Antimicrobial functions of inflammasomes

Candida auris uses metabolic strategies to escape and kill macrophages while avoiding robust activation of the NLRP3 inflammasome response

Metabolic adaptations regulate the response of macrophages to infection. The contributions of metabolism to macrophage interactions with the emerging fungal pathogen Candida auris are poorly understood. Here, we show that C. auris-infected macrophages undergo immunometabolic reprogramming and increase glycolysis but fail to activate a strong interleukin (IL)-1β cytokine response or curb C. auris growth. Further analysis shows that C. auris relies on its own metabolic capacity to escape from macrophages and proliferate in vivo. Furthermore, C. auris kills macrophages by triggering host metabolic stress through glucose starvation. However, despite causing macrophage cell death, C. auris does not trigger robust activation of the NLRP3 inflammasome. Consequently, inflammasome-dependent responses remain low throughout infection. Collectively, our findings show that C. auris uses metabolic regulation to eliminate macrophages while remaining immunologically silent to ensure its own survival. Thus, our data suggest that host and pathogen metabolism could represent therapeutic targets for C. auris infections.

Antiparasitic Effects and Cellular Mechanisms of Formononetin (a Natural Isoflavone) Against Hydatid Cyst Protoscoleces

Background: The chemical agents applied to reduce the complications of hydatid cyst surgery are not free of side effects. Formononetin (FMN), as a natural isoflavone, has been shown to have various therapeutic benefits. Objectives: We studied the in vitro and ex vivo protoscolicidal activity and cellular mechanisms of FMN against hydatid cyst protoscoleces. Methods: Formononetin at 75, 150, and 300 μg/mL was mixed with hydatid cyst protoscoleces (103/mL), and the viability was determined in 5 to 60 min through eosin staining assay. The effect of FMN on caspase-3 activity was tested through a commercial colorimetric protease kit. In addition, the penetrability of the plasma membrane of protoscoleces after exposure to FMN was also determined by the SYTOX assay. Results: Formononetin at 300 μg/mL completely destroyed hydatid cyst protoscoleces after 30 minutes. While at 150 μg/mL, after 60 minutes of contact, 100% of protoscoleces were eliminated. By ex vivo assay, FMN showed its antiparasitic effect for longer periods. Based on these results, FMN at 300 μg/mL concentration completely eliminated hydatid cyst protoscoleces after 60 minutes of exposure. While at a concentration of 150 μg/mL after 60 minutes, 89.6% of protoscoleces were destroyed. Formononetin noticeably increased (P < 0.001) the activity of caspase-3 and the permeability of protoscoleces dose-dependently. Conclusions: Formononetin as a natural product showed promising effects on the protoscoleces of hydatid cysts, indicating that it can be considered a valuable scolicidal agent. However, additional investigations are necessary to evaluate its efficacy in animal models and human subjects.

Neutrophils in Health and Disease: From Receptor Sensing to Inflammasome Activation

Neutrophils—polymorphonuclear cells (PMNs) are the cells of the initial immune response and make up the majority of leukocytes in the peripheral blood. After activation, these cells modify their functional status to meet the needs at the site of action or according to the agent causing injury. They receive signals from their surroundings and “plan” the course of the response in both temporal and spatial contexts. PMNs dispose of intracellular signaling pathways that allow them to perform a wide range of functions associated with the development of inflammatory processes. In addition to these cells, some protein complexes, known as inflammasomes, also have a special role in the development and maintenance of inflammation. These complexes participate in the proteolytic activation of key pro-inflammatory cytokines, such as IL-1β and IL-18. In recent years, there has been significant progress in the understanding of the structure and molecular mechanisms behind the activation of inflammasomes and their participation in the pathogenesis of numerous diseases. The available reports focus primarily on macrophages and dendritic cells. According to the literature, the activation of inflammasomes in neutrophils and the associated death type—pyroptosis—is regulated in a different manner than in other cells. The present work is a review of the latest reports concerning the course of inflammasome activation and inflammatory cytokine secretion in response to pathogens in neutrophils, as well as the role of these mechanisms in the pathogenesis of selected diseases.

Antiparasitic activity of Astragalus brachycalyx subsp. brachycalyx extract against hydatid cyst protoscoleces and its effect on induction of apoptosis: an in vitro and ex vivo study

Introduction: This study aims to evaluate in vitro and ex vivo antiparasitic activity of Astragalus brachycalyx subsp. brachycalyx. root chloroformic extract against hydatid cyst protoscoleces and its effect on induction of apoptosis. Methods: Various concentrations of the A. brachycalyx root chloroformic extract (56.25, 112.5, 225, and 450 mg/mL) were treated with hydatid cyst protoscoleces collected from the liver of infected sheep for 5-60minutes in vitro and ex vivo. Eosin exclusion test was also utilized to measure the mortality of protoscoleces. Moreover, the extract effect was assessed on apoptosis induction in hydatid cyst protoscoleces by caspase-3 activity measurement. Results: The mortality rate of protoscoleces in in vitro was 100% after being exposed to 450 and 225 mg/mL of A. brachycalyx extract for 20 and 30 minutes and in ex vivo for 30 and 60 minutes, respectively. Following 48 h treatment of protoscoleces, A. brachycalyx chloroformic extract at the doses of 56.25, 112.5, 225, and 450 mg/mL, dose-dependently motivated the caspase-3 enzyme ranging from 8.8% to 29.6% Conclusion: A. brachycalyx root chloroformic extract had a significant protoscolicidal effect; however, extra surveys are required to assess its efficacy and safety as a promising protoscolicidal agent in clinical settings.

