Upon microbial challenge, human neutrophils undergo rapid changes in nuclear architecture and chromatin folding to orchestrate an immediate inflammatory gene program.

Differentiating neutrophils undergo large-scale changes in nuclear morphology. How such alterations in structure are established and modulated upon exposure to microbial agents is largely unknown. Here, we found that prior to encounter with bacteria, an armamentarium of inflammatory genes was positioned in a transcriptionally passive environment suppressing premature transcriptional activation. Upon microbial exposure, however, human neutrophils rapidly (<3 h) repositioned the ensemble of proinflammatory genes toward the transcriptionally permissive compartment. We show that the repositioning of genes was closely associated with the swift recruitment of cohesin across the inflammatory enhancer landscape, permitting an immediate transcriptional response upon bacterial exposure. We found that activated enhancers, marked by increased deposition of H3K27Ac, were highly enriched for cistromic elements associated with PU.1, CEBPB, TFE3, JUN, and FOSL2 occupancy. These data reveal how upon microbial challenge the cohesin machinery is recruited to an activated enhancer repertoire to instruct changes in chromatin folding, nuclear architecture, and to activate an inflammatory gene program.

Erythrocyte sialoglycoproteins engage Siglec-9 on neutrophils to suppress activation.

Healthy blood neutrophils are functionally quiescent in the bloodstream, have a short lifespan, and exit the circulation to carry out innate immune functions, or undergo rapid apoptosis and macrophage-mediated clearance to mitigate host tissue damage. Limitation of unnecessary intravascular neutrophil activation is also important to prevent serious inflammatory pathologies. Because neutrophils become easily activated after purification, we carried out ex vivo comparisons with neutrophils maintained in whole blood. We found a difference in activation state, with purified neutrophils showing signs of increased reactivity: shedding of l-selectin, CD11b upregulation, increased oxidative burst, and faster progression to apoptosis. We discovered that erythrocytes suppressed neutrophil activation ex vivo and in vitro, including reduced l-selectin shedding, oxidative burst, chemotaxis, neutrophil extracellular trap formation, bacterial killing, and induction of apoptosis. Selective and specific modification of sialic acid side chains on erythrocyte surfaces with mild sodium metaperiodate oxidation followed by aldehyde quenching with 4-methyl-3-thiosemicarbazide reduced neutrophil binding to erythrocytes and restored neutrophil activation. By enzyme-linked immunosorbent assay and immunofluorescence, we found that glycophorin A, the most abundant sialoglycoprotein on erythrocytes, engaged neutrophil Siglec-9, a sialic acid-recognizing receptor known to dampen innate immune cell activation. These studies demonstrate a previously unsuspected role for erythrocytes in suppressing neutrophils ex vivo and in vitro and help explain why neutrophils become easily activated after separation from whole blood. We propose that a sialic acid-based "self-associated molecular pattern" on erythrocytes also helps maintain neutrophil quiescence in the bloodstream. Our findings may be relevant to some prior experimental and clinical studies of neutrophils.

Standard susceptibility testing overlooks potent azithromycin activity and cationic peptide synergy against MDR Stenotrophomonas maltophilia.

OBJECTIVES: The Gram-negative bacillus Stenotrophomonas maltophilia (SM) is an emerging MDR opportunistic pathogen. Recent studies identify a potentially relevant activity of azithromycin against Gram-negative bacteria overlooked in standard bacteriological testing. We investigated azithromycin activity against SM in testing conditions incorporating mammalian tissue culture medium and host defence factors. METHODS: MIC testing, chequerboard assays, time-kill assays and fluorescence microscopy were performed for azithromycin, the cationic peptide antibiotic colistin and the human defence peptide cathelicidin LL-37 alone or in combination in cation-adjusted Mueller-Hinton broth or mammalian tissue culture media. Azithromycin sensitization of SM to host immune clearance was tested in a human neutrophil killing assay and a murine pneumonia model. RESULTS: We observed potent bactericidal activity of azithromycin against SM in mammalian tissue culture medium absent in bacteriological medium. Colistin and LL-37 strongly potentiated azithromycin killing of SM by increasing drug entry. Additionally, azithromycin sensitized SM to neutrophil killing and increased SM clearance in the murine pneumonia model. CONCLUSIONS: Despite lack of activity in standard MIC testing, azithromycin synergizes with cationic peptide antibiotics to kill SM in medium mimicking tissue fluid conditions. Azithromycin, alone or in combination with colistin, merits further exploration in therapy of drug-resistant SM infections.

Role for streptococcal collagen-like protein 1 in M1T1 group A Streptococcus resistance to neutrophil extracellular traps.

