The streptococcal superantigen SMEZ exhibits wide allelic variation, mosaic structure, and significant antigenic variation

Closed-Tube Multiplex Real-Time PCR for the Detection of Group A Streptococcal Superantigens

Conventional PCR techniques are laborious and usually not suited for fast screening of large sample numbers in a clinical or research setting. Using this closed-tube multiplex real-time PCR, the presence of all 11 Streptococcus pyogenes superantigen (SAg) genes can be rapidly and accurately characterized. Identifying whether a strain contains a SAg can be done within 4 h compared to conventional methods which would take 11 times as long. This method provides an excellent diagnostic tool as well as a screening tool to help researchers clarify the role of SAgs in S. pyogenes infections.

Toxins and Superantigens of Group A Streptococci

Streptococcus pyogenes (i.e., the group A Streptococcus) is a human-restricted and versatile bacterial pathogen that produces an impressive arsenal of both surface-expressed and secreted virulence factors. Although surface-expressed virulence factors are clearly vital for colonization, establishing infection, and the development of disease, the secreted virulence factors are likely the major mediators of tissue damage and toxicity seen during active infection. The collective exotoxin arsenal of S. pyogenes is rivaled by few bacterial pathogens and includes extracellular enzymes, membrane active proteins, and a variety of toxins that specifically target both the innate and adaptive arms of the immune system, including the superantigens; however, despite their role in S. pyogenes disease, each of these virulence factors has likely evolved with humans in the context of asymptomatic colonization and transmission. In this article, we focus on the biology of the true secreted exotoxins of the group A Streptococcus, as well as their roles in the pathogenesis of human disease.

Streptococcal pharyngitis and rheumatic heart disease: the superantigen hypothesis revisited

Streptococcus pyogenes is a human-specific and globally prominent bacterial pathogen that despite causing numerous human infections, this bacterium is normally found in an asymptomatic carrier state. This review provides an overview of both bacterial and human factors that likely play an important role in nasopharyngeal colonization and pharyngitis, as well as the development of acute rheumatic fever and rheumatic heart disease. Here we highlight a recently described role for bacterial superantigens in promoting acute nasopharyngeal infection, and discuss how these immune system activating toxins could be crucial to initiate the autoimmune process in rheumatic heart disease.

Antigenic Variation and Immune Escape in the MTBC

Microbes that infect other organisms encounter host immune responses, and must overcome or evade innate and adaptive immune responses to successfully establish infection. Highly successful microbial pathogens, including M. tuberculosis, are able to evade adaptive immune responses (mediated by antibodies and/or T lymphocytes) and thereby establish long-term chronic infection. One mechanism that diverse pathogens use to evade adaptive immunity is antigenic variation, in which structural variants emerge that alter recognition by established immune responses and allow those pathogens to persist and/or to infect previously-immune hosts. Despite the wide use of antigenic variation by diverse pathogens, this mechanism appears to be infrequent in M. tuberculosis, as indicated by findings that known and predicted human T cell epitopes in this organism are highly conserved, although there are exceptions. These findings have implications for diagnostic tests that are based on measuring host immune responses, and for vaccine design and development.

Genome Analysis of Streptococcus pyogenes Associated with Pharyngitis and Skin Infections

Streptococcus pyogenes is a very important human pathogen, commonly associated with skin or throat infections but can also cause life-threatening situations including sepsis, streptococcal toxic shock syndrome, and necrotizing fasciitis. Various studies involving typing and molecular characterization of S. pyogenes have been published to date; however next-generation sequencing (NGS) studies provide a comprehensive collection of an organism’s genetic variation. In this study, the genomes of nine S. pyogenes isolates associated with pharyngitis and skin infection were sequenced and studied for the presence of virulence genes, resistance elements, prophages, genomic recombination, and other genomic features. Additionally, a comparative phylogenetic analysis of the isolates with global clones highlighted their possible evolutionary lineage and their site of infection. The genomes were found to also house a multitude of features including gene regulation systems, virulence factors and antimicrobial resistance mechanisms.

Stabilized plasmid-based system for bioluminescent labeling of multiple streptococcal species

OBJECTIVE

To determine if multiple streptococcal species can be easily labeled for biophotonic imaging using a toxin-antitoxin stabilized reporter plasmid containing the native firefly luciferase gene, originally developed for use in Group A Streptococcus.

RESULTS

A number of streptococcal species including Group B Streptococcus, Group C Streptococcus, Group G Streptococcus, S. iniae, S. vestibularis, and S. salivarius were successfully transformed with the reporter plasmid. In absence of antibiotic selection, the plasmid had variable stability amongst the six strains. The expression of firefly luciferase was highest in Group B Streptococcus and S. iniae, as observed by the brightest signal and lowest detection limits in vitro.

CONCLUSION

Multiple streptococcal species can be easily transformed with our toxin-antitoxin stabilized bioluminescent reporter plasmid. However, this plasmid shows variable stability and signal in different species, restricting its use for certain applications.

Prevalence and disease associations of superantigens szeF, szeN and szeP in the S. zooepidemicus population and possible functional redundancy of szeF

Streptococcus equi subspecies zooepidemicus (S. zooepidemicus) causes a variety of infections in a broad range of species. This study broadens prevalence data for three recently identified novel superantigens (szeF, szeN, and szeP) to define links between their presence and disease type. Screening of 437 strains across 190 sequence types (STs) revealed that 50% of strains contained superantigen genes. Results confirmed that the presence of S. zooepidemicus superantigen genes is significantly associated with non-Strangles lymph node abscessation in the horse (p-value = 0.003) and their absence is associated with uterine infection/abortion (p-value = 0.006). This study also investigated the lack of mitogenicity observed in szeF only. Results show that szeF is polymorphic, with 23 different alleles, and mutations altering the protein sequence. Gene expression differences are not responsible for lack of mitogenic activity in these strains. Taken together, these findings suggest that superantigens are important for S. zooepidemicus pathogenesis but SzeF probably has little involvement.

Emergence of Streptococcus pyogenes emm102 causing toxic shock syndrome in Southern Taiwan during 2005-2012

BACKGROUND

Streptococcal toxic shock syndrome (STSS) is an uncommon but life-threatening disease caused by Streptococcus pyogenes.

