Molecular basis for the recognition of two structurally different major histocompatibility complex/peptide complexes by a single T-cell receptor

Experimental Antiglomerular Basement Membrane GN Induced by a Peptide from Actinomyces

BACKGROUND

Antiglomerular basement membrane (anti-GBM) disease is associated with HLA-DRB1*1501 (the major predisposing genetic factor in the disease), with α3_127-148 as a nephritogenic T and B cell epitope. Although the cause of disease remains unclear, the association of infections with anti-GBM disease has been long suspected.

METHODS

To investigate whether microbes might activate autoreactive T and B lymphocytes via molecular mimicry in anti-GBM disease, we used bioinformatic tools, including BLAST, SYFPEITHI, and ABCpred, for peptide searching and epitope prediction. We used sera from patients with anti-GBM disease to assess peptides recognized by antibodies, and immunized WKY rats and a humanized mouse model (HLA-DR15 transgenic mice) with each of the peptide candidates to assess pathogenicity.

RESULTS

On the basis of the critical motif, the bioinformatic approach identified 36 microbial peptides that mimic human α3_127-148. Circulating antibodies in sera from patients with anti-GBM recognized nine of them. One peptide, B7, derived from Actinomyces species, induced proteinuria, linear IgG deposition on the GBM, and crescent formation when injected into WKY rats. The antibodies to B7 also targeted human and rat α3_127-148. B7 induced T cell activation from human α3_127-148-immunized rats. T cell responses to B7 were detected in rats immunized by Actinomyces lysate proteins or recombinant proteins. We confirmed B7's pathogenicity in HLA-DR15 transgenic mice that developed kidney injury similar to that observed in α3_135-145-immunized mice.

CONCLUSIONS

Sera from patients with anti-GBM disease recognized microbial peptides identified through a bioinformatic approach, and a peptide from Actinomyces induced experimental anti-GBM GN by T and B cell crossreactivity. These studies demonstrate that anti-GBM disease may be initiated by immunization with a microbial peptide.

Collagen Specific T-Cell Repertoire and HLA-DR Alleles: Biomarkers of Active Refractory Rheumatoid Arthritis

Rheumatoid arthritis (RA) is characterized by chronic joint inflammation and associates with HLA-DRB1*04. The Collagen IIp_261-273-specific T cell repertoire in the peripheral blood of DR4 + patients at the onset of the disease shows a restricted TCR-beta chain usage among which the most frequent is TRBV25.

To define whether this group of DR4-restricted collagen-specific shared T cell could represent markers of active-severe disease and response to therapy, 90 subjects affected by early-RA were enrolled in the study; peripheral blood mononuclear cells were cultured with or without the human collagen II peptide p261-273 and were examined by immunoscope analysis for the usage of the previously identified shared TCR-beta chains.

We report that the presence of T cells carrying rearrangement TRBV25 associated with HLA-DR haplotype and disease activity. HLA-DRB1* haplotypes 04–04, 04–01 and 04–11 were significantly associated with usage of TRBV25, higher disease activity at the onset of disease and poor response to DMARDs.

Finally, the HLA-DRB1* haplotype appeared complementary with current serologic tools to predict good and poor responders in a treat to target strategy. The data reported here offer clues to predict the course of the disease and to foresee personalized treatments in RA patients.

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In DR4 + RA patients disease activity is associated with detection of Collagen_261-273-specific T cells carrying TRBV25.

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HLA-DR 04/04, 04/01 and 04/11 alleles were associated with TRBV25, DAS at the onset, and poor response to DMARDs.

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These findings could lead to tailor the treatment in the subgroup of patients with an active refractory disease.

In the era of costly medical care with monoclonal antibodies and new molecules, and of an increasing request of a personalized medicine, a relevant socio-economic problem in the management of Rheumatoid Arthritis patients is the possible identification of the subgroups of poor responders to treatment. Our study aimed to detect the refractory active patients using an HLA-DR test (available in most hospital centers) combined with a relatively new biomarker of active disease expressed on the cell surface of autoreactive T cells. These tests appear complementary tools to identify the best and the poor responders to a “treat to target strategy”.

Combinatorial chemistry by ant colony optimization

Background: Prioritizing building blocks for combinatorial medicinal chemistry represents an optimization task. We present the application of an artificial ant colony algorithm to combinatorial molecular design (Molecular Ant Algorithm [MAntA]). Results: In a retrospective evaluation, the ant algorithm performed favorably compared with other stochastic optimization methods. Application of MAntA to peptide design resulted in new octapeptides exhibiting substantial binding to mouse MHC-I (H-2Kb). In a second study, MAntA generated a new functional factor Xa inhibitor by Ugi-type three-component reaction. Conclusion: This proof-of-concept study validates artificial ant systems as innovative computational tools for efficient building block prioritization in combinatorial chemistry. Focused activity-enriched compound collections are obtained without the need for exhaustive product enumeration.

MHC Class II Polymorphisms, Autoreactive T-Cells, and Autoimmunity

Major histocompatibility complex (MHC) genes, also known as human leukocyte antigen genes (HLA) in humans, are the prevailing contributors of genetic susceptibility to autoimmune diseases such as Type 1 Diabetes (T1D), multiple sclerosis, and rheumatoid arthritis, among others (1–3). Although the pathways through which MHC molecules afford autoimmune risk or resistance remain to be fully mapped out, it is generally accepted that they do so by shaping the central and peripheral T-cell repertoires of the host toward autoimmune proclivity or resistance, respectively. Disease-predisposing MHC alleles would both spare autoreactive thymocytes from central tolerance and bias their development toward a pathogenic phenotype. Protective MHC alleles, on the other hand, would promote central deletion of autoreactive thymocytes and skew their development toward non-pathogenic phenotypes. This interpretation of the data is at odds with two other observations: that in MHC-heterozygous individuals, resistance is dominant over susceptibility; and that it is difficult to understand how deletion of one or a few clonal autoreactive T-cell types would suffice to curb autoimmune responses driven by hundreds if not thousands of autoreactive T-cell specificities. This review provides an update on current advances in our understanding of the mechanisms underlying MHC class II-associated autoimmune disease susceptibility and/or resistance and attempts to reconcile these seemingly opposing concepts.

