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

V beta repertoire in rats and implications for endogenous superantigens

Endogenous superantigens of mice, encoded by mammary tumor virus proviral integrants, induce intrathymic deletion of entire T cell populations that express specific V beta gene products, a phenomenon proposed to be important in self-tolerance and prevention of toxic responses to exogenous microbial superantigens. Evidence for the presence of V beta selecting/deleting endogenous superantigens in other species is lacking. We report here that rats do not exhibit endogenous superantigen-induced V beta clonal deletions despite their strong response to bacterial superantigens. These findings indicate that endogenous superantigens are not obligatory in V beta repertoire shaping.

Mutations defining functional regions of the superantigen staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB) is both a superantigen and toxin. As a superantigen, SEB can bind to major histocompatibility complex (MHC) class II molecules to form a ligand for alpha/beta T cell receptors bearing particular V beta elements. As a toxin, SEB causes rapid weight loss in mice sometimes leading to death. We show here that both of these functions map to the NH2-terminal portion of the toxin. Three regions were identified: one important in MHC class II binding, one in T cell recognition, and one in both functions. These results support the conclusion that the toxicity of SEB is related to massive T cell stimulation and release of cytokine mediators and show that the residues interacting with MHC and the T cell receptor are intertwined.

Identification of HLA-DR1 beta chain residues critical for binding staphylococcal enterotoxins A and E

Superantigens are thought to make external contacts with major histocompatibility complex (MHC) class II molecules and with the V beta portion of a T cell antigen receptor (TCR), thereby stimulating entire families of T cells. The precise mapping of superantigen binding sites on class II molecules may provide valuable information on how TCR and MHC molecules interact. Two bacterial superantigens, staphylococcal enterotoxins A and E (SEA/SEE) bind well to most HLA-DR alleles, but poorly to HLA-DRw53. The sequences responsible for this binding were localized to the putative alpha helix of the DR beta chain by measuring toxin binding to a panel of chimeric class II molecules expressed on transfected cells. Binding of SEA/SEE to the DRw14 (Dw9) molecule suggested that the conserved histidine 81 in the beta chain of most DR molecules was important, whereas the tyrosine 81 in the DRw53 beta chain was detrimental for high-affinity binding. To prove this, reciprocal point mutations were introduced in the DR1 and DRw53 beta chains. Mutation of histidine 81 in the DR1 beta chain to tyrosine reduced SEA/SEE binding, but did not prevent recognition of two DR1- restricted peptides by six of eight antigen-specific T cell lines. Conversely, introduction to histidine at position 81 in the DRw53 beta chain restored normal levels of SEA/SEE binding. These data suggest that a binding site of SEA and SEE lies on the outer face of the beta chain alpha helix, pointing away from the antigen-binding groove.

Mouse autoreactive gamma/delta T cells. II. Molecular characterization of the T cell receptor

A panel of dendritic epidermal T cell (DETC) lines, and hybridomas derived from them, has been shown to spontaneously secrete lymphokines in the absence of added stimuli, which suggests that these cells are autoreactive. These cell lines are characterized by the expression of a V gamma 1.1C gamma 4/V delta 6 type T cell receptor (TcR), but several of the DETC lines also express a second TcR. Sequence analyses of these gamma/delta TcR revealed that the gamma chains were identical and that the delta chains, while not identical, were quite restricted in diversity, indicating that these receptors may recognize a common or closely related group of antigens. Analysis of hybridomas derived from newborn thymocytes identified six hybridomas that spontaneously secrete lymphokines. Five hybrids expressed a V gamma 1.1C gamma 4/V delta 6 receptor and one hybrid a V gamma 1.1C gamma 4/V delta 4 receptor that had a close structural relationship to the DETC gamma/delta TcR associated with spontaneous lymphokine secretion. gamma/delta TcR of the C gamma 4 type expressed by splenic hybridomas that did not spontaneously secrete lymphokines revealed no such relationship. Curiously, like the DETC, several of the thymocyte hybridomas that spontaneously secreted lymphokines expressed a second TcR, V gamma 2C gamma 1 or V gamma 3C gamma 1, apparently in association with the same delta chain that paired with the C gamma 4 chain. The presence of spontaneous lymphokine-secreting gamma/delta T cells with such highly homologous TcR in both the thymus and skin suggests a thymic origin for the autoreactive DETC and that these cells recognize a common or closely related group of self-antigens.

