Superantigens - powerful modifiers of the immune system

Synthetic Toll-like receptor 4 agonist enhances vaccine efficacy in an experimental model of toxic shock syndrome

The development of new protein subunit vaccines has stimulated the search for improved adjuvants to replace traditional aluminum-containing products. We investigated the adjuvant effects of a synthetic Toll-like receptor 4 (TLR4) agonist on vaccine efficacy in an experimental model of toxic shock syndrome. The TLR4 agonist E6020 has a simplified structure consisting of a hexa-acylated acyclic backbone. The vaccine examined is a recombinantly attenuated form of staphylococcal enterotoxin B (STEBVax). Using cells stably transfected with TLRs, E6020 transduced signals only through TLR4, suggesting monospecificity, while Escherichia coli 055:B5 lipopolysaccharide activated both the TLR2/6 heterodimer and TLR4. Coadministration of E6020 with STEBVax, by the intramuscular or intranasal route, induced significant levels of immunoglobulin G (IgG) in BALB/c mice. Further, increased IgG production resulted from the combination of E6020 with aluminum hydroxide adjuvant (AH). The antibody response to the vaccine coadministered with E6020 was a mixed Th1/Th2 response, as opposed to the Th2-biased response obtained with AH. Mice vaccinated with STEBVax coadministered with AH, TLR4 agonists, or a combination of both adjuvants were protected from toxic shock. Our data demonstrate the effectiveness of the synthetic TLR4 agonist E6020 as an alternative adjuvant for protein subunit vaccines that may also be used in combination with traditional aluminum-containing adjuvants.

Molecular and Clinical Characteristics of Invasive Group A Streptococcal Infection in Sweden

BACKGROUND

The incidence and severity of invasive group A streptococcal infection demonstrate great variability over time, which at least, in part, seems to be related to group A streptococcal type distribution among the human population.

METHODS

An enhanced surveillance study of invasive group A streptococcal infection (746 isolates) was performed in Sweden from April 2002 through December 2004. Noninvasive isolates from either the throat or skin (773 isolates) were collected in parallel for comparison. Clinical and epidemiological data were obtained from 88% of patients with invasive disease and were related to isolate characteristics, including T type, emm sequence type, and the presence of 9 superantigen genes, as well as pulsed-field gel electrophoresis pattern comparisons of selected isolates.

RESULTS

The annual incidence was 3.0 cases per 100,000 population. Among the patients with invasive disease, 11% developed streptococcal toxic shock syndrome, and 9.5% developed necrotizing fasciitis. The overall case-fatality rate was 14.5%, and 39% of the patients with streptococcal toxic shock syndrome died (P<.001). The T3/13/B3264 cluster accounted for 33% of invasive and 25% of noninvasive isolates. Among this most prevalent type cluster, emm types 89 and 81 dominated. Combined results from pulsed-field gel electrophoresis, emm typing, and superantigen gene profiling identified subgroups within specific emm types that are significantly more prone to cause invasive disease than were other isolates of the same type.

CONCLUSIONS

This study revealed a changing epidemiology of invasive group A streptococcal infection in Sweden, with emergence of new emm types that were previously not described. The results also suggest that some clones may be particularly prone to cause invasive disease.

Bacterial superantigens and T cell receptor beta-chain-bearing T cells in the immunopathogenesis of ulcerative colitis

Ulcerative colitis (UC) is a chronic relapsing-remitting inflammatory bowel disease (IBD) that affects the colon and the rectum producing debilitating symptoms, which impair ability to function and quality of life. The aetiology of IBD is incompletely understood, but within the lymphocyte population, specific T cell subsets are known to be major factors in the development of intestinal immune pathology while different subsets are essential regulators, controlling IBD. Hence, IBD is thought to reflect dysregulated T cell behaviour. This study was to investigate if the normal molecular configuration of the T cell receptor (TCR) repertoire is compromised in patients with UC. The percentage of T cell-bearing beta-chain 4 (TCRBV4) was high in patients with UC, and T cells showed polyclonal expansion in the presence of bacterial superantigens (SA) such as streptococcal mitogenic exotoxin Z-2 (SMEZ-2), indicating that bacterial SA promote specific TCRBV family expansion. Further, in patients with UC, the duration of UC was significantly longer in patients with skewed TCRBV4 compared with patients without TCRBV4 skewing, suggesting that long-term exposure to bacterial SA such as SMEZ-2 might promote systemic immune disorders like the remission-relapsing cycles seen in patients with UC. In conclusion, our observations in this study support the perception that the systemic activation of T cells by enteric bacterial SA might lead to a dysregulated, but exuberant immune activity causing the remission and flare-up cycle of mucosal inflammation in patients with UC. Future studies should strengthen our findings and increase understanding on the aetiology of IBD.

The superantigen staphylococcal enterotoxin A (SEA) and monoclonal antibody L243 share a common epitope but differ in their ability to induce apoptosis via MHC-II

Crosslinking of MHC class II (MHC-II) molecules by antibodies or by superantigens (SAg) induces a variety of functional responses in the antigen presenting cell. We were able to allocate K39 as the residue that is essential for binding of antibody L243 to the alpha chain of HLA-DR. K39 is also essential for binding of staphylococcal enterotoxin A (SEA). However, the functional responses of the two ligands differ considerably exemplified by the ability of L243 to induce apoptosis in monocytic cells and in B cells, whereas SEA is unable to activate the apoptosis pathway. Despite the differences in functional responses, both ligands induce cell aggregation in MonoMac-1 cells. The SEA molecule with its two different binding sites associates one MHC alpha chain with one beta chain as opposed to two alpha chains that are brought into close proximity by the two identical antigen binding sites of L243. We therefore conclude that the spatial orientation of dimerized MHC-II and their associated proteins is an important factor for the nature of the transduced signal and consequently the outcome of functional responses.

A role for IFNgamma in differential superantigen stimulation of conventional versus plasmacytoid DCs

Superantigens (SAgs) are known to play a role in food poisoning, toxic shock syndrome and have been identified as a potential mediator of autoimmunity. Although much is known about the effects of SAgs on T cells, by comparison few studies have investigated how SAgs influence innate immune cells. In particular no study has examined how SAgs affect murine plasmacytoid dendritic cells (pDC). We report that in vivo administration of staphylococcal enterotoxin A (SEA) increased the number of pDCs in secondary lymphoid organs, and induced CD86 and CD40 expression. Similar to SEA activation of conventional DCs (cDCs), pDCs relied on T cells, but not on CD40. Nonetheless, pDCs strictly required IFNgamma for upregulation of CD86 and CD40, but cDCs did not depend upon IFNgamma for activation. Further, even though IFNgamma deficient pDCs were not activated by SEA, they were still capable of producing wild-type levels of IFNalpha in response to CpG oligodeoxynucleotide (ODN). The source of IFNgamma for pDC activation was not T cells, nor did pDCs themselves have to synthesize or bind IFNgamma, but the presence of IFNgamma was essential. After SEA stimulation, IFNgamma deficient mice fail to induce expression of the pDC dependent chemokines CXCL9, and demonstrated a defect in recruitment of pDCs to marginal zones of lymphoid organs. Thus, SEA exerts its combined effect on pDC activation, recruitment and chemokine induction through the action of IFNgamma. This fundamental dichotomy of the effects of SAgs on pDCs versus cDCs show how a non-PAMP from bacteria, can selectively and indirectly stimulate innate cell subpopulations much in the same way that differential TLR expression influences cells of the innate immune system.

