Neutrophils Directly Recognize Group B Streptococci and Contribute to Interleukin-1β Production during Infection

Regulation of human neutrophil IL-1β secretion induced by Escherichia coli O157:H7 responsible for hemolytic uremic syndrome

Shiga-toxin producing Escherichia coli (STEC) infections can cause from bloody diarrhea to Hemolytic Uremic Syndrome. The STEC intestinal infection triggers an inflammatory response that can facilitate the development of a systemic disease. We report here that neutrophils might contribute to this inflammatory response by secreting Interleukin 1 beta (IL-1β). STEC stimulated neutrophils to release elevated levels of IL-1β through a mechanism that involved the activation of caspase-1 driven by the NLRP3-inflammasome and neutrophil serine proteases (NSPs). Noteworthy, IL-1β secretion was higher at lower multiplicities of infection. This secretory profile modulated by the bacteria:neutrophil ratio, was the consequence of a regulatory mechanism that reduced IL-1β secretion the higher were the levels of activation of both caspase-1 and NSPs, and the production of NADPH oxidase-dependent reactive oxygen species. Finally, we also found that inhibition of NSPs significantly reduced STEC-triggered IL-1β secretion without modulating the ability of neutrophils to kill the bacteria, suggesting NSPs might represent pharmacological targets to be evaluated to limit the STEC-induced intestinal inflammation.

Molecular regulation of neutrophil swarming in health and disease: Lessons from the phagocyte oxidase

Neutrophil swarming is a complex coordinated process in which neutrophils sensing pathogen or damage signals are rapidly recruited to sites of infections or injuries. This process involves cooperation between neutrophils where autocrine and paracrine positive-feedback loops, mediated by receptor/ligand pairs including lipid chemoattractants and chemokines, amplify localized recruitment of neutrophils. This review will provide an overview of key pathways involved in neutrophil swarming and then discuss the cell intrinsic and systemic mechanisms by which NADPH oxidase 2 (NOX2) regulates swarming, including modulation of calcium signaling, inflammatory mediators, and the mobilization and production of neutrophils. We will also discuss mechanisms by which altered neutrophil swarming in disease may contribute to deficient control of infections and/or exuberant inflammation. Deeper understanding of underlying mechanisms controlling neutrophil swarming and how neutrophil cooperative behavior can be perturbed in the setting of disease may help to guide development of tools for diagnosis and precision medicine.

Caspase-8 inhibition improves the outcome of bacterial infections in mice by promoting neutrophil activation

During differentiation, neutrophils undergo a spontaneous pro-inflammatory program that is hypothesized here to be under caspase-8 control. In mice, intraperitoneal administration of the caspase-8 inhibitor z-IETD-fmk is sufficient to unleash the production of pro-inflammatory cytokines and neutrophil influx in the absence of cell death. These effects are due to selective inhibition of caspase-8 and require tonic interferon-β (IFN-β) production and RIPK3 but not MLKL, the essential downstream executioner of necroptotic cell death. In vitro, stimulation with z-IETD-fmk is sufficient to induce significant cytokine production in murine neutrophils but not in macrophages. Therapeutic administration of z-IETD-fmk improves clinical outcome in models of lethal bacterial peritonitis and pneumonia by augmenting cytokine release, neutrophil influx, and bacterial clearance. Moreover, the inhibitor protects mice against high-dose endotoxin shock. Collectively, our data unveil a RIPK3- and IFN-β-dependent pathway that is constitutively activated in neutrophils and can be harnessed therapeutically using caspase-8 inhibition.

NADPH oxidase 2 limits amplification of IL-1β-G-CSF axis and an immature neutrophil subset in murine lung inflammation

The leukocyte NADPH oxidase 2 (NOX2) regulates inflammation independent of its antimicrobial activity. Inherited defects in NOX2 lead to chronic granulomatous disease (CGD), associated with recurrent bacterial and fungal infections, often with excessive neutrophilic inflammation that results in significant inflammatory burden and tissue damage. We previously showed that excessive leukotriene B4 (LTB4) production by NOX2-deficient mouse neutrophils was a key driver of elevated lung neutrophil infiltration in the initial response to pulmonary challenge with the model fungal particle zymosan. We now identify interleukin-1β (IL-1β) and downstream granulocyte colony-stimulating factor (G-CSF) as critical amplifying signals that augment and sustain neutrophil accrual in CGD mice. Neutrophils, delivered into the lung via LTB4, were the primary source of IL-1β within the airways, and their increased numbers in CGD lungs led to significantly elevated local and plasma G-CSF. Elevated G-CSF simultaneously promoted increased granulopoiesis and mobilized the release of higher numbers of an immature CD101- neutrophil subset from the marrow, which trafficked to the lung and acquired a significantly more proinflammatory transcriptome in CGD mice compared with wild-type mice. Thus, neutrophil-produced IL-1β and downstream G-CSF act sequentially but nonredundantly with LTB4 to deploy neutrophils and amplify inflammation in CGD mice after inhalation of zymosan. NOX2 plays a critical role in dampening multiple components of a feed-forward pipeline for neutrophil recruitment, and these findings highlight NOX2 as a key regulator of neutrophil number, subsets, and function at inflamed sites.