MHC I stabilizing potential of computer-designed octapeptides

Experimental results are presented for 180 in silico designed octapeptide sequences and their stabilizing effects on the major histocompatibility class I molecule H-2K^b. Peptide sequence design was accomplished by a combination of an ant colony optimization algorithm with artificial neural network classifiers. Experimental tests yielded nine H-2K^b stabilizing and 171 nonstabilizing peptides. 28 among the nonstabilizing octapeptides contain canonical motif residues known to be favorable for MHC I stabilization. For characterization of the area covered by stabilizing and non-stabilizing octapeptides in sequence space, we visualized the distribution of 100,603 octapeptides using a self-organizing map. The experimental results present evidence that the canonical sequence motives of the SYFPEITHI database on their own are insufficient for predicting MHC I protein stabilization.

Different strategies adopted by K(b) and L(d) to generate T cell specificity directed against their respective bound peptides

Mouse T cell clone 2C recognizes two different major histocompatibility (MHC) ligands, the self MHC K(b) and the allogeneic MHC L(d). Two distinct peptides, SIY (SIYRYYGL) and QL9 (QLSPFPFDL), act as strong and specific agonists when bound to K(b) and L(d), respectively. To explore further the mechanisms involved in peptide potency and specificity, here we examined a collection of single amino acid peptide variants of SIY and QL9 for 1) T cell activity, 2) binding to their respective MHC, and 3) binding to the 2C T cell receptor (TCR) and high affinity TCR mutants. Characterization of SIY binding to MHC K(b) revealed significant effects of three SIY residues that were clearly embedded within the K(b) molecule. In contrast, QL9 binding to MHC L(d) was influenced by the majority of peptide side chains, distributed across the entire length of the peptide. Binding of the SIY-K(b) complex to the TCR involved three SIY residues that were pointed toward the TCR, whereas again the majority of QL9 residues influenced binding of TCRs, and thus the QL9 residues had impacts on both L(d) and TCR binding. In general, the magnitude of T cell activity mediated by a peptide variant was influenced more by peptide binding to MHC than by binding the TCR, especially for higher affinity TCRs. Findings with both systems, but QL9-L(d) in particular, suggest that many single-residue substitutions, introduced into peptides to improve their binding to MHC and thus their vaccine potential, could impair T cell reactivity due to their dual impact on TCR binding.

Differential induction of experimental autoimmune encephalomyelitis by myelin basic protein molecular mimics in mice humanized for HLA-DR2 and an MBP(85-99)-specific T cell receptor

Multiple sclerosis (MS) is a chronic autoimmune neurological disease characterized by infiltration of peripheral inflammatory cells to the central nervous system (CNS) and demyelination of CNS white matter. Epidemiological evidence suggests a possible infectious trigger. One potential mechanism by which an infectious agent may trigger MS is via molecular mimicry wherein T cells generated against foreign epitopes cross-react with self-myelin epitopes, such as myelin basic protein (MBP), with sufficient sequence similarity. It has been previously reported that an MBP(85-99)-reactive T cell clone derived from an MS patient cross-reacted with multiple bacterial-derived mimic peptides in vitro. We show that the same mimic peptides can induce clinical disease in two different strains of mice transgenic for both a human MBP(85-99)-specific TCR and HLA-DR2 (MHC II), albeit with different disease patterns - relapsing-remitting vs. monophasic. Interestingly, clinical disease correlates with CNS infiltration of CD4(+) T cells and F4/80(+) macrophages, but not with in vitro proliferative or cytokine responses of splenocytes in response to either MBP(85-99) or its mimics.

Cross-reactive CD4+ T cells against one immunodominant tumor-derived epitope in melanoma patients

TCRs exhibit a high degree of specificity but may also recognize multiple and distinct peptide-MHC complexes, illustrating the so-called cross-reactivity of TCR-peptide-MHC recognition. In this study, we report the first evidence of CD4(+) T cells recognizing the same tumor peptide-epitope from NY-ESO-1, in the context of multiple HLA-DR and HLA-DP molecules. These cross-reactive CD4(+) T cells recognized not only autologous but also allogenic dendritic cells previously loaded with the relevant protein (i.e., the normally processed and presented epitope). Using clonotypic real-time RT-PCR, we have detected low frequencies of CD4(+) T cells expressing one cross-reactive TCR from circulating CD4(+) T cells of patients with stage IV melanoma either spontaneously or after immunization but not in normal donors. The maintenance of cross-reactive tumor Ag-specific CD4(+) T cells in PBLs of cancer patients required the presence of tumor Ag/epitope in the context of the MHC molecule used to prime the Ag-specific CD4(+) T cells. Our findings have significant implications for the optimization of TCR gene transfer immunotherapies widely applicable to cancer patients.

Extraordinary cross-reactivity of an autoimmune T-cell receptor recognizing specific peptides both on autologous and on allogeneic HLA class II molecules

A T-cell receptor's (TCR) recognition of a human leukocyte antigen (HLA)-peptide complex (pHLA) is normally described as being restricted by the HLA molecule and specific for the peptide. This is, however, not always true. Several TCRs have been described, which cross-react with other peptides bound to the restricting HLA molecule. This phenomenon has been considered a variant of molecular mimicry and is suggested to be one of the mechanisms behind autoimmunity. The positive selection of T cells in the thymus imposes low-affinity recognition of the TCRs toward self-pHLA, which increases the probability of the TCR to be promiscuous by nature, and further implies that the T-cell repertoire contains TCRs prone to be autoreactive and thus able to induce autoimmunity. We present an autoimmune TCR showing extreme cross-reactivity to several pHLA comprising both own HLA class II restriction element and allogeneic HLA class II restriction elements in complex with both self-derived and microbially derived peptides. The existence of such a significant cross-reactivity in the context of distinct HLA-DR molecules might be more common among autoimmune TCRs than previously anticipated and potentially reveals a new way of designing altered peptide ligands for therapeutic use.

Mimotope vaccines for cancer immunotherapy

Cancer vaccines need to be designed to effectively induce tumor-specific CD8(+) T cells, the key effector cells in immune responses against tumors. These T cells recognize peptides generated from cellular proteins by limited proteolysis, and bound and presented at cell surfaces by MHC class I molecules. Mimotopes, mimetics of T cell epitopes, have been derived from known epitopes by sequence modification, or developed de novo using combinatorial peptide libraries to scan the entire sequence space for peptides that induce the desired T cell responses. Mimotopes of both types have been tested in clinical vaccination trials for treatment of cancer.

