CHARACTERIZATION OF AN ALTERNATIVE SUPERANTIGEN BINDING-SITE EXPRESSED ON A RENAL FIBROBLAST CELL-LINE

It is well established that the bacterial superantigens bind to both T CR beta-chain and, with moderate affinity, to MHC class II molecules, Class II-bearing cells bind the superantigen and present the superanti gen to T cells expressing certain TCR beta-chain variable region allel es, We have observed that the superantigen staphylococcal enterotoxin (SE) B binds to COS-1, an African green monkey kidney fibroblast-like cell line, This cell line fails to express class II and the binding of SEB is saturable, Scatchard analysis of radiolabeled ligand binding d ata reveals a binding affinity for COS of 51 nM, while binding to DR1- transfected L cells is measured at 150 nM. Further analysis shows that SEB bound to COS cells, unlike SEB bound to DR1-bearing L cells, is n ot recognized by T cells in the presence or absence of accessory cytok ines or anti-CD28, Studies carried out with chemical cross-linking age nts show that radiolabeled SEB associates with a membrane protein of s imilar to 85 kDa, These studies suggest that the alternative superanti gen binding molecule (p85) binds SEB with an affinity roughly equivale nt to SEB binding by class II molecules, but at a site which does not permit recognition by the TCR.