Superantigens: mechanism of T-cell stimulation and role in immune responses

The crystal structures of EAP domains from Staphylococcus aureus reveal an unexpected homology to bacterial superantigens

The Eap (extracellular adherence protein) of Staphylococcus aureus functions as a secreted virulence factor by mediating interactions between the bacterial cell surface and several extracellular host proteins. Eap proteins from different Staphylococcal strains consist of four to six tandem repeats of a structurally uncharacterized domain (EAP domain). We have determined the three-dimensional structures of three different EAP domains to 1.8, 2.2, and 1.35 A resolution, respectively. These structures reveal a core fold that is comprised of an alpha-helix lying diagonally across a five-stranded, mixed beta-sheet. Comparison of EAP domains with known structures reveals an unexpected homology with the C-terminal domain of bacterial superantigens. Examination of the structure of the superantigen SEC2 bound to the beta-chain of a T-cell receptor suggests a possible ligand-binding site within the EAP domain (Fields, B. A., Malchiodi, E. L., Li, H., Ysern, X., Stauffacher, C. V., Schlievert, P. M., Karjalainen, K., and Mariuzza, R. (1996) Nature 384, 188-192). These results provide the first structural characterization of EAP domains, relate EAP domains to a large class of bacterial toxins, and will guide the design of future experiments to analyze EAP domain structure/function relationships.

Staphylococcus aureus cell envelope antigen is a new candidate for the induction of IgA nephropathy

BACKGROUND

IgA nephropathy is the most common form of glomerulonephritis worldwide. We previously reported a novel form of glomerulonephritis with glomerular IgA deposits following methicillin-resistant Staphylococcus aureus (S. aureus) infection. We investigated the role of S. aureus related antigens in the immunopathogenesis of IgA nephropathy by producing several monoclonal antibodies against S. aureus surface antigens and determining the epitopes of deposited antigens in patients with IgA nephropathy.

METHODS

Cell membrane proteins were isolated from cultured S. aureus. Mouse monoclonal antibodies against these proteins were generated, and their target epitopes were determined by antibody affinity chromatography and amino acid sequence analysis, and by monoclonal antibody screening of Escherichia coli clones transfected with plasmids from the Lambda S. aureus Genomic Library. Renal biopsy specimens from 116 patients with IgA nephropathy and 122 patients with other forms of renal disease were examined for glomerular antigen depositions by immunofluorescence microscopy.

RESULTS

. The major antigen recognized by monoclonal antibodies against S. aureus cell membrane was identified as the S. aureus cell envelope antigen designated 'probable adhesin' (ACCESSION AP003131-77, Protein ID; BAB41819.1). In 68.1% (79/116) of renal biopsy specimens from patients with IgA nephropathy, S. aureus cell envelope antigen was localized in the glomeruli, and the data confirmed that S. aureus cell envelope antigen was co-localized with IgA antibody in the glomeruli. No deposition of this antigen was detected in the glomeruli of patients with non-immune complex deposit forms of glomerulonephritis.

CONCLUSION

S. aureus cell envelope antigen is a new candidate for the induction of IgA nephropathy.

Peptide antagonists of superantigen toxins

Superantigens produced by Staphylococcus aureus and Streptococcus pyogenes are among the most lethal of toxins. Toxins in this large family trigger an excessive cellular immune response leading to toxic shock. Superantigens are secreted by the bacteria as diverse natural mixtures, a complexity that demands development of broad-spectrum countermeasures. We used a rational approach to design short peptides with homology to various domains in a typical superantigen (staphylococcal enterotoxin B) and screened each peptide for its ability to antagonize, in human peripheral blood mononuclear cells, superantigen-mediated induction of the genes encoding T helper 1 cytokines that mediate shock: interleukin-2, interferon-gamma and tumor necrosis factor. A dodecamer peptide proved a potent antagonist against widely different superantigens. This peptide protected mice from killing by superantigens and it was able to rescue mice undergoing toxic shock. The antagonist peptide shows homology to a beta-strand-hinge-alpha-helix domain that is structurally conserved among superantigens, yet currently of unknown function and remote from the binding sites for the known ligands essential for T cell activation, the major histocompatibility complex class II molecule and T cell receptor. The antagonist activity of this peptide thus identifies a novel domain in superantigens that is critical for their toxic action. The antagonist peptide provides a new tool for understanding the mechanism of excessive human immune response activation by superantigens that occurs during toxic shock and for identification of a novel target ligand that may interact with this superantigen domain.

Role of anti-CD3 in modulation of Th1-type immune response in Shigella dysenteriae infection

A murine model was used to evaluate the role of anti-CD3 in modulating a Th1-type response by restimulation of T-cells after immunization with the 57 kDa immunodominant antigen of Shigella dysenteriae 1 outer-membrane proteins (OMPs), followed by Shigella infection after immunization. To observe the effect of anti-CD3, other T-cell cultures were also established following anti-CD1, anti-IL2 and phytohaemagglutinin stimulation. Anti-CD3 stimulation of reconstituted T-cells showed 'mean' levels of CD4 and CD25 were enhanced by 34.5 and 31.1 % in immunized mice, which was comparable to 53.2 and 50.7 %, respectively, in challenged-immunized mice, and were dominant over CD8+ T-cells. Levels of IL2 generated by anti-CD3-stimulated T-cells of immunized mice were greater than those of unstimulated T-cells and were significantly elevated in challenged-immunized mice. The reactivity of T-cells indicated their complete responsiveness, as anti-CD3 antibody might not inhibit the migration of the macrophages but rather inhibit IL4. These macrophage factors synergistically act with anions towards an activated response, which in turn provokes IL2 secretion with a low degree of internalization of its receptor. Thus, sharing of IL2 to form a high-affinity receptor complex with CD4+ T-cells through motive signals suggested a generalized T-cell activation with increased humoral responses. Macrophage migration inhibition factor (MIF) and IL4 responses during anti-CD3 stimulation of immunized mice indicated that the role of anti-CD3 in generation of O2- is due to a synergistic effect by Th1 subsets of Th0 cells. The above findings should have implications for understanding the immunoregulatory role of anti-CD3 associated with 57 kDa antigen in immunoprophylactic measures.

