Recognition of multiple peptide cores by a single T cell receptor

Deconstructing the peptide-MHC specificity of T cell recognition

In order to survey a universe of major histocompatibility complex (MHC)-presented peptide antigens whose numbers greatly exceed the diversity of the T cell repertoire, T cell receptors (TCRs) are thought to be cross-reactive. However, the nature and extent of TCR cross-reactivity has not been conclusively measured experimentally. We developed a system to identify MHC-presented peptide ligands by combining TCR selection of highly diverse yeast-displayed peptide-MHC libraries with deep sequencing. Although we identified hundreds of peptides reactive with each of five different mouse and human TCRs, the selected peptides possessed TCR recognition motifs that bore a close resemblance to their known antigens. This structural conservation of the TCR interaction surface allowed us to exploit deep-sequencing information to computationally identify activating microbial and self-ligands for human autoimmune TCRs. The mechanistic basis of TCR cross-reactivity described here enables effective surveillance of diverse self and foreign antigens without necessitating degenerate recognition of nonhomologous peptides.

Intramolecular polyspecificity in CD4 T-cell recognition of Ad-restricted epitopes of proteoglycan aggrecan

T-cell recognition of MHC–peptide complexes shows a high degree of polyspecificity extending to recognition of a large number of structurally unrelated peptides. Examples of polyspecificity reported to date are confined to recognition of epitopes from distinct proteins or synthetic peptide libraries. Here we describe intramolecular polyspecificity of CD4 T cells specific for several epitopes within proteoglycan aggrecan, a structural glycoprotein of cartilage and candidate autoantigen in rheumatoid arthritis. T-cell hybridomas from aggrecan-immunized mice recognized four structurally unrelated epitopes from the G1 domain of aggrecan, but not other aggrecan epitopes or a variety of other peptide epitopes restricted by the same MHC class II allele. We also showed that the hierarchy of cross-reactivity broadly correlated with the strength of peptide binding to MHC class II. Similar polyspecificity was observed in responses of lymph node cells from peptide-immunized mice, suggesting polyspecificity of a significant proportion of the in vivo aggrecan specific T-cell repertoire. Polyspecific recognition of several epitopes within the same autoantigen may provide a novel mechanism to reach the activation threshold of low-affinity autoreactive T cells in the initiation of autoimmune diseases.

Sercarzian immunology--In memoriam. Eli E. Sercarz, 1934-2009

During his long career as a principal investigator and educator, Eli Sercarz trained over 100 scientists. He is best known for developing hen egg white lysozyme (HEL) as a model antigen for immunologic studies. Working in his model system Eli furthered our understanding of antigen processing and immunologic tolerance. His work established important concepts of how the immune system recognizes antigenic determinants processed from whole protein antigens; specifically he developed the concepts of immunodominance and crypticity. Later in his career he focused more on autoimmunity using a variety of established animal models to develop theories on how T cells can circumvent tolerance induction and how an autoreactive immune response can evolve over time. His theory of "determinant spreading" is one of the cornerstones of our modern understanding of autoimmunity. This review covers Eli's entire scientific career outlining his many seminal discoveries.

The CD4+ T-cell epitope-binding register is a critical parameter when generating functional HLA-DR tetramers with promiscuous peptides

Detection of CD4(+) T cells specific for tumor-associated antigens is critical to investigate the spontaneous tumor immunosurveillance and to monitor immunotherapy protocols in patients. We investigated the ability of HLA-DR 1101 multimers to detect CD4(+) T cells specific for three highly promiscuous MAGE-A3 derived peptides: MAGE-A3(191-205) (p39), MAGE-A3(281-295) (p57) and MAGE-A3(286-300) (p58). Tetramers stained specific CD4(+) T cells only when loaded with p39, although all peptides activated the specific T cells when presented by plastic-bound HLA-DR 1101 monomers. This suggested that tetramer staining ability was determined by the mode rather than the affinity of peptide binding to HLA-DR 1101. We hypothesized that peptides should bear a single P1 anchor residue to bind all arms of the multimer in a homogeneous register to generate peptide-HLA-DR conformers with maximal avidity. Bioinformatics analysis indicated that p39 contained one putative P1 anchor residue, whereas the other two peptides contained multiple ones. Designing p57 and p58 analogues containing a single anchor residue generated HLA-DR 1101 tetramers that stained specific CD4(+) T cells. Producing HLA-DR 1101 monomers linked with the optimized MAGE-A3 analogues, but not with the original epitopes, further improved tetramer efficiency. Optimization of CD4(+) T-cell epitope-binding registers is thus critical to generate functional HLA-DR tetramers.

Fine specificity of monoclonal antibodies against celiac disease-inducing peptides in the gluteome

BACKGROUND

In celiac disease patients, peptides derived from dietary gluten are recognized by HLA-DQ2-restricted CD4(+) T cells, which results in inflammation. Such immune-stimulatory peptides are found in both gliadins and glutenins. Monoclonal antibodies (mAbs) against these peptides can be used to screen food for the presence of such peptides.

OBJECTIVE

We aimed to determine the specificity of 5 mAbs raised against T cell stimulatory peptides found in alpha- and gamma-gliadins and in low- and high-molecular-weight glutenins and to compare it with the specificity of patient-derived T cells.

DESIGN

The reactivity of the mAbs with gluten peptides, enzymatic gluten digests, and intact gluten proteins was determined and compared with that of gluten-specific T cells by using a combination of immunologic and biochemical techniques. Furthermore, the reactivity of the mAbs with gluten homologues in barley, rye, and oat was determined.

RESULTS

The specificity of the mAbs largely overlaps with that of gluten-specific T cells. Moreover, mAbs detect several homologous peptides present in gluten proteins. All except the LMW-specific mAbs also detect storage proteins present in barley and rye, whereas the gamma-gliadin-specific mAbs also recognize oat proteins.

CONCLUSION

The mAbs raised against T cell stimulatory peptides in gliadins and glutenins allow a comprehensive screen for the presence of harmful gluten and gluten-like proteins and peptides in food products. They can thus be used to guarantee the safety of food for celiac disease patients.

