Human GBP1 Differentially Targets Salmonella and Toxoplasma to License Recognition of Microbial Ligands and Caspase-Mediated Death

Insights into the effect of guanylate-binding protein 1 on the survival of Brucella intracellularly

Brucellosis is a zoonotic disease that affects wild and domestic animals. It is caused by members of the bacterial genus Brucella. Guanylate-binding protein 1 (GBP1) is associated with microbial infections. However, the role of GBP1 during Brucella infection remains unclear. This investigation aimed to identify the association of GBP1 with brucellosis. Results showed that Brucella infection induced GBP1 upregulation in RAW 264.7 murine macrophages. Small interfering GBP1 targeting RNAs were utilized to explore how GBP1 regulates the survival of Brucella intracellularly. Results revealed that GBP1 knockdown promoted Brucella's survival ability, activated Nod-like receptor (NLR) containing a pyrin domain 3 (NLRP3) and absent in melanoma 2 (AIM2) inflammatory corpuscles, and induced pro-inflammatory cytokines IFN-γ and IL-1β. Furthermore, Brucella stimulated the expression of GBP1 in bone marrow-derived macrophages (BMDMs) and mice. During the inhibition of GBP1 in BMDMs, the intracellular growth of Brucella increased. In comparison, GBP1 downregulation enhanced the accumulation of Brucella-induced reactive oxygen species (ROS) in macrophages. Overall, the data indicate a significant role of GBP1 in regulating brucellosis and suggest the function underlying its suppressive effect on the survival and growth of Brucella intracellularly.

Inflammasomes in epithelial innate immunity: front line warriors

Our epithelium represents a battle ground against a variety of insults including pathogens and danger signals. It encodes multiple sensors that detect and respond to such insults, playing an essential role in maintaining and defending tissue homeostasis. One key set of defense mechanisms is our inflammasomes which drive innate immune responses including, sensing and responding to pathogen attack, through the secretion of pro-inflammatory cytokines and cell death. Identification of physiologically relevant triggers for inflammasomes has greatly influenced our ability to decipher the mechanisms behind inflammasome activation. Furthermore, identification of patient mutations within inflammasome components implicates their involvement in a range of epithelial diseases. This review will focus on exploring the roles of inflammasomes in epithelial immunity and cover: the diversity and differential expression of inflammasome sensors amongst our epithelial barriers, their ability to sense local infection and damage and the contribution of the inflammasomes to epithelial homeostasis and disease.

Strategies of bacterial detection by inflammasomes

Mammalian innate immunity is regulated by pattern-recognition receptors (PRRs) and guard proteins, which use distinct strategies to detect infections. PRRs detect bacterial molecules directly, whereas guards detect host cell manipulations by microbial virulence factors. Despite sensing infection through different mechanisms, both classes of innate immune sensors can activate the inflammasome, an immune complex that can mediate cell death and inflammation. Inflammasome-mediated immune responses are crucial for host defense against many bacterial pathogens and prevent invasion by non-pathogenic organisms. In this review, we discuss the mechanisms by which inflammasomes are stimulated by PRRs and guards during bacterial infection, and the strategies used by virulent bacteria to evade inflammasome-mediated immunity.

iNOS is necessary for GBP-mediated T. gondii clearance in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular parasite of rodents and humans. Interferon-inducible guanylate binding proteins (GBPs) are mediators of T. gondii clearance, however, this mechanism is incomplete. Here, using automated spatially targeted optical micro proteomics we demonstrate that inducible nitric oxide synthetase (iNOS) is highly enriched at GBP2+ parasitophorous vacuoles (PV) in murine macrophages. iNOS expression in macrophages is necessary to limit T. gondii load in vivo and in vitro. Although iNOS activity is dispensable for GBP2 recruitment and PV membrane ruffling; parasites can replicate, egress and shed GBP2 when iNOS is inhibited. T. gondii clearance by iNOS requires nitric oxide, leading to nitration of the PV and collapse of the intravacuolar network of membranes in a chromosome 3 GBP-dependent manner. We conclude that reactive nitrogen species generated by iNOS cooperate with GBPs to target distinct structures in the PV that are necessary for optimal parasite clearance in macrophages.

Host E3 ubiquitin ligase ITCH mediates Toxoplasma gondii effector GRA35-triggered NLRP1 inflammasome activation and cell-autonomous immunity

Toxoplasma gondii is an intracellular parasite that can activate the NLRP1 inflammasome leading to macrophage pyroptosis in Lewis rats, but the underlying mechanism is not well understood. In this study, we performed a genome-wide CRISPR screen and identified the dense granule proteins GRA35, GRA42, and GRA43 as the Toxoplasma effectors mediating cell death in Lewis rat macrophages. GRA35 localizes on the parasitophorous vacuole membrane, where it interacts with the host E3 ubiquitin ligase ITCH. Inhibition of proteasome activity or ITCH knockout prevented pyroptosis in Toxoplasma-infected Lewis rat macrophages, consistent with the "NLRP1 functional degradation model." However, there was no evidence that ITCH directly ubiquitinates or interacts with rat NLRP1. We also found that GRA35-ITCH interaction affected Toxoplasma fitness in IFNγ-activated human fibroblasts, likely due to ITCH's role in recruiting ubiquitin and the parasite-restriction factor RNF213 to the parasitophorous vacuole membrane. These findings identify a new role of host E3 ubiquitin ligase ITCH in mediating effector-triggered immunity, a critical concept that involves recognizing intracellular pathogens and initiating host innate immune responses.IMPORTANCEEffector-triggered immunity represents an innate immune defense mechanism that plays a crucial role in sensing and controlling intracellular pathogen infection. The NLRP1 inflammasome in the Lewis rats can detect Toxoplasma infection, which triggers proptosis in infected macrophages and eliminates the parasite's replication niche. The work reported here revealed that host E3 ubiquitin ligase ITCH is able to recognize and interact with Toxoplasma effector protein GRA35 localized on the parasite-host interface, leading to NLRP1 inflammasome activation in Lewis rat macrophages. Furthermore, ITCH-GRA35 interaction contributes to the restriction of Toxoplasma in human fibroblasts stimulated by IFNγ. Thus, this research provides valuable insights into understanding pathogen recognition and restriction mediated by host E3 ubiquitin ligase.

IFNs in host defence and parasite immune evasion during Toxoplasma gondii infections

Interferons (IFNs) are a family of cytokines with diverse functions in host resistance to pathogens and in immune regulation. Type II IFN, i.e. IFN-γ, is widely recognized as a major mediator of resistance to intracellular pathogens, including the protozoan Toxoplasma gondii. More recently, IFN-α/β, i.e. type I IFNs, and IFN-λ (type III IFN) have been identified to also play important roles during T. gondii infections. This parasite is a widespread pathogen of humans and animals, and it is a model organism to study cell-mediated immune responses to intracellular infection. Its success depends, among other factors, on the ability to counteract the IFN system, both at the level of IFN-mediated gene expression and at the level of IFN-regulated effector molecules. Here, I review recent advances in our understanding of the molecular mechanisms underlying IFN-mediated host resistance and immune regulation during T. gondii infections. I also discuss those mechanisms that T. gondii has evolved to efficiently evade IFN-mediated immunity. Knowledge of these fascinating host-parasite interactions and their underlying signalling machineries is crucial for a deeper understanding of the pathogenesis of toxoplasmosis, and it might also identify potential targets of parasite-directed or host-directed supportive therapies to combat the parasite more effectively.

