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

Molecular analysis of TCR and peptide/MHC interaction using P18-I10-derived peptides with a single D-amino acid substitution

For the structural analysis of T-cell receptor (TCR) and peptide/MHC interaction, a series of peptides with a single amino acid substitution by a corresponding D-amino acid, having the same weight, size, and charge, within P18-I10 (aa318-327: RGPGRAFVTI), an immunodominant epitope of HIV-1 IIIB envelope glycoprotein, restricted by the H-2Dd class I MHC molecule, has been synthesized. Using those peptides, we have observed that the replacement at positions 324F, 325V, 326T, and 327I with each corresponding D-amino acid induced marked reduction of the potency to sensitize targets for P18-I10-specific murine CD8+ cytotoxic T lymphocytes (CTLs), LINE-IIIB, recognition. To analyze further the role of amino acid at position 325, the most critical site for determining epitope specificity, we have developed a CTL line [LINE-IIIB(325D)] and its offspring clones specific for the epitope I-10(325v) having a D-valine (v) at position 325. Taking advantage of two distinct sets of CD8+ CTLs restricted by the same Dd, three-dimensional structural analysis on TCR and peptide/MHC complexes by molecular modeling was performed, which indicates that the critical amino acids within the TCRs for interacting with 325V or 325v appear to belong to the complementarity-determining region 1 but not to the complementarity-determining region 3 of Vbeta chain.

Complementarity-determining region 1 sequence requirements drive limited V alpha usage in response to influenza hemagglutinin 307-319 peptide

We have developed a T cell activation-based system that allows for the selection of TCRs with defined peptide/MHC specificities from libraries in which complementarity-determining region (CDR) sequences have been randomized by in vitro mutagenesis. Using this system, we have explored the sequence requirements for CDR1 and CDR2 of the TCR alpha-chain in a human T cell response characterized by restricted Valpha and Vbeta usage. Libraries of T cells expressing receptors built on the framework of a TCR specific for the influenza virus peptide hemagglutinin 307-319 presented by HLA-DR4, but with random sequences inserted at CDR1alpha or CDR2alpha, were selected for response to the same peptide/MHC ligand. A wide variety of CDR2alpha sequences were found to be permissive for recognition. Indeed, >25% of T cell clones chosen at random displayed a significant response. In contrast, a similar challenge of a randomized CDR1alpha library yielded only the parental sequence, and then only after multiple rounds of selection. T cell clones cross-reactive on closely related HLA alleles (subtypes of DR4) could be isolated from randomized libraries, but not clones restricted by more distantly related alleles such as HLA-DR1. These results indicate that, in the context of this T cell response, the structural requirements for recognition at CDR1alpha are significantly more restricted than at CDR2alpha. This system for mutation and selection of TCRs in vitro may be of use in engineering T cells with defined specificities for therapeutic applications.

Identification of key amino acids of the mouse mammary tumor virus superantigen involved in the specific interaction with T-cell receptor V(beta) domains

Mouse mammary tumor virus (MMTV) is a retrovirus encoding a superantigen that is recognized in association with major histocompatibility complex class II by the variable region of the beta chain (V(beta)) of the T-cell receptor. The C-terminal 30 to 40 amino acids of the superantigen of different MMTVs display high sequence variability that correlates with the recognition of particular T-cell receptor V(beta) chains. Interestingly, MMTV(SIM) and mtv-8 superantigens are highly homologous but have nonoverlapping T-cell receptor V(beta) specificities. To determine the importance of these few differences for specific V(beta) interaction, we studied superantigen responses in mice to chimeric and mutant MMTV(SIM) and mtv-8 superantigens expressed by recombinant vaccinia viruses. We show that only a few changes (two to six residues) within the C terminus are necessary to modify superantigen recognition by specific V(beta)s. Thus, the introduction of the MMTV(SIM) residues 314-315 into the mtv-8 superantigen greatly decreased its V(beta)12 reactivity without gain of MMTV(SIM)-specific function. The introduction of MMTV(SIM)-specific residues 289 to 295, however, induced a recognition pattern that was a mixture of MMTV(SIM)- and mtv-8-specific V(beta) reactivities: both weak MMTV(SIM)-specific V(beta)4 and full mtv-8-specific V(beta)11 recognition were observed while V(beta)12 interaction was lost. The combination of the two MMTV(SIM)-specific regions in the mtv-8 superantigen established normal MMTV(SIM)-specific V(beta)4 reactivity and completely abolished mtv-8-specific V(beta)5, -11, and -12 interactions. These new functional superantigens with mixed V(beta) recognition patterns allowed us to precisely delineate sites relevant for molecular interactions between the SIM or mtv-8 superantigen and the T-cell receptor V(beta) domain within the 30 C-terminal residues of the viral superantigen.