An Altered T Cell Repertoire in MECL-1-Deficient Mice

Immunoproteasome subunits low-molecular mass polypeptide (LMP)2 and LMP7 affect Ag presentation by MHC class I molecules. In the present study, we investigated the function of the third immunosubunit LMP10/multicatalytic endopeptidase complex-like (MECL)-1 (beta2i) in MECL-1 gene-targeted mice. The number of CD8+ splenocytes in MECL-1-/- mice was 20% lower than in wild-type mice. Infection with lymphocytic choriomeningitis virus (LCMV) elicited a markedly reduced cytotoxic T cell (CTL) response to the LCMV epitopes GP276-286/Db and NP205-212/Kb in MECL-1-/- mice. The weak CTL response to GP276-286/Db was not due to an impaired generation of this epitope but was attributed to a decreased precursor frequency of GP276-286/Db-specific T cells. The expansion of TCR-Vbeta10+ T cells, which contain GP276-286/Db-specific cells, was reduced in LCMV-infected MECL-1-/- mice. Taken together, our data reveal an in vivo function of MECL-1 in codetermining the T cell repertoire for an antiviral CTL response.

A structural basis for CD8+ T cell-dependent recognition of non-homologous peptide ligands: implications for molecular mimicry in autoreactivity

Molecular mimicry of self-epitopes by viral antigens is one possible pathogenic mechanism underlying induction of autoimmunity. A self-epitope, mDBM, derived from mouse dopamine beta-mono-oxygenase (KALYDYAPI) sharing 44% sequence identity with the lymphocytic choriomeningitis virus-derived immunodominant epitope gp33 (KAVYNFATC/M), has previously been identified as a cross-reactive self-ligand, presentation of which results in autoimmunity. A rat peptide homologue, rDBM (KALYNYAPI, 56% identity to gp33), which displayed similar properties to mDBM, has also been identified. We herein report the crystal structure of H-2Db.rDBM and a comparison with the crystal structures of the cross-reactive H-2Db.gp33 and non-cross-reactive H-2Db.gp33 (V3L) escape variant (KALYNFATM, 88% identity to gp33). Despite the large sequence disparity, rDBM and gp33 peptides are presented in nearly identical manners by H-2Db, with a striking juxtaposition of the central sections of both peptides from residues p3 to p7. The structural similarity provides H-2Db in complex with either a virus-derived or a dopamine beta-mono-oxygenase-derived peptide with a shared antigenic identity that conserves the positioning of the heavy chain and peptide residues that interact with the T cell receptor (TCR). This stands in contrast to the structure of H-2Db.gp33 (V3L), in which a single conserved mutation, also present in rDBM, induces large movements of both the peptide backbone and the side chains that interact with the TCR. The TCR-interacting surfaces of the H-2Db.rDBM and H-2Db.gp33 major histocompatibility complexes are very similar with regard to shape, topology, and charge distribution, providing a structural basis for CD8 T cell activation by molecular mimicry and potential subsequent development of autoreactivity.

How T cells 'see' antigen

T lymphocytes bearing alphabeta T cell receptors are pivotal in the immune response of most vertebrates. For example, helper T cells orchestrate antibody production by B cells as well as stimulating other cells, whereas cytotoxic T cells kill virally infected or abnormal cells. Regulatory T cells act to dampen responsiveness, and natural killer-like T cells monitor lipid metabolism. The specificity of these cells is governed by the alphabeta T cell receptors - antibody-like heterodimeric receptors that detect antigenic fragments (peptides) or lipids bound to histocompatibility molecules. Intriguing clues as to how these peculiar ligands are recognized have gradually emerged over the years and tell a remarkable story of biochemical and cellular novelty. Here we summarize some of the more recent work on alphabeta T cell receptor recognition and discuss the implications for activation.

A novel CEA vaccine stimulates T cell proliferation, γIFN secretion and CEA specific CTL responses

Carcinoembryonic antigen (CEA) is a cell surface protein over-expressed by a wide range of tumours. The mouse anti-idiotypic antibody, 708, mimics CEA and can induce both antibody and T cell responses that specifically recognise this antigen. Sequence analysis of 708 revealed homology with a previously identified HLA-A3 T cell epitope in CEA but not to other closely related molecules. 708 was chimerised to a human IgG1 to allow Fc targeting of APCs and was deimmunised to remove unwanted T cell epitopes. The chimerised and deimmunised, but not the mouse 708, could stimulate CTL, proliferation and gammaIFN responses in vitro in normal (HLA-A3, DR1) individuals. Furthermore, the CTLs killed tumour cells expressing CEA suggesting that this deimmunised antibody could be a useful vaccine for solid tumours.

Negative selection imparts peptide specificity to the mature T cell repertoire

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

Cytokine profile, HLA restriction and TCR sequence analysis of human CD4+ T clones specific for an immunodominant epitope of Mycobacterium tuberculosis 16-kDa protein

The identification of immunodominant and universal mycobacterial peptides could be applied to vaccine design and have an employment as diagnostic reagents. In this paper we have investigated the fine specificity, clonal composition and HLA class II restriction of CD4+ T cell clones specific for an immunodominant epitope spanning amino acids 91-110 of the 16-kDa protein of Mycobacterium tuberculosis. Twenty-one of the tested 28 clones had a Th1 profile, while seven clones had a Th0 profile. None of the clones had a Th2 profile. While the TCR AV gene usage of the clones was heterogeneous, a dominant TCR BV2 gene family was used by 18 of the 28 clones. The CDR3 regions of BV2+ T cell clones showed variation in lengths, but a putative common motif R-L/V-G/S-Y/W-E/D was detected in 13 of the 18 clones. Moreover, the last two to three residues of the putative CDR3 loops, encoded by conserved BJ sequences, could also play a role in peptide recognition. Antibody blockade and fine restriction analysis using HLA-DR homozygous antigen-presenting cells established that 16 of 18 BV2+ peptide-specific clones were DR restricted and two clones were DR-DQ and DR-DP restricted. Additionally, five of the 18 TCRBV2+ clones recognized peptide 91-110 in association with both parental and diverse HLA-DR molecules, indicating their promiscuous recognition pattern. The ability of peptide 91-110 to bind a wide range of HLA-DR molecules, and to stimulate a Th1-type interferon (IFN)-gamma response more readily, encourage the use of this peptide as a subunit vaccine component.

Systematic analysis of the combinatorial nature of epitopes recognized by TCR leads to identification of mimicry epitopes for glutamic acid decarboxylase 65-specific TCRs

Accumulating evidence indicates that recognition by TCRs is far more degenerate than formerly presumed. Cross-recognition of microbial Ags by autoreactive T cells is implicated in the development of autoimmunity, and elucidating the recognition nature of TCRs has great significance for revelation of the disease process. A major drawback of currently used means, including positional scanning synthetic combinatorial peptide libraries, to analyze diversity of epitopes recognized by certain TCRs is that the systematic detection of cross-recognized epitopes considering the combinatorial effect of amino acids within the epitope is difficult. We devised a novel method to resolve this issue and used it to analyze cross-recognition profiles of two glutamic acid decarboxylase 65-autoreactive CD4(+) T cell clones, established from type I diabetes patients. We generated a DNA-based randomized epitope library based on the original glutamic acid decarboxylase epitope using class II-associated invariant chain peptide-substituted invariant chains. The epitope library was composed of seven sublibraries, in which three successive residues within the epitope were randomized simultaneously. Analysis of agonistic epitopes indicates that recognition by both TCRs was significantly affected by combinations of amino acids in the antigenic peptide, although the degree of combinatorial effect differed between the two TCRs. Protein database searching based on the TCR recognition profile proved successful in identifying several microbial and self-protein-derived mimicry epitopes. Some of the identified mimicry epitopes were actually produced from recombinant microbial proteins by APCs to stimulate T cell clones. Our data demonstrate the importance of the combinatorial nature of amino acid residues of epitopes in molecular mimicry.