Function and mechanism of GBP1 in the development and progression of cervical cancer

Guanylate binding protein 1 (GBP1) is the most concerned member of the GBP family, which has a series of effects such as anti-infection and anti-angiogenesis. Its role in malignant tumors including cervical cancer is still controversial. We aim to explore the effects of GBP1 on cervical cancer through bioinformatics and related experiments. In this study, we first found that GBP1 was generally expressed in cervical cancer in various online databases and was closely related to immune invasion. Secondly, we used multicolor immunofluorescence technology to verify the expression of GBP1 in cervical cancer tissues and its relationship with immune invasion, and explored its relationship with the prognosis of patients with cervical cancer. Knockdown and overexpression assays of GBP1 in vitro were used to prove GBP1 as a potential oncogene of cervical cancer, and its carcinogenicity was verified by in vivo experiment. In order to explore the potential mechanism of GBP1 in promoting cancer, RNA-seq was performed on GBP1 overexpression and knockdown expression cell lines, and GBP1 knockdown and overexpression were found to be associated with many RNA alternative splicing events, suggesting that GBP1 maybe a RNA binding protein (RBP) which affect the biological characteristics of cervical cancer cells through the alternative splicing pathway. However, the later RNA binding protein immunoprecipitation (RIP) assay proved that GBP1 was not a direct alternative splicing factor, while the co-immunoprecipitation (CoIP)-mass spectroscopy (MS) assay combined with protein protein interaction (PPI) analysis proved that 8 alternative splicing factors including Heterogeneous Nuclear Ribonucleoprotein K (HNRNPK) were interacting proteins of GBP1. Combined with the existing reports and the results of RNA-seq alternative splicing analysis, it is speculated that GBP1 may regulate the alternative splicing of CD44 protein by binding to interacting protein-HNRNPK, and thus play a role in promoting cancer in cervical cancer.

Genome-wide and targeted CRISPR screens identify RNF213 as a mediator of interferon gamma-dependent pathogen restriction in human cells

To define cellular immunity to the intracellular pathogen Toxoplasma gondii, we performed a genome-wide CRISPR loss-of-function screen to identify genes important for (interferon gamma) IFN-γ-dependent growth restriction. We revealed a role for the tumor suppressor NF2/Merlin for maximum induction of Interferon Stimulated Genes (ISG), which are positively regulated by the transcription factor IRF-1. We then performed an ISG-targeted CRISPR screen that identified the host E3 ubiquitin ligase RNF213 as necessary for IFN-γ-mediated control of T. gondii in multiple human cell types. RNF213 was also important for control of bacterial (Mycobacterium tuberculosis) and viral (Vesicular Stomatitis Virus) pathogens in human cells. RNF213-mediated ubiquitination of the parasitophorous vacuole membrane (PVM) led to growth restriction of T. gondii in response to IFN-γ. Moreover, overexpression of RNF213 in naive cells also impaired growth of T. gondii. Surprisingly, growth inhibition did not require the autophagy protein ATG5, indicating that RNF213 initiates restriction independent of a previously described noncanonical autophagy pathway. Mutational analysis revealed that the ATPase domain of RNF213 was required for its recruitment to the PVM, while loss of a critical histidine in the RZ finger domain resulted in partial reduction of recruitment to the PVM and complete loss of ubiquitination. Both RNF213 mutants lost the ability to restrict growth of T. gondii, indicating that both recruitment and ubiquitination are required. Collectively, our findings establish RNF213 as a critical component of cell-autonomous immunity that is both necessary and sufficient for control of intracellular pathogens in human cells.

p97/VCP targets Toxoplasma gondii vacuoles for parasite restriction in interferon-stimulated human cells

IMPORTANCE

Toxoplasma gondii (Tg) is a ubiquitous parasitic pathogen, infecting about one-third of the global population. Tg is controlled in immunocompetent people by mechanisms that are not fully understood. Tg infection drives the production of the inflammatory cytokine interferon gamma (IFNγ), which upregulates intracellular anti-pathogen defense pathways. In this study, we describe host proteins p97/VCP, UBXD1, and ANKRD13A that control Tg at the parasitophorous vacuole (PV) in IFNγ-stimulated endothelial cells. p97/VCP is an ATPase that interacts with a network of cofactors and is active in a wide range of ubiquitin-dependent cellular processes. We demonstrate that PV ubiquitination is a pre-requisite for recruitment of these host defense proteins, and their deposition directs Tg PVs to acidification in endothelial cells. We show that p97/VCP universally targets PVs in human cells and restricts Tg in different human cell types. Overall, these findings reveal new players of intracellular host defense of a vacuolated pathogen.

Human GBP1 facilitates the rupture of the Legionella-containing vacuole and inflammasome activation

IMPORTANCE

Inflammasomes are essential for host defense against intracellular bacterial pathogens like Legionella, as they activate caspases, which promote cytokine release and cell death to control infection. In mice, interferon (IFN) signaling promotes inflammasome responses against bacteria by inducing a family of IFN-inducible GTPases known as guanylate-binding proteins (GBPs). Within murine macrophages, IFN promotes the rupture of the Legionella-containing vacuole (LCV), while GBPs are dispensable for this process. Instead, GBPs facilitate the lysis of cytosol-exposed Legionella. In contrast, the functions of IFN and GBPs in human inflammasome responses to Legionella are poorly understood. We show that IFN-γ enhances inflammasome responses to Legionella in human macrophages. Human GBP1 is required for these IFN-γ-driven inflammasome responses. Furthermore, GBP1 co-localizes with Legionella and/or LCVs in a type IV secretion system (T4SS)-dependent manner and promotes damage to the LCV, which leads to increased exposure of the bacteria to the host cell cytosol. Thus, our findings reveal species- and pathogen-specific differences in how GBPs function to promote inflammasome responses.

PIM1 controls GBP1 activity to limit self-damage and to guard against pathogen infection

Disruption of cellular activities by pathogen virulence factors can trigger innate immune responses. Interferon-γ (IFN-γ)-inducible antimicrobial factors, such as the guanylate binding proteins (GBPs), promote cell-intrinsic defense by attacking intracellular pathogens and by inducing programmed cell death. Working in human macrophages, we discovered that GBP1 expression in the absence of IFN-γ killed the cells and induced Golgi fragmentation. IFN-γ exposure improved macrophage survival through the activity of the kinase PIM1. PIM1 phosphorylated GBP1, leading to its sequestration by 14-3-3σ, which thereby prevented GBP1 membrane association. During Toxoplasma gondii infection, the virulence protein TgIST interfered with IFN-γ signaling and depleted PIM1, thereby increasing GBP1 activity. Although infected cells can restrain pathogens in a GBP1-dependent manner, this mechanism can protect uninfected bystander cells. Thus, PIM1 can provide a bait for pathogen virulence factors, guarding the integrity of IFN-γ signaling.

