Modulation of T cell development by an endogenous altered peptide ligand

NFM Cross-Reactivity to MOG Does Not Expand a Critical Threshold Level of High-Affinity T Cells Necessary for Onset of Demyelinating Disease

Of interest to the etiology of demyelinating autoimmune disease is the potential to aberrantly activate CD4⁺ T cells due to cross-recognition of multiple self-epitopes such as has been suggested for myelin oligodendrocyte glycoprotein epitope 35-55 (MOG_35-55) and neurofilament medium protein epitope 15-35 (NFM_15-35). NFM_15-35 is immunogenic in C57BL/6 mice but fails to induce demyelinating disease by polyclonal T cells despite having the same TCR contact residues as MOG_35-55, a known encephalitogenic Ag. Despite reported cross-reactivity with MOG-specific T cells, the polyclonal response to NFM_15-35 did not expand threshold numbers of MOG_38-49 tetramer-positive T cells. Furthermore, NFM lacked functional synergy with MOG to promote experimental autoimmune encephalomyelitis because NFM-deficient synonymous with knockout mice developed an identical disease course to wild-type mice after challenge with MOG_35-55 Single-cell analysis of encephalitogenic T cells using the peptide:MHC monomer-based two-dimensional micropipette adhesion frequency assay confirmed that NFM was not a critical Ag driving demyelinating disease because NFM_18-30-specific T cells in the CNS were predominantly reactive to MOG_38-49 The absence of NFM contribution to disease allowed mapping of the amino acids required for encephalitogenicity and expansion of high-affinity, MOG-specific T cells that defined the polyclonal response. Alterations of N-terminal residues outside of the NFM_15-35 core nonamer promoted expansion of high-affinity, MOG_38-49 tetramer-positive T cells and promoted consistent experimental autoimmune encephalomyelitis induction, unlike mice challenged with NFM_15-35 Although NFM_15-35 is immunogenic and cross-reactive with MOG at the polyclonal level, it fails to expand a threshold level of encephalitogenic, high-affinity MOG-specific T cells.

On Peptides and Altered Peptide Ligands: From Origin, Mode of Action and Design to Clinical Application (Immunotherapy)

T lymphocytes equipped with clonotypic T cell antigen receptors (TCR) recognize immunogenic peptides only when presented in the context of their own major histocompatibility complex (MHC) molecules. Peptide loading to MHC molecules occurs in intracellular compartments (ER for class I and MIIC for class II molecules) and relies on the interaction of the respective peptides and peptide binding pockets on MHC molecules. Those peptide residues not engaged in MHC binding point towards the TCR screening for possible peptide MHC complex binding partners. Natural or intentional modification of both MHC binding registers and TCR interacting residues of peptides - leading to the formation of altered peptide ligands (APLs) - might alter the way peptides interact with TCRs and hence influence subsequent T cell activation events, and consequently T cell effector functions. This review article summarizes how APLs were detected and first described, current concepts of how APLs modify T cellular signaling, which biological mechanisms might force the generation of APLs in vivo, and how peptides and APLs might be used for the benefit of patients suffering from allergic or autoimmune diseases.

Alloreactivity is limited by the endogenous peptide repertoire

A significant portion of the naive T-cell repertoire is capable of responding to allogeneic MHC, violating the paradigm of self-MHC restriction. Recent studies have demonstrated convincing evidence for germ-line affinity of T-cell receptors (TCR) for MHC, providing explanation for recognition of MHC not encountered during thymic development. However, although germ-line affinity proposes all TCR have inherent affinity for MHC, most T cells are not alloreactive to a given MHC. We propose that specific recognition of endogenous presented peptides, rather than inability to interact with allogeneic MHC molecules, is the primary determinant of alloreactivity. Here, we demonstrate that alloreactive and nonalloreactive TCR differ specifically in the CDR3 sequences responsible primarily for the peptide specificity of T-cell recognition. Limitations on alloreactivity imposed by a requirement for recognition of presented peptides are directly demonstrated by expansion of the alloreactive T-cell repertoire through the addition of peptide mimotopes enabling response to two distinct allogeneic MHC by otherwise nonalloreactive T cells. Responses to peptide mimotopes were specific and depended on TCR interaction with MHC. These results demonstrate that recognition of presented endogenous peptides, and not the inability to interact with allogeneic MHC, is the primary limiter on alloreactivity. This observation reconciles the concept of an inherently MHC-reactive TCR repertoire with observed frequencies of T cells responding to allogeneic stimulation and underscores the fundamental nature of TCR recognition of ligands, where both MHC and presented peptides contribute critically to T-cell recognition.

