Superantigens are thought to make external contacts with major histocompatibility complex (MHC) class II molecules and with the V-beta portion of a T cell antigen receptor (TCR), thereby stimulating entire families of T cells. The precise mapping of superantigen binding sites on class II molecules may provide valuable information on how TCR and MHC molecules interact. Two bacterial superantigens, staphylococcal enterotoxins A and E (SEA/SEE) bind well to most HLA-DR alleles, but poorly to HLA-DRw53. The sequences responsible for this binding were localized to the putative alpha-helix of the DR beta-chain by measuring toxin binding to a panel of chimeric class II molecules expressed on transfected cells. Binding of SEA/SEE to the DRw14 (Dw9) molecule suggested that the conserved histidine 81 in the beta-chain of most DR molecules was important, whereas the tyrosine 81 in the DRw53 beta-chain was detrimental for high-affinity binding. To prove this, reciprocal point mutations were introduced in the DR1 and DRw53 beta-chains. Mutation of histidine 81 in the DR1 beta-chain to tyrosine reduced SEA/SEE binding, but did not prevent recognition of two DR1-restricted peptides by six of eight antigen-specific T cell lines. Conversely, introduction of histidine at position 81 in the DRw53 beta-chain restored normal levels of SEA/SEE binding. These data suggest that a binding site of SEA and SEE lies on the outer face of the beta-chain alpha-helix, pointing away from the antigen-binding groove.
