Staphylococcal and streptococcal pyrogenic toxins involved in toxic shock syndrome and related illnesses

Toxic shock syndrome without rash in a young child: link with syndrome of hemorrhagic shock and encephalopathy?

A diffuse macular erythroderma and subsequent desquamation after 1 to 2 weeks are two of the five major diagnostic criteria of toxic shock syndrome (TSS). We present the case of a 15-month-old girl with TSS, but without erythroderma or desquamation. She was admitted with high fever, shock, and multiorgan involvement. Minimal or no cutaneous signs were present. Initially the diagnosis of the syndrome of hemorrhagic shock and encephalopathy was made. After 7 days, a TSS toxin 1-producing strain of Staphylococcus aureus was cultured from an inguinal lymph node, where inflammation had already been noticed on admission. Moreover, the girl had no antibodies against this toxin. The serum cytokine profile during the acute phase of her illness showed high levels of tumor necrosis factor-alpha, interleukin-6 and interferon-gamma, as is seen during activation of the immune system by TSS toxin 1. Other possible causes for the patient's illness were excluded. We conclude that the patient had TSS without rash. Without the evidence implicating a TSS toxin 1-producing strain of S. aureus as the cause of her disease, a diagnosis of syndrome of hemorrhagic shock and encephalopathy would have been made. It is possible that some cases of syndrome of hemorrhagic shock and encephalopathy represent a variant of TSS in small children.

Analysis of toxicity of streptococcal pyrogenic exotoxin A mutants

Streptococcal pyrogenic exotoxin A (SPE A) is secreted by some strains of Streptococcus pyogenes and is strongly associated with streptococcal toxic shock syndrome (STSS), a severe and often fatal illness. SPE A possesses a number of biological properties, some of which are shared with a group of exotoxins of streptococcal and staphylococcal origins, the pyrogenic toxin superantigens (PTSAgs). SPE A's most extensively studied property is superantigenicity. Superantigenic activation of T cells and monocytes stimulates the release of cytokines such as tumor necrosis factors alpha and beta, interleukin 1, and gamma interferon. These endogenous mediators are considered to be the primary cause of capillary leak, hypotension, and shock, the most severe manifestations of STSS. However, several studies have suggested that other properties of SPE A, such as ability to greatly enhance host susceptibility to endotoxin and ability to interact directly with endothelial cells, may play substantial roles in the syndrome. In this work we generated single- and double-site mutations of SPE A at residues K16, N20, C87, C90, C98, K157, S195, N20/C98, and N20/K157. The mutant SPE A's were analyzed in vivo for their lethal activity and in vitro for their superantigenic ability. Our results indicate that SPE A's ability to induce lethality and endotoxin enhancement does not require superantigenicity, and conversely superantigenicity does not necessarily lead to lethality. Thus, these properties and their relative contributions to the onset of hypotension and shock may be separable. Furthermore, evidence is presented that certain mutant toxins may be suitable for use as vaccine toxoids.

Refined structures of three crystal forms of toxic shock syndrome toxin-1 and of a tetramutant with reduced activity

The structure of toxic shock syndrome toxin-1 (TSST-1), the causative agent in toxic shock syndrome, has been determined in three crystal forms. The three structural models have been refined to R-factors of 0.154, 0.150, and 0.198 at resolutions of 2.05 A, 2.90 A, and 2.75 A, respectively. One crystal form of TSST-1 contains a zinc ion bound between two symmetry-related molecules. Although not required for biological activity, zinc dramatically potentiates the mitogenicity of TSST-1 at very low concentrations. In addition, the structure of the tetramutant TSST-1H [T69I, Y80W, E132K, I140T], which is nonmitogenic and does not amplify endotoxin shock, has been determined and refined in a fourth crystal form (R-factor = 0.173 to 1.9 A resolution).

Characterization of the canine type C enterotoxin produced by Staphylococcus intermedius pyoderma isolates

The type C staphylococcal enterotoxins (SECs) are a group of highly conserved proteins with substantial antigenic cross-reactivity. Although Staphylococcus intermedius and coagulase-positive species of staphylococci are reported to produce SEC and other SEs, toxins produced by species other than Staphylococcus aureus have not been previously characterized. In this study we report the molecular, biological, and immunological properties of the canine SEC (SECcanine) expressed by pathogenic isolates of S. intermedius. The mature form of SECcanine has 239 amino acid residues and a pI of 7.0. Typical of the SEs, purified SECcanine induces an emetic response in monkeys and the proliferation of T cells in a Vbeta-dependent manner. Although SECcanine has >95% sequence identity to previously described SEC variants, its sequence is most related to SEC2 and SEC3. In contrast to the sequence similarity, the Vbeta profile induced by SECcanine is typical of that induced by SEC1. This result is likely explained by the conservation of a cysteine residue at position 26 in SECcanine; residues at this position have been previously shown to determine subtype-dependent differences in T-cell receptor interactions of other SEs. Overall, these results show that superantigen toxins produced by the multiple members of the genus Staphylococcus are highly conserved in respect to biological and structural properties. Further, the frequent association of SECcanine with pyoderma in dogs supports the notion that the toxins are important for staphylococcal survival and pathogenesis.

