocytes progressing toward the CD8⁺ T-cell lineage, TSP CD8⁺ thymocytes increase their expression of CD3 and maintain high levels of Aiolos protein as the cells complete their maturation. In summary, we defined the TSP CD8⁺ developmental stage in human T-cell development and propose that this stage is where CD4/CD8 lineage commitment occurs. Ikaros, Helios and Aiolos each undergo a transient increase in protein levels at the TSP stage before diverging in their expression patterns at later stages.

Regulation of CD4 and CD8 coreceptor expression and CD4 versus CD8 lineage decisions

During blood cell development, hematopoietic stem cells generate diverse mature populations via several rounds of binary fate decisions. At each bifurcation, precursors adopt one fate and inactivate the alternative fate either stochastically or in response to extrinsic stimuli and stably maintain the selected fates. Studying of these processes would contribute to better understanding of etiology of immunodeficiency and leukemia, which are caused by abnormal gene regulation during the development of hematopoietic cells. The CD4(+) helper versus CD8(+) cytotoxic T-cell fate decision serves as an excellent model to study binary fate decision processes. These two cell types are derived from common precursors in the thymus. Positive selection of their TCRs by self-peptide presented on either MHC class I or class II triggers their fate decisions along with mutually exclusive retention and silencing of two coreceptors, CD4 and CD8. In the past few decades, extensive effort has been made to understand the T-cell fate decision processes by studying regulation of genes encoding the coreceptors and selection processes. These studies have identified several key transcription factors and gene regulatory networks. In this chapter, I will discuss recent advances in our understanding of the binary cell fate decision processes of T cells.

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.

Foxp3 transcription factor is proapoptotic and lethal to developing regulatory T cells unless counterbalanced by cytokine survival signals

Immune tolerance requires regulatory T (Treg) cells to prevent autoimmune disease, with the transcription factor Foxp3 functioning as the critical regulator of Treg cell development and function. We report here that Foxp3 was lethal to developing Treg cells in the thymus because it induced a unique proapoptotic protein signature (Puma⁺⁺⁺p-Bim⁺⁺p-JNK⁺⁺DUSP6⁻) and repressed expression of prosurvival Bcl-2 molecules. However, Foxp3 lethality was prevented by common gamma chain (γc)-dependent cytokine signals that were present in the thymus in limiting amounts sufficient to support only ∼1 million Treg cells. Consequently, most newly arising Treg cells in the thymus were deprived of this signal and underwent Foxp3-induced death, with Foxp3⁺CD25⁻ Treg precursor cells being the most susceptible. Thus, we identify Foxp3 as a proapoptotic protein that requires developing Treg cells to compete with one another for limiting amounts of γc-dependent survival signals in the thymus.

Epigenetic Thpok silencing limits the time window to choose CD4(+) helper-lineage fate in the thymus

CD4(+) helper and CD8(+) cytotoxic T cells differentiate from common precursors in the thymus after T-cell receptor (TCR)-mediated selection. Commitment to the helper lineage depends on persistent TCR signals and expression of the ThPOK transcription factor, whereas a ThPOK cis-regulatory element, ThPOK silencer, represses Thpok gene expression during commitment to the cytotoxic lineage. Here, we show that silencer-mediated alterations of chromatin structures in cytotoxic-lineage thymocytes establish a repressive state that is epigenetically inherited in peripheral CD8(+) T cells even after removal of the silencer. When silencer activity is enhanced in helper-lineage cells, by increasing its copy number, a similar heritable Thpok silencing occurs. Epigenetic locking of the Thpok locus may therefore be an independent event from commitment to the cytotoxic lineage. These findings imply that long-lasting TCR signals are needed to establish stable Thpok expression activity to commit to helper T-cell fate and that full commitment to the helper lineage requires persistent reversal of silencer activity during a particular time window.

