Seymour J. Klebanoff

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 consier 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.

Cutting Edge: Functional Interactions Between Toll-Like Receptor (TLR) 2 and TLR1 or TLR6 in Response to Phenol-Soluble Modulin

Toll-like receptor (TLR) 2 and TLR4 play important roles in the early, innate immune response to microbial challenge. TLR2 is preferentially involved in the inflammatory response to lipoteichoic acid, lipopeptides, and glycans from a variety of microbes, whereas TLR4 is essential for a complete response to LPSs. We report here that TLR2 transduces the response to phenol-soluble modulin, a factor secreted by Staphylococcus epidermidis. The TLR2-mediated response to this modulin was enhanced by TLR6 but inhibited by TLR1, indicating a functional interaction between these receptors. We also demonstrate that a response to phenol-soluble modulin mediated by TLR2 and TLR6 was more refractory to inhibition by TLR1 than one mediated by TLR2 alone.

Myeloperoxidase: Contribution to the Microbicidal Activity of Intact Leukocytes

Azide and, to a lesser extent, cyanide inhibit the microbicidal activity of myeloperoxidase and of intact normal leukocytes, but they have little or no effect on peroxidase-negative leukocytes. The contribution of the azide-sensitive (peroxidase-dependent?) systems to the total microbicidal activity of normal leukocytes is considerable. The azide-insensitive antimicrobial systems are more highly developed in peroxidase-negative leukocytes than in normal leukocytes, thus suggesting an adaptation.

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.

New mechanism for glomerular injury. Myeloperoxidase-hydrogen peroxide-halide system.

Reactive oxygen species, particularly hydrogen peroxide (H2O2), participate in neutrophil-mediated glomerulonephritis. However, the mechanism of H2O2 neptrotoxicity is unknown. Myeloperoxidase (MPO), a neutrophil cationic enzyme that localizes in glomeruli, can react with H2O2 and halides to form highly reactive products. We tested the hypothesis that the MPO-H2O2-halide system may induce glomerular injury by infusing MPO followed by H2O2 in a chloride-containing solution into the renal artery of rats. Controls received MPO or H2O2 alone. MPO-H2O2-perfused rats developed significant proteinuria, endothelial cell swelling, and epithelial cell foot process effacement, whereas control kidneys were normal. In the presence of free 125I, MPO-H2O2-perfused rats incorporated large amounts of 125I, localized to the glomerular basement membrane and mesangium by autoradiography, into glomeruli. Glomerular iodination was greatly decreased or absent in controls. The MPO-H2O2-halide system causes glomerular injury and may be important in neutrophil-mediated glomerulonephritis.

The relationship of hydrogen peroxide-producing lactobacilli to bacterial vaginosis and genital microflora in pregnant women.

Lactobacilli provide an important microbial defense against genital colonization by pathogens. The role of hydrogen peroxide (H2O2) in the control of genital microflora was explored in a cross-sectional study of 275 women in the second trimester of pregnancy. Vaginal cultures were obtained for detec

[52] Antimicrobial activity of myeloperoxidase

Publisher Summary Peroxidases when combined with H 2 O 2 and a halide (chloride, bromide, iodide, and pseudohalide thiocyanate) form a potent cytotoxic system, which contributes to the host defense against invading microorganisms and possibly tumor cells. Neutrophils and monocytes contain the same peroxidase (myeloperoxidase, MPO), and eosinophils a different peroxidase (eosinophil peroxidase, EPO), in cytoplasmic granules, and these enzymes are discharged into the phagosome following particle ingestion. Phagocytosis also is associated with a respiratory burst and much of the added oxygen consumed is converted to H 2 O 2 . Peroxidase, H 2 O 2 , and a halide interact in the phagosome to destroy the ingested organism. The components of the peroxidase system can also be released extracellularly where they may attack adjacent normal or malignant cells, uningested organisms, or soluble mediators. A variety of methods has been employed for the measurement of the toxicity of the peroxidase system. These methods depend on the nature of the target cell and include the measurement of replication in growth medium, Cr release, metabolic activity, and morphologic changes. This chapter focuses on bactericidal activity as measured by decrease in colony-forming units, using Escherichia coli as the target, MPO as the peroxidase, and chloride as the halide.

Reactive nitrogen intermediates and antimicrobial activity: Role of nitrite

The reactive nitrogen intermediate (RNI) nitric oxide (NO.) is formed from L-arginine by an NO. synthase and, following secondary reactions yielding additional toxic intermediates, nitrite (NO2-) and nitrate are formed. Nitrite, however, also has toxic properties. At acid pH, nitrous acid (HNO2) is bactericidal to Escherichia coli, in association with the loss of HNO2/NO2- and the uptake of oxygen, an effect which is increased by H2O2. Under conditions in which HNO2/NO2- +/- H2O2 were ineffective, the further addition of peroxidase (myeloperoxidase [MPO], eosinophil peroxidase, lactoperoxidase) or catalase resulted in bactericidal activity and the disappearance of HNO2/NO2-. Paradoxically, HNO2/NO2- also inhibited the bactericidal activity of MPO by the formation of a complex with MPO with a shift in the absorption spectrum, and by reaction with hypochlorous acid (HOCl) (the product of the chloride-supplemented MPO-H2O2 system), with loss of the bactericidal activity of HOCl and the disappearance of both HOCl and HNO2/NO2- from the reaction mixture. Thus, HNO2/NO2-, rather than being solely an end product of RNI formation, may influence antimicrobial activity either by acting alone, with H2O2, or with H2O2 and peroxidase as a source of toxic agents, or by inhibiting the peroxidase-mediated antimicrobial systems.

Peroxidase-Mediated Virucidal Systems

Peroxidase (myeloperoxidase or lactoperoxidase), hydrogen peroxide, and a halide such as iodide, bromide, or chloride form a potent virucidal system that is effective against polio and vaccinia viruis, particularly at a low pH. The peroxidase-halide-hydrogen peroxide system may contribute to the host defense against certain viral infections.

Human Formyl Peptide Receptor 2 Senses Highly Pathogenic Staphylococcus aureus

Virulence of emerging community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) and other highly pathogenic S. aureus strains depends on their production of phenol-soluble modulin (PSM) peptide toxins, which combine the capacities to attract and lyse neutrophils. The molecular basis of PSM-stimulated neutrophil recruitment has remained unclear. Here, we demonstrate that the human formyl peptide receptor 2 (FPR2/ALX), which has previously been implicated in control of endogenous inflammatory processes, senses PSMs at nanomolar concentrations and initiates proinflammatory neutrophil responses to CA-MRSA. Specific blocking of FPR2/ALX or deletion of PSM genes in CA-MRSA severely diminished neutrophil detection of CA-MRSA. Furthermore, a specific inhibitor of FPR2/ALX and of its functional mouse counterpart blocked PSM-mediated leukocyte infiltration in vivo in a mouse model. Thus, the innate immune system uses a distinct FPR2/ALX-dependent mechanism to specifically sense bacterial peptide toxins and detect highly virulent bacterial pathogens. FPR2/ALX represents an attractive target for new anti-infective or anti-inflammatory strategies.