Rhinosinusitis derived Staphylococcal enterotoxin B plays a possible role in pathogenesis of food allergy

BACKGROUND

Staphylococcal enterotoxin B (SEB) is a potent immunomodulator and implicated with pathogenesis of inflammatory diseases mediated by Th1 or Th2 dominant immune responses. The objective of this study is to determine a possible association between rhinosinusitis derived SEB and pathogenesis of food allergy (FA).

METHODS

The study included chronic rhinosinusitis (CRS) patients with FA (N = 46) or without FA (N = 33). Controls included FA patients without CRS (N = 26) and healthy volunteers (N = 25). In CRS patients, we assessed the parameters associated with FA including prick skin test (PST) reactivity to food allergens, serum levels of allergen-specific IgE and cytokines (IL-4, IL-13, IFN-I3), and the number/reactivity of food-allergen specific Th1/Th2 cells in the peripheral blood before and 2 months after sinus surgery. Changes of these parameters were evaluated in comparison with changes in SEB concentration in the sinus lavage and stool samples and also in vitro reactivity to SEB. In CRS patients with FA, we also assessed changes in reactivity to oral challenge of offending food before and after sinus surgery.

RESULTS

Two months following sinus surgery, we observed statistically significant reduction in PST and oral challenge reactivity in CRS patients with FA in parallel to decrease in serum levels of Th2 cytokines (IL-4 and IL-13) and allergen specific IgE. Improvement of reactivity to food allergens was positively associated with decline in SEB concentrations in the sinus lavage and stool samples. In vitro study results also indicated a role of SEB in aggravation of Th2 skewed responses to food allergens. Such changes were not observed in CRS-non FA patients or control FA patients.

CONCLUSION

The rhinosinusitis derived SEB plays a certain role in the pathogenesis of FA by augmenting and/or maintaining polarized Th2 responses. Removal of SEB-producing pathogens from the rhinosinuses may be beneficial for attenuating the FA symptoms in patients with CRS-FA.

TLR2 and MyD88 contribute to Lactobacillus casei extract-induced focal coronary arteritis in a mouse model of Kawasaki disease

BACKGROUND

Kawasaki disease is the most common cause of acquired cardiac disease and acute vasculitis in children, targets the coronary arteries, and can occasionally be fatal. The pathogenesis and the molecular mechanisms remain unknown. After injection of Lactobacillus casei cell-wall extract (LCCWE), mice develop a focal coronary arteritis that histopathologically resembles Kawasaki disease, but the mechanism remains unclear. Here, we tested the hypothesis that signaling by Toll-like receptors (TLRs) through their key downstream adaptor molecule myeloid differentiation factor 88 (MyD88) is required for the cellular activation and coronary arteritis produced by LCCWE.

METHODS AND RESULTS

Bone marrow-derived macrophages from TLR2- or MyD88-deficient mice were unresponsive to LCCWE-induced stimulation. In contrast, macrophages obtained from TLR4-deficient mice produced the same amount of interleukin-6 as macrophages from wild-type mice after stimulation with LCCWE. Intraperitoneal injection of LCCWE produced severe focal coronary arteritis in TLR4(-/-) and C57BL/6 control mice but not in TLR2(-/-) or MyD88(-/-) mice. Collectively, these results indicate that LCCWE is a potent inducer of nuclear factor-kappaB via TLR2 but not TLR4 and that this activation proceeds via the MyD88-dependent signaling pathway. In vivo studies suggest that TLR2(-/-) mice are protected from LCCWE-induced coronary arteritis and that this protection is mediated through the adaptor molecule MyD88.

CONCLUSIONS

Our results provide important insights into the molecular signaling in this mouse model of coronary arteritis. We show here that LCCWE-induced coronary arteritis is dependent on intact TLR2 and MyD88 signaling.

Expression, purification and crystallization of Streptococcus dysgalactiae-derived mitogen

Superantigens are bacterial or viral toxins with potent immunostimulatory properties. Streptococcus dysgalactiae-derived mitogen, a 25 kDa protein, is a recently discovered superantigen isolated from S. dysgalactiae culture supernatant. Sequence considerations suggest that it belongs to a new superantigen family distinct from other superantigens. The protein was expressed in Escherichia coli cells and purified to homogeneity. Crystals were grown at pH 4.2-4.4 in the presence of 18-20%(w/v) PEG 3350 and 0.4 M lithium nitrate. A complete data set to 2.4 A resolution was collected from a single crystal at liquid-nitrogen temperatures using synchrotron radiation. The crystals belong to space group P3/P3(1)/P3(2), with unit-cell parameters a = b = 52.7, c = 62.4 A, gamma = 120 degrees and one molecule in the crystallographic asymmetric unit.

Viable group A streptococci in macrophages during acute soft tissue infection

BACKGROUND

Group A streptococcal severe soft tissue infections, such as necrotizing fasciitis, are rapidly progressive infections associated with high mortality. Group A streptococcus is typically considered an extracellular pathogen, but has been shown to reside intracellularly in host cells.

METHODS AND FINDINGS

We characterized in vivo interactions between group A streptococci (GAS) and cells involved in innate immune responses, using human biopsies (n = 70) collected from 17 patients with soft tissue infections. Immunostaining and in situ image analysis revealed high amounts of bacteria in the biopsies, even in those collected after prolonged antibiotic therapy. Viability of the streptococci was assessed by use of a bacterial viability stain, which demonstrated viable bacteria in 74% of the biopsies. GAS were present both extracellularly and intracellularly within phagocytic cells, primarily within macrophages. Intracellular GAS were predominantly noted in biopsies from newly involved tissue characterized by lower inflammation and bacterial load, whereas purely extracellular GAS or a combination of intra- and extracellular GAS dominated in severely inflamed tissue. The latter tissue was also associated with a significantly increased amount of the cysteine protease streptococcal pyrogenic exotoxin SpeB. In vitro studies confirmed that macrophages serve as reservoirs for viable GAS, and infection with a speB-deletion mutant produced significantly lower frequencies of cells with viable GAS following infection as compared to the wild-type bacteria.

CONCLUSIONS

This is the first study to demonstrate that GAS survive intracellularly in macrophages during acute invasive infections. This intracellular presence may have evolved as a mechanism to avoid antibiotic eradication, which may explain our finding that high bacterial load is present even in tissue collected after prolonged intravenous antibiotic therapy. This new insight into the pathogenesis of streptococcal soft tissue infections highlights a need for alternative therapeutic strategies.

