Strength of signaling by CD4 and CD8 coreceptor tails determines the number but not the lineage direction of positively selected thymocytes

Structure, function, and immunomodulation of the CD8 co-receptor

Expressed on the surface of CD8+ T cells, the CD8 co-receptor is a key component of the T cells that contributes to antigen recognition, immune cell maturation, and immune cell signaling. While CD8 is widely recognized as a co-stimulatory molecule for conventional CD8+ αβ T cells, recent reports highlight its multifaceted role in both adaptive and innate immune responses. In this review, we discuss the utility of CD8 in relation to its immunomodulatory properties. We outline the unique structure and function of different CD8 domains (ectodomain, hinge, transmembrane, cytoplasmic tail) in the context of the distinct properties of CD8αα homodimers and CD8αβ heterodimers. We discuss CD8 features commonly used to construct chimeric antigen receptors for immunotherapy. We describe the molecular interactions of CD8 with classical MHC-I, non-classical MHCs, and Lck partners involved in T cell signaling. Engineered and naturally occurring CD8 mutations that alter immune responses are discussed. The applications of anti-CD8 monoclonal antibodies (mABs) that target CD8 are summarized. Finally, we examine the unique structure and function of several CD8/mAB complexes. Collectively, these findings reveal the promising immunomodulatory properties of CD8 and CD8 binding partners, not only to uncover basic immune system function, but to advance efforts towards translational research for targeted immunotherapy.

The CD4 Versus CD8 T Cell Fate Decision: A Multiomics-Informed Perspective

The choice of developing thymocytes to become CD8+ cytotoxic or CD4+ helper T cells has been intensely studied, but many of the underlying mechanisms remain to be elucidated. Recent multiomics approaches have provided much higher resolution analysis of gene expression in developing thymocytes than was previously achievable, thereby offering a fresh perspective on this question. Focusing on our recent studies using CITE-seq (cellular indexing of transcriptomes and epitopes) analyses of mouse thymocytes, we present a detailed timeline of RNA and protein expression changes during CD8 versus CD4 T cell differentiation. We also revisit our current understanding of the links between T cell receptor signaling and expression of the lineage-defining transcription factors ThPOK and RUNX3. Finally, we propose a sequential selection model to explain the tight linkage between MHC-I versus MHC-II recognition and T cell lineage choice. This model incorporates key aspects of previously proposed kinetic signaling, instructive, and stochastic/selection models.

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.

The order and logic of CD4 versus CD8 lineage choice and differentiation in mouse thymus

CD4 and CD8 mark helper and cytotoxic T cell lineages, respectively, and serve as coreceptors for MHC-restricted TCR recognition. How coreceptor expression is matched with TCR specificity is central to understanding CD4/CD8 lineage choice, but visualising coreceptor gene activity in individual selection intermediates has been technically challenging. It therefore remains unclear whether the sequence of coreceptor gene expression in selection intermediates follows a stereotypic pattern, or is responsive to signaling. Here we use single cell RNA sequencing (scRNA-seq) to classify mouse thymocyte selection intermediates by coreceptor gene expression. In the unperturbed thymus, Cd4+Cd8a- selection intermediates appear before Cd4-Cd8a+ selection intermediates, but the timing of these subsets is flexible according to the strength of TCR signals. Our data show that selection intermediates discriminate MHC class prior to the loss of coreceptor expression and suggest a model where signal strength informs the timing of coreceptor gene activity and ultimately CD4/CD8 lineage choice.

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.

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.

Survival of mature T cells depends on signaling through HOIP

T cell development in the thymus is controlled by a multistep process. The NF-κB pathway regulates T cell development as well as T cell activation at multiple differentiation stages. The linear ubiquitin chain assembly complex (LUBAC) is composed of Sharpin, HOIL-1L and HOIP, and it is crucial for regulating the NF-κB and cell death pathways. However, little is known about the roles of LUBAC in T-cell development and activation. Here, we show that in T-HOIP^Δlinear mice lacking the ubiquitin ligase activity of LUBAC, thymic CD4⁺ or CD8⁺ T cell numbers were markedly reduced with severe defects in NKT cell development. HOIP^Δlinear CD4⁺ T cells failed to phosphorylate IκBα and JNK through T cell receptor-mediated stimulation. Mature CD4⁺ and CD8⁺ T cells in T-HOIP^Δlinear mice underwent apoptosis more rapidly than control T cells, and it was accompanied by lower CD127 expression on CD4⁺CD24^low and CD8⁺CD24^low T cells in the thymus. The enforced expression of CD127 in T-HOIP^Δlinear thymocytes rescued the development of mature CD8⁺ T cells. Collectively, our results showed that LUBAC ligase activity is key for the survival of mature T cells, and suggest multiple roles of the NF-κB and cell death pathways in activating or maintaining T cell-mediated adaptive immune responses.