CodY Is a Global Transcriptional Regulator Required for Virulence in Group B Streptococcus

Group B Streptococcus (GBS) is a Gram-positive bacterium able to switch from a harmless commensal of healthy adults to a pathogen responsible for invasive infections in neonates. The signals and regulatory mechanisms governing this transition are still largely unknown. CodY is a highly conserved global transcriptional regulator that links nutrient availability to the regulation of major metabolic and virulence pathways in low-G+C Gram-positive bacteria. In this work, we investigated the role of CodY in BM110, a GBS strain representative of a hypervirulent lineage associated with the majority of neonatal meningitis. Deletion of codY resulted in a reduced ability of the mutant strain to cause infections in neonatal and adult animal models. The observed decreased in vivo lethality was associated with an impaired ability of the mutant to persist in the blood, spread to distant organs, and cross the blood-brain barrier. Notably, the codY null mutant showed reduced adhesion to monolayers of human epithelial cells in vitro and an increased ability to form biofilms, a phenotype associated with strains able to asymptomatically colonize the host. RNA-seq analysis showed that CodY controls about 13% of the genome of GBS, acting mainly as a repressor of genes involved in amino acid transport and metabolism and encoding surface anchored proteins, including the virulence factor Srr2. CodY activity was shown to be dependent on the availability of branched-chain amino acids, which are the universal cofactors of this regulator. These results highlight a key role for CodY in the control of GBS virulence.

Specific NLRP3 Inflammasome Assembling and Regulation in Neutrophils: Relevance in Inflammatory and Infectious Diseases

The NLRP3 inflammasome is a cytosolic multimeric protein platform that leads to the activation of the protease zymogen, caspase-1 (CASP1). Inflammasome activation mediates the proteolytic activation of pro-inflammatory cytokines (IL-1β and IL-18) and program cell death called pyroptosis. The pyroptosis is mediated by the protein executioner Gasdermin D (GSDMD), which forms pores at the plasma membrane to facilitate IL-1β/IL-18 secretion and causes pyroptosis. The NLRP3 inflammasome is activated in response to a large number of pathogenic and sterile insults. However, an uncontrolled inflammasome activation may drive inflammation-associated diseases. Initially, inflammasome-competent cells were believed to be limited to macrophages, dendritic cells (DC), and monocytes. However, emerging evidence indicates that neutrophils can assemble inflammasomes in response to various stimuli with functional relevance. Interestingly, the regulation of inflammasome in neutrophils appears to be unconventional. This review provides a broad overview of the role and regulation of inflammasomes—and more specifically NLRP3—in neutrophils.

Pathogen Stimulation of Interleukin-8 from Human Vaginal Epithelial Cells through CD40

Many bacterial and fungal pathogens cause disease across mucosal surfaces, and to a lesser extent through skin surfaces. Pathogens that potentially cause disease vaginally across epithelial cells include Staphylococcus aureus, group A and B streptococci, Escherichia coli, Neisseria gonorrhoeae, and Candida albicans. We have previously shown that staphylococcal and streptococcal superantigens induce inflammatory chemokines from vaginal epithelial cells through the immune costimulatory molecule CD40 through use of a CRISPR cas9 knockout mutant and complemented epithelial cell line. In this study, we show that the potential vaginal pathogens S. aureus, group A and B streptococci, E. coli, an Enterococcus faecalis strain, and C. albicans in part use CD40 to stimulate interleukin-8 (IL-8) production from human vaginal epithelial cells. In contrast, N. gonorrhoeae does not appear to use CD40 to signal IL-8 production. Normal flora Lactobacillus crispatus and an Enterococcus faecalis strain that produces reutericyclin do not induce IL-8. These data indicate that many potential pathogens, but no normal commensals, induce IL-8 to help disrupt the human vaginal epithelial barrier through CD40, thus providing a potential therapeutic target for drug development.

IMPORTANCE Most bacterial and fungal pathogens cause disease across mucosal, and to a lesser extent, skin barriers with the help of induced chemokines from epithelial cells. In this study, we showed that potential vaginal pathogens Staphylococcus aureus, group A and B streptococci, some Enterococcus faecalis strains, Escherichia coli, and Candida albicans use the immune costimulatory molecule CD40 to induce the chemokine interleukin-8 production. In contrast, Neisseria gonorrhoeae does not use CD40 to stimulate interleukin-8. Normal flora lactobacilli and at least one E. faecalis strain do not induce interleukin-8.

Neutrophils discriminate live from dead bacteria by integrating signals initiated by Fprs and TLRs

The mechanisms whereby neutrophils respond differentially to live and dead organisms are unknown. We show here that neutrophils produce 5- to 30-fold higher levels of the Cxcl2 chemokine in response to live bacteria, compared with killed bacteria or isolated bacterial components, despite producing similar levels of Cxcl1 or pro-inflammatory cytokines. Secretion of high levels of Cxcl2, which potently activates neutrophils by an autocrine mechanism, requires three signals. The first two signals are provided by two different sets of signal peptides released by live bacteria, which selectively activate formylated peptide receptor 1 (Fpr1) and Fpr2, respectively. Signal 3 originates from Toll-like receptor activation by microbial components present in both live and killed bacteria. Mechanistically, these signaling pathways converge at the level of the p38 MAP kinase, leading to activation of the AP-1 transcription factor and to Cxcl2 induction. Collectively, our data demonstrate that the simultaneous presence of agonists for Fpr1, Fpr2, and Toll-like receptors represents a unique signature associated with viable bacteria, which is sensed by neutrophils and induces Cxcl2-dependent autocrine cell activation.