Superantigens

Over the past three years superantigens have come to the forefront of immunological research. Studies in a number of laboratories have indicated that superantigens play a central role in shaping the T-cell repertoire in the development of tolerance, autoimmunity and immunodeficiency. This is in addition to their classic role in the pathogenesis of a number of infectious diseases.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

Oligoclonality of human intestinal intraepithelial T cells

T cells bearing the T cell receptor alpha/beta (TCR-alpha/beta) are the predominant lymphocyte population in the human intestinal epithelium. To examine if normal intestinal intraepithelial lymphocytes (IEL) have a TCR repertoire distinct from the TCR-alpha/beta repertoire in peripheral blood lymphocytes (PBL), comparative analysis of relative V beta gene usage in IEL and PBL was performed by quantitative polymerase chain reaction. In each of the six individuals examined, one to three V beta families made up more than 40% of the total V beta transcripts detected in the IEL, whereas there was a more even distribution of V beta gene usage in the paired PBL. The predominant V beta families, especially V beta 1, V beta 2, V beta 3, and V beta 6, were frequently shared among IEL of different individuals. PCR cloning and sequence analysis of the predominant V beta 6 family in two individuals revealed an identical V-D-J-C sequence in 13 of 21 clones obtained from one donor, and a different repeated sequence in 18 of 27 clones examined in the second donor. These data suggest that the V beta skewing in IEL is due to an oligoclonal T cell expansion and may reflect the response of the intestinal mucosal immune system to a restricted set of as yet undefined antigens present in the gut.

Retroviral super-antigens and T cells

For many years immunologists have been intrigued by a series of potent antigens encoded in the murine genome. These antigens, originally termed minor lymphocyte stimulating (Mls) antigens, are capable of inducing extremely strong T cell proliferative responses when presented in the context of MHC class II molecules. Recently, Mls antigens have been shown to stimulate T cells bearing particular T cell receptor V beta elements, leading to the designation of super-antigens. The endogenous expression of these super-antigens in mice results in the clonal elimination of large numbers of T cells in order to maintain self-tolerance. In this review we discuss the recent identification of endogenous super-antigens as retroviral gene products. In addition, we analyze the role of class II MHC molecules in the presentation of endogenous super-antigens to T cells. Finally, we discuss the dramatic effect of retroviral super-antigens on the T cell repertoire.

T cell receptors, immunoregulation, and autoimmunity

T cell receptor (TCR) peptide vaccines have proven useful in the prevention and treatment of autoimmune disease in animal models. Prospects for developing TCR peptide vaccines for human autoimmune disease are only now being explored. Preliminary indications provide cause for optimism that immunization with TCR peptides eventually will be a viable treatment option for autoimmune pathologies in humans. In the long term, development of this technology may permit reliable manipulation of T cell immunity, leading to treatments for autoimmunity, T lymphoproliferative disorders, and, in the broadest interpretation, any pathogenesis mediated by oligoclonal T cell populations.

Streptococcus associated toxic shock

In the past few years, there appears to have been a change in the spectrum of disease caused by group A beta-haemolytic streptococcus (GABHS), and a toxic shock-like syndrome caused by this organism has recently been described in adults. We report four children with an acute illness characterised by rapid progression of shock, erythematous rash, multisystem organ involvement, electrolyte derangements, and desquamation who fulfil the previously established diagnostic criteria for toxic shock syndrome. Three of the children had extensive cutaneous and soft tissue infection and the fourth had peritonitis. All four developed bacteraemia. Treatment included aggressive cardiovascular resuscitation and antibiotic therapy. Although no patient died, they suffered multiple and severe complications requiring prolonged treatment and hospitalisation. Streptococcal toxic shock syndrome is a separate and clearly defined entity occurring in previously healthy children.

