Abstract
Innate immune sensing of bacterial lipopolysaccharide (LPS) is critical to host defense, but a dysregulated response can lead to sepsis, a life-threatening condition associated with multiple organ failure and shock. LPS is sensed at the cell surface by Toll-Like Receptor 4 (TLR4) and in the cytosol by mouse caspase-11 (Casp11) or its human orthologs, caspase-4 and 5 (CASP4/5). In response to cytosolic LPS, Casp11 oligomerizes in a catalytically dependent manner into supramolecular organizing centers (SMOCs). We demonstrate that CASP4 and CASP5 also form catalytically dependent SMOCs in response to bacterial LPS in macrophages. Furthermore, we find that CASP4 and CASP5 differentially respond to distinct LPS structural variants that fail to activate CASP11 and that only activating LPS structures induce SMOC formation by the corresponding caspase. Notably, transgenic mice expressing CASP4 in lieu of Casp11 exert more effective control of Francisella novicida, a rodent pathogen whose LPS evades TLR4 and Casp11 detection. Conversely, CASP4-expressing mice are more susceptible to sepsis when exposed to E. coli expressing a lipid A variant that fails to activate TLR4 and Casp11. Together, these findings reveal human CASP4 and CASP5 have evolved to assemble into SMOCs in response to a broader range of LPS structures than mouse Casp11 and highlight the potential tradeoff between systemic inflammation and control of bacterial infection that may underline these differences. Our findings may inform the development of targeted therapeutic strategies for gram-negative bacterial infection and sepsis that are effective in humans.