METHODS

To understand the clinical and molecular characteristics of STSS, we analyzed clinical data and explored the emm types, superantigen genes, and pulsed-field gel electrophoresis of causative S. pyogenes isolates obtained between 2005 and 2012.

RESULTS

In total, 53 patients with STSS were included in this study. The median age of the patients was 57 years (range: 9-83 years), and 81.1% were male. The most prevalent underlying disease was diabetes mellitus (45.3%). Skin and soft-tissue infection accounted for 86.8% of STSS. The overall mortality rate was 32.1%. Underlying diseases had no statistical impact on mortality. A total of 19 different emm types were identified. The most prevalent emm type was emm102 (18.9%), followed by emm11 (17%), emm1 (11.3%), emm87 (9.4%), and emm89 (7.5%). There was no statistically significant association between emm type and a fatal outcome. Among the superantigen genes, speB was the most frequently detected one (92.5%), followed by smeZ (90.6%), speG (81.1%), speC (39.6%), and speF (39.6%). The majority of emm102 strains were found to have speB, speC, speG, and smeZ. The presence of speG was negatively associated with a fatal outcome (P = 0.045).

CONCLUSIONS

Our surveillance revealed the emergence of uncommon emm types, particularly emm102, causing STSS in southern Taiwan. Characterization of clinical, epidemiological, and molecular characteristics of STSS will improve our understanding of this life-threatening disease.

Streptococcal superantigens: categorization and clinical associations

Superantigens are key virulence factors in the immunopathogenesis of invasive disease caused by group A streptococcus. These protein exotoxins have also been associated with severe group C and group G streptococcal infections. A number of novel streptococcal superantigens have recently been described with some resulting confusion in their classification. In addition to clarifying the nomenclature of streptococcal superantigens and proposing guidelines for their categorization, this review summarizes the evidence supporting their involvement in various clinical diseases including acute rheumatic fever.

The contribution of group A streptococcal virulence determinants to the pathogenesis of sepsis

Streptococcus pyogenes (group A streptococcus, GAS) is responsible for a wide range of pathologies ranging from mild pharyngitis and impetigo to severe invasive soft tissue infections. Despite the continuing susceptibility of the bacterium to β-lactam antibiotics there has been an unexplained resurgence in the prevalence of invasive GAS infection over the past 30 years. Of particular importance was the emergence of a GAS-associated sepsis syndrome that is analogous to the systemic toxicosis associated with TSST-1 producing strains of Staphylococcus aureus. Despite being recognized for over 20 years, the etiology of GAS associated sepsis and the streptococcal toxic shock syndrome remains poorly understood. Here we review the virulence factors that contribute to the etiology of GAS associated sepsis with a particular focus on coagulation system interactions and the role of the superantigens in the development of streptococcal toxic shock syndrome.

Toxin-antitoxin-stabilized reporter plasmids for biophotonic imaging of Group A streptococcus

Bioluminescence is a rapid and cost-efficient optical imaging technology that allows the detection of bacteria in real-time during disease development. Here, we report a novel strategy to generate a wide range of bioluminescent group A streptococcus (GAS) strains by using a toxin-antitoxin-stabilized plasmid. The bacterial luciferin-luciferase operon (lux) or the firefly luciferase gene (ffluc) was introduced into GAS via a stabilized plasmid. The FFluc reporter gave significantly stronger bioluminescent signals than the Lux reporter, and was generally more stable. Plasmid-based luciferase reporters could easily be introduced into a variety of GAS strains and the signals correlated linearly with viable cell counts. Co-expression of the streptococcal ω-ε-ζ toxin-antitoxin operon provided segregational stability in the absence of antibiotics for at least 17 passages in vitro and up to 7 days in a mouse infection model. In addition, genome-integrated reporter constructs were also generated by site-specific recombination, but were found to be technically more challenging. The quick and efficient generation of various M-type GAS strains expressing plasmid-based luciferase reporters with comparable and quantifiable bioluminescence signals allows for comparative analysis of different GAS strains in vitro and in vivo.

Current insights in invasive group A streptococcal infections in pediatrics

A rising incidence of invasive group A Streptococcus infections (IGASI) has been noted in children in the past three decades. The relative frequency of the infection types showed marked differences to IGASI in adults, and severity of the disease resulted in a mortality rate usually comprising between 3.6% and 8.3%. The emm1-type group A Streptococcus (GAS) subclone displaying a particular pattern of virulence factors was widely disseminated and prevalent in children with IGASI while the emm3-type GAS subclone appeared as a recent emerging genotype. However, the implication of these hypervirulent clones in the increase of IGASI in children is still controversial. Recent advances in our knowledge on pathogenesis of IGASI underlined that deregulation of virulence factor production, individual susceptibility, as well as exuberant cytokine response are important factors that may account for the severity of the disease in children. Future changes in IGASI epidemiology are awaited from current prospects for a safe and effective vaccine against GAS. IGASI are complex infections associating septic, toxic, and immunological disorders. Treatment has to be effective on both the etiologic agent and its toxins, due to the severity of the disease associated to the spread of highly virulent bacterial clones. More generally, emergence of virulent clones responsible for septic and toxic disease is a matter of concern in pediatric infectiology in the absence of vaccination strategy.

Superantigenic activity of emm3 Streptococcus pyogenes is abrogated by a conserved, naturally occurring smeZ mutation

Streptococcus pyogenes M/emm3 strains have been epidemiologically linked with enhanced infection severity and risk of streptococcal toxic shock syndrome (STSS), a syndrome triggered by superantigenic stimulation of T cells. Comparison of S. pyogenes strains causing STSS demonstrated that emm3 strains were surprisingly less mitogenic than other emm-types (emm1, emm12, emm18, emm28, emm87, emm89) both in vitro and in vivo, indicating poor superantigenic activity. We identified a 13 bp deletion in the superantigen smeZ gene of all emm3 strains tested. The deletion led to a premature stop codon in smeZ, and was not present in other major emm-types tested. Expression of a functional non-M3-smeZ gene successfully enhanced mitogenic activity in emm3 S. pyogenes and also restored mitogenic activity to emm1 and emm89 S. pyogenes strains where the smeZ gene had been disrupted. In contrast, the M3-smeZ gene with the 13 bp deletion could not enhance or restore mitogenicity in any of these S. pyogenes strains, confirming that M3-smeZ is non-functional regardless of strain background. The mutation in M3-smeZ reduced the potential for M3 S. pyogenes to induce cytokines in human tonsil, but not during invasive infection of superantigen-sensitive mice. Notwithstanding epidemiological associations with STSS and disease severity, emm3 strains have inherently poor superantigenicity that is explained by a conserved mutation in smeZ.