Immunopotentiation of anti-viral and anti-tumor immune responses using anti-T cell receptor antibodies and mitogens

Although the immunosuppressive properties of anti-CD3 mAbs are now widely recognized, we have accumulated data characterizing the T cell activating properties of these antibodies. While in some situations these activating properties may be viewed as unwanted side-effects (for instance OKT3-mediated T cell activation may be responsible for some of the first dose toxicity seen with patients receiving OKT3 for suppression of allograft rejection), we have shown that anti-CD3 mAb therapy can augment host immune responses and provide protection against some tumors and viral infections. Importantly, this augmented response allows the development of long term, specific immunity. Because the immunosuppressive and activating properties of anti-CD3 mAbs are so closely overlapping, we have sought to identify other agents that are capable of activating T cell subsets selectively. We have found that SEB activates T cell subsets selectively in vivo and that this activation can be exploited to prevent the outgrowth of a malignant murine tumor. Studies currently in progress, including phenotypic and functional analysis of TILs and in vivo T cell subset depletions, should result in a more precise understanding of how SEB-induced T cell activation inhibits tumor growth.

Limited T-cell receptor beta-chain heterogeneity among interleukin 2 receptor-positive synovial T cells suggests a role for superantigen in rheumatoid arthritis

Rheumatoid arthritis (RA) is a disease affecting the synovial membranes of articulating joints that is thought to result from T-cell-mediated autoimmune phenomena. T cells responsible for the pathogenesis of RA are likely present in that fraction of synovial T cells that expresses the interleukin 2 receptor (IL-2R), one marker of T-cell activation. We report herein an analysis of T-cell receptor (TCR) beta-chain gene expression by IL-2R-positive synovial T cells. These T cells were isolated from uncultured synovial tissue specimens by using IL-2R-specific monoclonal antibodies and magnetic beads, and TCR beta-chain transcription was analyzed by PCR-catalyzed amplification using a panel of primers specific for the human TCR beta-chain variable region (V beta). Multiple V beta gene families were found to be transcribed in these patients samples; however, three gene families, V beta 3, V beta 14, and V beta 17, were found in a majority of the five synovial samples analyzed, suggesting that T cells bearing these V beta s had been selectively retained in the synovial microenvironment. In many instances, the V beta 3, V beta 14, or V beta 17 repertoires amplified from an individual patient were dominated by a single rearrangement, indicative of clonal expansion in the synovium and supportive of a role for these T cells in RA. Of note is a high sequence similarity between V beta 3, V beta 14, and V beta 17 polypeptides, particularly in the fourth complementarity-determining region (CDR). Given that binding sites for superantigens have been mapped to the CDR4s of TCR beta chains, the synovial localization of T cells bearing V beta s with significant CDR4 homology indicates that V beta-specific T-cell activation by superantigen may play a role in RA.

Profound alteration in an alpha beta T-cell antigen receptor repertoire due to polymorphism in the first complementarity-determining region of the beta chain

Amino acid residues that are critical in maintaining the framework structure of immunoglobulin heavy- and light-chain variable (V) regions are strongly conserved in the V alpha and V beta proteins of the alpha beta T-cell antigen receptor (TCR alpha beta). Consequently, it has been proposed that TCR alpha beta has a conformation similar to that of an immunoglobulin Fab fragment and that the regions of the TCR homologous to the three immunoglobulin complementarity-determining regions (CDRs 1, 2, and 3) bind to the peptide antigen-major histocompatibility complex (MHC) molecule ligand. A single amino acid substitution in the predicted CDR1 of the V beta 3 protein of certain mouse strains dramatically altered TCR alpha beta usage in an antigen-specific MHC-restricted immune response but did not abrogate V beta 3 specificity for the superantigens minor lymphocyte stimulatory locus (Mls)c and staphylococcal enterotoxin A (SEA). The results confirm the importance of the V beta CDR1 in antigen-MHC molecule recognition, supporting the Fab-like structural model of TCR alpha beta, and provide further evidence that conventional antigen-MHC recognition and superantigen recognition are mediated by distinct regions of the TCR beta chain. They also suggest that allelic polymorphism may be a significant source of diversity in the TCR repertoire.

