Staphylococcus aureusenterotoxins: a key in airway disease? (2002)

Staphylococcus aureus (S. aureus), one of three pathogenic species of the Gram-positive cocci, is often found as part of the normal microflora of the human skin, the upper respiratory tract, especially the vestibulum nasi, and the intestinal tract. Transmission of the organism is frequently achieved by direct contact with an infected individual, although it may also be airborne or infect via fomites. It is thought that about 25% of the population are permanent carriers of S. aureus and whilst approximately 20% of all human staphylococcal infections are autogenous, several factors have been identified that predispose the host to increased susceptibility to infection by S. aureus (1). These include injury to skin or mucous membrane, abnormal leukocyte function, viral infections (e.g., influenza), metabolic abnormalities (e.g., diabetes mellitus and uraemia) and miscellaneous conditions (e.g., malnutrition, old age, malignancies, etc.). Under appropriate conditions, the organism leads to a variety of clinical conditions, affecting the skin, lungs, heart, central nervous system (CNS), bones and joints, gastrointestinal tract and the blood system (1). Skin infections, lung abscesses, pneumonia, osteomyelitis, food poisoning, fever, scalded skin syndrome and toxic shock syndrome are among the most common conditions. Although the pathogenicity of S. aureus is closely correlated with the production of coagulase enzymes, and many conditions result as a consequence of invasion by the bacterium, these organisms also contain a number of cellular antigens and produce a variety of toxins with superantigenic properties (2, 3). Recent studies of these staphylococcal superantigens have demonstrated that they can influence the activity of both immuno-modulatory and pro-inflammatory effector cell types, and therefore may have a potentially important role in the pathogenesis of chronic inflammatory disease. Staphylococcal superantigens are a group of high-molecular weight pyrogenic proteins that have in common an extremely potent stimulatory activity for T lymphocytes, including CD4+ , CD8+ , and gamma delta + T cells, of several species (4). Several staphylococcal superantigens have been described, of which the staphylococcal enterotoxins (SEs) are the most widely studied. The SEs are a family of structurally related heat-stable proteins of approximately 27 kDa molecular mass comprising several major serological types: five prototypic SEs (types A to E) and three newly characterized SEs (types G to I) (2, 5). More recently a novel 26 kDa molecular mass SE, serological type K, which has many of the biological activities associated with other SEs, including superantigenicity, pyrogenicity and enhancement of endotoxin lethality, has been shown to be secreted from clinical isolates of S. aureus (6). Other studies have suggested the existence of variants of some of these major serological types. Abe and colleagues (5) have cloned a superantigen variant of SEG (SEGv) from clinical isolates of S. aureus and, using the polymerase chain reaction (PCR), have demonstrated that SEG or SEGv is one of the most frequently produced superantigens by S. aureus. Similarly, variants of SEA, SEC and SEE with superantigenic properties have also been demonstrated (7–9). More recently, the effect of a mutated variant of SEA with no measurable affinity for major histocompatibility complex (MHC) class II, has been shown to inhibit the wild type SEA-induced psoriasis in a severe combined immune-deficient (SCID)-hu xenogenic transplantation mouse model, suggesting that mutated variants of SEs with no superantigenic activity may have a therapeutic potential in the management of staphylococcal induced disease (10). Toxic shock syndrome toxin (TSST-1), a causative agent of toxic shock syndrome, is another staphylococcal toxin that has been widely investigated and shown to possess superantigenic properties, as indicated by its ability to activate T cells to proliferate and secrete lymphokines and to act as a nominal antigen to induce proliferation and immunoglobulin (Ig) secretion in human B cells (11, 12). Similar to the SEs, studies have demonstrated that there are naturally occurring variants of also this superantigen, which may or may not be fully effective as the wild type TSST-1 (13, 14). In addition to these most commonly occurring superantigenic toxins, some strains of S. aureus have also been shown to produce exfoliative toxins (ETs), which are the causative agents of staphylococcal scalded skin syndrome, a blistering skin disorder that predominantly