Molecular genetics of pyrogenic exotoxin "superantigens" of group A streptococci and Staphylococcus aureus

Tonsillitis and sore throat in children

Surgery of the tonsils is still one of the most frequent procedures during childhood. Due to a series of fatal outcomes after hemorrhage in children in Austria in 2006, the standards and indications for tonsillectomy have slowly changed in Germany. However, no national guidelines exist and the frequency of tonsil surgery varies across the country. In some districts eight times more children were tonsillectomized than in others. A tonsillectomy in children under six years should only be done if the child suffers from recurrent acute bacterially tonsillitis. In all other cases (i.e. hyperplasia of the tonsils) the low risk partial tonsillectomy should be the first line therapy. Postoperative pain and the risk of hemorrhage are much lower in partial tonsillectomy (=tonsillotomy). No matter whether the tonsillotomy is done by laser, radiofrequency, shaver, coblation, bipolar scissor or Colorado needle, as long as the crypts are kept open and some tonsil tissue is left behind. Total extracapsular tonsillectomy is still indicated in severely affected children with recurrent infections of the tonsils, allergy to antibiotics, PFAPA syndrome (periodic fever, aphthous stomatitis, pharyngitis, and cervical adenitis) and peritonsillar abscess. With regard to the frequency and seriousness of the recurrent tonsillitis the indication for tonsillectomy in children is justified if 7 or more well-documented, clinically important, adequately treated episodes of throat infection occur in the preceding year, or 5 or more of such episodes occur in each of the 2 preceding years (according to the paradise criteria). Diagnosis of acute tonsillitis is clinical, but sometimes it is hard to distinguish viral from bacterial infections. Rapid antigen testing has a very low sensitivity in the diagnosis of bacterial tonsillitis and swabs are highly sensitive but take a long time. In all microbiological tests the treating physician has to keep in mind, that most of the bacterials, viruses and fungi belong to the healthy flora and do no harm. Ten percent of healthy children even bear strepptococcus pyogenes all the time in the tonsils with no clinical signs. In these children decolonization is not necessary. Therefore, microbiological screening tests in children without symptoms are senseless and do not justify an antibiotic treatment (which is sometimes postulated by the kindergartens). The acute tonsillitis should be treated with steroids (e.g. dexamethasone), NSAIDs (e.g. ibuprofene) and betalactam antibiotics (e.g. penicillin or cefuroxime). With respect to the symptom reduction and primary healing the short-term late-generation antibiotic therapy (azithromycin, clarithromycin or cephalosporine for three to five days) is comparable to the long-term penicilline therapy. There is no difference in the course of healing, recurrence or microbiological resistance between the short-term penicilline therapy and the standard ten days therapy. On the other hand, only the ten days antibiotic therapy has proven to be effective in the prevention of rheumatic fever and glomerulonephritic diseases. The incidence of rheumatic heart disease is currently 0.5 per 100,000 children of school age. The main morbidity after tonsillectomy is pain and the late haemorrhage. Posttonsillectomy bleeding can occur till the whole wound is completely healed, which is normally after three weeks. Life-threatening haemorrhages occur often after smaller bleedings, which can spontaneously cease. That is why every haemorrhage, even the smallest, has to be treated properly and in ward. Patients and parents have to be informed about the correct behaviour in case of haemorrhage with a written consent before the surgery. The handout should contain important addresses, phone numbers and contact persons. Almost all cases of fatal outcome after tonsillectomy were due to false management of haemorrhage. Haemorrhage in small children can be especially life-threatening because of the lower blood volume and the danger of aspiration with asphyxia. A massive haemorrhage is an extreme challenge for every paramedic or emergency doctor because of the difficult airway management. Intubation is only possible with appropriate inflexible suction tubes. All different surgical techniques have the risk of haemorrhage and even the best surgeon will experience a postoperative haemorrhage. The lowest risk of haemorrhage is after cold dissection with ligature or suturing. All "hot" techniques with laser, radiofrequency, coblation, mono- or bipolar forceps have a higher risk of late haemorrhage. Children with a hereditary coagulopathy have a higher risk of haemorrhage. It is possible, that these children were not identified before surgery. Therefore it is recommended by the Society of paediatrics, anaesthesia and ENT, that a standardised questionnaire should be answered by the parents before tonsillectomy and adenoidectomy. This 17-point-checklist questionnaire is more sensitive and easier to perform than a screening with blood tests (e.g. INR and PTT). Unfortunately, a lot of surgeons still screen the children preoperatively by coagulative blood tests, although these tests are inappropriate and incapable of detecting the von Willebrand disease, which is the most frequent coagulopathy in Europe. The preoperative information about the surgery should be done with the child and the parents in a calm and objective atmosphere with a written consent. A copy of the consent with the signature of the surgeon and both custodial parents has to be handed out to the parents.