Peripheral T-cell and NK cell lymphoproliferative disorders: cell of origin, clinical and pathological implications

T-cell lymphoproliferative disorders are a heterogeneous group of neoplasms with distinct clinical-biological properties. The normal cellular counterpart of these processes has been postulated based on functional and immunophenotypic analyses. However, T lymphocytes have been proven to be remarkably capable of modulating their properties, adapting their function in relationship with multiple stimuli and to the microenvironment. This impressive plasticity is determined by the equilibrium among a pool of transcription factors and by DNA chromatin regulators. It is now proven that the acquisition of specific genomic defects leads to the enforcement/activation of distinct pathways, which ultimately alter the preferential activation of defined regulators, forcing the neoplastic cells to acquire features and phenotypes distant from their original fate. Thus, dissecting the landscape of the genetic defects and their functional consequences in T-cell neoplasms is critical not only to pinpoint the origin of these tumors but also to define innovative mechanisms to re-adjust an unbalanced state to which the tumor cells have become addicted and make them vulnerable to therapies and targetable by the immune system. In our review, we briefly describe the pathological and clinical aspects of the T-cell lymphoma subtypes as well as NK-cell lymphomas and then focus on the current understanding of their pathogenesis and the implications on diagnosis and treatment.

A Zap70-dependent feedback circuit is essential for efficient selection of CD4 lineage thymocytes

During positive selection of CD4(+), CD8(+) double positive (DP) thymocytes, expression of the tyrosine kinase Zap70 is subject to developmental regulation. Signalling downstream of T-cell receptor (TCR) induces Zap70 expression, forming a positive feedback circuit. Although previous studies show this circuit is required for generation of CD8 lineage cells, it is not known whether selection of CD4 T cells also depends on intact developmental regulation of Zap70. To address this, we analysed development of Class II-restricted thymocytes in mice lacking the Zap70 transcriptional circuitry. Rescue of Zap70 expression in Zap70(-/-) mice using a tetracycline-inducible Zap70 transgene, that is not subject to positive feedback by TCR signalling, restored positive selection of Class-II-restricted thymocytes. However, in conditions of static Zap70 expression, approximately half of selecting thymocytes failed to commit normally to the CD4 lineage. Instead, cells that failed to develop into CD4 T cells resembled CD8 lineage precursor DP thymocytes but failed to survive in vivo. Therefore, the Zap70 feedback circuit is essential to efficiently mediate the CD4 lineage differentiation programme in response to Class II selecting ligands.

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.

Cellular and molecular requirements for the selection of in vitro-generated CD8 T cells reveal a role for Notch

Differentiation of CD8 single-positive (SP) T cells is predicated by the ability of lymphocyte progenitors to integrate multiple signaling cues provided by the thymic microenvironment. In the thymus and the OP9-DL1 system for T cell development, Notch signals are required for progenitors to commit to the T cell lineage and necessary for their progression to the CD4(+)CD8(+) double-positive (DP) stage of T cell development. However, it remains unclear whether Notch is a prerequisite for the differentiation of DP cells to the CD8 SP stage of development. In this study, we demonstrate that Notch receptor-ligand interactions allow for efficient differentiation and selection of conventional CD8 T cells from bone marrow-derived hematopoietic stem cells. However, bone marrow-derived hematopoietic stem cells isolated from Itk(-/-)Rlk(-/-) mice gave rise to T cells with decreased IFN-γ production, but gained the ability to produce IL-17. We further reveal that positive and negative selection in vitro are constrained by peptide-MHC class I expressed on OP9 cells. Finally, using an MHC class I-restricted TCR-transgenic model, we show that the commitment of DP precursors to the CD8 T cell lineage is dependent on Notch signaling. Our findings further establish the requirement for Notch receptor-ligand interactions throughout T cell differentiation, including the final step of CD8 SP selection.