Role of Endosomal TLRs in Staphylococcus aureus Infection

Identification of the receptors involved in innate immune recognition of Staphylococcus aureus, a major cause of morbidity and mortality in humans, is essential to develop alternative strategies to treat infections caused by antibiotic-resistant strains. In the current study, we examine the role of endosomal TLRs, which sense the presence of prokaryotic-type nucleic acids, in anti-staphylococcal host defenses using infection models involving genetically defective mice. Single deficiencies in TLR7, 9, or 13 resulted in mild or no decrease in host defenses. However, the simultaneous absence of TLR7, 9, and 13 resulted in markedly increased susceptibility to cutaneous and systemic S. aureus infection concomitantly with decreased production of proinflammatory chemokines and cytokines, neutrophil recruitment to infection sites, and reduced production of reactive oxygen species. This phenotype was significantly more severe than that of mice lacking TLR2, which senses the presence of staphylococcal lipoproteins. Notably, the combined absence of TLR7, 9, and 13 resulted in complete abrogation of IL-12 p70 and IFN-β responses to staphylococcal stimulation in macrophages. Taken together, our data highlight the presence of a highly integrated endosomal detection system, whereby TLR7, 9, and 13 cooperate in sensing the presence of staphylococcal nucleic acids. We demonstrate that the combined absence of these receptors cannot be compensated for by cell surface-associated TLRs, such as TLR2, or cytosolic receptors. These data may be useful to devise strategies aimed at stimulating innate immune receptors to treat S. aureus infections.

Protective role of IL-18 in host defenses against group B Streptococcus

The aim of this study was to investigate the role of IL-18, a member of the IL-1 family, in group B Streptococcus (GBS) infection. Both in a neonatal and adult model of GBS infection, IL-18-deficient animals were significantly more susceptible to infection than WT animals. The lack of IL18 was associated with a marked reduction in IFN-γ-levels after bacterial stimulation but did not play a significant role in the recruitment of PMN to sites of GBS infection. Collectively, our data document a fundamental function of IL-18 signaling in boosting the host immune responses against GBS infection.

Hypoxia-Inducible Factor 1 Alpha Is Dispensable for Host Defense of Group B Streptococcus Colonization and Infection

Group B Streptococcus (GBS) is a leading cause of neonatal morbidity and mortality, and the primary source of exposure is the maternal vagina. Intrapartum antibiotic prophylaxis for GBS-positive mothers has reduced the incidence of GBS early-onset disease, however, potential long-lasting influence of an antibiotic-altered neonatal microbiota, and the frequent clinical sequelae in survivors of invasive GBS infection, compels alternative treatment options for GBS. Here, we examined the role of transcription factor hypoxia-inducible factor 1 alpha (HIF-1α), widely recognized as a regulator of immune activation during infection, in the host response to GBS. Given the importance of endogenous HIF-1α for innate immune defense, and the potential utility of HIF-1α stabilization in promoting bacterial clearance, we hypothesized that HIF-1α could play an important role in coordinating host responses to GBS in colonization and systemic disease. Counter to our hypothesis, we found that GBS infection did not induce HIF-1α expression in vaginal epithelial cells or murine macrophages, nor did HIF-1α deficiency alter GBS colonization or pathogenesis in vivo. Furthermore, pharmacological enhancement of HIF-1α did not improve control of GBS in pathogenesis and colonization models, while displaying inhibitory effects in vaginal epithelial cytokines and immune cell killing in vitro. Taken together, we conclude that HIF-1α is not a prominent aspect of the host response to GBS colonization or invasive disease, and its pharmacological modulation is unlikely to provide significant benefit against this important neonatal pathogen.

Phenotypical and Functional Characterization of Neutrophils in Two Pyrin-Associated Auto-inflammatory Diseases

PURPOSE

Familial Mediterranean Fever (FMF) and Pyrin-Associated Autoinflammation with Neutrophilic Dermatosis (PAAND) are clinically distinct autoinflammatory disorders caused by mutations in the pyrin-encoding gene MEFV. We investigated the transcriptional, phenotypical, and functional characteristics of patient neutrophils to explore their potential role in FMF and PAAND pathophysiology.

METHODS

RNA sequencing was performed to discover transcriptional aberrancies. The phenotypical features, degranulation properties, and phagocytic capacity of neutrophils were assessed by flow cytometry. Production of reactive oxygen species (ROS), myeloperoxidase (MPO) release, and chemotactic responses were investigated via chemiluminescence, ELISA, and Boyden chamber assays, respectively.

RESULTS

Neutrophils from PAAND and FMF patients showed a partially overlapping, activated gene expression profile with increased expression of S100A8, S100A9, S100A12, IL-4R, CD48, F5, MMP9, and NFKB. Increased MMP9 and S100A8/A9 expression levels were accompanied by high plasma concentrations of the encoded proteins. Phenotypical analysis revealed that neutrophils from FMF patients exhibited an immature character with downregulation of chemoattractant receptors CXCR2, C5aR, and BLTR1 and increased expression of Toll-like receptor 4 (TLR4) and TLR9. PAAND neutrophils displayed an increased random, but reduced CXCL8-induced migration. A tendency for enhanced random migration was observed for FMF neutrophils. PAAND neutrophils showed a moderately but significantly enhanced phagocytic activity as opposed to neutrophils from FMF patients. Neutrophils from both patient groups showed increased MPO release and ROS production.

CONCLUSIONS

Neutrophils from patients with FMF and PAAND, carrying different mutations in the MEFV gene, share a pro-inflammatory phenotype yet demonstrate diverse features, underscoring the distinction between both diseases.