Guanylate-binding proteins: mechanisms of pattern recognition and antimicrobial functions

Guanylate-binding proteins (GBPs) are a family of intracellular proteins which have diverse biological functions, including pathogen sensing and host defense against infectious disease. These proteins are expressed in response to interferon (IFN) stimulation and can localize and target intracellular microbes (e.g., bacteria and viruses) by protein trafficking and membrane binding. These properties contribute to the ability of GBPs to induce inflammasome activation, inflammation, and cell death, and to directly disrupt pathogen membranes. Recent biochemical studies have revealed that human GBP1, GBP2, and GBP3 can directly bind to the lipopolysaccharide (LPS) of Gram-negative bacteria. In this review we discuss emerging data highlighting the functional versatility of GBPs, with a focus on their molecular mechanisms of pattern recognition and antimicrobial activity.

Human guanylate-binding proteins in intracellular pathogen detection, destruction, and host cell death induction

Cell-intrinsic defense is an essential part of the immune response against intracellular pathogens regulated by cytokine-induced proteins and pathways. One of the most upregulated families of proteins in this defense system are the guanylate-binding proteins (GBPs), large GTPases of the dynamin family, induced in response to interferon gamma. Human GBPs (hGBPs) exert their antimicrobial activity through detection of pathogen-associated molecular patterns and/or damage-associated molecular patterns to execute control mechanisms directed at the pathogen itself as well as the vacuolar compartments in which it resides. Consequently, hGBPs are also inducers of canonical and noncanonical inflammasome responses leading to host cell death. The mechanisms are both cell-type and pathogen-dependent with hGBP1 acting as a pioneer sensor for intracellular invaders. This review focuses on the most recent functional roles of hGBPs in pathways of pathogen detection, destruction, and host cell death induction.

Collab or Cancel? Bacterial Influencers of Inflammasome Signaling

The immune system of multicellular organisms protects them from harmful microbes. To establish an infection in the face of host immune responses, pathogens must evolve specific strategies to target immune defense mechanisms. One such defense is the formation of intracellular protein complexes, termed inflammasomes, that are triggered by the detection of microbial components and the disruption of homeostatic processes that occur during bacterial infection. Formation of active inflammasomes initiates programmed cell death pathways via activation of inflammatory caspases and cleavage of target proteins. Inflammasome-activated cell death pathways such as pyroptosis lead to proinflammatory responses that protect the host. Bacterial infection has the capacity to influence inflammasomes in two distinct ways: activation and perturbation. In this review, we discuss how bacterial activities influence inflammasomes, and we discuss the consequences of inflammasome activation or evasion for both the host and pathogen.

Pyroptosis in defense against intracellular bacteria

Pathogenic microbes invade the human body and trigger a host immune response to defend against the infection. In response, host-adapted pathogens employ numerous virulence strategies to overcome host defense mechanisms. As a result, the interaction between the host and pathogen is a dynamic process that shapes the evolution of the host's immune response. Among the immune responses against intracellular bacteria, pyroptosis, a lytic form of cell death, is a crucial mechanism that eliminates replicative niches for intracellular pathogens and modulates the immune system by releasing danger signals. This review focuses on the role of pyroptosis in combating intracellular bacterial infection. We examine the cell type specific roles of pyroptosis in neutrophils and intestinal epithelial cells. We discuss the regulatory mechanisms of pyroptosis, including its modulation by autophagy and interferon-inducible GTPases. Furthermore, we highlight that while host-adapted pathogens can often subvert pyroptosis, environmental microbes are effectively eliminated by pyroptosis.

Inducible nitric oxide synthase (iNOS) is necessary for GBP-mediated T. gondii restriction in murine macrophages via vacuole nitration and intravacuolar network collapse

Toxoplasma gondii is an obligate intracellular, protozoan pathogen of rodents and humans. T. gondii's ability to grow within cells and evade cell-autonomous immunity depends on the integrity of the parasitophorous vacuole (PV). Interferon-inducible guanylate binding proteins (GBPs) are central mediators of T. gondii clearance, however, the precise mechanism linking GBP recruitment to the PV and T. gondii restriction is not clear. This knowledge gap is linked to heterogenous GBP-targeting across a population of vacuoles and the lack of tools to selectively purify the intact PV. To identify mediators of parasite clearance associated with GBP2-positive vacuoles, we employed a novel protein discovery tool automated spatially targeted optical micro proteomics (autoSTOMP). This approach identified inducible nitric oxide synthetase (iNOS) enriched at levels similar to the GBPs in infected bone marrow-derived myeloid cells. iNOS expression on myeloid cells was necessary for mice to control T. gondii growth in vivo and survive acute infection. T. gondii infection of IFNγ-primed macrophage was sufficient to robustly induce iNOS expression. iNOS restricted T. gondii infection through nitric oxide synthesis rather than arginine depletion, leading to robust and selective nitration of the PV. Optimal parasite restriction by iNOS and vacuole nitration depended on the chromosome 3 GBPs. Notably, GBP2 recruitment and ruffling of the PV membrane occurred in iNOS knockouts, however, these vacuoles contained dividing parasites. iNOS activity was necessary for the collapse of the intravacuolar network of nanotubular membranes which connects parasites to each other and the host cytosol. Based on these data we conclude reactive nitrogen species generated by iNOS cooperate with the chromosome 3 GBPs to target distinct biology of the PV that are necessary for optimal parasite clearance in murine myeloid cells.

Inflammasomes primarily restrict cytosolic Salmonella replication within human macrophages

Salmonella enterica serovar Typhimurium is a facultative intracellular pathogen that utilizes its type III secretion systems (T3SSs) to inject virulence factors into the host cell and colonize the host. In turn, a subset of cytosolic immune receptors respond to T3SS ligands by forming multimeric signaling complexes called inflammasomes, which activate caspases that induce interleukin-1 (IL-1) family cytokine release and an inflammatory form of cell death called pyroptosis. Human macrophages mount a multifaceted inflammasome response to Salmonella infection that ultimately restricts intracellular bacterial replication. However, how inflammasomes restrict Salmonella replication remains unknown. We find that caspase-1 is essential for mediating inflammasome responses to Salmonella and subsequent restriction of bacterial replication within human macrophages, with caspase-4 contributing as well. We also demonstrate that the downstream pore-forming protein gasdermin D (GSDMD) and ninjurin-1 (NINJ1), a mediator of terminal cell lysis, play a role in controlling Salmonella replication in human macrophages. Notably, in the absence of inflammasome responses, we observed hyperreplication of Salmonella within the cytosol of infected cells, and we also observed increased bacterial replication within vacuoles, suggesting that inflammasomes control Salmonella replication primarily within the cytosol and also within vacuoles. These findings reveal that inflammatory caspases and pyroptotic factors mediate inflammasome responses that restrict the subcellular localization of intracellular Salmonella replication within human macrophages.

