Regulation of T cell receptor expression in immature CD4+CD8+ thymocytes by p56lck tyrosine kinase: basis for differential signaling by CD4 and CD8 in immature thymocytes expressing both coreceptor molecules

The kinase occupancy of T cell coreceptors reconsidered

The sensitivity of the αβ T cell receptor (TCR) is enhanced by the coreceptors CD4 and CD8αβ, which are expressed primarily by cells of the helper and cytotoxic T cell lineages, respectively. The coreceptors bind to major histocompatibility complex (MHC) molecules and associate intracellularly with the Src-family kinase Lck, which catalyzes TCR phosphorylation during receptor triggering. Although coreceptor/kinase occupancy was initially believed to be high, a recent study suggested that most coreceptors exist in an Lck-free state, and that this low occupancy helps to effect TCR antigen discrimination. Here, using the same method, we found instead that the CD4/Lck interaction was stoichiometric (~100%) and that the CD8αβ/Lck interaction was substantial (~60%). We confirmed our findings in live cells using fluorescence cross-correlation spectroscopy (FCCS) to measure coreceptor/Lck codiffusion in situ. After introducing structurally guided mutations into the intracellular domain of CD4, we used FCCS to also show that stoichiometric coupling to Lck required an amphipathic α-helix present in CD4 but not CD8α. In double-positive cells expressing equal numbers of both coreceptors, but limiting amounts of kinase, CD4 outcompeted CD8αβ for Lck. In T cells, TCR signaling induced CD4/Lck oligomerization but did not affect the high levels of CD4/Lck occupancy. These findings help settle the question of kinase occupancy and suggest that the binding advantages that CD4 has over CD8 could be important when Lck levels are limiting.

MHC-independent αβT cells: Lessons learned about thymic selection and MHC-restriction

Understanding the generation of an MHC-restricted T cell repertoire is the cornerstone of modern T cell immunology. The unique ability of αβT cells to only recognize peptide antigens presented by MHC molecules but not conformational antigens is referred to as MHC restriction. How MHC restriction is imposed on a very large T cell receptor (TCR) repertoire is still heavily debated. We recently proposed the selection model, which posits that newly re-arranged TCRs can structurally recognize a wide variety of antigens, ranging from peptides presented by MHC molecules to native proteins like cell surface markers. However, on a molecular level, the sequestration of the essential tyrosine kinase Lck by the coreceptors CD4 and CD8 allows only MHC-restricted TCRs to signal. In the absence of Lck sequestration, MHC-independent TCRs can signal and instruct the generation of mature αβT cells that can recognize native protein ligands. The selection model thus explains how only MHC-restricted TCRs can signal and survive thymic selection. In this review, we will discuss the genetic evidence that led to our selection model. We will summarize the selection mechanism and structural properties of MHC-independent TCRs and further discuss the various non-MHC ligands we have identified.

Negative Regulation of Zap70 by Lck Forms the Mechanistic Basis of Differential Expression in CD4 and CD8 T Cells

Lck and Zap70, two non-receptor tyrosine kinases, play a crucial role in the regulation of membrane proximal TCR signaling critical for thymic selection, CD4/CD8 lineage choice and mature T cell function. Signal initiation upon TCR/CD3 and peptide/MHC interaction induces Lck-mediated phosphorylation of CD3 ITAMs. This is necessary for Zap70 recruitment and its phosphorylation by Lck leading to full Zap70 activation. In its native state Zap70 maintains a closed conformation creating an auto-inhibitory loop, which is relieved by Lck-mediated phosphorylation of Y315/Y319. Zap70 is differentially expressed in thymic subsets and mature T cells with CD8 T cells expressing the highest amount compared to CD4 T cells. However, the mechanistic basis of differential Zap70 expression in thymic subsets and mature T cells is not well understood. Here, we show that Zap70 is degraded relatively faster in DP and mature CD4 T cells compared to CD8 T cells, and inversely correlated with relative level of activated Zap70. Importantly, we found that Zap70 expression is negatively regulated by Lck activity: augmented Lck activity resulting in severe diminution in total Zap70. Moreover, Lck-mediated phosphorylation of Y315/Y319 was essential for Zap70 degradation. Together, these data shed light on the underlying mechanism of Lck-mediated differential modulation of Zap70 expression in thymic subsets and mature T cells.

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

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

CD146 bound to LCK promotes T cell receptor signaling and antitumor immune responses in mice

Initiation of T cell receptor (TCR) signaling involves the activation of the tyrosine kinase LCK; however, it is currently unclear how LCK is recruited and activated. Here, we have identified the membrane protein CD146 as an essential member of the TCR network for LCK activation. CD146 deficiency in T cells substantially impaired thymocyte development and peripheral activation, both of which depend on TCR signaling. CD146 was found to directly interact with the SH3 domain of coreceptor-free LCK via its cytoplasmic domain. Interestingly, we found CD146 to be present in both monomeric and dimeric forms in T cells, with the dimerized form increasing after TCR ligation. Increased dimerized CD146 recruited LCK and promoted LCK autophosphorylation. In tumor models, CD146 deficiency dramatically impaired the antitumor response of T cells. Together, our data reveal an LCK activation mechanism for TCR initiation. We also underscore a rational intervention based on CD146 for tumor immunotherapy.

