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

Adaptive immune responses during Shigella dysenteriae type 1 infection: an in vitro stimulation with 57 kDa major antigenic OMP in the presence of anti-CD3 antibody

An effort was made to understand the role of the 57 kDa major antigenic fraction of Shigella outer membrane protein (OMP) in the presence of T-cell antigen receptor in activation of adaptive immune responses of the cell mediated immune (CMI) restored patients. The expression of HLA-DR/CD4 out of CD3(+) T-cells was significantly dominant over the HLA-DR/CD8 and comparable to unstimulated cells of infected or healthy controls. CD4(+) T-cell activation together with HLA-DR is associated with the expression of CD25(+) (IL2Ralpha) for IL-2 growth factors with decreased IL-4 levels, required for maintaining the homeostasis of CD4(+) T cell. Furthermore, the positive expression of the CD45 antigen is possibly required for acquiring the memory for CD4(+) cells signals and facilitates the interaction with CD54 antigen. As a result, antigen-specific secondary signal is generated for B-cell activation to produce IgG2a and IgG2b. This suggests that antibody mediated-adaptive immune responses are generated due to anti-CD3 induced helper T-cell activity. The above mentioned findings reflect that the antigen alone might not exacerbate the selective T-cell responses. But these antigens in the presence of anti-CD3 antibody might help to elicit adaptive immune response via T-cell receptor (TCR) activation.

Potent neutralization of staphylococcal enterotoxin B by synergistic action of chimeric antibodies

Staphylococcal enterotoxin B (SEB), a shock-inducing exotoxin synthesized by Staphylococcus aureus, is an important cause of food poisoning and is a class B bioterrorism agent. SEB mediates antigen-independent activation of a major subset of the T-cell population by cross-linking T-cell receptors (TCRs) with class II major histocompatibility complex (MHC-II) molecules of antigen-presenting cells, resulting in the induction of antigen independent proliferation and cytokine secretion by a significant fraction of the T-cell population. Neutralizing antibodies inhibit SEB-mediated T-cell activation by blocking the toxin's interaction with the TCR or MHC-II and provide protection against the debilitating effects of this superantigen. We derived and searched a set of monoclonal mouse anti-SEB antibodies to identify neutralizing anti-SEB antibodies that bind to different sites on the toxin. A pair of non-cross-reactive, neutralizing anti-SEB monoclonal antibodies (MAbs) was found, and a combination of these antibodies inhibited SEB-induced T-cell proliferation in a synergistic rather than merely additive manner. In order to engineer antibodies more suitable than mouse MAbs for use in humans, the genes encoding the VL and VH gene segments of a synergistically acting pair of mouse MAbs were grafted, respectively, onto genes encoding the constant regions of human Igkappa and human IgG1, transfected into mammalian cells, and used to generate chimeric versions of these antibodies that had affinity and neutralization profiles essentially identical to their mouse counterparts. When tested in cultures of human peripheral blood mononuclear cells or splenocytes derived from HLA-DR3 transgenic mice, the chimeric human-mouse antibodies synergistically neutralized SEB-induced T-cell activation and cytokine production.

An LFA-1 (alphaLbeta2) small-molecule antagonist reduces inflammation and joint destruction in murine models of arthritis

LFA-1 appears to play a central role in normal immune responses to foreign Ags. In autoimmune or inflammatory diseases, there is increased expression of LFA-1 and/or its counterligand, ICAM-1. Others have demonstrated that the targeted disruption of LFA-1:ICAM interactions, either by gene deletion or Ab treatment in mice, results in reduced leukocyte trafficking, inflammatory responses, and inhibition of inflammatory arthritis in the K/BxN serum transfer model. However, there has been little success in finding a small-molecule LFA-1 antagonist that can similarly impact rodent models of arthritis. In this paper, we present the first reported example of an LFA-1 small-molecule antagonist, BMS-587101, that is efficacious in preclinical disease models. In vitro, BMS-587101 inhibited LFA-1-mediated adhesion of T cells to endothelial cells, T cell proliferation, and Th1 cytokine production. Because BMS-587101 exhibits in vitro potency, cross-reactivity, and oral bioavailability in rodents, we evaluated the impact of oral administration of this compound in two different models of arthritis: Ab-induced arthritis and collagen-induced arthritis. Significant impact of BMS-587101 on clinical score in both models was observed, with inhibition comparable or better than anti-mouse LFA-1 Ab. In addition, BMS-587101 significantly reduced cytokine mRNA levels in the joints of Ab-induced arthritis animals as compared with those receiving vehicle alone. In paws taken from the collagen-induced arthritis study, the bones of vehicle-treated mice had extensive inflammation and bone destruction, whereas treatment with BMS-587101 resulted in marked protection. These findings support the potential use of an LFA-1 small-molecule antagonist in rheumatoid arthritis, with the capacity for disease modification.

Diagnosis and management of chronic rhinosinusitis in adults

Chronic rhinosinusitis (CRS) is characterized by mucosal inflammation affecting both the nasal cavity and paranasal sinuses; its causes are potentially numerous, disparate, and frequently overlapping. The more common conditions that are associated with CRS are perennial allergic and nonallergic rhinitis, nasal polyps, and anatomical mechanical obstruction (septum/turbinate issues). Other less common etiologies include inflammation (eg, from superantigens), fungal sinusitis or bacterial sinusitis with or without associated biofilm formation, gastroesophageal reflux, smoke and other environmental exposures, immune deficiencies, genetics, and aspirin-exacerbated respiratory disease. A diagnosis of CRS is strongly suggested by a history of symptoms (eg, congestion and/or fullness; nasal obstruction, blockage, discharge, and/or purulence; discolored postnasal discharge; hyposmia/anosmia; facial pain and/or pressure) and their duration for > 3 months. A definitive diagnosis requires physical evidence of mucosal swelling or discharge appreciated during physical examination coupled with CT imaging if inflammation does not involve the middle meatus or ethmoid bulla. Multivariant causation makes the diagnosis of CRS and selection of treatment complex. Furthermore, various types of health care providers including ear, nose, and throat (ENT) specialists, allergists, primary care physicians, and pulmonologists treat CRS, and each is likely to have a different approach. A structured approach to the diagnosis and management of CRS can help streamline and standardize care no matter where patients present for evaluation and treatment. A 2008 Working Group on CRS in Adults, supported by the American Academy of Otolaryngic Allergy (AAOA), developed a series of algorithms for the differential diagnosis and treatment of CRS in adults, based on the evolving understanding of CRS as an inflammatory disease. The algorithms presented in this paper address an approach for all CRS patients as well as approaches for those with nasal polyps, edema observed on nasal endoscopy, purulence observed on nasal endoscopy, an abnormal history and physical examination, and an abnormal history and normal physical examination.

