Listeriolysin O-dependent bacterial entry into the cytoplasm is required for calpain activation and interleukin-1 alpha secretion in macrophages infected with Listeria monocytogenes

The group A Streptococcus interleukin-8 protease SpyCEP promotes bacterial intracellular survival by evasion of autophagy

Autophagy serves an innate immune function in defending the host against invading bacteria, including group A Streptococcus (GAS). Autophagy is regulated by numerous host proteins, including the endogenous negative regulator calpain, a cytosolic protease. Globally disseminated serotype M1T1 GAS strains associated with high invasive disease potential express numerous virulence factors and resist autophagic clearance. Upon in vitro infection of human epithelial cell lines with representative wild-type GAS M1T1 strain 5448 (M1.5448), we observed increased calpain activation linked to a specific GAS virulence factor, the IL-8 protease SpyCEP. Calpain activation inhibited autophagy and decreased capture of cytosolic GAS in autophagosomes. In contrast, the serotype M6 GAS strain JRS4 (M6.JRS4), which is highly susceptible to host autophagy-mediated killing, expresses low levels of SpyCEP and does not activate calpain. Overexpression of SpyCEP in M6.JRS4 stimulated calpain activation, inhibited autophagy and significantly decreased bacterial capture in autophagosomes. These paired loss- and gain-of-function studies reveal a novel role for the bacterial protease SpyCEP in enabling GAS M1 evasion of autophagy and host innate immune clearance.

Pneumolysin boosts the neuroinflammatory response to Streptococcus pneumoniae through enhanced endocytosis

In pneumococcal meningitis, bacterial growth in the cerebrospinal fluid results in lysis, the release of toxic factors, and subsequent neuroinflammation. Exposure of primary murine glia to Streptococcus pneumoniae lysates leads to strong proinflammatory cytokine and chemokine production, blocked by inhibition of the intracellular innate receptor Nod1. Lysates enhance dynamin-dependent endocytosis, and dynamin inhibition reduces neuroinflammation, blocking ligand internalization. Here we identify the cholesterol-dependent cytolysin pneumolysin as a pro-endocytotic factor in lysates, its elimination reduces their proinflammatory effect. Only pore-competent pneumolysin enhances endocytosis in a dynamin-, phosphatidylinositol-3-kinase- and potassium-dependent manner. Endocytic enhancement is limited to toxin-exposed parts of the membrane, the effect is rapid and pneumolysin permanently alters membrane dynamics. In a murine model of pneumococcal meningitis, mice treated with chlorpromazine, a neuroleptic with a complementary endocytosis inhibitory effect show reduced neuroinflammation. Thus, the dynamin-dependent endocytosis emerges as a factor in pneumococcal neuroinflammation, and its enhancement by a cytolysin represents a proinflammatory control mechanism.

Microfluidics Technology for the Design and Formulation of Nanomedicines

In conventional drug administration, drug molecules cross multiple biological barriers, distribute randomly in the tissues, and can release insufficient concentrations at the desired pathological site. Controlling the delivery of the molecules can increase the concentration of the drug in the desired location, leading to improved efficacy, and reducing the unwanted effects of the molecules under investigation. Nanoparticles (NPs), have shown a distinctive potential in targeting drugs due to their unique properties, such as large surface area and quantum properties. A variety of NPs have been used over the years for the encapsulation of different drugs and biologics, acting as drug carriers, including lipid-based and polymeric NPs. Applying NP platforms in medicines significantly improves the disease diagnosis and therapy. Several conventional methods have been used for the manufacturing of drug loaded NPs, with conventional manufacturing methods having several limitations, leading to multiple drawbacks, including NPs with large particle size and broad size distribution (high polydispersity index), besides the unreproducible formulation and high batch-to-batch variability. Therefore, new methods such as microfluidics (MFs) need to be investigated more thoroughly. MFs, is a novel manufacturing method that uses microchannels to produce a size-controlled and monodispersed NP formulation. In this review, different formulation methods of polymeric and lipid-based NPs will be discussed, emphasizing the different manufacturing methods and their advantages and limitations and how microfluidics has the capacity to overcome these limitations and improve the role of NPs as an effective drug delivery system.