Cutting edge: Highly alloreactive dual TCR T cells play a dominant role in graft-versus-host disease

Alloreactivity is the response of T cells to MHC molecules not encountered during thymic development. A small population (1-8%) of peripheral T cells in mice and humans express two TCRs due to incomplete allelic exclusion of TCRalpha, and we hypothesized they are highly alloreactive. FACS analysis of mouse T cell MLR revealed increased dual TCR T cells among alloreactive cells. Quantitative assessment of the alloreactive repertoire demonstrated a nearly 50% reduction in alloreactive T cell frequency among T cells incapable of expressing a secondary TCR. We directly demonstrated expansion of the alloreactive T cell repertoire at the single cell level by identifying a dual TCR T cell with distinct alloreactivities for each TCR. The importance of dual TCR T cells is clearly demonstrated in a parent-into-F(1) model of graft-vs-host disease, where dual TCR T cells comprised up to 60% of peripheral activated T cells, demonstrating a disproportionate contribution to disease.

The Tritope Model for restrictive recognition of antigen by T-cells II. Implications for ontogeny, evolution and physiology

Based on the Tritope Model of the TCR [Cohn, M., 2005c. The Tritope Model for restrictive recognition of antigen by T-cells. I. What assumptions about structure are needed to explain function? Mol. Immunol. 42, 1419-1443], a set of functional and evolutionary problems surrounding restrictive recognition of antigen are discussed. These include the origin of allele-specific recognition, the selection pressures for polygeneism and polymorphism, the TCR signaling interactions, the centrality of effector T-helper (eTh)-dependence for activation, the role of haplotype exclusion, "nonclassical" MHC-elements, alloreactivity versus xenoreactivity, etc. Further, a set of observations believed to support the Standard Model are reinterpreted.

Signalling events in the anergy induction of T helper 1 cells

T cells can interact productively with altered peptide ligands (APLs) resulting in different phenotypic outcomes. Stimulation of T helper 1 cells with an APL on live antigen-presenting cells results in the induction of anergy. We investigated the intracellular signalling events involved in generating this anergy by comparing protein tyrosine phosphorylation patterns after stimulation with the anergy-inducing APL or the immunogenic peptide. Stimulation by an APL resulted in a unique pattern of T cell receptor (TCR) phospho-zeta species, which was not observed with any dose of immunogenic peptide. This altered phospho-zeta pattern had a profound functional significance, in that the tyrosine kinase ZAP-70 was not activated. Thus, anergy can be induced by changing the constellation of intracellular signalling events in a T cell. These findings demonstrate that the TCR-CD3 complex can engage selective intracellular biochemical signalling pathways as a direct consequence of the nature of the ligand recognized and the initial phosphotyrosine pattern of the TCR-CD3 proteins. This then leads to different phenotypes.

HLA-DRB1*1101: a significant risk factor for sarcoidosis in blacks and whites

Sarcoidosis is a granulomatous disorder of unknown etiology, associated with an accumulation of CD4+ T cells and a TH1 immune response. Since previous studies of HLA associations with sarcoidosis were limited by serologic or low-resolution molecular identification, we performed high-resolution typing for the HLA-DPB1, HLA-DQB1, HLA-DRB1, and HLA-DRB3 loci and the presence of the DRB4 or DRB5 locus, to define HLA class II associations with sarcoidosis. A Case Control Etiologic Study of Sarcoidosis (ACCESS) enrolled biopsy-confirmed cases (736 total) from 10 centers in the United States. Seven hundred six (706) controls were case matched for age, race, sex, and geographic area. We studied the first 474 ACCESS patients and case-matched controls. The HLA-DRB1 alleles were differentially distributed between cases and controls (P<.0001). The HLA-DRB1*1101 allele was associated (P<.01) with sarcoidosis in blacks and whites and had a population attributable risk of 16% in blacks and 9% in whites. HLA-DRB1-F(47) was the amino acid residue most associated with sarcoidosis and independently associated with sarcoidosis in whites. The HLA-DPB1 locus also contributed to susceptibility for sarcoidosis and, in contrast to chronic beryllium disease, a non-E(69)-containing allele, HLA-DPB1*0101, conveyed most of the risk. Although significant differences were observed in the distribution of HLA class II alleles between blacks and whites, only HLA-DRB1*1501 was differentially associated with sarcoidosis (P<.003). In addition to being susceptibility markers, HLA class II alleles may be markers for different phenotypes of sarcoidosis (DRB1*0401 for eye in blacks and whites, DRB3 for bone marrow in blacks, and DPB1*0101 for hypercalcemia in whites). These studies confirm a genetic predisposition for sarcoidosis and present evidence for the allelic variation at the HLA-DRB1 locus as a major contributor.

