The I-A beta b region (65-85) is a binding site for the superantigen, staphylococcal enterotoxin A

Superantigens: The Good, the Bad, and the Ugly

Increasing evidence suggests that superantigens play a role in Immune-mediated diseases. Superantigens are potent activators of CD4* T cells, causing rapid and massive proliferation of cells and cytokine production. This characteristic of superantigens can be exploited in diseases where strong immunologic responses are required, such as in the B16F10 animal model of melanoma. Superantigen administration is able to significantly enhance Ineffective anti-tumor Immune responses, resulting in potent and long-lived protective anti-tumor immunity. However, superantigens are more well-known for the role they play in diseases. Studies using an animal model for neurologic demy-elinatlng diseases such as multiple sclerosis show that superantigens can induce severe relapses and activate auto-reactive T cells not involved in the Initial bout of disease. This may also involve epitope spreading of disease. Superantigens have also been implicated in acute diseases such as food poisoning and TSS, and in chronic diseases such as psoriasis and rheumatoid arthritis. Viral superantigens are also involved in the disease process, including superantigens derived from human Immunodeficiency virus and mouse mammary tumor virus. Finally, immunotherapies that ameliorate the role played by superantigens in disease are discussed.

Construction and characterization of a novel fusion protein MG7-scFv/SEB against gastric cancer

Antibody-targeted superantigen has been developed into a new strategy to treat many malignant tumors. In this study, for specific targeting to gastric cancer cell, superantigen SEB (Staphylococcal Enterotoxin B) was genetically fused to the single-chain variable fragment of gastric carcinoma-associated antibody MG7(MG7-scFv) that recognizes the MG7 antigen frequently expressed in gastric cancer cell. The recombinant MG7-scFv/SEB fusion proteins are expressed in E. coli as inclusion bodies, and the purified MG7-scFv/SEB retains high binding affinity with gastric cancer cell SGC-7901 (positive MG7 antigen expression). When incubated with effector cell-peripheral blood mononuclear cells (PBMCs), MG7-scFv/SEB could effectively inhibit the proliferation and induce apoptosis of SGC-7901. After being treated with MG7-scFv/SEB, PBMCs remarkably increased the production of Th1 cytokines (IFN-gamma, IL-2), and slightly increased the production of Th2 cytokines (IL-4, IL-10) in vitro. It was observed that gastric-tumor-bearing rats administrated with MG7-scFv/SEB showed more inflammatory cell infiltration, more significant tumor inhibition, and longer survival time than those of rats treated with SEB or NS (Normal Saline). The data indicated that MG7-scFv/SEB fusion protein could specifically target gastric cancer cell, enhance the activity of T cells and induce tumor cell apoptosis to exert the antitumor effect on gastric cancer.

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.

Modulation of disease by superantigens

Increasing evidence suggests that bacterial and viral superantigens are involved in immune-mediated disease. Studies using an animal model for multiple sclerosis show that superantigens can induce relapses and bring into play autoreactive T cells with restricted usage of T cell receptor V beta families that may be indirectly involved in the initial episode of disease. This may also involve epitope spreading. Superantigens have also been implicated in other autoimmune diseases such as rheumatoid arthritis and psoriasis. Superantigens encoded by viruses such as mouse mammary tumor virus play an important role in disease progression.

Functional activity of staphylococcal enterotoxin A requires interactions with both the alpha and beta chains of HLA-DR