Immune cell activation and subsequent epithelial dysfunction by Staphylococcus enterotoxin B is attenuated by the green tea polyphenol (-)-epigallocatechin gallate

Bacterial superantigens (SAg) are potent T cell activators and when delivered systemically elicit a self-limiting enteropathy in mice. Also, SAg-stimulated human peripheral blood mononuclear cells (PBMC) increase enteric epithelial cell monolayer permeability in vitro. Epigallocatechin gallate (EGCG), the major polyphenol component of green tea (Camilla sinesis) leaf, has been presented as an anti-inflammatory agent. We tested the hypothesis that EGCG (10-100 microM) would block PBMC activation by the SAg, Staphylococcus aureus enterotoxin B (SEB, 1 microg/ml), thus preventing disruption of the epithelial barrier. Pretreatment or co-treatment of human PBMC or murine lymphnode cells with EGCG significantly reduced SEB-induced proliferation and IL-2, IFNgamma, and TNFalpha production. ConA-induced proliferation was also inhibited by EGCG (50 microM) co-treatment. These effects of EGCG were not due to induction of immune cell apoptosis, and were independent of EGCGs anti-oxidant activity, and inhibition of NF-kappaB or AP-1 activation. Moreover, addition of exogenous IL-2 (20 ng/ml) to the cultures could not overcome the immunosuppressive effect of EGCG. Culture supernatant from PBMC stimulated in the presence of EGCG failed to increase the permeability of T84 epithelial cell monolayers: a finding consistent with the reduced IFNgamma and TNFalpha production by SAg+EGCG treated PBMC. These data promote EGCG as a suppressor of T cell activation, and given the prominent role that bacteria and T cells play in inflammatory disease we suggest that EGCG could be a useful addition to current treatments for enteric immune disorders and T cell driven immunopathologies.

Endothelial cells promote human immunodeficiency virus replication in nondividing memory T cells via Nef-, Vpr-, and T-cell receptor-dependent activation of NFAT

Human endothelial cells (ECs) enhance human immunodeficiency virus (HIV) replication within CD4(+) memory T cells by 50,000-fold in a Nef-dependent manner. Here, we report that EC-mediated HIV type 1 replication is also dependent on an intact vpr gene. Moreover, we demonstrate that despite a requirement for engaging major histocompatibility complex (MHC) class II molecules and costimulators, EC-stimulated virus-producing cells (p24(high) T cells) do not proliferate, nor are they arrested in the cell cycle. Rather, they are minimally activated, sometimes expressing CD69 but not CD25, HLA-DR, VLA-1, or effector cytokines. Blocking antibodies to interleukin 2 (IL-2), IL-6, IL-7, or tumor necrosis factor do not inhibit viral replication. Cyclosporine effectively inhibits viral replication, as does disruption of the NFAT binding site in the viral long terminal repeat. Furthermore, in the presence of ECs, suboptimal T-cell receptor (TCR) stimulation with phytohemagglutinin L supports efficient viral replication, and suboptimal stimulation with toxic shock syndrome toxin 1 leads to viral replication selectively in the TCR-stimulated, Vbeta2-expressing T cells. Collectively, these data indicate that ECs provide signals that promote Nef- and Vpr-dependent HIV replication in memory T cells that have been minimally activated through their TCRs. Our studies suggest a mechanism for HIV replication in vivo within the reservoir of circulating memory CD4(+) T cells that persist despite antiretroviral therapy and further suggest that maintenance of immunological memory by MHC class II-expressing ECs via TCR signaling may contribute to HIV rebound following cessation of antiretroviral therapy.

The structure of the C5a receptor-blocking domain of chemotaxis inhibitory protein of Staphylococcus aureus is related to a group of immune evasive molecules

The chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is a 121 residue excreted virulence factor. It acts by binding the C5a- (C5aR) and formylated peptide receptor (FPR) and thereby blocks specific phagocyte responses. Here, we report the solution structure of a CHIPS fragment consisting of residues 31-121 (CHIPS31-121). CHIPS31-121 has the same activity in blocking the C5aR compared to full-length CHIPS, but completely lacks FPR antagonism. CHIPS31-121 has a compact fold comprising an alpha-helix (residues 38-51) packed onto a four-stranded anti-parallel beta-sheet. Strands beta2 and beta3 are joined by a long loop with a relatively well-defined conformation. Comparison of CHIPS31-121 with known structures reveals striking homology with the C-terminal domain of staphylococcal superantigen-like proteins (SSLs) 5 and 7, and the staphyloccocal and streptococcal superantigens TSST-1 and SPE-C. Also, the recently reported structures of several domains of the staphylococcal extracellullar adherence protein (EAP) show a high degree of structural similarity with CHIPS. Most of the conserved residues in CHIPS and its structural homologues are present in the alpha-helix. A conserved arginine residue (R46 in CHIPS) appears to be involved in preservation of the structure. Site-directed mutagenesis of all positively charged residues in CHIPS31-121 reveals a major involvement of arginine 44 and lysine 95 in C5aR antagonism. The structure of CHIPS31-121 will be vital in the further unraveling of its precise mechanism of action. Its structural homology to S.aureus SSLs, superantigens, and EAP might help the design of future experiments towards an understanding of the relationship between structure and function of these proteins.

A defective viral superantigen-presenting phenotype in HLA-DR transfectants is corrected by CIITA

Activation of T lymphocytes by mouse mammary tumor virus superantigen (vSAg) requires binding to MHC class II molecules. The subcellular location where functional interactions occur between MHC class II molecules and vSAgs is still a matter of debate. To gain further insight into this issue, we have used human epithelial HeLa cells expressing HLA-DR1. Surprisingly, the human cells were unable to present transfected vSAg7 or vSAg9 to a series of murine T cell hybridomas. The defect is not related to a lack of vSAg processing, because these cells can indirectly activate T cells after coculture in the presence of B lymphocytes. However, after IFN-gamma treatment, the HeLa DR1(+) cells became apt at directly presenting the vSAg. Furthermore, transfection of CIITA was sufficient to restore presentation. Reconstitution experiments demonstrated the necessity of coexpressing HLA-DM and invariant chain (Ii) for efficient vSAg presentation. Interestingly, inclusion of a dileucine motif in the DRbeta cytoplasmic tail bypassed the need for HLA-DM expression and allowed the efficient presentation of vSAg7 in the presence of Ii. A similar trafficking signal was included in vSAg7 by replacing its cytoplasmic tail with the one of Ii. However, sorting of this chimeric Ii/vSAg molecule to the endocytic pathway completely abolished both its indirect and direct presentation. Together, our results suggest that functional vSAgs-DR complexes form after the very late stages of class II maturation, most probably at the cell surface.