Self-recognition and the biased mature repertoire in TCR beta transgenic mice: the exception that supports the rule

Detailed sequence analysis of the companion alpha chains in several T-cell receptor (TCR) beta chain transgenic mice have shown that the mature alpha chain repertoires of CD4+ T cells are biased toward the parental TCR alpha chain sequences. These studies further indicate that it is self-peptide-self-MHC recognition during positive intrathymic selection that biases the structure of the mature TCR alpha chain repertoire. To further establish the causative relationship, it is important to examine whether such a bias can be abolished when positive selection is absent. The human collagen IV-I-A(s) complex-specific KB TCR cannot be positively selected on the self-peptide-self-MHC complexes present in the I-A(s) strain. Therefore, the KB TCR beta chain transgenic mice offer a unique opportunity for addressing this issue.

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.

Impaired survival of T helper cells in the absence of CD4

Signal transduction in response to ligand recognition by T cell receptors regulates T cell fate within and beyond the thymus. Herein we examine the involvement of the CD4 molecule in the regulation of T helper cell survival. T helper cells that lack CD4 expression are prone to apoptosis and show diminished survival after adoptive transfer to irradiated recipients. The helper lineage in CD4(-/-) animals shows a higher than normal apparent rate of cell division and is also enriched for cells exhibiting a memory cell phenotype. Thus the data point to a necessary role for CD4 in the regulation of T helper cell survival and homeostasis.

Induction of T-cell response to cryptic MHC determinants during allograft rejection

The presentation of MHC peptides by recipient and donor antigen presenting cells is an essential element in allorecognition and allograft rejection. MHC proteins contains two sets of determinants: the dominant determinants that are efficiently processed and presented to T cells, and the cryptic determinants that are not presented sufficiently enough to induce T-cell responses in vivo. In transplanted mice, initial T-cell response to MHC peptides is consistently limited to a single or a few immunodominant determinants on donor MHC molecule. However, in this article we show that under appropriate circumstances the hierarchy of determinants on MHC molecules can be disrupted. First, we observed that gamma IFN can trigger de novo presentation of cryptic self-MHC peptides by spleen cells. Moreover, we showed that allotransplantation is associated with induction of T-cell responses to formerly cryptic determinants on both syngeneic and allogeneic MHC molecules. Our results suggest that cross-reactivity and inflammation are responsible for the initiation of these auto- and alloimmune responses after transplantation.

Review article: molecular signals and genetic reprogramming in peripheral T-cell differentiation

Rearrangement of gene segments occurs in T lymphocytes during thymic development as the T-cell receptor (TCR) is first expressed, allowing T cells to become central regulators of antigen specificity in the acquired immune system. However, further development of T cells occurs after population of peripheral lymphoid tissues, which can result in T-cell expansion and differentiation into effectors of various immune function, or progression to memory T cells, anergic cells or death by apoptosis. This review focuses on more recent developments concerning the choices that peripheral T cells make between first encountering antigen through TCR recognition and death. These decisions are associated with a process of genetic reprogramming that alters the behaviour of cells so that immune responses are appropriately regulated.

A NK1.1+ thymocyte-derived TCR beta-chain transgene promotes positive selection of thymic NK1.1+ alpha beta T cells

As a consequence of the peptide specificity of intrathymic positive selection, mice transgenic for a rearranged TCR beta-chain derived from conventional alphabeta T lymphocytes frequently carry mature T cells with significant skewing in the repertoire of the companion alpha-chain. To assess the generality of such an influence, we generated transgenic (Tg) mice expressing a beta-chain derived from nonclassical, NK1.1+ alphabeta T cells, the thymus-derived, CD1. 1-specific DN32H6 T cell hybridoma. Results of the sequence analysis of genomic DNA from developing DN32H6 beta Tg thymocytes revealed that the frequency of the parental alpha-chain sequence, in this instance the Valpha14-Jalpha281 canonical alpha-chain, is specifically and in a CD1.1-dependent manner, increased in the postselection thymocyte population. In accordance, we found phenotypic and functional evidence for an increased frequency of thymic, but interestingly not peripheral, NK1.1+ alphabeta T cells in DN32H6 beta Tg mice, possibly indicating a thymic determinant-dependent maintenance. Thus, in vivo expression of the rearranged TCR beta-chain from a thymus-derived NK1.1+ Valpha14+ T cell hybridoma promotes positive selection of thymic NK1.1+ alphabeta T cells. These observations indicate that the strong influence of productive beta-chain rearrangements on the TCR sequence and specificity of developing thymocytes, which operates through positive selection on self-determinants, applies to both classical and nonclassical alphabeta T cells and therefore represents a general phenomenon in intrathymic alphabeta T lymphocyte development.

