Preservation of the Specificity of Superantigen to T Cell Receptor Vβ Elements in the Absence of MHC Class II Molecules

Superantigens interact with specific Vβ elements of the T cell receptor and consequently activate all T cells bearing those elements. The ability of superantigens to stimulate T cells depends on the presence of APC that express MHC class II molecules on their surface. The question we are addressing is: do superantigens have to be seen in context of MHC class II molecules, or can they be recognized directly by T cell-receptor elements? We have previously shown that the APC requirement for the stimulation of T cells by the streptococcal superantigen, pep M5, can be bypassed by the addition of PMA and cytokines or by crosslinking CD28 molecules. Here we asked if the response of APC-depleted T cells to this superantigen is Vβ-restricted and whether in the presence of PMA and cytokines the specificity of pep M5 to Vβ elements is altered. We provide evidence that in the absence of APC, but in the presence of PMA and cytokines, the specificity of pep M5 to Vβ elements is identical to that observed when APC are present, with Vβ2, Vβ4, and Vβ8 being significantly expanded. In addition, we ruled out the possibility that the response is due to a minor contamination with APC or to the expression of DR molecules on T cells because anti-HLA class II monoclonal antibodies did not block the reconstituted response, whereas they totally abrogated the response in the presence of APC. We conclude that pep M5 does not have to complex with MHC class II molecules in order to interact with specific Vβ elements. In addition, we propose that the inhibitory effects of the anti-class II antibodies when APC are present may be due to preventing pep M5 from binding and activating APC, thereby blocking the production of costimulatory molecules necessary for T cell activation by this superantigen.