Comparative sequence analysis of the human T cell receptor TCRA and TCRB CDR3 regions

The T-cell receptor β chain CDR3 insights of bovine liver immune repertoire under heat stress

OBJECTIVE

The liver plays a dual role in regulating temperature and immune responses. Examining the influence of heat stress (HS) on liver T cells contributes significantly to understanding the intricate interplay between the immune system and hepatic tissues under thermal stress. This study focused on investigating the characteristics of the T-cell receptor (TCR) β chain CDR3 repertoire in bovine liver samples under both HS and pairfed (PF) environmental conditions.

METHODS

Sequencing data from six samples sourced from the GEO database underwent annotation. Utilizing immunarch and VDJtool software, the study conducted comprehensive analyses encompassing basic evaluation, clonality assessment, immune repertoire comparison, diversity estimation, gene usage profiling, VJ gene segment pairing scrutiny, clonal tracking, and Kmers analysis.

RESULTS

All four TCR chains, namely α, β, γ, and δ, were detected, with the α chains exhibiting the highest detection frequency, followed closely by the β chains. The prevalence of αβ TCRs in bovine liver samples underscored their crucial role in governing hepatic tissue's physiological functions. The TCR β CDR3 repertoire showcased substantial inter-individual variability, featuring diverse clonotypes exhibiting distinct amino acid lengths. Intriguingly, HS cattle displayed heightened diversity and clonality, suggesting potential peripheral T cell migration into the liver under environmental conditions. Notably, differential VJ gene pairings were observed in HS cattle compared to the PF, despite individual variations in V and J gene utilization. Additionally, while most high-frequency amino acid 5-mers remained consistent between the HS and PF, GELHF, and YDYHF were notably prevalent in the HS group. Across all samples, a prevalent trend of high-frequency 5mers skewed towards polar and hydrophobic amino acids was evident.

CONCLUSION

This study elucidates the characteristics of liver TCR β chain CDR3 repertoire under HS conditions, enhancing our understanding of HS implications.

Single-Cell Sequencing of T cell Receptors: A Perspective on the Technological Development and Translational Application

T cells recognize peptides bound to major histocompatibility complex (MHC) class I and class II molecules at the cell surface. This recognition is accomplished by the expression of T cell receptors (TCR) which are required to be diverse and adaptable in order to accommodate the various and vast number of antigens presented on the MHCs. Thus, determining TCR repertoires of effector T cells is necessary to understand the immunological process in responding to cancer progression, infection, and autoimmune development. Furthermore, understanding the TCR repertoires will provide a solid framework to predict and test the antigen which is more critical in autoimmunity. However, it has been a technical challenge to sequence the TCRs and provide a conceptual context in correlation to the vast number of TCR repertoires in the immunological system. The exploding field of single-cell sequencing has changed how the repertoires are being investigated and analyzed. In this review, we focus on the biology of TCRs, TCR signaling and its implication in autoimmunity. We discuss important methods in bulk sequencing of many cells. Lastly, we explore the most pertinent platforms in single-cell sequencing and its application in autoimmunity.

Revealing factors determining immunodominant responses against dominant epitopes

Upon recognition of peptide-MHC complexes by T cell receptors (TCR), the cognate T cells expand and differentiate into effector T cells to generate protective immunity. Despite the fact that any immune response generates a diverse set of TCR clones against a particular epitope, only a few clones are highly expanded in any immune response. Previous studies observed that the highest frequency clones usually control viral infections better than subdominant clones, but the reasons for this dominance among T cell clones are still unclear. Here, we used publicly available TCR amino acid sequences to study which factors determine whether a response becomes immunodominance (ID) per donor; we classified the largest T cell clone as the epitope-specific dominant clone and all the other clones as subdominant responses (SD). We observed a distinctively hydrophobic CDR3 in ID responses against a dominant epitope from influenza A virus, compared to the SD responses. The common V-J combinations were shared between ID and SD responses, suggesting that the biased V-J recombination events are restricted by epitope specificity; thus, the immunodominance is not directly determined by a bias combination of V and J genetic segments. Our findings reveal a close similarity of global sequence properties between dominant and subdominant clones of epitope-specific responses but detectable distinctive amino acid enrichments in ID. Taken together, we believe this first comparative study of immunodominant and subdominant TCR sequences can guide further studies to resolve factors determining the immunodominance of antiviral as well as tumor-specific T cell responses.