Tuning T cell activation threshold and effector function with cross-reactive peptide ligands

We have generated a panel of cross-reactive T cells by immunizing SJL mice (I-A(s)) with Q144 peptide, an analog of an autoantigenic peptide (W144) of myelin proteolipid protein (PLP) 139-151 (HSLGKWLGHPDKF) in which W was replaced by Q at position 144. Following immunization with Q144, T cells were expanded in vitro with W144, which is a cross-reactive, suboptimal ligand, for Q144-specific T cells. The T cell clones responded to both ligands and grew normally on the peptide W144, but were hyperstimulated when activated by Q144 in vitro. This hyperstimulation results in a heteroclitic proliferative response with secretion of additional cytokines not induced by W144. Thus expansion of T cells by a suboptimal cross-reactive ligand effectively lowers the activation threshold so that the immunizing antigen becomes a hyperstimulating ligand for the clones. Surprisingly, when the T cell clones are grown on the hyperstimulating ligand Q144, some adapt by increasing their activation threshold. This desensitization results in a loss of response to a number of cross-reactive ligands and the appearance of a more specific T cell response. Long-term culture with the hyperstimulating ligand is sometimes associated with down-regulation of CD4 expression. These results provide an explanation for the common finding of T cell heteroclicity, and suggest that although the specificity and hierarchy of the response of T cells to peptides is determined by the TCR, activation threshold and effector functions are modified by exposure to cross-reactive ligands. This observation has implications for the development and regulation of autoimmune disease.

CD4+ T cell division in irradiated mice requires peptides distinct from those responsible for thymic selection

We investigated the mechanism by which alpha/beta T cells expand upon transfer to T cell-deficient host mice by injecting carboxyfluorescein diacetate succinimidyl ester-labeled T cells into mice depleted of T cells by sublethal irradiation. We found that CD4+ T cells divided when transferred to irradiated hosts and that the division of more than half of these cells required class II expression. However, division of transferred CD4+ T cells did not occur in irradiated hosts that expressed class II molecules occupied solely by the peptide responsible for thymic selection, indicating that peptides distinct from those involved in thymic selection cause the division of CD4+ T cells in irradiated mice. These data establish that class II-bound peptides control the expansion of CD4+ T cells transferred to T cell-deficient hosts and suggest that the same peptides contribute to the maintenance of T cell numbers in normal mice.

MHC-peptide ligand interactions establish a functional threshold for antigen-specific T cell recognition

Antigen-specific T cell recognition is dependent on the functional density of the TCR-ligand, which consists of specific MHC molecules and a specifically bound peptide. We have examined the influence of the affinity and concentration of exogenous peptide and the density of specific MHC molecules on the proliferation of a CD4+, DQA1*0501/DQB1*0201 (DQ2.1)-restricted, HSV-2-specific T cell clone. Using antigen peptide analogs with different mutations of known DQ2-anchor residues, T cell response was reduced in an peptide-affinity and - concentration specific manner. The decrease using weaker binding peptides was gradual as stimulation with a peptide with intermediate affinity yielded intermediate T cell proliferation and the poorest binding peptide induced an even weaker T cell response. MHC class II density on the APC was modified using DQ2 homo- and heterozygous B-LCLs as APCs, however this variation of MHC concentration had no effect on T cell proliferation. We interpret this as a reflection of a low threshold for activation of the T cell clone, in which peptide-MHC avidity is the over-riding determinant of the strength of ligand signal.

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.

