Reduction in Mitochondrial Membrane Potential Is an Early Event in Fas-Independent CTL-Mediated Apoptosis

Cytotoxic T lymphocytes (CTL) can destroy target cells via the Fas-mediated pathway or the granule-mediated pathway. We used Fas-negative target cells to examine for target-cell reduction in mitochondrial membrane potential (ΔΨm) induced by intact CTL via the granule-mediated pathway. We find that reduction in ΔΨm is an early step in Fas-independent CTL killing of target cells that precedes phosphatidyl serine translocation, cytosolic protein release, or loss of plasma membrane integrity. Target-cell reduction in ΔΨm and cytoplasmic protein release in Fas-independent CTL killing were inhibited by N-carbobenzoxy-Ala-Pro-Phe chloromethyl ketone, but not by caspase inhibitors N-carbobenzoxy-Val-Ala-Asp fluoromethyl ketone (z-VAD-fmk) or N-carbobenzoxy-Asp-Glu-Val-Asp fluoromethyl ketone (z-DEVD-fmk). This contrasts with Fas-mediated apoptosis, in which the reduction in ΔΨm can be inhibited by z-VAD-fmk or z-DEVD-fmk. Assessing the changes in target-cell ΔΨm can provide for a sensitive and rapid means with which to monitor CTL activity.