A group A Streptococcus ADP-ribosyltransferase toxin stimulates a protective interleukin 1β-dependent macrophage immune response

Interplay between group A Streptococcus and host innate immune responses

SUMMARYGroup A Streptococcus (GAS), also known as Streptococcus pyogenes, is a clinically well-adapted human pathogen that harbors rich virulence determinants contributing to a broad spectrum of diseases. GAS is capable of invading epithelial, endothelial, and professional phagocytic cells while evading host innate immune responses, including phagocytosis, selective autophagy, light chain 3-associated phagocytosis, and inflammation. However, without a more complete understanding of the different ways invasive GAS infections develop, it is difficult to appreciate how GAS survives and multiplies in host cells that have interactive immune networks. This review article attempts to provide an overview of the behaviors and mechanisms that allow pathogenic GAS to invade cells, along with the strategies that host cells practice to constrain GAS infection. We highlight the counteractions taken by GAS to apply virulence factors such as streptolysin O, nicotinamide-adenine dinucleotidase, and streptococcal pyrogenic exotoxin B as a hindrance to host innate immune responses.

NLRP3 selectively drives IL-1β secretion by Pseudomonas aeruginosa infected neutrophils and regulates corneal disease severity

Macrophages infected with Gram-negative bacteria expressing Type III secretion system (T3SS) activate the NLRC4 inflammasome, resulting in Gasdermin D (GSDMD)-dependent, but GSDME independent IL-1β secretion and pyroptosis. Here we examine inflammasome signaling in neutrophils infected with Pseudomonas aeruginosa strain PAO1 that expresses the T3SS effectors ExoS and ExoT. IL-1β secretion by neutrophils requires the T3SS needle and translocon proteins and GSDMD. In macrophages, PAO1 and mutants lacking ExoS and ExoT (ΔexoST) require NLRC4 for IL-1β secretion. While IL-1β release from ΔexoST infected neutrophils is also NLRC4-dependent, infection with PAO1 is instead NLRP3-dependent and driven by the ADP ribosyl transferase activity of ExoS. Genetic and pharmacologic approaches using MCC950 reveal that NLRP3 is also essential for bacterial killing and disease severity in a murine model of P. aeruginosa corneal infection (keratitis). Overall, these findings reveal a function for ExoS ADPRT in regulating inflammasome subtype usage in neutrophils versus macrophages and an unexpected role for NLRP3 in P. aeruginosa keratitis.

Detection of Inflammasome Activation in Murine Bone Marrow-Derived Macrophages Infected with Group A Streptococcus

Inflammasomes are large multiprotein complexes that assemble mainly in innate immune cells after detection of microbial or sterile insults. Activation of inflammasomes is a key proinflammatory event during infection, and many pathogens have evolved specific evasion mechanisms to evade or inhibit inflammasome activation. One such pathogen is the common bacterium group A Streptococcus (GAS), which causes a wide range of diseases of varying severity. GAS secretes a multitude of virulence factors whereof the pore-forming protein streptolysin O (SLO) is the main inflammasome activation determinant. Here we provide a protocol for reliable evaluation of inflammasome activation in murine bone marrow-derived macrophages (BMDM) infected with GAS, including instructions for generating BMDMs and growing the bacterium. This protocol can easily be modified to other bacterial pathogens, or human macrophages.

Influenza A Virus Exacerbates Group A Streptococcus Infection and Thwarts Anti-Bacterial Inflammatory Responses in Murine Macrophages

Seasonal influenza epidemics pose a considerable hazard for global health. In the past decades, accumulating evidence revealed that influenza A virus (IAV) renders the host vulnerable to bacterial superinfections which in turn are a major cause for morbidity and mortality. However, whether the impact of influenza on anti-bacterial innate immunity is restricted to the vicinity of the lung or systemically extends to remote sites is underexplored. We therefore sought to investigate intranasal infection of adult C57BL/6J mice with IAV H1N1 in combination with bacteremia elicited by intravenous application of Group A Streptococcus (GAS). Co-infection in vivo was supplemented in vitro by challenging murine bone marrow derived macrophages and exploring gene expression and cytokine secretion. Our results show that viral infection of mice caused mild disease and induced the depletion of CCL2 in the periphery. Influenza preceding GAS infection promoted the occurrence of paw edemas and was accompanied by exacerbated disease scores. In vitro co-infection of macrophages led to significantly elevated expression of TLR2 and CD80 compared to bacterial mono-infection, whereas CD163 and CD206 were downregulated. The GAS-inducible upregulation of inflammatory genes, such as Nos2, as well as the secretion of TNFα and IL-1β were notably reduced or even abrogated following co-infection. Our results indicate that IAV primes an innate immune layout that is inadequately equipped for bacterial clearance.