Peripheral autoantigen induces regulatory T cells that prevent autoimmunity

Previous studies have shown that autoimmune thyroiditis can be induced in normal laboratory rats after thymectomy and split dose gamma-irradiation. Development of disease can be prevented by reconstitution of PVG rats shortly after their final irradiation with either peripheral CD4(+)CD45RC- T cells or CD4(+)CD8(-) thymocytes from syngeneic donors. Although the activity of both populations is known to depend on the activities of endogenously produced interleukin 4 and transforming growth factor beta, implying a common mechanism, the issue of antigen specificity of the cells involved has not yet been addressed. In this study, we show that the regulatory T cells that prevent autoimmune thyroiditis are generated in vivo only when the relevant autoantigen is also present. Peripheral CD4(+) T cells, from rats whose thyroids were ablated in utero by treatment with 131I, were unable to prevent disease development upon adoptive transfer into thymectomized and irradiated recipients. This regulatory deficit is specific for thyroid autoimmunity, since CD4(+) T cells from 131I-treated PVG.RT1(u) rats were as effective as those from normal donors at preventing diabetes in thymectomized and irradiated PVG.RT1(u) rats. Significantly, in contrast to the peripheral CD4(+) T cells, CD4(+)CD8(-) thymocytes from 131I-treated PVG donors were still able to prevent thyroiditis upon adoptive transfer. Taken together, these data indicate that it is the peripheral autoantigen itself that stimulates the generation of the appropriate regulatory cells from thymic emigrant precursors.

The Orientation and Nature of the Interaction Between Beef Insulin-Specific TCRs and the Insulin/Class II MHC Complex

Recent crystallographic studies suggest that TCR interact with peptide/class I MHC complexes in a single preferred orientation. Although similar studies have not been performed for class II-restricted TCR, it has been proposed that T cell recognition of peptide/class II complexes has similar orientational restrictions. This study represents a functional approach to systematic analysis of this question. Twenty-one mutant Aβd molecules were produced by alanine scanning mutagenesis and assessed for their ability to present species variants of insulin to a panel of beef insulin-specific T cell hybridomas with limited TCR α- and/or β-chain sequence differences. We demonstrate that all beef insulin-specific TCR have the same orientation on the insulin/Ad complex, such that the α-chain interacts with the carboxyl-terminal region of the Aβd α-helix, and the β-chain complementarity-determining region 3 interacts with the carboxyl-terminal portion of the peptide, consistent with that observed for crystallized TCR-peptide/class I complexes. Despite this structural constraint, even TCR that share structural similarity show remarkable heterogeneity in their responses to the panel of MHC mutants. This variability appears to result from conformational changes induced by binding of the TCR to the complex and the exquisite sensitivity of the threshold for T cell activation.

In Vivo Expression of a TCR Antagonist: T Cells Escape Central Tolerance But Are Antagonized in the Periphery

Transgenic 3.L2 T cells are stimulated by Hb(64–76)/I-Ek and are positively selected on I-Ek plus self-peptides. To this pool of self-peptides we have added a single, well-defined 3.L2 TCR antagonist (A72) in vivo. We find that mice expressing both the 3.L2 TCR and A72 have a minimal loss of T cells expressing the clonotypic TCR in the thymus and spleen. Importantly, the proliferative response of 3.L2 × A72 splenocytes is significantly reduced compared with splenocytes from 3.L2 mice. This reduced response can be attributed to peripheral antagonism. Thus we have identified a new class of self-ligands whose predominant effect is constitutive peripheral antagonism rather than negative selection. The net effect of these ligands is to avoid potential self-reactivity while maintaining as large a repertoire as possible.

Differential Activation of T Cells by Natural Antigen Peptide Analogues: Influence on Autoimmune and Alloimmune In Vivo T Cell Responses

Recent studies using synthetic altered peptide ligands (Analogues) have led to the fine dissection of TCR-mediated T cell functions elicited by Ag recognition. Certain Analogues behave as full agonists of the antigenic peptide while others are partial agonists in that they only trigger selected T cell functions. Additionally, peptide Analogues can behave as antagonists by inhibiting functions of T cell clones when coincubated with the wild-type peptide. In fetal thymic organ cultures, synthetic altered peptide ligands can impact T cell repertoire selection. However, the influence of naturally occurring peptide Analogues on T cell immunity in vivo remains hypothetical. We previously reported that, in B10.A mice, immunogenicity and tolerogenicity of the self-MHC class I peptide, Ld 61-80, were influenced by the presentation of a cross-reactive self-peptide, Kk 61-80. Here, we show that Kk 61-80 self-peptide represents a partial agonist of Ld 61-80 in that it induced the proliferation but not the lymphokine production of Ld 61-80-primed T cells. Next, we showed that presentation of Kk 61-80 Analogue peptide mediated T cell tolerance toward Ld 61-80 self-peptide. Alternatively, when Ld protein represented an alloantigen displayed on transplanted cells, immunization with Kk 61-80 Analogue sensitized recipient mice to Ld 61-80 peptide, thus inducing potent immune responses to donor cells. These results show that the presentation of natural Analogue peptides may represent an essential component of T cell responses involved in autoimmunity and transplant rejection.

