Abstract

Superantigens (SAgs) are a family of highly potent mitogens that share the ability to trigger excessive stimulation of human and other mammalian T lymphocytes. This leads to a massive release of T cell mediators and pro-inflammatory cytokines contributing to diseases such as toxic shock syndrome. In contrast to conventional peptides, SAgs bind as unprocessed molecules to major histocompatibility (MHC) class II molecules outside the peptide-binding groove and sequentially to the variable β-chain of the T cell receptor (TcRVβ). Currently, eleven Streptococcus pyogenes SAgs are described in the literature. Together with the SAgs produced by Staphylococcus aureus, they build a larger family of structurally related, heat-stable exotoxins.

This chapter provides a comprehensive overview of the discovery, biological function, and disease-associations of these remarkable proteins.

Introduction

Superantigens (SAgs) are a family of highly mitogenic exotoxins that are produced by a small number of bacterial species and some viruses (Fraser & Proft, 2008; Proft, Schrage, & Fraser, 2005) (Fraser & Proft, 2008; Proft, Schrage, & Fraser, 2005). The most common bacterial genus that produces SAgs is Streptococcus spp. and includes Streptococcus pyogenes (group A streptococcus), S. dysgalactiae (group C Streptococcus) and S. equi (group G streptococcus) (Proft, Schrage, & Fraser, 2005; Commons, Smeesters, Proft, Fraser, Robins-Browne, & Curtis, 2014; Proft & Fraser, 2003). A variety of SAgs are also found in Staphylococcus aureus and coagulase negative staphylococci, which together with the streptococcal SAgs, build a family of structurally related low molecular weight exotoxins, with secretion dependent on a cleavable signal peptide sequence (Fraser & Proft, 2008; Proft & Fraser, 2003; Baker & Acharya, 2004). Other, structurally non-related SAgs are produced by Mycoplasma arthritidis (Proft, Schrage, & Fraser, 2005; Rink & Kirchner, 1992) and Yersinia pseudotuberculosis (Proft, Schrage, & Fraser, 2005; Donadini & Fields, 2007).

A hallmark of SAgs is their ability to simultaneously bind to major histocompatibility complex (MHC) class II on antigen presenting cells and the T cell receptor on T cells (Fraser & Proft, 2008; Proft & Fraser, 2003; Proft & Fraser, 2007). In contrast to conventional peptide antigens, SAg binding is not restricted by polymorphic determinants of MHC class II molecules and occurs outside the peptide-binding groove. Furthermore, SAgs bind to the variable region of the TcRVβ chain, resulting in extensive heterogeneity in T cell clonal activation (Figure 1). The number of different TcR β-chains in the human T cell repertoire is restricted to less than 50 with only about 25 major Vβ types. Since SAgs generally bind more than one specific Vβ region, up to 25% of an individual’s T cell population can be activated, which is in sharp contrast to 1 in 10⁵-10⁶ naïve T cells that are stimulated in response to conventional peptide antigens. Consequently, each SAg is associated with a characteristic TcRVβ ‘fingerprint’ that is independent from MHC class II polymorphism (Table 1). For example, streptococcal pyrogenic exotoxin (SPE)-C triggers the activation and expansion of T cells carrying Vβ2.1, Vβ3.2, Vβ12.5 and Vβ15.1 with a strong preference for Vβ2.1, whereas streptococcal mitogenic exotoxin (SMEZ) shows specificity for Vβ2.1, Vβ4.1, Vβ7.3 and Vβ8.1 regions with a preference for Vβ4.1 and Vβ8.1 (Table 1). Due to the immense potency to stimulate human, and to a certain degree, other mammalian CD4 and CD8 T cells, the term ‘superantigen’ was introduced by Philippa Marrack and John Kappler in 1989 (White, et al., 1989). In response to the oligoclonal activation of T cells and antigen presenting cells massive amounts of proinflammatory cytokines, such as interleukin 1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), and T cell mediators, such as IL-2 are released. This ‘cytokine storm’ can lead to fever and shock. With half-maximum responses between 0.02 and 50 pg/ml for human T cells, SAgs are the most potent T cell mitogens ever discovered.

Figure 1.

Model of T cell activation by a conventional peptide antigen (Ag) and superantigen (SAg). APC, antigen-presenting cell; MHC II, major histocompatibility class II molecule; TcR, T cell receptor; TNF-α, tumor necrosis factor alpha; IFN-γ, (more...)

Table 1.

Functional properties of Streptococcus pyogenes superantigens

The discovery of Group A streptococcal superantigens

Over the last nine decades, eleven SAgs were discovered in Streptococcus pyogenes (Figure 2). It all started in 1924 when Dick and colleagues identified a toxin in culture filtrates from hemolytic streptococci isolated from patients with scarlet fever. This toxin was initially named ‘scarlet fever toxin’ (Dick & Dick, 1983). A second toxin was identified in 1934 and named toxin B (Hooker & Follensby, 1934) followed by the discovery of toxin C from a scarlet fever associated serotype M18 culture filtrate in 1960 (Watson, 1960). The three toxins were immunologically different, but shared several different biological activities, in particular the ability to induce fever when injected into rabbits (pyrogenicity) and the enhancement of susceptibility to endotoxic shock. Based on the strong pyrogenic effect, which was believed to be the primary characteristic of the toxins, Kim and Watson designated the toxins streptococcal pyrogenic exotoxins (SPE) A, B and C (Kim & Watson, 1970). During the 1980s, the toxin genes were cloned and recombinant proteins were produced in Escherichia coli and Bacillus subtilis, which allowed for a more careful study of the toxin functions in the absence of any contaminating proteins. It was found that SPE-A was identical to Blastogen A, a previously identified T cell mitogen (Schlievert & Gray, 1989), and it was able to activate murine T cells in a MHC class II-dependent and TcRVβ-specific mode (Imanishi, Igarashi, & Uchiyama, 1990). Similarly, the function as a SAg was also established for SPE-C when Leonard and co-workers showed MHC class II and TcRVβ-dependent T cell mitogenicity (Leonard, Lee, Jenkins, & Schlievert, 1991). In contrast, initial findings of SPE-B induced T cell stimulation were later disputed when experiments with recombinant toxin of very high purity could not detect any SAg activity (Gerlach, Reichardt, Fleischer, & Schmidt, 1994). Furthermore, sequencing of the speB gene from a serotype M12 S. pyogenes strain revealed identity with the gene encoding streptococcal cysteine protease (SCP) (Bohach, Hauser, & Schlievert, 1988).

Figure 2.

Phylogenetic tree of Group A streptococcal SAgs. The tree was created using ClustalW and is based on primary amino acid sequence homologies of the mature proteins.

The identification of SPE-F was reported in 1994 (Norrby-Teglund, Newton, Kotb, Holm, & Norgren, 1994), but it is now believed that the observed mitogenic activity, like in the case of SPE-B, was due to contamination with a powerful SAg. It was later shown that SPE-F is identical with streptococcal DNaseB (Sriskandan, Unnikrishnan, Krausz, & Cohen, 2000).

Musser and colleagues reported in 1993 the discovery of a novel SAg, which they found in the cell culture supernatant of a serotype M3 strain and named streptococcal superantigen (SSA) (Mollick, et al., 1993). Interestingly, SSA showed a higher degree of amino acid similarity to staphylococcal SAgs than to any other streptococcal SAg. Another SAg, called streptococcal mitogenic exotoxin Z (SMEZ), was found in 1997 in the culture supernatant of an M1/T1 S. pyogenes strain (Kamezawa, et al., 1997).

SMEZ was the last S. pyogenes SAg identified by conventional methods before the start of microbial genomics and the discovery of genes by database mining. The first S. pyogenes genome was sequenced from strain SF370, a serotype M1 strain, and raw DNA sequence data was made available on the researchers website at the University of Oklahoma for mining long before completion of the project (Ferretti, et al., 2001). Although the streptococcal (and also staphylococcal) SAgs often share only limited amino acid sequence homologies, they all possess the highly conserved “family signature motifs” Y-G-G-[LIV]-T-X(4)-N (Prosite entry PS00277) and K-X(2)-[LIVF]-X(4)-[LIVF]-D-X(2)-R-X(2)-L-X(5)-[LIV]-Y (PS00278). These motifs were used to mine the SF370 genome database, which resulted in the discovery of four novel sag genes, spe-G, spe-H, spe-I and spe-J. Recombinant forms of the toxins were generated in E. coli and functional analysis confirmed their role as SAgs (Proft, Arcus, Handley, Baker, & Fraser, 2001; Proft, Moffatt, Berkahn, & Fraser, 1999). Furthermore, SPE-J was shown to induce fever in rabbits and was lethal in two rabbit models of toxic shock syndrome (McCormick, Pragman, Stolpa, Leung, & Schlievert, 2001). The rapid discovery of novel SAgs by whole genome mining over the following years resulted in an increasingly confusing SAg nomenclature. Analysis of a complete S. pyogenes serotype M3 genome in the U. S. resulted in the discovery of a novel sag gene that was named speK (Beres, et al., 2002). However, this name had already been assigned to an incomplete sag gene on the SF370 genome (Ferretti, et al., 2001). In the same year, another group found the same sag gene on the genome of a Japanese S. pyogenes serotype M3 strain and named it speL (Ikebe, et al., 2002). Shortly after that, this gene was found on a serotype M89 isolate from New Zealand and was also named speL (Proft, Webb, Handley, & Fraser, 2003a). It was identified by PCR using specific primers for a previously identified orthologue on a Streptococcus equi subsp. zooepidemicus genome. The same strategy also led to the discovery of another novel sag gene from a serotype M80 isolate, which was named speM (Proft, Webb, Handley, & Fraser, 2003a). At about the same time, two sag genes were identified when a serotype M18 isolate genome was completed and named speL and speM (Smoot, et al., 2002a). SpeL is identical to speM found on the genome of the M80 isolate, whereas the speM gene from the serotype M18 genome had not been reported before. Mitogenic activity of the novel SAgs was confirmed after recombinant proteins were produced in E. coli and found to target T cells in a Vβ-specific and MHC class II-dependent mode (Proft, Webb, Handley, & Fraser, 2003a; Smoot, et al., 2002a).

Recently, a novel nomenclature for all streptococcal SAgs was proposed (Commons, et al., 2014). It was suggested to use the name SPE-K for the SAg identified by Beres and co-workers (Beres, et al., 2002), and the names SPE-L and SPE-M for the SAg identified by Smoot and colleagues (Smoot, et al., 2002a). In addition, the names SPE-N, SPE-O and SPE-P were reserved for potential orthologues of the superantigens SzeN, SzeF and SzeP that were recently found in Streptococcus equi subsp. zooepidemicus (Paillot, et al., 2010), although these toxins have not been found yet in S. pyogenes.

Superantigen orthologues in non-Group A streptococci

Group C Streptococcus (GCS) and Group G Streptococcus (GGS) are commonly regarded as commensals usually found in association with the normal flora of human skin, pharynx and intestine. However, there have been an increasing number of reports implicating GCS and GGS with severe invasive infections, such as necrotizing fasciitis and toxic shock syndrome (Oster & Bisno, 2006). Mitogenic activity in supernatants of clinical GCS and GGS isolates had been reported over several years, but SAgs had not been identified until 2002 when Timoney’s group identified two SAgs in Streptococcus equi, a bacterium that causes strangles in horses, but can also infect humans.

The Streptococcus equi pyrogenic exotoxins H and I (SePE-H, SePE-I) are highly homologous to SPE-H and SPE-I, (>98% amino acid sequence identities) indicating horizontal gene transfer from S. pyogenes to S. equi or vice versa (Artiushin, Timoney, Sheoran, & Muthupalani, 2002). Another two sag genes were identified by data mining of the S. equi genome at the Sanger Centre and named speL_Se and speM_Se, due to the homology to speL and speM (recently renamed to speK and speL, respectively (Commons, et al., 2014)) with 99% and 98.1% nucleotide identities, respectively (Proft et al., 2003b). Two SAgs have been identified from Streptococcus dysgalactiae subsp. equisimilis called Streptococcus dysgalactiae-derived mitogen (SDM) (Miyoshi-Akiyama, et al., 2003) and SPE-G^dys (Sachse, et al., 2002). SDM is 99% similar to SPE-M and SPE-G^dys is 86% similar to SPE-G.