Crystal structures reveal nucleotide-induced conformational changes in G motifs and distal regions in guanylate-binding protein 2

Guanylate-binding proteins (GBPs) are interferon-inducible GTPases that confer protective immunity against a variety of intracellular pathogens including bacteria, viruses, and protozoan parasites. GBP2 is one of the two highly inducible GBPs, yet the precise mechanisms underlying the activation and regulation of GBP2, in particular the nucleotide-induced conformational changes in GBP2, remain poorly understood. In this study, we elucidate the structural dynamics of GBP2 upon nucleotide binding through crystallographic analysis. GBP2 dimerizes upon GTP hydrolysis and returns to monomer state once GTP is hydrolyzed to GDP. By determining the crystal structures of GBP2 G domain (GBP2GD) in complex with GDP and nucleotide-free full-length GBP2, we unveil distinct conformational states adopted by the nucleotide-binding pocket and distal regions of the protein. Our findings demonstrate that the binding of GDP induces a distinct closed conformation both in the G motifs and the distal regions in the G domain. The conformational changes in the G domain are further transmitted to the C-terminal helical domain, leading to large-scale conformational rearrangements. Through comparative analysis, we identify subtle but critical differences in the nucleotide-bound states of GBP2, providing insights into the molecular basis of its dimer-monomer transition and enzymatic activity. Overall, our study expands the understanding of the nucleotide-induced conformational changes in GBP2, shedding light on the structural dynamics governing its functional versatility. These findings pave the way for future investigations aimed at elucidating the precise molecular mechanisms underlying GBP2's role in the immune response and may facilitate the development of targeted therapeutic strategies against intracellular pathogens.

Human GBP1 Is Involved in the Repair of Damaged Phagosomes/Endolysosomes

Mouse guanylate-binding proteins (mGBPs) are recruited to various invasive pathogens, thereby conferring cell-autonomous immunity against these pathogens. However, whether and how human GBPs (hGBPs) target M. tuberculosis (Mtb) and L. monocytogenes (Lm) remains unclear. Here, we describe hGBPs association with intracellular Mtb and Lm, which was dependent on the ability of bacteria to induce disruption of phagosomal membranes. hGBP1 formed puncta structures which were recruited to ruptured endolysosomes. Furthermore, both GTP-binding and isoprenylation of hGBP1 were required for its puncta formation. hGBP1 was required for the recovery of endolysosomal integrity. In vitro lipid-binding assays demonstrated direct binding of hGBP1 to PI4P. Upon endolysosomal damage, hGBP1 was targeted to PI4P and PI(3,4)P2-positive endolysosomes in cells. Finally, live-cell imaging demonstrated that hGBP1 was recruited to damaged endolysosomes, and consequently mediated endolysosomal repair. In summary, we uncover a novel interferon-inducible mechanism in which hGBP1 contributes to the repair of damaged phagosomes/endolysosomes.

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

Pyroptosis is an inflammatory form of cell death induced upon recognition of invading microbes. During an infection, pyroptosis is enhanced in interferon-gamma-exposed cells via the actions of members of the guanylate-binding protein (GBP) family. GBPs promote caspase-4 (CASP4) activation by enhancing its interactions with lipopolysaccharide (LPS), a component of the outer envelope of Gram-negative bacteria. Once activated, CASP4 promotes the formation of noncanonical 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 proposed 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, we find that additional IpaH9.8 targets, likely GBPs, promote CASP4 activation, even in the absence of GBP1. These observations suggest that by boosting LPS release, GBP1 provides CASP4-enhanced access to cytosolic LPS, thus promoting host cell death via pyroptosis.

IL-18BP mediates the balance between protective and pathological immune responses to Toxoplasma gondii

Interleukin-18 (IL-18) promotes natural killer (NK) and T cell production of interferon (IFN)-γ, a key factor in resistance to Toxoplasma gondii, but previous work has shown a limited role for endogenous IL-18 in control of this parasite. Although infection with T. gondii results in release of IL-18, the production of IFN-γ induces high levels of the IL-18 binding protein (IL-18BP). Antagonism of IL-18BP with a "decoy-to-the-decoy" (D2D) IL-18 construct that does not signal but rather binds IL-18BP results in enhanced innate lymphoid cell (ILC) and T cell responses and improved parasite control. In addition, the use of IL-18 resistant to IL-18BP ("decoy-resistant" IL-18 [DR-18]) is more effective than exogenous IL-18 at promoting innate resistance to infection. DR-18 enhances CD4+ T cell production of IFN-γ but results in CD4+ T cell-mediated pathology. Thus, endogenous IL-18BP restrains aberrant immune pathology, and this study highlights strategies that can be used to tune this regulatory pathway for optimal anti-pathogen responses.

Differential expression of interferon inducible protein: Guanylate binding protein (GBP1 & GBP2) in severe dengue

Dengue virus is reported to activate endothelial cells (EC), but the precise cause for severe dengue (SD) is not known. Guanylate binding proteins (GBPs) are IFN-inducible proteins secreted by ECs and are involved in the anti-oxidant and anti-viral response. The involvement of GBPs in the pathogenesis of dengue remains under explored. In the present study, we quantified the mRNA and protein levels of GBP1 and 2 during acute, defervescence and convalescent phase in SD-10, dengue without warning sign-15 and dengue with warning sign-25 compared to other febrile illnesses-10 and healthy controls-8 using RT-PCR and ELISA respectively. Lipid peroxidation in plasma samples were measured using the Kei Satoh method. Protein and DNA oxidation were determined by ELISA. The efficacy of the proteins in predicting disease severity was done by Support Vector Machine (SVM) model. A significant (P ≤ 0.01) decrease in the levels of mRNA and protein of both GBP1 and GBP2 was observed during defervescence in both SD and DWW cases. The levels were significantly (P ≤ 0.05) tapered off in SD cases from acute till critical phases compared to other study groups. DNA, protein and lipid oxidation markers showed an increasing trend in SD (P ≤ 0.01). Both GBP1 & 2 were found to be negatively associated plasma leakage and oxidative stress markers. EC's activated with SD serum showed a reduced expression of GBP 1 and 2. Nevertheless, the SVM model revealed that plasma levels of proteins along with clinical symptoms could predict the disease outcomes with higher precision. This is the first study reporting a downregulated expression of GBP1 & 2 and their association with oxidative stress and plasma leakage in dengue cases. This suggests the importance of GBPs in regulating disease manifestation. However, further investigations are required to ascertain its role as a biomarker or therapeutic target in dengue infection.

Noncanonical NLRP3 Inflammasome Activation: Standard Protocols

The canonical activation of multimeric inflammasomes usually occurs through caspase-1 activation, and it is characterized by the presence of extracellular IL-1β and IL-18 or measuring danger signal proteins, such as HMGB1 using enzyme-linked immunosorbent assay (ELISA) or Western blots; these assays differentiate non-cleaved and cleaved forms of these two cytokines (the cleaved form is the mature and active form). Similar techniques can be used to assess noncanonical inflammasome activation. Real-time PCR can measure the relative mRNA expression for a specific gene, whereas Western blots or immunocytochemistry can detect the presence of proteins by binding of specific antibodies to their antigens in biological samples. Moreover, noncanonical inflammasome activation can be evaluated through the cleavage of the amino and the carboxy terminals of one important component, gasdermin D (GSDMD), whose cleavage induces its pyroptotic activity. Thus, the analysis of cleaved GSDMD is an ideal pathway to study the noncanonical inflammasome. ELISA and immunoblot can be performed on cell culture supernatants or cell extracts.

