Superantigens: mechanism of T-cell stimulation and role in immune responses

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.

An alternative approach to the assessment of gamma delta T-cell clonality in celiac disease intestinal lesions through cDNA heteroduplex analysis of T-cell receptor VJ junctions

We have investigated the clonality of the gamma delta T lymphocytes infiltrating the intestinal mucosa of CD patients and control subjects by means of a simple and powerful method based on the heteroduplex analysis of the TCR VJ junctions. Each V-specific TCR chain, amplified either from fresh biopsy material or intestinal T-cell-line cDNA, is denatured and renatured to allow the random reshuffling of the various strands carrying different junctional sequences, coamplified in the same reaction. The mismatched chains (heteroduplexes) are separated from the matched ones (homoduplexes) through polyacrylamide gel electrophoresis, and whenever one or more T-cell clones are emerging over the polyclonal background, discrete bands are visible by ethidium-bromide staining. Through this method, we have estimated the diversity of the V delta 1-3 chains and a newly described V gene (V delta 8) whose homologue in mice is abundantly expressed in gamma delta iLs. We demonstrate that the well-documented expansion of V gamma 1+ gamma delta lymphocytes in the jejunum of CD patients is polyclonal. Overall, the heteroduplex analysis on fresh intestinal and peripheral blood lymphocytes from both healthy and affected subjects shows a polyclonal pattern of all the V delta+ subsets. In contrast, most intestinal T-cell lines produce oligoclonal patterns, suggesting a dramatic in vitro selection effect. The cell expansion in culture is generally not required for the TCR heteroduplex analysis, which can therefore be applied to rapidly monitor the T-cell response in a variety of physiologic and autoimmune reactions, substituting the standard approach of TCR cloning and multiple VJ sequencing.

Monoclonal antibodies defining functional sites on the toxin superantigen staphylococcal enterotoxin B

Four monoclonal antibodies (mAbs) were produced binding to four nonoverlapping epitopes on the superantigen staphylococcal enterotoxin B (SEB). The mAbs were tested for their ability to detect SEB bound to major histocompatibility complex (MHC) class II, to inhibit SEB binding to MHC class II, to inhibit SEB stimulation of T cell hybridomas, to bind to various nonfunctional mutants of SEB, and to capture and present SEB and its mutants to T cells in the absence of MHC class II. We concluded that two mAbs, B344 and B327, bound to epitopes not required for superantigen function, one mAb, 2B33, blocked an MHC interaction site on SEB, and the fourth mAb, B87, blocked the T cell recognition site on SEB. Moreover, two mAbs (B344 and 2B33) were capable of presenting SEB, although much less efficiently than APC, to CD4- but not CD4+ T cell hybridomas. The results confirm the functional domains on SEB originally defined by mutation and show that MHC class II is not always an essential component of the superantigen ligand.

Oral administration of lipopolysaccharides activates B-1 cells in the peritoneal cavity and lamina propria of the gut and induces autoimmune symptoms in an autoantibody transgenic mouse

About a half of the antierythrocyte autoantibody transgenic (autoAb Tg) mice, in which almost all B cells are detected in the spleen, lymph nodes, and Peyer's patches, but not in the peritoneal cavity, suffer from autoimmune hemolytic anemia. The occurrence of this disease is strongly linked to production of autoAb by activated peritoneal B-1 cells in the Tg mice. In this study, we have shown that oral administration of lipopolysaccharides (LPS) activated B-1 cells in the lamina propria of the gut as well as the peritoneal cavity in the healthy Tg mice and induced the autoimmune symptoms in all the Tg mice. The activation of peritoneal and lamina propria B-1 cells by enteric LPS is found not only in the anti-RBC autoAb Tg mice and normal mice but also in the aly mice which congenitally lack lymph nodes and Peyer's patches. These results suggest that B-1 cells in the two locations may form a common pool independent of Peyer's patches and lymph nodes, and can be activated by enteric thymus-independent antigens or polyclonal activators such as LPS. The induction of autoimmune hemolytic anemia in the Tg mice by enteric LPS through the activation of B-1 cells in the lamina propria of gut and in the peritoneal cavity suggests that B-1 cells and bacterial infection may play a pathogenic role in the onset of autoimmune diseases.

T-cell antigen receptors in rheumatoid arthritis

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

Altered antigen-presenting capacity of human monocytes after phagocytosis of bacteria

The antigen-presenting and accessory functions of monocytes were studied after phagocytosis of bacteria. Peripheral blood monocytes isolated from mononuclear cells by counterflow elutriation were incubated with suspensions of opsonized bacteria (Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, or Salmonella enteritidis) under conditions in which at least 80% of the monocytes engulfed microorganisms. Either the cells were pulsed with antigen (purified derivative of tuberculin or tetanus toxoid) and used as antigen-presenting cells for autologous T lymphocytes or the accessory function of the cells was examined in pokeweed mitogen-activated cultures of T cells. It has been found that phagocytosis of bacteria by monocytes reduces their ability to trigger antigen- and mitogen-induced proliferation. The reduced proliferative response of T lymphocytes was not due to a change of the kinetics of the response or triggering of suppressor mechanisms. Furthermore, antigen processing was not affected much after phagocytosis of bacteria since antigen-pulsed and paraformaldehyde-fixed cells containing bacteria were comparable to control cells in their antigen-presenting capacity. This phenomenon was observed after phagocytosis of both living and dead bacteria and was not correlated to the viability of monocytes, which were more affected after phagocytosis of living bacteria than of dead ones. As a result of phagocytosis of bacteria, reduced expression of CD54, CD14, and HLA-DQ, variable reduction of HLA-DP, CD58, and CD64, and reduced viability of monocytes were observed. In conclusion, phagocytosis of bacteria by monocytes affects their antigen-presenting and accessory functions presumably because of changes in the expression of molecules essential for monocyte-T-cell interactions and reduction of their viability.

