Secondary, tertiary, and quaternary structure of T-cell-specific immunoglobulin-like polypeptide chains

Identification, expression and molecular polymorphism of T-cell receptors α and β from the glacial relict Hucho bleekeri

The T-cell receptor (TCR) is a specific molecule on the surface of all T cells that mediates cellular adaptive immune responses to antigens. Hucho bleekeri is a critically endangered species and is regarded as a glacial relict that has the lowest-latitude distribution compared with any Eurasian salmonid. In the present study, two TCR genes, namely, TCR α and β, were identified and characterized in H. bleekeri. Both TCR α and TCR β have typical TCR structures, including the IgV domain, IgC domain, connecting peptide, transmembrane and cytoplasmic domains. The two TCR genes were constitutionally expressed in various tissues, with the highest expression found in the spleen for TCR α and in the trunk kidney for TCR β. Challenge of H. bleekeri with LPS or poly(I:C) resulted in significant upregulation of both TCR α and β expression in headkidney and spleen primary cells, indicating their potential roles in the immune response. Molecular polymorphism analysis of the whole ORF regions of TCR α and β in different individuals revealed high diversity of IgV domains of these two genes, especially in complementarity-determining region (CDR) 3. The ratio of nonsynonymous substitution occurred at a significantly higher frequency than synonymous substitution in the CDR of TCR α and β, demonstrating the existence of positive selection. The results obtained in the present study enhance our understanding of TCR roles in regulating immune mechanisms and provide new information for the study of TCR lineage diversity in fish.

Complementary DNA sequences of the constant regions of T-cell antigen receptors α, β and γ in mandarin fish, Siniperca chuatsi Basilewsky, and their transcriptional changes after stimulation with Flavobacterium columnare

In this study, the constant-region genes (Cα, Cβ and Cγ) that encode the T-cell antigen receptor (TCR) α, β and γ chains were cloned from mandarin fish, Siniperca chuatsi Basilewsky, an important freshwater fish species in China. The complementary DNA sequences of Cα, Cβ and Cγ were 843, 716 and 906 base pairs (bp) in length and had a 465-, 289- and 360-bp 3' untranslated region, encoding 125, 142 and 182 amino acids, respectively. The amino-acid sequences of the constant regions of mandarin fish TCR α, β and γ chains (encoded by Cα, Cβ and Cγ, respectively) were most similar to those of their teleost counterparts, showing 60% similarity with pufferfish, 48% similarity with Atlantic salmon and 57% similarity with flounder, respectively. The phylogenetic analysis revealed that the mandarin fish Cα, Cβ and Cγ were clustered, respectively, with their vertebrate counterparts. The mandarin fish Cα, Cβ and Cγ could also be separated into four domains: immunoglobulin; connecting peptide (CP); transmembrane (TM); and cytoplasmic tail. Several conserved features in mammalian TCRs were also found in those of mandarin fish, such as a conserved cysteine residue in the CP domain of Cα, necessary for creating an interchain disulphide bond with the TCR β chain, and a conserved antigen receptor TM motif in Cα and Cβ. Meanwhile, transcripts of Cα, Cβ and Cγ were detectable in all examined organs, with a stronger signal observed in lymphoid organs. In addition, the temporal transcriptional changes for Cα and Cγ were investigated, 1, 2, 3, 4, 5, 6 and 8 weeks after stimulation with Flavobacterium columnare, in head kidney, spleen, blood, thymus, gill and intestine, using real-time polymerase chain reaction. The results demonstrated stimulation-dependent up-regulations in almost all tissues examined, which indicates that T cells may play important roles in preventing mandarin fish from bacterial invasion. In particular, apart from thymus, T cells were distributed mainly in gill and intestine, where striking up-regulation of Cγ was also observed. These results will facilitate functional studies of teleost TCRs and T cells.

TCR Mechanobiology: Torques and Tunable Structures Linked to Early T Cell Signaling

Mechanotransduction is a basis for receptor signaling in many biological systems. Recent data based upon optical tweezer experiments suggest that the TCR is an anisotropic mechanosensor, converting mechanical energy into biochemical signals upon specific peptide-MHC complex (pMHC) ligation. Tangential force applied along the pseudo-twofold symmetry axis of the TCR complex post-ligation results in the αβ heterodimer exerting torque on the CD3 heterodimers as a consequence of molecular movement at the T cell-APC interface. Accompanying TCR quaternary change likely fosters signaling via the lipid bilayer predicated on the magnitude and direction of the TCR-pMHC force. TCR glycans may modulate quaternary change, thereby altering signaling outcome as might the redox state of the CxxC motifs located proximal to the TM segments in the heterodimeric CD3 subunits. Predicted alterations in TCR TM segments and surrounding lipid will convert ectodomain ligation into the earliest intracellular signaling events.

T cell receptor gene usage of BP180-specific T lymphocytes from patients with bullous pemphigoid and pemphigoid gestationis

BP180 is the autoantigen of different immunobullous diseases, including bullous pemphigoid (BP) and pemphigoid gestationis (PG). Previously, we demonstrated that the NC16A domain of this autoantigen harbors key epitopes of autoantibodies and T cells, indicating that it plays an essential role in the pathogenesis of diseases. Moreover, NC16A-specific T cell clones derived from these patients were shown to express a CD4+ memory T cell phenotype and secrete cytokines that may promote autoantibody production. In this study, we further characterize the properties of these T cells by analyzing their epitope specificity and T cell receptor (TCR) gene usage. We discovered that 83% of T cell clones obtained from BP patients preferentially express TCRBV13, while clones derived from a PG patient express the TCRBV3 gene. However, no preferential TCRBJ gene usage was identified. In conclusion, our results provide an advanced understanding of the characteristics of autoimmune T cells in immunobullous diseases.

T cell receptor gene usage in desmoglein-3-specific T lymphocytes from patients with pemphigus vulgaris

Pemphigus vulgaris (PV) is an autoimmune disease mediated by autoantibodies against desmoglein-3 (Dsg3). It has been documented that both humoral and cellular autoimmunity play essential roles in the development of PV. Recently, we identified that T cells from PV patients respond to three antigenic fragments on the ectodomain of Dsg3. These T cells are CD4 alpha/beta cells secreting a Th2-like cytokine profile, and responding of Dsg3 in a restriction to HLA-DRBI*0402 or 1401 alleles. Other characteristics of these cells, such as detailed epitope(s) and T cell receptors (TCRs) usage, however, have not been investigated. The purpose of this study is to determine detailed T cell epitope(s) and TCR genes utilized by Dsg3-specific T cells. Here, we found that Dsg3(AA145-192)-specific cells preferentially utilize the TCRVbeta13 gene, while Dsg3(AA240-303)- and Dsg3 (AA570-614)-specific cells utilize Vbeta7 and Vbeta17 genes, respectively. Analysis of TCRValpha gene expression, it appears that Valpha22 gene is expressed by Dsg3(AA145-192)-specific cells, whereas the Valpha10 gene is predominantly utilized by Dsg3(AA240-303)-specific T cells. There are no specific utilization of Valpha gene in the group of cells proliferate to Dsg3 (AA570-614). We believe that this information will further our understanding of the properties of autoimmune T cells in patients with PV.

