Divergent changes in the sensitivity of maturing T cells to structurally related ligands underlies formation of a useful T cell repertoire

A role for CD8 in the developmental tuning of antigen recognition and CD3 conformational change

TCR engagement by peptide-MHC class I (pMHC) ligands induces a conformational change (Deltac) in CD3 (CD3Deltac) that contributes to T cell signaling. We found that when this interaction took place between primary T lineage cells and APCs, the CD8 coreceptor was required to generate CD3Deltac. Interestingly, neither enhancement of Ag binding strength nor Src kinase signaling explained this coreceptor activity. Furthermore, Ag-induced CD3Deltac was developmentally attenuated by the increase in sialylation that accompanies T cell maturation and limits CD8 activity. Thus, both weak and strong ligands induced CD3Deltac in preselection thymocytes, but only strong ligands were effective in mature T cells. We propose that CD8 participation in the TCR/pMHC interaction can physically regulate CD3Deltac induction by "translating" productive Ag encounter from the TCR to the CD3 complex. This suggests one mechanism by which the developmentally regulated variation in CD8 sialylation may contribute to the developmental tuning of T cell sensitivity.

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.

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.

T cell activation enhancement by endogenous pMHC acts for both weak and strong agonists but varies with differentiation state

T cells are extremely sensitive in their ability to find minute amounts of antigenic peptide in the midst of many endogenous peptides presented on an antigen-presenting cell. The role of endogenous peptides in the recognition of foreign peptide and hence in T cell activation has remained controversial for CD8(+) T cell activation. We showed previously that in a CD8(+) T cell hybridoma, nonstimulatory endogenous peptides enhance T cell sensitivity to antigen by increasing the coreceptor function of CD8. However, others were not able to detect such enhancement in naive and activated CD8(+) T cells. Here, we show that endogenous peptides substantially enhance the ability of T cells to detect antigen, an effect measurable by up-regulation of activation or maturation markers and by increased effector function. This enhancement is most pronounced in thymocytes, moderate in naive T cells, and mild in effector T cells. The importance of endogenous peptides is inversely proportional to the agonist activity of the stimulatory peptide presented. Unlike for CD4(+) T cells, the T cell receptor of CD8(+) T cells does not distinguish between endogenous peptides for their ability to enhance antigen recognition.

A constant affinity threshold for T cell tolerance

T cell tolerance depends on the T cell receptor's affinity for peptide/major histocompatibility complex (MHC) ligand; this critical parameter determines whether a thymocyte will be included (positive selection) or excluded (negative selection) from the T cell repertoire. A quantitative analysis of ligand binding was performed using an experimental system permitting receptor–coreceptor interactions on live cells under physiological conditions. Using three transgenic mouse strains expressing distinct class I MHC–restricted T cell receptors, we determined the affinity that defines the threshold for negative selection. The affinity threshold for self-tolerance appears to be a constant for cytotoxic T lymphocytes.

Loss of N-terminal charged residues of mouse CD3 epsilon chains generates isoforms modulating antigen T cell receptor-mediated signals and T cell receptor-CD3 interactions

The antigen T cell receptor (TCR)-CD3 complexes present on the cell surface of CD4(+) T lymphocytes and T cell lines express CD3 epsilon chain isoforms with different isoelectric points (pI), with important structural and functional consequences. The pI values of the isoforms fit the predicted pI values of CD3 epsilon chains lacking one, two, and three negatively charged amino acid residues present in the N-terminal region. Different T cells have different ratios of CD3 epsilon chain isoforms. At a high pI, degraded CD3 epsilon isoforms can be better recognized by certain anti-CD3 monoclonal antibodies such as YCD3-1, the ability of which to bind to the TCR-CD3 complex is directly correlated with the pI of CD3 epsilon. The abundance of CD3 epsilon isoforms can be modified by treatment of T cells with the proteinase inhibitor phenanthroline. In addition, these CD3 epsilon isoforms have functional importance. This is shown, first, by the different structure of TCR-CD3 complexes in cells possessing different amounts of isoforms (as observed in surface biotinylation experiments), by their different antigen responses, and by the stronger interaction between low pI CD3 epsilon isoforms and the TCR. Second, incubation of cells with phenanthroline diminished the proportion of degraded high pI CD3 epsilon isoforms, but also the ability of the cells to deliver early TCR activation signals. Third, cells expressing mutant CD3 epsilon chains lacking N-terminal acid residues showed facilitated recognition by antibody YCD3-1 and enhanced TCR-mediated activation. Furthermore, the binding avidity of antibody YCD3-1 was different in distinct thymus populations. These results suggest that changes in CD3 epsilon N-terminal chains might help to fine-tune the response of the TCR to its ligands in distinct activation situations or in thymus selection.

