Role of inflammasomes in salmonella infection

Salmonella Enteritidis antitoxin DinJ inhibits NLRP3-dependent canonical inflammasome activation in macrophages

The inflammasome is a pivotal component of the innate immune system, acting as a multiprotein complex that plays an essential role in detecting and responding to microbial infections. Salmonella Enteritidis have evolved multiple mechanisms to regulate inflammasome activation and evade host immune system clearance. Through screening S. Enteritidis C50336ΔfliC transposon mutant library, we found that the insertion mutant of dinJ increased inflammasome activation. In this study, we demonstrated the genetic connection between the antitoxin DinJ and the toxin YafQ in S. Enteritidis, confirming their co-transcription. The deletion mutant ΔfliCΔdinJ increased cell death and IL-1β secretion in J774A.1 cells. Western blotting analysis further showed elevated cleaved Caspase-1 product (p10 subunits) and IL-1β secretion in cells infected with ΔfliCΔdinJ compared to cells infected with ΔfliC. DinJ was found to inhibit canonical inflammasome activation using primary bone marrow-derived macrophages (BMDMs) from Casp-/- C57BL/6 mice. Furthermore, DinJ specifically inhibited NLRP3 inflammasome activation, as demonstrated in BMDMs from Nlrp3-/- and Nlrc4-/- mice. Fluorescence resonance energy transfer (FRET) experiments confirmed the translocation of DinJ into host cells during infection. Finally, we revealed that DinJ could inhibit the secretion of IL-1β and IL-18 in vivo, contributing to S. Enteritidis evading host immune clearance. In summary, our findings provide insights into the role of DinJ in modulating the inflammasome response during S. Enteritidis infection, highlighting its impact on inhibiting inflammasome activation and immune evasion.

Serum cytokine profile of neonatal broiler chickens infected with Salmonella Typhimurium

The avian immune system responds to Salmonella infection by expressing cytokines and chemokines. We hypothesized that the immune status of Salmonella Typhimurium (ST) challenged neonatal broilers would differ from the uninfected treatment. The objective of this experiment was to evaluate 12 cytokines. Day of hatch male chicks were randomly allocated into a control or ST challenged group. At day three of age, sterile diluent or 5.0 × 108 CFU of ST was given orally to each chick. Blood was obtained 24 h post challenge and serum separated for later analysis (n = 30 chicks/treatment). Significant (p ≤ 0.05) increases in pro-inflammatory cytokines-interleukin-6 (IL-6), IL-16, and IL-21; anti-inflammatory cytokines- IL-10; chemokines-regulated on activation, normal T cell expressed and secreted (RANTES), macrophage inflammatory protein-1β (MIP-1β), and MIP-3α; colony stimulating factors-macrophage colony-stimulating factor (M-CSF); and growth factors-vascular endothelial growth factor (VEGF) were observed in the serum of the challenged chicks when compared to the control. No significant differences were observed in IL-2, interferon gamma (IFNγ), and IFNα. These data indicate the detection of mucosal immune responses in broiler chickens following ST infection. The heightened levels of pro-inflammatory cytokines, chemokines, and colony stimulating factors align with known inflammatory mechanisms, like the influx of immune cells. However, the elevation of IL-10 was unexpected, due to its immunoregulatory properties. Notably, the rise in VEGF levels is compelling, as it suggests the possibility of tissue repair and angiogenesis in ST infected birds.

Olive Mill Waste-Water Extract Enriched in Hydroxytyrosol and Tyrosol Modulates Host-Pathogen Interaction in IPEC-J2 Cells

