Mutations in the MHC class II binding domains of staphylococcal enterotoxin A differentially affect T cell receptor Vbeta specificity.

C-terminal residues of staphylococcal enterotoxin A (SEA), including H187, D225, and D227, are involved in moderate affinity binding to MHC class II beta-chain, whereas N-terminal residues, including F47, are involved in low affinity binding to MHC class II alpha-chain. The effect of alanine substitutions at residues D227 or F47 on induction of T cell proliferation and the expansion of specific TCR Vbeta families was determined. SEA wild type specifically activated T cells expressing Vbeta1, Vbeta5.2, Vbeta6, Vbeta7, Vbeta9, Vbeta18, and Vbeta22. Although SEA-D227A exhibited substantially reduced mitogenicity compared with SEA wild type, it expanded the same Vbeta-bearing T cells, except those expressing Vbeta1. By contrast, SEA-F47A, which was slightly less mitogenic than SEA wild type, induced expansion only of T cells expressing Vbeta6, Vbeta7, and to a lesser extent Vbeta22. Therefore, specific mutations affecting either MHC class II alpha or beta binding sites differentially affect the Vbeta specificity of this superantigen. The lack of expansion in four of seven Vbeta families by SEA-F47A suggests that the class II alpha binding site may position SEA on the MHC class II molecules in an appropriate conformation for interaction with certain Vbeta elements.