The human marrow produces approximately 1010 monocytes daily, and this production must be balanced by a similar rate of destruction. Monocytes/macrophages can undergo apoptosis after activating CD4+ T cells, suggesting one mechanism that may contribute to macrophage homeostasis. Previous reports indicate that Fas-Fas ligand interactions are the principle molecules mediating this response. However, D10, an Iak-restricted cloned Th2 line, will similarly induce apoptosis in Ag-presenting macrophages, and D10 cells lack Fas ligand. To confirm that D10 cells kill macrophages through Fas-independent pathways, D10 cells were shown to kill MRL lpr/lpr (Iak) macrophages in an Ag-dependent fashion, indicating additional mechanisms. Recent reports demonstrate that TNF-related apoptosis-inducing ligand (TRAIL), interacting with Apo2, and TNF-like weak inducer of apoptosis (TWEAK), interacting with Apo3, will induce apoptosis in some cells. Using Abs to TRAIL and an Apo3-IgG Fc fusion protein, we demonstrated that D10 cells express both TRAIL and TWEAK. The Apo3 fusion protein, but not human IgG, inhibited D10-induced macrophage apoptosis, as did anti-TRAIL. Further studies demonstrated that AE7, a cloned Th1 line, and splenic T cells express TWEAK, TRAIL, and Fas ligand, and inhibiting these molecules also inhibited macrophage killing. These results indicate that D10 cells induce macrophage apoptosis through TRAIL- and TWEAK-dependent pathways. Because normal T cells also express these molecules, these results support the concept that T cells have multiple pathways by which to induce macrophage apoptosis. These pathways may be important in immune processes such as macrophage homeostasis as well as in down-regulation of immune responses and elimination of macrophages infected with intracellular organisms.