Comparison of peptides bound to spleen and thymus class II

Quantitative impact of thymic clonal deletion on the T cell repertoire

Interactions between major histocompatibility complex (MHC) molecules expressed on stromal cells and antigen-specific receptors on T cells shape the repertoire of mature T lymphocytes emerging from the thymus. Some thymocytes with appropriate receptors are stimulated to undergo differentiation to the fully mature state (positive selection), whereas others with strongly autoreactive receptors are triggered to undergo programmed cell death before completing this differentiation process (negative selection). The quantitative impact of negative selection on the potentially available repertoire is currently unknown. To address this issue, we have constructed radiation bone marrow chimeras in which MHC molecules are present on radioresistant thymic epithelial cells (to allow positive selection) but absent from radiosensitive hematopoietic elements responsible for negative selection. In such chimeras, the number of mature thymocytes was increased by twofold as compared with appropriate control chimeras This increase in steady-state numbers of mature thymocytes was not related to proliferation, increased retention, or recirculation and was accompanied by a similar two- to threefold increase in the de novo rate of generation of mature cells. Taken together, our data indicate that half to two-thirds of the thymocytes able to undergo positive selection die before full maturation due to negative selection.

Thymic stromal cell specialization and the T-cell receptor repertoire

Ten years ago, we proposed a model for thymus function in which thymic epithelial cells are primarily responsible for imprinting major histocompatibility complex (MHC)-restricted specificity, and bone marrow-derived macrophages or dendritic cells are responsible for the induction of self-tolerance. Since then, transgenic and knockout models have allowed for a dissection of thymic stromal components in vivo, leading to a new understanding of their specialized functions. We have determined that with regard to class II-restricted CD4 T-cell development, two distinct subsets of thymic epithelium help shape the repertoire: Cortical epithelium appears solely responsible for positive selection, whereas a fucose-bearing subset of medullary epithelium is specialized for negative selection. This absolute separation of positive and negative selection into two distinct spatial and temporal compartments leads to a much simpler view of the process of repertoire selection. Finally, a novel view of the function of the thymic medulla is discussed.

T cell selection and autoimmunity: flexibility and tuning

Peptide-MHC interactions govern the fate of T cells in the thymus and the peripheral T cell repertoire. Recent progress has involved investigating how different peptides influence T cell selection and mature T cell function and the subsequent implications for tolerance and autoimmunity.

Differential expression of CLIP:MHC class II and conventional endogenous peptide:MHC class II complexes by thymic epithelial cells and peripheral antigen-presenting cells

Major histocompatibility complex (MHC) class II molecules expressed by thymic epithelial cells are involved in positive selection of CD4 T cells, whereas the high-avidity interaction of T cell receptors with the endogenous peptide: MHC class II complexes expressed on bone marrow (BM)-derived antigen-presenting cells (APC) and, to a lesser extent, on thymic epithelial cells mediate negative selection. To understand better the generation of the CD4 T cell repertoire both in the thymus and in the periphery we analyzed relative levels of expression of specific endogenous peptide: MHC class II complexes in thymic epithelial cells (TEC) and peripheral APC. Expression of E alpha52-68: I-A(b) and class II-associated invariant chain peptide (CLIP): I-A(b) complexes in thymic epithelial cells and in the bone-marrow derived splenic APC, i.e. B cells, was studied using YAe and 30-2 monoclonal antibodies which are specific for the corresponding complexes. To distinguish between expression of both complexes in radioresistant thymic epithelial elements and radiation sensitive BM-derived APC, radiation BM chimeras were constructed. Using immunohistochemical and immunochemical approaches we demonstrated that the level of expression of E alpha52-68: I-A(b) complexes in thymic epithelial cells is approximately 5-10% of that seen in splenic cells whereas total class II levels were comparable. In contrast, CLIP: I-A(b) complexes are expressed at substantially higher levels in TEC vs. splenic APC. This result demonstrates quantitative differences in expression of distinct peptide: MHC class II complexes in thymic epithelial cells and peripheral splenic APC.

