Function and Regulation of Noncanonical Caspase-4/5/11 Inflammasome

Inflammasomes are multiprotein complexes of the innate immune system that orchestrate development of inflammation by activating the secretion of proinflammatory cytokines, IL-1β and IL-18. The LPS of Gram-negative bacteria have been shown to activate a novel, noncanonical inflammasome by directly binding in the cytosol to human caspase-4 and mouse caspase-11. Activation of noncanonical inflammasome exerts two major effects: it activates the NLRP3-caspase-1-mediated processing and secretion of IL-1β and IL-18 and induces the inflammatory cell death, pyroptosis, via gasdermin D. This previously unexpected cytosolic LPS sensing of the innate immune system provides critical hints for host response to Gram-negative bacterial infections and development of different inflammatory diseases. However, many of its molecular regulatory mechanisms are yet to be discovered. In this review, we provide comprehensive analysis of current understanding of intracellular LPS detection and pyroptosis via noncanonical inflammasome and discuss the recently proposed mechanisms of its function and regulation.