Streptococcal collagen-like protein 1 (Scl-1) is one of the most highly expressed proteins in the invasive M1T1 serotype group A Streptococcus (GAS), a globally disseminated clone associated with higher risk of severe invasive infections. Previous studies using recombinant Scl-1 protein suggested a role in cell attachment and binding and inhibition of serum proteins. Here, we studied the contribution of Scl-1 to the virulence of the M1T1 clone in the physiological context of the live bacterium by generating an isogenic strain lacking the scl-1 gene. Upon subcutaneous infection in mice, wild-type bacteria induced larger lesions than the Δscl mutant. However, loss of Scl-1 did not alter bacterial adherence to or invasion of skin keratinocytes. We found instead that Scl-1 plays a critical role in GAS resistance to human and murine phagocytic cells, allowing the bacteria to persist at the site of infection. Phenotypic analyses demonstrated that Scl-1 mediates bacterial survival in neutrophil extracellular traps (NETs) and protects GAS from antimicrobial peptides found within the NETs. Additionally, Scl-1 interferes with myeloperoxidase (MPO) release, a prerequisite for NET production, thereby suppressing NET formation. We conclude that Scl-1 is a virulence determinant in the M1T1 GAS clone, allowing GAS to subvert innate immune functions that are critical in clearing bacterial infections.

Polyketide synthase (PKS) reduces fusion of Legionella pneumophila-containing vacuoles with lysosomes and contributes to bacterial competitiveness during infection.

L. pneumophila-containing vacuoles (LCVs) exclude endocytic and lysosomal markers in human macrophages and protozoa. We screened a L. pneumophila mini-Tn10 transposon library for mutants, which fail to inhibit the fusion of LCVs with lysosomes by loading of the lysosomal compartment with colloidal iron dextran, mechanical lysis of infected host cells, and magnetic isolation of LCVs that have fused with lysosomes. In silico analysis of the mutated genes, D. discoideum plaque assays and infection assays in protozoa and U937 macrophage-like cells identified well established as well as novel putative L. pneumophila virulence factors. Promising candidates were further analyzed for their co-localization with lysosomes in host cells using fluorescence microscopy. This approach corroborated that the O-methyltransferase, PilY1, TPR-containing protein and polyketide synthase (PKS) of L. pneumophila interfere with lysosomal degradation. Competitive infections in protozoa and macrophages revealed that the identified PKS contributes to the biological fitness of pneumophila strains and may explain their prevalence in the epidemiology of Legionnaires' disease.

The Effect of Retinoic Acid on Neutrophil Innate Immune Interactions With Cutaneous Bacterial Pathogens.

Vitamin A and its biologically active derivative, retinoic acid (RA), are important for many immune processes. RA, in particular, is essential for the development of immune cells, including neutrophils, which serve as a front-line defense against infection. While vitamin A deficiency has been linked to higher susceptibility to infections, the precise role of vitamin A/RA in host-pathogen interactions remains poorly understood. Here, we provided evidence that RA boosts neutrophil killing of methicillin-resistant Staphylococcus aureus (MRSA). RA treatment stimulated primary human neutrophils to produce reactive oxygen species, neutrophil extracellular traps, and the antimicrobial peptide cathelicidin (LL-37). Because RA treatment was insufficient to reduce MRSA burden in an in vivo murine model of skin infection, we expanded our analysis to other infectious agents. RA did not affect the growth of a number of common bacterial pathogens, including MRSA, Escherichia coli K1 and Pseudomonas aeruginosa; however, RA directly inhibited the growth of group A Streptococcus (GAS). This antimicrobial effect, likely in combination with RA-mediated neutrophil boosting, resulted in substantial GAS killing in neutrophil killing assays conducted in the presence of RA. Furthermore, in a murine model of GAS skin infection, topical RA treatment showed therapeutic potential by reducing both skin lesion size and bacterial burden. These findings suggest that RA may hold promise as a therapeutic agent against GAS and perhaps other clinically significant human pathogens.

Group A Streptococcal M1 Protein Provides Resistance against the Antimicrobial Activity of Histones.

Histones are essential elements of chromatin structure and gene regulation in eukaryotes. An unexpected attribute of these nuclear proteins is their antimicrobial activity. A framework for histone release and function in host defense in vivo was revealed with the discovery of neutrophil extracellular traps, a specialized cell death process in which DNA-based structures containing histones are extruded to ensnare and kill bacteria. Investigating the susceptibility of various Gram-positive pathogens to histones, we found high-level resistance by one leading human pathogen, group A Streptococcus (GAS). A screen of isogenic mutants revealed that the highly surface-expressed M1 protein, a classical GAS virulence factor, was required for high-level histone resistance. Biochemical and microscopic analyses revealed that the N-terminal domain of M1 protein binds and inactivates histones before they reach their cell wall target of action. This finding illustrates a new pathogenic function for this classic GAS virulence factor, and highlights a potential innate immune evasion strategy that may be employed by other bacterial pathogens.