Ikaros null mice display defects in T cell selection and CD4 versus CD8 lineage decisions

Previous evidence suggested that the hemopoietic-specific nuclear factor Ikaros regulates TCR signaling thresholds in mature T cells. In this study, we test the hypothesis that Ikaros also sets TCR signaling thresholds to regulate selection events and CD4 vs CD8 lineage determination in developing thymocytes. Ikaros null mice were crossed to three lines of TCR-transgenic mice, and positive selection, negative selection, and CD4 vs CD8 lineage decisions were analyzed. Mice expressing a polyclonal repertoire or a MHC class II-restricted TCR transgene exhibited enhanced positive selection toward the CD4 lineage. Moreover, in the absence of Ikaros, CD4 development can occur with decreased thresholds of TCR signaling. In addition, CD4 single-positive thymocytes were detected in MHC class I-restricted TCR-transgenic Ikaros null mice. To assess the role of Ikaros in negative selection, we analyzed deletion of T cells induced by conventional Ag or by endogenous superantigen. Surprisingly, negative selection was impaired in Ikaros null thymocytes despite evidence of high levels of TCR signal and no intrinsic defect in apoptosis ex vivo. To our knowledge, these data identify Ikaros as the first nuclear factor that plays a critical role in regulating negative selection as well as CD4 vs CD8 lineage decisions during positive selection.

Potential mechanisms of interferon-alpha induced autoimmunity

The type I interferons (IFN) are cytokines encoded by a multigene family comprising 13 closely related IFN-A genes, and a single IFN-B gene. These factors are rapidly induced upon viral infection, and have pleiotropic effects. Historically, the induction of a cell-autonomous state of antiviral resistance, the inhibition of cell growth, and the regulation of apoptosis were appreciated first. More recently, it became generally accepted that they can regulate immune effector functions. This latter feature led them to be reconsidered as signals linking innate and adaptive immunity, and potentially orchestrating autoimmunity associated with viral infection and IFN-alpha therapy. Common to almost all autoimmune diseases is their polygenic inheritance, incomplete penetrance, and evidence for the role of environmental factors, particularly viral infection. In addition, they are characterized by increased numbers of circulating autoreactive T- and B-cells. Endogenously produced or therapeutically applied IFN-alpha can tilt the usually tightly controlled balance towards activation of these autoreactive cells via a vast array of mechanisms. The genetic susceptibility factors determine which type of autoimmunity will develop. IFN-alpha induces numerous target genes in antigen presenting cells (APC), such that APC are stimulated and enhance humoral autoimmunity, promote isotype switching, and potently activate autoreactive T cells. Moreover, IFN-alpha can synergistically amplify T cell autoreactivity by directly promoting T cell activation and keeping activated T cells alive. In essence, type I IFNs may constitute one example of genes that have been conserved because they confer dominant disease resistance, but at the same time they can trigger autoimmunity in genetically susceptible individuals.

Role of 57 kDa major antigenic component of Shigella dysenteriae outer membrane proteins in induction of major histocompatibility complex II-restricted T-cell response

BACKGROUND

In the past, many Shigella surface antigens were used to activate both T and B lymphocytes but failed to induce antigen-specific responses in Shigellosis. Our objective was to identify in vitro T-cell components using 57 kDa major antigenic fraction of Shigella dysenteriae 1 (IPC-31) outer membrane proteins (OMPs) in modulating specific T-cell subset responses against Shigellosis.

METHODS

Antigen-specific T- and B-cell activation was studied in immunized Balb/c mice against 57 kDa antigen by proliferative responses using [3H]-thymidine incorporation and avidin-biotin complex (ABC) peroxidase staining for CD4, CD8, CD3, CD22, and CD25 followed by IL-2 and IL-4 estimation. Macrophage functional assays for migration inhibition factors (MIF) and superoxide (O2-) anions were also performed against 57 kDa antigen, whole OMPs, and phytohemagglutinin (PHA) stimulation.

RESULTS

Greater increase of lymphocyte proliferation was observed after 57 kDa antigen stimulation than post-OMP and -PHA stimulation. Proportionately, CD4+ and CD25+ expression of total CD3+ T-cells was significantly dominant (p >0.05) over CD8+ T-cells. On day 7 of this stimulation, it was found to increase % MIF and O2- anions with decrease of IL-2 leading to activation of MHC-II antigens. Later, on day 28 of immunization, IL-2 levels were more increased than on days 7 and 14 but insignificant with non-immunized mice stimulated with 57 kDa. Levels of IL-2 were also noted with low degree of internalization to its IL-2R receptors rather than to IL-4 receptors. In parallel, expression of CD22 was also recorded higher in this stimulation than in PHA, indicating a T-cell-dependent humoral response.

CONCLUSIONS

Our results suggested that 57 kDa major antigenic OMP is immunogenic for MHC II-restricted T-cell response to acquire host defense against Shigella infection.

Structural relationships and cellular tropism of staphylococcal superantigen-like proteins

The staphylococcal superantigen-like proteins (SSLs) are a family of polymorphic paralogs encoded in the Staphylococcus aureus genome whose function is unknown. The crystal structure of SSL7 was determined and compared to that of SSL5 and that of a classical superantigen, streptococcal pyrogenic exotoxin. Although the overall architecture of the superantigen family is retained in both SSL7 and SSL5, there are significant differences in the structures which suggest that the characteristic major histocompatibility complex binding site of superantigens has been lost. To complement these data, the abilities of SSL7 and a closely related paralog, SSL9, to interact with cells of the immune system were investigated. In populations of human white blood cells, both SSLs interacted selectively with monocytes via specific saturable but separate binding sites, which led to rapid uptake of the SSLs. In addition, SSLs were rapidly taken up by dendritic cells, but not by macrophages, into the same endosomal compartment as dextran. The ability of these secreted proteins to target antigen-presenting cells may enhance a misplaced antibody response against the proteins, which may facilitate bacterial colonization rather than contribute to host protection. Like classical superantigens, therefore, SSLs may distract the host's immune system, but they may do so via entirely different molecular mechanisms.

Stimulation of the immune system by different TBE-virus vaccines

Tick-borne encephalitis (TBE) is one of the most frequent arthropod-transmitted viral diseases in Europe. Different vaccines against TBE-virus have been developed; a thimerosal-free and also albumin-free vaccine [Ticovac (Baxter Hyland Immuno, Vienna)] was approved in 2000. Contrary to previous experience, 779 cases of fever occurred following the first vaccination of children under 15 years of age and in 62 children febrile convulsions were even observed. Consequently, the composition of the vaccine was changed and albumin was again added [FSME-Immun (Baxter Vaccines, Vienna)] in 2001. The new Encepur Kinder (Chiron-Behring, Marburg) from 2002 is a TBE-vaccine for children without any protein as stabilizer but with a relatively high concentration of sucrose, while the former vaccine Encepur K from 1991 contained polygeline as the stabilizer. The induction of the immune system by the different TBE virus vaccines was compared in an in vitro test in order to find an explanation for the unexpected fever attacks. Whole blood was stimulated with complete vaccine suspension, and TNF-alpha, IL-1beta, IL-6, and IL-8 were determined from heparin/EDTA-plasma and culture supernatants. It was shown that Ticovac and the new Encepur Kinder can induce relatively high amounts of TNF-alpha and lower amounts of IL-1beta. An increase of both cytokines was first observed following an incubation of 4 hours, with a maximum after 15 hours. Concentrations returned to base-line values within 26 hours. The behaviour of both cytokines correlates with the febrile phases in children up to two years old. Albumin or other proteins like polygeline and also immunoglobulins prevented a rise of cytokines.