Antigen targeting to major histocompatibility complex class II with streptococcal mitogenic exotoxin Z-2 M1, a superantigen-based vaccine carrier

Streptococcal mitogenic exotoxin Z-2 (SMEZ-2) is a streptococcal superantigen that primarily stimulates human T cells bearing Vβ8 and mouse T cells bearing Vβ11. Mutagenesis of T cell receptor (TCR)-binding residues (W75L, K182Q, D42C) produced a mutant called M1 that was >10(5)-fold less active toward human peripheral blood lymphocytes and splenocytes from transgenic mice that express human CD4 and either human HLA-DR3-DQ2 or HLA-DR4-DQ8. Similarly, cytokine production in response to M1 in lymphocyte culture was rendered undetectable, and no change in the frequency of Vβ11-bearing T cells in mice receiving M1 was observed. M1 toxoid was tested as a potential vaccine conjugate. Vaccination with 1 to 10 μg M1 conjugated to ovalbumin (M1-ovalbumin) resulted in more rapid and quantitatively higher levels of anti-ovalbumin IgG, with endpoint titers being 1,000- to 10,000-fold greater than those in animals immunized with unconjugated ovalbumin. Substantially higher levels of anti-ovalbumin IgG were observed in mice transgenic for human major histocompatibility complex (MHC) class II. Substitution of M1 with an MHC class II binding mutant (DM) eliminated enhanced immunity, suggesting that M1 enhanced the delivery of antigen via MHC class II-positive antigen-presenting cells that predominate within lymphoid tissue. Immunization of animals with a conjugate consisting of M1 and ovalbumin peptide from positions 323 to 339 generated levels of anti-peptide IgG 100-fold higher than those in animals immunized with peptide alone. Coupling of a TCR-defective superantigen toxoid presents a new strategy for conjugate vaccines with the additional benefit of targeted delivery to MHC class II-bearing cells.

A novel core genome-encoded superantigen contributes to lethality of community-associated MRSA necrotizing pneumonia

Bacterial superantigens (SAg) stimulate T-cell hyper-activation resulting in immune modulation and severe systemic illnesses such as Staphylococcus aureus toxic shock syndrome. However, all known S. aureus SAgs are encoded by mobile genetic elements and are made by only a proportion of strains. Here, we report the discovery of a novel SAg staphylococcal enterotoxin-like toxin X (SElX) encoded in the core genome of 95% of phylogenetically diverse S. aureus strains from human and animal infections, including the epidemic community-associated methicillin-resistant S. aureus (CA-MRSA) USA300 clone. SElX has a unique predicted structure characterized by a truncated SAg B-domain, but exhibits the characteristic biological activities of a SAg including Vβ-specific T-cell mitogenicity, pyrogenicity and endotoxin enhancement. In addition, SElX is expressed by clinical isolates in vitro, and during human, bovine, and ovine infections, consistent with a broad role in S. aureus infections of multiple host species. Phylogenetic analysis suggests that the selx gene was acquired horizontally by a progenitor of the S. aureus species, followed by allelic diversification by point mutation and assortative recombination resulting in at least 17 different alleles among the major pathogenic clones. Of note, SElX variants made by human- or ruminant-specific S. aureus clones demonstrated overlapping but distinct Vβ activation profiles for human and bovine lymphocytes, indicating functional diversification of SElX in different host species. Importantly, SElX made by CA-MRSA USA300 contributed to lethality in a rabbit model of necrotizing pneumonia revealing a novel virulence determinant of CA-MRSA disease pathogenesis. Taken together, we report the discovery and characterization of a unique core genome-encoded superantigen, providing new insights into the evolution of pathogenic S. aureus and the molecular basis for severe infections caused by the CA-MRSA USA300 epidemic clone.

Staphylococcus aureus is a global pathogen, responsible for an array of different illnesses in humans and animals. In particular, community-associated methicillin-resistant S. aureus (CA-MRSA) strains of the pandemic USA300 clone have the capacity to cause lethal human necrotizing pneumonia, but the molecular basis for the enhanced virulence remains unclear. Bacterial superantigens (SAg) stimulate T-cell hyper-activation resulting in severe systemic illnesses such as toxic shock syndrome (TSS). However, all S. aureus SAgs identified to date are encoded by mobile genetic elements found only in a proportion of clinical isolates. Here, we report the discovery of a unique core genome-encoded SAg (SElX) which was acquired by an ancestor of the S. aureus species and which has undergone genetic and functional diversification in pathogenic clones infecting humans and animals. Importantly, we report that SElX made by pandemic USA300 contributes to lethality in a rabbit model of human necrotizing pneumonia revealing a novel virulence determinant of severe CA-MRSA infection.

Selective pressure for allelic diversity in SeM of Streptococcus equi does not affect immunoreactive proteins SzPSe or Se18.9

Streptococcus equi, a clone or biovar of an ancestral Streptococcus zooepidemicus of Lancefield group C causes equine strangles, a highly contagious tonsillitis and lymphadenitis of the head and neck. At least 74 alleles based on N-terminal amino acid sequence of the anti-phagocytic SeM have been observed among isolates of S. equi from N. America, Europe and Japan. A d(N)/d(S) ratio of 5.93 for the 5' region of sem is indicative of positive selective pressure. The aim of this study was to determine whether variations in SeM were accompanied by variations in the surface exposed SzPSe and secreted Se18.9, both of which bind to equine tonsillar epithelium and, along with SeM, elicit strong nasopharyngeal IgA responses during convalescence. Sequences of genes for these proteins from 25 S. equi expressing 19 different SeM alleles isolated over 40 years in different countries were compared. No variation was observed in szpse, except for an Australian isolate with a deletion of a single repeat in the 3' end of the gene. Interestingly, only two SNP loci were detected in se18.9 compared to 93 and 55 in sem and szpse, respectively. The high frequency of nucleotide substitutions in szpse may be related to its mosaic structure since this gene in S. zooepidemicus exists in a variety of combinations of sequence segments and has a central hypervariable region that includes exogenous DNA sequence based on an atypical G-C percentage. In summary, the results of this study document very different responses of streptococcal genes for 3 immunoreactive proteins to selection pressure of the nasopharyngeal mucosal immune response.

