Staphylococcus-mediated T-cell activation and spontaneous natural killer cell activity in the absence of major histocompatibility complex class II molecules

Lipopolysaccharide is required for the lethal effects of enterotoxin B after D-galactosamine sensitization

We tested the D-galactosamine sensitization model with staphylococcal enterotoxin B (SEB). LPS was required for the lethal effects of SEB in D-galactosamine sensitized mice. Only two (2/62) among the C3HeB/FeJ (H-2k), Balb/c (H-2d) and C57BL/6J (H-2b) mice died in response to SEB in the absence of LPS whereas injection of SEB and minimally lethal concentrations of LPS became highly toxic. Similar to LPS, the lethal effect of SEB was dependent on the mouse strain used. Mouse strains more sensitive to the effects of LPS (Balb/c and C57BL/6J) were also more sensitive to the effects of SEB in comparison to C3H mice when equivalent doses of LPS and SEB were used. Among Balb/c and C3HeB/FeJ but not C57BL/6J mice, SEB (20 μg) potentiated the lethal effects of LPS at low doses (0.1 μg LPS), but had an apparent protective effect at high doses (1 μg LPS). Lastly, there was an inverse correlation between pathogenesis and serum IL-2 concentrations and splenic T cell activation in C3H mice. However, macrophage mobilization did correlate with lethality. Therefore, some questions remain about the mechanisms involved in the D-galactosamine/SEB pathogenesis model. We conclude that when sensitizing mice with D-galactosamine and assessing the lethal effects of SEB, endotoxin contamination must be assessed.

Defining the immune response to Ehrlichia species using murine models

Pathogenic bacteria belonging to the family Anaplasmataceae include species of the genera Ehrlichia and Anaplasma. Ehrlichia chaffeensis, first known as the causative agent of human monocytic ehrlichiosis, also infects several vertebrate hosts including white-tailed deer, dogs, coyotes and goats. E. chaffeensis is transmitted from the bite of an infected hard tick, such as Amblyomma americanum. E. chaffeensis and other tick-transmitted pathogens have adapted to both the tick and vertebrate host cell environments. Although E. chaffeensis persists in both vertebrate and tick hosts for long periods of time, little is known about that process. Immunological studies will be valuable in assessing how the pathogen persists in nature in both vertebrate and invertebrate hosts. Understanding the host immune response to the pathogen originating from dual host backgrounds is also important to develop effective methods of diagnosis, control and treatment. In this paper, we provide our perspective of the current understanding of the immune response against E. chaffeensis in relation to other related Anaplasmataceae pathogens.

In vivo staphylococcal superantigen-driven polyclonal Ig responses in mice: dependence upon CD4(+) cells and human MHC class II

Staphylococcal enterotoxin (SE) B and seven other staphylococcal superantigens (SAg), despite promoting vigorous Ig production in human peripheral blood mononuclear cell cultures, are exceedingly poor at eliciting Ig responses in cultures of spleen cells from C57BL/10J (B10) or C3H/HeJ mice. In contrast, SEB elicits Ig responses in cultures of spleen cells from human MHC class II-transgenic mice. Whereas i.p. administration of SEB (0.2-20 microg) to non-transgenic B10 mice elicits very weak in vivo Ig responses, identical treatment of CD4(+) cell-intact (but not CD4(+) cell-depleted) human MHC class II-transgenic mice elicits dramatic increases in both splenic Ig-secreting cells and serum Ig levels. Over a 2-week period, the SEB-induced in vivo Ig responses peak and then plateau or fall in association with a preferential increase in splenic CD8(+) cells. Nevertheless, in vivo depletion of CD8(+) cells has no sustained effect on SEB-driven Ig responses. Taken together, these observations demonstrate that the effects of SAg on in vivo humoral immune responses are highly CD4(+) cell dependent, are substantially CD8(+) cell independent and can be successfully investigated using human MHC class II-transgenic mice. This model system may be useful in investigating the polyclonally activating effects of microbial products (prototypic environmental insults) on the development of systemic autoimmunity.

What are the microbial components implicated in the pathogenesis of sepsis? Report on a symposium

Despite considerable efforts in the past quarter century to improve therapy for sepsis, mortality rates remain unacceptably high. Microbe-derived constituents can induce the host to produce many mediators that can contribute to immune dysregulation, tissue damage, and death. Although endotoxin-mediated events are clearly important in gram-negative infections, gram-positive bacteria can also play a dominant role. Understanding the interplay of microbial constituents and host immune or inflammatory responses prompted a meeting at Rockefeller University in May 1998. Participants discussed the relative merits of a "2-hit" hypothesis to explain the course of lethal septic shock and a "multihit" synergistic threshold hypothesis. Recommendations include the following: (1) developing animal models that closely mimic human sepsis; (2) further investigating antibiotic effects on bacteria; (3) assessing the relationships between endotoxin, prokaryotic DNA, and peptidoglycan (i.e., independent, additive, or synergistic) in inducing host responses; and (4) developing new strategies to improve outcomes. Studies are needed to better define which and how different microbial constituents lead to sepsis and to provide critical leads for therapeutic intervention.

