MUTATIONS IN THE MHC CLASS-II BINDING DOMAINS OF STAPHYLOCOCCAL-ENTEROTOXIN-A DIFFERENTIALLY AFFECT T-CELL RECEPTOR V-BETA SPECIFICITY

C-terminal residues of staphylococcal enterotoxin A (SEA), including H 187, D225, and D227, are involved in moderate affinity binding to MHC class II beta-chain, whereas N-terminal residues, including F47, are i nvolved in low affinity binding to MHC class II cy-chain. The effect o f alanine substitutions at residues D227 or F47 on induction of T cell proliferation and the expansion of specific TCR VP families was deter mined. SEA wild type specifically activated T cells expressing V beta 1, V beta 5.2, V beta 6, V beta 7, V beta 9, V beta 18, and V beta 22. Although SEA-D227A exhibited substantially reduced mitogenicity compa red with SEA wild type, it expanded the same V beta-bearing T cells, e xcept those expressing V beta 1. By contrast, SEA-F47A, which was slig htly less mitogenic than SEA wild type, induced expansion only of T ce lls expressing V beta 6, V beta 7, and to a lesser extent V beta 22. T herefore, specific mutations affecting either MHC class II alpha or be ta binding sites differentially affect the V beta specificity of this superantigen, The lack of expansion in four of seven Vp families by SE A-F47A suggests that tile class II alpha binding site may position SEA on the MHC class II molecules in an appropriate conformation for inte raction with certain V beta elements.