Activates the T-Cell Receptor Vb Repertoire In Vivo

The bacterial superantigen staphylococcal enterotoxin A (SEA) binds with high affinity to major histocom-patibility complex (MHC) class II molecules and subsequently activates T cells bearing particular T-cell receptor (TCR) Vb 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 a-chain, while the C-terminal residues H187, H225, and D227 coordinate a Zn21 ion and bind with moderate affinity to the b-chain. In order to analyze whether the SEA–MHC class II a-chain interaction plays a role in dictating the in vivo repertoire of T-cell subsets, we studied distinct Vb populations after stimulation with wild-type SEA [SEA(wt)] and SEA with an F47A mu-tation [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 Vb31 and Vb111 T cells were activated, while TCR Vb81 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 Vb111 T cells, whereas in vivo expansion and modulation of T-cell activation markers on TCR Vb31 T cells were similar to those for SEA(wt). A similar response to SEA(F47A) was seen among CD4 1 and CD81 T cells