IL-2 regulates SEB induced toxic shock syndrome in BALB/c mice

BACKGROUND

Toxic Shock Syndrome (TSS) is characterized by fever, rash, hypotension, constitutional symptoms, and multi-organ involvement and is caused by Staphylococcus aureus enterotoxins such as Staphylococcal Enterotoxin B (SEB). SEB binds to the MHC-IIalpha chain and is recognized by the TCRbeta chain of the Vbeta8 TCR(+) T cells. The binding of SEB to Vbeta chain results in rapid activation of T cells and production of inflammatory cytokines, such as Interleukin-2 (IL-2), Interferon-gamma and Tumor Necrosis Factor-alpha which mediate TSS. Although IL2 was originally identified as the T cell growth factor and was proposed to contribute to T cell differentiation, its role in TSS remains unexplored.

METHODOLOGY/PRINCIPAL FINDINGS

Mice were injected with D-Gal (25 mg/mouse). One hour after D-Galactosamine (D-Gal) injection each mouse was injected with SEB (20 microg/mouse. Mice were then observed for 72 hrs and death was recorded at different times. We tested Interleukin-12, IFNgamma, and IL-2 deficient mice (IL-2(-/-)), but only the IL-2 deficient mice were resistant to SEB induced toxic shock syndrome. More importantly reconstitution of IL-2 in IL-2 deficient mice restored the shock. Interestingly, SEB induced IL-2 production from T cells was dependent on p38MAPK activation in macrophages as inhibition of it in macrophages significantly inhibited IL-2 production from T cells.

CONCLUSION

This study shows the importance of IL -2 in TSS which has not been previously explored and it also shows that regulating macrophages function can regulate T cells and TSS.

Altered thymic selection by overexpressing cellular FLICE inhibitory protein in T cells causes lupus-like syndrome in a BALB/c but not C57BL/6 strain

Cellular FLICE inhibitory protein (c-FLIP) is an endogenous inhibitor of the caspase-8 pro-apoptotic signaling pathway downstream of death receptors. Recent evidence indicates that the long form of c-FLIP (c-FLIP_L) is required for proliferation and effector T cell development. However, the role of c-FLIP_L in triggering autoimmunity has not been carefully investigated. We now report that c-FLIP_L transgenic (Tg) mice develop splenomegaly, lymphadenopathy, multi-organ infiltration, high titers of autoantibodies, and proliferative glomerulonephritis with immune complex deposition in a strain-dependent fashion. The development of autoimmunity requires CD4⁺ T cells and may result from impaired thymic selection. At the molecular level, c-FLIP_L over-expression inhibits the ZAP-70 activation, thus impairing the signaling pathway derived from ZAP-70 required for thymic selection. Therefore, we have identified c-FLIP_L as a susceptibility factor under the influence of epistatic modifiers for the development of autoimmunity.

Mls: A Link Between Immunology and Retrovirology

The nature of the mysterious minor lymphocyte stimulating (Mls) antigens has recently been clarified. These molecules which were key elements for our current understanding of immune tolerance, have a strong influence on the mouse immune system and are encoded by the open reading frame (orf) of endogenous and exogenous mouse mammary tumor viruses (MMTV's). The knowledge that these antigens are encoded by cancerogenic retroviruses opens an interdisciplinary approach for understanding the mechanisms of immune responses and immune tolerance, retroviral carcinogenesis, and retroviral strategies for infection.

Gram-positive toxic shock syndromes

Toxic shock syndrome (TSS) is an acute, multi-system, toxin-mediated illness, often resulting in multi-organ failure. It represents the most fulminant expression of a spectrum of diseases caused by toxin-producing strains of Staphylococcus aureus and Streptococcus pyogenes (group A streptococcus). The importance of Gram-positive organisms as pathogens is increasing, and TSS is likely to be underdiagnosed in patients with staphylococcal or group A streptococcal infection who present with shock. TSS results from the ability of bacterial toxins to act as superantigens, stimulating immune-cell expansion and rampant cytokine expression in a manner that bypasses normal MHC-restricted antigen processing. A repetitive cycle of cell stimulation and cytokine release results in a cytokine avalanche that causes tissue damage, disseminated intravascular coagulation, and organ dysfunction. Specific therapy focuses on early identification of the illness, source control, and administration on antimicrobial agents including drugs capable of suppressing toxin production (eg, clindamycin, linezolid). Intravenous immunoglobulin has the potential to neutralise superantigen and to mitigate subsequent tissue damage.

Early infection termination affects number of CD8+ memory T cells and protective capacities in listeria monocytogenes-infected mice upon rechallenge

Here, we reevaluate the effects of early termination of infection on primary T cell expansion, subsequent memory cell development, and protective immunity. Using a murine Listeria monocytogenes (LM) infection model, we found the primary expansions of both CD4(+) and CD8(+) T cells were affected even when ampicillin was given as late as 60 h postinfection (p.i.). Subsequent development of CD8(+) memory T cells was also impaired, although to a lesser extent, and only mice that received ampicillin at 24 h p.i. revealed a significant decrease in memory CD8(+) T cells. Upon rechallenge with 1 x 10(5) CFU of LM, all ampicillin-treated mice cleared LM as effectively, and they generated similar amounts of Ag-specific CD8(+) T cells as with untreated mice. However, mice that received ampicillin at 24 h p.i. lost their protective abilities when rechallenged with 7.5 x 10(5) CFU of LM. Ampicillin treatment also revealed early down-regulation of B7.1 and B7.2, but not CD40, on dendritic cells 72 h p.i. Our results have several important implications: 1) they argue against the hypothesis that brief exposure of T cells to an Ag is sufficient for full-fledged primary T cell responses and subsequent memory T cell development in vivo; 2) they suggest the existence of a reservoir of memory T cells, more than the immune system can possibly expand during secondary infection; and 3) they suggest that protective capacity is correlated with the number of preexisting memory T cells and that secondary expanding T cells play a limited role, at least in murine LM infection.

Antigen-restricted gammadelta T-cell receptors?

