Residues of the variable region of the T-cell-receptor beta-chain that interact with S. aureus toxin superantigens

TCR peptide therapy in human autoimmune diseases

Inflammatory Th1 cells reacting to tissue/myelin derived antigens likely contribute to the pathogenesis of diseases such as multiple sclerosis (MS), rheumatoid arthritis (RA), and psoriasis. One regulatory mechanism that may be useful for treating autoimmune diseases involves an innate second set of Th2 cells specific for portions of the T cell receptor of clonally expanded pathogenic Th1 cells. These Th2 cells are programmed to respond to internally modified V region peptides from the T cell receptor (TCR) that are expressed on the Th1 cell surface in association with major histocompatibility molecules. Once the regulatory Th2 cells are specifically activated, they may inhibit inflammatory Th1 cells through a non-specific bystander mechanism. A variety of strategies have been used by us to identify candidate disease-associated TCR V genes present on pathogenic Th1 cells, including BV5S2, BV6S5, and BV13SI in MS, BV3, BV14, and BV17 in RA, and BV3 and BV13S1 in psoriasis. TCR peptides corresponding to the mid region of these BV genes were found to be consistently immunogenic in vivo when administered either i.d. in saline or i.m. in incomplete Freund's adjuvant (IFA). In MS patients, repeated injection of low doses of peptides (100-300 microg) significantly boosted the number of TCR-reactive Th2 cells. These activated cells secreted cytokines, including IL-10, that are known to inhibit inflammatory Th1 cells. Cytokine release could also be induced in TCR-reactive Th2 cells by direct cell-cell contact with Th1 cells expressing the target V gene. These findings indicate the potential of regulatory Th2 cells to inhibit not only the target Th1 cells, but also bystander Th1 cells expressing different V genes specific for other autoantigens. TCR peptide vaccines have been used in our studies to treat a total of 171 MS patients (6 trials), 484 RA patients (7 trials), and 177 psoriasis patients (2 trials). Based on this experience in 824 patients with autoimmune diseases, TCR peptide vaccination is safe and well tolerated, and can produce significant clinical improvement in a subset of patients that respond to immunization. TCR peptide vaccination represents a promising approach that is well-suited for treating complex autoimmune diseases.

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.

Staphylococcal enterotoxins

Staphylococcus aureus is a major human pathogen that produces a wide array of toxins, thus causing various types of disease symptoms. Staphylococcal enterotoxins (SEs), a family of nine major serological types of heat stable enterotoxins, are a leading cause of gastroenteritis resulting from consumption of contaminated food. In addition, SEs are powerful superantigens that stimulate non-specific T-cell proliferation. SEs share close phylogenetic relationships, with similar structures and activities. Here we review the structure and function of each known enterotoxin.

Identification of T cell ligands in a library of peptides covalently attached to HLA-DR4

While T cells have been clearly implicated in a number of disease processes including autoimmunity, graft rejection, and atypical immune responses, the precise Ags recognized by the pathogenic T cells have often been difficult to identify. This has particularly been true for MHC class II-restricted CD4+ T cells. Although such cells can be demonstrated to have undergone clonal expansion at sites of pathology, they are frequently difficult to establish as stable T cell clones. Furthermore, in general, larger peptides in higher concentrations are required to stimulate CD4+ T cells than CD8+ T cells, which makes some of the techniques developed to identify CD8+ T cell Ags impractical. To circumvent some of these problems, we developed a model system consisting of two parts. The first part involves the construction of an indicator T cell hybridoma expressing a chimeric TCR comprised of murine constant regions and human variable regions specific for influenza hemagglutinin 307-319 presented by DR4. The second part consists of a library of fibroblasts each expressing multiple peptides as amino terminal covalent extensions of the beta-chain of HLA-DR4 (DRA1*0101, DRB1*0401). Using this model system, we screened approximately 100, 000 peptides and identified three novel peptides stimulatory for the HA1.7 TCR. While there is some convergence at residues known to be important for T cell recognition, all three peptides differ markedly from each other and bear little resemblance to wild-type hemagglutinin 307-319.

Mouse xenoantigens contribute to rat T-cell Vbeta repertoire generation in mixed xenogeneic bone marrow chimeras

We previously demonstrated that rat bone-marrow-derived cells in mixed xenogeneic chimeras (rat + mouse --> mouse) contribute to peripheral selection of mouse T-cell receptor (TCR) variable betas (Vbetas) repertoire. In this study, we analysed rat T cells that developed in the chimeras to assess the contribution of mouse xenoantigens to the development of rat TCR repertoire. The expression of rat Vbetas was analysed using flow cytometry and a reverse transcription-polymerase chain reaction (RT-PCR) method that allows for both semiquantitative analysis of rat Vbeta gene expression and size heterogeneity of the complementarity determining region 3 (CDR3) domain. Three distinct patterns of Vbeta expression were detected. Partial deletion was observed for Vbeta5, 7, 12, 14, 16, 17 and 20 that exhibited reduced levels of peripheral expression by 3.4-, 1.8-, 8.7-, 2.0-, 7.8-, 9.5- and 1.8-fold, respectively, compared with the levels of Vbetas in naYve rats. Higher levels of peripheral expression were detected for three rat Vbeta genes; Vbeta6 (2.2-fold), Vbeta8.2 (3.2-fold), and Vbeta9 (1.7-fold). The relative expression of the other 10 known rat Vbeta families in chimeras was unchanged as compared with that of normal rats. We did not observe detectable changes in the pattern of CDR3 expression in chimeras, suggesting that the mouse xenogeneic environment exerted its influence on the development of rat T cells via the Vbeta-encoded CDR1/2 domains. Our data demonstrate that the rat T-cell repertoire in chimeras is shaped by both contractions as well as expansions of selected Vbetas and suggest that mouse xenoantigens and/or superantigens of endogenous mouse retroviruses may contribute as ligands for these selection processes

