The staphylococcal enterotoxins and their relatives

Staphylococcal enterotoxins and a group of related proteins made by Streptococci cause food poisoning and shock in man and animals. These proteins share an ability to bind to human and mouse major histocompatibility complex proteins. The complex ligand so formed has specificity for a particular part of T cell receptors, V beta, and by engaging V beta can stimulate many T cells. It is likely that some or all of the pathological effects of these toxins are caused by their ability to activate quickly so many T cells. It is also possible that encounters with such toxins have caused mice, at least, to evolve mechanisms for varying their T cell V beta repertoires, such that they are less susceptible to attack by the toxins.

Characterization of the murine antigenic determinant, designated L3T4a, recognized by monoclonal antibody GK1.5: expression of L3T4a by functional T cell clones appears to correlate primarily with class II MHC antigen-reactivity

We describe here the properties of mAb GK1.5, which recognizes a cell surface molecule designated L3T4; the determinant on L3T4 recognized by mAb GK1.5 is designated L3T4a. We present evidence here that: i) the expression of L3T4a by murine T cell clones correlates primarily with class II MHC antigen-reactivity; ii) mAb GK1.5 blocks all class II MHC antigen-specific functions (cytolysis, proliferation, release of lymphokines) by murine class II MHC antigen-reactive T cell clones, although there appears to be clonal heterogeneity in the degree to which these functions are blocked by mAb GK1.5; iii) mAb GK1.5 blocks class II MHC antigen-specific release of IL-2 from cloned T cell hybridomas by blocking class II MHC antigen-specific binding; and iv) L3T4 is very similar to the human Leu3/T4 antigen. The properties of mAb GK1.5 (complement fixation, reactivity with all mouse strains tested, profound blocking of all class II MHC antigen-specific functions by murine T cells, usefulness for FACS analyses, and usefulness for immuno-precipitation/SDS-PAGE analyses) make it suitable for investigating both the role of class II MHC antigen-reactive T cells in various immunological phenomena and the mechanistic basis, at the molecular level, of class II MHC antigen-reactivity by murine T cells.

The major histocompatibility complex-restricted antigen receptor on T cells. I. Isolation with a monoclonal antibody

An antibody-secreting B cell hybridoma, KJ1-26.1, has been prepared from mice immunized with the T cell hybridoma DO-11.10, which recognizes chicken ovalbumin in association with I-Ad (cOVA/I-Ad). KJ1-26.1 blocks I-restricted antigen recognition by DO-11.10 and a subclone of this T cell hybridoma, DO-11.10.24, which has the same specificity for cOVA/I-Ad as its parent. KJ1-26.1 does not block I-restricted antigen recognition by any other T cell hybridoma tested, including a number of T cell hybridomas closely related to DO-11.10, with similar, but not identical, specificities for antigen/I. Moreover, KJ1-26.1 binds to DO-11.10 and DO-11.10.24, but not to any other T cell hybridomas tested, including three subclones of DO-11.10 that have lost the ability to recognize cOVA/I-Ad. Thus, in every regard KJ1-26.1 appears to be binding to all or part of the receptors for antigen/I on the T cell hybridoma DO-11.10. KJ1-26.1 appears to bind to approximately 15,000 molecules/cell on the surface of DO-11.10. The antibody precipitates an 80,000 dimer from the cells, which on reduction migrates as 40-44,000 monomers. The receptor(s) for antigen/I on DO-11.10 therefore includes molecules with these properties.

Interaction of Staphylococcus aureus toxin "superantigens" with human T cells

A modification of the polymerase chain reaction has been used to establish the fact that a collection of Staphylococcus aureus toxins are "superantigens," each of which interacts with the T-cell alpha beta receptor of human T cells by means of a specific set of V beta elements.

Control of homeostasis of CD8+ memory T cells by opposing cytokines

Memory T cells maintain their numbers for long periods after antigen exposure. Here we show that CD8+ T cells of memory phenotype divide slowly in animals. This division requires interleukin-15 and is markedly increased by inhibition of interleukin-2 (IL-2). Therefore, the numbers of CD8+ memory T cells in animals are controlled by a balance between IL-15 and IL-2.

Antigen recognition by H-2-restricted T cells. I. Cell-free antigen processing

We examined the ability of a set of cloned chicken ovalbumin (cOVA)-specific, Id-restricted, T cell hybridomas to produce interleukin-2 in response to cOVA presented by the Ia+ B cell lymphoma line, A20-2J. Although viable A20-2J cells presented native, denatured, and fragmented cOVA more or less equally well, A20-2J cells that were glutaraldehyde-fixed could present only enzymatically or chemically fragmented cOVA. These results suggest that antigen fragmentation may be both necessary and sufficient to define accessory cell processing of soluble antigens so that they may be recognized in association with I-region molecules by T cells.