After more than two decades of investigation, the biological role of the gammadelta T-cell receptors (TCRs) remains elusive. In fact, a theory of ligand recognition is still lacking that accounts for their adaptable structure, their peripheral selection, and the observed responses of gammadelta T cells, which do not require immunization but only include cells sharing germline-encoded components of the TCR. Assuming that all gammadelta T cells recognize ligands by a common mechanism, we now propose that germline-encoded components of the gammadelta TCRs provide for the specific recognition of a select set of antigenic determinants (Ags) which appear on the cell surface in various molecular associations. Furthermore, we hypothesize that the adaptivity of the gammadelta TCRs serves to increase affinity for the molecules with which these Ags associate rather than for the Ags themselves. Here we outline this hypothetical mechanism and discuss its possible implications for thymic selection and potential for complementing known innate and adaptive mechanisms of immune defense.

First Streptococcus pyogenes signature-tagged mutagenesis screen identifies novel virulence determinants

The virulence of bacterial pathogens is a complex process that requires the dynamic expression of many genes for the pathogens to invade and circumvent host defenses, as well as to proliferate in vivo. In this study, we employed a large-scale screen, signature-tagged mutagenesis (STM), to identify Streptococcus pyogenes virulence genes important for pathogenesis within the host. Approximately 1,200 STM mutants were created and screened using the zebrafish infectious disease model. The transposon insertion site was identified for 29 of the 150 mutants that were considered attenuated for virulence. Previously reported streptococcal virulence genes, such as mga, hasA, amrA, smeZ, and two genes in the sil locus, were identified, confirming the utility of the model for revealing genes important for virulence. Multiple genes not previously implicated in virulence were also identified, including genes encoding putative transporters, hypothetical cytosolic proteins, and macrolide efflux pumps. The STM mutant strains display various levels of attenuation, and multiple separate insertions were identified in either the same gene or the same locus, suggesting that these factors are important for this type of acute, invasive infection. We further examined two such genes, silB and silC of a putative quorum-sensing regulon, and determined that they are significant virulence factors in our model of necrotizing fasciitis. sil locus promoter expression was examined under various in vitro conditions, as well as in zebrafish tissues, and was found to be differentially induced. This study was a unique investigation of S. pyogenes factors required for successful invasive infection.

Detection the Staphylococcus aureus producing enterotoxin isolated from skin infections in hospitalized patients

Staphylococcus aureus is a major human pathogen that produces a wide array of toxins, thus causing various type of disease symptoms. Staphyloceccal enterotoxins (SES), a family of 9 major serological types of heat-stable enterotoxins, are a main cause of gastroenteritis and skin infection. In this study to determine the extent of enterotoxin-producing S. aureus in skin infections of hospitalized patients, their samples were screened and the results showed that 42% of totally 200 patients studied in this research carried S. aureus and 45% of these S. aureus produced Staphylococcal enterotoxins. Twenty percent produced enterotoxin A, 25% produced enterotoxin B and 4.7% produced both enterotoxin A and B. The results demonstrated a high level of enterotoxigenic and multi drug resistance S. aureus in skin infections of hospitalized patients.

Superantigen-presentation by rat major histocompatibility complex class II molecules RT1.Bl and RT1.Dl

Rat major histocompatibility complex (MHC) class II molecules RT1.B(l) (DQ-like) and RT1.D(l) (DR-like) were cloned from the LEW strain using reverse transcription-polymerase chain reaction and expressed in mouse L929 cells. The transduced lines bound MHC class II-specific monoclonal antibodies in an MHC-isotype-specific manner and presented peptide antigens and superantigens to T-cell hybridomas. The T-cell-hybridomas responded well to all superantigens presented by human MHC class II, whereas the response varied considerably with rat MHC class II-transduced lines as presenters. The T-cell hybridomas responded to the pyrogenic superantigens Staphylococcus enterotoxin B (SEB), SEC1, SEC2 and SEC3 only at high concentrations with RT1.B(l)-transduced and RT1.D(l)-transduced cells as presenters. The same was true for streptococcal pyrogenic exotoxin A (SPEA), but this was presented only by RT1.B(l) and not by RT1.D(l). SPEC was recognized only if presented by human MHC class II. Presentation of Yersinia pseudotuberculosis superantigen (YPM) showed no MHC isotype preference, while Mycoplasma arthritidis superantigen (MAS or MAM) was presented by RT1.D(l) but not by RT1.B(l). Interestingly, and in contrast to RT1.B(l), the RT1.D(l) completely failed to present SEA and toxic shock syndrome toxin 1 even after transduction of invariant chain (CD74) or expression in other cell types such as the surface MHC class II-negative mouse B-cell lymphoma (M12.4.1.C3). We discuss the idea that a lack of SEA presentation may not be a general feature of RT1.D molecules but could be a consequence of RT1.D(l)beta-chain allele-specific substitutions (arginine 80 to lysine, asparagine 82 to aspartic acid) in the extremely conserved region flanking the Zn(2+)-binding histidine 81, which is crucial for high-affinity SEA-binding.

Regulatory T cells modulate staphylococcal enterotoxin B-induced effector T-cell activation and acceleration of colitis

Oral administration of bacterial superantigen Staphylococcus aureus enterotoxin B (SEB) activates mucosal T cells but does not cause mucosal inflammation. We examined the effect of oral SEB on the development of mucosal inflammation in mice in the absence of regulatory T (Treg) cells. SCID mice were fed SEB 3 and 7 days after reconstitution with CD4(+) CD45RB(high) or CD4(+) CD45RB(high) plus CD4(+) CD45RB(low) T cells. Mice were sacrificed at different time points to examine changes in tissue damage and in T-cell phenotypes. Feeding SEB failed to produce any clinical effect on SCID mice reconstituted with CD4(+) CD45RB(high) and CD4(+) CD45RB(low) T cells, but feeding SEB accelerated the development of colitis in SCID mice reconstituted with CD4(+) CD45RB(high) T cells alone. The latter was associated with an increase in the number of CD4(+) Vbeta8(+) T cells expressing CD69 and a significantly lower number of CD4(+) CD25(+) Foxp3(+) T cells. These changes were not observed in SCID mice reconstituted with both CD45RB(high) and CD45RB(low) T cells. In addition, SEB impaired the development of Treg cells in the SCID mice reconstituted with CD4(+) CD45RB(high) T cells alone but had no direct effect on Treg cells. In the absence of Treg cells, feeding SEB induced activation of mucosal T cells and accelerated the development of colitis. This suggests that Treg cells prevent SEB-induced mucosal inflammation through modulation of SEB-induced T-cell activation.