The recently proposed novel nomenclature for sag genes (see above) also included non- S. pyogenes SAgs and it was suggested to adapt the names of the S. pyogenes sag genes for all orthologues from non- S. pyogenes sag genes followed by the allele number (Commons, et al., 2014). For example, spe-G^dys would be named speG11 and sdm would become speM6. Based on this new nomenclature, a search at the National Center for Biotechnology Information Nucleotide (NCBI) database has found sag genes in S. dysgalactiae subsp. equisimilis (3 speA alleles, 1 speC allele, 16 speG alleles, 1 speH allele, 3 speK alleles, 1 speL allele and 1 speM allele), S. dysgalactiae subsp. dysgalactiae (1 speC allele, 6 speG alleles and 3 speM alleles), S. equi subsp. equi (2 speH alleles, 1 speI allele and 1 speK allele) and S. equi subsp. zooepidemicus (1 speK allele). Interestingly, no orthologues of ssa, smez or speJ were found on any non- S. pyogenes genomes. In case of smez and speJ this is most likely due to the fact that these genes are not associated with mobile DNA elements preventing them from horizontal gene transfer.

Allele diversity and frequency of sag genes

In general, S. pyogenes sag genes are well conserved and show only minor allelic variation, often not more than just a few nucleic acid differences. The exception is smez, of which more than 50 different alleles have been listed in the gene databases. The diversity ranges from single nucleic acid differences to changes in 36 positions (=5%) between smez-1 and smez-2. In addition, several smez genes contain nonsense mutations resulting in the expression of truncated and inactive forms of these toxins (Proft, et al., 2000; Turner, et al., 2012). In a recent study, the National Center for Biotechnology Information Nucleotide (NCBI) database was searched for all known streptococcal sag gene variants, including genes from non- S. pyogenes, and revealed a total of 145 unique alleles belonging to 14 groups. After excluding protein duplicates and truncated variants, a total of 91 unique SAg sequences were identified (Commons, et al., 2014). Currently known SAg variants in S. pyogenes are: six SPE-A, three SPE-C, six SPE-G, two SPE-H, two SPE-I, three SPE-J, one SPE-K, three SPE-L, four SPE-M, three SSA, and 56 SMEZ.

S. pyogenes sag genes are generally associated with bacteriophages, with the exception of speG, speJ and smez, which are chromosomally encoded. However, the speJ gene appears to be located on an instable genomic region and is absent in a number of S. pyogenes isolates from diverse linages (Friães, Pinto, Silva-Costa, Ramirez, & Melo-Cristino, 2013; Meisal, et al., 2010). It has been suggested that speJ has been acquired from a temperate phage that was later lost from the genome of descending S. pyogenes lineages (McMillan, et al., 2007). A recent comprehensive profiling of sag genes from 480 clinical S. pyogenes isolates by multiplex PCR revealed the following distribution: speA, 32.1%; speC, 51.5%; speG, 86.9%; speH, 17.1%; speI, 15.2%; speJ, 32.7%; speK, 24.6%; speL, 9.2%; speM, 9.2%; ssa, 35.4%; smez, 96% (Friães, Pinto, Silva-Costa, Ramirez, & Melo-Cristino, 2013). The frequencies of individual sag genes were generally in agreement with results from previous epidemiological studies (Proft, Webb, Handley, & Fraser, 2003a; Maripuu, Eriksson, & Norgren, 2008; Michaelsen, Andreasson, Langerud, & Caugant, 2011). The speL and speM genes were detected in only a small fraction of isolates, but were always found together suggesting a stable genetic linkage. Similarly, speH and speI are relatively rare and were found in association, as expected from their tandem location on a prophage (Commons, et al., 2008), but were also found independently in some isolates from different lineages supporting the idea that speI is occasionally lost during phage integration (Proft, Webb, Handley, & Fraser, 2003a; Friães, Pinto, Silva-Costa, Ramirez, & Melo-Cristino, 2013; Maripuu, Eriksson, & Norgren, 2008; Michaelsen, Andreasson, Langerud, & Caugant, 2011).

The origin of sag genes is still not entirely clear. The homology between sag genes in different streptococcal species, and also in Staphylococcus aureus, together with their mainly bacteriophage location suggests horizontal transfer between species. Direct evidence for horizontal gene transfer of a sag gene between S. pyogenes strains and also between streptococcal species has been provided by Vojtek and co-workers, who showed lysogenic conversion of several S. pyogenes M-serotypes and Streptococcus dysgalactiae subsp. equisimilis clinical isolates with S. pyogenes M12-derived prophage phi149 carrying the ssa gene (Vojtek, et al., 2008). A recent study has shown that the flanking regions of speG in S. pyogenes and in Streptococcus dysgalactiae subsp. equisimilis are conserved suggesting that both species descended from a common ancestor that carried an ancestral speG gene (Okumura, et al., 2012).

Regulation of S. pyogenes superantigen production

Streptococcus pyogenes SAgs are generally secreted in only small amounts, but little is known about the regulation of these SAgs. In growth medium, gene expression is the highest in the late logarithmic and early stationary phase (Unnikrishnan, Cohen, & Sriskandan, 1999). The production of SMEZ, a potent SAg, is so small that it can only be detected reliably using biological assays involving the detection of T cell mitogenicity (Proft, Sriskandan, Yang, & Fraser, 2003b). There are several lines of evidence showing significant upregulation of S. pyogenes SAgs after infection and host factors appear to play a role in this process. SPE-A expression increased after a diffusion chamber containing S. pyogenes was implanted subcutaneously into BALB/c mice. The increase was detected 7 days post-infection and was still high after 21 in vitro passages suggesting a stable switch of the speA gene (Kazmi, et al., 2001). The expression of SPE-C could be increased when a speC-carrying strain was co-cultured with human pharyngeal cells (Broudy, Pancholi, & Fischetti, 2001). In-vivo up-regulation of SAgs was also shown at the transcription level. In a genome-wide DNA microarray analysis, it was demonstrated that growth of a serotype M1 strain in human blood, compared to growth in growth medium, resulted in significant increase in speA, speG, speJ and smez transcripts (Graham, et al., 2005). Using the same methodology, sag transcription was analyzed during infection of cynomolgus macaques. The speA, speJ and smez genes were highly expressed in distinct phases of disease. Importantly, smez expression was 24-times higher than speA, despite the fact that SMEZ is about 10-times more potent in T cell stimulation compared to any other SAg. Furthermore, smez expression correlated with peak levels of C-reactive protein (an important inflammation marker) and was the most dominant acute-phase-correlated pro-inflammatory gene. However, there was no correlation of smez expression with pharyngitis or tonsillitis suggesting that SMEZ might play an important role in invasive S. pyogenes disease (Virtaneva, et al., 2005).

The human factors responsible for SAg upregulation are largely unknown. A study by Kansal et al. has shown that expression of SPE-A can be induced by human transferrin and lactoferrin. However, this was not because of a direct effect of these proteins, but rather due to their iron-scavenging activities, as iron deprivation also resulted in increased SPE-A expression, probably due to stress signals (Kansal, Aziz, & Kotb, 2005).

S. pyogenes SAg levels can also be regulated at the protein level. It was shown that SPE-B, a multifunctional cysteine protease is able to degrade SMEZ, whereas SPE-A and SPE-G were more resistant and SPE-J was completely resistant (Nooh, et al., 2006). Interestingly, SPE-B expression is significantly decreased in hypervirulent S. pyogenes strains that carry mutations in the two-component CovRS regulator, which suggests that SPE-B might have a role as a global regulator of SAg function through proteolysis (Walker, et al., 2007).

Superantigen protein structure

To date, the protein structures of six S. pyogenes SAgs have been solved by X-ray crystallography. These include SPE-A (Papageorgiou, et al., 1999), SPE-C (Roussel, Anderson, Baker, Fraser, & Baker, 1997), SPE-H (Arcus, et al., 2000), SPE-I (Brouillard, et al., 2007), SPE-J (Baker, et al., 2004) and SMEZ-2 (Arcus, et al., 2000). In addition, the protein structure of the Streptococcus dysgalactiae-derived mitogen (SDM) has been determined (Saarinen, Kato, Uchiyama, Miyoshi-Akiyama, & Papageorgiou, 2007). SDM shares 92% amino acid identity with SPE-M and has recently been renamed to SPE-M allele 6 (SPE-M6) (Commons, et al., 2014). All protein structures show a conserved two-domain architecture and the presence of a long, solvent-accessible α-helix that spans the center of the SAg molecule, a feature that is shared with the staphylococcal SAgs. The N-terminal domain is a mixed β-barrel with Greek key topology called an oligonucleotide/oligosaccharide binding (OB) fold, which consists of 8 superfamilies, including the ‘bacterial enterotoxin’ superfamily comprising the ‘SAg toxin N-terminal domain’ family and the ‘bacterial AB₅ toxin’ family. Members of the ‘SAg toxin N-terminal domain’ family also include the staphylococcal SAgs and the superantigen-like toxins (SSLs), which lack mitogenic activity (Arcus, 2002).

The larger C-terminal domain is a β-grasp fold and consists of a twisted β-sheet that is capped by the central α4-helix that packs against a four-strand antiparallel twisted sheet. SAgs are extremely stable proteins that resist denaturing by heat and acid and this is achieved by close packing of the N- and C-terminal domains. The structure is further stabilized by a section of the N-terminus that extends over the top of the C-terminal domain. Notably, the most conserved section of all streptococcal and staphylococcal SAgs, as well as the SSLs, is the region that builds the interface between the α4-helix and the inner side of the N-terminal OB-fold domain.

Molecular interactions of superantigens with host receptor molecules

A hallmark of SAgs is their ability to simultaneously bind to MHC class II molecules on antigen presenting cells and the Vβ-region of the T cell receptor on T cells (Figure 1). A variation in TcR –binding has been shown for the staphylococcal enterotoxin (SE)-H and the Mycoplasma arthritidis mitogen (MAM), which both recognize the variable region of the TcR α-chain (Wang, et al., 2007; Petersson, Pettersson, Skartved, Walse, & Forsberg, 2003). However, interaction with TcRVα has not been shown for any of the S. pyogenes SAgs. More recently, CD28 has been identified as an additional and essential receptor for cytokine production by both streptococcal and staphylococcal SAgs (Arad, et al., 2011; Kaempfer, Arad, Levy, Hillman, Nasie, & Rotfogel, 2013; Ramachandran, et al., 2013).

MHC class II binding

SAgs have developed a variety of ways for attachment to MHC class II, probably driven by a need to optimize the efficiency of individual SAgs. A very strong and stable binding of SAgs to MHC class II is a prerequisite for the extraordinary amplification in T cell signaling, as less SAg molecules are required for stimulation. This is supported by the fact that about four to five orders of magnitude less SAg molecules are required to stimulate human PBLs compared to mouse PBLs based on a slightly higher affinity towards human MHC class II. Further evidence was provided from experiments with transgenic mice expressing human MHC class II, which are significantly more sensitive to T cell stimulations compared to non-transgenic littermates (Nooh, El-Gengehi, Kansal, David, & Kotb, 2007; Sriskandan, et al., 2001).