Comparative analysis of CDR3 regions in paired human αβ CD8 T cells

The majority of human CD8 cytotoxic T lymphocytes express αβ T-cell receptors that recognize peptide-MHC class I complexes. Considerable attention has been devoted to TCR β repertoires, but study of TCR α chains has been limited. To gain a better understanding of the features of CDR3α and CDR3β in paired samples, we comprehensively analyzed 776 unique paired αβ TCR CDR3 regions in this study. We found that (I) the CDR3 length among paired αβ TCRs had a fairly narrow distribution due to random assortment of CDR3 length in alpha and beta chains; (II) nucleotide deletions among CDR3 regions were positively correlated with insertions in both α and β TCRs; (III) the CDR3 loops of both α and β chains contained an abundance of charged/polar residues and the CDR3 base regions contained a conserved motif; and (IV) the occurrence of Gly was CDR3 length- and position-dependent in both chains, whereas the frequency of Ser at positions 106 and 107 was positively correlated with CDR3 length in TCR β. Overall, the amino acids in CDR3 loop regions were significantly different between TCR α and β, which suggests a distinct role for each chain in the recognition of antigen-MHC complexes. Here, we have provided detailed information on CDR3 in paired TCRs expressed on human CD8+ T cells and established the basis of a reference set for αβ TCR repertoires in healthy humans.

Identification of multiple public TCR repertoires in chronic beryllium disease

Chronic beryllium disease (CBD) is a granulomatous lung disease characterized by the accumulation of beryllium (Be)-specific CD4(+) T cells in bronchoalveolar lavage. These expanded CD4(+) T cells are composed of oligoclonal T cell subsets, suggesting their recruitment to the lung in response to conventional Ag. In the current study, we noted that all bronchoalveolar lavage-derived T cell lines from HLA-DP2-expressing CBD patients contained an expansion of Be-responsive Vβ5.1(+) CD4(+) T cells. Using Be-loaded HLA-DP2-peptide tetramers, the majority of tetramer-binding T cells also expressed Vβ5.1 with a highly conserved CDR3β motif. Interestingly, Be-specific, Vβ5.1-expressing CD4(+) T cells displayed differential HLA-DP2-peptide tetramer staining intensity, and sequence analysis of the distinct tetramer-binding subsets showed that the two populations differed by a single conserved amino acid in the CDR3β motif. TCR Vα-chain analysis of purified Vβ5.1(+) CD4(+) T cells based on differential tetramer-binding intensity showed differing TCR Vα-chain pairing requirements, with the high-affinity population having promiscuous Vα-chain pairing and the low-affinity subset requiring restricted Vα-chain usage. Importantly, disease severity, as measured by loss of lung function, was inversely correlated with the frequency of tetramer-binding CD4(+) T cells in the lung. Our findings suggest the presence of a dominant Be-specific, Vβ5.1-expressing public T cell repertoire in the lungs of HLA-DP2-expressing CBD patients using promiscuous Vα-chain pairing to recognize an identical HLA-DP2-peptide/Be complex. Importantly, the inverse relationship between expansion of CD4(+) T cells expressing these public TCRs and disease severity suggests a pathogenic role for these T cells in CBD.

The molecular and functional characterization of clonally expanded CD8+ TCR BV T cells in eosinophilic granulomatosis with polyangiitis (EGPA)

In eosinophilic granulomatosis with polyangiitis (EGPA) clonally expanded T cells might concur in granuloma formation and vascular injury. The TCR β-variable (BV) chain repertoire and third complementarity determining region (CDR3) of peripheral CD4+ and CD8+ cells in EGPA patients and age-matched controls and the expression of cytokines and chemokine receptors were investigated. The CD8+ lymphocytes of EGPA patients showed an increased frequency of BV expansions with a skewed profile of BV CDR3 lengths, increased CCR5 and CXCR3 expression and increased INFγ and TNFα production. In two patients, the TCR CDR3 cDNA sequences of the expanded BV family were identified. The CD4+ lymphocytes of EGPA patients revealed a higher expression of CRTH2 and increased production of IL-5. In conclusion, CD4+ T cells display a Th2 profile and CD8+ T cells are clonally expanded in EGPA and have a proinflammatory phenotype, suggesting their pathogenic role in vasculitic damage.