Tolerance induction or sensitization in mice exposed to noninherited maternal antigens (NIMA)

Developmental exposure to noninherited maternal antigens (NIMA) exerts a tolerizing or sensitizing influence on clinical transplantation in humans and experimental animals. The aim of this study was to determine if strain and gender differences influence the NIMA effect. Six different mouse strain backcross matings of F(1) females with homozygous males ('NIMA backcross') and corresponding control breedings of F1 males with homozygous females were performed. H-2 homozygous offspring underwent heterotopic heart transplantation from fully allogeneic donors expressing noninherited H-2 antigens. A NIMA tolerizing effect on heart allograft outcome was found in three of six breeding models. In all three cases, the tolerizing antigens were from an H-2(d+) strain. The tolerogenic effect was greatest in male as compared with female recipients. Offspring from the three breeding models in which no tolerance was seen, appeared to be sensitized based on poorer graft survival, or enhanced T- or B-cell responses to the noninherited H-2(b or k) antigens. Significantly higher percentages of maternal antigen(+) cells were found in the peripheral blood of tolerant versus nontolerant strains of backcross mice prior to transplant. Our findings imply that transplants are predisposed to tolerance or rejection due to recipient developmental history and immunogenetic background.

Inhalation of Staphylococcus aureus enterotoxin A induces IFN-gamma and CD8 T cell-dependent airway and interstitial lung pathology in mice

Staphylococcus aureus, a primary source of bacterial superantigen (SAg), is known to colonize the human respiratory tract and has been implicated in airway inflammation. Studies have documented a role for SAgs in respiratory disorders, such as nasal polyps, chronic obstructive pulmonary disease, chronic rhinosinusitis, and asthma. However, cellular and molecular mediators involved in SAg-mediated pulmonary disease have not been clearly identified. In this study, we investigated the effect of intranasal staphylococcal enterotoxin A (SEA) exposure on murine lung. The pathological features in the lung resulting from SEA exposure had characteristics of both obstructive and restrictive pulmonary disorders. There was also an increase in bronchoalveolar lavage protein concentration and cellularity following SEA challenge. Massive CD8(+)Vbeta3(+) T cell accumulation observed in the lung was dependent on CD4 T cell help, both for recruitment and for IFN-gamma synthesis. The primary source of IFN-gamma synthesis was CD8 T cells, and depletion of these cells abrogated disease. IFN-gamma deficiency also prevented SEA-mediated disease, and this was by enhancing early recruitment of neutrophils as detected in the bronchoalveolar lavage. Thus, IFN-gamma appeared to selectively aid the recruitment of T cells to the lungs while preventing the neutrophil accumulation. Therefore, our results show that IFN-gamma-producing CD8 T cells mediated pulmonary alveolitis and inflammation, which were dependent upon CD4 T cells for their recruitment to the lung.

Toxigenicity in Staphylococcus aureus and Coagulase-Negative Staphylococci: Epidemiological and Molecular Aspects

Representatives of the Staphylococcus genus are the most common pathogens found in hospital environments, and they are etiological agents for a large variety of infections. Various virulence factors are responsible for the symptoms and severity of infections caused by Staphylococcus aureus. Among them are staphylococcal enterotoxins (SEs), which cause staphylococcal food poisoning, and toxic shock syndrome toxin-1 (TSST-1). Some reports indicate that TSST-1 and staphylococcal enterotoxins are also produced by coagulase-negative staphylococci (CNS). The present review aimed to discuss general aspects of staphylococcal toxins as well as the epidemiology, genetics and detection of toxins in Staphylococcus aureus and coagulase-negative staphylococci, since these microorganisms are becoming more and more frequent in nosocomial infections.

Recruitment and in situ renewal regulate rapid accumulation of CD11c+ cells in the lung following intranasal superantigen challenge

BACKGROUND

Staphylococcusaureus, a primary source of bacterial superantigen, is known to colonize the human respiratory tract and has been implicated in airway inflammation. The potential pathological effect of staphylococcal enterotoxins on the respiratory tract necessitates a detailed understanding of how they regulate innate immune cells, particularly CD11c-expressing dendritic cells (DCs).

METHODS

C57BL/6 mice were challenged intranasally with staphylococcal enterotoxin A (SEA) and at indicated time points lung tissue was perfused, digested and analyzed for CD11c+ expressing cells.

RESULTS

The pulmonary CD11c+ cells can be divided into two major populations based on their MHC II expression. One day following intranasal SEA challenge, there was rapid accumulation of CD11c+ cells expressing medium to high levels of MHC II. The peak accumulation of CD11c+ MHC II- population was observed 2 days after SEA challenge; however, careful examination of this cell population revealed that they were heterogeneous, being comprised of cells bearing CD3, CD19, NK1.1 and F4/80 along with varying levels of CD11c. Nevertheless, there was a 2-fold increase of CD11c+ MHC II- (CD3- CD19- NK1.1- F4/80-) cells in the lungs.

CONCLUSION

The mechanism of increase in the CD11c+ MHC II- immune progenitor population was mainly due to cellular division rather than migration from blood to lung. In contrast, the early and rapid accumulation of CD11c+ MHC II(hi) cells, conventionally known as DCs, in the lung on day 1 was mostly due to migration from blood. Thus this study examines the pulmonary innate immune response to a powerful immune stimulus.

Dynamic movement of the calcium sensor STIM1 and the calcium channel Orai1 in activated T-cells: puncta and distal caps

The proteins STIM1 and Orai1 are the long sought components of the store-operated channels required in T-cell activation. However, little is known about the interaction of these proteins in T-cells after engagement of the T-cell receptor. We found that T-cell receptor engagement caused STIM1 and Orai1 to colocalize in puncta near the site of stimulation and accumulate in a dense structure on the opposite side of the T-cell. FRET measurements showed a close interaction between STIM1 and Orai1 both in the puncta and in the dense cap-like structure. The formation of cap-like structures did not entail rearrangement of the entire endoplasmic reticulum. Cap formation depended on TCR engagement and tyrosine phosphorylation, but not on channel activity or Ca(2+) influx. These caps were very dynamic in T-cells activated by contact with superantigen pulsed B-cells and could move from the distal pole to an existing or a newly forming immunological synapse. One function of this cap may be to provide preassembled Ca(2+) channel components to existing and newly forming immunological synapses.

