A unified model for T cell antigen recognition and thymic selection of the T cell repertoire

Tolerogenic vaccines: Targeting the antigenic and cytokine niches of FOXP3+ regulatory T cells

FOXP3+ regulatory T cells (Tregs) constitute a critical barrier that enforces tolerance to both the self-peptidome and the extended-self peptidome to ensure tissue-specific resistance to autoimmune, allergic, and other inflammatory disorders. Here, we review intuitive models regarding how T cell antigen receptor (TCR) specificity and antigen recognition efficiency shape the Treg and conventional T cell (Tcon) repertoires to adaptively regulate T cell maintenance, tissue-residency, phenotypic stability, and immune function in peripheral tissues. Three zones of TCR recognition efficiency are considered, including Tcon recognition of specific low-efficiency self MHC-ligands, Treg recognition of intermediate-efficiency agonistic self MHC-ligands, and Tcon recognition of cross-reactive high-efficiency agonistic foreign MHC-ligands. These respective zones of TCR recognition efficiency are key to understanding how tissue-resident immune networks integrate the antigenic complexity of local environments to provide adaptive decisions setting the balance of suppressive and immunogenic responses. Importantly, deficiencies in the Treg repertoire appear to be an important cause of chronic inflammatory disease. Deficiencies may include global deficiencies in Treg numbers or function, subtle 'holes in the Treg repertoire' in tissue-resident Treg populations, or simply Treg insufficiencies that are unable to counter an overwhelming molecular mimicry stimulus. Tolerogenic vaccination and Treg-based immunotherapy are two therapeutic modalities meant to restore dominance of Treg networks to reverse chronic inflammatory disease. Studies of these therapeutic modalities in a preclinical setting have provided insight into the Treg niche, including the concept that intermediate-efficiency TCR signaling, high IFN-β concentrations, and low IL-2 concentrations favor Treg responses and active dominant mechanisms of immune tolerance. Overall, the purpose here is to assimilate new and established concepts regarding how cognate TCR specificity of the Treg repertoire and the contingent cytokine networks provide a foundation for understanding Treg suppressive strategy.

TCR affinity promotes CD8+ T cell expansion by regulating survival

Ligation with high affinity ligands are known to induce T lymphocytes to become fully activated effector cells while ligation with low affinity ligands (or partial agonists) may result in a delayed or incomplete response. We have examined the quantitative features of CD8(+) T cell proliferation induced by peptides of different TCR affinities at a range of concentrations in the mouse OT-I model. Both the frequency of cells responding and the average time taken for cells to reach their first division are affected by peptide concentration and affinity. Consecutive division times, however, remained largely unaffected by these variables. Importantly, we identified affinity to be the sole regulator of cell death in subsequent division. These results suggest a mechanism whereby TCR affinity detection can modulate the subsequent rate of T cell growth and ensure the dominance of higher affinity clones over time.

T cell allorecognition and MHC restriction--A case of Jekyll and Hyde?

A great paradox in cellular immunology is how T cell allorecognition exists at high frequencies (up to 10%) despite the stringent requirements of discriminating 'self' from 'non-self' imposed by MHC restriction. Thus, in tissue transplantation, a substantial proportion of the recipient's T cells will have the ability to recognize the graft and instigate an immune response against the transplanted tissue, ultimately resulting in graft rejection--a manifestation of T cell alloreactivity. Transplantation of human organs and lymphoid cells as treatment for otherwise life-threatening diseases has become a more routine medical procedure making this problem of great importance. Immunologists have gained important insights into the mechanisms of T cell alloreactivity from cytotoxic T cell assays, affinity-avidity studies, and crystal structures of peptide-MHC (pMHC) molecules and T cell receptors (TCRs) both alone and in complex. Despite the clinical significance of alloreactivity, the crystal structure of an alloreactive human TCR in complex with both cognate pMHC and an allogeneic pMHC complex has yet to be determined. This review highlights some of the important findings from studies characterizing the way in which alloreactive T cell receptors and pMHC molecules interact in an attempt to resolve this great irony of the cellular immune response.

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.