Expansion of the antigenic repertoire of a single T cell receptor upon T cell activation

Activated T cells and their naive precursors display different functional avidities for peptide/MHC, but are thought to have identical antigenic repertoires. We show that, following activation with a cognate mimotope (NRP), diabetogenic CD8(+) T cells expressing a single TCR (8.3) respond vigorously to numerous peptide analogs of NRP that were unable to elicit any responses from naive 8.3-CD8(+) T cells, even at high concentrations. The NRP-reactive, in vivo activated CD8(+) cells arising in pancreatic islets of nonobese diabetic mice are similarly promiscuous for peptide/MHC, and paradoxically this promiscuity expands as the aviditiy of the T cell population for NRP/MHC increases with age. Thus, activation and avidity maturation of T lymphocyte populations can lead to dramatic expansions in the range of peptides that elicit functional T cell responses.

Flexibility in MHC and TCR Recognition: Degenerate Specificity at the T Cell Level in the Recognition of Promiscuous Th Epitopes Exhibiting No Primary Sequence Homology

Recognition of peptide Ags by T cells through the TCR can be highly specific. In this report we show the degeneracy of Ag recognition at both MHC and TCR levels. We present evidence that unrelated promiscuous Th cell epitopes from various protein sources exhibit sufficient structural homology, despite minimal structural identity, to elicit cross-reactive proliferative responses at the bulk T cell level. This epitopic mimicry was also observed when peptide (CS.T3(378-395) and TT(830-844))-specific CD4+ T cell lines and T cell hybridoma clones were used in proliferation and Ag presentation assays. A scrambled CS.T3(378-395) peptide did not show any proliferation, indicating that the specificity of the cross-reactive responses may be linked with the primary structure of the peptides. Blocking of CS.T3(378-395)-specific CD4+ T cell proliferation by anti-MHC class II mAb showed that recognition of promiscuous T cell epitopes is largely in association with MHC class II molecules. These findings suggest that promiscuous Th epitopes may be useful in designing peptide-based vaccine constructs. At the same time these results show that at the T cell level there may be a great deal of immunological cross-reactivity between heterologous pathogens, and because of this the host's response to a pathogen may be modified by its previous experience with other unrelated pathogens.

Responses of Coxsackievirus B4-Specific T-Cell Lines to 2C Protein—Characterization of Epitopes with Special Reference to the GAD65 Homology Region

Coxsackie B viruses (CBV) have been indicated as environmental triggers initiating autoimmune destruction of insulin-producing pancreatic beta-cells, and molecular mimicry might be the mechanism. A prime candidate for inducing cross-reactive immune responses is a homology sequence, PEVKEK, found both in CBV4 2C protein and in GAD65. To characterize the CBV4-specific T-cell epitopes, overlapping peptides covering the 2C protein were synthesized and CBV4-specific T-cell lines were established from healthy and diabetic subjects. The T-cell epitopes were dependent on the HLA-DR genotype of the T-cell donor, but no difference between diabetic and healthy subjects could be detected. Peptide p4, which included the PEVKEK sequence, contained an HLA-DR1-restricted T-cell epitope. Three randomly selected CBV4-specific T-cell lines, which responded to peptide p4, failed to recognize GAD65 protein or GAD65 peptides containing the PEVKEK sequence. We conclude that the CBV4 2C protein is strongly immunogenic for T-cells and PEVKEK is included in a T-cell epitope. However, presentation of this epitope in the context of neutral HLA-DR1 allele does not support its role in pathogenesis of type 1 diabetes.

Structural basis of molecular mimicry

Infectious agents are thought to play an important role in the development of autoimmune diseases. Sequence similarity between infectious agents and self-proteins (molecular mimicry) has been proposed as a mechanism for the induction of autoimmunity [1]. However, it has been difficult to identify microbial peptides that activate autoreactive T cells using conventional sequence alignments. This chapter reviews progress made in the identification of such microbial peptides based on the analysis of structural features that are important for TCR recognition of MHC-bound peptides [2].

"Classic" myelin basic proteins are expressed in lymphoid tissue macrophages

"Classic" myelin basic proteins (MBPs) are demonstrated in lymph nodes of SJL mice by western blot and RT-PCR. Interestingly, expression of these "classic" MBPs was increased during the late relapsing phase of adoptive experimental autoimmune encephalomyelitis (EAE). When splenocytes from SJL mice were separated into macrophage versus B lymphocyte-enriched populations, intact MBP isoforms were demonstrated in the macrophage-enriched population while undetectable in the B lymphocyte-enriched population. RT-PCR demonstrated "classic" MBP transcripts in splenic macrophages, as well as in a macrophage cell line (RAW). The expression of "classic" MBPs in lymphoid tissue macrophages raises the possibility that MBP-specific T cells may be exposed to autoantigen outside the central nervous system (CNS).

Antigenic mimicry, clonal selection and autoimmunity

The triggering of autoimmunity by infection or immunization is often blamed on antigenic mimicry. But the concept of antigen mimicry impinges on our understanding of adaptive immunity in general, and not only on autoimmunity. Here are some thoughts about the consequences of immune mimicry.