Lack of association of T cell receptor V beta 8 polymorphism with rheumatoid arthritis in United Kingdom and Italian white patients

OBJECTIVES

To study if the reported association of a BamH I 2kb RFLP of the T cell receptor V beta 8 gene with DR4+ rheumatoid arthritis patients is found in non-American white populations.

METHODS

The frequency of this RFLP in two different populations was analysed. Eighty one northern Italians were studied for HLA-DR genotypes and V beta 8 polymorphism, and 29 DR4+ British white patients were studied for V beta 8 polymorphism.

RESULTS

No association between the V beta 8 RFLP and DR4 was found with rheumatoid arthritis in both groups.

CONCLUSION

The reported V beta 8-DR4 association is not generally applicable. The lack of association in our populations may be due to genetic differences, or to differences in factors which shaped the T cell repertoire.

Virus-specific CD8+ T-cell responses in mice transgenic for a T-cell receptor beta chain selected at random

The consequences of severely limiting the T-cell receptor (TCR) repertoire available for the response to intranasal infection with an influenza A virus or with Sendai virus have been analyzed by using H-2k mice (TG8.1) transgenic for a TCR beta-chain gene (V beta 8.1D beta 2J beta 2.3C beta 2). Analyzing the prevalence of V beta 8.1+ CD8+ T cells in lymph node cultures from nontransgenic (non-TG) H-2k controls primed with either virus and then stimulated in vitro with the homologous virus or with anti-CD3 epsilon showed that this TCR is not normally selected from the CD8+ T-cell repertoire during these infections. However, the TG8.1 mice cleared both viruses and generated virus-specific effector cytotoxic T lymphocytes (CTL) and memory CTL precursors, though the responses were delayed compared with the non-TG controls. Depletion of the CD4+ T-cell subset had little effect on the course of influenza virus infection but substantially slowed the development of the Sendai virus-specific CTL response and virus elimination in both the TG8.1 and non-TG mice, indicating that CD4+ helpers are promoting the CD8+ T-cell response in the Sendai virus model. Even so, restricting the available T-cell repertoire to lymphocytes expressing a single TCR beta chain still allows sufficient TCR diversity for CD8+ T cells (acting in the presence or absence of the CD4+ subset) to limit infection with an influenza A virus and a parainfluenza type 1 virus.

Maternal transfer of infectious mouse mammary tumor retroviruses does not depend on clonal deletion of superantigen-reactive V beta 14+ T cells

Female C3H/HeJ mice maternally transmit through their milk an infectious mouse mammary tumor retrovirus (MMTV) which causes clonal deletion of T cell receptor (TcR)V beta 14+ T cells reactive to the retroviral superantigen (SAG). To test whether CD4+ or CD8+ T cells are crucial for intestinal infection and maternal transfer of exogenous retroviruses, newborn mice lacking CD4 or CD8 molecules after gene targetting were raised by surrogate C3H/HeJ mothers. In CD8-/- mice, clonal deletion of TcRV beta 14+ cells reactive to the SAG from this exogenous MMTV occurred with delayed kinetics. Deletion of TcRV beta 14+ cells was not observed in CD4-/- mice up to 12 months after exposure to the retrovirus. In both CD4-/- and CD8-/- mice TcRV beta 5+ and TcRV beta 11+ T cells were deleted in the presence of genomically integrated endogenous MMTV (Mtv), indicating that the lack of SAG-induced clonal deletion was not due to a general defect in these mutant mouse strains. Although TcRV beta 14+ T cells were not deleted in CD4-/- mice, female CD4-/- mice nursed on C3H/HeJ milk maternally transmitted the retrovirus to their offspring, albeit with delayed kinetics. These data demonstrate that CD4+ and CD8+ lymphocytes influence clonal deletion events and that the mechanisms responsible for clonal deletion of SAG-reactive TcRV beta 14+ T cells may be different from mechanisms which allow the mammary tumor virus to enter the mammary gland and complete its infectious cycle.

Clonal expansion of T lymphocytes causes arthritis and mortality in mice infected with toxic shock syndrome toxin-1-producing staphylococci

Erosive arthritis is a common and feared complication of staphylococcal infection. The reason(s) for the progressive course of the arthritis is unknown. It has been recently established that enterotoxins produced by Staphylococcus aureus display superantigen properties leading to stimulation of T cells carrying distinct T cell receptor V beta elements. This finding provides a potential connection between staphylococcal exoproteins and endogenous immune mechanisms participating in the infectious process. We have recently describe successful induction of infections arthritis in mice after intravenous inoculation of a toxic shock syndrome toxin-1 (TSST-1)-producing S. aureus LS-1 strain. Using this model we have now found a clonal expansion of T cells expressing V beta 11+ T cell receptor in the synovial tissue of arthritic mice. The role of TSST-1 as a superantigen inducing oligoclonal expansion was confirmed in an in vitro culture system. The expansion of V beta 11+ T cells proved to be of arthritogenic significance since mice genomically deleted of the V beta 11+ T cells did not develop arthritis and since pretreatment of healthy mice with anti-CD4 or anti-V beta 11 monoclonal antibodies inhibited arthritis. In addition, CD4+ and V beta 11+ T cells showed themselves to be of pathogenic significance in staphylococcal-induced mortality, since mice depleted of such populations showed increased survival. We propose that in hematogenously spread S. aureus-induced arthritis the TSST-1-dependent clonal expansion of CD4+ V beta 11+ T cells is a driving pathogenic force.

Three-dimensional structure of a human class II histocompatibility molecule complexed with superantigen

The structure of a bacterial superantigen, Staphylococcus aureus enterotoxin B, bound to a human class II histocompatibility complex molecule (HLA-DR1) has been determined by X-ray crystallography. The superantigen binds as an intact protein outside the conventional peptide antigen-binding site of the class II major histocompatibility complex (MHC) molecule. No large conformational changes occur upon complex formation in either the DR1 or the enterotoxin B molecules. The structure of the complex helps explain how different class II molecules and superantigens associate and suggests a model for ternary complex formation with the T-cell antigen receptor (TCR), in which unconventional TCR-MHC contacts are possible.

