Involvement of Fas-dependent apoptosis in renal tubular epithelial cell deletion in chronic renal failure. Renal tubular atrophy predicts a poor prognosis in chronic renal failure, but the molecular mechanisms that regulate tubular atrophy are unknown. Because the Fas apoptosis pathway has been implicated in disease pathogenesis and Fas is expressed in kidney, we hypothesized that Fas-mediated renal tubule epithelial cell (RTC) apoptosis contributes to tubular atrophy in chronic renal failure. Immunohistochemical analyses of renal sections from two murine models of progressive renal disease revealed increases in RTC Fas expression and apoptosis compared with tissue sections from age-matched control kidneys. Increased RTC apoptosis was not accompanied by compensatory hyperplasia, suggesting that RTCs targeted for Fas-dependent apoptotic deletion contribute to tubular atrophy. These data are supported by in vitro studies that showed that interleukin-1α or tumor necrosis factor-α, cytokines that are secreted in chronic renal failure, stimulated increases in Fas expression in cultured RTCs. Both murine kidney cortex and RTCs in culture demonstrated constitutive expression of Fas ligand, a feature that is characteristically restricted to lymphocytes and immune-privileged tissues and previously unrecognized in RTCs. Functional studies revealed that interleukin-1α–stimulated RTC Fas expression was accompanied by increased apoptosis, which was inhibited by blocking anti-Fas ligand antibodies. The data suggest that up-regulated RTC Fas binds to Fas ligand on adjacent RTCs, which then leads to RTC death by fratricide. We propose this pathway as an initiating mechanism of tubular atrophy.