Definition of the mitogenic factor (MF) as a novel streptococcal superantigen that is different from streptococcal pyrogenic exotoxins A, B, and C

Aetiological Significance of Infectious Stimuli in Kawasaki Disease

Kawasaki disease (KD) is a pediatric vasculitis syndrome that is often involves coronary artery lesions (e. g., coronary artery aneurysms). Although its causal factors and entire pathogenesis remain elusive, the available evidence indicates that the pathogenesis of KD is closely associated with dysregulation of immune responses to various viruses or microbes. In this short review, we address several essential aspects of the etiology of KD with respect to the immune response to infectious stimuli: 1) the role of viral infections, 2) the role of bacterial infections and the superantigen hypothesis, 3) involvement of innate immune response including pathogens/microbe-associated molecular patterns and complement pathways, and 4) the influence of genetic background on the response to infectious stimuli. Based on the clinical and experimental evidence, we discuss the possibility that a wide range of microbes and viruses could cause KD through common and distinct immune processes.

Highly efficient recombinant production and purification of streptococcal cysteine protease streptopain with increased enzymatic activity

Streptococcus pyogenes produces the cysteine protease streptopain (SpeB) as a critical virulence factor for pathogenesis. Despite having first been described seventy years ago, this protease still holds mysteries which are being investigated today. Streptopain can cleave a wide range of human proteins, including immunoglobulins, the complement activation system, chemokines, and structural proteins. Due to the broad activity of streptopain, it has been challenging to elucidate the functional results of its action and precise mechanisms for its contribution to S. pyogenes pathogenesis. To better study streptopain, several expression and purification schemes have been developed. These methods originally involved isolation from S. pyogenes culture but were more recently expanded to include recombinant Escherichia coli expression systems. While substantially easier to implement, the latter recombinant approach can prove challenging to reproduce, often resulting in mostly insoluble protein and poor purification yields. After extensive optimization of a wide range of expression and purification conditions, we applied the autoinduction method of protein expression and developed a two-step column purification scheme that reliably produces large amounts of purified soluble and highly active streptopain. This method reproducibly yielded 3 mg of streptopain from 50 mL of expression culture at >95% purity, with an activity of 5306 ± 315 U/mg, and no remaining affinity tags or artifacts from recombinant expression. This improved method therefore enables the facile production of the important virulence factor streptopain at higher yields, with no purification scars that might bias functional studies, and with an 8.1-fold increased enzymatic activity compared to previously described procedures.

Specificity of Streptococcus pyogenes NAD(+) glycohydrolase in cytolysin-mediated translocation

The mechanism by which the cytolysin-mediated translocation (CMT) pathway of the Gram-positive pathogen Streptococcus pyogenes injects effector proteins into the cytosol of an infected host cell via the pore-forming protein streptolysin O is unknown. Key questions include whether the pathway can discriminate between different substrates for translocation, and whether the effector protein plays an active or passive role in the translocation process. Here we show that CMT can discriminate between a known effector of the pathway, the S. pyogenes NAD(+) glycohydrolase (SPN), and a second secreted protein, the mitogenic factor (MF), routing the former into the host cell cytosol and the latter into the extracellular milieu. Residues within the amino-terminal 190 residues of SPN were essential for discrimination, as deletions within this domain produced proteins that retained full enzymatic activity, but were completely uncoupled from the translocation pathway. The enzymatic domain itself played a pivotal role in the discrimination as deletions within this domain also produced translocation incompetent proteins and the conversion of MF to a translocation-competent form required fusion with both SPN domains in a contiguous orientation. These data establish that CMT is discriminatory, and that SPN is a multidomain protein that plays an active role in its translocation.