Type I interferons keep activated T cells alive

Antigen injection into animals causes antigen-specific T cells to become activated and, rapidly thereafter, die. This antigen-induced death is inhibited by inflammation. To find out how inflammation has this effect, various cytokines were tested for their ability to interfere with the rapid death of activated T cells. T cells were activated in vivo, isolated, and cultured with the test reagents. Two groups of cytokines were active, members of the interleukin 2 family and the interferons (IFNs) alpha and beta. This activity of IFN-alpha/beta has not been described previously. It was due to direct effects of the IFNs on the T cells and was not mediated by induction of a second cytokine such as interleukin 15. IFN-gamma did not slow the death of activated T cells, and therefore the activity of IFN-alpha/beta was not mediated only by activation of Stat 1, a protein that is affected by both classes of IFN. IFN-alpha/beta did not raise the levels of Bcl-2 or Bcl-XL in T cells. Therefore, their activity was distinct from that of members of the interleukin 2 family or CD28 engagement. Since IFN-alpha/beta are very efficiently generated in response to viral and bacterial infections, these molecules may be among the signals that the immune system uses to prevent activated T cell death during infections.

V beta-specific stimulation of human T cells by staphylococcal toxins

The staphylococcal toxins are responsible for a number of diseases in man and other animals. Many of them have also long been known to be powerful T cell stimulants. They do not, however, stimulate all T cells. On the contrary, each toxin reacts with human T cells bearing particular V beta sequences as part of their receptors for major histocompatibility complex protein-associated antigen. The specificity of these toxins for V beta s puts them in the recently described class of superantigens and may account for the differential sensitivity of different individuals to the toxic effects of these proteins.

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.

The role of the T cell receptor in positive and negative selection of developing T cells

Although many combinations of alpha beta T cell receptors are available to the T cells in any given organism, far fewer are actually used by mature T cells. The combinations used are limited by two selective processes, positive selection of T cells bearing receptors that will be useful to the host, and clonal elimination or inactivation of T cells bearing receptors that will be damaging to the host. The ways in which these two apparently contradictory processes occur, and the hypotheses that have been suggested to reconcile them, are discussed.

Detection of antigen-specific T cells with multivalent soluble class II MHC covalent peptide complexes

Multimeric soluble MHC class II molecules stably occupied with covalently attached peptides bind with appropriate specificity to T cell hybridomas and T cells from T cell receptor transgenic mice. There is a direct correlation between soluble T cell receptor affinity for monomeric MHC/peptide and level of binding of multimeric MHC/peptide to T cells. While binding of the multimeric MHC/peptide complex is proportional to T cell receptor affinity and expression level, there is little influence of T cell CD4.

Reactive oxygen species regulate activation-induced T cell apoptosis

Reactive oxygen species (ROS) mediate apoptosis in a number of cell types. We studied the role that ROS play in activated T cell apoptosis by activating T cells in vivo and then culturing them for a short time. Activated T cells died independently of Fas and TNF alpha. Their death was characterized by rapid loss of mitochondrial transmembrane potential (delta psi(m)), caspase-dependent DNA fragmentation, and superoxide generation. A superoxide dismutase mimetic, Mn (III) tetrakis (5, 10, 15, 20-benzoic acid) porphyrin (MnTBAP), protected T cells from superoxide generation, caspase-dependent DNA loss, loss of delta psi(m), and cell death. These results indicate that ROS can regulate signals involved in caspase activation and apoptosis and may contribute to peripheral T cell deletion.

Autoimmune disease: why and where it occurs

Autoimmune disease is controlled by genetic and environmental factors. Both of these affect susceptibility to autoimmunity at three levels: the overall reactivity of the immune system, the specific antigen and its presentation, and the target issue.

The T cell receptor

The primary structure of T cell receptor proteins and genes is well understood. Immunologists are now trying to understand the properties of these interesting molecules. Evidence suggests that T cell alpha beta receptors recognize a complex of an antigen-derived peptide bound to one of the cell-surface products of the major histocompatibility complex (MHC) genes. It is likely that alpha beta receptors and MHC proteins have coevolved to have some affinity for each other. During T cell development in the thymus, cells bearing self-reactive receptors are deleted by the mechanisms of tolerance, and cells are preferentially allowed to mature if they bear receptors that will be able to recognize antigen plus self-MHC after they have become full-fledged T cells. Some explanations for these phenomena have been tested, but no satisfactory theory can yet be proposed to account for them.

