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Chapter 21 : Staphylococcal and Streptococcal Superantigens
This chapter discusses the superantigens (SAgs) of Staphylococcus aureus and Streptococcus pyogenes. The classic SAgs form a large family of related proteins that lack detectable enzymatic function but rather function to cross-link major histocompatibility complex (MHC) class II molecules on antigen-presenting cells (APCs) with certain T-cell receptors (TCRs), primarily those of the CD4 lineage. Staphylococcal and streptococcal SAgs are single-polypeptide-chain, nonglycosylated proteins of 22 to 30 kDa. TSST-1 is encoded on SaPIs 1, 2, and bovine. SaPIs are approximately 15.2 kb in size and are present in a single, but not necessarily the same, site in the chromosome. Mutagenesis studies provided evidence that the cystine loop of SE C1 is required for emesis. SAgs interact with TCR outside the typical TCR region for contact with antigenic peptide and MHC class II. The massive release of these cytokines can cause hypotension through capillary leak mediated by the nitric oxide pathway, with consequent effects on the endothelium. SAgs are most often associated with TSS illnesses. Staphylococci and streptococci are common pathogens of humans, and as such have myriad cell surface virulence factors that contribute to colonization and immune avoidance. Most of the SAgs are made during the postexponential phase of growth (excluding SE A, K, and Q, which are exponential phase regulated), when cell numbers are highest and the organisms are most likely to spread to new hosts.