Defective bactericidal activity in myeloperoxidase-deficient human neutrophils

The formation and function of the neutrophil phagosome

Neutrophils are the most abundant circulating leukocyte and are crucial to the initial innate immune response to infection. One of their key pathogen-eliminating mechanisms is phagocytosis, the process of particle engulfment into a vacuole-like structure called the phagosome. The antimicrobial activity of the phagocytic process results from a collaboration of multiple systems and mechanisms within this organelle, where a complex interplay of ion fluxes, pH, reactive oxygen species, and antimicrobial proteins creates a dynamic antimicrobial environment. This complexity, combined with the difficulties of studying neutrophils ex vivo, has led to gaps in our knowledge of how the neutrophil phagosome optimizes pathogen killing. In particular, controversy has arisen regarding the relative contribution and integration of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-derived antimicrobial agents and granule-delivered antimicrobial proteins. Clinical syndromes arising from dysfunction in these systems in humans allow useful insight into these mechanisms, but their redundancy and synergy add to the complexity. In this article, we review the current knowledge regarding the formation and function of the neutrophil phagosome, examine new insights into the phagosomal environment that have been permitted by technological advances in recent years, and discuss aspects of the phagocytic process that are still under debate.

The effects of neutrophil-generated hypochlorous acid and other hypohalous acids on host and pathogens

Neutrophils are predominant immune cells that protect the human body against infections by deploying sophisticated antimicrobial strategies including phagocytosis of bacteria and neutrophil extracellular trap (NET) formation. Here, we provide an overview of the mechanisms by which neutrophils kill exogenous pathogens before we focus on one particular weapon in their arsenal: the generation of the oxidizing hypohalous acids HOCl, HOBr and HOSCN during the so-called oxidative burst by the enzyme myeloperoxidase. We look at the effects of these hypohalous acids on biological systems in general and proteins in particular and turn our attention to bacterial strategies to survive HOCl stress. HOCl is a strong inducer of protein aggregation, which bacteria can counteract by chaperone-like holdases that bind unfolding proteins without the need for energy in the form of ATP. These chaperones are activated by HOCl through thiol oxidation (Hsp33) or N-chlorination of basic amino acid side-chains (RidA and CnoX) and contribute to bacterial survival during HOCl stress. However, neutrophil-generated hypohalous acids also affect the host system. Recent studies have shown that plasma proteins act not only as sinks for HOCl, but get actively transformed into modulators of the cellular immune response through N-chlorination. N-chlorinated serum albumin can prevent aggregation of proteins, stimulate immune cells, and act as a pro-survival factor for immune cells in the presence of cytotoxic antigens. Finally, we take a look at the emerging role of HOCl as a potential signaling molecule, particularly its role in neutrophil extracellular trap formation.

Reactive oxygen and nitrogen species and innate immune response

The innate immune system is the first line of defense against pathogens and is characterized by its fast but nonspecific response. One important mechanism of this system is the production of the biocidal reactive oxygen and nitrogen species, which are widely distributed within biological systems, including phagocytes and secretions. Reactive oxygen and nitrogen species are short-lived intermediates that are biochemically synthesized by various enzymatic reactions in aerobic organisms and are regulated by antioxidants. The physiological levels of reactive species play important roles in cellular signaling and proliferation. However, higher concentrations and prolonged exposure can fight infections by damaging important microbial biomolecules. One feature of the reactive species generation system is the interaction between its components to produce more biocidal agents. For example, the phagocytic NADPH oxidase complex generates superoxide, which functions as a precursor for antimicrobial hydrogen peroxide synthesis. Peroxide is then used by myeloperoxidase in the same cells to generate hypochlorous acid, a highly microbicidal agent. Studies on animal models and microorganisms have shown that deficiency of these antimicrobial agents is associated with severe recurrent infections and immunocompromised diseases, such as chronic granulomatous disease. There is accumulating evidence that reactive species have important positive aspects on human health and immunity; however, some important promising features of this system remain obscure.

The Role of Neutrophils in the Immune System: An Overview

Neutrophils, also known as polymorphonuclear neutrophils (PMNs), have long been considered as the short-lived, nonspecific white cells that form pus-and also happen to kill invading microbes. Indeed, neutrophils were often neglected (and largely not considered) as immune cells. This historic view of neutrophils has changed considerably over the past several decades, and we now know that in addition to playing the predominant role in the clearance of bacteria and fungi, they have a major role in shaping the host response to infection and immune system homeostasis. The change in our view of the role of neutrophils in the immune system has been due in large part to the study of these cells in vitro. Such work has been made possible by new and/or improved methods and approaches used to investigate neutrophils. These methods are the focus of this volume.