Delineation of the minimal encephalitogenic epitope within the immunodominant region of myelin oligodendrocyte glycoprotein: diverse V beta gene usage by T cells recognizing the core epitope encephalitogenic for T cell receptor V beta b and T cell receptor V beta a H-2b mice

The nature of the autoimmune T cell response to myelin oligodendrocyte glycoprotein (MOG), recently recognized as a potential target antigen in multiple sclerosis (MS), has not yet been characterized, in contrast to the T cell reactivity to other potential target antigens in MS such as myelin basic protein and proteolipid protein. Here, we show that the encephalitogenicity of the recombinant Ig-like domain of human MOG is associated, in H-2 b mice, with an immunodominant T cell reactivity against a single region of MOG spanning amino acids 35-55, accounting for the previously reported strong encephalitogenic activity of pMOG 35-55. A single injection of pMOG 35-55 with or without administration of pertussis toxin was sufficient to induce severe clinical experimental autoimmune encephalomyelitis (EAE) in H-2 b mice. Encephalitogenic pMOG 35-55-specific T cell lines derived from C3H.SW (V beta b) mice were diverse in their TCR V beta gene usage (V beta 1, V beta 6, V beta 8 and V beta 15), although V beta 8.2 was most predominantly expressed (48%). However, V beta 8 + T cells may only be part of the encephalitogenic MOG-specific T cell repertoire in H-2 b mice, as demonstrated by the susceptibility of C57L (V beta a) mice to disease induced by pMOG 35-55. Encephalitogenic T cell lines from V beta a mice were also diverse in their TCR V beta gene usage (V beta 1, V beta 2, V beta 6, V beta 14 and V beta 16). Such a heterogeneous TCT V beta gene expression by pMOG 35-55/I-A b-reactive T cells from both V beta a and V beta b H-2 b mice suggested multiple epitopes within pMOG 35-55. Analysis of the pattern of reactivity by pMOG 35-55-reactive T cells to a set of truncated peptides was not commensurate with independent nested epitopes, but revealed a requirement for recognition of a core sequence, YRSPFSRVV (pMOG 40-48). However, optimal stimulation was obtained with longer peptides, with each additional amino acid flanking either the N or the C terminus differentially increasing the stimulatory capacity of pMOG 40-48. Nonetheless, pMOG 40-48 was the minimal encephalitogenic epitope for both V beta a and V beta b mice. Thus, the T cell reactivity against the immunodominant encephalitogenic region of MOG is characterized by a diverse V beta gene usage and a requirement for the same core epitope. This pattern of reactivity may favor epitope-directed, rather than TCR-targeted, approaches to immunospecific therapy for MOG-related autoimmune disease.

Direct identification of naturally processed autoantigen-derived peptides bound to HLA-DR15

Biochemical analysis of HLA class II-associated peptides from antigen-pulsed cells is a potentially useful approach to the analysis of antigen processing and presentation because it examines directly which antigen-derived peptides are presented. This is especially advantageous in the analysis of self-antigen presentation where conventional approaches utilizing antigen-specific T cells may be biased by the presence of self-tolerance. However, successful biochemical analysis has been reported for only one exogenous antigen and no autoantigens. We have used a novel analytical approach coupling biochemical data with the reported properties of class II-associated peptides to characterize the peptides derived from a clinically relevant autoantigen presented on the disease-associated class II type. Incubating the target of autoimmune attack in patients with Goodpasture's disease, the 230-amino acid NC1 domain of the alpha3 chain of type IV collagen (Goodpasture antigen, alpha3(IV)NC1), with human B cells homozygous for HLA-DR15, the allele carried by 80% of patients, we find that alpha3(IV)NC1 is presented as at least two sets of three to five peptides centered on common core sequences (nested sets). Synthetic peptides containing these core sequences bind to HLA-DR15 with intermediate affinity (IC50, 1.1-6 microM).