Canonical T cell receptor docking on peptide-MHC is essential for T cell signaling

T cell receptor (TCR) recognition of peptide-major histocompatibility complexes (pMHCs) is characterized by a highly conserved docking polarity. Whether this polarity is driven by recognition or signaling constraints remains unclear. Using "reversed-docking" TCRβ-variable (TRBV) 17⁺ TCRs from the naïve mouse CD8⁺ T cell repertoire that recognizes the H-2D^b-NP₃₆₆ epitope, we demonstrate that their inability to support T cell activation and in vivo recruitment is a direct consequence of reversed docking polarity and not TCR-pMHCI binding or clustering characteristics. Canonical TCR-pMHCI docking optimally localizes CD8/Lck to the CD3 complex, which is prevented by reversed TCR-pMHCI polarity. The requirement for canonical docking was circumvented by dissociating Lck from CD8. Thus, the consensus TCR-pMHC docking topology is mandated by T cell signaling constraints.

Classical MHC expression by DP thymocytes impairs the selection of non-classical MHC restricted innate-like T cells

Conventional T cells are selected by peptide-MHC expressed by cortical epithelial cells in the thymus, and not by cortical thymocytes themselves that do not express MHC I or MHC II. Instead, cortical thymocytes express non-peptide presenting MHC molecules like CD1d and MR1, and promote the selection of PLZF⁺ iNKT and MAIT cells, respectively. Here, we report an inducible class-I transactivator mouse that enables the expression of peptide presenting MHC I molecules in different cell types. We show that MHC I expression in DP thymocytes leads to expansion of peptide specific PLZF⁺ innate-like (PIL) T cells. Akin to iNKT cells, PIL T cells differentiate into three functional effector subsets in the thymus, and are dependent on SAP signaling. We demonstrate that PIL and NKT cells compete for a narrow niche, suggesting that the absence of peptide-MHC on DP thymocytes facilitates selection of non-peptide specific lymphocytes.

Lck bound to coreceptor is less active than free Lck

Src family kinase Lck plays critical roles during T cell development and activation, as it phosphorylates the TCR/CD3 complex to initiate TCR signaling. Lck is present either in coreceptor-bound or coreceptor-unbound (free) forms, and we here present evidence that the two pools of Lck have different molecular properties. We discovered that the free Lck fraction exhibited higher mobility than CD8α-bound Lck in OT-I T hybridoma cells. The free Lck pool showed more activating Y394 phosphorylation than the coreceptor-bound Lck pool. Consistent with this, free Lck also had higher kinase activity, and free Lck mediated higher T cell activation as compared to coreceptor-bound Lck. Furthermore, the coreceptor-Lck coupling was independent of TCR activation. These findings give insights into the initiation of TCR signaling, suggesting that changes in coreceptor-Lck coupling constitute a mechanism for regulation of T cell sensitivity.

Single-cell transcriptomics identifies multiple pathways underlying antitumor function of TCR- and CD8αβ-engineered human CD4+ T cells

Transgenic coexpression of a class I-restricted tumor antigen-specific T cell receptor (TCR) and CD8αβ (TCR8) redirects antigen specificity of CD4+ T cells. Reinforcement of biophysical properties and early TCR signaling explain how redirected CD4+ T cells recognize target cells, but the transcriptional basis for their acquired antitumor function remains elusive. We, therefore, interrogated redirected human CD4+ and CD8+ T cells by single-cell RNA sequencing and characterized them experimentally in bulk and single-cell assays and a mouse xenograft model. TCR8 expression enhanced CD8+ T cell function and preserved less differentiated CD4+ and CD8+ T cells after tumor challenge. TCR8+CD4+ T cells were most potent by activating multiple transcriptional programs associated with enhanced antitumor function. We found sustained activation of cytotoxicity, costimulation, oxidative phosphorylation- and proliferation-related genes, and simultaneously reduced differentiation and exhaustion. Our study identifies molecular features of TCR8 expression that can guide the development of enhanced immunotherapies.

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.

What Happens in the Thymus Does Not Stay in the Thymus: How T Cells Recycle the CD4+-CD8+ Lineage Commitment Transcriptional Circuitry To Control Their Function

MHC-restricted CD4(+) and CD8(+) T cells are at the core of most adaptive immune responses. Although these cells carry distinct functions, they arise from a common precursor during thymic differentiation, in a developmental sequence that matches CD4 and CD8 expression and functional potential with MHC restriction. Although the transcriptional control of CD4(+)-CD8(+) lineage choice in the thymus is now better understood, less was known about what maintains the CD4(+) and CD8(+) lineage integrity of mature T cells. In this review, we discuss the mechanisms that establish in the thymus, and maintain in postthymic cells, the separation of these lineages. We focus on recent studies that address the mechanisms of epigenetic control of Cd4 expression and emphasize how maintaining a transcriptional circuitry nucleated around Thpok and Runx proteins, the key architects of CD4(+)-CD8(+) lineage commitment in the thymus, is critical for CD4(+) T cell helper functions.