Pairing of Vbeta6 with certain Valpha2 family members prevents T cell deletion by Mtv-7 superantigen

Superantigens (SAg) are proteins of bacterial or viral origin able to activate T cells by forming a trimolecular complex with both MHC class II molecules and the T cell receptor (TCR), leading to clonal deletion of reactive T cells in the thymus. SAg interact with the TCR through the beta chain variable region (Vbeta), but the TCR alpha chain has been shown to have an influence on the T cell reactivity. We have investigated here the role of the TCR alpha chain in the modulation of T cell reactivity to Mtv-7 SAg by comparing the peripheral usage of Valpha2 in Vbeta6(+) (SAg-reactive) and Vbeta8.2(+) (SAg non-reactive) T cells, in either BALB/D2 (Mtv-7(+)) or BALB/c (Mtv-7(-)) mice. The results show, first, that pairing of Vbeta6 with certain Valpha2 family members prevents T cell deletion by Mtv-7 SAg. Second, there is a strikingly different distribution of the Valpha2 family members in CD4 and CD8 populations of Vbeta6 but not of Vbeta8.2 T cells, irrespective of the presence of Mtv-7 SAg. Third, the alpha chain may play a role in the overall stability of the TCR/SAg/MHC complex. Taken together, these results suggest that the Valpha domain contributes to the selective process by its role in the TCR reactivity to SAg/MHC class II complexes, most likely by influencing the orientation of the Vbeta domain in the TCR alphabeta heterodimer.

Superantigens: mechanisms by which they may induce, exacerbate and control autoimmune diseases

Superantigens are polypeptide molecules produced by a broad range of infectious microorganisms which elicit excessive and toxic T-cell responses in mammalian hosts. In light of this property and the fact that autoimmune diseases are frequently the sequelae of microbial infections, it has been suggested that superantigens may be etiologic agents of autoreactive immunological responses resulting in initiation, exacerbation or relapse of autoimmune diseases. This article relates the biology of superantigens to possible mechanisms by which they may exert these activities and reviews the evidence for their roles in various human and animal models of autoimmune disease. Finally, a mechanism of active suppression by superantigen-activated CD4+ T-cells that could be exploited for therapy as well as prophylaxis of human autoimmune diseases is proposed.

Systematic Mutagenesis of TCR Complementarity-Determining Region 3 Residues: A Single Conservative Substitution Dramatically Improves Response to Both Multiple HLA-DR Alleles and Peptide Variants

To define the relative contributions of HLA and peptide contacts with TCR complementarity-determining region (CDR) 3 residues in T cell recognition, systematic mutagenesis and domain swapping was conducted on two highly similar TCRs that both respond to the influenza hemagglutinin (HA) peptide, HA307–319, but with different HLA restrictions. Despite the primary sequence similarity of these TCRs, exchange of as little as two CDR3 residues between them completely abrogated responsiveness. At position 95 within CDR3α, various substitutions still allowed for some degree of recognition. One modest substitution, alanine for glycine (essentially the addition of a methyl group), significantly broadened the specificity of the TCR. Transfectants expressing this mutant TCR responded strongly in the context of multiple HLA-DR alleles and to HA peptide variants with substitutions at each TCR contact residue. These results suggest that the conformations of CDR3 loops are crucial to TCR specificity and that it may not be reliable to extrapolate from primary sequence similarities in TCRs to similarities in specificity. The ease with which a broad specificity is induced in this mutant TCR has implications for the mechanisms and frequency of alloreactivity and promiscuity in T cell responses.

TCR Reactivity in Human Nickel Allergy Indicates Contacts with Complementarity-Determining Region 3 but Excludes Superantigen-Like Recognition

Nickel is the most common inducer of contact sensitivity in humans. We previously found that overrepresentation of the TCRBV17 element in Ni-induced CD4+ T cell lines of Ni-allergic patients relates to the severity of the disease. Amino acid sequences of these β-chains suggested hypothetical contact points for Ni2+ ions in complementarity-determining region (CDR) 1 and CDR3. To specifically address the molecular requirements for Ni recognition by TCR, human TCR α- and β-chains of VB17+ Ni-reactive T cell clones were functionally expressed together with the human CD4 coreceptor in a mouse T cell hybridoma. Loss of CD4 revealed complete CD4 independence for one of the TCR studied. Putative TCR/Ni contact points were tested by pairing of TCR chains from different clones, also with different specificity. TCRBV17 chains with different J regions, but similar CDR3 regions, could be functionally exchanged. Larger differences in the CDR3 region were not tolerated. Specific combinations of α- and β-chains were required, excluding a superantigen-like activation by Ni. Mutation of amino acids in CDR1 of TCRBV17 did not affect Ag recognition, superantigen activation, or HLA restriction. In contrast, mutation of Arg95 or Asp96, conserved in many CDR3B sequences of Ni-specific, VB17+ TCR, abrogated Ni recognition. These results define specific amino acids in the CDR3B region of a VB17+ TCR to be crucial for human nickel recognition. CD4 independence implies a high affinity of such receptor types for the Ni/MHC complex. This may point to a dominant role of T cells bearing such receptors in the pathology of contact dermatitis.