Superantigen antagonist protects against lethal shock and defines a new domain for T-cell activation

Superantigens trigger an excessive cellular immune response, leading to toxic shock. We have designed a peptide antagonist that inhibits superantigen-induced expression of human genes for interleukin-2, gamma interferon and tumor necrosis factor-b, which are cytokines that mediate shock. The peptide shows homology to a b-strand-hinge-a-helix domain that is structurally conserved in superantigens, yet is remote from known binding sites for the major histocompatibility class II molecule and T-cell receptor. Superantigens depend on this domain for T-cell activation. The peptide protected mice against lethal challenge with staphylococcal and streptococcal superantigens. Moreover, it rescued mice undergoing toxic shock. Surviving mice rapidly developed protective antibodies against superantigen that rendered them resistant to further lethal challenges, even with different superantigens. Thus, the lethal effect of superantigens can be blocked with a peptide antagonist that inhibits their action at the beginning of the toxicity cascade, before activation of T cells takes place.

Mapping the energy of superantigen Staphylococcus enterotoxin C3 recognition of an alpha/beta T cell receptor using alanine scanning mutagenesis

Binding of the T cell receptor (TCR) to a bacterial superantigen (SAG) results in stimulation of a large population of T cells and subsequent inflammatory reactions. To define the functional contribution of TCR residues to SAG recognition, binding by 24 single-site alanine substitutions in the TCR Vbeta domain to Staphylococcus aureus enterotoxin (SE) C3 was measured, producing an energy map of the TCR-SAG interaction. The results showed that complementarity determining region 2 (CDR2) of the Vbeta contributed the majority of binding energy, whereas hypervariable region 4 (HV4) and framework region 3 (FR3) contributed a minimal amount of energy. The crystal structure of the Vbeta8.2-SEC3 complex suggests that the CDR2 mutations act by disrupting Vbeta main chain interactions with SEC3, perhaps by affecting the conformation of CDR2. The finding that single Vbeta side chain substitutions had significant effects on binding and that other SEC3-reactive Vbeta are diverse at these same positions indicates that SEC3 binds to other TCRs through compensatory mechanisms. Thus, there appears to be strong selective pressure on SAGs to maintain binding to diverse T cells.

The spectral and thermodynamic properties of staphylococcal enterotoxin A, E, and variants suggest that structural modifications are important to control their function

The superantigens staphylococcal enterotoxin A and E (SEA and SEE) can activate a large number of T-cells. SEA and SEE have approximately 80% sequence identity but show some differences in their biological function. Here, the two superantigens and analogues were characterized biophysically. SEE was shown to have a substantially higher thermal stability than SEA. Both SEA and SEE were thermally stabilized by 0.1 mM Zn(2+) compared with Zn(2+)-reduced conditions achieved using 1 mM EDTA or specific replacements that affect Zn(2+) coordination. The higher stability of SEE was only partly caused by the T-cell receptor (TCR) binding regions, whereas regions in the vicinity of the major histocompatibility complex class II binding sites affected the stability to a greater extent. SEE exhibited a biphasic denaturation between pH 5.0-6.5, influenced by residues in the TCR binding regions. Interestingly, enzyme-linked immunosorbent assay, isoelectric focusing, and circular dichroism analysis indicated that conformational changes had occurred in the SEA/E chimerical constructs relative to SEA and SEE. Thus, it is proposed that the Zn(2+) binding site is very important for the stability and potency of SEA and SEE, whereas residues in the TCR binding site have a substantial influence on the molecular conformation to control specificity and function.

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.

Staphylococcal Enterotoxin H Displays Unique MHC Class II-Binding Properties

Staphylococcal enterotoxin H (SEH) has been described as a superantigen by sequence homology with the SEA subfamily and briefly characterized for its in vivo activity. In this study, we demonstrate that SEH is a potent T cell mitogen and inducer of T cell cytotoxicity that possesses unique MHC class II-binding properties. The apparent affinity of SEH for MHC class II molecules is the highest affinity ever measured for a staphylococcal enterotoxin (Bmax1/2 ∼ 0.5 nM for MHC class II expressed on Raji cells). An excess of SEA or SEAF47A, which has reduced binding to the MHC class II α-chain, is able to compete for binding of SEH to MHC class II, indicating an overlap in the binding sites at the MHC class II β-chain. The binding of SEH to MHC class II is like SEA, SED, and SEE dependent on the presence of zinc ions. However, SEH, in contrast to SEA, binds to the alanine-substituted DR1 molecule, βH81A, believed to have impaired zinc-bridging capacity. Furthermore, alanine substitution of residues D167, D203, and D208 in SEH decreases the affinity for MHC class II as well as its in vitro potency. Together, this indicates an MHC class II binding site on SEH with a different topology as compared with SEA. These unique binding properties will be beneficial for SEH to overcome MHC class II isotype variability and polymorphism as well as to allow an effective presentation on APCs also at low MHC class II surface expression.