Effects of Myeloperoxidase on Methicillin-Resistant Staphylococcus aureus-Colonized Burn Wounds in Rats

Objective: To achieve better therapeutic results in burn wound infections and to examine alternatives to antibiotics, we designed this study to elaborate the role of myeloperoxidase (MPO) on infected burn wounds in rats. Approach: We compared chemical properties as well as bacteriostatic ability of MPO in different concentrations with NeutroPhase. Subsequently, we applied MPO (MPO group), NeutroPhase (NeutroPhase group), NaCl+H₂O₂ (NaCl+H₂O₂ group), or NaCl (control group) on rat dorsal burn wounds inoculated with methicillin-resistant Staphylococcus aureus (MRSA). Their effects on MRSA-colonized wounds were evaluated by microscopy, histologic section, and Western blot. Results: MPO produced more H⁺ and HClO^-, leading to a more acidic environment. Moreover, MPO inhibited the growth of MRSA more intensely after 6 h of inoculation ex vivo. In vivo the open wound rate in the MPO group was significantly lower, while the contraction rate and epithelialization rate of MPO group were higher than that of the control group, NaCl+H₂O₂ group, and NeutroPhase group on day 20. The hematoxylin and eosin staining of MPO group showed better wound healing than other groups. More vascular endothelial growth factor (VEGF) was expressed in wound tissue of MPO group by Western blot. Innovation: This is the first study to use MPO for MRSA-colonized burn wound therapy. Conclusion: MPO displayed more effective bacteriostatic ability, possibly beneficial for MRSA-colonized wound healing.

Myeloperoxidase-deficient zebrafish show an augmented inflammatory response to challenge with Candida albicans

Myeloperoxidase is a key component of neutrophil granules involved in killing engulfed microorganisms. We obtained a zebrafish mutant (smu681) lacking Sudan black staining by large-scale screening, which was a neutrophil-replete but myeloperoxidase-deficient mutant. When infiltrated with Candida albicans, smu681 embryos and sibling embryos showed similar survival after infection. Proliferation of C. albicans was more rapid in smu681 embryos than in sibling embryos, although it was eventually suppressed. In addition, the number of neutrophils accumulating at the site of infection was significantly larger in mutant embryos than in sibling embryos, and mutant embryos showed increased expression of several inflammatory cytokines after C. albicans infection. These findings indicate that myeloperoxidase deficiency alters the inflammatory response to fungal infection.

Salmonella evades D-amino acid oxidase to promote infection in neutrophils

Neutrophils engulf and kill bacteria using oxidative and nonoxidative mechanisms. Despite robust antimicrobial activity, neutrophils are impaired in directing Salmonella clearance and harbor viable intracellular bacteria during early stages of infection that can subsequently escape to more-permissive cell types. The mechanisms accounting for this immune impairment are not understood. We report that Salmonella limits exposure to oxidative damage elicited by D-amino acid oxidase (DAO) in neutrophils by expressing an ABC importer specific for D-alanine, a DAO substrate found in peptidoglycan stem peptides. A Salmonella dalS mutant defective for D-alanine import was more susceptible to killing by DAO through exposure to greater oxidative stress during infection. This fitness defect was reversed by selective depletion of neutrophils or by inhibition of DAO in vivo with a small-molecule inhibitor. DalS-mediated subversion of neutrophil DAO is a novel host-pathogen interaction that enhances Salmonella survival during systemic infection.

IMPORTANCE

Neutrophils engulf Salmonella during early stages of infection, but bacterial killing is incomplete. Very little is known about how Salmonella survives in neutrophils to gain access to other cell types during infection. In this study, we show that D-amino acid oxidase (DAO) in neutrophils consumes D-alanine and that importing this substrate protects Salmonella from oxidative killing by neutrophil DAO. Loss of this importer results in increased bacterial killing in vitro, in neutrophils, and in a mouse model of infection, all phenotypes that are lost upon inhibition of DAO. These findings add mechanistic insight into a novel host-pathogen interaction that has consequences on infection outcome.

Neutrophil-mediated phagocytic host defense defect in myeloid Cftr-inactivated mice

Cystic fibrosis (CF) is a common and deadly inherited disease, caused by mutations in the CFTR gene that encodes a cAMP-activated chloride channel. One outstanding manifestation of the disease is the persistent bacterial infection and inflammation in the lung, which claims over 90% of CF mortality. It has been debated whether neutrophil-mediated phagocytic innate immunity has any intrinsic defect that contributes to the host lung defense failure. Here we compared phagosomal CFTR targeting, hypochlorous acid (HOCl) production, and microbial killing of the neutrophils from myeloid Cftr-inactivated (Myeloid-Cftr-/-) mice and the non-inactivated control (Cftrfl10) mice. We found that the mutant CFTR that lacked Exon-10 failed to target to the neutrophil phagosomes. This dysfunction resulted in impaired intraphagosomal HOCl production and neutrophil microbial killing. In vivo lung infection with a lethal dose of Pseudomonas aeruginosa caused significantly higher mortality in the myeloid CF mice than in the controls. The myeloid-Cftr-/- lungs were deficient in bacterial clearance, and had sustained neutrophilic inflammation and stalled transition from early to late immunity. These manifestations recapitulated the symptoms of human CF lungs. The data altogether suggest that myeloid CFTR expression is critical to normal host lung defense. CFTR dysfunction in neutrophils compromises the phagocytic innate immunity, which may predispose CF lungs to infection.

