Clarifying the Mechanism of Superantigen Toxicity

Superantigens are bacterial proteins that generate a powerful immune response by binding to Major Histocompatibility Complex class II molecules on antigen-presenting cells and T cell receptors on T cells. A recent article reveals that at least one of the superantigens, staphylococcal enterotoxin B (SEB), also binds the co-stimulatory molecule CD28, suggesting that a much larger and potentially more stable complex is formed at the immunological synapse than was previously thought. This revelation greatly clarifies some of the mystery surrounding how and why these toxins are able to elicit such a toxic immune response at extremely low concentrations. These findings also highlight a novel role for CD28 in microbial pathogenicity. Bacterial superantigens (Sags) constitute a family of very stable bacterial proteins that are the most potent known activators of the immune system. They can cause food poisoning or, if they occur at sufficient concentration in the blood or lymphoid tissue, systemic shock [1]. Those unfortunate enough to eat food contaminated with Staphylococcus aureus will experience a brief but violent episode of vomiting and diarrhoea just a few hours later—the gut’s attempt to expel the Sag before it wreaks havoc with the immune system. If a Sag does get into the bloodstream, and if the patient has no neutralising antibody from previous exposure, then the Sag will induce a sudden and profound T cell stimulation that generates a cascade of cytokines, resulting in symptoms that include high fever, headache, vomiting, hypotension, aches, and rash, causing the condition known as Toxic Shock Syndrome. This life-threatening illness is often associated with young females who have developed an intra-vaginal infection of a staphylococcal strain producing the Sag Toxic Shock Syndrome Toxin (TSST) [2,3]. Deep tissue infections by Streptococcus pyogenes can also produce similarly powerful Sags capable of causing lethal shock [1]. Interestingly, the Saginduced immune response is not targeted at the bacteria themselves, but rather Sags function to direct a nonspecific T celland cytokine-mediated immune response that somehow assists in bacterial survival. Although many cytokines are produced in response to a single Sag, acute toxicity is blamed on the excessive production of three T cell cytokines— Interleukin-2 (IL-2), Interferon-c (INF-c), and particularly Tumour necrosis Factor a (TNF-a) [4,5]. Perhaps the most notable feature of Sags is their extreme potency: many of the more than 30 different staphylococcal and streptococcal Sags stimulate profound proliferation and cytokine production in up to 20% of all peripheral T cells, at concentrations in culture that approach 1 femtomolar (10 moles/l). This is especially remarkable since T cells are not directly involved in the immediate defence against these bacteria. Why S. aureus and S. pyogenes should produce such a powerful T cell response has never been clearly resolved. One hypothesis proposes that Sags are important in the very early stages of colonisation when the bacteria are struggling to establish a niche. By stimulating local T cells, Sags may suppress the recruitment and activation of their real enemy—neutrophils and macrophages, which would destroy the bacteria. Sags must therefore be effective at vanishingly low concentrations and it is only when the bacterial colony becomes well established and fails to shut off Sag production that the toxic sequelae arise—a state that can be of little benefit to the bacteria and even less benefit to the host. Normally, microbial antigens are internalized by antigen presenting cells, digested, and then presented as small peptides on the cell surface bound together with Major Histocompatability Complex class II (MHC class II) molecules; the combination of MHC and peptide is then recognized by T cell Receptors (TcRs) expressed on T cells, thus stimulating an immune response specific to that peptide antigen. However, over the past two decades many elegant studies have revealed that all Sags do one thing very well—they hijack T cell antigen recognition by directly cross-linking MHC class II and TcR, thus bypassing the antigenpresenting stage and stimulating a much larger, inappropriate immune response. All Sags therefore have at least two separate binding sites—one for MHC class II and another for the b-chain of TcR [6]. Mutagenesis studies have mapped these sites for a number of different Sags and co-crystal structures of Sags bound to either MHC class II or TcR have confirmed their location [7,8]. What has been most surprising is the variety of binding modes used by individual Sags (Figure 1). For example Staphylococcal enterotoxin B (SEB) and C (SEC) cross-link MHC class II a-chain and TcR b-chain. Streptococcal pyrogenic exotoxin C (SPEC) binds to the other side of MHC class II and cross-links TcR b-chain while Staphylococcal Enterotoxin A (SEA) binds both aand b-chain binding sites on MHC class II to cross-link TcR b-chain. Staphylococcal enterotoxin H (SEH) is the only Sag that binds to a TcR a-chain (Figure 1) [9]. Thus although Sags share a very similar protein structure, each has evolved its own way of binding to MHC and TcR—and this remarkable binding diversity clearly offers S. aureus and S. pyogenes an important survival advantage. Although this Sag-MHC-TcR trimer model of Sag activation is universally accepted [6], there are several perplexing aspects of Sag behaviour that have never quite gelled, hinting that something else might better explain their extreme potency and