Activation-induced cell death (AICD) has been demonstrated in T-cell hybridomas, immature thymocytes, and activated mature T cells. However, the molecular mechanisms of AICD and its physiological role in T-helper-cell differentiation remain uncertain. Recently, we have shown that Th1 and Th2 cells have distinct mechanisms of AICD. Our findings suggest that signaling from cytokines initiates the differentiation program, but that the selective action of death effectors determines the fate of differentiating T-helper cells, and thus, the ultimate balance between T-helper subpopulations. Among T cells, activation-induced expression of the tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is observed exclusively in Th2 clones and primary T-helper cells differentiated under Th2 conditions, while the expression of CD95L (Fas ligand) occurs mainly in Th1 cells. Furthermore, Th1 cells are more susceptible than Th2 cells to apoptosis induced through either TRAIL or CD95L, and radiolabeled Th1 cells can be induced into apoptosis via fratricide by both Th1 and Th2 cells, while Th2 cells are spared. The pan-caspase inhibitor, z-VAD, prevents AICD in Th1 cells, but not Th2 cells, indicating different mechanisms of AICD in each T-helper subtype. Antibody blockade of TRAIL and CD95L significantly boosts interferon-γ (IFN-γ) production in vitro. Also, young mice with mutant CD95 (MRL/MpJ-lpr/lpr) have a stronger Th1 response to ovalbumin immunization than do controls. We conclude that apoptosis mediated by CD95L and TRAIL is critical in the selective removal of differentiating T helper cells.