Immunoregulatory effects of superantigens: interactions of staphylococcal enterotoxins with host MHC and non-MHC products

Conformational variants of I-Ak MHC class II molecules carry distinct immunopeptidomes

Major histocompatibility (MHC) class II molecules can exist in two distinct conformational states based on alternative pairing of transmembrane domain GxxxG dimerization motifs (i.e., M1- and M2-paired MHC class II). M1- and M2-paired MHC class II molecules drive different levels of T cell activation and B cell signaling; consequently, differential peptide loading would impact the level of immune response elicited by various antigens/epitopes. In previous studies of a single model antigen, we show that while peptide from BCR-bound antigen is selectively loaded onto M1-paired I-Ak class II, peptide from fluid phase processing of the same antigen is loaded onto both M1- and M2-paired I-Ak. To expand this analysis, we determined the immunnopeptidomes of M1-paired vs. total I-Ak class II molecules isolated from murine B cells. By comparing the two immunopeptidomes as well as the source proteins (antigens), a picture emerges highlighting the unique access each class II conformer has to antigens from different subcellular compartments. Sequence analysis of the two immunopeptidomes suggests a high degree of similarity between the peptide binding grooves of the two class II conformers. Analysis of class II-associated invariant chain (Ii)-derived peptides reveals the robust presence of a nested set of non-CLIP peptides that associate primarily with M1-paired class II, likely outside of the canonical peptide binding groove. In total, these results further highlight the differential peptide loading of M1- vs. M2-paired MHC class II molecules and support the idea that differential peptide loading could impact overall immune responsiveness.

A Few Key Historical Events in the Antibody Field: The Alacritous Antibody

We have coined the term "alacrity" to describe the extraordinary diversity of B cell activation potentials, even among cells in a single B cell clone responding to a single antigen. The discovery of methodologies for B cell culture in limiting dilution allowed scientists to identify the source of cellular heterogeneity among cells of the immune system. Analyses of individual B cells set the stage for more detailed descriptions of the factors that diversify B cell functions, some of which will be expanded upon by partner articles in this B cell issue.

Targeting fundamental pathways to disrupt Staphylococcus aureus survival: clinical implications of recent discoveries

The emergence of community-associated methicillin-resistant Staphylococcus aureus during the past decade along with an impending shortage of effective antistaphylococcal antibiotics have fueled impressive advances in our understanding of how S. aureus overcomes the host environment to establish infection. Backed by recent technologic advances, studies have uncovered elaborate metabolic, nutritional, and virulence strategies deployed by S. aureus to survive the restrictive and hostile environment imposed by the host, leading to a plethora of promising antimicrobial approaches that have potential to remedy the antibiotic resistance crisis. In this Review, we highlight some of the critical and recently elucidated bacterial strategies that are potentially amenable to intervention, discuss their relevance to human diseases, and address the translational challenges posed by current animal models.

O-Acetylation of Peptidoglycan Limits Helper T Cell Priming and Permits Staphylococcus aureus Reinfection

Humans do not usually develop effective immunity to Staphylococcus aureus reinfection. Using a murine model that mimics human infection, we show that lack of protective immunity to S. aureus systemic reinfection is associated with robust interleukin-10 (IL-10) production and impaired protective Th17 responses. In dendritic cell co-culture assays, priming with S. aureus promotes robust T cell proliferation, but limits Th cells polarization and production of IL-1β and other cytokines important for Th1 and Th17 differentiation. We show that O-acetylation of peptidoglycan, a mechanism utilized by S. aureus to block bacterial cell wall breakdown, limits the induction of pro-inflammatory signals required for optimal Th17 polarization. IL-10 deficiency in mice restores protective immunity to S. aureus infection, and adjuvancy with a staphylococcal peptidoglycan O-acetyltransferase mutant reduces IL-10, increases IL-1β, and promotes development of IL-17-dependent, Th cell-transferable protective immunity. Overall, our study suggests a mechanism whereby S. aureus modulates cytokines critical for induction of protective Th17 immunity.

