Staphylococcal enterotoxins (SE) are the etiologic agents of staphylococcal food poisoning and have been implicated in the pathopysiology of other diseases, such as toxic shock syndrome. The SE act as potent immunomodulatory agents that stimulate T lymphocytes by directly cross-linking variable regions of the TCR β chain with MHC class II molecules on the surface of APC, and result in the release of cytokines that may induce shock. We are currently investigating an unconventional model of enterotoxinmediated T cell activation in which T cells proliferate in response to SE in the presence of MHC class Il-negative cells derived from the human colorectal carcinoma cell line SW480. We found that paraformaldehyde-fixed SW480 cells presented SEB and SED, but not SEA or SEE, to purified human T cells with an EC50 of 5 ng/ml, yet failed to express HLA-DR, DQ or DP protein or mRNA as assessed by immunoprecipitation and RT-PCR. The TCR V profile of T cells stimulated by SEB in association with SW480 cells was found to be qualitatively similar to the Vβ profile of T cells stimulated by SEB plus MHC class II-positive APC, indicating that TCR Vβ recognition of SEB in the absence of MHC class II molecules was not altered. SW480 cells, however, supported a greater level of Vβl2-specific T cell expansion than MHC class II positive APC. SEB bound to SW480 cells in a dose-dependent and saturable manner, and could be significantly inhibited by the addition of a 10 fold excess of unlabeled SEB, suggesting that the binding of SEB to SW480 cells is specific. Taken together, these data suggest that functionally active receptors for SE exist on human cells that are distinct from conventional MHC class II molecules. These putative receptors for SE may normally be involved in the uptake, transcytosis, and presentation of proteins to elements of the gutassociated lymphatics, and may underlie emesis and other pathological processes of SE in humans.