Death receptor/death ligand interactions have been recognized as important triggers of cell death in various cell types. So far, best understood are the signaling events that are activated upon binding of the Fas ligand to the Fas (CD95) receptor, and subsequently lead to apoptosis in the target cell. Fas-mediated apoptosis plays a crucial role in many physiological and pathophysiological events, such as immune homeostasis, immune privilege, tissue turnover, and cellmediated cytotoxicity. 1, 2 Similar to Fas ligand, TRAIL/Apo-2 ligand (Tumor Necrosis Factor-related apoptosis-inducing ligand) has been proposed to represent an important cytotoxic effector molecule, by which cytotoxic T cells and natural killer cells eliminate their target cells. TRAIL has particularly received strong scientific attention due to its proposed role in tumor immune surveillance and its potential therapeutic application in cancer patients. 3 Apart from tumor immunity, however, TRAIL has also been implicated in various other aspects of immune cell regulation. For example, neutralization of TRAIL causes an exacerbation of experimental rheumatoid arthritis, autoimmune encephalomyelitis and diabetes, suggesting that TRAIL-induced apoptosis or effector cell modulation represents an important regulatory process in the pathogenesis of autoimmune diseases. Recently, Lamhamedi-Cherradi et al. 4 have proposed TRAIL/TRAIL receptor-mediated cell death as an underlying mechanism of thymic negative selection. Thymocytes from TRAIL-deficient mice were found to be more resistant to antigen-or CD3/T cell receptor (TCR)-mediated apoptosis, and soluble TRAIL receptor was reported to block thymic cell death. This finding, however, is quite controversial and has therefore been questioned by other investigators. 5–7 For example, Simon et al. 5 have not been able to block thymic negative selection by soluble TRAIL receptor, and Cretney et al. 6 have found normal antigen-driven negative selection in the very same TRAIL-deficient mice. Furthermore, CD3/TCR-mediated apoptosis in vivo has been reported to require the release of systemic glucocorticoids and may thus not necessarily reflect the biology of negative selection. 8 Particularly incompatible with this proposed role of TRAIL in thymic negative selection is the observation that the adaptor molecule FADD (Fas-associated death domain) is a crucial element in the TRAIL-induced signaling pathways leading to apoptosis, however, overexpression of dominant-negative FADD does not alter thymic negative selection. 7, 9 Thus, the role of TRAIL–TRAIL receptor interaction in the establishment of central tolerance is far from being well accepted. The different publications describing TRAIL or TRAIL-R expression in the thymus, or their involvement in thymic negative selection are summarized in Table 1. They demonstrate that TRAIL and TRAIL receptor are expressed in the thymus and thymocytes, and that thymocytes are sensitive to TRAIL, yet come to different conclusions regarding the role of TRAIL–TRAIL receptor interaction in TCR-induced thymocyte apoptosis. The discrepancy between these different reports is difficult to reconcile and may partially depend on the different experimental systems used. We have thus reinvestigated the role of TRAIL in activation-induced apoptosis in thymocytes using TRAIL-deficient mice, and have extended our experiments to other apoptosis triggers. Both, in vitroandin vivoactivation of thymocytes by CD3/TCR ligation using anti-CD3ε-crosslinking antibodies caused massive induction of apoptosis in CD4+ CD8+ immature thymocytes (Figure 1a and b). Importantly, double-positive thymocytes from wild …