Dysregulated hemolysin liberates bacterial outer membrane vesicles for cytosolic lipopolysaccharide sensing

Abstract

Inflammatory caspase-11/4/5 recognize cytosolic LPS from invading Gram-negative bacteria and induce pyroptosis and cytokine release, forming rapid innate antibacterial defenses. Since extracellular or vacuole-constrained bacteria are thought to rarely access the cytoplasm, how their LPS are exposed to the cytosolic sensors is a critical event for pathogen recognition. Hemolysin is a pore-forming bacterial toxin, which was generally accepted to rupture cell membrane, leading to cell lysis. Whether and how hemolysin participates in non-canonical inflammasome signaling remains undiscovered. Here, we show that hemolysin-overexpressed enterobacteria triggered significantly increased caspase-4 activation in human intestinal epithelial cell lines. Hemolysin promoted LPS cytosolic delivery from extracellular bacteria through dynamin-dependent endocytosis. Further, we revealed that hemolysin was largely associated with bacterial outer membrane vesicles (OMVs) and induced rupture of OMV-containing vacuoles, subsequently increasing LPS exposure to the cytosolic sensor. Accordingly, overexpression of hemolysin promoted caspase-11 dependent IL-18 secretion and gut inflammation in mice, which was associated with restricting bacterial colonization in vivo. Together, our work reveals a concept that hemolysin promotes noncanonical inflammasome activation via liberating OMVs for cytosolic LPS sensing, which offers insights into innate immune surveillance of dysregulated hemolysin via caspase-11/4 in intestinal antibacterial defenses.

Identification of Microbial Genetic Capacities and Potential Mechanisms Within the Rumen Microbiome Explaining Differences in Beef Cattle Feed Efficiency

“…genes encoding for hemolysin which were also associated with low RFI (K06442) and low FCR (K11068). Although hemolysin is generally produced by pathogenic bacteria for nutrient acquisition by initiating host cell lysis, it has been suggested that hemolysin can promote activation of inflammasome signals reducing pathogen colonization in the intestine (Chen et al, 2018). Results related to Eubacterium might be a good example of possible gastrointestinal cross-talk from bacteria detected in the rumen having a possible effect on the lower gut.…”

Variation in hemolysin A expression between uropathogenic Escherichia coli isolates determines NLRP3‐dependent vs . ‐independent macrophage cell death and host colonization

“…Another study revealed that CFT073 lacking D-serine deaminase had enhanced expression of hlyA and fimbriae genes, thereby contributing to increased bacterial colonization (60). Finally, a recent study demonstrated that, in E. tarda, OMV-associated HlyA enabled delivery of LPS to the cytoplasm and noncanonical inflammasome activation, as well as bacterial clearance in an oral challenge model (58). These studies support our findings that HlyA is an important regulator of bladder colonization and are consistent with the view that UPEC employs active mechanisms to limit HlyA expression to prevent host-protective inflammasome responses.…”

“…One possibility is that the levels of HlyA produced by a particular UPEC strain influence the distribution of this toxin to outer membrane vesicles (OMVs), which in some studies using E. coli have been shown to contain HlyA (56,57). Moreover, OMV-associated HlyA has been shown to initiate host cell death responses in the case of enterohemorrhagic E. coli (57), as well as the enteric pathogen Edwardsiella tarda (58). Because OMVs can influence numerous other immune cell functions (59), differential effects of HlyA expression levels on host protection vs. colonization may relate to variation in HlyA distribution to OMVs.…”

Bacterial Outer Membrane Vesicle-Mediated Cytosolic Delivery of Flagellin Triggers Host NLRC4 Canonical Inflammasome Signaling

“…Bacterial OMVs can also trigger host innate sensing machinery (e.g., inflammasome pathways), which helps to increase bacterial clearance (17). In particular, bacteria-released OMVs promote caspase-11mediated non-canonical inflammasome responses via OMVbound LPS (18,24,25). OMV-mediated intracellular delivery of LPS results in the activation of caspase-11, a cytosolic LPS sensor, with the help of GBPs (19).…”