The role of nitric oxide in experimental murine sepsis due to pyrogenic exotoxin A-producing Streptococcus pyogenes

Nitric oxide (NO) produced by inducible NO synthase (iNOS) mediates hypotension in endotoxemia. In this study, NO induction by a toxin-producing Streptococcus pyogenes isolate, H250, and by recombinant streptococcal pyrogenic exotoxin A (rSPEA) has been examined, both in vitro and in vivo. Streptococcal supernatants, but not rSPEA, induce production of nitrite by murine macrophages when both are coincubated with gamma interferon. Intraperitoneal injection of rSPEA did not cause significant production of NO. However, an elevated level of nitrate in serum was detected in a model of streptococcal fasciitis due to live H250. iNOS was localized to Kupffer cells, hepatocytes, and renal tubular cells by immunostaining. Administration of a NOS inhibitor, N(G)-monomethyl-L-arginine (L-NMMA), reduced peak concentrations of nitrate in serum but did not affect survival. NO is induced by H250, both in vitro and in vivo, mainly via SPEA-independent mechanisms. In this model, iNOS is expressed predominantly in the liver. Furthermore, in this model L-NMMA is not protective.

Transcytosis of staphylococcal superantigen toxins

Staphylococcus aureus produces a set of proteins (e.g., staphylococcal enterotoxin A [SEA], SEB, toxic shock syndrome toxin 1 [TSST-1]) which act both as superantigens (SAgs) and toxins. Although their mode of action as SAgs is well understood, little is known about how they enter the body via the intestine and cause food poisoning. To examine this problem we used an in vitro culture system to study the capacity of class II MHC-negative human intestinal epithelial cells (Caco-2) to transcytose several staphylococcal toxins. We found that Caco-2 cells are capable of dose-dependent, facilitated transcytosis of SEB and TSST-1, but not SEA. We extended these studies in vivo in mice by showing that ingested SEB appears in the blood more efficiently than SEA. Our data suggest that these toxins can cross the epithelium in an immunologically intact form. These results may have important implications for the pathogenesis of food poisoning.

Clinical role for a superantigen in Yersinia pseudotuberculosis infection

Yersinia pseudotuberculosis is an enteric pathogen that causes a variety of clinical symptoms in the human. Recently, we reported the production of a superantigen (Y. pseudotuberculosis-derived mitogen, YPM) by this organism and characterized the gene structure of ypm. To further study the potential pathogenic role of YPM in Y. pseudotuberculosis infection, we assayed IgG anti-YPM antibodies and T cell antigen receptor-Vbeta expression of the T cells in peripheral blood and in mesenteric lymph node in patients acutely infected with Y. pseudotuberculosis. 20 out of 33 patients (61%) had an elevated antibody titer compared with healthy controls (P = 0.0001). Patients with systemic symptoms such as lymphadenopathy, transient renal dysfunction, and arthritis had significantly higher titers of anti-YPM than patients with gastrointestinal tract symptoms alone. T cells bearing the Vbeta3 gene segment were significantly increased (P = 0.009) among acute phase patients compared with healthy children. During the convalescence phase of the illness, there was a reduction in the abnormal level of Vbeta3 T cells. Moreover, in the mesenteric lymph node, an elevated level of Vbeta3 T cells compared with peripheral blood and a sequence diversity in the junctional region of the T cell antigen receptor beta-chain containing Vbeta3 element was observed in one patient. Together, these findings suggest that YPM was produced in vivo and played an important role in the pathogenesis of Y. pseudotuberculosis infection.

Pediatric dermatology: advances in therapy

Several recent therapeutic advances in pediatric dermatology have been made. Of particular importance are new developments in the use of antimicrobials, antivirals, antifungals, retinoids, calcipotriene, and intravenous gamma globulin. We review safety and efficacy data of these drugs in their use in children with cutaneous disease.

The structure of the superantigen exfoliative toxin A suggests a novel regulation as a serine protease

Exfoliative toxin A (ETA) causes staphylococcal scalded skin syndrome which is characterized by a specific intraepidermal separation of layers of the skin. The mechanism by which ETA causes skin separation is unknown although protease or superantigen activity has been implicated. The X-ray crystal structure of ETA has been solved in two crystal forms to 2.1 and 2.3 A resolution and R-factors of 17% and 19%, respectively. The structures indicate that ETA belongs to the chymotrypsin-like family of serine proteases and cleaves substrates after acidic residues. The conformation of a loop adjacent to the catalytic site is suggested to be key in regulating the proteolytic activity of ETA through controlling whether the main chain carbonyl group of Pro192 occupies the oxyanion hole. A unique amino-terminal domain containing a 15-residue amphipathic alpha helix may also be involved in protease activation through binding a specific receptor. Substitution of the active site serine residue with cysteine abolishes the ability of ETA to produce the characteristic separation of epidermal layers but not its ability to induce T cell proliferation.

Staphylococcal enterotoxin B mutants (N23K and F44S): biological effects and vaccine potential in a mouse model

Superantigens produced by Staphylococcus aureus can cause food poisoning and toxic shock syndrome. The biological activities and vaccine potential of mutant staphylococcal enterotoxin B (SEB) proteins, N23K and F44S, were studied in a lipopolysaccharide-potentiated mouse model. Although 10 micrograms of SEB per mouse is equivalent to 30 LD50, the same intraperitoneal dose of either mutant protein was nonlethal and did not elevate serum levels of tumor necrosis factors (TNF). N23K, F44S, and SEB were serologically identical in an enzyme-linked immunosorbent assay with polyclonal anti-SEB. Immunization with alum containing N23K, F44S, or SEB elicited an anti-SEB response that protected 80-87% of the mice against a 10 micrograms SEB challenge. Controls lacking an anti-SEB titer did not survive. Pooled sera from immunized mice effectively blocked SEB-induced T-cell proliferation in vitro. Naive mice survived a lethal SEB challenge when given pooled antisera 1, 2, or 4 h later, whereas the antisera failed to protect animals when administered 6 or 8 h after the toxin. Lethality at the later times was consistent with increased serum levels of TNF observed 6 h after SEB injection. These studies suggest that the N23K and F44S mutant proteins of SEB are less biologically active than the wild-type toxin, yet retain epitopes useful for eliciting a protective antibody response.