Gasdermin D protects against Streptococcus equi subsp. zooepidemicus infection through macrophage pyroptosis

Streptococcus equi subsp. zooepidemicus (S. zooepidemicus, SEZ) is an essential zoonotic bacterial pathogen that can cause various inflammation, such as meningitis, endocarditis, and pneumonia. Gasdermin D (GSDMD) is involved in cytokine release and cell death, indicating an important role in controlling the microbial infection. This study investigated the protective role of GSDMD in mice infected with SEZ and examined the role of GSDMD in peritoneal macrophages in the infection. GSDMD-deficient mice were more susceptible to intraperitoneal infection with SEZ, and the white pulp structure of the spleen was seriously damaged in GSDMD-deficient mice. Although the increased proportion of macrophages did not depend on GSDMD in both spleen and peritoneal lavage fluid (PLF), deficiency of GSDMD caused the minor release of the pro-inflammatory cytokines interleukin-1β (IL-1β) and interleukin-18 (IL-18) during the infection in vivo. In vitro, SEZ infection induced more release of IL-1β, IL-18, and lactate dehydrogenase (LDH) in wild-type macrophages than in GSDMD-deficient macrophages. Finally, we demonstrated that pore formation and pyroptosis of macrophages depended on GSDMD. Our findings highlight the host defense mechanisms of GSDMD against SEZ infection, providing a potential therapeutic target in SEZ infection.

Streptolysins are the primary inflammasome activators in macrophages during Streptococcus pyogenes infection

Group A Streptococcus (GAS) is a Gram-positive bacterial pathogen that causes an array of infectious diseases in humans. Accumulating clinical evidence suggests that proinflammatory interleukin (IL)-1β signaling plays an important role in GAS disease progression. The host regulates the production and secretion of IL-1β via the cytosolic inflammasome pathway. Activation of the NLR family pyrin domain-containing 3 (NLRP3) inflammasome complex requires two signals: a priming signal that stimulates increased transcription of genes encoding the components of the inflammasome pathway, and an activating signal that induces assembly of the inflammasome complex. Here we show that GAS-derived lipoteichoic acid can provide a priming signal for NLRP3 inflammasome activation. As only few GAS-derived proteins have been associated with inflammasome-dependent IL-1β signaling, we investigated novel candidates that might play a role in activating the inflammasome pathway by infecting mouse bone marrow-derived macrophages and human THP-1 macrophage-like cells with a panel of isogenic GAS mutant strains. We found that the cytolysins streptolysin O (SLO) and streptolysin S are the main drivers of IL-1β release in proliferating logarithmic phase GAS. Using a mutant form of recombinant SLO, we confirmed that bacterial pore formation on host cell membranes is a key mechanism required for inflammasome activation. Our results suggest that streptolysins are major determinants of GAS-induced inflammation and present an attractive target for therapeutic intervention.

Inflammasome activation and IL-1β signalling in group A Streptococcus disease

Group A Streptococcus (GAS) is a Gram-positive bacterial pathogen that causes significant morbidity and mortality worldwide. Recent clinical evidence suggests that the inflammatory marker interleukin-1β (IL-1β) plays an important role in GAS disease progression, and presents a potential target for therapeutic intervention. Interaction with GAS activates the host inflammasome pathway to stimulate production and secretion of IL-1β, but GAS can also stimulate IL-1β production in an inflammasome-independent manner. This review highlights progress that has been made in understanding the importance of host cell inflammasomes and IL-1 signalling in GAS disease, and explores challenges and unsolved problems in this host-pathogen interaction. TAKE AWAY: Inflammasome signalling during GAS infection is an emerging field of research. GAS modulates the NLRP3 inflammasome pathway through multiple mechanisms. SpeB contributes to IL-1β production independently of the inflammasome pathway. IL-1β signalling can be host-protective, but also drive severe GAS disease.

The Pseudomonas aeruginosa protease LasB directly activates IL-1β

BACKGROUND

Pulmonary damage by Pseudomonas aeruginosa during cystic fibrosis lung infection and ventilator-associated pneumonia is mediated both by pathogen virulence factors and host inflammation. Impaired immune function due to tissue damage and inflammation, coupled with pathogen multidrug resistance, complicates the management of these deep-seated infections. Pathological inflammation during infection is driven by interleukin-1β (IL-1β), but the molecular processes involved are not fully understood.

METHODS

We examined IL-1β activation in a pulmonary model infection of Pseudomonas aeruginosa and in vitro using genetics, specific inhibitors, recombinant proteins, and targeted reporters of protease activity and IL-1β bioactivity.

FINDINGS

Caspase-family inflammasome proteases canonically regulate maturation of this proinflammatory cytokine, but we report that plasticity in IL-1β proteolytic activation allows for its direct maturation by the pseudomonal protease LasB. LasB promotes IL-1β activation, neutrophilic inflammation, and destruction of lung architecture characteristic of severe P. aeruginosa pulmonary infection.

INTERPRETATION

Preservation of lung function and effective immune clearance may be enhanced by selectively controlling inflammation. Discovery of this IL-1β regulatory mechanism provides a distinct target for anti-inflammatory therapeutics, such as matrix metalloprotease inhibitors that inhibit LasB and limit inflammation and pathology during P. aeruginosa pulmonary infections.

FUNDING

Full details are provided in the Acknowledgements section.

Group A Streptococcal S Protein Utilizes Red Blood Cells as Immune Camouflage and Is a Critical Determinant for Immune Evasion

Group A Streptococcus (GAS) is a human-specific pathogen that evades the host immune response through the elaboration of multiple virulence factors. Although many of these factors have been studied, numerous proteins encoded by the GAS genome are of unknown function. Herein, we characterize a biomimetic red blood cell (RBC)-captured protein of unknown function—annotated subsequently as S protein—in GAS pathophysiology. S protein maintains the hydrophobic properties of GAS, and its absence reduces survival in human blood. S protein facilitates GAS coating with lysed RBCs to promote molecular mimicry, which increases virulence in vitro and in vivo. Proteomic profiling reveals that the removal of S protein from GAS alters cellular and extracellular protein landscapes and is accompanied by a decrease in the abundance of several key GAS virulence determinants. In vivo, the absence of S protein results in a striking attenuation of virulence and promotes a robust immune response and immunological memory.