IL-7 signaling must be intermittent, not continuous, during CD8⁺ T cell homeostasis to promote cell survival instead of cell death

The maintenance of naive CD8(+) T cells is necessary for lifelong immunocompetence but for unknown reasons requires signaling via both interleukin 7 (IL-7) and the T cell antigen receptor (TCR). We now report that naive CD8(+) T cells required IL-7 signaling to be intermittent, not continuous, because prolonged IL-7 signaling induced naive CD8(+) T cells to proliferate, produce interferon-γ (IFN-γ) and undergo IFN-γ-triggered cell death. Homeostatic engagement of the TCR interrupted IL-7 signaling and thereby supported the survival and quiescence of CD8(+) T cells. However, CD8(+) T cells with insufficient TCR affinity for self ligands received prolonged IL-7 signaling and died during homeostasis. In this study we identified regulation of the duration of IL-7 signaling by homeostatic engagement of the TCR as the basis for in vivo CD8(+) T cell homeostasis.

E protein transcription factors are required for the development of CD4(+) lineage T cells

The double-positive (DP) to single-positive (SP) transition during T cell development is initiated by downregulation of the E protein transcription factors HEB and E2A. Here, we have demonstrated that in addition to regulating the onset of this transition, HEB and E2A also play a separate role in CD4(+) lineage choice. Deletion of HEB and E2A in DP thymocytes specifically blocked the development of CD4(+) lineage T cells. Furthermore, deletion of the E protein inhibitors Id2 and Id3 allowed CD4(+) T cell development but blocked CD8(+) lineage development. Analysis of the CD4(+) lineage transcriptional regulators ThPOK and Gata3 placed HEB and E2A upstream of CD4(+) lineage specification. These studies identify an important role for E proteins in the activation of CD4(+) lineage differentiation as thymocytes undergo the DP to SP transition.

Coreceptor gene imprinting governs thymocyte lineage fate

Double-positive (CD4⁺CD8⁺) thymocytes differentiate into CD4⁺ helper T cells and CD8⁺ cytotoxic T cells. A knock-in approach replacing CD8-coding sequences with CD4 cDNA shows that it is the expression kinetics of CD8, and not the identity of the coreceptor, that governs thymocyte-lineage fate.

Immature thymocytes are bipotential cells that are signalled during positive selection to become either helper- or cytotoxic-lineage T cells. By tracking expression of lineage determining transcription factors during positive selection, we now report that the Cd8 coreceptor gene locus co-opts any coreceptor protein encoded within it to induce thymocytes to express the cytotoxic-lineage factor Runx3 and to adopt the cytotoxic-lineage fate, findings we refer to as ‘coreceptor gene imprinting'. Specifically, encoding CD4 proteins in the endogenous Cd8 gene locus caused major histocompatibility complex class II-specific thymocytes to express Runx3 during positive selection and to differentiate into CD4⁺ cytotoxic-lineage T cells. Our findings further indicate that coreceptor gene imprinting derives from the dynamic regulation of specific cis Cd8 gene enhancer elements by positive selection signals in the thymus. Thus, for coreceptor-dependent thymocytes, lineage fate is determined by Cd4 and Cd8 coreceptor gene loci and not by the specificity of T-cell antigen receptor/coreceptor signalling. This study identifies coreceptor gene imprinting as a critical determinant of lineage fate determination in the thymus.

Expanding roles for ThPOK in thymic development

The role of the zinc finger transcription factor ThPOK (T-helper-inducing POZ-Kruppel-like factor) in promoting commitment of αβ T cells to the CD4 lineage is now well established. New results indicate that ThPOK is also important for the development and/or acquisition of effector functions by other T cell subsets, including several not marked by CD4 expression, i.e. double-negative invariant natural killer T (iNKT) cells, γδ cells, and even memory CD8(+) T cells. There is compelling evidence that ThPOK expression in most or all of these cases is dependent on T-cell receptor signaling and that differences in relative TCR signal strength/length may induce different levels of ThPOK expression. The developmental consequences of ThPOK expression vary according to cell type, which may partly reflect differences in ThPOK levels and/or in transcriptional networks between cell types.