Epstein-Barr virus latent membrane protein LMP-2A is sufficient for transactivation of the human endogenous retrovirus HERV-K18 superantigen

Superantigens are microbial proteins that strongly stimulate T cells. We described previously that the Epstein-Barr virus (EBV) transactivates a superantigen encoded by the human endogenous retrovirus, HERV-K18. We now report that the transactivation is dependent upon the EBV latent cycle proteins. Moreover, LMP-2A is sufficient for induction of HERV-K18 superantigen activity.

Superantigens: structure-function relationships

Superantigens are a class of highly potent immuno-stimulatory molecules produced by Staphylococcus aureus and Streptococcus pyogenes. These toxins possess the unique ability to interact simultaneously with MHC class II molecules and T-cell receptors, forming a trimolecular complex that induces profound T-cell proliferation. The resultant massive cytokine release causes epithelial damage and leads to capillary leak and hypotension. The staphylococcal superantigens are designated staphylococcal enterotoxins A, B, C (and antigenic variants), D, E, and the recently discovered enterotoxins G to Q, and toxic shock syndrome toxin-1. The streptococcal superantigens include the pyrogenic exotoxins A (and antigenic variants), C, G-J, SMEZ, and SSA. Superantigens are implicated in several diseases including toxic shock syndrome, scarlet fever and food poisoning; and their function appears primarily to debilitate the host sufficiently to permit the causation of disease. Structural studies over the last 10 years have provided a great deal of information regarding the complex interactions of these molecules with their receptors. This, combined with the wealth of new information from genomics initiatives, have shown that, despite their common molecular architecture, superantigens are able to crosslink MHC class II molecules and T-cell receptors by a variety of subtly different ways through the use of various structural regions within each toxin.

Deficiency of CD11b or CD11d Results in Reduced Staphylococcal Enterotoxin-Induced T Cell Response and T Cell Phenotypic Changes

The beta(2) integrin CD11a is involved in T cell-APC interactions, but the roles of CD11b, CD11c, and CD11d in such interactions have not been examined. To evaluate the roles of each CD11/CD18 integrin in T cell-APC interactions, we tested the ability of splenocytes of CD11-knockout (KO) mice to respond to staphylococcal enterotoxins (SEs), a commonly used superantigen. The defect in T cell proliferation with SEA was more severe in splenocytes from mice deficient in CD18, CD11b, or CD11d than in CD11a-deficient splenocytes, with a normal response in CD11c-deficient splenocytes. Mixing experiments showed that the defect of both CD11b-KO and CD11d-KO splenocytes was, unexpectedly, in T cells rather than in APC. Cytometric analysis failed to detect CD11b or CD11d on resting or activated T cells or on thymocytes of wild-type adult mice, nor did Abs directed to these integrins block responses in culture, suggesting that T cells educated in CD11b-KO or CD11d-KO mice were phenotypically altered. Consistent with this hypothesis, T cells from CD11b-KO and CD11d-KO splenocytes exhibited reduced intensity of CD3 and CD28 expression and decreased ratios of CD4/CD8 cells, and CD4(+) T cells were reduced among CD11b-KO and CD11d-KO thymocytes. CD11b and CD11d were coexpressed on a subset of early wild-type fetal thymocytes. We postulate that transient thymocyte expression of both CD11b and CD11d is nonredundantly required for normal thymocyte and T cell development, leading to phenotypic changes in T cells that result in the reduced response to SE stimulation.

Simultaneous analysis of multiple staphylococcal enterotoxin genes by an oligonucleotide microarray assay

Staphylococcal enterotoxins (SEs) are a family of 17 major serological types of heat-stable enterotoxins that are one of the leading causes of gastroenteritis resulting from consumption of contaminated food. SEs are considered potential bioweapons. Many Staphylococcus aureus isolates contain multiple SEs. Because of the large number of SEs, serological typing and PCR typing are laborious and time-consuming. Furthermore, serological typing may not always be practical because of antigenic similarities among enterotoxins. We report on a microarray-based one-tube assay for the simultaneous detection and identification (genetic typing) of multiple enterotoxin (ent) genes. The proposed typing method is based on PCR amplification of the target region of the ent genes with degenerate primers, followed by characterization of the PCR products by microchip hybridization with oligonucleotide probes specific for each ent gene. We verified the performance of this method by using several other techniques, including PCR amplification with gene-specific primers, followed by gel electrophoresis or microarray hybridization, and sequencing of the enterotoxin genes. The assay was evaluated by analysis of previously characterized staphylococcal isolates containing 16 ent genes. The microarray assay revealed that some of these isolates contained additional previously undetected ent genes. The use of degenerate primers allows the simultaneous amplification and identification of as many as nine different ent genes in one S. aureus strain. The results of this study demonstrate the usefulness of the oligonucleotide microarray assay for the analysis of multitoxigenic strains, which are common among S. aureus strains, and for the analysis of microbial pathogens in general.

Burn Injury Initiates a Shift in Superantigen-Induced T Cell Responses and Host Survival

Severe injury induces a temporal shift in immune reactivity that can cause serious complications or even death. We previously reported that mice exposed to bacterial superantigen (SAg) early after injury undergo a strong SAg response with lethal consequences. This study compares the early and late effects of burn injury on SAg reactivity in vivo to establish how injury influences adaptive immune responses. We found that mice challenged with ordinarily sublethal doses of staphylococcal enterotoxin A or staphylococcal enterotoxin B at 1 day after burn injury exhibited high mortality, whereas no mortality occurred at 7 days after injury. This shift in mortality correlated with higher Th2-type cytokines (IL-4 and IL-10) being expressed by CD4(+) and CD8(+) T cells from burn as opposed to sham mice at 7 days after injury. Lymph node cells from burn-injured mice also produced higher levels of Th2-type cytokines at 7 days after injury. The results of cell-mixing studies using CD4(+) and CD8(+) T cells mixed with APCs from sham or burn mice suggested that changes in both T cells and APCs are involved in the altered SAg response. Finally, the biological significance of altered SAg reactivity following injury was shown by demonstrating that blocking IL-10 activity in vivo caused higher SAg-induced mortality at 7 days after injury. These findings support the idea that injury promotes a Th2-type shift in adaptive immune reactivity. Although prior studies link this counterinflammatory-type response to lowered resistance to infection, the present results suggest it may sometimes benefit the injured host.