Faecalibacterium prausnitzii Attenuates DSS-Induced Colitis by Inhibiting the Colonization and Pathogenicity of Candida albicans

SCOPE

Intestinal commensal microbiota interactions play critical roles in the inflammatory bowel disease (IBD) development. Candida albicans (CA) can aggravate intestinal inflammation; however, whether Faecalibacterium prausnitzii (FP) can antagonize CA is unknown.

METHODS AND RESULTS

CA are co-cultured with bacteria (FP and Escherichia coli (EC)), bacterial supernatant, and bacterial medium, respectively. Then, the CA hyphae-specific genes' expression and CA cells' morphology are investigated. The Nod-like receptor pyrin-containing protein 6 (NLRP6) inflammasome, inflammatory cytokines, and antimicrobial peptides (AMPs) production are evaluated in intestinal epithelial cells pre-treated with bacteria, bacterial med, and bacterial supernatant and exposed without or with CA. Both bacteria significantly prohibit CA numbers, while only FP and FP supernatant prohibit the transformation and virulence factors (extracellular phospholipase, secreted aspartyl proteinase, and hemolysin) secretion of CA in a co-culture system compared with media controls. Further, FP and FP supernatant promote the production of the NLRP6 inflammasome, interleukin (IL)-1β, IL-18, and antibacterial peptides (β-defensin (BD)-2 and BD-3) and inhibit in vitro and in vivo CA growth and pathogenicity, and alleviate DSS-colitis in mice, while EC do not show the similar effect.

CONCLUSION

FP improve intestinal inflammation by inhibiting CA reproduction, colonization, and pathogenicity and inducing AMP secretion in the gut. This study uncovers new relationships between intestinal microbes and fungi in IBD patients.

Longitudinal Assessment of Cytokine Expression and Plasminogen Activation in Hantavirus Cardiopulmonary Syndrome Reveals Immune Regulatory Dysfunction in End-Stage Disease

Pathogenic New World orthohantaviruses cause hantavirus cardiopulmonary syndrome (HCPS), a severe immunopathogenic disease in humans manifested by pulmonary edema and respiratory distress, with case fatality rates approaching 40%. High levels of inflammatory mediators are present in the lungs and systemic circulation of HCPS patients. Previous studies have provided insights into the pathophysiology of HCPS. However, the longitudinal correlations of innate and adaptive immune responses and disease outcomes remain unresolved. This study analyzed serial immune responses in 13 HCPS cases due to Sin Nombre orthohantavirus (SNV), with 11 severe cases requiring extracorporeal membrane oxygenation (ECMO) treatment and two mild cases. We measured viral load, levels of various cytokines, urokinase plasminogen activator (uPA), and plasminogen activator inhibitor-1 (PAI-1). We found significantly elevated levels of proinflammatory cytokines and PAI-1 in five end-stage cases. There was no difference between the expression of active uPA in survivors' and decedents' cases. However, total uPA in decedents' cases was significantly higher compared to survivors'. In some end-stage cases, uPA was refractory to PAI-1 inhibition as measured by zymography, where uPA and PAI-1 were strongly correlated to lymphocyte counts and IFN-γ. We also found bacterial co-infection influencing the etiology and outcome of immune response in two cases. Unsupervised Principal Component Analysis and hierarchical cluster analyses resolved separate waves of correlated immune mediators expressed in one case patient due to a sequential co-infection of bacteria and SNV. Overall, a robust proinflammatory immune response, characterized by an imbalance in T helper 17 (Th17) and regulatory T-cells (Treg) subsets, was correlated with dysregulated inflammation and mortality. Our sample size is small; however, the core differences correlated to survivors and end-stage HCPS are instructive.

T6SS Mediated Stress Responses for Bacterial Environmental Survival and Host Adaptation

The bacterial type VI secretion system (T6SS) is a protein secretion apparatus widely distributed in Gram-negative bacterial species. Many bacterial pathogens employ T6SS to compete with the host and to coordinate the invasion process. The T6SS apparatus consists of a membrane complex and an inner tail tube-like structure that is surrounded by a contractile sheath and capped with a spike complex. A series of antibacterial or antieukaryotic effectors is delivered by the puncturing device consisting of a Hcp tube decorated by the VgrG/PAAR complex into the target following the contraction of the TssB/C sheath, which often leads to damage and death of the competitor and/or host cells. As a tool for protein secretion and interspecies interactions, T6SS can be triggered by many different mechanisms to respond to various physiological conditions. This review summarizes our current knowledge of T6SS in coordinating bacterial stress responses against the unfavorable environmental and host conditions.

Metabolic competition between host and pathogen dictates inflammasome responses to fungal infection

The NLRP3 inflammasome has emerged as a central immune regulator that senses virulence factors expressed by microbial pathogens for triggering inflammation. Inflammation can be harmful and therefore this response must be tightly controlled. The mechanisms by which immune cells, such as macrophages, discriminate benign from pathogenic microbes to control the NLRP3 inflammasome remain poorly defined. Here we used live cell imaging coupled with a compendium of diverse clinical isolates to define how macrophages respond and activate NLRP3 when faced with the human yeast commensal and pathogen Candida albicans. We show that metabolic competition by C. albicans, rather than virulence traits such as hyphal formation, activates NLRP3 in macrophages. Inflammasome activation is triggered by glucose starvation in macrophages, which occurs when fungal load increases sufficiently to outcompete macrophages for glucose. Consistently, reducing Candida’s ability to compete for glucose and increasing glucose availability for macrophages tames inflammatory responses. We define the mechanistic requirements for glucose starvation-dependent inflammasome activation by Candida and show that it leads to inflammatory cytokine production, but it does not trigger pyroptotic macrophage death. Pyroptosis occurs only with some Candida isolates and only under specific experimental conditions, whereas inflammasome activation by glucose starvation is broadly relevant. In conclusion, macrophages use their metabolic status, specifically glucose metabolism, to sense fungal metabolic activity and activate NLRP3 when microbial load increases. Therefore, a major consequence of Candida-induced glucose starvation in macrophages is activation of inflammatory responses, with implications for understanding how metabolism modulates inflammation in fungal infections.