The V beta 17+ T cell repertoire: skewed J beta usage after thymic selection; dissimilar CDR3s in CD4+ versus CD8+ cells

To ascertain how the actual repertoire of T cell receptors (TCRs) deviates from the theoretical, we have generated a large number of junctional region sequences from TCRs carrying the V beta 17 variable region. The greater than 600 sequences analyzed represent transcripts from nine different cell populations, permitting several comparisons: transcripts from an expressed vs. a non-expressed V beta 17 allele, those from E+ vs. E- mice, transcripts from immature vs. mature thymocytes, those from thymic vs. peripheral T cells, and those from CD4+ vs. CD8+ cells. These comparisons have allowed us to distinguish between the influence of molecular events involved in TCR gene rearrangement and that of various selection events that shape the T cell repertoire. Our most striking findings are: (a) that J beta usage is markedly skewed, partly due to recombination mechanics and partly due to selection forces: in particular, those mediated by the class II E molecule in the thymus; and (b) that TCRs on CD4+ and CD8+ cells show intriguing dissimilarities. In addition, we present evidence that N nucleotide additions occur with clear biases, probably due to idiosyncrasies of the recombination enzymes, and provide arguments that TCR and immunoglobulin CDR3s have distinct structures.

Crosslinked staphylococcal enterotoxin B stimulates CD8+ T cells only in the presence of unlinked costimulator signals

The superantigen staphylococcal enterotoxin B (SEB) binds to class II MHC expressing cells and subsequently causes selective activation of T cells carrying appropriate T cell receptor (TCR) V beta chains. Apparently SEB acts as a bifunctional molecule by bridging class II MHC structures with the appropriate TCR-V beta chains. This assumption predicts that immobilized SEB ought to stimulate purified, class II MHC negative murine T cells. We show here that immobilized SEB lacks the ability to trigger murine CD8 T cells. Responsiveness obtained at a high T cell concentration is due to contaminating class II MHC-positive lymphocytes. Complementation of the culture system with syngeneic irradiated B cells blasts effectively restores responsiveness. The proliferating cells exhibit SEB specific cytotoxicity and a bias for V beta 8 expression. Since no evidence for leakiness of SEB covalently bound to sephadex beads was obtained, the data imply that immobilized SEB in fact binds to the TCR of T cells expressing the appropriate V beta chains. However, for primary activation additional costimulatory signals are required which can be provided in an unlinked fashion by activated B cells. Resting B cells are activated by immobilized SEB to cells expressing high costimulator activity. As such, the data point out a third function of SEB.

Evidence for quantitative and qualitative differences in functional activation of Mls-reactive T cell clones and hybridomas by antigen or TcR/CD3 antibodies

In this study, we demonstrated that some V beta 6+, CD4+, Mls-1a-specific T cell clones had cytolytic activity when stimulated with anti-T cell receptor(TcR)/CD3 monoclonal antibodies (mAb), but not with targets expressing Mls-1a, although they produced lymphokines (interleukin 2 and interferon-gamma) in response to both types of stimuli. To examine the possibility that lack of cytolysis resulted from expression of the Mls-1a antigen on merely a fraction of splenic B blasts, we (a) used the B cell lymphoma LBB.3.4.16 and (b) measured esterase secretion which is generally concurrent with cytotoxic T lymphocyte (CTL) activity. The B cell lymphoma maximally stimulated the T cell clone for interferon-gamma production when responding and stimulating cells were incubated at a 1:1 ratio, but it was never killed by the Mls-1a-specific T cell clone unless TcR/CD3-specific mAb were added. Furthermore, a fivefold excess of the Mls-1a B cell lymphoma did not induce any secretion of esterase, which was observed only in the presence of the TcR/CD3-specific mAb. Comparison of the reactivity of two Mls-1a-specific T cell hybridomas expressing the same TcR at similar surface density, revealed both quantitative and qualitative differences between CD3-specific mAb and Mls stimulation of the hybridomas. A small quantitative difference in the sensitivity of hybridoma FJ22.5 to stimulation with V beta 6 or CD3-specific mAb resulted in a marked decrease in efficiency of stimulation by Mls-1a for interleukin 2 production and to inability to detect growth inhibition by Mls-expressing cells. A qualitative difference was observed when analyses of inositol phosphate production were performed under optimal conditions of stimulation of the highly responsive T cell hybridoma (FJ8.1): only stimulation with CD3-specific mAb, but not Mls-expressing cells, could induce detectable inositol phosphate production. Lack of cytolysis of Mls-1a class II-expressing B cells may have evolutionary significance in view of the recent mapping of Mls to mouse mammary tumor virus genes.