Apparent MHC-Independent Stimulation of CD8+ T Cells In Vivo During Latent Murine Gammaherpesvirus Infection

Like EBV-infected humans with infectious mononucleosis, mice infected with the rodent gammaherpesvirus MHV-68 develop a profound increase in the number of CD8+ T cells in the circulation. In the mouse model, this lymphocytosis consists of highly activated CD8+ T cells strikingly biased toward Vβ4 TCR expression. Moreover, this expansion of Vβ4+CD8+ T cells does not depend on the MHC haplotype of the infected animal. Using a panel of lacZ-inducible T cell hybridomas, we have detected Vβ4-specific T cell stimulatory activity in the spleens of MHV-68-infected mice. We show that the appearance and quantity of this activity correlate with the establishment and magnitude of latent viral infection. Furthermore, on the basis of Ab blocking studies as well as experiments with MHC class II, β2-microglobulin (β2m) and TAP1 knockout mice, the Vβ4-specific T cell stimulatory activity does not appear to depend on conventional presentation by classical MHC class I or class II molecules. Taken together, the data indicate that during latent infection, MHV-68 may express a T cell ligand that differs fundamentally from both conventional peptide Ags and classical viral superantigens.

Major histocompatibility class I molecules present Urtica dioica agglutinin, a superantigen of vegetal origin, to T lymphocytes

The Urtica dioica agglutinin (UDA) shares with the superantigens the property of activating T cell subsets bearing particular Vbeta segments of the TCR. However, UDA is a lectin capable of binding to many glycoproteins on cell membranes. The implication of MHC versus other glycoproteins in UDA presentation was presently studied. Using mutant mice lacking MHC class I (MHC-I), MHC class II (MHC-II) or both MHC antigens, we provided evidence that MHC-I and MHC-II molecules serve as UDA receptors. Presentation by either one of these molecules ensured similar T cell responses and co-stimulatory signals were mandatory for optimal T cell activation and proliferation both in MHC-I and MHC-II contexts. Remarkably, in the absence of MHC molecules, UDA could not be efficiently presented to T cells by other glycosylated proteins. Surface plasmon resonance studies were used to confirm the binding of UDA to MHC-I molecules using a fusion protein consisting of MHC-I domains and beta2-microglobulin. The results indicated that the interaction between UDA and MHC-I molecules implicated lectin-binding site(s) of UDA. Taken together, our data demonstrate that, in addition to MHC-II antigens, MHC-I molecules serve as an alternative ligand for UDA.

Unique Superantigen Activity of Staphylococcal Exfoliative Toxins

Certain strains of Staphylococcus aureus express one or both of two related, but immunologically distinct, exfoliative toxins (ETA and ETB). These toxins induce the symptoms associated with staphylococcal scalded skin syndrome. Both ETs have been shown to stimulate T cell proliferation. Recently, it was reported that ETA is a superantigen that stimulates T cells bearing human Vβ2 or several murine Vβs. However, other investigators have proposed that the superantigenicity reported for ETA resulted from contaminants in commercial preparations. This present study addresses those conflicting reports by assessing the biological and immunologic activities of highly purified rETs. ETA and ETB required APCs to induce selective polyclonal expansion of several human Vβs (huVβs), although, neither toxin expanded huVβ2. ETB induced expansion of murine T cells bearing Vβs 7 and 8, those that have the highest homology to the huVβs expanded by ETA and ETB. Although flow cytometry of ETB-stimulated T cells matched PCR results, stimulation by ETA reduced percentages of T cells positive for several huVβs that had been shown to have increased levels of mRNA transcripts. ETA and ETB induced contrasting reactions in vivo. In rabbits, ETB was moderately pyrogenic and enhanced susceptibility to lethal shock, while ETA lacked both activities. Predictions based on comparisons with other superantigens suggest molecular regions potentially involved in receptor binding in the ETA crystal structure and a modeled ETB three-dimensional structure. These results show that ETs are superantigens with unique properties that could account for the discrepancies reported.

Clinical, microbial, and biochemical aspects of the exfoliative toxins causing staphylococcal scalded-skin syndrome

The exfoliative (epidermolytic) toxins of Staphylococcus aureus are the causative agents of the staphylococcal scalded-skin syndrome (SSSS), a blistering skin disorder that predominantly affects children. Clinical features of SSSS vary along a spectrum, ranging from a few localized blisters to generalized exfoliation covering almost the entire body. The toxins act specifically at the zona granulosa of the epidermis to produce the characteristic exfoliation, although the mechanism by which this is achieved is still poorly understood. Despite the availability of antibiotics, SSSS carries a significant mortality rate, particularly among neonates with secondary complications of epidermal loss and among adults with underlying diseases. The aim of this article is to provide a comprehensive review of the literature spanning more than a century and to cover all aspects of the disease. The epidemiology, clinical features, potential complications, risk factors, susceptibility, diagnosis, differential diagnoses, investigations currently available, treatment options, and preventive measures are all discussed in detail. Recent crystallographic data on the toxins has provided us with a clearer and more defined approach to studying the disease. Understanding their mode of action has important implications in future treatment and prevention of SSSS and other diseases, and knowledge of their specific site of action may provide a useful tool for physiologists, dermatologists, and pharmacologists.