Wierzbicki et al. show that S protein is a major group A Streptococcus (GAS) virulence factor that facilitates bacterial coating with lysed red blood cells to promote molecular mimicry, which increases virulence in vitro and in vivo. Removal of S protein reduces the abundance of multiple virulence factors and attenuates virulence.

T6SS and ExoA of flesh-eating Aeromonas hydrophila in peritonitis and necrotizing fasciitis during mono- and polymicrobial infections

An earlier report described a human case of necrotizing fasciitis (NF) caused by mixed infection with 4 Aeromonas hydrophila strains (NF1-NF4). While the NF2, NF3, and NF4 strains were clonal and possessed exotoxin A (ExoA), the NF1 strain was determined to be phylogenetically distinct, harboring a unique type 6 secretion system (T6SS) effector (TseC). During NF1 and NF2 mixed infection, only NF1 disseminated, while NF2 was rapidly killed by a contact-dependent mechanism and macrophage phagocytosis, as was demonstrated by using in vitro models. To confirm these findings, we developed 2 NF1 mutants (NF1ΔtseC and NF1ΔvasK); vasK encodes an essential T6SS structural component. NF1 VasK and TseC were proven to be involved in contact-dependent killing of NF2 in vitro, as well as in its elimination at the intramuscular injection site in vivo during mixed infection, with overall reduced mouse mortality. ExoA was shown to have an important role in NF by both NF1-exoA (with cis exoA) and NF2 during monomicrobial infection. However, the contribution of ExoA was more important for NF2 than NF1 in the murine peritonitis model. The NF2∆exoA mutant did not significantly alter animal mortality or NF1 dissemination during mixed infection in the NF model, suggesting that the ExoA activity was significant at the injection site. Immunization of mice to ExoA protected animals from NF2 monomicrobial challenge, but not from polymicrobial infection because of NF2 clearance. This study clarified the roles of T6SS and ExoA in pathogenesis caused by A. hydrophila NF strains in both mouse peritonitis and NF models in monomicrobial and polymicrobial infections.

The Role of Streptococcal and Staphylococcal Exotoxins and Proteases in Human Necrotizing Soft Tissue Infections

Necrotizing soft tissue infections (NSTIs) are critical clinical conditions characterized by extensive necrosis of any layer of the soft tissue and systemic toxicity. Group A streptococci (GAS) and Staphylococcus aureus are two major pathogens associated with monomicrobial NSTIs. In the tissue environment, both Gram-positive bacteria secrete a variety of molecules, including pore-forming exotoxins, superantigens, and proteases with cytolytic and immunomodulatory functions. The present review summarizes the current knowledge about streptococcal and staphylococcal toxins in NSTIs with a special focus on their contribution to disease progression, tissue pathology, and immune evasion strategies.

Targeting apoptosis pathways in infections

The programmed cell death pathway of apoptosis is essential for mammalian development and immunity as it eliminates unwanted and dangerous cells. As part of the cellular immune response, apoptosis removes the replicative niche of intracellular pathogens and enables the resolution of infections. To subvert apoptosis, pathogens have evolved a diverse range of mechanisms. In some circumstances, however, pathogens express effector molecules that induce apoptotic cell death. In this review, we focus on selected host-pathogen interactions that affect apoptotic pathways. We discuss how pathogens control the fate of host cells and how this determines the outcome of infections. Finally, small molecule inhibitors that activate apoptosis in cancer cells can also induce apoptotic cell death of infected cells. This suggests that targeting host death factors to kill infected cells is a potential therapeutic option to treat infectious diseases.

Group A Streptococcus-Mediated Host Cell Signaling

In the past decade, the field of the cellular microbiology of group A Streptococcus (S. pyogenes) infection has made tremendous advances and touched upon several important aspects of pathogenesis, including receptor biology, invasive and evasive phenomena, inflammasome activation, strain-specific autophagic bacterial killing, and virulence factor-mediated programmed cell death. The noteworthy aspect of S. pyogenes-mediated cell signaling is the recognition of the role of M protein in a variety of signaling events, starting with the targeting of specific receptors on the cell surface and on through the induction and evasion of NETosis, inflammasome, and autophagy/xenophagy to pyroptosis and apoptosis. Variations in reports on S. pyogenes-mediated signaling events highlight the complex mechanism of pathogenesis and underscore the importance of the host cell and S. pyogenes strain specificity, as well as in vitro/in vivo experimental parameters. The severity of S. pyogenes infection is, therefore, dependent on the virulence gene expression repertoire in the host environment and on host-specific dynamic signaling events in response to infection. Commonly known as an extracellular pathogen, S. pyogenes finds host macrophages as safe havens wherein it survives and even multiplies. The fact that endothelial cells are inherently deficient in autophagic machinery compared to epithelial cells and macrophages underscores the invasive nature of S. pyogenes and its ability to cause severe systemic diseases. S. pyogenes is still one of the top 10 causes of infectious mortality. Understanding the orchestration of dynamic host signaling networks will provide a better understanding of the increasingly complex mechanism of S. pyogenes diseases and novel ways of therapeutically intervening to thwart severe and often fatal infections.