Activation of the immune regulator NLRP3 inflammasome by microbial pathogens has been shown to play both protective and destructive roles in infection, underscoring the importance of tight control over NLRP3-driven inflammation to ensure host health. A key microbe recognised by NLRP3 is the human yeast commensal and pathogen Candida albicans, which is responsible for mucosal and invasive infections. We demonstrate that innate immune cells sense their metabolic status to trigger NLRP3 activation only when microbial numbers have reached dangerous levels. This regulation is a consequence of metabolic competition between C. albicans and macrophages for an essential nutrient–glucose. The NLRP3 inflammasome is activated when increased fungal load in the infection microenvironment drives down glucose levels, thereby causing glucose starvation in macrophages. Restoring glucose homeostasis in macrophages reduced NLRP3 activation and production of the proinflammatory cytokine IL-1β, suggesting that metabolism regulates NLRP3 inflammasome activity in fungal infections.

Inflammasome signaling and regulation of interleukin-1 family cytokines

Specific IL-1 family cytokines are initially expressed as inactive, cytosolic pro-forms. Chan and Schroder review inflammasome signaling and cell death decisions, mechanisms underpinning IL-1α, IL-1β, IL-18, and IL-37 maturation and release, and the functions of these cytokines in protective and pathological inflammation.

Specific IL-1 family cytokines are expressed by cells as cytosolic pro-forms that require cleavage for their activity and cellular release. IL-1β, IL-18, and IL-37 maturation and secretion is governed by inflammatory caspases within signaling platforms called inflammasomes. By inducing pyroptosis, inflammasomes can also drive the release of the alarmin IL-1α. Recent advances have transformed our mechanistic understanding of inflammasome signaling, cell death decisions, and cytokine activation and secretion. Here, we provide an updated view of inflammasome signaling; mechanisms underpinning IL-1α, IL-1β, IL-18, and IL-37 maturation and release; and the functions of these cytokines in protective and pathological inflammation.

Primary immunodeficiencies in cytosolic pattern-recognition receptor pathways: Toward host-directed treatment strategies

In the last decade, the paradigm of primary immunodeficiencies (PIDs) as rare recessive familial diseases that lead to broad, severe, and early-onset immunological defects has shifted toward collectively more common, but sporadic autosomal dominantly inherited isolated defects in the immune response. Patients with PIDs constitute a formidable area of research to study the genetics and the molecular mechanisms of complex immunological pathways. A significant subset of PIDs affect the innate immune response, which is a crucial initial host defense mechanism equipped with pattern-recognition receptors. These receptors recognize pathogen- and damage-associated molecular patterns in both the extracellular and intracellular space. In this review, we will focus on primary immunodeficiencies caused by genetic defects in cytosolic pattern-recognition receptor pathways. We discuss these PIDs organized according to their mutational mechanisms and consequences for the innate host response. The advanced understanding of these pathways obtained by the study of PIDs creates the opportunity for the development of new host-directed treatment strategies.

Transcriptional analysis of THP-1 cells infected with Leishmania infantum indicates no activation of the inflammasome platform

Leishmaniasis is caused by intracellular parasites transmitted to vertebrates by sandfly bites. Clinical manifestations include cutaneous, mucosal or visceral involvement depending upon the host immune response and the parasite species. To assure their survival inside macrophages, these parasites developed a plethora of highly successful strategies to manipulate various immune system pathways. Considering that inflammasome activation is critical for the establishment of a protective immune response in many parasite infections, in this study we determined the transcriptome of THP-1 cells after infection with L. infantum, with a particular focus on the inflammasome components. To this end, the human cell line THP-1, previously differentiated into macrophages by PMA treatment, was infected with L. infantum promastigotes. Differentiated THP-1 cells were also stimulated with LPS to be used as a comparative parameter. The gene expression signature was determined 8 hours after by RNA-seq technique. Infected or uninfected THP-1 cells were stimulated with nigericin (NIG) to measure active caspase-1 and TNF-α, IL-6 and IL-1β levels in culture supernatants after 8, 24 and 48 hours. L. infantum triggered a gene expression pattern more similar to non-infected THP-1 cells and very distinct from LPS-stimulated cells. Some of the most up-regulated genes in L. infantum-infected cells were CDC20, CSF1, RPS6KA1, CD36, DUSP2, DUSP5, DUSP7 and TNFAIP3. Some up-regulated GO terms in infected cells included cell coagulation, regulation of MAPK cascade, response to peptide hormone stimulus, negative regulation of transcription from RNA polymerase II promoter and nerve growth factor receptor signaling pathway. Infection was not able to induce the expression of genes associated with the inflammasome signaling pathway. This finding was confirmed by the absence of caspase-1 activation and IL-1β production after 8, 24 and 48 hours of infection. Our results indicate that L. infantum was unable to activate the inflammasomes during the initial interaction with THP-1 cells.

