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

Monkey Feeding Assay for Testing Emetic Activity of Staphylococcal Enterotoxin

Staphylococcal enterotoxins (SEs) are unique bacterial toxins that cause gastrointestinal toxicity as well as superantigenic activity. Since systemic administration of SEs induces superantigenic activity leading to toxic shock syndrome that may mimic enterotoxic activity of SEs such as vomiting and diarrhea, oral administration of SEs in the monkey feeding assay is considered as a standard method to evaluate emetic activity of SEs. This chapter summarizes and discusses practical considerations of the monkey feeding assay used in studies characterizing classical and newly identified SEs.

Detection of the staphylococcal enterotoxin D-like gene from staphylococcal food poisoning isolates over the last two decades in Tokyo

The plasmid is a very well-known mobile genetic element that participates in the acquisition of virulence genes, such as staphylococcal enterotoxins (SEs), via horizontal transfer. SEs are emetic toxins and causative agents in staphylococcal food poisoning (SFP). We herein identified the types of plasmids harbored by seven SFP isolates and examined their production of plasmid-related SE/SEl to determine whether the new types of plasmid-related SE or SE-like (SEl) toxins (i.e. SElJ and SER) were involved in SFP. These isolates harbored pIB485-like plasmids, and all, except for one isolate, produced SElJ and SER. The amount of SER produced by each isolate accounted for the highest or second highest percentage of the total amount of SE/SEl produced. These new types of plasmid-related SE/SEls as well as classical SE may play a role in SFP. The seven isolates were classified into two SED-production types; a high SED-production type (>500 ng/ml) and no SED-production type. A nucleotide sequencing analysis revealed that three plasmids harbored by the SED-non-producing isolates had a single-base deletion in the sed gene with a resulting stop codon (from 233 amino acids of the intact SED to 154 amino acids of the mutant SED (mSED)). A real-time reverse transcription-PCR analysis showed that the mRNA of the msed gene was transcribed in the isolates. If the msed gene was translated as a protein, mSED may act as an emetic toxin instead of intact SED.

Establishment of Highly Specific and Quantitative Immunoassay Systems for Staphylococcal Enterotoxin A, B, and C Using Newly-Developed Monoclonal Antibodies

Staphylococcal enterotoxin (SE) activities remain after boiling or treating with proteases. The main symptoms such as vomiting and diarrhea, are caused by the ingestion of SEs. Among SEs, SEA has been reported to be the major and most toxic protein. A highly specific and simple assay system is required to diagnose staphylococcal food poisoning. Therefore, the development of a suitable assay system is strongly anticipated. In this study, we have established a highly specific and sensitive avidin-biotin sandwich ELISA (ABS-ELISA) system for SEA, SEB, and SEC1 using newly-developed monoclonal antibodies. The linearity of these systems obtained was in the range of 0.78-25 ng/ml for each SE, and furthermore, the lower concentrations of SEs could also be detected. The recoveries of SEs from murine serum, skim milk solution, and raw milk were found to be over 90%, suggesting that our systems could detect SEs without any interventions, such as these from milk or serum proteins. We were also able to quantify SEs in 22 specimens of culture supernatants of S. aureus isolated in past occurrences. Our established system should be very useful not only in the clinical field but also in various fields of investigation because of its quantifi-cation and simplicity in detecting SEs.

Inhibition of emetic and superantigenic activities of staphylococcal enterotoxin A by synthetic peptides

Staphylococcus aureus is a major human pathogen producing different types of toxins. Enterotoxin A (SEA) is the most common type among clinical and food-related strains. The aim of the present study was to estimate functional regions of SEA that are responsible for emetic and superantigenic activities using synthetic peptides. A series of 13 synthetic peptides corresponding to specific regions of SEA were synthesized, and the effect of these peptides on superantigenic activity of SEA including interferon γ (IFN-γ) production in mouse spleen cells, SEA-induced lethal shock in mice, spleen cell proliferation in house musk shrew, and emetic activity in shrews were assessed. Pre-treatment of spleen cells with synthetic peptides corresponding to the regions 21-40, 35-50, 81-100, or 161-180 of SEA significantly inhibited SEA-induced IFN-γ production and cell proliferation. These peptides also inhibited SEA-induced lethal shock. Interestingly, peptides corresponding to regions 21-40, 35-50 and 81-100 significantly inhibited SEA-induced emesis in house musk shrews, but region 161-180 did not. These findings indicated that regions 21-50 and 81-100 of SEA are important for both superantigenic and emetic activities of SEA molecule while region 161-180 is involved in superantigenic activity but not emetic activity of SEA. These regions could be important targets for therapeutic intervention against SEA exposure.