affects children (15). Although there have been conflicting reports on the superantigenic nature of these agents, a recent study has confirmed this by the use of highly purified ETA and ETB to demonstrate that these agents induce selective polyclonal expansion of several human T cells (16). Additionally, ETB was also shown to be moderately pyrogenic and enhanced susceptibility to lethal shock in rabbits. Studies of staphylococcal virulence factor, protein A, have suggested that this toxin also has superantigenic properties because it directly influences the activity of a subset of B cells and leads to a T-cell-independent depletion of these cells in vivo (17, 18). Structural studies have demonstrated that pyrogenic superantigens produced by S. aureus, and indeed Streptococcus pyogenes, are all similar in size and have a conserved two domain tertiary fold, of which the structure of domain 1 is similar to the Ig-binding motif of streptococcal proteins G and L, and the structure of domain 2 is similar to the oligosaccharide/oligonucleotide-binding motifs of the AB[5] heat-labile enterotoxins, cholera toxin, pertussis toxin, and verotoxin (19). The close structural homology amongst the bacterial superantigens suggests that they may have evolved through the recombination of two smaller beta-strand motifs. Mechanistic studies have shown that superantigens stimulate the T cells by cross-linking the variable part on the beta chain of the T-cell receptor (TCR) with MHC class II molecules on accessory or target T cells, outside the peptide-binding groove area (20, 21), although binding to MHC class I molecule has also been observed in a MHC class II-negative epidermal cell line (22). This leads to stimulation of up to 20–25% of the naive T-cell population in a nonspecific way, compared with stimulation of only about 0.1% of the T-cell population via the conventional allergen-specific MHC-restricted route utilizing both TCR-Vα and β chains (23, 24). Additionally, mutational analysis of the TCR variable beta domain, combined with the crystal structure of the TCR beta chain, have demonstrated that the recognition sites for these bacterial superantigens are distinct from those recognized by viral superantigens, which also exaggerate T-cell activity in a similar MHC-unrestricted manner (25). Crystallographic studies have also shown that there is considerable homology between the superantigen-binding site and peptide/MHC-binding sites of the TCR, which allows the superantigen to act as a ‘wedge’ between the TCR and MHC (8). This leads to displacement of the antigenic peptide away from the TCR-binding site and results in the superantigen circumventing the normal mechanism for T-cell activation by specific peptide/MHC complexes. Similarly, a study employing the use of a plasmon resonance biosensor to characterize the kinetics of the trimolecular interaction between TCR, MHC class II and the superantigen SEA has demonstrated that although the binding between the individual components is very weak, the stability of the trimolecular complex is significantly increased, to a level similar to that seen for the interaction between TCR and MHC : peptide ligand (26). Consequently, it has been suggested that the potency of the SEA, and likely the other SEs, in activating the T cell is owing to the co-operative effect of interactions in the TCR-superantigen-MHC class II trimolecular complex rather than as a result of particularly strong affinities between the various proteins. A study investigating the interaction between TSST-1 and MHC class II, however, has suggested that this is controlled by the carboxyl (C)-terminal residues of the class II associated peptides (27). It appears that binding/presentation of TSST-1 to MHC class II does not involve cognate interaction between the peptide and TSST-1, but is dependent on the length of the C-terminal region. Thus, many naturally processed class II/peptide complexes with ‘inappropriate’ C-terminal regions may not necessarily present TSST-1 to T-cells. Studies investigating the effect of staphylococcal superantigens, in particular enterotoxins A to E (SEA to SEE) and TSST-1, on T-cell function have been pivotal in understanding the etiology and pathogenesis of S. aureus-induced disease. They, particularly in transgenic KO mice, have demonstrated that injection of these superantigens lead to T-cell proliferation/polyclonal expansion, pro-inflammatory cytokine production, and programmed cell death (apoptosis), as well as T-cell anergy in vivo (28). Compared with normal IL-(interleukin)-4 + mice, SEB increased the synthesis of interferon-(IFNγ) and decreased