Caspase-8 inactivation drives autophagy-dependent inflammasome activation in myeloid cells

Area of Science:

• Immunology
• Cell Biology
• Molecular Mechanisms of Cell Death

Background:

• Caspase-8 activity regulates cell death pathways, including pyroptosis, apoptosis, and necroptosis.
• The mechanisms by which caspase-8 inactivation triggers inflammasome activation remain incompletely understood.

Purpose of the Study:

• To elucidate the molecular pathways involved in inflammasome activation upon caspase-8 inhibition in the absence of apoptosis and necroptosis.
• To investigate the roles of autophagy, cathepsin-B, and other signaling molecules in this process.

Main Methods:

• Utilized Toll-like receptor (TLR)-primed Fadd^-/-Ripk3^-/- myeloid cells and macrophages.
• Administered caspase-8 inhibitor (IETD) and assessed inflammasome activation, cytokine production (IL-1β, IL-18), and cell death.
• Investigated the necessity of caspase-8, caspase-1/11, GSDMD, NLRP3, RIPK1, autophagy, and cathepsin-B using genetic knockouts and pharmacological inhibitors.
• Observed septic shock models in Fadd^-/-Ripk3^-/- mice.

Main Results:

• Caspase-8 inhibition (IETD) in TLR-primed Fadd^-/-Ripk3^-/- myeloid cells induced IL-1β and IL-18 production via inflammasome activation.
• Essential components for IETD-triggered inflammasome activation included caspase-8, caspase-1/11, and GSDMD, but not NLRP3 or RIPK1.
• Autophagy was upregulated and crucial for IETD-induced cell death and cytokine production in macrophages.
• Cathepsin-B activity contributed to IETD-mediated inflammasome activation and IL-1β production.
• Inhibition of GSDMD or autophagy did not prevent IETD-induced septic shock in mice, suggesting distinct mechanisms in vivo.

Conclusions:

• The autophagy and cathepsin-B axis is a key pathway for atypical inflammasome activation when apoptosis and necroptosis are suppressed and caspase-8 is inhibited in myeloid cells.
• These findings reveal novel mechanisms of inflammasome regulation beyond canonical pathways.
• Distinct cell-type-specific or in vivo mechanisms may contribute to septic shock development under these conditions.