Structural, antigenic and evolutionary analyses of immunoglobulins and T cell receptors

We have had the pleasure of collaborating with Allen Edmundson for the past 15 years on the structure, binding properties and evolution of immunoglobulins and T cell receptors. Among the most significant contributions of our joint efforts were: (1) the predictive use of structural features of immunoglobulin domains to model the three-dimensional structures of the immunoglobulin domains of human T-cell receptor alpha and beta chains as well as shark light chains and V(H) domains; (2) the finding that normal humans and other vertebrates express autoantibodies against combining site epitopes of their own T cell receptors; (3) the mapping of the peptide autoepitopes recognized in health, autoimmunity and retroviral infection; and (4) the determination that epitope recognition promiscuity is a characteristic property of the combining sites of IgM immunoglobulins ranging from those of sharks to those of humans. We briefly review the salient findings and status of these studies and indicate the future directions that we will pursue in their continuation.

Structure-based design of a bispecific receptor mimic that inhibits T cell responses to a superantigen

Key surface proteins of pathogens and their toxins bind to the host cell receptors in a manner that is quite different from the way the natural ligands bind to the same receptors and direct normal cellular responses. Here we describe a novel strategy for "non-antibody-based" pathogen countermeasure by targeting the very same "alternative mode of host receptor binding" that the pathogen proteins exploit to cause infection and disease. We have chosen the Staphylococcus enterotoxin B (SEB) superantigen as a model pathogen protein to illustrate the principle and application of our strategy. SEB bypasses the normal route of antigen processing by binding as an intact protein to the complex formed by the MHC class II receptor on the antigen-presenting cell and the T cell receptor. This alternative mode of binding causes massive IL-2 release and T cell proliferation. A normally processed antigen requires all the domains of the receptor complex for its binding, whereas SEB requires only the alpha1 subunit (DRalpha) of the MHC class II receptor and the variable beta subunit (TCRVbeta) of the T cell receptor. This prompted us to design a bispecific chimera, DRalpha-linker-TCRVbeta, that acts as a receptor mimic and prevents the interaction of SEB with its host cell receptors. We have adopted (GSTAPPA)(2) as the linker sequence because it supports synergistic binding of DRalpha and TCRVbeta to SEB and thereby makes DRalpha-(GSTAPPA)(2)-TCRVbeta as effective an SEB binder as the native MHC class II-T cell receptor complex. Finally, we show that DRalpha-(GSTAPPA)(2)-TCRVbeta inhibits SEB-induced IL-2 release and T cell proliferation at nanomolar concentrations.

The crystal structure of a T cell receptor in complex with peptide and MHC class II

The crystal structure of a complex involving the D10 T cell receptor (TCR), 16-residue foreign peptide antigen, and the I-Ak self major histocompatibility complex (MHC) class II molecule is reported at 3.2 angstrom resolution. The D10 TCR is oriented in an orthogonal mode relative to its peptide-MHC (pMHC) ligand, necessitated by the amino-terminal extension of peptide residues projecting from the MHC class II antigen-binding groove as part of a mini beta sheet. Consequently, the disposition of D10 complementarity-determining region loops is altered relative to that of most pMHCI-specific TCRs; the latter TCRs assume a diagonal orientation, although with substantial variability. Peptide recognition, which involves P-1 to P8 residues, is dominated by the Valpha domain, which also binds to the class II MHC beta1 helix. That docking is limited to one segment of MHC-bound peptide offers an explanation for epitope recognition and altered peptide ligand effects, suggests a structural basis for alloreactivity, and illustrates how bacterial superantigens can span the TCR-pMHCII surface.

Molecular interactions between extracellular components of the T-cell receptor signaling complex

The structural and biochemical basis of antigen recognition by the T-cell receptor (TCR)-CD3 signaling complex has been illuminated greatly over the past few years. Structural biology has contributed enormously to this understanding through the determination of crystal structures of many of the individual components of this complex, and some of the complexes. A number of general principles can be derived for the structure of the alpha beta TCR and its interaction with peptide-major histocompatibility complex (pMHC) in class I systems, as well as interaction of the CD8 co-receptor with MHC. Large buried surface areas within the protein-protein interfaces, and varying degrees of shape complementarity appear critical for modulating the stability of the multicomponent, low-affinity macromolecular complexes consisting of TCR, pMHC, CD8 or CD4, and CD3 gamma, delta, epsilon and zeta. Significant structural alterations in TCR and pMHC, upon complex formation, hint at an as yet unclear role for conformational change in both recognition and activation. Subtle chemical alterations in key peptide residues which contact the TCR can have dramatic agonist or antagonist effects on receptor activation, which correlate only loosely with the TCR/pMHC complex affinity, implying an ability of the signaling complex to "sense" fine differences in the interface. The stoichiometry of an activated TCR signaling complex is still an unresolved issue, as is the structure and disposition of the CD3 components. However, functional experiments are bridging this gap and providing us with preliminary working models of the multimeric assemblies.

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.

T-cell receptor peptide-MHC interactions: biological lessons from structural studies

Fifteen years have passed since T-cell receptor (TCR) genes were identified (reviewed in [1]). Unlike the situation for antibodies, no direct structural information on the TCR proteins has been available for most of this time. Recently, however, the crystal structures of isolated alpha and beta chains were determined, shortly followed by the determination of the structure of an alpha beta heterodimer. Subsequently, the structures of two TCR peptide-MHC (pMHC) complexes have been reported. The windfall of this, and other more recent structural information, has elucidated some generalizations for TCR binding and recognition of pMHC. The crystal structures have, however, given us very little insight into the mechanisms of signal transduction by the TCR complex and the subsequent events which lead to activation of a T cell. Ultimately, the crystallographio results will be reconciled with experiments from other disciplines for a complete understanding of the molecular events of T cell activation.

Point mutations in the beta chain CDR3 can alter the T cell receptor recognition pattern on an MHC class I/peptide complex over a broad interface area

To study how the T cell receptor interacts with its cognate ligand, the MHC/peptide complex, we used site directed mutagenesis to generate single point mutants that alter amino acids in the CDR3beta loop of a H-2Kb restricted TCR (N30.7) specific for an immunodominant peptide N52-N59 (VSV8) derived from the vesicular stomatitis virus nucleocapsid. The effect of each mutation on antigen recognition was analyzed using wild type H-2Kb and VSV8 peptide, as well as H-2Kb and VSV8 variants carrying single replacements at residues known to be exposed to the TCR. These analyses revealed that point mutations at some positions in the CDR3beta loop abrogated recognition entirely, while mutations at other CDR3beta positions caused an altered pattern of antigen recognition over a broad area on the MHC/peptide surface. This area included the N-terminus of the peptide, as well as residues of the MHC alpha1 and alpha2 helices flanking this region. Assuming that the N30 TCR docks on the MHC/peptide with an orientation similar to that recently observed in two different TCR-MHC/peptide crystal structures, our findings would suggest that single amino acid alterations within CDR3beta can affect the interaction of the TCR with an MHC surface region distal from the predicted CDR3beta-Kb/VSV8 interface. Such unique recognition capabilities are generated with minimal alterations in the CDR3 loops of the TCR. These observations suggest the hypothesis that extensive changes in the recognition pattern due to small perturbations in the CDR3 structure appears to be a structural strategy for generating a highly diversified TCR repertoire with specificity for a wide variety of antigens.

