Synergy between T cell receptor and Fas (CD95/APO-1) signaling in mouse thymocyte death

Administration of anti-TCR/CD3 epsilon antibody in vivo or in thymic organ culture results in the apoptotic death of CD4(+)/CD8(+) thymocytes. In contrast, purified thymocytes in suspension culture are resistant to TCR/CD3 epsilon-induced apoptotic death. We show that induction of thymocyte death, in suspension culture, can be induced by the combination of TCR/CD3 epsilon and Fas (CD95/Apo-1) signaling. No significant thymocyte death was observed after in vitro Fas cross-linking unless TCR/CD3 epsilon was simultaneously co-cross-linked or metabolic inhibitors such as actinomycin D were added. Furthermore, TCR/CD3 epsilon and Fas synergy did not operate through upregulation of Fas but by facilitation of the Fas-mediated death signal. Both TCR(mid/lo)/HSA(hi)/CD4(+)/CD8(+) (double positive) and TCR(hi)/HSA(lo)/CD4(+)/CD8(-) or CD4(-)/CD8(+) (single positive) thymocytes were susceptible to death induced by co-cross-linking of TCR/CD3 epsilon and Fas. Our results reveal a signaling synergy between the Fas and TCR/CD3 epsilon complex that has important implications for our understanding of in vivo vs in vitro models of thymocyte deletion. (C) 1996 Academic Press, Inc.