Behavioural endocrine immune-conditioned response is induced by taste and superantigen pairing

Administration of bacterial superantigen, such as staphylococcal enterotoxin B (SEB), induces in vivo stimulation of T cell proliferation and cytokine production such as interleukin-2 (IL-2). It has been previously reported that SEB administration induces fever, c-Fos expression in the brain, and hypothalamus-pituitary-adrenal axis activation, demonstrating that the brain is able to sense and respond to SEB. Previously it had been shown that immune functions can be behaviourally conditioned pairing a novel gustatory stimulus together with an immunomodulatory drug or an antigen. We designed an experimental protocol using Dark Agouti rats in which saccharin taste, as conditioned stimulus, was paired with an i.p. injection of SEB (2 mg/kg), as unconditioned stimulus. Six days later, when conditioned animals were re-exposed to the conditioned stimulus they displayed strong conditioned taste avoidance to the saccharin. More importantly, re-exposure to the conditioned stimulus significantly increased IL-2, interferon-gamma and corticosterone plasma levels, in comparison with conditioned animals which had not been re-exposed to saccharin taste. These results demonstrate a behavioural-immune-endocrine conditioned response using a superantigen as unconditioned stimulus. In addition, they illustrate the brain abilities to mimic the unconditioned effects of a superantigen by yet unknown mechanisms.

Effects of bacterial superantigens on behavior of mice in the elevated plus maze and light-dark box

Bacterial superantigens, such as staphylococcal enteroxins A and B (SEA/SEB) stimulate T cells to produce high levels of cytokines in blood. Previously it had been shown that these toxins were capable of stimulating increased neuroendocrine activity and enhanced behavioral reactivity to novel gustatory and non-gustatory stimuli. Therefore, it was suggested that these superantigens may promote anxiety-like behavior. In the current set of experiments, BALB/cByJ and C57BL/6J male mice were challenged with either SEB (50 microg) or SEA (5 or 10 microg) and tested for behavior in the elevated plus maze (EPM). Results suggested an absence of increased anxiety-like behavior, with exploration of the open arms being enhanced by SEA or SEB treatment. In another test of anxiety, the light-dark box, SEB challenge of BALB/cByJ mice 90 min prior to testing, did not alter exit latency, activity nor time spent in the dark. However, in a second experiment, it was found that if animals were first tested for consumption, followed by testing in the light-dark box, SEB challenged animals displayed increased exit latency and reduced exploration. These studies suggest that in standard tests of rodent anxiety-like behavior, evidence for the induction of anxiety-like processes subsequent to challenge with SEA or SEB is not patently discernable. However, neurobiological events induced by immunological challenge might synergize with reactivity to psychogenic and/or gustatory stimuli, thereby resulting in increased anxiety-like behavior that could be unmasked by standard behavioral tests such as the light-dark box or EPM.

Identification of a secondary zinc-binding site in staphylococcal enterotoxin C2. Implications for superantigen recognition

The previously determined crystal structure of the superantigen staphylococcal enterotoxin C2 (SEC2) showed binding of a single zinc ion located between the N- and C-terminal domains. Here we present the crystal structure of SEC2 determined to 2.0 A resolution in the presence of additional zinc. The structure revealed the presence of a secondary zinc-binding site close to the major histocompatibility complex (MHC)-binding site of the toxin and some 28 A away from the primary zinc-binding site of the toxin found in previous studies. T cell stimulation assays showed that varying the concentration of zinc ions present affected the activity of the toxin and we observed that high zinc concentrations considerably inhibited T cell responses. This indicates that SEC2 may have multiple modes of interaction with the immune system that are dependent on serum zinc levels. The potential role of the secondary zinc-binding site and that of the primary one in the formation of the TCR.SEC2.MHC complex are considered, and the possibility that zinc may regulate the activity of SEC2 as a toxin facilitating different T cell responses is discussed.

Cardiovascular diabetology in the core of a novel interleukins classification: the bad, the good and the aloof

Background

The impressive correlation between cardiovascular disease and glucose metabolism alterations has raised the likelihood that atherosclerosis and type 2 diabetes may share common antecedents. Inflammation is emerging as a conceivable etiologic mechanism for both. Interleukins are regulatory proteins with ability to accelerate or inhibit inflammatory processes.

Presentation of the hypothesis

A novel interleukins classification is described, based on their role in diabetes and atherosclerosis, hypothesizing that each interleukin (IL) acts on both diseases in the same direction – regardless if harmful, favorable or neutral.

Testing the hypothesis

The 29 known interleukins were clustered into three groups: noxious (the "bad", 8 members), comprising IL-1, IL-2, IL-6, IL-7, IL-8, IL-15, IL-17 and IL-18; protective (the "good", 5 members), comprising IL-4, IL-10, IL-11, IL-12 and IL-13; and "aloof", comprising IL-5, IL-9, IL-14, IL-16 and IL-19 through IL-29 (15 members). Each group presented converging effects on both diseases. IL-3 was reluctant to clustering.

Implications

These observations imply that 1) favorable effects of a given IL on either diabetes or atherosclerosis predicts similar effects on the other; 2) equally, harmful IL effects on one disease can be extrapolated to the other; and 3) absence of influence of a given IL on one of these diseases forecasts lack of effects on the other. These facts further support the unifying etiologic theory of both ailments, emphasizing the importance of a cardiovascular diabetologic approach to interleukins for future research. Pharmacologic targeting of these cytokines might provide an effective means to simultaneously control both atherosclerosis and diabetes.

A DNA Spiegelmer to staphylococcal enterotoxin B

Bacterial staphylococcal enterotoxin B is involved in several severe disease patterns and it was therefore used as a target for the generation of biologically stable mirror-image oligonucleotide ligands, so called Spiegelmers. The toxin is a 28 kDa protein consisting of 239 amino acids. Since the full-length protein is not accessible to chemical peptide synthesis, a stable domain of 25 amino acids was identified as a suitable selection target. DNA in vitro selection experiments were carried out against the equivalent mirror-image D-peptide domain resulting in high affinity D-DNA aptamers. As expected, the corresponding enantiomeric L-DNA Spiegelmer showed comparable binding characteristics to the L-peptide domain. Moreover, the Spiegelmer bound the whole protein target with only slightly reduced affinity. Dissociation constants of both peptide-oligonucleotide complexes were measured in the range of 200 nM, whereas the Spiegelmer binding to the full-length protein was determined at approximately 420 nM. These data demonstrate the possibility to identify Spiegelmers against large protein targets by a domain approach.

