Introduction: cell-autonomous immunity

Direct binding of polymeric GBP1 to LPS disrupts bacterial cell envelope functions

In the outer membrane of gram‐negative bacteria, O‐antigen segments of lipopolysaccharide (LPS) form a chemomechanical barrier, whereas lipid A moieties anchor LPS molecules. Upon infection, human guanylate binding protein‐1 (hGBP1) colocalizes with intracellular gram‐negative bacterial pathogens, facilitates bacterial killing, promotes activation of the lipid A sensor caspase‐4, and blocks actin‐driven dissemination of the enteric pathogen Shigella. The underlying molecular mechanism for hGBP1's diverse antimicrobial functions is unknown. Here, we demonstrate that hGBP1 binds directly to LPS and induces “detergent‐like” LPS clustering through protein polymerization. Binding of polymerizing hGBP1 to the bacterial surface disrupts the O‐antigen barrier, thereby unmasking lipid A, eliciting caspase‐4 recruitment, enhancing antibacterial activity of polymyxin B, and blocking the function of the Shigella outer membrane actin motility factor IcsA. These findings characterize hGBP1 as an LPS‐binding surfactant that destabilizes the rigidity of the outer membrane to exert pleiotropic effects on the functionality of gram‐negative bacterial cell envelopes.

Detection of Cytosolic Shigella flexneri via a C-Terminal Triple-Arginine Motif of GBP1 Inhibits Actin-Based Motility

Dynamin-like guanylate binding proteins (GBPs) are gamma interferon (IFN-γ)-inducible host defense proteins that can associate with cytosol-invading bacterial pathogens. Mouse GBPs promote the lytic destruction of targeted bacteria in the host cell cytosol, but the antimicrobial function of human GBPs and the mechanism by which these proteins associate with cytosolic bacteria are poorly understood. Here, we demonstrate that human GBP1 is unique among the seven human GBP paralogs in its ability to associate with at least two cytosolic Gram-negative bacteria, Burkholderia thailandensis and Shigella flexneri. Rough lipopolysaccharide (LPS) mutants of S. flexneri colocalize with GBP1 less frequently than wild-type S. flexneri does, suggesting that host recognition of O antigen promotes GBP1 targeting to Gram-negative bacteria. The targeting of GBP1 to cytosolic bacteria, via a unique triple-arginine motif present in its C terminus, promotes the corecruitment of four additional GBP paralogs (GBP2, GBP3, GBP4, and GBP6). GBP1-decorated Shigella organisms replicate but fail to form actin tails, leading to their intracellular aggregation. Consequentially, the wild type but not the triple-arginine GBP1 mutant restricts S. flexneri cell-to-cell spread. Furthermore, human-adapted S. flexneri, through the action of one its secreted effectors, IpaH9.8, is more resistant to GBP1 targeting than the non-human-adapted bacillus B. thailandensis. These studies reveal that human GBP1 uniquely functions as an intracellular “glue trap,” inhibiting the cytosolic movement of normally actin-propelled Gram-negative bacteria. In response to this powerful human defense program, S. flexneri has evolved an effective counterdefense to restrict GBP1 recruitment.

Several pathogenic bacterial species evolved to invade, reside in, and replicate inside the cytosol of their host cells. One adaptation common to most cytosolic bacterial pathogens is the ability to coopt the host’s actin polymerization machinery in order to generate force for intracellular movement. This actin-based motility enables Gram-negative bacteria, such as Shigella species, to propel themselves into neighboring cells, thereby spreading from host cell to host cell without exiting the intracellular environment. Here, we show that the human protein GBP1 acts as a cytosolic “glue trap,” capturing cytosolic Gram-negative bacteria through a unique protein motif and preventing disseminated infections in cell culture models. To escape from this GBP1-mediated host defense, Shigella employs a virulence factor that prevents or dislodges the association of GBP1 with cytosolic bacteria. Thus, therapeutic strategies to restore GBP1 binding to Shigella may lead to novel treatment options for shigellosis in the future.

Interferon-inducible guanylate-binding proteins at the interface of cell-autonomous immunity and inflammasome activation

Guanylate-binding proteins (GBPs) are essential components of cell-autonomous immunity. In response to IFN signaling, GBPs are expressed in the cytoplasm of immune and nonimmune cells, where they unleash their antimicrobial activity toward intracellular bacteria, viruses, and parasites. Recent studies have revealed that GBPs are essential for mediating activation of the caspase-1 inflammasome in response to the gram-negative bacteria Salmonella enterica serovar Typhimurium, Francisella novicida, Chlamydia muridarum, Chlamydia trachomatis, Legionella pneumophila, Vibrio cholerae, Enterobacter cloacae, and Citrobacter koseri During infection with vacuolar-restricted gram-negative bacteria, GBPs disrupt the vacuolar membrane to ensure liberation of LPS for cytoplasmic detection by caspase-11 and the noncanonical NLRP3 inflammasome. In response to certain cytosolic bacteria, GBPs liberate microbial DNA for activation of the DNA-sensing AIM2 inflammasome. GBPs also promote the recruitment of antimicrobial proteins, including NADPH oxidase subunits and autophagy-associated proteins to the Mycobacterium-containing vacuole to mediate intracellular bacterial killing. Here, we provide an overview on the emerging relationship between GBPs and activation of the inflammasome in innate immunity to microbial pathogens.

Regulatory interactions between IRG resistance GTPases in the cellular response to Toxoplasma gondii

Members of the immunity-related GTPase (IRG) family are interferon-inducible resistance factors against a broad spectrum of intracellular pathogens including Toxoplasma gondii. The molecular mechanisms governing the function and regulation of the IRG resistance system are largely unknown. We find that IRG proteins function in a system of direct, nucleotide-dependent regulatory interactions between family members. After interferon induction but before infection, the three members of the GMS subfamily of IRG proteins, Irgm1, Irgm2 and Irgm3, which possess an atypical nucleotide-binding site, regulate the intracellular positioning of the conventional GKS subfamily members, Irga6 and Irgb6. Following infection, the normal accumulation of Irga6 protein at the parasitophorous vacuole membrane (PVM) is nucleotide dependent and also depends on the presence of all three GMS proteins. We present evidence that an essential role of the GMS proteins in this response is control of the nucleotide-bound state of the GKS proteins, preventing their GTP-dependent activation before infection. Accumulation of IRG proteins at the PVM has previously been shown to be associated with a block in pathogen replication: our results relate for the first time the enzymatic properties of IRG proteins to their role in pathogen resistance.