The role of Rab27a in the regulation of neutrophil function

Neutrophils are central regulators of the innate immune response and help shape the adaptive immune response. Malfunction and unregulated neutrophil activation leads to disease and inflammation. During the host response to infection, neutrophils display several mechanisms of defense mediated by their arsenal of granular proteins. Regulation of granular trafficking, docking and fusion is at the core of the neutrophil defense response to pathogens. The small GTPase Rab27a has emerged as a central regulator of the neutrophil response through its tight control of vesicular trafficking and degranulation. This review focuses on the latest research that has led to the characterization of Rab27a as an essential regulator of neutrophil function.

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.

The role of neutrophils in the immune system: an overview

Neutrophils, also known as polymorphonuclear leukocytes (PMNs), have long been considered as the short-lived, nonspecific white cells that form pus-and also happen to kill invading microbes. Indeed, neutrophils were often neglected (and largely not considered) as immune cells. This historic view of neutrophils has changed considerably over the past several decades, and we know now that, in addition to playing the predominant role in the clearance of bacteria and fungi, they play a major role in shaping the host response to infection and immune system homeostasis. The change in our view of the role of neutrophils in the immune system has been due in large part to the study of these cells in vitro. Such work has been made possible by new and/or improved methods and approaches used to investigate neutrophils. These methods are the focus of this volume.

Myeloperoxidase: a leukocyte-derived protagonist of inflammation and cardiovascular disease

SIGNIFICANCE

The heme-enzyme myeloperoxidase (MPO) is one of the major neutrophil bactericidal proteins and is stored in large amounts inside azurophilic granules of neutrophils. Upon cell activation, MPO is released and extracellular MPO has been detected in a wide range of acute and chronic inflammatory conditions. Recent ADVANCES AND CRITICAL ISSUES: Apart from its role during infection, MPO has emerged as a critical modulator of inflammation throughout the last decade and is currently discussed in the initiation and propagation of cardiovascular diseases. MPO-derived oxidants (e.g., hypochlorous acid) interfere with various cell functions and contribute to tissue injury. Recent data also suggest that MPO itself exerts proinflammatory properties independent of its catalytic activity. Despite advances in unraveling the complex action of MPO and MPO-derived oxidants, further research is warranted to determine the precise nature and biological role of MPO in inflammation.

FUTURE DIRECTIONS

The identification of MPO as a central player in inflammation renders this enzyme an attractive prognostic biomarker and a potential target for therapeutic interventions. A better understanding of the (patho-) physiology of MPO is essential for the development of successful treatment strategies in acute and chronic inflammatory diseases.

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.

α-Defensins in human innate immunity

Defensins are small, multifunctional cationic peptides. They typically contain six conserved cysteines whose three intramolecular disulfides stabilize a largely β-sheet structure. This review of human α-defensins begins by describing their evolution, including their likely relationship to the Big Defensins of invertebrates, and their kinship to the β-defensin peptides of many if not all vertebrates, and the θ-defensins found in certain non-human primates. We provide a short history of the search for leukocyte-derived microbicidal molecules, emphasizing the roles played by luck (good), preconceived notions (mostly bad), and proper timing (essential). The antimicrobial, antiviral, antitoxic, and binding properties of human α-defensins are summarized. The structural features of α-defensins are described extensively and their functional contributions are assessed. The properties of HD6, an enigmatic Paneth cell α-defensin, are contrasted with those of the four myeloid α-defensins (HNP1-4) and of HD5, the other α-defensin of human Paneth cells. The review ends with a decalogue that may assist researchers or students interested in α-defensins and related aspects of neutrophil function.

Staphylococcal response to oxidative stress

Staphylococci are a versatile genus of bacteria that are capable of causing acute and chronic infections in diverse host species. The success of staphylococci as pathogens is due in part to their ability to mitigate endogenous and exogenous oxidative and nitrosative stress. Endogenous oxidative stress is a consequence of life in an aerobic environment; whereas, exogenous oxidative and nitrosative stress are often due to the bacteria's interaction with host immune systems. To overcome the deleterious effects of oxidative and nitrosative stress, staphylococci have evolved protection, detoxification, and repair mechanisms that are controlled by a network of regulators. In this review, we summarize the cellular targets of oxidative stress, the mechanisms by which staphylococci sense oxidative stress and damage, oxidative stress protection and repair mechanisms, and regulation of the oxidative stress response. When possible, special attention is given to how the oxidative stress defense mechanisms help staphylococci control oxidative stress in the host.

