Shigella OspC3 suppresses murine cytosolic LPS sensing

Abstract

Summary Shigella flexneri , a cytosol-invasive gram-negative pathogen, deploys an array of type III-secreted effector proteins to evade host cell defenses. Caspase-11 and its human ortholog caspase-4 detect cytosolic lipopolysaccharide (LPS) and trigger gasdermin D-mediated pyroptosis to eliminate intra-cytoplasmic bacterial threats . However, the role of caspase-11 in combating S. flexneri is unclear. The Shigella T3SS effector OspC3 reportedly suppresses cytosolic LPS sensing by inhibiting caspase-4 but not caspase-11 activity. Surprisingly, we found that S. flexneri also uses OspC3 to inhibit murine caspase-11 activity. Mechanistically, we found that OspC3 binds only to primed caspase-11. Importantly, we demonstrate that S. flexneri employs OspC3 to prevent caspase-11-mediated pyroptosis in neutrophils, enabling bacteria to disseminate and evade clearance following intraperitoneal challenge. In contrast, S. flexneri lacking OspC3 is attenuated in a caspase-11- and gasdermin D-dependent fashion. Overall, our study reveals that OspC3 suppresses cytosolic LPS detection in a broad array of mammals.

A hierarchy of cell death pathways confers layered resistance to shigellosis in mice

“…Shigella also deploys an effector, IpaH7.8, which degrades human (but not mouse) GSDMD to block pyroptosis, further underscoring the importance of this axis in defense (Luchetti et al, 2021). We note that CASP11-dependent protection is not sufficient to render ospC3-infected B6.Nlrc4 -/mice completely resistant to disease symptoms, perhaps because the priming required to induce CASP11 expression might delay its protective response (Oh et al, 2021).…”

“…Shigella flexneri protein OspC3 is a T3SS-secreted effector that inhibits both human Caspase-4 and mouse Caspase-11 to suppress pyroptosis (Kobayashi et al, 2013; Li et al, 2021; Mou et al, 2018; Oh et al, 2021). While OspC3 has been shown to be required for virulence during intraperitoneal mouse infection (Li et al, 2021; Oh et al, 2021) and for intestinal lumenal colonization in wild-type mice (Alphonse et al, 2022), the role of this effector has not been studied in an oral mouse model of infection where Shigella invades and replicates within the intestinal epithelium. Indeed, our results indicating a role for Caspase-11 in defense against wild-type Shigella (see above, Figures 1 and 2) suggested that the inhibition of Caspase-11 by OspC3 could be incomplete in epithelial cells.…”

“…Shigella is an example of a pathogen in intense conflict with host cell death pathways (Ashida et al, 2021). Shigella encodes multiple effectors to prevent cell death in human cells, including OspC3 to block Caspase-4 inflammasome activation (Kobayashi et al, 2013; Li et al, 2021; Mou et al, 2018; Oh et al, 2021), IpaH7.8 to inhibit Gasdermin D-dependent pyroptosis (Luchetti et al, 2021), OspC1 to suppress Caspase-8-dependent apoptosis (Ashida et al, 2020), and OspD3 to block necroptosis (Ashida et al, 2020). The antagonism of these pathways (and perhaps others that are yet undiscovered) and the resulting maintenance of the epithelial niche appears sufficient to render humans susceptible to Shigella infection.…”

A family of conserved bacterial virulence factors dampens interferon responses by blocking calcium signaling

“…Expression of several effectors (blue bars) led to IFN inhibition at a level comparable with that mediated by NSP2 (Figure 1D). We chose to follow up on the OspC effector family that had previously been shown to modulate immune pathways (Ashida et al, 2020;Kobayashi et al, 2013;Li et al, 2021;Miller et al, 2018;Mou et al, 2018;Oh et al, 2021;Zurawski et al, 2006).…”

Autophagy May Allow a Cell to Forbear Pyroptosis When Confronted With Cytosol-Invasive Bacteria

“…This illustrates the potent defense conferred by pyroptosis and autophagy against pathogens that have intermediate or even high virulence potential, but which lack the ability to inhibit these defenses. Shigella is a pathogen with high virulence that additionally can inhibit these defenses ( 40 , 41 ). Shigella dysenteriae and Shigella flexneri cause the disease Shigellosis, which is characterized by hemorrhagic diarrhea and is particularly dangerous to young children ( 42 , 43 ).…”