HLA-DR alpha 2 mediates negative signalling via binding to Tirc7 leading to anti-inflammatory and apoptotic effects in lymphocytes in vitro and in vivo

Classically, HLA-DR expressed on antigen presenting cells (APC) initiates lymphocyte activation via presentation of peptides to TCR bearing CD4+ T-Cells. Here we demonstrate that HLA-DR alpha 2 domain (sHLA-DRα2) also induces negative signals by engaging TIRC7 on lymphocytes. This interaction inhibits proliferation and induces apoptosis in CD4+ and CD8+ T-cells via activation of the intrinsic pathway. Proliferation inhibition is associated with SHP-1 recruitment by TIRC7, decreased phosphorylation of STAT4, TCR-ζ chain & ZAP70, and inhibition of IFN-γ and FasL expression. HLA-DRα2 and TIRC7 co-localize at the APC-T cell interaction site. Triggering HLA-DR - TIRC7 pathway demonstrates that sHLA-DRα2 treatment inhibits proinflammatory-inflammatory cytokine expression in APC & T cells after lipopolysaccaride (LPS) stimulation in vitro and induces apoptosis in vivo. These results suggest a novel antiproliferative role for HLA-DR mediated via TIRC7, revise the notion of an exclusive stimulatory interaction of HLA-DR with CD4+ T cells and highlights a novel physiologically relevant regulatory pathway.

The immunology of sepsis

Sepsis, the systemic inflammatory response to infection, is considered the major cause of death among critically ill patients in the developed world. While there is a general view that this reflects contributions from both the pathogen and the host with respect to an inappropriate inflammatory response, there is a lack of agreement as to the key immune mechanisms. This has been reflected in the diverse range of immunotherapies tested in clinical trials, often with rather marginal effects. The case has been made for a pathogenic role of excessive immunity, the so-called 'cytokine storm', and for a role of too little immunity through immune paralysis. Apoptosis is implicated as a key mechanism in both this immune paralysis and the multi-organ failure that is a feature of severe sepsis. A number of polymorphisms have been implicated in susceptibility to sepsis, including cytokine genes, HLA class II and caspase-12. In this review we focus in particular on the role of group A streptococci in severe sepsis. Here the effect of bacterial superantigens appears to be a correlate of inflammatory activation, although the precise evolutionary role of the superantigens remains unclear.

Impaired cytokine production by peripheral blood mononuclear cells in type 1 diabetic patients

AIMS

The objective of the present investigation was to study the production of IL-1, IL-6, IL-10, IFNgamma and TNFalpha in cultures of peripheral blood mononuclear cells (PBMC) taken from type 1 diabetic patients with inadequate metabolic control.

METHODS

Seventeen type 1 diabetic patients and a gender- and age-matched group of 17 healthy individuals were studied. PBMC cultures were stimulated with phytohemagglutinin (PHA; 20 microg/ml) and lipopolysaccharide (LPS; 10 microg/ml), and enzyme immunoassay (Elisa) was used to measure IL-1, IL-6, IL-10, IFNgamma and TNFalpha in the cell-culture supernatants.

RESULTS

IFNgamma levels in PHA-stimulated cultures were lower in the type 1 diabetics than in the non-diabetic controls (P<0.0001) while, in contrast, IL-10 levels were increased in the PHA-stimulated culture supernatants of the diabetics compared with the controls (P<0.0001). In addition, supernatant levels of the cytokines IL-1, IL-6 and TNFalpha released in the presence of LPS in the cell cultures from the diabetic patients were significantly lower than in the non-diabetic subjects (P<0.0001, P<0.0001 and P<0.03, respectively).

CONCLUSIONS

The impaired production of IL-1, IL-6, TNFalpha and IFNgamma, and the increased production of IL-10, in PBMC cultures from type 1 diabetics with inadequate metabolic control compared with healthy subjects may be an indication of a deficiency in mononuclear cell activation and, consequently, a deficient immune cellular adaptive response that, in turn, may be the cause of the increased incidence of infections in people with type 1 diabetes.

Age-dependent TCR revision mediated by interaction between alphabeta TCR and self-antigens

Interactions between TCR and self-peptide/MHC complex play an important role in homeostasis and Ag reactivity of mature peripheral T cells. In this report, we demonstrate that the interactions between mature peripheral T cells and endogenous Ags have a negative impact on the maintenance of foreign Ag-specific T cells in an age-dependent manner. This is mediated by RAG-dependent secondary rearrangement of the TCR alpha-chain (receptor revision). The TCR revision in mature T cells is readily observed in mouse expressing transgenic TCR alpha-chain inserted into the physiological locus (knockin mouse) but not in conventional transgenic mouse with an identical TCR alpha-chain. Thus, our results suggest that under physiological conditions in which all TCR alpha-chains are susceptible to deletion by secondary rearrangement, TCR revision in mature peripheral T cells is an ongoing process in adult animals and contributes to age-dependent changes in T cell function and repertoire.

Mycoplasma arthritidis-derived superantigen (MAM) displays DNase activity

Bacterial superantigens are potent stimulators of the immune system. In this study, we expressed recombinant superantigens, which were then affinity purified and used for growth curves and DNase activity assays. Overexpression of Mycoplasma arthritidis-derived superantigen in Escherichia coli reduced bacterial growth. This is unique, as staphylococcal enterotoxin A and toxic shock syndrome toxin-1, expressed in the same vector system, showed no growth impairment. The observed growth inhibition was caused by the DNase activity of recombinant M. arthritidis-derived superantigen, thus describing the first superantigen showing enzymatic activity, which may be a result of the separate evolution of this toxin.