Studies on the functional site on staphylococcal enterotoxin A responsible for production of murine gamma interferon

To identify the functional region(s) associated with induction of gamma interferon on the staphylococcal enterotoxin A molecule, native staphylococcal enterotoxin A molecules and 12 various synthetic peptides corresponding to different regions of entire staphylococcal enterotoxin A were compared to induce gamma interferon production in murine spleen cells. The native staphylococcal enterotoxin A molecule induced gamma interferon production, whereas all of the 12 synthetic peptides did not. Pre-treatment of the murine spleen cells with synthetic peptide A-9 (corresponding to amino acid residues 161-180) significantly inhibited the staphylococcal enterotoxin A-induced gamma interferon production, whereas those with other synthetic peptides did not. When native staphylococcal enterotoxin A was pre-treated with either anti-staphylococcal enterotoxin A serum or anti-peptide sera, anti-staphylococcal enterotoxin A serum and antisera to peptides A-1 (1-20), A-7 (121-140), A-8 (141-160), A-9 (161-180) and A-10 (181-200) inhibited the staphylococcal enterotoxin A-induced gamma interferon production. From these findings, the amino acid residues 161-180 on the staphylococcal enterotoxin A molecule may be an essential region for murine gamma interferon production. Furthermore, the neutralizing epitopes may be also located on regions of amino acid residues 1-20, 121-140, 141-160 and 181-200 on the staphylococcal enterotoxin A molecule.

Impaired cytotoxic T lymphocyte activity in systemic lupus erythematosus following in vitro polyclonal T cell stimulation: a contributory role for non-T cells

To determine whether non-T cells contribute to impaired generation of nonrestricted cytotoxic T lymphocyte (CTL) activity in human SLE, peripheral blood mononuclear cells (PBMC) and sort-purified T cells from normal subjects and SLE patients were stimulated with anti-CD3 mAb, maintained in IL2, and assayed for cytolytic activity against 51Cr-labeled Daudi target cells. In addition, T cell and non-T cell fractions were isolated from nine pairs of monozygotic (MZ) twins discordant for SLE, reconstituted in a criss-cross pattern, and stimulated and assayed for cytolytic activity. Cytolytic responses were significantly lower in SLE PBMC cultures than in normal PBMC cultures. Addition of SLE serum to normal PBMC cultures did not inhibit generation of normal cytolytic responses, and neither 'resting' SLE PBMC prior to stimulation nor addition of neutralizing anti-IL10 mAb or costimulating anti-CD28 mAb restored generation of SLE cytolytic responses to normal. Nevertheless, despite the significantly greater cytolytic responses in normal PBMC cultures than in SLE PBMC cultures, cytolytic responses in normal purified T cell cultures were only modestly and insignificantly greater than those in SLE purified T cell cultures. Moreover, substitution of 'healthy' non-T cells for SLE non-T cells in four of the nine MZ twin-pairs appreciably enhanced cytolytic responses, and substitution of SLE non-T cells for 'healthy' non-T cells in five of the seven twin-pairs tested appreciably diminished cytolytic responses. Taken together, these results indicate that, in addition to any inherent SLE T cell abnormalities, impaired function of SLE non-T cells contributes to impaired generation of nonrestricted CTL activity.

Cutting Edge: Trimolecular Interaction of TCR with MHC Class II and Bacterial Superantigen Shows a Similar Affinity to MHC:Peptide Ligands

Bacterial superantigens such as Staphylococcus aureus enterotoxin A (SEA) are very potent stimulators of T cells. They bind to the Vβ region of the TCR and to MHC class II, stimulating T cells at nanomolar concentrations. Using surface plasmon resonance measurements, we find that binding between the individual components of the complex (TCR-class II, TCR-SEA, SEA-class II) is very weak, but that the stability of the trimolecular complex is considerably enhanced, reaching an affinity similar to that found for TCR interactions with MHC:peptide ligand. Thus, the potency of SEA in stimulation of T cells is not due to particularly strong affinities between the proteins, but to a cooperative effect of interactions in the TCR-SEA-MHC class II trimolecular complex that brings the kinetics into a similar range to binding of conventional Ags. This range may be the optimum for T cell activation.

Diverse Fine Specificity and Receptor Repertoire of T Cells Reactive to the Major VP1 Epitope (VP1230–250) of Theiler’s Virus: Vβ Restriction Correlates with T Cell Recognition of the C-Terminal Residue