T-cell receptor antagonist modifies cytokine secretion profile of naive CD4+ T cells and their differentiation into type-1 and type-2 helper T cells

A T-cell receptor (TCR) antagonist is an analog of a peptide ligand for TCR that inhibits T-cell responses to the original peptide. We investigated the effects of a TCR antagonist on cytokine secretion of naive CD4+ T cells and their differentiation into type-1 and type-2 helper T cells (Th1 and Th2) induced by stimulation with varying doses of an antigenic peptide. In the presence of a TCR antagonist peptide, proliferation of naive CD4+ T cells and antigen dose-dependent secretion of interferon-gamma, a typical Th1-type cytokine, by these cells was down-regulated. With respect to the secretion of interleukin-4 (IL-4), a typical Th2-type cytokine, the TCR antagonist raised the concentration of the antigenic peptide required to elicit maximal IL-4 production and, surprisingly, significantly increased the maximum level of IL-4 secretion. Similar effects induced by the TCR antagonist were observed on the Th1/Th2 differentiation of naive CD4+ T cells. These results clearly indicate that, for naive CD4+ T cells, a TCR antagonist has the potential to change the balance of Th1/Th2 cytokine secretion and even enhance Th2 responses.

Could TCR antagonism explain associations between MHC genes and disease?

Alleles of major histocompatibility complex (MHC) loci are associated with certain types of diseases, including those of infectious and autoimmune origin. MHC products can promote susceptibility or resistance to disease by stimulating or inhibiting immune responses. Recent evidence suggests that MHC-associated peptides derived from self-proteins can act as antagonists of T-cell activation, thereby inhibiting immune responses to antigens. We suggest that self-peptide-promoted antagonism might explain some associations between MHC alleles and particular chronic diseases.

Genomics-based identification of self-ligands with T cell receptor-specific biological activity

Self-peptide/major histocompatibility complex (MHC) complexes profoundly influence the biology of T lymphocytes. They promote the selection of the T cell receptor (TCR) repertoire in the thymus, maintain the homeostasis of peripheral T cells prior to encounter with antigen, and modify the responsiveness of T cells to foreign antigens. In addition, they can serve as antigens for autoaggressive T cells that induce autoimmune diseases. The complete sequencing of the genomes of human, mouse, and many pathogenic organisms now provides us with a comprehensive list of all possible proteins that may be the source of foreign antigenic and self-peptides. A computational approach using profile-based similarity searches on potential self-MHC-binding peptides can be used to efficiently predict self-peptides with biological activities. The common feature of the identified peptides is similarity to antigen. Thus, self-peptides may form 'hazy' images of the universe of antigens that are used as templates to create and maintain the TCR repertoire.

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.

A reliable and safe T cell repertoire based on low-affinity T cell receptors

Antigens are presented to T cells as short peptides bound to MHC molecules on the surface of body cells. The binding between MHC/peptides and T cell receptors (TCRs) has a low affinity and is highly degenerate. Nevertheless, TCR-MHC/peptide recognition results in T cell activation of high specificity. Moreover, the immune system is able to mount a cellular response when only a small fraction of the MHC molecules on an antigen-presenting cell is occupied by foreign peptides, while autoimmunity remains relatively rare. We consider how to reconcile these seemingly contradictory facts using a quantitative model of TCR signalling and T cell activation. Taking into account the statistics of TCR recognition and antigen presentation, we show that thymic selection can produce a working T cell repertoire which will produce safe and effective responses, that is, recognizes foreign antigen presented at physiological levels while tolerating self. We introduce "activation curves" as a useful tool to study the repertoire's statistical activation properties.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.

The dynamics of T cell receptor signaling: complex orchestration and the key roles of tempo and cooperation

T cells constantly sample their environment using receptors (TCR) that possess both a germline-encoded low affinity for major histocompatibility complex (MHC) molecules and a highly diverse set of CDR3 regions contributing to a range of affinities for specific peptides bound to these MHC molecules. The decision of a T cell "to sense and to respond" with proliferation and effector activity rather than "to sense, live on, but not respond" is dependent on TCR interaction with a low number of specific foreign peptide:MHC molecule complexes recognized simultaneously with abundant self peptide-containing complexes. Interaction with self-complexes alone, on the other hand, generates a signal for survival without a full activation response. Current models for how this distinction is achieved are largely based on translating differences in receptor affinity for foreign versus self ligands into intracellular signals that differ in quality, intensity, and/or duration. A variety of rate-dependent mechanisms involving assembly of molecular oligomers and enzymatic modification of proteins underlie this differential signaling. Recent advances have been made in measuring TCR:ligand interactions, in understanding the biochemical origin of distinct proximal and distal signaling events resulting from TCR binding to various ligands, and in appreciating the role of feedback pathways. This new information can be synthesized into a model of how self and foreign ligand recognition each evoke the proper responses from T cells, how these two classes of signaling events interact, and how pathologic responses may arise as a result of the underlying properties of the system. The principles of signal spreading and stochastic resonance incorporated into this model reveal a striking similarity in mechanisms of decision-making among T cells, neurons, and bacteria.

