Specific receptor binding of staphylococcal enterotoxins by murine splenic lymphocytes

We describe a reliable assay to measure the specific binding of 125I-labeled staphylococcal enterotoxin A (SEA) by murine spleen cells. Toxin binding by lymphocytes was specific in that it was inhibited by unlabeled SEA but not by unrelated proteins. The biological activity of SEA (T-lymphocyte mitogenesis) correlated with toxin binding to splenic lymphocytes. In the presence of high concentrations of [125I]SEA, specific binding increased rapidly and approached saturation after 2 h. Toxin binding was sensitive to temperature and pH and was directly proportional to the concentration of spleen cells in the incubation mixture. We estimated that there was a single class of toxin-binding sites, which had an apparent equilibrium dissociation constant (Kd) of 8 x 10(-7) M and numbered 3,600 sites per cell. SEA and the antigenically distinct compounds staphylococcal enterotoxins B and E in excess competitively inhibited binding of [125I]SEA to mouse spleen cells. Our data suggest a common class of binding sites for the three staphylococcal enterotoxins.