Staphylococcus aureus enterotoxin mediated specific non-responsiveness of human T cells

Exotoxins produced by certain strains of Staphylococcus aureus are able to both stimulate and induce non-responsiveness in T cells expressing specific T cell antigen receptor V beta gene elements. The exposure of human CD4+ T cells to the appropriate enterotoxin rendered them anergic to restimulation with their natural ligand, although responsiveness to exogenous IL-2 remained intact. The loss of antigen-dependent proliferation was associated with the down-regulation of the TCR complex that was paralleled by enhanced cell surface CD2 and CD25. Further analysis of the phenotypic changes revealed that membrane levels of CD28 were increased only on activation, suggesting a differential expression of this protein on activated and anergic T cells. During the induction of anergy it was observed that the synthesis of the lymphokines IL-2, IL-4 and IFN-gamma was differentially regulated. IL-4 and IFN-gamma, but not IL-2 were detected in the supernatants of overnight cultures of T cells exposed to tolerising concentrations of toxin. Transcription of IL-4, as determined by polymerase chain reaction at selected intervals, was elevated during the induction of anergy and accounted for the presence of the protein in the supernatants. In contrast, no tight coupling was observed between protein and mRNA levels for IL-2, suggesting post-translational regulation.

A method for production of antibodies to human T-cell receptor beta-chain variable regions

Mouse T-cell hybridomas bearing human V beta elements were produced by transfection of human/mouse hybrid T-cell receptor beta-chain genes into a mouse T-cell hybridoma lacking an endogenous beta-chain gene. These hybridomas were entirely mouse in origin except for the human V beta region. These cells were used to immunize mice against human V beta elements. Mouse monoclonal antibodies have thus been generated against human V beta 13.1 and -13.2. We expect that the method outlined in this paper will be useful in the production of monoclonal antibodies specific for other human V beta or V alpha elements.

Conventional antigen and superantigen may be coupled to distinct and cooperative T-cell activation pathways

CD4+ T cells are equipped to detect two major classes of ligands. Infectious microbial agents, including bacteria and retroviruses, carry a class of proteins termed superantigens that are recognized by the T-cell receptor in association with class II products of the major histocompatibility complex. Proteins expressed by other cells and organisms are processed by macrophages into peptides that are presented to CD4+ T cells by class II molecules. We have examined CD4+ T-cell clones that proliferate vigorously in response both to conventional peptide antigens and to bacterial or retroviral superantigens. The response to peptide antigen is characterized by a rapid and sustained increase in the levels of intracellular free Ca2+ and a vigorous cytokine response. In contrast, the proliferative response of these clones to bacterial or retroviral superantigen is not accompanied by detectable increases in intracellular Ca2+ or by significant cytokine production. Further analysis of T-cell activation indicates that interaction of a single T-cell receptor with the two types of ligand may be coupled to functionally distinct signaling pathways that interact in a synergistic fashion to achieve T-cell activation.

An additional hypervariable region encoded by V gene segments occurs in Tcr V beta at a location compatible with its involvement in Tcr active site--a general model for alloreactivity

The number of V alpha and V beta sequences of T cell receptors now available allows a meaningful analysis of their variability profiles. Variability plots were derived using a modified form of Wu and Kabat's algorithm: variability is not computed as a proportion of the number of different residues occurring at a position, but rather proportionally to the physicochemical differences between the different residues. Results show that the classical hypervariable regions occurring in immunoglobulins also occur in T cell receptors at equivalent positions. Contrary to immunoglobulins the framework of Tcr V regions displays many relatively variable regions and positions. This phenomenon can be connected with the genetic organization of V genes of T cell receptors which seem to avoid any framework homogenization and the resulting gene conversion. More importantly an additional hypervariable region was detected in V beta but not in V alpha. This fourth hypervariable region is located between the second and the D hypervariable CDR. The predicted three-dimensional location of this additional hypervariable region is compatible with a possible role in antigen recognition and therefore also in positive and/or negative selection. Furthermore our data suggest that this fourth hypervariable region is involved in the recognition of superantigens like bacterial enterotoxins. Indeed this additional hypervariable region is not detected when variability is derived using an alignment of the V beta subgroups stimulated by one toxin of S. aureus. Finally we propose a new and simple molecular model to explain alloreactivity as crossreactivity between the universe of shapes (isomers of conformation) of different MHC haplotypes.