Theiler’s murine encephalomyelitis virus induces chronic demyelinating disease in genetically susceptible mice. The histopathological and immunological manifestation of the disease closely resembles human multiple sclerosis, and, thus, this system serves as a relevant infectious model for multiple sclerosis. The pathogenesis of demyelination appears to be mediated by the inflammatory Th1 response to viral epitopes. In this study, T cell repertoire reactive to the major pathogenic VP1 epitope region (VP1233–250) was analyzed. Diverse minimal T cell epitopes were found within this region, and yet close to 50% of the VP1-reactive T cell hybridomas used Vβ16. The majority (8/11) of the Vβ16+ T cells required the C-terminal amino acid residue on the epitope, valine at position 245, and every T cell hybridoma recognizing this C-terminal residue expressed Vβ16. However, the complementarity-determining region 3 sequences of the Vβ16+ T cell hybridomas were markedly heterogeneous. In contrast, such a restriction was not found in the Vα usage. Only restricted residues at this C-terminal position allowed for T cell activation, suggesting that Vβ16 may recognize this terminal residue. Further functional competition analysis for TCR and MHC class II-contacting residues indicate that many different residues can be involved in the class II and/or TCR binding depending on the T cell population, even if they recognize the identical minimal epitope region. Thus, recognition of the C-terminal residue of a minimal T cell epitope may associate with a particular Vβ (but not Vα) subfamily-specific sequence, resulting in a highly restricted Vβ repertoire of the epitope-specific T cells.

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.

Understanding the mechanism of action of bacterial superantigens from a decade of research

In the face of the unique diversity and plasticity of the immune system pathogenic organisms have developed multiple mechanisms in adaptation to their hosts, including the expression of a particular class of molecules called superantigens. Bacterial superantigens are the most potent stimulators of T cells. The functional consequences of the expression of superantigens by bacteria can be extended not only to T lymphocytes, but also to B lymphocytes and to cells of the myeloid compartment, including antigen-presenting cells and phagocytes. The biological effects of bacterial superantigens as well as their molecular aspects have now been studied for a decade. Although there is still a long way to go to clearly understand the role these molecules play in the establishment of disease, recently acquired knowledge of their biochemistry now offers unique experimental opportunities in defining the molecular rules of T-cell activation. Here, we present some of the most recent functional and molecular aspects of the interaction of bacterial superantigens with MHC class II molecules and the T-cell receptor.

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.

Molecular requirements for T cell recognition by a major histocompatibility complex class II-restricted T cell receptor: the involvement of the fourth hypervariable loop of the Valpha domain

The role of two central residues (K68, E69) of the fourth hypervariable loop of the Valpha domain (HV4alpha) in antigen recognition by an MHC class II-restricted T cell receptor (TCR) has been analyzed. The TCR recognizes the NH2-terminal peptide of myelin basic protein (Ac1-11, acetylated at NH2 terminus) associated with the class II MHC molecule I-Au. Lysine 68 (K68) and glutamic acid 69 (E69) of HV4alpha have been mutated both individually and simultaneously to alanine (K68A, E69A). The responsiveness of transfectants bearing wild-type and mutated TCRs to Ac1-11-I-Au complexes has been analyzed in the presence and absence of expression of the coreceptor CD4. The data demonstrate that in the absence of CD4 expression, K68 plays a central role in antigen responsiveness. In contrast, the effect of mutating E69 to alanine is less marked. CD4 coexpression can partially compensate for the loss of activity of the K68A mutant transfectants, resulting in responses that, relative to those of the wild-type transfectants, are highly sensitive to anti-CD4 antibody blockade. The observations support models of T cell activation in which both the affinity of the TCR for cognate ligand and the involvement of coreceptors determine the outcome of the T cell-antigen-presenting cell interaction.

Staphylococcal enterotoxin-A directly stimulates signal transduction and interferon-gamma production in psoriatic T-cell lines

Bacterial superantigens such as staphylococcal enterotoxin-A (SEA) have been implicated in the pathogenesis of psoriasis vulgaris. Major histocompatibility complex (MHC) class II molecules are high affinity receptors for SEA, and T cells found in psoriatic skin lesions express high levels of MHC class II. Here we address the question of whether SEA can directly activate psoriatic T cells in the absence of professional antigen-presenting cells. We show that SEA induces i) tyrosine phosphorylation of several proteins, ii) downregulation of the T-cell receptor (TCR), and iii) production of interferon-gamma (IFN-gamma), but not autocrine mitogenesis in CD8-positive T clones obtained from skin lesions of a patient with psoriasis vulgaris. Psoriatic T cells do not respond to SEA molecules if mutations are introduced in the TCRbeta- or in both the two MHC class II alpha- and beta-binding sites of SEA. Mutations in only one of the two MHC class II binding sites of SEA has different effects on T-cell activation. Thus, SEA molecules with a mutation in the MHC class II beta-binding site induce protein tyrosine phosphorylation, but not IFN-gamma production or co-stimulation of cytokine-mediated proliferation. In contrast, SEA with a mutation in the MHC class II alpha-binding site induces IFN-gamma and a qualitatively changed tyrosine phosphorylation profile. Both mutations delete the co-stimulatory effect on cytokine-mediated proliferation. This suggests that both MHC class II binding sites are involved in the autopresentation of SEA by psoriatic T cells. In conclusion, we provide evidence that SEA directly activates MVHC class H-positive psoriatic T-cell lines to produce IFN-gamma, a key cytokine in the pathogenesis of psoriasis vulgaris.

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.