Timing and duration of MHC I positive selection signals are adjusted in the thymus to prevent lineage errors

Major histocompatibility complex class I (MHC I) positive selection of CD8⁺ T cells in the thymus requires that T cell antigen receptor (TCR) signaling end in time for cytokines to induce Runx3d, the CD8-lineage transcription factor. We examined the time required for these events and found that the overall duration of positive selection was similar for all CD8⁺ thymocytes in mice, despite markedly different TCR signaling times. Notably, prolonged TCR signaling times were counter-balanced by accelerated Runx3d induction by cytokines and accelerated differentiation into CD8⁺ T cells. Consequently, lineage errors did not occur except when MHC I-TCR signaling was so prolonged that the CD4-lineage-specifying transcription factor ThPOK was expressed, preventing Runx3d induction. Thus, our results identify a compensatory signaling mechanism that prevents lineage-fate errors by dynamically modulating Runx3d induction rates during MHC I positive selection.

Regulation of SATB1 during thymocyte development by TCR signaling

T lymphocyte development and differentiation is a multi-step process that begins in the thymus and completed in the periphery. Sequential development of thymocytes is dependent on T cell receptor (TCR) signaling and an array of transcription factors. In this study we show that special AT-rich binding protein 1 (SATB1), a T lineage-enriched chromatin organizer and regulator, is induced in response to TCR signaling during early thymocyte development. SATB1 expression profile coincides with T lineage commitment and upregulation of SATB1 correlates with positive selection of thymocytes. CD4 thymocytes exhibit a characteristic bimodal expression pattern that corresponds to immature and mature CD4 thymocytes. We also demonstrate that GATA3, the key transcriptional regulator of αβ T cells positively regulates SATB1 expression in thymocytes suggesting an important role for SATB1 during T cell development.

Codification of bidentate pMHC interaction with TCR and its co-receptor

A 1983 Immunology Today rostrum hypothesized that each T cell has two recognition units: a T cell receptor (TCR) complex, which binds antigen associated with a polymorphic region of a MHC molecule (pMHC), and a CD4 or CD8 molecule that binds to a conserved region of that same MHC gene product (class II or I, respectively). Structural biology has since precisely revealed those bidentate pMHC interactions. TCRαβ ligates the membrane-distal antigen-binding MHC platform, whereas CD8 clamps a membrane-proximal MHCI α3 domain loop and CD4 docks to a hydrophobic crevice between MHCII α2 and β2 domains. Here, we review how MHC class-restricted binding impacts signaling and lineage commitment, discussing TCR force-driven conformational transitions that may optimally expose the co-receptor docking site on MHC.

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.

Coreceptor scanning by the T cell receptor provides a mechanism for T cell tolerance

In the thymus, high-affinity, self-reactive thymocytes are eliminated from the pool of developing T cells, generating central tolerance. Here, we investigate how developing T cells measure self-antigen affinity. We show that very few CD4 or CD8 coreceptor molecules are coupled with the signal-initiating kinase, Lck. To initiate signaling, an antigen-engaged T cell receptor (TCR) scans multiple coreceptor molecules to find one that is coupled to Lck; this is the first and rate-limiting step in a kinetic proofreading chain of events that eventually leads to TCR triggering and negative selection. MHCII-restricted TCRs require a shorter antigen dwell time (0.2 s) to initiate negative selection compared to MHCI-restricted TCRs (0.9 s) because more CD4 coreceptors are Lck-loaded compared to CD8. We generated a model (Lck come&stay/signal duration) that accurately predicts the observed differences in antigen dwell-time thresholds used by MHCI- and MHCII-restricted thymocytes to initiate negative selection and generate self-tolerance.

CD3ζ-based chimeric antigen receptors mediate T cell activation via cis- and trans-signalling mechanisms: implications for optimization of receptor structure for adoptive cell therapy

Chimeric antigen receptors (CARs) can mediate redirected lysis of tumour cells in a major histocompatibility complex (MHC)-independent manner, thereby enabling autologous adoptive T cell therapy for a variety of malignant neoplasms. Currently, most CARs incorporate the T cell receptor (TCR) CD3ζ signalling chain; however, the precise mechanisms responsible for CAR-mediated T cell activation are unclear. In this study, we used a series of immunoreceptor tyrosine-based activation motif (ITAM)-mutant and transmembrane-modified receptors to demonstrate that CARs activate T cells both directly via the antigen-ligated signalling chain and indirectly via associated chains within the TCR complex. These observations allowed us to generate new receptors capable of eliciting polyfunctional responses in primary human T cells. This work increases our understanding of CAR function and identifies new avenues for the optimization of CAR-based therapeutic interventions.

