Mature T cell reactivity altered by peptide agonist that induces positive selection

TCR tuning of T cell subsets

After selection in the thymus, the post-thymic T cell compartments comprise heterogenous subsets of naive and memory T cells that make continuous T cell receptor (TCR) contact with self-ligands bound to major histocompatibility complex (MHC) molecules. T cell recognition of self-MHC ligands elicits covert TCR signaling and is particularly important for controlling survival of naive T cells. Such tonic TCR signaling is tightly controlled and maintains the cells in a quiescent state to avoid autoimmunity. Here, we review how naive and memory T cells are differentially tuned and wired for TCR sensitivity to self and foreign ligands.

TCR Signal Strength and T Cell Development

Thymocyte selection involves the positive and negative selection of the repertoire of T cell receptors (TCRs) such that the organism does not suffer autoimmunity, yet has the benefit of the ability to recognize any invading pathogen. The signal transduced through the TCR is translated into a number of different signaling cascades that result in transcription factor activity in the nucleus and changes to the cytoskeleton and motility. Negative selection involves inducing apoptosis in thymocytes that express strongly self-reactive TCRs, whereas positive selection must induce survival and differentiation programs in cells that are more weakly self-reactive. The TCR recognition event is analog by nature, but the outcome of signaling is not. A large number of molecules regulate the strength of the TCR-derived signal at various points in the cascades. This review discusses the various factors that can regulate the strength of the TCR signal during thymocyte development.

PTPN2 attenuates T-cell lymphopenia-induced proliferation

When the peripheral T-cell pool is depleted, T cells undergo homoeostatic expansion. This expansion is reliant on the recognition of self-antigens and/or cytokines, in particular interleukin-7. The T cell-intrinsic mechanisms that prevent excessive homoeostatic T-cell responses and consequent overt autoreactivity remain poorly defined. Here we show that protein tyrosine phosphatase N2 (PTPN2) is elevated in naive T cells leaving the thymus to restrict homoeostatic T-cell proliferation and prevent excess responses to self-antigens in the periphery. PTPN2-deficient CD8(+) T cells undergo rapid lymphopenia-induced proliferation (LIP) when transferred into lymphopenic hosts and acquire the characteristics of antigen-experienced effector T cells. The enhanced LIP is attributed to elevated T-cell receptor-dependent, but not interleukin-7-dependent responses, results in a skewed T-cell receptor repertoire and the development of autoimmunity. Our results identify a major mechanism by which homoeostatic T-cell responses are tuned to prevent the development of autoimmune and inflammatory disorders.

Dynamic tuning of lymphocytes: physiological basis, mechanisms, and function

Dynamic tuning of cellular responsiveness as a result of repeated stimuli improves the ability of cells to distinguish physiologically meaningful signals from each other and from noise. In particular, lymphocyte activation thresholds are subject to tuning, which contributes to maintaining tolerance to self-antigens and persisting foreign antigens, averting autoimmunity and immune pathogenesis, but allowing responses to strong, structured perturbations that are typically associated with acute infection. Such tuning is also implicated in conferring flexibility to positive selection in the thymus, in controlling the magnitude of the immune response, and in generating memory cells. Additional functional properties are dynamically and differentially tuned in parallel via subthreshold contact interactions between developing or mature lymphocytes and self-antigen-presenting cells. These interactions facilitate and regulate lymphocyte viability, maintain their functional integrity, and influence their responses to foreign antigens and accessory signals, qualitatively and quantitatively. Bidirectional tuning of T cells and antigen-presenting cells leads to the definition of homeostatic set points, thus maximizing clonal diversity.

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.

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.

Signal transduction via the T cell antigen receptor in naïve and effector/memory T cells

T cells play an indispensable role in immune defense against infectious agents, but can also be pathogenic. These T cells develop in the thymus, are exported into the periphery as naïve cells and participate in immune responses. Upon recognition of antigen, they are activated and differentiate into effector and memory T cells. While effector T cells carry out the function of the immune response, memory T cells can last up to the life time of the individual, and are activated by subsequent antigenic exposure. Throughout this life cycle, the T cell uses the same receptor for antigen, the T cell Receptor, a complex multi-subunit receptor. Recognition of antigen presented by peptide/MHC complexes on antigen presenting cells unleashes signaling pathways that control T cell activation at each stage. In this review, we discuss the signals regulated by the T cell receptor in naïve and effector/memory T cells.