Stage-dependent reactivity of thymocytes to self-peptide--MHC complexes

In mice that express a transgene for the 2C T cell antigen-receptor (TCR) and lack a recombinase-activating gene (2C(+)RAG(-/-) mice) most of the peripheral T cells are CD8(+), a few are CD4(+), and a significant fraction are CD4(-)CD8(-) [double negative (DN)]. The DN 2C cells, like DN T cells that are abundant in various other alphabeta TCR-transgenic mice, appear to be derived directly from DN thymocytes that prematurely express the TCR transgene. The DN 2C cells are virtually absent in mice deficient in major histocompatibility complex class II (MHC-II) but more abundant in mice deficient in MHC-I, suggesting that the DN 2C thymocytes are positively selected by self-peptide-MHC-II (pMHC-II) complexes and negatively selected by self-pMHC-I complexes. The pMHC-I complexes, however, positively select CD8(+) 2C T cells in the same mice. The different effects of thymic pMHC-I on DN and CD8(+) thymocytes are consistent with the finding that DN 2C thymocytes are more sensitive than more mature CD4(+)CD8(+) [double positive (DP)] thymocytes to a weak pMHC-I agonist for the 2C TCR. Together with previous evidence that DP thymocytes respond more sensitively than T cells in the periphery to weak pMHC agonists, the findings suggest progressive decreases in responsiveness to self-pMHC-I complexes as thymocytes develop from DN to DP thymocytes and then to mature naïve T cells in the periphery.

T cells with low avidity for a tissue-restricted antigen routinely evade central and peripheral tolerance and cause autoimmunity

T cells causing autoimmunity must escape tolerance. We observed that CD8(+) T cells with high avidity for an antigen expressed in the pancreas, kidney, and thymic medulla were efficiently removed from a polyclonal repertoire by central and peripheral tolerance mechanisms. However, both mechanisms spared low-avidity T cells from elimination. Neither the introduction of activated, self-antigen-specific CD4(+) helper T cells nor a global inflammatory stimulus were sufficient to activate the low-avidity CD8(+) T cells and did not break tolerance. In contrast, challenge with a recombinant bacterium expressing the self antigen primed the low-avidity T cells, and the animals rapidly developed autoimmune diabetes. We suggest that whereas thymic and peripheral tolerance mechanisms remove cells that can be primed by endogenous amounts of self antigen, they do not guard against tissue destruction by low-avidity effector T cells, which have been primed by higher amounts of self antigen or by crossreactive antigens.

The sialyltransferase ST3Gal-I is not required for regulation of CD8-class I MHC binding during T cell development

The CD8 coreceptor plays a crucial role in thymocyte and T cell sensitivity by binding to class I MHC and recruiting downstream signaling molecules to the TCR. Previous studies reported considerable changes in TCR-independent CD8/class I MHC binding (i.e., CD8 noncognate interactions) during T cell development, changes that correlated with altered glycosylation of surface molecules. In particular, expression of the sialyltransferase ST3Gal-I has been proposed as a critical factor regulating the attenuation of CD8 avidity during the double-positive to CD8 single-positive progression. This hypothesis is strengthened by the fact that ST3Gal-I(-/-) animals show a profound disregulation of CD8 T cell homeostasis. In contrast to this model, however, we report in this study that ST3Gal-I deficiency had no detectable impact on CD8 noncognate binding to multimeric peptide/MHC class I ligands at any stage of thymocyte development. We also found that the susceptibility to CD8-induced cell death is not markedly influenced by ST3Gal-I deficiency. Thus, the profound effects of ST3Gal-I on CD8 T cell survival evidently do not involve a role for this enzyme in controlling CD8-class I binding.

Cooperation between CD4+ and CD8+ T cells: when, where, and how

Concepts of cell-cell interactions in adaptive immunity have alternated between the simple and the complex. The notion that one population of small, circulating lymphocytes is responsible for adaptive immunity was sequentially supplanted by the concept of separate T and B lymphocyte populations that cooperate to produce IgG antibody responses, by a three-cell model in which a myeloid APC initiates these cooperative lymphoid responses, by the recognition of T cell subsets, and by the idea that CD8+ T cell subset responses to graft antigens depend on CD4+ T cell subset activity. Simplicity was reintroduced with the revelation that CD8+ T cells can act independently of CD4+ T cells against acute viral infections. The pendulum has swung again toward complexity with recognition of the distinct and conjoint contributions of innate stimuli, APCs, NK and NKT cells, Tregs, and CD4+ helper T cells to CD8+ T cell behavior during acute and chronic infections or as memory cells. The renewed appreciation that multiple, sometimes rare cell types must communicate during cell-mediated immune responses has led to questions about how such interactions are orchestrated within organized lymphoid tissues. We review recent advances in deciphering the specific contribution of CD4+ T cells to physiologically useful CD8+ T cell responses, the signals involved in producing acute effectors versus long-lived memory cells, and the mechanisms underlying the cell-cell associations involved in delivery of such signals. We propose a model based on these new findings that may serve as a general paradigm for cellular interactions that occur in an inflamed lymph node during the initiation of immune responses.