Anaplasma phagocytophilum Induces TLR- and MyD88-Dependent Signaling in In Vitro Generated Murine Neutrophils

Anaplasma phagocytophilum is a tick-transmitted obligate intracellular Gram-negative bacterium that replicates in neutrophils. It elicits febrile disease in humans and in animals. In a mouse model, elimination of A. phagocytophilum required CD4⁺ T cells, but was independent of IFN-γ and other classical antibacterial effector mechanisms. Further, mice deficient for immune recognition and signaling via Toll-like receptor (TLR) 2, TLR4 or MyD88 were unimpaired in pathogen control. In contrast, animals lacking adaptor molecules of Nod-like receptors (NLR) such as RIP2 or ASC showed delayed clearance of A. phagocytophilum. In the present study, we investigated the contribution of further pattern recognition receptor (PRR) pathways to the control of A. phagocytophilum in vivo. Mice deficient for the NLR NOD2 had elevated bacterial loads in the early phase of infection, but were unimpaired in pathogen elimination. In contrast, animals lacking adaptor proteins of different C-type lectin receptors (CLR) such as DAP12, Fc-receptor γ-chain (FcRγ) and SYK controlled A. phagocytophilum as efficiently as wild-type mice. Further, we investigated which PRR pathways are involved in the sensing of A. phagocytophilum by in vitro generated Hoxb8 murine neutrophils. In vitro, recognition of A. phagocytophilum by murine neutrophils was dependent on TLR- and MyD88 signaling. However, it remained intact in the absence of the NLR NOD1, NOD2 and NALP3 and of the CLR adaptor molecules DAP12 and FcRγ. From these results, we conclude that TLR rather than NLR or CLR are critical for the detection of A. phagocytophilum by neutrophils although in vivo defective TLR-signaling is compensated probably because of the redundancy of the immune system.

A Novel Approach for Detecting Unique Variations among Infectious Bacterial Species in Endocarditic Cardiac Valve Vegetation

Infectious endocarditis (IE) remains one of the deadliest heart diseases with a high death rate, generally following thrombo-embolic events. Today, therapy is based on surgery and antibiotic therapy. When thromboembolic complications in IE patients persist, this is often due to our lack of knowledge regarding the pathophysiological development and organization of cells in the vegetation, most notably the primordial role of platelets and further triggered hemostasis, which is related to the diversity of infectious microorganisms involved. Our objective was to study the organization of IE vegetations due to different bacteria species in order to understand the related pathophysiological mechanism of vegetation development. We present an approach for ultrastructural analysis of whole-infected heart valve tissue based on scanning electron microscopy and energy-dispersive X-ray spectroscopy. Our approach allowed us to detect differences in cell organization between the analyzed vegetations and revealed a distinct chemical feature in viridans Streptococci ones. Our results illustrate the benefits that such an approach may bring for guiding therapy, considering the germ involved for each IE patient.

Pro-Inflammatory Response of Bovine Polymorphonuclear Cells Induced by Mycoplasma mycoides subsp. mycoides

Mycoplasma mycoides subsp. mycoides (Mmm) is the etiological agent of contagious bovine pleuropneumonia (CBPP), one of the major diseases affecting cattle in sub-Saharan Africa. Some evidences suggest that the immune system of the host (cattle) plays an important role in the pathogenic mechanism of CBPP, but the factors involved in the process remain largely unknown. The present study aimed to investigate the cell response of bovine polymorphonuclear neutrophils (PMNs) after Mmm in vitro exposure using one step RT-qPCR and Western blotting. Data obtained indicate that gene and protein expression levels of some pro-inflammatory factors already change upon 30 min of PMNs exposure to Mmm. Of note, mRNA expression level in Mmm exposed PMNs increased in a time-dependent manner and for all time points investigated; targets expression was also detected by Western blotting in Mmm exposed PMNs only. These data demonstrate that when bovine PMN cells are triggered by Mmm, they undergo molecular changes, upregulating mRNA and protein expression of specific pro-inflammatory factors. These results provide additional information on host-pathogen interaction during CBPP infection.

Nucleic Acid-Sensing Toll-Like Receptors Play a Dominant Role in Innate Immune Recognition of Pneumococci

Streptococcus pneumoniae (or pneumococcus) is a highly prevalent human pathogen. Toll-like receptors (TLRs) function as immune sensors that can trigger host defenses against this bacterium. Defects in TLR-activated signaling pathways, including deficiency in the adaptor protein myeloid differentiation factor 88 (MyD88), are associated with markedly increased susceptibility to infection. However, the individual MyD88-dependent TLRs predominantly involved in antipneumococcal defenses have not been identified yet. Here we find that triple knockout mice simultaneously lacking TLR7, TLR9, and TLR13, which sense the presence of bacterial DNA (TLR9) and RNA (TLR7 and TLR13) in the phagolysosomes of phagocytic cells, display a phenotype that largely resembles that of MyD88-deficient mice and rapidly succumb to pneumococcal pneumonitis due to defective neutrophil influx into the lung. Accordingly, TLR7/9/13 triple knockout resident alveolar macrophages were largely unable to respond to pneumococci with the production of neutrophil-attracting chemokines and cytokines. Mice with single deficiencies of TLR7, TLR9, or TLR13 showed unaltered ability to control lung infection but were moderately more susceptible to encephalitis, in association with a decreased ability of microglia to mount cytokine responses in vitro Our data point to a dominant, tissue-specific role of nucleic acid-sensing pathways in innate immune recognition of S. pneumoniae and also show that endosomal TLRs are largely capable of compensating for the absence of each other, which seems crucial to prevent pneumococci from escaping immune recognition. These results may be useful to develop novel strategies to treat infections by antibiotic-resistant pneumococci based on stimulation of the innate immune system.IMPORTANCE The pneumococcus is a bacterium that frequently causes infections in the lungs, ears, sinus cavities, and meninges. During these infections, body defenses are triggered by tissue-resident cells that use specialized receptors, such as Toll-like receptors (TLRs), to sense the presence of bacteria. We show here that pneumococci are predominantly detected by TLRs that are located inside intracellular vacuoles, including endosomes, where these receptors can sense the presence of nucleic acids released from ingested bacteria. Mice that simultaneously lacked three of these receptors (specifically, TLR7, TLR9, and TLR13) were extremely susceptible to lung infection and rapidly died after inhalation of pneumococci. Moreover, tissue-resident macrophages from these mice were impaired in their ability to respond to the presence of pneumococci by producing inflammatory mediators capable of recruiting polymorphonuclear leucocytes to infection sites. This information may be useful to develop drugs to treat pneumococcal infections, particularly those caused by antibiotic-resistant strains.

