Neutrophil defect and lung pathogen selection in cystic fibrosis

Cystic fibrosis is a life-threatening genetic disorder caused by mutations in the CFTR chloride channel. Clinically, over 90% of patients with cystic fibrosis succumb to pulmonary complications precipitated by chronic bacterial infections, predominantly by Pseudomonas aeruginosa and Staphylococcus aureus. Despite the well-characterized gene defect and clearly defined clinical sequelae of cystic fibrosis, the critical link between the chloride channel defect and the host defense failure against these specific pathogens has not been established. Previous research from us and others has uncovered that neutrophils from patients with cystic fibrosis are defective in phagosomal production of hypochlorous acid, a potent microbicidal oxidant. Here we report our studies to investigate if this defect in hypochlorous acid production provides P. aeruginosa and S. aureus with a selective advantage in cystic fibrosis lungs. A polymicrobial mixture of cystic fibrosis pathogens (P. aeruginosa and S. aureus) and non-cystic fibrosis pathogens (Streptococcus pneumoniae, Klebsiella pneumoniae, and Escherichia coli) was exposed to varied concentrations of hypochlorous acid. The cystic fibrosis pathogens withstood higher concentrations of hypochlorous acid than did the non-cystic fibrosis pathogens. Neutrophils derived from F508del-CFTR HL-60 cells killed P. aeruginosa less efficiently than did the wild-type counterparts in the polymicrobial setting. After intratracheal challenge in wild-type and cystic fibrosis mice, the cystic fibrosis pathogens outcompeted the non-cystic fibrosis pathogens and exhibited greater survival in the cystic fibrosis lungs. Taken together, these data indicate that reduced hypochlorous acid production due to the absence of CFTR function creates an environment in cystic fibrosis neutrophils that provides a survival advantage to specific microbes-namely, S. aureus and P. aeruginosa-in the cystic fibrosis lungs.

Human neutrophils ≠ murine neutrophils: Does it matter?

Human and murine neutrophils differ with respect to representation in blood, receptors, nuclear morphology, signaling pathways, granule proteins, NADPH oxidase regulation, magnitude of oxidant and hypochlorous acid production, and their repertoire of secreted molecules. These differences often matter and can undermine extrapolations from murine studies to clinical care, as illustrated by several failed therapeutic interventions based on mouse models. Likewise, coevolution of host and pathogen undercuts fidelity of murine models of neutrophil-predominant human infections. However, murine systems that accurately model the human condition can yield insights into human biology difficult to obtain otherwise. The challenge for investigators who employ murine systems is to distinguish models from pretenders and to know when the mouse provides biologically accurate insights. Testing with human neutrophils observations made in murine systems would provide a safeguard but is not always possible. At a minimum, studies that use exclusively murine neutrophils should have accurate titles supported by data and restrict conclusions to murine neutrophils and not encompass all neutrophils. For now, the integration of evidence from studies of neutrophil biology performed using valid murine models coupled with testing in vitro of human neutrophils combines the best of both approaches to elucidate the mysteries of human neutrophil biology.

HDAC7 is an immunometabolic switch triaging danger signals for engagement of antimicrobial versus inflammatory responses in macrophages

The immune system must be able to respond to a myriad of different threats, each requiring a distinct type of response. Here, we demonstrate that the cytoplasmic lysine deacetylase HDAC7 in macrophages is a metabolic switch that triages danger signals to enable the most appropriate immune response. Lipopolysaccharide (LPS) and soluble signals indicating distal or far-away danger trigger HDAC7-dependent glycolysis and proinflammatory IL-1β production. In contrast, HDAC7 initiates the pentose phosphate pathway (PPP) for NADPH and reactive oxygen species (ROS) production in response to the more proximal threat of nearby bacteria, as exemplified by studies on uropathogenic Escherichia coli (UPEC). HDAC7-mediated PPP engagement via 6-phosphogluconate dehydrogenase (6PGD) generates NADPH for antimicrobial ROS production, as well as D-ribulose-5-phosphate (RL5P) that both synergizes with ROS for UPEC killing and suppresses selective inflammatory responses. This dual functionality of the HDAC7-6PGD-RL5P axis prioritizes responses to proximal threats. Our findings thus reveal that the PPP metabolite RL5P has both antimicrobial and immunomodulatory activities and that engagement of enzymes in catabolic versus anabolic metabolic pathways triages responses to different types of danger for generation of inflammatory versus antimicrobial responses, respectively.

Scylla, Charybdis, and navigating antimicrobial action in the neutrophil phagosome

The text extracted from the initial paragraph of a paper coauthored by Zanvil Cohn, one of the pioneers in the study of leukocyte biology, highlights two phenomena that stimulated investigations of innate immunity in the middle of the last century, namely phagocytosis and intracellular antimicrobial activity. Although many features of phagocytosis have been characterized since that time, fundamental aspects of the antimicrobial action of neutrophils remain unknown. The report by Ashby et al. provides a refined and nuanced look at the interface between an ingested microbe, Staphylococcus aureus, and HOCl generated by the myeloperoxidase (MPO)-H₂ O₂ -chloride system in neutrophil phagosomes and represents a holistic approach to the analysis of bactericidal mechanisms that recognizes contributions from both phagocyte and its ingested prey.

Neutrophil dysfunction in the pathogenesis of cystic fibrosis

Polymorphonuclear neutrophils (PMNs) figure prominently in host defense against infection and in noninfectious inflammation. Mobilized early in an inflammatory response, PMNs mediate immediate cellular defense against microbes and orchestrate events that culminate in cessation of inflammation and restoration of homeostasis. Failure to terminate the inflammatory response and its causes can fuel exuberant inflammation characteristic of many human diseases, including cystic fibrosis (CF), an autosomal recessive genetic disease caused by mutations in the CF transmembrane conductance regulator. CF affects multiple end organs, with persistent bacterial infection and chronic neutrophilic inflammation in airways predominating the clinical picture. To match the diverse microbial challenges that they may encounter, PMNs possess a variety of antimicrobial systems to slow or kill invading microorganisms confined in their phagosomes. Prominent among PMN defense systems is their ability to generate hypochlorous acid, a potent microbicide, by reacting oxidants generated by the NADPH oxidase with myeloperoxidase (MPO) released from azurophilic granules in the presence of chloride (Cl-). Products of the MPO-H2O2-Cl system oxidize susceptible biomolecules and support robust antimicrobial action against many, but not all, potential human pathogens. Underscoring that the MPO-H2O2-Cl system is integral to optimal host defense and proper regulation of inflammation, individuals with defects in any component of this system, as seen in chronic granulomatous disease or MPO deficiency, incur increased rates or severity of infection and signs of dysregulated inflammatory responses. We focus attention in this review on the molecular basis for and the clinical consequences of defects in the MPO-H2O2-Cl system because of the compromised Cl transport seen in CF. We will discuss first how the MPO-H2O2-Cl system in healthy PMNs participates in host defense and resolution of inflammation and then review how a defective MPO-H2O2-Cl system contributes to the increased susceptibility to infection and dysregulated inflammation associated with the clinical manifestations of CF.

