Exotoxins of Staphylococcus aureus belong to a family of bacterial proteins that act as superantigens by activating a large subset of the T-cell population, causing massive release of inflammatory cytokines. This cascade can ultimately result in toxic shock syndrome and death. Therapeutics targeting the early stage of the pathogenic process, when the superantigen binds to its receptor, could limit the severity of disease. We engineered picomolar binding affinity agents to neutralize the potent toxin staphylococcal enterotoxin B (SEB). A single immunoglobulin-like domain of the T-cell receptor (variable region, Vbeta) was subjected to multiple rounds of directed evolution using yeast display. Soluble forms of the engineered Vbeta proteins produced in Escherichia coli were effective inhibitors of SEB-mediated T-cell activation and completely neutralized the lethal activity of SEB in animal models. These Vbeta proteins represent an easily produced potential treatment for diseases mediated by bacterial superantigens.

Background The CDC et al. reported methicillin-resistant S. aureus (MRSA) are significant causes of serious human infections, including pulmonary illnesses. We investigated the role of superantigens (SAgs) in lung-associated lethal illness in rabbits. Methods A rabbit model was established to investigate the potential role of SAgs, staphylococcal enterotoxin (SE) B and SEC, and toxic shock syndrome toxin-1 (TSST-1). Rabbits received intra-bronchial community-associated (CA) MRSA strains USA200 (TSST-1+), MW2 (SEC+), or c99-529 (SEB+), or purified SAgs. Some rabbits were pre-immunized against SAgs or treated with soluble high-affinity T cell receptors (Vβ-TCR) to neutralize SEB and then challenged intra-bronchially with CA-MRSA or SAgs. Results Rabbits challenged with CA-MRSA or SAgs developed fatal, pulmonary illnesses. Animals pre-immunized against purified SAgs, or treated passively with Vβ-TCRs, and then challenged with CA-MRSA or SAgs, survived. Lung histology indicated non-immune animals developed lesions consistent with necrotizing pneumonia after challenge with CA-MRSA or purified SAgs. SAg immune animals or animals treated with soluble Vβ-TCRs did not develop pulmonary lesions. Conclusions SAgs contribute to lethal pulmonary illneses due to CA-MRSA; pre-existing immunity to SAgs prevents lethality. Administration of high-affinity Vβ-TCR with specificity for SEB to non-immune animals protects from lethal pulmonary illness due to SEB+ CA-MRSA and SEB.

Although cellular processes depend on protein-protein interactions, our understanding of molecular recognition between proteins remains far from comprehensive. Protein-protein interfaces are structural and energetic mosaics in which a subset of interfacial residues, called hot spots, contributes disproportionately to the affinity of the complex. These hot-spot residues can be further clustered into hot regions. It has been proposed that binding energetics between residues within a hot region are cooperative, whereas those between hot regions are strictly additive. If this idea held true for all protein-protein interactions, then energetically significant long-range conformational effects would be unlikely to occur. In the present study, we show cooperative binding energetics between distinct hot regions that are separated by >20 Å. Using combinatorial mutagenesis and surface plasmon resonance binding analysis to dissect additivity and cooperativity in a complex formed between a variable domain of a T cell receptor and a bacterial superantigen, we find that combinations of mutations from each of two hot regions exhibited significant cooperative energetics. Their connecting sequence is composed primarily of a single ␤-strand of the T cell receptor variable Ig domain, which has been observed to undergo a strand-switching event and does not form an integral part of the stabilizing core of this Ig domain. We propose that these cooperative effects are propagated through a dynamic structural network. Cooperativity between hot regions has significant implications for the prediction and inhibition of protein-protein interactions.binding energy ͉ cooperativity ͉ protein-protein interaction ͉ surface plasmon resonance ͉ T cell activity

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Superantigens (SAGs) bind simultaneously to major histocompatibility complex (MHC) and T-cell receptor (TCR) molecules, resulting in the massive release of inflammatory cytokines that can lead to toxic shock syndrome (TSS) and death. A major causative agent of TSS is toxic shock syndrome toxin-1 (TSST-1), which is unique relative to other bacterial SAGs owing to its structural divergence and its stringent TCR specificity. Here, we report the crystal structure of TSST-1 in complex with an affinity-matured variant of its wild-type TCR ligand, human T-cell receptor b chain variable domain 2.1. From this structure and a model of the wild-type complex, we show that TSST-1 engages TCR ligands in a markedly different way than do other SAGs. We provide a structural basis for the high TCR specificity of TSST-1 and present a model of the TSST-1-dependent MHC-SAG-TCR T-cell signaling complex that is structurally and energetically unique relative to those formed by other SAGs. Our data also suggest that protein plasticity plays an exceptionally significant role in this affinity maturation process that results in more than a 3000-fold increase in affinity.