Cytotoxic herpes simplex type 2-specific, DQ0602-restricted CD4 T+-cell clones show alloreactivity to DQ0601

Alloreactivity is one of the most serious problems in organ transplantation. It has been hypothesized that pre-existing alloreactive T cells are actually cross-reacting cells that have been primed by the autologous major histocompatibility complex (MHC) and a specific peptide. CD8+ cytotoxic T lymphocytes that are alloreactive and recognize a virus-peptide that is presented by the autologous MHC have been reported. Here we demonstrate a cross-reactivity that exists between DQ0602 restricted, herpes simplex type 2 VP16 40-50 specific CD4+ T-cell clones, which can be alloreactive to DQ0601. Though most of the DQ0602 restricted T-cell clones we isolated from two different donors were not alloreactive, weakly cross-reacting T-cell clones could be isolated from both donors. Two strongly cross-reacting T-cell clones with high affinity interaction of their T-cell receptor (TCR) with both DQ0602/VP16 40-50 and DQ0601 could be isolated from one donor. DNA sequencing of the a fragment of the Vbeta gene used in their TCR confirmed that these two T cells indeed are two independent clones. These clones are cytotoxic and produce cytokines of a T helper 2-like pattern. Possible implications in a DR-matched transplantation setting are discussed.

Redundancy in antigen-presenting function of the HLA-DR and -DQ molecules in the multiple sclerosis-associated HLA-DR2 haplotype

The three HLA class II alleles of the DR2 haplotype, DRB1*1501, DRB5*0101, and DQB1*0602, are in strong linkage disequilibrium and confer most of the genetic risk to multiple sclerosis. Functional redundancy in Ag presentation by these class II molecules would allow recognition by a single TCR of identical peptides with the different restriction elements, facilitating T cell activation and providing one explanation how a disease-associated HLA haplotype could be linked to a CD4+ T cell-mediated autoimmune disease. Using combinatorial peptide libraries and B cell lines expressing single HLA-DR/DQ molecules, we show that two of five in vivo-expanded and likely disease-relevant, cross-reactive cerebrospinal fluid-infiltrating T cell clones use multiple disease-associated HLA class II molecules as restriction elements. One of these T cell clones recognizes >30 identical foreign and human peptides using all DR and DQ molecules of the multiple sclerosis-associated DR2 haplotype. A T cell signaling machinery tuned for efficient responses to weak ligands together with structural features of the TCR-HLA/peptide complex result in this promiscuous HLA class II restriction.

HLA-DRB5*0101 and -DRB1*1501 expression in the multiple sclerosis-associated HLA-DR15 haplotype

The HLA region, and particularly the DR15 haplotype (containing the two DRB* genes DRB1*1501 and DRB5*0101 and the tightly linked DQ alleles DQA*0102 and DQB1*0602, which together form the DQw6 molecule) in Caucasians, shows the strongest genetic association with multiple sclerosis (MS). In the DR15 haplotype, two beta-chains HLA-DRB1*1501 and -DRB5*0101 are co-expressed resulting in two different surface HLA-DR alphabeta heterodimers, DR2b and DR2a. Most previous studies focused on DRB1*1501, however, both DR2a and DR2b may contribute to MS pathogenesis via antigen presentation to myelin-specific T lymphocytes. We therefore analyzed the expression of the two DR15 genes in various antigen presenting cells (APCs), central nervous system and thymic tissues. Transcript levels were higher for DRB5*0101 in all cell types and tissues. Both HLA-DR heterodimers were expressed at significant levels on the cell surface, where they showed a differential expression pattern in different APCs. They were similarly regulated after stimulation with interferon-gamma and interleukin-4. Finally, immunohistochemistry experiments indicated that both molecules were expressed in thymic tissue. Our results encourage future research to investigate the potential functional relevance of both genes for the pathogenesis of MS.

Characterizing the impact of CD8 antibodies on class I MHC multimer binding

Many studies have suggested that CD8 Abs affect the binding of class I MHC tetramers/multimers to CD8(+) T cells, which has led to the interpretation that CD8 participates directly in multimer binding. In contrast, a recent publication has argued that CD8 Abs instead cause reorganization of TCR distribution and hence have an indirect effect on multimer binding to the TCR alone. We address these issues by testing the role of CD8 and the impact of CD8 Abs on the binding of normal and mutant multimers to Ag-specific mouse T cells. Our data suggest that, in this system, CD8 Abs act directly on CD8 and only mediate their effects on multimer binding when CD8 is capable of binding to the multimer. These data reinforce the paradigm that CD8 plays an active and direct role in binding of class I MHC multimers.

Mimotopes for alloreactive and conventional T cells in a peptide-MHC display library

The use of peptide libraries for the identification and characterization of T cell antigen peptide epitopes and mimotopes has been hampered by the need to form complexes between the peptides and an appropriate MHC molecule in order to construct a complete T cell ligand. We have developed a baculovirus-based peptide library method in which the sequence encoding the peptide is embedded within the genes for the MHC molecule in the viral DNA, such that insect cells infected with virus encoding a library of different peptides each displays a unique peptide-MHC complex on its surface. We have fished in such a library with two different fluorescent soluble T cell receptors (TCRs), one highly peptide specific and the other broadly allo-MHC specific and hypothesized to be much less focused on the peptide portion of the ligand. A single peptide sequence was selected by the former alphabetaTCR that, not unexpectedly, was highly related to the immunizing peptide. As hypothesized, the other alphabetaTCR selected a large family of peptides, related only by a similarity to the immunizing peptide at the p5 position. These findings have implications for the relative importance of peptide and MHC in TCR ligand recognition. This display method has broad applications in T cell epitope identification and manipulation and should be useful in general in studying interactions between complex proteins.

Human CTLs to Wild-Type and Enhanced Epitopes of a Novel Prostate and Breast Tumor-Associated Protein, TARP, Lyse Human Breast Cancer Cells

Vaccine therapy for prostate and breast cancer may have potential for treating these major causes of death in males and females, respectively. Critical to the development of tumor-specific vaccines is finding and characterizing novel antigens to be recognized by CD8(+) T cells. To define new CD8(+) T-cell tumor antigens, we determined two wild-type HLA-A2 epitopes from a recently found tumor-associated protein, TARP (T-cell receptor gamma alternate reading frame protein), expressed in prostate and breast cancer cells. We were also able to engineer epitope-enhanced peptides by sequence modifications. Both wild-type and enhanced epitopes induced peptide-specific CD8(+) T-cell responses in A2K(b) transgenic mice. In vitro restimulation of human CD8(+) T cells from a prostate cancer patient resulted in CD8(+) T cells reactive to the peptide epitopes that could lyse HLA-A2(+) human breast cancer cells (MCF-7) expressing TARP. Epitope-specific human CD8(+) T cells were also enumerated in patients' peripheral blood by tetramer staining. Our data suggest that HLA-A2-binding TARP epitopes and enhanced epitopes discovered in this study could be incorporated into a potential vaccine for both breast and prostate cancer.