Subcellular distribution of Lck during CD4 T-cell maturation in the thymic medulla regulates the T-cell activation threshold

Mature peripheral T cells respond to foreign but not to self-antigens. During development in the thymus, deletion of high-affinity self-reactive immature thymocytes contributes to tolerance of mature T cells. However, double-positive thymocytes are positively selected to survive if they respond to self-peptide-MHC complexes; thus, there must be mechanisms to prevent overt reactivity to those same complexes in the periphery. "Developmental tuning" is the active process through which T-cell receptor (TCR)-associated signaling pathways of single-positive (SP) thymocytes are attenuated to respond appropriately to self-peptide-MHC complexes in the periphery. We previously showed that MHC class II expression in the thymic medulla was necessary to tune CD4(+) SP (CD4 SP) thymocytes. CD4 SP thymocytes from mice lacking medullary MHC class II expression had inappropriately enhanced proximal TCR signaling to low-affinity self-ligands that was associated with altered cellular distribution of the tyrosine kinase Lck. Now, we report that activation of both tuned and untuned CD4 SP thymocytes is Lck-dependent. Untuned CD4 SP cells contain a pool of Lck with increased basal phosphorylation that is not associated with the CD4 coreceptor. Phosphorylation of this pool of Lck decreases with tuning. Immunogold transmission electron microscopy of membrane sheets permitted direct visualization of Lck. In the absence of tuning, a significant proportion of Lck and the TCR subunit CD3ζ are expressed on the same protein island; this close association of Lck and the TCR probably explains the enhanced activation of untuned CD4 SP cells. Thus, changes in membrane topography during thymic maturation determine the set point for TCR responsiveness.

How the TCR balances sensitivity and specificity for the recognition of self and pathogens

The T cell repertoire is generated during thymic development in preparation for the response to antigens from pathogens. The T cell repertoire is shaped by positive selection, which requires recognition by the T cell antigen receptor (TCR) of complexes of self peptide and major histocompatibility complex proteins (self-pMHC) with low affinity, and negative selection, which eliminates T cells with TCRs that recognize self-pMHC with high affinity. This generates a repertoire with low affinity for self-pMHC but high affinity for foreign antigens. The TCR must successfully engage both of these ligands for development, homeostasis and immune responses. This review discusses mechanisms underlying the interaction of the TCR with peptide-major histocompatibility complex ligands of varying affinity and highlights signaling mechanisms that enable the TCR to generate different responses to very distinct ligands.

Normal T cell homeostasis: the conversion of naive cells into memory-phenotype cells

Weak T cell antigen receptor (TCR) signals from contact with self ligands act in synergy with antiapoptotic signals induced by interleukin 7 (IL-7) to promote the survival of naive T cells in a resting state. The amount of background TCR signaling in naive T cells is set by post-thymic TCR tuning and operates at an intensity just below that required to induce entry into the cell cycle. Costimulation from higher concentrations of IL-7 and other common γ-chain cytokines can induce T cells to undergo homeostatic proliferation and conversion into cells with a memory phenotype; many of these memory phenotype cells may be the progeny of cells responding to self antigens. The molecular mechanisms that control the conversion of naive resting T cells into memory-phenotype cells TCR-dependent in normal animals are beginning to be understood.

Signaling in thymic selection

T cell receptor signaling allows the developing thymocyte to undergo positive or negative selection, which is required for the formation of a useful mature T cell repertoire. Recent developments include the finding that much of the Lck kinase (required to initiate T cell signaling) is already in an active configuration before signaling. The analog strength of antigen binding to the T cell receptor binding may be translated into a digital signal by the amount of time the TCR is paired with a co-receptor carrying Lck. Downstream, the cellular localization of MAP kinase signaling is determined by the strength of the signal and in turn predicts positive or negative selection. A novel protein, Themis, is important in crossing the positive selection developmental checkpoint, but its mode of action is still uncertain. Commitment to the CD4 or CD8 lineage is influenced by the amount of ZAP-70 signaling and also by closely regulated responsiveness to intrathymic cytokines such as IL7.

Death, adaptation and regulation: The three pillars of immune tolerance restrict the risk of autoimmune disease caused by molecular mimicry

Extensive cross-reactivity in T cell receptor (TCR) recognition of peptide-MHC (pMHC) complexes seems to be essential to give sufficient immune surveillance against invading pathogens. This carries with it an inherent risk that T cells activated during a response to clear an infection can, perhaps years later, respond to a self pMHC of sufficient similarity. This lies at the heart of the molecular mimicry theory. Here we discuss our studies on the disease-causing potential of altered peptide ligands (APL) based on the sequence of a single autoantigenic epitope, the Ac1-9 peptide of myelin basic protein that induces experimental autoimmune encephalomyelitis in mice. These show that the window of similarity to self for induction of disease by cross-reactive non-self peptides is actually quite restricted. We show that each of the three pillars of immune tolerance (death, anergy/adaptation and regulation) has a role in limiting the risk of molecular mimicry by maintaining a threshold for harm.

CD8+ T cell activation is governed by TCR-peptide/MHC affinity, not dissociation rate

Binding of peptide/MHC (pMHC) complexes by TCR initiates T cell activation. Despite long interest, the exact relationship between the biochemistry of TCR/pMHC interaction (particularly TCR affinity or ligand off-rate) and T cell responses remains unresolved, because the number of complexes examined in each independent system has been too small to draw a definitive conclusion. To test the current models of T cell activation, we have analyzed the interactions between the mouse P14 TCR and a set of altered peptides based on the lymphocytic choriomeningitis virus epitope gp33-41 sequence bound to mouse class I MHC D(b). pMHC binding, TCR-binding characteristics, CD8+ T cell cytotoxicity, and IFN-gamma production were measured for the peptides. We found affinity correlated well with both cytotoxicity and IFN-gamma production. In contrast, no correlation was observed between any kinetic parameter of TCR-pMHC interaction and cytotoxicity or IFN-gamma production. This study strongly argues for an affinity threshold model of T cell activation.