The role of CXC chemokine receptor 2 in Staphylococcus aureus keratitis

Staphylococcus aureus is a leading cause of corneal infection. CXC receptor 2 binding chemokines have been implicated in the pathogenesis of Pseudomonas aeruginosa keratitis. The role of this receptor in immune responses during Staphylococcus keratitis remains to be fully understood. Corneas of CXC receptor 2 knockout and wild-type mice (Cmkar -/- & Cmkar +/+) were scratched and 1 × 10(8) cfu/ml of strain Staph 38 applied. Twenty-four hours post-infection, mice were sacrificed and eyes harvested for enumeration of bacteria and measurement of myeloperoxidase levels. Production of inflammatory mediators, cellular adhesion molecules and chemokines in response to infection were investigated by ELISA, and PCR. 24 h after challenge with S. aureus, Cmkar -/- mice had developed a more severe response with a 50-fold higher bacterial load than WT mice. PMNs failed to penetrate the corneas of Cmkar -/- mice. However, concentrations of KC, MIP-2, IL-1β and IL-6 were significantly elevated (6-13 fold) in Cmkar-/- mice. The concentration of LTB4 was decreased (2 fold). Cmkar-/- mice failed to upregulate mRNA for VCAM-1 or PECAM-1 in response to infection, but had constitutively higher levels of ICAM-1. A lack of CXC receptor 2 lead to an inability to control bacterial numbers as a result of failure of PMNs to penetrate the cornea to the site of infection, even when chemokines were more highly produced. These results imply that CXCR2-mediated signaling through upregulation of adhesion molecules is essential to margination of PMNs in this infection model.

The effect of treating infected skin grafts with Acticoat™ on immune cells

A study was conducted to determine the effect of Acticoat placed on an infected skin graft on parameters of immunity. Two partial thickness wounds (2 cm x 4 cm) were created on the dorsal midline of Hartley guinea pigs (n=28). Wounds were covered with autologous skin graft and maintained either aseptically (Noninoculated, n=8), inoculated with Staphylococcus aureus (Surgery-Inoculated, n=8) with or without Acticoat bandage (Surgery-Inoculated-Acticoat, n=6). Five days later, splenocytes and blood were collected to estimate natural killer cell (NK) cytotoxicity, proliferative response to T and B cell mitogens and neutrophil oxidative burst. Animals that did not undergo surgery were included as a nonsurgery control group. [(3)H]-thymidine incorporation in response to a variety of T and B cell mitogens was significantly lower for all groups undergoing surgery compared to the nonsurgery control group (p<0.0001) and no additional effect was observed on this immune measure by applying the Acticoat bandage. The Surgery-Inoculated-Acticoat group exhibited greater NK cytotoxic activity (as assessed as the ability to lyse K562 tumor cells) compared to the Surgery-Inoculated group (p<0.006). The Surgery-Inoculated-Acticoat group had higher neutrophil oxidative burst at 5 min post stimulation, but was not different from controls after 15 min. In conclusion, the application of an Acticoat bandage to an inoculated surgery wound did not alter the low cell-mediated immune response that followed surgery, but appeared to increase parameters (NK cytotoxic activity and neutrophil function) of innate immunity.

Large-scale early in vitro response to actinobacillus actinomycetemcomitans suggests superantigenic activation of T-cells

The mode of T-cell response to Actinobacillus actinomycetemcomitans is largely unknown. The present study sought to investigate the hypothesis that A. actinomycetemcomitans expresses superantigens, capable of antigen-non-specific T-cell activation. To that end, peripheral blood mononuclear cells were stimulated with A. actinomycetemcomitans, and T-cell expression of the early activation marker, CD69, was determined by flow cytometry. Results showed that A. actinomycetemcomitans activated a large number of T-cells with magnitude similar to that of staphylococcal enterotoxin superantigens. A. actinomycetemcomitans sonicate preferentially activated T-cells expressing Vbeta5.1 and Vbeta8, while the extracellular preparation activated Vbeta5.1+, Vbeta8+, and Vbeta12+ T-cells. T-cell response to A. actinomycetemcomitans was observed in the presence of autologous, as well as heterologous, antigen-presenting cells, suggesting a MHC-non-restricted response. Thus, the in vitro response to A. actinomycetemcomitans is characterized by large-scale T-cell activation in a Vbeta-specific and MHC-non-restricted manner, consistent with the involvement of superantigens.