Purification, crystallization and preliminary crystallographic analysis of the minor pilin FctB from Streptococcus pyogenes

The minor pilin FctB is an integral part of the pilus assembly expressed by Streptococcus pyogenes. Since it is located at the cell wall, it can be hypothesized that it functions as a cell-wall anchor for the streptococcal pilus. In order to elucidate its structure, the genes for FctB from the S. pyogenes strains 90/306S and SF370 were cloned for overexpression in Escherichia coli. FctB from strain 90/306S was crystallized by the sitting-drop vapour-diffusion method using sodium citrate as a precipitant. The hexagonal FctB crystals belonged to space group P6(1) or P6(5), with unit-cell parameters a = b = 95.15, c = 100.25 A, and diffracted to 2.9 A resolution.

Superantigen gene profile diversity among clinical group A streptococcal isolates

This study examines the diversity of superantigen gene profiles between and within emm-genotypes of 92 clinical group A streptococcal isolates (30 STSS, 24 sepsis, 25 erysipelas, and 12 tonsillitis) collected in Sweden between 1986 and 2001. The emm-genotype and the distribution of smeZ, speG, speJ, speA, speC, speH, speI, speK/L, speL/M, speM, and ssa genes, and the smeZ allelic variant were determined using PCR and DNA sequencing. Forty-five emm1 isolates revealed 10 superantigen gene profiles. One profile dominated and was identified in 22 isolates collected over 14 years. The results indicate that a selective advantage maintained this genotype in circulation. The superantigen content among the emm1 isolates ranged from three to seven, with smeZ-1, speG, and speA present in all but one profile. The 47 isolates of 27 other emm-genotypes exhibited 29 superantigen gene profiles. Thus, the distribution of superantigen genes was highly variable within isolates regardless of emm-genotype. Two novel emm1 subtypes and 14 novel smeZ allelic variants were identified. The 22 smeZ alleles were generally linked to the emm-genotype. The results of the investigation show that superantigen gene profiling is useful for tracking spread of clones in the community.

Cloning, expression, and characterization of the superantigen streptococcal pyrogenic exotoxin G from Streptococcus dysgalactiae

We identified seven novel variants of streptococcal pyrogenic exotoxin G (SPEGG), a superantigen, in Streptococcus dysgalactiae subsp. dysgalactiae or equisimilis isolates from clinical cases of infection in humans and animals. Phylogenetic analysis of the SPEGG variants indicated two clades in the dendrogram: one composed of variants derived from the bacteria isolated from the humans and the other composed of variants from the bacteria isolated from the animals. Bovine peripheral blood mononuclear cells (PBMCs) were stimulated effectively by recombinant SPEGGs (rSPEGGs) expressed in Escherichia coli, while human PBMCs were not stimulated well by any of the rSPEGGs tested. SPEGGs selectively stimulated bovine T cells bearing Vbeta1,10 and Vbeta4. Bovine serum showed reactivity to the rSPEGG proteins. These results indicated that SPEGGs have properties as superantigens, and it is likely that SPEGGs play a pathogenic role in animals.

Streptococcus pyogenes: Insight into the function of the streptococcal superantigens

The group A streptococcus produces a number of highly potent exoproteins that act as superantigens. The cascade of pro-inflammatory events that follow invasive streptococcal infection is greatly enhanced by production of such toxins, leading to profound hypotension and multi-organ failure in some cases. Superantigens such as streptococcal mitogenic exotoxin Z (SMEZ) interact with host MHC class II and the T cell receptor, leading to activation events in both cells. In vitro, these interactions lead to expansion and cytokine production by specified T cell subsets. Studies using humanized HLA class II transgenic mice and isogenic streptococcal strains have characterised the in vivo responses to superantigens produced in the context of live infection. Notwithstanding the obvious deleterious role of superantigens in toxic shock, the evolutionary advantage conferred by these toxins remains a subject of speculation.

Different Preparations of Intravenous Immunoglobulin Vary in Their Efficacy to Neutralize Streptococcal Superantigens: Implications for Treatment of Streptococcal Toxic Shock Syndrome

Eight different batches of intravenous immunoglobulin from 3 different manufacturers were tested for neutralizing activities against all currently known streptococcal superantigens. Statistically significant variation among different intravenous immunoglobulin preparations (P<.0001) and between individual streptococcal superantigens (P<.0001) was observed. These results might be helpful for optimizing the type and dose of intravenous immunoglobulin used in adjunctive therapy for severe invasive streptococcal disease.

Superantigen gene profile, emm type and antibiotic resistance genes among group A streptococcal isolates from Barcelona, Spain

Group A streptococcus (GAS) has been described as an emerging cause of severe invasive infections. A retrospective hospital-based study was conducted, including GAS isolates causing invasive or non-invasive infections from January 1999 to June 2003 in Barcelona. Demographic and clinical information on the invasive cases was obtained from medical files. GAS isolates collected from 27 patients with invasive infections and 99 patients with non-invasive infections were characterized by emm type and subtype, superantigen (SAg) gene profile (speA–C, speF–J, speL, speM, ssa and smeZ), allelic variants of speA and smeZ genes, antibiotic susceptibility and genetic resistance determinants. The most prevalent emm type was emm1 (17.5 %), followed by emm3 (8.7 %), emm4 (8.7 %), emm12 (7.1 %) and emm28 (7.1 %). The smeZ allele and SAg gene profiles were closely associated with the emm type. The speA2, speA3 and speA4 alleles were found in emm1, emm3 and emm6 isolates, respectively. Overall, 27.8, 25.4 and 11.9 % of isolates were resistant to erythromycin, tetracycline or both agents, respectively. Reduced susceptibility to ciprofloxacin and levofloxacin (MIC 2–4 μg ml−1) was found in 3.2 % of isolates. mef(A)-positive emm types 4, 12 and 75, and erm(B)-positive emm types 11 and 25 were responsible for up to 80 % of the erythromycin-resistant isolates. No significant differences in emm-type distribution, SAg gene profile or resistance rates were found between invasive and non-invasive isolates. The SAg and antibiotic resistance genes appeared to be associated with the emm type and were independent of the disease type.

