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

Abstract

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) Vβ 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 α-chain, while the C-terminal residues H187, H225, and D227 coordinate a Zn2+ ion and bind with moderate affinity to the β-chain. In order to analyze whether the SEA-MHC class II α-chain interaction plays a role in dictating the in vivo repertoire of T-cell subsets, we studied distinct Vβ 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 Vβ3+ and Vβ11+ T cells were activated, while TCR Vβ8+ 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 Vβ11+ T cells, whereas in vivo expansion and modulation of T-cell activation markers on TCR Vβ3+ 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 Vβ3+ and TCR Vp11+ T-cell hybridomas confirmed that SEA((F47A)) activates TCR Vβ3+ but not TCR Vβ11+ T cells. The data support the view that the SEA-N-terminal MHC class II α-chain interaction defines a topology that is required for engagement of certain TCR Vβ chains in vivo.