CD81 interacts with the T cell receptor to suppress signaling

CD81 (TAPA-1) is a ubiquitously expressed tetraspanin protein identified as a component of the B lymphocyte receptor (BCR) and as a receptor for the Hepatitis C Virus. In an effort to identify trans-membrane proteins that interact with the T-cell antigen receptor (TCR), we performed a membrane yeast two hybrid screen and identified CD81 as an interactor of the CD3delta subunit of the TCR. We found that in the absence of CD81, in thymocytes from knockout mice, TCR engagement resulted in stronger signals. These results were recapitulated in T cell lines that express low levels of CD81 through shRNA mediated silencing. Increased signaling did not result from alterations in the levels of TCR on the surface of T lymphocytes. Although CD81 is not essential for normal T lymphocyte development, it plays an important role in regulating TCR and possibly pre-TCR signal transduction by controlling the strength of signaling. CD81 dependent alterations in thymocyte signaling are evident in increased CD5 expression on CD81 deficient double positive (DP) thymocytes. We conclude that CD81 interacts with the T cell receptor to suppress signaling.

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.

Development of thymically derived natural regulatory T cells

Natural regulatory T cells (nTregs) are defined by their inherent ability to establish and maintain peripheral self-tolerance. In recent years, the development of nTregs has come under close examination with the advent of Forkhead Box P3 protein (FOXP3)-green fluorescent protein reporter mice that pinpointed the initiation of FOXP3 expression within the thymus. The mechanism and pathway of nTreg development has only recently been studied in detail and to a large degree remains unclear. In this review, we will discuss our current understanding of nTreg lineage choice and development from a cellular and intracellular standpoint.

Functional development of the T cell receptor for antigen

For over three decades now, the T cell receptor (TCR) for antigen has not ceased to challenge the imaginations of cellular and molecular immunologists alike. T cell antigen recognition transcends every aspect of adaptive immunity: it shapes the T cell repertoire in the thymus and directs T cell-mediated effector functions in the periphery, where it is also central to the induction of peripheral tolerance. Yet, despite its central position, there remain many questions unresolved: how can one TCR be specific for one particular peptide-major histocompatibility complex (pMHC) ligand while also binding other pMHC ligands with an immunologically relevant affinity? And how can a T cell's extreme specificity (alterations of single methyl groups in their ligand can abrogate a response) and sensitivity (single agonist ligands on a cell surface are sufficient to trigger a measurable response) emerge from TCR-ligand interactions that are so low in affinity? Solving these questions is intimately tied to a fundamental understanding of molecular recognition dynamics within the many different contexts of various T cell-antigen presenting cell (APC) contacts: from the thymic APCs that shape the TCR repertoire and guide functional differentiation of developing T cells to the peripheral APCs that support homeostasis and provoke antigen responses in naïve, effector, memory, and regulatory T cells. Here, we discuss our recent findings relating to T cell antigen recognition and how this leads to the thymic development of foreign-antigen-responsive alphabetaT cells.

Structural basis of the CD8 alpha beta/MHC class I interaction: focused recognition orients CD8 beta to a T cell proximal position

In the immune system, B cells, dendritic cells, NK cells, and T lymphocytes all respond to signals received via ligand binding to receptors and coreceptors. Although the specificity of T cell recognition is determined by the interaction of T cell receptors with MHC/peptide complexes, the development of T cells in the thymus and their sensitivity to Ag are also dependent on coreceptor molecules CD8 (for MHC class I (MHCI)) and CD4 (for MHCII). The CD8alphabeta heterodimer is a potent coreceptor for T cell activation, but efforts to understand its function fully have been hampered by ignorance of the structural details of its interactions with MHCI. In this study we describe the structure of CD8alphabeta in complex with the murine MHCI molecule H-2D(d) at 2.6 A resolution. The focus of the CD8alphabeta interaction is the acidic loop (residues 222-228) of the alpha3 domain of H-2D(d). The beta subunit occupies a T cell membrane proximal position, defining the relative positions of the CD8alpha and CD8beta subunits. Unlike the CD8alphaalpha homodimer, CD8alphabeta does not contact the MHCI alpha(2)- or beta(2)-microglobulin domains. Movements of the CD8alpha CDR2 and CD8beta CDR1 and CDR2 loops as well as the flexibility of the H-2D(d) CD loop facilitate the monovalent interaction. The structure resolves inconclusive data on the topology of the CD8alphabeta/MHCI interaction, indicates that CD8beta is crucial in orienting the CD8alphabeta heterodimer, provides a framework for understanding the mechanistic role of CD8alphabeta in lymphoid cell signaling, and offers a tangible context for design of structurally altered coreceptors for tumor and viral immunotherapy.