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

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

Bimodal distribution of V beta 2+CD4+ T cells in human peripheral blood

The distribution of T cells using the V beta 2 gene was studied in a group of 99 humans. The distribution of V beta 2+CD4+ levels was bimodal. Twelve individuals had levels of V beta 2+CD4+ less than 2% and 86 others had values greater than 5%. Only one individual had a value between 2% and 5%. The V beta 2 low (mean 1.3 +/- 0.49) and V beta 2 high (mean 8.2 +/- 1.65) phenotypes were stable. The V beta 2 low phenotype is inherited and not limited to HLA or T cell receptor V beta gene complexes. The CD8V beta 2 levels of CD4V beta 2 low individuals are also low. The residual V beta 2+ T cells in V beta 2 low individuals were not anergic to V beta 2-specific stimulation. These data are compatible with the effects of an endogenous superantigen.

Superantigens and retroviral infection: insights from mouse mammary tumor virus

Superantigens induce a vigorous immune response by stimulating T cells that express particular T-cell receptor V beta chains. Mouse mammary tumor virus is a milk-transmitted retrovirus that encodes such a superantigen. Paradoxically, as discussed by Werner Held and colleagues, the strong superantigen-induced immune response permits the survival of the virus via T-cell dependent clonal expansion of infected B cells.

Antagonism of superantigen-stimulated helper T-cell clones and hybridomas by altered peptide ligand

T-cell activation by an immunogenic peptide can be antagonized by nonstimulatory analogs of that peptide. We investigated this T-cell receptor antagonism by using staphylococcal enterotoxin superantigen to stimulate hemoglobin-specific helper T (Th) cells because its activation pathway may differ from that of conventional antigen. Interestingly, superantigen activation of these Th cells was antagonized by hemoglobin peptide analogs even though agonist (superantigen) and antagonist (analog peptide) bind at different sites on the major histocompatibility complex-encoded molecule and the T-cell receptor. The antagonism appeared to be a fundamental block in T-cell activation, as phosphoinositol generation, cytokine production, and proliferation were reduced in Th1 clones, and, similarly, proliferative and cytokine responses were inhibited in Th2 cells. Even T-cell hybridoma activation (cytokine production and apoptosis) was inhibited by peptide antagonists. Furthermore, analog peptides that functioned as partial agonists for these Th cells also antagonized superantigen-induced proliferation and thus were a subset of the peptide antagonists. In summary, our results demonstrate that analogs of immunogenic peptide are potent antagonists for Th cell responses induced by superantigen as well as immunogenic peptide.

Murine AIDS is initiated in the lymph nodes draining the site of inoculation, and the infected B cells influence T cells located at distance, in noninfected organs

The infection of cells which belong to the B-cell lineage is thought to be the primary event leading to the phenotypic and functional alterations seen in the murine AIDS (M. Huang, C. Simard, D. Kay, and P. Jolicoeur, J. Virol. 65:6562-6571, 1991). Using in situ hybridization, we studied the time course of the anatomic distribution of the murine AIDS-infected B cells in C57BL/6 mice inoculated intraperitoneally or in the foot pad with helper-free stocks of the defective murine AIDS virus. The local lymph nodes draining the injection site (the mediastinal or popliteal lymph nodes) were the primary organs in which infected B cells could be detected. From this initial site, the proliferating infected B cells were found to migrate progressively to most of the other lymph nodes and to the spleen. The bone marrow cells (containing the precursor B cells) were not found to be infected by the virus. These results suggest that the defective murine AIDS virus infects mature Ly-1- B cells present in lymph nodes. We compared the concanavalin A response of the T cells at an early time postinoculation, before all lymphoid organs are infiltrated with infected B cells. In lymphoid organs free of infected B cells, T cells were found to be hyperresponsive. In lymphoid organs in which infected B cells were present, T cells were hyporesponsive. These data suggest that infected B cells influence distant T cells, maybe by the release of a circulating factor or through another uninfected cell population activated by the infected B cells.

Binding sites for bacterial and endogenous retroviral superantigens can be dissociated on major histocompatibility complex class II molecules

Bacterial and retroviral superantigens (SAGs) interact with major histocompatibility complex (MHC) class II molecules and stimulate T cells upon binding to the V beta portion of the T cell receptor. Whereas both types of molecules exert similar effects on T cells, they have very different primary structures. Amino acids critical for the binding of bacterial toxins to class II molecules have been identified but little is known of the molecular interactions between class II and retroviral SAGs. To determine whether both types of superantigens interact with the same regions of MHC class II molecules, we have generated mutant HLA-DR molecules which have lost the capacity to bind three bacterial toxins (Staphylococcus aureus enterotoxin A [SEA], S. aureus enterotoxin B [SEB], and toxic shock syndrome toxin 1 [TSST-1]). Cells expressing these mutated class II molecules efficiently presented two retroviral SAGs (Mtv-9 and Mtv-7) to T cells while they were unable to present the bacterial SAGs. These results demonstrate that the binding sites for both types of SAGs can be dissociated.

Cytoplasmic membrane-associated protein (CAP) isolated from Streptococcus pyogenes: as a new bacterial superantigen

A protein isolated from the cytoplasmic membranes of Streptococcus pyogenes (cytoplasmic membrane-associated protein, CAP) stimulated human T cells in vitro to induce their mitogenic response. This CAP-induced T cell proliferation required the presence of nylon-adherent accessory cells (AC) of either autologous or allogeneic origin in the reaction mixtures. In addition, the reaction was inhibited by monoclonal antibodies (mAbs) against major histocompatibility complex (MHC) class II molecules, HLA-DR and -DQ, but not -DP. Human lymphoid cell lines positive for HLA-DR but not those lacking it were also effective as AC for the reaction. A binding test using fluorescein-labeled protein revealed that CAP bound to the adherent monocytes and HLA-DR+ but not to -DR- lymphoid cell lines. The proliferative response of T cells to CAP was, however, not inhibited by the addition of the lysosomotrophic agent NH4Cl to the reaction mixtures. These results suggest that the presentation of CAP by AC to human T cells is mediated through binding of the protein to the MHC class II molecules but without being processed in the AC. The proliferative response of T cells was also found to be inhibited by addition of anti-CD2, -CD3 or -T cell receptor (TcR) mAbs. A major population responding to CAP was CD3+4+8- T cells. CAP also appears to stimulate T cells bearing V beta 8 sequences much more selectively than T cells bearing other V beta s. These results indicate that this streptococcal membrane protein, CAP, may be a new protein belonging to a group of bacterial superantigens.