The staphylococcal enterotoxins, SEA and SEE, bind one zinc atom per molecule of protein. The presence of this metal atom enhances the binding of the toxins to MHC class II molecules, presumably through an interaction with histidine 81 of the beta chain. L cell transfectants expressing HLA-DR1 and HLA-DR7 molecules, with mutations in either the alpha1 or beta1 domains, were tested for their ability to bind SEA and present it to T cells. Cells expressing DR1 molecules with alanine at positions 77, 78, 80, 83, 84 and 85, or serine at position 79 could all bind SEA and present it to either polyclonal or monoclonal T cells. Most point mutations within the alpha-helical portion of the DR7 beta chain had no effect on binding and presentation. However, substitution of histidine 81 with alanine, glutamate, or aspartate, abrogated SEA binding as well as T cell stimulation by the superantigen. This effect was also observed when the non-polymorphic aspartate, at position 76 was changed to alanine. Mutation of the asparagine at position 82 had an intermediate effect. Point mutations of the DR alpha chain had little effect on binding of SEA as determined by a flow cytometric assay. However, mutation of lysine at position 39 of the alpha chain and, to a lesser extent methionine at position 36, markedly decreased the ability of SEA to stimulate toxin-responsive mouse T cell hybridomas. Finally, the monoclonal antibody, L243 binds to the alpha chain of HLA-DR, and was able to block T cell activation by SEA without blocking SEA binding. These data support the model whereby HLA-DR has two binding sites for SEA. A low affinity site, present on the alpha chain, is required for T cell stimulation by the superantigen, but is insufficient to mediate toxin binding. High affinity binding of HLA-DR to SEA occurs solely through residues on the beta chain, including both histidine 81 and aspartate 76.

The Staphylococcus aureus enterotoxin B superantigen induces specific T cell receptor down-regulation by increasing its internalization

Superantigens are able to stimulate T lymphocyte populations expressing T cell antigen receptors (TCR) belonging to particular V beta families. Moreover, the presence of these superantigens may induce long term unresponsiveness (anergy) of these sensitive cells. Some bacterial toxins are potent superantigens. We have analyzed in vitro the capacity of some Staphylococcus aureus enterotoxin superantigens to modulate T cell antigen receptor expression and the cellular mechanisms involved. Staphylococcus enterotoxin B (SEB) induced rapid down-regulation of surface T cell antigen receptors in V beta 3-expressing T lymphocytes, as assessed by flow cytometry. This phenomenon was a consequence of the direct interaction between the toxin and the TCR since it was observed in the absence of cells expressing major histocompatibility complex class II molecules. The cellular mechanism involved in SEB-induced down-regulation of TCR was further investigated. Immunofluorescence and confocal microscopy experiments showed that toxin B induced intracellular accumulation of TCR.CD3 in endocytic vesicles. Moreover, SEB induced an increase in T cell receptor endocytosis as measured using radiolabeled Fab fragments of an anti-CD3 monoclonal antibody. Taken together, our observations indicate that Staphylococcus enterotoxin B superantigen induced changes in the dynamics of surface T cell receptors, which resulted in the fast reduction of membrane receptor numbers.

Predictions of T-cell receptor- and major histocompatibility complex-binding sites on staphylococcal enterotoxin C1

We have focused on regions of staphylococcal enterotoxin C1 (SEC1) causing immunomodulation. N-terminal deletion mutants lacking residues 6 through 13 induced T-cell proliferation similar to that induced by native toxin. However, mutants with residues deleted between positions 19 and 33, although nonmitogenic themselves, were able to inhibit both SEC1-induced T-cell proliferation and binding of the native toxin to major histocompatibility complex (MHC) class II. Presumably, these deletions define a part of SEC1 that interacts with the T-cell receptor. Three synthetic peptides containing residues located in a region analogous to the alpha 5 groove of SEC3 had residual mitogenic activity or blocked T-cell proliferation induced by SEC1 and appear to recognize the same site as SEC1 on a receptor for the toxin, presumably MHC class II. We conclude that isolated portions of the SEC1 molecule can retain residual mitogenic activity but that the entire protein is needed to achieve maximal superantigenic stimulation. Our results, together with the results of other investigators, support a model in which SEC1 binds to an alpha helix of MHC class II through a central groove in the toxin and thereby promotes or stabilizes the interaction between antigen-presenting cells and T cells.