A basis for alloreactivity: MHC helical residues broaden peptide recognition by the TCR

The high frequency of alloreactive T cells is a major hindrance for transplantation; however, the molecular basis for alloreactivity remains elusive. We examined the I-Ep alloreactivity of a well-characterized Hb(64-76)/I-Ek-specific murine T cell. Using a combinatorial peptide library approach, we identified a highly stimulatory alloepitope mimic and observed that the recognition of the central TCR contact residues (P3 and P5) was much more flexible than that seen with Hb(64-76)/I-Ek, but still specific. Therefore, alloreactive T cells can recognize a self-peptide/MHC surface; however, the allogeneic MHC molecule changes the recognition requirements for the central region of the peptide, allowing a more diverse repertoire of ligands to be recognized.

A roadmap for HLA-DR peptide binding specificities

Peptide residue positional environments have previously been defined for class I MHC allelic products. These environments provide a less restrictive description of the traditional peptide binding pockets of class I molecules. When combined with the peptide anchor motifs that have been identified for some class I molecules, predictions as to likely motifs for other MHC molecules, which share the same potential environment can be made. Here, the same approach is used to derive peptide residue positional environments for class II MHC molecules. The environments are used to make predictions as to likely binding motifs for HLA-DR allelic products. The predictions are presented in the form of a Table and shown to have concordance with experimental results.

The imprint of intrathymic self-peptides on the mature T cell receptor repertoire

The analysis of T cell receptor alpha (TCR alpha) chains in mice transgenic for a TCR beta chain has allowed us to demonstrate a central role for self-peptides in the positive intrathymic selection of major histocompatibility complex (MHC) class II-restricted T cells. Analysis of specific V alpha-J alpha joins in mature CD4+ TCRhigh thymocytes and in peripheral CD4+ T cells revealed a limitation in amino-acid sequences. By analysis of immature thymocytes, we could show that this limited repertoire was selected from a more diverse repertoire. By analysis of the same beta chain-transgenic mice bred to H-2Ma-deficient mice that express one or a very limited number of peptides, we could demonstrate that the V alpha-J alpha join repertoire was now altered and much more limited. Together, these data provide molecular and genetic evidence that the intrathymic positive selection of the TCR repertoire is critically affected by self-peptides presented by MHC class II molecules, most likely on thymic cortical epithelial cells.

T cell tolerance to kappa light chain (L kappa): identification of a naturally processed self-C kappa-peptidic region by specific CD4+ T cell hybridomas obtained in L kappa-deficient mice

In contrast to H-2d kappa light chain-deficient mice (kappa-/-), BALB/c (kappa+/+) mice fail to respond to kappa light chains (L kappa). This suggests that C kappa-specific T cells are tolerant to this self-antigen in kappa+/+ mice. To get insights into the cellular and molecular basis of this tolerance, we first characterized the presented L kappa-derived C kappa-peptidic region(s). Among a library of overlapping peptides spanning the whole C kappa sequence, only three consecutive peptides are recognized by CD4+ T cell hybridomas obtained in L kappa-immunized kappa-/- mice. This C kappa-peptidic region, which is also the only one containing the I-Ed-binding consensus motif, is immunogenic since it is able to prime lymph node cells of kappa-/- mice to subsequent in vitro proliferative response to either L kappa or kappa+/+ APC. Conversely, no kappa+/+ T cell proliferation is observed under the same conditions. Activation of our hybridomas by cells from central and peripheral lymphoid tissues reveals that this C kappa region is naturally expressed on BALB/c kappa+/+ APC. In addition to B cells, macrophages and dendritic cells are able to present this region. Taken together our data suggest that the described self-C kappa region is implicated in the C kappa-specific CD4+ T cell tolerization in BALB/c mice.