the synthesis of IL-5 in IL-4 knockout (KO) mice, in which the IL-4 gene was deleted (29). Additionally, this was accompanied by a partial deletion and anergy of the remaining SEB-reactive T cells, in particular the CD8Vbeta8 + T cells, and increased susceptibility to low-dose D-galactosamine-induced shock in the KO mice, but not the normal mice, suggesting that IL-4 is essential in the rescue of CD8+ T-cells from SEB-induced apoptosis and anergy. Indeed, studies of patients who develop glomerulonephritis following methicillin-resistant S. aureus infection have also demonstrated that there is a marked increase in CD4+ , CD8+ , and TCR Vbeta + T cells and a variety of cytokines, including IL-1β, IL-2, IL-6, IL-8, IL-10, and tumour necrosis factor-alpha (TNFα), compared with patients not developing glomerulonephritis or normal healthy subjects (30). These findings for staphylococcal superantigen-induced human T-cell proliferation and pro-inflammatory cytokine release from these cells have been confirmed in vitro by several groups of workers. Many of these studies have additionally provided useful information with respect to the mechanisms underlying these effects. Investigation of cell-surface molecules that can mediate staphylococcal TSST-1 and enterotoxin SEB/SEC1/SED-induced T-cell proliferation and production of cytokines have indicated that CD2, CD11a, CD18, CD28, CD44, CD58 and ICAM-1 are likely to be involved, because these effects could be inhibited by the treatment of the cells with monoclonal antibodies (MoAbs) to these molecules (31–33). A more recent study has suggested that T-cell anergy may be mediated by the direct effect of IL-10 on the CD28-signaling pathway (34). The study demonstrated that IL-10 inhibited the phosphorylation of CD28 and subsequently inhibited the production of the Th1 and Th2 cytokines, including IL-2, IFN-γ, IL-4, IL-5 and IL-13. In contrast, the treatment of the cells with anti-IL-10 MoAbs significantly increased both the T-cell proliferation and the secretion of Th1 and Th2 cytokines. Furthermore, the high viability and the unresponsive state of T cells tolerized to IL-10 was reversed by the stimulation with anti-CD3 MoAb and IL-12, but not by stimulation with anti-CD28 antibody. Although a large number of studies investigating the effects of staphylococcal superantigens have focused on the T-cell function, there is increasing evidence that the staphylococcal superantigens can also directly affect the frequency and activation of B cells, and subsequently influence the expression of the B-cell repertoire. Studies, predominantly with S. aureus protein A, have demonstrated that this superantigen influences the activity of B cells by binding the Fab region of human Igs whose heavy chains are encoded by V (H) clan III genes (17, 35). More recently, the interaction between S. aureus protein A and the Fab fragment of human IgM antibody has been investigated by crystallography, and shown to occur between specific regions in domain D of protein A and the variable region of the Fab heavy chain, without involvement of the hypervariable regions (36). Similarly, sequence analysis of VH3 gene products and binding with the Ig VH3 domain have indicated that there are several sites that can interact with SEA to stimulate B-cell activity (37). Functional studies in B cells have shown that S. aureus protein A induces proliferation of these cells, following the signal transduction involving protein kinase C (PKC), mitogen-activated protein kinase (MAP kinase), serum responsive factor (SRF) and Bcl-2 gene expression (38). SEB and SED, however, stimulate human B cells expressing VH3- and VH4-containing IgM, respectively (37, 39). Whilst SEB does not induce T-cell independent proliferation nor differentiation of VH3-expressing cells, it enhances the survival of these B cells. In contrast, SED induces the T-cell dependent polyclonal proliferation and differentiation of VH4-expressing cells, and additionally enhances the T-cell-independent survival of these cells, suggesting that both these staphylococcal enterotoxins may function as unique B-cell superantigens by rescuing them from apoptosis. Studies with TSST-1 have shown that this staphylococcal superantigen may play an important role in the modulation of allergic disease, because it can augment isotype switching and synthesis of IgE both in vitro (40, 41) and in vivo, in a SCID-mouse model (42). Although TSST-1-induced activation of B cells in vitro is indirect and dependent on the increased expression of CD40 ligand on T cells, a more recent study has provided additional evidence for a direct effect by