Neutrophil-derived extracellular vesicles modulate the phenotype of naïve human neutrophils

Neutrophils (PMN) regulate inflammation in many ways, including communication with other immune cells via extracellular vesicles (EVs). EVs released by human neutrophils activated with N-formylmethionyl-leucyl-phenylalanine (fMLF) (PMN-fMLF EVs) had an outside-out orientation and contained functionally important neutrophil plasma membrane proteins, including flavocytochrome b558, and enzymatically active granule proteins, elastase, and myeloperoxidase. Treatment of naïve PMN with PMN-fMLF EVs primed fMLF-stimulated NADPH oxidase activity, increased surface expression of the complement receptors CD11b/CD18 and CD35, the specific granule membrane protein CD66, and flavocytochrome b₅₅₈ , and promoted phagocytosis of serum-opsonized Staphylococcus aureus. The primed oxidase activity reflected increased surface expression of flavocytochrome b558 and phosphorylation of SER345 in p47^phox , two recognized mechanisms for oxidase priming. Taken together, these data demonstrate that stimulated PMN released EVs that altered the phenotype of naïve phagocytes by priming of the NADPH oxidase activity and augmenting phagocytosis, two responses that are integral to optimal PMN host defense.

Cathepsin G Degrades Staphylococcus aureus Biofilms

Polymorphonuclear leukocytes (PMN) phagocytose and kill individual bacteria but are far less efficient when challenged with bacterial aggregates. Consequently, growth within a biofilm affords Staphylococcus aureus some protection but PMN penetrate S. aureus biofilms and phagocytose bacteria, suggesting that enzymes released through neutrophil degranulation degrade biofilms into fragments small enough for phagocytosis. Here we show that the capacity of PMN to invade biofilms depended largely on the activity of secreted cathepsin G.

Modulation of phagocytosis-induced cell death of human neutrophils by Neisseria gonorrhoeae

Optimal innate immune response to infection includes eradication of potential pathogens, resolution of associated inflammation, and restitution of homeostasis. Phagocytosing human polymorphonuclear leukocytes (hPMN) undergo accelerated apoptosis, a process referred to as phagocytosis-induced cell death (PICD) and an early step in their clearance from inflammatory sites. Among human pathogens that modulate hPMN apoptosis, Neisseria gonorrhoeae delays PICD, which may contribute to the exuberant neutrophilic inflammation that characterizes gonorrhea. To elucidate the mechanisms underlying delayed PICD, we compared features of hPMN cell death that followed phagocytosis of N. gonorrhoeae FA1090 wild-type (GC) or serum-opsonized zymosan (OPZ), a prototypical stimulus of PICD. Phosphatidylserine externalization required NADPH oxidase activity after ingestion of GC or OPZ, and annexin V staining and DNA fragmentation were less after phagocytosis of GC compared to OPZ. Caspase 3/7 and caspase 9 activities after phagocytosis of GC were less than that seen after ingestion of OPZ, but caspase 8 activity was the same after ingestion of GC or OPZ. When hPMN sequentially ingested GC followed by OPZ, both caspase 3/7 and 9 activities were less than that seen after OPZ alone, and the inhibition was dose dependent for GC, suggesting that ingestion of GC actively inhibited PICD. Sequential phagocytosis did not block caspase 8 activity, mitochondrial depolarization, or annexin V/propidium iodide staining compared to responses of hPMN fed OPZ alone, despite inhibition of caspases 3/7 and 9. Taken together, these data suggest that active inhibition of the intrinsic pathway of apoptosis contributes to the delay in PICD after hPMN ingestion of N. gonorrhoeae.

Myeloid CFTR loss-of-function causes persistent neutrophilic inflammation in cystic fibrosis

Persistent neutrophilic inflammation is a hallmark of cystic fibrosis (CF). However, the mechanisms underlying this outstanding pathology remain incompletely understood. Here, we report that CFTR in myeloid immune cells plays a pivotal role in control of neutrophilic inflammation. Myeloid CFTR-Knockout (Mye-Cftr-/-) mice and congenic wild-type (WT) mice were challenged peritoneally with zymosan particles at different doses, creating aseptic peritonitis with varied severity. A high-dose challenge resulted in significantly higher mortality in Mye-Cftr-/- mice, indicating an intrinsic defect in host control of inflammation in mice whose myeloid cells lack CF. The low-dose challenge demonstrated an impaired resolution of inflammation in Mye-Cftr-/- mice, reflected by a significant overproduction of proinflammatory cytokines, including neutrophil chemokines MIP-2 and KC, and sustained accumulation of neutrophils. Tracing neutrophil mobilization in vivo demonstrated that myeloid CF mice recruited significantly more neutrophils than did WT mice. Pulmonary challenge with zymosan elicited exuberant inflammation in the lung and recapitulated the findings from peritoneal challenge. To determine the major type of cell that was primarily responsible for the over-recruitment of neutrophils, we purified and cultured ex vivo zymosan-elicited peritoneal neutrophils and macrophages. The CF neutrophils produced significantly more MIP-2 than did the WT counterparts, and peripheral blood neutrophils isolated from myeloid CF mice also produced significantly more MIP-2 after zymosan stimulation in vitro. These data altogether suggest that CFTR dysfunction in myeloid immune cells, especially neutrophils, leads to hyperinflammation and excessive neutrophil mobilization in the absence of infection. Thus, dysregulated inflammation secondary to abnormal or absent CFTR in myeloid cells may underlie the clinically observed neutrophilic inflammation in CF.

Global Network Analysis of Neisseria gonorrhoeae Identifies Coordination between Pathways, Processes, and Regulators Expressed during Human Infection

Neisseria gonorrhoeae is a Gram-negative diplococcus that is responsible for the sexually transmitted infection gonorrhea, a high-morbidity disease in the United States and worldwide. Over the past several years, N. gonorrhoeae strains resistant to antibiotics used to treat this infection have begun to emerge across the globe. Thus, new treatment strategies are needed to combat this organism. Here, we utilized N. gonorrhoeae transcriptomic data sets, including those obtained from natural infection of the human genital tract, to infer the first global gene coexpression network of this pathogen. Interrogation of this network revealed genes central to the network that are likely critical for gonococcal growth, metabolism, and virulence, including genes encoding hypothetical proteins expressed during mucosal infection. In addition, network analysis revealed overlap in the response of N. gonorrhoeae to incubation with neutrophils and exposure to hydrogen peroxide stress in vitro Network analysis also identified new targets of the gonococcal global regulatory protein Fur, while examination of the network neighborhood of genes allowed us to assign additional putative categories to several proteins. Collectively, the characterization of the first gene coexpression network for N. gonorrhoeae described here has revealed new regulatory pathways and new categories for proteins and has shown how processes important to gonococcal infection in both men and women are linked. This information fills a critical gap in our understanding of virulence strategies of this obligate human pathogen and will aid in the development of new treatment strategies for gonorrhea.IMPORTANCE Neisseria gonorrhoeae is the causative agent of the sexually transmitted infection (STI) gonorrhea, a disease with high morbidity worldwide with an estimated 87 million cases annually. Current therapeutic and pharmacologic approaches to treat gonorrhea have been compromised by increased antibiotic resistance worldwide, including to the most recent FDA-approved antibiotic. New treatment strategies are urgently needed to combat this organism. In this study, we used network analysis to interrogate and define the coordination of pathways and processes in N. gonorrhoeae An analysis of the gonococcal network was also used to assign categories to genes and to expand our understanding of regulatory strategies. Network analysis provides important insights into pathogenic mechanisms of this organism that will guide the design of new strategies for disease treatment.