Interactions of Autophagy and the Immune System in Health and Diseases

Autophagy is a highly conserved process that utilizes lysosomes to selectively degrade a variety of intracellular cargo, thus providing quality control over cellular components and maintaining cellular regulatory functions. Autophagy is triggered by multiple stimuli ranging from nutrient starvation to microbial infection. Autophagy extensively shapes and modulates the inflammatory response, the concerted action of immune cells, and secreted mediators aimed to eradicate a microbial infection or to heal sterile tissue damage. Here, we first review how autophagy affects innate immune signaling, cell-autonomous immune defense, and adaptive immunity. Then, we discuss the role of non-canonical autophagy in microbial infections and inflammation. Finally, we review how crosstalk between autophagy and inflammation influences infectious, metabolic, and autoimmune disorders.

Pyroptosis and pyroptosis-inducing cancer drugs

Pyroptosis, an inflammatory form of lytic cell death, is a type of cell death mediated by the gasdermin (GSDM) protein family. Upon recognizing exogenous or endogenous signals, cells undergo inflammasome assembly, GSDM cleavage, the release of proinflammatory cytokines and other cellular contents, eventually leading to inflammatory cell death. In this review, we discuss the roles of the GSDM family for anti-cancer functions and various antitumor drugs that could activate the pyroptosis pathways.

Lipid A Variants Activate Human TLR4 and the Noncanonical Inflammasome Differently and Require the Core Oligosaccharide for Inflammasome Activation

Detection of Gram-negative bacterial lipid A by the extracellular sensor, myeloid differentiation 2 (MD2)/Toll-like receptor 4 (TLR4), or the intracellular inflammasome sensors, CASP4 and CASP5, induces robust inflammatory responses. The chemical structure of lipid A, specifically its phosphorylation and acylation state, varies across and within bacterial species, potentially allowing pathogens to evade or suppress host immunity. Currently, it is not clear how distinct alterations in the phosphorylation or acylation state of lipid A affect both human TLR4 and CASP4/5 activation. Using a panel of engineered lipooligosaccharides (LOS) derived from Yersinia pestis with defined lipid A structures that vary in their acylation or phosphorylation state, we identified that differences in phosphorylation state did not affect TLR4 or CASP4/5 activation. However, the acylation state differentially impacted TLR4 and CASP4/5 activation. Specifically, all tetra-, penta-, and hexa-acylated LOS variants examined activated CASP4/5-dependent responses, whereas TLR4 responded to penta- and hexa-acylated LOS but did not respond to tetra-acylated LOS or penta-acylated LOS lacking the secondary acyl chain at the 3' position. As expected, lipid A alone was sufficient for TLR4 activation. In contrast, both core oligosaccharide and lipid A were required for robust CASP4/5 inflammasome activation in human macrophages, whereas core oligosaccharide was not required to activate mouse macrophages expressing CASP4. Our findings show that human TLR4 and CASP4/5 detect both shared and nonoverlapping LOS/lipid A structures, which enables the innate immune system to recognize a wider range of bacterial LOS/lipid A and would thereby be expected to constrain the ability of pathogens to evade innate immune detection.

Pyroptosisin periodontitis: From the intricate interaction with apoptosis, NETosis, and necroptosis to the therapeutic prospects

Periodontitis is highly prevalent worldwide. It is characterized by periodontal attachment and alveolar bone destruction, which not only leads to tooth loss but also results in the exacerbation of systematic diseases. As such, periodontitis has a significant negative impact on the daily lives of patients. Detailed exploration of the molecular mechanisms underlying the physiopathology of periodontitis may contribute to the development of new therapeutic strategies for periodontitis and the associated systematic diseases. Pyroptosis, as one of the inflammatory programmed cell death pathways, is implicated in the pathogenesis of periodontitis. Progress in the field of pyroptosis has greatly enhanced our understanding of its role in inflammatory diseases. This review first summarizes the mechanisms underlying the activation of pyroptosis in periodontitis and the pathological role of pyroptosis in the progression of periodontitis. Then, the crosstalk between pyroptosis with apoptosis, necroptosis, and NETosis in periodontitis is discussed. Moreover, pyroptosis, as a novel link that connects periodontitis with systemic disease, is also reviewed. Finally, the current challenges associated with pyroptosis as a potential therapeutic target for periodontitis are highlighted.

Meeting report - Cell dynamics: host-pathogen interface

Two years into the most significant infectious disease event of our generation, infections have populated every conversation and in-depth understanding of host-pathogen interactions has, perhaps, never been more important. In a successful return to in-person conferences, the host-pathogen interface was the focus of the third Cell Dynamics meeting, which took place at the glorious Wotton House in Surrey, UK. The meeting organised by Michaela Gack, Maximiliano Gutierrez, Dominique Soldati-Favre and Michael Way gathered an international group of scientists who shared their recent discoveries and views on numerous aspects, including cell-autonomous defence mechanisms, pathogen interactions with host cytoskeletal or membrane dynamics, and cellular immune regulation. More than 30 years into the beginning of cellular microbiology as a field, the meeting exhibited the unique aspect of the host-pathogen interface in uncovering the fundamentals of both pathogens and their hosts.

Pathogen-selective killing by guanylate-binding proteins as a molecular mechanism leading to inflammasome signaling

Inflammasomes are cytosolic signaling complexes capable of sensing microbial ligands to trigger inflammation and cell death responses. Here, we show that guanylate-binding proteins (GBPs) mediate pathogen-selective inflammasome activation. We show that mouse GBP1 and GBP3 are specifically required for inflammasome activation during infection with the cytosolic bacterium Francisella novicida. We show that the selectivity of mouse GBP1 and GBP3 derives from a region within the N-terminal domain containing charged and hydrophobic amino acids, which binds to and facilitates direct killing of F. novicida and Neisseria meningitidis, but not other bacteria or mammalian cells. This pathogen-selective recognition by this region of mouse GBP1 and GBP3 leads to pathogen membrane rupture and release of intracellular content for inflammasome sensing. Our results imply that GBPs discriminate between pathogens, confer activation of innate immunity, and provide a host-inspired roadmap for the design of synthetic antimicrobial peptides that may be of use against emerging and re-emerging pathogens.

Guanylate-binding proteins (GBP) have a function in inflammasome formation and pathogen defence. Here the authors show that these GBP proteins are able to kill certain bacteria and promote selective inflammasome activation and that this is mediated by specific GBP protein regions.

Role of GBP1 in innate immunity and potential as a tuberculosis biomarker

Tuberculosis (TB) is a global health problem of major concern. Identification of immune biomarkers may facilitate the early diagnosis and targeted treatment of TB. We used public RNA-sequencing datasets of patients with TB and healthy controls to identify differentially expressed genes and their associated functional networks. GBP1 expression was consistently significantly upregulated in TB, and 4492 differentially expressed genes were simultaneously associated with TB and high GBP1 expression. Weighted gene correlation analysis identified 12 functional modules. Modules positively correlated with TB and high GBP1 expression were associated with the innate immune response, neutrophil activation, neutrophil-mediated immunity, and NOD receptor signaling pathway. Eleven hub genes (GBP1, HLA-B, ELF4, HLA-E, IFITM2, TNFRSF14, CD274, AIM2, CFB, RHOG, and HORMAD1) were identified. The least absolute shrinkage and selection operator model based on hub genes accurately predicted the occurrence of TB (area under the receiver operating characteristic curve = 0.97). The GBP1-module-pathway network based on the STRING database showed that GBP1 expression correlated with the expression of interferon-stimulated genes (GBP5, BATF2, EPSTI1, RSAD2, IFI44L, IFIT3, and OAS3). Our study suggests GBP1 as an optimal diagnostic biomarker for TB, further indicating an association of the AIM2 inflammasome signaling pathway in TB pathology.