The repertoire of T cells shaped by a single MHC/peptide ligand

Although the thymus produces many immature thymocytes, few of these cells mature. Positive selection has been thought to limit thymocyte development. In thymuses expressing a single MHC/peptide combination, however, surprisingly large numbers of thymocytes are selected to mature. Many of these react with the selecting MHC, bound to other self-peptides. Therefore, the number of thymocytes that mature is limited by the fact that positively selected cells die because they react too well with MHC bound to self-peptides that are not identical to those involved in positive selection. T cells that mature in thymuses expressing a single MHC/peptide ligand react frequently with foreign MHC, suggesting that the repertoire of alpha beta receptors may be more biased toward reaction with MHC than was previously thought.

T cells compete for access to antigen-bearing antigen-presenting cells

These studies tested whether antigenic competition between T cells occurs. We generated CD8(+) T cell responses in H-2(b) mice against the dominant ovalbumin epitope SIINFEKL (ova8) and subdominant epitope KRVVFDKL, using either vaccinia virus expressing ovalbumin (VV-ova) or peptide-pulsed dendritic cells. CD8(+) T cell responses were visualized by major histocompatibility complex class I-peptide tetrameric molecules. Transfer of transgenic T cells with high affinity for ova8 (OT1 T cells) completely inhibited the response of host antigen-specific T cells to either antigen, demonstrating that T cells can directly compete with each other for response to antigen. OT1 cells also inhibited CD8(+) T cell responses to an unrelated peptide, SIYRYGGL, providing it was presented on the same dendritic cells as ova8. These inhibitions were not due to a more rapid clearance of virus or antigen-presenting cells (APCs) by the OT1 cells. Rather, the inhibition was caused by competition for antigen and antigen-bearing cells, since it could be overcome by the injection of large numbers of antigen-pulsed dendritic cells. These results imply that common properties of T cell responses, such as epitope dominance and secondary response affinity maturation, are the result of competitive interactions between antigen-bearing APC and T cell subsets.

The antigen-specific, major histocompatibility complex-restricted receptor on T cells. VI. An antibody to a receptor allotype

We have prepared a monoclonal antibody, KJ16-133, from the cells of a rat immunized with the purified receptor for antigen plus I-A of a BALB/c T cell hybridoma, DO-11.10. Unlike most other monoclonal anti-receptor antibodies that have been described before, KJ16-133 is not clone specific. It reacts with approximately 20% of the receptors on T cells of normal BALB/c mice. It also reacts with about the same percentage of antigen-specific, major histocompatibility complex (MHC)-restricted or allogeneic I-region specific T cell hybridomas. Reaction of KJ16-133 with a given T cell hybridoma does not seem to depend on the antigen specificity or MHC-restricting element of the T cell in question. The determinant recognized by KJ16-133 has some unexpected properties. It is absent in several strains of mice including SJL/J and SJA/20, but present on the T cells of most other commonly used strains. The determinant recognized therefore does not map to Igh. Our experiments suggest that a clone-specific "antiidiotypic" antibody and KJ16-133 recognize determinants on different parts of the receptor. For example, the binding of a clone-specific antibody to target T cells is relatively temperature insensitive, whereas KJ16-133 binds well to cells at 37 degrees C but poorly to cells at 4 degrees C. The determinant recognized by a clone-specific antibody is sensitive to reduction and alkylation of the receptor, whereas KJ16-133 reactivity is not. Finally, binding of KJ16-133 at saturating concentrations to target T cells does not block the binding of a clone-specific antibody. Similarly, binding of a clone-specific antibody only marginally inhibits binding of KJ16-133. Taken together, these results suggest that KJ16-133 is directed against an allelic determinant on T cells that may be close to the membrane, and not in the receptor binding site for antigen plus MHC. The antibody may recognize an allele of a constant region isotype, or an allele of a J region.

Superantigens interact with MHC class II molecules outside of the antigen groove

Superantigens, including the staphylococcal enterotoxins and the minor lymphocyte stimulatory antigens, are highly potent immunostimulatory molecules, capable of activating virtually all T cells that express particular T cell receptor (TCR) variable regions. Superantigen stimulation of T lymphocytes depends on major histocompatibility complex (MHC) class II molecules, so there has been some debate as to whether superantigens interact with the antigen binding "groove" on class II complexes, just like conventional peptide antigens, or whether they bind elsewhere and serve as TCR coligands. We compared the presentation of peptide antigens and superantigens by a panel of mutant-presenting cell lines, each displaying an A kappa alpha chain with a single alanine replacement along the alpha helix proposed to form one face of the groove. The negligible effect of these 30 mutations on superantigen presentation, versus their drastic consequences for peptide presentation, prompts us to conclude that superantigens interact with MHC class II molecules outside the groove.