Isolation of Human Neutrophils from Venous Blood

Venous blood provides a ready source of large numbers of unstimulated granulocytes and mononuclear cells. Exploiting the differences in the relative densities of the leukocytes circulating in venous blood, one can separate leukocytes from erythrocytes as well as isolate the individual leukocyte populations in high purity for use in ex vivo studies. For selected functional studies, such as transcriptional analysis or cytokine quantitation, addition of an immunomagnetic negative selection step to the standard isolation protocol can yield highly purified human neutrophils.

Don't Let Its Name Fool You: Relapsing Thoracic Actinomycosis Caused by Pseudopropionibacterium propionicum (Formerly Propionibacterium propionicum)

Patient: Male, 71-year-old

Final Diagnosis: Thoracic actinomycosis caused by Propionibacterium propionicum

Symptoms: Fever

Medication: —

Clinical Procedure: —

Specialty: Infectious Diseases

The phagocyte NOX2 NADPH oxidase in microbial killing and cell signaling

The phagocyte NADPH oxidase possesses a transmembrane electron transferase comprised of gp91phox (aka NOX2) and p22phox and two multicomponent cytosolic complexes, which in stimulated phagocytes translocate to assemble a functional enzyme complex at plasma or phagosomal membranes. The NOX2-centered NADPH oxidase shuttles electrons from cytoplasmic NADPH to molecular oxygen in phagosomes or the extracellular space to produce oxidants that support optimal antimicrobial activity by phagocytes. Additionally, NOX2-generated oxidants have been implicated in both autocrine and paracrine signaling in a variety of biological contexts. However, when interpreting experimental results, investigators must recognize the complexity inherent in the biochemistry of oxidant-mediated attack of microbial targets and the technical limitations of the probes currently used to detect intracellular oxidants.

Coinfection with Leishmania major and Staphylococcus aureus enhances the pathologic responses to both microbes through a pathway involving IL-17A

Cutaneous leishmaniasis (CL) is a parasitic disease causing chronic, ulcerating skin lesions. Most humans infected with the causative Leishmania protozoa are asymptomatic. Leishmania spp. are usually introduced by sand flies into the dermis of mammalian hosts in the presence of bacteria from either the host skin, sand fly gut or both. We hypothesized that bacteria at the dermal inoculation site of Leishmania major will influence the severity of infection that ensues. A C57BL/6 mouse ear model of single or coinfection with Leishmania major, Staphylococcus aureus, or both showed that single pathogen infections caused localized lesions that peaked after 2–3 days for S. aureus and 3 weeks for L. major infection, but that coinfection produced lesions that were two-fold larger than single infection throughout 4 weeks after coinfection. Coinfection increased S. aureus burdens over 7 days, whereas L. major burdens (3, 7, 28 days) were the same in singly and coinfected ears. Inflammatory lesions throughout the first 4 weeks of coinfection had more neutrophils than did singly infected lesions, and the recruited neutrophils from early (day 1) lesions had similar phagocytic and NADPH oxidase capacities. However, most neutrophils were apoptotic, and transcription of immunomodulatory genes that promote efferocytosis was not upregulated, suggesting that the increased numbers of neutrophils may, in part, reflect defective clearance and resolution of the inflammatory response. In addition, the presence of more IL-17A-producing γδ and non-γδ T cells in early lesions (1–7 days), and L. major antigen-responsive Th17 cells after 28 days of coinfection, with a corresponding increase in IL-1β, may recruit more naïve neutrophils into the inflammatory site. Neutralization studies suggest that IL-17A contributed to an enhanced inflammatory response, whereas IL-1β has an important role in controlling bacterial replication. Taken together, these data suggest that coinfection of L. major infection with S. aureus exacerbates disease, both by promoting more inflammation and neutrophil recruitment and by increasing neutrophil apoptosis and delaying resolution of the inflammatory response. These data illustrate the profound impact that coinfecting microorganisms can exert on inflammatory lesion pathology and host adaptive immune responses.

Cutaneous leishmaniasis (CL) is a vector-borne ulcerating skin disease affecting several million people worldwide. The causative Leishmania spp. protozoa are transmitted by infected phlebotomine sand flies. During a sand fly bite, bacteria can be coincidentally inoculated into the dermis with the parasite. Staphylococcus aureus is the most common bacterium in CL skin lesions. Symptomatic CL is characterized by papulonodular skin lesions that ulcerate and resolve with scarring, although most cutaneous Leishmania infections are asymptomatic. We sought to explore factors that determine whether infection with a cutaneous Leishmania species would result in symptomatic CL rather than asymptomatic infection. We hypothesized that local bacteria promote the development of symptomatic CL lesions during infection with Leishmania major. We discovered that cutaneous lesions were significantly larger in mice inoculated simultaneously with S. aureus and L. major than in mice infected with either organism alone. Coinfection led to increased S. aureus growth in skin lesions, whereas L. major parasite numbers were unchanged by coinfection. The size of the exacerbated lesion correlated with early increased numbers of neutrophils and elevated levels of proinflammatory cytokines IL-1β and IL-17A during the first 7 days, and with sustained increases in IL-17A through 28 days of coinfection. Neutralizing antibody experiments suggested IL-17A was partially responsible for lesion exacerbation during coinfection, whereas IL-1β was important for both control of early lesion exacerbation and promotion of IL-17A production. These data suggest that treatment of symptomatic CL targeting the parasite, local commensal bacteria, and host proinflammatory IL-17A immune responses might improve the outcome of CL.

Spectroscopy of NOX Protein Family Members

All members of the NOX protein family contain a unique b-type cytochrome that mediates the electron transport that characterizes the activity of the multicomponent oxidase complexes. Referred to as cytochrome b558, because of its signature spectral absorbance at 558 nm in reduced-minus-oxidized difference spectroscopy, or cytochrome b(-245), because of its very low midpoint potential of -245 mV at pH 7.0, the protein possesses two stacked inequivalent hemes ligated by pairs of histidine residues in membrane helices h3 and h5. In a flavin-dependent fashion, cytochrome b558 shuttles electrons from cytoplasmic NADPH across membranes to molecular oxygen and thereby generates superoxide anion. By performing reduced-minus-oxidized difference spectroscopy and using the millimolar extinction coefficient, E _559-540 nm = 21.6 cm^-1 mM^-1, one can calculate the amount of cytochrome b558 in intact cells or partially purified membrane preparations. Measurements in samples where cytochrome b558 is relatively high and the presence of unrelated heme-containing proteins low, as in neutrophils, are straightforward. However, low levels of cytochrome b558 expression combined with an abundance of mitochondria and other sources of heme proteins make spectral detection of cytochrome b558 in non-phagocytic cells extremely challenging.