Single Amino Acid Replacements in an Antigenic Peptide Are Sufficient to Alter the TCR Vβ Repertoire of the Responding CD8+ Cytotoxic Lymphocyte Population

Cytotoxic CD8+ T lymphocytes are activated upon the engagement of their Ag-specific receptors by MHC class I molecules loaded with peptides 8–11 amino acids long. T cell responses triggered by certain antigenic peptides are restricted to a limited number of TCR Vβ elements. The precise role of the peptide in causing this restricted TCR Vβ expansion in vivo remains unclear. To address this issue, we immunized C57BL/6 mice with the immunodominant peptide of the vesicular stomatitis virus (VSV) and several peptide variants carrying single substitutions at TCR-contact residues. We observed the expansion of a limited set of TCR Vβ elements responding to each peptide variant. To focus our analysis solely on the TCR β-chain, we created a transgenic mouse expressing exclusively the TCR α-chain from a VSV peptide-specific CD8+ T cell clone. These mice showed an even more restricted TCR Vβ usage consequent to peptide immunization. However, in both C57BL/6 and TCRα transgenic mice, single amino acid replacements in TCR-contact residues of the VSV peptide could alter the TCR Vβ usage of the responding CD8+ T lymphocytes. These results provide in vivo evidence for an interaction between the antigenic peptide and the germline-encoded complementarity-determining region-β loops that can influence the selection of the responding TCR repertoire. Furthermore, only replacements at residues near the C terminus of the peptide were able to alter the TCR Vβ usage, which is consistent with the notion that the TCR β-chain interacts in vivo preferentially with this region of the MHC/peptide complex.

Role of Peptide Backbone in T Cell Recognition

T cells recognize self and nonself peptides presented by molecules of the MHC. Amino acid substitutions in the antigenic peptide showed that T cell specificity is highly degenerate. Recently, determination of the crystal structure of several TCR/MHC-peptide complexes suggested that the peptide backbone may significantly contribute to the interaction with the TCR. To directly investigate the role of the peptide backbone in T cell recognition, we performed a methylene-amino scan on the backbone of an antigenic peptide and measured the capacity of such pseudopeptides to bind their cognate MHC molecule, to sensitize target cells for T cell lysis, and to stimulate IL-2 secretion by two T cell hybridomas. For one of these pseudopeptides, we prepared fluorescent tetramers of MHC molecules and compared the staining of two T cell hybridomas. Our results demonstrate that the peptide backbone has an important contribution to TCR binding and suggest that some interactions between the peptide backbone and the TCR may be partially conserved. We discuss this finding in the perspective of TCR plasticity and T cell function.

Dramatic Influence of Vβ Gene Polymorphism on an Antigen-Specific CD8+ T Cell Response In Vivo

According to recent crystallographic studies, the TCR-αβ contacts MHC class I-bound antigenic peptides via the polymorphic V gene-encoded complementarity-determining region 1β (CDR1β) and the hypervariable (D)J-encoded CDR3β and CDR3α domains. To evaluate directly the relative importance of CDR1β polymorphism on the fine specificity of T cell responses in vivo, we have taken advantage of congenic Vβa and Vβb mouse strains that differ by a CDR1 polymorphism in the Vβ10 gene segment. The Vβ10-restricted CD8+ T cell response to a defined immunodominant epitope was dramatically reduced in Vβa compared with Vβb mice, as measured either by the expansion of Vβ10+ cells or by the binding of MHC-peptide tetramers. These data indicate that Vβ polymorphism has an important impact on TCR-ligand binding in vivo, presumably by modifying the affinity of CDR1β-peptide interactions.