Cytokine-induced survival of activated T cells in vitro and in vivo

Many antigen-specific T cells die after exposure to antigen in animals. These cells also die if they are isolated from animals shortly after activation and cultured. Various cytokines were tested for their ability to interfere with this in vitro death. Surprisingly, tumor necrosis factor alpha and other inflammatory cytokines did not prevent the in vitro death of activated T cells, even though these cytokines do prevent activated T cell death in animals. Therefore, the inflammatory cytokines probably act on T cells in vivo via an intermediary factor. Four cytokines, interleukin (IL)-2, IL-4, IL-7, and IL-15, did prevent activated T cell death in vitro, with IL-4 and IL-15 more effective than IL-2 or IL-7. These cytokines share a component of their receptors, the common gamma chain, gammac. Therefore, their collective ability to protect activated T cells from death may be mediated by signals involving gammac. To assess their activity in vivo, two of the cytokines, IL-2 and IL-4, were expressed in animals at local sites of superantigen responses. Both cytokines increased the numbers of T cells found at the local sites 14 days later. Interleukin 4 was more effective than IL-2, even though IL-2 stimulates T cell proliferation better than IL-4. This result suggested that IL-4 and related cytokines can promote T cell survival in vivo as well as in vitro. The ability of these cytokines to prevent the death of activated T cells may be important at certain stages of immune responses in animals.

The major histocompatibility complex-restricted antigen receptor on T cells. II. Role of the L3T4 product

We have examined the role of the murine homologue of Leu-3 T4, L3T4, in recognition of antigen in association with products of the major histocompatibility complex (Ag/MHC) by murine T cell hybridomas. A series of ovalbumin (OVA)/I-Ad-specific T cell hybridomas were ranked in their sensitivity to Ag/I by measuring their ability to respond to low doses of OVA, or their sensitivity to inhibition by anti-I-Ad antibodies. T cell hybridomas with low apparent avidity for OVA/I-Ad, i.e. that did not respond well to low concentrations of OVA and were easily inhibited by anti-I-Ad, were also easily inhibited by anti-L3T4 antibodies. The reverse was true for T cell hybridomas with apparent high avidity for Ag/MHC. We found that the presence of low doses of anti-L3T4 antibodies caused T cell hybridomas to respond less well to low doses of Ag, and to be more easily inhibited by anti-I-Ad antibodies. These results suggested that the role of the L3T4 molecule is to increase the overall avidity of the reaction between T cells and Ag-presenting cells. In support of this idea was the discovery of several L3T4- subclones of one of our L3T4+ T cell hybridomas, D0.11.10. The L3T4- subclones had the same amount of receptor for OVA/I-Ad as their L3T4+ parent, as detected by an anti-receptor monoclonal antibody. The L3T4- subclones, however, responded less well to low doses of OVA, and were more easily inhibited by anti-I-Ad antibodies than their L3T4/ parent. These results showed that the L3T4 molecule was not required for surface expression of, or functional activity of, the T cell receptor for Ag/MHC. The L3T4 molecule did, however, increase the sensitivity with which the T cell reacted with Ag/MHC on Ag-presenting cells.

Selective expansion of T cells expressing V beta 2 in toxic shock syndrome

Infection with Staphylococcus aureus and the production of toxic shock syndrome toxin-1 (TSST-1) have been implicated in the pathogenesis of toxic shock syndrome. Previous in vitro studies have demonstrated that TSST-1 is a powerful but selective stimulator of human T cells, and that the majority of activated cells express the TCR V beta 2 gene segment. We therefore studied patients with toxic shock syndrome using a modification of the PCR to determine if expansion of V beta 2+ T cells is a marker of the in vivo disease process. Five of eight patients studied demonstrated markedly elevated levels of circulating V beta 2+ T cells, whereas none showed significantly elevated levels of T cells expressing other V beta gene segments. The results suggest that toxin-mediated T cell activation, which involves a large fraction of the human T cell repertoire, may be critical in the pathogenesis of this disease.