Interaction of Macrophages and Cholesterol-Dependent Cytolysins: The Impact on Immune Response and Cellular Survival

Cholesterol-dependent cytolysins (CDCs) are key virulence factors involved in many lethal bacterial infections, including pneumonia, necrotizing soft tissue infections, bacterial meningitis, and miscarriage. Host responses to these diseases involve myeloid cells, especially macrophages. Macrophages use several systems to detect and respond to cholesterol-dependent cytolysins, including membrane repair, mitogen-activated protein (MAP) kinase signaling, phagocytosis, cytokine production, and activation of the adaptive immune system. However, CDCs also promote immune evasion by silencing and/or destroying myeloid cells. While there are many common themes between the various CDCs, each CDC also possesses specific features to optimally benefit the pathogen producing it. This review highlights host responses to CDC pathogenesis with a focus on macrophages. Due to their robust plasticity, macrophages play key roles in the outcome of bacterial infections. Understanding the unique features and differences within the common theme of CDCs bolsters new tools for research and therapy.

Intracellular Pathogens: Host Immunity and Microbial Persistence Strategies

Infectious diseases caused by pathogens including viruses, bacteria, fungi, and parasites are ranked as the second leading cause of death worldwide by the World Health Organization. Despite tremendous improvements in global public health since 1950, a number of challenges remain to either prevent or eradicate infectious diseases. Many pathogens can cause acute infections that are effectively cleared by the host immunity, but a subcategory of these pathogens called "intracellular pathogens" can establish persistent and sometimes lifelong infections. Several of these intracellular pathogens manage to evade the host immune monitoring and cause disease by replicating inside the host cells. These pathogens have evolved diverse immune escape strategies and overcome immune responses by residing and multiplying inside host immune cells, primarily macrophages. While these intracellular pathogens that cause persistent infections are phylogenetically diverse and engage in diverse immune evasion and persistence strategies, they share common pathogen type-specific mechanisms during host-pathogen interaction inside host cells. Likewise, the host immune system is also equipped with a diverse range of effector functions to fight against the establishment of pathogen persistence and subsequent host damage. This article provides an overview of the immune effector functions used by the host to counter pathogens and various persistence strategies used by intracellular pathogens to counter host immunity, which enables their extended period of colonization in the host. The improved understanding of persistent intracellular pathogen-derived infections will contribute to develop improved disease diagnostics, therapeutics, and prophylactics.

Rapid Remodeling of the Host Epithelial Cell Proteome by the Listeriolysin O (LLO) Pore-forming Toxin

Bacterial pathogens use various strategies to interfere with host cell functions. Among these strategies, bacteria modulate host gene transcription, thereby modifying the set of proteins synthetized by the infected cell. Bacteria can also target pre-existing host proteins and modulate their post-translational modifications or trigger their degradation. Analysis of protein levels variations in host cells during infection allows to integrate both transcriptional and post-transcriptional regulations induced by pathogens. Here, we focused on host proteome alterations induced by the toxin Listeriolysin O (LLO), secreted by the bacterial pathogen Listeria monocytogenes. We showed that a short-term treatment with LLO remodels the host cell proteome by specifically decreasing the abundance of 149 proteins. The same decrease in host protein levels was observed in different epithelial cell lines but not in macrophages. We show in particular that this proteome remodeling affects several ubiquitin and ubiquitin-like ligases and that LLO leads to major changes in the host ubiquitylome. Strikingly, this toxin-induced proteome remodeling involves only post-transcriptional regulations, as no modification in the transcription levels of the corresponding genes was observed. In addition, we could show that Perfringolysin O, another bacterial pore-forming toxin similar to LLO, also induces host proteome changes. Taken together, our data reveal that different bacterial pore-forming toxins induce important host proteome remodeling, that may impair epithelial cell functions.