Differential CD4/CD8 subset-specific expression of highly homologous rat Tcrb-V8 family members suggests a role of CDR2 and/or CDR4 (HV4) in MHC class-specific thymic selection

Different rat Tcrb haplotypes express either TCR beta variable segment (Tcrb-V) 8.2l or 8.4a. Both V segments bind the mAb R78 but differ by one conservative substitution (L14V) and clusters of two and four substitutions in the complementarity-determining region (CDR) 2 and CDR4 [hypervariable loop 4 (HV4)]. Independently of MHC alleles numbers of R78+ CD4+ cells are lower in Tcrb-V8.2l-expressing than in Tcrb-V8.4a-expressing strains. Expression of R78+ TCR during T cell development, analysis of backcross populations and generation of a Tcrb congenic strain [LEW.TCRB(AS)] define two mechanisms how Tcrb haplotypes affect the frequency of R78+ cells, one acting prior to thymic selection leading to up to 2-fold higher frequency of Tcrb-V8.4a versus Tcrb-V8.2l in unselected thymocytes and another occurring between the TCRlow and the CD4/CD8 single-positive stage. The latter leads to a 50% reduction of frequency of Tcrb-V8.4a CD8+ cells but not CD4+ cells and does not affect either subset of Tcrb-V8.2l cells. A comparison of rat classical class I MHC (RT1.A) sequences and current models of TCR-MHC-peptide interaction suggests that this reduction in frequency of Tcrb-V8.4a CD8 cells may be a consequence of differential selection of Tcrb-V8.2l versus Tcrb-V8.4a TCR by differential binding of CDR2beta to highly conserved areas of C-terminal parts of the alpha helices of class I MHC molecules.

The Qa-1b molecule binds to a large subpopulation of murine NK cells

Recent studies on human NK cells have demonstrated that the NK cell CD94/NKG2 receptors bind to the nonclassical MHC class I molecule HLA-E. A functional CD94/NKG2 complex has not yet been identified in rodents, but cDNA encoding rat and mouse CD94 and NKG2 have recently been cloned, suggesting that CD94/NKG2 receptors may exist in species other than man. The mouse nonclassical MHC class I molecule Qa-1 shares several features with HLA-E. This suggests that Qa-1 may be similarly recognized by murine NK cells. To study the ability of Qa-1 to bind to murine NK cells, we have produced a soluble tetrameric form of Qa-1b. In the present study, we demonstrate that Qa-1b tetramers distinctly bind to a large subset of fresh or IL-2-activated NK1.1+/CD3- splenocytes independently of the expression of Ly49 inhibitory receptors. Binding occurs whether NK cells have evolved in an MHC class I-expressing or in an MHC class I-deficient environment. Our data suggest the existence of a Qa-1-recognizing structure on a large subpopulation of murine NK cells that may be similar to the human CD94/NKG2 heterodimeric complex.

Differential reactivity of TCR Vbeta10 alleles to a mouse mammary tumor virus superantigen

Mouse mammary tumor virus (MMTV) expresses a superantigen (SAg) which plays a critical role in the viral life cycle. We have recently described the new infectious MMTV (SIM) encoding a Vbeta4-specific SAg in mice with a TCR-Vbeta(b) haplotype. We have now compared the SAg activity of this virus in BALB/c mice harboring the TCR-Vbeta(a), TCR-Vbeta(b) or TCR-Vbeta(c) haplotypes which differ by a central deletion in the TCR-Vbeta(a) and TCR-Vbeta(c) locus and by mutations in some of the remaining Vbeta elements. Injection of MMTV (SIM) led to a strong stimulation of Vbeta4+ CD4+ T cells in TCR-Vbeta(b) mice, but only to a weak stimulation of these cells in TCR-Vbeta(a) or TCR-Vbeta(c) mice. A large increase in the percentage of Vbeta10+ cells was observed among CD4+ T cells in mice with the Vbeta(a) or Vbeta(c), but not the Vbeta(b) TCR-Vbeta haplotype. Vbeta10+ cells dominated the response when Vbeta10(a/c) and Vbeta4 subsets were present together. This is the first report of a viral SAg interacting with murine Vbeta10+ cells. Six amino acid differences between Vbeta10(a/c) and Vbeta10(b) could account for the gain of reactivity of Vbeta10(a/c) to the MMTV(SIM) SAg. No mutations were found in the hypervariable region 4 (HV4) of the TCR. Mutations at positions 22 and 28 introduce into Vbeta10(a/c) the same amino acids which are found at these positions in the MMTV(SIM)-reactive Vbeta4. Tridimensional models indicated that these amino acids lie close to HV4 and are likely to be important for the interaction of the SAg with the TCR.

