While the toxic effects of TCDD on T cells in vivo have been well characterized, attempts to reproduce these findings in vitro have not been successful. In the current study, we examined whether activation or the presence of dendritic cells (DCs) would make primary naÜ¯ve T cells from C57BL/6 mice susceptible to TCDD-induced apoptosis in vitro. While nonactivated primary T cells cultured with TCDD (10-1000 nM) were relatively resistant to apoptosis, they became sensitive to apoptosis upon activation with ConA. Moreover, ConA-activated T cells cultured in the presence of DCs showed highest levels of TCDD-induced apoptosis. Similarly, primary T cells from OT II.2a mice cultured with specific Ova peptide and syngeneic DCs showed higher levels of apoptosis when compared to similar nonactivated T cells. T cell activation led to upregulation of AhR, Fas, and FasL expression. Also, DC maturation and culture with TCDD caused significant induction of FasL. TCDD-mediated apoptosis in activated peripheral T cells was AhR-dependent. Analysis of why nonactivated T cells are more resistant while activated T cells are sensitive to TCDD-induced apoptosis revealed that TCDD treatment of activated but not nonactivated T cells led to down-regulation of c-FLIP an inhibitor of apoptosis. Moreover, down-regulation of c-FLIP using siRNA in unactivated T cells made them sensitive to TCDD-induced apoptosis. The current study demonstrates for the first time that TCDD can induce apoptosis in vitro in peripheral T cells upon activation and in the presence of DCs and that this may be mediated by down-regulation of c-FLIP.