In general, streptococcal SAgs bind to MHC class II either via the invariant α-chain or the polymorphic β-chain. The staphylococcal SAgs toxic shock syndrome toxin-1 (TSST-1) and staphylococcal enterotoxin (SE)-B are the prototype SAgs for binding to the MHC II α-chain. Co-crystallization studies with these SAgs bound to HLA-DR have revealed an exposed hydrophobic loop region within the N-terminal β-barrel domain that binds to a hydrophobic groove located in the distal region of the DR α1-domain with binding affinities of 10^-5 M (Jardetzky, et al., 1994; Kim, Urban, Strominger, & Wiley, 1994). This region on the MHC class II molecule has been referred to as the ‘generic’ or ‘low-affinity’ binding site for SAgs. Only two of the eleven S. pyogenes SAgs, SPE-A and SSA, use this binding mode (Figure 3A). SPE-A competes with SEB for binding to HLA-DR molecules suggesting common recognition sites for MHC class II. However, the binding sites appear to be non-identical, as SPE-A shows higher affinity towards HLA-DQ compared to HLA-DR and HLA-DP, whereas SEB preferentially binds to HLA-DR (Hartwig, Gerlach, & Fleischer, 1994). The other nine S. pyogenes SAgs all bind to the MHC class II β-chain via a single, highly conserved histidine residue (His81) in an otherwise highly polymorphic MHC class II molecule. This interaction is based on the formation of a tetravalent zinc complex that includes three residues within the C-terminal domain of the SAgs, also known as the zinc-binding site, in addition to the His81 of MHC class II β-chain. The relative binding affinity of this interaction is about 100-times higher than the generic low affinity site (10^-7 M) and has been referred to as the ‘high-affinity’ binding site. SPE-C was the first streptococcal SAg for which this binding mode was shown. SPE-C-binding to the MHC class II β-chain can be completely abolished by adding EDTA and can be restored by excess of Zn²⁺ over EDTA (Li, Tiedemann, Moffatt, & Fraser, 1997). Structural analysis of SPE-C revealed residues His167, His201 and Asp203 as the zinc-binding residues (Roussel, Anderson, Baker, Fraser, & Baker, 1997). The complete zinc coordinated binding was later confirmed by structural analysis of SPE-C in complex with HLA-DR2a bearing a peptide derived from myelin basic protein (Figure 3B). Interestingly, the co-crystal structure also revealed extensive interaction of SPE-C with the bound peptide (Li, et al., 2001). Structural analysis of SPE-H, SPE-J and SMEZ-2, and computer-generated models of SPE-G, SPE-I, SPE-K, SPE-L and SPE-M showed the conserved zinc-binding site in the C-terminal domain, but absence of a generic MHC class II α-chain binding region. This was confirmed in biochemical assays when removal of Zn²⁺ by EDTA completely abolished MHC class II binding. Interestingly, Scatchard plot analysis of SPE-G, SPE-H, SMEZ and SMEZ-2 revealed a range of different binding affinities (from nanomolar to micromolar) towards MHC class II for each of the toxins. Based on the fact that the generic low-affinity binding site is absent in these toxins and the observation of the extensive SPE-C - peptide interaction, it was suggested that some SAgs might have a more restricted MHC class II repertoire defined by the bound peptide antigen (Proft, Moffatt, Berkahn, & Fraser, 1999). Fernandez and colleagues have shown that despite the important role of the zinc complex in MHC class II binding, about 25% of the contacts are made to the antigenic peptide. However, the interactions are mainly with the peptide backbone atoms rather than the side-chain atoms. Furthermore, SAgs interact with the MHC class II-bound peptides at their conformationally conserved N-terminal regions, minimizing sequence-specific interactions with peptide residues to enhance cross-reactivity (Fernández, Guan, Swaminathan, Malchiodi, & Mariuzza, 2006).

Figure 3.

Protein structures of SAg bound to human MHC class II. A. Structural model of the SPE-A – DR complex in which the SPE-A structure (1B1Z) was superimposed onto the SEB – DR1 structure (1SEB). SPE-A (red) binds to the α-chain of (more...)

Several SAgs are capable of forming dimers in solution. For example, SPE-C forms a homodimer using a secondary zinc-binding site, which is located within the N-terminal domain. Consequently, the dimer interface is located opposite the high-affinity HLA-DR β-chain binding site and dimer formation might result in DRβ - SPE-C - SPE-C - DRβ complexes (Roussel, Anderson, Baker, Fraser, & Baker, 1997; Li, Tiedemann, Moffatt, & Fraser, 1997). However, zinc-binding and dimerization of SPE-C are not essential for T cell stimulation (Swietnicki, Barnie, Dyas, & Ulrich, 2003). The biological function of SPE-C dimerization is unknown, but one might speculate that MHC class II crosslinking leads to increased expression of co-stimulatory molecules, such as B7, and cell adhesion molecules on antigen presenting cells. It has previously been shown that cross-linking of MHC class II by staphylococcal enterotoxin A (SEA) is necessary for inflammatory cytokine expression (Mehindate, et al., 1995). Dimer formation has also been demonstrated for SPE-J and SSA (Baker, et al., 2004; De Marzí, et al., 2004). However, in both cases the formation of homodimers would prevent the toxins from binding to the TcR. An alternative function, apart from T cell activation has been suggested, but this has never been confirmed.

TcR binding

SAgs bind to TcR molecules primarily by engaging with the variable region of the β-chain (Vβ-domain). This results in an oligoclonal stimulation of a defined T cell repertoire and the potential activation of >20% of all T cells. The first two co-crystal structures of a streptococcal SAg bound to a TcR β-chain were published in 2002 and showed SPE-A in complex with murine TcRVβ8.2 and SPE-C bound to human TcRVβ2.1 (Sundberg, et al., 2002) (Figure 4A). Considering the structural homology between SPE-A and SEB it was not surprising that the SPE-A - mVβ8.2 complex showed strong similarity to the previously solved SEB - mVβ8.2 structure (Li, et al., 1998a). Residues from the complementarity-determining region 2 (CDR2), framework region 2 (FR2) and, to a lesser extent, hypervariable region 4 and FR3 play a role in the interaction of mTcRVβ8.2 with SAgs. Binding to the CDR2 loop appears to be a requirement for all streptococcal and staphylococcal SAgs, whereas binding to other Vβ domains seems to be responsible for Vβ-specificity (Sundberg, Deng, & Mariuzza, 2007). However, there are also some differences in TcR binding of SPE-A compared to SEB. In addition to intermolecular interaction with CDR2, FR3 and hypervariable region 4, SPE-A also binds to the CDR1 loop of the mVβ8 TcR. In addition, there are several hydrogen bonds between SPE-A and mVβ8.2 that involve side chain atoms, whereas the SEB - mVβ8.2 complex shows exclusively main chain contacts.

Figure 4.

Protein structures of SAgs bound to the T cell receptor. A. Crystal structure of SPE-C bound to the human TcRVβ2 chain (1KTK). SPE-C (red) shows high specificity for hVβ2.1 (blue) due to extensive interactions involving all hypervariable (more...)

SPE-C displays a much higher specificity towards T cells targeting mainly TcRVβ2.1 compared to e.g. SPE-A, which binds to Vβ2.1, Vβ12.2, Vβ14.1 and Vβ15.1. This can be explained by a significantly larger buried surface area and the involvement of all Vβ hypervariable loops, including CDR1, CDR2, CDR3, and HV4 (Li, Llera, & Mariuzza, 1998). In addition, residues on the CDR1 and CDR2 loops are involved in extensive intermolecular contacts. Another variation in TcRVβ-binding has been suggested for SPE-I (Figure 4B), which possesses a unique extension (α3-β8 loop) (Brouillard, et al., 2007). A similar extension has been found in the staphylococcal toxins SEI and SEK and the crystal structure of SEK bound to human TcRVβ5.1 has revealed that residues within the α3-β8 loop make intermolecular contacts with the apical loop of framework region 4 (FR4) (Günther, et al., 2007).

Four categories of SAg - TcR interactions have been proposed: a) highly promiscuous T cell binders, including SEB, that bind to TcRVβ in a simple conformation-dependent mode and only interact with a single CDR2 loop (CDR2); b) moderately promiscuous molecules, including SPE-A, that have direct side chain/side chain contacts in addition to the conformation dependence; c) highly selective T cell activators, like SPE-C, that bind to TcRVβ with the highest degree of structural dissimilarity, and the usage of all three CDR loops (Sundberg, Li, & Mariuzza, 2002) and d) SAgs, like SPE-I, containing the α3-β8 loop and extending the TcRVβ domain binding site into the FR4 region.

Recently, the first ternary complex of a SAg with MHC class II and the TcR was solved. The protein structure of SEB in complex with HLA-DR1 and TcRVα22/Vβ19 confirmed that the SAg adopts a wedge-like position when binding to the TcRVβ-chain, allowing for an interaction between the Vα chain and MHC class II. This binding mode also circumvents contact between TcR and the presented peptide allowing the SAg to trigger a peptide-independent activation of T cells (Rödström, Elbing, & Lindkvist-Petersson, 2014).

CD28 binding

Over two decades it was believed that the simultaneous interaction of SAg with MHC class II and the TcR was not only necessary, but also sufficient to induce a strong mitogenic activity and the production of large amounts of pro-inflammatory cytokines. This classical view has recently been challenged when it was shown that SAgs can also bind to CD28 (Arad, et al., 2011). CD28 is the general co-stimulatory receptor, which is constitutively expressed on T cells and interacts with B7 molecules (CD80 and CD86) (Riley & June, 2005). It was previously shown that small synthetic peptides mimicking a region that is highly conserved among SAgs (β-strand/hinge/α-helix domain) were strong inhibitors of staphylococcal enterotoxin B (SEB), staphylococcal toxic-shock syndrome toxin-1 (TSST-1), and SPE-A and were protective in mice against a lethal challenge with those SAgs. Notably, the peptide region was neither involved in MHC class II binding, nor in binding to TcR. In contrast, synthetic peptides of regions known to interact with MHC class II or TcR failed to reduce a cytokine response (Arad, Levy, Hillman, & Kaempfer, 2000). It was later shown that the synthetic peptide successfully competed with a monoclonal anti-CD28 antibody for binding to CD28 without directly binding to the antibody. This suggested an interaction of the peptide with CD28. Direct binding of the peptide and of staphylococcal enterotoxin B to CD28 with micromolar affinity was demonstrated by surface plasmon-resonance analysis and this interaction is essential for the induction of pro-inflammatory cytokine genes (Arad, et al., 2011). A structural model was suggested that shows a possible binding interface between the N-terminal 118-residue region of the extracellular domain of CD28 (1yjd) with a freely accessible β-strand/hinge/α-helix domain of staphylococcal enterotoxin C3 (SEC3) in complex with MHC class II α-chain and mTcRVβ8.2 (1jck) (Arad, et al., 2011). More recently, it was demonstrated that a CD28 mimetic peptide protects mice from a lethal challenge with SPE-A, as well as from a lethal S. pyogenes infection in a mouse necrotizing soft tissue infection model providing further evidence for the importance of a SAg-CD28 interaction in SAg-mediated disease (Ramachandran, et al., 2013).

It should be mentioned that all the research described above was carried out with SAgs that bind to the MHC class II α-chain. However, nine of the eleven SAgs produced by S. pyogenes bind to the MHC class II β-chain via the zinc-binding site in the C-terminal domain and this binding mode would sterically hinder the suggested interaction with CD28. Therefore, it is currently unclear, if the activity of those SAgs is CD28-independent, or if there is another binding site for CD28.

Consequences of SAg binding to host receptors

Engagement of SAg with its receptors results in rapid release of TNF-α and TNF-β, followed sequentially by IL-2, IL-6, IL-1 and IFN-γ. Animal studies with mice have shown a dramatic increase of TNF-α within the first hour of SAg exposure and T cells within the spleen were found to be the major source for the early release of TNF-α (Faulkner, Cooper, Fantino, Altmann, & Sriskandan, 2005). A comparative study with SPE-A and SMEZ showed that the cytokine-inducing capacity of SMEZ was approximately 10-fold higher than observed with SPE-A (Müller-Alouf, et al., 2001). Furthermore, disruption of the smez gene in an M89 strain completely abolished cytokine production of a S. pyogenes culture supernatant in vitro (Unnikrishnan, et al., 2002).