B-cell superantigens: molecular and cellular implications

B cell superantigens are proteins that are capable of immunoglobulin variable region mediated binding interactions with the naive B cell repertoire at frequencies that are orders of magnitude greater than occur for conventional antigens. Within this review we discuss recent observations regarding the molecular basis of these interactions and the distribution of superantigen binding capacities in different human B cell populations. These findings and current predictions regarding the relevance of these proteins to the physiologic development of immune repertoires are also discussed.

Searching for superantigens

Superantigens comprise a large group of viral and bacterial proteins that stimulate T lymphocyte proliferation without regard for the antigenic specificity of the T cells but dependent on the composition of the variable part of the beta chain of the T cell receptor. Superantigens induce T cell proliferation dependent on class II MHC molecules on antigen presenting cells but do not require processing. Major subfamilies of superantigens include the viral superantigens, the bacterial pyrogenic toxin superantigens, and other bacterial superantigens. Two major approaches have been taken to identify superantigen association with human diseases: a) assessing V beta T cell receptor skewing in peripheral blood or tissues of patients with illnesses, b) recognition of toxic shock syndrome and related illnesses which are likely to be caused by superantigens.

Is the nasopharynx warmer in children than in adults?

Recent studies on the aetiology of the Sudden Infant Death Syndrome (SIDS) have suggested that some of these deaths are the consequence of an overwhelming inflammatory response to the production of pyrogenic toxins from bacteria colonizing the upper respiratory tract, particularly the nasopharynx. The pyrogenic toxins of Staphlococcus aureus, one of the likelier bacterial candidates, are only produced in temperatures of over 37 degrees C. This study examined nasopharyngeal temperatures in children. It is a preliminary study to develop an accurate means to measure how close to 37 degrees C the nasopharyngeal temperature lies in infants at the age when SIDS deaths occur. Following a pilot study and power calculation, measurements of nasopharyngeal temperature were made on 30 apyrexial children aged 4-10 years and 30 adults with no nasal pathology, undergoing surgery under general anaesthesia, using an accurately sited thermocouple probe. The mean temperature in children (35.64 degrees C) was significantly higher than in adults (34.01 degrees C). Comparable measurements attempted with the same subjects awake gave similar results.

Crystal structure of a T-cell receptor beta-chain complexed with a superantigen

Superantigens (SAgs) are viral or bacterial proteins that act as potent T-cell stimulants and have been implicated in a number of human diseases, including toxic shock syndrome, diabetes mellitus and multiple sclerosis. The interaction of SAgs with the T-cell receptor (TCR) and major histocompatibility complex (MHC) proteins results in the stimulation of a disproportionately large fraction of the T-cell population. We report here the crystal structures of the beta-chain of a TCR complexed with the Staphylococcus aureus enterotoxins C2 and C3 (SEC2, SEC3). These enterotoxins, which cause both toxic shock and food poisoning, bind in an identical way to the TCR beta-chain. The complementarity-determining region 2 (CDR2) of the beta-chain and, to lesser extents, CDR1 and hypervariable region 4 (HV4), bind in a cleft between the two domains of the SAgs. Thus, there is considerable overlap between the SAg-binding site and the peptide/MHC-binding sites of the TCR. A model of a TCR-SAg-MHC complex constructed from the crystal structures of (1) the beta-chain-SEC3 complex, (2) a complex between staphylococcal enterotoxin B (SEB) and an MHC molecule, and (3) a TCR V(alpha) domain, reveals that the SAg acts as a wedge between the TCR and MHC to displace the antigenic peptide away from the TCR combining site. In this way, the SAg is able to circumvent the normal mechanism for T-cell activation by specific peptide/MHC complexes.

A new procedure for the purification of streptococcal pyrogenic exotoxin A from Streptococcus pyogenes supernatant

An important role in the pathogenesis of invasive group A streptococcal disease has been ascribed to the production of streptococcal pyrogenic exotoxin A. We present a new technique for the purification of streptococcal pyrogenic exotoxin A from Streptococcus pyogenes NY-5 supernate, which is highly efficient with respect to yield (35%), purity (> or = 99%), and time.

Differential effects of staphylococcal toxic shock syndrome toxin-1 on B cell apoptosis

Superantigens, such as toxic shock syndrome toxin 1 (TSST-1), have been implicated in the pathogenesis of several autoimmune and allergic diseases associated with polyclonal B cell activation. In this report, we studied the in vitro effects of TSST-1 on B cell activation. We show herein that TSST-1 produced antagonistic effects on Ig synthesis by peripheral blood mononuclear cells (PBMC) from normal subjects, depending on the concentration used; Ig production was inhibited at 1000 pg/ml (P < 0.01) and enhanced at 1 and 0.01 pg/ml (P < 0.01) of toxin. Cultures of PBMC were then examined for morphologic features and DNA fragmentation characteristic for apoptosis. B cells exhibited a significantly higher (P < 0.01) incidence of apoptosis after stimulation with 1000 pg/ml of TSST-1 compared with 1 or 0.01 pg/ml of toxin or medium alone. Abundant expression of Fas, a cell surface protein that mediates apoptosis, was detected on B cells after stimulation with 1000 pg/ml of TSST-1 and was significantly higher on B cells undergoing apoptosis than on live cells (P = 0.01). Additionally, increased Fas expression and B cell death occurred at concentrations of TSST-1 inducing the production of high amounts of gamma interferon (IFN-gamma), and both events could be blocked by neutralizing anti-IFN-gamma antibody. These findings suggest that high concentrations of TSST-1 can induce IFN-gamma-dependent B cell apoptosis, whereas at low concentrations it stimulates Ig synthesis by PBMC from normal subjects. These findings support the concept that staphylococcal toxins have a role in B cell hyperactivity in autoimmunity and allergy.