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.

T-cell allorecognition and transplant rejection: a summary and update

Transplant biologists have made significant progress over the last 20 years towards unraveling the immunologic intricacies of allograft rejection. This large body of work has resulted in an improved understanding of T-cell allorecognition at a molecular level and has provided new insight into the functional consequences resulting from the allorecognition events. The findings suggest that the survival and the histologic features of a transplanted organ are influenced not only by the T-cell recognition pathway, but also by the frequency, the induced effector functions and the specific cellular targets of the alloreactive T-cell repertoire.

Mimicking the way to autoimmunity: an evolving theory of sequence and structural homology

Although the etiology of autoimmune diseases remains largely unknown, a prevailing theory concerns the infection-induced activation of self-reactive lymphocytes via the process of molecular mimicry. Here, we discuss the theory of molecular mimicry and its continued evolution from the initial basic considerations of sequence similarity to the current theories of structural homology. Such findings serve to further our understanding of T-cell receptor degeneracy and might one day provide a direct link between infection and autoimmune disease.

Anti-idiotype antibody vaccine therapy for cancer

The use of anti-idiotype (Id) antibodies as vaccines to stimulate antitumour immunity is one of several promising immunologic approaches to the therapy of cancer. Extensive studies in animal tumour models have demonstrated the efficacy of anti-Id vaccines in preventing tumour growth and curing mice with established tumours. A number of monoclonal anti-Id antibodies that mimic distinct human tumour-associated antigens (TAAs) have been developed and tested in the clinic, and demonstrate encouraging results. In general, the antigen mimicry by anti-Id antibodies has reflected structural homology in the majority of the cases, and amino acid sequence homology in a few of them. The greatest challenge of immunotherapy by means of anti-Id vaccines is to identify the optimal anti-Id antibody that will function as a true surrogate antigen for a TAA system, and ideally will generate both humoral and cellular immune responses. Although several clinical studies have shown enhanced survival of patients receiving anti-Id vaccines, the efficacy of these vaccines will depend on the results of several randomised Phase III clinical trials that are currently planned or ongoing.

Different qualitative and quantitative regulation of V beta TCR transcripts during early acute allograft rejection and tolerance induction

Recently, using a global method of T cell repertoire analysis, we showed that purified naive T cells confronted in vitro with allogeneic APCs in a direct pathway-restricted MLR up-regulate their Vbeta mRNAs without exhibiting skewing of complementarity-determining region 3 (CDR3) length distribution. In this report, using this approach, we show in vivo that Vbeta transcript regulation and CDR3 length distribution follow the same pattern during acute rejection of MHC-incompatible heart allografts. In contrast, in tolerance induction by priming of recipients with donor cells, the vigorous Vbeta mRNA accumulation with Gaussian CDR3 length distribution is abolished, providing a possible explanation for the down-regulation of activated T cells in tolerant animals. In addition, tolerated grafts harbor T cells with a highly altered repertoire, suggestive of self-restricted presentation with some patterns corresponding to previously identified regulatory cells.

Mimetics of a T cell epitope based on poly-N-acylated amine backbone structures induce T cells in vitro and in vivo

Peptidomimetics of the major histocompatibility complex (MHC) class I-restricted ovalbumin-derived T cell epitope SIINFEKL were generated by replacing parts of the peptide backbone by a poly-N-acylated amine (PAA) backbone with aromatic, heteroaromatic, and pseudoaromatic side chains that branch off of the main chain at the amine nitrogen. The structure of the PAAs was designed to position this side chain in the central epitope anchor pocket of the MHC molecule. A number of biologically active PAAs were found that induced cytolysis by the mouse cytotoxic T cell clone 4G3. Competition experiments with independent peptides that are known to bind to the restricting MHC molecule H-2K(b) suggest that the PAAs are bound by the MHC molecules at the same site as conventional peptide epitopes. The PAAs were active also in vivo and induced primary cytotoxic T cell responses in mice.

T cell selection and differential activation on structurally related HLA-DR4 ligands

Plasticity of TCR interactions during CD4(+) T cell activation by an MHC-peptide complex accommodates variation in the peptide or MHC contact sites in which recognition of an altered ligand by the T cell can modify the T cell response. To explore the contribution of this form of TCR cross-recognition in the context of T cell selection on disease-associated HLA molecules, we have analyzed the relationship between TCR recognition of the DRB1*0401- and DRB1*0404-encoded HLA class II molecules associated with rheumatoid arthritis. Thymic reaggregation cultures demonstrated that CD4(+) T cells selected on either DRB1*0401 or DRB1*0404 could be subsequently activated by the other MHC molecule. Using HLA tetramer technology we identify hemagglutinin residue 307-319-specific T cells restricted by DRB1*0401, but activated by hemagglutinin residues 307-319, in the context of DRB1*0404. One such clone exhibits an altered cytokine profile upon activation with the alternative MHC ligand. This altered phenotype persists when both class II molecules are present. These findings directly demonstrate that T cells selected on an MHC class II molecule carry the potential for activation on altered self ligands when encountering Ags presented on a related class II molecule. In individuals heterozygous for these alleles the possibility of TCR cross-recognition could lead to an aberrant immune response.