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.

'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.

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.

Overlapping functions of human CD3delta and mouse CD3gamma in alphabeta T-cell development revealed in a humanized CD3gamma-mouse

Humans lacking the CD3gamma subunit of the pre-TCR and TCR complexes exhibit a mild alphabeta T lymphopenia, but have normal T cells. By contrast, CD3gamma-deficient mice are almost devoid of mature alphabeta T cells due to an early block of intrathymic development at the CD4(-)CD8(-) double-negative (DN) stage. This suggests that in humans but not in mice, the highly related CD3delta chain replaces CD3gamma during alphabeta T-cell development. To determine whether human CD3delta (hCD3delta) functions in a similar manner in the mouse in the absence of CD3gamma, we introduced an hCD3delta transgene in mice that were deficient for both CD3delta and CD3gamma, in which thymocyte development is completely arrested at the DN stage. Expression of hCD3delta efficiently supported pre-TCR-mediated progression from the DN to the CD4(+)CD8(+) double-positive (DP) stage. However, alphabetaTCR-mediated positive and negative thymocyte selection was less efficient than in wild-type mice, which correlated with a marked attenuation of TCR-mediated signaling. Of note, murine CD3gamma-deficient TCR complexes that had incorporated hCD3delta displayed abnormalities in structural stability resembling those of T cells from CD3gamma-deficient humans. Taken together, these data demonstrate that CD3delta and CD3gamma play a different role in humans and mice in pre-TCR and TCR function during alphabeta T-cell development.

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.

Commitment issues: linking positive selection signals and lineage diversification in the thymus

The thymus is responsible for the production of CD4+ helper and CD8+ cytotoxic T cells, which constitute the cellular arm of the immune system. These cell types derive from common precursors that interact with thymic stroma in a T-cell receptor (TCR)-specific fashion, generating intracellular signals that are translated into function-specific changes in gene expression. This overall process is termed positive selection, but it encompasses a number of temporally distinct and possibly mechanistically distinct cellular changes, including rescue from apoptosis, initiation of cell differentiation, and commitment to the CD4+ or CD8+ T-cell lineage. One of the puzzling features of positive selection is how specificity of the TCR controls lineage commitment, as both helper and cytolytic T cells utilize the same antigen-receptor components, with the exception of the CD4 or CD8 coreceptors themselves. In this review, we focus on the signals required for positive selection, particularly as they relate to lineage commitment. Identification of genes encoding transcriptional regulators that play a role in T-cell development has led to significant recent advances in the field. We also provide an overview of nuclear factors in this context and, where known, how their regulation is linked to the same TCR signals that have been implicated in initiating and regulating positive selection.

Receptor signals and nuclear events in CD4 and CD8 T cell lineage commitment

MHC specificity in positive selection is a major determinant in the CD4/CD8 T cell lineage decision. Previous studies support the view that quantitative differences in T cell receptor (TCR) signaling in immature CD4+CD8+ double positive thymocytes leads to an instructive bias in CD4/CD8 T cell lineage commitment that must be re-inforced in subsequent selection steps to ensure that MHC-restricted antigen recognition is linked to appropriate effector functions in mature T cells. Recent work has further defined the TCR signaling pathways involved in this process, but a major effort has been made to identify transcription factors and other regulators of CD4 and CD8 T cell lineage commitment. Methods and screens for detecting changes in gene expression, associated with TCR signaling in positive selection and lineage determination, are starting to provide a better understanding of these complex developmental processes.

