Extended Data Fig. 9: Model for noncanonical activation of the NLRP3 inflammasome by live Gram-negative bacteria.
From: Caspase-11 interaction with NLRP3 potentiates the noncanonical activation of the NLRP3 inflammasome
Following phagocytosis of live Gram-negative bacteria, two classes of PAMPs are exposed to cytoplasmic pattern recognition receptors: the classical PAMP LPS, shared by live and killed bacteria, and the vita-PAMP bacterial mRNA (mRNAbac), present only in live bacteria. Coincident detection of mRNAbac and LPS from virulent or avirulent Gram-negative bacteria alike promotes a physical and mutually exclusive interaction between NLRP3 and the intracellular LPS receptor pro-caspase-11. This interaction localizes to the dispersed Trans-Golgi Network (TGN) and is mediated through the pro-caspase-11 SCAF domain and the LRR and PYD domains of NLRP3. The interaction of NLRP3 and pro-caspase-11 is upstream of their activation: It does not require the ability of LPS to activate caspase-11 and can still occur in the absence of GSDMD. It also does not require ASC and caspase-1 which are important for NLRP3 activation. Besides their interaction, NLRP3 and pro-caspase-11 are reciprocally required for their function: LPS binding to but not activation of pro-caspase-11, is necessary for mRNAbac-mediated NLRP3 inflammasome assembly. Reciprocally, NLRP3 and ASC but not caspase-1 are required for pro-caspase-11 activation, indicating the necessity for ‘nascent’ NLRP3 inflammasome assembly upon sensing the viability of Gram-negative bacteria (detection of mRNAbac) and irrespective of bacterial virulence factor expression. Although NLRP3-ASC oligomers can form in the absence of pro-caspase-1, these oligomers are unstable indicating stabilization of the ‘nascent’ assembled NLRP3 inflammasome upon pro-caspase-1 recruitment. Furthermore, higher concentrations of intracellular LPS, likely due to virulence factor activity during infection with virulent cell-invasive Gram-negative bacteria, trigger faster kinetics of plasma membrane permeabilization/pyroptosis compared with avirulent bacteria, and independently of NLRP3 and ASC2,9. Collectively, the model that emerges demonstrates two modes of pro-caspase-11 activation by LPS, a fast NLRP3-independent mode triggered by the concurrent expression of bacterial virulence factors, and a slower NLRP3-dependent mode triggered by coincident detection of the vita-PAMP mRNAbac that signifies bacterial viability. (SCAF, scaffold; PYD, Pyrin; LRR, leucine rich repeat).