A novel Zap70 mutation with reduced protein stability demonstrates the rate-limiting threshold for Zap70 in T-cell receptor signalling

Loss of ζ-associated protein 70 (Zap70) results in severe immunodeficiency in humans and mice because of the critical role of Zap70 in T-cell receptor (TCR) signalling. Here we describe a novel mouse strain generated by N-ethyl-N-nitrosourea mutagenesis, with the reduced protein stability (rps) mutation in Zap70. The A243V rps mutation resulted in decreased Zap70 protein and a reduced duration of TCR-induced calcium responses, equivalent to that induced by a 50% decrease in catalytically active Zap70. The reduction of signalling through Zap70 was insufficient to substantially perturb thymic differentiation of conventional CD4 and CD8 T cells, although Foxp3(+) regulatory T cells demonstrated altered thymic production and peripheral homeostasis. Despite the mild phenotype, the Zap70(A243V) variant lies just above the functional threshold for TCR signalling competence, as T cells relying on only a single copy of the Zap70(rps) allele for TCR signalling demonstrated no intracellular calcium response to TCR stimulation. This addition to the Zap70 allelic series indicates that a rate-limiting threshold for Zap70 protein levels exists at which signalling capacity switches from nearly intact to effectively null.

LAPTM5 promotes lysosomal degradation of intracellular CD3ζ but not of cell surface CD3ζ

The lysosomal protein LAPTM5 has been shown to negatively regulate cell surface T cell receptor (TCR) expression and T-cell activation by promoting CD3ζ degradation in lysosomes, but the mechanism remains largely unknown. Here we show that LAPTM5 promotes lysosomal translocation of intracellular CD3ζ but not of the cell surface CD3ζ associated with the mature TCR complex. Kinetic analysis of the subcellular localization of the newly synthesized CD3ζ suggests that LAPTM5 targets CD3ζ in the Golgi apparatus and promotes its lysosomal translocation. Consistently, a Golgi-localizing mutant CD3ζ can be transported to and degraded in the lysosome by LAPTM5. A CD3ζ YF mutant in which all six tyrosine residues in the immunoreceptor tyrosine-based activation motif are mutated to phenylalanines is degraded as efficiently as is wild type CD3ζ, further suggesting that TCR signaling-triggered tyrosine phosphorylation of CD3ζ is dispensable for LAPTM5-mediated degradation. Previously, Src-like adapter protein (SLAP) and E3 ubiquitin ligase c-Cbl have been shown to mediate the ubiquitination of CD3ζ in the internalized TCR complex and its subsequent lysosomal degradation. We show that LAPTM5 and SLAP/c-Cbl function in distinct genetic pathways to negatively regulate TCR expression. Collectively, these results suggest that CD3ζ can be degraded by two pathways: SLAP/c-Cbl, which targets internalized cell surface CD3ζ dependent on TCR signaling, and LAPTM5, which targets intracellular CD3ζ independent of TCR signaling.

Strict Major Histocompatibility Complex Molecule Class-Specific Binding by Co-Receptors Enforces MHC-Restricted αβ TCR Recognition during T Lineage Subset Commitment

Since the discovery of co-receptor dependent αβTCR recognition, considerable effort has been spent on elucidating the basis of CD4 and CD8 lineage commitment in the thymus. The latter is responsible for generating mature CD4 helper and CD8αβ cytotoxic T cell subsets. Although CD4⁺ and CD8⁺ T cell recognition of peptide antigens is known to be MHC class II- and MHC class I-restricted, respectively, the mechanism of single positive (SP) thymocyte lineage commitment from bipotential double-positive (DP) progenitors is not fully elucidated. Classical models to explain thymic CD4 vs. CD8 fate determination have included a stochastic selection model or instructional models. The latter are based either on strength of signal or duration of signal impacting fate. More recently, differential co-receptor gene imprinting has been shown to be involved in expression of transcription factors impacting cytotoxic T cell development. Here, we address commitment from a structural perspective, focusing on the nature of co-receptor binding to MHC molecules. By surveying 58 MHC class II and 224 MHC class I crystal structures in the Protein Data Bank, it becomes clear that CD4 cannot bind to MHC I molecules, nor can CD8αβ or CD8αα bind to MHC II molecules. Given that the co-receptor delivers Lck to phosphorylate exposed CD3 ITAMs within a peptide/MHC (pMHC)-ligated TCR complex to initiate cell signaling, this strict co-receptor recognition fosters MHC class-restricted SP thymocyte lineage commitment at the DP stage even though both co-receptors are expressed on a single cell. In short, the binding preference of an αβTCR for a peptide complexed with an MHC molecule dictates which co-receptor subsequently binds, thereby supporting development of that subset lineage. How function within the lineage is linked further to biopotential fate determination is discussed.

