Caspase 8 activity in membrane blebs after anti-Fas ligation

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• Fas cross-linking is a known inducer of apoptosis via caspase cascades.
• Previous studies indicated caspase 8 activation as the earliest event in Fas-mediated apoptosis.
• Flow cytometry underestimated early caspase activity in certain apoptosis models.

Purpose of the Study:

• To investigate the precise timing and location of caspase activation during anti-Fas-induced apoptosis in cytotoxic T lymphocytes (CTL).
• To compare caspase activation pathways induced by anti-Fas treatment versus IL-2 withdrawal in CTL.
• To elucidate the role of plasma membrane vesiculation in early caspase activity.

Main Methods:

• Utilized cell-permeable fluorogenic peptide substrates to measure caspase activity.
• Employed flow cytometry and confocal microscopy to analyze caspase activation.
• Induced apoptosis in CTL via anti-Fas cross-linking and IL-2 withdrawal.

Main Results:

• Anti-Fas treatment in CTL showed early caspase 9, 1, and 6 activation preceding caspase 8 (IETDase) activity, contrary to prior expectations.
• Confocal microscopy revealed early caspase 8-containing plasma membrane vesicles released from CTL surfaces upon anti-Fas stimulation.
• IL-2 withdrawal induced apoptosis with early cytoplasmic caspase 8 activation and no membrane vesiculation.

Conclusions:

• Anti-Fas-induced apoptosis in CTL involves early caspase 8 activation localized to plasma membrane vesicles.
• The release of these vesicles leads to an underestimation of early caspase 8 activity by standard flow cytometry.
• Caspase activation mechanisms differ significantly between anti-Fas and IL-2 withdrawal apoptotic pathways in CTL.