Environmental modulation of autoimmune arthritis involves the spontaneous microbial induction of T cell responses to regulatory determinants within heat shock protein 65

Both genetic and environmental factors are believed to be involved in the induction of autoimmune diseases. Adjuvant arthritis (AA) is inducible in susceptible rat strains by injection of Mycobacterium tuberculosis, and arthritic rats raise T cell responses to the 65-kDa mycobacterial heat-shock protein (Bhsp65). We observed that Fischer 344 (F344) rats raised in a barrier facility (BF-F344) are susceptible to AA, whereas F344 rats maintained in a conventional facility (CV-F344) show significantly reduced incidence and severity of AA, despite responding well to the arthritogenic determinant within Bhsp65. The acquisition of protection from AA can be circumvented if rats are maintained on neomycin/acidified water. Strikingly, naive unimmunized CV-F344 rats but not BF-F344 rats raised T cell responses to Bhsp65 C-terminal determinants (BCTD) (we have previously shown that BCTD are involved in regulation of acute AA in the Lewis rat); however, T cells of naive CV-F344 and BF-F344 gave a comparable level of proliferative response to a mitogen, but no response at all to an irrelevant Ag. Furthermore, adoptive transfer into naive BF-F344 rats of splenic cells of naive CV-F344 rats (restimulated with BCTD in vitro) before induction of AA resulted in a considerably reduced severity of AA. These results suggest that spontaneous (inadvertent) priming of BCTD-reactive T cells, owing to determinant mimicry between Bhsp65 and its homologues in microbial agents in the conventional environment, is involved in modulating the severity of AA in CV-F344 rats. These results have important implications in broadening understanding of the host-microbe interaction in human autoimmune diseases.

Characterization of T cell receptor (TCR) of organ-specific autoimmune disease-inducing T cells and TCR-based immunotherapy with DNA vaccines

Organ-specific autoimmune diseases and their animal models are characterized by the finding that the development of the diseases is closely associated with, or induced by, T cells reactive to organ-specific antigens. Therefore, the identification of T cell receptors (TCR) used by disease-inducing T cells within a short period of time is a key factor for designing TCR-based immunotherapy. The findings introduced in this article show that TCR associated with the development of multiple sclerosis and experimental autoimmune diseases including encephalomyelitis (EAE), neuritis (EAN) and carditis (EAC) are identifiable by complementarity-determining region 3 (CDR3) spectratyping analysis and subsequent sequencing of the CDR3 region of spectratype-derived TCR clones. It is also demonstrated that immunotherapy targeting disease-associated TCR using monoclonal antibodies and DNA vaccines significantly reduced the histological severity, and completely suppressed the inflammation in some animals. Since depletion or suppression of one of several types of effector cells does not significantly improve the severity of the disease, combined TCR-based immunotherapy should be considered as a primary therapy for T cell-mediated autoimmune diseases. TCR-based immunotherapy after rapid identification of autoimmune disease-associated TCR by CDR3 spectratyping can be applicable, not only to animal, but also to human autoimmune diseases whose pathomechanism is poorly understood.

Compositional bias and mimicry toward the nonself proteome in immunodominant T cell epitopes of self and nonself antigens

We investigated whether and how molecular mimicry affects the shaping of the helper T cell repertoire. We implemented an algorithm that measures the probability of mimicry between epitopes of known immunogenicity and self or nonself proteomes. This algorithm yields 'similarity profiles', which represent the probability of matching between all contiguous overlapping peptides of the antigen under examination and those in the proteome(s) considered. Similarity profiles between helper T cell epitopes (of self or microbial antigens and allergens) and human or microbial SWISSPROT collections were produced. For each antigen, both collections yielded largely overlapping profiles, demonstrating that self-nonself discrimination does not rely on qualitative features that distinguish human from microbial peptides. However, epitopes whose probability of mimicry with self or nonself prevails are, respectively, tolerated or immunodominant and coexist within the same (auto-)antigen regardless of its self/nonself nature. Epitopes (on self and nonself antigens) can cross-stimulate T cells at increasing potency as their similarity with nonself augments. Mimicry, rather than complicating self-nonself discrimination, assists in the shaping of the immune repertoire and helps define the defensive or autoreactive potential of a T cell. Being a predictor of epitope immunogenicity, it bears relevance to vaccine design.

MEK activity regulates negative selection of immature CD4+CD8+ thymocytes

CD4+CD8+ thymocytes are either positively selected and subsequently mature to CD4 single positive (SP) or CD8 SP T cells, or they die by apoptosis due to neglect or negative selection. This clonal selection is essential for establishing a functional self-restricted T cell repertoire. Intracellular signals through the three known mitogen-activated protein (MAP) kinase pathways have been shown to selectively guide positive or negative selection. Whereas the c-Jun N-terminal kinase and p38 MAP kinase regulate negative selection of thymocytes, the extracellular signal-regulated kinase (ERK) pathway is required for positive selection and T cell lineage commitment. In this paper, we show that the MAP/ERK kinase (MEK)-ERK pathway is also involved in negative selection. Thymocytes from newborn TCR transgenic mice were cultured with TCR/CD3epsilon-specific Abs or TCR-specific agonist peptides to induce negative selection. In the presence of the MEK-specific pharmacological inhibitors PD98059 or UO126, cell recovery was enhanced and deletion of DP thymocytes was drastically reduced. Furthermore, development of CD4 SP T cells was blocked, but differentiation of mature CD8 SP T cells proceeded in the presence of agonist peptides when MEK activity was blocked. Thus, our data indicate that the outcome between positively and negatively selecting signals is critically dependent on MEK activity.

Cancer immunotherapy targeting Wilms' tumor gene WT1 product

The Wilms' tumor gene WT1 is expressed at high levels not only in acute myelocytic and lymphocytic leukemia and in chronic myelocytic leukemia but also in various types of solid tumors including lung cancers. To determine whether the WT1 protein can serve as a target Ag for tumor-specific immunity, three 9-mer WT1 peptides (Db126, Db221, and Db235), which contain H-2Db-binding anchor motifs and have a comparatively higher binding affinity for H-2Db molecules, were tested in mice (C57BL/6, H-2Db) for in vivo induction of CTLs directed against these WT1 peptides. Only one peptide, Db126, with the highest binding affinity for H-2Db molecules induced vigorous CTL responses. The CTLs specifically lysed not only Db126-pulsed target cells dependently upon Db126 concentrations but also WT1-expressing tumor cells in an H-2Db-restricted manner. The sensitizing activity to the Db126-specific CTLs was recovered from the cell extract of WT1-expressing tumor cells targeted by the CTLs in the same retention time as that needed for the synthetic Db126 peptide in RP-HPLC, indicating that the Db126-specific CTLs recognize the Db126 peptide to kill WT1-expressing target cells. Furthermore, mice immunized with the Db126 peptide rejected challenges by WT1-expressing tumor cells and survived for a long time with no signs of autoaggression by the CTLs. Thus, the WT1 protein was identified as a novel tumor Ag. Immunotherapy targeting the WT1 protein should find clinical application for various types of human cancers.