Selective depletion of Vbeta2+CD8+ T cells in peripheral blood from rheumatic heart disease patients

Acute rheumatic fever (ARF) and its chronic valvular sequelae are the delayed consequence of a pharyngeal infection with group A Streptococcus (GAS). Several GAS proteins have been shown to be superantigens, raising the possibility that the expansion or deletion of T cells expressing specific Vbeta regions might play a role in the pathogenesis of ARF or chronic rheumatic heart disease (RHD). We therefore analyzed by four-color flow cytometry, the Vbeta repertoire on CD3, CD4 and CD8 T cells from four ARF patients, 10 RHD patients and also nine healthy controls. A selective depletion of Vbeta2+ T cells was found only in the CD8 subset of chronic RHD patients. This is of interest since a number of GAS superantigens exert their effects on Vbeta2+ cells and because only CD8+ T cells from ARF and RHD patients undergo anergy in response to GAS superantigens. Our results suggests that an ongoing immune process is present in RHD patients and that CD8+ T cells may have an important immunoregulatory role in the pathogenesis of the disease.

Relation of streptococcal pyrogenic exotoxin C as a causative superantigen for Kawasaki disease

We previously reported that the frequency of TCRBV2 and TCRBV6S5-bearing T-cells was high in patients in the acute phase of Kawasaki disease (KD) and that streptococcal pyrogenic exotoxin C (SPE-C) was a potent stimulator of these TCRBV-bearing T-cells. To further elucidate the pathogenesis of KD, we examined the T-cell receptor (TCR) repertoire, human leukocyte antigen (HLA)-DRB1 genotype, and antibody responses to recombinant(r) SPE-C in patients with KD. We also performed in vitro stimulation with rSPE-A and rSPE-C of peripheral blood mononuclear cells from healthy donors and characterized the reacting T-cells. The percentage of T-cells bearing TCRBV2 and TCRBV6S5 was high in patients in the acute stage of KD. rSPE-C stimulation of PBMC from healthy donors induced expansion of TCRBV2 and TCRBV6S5-bearing T-cells. Furthermore, serum levels of anti-SPEC antibodies, which did not display antimitogenic activity, were higher in patients with acute KD than in age-matched controls. The frequencies of the DRB1*04051, 0406, and 0901 were high, whereas that of the DRB1*1101 was low among patients with KD as compared with the healthy adults.

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.

Cysteine protease activity and histamine release from the human mast cell line HMC-1 stimulated by recombinant streptococcal pyrogenic exotoxin B/streptococcal cysteine protease

We constructed the expression vector pSK-SCP containing the streptococcal exotoxin B gene (spe b) which expressed protease activity. We showed that the recombinant streptococcal pyogenic exotoxin B/streptococcal cysteine protease (rSPE B/SCP) was secreted into the culture supernatant of the transformant and retained its SCP activity, which was equivalent to or greater than that of the naturally occurring molecule. The secreted rSPE B/SCP induced histamine release and degranulation of the human mast cell line HMC-1. This study may contribute to the understanding of the pathogenic role of SPE B/SCP in streptococcal infection and streptococcal toxic shock syndrome.

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.

[Cloning of group A streptococcal pyrogenic exotoxin-B gene and its recombinant protein expression in culture supernatant]

Streptococcal pyrogenic exotoxin B, a conserved cysteine protease (SPE B/SCP) released by group A Streptococcus (GAS) strains, is considered to be an important virulence factor of this bacterium. This paper reports the cloning of gene encoding SPE B/SCP. For production of recombinant SPE B/SCP (rSPE B/SCP), the primers specific for the SPE B/SCP gene (spe b) were designed based on its nucleotide sequence. Polymerase chain reaction (PCR) was performed with the genomic DNA of GAS strain NZ131 as a template. The amplified PCR products were purified and cloned into the pBluescript II SK(+) plasmid vector. The vector was transformed into Escherichia coli (E. coli) JM109. The rSPE B/SCP and its recombinant proenzyme (rzym) were secreted in the culture supernate of the transformant. The rSPE B/SCP was purified from the supernatant by sequential chromatography on DEAE-Sepharose, matrix gel Red A and Sephadex G-50 columns. The purified rzym and rSPE B/SCP, respectively, gave a single band with a molecular weight approximately 40 kDa and 27 kDa on SDS-polyacrylamide gel electrophoresis, and reacted with anti-SPE B/SCP antibodies in Western Blot analysis. This is the first report in which rSPE B/SCP was obtained from the culture supernate of the transformant.