Neutrophils Enhance Their Own Influx to Sites of Bacterial Infection via Endosomal TLR-Dependent Cxcl2 Production

The influx of neutrophils to infection sites is a fundamental step in host defenses against the frequent human pathogen group B Streptococcus (GBS) and other extracellular bacteria. Using a mouse model of GBS-induced peritonitis, we show in this study that the chemokines Cxcl1 and Cxcl2 play distinctive roles in enhancing the recruitment and the antibacterial activities of neutrophils in a manner that is linked to differences in the cellular sources of these mediators. Cell depletion experiments demonstrated that neutrophils make a significant contribution to the in vivo production of Cxcl2 but not Cxcl1. In vitro, neutrophils responded weakly to LPS but released high levels of Cxcl2 after stimulation with GBS or other bacteria. Neutrophil-derived Cxcl2 acted in an autocrinous manner to increase its own production and to enhance antibacterial activities, including the release of oxygen radicals. In both neutrophils and macrophages, the production of Cxcl1/2 largely required the presence of functional UNC93B1, a chaperone protein involved in signaling by endosomal TLRs. Moreover, the phenotype of UNC93B1-defective phagocytes could be recapitulated by the simultaneous absence of TLR7, 9, and 13 but not by the absence of individual TLRs. Collectively, our data show that neutrophils recognize Gram-positive and Gram-negative bacteria by means of multiple phagosomal TLRs, resulting in de novo synthesis of Cxcl2, amplification of neutrophil recruitment, and potentiation of their antibacterial activities. These data may be useful to devise alternative therapeutic strategies aimed at enhancing the recruitment and the functional activities of polymorphonuclear leukocytes during infections caused by antibiotic-resistant bacteria.

Toll-Like Receptors 2 and 4 Modulate Pulmonary Inflammation and Host Factors Mediated by Outer Membrane Vesicles Derived from Acinetobacter baumannii

Pneumonia due to Gram-negative bacteria is associated with high mortality. Acinetobacter baumannii is a Gram-negative bacterium that is associated with hospital-acquired and ventilator-associated pneumonia. Bacteria have been described to release outer membrane vesicles (OMVs) that are capable of mediating systemic inflammation. The mechanism by which A. baumannii OMVs mediate inflammation is not fully defined. We sought to investigate the roles that Toll-like receptors (TLRs) play in A. baumannii OMV-mediated pulmonary inflammation. We isolated OMVs from A. baumannii cultures and intranasally introduced the OMVs into mice. Intranasal introduction of A. baumannii OMVs mediated pulmonary inflammation, which is associated with neutrophil recruitment and weight loss. In addition, A. baumannii OMVs increased the release of several chemokines and cytokines in the mouse lungs. The proinflammatory responses were partially inhibited in TLR2- and TLR4-deficient mice compared to those of wild-type mice. This study highlights the important roles of TLRs in A. baumannii OMV-induced pulmonary inflammation in vivo.

Interleukin-1 signaling induced by Streptococcus suis serotype 2 is strain-dependent and contributes to bacterial clearance and inflammation during systemic disease in a mouse model of infection

Streptococcus suis serotype 2 is an important porcine pathogen and zoonotic agent causing sudden death, septic shock and meningitis, with exacerbated inflammation being a hallmark of the infection. A rapid, effective and balanced innate immune response against S. suis is critical to control bacterial growth without causing excessive inflammation. Even though interleukin (IL)-1 is one of the most potent and earliest pro-inflammatory mediators produced, its role in the S. suis pathogenesis has not been studied. We demonstrated that a classical virulent European sequence type (ST) 1 strain and the highly virulent ST7 strain induce important levels of IL-1 in systemic organs. Moreover, bone marrow-derived dendritic cells and macrophages contribute to its production, with the ST7 strain inducing higher levels. To better understand the underlying mechanisms involved, different cellular pathways were studied. Independently of the strain, IL-1β production required MyD88 and involved recognition via TLR2 and possibly TLR7 and TLR9. This suggests that the recognized bacterial components are similar and conserved between strains. However, very high levels of the pore-forming toxin suilysin, produced only by the ST7 strain, are required for efficient maturation of pro-IL-1β via activation of different inflammasomes resulting from pore formation and ion efflux. Using IL-1R^−/− mice, we demonstrated that IL-1 signaling plays a beneficial role during S. suis systemic infection by modulating the inflammation required to control and clear bacterial burden, thus promoting host survival. Beyond a certain threshold, however, S. suis-induced inflammation cannot be counterbalanced by this signaling, making it difficult to discriminate its role.