Loss of a gimap/ian gene leads to activation of NF-kappaB through a MAPK-dependent pathway

The diabetes-prone biobreeding (BB-DP) rat contains the lyp mutation which results in lymphopenia and promotes the progression of a T cell-mediated autoimmune attack of the pancreas in certain rat strains. This mutation has been mapped to a gene which bears homology to human Gimap5/Ian5 and results in the truncation and loss of activity of this protein. The lymphopenic state induced by the loss of this protein has led to the proposal that Gimap5 has an anti-apoptotic function. Previously we described an additional phenotype of incomplete activation mediated by the loss of Gimap5 function. Here we further characterize this incomplete activation phenotype and map a potential signal transduction pathway leading to activation. We show that CD5 expression on peripheral T cells is elevated in Gimap5 animals, while thymocyte expression remains similar between the two strains. Additionally, we show that NF-kappaB but not NFAT is activated in unstimulated Gimap5 mutant T cells as compared to unstimulated wild type T cells. Mapping this activation to its upstream source we show that activation of NF-kappaB is correlated with an activation of IKK. Using a variety of kinase inhibitors we further map this increase in IKK to an increase in MEK activation. Finally, to counter the possibility that activation is an indirect consequence of the lymphopenic environment, we created bone marrow chimeras in which Gimap5 mutant T cells developed in a normal environment and show that these cells retain their activated phenotype. Together, we interpret these data as demonstrating that the activation caused by loss of Gimap5 is a cell intrinsic phenomenon caused, in part, by a MEK-dependent activation of IKK. This, in turn, would suggest that Gimap5 functions to promote both T cell survival and quiescence and that these pathways are biochemically linked.

Adaptive tolerance and clonal anergy are distinct biochemical states

Adaptive tolerance is a process by which T cells become desensitized when Ag stimulation persists following an initial immune response in vivo. To examine the biochemical changes in TCR signaling present in this state, we used a mouse model in which Rag2(-/-) TCR-transgenic CD4(+) T cells were transferred into CD3epsilon(-/-) recipients expressing their cognate Ag. Compared with naive T cells, adaptively tolerant T cells had normal levels of TCR and slightly increased levels of CD4. Following activation with anti-TCR and anti-CD4 mAbs, the predominant signaling block in the tolerant cells was at the level of Zap70 kinase activity, which was decreased 75% in vitro. Phosphorylations of the Zap70 substrates (linker of activated T cells and phospholipase Cgamma1 were also profoundly diminished. This proximal defect impacted mostly on the calcium/NFAT and NF-kappaB pathways, with only a modest decrease in ERK1/2 phosphorylation. This state was contrasted with T cell clonal anergy in which the RAS/MAPK pathway was preferentially impaired and there was much less inhibition of Zap70 kinase activity. Both hyporesponsive states manifested a block in IkappaB degradation. These results demonstrate that T cell adaptive tolerance and clonal anergy are distinct biochemical states, possibly providing T cells with two molecular mechanisms to curtail responsiveness in different biological circumstances.

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.

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.

Molecular signature of recent thymic selection events on effector and regulatory CD4+ T lymphocytes

Natural CD4+CD25+ regulatory T lymphocytes (Treg) are key protagonists in the induction and maintenance of peripheral T cell tolerance. Their thymic origin and biased repertoire continue to raise important questions about the signals that mediate their development. We validated analysis of MHC class II capture by developing thymocytes from thymic stroma as a tool to study quantitative and qualitative aspects of the cellular interactions involved in thymic T cell development and used it to analyze Treg differentiation in wild-type mice. Our data indicate that APCs of bone marrow origin, but, surprisingly and importantly, not thymic epithelial cells, induce significant negative selection among the very autoreactive Treg precursors. This fundamental difference between thymic development of regulatory and effector T lymphocytes leads to the development of a Treg repertoire enriched in cells specific for a selected subpopulation of self-Ags, i.e., those specifically expressed by thymic epithelial cells.