Direct recognition of foreign MHC determinants by naive T cells mobilizes specific Vbeta families without skewing of the complementarity-determining region 3 length distribution

The capacity of T cells to interact with nonself-APC, also referred to as direct allorecognition, is an essential feature of the cellular response involved in graft rejection. However, there is no study on TCR repertoire biases associated with direct restricted T cell activation. In this paper, we have addressed the impact of direct recognition on the whole naive T cell repertoire, using a new approach that provides, for the first time, an integrated depiction of the quantitative and qualitative alterations in the TCR Vbeta transcriptome. This method can differentiate resting patterns from polyclonally activated ones, as evidenced by superantigen usage. According to this new readout, we show that direct recognition of nonself-MHC molecules triggers mRNA accumulation of several TCR Vbeta families, specific to the combination studied. Moreover, in marked contrast to the situation that prevails in indirect allorecognition, T cell activation through the direct presentation pathway was not associated with skewing of the complementarity determining region (CDR) 3 length distribution. Altogether, these data argue for the significance of TCR contacts with the MHC framework in direct allorecognition. In addition, the TCR diversity mobilized by this interaction and the massive TCRbeta mRNA accumulation observed after a few days of culture suggest that a significant proportion of naive T cells receive a signal leading to TCRbeta transcriptional activation even though only a few of them engage in mitosis.

Characterization of a new H-2D(k)-restricted epitope prominent in primary influenza A virus infection

Influenza A virus infection of mice has been used extensively as a model to investigate the mechanisms of antigen presentation to cytotoxic T lymphocytes (CTL) and the phenomenon of immunodominance in antiviral CTL responses. The different virus-encoded epitopes that are recognized in H-2(b) and H-2(d) mice have been characterized and their relative immunodominance has been well-studied. In H-2(k) mice, four different K(k)-restricted influenza virus epitopes have been described, but the dominance hierarchy of these epitopes is unknown and there is also an uncharacterized D(k)-restricted response against the virus. In this study, a D(k)-restricted epitope derived from the influenza virus A/PR/8/34 polymerase protein PB1, corresponding to amino acid residues 349-357 (ARLGKGYMF), was identified. This peptide is the major epitope within the PB1 polymerase and is at least as dominant as any of the four K(k)-restricted epitopes that are recognized in CBA mice following primary influenza virus infection. The PB1 epitope is only the fourth D(k)-presented peptide to be reported and the sequence of this epitope confirms a D(k)-restricted peptide motif, consisting of arginine at position two, arginine or lysine at position five and a hydrophobic residue at the carboxy terminus.

Mimotopes for tumor-specific T lymphocytes in human cancer determined with combinatorial peptide libraries

Mimotopes of a tumor-associated T cell epitope were determined using randomized and combinatorial peptide libraries and a CD8(+) T cell clone specific for the cutaneous T cell lymphoma cell line MyLa. Antigen recognition by this clone was found to be HLA-B8 restricted. More than 80 % of HLA-matched patients with cutaneous T cell lymphoma had mimotope-specific CD8(+) T cells in their peripheral blood. Mimotope-specific T cells isolated and expanded from a patient lysed MyLa cells in in vitro assays thus demonstrating their cytolytic capacity and tumor specificity. Mimotope vaccination of a patient without detectable mimotope-specific T cells induced frequencies of these cells of up to 1.82 % of the peripheral blood CD8(+) T cells.

Structural relatedness of distinct determinants recognized by monoclonal antibody TP25.99 on beta 2-microglobulin-associated and beta 2-microglobulin-free HLA class I heavy chains

The association of HLA class I heavy chains with beta2-microglobulin (beta2m) changes their antigenic profile. As a result, Abs react with either beta2m-free or beta2m-associated HLA class I heavy chains. An exception to this rule is the mAb TP25.99, which reacts with both beta2m-associated and beta2m-free HLA class I heavy chains. The reactivity with beta2m-associated HLA class I heavy chains is mediated by a conformational determinant expressed on all HLA-A, -B, and -C Ags. This determinant has been mapped to amino acid residues 194-198 in the alpha3 domain. The reactivity with beta2m-free HLA class I heavy chains is mediated by a linear determinant expressed on all HLA-B Ags except the HLA-B73 allospecificity and on <50% of HLA-A allospecificities. The latter determinant has been mapped to amino acid residues 239-242, 245, and 246 in the alpha3 domain. The conformational and the linear determinants share several structural features, but have no homology in their amino acid sequence. mAb TP25.99 represents the first example of a mAb recognizing two distinct and spatially distant determinants on a protein. The structural homology of a linear and a conformational determinant on an antigenic entity provides a molecular mechanism for the sharing of specificity by B and TCRs.

The alloreactive and self-restricted CD4+ T cell response directed against a single MHC class II/peptide combination

The cellular basis for allograft rejection derives from the strong T cell response to cells bearing foreign MHC. While it was originally assumed that alloreactive T cells focus their recognition on the polymorphic residues that differ between syngeneic and allogeneic MHC molecules, studies with MHC class I-restricted CTL have shown that MHC-bound peptides play a critical role in allorecognition. It has been suggested that alloreactive T cells depend more strongly on interactions with the MHC molecule than with the associated peptide, but there is little evidence to support this idea. Here we have studied the alloreactive and self-restricted response directed against the class II H2-Ab molecule bound with a single peptide, Ep, derived from the H2-Ealpha chain. This MHC class II-peptide combination was a poor target and stimulator of alloreactive CD4+ T cell responses, indicating that MHC-bound peptides are as important for alloreactive CD4+ T cells as they are for alloreactive CTL. We also generated alloreactive T cells with exquisite specificity for the Ab/Ep complex, and compared their reactivity with self-restricted T cells specific for the same Ab/Ep complex. Our results showed that peptide-specific alloreactive T cells, as compared with self-restricted T cells, were more sensitive to peptide stimulation, but equally sensitive to amino acid substitutions in the peptide. These findings indicate that alloreactive and self-restricted T cells interact similarly with their MHC/peptide ligand.

Mistaken self, a novel model that links microbial infections with myelin-directed autoimmunity in multiple sclerosis

Several findings indicate that infectious events play a role in the pathogenesis of multiple sclerosis (MS). At the same time, T-cell autoimmunity to myelin antigens is widely believed to be crucial to the development of MS lesions. Several mechanisms have been put forward to explain the presumed link between microbial infections and myelin-directed autoimmunity. These include molecular mimicry, bystander activation including epitope spreading and superantigenic activation of T cells. Evidence that either one of these mechanisms actually occurs in MS patients, however, is still weak. Also, none of the above mechanisms explain why MS is unique to humans. We propose an alternative link between microbial infection and myelin autoimmunity, which we refer to as 'mistaken self'. In this mechanism, peripheral microbial infections of lymphoid cells prime the human T-cell repertoire not only to microbial antigens but also to the stress protein alpha B-crystallin that is expressed de novo in infected lymphoid cells. Subsequently, stress-induced accumulation of this self antigen in oligodendocytes/myelin can provoke pro-inflammatory responses as the recruited memory T-cell repertoire then mistakes the self protein for a microbial antigen. In this paper we review the currently available evidence that 'mistaken self' centering on alpha B-crystallin represents a powerful source of anti-myelin autoimmunity in a way that is unique to humans.

