Paracrine transfer of mouse mammary tumor virus superantigen

Transfer of vSAG7, the endogenous superantigen encoded in the Mtv7 locus, from MHC class II- to MHC class II+ cells has been suggested to occur both in vivo and in vitro. This transfer usually leads to the activation and deletion of T cells expressing responsive V beta s. However, there is no direct molecular evidence for such a transfer. We have developed an in vitro system which confirms this property of vSAGs. vSAG7 was transfected into a class II- murine fibroblastic line. Coculture of these cells with class II+ cells and murine T cell hybridomas expressing the specific V beta s led to high levels of IL-2 production which was specifically inhibited by vSAG7- and MHC class II-specific mAbs. Moreover, injection of vSAG7(+) class II- cells in mice led to expansion of V beta 6(+) CD4(+) cells. We show that this transfer activity is paracrine but does not require cell-to-cell contact. Indeed, vSAG7 was transferred across semi-permeable membranes. Transfer can occur both from class II- and class II+ cells, indicating that MHC class II does not sequester vSAC7. Finally, competition experiments using bacterial toxins with well defined binding sites showed that the transferred vSAG7 fragment binds to the alpha 1 domain of HLA-DR.