Caspase-8 as an Effector and Regulator of NLRP3 Inflammasome Signaling

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• The NLRP3 inflammasome is a key regulator of inflammation, typically activating caspase-1 for IL-1β processing and pyroptosis.
• Caspase-8 has been implicated in noncanonical inflammasome pathways, but its precise role in NLRP3 inflammasome activation remains under investigation.

Purpose of the Study:

• To investigate the role of caspase-8 in IL-1β processing and cell death mediated by the NLRP3 inflammasome, particularly in the absence of caspase-1 and -11.
• To elucidate the mechanism by which caspase-8 is activated and functions within the NLRP3 inflammasome complex.

Main Methods:

• Utilized murine bone marrow-derived dendritic cells (BMDC) from wild-type, caspase-1/11-deficient, Nlrp3-deficient, Asc-deficient, and caspase-8-deficient mice.
• Stimulated BMDC with lipopolysaccharide (LPS) followed by nigericin, a canonical NLRP3 agonist.
• Assessed IL-1β processing, caspase activation (caspase-8, caspase-1), cell death (pyroptosis, apoptosis), and protein complex formation via biochemical analyses.

Main Results:

• Activated NLRP3 inflammasomes recruit and activate caspase-8 to process IL-1β in caspase-1/11-deficient BMDC, independent of caspase-1.
• Caspase-8 activation and IL-1β processing are dependent on NLRP3 and ASC, and can be inhibited by caspase-8 specific inhibitors.
• In the absence of caspase-1, NLRP3 inflammasome activation leads to delayed apoptosis mediated by caspase-8, contrasting with rapid pyroptosis in wild-type cells.
• Caspase-8 is processed within ASC-enriched complexes before its release during nigericin stimulation.
• Caspase-8 contributes to IL-1β production and regulated cell death signaling via NLRP3 inflammasomes.

Conclusions:

• In the absence of caspase-1, the NLRP3 inflammasome directly utilizes caspase-8 as a major IL-1β-converting enzyme and a pro-apoptotic initiator.
• Caspase-8 acts as a positive modulator of NLRP3-dependent caspase-1 signaling, enhancing IL-1β production and pyroptotic cell death when caspase-1 is present.