Differential Presentation of an Altered Peptide Within Fetal Central and Peripheral Organs Supports an Avidity Model for Thymic T Cell Development and Implies a Peripheral Readjustment for Activation

Altered self peptides may drive T cell development by providing avidity of interactions low enough to potentiate positive selection but not powerful enough to trigger programmed cell death. Since the peptide repertoire in both central and peripheral organs is nearly the same, interactions of these peptides with T cells in the thymus would have to be different from those taking place in the periphery; otherwise, T cell development and maturation would result in either autoimmunity or T cell deficiency. Herein, a self and an altered self peptide were delivered to fetuses, and their presentation as well as the consequence of such presentation on T cell development were assessed. The results indicate that the self peptide was presented in both central and peripheral fetal organs and that such presentation abolished T cell responses to both peptides during adult life. However, the altered peptide, although presented in vivo as well as in vitro by splenic cells, was unable to stimulate a specific T cell clone when the presenting cells were of thymic origin and allowed offspring to be responsive to both peptides. These findings indicate that central and peripheral organs accommodate selection and peripheral survival of T cells by promoting differential altered peptide presentation.

Partial agonism elicits an enduring phase of T-cell-medicated antigen presentation

Previous studies have shown that the anti-CD4 mAb W3/25 strongly enhances T cell APC (T-APC) activity. In this study, single positive CD4+ and double negative (DN) (CD4-CD8-) T-helper cells specific for the 55-69 or 72-86 sequence of guinea pig (GP) myelin basic protein (GPMBP) were used to study CD4 regulation of T-APC activity. Clones were cultured with irradiated SPL and GPMBP or rat (R) MBP for 2-3 days, were propagated in IL-2 for another 1-3 days, were irradiated, and were used as T-APC. DN T cells specific for GP55-69 effectively presented GPMBP and were superior APC compared to other CD4+ T cells for presentation of this antigen. In contrast, DN T cells specific for the dominant encephalitogenic 72-86 determinant did not effectively present the agonist GPMBP but potently presented the partial agonist RMBP. The heightened APC activity of DN T cells reflected the lack of CD4 because the anti-CD4 mAb W3/25 promoted T-APC activity of CD4+ T cells to those levels expressed by DN T cells. Overall, T cells with potent reactivity to GPMBP or RMBP were subsequently unable to present that antigen, whereas T cells exhibiting partial or low antigen reactivities were highly effective APC for presentation of that antigen. The unrelated antigen conalbumin was presented by MBP-specific clones only when added to culture with a specific partial agonist. Together, these data indicate that partially agonistic MHC ligands promote prolonged expression of T-APC activity and that DN T cells may be specialized to mediate postactivational antigen presentation.

Class II MHC/peptide complexes on T cell antigen-presenting cells: agonistic antigen recognition inhibits subsequent antigen presentation

Previous studies have shown that tolerogenic anti-CD4 (W3/25) and anti-LFA-1 mAb (LRTC1) which block T cell activation paradoxically enhance T cell-mediated antigen presentation. Lasting T cell APC (T-APC) activity requires and initial exposure of T cells to these mAb in the presence of professional APC and antigen. This study revealed a central mechanism regulating the duration of T-APC activity. T cell recognition of class II MHC complexes of T-APC catalyzed a rapid decay in the presentation of agonistic antigens, whereas partial agonistic signals decayed at a shower rate. Likewise, blockade of agonistic T-T cell autorecognition by these mAb led to the persistence of agonistic MHC/antigen on T-APC. The best predictor of T-APC activity was related to the ability of clonal T cells to respond to antigen presented by neighboring T cells. Strong responders were inefficient T-APC, whereas inefficient responders were strong T-APC. Addition of irradiated myelin basic protein (MBP0-specific responders to T-APC cultures specifically inhibited the subsequent presentation of MBP but not conalbumin, and vice versa. T-APC presentation of antigen to responder T cells also resulted in reduced surface expression of class II MHC I-A glycoproteins on T-APC. These findings indicate that agonistic recognition of antigen of T-APC specifically inhibits subsequent presentation of that antigen, whereas antagonistic MHC/antigen complexes are preserved for an enduring T-APC activity.