Modeling T cell antigen discrimination based on feedback control of digital ERK responses

T-lymphocyte activation displays a remarkable combination of speed, sensitivity, and discrimination in response to peptide-major histocompatibility complex (pMHC) ligand engagement of clonally distributed antigen receptors (T cell receptors or TCRs). Even a few foreign pMHCs on the surface of an antigen-presenting cell trigger effective signaling within seconds, whereas 1 x 10(5)-1 x 10(6) self-pMHC ligands that may differ from the foreign stimulus by only a single amino acid fail to elicit this response. No existing model accounts for this nearly absolute distinction between closely related TCR ligands while also preserving the other canonical features of T-cell responses. Here we document the unexpected highly amplified and digital nature of extracellular signal-regulated kinase (ERK) activation in T cells. Based on this observation and evidence that competing positive- and negative-feedback loops contribute to TCR ligand discrimination, we constructed a new mathematical model of proximal TCR-dependent signaling. The model made clear that competition between a digital positive feedback based on ERK activity and an analog negative feedback involving SH2 domain-containing tyrosine phosphatase (SHP-1) was critical for defining a sharp ligand-discrimination threshold while preserving a rapid and sensitive response. Several nontrivial predictions of this model, including the notion that this threshold is highly sensitive to small changes in SHP-1 expression levels during cellular differentiation, were confirmed by experiment. These results combining computation and experiment reveal that ligand discrimination by T cells is controlled by the dynamics of competing feedback loops that regulate a high-gain digital amplifier, which is itself modulated during differentiation by alterations in the intracellular concentrations of key enzymes. The organization of the signaling network that we model here may be a prototypic solution to the problem of achieving ligand selectivity, low noise, and high sensitivity in biological responses.

Naive CD4+ T Cells from Lupus-Prone Fas-Intact MRL Mice Display TCR-Mediated Hyperproliferation Due to Intrinsic Threshold Defects in Activation

Autoreactive T cell activation is a consistent feature of murine lupus; however, the mechanism of such activation remains unclear. We hypothesized that naive CD4+ T cells in lupus have a lower threshold of activation through their TCR-CD3 complex that renders them more susceptible to stimulation with self-Ags. To test this hypothesis, we compared proliferation, IL-2 production, and single cell calcium signaling of naive CD4+ T cells isolated from Fas-intact MRL/+(Fas-lpr) mice with H-2k-matched B10.BR and CBA/CaJ controls, following anti-CD3 stimulation in the presence or absence of anti-CD28. We also assessed the responsiveness of naive CD4+ T cells isolated from Fas-intact MRL and control mice bearing a rearranged TCR specific for amino acids 88-104 of pigeon cytochrome c to cognate and low affinity peptide Ags presented by bone marrow-matured dendritic cells. TCR transgenic and wild-type CD4+ T cells from MRL mice displayed a lower threshold of activation than control cells, a response that was class II MHC dependent. The rise in intracellular calcium in MRL vs controls was enhanced and prolonged following anti-CD3 triggering, suggestive of proximal defects in TCR-engendered signaling as the mechanism for the observed hyperactivity. These findings were observed as early as 1-2 mo postweaning and, based on analysis of F1 T cells, appeared to be dominantly expressed. This genetically altered threshold for activation of MRL T cells, a consequence of a proximal defect in CD3-mediated signal transduction, may contribute to the abrogation of T cell tolerance to self-Ags in lupus.

Defining the parameters necessary for T-cell recognition of ligands that vary in potency

Identification of the mechanisms by which a T cell is able to sense ligands of varying strength, such as those that mediate tumor growth, viral evasion, and autoimmunity, is a major goal of T-cell activation studies. In recent years, parameters important for T-cell activation by strong ligands (agonists) are beginning to be characterized. Here, we review our current work on the factors that are critical for T-cell activation by ligands that differ in potency, typified by full agonists, weak agonists, partial agonists, and antagonists. Furthermore, we discuss mechanisms contributing to the lack of a full range of effector functions observed in T cells following their stimulation by suboptimal ligands. Finally, we present strategies for the design of peptide-based therapies to control activation of polyclonal, autoreactive T-cell populations.

Activation of naive CD4+ T cells in vivo by a self-peptide mimic: mechanism of tolerance maintenance and preservation of immunity

Intrathymic selection generates a peripheral repertoire of CD4(+) T cells with receptors that retain low affinity for self-peptide MHC complexes. Despite self-recognition, T cells remain tolerant even in the setting of microbial challenge and resultant costimulatory signals. We demonstrate here a novel mechanism for tolerance maintenance under conditions of self-recognition and strong costimulation. TCR engagement in vivo with a low-avidity peptide, as a mimic of self, provided with poly(I:C) (dsRNA) led to division of naive T cells that was dependent upon costimulatory signals; however, the dividing cells rapidly underwent deletion. By contrast, the surviving cells that were activated as evidenced by up-regulation of CD69 did not become effectors upon restimulation with the same ligand and maintained an effective response against agonist peptide. We suggest TCR engagement with self-peptide MHC complexes promotes tolerance maintenance during pathogen challenge, while preserving efficient reactivity for subsequent encounter with foreign Ags.

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.