The past, present, and future of immune repertoire biology - the rise of next-generation repertoire analysis

T and B cell repertoires are collections of lymphocytes, each characterized by its antigen-specific receptor. We review here classical technologies and analysis strategies developed to assess immunoglobulin (IG) and T cell receptor (TR) repertoire diversity, and describe recent advances in the field. First, we describe the broad range of available methodological tools developed in the past decades, each of which answering different questions and showing complementarity for progressive identification of the level of repertoire alterations: global overview of the diversity by flow cytometry, IG repertoire descriptions at the protein level for the identification of IG reactivities, IG/TR CDR3 spectratyping strategies, and related molecular quantification or dynamics of T/B cell differentiation. Additionally, we introduce the recent technological advances in molecular biology tools allowing deeper analysis of IG/TR diversity by next-generation sequencing (NGS), offering systematic and comprehensive sequencing of IG/TR transcripts in a short amount of time. NGS provides several angles of analysis such as clonotype frequency, CDR3 diversity, CDR3 sequence analysis, V allele identification with a quantitative dimension, therefore requiring high-throughput analysis tools development. In this line, we discuss the recent efforts made for nomenclature standardization and ontology development. We then present the variety of available statistical analysis and modeling approaches developed with regards to the various levels of diversity analysis, and reveal the increasing sophistication of those modeling approaches. To conclude, we provide some examples of recent mathematical modeling strategies and perspectives that illustrate the active rise of a "next-generation" of repertoire analysis.

Multiple glycines in TCR alpha-chains determine clonally diverse nature of human T cell memory to influenza A virus

Detailed assessment of how the structural properties of T cell receptors affect clonal repertoires of Ag-specific cells is a prerequisite for a better understanding of human antiviral immunity. Herein we examine the alpha TCR repertoires of CD8 T cells reactive against the influenza A viral epitope M1(58-66), restricted by HLA-A2.1. Using molecular cloning, we systematically studied the impact of alpha-chain usage in the formation of T cell memory and revealed that M1(58-66)-specific, clonally diverse VB19 T cells express alpha-chains encoded by multiple AV genes with different CDR3 sizes. A unique feature of these alpha TCRs was the presence of CDR3 fitting to an AGA(G(n))GG-like amino acid motif. This pattern was consistent over time and among different individuals. Further molecular assessment of human CD4(+)CD8(-) and CD4(-)CD8(+) thymocytes led to the conclusion that the poly-Gly/Ala runs in CDR3alpha were a property of immune, but not naive, repertoires and could be attributed to influenza exposure. Repertoires of T cell memory are discussed in the context of clonal diversity, where poly-Gly/Ala runs in the CDR3 of alpha- and beta-chains might provide high levels of TCR flexibility during Ag recognition while gene-encoded CDR1 and CDR2 contribute to the fine specificity of the TCR-peptide MHC interaction.

New perspectives for large-scale repertoire analysis of immune receptors

In vertebrates, the world of antigenic motifs is matched to large populations of lymphocytes through specific recognition of an epitope by a given receptor unique to a lymphocyte clone. The concept of immune repertoire was proposed to describe this diversity of lymphocyte receptors - Ig and TCR - required by the network of interactions. The immune repertoires became useful tools to describe lymphocyte and receptor populations through the development of the immune system and in pathological situations. Recently, the development of mass technologies made possible a comprehensive survey of immune repertoires at the genome, transcript and protein levels, and some of these techniques have been already adapted to TCR and Ig repertoire analyses. Such approaches generate very big datasets, which necessitates complex and multi-parametric annotations in dedicated databases. They also require new analysis methods, leading to the integration of structure and dynamics of the immune repertoires, at different time scales (immune response, development of the individual, evolution of the species). Such methods may be extended to the analysis of new classes of adaptive-like receptors, which were recently discovered in different invertebrates and in agnathans. Ultimately, they may allow a parallel monitoring of pathogen and immune repertoires addressing the reciprocal influences that decide for the host survival or death. In this review, we first study the characteristics of Ig and TCR repertoires, and we examine several systematic approaches developed for the analysis of these "classical" immune repertoires at different levels. We then consider examples of the recent developments of modeling and statistical analysis, and we discuss their relevance and their importance for the study of the immune diversity. An extended view of immune repertoires is proposed, integrating the diversity of other receptors involved in immune recognition. Also, we discuss how repertoire studies could link pathogen variation and immune diversity to reveal regulatory patterns and rules driving their co-diversification race.