A role for the B7-1/B7-2:CD28/CTLA-4 pathway during negative selection

Although costimulation plays an important role in activating naive T cells, its role in negative selection is controversial. By following thymocyte deletion induced by endogenous superantigens in mice lacking B7-1 and/or B7-2, we have identified a role for both B7-1 and B7-2 in negative selection. Studies using CD28-deficient and CD28/CTLA-4-double-deficient mice have revealed that either CD28 or another as yet undefined coreceptor can mediate these B7-dependent signals that promote negative selection. Finally, CTLA-4 delivers signals that inhibit selection, suggesting that CTLA-4 and CD28 have opposing functions in thymic development. Combined, the data demonstrate that B7-1/B7-2-dependent signals help shape the T cell repertoire.

Therapy and prophylaxis of inhaled biological toxins

This review highlights the current lack of therapeutic and prophylactic treatments for use against inhaled biological toxins, especially those considered as potential biological warfare (BW) or terrorist threats. Although vaccine development remains a priority, the use of rapidly deployable adjunctive therapeutic or prophylactic drugs could be life-saving in severe cases of intoxication or where vaccination has not been possible or immunity not established. The current lack of such drugs is due to many factors. Thus, methods involving molecular modelling are limited by the extent to which the cellular receptor sites and mode of action and structure of a toxin need to be known. There is also our general lack of knowledge of what effect individual toxins will have when inhaled into the lungs - whether and to what extent the action will be cell specific and cytotoxic or rather an acute inflammatory response requiring the use of immunomodulators. Possible sources of specific high-affinity toxin antagonists being investigated include monoclonal antibodies, selected oligonucleotides (aptamers) and derivatized dendritic polymers (dendrimers). The initial selection of suitable agents of these kinds can be made using cytotoxicity assays involving cultured normal human lung cells and a range of suitable indicators. The possibility that a mixture of selected antibody, aptamer or dendrimer-based materials for one or more toxins could be delivered simultaneously as injections or as inhaled aerosol sprays should be investigated.

Interaction of mycoplasmas with host cells

The mycoplasmas form a large group of prokaryotic microorganisms with over 190 species distinguished from ordinary bacteria by their small size, minute genome, and total lack of a cell wall. Owing to their limited biosynthetic capabilities, most mycoplasmas are parasites exhibiting strict host and tissue specificities. The aim of this review is to collate present knowledge on the strategies employed by mycoplasmas while interacting with their host eukaryotic cells. Prominant among these strategies is the adherence of mycoplasma to host cells, identifying the mycoplasmal adhesins as well as the mammalian membrane receptors; the invasion of mycoplasmas into host cells including studies on the role of mycoplasmal surface molecules and signaling mechanisms in the invasion; the fusion of mycoplasmas with host cells, a novel process that raises intriguing questions of how microinjection of mycoplasma components into eukaryotic cells subvert and damage the host cells. The observations of diverse interactions of mycoplasmas with cells of the immune system and their immunomodulatory effects and the discovery of genetic systems that enable mycoplasmas to rapidly change their surface antigenic composition have been important developments in mycoplasma research over the past decade, showing that mycoplasmas possess an impressive capability of maintaining a dynamic surface architecture that is antigenically and functionally versatile, contributing to the capability of the mycoplasmas to adapt to a large range of habitats and cause diseases that are often chronic in nature.

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.

Analysis of human CD4 T lymphocyte proliferation induced by porcine lymphoblastoid B cell lines

BACKGROUND

This study was undertaken to characterize the two porcine lymphoblastoid cell lines L23 and L35, derived from a pig inoculated by the retrovirus Tsukuba-1, and to determine how they induce a strong human lymphocyte proliferation.

METHODS

Phenotypic characterization was performed by flow cytometry and reverse transcriptase-polymerase chain reaction analyses. Xenogeneic mixed lymphocyte reactions (XMLR) were performed using unfractionated human peripheral blood mononuclear cells (huPBMC) and purified CD4+ T lymphocytes as responding cells, in the presence of blocking antibodies and fusion proteins.

RESULTS

The immunoglobulin genes were demonstrated to be rearranged in L23 and L35 cell lines, in agreement with the expression of a B cell phenotype. Both induced a similar proliferation of huPBMCs and purified human CD4+ lymphocytes from adult or cord blood (naïve cells). Proliferation of CD4+ T lymphocytes was completely blocked by anti-SLA-DR plus anti-SLA-DQ mAbs, excluding human lymphocyte transformation by porcine viruses. The frequency of proliferative precursors was inconsistent with that induced by a retroviral superantigen but similar to classical direct xenoantigen presentation as observed with other porcine antigen-presenting cells. Extensive analysis of costimulatory signals led to the identification of the CD28 pathway, in agreement with membrane expression of B7 molecules on L23 and L35 cells, and of the CD2 pathway in L35 cells.

CONCLUSION

These two porcine lymphoblastoid cell lines have been further characterized and clearly identified as belonging to the B cell lineage. By expressing major histocompatibility complex class II antigens and costimulatory molecules, they induce a vigorous proliferative response of human CD4+ lymphocytes through a direct presentation pathway.

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.

Burn injury-mediated alterations in cell cycle progression in lymphoid organs of mice

A key event in the cellular and molecular pathogenesis of multiple organ failure (MOF) after burn injury may be the change in profiles of the cell cycle progression in affected organs. We investigated the effects of burn injury on cell cycle progression in immune organs. Cell cycle analysis in the lymphoid tissues of mice after 18% burn injury revealed that S phase entry was temporarily arrested in the thymus 1 day after injury, whereas the spleen had substantially increased S phase entry at day 8. This mode of cell cycle regulation was reproduced in different age groups and strains of mice. Furthermore, the reactivity to the Ki-67 antibody (indicative of proliferation) was markedly reduced in the thymic cortex at day 1. There was a distinct pattern of hematopoietic foci formation and increased reactivities to the Ki-67 antibody in myelogenous cells in the red pulp of spleen at day 7, consistent with the elevated S phase entry. These data suggest that differential regulation of cell cycle progression may play a crucial role in the phenotypic changes in immune organs after burn injury.