Use of staphylococcal enterotoxin A-induced lymphoproliferation and interleukin 2 production as indicators of immunotoxicity

Suppression of mitogen-induced splenocyte lymphoproliferation and interleukin 2 (IL-2) production can be used as indicators of immunotoxicity. Staphylococcal enterotoxin A (SEA) is both a potent mitogen and the most potent in vitro inducer of IL-2 production that has been described. An in vitro system was used to measure impairment of SEA-induced lymphoproliferation and IL-2 production using splenocytes from female C57BL/6 mice dosed with either cyclosporin A (30 mg/kg/day, 14 days), benzene (220, 440, or 880 mg/kg/day, 14 days), or vehicle. Splenocytes were stimulated with either concanavalin A (con A) or SEA. Benzene- and cyclosporin A-treated mice demonstrated significant decreases in splenocyte proliferation. IL-2 production was determined by incubating splenocyte culture supernatants with IL-2 dependent cytotoxic T-cells (CTLL-2), pulsing with 3H-thymidine, and determining amount of incorporated label. Cell proliferation and IL-2 production were inhibited by both benzene and cyclosporin A, effects more clearly demonstrated using SEA than con A. SEA was a superior mitogen compared to con A in the assays evaluated here.

Stimulation of rat spleen cells by staphylococcal enterotoxins

There is much interest in staphylococcal enterotoxins as T cell mitogens in humans, mice and rabbits. Rat spleen cells were shown to proliferate in response to staphylococcal enterotoxins A and B and toxic shock syndrome toxin-1 at concentrations (5 to 500 ng ml-1) which also stimulate mouse spleen cells. The proliferative response to all these enterotoxins was inhibited by cyclosporin A, indicating the response to be predominantly that of T cells. These results indicate that the rat provides another convenient model for the analysis of T cell responses to enterotoxins.

Bacterial and retroviral superantigens share a common binding region on class II MHC antigens

Staphylococcal enterotoxin A (SEA), one of the most potent T-cell mitogens known, has been classified as a bacterial superantigen on the basis of ability to stimulate V beta-specific T-cell subsets. SEA interacts with class II major histocompatibility complex (MHC) antigens on antigen-presenting cells and the T-cell antigen receptor (TCR) on T cells, resulting in a ternary complex of MHC-SEA-TCR. Mls antigens are known to be products of mouse mammary tumour virus (MMTV), and it has been reported that two exogenous strains of MMTV encode retroviral superantigens in the open reading frames of the 3' long terminal repeat of the viral genome; however, no binding of the putative MMTV superantigen to either MHC antigens or TCR has been demonstrated. Here we use synthetic peptides to identify a site on the MMTV-1 superantigen that binds to class II MHC antigens. The site is encompassed by amino-acid residues 76-119 of the MMTV-1 superantigen. Direct binding and competition experiments show that the MMTV superantigen and SEA bind to at least one common region on class II MHC antigens.

The bacterial and mouse mammary tumor virus superantigens; two different families of proteins with the same functions

In conclusion, the bacterial toxins are completely unlike the MTV superantigens in primary sequence and structure. The former are soluble globular proteins which do not have to be proteolytically cleaved before they act. The latter are synthesized as type II membrane proteins and may be clipped before they reach the cell surface and act to stimulate T cells. Table III summarizes the similarities and differences between the two sets of superantigens. The most notable quality of these molecules is that both sets of families have developed strategies whereby they bind to Class II and engage V beta. As far as the microorganisms which produce them are concerned, these two properties appear to be essential since they are absolutely conserved over proteins of a number of different structures. Several questions can now be addressed as follows. a. Why do all known superantigens bind to Class II? For the microorganism which produces them, the function of superantigens appears to be T-cell and perhaps directly or indirectly B-cell and macrophage stimulation. Activation of virgin T cells requires engagement with antigen plus MHC on professional antigen-presenting cells. Unlike other cell surface proteins, for example Class I, most Class II in animals is expressed on such cells. Therefore it is likely that superantigens have evolved to engage Class II because presentation to T cells by Class II-bearing cells offers the superantigen the best chance of activating its target T cells. b. Why do superantigens engage TCR V beta and not V alpha or CD3? It is possible that superantigens bind to the V beta portion of the TCR rather than V alpha because the latter does not have a consistently well exposed face for engagement. The fact that it is perhaps relatively easier to produce anti-V beta rather than anti-V alpha antibodies supports this idea. We have shown that N-glycosylation of V beta can interfere with recognition by vSAGs (Pullen et al. 1991), perhaps glycosylation of V alpha tends to conceal otherwise available sites. As far as C beta, C alpha or CD3 engagement is concerned, this may be just too dangerous for MTVs. The role of MTVs SAgs in the life history of the virus seems to be to stimulate T cells in the suckling recipient and thereby create a pool of activated lymphocytes in which the virus may survive until the mouse gives birth and transmits the virus to her own progeny (Hainaut et al. 1990, Golovkina et al. 1992).(ABSTRACT TRUNCATED AT 400 WORDS)