Neutrophils in innate host defense against Staphylococcus aureus infections

Staphylococcus aureus has been an important human pathogen throughout history and is currently a leading cause of bacterial infections worldwide. S. aureus has the unique ability to cause a continuum of diseases, ranging from minor skin infections to fatal necrotizing pneumonia. Moreover, the emergence of highly virulent, drug-resistant strains such as methicillin-resistant S. aureus in both healthcare and community settings is a major therapeutic concern. Neutrophils are the most prominent cellular component of the innate immune system and provide an essential primary defense against bacterial pathogens such as S. aureus. Neutrophils are rapidly recruited to sites of infection where they bind and ingest invading S. aureus, and this process triggers potent oxidative and non-oxidative antimicrobial killing mechanisms that serve to limit pathogen survival and dissemination. S. aureus has evolved numerous mechanisms to evade host defense strategies employed by neutrophils, including the ability to modulate normal neutrophil turnover, a process critical to the resolution of acute inflammation. Here we provide an overview of the role of neutrophils in host defense against bacterial pathogens and discuss strategies employed by S. aureus to circumvent neutrophil function.

Respiratory syncytial virus infection is associated with an altered innate immunity and a heightened pro-inflammatory response in the lungs of preterm lambs

Introduction

Factors explaining the greater susceptibility of preterm infants to severe lower respiratory infections with respiratory syncytial virus (RSV) remain poorly understood. Fetal/newborn lambs are increasingly appreciated as a model to study key elements of RSV infection in newborn infants due to similarities in lung alveolar development, immune response, and susceptibility to RSV. Previously, our laboratory demonstrated that preterm lambs had elevated viral antigen and developed more severe lesions compared to full-term lambs at seven days post-infection. Here, we compared the pathogenesis and immunological response to RSV infection in lungs of preterm and full-term lambs.

Methods

Lambs were delivered preterm by Caesarian section or full-term by natural birth, then inoculated with bovine RSV (bRSV) via the intratracheal route. Seven days post-infection, lungs were collected for evaluation of cytokine production, histopathology and cellular infiltration.

Results

Compared to full-term lambs, lungs of preterm lambs had a heightened pro-inflammatory response after infection, with significantly increased MCP-1, MIP-1α, IFN-γ, TNF-α and PD-L1 mRNA. RSV infection in the preterm lung was characterized by increased epithelial thickening and periodic acid-Schiff staining, indicative of glycogen retention. Nitric oxide levels were decreased in lungs of infected preterm lambs compared to full-term lambs, indicating alternative macrophage activation. Although infection induced significant neutrophil recruitment into the lungs of preterm lambs, neutrophils produced less myeloperoxidase than those of full-term lambs, suggesting decreased functional activation.

Conclusions

Taken together, our data suggest that increased RSV load and inadequate immune response may contribute to the enhanced disease severity observed in the lungs of preterm lambs.

Metabolic event involved in the bactericidal activity of normal mouse macrophages

The intracellular bactericidal activity of normal mouse peritoneal macrophages requires a viable, actively metabolizing cell. The killing mechanism is apparently dependent upon an intact respiratory electron chain since inhibition of activity is achieved by anaerobiosis, cyanide, antimycin A, or amytal. By way of contrast, inhibitors of oxidative phosphorylation, glycolysis, the Krebs citric acid cycle, and the phosphogluconate oxidative pathway are without influence on this antibacterial activity. Various dyes and electron acceptors with selected reduction-oxidation values also inhibit killing. Although the bactericidal substance was not identified, the data are consistent with the hypothesis that the primary agent is hydrogen peroxide.

Myeloperoxidase: molecular mechanisms of action and their relevance to human health and disease

Myeloperoxidase (MPO) is a heme-containing peroxidase abundantly expressed in neutrophils and to a lesser extent in monocytes. Enzymatically active MPO, together with hydrogen peroxide and chloride, produces the powerful oxidant hypochlorous acid and is a key contributor to the oxygen-dependent microbicidal activity of phagocytes. In addition, excessive generation of MPO-derived oxidants has been linked to tissue damage in many diseases, especially those characterized by acute or chronic inflammation. It has become increasingly clear that MPO exerts effects that are beyond its oxidative properties. These properties of MPO are, in many cases, independent of its catalytic activity and affect various processes involved in cell signaling and cell-cell interactions and are, as such, capable of modulating inflammatory responses. Given these diverse effects, an increased interest has emerged in the role of MPO and its downstream products in a wide range of inflammatory diseases. In this article, our knowledge pertaining to the biologic role of MPO and its downstream effects and mechanisms of action in health and disease is reviewed and discussed.