Listeriolysin O: from bazooka to Swiss army knife

Listeria monocytogenes (Lm) is a Gram-positive facultative intracellular pathogen. Infections in humans can lead to listeriosis, a systemic disease with a high mortality rate. One important mechanism of Lm dissemination involves cell-to-cell spread after bacteria have entered the cytosol of host cells. Listeriolysin O (LLO; encoded by the hly gene) is a virulence factor present in Lm that plays a central role in the cell-to-cell spread process. LLO is a member of the cholesterol-dependent cytolysin (CDC) family of toxins that were initially thought to promote disease largely by inducing cell death and tissue destruction-essentially acting like a 'bazooka'. This view was supported by structural studies showing CDCs can form large pores in membranes. However, it is now appreciated that LLO has many subtle activities during Lm infection of host cells, and many of these likely do not involve large pores, but rather small membrane perforations. It is also appreciated that membrane repair pathways of host cells play a major role in limiting membrane damage by LLO and other toxins. LLO is now thought to represent a 'Swiss army knife', a versatile tool that allows Lm to induce many membrane alterations and cellular responses that promote bacterial dissemination during infection.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Pneumolysin-Dependent Calpain Activation and Interleukin-1α Secretion in Macrophages Infected with Streptococcus pneumoniae

Pneumolysin (PLY), a major virulence factor of Streptococcus pneumoniae, is a pore-forming cytolysin that modulates host innate responses contributing to host defense against and pathogenesis of pneumococcal infections. Interleukin-1α (IL-1α) has been shown to be involved in tissue damage in a pneumococcal pneumonia model; however, the mechanism by which this cytokine is produced during S. pneumoniae infection remains unclear. In this study, we examined the role of PLY in IL-1α production. Although the strains induced similar levels of pro-IL-1α expression, wild-type S. pneumoniae D39, but not a deletion mutant of the ply gene (Δply), induced the secretion of mature IL-1α from host macrophages, suggesting that PLY is critical for the maturation and secretion of IL-1α during S. pneumoniae infection. Further experiments with calcium chelators and calpain inhibitors indicated that extracellular calcium ions and calpains (calcium-dependent proteases) facilitated the maturation and secretion of IL-1α from D39-infected macrophages. Moreover, we found that PLY plays a critical role in calcium influx and calpain activation, as elevated intracellular calcium levels and the degradation of the calpain substrate α-fodrin were detected in macrophages infected with D39 but not the Δply strain. These results suggested that PLY induces the influx of calcium in S. pneumoniae-infected macrophages, followed by calpain activation and subsequent IL-1α maturation and secretion.

NLRP3 inflammasome activation contributes to Listeria monocytogenes-induced animal pregnancy failure

Background

Listeria monocytogenes (LM), a foodborne pathogen, can cause pregnancy failure in animals, especially in ruminants. Recent studies have shown that LM activates inflammasomes to induce IL-1β release in macrophages, however, whether the inflammasome activation regulates LM-induced pregnancy failure remains largely unknown. Here we used mouse model to investigate the molecular mechanism by which LM-induced inflammsome activation contributes to LM-associated pregnancy failure

Results

We showed that wild-type, but not Listeriolysin O-deficient (Δhly) LM, significantly reduced mouse embryo survival, accompanied by the increase of IL-1β release and caspase-1 activation. IL-1β neutralization significantly reduced the LM-induced embryo losses, suggesting that LM-induced pregnancy failure was associated with LLO-induced inflammasome activation. To dissect the inflammasome sensor and components responsible for LM-induced caspase-1 activation and IL-1β production, we used wild-type and NLRP3^−/−, AIM2^−/−, NLRC4^−/−, ASC^−/−, caspase-1^−/− and cathepsin B^−/− mouse macrophages to test the roles of these molecules in LM-induce IL-1β production. We found that NLRP3 inflammasome was the main pathway in LM-induced caspase-1 activation and IL-1β production. To explore the mechanism of LM-induced pregnancy failure, we investigated the effects of LM-infected macrophages on SM9-1 mouse trophoblasts. We found that the conditioned medium from LM-infected-macrophage or the recombinant IL-1β significantly up-regulated TNFα, IL-6 and IL-8 productions in trophoblasts, suggesting that the LM-induced macrophage inflammasome activation increased trophoblast pro-inflammatory cytokine production, which was adverse to the animal pregnancy maintenance.