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.

Basic streptococcal superantigens (SPEX/SMEZ or SPEC) are responsible for the mitogenic activity of the so-called mitogenic factor (MF)

The mitogenic factor (MF) of group A streptococci has been reported to be a superantigen stimulating human T cells carrying Vbeta2, 4 and 8 and has been designated streptococcal pyrogenic exotoxin F (SPEF). MF was also shown to possess DNase activity. Here we have purified MF from culture supernatants of different Streptococcus pyogenes strains. Surprisingly, the MF preparations from different strains showed different Vbeta specificities depending on the expression of SPEC or SMEZ3 by the producing strain. Their mitogenic activity decreased upon further purification. In addition, the mitogenic activity could be only neutralized by antibodies against the basic streptococcal superantigens SPEC or SPEX (SMEZ3) but not by antibodies against MF itself although the latter were able to neutralize completely the DNase activity of MF. We found that streptodornase type B (SDB) was expressed in two molecular forms (SDBI and SDBII), differing only by one additional N-terminal arginine at SDBI. MF was found identical to the enzyme SDBII but is devoid of superantigenic properties and should no longer be called a superantigen or a pyrogenic exotoxin.

Mitogenic factor (MF) is the major DNase of serotype M89 Streptococcus pyogenes

To investigate the role of mitogenic factor (MF) in streptococcal pathogenesis, the structural gene (mf) encoding this protein was disrupted in a clinical isolate of Streptococcus pyogenes H293, to yield the isogenic mutant H363. Growth in enriched broth and on blood agar was unaffected by disruption of mf. Cell-free broth supernatants from H293 and H363 demonstrated identical promitogenic activities when co-incubated with human peripheral blood mononuclear cells, even when diluted 100000-fold, showing that MF is not a major streptococcal mitogen compared with other secreted superantigens. Disruption of mf resulted in complete loss of DNase B production and detectable DNase activity in H363 compared with the parent strain, confirming that the single gene mf, which is present in all group A streptococcal M serotypes studied, encodes DNase B. Despite loss of DNase activity, the virulence of S. pyogenes in a mouse model of necrotizing fasciitis and myositis was unaffected.

Purification and biochemical characterization of a basic superantigen (SPEX/SMEZ3) from Streptococcus pyogenes

A potent basic superantigen (designated streptococcal pyrogenic exotoxin X, SPEX/SMEZ3) was purified to homogeneity from culture supernatants of a Streptococcus pyogenes scarlatina strain of type 12 (genotype speA(-), speC(-)) and characterized. Sequence alignments revealed SPEX to be an allele of the streptococcal mitogens type Z (SMEZ). The N-terminal amino acid sequence of SPEX was found with LEVDNNSLLR to be identical to the recently described acidic superantigen SMEZ. Although SPEX/SMEZ genes were present in all of the streptococcal strains tested, a toxin production could only be detected in a small number of strains. The produced toxin concentration in the culture supernatants of positive strains differed between 0 and 20 ng ml(-1). The purified SPEX stimulated human T-lymphocytes with Vbeta8 specificity at extremely low concentrations (lower than 100 pg ml(-1)).

Streptococcal pyrogenic exotoxin F (SpeF) causes permeabilization of lung blood vessels

Acute respiration distress syndrome (ARDS) is a typical complication in toxic shock-like syndrome (TSLS) caused by Streptococcus pyogenes. An isolated perfused rat lung model was used to identify the causative agent of ARDS in TSLS in this study. Some crude preparations separated from the culture supernatants of S. pyogenes isolates caused rapid increases in the weight of perfused lungs, indicating vascular permeabilization. Six samples from M type 1 and 3 isolates from TSLS and pharyngitis patients showed strong permeabilization activity, whereas preparations from isolates of other M types (although the number of isolates examined was limited) were negative. The active substance was purified to a single band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis using various columns, and the N-terminal amino acid sequence was determined. The resultant sequence of eight amino acids was identical to SpeF (mitogenic factor). Moreover, the vascular permeabilization activity of the purified band was abolished with anti-SpeF antiserum prepared by immunizing with the purified SpeF. This activity was also neutralized by the antiserum prepared by immunizing with a synthetic peptide derived from the published SpeF sequence. These results suggested that streptococcal SpeF is a major cause of permeabilization of lung blood vessels and sufficient for the pathogenesis of ARDS under the conditions of TSLS caused by S. pyogenes.