Induction of T-cell development by Delta-like 4-expressing fibroblasts

The thymus provides a unique environment for the induction of T-cell lineage commitment and differentiation, which is predicted by specific Notch ligand-receptor interactions on epithelial cells and lymphoid progenitors, respectively. Accordingly, a bone marrow-derived stromal cell line (OP9) ectopically expressing the Notch ligand Delta-like 1 (Dll1) or Dll4 (OP9-DL1 and OP9-DL4, respectively) gains the ability to recapitulate thymus-like function, supporting T-cell differentiation of both mouse and human progenitors. In this study, we extend these findings by demonstrating that, unlike the NIH3T3 cell line, mouse primary fibroblasts made to express Dll4 (mFibro-DL4) acquire the capacity to promote and support T-cell development from hematopoietic stem cells (HSCs) into TCRαβ(+), CD4(+) and CD8(+) T-lineage cells. However, mFibro-DL4 cells showed a lower efficiency of T-cell generation than OP9-DL4 cells did. Nevertheless, progenitor T-cells (CD117(+) CD44(+) CD25(+)) generated in HSC/mFibro-DL4 co-cultures, when intravenously transferred into immunodeficient (Rag2(-/-) γc(-/-)) mice, home to the thymus, undergo differentiation, and give rise to mature T-cells that go on to populate the periphery. Surprisingly, primary human fibroblast cells expressing Dll4 showed very low efficiency in supporting human T-lineage differentiation, which could not be improved by blocking myelopoiesis. Nevertheless, mFibro-DL4 cells could support human T-cell lineage differentiation. Our results provide a functional framework for the induction of T-cell development using easily accessible fibroblasts made to express Dll4. These cells, which are amenable for in vitro applications, can be further utilized in the design of individualized systems for T-cell production, with implications for the treatment of immunodeficiencies.

Self-peptides in TCR repertoire selection and peripheral T cell function

The vertebrate antigen receptors are anticipatory in their antigen recognition and display a vast diversity. Antigen receptors are assembled through V(D)J recombination, in which one of each Variable, (Diverse), and Joining gene segment are randomly utilized and recombined. Both gene rearrangement and mutational insertion are generated through randomness; therefore, the process of antigen receptors generation requires a rigorous testing system to select every receptor which is useful to recognize foreign antigens, but which would cause no harm to self cells. In the case of T cell receptors (TCR), such a quality control responsibility rests in thymic positive and negative selection. In this review, we focus on the critical involvement of self-peptides in the generation of a T cell repertoire, discuss the role of T cell thymic development in shaping the specificity of TCR repertoire, and directing function fitness of mature T cells in periphery. Here, we consider thymic positive selection to be not merely a one-time maturing experience for an individual T cell, but a life-long imprinting which influences the function of each individual T cell in periphery.

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.

ZAP-70: an essential kinase in T-cell signaling

ZAP-70 is a cytoplasmic protein tyrosine kinase that plays a critical role in the events involved in initiating T-cell responses by the antigen receptor. Here we review the structure of ZAP-70, its regulation, its role in development and in disease. We also describe a model experimental system in which ZAP-70 function can be interrupted by a small chemical inhibitor.

Two receptors, two kinases, and T cell lineage determination

The T cell antigen receptor (TCR) serves as a paradigm for how membrane receptors transmit signals to the cytoplasm because it controls many aspects of T cell differentiation and function by detecting atom-sized variations in the quality of the ligand that is recognized. The mechanisms that underlie the different signaling outcomes are unclear. Studies that suggest a ligand-tailored, qualitatively different signal are confronted with evidence that favors a quantitative model, and studies of TCR-dependent T cell differentiation in the thymus are no exception. Mature T cells with an alphabeta TCR are classified according to two major distinct subsets based on the mutually exclusive presence of the co-receptors CD4 and CD8, which play essential roles in recognition of the major histocompatibility complex (MHC) class II and I ligands, respectively, and in the recruitment of the tyrosine kinase Lck to the TCR complex. Mature CD4(+) and CD8(+) T cells derive from a common precursor in the thymus, a double-positive (DP) thymocyte, which has both co-receptors. Early signaling models suggested that the differential capacity of CD4 and CD8 to recruit Lck to the TCR underlay lineage decision. A study now shows that differentiation into the CD8(+) lineage requires the TCR-induced increased abundance of the tyrosine kinase zeta chain-associated protein kinase of 70 kD (Zap70). This finding, together with the known importance of Lck in the determination of CD4(+) and CD8(+) lineages, enables us to propose that a balance between the activation of these two kinases by the TCR determines lineage decisions.

Cutting edge: inhibitory effects of CD4 and CD8 on T cell activation induced by high-affinity noncognate ligands

It has been proposed that MHC restriction during thymocyte selection is controlled by coreceptor (CD4 or CD8) sequestration of the signaling molecule Lck. We explored this model as a mechanism for preventing peripheral T cell activation due to non-MHC ligand cross-reactivities of TCRs. TCRs that have a range of affinities for a class I MHC ligand were transduced into a T cell hybridoma in the absence or presence of coreceptors. High and intermediate affinity TCRs (K(D) = 17 and 540 nM) did not require CD8 for T cell activity, but CD4 acted as a potent inhibitor of the intermediate affinity TCR. These and other findings support the view that even high-affinity TCR:ligand interactions can be influenced by coreceptor sequestration of Lck. Thus, CD4 and CD8 act as "coreceptor inhibitors" to maintain appropriate TCR-mediated MHC restriction in peripheral T cell activity.