Understanding the mechanism of action of bacterial superantigens from a decade of research

In the face of the unique diversity and plasticity of the immune system pathogenic organisms have developed multiple mechanisms in adaptation to their hosts, including the expression of a particular class of molecules called superantigens. Bacterial superantigens are the most potent stimulators of T cells. The functional consequences of the expression of superantigens by bacteria can be extended not only to T lymphocytes, but also to B lymphocytes and to cells of the myeloid compartment, including antigen-presenting cells and phagocytes. The biological effects of bacterial superantigens as well as their molecular aspects have now been studied for a decade. Although there is still a long way to go to clearly understand the role these molecules play in the establishment of disease, recently acquired knowledge of their biochemistry now offers unique experimental opportunities in defining the molecular rules of T-cell activation. Here, we present some of the most recent functional and molecular aspects of the interaction of bacterial superantigens with MHC class II molecules and the T-cell receptor.

Anti-Vbeta8 antibodies induce and maintain staphylococcal enterotoxin B-triggered Vbeta8+ T cell anergy

The mechanism involved in the maintenance of staphylococcal enterotoxin B (SEB)-induced T cell anergy is poorly understood. We demonstrated earlier that B cells play an important role in the maintenance of SEB-induced T cell anergy in vivo and in vitro. Here, we demonstrate that B cells are not essential in SEB-induced T cell activation, but are important for the maintenance of T cell memory phenotype and anergy in vivo. Studying the activated B cell repertoire, we observe that SEB treatment increases serum anti-Vbeta8 antibody titer as detected by enzyme-linked immunosorbent assay using soluble Vbeta8 chains as antigens, and by staining of a Vbeta8-expressing thymoma. These antibodies disappear gradually after immunization with SEB, whereas the capacity of the T cells to respond to SEB in vitro is restored. Anti-Vbeta8 monoclonal antibody treatment causes Vbeta8+ T cell unresponsiveness to SEB in vitro (anergy), without affecting CD4Vbeta8+ T cell frequency. Together, these results suggest a new mechanism to explain the maintenance of SEB-induced T cell anergy, which is dependent on B cells and on anti-Vbeta8 antibody that specifically interacts with Vbeta8+ T cells.

Macrophage cell lines derived from major histocompatibility complex II-negative mice

Two bone-marrow-derived macrophage cell lines, C2D and C2Dt, were isolated from major histocompatibility class II negative knock-out mice. The C2D cell line was stabilized by continuous culture in colony-stimulating factor-1 and the C2Dt cell line was transformed with SV40 virus large T antigen. These cells exhibited phenotypic properties of macrophages including morphology and expression of Mac 1 and Mac 2 cell surface molecules. These cells also had comparable growth to the bone-marrow-derived macrophage cell line B6MP102. These new cell lines were not spontaneously cytotoxic and were only capable of modest killing of F5b tumor cells when stimulated with LPS and interferon-gamma, but not when stimulated with LPS alone or with staphylococcal exotoxin. C2D and C2Dt cells phagocytosed labeled Staphylococcus aureus similarly to B6MP102 cells but less well than C2D peritoneal macrophages. These cell lines secreted interleukin-6, but not tumor necrosis factor or nitric oxide in response to LPS or staphlococcal enterotoxins A or B C2D(t) cells were tumorigenic in C2D and C57BL/6J mice but C2D cells were not. These data suggest that macrophage cell lines can be established from bone marrow cells of major histocompatibility complex II-negative mice.

Cutaneous exposure to the superantigen staphylococcal enterotoxin B elicits a T-cell-dependent inflammatory response