Severe bacterial infections of the skin: uncommon presentations

Although most bacterial infections of the skin prove to be minor in nature, a few such dermatologic entities are significant, to the point of even being fatal. Their course can be extremely rapid and can lead to dreadful complications. The mortality rate is usually up to 30% to 50% and depends upon the type of infection, underlying disease, and immune status. Patients suffering them usually need to be hospitalized, sometimes in intensive care or burn units. They should be treated systemically with appropriate antimicrobial therapy plus aggressive supportive care. The two life-threatening skin infections which are most commonly experienced are toxin-mediated staphylococcal and streptococcal disorders; one could overlap the other. Several other related entities will also be discussed.

Involvement of streptococcal mitogenic exotoxin Z in streptococcal toxic shock syndrome

We report a nonfatal case of streptococcal toxic shock syndrome (STSS) caused by a Streptococcus pyogenes emm118 strain encoding a novel variant of streptococcal mitogenic exotoxin Z (SMEZ-34). This variant was responsible for the major mitogenic activity in the cell culture supernatant. Patient sera showed seroconversion toward SMEZ, implying a role for this toxin in STSS.

Variations in emm type among group A streptococcal isolates causing invasive or noninvasive infections in a nationwide study

Since the late 1980s several studies have described the increased incidence and severity of invasive group A streptococcal (GAS) infections. However, most studies on GAS pathogenesis have focused on information obtained during outbreaks. We analyzed isolate distribution and host susceptibility as part of a nationwide prospective surveillance study performed between January 2001 and August 2002. GAS isolates collected from 201 patients with invasive infections, 335 patients with noninvasive infections, and 17 asymptomatic carriers were characterized with respect to their emm types and superantigen genotypes. The superantigen-neutralizing capacity and levels of antibodies against streptolysin O and DNAse B were determined for isolates from the sera from 36 invasive cases and 91 noninvasive cases. emm type 1 (emm-1) isolates were significantly more common among invasive cases, whereas emm-4, emm-6, and emm-12 dominated among the noninvasive cases. The distributions of the phage-associated superantigen genes (speA, speC, speH, speI, ssa) differed among invasive and noninvasive isolates, mainly due to their linkage to certain emm types. No significant differences in serum superantigen-neutralizing capacities were observed. The levels of anti-streptolysin O and anti-DNAse B antibodies were highest in the sera from invasive cases. Our study emphasizes the importance of obtaining data during years with stable incidences, which will enable evaluation of future outbreak data.

egc-Encoded superantigens from Staphylococcus aureus are neutralized by human sera much less efficiently than are classical staphylococcal enterotoxins or toxic shock syndrome toxin

PCR was employed to determine the presence of all known superantigen genes (sea, seq, and tst) and of the exotoxin-like gene cluster (set) in 40 Staphylococcus aureus isolates from blood cultures and throat swabs; 28 isolates harbored superantigen genes, five on average, and this strictly correlated with their ability to stimulate T-cell proliferation. In contrast, the set gene cluster was detected in every S. aureus strain, suggesting a nonredundant function for these genes which is different from T-cell activation. No more than 10% of normal human serum samples inhibited the T-cell stimulation elicited by egc-encoded enterotoxins (staphylococcal enterotoxins G, I, M, N, and O), whereas between 32 and 86% neutralized the classical superantigens. Similarly, intravenous human immunoglobulin G preparations inhibited egc-encoded superantigens with 10- to 100-fold-reduced potency compared with the classical enterotoxins. Thus, there are surprisingly large gaps in the capacity of human serum samples to neutralize S. aureus superantigens.

Interplay Between Superantigens and Immunoreceptors

Superantigens (SAGs) cause a massive T-cell proliferation by simultaneously binding to major histocompatibility complex (MHC) class II on antigen-presenting cells and T-cell receptors (TCRs) on T cells. These T-cell mitogens can cause disease in host, such as food poisoning or toxic shock. The best characterized groups of SAGs are the bacterial SAGs secreted by Staphylococcus aureus and Streptococcus pyogenes. Despite a common overall three-dimensional fold of these SAGs, they have been shown to bind to MHC class II in different ways. Recently, it has also been shown that SAGs have individual preferences in their binding to the TCRs. They can interact with various regions of the variable beta-chain of TCRs and at least one SAG seems to bind to the alpha-chain of TCRs. In this review, different subclasses of SAGs are classified based upon their binding mode to MHC class II, and models of trimolecular complexes of MHC-SAG-TCR molecules are described in order to reveal and understand the complexity of SAG-mediated T-cell activation.

Site-specific manifestations of invasive group a streptococcal disease: type distribution and corresponding patterns of virulence determinants

As part of a national surveillance program on invasive group A streptococci (GAS), isolates that caused specific manifestations of invasive GAS disease in The Netherlands were collected between 1992 and 1996. These site-specific GAS infections involved meningitis, arthritis, necrotizing fasciitis, and puerperal sepsis. An evaluation was performed to determine whether GAS virulence factors correlate with these different disease manifestations. PCRs were developed to detect 9 genes encoding exotoxins and 12 genes encoding fibronectin binding proteins. The genetic backgrounds of all isolates were determined by M genotyping and pulsed-field gel electrophoresis (PFGE) analysis. The predominant M types included M1, M2, M3, M4, M6, M9, M12, and M28. Most M types were associated with all manifestations of GAS disease. However, M2 was found exclusively in patients with puerperal sepsis, M6 predominated in patients with meningitis, and M12 predominated in patients with GAS arthritis. While characteristic gene profiles were detected in most M types, the resolution of detection of different gene profiles within M genotypes was enhanced by PFGE analysis, which clearly demonstrated the existence of some clonal lineages among invasive GAS isolates in The Netherlands. M1 isolates comprised a single clone carrying highly mitogenic toxin genes (speA, smeZ) and were associated with toxic shock-like syndrome. Toxin profiles were highly conserved among the most virulent strains, such as M1 and M3.