A dominant V beta bias in the CTL response after HSV-1 infection is determined by peptide residues predicted to also interact with the TCR beta-chain CDR3

Many T cell responses are dominated by restricted TCR expression and can range from repeated usage of particular TCR Vbeta- and/or Valpha-elements, to the preferential usage of both V- and J-elements, often in conjunction with conserved V-D-J or V-J junctional sequences. Cytotoxic T lymphocytes specific for a Kb-restricted determinant from the herpes simplex virus glycoprotein B (gB) preferentially express a dominant TCRBV10 beta-chain with sequence conservation of a tryptophan-glycine located in the V-D junction. Here we have examined whether immunisation of C57BL/6 mice with the gB-peptide can mimic the CTL response seen after HSV-1 infection. Immunisation with the gB-peptide resulted in the generation of gB-specific CTL that showed a similar TCRBV10 bias to that observed after HSV-1 infection. When the gB-determinant was expressed as a part of a fusion protein, immunised mice again exhibited the TCRBV10 bias with the junctional sequence conservation in the responding CTL. C57BL/6 mice were then immunised with variants of the gB-peptide that contained amino acid substitutions at positions previously predicted to contact the TCR beta-chain CDR3. Analysis of the TCRBV usage of variant specific CTL lines showed that substitutions at the TCR-contact positions 4, 6 and 7 of the gB-peptide resulted in a loss of the TCRBV10 bias. These results suggest that the TCRBV10 bias seen in gB-specific CTL after HSV-1 infection is due to antigenic selection by the minimal peptide and is determined by residues proposed to contact the TCR beta-chain CDR3.

Atomic structure of an alphabeta T cell receptor (TCR) heterodimer in complex with an anti-TCR fab fragment derived from a mitogenic antibody

Each T cell receptor (TCR) recognizes a peptide antigen bound to a major histocompatibility complex (MHC) molecule via a clonotypic alphabeta heterodimeric structure (Ti) non-covalently associated with the monomorphic CD3 signaling components. A crystal structure of an alphabeta TCR-anti-TCR Fab complex shows an Fab fragment derived from the H57 monoclonal antibody (mAb), interacting with the elongated FG loop of the Cbeta domain, situated beneath the Vbeta domain. This loop, along with the partially exposed ABED beta sheet of Cbeta, and glycans attached to both Cbeta and Calpha domains, forms a cavity of sufficient size to accommodate a single non-glycosylated Ig domain such as the CD3epsilon ectodomain. That this asymmetrically localized site is embedded within the rigid constant domain module has implications for the mechanism of signal transduction in both TCR and pre-TCR complexes. Furthermore, quaternary structures of TCRs vary significantly even when they bind the same MHC molecule, as manifested by a unique twisting of the V module relative to the C module.

Conformational integrity and ligand binding properties of a single chain T-cell receptor expressed in Escherichia coli

We recently showed that a soluble, heterodimeric murine D10 T-cell receptor (TCR) (Valpha2Calpha, Vbeta8.2Cbeta) expressed in insect cells binds both Vbeta8.2-specific bacterial superantigen staphylococcal enterotoxin C2 (SEC2) and a soluble, heterodimeric major histocompatibility complex class II I-Ak.conalbumin peptide complex with a low micromolar affinity. To define further the structural requirements for the TCR/ligand interactions, we have produced in Escherichia coli a soluble, functional D10 single chain (sc) TCR molecule in which the Valpha and Vbeta domains are connected by a flexible peptide linker. Purified and refolded D10 scTCR bound to SEC2 and murine major histocompatibility complex class II I-Ak.conalbumin peptide complex with thermodynamic and kinetic binding constants similar to those measured for the baculovirus-derived heterodimeric D10 TCR suggesting that neither the TCR constant domains nor potential N- or O-linked carbohydrate moieties are necessary for ligand recognition and for expression and proper folding of the D10 scTCR. Purified D10 scTCR remained soluble at concentrations up to 1 mM. Circular dichroism and NMR spectroscopy indicated that D10 scTCR is stabilized predominantly by beta-sheet secondary structure, consistent with its native-like conformation. Because of its limited size, high solubility, and structural integrity, purified D10 scTCR appears to be suitable for structural studies by multidimensional NMR spectroscopy.

T-cell receptor structure and TCR complexes

The first crystal structures of intact T-cell receptors (TCRs) and their complexes with MHC peptide antigens (pMHC) were reported during the past year, along with those of a single-chain TCR Fv fragment and a beta-chain complexed with two different bacterial superantigens. These structures have shown the similarities and differences in the architecture of the antigen-binding regions of TCRs and antibodies, and how the TCR interacts with pMHC ligands as well as with superantigens.

Topology of T cell receptor-peptide/class I MHC interaction defined by charge reversal complementation and functional analysis

The molecular interactions between the CD8 co-receptor dependent N15 and N26 T cell receptors (TCRs) and their common ligand, the vesicular stomatitis virus octapeptide (VSV8) bound to H-2Kb, were studied to define the docking orientation(s) of MHC class I restricted TCRs during immune recognition. Guided by the molecular surfaces of the crystallographically defined peptide/MHC and modeled TCRs, a series of mutations in exposed residues likely contacting the TCR ligand were analyzed for their ability to alter peptide-triggered IL-2 production in T cell transfectants. Critical residues which diminished antigen recognition by 1000 to 10,000-fold in molar terms were identified in both N15 Valpha (alphaE94A or alphaE94R, Y98A and K99) and Vbeta (betaR96A, betaW97A and betaD99A) CDR3 loops. Mutational analysis indicated that the Rp1 residue of VSV8 is critical for antigen recognition of N15 TCR, but R62 of H-2Kb is less critical. More importantly, the alphaE94R mutant could be fully complemented by a reciprocal charge reversal at Kb R62 (R62E). This result suggests a direct interaction between N15 TCR Valpha E94R and Kb R62E residues. As Rp1 of VSV8 is adjacent to R62 in the VSV8/Kb complex and essential for T cell activation, this orientation implies that the N15 Valpha CDR3 loop interacts with the N-terminal residues of VSV8 with the Valpha domain docking to the Kb alpha2 helix while the N15 Vbeta CDR3 loop interacts with the more C-terminal peptide residues and the Vbeta domain overlies the Kb alpha1 helix. An equivalent orientation is suggested for N26, a second VSV8/Kb specific TCR. Given that genetic analysis of two different class II MHC-restricted TCRs and two crystallographic studies of class I restricted TCRs offers a similar overall orientation of V domains relative to alpha-helices, these data raise the possibility of a common docking mode between TCRs and their ligands regardless of MHC restriction.