Superantigen natural affinity maturation revealed by the crystal structure of staphylococcal enterotoxin G and its binding to T-cell receptor Vβ8.2

The illnesses associated with bacterial superantigens (SAgs) such as food poisoning and toxic shock syndrome, as well as the emerging threat of purpura fulminans and community-associated methicillin-resistant S. aureus producer of SAgs, emphasize the importance of a better characterization of SAg binding to their natural ligands, which would allow the development of drugs or biological reagents able to neutralize their action. SAgs are toxins that bind major histocompatibility complex class II molecules (MHC-II) and T-cell receptors (TCR), in a nonconventional manner, inducing T-cell activation that leads to production of cytokines such as tumor necrosis factor and interleukin-2, which may result in acute toxic shock. Previously, we cloned and expressed a new natural variant of staphylococcal enterotoxin G (SEG) and evaluated its ability to stimulate in vivo murine T-cell subpopulations. We found an early, strong, and widespread stimulation of mouse Vbeta8.2 T-cells when compared with other SAgs member of the SEB subfamily. In search for the reason of the strong mitogenic potency, we determined the SEG crystal structure by X-ray crystallography to 2.2 A resolution and analyzed SEG binding to mVbeta8.2 and MHC-II. Calorimetry and SPR analysis showed that SEG has an affinity for mVbeta8.2 40 to 100-fold higher than that reported for other members of SEB subfamily, and the highest reported for a wild type SAg-TCR couple. We also found that mutations introduced in mVbeta8.2 to produce a high affinity mutant for other members of the SEB subfamily do not greatly affect binding to SEG. Crystallographic analysis and docking into mVbeta8.2 in complex with SEB, SEC3, and SPEA showed that the deletions and substitution of key amino acids remodeled the putative surface of the mVbeta8.2 binding site without affecting the binding to MHC-II. This results in a SAg with improved binding to its natural ligands, which may confer a possible evolutionary advantage for bacterial strains expressing SEG.

The PI3K p110delta controls T-cell development, differentiation and regulation

PI3Ks (phosphoinositide 3-kinases) regulate diverse cellular functions such as metabolism, growth, gene expression and migration. The p110delta isoform of PI3K is mainly expressed in cells of the immune system and contributes to cellular and humoral immunity. In the thymus, p110delta and p110gamma play complementary roles in regulating the transition through key developmental checkpoints. In addition, p110delta regulates the differentiation of peripheral Th (helper T-cells) towards the Th1 and Th2 lineages. Moreover, p110delta is critical for Treg (regulatory T-cell) function. Here, we review the role of PI3Ks in T-cell development and function.

Nonmalignant T cells stimulate growth of T-cell lymphoma cells in the presence of bacterial toxins

Bacterial toxins including staphylococcal enterotoxins (SEs) have been implicated in the pathogenesis of cutaneous T-cell lymphomas (CTCLs). Here, we investigate SE-mediated interactions between nonmalignant T cells and malignant T-cell lines established from skin and blood of CTCL patients. The malignant CTCL cells express MHC class II molecules that are high-affinity receptors for SE. Although treatment with SE has no direct effect on the growth of the malignant CTCL cells, the SE-treated CTCL cells induce vigorous proliferation of the SE-responsive nonmalignant T cells. In turn, the nonmalignant T cells enhance proliferation of the malignant cells in an SE- and MHC class II-dependent manner. Furthermore, SE and, in addition, alloantigen presentation by malignant CTCL cells to irradiated nonmalignant CD4(+) T-cell lines also enhance proliferation of the malignant cells. The growth-promoting effect depends on direct cell-cell contact and soluble factors such as interleukin-2. In conclusion, we demonstrate that SE triggers a bidirectional cross talk between nonmalignant T cells and malignant CTCL cells that promotes growth of the malignant cells. This represents a novel mechanism by which infections with SE-producing bacteria may contribute to pathogenesis of CTCL.

Foxp3 controls autoreactive T cell activation through transcriptional regulation of early growth response genes and E3 ubiquitin ligase genes, independently of thymic selection

To elucidate the mechanisms of autoreactive T cell activation and expansion, we used endogenous viral superantigens (VSAg)-reactive T cells as a model of self-antigens in two strains of Foxp3-mutant mice. These two strains, together with wild-type mice, provided us with an advantage to simultaneously study the positively and negatively selected as well as rescued autoreactive T cells. We show here that while both VSAg-reactive and non-VSAg-reactive T cells are equally activated in Foxp3-mutant mice, only the VSAg-reactive T cells are preferentially expanded independently of their selected states in the thymus. The T cell activation appears to be controlled by Foxp3 through transcriptional regulation of early growth response (Egr) genes Egr-2 and Egr-3, and E3 ubiquitin (Ub) ligase genes Cblb, Itch and GRAIL, subsequently affecting degradation of two key signaling proteins, PLCgamma1 and PKC-theta. Physiologically, the positively, but not negatively selected VSAg-reactive T cells are spontaneously activated without significant expansion. The results suggest that autoreactive T cell activation is controlled by Foxp3 through transcriptional regulation of early growth response genes and E3 ubiquitin ligase genes, independently of thymic selection.

Direct impact of inactivated HIV-1 virions on B lymphocyte subsets

Although there is no convincing evidence that HIV infects primary B cells, marked changes in B cell responses have been described in HIV-1-infected subjects, including B cell repertoire perturbations, depression of B cell memory and paucity of CD5(+) B cells. As it is hard to assess the consequences of these in vitro and ex vivo observations in patients, the pathogenic mechanisms responsible for the B cell deficit are unclear, and direct and indirect effects of HIV-1 remain possible. To gain further insight into the impact of HIV-1 on the B cell compartment in vivo, we used XenoMouse mice, mice genetically engineered to express human antibodies with an absence of mouse antibody expression. In these transgenic animals, B cells expressing a virtually full human Ig repertoire develop, which allows investigation of the in vivo consequences of confronting B cells expressing human immunoglobulins with HIV-1. We found that soluble gp120 induced an inversion in the B-1a/B-1b cell ratios, without impacting B-2 cells or affecting substantially the T cell compartment. Virion treatment specifically and dramatically depressed B-1a cells, which represent the majority of B-1 cells in normal mice. The observed B cell changes were associated with a functional alteration of the humoral response to tetanus toxoid. Thus, the results reveal a capacity of HIV-1 to specifically impact a highly specialized B cell subpopulation. Because there is evidence that human IgM memory B cells are functionally equivalent to murine B-1a cells, our findings suggest that gp120 may have a direct deleting activity on B cell memory.

Cutting edge: the phosphoinositide 3-kinase p110 delta is critical for the function of CD4+CD25+Foxp3+ regulatory T cells

CD4+CD25+Foxp3+ regulatory T cells (Tregs) contribute to the maintenance of peripheral tolerance by inhibiting the expansion and function of conventional T cells. Treg development and homeostasis are regulated by the Ag receptor, costimulatory receptors such as CD28 and CTLA-4, and cytokines such as IL-2, IL-10, and TGF-beta. Here we show that the proportions of Tregs in the spleen and lymph nodes of mice with inactive p110delta PI3K (p110deltaD910A/D910A) are reduced despite enhanced Treg selection in the thymus. p110deltaD910A/D910A CD4+CD25+Foxp3+ Tregs showed attenuated suppressor function in vitro and failed to secrete IL-10. In adoptive transfer experiments, p110deltaD910A/D910A T cells failed to protect against experimental colitis. The identification of p110delta as an intracellular signaling protein that regulates the activity of CD4+CD25+Foxp3+ Tregs may facilitate the further elucidation of the molecular mechanisms responsible for Treg-mediated suppression.