Signals and signs for lymphocyte responses

The adaptive immune response protects us from infection in a world of pathogens that is forever evolving new variants. As the system is built on the generation of an open repertoire of receptors, the recognition of self is unavoidable, and is guarded against by deletion during lymphocyte development of those cells that are specific for ubiquitous self antigens, and the silencing of those that are specific for self antigens only encountered after cells achieve functional maturity in the periphery. This silencing occurs when lymphocytes recognize antigens in the absence of suitable costimulatory molecules. By contrast, when the same cell encounters the same ligand on a cell that expresses costimulatory molecules, it will proliferate and differentiate into an effector cell. These effector cells mediate protective immunity when the antigen is carried by a pathogen, but they can mount autoimmune responses if the antigen is derived from self. The major costimulatory molecules for CD4 T cells appear to be B7 and B7.2 that bind to the CD28 and CTLA-4 receptors on the T cell. The signals from the TCR appear to be integrated with those from the costimulator receptor, and the T cell response depends on the precise nature of these signals, further conditioned by cytokines present in the environment of the responding cell. B cells can be viewed in a similar way, with the costimulatory molecule CD40 ligand and cytokines coming mainly from CD4 helper T cells determining the fate of the responding B cell. The TCR is not simply an on and off switch, since the precise way in which the TCR is ligated determines the differentiation of the T cell and can alter the effector responses of established T cell lines. Thus, the response capabilities of T cells are more flexible than originally believed, and much of this flexibility comes from the interplay of TCR signals and signs from the environment. If the biochemical nature of these differential signaling pathways were known, it might be possible to develop simple pharmacological agents capable of diverting T cell responses from harmful to innocuous by getting the T cell to reinterpret the signals it is receiving via its receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Negative selection of lymphocytes

Almost by definition, negative selection of T and B lymphocytes cannot be absolute. Given that both sets of receptors are derived by stochastic processes, recognition of epitopes by lymphocyte receptors will not be an all or none affair but a relative one. Too effective a mechanism of negative selection would have resulted in deletion of the whole repertoire, as all specificities would have cross-reacted with some self-epitope at least to some degree. This review has documented some of the influences impacting on emerging T and B cell repertoires that result in a removal of the most dangerous self-reactive cells and the progressive quantitative and qualitative increase, through positive clonal selection, of other cells specific for the actual foreign antigens encountered by each individual. T and B lymphocytes pass through a stage where their natural reaction to antigen is one of negative selection and on to a later stage where the cell is more likely to become activated. Geography plays a role in this; the primary lymphoid organs are designed largely to exclude foreign antigens and to present self-antigens, whereas the secondary lymphoid organs are designed to filter out and concentrate foreign material and to promote costimulatory intercellular immune interactions. Ontogeny of individual cells also plays its role, probably through the progressive assembly of the full receptor signaling machinery, incomplete arrays promoting a negative rather than a positive signal. However, the differing susceptibilities of immature and mature cells to silencing by deletion or anergy are relative rather than absolute. Negative signaling may involve immediate or somewhat delayed death of the anti-self-cell, and in some cases the bad cell may have the chance of editing its receptor to create one lacking anti-self-reactivity. Alternatively, the cell may receive a nonlethal down-regulatory signal and may be induced into a state of anergy. Such anergic cells may have a reduced life span, showing that anergy and deletion may shade into each other. Upon the removal of antigen (an unlikely event for cells anergic to an authentic self-antigen), the state of anergy is reversible. The strength of the receptor cross-linking signal may chiefly determine whether deletion (strong cross-linking) or anergy (weaker cross-linking) supervenes. Some lymphocytes with self-reactivity ignore the self-antigen in question because of low affinity, poor accessibility, lack of suitable presentation, or absence of appropriate help.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of the peripheral T-cell receptor V beta repertoire in newly diagnosed patients with type I diabetes

Type I diabetes is an autoimmune disease characterised by a marked activation of peripheral T cells around the time of clinical diagnosis. Studies of T-cell antigen receptor V beta (TCRBV) gene usage in type I diabetes have been conflicting. Using a semi-quantitative polymerase chain reaction technique and flow cytometry we have investigated the TCRBV gene usage of 13 newly diagnosed patients with type I diabetes and 11 normal healthy controls. No preferential TCRBV gene usage was found between patients and controls even after matching for HLA-DR3 and/or -DR4. In addition, no significant differences in TCRBV gene usage were found between sequential samples taken over a period of up to 7 months following diagnosis. These results suggest that the TCR repertoire of these patients is heterogeneous and it is unlikely that a single 'pathogenic' T-cell clone is dominant at the clinical onset of the disease.

Linomide inhibits programmed cell death of peripheral T cells in vivo

Programmed cell death (PCD) is involved in the physiological regulation of lymphocyte turnover, as well in the antigen-driven selection of T and B cells. Here it is shown that the immunomodulator linomide (quinoline-3-carboxamide) inhibits the apoptotic decay of peripheral T lymphocytes in response to three different stimuli. First, linomide reduces the superantigen-mediated apoptosis and deletion of specific T lymphocytes of both the CD4+ and the CD8+ subsets without affecting other superantigen-triggered phenomena such as T cell expansion and anergy. Second, linomide abolishes the T lymphopenia and inhibits PCD of splenic CD4+ and CD8+ T cells induced by exogenous glucocorticoids. This effect is restricted to peripheral T lymphocytes and does not concern thymocytes. Finally, linomide abolishes the development of lymphopenia that follows infection with vaccinia virus, while reducing PCD of CD4+ and CD8+ peripheral T cells. The anti-apoptotic effect of linomide could account for its immunostimulatory properties and might be relevant to the treatment of immunodeficiencies associated with an increased apoptotic decay of T lymphocytes.