Intercellular Adhesion Molecule-1 and Leukocyte Function-Associated Antigen-3 Provide Costimulation for Superantigen-Induced T Lymphocyte Proliferation in the Absence of a Specific Presenting Molecule

Bacterial superantigens can bind TCR in the absence of MHC class II molecules and activate T lymphocytes when cocultured with certain class II-deficient accessory cells. It has not been determined, however, whether these accessory cells provide direct costimulation to the T cell or serve to present superantigens via a nonconventional ligand. We have identified a human adenocarcinoma cell line, SW480, that assists in the activation of human T cells by the staphylococcal enterotoxins B (SEB), C1 (SEC1), and D (SED), but not SEA, SEC2, SEC3, or SEE. SW480 cells did not express class II molecules, and anti-class II mAbs did not inhibit T cell proliferation, supporting the hypothesis that class II is not absolutely required for enterotoxin-mediated T cell activation. The TCR Vβ profile of T cells stimulated by SEB plus SW480 cells was similar to that of T cells stimulated by SEB plus class II+ APC, indicating that TCR-SEB interactions were preserved in the absence of class II molecules. Binding studies failed to detect specific association of SEB with SW480 cells, suggesting that SW480 cells do not express receptors for enterotoxin. SEB coupled to beads, however, stimulated T cell proliferation, but only in the presence of SW480 cells. SW480 cells express both ICAM-1 and LFA-3 molecules, and the addition of Abs to these receptors inhibited T cell proliferation. These findings support a model in which certain enterotoxins engage the TCR independent of MHC class II or other specific presenting molecules and induce T cell proliferation with signals provided by nonconventional accessory cells.

Delineation by use of specific monoclonal antibodies of the T-cell receptor and major histocompatibility complex interaction sites on the superantigen toxic shock syndrome toxin 1

Murine monoclonal antibodies (MAbs) specific for toxic shock syndrome toxin 1 (TSST-1), a bacterial superantigen, showed the ability either to detect TSST-1 bound to histocompatibility locus antigen (HLA)-DR molecules or to inhibit TSST-1 binding to HLA-DR. A MAb capable of detecting DR-bound TSST-1 could also inhibit the toxin-induced activation of a T-cell receptor Vbeta15-expressing murine T-cell hybridoma. Alternatively, MAbs with specificity for the HLA-DR association site could present TSST-1 in vitro, stimulating CD4+ human T cells to proliferate. These functional activities correlated directly with with MAb specificity for HLA-DR versus T-cell receptor Vbeta interaction sites on TSST-1 as determined by reactivity with a panel of recombinant TSST-1 mutant molecules. Therefore, these MAbs discriminate the superantigen functional sites on the TSST-1 molecule and constitute reagents with the property of being potent modulators of the toxic activity of TSST-1.

Superantigens and pseudosuperantigens of gram-positive cocci

Superantigens use an elaborate and unique mechanism of T lymphocyte stimulation. Prototype superantigen are the pyrogenic exotoxins produced by Staphylococcus aureus and Streptococcus pyogenes. Many candidate proteins of bacterial, viral and protozoal origin have recently been reported to be superantigens. In most cases the evidence that these proteins are in fact superantigens is highly indirect. In this review the evidence that gram-positive cocci produce superantigens other than the pyrogenic exotoxins is critically discussed. Evidence in described demonstrating that the epidermolytic toxins of Staphylococcus aureus and the pyrogenic exotoxin B and M-proteins of Streptococcus pyrogenes are not superantigens. Criteria are described for acceptance of a candidate as a superantigen.

Monoclonal antibodies defining functional sites on the toxin superantigen staphylococcal enterotoxin B

Four monoclonal antibodies (mAbs) were produced binding to four nonoverlapping epitopes on the superantigen staphylococcal enterotoxin B (SEB). The mAbs were tested for their ability to detect SEB bound to major histocompatibility complex (MHC) class II, to inhibit SEB binding to MHC class II, to inhibit SEB stimulation of T cell hybridomas, to bind to various nonfunctional mutants of SEB, and to capture and present SEB and its mutants to T cells in the absence of MHC class II. We concluded that two mAbs, B344 and B327, bound to epitopes not required for superantigen function, one mAb, 2B33, blocked an MHC interaction site on SEB, and the fourth mAb, B87, blocked the T cell recognition site on SEB. Moreover, two mAbs (B344 and 2B33) were capable of presenting SEB, although much less efficiently than APC, to CD4- but not CD4+ T cell hybridomas. The results confirm the functional domains on SEB originally defined by mutation and show that MHC class II is not always an essential component of the superantigen ligand.