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

For developing T cells, coreceptor choice is matched to T cell antigen receptor (TCR) MHC specificity during positive selection in the thymus, but the mechanism remains uncertain. Here, we document that TCR-mediated positive selection signals inactivate the immature CD8(III) enhancer in double positive (DP) thymocytes, explaining in part the cessation of CD8 coreceptor transcription that occurs during positive selection. More importantly, by placing CD4 protein expression under the control of CD8 transcriptional regulatory elements, we demonstrate that cessation of CD4 coreceptor transcription during positive selection results in precisely the same lineage fate as cessation of CD8 coreceptor transcription. That is, MHC-II-signaled DP thymocytes differentiated into CD8-lineage cytotoxic T cells, despite the MHC-II specificity and CD4 dependence of their TCRs. This study demonstrates that termination of coreceptor transcription during positive selection promotes CD8-lineage fate, regardless of TCR specificity or coreceptor protein identity.

An antagonist peptide mediates positive selection and CD4 lineage commitment of MHC class II-restricted T cells in the absence of CD4

The CD4 coreceptor works together with the T cell receptor (TCR) to deliver signals to the developing thymocyte, yet its specific contribution to positive selection and CD4 lineage commitment remains unclear. To resolve this, we used N3.L2 TCR transgenic, RAG-, and CD4-deficient mice, which are severely impaired in positive selection, and asked whether altered peptide ligands can replace CD4 function in vivo. Remarkably, in the presence of antagonist ligands that normally deleted CD4⁺ T cells in wild-type mice, we induced positive selection of functional CD4 lineage T cells in mice deficient in CD4. We show that the kinetic threshold for positive and negative selection was lowered in the absence of CD4, with no evident skewing toward the CD8 lineage with weaker ligands. These results suggest that CD4 is dispensable as long as the affinity threshold for positive selection is sustained, and strongly argue that CD4 does not deliver a unique instructional signal for lineage commitment.

Gfi1:Green Fluorescent Protein Knock-in Mutant Reveals Differential Expression and Autoregulation of the Growth Factor Independence 1 (Gfi1) Gene during Lymphocyte Development

The Gfi1 gene encodes a 55-kDa transcriptional repressor protein with important functions in T-cell development, in granulopoiesis, and in the regulation of the innate immune response. To follow expression of the Gfi1 gene during the differentiation of specific immune cells, we have generated a mouse mutant in which the Gfi1 coding region is replaced by the gene for the green fluorescent protein (GFP). We found that Gfi1 gene expression is highest in early B-cell subpopulation and differentially expressed during T-cell development with peak levels at stages where pre-TCR or positive/negative selection takes place. Gfi1 is absent in mature B-cells, whereas in peripheral T-cells Gfi1 gene expression is low but rises significantly upon T-cell receptor triggering and decreases again in T-memory cells. Constitutive expression of an lck promoter-driven Gfi1 transgene led to transcriptional silencing of the Gfi1:GFP allele in T-cells. Because Gfi1 was found to occupy genomic sites of its own promoter in thymocytes and was able to repress its own transcription in vitro we propose that transcription of the Gfi1 gene is regulated through an autoregulatory feedback loop.

Thymocyte-intrinsic genetic factors influence CD8 T cell lineage commitment and affect selection of a tumor-reactive TCR

Selection of immature CD4CD8 double-positive (DP) thymocytes for CD4 or CD8-lineage commitment is controlled by the interaction of the TCR with stromal cell-expressed peptide/MHC. We show that thymocyte-intrinsic genes influence the pattern of expression of a MHC class I-restricted transgenic (tg) TCR so that in DBA/2 mice, DP thymocytes with a characteristically high expression of tg TCR, infrequently transit to CD8 single-positive thymocytes. In contrast, in B10.D2 mice, the same tg TCR is expressed at lower levels on a subpopulation of DP thymocytes that more frequently transit to CD8 single-positive thymocytes. These characteristics were not influenced by thymic stromal components that control positive selection. Radiation chimeras reconstituted with a mixture of BM from tg TCR mice of the two genetic backgrounds revealed that the relative frequency of transit to the CD8 lineage remained thymocyte-intrinsic. Identifying the gene products whose polymorphism controls CD8 T cell development may shed new light on the mechanisms controlling T cell commitment/selection in mice other than the most studied "C57BL/6"-based strains.