The dietary supplementation of olive oil by-products, including olive mill waste-water (OMWW) in animal diets, is a novel application that allows for their re-utilization and recycling and could potentially decrease the use of antibiotics, antimicrobial resistance risk in livestock species, and the occurrence of intestinal diseases. Salmonella serovar typhimurium is one of the most widespread intestinal pathogens in the world, causing enterocolitis in pigs. The aim of this study was to investigate the effect of an OMWW extract enriched in polyphenols (hydroxytyrosol and tyrosol) in the immune response of an intestinal porcine epithelial cell line (IPEC-J2) following S. typhimurium infection. Cells were pre-treated with OMWW-extract polyphenols (OMWW-EP, 0.35 and 1.4 µg) for 24 h and then infected with S. typhimurium for 1 h. We evaluated bacterial invasiveness and assayed IPEC-J2 gene expression with RT-qPCR and cytokine release with an ELISA test. The obtained results showed that OMWW-EP (1.4 µg) significantly reduced S. typhimurium invasiveness; 0.35 µg decreased the IPEC-J2 gene expression of IL1B, MYD88, DEFB1 and DEFB4A, while 1.4 µg down-regulated IL1B and DEFB4A and increased TGFB1. The cytokine content was unchanged in infected cells. This is the first study demonstrating the in vitro immunomodulatory and antimicrobial activity of OMWW extracts enriched in polyphenols, suggesting a protective role of OMWW polyphenols on the pig intestine and their potential application as feed supplements in farm animals such as pigs.

Two Additional Connections between the Transcriptional Programs Controlling Invasion and Intracellular Replication of Salmonella: HilD-SprB Positively Regulates phoP and slyA

Salmonella virulence relies on the ability of this bacterium to invade the intestinal epithelium and to replicate inside macrophages, which are functions mainly encoded in Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2), respectively. Complex regulatory programs control the expression of SPI-1 and SPI-2 and functionally related genes, involving the integration of ancestral regulators and regulators that Salmonella has acquired during its evolution. Interestingly, some previous studies have revealed cross talk between the regulatory programs for SPI-1 and SPI-2. Here, we report two additional connections between the regulatory programs controlling the expression of genes for invasion and intracellular replication. Our results show that the acquired regulators HilD and SprB, both encoded in SPI-1, induce, in a cascade fashion, the expression of PhoP and SlyA, two ancestral regulators that activate the expression of SPI-2 and other genes required for intracellular replication. We provide evidence supporting that the regulation of phoP and slyA by HilD-SprB was adapted during the divergence of Salmonella from its closer species, Escherichia coli, with the acquisition of SPI-1 and thus the gain of HilD and SprB, as well as through cis-regulatory evolution of phoP and slyA. Therefore, our study further expands the knowledge about the intricate regulatory network controlling the expression of virulence genes in Salmonella. IMPORTANCE Bacteria have developed diverse regulatory mechanisms to control genetic expression, in the case of pathogenic bacteria, to induce the expression of virulence genes in particular niches during host infection. In Salmonella, an intricate regulatory network has been determined, which controls the spatiotemporal expression of the SPI-1 and SPI-2 gene clusters that mediate the invasion to and the replication inside host cells, respectively. In this study, we report two additional pathways of cross talk between the transcriptional programs for SPI-1 and SPI-2. Additionally, our results support that these additional regulatory pathways were adapted during the divergence of Salmonella from its closer species, Escherichia coli. This study further expands the knowledge about the mechanisms determining the Salmonella virulence.

Inflammasome activation, NLRP3 engagement and macrophage recruitment to tumor microenvironment are all required for Salmonella antitumor effect