Intersecting Stories of the Phagocyte NADPH Oxidase and Chronic Granulomatous Disease

Neutrophils serve as the circulating cells that respond early and figure prominently in human host defense to infection and in inflammation in other settings. Optimal oxidant-dependent antimicrobial activity by neutrophils relies on the ability of stimulated phagocytes to utilize a multicomponent NADPH oxidase to generate oxidants. The frequent, severe, and often fatal infections experienced by individuals with chronic granulomatous disease (CGD), an inherited disorder in which one of the NADPH oxidase components is absent or dysfunctional, underscore the link between a functional phagocyte NADPH oxidase and robust host protection against microbial infection.The history of the discovery and characterization of the normal neutrophil NADPH oxidase and the saga of recognizing CGD and its underlying causes together illustrate how the observations of astute clinicians and imaginative basic scientists synergize to forge new understanding of both basic cell biology and pathogenesis of human disease.In this chapter, we review the events in the stepwise evolution of our understanding of the phagocyte NADPH oxidase, both in the context of normal human neutrophil function and in the setting of CGD. The phagocyte oxidase complex employs a heterodimeric transmembrane protein composed of gp91phox and p22phox to relay electrons from NADPH to molecular oxygen, while other cofactors contribute to localization and regulation of the activity of the assembled oxidase. The b-type cytochrome gp91phox, also known as NOX2, serves as the catalytic component of this multicomponent enzyme complex. Although many of the features of the composition and regulation of the phagocyte oxidase may apply as well to NOX2 expressed in non-phagocytes and to other members of the NOX protein family, exceptions exist and pose special challenges to investigators exploring the biology of NADPH oxidases.

Frontline Science: Staphylococcus aureus promotes receptor-interacting protein kinase 3- and protease-dependent production of IL-1β in human neutrophils

Microbial infection elicits robust immune responses that initially depend on polymorphonuclear neutrophils (PMN), which ingest and kill invading bacteria. However, community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) remain viable within PMN and prompt their lysis with concomitant release of damage-associated molecular patterns and proinflammatory cytokines that promote additional inflammation. Here, we show that ultrapure human PMN (>99.8% pure) that have ingested CA-MRSA released interleukin (IL)-1β but not IL-18. The ingested CA-MRSA needed to be viable, and phagocytosis alone was insufficient to stimulate IL-1β secretion from PMN fed CA-MRSA. In contrast to PMN response to the canonical NLRP3 inflammasome agonist nigericin, IL-1β secretion by PMN fed CA-MRSA occurred independently of NLRP3 inflammasome or caspase-1 activation and required instead active receptor-interacting protein kinase 3 (RIPK3) but not RIPK1. Furthermore, inhibition of neutrophil serine proteases blocked pro-IL-1β cleavage in PMN fed CA-MRSA. Taken together, our data suggest that with respect to secretion of IL-1β and IL-18, PMN differ from human macrophages and exhibit agonist-specific responses. After phagocytosis of CA-MRSA, human PMN secreted IL-1β through a previously unrecognized mechanism dependent on RIPK3 and serine proteases but independent of canonical NLRP3 inflammasome and caspase-1 activation.

A structurally dynamic N-terminal region drives function of the staphylococcal peroxidase inhibitor (SPIN)

The heme-containing enzyme myeloperoxidase (MPO) is critical for optimal antimicrobial activity of human neutrophils. We recently discovered that the bacterium Staphylococcus aureus expresses a novel immune evasion protein, called SPIN, that binds tightly to MPO, inhibits MPO activity, and contributes to bacterial survival following phagocytosis. A co-crystal structure of SPIN bound to MPO suggested that SPIN blocks substrate access to the catalytic heme by inserting an N-terminal β-hairpin into the MPO active-site channel. Here, we describe a series of experiments that more completely define the structure/function relationships of SPIN. Whereas the SPIN N terminus adopts a β-hairpin confirmation upon binding to MPO, the solution NMR studies presented here are consistent with this region of SPIN being dynamically structured in the unbound state. Curiously, whereas the N-terminal β-hairpin of SPIN accounts for ∼55% of the buried surface area in the SPIN-MPO complex, its deletion did not significantly change the affinity of SPIN for MPO but did eliminate the ability of SPIN to inhibit MPO. The flexible nature of the SPIN N terminus rendered it susceptible to proteolytic degradation by a series of chymotrypsin-like proteases found within neutrophil granules, thereby abrogating SPIN activity. Degradation of SPIN was prevented by the S. aureus immune evasion protein Eap, which acts as a selective inhibitor of neutrophil serine proteases. Together, these studies provide insight into MPO inhibition by SPIN and suggest possible functional synergy between two distinct classes of S. aureus immune evasion proteins.

Structure of human promyeloperoxidase (proMPO) and the role of the propeptide in processing and maturation

Myeloperoxidase (MPO) is synthesized by neutrophil and monocyte precursor cells and contributes to host defense by mediating microbial killing. Although several steps in MPO biosynthesis and processing have been elucidated, many questions remained, such as the structure-function relationship of monomeric unprocessed proMPO versus the mature dimeric MPO and the functional role of the propeptide. Here we have presented the first and high resolution (at 1.25 Å) crystal structure of proMPO and its solution structure obtained by small-angle X-ray scattering. Promyeloperoxidase hosts five occupied glycosylation sites and six intrachain cystine bridges with Cys-158 of the very flexible N-terminal propeptide being covalently linked to Cys-319 and thereby hindering homodimerization. Furthermore, the structure revealed (i) the binding site of proMPO-processing proconvertase, (ii) the structural motif for subsequent cleavage to the heavy and light chains of mature MPO protomers, and (iii) three covalent bonds between heme and the protein. Studies of the mutants C158A, C319A, and C158A/C319A demonstrated significant differences from the wild-type protein, including diminished enzymatic activity and prevention of export to the Golgi due to prolonged association with the chaperone calnexin. These structural and functional findings provide novel insights into MPO biosynthesis and processing.

Methylotroph Infections and Chronic Granulomatous Disease

Disease caused by these environmental bacteria is almost exclusively limited to patients with this condition.

Chronic granulomatous disease (CGD) is a primary immunodeficiency caused by a defect in production of phagocyte-derived reactive oxygen species, which leads to recurrent infections with a characteristic group of pathogens not previously known to include methylotrophs. Methylotrophs are versatile environmental bacteria that can use single-carbon organic compounds as their sole source of energy; they rarely cause disease in immunocompetent persons. We have identified 12 infections with methylotrophs (5 reported here, 7 previously reported) in patients with CGD. Methylotrophs identified were Granulibacter bethesdensis (9 cases), Acidomonas methanolica (2 cases), and Methylobacterium lusitanum (1 case). Two patients in Europe died; the other 10, from North and Central America, recovered after prolonged courses of antimicrobial drug therapy and, for some, surgery. Methylotrophs are emerging as disease-causing organisms in patients with CGD. For all patients, sequencing of the 16S rRNA gene was required for correct diagnosis. Geographic origin of the methylotroph strain may affect clinical management and prognosis.