Lipopolysaccharide-dependent down-regulation of CD27 expression on T cells activated with superantigen

To investigate the mechanisms underlying T-cell responses during superantigen (SAg) stimulation, we analysed the effects of SAg on CD27 expression with or without lipopolysaccharide (LPS) as a novel regulator of T-cell function. CD27 is expressed on the majority of resting peripheral blood T cells (CD27low). Activation of T cells by SAg induces high levels of CD27 surface expression (CD27high) accompanied with simultaneous CD30 receptor expression. After prolonged activation in vitro, the level of CD27 expression became intermediate. The effects of LPS on down-regulation of CD27high expression on CD30+ T cells were dose-dependent. Separating LPS-stimulated monocytes from T cells by mechanical dispersion abolished its inhibitory effect, indicating the requirement for direct interactions between monocytes and T cells. We also found that SAg up-regulated CD80 expression on CD14+ monocytes and LPS inhibited SAg-induced CD80 expression after 24 hr of stimulation. Up-regulation of CD152 (CTLA-4) was selective, since it was found to be preferentially expressed on the CD30+ population. Competitive experiments using soluble blocking peptides showed that addition of CD28 or CD80 peptide recovered LPS-induced down-regulation of CD27high expression on CD30+ T cells. These observations suggested that the presence of low levels of CD80 on monocytes may partially inhibit CD27 expression due to inefficient delivery of positive signals via CD28/CD80 interaction, and that the increased levels of CD80 enhance the inhibition through interactions with CD152 which is expressed at the highest levels after 48 hr of activation.

Cytokine induction by Mycoplasma arthritidis-derived superantigen (MAS), but not by TSST-1 or SEC-3, is correlated to certain HLA-DR types

Superantigens bind to major histocompatibility complex (MHC) class II molecules on antigen presenting cells and T cells in a V beta-restricted manner. Both cell types are activated resulting in cytokine production. Although the MHC-II binding site for superantigens has been well described, little is known as to whether this binding complex has an influence on cytokine induction. In order to assess superantigen induced cytokine production and its correlation to HLA-DR types, the authors stimulated peripheral blood from 40 subjects with superantigens toxic shock syndrome toxin-1 (TSST-1), staphylococcal enterotoxin C-3 (SEC-3) and Mycoplasma arthritidis-derived superantigen (MAS), and measured cytokine levels thereafter. The HLA-DR type was determined in each subject. A statistical evaluation was carried out between the highest superantigen cytokine induction and the presence of certain HLA-DR types. Whereas MAS presented a statistical association between the highest cytokine production with HLA-DR4, DR7 and DR12, no such associations were observed for TSST-1 and SEC-3. These results demonstrate that T cell stimulation, and consequently its cytokine production by MAS but not by TSST-1 and SEC-3, depends on the presenting HLA-DR type. Because the diverse HLA-DR specificities are given according to the variability of the beta chain of the HLA-DR molecule, the data suggest the participation of the human MHC-II beta chain in the MAS/MHC-II binding.

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.

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.