Structures of an MHC class II molecule with covalently bound single peptides

The high-resolution x-ray crystal structures of the murine major histocompatibility complex (MHC) class II molecule, I-E(k), occupied by either of two antigenic peptides were determined. They reveal the structural basis for the I-E(k) peptide binding motif and suggest general principles for additional alleles. A buried cluster of acidic amino acids in the binding groove predicted to be conserved among all murine I-E and human DR MHC class II molecules suggests how pH may influence MHC binding or exchange of peptides. These structures also complement mutational studies on the importance of individual peptide residues to T cell receptor recognition.

The myelin basic protein-specific T cell repertoire in (transgenic) Lewis rat/SCID mouse chimeras: preferential V beta 8.2 T cell receptor usage depends on an intact Lewis thymic microenvironment

In the Lewis rat, myelin basic protein (MBP)-specific, encephalitogenic T cells preferentially recognize sequence 68-88, and use the V beta 8.2 gene to encode their T cell receptors. To analyze the structural prerequisites for the development of the MBP-specific T cell repertoire, we reconstituted severe-combined immunodeficient (SCID) mice with fetal (embryonic day 15-16) Lewis rat lymphoid tissue, and then isolated MBP-specific T cell lines from the adult chimeras after immunization. Two types of chimera were constructed: SCID mice reconstituted with rat fetal liver cells only, allowing T cell maturation within a chimeric SCID thymus consisting of mouse thymic epithelium and rat interdigitating dendritic cells, and SCID mice reconstituted with rat fetal liver cells and rat fetal thymus grafts, allowing T cell maturation within the chimeric SCID and the intact Lewis rat thymic microenvironment. Without exception, the T cell lines isolated from MBP-immunized SCID chimeras were restricted by MHC class II of the Lewis rat (RT1.B1), and none by I-Ad of the SCID mouse. Most of the T cell lines recognized the immunodominant MBP epitope 68-88. In striking contrast to intact Lewis rats, in SCID mice reconstituted by rat fetal liver only, MBP-specific T cell clones used a seemingly random repertoire of V beta genes without a bias for V beta 8.2. In chimeras containing fetal Lewis liver plus fetal thymus grafted under the kidney capsule, however, dominant utilization of V beta 8.2 was restored. The migration of liver-derived stem cells through rat thymus grafts was documented by combining fetal tissues from wild-type and transgenic Lewis rats. The results confirm that the recognition of the immunodominant epitope 68-88 by MBP-specific encephalitogenic T cells is a genetically determined feature of the Lewis rat T cell repertoire. They further suggest that the formation of the repertoire requires T cell differentiation in a syngeneic thymic microenvironment.

Immune responses to self peptides naturally presented by murine class II major histocompatibility complex molecules

Peptides eluted from murine Major Histocompatibility Complex (MHC) class II molecules are predominantly fragments of self proteins, which include apolipoprotein E, cystatin-c, transferrin receptor, MHC class II and Ii chains. These naturally processed self peptides are expected to be presented during ontogeny. Therefore, immune responses to these peptides in syngeneic hosts may be under physiological control so as to modulate auto-reactivity. As would be expected from our current understanding, T cells reactive to such antigens should be deleted or clonally anergized. To explore this possibility, we investigated the immunogenicity of a number of these self peptides in mice that express MHC class II, from which these peptides were eluted. T cell and antibody responses were measured following immunization of mice with the appropriate peptide. Surprisingly, many of these peptides were highly immunogenic in normal mice. T cells reactive to these self peptides are restricted by syngeneic MHC class II and were blocked by alpha CD4 antibodies. T cells primed with the native protein in vivo could be challenged with the appropriate self peptide in vitro. Some of the self epitopes induce Th1 cells as indicated by IFN-gamma but not IL-4 production and others induce Th2 cells. Antipeptide antibodies were detected only at higher doses of antigen. Our results suggest that T cells specific for many of the naturally processed self peptides are not deleted but tolerance to these peptides is still maintained in vivo. Presumably the high-affinity self-reactive T cells are deleted in the thymus and the low-affinity self peptide reactive T cells remain unresponsive to antigen challenge in vitro. Upon antigen priming in vivo, many of these self-reactive T cells become activated and readily respond to antigen challenge in vitro. These results point to the physiological control of the maintenance of tolerance to naturally processed self peptides.