In situ stimulation of a T helper cell hybridoma with a cellulose-bound peptide antigen

Many enzyme-linked immunosorbent assays take advantage of immobilized antigens for the identification of antibody binding sites. Generally, the analysis of cellulose membrane-bound B-cell epitopes is currently considered of high utility. We adapted this methodology for the stimulation of a T helper cell hybridoma with known specificity. Forty overlapping peptides corresponding to the entire rabies virus nucleoprotein were synthesized in duplicates on a single sheet of 90x130 mm size amino-modified paper. The efficacy of the peptide assembly was monitored by color staining of the unreacted amino groups. After completion of the synthesis, the side-chain protecting groups were removed, and the membrane was thoroughly cleaned of all organic and inorganic contaminants. The membrane was cut into pieces, and a standard lymphokine release assay was performed directly from the paper-bound antigens. From all the 40 peptide spots only peptide 31D stimulated the proliferation of the 9C5.D8-H T-cell hybridoma, known to react to this peptide. By using this protocol, as little as 0.4 microgram (approximately 200 pmole) of peptide could be detected. According to mass spectrometry the T-cell stimulation proceeded as a true solid-phase assay. The peptide neither leached from the membrane nor was cleaved by the medium-splenocyte mixture. Additionally, tryptic digestion of the cellulose membrane released the expected peptide fragments.

Ovalbumin(323-339) peptide binds to the major histocompatibility complex class II I-A(d) protein using two functionally distinct registers

Proteins of the class II major histocompatibility complex (MHC) bind antigenic peptides that are subsequently presented to T cells. Previous studies have shown that most of the residues required for binding of the chicken ovalbumin (Ova) 323-339 peptide to the I-A(d) MHC class II protein are contained within the shorter 325-336 peptide. This observation is somewhat inconsistent with the X-ray structure of the Ova peptide covalently attached to I-A(d) ( structure) in which residues 323 and 324 form binding interactions with the protein. A second register for the Ova(325-336) peptide is proposed where residues 326 and 327 occupy positions similar to residues 323 and 324 in the structure. Two Ova peptides that minimally encompass the and alternate registers, Ova(323-335) and Ova(325-336), respectively, were found to dissociate from I-A(d) with distinct kinetics. The dissociation rates for both peptides were enhanced when the His81 residue of the MHC beta-chain was replaced with an asparagine. In the structure the betaH81 residue forms a hydrogen bond to the backbone carbonyl of I323. If the Ova(325-336) peptide were also bound in the register, there would be no comparable hydrogen-bond acceptor for the betaH81 side chain that could explain this peptide's sensitivity to the betaH81 replacement. The Ova(323-335) peptide that binds in the register does not stimulate a T-cell hybridoma that is stimulated by Ova(325-336) bound in the alternate register. These results demonstrate that a single peptide can bind to an MHC peptide in alternate registers producing distinct T-cell responses.

The Self-Directed T Cell Repertoire Against Mouse Lysozyme Reflects the Influence of the Hierarchy of Its Own Determinants and Can Be Engaged by a Foreign Lysozyme

The T cell repertoire is shaped by the processes of positive and negative selection. We have previously shown that mice are tolerant to a native self-Ag, mouse lysozyme (ML), but they respond vigorously when challenged with different ML peptides (“cryptic” self-determinants). In this study, we have addressed the issue of the physiological significance of both the hierarchy (dominance/crypticity) of self-determinants within ML and the anti-cryptic, self (ML)-directed T cell repertoire. Our results demonstrate that there are several ML peptides that bind well to MHC but are totally nonimmunogenic when tested for proliferative T cell response and cytokine secretion: a subset of these peptides presumably represent the originally dominant self-determinants of ML, which have rendered the T cells tolerant during thymic selection. Other ML peptides, which bind well to MHC and are immunogenic, correspond to the cryptic determinants of ML: T cells against cryptic ML determinants escape tolerance induction. Thus, the mature T cell repertoire against ML bears the direct imprint of the hierarchy of self (ML)-determinants. Interestingly, hen egg white lysozyme could prime T cells in vivo that were cross-reactive with certain cryptic ML determinants, and vice versa, without requiring any coimmunization with the foreign lysozyme and ML peptide(s). Moreover, repeated, deliberate priming and expansion of T cells by hen egg white lysozyme immunization concomitantly enhanced T cell response to such cross-reactive ML determinants. This reciprocal self-foreign determinant cross-reactivity may play a previously unrecognized, but crucial, role in the expansion and diversification of self-reactive clones in the autoimmune response.

Cross-Reactivity of Myelin Basic Protein-Specific T Cells with Multiple Microbial Peptides: Experimental Autoimmune Encephalomyelitis Induction in TCR Transgenic Mice

Activation of autoreactive T cells is a crucial event in the pathogenesis of autoimmune diseases. Cross-reactivity between microbial and self Ags (molecular mimicry) is one hypothesis that could explain the activation of autoreactive T cells. We have systematically examined this hypothesis in experimental autoimmune encephalomyelitis using mice bearing exclusively myelin basic protein (MBP)-specific T cells (designated T+ α−). A peptide substitution analysis was performed in which each residue of the MBPAc1–11 peptide was exchanged by all 20 naturally occurring amino acids. This allowed the definition of the motif (supertope) that is recognized by the MBPAc1–11-specific T cells. The supertope was used to screen protein databases (SwissProt and TREMBL). By the search, 832 peptides of microbial origin were identified and synthesized. Of these, 61 peptides induced proliferation of the MBPAc1–11-specific transgenic T cells in vitro. Thus, the definition of a supertope by global amino acid substitution can identify multiple microbial mimic peptides that activate an encephalitogenic TCR. Peptides with only two native MBP-residues were sufficient to activate MBPAc1–11-specific T cells in vitro, and experimental autoimmune encephalomyelitis could be induced by immunizing mice with a mimic peptide with only four native MBP residues.

Identification of a cross-reactive self ligand in virus-mediated autoimmunity

Molecular mimicry has been considered to be one of the potential mechanisms underlying the induction of autoimmune diseases. Using a TCR-transgenic model specific for lymphocytic choriomeningitis virus (LCMV) we have examined the potential for cross-reactive recognition of tissue-restricted self peptides. Several peptides were identified that were able to cross-react with the TCR-transgenic virus-specific T cells in vitro. One peptide was derived from dopamine beta-mono-oxygenase, an enzyme expressed in the adrenal medulla. Interestingly, after activation of the transgenic T cells with LCMV glycoprotein peptides or viruses, infiltration of the adrenal medulla was detected in conjunction with alterations in dopamine metabolism. However, complete destruction of the adrenal medulla was not observed. This suggests that molecular mimicry may be sufficient for self recognition and infiltration, but other factors clearly contribute to chronic autoimmune disease.

