Inflammatory caspases are innate immune receptors for intracellular LPS

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• The non-canonical inflammasome, involving murine caspase-11, responds to bacterial infections.
• Caspase-11 activation leads to pyroptosis and endotoxic shock via cytoplasmic lipopolysaccharide (LPS) sensing, but the receptor remains unknown.

Purpose of the Study:

• To elucidate the mechanism of cytosolic LPS sensing and identify the pattern recognition receptor responsible for LPS-induced cell death.

Main Methods:

• Investigated LPS-induced cytotoxicity in human cells (monocytes, epithelial, keratinocytes) upon cytoplasmic delivery.
• Utilized functional complementation assays between human caspase-4 and murine caspase-11.
• Performed direct binding assays of caspases (caspase-4/11, caspase-5) to LPS and lipid A.
• Examined caspase oligomerization and activation upon LPS binding using purified insect-cell caspases.
• Generated and tested binding-deficient CARD-domain point mutants in LPS electroporation and bacterial infection models.

Main Results:

• Human cells undergo necrosis upon cytoplasmic LPS delivery, mediated by human caspase-4, which functionally complements murine caspase-11.
• Human caspase-4/11 and caspase-5 directly bind LPS and lipid A with high affinity.
• LPS binding induces oligomerization and activation of caspase-4/11, a process dependent on the CARD domain.
• Underacylated LPS variants failed to induce caspase activation, indicating specificity.
• CARD-domain mutants lost the ability to oligomerize, activate, and induce pyroptosis.

Conclusions:

• Human caspase-4/11 and caspase-5 act as direct LPS sensors, mediating pyroptosis and necrosis.
• This represents a novel pattern recognition mechanism in immunity and a new pathway for caspase activation.