Duration of TCR signaling controls CD4-CD8 lineage differentiation in vivo

The duration of T cell receptor (TCR) signaling is thought to be important for thymocyte differentiation into the CD4 or CD8 lineage. However, the in vivo relevance of this hypothesis is unclear. Here we divided T cell positive selection into genetically separable developmental steps by confining TCR signal transduction to discrete thymocyte developmental windows. TCR signals confined to the double-positive thymocyte stage promoted CD8, but not CD4, lineage differentiation. Major histocompatibility complex (MHC) class II-restricted thymocytes were, instead, redirected into the CD8 lineage. These findings support the hypothesis that distinct kinetics of MHC class I- and MHC class II-induced TCR signals direct intrathymic developmental decisions.

Signaling but not trafficking function of HIV-1 protein Nef is essential for Nef-induced defects in human intrathymic T-cell development

The HIV-1 gene nef is important for progression toward AIDS and cellular depletion of the infected thymus. Expression of the Nef protein alone impairs human thymopoiesis. Here, we performed a structure-function analysis of the Nef protein by comparing the effect on T-cell development of different nef alleles, either wild type or defective for selected functions, expressed by human thymocytes. We show that Nef-mediated impaired thymopoiesis is not due to altered surface marker trafficking, nor dependent on oligomerization of Nef. By contrast, mutations in the myristoylation site and in signaling sites of Nef, ie, sites important for interaction with phosphofurin acidic cluster sorting protein-1 (PACS-1), Src homology domain 3 (SH3) domains, and p21-activated kinase 2 (PAK2), were found to be critical for its effect on T-cell development. These results point to sites in Nef to target therapeutically for restoration of thymopoiesis in HIV infection.

Stalk region of beta-chain enhances the coreceptor function of CD8

CD8 glycoproteins are expressed as either alphaalpha homodimers or alphabeta heterodimers on the surface of T cells. CD8alphabeta is a more efficient coreceptor than the CD8alphaalpha for peptide Ag recognition by TCR. Each CD8 subunit is composed of four structural domains, namely, Ig-like domain, stalk region, transmembrane region, and cytoplasmic domain. In an attempt to understand why CD8alphabeta is a better coreceptor than CD8alphaalpha, we engineered, expressed, and functionally tested a chimeric CD8alpha protein whose stalk region is replaced with that of CD8beta. We found that the beta stalk region enhances the coreceptor function of chimeric CD8alphaalpha to a level similar to that of CD8alphabeta. Surprisingly, the beta stalk region also restored functional activity to an inactive CD8alpha variant, carrying an Ala mutation at Arg(8) (R8A), to a level similar to that of wild-type CD8alphabeta. Using the R8A variant of CD8alpha, a panel of anti-CD8alpha Abs, and three MHC class I (MHCI) variants differing in key residues known to be involved in CD8alpha interaction, we show that the introduction of the CD8beta stalk leads to a different topology of the CD8alpha-MHCI complex without altering the overall structure of the Ig-like domain of CD8alpha or causing the MHCI to employ different residues to interact with the CD8alpha Ig domain. Our results show that the stalk region of CD8beta is capable of fine-tuning the coreceptor function of CD8 proteins as a coreceptor, possibly due to its distinct protein structure, smaller physical size and the unique glycan adducts associated with this region.

Unraveling a revealing paradox: Why major histocompatibility complex I-signaled thymocytes "paradoxically" appear as CD4+8lo transitional cells during positive selection of CD8+ T cells

The mechanism by which T cell receptor specificity determines the outcome of the CD4/CD8 lineage decision in the thymus is not known. An important clue is the fact that major histocompatibility complex (MHC)-I-signaled thymocytes paradoxically appear as CD4+8lo transitional cells during their differentiation into CD8+ T cells. Lineage commitment is generally thought to occur at the CD4+8+ (double positive) stage of differentiation and to result in silencing of the opposite coreceptor gene. From this perspective, the appearance of MHC-I-signaled thymocytes as CD4+8lo cells would be due to effects on CD8 surface protein expression, not CD8 gene expression. But contrary to this perspective, this study demonstrates that MHC-I-signaled thymocytes appear as CD4+8lo cells because of transient down-regulation of CD8 gene expression, not because of changes in CD8 surface protein expression or distribution. This study also demonstrates that initial cessation of CD8 gene expression in MHC-I-signaled thymocytes is not necessarily indicative of commitment to the CD4+ T cell lineage, as such thymocytes retain the potential to differentiate into CD8+ T cells. These results challenge classical concepts of lineage commitment but fulfill predictions of the kinetic signaling model.

