MOLECULAR CHARACTERIZATION AND ROLE IN T-CELL ACTIVATION OF STAPHYLOCOCCAL-ENTEROTOXIN A BINDING TO THE HLA-DR ALPHA-CHAIN

Superantigens bind to MHC class II-positive cells and stimulate T lymp hocytes expressing specific V beta regions of the TCR. Two distinct re gions of staphylococcal enterotoxin A superantigen (SEA) have been sho wn to affect the binding to MHC class II molecules. Results presented here demonstrate for the first time that the SEA-DR interaction can be affected by mutations on the class II alpha-chain. Furthermore, we ha ve precisely mapped the interaction of the SEA N-terminal domain with the alpha 1 domain of HLA-DR. Scatchard analysis using DAP cells trans fected with mutant class II molecules showed a role for residue DR alp ha K39 in the binding of SEA. Also, complementation experiments using mutant SEA molecules revealed an interaction between SEA residue F47 a nd position alpha Q18 on an outer loop of HLA-DR. These interactions b etween SEAF47 and the DR alpha-chain are critical, as they allow the r ecognition by an otherwise nonreactive V beta 1(+) T cell hybridoma an d induction of tyrosine phosphorylation through the TCR.