In vivo and in vitro antitumor effects of a staphylococcal enterotoxin A mutant (SEA-H61D)

In this study, we evaluated SEA-H61D, a staphylococcal enterotoxin A mutant without emetic activity, as an antitumor agent in vitro and in vivo. It showed that SEA-H61D could significantly inhibit the growth of many cancer cell lines in vitro at very low concentrations by activating human peripheral blood mononuclear cells (PBMCs). CD4+ and CD8+ T lymphocytes could be activated at a dose between 125 and 500 μg/kg. Systemic administration of SEA-H61D in vivo significantly inhibited tumor growth, with the treated group undergoing tumor necrosis and showing a strong infiltration of lymphocytes to the tumor area.

Multiple roles of Staphylococcus aureus enterotoxins: pathogenicity, superantigenic activity, and correlation to antibiotic resistance

Heat-stable enterotoxins are the most notable virulence factors associated with Staphylococcus aureus, a common pathogen associated with serious community and hospital acquired diseases. Staphylococcal enterotoxins (SEs) cause toxic shock-like syndromes and have been implicated in food poisoning. But SEs also act as superantigens that stimulate T-cell proliferation, and a high correlation between these activities has been detected. Most of the nosocomial S. aureus infections are caused by methicillin-resistant S. aureus (MRSA) strains, and those resistant to quinolones or multiresistant to other antibiotics are emerging, leaving a limited choice for their control. This review focuses on these diverse roles of SE, their possible correlations and the influence in disease progression and therapy.

Staphylococcus aureus isolates from Irish domestic refrigerators possess novel enterotoxin and enterotoxin-like genes and are clonal in nature

A previous study carried out by the National Food Centre in Dublin on bacterial contamination of Irish domestic refrigeration systems revealed that 41% were contaminated with Staphylococcus aureus. One hundred fifty-seven S. aureus isolates were screened by multiplex PCR analysis for the presence of 15 staphylococcal enterotoxin and enterotoxin-like genes (sea-see, seg-sei, selj-selo, and selq) and the toxic shock toxin superantigen tst gene. Of the refrigerator isolates, 64.3% possessed more than one staphylococcal enterotoxin or staphylococcal enterotoxin-like gene. All bar one of the 101 staphylococcal enterotoxin or staphylococcal enterotoxin-like gene-positive strains possessed the egc locus bearing the seg, sei, selm, seln, and selo genes. Twelve random amplified polymorphic DNA (RAPD) types accounted for 119 (75.8%) of the strains, two of these types accounting for 25 (RAPD type 1, 15.9%) and 52 (RAPD type 5, 33.1%), respectively. All of the RAPD type 5 isolates possessed the egc gene cluster only. The RAPD type 5 amplicon profile was identical to that of S. aureus isolates associated with osteomyelitis in broiler chickens in Northern Ireland that also possessed the egc locus only. However, the RAPD type 5 domestic refrigerator and chicken isolates differed in penicillin G sensitivity, production of Protein A and staphylokinase, and crystal violet agar growth type. These findings highlight that the average Irish household refrigerator harbors potential enterotoxin-producing S. aureus that may or may not be of animal origin and, accordingly, is a potential reservoir for staphylococcal food poisoning.