Sex-specific maternofetal innate immune responses triggered by group B Streptococci

Group B Streptococcus (GBS) is one of the most common bacteria isolated in human chorioamnionitis, which is a major risk factor for premature birth and brain injuries. Males are at greater risk than females for developing lifelong neurobehavioural disorders, although the origins of this sex bias remain poorly understood. We previously showed that end-gestational inflammation triggered by GBS led to early neurodevelopmental impairments mainly in the male rat progeny. Identifying key inflammatory players involved in maternofetal immune activation by specific pathogens is critical to develop appropriate novel therapeutic interventions. We aimed to map out the GBS-induced profile of innate immune biomarkers in the maternal-placental-fetal axis, and to compare this immune profile between male and female tissues. We describe here that the GBS-induced immune signalling involved significantly higher levels of interleukin (IL)-1β, cytokine-induced neutrophil chemoattractant-1 (CINC-1/CXCL1) and polymorphonuclear cells (PMNs) infiltration in male compared to female maternofetal tissues. Although male - but not female - fetuses presented increased levels of IL-1β, fetuses from both sexes in-utero exposed to GBS had increased levels of TNF-α in their circulation. Levels of IL-1β detected in fetal sera correlated positively with the levels found in maternal circulation. Here, we report for the first time that the maternofetal innate immune signalling induced by GBS presents a sexually dichotomous profile, with more prominent inflammation in males than females. These sex-specific placental and fetal pro-inflammatory responses are in keeping with the higher susceptibility of the male population for preterm birth, brain injuries and neurodevelopmental disorders such as cerebral palsy and autism spectrum disorders.

Siglec-7 engagement by GBS β-protein suppresses pyroptotic cell death of natural killer cells

Natural killer (NK) cells are innate immune lymphocytes that recognize and destroy abnormal host cells, such as tumor cells or those infected by viral pathogens. To safely accomplish these functions, NK cells display activating receptors that detect stress molecules or viral ligands displayed at the cell surface, balanced by inhibitory receptors that bind to self-molecules. To date, such activating and inhibitory receptors on NK cells are not known to recognize bacterial determinants. Moreover, NK cell responses to direct interactions with extracellular bacteria are poorly explored. In this study, we observed the human neonatal pathogen group B Streptococcus (GBS) can directly engage human NK cells. The interaction was mediated through the B6N segment of streptococcal β-protein, binding to the inhibitory receptor Siglec-7 via its amino-terminal V-set domain. Unlike classical Siglec binding, the interaction is also independent of its sialic acid recognition property. In contrast to WT GBS, mutants lacking β-protein induced efficient pyroptosis of NK cells through the NLRP3 inflammasome, with production and secretion of the proinflammatory cytokine IL-1β and dissemination of the cytotoxic molecule granzyme B. We postulate that GBS evolved β-protein engagement of inhibitory human Siglec-7 to suppress the pyroptotic response of NK cells and thereby block recruitment of a broader innate immune response, i.e., by "silencing the sentinel."

Staphylococcus epidermidis from prosthetic joint infections induces lower IL-1β release from human neutrophils than isolates from normal flora

The aim of this study was to test the hypothesis that Staphylococcus epidermidis isolated from prosthetic joint infections (PJIs) differs from S. epidermidis isolated from normal flora in terms of its capacity to induce activation of caspase-1 and release of IL-1β in human neutrophils. The amount of active caspase-1 was determined over 6 h by detecting Ac-YVAD-AMC fluorescence in human neutrophils incubated with S. epidermidis isolates from PJIs (ST2) or normal flora. The amount of IL-1β was detected by ELISA in neutrophil supernatants after 6 h of incubation. Mean IL-1β release was lower after incubation with S. epidermidis from PJIs compared to isolates from normal flora, but no statistically significant difference was found in active caspase-1. Substantial inter-individual differences in both active caspase-1 and IL-1β were noted. These results suggest that evasion of innate immune response, measured as reduced capacity to induce release of IL-1β from human neutrophils, might be involved in the predominance of ST2 in S. epidermidis PJIs, but that other microbe-related factors are probably also important.

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.

Neonatal neutrophils stimulated by group B Streptococcus induce a proinflammatory T-helper cell bias

BackgroundGroup B Streptococcus (GBS) infection causes inflammatory comorbidities in newborns. While the mechanisms remain unclear, evidence suggests that impaired innate-adaptive immune interactions may be contributory. We hypothesized that GBS-stimulated neonatal neutrophils provide a milieu that may drive proinflammatory T-helper (Th) cell programming.MethodsNeutrophils were stimulated with Type III GBS (COH1); supernatants or intact neutrophils were cocultured with CD4⁺ T cells or regulatory T cells (Tregs). Resulting intracellular cytokines and nuclear transcription factors were determined by multicolor flow cytometry.ResultsGBS-stimulated neutrophils released soluble mediators that induced greater interleukin-17 (IL-17) responses in neonatal vs. adult CD4⁺ T cells in the absence of added polarizing cytokines. GBS-stimulated neonatal neutrophils also induced robust expression of the canonical nuclear transcription factors for Th1 (Tbet) and Th17 (IL-17) cells in CD4⁺ T cells. Following GBS stimulation, both intact neutrophils and neutrophil-derived mediators promoted the generation of Tregs with Th1 and Th17 characteristics.ConclusionGBS-stimulated neonatal neutrophils bias the in vitro Th differentiation program of neonatal CD4⁺ T cells and promote proinflammatory Th1 and Th17 phenotypes in Tregs. Our data suggest that developmental modifications of innate-adaptive immune cross-talk mechanisms may contribute to the inflammatory complications associated with neonatal GBS infection.

