Human CD4+ cytolytic T cells kill antigen-pulsed target T cells by induction of apoptosis

Stimulation of human CD4+ T cell clones with appropriate specific peptides or with lectins in the absence of APC induces a substantial degree of cell death. We have investigated the mechanisms of induction of this cell death and show that it occurs by apoptosis, identified by morphology and the characteristic pattern of DNA degradation. We also investigated whether this T cell death was a result of a suicide process activated in the T cell after "inappropriate" recognition of Ag on the surface of another T cell clone (and in the absence of other accessory signals), or was due to a conventional lethal hit delivered by one cytolytic T cell to another "target" T cell. Our results strongly suggest that peptide-induced cell death of human CD4+ CTL is due to active killing of Ag-presenting target T cells by effector T cells of the same clone. The cell death that is induced in the target T cell occurs via apoptosis that requires de novo RNA transcription and translation in the effector T cell.