We analyzed the impact of superantigens secreted by skin-colonizing Staphylococci on the skin and the associated lymphoid tissue following epicutaneous application and intracutaneous injection of small amounts of staphylococcal enterotoxin B (SEB). A single intracutaneous injection of 50 ng of SEB elicited a strong inflammatory response in the skin of BALB/c mice. Three to 6 h later, we observed langerhans cell activation, mast cell degranulation, vasodilation, upregulation of ICAM-1, and induction of VCAM-1 on dermal blood vessels, with vascular adhesion of granulocytes. by 12 to 24 h, cell infiltration of the dermis increased, reaching the epidermis. Among the infiltrating leukocytes, a substantial number of eosinophils was found. After 48 h, the infiltrate was dominated by mononuclear cells. The response to SEB was dose-dependent, and signs of inflammation slowly disappeared over 5 to 7 days. Although the induction of VCAM-1 on dermal blood vessels suggested a role for interleukin-1/tumor necrosis factor-alpha in this reaction, the activation of monocytes/macrophages was not able to substitute for lymphocytes, as severe combined immunodeficiency (SCID) mice (which are lymphocyte-deficient) did not mount an inflammatory skin response to intradermal injection of SEB. The fact that nude mice (T-cell-deficient) also did not mount an inflammatory response to SEB indicated the T-cell dependency of the response. The V beta specificity of the SEB effect was demonstrated by the fact that SJL/J mice, which lack V beta 8+ T cells (the major SEB-reactive T cell population in mice), exhibited much weaker responses. Deletion or tolerization of SEB-reactive V beta T cells was not observed after a single intradermal injection of such minute amounts of SEB.

Analysis of the superantigenic activity of mutant and allelic forms of streptococcal pyrogenic exotoxin A

Infections with Streptococcus pyogenes (group A streptococcus) can result in the recently described streptococcal toxic shock syndrome (STSS), which is characterized by rashes, hypotension, multiorgan failure, and a high mortality rate. S. pyogenes isolates associated with STSS usually produce streptococcal pyrogenic exotoxin A (SpeA), a bacterial superantigen capable of stimulating host immune cells. Most of the symptoms of STSS are believed to result from cytokine release by the stimulated cells. To better understand the pathogenesis of STSS, we began studies on the SpeA-immune cell interaction. We generated 20 mutant forms of SpeA1 (SpeA encoded by allele 1), and the mutant toxins were analyzed for mitogenic stimulation of human peripheral blood mononuclear cells, affinity for class II major histocompatibility complex molecules (DQ), and disulfide bond formation. Residues necessary for each of these functions were identified. There are four alleles of speA, and STSS strains usually contain either allele 2 or allele 3. The product of allele 2, SpeA2, had slightly higher affinity for the class II MHC molecule compared with SpeA1 but not significantly greater mitogenic activity. SpeA3, however, was significantly increased in mitogenic activity and affinity for class II MHC compared with SpeA1. Thus, we have evidence that the toxin encoded by some of the highly virulent S. pyogenes STSS-associated isolates is a more active form of SpeA.

Staphylococcal enterotoxins bind H-2Db molecules on macrophages

We screened a panel of monoclonal antibodies against selected macrophage cell surface molecules for their ability to inhibit enterotoxin binding to major histocompatibility complex class II-negative C2D (H-2b) macrophages. Two monoclonal antibodies, HB36 and TIB126, that are specific for the alpha 2 domain of major histocompatibility complex class I, blocked staphylococcal enterotoxins A and B (SEA and SEB, respectively) binding to C2D macrophages in a specific and concentration-dependent manner. Inhibitory activities were haplotype-specific in that SEA and SEB binding to H-2k or H-2d macrophages was not inhibited by either monoclonal antibody. HB36, but not TIB126, inhibited enterotoxin-induced secretion of cytokines by H-2b macrophages. Lastly, passive protection of D-galactosamine-sensitized C2D mice by injection with HB36 antibody prevented SEB-induced death. Therefore, SEA and SEB binding to the alpha 2 domain of the H-2Db molecule induces biological activity and has physiological consequences.

Bacterial pyrogenic exotoxins as superantigens

The recent discovery of the mode of interaction between a group of microbial proteins known as superantigens and the immune system has opened a wide area of investigation into the possible role of these molecules in human diseases. Superantigens produced by certain viruses and bacteria, including Mycoplasma species, are either secreted or membrane-bound proteins. A unique feature of these proteins is that they can interact simultaneously with distinct receptors on different types of cells, resulting in enhanced cell-cell interaction and triggering a series of biochemical reactions that can lead to excessive cell proliferation and the release of inflammatory cytokines. However, although superantigens share many features, they can have very different biological effects that are potentiated by host genetic and environmental factors. This review focuses on a group of secreted pyrogenic toxins that belong to the superantigen family and highlights some of their structural-functional features and their roles in diseases such as toxic shock and autoimmunity. Deciphering the biological activities of the various superantigens and understanding their role in the pathogenesis of microbial infections and their sequelae will enable us to devise means by which we can intervene with their activity and/or manipulate them to our advantage.