Visceral leishmaniasis, caused by Leishmania infantum, is a disease that affects millions of people worldwide. The entry of microorganisms into the host is commonly associated with activation of a multiprotein platform called inflammasome whose assembly culminates in caspase-1 activation and IL-1β production. ILβ activates other cells and effector mechanisms leading to clearance of pathogens. However, the involvement of inflammasomes in the human infection with L. infantum is poorly known. To investigate the parasite-host interaction is fundamental to understand the immunopathogenesis of visceral leishmaniasis and to allow the development of new therapeutic strategies. In this study, we used RNA-seq, a tool that allowed to investigate the global gene expression of THP-1 cells, which is a macrophage-like human cell line, infected with L. infantum. By using computational analysis, this approach allowed us to evaluate the expression of genes that compose the inflammasomes pathway and other gene networks and signaling pathways triggered after infection. This analysis indicated that, unlike species causing cutaneous leishmaniasis, L. infantum did not induce the expression of genes of inflammasome pathways, nor caspase-1 activation or IL-1β production, possibly reflecting a parasite strategy to manipulate immune system and therefore, to allow its survival inside the cells.

Feeding Grass Hay Before Concentrate Mitigates the Effect of Grain-Based Concentrates on Postprandial Plasma Interleukin-1β

When fed to horses, high-starch diets elevate plasma concentrations of interleukin-1β (IL-1β) as soon as 1 hour posteating. This increase in IL-β is possibly because of changes in intestinal pH that result from rapid bacterial fermentation of starches and sugars in the digestive tract. The purpose of this research was to investigate the effect of feeding 0.9 kg of grass hay 30 minutes before feeding a concentrate meal on the postprandial rise in IL-1β, compared with control horses receiving the same concentrate without hay first. Six mature geldings were used in a switchback design. Horses were fasted overnight before being offered a concentrate feed that provided 1.2 g/kg bodyweight of nonstructural carbohydrates. Plasma was harvested 30 minutes before hay feeding and 1, 2, 4, 6, and 8 hours postfeeding. Concentrations of IL-1β and d-lactate were analyzed by repeated measures analysis of variance. The hay-first treatment reduced (P = .034) postprandial concentrations of IL-1β at all time points compared with the control horses. An interaction between hour and treatment was detected for mean d-lactate concentrations (P = .037), with lower concentrations in hay-first fed horses at postfeeding hours 1, 2, and 4, compared with control horses. Given these findings, we believe that feeding a small amount of hay before feeding a meal of moderate starch and sugar content reduced the negative effects of rapid starch and sugar fermentation in the equine digestive tract, evidenced by reduced postprandial d-lactate and IL-1β concentrations.

Candida albicans SC5314 inhibits NLRP3/NLRP6 inflammasome expression and dampens human intestinal barrier activity in Caco-2 cell monolayer model

Candida albicans is an opportunistic fungal pathogen that colonizes human gastro-intestinal mucosal tissues. Its effect on the immune response in intestinal epithelial cells and on the intestinal mucosal barrier are not yet fully understood. In this study, we investigated Caco-2 cells, a monolayer model of intestinal epithelial cells, with or without treatment with C. albicans SC5314 (CA) or heat-inactivated CA (CA-inact). RNA sequencing was conducted, and the mRNA and protein levels of NOD-like receptor pyrin domain-containing protein 3 (NLRP3) or NLRP6/ASC/caspase-1 inflammasome signaling pathway components, inflammatory cytokines (interleukin-18 [IL-18] and IL-1β), anti-microbial peptides (AMPs; β-defensin-2 [BD-2], BD-3, and LL-37), and tight junction proteins (occludin and zona occludens-1 [ZO-1]) were examined by real-time PCR, western blotting, and/or immunofluorescence microscopy. Lactase dehydrogenase (LDH) activity in the Caco-2 cell supernatant were measured by enzyme kinetics analysis. Our results showed that the NOD-like receptor signaling pathway participates in the CA- and CA-inact-infected Caco-2 cells, as shown by microarray analysis of total mRNA expression. The expression of NLRP3, NLRP6, ASC, BD-2, BD-3, occludin, and ZO-1 were significantly decreased in Caco-2 cells infected with CA and CA-inact compared to that in the untreated control. IL-1β expression was decreased in the Caco-2 cells in both the CA- and CA-inact-infected groups compared to that in the control. Caspase-1 and IL-18 levels were not markedly affected by CA or CA-inact in Caco-2 cells. Our findings indicate that CA can inhibit the NLRP3 and NLRP6 pathways and dampen human intestinal mucosal barrier activity by decreasing the production of AMPs and tight junction proteins, independent of CA activity.

Antibody and DNA sensing pathways converge to activate the inflammasome during primary human macrophage infection

Inflammasomes are potent innate immune signalling complexes that couple cytokine release with pro-inflammatory cell death. However, pathogens have evolved strategies to evade this cell autonomous system. Here, we show how antibodies combine with innate sensors in primary human macrophages to detect viral infection and activate the inflammasome. Our data demonstrate that antibody opsonisation of virions can activate macrophages in multiple ways. In the first, antibody binding of adenovirus causes lysosomal damage, activating NLRP3 to drive inflammasome formation and IL-1β release. Importantly, this mechanism enhances virion capture but not infection and is accompanied by cell death, denying the opportunity for viral replication. Unexpectedly, we also find that antibody-coated viruses, which successfully escape into the cytosol, trigger a second system of inflammasome activation. These viruses are intercepted by the cytosolic antibody receptor TRIM21 and the DNA sensor cGAS. Together, these sensors stimulate both NLRP3 inflammasome formation and NFκB activation, driving dose-dependent IL-1β and TNF secretion, without inducing cell death. Our data highlight the importance of cooperativity between multiple sensing networks to expose viruses to the inflammasome pathway, which is particularly important for how our innate immune system responds to infection in the presence of pre-existing immunity.