Conclusions

Our data demonstrated that the LLO-induced NLRP3 inflammasome activation plays a key role in LM-induced pregnancy failure, and inflammasome-mediated macrophage dysregulation on trophoblasts might be involved in the pregnancy failure.

The diversity of receptor recognition in cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins (CDCs) are bacterial pore-forming toxins secreted mainly by pathogenic Gram-positive bacteria. CDCs generally recognize and bind to membrane cholesterol to create pores and lyse target cells. However, in contrast to typical CDCs such as streptolysin O, several atypical CDCs have been reported. The first of these was intermedilysin, which is secreted by Streptococcus intermedius and has human cell-specificity, human CD59 (huCD59) being its receptor. In the study reported here, the diversity of receptor recognition among CDCs was investigated and multi-receptor recognition characteristics were identified within this toxin family. Streptococcus mitis-derived human platelet aggregation factor (Sm-hPAF) secreted by S. mitis strain Nm-65 isolated from a patient with Kawasaki disease was previously shown to hemolyze erythrocytes in a species-dependent manner, its maximum activity being in human cells. In the present study, it was found that Sm-hPAF recognizes both membrane cholesterol and huCD59 as receptors for triggering pore-formation. Moreover, vaginolysin (VLY) of Gardnerella vaginalis showed similar characteristics to Sm-hPAF regarding receptor recognition. On the basis of the results presented here, the mode of receptor recognition of CDCs can be categorized into the following three groups: (i) Group I, comprising typical CDCs with high affinity to cholesterol and no or very little affinity to huCD59; (ii) Group II, including atypical CDCs such as ILY, with no or very little affinity to cholesterol and high affinity to huCD59; and (iii) Group III, which contains atypical CDCs such as Sm-hPAF and VLY with affinity to both cholesterol and huCD59.

Redox control of the senescence regulator interleukin-1α and the secretory phenotype

Senescent cells accumulate in aged tissue and are causally linked to age-associated tissue degeneration. These non-dividing, metabolically active cells are highly secretory and alter tissue homeostasis, creating an environment conducive to metastatic disease progression. IL-1α is a key senescence-associated (SA) proinflammatory cytokine that acts as a critical upstream regulator of the SA secretory phenotype (SASP). We established that SA shifts in steady-state H2O2 and intracellular Ca(2+) levels caused an increase in IL-1α expression and processing. The increase in intracellular Ca(2+) promoted calpain activation and increased the proteolytic cleavage of IL-1α. Antioxidants and low oxygen tension prevented SA IL-1α expression and restricted expression of SASP components IL-6 and IL-8. Ca(2+) chelation or calpain inhibition prevented SA processing of IL-1α and its ability to induce downstream cytokine expression. Conditioned medium from senescent cells treated with antioxidants or Ca(2+) chelators or cultured in low oxygen markedly reduced the invasive capacity of proximal metastatic cancer cells. In this paracrine fashion, senescent cells promoted invasion by inducing an epithelial-mesenchymal transition, actin reorganization, and cellular polarization of neighboring cancer cells. Collectively, these findings demonstrate how SA alterations in the redox state and Ca(2+) homeostasis modulate the inflammatory phenotype through the regulation of the SASP initiator IL-1α, creating a microenvironment permissive to tumor invasion.