Early studies have shown that a combination of sub-lethal doses of SAg and LPS can act synergistically to cause shock in rodents, although only when D-galactosamine was used as a sensitizing agent (Bohach, Fasdt, Nelson, & Schlievert, 1990). Similarly, primary human monocytes that were pre-exposed to SAgs for 3 hours showed highly exaggerated TNF-α responses after exposure to LPS. It has been suggested that this synergy results from enhanced pattern recognition of LPS and this is based on the observation that SAg signaling increases expression of toll-like receptor 4 (TLR4), the pattern recognition receptor for LPS (Hopkins, et al., 2005). Furthermore, several SAgs, including the streptococcal SAgs SPE-A and SMEZ, were able to up-regulate TLR2 on the surface of primary human monocytes. This was dependent on SAg-binding to MHC class II, but did not involve signaling by ligation to TLR2. TLR2 up-regulation was associated with an increase in the pro-inflammatory response to TLR2 ligands, but only at high ligand concentration (Hopkins, et al., 2008).

In contrast to the classical AB family toxins, SAgs are believed to remain extracellular and function by signaling inside the host cell. However, a recent study by Ganem et al. has demonstrated an uptake of SAgs by mouse dendritic cells (DCs) without triggering DC maturation. This was followed by SAg recycling to the cell membrane of DCs and the SAg-loaded DCs were capable of triggering a strong lymphocyte proliferation. The authors suggested that intracellular trafficking of SAgs might increase the local concentrations of SAgs and promote their encounter with MHC class II on APCs and the TcR on T cells in lymph nodes (Ganem, et al., 2013).

Streptococcus pyogenes superantigens and disease

SAgs have been implicated in a range of S. pyogenes diseases, including invasive infections such as necrotizing fasciitis and streptococcal toxic shock syndrome (STSS), Kawasaki disease, psoriasis and acute rheumatic fever. The potential involvement of SAgs in these diseases has been demonstrated mainly by epidemiological studies, clinical studies and animal infections models. In addition, several studies have shown specific skewing of the TcRVβ-repertoire in stimulated T cells consistent with SAg activity. However, direct evidence for the involvement of SAgs in S. pyogenes disease remains inconclusive.

Invasive Streptococcus pyogenes disease

Kawasaki Disease

Kawasaki disease (KD) is an acute multisystem vasculitis of unknown etiology that affects mostly young children leading to coronary artery damage (Takahashi, Oharaseki, & Yokouchi, 2014). Streptococcal SAgs have been proposed as etiological agents in the pathogenesis of KD. Multiple studies reported the selective expansion of T cells bearing TcRVβ2 pointing to a possible involvement of SAgs in the disease, in particular SPE-C and SPE-J which both preferentially stimulate the TcRVβ2 T cell subset (Abe, et al., 1992; Konishi, et al., 1997). Yoshioka et al. reported polyclonal expansion of TcRVβ2- and TcRVβ6-bearing T cells and elevated plasma levels of IL-1β, IL-2, IL-6, IL-8, IL-10, IFN-γ, and TNF-α in the acute phase of KD. Moreover, anti-SPE-C antibody levels were significantly higher in acute and convalescent serum from KD patients compared to age-matched controls (Yoshioka, et al., 1999). High levels of anti-SPE-A IgM antibodies were also found in KD patients and increased with the clinical weeks reaching 43% of KD subjects at the fourth week (Matsubara, et al., 2006).

PCR-analysis of speA, speC, speG, and speJ, in stool specimen obtained from 60 patients with KD and 62 age-matched children showed higher prevalence of sag genes in KD patients compared to controls (Suenaga, Suzuki, Shibuta, Takeuchi, & Yoshikawa, 2009). Two studies that investigated the T cell repertoire in KD patients also provided evidence for a role of SAgs in KD. TcRVβ restricted CD4 and/or CD8 activation was observed in eight of 11 (72%) of the KD patients, a finding not observed in healthy controls. Moreover, 81% children with KD had evidence of either TcRVβ skewing (particularly CD4 Vβ2 and Vβ5.1) and/or TcRVβ restricted activation (Brogan, Shah, Clarke, Dillon, & Klein, 2008). Nagata and colleagues identified 18 strains of Gram-positive cocci from the upper gastrointestinal tract from patients with KD that had superantigenic properties and which induced the expansion of TcRVβ2 T cells in vitro (Nagata, et al., 2009).

However, other investigations have failed to show any evidence for SAg involvement in KD. In particular, data from several serological studies showed no significant difference in the prevalence of SAg antibodies between KD patients and control subjects (Gupta-Malhotra, Viteri-Jackson, Thomas, & Zabriskie, 2004; Morita, Imada, Igarashi, & Yutsudo, 1997; Nomura, Yoshinaga, Masuda, Takei, & Miyata, 2002). Furthermore, IgM transcripts expressed by the B cells in the peripheral blood of KD patients in the acute phase of the disease clearly showed an oligoclonal expansion, suggesting that KD is caused not by stimulation of a SAg, but rather by a conventional antigen (Lee, Shin, Kim, & Park, 2009).

Psoriasis

Psoriasis is a chronic inflammatory multi organ disease with well-characterized pathology occurring in the skin and often the joints (Raychaudhuri, Maverakis, & Raychaudhuri, 2014). It has been reported that a particular form of psoriasis, guttate psoriasis, is triggered by S. pyogenes throat infections in 2/3 patients (Nahary, et al., 2008). The causes of psoriasis are not fully understood, but several lines of evidence point to an involvement of SAgs in the disease mechanism. Some studies have demonstrated a TcRVβ-restricted T cell stimulation in psoriasis patients. Leung and co-workers have shown T cell expansion consistent with SAg activity in skin biopsies from two patients with psoriasis, but not in peripheral blood. Skin biopsies from 10 out of 10 patients with acute guttate psoriasis, but not skin biopsies from 12 patients with acute atopic dermatitis or inflammatory skin lesions induced in normal subjects with sodium lauryl sulfate, demonstrated selective accumulation of TcRVβ2 T cells, which occurred in both the CD4+ and the CD8+ T cell subsets. Moreover, the TcR showed extensive junctional region diversity suggesting SAg-induced stimulation of T cells (Leung, et al., 1995). Other studies demonstrated an increase of TcRVβ2 and Vβ5.1 T cells in the skin of patients with guttate and chronic plaque psoriasis compared with peripheral blood (Lewis, et al., 1993) and an increase of TcRVβ2 and Vβ17 cutaneous T cells in patients with guttate psoriasis, but not in control patients (Davison, Allen, Mallon, & Barker, 2001). On the other hand, since 1994, at least 14 studies reported by nine independent groups have indicated that chronic psoriasis lesions are infiltrated by oligoclonal T cells suggesting stimulation by conventional antigens rather than SAgs (reviewed by (Valdimarsson, Thorleifsdottir, Sigurdardottir, Gudjonsson, & Johnston, 2009).

Acute Rheumatic Fever

Acute rheumatic fever (ARF) is a post-streptococcal autoimmune disease. Multiple episodes can result in rheumatic heart disease (RHD), which is the leading cause of preventable pediatric heart disease. It mainly occurs in school age children and young adults after pharyngeal infection with S. pyogenes (Carapetis, Steer, Mulholland, & Weber, 2005). Cross-reactive immune responses to cardiac tissue and joints are responsible for inflammation in the host and it has been suggested that SAgs might stimulate the reactive T cells. There is a correlation between M18 isolates associated with ARF in the USA and speL and speM. These sag genes were found in all M18/emm18 isolates collected over a 69-year period (Smoot, et al., 2002b). Antibodies against SPE-L and SPE-M were more common in convalescent sera from ARF patients compared to pharyngitis patients (Smoot, et al., 2002a). However, serum antibodies against SAgs did not predict the susceptibility of Aboriginal Australians (Yang, et al., 2006).

Therapeutic Interventions

What are SAgs doing for the bacteria?

After more than two decades of intensive research, the question of why SAgs are important for the bacteria remains largely unanswered. There are currently 11 SAgs found in S. pyogenes and many of them have orthologues in other streptococci, all of them sharing a common protein fold and the same target receptors on host cells, MHC class II and TcR. The evolutionary advantage of SAg production seems therefore eminent and is supported by the fact that SAg-producing streptococci only infect hosts with adaptive immunity. Furthermore, most SAgs show allelic variation, in particular SMEZ with >50 variants, that results in antigenic rather than functional differences. This confirms that SAg evolution is mainly driven by host immunity. It is almost certain that the role of SAgs is not to induce systemic lethal shock in the host. Significant antibody responses to bacterial SAgs are commonly found in healthy adults, indicating that SAg exposure must occur during either non-severe infections or asymptomatic colonization (Basma, et al., 1999). A possible advantage of SAg production might involve the corruption of the host immune response. SAgs interfere with the adaptive immune system resulting in profound Th1 type responses with non-specific T cell proliferation and massive release of type 1 cytokines, such as IL-2, IFN-γ and TNF-α. This might suppress local inflammation at the site of infection, although there is no evidence that SAgs directly enhance colonization. By promoting a Th1 type response, SAgs might also suppress a type 2 response and prevent the production of high-affinity cytotoxic antibodies. Another possible mechanism of how SAgs corrupt the immune system might be their ability to induce T cell anergy, a non-responsive state that results from the systemic stimulation of T cells by SAgs. Anergic T cells are unable to produce IL-2 and therefore SAg stimulation might lead to local IL-2 deficiency, which could limit the expansion of antigen-specific T cells (Lavoie, Thibodeau, Erard, & Sékaly, 1999; Miller, Ragheb, & Schwartz, 1999; O'Hehir & Lamb, 1990). More recently, Llewelyn and colleagues suggested that SAgs are able to induce a regulatory T cell phenotype restricted only by the Vβ specificity of the toxin or toxins produced. They showed that stimulation of PBMCs with SPE-K (previously SPE-K/L) resulted in the rise of CD4(+) CD25(+) T regulatory T cells (Tregs) from CD4(+) CD25(-) T cells. This was Vβ-specific and required APCs. Furthermore, the Tregs expressed the anti-inflammatory cytokine IL-10 at lower SAg concentrations than was required to trigger IFN-γ production (Taylor & Llewelyn, 2010). It was later shown that SAgs are also potent inducers of human regulatory CD8(+) T cells, which were able to suppress the proliferation of CD4(+) CD25(-) T cells in response to anti-CD3 stimulation in a cell contact dependent mode. SAg induced stimulation of Tregs might therefore be a feature of acute bacterial infections contributing to immune evasion by the microbe and disease pathogenesis (Taylor, Cross, & Llewelyn, 2012). In a recent study, Kasper et al. have shown that acute S. pyogenes infection in the nasopharynx of mice is dependent upon both bacterial SAgs and host MHC class II molecules (Kasper, et al., 2014). S. pyogenes was rapidly cleared from the nasal cavity of wild-type C57BL/6 (B6) mice, but infection was enhanced up to ~10,000-fold in B6 mice that express the human MHC class II molecule HLA-DQ8. This infection phenotype was dependent on the production of SPE-A since an speA^- isogenic strain showed markedly reduced infection in the noses of the B6–DQ8 transgenic mouse. Moreover, pre-vaccination with an MHC class II binding mutant toxoid of SPE-A inhibited infection. This is the first study to show that survival of S. pyogenes in a common niche is indeed enhanced by the production of a SAg, and gives some credence to the long held notion that SAgs are indeed virulence as well as pathogenicity factors.

Acknowledgements

The authors are supported by the Health Research Council New Zealand.

