Classically, naive T cells recognize a specific peptide-MHC complex resulting in their activation and differentiation. However, it is known that T cells also have the ability to interact productively with variant ligands, indicating a flexibility in TCR Ag recognition. These altered peptide ligands have been shown to trigger responses ranging from complete activation to full inhibition of T cell responses, and thus may play an important role in initiating or sustaining T cell-mediated immunity. We have found that influenza virus-specific CD8(+) TCR transgenic T cells differentially respond to a native (agonist) and variant viral epitope, differing in two amino acids that are thought to alter TCR recognition. In response to stimulation with the agonist epitope, these cells activate, proliferate, and differentiate into effector CTLs. Conversely, stimulation with the variant epitope results in activation, proliferation, and development of effector activity followed by rapid and extensive apoptotic cell death. Stimulation of the T cells with the altered ligand results in an inability to sustain the expression of the prosurvival molecules, Bcl-2 and Bcl-xL. These data suggest that the response to the agonist and variant epitopes may reflect TCR avidity-dependent differential signaling through the TCR, resulting either in activation-dependent T cell proliferative expansion and survival or in the accelerated death of acutely activated differentiating T cells. This process of CD8(+) T cell activation, proliferation, and differentiation followed by rapid cell death may represent a novel mechanism of altered peptide ligand-induced apoptosis programmed by initial Ag receptor engagement.