Salmonella-based cancer therapies show great potential in preclinical models, but for most cases the observed antitumor effect is transient. Understanding the basis of the antitumor efficacy might guide the design of improved strains that elicit long-lasting effects, paving the wave for clinical use.  Here, we deepened into the role of macrophages and inflammasome activation in the context of Salmonella anti-melanoma effect. We showed inflammasome activation in melanoma cells upon infection, which correlated with cell surface exposure of gasdermin-D (GSDM-D) and calreticulin (CRT) and High mobility group box 1 protein (HMGB-1) release, suggesting immunogenic cell death, particularly pyroptosis. Salmonella infection upregulated levels of Caspase-11 (Casp11) mRNA, but not Nlrp3 or Nlrc4 mRNA, the only described inflammasome receptors engaged by Salmonella, suggesting that non-canonical inflammasome activation could be occurring in melanoma cells. Intratumoral administration of Salmonella to melanoma-bearing mice elicited local inflammasome activation and interleukin-1β (IL-1β) production together with tumor growth retardation and extended survival in wild type but not Caspase-1/11 (Casp1/11) knockout mice despite similar levels of intratumoral IL-1β in the later. Salmonella antitumor activity was also suppressed in melanoma bearing Nlrp3 knockout mice. Salmonella induced macrophage recruitment to the tumor site and infiltrating cells exhibited inflammasome activation. Depletion experiments confirmed that macrophages are also essential for Salmonella anti-melanoma effect. Intratumoral macrophages showed a marked M2/M1 shift soon after treatment but this inflammatory profile is then lost, which could explain the transient effect of therapy.  All in all, our results highlight CASP-1/11 axis and macrophages as essential players in Salmonella-based cancer immunotherapy and suggest a possible target for future interventions.

NLRC4 inflammasome-dependent cell death occurs by a complementary series of three death pathways and determines lethality in mice

Inflammasome is an innate immune defense mechanism, but its overactivation can lead to host death. Here, we show that cell death dictates mouse death caused by NLRC4 inflammasome overactivation. To execute NLRC4-dependent cell death, three death pathways complement each other in a specific order: Pyroptosis pathway requiring caspase-1 and GSDMD is the default path; impairment of it initiates ASC-mediated caspase-8–dependent apoptosis; when these two pathways are blocked, caspase-1 triggers intrinsic apoptotic pathway. Blocking one or two of these death pathways inhibits induction of various cytokines and lipid mediators, but mice still succumb, and only genetic deletions that block all death paths prevent NLRC4-mediated cell death, tissue damage, and mice death. In addition, infection of nonpropagative Salmonella-caused mice death is attenuated by blocking these death pathways. Thus, to reduce the lethality of infection-related diseases, preventing cell death might be necessary when propagation of infected pathogen was controlled by other means.

Avoidance of the NLRP3 Inflammasome by the Stealth Pathogen, Coxiella burnetii

Coxiella burnetii, a highly adapted obligate intracellular bacterial pathogen and the cause of the zoonosis Q fever, is a reemerging public health threat. C. burnetii employs a Type IV secretion system (T4SS) to establish and maintain its intracellular niche and modulate host immune responses including the inhibition of apoptosis. Interactions between C. burnetii and caspase-1-mediated inflammasomes are not fully elucidated. This study confirms that C. burnetii does not activate caspase-1 during infection of mouse macrophages in vitro. C. burnetii-infected cells did not develop NLRP3 and ASC foci indicating its ability to avoid cytosolic detection. C. burnetii is unable to inhibit the pyroptosis and IL-1β secretion that is induced by potent inflammasome stimuli but rather enhances these caspase-1-mediated effects. We found that C. burnetii upregulates pro-IL-1β and robustly primes NLRP3 inflammasomes via TLR2 and MyD88 signaling. As for wildtype C. burnetii, T4SS-deficient mutants primed and potentiated NLRP3 inflammasomes. An in vivo model of pulmonary infection in C57BL/6 mice was developed. Mice deficient in NLRP3 or caspase-1 were like wildtype mice in the development and resolution of splenomegaly due to red pulp hyperplasia, and histologic lesions and macrophage kinetics, but had slightly higher pulmonary bacterial burdens at the greatest measured time point. Together these findings indicate that C. burnetii primes but avoids cytosolic detection by NLRP3 inflammasomes, which are not required for the clinical resistance of C57BL/6 mice. Determining mechanisms employed by C. burnetii to avoid cytosolic detection via NLRP3 inflammasomes will be beneficial to the development of preventative and interventional therapies for Q fever.