Superantigens and their potential role in human disease

In the past few years, there has been a virtual explosion of information on the viral and bacterial molecules now known as superantigens. Some structures have been defined and the mechanism by which they interact with MHC class II and the V beta region of the T cell receptor is being clarified. Data are accumulating regarding the importance of virally encoded superantigens in infectivity, viral replication, and the life cycle of the virus. In the case of MMTV, evidence also suggests that superantigens encoded by a provirus may be maintained by the host to protect against future exogenous MMTV infection. Experiments in animals have also begun to elucidate the dramatic and variable effects of superantigens on responding T cells and other immune processes. Finally, the role of superantigens in certain human diseases such as toxic shock syndrome, some autoimmune diseases like Kawasaki syndrome, and perhaps some immunodeficiency disease such as that secondary to HIV infection is being addressed and mechanisms are being defined. Still, numerous important questions remain. For example, it is not clear how superantigens with such different structures, for example, SEB, TSST-1, and MMTV vSAG, can interact with MHC and a similar region of the TCR in such basically similar ways. It remains to be determined whether there are human equivalents of the endogenous murine MMTV superantigens. The functional role of bacterial superantigens also remains to be explained. Serious infection and serious consequences from toxin-producing bacteria are relatively rare events, and it is questionable whether such events are involved in the selection pressure to maintain production of a functional superantigen. Hypotheses to explain these molecules, which can differ greatly in structure, include T cell stimulation-mediated suppression of host responses or enhancement of environments for bacterial growth and replication, but substantiating data for these ideas are mostly absent. It also seems likely that only the tip of the iceberg has been uncovered in terms of the role of superantigens in human disease. Unlike toxic shock syndrome, other associations, especially with viral superantigens, may be quite subtle and defined only after considerable effort. The definition of these molecules and mechanisms of disease may result in new therapeutic strategies. Finally, it is apparent that superantigens have dramatic effects on the immune system. One wonders whether these molecules or modifications of them can be used as specific modulators of the immune system to treat disease.

T-cell antigen receptor binding sites for the microbial superantigen staphylococcal enterotoxin A

We have examined the interaction of the microbial superantigen staphylococcal enterotoxin A (SEA) with peptides corresponding to overlapping regions of the T-cell antigen receptor beta chain variable region V beta 3. SEA is known to stimulate murine T cells bearing certain V beta elements, among them V beta 3. Five peptides were synthesized representing amino acids 1-24, 20-44, 39-60, 57-77, and 74-95 of V beta 3. We demonstrate here that soluble V beta 3-bearing beta chains can bind to a complex of SEA and major histocompatibility complex class II and that the synthetic peptide V beta 3-(57-77) blocked this interaction. The peptide V beta 3-(57-77) also inhibited SEA-induced interferon-gamma production and SEA-induced proliferation of B10.BR spleen cells. Conversely, the peptide corresponding to amino acids 57-77 of V beta 8.2, a V beta element that is not recognized by SEA, decreased staphylococcal enterotoxin C-2-induced proliferation but did not affect SEA-induced proliferation. The peptide inhibition of SEA-induced function was due at least in part to inhibition of V beta 3-bearing T-cell activity, since the percentage of T cells reactive with an anti-V beta 3 monoclonal antibody was significantly reduced by V beta 3-(57-77). These data suggest that the region of V beta 3 encompassing amino acids 57-77 is an area that displays the appropriate sequence and conformation for binding of the SEA molecule and blocking of the resultant interaction with the T-cell antigen receptor.

