GSDMD pore formation regulates caspase-4 cleavage to limit IL-18 production in the intestinal epithelium

Abstract

Epithelial inflammasomes induce pyroptosis and release cytokines to defend against cytosolic pathogens. However, pyroptosis in epithelial barriers must be carefully regulated to facilitate elimination of infected cells while limiting widespread pyroptosis to preserve the single cell barrier. How epithelial cells achieve this is unknown. In this study, we describe a novel epithelial caspase regulation mechanism. By examining caspase-4 activation in human epithelial cells, we discovered that GSDMD pore formation serves as a signal to terminate caspase-4 activity thus facilitating epithelial cell expulsion while controlling cytokine secretion. Inhibition of epithelial pyroptosis led to IL-18 hyperproduction, likely as a mechanism to combat increased pathogen burden and initiate a wider immune response. Moreover, we demonstrate that full-length, rather than cleaved caspase-4 is active against IL-18 and propose that GSDMD pore formation facilitates cleavage of caspase-4 to terminate its catalytic activity. By comparing human cells and murine epithelial organoids to immune cells, we show that GSDMD pore mediated inhibition of caspase activity is largely specific to epithelial cells. Overall, these studies characterise a novel, epithelial-specific negative feedback loop that modulates inflammasome activity and challenge the dogma that autocatalytic caspase cleavage is required for caspase activity against substrates. Graphical AbstractIn intestinal epithelial cells, caspase activation simultaneously leads to GSDMD pore formation and IL-18 release. GSDMD pore formation provides a signal to terminate caspase activity and limit cytokine production. In GSDMD deficient cells, lack of an inhibition signal leads to caspase mediated IL-18 hyperproduction. Upon cell death this leads to release of massive amounts of IL-18. Created with BioRender.com O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=103 SRC="FIGDIR/small/578487v1_ufig1.gif" ALT="Figure 1"> View larger version (24K): org.highwire.dtl.DTLVardef@46f768org.highwire.dtl.DTLVardef@11cf918org.highwire.dtl.DTLVardef@125be35org.highwire.dtl.DTLVardef@ea9ead_HPS_FORMAT_FIGEXP M_FIG C_FIG