Influence of the NH2-terminal amino acid of the T cell receptor alpha chain on major histocompatibility complex (MHC) class II + peptide recognition

The alpha/beta T cell receptor (TCR) recognizes peptide fragments bound in the groove of major histocompatibility complex (MHC) molecules. We modified the TCR alpha chain from a mouse T cell hybridoma and tested its ability to reconstitute TCR expression and function in an alpha chain-deficient variant of the hybridoma. The modified alpha chain differed from wild type only in its leader peptide and mature NH2-terminal amino acid. Reconstituted cell surface TCR complexes reacted normally with anti-TCR and anti-CD3 antibodies. Although cross-linking of this TCR with an antibody to the TCR idiotype elicited vigorous T cell hybridoma activation, stimulation with its natural MHC + peptide ligand did not. We demonstrated that this phenotype could be reproduced simply by substituting the glutamic acid (E) at the mature NH2 terminus of the wild type TCR alpha chain with aspartic acid (D). The substitution also dramatically reduced the affinity of soluble alpha/beta-TCR heterodimers for soluble MHC + peptide molecules in a cell-free system, suggesting that it did not exert its effect simply by disrupting TCR interactions with accessory molecules on the hybridoma. These results demonstrate for the first time that amino acids which are not in the canonical TCR complementarity determining regions can be critical in determining how the TCR engages MHC + peptide.

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.

Major histocompatibility complex recognition by immune receptors: differences among T cell receptor versus antibody interactions with the VSV8/H-2Kb complex

The surface residues of the VSV8/Kb complex important for recognition by N15 and N26 alphabeta T cell receptors (TCR) were mapped by mutational analysis and compared to each other and with epitopes of well-characterized Kb specific monoclonal antibodies (mAb). Three features of immune receptor recognition emerge. First, the footprints of the two TCR on VSV8/Kb are similar with more than 80 % overlap between sites. Given that only 8 of 14 surface exposed VSV8/Kb residues identified as critical for TCR interaction are in common, the chemical basis of the N15 and N26 interactions is nevertheless distinct. Second, the cognate peptide is a major focus of TCR recognition: mutation at any of the three exposed side chains (at p1, p4 or p6) abrogates interaction of both TCR as measured by functional T cell activation. Third, in contrast to TCR, mAb bind to discrete segments on the periphery of the alpha1 and/or alpha2 helices without orientational restriction. These findings suggest that unlike soluble antibodies, surface membrane receptor-ligand interactions on opposing cells (i.e. TCR-peptide/ MHC, CD8-MHC) limit the orientational freedom of the TCR in the immune recognition process.

Crystallization of a deglycosylated T cell receptor (TCR) complexed with an anti-TCR Fab fragment

A strategy to overexpress T cell receptors (TCRs) in Lec3.2.8.1 cells has been developed using the "Velcro" leucine zipper sequence to facilitate alpha-beta pairing. Upon secretion in culture media, the VSV-8-specific/H2-Kb-restricted N15 TCR could be readily immunopurified using the anti-leucine zipper monoclonal antibody 2H11, with a yield of 5-10 mg/liter. Mass spectrometry analysis revealed that all attached glycans were GlcNAc2-Man5. Following Superdex 200 gel filtration to remove aggregates, wild-type N15 or N15(s), a C183S variant lacking the unpaired cysteine at amino acid residue 183 in the Cbeta domain, was thrombin-cleaved and endoglycosidase H-digested, and the two derivatives were termed iN15DeltaH and N15(s)DeltaH, respectively, and sized by Superdex 75 chromatography to high purity. N-terminal and C-terminal microsequencing analysis showed the expected unique termini of N15 alpha and beta subunits. Nevertheless, neither protein crystallized under a wide range of conditions. Subsequently, we produced a Fab fragment of the murine TCR Cbeta-specific hamster monoclonal antibody H57 and complexed the Fab fragment with iN15DeltaH and N15(s)DeltaH. Both N15(s)DeltaH-Fab[H57] and iN15DeltaH-Fab[H57] complexes crystallize, with the former diffracting to 2.8-A resolution. These findings show that neither intact glycans nor the conserved and partially exposed Cys-183 is required for protein stability. Furthermore, our results suggest that the H57 Fab fragment aids in the crystallization of TCRs by altering their molecular surface and/or stabilizing inherent conformational mobility.

An alphabeta T cell receptor structure at 2.5 A and its orientation in the TCR-MHC complex

The central event in the cellular immune response to invading microorganisms is the specific recognition of foreign peptides bound to major histocompatibility complex (MHC) molecules by the alphabeta T cell receptor (TCR). The x-ray structure of the complete extracellular fragment of a glycosylated alphabeta TCR was determined at 2.5 angstroms, and its orientation bound to a class I MHC-peptide (pMHC) complex was elucidated from crystals of the TCR-pMHC complex. The TCR resembles an antibody in the variable Valpha and Vbeta domains but deviates in the constant Calpha domain and in the interdomain pairing of Calpha with Cbeta. Four of seven possible asparagine-linked glycosylation sites have ordered carbohydrate moieties, one of which lies in the Calpha-Cbeta interface. The TCR combining site is relatively flat except for a deep hydrophobic cavity between the hypervariable CDR3s (complementarity-determining regions) of the alpha and beta chains. The 2C TCR covers the class I MHC H-2Kb binding groove so that the Valpha CDRs 1 and 2 are positioned over the amino-terminal region of the bound dEV8 peptide, the Vbeta chain CDRs 1 and 2 are over the carboxyl-terminal region of the peptide, and the Valpha and Vbeta CDR3s straddle the peptide between the helices around the central position of the peptide.

A functionally significant allelic polymorphism in a T cell receptor V beta gene segment

The effect of an allelic polymorphism in the BV1S1 gene segment on recognition of major histocompatibility complex (MHC)-peptide complexes by a specific T cell receptor (TCR) was studied using RBL 2H3 cells transfected with TCR-CD3 zeta chimeric receptors. An HLA-A2-restricted human immunodeficiency virus (HIV) pol-specific cytotoxic T lymphocyte (CTL) clone utilizing the BV1S1A2 gene in combination with AV2S1A2 was identified and the extracellular domains of the TCR were fused to CD3 zeta. In degranulation assays RBL 2H3 transfectants expressing this receptor maintained the specificity of the parental CTL clone. The allelic variant BV1S1A1N1 containing a glutamine for histidine substitution at position 48 in the loop of the second complementarity-determining region was generated by site-directed mutagenesis. Transfection of this molecule as a CD3 zeta chimera together with the original AV2S1A2 CD3 zeta molecule resulted in cell surface expression of both chains but a loss of recognition of HLA-A2 HIV pol peptide-pulsed targets. The effect of this polymorphism on MHC-peptide recognition supports current models of TCR MHC-peptide interaction and provides evidence for a functional role for polymorphism in the TCRV genes.