Sharing of T cell receptors in antigen-specific responses is driven by convergent recombination

Public responses where identical T cell receptors (TCRs) are clonally dominant and shared between different individuals are a common characteristic of CD8(+) T cell-mediated immunity. Focusing on TCR sharing, we analyzed approximately 3,400 TCR beta chains (TCRbetas) from mouse CD8(+) T cells responding to the influenza A virus D(b)NP(366) and D(b)PA(224) epitopes. Both the "public" D(b)NP(366)-specific and "private" D(b)PA(224)-specific TCR repertoires contain a high proportion ( approximately 36%) of shared TCRbetas, although the numbers of mice sharing TCRbetas in each repertoire varies greatly. Sharing of both the TCRbeta amino acid and TCRbeta nucleotide sequence was negatively correlated with the prevalence of random nucleotide additions in the sequence. However, the extent of TCRbeta amino acid sequence sharing among mice was strongly correlated with the level of diversity in the encoding nucleotide sequences, suggesting that a key feature of public TCRs is that they can be made in a variety of ways. Using a computer simulation of random V(D)J recombination, we estimated the relative production frequencies and variety of production mechanisms for TCRbeta sequences and found strong correlations with the sharing of both TCRbeta amino acid sequences and TCRbeta nucleotide sequences. The overall conclusion is that "convergent recombination," rather than a bias in recombination or subsequent selection, provides the mechanistic basis for TCR sharing between individuals responding to identical peptide plus MHC class I glycoprotein complexes.

Antigen-recognition sites of micromanipulated T cells in patients with acquired aplastic anemia

OBJECTIVE

Acquired aplastic anemia (AA) is a rare disorder characterized by pancytopenia and hypocellular bone marrow. Though experimental and clinical data suggest that AA represents a T cell-mediated disease, neither the immune response nor the nature of inciting antigen(s) have been characterized so far. The identification of a restricted T cell repertoire by PCR techniques in total lymphocyte populations supports an antigen-driven T cell response. In order to investigate the clonal composition, we analyzed the gene rearrangements of the T cell receptor (TCR) variable beta chain (Vbeta) at the single-cell level.

PATIENTS AND METHODS

CD3(+) T lymphocytes were micromanipulated from peripheral blood and bone marrow samples of 8 AA patients and healthy controls. Subsequently amplified VDJ gene segments of the TCRVbeta chain were analyzed for functional rearrangements. More than 500 functionally rearranged TCR loci were studied for Vbeta/Jbeta gene segment usage and molecular composition of the complementary-determining region 3 (CDR3).

RESULTS

In comparison to healthy controls, the Vbeta sequences confirmed a highly restricted T cell repertoire in AA patients at the single-cell level. Both in bone marrow and peripheral blood a predominance of Vbeta13 and Jbeta2S7 was observed. Furthermore, individual clonal T-cell expansion was identified in the majority of patients. However, deduced CDR3 amino acid sequences revealed a high variability without common motifs among the 8 patients.

CONCLUSION

Individual clonal T-cell expansion with high diversity of the antigen-binding sites among the analyzed patients argues for the predominance of private inciting epitopes in AA.

HIV-specific cytotoxic T cells from long-term survivors select a unique T cell receptor

HIV-specific cytotoxic T lymphocytes (CTL) are important in controlling HIV replication, but the magnitude of the CTL response does not predict clinical outcome. In four donors with delayed disease progression we identified Vβ13.2 T cell receptors (TCRs) with very similar and unusually long β-chain complementarity determining region 3 (CDR3) regions in CTL specific for the immunodominant human histocompatibility leukocyte antigens (HLA)-B8–restricted human immunodeficiency virus-1 (HIV-1) nef epitope, FLKEKGGL (FL8). CTL expressing Vβ13.2 TCRs tolerate naturally arising viral variants in the FL8 epitope that escape recognition by other CTL. In addition, they expand efficiently in vitro and are resistant to apoptosis, in contrast to FL8–specific CTL using other TCRs. Selection of Vβ13.2 TCRs by some patients early in the FL8-specific CTL response may be linked with better clinical outcome.