Potentiation of lethal endotoxin shock by streptococcal pyrogenic exotoxin in rabbits: possible relevance of hyperreactivity of macrophages to endotoxin

Streptococcal pyrogenic exotoxin (SPE) potentiates lethal shock induced by endotoxin. We have previously reported that macrophages derived from SPE-treated rabbits showed hyperreactivity to endotoxin, and that the effect of SPE on macrophages was mediated by a lymphokine(s). Here we show that culture supernatants of SPE-stimulated lymphocytes, when administered into rabbits three hours before or together with endotoxin, potentiate a variety of endotoxin-induced pathophysiological changes and even lethal shock. These results suggest that SPE-induced lymphokine(s) mediates the potentiating effect of SPE on the lethal endotoxin shock through enhancing endotoxin reactivity of macrophages which play the central role in mediating endotoxin toxicity.

Delineation by use of specific monoclonal antibodies of the T-cell receptor and major histocompatibility complex interaction sites on the superantigen toxic shock syndrome toxin 1

Murine monoclonal antibodies (MAbs) specific for toxic shock syndrome toxin 1 (TSST-1), a bacterial superantigen, showed the ability either to detect TSST-1 bound to histocompatibility locus antigen (HLA)-DR molecules or to inhibit TSST-1 binding to HLA-DR. A MAb capable of detecting DR-bound TSST-1 could also inhibit the toxin-induced activation of a T-cell receptor Vbeta15-expressing murine T-cell hybridoma. Alternatively, MAbs with specificity for the HLA-DR association site could present TSST-1 in vitro, stimulating CD4+ human T cells to proliferate. These functional activities correlated directly with with MAb specificity for HLA-DR versus T-cell receptor Vbeta interaction sites on TSST-1 as determined by reactivity with a panel of recombinant TSST-1 mutant molecules. Therefore, these MAbs discriminate the superantigen functional sites on the TSST-1 molecule and constitute reagents with the property of being potent modulators of the toxic activity of TSST-1.

Fatal group A streptococcal infection with toxic shock syndrome: complicating minor orthopedic trauma

Since 1987, reports have appeared of a streptococcal toxic shock syndrome in various clinical settings. None have appeared in the orthopaedic literature. Between 1989 and 1991 at our institution three patients with relatively minor orthopaedic injuries or procedures died of group A streptococcal infections complicated by toxic shock syndrome. The manifestations of this syndrome included rapid progression of systemic sepsis, necrotizing soft-tissue infections, acute renal failure, adult respiratory distress syndrome, and coagulopathy. All three patients died despite aggressive resuscitative measures and surgical debridement. Optimal treatment of this life-threatening process requires early recognition, aggressive surgical debridement, appropriate antibiotic management, and intensive care unit support.

Analysis of the superantigenic activity of mutant and allelic forms of streptococcal pyrogenic exotoxin A

Infections with Streptococcus pyogenes (group A streptococcus) can result in the recently described streptococcal toxic shock syndrome (STSS), which is characterized by rashes, hypotension, multiorgan failure, and a high mortality rate. S. pyogenes isolates associated with STSS usually produce streptococcal pyrogenic exotoxin A (SpeA), a bacterial superantigen capable of stimulating host immune cells. Most of the symptoms of STSS are believed to result from cytokine release by the stimulated cells. To better understand the pathogenesis of STSS, we began studies on the SpeA-immune cell interaction. We generated 20 mutant forms of SpeA1 (SpeA encoded by allele 1), and the mutant toxins were analyzed for mitogenic stimulation of human peripheral blood mononuclear cells, affinity for class II major histocompatibility complex molecules (DQ), and disulfide bond formation. Residues necessary for each of these functions were identified. There are four alleles of speA, and STSS strains usually contain either allele 2 or allele 3. The product of allele 2, SpeA2, had slightly higher affinity for the class II MHC molecule compared with SpeA1 but not significantly greater mitogenic activity. SpeA3, however, was significantly increased in mitogenic activity and affinity for class II MHC compared with SpeA1. Thus, we have evidence that the toxin encoded by some of the highly virulent S. pyogenes STSS-associated isolates is a more active form of SpeA.

Streptococcal toxic shock syndrome in a postpartum woman. Case report and review of the literature

A case of streptococcal toxic shock syndrome in a 29-year-old postpartum woman is described. The patient presented with hypotension, coagulation defects, adult respiratory distress syndrome and scarlet exanthema as a complication of hemolytic group A streptococcal endometritis. One hundred and twenty-two other cases of streptococcal toxic shock syndrome in the literature are reviewed and the criteria of the syndrome discussed. Streptococcal toxic shock syndrome seems to be increasing along with more frequent invasive streptococcal infections in several countries. It is therefore important to recognize and treat the infection as early as possible to minimize the risk of mortality.