Burn injury induces an early activation response by lymph node CD4+ T cells

Several reports have shown that burn injury primes the immune system for an early and vigorous proinflammatory CD4 T cell response, suggesting that injury might signal CD4 T cell activation. We addressed this possibility by investigating changes in CD4 T cell activation marker expression, proliferation, and T cell receptor (TCR) usage at several early time points after burn injury. Using a sensitive flow cytometry approach to measure changes in the expression of Ki-67 antigen, a nuclear protein detected only in proliferating cells, we observed an early burst of proliferation by lymph node, but not spleen, CD4 T cells 12 h after burn injury. In contrast, mice that were treated with the bacterial superantigen staphylococcal enterotoxin B (SEB) as a positive control for in vivo T cell activation did not show this early proliferation. Instead, we observed a significant increase in proliferating lymph node and spleen CD4 and CD8 T cells by 3 days after SEB treatment. Burn injury induced higher cell surface CD25 and CD152 expression on lymph node CD4 T cells, whereas SEB treatment increased CD25 and CD69 expression on CD4 and CD8 T cells. Finally, we found that burn injury induced a proliferative response at 12 h by an oligoclonal subset of TCR Vbeta-chain-expressing CD4 T cells (Vbeta4, Vbeta6, Vbeta11, and Vbeta14). Interestingly, CD4 T cells expressing the Vbeta11-TCR remained significantly increased in the lymph nodes 3 days after burn injury. Taken together, these findings indicate that burn injury induces an early proliferation and activation of CD4 T cells in the regional lymph nodes and that these proliferating cells show restricted TCR Vbeta-chain usage consistent with the idea that injury triggers an early T cell activation signal.

Effect of metabolic control on the in vitro proliferation of peripheral blood mononuclear cells in type 1 and type 2 diabetic patients

CONTEXT AND OBJECTIVE

Diabetes mellitus is a clinical syndrome that frequently leads to the development of chronic complications and high susceptibility to infections. It is probably due to defective immunological defense, which may be related to metabolic control of the disease. The aim of this study was to evaluate the effect of metabolic control on immune-cell behavior in type 1 and type 2 diabetic patients. For this, the in vitro proliferation of peripheral blood mononuclear cells (PBMC) was analyzed in patients with inadequate and adequate metabolic control.

DESIGN AND SETTING

Experimental/laboratory study at a university hospital.

METHODS

Eleven type 1 and thirteen type 2 diabetic patients were studied, together with 21 healthy individuals divided in two groups (11/10), who were matched by sex and age with those diabetic patients. PBMC cultures stimulated with concanavalin-A (Con-A) were used to measure 3H-thymidine incorporation after 72 hours of cell culturing. For patients with inadequate metabolic control, culturing was performed on the first day of patient hospitalization and again after intensive treatment to achieve adequate control.

RESULTS

The proliferation index for Con-A-stimulated cultures from type 1 diabetic patients was significantly greater than that for cultures from healthy individuals and type 2 diabetic patients, independent of metabolic control. A negative correlation between the proliferation cell index and body mass index and serum C-reactive protein levels was also observed.

CONCLUSION

The increase in the proliferation capacity of type 1 diabetic T lymphocytes was probably not caused by hyperglycemia and/or insulinopenia related to inadequate metabolic control.

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.

Regulation of Apoptosis by Gram-Positive Bacteria: Mechanistic Diversity and Consequences for Immunity

Apoptosis, or programmed cell death (PCD), is an important physiological mechanism, through which the human immune system regulates homeostasis and responds to diverse forms of cellular damage. PCD may also be involved in immune counteraction to microbial infection. Over the past decade, the amount of research on bacteria-induced PCD has grown tremendously, and the implications of this mechanism on immunity are being elucidated. Some pathogenic bacteria actively trigger the suicide response in critical lineages of leukocytes that orchestrate both the innate and adaptive immune responses; other bacteria proactively prevent PCD to benefit their own survival and persistence. Currently, the microbial virulence factors, which represent the keys to unlocking the suicide response in host cells, are a primary focus of this field. In this review, we discuss these bacterial "apoptosis regulatory molecules" and the apoptotic events they either trigger or prevent, the host target cells of this regulatory activity, and the possible ramifications for immunity to infection. Gram-positive pathogens including Staphylococcus, Streptococcus, Bacillus, Listeria, and Clostridia species are discussed as important agents of human infection that modulate PCD pathways in eukaryotic cells.

CNS activational responses to staphylococcal enterotoxin B: T-lymphocyte-dependent immune challenge effects on stress-related circuitry

Staphylococcal enterotoxin B (SEB) is a bacterial superantigen that engages the immune system in a T-lymphocyte-dependent manner and induces a cytokine profile distinct from that elicited by the better-studied bacterial pathogen analog, lipopolysaccharide (LPS). Because of reports of SEB recruiting central nervous system (CNS) host defense mechanisms via pathways in common with LPS, we sought to further characterize central systems impacted by this agent. Rats were treated with SEB at doses of 50-5,000 mug/kg, and killed 0.5-6 hours thereafter. SEB injection produced a discrete pattern of Fos induction in brain that peaked at 2-3 hours postinjection and whose strength was dose-related. Induced Fos expression was predominantly subcortical and focused in a set of interconnected central autonomic structures, including aspects of the bed n. of the stria terminalis, central amygdala and lateral parabrachial nuclei; functionally related (and LPS-responsive) cell groups in the n. solitary tract, ventrolateral medulla, and paraventricular hypothalamic n. (PVH) were, by contrast, weakly responsive. SEB also activated cell groups in the limbic forebrain (lateral septal n, medial prefrontal cortex) and hypothalamic GABAergic neurons, which could account for its failure to elicit reliable increases in Fos-ir or corticotropin-releasing factor (CRF) mRNA in the PVH. SEB nevertheless did provoke reliable pituitary-adrenal secretory responses. The identification of subsets of central autonomic and limbic forebrain structures that are sensitive to SEB provides a basis for a systems-level understanding of the physiological and behavioral effects attributed to the superantigen. Core SEB-responsive cell groups exclude a medullary-PVH circuit implicated in pituitary-adrenal responses to LPS.