Guanylate-Binding Protein 1 Regulates Infection-Induced Autophagy through TBK1 Phosphorylation

Invading bacteria can be degraded by selective autophagy, known as xenophagy. Recent studies have shown that the recruitment of autophagy adaptor proteins such as p62 to bacteria and its regulation by activated TANK-binding kinase 1 (TBK1) are required to overcome bacterial infection. However, the detailed molecular mechanisms behind this are not yet fully understood. Here, we show that the human guanylate-binding protein (GBP) family, especially GBP1, directs xenophagy against invading Group A Streptococcus (GAS) by promoting TBK1 phosphorylation. GBP1 exhibits a GAS-surrounding localization response to bacterially caused membrane damage mediated by the membrane damage sensor galectin-3. We found that GBP1 knockout attenuated TBK1 activation, followed by reduced p62 recruitment and lower bactericidal activity by xenophagy. Furthermore, GBP1-TBK1 interaction was detected by immunoprecipitation. Our findings collectively indicate that GBP1 contributes to GAS-targeted autophagy initiated by membrane damage detection by galectin-3 via TBK1 phosphorylation.

Evaluation of Host Protein Biomarkers by ELISA From Whole Lysed Peripheral Blood for Development of Diagnostic Tests for Active Tuberculosis

Tuberculosis (TB) remains a significant global health crisis and the number one cause of death for an infectious disease. The health consequences in high-burden countries are significant. Barriers to TB control and eradication are in part caused by difficulties in diagnosis. Improvements in diagnosis are required for organisations like the World Health Organisation (WHO) to meet their ambitious target of reducing the incidence of TB by 50% by the year 2025, which has become hard to reach due to the COVID-19 pandemic. Development of new tests for TB are key priorities of the WHO, as defined in their 2014 report for target product profiles (TPPs). Rapid triage and biomarker-based confirmatory tests would greatly enhance the diagnostic capability for identifying and diagnosing TB-infected individuals. Protein-based test methods e.g. lateral flow devices (LFDs) have a significant advantage over other technologies with regard to assay turnaround time (minutes as opposed to hours) field-ability, ease of use by relatively untrained staff and without the need for supporting laboratory infrastructure. Here we evaluate the diagnostic performance of nine biomarkers from our previously published biomarker qPCR validation study; CALCOCO2, CD274, CD52, GBP1, IFIT3, IFITM3, SAMD9L, SNX10 and TMEM49, as protein targets assayed by ELISA. This preliminary evaluation study was conducted to quantify the level of biomarker protein expression across latent, extra-pulmonary or pulmonary TB groups and negative controls, collected across the UK and India, in whole lysed blood samples (WLB). We also investigated associative correlations between the biomarkers and assessed their suitability for ongoing diagnostic test development, using receiver operating characteristic/area under the curve (ROC) analyses, singly and in panel combinations. The top performing single biomarkers for pulmonary TB versus controls were CALCOCO2, SAMD9L, GBP1, IFITM3, IFIT3 and SNX10. TMEM49 was also significantly differentially expressed but downregulated in TB groups. CD52 expression was not highly differentially expressed across most of the groups but may provide additional patient stratification information and some limited use for incipient latent TB infection. These show therefore great potential for diagnostic test development either in minimal configuration panels for rapid triage or more complex formulations to capture the diversity of disease presentations.

Activation and Function of NLRP3 Inflammasome in Bone and Joint-Related Diseases

Inflammation is a pivotal response to a variety of stimuli, and inflammatory molecules such as cytokines have central roles in the pathogenesis of various diseases, including bone and joint diseases. Proinflammatory cytokines are mainly produced by immune cells and mediate inflammatory and innate immune responses. Additionally, proinflammatory cytokines accelerate bone resorption and cartilage destruction, resulting in the destruction of bone and joint tissues. Thus, proinflammatory cytokines are involved in regulating the pathogenesis of bone and joint diseases. Interleukin (IL)-1 is a representative inflammatory cytokine that strongly promotes bone and cartilage destruction, and elucidating the regulation of IL-1 will advance our understanding of the onset and progression of bone and joint diseases. IL-1 has two isoforms, IL-1α and IL-1β. Both isoforms signal through the same IL-1 receptor type 1, but the activation mechanisms are completely different. In particular, IL-1β is tightly regulated by protein complexes termed inflammasomes. Recent research using innovative technologies has led to a series of discoveries about inflammasomes. This review highlights the current understanding of the activation and function of the NLRP3 (NOD-like receptor family, pyrin domain-containing 3) inflammasome in bone and joint diseases.

Activation and manipulation of inflammasomes and pyroptosis during bacterial infections

Following detection of pathogen infection and disrupted cellular homeostasis, cells can activate a range of cell death pathways, such as apoptosis, necroptosis and pyroptosis, as part of their defence strategy. The initiation of pro-inflammatory, lytic pyroptosis is controlled by inflammasomes, which respond to a range of cellular perturbations. As is true for many host defence pathways, pathogens have evolved multiple mechanisms to subvert this pathway, many of which have only recently been described. Herein, we will discuss the mechanisms by which inflammasomes sense pathogen invasion and initiate pyroptosis and the effector mechanisms used by pathogens to suppress this pathway and preserve their niche.

Mechanisms and Consequences of Noncanonical Inflammasome-Mediated Pyroptosis

The noncanonical inflammasome, comprising inflammatory caspases 4, 5, or 11, monitors the cytosol for bacterial lipopolysaccharide (LPS). Intracellular LPS-elicited autoproteolysis of these inflammatory caspases leads to the cleavage of the pore-forming protein gasdermin D (GSDMD). GSDMD pore formation induces a lytic form of cell death known as pyroptosis and the release of inflammatory cytokines and DAMPs, thereby promoting inflammation. The noncanonical inflammasome-dependent innate sensing of cytosolic LPS plays important roles in bacterial infections and sepsis pathogenesis. Exciting studies in the recent past have significantly furthered our understanding of the biochemical and structural basis of the caspase-4/11 activation of GSDMD, caspase-4/11's substrate specificity, and the biological consequences of noncanonical inflammasome activation of GSDMD. This review will discuss these recent advances and highlight the remaining gaps in our understanding of the noncanonical inflammasome and pyroptosis.