T Cells Expressing Receptors of Different Affinity for Antigen Ligands Reveal a Unique Role for p59fyn in T Cell Development and Optimal Stimulation of T Cells by Antigen

Signaling from the TCR involves the protein tyrosine kinase p59fyn (Fyn). Previous studies have shown that T cell development occurs normally in Fyn−/− mice. In this study, we investigated the requirement for Fyn in the development and function of T cells expressing either the transgenic 2C TCR, with high affinity for its Ag ligand, or the transgenic H-Y TCR, representative of a low affinity TCR. Although Fyn was not essential for positive selection of thymocytes expressing either the 2C or the H-Y TCR, it facilitated the down-regulation of the heat-stable Ag in positively selected CD4−CD8+ thymocytes in both 2C and H-Y mice. Negative selection of thymocytes expressing the H-Y TCR also occurs efficiently in Fyn−/− mice. However, in Fyn−/− mice, there was a preferential survival of thymocytes that expressed higher levels of the CD8 coreceptor and the transgenic TCR. Positively selected CD4−CD8+ thymocytes and peripheral T cells expressing either the 2C or the H-Y TCR differed in their requirement of Fyn for optimal proliferation responses to stimulation by antigenic ligands. Whereas 2C Fyn−/− or 2C Fyn+/+ thymocytes and peripheral T cells responded optimally to stimulation by the specific Ag, H-Y Fyn−/− thymocytes and peripheral T cells were hyporesponsive compared with Fyn+/+ cells. Significantly, in response to a defined low affinity ligand, both 2C Fyn−/− thymocytes and peripheral T cells required Fyn for optimal response to Ag stimulation. Thus, Fyn plays a role during thymocyte development and is required for optimal responses to low affinity/avidity ligands.

The tetramer model: a new view of class II MHC molecules in antigenic presentation to T cells

Crystallographic studies suggest a plausible divalent interaction between T-cell receptor (TCR) and MHC class II molecules. In addition, biochemical data suggest that these divalent MHC molecules are preformed at the membrane of the antigen-presenting cell. The tetramer model is based on these preformed tetrameric class II molecules that can be loaded with identical or different peptides in their two grooves. This enables divalent class II molecules to deliver two different messages to T cell: 1) a two-peptide message, in which the tetramer with two identical peptides is able to cross-link two TCRs triggering full activation of a T cell. At the thymic level we propose that this message induces negative selection; or 2) a one-peptide message: only one of the peptides loaded in the class II tetramer is able to interact with that TCR. This message would be involved in triggering partial activation phenomena in mature lymphocytes, whereas in thymocytes this message would mediate positive selection. Since high concentrations of a peptide would favor the load of tetramers with identical peptides, the tetramer could therefore be viewed as a quantitative-qualitative transducer that would trigger different responses depending on the concentration of antigenic peptides.

Peptide-induced positive selection of TCR transgenic thymocytes in a coreceptor-independent manner

T cell receptor (TCR) transgenic thymocytes specific for the LCMV gp peptide are normally positively selected to the CD8 lineage. Transgenic thymocyte development was substantially reduced in the absence of these CD8 coreceptors. However, efficient positive selection was restored when TCR transgenic CD8-/- fetal thymic lobes were cultured with a peptide variant of the wild-type ligand. These mature thymocytes were functional, as shown by their ability to respond against strong peptide agonists. Additional experiments demonstrated that transgenic positive selection was peptide-specific. These results prove that CD8 does not possess essential signaling properties that are necessary for T cell development. In addition, the unilateral commitment of transgenic thymocytes to mature CD4-TCR(hi) T cells expressing intracellular perforin suggests that there must be some instructive component to CD4 down-regulation and lineage commitment during thymocyte selection.

The agonist-antagonist balance in positive selection

Peptide antigens expressed in the thymus, in combination with self major histocompatibility complex molecules play a crucial role in thymocyte selection and shaping of the mature T-cell repertoire. Here, it is proposed that a single thymocyte may be exposed to numerous different peptide ligands as it matures, such that its fate is determined by the sum of signals produced by these interactions.