The transcriptional repressor Gfi1 affects development of early, uncommitted c-Kit+ T cell progenitors and CD4/CD8 lineage decision in the thymus

In the thymus, several steps of proliferative expansion and selection coordinate the maturation of precursors into antigen-specific T cells. Here we identify the transcriptional repressor Gfi1 as an important regulator of this maturation process. Mice lacking Gfi1 show reduced thymic cellularity due to an increased cell death rate, lack of proliferation, and a differentiation block in the very early uncommitted CD4-/CD8-/c-Kit+ cytokine-dependent T cell progenitors that have not yet initiated VDJ recombination. In addition, Gfi1-deficient mice show increased major histocompatibility complex class I-restricted positive selection and develop significantly more CD8+ cells suggesting a requirement of Gfi1 for a correct CD4/CD8 lineage decision. Absence of Gfi1 correlates with high level expression of the genes for lung Krüppel-like factor (LKLF), inhibitor of DNA binding (Id)1 and Id2, suggesting the existence of new regulatory pathways in pre-T cell development and thymic selection in which Gfi1 acts upstream of LKLF as well as the E-proteins, which are negatively regulated by Id1 and Id2.

In vitro evidence that cytokine receptor signals are required for differentiation of double positive thymocytes into functionally mature CD8+ T cells

CD4(+)8(+) double positive (DP) thymocytes differentiate into CD4(+) and CD8(+) mature T cells in response to TCR signals. However, TCR signals that are initiated in DP thymocytes are unlikely to persist throughout all subsequent differentiation steps, suggesting that other signals must sustain thymocyte differentiation after TCR signaling has ceased. Using an in vitro experimental system, we now demonstrate that cytokine receptor signals, such as those transduced by IL-7 receptors, are required for differentiation of signaled DP thymocytes into functionally mature CD8(+) T cells as they: (a) up-regulate Bcl-2 expression to maintain thymocyte viability; (b) enhance CD4 gene silencing; (c) promote functional maturation;and (d) up-regulate surface expression of glucose transporter molecules, which improve nutrient uptake and increase metabolic activity. IL-7Rs appear to be unique among cytokine receptors in maintaining the viability of newly generated CD4(-)8(+) thymocytes, whereas several different cytokine receptors can provide the trophic/differentiative signals for subsequent CD8(+) thymocyte differentiation and maturation. Thus, cytokine receptors provide both survival and trophic/differentiative signals with varying degrees of redundancy that are required for differentiation of signaled DP thymocytes into functionally mature CD8(+) T cells.

Restricting Zap70 expression to CD4+CD8+ thymocytes reveals a T cell receptor-dependent proofreading mechanism controlling the completion of positive selection

Although T cell receptor (TCR) signals are essential for intrathymic T cell-positive selection, it remains controversial whether they only serve to initiate this process, or whether they are required throughout to promote thymocyte differentiation and survival. To address this issue, we have devised a novel approach to interfere with thymocyte TCR signaling in a developmental stage-specific manner in vivo. We have reconstituted mice deficient for Zap70, a tyrosine kinase required for TCR signaling and normally expressed throughout T cell development, with a Zap70 transgene driven by the adenosine deaminase (ADA) gene enhancer, which is active in CD4(+)CD8(+) thymocytes but inactive in CD4(+) or CD8(+) single-positive (SP) thymocytes. In such mice, termination of Zap70 expression impaired TCR signal transduction and arrested thymocyte development after the initiation, but before the completion, of positive selection. Arrested thymocytes had terminated Rag gene expression and up-regulated TCR and Bcl-2 expression, but failed to differentiate into mature CD4 or CD8 SP thymocytes, to be rescued from death by neglect or to sustain interleukin 7R alpha expression. These observations identify a TCR-dependent proofreading mechanism that verifies thymocyte TCR specificity and differentiation choices before the completion of positive selection.