Lipoteichoic acid anchor triggers Mincle to drive protective immunity against invasive group A Streptococcus infection

Group A Streptococcus (GAS) is a Gram-positive bacterial pathogen that causes a range of diseases, including fatal invasive infections. However, the mechanisms by which the innate immune system recognizes GAS are not well understood. We herein report that the C-type lectin receptor macrophage inducible C-type lectin (Mincle) recognizes GAS and initiates antibacterial immunity. Gene expression analysis of myeloid cells upon GAS stimulation revealed the contribution of the caspase recruitment domain-containing protein 9 (CARD9) pathway to the antibacterial responses. Among receptors signaling through CARD9, Mincle induced the production of inflammatory cytokines, inducible nitric oxide synthase, and reactive oxygen species upon recognition of the anchor of lipoteichoic acid, monoglucosyldiacylglycerol (MGDG), produced by GAS. Upon GAS infection, Mincle-deficient mice exhibited impaired production of proinflammatory cytokines, severe bacteremia, and rapid lethality. GAS also possesses another Mincle ligand, diglucosyldiacylglycerol; however, this glycolipid interfered with MGDG-induced activation. These results indicate that Mincle plays a central role in protective immunity against acute GAS infection.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.

THE ROLE OF ARGININE DEIMINASE FROM STREPTOCOCCUS PYOGENES IN INHIBITION MACROPHAGES NITROGEN MONOXIDE (NO) SYNTHESIS

The protective role of macrophages closely related to the production of bactericidal molecules, in which nitrogen monoxide (NO) play an  important role. Arginine serves as a substrate for inducible NO  synthase (iNOS) in course of NO production. Expression and activity  of iNOS are regulated by the availability of the substrate (arginine)  in the intercellular space. The bacterial enzyme arginine deiminase  also uses arginine as a substrate, causing its deficiency for host  cells. The aim of this study was to confirm the possible role of  arginine deiminase from S. pyogenes in inhibiting NO synthesis by  macrophages. For this purpose, a comparative study was made of  the effect on the synthesis of NO by macrophages of the products of  destruction of two strains: the initial S. pyogenes M49-16 and the  isogenic mutant S. pyogenes M49-16 delArcA with the inactivated  arginine deiminase gene (arcA). It has been shown that the ability of S. pyogenes M49-16 to inhibit production of NO by macrophages  depends on its arginine deiminase activity because the isogenous mutant of S. pyogenes M49-16 delArcA with the  inactivated gene arcA has lost its ability to inhibit NO synthesis. This allows us to consider the effects of S. pyogenes M49-16 as  effects of arginine deiminase. An analysis of the inhibitory mechanisms of the enzyme showed that suppression of NO synthesis was not associated with the effect of destruction products  of S. pyogenes M49-16 on the viability of macrophages. According to data of flow cytometry, incubation of cells in the presence of S.  pyogenes destruction products of the original and mutant strains did  not affect the level of iNOS expression, i.e. did not alter synthesis or  stability of this enzyme. At the same time, the decrease in NO  production under the influence of the original S. pyogenes strain  M49-16 correlated with a decrease in the content of arginine in the  culture medium. When exogenous arginine to the culture medium  was added, the effect of the original strain of the suppression of NO  production was declined. This confirms that the depletion of arginine  is the main mechanism of the inhibitory effect of arginine deiminase  on the production of NO by macrophages. The deficiency of NO  production in the course of streptococcal infection can lead to a  weakening of bactericidal activity of macrophages and to a decrease  in the effectiveness of antimicrobial protection.

The Heme Transporter HtsABC of Group A Streptococcus Contributes to Virulence and Innate Immune Evasion in Murine Skin Infections

Group A Streptococcus (GAS) requires iron for growth, and heme is an important source of iron for GAS. Streptococcus heme transporter A (HtsA) is the lipoprotein component of the GAS heme-specific ABC transporter (HtsABC). The objective of this study is to examine the contribution of HtsABC to virulence and host interaction of hypervirulent M1T1 GAS using an isogenic htsA deletion mutant (ΔhtsA). The htsA deletion exhibited a significantly increased survival rate, reduced skin lesion size, and reduced systemic GAS dissemination in comparison to the wild type strain. The htsA deletion also decreased the GAS adhesion rate to Hep-2 cells, the survival in human blood and rat neutrophils, and increased the production of cytokine IL-1β, IL-6, and TNF-α levels in air pouch exudate of a mouse model of subcutaneous infection. Complementation of ΔhtsA restored the wild type phenotype. These findings support that the htsA gene is required for GAS virulence and that the htsA deletion augments host innate immune responses.

Defining Host Responses during Systemic Bacterial Infection through Construction of a Murine Organ Proteome Atlas

Group A Streptococcus (GAS) remains one of the top 10 deadliest human pathogens worldwide despite its sensitivity to penicillin. Although the most common GAS infection is pharyngitis (strep throat), it also causes life-threatening systemic infections. A series of complex networks between host and pathogen drive invasive infections, which have not been comprehensively mapped. Attempting to map these interactions, we examined organ-level protein dynamics using a mouse model of systemic GAS infection. We quantified over 11,000 proteins, defining organ-specific markers for all analyzed tissues. From this analysis, an atlas of dynamically regulated proteins and pathways was constructed. Through statistical methods, we narrowed organ-specific markers of infection to 34 from the defined atlas. We show these markers are trackable in blood of infected mice, and a subset has been observed in plasma samples from GAS-infected clinical patients. This proteomics-based strategy provides insight into host defense responses, establishes potentially useful targets for therapeutic intervention, and presents biomarkers for determining affected organs during bacterial infection.

