An MHC class II-expressing T cell clone presenting conventional antigen lacks the ability to present bacterial superantigen.

We have analyzed the response of rat T cells to myelin basic protein (MBP) and the bacterial superantigen, staphylococcal enterotoxin E (SEE). Rat T cells reactive with MBP can respond to SEE presented by spleen cells but not to SEE presented by LOA, a rat T cell clone that expresses both I-A and I-E MHC class II molecules, even though LOA is much more efficient than splenic APC in the presentation of MBP. The inability of LOA to present superantigen is not due to a structural difference in MHC II molecules between LOA and the splenic APC or to differential expression of major accessory/adhesion molecules, including CD2, CD5, CD4 and CD44, on LOA. The non-responsiveness of SEE/LOA-induced T cells differs from anergy, in that such cells do not lose their subsequent responsiveness to either MBP or SEE. Our results demonstrate that: (i) MHC class II molecules (I-A and I-E) alone are insufficient for the activation of T cells by bacterial superantigen, (ii) failure to respond to antigen presented upon inappropriate APC or in inadequate doses may not necessarily represent anergy, and (iii) the quality of the T cell response towards certain ligands can be strongly influenced by the nature of the APC.