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Chapter 8 : Superantigens: Structure, Function, and Diversity
Category: Bacterial Pathogenesis; Immunology
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Bacterial superantigens (SAgs) are powerful T-cell stimulatory molecules produced primarily by Staphylococcus aureus and Streptococcus pyogenes. Bacterial SAgs possess the unique ability to cross-link major histocompatibility complex (MHC) class II molecules and T-cell receptors, which in turn is responsible for their ability to illicit an immune response several orders of magnitude greater than that of conventional peptide antigens. The division of staphylococcal and streptococcal SAgs into subfamilies based on amino acid sequence, structure, and physiological information has caused some disagreement in the scientific community. Conventional antigens are processed internally by antigen-presenting cells (APCs) and displayed as discrete peptides on the cell surface by MHC class II molecules. These peptide antigens are then recognized by T-cell receptors (TCRs) specific to that peptide. Crystal structures of SAgs in complex with MHC class II molecules via both the generic site (SEB and TSST-1 in complex with HLA-DR1) and the high-affinity site SpeC in complex with HLA-DR2 and SEH in complex with HLA-DR1 have allowed a detailed examination of these interactions. A zinc ion plays a critical role in the binding of SME-Z2, SpeG, and SpeH to MHC class II molecules, as the binding of all three of these toxins to LG-2 cells is significantly reduced by chelating the zinc. The chapter talks about binding to the t-cell receptor, formation of the trimeric complex for signal transduction, and other structural features and idiosyncrasies.
Role of zinc ion in superantigen function