The T cell receptor repertoire of CD4-8+ thymocytes is altered by overexpression of the BCL-2 protooncogene in the thymus

The bcl-2 gene encodes an intracellular, membrane-associated protein that protects immature cortical thymocytes from a wide variety of apoptotic stimuli, including glucocorticoids, radiation, and anti-CD3 treatment. Since cortical thymocytes are the primary target cells for thymic positive and negative selection processes, and since these processes are associated with cell death, we evaluated the role of bcl-2 in T cell development in two ways. In the first approach, transgenic mice expressing high levels of Bcl-2 in cortical thymocytes were mated with H-Y T cell receptor (TCR) transgenic mice, the latter being a well-defined system for the study of positive and negative selection of T cells. We found that the bcl-2 transgene had a dramatic effect on positive selection. This was manifested by a greatly increased production of mature thymocytes that were highly skewed towards the CD4-8+ lineage. The change involving CD4-8+ thymocytes occurred not only in bcl-2 transgenic mice, but was also observed in H-Y TCR/bcl-2 doubly transgenic mice, regardless of whether the H-Y TCR was expressed in the selecting (H-2b) or nonselecting (H-2d) environments. Furthermore, a large proportion of CD4-8+ thymocytes produced in H-2b H-Y TCR/bcl-2 doubly transgenic female mice expressed endogenous TCR alpha chains rather than the transgenic TCR alpha chain. These observations are consistent with the model that high expression of Bcl-2 in cortical thymocytes overrides the normal apoptotic pathway. This then allows the selection of CD4-8+ thymocytes expressing TCRs that are otherwise nonselectable. However, the bcl-2 transgene did not protect CD4+8+ thymocytes expressing the male-specific TCR from deletion in male doubly transgenic mice. In the second approach, we determined the level of bcl-2 mRNA expression in populations of thymocytes defined by their CD4/CD8 phenotypes using quantitative reversed transcriptase PCR techniques. Our results indicate that bcl-2 mRNA was expressed at a high level in immature CD4-8- thymocytes and in mature CD4+8- thymocytes. There is a dramatic downregulation of bcl-2 mRNA in CD4+8+ thymocytes, particularly those expressing a low level of TCR. CD4+8+ thymocytes that upregulated their TCR, likely as a result of receiving positive selection signals, also upregulated bcl-2 mRNA. This observation suggests that rescue of immature thymocytes from the programmed cell death pathway by positive selection signals is accompanied by the upregulation of bcl-2 mRNA.

Human T-cell responses to Mycoplasma arthritidis-derived superantigen

When injected into mice, Mycoplasma arthritidis causes a chronic arthritis that resembles rheumatoid arthritis, histologically. The organism produces a superantigen termed Mycoplasma arthritidis mitogen or MAM, that in humans preferentially expands T cells whose antigen receptors express V beta 17. T cells with this phenotype appear to be increased in rheumatoid synovial effusions. We describe a novel approach to isolating and characterizing human MAM-reactive T-cell lines and determining their T-cell receptor (TCR) V beta usage. Lines were prepared from T cells that clustered with dendritic cells during a 2-day exposure to MAM. Cluster and noncluster fractions of T cells were then expanded by using feeder cells and a polyclonal mitogen. Most of the MAM reactivity was found in dendritic T-cell clusters, as were most of the T cells expressing TCR V beta 17. After expansion, 76% of the cluster-derived T-cell lines were MAM reactive, while no reactivity was seen in cell lines derived from the noncluster fraction. Of the MAM-reactive lines, 49% expressed V beta 17 on some or all of the cells. Cell lines from both cluster and noncluster fractions were analyzed for TCR V beta mRNA expression by PCR amplification. Other V beta genes (5.1, 7, 8, 12, and 20) were found to be expressed by lines that were MAM reactive, although these were not a major component of the cluster-derived T cells. Some non-cluster-derived lines expressed V beta s 17, 12, and 7, but these proved to be nonreactive to MAM. Therefore, dendritic cells can be used to immunoselect and characterize T cells that express superantigen-reactive TCRs.

Natural and acquired resistance to Leishmania: cellular activation by Leishmania aethiopica of mononuclear cells from unexposed individuals is through the stimulation of natural killer (NK) cells

Cells from normal non-Leishmania-exposed individuals could respond in vitro by proliferation and interferon-gamma (IFN-gamma) production to Leishmania aethiopica stimulation. The main cell type that appeared to be activated following such stimulation was CD3-, CD16+/56+, i.e. NK cells. Of the few CD3+ cells responding, an involvement of CD8+ cells was evident in the absence of activation of CD4+ cells in normal individuals, while a different feature was observed when patients' cells were investigated. Cells from patients with L. aethiopica infection did not show this NK response, but rather the CD4+ cells were the prominent responding cells. No evidence of the involvement of superantigens or cells utilizing the gamma delta T cell receptor (gamma delta cells) in the response of unexposed individuals was noted.

CD4+ T-cell receptor V beta usage during the immune response to Schistosoma mansoni

T-cell receptor V beta usage by CD4+ cells during the immune response of C57BL/6J mice to Schistosoma mansoni was examined. During acute infection, the distribution of splenic CD4+ cells utilizing V beta s 3, 5, 6, 7, 8.1/8.2, 9, and 11 was not significantly different from that in uninfected mice. Preferential V beta usage or deletion was not detected when either the primary or the memory immune response to parasite eggs was examined.

In Mls-1a mice, fetal-type beta-gene rearrangements are frequent among self-anergic V beta 6 T cells

T lymphocytes generated in the fetal and neonatal period are characterized by T cell receptor (TCR) gene rearrangements that lack N region nucleotides (fetal-type TCR). Using fetal-type TCR as a lineage marker, we show that such T cells are long-lived and persist in the periphery of adult mice. Moreover, in both neonatal and adult environments, upon encounter with self-antigens, they are less likely to be deleted. Inefficient clonal deletion could be due to the intrinsic properties of the T cells generated during this period, or to yet unknown properties of the perinatal thymus. Such anergic T cells constitute a subset that can further expand in vivo in an antigen-independent fashion, leaving open the possibility for self-aggression under the appropriate triggering conditions.