Influence of staphylococcal enterotoxin B (SEB) on the course of murine listeriosis

Confrontation of the immune system with bacterial superantigens leads to an initial activation of the immune system followed by a state of profound immunosuppression. To investigate the role of a superantigen in an acute infection with a facultatively intracellular bacterium, we have studied the effect of staphylococcal enterotoxin B on the course of murine listeriosis. Intraperitoneal injection of SEB led to a statistically significant growth restriction of Listeria monocytogenes in the organs of mice infected intravenously or intraperitoneally when treatment with SEB and infection with L. monocytogenes were given simultaneously or when the mice were treated two days before infection. No effect of SEB on murine listeriosis was found when SEB was given more than two days before infection or one or more days after infection. We conclude that initial immunostimulation by SEB which is indicated by a massive liberation of all interleukins measured (IL1 alpha, IL6, TNF alpha, IL2, IFN gamma, IL4) is responsible for the growth restriction of L. monocytogenes in the organs of treated mice. Apoptosis of V beta 8 positive T cells which was accompanied by a 30% reduction of these cells at day 7 after treatment seems to be totally compensated.

Staphylococcal enterotoxin A and toxic shock syndrome toxin compete with CD4 for human major histocompatibility complex class II binding

We have examined the role of the CD4 molecule in primary T-lymphocyte responses to the staphylococcal enterotoxins SEA, SEB, SEC1, and the toxic shock syndrome toxin TSST-1. Proliferating cells were predominantly CD4+; however, the responses to SEA and TSST-1 were most sensitive to inhibition by the anti-CD4 antibody Leu-3a. T-lymphocyte responses to the bacterial superantigens were inhibited by site-directed mutations of residues in the DR beta membrane-proximal domain (DR beta 2) that are also known to be important for interactions with CD4. SEA and TSST-1 binding to DR was reduced by the DR beta 2 mutations and by competition with soluble recombinant CD4. We propose that bacterial superantigens sequentially, or simultaneously with CD4, stabilize complexes of T-cell antigen receptors and major histocompatibility complex class II molecules. The superantigen qualities of these toxins may be due, in part, to a molecular mimicry of CD4 and other adhesion molecules.

Molecular mimicry in the pathogenesis of AIDS: the HIV/MHC/mycoplasma triangle

The immune defects characterizing infection with the human immunodeficiency virus (HIV) and culminating in the acquired immune deficiency syndrome (AIDS) are the result of a multifactorial disease process, components of which are the occurrence of autoimmune phenomena and opportunistic infection. In this discussion, the observation that both the HIV-1 gp 120 envelope and Mycoplasma genitalium adhesin proteins share an area of significant similarity with the CD4-binding site of the class II major histocompatibility complex (MHC) proteins is placed in this perspective and mechanisms by which interaction within this triad could contribute to the T-cell dysfunction, T-cell depletion, Th1-cell-->Th2-cell shift, B-cell proliferation, hyperglobulinemia and antigen-presenting cell dysfunction observed during the development of AIDS are proposed.

Binding and activation of major histocompatibility complex class II-deficient macrophages by staphylococcal exotoxins

Macrophages from C2D transgenic mice deficient in the expression of major histocompatibility complex (MHC) class II proteins were used to identify binding sites for superantigens distinct from the MHC class II molecule. Iodinated staphylococcal enterotoxins A and B (SEA and SEB) and exfoliative toxins A and B (ETA and ETB) bound to C2D macrophages in a concentration-dependent and competitive manner. All four toxins increased F-actin concentration within 30 s of their addition to C2D macrophages, indicating that signal transduction occurred in response to toxin in the absence of class II MHC. Furthermore, ETA, ETB, SEA, and, to a lesser extent, SEB induced C2D macrophages to produce interleukin 6. Several molecular species on C2D macrophages with molecular masses of 140, 97, 61, 52, 43, and 37 kDa bound SEA in immunoprecipitation experiments. These data indicate the presence of novel, functionally active toxin binding sites on murine macrophages distinct from MHC class II molecules.