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.

The specificity and orientation of a TCR to its peptide-MHC class II ligands

A T cell-mediated immune response is mainly determined by the 3-5 aa residues that protrude upwards from a peptide bound to an MHC molecule. Alterations of these peptide residues can diminish, eliminate or radically alter the signal that the T cell receives through its T cell receptor (TCR). We have used peptide immunizations of normal mice and mice carrying alpha or beta chain TCR transgenes to identify three distinct peptide contact points. One, near the carboxyl terminus of the peptide, involves the beta chain CDR3 region; the second was centrally located and interacted with both the alpha and beta chain CDR3 loops; the third was near the amino terminus of the peptide, and affected V alpha gene usage, but not the structure of CDR3 of either TCR chain. Based on these results, we propose an orientation for the TCR of this cloned line and argue for its generality.

A model for developmentally acquired thymus-dependent tolerance to central and peripheral antigens

Current models of tolerance to peripheral, tissue-specific antigens contain some major caveats. First, they consider peripheral tolerance independently from intrathymic T cell selection, a dichotomy that is challenged by observations on TE-induced tolerance. Second, they do not account for the fact that vertebrates are more readily tolerised in development than in adult life. Third, they do not explain the fact that embryonic/neonatal tolerance to foreign tissues can only be induced by HC or TE. A model of thymic selection and peripheral tolerance is developed here that resolves those problems, by assuming two classes of T cell effector functions, one being regulatory and the other aggressive. Three postulates are required: (1) both epithelial and hemopoietic cellular compartments of the thymic stroma can support both positive and negative selection of T cells, but with vastly different avidity requirements and efficiency; (2) positively selected T cells with the highest avidity that escape deletion are activated intrathymically and irreversibly committed for regulatory effector functions; (3) the functional phenotype of all other thymic emigrants is determined in the periphery upon encounter with antigen. Functional commitment in the periphery depends on the maturity stage (RTE or PMR) of the immunocompetent cell, on the nature of the antigen-presenting cells, and on the effector classes of other T lymphocytes interacting on the same presenting cell. This model explains a number of observations on experimental autoimmune disease and transplantation tolerance, and it contains several readily testable predictions.

Multiple binding sites for bacterial superantigens on soluble class II MHC molecules

We used surface plasmon resonance to study the binding of a set of soluble mouse I-E class II major histocompatibility molecules, each occupied by a different single peptide, to the staphylococcal enterotoxin superantigens, SEA and SEB. The rates of association and dissociation to SEA varied greatly depending on the I-E-bound peptide. By contrast, binding to SEB yielded fast association and dissociation rates, which were relatively peptide independent. The results also indicated nonoverlapping binding sites for SEB and SEA on class II and raised the possibility of enhanced SAg presentation to T cells by cross-linking of cell surface class II.

Strong agonist ligands for the T cell receptor do not mediate positive selection of functional CD8+ T cells

Positive selection of functional CD8+ T cells expressing an MHC class I-restricted T cell receptor can be induced in fetal thymus organ culture by class I-binding peptides related to the antigenic peptide ligand. Peptides that act as antagonist or weak agonist/antagonist ligands for mature T cells work efficiently in this regard. In the present study, we have investigated whether low concentrations of the original agonist peptide, or variants that still have a strong agonist activity can also mediate positive selection. The antigenic peptide did not induce positive selection at any concentration tested. A strong agonist variant was capable of stimulating the differentiation of TCRhi CD8+ cells, giving the appearance of phenotypic positive selection. However, these cells lacked biological function, since they could not proliferate in response to antigen. The most efficient positive selection resulted with ligands that did not activate mature T cells or stimulate negative selection.