Flare-up reaction on murine contact hypersensitivity. III. Effect of staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB), a bacterial superantigen, is known as an immunomodulator because it activates an extremely large number of T-cells, and induces the production of large amounts of cytokines. In this study, we examined the effects of SEB on the contact hypersensitivity reaction (CHR). BALB/c mice were first sensitized through haptens applied to the back, and CHR was then induced through challenge to the left ear using the same haptens. SEB was administered intravenously 4 weeks later, causing a flare-up, peaking at 24 h post-administration, in the left ear that had previously exhibited CHR. This flare-up reaction was hapten non-specific, and was inhibited by anti-mouse tumor necrosis factor (TNF)-alpha antibodies. The flare-up was also suppressed by the oral administration of cyclosporin A prior to the administration of SEB. These results suggest that SEB induces a flare-up of CHR via the production of TNF-alpha.

Restriction in the usage of variable beta regions in T-cells infiltrating valvular tissue from rheumatic heart disease patients

Rheumatic Heart Disease (RHD) is a delayed consequence of a pharyngeal infection with group A streptococcus (GAS), usually ascribed to a cross-reactive immune response to the host's cardiac tissues. Several GAS proteins have been reported to be superantigens, also raising the possibility that T cells in RHD could be driven by superantigens. We therefore analysed the variable beta (V beta) repertoire of T cells infiltrating heart valves from chronic RHD patients undergoing elective valvular surgery. We analysed 15 valve specimens from patients with longstanding quiescent RHD and control valves from four non-rheumatic individuals. Total RNA was extracted from fresh valve tissue and employed to amplify 22 V beta genes by RT-PCR. In valvular tissue, a restricted number of only 2 to 9 V beta regions were detected as opposed to the findings in control valves. In 8 RHD valves, the expression of V beta1, 2, 3, 5.1, 7, 8, 9 or 14 was marked. These V beta regions have been related to GAS superantigens. Our results evidence the presence of a restricted set of T lymphocytes in valvular tissue from a majority of patients with chronic RHD and suggest that valvular sequelae in these patients might be related to a local antigen or superantigen driven inflammatory process that persists even many years after the initial triggering event.

Characterization of a novel staphylococcal enterotoxin-like superantigen, a member of the group V subfamily of pyrogenic toxins

Staphylococcus aureus is an important human pathogen, causing a variety of diseases. Major virulence factors of this organism include staphylococcal enterotoxins (SEs) that cause food poisoning and toxic shock syndrome. Our study identified a novel enterotoxin-like protein that is a member of the new subfamily (group V) of pyrogenic toxin superantigens (PTSAgs) and examined its biochemical and immunobiological properties. The gene encoding the SE-like protein is directly 5' of another recently identified PTSAg, SEK. The SE-like protein had a molecular weight of 26000 and an experimentally determined isoelectric point between 7.5 and 8.0. We demonstrated that the PTSAg had many of the biological activities associated with SEs, including superantigenicity, pyrogenicity, and ability to enhance endotoxin shock, but lacked both lethality in rabbits when administered in subcutaneous miniosmotic pumps and emetic activity in monkeys. Recombinant protein stimulated human CD4 and CD8 T cells in a T cell receptor variable region, beta chain (TCRVbeta) specific manner. T cells bearing TCRVbeta 2, 5.1, and 21.3 were significantly stimulated.

Jak3-regulated genes: DNA array analysis of concanavalin a-interleukin-2-activated chicken T cells treated with a specific jak3 inhibitor

Janus kinase 3 (Jak3) is important in the activation and proliferation of lymphoid cells and binds to the common gamma subunit of several cytokine receptors, including the interleukin-2 (IL-2) receptor (IL-2R). DNA arrays were used to measure mRNA levels of a large number of genes regulated by signaling through the Jak3 tyrosine kinase pathway by blocking concanavalin A (ConA)-IL-2-activated chicken splenic T cells with a specific Jak3 inhibitor (WHI-P154). Of the 635 genes detected by arrays containing about 1200 cDNAs, 12 were upregulated in control cells compared with inhibitor-treated cells, and 6 were expressed at higher levels in the inhibitor-treated group. By identifying genes that are directly or indirectly regulated by Jak3, we can gain insight into the roles of this key intermediate in avian T cell activation and further our understanding of intracellular signaling networks in the immune response.

CD4 regulatory T cells prevent lethal autoimmunity in IL-2Rbeta-deficient mice. Implications for the nonredundant function of IL-2

Lethal autoimmunity associated with IL-2Rbeta-deficient mice is prevented after thymic transgenic expression of wild-type IL-2Rbeta in IL-2Rbeta(-/-) mice (Tg -/- mice). Here, we show that CD4(+)CD25(+) regulatory T cells were not readily detected in IL-2Rbeta(-/-) mice, but the production of functional CD4(+)CD25(+) T cells was reconstituted in Tg -/- mice. Adoptive transfer of normal CD4(+)CD25(+) T cells into neonatal IL-2Rbeta-deficient mice prevented this lethal autoimmune syndrome. The CD4(+)CD25(+) T cells in disease-free adult IL-2Rbeta-deficient recipient mice were present at a near normal frequency, were solely donor-derived, and depended on IL-2 for expansion. These observations indicate that the essential function of the IL-2/IL-2R system primarily lies at the level of the production of CD4(+)CD25(+) regulatory T cells.

Differential expansion of T-cell receptor variable beta subsets after antigenic stimulation in patients with different outcomes of hepatitis C infection

Persistent antigenic stimulation during chronic hepatitis C may alter the T-cell receptor variable chain beta (TCR BV) repertoire as well as the cytokine responses of hepatitis C virus (HCV)-specific T lymphocytes. We analysed the distribution of the TCR BV subsets 2.1, 3.1, 5.1, 6.1, 8, 13.1, 13.6, 14.1, 17.1, 21.3 in relation to intracytoplasmic expression of interleukin-2, interferon-gamma, interleukin-4 and interleukin-10. Using flow cytometry, CD45RO+ memory T cells of 27 patients with chronic hepatitis C, eight patients with resolved HCV infection and 16 non-HCV-related controls were studied with and without stimulation by the HCV core, NS3, NS4, NS5a and NS5b proteins. Patients with chronic and resolved hepatitis C differed by larger basal TCR BV2.1+, BV6.1+, BV17.1+ and BV21.3+ subsets in chronic hepatitis C, which were correlated to the numbers of T cells with spontaneous interleukin-2 and interferon-gamma production (r=0.51-0.73, P<0.05). Upon HCV-specific stimulation these subsets did not expand, whereas a marked in vitro expansion of TCR BV8+ T cells in response to all HCV proteins was selectively noted in chronic hepatitis C (P<0.05). This expansion of TCR BV8+ memory T cells was significantly correlated to HCV-induced interleukin-10 expression (r=0.58-0.98, P<0.01). Thus, differential involvement of selected TCR BV subsets may be related to the outcome of HCV infection.