Staphylococcal Enterotoxin A Induces Survival of VH3-Expressing Human B Cells by Binding to the VH Region with Low Affinity

Staphylococcal enterotoxins (SE) are bacterial superantigens that bind to MHC class II molecules and to the Vβ-chain of the TCR, and subsequently activate T cells expressing specific Vβ regions. In this study, we have studied the effects of SEA on human B cell activation, and specifically the capacity of SEA to function as a B cell superantigen in vitro. We show herein that SEA failed to induce B cell proliferation and differentiation in the absence of T cells. However, SEA induced survival of B cells uniquely expressing VH3-containing IgM, independently of light chain utilization. The sequences of VH3 IgM gene products were determined and found to include a number of members of the VH3 family with a variety of different D and JH gene segments. Analysis of the sequences of VH3 gene products revealed possible sites in framework region 1 and/or framework region 3 that could be involved in SEA-mediated activation of VH3-expressing B cells. Binding studies showed that SEA interacts with the VH3 domain of Ig with low, but detectable affinity. These results indicate that SEA functions as a B cell superantigen by interacting with VH3 gene segments of Ig.

Ligand recognition by alpha beta T cell receptors

While still incomplete, the first data concerning the biochemistry of T cell receptor-ligand interactions in cell-free systems seem to have considerable predictive value regarding whether a T cell response is strong or weak or suppressive. This data will help considerably in elucidating the mechanisms behind T cell responsiveness. Also of great interest are the first structures of T cell receptor molecules and, particularly, TCR-ligand complexes. These appear to confirm earlier suggestions of a fixed orientation for TCR engagement with peptide/MHC and should form the basis for understanding higher oligomers, evidence for which has also just emerged. We conclude with an analysis of the highly diverse CDR3 loops found in all antigen receptor molecules and suggest that such regions form the core of both TCR and antibody specificity.

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.

Serologic evidence that streptococcal superantigens are not involved in the pathogenesis of Kawasaki disease

Kawasaki disease (KD) is an acute multisystem vasculitis of unknown etiology and is associated with marked activation of T cells and monocyte macrophages, leading to the assumption that superantigens are involved in its pathogenesis. To determine if an association exists between streptococcal superantigens and KD, we examined serum antibody responses to superantigens in sera from 50 paired acute and convalescent KD patients using purified recombinant streptococcal superantigens, such as SPEA, SPEC, SSA and MF. We found a very low frequency of detection of anti-superantigen antibodies by ELISA and no marked IgG seroconversion to each superantigen, indicating the absence of a serological relationship between toxin-producing streptococcal infection and the onset of KD.

A mutation of F47 to A in staphylococcus enterotoxin A activates the T-cell receptor Vbeta repertoire in vivo

The bacterial superantigen staphylococcal enterotoxin A (SEA) binds with high affinity to major histocompatibility complex (MHC) class II molecules and subsequently activates T cells bearing particular T-cell receptor (TCR) Vbeta chains. Structural and mutational studies have defined two distinct MHC class II binding sites located in the N-terminal and C-terminal domains of SEA. The N-terminal F47 amino acid is critically involved in a low-affinity interaction to the MHC class II alpha-chain, while the C-terminal residues H187, H225, and D227 coordinate a Zn2+ ion and bind with moderate affinity to the beta-chain. In order to analyze whether the SEA-MHC class II alpha-chain interaction plays a role in dictating the in vivo repertoire of T-cell subsets, we studied distinct Vbeta populations after stimulation with wild-type SEA [SEA(wt)] and SEA with an F47A mutation [SEA(F47A)]. Injections of SEA(wt) in C57BL/6 mice induced cytokine release in serum, strong cytotoxic T-lymphocyte activity, expansion of T-cell subsets, and modulated expression of the T-cell activation antigens CD25, CD11a, CD44, CD62L, and CD69. SEA-reactive TCR Vbeta3+ and Vbeta11+ T cells were activated, while TCR Vbeta8+ T cells remained unaffected. The SEA(F47A) mutant protein induced a weaker T-cell response and failed to induce substantial interleukin-6 production compared to SEA(wt). Notably, SEA(F47A) failed to activate TCR Vbeta11+ T cells, whereas in vivo expansion and modulation of T-cell activation markers on TCR Vbeta3+ T cells were similar to those for SEA(wt). A similar response to SEA(F47A) was seen among CD4+ and CD8+ T cells. Activation of TCR Vbeta3+ and TCR Vbeta11+ T-cell hybridomas confirmed that SEA(F47A) activates TCR Vbeta3+ but not TCR Vbeta11+ T cells. The data support the view that the SEA-N-terminal MHC class II alpha-chain interaction defines a topology that is required for engagement of certain TCR Vbeta chains in vivo.

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.

Vbeta-dependent stimulation of bovine and human T cells by host-specific staphylococcal enterotoxins

Staphylococcus aureus isolates from bovine and ovine species produce unique molecular variants of type C staphylococcal enterotoxin (SEC). The SEC animal variants have greater than 98% amino acid sequence identity with SEC1, a human-associated SEC. The two SEC animal variants have been designated SEC(bovine) and SEC(ovine) according to their corresponding host species. We showed previously that these toxins induce quantitatively different levels of T-cell stimulation in several animal species. The present study compared the abilities of these closely related host-specific SEC variants to stimulate Vbeta-bearing T cells from bovine and human donors. All three toxins expanded human T cells bearing T-cell receptor Vbeta elements (huVbeta) 3, 12, 13.2, 14, 15, 17, and 20. However, SEC1 resulted in greater expansion of hyVbeta12 than either SEC(bovine) or SEC(ovine). In addition, bovine T cells proliferate in a Vbeta-dependent manner in response to these superantigens (SAgs). All three toxins induced the proliferation of bovine T cells bearing the previously sequenced Vbeta element (boVbeta) from the bovine T-cell clone BTB13 (boVbetaBTB13). SEC1 and SEC(ovine) also were able to induce proliferation of bovine T cells bearing boVbetaBTB35, which SEC(bovine) failed to stimulate. The species-specific differences in T-cell proliferation exhibited by these closely related SEC variants may reflect the evolutionary adaptation of S. aureus, presumably to increase its host range by the manipulation of the immune system in a host-specific manner.