Site-specific glycosylation regulates the form and function of the intermediate filament cytoskeleton

Intermediate filaments (IF) are a major component of the metazoan cytoskeleton and are essential for normal cell morphology, motility, and signal transduction. Dysregulation of IFs causes a wide range of human diseases, including skin disorders, cardiomyopathies, lipodystrophy, and neuropathy. Despite this pathophysiological significance, how cells regulate IF structure, dynamics, and function remains poorly understood. Here, we show that site-specific modification of the prototypical IF protein vimentin with O-linked β-N-acetylglucosamine (O-GlcNAc) mediates its homotypic protein-protein interactions and is required in human cells for IF morphology and cell migration. In addition, we show that the intracellular pathogen Chlamydia trachomatis, which remodels the host IF cytoskeleton during infection, requires specific vimentin glycosylation sites and O-GlcNAc transferase activity to maintain its replicative niche. Our results provide new insight into the biochemical and cell biological functions of vimentin O-GlcNAcylation, and may have broad implications for our understanding of the regulation of IF proteins in general.

Like the body's skeleton, the cytoskeleton gives shape and structure to the inside of a cell. Yet, unlike a skeleton, the cytoskeleton is ever changing. The cytoskeleton consists of many fibers each made from chains of protein molecules. One of these proteins is called vimentin and it forms intermediate filaments in the cytoskeleton. Many different types of cells contain vimentin and a lot of it is found in cancer cells that have spread beyond their original location to other sites in the body.

Cells use chemical modifications to regulate cytoskeleton proteins. For example, through a process called glycosylation, cells can reversibly attach a sugar modification called O-GlcNAc to vimentin. O-GlcNAc can be attached to several different parts of vimentin and each location may have a different effect. It is not currently clear how cells control their vimentin filaments or what role O-GlcNAc plays in this process.

Using genetic engineering, Tarbet et al. produced human cells in the laboratory with modified vimentin proteins. These altered proteins lacked some of the sites for O-GlcNAc attachment. The goal was to see whether the loss of O-GlcNAc at a specific location would affect fiber formation and cell behavior. The results showed one site where vimentin needs O-GlcNAc to form fibers. Without O-GlcNAc at this site, cells could not migrate towards chemical signals. In addition, in normal human cells, Chlamydia bacteria hijack vimentin and rearrange the filaments to form a cage around themselves for protection. However, the cells lacking O-GlcNAc on vimentin were resistant to infection by Chlamydia bacteria.

These findings highlight the importance of O-GlcNAc on vimentin in healthy cells and during infection. Vimentin’s contribution to cell migration may also help to explain its role in the spread of cancer. The importance of O-GlcNAc suggests it could be a new target for therapies. Yet, it also highlights the need for caution due to the delicate balance between the activity of vimentin in healthy and diseased cells. In addition, human cells produce about 70 other vimentin-like proteins and further work will examine if they are also affected by O-GlcNAc.

Group A Streptococcus encounters with host macrophages

Group A Streptococcus (GAS) is a leading human bacterial pathogen with diverse clinical manifestations. Macrophages constitute a critical first line of host defense against GAS infection, using numerous surface and intracellular receptors such as Toll-like receptors and inflammasomes for pathogen recognition and activation of inflammatory signaling pathways. Depending on the intensity of the GAS infection, activation of these signaling cascades may provide a beneficial early alarm for effective immune clearance, or conversely, may cause hyperinflammation and tissue injury during severe invasive infection. Although traditionally considered an extracellular pathogen, GAS can invade and replicate within macrophages using specific molecular mechanisms to resist phagolysosomal and xenophagic killing. Unraveling GAS-macrophage encounters may reveal new treatment options for this leading agent of infection-associated mortality. [Formula: see text].

Group A streptococcal M protein activates the NLRP3 inflammasome

Group A Streptococcus (GAS) is among the top ten causes of infection-related mortality in humans. M protein is the most abundant GAS surface protein, and M1 serotype GAS strains are associated with invasive infections, including necrotizing fasciitis and toxic shock syndrome. Here, we report that released, soluble M1 protein triggers programmed cell death in macrophages (Mϕ). M1 served as a second signal for caspase-1-dependent NLRP3 inflammasome activation, inducing maturation and release of proinflammatory cytokine interleukin-1β (IL-1β) and macrophage pyroptosis. The structurally dynamic B-repeat domain of M1 was critical for inflammasome activation, which involved K⁺ efflux and M1 protein internalization by clathrin-mediated endocytosis. Mouse intraperitoneal challenge showed that soluble M1 was sufficient and specific for IL-1β activation, which may represent an early warning to activate host immunity against the pathogen. Conversely, in systemic infection, hyperinflammation associated with M1-mediated pyroptosis and IL-1β release could aggravate tissue injury.

