At the end of the T cell response, the majority of the activated T cells die. We activated Vbeta8(+) T cells with staphylococcal enterotoxin B (SEB) in vivo and monitored the expansion and deletion of Vbeta8(+) T cells. We found that, in response to SEB, activated T cells died in vivo in the absence of Fas or TNF-R signaling but not when they overexpressed human Bcl-2. We also found that Vbeta8(+) T cells from Bim-deficient mice are resistant to SEB-induced deletion. While Bim levels did not change, endogenous Bcl-2 levels within Vbeta8(+) T cells decrease following SEB injection. Thus, the death of superantigen-stimulated T cells in vivo is mediated by Bim and may be modulated by a decrease in Bcl-2.

After antigen-driven expansion, the majority of T cells involved in an immune response die rapidly by apoptosis dependent on the Bcl-2 related proteins, Bim and Bax or Bak. The details of how these proteins are activated and interact are still unclear. The crystal structure of mouse Bcl-x(L) bound to a long helical fragment of Bim indicates that the structure of Bim is very different from proteins with a Bcl-2-like fold and may leave the BH3 region of Bim constitutively exposed. Based on the structural homology between Bcl-x(L) and Bax, we predicted that binding of Bim to Bax would require displacement of the Bax penultimate alpha helix. Consistent with this prediction, truncation of this short helix was required for Bim/Bax interaction and led to spontaneous activation of Bax. Our results suggest a way in which both Bim and Bax/Bak might be required for activated T cell apoptosis.