Shigella IpaH9.8 limits GBP1-dependent LPS release from intracytosolic bacteria to suppress caspase-4 activation

Abstract

Pyroptosis is an inflammatory form of cell death induced upon the recognition of invading microbes. During an infection, pyroptosis is enhanced in interferon-gamma (IFN{gamma}) exposed cells due to the actions of members of the guanylate binding protein (GBP) family. GBPs promote activation of caspase-4 (CASP4) by enhancing its interactions with lipopolysaccharide (LPS), a component of the outer envelope of Gram-negative bacteria. Once activated, CASP4 promotes the formation of non-canonical inflammasomes, signaling platforms that mediate pyroptosis. To establish an infection, intracellular bacterial pathogens, like Shigella species, inhibit pyroptosis. The pathogenesis of Shigella is dependent on its type III secretion system, which injects ~30 effector proteins into host cells. Upon entry into host cells, Shigella are encapsulated by GBP1, followed by GBP2, GBP3, GBP4, and in some cases, CASP4. It has been suggested that the recruitment of CASP4 to bacteria leads to its activation. Here, we demonstrate that two Shigella effectors, OspC3 and IpaH9.8, cooperate to inhibit CASP4-mediated pyroptosis. We show that in the absence of OspC3, an inhibitor of CASP4, IpaH9.8 inhibits pyroptosis via its known degradation of GBPs. We find that, while some LPS is present within the host cell cytosol of epithelial cells infected with wild type Shigella, in the absence of IpaH9.8, increased amounts are shed in a GBP1-dependent manner. Furthermore, that additional IpaH9.8 targets, likely GBPs, promote CASP4 activation, even in the absence of GBP1. These observations suggest that by promoting LPS release, GBP1 provides CASP4 access to LPS, thus promoting host cell death via pyroptosis. Significance StatementNon-canonical inflammasomes mediate the death of intestinal epithelial cells in response to invading Gram-negative bacterial pathogens. CASP4 is activated upon binding to LPS. Recent studies have suggested that GBP1 recruits and enables the activation of CASP4 on the surface of intracytosolic bacteria. This has led to the proposal that non-canonical inflammasomes assemble on the bacterial surface due to GBP1-mediated access of CASP4 to the membrane-embedded lipid A component of LPS. Here, we show that by targeting the degradation of GBPs, Shigella IpaH9.8 limits the release of LPS from intracellular Shigella into the host cell cytosol in a GBP1-dependent manner. Furthermore, we find evidence to suggest that in the absence of GBP1, other members of the GBP family promote CASP4 activation.