Interferon gamma (IFN-gamma) is necessary for the genesis of acetylcholine receptor-induced clinical experimental autoimmune myasthenia gravis in mice

Experimental autoimmune myasthenia gravis (EAMG) is an animal model of human myasthenia gravis (MG). In mice, EAMG is induced by immunization with Torpedo californica acetylcholine receptor (AChR) in complete Freund's adjuvant (CFA). However, the role of cytokines in the pathogenesis of EAMG is not clear. Because EAMG is an antibody-mediated disease, it is of the prevailing notion that Th2 but not Th1 cytokines play a role in the pathogenesis of this disease. To test the hypothesis that the Th1 cytokine, interferon (IFN)-gamma, plays a role in the development of EAMG, we immunized IFN-gamma knockout (IFN-gko) (-/-) mice and wild-type (WT) (+/+) mice of H-2(b) haplotype with AChR in CFA. We observed that AChR-primed lymph node cells from IFN-gko mice proliferated normally to AChR and to its dominant pathogenic alpha146-162 sequence when compared with these cells from the WT mice. However, the IFN-gko mice had no signs of muscle weakness and remained resistant to clinical EAMG at a time when the WT mice exhibited severe muscle weakness and some died. The resistance of IFN-gko mice was associated with greatly reduced levels of circulating anti-AChR antibody levels compared with those in the WT mice. Comparatively, immune sera from IFN-gko mice showed a dramatic reduction in mouse AChR-specific IgG1 and IgG2a antibodies. However, keyhole limpet hemocyanin (KLH)-priming of IFN-gko mice readily elicited both T cell and antibody responses, suggesting that IFN-gamma regulates the humoral immune response distinctly to self (AChR) versus foreign (KLH) antigens. We conclude that IFN-gamma is required for the generation of a pathogenic anti-AChR humoral immune response and for conferring susceptibility of mice to clinical EAMG.

Presentation of a T cell receptor antagonist peptide by immunoglobulins ablates activation of T cells by a synthetic peptide or proteins requiring endocytic processing

T cell receptor (TCR) antagonism is being considered for inactivation of aggressive T cells and reversal of T cell-mediated autoimmune diseases. TCR antagonist peptides silence aggressive T cells and reverse experimental allergic encephalomyelitis induced with free peptides. However, it is not clear whether free antagonist peptides could reverse natural disease where the antigen is presumably available for endocytic processing and peptides gain access to newly synthesized class II MHC molecules. Using an efficient endocytic presentation system, we demonstrate that a proteolipid protein (PLP) TCR antagonist peptide (PLP-LR) presented on an Ig molecule (Ig-PLP-LR) abrogates the activation of T cells stimulated with free encephalitogenic PLP peptide (PLP1), native PLP, or an Ig containing PLP1 peptide (Ig-PLP1). Free PLP-LR abolishes T cell activation when the stimulator is free PLP1 peptide, but has no measurable effect when the stimulator is the native PLP or Ig-PLP1. In vivo, Ig-PLP1 induces a T cell response to PLP1 peptide. However, when coadministered with Ig-PLP-LR, the response to PLP1 peptide is markedly reduced whereas the response to PLP-LR is normal. Free PLP-LR coadministered with Ig-PLP1 has no effect on the T cell response to PLP1. These findings indicate that endocytic presentation of an antagonist peptide by Ig outcompete both external and endocytic agonist peptides whereas free antagonist hinders external but not endocytic agonist peptide. Direct contact with antagonist ligand and/or trans-regulation by PLP-LR-specific T cells may be the operative mechanism for Ig-PLP-LR-mediated downregulation of PLP1-specific T cells in vivo. Efficient endocytic presentation of antagonist peptides, which is the fundamental event for either mechanism, may be critical for reversal of spontaneous T cell-mediated autoimmune diseases where incessant endocytic antigen processing could be responsible for T cell aggressivity.