Divergent Approaches to Virulence in C. albicans and C. glabrata: Two Sides of the Same Coin

Candida albicans and Candida glabrata are the two most prevalent etiologic agents of candidiasis worldwide. Although both are recognized as pathogenic, their choice of virulence traits is highly divergent. Indeed, it appears that these different approaches to fungal virulence may be equally successful in causing human candidiasis. In this review, the virulence mechanisms employed by C. albicans and C. glabrata are analyzed, with emphasis on the differences between the two systems. Pathogenesis features considered in this paper include dimorphic growth, secreted enzymes and signaling molecules, and stress resistance mechanisms. The consequences of these traits in tissue invasion, biofilm formation, immune system evasion, and macrophage escape, in a species dependent manner, are discussed. This review highlights the observation that C. albicans and C. glabrata follow different paths leading to a similar outcome. It also highlights the lack of knowledge on some of the specific mechanisms underlying C. glabrata pathogenesis, which deserve future scrutiny.

Extrinsic and intrinsic apoptosis activate pannexin-1 to drive NLRP3 inflammasome assembly

Pyroptosis is a form of lytic inflammatory cell death driven by inflammatory caspase-1, caspase-4, caspase-5 and caspase-11. These caspases cleave and activate the pore-forming protein gasdermin D (GSDMD) to induce membrane damage. By contrast, apoptosis is driven by apoptotic caspase-8 or caspase-9 and has traditionally been classified as an immunologically silent form of cell death. Emerging evidence suggests that therapeutics designed for cancer chemotherapy or inflammatory disorders such as SMAC mimetics, TAK1 inhibitors and BH3 mimetics promote caspase-8 or caspase-9-dependent inflammatory cell death and NLRP3 inflammasome activation. However, the mechanism by which caspase-8 or caspase-9 triggers cell lysis and NLRP3 activation is still undefined. Here, we demonstrate that during extrinsic apoptosis, caspase-1 and caspase-8 cleave GSDMD to promote lytic cell death. By engineering a novel Gsdmd D88A knock-in mouse, we further demonstrate that this proinflammatory function of caspase-8 is counteracted by caspase-3-dependent cleavage and inactivation of GSDMD at aspartate 88, and is essential to suppress GSDMD-dependent cell lysis during caspase-8-dependent apoptosis. Lastly, we provide evidence that channel-forming glycoprotein pannexin-1, but not GSDMD or GSDME promotes NLRP3 inflammasome activation during caspase-8 or caspase-9-dependent apoptosis.

Leptospira species promote a pro-inflammatory phenotype in human neutrophils

Leptospirosis is a global zoonosis caused by pathogenic Leptospira. Neutrophils are key cells against bacterial pathogens but can also contribute to tissue damage. Because the information regarding the role of human neutrophils in leptospirosis is scant, we comparatively analysed the human neutrophil's response to saprophytic Leptospira biflexa serovar Patoc (Patoc) and the pathogenic Leptospira interrogans serovar Copenhageni (LIC). Both species triggered neutrophil responses involved in migration, including the upregulation of CD11b expression, adhesion to collagen, and the release of IL-8. In addition, both species increased levels of pro-inflammatory IL-1β and IL-6 associated with the inflammasome and NFκB pathway activation and delayed neutrophil apoptosis. LIC was observed on the neutrophil surface and not phagocytized. In contrast, Patoc generated intracellular ROS associated with its uptake. Neutrophils express the TYRO3, AXL, and MER receptor protein tyrosine kinases (TAM), but only LIC selectively increased the level of AXL. TLR2 but not TLR4-blocking antibodies abrogated the IL-8 secretion triggered by both Leptospira species. In summary, we demonstrate that Leptospira species trigger a robust neutrophil activation and pro-inflammatory response. These findings may be useful to find new diagnostic markers and therapeutic strategies against leptospirosis.

NLRC4 regulates caspase-1 and IL-1beta production in a CD11blowLy6Glow population of cells required for resistance to Pseudomonas aeruginosa keratitis

Psbetaeudomonas (P.) aeruginosa infection of the cornea in BALB/c mice does not result in perforation and the mice have been classified as resistant. However, regulation of this response via inflammasome activation remained untested. Therefore, BALB/c mice were infected with P. aeruginosa ATCC strain 19660 and NLRP3 and NLRC4 protein tested by ELISA. Since NLRC4 vs NLRP3 protein levels were significantly higher in the corneas of BALB/c at 1 and 5 days postinfection we used silencing to knockdown NLRC4. Silencing NLRC4 vs scrambled siRNA treatment exacerbated disease in BALB/c mice, reduced myeloperoxidase levels and elevated bacterial plate counts at 5 days postinfection. It also increased pro IL-1beta, but reduced total protein for IL-1beta and IL-18 at 5 days postinfection. Flow cytometry to identify cells affected by silencing, showed reduced caspase-1 levels in a CD11b^lowLy6G^low population of cells, (but not PMN or macrophages) from the infected cornea of siNLRC4 treated mice that produced less mature IL-1beta. These data provide evidence that the NLRC4 inflammasome contributes to resistance through regulation of caspase-1, IL-1beta and IL-18 in a CD11b^lowLy6G^low population of cells.