Peptide fine specificity of anti-glycoprotein 100 CTL is preserved following transfer of engineered TCR alpha beta genes into primary human T lymphocytes

TCR with known antitumor reactivity can be genetically introduced into primary human T lymphocytes and provide promising tools for immunogene therapy of tumors. We molecularly characterized two distinct TCRs specific for the same HLA-A2-restricted peptide derived from the melanocyte differentiation Ag gp100, yet exhibiting different stringencies in peptide requirements. The existence of these two distinct gp100-specific TCRs allowed us to study the preservation of peptide fine specificity of native TCRalphabeta when engineered for TCR gene transfer into human T lymphocytes. Retroviral transduction of primary human T lymphocytes with either one of the two sets of TCRalphabeta constructs enabled T lymphocytes to specifically kill and produce TNF-alpha when triggered by native gp100(pos)/HLA-A2(pos) tumor target cells as well as gp100 peptide-loaded HLA-A2(pos) tumor cells. Peptide titration studies revealed that the cytolytic efficiencies of the T lymphocyte transductants were in the same range as those of the parental CTL clones. Moreover, primary human T lymphocytes expressing either one of the two engineered gp100-specific TCRs show cytolytic activities in response to a large panel of peptide mutants that are identical with those of the parental CTL. The finding that two gp100-specific TCR, derived from two different CTL, can be functionally introduced into primary human T lymphocytes without loss of the Ag reactivity and peptide fine specificity, holds great promise for the application of TCR gene transfer in cancer immunotherapy.

Increased activation and oligoclonality of peripheral CD8(+) T cells in the chronic human helminth infection alveolar echinococcosis

Alveolar echinococcosis (AE) in humans is a chronic disease characterized by slowly expanding liver lesions. Cellular immunity restricts the spreading of the extracellular pathogen, but functional contributions of CD4(+) and CD8(+) T cells are not defined. Here we studied ex vivo the phenotype and function of circulating T-cell subsets in AE patients by means of flow cytometry, T-cell receptor spectratyping, and lymphocyte proliferation. AE patients with parasitic lesions displayed a significant increase of activation of predominantly CD8(+) T cells compared to healthy controls and AE patients without lesions. In vitro, proliferative T-cell responses to polyclonal stimulation with recall antigens and Echinococcus multilocularis vesicular fluid antigen were sustained during chronic persisting infection in all AE patients. Only in AE patients with parasitic lesions did T-cell receptor spectratyping reveal increased oligoclonality of CD8(+) but not CD4(+) T cells, suggesting a persistent antigenic drive for CD8(+) T cells with subsequent proliferation of selected clonotypes. Thus, our data provide strong evidence for an active role of CD8(+) T cells in AE.

Prevalence of an ulcerative colitis-associated CD8+ T cell receptor beta-chain CDR3-region motif and its association with disease activity

The normal human intestinal mucosa contains clonal T cell expansions. Clonal populations of T cells can be determined through evaluation of the idiotypic, hypervariable region of their T cell receptor (TCR). We have previously reported that there exists a highly conserved TCR pattern among intestinal CD8+ T cells in the majority of ulcerative colitis (UC) patients undergoing colectomy that was not present in normal control individuals. This TCR pattern, or motif, was characterized by particular beta-chain usage (TCRBV3 and TCRBJ1S6) and a defined length in the hypervariable third complementarity determining region (CDR3). The aim of this study was to assess the motif's relationship to disease activity. Subjects were 66 with UC, 19 with Crohn's disease, 14 inflammatory controls, and 6 normal controls. cDNA and gDNA were prepared from colonic biopsies and paraffin blocks, respectively, obtained from study subjects and used to assess TCRBV CDR3 region length and usage, as well as for cloning and sequencing of TCRs. The TCRBV CDR3 region was present in 25 of a series of 48 UC subjects but only 3 of 19 Crohn's disease patients and 3 of 14 inflammatory controls. The motif was more common in UC than either Crohn' s disease or inflammatory controls (chi2 = 7.5, P = 0.006, and chi2 = 4.1, P = 0.04, respectively). The motifs presence was not dependent upon histologic disease activity (either active or inactive UC). Clinical UC disease activity was also not significantly associated with an increased presence of the motif in 14 paired biopsies, which were taken during times of clinical activity or inactivity. There was a trend toward persistence of the motif, as it was present in 6 of 14 subjects over a 3- to 6-month time period. The previously described UC-associated TCRBV CDR3 region motif located in the intestinal CD8+ T-cell subset is found in a significant proportion of UC subjects. The TCR motif does not significantly discriminate active from inactive disease states. The persistent and diffuse nature of this TCR-associated motif in UC suggests that an ongoing T-cell response to a particular antigen(s) is occuring in this disorder.