Whole-Genome Sequencing of Invasion-Resistant Cells Identifies Laminin α2 as a Host Factor for Bacterial Invasion.

To understand the role of glycosaminoglycans in bacterial cellular invasion, xylosyltransferase-deficient mutants of Chinese hamster ovary (CHO) cells were created using clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR-associated gene 9 (CRISPR-cas9) gene targeting. When these mutants were compared to the pgsA745 cell line, a CHO xylosyltransferase mutant generated previously using chemical mutagenesis, an unexpected result was obtained. Bacterial invasion of pgsA745 cells by group B Streptococcus (GBS), group A Streptococcus, and Staphylococcus aureus was markedly reduced compared to the invasion of wild-type cells, but newly generated CRISPR-cas9 mutants were only resistant to GBS. Invasion of pgsA745 cells was not restored by transfection with xylosyltransferase, suggesting that an additional mutation conferring panresistance to multiple bacteria was present in pgsA745 cells. Whole-genome sequencing and transcriptome sequencing (RNA-Seq) uncovered a deletion in the gene encoding the laminin subunit α2 (Lama2) that eliminated much of domain L4a. Silencing of the long Lama2 isoform in wild-type cells strongly reduced bacterial invasion, whereas transfection with human LAMA2 cDNA significantly enhanced invasion in pgsA745 cells. The addition of exogenous laminin-α2ß1γ1/laminin-α2ß2γ1 strongly increased bacterial invasion in CHO cells, as well as in human alveolar basal epithelial and human brain microvascular endothelial cells. Thus, the L4a domain in laminin α2 is important for cellular invasion by a number of bacterial pathogens. IMPORTANCE: Pathogenic bacteria penetrate host cellular barriers by attachment to extracellular matrix molecules, such as proteoglycans, laminins, and collagens, leading to invasion of epithelial and endothelial cells. Here, we show that cellular invasion by the human pathogens group B Streptococcus, group A Streptococcus, and Staphylococcus aureus depends on a specific domain of the laminin α2 subunit. This finding may provide new leads for the molecular pathogenesis of these bacteria and the development of novel antimicrobial drugs.

The Selective Estrogen Receptor Modulator Raloxifene Inhibits Neutrophil Extracellular Trap Formation.

Raloxifene is a selective estrogen receptor modulator typically prescribed for the prevention/treatment of osteoporosis in postmenopausal women. Although raloxifene is known to have anti-inflammatory properties, its effects on human neutrophils, the primary phagocytic leukocytes of the immune system, remain poorly understood. Here, through a screen of pharmacologically active small molecules, we find that raloxifene prevents neutrophil cell death in response to the classical activator phorbol 12-myristate 13-acetate (PMA), a compound known to induce formation of DNA-based neutrophil extracellular traps (NETs). Inhibition of PMA-induced NET production by raloxifene was confirmed using quantitative and imaging-based assays. Human neutrophils from both male and female donors express the nuclear estrogen receptors ERα and ERß, known targets of raloxifene. Similar to raloxifene, selective antagonists of these receptors inhibit PMA-induced NET production. Furthermore, raloxifene inhibited PMA-induced ERK phosphorylation, but not reactive oxygen species production, pathways known to be key modulators of NET production. Finally, we found that raloxifene inhibited PMA-induced, NET-based killing of the leading human bacterial pathogen, methicillin-resistant Staphylococcus aureus. Our results reveal that raloxifene is a potent modulator of neutrophil function and NET production.

Mechanisms of group A Streptococcus resistance to reactive oxygen species.

Streptococcus pyogenes, also known as group A Streptococcus (GAS), is an exclusively human Gram-positive bacterial pathogen ranked among the 'top 10' causes of infection-related deaths worldwide. GAS commonly causes benign and self-limiting epithelial infections (pharyngitis and impetigo), and less frequent severe invasive diseases (bacteremia, toxic shock syndrome and necrotizing fasciitis). Annually, GAS causes 700 million infections, including 1.8 million invasive infections with a mortality rate of 25%. In order to establish an infection, GAS must counteract the oxidative stress conditions generated by the release of reactive oxygen species (ROS) at the infection site by host immune cells such as neutrophils and monocytes. ROS are the highly reactive and toxic byproducts of oxygen metabolism, including hydrogen peroxide (H2O2), superoxide anion (O2•(-)), hydroxyl radicals (OH•) and singlet oxygen (O2*), which can damage bacterial nucleic acids, proteins and cell membranes. This review summarizes the enzymatic and regulatory mechanisms utilized by GAS to thwart ROS and survive under conditions of oxidative stress.

Group A Streptococcal M1 Protein Sequesters Cathelicidin to Evade Innate Immune Killing.