Do holes in the T-cell repertoire have a center-surround regulatory structure? A rationale for the bifurcation of the Th1 and Th2 pathways of differentiation

Regulatory strategies controlling the balance of Th1 versus Th2 T-helper cell responses have been a long-standing mystery with important consequences for immunological disease. A novel model is presented to explain the comparative differentiation of Th1 and Th2 T cells as part of a mechanism to ensure self-tolerance. This model is based on the assumption that thymic interactions of T cell antigen receptors with self major histocompatibility complex ligands may vary in efficacy. By this model, fully agonistic major histocompatibility complex ligands elicit apoptosis during thymic selection to generate 'holes' in the repertoire. Conversely, major histocompatibility complex ligands having some degree of partial efficacy (i.e. a mixed agonist/antagonist) may promote Th2 differentiation whereas fully antagonistic major histocompatibility complex ligands elicit Th0 differentiation. Differentiation of Th2 T cells may continue in the extrathymic tissues upon continued interactions with self major histocompatibility complex ligands having mixed agonist/ antagonist properties. By this mechanism, each 'hole' in the repertoire will develop an inhibitory surround comprised to Th2 T cell clones having partial reactivities to a particular self major histocompatibility complex ligand. During immune responses to self-mimicking foreign antigens, the more numerous Th2 T cells of the inhibitory surround would prevent clonal expansion and Th1 differentiation of any fully autoreactive T cell.

The role of CD4-Lck in T-cell receptor antagonism: evidence for negative signaling

Small changes in the complex between a peptide and a molecule of the major histocompatibility complex generate ligands able to partially activate (partial agonist) or even inhibit (antagonist) T-cell functions. T-cell receptor engagement of antagonist complex results in a partial zeta chain phosphorylation without activation of the associated ZAP-70 kinase. Herein we show that, despite a strong inhibition of both inositol phospholipid hydrolysis and extracellular increasing antagonist concentrations increased the activity of the CD4-Lck kinase. Addition of anti-CD4 antibody to culture medium prevented inhibitory effects induced by antagonist ligand. We propose that CD4-Lck activation triggered by antagonist complexes may act in a dominant negative mode, thus overriding stimulatory signals coming from agonist ligand. These findings identify a new T-cell signaling profile that may explain the ability of some T-cell receptor variant ligands to inhibit specific biological activities or trigger alternative activation programs.

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

Recent studies have investigated how defined peptides influence T cell development. Using a T cell receptor-transgenic beta2-microglobulin-deficient model, we have examined T cell maturation in fetal thymic organ cultures in the presence of various peptides containing single-alanine substitutions of the strong peptide agonist, p33. Cocultivation with the peptide A4Y, which contains an altered T cell contact residue, resulted in efficient positive selection. Several in vitro assays demonstrated that A4Y was a moderate agonist relative to p33. Although A4Y promoted positive selection over a wide concentration range, high doses of this peptide could not induce clonal deletion. Thymocytes maturing in the presence of A4Y were no longer able to respond to A4Y, but could proliferate against p33. These studies demonstrate that (a) peptides that induce efficient positive selection at high concentrations are not exclusively antagonists; (b) some agonists do not promote clonal deletion; (c) positive selection requires a unique T cell receptor-peptide-major histocompatibility complex interaction; and (d) interactions with selecting peptides during T cell ontogeny may define the functional reactivity of mature T cells.

Specific T cell recognition of minimally homologous peptides: evidence for multiple endogenous ligands

The T cell receptor (TCR) can interact with a spectrum of peptides as part of its ligand, including the immunogenic peptide, variants of this peptide,and apparently unrelated peptides. The basis of this broad specificity for ligand was investigated by substitution analysis of a peptide antigen and functional testing using a B cell apoptosis assay. A peptide containing as few as 1 aa in common with this peptide could stimulate a specific T cell response. Two endogenous ligands, an agonist and a partial agonist, were readily identified from a search of the SwissProt database, indicating that multiple endogenous ligands likely exist for a given T cell. These findings strongly support the concept that one TCR has the ability to interact productively with multiple different ligands, and provide evidence that such ligands exist in the endogenous peptide repertoire.

Ligands for the T-cell receptor: hard times for avidity models

T-cell activation occurs when the T-cell receptor (TCR) binds to a self major histocompatibility complex (MHC) molecule carrying a specific peptide. Small changes in peptide structure can alter or inhibit this response. Although this phenomenon is normally attributed to a lowering of the avidity of the ligand for its receptor, recent data suggest that conformational changes in the receptor itself may be playing a critical role, as discussed here by Charles Janeway.