The structure of the T cell antigen receptor

Recent crystallographic studies of T cell antigen receptor (TCR) fragments from the alpha and beta chains have now confirmed the expected structural similarity to corresponding immunoglobulin domains. Although the three-dimensional structure of a complete TCR alpha beta heterodimer has not yet been determined, these results support the view that the extracellular region should resemble an immunoglobulin Fab fragment with the antigen-binding site formed from peptide loops homologous to immunoglobulin complementarity-determining regions (CDR). These preliminary results suggest that CDR1 and CDR2 may be less variable in structure than their immunoglobulin counterparts, consistent with the idea that they may interact preferentially with the less polymorphic regions of the molecules of the major histocompatibility complex. The region on the variable beta domain responsible for superantigen recognition is analyzed in detail. The implications for T cell activation from the interactions observed between domains of the alpha and beta chains are also discussed in terms of possible dimerization and allosteric mechanisms.

Characterization of diverse primary herpes simplex virus type 1 gB-specific cytotoxic T-cell response showing a preferential V beta bias

The glycoprotein B (gB) from herpes simplex virus type I is a major target of cytotoxic T lymphocytes (CTL) in C57BL/6 mice. The majority of these T cells are directed to a single Kb-restricted determinant, gB498-505. We have analyzed the T-cell receptor (TCR) usage in gB-specific CTL lines derived shortly after virus infection. The CTL populations preferentially used two V beta regions, a dominant V beta 10 element and a subdominant V beta 8 element. Detailed sequence analysis revealed considerable TCR beta-chain heterogeneity despite a striking level of predicted amino acid conservation at the V beta-D beta junction. This junction forms part of the third hypervariable loop of the TCR thought to directly contact the major histocompatibility complex-bound antigenic peptide. The results reveal considerable diversity within the primary T cells responding to a single viral determinant while still maintaining a high degree of TCR V beta bias and sequence conservation at the V-D-J junction.

Molecular modeling of a T-cell receptor bound to a major histocompatibility complex molecule: implications for T-cell recognition

The main functions of the T-cell receptor (TCR) involve its specific interaction with short and linear antigenic peptides bound to the major histocompatibility complex (MHC) molecules. In the absence of a 3D structure for TCR and for the TCR/peptide/MHC complex, several attempts to characterize the structural components of the TCR/peptide/MHC interaction have been made. However, this subject is still troublesome. In this paper a computer-based 3D model for a TCR/peptide/MHC complex (5C.C7/moth cytochrome c [MCC] peptide 93-103/I-Ek) was obtained. The complex surface shows a high complementarity between the 5C.C7 structure and the peptide/I-Ek molecule. The mapping of residues involved in the TCR/peptide/MHC interaction shows close agreement with mutational experiments (Jorgensen JL, Reay PA, Ehrich EW, Davis MM, 1992b, Annu Rev Immunol 10:835-873). Moreover, the results are consistent with a recent variability analysis of TCR sequences using three variability indexes (Almagro JC, Zenteno-Cuevas R, Vargas-Madrazo E, Lara-Ochoa F, 1995b, Int J Pept Protein Res 45:180-186). Accordingly, the 3D model of the 5C.C7/MCC peptide 93-103/I-Ek complex provides a framework to generate testable hypotheses about TCR recognition. Thus, starting from this model, the role played by each loop that forms the peptide/MHC binding site of the TCR is discussed.

Crystal structure of the beta chain of a T cell antigen receptor

The crystal structure of the extracellular portion of the beta chain of a murine T cell antigen receptor (TCR), determined at a resolution of 1.7 angstroms, shows structural homology to immunoglobulins. The structure of the first and second hypervariable loops suggested that, in general, they adopt more restricted sets of conformations in TCR beta chains than those found in immunoglobulins; the third hypervariable loop had certain structural characteristics in common with those of immunoglobulin heavy chain variable domains. The variable and constant domains were in close contact, presumably restricting the flexibility of the beta chain. This may facilitate signal transduction from the TCR to the associated CD3 molecules in the TCR-CD3 complex.

Variability analysis of the T-cell receptors using three variability indexes

In the absence of a three-dimensional structure for TCR molecules, several attempts to identify their hypervariable regions by variability methods have been made; this subjects is still troublesome. In this paper three different variability indexes were used: (i) the Kabat index, which is the classical measure of sequence variability, (ii) the modified Kabat index, successfully used in the beta-chain of T-cell receptors and (iii) an information-theoretical entropy concept, recently proposed as an improved measure of the variability. In order to identify the hypervariable regions in the TCR sequences, a Fourier filtering was applied on each variability profile. Results show that the three variability indexes have distinct resolutions for different levels of variability. Thus, the simultaneous use of these indexes compensates for the deficiency of any one of them in estimating variability. Applying the Fourier filtering, it is found that the hypervariable regions here identified, roughly coincide with the defined CDR-2 and CDR-3 in TCR by analogy with Ig. However, no hypervariable in the CDR-1 of alpha- and beta-chains was found. The study on the influence of sample size in variability analysis, indicates that results are independent of the sample size. Considering current structural models of TCR-peptide-MHC interaction, one can suggest that the low-variability characteristics of these regions is inherently related to the interaction with relatively conserved region on the alpha-helices of MHC.

Topology of the CD2-CD48 cell-adhesion molecule complex: implications for antigen recognition by T cells

BACKGROUND

The T-lymphocyte cell-surface molecule, CD2, was the first heterophilic cell-adhesion molecule to be discovered and has become an important paradigm for understanding the structural basis of cell adhesion. Interaction of CD2 with its ligands. CD58 (in humans) and CD48 (in mice and rats), contributes to antigen recognition by T cells. CD2, CD48 and CD58 are closely related members of the immunoglobulin superfamily and their extracellular regions are predicted to have very similar structures. The three-dimensional crystal structure of this region of CD2 has been determined, revealing two immunoglobulin domains with the ligand-binding site situated on an exposed beta sheet in the membrane-distal domain. This GFCC'C" beta sheet is also involved in a homophilic 'head-to-head' interaction in the CD2 crystal lattice, which has been proposed to be a model for the interactions of CD2 with its ligands.

RESULTS

We show that the CD2-binding site on rat CD48 lies on the equivalent beta-sheet of its membrane-distal immunoglobulin domain. By making complementary mutations, we have shown that two charged residues in the CD48 ligand-binding site interact directly with two oppositely charged residues in CD2's ligand-binding site. These results indicate that the amino-terminal immunoglobulin domains of CD2 and CD48 bind each other in the same orientation as the CD2-CD2 crystal lattice interaction, strongly supporting the suggestion that CD2 interacts head-to-head with its ligand. Modelling CD48 onto the CD2 structure reveals that the CD2-CD48 complex spans approximately the same distance (134 A) as predicted for the complex between the T-cell receptor and the peptide-bound major histocompatibility complex (MHC) molecule.