The superantigenic activity of streptococcal pyrogenic exotoxin B is independent of the protease activity

The nature of the mitogenic activity of pyrogenic streptococcal exotoxin B, also known as streptococcal cysteine protease, has been debated in the literature. Streptococcal exotoxin B has been shown to cleave interleukin-1beta precursor and create biologically active interleukin-1beta, a major cytokine mediating inflammation and shock. This activity could mimic the mitogenicity and cytokine release induced by superantigens in lymphocyte stimulating experiments. In this study, the protease activity of streptococcal exotoxin B was irreversibly inhibited by covalent binding of a tripeptide and the superantigenic properties of streptococcal exotoxin B were found not to be influenced by this inactivation. Native as well as protease-inactivated streptococcal exotoxin B was shown to stimulate T-cell proliferation without a need of metabolically active antigen presenting cells. Furthermore, streptococcal exotoxin B-induced T-cell proliferation was shown to require HLA-DQ since addition of HLA-DQ monoclonal antibodies totally inhibited the mitogenic activity of streptococcal exotoxin B, indicating that streptococcal exotoxin B, as other superantigens, makes direct contact with the T-cell receptor via HLA class II. The aim of this study was to characterize the relationship between the proteolytic and superantigenic properties of streptococcal exotoxin B.

Molecular analysis of the role of streptococcal pyrogenic Exotoxin A (SPEA) in invasive soft-tissue infection resulting from Streptococcus pyogenes

Epidemiological studies strongly implicate the bacterial superantigen, streptococcal pyrogenic exotoxin A (SPEA), in the pathogenesis of necrotizing soft-tissue infection and toxic shock syndrome resulting from Streptococcus pyogenes. SPEA can act as a superantigen and cellular toxin ex vivo, but its role during invasive streptococcal infection is unclear. We have disrupted the wild-type spea gene in an M1 streptococcal isolate. Supernatants from toxin-negative mutant bacteria demonstrated a 50% reduction in pro-mitogenic activity in HLA DQ-positive murine splenocyte culture, and up to 20% reduction in activity in human PBMC culture. Mutant and wild-type bacteria were then compared in mouse models of bacteraemia and streptococcal muscle infection. Disruption of spea was not associated with attenuation of virulence in either model. Indeed, a paradoxical increase in mutant strain-induced mortality was seen after intravenous infection. Intramuscular infection with the SPEA-negative mutant led to increased bacteraemia at 24 h and a reduction in neutrophils at the site of primary muscle infection. Purified SPEA led to a dose-dependent increase in peritoneal neutrophils 6 h after administration. SPEA is not a critical virulence factor in invasive soft-tissue infection or bacteraemia caused by S. pyogenes, and it could have a protective role in murine immunity to pyogenic infection. The role of this toxin may be different in hosts with augmented superantigen responsiveness.

Polyclonal expansion of TCRBV2- and TCRBV6-bearing T cells in patients with Kawasaki disease

We examined T-cell receptor (TCR) usage, cytokine production and antibody responses to superantigens in patients with Kawasaki disease (KD) to facilitate a better understanding of the immunopathogenesis of KD. The mean percentage of VB2- or VB6. 5-bearing T cells in peripheral blood mononuclear cells (PBMC) of patients with acute-phase KD was significantly higher than that of patients in the convalescent phase of KD or in healthy donors. Expansion of VB2- or VB6.5-bearing T cells was polyclonal because DNA sequences in the complementarity determining region 3 of VB2- and VB6.5-positive cDNA clones were all different from each other. The plasma levels of interleukin (IL)-1beta, IL-2, IL-6, IL-8, IL-10, interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha) and granulocyte colony-stimulating factor (G-CSF) were elevated in the acute phase of KD. We previously reported that streptococcal pyrogenic exotoxin C (SPEC) was a potent stimulator of VB2- and VB6.5-positive T cells and, furthermore, serum levels of anti-SPEC antibodies were significantly higher in patients with acute and convalescent KD than in age-matched controls. The results of the present study, together with those of our previous report, suggest that SPEC induces activation and polyclonal expansion of VB2- and VB6.5-positive T cells, and that SPEC-induced activation of T cells may lead to the pathogenesis of KD.