The differential regulation of Lck kinase phosphorylation sites by CD45 is critical for T cell receptor signaling responses

The molecular mechanisms whereby the CD45 tyrosine phosphatase (PTPase) regulates T cell receptor (TCR) signaling responses remain to be elucidated. To investigate this question, we have reconstituted CD45 (encoded by Ptprc)-deficient mice, which display severe defects in thymic development, with five different expression levels of transgenic CD45RO, or with mutant PTPase null or PTPase-low CD45R0. Whereas CD45 PTPase activity was absolutely required for the reconstitution of thymic development, only 3% of wild-type CD45 activity restored T cell numbers and normal cytotoxic T cell responses. Lowering the CD45 expression increased CD4 lineage commitment. Peripheral T cells with very low activity of CD45 phosphatase displayed reduced TCR signaling, whereas intermediate activity caused hyperactivation of CD4+ and CD8+ T cells. These results are explained by a rheostat mechanism whereby CD45 differentially regulates the negatively acting pTyr-505 and positively acting pTyr-394 p56(lck) tyrosine kinase phosphorylation sites. We propose that high wild-type CD45 expression is necessary to dephosphorylate p56(lck) pTyr-394, suppressing CD4 T+ cell lineage commitment and hyperactivity.

Potent T cell agonism mediated by a very rapid TCR/pMHC interaction

The interaction between T cell receptors (TCR) and peptide-major histocompatibility complex (pMHC) antigens can lead to varying degrees of agonism (T cell activation), or antagonism. The P14 TCR recognises the lymphocytic choriomeningitis virus (LCMV)-derived peptide, gp33 residues 33–41 (KAVYNFATC), presented in the context of H-2D^b. The cellular responses to various related H-2D^b peptide ligands are very well characterised, and P14 TCR-transgenic mice have been used extensively in models of virus infection, autoimmunity and tumour rejection. Here, we analyse the binding of the P14 soluble TCR to a broad panel of related H-2D^b-peptide complexes by surface plasmon resonance, and compare this with their diverse cellular responses. P14 TCR binds H-2D^b-gp33 with a K_D of 3 µM (±0.5 µM), typical of an immunodominant antiviral TCR, but with unusually fast kinetics (k_off=1 s⁻¹), corresponding to a half-life of 0.7 s at 25°C, outside the range previously observed for murine agonist TCR/pMHC interactions. The most striking feature of these data is that a very short half-life does not preclude the ability of a TCR/pMHC interaction to induce antiviral immunity, autoimmune disease and tumour rejection.

Cytotoxic T cell-mediated diabetes in RIP-CD80 transgenic mice: autoantigen peptide sensitivity and fine specificity

Rodent immune-mediated diabetes model studies have advanced understanding of beta cell-specific T cell responses, and the testing of therapeutic approaches. We have used an inducible diabetes model based on rat insulin promotor (RIP)-driven expression of CD80 (B7-1) on pancreatic beta cells. Using these mice, we have established that immunizing with a single autoantigen can promote progressive islet inflammation and eventually T cell-mediated diabetes. We now describe a potent immunization protocol using peptide-pulsed mature dendritic cells (DCs) to examine peptide epitopes derived from endogenous (preproinsulin) and transgenically expressed beta cell antigens, namely lymphocytic choriomeningitis virus glycoprotein (LCMV-GP). LCMV-GP epitopes efficiently promote beta cell destruction, and the autoantigenic peptide concentration used to load the DCs correlates directly with diabetes onset. The system allowed us to assess cytotoxic T cell (CTL) fine specificity by immunizing with DCs presenting altered peptide ligands (APLs) of the dominant LCMV-GP epitope, gp33. Finally, using an adoptive transfer system, we tested alternative in vitro T cell activation conditions, including APLs and mitogens, for their impact on T cell effector function and diabetes onset. Our studies revealed a marked discrepancy between (inflammatory) effector functions and diabetes progression, thus emphasizing the importance of structural identity between sensitizing and target epitope and the context of initial T cell activation.

T-cell avidity and tuning: the flexible connection between tolerance and autoimmunity

Thymic T-cell selection mechanisms generate a cross-reactive, self-MHC restricted peripheral T-cell pool. Affinity and avidity are of profound influence on this selection and the generation of immunity. Autoreactive T cells can escape thymic deletion by lowering their avidity and retain this "tuned" state in the periphery. Upon activation, tuned T cells can cause autoimmunity, while immunotherapeutic strategies may be hampered by existing T-cell tolerance. The regulation of T-cell avidity and tuning therefore determines the balance between tolerance and autoimmunity and should be taken into account in the design of therapeutic strategies aimed at T-cell reactivity.