Inhibition of Inflammasome-Dependent Interleukin 1β Production by Streptococcal NAD+-Glycohydrolase: Evidence for Extracellular Activity

Group A Streptococcus (GAS) is a common human pathogen and the etiologic agent of a large number of diseases ranging from mild, self-limiting infections to invasive life-threatening conditions. Two prominent virulence factors of this bacterium are the genetically and functionally linked pore-forming toxin streptolysin O (SLO) and its cotoxin NAD⁺-glycohydrolase (NADase). Overexpression of these toxins has been linked to increased bacterial virulence and is correlated with invasive GAS disease. NADase can be translocated into host cells by a SLO-dependent mechanism, and cytosolic NADase has been assigned multiple properties such as protection of intracellularly located GAS bacteria and induction of host cell death through energy depletion. Here, we used a set of isogenic GAS mutants and a macrophage infection model and report that streptococcal NADase inhibits the innate immune response by decreasing inflammasome-dependent interleukin 1β (IL-1β) release from infected macrophages. Regulation of IL-1β was independent of phagocytosis and ensued also under conditions not allowing SLO-dependent translocation of NADase into the host cell cytosol. Thus, our data indicate that NADase not only acts intracellularly but also has an immune regulatory function in the extracellular niche.

In the mid-1980s, the incidence and severity of invasive infections caused by serotype M1 GAS suddenly increased. The results of genomic analyses suggested that this increase was due to the spread of clonal bacterial strains and identified a recombination event leading to enhanced production of the SLO and NADase toxins in these strains. However, despite its apparent importance in GAS pathogenesis, the function of NADase remains poorly understood. In this study, we demonstrate that NADase inhibits inflammasome-dependent IL-1β release from infected macrophages. While previously described functions of NADase pertain to its role upon SLO-mediated translocation into the host cell cytosol, our data suggest that the immune regulatory function of NADase is exerted by nontranslocated enzyme, identifying a previously unrecognized extracellular niche for NADase functionality. This immune regulatory property of extracellular NADase adds another possible explanation to how increased secretion of NADase correlates with bacterial virulence.

The Influence of Programmed Cell Death in Myeloid Cells on Host Resilience to Infection with Legionella pneumophila or Streptococcus pyogenes

Pathogen clearance and host resilience/tolerance to infection are both important factors in surviving an infection. Cells of the myeloid lineage play important roles in both of these processes. Neutrophils, monocytes, macrophages, and dendritic cells all have important roles in initiation of the immune response and clearance of bacterial pathogens. If these cells are not properly regulated they can result in excessive inflammation and immunopathology leading to decreased host resilience. Programmed cell death (PCD) is one possible mechanism that myeloid cells may use to prevent excessive inflammation. Myeloid cell subsets play roles in tissue repair, immune response resolution, and maintenance of homeostasis, so excessive PCD may also influence host resilience in this way. In addition, myeloid cell death is one mechanism used to control pathogen replication and dissemination. Many of these functions for PCD have been well defined in vitro, but the role in vivo is less well understood. We created a mouse that constitutively expresses the pro-survival B-cell lymphoma (bcl)-2 protein in myeloid cells (CD68(bcl2tg), thus decreasing PCD specifically in myeloid cells. Using this mouse model we explored the impact that decreased cell death of these cells has on infection with two different bacterial pathogens, Legionella pneumophila and Streptococcus pyogenes. Both of these pathogens target multiple cell death pathways in myeloid cells, and the expression of bcl2 resulted in decreased PCD after infection. We examined both pathogen clearance and host resilience and found that myeloid cell death was crucial for host resilience. Surprisingly, the decreased myeloid PCD had minimal impact on pathogen clearance. These data indicate that the most important role of PCD during infection with these bacteria is to minimize inflammation and increase host resilience, not to aid in the clearance or prevent the spread of the pathogen.

SpyB, a Small Heme-Binding Protein, Affects the Composition of the Cell Wall in Streptococcus pyogenes

Streptococcus pyogenes (Group A Streptococcus or GAS) is a hemolytic human pathogen associated with a wide variety of infections ranging from minor skin and throat infections to life-threatening invasive diseases. The cell wall of GAS consists of peptidoglycan sacculus decorated with a carbohydrate comprising a polyrhamnose backbone with immunodominant N-acetylglucosamine side-chains. All GAS genomes contain the spyBA operon, which encodes a 35-amino-acid membrane protein SpyB, and a membrane-bound C3-like ADP-ribosyltransferase SpyA. In this study, we addressed the function of SpyB in GAS. Phenotypic analysis of a spyB deletion mutant revealed increased bacterial aggregation, and reduced sensitivity to β-lactams of the cephalosporin class and peptidoglycan hydrolase PlyC. Glycosyl composition analysis of cell wall isolated from the spyB mutant suggested an altered carbohydrate structure compared with the wild-type strain. Furthermore, we found that SpyB associates with heme and protoporphyrin IX. Heme binding induces SpyB dimerization, which involves disulfide bond formation between the subunits. Thus, our data suggest the possibility that SpyB activity is regulated by heme.

IL-1β is an innate immune sensor of microbial proteolysis

Interleukin-1β (IL-1β) is a key proinflammatory cytokine that drives antimicrobial immune responses. IL-1β is aberrantly activated in autoimmune diseases, and IL-1β inhibitors are used as therapeutic agents to treat patients with certain autoimmune disorders. Review of postmarketing surveillance of patients receiving IL-1β inhibitors found a disproportionate reporting of invasive infections by group A Streptococcus (GAS). IL-1β inhibition increased mouse susceptibility to GAS infection, but IL-1β was produced independent of canonical inflammasomes. Newly synthesized IL-1β has an amino-terminal prodomain that blocks signaling activity, which is usually proteolytically removed by caspase-1, a protease activated within the inflammasome structure. In place of host caspases, the secreted GAS cysteine protease SpeB generated mature IL-1β. During invasive infection, GAS isolates may acquire pathoadaptive mutations eliminating SpeB expression to evade detection by IL-1β. Pharmacological IL-1β inhibition alleviates this selective pressure, allowing invasive infection by nonpathoadapted GAS. Thus, IL-1β is a sensor that directly detects pathogen-associated proteolysis through an independent pathway operating in parallel with host inflammasomes. Because IL-1β function is maintained across species, yet cleavage by caspases does not appear to be, detection of microbial proteases may represent an ancestral system of innate immune regulation.

