Rituximab (anti-CD20 mAb) mediates antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and apoptosis in B-NHL cells. The contribution of other host-mediated cytotoxic effects has not been examined. The expression of death-inducing ligands (e.g. TRAIL) by host effector cells may contribute to the mechanism of tumor cell destruction in vivo by rituximab-mediated sensitization of resistant B-cell non-Hodgkin lymphoma (B-NHL) cells. We have examined the sensitizing activity of rituximab on B-NHL cell lines resistant to TRAIL (as model) and natural killer (NK)-induced apoptosis. Treatment of TRAIL-resistant B-NHL cell lines with rituximab sensitized the cells to TRAIL apoptosis and synergy was achieved via activation of the type II mitochondrial pathway for apoptosis. Further, rituximab (Fab')(2)-treated tumor cells were killed by purified peripheral blood-derived NK cells via TRAIL. Treatment of B-NHL cells with rituximab inhibited both YY1 DNA-binding activity and expression. Rituximab-mediated sensitization to TRAIL apoptosis was due, in large part, to rituximab-mediated inhibition of the transcription factor Yin Yang 1 (YY1). The direct role of YY1 in TRAIL sensitization by rituximab was shown in cells transfected with YY1 siRNA, and such cells mimicked rituximab and became sensitive to TRAIL-induced apoptosis. These data suggest that, in vivo, host effector cells expressing TRAIL may contribute to rituximab-mediated depletion of B-NHL cells.