Homodimerization and heterodimerization of minimal zinc(II)-binding-domain peptides of T-cell proteins CD4, CD8alpha, and Lck

Metal-mediated protein oligomerization is an emerging mode of protein-protein interaction. The C-terminal cytosolic domains of T-cell coreceptors CD4 and CD8alpha form zinc-bridged heterodimers with the N-terminal region of the kinase Lck, with each protein contributing two cysteinate ligands to the complex. Using size exclusion chromatography, (1)H NMR, and UV/visible absorption spectroscopy with cobalt(II) as a spectroscopic probe, we demonstrate that small peptides derived from these regions form metal-bridged heterodimers but also homodimers, in contrast to previous reports. The Lck-CD4 and Lck-CD8alpha cobalt(II)-bridged heterodimer complexes are more stable than the corresponding (Lck)(2)cobalt(II) complex by factors of 11 +/- 4 and 22 +/- 9, respectively. These studies were aided by the discovery that cobalt(II) complexes with a cobalt(II)(-Cys-X-X-Cys-)(-Cys-X-Cys-) chromophore show unusual optical spectra with one component of the visible d-d ((4)A(2)-to-(4)T(1)(P)) transition red-shifted and well separated from the other components. These results provide insights into the basis of specificity of metal-bridged complex formation and on the potential biological significance of metal-bridged homodimers in T-cells.

Expression of fully assembled TCR-CD3 complex on double positive thymocytes: synergistic role for the PRS and ER retention motifs in the intra-cytoplasmic tail of CD3epsilon

TCR expression on double-positive (DP) thymocytes is a prerequisite for thymic selection that results in the generation of mature CD4(+) and CD8(+) single-positive T cells. TCR is expressed at very low level on preselection DP thymocytes and is dramatically up-regulated on positively selected thymocytes. However, mechanism governing TCR expression on developing thymocytes is not understood. In the present report, we demonstrate that the intra-cytoplasmic (IC) domain of CD3epsilon plays a critical role in regulating TCR expression on DP thymocytes. We provide genetic and biochemical evidence to show that the CD3epsilon IC domain mutations result in elevated expression of fully assembled TCR on DP thymocytes. We also demonstrate that TCR up-regulation on DP thymocytes in these transgenic mice occurs in a ligand-independent manner. Further, we show that the proline-rich sequence and endoplasmic reticulum (ER) retention motifs in the IC domain of CD3epsilon play synergistic role in regulating TCR surface expression on DP thymocytes.

A new function for LAT and CD8 during CD8-mediated apoptosis that is independent of TCR signal transduction

The majority (>95%) of thymocytes undergo apoptosis during selection in the thymus. Several mechanisms have been proposed to explain how apoptosis of thymocytes that are not positively selected occurs; however, it is unknown whether thymocytes die purely by "neglect" or whether signaling through a cell-surface receptor initiates an apoptotic pathway. We have previously demonstrated that on double positive thymocytes the ligation of CD8 in the absence of TCR engagement results in apoptosis and have postulated this is a mechanism to remove thymocytes that have failed positive selection. On mature single positive T cells CD8 acts as a co-receptor to augment signaling through the TCR that is dependent on the phosphorylation of the adaptor protein, linker for activation of T cells (LAT). Here, we show that during CD8-mediated apoptosis of double positive thymocytes there is an increase in the association of CD8 with LAT and an increase in LAT tyrosine phosphorylation. Decreasing LAT expression and mutation of tyrosine residues of LAT reduced apoptosis upon crosslinking of CD8. Our results identify novel functions for both CD8 and LAT that are independent of TCR signal transduction and suggest a mechanism for signal transduction leading to apoptosis upon CD8 crosslinking.

The structure, regulation, and function of ZAP-70

The tyrosine ZAP-70 (zeta-associated protein of 70 kDa) kinase plays a critical role in activating many downstream signal transduction pathways in T cells following T-cell receptor (TCR) engagement. The importance of ZAP-70 is evidenced by the severe combined immunodeficiency that occurs in ZAP-70-deficient mice and humans. In this review, we describe recent analyses of the ZAP-70 crystal structure, revealing a complex regulatory mechanism of ZAP-70 activity, the differential requirements for ZAP-70 and spleen tyrosine kinase (SyK) in early T-cell development, as well as the role of ZAP-70 in chronic lymphocytic leukemia and autoimmunity. Thus, the critical importance of ZAP-70 in TCR signaling and its predominantly T-cell-restricted expression pattern make ZAP-70 an attractive drug target for the inhibition of pathological T-cell responses in disease.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

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.