Modeling of receptor mimics that inhibit superantigen pathogenesis

Staphylococcal enterotoxins SEB and SEC3 and toxic shock syndrome toxin TSST-1 act as superantigens by overstimulating the human immune system and thereby compromise host defense. The mechanism of pathogenesis is explained on the basis of superantigen binding to the MHC class II receptor on the antigen presenting cell and to the T cell receptor (TcR) on the T cell. SEB, SEC3 and TSST-1 bind as intact proteins and make contacts with the alpha1 subdomain (DRalpha) of MHC class II and Vbeta subdomain of TcR. SEB, SEC3 and TSST-1 show specificities for different TcRVbeta isoforms. We have designed three different chimeras linking the same DRalpha with different TcRVbeta isoforms to specifically target SEB, SEC3 and TSST-1 and inhibit their pathogenesis. Here, we show by molecular modeling that the DRalpha, TcRVbeta and linker of a given chimera interact with the target superantigen in a type-specific manner. An initial model of the complex is constructed on the basis of observed inter-molecular contacts between DRalpha/TcRVbeta and the superantigens. A constant temperature (300 K) 200 ps molecular dynamics is performed to sample different conformations of a chimera-superantigen complex by utilizing the flexibility of the (GSTAPPA)(2) linker while maintaining the native folds of superantigen, DRalpha and TcRVbeta and the observed intermolecular contacts. After equilibration, 100 molecular dynamic snapshots are minimized and analyzed. This provides descriptions of various pairwise interactions at the contact interface in the complex and important clues on single site mutations on the chimera that may enhance the stability of a given superantigen-chimera complex.

The prevalence of theStaphylococcus aureus tstgene among community- and hospital-acquired strains and isolates from Wegener's Granulomatosis patients

To allow rapid identification of toxic shock syndrome toxin-1 (TSST-1)-producing Staphylococcus aureus strains, a real-time PCR assay for the detection of the tst gene, which encodes TSST-1, was developed. The assay was applied to S. aureus isolates from patients with Wegener's Granulomatosis (WG), as well as isolates that were classified as either community- (CA) or hospital-acquired (HA). No significant difference in the percentage of tst-positive strains was observed between isolates from WG patients and CA isolates (24% and 25%, respectively). In contrast, only 14% of the HA isolates were tst-positive (p<0.05). Investigation of the clonal relationship between tst-positive CA and HA strains could indicated the recent emergence of a virulent S. aureus clone in the community.

Structure-based preliminary analysis of immunity and virulence of SARS coronavirus

The research on SARS-associated coronavirus (SARS-CoV) has not stopped since its discovery, but the pathogenesis of SARS is still unclear. To explore the possible molecular mechanisms of the invasion and virulence of SARS-CoV, we investigated the structural basis of the viral proteins using computational biology. Forty-five motifs relating to superantigens, toxins and other bioactive molecules were detected in the proteins of SARS-CoV. The results showed that the distribution of the motifs varied in different proteins. Enzyme-like motifs were located in the R protein, while ICAM- 1-like and toxin-like molecules were located in the spike, envelop, nucleocapsid, PUP1, PUP 2 and PUP 4 proteins. Comparison of SARS-CoV with other viruses (OC43, PEDV, HRSV, HHerpV and HAdenoV) showed that each group of motifs was different for each type of virus. Data suggest that the proteins of SARS-CoV with toxic motifs might play crucial roles in targeting host cells and interfering with the immune system. This study provides new information for drug and vaccine design, as well as therapeutic strategies against SARS.

The Staphylococcal Superantigen-Like Protein 7 Binds IgA and Complement C5 and Inhibits IgA-FcαRI Binding and Serum Killing of Bacteria

The staphylococcal superantigen-like proteins (SSLs) are close relatives of the superantigens but are coded for by a separate gene cluster within a 19-kb region of the pathogenicity island SaPIn2. rSSL7 (formally known as SET1) bound with high affinity (K(D), 1.1 nM) to the monomeric form of human IgA1 and IgA2 plus serum IgA from primate, pig, rat, and horse. SSL7 also bound the secretory form of IgA found in milk from human, cow, and sheep, and inhibited IgA binding to cell surface FcalphaRI (CD89) and to a soluble form of the FcalphaRI protein. In addition to IgA, SSL7 bound complement factor C5 from human (K(D), 18 nM), primate, sheep, pig, and rabbit serum, and inhibited complement-mediated hemolysis and serum killing of a Gram-negative organism Escherichia coli. SSL7 is a superantigen-like protein secreted from Staphylococcus aureus that blocks IgA-FcR interactions and inhibits complement, leading to increased survival of a sensitive bacterium in blood.

Organization of secondary lymphoid tissue and local IgE formation to Staphylococcus aureus enterotoxins in nasal polyp tissue

BACKGROUND

Bilateral nasal polyposis (NP) is characterized by high concentrations of IgE in NP tissue, which show no relation to the atopic status. We aimed to study the relationship between systemic and local IgE formation, nasal carriage of Staphylococcus aureus and nasal polyposis.

METHODS

In serum and nasal tissue homogenates from 24 NP patients and 12 controls, we determined concentrations of total IgE and IgE antibodies to inhalant allergens and S. aureus enterotoxins (SAEs; A,B,C,D,E,TSST) by ImmunoCAP. Tissue cryosections were stained for CD3, CD20, CD38, CD23, FcepsilonRI, IgE and SEA/SEB.

RESULTS

We demonstrated a higher incidence of S. aureus colonization (17/24) and IgE antibodies to SAEs in NP tissue (12/24) compared with controls (3/12 and 0/12, respectively). Total IgE and IgE antibodies in serum and NP tissue were dissociated because of local polyclonal IgE formation in NP tissue. Staining of NP tissue revealed follicular structures characterized by B and T cells, and lymphoid accumulations with diffuse plasma cell infiltration.

CONCLUSIONS

We demonstrated the organization of secondary lymphoid tissue in polyp tissue and a polyclonal hyper-immunoglobulinemia E associated with the presence of IgE antibodies to SAEs, colonization with S. aureus, and tissue eosinophilia in a relevant subgroup of polyp patients.

Detection of Staphylococcus aureus enterotoxin A and B genes with PCR-EIA and a hand-held electrochemical sensor

Two electrochemical assays for detecting Staphylococcus aureus enterotoxin A and B genes were developed. The assays are based on PCR amplification with biotinylated primers, hybridization to a fluorescein-labeled probe, and detection with horseradish peroxidase-conjugated anti-fluorescein antibody using a hand-held electrochemical detector. The limit of detection (LOD) for both assays was approximately 16 copies of the sea and seb genes. The assays were evaluated in blinded studies, each with 81 samples that included genomic and cloned S. aureus DNA, and genomic DNA from Alcaligens, Bacillus, Bacteroides, Bordetella, Borkholderia, Clostridium, Comanonas, Enterobacter, Enterococcus, Escherichia, Francisella, Haemophilus, Klebsiella, Listeria, Moraxella, Neisseria, Proteus, Pseudomonas, Salmonella, Serratia, Shigella, Streptococcus, Vibrio and Yersinia species. Both assays showed 100% sensitivity. The specificity was 96% for the SEA assay and 98% for the SEB assay. These results demonstrate the feasibility of performing probe-based detection of PCR products with a low-cost, hand-held, electrochemical detection device as a viable alternative to colorimetric enzyme-linked assays of PCR products.

