SEQUENCES IN BOTH CLASS-II MAJOR HISTOCOMPATIBILITY COMPLEX ALPHA AND BETA CHAINS CONTRIBUTE TO THE BINDING OF THE SUPERANTIGEN TOXIC SHOCK SYNDROME TOXIN-1

Class II major histocompatibility complex (MHC) molecules present peptides derived from processed antigen to antigen-specific CD4-positive T cells. In addition, class II molecules bind with high affinity another class of antigens, termed superantigens. T cell stimulation by superantigens depends almost exclusively on the V-beta segment expressed by the T cell receptor (TCR). Mapping of the superantigen binding site on class II molecules should provide valuable information on how MHC and TCR molecules interact. Recombinant mouse I-A class II molecules expressed on transfected L cells were analyzed for their ability to bind the toxic shock syndrome toxin 1. Polymorphic residues in the alpha-helices of both the alpha and beta-chains of I-A contributed to quantitative toxin binding, suggesting that the toxin binds to either a combinatorial or a conformational site on class II MHC molecules.