Superantigen concomitantly induces Th1 cytokine genes and the ability to shut off their expression on re-exposure to superantigen

Superantigens, exemplified by staphylococcal enterotoxin B (SEB), are the strongest known inducers of a cellular immune response; they elicit the production of excessive amounts of Th1 cytokines, IL-2, IFN-gamma and TNF, leading to toxic shock. We show that increasing doses of SEB cause not only a greater induction but also a more rapid cessation of IL-2 gene expression. Remarkably, exposure of human PBMC to a second dose of SEB, even at concentrations 10- or 100-fold lower than the initial inducing dose and even within 2 h after the first exposure to SEB, resulted in an immediate and essentially complete shutoff of the induced IL-2 and IFN-gamma mRNA expression. The shutoff response was observed when primary induction of IL-2 and IFN-gamma gene expression was by SEB but not when it was by phytohemaggutinin-P. Signaling by a superantigen thus results not only in a vigorous induction of Th1 cytokine genes but concomitantly induces the ability to shut off their expression upon re-exposure to superantigen. Without induction of this negative control mechanism, the cellular immune response to a superantigen would be even more pronounced.

Involvement of noradrenergic nerves in the activation and clonal deletion of T cells stimulated by superantigen in vivo

Superantigens, like staphylococcal enterotoxin B (SEB), induce a strong proliferative response followed by clonal deletion of a substantial portion of defined Vbeta T cells. The remaining cells display in vitro anergy. We found that the immune response to SEB was paralleled by biphasic changes in the activity of the sympathetic nervous system. Furthermore, sympathetic denervation resulted in decreased SEB-induced cell proliferation and IL-2 production, and impeded the specific deletion of splenic CD4Vbeta8 cells observed in intact animals without affecting anergy. These studies provide the first evidence of an immunoregulatory cross-talk between sympathetic nerves and superantigen-activated immune cells.

The in vitro interaction of Streptococcus pyogenes with human pharyngeal cells induces a phage-encoded extracellular DNase

The role lysogenic bacteriophage play in the pathogenesis of the host bacterium is poorly understood. In a previous study, we found that streptococcal coculture with human pharyngeal cells resulted in the induction of lysogenic bacteriophage as well as the phage-associated streptococcal pyrogenic exotoxin C (SpeC). In this study, we have determined that in addition to SpeC induction, a number of other streptococcal proteins are also released by the bacteria during coculture with pharyngeal cells. Among these, we identified and characterized a novel 27-kDa secreted protein. Sequence analysis of this novel protein demonstrated it to be encoded by the same lysogenic bacteriophage which harbors speC. Protein sequence analysis revealed varied homologies with several streptococcal DNases. Further biochemical characterization of the recombinantly expressed protein verified it to be a divalent cation-dependent streptococcal phage-encoded DNase (Spd1). Although functionally distinct, SpeC and Spd1 are associated by a number of parameters, including genetic proximity and transcriptional regulation. Finally, we speculate on the induction of phage-encoded DNase (Spd1) enhancing the fitness of both bacteria and phage.

Toll-like receptor-4 signaling mediates pulmonary neutrophil sequestration in response to gram-positive bacterial enterotoxin

BACKGROUND

Toll-like receptors (TLRs) serve as mediators of innate immune responses to pathogen-associated molecular patterns (PAMPs) which include lipopolysaccharide (LPS) and staphylococcal enterotoxin B (SEB). TLR-4 is thought to act as the primary effector of LPS recognition and TLR-2 is thought to mediate responses to Gram-positive bacterial proteins. Chemokines such as macrophage inflammatory protein (MIP-2) are peptides that are responsible for lung neutrophil (PMN) sequestration following an infectious or inflammatory insult. Given the Gram-positive origin of SEB, we hypothesized that mice with altered TLR-4 signaling would exhibit no difference in lung PMN sequestration following SEB when compared to wild-type mice.

METHODS

Wild-type and TLR-4 mutant mice were administered intratracheal saline, LPS (Escherichia coli 0.1 mg/kg), or SEB (1 mg/kg). After 24 h, lung PMN accumulation was determined by myeloperoxidase (MPO) assay and bronchoalveolar lavage fluid cell count (BALfcc). Total lung and BALf MIP-2 was measured by enzyme-linked immunosorbent assay.

RESULTS

There was an increase in lung PMN accumulation (by both MPO and BALfcc) and MIP-2 following LPS and SEB in wild-type mice compared to saline-treated controls. In contrast, TLR-4 mice failed to exhibit an increase in lung MIP-2 or PMN accumulation following either LPS or SEB compared to wild-type mice.

CONCLUSIONS

TLR-4 mutant mice are unresponsive to intratracheal LPS. SEB elicited an increase in lung MIP-2 and PMN accumulation in wild-type mice. However, TLR-4 mutant mice were protected from this process. This suggests that TLR-4 signaling may mediate the responses to other PAMPs in addition to LPS.

Cutting edge: selective impairment of CD8+ T cell function in mice lacking the TNF superfamily member LIGHT

Interactions of LIGHT and its receptors, herpesvirus entry mediator on T cells and lymphotoxin beta receptor on stromal cells, are implicated in the regulation of lymphoid organogenesis, costimulation of T cells, and activation of dendritic cells. In this work we report that LIGHT-deficient mice had normal lymphoid organs with T cells and APCs that normally responded to Ag stimulation and normally stimulated T cells. Although the number of Vbeta8(+) T cells in naive LIGHT(+/+) and LIGHT(-/-) mice was identical, Vbeta8(+)CD8(+) T cell proliferation in response to staphylococcal enterotoxin B was significantly lower in LIGHT(-/-) mice. Consistently, induction and cytokine secretion of CD8(+) CTL to MHC class I-restricted peptide was also reduced in LIGHT(-/-) mice. However, the proliferative response of Vbeta8(+)CD4(+) T cells to staphylococcal enterotoxin B was comparable in LIGHT(-/-) and LIGHT(+/+) mice. Our results suggest that LIGHT is required for activation of normal CD8(+) T cells but not CD4(+) T cells.

Fulminant infection and toxic shock syndrome caused by Streptococcus pyogenes

Two patients presented to the Emergency Department (ED) with features of toxic shock syndrome, including hypotension, acute respiratory distress syndrome (ARDS), renal and hepatic insufficiency and disseminated intravascular coagulation (DIC). Computed tomography (CT) scan identified the source of infection in one patient. At laparotomy, pelvic peritonitis and massive edema of the pelvic retroperitoneal tissue was found. The other patient had myonecrosis of the forearm necessitating amputation. Intra-operative cultures of tissue in each case yielded Streptococcus pyogenes, Group A. These patients were treated early with clindamycin and intensive supportive care as well as surgery, and both made a full recovery. Because of the necessity of early recognition of the varied presentation of these infections, the clinical features as well as essential interventions are emphasized. We review the pathophysiology of invasive Group A streptococcal infection to increase awareness of these uncommon but fulminant and often lethal infections.