References

• Abe J., Kotzin B. L., Jujo K., Melish M. E., Glode M. P., Kohsaka T., et al. Selective expansion of T cells expressing T-cell receptor variable regions V beta 2 and V beta 8 in Kawasaki disease. Proceedings of the National Academy of Sciences of the United States of America. 1992;89(9):4066–4070. [PMC free article: PMC525633] [PubMed: 1315049]
• Arad G., Levy R., Hillman D., Kaempfer R. Superantigen antagonist protects against lethal shock and defines a new domain for T-cell activation. Nature Medicine. 2000;6(4):414–421. [PubMed: 10742148]
• Arad G., Levy R., Nasie I., Hillman D., Rotfogel Z., Barash U., et al. Binding of superantigen toxins into the CD28 homodimer interface is essential for induction of cytokine genes that mediate lethal shock. PLoS Biology. 2011;13(8):e1001149. [PMC free article: PMC3172200] [PubMed: 21931534]
• Arcus V. OB-fold domains: a snapshot of the evolution of sequence, structure and function. Current Opinion in Structural Biology. 2002;12(6):794–801. [PubMed: 12504685]
• Arcus V. L., Proft T., Sigrell J. A., Baker H. M., Fraser J. D., Baker E. N. Conservation and variation in superantigen structure and activity highlighted by the three-dimensional structures of two new superantigens from Streptococcus pyogenes. Journal of Molecular Biology. 2000;299(1):157–168. [PubMed: 10860729]
• Artiushin S. C., Timoney J. F., Sheoran A. S., Muthupalani S. K. Characterization and immunogenecity of pyrogenic mitogens SePE-H and SePE-I of Streptococcus equi. Microbial Pathogenesis. 2002;32(2):71–85. [PubMed: 11812213]
• Baker H. M., Proft T., Webb P. D., Arcus V. L., Fraser J. D., Baker E. N. Crystallographic and mutational data show that the streptococcal pyrogenic exotoxin J can use a common binding surface for T cell receptor binding and dimerization. The Journal of Biological Chemistry. 2004;279(37):38571–38576. [PubMed: 15247241]
• Baker M. D., Acharya K. R. Superantigens: structure-function relationships. International Journal of Medical Microbiology. 2004;293(7-8):529–537. [PubMed: 15149028]
• Basma H., Norrby-Teglund A., Guedez Y., McGeer A., Low D. E., El-Ahmedy O., et al. Risk factors in the pathogenesis of invasive group A streptococcal infections: role of protective humoral immunity. Infection and Immunity. 1999;67(4):1871–1877. [PMC free article: PMC96540] [PubMed: 10085030]
• Beres S. B., Sylva G. L., Barbian K. D., Lei B., Hoff J. S., Mammarella N. D., et al. Genome sequence of a serotype M3 strain of group A Streptococcus: phage-encoded toxins, the high-virulence phenotype, and clone emergence. Proceedings of the National Academy of Sciences of the United States of America. 2002;99(15):10078–10083. [PMC free article: PMC126627] [PubMed: 12122206]
• Bohach G. A., Fasdt D. J., Nelson R. D., Schlievert P. M. Staphylococcal and streptococcal pyrogenic toxins involved in toxic shock syndrome and related illnesses. Critical Reviews in Microbiology. 1990;17(4):251–272. [PubMed: 2206394]
• Bohach G. A., Hauser A. R., Schlievert P. M. Cloning of the gene, speB, for streptococcal pyrogenic exotoxin type B in Escherichia coli. Infection and Immunity. 1988;56(6):1665–1667. [PMC free article: PMC259455] [PubMed: 3286506]
• Brogan P. A., Shah V., Clarke L. A., Dillon M. J., Klein N. T cell activation profiles in Kawasaki syndrome. Clinical & Experimental Immunology. 2008;151(2):267–274. [PMC free article: PMC2276940] [PubMed: 18070150]
• Broudy T. B., Pancholi V., Fischetti V. A. Induction of Lysogenic Bacteriophage and Phage-Associated Toxin from group A streptococci during Coculture with Human Pharyngeal Cells. Infection and Immunity. 2001;69(3):1440–1443. [PMC free article: PMC98039] [PubMed: 11179310]
• Brouillard J. N., Günther S., Varma A. K., Gryski I., Herfst C. A., Rahman A. K., et al. Crystal structure of the streptococcal superantigen SpeI and functional role of a novel loop domain in T cell activation by group V superantigens. Journal of Molecular Biology. 2007;367(4):925–934. [PubMed: 17303163]
• Buonpane R. A., Churchill H. R., Moza B., Sundberg E. J., Peterson M. L., Schlievert P. M., et al. Neutralization of staphylococcal enterotoxin B by soluble, high-affinity receptor antagonists. Nature Medicine. 2007;13(6):725–729. [PubMed: 17515896]
• Carapetis J. R., Steer A. C., Mulholland E. K., Weber M. The global burden of group A streptococcal diseases. The Lancet Infectious Diseases. 2005;5(11):685–694. [PubMed: 16253886]
• Carapetis J., Robins-Browne R., Martin D., Shelby-James T., Hogg G. Increasing severity of invasive group A Streptococcal disease in Australia: clinical and molecular epidemiological features and identification of a new virulent M-nontypeable clone. Clinical Infectious Diseases. 1995;21(5):1220–1227. [PubMed: 8589146]
• Cleary P. P., Kaplan E. L., Handley J. P., Wlazlo A., Kim M. H., Hauser A. R., et al. Clonal basis for resurgence of serious Streptococcus pyogenes disease in the 1980s. Lancet. 1992;339(8792):518–521. [PubMed: 1346879]
• Commons R. J., Smeesters P. R., Proft T., Fraser J. D., Robins-Browne R., Curtis N. Streptococcal superantigens: categorization and clinical associations. Trends in Molecular Medicine. 2014;20(1):48–62. [PubMed: 24210845]
• Commons R., Rogers S., Gooding T., Danchin M., Carapetis J., Robins-Browne R., et al. Superantigen genes in group A streptococcal isolates and their relationship with emm types. Journal of Medical Microbiology. 2008;57(Pt 10):1238–1246. [PubMed: 18809552]
• Darenberg J., Söderquist B., Normark B. H., Norrby-Teglund A. Differences in potency of intravenous polyspecific immunoglobulin G against streptococcal and staphylococcal superantigens: implications for therapy of toxic shock syndrome. Clinical Infectious Diseases. 2004;38(6):836–842. [PubMed: 14999628]
• Davison S. C., Allen M. H., Mallon E., Barker J. N. Contrasting patterns of streptococcal superantigen-induced T-cell proliferation in guttate vs. chronic plaque psoriasis. British Journal of Dermatology. 2001;145(2):245–251. [PubMed: 11531786]
• De Marzí M. C., Fernández M. M., Sundberg E. J., Molinero L., Zwirner N. W., Llera A. S., et al. Cloning, expression, and interaction of human T cell receptors with the bacterial superantigen SSA. European Journal of Biochemistry. 2004;271(20):4075–4083. [PubMed: 15479236]
• Descheemaeker P., Van Loock F., Hauchecorne M., Vandamme P., Goossens H. Molecular characterisation of group A streptococci from invasive and non-invasive disease episodes in Belgium during 1993-1994. Journal of Medical Microbiology. 2000;49(5):467–471. [PubMed: 10798560]
• Dick G. F., Dick G. H. Landmark article Jan 26, 1924: The etiology of scarlet fever. The Journal of the American Medical Association. 1983;250(22):3096. [PubMed: 6358561]
• Dickgreber N., Stoitzner P., Bai Y., Price K. M., Farrand K. J., Manning K., et al. Targeting antigen to MHC class II molecules promotes efficient cross-presentation and enhances immunotherapy. The Journal of Immunology. 2009;182(3):1260–1269. [PubMed: 19155471]
• Donadini R., Fields B. A. Yersinia pseudotuberculosis superantigens. Chemical Immunology and Allergy. 2007;93:77–91. [PubMed: 17369701]
• Eriksson B. K., Andersson J., Holm S. E., Norgren M. Invasive group A streptococcal infections: T1M1 isolates expressing pyrogenic exotoxins A and B in combination with selective lack of toxin-neutralizing antibodies are associated with increased risk of streptococcal toxic shock syndrome. The Journal of Infectious Diseases. 1999;180(2):410–418. [PubMed: 10395857]
• Faulkner L., Cooper A., Fantino C., Altmann D. M., Sriskandan S. The mechanism of superantigen-mediated toxic shock: not a simple Th1 cytokine storm. The Journal of Immunology. 2005;175(10):6870–6877. [PubMed: 16272345]
• Fernández M. M., Guan R., Swaminathan C. P., Malchiodi E. L., Mariuzza R. A. Crystal structure of staphylococcal enterotoxin I (SEI) in complex with a human major histocompatibility complex class II molecule. Journal of Biological Chemistry. 2006;281(35):25356–25364. [PMC free article: PMC2730046] [PubMed: 16829512]
• Ferretti J. J., McShan W. M., Ajdic D., Savic D. J., Savic G., Lyon K., et al. Complete genome sequence of an M1 strain of Streptococcus pyogenes. Proceedings of the National Academy of Sciences of the United States of America. 2001;98(8):4658–4663. [PMC free article: PMC31890] [PubMed: 11296296]
• Fraser J. D., Proft T. The bacterial superantigen and superantigen-like proteins. Immunological Reviews. 2008;225:226–243. [PubMed: 18837785]
• Friães A., Pinto F. R., Silva-Costa C., Ramirez M., Melo-Cristino J. Superantigen gene complement of Streptococcus pyogenes--relationship with other typing methods and short-term stability. European Journal of Clinical Microbiology & Infectious Diseases. 2013;32(1):115–125. [PubMed: 22936424]
• Friães A., Pinto F. R., Silva-Costa C., Ramirez M., Melo-Cristino J., Infections P. G. Group A streptococci clones associated with invasive infections and pharyngitis in Portugal present differences in emm types, superantigen gene content and antimicrobial resistance. BMC Microbiology. 2012;12:280. [PMC free article: PMC3543706] [PubMed: 23181337]
• Ganem M. B., De Marzí M. C., Fernández-Lynch M. J., Jancic C., Vermeulen M., Geffner J., et al. Uptake and intracellular trafficking of superantigens in dendritic cells. PLoS One. 2013;8:e66244. [PMC free article: PMC3682983] [PubMed: 23799083]
• Gerlach D., Reichardt W., Fleischer B., Schmidt K. H. Separation of mitogenic and pyrogenic activities from so-called erythrogenic toxin type B (Streptococcal proteinase). Zentralbl Bakteriol. 1994;280(4):507–514. [PubMed: 8061411]
• Graham M. R., Virtaneva K., Porcella S. F., Barry W. T., Gowen B. B., Johnson C. R., et al. Group A Streptococcus transcriptome dynamics during growth in human blood reveals bacterial adaptive and survival strategies. The American Journal of Pathology. 2005;166(2):455–465. [PMC free article: PMC1602339] [PubMed: 15681829]
• Günther S., Varma A. K., Moza B., Kasper K. J., Wyatt A. W., Zhu P., et al. A novel loop domain in superantigens extends their T cell receptor recognition site. Journal of Molecular Biology. 2007;371(1):210–221. [PMC free article: PMC2949350] [PubMed: 17560605]
• Gupta-Malhotra M., Viteri-Jackson A., Thomas W., Zabriskie J. B. Antibodies to highly conserved peptide sequence of staphylococcal and streptococcal superantigens in Kawasaki disease. Experimental and Molecular Pathology. 2004;76(2):117–121. [PubMed: 15010289]
• Hartwig U. F., Gerlach D., Fleischer B. Major histocompatibility complex class II binding site for streptococcal pyrogenic (erythrogenic) toxin A. Medical Microbiology and Immunology. 1994;183(5):257–264. [PubMed: 7715537]
• Haukness H. A., Tanz R. R., Thomson R. B. Jr, Pierry D. K., Kaplan E. L., Beall B., et al. The heterogeneity of endemic community pediatric group a streptococcal pharyngeal isolates and their relationship to invasive isolates. The Journal of Infectious Diseases. 2002;185(7):915–920. [PubMed: 11920315]