Cloning, expression and interaction of human T-cell receptors with the bacterial superantigen SSA

Superantigens (SAgs) are a class of disease-causing and immunostimulatory proteins of bacterial or viral origin that activate a large number of T-cells through interaction with the Vbeta domain of T-cell receptors (TCRs). In this study, recombinant TCR beta chains were constructed with human variable domains Vbeta5.2, Vbeta1 and Vbeta2.1, expressed as inclusion bodies, refolded and purified. The Streptococcus pyogenes SAg SSA-1 was cloned and expressed as a soluble periplasmic protein. SSA-1 was obtained both as a monomer and a dimer that has an intermolecular disulfide bond. We analyzed the biological activity of the recombinant SAgs by proliferation assays. The results suggest that SSA dimerization occludes the TCR interaction site. Naturally occurring SSA dimerization was also observed in supernatants of S. pyogenes isolates. An SSA mutant [SSA(C26S)] was produced to eliminate the Cys responsible for dimerization. Affinity assays using a resonant biosensor showed that both the mutant and monomeric wild type SSA have affinity for human Vbeta5.2 and Vbeta1 with Kd of 9-11 microm with a fast kass and a moderately fast kdiss. In spite of the reported stimulation of Vbeta2.1 bearing T-cells by SSA, we observed no measurable interaction.

In silico analysis of gene expression profiles in the olfactory mucosae of aging senescence-accelerated mice

We utilized high-density Affymetrix oligonucleotide arrays to investigate gene expression in the olfactory mucosae of near age-matched aging senescence-accelerated mice (SAM). The senescence-prone (SAMP) strain has a significantly shorter lifespan than does the senescence-resistant (SAMR) strain. To analyze our data, we applied biostatistical methods that included a correlation analysis to evaluate sources of methodologic and biological variability; a two-sided t-test to identify a subpopulation of Present genes with a biologically relevant P-value <0.05; and a false discovery rate (FDR) analysis adjusted to a stringent 5% level that yielded 127 genes with a P-value of <0.001 that were differentially regulated in near age-matched SAMPs (SAMP-Os; 13.75 months) compared to SAMRs (SAMR-Os, 12.5 months). Volcano plots related the variability in the mean hybridization signals as determined by the two-sided t-test to fold changes in gene expression. The genes were categorized into the six functional groups used previously in gene profiling experiments to identify candidate genes that may be relevant for senescence at the genomic and cellular levels in the aging mouse brain (Lee et al. [2000] Nat Genet 25:294-297) and in the olfactory mucosa (Getchell et al. [2003] Ageing Res Rev 2:211-243), which serves several functions that include chemosensory detection, immune barrier function, xenobiotic metabolism, and neurogenesis. Because SAMR-Os and SAMP-Os have substantially different median lifespans, we related the rate constant alpha in the Gompertz equation on aging to intrinsic as opposed to environmental mechanisms of senescence based on our analysis of genes modulated during aging in the olfactory mucosa.

Crystallographic and Mutational Data Show That the Streptococcal Pyrogenic Exotoxin J Can Use a Common Binding Surface for T-cell Receptor Binding and Dimerization

The protein toxins known as superantigens (SAgs), which are expressed primarily by the pathogenic bacteria Staphylococcus aureus and Streptococcus pyogenes, are highly potent immunotoxins with the ability to cause serious human disease. These SAgs share a conserved fold but quite varied activities. In addition to their common role of cross-linking T-cell receptors (TCRs) and major histocompatibility complex class II (MHC-II) molecules, some SAgs can cross-link MHC-II, using diverse mechanisms. The crystal structure of the streptococcal superantigen streptococcal pyrogenic exotoxin J (SPE-J) has been solved at 1.75 A resolution (R = 0.209, R(free) = 0.240), both with and without bound Zn(2+). The structure displays the canonical two-domain SAg fold and a zinc-binding site that is shared by a subset of other SAgs. Most importantly, in concentrated solution and in the crystal, SPE-J forms dimers. These dimers, which are present in two different crystal environments, form via the same face that is used for TCR binding in other SAgs. Site-directed mutagenesis shows that this face is also used for TCR binding SPE-J. We infer that SPE-J cross-links TCR and MHC-II as a monomer but that dimers may form on the antigen-presenting cell surface, cross-linking MHC-II and eliciting intracellular signaling.

Kawasaki disease and toxic shock syndrome--at last the etiology is clear?

A decade after the superantigen hypothesis for KD was first suggested, it has still not been either proven or refuted conclusively. Although initial optimism for the hypothesis was quashed by a series of published papers apparently refuting the idea, in the last few years there have been a number of good studies providing evidence in support of the superantigen hypothesis. Whether this renewed enthusiasm is justified will hopefully become clear in the near future. Ultimately, accurate diagnosis, more targeted treatment, and preventative strategies depend on the unraveling of the immunopathogenesis of this disease.

T lymphocyte-endothelial cell interactions

Human vascular endothelial cells (EC) basally display class I and II MHC-peptide complexes on their surface and come in regular contact with circulating T cells. We propose that EC present microbial antigens to memory T cells as a mechanism of immune surveillance. Activated T cells, in turn, provide both soluble and contact-dependent signals to modulate normal EC functions, including formation and remodeling of blood vessels, regulation of blood flow, regulation of blood fluidity, maintenance of permselectivity, recruitment of inflammatory leukocytes, and antigen presentation leading to activation of T cells. T cell interactions with vascular EC are thus bidirectional and link the immune and circulatory systems.

Differences in Potency of Intravenous Polyspecific Immunoglobulin G against Streptococcal and Staphylococcal Superantigens: Implications for Therapy of Toxic Shock Syndrome

Administration of intravenous polyspecific immunoglobulin G (IVIG) has been proposed as adjunctive therapy for toxic shock syndrome caused by Streptococcus pyogenes or Staphylococcus aureus. We investigated whether superantigen-containing culture supernatants prepared from streptococcal isolates (n=21) and staphylococcal isolates (n=20) from cases of severe sepsis were inhibited to an equal extent by IVIG in proliferation experiments that used human peripheral blood mononuclear cells. All 3 IVIG preparations tested were highly efficient in neutralizing the superantigens, and most supernatants were completely inhibited at concentrations ranging from 0.05 to 2.5 mg IVIG/mL. An important finding was that culture supernatants from S. pyogenes isolates were consistently inhibited to a greater extent than those of S. aureus isolates (P<.01). The findings demonstrate that staphylococcal superantigens are not inhibited as efficiently as streptococcal superantigens by IVIG, and, hence, a higher dose of IVIG may be required for therapy of staphylococcal toxic shock syndrome in order to achieve protective titers and clinical efficacy.

Crystallization and preliminary crystallographic analysis of Mycoplasma arthritidis-derived mitogen complexed with peptide/MHC class II antigen

Mycoplasma arthritidis-derived mitogen (MAM), a bacterial superantigen, has been crystallized in complex with its human receptor, major histocompatibility complex (MHC) class II antigen, by the hanging-drop vapor-diffusion method. Crystals were obtained under three conditions, with ammonium sulfate, phosphate salt and PEG 8000 as the precipitant. The crystals grown under these conditions all belong to space group I222, with the same unit-cell parameters: a = 137.4, b = 178.2, c = 179.6 A. Diffraction data were collected to 3.3 and 3.4 A resolution from crystals of native and selenomethionylated MAM-MHC complexes, respectively. Self- and cross-rotation function calculations suggest the presence of two complex molecules in the asymmetric unit, resulting in a V(M) of 4.0 and a solvent content of 69%. An interpretable electron-density map was produced using a combination of molecular replacement and SAD phasing.

