Synergy between T Cell Receptor and Fas (CD95/APO-1) Signaling in Mouse Thymocyte Death

Administration of anti-TCR/CD3ϵ antibodyin vivoor in thymic organ culture results in the apoptotic death of CD4+/CD8+thymocytes. In contrast, purified thymocytes in suspension culture are resistant to TCR/CD3ϵ-induced apoptotic death. We show that induction of thymocyte death, in suspension culture, can be induced by the combination of TCR/CD3ϵ and Fas (CD95/Apo-1) signaling. No significant thymocyte death was observed afterin vitroFas cross-linking unless TCR/CD3ϵ was simultaneously co-cross-linked or metabolic inhibitors such as actinomycin D were added. Furthermore, TCR/CD3ϵ and Fas synergy did not operate through upregulation of Fas but by facilitation of the Fas-mediated death signal. Both TCRmid/lo/HSAhi/CD4+/CD8+(double positive) and TCRhi/HSAlo/CD4+/CD8−or CD4−/CD8+(single positive) thymocytes were susceptible to death induced by co-cross-linking of TCR/CD3ϵ and Fas. Our results reveal a signaling synergy between the Fas and TCR/CD3ϵ complex that has important implications for our understanding ofin vivovsin vitromodels of thymocyte deletion.