Mycoplasma superantigen is a CDR3-dependent ligand for the T cell antigen receptor

Superantigens are defined as proteins that activate a large number of T cells through interaction with the Vbeta region of the T cell antigen receptor (TCR). Here we demonstrate that the superantigen produced by Mycoplasma arthritidis (MAM), unlike six bacterial superantigens tested, interacts not only with the Vbeta region but also with the CDR3 (third complementarity-determining region) of TCR-beta. Although MAM shares typical features with other superantigens, direct interaction with CDR3-beta is a feature of nominal peptide antigens situated in the antigen groove of major histocompatibility complex (MHC) molecules rather than superantigens. During peptide recognition, Vbeta and Valpha domains of the TCR form contacts with MHC and the complex is stabilized by CDR3-peptide interactions. Similarly, recognition of MAM is Vbeta-dependent and is apparently stabilized by direct contacts with the CDR3-beta region. Thus, MAM represents a new type of ligand for TCR, distinct from both conventional peptide antigens and other known superantigens.

Enrichment of antigen-specific T lymphocytes by panning on immobilized MHC-peptide complexes

Numerous studies have focused on characterizing and monitoring antigen-specific T cells during the course of an immune response. Mostly indirect methods were used to circumvent the low frequency of T cell precursors and the inherent complexity of T cell receptor (TcR)-MHC-peptide interactions. Here, we took advantage of peptide-specific adhesion induced by immobilized MHC-peptide complexes. We describe a simple technique which allows enrichment in antigen-specific T lymphocytes among a heterogeneous CD8+ T cell population. Enrichment of T cells according to their specificity should facilitate their characterization and provide an attractive tool for immunotherapy.

Dominance of the BV17 element in nickel-specific human T cell receptors relates to severity of contact sensitivity

Hypersensitivity to nickel (Ni) represents the most common manifestation of contact allergy in humans. The role of metal-specific T cells in this disease is well established, but the molecular interactions involved in their activation are poorly understood. We examined the T cell receptor (TCR) repertoire in T cells activated with either NiSO4 or NiSO4-treated human serum albumin from six allergic patients. For the three most hyperreactive donors, we found a strong over-representation of the TCR BV17 element. TCR sequencing for one of these donors revealed an additional skewing for AV1 as well as a selection for an N region encoded argine at position 95 of the BV17 complementarity determining region (CDR)3. Since Arg is not known to participate in Ni complexing, we suppose that this selection is driven by contacts with peptide rather than nickel. However, the CDR1 of BV17 contains a unique combination of amino acids (HDA) that bears similarities to known motifs in Ni-binding proteins or peptides. We therefore propose that the severe hypersensitivity reactions found in BV17 over-expressors may be the result of Ni2+ ions bridging the germ-line-encoded BV17 CDR1 loop to corresponding sites in the major histocompatibility complex/peptide complex and thereby creating a superantigen-like enhancement of weak TCR-peptide contacts.

Differential fate of glycoproteins carrying a monoglucosylated form of truncated N-glycan in a new CHO line, MadIA214214, selected for a thermosensitive secretory defect

A temperature sensitive secretory line, MadIA214, was selected from mutagenized Chinese hamster ovary cells that express two heterologous export marker proteins: a secretory form of the human placental alkaline phosphatase (SeAP), and the Kd heavy chain of mouse MHC class I. SeAP secretion in MadIA214 was extremely reduced at elevated temperature (40 degrees C), while the export of functional H-2Kd molecules to the plasma membrane was only slightly affected. This mutant constitutively transferred onto newly synthesized proteins a truncated oligosaccharide core, Man5GlcNAc2, which was monoglucosylated in the protein-bound form. Nevertheless, the final oligosaccharide-structures associated to mature SeAP and H-2Kd were similar in mutant and wild-type glycoproteins. The inaccessibility in MadIA214 endoplasmic reticulum (ER) of one or more components required for oligosaccharide chain elongation is supported by the reconstitution of a correct core structure, obtained after disruption of cellular compartments, but not after cell permeabilisation or blocking ER-to-Golgi transport. The increased association of the ER-chaperone BiP with immature SeAP correlated with the thermodependent decrease in SeAP secretion. The retention of incompletely folded polypeptides in MadIA214 parallels both a marked ER-dilation and an important glycoprotein degradation documented by the formation of soluble oligomannosides with one GlcNAc residue. Our data provide the first in vivo evidence that the initial step in N-glycosylation differentially governs glycoprotein maturation, transport and degradation.