Analysis of the epitopes on staphylococcal enterotoxin A responsible for emetic activity

To identify which region of staphylococcal enterotoxin A (SEA) is responsible for the emetic activity, twelve synthetic peptides corresponding to the entire SEA amino acid sequence and their respective anti-peptide antibodies were prepared and tested. The anti-peptide antibodies were tested for neutralization of SEA-induced emesis in Suncus murinus (Shrew mouse). The results indicate that SEA-induced emesis was neutralized by the mixture of three anti-peptide antibodies to A-7 (corresponding to amino acid residues 121-140), A-8 (141-160) and A-9 (160-180). These findings suggest that the regions corresponding to residues 121-180 may be the epitopes responsible for the emetic activity of SEA.

Staphylococcal enterotoxins

Staphylococcus aureus is a major human pathogen that produces a wide array of toxins, thus causing various types of disease symptoms. Staphylococcal enterotoxins (SEs), a family of nine major serological types of heat stable enterotoxins, are a leading cause of gastroenteritis resulting from consumption of contaminated food. In addition, SEs are powerful superantigens that stimulate non-specific T-cell proliferation. SEs share close phylogenetic relationships, with similar structures and activities. Here we review the structure and function of each known enterotoxin.

Pyrogenic toxin superantigen site specificity in toxic shock syndrome and food poisoning in animals

Staphylococcus aureus and Streptococcus pyogenes express pyrogenic toxin superantigens (PTSAgs) that are associated with toxic shock syndrome (TSS) and staphylococcal food poisoning (SFP). Most PTSAgs cause TSS in deep-tissue infections, whereas only TSS toxin 1 (TSST-1) is associated with menstrual, vaginal TSS. In contrast, SFP has been linked only with staphylococcal enterotoxins (SEs). Because of the differential abilities of PTSAgs to cause systemic or localized symptoms in a site-dependent manner, the present study was undertaken to assess the toxins' abilities to cross mucosal barriers. The activity of three PTSAgs when delivered orally, vaginally, or intravenously to rabbits and orally to monkeys was investigated. TSST-1 induced shock via all three routes in rabbits. Although active when administered intravenously, SEC1 and streptococcal pyrogenic exotoxin A (SPEA) did not cause symptoms when administered orally or vaginally. Only SEC1 induced emesis in the monkey feeding assay. TSST-1, albeit less stable than SEC1 and SPEA to pepsin, induced diarrhea in monkeys. Our results may explain the unique association of TSST-1 with menstrual TSS and why SPEA is only rarely associated with TSS after pharyngitis, despite being highly associated with TSS after subcutaneous infections. Finally, our studies indicate that enterotoxicity in SFP is not the result of superantigenicity.

Mutational analysis of superantigen activity responsible for the induction of skin erythema by streptococcal pyrogenic exotoxin C

Streptococcal pyrogenic exotoxin C (SPEC), when injected intradermally, induces erythema in unsensitized rabbits. In the present study, we examined whether this erythema induction is due to the T-cell stimulatory activity of SPEC as a superantigen. Analysis by using single-residue mutant SPECs indicated that mutant SPECs Y15I, A16E, and Y17I, in which tyrosine 15, alanine 16, and tyrosine 17 were replaced with isoleucine, glutamic acid, and isoleucine, respectively, exhibited significantly reduced mitogenic activity for Vbeta2(+) human T cells in vitro, and Y15I showed as much as a 1, 000-fold reduction. Y15I mutant SPEC, however, retained the ability to bind to major histocompatibility complex class II antigen and to form a homodimer, implying that residue 15 is critically important for the interaction of SPEC with T-cell antigen receptor beta chains. When injected intradermally into normal rabbits, wild-type SPEC induced a characteristic erythema after 3 h in a dose-dependent fashion, which was associated with polymorphonuclear and mononuclear cell infiltration. This erythema formation was found to be severely suppressed by systemic pretreatment with cyclosporin A, suggesting the involvement of host T cells. Y15I mutant SPEC exhibited nearly 1, 000-fold less erythema induction in vivo than wild-type SPEC. Altogether, the present results strongly suggest that erythema induction in rabbits by SPEC is attributable mostly to its T-cell stimulatory activity as a superantigen.