Guanylate Binding Protein 1 (GBP1): A Key Protein in Inflammatory Pyroptosis

Scientists recently made a significant breakthrough in the recognition of pathogens via guanylate binding protein 1 (GBP1). Wandel et al. [1] in Nature Immunology described their findings where GBP1 acts as a pattern recognition receptor that directly connects to lipopolysaccharide (LPS). GBP1 identifies gram-negative bacteria such as the enteric pathogen, Salmonella enterica serovar Typhimurium, that enter the cytoplasm of the host cell. GBP1 then quickly connects to LPS and stimulates the assembly of more GBPs in the order of GBP2, GBP3, and GBP4. Subsequently, inflammatory caspase-4 arrives at the GBP1-4 activation platform. Next, the activated caspase-4 drives the cleavage of Gasdermin D, triggering the release of the pro-inflammatory cytokine, interleukin-18 (IL-18) leading to inflammatory pyroptosis and cell death. Not only do these remarkable results expand our current understanding of GBP1, but they also carry the potential to develop therapeutic targets for inflammasome-mediated human disorders.

Toxoplasma-proximal and distal control by GBPs in human macrophages

Human guanylate binding proteins (GBPs) are key players of interferon-gamma (IFNγ)-induced cell intrinsic defense mechanisms targeting intracellular pathogens. In this study, we combine the well-established Toxoplasmagondii infection model with three in vitro macrophage culture systems to delineate the contribution of individual GBP family members to control this apicomplexan parasite. Use of high-throughput imaging assays and genome engineering allowed us to define a role for GBP1, 2 and 5 in parasite infection control. While GBP1 performs a pathogen-proximal, parasiticidal and growth-restricting function through accumulation at the parasitophorous vacuole of intracellular Toxoplasma, GBP2 and GBP5 perform a pathogen-distal, growth-restricting role. We further find that mutants of the GTPase or isoprenylation site of GBP1/2/5 affect their normal function in Toxoplasma control by leading to mis-localization of the proteins.

Activation of the Non-canonical Inflammasome in Mouse and Human Cells

The non-canonical inflammasome is a signaling platform that allows for the detection of cytoplasmic lipopolysaccharides (LPS) in immune and non-immune cells. Upon detection of LPS, this inflammasome activates the signaling proteases caspase-4 and -5 (in humans) and caspase-11 (in mice). Inflammatory caspases activation leads to caspase self-processing and the cleavage of the pore-forming protein Gasdermin D (GSDMD). GSDMD N-terminal fragments oligomerize and form pores at the plasma membranes, leading to an inflammatory form of cell death called pyroptosis. Here, we describe a simple method to activate the non-canonical inflammasome in myeloid and epithelial cells and to measure its activity using cell death assay and immunoblotting.

Gasdermin D and Beyond - Gasdermin-mediated Pyroptosis in Bacterial Infections

The discovery of pyroptosis and its subsequent implications in infection and immunity has uncovered a new angle of host-defence against pathogen assault. At its most simple, gasdermin-mediated pyroptosis in bacterial infection would be expected to remove pathogens from the relative safety of the cytosol or pathogen containing vacuole/phagosome whilst inducing a rapid and effective immune response. Differences in gasdermin-mediated pyroptosis between cell types, stimulation conditions, pathogen and even animal species, however, make things more complex. The excessive inflammation associated with the pathogen-induced gasdermin-mediated pyroptosis contributes to a downward spiral in sepsis. With no currently approved effective treatment options for sepsis understanding how gasdermin-mediated pyroptotic pathways are regulated provides an opportunity to identify novel therapeutic candidates against this complex disease. In this review we cover recent advances in the field of gasdermin-mediated pyroptosis with a focus on bacterial infection and sepsis models in the context of humans and other animal species. Importantly we also consider why there is considerable redundancy set into these ancient immune pathways.

Cytosolic detection of phagosomal bacteria-Mechanisms underlying PAMP exodus from the phagosome into the cytosol

The metazoan innate immune system senses bacterial infections by detecting highly conserved bacterial molecules, termed pathogen-associated molecular patterns (PAMPs). PAMPs are detected by a variety of host pattern recognition receptors (PRRs), whose function is to coordinate downstream immune responses. PRR activities are, in part, regulated by their subcellular localizations. Accordingly, professional phagocytes can detect extracellular bacteria and their PAMPs via plasma membrane-oriented PRRs. Conversely, phagocytosed bacteria and their PAMPs are detected by transmembrane PRRs oriented toward the phagosomal lumen. Even though PAMPs are unable to passively diffuse across membranes, phagocytosed bacteria are also detected by PRRs localized within the host cell cytosol. This phenomenon is explained by phagocytosis of bacteria that specialize in phagosomal escape and cytosolic residence. Contrary to this cytosolic lifestyle, most bacteria studied to date spend their entire intracellular lifestyle contained within phagosomes, yet they also stimulate cytosolic PRRs. Herein, we will review our current understanding of how phagosomal PAMPs become accessible to cytosolic PRRs, as well as highlight knowledge gaps that should inspire future investigations.

Lessons from Toxoplasma: Host responses that mediate parasite control and the microbial effectors that subvert them

The intracellular parasite Toxoplasma gondii has long provided a tractable experimental system to investigate how the immune system deals with intracellular infections. This review highlights the advances in defining how this organism was first detected and the studies with T. gondii that contribute to our understanding of how the cytokine IFN-γ promotes control of vacuolar pathogens. In addition, the genetic tractability of this eukaryote organism has provided the foundation for studies into the diverse strategies that pathogens use to evade antimicrobial responses and now provides the opportunity to study the basis for latency. Thus, T. gondii remains a clinically relevant organism whose evolving interactions with the host immune system continue to teach lessons broadly relevant to host–pathogen interactions.

Human guanylate binding proteins: nanomachines orchestrating host defense

Disease-causing microorganisms not only breach anatomical barriers and invade tissues but also frequently enter host cells, nutrient-enriched environments amenable to support parasitic microbial growth. Protection from many infectious diseases is therefore reliant on the ability of individual host cells to combat intracellular infections through the execution of cell-autonomous defense programs. Central players in human cell-autonomous immunity are members of the family of dynamin-related guanylate binding proteins (GBPs). The importance of these interferon-inducible GTPases in host defense to viral, bacterial, and protozoan pathogens has been established for some time; only recently, cell biological and biochemical studies that largely focused on the prenylated paralogs GBP1, GBP2, and GBP5 have provided us with robust molecular frameworks for GBP-mediated immunity. Specifically, the recent characterization of GBP1 as a bona fide pattern recognition receptor for bacterial lipopolysaccharide (LPS) disrupting the integrity of bacterial outer membranes through LPS aggregation, the discovery of a link between hydrolysis-induced GMP production by GBP1 and inflammasome activation, and the classification of GBP2 and GBP5 as inhibitors of viral envelope glycoprotein processing via suppression of the host endoprotease furin have paved the way for a vastly improved conceptual understanding of the molecular mechanisms by which GBP nanomachines execute cell-autonomous immunity. The herein discussed models incorporate our current knowledge of the antimicrobial, proinflammatory, and biochemical properties of human GBPs and thereby provide testable hypotheses that will guide future studies into the intricacies of GBP-controlled host defense and their role in human disease.

