Structural insights into cytokine cleavage by inflammatory caspase-4

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• Inflammatory caspases are crucial for innate immunity, processing cytokines like IL-1 family members.
• Canonical inflammasomes and caspase-1 are traditionally known for cleaving pro-IL-1β and pro-IL-18.
• The structural basis for inflammatory caspase-mediated cytokine processing remained largely undefined.

Purpose of the Study:

• To investigate the role of human caspase-4 in pro-IL-18 processing.
• To elucidate the structural mechanisms of caspase-4 interacting with its substrates.
• To understand how caspase-4 contributes to IL-18 release independently of canonical inflammasomes.

Main Methods:

• Biochemical assays to assess caspase-4's cleavage activity on pro-IL-18.
• Cryogenic electron microscopy (cryo-EM) to determine the structure of the caspase-4-pro-IL-18 complex.
• Comparative analysis of substrate binding and cleavage mechanisms with gasdermin D.

Main Results:

• Human caspase-4 efficiently cleaves pro-IL-18, similar to caspase-1.
• Caspase-4's ability to cleave both pro-IL-18 and gasdermin D allows IL-18 release without canonical inflammasomes.
• Cryo-EM revealed two distinct interaction interfaces between caspase-4 and pro-IL-18, including active site and exosite interactions.
• The identified mechanisms for cytokine substrate capture and cleavage differ from those of gasdermin D.

Conclusions:

• Human caspase-4 possesses significant pro-IL-18 processing activity, offering an alternative pathway for IL-18 release.
• The structural insights provide a framework for understanding caspase-4 function in innate immunity and inflammatory diseases.
• These findings challenge the exclusive role of caspase-1 in IL-18 processing and highlight caspase-4's broader substrate specificity.