Identification and characterization of staphylococcal enterotoxin types G and I from Staphylococcus aureus

Staphylococcal enterotoxins are exotoxins produced by Staphylococcus aureus that possess emetic and superantigenic properties. Prior to this research there were six characterized enterotoxins, staphylococcal enterotoxin types A to E and H (referred to as SEA to SEE and SEH). Two new staphylococcal enterotoxin genes have been identified and designated seg and sei (staphylococcal enterotoxin types G and I, respectively). seg and sei consist of 777 and 729 nucleotides, respectively, encoding precursor proteins of 258 (SEG) and 242 (SEI) deduced amino acids. SEG and SEI have typical bacterial signal sequences that are cleaved to form toxins with 233 (SEG) and 218 (SEI, predicted) amino acids, corresponding to mature proteins of 27,043 Da (SEG) and 24,928 Da (SEI). Biological activities for SEG and SEI were determined with recombinant S. aureus strains. SEG and SEI elicited emetic responses in rhesus monkeys upon nasogastric administration and stimulated murine T-cell proliferation with the concomitant production of interleukin 2 (IL-2) and gamma interferon (IFN-gamma), as measured by cytokine enzyme-linked immunoassays. SEG and SEI are related to other enterotoxins of S. aureus and to streptococcal pyrogenic exotoxin A (SpeA) and streptococcal superantigen (SSA) of Streptococcus pyogenes. Phylogenetic analysis and comparisons of amino acid and nucleotide sequence identities were performed on related staphylococcal and streptococcal protein toxins to group SEG and SEI among the characterized toxins. SEG is most similar to SpeA, SEB, SEC, and SSA (38 to 42% amino acid identity), while SEI is most similar to SEA, SEE, and SED (26 to 28% amino acid identity). Polyclonal antiserum was generated against purified histidine-tagged SEG and SEI (HisSEG and HisSEI). Immunoblot analysis of the enterotoxins, toxic-shock syndrome toxin 1, and SpeA with antiserum prepared against HisSEG and HisSEI revealed that SEG shares some epitopes with SEC1 while SEI does not.

Molecular characterization of the putative T-cell receptor cavity of the superantigen staphylococcal enterotoxin B

A number of investigators have utilized a variety of methods to identify the structural basis for the interaction of superantigens with the T-cell receptor beta-chain. The previous studies strongly suggest that a region of the toxin near residues N23, Y61, Y91 and D209 is important for this binding activity. Examination of crystal structure data shows that these residues line the rim of one side of a shallow cavity in the toxin. In an attempt further to define the face of the staphylococcal enterotoxin B (SEB) molecule involved in the interaction with the beta-chain, we have employed a polymerase chain reaction (PCR)-based, site-specific mutagenesis method to generate amino acid substitutions of residues on the opposite side of this putative T-cell receptor interaction cavity. Our results show that Y175 and N179 appear to be involved in the function of this superantigen, since each of several substitutions at this position exhibits a significantly reduced ability to induce T-cell proliferation. At the same time, mutation of the proximal Y186 does not alter the superantigen activity of SEB. Binding analysis of these mutants shows that class II binding activity is not significantly altered. Analysis of the responding T cells shows that the mutant toxins maintain T-cell receptor V beta selectivity. However, responses of T cells bearing the V beta 8.1 allele appear to be particularly diminished. When viewed in the context of other results reported in the literature, our results suggest that the T-cell receptor interaction site involves SEB residues which ring both the Y175/N179-side and the N23-side of a cavity on one side of the toxin molecule.

Superantigens of gram-positive bacteria: structure-function analyses and their implications for biological activity

Just as we thought that we know everything about superantigens, new molecular and structural studies indicate that we have only just begun to unravel the secrets of these fascinating molecules. Recent structure-function analysis of superantigens from Gram-positive bacteria, with emphasis on their interaction with major histocompatibility complex molecules, could help us decipher the role of superantigens in disease, identify host factors that potentiate their effects and design drugs that specifically block their activity.