Double-positive T cell receptor(high) thymocytes are resistant to peptide/major histocompatibility complex ligand-induced negative selection

To investigate negative selection events during intrathymic ontogeny, we established T cell receptor (TCR)-transgenic mice [N15tg/RAG-2-/- (H-2b)] expressing a single TCR specific for vesicular stomatitis virus nuclear octapeptide N52-59 (VSV8) in the context of the major histocompatibility complex (MHC) class I molecule, K(b). Administration of VSV8 in vivo induced apoptosis in less than 4 h, deleting the majority of immature double-positive (DP) thymocytes by 24 h. In contrast, DP TCRhigh as well as single-positive (SP) thymocytes were refractory to this death process. Moreover, DP TCRhigh cells differentiated into SP thymocytes in vitro and in vivo, maturing into functional cytotoxic T lymphocytes upon intrathymic transfer to beta RAG 2-/- recipients. Hence, negative selection processes involving MHC-bound peptide ligands are operative only prior to the late DP thymocyte stage in this MHC class I-restricted TCR transgene system.

Crystal structure of the streptococcal superantigen SPE-C: dimerization and zinc binding suggest a novel mode of interaction with MHC class II molecules

Bacterial superantigens are small proteins that have a very potent stimulatory effect on T lymphocytes through their ability to bind to both MHC class II molecules and T-cell receptors. We have determined the three-dimensional structure of a Streptococcal superantigen, SPE-C, at 2.4 A resolution. The structure shows that SPE-C has the usual superantigen fold, but that the surface that forms a generic, low-affinity MHC-binding site in other superantigens is here used to create a SPE-C dimer. Instead, MHC class II binding occurs through a zinc binding site that is analogous to a similar site in staphylococcal enterotoxin A. Consideration of the SPE-C dimer suggests a novel mechanism for promotion of MHC aggregation and T-cell activation.

A structural and functional comparison of staphylococcal enterotoxins A and C2 reveals remarkable similarity and dissimilarity

Staphylococcal enterotoxins and toxic shock syndrome toxin-1 are known as superantigens due to their ability to activate a large number of T-cells by crosslinking the major histocompatibility complex class II molecules with the T-cell receptor. Although superantigens seem to act by a common mechanism, they vary in many of their specific interactions and biological properties. A structural comparison of staphylococcal enterotoxins A and C2, members of the staphylococcal superantigens, has shown large conformational differences at the putative TcR interaction site (loops between alphaN-alpha2, alpha4-beta9 and beta10-alpha5 in staphylococcal enterotoxin A) that could explain the variability in their T-cell receptor specificity. A common Zn2(+)-binding site was identified in both staphylococcal enterotoxin A and C2 that is superimposable but differs somewhat in its coordination geometry between the two molecules.

Activation of Human T Cells by Major Histocompatability Complex Class II Expressing Neutrophils: Proliferation in the Presence of Superantigen, But Not Tetanus Toxoid

The primary function of polymorphonuclear neutrophils (PMN) in the immune response appears to be acute phagocytic clearance of foreign pathogens and release of inflammatory mediators. Consistent with their assumed lack of major histocompatibility complex (MHC) class II expression, PMN have not been considered to play a role in antigen presentation and T-cell activation. However, recent reports have shown that human PMN can express MHC class II molecules both in vitro and in vivo after stimulation with either granulocyte-macrophage colony-stimulating factor (GM-CSF ) or interferon-γ (IFN-γ). Thus, under appropriate conditions, PMN could play a significant role in immune regulation, including T-cell activation. In this report, we demonstrate that human class II–expressing PMN can serve as accessory cells in superantigen (SAg)-mediated T-cell activation. This accessory activity for SAg presentation was present only after induction of MHC class II expression, and was especially pronounced following culture of PMN with GM-CSF plus IFN-γ, which acted synergistically to induce MHC class II molecules on PMN. Moreover, the level of MHC class II expression and the magnitude of SAg-induced T-cell responses were found to be highly correlated and distinctly donor dependent, with PMN from some donors repeatedly showing fivefold higher responses than PMN from other donors. On the other hand, culture of PMN with GM-CSF plus IFN-γ under conditions that resulted in optimal MHC class II expression did not enable them to function as antigen-presenting cells for either intact tetanus toxoid (TT) or for a TT peptide. These results delineate a new pathway for T-cell activation by SAg that may play an important role in the severity of SAg-induced inflammatory responses. They also identify a donor-specific polymorphism for induction of PMN MHC class II expression which may be of significance for therapies involving GM-CSF and IFN-γ.