The RD1 locus in the Mycobacterium tuberculosis genome contributes to the maturation and secretion of IL-1α from infected macrophages through the elevation of cytoplasmic calcium levels and calpain activation

Region of difference 1 (RD1) is a genomic locus in the Mycobacterium tuberculosis genome that has been shown to participate in the virulence of the bacterium, induction of cell death, and cytokine secretion in infected macrophages. In this study, we investigated the role of RD1 in interleukin-1α (IL-1α) secretion. M. tuberculosis H37Rv strain, but not a mutant strain deficient for RD1 (∆RD1), significantly induced IL-1α secretion from infected macrophages. Although IL-1α secretion was only observed in H37Rv-infected macrophages, there was no difference in the level of IL-1α transcription and pro-IL1α synthesis after infection with H37Rv and ∆RD1. Interestingly, ∆RD1 infection did not increase intracellular Ca(2+) levels, and Ca(2+) chelators markedly inhibited IL-1α secretion in response to H37Rv infection. Moreover, the inability of ∆RD1 to induce IL-1α secretion was restored by treatment with the calcium ionophore A23187. A significant increase in calpain activity was detected in macrophages infected with H37Rv, but not with ∆RD1, and calpain inhibitors abrogated IL-1α secretion. Taken together, these results suggest that in M. tuberculosis-infected macrophages, RD1 contributed to maturation and secretion of IL-1α by enhancing the influx of Ca(2+) followed by calpain activation.

IL-1α signaling initiates the inflammatory response to virulent Legionella pneumophila in vivo

Legionella pneumophila is an intracellular bacterial pathogen that is the cause of a severe pneumonia in humans called Legionnaires' disease. A key feature of L. pneumophila pathogenesis is the rapid influx of neutrophils into the lungs, which occurs in response to signaling via the IL-1R. Two distinct cytokines, IL-1α and IL-1β, can stimulate the type I IL-1R. IL-1β is produced upon activation of cytosolic sensors called inflammasomes that detect L. pneumophila in vitro and in vivo. Surprisingly, we find no essential role for IL-1β in neutrophil recruitment to the lungs in response to L. pneumophila. Instead, we show that IL-1α is a critical initiator of neutrophil recruitment to the lungs of L. pneumophila-infected mice. We find that neutrophil recruitment in response to virulent L. pneumophila requires the production of IL-1α specifically by hematopoietic cells. In contrast to IL-1β, the innate signaling pathways that lead to the production of IL-1α in response to L. pneumophila remain poorly defined. In particular, although we confirm a role for inflammasomes for initiation of IL-1β signaling in vivo, we find no essential role for inflammasomes in production of IL-1α. Instead, we propose that a novel host pathway, perhaps involving inhibition of host protein synthesis, is responsible for IL-1α production in response to virulent L. pneumophila. Our results establish IL-1α as a critical initiator of the inflammatory response to L. pneumophila in vivo and point to an important role for IL-1α in providing an alternative to inflammasome-mediated immune responses in vivo.

Redox-control of the alarmin, Interleukin-1α

The pro-inflammatory cytokine Interleukin-1α (IL-1α) has recently emerged as a susceptibility marker for a wide array of inflammatory diseases associated with oxidative stress including Alzheimer's, arthritis, atherosclerosis, diabetes and cancer. In the present study, we establish that expression and nuclear localization of IL-1α are redox-dependent. Shifts in steady-state H₂O₂ concentrations (SS-[H₂O₂]) resulting from enforced expression of manganese superoxide dismutase (SOD2) drive IL-1α mRNA and protein expression. The redox-dependent expression of IL-1α is accompanied by its increased nuclear localization. Both IL-1α expression and its nuclear residency are abrogated by catalase co-expression. Sub-lethal doses of H₂O₂ also cause IL-1α nuclear localization. Mutagenesis revealed IL-1α nuclear localization does not involve oxidation of cysteines within its N terminal domain. Inhibition of the processing enzyme calpain prevents IL-1α nuclear localization even in the presence of H₂O₂. H₂O₂ treatment caused extracellular Ca²⁺ influx suggesting oxidants may influence calpain activity indirectly through extracellular Ca²⁺ mobilization. Functionally, as a result of its nuclear activity, IL-1α overexpression promotes NF-kB activity, but also interacts with the histone acetyl transferase (HAT) p300. Together, these findings demonstrate a mechanism by which oxidants impact inflammation through IL-1α and suggest that antioxidant-based therapies may prove useful in limiting inflammatory disease progression.