L1 adhesion molecule on mouse leukocytes: regulation and involvement in endothelial cell binding

L1 is a cell surface glycoprotein of the immunoglobulin superfamily which was initially shown to mediate adhesion between neural cells. Recently we have reported that L1 is expressed by bone marrow cells and the majority of mature lymphocytes (Kowitz et al., Eur. J. Immunol. 1992. 22: 1199-1205). To analyze the function of L1 on leukocytes we studied its regulation following cell activation. In vitro activation of B lymphocytes with lipopolysaccharide or T lymphocytes with phorbol 12-myristate 13-acetate/Ca2+ ionophore, concanavalin A or anti-CD3 monoclonal antibody as well as in vivo activation of V beta 8+ T cells with staphylococcal enterotoxin B (SEB) revealed a down-regulation of L1 within 48 h. A rapid loss of L1 expression was seen when mouse neutrophils were activated with PMA alone. This rapid loss paralleled the shedding of L-selectin. We also studied a possible role of L1 in the binding of leukocytes to endothelial cells. ESb-MP lymphoma cells with a high expression of L1 (L1hi) could bind to bend3 endothelioma cells without prior activation with inflammatory cytokines. The interaction was inhibited by anti-L1 antibodies. In contrast, ESb-MP cells with low L1 expression (L1lo) were only marginally bound. Latex beads coated with affinity-isolated L1 antigen were also able to bind to the endothelioma cells in a specific fashion. The binding of ESb-MP lymphoma cells required Ca2+ and Mg2+ ions and was sensitive to cold temperature. Since the endothelioma cells did not express L1 the binding mechanism studied here is distinct from the established L1-L1 homotypic interaction. It is possible that the novel L1-mediated adhesion pathway involves an unidentified ligand and could play a role in leukocyte migration.

Superantigen mediated shock: a cytokine release syndrome

Treatment of animals with superantigens results in profound immunological changes. A major fraction of all peripheral T cells becomes activated in vivo. Subsequently, successive waves of cytokines are produced with TNF playing a central pathophysiologic role. In addition, if the liver is damaged by an as yet poor defined mechanism the consequences of the cytokine syndrome are life threatening. However, TNF alone is not sufficient to cause death, instead synergizing interactions with cytokines like IL-1, IL-6, and IFN-gamma are probably involved. On the other hand, certain experimental conditions prevent these waves of cytokines and consequently lethal shock. Furthermore, a significant fraction of SA reactive T cells are deleted by programmed cell death 10 to 24 hours after treatment. Thereafter the surviving cells proliferate vigorously until day 2 or 3, followed by a second wave of apoptosis resulting in reduced SA reactive T cell numbers as compared to pretreatment levels. Of course, many aspects of the complicated events are only marginally understood and deserve further investigation.

Distribution and kinetics of superantigen-induced cytokine gene expression in mouse spleen

The polyclonal stimulation of T cells by bacterial superantigens is involved in the pathogenesis of the toxic shock syndrome in certain staphylococcal and streptococcal infections. Here we describe the onset and kinetics of superantigen-induced cytokine production in situ in spleens of normal BALB/c mice monitored at the level of cytokine mRNA expression by in situ hybridization. Messenger RNAs for interleukin 2 (IL-2), interferon gamma, and tumor necrosis factors (TNF) alpha and beta were not expressed at detectable levels in spleens of unstimulated animals but became visible already 30 min after intraperitoneal application of 50 micrograms staphylococcal enterotoxin B. All mRNA levels showed peak expression approximately 3 h after injection and a slow decrease up to 24 h after injection. Expression of the mRNAs was restricted to the T cell-dependent area of the periarteriolar lymphatic sheets of the spleen. Interestingly, TNF-alpha mRNA showed a biphasic response, the early appearing mRNA had the same localization as the other mRNAs, whereas after 3 h TNF-alpha mRNA showed a broader distribution indicating a second cell population producing TNF-alpha. The expression of IL-2 and TNF proteins in the serum increased in parallel to the observed mRNA changes with a slight delay. The presence of macrophages was not required for the expression of the cytokine mRNAs in the spleen as the expression was unchanged in macrophage-depleted mice. Only the second phase of TNF-alpha mRNA expression was abrogated in such animals. The expression of all mRNAs was completely suppressed by prior administration of cyclosporin A. These data show that nonphagocytic cells are the essential superantigen-presenting cells in vivo and indicate that at least part of the pathogenetic TNF-alpha is T cell derived.

Activated keratinocytes present bacterial-derived superantigens to T lymphocytes: relevance to psoriasis

In the past, epidermal keratinocytes were felt to be primarily concerned with the barrier function of skin. During inflammatory and immune-mediated skin diseases, keratinocytes were only portrayed as being passive/inert targets for noxious agents produced by infiltrating leukocytes. This innocent bystander and/or 'brick and mortar' conceptualization of the keratinocyte must now be significantly modified to take into account the growing body of experimental in vitro and in vivo results that substantiate re-classification of keratinocytes as fully fledged members of the immune system (i.e. immunocytes). Because keratinocytes produce important primary cytokines, adhesion molecules, and mononuclear cell chemotactic factors; as well as functioning as accessory cells for resting T lymphocytes, they can initiate and perpetuate the inflammatory and immunological reactions in the skin which contribute to the pathobiology of psoriasis. This review will emphasize the dynamic contribution that epidermal keratinocytes make to cutaneous immunohomeostasis, with particular focus on the potential role of bacterial derived superantigens and their ability to stimulate resting T cell proliferation when presented by cytokine-activated keratinocytes.