Altered Peptide Ligands Induce Delayed CD8-T Cell Receptor Interaction—a Role for CD8 in Distinguishing Antigen Quality

How T cells translate T cell receptor (TCR) recognition of almost identical pMHC ligands into distinct biological responses has remained enigmatic. Although differences in affinity or off rate are important, they offer at best an incomplete explanation. By using Förster resonance energy transfer (FRET), we have visualized the ligand-induced interaction between OT-I TCR and CD8. We found that both recruitment of TCR to the immunological synapse and the TCR-CD8 interaction induced by weak agonists (positive-selecting ligands) was delayed but not necessarily weaker than strong agonists (negative selectors). A delayed and perhaps longer lasting CD8-TCR interaction results in delayed phospho-ERK recruitment to the synapse. The kinetics of the TCR-CD8 interaction can reconcile previously anomalous data, where biological activity did not correlate with TCR-pMHC binding kinetics for certain ligands. Our findings indicate that the T cell translates antigen recognition into T cell responses by differential recruitment of CD8 to the TCR.

CD8 T cell competition for dendritic cells in vivo is an early event in activation

T cell responses against an antigen are often focused on a small fraction of potentially immunogenic determinants, a phenomenon known as immunodominance. Immunodominance can be established at several stages of antigen presentation, including antigen processing, binding of peptides to MHC, and competition between T cells for dendritic cells (DCs). The mechanism of this T cell competition is unclear, but may include competition for physical access to DCs, competition for limiting soluble DC-derived factors, or a suppressive effect of one T cell population on the other(s). To model DC-specific T cell competition, normal mice were injected with one or two T cell receptor transgenic CD8 T cell populations, each with high affinity for different peptides presented by different class I MHC complexes. These mice were immunized with DCs pulsed with peptides that are recognized by the transferred T cells. Competition was detectable when both T cell populations were transferred and their target peptides were present on the same DCs. The competition resulted in fewer cells entering the response, but had no effect on the level of activation of the cells that did respond. The effect was evident very early in the response, and in fact the competing T cells needed to be present in the first 5 h of the response for competition to occur. Thus, some aspect of DCs other than peptide/MHC complexes is limiting in the earliest stages of the CD8 T cell response. These results have implications for the design of multivalent vaccines.

Self-peptide/MHC and TCR antagonism: physiological role and therapeutic potential

TCR antagonists are peptides that bind MHC molecules and can specifically inhibit T cell activation induced by antigens. Studying TCR antagonism has taken an important place in immunology for both theoretical and practical reasons. Deciphering the mechanism(s) of action of TCR antagonists can yield important information about interactions of the TCR with ligands, T cell development, and TCR signaling. Moreover, microorganisms may employ TCR antagonism to elude the attention of the immune system. Finally, specificity of inhibition makes TCR antagonists an ideal tool to seek antigen-specific immunomodulation. Present state of knowledge on these topics is reviewed.

Plasticity in the positive selection of T cells: affinity of the selecting antigen and IL-7 affect T cell responsiveness

The current study examines how responsiveness of T cells is affected by the avidity of the peptide/MHC engaged during positive selection of their thymocyte precursors. We used a thymus reaggregate culture system in which CD4(+)CD8(+) thymocytes from AND TCR transgenic mice were induced to undergo positive selection by pigeon cytochrome c (PCC) peptide or its analogs presented by I-E(k) class II MHC on a thymic epithelial cell line. When low-affinity peptide analogs drove positive selection, up to 100 microM was needed to produce >50% CD4(+) T cells, and these cells were highly responsive to PCC. In contrast, <0.2 microM high-affinity peptides was required to achieve similar selection efficiency, but the resultant cells failed to respond to PCC. However, these cells were not dead based on dye exclusion and capacity to respond to phorbal ester and to agonist if IL-2 was also present, supporting the view that non-responsiveness of cells selected on high-affinity peptides is a form of central T cell tolerance distinct from deletion. Cells selected on intermediate-affinity peptides showed variable responsiveness which was suppressed 5- to 10-fold by addition during reaggregate culture of antibody to the IL-7R. Similarly, supplementary IL-7 in the reaggregate culture produced CD4(+) T cells that were promiscuously responsive. Overall, this study demonstrates that the responsiveness of T cells is not rigidly controlled and that the presence of IL-7 during T cell development has the potential to negate central T cell tolerance and produce autoreactive T cells.

Distinct requirements for deletion versus anergy during CD8 T cell peripheral tolerance in vivo

Activation of naive T cells by quiescent APCs results in tolerance through deletion and anergy. The underlying basis for these distinct fates is unclear. Using clone 4 TCR transgenic animals as a source of naive CD8 T cells, we examined the requirements for peripheral deletion in vivo. Our results demonstrate that independent of the amount of Ag used for stimulation, a single dose was insufficient to achieve complete clonal deletion. Instead, further antigenic exposure was required to completely eliminate all of the activated T cells. Additionally, consecutive stimulations with low doses of Ag were highly effective in promoting deletion. In contrast, although stimulation with high doses of Ag initially led to the apoptosis of many of the activated T cells, it induced hyporesponsiveness in a portion of the responding cells, thereby sparing them from further activation and deletion. These data explain why some conditions promote tolerance through clonal deletion whereas others promote anergy. Furthermore, these data provide a framework to devise protocols for effective deletion of potentially autoreactive T cells.