Activation-threshold tuning in an affinity model for the T-cell repertoire

Naive T cells respond to peptides from foreign proteins and remain tolerant to self peptides from endogenous proteins. It has been suggested that self tolerance comes about by a 'tuning' mechanism, i.e. by increasing the T-cell activation threshold upon interaction with self peptides. Here, we explore how such an adaptive mechanism of T-cell tolerance would influence the reactivity of the T-cell repertoire to foreign peptides. We develop a computer simulation model in which T cells are tolerized by increasing their activation-threshold dependent on the affinity with which they see self peptides presented in the thymus. Thus, different T cells acquire different activation thresholds (i.e. different cross-reactivities). In previous mathematical models, T-cell tolerance was deletional and based on a fixed cross-reactivity parameter, which was assumed to have evolved to an optimal value. Comparing these two different tolerance-induction mechanisms, we found that the tuning model performs somewhat better than an optimized deletion model in terms of the reactivity to foreign antigens. Thus, evolutionary optimization of clonal cross-reactivity is not required. A straightforward extension of the tuning model is to delete T-cell clones that obtain a too high activation threshold, and to replace these by new clones. The reactivity of the immune repertoires of such a replacement model is enchanced compared with the basic tuning model. These results demonstrate that activation-threshold tuning is a functional mechanism for self tolerance induction.

Dynamics of T cell activation threshold tuning

T lymphocytes are believed to alter their sensitivity to TCR stimulation by means of a tunable cellular activation threshold. We present two modelling examples which show that the concept of a tunable threshold can be made mechanistically plausible. The tunable threshold is treated as an emergent property of the dynamics of the T cell's signalling machinery. In addition, we discuss how the dynamic properties of activation threshold tuning can be determined experimentally with the aid of these two models. We propose a novel 'avidity selection' mechanism for the initial stages of the immune response, based on the properties of the T cell activation threshold tuning mechanism we propose for the commitment to differentiation. Our main finding is that activation threshold tuning allows T cells to respond to relevant ligands with a detection threshold that is (i) uniform across both the T cell repertoire and the secondary lymphoid tissues, while (ii) retaining tolerance to autostimulation. Our analysis indicates that central tolerance enhances the efficiency of peripheral tolerance, casting new light on the role of negative selection in the thymus.

The impact of self-tolerance on the polyclonal CD8+ T cell repertoire

TCRs possess considerable cross-reactivity toward structurally related Ags. Because the signaling threshold for negative selection is lower than that required for activation of mature T cells, the question arises as to which extent thymic deletion of self-specific T cells affects T cell responsiveness toward foreign peptides. In this study we show, in three different mouse models systems, that the polyclonal CD8(+) T cell repertoire has a marked ability to react against the majority of Ags related to self despite self-tolerance, even in cases where self and foreign differ only marginally at a single TCR-contact residue. Thus, while individual T cells are markedly cross-reactive, the ability to distinguish between closely related Ags is introduced at the polyclonal T cell level.

Ligand-dependent regulation of T cell development and activation

Mature CD4+ and CD8 + T lymphocytes develop in the thymus from precursors with diverse clonally distributed receptors, possessing binding sites with negligible, intermediate, or high affinity for selfpeptide major histocompatibility complex (MHC) ligands. Positive- and negative-selection processes acting on this precursor pool yield a peripheral T cell population comprised of cells with receptors (TCR) capable of self-peptide MHC ligand recognition, but largely depleted of those able to mediate overt self-responsiveness. The Lymphocyte Biology Section of the Laboratory of Immunology studies how self-ligand recognition guides T cell development in the thymus and influences the functionality of naive and activated T cells in the periphery. It also seeks to define the molecular basis for the discrimination between self-ligands and foreign antigens that controls T cell activation to effector function. Finally, it uses a combination of conventional cellular immunological methods, biochemical and biophysical studies, and advanced imaging techniques to visualize, quantitate, and model the various steps in the development of primary and memory T cell immune responses.

Thymocyte sensitivity and supramolecular activation cluster formation are developmentally regulated: a partial role for sialylation

TCR reactivity is tuned during thymic development. Immature thymocytes respond to low-affinity self-ligands resulting in positive selection. Following differentiation, T cells no longer respond to low-affinity ligands, but respond well to high-affinity (foreign) ligands. We show in this study that this response includes integrin activation, supramolecular activation cluster formation, Ca(2+) flux, and CD69 expression. Because glycosylation patterns are known to change during T cell development, we tested whether alterations in sialylation influence CD8 T cell sensitivity to low affinity TCR ligands. Using neuraminidase treatment or genetic deficiency in the ST3Gal-I sialyltransferase, we show that desialylation of mature CD8 T cells enhances their sensitivity to low-affinity ligands, although these treatments do not completely recapitulate the dynamic range of immature T cells. These studies identify sialylation as one of the factors that regulate CD8 T cell tuning during development.