CONCLUSIONS

Our results, together with recent structural studies of CD2, provide the first indication of the specific topology of a cell-adhesion molecule complex. The similar dimensions predicted for the CD2-CD48 complex and the complex between the T-cell receptor and the peptide-bound MHC molecule suggest that one of the functions of CD2 may be to position the plasma membranes of the T cell and the antigen-presenting (or target) cell at the optimal distance for the low-affinity interaction between the T-cell receptor and the peptide-bound MHC molecule.

Molecular analysis of the helper T cell response in murine interstitial nephritis. T cells recognizing an immunodominant epitope use multiple T cell receptor V beta genes with similarities across CDR3

Anti-tubular basement membrane disease (alpha TBM disease) produces T cell-mediated interstitial nephritis in SJL mice after immunization with renal tubular antigen. Initial mononuclear infiltrates appear in vivo after several weeks, with the subsequent progression to renal fibrosis and end stage renal disease over many months. We have analyzed the fine specificity of the autoreactive helper T cell repertoire in alpha TBM disease through the isolation and characterization of a panel of CD4+ Th1 clones harvested after 1-2 wk from animals immunized to produce disease. All clones capable of mediating alpha TBM disease are directed towards a 14-residue immunodominant epitope (STMSAEVPEAASEA) contained within the target antigen, 3M-1. Evaluation of the T cell receptor (TCR) V beta repertoire used by these autoreactive T cells reveals the use of several V beta genes, but with some preference for V beta 14. Sequencing across the putative CDR3 region of the TCR beta chains suggests that common amino acids at the V beta(N)D beta junction and the D beta(N)J beta junction may contribute to the specific ability of these cells to recognize the immunodominant epitope.

Binding of soluble natural ligands to a soluble human T-cell receptor fragment produced in Escherichia coli

An Escherichia coli expression system has been developed to produce milligram quantities of the variable domains of a human T-cell receptor from a cytotoxic T cell that recognizes the HLA-A2-influenza matrix peptide complex as a single polypeptide chain. The recombinant protein was purified by metal-chelate chromatography and then refolded in a redox buffer system. The refolded protein was shown to directly bind both Staphylococcus aureus enterotoxin B and the major histocompatibility complex protein-peptide complex using a BIAcore biosensor. Thus this preparation of a single-chain, variable-domain, T-cell receptor fragment can bind both of its natural ligands and some of it is therefore a functional fragment of the receptor molecule.

Design of interchain disulfide bonds in the framework region of the Fv fragment of the monoclonal antibody B3

The Fv fragments are the smallest units of antibodies that retain the specific antigen binding characteristics of the whole molecule and are being used for the diagnosis and therapy of human diseases. These are noncovalently associated heterodimers of the heavy (VH) and the light (VL) chain variable domains, which, without modification, tend to dissociate, unfold, and/or nonspecifically aggregate. The fragment is usually stabilized by producing it as a single chain recombinant molecule in which the two chains are linked by means of a short polypeptide linker. An alternative strategy is to connect the two chains by means of an interchain disulfide bond. We used molecular graphics and other modeling tools to identify two possible interchain disulfide bond sites in the framework region of the Fv fragment of the monoclonal mouse antibody (mAb) B3. The mAb B3 binds to many human cancer cells and is being used in the development of a new anticancer agent. The two sites identified are VH44-VL105 and VH111-VL48. (VH44-VL100 and VH105-VL43 in the numbering scheme of Kabat et al., "Sequence of Proteins of Immunological Interest," U.S. DHHS, NIH publication No. 91-3242, 1991). This design was recently tested using the chimeric protein composed of a truncated form of Pseudomonas exotoxin and the Fv fragment of mAb B3 with the engineered disulfide bond at VH44-VL105 (Brinkmann et al., Proc. Natl. Acad. Sci. U.S.A. 90:7538, 1993). The chimeric toxin was found to be just as active as the corresponding single chain counterpart and considerably more stable. Because these disulfide bond sites are in the framework region, they can be located from sequence alignment alone. We expect that the disulfide bond at these sites will stabilize the Fv fragment of most antibodies and the antigen-specific portion of the T-cell receptors, which are homologous.

Cell surface recognition and the immunoglobulin superfamily

Immunoglobulins serve as humoral recognition and effector molecules and as antigen-specific cell surface receptors on B and T cells. These molecules are constructed according to a characteristic domain pattern. Variable and constant domains diverged from one another early in vertebrate evolution, and they are joined by a "switch peptide" specified by the joining gene segments. Peptides specified by J-gene segments are strongly conserved in evolution in comparison among Ig light chains and T-cell receptors. Molecules less strongly related to Ig domains have been assembled into an Ig "superfamily" where the identities to classical IgC or V domains are < or = 20%. Among these are cell surface adhesion molecules, receptors for cytokines, and Fc receptors. Moreover, MHC antigens have an Ig-like membrane-proximal domain significantly related to IgC regions. We will analyze putative evolutionary relationships among canonical Igs and members of the Ig superfamily using highly conserved sequences from light and heavy chains of primitive vertebrates (e.g., the sandbar shark) as prototypes to ascertain similarities between Ig-related molecules of vertebrates and invertebrates.

The V beta complementarity determining region 1 of a major histocompatibility complex (MHC) class I-restricted T cell receptor is involved in the recognition of peptide/MHC I and superantigen/MHC II complex

We investigated the role of the complementarity determining region 1 (CDR1) of T cell receptor (TCR) beta chain both in antigen/major histocompatibility complex I (MHC I) and in superantigen (SAg)/MHC II complex recognition. Residues 26 to 31 of the V beta 10 domain of a TCR derived from an H-2Kd-restricted cytotoxic clone were individually changed to alanine, using site-directed mutagenesis, and the mutated TCR beta chains were transfected along with the wild-type TCR alpha chain into a TCR alpha-beta-T hydridoma. These mutations affected antigen/H-2Kd complex recognition, although to a different extent, as estimated by interleukin 2 production. Certain mutations also affected differently the recognition of two Staphylococcal toxins, exfoliative toxin and Staphylococcal enterotoxin C2, presented by HLA-DR1. Whereas mutation of residues D30 or T31 affect the recognition of both toxins, residues T26, L27, and H29 are critical for the recognition of only one of the SAgs. These observations demonstrate the participation of the CDR1 region in the recognition of peptide/MHC class I as well as SAg/MHC II complexes.