Clonal anergy to staphylococcal enterotoxin B in vivo: selective effects on T cell subsets and lymphokines

Injection of bacterial superantigens such as staphylococcal enterotoxin B (SEB) in adult mice results in initial proliferation of SEB-responsive V beta 8+ T cells followed by induction of a state of non-responsiveness frequently referred to as clonal anergy. We show here that SEB-induced anergy involves selective changes in lymphokine production and that it affects CD4+ V beta 8+ and CD8+ V beta 8+ T cells in different fashions. Whereas both CD4+ V beta 8+ and CD8+ V beta 8+ cells from anergic mice exhibit strongly reduced proliferative capacity and interleukin(IL)-2 production upon restimulation with SEB either in vivo or in vitro the CD8+ subset from SEB-injected mice produces other lymphokines (such as interferon(IFN)-gamma) at normal or slightly increased levels in response to SEB. Changes in the levels of production of IL-2 and IFN-gamma protein correlated well with mRNA accumulation both in vivo and in vitro. Collectively these data suggest that superantigen-induced anergy involves selective changes in signal transduction and/or gene regulation in T lymphocytes.

Plasmodium berghei-specific T cells respond to non-processed sporozoites presented by B cells

The mechanism of malaria protective immunity induced by immunization with radiation-attenuated Plasmodium sporozoites (SPZ) is only partially understood. For example, B and T cell responses specific for the circumsporozoite (CS) protein, a 46 kDa SPZ surface protein, have been characterized; however, events leading to SPZ-specific T cell activation, i.e., processing and presentation of SPZ by antigen-presenting cells have not been investigated. In the present study we describe the in vitro analysis of requirements for accessory cell function in the presentation of SPZ to SPZ-immune T cells. The results establish that SPZ-induced proliferative T cells are reactive to non-processed SPZ presented by activated B cells and, thus, imply that the non-processed form of the SPZ-associated CS protein restricts the induction of the potential CS protein T cell repertoire.

HIV glycoprotein as a superantigen. A mechanism of autoimmunity and implications for a vaccination strategy

The pathogenic effects of HIV may reflect mimicry of several key immunological molecules. The surface glycoprotein of HIV has superantigenic properties responsible for the sequential deletion of T-cell clones. In addition, the glycoprotein has several regions sharing homology with class II MHC products. It can elicit cross-reactive antibodies which block replenishment of these T-cells in the thymus. The usefulness of conventional vaccination strategies in the fight against AIDS has been subject to debate. Based on these considerations, we argue that effective vaccines should avoid viral sequences homologous to class II MHC proteins which might exacerbate this process.

Cytotoxic and interferon gamma-producing activities of gamma delta T cells in the mouse intestinal epithelium are strain dependent

We have analyzed the cytolytic activity of freshly isolated intraepithelial T cells (i-IEL) from the intestines of several different mouse strains in an anti-T-cell receptor monoclonal antibody-mediated redirected lysis assay. The cytolytic activity of gamma delta i-IEL but not that of alpha beta i-IEL was strain dependent. Mouse strains could be divided into high (H), marginal (M), and null (N) strains. The anti-gamma delta T-cell receptor monoclonal antibody-induced interferon gamma production showed the same strain-dependent variability, but the proliferative responses to gamma delta T-cell receptor crosslinking did not show this variability. The N phenotype of gamma delta i-IEL was found to be dominant in (H x N)F1 mice. In radiation bone-marrow chimeras the H/N phenotype was determined by the genotype of the reconstituting bone-marrow-derived cells but was not determined by the genotype of the radioresistant host cells. Analysis of (H x N)F1 backcross animals indicated that at least two genes are involved in determination of the H/N phenotype. One of these genes is major-histocompatibility-complex linked. No difference in the use of the variable region segment of the gamma-chain or delta-chain was seen between the gamma delta i-IEL from H and N strains. Various models that might explain the strain-dependent gamma delta i-IEL phenotypes are discussed.

Purification and partial characterization of a product from Yersinia pseudotuberculosis with the ability to activate human T cells

We tried to purify a substance exhibiting mitogenicity for human peripheral blood lymphocytes from Yersinia pseudotuberculosis isolated from patients with Y. pseudotuberculosis infection manifesting acute and systemic clinical symptoms. The supernatant of a suspension of the bacteria disrupted by sonication was serially chromatographed on DEAE-Sepharose fast-flow, Sephacryl S-100 HR, and TSK-gel G2000SW high-pressure liquid chromatography columns. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the purified mitogenically active substance migrated as a single band corresponding to a molecular mass of 21 kDa. We designated the purified substance Y. pseudotuberculosis-derived mitogen (YPM). YPM stimulated human peripheral T cells to proliferate and produce interleukin-2 at 0.1 ng/ml or more. YPM-induced T-cell activation required the expression of HLA class II molecules on accessory cells.

Autoimmune gld mutation uncouples suicide and cytokine/proliferation pathways in activated, mature T cells

Antigen receptor-directed suicide plays an important role in the elimination of potentially autoaggressive immature T cells during thymic differentiation. Here we demonstrated evidence for a second pathway of receptor-directed suicide in mature T cells that is missing in a mutant strain (gld) of mice with an "autoimmune" lymphoproliferative syndrome. The defect is evident within the gld activated T cell and does not require the presence of an antigen-presenting cell for its expression. Receptor-driven suicide is intact in immature T cells of animals with this mutation. These results support the significance of receptor-directed suicide in the mature T cell compartment and suggest that the immune system may use three independent pathways for regulating programmed cell death in shaping and controlling the immune response.

Superantigen-induced immune stimulation amplifies mouse mammary tumor virus infection and allows virus transmission

Endogenous and infectious mouse mammary tumor viruses (MMTVs) encode in their 3' long terminal repeat a protein that exerts superantigen activity; that is, it is able to interact with T cells via the variable domain of the T cell receptor (TCR) beta chain. We show here that transmission of an infectious MMTV is prevented when superantigen-reactive cells are absent through either clonal deletion due to the expression of an endogenous MTV with identical superantigen specificity or exclusion due to expression of a transgenic TCR beta chain that does not interact with the viral superantigen. A strict requirement for superantigen-reactive T cells is also seen for a local immune response following MMTV infection. This immune response locally amplifies the number of MMTV-infected B cells, most likely owing to their clonal expansion. Collectively, our data indicate that a superantigen-induced immune response is critical for the MMTV life cycle.