CD3 delta establishes a functional link between the T cell receptor and CD8

T cells expressing T cell receptor (TCR) complexes that lack CD3 delta, either due to deletion of the CD3 delta gene, or by replacement of the connecting peptide of the TCR alpha chain, exhibit severely impaired positive selection and TCR-mediated activation of CD8 single-positive T cells. Because the same defects have been observed in mice expressing no CD8 beta or tailless CD8 beta, we examined whether CD3 delta serves to couple TCR.CD3 with CD8. To this end we used T cell hybridomas and transgenic mice expressing the T1 TCR, which recognizes a photoreactive derivative of the PbCS 252-260 peptide in the context of H-2K(d). We report that, in thymocytes and hybridomas expressing the T1 TCR.CD3 complex, CD8 alpha beta associates with the TCR. This association was not observed on T1 hybridomas expressing only CD8 alpha alpha or a CD3 delta(-) variant of the T1 TCR. CD3 delta was selectively co-immunoprecipitated with anti-CD8 antibodies, indicating an avid association of CD8 with CD3 delta. Because CD8 alpha beta is a raft constituent, due to this association a fraction of TCR.CD3 is raft-associated. Cross-linking of these TCR-CD8 adducts results in extensive TCR aggregate formation and intracellular calcium mobilization. Thus, CD3 delta couples TCR.CD3 with raft-associated CD8, which is required for effective activation and positive selection of CD8(+) T cells.

Positive selection by the pre-TCR yields mature CD8+ T cells

It has been of much interest whether there is functional redundancy between the constitutively signaling pre-Talpha/TCRbeta (pre-TCR) and ligated TCRalphabeta complexes, which independently operate the two distinct checkpoints during thymocyte development, i.e., the pre-TCR involved in beta-selection at the CD4(-)CD8(-) double-negative stage and the TCRalphabeta being crucial for positive/negative selection at the CD4(+)CD8(+) double-positive stage. We found that the pre-TCR expressed on double-positive cells in TCRalpha-deficient (TCRalpha(-/-)) mice produced a small number of mature CD8(+) T cells. Surprisingly, when pre-Talpha was overexpressed, resulting in augmentation of pre-TCR expression, there was a striking increase of the CD8(+) T cells. In addition, even in the absence of up-regulation of pre-TCR expression, a similar increase of CD8(+) T cells was also observed in TCRalpha(-/-) mice overexpressing Egr-1, which lowers the threshold of signal strength required for positive selection. In sharp contrast, the CD8(+) T cells drastically decreased in the absence of pre-Talpha on a TCRalpha(-/-) background. Thus, the pre-TCR appears to functionally promote positive selection of CD8(+) T cells. The biased production of CD8(+) T cells via the pre-TCR might also support the potential involvement of signal strength in CD4/CD8 lineage commitment.

New perspectives on a developmental dilemma: the kinetic signaling model and the importance of signal duration for the CD4/CD8 lineage decision

Double-positive thymocytes are short-lived bipotential cells whose developmental fate is determined by the specificity of their TCRs. A relatively small number of double-positive thymocytes undergo positive selection in the thymus and these are signaled to differentiate either into CD4(+) or CD8(+) mature T cells. The mechanism by which double-positive thymocytes determine their appropriate CD4/CD8 fate has been the subject of intense theoretical debate and rigorous experimental analysis. In the last year, 'signal duration' has been offered as a replacement for 'signal strength' as a major determinant of the CD4/CD8 decision, a deceptively minor refinement that requires a major change in our understanding of how signaled double-positive thymocytes differentiate into mature T cells. Indeed, the kinetic signaling model provides a radically new perspective on the mechanism by which the CD4/CD8 lineage decision is made.

Deletion of the CD4 silencer element supports a stochastic mechanism of thymocyte lineage commitment

The mechanism of T cell lineage commitment remains controversial; to examine it we deleted the CD4-silencer element in the germ line of a mouse using a combination of gene targeting and Cre/LoxP-mediated recombination. We found that these mice were unable to extinguish CD4 expression either in immature thymocytes or mature CD8+ cytotoxic T cells (CTLs), which resulted in the development of major histocompatibility complex class II-restricted double-positive CTLs in the periphery. This finding strongly supports a stochastic over an instructive mechanism of coreceptor down-regulation.