Description of an elasmobranch TCR coreceptor: CD8α from Rhinobatos productus

Cell-mediated immunity plays an essential role for the control and eradication of intracellular pathogens. To learn more about the evolutionary origins of the first signal (Signal 1) for T-cell activation, we cloned CD8α from an elasmobranch, Rhinobatos productus. Similar to full-length CD8α cDNAs from other vertebrates, Rhpr-CD8α (1800bp) encodes a 219 amino acid open reading frame composed of a signal peptide, an extracellular IgSF V domain and a stalk/hinge region followed by a well-conserved transmembrane domain and cytoplasmic tail. Overall, the mature Rhpr-CD8α protein (201 aa) displays ∼ 30% amino acid identity with mammalian CD8α including absolute conservation of cysteine residues involved in the IgSf V domain fold and dimerization of CD8αα and CD8αβ. One prominent feature is the absence of the LCK association motif (CXC) that is needed for achieving signal 1 in tetrapods. Both elasmobranch and teleost CD8α protein sequences possess a similar but distinctly different motif (CXH) in the cytoplasmic tail. The overall genomic structure of CD8α has been conserved during the course of vertebrate evolution both for the number of exons and phase of splicing. Finally, quantitative RTPCR demonstrated that elasmobranch CD8α is expressed in lymphoid-rich tissues similar to CD8 in other vertebrates. The results from this study indicate the existence of CD8 prior to the emergence of the gnathostomes (>450 MYA) while providing evidence that the canonical LCK association motif in mammals is likely a derived characteristic of tetrapod CD8α, suggesting potential differences for T-cell education and activation in the various gnathostomes.

The epigenetic landscape of lineage choice: lessons from the heritability of CD4 and CD8 expression

Developing αβ T cells choose between the helper and cytotoxic lineages, depending upon the specificity of their T cell receptors for MHC molecules. The expression of the CD4 co-receptor on helper cells and the CD8 co-receptor on cytotoxic cells is intimately linked to this decision, and their regulation at the transcriptional level has been the subject of intense study to better understand lineage choice. Indeed, as the fate of developing T cells is decided, the expression status of these genes is accordingly locked. Genetic models have revealed important transcriptional elements and the ability to manipulate these elements in the framework of development has added a new perspective on the temporal nature of their function and the epigenetic maintenance of gene expression. We examine here novel insights into epigenetic mechanisms that have arisen through the study of these genes.

Transcriptional and epigenetic regulation of CD4/CD8 lineage choice

The helper versus cytotoxic-lineage choice of CD4(+)CD8(+) DP thymocytes correlates with MHC restriction of their T cell receptors and the termination of either CD8 or CD4 coreceptor expression. It has been hypothesized that transcription factors regulating the expression of the Cd4/Cd8 coreceptor genes must play a role in regulating the lineage decision of DP thymocytes. Indeed, progress made during the past decade led to the identification of several transcription factors that regulate CD4/CD8 expression that are as well important regulators of helper/cytotoxic cell fate choice. These studies provided insight into the molecular link between the regulation of coreceptor expression and lineage decision. However, studies initiated by the identification of ThPOK, a central transcription factor for helper T cell development, have offered another perspective on the cross-regulation between these two processes. Here, we review advances in our understanding of regulatory circuits composed of transcription factors and their link to epigenetic mechanisms, which play essential roles in specifying and sealing cell lineage identity during the CD4/CD8 commitment process of DP thymocytes.

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

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

Decision checkpoints in the thymus

The development of T cells in the thymus involves several differentiation and proliferation events, during which hematopoietic precursors give rise to T cells ready to respond to antigen stimulation and undergo effector differentiation. This review addresses signaling and transcriptional checkpoints that control the intrathymic journey of T cell precursors. We focus on the divergence of alphabeta and gammadelta lineage cells and the elaboration of the alphabeta T cell repertoire, with special emphasis on the emergence of transcriptional programs that direct lineage decisions.