The toxic shock syndrome toxin-1 induces anergy in human T cells in vivo

TSST-1 is a Staphylococcus aureus-derived superantigen which has been implicated in the pathogenesis of toxic shock syndrome. In mice, superantigen-induced proliferation is followed by deletion or anergy of reactive T cells. So far, superantigen-induced T-cell anergy has not been observed in humans. We therefore examined PBMCs derived from a 15-year-old patient suffering from severe toxic shock syndrome. Markedly elevated levels of circulating TSST-1-reactive T cells were found by cytofluorometric analysis. Upon in vitro restimulation with TSST-1, hyporesponsiveness of TSST-1-responsive V beta 2+ T cells was detected, thus confirming results obtained in the murine system.

Do monoclonal antibodies and anticytokines still have a future in infectious diseases?

The continuing high mortality of septic shock has prompted a major effort by the research community to identify novel therapeutic targets. These targets can be conveniently grouped into (1) those derived from microbial components or products; (2) inflammatory mediators; and (3) effector molecules. Many of the experimental, so-called adjunctive agents developed have been monoclonal antibodies or anticytokine molecules of various kinds, and some have progressed into clinical trial. Unfortunately, these trials have failed to show unequivocal survival benefit for patients in shock, prompting a reappraisal of our approach to these agents. In this article, we discuss the possible reasons for these failures: (1) the targets are wrong; (2) the agents are inappropriate; or (3) the trial design is flawed. It would be premature to conclude that adjunctive agents have no future in the therapy of sepsis, but identifying the correct agent, and perhaps more importantly, the correct target population, is going to be more difficult than was at first believed.

Superantigen binding to a T cell receptor beta chain of known three-dimensional structure

The three-dimensional structure of an unglycosylated T cell antigen receptor (TCR) beta chain has recently been determined to 1.7 A resolution. To investigate whether this soluble beta chain (murine V beta 8.2J beta 2.1C beta 1) retains superantigen (SAG)-binding activity, we measured its affinity for various bacterial SAGs in the absence of MHC class II molecules. Dissociation constants (KDs) were determined using two independent techniques: surface plasmon resonance detection and sedimentation equilibrium. Specific binding was demonstrated to staphylococcal enterotoxins (SEs) B, C1, C2, and C3 and to streptococcal pyrogenic exotoxin A (SPEA), consistent with the known proliferative effects of these SAGs on T cells expressing V beta 8.2. In contrast, SEA, which does not stimulate V beta 8.2-bearing cells, does not bind the recombinant beta chain. Binding of the beta chain to SAGs was characterized by extremely fast dissociation rates (> 0.1 s-1), similar to those reported for certain leukocyte adhesion molecules. Whereas the beta chain bound SEC1, 2, and 3 with KDs of 0.9-2.5 microM, the corresponding value for SEB was approximately 140 microM. The much weaker binding to SEB than to SEC1, 2, or 3 was surprising, especially since SEB was found to actually be 3- to 10-fold more effective, on a molar basis, than the other toxins in stimulating the parental T cell hybridoma. We interpret these results in terms of the ability of SEC to activate T cells independently of MHC, in contrast to SEB. We have also measured SE binding to the glycosylated form of the beta chain and found that carbohydrate apparently does not contribute to recognition, even though the N-linked glycosylation sites at V beta 8.2 residues Asn24 and Asn74 are at or near the putative SAG-binding site. This result, along with the structural basis for the V beta specificity of SEs, are discussed in relation to the crystal structure of the unglycosylated beta chain.

Upregulation of IgE synthesis by staphylococcal toxic shock syndrome toxin-1 in peripheral blood mononuclear cells from patients with atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is a chronic skin disease associated with increased IgE synthesis and colonization with Staphylococcus aureus secreting exotoxins, such as Toxic Shock Syndrome Toxin-1 (TSST-1).

OBJECTIVES

In this study, we were interested in determining the in vitro effects of TSST-1 on IgE synthesis in peripheral blood mononuclear cells from patients with AD.

METHODS

We stimulated peripheral blood mononuclear cells (PBMC) from AD patients with a wide range of TSST-1 concentrations and measured IgE synthesis by enzyme-linked immunosorbent assay (ELISA) after 14 days.

RESULTS

We show herein that TSST-1 produced antagonistic effects on IgE synthesis by PBMC from AD patients, depending on the concentration used: IgE synthesis was inhibited at 1000 pg/mL (P < 0.05) and enhanced at 0.01 pg/mL (P < 0.01) of toxin. TSST-1 was found to induce the production of much higher amounts of interferon-gamma (IFN gamma) at 1000 pg/mL than at 0.01 pg/mL of toxin (P = 0.0001). More importantly, immunoglobulin E (IgE) synthesis was enhanced by TSST-1 at 1 pg/mL in the presence of antibodies blocking IFN gamma activity. The other immunoglobulin (Ig) isotypes were also increased after TSST-1 stimulation suggesting that the enhanced IgE synthesis was secondary to a polyclonal B cell activation rather than an isotype switch. TSST-1-stimulated IgE synthesis was T cell-dependent because purified tonsil B cells were only able to synthesize increased amounts of IgE when small numbers of T cells were added to the cultures. Anti-HLA-DR and anti-LFA-1 monoclonal antibodies (MoAb) inhibited TSST-1-enhanced IgE synthesis, suggesting that the bridging of the T cell receptor (TCR) and major histocompatibility complex (MHC) class II on B cells was necessary for activation of B cell differentiation.

CONCLUSION

These data indicate that staphylococcal superantigens are able, at concentrations inducing low amounts of IFN gamma, to stimulate IgE synthesis by PBMC from AD patients, and suggest that staphylococcal toxins may contribute to elevated IgE synthesis in AD.