Molecular basis for recognition of an arthritic peptide and a foreign epitope on distinct MHC molecules by a single TCR

KRN TCR transgenic T cells recognize two self-MHC molecules: a foreign peptide, bovine RNase 42-56, on I-Ak and an autoantigen, glucose-6-phosphate isomerase 282-294, on I-Ag7. Because the latter recognition event initiates a disease closely resembling human rheumatoid arthritis, we investigated the structural basis of this pathogenic TCR's dual specificity. While peptide recognition is altered to a minor degree between the MHC molecules, we show that the receptor's cross-reactivity critically depends upon a TCR contact residue completely conserved in the foreign and self peptides. Further, the altered recognition of peptide derives from discrete differences on the MHC recognition surfaces and not the disparate binding grooves. This work provides a detailed structural comparison of an autoreactive TCR's interactions with naturally occurring peptides on distinct MHC molecules. The capacity to interact with multiple self-MHCs in this manner increases the number of potentially pathogenic self-interactions available to a T cell.

The role of peptides in T cell alloreactivity is determined by self-major histocompatibility complex molecules

By analyzing T cell responses against foreign major histocompatibility complex (MHC) molecules loaded with peptide libraries and defined self- and viral peptides, we demonstrate a profound influence of self-MHC molecules on the repertoire of alloreactive T cells: the closer the foreign MHC molecule is related to the T cell's MHC, the higher is the proportion of peptide-specific, alloreactive ("allorestricted") T cells versus T cells recognizing the foreign MHC molecule without regard to the peptide in the groove. Thus, the peptide repertoire of alloreactive T cells must be influenced by self-MHC molecules during positive or negative thymic selection or peripheral survival, much like the repertoire of the self-restricted T cells. In consequence, allorestricted, peptide-specific T cells (that are of interest for clinical applications) are easier to obtain if T cells and target cells express related MHC molecules.

A quantitative theory of affinity-driven T cell repertoire selection

Binding of the T cell antigen receptor (TCR) to peptides presented on molecules encoded by major histocompatibility complex (MHC) genes is the key event driving T cell development and activation. Selection of the T cell repertoire in the thymus involves two steps. First, positive selection promotes the survival of cells binding thymic self-MHC-peptide complexes with sufficient affinity. The resulting repertoire is self-MHC restricted: it recognizes foreign peptides presented on self, but not foreign MHC. Second, negative selection deletes cells which may be potentially harmful because their receptors interact with self-MHC-peptide complexes with too high an affinity. The mature repertoire is also highly alloreactive: a large fraction of T cells respond to tissues harboring foreign MHC. We derive mathematical expressions giving the frequency of alloreactivity, the level of self-MHC restriction, and the fraction of the repertoire activated by a foreign peptide, as a function of the parameters driving the generation and selection of the repertoire: self-MHC and self-peptide diversity, the stringencies of positive and negative selection, and the number of peptide and MHC polymorphic residues that contribute to T cell receptor binding. Although the model is based on a simplified digit string representation of receptors, all the parameters but one relate directly to experimentally determined quantities. The only parameter without a biological counterpart has no effect on the model's behavior besides a trivial and easily preventable discretization effect. We further analyse the role of the MHC and peptide contribution to TCR binding, and find that their relative, rather than absolute value, is important in shaping the mature repertoire. This result makes it possible to adopt different physical interpretations for the digit string formalism. We also find that the alloreactivity level can be inferred directly from data on the stringency of selection, and that, in agreement with recent experiments, it is not affected by thymic selection.

Developing antigen-specific therapies in multiple sclerosis: a tale of Tantalus or Ulysses?

Autoreactive T-cell responses directed to myelin proteins in the central nervous system are widely believed to be crucial in the pathology of multiple sclerosis (MS). However, effective ways of selectively targeting these T-cells in order to alter the clinical course of MS in a predictable manner has yet to be demonstrated. This review discusses two recent developments of crucial importance to the rational development of antigen-specific therapy in MS. The very idea of antigen-specific therapy in MS has long faced the challenge of determinant spreading, i.e., the development of novel autoimmune responses as the consequence of tissue damage. This phenomenon has led many to expect that in ongoing MS, many different pathogenic specificities would accumulate. Obviously, this would render antigen-specific therapy very difficult. Recent data now suggest that determinant spreading is most likely to be a transient phenomenon limited only to the first stages of tissue damage. A second development has changed our perspective on the specificity of individual T-cells and, thus, on the suitability of various ways to implement antigen-specific therapy. Evidence is rapidly accumulating that T-cell receptors are much more cross-reactive than previously assumed. This notion poses unexpected challenges to therapeutic approaches in MS that are based on selective targeting of autoreactive TCR. Vaccination with TCR peptides, administration of anti-TCR antibodies and development of therapeutically altered peptide ligands all depend on a significant level of predictability of pathogenic TCR. With such predictability now turning out to be much lower than was previously hoped, selective TCR-directed strategies for intervention may therefore turn out to be much less effective than anticipated. In the development of antigen-specific therapies, the use of whole protein tolerogens now seems to be the most promising route. Oral, intranasal or iv. administration of antigen remain viable options to re-establish selective tolerance that ought to be further developed for the benefit of MS patients.

Alloreactive and Syngeneic CTL Are Comparably Dependent on Interaction with MHC Class I α-Helical Residues

The molecular basis for the difference in the strength of T cell responses to self vs alloantigens is unknown, but may reflect how T cells are selected in the thymus. Because T cells with a high affinity for foreign as opposed to self MHC molecules are able to mature, it has been proposed that alloreactive T cells may be more strongly dependent upon interaction with MHC residues than are self-restricted T cells. This study was undertaken to rigorously address this hypothesis. Whereas other studies have compared self vs alloantigen recognition of different MHC alleles by a single T cell clone, we have compared self vs alloantigen recognition of a single MHC allele, H-2Ld, by a large panel of self-restricted and alloreactive T cell clones. Target cells expressing Ld molecules mutated at several different potential TCR contact residues were analyzed to determine which residues are important for recognition by self-restricted vs alloreactive T cells. We unequivocally demonstrate that self-restricted and alloreactive T cells do not differ, but rather are comparably dependent on interaction with MHC residues. Importantly, both self-restricted and alloreactive T cells are dependent upon the same MHC residues as primary contacts and, in addition, share a common recognition pattern of Ld. Furthermore, our analysis enables us to provide a model for allotype-specific T cell recognition of Ld vs Kb class I molecules.