Cytotoxic T lymphocytes define multiple peptide isoforms derived from the melanoma-associated antigen MART-1/Melan-A

Peptides derived from the melanoma-associated MART-1/Melan-A antigen are currently implemented in immunotherapy for inducing or augmenting T-cell responses directed against peptides expressed by autologous tumor cells in HLA-A2+ patients with melanoma. Here, we describe the specificity of the T-cell clone SK29-FFM1.1, which secretes GM-CSF in response to a panel of synthetic MART-1/Melan-A-derived peptides, including the naturally presented ILTVILGVL(32-40), but exhibits cytotoxicity and IFN-gamma secretion exclusively to the MART-1/Melan-A derived peptide AAGIGILTV(27-35). In addition, cytotoxic T-lymphocyte (CTL) clone SK29-FFM1.1 recognizes 3 different naturally processed and presented peptides on HLA-A2+ MART-1/Melan-A+ melanoma cells, as defined by cytotoxicity and IFN-gamma and GM-CSF secretion. Processing and presentation of MART-1/Melan-A peptides appears to be different in cells of non-melanocytic origin, as shown by the characterization of naturally presented peptides displayed by HLA-A2+ colorectal cancer cells transduced with a MART-1/Melan-A gene-containing retrovirus. Our data suggest that multiple epitopes, including ILTVILGVL and different isoforms of AAGIGILTV derived from MART-1/Melan-A may be naturally presented by melanoma cells to the immune system.

Structural basis of T cell recognition

Exciting breakthroughs in the last two years have begun to elucidate the structural basis of cellular immune recognition. Crystal structures have been determined for full-length and truncated forms of alpha beta T cell receptor (TCR) heterodimers, both alone and in complex with their peptide-MHC (pMHC) ligands or with anti-TCR antibodies. In addition, a truncated CD8 coreceptor has been visualized with a pMHC. Aided in large part by the substantial body of knowledge accumulated over the last 25 years on antibody structure, a number of general conclusions about TCR structure and its recognition of antigen can already be derived from the relatively few TCR structures that have been determined. Small, but important, variations between TCR and antibody structures bear on their functional differences as well as on their specific antigen recognition requirements. As observed in antibodies, canonical CDR loop structures are already emerging for some of the TCR CDR loops. Highly similar docking orientations of the TCR V alpha domains in the TCR/pMHC complex appear to play a primary role in dictating orientation, but the V beta positions diverge widely. Similar TCR contact positions, but whose exact amino acid content can vary, coupled with relatively poor interface shape complementarity, may explain the flexibility and short half-lives of many TCR interactions with pMHC. Here we summarize the current state of this field, and suggest that the knowledge gap between the three-dimensional structure and the signaling function of the TCR can be bridged through a synthesis of molecular biological and biophysical techniques.

Highly Cross-Reactive T Cell Responses to Myelin Basic Protein Epitopes Reveal a Nonpredictable Form of TCR Degeneracy

We identified two nonoverlapping epitopes in myelin basic protein presented by I-Au that are responsible for mediating tolerance induction to this self-Ag. A large number of T cells expressing diverse TCRs are strongly cross-reactive to both epitopes. Surprisingly, the TCR contact residues in each peptide are highly dissimilar. Furthermore, functional TCR contacts cannot be interchanged between the two epitopes, indicating that the TCR contacts in each peptide can only be recognized within the context of the other amino acids present in that peptide’s sequence. This observation indicates that both buried and exposed residues of each peptide contribute to the sculpting of completely distinct antigenic surfaces. We propose that the cross-reactive TCRs adopt mutually exclusive conformations to recognize these dissimilar epitopes, adding a new dimension to TCR degeneracy. This unpredictable TCR plasticity indicates that using just the TCR contacts on a single epitope to define other cross-reactive peptides will identify only a subset of the complete repertoire of cross-reactive epitopes.

A Single Specific Amino Acid Residue in Peptide Antigens Is Sufficient to Activate Memory CTL: Potential Role of Cross-Reactive Peptides in Memory T Cell Maintenance

In the present study, we examined the structural requirements of peptide Ags for productive interactions with the TCR of CTL. For this purpose, we used as a model a previously identified immunodominant epitope that represents the target of EBV-specific HLA-A11-restricted CTL responses. By the use of peptides having minimal sequence homology with the wild-type epitope, we demonstrated that it is possible to selectively expand and reactivate memory CTL precursors without triggering the lytic mechanisms of wild-type specific effectors. In fact, stimulation of PBL from EBV-seropositive donors by polyalanine analogues, sharing only the putative TCR contact residue with the natural epitope, exclusively induced clonal expansion and reactivation of EBV-specific memory CTL precursors. Interestingly, these polyalanine peptides failed to trigger the cytotoxic function of CTLs specific for the wild-type viral epitope. This clearly indicates that reactivation of memory CTL precursors and triggering of the cytotoxic function have different requirements. The same phenomenon was observed using as stimulators naturally occurring peptides carrying the appropriate TCR contact residue. These data strongly suggest that cross-reactive peptides may play an important role in the expansion and reactivation of CTL clones from the memory T cell pool, and may be involved in long-term maintenance of T cell memory.

Definition of Natural T Cell Antigens with Mimicry Epitopes Obtained from Dedicated Synthetic Peptide Libraries

Progress has recently been made in the use of synthetic peptide libraries for the identification of T cell-stimulating ligands. T cell epitopes identified from synthetic libraries are mimics of natural epitopes. Here we show how the mimicry epitopes obtained from synthetic peptide libraries enable unambiguous identification of natural T cell Ags. Synthetic peptide libraries were screened with Mycobacterium tuberculosis-reactive and -autoreactive T cell clones. In two cases, database homology searches with mimicry epitopes isolated from a dedicated synthetic peptide library allowed immediate identification of the natural antigenic protein. In two other cases, an amino acid pattern that reflected the epitope requirements of the T cell was determined by substitution and omission mixture analysis. Subsequently, the natural Ag was identified from databases using this refined pattern. This approach opens new perspectives for rapid and reliable Ag definition, representing a feasible alternative to the biochemical and genetic approaches described thus far.

Persisting viruses and chronic inflammation: understanding their relation to autoimmunity

Viral infections may induce and sustain autoimmune processes via several and overlapping mechanisms. We outline how chronic inflammation, sustained by persisting viruses, may be "the prerequisite" for initiation and maintenance of the multistep process leading to autoimmunity. Chronic inflammation may favour priming of autoreactive T cells which have escaped thymic tolerance and are able to mount a cross-reactive response to self-mimicking antigens carried by viruses in the periphery. Moreover, chronic inflammation and persisting viruses can synergistically support autoimmunity through other relevant mechanisms: unveiling of cryptic self-epitopes, determinant spreading, activation of dendritic cells, constant priming of new autoreactive T cells, and efficient generation and restimulation of memory cells. Therefore, viruses seem to play a key role among the many environmental factors which, together with the genetic background, have been implicated in the pathogenesis of autoimmune diseases. We will also discuss some hypotheses explaining why autoimmunity is a rare event.