Deletion of the lon gene augments expression of Salmonella Pathogenicity Island (SPI)-1 and metal ion uptake genes leading to the accumulation of bactericidal hydroxyl radicals and host pro-inflammatory cytokine-mediated rapid intracellular clearance

In the present study, we characterized the involvement of Lon protease in bacterial virulence and intracellular survival in Salmonella under abiotic stress conditions resembling the conditions of a natural infection. Wild type (JOL401) and the lon mutant (JOL909) Salmonella Typhimurium were exposed to low temperature, pH, osmotic, and oxidative stress conditions and changes in gene expression profiles related to virulence and metal ion uptake were investigated. Expression of candidate genes invF and hilC of Salmonella Pathogenicity Island (SPI)-1 and sifA and sseJ of SPI-2 revealed that Lon protease controls SPI-1 genes and not SPI-2 genes under all stress conditions tested. The lon mutant exhibited increased accumulation of hydroxyl (OH·) ions that lead to cell damage due to oxidative stress. This oxidative damage can also be linked to an unregulated influx of iron due to the upregulation of ion channel genes such as fepA in the lon mutant. The deletion of lon from the Salmonella genome causes oxidative damage and increased expression of virulence genes. It also prompts the secretion of host pro-inflammatory cytokines leading to early clearance of the bacteria from host cells. We conclude that poor bacterial recovery from mice infected with the lon mutant is a result of disrupted bacterial intracellular equilibrium and rapid activation of cytokine expression leading to bacterial lysis.

Short-chain fatty acids bind to apoptosis-associated speck-like protein to activate inflammasome complex to prevent Salmonella infection

Short-chain fatty acids (SCFAs) produced by gastrointestinal microbiota regulate immune responses, but host molecular mechanisms remain unknown. Unbiased screening using SCFA-conjugated affinity nanobeads identified apoptosis-associated speck-like protein (ASC), an adaptor protein of inflammasome complex, as a noncanonical SCFA receptor besides GPRs. SCFAs promoted inflammasome activation in macrophages by binding to its ASC PYRIN domain. Activated inflammasome suppressed survival of Salmonella enterica serovar Typhimurium (S. Typhimurium) in macrophages by pyroptosis and facilitated neutrophil recruitment to promote bacterial elimination and thus inhibit systemic dissemination in the host. Administration of SCFAs or dietary fibers, which are fermented to SCFAs by gut bacteria, significantly prolonged the survival of S. Typhimurium–infected mice through ASC-mediated inflammasome activation. SCFAs penetrated into the inflammatory region of the infected gut mucosa to protect against infection. This study provided evidence that SCFAs suppress Salmonella infection via inflammasome activation, shedding new light on the therapeutic activity of dietary fiber.

This study shows that short-chain fatty acids (SCFAs) bind to the inflammasome adaptor protein, apoptosis-associated speck-like protein (ASC). SCFAs thereby promote inflammasome activation in macrophages and protect against Salmonella infection via bacterial elimination in gut, shedding new light on the therapeutic activity of dietary fiber.

Manipulation of Salmonella Typhi Gene Expression Impacts Innate Cell Responses in the Human Intestinal Mucosa

Although immunity induced by typhoid fever is moderated and short-lived, typhoid vaccination with the attenuated Ty21a oral vaccine generates long-lasting protection rates reaching up to 92%. Thus, there are important differences on how wild-type Salmonella and typhoid vaccine strains stimulate host immunity. We hypothesize that vaccine strains with different mutations might affect gut inflammation and intestinal permeability by different mechanisms. To test this hypothesis, we used an in vitro organotypic model of the human intestinal mucosa composed of human intestinal epithelial cells, lymphocytes/monocytes, endothelial cells, and fibroblasts. We also used six Salmonella enterica serovar Typhi (S. Typhi) strains: the licensed Ty21a oral vaccine, four typhoid vaccine candidates (i.e., CVD 908, CVD 909, CVD 910, and CVD 915) and the wild-type Ty2 strain. We found that genetically engineered S. Typhi vaccine strains elicit differential host changes not only in the intestinal permeability and secretion of inflammatory cytokines, but also in the phenotype and activation pathways of innate cells. These changes were distinct from those elicited by the parent wild-type S. Typhi and depended on the genetic manipulation. In sum, these results emphasize the importance of carefully selecting specific manipulations of the Salmonella genome in the development of typhoid vaccines.