DNase I inhibits a late phase of reactive oxygen species production in neutrophils

Neutrophils kill bacteria on extracellular complexes of DNA fibers and bactericidal proteins known as neutrophil extracellular traps (NETs). The NET composition and the bactericidal mechanisms they use are not fully understood. Here, we show that treatment with deoxyribonuclease (DNase I) impairs a late oxidative response elicited by Gram-positive and Gram-negative bacteria and also by phorbol ester. Isoluminol-dependent chemiluminescence elicited by opsonized Listeria monocytogenes-stimulated neutrophils was inhibited by DNase I, and the DNase inhibitory effect was also evident when phagocytosis was blocked, suggesting that DNase inhibits an extracellular mechanism of reactive oxygen species (ROS) generation. The DNase inhibitory effect was independent of actin polymerization. Phagocytosis and cell viability were not impaired by DNase I. Immunofluorescence analysis shows that myeloperoxidase is present on NETs. Furthermore, granular proteins were detected in NETs from Rab27a-deficient neutrophils which have deficient exocytosis, suggesting that exocytosis and granular protein distribution on NETs proceed by independent mechanisms. NADPH oxidase subunits were also detected on NETs, and the detection of extracellular trap-associated NADPH oxidase subunits was abolished by treatment with DNase I and dependent on cell stimulation. In vitro analyses demonstrate that MPO and NADPH oxidase activity are not directly inhibited by DNase I, suggesting that its effect on ROS production depends on NET disassembly. Altogether, our data suggest that inhibition of ROS production by microorganism-derived DNase would contribute to their ability to evade killing.

Neutrophil bleaching of GFP-expressing staphylococci: probing the intraphagosomal fate of individual bacteria

Successful host defense against bacteria such as Staphylococcus aureus (SA) depends on a prompt response by circulating polymorphonuclear leukocytes (PMN). Stimulated PMN create in their phagosomes an environment inhospitable to most ingested bacteria. Granules that fuse with the phagosome deliver an array of catalytic and noncatalytic antimicrobial peptides, while activation of the NADPH oxidase at the phagosomal membrane generates reactive oxygen species within the phagosome, including hypochlorous acid (HOCl), formed by the oxidation of chloride by the granule protein myeloperoxidase in the presence of H(2)O(2). In this study, we used SA-expressing cytosolic GFP to provide a novel probe of the fate of SA in human PMN. PMN bleaching of GFP in SA required phagocytosis, active myeloperoxidase, H(2)O(2) from the NADPH oxidase, and chloride. Not all ingested SA were bleached, and the number of cocci within PMN-retaining fluorescent GFP closely correlated with the number of viable bacteria remaining intracellularly. The percent of intracellular fluorescent and viable SA increased at higher multiplicity of infection and when SA presented to PMN had been harvested from the stationary phase of growth. These studies demonstrate that the loss of GFP fluorescence in ingested SA provides a sensitive experimental probe for monitoring biochemical events within individual phagosomes and for identifying subpopulations of SA that resist intracellular PMN cytotoxicity. Defining the molecular basis of SA survival within PMN should provide important insights into bacterial and host properties that limit PMN antistaphylococcal action and thus contribute to the pathogenesis of staphylococcal infection.

Attenuation of Allergic Contact Dermatitis Through the Endocannabinoid System

Allergic contact dermatitis affects about 5% of men and 11% of women in industrialized countries and is one of the leading causes for occupational diseases. In an animal model for cutaneous contact hypersensitivity, we show that mice lacking both known cannabinoid receptors display exacerbated allergic inflammation. In contrast, fatty acid amide hydrolase-deficient mice, which have increased levels of the endocannabinoid anandamide, displayed reduced allergic responses in the skin. Cannabinoid receptor antagonists exacerbated allergic inflammation, whereas receptor agonists attenuated inflammation. These results demonstrate a protective role of the endocannabinoid system in contact allergy in the skin and suggest a target for therapeutic intervention.

How neutrophils kill microbes

Neutrophils provide the first line of defense of the innate immune system by phagocytosing, killing, and digesting bacteria and fungi. Killing was previously believed to be accomplished by oxygen free radicals and other reactive oxygen species generated by the NADPH oxidase, and by oxidized halides produced by myeloperoxidase. We now know this is incorrect. The oxidase pumps electrons into the phagocytic vacuole, thereby inducing a charge across the membrane that must be compensated. The movement of compensating ions produces conditions in the vacuole conducive to microbial killing and digestion by enzymes released into the vacuole from the cytoplasmic granules.

Myeloperoxidase plays critical roles in killing Klebsiella pneumoniae and inactivating neutrophil elastase: effects on host defense

Activated neutrophils use myeloperoxidase (MPO) to generate an array of potent toxic oxidants. In the current studies we used genetically altered mice deficient in MPO to investigate the role of the enzyme in host defense against the Gram-negative bacterium Klebsiella pneumoniae, an important human pathogen. For comparison, we used mice deficient in the antimicrobial molecule, neutrophil elastase (NE). When challenged i.p., mice deficient in either MPO or NE were markedly more susceptible to bacterial infection and death. In vitro studies suggested that MPO impairs the morphology of bacteria in a distinctive way. Of importance, our in vitro studies found that MPO mediated oxidative inactivation of NE, an enzyme that has been widely implicated in the pathogenesis of various tissue-destructive diseases. This pathway of oxidative inactivation may be physiologically relevant, because activated neutrophils isolated from MPO-deficient mice exhibited increased elastase activity. Our observations provide strong evidence that MPO, like NE, is a key player in the killing of K. pneumoniae bacteria. They also suggest that MPO may modulate NE to protect the host from the tissue-degrading activity of this proteinase.