Association of human leukocyte antigen with outcomes of infectious diseases: the streptococcal experience

The role of host genetic factors in determining susceptibility to infections has become more evident. Certain individuals appear to be predisposed to certain infections, whereas others are protected. By studying the immune response and the genetic makeup of susceptible and resistant individuals a better understanding of the disease process can be achieved. Infections caused by group A streptococci offer an excellent model to study host-pathogen interactions and how the host genetic variation can influence the infection outcome. These studies showed that the same clone of these bacteria can cause severe or non-severe invasive disease. This difference was largely related to the human leukocyte antigen class 11 type of the patient. Certain class II haplotypes present the streptococcal superantigens in a way that results in responses, whereas others present the same superantigens in a way that elicits very potent inflammatory responses that can lead to organ failure and shock. These findings underscore the role of host genetic factors in determining the outcome of serious infections and warrants further investigations into how the same or different genetic factors affect susceptibility to other emerging and re-emerging pathogens.

Identification of superantigen genesspeM,ssa, andsmeZin invasive strains of beta-hemolytic group C and G streptococci recovered from humans

Group C and G Streptococcus dysgalactiae subspecies equisimilis (GCSE and GGSE) cause a substantial percentage of invasive disease caused by beta-hemolytic streptococci. To determine whether Streptococcus pyogenes superantigen (SAg) genes commonly exist within these organisms, 20 recent invasive GCSE and GGSE human isolates and one group G Streptococcus canis human isolate were tested for the presence of SAg genes speH, speJ, speL, speM, ssa and smeZ by polymerase chain reaction (PCR). Prior to this work, sequence-based evidence of the speM, ssa, and smeZ genes in GCSE, GGSE, and S. canis had not been documented. Eleven of the 21 isolates were PCR-positive for the presence of one to two of the SAgs speM, ssa, or smeZ, with four of these isolates carrying ssa+speM or ssa+smeZ. No isolate was positive for speH, speJ and speL. All six ssa-positive GGSE strains harbored the ssa3 allele, previously only found among S. pyogenes strains. All three smeZ-positive GGSE isolates carried one of two smeZ alleles previously only found within S. pyogenes, however the single S. canis isolate carried a new smeZ allele. All five GCSE and GGSE speM-positive isolates harbored a newly discovered speM allele. The identification of these SAgs within S. dysgalactiae subsp. equisimilis and S. canis with identical or near-identical sequences to their counterparts in S. pyogenes suggests frequent interspecies gene exchange between the three beta-hemolytic streptococcal species.

Two novel superantigens found in both group A and group C Streptococcus

Two novel streptococcal superantigen genes (speL(Se) and speM(Se)) were identified from the Streptococcus equi genome database at the Sanger Center. Genotyping of 8 S. equi isolates and 40 Streptococcus pyogenes isolates resulted in the detection of the orthologous genes speL and speM in a restricted number of S. pyogenes isolates (15 and 5%, respectively). Surprisingly, the novel superantigen genes could not be found in any of the analyzed S. equi isolates. The results suggest that both genes are located on a mobile element that enables gene transfer between individual isolates and between streptococci from different Lancefield groups. S. equi pyrogenic exotoxin L (SPE-L(Se))/streptococcal pyrogenic exotoxin L (SPE-L) and SPE-M(Se)/SPE-M are most closely related to SMEZ, SPE-C, SPE-G, and SPE-J, but build a separate branch within this group. Recombinant SPE-L (rSPE-L) and rSPE-M were highly mitogenic for human peripheral blood lymphocytes, with half-maximum responses at 1 and 10 pg/ml, respectively. The results from competitive binding experiments suggest that both proteins bind major histocompatibility complex class II at the beta-chain, but not at the alpha-chain. The most common targets for both toxins were human Vbeta1.1 expressing T cells. Seroconversion against SPE-L and SPE-M was observed in healthy blood donors, suggesting that the toxins are expressed in vivo. Interestingly, the speL gene is highly associated with S. pyogenes M89, a serotype that is linked to acute rheumatic fever in New Zealand.

Staphylococcal and streptococcal superantigens: molecular, biological and clinical aspects

Superantigens (SAgs) include a class of certain bacterial and viral proteins exhibiting highly potent lymphocyte-transforming (mitogenic) activity towards human and or other mammalian T lymphocytes. Unlike conventional antigens, SAgs bind to certain regions of major histocompatibility complex (MHC) class II molecules of antigen-presenting cells (APCs) outside the classical antigen-binding groove and concomitantly bind in their native form to T cells at specific motifs of the variable region of the beta chain (Vbeta) of the T cell receptor (TcR). This interaction triggers the activation (proliferation) of the targeted T lymphocytes and leads to the in vivo or in vitro release of high amounts of various cytokines and other effectors by immune cells. Each SAg interacts specifically with a characteristic set of Vbeta motifs. The review summarizes our current knowledge on S. aureus and S. pyogenes superantigen proteins. The repertoire of the staphylococcal and streptococcal SAgs comprises 24 and 8 proteins, respectively. The staphylococcal SAgs include (i) the classical enterotoxins A, B, C (and antigenic variants), D, E, and the recently discovered enterotoxins G to Q, (ii) toxic shock syndrome toxin-1, (iii) exfoliatins A and B. The streptococcal SAgs include the classical pyrogenic exotoxins A and C and the newly identified pyrogenic toxins, G, H, I, J, SMEZ, and SSA. The structural and genomic aspects of these toxins and their molecular relatedness are described as well as the available 3-D crystal structure of some of them and that of certain of their complexes with MHC class II molecules and the TcR, respectively. The pathophysiological properties and clinical disorders related to these SAgs are reviewed.