Functional interaction between human T-cell protein CD4 and the major histocompatibility complex HLA-DR antigen

Mature T cells segregate phenotypically into one of two classes: those that express the surface glycoprotein CD4, and those that express the glycoprotein CD8. The CD4 molecule is expressed primarily on helper T cells whereas CD8 is found on cytotoxic and suppressor cells. A more stringent association exists, however, between these T-cell subsets and the major histocompatibility complex (MHC) gene products recognized by their T-cell receptors (TCRs). CD8+ lymphocytes interact with targets expressing class I MHC gene products, whereas CD4+ cells interact with class II MHC-bearing targets. To explain this association, it has been proposed that these 'accessory' molecules bind to monomorphic regions of the MHC proteins on the target cell, CD4 to class II and CD8 to class I products. This binding could hold the T cell and its target together, thus improving the probability of the formation of the trimolecular antigen: MHC: TCR complex. Because the TCR on CD4+ cells binds antigen in association with class II MHC, it has been difficult to design experiments to detect the association of CD4 with a class II molecule. To address this issue, we devised a xenogeneic system in which human CD4 complementary DNA was transfected into the murine CD4-, CD8- T-cell hybridoma 3DT-52.5.8, the TCR of which recognizes the murine class I molecule H-2Dd. The murine H-2Dd-bearing target cell line, P815, was cotransfected with human class II HLA-DR alpha, beta and invariant chain cDNAs. Co-culture of the parental T-cell and P815 lines, or of one parental and one transfected line resulted in a low baseline response. In contrast, a substantial increase in response was observed when CD4+ 3DT-52.5.8 cells were co-cultured with HLA-DR+ P815 cells. This result strongly indicates that CD4:HLA-DR binding occurs in this system and that this interaction augments T-cell activation.

A maternally inherited superantigen encoded by a mammary tumour virus

A collection of superantigens, molecules which in combination with class II major histocompatibility complex (MHC) engage T cells bearing particular V beta chains as part of their alpha beta receptors, have recently been described. The mouse self superantigen, Mls-1a, for example, in conjunction with many MHC class II proteins, engages mouse T cells bearing V beta 6, 7, 8.1 and 9, almost regardless of the sequences of the other variable components of the receptors on the T cells. Two types of superantigen have been identified so far: first, superantigens encoded in the mouse genome, such as Mls-1a; second, superantigens produced by bacteria, such as the staphylococcal enterotoxins. Although the latter type of superantigens are in many cases known to be proteins of about 220 amino acids, nothing is known about the structures of any of the superantigens encoded in mouse. Here we describe the properties of a new mouse superantigen. The antigen is maternally transmitted in milk and is probably encoded by a mammary tumour virus (MTV). Given the known genetic linkage between at least one of the mouse genomic superantigens and endogenous MTV integration sites, it is tempting to speculate that the superantigen described here and some of the endogenous mouse superantigens are encoded by MTVs.

The toxicity of staphylococcal enterotoxin B in mice is mediated by T cells

Staphylococcal enterotoxin B (SEB) has been shown in the past to be a potent T cell stimulant in mouse or man. The toxin acts as a superantigen that is, it binds to class II MHC proteins and, as such a complex, stimulates T cells bearing particular V beta s as part of their receptors. The toxin also has several pathological effects, causing, in mice, rapid weight loss, thymus atrophy, immunosuppression, and, at high doses, death. The data in this paper show that at least one of these effects, weight loss, is T cell mediated. Staphylococcal enterotoxin-mediated weight loss is MHC dependent, and is almost absent in animals expressing MHC class II molecules, which, complexed with SEB, are poor T cell stimulants. Also, mice that lack T cell function, genetically or because of cyclosporin A treatment, lose no or less weight than controls in response to SEB. Finally, animals bred such that they express few T cells bearing V beta s with which SEB can interact lose much less weight in response to the toxin than littermate controls that have higher numbers of reactive T cells. It is therefore suggested that the pathological effects of the staphylococcal, T cell-stimulating toxins in mouse and man may be partially or wholly the consequence of massive T cell stimulation.

The effect of thymus environment on T cell development and tolerance

During development in the thymus, T cells are deleted if their receptors are able to recognize self major histocompatibility complex (MHC) proteins. We show that such clonal deletion can occur because of interaction between receptors on T cells and MHC expressed on bone marrow-derived cells. In addition, development in the thymus picks out T cells to mature if their receptors will be restricted for antigen recognition in association with self MHC alleles expressed on thymus epithelial cells. This process is usually thought to involve positive selection of T cells bearing receptors with high and low affinity for MHC on thymus epithelium, and subsequent deletion of high affinity cells by interaction with bone marrow-derived cells. Our data do not fit such a model, but rather suggest that MHC molecules on thymus epithelium and bone marrow-derived cells may not be seen identically by T cell receptors.