IFN-γ targets macrophage-mediated immune responses toward Staphylococcus aureus

Infections, especially with Staphylococcus aureus (SA), commonly cause morbidity and mortality in patients with chronic granulomatous disease (CGD), a condition characterized by a defective phagocyte oxidase. IFN-γ reduces the frequency and consequences of infection in CGD by mechanisms that remain unknown. As IFN-γ promotes bacterial killing, efferocytosis of effete polymorphonuclear neutrophils (PMN), and cytokine production in macrophages-the same macrophage effector functions that are impaired in response to SA-we hypothesized that IFN-γ may reverse these defects and thereby, augment macrophage control of SA during infection. IFN-γ primed activation of the NADPH oxidase in a time-dependent manner, enhanced killing of ingested SA independent of any effects on phagocytosis, and increased binding of SA-laden neutrophils (PMN-SA) to macrophages. However, IFN-γ did not increase the percentage of apoptotic PMN or PMN-SA internalized by macrophages. Under conditions in which viable SA were eliminated, PMN-SA primed the inflammasome for subsequent activation by silica but did not induce IL-1β production by macrophages. IFN-γ enhanced IL-6 production in response to SA or PMN-SA but did not increase inflammasome activation in response to either agonist. In summary, IFN-γ augmented direct killing of SA by macrophages, promoted engagement of PMN-SA, and enhanced macrophage-mediated cytokine responses that could collectively augment control of SA infection. Together, these findings support the hypothesis that IFN-γ improves responsiveness of macrophages to SA and provides insights into the mechanism of the clinical benefits of IFN-γ.

Pondering neutrophil extracellular traps with healthy skepticism

The authors engage in a dialogue that evaluates critically the state of the study of neutrophil extracellular traps (NETs), a phenomenon currently the object of considerable interest, with the goal of identifying those aspects that merit clarification in order to assign the process its proper place in our current understanding of cell biology. Since the seminal observations in the Zychlinsky laboratory that described the extrusion of filaments of nuclear DNA associated with histones and granule proteins from neutrophils stimulated in vitro, many investigators have examined the phenomenon of NET formation in numerous and diverse settings. However, an overview of work in this rapidly growing field prompts several fundamental questions about NETs, including their precise composition, the mechanisms by which they arise, their clinical relevance, and the interrelationship of those observed in vitro and in vivo. In this discussion, the authors challenge interpretation of data from some experimental settings and provide recommendations for specific studies that would address the concerns raised, improve understanding of the biological relevance of NETs, and strengthen the field.

Salt, chloride, bleach, and innate host defense

Salt provides 2 life-essential elements: sodium and chlorine. Chloride, the ionic form of chlorine, derived exclusively from dietary absorption and constituting the most abundant anion in the human body, plays critical roles in many vital physiologic functions, from fluid retention and secretion to osmotic maintenance and pH balance. However, an often overlooked role of chloride is its function in innate host defense against infection. Chloride serves as a substrate for the generation of the potent microbicide chlorine bleach by stimulated neutrophils and also contributes to regulation of ionic homeostasis for optimal antimicrobial activity within phagosomes. An inadequate supply of chloride to phagocytes and their phagosomes, such as in CF disease and other chloride channel disorders, severely compromises host defense against infection. We provide an overview of the roles that chloride plays in normal innate immunity, highlighting specific links between defective chloride channel function and failures in host defense.

Nfu facilitates the maturation of iron-sulfur proteins and participates in virulence in Staphylococcus aureus

The acquisition and metabolism of iron (Fe) by the human pathogen Staphylococcus aureus is critical for disease progression. S. aureus requires Fe to synthesize inorganic cofactors called iron-sulfur (Fe-S) clusters, which are required for functional Fe-S proteins. In this study we investigated the mechanisms utilized by S. aureus to metabolize Fe-S clusters. We identified that S. aureus utilizes the Suf biosynthetic system to synthesize Fe-S clusters and we provide genetic evidence suggesting that the sufU and sufB gene products are essential. Additional biochemical and genetic analyses identified Nfu as an Fe-S cluster carrier, which aids in the maturation of Fe-S proteins. We find that deletion of the nfu gene negatively impacts staphylococcal physiology and pathogenicity. A nfu mutant accumulates both increased intracellular non-incorporated Fe and endogenous reactive oxygen species (ROS) resulting in DNA damage. In addition, a strain lacking Nfu is sensitive to exogenously supplied ROS and reactive nitrogen species. Congruous with ex vivo findings, a nfu mutant strain is more susceptible to oxidative killing by human polymorphonuclear leukocytes and displays decreased tissue colonization in a murine model of infection. We conclude that Nfu is necessary for staphylococcal pathogenesis and establish Fe-S cluster metabolism as an attractive antimicrobial target.

Proteases, neutrophils, and periodontitis: the NET effect

Neutrophils exert potent antimicrobial activities in their role as first-line cellular defenders against infection. The synergistic and collective actions of oxidants and granule proteins, including serine proteases, support the microbial killing in phagosomes, where most neutrophil-mediated antimicrobial action occurs. In addition to phagocytosis, specific stimuli prompt neutrophils to extrude a matrix of DNA, histones, and granule proteins to produce neutrophil extracellular traps (NETs), which can trap microbes. Mice lacking the serine proteases necessary for NET production are more susceptible to infection, an observation suggesting that functional NETs are required for host protection. In this issue of the JCI, Sørensen and colleagues characterize neutrophils from a patient with Papillon-Lefèvre syndrome. The patient has an inactivating mutation in the gene encoding dipeptidyl peptidase I, resulting in neutrophils lacking elastase, a serine protease required for NET production. Despite the inability to form NETS, neutrophils from this patient killed pathogens in vitro, and the patient did not exhibit evidence of an increased propensity toward bacterial infections. Together, these results suggest that proteases in human neutrophils are dispensable for protection against bacterial infection and that the ability to generate NETs in vitro does not compromise host defense.

Neutrophils at work

In this Review we discuss data demonstrating recently recognized aspects of neutrophil homeostasis in the steady state, granulopoiesis in 'emergency' conditions and interactions of neutrophils with the adaptive immune system. We explore in vivo observations of the recruitment of neutrophils from blood to tissues in models of blood-borne infections versus bacterial invasion through epithelial linings. We examine data on novel aspects of the activation of NADPH oxidase and the heterogeneity of phagosomes and, finally, consider the importance of two neutrophil-derived biological agents: neutrophil extracellular traps and ectosomes.

Myeloperoxidase in human neutrophil host defence

Human neutrophils represent the predominant leucocyte in circulation and the first responder to infection. Concurrent with ingestion of microorganisms, neutrophils activate and assemble the NADPH oxidase at the phagosome, thereby generating superoxide anion and hydrogen peroxide. Concomitantly, granules release their contents into the phagosome, where the antimicrobial proteins and enzymes synergize with oxidants to create an environment toxic to the captured microbe. The most rapid and complete antimicrobial action by human neutrophils against many organisms relies on the combined efforts of the azurophilic granule protein myeloperoxidase and hydrogen peroxide from the NADPH oxidase to oxidize chloride, thereby generating hypochlorous acid and a host of downstream reaction products. Although individual components of the neutrophil antimicrobial response exhibit specific activities in isolation, the situation in the environment of the phagosome is far more complicated, a consequence of multiple and complex interactions among oxidants, proteins and their by-products. In most cases, the cooperative interactions among the phagosomal contents, both from the host and the microbe, culminate in loss of viability of the ingested organism.