Expression of bacterial superantigen genes in mice induces localized mononuclear cell inflammatory responses

Bacterial superantigens are potent T cell activators, and superantigen proteins have been injected into mice and other animals to study T cell responses in vivo. When superantigen proteins are injected, however, the T cell stimulatory effects cannot be confined to specific tissues. Therefore, to target superantigen expression to specific tissues, we used gene transfer techniques to express bacterial superantigen genes in mammalian cells in vitro and in tissues in vivo. Murine, human, and canine cells transfected with superantigen genes in vitro all produced superantigen proteins both intracellularly and extracellularly, as assessed by bioassay, immunocytochemistry, and antigen ELISA. Superantigens produced by transfected eukaryotic cells retained their biologic specificity for T cell receptor binding. Intramuscular injection of superantigen plasmid DNA in vivo induced an intense intramuscular mononuclear cell infiltrate, an effect that could not be reproduced by intramuscular injection of superantigen protein. Intradermal and intravenous injection of superantigen DNA induced cutaneous and intrapulmonary mononuclear cell inflammatory responses, respectively. Thus, superantigen genes can be expressed by mammalian cells in vivo. Superantigen gene therapy represents a novel method of targeting localized T cell inflammatory reactions, with potential application to treatment of cancer and certain infectious diseases.

B-cell superantigens: molecular and cellular implications

B cell superantigens are proteins that are capable of immunoglobulin variable region mediated binding interactions with the naive B cell repertoire at frequencies that are orders of magnitude greater than occur for conventional antigens. Within this review we discuss recent observations regarding the molecular basis of these interactions and the distribution of superantigen binding capacities in different human B cell populations. These findings and current predictions regarding the relevance of these proteins to the physiologic development of immune repertoires are also discussed.

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.

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).

Immune response during tumor therapy with antibody-superantigen fusion proteins

To engineer superantigens (SAg) to express tumor reactivity, we genetically fused the Fab-part of the tumor-reactive MAb C215 and the bacterial SAg staphylococcal enterotoxin A (SEA). Treatment of mice carrying established lung micrometastases of the C215-transfected syngeneic B16 melanoma with 3-4 daily injections of C215Fab-SEA resulted in strong antitumor effects, while only moderate effects were seen when treatment was given every 4th day (intermittent treatment). High serum levels of IL-2, TNF-alpha, IFN-gamma and strong induction of CTLs (cytotoxic T lymphocytes) were noted after priming with the fusion protein. T cells responded well to 3 daily injections of C215Fab-SEA and then gradually entered a hyporesponsive state, characterized by a reduced ability to produce IL-2, TNF-alpha and IFN-gamma and failure to mediate cytotoxicity in vitro. Intermittent treatment was characterized by increased levels of IL-10, concomitant with accentuated loss of IL-2, TNF-alpha and IFN-gamma production. A 10-fold increase in SEA-reactive TCR V(beta)3+ CD4+ cells was observed in the spleen, while a loss of TCR V(beta)3+ CD8+ and V(beta)11+ CD8+ cells was noted. This is in striking contrast to injections of native SEA which induced a marked deletion of TCR V(beta)3+ CD4+ T cells, but not of CD8+ cells. Recovery of the TH1 cytokine profile occurred within 1-2 weeks, while restoration of cytotoxicity required several months and correlated with recovery of TCR V(beta)3+ CD8+ and TCR V(beta)11+ CD8+ T cells. These results show that the temporal relationship of SAg stimulations dictates the cytokine profile. Moreover, different mechanisms appear to regulate hyporesponsiveness in CD4+ and CD8+ T cells.

CD30/TNF receptor-associated factor interaction: NF-kappa B activation and binding specificity

CD30 is a member of the tumor necrosis factor (TNF) receptor superfamily. CD30 is expressed on normal activated lymphocytes, on several virally transformed T- or B-cell lines and on neoplastic cells of Hodgkin's lymphoma. The interaction of CD30 with its ligand induces pleiotropic effects on cells resulting in proliferation, differentiation, or death. The CD30 cytoplasmic tail interacts with TNF receptor-associated factors (TRAFs), which have been shown to transduce signals mediated by TNF-R2 and CD40. We demonstrate here that TRAF2 also plays an important role in CD30-induced NF-kappa B activation. We also show that TRAF2-mediated activation of NF-kappa B plays a role in the activation of HIV transcription induced by CD30 cross-linking. Detailed site-directed mutagenesis of the CD30 cytoplasmic tail reveals that there are two independent binding sites for TRAF, each interacting with a different domain of TRAF. Furthermore, we localized the TRAF-C binding site in CD30 to a 5-7 amino acid stretch.

An Epstein-Barr virus-associated superantigen

More than 90% of adults are latently infected with Epstein-Barr virus (EBV), the causative agent of infectious mononucleosis, a self-limiting lymphoproliferative disease characterized by extensive T cell activation. Reactivation of this herpesvirus during immunosuppression is often associated with oncogenesis. These considerations led us to analyze the early events that occur after exposure of the immune system to EBV. Strong major histocompatibility complex (MHC) class II-dependent but not MHC-restricted, T cell proliferation was observed in vitro in response to autologous, lytically infected EBV-transformed B cells. By measuring the appearance of the early activation marker CD69 on individual T cell V beta subsets, we could demonstrate selective activation of human V beta 13- T cells. This was confirmed with murine T cell hybridomas expressing various human BV genes. While EBV- Burkitt's lymphoma cells were nonstimulatory, they induced V beta-restricted T cell activation after EBV infection. EBV specific activation was also demonstrated in cord blood cells, excluding a recall-antigen response. Thus, all of the characteristics of a superantigen-stimulated response are seen, indicating that induction of the EBV lytic cycle is associated with the expression of a superantigen in B cells. A model is presented proposing a role for the superantigen in infection, latency, and oncogenesis.