The strength of persistent antigenic stimulation modulates adaptive tolerance in peripheral CD4+ T cells

The quantitative adaptation of receptor thresholds allows cells to tailor their responses to changes in ambient ligand concentration in many biological systems. Such a cell-intrinsic calibration of T cell receptor (TCR) sensitivity could be involved in regulating responses to autoantigens, but this has never been demonstrated for peripheral T cells. We examined the ability of monoclonal naive T cells to modulate their responsiveness differentially after exposure to fourfold different levels of persistent antigen stimulation in vivo. T cells expanded and entered a tolerant state with different kinetics in response to the two levels of stimulation, but eventually adjusted to a similar slow rate of turnover. In vivo restimulation revealed a greater impairment in the proliferative ability of T cells resident in a higher antigen presentation environment. We also observed subtle differences in TCR signaling and in vitro cytokine production consistent with differential adaptation. Unexpectedly, the system failed to similarly compensate to the persistent stimulus in vivo at the level of CD69 expression and actin polymerization. This greater responsiveness of T cells residing in a host with a lower level of antigen presentation allows us to demonstrate for the first time an intrinsic tuning process in mature T lymphocytes, albeit one more complex than current theories predict.

Negative selection imparts peptide specificity to the mature T cell repertoire

The T cell alphabeta receptor (TCR) recognizes foreign peptide antigens bound to proteins encoded in the MHC. The MHC portion of this complex contributes much to the footprint of the TCR on the ligand, yet T cells are usually very specific for individual foreign peptides. Here, we show that the development of peptide-specific T cells is not intrinsic to thymocytes that undergo thymic-positive selection but is an outcome of eliminating, through negative selection, thymocytes bearing TCRs with extensive peptide cross-reactivity. Hence, thymic-negative selection imposes peptide specificity on the mature T cell repertoire.

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.

The influence of the thymic environment on the CD4-versus-CD8 T lineage decision

T cell receptor signaling is an essential factor regulating thymocyte selection, but the function of the thymic environment in this process is not clear. In mice transgenic for major histocompatibility complex class II-restricted T cell receptors, every thymocyte is potentially selectable for maturation in the CD4 lineage. To address whether selection frequency affects positive selection, we created hematopoietic chimeras with mixtures of selectable and nonselectable precursors. With increased proportions of nonselectable thymocytes, positive selection of MHC class II-specific precursors was enhanced, generating not only CD4 but also CD8 thymocytes. These results indicate that the CD4 versus CD8 fate of selectable precursors can be influenced by the selection potential of its neighbors.

Self-recognition and the regulation of CD4+ T cell survival

CD4+ T cells differentiate in the thymus from committed precursors to mature naive cells ready for peripheral circulation. Successful maturation depends on adequate but not excessive signaling upon T cell receptor (TCR) engagement of self-peptide/MHC class II molecule ligands present in the thymic environment. Persistent TCR signaling throughout development from the CD4+CD8+ to the CD4+ state is required for completion of the developmental process. Recent work has suggested that a continuation of this signaling is essential for sustained survival of CD4+ T cells once they leave the thymus but our studies suggest otherwise. Although we found clear evidence for active TCR signaling involving recognition of self-ligands in peripheral lymphoid tissues, we did not see a substantial effect of loss of such signaling on the life-time of naive CD4+ T cells. Based on a careful review of the literature, we conclude that essentially all previous claims that MHC class II recognition plays a significant role in the survival of CD4+ T cells can be reinterpreted as an effect of self-recognition on proliferation in lymphopenic environments, maintaining population numbers without a marked effect on individual cell viability. We propose a possible explanation for why, in contrast, the viability of naive CD8+ T cells appears to show such self-MHC dependence and suggest that a primary function of self-recognition by T cells may be to enhance responses to foreign antigen.

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.

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

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

TCR reserve: a novel principle of CD4 T cell activation by weak ligands

Some ligand-receptor systems have a receptor reserve where a maximal response can be achieved by occupation of a fraction of available receptors. An implication of a receptor reserve is the expansion of the number of ligands for response. To determine whether T cells follow receptor reserve, we have characterized the effect of reducing TCR levels on CD4 T cell responses elicited by altered peptide ligands that vary in potency. Agonist peptide is unaffected by a 90% reduction in TCR level while proliferation to weak agonists is significantly inhibited when TCR expression is reduced by 40%. Thymocyte-negative selection similarly demonstrates a differential requirement of TCR for response to agonist, weak agonist, and partial agonist. Therefore, our data demonstrate receptor reserve as a novel principle of T cell activation in which excess TCRs expand the antigenic repertoire to include less potent ligands.