Molecular structure of staphylococcus and streptococcus superantigens

Staphylococcus aureus and streptococci, notably those belonging to group A, make up a large family of true exotoxins referred to as pyrogenic toxin superantigens. These toxins cause toxic shock-like syndromes and have been implicated in several allergic and autoimmune diseases. Included within this group of proteins are the staphylococcal enterotoxins, designated serotypes A, B, Cn, D, E, and G; two forms of toxic shock syndrome toxin-1 also made by Staphylococcus aureus; the group A streptococcal pyrogenic exotoxins, serotypes A, B, and C; and recently described toxins associated with groups B, C, F, and G streptococci. The nucleotide sequences of the genes for all of the toxins except those from the groups B, C, F, and G streptococcal strains have been sequenced. The sequencing studies indicate that staphylococcal enterotoxins B and C and streptococcal pyrogenic exotoxin A share highly significant sequence similarity; staphylococcal enterotoxins A, D, and E share highly significant sequence similarity; and toxic shock syndrome toxin-1 and streptococcal pyrogenic exotoxin B and C share little, if any, sequence similarity with any of the toxins. Despite the dissimilarities seen in primary amino acid sequence among some members of the toxin family, it was hypothesized that there was likely to be significant three-dimensional structure similarity among all the toxins. The three-dimensional structures of three of the pyrogenic toxin superantigens have been determined recently. The structural features of two of these, toxic shock syndrome toxin-1 and enterotoxin C3, are presented. Toxic shock syndrome-1 exists as a protein with two major domains, referred to as A and B. The molecule begins with a short N-terminal alpha-helix that then leads into a clawshaped structure in domain B that is made up of beta strands.

The cell wall components peptidoglycan and lipoteichoic acid from Staphylococcus aureus act in synergy to cause shock and multiple organ failure

Although the incidence of Gram-positive sepsis has risen strongly, it is unclear how Gram-positive organisms (without endotoxin) initiate septic shock. We investigated whether two cell wall components from Staphylococcus aureus, peptidoglycan (PepG) and lipoteichoic acid (LTA), can induce the inflammatory response and multiple organ dysfunction syndrome (MODS) associated with septic shock caused by Gram-positive organisms. In cultured macrophages, LTA (10 micrograms/ml), but not PepG (100 micrograms/ml), induces the release of nitric oxide measured as nitrite. PepG, however, caused a 4-fold increase in the production of nitrite elicited by LTA. Furthermore, PepG antibodies inhibited the release of nitrite elicited by killed S. aureus. Administration of both PepG (10 mg/kg; i.v.) and LTA (3 mg/kg; i.v.) in anesthetized rats resulted in the release of tumor necrosis factor alpha and interferon gamma and MODS, as indicated by a decrease in arterial oxygen pressure (lung) and an increase in plasma concentrations of bilirubin and alanine aminotransferase (liver), creatinine and urea (kidney), lipase (pancreas), and creatine kinase (heart or skeletal muscle). There was also the expression of inducible nitric oxide synthase in these organs, circulatory failure, and 50% mortality. These effects were not observed after administration of PepG or LTA alone. Even a high dose of LTA (10 mg/kg) causes only circulatory failure but no MODS. Thus, our results demonstrate that the two bacterial wall components, PepG and LTA, work together to cause systemic inflammation and multiple systems failure associated with Gram-positive organisms.

Crystal structure of the superantigen enterotoxin C2 from Staphylococcus aureus reveals a zinc-binding site

BACKGROUND

Staphylococcus aureus enterotoxin C2 (SEC2) belongs to a family of proteins, termed 'superantigens', that form complexes with class II MHC molecules enabling them to activate a substantial number of T cells. Although superantigens seem to act by a common mechanism, they vary in many of their specific interactions and biological properties. Comparison of the structure of SEC2 with those of two other superantigens--staphylococcal enterotoxin B (SEB) and toxic shock syndrome toxin-1 (TSST-1)--may provide insight into their mode of action.

RESULTS

The crystal structure of SEC2 has been determined at 2.0 A resolution. The overall topology of the molecule resembles that of SEB and TSST-1, and the regions corresponding to the MHC class II and T-cell receptor binding sites on SEB are quite similar in SEC2. A unique feature of SEC2 is the presence of a zinc ion located in a solvent-exposed region at the interface between the two domains of the molecule. The zinc ion is coordinated to Asp83, His118, His122 and Asp9* (from the neighbouring molecule in the crystal lattice). Atomic absorption spectrometry demonstrates that zinc is also bound to SEC2 in solution.

CONCLUSIONS

SEC2 appears to be capable of binding to MHC class II molecules in much the same manner as SEB. However, structure-function studies have suggested an alternative binding mode that involves a different site on the toxin. The zinc ion of SEC2 lies within this region and thus may be important for complex formation, for example by acting as a bridge between the two molecules. Other possible roles for the metal cation, including a catalytic one, are also considered.

The role of superantigens in skin disease

Staphylococcus aureus and streptococci secrete a large family of exotoxins involved in the pathogenesis of toxic-shock-like syndromes and have been implicated in several autoimmune disorders. These toxins act as prototypic superantigens capable of binding to major histocompatibility complex proteins on antigen-presenting cells outside the antigen peptide-binding groove and can thereby stimulate cytokine release from macrophages. The superantigen-major histocompatibility complex unit is recognized primarily by the variable region of the T-cell receptor beta chain, and by engaging this region, can activate a large portion of the T-cell repertoire. It is thought that the capacity of these toxins to cause the massive stimulation of T cells and accessory cells such as macrophages, Langerhans cells, and activated keratinocytes accounts for most of their pathologic effects. The current review examines the evidence that implicates a role for these superantigens in the pathogenesis of certain skin diseases.

