Modulation of T cell responses through CD95 in naïve versus activated T cells. The activation state of a given T cell (population) defines the signal threshold for pro- or non- apoptotic CD95 signaling. At the next level, the signal strength passing through CD95 determines whether signal transduction results in cell death, survival, cell cycle arrest or enhanced proliferation. In naïve CD95-resistant T cells, CD95 acts as a potent costimulatory receptor that can transduce activatory or inhibitory signals depending on the dose of CD95 agonists to modulate TCR/CD3 signal induction. Activated T cells are CD95-sensitive and undergo apoptosis when exposed to high concentrations of CD95L. In contrast, a weak CD95 stimulus (again below a certain threshold level) might induce survival signaling in the absence of detectable cell death.