Identification and characterization of novel superantigens from Streptococcus pyogenes

Three novel streptococcal superantigen genes (spe-g, spe-h, and spe-j) were identified from the Streptococcus pyogenes M1 genomic database at the University of Oklahoma. A fourth novel gene (smez-2) was isolated from the S. pyogenes strain 2035, based on sequence homology to the streptococcal mitogenic exotoxin z (smez) gene. SMEZ-2, SPE-G, and SPE-J are most closely related to SMEZ and streptococcal pyrogenic exotoxin (SPE)-C, whereas SPE-H is most similar to the staphylococcal toxins than to any other streptococcal toxin. Recombinant (r)SMEZ, rSMEZ-2, rSPE-G, and rSPE-H were mitogenic for human peripheral blood lymphocytes with half-maximal responses between 0.02 and 50 pg/ml (rSMEZ-2 and rSPE-H, respectively). SMEZ-2 is the most potent superantigen (SAg) discovered thus far. All toxins, except rSPE-G, were active on murine T cells, but with reduced potency. Binding to a human B-lymphoblastoid line was shown to be zinc dependent with high binding affinity of 15-65 nM. Evidence from modeled protein structures and competitive binding experiments suggest that high affinity binding of each toxin is to the major histocompatibility complex class II beta chain. Competition for binding between toxins was varied and revealed overlapping but discrete binding to subsets of class II molecules in the hierarchical order (SMEZ, SPE-C) > SMEZ-2 > SPE-H > SPE-G. The most common targets for the novel SAgs were human Vbeta2.1- and Vbeta4-expressing T cells. This might reflect a specific role for this subset of Vbetas in the immune defense of gram-positive bacteria.

Serologic evidence that streptococcal superantigens are not involved in the pathogenesis of Kawasaki disease

Kawasaki disease (KD) is an acute multisystem vasculitis of unknown etiology and is associated with marked activation of T cells and monocyte macrophages, leading to the assumption that superantigens are involved in its pathogenesis. To determine if an association exists between streptococcal superantigens and KD, we examined serum antibody responses to superantigens in sera from 50 paired acute and convalescent KD patients using purified recombinant streptococcal superantigens, such as SPEA, SPEC, SSA and MF. We found a very low frequency of detection of anti-superantigen antibodies by ELISA and no marked IgG seroconversion to each superantigen, indicating the absence of a serological relationship between toxin-producing streptococcal infection and the onset of KD.

Mitogenic factor secreted by Streptococcus pyogenes is a heat-stable nuclease requiring His122 for activity

The gene encoding a mitogenic factor, termed MF, was cloned from Streptococcus pyogenes and the recombinant MF was overexpressed in Escherichia coli. Both the natural and recombinant MF had heat-resistant nuclease activity. The nuclease activity of MF was characterized using the recombinant protein. MF showed endonuclease activity, digesting ssDNA, dsDNA and tRNA. The optimal pH for the DNase activity of MF was 9.5. The DNase activity was enhanced approximately tenfold by the simultaneous presence of two divalent cations, Mg2+ and Ca2+, compared to either alone and was inhibited by EDTA or NaCl. The heat stability of MF was biphasic; the DNase activity was heat-stable from 0 to 50 degrees C over 80 degrees C but very unstable at around 60 degrees C. DNA digested by MF possessed 5'-phosphorylated and 3'-hydroxylated termini, identical to those obtained by digestion of DNA by pancreatic deoxyribonuclease I. A mutant clone revealed that His122 was a residue essential to the nuclease activity.