Genetic Architecture of Group A Streptococcal Necrotizing Soft Tissue Infections in the Mouse

Host genetic variations play an important role in several pathogenic diseases, and we have previously provided strong evidences that these genetic variations contribute significantly to differences in susceptibility and clinical outcomes of invasive Group A Streptococcus (GAS) infections, including sepsis and necrotizing soft tissue infections (NSTIs). Our initial studies with conventional mouse strains revealed that host genetic variations and sex differences play an important role in orchestrating the severity, susceptibility and outcomes of NSTIs. To understand the complex genetic architecture of NSTIs, we utilized an unbiased, forward systems genetics approach in an advanced recombinant inbred (ARI) panel of mouse strains (BXD). Through this approach, we uncovered interactions between host genetics, and other non-genetic cofactors including sex, age and body weight in determining susceptibility to NSTIs. We mapped three NSTIs-associated phenotypic traits (i.e., survival, percent weight change, and lesion size) to underlying host genetic variations by using the WebQTL tool, and identified four NSTIs-associated quantitative genetic loci (QTL) for survival on mouse chromosome (Chr) 2, for weight change on Chr 7, and for lesion size on Chr 6 and 18 respectively. These QTL harbor several polymorphic genes. Identification of multiple QTL highlighted the complexity of the host-pathogen interactions involved in NSTI pathogenesis. We then analyzed and rank-ordered host candidate genes in these QTL by using the QTLminer tool and then developed a list of 375 candidate genes on the basis of annotation data and biological relevance to NSTIs. Further differential expression analyses revealed 125 genes to be significantly differentially regulated in susceptible strains compared to their uninfected controls. Several of these genes are involved in innate immunity, inflammatory response, cell growth, development and proliferation, and apoptosis. Additional network analyses using ingenuity pathway analysis (IPA) of these 125 genes revealed interleukin-1 beta network as key network involved in modulating the differential susceptibility to GAS NSTIs.

GAS bacteria are major human pathogens that are responsible for millions of infections worldwide, including severe and deadly NSTIs. Several studies have identified numerous GAS secreted virulence factors including proteases, DNases, and superantigens, which mediate several pathologic features of GAS NSTIs. However, the exact role of host genetic and/or nongenetic factors in GAS NSTIs has not been studied so far. To understand these contributions, we undertook the present study utilizing the ARI panel of BXD strains. We found that host genetic context and sex differences can modulate host-pathogen interplay and accordingly potentiate disease severity, manifestations, and outcomes. We also mapped the genetic susceptibility loci of GAS NSTIs to four mouse chromosomes, namely 2, 6, 7 and 18, harboring several polymorphic genes. We believe that these findings will be helpful in uncovering further regulatory events of host-mediated GAS pathogenesis that may occur once the pathogen becomes invasive.

An Entamoeba histolytica ADP-ribosyl transferase from the diphtheria toxin family modifies the bacterial elongation factor Tu

ADP-ribosyl transferases are enzymes involved in the post-translational modification of proteins; they participate in multiple physiological processes, pathogenesis and host-pathogen interactions. Several reports have characterized the functions of these enzymes in viruses, prokaryotes and higher eukaryotes, but few studies have reported ADP-ribosyl transferases in lower eukaryotes, such as parasites. The locus EHI_155600 from Entamoeba histolytica encodes a hypothetical protein that possesses a domain from the ADP-ribosylation superfamily; this protein belongs to the diphtheria toxin family according to a homology model using poly-ADP-ribosyl polymerase 12 (PARP12 or ARTD12) as a template. The recombinant protein expressed in Escherichia coli exhibited in vitro ADP-ribosylation activity that was dependent on the time and temperature. Unlabeled βNAD(+), but not ADP-ribose, competed in the enzymatic reaction using biotin-βNAD(+) as the ADP-ribose donor. The recombinant enzyme, denominated EhToxin-like, auto-ADP-ribosylated and modified an acceptor from E. coli that was identified by MS/MS as the elongation factor Tu (EF-Tu). To the best of our knowledge, this is the first report to identify an ADP-ribosyl transferase from the diphtheria toxin family in a protozoan parasite. The known toxins from this family (i.e., the diphtheria toxin, the Pseudomonas aeruginosa toxin Exo-A, and Cholix from Vibrio cholerae) modify eukaryotic elongation factor two (eEF-2), whereas the amoeba EhToxin-like modified EF-Tu, which is another elongation factor involved in protein synthesis in bacteria and mitochondria.