Neutrophil pyroptosis mediates pathology of P. aeruginosa lung infection in the absence of the NADPH oxidase NOX2

Nod-like receptor family, CARD domain-containing 4 (NLRC4) inflammasome activation is required for efficient clearance of intracellular pathogens through caspsase-1-dependent pyroptosis in macrophages. Although neutrophils have a critical role in protection from Pseudomonas aeruginosa infection, the mechanisms regulating inflammasome-mediated pyroptosis in neutrophils and its physiological role are largely unknown. We sought to determine the specific mechanisms regulating neutrophil pyroptosis in P. aeruginosa strain PAO1 (PAO1) lung infection and to identify the pathological role of this process. Nox2^-/- models with reduced neutrophil antibacterial activity exhibited increased neutrophil pyroptosis, which was mediated by flagellin, a pathogenic PAO1 component. We also demonstrate that PAO1-induced pyroptosis depended on NLRC4 and Toll-like receptor 5 (TLR5) in neutrophils generated from Nlrc4^-/- or Tlr5^-/- mice. Our study reveals previously unknown mechanisms and physiological role of neutrophil pyroptosis during P. aeruginosa lung infection. Furthermore, our findings regarding neutrophil pyroptosis in the context of neutrophil dysfunction may explain the causes of acute and/or chronic infectious diseases discovered in immune-compromised patients.

Emerging Roles for Epigenetic Programming in the Control of Inflammatory Signaling Integration in Heath and Disease

Macrophages and dendritic cells initiate the innate immune response to infection and injury and contribute to inflammatory signaling to maintain the homeostasis of various tissues, which includes resident macrophages for the elimination of invading microorganisms and tissue damage. Inappropriate inflammatory signaling can lead to persistent inflammation and further develop into autoimmune and inflammation-associated diseases. Inflammatory signaling pathways have been well characterized, but how these signaling pathways are converted into sustained and diverse patterns of expression of cytokines, chemokines, and other genes in response to environmental challenges is unclear. Emerging evidence suggests the important role of epigenetic mechanisms in finely tuning the outcome of the host innate immune response. An understanding of epigenetic regulation of innate immune cell identity and function will enable the identification of the mechanism between gene-specific host defenses and inflammatory disease and will also allow for exploration of the program of innate immune memory in health and disease. This information could be used to develop therapeutic agents to enhance the host response, preventing chronic inflammation through preserving tissues and signaling integrity.

Questions and controversies in innate immune research: what is the physiological role of NLRP3?

The NLRP3 inflammasome is a key component of the innate immune system that induces pro-inflammatory cytokine production and cell death. Although NLRP3 is activated by many pathogens, it only appears to be critical for host defense for a limited number of specific infections. NLRP3 is however strongly associated with the initiation and pathology of many inflammatory diseases. If NLRP3 function is largely redundant for host defense, but drives a number of inflammatory diseases, this raises the important question of why evolution has elected to maintain NLRP3 function. We propose that the primary physiological functions of NLRP3 in health are to engage pathways to clear noxious substances (e.g. protein aggregates and crystals), and to regulate metabolism. We discuss the newly identified functions for NLRP3 in metabolic homeostasis, and how NLRP3 beneficial functions in homeostasis may become detrimental during the onset of inflammatory and metabolic diseases. A common feature of most NLRP3-driven diseases is that they are associated with ageing or metabolic excess, and indeed, Nlrp3 deficiency promotes 'healthspan' in ageing mice. This suggests that beneficial functions of NLRP3 in youth may become increasingly countered by NLRP3-dependent pathology as an individual ages, and we propose a general model by which ageing or nutrient excess may provide a tipping point to switch NLRP3 function from beneficial to pathological. The physiological role of NLRP3 in healthy individuals remains incompletely understood and future research will need to address this if NLRP3 is to become a successful therapeutic target for the clinical management of inflammatory diseases.

Defensin γ-thionin from Capsicum chinense has immunomodulatory effects on bovine mammary epithelial cells during Staphylococcus aureus internalization

β-Defensins are members of the antimicrobial peptide superfamily that are produced in various species from different kingdoms, including plants. Plant defensins exhibit primarily antifungal activities, unlike those from animals that exhibit a broad-spectrum antimicrobial action. Recently, immunomodulatory roles of mammal β-defensins have been observed to regulate inflammation and activate the immune system. Similar roles for plant β-defensins remain unknown. In addition, the regulation of the immune system by mammalian β-defensins has been studied in humans and mice models, particularly in immune cells, but few studies have investigated these peptides in epithelial cells, which are in intimate contact with pathogens. The aim of this work was to evaluate the effect of the chemically synthesized β-defensin γ-thionin from Capsicum chinense on the innate immune response of bovine mammary epithelial cells (bMECs) infected with Staphylococcus aureus, the primary pathogen responsible for bovine mastitis, which is capable of living within bMECs. Our results indicate that γ-thionin at 0.1 μg/ml was able to reduce the internalization of S. aureus into bMECs (∼50%), and it also modulates the innate immune response of these cells by inducing the mRNA expression (∼5-fold) and membrane abundance (∼3-fold) of Toll-like receptor 2 (TLR2), as well as by inducing genes coding for the pro-inflammatory cytokines TNF-α and IL-1β (∼14 and 8-fold, respectively) before and after the bacterial infection. γ-Thionin also induces the expression of the mRNA of anti-inflammatory cytokine IL-10 (∼12-fold). Interestingly, the reduction in bacterial internalization coincides with the production of other antimicrobial products by bMECs, such as NO before infection, and the secretion into the medium of the endogenous antimicrobial peptide DEFB1 after infection. The results from this work support the potential use of β-defensins from plants as immunomodulators of the mammalian innate immune response.