Known and potential functions for the SLP-76 adapter protein in regulating T-cell activation and development

The hematopoietic adapter protein SLP-76 is a critical component of multiple biochemical signaling 'circuits' in T cells that integrate proximal signaling events initiated by ligation of the T-cell receptor (TCR) into more distal pathways. Given the important role ascribed to TCR signaling in directing the outcome of thymocyte selection, it seems likely that SLP-76 may also function in signaling pathways that ultimately impact the establishment of the peripheral T-cell repertoire. It is generally accepted that the peripheral T-cell repertoire is selected in large part during T-cell development in the thymus. Molecular interactions between the TCR and self-peptide/major histocompatibility complexes expressed on thymic stromal elements dictate the fate of developing thymocytes. Thymocyte survival and further maturation (positive selection) require an active signal delivered to the cell as a consequence of TCR ligation. This raises the intriguing question of how a thymocyte can, for a narrow window of developmental time, obtain responsiveness to self while maintaining tolerance to these same determinants upon export to the periphery. This article reviews the current literature describing SLP-76-dependent signaling pathways in mature T cells and developing thymocytes. A potential role for this critical signaling intermediate in integrating signals leading to positive and negative selection of the peripheral T-cell repertoire is also discussed.

Type 1 cytokines polarize thymocytes during T cell development in adult thymus organ cultures

Peripheral T cells can be polarized towards type 1 or type 2 cytokine immune responses during TCR engagement. Because T cell selection by peptide plus self-MHC in the thymus requires TCR engagement, we hypothesized that type 1 cytokines may polarize developing T cells. We cultured thymi from BBDR rats in adult thymus organ cultures (ATOC) under type 1 cytokine conditions in the absence of exogenous antigen. Type 1 cytokine-conditioned ATOC generated cells that spontaneously secreted high levels of IFNgamma, but not IL-4. A second exposure to type 1 cytokines further increased IFNgamma secretion by these cells, most of which were blasts that expressed the activation markers CD25, CD71, CD86, and CD134. Studies using blocking antibodies and pharmacological inhibitors suggested that both IL-18 and cognate TCR-MHC/ligand interactions were important for activation. Blocking anti-MHC class I plus anti-MHC class II antibodies, neutralizing anti-IL-18 antibody, and the p38 MAP-kinase inhibitor SB203580 each reduced IFNgamma production by approximately 75-80%. Cyclosporin A, which prevents TCR signaling, inhibited IFNgamma production by approximately 50%. These data demonstrate that exposure to type 1 cytokines during intrathymic development can polarize differentiating T cells, and suggest a mechanism by which intrathymic exposure to type 1 cytokines may modulate T cell development.

On the role of self-recognition in T cell responses to foreign antigen

The key role of the thymus in shaping the peripheral T cell receptor (TCR) repertoire has been appreciated for nearly a quarter of a century. For most of that time, a single model has dominated thinking about the physiological role of the positive selection process mediated by TCR recognition of self-peptides and major histocompatibility complex (MHC) molecules. This developmental filter was believed to populate secondary lymphoid tissues with T cells bearing receptors best able to recognize unknown foreign peptides associated with the particular allelic forms of the MHC molecules present in an individual. More recently, self-recognition has been suggested to regulate the viability of naïve T cells. Here we focus on new results indicating that a critical contribution of positive selection to host defense is insuring that each peripheral T cell can use self-recognition to (i) enhance TCR signaling sensitivity upon foreign antigen recognition and (ii) augment the clonal expansion that accompanies limiting foreign antigen display at early points in an infectious process. We also detail new insights into the intracellular signaling circuitry that underlies the effective discrimination between low- and high-quality ligands of the TCR and speculate on how this design might facilitate an additional contribution of self-recognition to T cell activation in the presence of foreign stimuli.

A serine/threonine phosphorylation site in the ectodomain of a T cell receptor beta chain is required for activation by superantigen

The presence of consensus phosphorylation sites in the ectodomains of cell surface proteins suggests that such post-translational modification may be important in regulation of surface receptor activity. To date, the only cell surface receptor for which such ectodomain phosphorylation has been conclusively demonstrated is the clonally expressed T cell antigen receptor (TCR). Attempts to conclusively identify individual phosphorylated residues in TCR alpha and beta chains and determine their functional significance by biochemical approaches failed due to insufficient quantities of purified molecules. Here we present the results of an alternative approach where survey of phosphorylation sites in the TCR alpha and beta chains was accomplished using site-directed mutagenesis and retroviral vector expression, as well as in vitro phosphorylation of synthetic peptide substrates. All mutants studied directed the cell surface expression of normal amounts of TCR, and all transfectants could be stimulated to produce IL-2 in response to substrate-immobilized antibody to TCR. However, mutation of serine-88 in the protein kinase A phosphorylation site of the TCR beta chain resulted in a complete lack of response to the superantigen staphylococcal enterotoxin B (SEB). In addition, this mutation abolished TCR-associated tyrosine phosphorylation, consistent with the impairment of cell signaling. Reversion of the serine-88/alanine mutation with phosphorylatable threonine completely restored the SEB recognition by TCR. These results, interpreted in the context of the known three-dimensional structure of the complex of SEB and TCR, are consistent with the view that serine-88 is important for the contact of the TCR beta chain with SEB.