Analysis of toxicity of streptococcal pyrogenic exotoxin A mutants

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

Mutations affecting the superantigen activity of staphylococcal enterotoxin B

As a superantigen, staphylococcal enterotoxin B (SEB) possesses the ability to bind to major histocompatibility complex class II molecules and be recognized by T cells bearing certain T-cell receptor (TCR) V beta alleles. Other investigators have utilized site-specific mutagenesis to generate amino acid substitutions to identify residues that may be involved in the interaction with the TCR beta-chain. In an attempt further to define the face of the SEB molecule involved in the interaction with the beta-chain, we have employed a polymerase chain reaction (PCR)-based, site-specific mutagenesis method to generate amino acid substitutions with altered superantigen activity. Our results show that valine at position 169 appears to be involved in the function of this superantigen, since each of several substitutions at this position exhibit a significantly reduced ability to induce T-cell proliferation. Analysis of the responding T cells to the residue 169 substitution shows that the mutant toxins maintain TCR V beta selectivity. At the same time, mutation of the proximal histidine at position 166 does not alter the superantigen activity of SEB. Radiolabelled binding analysis of these H166 and V169 mutants shows that class II-binding activity is not significantly altered. When viewed in the context of other results reported in the literature, combined with the crystal structure of the toxin, our results suggest that the interaction with the TCR probably involves SEB residues which ring a cavity along one side of the toxin molecule.

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

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

Mitogenic activities of amino acid substitution mutants of staphylococcal enterotoxin B in human and mouse lymphocyte cultures

Site-directed mutagenesis has been used to introduce amino acid substitutions at specific residues of the staphylococcal enterotoxin B (SEB) gene cloned from Staphylococcus aureus 10-275. The mitogenic activities of these derivatives were determined in two assay systems: (i) mouse spleen cells and (ii) a mixture of human peripheral blood mononuclear cells and lymphocytes. Substitution of either His-12, His-32, His-121, His-166, Lys-152, or Gly-205 did not significantly alter the mitogenic activity from that of the wild-type toxin in either proliferation assay. Substitution of either residue Asn-23, Phe-44, or Cys-93 reduced the mitogenicity of SEB by a degree that depended upon the assay system used. Similar to the results reported by others measuring toxin activation of mouse lymphoid cells, we found that substitutions of these three residues of SEB caused at least 800-fold reductions of mitogenic activity from that of the wild-type toxin. When tested for toxicity in vivo in D-galactosamine-treated mice, the reduced activities of these mutant toxins, however, were not as pronounced. In contrast, when tested in the human cell mitogenicity assay, these mutant toxins were active. Small alterations in activity (two- to fivefold reduction) were observable only at low concentrations. Our findings reveal the importance of using human lymphocytes in addition to the traditional mouse spleen cell assay when assessing biological activities of staphylococcal enterotoxins.

Biochemical and mutational analysis of the histidine residues of staphylococcal enterotoxin A

The goal of this study was to examine the role of histidine residues in the biological activities of staphylococcal enterotoxin A (SEA). Carboxymethylated SEA was unable to stimulate murine T-cell proliferation but was resistant to monkey stomach lavage fluid degradation, suggesting that native conformation was intact. Site-directed mutagenesis of the histidine residues of SEA was subsequently performed. SEA-H44A (SEA with histidine 44 replaced with alanine), SEA-H44D, SEA-H50A, SEA-H50D, SEA-H114A, SEA-H114D, SEA-H187A, and SEA-H187D retained superantigen and emetic activities, whereas SEA-H225A and SEA-H225D were defective in the ability to stimulate T-cell proliferation. These mutants were unable to compete with SEA for binding to Raji cells, suggesting that the defect in SEA-H225A and SEA-H225D is due to impaired major histocompatibility complex class II binding. SEA-H225D provoked an emetic response in monkeys only if fed at high doses, while SEA-H225A did not provoke an emetic response at low or high doses. In comparison, SEA-H61A and SEA-H61D were defective in emetic activity but not in the ability to stimulate murine T-cell proliferation. Overall, these studies show that the carboxy-terminal histidine at residue position 225 of SEA is important for both the superantigen and emetic activities of this enterotoxin. Histidine 61 appears to be important for emetic activity but not for superantigen activity, consistent with the hypothesis that the two activities are separable in staphylococcal enterotoxins.