The antimicrobial peptide LL-37 is generated upon proteolytic cleavage of cathelicidin and limits invading pathogens by directly targeting microbial membranes as well as stimulating innate immune cell function. However, some microbes evade LL-37-mediated defense. Notably, group A Streptococcus (GAS) strains belonging to the hypervirulent M1T1 serogroup are more resistant to human LL-37 than other GAS serogroups. We show that the GAS surface-associated M1 protein sequesters and neutralizes LL-37 antimicrobial activity through its N-terminal domain. M1 protein also binds the cathelicidin precursor hCAP-18, preventing its proteolytic maturation into antimicrobial forms. Exogenous M1 protein rescues M1-deficient GAS from killing by neutrophils and within neutrophil extracellular traps and neutralizes LL-37 chemotactic properties. M1 also binds murine cathelicidin, and its virulence contribution in a murine model of necrotizing skin infection is largely driven by its ability to neutralize this host defense peptide. Thus, cathelicidin resistance is essential for the pathogenesis of hyperinvasive M1T1 GAS.

Streptolysin O Rapidly Impairs Neutrophil Oxidative Burst and Antibacterial Responses to Group A Streptococcus.

Group A Streptococcus (GAS) causes a wide range of human infections, ranging from simple pharyngitis to life-threatening necrotizing fasciitis and toxic shock syndrome. A globally disseminated clone of M1T1 GAS has been associated with an increase in severe, invasive GAS infections in recent decades. The secreted GAS pore-forming toxin streptolysin O (SLO), which induces eukaryotic cell lysis in a cholesterol-dependent manner, is highly upregulated in the GAS M1T1 clone during bloodstream dissemination. SLO is known to promote GAS resistance to phagocytic clearance by neutrophils, a critical first element of host defense against invasive bacterial infection. Here, we examine the role of SLO in modulating specific neutrophil functions during their early interaction with GAS. We find that SLO at subcytotoxic concentrations and early time points is necessary and sufficient to suppress neutrophil oxidative burst, in a manner reversed by free cholesterol and anti-SLO blocking antibodies. In addition, SLO at subcytotoxic concentrations blocked neutrophil degranulation, interleukin-8 secretion and responsiveness, and elaboration of DNA-based neutrophil extracellular traps, cumulatively supporting a key role for SLO in GAS resistance to immediate neutrophil killing. A non-toxic SLO derivate elicits protective immunity against lethal GAS challenge in a murine infection model. We conclude that SLO exerts a novel cytotoxic-independent function at early stages of invasive infections (<30 min), contributing to GAS escape from neutrophil clearance.

IgG protease Mac/IdeS is not essential for phagocyte resistance or mouse virulence of M1T1 group A Streptococcus.

UNLABELLED: The Mac/IdeS protein of group A Streptococcus (GAS) is a secreted cysteine protease with cleavage specificity for IgG and is highly expressed in the GAS serotype M1T1 clone, which is the serotype most frequently isolated from patients with life-threatening invasive infections. While studies of Mac/IdeS with recombinant protein have shown that the protein can potentially prevent opsonophagocytosis of GAS by neutrophils, the role of the protein in immune evasion as physiologically produced by the living organism has not been studied. Here we examined the contribution of Mac/IdeS to invasive GAS disease by generating a mutant lacking Mac/IdeS in the hyperinvasive M1T1 background. While Mac/IdeS was highly expressed and proteolytically active in the hyperinvasive strain, elimination of the bacterial protease did not significantly influence GAS phagocytic uptake, oxidative-burst induction, cathelicidin sensitivity, resistance to neutrophil or macrophage killing, or pathogenicity in pre- or postimmune mouse infectious challenges. We conclude that in the highly virulent M1T1 background, Mac/IdeS is not essential for either phagocyte resistance or virulence. Given the conservation of Mac/IdeS and homologues across GAS strains, it is possible that Mac/IdeS serves another important function in GAS ecology or contributes to virulence in other strain backgrounds. IMPORTANCE: Group A Streptococcus (GAS) causes human infections ranging from strep throat to life-threatening conditions such as flesh-eating disease and toxic shock syndrome. Common disease-associated clones of GAS can cause both mild and severe infections because of a characteristic mutation and subsequent change in the expression of several genes that develops under host immune selection. One of these genes encodes Mac/IdeS, a protease that has been shown to cleave antibodies important to the immune defense system. In this study, we found that while Mac/IdeS is highly expressed in hypervirulent GAS, it does not significantly contribute to the ability of the bacteria to survive white blood cell killing or produce invasive infection in the mouse. These data underscore the importance of correlating studies on virulence factor function with physiologic expression levels and the complexity of streptococcal pathogenesis and contribute to our overall understanding of how GAS causes disease.