The bacterial superantigen streptococcal mitogenic exotoxin Z is the major immunoactive agent of Streptococcus pyogenes

The gene encoding streptococcal mitogenic exotoxin Z (SMEZ) was disrupted in Streptococcus pyogenes. Despite the presence of other superantigen genes, mitogenic responses in human and murine HLA-DQ transgenic cells were abrogated when cells were stimulated with supernatant from the smez(-) mutant compared with the parent strain. Remarkably, disruption of smez led to a complete inability to elicit cytokine production (TNF-alpha, lymphotoxin-alpha, IFN-gamma, IL-1 and -8) from human cells, when cocultured with streptococcal supernatants. The potent effects of SMEZ were apparent even though transcription and expression of SMEZ were barely detectable. Human Vbeta8(+) T cell proliferation in response to S. pyogenes was SMEZ-dependent. Cells from HLA-DQ8 transgenic mice were 3 logs more sensitive to SMEZ-13 than cells from HLA-DR1 transgenic or wild-type mice. In the mouse, SMEZ targeted the human Vbeta8(+) TCR homologue, murine Vbeta11, at the expense of other TCR T cell subsets. Expression of SMEZ did not affect bacterial clearance or survival from peritoneal streptococcal infection in HLA-DQ8 mice, though effects of SMEZ on pharyngeal infection are unknown. Infection did lead to a rise in Vbeta11(+) T cells in the spleen which was partly reversed by disruption of the smez gene. Most strikingly, a clear rise in murine Vbeta4(+) cells was seen in mice infected with the smez(-) mutant S. pyogenes strain, indicating a potential role for SMEZ as a repressor of cognate anti-streptococcal responses.

The Three-dimensional structure of a superantigen-like protein, SET3, from a pathogenicity island of the Staphylococcus aureus genome

The staphylococcal enterotoxin-like toxins (SETs) are a family of proteins encoded within the Staphylococcus aureus genome that were identified by their similarity to the well described bacterial superantigens. The first crystal structure of a member of the SET family, SET3, has been determined to 1.9 A (R = 0.205, R(free) = 0.240) and reveals a fold characteristic of the superantigen family but with significant differences. The SET proteins are secreted at varying levels by staphylococcal isolates, and seroconversion studies of normal individuals indicate that they are strongly antigenic to humans. Recombinant SETs do not exhibit any of the properties expected of superantigens such as major histocompatibility complex class II binding or broad T-cell activation, suggesting they have an entirely different function. The fact that the whole gene family is clustered within the pathogenicity island SaIn2 of the S. aureus genome suggests that they are involved in host/pathogen interactions.

Superantigens: microbial agents that corrupt immunity

Microbial superantigens are a family of protein exotoxins that share the ability to trigger excessive and aberrant activation of T cells. The best characterised are the staphylococcal enterotoxins and the streptococcal pyrogenic exotoxins that trigger the staphylococcal and streptococcal toxic shock syndromes. It is now apparent that superantigens have a wider role in the pathology of infectious diseases than has previously been appreciated. Staphylococcus aureus and Streptococcus pyogenes together produce 19 different superantigens. The range of microorganisms known to produce superantigens has expanded to include Gram negative bacteria, mycoplasma, and viruses. Research is beginning to shed light on the more subtle parts these molecules play in causing disease and to produce some real possibilities for specific treatment of superantigen-induced toxicity. We aim to highlight these new developments and review the science behind these fascinating molecules.

So many ways of getting in the way: diversity in the molecular architecture of superantigen-dependent T-cell signaling complexes

Superantigens (SAGs) elicit massive T-cell proliferation through simultaneous interaction with MHC and TCR molecules. SAGs have been implicated in toxic shock syndrome and food poisoning, and they may also play a pathogenic role in autoimmune diseases. The best-characterized group of SAGs are the pyrogenic bacterial SAGs, which utilize a high degree of genetic variation on a common structural scaffold to achieve a wide range of MHC-binding and T-cell-stimulating effects while assisting pathogen evasion of the adaptive immune response. Several new structures of SAG-MHC and SAG-TCR complexes have significantly increased understanding of the molecular bases for high-affinity peptide/MHC binding by SAGs and for TCR Vbeta domain specificity of SAGs. Using the currently available SAG-MHC and SAG-TCR complex structures, models of various trimolecular MHC-SAG-TCR complexes may be constructed that reveal wide diversity in the architecture of SAG-dependent T-cell signaling complexes, which nevertheless may result in similar signaling outcomes.

Toxic shock syndrome and bacterial superantigens: an update

Toxic shock syndrome (TSS) is an acute onset illness characterized by fever, rash formation, and hypotension that can lead to multiple organ failure and lethal shock, as well as desquamation in patients that recover. The disease is caused by bacterial superantigens (SAGs) secreted from Staphylococcus aureus and group A streptococci. SAGs bypass normal antigen presentation by binding to class II major histocompatibility complex molecules on antigen-presenting cells and to specific variable regions on the beta-chain of the T-cell antigen receptor. Through this interaction, SAGs activate T cells at orders of magnitude above antigen-specific activation, resulting in massive cytokine release that is believed to be responsible for the most severe features of TSS. This review focuses on clinical and epidemiological aspects of TSS, as well as important developments in the genetics, biochemistry, immunology, and structural biology of SAGs. From the evolutionary relationships between these important toxins, we propose that there are five distinct groups of SAGs.

Superantigen bacterial toxins: state of the art

Superantigens (SAgs) are viral and bacterial proteins exhibiting a highly potent polyclonal lymphocyte-proliferating activity for CD4(+), CD8(+) and sometimes gammadelta(+) T cells of human and (or) various animal species. Unlike conventional antigens, SAgs bind as unprocessed proteins to invariant regions of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APCs) and to particular motifs of the variable region of the beta chain (Vbeta) of T-cell receptor (TcR) outside the antigen-binding groove. As a consequence, SAgs stimulate at nano-to picogram concentrations up to 10 to 30% of host T-cell repertoire while only one in 10(5)-10(6) T cells (0.01-0.0001%) are activated upon conventional antigenic peptide binding to TcR. SAg activation of an unusually high percentage of T lymphocytes initiates massive release of pro-inflammatory and other cytokines which play a pivotal role in the pathogenesis of the diseases provoked by SAg-producing microorganisms. We briefly describe in this review the molecular and biological properties of the bacterial superantigen toxins and mitogens identified in the past decade.