Bacterial pyrogenic exotoxins as superantigens

The recent discovery of the mode of interaction between a group of microbial proteins known as superantigens and the immune system has opened a wide area of investigation into the possible role of these molecules in human diseases. Superantigens produced by certain viruses and bacteria, including Mycoplasma species, are either secreted or membrane-bound proteins. A unique feature of these proteins is that they can interact simultaneously with distinct receptors on different types of cells, resulting in enhanced cell-cell interaction and triggering a series of biochemical reactions that can lead to excessive cell proliferation and the release of inflammatory cytokines. However, although superantigens share many features, they can have very different biological effects that are potentiated by host genetic and environmental factors. This review focuses on a group of secreted pyrogenic toxins that belong to the superantigen family and highlights some of their structural-functional features and their roles in diseases such as toxic shock and autoimmunity. Deciphering the biological activities of the various superantigens and understanding their role in the pathogenesis of microbial infections and their sequelae will enable us to devise means by which we can intervene with their activity and/or manipulate them to our advantage.

Biological activities of staphylococcal enterotoxin type A mutants with N-terminal substitutions

The purpose of this study was to examine the importance of certain N-terminal amino acid residues of staphylococcal enterotoxin type A (SEA) for biological activity. The results confirm our previous observation that Asn-25, Phe-47, and Leu-48 are important for SEA's emetic and superantigen activities. Substitutions at six other sites (Leu-12, Lys-14, Ser-16, Asp-45, Gln-46, and Thr-51) did not reveal any additional residues required for biological activity. Mutant SEAs with substitutions at 25, 47, or 48 all had decreased T-cell stimulatory activity, with the mutants at position 47 being the most defective. Results of a competition assay for binding to the major histocompatibility complex (MHC) class II-expressing cell line Raji suggested that the decreased superantigen activities of the mutants with substitutions at positions 47 and 48 are due to poor interactions with MHC class II molecules, whereas the defects of the mutants at position 25 are a consequence of faulty interactions with T-cell receptors. With respect to emetic activity in rhesus monkeys, the mutants at position 25 or 48 exhibited decreased but significant activity. Interestingly, the two mutants at position 47 had different emetic activities; SEA-F47G was nonemetic when administered intragastrically at 500 micrograms per animal, whereas SEA-F47S was emetic at this dosage. Since the mutants at position 47 were equally defective for superantigen activity, this further supports our previous suggestion of an incomplete correlation between SEA's emetic and superantigen activities.

Infectious agents, the inflammatory responses of infants and sudden infant death syndrome (SIDS)

There is no convincing epidemiological or pathological evidence that particular infectious agents cause sudden infant death syndrome (SIDS); therefore, we have explored the concept that synergy between bacterial endotoxins, exotoxins or viruses might elicit inflammatory responses during a period when the infant's endocrine system is less able to 'damp down' the effects of powerful mediators such as tumour necrosis factor or to maintain glucose homoeostasis which is affected by these mediators. This hypothesis is discussed with reference to the recent decline in the number of cot deaths.

Biological activity of toxic shock syndrome toxin 1 and a site-directed mutant, H135A, in a lipopolysaccharide-potentiated mouse lethality model

A recombinant of toxic shock syndrome toxin 1 (TSST-1) which contains a single histidine-to-alanine mutation at residue 135 (H135A) was analyzed for toxicity and vaccine potential in a lipopolysaccharide (LPS)-potentiated mouse lethality model. The 50% lethal dose (LD50) of TSST-1 in BALB/c mice was 47.2 micrograms/kg, but H135A was not lethal when tested at a dose equivalent to 10 LD50s of TSST-1. Levels of tumor necrosis factor (TNF) and gamma interferon (IFN-gamma) in serum were, respectively, 10- and 50-fold higher in LPS-potentiated mice injected with 15 LD50s of TSST-1 than in mice given H135A. Mice injected with only TSST-1 did not have elevated levels of TNF or IFN-gamma in serum, while H135A plus LPS or LPS alone elicited identical, yet very low, levels of TNF and IFN-gamma. An enzyme-linked immunosorbent assay of H135A and TSST-1 with anti-TSST-1 serum yielded very similar dose-response curves, which strongly suggests that H135A serologically and conformationally resembles the native toxin. Mice immunized with H135A developed antibodies that recognized TSST-1 in an enzyme-linked immunosorbent assay and afforded protection against a 15-LD50 challenge of TSST-1 plus LPS. The pooled sera of mice immunized with either TSST-1 or H135A also prevented lymphocyte proliferation due to TSST-1.