• Hauser A. R., Stevens D. L., Kaplan E. L., Schlievert P. M. Molecular analysis of pyrogenic exotoxins from Streptococcus pyogenes isolates associated with toxic shock-like syndrome. Journal of Clinical Microbiology. 1991;29(8):1562–1567. [PMC free article: PMC270163] [PubMed: 1684795]
• Hooker S. B., Follensby E. M. Studies on scarlet fever. II. Different toxins produced by hemolytic streptococci of scarlatinal origin. The Journal of Immunology. 1934;27(2):177–193.
• Hopkins P. A., Fraser J. D., Pridmore A. C., Russell H. H., Read R. C., Sriskandan S. Superantigen recognition by HLA class II on monocytes up-regulates toll-like receptor 4 and enhances proinflammatory responses to endotoxin. Blood. 2005;105(9):3655–3662. [PubMed: 15644417]
• Hopkins P. A., Pridmore A. C., Ellmerich S., Fraser J. D., Russell H. H., Read R. C., et al. Increased surface toll-like receptor 2 expression in superantigen shock. Critical Care Medicine. 2008;36(4):1267–1276. [PubMed: 18379254]
• Hsueh P. R., Wu J. J., Tsai P. J., Liu J. W., Chuang Y. C., Luh K. T. Invasive group A streptococcal disease in Taiwan is not associated with the presence of streptococcal pyrogenic exotoxin genes. Clinical Infectious Diseases. 1998;26(3):584–589. [PubMed: 9524827]
• Ikebe T., Wada A., Inagaki Y., Sugama K., Suzuki R., Tanaka D., et al. Dissemination of the Phage-Associated Novel Superantigen gene speL in recent Invasive and Noninvasive Streptococcus pyogenes M3/T3 isolates in Japan. Infection and Immunity. 2002;70(6):3227–3233. [PMC free article: PMC128029] [PubMed: 12011018]
• Imanishi K., Igarashi H., Uchiyama T. Activation of murine T cells by streptococcal pyrogenic exotoxin type A. Requirement for MHC class II molecules on accessory cells and identification of V beta elements in T cell receptor of toxin-reactive T cells. The Journal of Immunology. 1990;145(10):3170–3176. [PubMed: 2230113]
• Jardetzky T. S., Brown J. H., Gorga J. C., Stern L. J., Urban R. G., Chi Y. I., et al. Three-dimensional structure of a human class II histocompatibility molecule complexed with superantigen. Nature. 1994;368(6473):711–718. [PubMed: 8152483]
• Kaempfer R., Arad G., Levy R., Hillman D., Nasie I., Rotfogel Z. CD28: direct and critical receptor for superantigen toxins. Toxins. 2013;2013;(9):1531–1542. [PMC free article: PMC3798871] [PubMed: 24022021]
• Kamezawa Y., Nakahara T., Nakano S., Abe Y., Nozaki-Renard J., Isono T. Streptococcal mitogenic exotoxin Z, a novel acidic superantigenic toxin produced by a T1 strain of Streptococcus pyogenes. Infection and Immunity. 1997;65(9):3828–3833. [PMC free article: PMC175546] [PubMed: 9284159]
• Kansal R. G., Aziz R. K., Kotb M. Modulation of expression of superantigens by human transferrin and lactoferrin: a novel mechanism in host-Streptococcus interactions. The Journal of Infectious Diseases. 2005;191(12):2121–2129. [PubMed: 15897999]
• Kasper K. J., Zeppa J. J., Wakabayashi A. T., Xu S. X., Mazzuca D. M., Welch I., et al. Bacterial Superantigens Promote Acute Nasopharyngeal Infection by Streptococcus pyogenes in a Human MHC Class II-Dependent Manner. PLoS Pathogens. 2014;10(5):e1004155. [PMC free article: PMC4038607] [PubMed: 24875883]
• Kazmi S. U., Kansal R., Aziz R. K., Hooshdaran M., Norrby-Teglund A., Low D. E., et al. Reciprocal, temporal expression of SpeA and SpeB by invasive M1T1 group a streptococcal isolates in vivo. Infection and Immunity. 2001;69(8):4988–4995. [PMC free article: PMC98591] [PubMed: 11447177]
• Kim J., Urban R. G., Strominger J. L., Wiley D. C. Toxic shock syndrome toxin-1 complexed with a class II major histocompatibility molecule HLA-DR1. Science. 1994;266(5192):1870–1874. [PubMed: 7997880]
• Kim Y. B., Watson D. W. Apurified group A streptococcal pyrogenic exotoxin. Physiochemical and biological properties, including the enhancement of susceptibility to endotoxin lethal shock. The Journal of Experimental Medicine. 1970;131(3):611–622. [PMC free article: PMC2138823] [PubMed: 4905084]
• Kittang B. R., Skrede S., Langeland N., Haanshuus C. G., Mylvaganam H. emm gene diversity, superantigen gene profiles and presence of SlaA among clinical isolates of group A, C and G streptococci from western Norway. European Journal of Clinical Microbiology. 2011;30(3):423–433. [PMC free article: PMC3034890] [PubMed: 21103900]
• Konishi N., Baba K., Abe J., Maruko T., Waki K., Takeda N., et al. A case of Kawasaki disease with coronary artery aneurysms documenting Yersinia pseudotuberculosis infection. Acta Paediatrica. 1997;86(6):661–664. [PubMed: 9202805]
• Kotb M., Norrby-Teglund A., McGeer A., El-Sherbini H., Dorak M. T., Khurshid A., et al. An immunogenetic and molecular basis for differences in outcomes of invasive group A streptococcal infections. Nature Medicine. 2002;8(12):1398–1404. [PubMed: 12436116]
• Lavoie P. M., Thibodeau J., Erard F., Sékaly R. P. Understanding the mechanism of action of bacterial superantigens from a decade of research. Immunological Reviews. 1999;168:257–269. [PubMed: 10399079]
• Lee H. H., Shin J.-S., Kim D. S., Park I. H. Immunoglobulin V(H) chain gene analysis of peripheral blood IgM-producing B cells in patients with Kawasaki disease. Yonsei Medical Journal. 2009;50(4):493–504. [PMC free article: PMC2730610] [PubMed: 19718396]
• Lehnert N. M., Allen D. L., Allen B. L., Catasti P., Shiflett P. R., Chen M., et al. Structure-based design of a bispecific receptor mimic that inhibits T cell responses to a superantigen. Biochemistry. 2001;40(14):4222–4228. [PubMed: 11284677]
• Leonard B. A., Lee P. K., Jenkins M. K., Schlievert P. M. Cell and receptor requirements for streptococcal pyrogenic exotoxin T cell mitogenicity. Infection and Immunity. 1991;59(3):1210–1214. [PMC free article: PMC258393] [PubMed: 1997426]
• Leung D. Y., Travers J. B., Giorno R., Norris D. A., Skinner R., Aelion J., et al. Evidence for a streptococcal superantigen-driven process in acute guttate psoriasis. The Journal of Clinical Investigation. 1995;96(5):2106–2112. [PMC free article: PMC185858] [PubMed: 7593594]
• Lewis H. M., Baker B. S., Bokth S., Powles A. V., Garioch J. J., Valdimarsson H., et al. Restricted T-cell receptor V beta gene usage in the skin of patients with guttate and chronic plaque psoriasis. British Journal of Dermatology. 1993;129(5):514–520. [PubMed: 8251347]
• Li H., Llera A., Mariuzza R. A. Structure-function studies of T-cell receptor-superantigen interactions. Immunological Reviews. 1998b;163:177–186. [PubMed: 9700510]
• Li H., Llera A., Tsuchiya D., Leder L., Ysern X., Schlievert P. M., et al. Three-dimensional structure of the complex between a T cell receptor beta chain and the superantigen staphylococcal enterotoxin B. Immunity. 1998a;9(6):807–816. [PubMed: 9881971]
• Li P.-L., Tiedemann R. E., Moffatt S. L., Fraser J. D. The superantigen streptococcal pyrogenic exotoxin C (SPE-C) exhibits a novel mode of action. The Journal of Experimental Medicine. 1997;186(3):375–383. [PMC free article: PMC2199005] [PubMed: 9236189]
• Li Y., Li H., Dimasi N., McCormick J. K., Martin R., Schuck P., et al. Crystal structure of a superantigen bound to the high-affinity, zinc-dependent site on MHC class II. Immunity. 2001;14(1):93–104. [PubMed: 11163233]
• Lintges M., van der Linden M., Hilgers R. D., Arlt S., Al-Lahham A., Reinert R. R., et al. Superantigen genes are more important than the emm type for the invasiveness of group A Streptococcus infection. Journal of Infectious Diseases. 2010;202(1):20–28. [PubMed: 20497047]
• Llewelyn M., Sriskandan S., Peakman M., Ambrozak D. R., Douek D. C., Kwok W. W., et al. HLA class II polymorphisms determine responses to bacterial superantigens. The Journal of Immunology. 2004;172(3):1719–1726. [PubMed: 14734754]
• Maripuu L., Eriksson A., Norgren M. Superantigen gene profile diversity among clinical group A streptococcal isolates. FEMS Immunology and Medical Microbiology. 2008;54(2):236–244. [PubMed: 18754783]
• Matsubara K., Fuyaka T., Miwa K., Shibayama N., Nigami H., Harigaya H., et al. Development of serum IgM antibodies against superantigens of Staphylococcus aureus and Streptococcus pyogenes in Kawasaki disease. Clinical & Experimental Immunology. 2006;143(3):427–434. [PMC free article: PMC1809617] [PubMed: 16487241]
• McCormick J. K., Pragman A. A., Stolpa J. C., Leung D. Y., Schlievert P. M. Functional characterization of streptococcal pyrogenic exotoxin J, a novel superantigen. Infection and Immunity. 2001;69(3):1381–1388. [PMC free article: PMC98031] [PubMed: 11179302]
• McCormick J. K., Tripp T. J., Olmsted S. B., Matsuka Y. V., Gahr P. J., Ohlendorf D. H., et al. Development of streptococcal pyrogenic exotoxin C vaccine toxoids that are protective in the rabbit model of toxic shock syndrome. The Journal of Immunology. 2000;165(4):2306–2312. [PubMed: 10925320]
• McIntosh J. D., Manning K., Chokshi S., Naoumov N. V., Fraser J. D., Dunbar P. R., et al. An engineered non-toxic superantigen increases cross presentation of hepatitis B virus nucleocapsids by human dendritic cells. PLoS One. 2014;9(4):e93598. [PMC free article: PMC3972192] [PubMed: 24690680]
• McMillan D. J., Geffers R., Buer J., Vlaminckx B. J., Sriprakash K. S., Chhatwal G. S. Variations in the distribution of genes encoding virulence and extracellular proteins in group A streptococcus are largely restricted to 11 genomic loci. Microbes and Infection. 2007;9(3):259–270. [PubMed: 17307378]
• Mehindate K., Thibodeau J., Dohlsten M., Kalland T., Sékaly R. P., Mourad W. Cross-linking of major histocompatibility complex class II molecules by staphylococcal enterotoxin A superantigen is a requirement for inflammatory cytokine gene expression. The Journal of Experimental Medicine. 1995;182(5):1573–1577. [PMC free article: PMC2192187] [PubMed: 7595227]
• Meisal R., Andreasson I. K., Høiby E. A., Aaberge I. S., Michaelsen T. E., Caugant D. A. Streptococcus pyogenes isolates causing severe infections in Norway in 2006 to 2007: emm types, multilocus sequence types, and superantigen profiles. Journal of Clinical Microbiology. 2010;48(3):842–851. [PMC free article: PMC2832411] [PubMed: 20042624]
• Michaelsen T. E., Andreasson I. K., Langerud B. K., Caugant D. A. Similar superantigen gene profiles and superantigen activity in norwegian isolates of invasive and non-invasive group a streptococci. Scandinavian Journal of Immunology. 2011;74(5):423–429. [PubMed: 21707691]
• Michie C., Scott A., Cheesbrough J., Beverley P., Pasvol G. Streptococcal toxic shock-like syndrome: evidence of superantigen activity and its effects on T lymphocyte subsets in vivo. Clinical & Experimental Immunology. 1994;98(1):140–144. [PMC free article: PMC1534184] [PubMed: 7923873]