The transcriptional regulator SsrB is involved in a molecular switch controlling virulence lifestyles of Salmonella

The evolution of bacterial pathogenicity, heavily influenced by horizontal gene transfer, provides new virulence factors and regulatory connections that alter bacterial phenotypes. Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2) are chromosomal regions that were acquired at different evolutionary times and are essential for Salmonella virulence. In the intestine of mammalian hosts, Salmonella expresses the SPI-1 genes that mediate its invasion to the gut epithelium. Once inside the cells, Salmonella down-regulates the SPI-1 genes and induces the expression of the SPI-2 genes, which favor its intracellular replication. The mechanism by which the invasion machinery is deactivated following successful invasion of host cells is not known. Here, we show that the SPI-2 encoded transcriptional regulator SsrB, which positively controls SPI-2, acts as a dual regulator that represses expression of SPI-1 during intracellular stages of infection. The mechanism of this SPI-1 repression by SsrB was direct and acts upon the hilD and hilA regulatory genes. The phenotypic effect of this molecular switch activity was a significant reduction in invasion ability of S. enterica serovar Typhimurium while promoting the expression of genes required for intracellular survival. During mouse infections, Salmonella mutants lacking SsrB had high levels of hilA (SPI-1) transcriptional activity whereas introducing a constitutively active SsrB led to significant hilA repression. Thus, our results reveal a novel SsrB-mediated mechanism of transcriptional crosstalk between SPI-1 and SPI-2 that helps Salmonella transition to the intracellular lifestyle.

Salmonella infect humans and a wide range of mammalian hosts. Successful infection requires the bacteria to sense their surroundings and regulate gene expression in a way that maximizes fitness in that particular environment. The two major lifestyles of Salmonella include extracellular stages and intracellular stages of host cell infection; however, the molecular mechanisms of how Salmonella transitions between these two lifestyles are not completely understood. Here we show that the transcriptional regulator SsrB functions in a dual capacity, activating genes required for intracellular survival while simultaneously repressing genes needed for extracellular stages of infection. Our data highlight how regulatory crosstalk is selective during infection, presumably because it helps facilitate rapid transitions in bacterial lifestyles that ultimately promote bacterial survival and replication.

Anti-inflammatory and antiatherogenic effects of the NLRP3 inflammasome inhibitor arglabin in ApoE2.Ki mice fed a high-fat diet

BACKGROUND

This study was designed to evaluate the effect of arglabin on the NLRP3 inflammasome inhibition and atherosclerotic lesion in ApoE2Ki mice fed a high-fat Western-type diet.

METHODS AND RESULTS

Arglabin was purified, and its chemical identity was confirmed by mass spectrometry. It inhibited, in a concentration-dependent manner, interleukin (IL)-1β and IL-18, but not IL-6 and IL-12, production in lipopolysaccharide and cholesterol crystal-activated cultured mouse peritoneal macrophages, with a maximum effect at ≈50 nmol/L and EC50 values for both cytokines of ≈ 10 nmol/L. Lipopolysaccharide and cholesterol crystals did not induce IL-1β and IL-18 production in Nlrp3(-/-) macrophages. In addition, arglabin activated autophagy as evidenced by the increase in LC3-II protein. Intraperitoneal injection of arglabin (2.5 ng/g body weight twice daily for 13 weeks) into female ApoE2.Ki mice fed a high-fat diet resulted in a decreased IL-1β plasma level compared with vehicle-treated mice (5.2±1.0 versus 11.7±1.1 pg/mL). Surprisingly, arglabin also reduced plasma levels of total cholesterol and triglycerides to 41% and 42%, respectively. Moreover, arglabin oriented the proinflammatory M1 macrophages into the anti-inflammatory M2 phenotype in spleen and arterial lesions. Finally, arglabin treatment markedly reduced the median lesion areas in the sinus and whole aorta to 54% (P=0.02) and 41% (P=0.02), respectively.