A Viral Peptide with Limited Homology to a Self Peptide Can Induce Clinical Signs of Experimental Autoimmune Encephalomyelitis

Molecular mimicry has been suggested as a mode of autoreactive T cell stimulation in autoimmune diseases. Myelin basic protein (MBP) peptide 1–11 induces experimental autoimmune encephalomyelitis (EAE) in susceptible strains of mice. Here we show that a herpesvirus Saimiri (HVS) peptide, AAQRRPSRPFA, with a limited homology to MBP1–11 peptide, ASQKRPSQRHG (underlined letters showing homology), can stimulate a panel of MBP1–11-specific T cell hybridomas and more importantly cause EAE in mice. We demonstrate that this is due to cross-recognition of these two peptides by TCRs. Results presented in this communication are the first demonstration that a viral peptide with homology at just 5 amino acids with a self peptide can induce clinical signs of EAE in mice. These findings have important implications in understanding the breakdown of T cell tolerance to self Ags in autoimmune diseases by means of cross-reactivity with unrelated peptides.

Interactions with multiple peptide ligands determine the fate of developing thymocytes

Thymocytes are positively or negatively selected depending on interactions between their T cell receptors (TCR) and peptides presented by major histocompatibility complex molecules. We have previously shown that apoptosis of thymocytes from an alpha beta TCR-transgenic mouse (F5), induced by antigenic peptide, can be inhibited specifically by an antagonist peptide variant in an in vitro culture model. We have now extended these experiments by demonstrating that the antagonist peptide can inhibit natural negative selection of maturing thymocytes, induced by endogenously expressed antigen, in fetal thymic organ cultures (FTOC). This inhibition resulted in the rescue and maturation of thymocytes that would otherwise have been deleted. Mature T cells generated in these cultures were able to respond to antigen by producing limited quantities of interferon-gamma, but unlike T cells from control FTOC, they required exogenous interleukin-2 to generate cytolytic effector cells. Interestingly, the antagonist peptide also accelerated the development of F5 thymocytes in the absence of the negatively selecting ligand. These data suggest that the developmental fate of a thymocyte may be determined by the recognition of multiple distinct peptide ligands during thymic selection. Alterations in the profiles of selecting peptides presented in the thymus would thus have profound effects on the size and autoreactive potential of the T cell repertoire generated.

Differential tolerance is induced in T cells recognizing distinct epitopes of myelin basic protein

Experimental allergic encephalomyelitis (EAE) is induced by T cell-mediated immunity to central nervous system antigens. In H-2u mice, EAE is mediated primarily by T cells specific for residues 1-11 of myelin basic protein (MBP). We demonstrate that differential tolerance to MBP1-11 versus epitopes in MBP121-150 is induced by expression of endogenous MBP, reflecting extreme differences in stability of peptide/MHC complexes. The diverse MBP121-150-specific TCR repertoire can be divided into three fine specificity groups. Two groups were identified in wild-type mice despite extensive tolerance, but the third group was not detected. Activated MBP121-150-specific T cells induce EAE in wild-type mice. Thus, encephalitogenic T cells that escape tolerance either recognize short-lived peptide/MHC complexes or express TCRs with unique specificities for stable complexes.

High frequency of T-cell lines responsive to immunodominant epitopes of thyrotropin receptor in healthy subjects

In this study we analyzed the proliferative response to the extracellular domain of thyrotropin receptor (TSHR-ECD) of T-cell lines raised from healthy subjects. We found high frequencies of cell lines reactive to TSHR-ECD, ranging from 12% to 37%. The response of the cell lines to a set of overlapping peptides of TSHR-ECD showed that the most recognized epitopes by T lymphocytes are on the C-terminal portion. In particular, the regions of residues 360-396 and 258-277 are immunodominant in T-lymphocyte reactivity. A group of cell lines specific for the peptides of TSHR-ECD lost the response to the peptides during time in culture. However, these lines were still responsive to TSHR extracellular domain. The cloning of one of these lines showed three types of T-cell clones: (1) CD4+ clones (n = 4) highly responsive to the TSHR-ECD; (2) CD4+ clones (n = 4) low responsive to TSHR-ECD; (3) CD8+ clones (n = 9) not responsive to TSHR-ECD. The first group of clones was stable during time in culture, while the second group was characterized by the loss of the specific response to TSHR-ECD after some weeks from the first analysis. The observation of a spontaneous anergy in the second group of CD4+ clones suggests that mechanisms of control of the lymphocyte response to TSHR-ECD could be activated in vitro.

Structural basis of plasticity in T cell receptor recognition of a self peptide-MHC antigen

The T cell receptor (TCR) inherently has dual specificity. T cells must recognize self-antigens in the thymus during maturation and then discriminate between foreign pathogens in the periphery. A molecular basis for this cross-reactivity is elucidated by the crystal structure of the alloreactive 2C TCR bound to self peptide-major histocompatibility complex (pMHC) antigen H-2Kb-dEV8 refined against anisotropic 3.0 angstrom resolution x-ray data. The interface between peptide and TCR exhibits extremely poor shape complementarity, and the TCR beta chain complementarity-determining region 3 (CDR3) has minimal interaction with the dEV8 peptide. Large conformational changes in three of the TCR CDR loops are induced upon binding, providing a mechanism of structural plasticity to accommodate a variety of different peptide antigens. Extensive TCR interaction with the pMHC alpha helices suggests a generalized orientation that is mediated by the Valpha domain of the TCR and rationalizes how TCRs can effectively "scan" different peptides bound within a large, low-affinity MHC structural framework for those that provide the slight additional kinetic stabilization required for signaling.