Clonal expansions of CD8(+) T cells dominate the T cell infiltrate in active multiple sclerosis lesions as shown by micromanipulation and single cell polymerase chain reaction

Clonal composition and T cell receptor (TCR) repertoire of CD4(+) and CD8(+) T cells infiltrating actively demyelinating multiple sclerosis (MS) lesions were determined with unprecedented resolution at the level of single cells. Individual CD4(+) or CD8(+) T cells were isolated from frozen sections of lesional tissue by micromanipulation and subjected to single target amplification of TCR-beta gene rearrangements. This strategy allows the assignment of a TCR variable region (V region) sequence to the particular T cell from which it was amplified. Sequence analysis revealed that in both cases investigated, the majority of CD8(+) T cells belonged to few clones. One of these clones accounted for 35% of CD8(+) T cells in case 1. V region sequence comparison revealed signs of selection for common peptide specificities for some of the CD8(+) T cells in case 1. In both cases, the CD4(+) T cell population was more heterogeneous. Most CD4(+) and CD8(+) clones were represented in perivascular infiltrates as well as among parenchymal T cells. In case 2, two of the CD8(+) clones identified in brain tissue were also detected in peripheral blood. Investigation of the antigenic specificities of expanded clones may help to elucidate their functional properties.

Clonal Expansion Within CD4+ and CD8+ T Cell Subsets in Human T Lymphotropic Virus Type I-Infected Individuals

To investigate the diversity of the T cell repertoire involved in human T lymphotropic virus type I (HTLV-I) infections, peripheral blood T cell subsets were analyzed by using a PCR-based assay that permits determination of complementarity-determining region 3 (CDR3) length variation in TCR Vβ transcripts. In two of four asymptomatic HTLV-I carriers and in four of five patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP), mono- or oligoclonal expansions were detected in the CD4+ T cell subset. In one patient with adult T cell leukemia, a specific clone bearing Vβ7 was detected in the CD4+ T cell subset. In contrast, clonal expansion was not observed in the CD4 T cell subsets of three individuals with asymptomatic HTLV-II infection or in our previous studies of a large number of uninfected individuals. Oligoclonal expansions in the CD8+ T cell subset were detected in all subjects, including the patient with adult T cell leukemia. No differences in the number of expanded clones were noted between asymptomatic carriers and in patients with HAM/TSP and there was no obvious restriction in the TCR V region usage. Direct sequencing revealed no significant bias in the CDR3 motifs utilized by the predominant clones. This report is the first direct demonstration of clonal expansions within fractionated T cell subsets (CD4+ and CD8+) in HTLV-I infections and suggests that 1) clonal expansion of CD4+ T lymphocytes likely occurs as a direct result of infection and 2) polyclonal CD8+ T cell expansion occurs frequently and independently of disease association.

Structural Aspects of the Interaction Between Heterogeneic Human Papillomavirus Type 1 E4-Specific T Cell Receptors and the Same Peptide/HLA-DQ8 Complex

TCR usage has been studied in a panel of Th cell clones specific for the same peptide epitope (P N S Q D R G R P R R S D), derived from the human papillomavirus type 1 (HPV1) E4 protein, and restricted through HLA-DQ8. After identifying the V, D, and J genes used by the TCRs and sequencing across the V(D)J junctions, five different α-chain sequences and five different β-chain sequences, comprising six independent clones, were identified. A structural model of our E4 peptide/HLA-DQ8 complex predicted that the guanidinyl side chain on the arginine residue at position 6 of the peptide could exist in different orientations. An intramolecular interaction between this arginine and the glutamine residue at position four appeared to control this orientation. Interacting HPV1 E4-specific TCRs would therefore have to recognize the complex in different conformations, and molecular modeling of the TCRs suggested that this could be achieved by changing the dimensions of the central pocket formed where the CDR3 loops of the TCR α- and β-chains converge. It is known that interactions between bound peptide and amino acid residues lining the peptide-binding cleft of HLA molecules are important for determining the conformation and orientation of the peptide/MHC complex. The suggestion here that intramolecular interactions between amino acids of close proximity on the bound peptide are also important adds a further level of complexity to the mechanism by which TCRs interact with Ag.