Phagocytosis of Staphylococcus aureus by human neutrophils prevents macrophage efferocytosis and induces programmed necrosis

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) pose a significant threat to human health. Polymorphonuclear leukocytes (PMN) are the first responders during staphylococcal infection, but 15-50% of the initial ingested inoculum survives within the PMN phagosome and likely contributes directly or indirectly to disease pathogenesis. We hypothesize that surviving intracellular CA-MRSA undermine effective phagocyte-mediated defense by causing a decrease in macrophage uptake of PMN containing viable S. aureus and by promoting PMN lysis. In support of this hypothesis, PMN harboring viable CA-MRSA strain USA300 (PMN-SA) upregulated the "don't eat me" signal CD47, remained bound to the surface, and were inefficiently ingested by macrophages. In addition, coculture with PMN-SA altered the macrophage phenotype. Compared to macrophages fed USA300 alone, macrophages challenged with PMN-SA produced more IL-8 and less IL-1 receptor antagonist, TNF-α, activated caspase-1, and IL-1β. Although they exhibited some features of apoptosis within 3 h following ingestion of S. aureus, including phosphatidylserine exposure and mitochondrial membrane depolarization, PMN-SA had sustained levels of proliferating cell nuclear Ag expression, absence of caspase activation, and underwent lysis within 6 h following phagocytosis. PMN lysis was dependent on receptor-interacting protein 1, suggesting that PMN-SA underwent programmed necrosis or necroptosis. These data are the first demonstration, to our knowledge, that bacteria can promote sustained expression of proliferating cell nuclear Ag and that human PMN undergo necroptosis. Together, these findings demonstrate that S. aureus surviving within PMN undermine the innate immune response and may provide insight into the pathogenesis of S. aureus disease.

Characterization of N-glycosylation sites on the extracellular domain of NOX1/NADPH oxidase

Extensive evidence demonstrates the pathophysiological importance of NOX1, the catalytic subunit of superoxide-generating enzyme NADPH oxidase, as a source of reactive oxygen species in nonphagocytic cells. However, the biochemical properties of NOX1 have not been extensively characterized due to a lack of specific immunological tools. We used a newly raised NOX1 polyclonal antibody to investigate posttranslational modifications of NOX1 overexpressed in cultured cells and in the colon, where endogenous NOX1 is highly expressed. Immunoblots of lysates from cells expressing NOX1 revealed a doublet of 56 and 60kDa accompanied by a broad band of 60-90kDa. Based on differential sensitivity to glycosidases, the doublet was identified as two high-mannose-type glycoforms of NOX1, whereas the broad band represented NOX1 with complex-type N-linked oligosaccharides. Deglycosylated NOX1 migrated at ~53kDa and N-glycosylation was demonstrated in NOX1 derived from both rat and human. Site-directed mutagenesis identified N-glycosylation sites at Asn(161) and Asn(241) on the extracellular loop of mouse NOX1. Elimination of N-glycosylation on NOX1 did not affect its electron transferase activity, protein stability, targeting to the cell surface, or localization in F-actin-positive membrane protrusions. Taken together, these data identify the two specific sites of N-linked glycosylation of murine NOX1 and demonstrate that they are not required for normal enzyme activity, protein stability, and membrane trafficking. As is true for NOX2, the contribution of glycosylation in NOX1 to its biologic function(s) merits further study.

Abstract W MP35: Localized Increase of Myeloperoxidase in the Lumen of Human Cerebral Aneurysms

The neutrophil granule protein myeloperoxidase (MPO) reacts with H 2 O 2 to oxidize chloride, thereby generating hypochlorous acid (HOCl), a highly reactive product. Circulating levels of MPO are increased in many inflammatory diseases, and have been linked to oxidative stress and elevated risk for vascular disease in humans. We hypothesized that MPO is increased locally in human intracranial aneurysms. Blood, both from the lumen of the aneurysm and femoral artery, and tissue, both from the aneurysm wall and from a superficial temporal artery, were sampled from seventeen patients with intracranial aneurysms who underwent microsurgical clipping. Plasma concentrations of MPO (ELISA) were 3-fold higher in aneurysm (100±15 ng/ml) (mean±SE) than in femoral blood (33±10; p=0.0007). Plasma concentrations of vascular peroxidase 1 (VPO1), a homologue of MPO that is expressed in endothelial cells, were not increased in aneurysm (384±51 μg/ml) compared to femoral blood (513±65; p=0.12). mRNA expression (RT-qPCR) was similar in leukocytes recovered from aneurysmal blood (2005±377 copies/50 ng RNA for MPO and 342±98 for VPO1) and femoral blood (2129±313 for MPO and 381±61 for VPO1; p=0.5 and 0.73, respectively). There were significantly more MPO-positive cells (immunohistochemistry) in aneurysm tissue (69±8 positive cells/field, 32%±3% of total cells) than superficial temporal artery (6±3 positive cells/field, 2%±1%; p<0.001). These findings suggest that MPO is increased specifically and locally in aneurysm blood and tissue, and the increase is contributed by MPO-positive cells in aneurysm tissue. We speculate that local MPO may contribute to the formation and/or rupture of intracranial aneurysms, and thus, could be a therapeutic target.

Isolation of human neutrophils from venous blood

Venous blood provides a ready source of large numbers of unstimulated granulocytes and mononuclear cells. Exploiting the differences in the relative densities of the leukocytes circulating in venous blood, one can separate leukocytes from erythrocytes as well as isolate the individual leukocyte populations in high purity for use in ex vivo studies.

Methionine sulfoxide reductases protect against oxidative stress in Staphylococcus aureus encountering exogenous oxidants and human neutrophils

To establish infection successfully, Staphylococcus aureus must evade clearance by polymorphonuclear neutrophils (PMN). We studied the expression and regulation of the methionine sulfoxide reductases (Msr) that are involved in the repair of oxidized staphylococcal proteins and investigated their influence on the fate of S. aureus exposed to oxidants or PMN. We evaluated a mutant deficient in msrA1 and msrB for susceptibility to hydrogen peroxide, hypochlorous acid and PMN. The expression of msrA1 in wild-type bacteria ingested by human PMN was assessed by real-time PCR. The regulation of msr was studied by screening a library of two-component regulatory system (TCS) mutants for altered msr responses. Relative to the wild-type bacteria, bacteria deficient in Msr were more susceptible to oxidants and PMN. Upregulation of staphylococcal msrA1 occurred within the phagosomes of normal PMN and PMN deficient in NADPH oxidase activity. Furthermore, PMN granule-rich extract stimulated the upregulation of msrA1. Modulation of msrA1 within PMN was shown to be partly dependent on the VraSR TCS. Msr contributes to staphylococcal responses to oxidative attack and PMN. Our study highlights a novel interaction between the oxidative protein repair pathway and the VraSR TCS that is involved in cell wall homeostasis.