The proline-rich sequence of CD3epsilon controls T cell antigen receptor expression on and signaling potency in preselection CD4+CD8+ thymocytes

Antigen recognition by T cell antigen receptors (TCRs) is thought to 'unmask' a proline-rich sequence (PRS) present in the CD3epsilon cytosolic segment, which allows it to trigger T cell activation. Using 'knock-in' mice with deletion of the PRS, we demonstrate here that elimination of the CD3epsilon PRS had no effect on mature T cell responsiveness. In contrast, in preselection CD4+CD8+ thymocytes, the CD3epsilon PRS acted together with the adaptor protein SLAP to promote CD3zeta degradation, thereby contributing to downregulation of TCR expression on the cell surface. In addition, analysis of CD4+CD8+ thymocytes of TCR-transgenic mice showed that the CD3epsilon PRS enhanced TCR sensitivity to weak ligands. Our results identify previously unknown functions for the evolutionarily conserved CD3epsilon PRS at the CD4+CD8+ developmental stage and suggest a rather limited function in mature T cells.

Impaired thymic selection in mice expressing altered levels of the SLP-76 adaptor protein

Intracellular signaling initiated by ligation of the TCR influences cell fate at multiple points during the lifespan of a T cell. This is especially evident during thymic selection, where the nature of TCR-dependent signaling helps to establish a MHC-restricted, self-tolerant T cell repertoire. The Src homology 2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76) adaptor protein is a required intermediate in multiple signaling pathways triggered by TCR engagement, several of which have been implicated in dictating the outcome of thymic selection (e.g., intracellular calcium flux and activation of ERK family MAPKs). To determine if thymocyte maturation and selection at later stages of development are sensitive to perturbations in SLP-76 levels, we analyzed these crucial events using several transgenic (Tg) lines of mice expressing altered levels of SLP-76 in the thymus. In Tg mice expressing low levels of SLP-76 in preselection thymocytes, the CD4:CD8 ratio in the thymus and spleen was skewed in a manner consistent with impaired selection and/or maturation of CD4+ thymocytes. Low SLP-76 expression also correlated with reduced CD5 expression on immature thymocytes, consistent with reduced TCR signaling potential. In contrast, reconstitution of SLP-76 at higher levels resulted in normal thymic CD5 expression and CD4:CD8 ratios in the thymus and periphery. It is curious that thymic deletion of TCR-Tg (HY) thymocytes was markedly impaired in both lines of Tg-reconstituted SLP-76-/- mice. Studies using chimeric mice indicate that the defect in deletion of HY+ thymocytes is intrinsic to the developing thymocyte, suggesting that maintenance of sufficient SLP-76 expression from the endogenous locus is a key element in the selection process.

'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.

Induction of autoimmune disease in CTLA-4-/- mice depends on a specific CD28 motif that is required for in vivo costimulation

CTLA-4-deficient mice develop a lethal autoimmune lymphoproliferative disorder that is strictly dependent on in vivo CD28 costimulation. Nevertheless, it is not known whether there is a specific site on the CD28 molecule that is required for induction of autoimmunity. Using CTLA-4-deficient mice expressing CD28 molecules with various point mutations in the CD28 cytosolic tail, the present study documents that in vivo costimulation for induction of autoimmune disease strictly requires an intact C-terminal proline motif that promotes lymphocyte-specific protein tyrosine kinase Lck binding to the CD28 cytosolic tail, because point mutations in C-terminal proline residues (Pro-187 and Pro-190) completely prevented disease induction. In contrast, in vivo costimulation for disease induction did not require either an intact YMNM motif or an intact N-terminal proline motif, which, respectively, promote phosphoinositide 3-kinase and IL2-inducible T cell kinase binding to the CD28 cytosolic tail. Thus, in vivo CD28 costimulation for induction of autoimmune disease is strictly and specifically dependent on an intact C-terminal proline motif that serves as a lymphocyte-specific protein tyrosine Lck kinase binding site in the CD28 cytosolic tail.

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.

Generation of T cell help through a MHC class I-restricted TCR

CD4+ T cells that are activated by a MHC class II/peptide encounter can induce maturation of APCs and promote cytotoxic CD8+ T cell responses. Unfortunately, the number of well-defined tumor-specific CD4+ T cell epitopes that can be exploited for adoptive immunotherapy is limited. To determine whether Th cell responses can be generated by redirecting CD4+ T cells to MHC class I ligands, we have introduced MHC class I-restricted TCRs into postthymic murine CD4+ T cells and examined CD4+ T cell activation and helper function in vitro and in vivo. These experiments indicate that Ag-specific CD4+ T cell help can be induced by the engagement of MHC class I-restricted TCRs in peripheral CD4+ T cells but that it is highly dependent on the coreceptor function of the CD8beta-chain. The ability to generate Th cell immunity by infusion of MHC class I-restricted Th cells may prove useful for the induction of tumor-specific T cell immunity in cases where MHC class II-associated epitopes are lacking.

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.