Guanylate-Binding Protein-Dependent Noncanonical Inflammasome Activation Prevents Burkholderia thailandensis-Induced Multinucleated Giant Cell Formation

Inflammasomes are cytosolic multiprotein signaling complexes that are activated upon pattern recognition receptor-mediated recognition of pathogen-derived ligands or endogenous danger signals. Their assembly activates the downstream inflammatory caspase-1 and caspase-4/5 (human) or caspase-11 (mouse), which induces cytokine release and pyroptotic cell death through the cleavage of the pore-forming effector gasdermin D. Pathogen detection by host cells also results in the production and release of interferons (IFNs), which fine-tune inflammasome-mediated responses. IFN-induced guanylate-binding proteins (GBPs) have been shown to control the activation of the noncanonical inflammasome by recruiting caspase-4 on the surface of cytosolic Gram-negative bacteria and promoting its interaction with lipopolysaccharide (LPS). The Gram-negative opportunistic bacterial pathogen Burkholderia thailandensis infects epithelial cells and macrophages and hijacks the host actin polymerization machinery to spread into neighboring cells. This process causes host cell fusion and the formation of so-called multinucleated giant cells (MNGCs). Caspase-1- and IFN-regulated caspase-11-mediated inflammasome pathways play an important protective role against B. thailandensis in mice, but little is known about the role of IFNs and inflammasomes during B. thailandensis infection of human cells, particularly epithelial cells. Here, we report that IFN-γ priming of human epithelial cells restricts B. thailandensis-induced MNGC formation in a GBP1-dependent manner. Mechanistically, GBP1 does not promote bacteriolysis or impair actin-based bacterial motility but acts by inducing caspase-4-dependent pyroptosis of the infected cell. In addition, we show that IFN-γ priming of human primary macrophages confers a more efficient antimicrobial effect through inflammasome activation, further confirming the important role that interferon signaling plays in restricting Burkholderia replication and spread.

New Insights on the Early Interaction Between Typhoid and Non-typhoid Salmonella Serovars and the Host Cells

Salmonella enterica is a common source of food and water-borne infections, causing a wide range of clinical ailments in both human and animal hosts. Immunity to Salmonella involves an interplay between different immune responses, which are rapidly initiated to control bacterial burden. However, Salmonella has developed several strategies to evade and modulate the host immune responses. In this sense, the main knowledge about the pathogenicity of this bacterium has been obtained by the study of mouse models with non-typhoidal serovars. However, this knowledge is not representative of all the pathologies caused by non-typhoidal serovars in the human. Here we review the most important features of typhoidal and non-typhoidal serovars and the diseases they cause in the human host, describing the virulence mechanisms used by these pathogens that have been identified in different models of infection.

Great balls of fire: activation and signalling of inflammatory caspases

Innate immune responses are tightly regulated by various pathways to control infections and maintain homeostasis. One of these pathways, the inflammasome pathway, activates a family of cysteine proteases called inflammatory caspases. They orchestrate an immune response by cleaving specific cellular substrates. Canonical inflammasomes activate caspase-1, whereas non-canonical inflammasomes activate caspase-4 and -5 in humans and caspase-11 in mice. Caspases are highly specific enzymes that select their substrates through diverse mechanisms. During inflammation, caspase activity is responsible for the secretion of inflammatory cytokines and the execution of a form of lytic and inflammatory cell death called pyroptosis. This review aims to bring together our current knowledge of the biochemical processes behind inflammatory caspase activation, substrate specificity, and substrate signalling.

Interferon-induced GTPases orchestrate host cell-autonomous defence against bacterial pathogens

Interferon (IFN)-induced guanosine triphosphate hydrolysing enzymes (GTPases) have been identified as cornerstones of IFN-mediated cell-autonomous defence. Upon IFN stimulation, these GTPases are highly expressed in various host cells, where they orchestrate anti-microbial activities against a diverse range of pathogens such as bacteria, protozoan and viruses. IFN-induced GTPases have been shown to interact with various host pathways and proteins mediating pathogen control via inflammasome activation, destabilising pathogen compartments and membranes, orchestrating destruction via autophagy and the production of reactive oxygen species as well as inhibiting pathogen mobility. In this mini-review, we provide an update on how the IFN-induced GTPases target pathogens and mediate host defence, emphasising findings on protection against bacterial pathogens.

Necroptosis, pyroptosis and apoptosis: an intricate game of cell death

Cell death is a fundamental physiological process in all living organisms. Its roles extend from embryonic development, organ maintenance, and aging to the coordination of immune responses and autoimmunity. In recent years, our understanding of the mechanisms orchestrating cellular death and its consequences on immunity and homeostasis has increased substantially. Different modalities of what has become known as 'programmed cell death' have been described, and some key players in these processes have been identified. We have learned more about the intricacies that fine tune the activity of common players and ultimately shape the different types of cell death. These studies have highlighted the complex mechanisms tipping the balance between different cell fates. Here, we summarize the latest discoveries in the three most well understood modalities of cell death, namely, apoptosis, necroptosis, and pyroptosis, highlighting common and unique pathways and their effect on the surrounding cells and the organism as a whole.

The GBP1 microcapsule interferes with IcsA-dependent septin cage assembly around Shigella flexneri

Many cytosolic bacterial pathogens hijack the host actin polymerization machinery to form actin tails that promote direct cell-to-cell spread, enabling these pathogens to avoid extracellular immune defenses. However, these pathogens are still susceptible to intracellular cell-autonomous immune responses that restrict bacterial actin-based motility. Two classes of cytosolic antimotility factors, septins and guanylate-binding proteins (GBPs), have recently been established to block actin tail formation by the human-adapted bacterial pathogen Shigella flexneri. Both septin cages and GBP1 microcapsules restrict S. flexneri cell-to-cell spread by blocking S. flexneri actin-based motility. While septins assemble into cage-like structures around immobile S. flexneri, GBP1 forms microcapsules around both motile and immobile bacteria. The interplay between these two defense programs remains elusive. Here, we demonstrate that GBP1 microcapsules block septin cage assembly, likely by interfering with the function of S. flexneri IcsA, the outer membrane protein that promotes actin-based motility, as this protein is required for septin cage formation. However, S. flexneri that escape from GBP1 microcapsules via the activity of IpaH9.8, a type III secreted effector that promotes the degradation of GBPs, are often captured within septin cages. Thus, our studies reveal how septin cages and GBP1 microcapsules represent complementary host cell antimotility strategies.

Interferons at the crossroad of cell death pathways during gastrointestinal inflammation and infection

Interferons (IFNs) are pleiotropic immune-modulatory cytokines that are well known for their essential role in host defense against viruses, bacteria, and other pathogenic microorganisms. They can exert both, protective or destructive functions depending on the microorganism, the targeted tissue and the cellular context. Interferon signaling results in the induction of IFN-stimulated genes (ISGs) influencing different cellular pathways including direct anti-viral/anti-bacterial response, immune-modulation or cell death. Multiple pathways leading to host cell death have been described, and it is becoming clear that depending on the cellular context, IFN-induced cell death can be beneficial for both: host and pathogen. Accordingly, activation or repression of corresponding signaling mechanisms occurs during various types of infection but is also an important pathway for gastrointestinal inflammation and tissue damage. In this review, we summarize the role of interferons at the crossroad of various cell death pathways in the gut during inflammation and infection.