Bacterial superantigens induce T cell expression of the skin-selective homing receptor, the cutaneous lymphocyte-associated antigen, via stimulation of interleukin 12 production

T lymphocyte infiltration is a prominent feature of the skin inflammation associated with infections by toxin (superantigen)-secreting Staphylococcus aureus or Streptococcus bacteria. The cutaneous lymphocyte-associated antigen (CLA) has been hypothesized to be a homing receptor (HR) involved in selective migration of memory/effector T cells to the skin. Since the expression of this putative skin-selective HR is known to be under strict microenvironmental control, we sought to determine the effect of staphylococcal and streptococcal toxins on T cell expression of CLA. After in vitro stimulation of peripheral blood mononuclear cells with staphylococcal enterotoxin B, toxic shock syndrome toxin-1, and streptococcal pyrogenic exotoxins A and C, there was a significant increase in the numbers of CLA+ T cell blasts (p < 0.01), but not blasts bearing the mucosa-associated adhesion molecule alpha e beta 7-integrin, compared with T cells stimulated with phytohemaglutinin (PHA) or anti-CD3. Bacterial toxins were also found to specifically induce interleukin (IL) 12 production. More importantly, induction of toxin-induced CLA expression was blocked by anti-IL-12, and the addition of IL-12 to PHA-stimulated T cells induced CLA, but not alpha e beta 7-integrin, expression. These data suggest that bacterial toxins induce the expansion of skin-homing CLA+ T cells in an IL-12-dependent manner, and thus may contribute to the development of skin rashes in superantigen-mediated diseases.

Production and partial characterization of monoclonal antibodies against erythrogenic toxins type A and C from Streptococcus pyogenes

Hybridoma cell lines producing monoclonal antibodies against streptococcal erythrogenic toxins type A and C were established from fusions of immunized BALB/c mice splenocytes with P3X63-Ag8.653 myeloma cells. Six MAbs recognize ETA and 11 MAbs bind to ETC. Two MAbs (designated ETA-2 and ETC-10) were produced in ascitic fluid and further characterized. ETA-2 (IgG2a) binds to ETA with an affinity constant of 1.8 x 10(10) M-1 and ETC-10 (IgG1) binds to ETC with an affinity constant of 3.5 x 10(9) M-1. The specificities of the MAbs were evaluated by ELISA and immunoblotting. Both MAbs ETA-2 and ETC-10 are useful in developing specific double-sandwich ELISAs, in which the MAbs were used as solid-phase capture antibodies for the quantitative determinations of ETA and ETC.

The gene encoding a new mitogenic factor in a Streptococcus pyogenes strain is distributed only in group A streptococci

We recently cloned a gene encoding a new mitogenic factor (MF) from Streptococcus pyogenes NY-5. In the present study, we determined the distribution of this MF gene (mf) by PCR based upon its sequence. Of 371 streptococcal group A strains isolated from clinical specimens, 370 (99.7%) were positive for mf. The strain that was negative for the MF gene was also negative for the streptolysin O gene (slo). Some streptococcal strains belonging to groups C and G were negative for mf but positive for slo. Group B strains were negative for both. Furthermore, we examined the presence of mf in 54 strains belonging to 28 families and found mf only in group A streptococci. These results indicate that mf is distributed specifically in group A streptococci and the presence of mf in clinical samples strongly suggests infection with group A streptococci.

Investigation of the role of the disulphide bond in the activity and structure of staphylococcal enterotoxin C1

The goal of this study was to investigate the role of the disulphide bond of staphylococcal enterotoxin C1 (SEC1) in the structure and activity of the toxin. Mutants unable to form a disulphide bond were generated by substituting alanine or serine for cysteine at positions 93 and/or 110. Although we did not directly investigate the residues between the disulphide linkage, tryptic lability showed that significant native structure in the cystine loop is preserved in the absence of covalent bonding between residues 93 and 110. Since no correlation was observed between the behaviour of these mutants with regard to toxin stability, emesis and T cell proliferation we conclude that SEC1-induced emesis and T cell proliferation are dependent on separate regions of the molecule. The disulphide bond itself is not an absolute requirement for either activity. However, conformation within or adjacent to the loop is important for emesis. Although mutants with alanine substitutions were not emetic, those with serine substitutions retained this activity, suggesting that the disulphide linkage stabilizes a crucial conformation but can be replaced by residues which hydrogen bond.

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.

The role of infectious agents in sudden infant death syndrome

Epidemiological factors associated with susceptibility to respiratory infections are similar to those associated with Sudden Infant Death Syndrome. Here we review the evidence that respiratory pathogens might be involved in some cases of Sudden Infant Death Syndrome in the context of factors identified in epidemiological studies of cot deaths: the age range affected; mother' smoking; respiratory viral infections; immunisation status. Both laboratory and epidemiological evidence suggests that vulnerability of infants to infectious agents depends on interactions between genetic, developmental and environmental factors that contribute to colonisation by microorganisms, the inflammatory and specific immune responses and the infants' physiological responses to inflammatory mediators. A model is proposed to explain how microorganisms might trigger a series of events resulting in some of these unexpected deaths and discusses how the the present recommendations regarding child care practices might help reduce the numbers of Sudden Infant Death Syndrome cases associated with infectious agents.

Dual infections with Staphylococcus aureus and Streptococcus pyogenes causing toxic shock syndrome. Possible synergistic effects of toxic shock syndrome toxin 1 and streptococcal pyrogenic exotoxin C

We describe a 35-year-old woman with clinical, microbiologic, and serologic findings suggesting that the patient developed toxic shock syndrome as a result of dual infections caused by toxin-producing strains of Staphylococcus aureus and Streptococcus pyogenes. Certain aspects of the pathogenesis of this toxin-related syndrome are reviewed.

Case report: toxic shock syndrome arising from cellulitis

Toxic shock syndrome is a febrile, multiorgan illness related to toxins elaborated by staphylococcal or streptococcal infections. In the 1980s, most cases were associated with menstruation. More recently, many cases now are unrelated to menses. In this article, the authors describe a case of a nonmenstruating woman with toxic shock syndrome, associated with cellulitis of her arm. Cultures of the arm grew Staphylococcal aureus, which produced enterotoxin B.