CDR3 length in antigen-specific immune receptors

In both immunoglobulins (Ig) and T cell receptors (TCR), the rearrangement of V, D, and J region sequence elements during lymphocyte maturation creates an enormous degree of diversity in an area referred to as the complementarity determining region 3 (CDR3) loop. Variations in the particular V, D, and J elements used, precise points of recombination, and random nucleotide addition all lead to extensive length and sequence heterogeneity. CDR3 loops are often critical for antigen binding in Igs and appear to provide the principal peptide binding residues in TCRs. To better understand the physical and selective constraints on these sequences, we have compiled information on CDR3 size variation for Ig H, L (kappa and lambda) and TCR alpha, beta, gamma, and delta. Ig H and TCR delta CDR3s are the most variable in size and are significantly longer than L and gamma chains, respectively. In contrast, TCR alpha and beta chain distributions are highly constrained, with nearly identical average CDR3 lengths, and their length distributions are not altered by thymic selection. Perhaps most significantly, these CDR3 length profiles suggest that gamma/delta TCRs are more similar to Igs than to alpha/beta TCRs in their putative ligand binding region, and thus gamma/delta and alpha/beta T cells may have fundamentally different recognition properties.

Identification of conserved T cell receptor CDR3 residues contacting known exposed peptide side chains from a major histocompatibility complex class I-bound determinant

We have analyzed the T cell receptor (TCR) repertoire found in the major histocompatibility complex class I-restricted cytotoxic T lymphocyte (CTL) response to the protein ovalbumin (OVA). Despite skewing towards the expression of V beta 5.2+TCR by OVA-specific CTL from C57BL/6 mice, we found a relatively high degree of diversity in V(D)J usage in both TCR alpha- and beta-chains. Closer examination showed that the majority of these sequences encoded negatively and positively charged residues at their respective TCR alpha- and beta-chain VJ or VDJ junctions. These junctions form the third complementarity-determining regions (CDR3) of the TCR polypeptides involved in the direct interaction with the class I-bound peptide. Crystallographic analyses of Kb-peptide complexes predict that the major determinant from OVA, peptide OVA257-264 (SIINFEKL), contains two exposed charged side chains which can contact the TCR. These are the negatively charged glutamic acid at determinant position 6 (P6) and the positively charged lysine at P7. To examine whether the TCR alpha-chain makes contact with P7 lysine, we established a single chain TCR transgenic C57BL/6 mouse line where all T cells express a TCR beta-chain derived from the V beta 5.2+ clone B3. OVA-specific T cells derived from in vivo primed transgenic mice preferentially expressed TCR alpha-chains that also contained negatively charged junctional residues despite some further variation in V alpha and J alpha sequences. Stimulation of naive TCR beta-chain transgenic T cells with a P7 substitution peptide analogue induced a T cell response that was no longer cross-reactive with the wild-type OVA257-264 determinant, suggesting that the TCR alpha-chain from the T cell clone B3 can determine the specificity for this residue. Consequently, these results reveal the existence of conserved residues in the CDR3 of TCR alpha- and beta-chains specific for OVA257-264 and identify their possible orientation over the peptide-class I complex.

Conserved structure of amphibian T-cell antigen receptor beta chain

All jawed vertebrates possess well-differentiated thymuses and elicit T-cell-like cell-mediated responses; however, no surface T-cell receptor (TCR) molecules or TCR genes have been identified in ectothermic vertebrate species. Here we describe cDNA clones from an amphibian species, Ambystoma mexicanum (the Mexican axolotl), that have sequences highly homologous to the avian and mammalian TCR beta chains. The cloned amphibian beta chain variable region (V beta) shares most of the structural characteristics with the more evolved vertebrate V beta and presents approximately 56% amino acid identities with the murine V beta 14 and human V beta 18 families. The two different cloned axolotl beta chain joining regions (J beta) were found to have conserved all the invariant mammalian J beta residues, and in addition, the presence of a conserved glycine at the V beta-J beta junction suggests the existence of diversity elements. The extracellular domains of the two axolotl beta chain constant region isotypes C beta 1 and C beta 2 show an impressively high degree of identity, thus suggesting that a very efficient mechanism of gene correction has been in operation to preserve this structure at least from the early tetrapod evolution. The transmembrane axolotl C beta domains have been less well conserved when compared to the mammalian C beta but they do maintain the lysine residue that is thought to be involved in the charged interaction between the TCR alpha beta heterodimer and the CD3 complex.

The sizes of the CDR3 hypervariable regions of the murine T-cell receptor beta chains vary as a function of the recombined germ-line segments

A method using PCR amplification and primer extension with fluorescent oligonucleotides was developed to analyze T-cell repertoires. The sizes of the hypervariable CDR3-like regions of the murine T-cell antigen receptor beta chains were measured for all possible V beta-J beta combinations. This analysis shows that beta chains are distributed into at least 2000 groups, a value that provides a lower limit to their complexity. The CDR3 sizes appear to be dependent on the J beta and especially the V beta segment used and correlates with amino acid sequence motifs in the corresponding CDR1 region. This feature of T-cell receptors is discussed.

A bacterially expressed single-chain Fv construct from the 2B4 T-cell receptor

A single-chain Fv construct of the 2B4 T-cell receptor has been made and expressed in Escherichia coli as bacterial inclusion bodies. After solubilization in 6 M guanidine hydrochloride and formation of mixed disulfides with glutathione, the protein was refolded by diluting out the denaturant and allowing intramolecular disulfide bridges to form by disulfide exchange. Approximately 65-100 mg of refolded protein was obtained from 1 liter of bacterial culture, an appreciable fraction of which was monomeric in nondenaturing solvents. This protein bound to three monoclonal antibodies specific for allotypic or idiotypic determinants on the native 2B4 variable region but did not bind several other anti-T-cell-receptor monoclonal antibodies that lacked such specificity. These experiments show that T-cell-receptor variable regions, like the V regions of antibodies, can form a well-behaved single-chain Fv molecule and provide large amounts of recombinant single-chain Fv T-cell receptor that can be used to study T-cell function.

Changes at peptide residues buried in the major histocompatibility complex (MHC) class I binding cleft influence T cell recognition: a possible role for indirect conformational alterations in the MHC class I or bound peptide in determining T cell recognition

Recent crystallographic studies on two peptide complexes with the mouse Kb molecule have shown that peptide binding appears to alter the conformation of the class I alpha-helical regions that flank the antigen binding cleft. Given that this study also showed that much of the foreign peptide is buried within the class I binding cleft with only a small portion accessible for direct interaction with the components of the T cell receptor, this finding suggests that at least some component of T cell specificity may arise as a consequence of peptide-induced conformational changes in the class I structure. To assess this possibility, we have made systematic substitutions at residues within the Kb-restricted determinant from ovalbumin (OVA257-264) that are thought to be buried on binding to the class I molecule. We have found that changes in this determinant at the completely buried second residue (P2) can influence T cell recognition without affecting binding to Kb, suggesting that the substitutions may indirectly determine T cell recognition by altering the conformation of the class I molecule or the bound peptide.