CONCLUSIONS

Arglabin reduces inflammation and plasma lipids, increases autophagy, and orients tissue macrophages into an anti-inflammatory phenotype in ApoE2.Ki mice fed a high-fat diet. Consequently, a marked reduction in atherosclerotic lesions was observed. Thus, arglabin may represent a promising new drug to treat inflammation and atherosclerosis.

The thioredoxin system as a therapeutic target in human health and disease

The thioredoxin (Trx) system comprises Trx, truncated Trx (Trx-80), Trx reductase, and NADPH, besides a natural Trx inhibitor, the thioredoxin-interacting protein (TXNIP). This system is essential for maintaining the balance of the cellular redox status, and it is involved in the regulation of redox signaling. It is also pivotal for growth promotion, neuroprotection, inflammatory modulation, antiapoptosis, immune function, and atherosclerosis. As an ubiquitous and multifunctional protein, Trx is expressed in all forms of life, executing its function through its antioxidative, protein-reducing, and signal-transducing activities. In this review, the biological properties of the Trx system are highlighted, and its implications in several human diseases are discussed, including cardiovascular diseases, heart failure, stroke, inflammation, metabolic syndrome, neurodegenerative diseases, arthritis, and cancer. The last chapter addresses the emerging therapeutic approaches targeting the Trx system in human diseases.

Interleukin-18 in pulmonary inflammatory diseases

The proinflammatory cytokine interleukin (IL)-18 was originally discovered as an interferon-γ-inducing factor in 1995. IL-18 is known to play an important role in Th1/Tc1 polarization and promoting the production of Th2 cytokines (e.g., IL-4, IL-5, IL-9, and IL-13) by T cells, NK cells, basophils, and mast cells. IL-18 can act as a cofactor for Th2 cell development and IgE production, and also plays an important role in the differentiation of Th17 cells. IL-18 is a key player in the pathogenesis of inflammatory diseases such as atopic dermatitis, rheumatoid arthritis, adult-onset Still's disease, Sjögren's syndrome, and inflammatory bowel diseases. Furthermore, many lines of evidence suggest that IL-18 plays a key role in the pathogenesis of pulmonary inflammatory diseases, including bronchial asthma and chronic obstructive pulmonary disease. Here, we review the pathological roles of IL-18 in pulmonary inflammatory diseases.

NLRC4-driven production of IL-1β discriminates between pathogenic and commensal bacteria and promotes host intestinal defense

Intestinal phagocytes transport oral antigens and promote immune tolerance, but their role in innate immune responses remains unclear. Here we report that intestinal phagocytes are anergic to Toll-like receptor ligands or commensals, but constitutively express pro-interleukin-1β (proIL-1β). Upon infection with pathogenic Salmonella or Pseudomonas, intestinal phagocytes produce mature IL-1β through the NLRC4 inflammasome, but not tumor necrosis factor or IL-6. Mice deficient in NLRC4 or IL-1 receptor on a Balb/c background were highly susceptible to orogastric but not intraperitoneal infection with Salmonella. Increased lethality was preceded by impaired expression of endothelial adhesion molecules, lower neutrophil recruitment, and poor intestinal pathogen clearance. Thus, NLRC4-dependent IL-1β production by intestinal phagocytes represents a specific response discriminating pathogenic from commensal bacteria and contributes to host defense in the intestine.

Sensing and reacting to microbes through the inflammasomes

Inflammasomes are multiprotein complexes that activate caspase-1, which leads to maturation of the proinflammatory cytokines interleukin 1β (IL-1β) and IL-18 and the induction of pyroptosis. Members of the Nod-like receptor (NLR) family, including NLRP1, NLRP3 and NLRC4, and the cytosolic receptor AIM2 are critical components of inflammasomes and link microbial and endogenous danger signals to the activation of caspase-1. In response to microbial infection, activation of the inflammasomes contributes to host protection by inducing immune responses that limit microbial invasion, but deregulated activation of inflammasomes is associated with autoinflammatory syndromes and other pathologies. Thus, understanding inflammasome pathways may provide insight into the mechanisms of host defense against microbes and the development of inflammatory disorders.