Effect of isotypic and allotypic variations of MHC class II molecules on staphylococcal enterotoxin presentation to murine T cells

Staphylococcal enterotoxins (SE) are known to be potent T cell activators, stimulating +/- proliferation and lymphokine production. These toxins have recently have been termed "superantigens" because of their ability to bind directly to class II molecules forming a ligand that interacts with particular V beta gene elements within the TCR complex. This interaction between SE and MHC class II molecules plays a central role in toxin-induced mitogenesis. In the present study we have examined the effect of polymorphism on the ability of MHC class II molecules to bind and present SE. Through the use of H-2 congenic mouse strains, it was possible to look directly at haplotype differences within the MHC and their effect on SE presentation to a panel of responsive V beta-bearing T cells. The results demonstrate that toxin presentation by class II-bearing accessory cells to murine T cells is greatly affected by polymorphisms within the H-2 complex. Toxin-pulsed accessory cells obtained from mice of an H-2k and H-2u haplotype were found to be less efficient in activating a variety of T cell clones and hybridomas. However, one T cell clone responded similarly to the enterotoxins presented on all H-2 haplotypes, suggesting that differences in responses of T cells are not simply a function of the degree of binding of these toxins to various class II molecules. Neutralization analysis with monoclonal anti-class II antibodies demonstrates that both I-A and I-E molecules play a significant role in SEA and SEB presentation to murine T cells. These results suggest that the differential activation of T cells by a particular enterotoxin may reflect a difference in recognition of an SE:class II ligand by a surface T cell receptor complex.

Direct activation of murine T cells by staphylococcal enterotoxins

The capacity of staphylococcal enterotoxins to stimulate all T cells bearing certain TCR variable region alleles has generated a great deal of interest. This stimulation appears to involve specific binding of the toxin to class II molecules and subsequent stimulation of the T cell via the TCR V beta elements. Recent studies from our laboratory have focused on the ability of staphylococcal enterotoxins to directly activate purified lymph node T cells and a panel of T cell clones and hybridomas. A T cell costimulation assay was performed to assess cellular activation requirements and cytokine receptor expression. Activation of highly purified lymph node T cells by staphylococcal enterotoxin B (SEB) required costimulatory signals which could be provided by IL-1, IL-2, IL-4, or IL-6, whereas SEB alone demonstrated no significant proliferative response. Using a panel of TH1 and TH2 cell clones and T cell hybridomas possessing various responsive and nonresponsive V beta alleles, it was possible to demonstrate that SEA and SEB costimulate T cells via the TCR complex. Additionally, enterotoxin-pretreated T cells demonstrated a significant proliferative response upon exposure to class II-bearing accessory cells, suggesting that these toxins bind directly to T cells. Highly purified T cells cultured with both SEB and IL-1 exhibit significantly increased levels of IL-2 receptor, whereas cells cultured with SEB or IL-1 alone demonstrated low levels of this receptor. These results do not exclude an association of the staphylococcal enterotoxins with class II molecules in a manner which results in a high avidity binding to the TCR required for transduction of the appropriate activation signals. In the absence of class II molecules, however, these superantigens can still bind to T cells, and the activation signal is delivered in the presence of cytokines that trigger T cell growth and lymphokine production.

Evidence for the alpha-helicity of class II MHC molecular binding sites for the superantigen, staphylococcal enterotoxin A

Circular dichroism (CD) spectra of class II MHC peptides revealed the alpha-helical conformation of superantigen-binding peptides I-A beta b(60-90), I-A beta b(65-85), and I-A alpha b(51-80), but not the nonbinding peptide I-A beta b(80-100). These CD spectra provide biophysical evidence for the alpha-helicity of class II MHC molecular binding sites for the superantigen, staphylococcal enterotoxin A (SEA). Alanine-substituted analogs of the SEA binding-site peptide, I-A beta b(65-85), were used to implicate beta-chain residues 72 and 80 in class II MHC-SEA binding. The data support SEA binding away from the class II antigen binding cleft along the faces of the alpha-helices.