Developmental alterations in thymocyte sensitivity are actively regulated by MHC class II expression in the thymic medulla

Developing thymocytes are positively selected if they respond to self-MHC-peptide complexes, yet mature T cells are not activated by those same self-complexes. To avoid autoimmunity, positive selection must be followed by a period of maturation when the cellular response to TCR signals is altered. The mechanisms that mediate this postselection developmental tuning remain largely unknown. Specifically, it is unknown whether developmental tuning is a preprogrammed outcome of positive selection or if it is sensitive to ongoing interactions between the thymocyte and the thymic stroma. We probed the requirement for MHC class II-TCR interactions in postselection maturation by studying single positive (SP) CD4 thymocytes from K14/A(beta)(b) mice, in which CD4 T cells cannot interact with MHC class II in the thymic medulla. We report here that SP CD4 thymocytes must receive MHC class II signals to avoid hyperactive responses to TCR signals. This hyperactivity correlates with decreased expression of CD5; however, developmental tuning can occur independently of CD5, correlating instead with differences in the distribution of Lck. Thus, the maturation of postselection SP CD4 thymocytes is an active process mediated by ongoing interactions between the T cell and MHC class II molecules. This represents a novel mechanism by which the thymic medulla prevents autoreactivity.

Functional hierarchy of the N-terminal tyrosines of SLP-76

The adaptor protein Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) plays a central role in T cell activation and T cell development. SLP-76 has three functional modules: an acidic domain with three key tyrosines, a central proline-rich domain, and a C-terminal Src homology 2 domain. Of these, mutation of the three N-terminal tyrosines (Y112, Y128, and Y145) results in the most profound effects on T cell development and function. Y112 and Y128 associate with Vav and Nck, two proteins shown to be important for TCR-induced phosphorylation of proximal signaling substrates, Ca(2+) flux, and actin reorganization. Y145 has been shown to be important for optimal association of SLP-76 with inducible tyrosine kinase, a key regulator of T cell function. To investigate further the role of the phosphorylatable tyrosines of SLP-76 in TCR signaling, cell lines and primary T cells expressing SLP-76 with mutations in individual or paired tyrosine residues were analyzed. These studies show that Tyr(145) of SLP-76 is the most critical tyrosine for both T cell function in vitro and T cell development in vivo.

Inefficient cell spreading and cytoskeletal polarization by CD4+CD8+ thymocytes: regulation by the thymic environment

CD4(+)CD8(+) double-positive (DP) thymocytes express a lower level of surface TCR than do mature T cells or single-positive (SP) thymocytes. Regulation of the TCR on DP thymocytes appears to result from intrathymic signaling, as in vitro culture of these cells results in spontaneous TCR up-regulation. In this study, we examined cell spreading and cytoskeletal polarization responses that have been shown to occur in response to TCR engagement in mature T cells. Using DP thymocytes stimulated on lipid bilayers or nontransgenic thymocytes added to anti-CD3-coated surfaces, we found that cell spreading and polarization of the microtubule organizing center and the actin cytoskeleton were inefficient in freshly isolated DP thymocytes, but were dramatically enhanced after overnight culture. SP (CD4(+)) thymocytes showed efficient responses to TCR engagement, suggesting that releasing DP thymocytes from the thymic environment mimics some aspects of positive selection. The poor translation of a TCR signal to cytoskeletal responses could limit the ability of DP thymocytes to form stable contacts with APCs and may thereby regulate thymocyte selection during T cell development.

Peripheral CD8+CD25+ T lymphocytes from MHC class II-deficient mice exhibit regulatory activity

We characterized CD8(+) T cells constitutively expressing CD25 in mice lacking the expression of MHC class II molecules. We showed that these cells are present not only in the periphery but also in the thymus. Like CD4(+)CD25(+) T cells, CD8(+)CD25(+) T cells appear late in the periphery during ontogeny. Peripheral CD8(+)CD25(+) T cells from MHC class II-deficient mice also share phenotypic and functional features with regulatory CD4(+)CD25(+) T cells: in particular, they strongly express glucocorticoid-induced TNFR family-related gene, CTLA-4 and Foxp3, produce IL-10, and inhibit CD25(-) T cell responses to anti-CD3 stimulation through cell contacts with similar efficiency to CD4(+)CD25(+) T cells. However, unlike CD4(+)CD25(+) T cells CD8(+)CD25(+) T cells from MHC class II-deficient mice strongly proliferate and produce IFN-gamma in vitro in response to stimulation in the absence of exogenous IL-2.

CD4-CD8 lineage commitment: an inside view

The mechanism of CD4-CD8 lineage commitment, which ensures the correlation between T cell receptor specificity and adoption of the T killer or T helper phenotype, has long been the subject of intense debate. Various approaches are slowly elucidating the underlying molecular pathways. Analysis of the function of T cell receptor signaling (the 'top-down' approach) supports the view that differences in signal strength and/or duration 'instruct' alternative commitment. Analysis of the transcriptional regulation of the genes encoding CD4 and CD8 (the 'bottom-up' approach) has identified critical cis-acting elements and their interacting factors. Finally, identification of the transcription factor Th-POK as a central component of the CD4 lineage-determining pathway has provided a new starting point from which to unravel this intriguing process 'from the inside out'.

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.