Monocyte deactivation in severe human sepsis or following cardiopulmonary bypass

We investigated the specificity for gram-negative stimuli as well as the contribution of signal transduction pathways for leukocyte hyporesponsiveness in sepsis or following cardiopulmonary bypass (CPB). Whole blood of nine patients undergoing CPB and 25 patients with severe sepsis was stimulated ex vivo with LPS (E. coli O111:B4) or with Staphylococcus aureus Cowan strain I (SAC-I) lysate in the absence or presence of inhibitors of protein kinase C (PKC), protein-tyrosine kinase (PTK), or protein-tyrosine phosphatase (PTP). Both toxins stimulated a TNF-alpha response through PTK signaling. Although suppression of the cytokine response was similar for LPS and SAC-I after CPB, it was significantly more pronounced for SAC-I in sepsis. Inhibition of PTP failed to increase TNF-alpha upon LPS, whereas a moderate increase was observed with SAC-I. Impaired TNF-alpha responses occur in sepsis and after CPB. Although this has primarily been reported for gram-negative stimuli, our data suggest that this is even more pronounced for gram-positive stimuli in severe sepsis. Although PTK was the predominant signaling pathway, inhibition of PTP only partially restored the TNF-alpha response to SAC-I. Our results suggest that cellular mechanisms underlying monocyte deactivation are different in sepsis or following CPB and are discriminate for gram-positive and gram-negative toxins.

Ocular toxoplasmosis: more than just what meets the eye

Toxoplasma gondii is an intracellular parasite whose life cycle may include the man as an intermediate host. Close to a billion people are infected with this parasite worldwide. Ocular lesions may occur in up to 25% of those individuals infected. The infection may occur intra-uterus, through the placenta when the mother is infected during pregnancy. The parasite may also infect adults after the ingestion of contaminated food products, most notably meats or water. We have shown that although congenital and post-natal (acquired) infection results in similar ocular lesions, the immunological mechanisms behind the development of disease are different. On the other hand, contrary to published data obtained in mice, we were unable to find evidence that the T. gondii express superantigen activity for human lymphocytes. Our findings are important because they suggest that superantigen activity is not important as a pathological mechanism in human disease. Our data also suggest that, whereas the ocular lesion caused by infection after birth is the result of an excessive or dysfunctional immune response, the lesions caused by congenital infection may be due to a lack of an appropriate response to the parasite.

CD40 ligand functions non-cell autonomously to promote deletion of self-reactive thymocytes

CD40 ligand (CD40L)-deficient mice have been shown to have a defect in negative selection of self-reactive T cells during thymic development. However, the mechanism by which CD40L promotes deletion of autoreactive thymocytes has not yet been elucidated. We have studied negative selection in response to endogenous superantigens in CD40L-deficient mice and, consistent with previous reports, have found a defect in negative selection in these mice. To test the requirement for expression of CD40L on T cells undergoing negative selection, we have generated chimeric mice in which CD40L wild-type and CD40L-deficient thymocytes coexist. We find that both CD40L wild-type and CD40L-deficient thymocytes undergo equivalent and efficient negative selection when these populations coexist in chimeric mice. These results indicate that CD40L can function in a non-cell-autonomous manner during negative selection. Deletion of superantigen-reactive thymocytes was normal in B7-1/B7-2 double-knockout mice, indicating that CD40-CD40L-dependent negative selection is not solely mediated by B7 up-regulation and facilitation of B7-dependent T cell signaling. Finally, although the absence of CD40-CD40L interactions impairs negative selection of autoreactive CD4(+) and CD8(+) cells during thymic development, we find that self-reactive T cells are deleted in the mature CD4(+) population through a CD40L-independent pathway.

Structural basis for abrogated binding between staphylococcal enterotoxin A superantigen vaccine and MHC-IIalpha

Staphylococcal enterotoxins (SEs) are superantigenic protein toxins responsible for a number of life-threatening diseases. The X-ray structure of a staphylococcal enterotoxin A (SEA) triple-mutant (L48R, D70R, and Y92A) vaccine reveals a cascade of structural rearrangements located in three loop regions essential for binding the alpha subunit of major histocompatibility complex class II (MHC-II) molecules. A comparison of hypothetical model complexes between SEA and the SEA triple mutant with MHC-II HLA-DR1 clearly shows disruption of key ionic and hydrophobic interactions necessary for forming the complex. Extensive dislocation of the disulfide loop in particular interferes with MHC-IIalpha binding. The triple-mutant structure provides new insights into the loss of superantigenicity and toxicity of an engineered superantigen and provides a basis for further design of enterotoxin vaccines.

CDP binding to multiple sites in the mouse mammary tumor virus long terminal repeat suppresses basal and glucocorticoid-induced transcription

Mouse mammary tumor virus (MMTV) is transcribed at high levels in the lactating mammary gland to ensure transmission of virus from the milk of infected female mice to susceptible offspring. We previously have shown that the transcription factor CCAAT displacement protein (CDP) is expressed in high amounts in virgin mammary gland, yet DNA-binding activity for the MMTV long terminal repeat (LTR) disappears as mammary tissue differentiates during lactation. CDP is a repressor of MMTV expression and, therefore, MMTV expression is suppressed during early mammary gland development. In this study, we have shown using DNase I footprinting and electrophoretic mobility shift assays that there are at least five CDP-binding sites in the MMTV LTR upstream of those previously described in the promoter-proximal negative regulatory element (NRE). Single mutations in two of these upstream sites (+691 or +692 and +735 relative to the first base of the LTR) reduced CDP binding to the cognate sites and elevated reporter gene expression from the full-length MMTV LTR. Combination of a mutation in the promoter-distal NRE with a mutation in the proximal NRE gave approximately additive increases in LTR-reporter gene activity, suggesting that these binding sites act independently. Mutations in several different CDP-binding sites allowed elevation of reporter gene activity from the MMTV promoter in the absence and presence of glucocorticoids, hormones that contribute to high levels of MMTV transcription during lactation by activation of hormone receptor binding to the LTR. In addition, overexpression of CDP in transient-transfection assays suppressed both basal and glucocorticoid-induced LTR-mediated transcription in a dose-dependent manner. These data suggest that multiple CDP-binding sites contribute independently to regulate binding of positive factors, including glucocorticoid receptor, to the MMTV LTR during mammary gland development.