Mitochondrial cardiolipin is required for Nlrp3 inflammasome activation

Nlrp3 inflammasome activation occurs in response to numerous agonists but the specific mechanism by which this takes place remains unclear. All previously evaluated activators of the Nlrp3 inflammasome induce the generation of mitochondrial reactive oxygen species (ROS), suggesting a model in which ROS is a required upstream mediator of Nlrp3 inflammasome activation. Here we have identified the oxazolidinone antibiotic linezolid as a Nlrp3 agonist that activates the Nlrp3 inflammasome independently of ROS. The pathways for ROS-dependent and ROS-independent Nlrp3 activation converged upon mitochondrial dysfunction and specifically the mitochondrial lipid cardiolipin. Cardiolipin bound to Nlrp3 directly and interference with cardiolipin synthesis specifically inhibited Nlrp3 inflammasome activation. Together these data suggest that mitochondria play a critical role in the activation of the Nlrp3 inflammasome through the direct binding of Nlrp3 to cardiolipin.

Detection of superoxide anion and hydrogen peroxide production by cellular NADPH oxidases

BACKGROUND

The recent recognition that isoforms of the cellular NADPH-dependent oxidases, collectively known as the NOX protein family, participate in a wide range of physiologic and pathophysiologic processes in both the animal and plant kingdoms has stimulated interest in the identification, localization, and quantitation of their products in biological settings. Although several tools for measuring oxidants released extracellularly are available, the specificity and selectivity of the methods for reliable analysis of intracellular oxidants have not matched the enthusiasm for studying NOX proteins.

SCOPE OF REVIEW

Focusing exclusively on superoxide anion and hydrogen peroxide produced by NOX proteins, this review describes the ideal probe for analysis of O2(-) and H2O2 generated extracellularly and intracellularly by NOX proteins. An overview of the components, organization, and topology of NOX proteins provides a rationale for applying specific probes for use and a context in which to interpret results and thereby construct plausible models linking NOX-derived oxidants to biological responses. The merits and shortcomings of methods currently in use to assess NOX activity are highlighted, and those assays that provide quantitation of superoxide or H2O2 are contrasted with those intended to examine spatial and temporal aspects of NOX activity.

MAJOR CONCLUSIONS

Although interest in measuring the extracellular and intracellular products of the NOX protein family is great, robust analytical probes are limited.

GENERAL SIGNIFICANCE

The widespread involvement of NOX proteins in many biological processes requires rigorous approaches to the detection, localization, and quantitation of the oxidants produced. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Abstract 2916: NOXX: Friend or foe for cellular proliferation

Reactive oxygen species (ROS) are signaling molecules that determine cellular fate. In tumor progression, ROS have been suggested to function as a dual cell controller. They could induce cellular proliferation in low levels while inhibiting cellular proliferation in high levels. ROS can be produced by NOXX, a member of NADPH oxidase family. Here, we present that an increase in NOXX levels by activation of STAT5 are critical in tumor progression by comparing the NOXX levels of breast and lung tumor tissues to those of normal tissues. Consistently, depletion of NOXX causes tumor suppression of lung and breast cancer cells, and expression of NOXX induces proliferation of normal lung cells. Notably, we found that cisplatin, an antineoplastic agent, increases NOXX activity and inhibits cellular proliferation. Specifically, cisplatin activates NOXX to induce a high level of ROS in multiple ways through (i) an increase in NOXX levels, (ii) an increase in levels of c-abl which is an activator of NOXX, and (iii) a release of Ca2+, the other activator of NOXX. Together, NOXX is a double-edged sword for tumorigenesis in lung and breast cancers by modulating ROS levels, and will be a promising target for cancer therapy.

Citation Format: So Hee Dho, Ji Young Kim, Chang-Jin Kim, William M. Nauseef, So-Young Choi, Kwang-Pyo Lee, Ki-Sun Kwon. NOXX: Friend or foe for cellular proliferation. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2916. doi:10.1158/1538-7445.AM2013-2916

β2 integrin-mediated cell-cell contact transfers active myeloperoxidase from neutrophils to endothelial cells

Atherosclerosis and vasculitis both feature inflammation mediated by neutrophil-endothelial cell (EC) contact. Neutrophil myeloperoxidase (MPO) can disrupt normal EC function, although the mechanism(s) by which MPO is transferred to ECs are unknown. We tested the hypothesis that close, β2 integrin-dependent neutrophil-EC contact mediates MPO transfer from neutrophils to ECs. We used sensitive MPO assays and flow cytometry to detect MPO in ECs and demonstrate that ECs acquired MPO when contacted by neutrophils directly but not when ECs and neutrophils were separated in Transwells. The transfer was dependent on neutrophil number, exposure time, and incubation temperature. Transfer occurred in several EC types, increased with endotoxin, was not accompanied by MPO release into the medium, and was not abrogated by inhibiting degranulation to secretagogues. Confocal microscopy showed MPO internalization by ECs with cytoplasmic and nuclear staining. Neutrophils and ECs formed intimate contact sites demonstrated by electron microscopy. Blocking CD11b or CD18 β2 integrin chains, or using neutrophils from CD11b gene-deleted mice, reduced MPO transfer. EC-acquired MPO was enzymatically active, as demonstrated by its ability to oxidize the fluorescent probe aminophenyl fluorescein in the presence of a hydrogen peroxide source. The data suggest an alternative to EC uptake of soluble MPO, namely the cell contact-dependent, β2 integrin-mediated transfer from neutrophils. The findings could be of therapeutic relevance in atherosclerosis and vasculitis.

Cystic fibrosis transmembrane conductance regulator recruitment to phagosomes in neutrophils

Optimal microbicidal activity of human polymorphonuclear leukocytes (PMN) relies on the generation of toxic agents such as hypochlorous acid (HOCl) in phagosomes. HOCl formation requires H2O2 produced by the NADPH oxidase, myeloperoxidase derived from azurophilic granules, and chloride ion. Chloride transport from cytoplasm into phagosomes requires chloride channels which include cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated chloride channel. However, the phagosomal targeting of CFTR in PMN has not been defined. Using human peripheral blood PMN, we determined that 95-99% of lysosomal-associated membrane protein 1 (LAMP-1)-positive mature phagosomes were CFTR positive, as judged by immunostaining and flow cytometric analysis. To establish a model cell system to evaluate CFTR phagosomal recruitment, we stably expressed enhanced green fluorescent protein (EGFP) alone, EGFP-wt-CFTR and EGFP-DF508-CFTR fusion proteins in promyelocytic PLB-985 cells, respectively. After differentiation into neutrophil-like cells, CFTR presentation to phagosomes was examined. EGFP-wt-CFTR was observed to associate with phagosomes and colocalize with LAMP-1. Flow cytometric analysis of the isolated phagosomes indicated that such a phagosomal targeting was determined by the CFTR portion of the fusion protein. In contrast, significantly less EGFP-DF508-CFTR was found in phagosomes, indicating a defective targeting of the molecule to the organelle. Importantly, the CFTR corrector compound VRT-325 facilitated the recruitment of DF508-CFTR to phagosomes. These data demonstrate the possibility of pharmacologic correction of impaired recruitment of mutant CFTR, thereby providing a potential means to augment chloride supply to the phagosomes of PMN in patients with cystic fibrosis to enhance their microbicidal function.