Sterile signals generate weaker and delayed macrophage NLRP3 inflammasome responses relative to microbial signals

Inflammation is the host response to microbial infection or sterile injury that aims to eliminate the insult, repair the tissue and restore homeostasis. Macrophages and the NLRP3 inflammasome are key sentinels for both types of insult. Although it is well established that the NLRP3 inflammasome is activated by microbial products and molecules released during sterile injury, it is unclear whether the responses elicited by these different types of signals are distinct. In this study, we used lipopolysaccharide and tumor necrosis factor as prototypical microbial and sterile signal 1 stimuli, respectively, to prime the NLRP3 inflammasome. We then used the bacterial toxin nigericin and a common product released from necrotic cells, ATP, as prototypical microbial and sterile signal 2 stimuli, respectively, to trigger the assembly of the NLRP3 inflammasome complex in mouse and human macrophages. We found that NLRP3 inflammasome responses were weakest when both signal 1 and signal 2 were sterile, but responses were faster and stronger when at least one of the two signals was microbial. Ultimately, the most rapid and potent responses were elicited when both signals were microbial. Together, these data suggest that microbial versus sterile signals are distinct, both kinetically and in magnitude, in their ability to generate inflammasome-dependent responses. This hierarchy of NLRP3 responses to sterile versus microbial stimuli likely reflects the urgent need for the immune system to respond rapidly to the presence of infection to halt pathogen dissemination.

Non-classical effects of prolactin on the innate immune response of bovine mammary epithelial cells: Implications during Staphylococcus aureus internalization

Staphylococcus aureus has the ability to invade mammary epithelial cells (bMECs) causing mastitis. This event depends primarily on the α5β1 integrin in the host cell. In addition, bMECs are a target for the hormone prolactin (PRL), which can regulate β1 integrin-dependent actions related to differentiation and lactation. Previously, we demonstrated that bovine PRL (bPRL, 5 ng/ml) stimulates S. aureus internalization into bMECs. TLR2 is important during S. aureus infections, but its activation by PRL has not yet been established. The objective of this study was to determine the role of α5β1 integrin and TLR2 during S. aureus internalization into bMECs stimulated with bPRL. We demonstrated that the prolactin-stimulated internalization of S. aureus decreases in response to the blockage of α5β1 integrin (∼ 80%) and TLR2 (∼ 80%). bPRL increases the membrane abundance (MA) of α5β1 integrin (∼ 20%) and induces TLR2 MA (∼ 2-fold). S. aureus reduces the α5β1 integrin MA in bMECs treated with bPRL (∼ 75%) but induces TLR2 MA in bMECs (∼ 3-fold). Bacteria and bPRL did not modify TLR2 MA compared with the hormone alone. S. aureus induces the activation of the transcription factor AP-1, which was inhibited in bMECs treated with bPRL and infected. In general, bPRL induces both pro- and anti-inflammatory responses in bMECs, which are abated in response to bacterial challenge. Interestingly, the canonical Stat-5 transcription factor was not activated in the challenged bMECs and/or treated with bPRL. Taken together, these results support novel functions of prolactin as a modulator of the innate immune response that do not involve the classical prolactin pathway.

Deficient NLRP3 and AIM2 Inflammasome Function in Autoimmune NZB Mice

Inflammasomes are protein complexes that promote caspase activation, resulting in processing of IL-1β and cell death, in response to infection and cellular stresses. Inflammasomes have been anticipated to contribute to autoimmunity. The New Zealand Black (NZB) mouse develops anti-erythrocyte Abs and is a model of autoimmune hemolytic anemia. These mice also develop anti-nuclear Abs typical of lupus. In this article, we show that NZB macrophages have deficient inflammasome responses to a DNA virus and fungal infection. Absent in melanoma 2 (AIM2) inflammasome responses are compromised in NZB by high expression of the AIM 2 antagonist protein p202, and consequently NZB cells had low IL-1β output in response to both transfected DNA and mouse CMV infection. Surprisingly, we also found that a second inflammasome system, mediated by the NLR family, pyrin domain containing 3 (NLRP3) initiating protein, was completely lacking in NZB cells. This was due to a point mutation in an intron of the Nlrp3 gene in NZB mice, which generates a novel splice acceptor site. This leads to incorporation of a pseudoexon with a premature stop codon. The lack of full-length NLRP3 protein results in NZB being effectively null for Nlrp3, with no production of bioactive IL-1β in response to NLRP3 stimuli, including infection with Candida albicans. Thus, this autoimmune strain harbors two inflammasome deficiencies, mediated through quite distinct mechanisms. We hypothesize that the inflammasome deficiencies in NZB alter the interaction of the host with both microflora and pathogens, promoting prolonged production of cytokines that contribute to development of autoantibodies.