Vimentin in Bacterial Infections

Despite well-studied bacterial strategies to target actin to subvert the host cell cytoskeleton, thus promoting bacterial survival, replication, and dissemination, relatively little is known about the bacterial interaction with other components of the host cell cytoskeleton, including intermediate filaments (IFs). IFs have not only roles in maintaining the structural integrity of the cell, but they are also involved in many cellular processes including cell adhesion, immune signaling, and autophagy, processes that are important in the context of bacterial infections. Here, we summarize the knowledge about the role of IFs in bacterial infections, focusing on the type III IF protein vimentin. Recent studies have revealed the involvement of vimentin in host cell defenses, acting as ligand for several pattern recognition receptors of the innate immune system. Two main aspects of bacteria-vimentin interactions are presented in this review: the role of vimentin in pathogen-binding on the cell surface and subsequent bacterial invasion and the interaction of cytosolic vimentin and intracellular pathogens with regards to innate immune signaling. Mechanistic insight is presented involving distinct bacterial virulence factors that target vimentin to subvert its function in order to change the host cell fate in the course of a bacterial infection.

Cytosolic Replication of Group A Streptococcus in Human Macrophages

As key components of innate immune defense, macrophages are essential in controlling bacterial pathogens, including group A Streptococcus(GAS). Despite this, only a limited number of studies have analyzed the recovery of GAS from within human neutrophils and macrophages. Here, we determined the intracellular fate of GAS in human macrophages by using several quantitative approaches. In both U937 and primary human macrophages, the appearance over time of long GAS chains revealed that despite GAS-mediated cytotoxicity, replication occurred in viable, propidium iodide-negative macrophages. Whereas the major virulence factor M1 did not contribute to bacterial growth, a GAS mutant strain deficient in streptolysin O (SLO) was impaired for intracellular replication. SLO promoted bacterial escape from the GAS-containing vacuole (GCV) into the macrophage cytosol. Up to half of the cytosolic GAS colocalized with ubiquitin and p62, suggesting that the bacteria were targeted by the autophagy machinery. Despite this, live imaging of U937 macrophages revealed proficient replication of GAS after GCV rupture, indicating that escape from the GCV is important for growth of GAS in macrophages. Our results reveal that GAS can replicate within viable human macrophages, with SLO promoting GCV escape and cytosolic growth, despite the recruitment of autophagy receptors to bacteria.

IMPORTANCE

Classically regarded as an extracellular pathogen, GAS can persist within human epithelial cells, as well as neutrophils and macrophages. Some studies suggest that GAS can modulate its intracellular vacuole to promote survival and perhaps replicate in macrophages. However, an in-depth single-cell analysis of the dynamics of survival and replication is lacking. We used macrophage-like cell lines and primary macrophages to measure the intracellular growth of GAS at both the population and single-cell levels. While CFU counts revealed no increase in overall bacterial growth, quantitative fluorescence microscopy, flow cytometry, and time-lapse imaging revealed bacterial replication in a proportion of infected macrophages. This study emphasizes the importance of single-cell analysis especially when studying the intracellular fate of a pathogen that is cytotoxic and displays heterogeneity in terms of intracellular killing and growth. To our knowledge, this study provides the first direct visualization of GAS replication inside human cells.

Type I Interferon Signaling Prevents IL-1β-Driven Lethal Systemic Hyperinflammation during Invasive Bacterial Infection of Soft Tissue

Type I interferons (IFN-Is) are fundamental for antiviral immunity, but their role in bacterial infections is contradictory and incompletely described. Streptococcus pyogenes activates IFN-I production in innate immune cells, and IFN-I receptor 1 (Ifnar1)-deficient mice are highly susceptible to S. pyogenes infection. Here we report that IFN-I signaling protects the host against invasive S. pyogenes infection by restricting inflammation-driven damage in distant tissues. Lethality following infection in Ifnar1-deficient mice is caused by systemically exacerbated levels of the proinflammatory cytokine IL-1β. Critical cellular effectors of IFN-I in vivo are LysM+ and CD11c+ myeloid cells, which exhibit suppression of Il1b transcription upon Ifnar1 engagement. These cells are also the major source of IFN-β, which is significantly induced by S. pyogenes 23S rRNA in an Irf5-dependent manner. Our study establishes IL-1β and IFN-I levels as key homeostatic variables of protective, yet tuned, immune responses against severe invasive bacterial infection.

Streptococcal toxins: role in pathogenesis and disease

Group A Streptococcus (Streptococcus pyogenes), group B Streptococcus (Streptococcus agalactiae) and Streptococcus pneumoniae (pneumococcus) are host-adapted bacterial pathogens among the leading infectious causes of human morbidity and mortality. These microbes and related members of the genus Streptococcus produce an array of toxins that act against human cells or tissues, resulting in impaired immune responses and subversion of host physiological processes to benefit the invading microorganism. This toxin repertoire includes haemolysins, proteases, superantigens and other agents that ultimately enhance colonization and survival within the host and promote dissemination of the pathogen.

Inflammasome/IL-1β Responses to Streptococcal Pathogens

Inflammation mediated by the inflammasome and the cytokine IL-1β are some of the earliest and most important alarms to infection. These pathways are responsive to the virulence factors that pathogens use to subvert immune processes, and thus are typically activated only by microbes with potential to cause severe disease. Among the most serious human infections are those caused by the pathogenic streptococci, in part because these species numerous strategies for immune evasion. Since the virulence factor armament of each pathogen is unique, the role of IL-1β and the pathways leading to its activation varies for each infection. This review summarizes the role of IL-1β during infections caused by streptococcal pathogens, with emphasis on emergent mechanisms and concepts countering paradigms determined for other organisms.