Autophagy Mediates Interleukin-1β Secretion in Human Neutrophils

Interleukin-1β (IL-1β), a major pro-inflammatory cytokine, is a leaderless cytosolic protein whose secretion does not follow the classical endoplasmic reticulum-to-Golgi pathway, and for which a canonical mechanism of secretion remains to be established. Neutrophils are essential players against bacterial and fungi infections. These cells are rapidly and massively recruited from the circulation into infected tissues and, beyond of displaying an impressive arsenal of toxic weapons effective to kill pathogens, are also an important source of IL-1β in infectious conditions. Here, we analyzed if an unconventional secretory autophagy mechanism is involved in the exportation of IL-1β by these cells. Our findings indicated that inhibition of autophagy with 3-methyladenine and Wortmannin markedly reduced IL-1β secretion induced by LPS + ATP, as did the disruption of the autophagic flux with Bafilomycin A1 and E64d. These compounds did not noticeable affect neutrophil viability ruling out that the effects on IL-1β secretion were due to cell death. Furthermore, VPS34IN-1, a specific autophagy inhibitor, was still able to reduce IL-1β secretion when added after it was synthesized. Moreover, siRNA-mediated knockdown of ATG5 markedly reduced IL-1β secretion in neutrophil-differentiated PLB985 cells. Upon LPS + ATP stimulation, IL-1β was incorporated to an autophagic compartment, as was revealed by its colocalization with LC3B by confocal microscopy. Overlapping of IL-1β-LC3B in a vesicular compartment peaked before IL-1β increased in culture supernatants. On the other hand, stimulation of autophagy by cell starvation augmented the colocalization of IL-1β and LC3B and then promoted neutrophil IL-1β secretion. In addition, specific ELISAs indicated that although both IL-1β and pro-IL-1β are released to culture supernatants upon neutrophil stimulation, autophagy only promotes IL-1β secretion. Furthermore, the serine proteases inhibitor AEBSF reduced IL-1β secretion. Moreover, IL-1β could be also found colocalizing with elastase, suggesting both some vesicles containing IL-1β intersect azurophil granules content and that serine proteases also regulate IL-1β secretion. Altogether, our findings indicate that an unconventional autophagy-mediated secretory pathway mediates IL-1β secretion in human neutrophils.

Group B Streptococcal Maternal Colonization and Neonatal Disease: Molecular Mechanisms and Preventative Approaches

Group B Streptococcus (GBS) colonizes the gastrointestinal and vaginal epithelium of a significant percentage of healthy women, with potential for ascending intrauterine infection or transmission during parturition, creating a risk of serious disease in the vulnerable newborn. This review highlights new insights on the bacterial virulence determinants, host immune responses, and microbiome interactions that underpin GBS vaginal colonization, the proximal step in newborn infectious disease pathogenesis. From the pathogen perspective, the function GBS adhesins and biofilms, β-hemolysin/cytolysin toxin, immune resistance factors, sialic acid mimicry, and two-component transcriptional regulatory systems are reviewed. From the host standpoint, pathogen recognition, cytokine responses, and the vaginal mucosal and placental immunity to the pathogen are detailed. Finally, the rationale, efficacy, and potential unintended consequences of current universal recommended intrapartum antibiotic prophylaxis are considered, with updates on new developments toward a GBS vaccine or alternative approaches to reducing vaginal colonization.

Mycobacterial infection induces higher interleukin-1β and dysregulated lung inflammation in mice with defective leukocyte NADPH oxidase

Granulomatous inflammation causes severe tissue damage in mycobacterial infection while redox status was reported to be crucial in the granulomatous inflammation. Here, we used a NADPH oxidase 2 (NOX2)-deficient mice (Ncf1^-/-) to investigate the role of leukocyte-produced reactive oxygen species (ROS) in mycobacterium-induced granulomatous inflammation. We found poorly controlled mycobacterial proliferation, significant body weight loss, and a high mortality rate after M. marinum infection in Ncf1^-/- mice. Moreover, we noticed loose and neutrophilic granulomas and higher levels of interleukin (IL)-1β and neutrophil chemokines in Ncf1^-/- mice when compared with those in wild type mice. The lack of ROS led to reduced production of IL-1β in macrophages, whereas neutrophil elastase (NE), an abundant product of neutrophils, may potentially exert increased inflammasome-independent protease activity and lead to higher IL-1β production. Moreover, we showed that the abundant NE and IL-1β were present in the caseous granulomatous inflammation of human TB infection. Importantly, blocking of IL-1β with either a specific antibody or a recombinant IL-1 receptor ameliorated the pulmonary inflammation. These findings revealed a novel role of ROS in the early pathogenesis of neutrophilic granulomatous inflammation and suggested a potential role of IL-1 blocking in the treatment of mycobacterial infection in the lung.