Design of Chimeric Receptor Mimics with Different TcRVβ Isoforms

The Staphylococcus aureus enterotoxins (S.E.) A-I, and toxic-shock syndrome toxin TSST-1 act as superantigens to cause overstimulation of the host immune system, leading to the onset of various diseases including food poisoning and toxic shock syndrome. SAgs bind as intact proteins to the DRalpha1 domain of the MHC class II receptor and the TcRVbeta domain from the T cell receptor and cause excessive release of cytokines such as IL-2, TNF-alpha, and IFN-gamma, and hyperproliferation of T cells. In addition, different SAgs bind and activate different TcRVbeta isoforms during pathogenesis of human immune cells. These two properties of SAgs prompted us to design several chimeric DRalpha1-linker-TcRVbeta proteins using different TcRVbeta isoforms to create chimeras that would specifically inhibit the pathogenesis of SAgs against which they were designed. In this study, we compare the design, interaction, and inhibitory properties of three different DRalpha1-linker-TcRVbeta chimeras targeted against three different SAgs, SEB, SEC3, and TSST-1. The inhibitory properties of the chimeras were tested by monitoring IL-2 release and T cell proliferation using a primary human cell model. We demonstrate that the three chimeras specifically inhibit the pathogenesis of their target superantigen. We performed molecular modeling to analyze the structural basis of the type specificity exhibited by different chimeras designed against their target SAgs, examine the role of the linker in determining binding and specificity, and suggest site-specific mutations in the chimera to enhance binding affinity. The fact that our strategy works equally well for SEB and TSST-1, two widely different phylogenic variants, suggests that the DRalpha1-linker-TcRVbeta chimeras may be developed as a general therapy against a broad spectrum of superantigens released during Staphylococcal infection.

Identification of the antigenic epitopes in staphylococcal enterotoxins A and E and design of a superantigen for human cancer therapy

Monoclonal antibodies have a potential for cancer therapy that may be further improved by linking them to effector molecules such as superantigens. Tumor targeting of a superantigen leads to a powerful T cell attack against the tumour tissue. Encouraging results have been observed preclinically and in patients using the superantigen staphylococcal enterotoxin A, SEA. To further improve the concept, we have reduced the reactivity to antibodies against superantigens, which is found in all individuals. Using epitope mapping, antibody binding sites in SEA and SEE were found around their MHC class II binding sites. These epitopes were removed genetically and a large number of synthetic superantigens were produced in an iterative engineering procedure. Properties such as decreased binding to anti-SEA as well as higher selectivity to induce killing of tumour cells compared to MHC class II expressing cells, were sequentially improved. The lysine residues 79, 81, 83 and 84 are all part of major antigenic epitopes, Gln204, Lys74, Asp75 and Asn78 are important for optimal killing of tumour cells while Asp45 affects binding to MHC class II. The production properties were optimised by further engineering and a novel synthetic superantigen, SEA/E-120, was designed. It is recognised by approximately 15% of human anti-SEA antibodies and have more potent tumour cell killing properties than SEA. SEA/E-120 is likely to have a low toxicity due to its reduced capacity to mediate killing of MHC class II expressing cells. It is produced as a Fab fusion protein at approximately 35 mg/l in Escherichia coli.

Colonic bacterial superantigens evoke an inflammatory response and exaggerate disease in mice recovering from colitis

BACKGROUND & AIMS

There is renewed interest in commensal bacteria as triggers of idiopathic disease, a concept that is prominent in inflammatory bowel disease (IBD). Here the effect of intracolonic instillation of Staphylococcus aureus enterotoxin B (SEB), a model superantigen (SAgs: potent T-cell stimuli), into mice was examined.

METHODS

Mice (Balb/c, severe combined immunodeficient [SCID], V beta 8(+) ovalbumin transgenic [OVA-Tg], interleukin 10 [IL-10] knockout [KO]) received a single intrarectal (IR) dose of SAg and colonic form (histology, myeloperoxidase [MPO] activity) and function (ion transport) were assessed 12-72 hours later. In subsequent studies the potential for SEB to reactivate disease in mice recovering from dextran sodium sulfate (DSS)-induced colitis (5 days at 4% [wt/vol] followed by 14 days normal water) was examined.

RESULTS

SEB-treated Balb/c mice displayed a time- and dose-dependent colonic inflammation (increased MPO, histologic damage score, and macrophage number). Similar events occurred in response to other SAgs, namely S. aureus enterotoxin A (SEA) and Yersinia pseudotuberculosis mitogen. Ion transport, the driving force for water movement, was unaffected by SEB treatment. SCID mice developed no inflammation after IR SEB delivery, whereas OVA Tg mice displayed enhanced responsiveness. Although SEB treatment of IL-10 KO mice did elicit a response, the inflammation was transitory and did not hasten the spontaneous colitis seen in these mice. Finally, mice recovering from DSS-induced colitis showed a worsening of the disease when challenged with SEB; IR SEB evoked significant increases in MPO, macrophage infiltration, T-cell activation (i.e., CD25 expression), and perturbed epithelial ion transport.

CONCLUSIONS

Lumen-derived bacterial SAgs can elicit a local inflammation and aggravate enteric inflammatory disorders in which they were not the causative agent.

Early activation events differentiate the reactivity of two T-cell families to Staphylococcus enterotoxin A

Analysis of early activation events in two SEA responsive T-cell families demonstrated that low doses of SEA induced CD4+Vbeta22 T-cells to down-regulate their TCR and express CD69, considerably earlier than CD4+Vbeta5 T-cells. The rapid down-regulation of Vbeta22 TCR led to its proliferation, whereas even a 10-fold higher dose of toxin induced only a partial down-regulation of Vbeta5 TCR. Stimulation with SEA induced a significantly higher percentage of Vbeta22 T-cells to produce IFN-gamma compared to Vbeta5 T-cells. SEAF47A, a mutant of SEA, known to have a lower binding affinity for the MHC class II molecule, failed to activate Vbeta5 T-cells whereas Vbeta22 T-cell activation was slightly decreased. Hence, early activation events highlighted the differential requirements of T-cell families to respond to SEA.