demonstrating TSST-1-induced expression on B cells of B7.2 (43), a molecule that has been shown to enhance Th2 responses and to be involved in IgE regulation. Compared with studies on lymphocytes, there are comparatively fewer studies investigating the effects of staphylococcal superantigens on other pro-inflammatory cell types, in particular eosinophils, macrophages, mast cells, and epithelial cells, which are known to play key roles in the pathogenesis of inflammatory airway disease. Studies on eosinophils have mostly described the role of these cells as accessory cells, which stimulate T cells by presenting staphylococcal superantigens via the MHC class II molecules expressed on their cell surface. Incubation of human eosinophils in the presence of granulocyte macrophage-colony stimulating factor (GM-CSF), leads to an increased expression of HLA-DR (a MHC class II molecule) and a subsequent proliferation of resting CD4+ T cells, in response to SEA, SEB and SEE (44). Although eosinophil-mediated T-cell proliferation is correlated with the proportion of HLA-DR-expressing eosinophils, eosinophils are not as efficient as macrophages in inducing proliferation of T cells in response to the staphylococcal superantigens (44). A preliminary study has recently suggested that SEA, SEB, SEC, SED and TSST-1 may also directly affect the eosinophil activity by inhibiting apoptosis and modulating important cell-surface antigens, including the upregulation of CD11b, CD45 and CD69, the and downregulation of CD34 and CD54 expression (45). Macrophages also act as accessory cells for a T-cell activation/proliferation response to staphylococcal superantigens (44, 46, 47), and direct effects of superantigens on macrophage activity have been documented, in particular the production of cytokines. Studies of human alveolar macrophages have shown that the with SEA leads to a increase in synthesis and secretion of and Incubation of a line with SEA, however, to the release of of SEA-induced IL-8, which increased by treatment of the cell line with suggesting that of the cell to the macrophage the release of IL-8, and likely other cytokines of MHC class I with class I antibodies or MHC class I SEA with in MHC class macrophages, also leads to the release of and from these cells suggesting that SEA may activate macrophages via both MHC class I and MHC class II Studies with SEB have also demonstrated that this superantigen induces production in macrophages of the gene in macrophages signal transduction from binding of SEB with MHC class II which lead to the activation of and by the activation and of cells can be directly by staphylococcal superantigens and also act as accessory cells for the activation of T cells, similar to eosinophils and Staphylococcal protein A is of cross-linking IgE molecules on mast cells by a structure the of the and colleagues have investigated the effect of staphylococcal protein A on the release of several from mast cells from human and demonstrated that this superantigen to the increased release of and of the cells with monoclonal IgM V + inhibited the protein with protein A in to a subsequent with suggesting that an interaction between protein A and IgE on the mast cells was important in the effect of the In contrast, SEB was not shown to release from a human line and indeed to a of IL-4 release More recently, however, one study of human mast cells and a line expressing MHC class II antigens demonstrated that SEB and TSST-1 CD4+ T-cell and induced the release of from the mast cells The superantigen-induced release of was dependent on contact between the mast cells and T cells and was inhibited by MHC class II suggesting an indirect activation of mast cells by the cells have also been shown to release pro-inflammatory cytokines, including and IL-8, following treatment with SEB of epithelial cells with T cells also leads to indirect effects on epithelial cell activity and function, as a result of epithelial accessory cell function for superantigen-induced T-cell the most and important effects of this interaction are on epithelial and which are by SEB and thought to be owing to the increased release of and from the T cells It has been demonstrated that these epithelial effects may be by IL-10 and factor In of the increasing evidence for the effects of staphylococcal superantigens on immuno-modulatory and pro-inflammatory cells, it is likely that there is an between staphylococcal infection and pathogenesis of as and which is by these cells. The role of staphylococcal superantigens in to the to as has recently been by and colleagues and therefore be only in the present to with airway