Myeloperoxidase: friend and foe

Neutrophilic polymorphonuclear leukocytes (neutrophils) are highly specialized for their primary function, the phagocytosis and destruction of microorganisms. When coated with opsonins (generally complement and/or antibody), microorganisms bind to specific receptors on the surface of the phagocyte and invagination of the cell membrane occurs with the incorporation of the microorganism into an intracellular phagosome. There follows a burst of oxygen consumption, and much, if not all, of the extra oxygen consumed is converted to highly reactive oxygen species. In addition, the cytoplasmic granules discharge their contents into the phagosome, and death of the ingested microorganism soon follows. Among the antimicrobial systems formed in the phagosome is one consisting of myeloperoxidase (MPO), released into the phagosome during the degranulation process, hydrogen peroxide (H2O2), formed by the respiratory burst and a halide, particularly chloride. The initial product of the MPO-H2O2-chloride system is hypochlorous acid, and subsequent formation of chlorine, chloramines, hydroxyl radicals, singlet oxygen, and ozone has been proposed. These same toxic agents can be released to the outside of the cell, where they may attack normal tissue and thus contribute to the pathogenesis of disease. This review will consider the potential sources of H2O2 for the MPO-H2O2-halide system; the toxic products of the MPO system; the evidence for MPO involvement in the microbicidal activity of neutrophils; the involvement of MPO-independent antimicrobial systems; and the role of the MPO system in tissue injury. It is concluded that the MPO system plays an important role in the microbicidal activity of phagocytes.

The NADPH oxidase of professional phagocytes--prototype of the NOX electron transport chain systems

The NADPH oxidase is an electron transport chain in "professional" phagocytic cells that transfers electrons from NADPH in the cytoplasm, across the wall of the phagocytic vacuole, to form superoxide. The electron transporting flavocytochrome b is activated by the integrated function of four cytoplasmic proteins. The antimicrobial function of this system involves pumping K+ into the vacuole through BKCa channels, the effect of which is to elevate the vacuolar pH and activate neutral proteases. A number of homologous systems have been discovered in plants and lower animals as well as in man. Their function remains to be established.

Nonmammalian vertebrate antibiotic peptides

With the exception of cyclostomes, all vertebrates share the common immune strategy of adaptive, highly specific immunity, based on the products of recombination-activating genes and recombined noninherited receptors for antigens. In addition, they have retained ancient vectors of innate immunity, such as antimicrobial peptides, which are widespread in all eukaryotic organisms and show a high degree of structural homology across most animal taxa. Recently, these substances have become the objects of intensive study for their outstanding bioactive properties with the aim to be applied as very efficient antibiotics, antimicrobials, and even cancerostatics in clinical practice.

Increased atherosclerosis in myeloperoxidase-deficient mice

Myeloperoxidase (MPO), a heme enzyme secreted by activated phagocytes, generates an array of oxidants proposed to play critical roles in host defense and local tissue damage. Both MPO and its reaction products are present in human atherosclerotic plaque, and it has been proposed that MPO oxidatively modifies targets in the artery wall. We have now generated MPO-deficient mice, and show here that neutrophils from homozygous mutants lack peroxidase and chlorination activity in vitro and fail to generate chlorotyrosine or to kill Candida albicans in vivo. To examine the potential role of MPO in atherosclerosis, we subjected LDL receptor-deficient mice to lethal irradiation, repopulated their marrow with MPO-deficient or wild-type cells, and provided them a high-fat, high-cholesterol diet for 14 weeks. White cell counts and plasma lipoprotein profiles were similar between the two groups at sacrifice. Cross-sectional analysis of the aorta indicated that lesions in MPO-deficient mice were about 50% larger than controls. Similar results were obtained in a genetic cross with LDL receptor-deficient mice. In contrast to advanced human atherosclerotic lesions, the chlorotyrosine content of aortic lesions from wild-type as well as MPO-deficient mice was essentially undetectable. These data suggest an unexpected, protective role for MPO-generated reactive intermediates in murine atherosclerosis. They also identify an important distinction between murine and human atherosclerosis with regard to the potential involvement of MPO in protein oxidation.