Modulation of T cell development by an endogenous altered peptide ligand

T cells potentially encounter numerous endogenous peptides during selection in the thymus and in the periphery. We examined the impact of an endogenous peptide on in vivo T cell development, using a TCR transgenic mouse model based on a hemoglobin-specific T cell clone. In these mice, the transgenic beta chains paired with endogenous alpha chains. This led to a serendipitous primary reactivity to Ser69 peptide, an altered peptide ligand of the Hbd (64-76) epitope of the parent clone. Two Ser69-reactive T cell populations were identified. A smaller population of the Ser69-reactive T cells responded both to Ser69 and Hbd (64-76). A majority reacted only to Ser69, and not to Hbd(64-76); in fact, Hbd(64-76) was a specific TCR antagonist for these Ser69-only-reactive T cells. Thus, in this unique experimental system, Ser69 became an agonist, and Hbd (64-76) was an antagonist. Endogenous presentation of the antagonist ligand in the thymus selectively eliminated the high-avidity cells, while sparing low-avidity cells in the Ser69-reactive T cell repertoire. These results highlight how specificity guides developing T cells through a network of ligands and indicate that the endogenous peptide pool has a profound effect on T cell development and repertoire.

Both high and low avidity antibodies to the T cell receptor can have agonist or antagonist activity

Anti-TCR antibodies can activate or block the activation of T cells. In the present experiments, we have shown that different monoclonal antibodies to the same TCR can have either agonist or antagonist activity, and we have examined the relationship between these functional effects and the avidity of the antibody for the TCR. We show here that it is not the avidity of an anti-TCR antibody that determines whether it acts as an agonist or an antagonist. Moreover, we show that monovalent Fab fragments of agonist antibodies produce detectable changes in T cell behavior. These data suggest that T cell activation may involve not just aggregation of the TCR but also some induced change in individual ligated receptors, and that agonists may produce this change while antagonists do not. We argue that similar effects may apply to peptide-MHC ligands as well.

T cell receptor antagonist peptides induce positive selection

We have used organ culture of fetal thymic lobes from T cell receptor (TCR) transgenic beta 2M(-/-) mice to study the role of peptides in positive selection. The TCR used was from a CD8+ T cell specific for ovalbumin 257-264 in the context of Kb. Several peptides with the ability to induce positive selection were identified. These peptide-selected thymocytes have the same phenotype as mature CD8+ T cells and can respond to antigen. Those peptides with the ability to induce positive selection were all variants of the antigenic peptide and were identified as TCR antagonist peptides for this receptor. One peptide tested, E1, induced positive selection on the beta 2M(-/-) background but negative selection on the beta 2M(+/-) background. These results show that the process of positive selection is exquisitely peptide specific and sensitive to extremely low ligand density and support the notion that low efficacy ligands mediate positive selection.

Immune discrimination of self and nonself: a unified theory for the induction of self tolerance among thymocytes and mature peripheral T cells

A comprehensive model of immunological self tolerance is described which is based on the unique tenet that interactions between T cell antigen receptors (TcR) and specific MHC ligands may vary in efficacy (the ability of an MHC ligand to catalyze TcR-mediated activation). Based on this postulate, two interrelated mechanisms are described to explain how self tolerance is induced among immature thymocytes and mature peripheral T cells, respectively. In the thymus, APC apparently present a diverse array of self MHC ligands (complexes of self peptides and MHC glycoproteins) to clonotypic T cells. According to the first mechanism, immature thymocytes that efficaciously bind specific MHC ligands undergo TcR-mediated activation and programmed cell death whereas those that nonefficaciously bind MHC ligands are not activated and thereby escape negative selection. The latter T cells undergo positive selection and eventually constitute the mature T cell repertoire. This model of thymic selection ensures that interactions of mature T cells with self in peripheral tissues are predominantly nonefficacious. According to the second mechanism, clonotypically diverse T cells and individual APC comprise an integrative unit that measures antigenic complexity of the local environment as a basis to enable or disable immunogenic responses by mature T cells. T cells recognize efficacious MHC ligands (E) via the TcR/CD3 complex but are also able to detect nonefficacious MHC ligands (N) by conserved signal transduction pathways that are initiated upon cell-cell contact with APC. Clonotypic T cells relay E or N signals by conserved feedback pathways back to APC. APC integrate and compare large numbers of E or N signals to derive an E/N ratio.(ABSTRACT TRUNCATED AT 250 WORDS)