Mechanisms underlying the formation of the T cell receptor repertoire in rheumatoid arthritis

The contributions of germline-encoded T cell receptor segments and of HLA-DR polymorphisms in shaping the repertoire of human CD4+ CD45RO- T cells were investigated in healthy unrelated individuals and in patients with rheumatoid arthritis, an HLA-DRB1 04-associated disease. By comparing frequencies of V beta-J beta combinations, healthy individuals segregated into independent clusters, which strongly correlated with the HLA-DRB1 allele expression. The repertoire fingerprint imposed by the HLA-DRB1 alleles involved only a selected group of J beta elements, whereas the distribution of the other J beta segments was HLA independent. The HLA-restricted J beta elements are characterized by a Gly-Pro-Gly sequence within the conserved Phe-Gly-X-Gly motif, which induces rigidity in an otherwise more flexible protein backbone. The T cell receptor repertoire distinguished patients with RA from healthy HLA-DR-matched individuals, suggesting that patients share a selection mechanism that significantly distorts the composition of the T cell receptor repertoire.

Intrathymic T cell receptor (TcR) targeting in mice lacking CD4 or major histocompatibility complex (MHC) class II: rescue of CD4 T cell lineage without co-engagement of TcR/CD4 by MHC class II

A critical step during intrathymic T cell development, termed positive selection, is associated with rescue of short-lived, immature thymocytes from programmed cell death, T cell lineage commitment, and induction of lineage-specific differentiation programs. T cell receptor (TcR)-major histocompatibility complex (MHC) interactions during positive selection can be closely mimicked by targeting TcR on immature thymocytes to cortical epithelial cells in situ via hybrid antibodies. Here, we show that antibody-mediated TcR signaling in mice deficient for CD4 or MHC class II expression induces polyclonal differentiation of the CD4 T cell lineage. Following a single TcR signal pulse in situ, a temporal sequence of phenotype changes can be discerned: CD69 up-regulation (< 1 day), CD8 down-regulation, TcR up-regulation (1-1.5 days) and down-regulation of the heat-stable antigen (1.5-2 days). Differentiation of phenotypically and functionally mature CD4 T cells in situ is attained within 3 days. Rescue of CD4 lineage T cells in the absence of TcR/CD4 co-engagement by MHC class II in this experimental system supports the stochastic/selective model of T cell lineage commitment.

Intrathymic and extrathymic clonal deletion of T cells

Clonal elimination accounts for self-tolerance induction in the thymus and also affects mature T cells responding to exogenous antigens in the periphery. Recent evidence on the microenvironments, cell-cell interactions and signalling requirements for clonal deletion of immature and mature T cells is discussed.

Identification of a major I-Ek-restricted determinant of hen egg lysozyme: limitations of lymph node proliferation studies in defining immunodominance and crypticity

We have chemically analyzed the peptides presented by I-Ek molecules after processing of hen egg lysozyme (HEL) by a murine B-lymphoma line or by splenocytes. In both cases, the identified peptides were derived from a single region of HEL, containing the core residues 85-96 with heterogeneous N and C termini. This was a surprising result because this determinant had previously been described as cryptic--i.e., not presented after processing of intact HEL. Examination of the specificities of T hybridomas isolated after immunization with either HEL or 84-96 peptide (p84-96) provided an explanation for this controversy. Whereas hybridomas induced by immunization with HEL responded equally well to HEL and p84-96, those induced by peptide immunization showed a marked preference for p84-96 over intact HEL. In other words, hybridomas isolated after p84-96 immunization responded poorly to forms of the 84-96 determinant produced by natural processing, leading to the possible erroneous interpretation that 84-96 is a hidden determinant. We conclude that (i) p84-96 is efficiently presented on I-Ek molecules after processing of HEL, (ii) the explanation for the weak lymph node response to this epitope after immunization with HEL lies at the level of the T cell, not the antigen-presenting cell, and (iii) crypticity cannot be defined on the basis of T-cell proliferation studies alone.