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.

The fourth way? Harnessing aggressive tendencies in the thymus

During late stages of thymic development, T cells must chose between different fates, dictated by their TCR specificity. Typically, this is thought of as a choice between three alternatives (being positive selection for useful T cells vs negative selection or neglect for harmful or useless T cells). However, there is growing evidence for a fourth alternative, in which T cells are positively selected by agonist ligands, which would normally be expected to induce T cell deletion. In this review, we will discuss where and when agonist selection is induced and whether this should be considered as a novel form of thymic selection or as an alternative differentiation state for Ag-exposed T cells.

Delineation of Signals Required for Thymocyte Positive Selection

Peptide/MHC complexes capable of inducing positive selection in mouse fetal thymic organ cultures fail to do so in suspension culture. Furthermore, this type of culture does not promote initial stages of differentiation, such as coreceptor down-modulation, unless peptides used for stimulation have (at least) weak agonist activity. We show in this study that signals provided in suspension culture by nonagonist peptide/MHC complexes on the surface of macrophages, even though apparently silent, are sufficient to promote complete phenotypic differentiation when CD4+CD8+ thymocytes are subsequently placed in a proper anatomical setting. Furthermore, the synergistic actions of suboptimal concentrations of phorbol esters and nonagonist peptide/MHC complexes can make the initial stages of positive selection visible, without converting maturation into negative selection. Thus, the correlation between efficiency of positive selection and the degree of coreceptor down-modulation on CD4+CD8+ thymocytes is not linear. Furthermore, these results suggest that the unique role of thymic stromal cells in positive selection is related not to presentation of self-peptide/MHC complexes, but most likely to another ligand.

2,3,7,8-Tetrachlorodibenzo-p-dioxin Enhances Negative Selection of T Cells in the Thymus but Allows Autoreactive T Cells to Escape Deletion and Migrate to the Periphery

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an environmental pollutant, has been shown to cause thymic atrophy and apoptosis. However, whether TCDD alters the process of T-cell selection in the thymus is not clear. To this end, we investigated the effects of TCDD in the context of the HY-T-cell receptor (TCR) transgenic (Tg) mouse model. We noted that negatively selecting male HY-TCR Tg mice were significantly more sensitive to the thymotoxic effects of TCDD relative to positively selecting female HY-TCR Tg mice, including increased reduction in cellularity and increased induction of apoptosis. TCDD exposure also altered the thymocyte subset composition in HY-TCR Tg male but not female mice. In addition, TCDD treatment resulted in increased extracellularly regulated kinase phosphorylation and lymphocyte-specific protein tyrosine kinase expression in thymocytes of HY-TCR Tg male but not female mice. The increase in proportion of CD8+ mature thymocytes noted in HY-TCR Tg male mice was reflected in the periphery, with TCDD-exposed HY-TCR Tg male mice having increased numbers of CD8+ T cells. Finally, we noted that the proliferative response of HY-TCR Tg male T cells to HY(self)-Ag was enhanced after exposure to TCDD, whereas that of HY-TCR Tg female mice was decreased. Taken together, these data suggest that TCDD alters the process of thymic selection, possibly by enhancing negative thymocyte selection, whereas at the same time allowing autoreactive T cells to escape deletion in the thymus and immigrate to the periphery.

Distinct footprints of TCR engagement with highly homologous ligands

T cell receptor engagement promotes proliferation, differentiation, survival, or death of T lymphocytes. The affinity/avidity of the TCR ligand and the maturational stage of the T cell are thought to be principal determinants of the outcome of TCR engagement. We demonstrate in this study that the same mouse TCR preferentially uses distinct residues of homologous peptides presented by the MHC molecules to promote specific cellular responses. The preference for distinct TCR contacts depends on neither the affinity/avidity of TCR engagement (except in the most extreme ranges), nor the maturity of engaged T cells. Thus, different portions of the TCR ligand appear capable of biasing T cells toward specific biological responses. These findings explain differences in functional versatility of TCR ligands, as well as anomalies in the relationship between affinity/avidity of the TCR for the peptide/MHC and cellular responses of T cells.

Essential Role of the E3 Ubiquitin Ligase Cbl-b in T Cell Anergy Induction

Antigen-specific immunotolerance limits the expansion of self-reactive T cells involved in autoimmune diseases. Here, we show that the E3 ubiquitin ligase Cbl-b is upregulated in T cells after tolerizing signals. Loss of Cbl-b in mice results in impaired induction of T cell tolerance both in vitro and in vivo. Importantly, rechallenge of Cbl-b mutant mice with the tolerizing antigen results in massive lethality. Moreover, ablation of Cbl-b resulted in exacerbated autoimmunity. Mechanistically, loss of Cbl-b rescues reduced calcium mobilization of anergic T cells, which was attributed to Cbl-b-mediated regulation of PLCgamma-1 phosphorylation. Our results show a critical role for Cbl-b in the regulation of peripheral tolerance and anergy of T cells.