• Miller C., Ragheb J. A., Schwartz R. H. Anergy and cytokine-mediated suppression as distinct superantigen-induced tolerance mechanisms in vivo. The Journal of Experimental Medicine. 1999;190(1):53–64. [PMC free article: PMC2195559] [PubMed: 10429670]
• Miyoshi-Akiyama T., Zhao J., Kato H., Kikuchi K., Totsuka K., Kataoka Y., et al. Streptococcus dysgalactiae-derived mitogen (SDM), a novel bacterial superantigen: characterization of its biological activity and predicted tertiary structure. Molecular Microbiology. 2003;47(6):1589–1599. [PubMed: 12622814]
• Mollick J. A., Miller G. G., Musser J. M., Cook R. G., Grossman D., Rich R. R. A novel superantigen isolated from pathogenic strains of Streptococcus pyogenes with aminoterminal homology to staphylococcal enterotoxins B and C. Journal of Clinical Investigation. 1993;92(2):710–719. [PMC free article: PMC294905] [PubMed: 8349810]
• Morita A., Imada Y., Igarashi H., Yutsudo T. Serologic evidence that streptococcal superantigens are not involved in the pathogenesis of Kawasaki disease. Microbiology and Immunology. 1997;41(11):895–900. [PubMed: 9444333]
• Müller-Alouf H., Proft T., Zollner T. M., Gerlach D., Champagne E., Desreumaux P., et al. Pyrogenicity and cytokine-inducing properties of Streptococcus pyogenes superantigens: comparative study of streptococcal mitogenic exotoxin Z and pyrogenic exotoxin A. Infection and Immunity. 2001;69(6):4141–4145. [PMC free article: PMC98482] [PubMed: 11349089]
• Murakami J., Kawabata S., Terao Y., Kikuchi K., Totsuka K., Tamaru A., et al. Distribution of emm genotypes and superantigen genes of Streptococcus pyogenes isolated in Japan, 1994-9. Epidemiology and Infection. 2002;128(3):397–404. [PMC free article: PMC2869835] [PubMed: 12113483]
• Mylvaganam H., Bjorvatn B., Osland A. Distribution and sequence variations of selected virulence genes among group A streptococcal isolates from western Norway. APMIS. 2000;108(11):771–778. [PubMed: 11211972]
• Nagata S., Yamashiro Y., Ohtsuka Y., Shimizu T., Sakurai Y., Misawa S., et al. Heat shock proteins and superantigenic properties of bacteria from the gastrointestinal tract of patients with Kawasaki disease. Immunology. 2009;128(4):511–520. [PMC free article: PMC2792135] [PubMed: 19950419]
• Nahary L., Tamarkin A., Kayam N., Sela S., Fry L., Baker B., et al. An investigation of antistreptococcal antibody responses in guttate psoriasis. Archives of Dermatological Research. 2008;300(8):441–449. [PubMed: 18648827]
• Nasser W., Beres S. B., Olsen R. J., Dean M. A., Rice K. A., Long S. W., et al. Evolutionary pathway to increased virulence and epidemic group A Streptococcus disease derived from 3,615 genome sequences. Proceedings of the National Academy of Sciences of the United States of America. 2014;111(17):E1768–E1776. [PMC free article: PMC4035937] [PubMed: 24733896]
• Nomura Y., Yoshinaga M., Masuda K., Takei S., Miyata K. Maternal antibody against toxic shock syndrome toxin-1 may protect infants younger than 6 months of age from developing Kawasaki syndrome. The Journal of Infectious Diseases. 2002;185(11):1677–1680. [PubMed: 12023778]
• Nooh M. M., Aziz R. K., Kotb M., Eroshkin A., Chuang W. J., Proft T., et al. Streptococcal mitogenic exotoxin, SmeZ, is the most susceptible M1T1 streptococcal superantigen to degradation by the streptococcal cysteine protease, SpeB. The Journal of Biological Chemistry. 2006;281(46):35281–35288. [PubMed: 16980693]
• Nooh M. M., El-Gengehi N., Kansal R., David C. S., Kotb M. HLA transgenic mice provide evidence for a direct and dominant role of HLA class II variation in modulating the severity of streptococcal sepsis. The Journal of Immunology. 2007;178(5):3076–3083. [PubMed: 17312154]
• Nooh M. M., Nookala S., Kansal R., Kotb M. Individual genetic variations directly effect polarization of cytokine responses to superantigens associated with streptococcal sepsis: implications for customized patient care. The Journal of Immunology. 2011;186(5):3156–3163. [PubMed: 21282506]
• Norrby-Teglund, A., Low, D. E., & Kotb, M. (2007). Intravenous immunoglobulin therapy in superantigen-mediated toxic shock syndrome. In M. Kotb, & J. D. Fraser (Eds.), Superantigens: Molecular Basis for Their Role in Human Diseases (pp. 197-216). Washington, DC: ASM Press.
• Norrby-Teglund A., Nepom G. T., Kotb M. Differential presentation of group A streptococcal superantigens by HLA class II DQ and DR alleles. European Journal of Immunology. 2002;32(9):2570–2577. [PubMed: 12207341]
• Norrby-Teglund A., Newton D., Kotb M., Holm S. E., Norgren M. Superantigenic properties of the group A streptococcal exotoxin SpeF (MF). Infection and Immunity. 1994;62(12):5227–5233. [PMC free article: PMC303258] [PubMed: 7960098]
• O'Hehir R. E., Lamb J. R. Induction of specific clonal anergy in human T lymphocytes by Staphylococcus aureus enterotoxins. Proceedings of the National Academy of Sciences of the United States of America. 1990;87(22):8884–8888. [PMC free article: PMC55064] [PubMed: 1978940]
• Okumura K., Shimomura Y., Murayama S. Y., Yagi J., Ubukata K., Kirikae T., et al. Evolutionary paths of streptococcal and staphylococcal superantigens. BMC Genomics. 2012;13:404. [PMC free article: PMC3538662] [PubMed: 22900646]
• Oster, H., & Bisno, A. (2006). Group C and G streptococcal infections: epidemiologic and clinical aspects. In V. A. Fischetti, R. P. Novick, J. J. Ferretti, & D. A. Portnoy (Eds.), Gram-Positive Pathogens (pp. 184-190). Washington, DC: ASM Press.
• Paillot R., Darby A. C., Robinson C., Wright N. L., Steward K. F., Anderson E., et al. Identification of three novel superantigen-encoding genes in Streptococcus equi subsp. zooepidemicus, szeF, szeN, and szeP. Infection and Immunity. 2010;78(11):4817–4827. [PMC free article: PMC2976332] [PubMed: 20713629]
• Papageorgiou A. C., Collins C. M., Gutman D. M., Kline J. B., O'Brien S. M., Tranter H. S., et al. Structural basis for the recognition of superantigen streptococcal pyrogenic exotoxin A (SpeA) by MHC class II molecules and T cell receptors. The EMBO Journal. 1999;18(1):9–21. [PMC free article: PMC1171097] [PubMed: 9878045]
• Petersson K., Pettersson H., Skartved N. J., Walse B., Forsberg G. Staphylococcal enterotoxin H induces V alpha-specific expansion of T cells. The Journal of Immunology. 2003;170(8):4148–4154. [PubMed: 12682246]
• Proft T., Fraser J. D. Bacterial superantigens. Clinical & Experimental Immunology. 2003;133(3):299–306. [PMC free article: PMC1808794] [PubMed: 12930353]
• Proft T., Fraser J. D. Streptococcal superantigens. Chemical Immunology and Allergy. 2007;93:1–23. [PubMed: 17369697]
• Proft T., Arcus V. L., Handley V., Baker E. N., Fraser J. D. Immunological and Biochemical Characterization of Streptococcal Pyrogenic Exotoxins I and J (SPE-I and SPE-J) from Streptococcus pyogenes. The Journal of Immunology. 2001;166(11):6711–6719. [PubMed: 11359827]
• Proft T., Moffatt S. L., Berkahn C. J., Fraser J. D. Identification and characterization of novel superantigens from Streptococcus pyogenes. The Journal of Experimental Medicine. 1999;189(1):89–102. [PMC free article: PMC1887688] [PubMed: 9874566]
• Proft T., Moffatt S. L., Weller K. D., Paterson A., Martin D., Fraser J. D. The streptococcal superantigen SMEZ exhibits wide allelic variation, mosaic structure, and significant antigenic variation. The Journal of Experimental Medicine. 2000;191(10):1765–1776. [PMC free article: PMC2193151] [PubMed: 10811869]
• Proft, T., Schrage, B., & Fraser, J. D. (2005). uperantigens: Microbial Toxins that Target the Immune System. In T. Proft, Microbial Toxins: Molecular and Cellular Biology (pp. 179-214). New York: Taylor & Francis.
• Proft T., Sriskandan S., Yang L., Fraser J. D. Superantigens and streptococcal toxic shock syndrome. Emerging Infectious Diseases. 2003b;9(10):1211–1218. [PMC free article: PMC3033064] [PubMed: 14609454]
• Proft T., Webb P. D., Handley V., Fraser J. D. Two Novel Superantigens Found in Both Group A and Group C Streptococcus. Infection and Immunity. 2003a;71(3):1361–1369. [PMC free article: PMC148831] [PubMed: 12595453]
• Radcliff F. J., Loh J. M., Ha B., Schuhbauer D., McCluskey J., Fraser J. D. Antigen targeting to major histocompatibility complex class II with streptococcal mitogenic exotoxin Z-2 M1, a superantigen-based vaccine carrier. Clinical and Vaccine Immunology. 2012;19(4):574–586. [PMC free article: PMC3318283] [PubMed: 22301693]
• Rajagopalan G., Polich G., Sen M. M., Singh M., Epstein B. E., Lytle A. K., et al. Evaluating the role of HLA-DQ polymorphisms on immune response to bacterial superantigens using transgenic mice. Tissue Antigens. 2008;71(2):135–145. [PubMed: 18086265]
• Ramachandran G., Tulapurkar M. E., Harris K. M., Arad G., Shirvan A., Shemesh R., et al. A peptide antagonist of CD28 signaling attenuates toxic shock and necrotizing soft-tissue infection induced by Streptococcus pyogenes. The Journal of Infectious Diseases. 2013;207(12):1869–1877. [PMC free article: PMC3654752] [PubMed: 23493729]
• Raychaudhuri S. K., Maverakis E., Raychaudhuri S. P. Diagnosis and classification of psoriasis. Autoimmunity Reviews. 2014;13(4-5):490–495. [PubMed: 24434359]
• Reichardt W., Müller-Alouf H., Alouf J. E., Köhler W. Erythrogenic toxins A, B, and C: occurrence of the genes and exotoxin formation from clinical Streptococcus pyogenes strains associated with streptococcal toxic shock-like syndrome. FEMS Microbiology Letters. 1992;100(1-3):313–322. [PubMed: 1478466]
• Riley J. L., June C. H. The CD28 family: a T-cell rheostat for therapeutic control of T-cell activation. Blood. 2005;105(1):13–21. [PubMed: 15353480]
• Rink L., Kirchner H. Mycoplasma arthritidis-derived superantigen. Chemical Immunology and Allergy. 1992;55:137–145. [PubMed: 1418615]
• Rödström K. E., Elbing K., Lindkvist-Petersson K. Structure of the Superantigen Staphylococcal Enterotoxin B in Complex with TCR and Peptide-MHC Demonstrates Absence of TCR-Peptide Contacts. The Journal of Immunology. 2014;193(4):1998–2004. [PubMed: 25015819]
• Roggiani M., Stoehr J. A., Olmsted S. B., Matsuka Y. V., Pillai S., Ohlendorf D. H., et al. Toxoids of streptococcal pyrogenic exotoxin A are protective in rabbit models of streptococcal toxic shock syndrome. Infection and Immunity. 2000;68(9):5011–5017. [PMC free article: PMC101724] [PubMed: 10948118]
• Roussel A., Anderson B. F., Baker H. M., Fraser J. D., Baker E. N. Crystal structure of the streptococcal superantigen SPE-C: dimerization and zinc binding suggest a novel mode of interaction with MHC class II molecules. Nature Structural Biology. 1997;4:635–643. [PubMed: 9253413]
• Saarinen S., Kato H., Uchiyama T., Miyoshi-Akiyama T., Papageorgiou A. C. Crystal structure of Streptococcus dysgalactiae-derived mitogen reveals a zinc-binding site and alterations in TcR binding. Journal of Molecular Biology. 2007;373(5):1089–1097. [PubMed: 17900619]