Selective T cell receptor Vbeta gene usage by alloreactive T cells responding to defined HLA-DR alleles

Previous studies have provided strong evidence for restricted Vbeta gene usage in response to DR synthetic peptides presented in context by self MHC molecules, i.e., via the indirect pathway of allorecognition. Although numerous studies have suggested a role for the T cell receptor (TCR) beta chain in the direct pathway of allorecognition, it is not clear whether a particular HLA allele elicits a consistent pattern of restricted Vbeta gene usage by resting T cells from different individuals. To address this problem, HLA class II homozygous cell lines that do not express class I antigens were used to study the role of the TCR Vbeta elements in direct recognition of specific DR alleles. Analysis of TCR V gene usage revealed that each DR allele tested elicited the same pattern of preferential Vbeta gene usage in all individuals studied. For example, the DRB1*0101 allele was preferentially recognized by T cells expressing Vbeta2, Vbeta13.1, Vbeta18, and Vbeta20, and the DRB1*1301 caused expansion of Vbeta4-, Vbeta6-, Vbeta8-, and Vbeta18-bearing T cells. Similarly, analysis of TCR V gene usage in response to defined DR alleles expressed on homozygous cell lines that express class I was also made possible by using anti-class I antibodies to block class I recognition and focus the response on the DR molecule. The results showed that the DRB1*1501 allele expressed on two distinct homozygous lines elicited the preferential expansion of Vbeta2, Vbeta8, and Vbeta13.2 T cells. Similarly, the DRB1*0301 allele expressed on normal, Epstein-Barr virus-transformed, or transfected fibroblasts was found to elicit the same pattern of Vbeta expansion and to selectively stimulate T cells bearing Vbeta2, Vbeta16, and Vbeta17 elements. In contrast to this highly reproducible pattern of Vbeta gene usage elicited by specific DR alleles, extensive heterogeneity in the CDR3 region was found and no preferential Jbeta or Valpha gene usage was observed in response to any of the DR alleles tested. The data suggest that, similar to the situation with the indirect pathway, TCR Vbeta elements are involved in the direct pathway of allorecognition. Although some overlap may exist, different sets of Vbeta elements may be preferentially used for each of these pathways. Inasmuch as HLA-identical siblings, but not HLA-identical unrelated individuals, express a very similar TCR repertoire, our data suggest that additional factors are involved in shaping the repertoire. Preferential activation of T-cell subsets by specific DR alleles may play an important role in primary alloresponses, e.g., in mixed lymphocyte reactions, and organ transplantation. Elucidation of the Vbeta specificity of each DR allele may have an impact on therapeutic strategies aimed at blocking specific alloresponse and prolonging graft survival in transplant recipients while avoiding the hazardous sequelae of nonspecific immunosuppression.

TCR binding differs for a bacterial superantigen (SEE) and a viral superantigen (Mtv-9)

Both superantigens (SAG) and many anti-TCR monoclonal antibodies (mAb) have specificity for the V beta region of the TCR encoded by TCRBV genes. For instance the bacterial SAG staphylococcal enterotoxin E (SEE), the retroviral SAG MTV-9 and the mAb OT145 each react with human T cells expressing BV6S7. This BV gene encodes two common alleles. We found that SEE and the mAb preferentially activate T cells expressing BV6S7*1 as opposed to BV6S7*2, but Mtv-9 activates T cells expressing either allele. Thus binding to the TCR differs between the two SAGs. A mutation in the TCR HVR-4 region of BV6S7*1 (G72E), where the two BV6S7 alleles differ, indicated that HVR-4 is a component of the binding site for SEE and for the mAb OT145. BV6S7*2 has a charged E72 which may result in electrostatic repulsion of SEE, as SEE contains a similarly acidic aspartic acid residue at a TCR interaction site (204D).

Role of the T cell receptor alpha-chain in superantigen recognition

Superantigens bind to antigen-presenting cells on the outside of the major histocompatibility complex (MHC) class II molecule and to T cells via the external face of the T cell receptor (TCR) V beta element. As a consequence, superantigens stimulate populations of T cells in a V beta-specific, non-MHC-restricted manner. However, accumulating evidence has shown an additional contribution of the TCR alpha-chain and polymorphic residues of the MHC molecule to superantigen recognition by some T cells. These data suggest that the TCR and MHC come into contact during superantigen engagement and indirectly modulate the superantigen reactivity. Thus, additional interactions between non-V beta elements of the TCR and MHC play a role in the overall stability of the superantigen/MHC/TCR complex, explaining the influence of the TCR alpha-chain. It is likely that this additional interaction is of greater consequence for weakly reactive T cells. This modulation of superantigen reactivity in individual T cells may have physiological consequences, for example, in the induction of autoimmunity.