The role of ThPOK in control of CD4/CD8 lineage commitment

During alphabeta T cell development, cells diverge into alternate CD4 helper and CD8(+) cytotoxic T cell lineages. The precise correlation between a T cell's CD8 and CD4 choice and its TCR specificity to class I or class II MHC was noted more than 20 years ago, and establishing the underlying mechanism has remained a focus of intense study since then. This review deals with three formerly discrete topics that are gradually becoming interconnected: the role of TCR signaling in lineage commitment, the regulation of expression of the CD4 and CD8 genes, and transcriptional regulation of lineage commitment. It is widely accepted that TCR signaling exerts a decisive influence on lineage choice, although the underlying mechanism remains intensely debated. Current evidence suggests that both duration and intensity of TCR signaling may control lineage choice, as proposed by the kinetic signaling and quantitative instructive models, respectively. Alternate expression of the CD4 and CD8 genes is the most visible manifestation of lineage choice, and much progress has been made in defining the responsible cis elements and transcription factors. Finally, important clues to the molecular basis of lineage commitment have been provided by the recent identification of the transcription factor ThPOK as a key regulator of lineage choice. ThPOK is selectively expressed in class II-restricted cells at the CD4(+)8(lo) stage and is necessary and sufficient for development to the CD4 lineage. Given the central role of ThPOK in lineage commitment, understanding its upstream regulation and downstream gene targets is expected to reveal further important aspects of the molecular machinery underlying lineage commitment.

CD4-CD8 lineage differentiation: Thpok-ing into the nucleus

The mature alphabeta T cell population is divided into two main lineages that are defined by the mutually exclusive expression of CD4 and CD8 surface molecules (coreceptors) and that differ in their MHC restriction and function. CD4 T cells are typically MHC-II restricted and helper (or regulatory), whereas CD8 T cells are typically cytotoxic. Several transcription factors are known to control the emergence of CD4 and CD8 lineages, including the zinc finger proteins Thpok and Gata3, which are required for CD4 lineage differentiation, and the Runx factors Runx1 and Runx3, which contribute to CD8 lineage differentiation. This review summarizes recent advances on the function of these transcription factors in lineage differentiation. We also discuss how the "circuitry" connecting these factors could operate to match the expression of the lineage-committing factors Thpok and Runx3, and therefore lineage differentiation, to MHC specificity.

Upregulation of CD4 expression during MHC class II-specific positive selection is essential for error-free lineage choice

The lineage fate of developing thymocytes is determined by the persistence or cessation of T cell receptor (TCR) signaling during positive selection, with persistent TCR signaling required for CD4 lineage choice. We show here that transcriptional upregulation of CD4 expression is essential for error-free lineage choice during major histocompatibility complex class II (MHC II)-specific positive selection and is critical for error-free lineage choice in TCR-transgenic mice whose thymocytes compete for the identical selecting ligand. CD4 upregulation occurred for endogenously encoded CD4 coreceptors, but CD4 transgenes were downregulated during positive selection, disrupting MHC II-specific TCR signaling and causing lineage errors regardless of the absolute number or signaling strength of transgenic CD4 proteins. Thus, the kinetics of CD4 coreceptor expression during MHC II-specific positive selection determines the integrity of CD4 lineage choice, revealing an elegant symmetry between coreceptor kinetics and lineage choice.

GATA3 and the T-cell lineage: essential functions before and after T-helper-2-cell differentiation

Many advances in our understanding of the molecules that regulate the development, differentiation and function of T cells have been made over the past few years. One important regulator of T-cell differentiation is the transcription factor GATA-binding protein 3 (GATA3). Although the main function of GATA3 is to act as a master transcription factor for the differentiation of T helper 2 (T(H)2) cells, new research has helped to uncover crucial functions of GATA3 in T cells that go beyond T(H)2-cell differentiation and that are important at earlier stages of haematopoietic and lymphoid-cell development. This Review focuses on the functions of GATA3 from early thymocyte development to effector T-cell differentiation. In addition, we discuss the interactions between GATA3 and other transcription factors and signalling pathways, and highlight the functional significance of the GATA3 protein structure.

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.

Cascading suppression of transcriptional silencers by ThPOK seals helper T cell fate

CD4 and the transcription factor ThPOK are essential for the differentiation of major histocompatibility complex class II-restricted thymocytes into the helper T cell lineage; their genes (Cd4 and Zbtb7b (called 'ThPOK' here)) are repressed by transcriptional silencer elements in cytotoxic T cells. The molecular mechanisms regulating expression of these genes during helper T cell lineage differentiation remain unknown. Here we showed that inefficient upregulation of ThPOK, induced by removal of the proximal enhancer from the ThPOK locus, resulted in the transdifferentiation of helper lineage-specified cells into the cytotoxic T cell lineage. Furthermore, direct antagonism by ThPOK of the Cd4 and ThPOK silencers generated two regulatory loops that initially inhibited Cd4 downregulation and later stabilized ThPOK expression. Our results show how an initial lineage-specification signal can be amplified and stabilized during the lineage-commitment process.