• Sachse S., Seidel P., Gerlach D., Günther E., Rödel J., Straube E., et al. Superantigen-like gene(s) in human pathogenic Streptococcus dysgalactiae, subsp equisimilis: genomic localisation of the gene encoding streptococcal pyrogenic exotoxin G (speG(dys)). FEMS Immunology and Medical Microbiology. 2002;34(2):159–167. [PubMed: 12381468]
• Schlievert P. M., Gray E. D. Group A streptococcal pyrogenic exotoxin (scarlet fever toxin) type A and blastogen A are the same protein. Infection and Immunity. 1989;57(6):1865–1867. [PMC free article: PMC313370] [PubMed: 2498210]
• Schlievert P. M., Assimacopoulos A. P., Cleary P. Severe invasive group Astreptococcal disease: clinical description and mechanisms of pathogenesis. Journal of Laboratory and Clinical Medicine. 1996;127(1):13–22. [PubMed: 8592092]
• Schrage B., Duan G., Yang L. P., Fraser J. D., Proft T. Different preparations of intravenous immunoglobulin vary in their efficacy to neutralize streptococcal superantigens: implications for treatment of streptococcal toxic shock syndrome. Clinical Infectious Diseases. 2006;43(6):743–746. [PubMed: 16912949]
• Sharma P., Wang N., Kranz D. M. Soluble T Cell Receptor Vβ Domains Engineered for High-Affinity Binding to Staphylococcal or Streptococcal Superantigens. Toxins (Basel). 2014;6(2):556–574. [PMC free article: PMC3942751] [PubMed: 24476714]
• Slaney C. Y., Toker A., Fraser J. D., Harper J. L., Bäckström B. L. A modified superantigen rescues Ly6G- CD11b+ blood monocyte suppressor function and suppresses antigen-specific inflammation in EAE. Autoimmunity. 2013;46(4):269–278. [PubMed: 23374140]
• Smoot J. C., Barbian K. D., Van Gompel J. J., Smoot L. M., Chaussee M. S., Sylva G. L., et al. Genome sequence and comparative microarray analysis of serotype M18 group A Streptococcus strains associated with acute rheumatic fever outbreaks. Proceedings of the National Academy of Sciences of the United States of America. 2002b;99(7):4668–4673. [PMC free article: PMC123705] [PubMed: 11917108]
• Smoot L. M., McCormick J. K., Smoot J. C., Hoe N. P., Strickland I., Cole R. L., et al. Characterization of Two Novel pyrogenic toxin Superantigens Made by an Acute Rheumatic Fever Clone of Streptococcus pyogenes Associated with Multiple Disease Outbreaks. Infection and Immunity. 2002a;70(12):7095–7104. [PMC free article: PMC133074] [PubMed: 12438391]
• Sriskandan S., Moyes D., Cohen J. Detection of circulating bacterial superantigen and lymphotoxin-a in patients with streptococcal toxic shock syndrome. Lancet. 1996a;348(9037):1315–1316. [PubMed: 8909404]
• Sriskandan S., Moyes D., Buttery L. K., Krausz T., Evans T. J., Polak J., et al. Streptococcal pyrogenic exotoxin A release, distribution, and role in a murine model of fasciitis and multiorgan failure due to Streptococcus pyogenes. The Journal of Infectious Diseases. 1996b;173(6):1399–1407. [PubMed: 8648212]
• Sriskandan S., Unnikrishnan M., Krausz T., Cohen J. Mitogenic factor (MF) is the major DNase of serotype M89 Streptococcus pyogenes. Microbiology. 2000;146(Pt 11):2785–2792. [PubMed: 11065357]
• Sriskandan S., Unnikrishnan M., Krausz T., Dewchand H., Van Noorden S., Cohen J., et al. Enhanced susceptibility to superantigen-associated streptococcal sepsis in human leukocyte antigen-DQ transgenic mice. The Journal of Infectious Diseases. 2001;184(2):166–173. [PubMed: 11424013]
• Stevens D. L., Bryant A. E., Hackett S. P., Chang A., Peer G., Kosanke S., et al. Group A streptococcal bacteremia: the role of tumor necrosis factor in shock and organ failure. The Journal of Infectious Diseases. 1996;173(3):619–626. [PubMed: 8627025]
• Suenaga T., Suzuki H., Shibuta S., Takeuchi T., Yoshikawa N. Detection of multiple superantigen genes in stools of patients with Kawasaki disease. The Journal of Pediatrics. 2009;155(2):266–270. [PubMed: 19446844]
• Sundberg E. J., Deng L., Mariuzza R. A. TCR recognition of peptide/MHC class II complexes and superantigens. Seminars in Immunology. 2007;19(4):262–271. [PMC free article: PMC2949352] [PubMed: 17560120]
• Sundberg E. J., Li H., Llera A. S., McCormick J. K., Tormo J., Schlievert P. M., et al. Structures of Two Streptococcal Superantigens Bound to TCR b Chains Reveal Diversity in the Architecture of T cell Signalling Complex. Structure. 2002a;10(5):687–699. [PubMed: 12015151]
• Sundberg E. J., Li Y., Mariuzza R. A. So many ways of getting in the way: diversity in the molecular architecture of superantigen-dependent T cell signaling complexes. Current Opinion in Immunology. 2002b;14(1):36–44. [PubMed: 11790531]
• Swietnicki W., Barnie A. M., Dyas B. K., Ulrich R. G. Zinc binding and dimerization of Streptococcus pyogenes pyrogenic exotoxin C are not essential for T-cell stimulation. Journal of Biological Chemistry. 2003;278(11):9885–9895. [PubMed: 12473669]
• Takahashi K., Oharaseki T., Yokouchi Y. Update on etio and immunopathogenesis of Kawasaki disease. Current Opinion in Rheumatology. 2014;26(1):31–36. [PubMed: 24247115]
• Talkington D. F., Schwartz B., Black C. M., Todd J. K., Elliott J., Breiman R. F., et al. Association of phenotypic and genotypic characteristics of invasive Streptococcus pyogenes isolates with clinical components of streptococcal toxic shock syndrome. Infection and Immunity. 1993;61(8):3369–3374. [PMC free article: PMC281012] [PubMed: 8335368]
• Taylor A. L., Llewelyn M. J. Superantigen-induced proliferation of human CD4+CD25- T cells is followed by a switch to a functional regulatory phenotype. The Journal of Immunology. 2010;185(11):6591–6598. [PubMed: 21048104]
• Taylor A. L., Cross E. L., Llewelyn M. J. Induction of contact-dependent CD8(+) regulatory T cells through stimulation with staphylococcal and streptococcal superantigens. Immunology. 2012;135(2):158–167. [PMC free article: PMC3277718] [PubMed: 22043981]
• Thomas D., Perpoint T., Dauwalder O., Lina G., Floccard B., Richard J. C., et al. In vivo and in vitro detection of a superantigenic toxin Vbeta signature in two forms of streptococcal toxic shock syndrome. European Journal of Clinical Microbiology. 2009;28(6):671–676. [PubMed: 19020908]
• Turner C. E., Sommerlad M., McGregor K., Davies F. J., Pichon B., Chong D. L., et al. Superantigenic activity of emm3 Streptococcus pyogenes is abrogated by a conserved, naturally occurring smeZ mutation. PLoS One. 2012;7(10):e46376. [PMC free article: PMC3462185] [PubMed: 23049698]
• Unnikrishnan M., Altmann D., Proft T., Wahid F., Cohen J., Fraser J. D., et al. The bacterial superantigen streptococcal mitogenic exotoxin Z is the major immunoactive agent of Streptococcus pyogenes. The Journal of Immunology. 2002;169(5):2561–2569. [PubMed: 12193726]
• Unnikrishnan M., Cohen J., Sriskandan S. Growth-phase-dependent expression of virulence factors in an M1T1 clinical isolate of Streptococcus pyogenes. Infection and Immunity. 1999;67(10):5495–5499. [PMC free article: PMC96913] [PubMed: 10496938]
• Valdimarsson H., Thorleifsdottir R. H., Sigurdardottir S. L., Gudjonsson J. E., Johnston A. Psoriasis – as an autoimmune disease caused by molecular mimicry. Trends in Immunology. 2009;30(10):494–501. [PubMed: 19781993]
• Virtaneva K., Porcella S. F., Graham M. R., Ireland R. M., Johnson C. A., Ricklefs S. M., et al. Longitudinal analysis of the group A Streptococcus transcriptome in experimental pharyngitis in cynomolgus macaques. Proceedings of the National Academy of Sciences of the United States of America. 2005;102(25):9014–9019. [PMC free article: PMC1150296] [PubMed: 15956184]
• Vlaminckx B. J., Mascini E. M., Schellekens J., Schouls L. M., Paauw A., Fluit A. C., et al. Site-specific manifestations of invasive group a streptococcal disease: type distribution and corresponding patterns of virulence determinants. Journal of Clinical Microbiology. 2003;41(11):4941–4949. [PMC free article: PMC262503] [PubMed: 14605121]
• Vojtek I., Pirzada Z. A., Henriques-Normark B., Mastny M., Janapatla R. P., Charpentier E. Lysogenic transfer of group A Streptococcus superantigen gene among Streptococci. The Journal of Infectious Diseases. 2008;197(2):225–234. [PMC free article: PMC3030952] [PubMed: 18179387]
• Walker M. J., Hollands A., Sanderson-Smith M. L., Cole J. N., Kirk J. K., Henningham A., et al. DNase Sda1 provides selection pressure for a switch to invasive group A streptococcal infection. Nature Medicine. 2007;13(8):981–985. [PubMed: 17632528]
• Wang L., Zhao Y., Li Z., Guo Y., Jones L. L., Kranz D. M., et al. Crystal structure of a complete ternary complex of TCR, superantigen and peptide-MHC. Nature Structural & Molecular Biology. 2007;14:169–171. [PMC free article: PMC3923521] [PubMed: 17220897]
• Wang N., Mattis D. M., Sundberg E. J., Schlievert P. M., Kranz D. M. A single, engineered protein therapeutic agent neutralizes exotoxins from both Staphylococcus aureus and Streptococcus pyogenes. Clinical and Vaccine Immunology. 2010;17(11):1781–1789. [PMC free article: PMC2976085] [PubMed: 20861327]
• Watanabe-Ohnishi R., Low D. E., McGeer A., Stevens D. L., Schlievert P. M., Newton D., et al. Selective depletion of V beta-bearing T cells in patients with severe invasive group A streptococcal infections and streptococcal toxic shock syndrome. Ontario Streptococcal Study Project. The Journal of Infectious Diseases. 1995;171(1):74–84. [PubMed: 7798684]
• Watson D. W. Host-parasite factors in group A streptococcal infections. Pyrogenic and other effects of immunologic distinct exotoxins related to scarlet fever toxins. The Journal of Experimental Medicine. 1960;111:255–284. [PMC free article: PMC2137249] [PubMed: 13783427]
• White J., Herman A., Pullen A. M., Kubo R., Kappler J. W., Marrack P. The V beta-specific superantigen staphylococcal enterotoxin B: stimulation of mature T cells and clonal deletion in neonatal mice. Cell. 1989;56(1):27–35. [PubMed: 2521300]
• Yang L. P., Eriksson B. K., Harrington Z., Curtis N., Lang S., Currie B. J., et al. Variations in the protective immune response against streptococcal superantigens in populations of different ethnicity. Medical Microbiology and Immunology. 2006;195(1):37–43. [PubMed: 15988608]
• Yang L., Thomas M., Woodhouse A., Martin D., Fraser J. D., Proft T. Involvement of streptococcal mitogenic exotoxin Z in streptococcal toxic shock syndrome. Journal of Clinical Microbiology. 2005;43(7):3570–3573. [PMC free article: PMC1169092] [PubMed: 16000510]
• Yoshioka T., Matsutani T., Iwagami S., Toyosaki-Maeda T., Yutsudo T., Tsuruta Y., et al. Polyclonal expansion of TCRBV2- and TCRBV6-bearing T cells in patients with Kawasaki disease. Immunology. 1999;96(3):465–472. [PMC free article: PMC2326769] [PubMed: 10233729]
• Yu C. E., Ferretti J. J. Molecular epidemiologic analysis of the type A streptococcal exotoxin (erythrogenic toxin) gene (speA) in clinical Streptococcus pyogenes strains. Infection and Immunity. 1989;57(12):3715–3719. [PMC free article: PMC259895] [PubMed: 2553612]