Modulation of promiscuous T cell receptor recognition by mutagenesis of CDR2 residues

The T cell receptor (TCR) recognizes a ligand composed of a major histocompatibility complex (MHC) molecule and a peptide antigen. Prior studies of murine T cell clones have demonstrated that residues in the CDR3 region of TCR interact with amino acids in the peptide during MHC-restricted antigen recognition. However, the questions of whether direct TCR MHC contacts are made and where such contact sites might map in the TCR have not been resolved. In this study, we have taken advantage of the promiscuous recognition of a peptide from influenza virus (HA 307-319) by human T cell clones to map sites in the TCR that mediate differences in human leukocyte antigen-D related (HLA-DR) restriction in the presence of a common peptide antigen. Site-specific mutagenesis of cloned TCR genes and transfection into Jurkat cells were used to demonstrate that single amino acid substitutions in CDR2 of the TCR-alpha chain controlled whether a T cell was restricted by the product of a single DR allele (DR7) or would respond to the HA 307-319 peptide when presented by the products of one of several different DR alleles (DR1, DR4, DR5, or DR7). Because the relevant DR alleles are defined by polymorphism in the DR-beta chain, these results also suggest a rotational orientation for recognition in which TCR-alpha interacts with DR beta.

Different superantigens interact with distinct sites in the Vbeta domain of a single T cell receptor

CD4 T cell receptors (TCRs) recognize antigenic peptides presented by self major histocompatibility complex (MHC) class II molecules as well as non-self MHC class II molecules. The TCRs can also recognize endogenous retroviral gene products and bacterial toxins known collectively as superantigens (SAGs) that act mainly on the Vbeta gene segment-encoded portion of the Vbeta domain; most SAGs also require MHC II class for presentation. We have studied the interaction of the TCR from a well-characterized CD4 T cell line with SAGs by mutational analysis of its Vbeta domain. This appears to separate viral (v)SAG from bacterial (b)SAG recognition. T cells having a TCR with glycine to valine mutation in amino acid residue 51 (G51V) in complementarity determining region 2 of the TCR Vbeta domain fail to respond the bSAGs staphylococcal enterotoxin B (SEB), SEC1, SEC2, and SEC3, whereas they retain the ability to respond to non-self MHC class II molecules and to foreign peptides presented by self MHC class II molecules. It is interesting to note that T cells expressing mutations of both G51V and G53D of V beta regain the response to SEB and partially that to SEC1, but do not respond to SEC2, and SEC3, suggesting that different bacterial SAGs are viewed differently by the same TCR. These results are surprising, because it has been generally believed that SAG recognition by T cells is mediated exclusively by hypervariable region 4 on the exposed, lateral face of the TCR Vbeta domain. Response to the vSAG Mtv-7 was generated by mutation in Vbeta residue 24 (N24H), confirming previously published data. These data show that the vSAG Mtv-7 and bSAGs are recognized by different regions of the TCR Vbeta domain. In addition, various bSAGs are recognized differently by the same TCR. Thus, these mutational data, combined with the crystal structure of the TCR beta chain, provide evidence for distinct recognition sites for vSAG and bSAG.

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.

T cell receptor (TCR) repertoire in alloimmune responses

A large number of alloantigenic determinants could be generated by both the direct and indirect alloantigen presentation pathways. Hence, a heterogeneous population of T cells expressing a wide variety of receptors would be expected to respond to this diverse array of alloantigenic determinants. However, T cells expressing highly restricted T cell receptor (TCR) variable genes have been reported in a variety of alloimmune responses. A similar phenomenon has been observed in a wide variety of other immune responses, from those induced by superantigens, to very specific responses induced by a single peptide presented by a single MHC molecule. Given this scenario, the limited number of T cell clones which dominate an allograft rejection response, or for that matter an autoimmune response or a tumor specific response, could be therapeutically targeted by virtue of the selected TCR expression.

Immunopathogenesis of juvenile rheumatoid arthritis: role of T cells and MHC

Juvenile rheumatoid arthritis (JRA) is defined as chronic arthritis of unknown etiology appearing in patients less than 16 years of age. The disease is heterogeneous and is classified as pauciarticular, polyarticular, or systemic-onset disease. A few lines of evidence suggest that T cells are involved in the pathogenesis of the disease. T cells infiltrating the synovial membrane bear markers of activation and produce cytokines. The association of particular subtypes of JRA with certain HLA class II alleles provides strong evidence in favor of T cell involvement through an HLA-peptide-T cell receptor complex. Limited data from a few patients with JRA on T cell receptor transcripts from synovial membrane or synovial fluid cells point towards oligoclonality. This further supports the concept that T cells infiltrating the synovial membrane or extravasating into synovial fluid in patients with JRA reflect antigen-driven T cell proliferation.

In vivo effects of superantigens

Superantigens are potent immunostimulatory molecules that activate both T cells and antigen presenting cells. The consequences of superantigen exposure range from induction of T cell proliferation, massive cytokine release and systemic shock to immunosuppression and tolerance. Superantigens have been directly implicated in a number of human conditions including food poisoning and toxic shock. In addition, there is evidence to suggest that superantigens are involved in the initiation of autoimmunity, and the immune dysfunction associated with HIV infection. Because of their possible role in human disease, and their potential use in immune therapy, it is important that we more completely understand the in vivo effects of superantigens.