Selective activation of Fas/Fas ligand-mediated cytotoxicity by a self peptide

To study how MHC-associated self antigens may regulate the function of T cells in the periphery, we generated CD8+ T cell lines specific for a single residue variant of a self peptide. The self peptide (GAYEFTTL) was isolated from H-2-Kb class I MHC molecules immunopurified from tumor cells. CD8+ CTL lines from H-2b mice were generated against a variant peptide, pE4R, (arginine for glutamic acid at the TCR contact position 4). In short-term 51Cr-release assays, these CTL lysed H-2Kb targets that were pulsed with picomolar levels of pE4R but did not lyse target cells coated with the self peptide at micromolar levels. However, in overnight assays the CTL lysed Fas-positive target cells in the presence of nanomolar levels of the self peptide. This killing was shown to be entirely Fas/Fas ligand mediated by blocking with anti-Fas antibody and Fas-Fc chimeric molecules. While the self peptide was unable to induce serine esterase release from the CTL, it did induce secretion of IFN-gamma. By these criteria then, the unmodified self ligand served as a partial agonist for the CTL raised against a single-residue variant. CD8+ T cell lines raised by in vitro stimulation with the self peptide were likewise unable to kill self peptide-coated targets via the perforin pathway but did lyse targets via Fas. These and similar data from other groups show that self antigens (i.e., MHC/peptide complexes) may be recognized by mature peripheral T cells. The T cell population is tolerant of the self antigen in the sense that they do not respond to physiological levels of the MHC/peptide complex. However, when the level of self antigen is increased (by using synthetic peptide loading) CD8+ T cells may respond by proliferation, IFN-gamma secretion, Fas ligand upregulation, and Fas-mediated cytolysis but are still unable to respond by perforin-mediated cytolysis or granzyme release. The physiological significance of such partial activation in regulation of the immune system remains to be demonstrated.

Endogenous altered peptide ligands can affect peripheral T cell responses

T cells potentially encounter a large number of endogenous self-peptide/MHC ligands in the thymus and the periphery. These endogenous ligands are critical to both positive and negative selection in the thymus; however, their effect on peripheral T cells has not been directly ascertained. Using the murine allelic Hbd (64-76)/I-Ek self-antigen model, we have previously identified altered peptide ligands (APLs) which are able to stimulate some but not all TCR-mediated effector functions. To determine directly the effect of endogenously synthesized APL/MHC complexes on peripheral T cells, we used a TCR transgenic mouse which had reversed our normal antigen system, with Ser69 peptide now being the agonist and Hbd(64-76) being the APL. In this report, we show that the constitutive level of endogenous Hbd(64-76)/I-Ek complexes presented by APCs in vivo is too low to affect the response of Ser69 reactive T cells. However, by increasing the number of Hbd(64-76)/I-Ek complexes expressed by the APCs, TCR antagonism is observed for both primary T cells and T cell hybridomas. In addition, the level of the CD4 coreceptor expressed on T cells and T cell hybridomas. In addition, the level of the CD4 coreceptor expressed on T cells changes the response pattern to endogenously presented Hbd(64-76)/I-Ek ligand. These findings demonstrate that T cells are selected to ignore the constitutive levels of endogenous complexes they encounter in the periphery. T cell responses can be affected by endogenous APLs in the periphery under limited but attainable circumstances which change the efficacy of the TCR/ligand interaction. Thus, endogenous APLs play a role in both the selection of T cells in the thymus and the responses of peripheral T cells.

Positive selection of CD4+ and CD8+ T cells

Significant progress has been made in characterizing intermediates and defining individual steps of positive selection, providing important insights into mechanisms of CD4/CD8 lineage commitment. New evidence suggests that specific recognition of peptides may be important for positive selection of CD4+ T cells. Several studies have defined signal-transduction pathways important for positive selection and have provided evidence that distinct signaling pathways may regulate positive versus negative selection.

Altered peptide ligand-induced partial T cell activation: molecular mechanisms and role in T cell biology

The elucidation of the phenomena of T cell antagonism and partial activation by altered peptide ligands has necessitated a revision in the traditional concepts of TCR recognition of antigen and subsequent signal transduction. Whereas previous models supported a single ligand specificity for any particular T cell, many studies using analogs of immunogenic peptides have now demonstrated a flexibility in this recognition. Moreover, interaction with such altered peptide ligands can result in dramatically different phenotypes of the T cells, ranging from inducing selective stimulatory functions to completely turning off their functional capacity. Investigations of the biochemical basis leading to these phenotypes have shown that altered peptide ligands can induce a qualitatively different pattern of signal transduction events than does any concentration of the native ligand. Such observations imply that several signaling modules are directly linked to the TCR/CD3 complex and that they can be dissociated from each other as a direct result of the nature of the ligand bound. Interestingly, many in vivo models of T cell activation are compatible with a selective signaling model, and several studies have shown that peptide analogs can play a role in various T cell biologic phenomena. These data strongly suggest that naturally occurring altered peptide ligands for any TCR exist in the repertoire of self-peptides or, in nature, derived from pathogens, and recent reports provide compelling evidence that this is indeed the case. The concept of altered peptide ligands, their effects on T cell signaling, the hypothesized mechanisms by which they exert their effects, and their possible roles in shaping the T cell immune response are the scope of this review.

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.