TCR binding kinetics measured with MHC class I tetramers reveal a positive selecting peptide with relatively high affinity for TCR

The interaction between TCR and peptide-MHC (pMHC) complexes is crucial for the activation of T cells as well as for positive and negative selection in the thymus. The kinetics and affinity of this interaction and the densities of TCR and pMHC complexes on the cell surface are determining factors for different outcomes during thymic selection. In general, it is thought that agonist pMHC, which cause negative selection, have higher affinities and, in particular, slower off-rates than partial or weak agonists and antagonists, which cause positive selection. In this study, we have used pMHC tetramers to investigate the kinetics of TCR-pMHC interaction for agonist, weak agonist, and antagonist ligands of the anti-lymphocytic choriomeningitis virus P14 TCR. Kinetics determined on the cell surface may be biologically more relevant than methods using soluble proteins. We can distinguish between agonists and weak agonists or antagonists based on the half-life and the avidity of tetramer-TCR interaction. Furthermore, we show that a weak agonist self-peptide that positively selects P14 TCR(+) thymocytes has a tetramer half-life and avidity only slightly weaker than strong agonists. We show that, in fact, it can act as quite a strong agonist, but that its poor ability to stabilize MHC causes it instead to have a weak agonist phenotype.

Positive and negative selection of T cells

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

T cell antagonism is functionally uncoupled from the 21- and 23-kDa tyrosine-phosphorylated TCR zeta subunits

The functional effects of altered peptide ligands on T cells is proposed to involve differential intracellular signaling mediated by the 21- and 23-kDa tyrosine-phosphorylated derivatives of the TCR zeta subunit (p21 and p23). To understand the functional contribution of p21 and p23 to T cell development and T cell antagonism, we generated selected TCR zeta transgenic mice maintained on the P14 alphabeta TCR transgenic line such that p23 or both p21 and p23 were selectively eliminated. Importantly, one line (YF1,2) retains the constitutively tyrosine-phosphorylated p21 in the complete absence of inducible p23. We determined that T cell development was uncoupled from p21 and/or p23. Using a series of agonist, weak agonist, and antagonist peptides, we analyzed the role of each of the phosphorylated forms of TCR zeta on T cell activation and antagonism. In this study, we report that the proliferative responses of alphabeta P14 T cells to agonist peptides and the inhibition of proliferation resulting from antagonist peptide treatments was functionally uncoupled from p21 and/or p23. These results suggest that the mechanism of T cell antagonism is independent of the two phosphorylated TCR zeta derivatives.

Homology between an alloantigen and a self MHC allele calibrates the avidity of the alloreactive T cell repertoire independent of TCR affinity

The self-restricted T cell repertoire exhibits a high frequency of alloreactivity. Because these alloreactive T cells are derived from the pool of cells selected on several different self MHC alleles, it is unknown how development of the alloantigenic repertoire is influenced by homology between a self MHC allele and an alloantigen. To address this, we used the 2C transgenic TCR that is selected by K(b), is alloreactive for L(d), and cross-reacts with L(q). L(q) is highly homologous to L(d) and binds several of the same peptide ligands, including p2Ca, the peptide recognized by 2C. We find that L(d)/p2Ca is a high avidity agonist ligand, whereas L(q)/p2Ca is a low avidity agonist ligand for 2C T cells. When mice transgenic for the 2C TCR are bred to L(q)-expressing mice, 2C(+) T cells develop; however, they express lower levels of either the 2C TCR or CD8 and require a higher L(d)/p2Ca ligand density to be activated than 2C(+) T cells selected by K(b). Furthermore, the 2C T cells selected in the presence of L(q) fail to detect L(q)/p2Ca complexes even at high ligand density. Thus, despite possessing the identical TCR, there is a functional avidity difference between 2C(+) T cells selected in the presence of L(q) vs K(b). These data provide evidence that homology between the selecting ligand and an alloantigen can influence the avidity of the T cell repertoire for the alloantigen, and suggest that thymic selection can fine tune T cell avidity independent of intrinsic TCR affinity.

Receptor sensitivity: when T cells lose their sense of self

The T-cell antigen receptor binds to self-MHC molecules with low affinity. Recent reports disagree as to whether this interaction sensitizes or desensitizes the receptor. Here we discuss how these findings might be reconciled.

The formation and functions of the 21- and 23-kDa tyrosine-phosphorylated TCR zeta subunits

The interaction between the T cell receptor (TCR) and its cognate antigen/major histocompatibility complex (MHC) complex activates a cascade of intracellular protein phosphorylations within the T cell. The signals are initiated by the specific phosphorylation of two tyrosine residues located in a conserved sequence motif termed an ITAM (immune receptor-based tyrosine activation motif). There are 10 ITAMs in the TCR complex, and 6 of these ITAMs are present in the TCR zeta homodimer. Following TCR stimulation, the TCR zeta subunit forms two tyrosine-phosphorylated intermediates of 21- and 23-kDa, respectively. The dramatic and diverse biological responses of T cells are proposed to be partly regulated by the relative ratios of the 21- vs. 23-kDa phosphorylated forms of TCR zeta that are induced following TCR ligation. In this review, we describe a stepwise model of zeta phosphorylation required for the formation of these two phosphorylated derivatives. We describe the kinases and phosphatases controlling these phosphorylation processes. In addition, we present some preliminary findings from ongoing studies that discuss the contributions of each phosphorylated form of zeta on T cell development, TCR signaling, T cell anergy induction, and T cell survival.