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.

Reduced self-reactivity of an autoreactive T cell after activation with cross-reactive non-self-ligand

Autoreactive CD4(+) T lymphocytes are critical to the induction of autoimmune disease, but because of the degenerate nature of T cell receptor (TCR) activation such receptors also respond to other ligands. Interaction of autoreactive T cells with other non-self-ligands has been shown to activate and expand self-reactive cells and induce autoimmunity. To understand the effect on the autoreactivity of naive cross-reactive T cells of activation with a potent nonself ligand, we have generated a TCR transgenic mouse which expresses a TCR with a broad cross-reactivity to a number of ligands including self-antigen. The activation of naive transgenic recombination activating gene (Rag)2(-)(/)(-) T cells with a potent non-self-ligand did not result in a enhancement of reactivity to self, but made these T cells nonresponsive to the self-ligand and anti-CD3, although they retained a degree of responsiveness to the non-self-ligand. These desensitized cells had many characteristics of anergic T cells. Interleukin (IL)-2 production was selectively reduced compared with interferon (IFN)-gamma. p21(ras) activity was reduced and p38 mitogen-activated protein kinase (MAPK) was relatively spared, consistent with known biochemical characteristics of anergy. Surprisingly, calcium fluxes were also affected and the anergic phenotype could not be reversed by exogenous IL-2. Therefore, activation with a hyperstimulating non-self-ligand changes functional specificity of an autoreactive T cell without altering the TCR. This mechanism may preserve the useful reactivity of peripheral T cells to foreign antigen while eliminating responses to self.

Sweet 'n' sour: the impact of differential glycosylation on T cell responses

The fate and functional activity of T lymphocytes depend largely on the precise timing of gene expression and protein production. However, it is clear that post-translational modification of proteins affects their functional properties. Although modifications such as phosphorylation have been intensely studied by immunologists, less attention has been paid to the impact that changes in glycosylation have on protein function. However, there is considerable evidence that glycosylation plays a key role in immune regulation. We will focus here on examples in which differential glycosylation affects the development, survival or reactivity of T cells.

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").

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.

Contributions of the T cell receptor-associated CD3gamma-ITAM to thymocyte selection

The immunoreceptor tyrosine-based activation motifs (ITAMs) in the CD3 chains associated with the T cell receptor (TCR) are crucial for TCR signaling. To probe the role of the CD3gamma-ITAM in T cell development, we created knock-in mice in which the CD3gamma chain of the TCR complex is replaced by a mutant signaling-deficient CD3gamma chain, lacking the CD3gamma-ITAM. This mutation results in considerable impairment in positive selection in the polyclonal TCR repertoire. When CD3gamma-deltaITAM mice are crossed to mice expressing transgenic F5 TCRs, their thymocytes are completely unable to perform positive selection in vivo in response to intrathymic ligands. Also, the in vitro positive selection response of double-positive (DP) thymocytes with F5-CD3gamma-deltaITAM mutant receptors to their agonist ligand and many of its variants is severely impaired or abrogated. Yet, the binding and dissociation constants of agonist ligands for the F5 receptor are not affected by the CD3gamma-deltaITAM mutation. Furthermore, DP thymocytes with mutant receptors can respond to agonist ligand with normal antigen sensitivity and to normal levels, as shown by their ability to induce CD69 up-regulation, TCR down-regulation, negative selection, and ZAP70 and c-Jun NH2-terminal kinase activation. In sharp contrast, induction of extracellular signal-regulated kinase (ERK) activation and linker for activation of T cells (LAT) phosphorylation are severely impaired in these cells. Together, these findings underscore that intrinsic properties of the TCR-CD3 complex regulate selection at the DP checkpoint. More importantly, this analysis provides the first direct genetic evidence for a role of the CD3gamma-ITAM in TCR-driven thymocyte selection.

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.

A T-cell functional phenotype common among autoimmune-prone rodent strains

The genetic basis and familial clustering of autoimmunity suggest that common phenotypic traits predispose individuals to disease. We found a hyporesponsive T-cell phenotype that was shared by all autoimmune-prone mouse and rat strains tested, including MRL, nonobese diabetic (NOD), NZB, NZW, NZB/W F1, SJL and SWR mice, as well as DA and BB rats, but was not evident in nonautoimmune-prone rodents. This T-cell intrinsic, age-independent hyporesponsiveness is measured as an increased activation threshold for upregulation of activation markers upon T-cell receptor (TCR) cross-linking both in vitro and in vivo. Inefficient deletion of CD4 and CD8 single-positive, heat stable antigen (HSA)hi medullary thymocytes was also observed in hyporesponsive donors. We interpret these data to suggest that increased TCR-mediated signalling thresholds in autoimmune-prone individuals may contribute to the escape of autoreactive thymocytes from negative selection.