Resistance to Polyoma Virus-Induced Tumors Correlates with CTL Recognition of an Immunodominant H-2Dk-Restricted Epitope in the Middle T Protein

The natural mouse pathogen polyoma virus is highly oncogenic in H-2k mice carrying the endogenous superantigen encoded by the mouse mammary tumor provirus Mtv-7. This superantigen results in deletion of Vβ6 TCR-expressing polyoma-specific CD8+ CTL, which appear to be critical effectors against polyoma tumorigenesis. Here we have isolated cloned lines of CD8+ T cells from resistant (i.e., Mtv-7−) H-2k mice that specifically lyse syngeneic polyoma virus-infected cells and polyoma tumor cells. Nearly all these CTL clones express Vβ6 and are restricted in their recognition of virus-infected cells by H-2Dk. Screening a panel of synthetic peptides predicted to bind to Dk, for which no consensus peptide binding motif is known, we identified a peptide corresponding to a nine-amino acid sequence in the carboxyl-terminus of the middle T (MT) protein (amino acids 389–397) that was recognized by all the Vβ6+CD8+ CTL clones. The inability of MT389–397-reactive CTL to recognize cells infected with a mutant polyoma virus encoding a MT truncated just proximal to this sequence indicates that MT389-397 is a naturally processed peptide. The frequencies of precursor CTL specific for polyoma virus and MT389–397 peptide were similar, indicating that MT389–397 is the immunodominant epitope in H-2k mice. In addition, polyoma-infected resistant mice possess a 10- to 20-fold higher MT389-397-specific precursor CTL frequency than susceptible mice. This highly focused CTL response to polyoma virus provides a valuable animal model to investigate the in vivo activity of CTL against virus-induced neoplasia.

Promiscuous presentation and recognition of nucleosomal autoepitopes in lupus: role of autoimmune T cell receptor alpha chain

T cells specific for nucleosomal autoepitopes are selectively expanded in lupus mice and these Th cells drive autoimmune B cells to produce pathogenic antinuclear antibodies. We transfected the TCR-alpha and -beta chain genes of a representative, pathogenic autoantibody-inducing Th clone specific for the nucleosomal core histone peptide H471-94 into TCR-negative recipient cells. Although the autoimmune TCRs were originally derived from SNF1 (I-Ad/q) mice, the transfectants could recognize the nucleosomal autoepitope presented by APC-bearing I-A molecules of all haplotypes tested, as well as human DR molecules. Competition assays indicated that the autoepitopes bound to the MHC class II groove. Most remarkably, MHC-unrestricted recognition of the nucleosomal peptide epitope was conferred by the lupus TCR-alpha chain even when it paired with a TCR-beta chain of irrelevant specificity. Several other disease-relevant Th clones and splenic T cells of lupus mice had similar properties. The TCR-alpha chains of these murine lupus Th clones shared related motifs and charged residues in their CDRs, and similar motifs were apparent even in TCR-alpha chains of human lupus Th clones. The lupus TCR-alpha chains probably contact the nucleosomal peptide complexed with MHC with relatively high affinity/avidity to sustain TCR signaling, because CD4 coreceptor was not required for promiscuous recognition. Indeed, pathogenic autoantibody-inducing, CD4-negative, TCR-alphabeta+ Th cells are expanded in systemic lupus erythematosus. These results have implications regarding thymic selection and peripheral expansion of nucleosome-specific T cells in lupus. They also suggest that universally tolerogenic epitopes could be designed for therapy of lupus patients with diverse HLA alleles. We propose to designate nucleosomes and other antigens bearing universal epitopes "Pantigens" (for promiscuous antigens).

Degenerate Recognition of Alloantigenic Peptides on a Positive-Selecting Class I Molecule

The well-defined 2C T cell was used to investigate alloreactive degeneracy. A panel of class I molecules that are known ligands for the 2C TCR were sensitized with three known peptide ligands, p2Ca (LSPFPFDL), dEV-8 (EQYKFYSV), and SIYR-8 (SIYRYYGL). The peptide p2Ca was originally identified as the allopeptide seen in the Ld class I molecule by 2C T cells, 2C recognizes the dEV-8 peptide as the ligand in the Kbm3 class I molecule, and SIYR-8 was recently identified as a peptide ligand for 2C in the context of the Kb class I molecule. Strong recognition of all three Ag-presenting molecules occurred in the context of their respective allopeptides, but 2C recognized all three peptides to a measurable extent in the context of Kb. Molecular modeling of these Kb/peptide complexes revealed a high degree of similarity between dEV-8 and SIYR-8, but very little conformational similarity of either of these peptides with p2Ca. Furthermore, the structural changes in the mutant Kbm3 binding site resulted in generalized changes in the conformation of each of five bound peptides compared with those of the same peptides bound to Kb. The finding that degenerate recognition occurs on Kb, the restriction element responsible for selecting 2C T cells, suggests a unique relationship between a TCR and the Ag-presenting molecule that mediates its positive selection.

The identification of CD4+ T cell epitopes with dedicated synthetic peptide libraries

For a large number of T cell-mediated immunopathologies, the disease-related antigens are not yet identified. Identification of T cell epitopes is of crucial importance for the development of immune-intervention strategies. We show that CD4+ T cell epitopes can be defined by using a new system for synthesis and screening of synthetic peptide libraries. These libraries are designed to bind to the HLA class II restriction molecule of the CD4+ T cell clone of interest. The screening is based on three selection rounds using partial release of 14-mer peptides from synthesis beads and subsequent sequencing of the remaining peptide attached to the bead. With this approach, two peptides were identified that stimulate the beta cell-reactive CD4+ T cell clone 1c10, which was isolated from a newly diagnosed insulin-dependent diabetes mellitus patient. After performing amino acid-substitution studies and protein database searches, a Haemophilus influenzae TonB-derived peptide was identified that stimulates clone 1c10. The relevance of this finding for the pathogenesis of insulin-dependent diabetes mellitus is currently under investigation. We conclude that this system is capable of determining epitopes for (autoreactive) CD4+ T cell clones with previously unknown peptide specificity. This offers the possibility to define (auto)antigens by searching protein databases and/or to induce tolerance by using the peptide sequences identified. In addition the peptides might be used as leads to develop T cell receptor antagonists or anergy-inducing compounds.

Engagement of a T cell receptor by major histocompatibility complex irrespective of peptide

T cell receptors (TCR) identify target cells presenting a ligand consisting of a major histocompatibility complex molecule (MHC) and an antigenic peptide. A considerable amount of evidence indicates that the TCR contacts both the peptide and the MHC components of the ligand. In fully differentiated T cells the interaction between the peptide and the TCR makes the critical contribution to eliciting a cellular response. However, during the positive selection of thymocytes the contribution of peptide relative to MHC is less well established. Indeed it has been suggested that the critical interaction for positive selection is between the TCR and the MHC molecule and that peptides can be viewed as either allowing or obstructing this contact. This predicts that a given TCR is capable of engaging multiple MHC/peptide complexes. In this study a system is described which detects simply engagement of the TCR by MHC/peptide complexes rather than the functional outcome of such interactions. Using this approach the extent to which peptides can influence contacts between the TCR and the MHC molecule has been examined. The results show that the TCR does in fact engage a wide range of ligands in an MHC-restricted but largely peptide-independent manner, suggesting that only a few peptides are able to prevent the TCR from contacting the MHC molecule.