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Sod2-dependent increases in steady-state H₂O₂ promote IL-1α expression.

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H₂O₂ causes nuclear localization of IL-1α and extracellular Ca²⁺ influx.

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Inhibition of the Ca²⁺ regulated calpain prevents H₂O₂ dependent IL-1α nuclear localization.

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Nuclear IL-1α interacts with p300 and promotes NF-κB activity.

Encephalomyocarditis virus viroporin 2B activates NLRP3 inflammasome

Nod-like receptors (NLRs) comprise a large family of intracellular pattern- recognition receptors. Members of the NLR family assemble into large multiprotein complexes, termed the inflammasomes. The NLR family, pyrin domain-containing 3 (NLRP3) is triggered by a diverse set of molecules and signals, and forms the NLRP3 inflammasome. Recent studies have indicated that both DNA and RNA viruses stimulate the NLRP3 inflammasome, leading to the secretion of interleukin 1 beta (IL-1β) and IL-18 following the activation of caspase-1. We previously demonstrated that the proton-selective ion channel M2 protein of influenza virus activates the NLRP3 inflammasome. However, the precise mechanism by which NLRP3 recognizes viral infections remains to be defined. Here, we demonstrate that encephalomyocarditis virus (EMCV), a positive strand RNA virus of the family Picornaviridae, activates the NLRP3 inflammasome in mouse dendritic cells and macrophages. Although transfection with RNA from EMCV virions or EMCV-infected cells induced robust expression of type I interferons in macrophages, it failed to stimulate secretion of IL-1β. Instead, the EMCV viroporin 2B was sufficient to cause inflammasome activation in lipopolysaccharide-primed macrophages. While cells untransfected or transfected with the gene encoding the EMCV non-structural protein 2A or 2C expressed NLRP3 uniformly throughout the cytoplasm, NLRP3 was redistributed to the perinuclear space in cells transfected with the gene encoding the EMCV 2B or influenza virus M2 protein. 2B proteins of other picornaviruses, poliovirus and enterovirus 71, also caused the NLRP3 redistribution. Elevation of the intracellular Ca(2+) level, but not mitochondrial reactive oxygen species and lysosomal cathepsin B, was important in EMCV-induced NLRP3 inflammasome activation. Chelation of extracellular Ca(2+) did not reduce virus-induced IL-1β secretion. These results indicate that EMCV activates the NLRP3 inflammasome by stimulating Ca(2+) flux from intracellular storages to the cytosol, and highlight the importance of viroporins, transmembrane pore-forming viral proteins, in virus-induced NLRP3 inflammasome activation.

Inhibition of calpain blocks the phagosomal escape of Listeria monocytogenes

Listeria monocytogenes is a gram-positive facultative intracellular bacterium responsible for the food borne infection listeriosis, affecting principally the immunocompromised, the old, neonates and pregnant women. Following invasion L. monocytogenes escapes the phagosome and replicates in the cytoplasm. Phagosome escape is central to L. monocytogenes virulence and is required for initiating innate host-defence responses such as the secretion of the cytokine interleukin-1. Phagosome escape of L. monocytogenes is reported to depend upon host proteins such as γ-interferon-inducible lysosomal thiol reductase and the cystic fibrosis transmembrane conductance regulator. The host cytosolic cysteine protease calpain is required in the life cycle of numerous pathogens, and previous research reports an activation of calpain by L. monocytogenes infection. Thus we sought to determine whether host calpain was required for the virulence of L. monocytogenes. Treatment of macrophages with calpain inhibitors blocked escape of L. monocytogenes from the phagosome and consequently its proliferation within the cytosol. This was independent of any direct effect on the production of bacterial virulence factors or of a bactericidal effect. Furthermore, the secretion of interleukin-1β, a host cytokine whose secretion induced by L. monocytogenes depends upon phagosome escape, was also blocked by calpain inhibition. These data indicate that L. monocytogenes co-opts host calpain to facilitate its escape from the phagosome, and more generally, that calpain may represent a cellular Achilles heel exploited by pathogens.