Binding of major histocompatibility complex class II to the invariant chain-derived peptide, CLIP, is regulated by allelic polymorphism in class II

Major histocompatibility complex class II-associated invariant chain (Ii) provides several important functions that regulate class II expression and function. One of these is the ability to inhibit class II peptide loading early in biosynthesis. This allows for efficient class II folding and egress from the endoplasmic reticulum, and protects the class II peptide binding site from loading with peptides before entry into endosomal compartments. The ability of Ii to interact with class II and interfere with peptide loading has been mapped to Ii exon 3, which encodes amino acids 82-107. This same region of Ii has been described as a nested set of class II-associated Ii peptides (CLIPs) that are transiently associated with class II in normal cells and accumulate in human histocompatibility leukocyte antigen-DM-negative cell lines. Currently it is not clear how CLIP and the CLIP region of Ii blocks peptide binding. CLIP may bind directly to the class II peptide binding site, or may bind elsewhere on class II and modulate class II peptide binding allosterically. In this report, we show that CLIP can interact with many different murine and human class II molecules, but that the affinity of this interaction is controlled by polymorphic residues in the class II chains. Likewise, structural changes in CLIP also modulate class II binding in an allele-dependent manner. Finally, the specificity and kinetics of CLIP binding to class II molecule is similar to antigenic peptide binding to class II. These data indicate that CLIP binds to class II in an analogous fashion as conventional antigenic peptides, suggesting that the CLIP segment of Ii may actually occupy the class II peptide binding site.

Thymic epithelium induces full tolerance to skin and heart but not to B lymphocyte grafts

Athymic nude mice reconstituted at birth with allogeneic thymic epithelia (TE) from day 10 embryos (E10), show life-long specific tolerance to skin and heart grafts, but eliminate B lymphocytes of the TE donor haplotype, nearly as well as those from a third strain. Previous immunizations with B cells do not alter the state of tolerance to skin grafts, but specifically accelerate elimination of lymphocytes. In contrast, transplantation of E15 allogeneic thymuses already seeded by hematopoietic cells resulted in chimeras tolerant to both skin and B lymphocytes. In vitro reactivities towards stimulator spleen cells of the haplotype of the thymus were observed in both E10 TE and E15 thymus chimeras. We conclude that induction of full in vivo tolerance to B cells requires hematopoietic cells, while this is not the case for induction of tolerance to skin and heart tissues; furthermore, in vitro reactivity to stimulator spleen cells of the tolerized haplotype is independent of in vivo tolerance.

Chemistry of peptides associated with MHC class I and class II molecules

Recent developments have led to a clearer understanding of the association between peptides and MHC molecules. It is now clear that the peptides presented by MHC class I or class II molecules follow stringent rules that are different for each allelic product. The allele-specific interaction usually involves a sequence of nine amino acids spanning the MHC groove. For class I molecules, the entire peptide ligand is involved in allele-specific interaction with MHC but for class II, the peptides are longer and the nine amino acid sequence is roughly central to the peptide. Allele-specific interactions are brought about by anchoring peptide side chains in complementary pockets in the MHC groove. The sum of allele-specific peptide-MHC interaction requirements can be described as a motif, characterized by number, spacing and specificities of anchors, as well as the more degenerate preferences at non-anchor positions within the nonamer stretches. Such information is useful for T-cell epitope predictions.

Central tolerance of T cells

The immune system is constructed to tolerate self antigens but give vigorous responses to foreign antigens. How this state of self/nonself discrimination is maintained is controversial. In the case of T cells, many self antigens are transported to the thymus via the bloodstream and induce tolerance (clonal deletion) of self-reactive thymocytes in situ. Although such central tolerance in the thymus is well documented, it is often argued that full induction of tolerance requires peripheral mechanisms such as suppression or induction of anergy. This article proposes that steady-state tolerance of T cells to self components is due solely to central tolerance to circulating self antigens combined with sequestration of tissue-specific antigens. Backup mechanisms for tolerance do exist but such immunoregulation only operates when self tolerance breaks. This scheme allows the immune system to give unrestricted primary responses to foreign antigens.