T-cell recognition of myelin basic protein

Multiple sclerosis is a chronic inflammatory disease of the central nervous system which has been hypothesized to be autoimmune in nature. To test whether this is the case, Kai Wucherpfennig and colleagues have developed a set of criteria that must be met to satisfy the hypothesis. Here, they present these criteria and assess the extent to which studies to date satisfy them.

Measurement of ligand-induced activation in single viable T cells using the lacZ reporter gene

We have used the bacterial beta-galactosidase gene (lacZ) as a reporter gene for the rapid measurement of T-cell antigen receptor (TCR)-mediated activation of individual T cells. The reporter construct contained the lacZ gene under the control of the nuclear factor of activated T cells (NF-AT) element of the human interleukin 2 enhancer [Fiering, S., Northrop, J. P., Nolan, G. P., Matilla, P., Crabtree, G. R. & Herzenberg, L. A. (1990) Genes Dev. 4, 1823-1834]. The activity of the intracellular lacZ enzyme was analyzed by flow cytometric measurement of fluorescein accumulation in cells loaded with the fluorogenic beta-galactosidase substrate fluorescein di-beta-D-galactopyranoside. As a model system, the T-cell hybridoma BO4H9.1, which is specific for the lysozyme peptide (amino acids 74-88)/Ab complex, was transfected with the NF-AT-lacZ construct. lacZ activity was induced in 50-100% of the transfectant cells following exposure to pharmacological agents, to the physiological peptide/major histocompatibility complex ligand, or to other TCR-specific stimuli. Interestingly, increasing concentrations of the stimulus increased the fraction of lacZ+ cells, but not the level of lacZ activity per cell. Even under widely varying levels of stimulus, the level of lacZ activity in individual lacZ+ cells remained within a remarkably narrow range. These results demonstrate that TCR-mediated activation can be readily measured in single T cells and strongly suggest that, once committed to activation, the level of NF-AT transcriptional activity in individual T cells is independent of the form or concentration of stimulus. This assay is likely to prove useful for the study of early activation events in individual T cells and of TCR ligands.

T lymphocyte-stimulating microbial toxins as "superantigens"

Infectious pathogens generally have to cope with the host's adaptive immune system, i.e., T and B lymphocytes. Common evasion mechanisms in this complex interaction are antigenic variations, the escape to immunologically priviledged sites or the use of immunosuppressive mechanisms. Many bacteria and other microorganisms eleborate soluble factors or toxins that act suppressively on cells of the immune system, such as pore-forming molecules or proteins that interfere with the function of G proteins. Gram-positive cocci and a mycoplasma have developed an extremely potent mechanism of T cell stimulation by closely mimicking recognition of specific antigen. From the functional similarity to antigen recognition and the multiclonal activation of T cells the designation "superantigens" has been suggested for these molecules.

Analysis of the interaction site for the self superantigen Mls-1a on T cell receptor V beta

Superantigen bound to major histocompatibility complex (MHC) products have been shown to stimulate T cells in a V beta-specific manner. Mouse T cells bearing V beta 8.1 usually respond to the self superantigen, Mls-1a, whereas T cells bearing V beta 8.2a do not. Previously, using site-directed mutational analysis, we identified the residues of natural variants of T cell receptor (TCR) V beta 8.2 that conferred Mls-1a reactivity. These residues are predicted to lie on a beta-pleated sheet of the TCR V beta element, well away from the expected binding site for antigen and MHC proteins. This study was undertaken to determine the effect of glycosylation on this beta-pleated sheet on Mls-1a reactivity and to map the extent of the interaction site on V beta 8.2 for Mls-1a. to Mls-1a, as well as to peptides derived from the conventional protein antigen, chicken ovalbumin. Here we demonstrate that first, N-linked carbohydrate on the lateral surface of V beta blocks the interaction of the TCR V beta with the self superantigen, Mls-1a, but has no effect on the TCR interaction with peptide antigen and MHC, second, that the interaction site for Mls-1a extends over the surface of the solvent-exposed beta-pleated sheet on the side of the TCR, and third, that mutations which affect both superantigen and peptide antigen reactivity lie at the beginning of the first complementarity determining region of V beta, consistent with models of the trimolecular complex of TCR-peptide-MHC.