Identification of the staphylococcal enterotoxin A superantigen binding site in the beta 1 domain of the human histocompatibility antigen HLA-DR

The staphylococcal enterotoxin A (SEA) is a superantigen that must bind to class II molecules of the major histocompatibility complex to be recognized by T cells. In humans, most HLA-DR class II allelic and isotypic forms, such as DR1, bind SEA well. DRw53 is an exception, binding SEA very poorly. We have localized this difference to a single residue (amino acid 81) in the beta 1 domain. A highly conserved histidine at residue 81 allows SEA binding, but a tyrosine does not. Residue 81 is predicted to lie in an alpha-helix on the surface of the molecule, with its side chain pointing up out of the pocket associated with binding of conventional peptide antigens. This finding supports the hypothesis that superantigens and conventional antigens bind to different sites on the class II molecule.

Both alpha-helices along the major histocompatibility complex binding cleft are required for staphylococcal enterotoxin A function

The superantigen staphylococcal enterotoxin A (SEA) requires interaction with class II major histocompatibility complex (MHC) molecules to activate T cells. We have previously used the synthetic peptide approach to establish one side of the hypothetical class II foreign-antigen binding cleft, alpha-helical region 65-85 of the beta chain, as a binding site involved in accessory cell presentation of SEA to T cells. To further characterize the structural basis for MHC-SEA interaction we have examined the role of the alpha-helical regions of the class II alpha and beta chains in SEA function. Using the synthetic peptide approach, we have found that both alpha-helical regions are required for SEA-induced proliferation. Their corresponding peptides directly bound SEA. Although the beta-chain peptides were able to inhibit SEA binding to human and mouse cells, the alpha-chain peptides were not. The data suggest that the alpha-helices along both sides of the hypothetical class II MHC molecule binding cleft are required for SEA-induced function, whereas the beta-chain alpha-helix is sufficient for SEA binding. A model of superantigen presentation is proposed wherein the MHC beta chain, possibly region 70-80, interacts with SEA region 1-45, whereas another region of SEA binds region 51-80 of the alpha chain.

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.

Structural basis for differential binding of staphylococcal enterotoxin A and toxic shock syndrome toxin 1 to class II major histocompatibility molecules

The related staphylococcal toxins staphylococcal enterotoxin A (SEA) and toxic shock syndrome toxin 1 (TSST-1) are microbial superantigens. They require interaction with class II major histocompatibility complex (MHC) molecules to activate T cells. We have previously identified a binding site on SEA, the N-terminal 45 amino acids, as well as its corresponding receptor on the MHC antigen, residues 65-85 of the beta chain. To further characterize the structural basis for SEA binding to class II MHC molecules we have examined its relationship to TSST-1 binding. Both toxins bound similarly to murine A20 cells, but blockage of binding was observed only with the homologous toxin, which suggests that the binding sites for the two toxins on A20 cells are distinct. In contrast, specific binding of SEA was greater than that of TSST-1 on human Raji cells. Further, SEA was a better inhibitor of TSST-1 binding than was TSST-1 itself at low concentrations, but TSST-1 only minimally inhibited SEA binding. The data suggest that TSST-1 interacts with Raji cells at an SEA binding site, but with a lower affinity. The peptides SEA-(1-45) and I-A beta b-(65-85) were capable of blocking SEA binding on both A20 and Raji cells, but blockage was more effective on A20 cells. Neither peptide was capable of blocking TSST-1 binding on either cell line. The data are compatible with a model in which SEA has a binding site on A20 cells involving SEA-(1-45) and I-A beta b-(65-85) which is distinct from that which binds TSST-1, while at least two binding sites are present on Raji cells. One site involves predominantly the residue 1-45 region on SEA and the 65-85 region of the MHC beta chain, while the other site involves both a different region on the SEA molecule and a different site on the class II MHC molecule to which it binds. This latter site also binds TSST-1.