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.

SATB1 is required for CD8 coreceptor reversal

Intrathymic signals induce the differentiation of immature CD4(+)CD8(+) double positive (DP) thymocytes into mature CD4(+) or CD8(+) single positive (SP) T cells. The transcriptional mechanism by which CD8 lineage is determined is not fully understood. The best evidence, which favors the kinetic signaling/coreceptor reversal model, indicates that signaled DP thymocytes terminate CD8 transcription prior to their subsequent re-initiation of CD8 transcription and ultimate differentiation into CD8SP T cells. We and others have shown that CD8 lineage commitment is severely perturbed in mice in which expression of the transcription factor SATB1 is either conventionally knocked out or T cell-specifically knocked down. Here, we demonstrate that, as with normal thymocytes, cultured SATB1-deficient DP thymocytes inactivate CD8 coreceptor transcription following receipt of signals (PMA plus ionomycin) that mimic TCR-mediated positive selection. However, this terminated CD8 transcription is not re-initiated by signals (IL-7) conducive to CD8 differentiation in SATB1-deficient DP. We show that SATB1 specifically binds to a cis-regulatory element within the CD8 enhancer (E8(III)) known to be required for coreceptor reversal. A requirement in CD8 coreceptor reversal identifies SATB1 as an essential trans-regulator of CD8 lineage fate, whose action may be mediated via recruitment to the E8(III) DP enhancer.

Thymus lineage commitment: a single switch

In this issue of Immunity, He et al. (2008) establish the logic and circuitry that determine CD4-CD8 lineage specification. It all comes down to an eighty base pair silencer switch.

Repression of the transcription factor Th-POK by Runx complexes in cytotoxic T cell development

Mouse CD4+CD8+ double-positive (DP) thymocytes differentiate into CD4+ helper-lineage cells upon expression of the transcription factor Th-POK but commit to the CD8+ cytotoxic lineage in its absence. We report the redirected differentiation of class I-restricted thymocytes into CD4+CD8- helper-like T cells upon loss of Runx transcription factor complexes. A Runx-binding sequence within the Th-POK locus acts as a transcriptional silencer that is essential for Th-POK repression and for development of CD8+ T cells. Thus, Th-POK expression and genetic programming for T helper cell development are actively inhibited by Runx-dependent silencer activity, allowing for cytotoxic T cell differentiation. Identification of the transcription factors network in CD4 and CD8 lineage choice provides insight into how distinct T cell subsets are developed for regulating the adaptive immune system.

Development of all CD4 T lineages requires nuclear factor TOX

CD8(+) cytotoxic and CD4(+) helper/inducer T cells develop from common thymocyte precursors that express both CD4 and CD8 molecules. Upon T cell receptor signaling, these cells initiate a differentiation program that includes complex changes in CD4 and CD8 expression, allowing identification of transitional intermediates in this developmental pathway. Little is known about regulation of these early transitions or their specific importance to CD4 and CD8 T cell development. Here, we show a severe block at the CD4(lo)CD8(lo) transitional stage of positive selection caused by loss of the nuclear HMG box protein TOX. As a result, CD4 lineage T cells, including regulatory T and CD1d-dependent natural killer T cells, fail to develop. In contrast, functional CD8(+) T cells develop in TOX-deficient mice. Our data suggest that TOX-dependent transition to the CD4(+)CD8(lo) stage is required for continued development of class II major histocompatibility complex-specific T cells, regardless of ultimate lineage fate.

'Coreceptor tuning': cytokine signals transcriptionally tailor CD8 coreceptor expression to the self-specificity of the TCR

T cell immunity requires the long-term survival of T cells that are capable of recognizing self antigens but are not overtly autoreactive. How this balance is achieved remains incompletely understood. Here we identify a homeostatic mechanism that transcriptionally tailors CD8 coreceptor expression in individual CD8+ T cells to the self-specificity of their clonotypic T cell receptor (TCR). 'Coreceptor tuning' results from interplay between cytokine and TCR signals, such that signals from interleukin 7 and other common gamma-chain cytokines transcriptionally increase CD8 expression and thereby promote TCR engagement of self ligands, whereas TCR signals impair common gamma-chain cytokine signaling and thereby decrease CD8 expression. This dynamic interplay induces individual CD8+ T cells to express CD8 in quantities appropriate for the self-specificity of their TCR, promoting the engagement of self ligands, yet avoiding autoreactivity.