Correlation between the number of T cell receptors required for T cell activation and TCR-ligand affinity

The number of T cell receptors on CTL clone 2C that are required for recognition of various peptide-MHC or superantigen-MHC ligands were measured as a function of both the ligand density on target cells and the binding affinity of the TCR. Quantitative inverse correlations were determined between the number of TCRs required for recognition and the number of ligands on target cells, and the number of TCR required and the Ka of the TCR for the ligand. We propose and test predictive uses of these relationships to determine the number of endogenous peptide-MHC complexes on a target cell (when TCR affinity is known) or to determine the affinity of the TCR (when the number of ligands is known).

Do superantigens play a role in lymphoproliferation?

The T cell superantigens are infectious agents that interact with the T cell receptor and the MHC molecules outside their normal antigen-specific sites, with products of conserved sequences of the variable region chains. This non-specific interaction results in the massive stimulation of T cells (up to 20% of the total) as opposed to conventional antigenic stimulation, which is specific and limited to about 10,000 cells. B cell superantigens have recently been described, stimulating a restricted subset of B cells, those expressing the VH3 subgroup in their rearranged immunoglobulin genes. A number of murine malignancies have been shown to be due to T cell superantigens acting either on T cells or on B cells: the RCS B cell lymphomas in SJL mice, the radiation leukemia virus-induced T cell thymic lymphomas in C57BL/Ka mice and B cell lymphomas in the murine AIDS. We propose that some human B cell malignancies can be due to a similar type of interaction. B cell lymphomas in AIDS patients were recently suggested to be due to the HIV gp120 envelope glycoprotein, a newly recognized superantigen. It can be speculated that the low grade B cell gastric lymphomas of mucosa-associated lymphoid tissue (MALT) are the result of exposure to the H. pylori pathogen. EBV-related lymphocytic proliferation has also been shown to be related with a restricted repertoire and may constitute another example of superantigen driven proliferation. A classification of the various superantigen-driven lymphoproliferative states is proposed.

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.

Major histocompatibility complex class I molecule serves as a ligand for presentation of the superantigen staphylococcal enterotoxin B to T cells

Superantigens, such as staphylococcal enterotoxin B (SEB), elicit a strong proliferative response in T cells when presented in the context of major histocompatibility complex (MHC) class II molecules. We observed a similar T-cell response, when MHC class II-negative epidermal cell lines were employed as antigen-presenting cells. Immunoprecipitation studies indicated that the ligand to which SEB bound had a molecular mass of 46 kDa. Radiolabeled SEB could be immunoprecipitated from isolated membrane proteins on the SCC13 epidermal cell line with a monoclonal antibody directed against the MHC class I molecule, and transfection of the K-562 cell line with MHC class I molecules showed a 75% increased SEB-binding capacity compared with the nontransfected MHC class I- and class II-negative counterpart. In functional studies, antibodies to the MHC class I molecule inhibited T-cell proliferation by at least 50%. From these studies, we conclude that MHC class I molecules on malignant squamous cell carcinomas serve as ligands for SEB, which, given the appropriate costimulatory signals, is sufficient to allow for superantigen-induced T-cell proliferation.

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.

Reactivity of mouse T-cell hybridomas expressing human Vbeta gene segments with staphylococcal and streptococcal superantigens

A panel of 15 mouse T-cell hybridomas, each expressing a different human Vbeta gene segment (hVbeta) in an otherwise mouse T-cell receptor (i.e., mouse alpha chain and CD3 complex), was constructed by transfection of hVbeta/mouse Cbeta chimeric T-cell receptor (TCR)-beta genes into a mouse T-cell hybridoma recipient lacking the endogenous TCR-beta chain. Several qualities that are conferred by the hVbeta chain of the TCR are retained in the chimeric human-mouse TCR complex: a large panel of hVbeta-specific antibodies specifically stained the hVbeta expressed by the mouse T-cell hybridomas. Moreover, hVbeta-transfected mouse cells could readily produce interleukin 2 when stimulated by superantigens presented by antigen-presenting cells. These characteristics made it possible to refine the reactivity of 17 superantigen preparations with the available transfected Vbetas. Each superantigen gave a characteristic pattern of reactivity on the transfectants. Positive reactivities with some of these transfectants, which differ only by the expressed hVbeta, demonstrate unambiguously the superantigenic character of a protein or fraction and its potential to react with the corresponding Vbetas. Therefore, these hVbeta-transfected cells constituted a valuable tool for determining "specificity fingerprints" of known or putative superantigens. First, commonly used, commercially available superantigens such as staphylococcal enterotoxin B and toxic shock syndrome toxin-1 (TSST-1) showed additional Vbeta reactivities, compared with those of their recombinant counterparts. This stresses the importance of using defined preparations of superantigens for the definition of Vbeta specificities. Second, the stimulatory pattern of a strain of Streptococcus pyogenes demonstrated that this strain, unlike others, produces a potent Vbeta 8-specific superantigen that is an yet undefined at the molecular level.