Lack of selective V beta deletion in peripheral CD4+ T cells of human immunodeficiency virus-infected infants

To investigate the possibility of super-antigen-mediated deletions of T cells expressing particular T cell receptor V beta (TcR V beta) gene segments during human immunodeficiency virus (HIV) infection, TcR V beta usage in CD4+ and CD8+ subsets was analyzed in a cohort of infants maternally infected by HIV and in a group of healthy neonates. We used a semi-quantitative anchored polymerase chain reaction technique together with cytofluorographic analysis with anti-V beta monoclonal antibodies. The representation of the 24 V beta families in CD4+ and CD8+ T cells from normal neonates was very similar to that in adults. Preferential expression of V beta 2 in the CD4+ subset was observed in both the neonates and in healthy adults. The representation of the 24 V beta families in peripheral CD4+ T cells from the HIV-infected infants showed no selective V beta deletion, even when the CD4+ subset was globally depleted. Moreover, the main characteristics of the control group (predominance of certain V beta families and V beta 2 skewing towards the CD4+ subset) were also present in all the HIV-infected infants.

Direct evidence for the role of COOH terminus of mouse mammary tumor virus superantigen in determining T cell receptor V beta specificity

It has recently been shown that open reading frames in the 3' long terminal repeats of mouse mammary tumor viruses encode superantigens. These viral superantigens (vSAGs) stimulate most T cells expressing appropriate V beta s almost regardless of the rest of the variable components of the T cell receptors (TCR) expressed by those cells. vSAGs produce a type II integral membrane protein with a nonessential short cytoplasmic domain and a large glycosylated extracellular COOH-terminal domain, which is predicted to interact with major histocompatibility complex class II molecules and the TCR. The transmembrane region of vSAG also has an internal positively charged lysine residue of unknown significance. A set of chimeric and mutant vSAG genes has been used in transfection experiments to show that only the extreme COOH-terminal portion of vSAGs determine their TCR V beta specificities, and to show that the lysine residue in the transmembrane domain is not essential for the function of vSAG.

Lack of complete correlation between emetic and T-cell-stimulatory activities of staphylococcal enterotoxins

This study examined the emetic activity of several staphylococcal enterotoxin type A and B (SEA and SEB, respectively) mutants that had either one or two amino acid residue substitutions. New sea gene mutations were constructed by site-directed mutagenesis; gene products were obtained with glycine residues at position 25, 47, 48, 81, 85, or 86 of mature SEA. Culture supernatants from Staphylococcus aureus RN4220, or derivatives containing either sea or a sea mutation, were analyzed for the ability to stimulate proliferation of murine splenocytes, as determined by incorporation of [3H]thymidine. Culture supernatants containing SEA-N25G (a SEA mutant with a substitution of glycine for the asparagine residue at position 25), SEA-F47G, or SEA-L48G did not stimulate T-cell proliferation, unlike supernatants containing the other substitution mutants. Purified preparations of SEA-N25G had weak activity and those of SEA-F47G and SEA-L48G had essentially no activity in the T-cell proliferation assay. All mutants except SEA-V85G, which was degraded by monkey stomach lavage fluid in vitro, were tested for emetic activity. SEA-C106A and two SEB mutants, SEB-D9N/N23D and SEB-F44S (previously referred to as BR-257 and BR-358, respectively), whose construction and altered immunological properties have been reported previously, were also tested in the emetic assay. Each mutant was initially administered intragastrically at doses of 75 to 100 micrograms per animal; if none of the animals responded, the dose was increased four-to fivefold. SEA-F47G, SEA-C106A, and SEB-D9N/N23D were the only mutants that did not induce vomiting at either dose tested; these three mutants had reduced immunological activity. However, there was not a perfect correlation between immunological and emetic activities; SEA-L48G and SEB-F44S retained emetic activity, although they had essentially no T-cell-stimulatory activity. These studies suggest that these two activities can be dissociated.

A novel superantigen isolated from pathogenic strains of Streptococcus pyogenes with aminoterminal homology to staphylococcal enterotoxins B and C

Streptococcus pyogenes (group A Streptococcus) has re-emerged in recent years as a cause of severe human disease. Because extracellular products are involved in streptococcal pathogenesis, we explored the possibility that a disease isolate expresses an uncharacterized superantigen. We screened culture supernatants for superantigen activity with a major histocompatibility complex class II-dependent T cell proliferation assay. Initial fractionation with red dye A chromatography indicated production of a class II-dependent T cell mitogen by a toxic shock-like syndrome (TSLS) strain. The amino terminus of the purified streptococcal superantigen was more homologous to the amino termini of staphylococcal enterotoxins B, C1, and C3 (SEB, SEC1, and SEC3), than to those of pyrogenic exotoxins A, B, C or other streptococcal toxins. The molecule, designated SSA, had the same pattern of class II isotype usage as SEB in T cell proliferation assays. However, it differed in its pattern of human T cell activation, as measured by quantitative polymerase chain reaction with V beta-specific primers. SSA activated human T cells that express V beta 1, 3, 15 with a minor increase of V beta 5.2-bearing cells, whereas SEB activated V beta 3, 12, 15, and 17-bearing T cells. Immunoblot analysis of 75 disease isolates from several localities detected SSA production only in group A streptococci, and found that SSA is apparently confined to only three clonal lineages as defined by multilocus enzyme electrophoresis typing. Isolates of one of these lineages, (electrophoretic type 2) are strongly associated with TSLS. The data identify SSA as a novel streptococcal superantigen that appears to be more related structurally to staphylococcal enterotoxins than to streptococcal exotoxins. Because abundant SSA production is apparently confined to only three streptococcal clonal lineages, the data also suggest that the SSA gene has only recently been acquired by S. pyogenes.