disease. studies of patients with have shown that there is S. aureus in the skin of patients than in the skin of normal healthy subjects and that the organism often up to of the normal in the S. aureus from the skin of of the patients secrete superantigens SEA, SEB, SEC, SED and TSST-1, which the and they interact with cells of the to a The is by the superantigens by one of two polyclonal activation of T cells and of T cells that are to the of IgE which subsequently play a role in The superantigens also influence by the activity of both and cells by binding to MHC class I and class II which results in the increased expression of molecules and inflammatory cytokines in these cells. Compared with studies have the role of S. aureus or its superantigens in allergic or airway disease. suggested an to as an important of of disease in patients from allergic airway disease the involved bacterial and highly and no found among these have recently investigated from patients with which are characterized by a severe and often with for specific IgE to a variety of and S. aureus enterotoxins in to demonstrated that in about of the specific IgE to Staphylococcus aureus enterotoxins SEA SEB was and was to a polyclonal IgE is in IgE compared to the was significantly in of synthesis of and and these patients more often from Mechanistic studies have shown that S. aureus can induce the release of from of both and and additionally release of in to It appears that the expression of T cells in of patients with controlled is also significantly increased, compared with cells in of patients with and normal suggesting that the staphylococcal superantigens are a potential of T-cell activation in controlled More recent however, have suggested that the staphylococcal superantigens may lead to disease because they can induce in blood cells Compared with the level of in proliferation of in response to the level of in proliferation of in response to treatment with and TSST-1 was Furthermore, SEB also significantly increased the expression of receptor compared with and cells. Similarly, a of normal B cells B cells has shown that although both cell efficient in presenting S. aureus superantigen to an T-cell the activity of the B cells, but not B cells, was inhibited by the treatment with Whilst the effect of superantigens has not been investigated on airway in for studies in have shown that SEB airway of several pro-inflammatory cell TCR Vbeta and TCR Vbeta T cells, eosinophils, and and release of cytokines IL-4 and but not which are associated with increased airway in these the increase in airway was particularly CD4+ and was observed in the of or allergen-specific Although particularly in patients from have the presence of S. aureus as one of the more common and in the there is a very marked of studies an between S. aureus or its superantigens and of upper disease, particularly allergic or chronic recent however, has provided some which suggests that S. aureus superantigens may disease in patients from allergic The of of and S. aureus found to be significantly in compared with More significantly in S. compared with S. although no was observed between and superantigen Furthermore, in vitro of the response of blood to SEB or TSST-1, indicated that the of patients to a significantly than the of and also produced significantly of IL-4 and IL-5 in a In contrast, the seen to produce significantly of IFN-γ, compared with from Although the presence of S. aureus has also been as a common in there is or no information on the role of staphylococcal superantigens in this disease. however, have demonstrated that the of and SEB-induced of in both and although the of disease was in the strains however, was by the of cells the alveolar and and an increase in in is increasing evidence that staphylococcal toxins act as both superantigens and and therefore play a role in modulating chronic inflammatory airway disease, both via and Studies of patients predominantly from have demonstrated between the of disease and the with S. aureus or presence of staphylococcal superantigens, a causative role for S. aureus in the pathogenesis of Similarly, there is increasing evidence that S. aureus superantigens may also have the potential to influence the of upper and airway disease, including and The evidence for the role of staphylococcal superantigens in the etiology of allergic disease, however, is preliminary and the of Consequently, more studies and controlled clinical are to the role of S. aureus and its superantigens in the etiology of these Additionally, understanding the superantigen function may also novel therapeutic for these