Inefficient clustering of tyrosine-phosphorylated proteins at the immunological synapse in response to an antagonist peptide

Interactions of T cells with MHC plus peptide in the peripheral lymphoid system are important for their survival. In this study we investigated further the molecular consequences of such interactions using F5 TCR transgenic mice and peptides previously shown to induce either negative or positive selection in the thymus. Following TCR ligation with the negatively selecting agonist peptide, mature CD8(+) cells proliferated and up-regulated the activation marker CD69. Interestingly, ligation of this TCR with MHC molecules loaded with high concentrations of the positively selecting peptide also resulted in the aforementioned changes, but with slower kinetics. Analysis of the biochemical changes that occur following stimulation with these peptides showed that phosphorylation of key signaling molecules, such as ZAP-70, CD3zeta, Vav, SLP-76, LAT, and ERK-1 and 2, could be detected after exposure to agonist but not antagonist peptide. Confocal microscopy, however, revealed infrequent phosphorylation 'patches' at the site of contact between T cells and APC presenting the antagonist peptide. Our data suggest that peptides capable of inducing positive selection in the thymus can be recognized by mature T cells and cause proliferation, up-regulation of CD69 and accumulation of phosphorylated proteins at the immunological synapse with low efficiency; however no phosphorylation of signaling molecules can be detected using conventional biochemical assays.

Peripheral expression of self-MHC-II influences the reactivity and self-tolerance of mature CD4(+) T cells: evidence from a lymphopenic T cell model

While intrathymic MHC expression influences the specificity of developing thymocytes, we considered that peripheral MHC expression might influence the reactivity of postthymic T cells. We now report for CD4(+) T cells that peripheral MHC-II expression does influence their reactivity and self-tolerance. Upon transfer into MHC-II-deficient lymphopenic hosts, mature CD4(+) T cells were found to acquire an activated memory phenotype and to become: (1) autoreactive against syngeneic MHC-II(+) skin grafts, (2) hyperreactive against third-party MHC-II(+) skin grafts, and (3) functionally dysregulated, resulting in a lymphoproliferative disorder characterized by intraepithelial infiltrations. Peripheral MHC-II expression appeared to influence CD4(+) T cell reactivity by two complementary mechanisms: maintenance of CD4(+)CD25(+) regulatory T cells ("suppression") and direct dampening of CD4(+) T cell reactivity ("tuning").

CD28-dependent activation of protein kinase B/Akt blocks Fas-mediated apoptosis by preventing death-inducing signaling complex assembly

The T cell costimulatory molecule CD28 is important for T cell survival, yet both the signaling pathways downstream of CD28 and the apoptotic pathways they antagonize remain poorly understood. Here we demonstrate that CD4(+) T cells from CD28-deficient mice show increased susceptibility to Fas-mediated apoptosis via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. Protein kinase B (PKBalpha/Akt1) is an important serine/threonine kinase that promotes survival downstream of PI3K signals. To understand how PI3K-mediated signals downstream of CD28 contribute to T cell survival, we examined Fas-mediated apoptosis in T cells expressing an active form of PKBalpha. Our data demonstrate that T cells expressing active PKB are resistant to Fas-mediated apoptosis in vivo and in vitro. PKB transgenic T cells show reduced activation of caspase-8, BID, and caspase-3 due to impaired recruitment of procaspase-8 to the death-inducing signaling complex (DISC). Similar alterations are seen in T cells from mice which are haploinsufficient for PTEN, a lipid phosphatase that regulates phosphatidylinositol-3,4,5-trisphosphate (PIP(3)) and influences PKBalpha activity. These findings provide a novel link between CD28 and an important apoptosis pathway in vivo, and demonstrate that PI3K/PKB signaling prevents apoptosis by inhibiting DISC assembly.

Rare, structurally homologous self-peptides promote thymocyte positive selection

Although it is clear that positive selection of T cells involves recognition of specific self-peptide/MHC complexes, the nature of these self-ligands and their relationship to the cognate antigen are controversial. Here we used two complementary strategies to identify naturally occurring self-peptides able to induce positive selection of T cells bearing a specific T cell receptor, OT-I. Both the bioassay- and bioinformatics-based strategies identified the same self-peptides, derived from F-actin capping protein and beta-catenin. These peptides displayed charge conservation at two key TCR contact residues. The biological activity of 43 other self-peptides and of complex peptide libraries directly correlated to the extent of conservation at TCR contact residues. These results demonstrate that selecting self-peptides are rare and can be identified by homology-based search strategies.

Selection and fine-tuning of the autoimmune T-cell repertoire

The immune system must avoid aggressive T-cell responses against self-antigens. But, paradoxically, exposure to self-peptides seems to have an important role in positive selection in the thymus and the maintenance of a broad T-cell repertoire in the periphery. Recent experiments have highlighted situations that allow high-avidity self-reactive T cells to avoid negative selection in the thymus. Accumulating evidence indicates that other, non-deleting mechanisms control the avidity with which T cells recognize self-antigens--a phenomenon that is known as 'tuning'. This might maximize the peripheral T-cell repertoire by allowing the survival of T cells that can respond to self, but only at concentrations that are not normally reached in vivo.