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.

Dimerization of soluble major histocompatibility complex-peptide complexes is sufficient for activation of T cell hybridoma and induction of unresponsiveness

Major histocompatibility complex (MHC) class I molecules are cell-surface proteins that present peptides to CD8+ T cells. These peptides are mostly derived from endogenously synthesized protein. Recombinant, soluble MHC class I molecules were produced, purified, and loaded homogeneously with synthetic peptide. These MHC-peptide complexes were used to activate a T cell hybridoma. While monomers of MHC-peptide bound to the T cell, they showed no stimulatory activity. Dimers fully triggered the T cell hybridoma to secrete interleukin 2. This response was followed by a state in which the T cell was refractory to restimulation as a result of defective signal transduction through the T cell receptor.

Bacterial pyrogenic exotoxins as superantigens

The recent discovery of the mode of interaction between a group of microbial proteins known as superantigens and the immune system has opened a wide area of investigation into the possible role of these molecules in human diseases. Superantigens produced by certain viruses and bacteria, including Mycoplasma species, are either secreted or membrane-bound proteins. A unique feature of these proteins is that they can interact simultaneously with distinct receptors on different types of cells, resulting in enhanced cell-cell interaction and triggering a series of biochemical reactions that can lead to excessive cell proliferation and the release of inflammatory cytokines. However, although superantigens share many features, they can have very different biological effects that are potentiated by host genetic and environmental factors. This review focuses on a group of secreted pyrogenic toxins that belong to the superantigen family and highlights some of their structural-functional features and their roles in diseases such as toxic shock and autoimmunity. Deciphering the biological activities of the various superantigens and understanding their role in the pathogenesis of microbial infections and their sequelae will enable us to devise means by which we can intervene with their activity and/or manipulate them to our advantage.

Superantigenic characteristics of mouse mammary tumor viruses play a critical role in susceptibility to infection in mice

Mouse mammary tumor viruses (MMTV) are retroviruses that induce mammary carcinomas. An interesting feature of these viruses is the superantigen (SAg) encoded in an open reading frame within the 3' long terminal repeat. The mechanism by which ingestion of milk-borne virus results in infection of the host mammary tissue remains incompletely understood. However, a working model has been proposed in which the interaction between viral SAg, T-cell receptor and MHC class II I-E facilitates viral replication and hence infectivity. In this review we summarize current studies demonstrating the role of SAg stimulation in susceptibility to MMTV infection.

Cells expressing a major histocompatibility complex class I molecule with a single covalently bound peptide are highly immunogenic

The major histocompatibility complex (MHC) class I molecules expressed at the cell surface are associated with a large number of different peptides so that the density of a given MHC-peptide complex is relatively low. Here we describe the properties of MHC class I molecules genetically attached to a single antigenic peptide. Cells expressing these fusion proteins are recognized by T cells specific for the particular MHC-peptide complex. Coculture of naive splenocytes with cells expressing these MHC-peptide fusion proteins and the B7.1 antigen allows the induction of primary cytotoxic T lymphocytes (CTL) in vitro. Injection of these cells into naive mice enhances the frequency of specific CTL precursors and protects against a subsequent challenge with a tumor or a virus bearing the antigenic peptide. Soluble MHC-peptide fusions were also produced in which all three components, that is, the heavy chain, beta 2-microglobulin and the peptide, have fused into a single-chain protein. The availability of MHC class I molecules bound to a single peptide provides valuable tools for the manipulation of CTL responses and the analysis of the selection processes in the thymus.

T-cell antigen receptors in rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic disease of unknown etiology characterized by chronic inflammation mainly in the joints. Several lines of evidence suggest that T cells are involved in the pathogenesis of the disease. RA is associated with certain HLA-DR alleles. Studies analyzing T-cell receptor transcripts in RA have found biased or preferential usage of certain V alpha and/or V beta gene segments by T cells infiltrating the synovial membrane or extravasating into the synovial fluid compared to peripheral blood. In certain patients few T-cell antigen receptor (TCR) clones dominated the infiltrating T cells, suggesting that T cells from the synovial membrane or the synovial fluid comprise oligoclonal populations of T cells. However, other studies have found a polyclonal population of T cells. In interpreting these results the phase of the disease (early vs. late RA), the source of T cells and the limitations of the methods used in these studies should be taken into consideration. However, it appears that synovial T cells comprise oligoclonal populations of T cells and that there is a bias towards particular TCR gene segments, although a specific TCR gene segment in RA has not emerged.