Effects of pH and chloride concentration on resonance Raman spectra of human myeloperoxidase and Raman microspectroscopic analysis of enzyme state in azurophilic granules

Resonance Raman spectra of human myeloperoxidase were examined at pH 3.3-10.5 in the absence and presence of chloride ions. Among the porphyrin vibrational bands, the core-size marker bands showed particularly large wavenumber downshifts on going from pH 8.7 to 5.3 with a transition midpoint at pH 6.5 in the absence of chloride ions. The chloride ions did not affect the spectrum at a pH below 5.3 and above 8.7 whereas an increase of chloride concentration at neutral pH caused spectral changes similar to those observed upon pH lowering. Analogous effects were also observed on the Raman intensity. In addition, the stretching mode of the bond between the heme Fe and proximal histidine shifted by -2 cm(-1) on going from pH 8.7 to 5.3. Decomposition of the nu(3) band revealed the presence of two components, which was confirmed by an isosbestic point in the absorption spectra. The observed spectral changes indicated the existence of alkaline and acidic forms of the enzyme. The pK of interconversion was 6.5, and it was increased by binding of chloride ions. The porphyrin core was slightly expanded in the acidic form compared to that in the alkaline form. A molecular mechanism of the porphyrin core expansion was proposed on the basis of the X-ray crystal structure. The pH-spectrum relationships obtained for the isolated enzyme were applied to in situ analysis of the state of myeloperoxidase in azurophilic granules of living neutrophils. The enzyme was stored in the acidic form and kept inactive in catalyzing HOCl production.

A comparative study on certain enzymes of the granulocyte from different ruminant species

In the present study the level of enzyme hydrolases (alkaline phosphatase, myeloperoxidase, elastase, arginase, lysozyme and beta-galactosidase) of polymorphonuclear cell (PMN) granules in different ruminant species and their release in response to activation was studied. Buffalo PMN alkaline phosphatase activity was higher (P < 0.01) than in PMNs of cattle and goats. Interestingly, myeloperoxidase was higher in cattle PMNs and least in goat PMNs (P < 0.01), a similar pattern was observed in the distribution of enzyme arginase. As far as lysozyme is concerned, its activity was significantly higher (P < 0.01) in PMNs of buffaloes than in the case of cattle and goat PMNs. On activation, these cells released MPO and elastase, in all the species studied, while lysozyme was secreted only in buffalo PMN cells. Activity of certain enzymes related to oxidant defence systems such as glutathione peroxidase and glutathione reductase were higher in cattle and goats compared to that in buffaloes. These observations are likely to have bearing on immunodefense roles played by PMNs and reflected differences among the ruminant species studied.

Involvement of superoxide and myeloperoxidase in oxygen-dependent killing of Staphylococcus aureus by neutrophils

We have used a quantitative assay that measures independent rate constants for phagocytosis and killing of Staphylococcus aureus to investigate the involvement of superoxide and myeloperoxidase in bacterial killing by human neutrophils. To inhibit superoxide-dependent processes, superoxide dismutase was cross-linked to immunoglobulin G and the conjugate was attached to the surface of S. aureus via protein A in its cell wall. Myeloperoxidase was inhibited with azide, and myeloperoxidase-deficient neutrophils were used. Adding the NADPH oxidase inhibitor diphenyleneiodonium, to prevent superoxide production, decreased the killing rate to 25%, indicating that oxidative killing mechanisms predominate in this system. The rate constant for killing of S. aureus with superoxide dismutase attached was 70% of that for control bacteria linked to inactivated enzyme. Superoxide dismutase had no effect in the presence of diphenyleneiodonium. The rate of killing was decreased to 33% in the presence of azide and to 40% with myeloperoxidase-deficient neutrophils. Superoxide dismutase had no effect in the presence of azide. On the assumption that the oxidative and nonoxidative components of killing can be considered separately, the oxidative rate was decreased by almost half by superoxide dismutase and was about six times lower when myeloperoxidase was inactive. We conclude that myeloperoxidase-dependent processes are strongly favored by human neutrophils as their prime mechanism of oxidative killing of S. aureus and that superoxide makes a direct contribution to killing. Our results also suggest that superoxide acts in conjunction with a myeloperoxidase-dependent pathway.

Neutrophil defects as risk factors for periodontal diseases

There are several hypotheses proposed for the etiologic mechanisms causing periodontal diseases. These include a paradigm in which all individuals are equally susceptible to one or several pathogenic bacteria; a second paradigm that holds that all bacteria are equally virulent and that host susceptibility determines onset of disease; or a combination of the above. In this review, we analyze the role of neutrophil dysfunction as a risk factor for the onset of periodontitis. Both intrinsic or genetically inherited abnormalities of neutrophils and acquired neutrophil abnormalities are considered. While a large body of data implicates neutrophil dysfunction, either intrinsic or acquired (bacterially or extrinsically induced), as a significant risk factor for the periodontal diseases, clear, prospective, longitudinal epidemiologic studies to evaluate this association remain to be performed.