Modulation of CD4 co-receptor limits spontaneous autoimmunity when high-affinity transgenic TCR specific for self-antigen is expressed on a genetically resistant background

Myelin proteolipid protein (PLP) 139-151 is an immunodominant peptide that induces experimental autoimmune encephalomyelitis (EAE) in H-2(s) SJL/J mice. While PLP 139-151-specific TCR transgenic (tg) 4E3 mice develop fulminant spontaneous disease on the susceptible SJL/J background, spontaneous EAE is dramatically reduced on the H-2(s) congenic B10.S background. On this resistant background, we observed a high frequency of positively selected tg CD4-CD8- (DN) thymocytes and peripheral DN tg T cells. Splenic DN tg T cells responded to anti-CD3 stimulation similarly to CD4+ cells, but proliferative and cytokine responses to PLP 139-151 were blunted, implying that CD4 co-receptor down-regulation modulated T cell responses to the self-antigen in vitro. Adoptive transfer of tg DN CD3hi cells into RAG-deficient wild-type (WT) recipients induced EAE less efficiently than transfer of CD4+ T tg cells indicating the blunted responses of DN tg T cells to self-antigen in vivo. The frequency of tg DN T cells was irrespective of thymic expression of the autoantigen. These data implicate that down-regulation of CD4 co-receptor in the thymus, which is independent from the expression of thymic autoantigen, results in a blunted response to the autoantigen in the periphery and limits the incidence of spontaneous autoimmunity in genetically resistant mice bearing a large autoreactive tg T cell repertoire.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Signalling in T-cell development: is it all location, location, location?

During thymocyte development, signals through the pre-T-cell receptor induce proliferation and differentiation at early stages of maturation; thereafter, signals through the alphabeta T-cell receptor complex mediate positive and negative selection and commitment to the CD4 or CD8 lineage. How these signals are initiated, the transduction pathways involved, and ultimately how gene transcription is regulated are current active areas of research. Recent literature highlights the importance of geography to thymocyte differentiation: first, in relation to the ability of the developing thymocyte to traffic to localities within the thymus in which particular selection events occur and which are crucial for successful T-cell maturation; and, second, in respect to the subcellular localisation of intracellular signalling molecules, which might provide the key to understanding how similar signals can be translated into dramatically distinct fates, such as positive selection, lineage commitment and negative selection.

Coreceptor signal strength regulates positive selection but does not determine CD4/CD8 lineage choice in a physiologic in vivo model

TCR signals drive thymocyte development, but it remains controversial what impact, if any, the intensity of those signals have on T cell differentiation in the thymus. In this study, we assess the impact of CD8 coreceptor signal strength on positive selection and CD4/CD8 lineage choice using novel gene knockin mice in which the endogenous CD8alpha gene has been re-engineered to encode the stronger signaling cytoplasmic tail of CD4, with the re-engineered CD8alpha gene referred to as CD8.4. We found that stronger signaling CD8.4 coreceptors specifically improved the efficiency of CD8-dependent positive selection and quantitatively increased the number of MHC class I (MHC-I)-specific thymocytes signaled to differentiate into CD8+ T cells, even for thymocytes expressing a single, transgenic TCR. Importantly, however, stronger signaling CD8.4 coreceptors did not alter the CD8 lineage choice of any MHC-I-specific thymocytes, even MHC-I-specific thymocytes expressing the high-affinity F5 transgenic TCR. This study documents in a physiologic in vivo model that coreceptor signal strength alters TCR-signaling thresholds for positive selection and so is a major determinant of the CD4:CD8 ratio, but it does not influence CD4/CD8 lineage choice.

