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The bacterial superantigen staphylococcal enterotoxin A (SEA) binds with high affinity to major histocompatibility complex (MHC) class II molecules and subsequently activates T cells bearing particular T-cell receptor (TCR) Vbeta chains. Structural and mutational studies have defined two distinct MHC class II binding sites located in the N-terminal and C-terminal domains of SEA. The N-terminal F47 amino acid is critically involved in a low-affinity interaction to the MHC class II alpha-chain, while the C-terminal residues H187, H225, and D227 coordinate a Zn2+ ion and bind with moderate affinity to the beta-chain. In order to analyze whether the SEA-MHC class II alpha-chain interaction plays a role in dictating the in vivo repertoire of T-cell subsets, we studied distinct Vbeta populations after stimulation with wild-type SEA [SEA(wt)] and SEA with an F47A mutation [SEA(F47A)]. Injections of SEA(wt) in C57BL/6 mice induced cytokine release in serum, strong cytotoxic T-lymphocyte activity, expansion of T-cell subsets, and modulated expression of the T-cell activation antigens CD25, CD11a, CD44, CD62L, and CD69. SEA-reactive TCR Vbeta3+ and Vbeta11+ T cells were activated, while TCR Vbeta8+ T cells remained unaffected. The SEA(F47A) mutant protein induced a weaker T-cell response and failed to induce substantial interleukin-6 production compared to SEA(wt). Notably, SEA(F47A) failed to activate TCR Vbeta11+ T cells, whereas in vivo expansion and modulation of T-cell activation markers on TCR Vbeta3+ T cells were similar to those for SEA(wt). A similar response to SEA(F47A) was seen among CD4+ and CD8+ T cells. Activation of TCR Vbeta3+ and TCR Vbeta11+ T-cell hybridomas confirmed that SEA(F47A) activates TCR Vbeta3+ but not TCR Vbeta11+ T cells. The data support the view that the SEA-N-terminal MHC class II alpha-chain interaction defines a topology that is required for engagement of certain TCR Vbeta chains in vivo.

A mutation of F47 to A in staphylococcus enterotoxin A activates the T-cell receptor Vbeta repertoire in vivo