Immunological and biochemical characterization of streptococcal pyrogenic exotoxins I and J (SPE-I and SPE-J) from Streptococcus pyogenes

Recently, we described the identification of novel streptococcal superantigens (SAgs) by mining the Streptococcus pyogenes M1 genome database at Oklahoma University. Here, we report the cloning, expression, and functional analysis of streptococcal pyrogenic exotoxin (SPE)-J and another novel SAg (SPE-I). SPE-I is most closely related to SPE-H and staphylococcal enterotoxin I, whereas SPE-J is most closely related to SPE-C. Recombinant forms of SPE-I and SPE-J were mitogenic for PBL, both reaching half maximum responses at 0.1 pg/ml. Evidence from binding studies and cell aggregation assays using a human B-lymphoblastoid cell line (LG-2) suggests that both toxins exclusively bind to the polymorphic MHC class II beta-chain in a zinc-dependent mode but not to the generic MHC class II alpha-chain. The results from analysis by light scattering indicate that SPE-J exists as a dimer in solution above concentrations of 4.0 mg/ml. Moreover, SPE-J induced a rapid homotypic aggregation of LG-2 cells, suggesting that this toxin might cross-link MHC class II molecules on the cell surface by building tetramers of the type HLA-DRbeta-SPE-J-SPE-J-HLA-DRbeta. SPE-I preferably stimulates T cells bearing the Vbeta18.1 TCR, which is not targeted by any other known SAG: SPE-J almost exclusively stimulates Vbeta2.1 T cells, a Vbeta that is targeted by several other streptococcal SAgs, suggesting a specific role for this T cell subpopulation in immune defense. Despite a primary sequence diversity of 51%, SPE-J is functionally indistinguishable from SPE-C and might play a role in streptococcal disease, which has previously been addressed to SPE-C.

Multilocus analysis of extracellular putative virulence proteins made by group A Streptococcus: population genetics, human serologic response, and gene transcription

Species of pathogenic microbes are composed of an array of evolutionarily distinct chromosomal genotypes characterized by diversity in gene content and sequence (allelic variation). The occurrence of substantial genetic diversity has hindered progress in developing a comprehensive understanding of the molecular basis of virulence and new therapeutics such as vaccines. To provide new information that bears on these issues, 11 genes encoding extracellular proteins in the human bacterial pathogen group A Streptococcus identified by analysis of four genomes were studied. Eight of the 11 genes encode proteins with a LPXTG(L) motif that covalently links Gram-positive virulence factors to the bacterial cell surface. Sequence analysis of the 11 genes in 37 geographically and phylogenetically diverse group A Streptococcus strains cultured from patients with different infection types found that recent horizontal gene transfer has contributed substantially to chromosomal diversity. Regions of the inferred proteins likely to interact with the host were identified by molecular population genetic analysis, and Western immunoblot analysis with sera from infected patients confirmed that they were antigenic. Real-time reverse transcriptase-PCR (TaqMan) assays found that transcription of six of the 11 genes was substantially up-regulated in the stationary phase. In addition, transcription of many genes was influenced by the covR and mga trans-acting gene regulatory loci. Multilocus investigation of putative virulence genes by the integrated approach described herein provides an important strategy to aid microbial pathogenesis research and rapidly identify new targets for therapeutics research.

Pyrogenicity and cytokine-inducing properties of Streptococcus pyogenes superantigens: comparative study of streptococcal mitogenic exotoxin Z and pyrogenic exotoxin A

Streptococcal mitogenic exotoxin Z (SMEZ), a superantigen derived from Streptococcus pyogenes, provoked expansion of human lymphocytes expressing the Vbeta 2, 4, 7 and 8 motifs of T-cell receptor. SMEZ was pyrogenic in rabbits and stimulated the expression of the T-cell activation markers CD69 and cutaneous lymphocyte-associated antigen. A variety of cytokines was released by human mononuclear leukocytes stimulated with SMEZ, which was 10-fold more active than streptococcal pyrogenic exotoxin A. Th2-derived cytokines were elicited only by superantigens and not by streptococcal cells.

Functional characterization of streptococcal pyrogenic exotoxin J, a novel superantigen

Streptococcal toxic shock syndrome (STSS) is a highly lethal, acute-onset illness that is a subset of invasive streptococcal disease. The majority of clinical STSS cases have been associated with the pyrogenic toxin superantigens (PTSAgs) streptococcal pyrogenic exotoxin A or C (SPE A or C), although cases have been reported that are not associated with either of these exotoxins. Recent genome sequencing projects have revealed a number of open reading frames that potentially encode proteins with similarity to SPEs A and C and to other PTSAgs. Here, we describe the cloning, expression, purification, and functional characterization of a novel exotoxin termed streptococcal pyrogenic exotoxin J (SPE J). Purified recombinant SPE J (rSPE J) expressed from Escherichia coli stimulated the expansion of both rabbit splenocytes and human peripheral blood lymphocytes, preferentially expanded human T cells displaying Vbeta2, -3, -12, -14, and -17 on their T-cell receptors, and was active at concentrations as low as 5 x 10(-6) microg/ml. Furthermore, rSPE J induced fevers in rabbits and was lethal in two models of STSS. Biochemically, SPE J had a predicted molecular weight of 24,444 and an isoelectric point of 7.7 and lacked the ability to form the cystine loop structure characteristic of many PTSAgs. SPE J shared 19.6, 47.1, 38.8, 18.1, 19.6, and 24.4% identity with SPEs A, C, G, and H, streptococcal superantigen, and streptococcal mitogenic exotoxin Z-2, respectively, and was immunologically cross-reactive with SPE C. The characterization of a seventh functional streptococcal PTSAg raises important questions relating to the evolution of the streptococcal superantigens.