Subsets of HLA-DR1 molecules defined by SEB and TSST-1 binding

Superantigens bind to major histocompatibility complex class II molecules on antigen-presenting cells and stimulate T cells. Staphylococcus aureus enterotoxin B (SEB) and toxic shock syndrome toxin-1 (TSST-1) bind to the same region of human lymphocyte antigen (HLA)-DR1 but do not compete with each other, which indicates that they bind to different subsets of DR1 molecules. Here, a mutation in the peptide-binding groove disrupted the SEB and TSST-1 binding sites, which suggests that peptides can influence the interaction with bacterial toxins. In support of this, the expression of the DR1 molecule in various cell types differentially affected the binding of these toxins.

The expression of self antigenic determinants: implications for tolerance and autoimmunity

T lymphocytes respond to small peptides in the context of major histocompatibility molecules and a host of other cell-surface proteins on antigen-presenting cells. By design, therefore, T-cell responses are dependent on the efficient and accurate processing of both foreign and self peptides by antigen-presenting cells. This review examines the functions of T cells that may be specific for self peptides processed and presented under less than ideal conditions or outside the normal pathways of antigen processing. Do these T cells survive selection events and remain in the repertoire of normal lymphocytes? Moreover, can these cells become activated and are they important in the pathogenesis of autoimmunity?

Production of soluble MHC class II proteins with covalently bound single peptides

The alpha beta T-cell receptors (TCRs) react with complex ligands composed of peptides bound to major histocompatibility complex (MHC) proteins. In the absence of foreign antigens the peptides bound to MHC molecules come from the proteins of the host itself. Interactions between TCRs and these self-peptide-MHC ligands work positively to drive T-cell development in the thymus and negatively to delete or inactivate T cells with potential self-reactivity. On the cell surface, MHC proteins are associated with many different self peptides, making it impossible to know which self peptide was involved in positive or negative interactions with a particular T cell. These studies as well as in vitro studies on TCR-peptide-MHC interactions would be aided by a means of producing MHC molecules containing a single peptide. We have tackled this problem for MHC class II proteins by genetically attaching the peptide by a flexible peptide linker to the amino terminus of the class II beta-chain. Here we report that a secreted, soluble form of this covalent peptide-MHC complex can be expressed in insect cells. The peptide is engaged by the peptide-binding groove of the secreted MHC molecule and this complex is recognized by T cells bearing receptors specific for that combination.

The ligand for positive selection of T lymphocytes in the thymus

T cells are spared from programmed cell death in the thymus after an appropriate interaction between the T-cell receptor and a self peptide/MHC complex; this step is referred to as positive selection. Recent work has focused on precise identification of the positively selecting ligand, and the cell that presents it. First, it was shown that bone marrow derived cells or fibroblasts can substitute for epithelial cells in providing the ligand for positive selection. Second, in a T-cell receptor transgenic system, variants of the antigenic peptides were found to induce positive selection. Peptides that served as antagonists or weak agonists for the T-cell receptor efficiently selected immature thymocytes for survival. It appears that the peptide ligands for positive selection of T cells are self peptides, which serve as mimics or look alikes for the universe of pathogen peptides. The challenge remains to identify a naturally occurring thymic self peptide that can cause positive selection and determine the range of reactivities to foreign peptides which it can select.

Positive and negative thymocyte selection induced by different concentrations of a single peptide

T lymphocyte maturation is dependent on interactions between the T cell receptor (TCR) expressed on the developing thymocyte and intrathymic major histocompatibility complex (MHC)-peptide ligands. The relation between the peptide-MHC complex that results in negative or positive selection has not been identified. Here, the requirements for the maturation of thymocytes expressing a defined transgenic TCR specific for a viral peptide are studied in fetal thymic organ culture. Low concentrations of the viral peptide antigen recognized by this transgenic TCR can mediate positive selection, whereas high concentrations result in thymocyte tolerance. These findings support the affinity-avidity model of thymocyte selection.