Neutrophil function and oral disease

The pathological sequela of reduced neutrophil function in the oral cavity and the mechanisms behind dysfunction have added to our understanding of infectious diseases. Numerous examples have been given, and the overriding conclusion must be that any impairment of neutrophil function will lead to some degree of increased susceptibility to infection. Perhaps the tissue most sensitive to pathological changes in the oral cavity is the periodontium. In cases of severe neutrophil dysfunction, there is severe periodontal breakdown, but also in cases of "mild" neutrophil dysfunction, where there is no other infection, such as in individuals with LJP, there is severe periodontal breakdown. The molecular basis of neutrophil dysfunction is beginning to be understood in individuals with LJP, LAD, CGD, and AIDS. It is our hope that further research in this area will help to delineate the pathogenesis of these and other oral diseases.

The bactericidal effects of the respiratory burst and the myeloperoxidase system isolated in neutrophil cytoplasts

Neutrophil polymorphonuclear leucocytes kill bacteria by oxygen-dependent and oxygen-independent mechanisms. Many potentially toxic mechanisms have been described, but the complexity of the phagosomal environment and the synergy between oxidative and non-oxidative systems hamper the investigation of individual bactericidal mechanism in whole cells. Neutrophil cytoplasts are greatly depleted of granule proteins and permit the investigation of the bactericidal effects of the respiratory burst in isolation. In this study they have been used to examine the role of the respiratory burst and myeloperoxidase in oxygen-dependent killing of Staphylococcus aureus. Cytoplasts generated oxygen radicals at comparable rates to human neutrophils and phagocytosed but did not kill S. aureus. The selective reconstitution of the myeloperoxidase-hydrogen peroxide-halide system by coating bacteria with myeloperoxidase conferred on cytoplasts the ability to kill intracellular bacteria. However, extracellular killing by diffusible bactericidal factors was not detected in this system.

Primary Candida meningitis and chronic granulomatous disease

The occurrence of two rare entities in a single patient can be fortuitous or may signify some deeper relationship. A young boy was recently treated for primary Candida meningitis. Autopsy findings suggested to an experienced pathologist the presence of chronic granulomatous disease (CGD), unrecognized during his life. The patient's identical twin brother was tested and found to have the typical laboratory features of CGD. The literature on Candida meningitis was reviewed and 15 cases were discovered that apparently arose in the absence of recognized predisposing causes. All but one of these cases occurred in males, and most occurred during the first three decades of life. The case reports and literature review presented herein suggest that CGD should be suspected when a case of "primary" Candida meningitis is encountered.

Similarity of kinetics of three types of myeloperoxidase from human leukocytes and four types from HL-60 cells

Km values for H2O2 and Vmax values for three types of myeloperoxidase (MPO) from human leukocytes (MPO-I, -II, and -III) and four types from human myeloid leukemia HL-60 cells (MPO-IA, -IB, -II, and -III) were determined. Km values of human leukocyte MPOs decreased with increasing pH from 4.4 to 6.2 and increased with increasing NaCl concentration from 0.025 to 0.14 M. There was no significant difference among Km values of leukocyte MPO-I, -II, and -III. NaBr also showed a tendency similar to that of NaCl with regard to the effects of pH and halide concentration on Km values. However, Km values in the presence of NaBr were lower than those in the presence of NaCl. Effects of pH and NaCl concentration on Vmax values of MPO-I, -II, and -III were also examined. Vmax values of MPO-I, -II, and -III were higher at pH 4.9 and 5.4 and increased with increasing NaCl concentration. In addition, no difference was observed between Km values of leukocyte and those of HL-60 cells. MPO-IB, the half-molecular-weight enzyme of HL-60 cells, also had the same Km values as the others. Furthermore, inhibition of the activities of seven MPOs of leukocytes and HL-60 cells by H2O2 was similarly observed at concentrations above 1 mM at pH 5.4 but not at pH 4.4. These results indicate that there is no difference in the affinity to H2O2 among leukocyte MPO-I, -II, and -III and HL-60 cell MPO-IA, -IB, -II, and -III.

Neutrophil function and oral disease

The pathologic sequela of reduced neutrophil function have been reviewed. In each case, the mechanism for the reduction in function has been elaborated when known. Special emphasis has been placed upon the pathologic changes in the oral cavity as a result of neutrophil dysfunction. Numerous examples have been given, and the overriding conclusion must be that any impairment of neutrophil function will lead to some degree of increased susceptibility to infection. Perhaps the tissue most sensitive to pathologic changes in the oral cavity is the periodontium. In cases of severe neutrophil dysfunction there is severe periodontal breakdown. But also in cases of "mild" neutrophil dysfunction, where there is no other infection, such as in individuals with LJP, there is severe periodontal breakdown. The molecular basis of neutrophil dysfunction is beginning to be understood in individuals with LJP. It is our hope that further research in this area will help to delineate the pathogenesis of periodontal diseases.