The T cell receptor alpha beta V-J shuffling shows lack of autonomy between the combining site and the constant domain of the receptor chains

In order to assess the structural independence of the T cell receptor (TCR) combining site from the rest of the molecule we have generated two recombinant chains consisting of a TCR V-J alpha region linked to the C beta and a TCR V-J beta linked to the C alpha. If the V and C domains of the TCR form independent domains, as has been shown for the Ig molecules, we would expect to obtain a functional chimeric TCR. Interestingly, it was found that the shuffled molecules are produced intracellularly in T cell hybridomas, but are not expressed on the cell surface. To explain this failure of the shuffled molecules we propose that the TCR has a more compact structure, compared to the Ig, and that it is indispensable to keep a longitudinal inter-domain contact between the V-J and C portion to have a functional molecule.

Similarity between fluorescein-specific T-cell receptor and antibody in chemical details of antigen recognition

A computer-generated model of the single-chain variable V alpha V beta fragment of the RFL3.8 T-cell receptor (TCR) specific for fluorescein served as a starting point for mutagenesis aimed at identification of its antigen-contacting residues. Selected backbone segments of the model representing regions of prominent sequence similarity between antibodies and TCRs were least-squares superimposed onto the corresponding segments of the crystallographically resolved 4-4-20 antibody complexed with its antigen, fluorescein. The superimposition placed the antibody-bound fluorescein molecule close to a cavity on the surface of the TCR model formed by the complementarity-determining region (CDR) loops. Some of the TCR cavity forming loops displayed sequence motifs related to canonical CDR loops previously found in antibodies. Six putative amino acid contacts were identified and single-chain TCRs with mutations at each of these positions were expressed in Escherichia coli, purified, refolded, and assayed for fluorescein binding. Five of the six mutations resulted in a loss of detectable binding. These RFL3.8 antigen combining site residues are distributed among the beta 3, alpha 1, and alpha 2 CDR loops and show striking chemical similarity to the known fluorescein contact residues on 4-4-20. Thus, antibodies and TCRs are similar both in their overall architecture and in the chemical details of specific antigen recognition.

Functional analysis of the antigen binding site on the T cell receptor alpha chain

We have identified residues on a T cell receptor (TCR) alpha chain that are important for interaction with antigen/major histocompatibility complex (MHC). Using site-directed mutagenesis, we modified DNA encoding the postulated antigen/MHC binding loops on the TCR alpha chain expressed by the T cell clone D5, which recognizes p-azobenzenearsonate-conjugated antigens presented by cells bearing I-Ad. These variant TCR alpha chains were expressed in conjunction with the wild-type D5 TCR beta chain on the surface of hybridoma cells, and were tested for the ability to recognize hapten-conjugated antigens presented by I-Ad. Individual amino acid substitutions in each of the three antigen binding loops (alpha 1, alpha 2, alpha 3) of the D5 TCR alpha chain affected antigen recognition, demonstrating that all three loops are important in recognition of antigen/MHC. A subset of the single amino acid substitutions completely eliminated antigen recognition, thus identifying the residues that are particularly important in the recognition of antigenic peptide/MHC by the D5 TCR. Because the wild-type D5 TCR recognizes arsonate and certain structural analogues of arsonate conjugated to a variety of protein antigens, we were able to test whether the TCR substitutions affected the specificity of the D5 TCR for hapten or carrier antigen. One substitution introduced into antigen binding loop alpha 3 markedly altered the pattern of carrier recognition. Together, these results verify the Ig model for the TCR and are consistent with the proposition that residues forming the first and second antigen binding loops of the TCR contact the MHC, while those forming the third loop contact mainly antigenic peptides.

Characterization of a single-chain T-cell receptor expressed in Escherichia coli

Despite progress in defining the nature of major histocompatibility complex products that are recognized by the T-cell antigen receptor, the binding properties and structure of the receptor have not been solved. The primary problem has been the difficulty in obtaining sufficient quantities of active receptor. In this report we show that a single-chain T-cell receptor gene can be expressed in Escherichia coli. The protein consists of the variable (V) regions of the alpha and beta chains (V alpha and V beta) encoded by the cytotoxic T-lymphocyte clone 2C (a H-2b anti-H-2d alloreactive cell line) linked by a 25-amino acid flexible peptide. Solubilized extracts that contain the 27-kDa V alpha 3V beta 8 protein are positive in solid-phase immunoassays with the anti-V beta 8 antibody KJ16 and the anti-clonotypic antibody 1B2. Approximately 1% of the protein can be specifically purified on a 1B2-conjugated column. These results indicate that a fraction of the protein is able to fold into a native conformation and that single-chain proteins should be useful not only as immunogens for eliciting anti-T-cell receptor antibodies but in the study of T-cell receptor structure and function.

Preferential V beta gene usage and lack of junctional sequence conservation among human T cell receptors specific for a tetanus toxin-derived peptide: evidence for a dominant role of a germline-encoded V region in antigen/major histocompatibility complex recognition

To investigate the structural and genetic basis of the T cell response to defined peptide/major histocompatibility (MHC) class II complexes in humans, we established a large panel of T cell clones (61) from donors of different HLA-DR haplotypes and reactive with a tetanus toxin-derived peptide (tt830-844) recognized in association with most DR molecules (universal peptide). By using a bacterial enterotoxin-based proliferation assay and cDNA sequencing, we found preferential use of a particular V beta region gene segment, V beta 2.1, in three of the individuals studied (64%, n = 58), irrespective of whether the peptide was presented by the DR6wcI, DR4w4, or DRw11.1 and DRw11.2 alleles, demonstrating that shared MHC class II antigens are not required for shared V beta gene use by T cell receptors (TCRs) specific for this peptide. V alpha gene use was more heterogeneous, with at least seven different V alpha segments derived from five distinct families encoding alpha chains able to pair with V beta 2.1 chains to form a tt830-844/DR-specific binding site. Several cases were found of clones restricted to different DR alleles that expressed identical V beta and (or very closely related) V alpha gene segments and that differed only in their junctional sequences. Thus, changes in the putative complementary determining region 3 (CDR3) of the TCR may, in certain cases, alter MHC specificity and maintain peptide reactivity. Finally, in contrast to what has been observed in other defined peptide/MHC systems, a striking heterogeneity was found in the junctional regions of both alpha and beta chains, even for TCRs with identical V alpha and/or V beta gene segments and the same restriction. Among 14 anti-tt830-844 clones using the V beta 2.1 gene segment, 14 unique V beta-D-J beta junctions were found, with no evident conservation in length and/or amino acid composition. One interpretation for this apparent lack of coselection of specific junctional sequences in the context of a common V element, V beta 2.1, is that this V region plays a dominant role in the recognition of the tt830-844/DR complex.

Mapping T-cell receptor-peptide contacts by variant peptide immunization of single-chain transgenics

To test models of T-cell recognition, mice transgenic for T-cell receptor alpha or beta chain have been immunized with variant peptides that force changes in the resulting T-cell response. In particular, charge substitutions on the peptide often elicit reciprocal charges in the junctional (CDR3) sequences of T-cell receptor V alpha or V beta chains, indicating direct T-cell receptor-peptide contact, and allowing derivation of a topology for the T-cell receptor-MHC interaction. At one position on the peptide, variants transformed a homogeneous V beta response into a very heterogeneous one.