Rap1A positively regulates T cells via integrin activation rather than inhibiting lymphocyte signaling

T cell receptor (TCR) stimulation activates the small GTPase Rap1A, which is reported to antagonize Ras signaling and induces T cell anergy. To address its role in vivo, we generated transgenic mice that constitutively expressed active Rap1A within the T cell lineage. We found that active Rap1A did not interfere with the Ras signaling pathway or antagonize T cell activation. Instead of anergy, the T lymphocytes that constitutively expressed active Rap1A showed enhanced TCR-mediated responses, both in thymocytes and mature T cells. In addition, Rap1A activation was sufficient to induce strong activation of the beta1 and beta2 integrins via an avidity-modulation mechanism. This shows that, far from playing an inhibitory role during T cell activation, Rap1A positively influences T cells by augmenting lymphocyte responses and directing integrin activation.

Tumor growth enhances cross-presentation leading to limited T cell activation without tolerance

Using a tumor model of spontaneously arising insulinomas expressing a defined tumor-associated antigen, we investigated whether tumor growth promotes cross-presentation and tolerance of tumor-specific T cells. We found that an advanced tumor burden enhanced cross-presentation of tumor-associated antigens to high avidity tumor-specific T cells, inducing T cell proliferation and limited effector function in vivo. However, contrary to other models, tumor-specific T cells were not tolerized despite a high tumor burden. In fact, in tumor-bearing mice, persistence and responsiveness of adoptively transferred tumor-specific T cells were enhanced. Accordingly, a potent T cell-mediated antitumor response could be elicited by intravenous administration of tumor-derived peptide and agonistic anti-CD40 antibody or viral immunization and reimmunization. Thus, in this model, tumor growth promotes activation of high avidity tumor-specific T cells instead of tolerance. Therefore, the host remains responsive to T cell immunotherapy.

Autoreactivity, dynamic tuning and selectivity

The immune system adjusts its response to the context in which antigens, including self-antigens, are recognized. Recent observations support a conceptual framework for understanding how this may be achieved at the cellular and cell-population levels. At both levels, 'perturbations' elicit competition between excitation and de-excitation, resulting either in adaptation or in various responses. The responsiveness of individual cells is dynamically tuned, reflecting their recent experience. The tuning of T-cell activation thresholds by self-ligands facilitates positive selection and continuously regulates the level of autoreactivity in the periphery. Autoreactivity appears to be involved in regulation of the immune response, homeostasis, maintaining of the functional integrity of naïve and memory cells, and in other physiological functions.

Cutting edge: positive selection induced by a self-peptide with TCR antagonist activity

Antagonist-like engagement of the TCR has been proposed to induce T cell selection in the thymus. However, no natural TCR ligand with TCR antagonist activity is presently known. Using a combination of bioinformatics and functional testing we identified the first self-peptide that can both deliver antagonist-like signals and promote T cell selection in the thymus. The peptide is presented by appropriate MHC class I molecules in vivo. Thus, endogenous antagonist peptides exist and may be involved in TCR repertoire selection.

Inefficient ZAP-70 phosphorylation and decreased thymic selection in vivo result from inhibition of NF-kappaB/Rel

Signaling from the TCR regulates T lymphoid survival, deletion by apoptosis, and selective clonal expansion. One set of signaling pathways activated during thymic selection leads to degradation of a cytosolic retention protein, the inhibitor of kappaB (IkappaB)alpha, followed by nuclear translocation of the NF-kappaB/Rel family of transcription factors. It has been found previously that NF-kappaB proteins mediate a pathway signaling the survival of mature T cells and protection of thymocytes against TNF-induced apoptosis. In contrast, we show in this study that a transgenic inhibitor of NF-kappaB/Rel signaling interferes with the negative selection of immature thymocytes by endogenous MHC ligands in vivo. Positive selection of the H-Y TCR also was diminished. This attenuation of thymic selection efficiency was associated with decreased ZAP-70 phosphorylation and TCR signaling of CD69 induction. These findings demonstrate that the NF-kappaB transcriptional pathway plays an important role in normal processes of clonal deletion and they indicate that the NF-kappaB/IkappaB axis can regulate the efficiency of TCR signaling.

Sensory adaptation in naive peripheral CD4 T cells

T cell receptor interactions with peptide/major histocompatibility complex (pMHC) ligands control the selection of T cells in the thymus as well as their homeostasis in peripheral lymphoid organs. Here we show that pMHC contact modulates the expression of CD5 by naive CD4 T cells in a process that requires the continued expression of p56(lck). Reduced CD5 levels in T cells deprived of pMHC contact are predictive of elevated Ca(2)+ responses to subsequent TCR engagement by anti-CD3 or nominal antigen. Adaptation to peripheral pMHC contact may be important for regulating naive CD4 T cell responsiveness.