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.

Cutting Edge: A Test of the Dominant Negative Signal Model for TCR Antagonism

The mechanism by which TCR antagonists interfere with T cell activation is unclear. One popular hypothesis is that incomplete early signaling events induced by these ligands dominantly inhibit the T cell’s ability to respond to a copresented agonist ligand. Here we test this “dominant negative” signal hypothesis by studying T cells expressing two distinct MHC class I-restricted TCRs (2C and OT-I). Although responses through each TCR can be efficiently inhibited by their specific antagonists, we found no evidence for “cross-antagonism” in which an antagonist for receptor “A” blocks responses through receptor “B.” Such inhibition would have been expected were the dominant negative signaling hypothesis correct, and alternative models for TCR antagonism are discussed.

Cross-Recognition of Two Middle T Protein Epitopes by Immunodominant Polyoma Virus-Specific CTL

We recently identified the immunodominant epitope for polyoma virus-specific CTL as the Dk-associated peptide MT389–397 derived from the middle T (MT) viral oncoprotein. Another Dk-restricted peptide corresponding to residues 236–244 of MT was recognized by nearly all MT389–397-reactive CTL clones, but required concentrations at least 2 logs higher to sensitize syngeneic target cells for lysis. Except for identity at the three putative Dk-peptide anchor residues, MT236–244 shares no homology with MT389–397. Using a novel europium-based class I MHC-peptide binding immunoassay, we determined that MT236–244 bound Dk 2–3 logs less well than MT389–397. Infection with a mutant polyoma virus whose MT is truncated just before the MT389–397 epitope or immunization with MT389–397 or MT236–244 peptides elicited CTL that recognized both MT389–397 and MT236–244. Importantly, infection with a polyoma virus lacking MT389–397 and mutated in an MT236–244 Dk anchor position induced polyoma virus-specific CTL recognizing neither MT389–397 nor MT236–244 epitopes. Despite predominant usage of the Vβ6 gene segment, MT389–397/MT236–244 cross-reactive CTL clones possess diverse complementarity-determining region 3β domains; this is functionally reflected in their heterogeneous recognition patterns of alanine-monosubstituted MT389–397 peptides. Using Dk/MT389–397 tetramers, we directly visualized MT236–244 peptide-induced TCR down-modulation of virtually all MT389–397-specific CD8+ T cells freshly explanted from polyoma-infected mice, suggesting that a single TCR recognizes both Dk-restricted epitopes. The availability of immunodominant epitope-specific CTL capable of recognizing a second epitope in MT, a viral protein essential for tumorigenesis, may serve to amplify the CTL response to the immunodominant epitope and prevent the emergence of immunodominant epitope-loss viruses and virus-induced tumors.

Structural evidence of T cell xeno-reactivity in the absence of molecular mimicry

The T cell receptor (TCR), from a xeno-reactive murine cytotoxic T lymphocyte clone AHIII12.2, recognizes murine H-2Db complexed with peptide p1027 (FAPGVFPYM), as well as human HLA-A2.1 complexed with peptide p1049 (ALWGFFPVL). A commonly proposed model (the molecular mimicry model) used to explain TCR cross-reactivity suggests that the molecular surfaces of the recognized complexes are similar in shape, charge, or both, in spite of the primary sequence differences. To examine the mechanism of xeno-reactivity of AHIII12.2, we have determined the crystal structures of A2/p1049 and Db/p1027 to 2.5 A and 2.8 A resolution, respectively. The crystal structures show that the TCR footprint regions of the two class I complexes are significantly different in shape and charge. We propose that rather than simple molecular mimicry, unpredictable arrays of common and differential contacts on the two class I complexes are used for their recognition by the same TCR.

Crystal structures of two H-2Db/glycopeptide complexes suggest a molecular basis for CTL cross-reactivity

Two synthetic O-GlcNAc-bearing peptides that elicit H-2Db-restricted glycopeptide-specific cytotoxic T cells (CTL) have been shown to display nonreciprocal patterns of cross-reactivity. Here, we present the crystal structures of the H-2Db glycopeptide complexes to 2.85 A resolution or better. In both cases, the glycan is solvent exposed and available for direct recognition by the T cell receptor (TCR). We have modeled the complex formed between the MHC-glycopeptide complexes and their respective TCRs, showing that a single saccharide residue can be accommodated in the standard TCR-MHC geometry. The models also reveal a possible molecular basis for the observed cross-reactivity patterns of the CTL clones, which appear to be influenced by the length of the CDR3 loop and the nature of the immunizing ligand.

Structural Basis of Specificity and Degeneracy of T Cell Recognition: Pluriallelic Restriction of T Cell Responses to a Peptide Antigen Involves Both Specific and Promiscuous Interactions Between the T Cell Receptor, Peptide, and HLA-DR

TCR engagement of peptide-MHC class II ligands involves specific contacts between the TCR and residues on both the MHC and peptide molecules. We have used molecular modeling and assays of peptide binding and T cell function to characterize these interactions for a CD4+ Th1 cell clone, ESL4.34, which recognizes a peptide epitope of the herpes simplex type 2 virus virion protein, VP16 393–405, in the context of several HLA-DR alleles. This clone responded to VP16 393–405 in proliferation and cytotoxicity assays when presented by DRB1*0402, DRB1*1102, and DRB1*1301, which share a common amino acid sequence, ILEDE, at residues 67–71 in the α-helical portion of the DRβ polypeptide, but not when presented by other DR4, DR11, and DR13 alleles that are negative for this sequence. Using a panel of APCs expressing DR4 molecules that were mutagenized in vitro at individual residues within this shared epitope and using peptide analogues with single amino acid substitutions of predicted MHC and TCR contact residues, a unit of recognition was identified dependent on DRβ residues 67–71 and relative position 4 (P4) of the VP16 393–405 peptide. The interactions of this portion of the peptide-DR ligand with the ESL4.34 TCR support a structural model for MHC-biased recognition in some Ag-specific and alloreactive T cell responses and suggest a possible mechanism for autoreactive T cell selection in rheumatoid arthritis.