Superantigens and pseudosuperantigens of gram-positive cocci

Superantigens use an elaborate and unique mechanism of T lymphocyte stimulation. Prototype superantigen are the pyrogenic exotoxins produced by Staphylococcus aureus and Streptococcus pyogenes. Many candidate proteins of bacterial, viral and protozoal origin have recently been reported to be superantigens. In most cases the evidence that these proteins are in fact superantigens is highly indirect. In this review the evidence that gram-positive cocci produce superantigens other than the pyrogenic exotoxins is critically discussed. Evidence in described demonstrating that the epidermolytic toxins of Staphylococcus aureus and the pyrogenic exotoxin B and M-proteins of Streptococcus pyrogenes are not superantigens. Criteria are described for acceptance of a candidate as a superantigen.

Bacterial stimulators of macrophages

Our current understanding of the interaction between bacteria and macrophages, cells of the immune system that play a major role in the defense against infection, is summarized. Cell-surface structures of Gram-negative and Gram-positive bacteria that account for these interactions are described in detail. Besides surface structures, soluble bacterial molecules, toxins that are derived from pathogenic bacteria, are also shown to modulate macrophage functions. In order to affect macrophage functions, bacterial surface structures have to be recognized by the macrophage and toxins have to be taken up. Subsequently, signal transduction mechanisms are initiated that enable the macrophage to respond to the invading bacteria. To destroy bacteria, macrophages employ many strategies, among which antigen processing and presentation to T cells, phagocytosis, chemotaxis, and different bactericidal mechanisms are considered to be the main weapons.

Frequency of toxic shock syndrome toxin- and enterotoxin-producing clinical isolates of Staphylococcus aureus

In 183 clinical isolates of Staphylococcus aureus, toxic shock syndrome toxin was detected in 13.7%, enterotoxin A in 20.2%, enterotoxin B in 7.7%, enterotoxin C in 5.5% and enterotoxin D in 3.3%. Seventy-three (39.9%) of the strains were found to produce one or more toxins. Multiple secretion of toxins was rare (< 1% of all strains) except for the combinations enterotoxin A with toxic shock syndrome toxin and enterotoxin A with enterotoxin D. These combinations were observed at significantly higher frequencies (4.9% and 2.2%, respectively) than would have been expected from values calculated from respective individual frequencies (0.88% and 0.06%) (p < 0.0001). In comparison with Staphylococcus aureus strains isolated from miscellaneous material, Staphylococcus aureus blood culture isolates produced enterotoxin D in significantly larger amounts and at higher frequencies (p = 0.01 in both cases). These toxins might play an important pathogenic role in Staphylococcus aureus infections.

Superantigens produced by infectious pathogens: molecular mechanism of action and biological significance

"Superantigens" have in common an extremely potent stimulatory activity for CD4+, CD8+, and some gamma delta+ T lymphocytes. Superantigens use a unique mechanism: they crosslink variable parts of the T cell receptor with MHC class II molecules on accessory or target cells. The interaction site on the T cell receptor is the variable part of the beta-chain (V beta). There are several reasons why these molecules have aroused such tremendous interest in recent years. First, they have provided key information on tolerance mechanisms, both on the deletion of T cells in the thymus and on the induction of peripheral tolerance by anergy and apoptosis. Second, of all polyclonal T cell stimulators they are the ones that most closely mimic the recognition of specific antigen. Finally, they have been recognized as important factors in the pathogenicity of the producing pathogens, inducing shock and immunosuppression. Moreover, it has been postulated that superantigens could be involved in the pathogenesis of certain human diseases.

Growth and analysis of crystal forms of toxic shock syndrome toxin 1

Native toxic shock syndrome toxin 1 (TSST-1) purified from Staphylococcus aureus has been crystallized in four different forms. The highest resolution data (2.05 A) was collected from orthorhombic crystals belonging to the space group C222(1). The unit cell dimensions are a = 108.7 A, b = 177.5 A, c = 97.6 A. Rotation function analysis of this form indicates that there is trimer of toxin molecules in the asymmetric unit with a local 3-fold axis parallel to the crystallographic c axis. Crystals of a double mutant of TSST-1 have been grown which has a single molecule in the asymmetric unit and diffract to 1.9 A. The space group is P2(1) with unit cell parameters of a = 44.4 A, b = 34.0 A, c = 55.2 A, beta = 93.0 degrees.