Cross-positive selection of thymocytes expressing a single TCR by multiple major histocompatibility complex molecules of both classes: implications for CD4+ versus CD8+ lineage commitment

This study has investigated the cross-reactivity upon thymic selection of thymocytes expressing transgenic TCR derived from a murine CD8+ CTL clone. The Idhigh+ cells in this transgenic mouse had been previously shown to mature through positive selection by class I MHC, Dq or Lq molecule. By investigating on various strains, we found that the transgenic TCR cross-reacts with three different MHCs, resulting in positive or negative selection. Interestingly, in the TCR-transgenic mice of H-2q background, mature Idhigh+ T cells appeared among both CD4+ and CD8+ subsets in periphery, even in the absence of RAG-2 gene. When examined on beta2-microglobulin-/- background, CD4+, but not CD8+, Idhigh+ T cells developed, suggesting that maturation of CD8+ and CD4+ Idhigh+ cells was MHC class I (Dq/Lq) and class II (I-Aq) dependent, respectively. These results indicated that this TCR-transgenic mouse of H-2q background contains both classes of selecting MHC ligands for the transgenic TCR simultaneously. Further genetic analyses altering the gene dosage and combinations of selecting MHCs suggested novel asymmetric effects of class I and class II MHC on the positive selection of thymocytes. Implications of these observations in CD4+/CD8+ lineage commitment are discussed.

CD4/CD8 lineage commitment: light at the end of the tunnel?

Two surprisingly clear results have emerged in the past year that suggest that the seemingly intractable problem of CD4/CD8 lineage commitment might eventually be resolved. Manipulating expression of the CD4 and CD8 coreceptors has long been a favorite method to examine the influence of T-cell receptor signalling on lineage commitment. An elegant new twist on this approach now shows that it is all a matter of timing. Thus, termination of CD4 expression after the initiation of positive selection is sufficient to cause complete redirection of class II-restricted thymocytes to the CD8 lineage, which strongly supports quantitative instructive models of lineage commitment. Progress in the field has been significantly hampered by ignorance of the underlying intracellular pathways. Two independent groups, which employed old-fashioned genetics versus new-fangled microarray technology, have now identified the same transcription factor, Th-POK, as a key regulator of alternate lineage commitment. The presence of this factor directs positively selected thymocytes to the CD4 lineage, whereas its absence causes default development to the CD8 lineage.

Role of the transcription factor Th-POK in CD4:CD8 lineage commitment

The molecular basis of CD4:CD8 lineage commitment, in particular the mechanism by which the precise correlation between lineage choice and T-cell receptor (TCR) specificity toward class I or II major histocompatibility complex is achieved, remains controversial. Both stochastic/selective and instructive models in various forms have been proposed to explain this correlation. The two main experimental approaches previously employed to elucidate this process have focused on the beginning and end of the process, i.e. the influence of TCR signaling and the alternate transcriptional control of the CD4 and CD8 loci during commitment. The recent finding that the transcription factor Th-POK is necessary and sufficient for CD4 commitment has now provided a direct entry point for studying the intracellular pathways that govern lineage commitment. Here, we review data leading to the identification and characterization of this factor and discuss the implications of these studies in the context of current models of lineage commitment.

The role of erk1 and erk2 in multiple stages of T cell development

Activation of extracellular-signal-regulated protein kinase (Erk) is central to growth-factor-receptor-mediated signaling including that originating from the T cell antigen receptor. It integrates cytoplasmic signals to effect changes in transcription associated with differentiation, proliferation, and survival. In this report, we present an analysis of mice with targeted deletions in Erk1 and Erk2 to assess the relationship between Erk activity and cell-cycle progression, thymocyte development, and lineage commitment. These studies show that Erk is selectively retained during beta selection-driven proliferation, and yet Erk1/2 are not required to complete differentiation to CD4+CD8+ preselection stage of development. Erk activity is essential for the process of positive selection, and it differentially affects CD4 and CD8 T cell maturation; yet, diminished expression itself is not sufficient to alter lineage commitment.

Selection and lineage specification in the thymus: commitment 4-stalled

How CD4(+)CD8(+) thymocytes commit to CD4 helper versus CD8 cytotoxic lineages is a central unresolved question in developmental immunology. In this issue, show that engineering CD4 for shutoff immediately after positive selection misdirects cells to the cytotoxic lineage. The result highlights the distinction between positive selection and lineage commitment and provides new impetus for reexamining lineage models.