Staphylococcal-enterotoxin-dependent cell-mediated cytotoxicity

T cells equipped with sophisticated TCR and MHC recognition structures, an efficient cytokine communication network and lethal cytotoxic effector functions constitute one of the bulwarks of the mammalian immune system. However, infective agents have developed strategies to undermine T-cell immunity; for example, certain bacterial toxins serve as 'superantigens' by binding to preserved determinants on MHC class-II-encoded proteins and activating T cells expressing particular sequences of TCR V beta gene products. In this paper, Mikael Dohlsten and colleagues present evidence suggesting that these bacterial superantigens direct T cells to eradicate MHC class-II-expressing antigen-presenting cells, thus counteracting specific T-cell functions.

In vivo T-cell activation by staphylococcal enterotoxin B prevents outgrowth of a malignant tumor

Treatment of T cells with staphylococcal enterotoxins in vitro is known to activate T cells in a subset restricted manner based on beta-chain variable region (V beta) gene expression. In particular, staphylococcal enterotoxin B (SEB) activates T cells bearing V beta 7 or V beta 8. We examined the ability of SEB to activate T cells in vivo. Treatment of C3H mice with doses of SEB ranging from 5 to 250 micrograms resulted in a dose-dependent activation of V beta 8+ T cells as reflected by increased interleukin 2 receptor (IL-2R) expression, proliferation to exogenous IL-2 and allogeneic cells, and production of gamma interferon. SEB also caused proliferation of the CD8+ subset of V beta 8+ cells in vivo. Thus, T-cell activation by SEB in vivo appears to be specific since V beta 2+ cells (non-SEB reactive) did not show increases in IL-2R expression similar to those seen with V beta 8+ cells nor did they proliferate. We then studied the ability of these activated cells to potentiate the immune response to a malignant progressor tumor. Treatment of C3H mice with 50 micrograms of SEB at the time of inoculation with tumor fragments resulted in a statistically significant decrease in the frequency of tumor outgrowth. These data demonstrate that treatment of C3H mice with SEB results in specific activation of V beta 8+ cells in vivo and that these activated cells are capable of preventing the outgrowth of a malignant tumor.

Direct binding of secreted T-cell receptor beta chain to superantigen associated with class II major histocompatibility complex protein

The interaction of the T-cell receptor (TCR) with peptide antigen plus major histocompatibility complex (MHC) protein requires both alpha and beta chains of the TCR. The "superantigens" are a group of molecules that are recognized in association with MHC class II but that do not appear to conform to this pattern. Superantigens are defined as such because they cause the activation or thymic deletion of many or all T cells bearing specific TCR beta-chain variable region (V beta) elements. The strong association of particular V beta S with T-cell responses to superantigens suggests that their interaction with the TCR is fundamentally different from that of most antigens. We have directly investigated the involvement of the beta chain in recognition of a superantigen by using a secreted, truncated TCR beta chain and the bacterial superantigen staphylococcal enterotoxin A complexed to cell-surface MHC class II. We demonstrate that this interaction is specific for the enterotoxin and is dependent on MHC class II expression by the cell. The reaction can be inhibited by antibodies against the three components of the reaction: V beta, enterotoxin, and class II. This shows that the TCR beta chain is sufficient to mediate the interaction with a superantigen-class II complex. The TCR alpha chain and co-receptors such as CD4 are not required.

Allelic variations of human TCR V gene products

A central problem confronting the immune system is how to discriminate among vast numbers of antigens. Novel genetic ploys that aid the discriminative process, including complex gene rearrangements (in antibody and T-cell receptor (TCR) genes) and extensive allelic polymorphism (in major histocompatibility complex (MHC) genes), have been described. Recent evidence has suggested a further level of diversity; TCR V gene allelic variation. In this article David Posnett summarizes evidence in favour of this possibility and speculates on the possible functional consequences of TCR allelism.