MHC class II signaling in B-cell activation

The cognate interaction between T cells and antigen-presenting cells (APCs), mediated by major histocompatibility complex (MHC) class II molecules, results in the delivery of activation signals to the APC. These signals contribute to the expression of co-stimulatory activity by APCs and have important consequences for cell effector function. MHC class II molecules also serve as receptors for B-cell stimulation by microbial superantigens. In this review, Paul Scholl and Raif Geha discuss recent advances in our understanding of mechanisms of MHC class II signaling and analyse their role in human B-cell activation.

Lipopolysaccharide-stimulated events in B cell activation

High concentrations of thymus-independent (TI) antigens are capable of inducing polyclonal B cell activation by their intrinsic mitogenic properties, irrespective of the specificity of the Ig receptors. Due to a genetic defect on the 4th chromosome, B cells from C3H/HeJ mice do not respond to lipopolysaccharide (LPS). In order to define at which step the mutation affects the signalling pathway, we compared B cells from C3H/HeJ and CBA mice with regard to changes of three events, namely cell size, and MHC class I and II antigen expression after LPS stimulation. We found that cell size and expression of MHC antigens increase in B cells from CBA mice after LPS stimulation, whereas B cells from C3H/HeJ mice do not respond at all. This suggests that the defect in C3H/HeJ mice interferes with early events in the signalling pathway, either due to the absence of a LPS receptor on B cell surface or the lack of an initial component necessary for effective signal transmission subsequent to LPS receptor binding. Our results also have shown that stimulation of anti-Ig antibodies and LPS differ in some signalling events and have different final effects on B cells, which suggests that they may function differently via distinct signalling pathways.

Toxicity of staphylococcal enterotoxins potentiated by lipopolysaccharide: major histocompatibility complex class II molecule dependency and cytokine release

The biological effects of staphylococcal enterotoxins (SE), potentiated by bacterial lipopolysaccharide (LPS), were studied with mice. Control animals survived the maximum dose of either SE or LPS, while mice receiving both agents died. SEA was 43-fold more potent than SEB and 20-fold more potent than SEC1. The mechanism of toxicity was further examined with transgenic mice deficient in major histocompatibility complex class I or II expression. Class II-deficient mice were resistant to SEA or SEB. However, class I-deficient animals were less susceptible to SEA (30% lethality) than wild-type mice (93% lethality). In vitro stimulation of T cells from the three mouse phenotypes by SEA correlated well with toxicity. T cells from transgenic or wild-type mice were similarly responsive to SEA when presented by irradiated, wild-type mononuclear cells. These data confirmed that the toxicity of SE was mainly exerted through a mechanism dependent on the expression of major histocompatibility complex class II molecules. Toxicity was also linked to stimulated cytokine release. Levels in serum of tumor necrosis factor alpha, interleukin-6, and gamma interferon peaked 2 to 4 h after the potentiating dose of LPS but returned to normal within 10 h. Concentrations of interleukin-1 alpha were also maximal after 2 h but remained above the background for up to 22 h. Relative to the levels in mice given only SEA or LPS, the levels in serum of tumor necrosis factor alpha, interleukin-6, and gamma interferon increased 5-, 10-, and 15-fold, respectively, after injections of SEA plus LPS. There was only an additive effect of SEA and LPS on interleukin-1 alpha concentrations.

Superantigen mediated shock: a cytokine release syndrome

Treatment of animals with superantigens results in profound immunological changes. A major fraction of all peripheral T cells becomes activated in vivo. Subsequently, successive waves of cytokines are produced with TNF playing a central pathophysiologic role. In addition, if the liver is damaged by an as yet poor defined mechanism the consequences of the cytokine syndrome are life threatening. However, TNF alone is not sufficient to cause death, instead synergizing interactions with cytokines like IL-1, IL-6, and IFN-gamma are probably involved. On the other hand, certain experimental conditions prevent these waves of cytokines and consequently lethal shock. Furthermore, a significant fraction of SA reactive T cells are deleted by programmed cell death 10 to 24 hours after treatment. Thereafter the surviving cells proliferate vigorously until day 2 or 3, followed by a second wave of apoptosis resulting in reduced SA reactive T cell numbers as compared to pretreatment levels. Of course, many aspects of the complicated events are only marginally understood and deserve further investigation.