Toll-Like Receptor 8 Is a Major Sensor of Group B Streptococcus But Not Escherichia coli in Human Primary Monocytes and Macrophages

TLR8 is the major endosomal sensor of degraded RNA in human monocytes and macrophages. It has been implicated in the sensing of viruses and more recently also bacteria. We previously identified a TLR8-IFN regulatory factor 5 (IRF5) signaling pathway that mediates IFNβ and interleukin-12 (IL-12) induction by Staphylococcus aureus and is antagonized by TLR2. The relative importance of TLR8 for the sensing of various bacterial species is however still unclear. We here compared the role of TLR8 and IRF5 for the sensing of Group B Streptococcus (GBS), S. aureus, and Escherichia coli in human primary monocytes and monocyte-derived macrophages (MDM). GBS induced stronger IFNβ and TNF production as well as IRF5 nuclear translocation compared to S. aureus grown to the stationary phase, while S. aureus in exponential growth appeared similarly potent to GBS. Cytokine induction in primary human monocytes by GBS was not dependent on hemolysins, and induction of IFNβ and IL-12 as well as IRF5 activation were reduced with TLR2 ligand costimulation. Heat inactivation of GBS reduced IRF5 and NF-kB translocation, while only the viable E. coli activated IRF5. The attenuated stimulation correlated with loss of bacterial RNA integrity. The E. coli-induced IRF5 translocation was not inhibited by TLR2 costimulation, suggesting that IRF5 was activated via a TLR8-independent mechanism. Gene silencing of MDM using siRNA revealed that GBS-induced IFNβ, IL-12-p35, and TNF production was dependent on TLR8 and IRF5. In contrast, cytokine induction by E. coli was TLR8 independent but still partly dependent on IRF5. We conclude that TLR8-IRF5 signaling is more important for the sensing of GBS than for stationary grown S. aureus in human primary monocytes and MDM, likely due to reduced resistance of GBS to phagosomal degradation and to a lower production of TLR2 activating lipoproteins. TLR8 does not sense viable E. coli, while IRF5 still contributes to E. coli-induced cytokine production, possibly via a cytosolic nucleic acid sensing mechanism.

Perinatal Group B Streptococcal Infections: Virulence Factors, Immunity, and Prevention Strategies

Group B streptococcus (GBS) or Streptococcus agalactiae is a β-hemolytic, Gram-positive bacterium that is a leading cause of neonatal infections. GBS commonly colonizes the lower gastrointestinal and genital tracts and, during pregnancy, neonates are at risk of infection. Although intrapartum antibiotic prophylaxis during labor and delivery has decreased the incidence of early-onset neonatal infection, these measures do not prevent ascending infection that can occur earlier in pregnancy leading to preterm births, stillbirths, or late-onset neonatal infections. Prevention of GBS infection in pregnancy is complex and is likely influenced by multiple factors, including pathogenicity, host factors, vaginal microbiome, false-negative screening, and/or changes in antibiotic resistance. A deeper understanding of the mechanisms of GBS infections during pregnancy will facilitate the development of novel therapeutics and vaccines. Here, we summarize and discuss important advancements in our understanding of GBS vaginal colonization, ascending infection, and preterm birth.

Age-Appropriate Functions and Dysfunctions of the Neonatal Neutrophil

Neonatal and adult neutrophils are distinctly different from one another due to well-defined and documented deficiencies in neonatal cells, including impaired functions, reduced concentrations of microbicidal proteins and enzymes necessary for pathogen destruction, and variances in cell surface receptors. Neutrophil maturation is clearly demonstrated throughout pregnancy from the earliest hematopoietic precursors in the yolk sac to the well-developed myeloid progenitor cells in the bone marrow around the seventh month of gestation. Notable deficiencies of neonatal neutrophils are generally correlated with gestational age and clinical condition, so that the least functional neutrophils are found in the youngest, sickest neonates. Interruption of normal gestation secondary to preterm birth exposes these shortcomings and places the neonate at an exceptionally high rate of infection and sepsis-related mortality. Because the fetus develops in a sterile environment, neonatal adaptive immune responses are deficient from lack of antigen exposure in utero. Newborns must therefore rely on innate immunity to protect against early infection. Neutrophils are a vital component of innate immunity since they are the first cells to respond to and defend against bacterial, viral, and fungal infections. However, notable phenotypic and functional disparities exist between neonatal and adult cells. Below is review of neutrophil ontogeny, as well as a discussion regarding known differences between preterm and term neonatal and adult neutrophils with respect to cell membrane receptors and functions. Our analysis will also explain how these variations decrease with postnatal age.

Group B Streptococcus circumvents neutrophils and neutrophil extracellular traps during amniotic cavity invasion and preterm labor

Preterm birth is a leading cause of neonatal morbidity and mortality. Although microbial invasion of the amniotic cavity (MIAC) is associated with the majority of early preterm births, the temporal events that occur during MIAC and preterm labor are not known. Group B Streptococci (GBS) are β-hemolytic, gram-positive bacteria, which commonly colonize the vagina but have been recovered from the amniotic fluid in preterm birth cases. To understand temporal events that occur during MIAC, we utilized a unique chronically catheterized nonhuman primate model that closely emulates human pregnancy. This model allows monitoring of uterine contractions, timing of MIAC and immune responses during pregnancy-associated infections. Here, we show that adverse outcomes such as preterm labor, MIAC, and fetal sepsis were observed more frequently during infection with hemolytic GBS when compared to nonhemolytic GBS. Although MIAC was associated with systematic progression in chorioamnionitis beginning with chorionic vasculitis and progressing to neutrophilic infiltration, the ability of the GBS hemolytic pigment toxin to induce neutrophil cell death and subvert killing by neutrophil extracellular traps (NETs) in placental membranes in vivo facilitated MIAC and fetal injury. Furthermore, compared to maternal neutrophils, fetal neutrophils exhibit decreased neutrophil elastase activity and impaired phagocytic functions to GBS. Collectively, our studies demonstrate how a unique bacterial hemolytic lipid toxin enables GBS to circumvent neutrophils and NETs in placental membranes to induce fetal injury and preterm labor.