Generation of protective immunity by inactivated recombinant staphylococcal enterotoxin B vaccine in nonhuman primates and identification of correlates of immunity

At this time there are no vaccines or therapeutics to protect against staphylococcal enterotoxin B (SEB) exposure. Here, we report vaccine efficacy of an attenuated SEB in a nonhuman primate model following lethal aerosol challenge and identify several biomarkers of protective immunity. Initial in vitro results indicated that the mutation of key amino acid residues in the major histocompatibility complex (MHC) class II binding sites of SEB produced a nontoxic form of SEB, which had little to no detectable binding to MHC class II molecules, and lacked T-cell stimulatory activities. When examined in a mouse model, we found that the attenuated SEB retained antigenic structures and elicited protective immune responses against wild-type SEB challenge. Subsequently, a vaccine regimen against SEB in a nonhuman primate model was partially optimized, and investigations of immune biomarkers as indicators of protection were performed. SEB-naïve rhesus monkeys were vaccinated two or three times with 5 or 20 microg of the attenuated SEB and challenged by aerosol with wild-type SEB toxin. Unlike exposure to the native toxin, the vaccine did not trigger the release of inflammatory cytokines (TNF alpha, IL6, or IFN gamma). All rhesus monkeys that developed anti-SEB serum titers > or = 10(4) and elicited high levels of neutralizing antibody survived the aerosol challenge. These findings suggest that the attenuated SEB is fully protective against aerosolized toxin when administered to unprimed subjects. Moreover, experiments presented in this study identified various biomarkers that showed substantial promise as correlates of immunity and surrogate endpoints for assessing in vivo biological responses in primates, and possibly in humans, to vaccines against SEs.

Staphylococcal enterotoxin H induces V alpha-specific expansion of T cells

Staphylococcal enterotoxin H (SEH) is a bacterial superantigen secreted by Staphylococcus aureus. Superantigens are presented on the MHC class II and activate large amounts of T cells by cross-linking APC and T cells. In this study, RT-PCR was used to show that SEH stimulates human T cells via the Valpha domain of TCR, in particular Valpha10 (TRAV27), while no TCR Vbeta-specific expansion was seen. This is in sharp contrast to all other studied bacterial superantigens, which are highly specific for TCR Vbeta. It was further confirmed by flow cytometry that SEH stimulation does not alter the levels of certain TCR Vbeta. In a functional assay addressing cross-reactivity, Vbeta binding superantigens were found to form one group, whereas SEH has different properties that fit well with Valpha reactivity. As SEH binds on top of MHC class II, an interaction between MHC and TCR upon SEH binding is not likely. This concludes that the specific expansion of TCR Valpha is not due to contacts between MHC and TCR, instead we suggest that SEH directly interacts with the TCR Valpha domain.

Staphylococcal enterotoxin B induces anergy to conventional peptide in memory T cells

Microbial superantigens can alter host immunity through aberrant activation and subsequent anergy of responding naive T cells. We show here that the superantigen, staphylococcal enterotoxin B (SEB), directly induces tolerance in memory CD4 T cells. Murine naive and memory CD4(+) T cells were labeled with the fluorescent dye CFSE and the cells were exposed to SEB before they were cultured with specific peptide antigen. Memory, but not naive, T cells became anergic and did not respond to their cognate peptide antigen. The extent and duration of T cell receptor (TCR) clustering was similar to promote naive T cell activation and memory T cell anergy, suggesting similar TCR-SEB interactions led to distinct intracellular signaling processes in the two cell types. Like SEB, soluble anti-CD3 mAb does not stimulate memory cell proliferation. However, unlike SEB, soluble anti-CD3 mAbs did not induce anergy to cognate peptide. Anergy was directly visualized in vivo. CD4(+) memory T cells were identified in mice that had been administered SEB. The cells failed to proliferate in response to subsequent immunization with their cognate recall antigen. Hence, one mode of pathogen survival is the modulation of host immunity through selective elimination of memory T cell responses.

T cell Vbeta repertoires in childhood vasculitides

Superantigens (SAgs) are potent stimulators of T cells bearing specific Vbeta T cell receptors (TCR) and may play a role in the aetiopathogenesis of systemic vasculitis, although this remains contentious. To investigate the possible aetiological role of SAgs, this study examined peripheral blood T cell Vbeta repertoires in children with systemic vasculitis. FACS analysis of 17 different peripheral blood T cell Vbeta families was performed in 20 healthy control children, 27 disease control children with nonvasculitic inflammatory disease, 25 children with primary systemic vasculitis, six patients with Kawasaki disease (KD) and six patients with Henoch-Schönlein purpura (HSP). There was a significantly increased variance of CD4 Vbeta12 and Vbeta17, and CD8 Vbeta1 in the primary systemic vasculitis group compared to control and disease controls. Moreover, 80% of the primary systemic vasculitis children had one or more CD4 Vbeta expansions or deletions, compared with 30% of controls (P < 0.002), and 37% of the disease controls (P < 0.002). In the KD group, the mean percentage of CD4 Vbeta2 T cells was higher than in controls or disease controls. In the HSP group, there was no consistent skewing of the T cell Vbeta repertoire. We have observed changes in the T cell Vbeta repertoire in children with vasculitis over and above those observed in disease controls. While these data provide impetus for further research into this contentious field, they do not resolve unequivocally the question of the role of SAgs in childhood vasculitic syndromes.

Streptococcus dysgalactiae-derived mitogen (SDM), a novel bacterial superantigen: characterization of its biological activity and predicted tertiary structure

A mitogenic substance, designated Streptococcus dysgalactiae-derived mitogen (SDM), was purified from S. dysgalactiae culture supernatant, and the gene encoding the mitogen was cloned. Both native and recombinant SDM expressed in Escherichia coli significantly activated human V beta 1+ and V beta 23+ T cells in association with major histocompatibility complex (MHC) class II molecules on accessory cells, indicating that SDM possesses superantigenic properties. The sdm gene consists of two segments encoding a signal peptide and a mature 25 kDa protein composed of 212 amino acids. Three of 34 S. dysgalactiae strains but none of 28 Streptococcus pyogenes strains examined carried sdm. Phylogenetic analysis indicated that SDM belongs to a family distinct from established bacterial superantigens. SDM showed around 30% homology with other superantigens at the amino acid sequence level. The tertiary structure of SDM was predicted by modelling onto streptococcal pyrogenic exotoxin C and streptococcal mitogenic exotoxin Z-2, both of which share highly homologous structure-determining regions. SDM showed overall structural similarity to both these superantigens. This is the first study to characterize fully a bacterial superantigen from S. dysgalactiae.

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.

Therapeutic approaches to superantigen-based diseases: a review

Superantigens secreted by the bacterial pathogen Staphyloccocus aureus are extremely potent toxins that overstimulate the host immune system by binding to the MHC class II and T cell receptors and activating a large population of T cells. Superantigen infection has been shown to be the causative agents in acute diseases, food poisoning and toxic shock syndrome, and in more chronic conditions such as inflammatory skin diseases. In addition to the toll on public health, S. aureus superantigens also represent a potential biothreat to our national security. To address these risks, a number of different therapeutic strategies have been developed that target different aspects of the pathogenic mechanism of S. aureus and superantigen infection. These therapies, which encompass strategies as diverse as production of neutralizing antibodies, inhibitory peptide/receptor design and blockage of superantigen gene transcription, are being tested for treatment of established S. aureus infections in pre- and post-exposure scenarios. In this review, we will describe these different strategies and their efficacies in inhibition of superantigen-induced effects in the host, and present the future outlook for successfully producing therapies for superantigen-based disease.