Mechanisms of unconventional secretion of IL-1 family cytokines

One of the most poorly understood processes in cell biology is the peculiar ability of specific leaderless proteins to be secreted via ER/Golgi-independent mechanisms ('unconventional protein secretion'). One such leaderless protein is the major immune-activating cytokine, interleukin-1β (IL-1β). Unusual amongst cytokines, IL-1β is expressed in the cytosol as an inactive precursor protein. It requires maturation by the caspase-1 protease, which itself requires activation upon immune cell sensing of infection or cell stress. Despite 25 years of intensive research into IL-1β secretory mechanisms, how it exits the cell is still not well understood. Here we will review the various mechanisms by which macrophages have been proposed to secrete IL-1 family cytokines, and the potential involvement of caspase-1 therein. Since aberrant IL-1β production drives inherited and acquired human diseases (e.g. autoinflammatory diseases, arthritic diseases, gout, Alzheimer's disease), elucidation of the IL-1β secretory pathway may offer new therapeutic opportunities for treatment across this wide range of human conditions.

Structural aspects of molecular recognition in the immune system. Part II: Pattern recognition receptors (IUPAC Technical Report)

The vertebrate immune system uses pattern recognition receptors (PRRs) to detect a large variety of molecular signatures (pathogen-associated molecular patterns, PAMPs) from a broad range of different invading pathogens. The PAMPs range in size from relatively small molecules, to others of intermediate size such as bacterial lipopolysaccharide, lipopeptides, and oligosaccharides, to macromolecules such as viral DNA, RNA, and pathogen-derived proteins such as flagellin. Underlying this functional diversity of PRRs is a surprisingly small number of structurally distinct protein folds that include leucine-rich repeats in Toll-like receptors (TLRs) and NOD-like receptors (NLRs), the DExH box helicase domain in RIG-like receptors (RLRs), and C-type lectin domains (CTLDs) in the C-type lectins. Following PAMP recognition by the PRRs, downstream signaling pathways activate the innate immune system to respond to invading pathogenic organisms. The resulting stimulatory response is also vital for a balanced adaptive immune response to the pathogen, mediated by circulating antibodies and/or cytotoxic T cells. However, an aberrant stimulation of the innate immune system can also lead to excessive inflammatory and toxic stress responses. Exciting opportunities are now arising for the design of small synthetic molecules that bind to PRRs and influence downstream signaling pathways. Such molecules can be useful tools to modulate immune responses, for example, as adjuvants to stimulate adaptive immune responses to a vaccine, or as therapeutic agents to dampen aberrant immune responses, such as inflammation. The design of agonists or antagonists of PRRs can now benefit from a surge in knowledge of the 3D structures of PRRs, many in complexes with their natural ligands. This review article describes recent progress in structural studies of PRRs (TLRs, NLRs, CTLs, and RLRs), which is required for an understanding of how they specifically recognize structurally diverse “foreign” PAMPs amongst a background of other “self” molecules, sometimes closely related in structure, that are present in the human body.

The neutrophil NLRC4 inflammasome selectively promotes IL-1β maturation without pyroptosis during acute Salmonella challenge

The macrophage NLRC4 inflammasome drives potent innate immune responses against Salmonella by eliciting caspase-1-dependent proinflammatory cytokine production (e.g., interleukin-1β [IL-1β]) and pyroptotic cell death. However, the potential contribution of other cell types to inflammasome-mediated host defense against Salmonella was unclear. Here, we demonstrate that neutrophils, typically viewed as cellular targets of IL-1β, themselves activate the NLRC4 inflammasome during acute Salmonella infection and are a major cell compartment for IL-1β production during acute peritoneal challenge in vivo. Importantly, unlike macrophages, neutrophils do not undergo pyroptosis upon NLRC4 inflammasome activation. The resistance of neutrophils to pyroptotic death is unique among inflammasome-signaling cells so far described and allows neutrophils to sustain IL-1β production at a site of infection without compromising the crucial inflammasome-independent antimicrobial effector functions that would be lost if neutrophils rapidly lysed upon caspase-1 activation. Inflammasome pathway modification in neutrophils thus maximizes host proinflammatory and antimicrobial responses during pathogen challenge.

Candida albicans triggers NLRP3-mediated pyroptosis in macrophages

Pyroptosis is an inflammasome-mediated programmed cell death pathway triggered in macrophages by a variety of stimuli, including intracellular bacterial pathogens. Activation of pyroptosis leads to the secretion of interleukin-1β (IL-1β) and pore-mediated cell lysis. Although not considered an intracellular pathogen, Candida albicans is able to kill and, thereby, escape from macrophages. Here, we show that C. albicans-infected bone marrow-derived macrophages (BMDM) and murine J774 macrophages undergo pyroptotic cell death that is suppressed by glycine and pharmacologic inhibition of caspase-1. Infection of BMDM harvested from mice lacking components of the inflammasome revealed that pyroptosis was dependent on caspase-1, ASC, and NLRP3 and independent of NLRC4. In contrast to its role during intracellular bacterial infection, pyroptosis does not restrict C. albicans replication. Nonfilamentous Candida spp. did not trigger pyroptosis, while Candida krusei, which forms pseudohyphae in macrophages, triggered much lower levels than did C. albicans. Interestingly, a Saccharomyces cerevisiae strain from the filamentous background Σ1278 also triggered low, but significant, levels of pyroptosis. We have found that deletion of the transcription factor UPC2 decreases pyroptosis but has little effect on filamentation in the macrophage. In addition, a gain-of-function mutant of UPC2 induces higher levels of pyroptosis than does a matched control strain. Taken together, these data are most consistent with a model in which filamentation is necessary but not sufficient to trigger NLRP3 inflammasome-mediated pyroptosis. This is the first example of a fungal pathogen triggering pyroptosis and indicates that C. albicans-mediated macrophage damage is not solely due to hypha-induced physical disruption of cellular integrity.