Probing the activation requirements for naive CD8+ T cells with Drosophila cell transfectants as antigen presenting cells

Activation of T cells involves multiple receptor-ligand interactions between T cells and antigen presenting cells (APC). At least two signals are required for T-cell activation: Signal 1 results from recognition of MHC/peptide complexes on the APC by cell surface T-cell receptors (TCR), whereas Signal 2 is induced by the interactions of co-stimulatory molecules on APC with their complementary receptors on T cells. This review focuses on our attempts to understand these various signals in a model system involving the 2C TCR. The structural basis of Signal 1 was investigated by determining the crystal structure of 2C TCR alone and in complex with MHC/peptide. Analysis of these structures has provided some basic rules for how TCR and MHC/peptide interact; however, the critical question of how this interaction transduces Signal 1 to T cells remains unclear. The effects of Signal 1 and Signal 2 on T-cell activation were examined with naive T cells from the 2C TCR transgenic mice, defined peptides as antigen and transfected Drosophila cells as APC. The results suggest that, except under extreme conditions, Signal 1 alone is unable to activate naive CD8 T cells despite the induction of marked TCR downregulation. Either B7 or intercellular adhesion molecule (ICAM)-1 can provide the second signal for CD8 T-cell activation. However, especially at low MHC/peptide densities, optimal activation and differentiation of CD8 T cells required interaction with both B7 and ICAM-1 on the same APC. Thus, the data suggest that at least two qualitatively different co-stimulation signals are required for full activation of CD8 T cells under physiological conditions.

The T cell repertoire primed by antiviral vaccination is influenced by self-tolerance

Vaccination can elicit CD8(+) cytotoxic T lymphocytes (CTL) that recognize peptides presented by class I MHC molecules. Relatively little is known, however, about the genetic factors that shape the repertoire of T cell clonotypes responding to any given epitope. We report here that H-2(b) mice immunized with a plasmid DNA vaccine or vaccinia virus encoding for HIV-1SF2p55gag elicit CD8(+) CTL against the H-2Db-restricted immunodominant epitope (pgagb). This response involved three different T cell populations based on their recognition of alloantigens: one that cross-reacted with the alloantigen H-2Ld, one that cross-reacted with H-2Kd, and one that did not cross-react with either H-2(d) or H-2(k) molecules. Using the TAP-deficient cell line T2-Ld, we showed that pgagb-specific CTL cross-react with H-2Ld and a yet unidentified self-peptide. In mice expressing H-2(b) and H-2(d) allotypes, we investigated whether tolerance to H-2(d) influenced the HIVp55gag-specific CTL repertoire as a consequence of thymic deletion of the cross-reactive CTL repertoire. In (H-2(dxb))F1 mice heterogygosity at the MHC-I level prevented maturation of some but not all TCR combinations specific for H-2Db+pgagb, illustrating the concept that self-tolerance can influence the repertoire of antiviral T cells.

Allo- and self-restricted cytotoxic T lymphocytes against a peptide library: evidence for a functionally diverse allorestricted T cell repertoire

BALB/c-derived spleen cells were depleted of cytotoxic T lymphocytes (CTL) recognizing allogeneic (H2b) and TAP-negative cells followed by stimulation with the same cells loaded with a synthetic library binding to H2-Kb. The resulting CTL lines were found to differ widely in peptide specificity and to exhibit an avidity towards the library as that demonstrated for syngeneic CTL. These results demonstrate that positive selection in the context of a certain MHC molecule does not seem to be required for generating high-avidity TCR that are restricted by the same molecule. However, positive selection increases the frequency of such CTL. By raising T cell lines from a repertoire which did not undergo negative selection by the restriction element in question, it becomes possible to produce effective self-peptide/ MHC as well as nonself-peptide/MHC-specific CTL as tools for adoptive tumor immunotherapy.

Structural basis of 2C TCR allorecognition of H-2Ld peptide complexes

MHC class I H-2Ld complexed with peptide QL9 (or p2Ca) is a high-affinity alloantigen for the 2C TCR. We used the crystal structure of H-2Ld with a mixture of bound peptides at 3.1 A to construct a model of the allogeneic 2C-Ld/QL9 complex for comparison with the syngeneic 2C-Kb/dEV8 structure. A prominent ridge on the floor of the Ld peptide-binding groove, not present in Kb, creates a C-terminal bulge in Ld peptides that greatly increases interactions with the 2C beta-chain. Furthermore, weak electrostatic complementarity between Asp77 on the alpha1 helix of Kb and 2C is enhanced in the allogeneic complex by closer proximity of QL9 peptide residue AspP8 to the 2C HV4 loop.

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

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

High-resolution characterization of cytokine-producing alloreactivity in naive and allograft-primed mice

BACKGROUND

Whether alloreactive T cells in a naive host derive from naive or memory T cells remains unclear. It is also unclear whether graft rejection alters the phenotype of these T cells. Proliferation assays and cytokine enzyme-linked immunosorbent assays performed on culture supernatants do not differentiate primary T-cell alloreactivity from recall responses in allograft-primed mice, suggesting that these methods are inadequate measures of the alloreactive immune repertoire.

METHODS

To better characterize alloreactivity in naive and skin allograft-primed mice, we used a modified, high-resolution cytokine enzyme-linked immunosorbent spot assay capable of detecting cytokine production over short time periods.

RESULTS

Twenty-four-hour analysis of alloreactivity in mice that rejected fully MHC-disparate skin allografts revealed a high frequency of interferon (IFN)-gamma- and interleukin (IL)-4-producing, L-selectin-negative T cells, consistent with a memory phenotype. In contrast, 24-hr allostimulation of T cells from naive mice resulted in IL-2 production with minimal secretion of IFN-gamma or IL-4. The frequency of IL-2 producers was low and their phenotype was L-selectin positive, suggesting that they were naive and not memory T cells. When maintained in culture for 48 hr, however, the T cells from the primary mixed lymphocyte reaction began producing IFN-gamma, consistent with in vitro priming.

CONCLUSIONS

The primary alloresponse does not seem to involve clones that have been preprimed by environmental antigens, but instead behaves similarly to self-MHC-restricted immunity directed toward prototypic protein antigens: T cells with a naive phenotype are specifically induced to differentiate into high-frequency memory populations. These findings may have important implications for therapeutic induction of allograft tolerance.