Chronic rhinosinusitis and superantigens

This article discusses the potential role of bacterial superantigens (SAgs) in chronic rhinosinusitis with nasal polyposis (CRS/NP). First, it briefly describes SAgs, focusing on how they interact with the immune system by binding to T-cell receptors (TCR) and major histocompatibility complex (MHC) class II molecules. Second, it discusses the role of SAgs in other chronic inflammatory diseases.Finally, it presents evidence for the role of SAgs in the pathogenesis and maintenance of CRS/NP focusing on current research and future considerations.

The role of the MHC on resistance to group a streptococci in mice

The severity of infection with Streptococcus pyogenes is strongly influenced by the host's genetics. This observation extends to the murine model of streptococcal infection, where the background of the mouse strain determines the infection outcome (BALB/c are resistant, whereas C3H/HeN are susceptible). To determine the extent to which the MHC complex (H2) contributed to diseases susceptibility, the response to S. pyogenes of congenic BALB mice from a resistant background (BALB/c), but carrying the H2(k) region of susceptible C3H/HeN mice (BALB/k), was examined. BALB/k were as susceptible as the H2 donor strain (C3H/HeN). Linkage analysis performed in F(2) backcross ([BALB/c x C3H/HeN] x BALB/c) mice confirmed the presence of a susceptibility locus within the H2 region on proximal chromosome 17. The possibility that modulation of T cell responses to streptococcal superantigens (GAS-SAgs) by different H2 haplotypes may influence disease severity was examined. BALB/k exhibited a significantly stronger response at the level of cell proliferation and cytokine production to GAS-SAgs than did BALB/c mice. However, the fact that T cell-deficient SCID-C3H/HeN mice also exhibited a susceptible phenotype suggests a more important contribution of innate effector cells to disease susceptibility. Lower transcriptional levels of certain inflammation-related regulatory genes located on chromosome 17 were detected in macrophages from susceptible than in those from resistant mice in response to infection. These results suggest that susceptibility to S. pyogenes may be associated with an altered transcription of specific genes that may compromise the endogenous regulatory processes controlling the inflammatory cascade and favor the progression to sepsis.

Natural killer T cells are required for the development of a superantigen-driven T helper type 2 immune response in mice

We show, here, that one single injection or weekly injections of staphylococcal enterotoxin B (SEB), starting in 1-day-old newborn mice, induced a powerful immune response with a T helper type 2 (Th2) pattern, as judged by the isotype and cytokine profile, with the production of large amounts of SEB-specific immunoglobulin G1 (IgG1), detectable levels of SEB-specific IgE and increased production of interleukin-4 by spleen cells. These protocols also induced an increase in the levels of total IgE in the serum. Memory of SEB was transferred to secondary recipients by using total spleen cells from primed animals. The secondary humoral response in transferred mice was diminished if spleen cells from SEB-treated mice were previously depleted of CD3+ or Vbeta8+ T cells or NK1.1+ cells. In vivo depletion of NK1.1+ cells in adult mice resulted in a marked reduction in the SEB-specific antibody response in both the primary and secondary immune responses. Additionally, purified NK1.1+ T cells were able to perform SEB-specific helper B-cell actions in vitro and in vivo. These results suggest that NK1.1+ T cells are required for the full development of humoral immunological memory, whilst making neonatal tolerance to SEB unachievable.

Endogenous superantigens shape response to exogenous superantigens

Endogenous superantigen-mediated thymic negative selection resulted in a paucity of mature T cells bearing T-cell receptor (TCR) Vbeta8 in the periphery. Consequently, the magnitude of immune response to exogenous superantigen staphylococcal enterotoxin B, which activates TCR Vbeta8(+) T cells, was significantly reduced and conferred protection from superantigen-induced mortality.

Involvement of streptococcal mitogenic exotoxin Z in streptococcal toxic shock syndrome

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

Variations in the protective immune response against streptococcal superantigens in populations of different ethnicity

Superantigens (SAgs) from group A streptococcus (GAS) are potent T cell mitogens, and have been suggested to play a role in severe streptococcal disease. Neutralizing antibodies protect against SAg-mediated disease and their levels should therefore be inversely related to severe streptococcal infection. Neutralizing anti-SAg titers in patients with severe GAS infection and patients without disease were compared in two separate groups. The first group comprised patients with invasive GAS disease from New Zealand European, Maori, and Pacific Island descent. The second group comprised Aboriginal Australian individuals with rheumatic heart disease and/or a past history of acute rheumatic fever. Patients sera were tested for their ability to neutralize T cell mitogenicity of recombinant streptococcal SAgs as a measure of functional SAg-neutralizing antibody concentration. In both studies, no inverse correlation was observed between disease and the level of serum SAg-neutralizing activity. Notably, much higher levels of natural immunity to all streptococcal SAgs were found in New Zealand Maori, New Zealand Pacific Island, and Aboriginal Australian individuals, suggesting a high degree of natural exposure and seroconversion in these groups compared to the New Zealand European cohort. Levels of serum antibodies against SAgs could not be used to predict disease susceptibility in groups with existing high levels of SAg-neutralizing antibodies.

Negative thymocyte selection to HERV-K18 superantigens in humans

An experimental system to explore central tolerance in humans is unavailable. However, the human endogenous retrovirus K-18 (HERV-K18) region on chromosome 1 provides an excellent model: HERV-K18 encodes a superantigen (SAg) stimulating Vβ7CD4 T cells that is implicated in type 1 diabetes and Epstein-Barr virus persistence. In this study, we have addressed thymic HERV-K18 SAg expression, the capacity of SAg to induce negative selection, and the consequences of this for peripheral tolerance compared with SAg reactivity. We demonstrate that thymic HERV-K18 SAg expression is constitutive and is restricted in time and space such that it can induce negative selection. We developed an in vitro assay capable of detecting negative human thymocyte selection by bacterial SAgs presented on extrathymic antigen-presenting cells (APCs). Using this assay, the HERV-K18 SAg is necessary and sufficient for negative selection of immature or semimature Vβ7CD4 thymocytes. Decreases of SAg reactive Vβ7CD4 T cells generated in the thymus predict low or absent SAg reactivity. Therefore, these results indicate that negative thymic selection to HERV-K18 SAgs constitutes a first checkpoint controlling peripheral tolerance compared with SAg reactivity. This study now offers a framework to dissect negative selection and its interplay with viral persistence and autoimmunity in humans.