Myeloperoxidase: a front-line defender against phagocytosed microorganisms

Successful immune defense requires integration of multiple effector systems to match the diverse virulence properties that members of the microbial world might express as they initiate and promote infection. Human neutrophils--the first cellular responders to invading microbes--exert most of their antimicrobial activity in phagosomes, specialized membrane-bound intracellular compartments formed by ingestion of microorganisms. The toxins generated de novo by the phagocyte NADPH oxidase and delivered by fusion of neutrophil granules with nascent phagosomes create conditions that kill and degrade ingested microbes. Antimicrobial activity reflects multiple and complex synergies among the phagosomal contents, and optimal action relies on oxidants generated in the presence of MPO. The absence of life-threatening infectious complications in individuals with MPO deficiency is frequently offered as evidence that the MPO oxidant system is ancillary rather than essential for neutrophil-mediated antimicrobial activity. However, that argument fails to consider observations from humans and KO mice that demonstrate that microbial killing by MPO-deficient cells is less efficient than that of normal neutrophils. We present evidence in support of MPO as a major arm of oxidative killing by neutrophils and propose that the essential contribution of MPO to normal innate host defense is manifest only when exposure to pathogens overwhelms the capacity of other host defense mechanisms.

Conserved cysteine residues provide a protein-protein interaction surface in dual oxidase (DUOX) proteins

Intramolecular disulfide bond formation is promoted in oxidizing extracellular and endoplasmic reticulum compartments and often contributes to protein stability and function. DUOX1 and DUOX2 are distinguished from other members of the NOX protein family by the presence of a unique extracellular N-terminal region. These peroxidase-like domains lack the conserved cysteines that confer structural stability to mammalian peroxidases. Sequence-based structure predictions suggest that the thiol groups present are solvent-exposed on a single protein surface and are too distant to support intramolecular disulfide bond formation. To investigate the role of these thiol residues, we introduced four individual cysteine to glycine mutations in the peroxidase-like domains of both human DUOXs and purified the recombinant proteins. The mutations caused little change in the stabilities of the monomeric proteins, supporting the hypothesis that the thiol residues are solvent-exposed and not involved in disulfide bonds that are critical for structural integrity. However, the ability of the isolated hDUOX1 peroxidase-like domain to dimerize was altered, suggesting a role for these cysteines in protein-protein interactions that could facilitate homodimerization of the peroxidase-like domain or, in the full-length protein, heterodimeric interactions with a maturation protein. When full-length hDUOX1 was expressed in HEK293 cells, the mutations resulted in decreased H2O2 production that correlated with a decreased amount of the enzyme localized to the membrane surface rather than with a loss of activity or with a failure to synthesize the mutant proteins. These results support a role for the cysteine residues in intermolecular disulfide bond formation with the DUOX maturation factor DUOXA1.

Editorial: Gazing forward while looking back

Discussion of how transcriptional responses of neutrophils contribute to the resolution of inflammation, and direct studies of human innate immune responses.

Proconvertase proteolytic processing of an enzymatically active myeloperoxidase precursor

Optimal and efficient killing of ingested microbes by human neutrophils is mediated in large part by the action of hypochlorous acid produced by the myeloperoxidase-H(2)O(2)-chloride system in phagosomes. Myeloperoxidase gene transcription is limited to early myeloid precursors in the bone marrow, when myeloperoxidase is synthesized and stored in azurophilic granules for subsequent release from stimulated neutrophils. Promyeloperoxidase, the 90 kDa myeloperoxidase precursor synthesized in the endoplasmic reticulum (ER), contains a 125-amino acid pro-region whose function and fate during myeloperoxidase biosynthesis are unknown. Promyeloperoxidase has two fates during myeloperoxidase biosynthesis; the majority undergoes proteolytic processing to generate mature myeloperoxidase, while the remainder is constitutively secreted from the cells in bone marrow. We used a promyelocytic cell line that produces endogenous myeloperoxidase as well as human embryonic kidney cells stably expressing normal and mutant forms of myeloperoxidase to examine proteolytic processing of promyeloperoxidase. We demonstrated that CMK-RVKR, an inhibitor of subtilisin-like proteinases, blocked cleavage of the pro-peptide of promyeloperoxidase in a post-ER compartment. Mutants with alanine substitution of basic residues in the predicted proteinase cleavage site failed to undergo maturation to normal myeloperoxidase subunits and were arrested at the promyeloperoxidase stage. Whereas specific mutants varied as to their stability, secreted promyeloperoxidase from the mutants retained the capacity to generate hypochlorous acid. Taken together, these studies demonstrate proconvertase-dependent cleavage of promyeloperoxidase as an essential step in normal proteolytic processing and granule targeting of myeloperoxidase. Furthermore, although mutations in the proteinase cleavage site reduced intracellular stability of the mutants, the integrity of the heme group was not compromised, as chlorinating activity was retained in the secreted promyeloperoxidase.

Functional consequence of positive selection revealed through rational mutagenesis of human myeloperoxidase

Myeloperoxidase (MPO) is a member of the mammalian heme peroxidase (MHP) multigene family. Whereas all MHPs oxidize specific halides to generate the corresponding hypohalous acid, MPO is unique in its capacity to oxidize chloride at physiologic pH to produce hypochlorous acid (HOCl), a potent microbicide that contributes to neutrophil-mediated host defense against infection. We have previously resolved the evolutionary relationships in this functionally diverse multigene family and predicted in silico that positive Darwinian selection played a major role in the observed functional diversities (Loughran NB, O'Connor B, O'Fagain C, O'Connell MJ. 2008. The phylogeny of the mammalian heme peroxidases and the evolution of their diverse functions. BMC Evol Biol. 8:101). In this work, we have replaced positively selected residues asparagine 496 (N496), tyrosine 500 (Y500), and leucine 504 (L504) with the amino acids present in the ancestral MHP and have examined the effects on the structure, biosynthesis, and activity of MPO. Analysis in silico predicted that N496F, Y500F, or L504T would perturb hydrogen bonding in the heme pocket of MPO and thus disrupt the structural integrity of the enzyme. Biosynthesis of the mutants stably expressed in human embryonic kidney 293 cells yielded apoproMPO, the heme-free, enzymatically inactive precursor of MPO, that failed to undergo normal maturation or proteolytic processing. As a consequence of the maturational arrest at the apoproMPO stage of development, cells expressing MPO with mutations N496F, Y500F, L504T, individually or in combination, lacked normal peroxidase or chlorinating activity. Taken together, our data provide further support for the in silico predictions of positive selection and highlight the correlation between positive selection and functional divergence. Our data demonstrate that directly probing the functional importance of positive selection can provide important insights into understanding protein evolution.