Crystal structure of a SEA variant in complex with MHC class II reveals the ability of SEA to crosslink MHC molecules

Although the biological properties of staphylococcal enterotoxin A (SEA) have been well characterized, structural insights into the interaction between SEA and major histocompatibilty complex (MHC) class II have only been obtained by modeling. Here, the crystal structure of the D227A variant of SEA in complex with human MHC class II has been determined by X-ray crystallography. SEA(D227A) exclusively binds with its N-terminal domain to the alpha chain of HLA-DR1. The ability of one SEA molecule to crosslink two MHC molecules was modeled. It shows that this SEA molecule cannot interact with the T cell receptor (TCR) while a second SEA molecule interacts with MHC. Because of its relatively low toxicity, the D227A variant of SEA is used in tumor therapy.

Characterization of two novel pyrogenic toxin superantigens made by an acute rheumatic fever clone of Streptococcus pyogenes associated with multiple disease outbreaks

The pathogenesis of acute rheumatic fever (ARF) is poorly understood. We identified two contiguous bacteriophage genes, designated speL and speM, encoding novel inferred superantigens in the genome sequence of an ARF strain of serotype M18 group A streptococcus (GAS). speL and speM were located at the same genomic site in 33 serotype M18 isolates, and no nucleotide sequence diversity was observed in the 33 strains analyzed. Furthermore, the genes were absent in 13 non-M18 strains tested. These data indicate a recent acquisition event by a distinct clone of serotype M18 GAS. speL and speM were transcribed in vitro and upregulated in the exponential phase of growth. Purified SpeL and SpeM were pyrogenic and mitogenic for rabbit splenocytes and human peripheral blood mononuclear cells in picogram amounts. SpeL preferentially expanded human T cells expressing T-cell receptors Vbeta1, Vbeta5.1, and Vbeta23, and SpeM had specificity for Vbeta1 and Vbeta23 subsets, indicating that both proteins had superantigen activity. SpeL was lethal in two animal models of streptococcal toxic shock, and SpeM was lethal in one model. Serologic studies indicated that ARF patients were exposed to serotype M18 GAS, SpeL, and SpeM. The data demonstrate that SpeL and SpeM are pyrogenic toxin superantigens and suggest that they may participate in the host-pathogen interactions in some ARF patients.

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.

Induction of tolerance in autoimmune diseases by hematopoietic stem cell transplantation: getting closer to a cure?

Hematopoietic stem cells (HSCs) are the earliest cells of the immune system, giving rise to B and T lymphocytes, monocytes, tissue macrophages, and dendritic cells. In animal models, adoptive transfer of HSCs, depending on circumstances, may cause, prevent, or cure autoimmune diseases. Clinical trials have reported early remission of otherwise refractory autoimmune disorders after either autologous or allogeneic hematopoietic stem cell transplantation (HSCT). By percentage of transplantations performed, autoimmune diseases are the most rapidly expanding indication for stem cell transplantation. Although numerous editorials or commentaries have been previously published, no prior review has focused on the immunology of transplantation tolerance or development of phase 3 autoimmune HSCT trials. Results from current trials suggest that mobilization of HSCs, conditioning regimen, eligibility and exclusion criteria, toxicity, outcome, source of stem cells, and posttransplantation follow-up need to be disease specific. HSCT-induced remission of an autoimmune disease allows for a prospective analysis of events involved in immune tolerance not available in cross-sectional studies.

Protection against bacterial superantigen staphylococcal enterotoxin B by passive vaccination

We investigated the ability of two overlapping fragments of staphylococcal enterotoxin B (SEB), which encompass the whole toxin, to induce protection and also examined if passive transfer of chicken anti-SEB antibodies raised against the holotoxin could protect rhesus monkeys against aerosolized SEB. Although both fragments of SEB were highly immunogenic, the fragments failed to protect mice whether they were injected separately or injected together. Passive transfer of antibody generated in chickens (immunoglobulin Y [IgY]) against the whole toxin suppressed cytokine responses and was protective in mice. All rhesus monkeys treated with the IgY specific for SEB up to 4 h after challenge survived lethal SEB aerosol exposure. These findings suggest that large fragments of SEB may not be ideal for productive vaccination, but passive transfer of SEB-specific antibodies protects nonhuman primates against lethal aerosol challenge. Thus, antibodies raised in chickens against the holotoxin may have potential therapeutic value within a therapeutic window of opportunity after SEB encounter.

T cell-dependent maturation of dendritic cells in response to bacterial superantigens

Dendritic cells (DC) express a set of germline-encoded transmembrane Toll-like receptors that recognize shared microbial products, such as Escherichia coli LPS, termed pathogen-associated molecular patterns. Analysis of the in vivo response to pathogen-associated molecular patterns has uncovered their ability to induce the migration and the maturation of DC, favoring thus the delivery of Ag and costimulatory signals to naive T cells in vivo. Bacterial superantigens constitute a particular class of pathogen-derived molecules known to induce a potent inflammatory response in vivo, secondary to the activation of a large repertoire of T cells. We demonstrate in this work that Staphylococcal superantigens induce migration and maturation of DC populations in vivo. However, in contrast to LPS, superantigens failed to induce DC maturation in RAG or MHC class II-deficient mice, suggesting that T cell activation was a prerequisite for DC maturation. This conclusion was further supported by the finding that T cell activation induced by 1) mitogenic anti-CD3 mAbs, 2) allo-MHC determinants, or 3) nominal Ag in a TCR-transgenic model induces DC maturation in vivo. These studies also revealed that DC that matured in response to T cell mitogens display, comparatively to LPS, a distinctive phenotype characterized by high expression of the MHC class II, CD40, and CD205 markers, but only moderate (CD86) to minimal (CD80) expression of CD28/CTLA4 ligands. This work demonstrates that activation of a sufficient number of naive T cells in vivo constitutes a novel form of immune danger, functionally linked to DC maturation.

Staphylococcal enterotoxin H contrasts closely related enterotoxins in species reactivity

Staphylococcus aureus enterotoxin H (SEH) belongs to the staphylococcal enterotoxin (SE) family of superantigens (SAgs). SEH has structural similarities to other SE; however, its biological properties are less well characterized. SEH binds with high affinity to human major histocompatibility complex (MHC) class II and exhibits strong mitogenic activity in human T cells, although it was found to be less potent than the related SEA. Surprisingly and in sharp contrast to related SEs, SEH did not possess superantigen activity in murine T cells and T cells from three investigated rat strains. However, SEH bound to a high extent to murine MHC class II expressing cells and when presented by these cells SEH stimulated human T cells to proliferate. Thus, SEH interacts with the murine MHC class II molecule in a functional manner. Notably, SEH had an inhibitory effect on murine SEA response, demonstrating that SEH interferes with the SEA interactions with murine cells. Despite this, murine T cells did not proliferate regardless of whether SEH was presented on human or murine MHC class II expressing cells. Consequently, SEH differs in species reactivity as compared to related SEs and lacks critical properties for T-cell activation in mice. We propose that unlike other SEs, SEH does not interact with murine T cells since it is not recognized by murine T-cell receptors.