A MUTATION OF F47 TO A IN STAPHYLOCOCCUS ENTEROTOXIN-A ACTIVATES THE T-CELL RECEPTOR V-BETA REPERTOIRE IN-VIVO

The bacterial superantigen staphylococcal enterotoxin A (SEA) binds wi th high affinity to major histocompatibility complex (MHC) class II mo lecules and subsequently activates T cells bearing particular T-cell r eceptor (TCR) V beta chains, Structural and mutational studies have de fined two distinct MHC class II binding sites located in the N-termina l and C-terminal domains of SEA. The N-terminal F47 amino acid is crit ically involved in a low-affinity interaction to the MHC class II alph a-chain, while the C-terminal residues H187, H225, and D227 coordinate a Zn2+ ion and bind with moderate affinity to the beta-chain. In orde r to analyze whether the SEA-MHC class II alpha-chain interaction play s a role in dictating the in vivo repertoire of T-cell subsets, rye st udied distinct V beta populations after stimulation with wild-type SEA [SEA((wt))] and SEA with an F47A mutation [SEA((F47A))]. Injections o f SEA((wt)) in C57BL/6 mice induced cytokine release in serum, strong cytotoxic T-lymphocyte activity, expansion of T-cell subsets, and modu lated expression of the T-cell activation antigens CD25, CD11a, CD44, CD62L, and CD69. SEA-reactive TCR V beta 3(+) and V beta 11(+) T cells were activated, while TCR V beta 8(+) T cells remained unaffected. Th e SEA((F47A)) mutant protein induced a weaker T-cell response and fail ed to induce substantial interleukin-6 production compared to SEA((wt) ). Notably, SEA((F47A)) failed to activate TCR V beta 11(+) T cells, w hereas in vivo expansion and modulation of T-cell activation markers o n TCR V beta 3(+) T cells were similar to those for SEA((wt)). A simil ar response to SEA((F47A)) was seen among CD4+ and CD8+ T cells. Activ ation of TCR V beta 3(+) and TCR V beta 11(+) T-cell hybridomas confir med that SEA((F47A)) activates TCR V beta 3(+) but not TCR V beta 11() 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 en gagement of certain TCR V beta chains in vivo.