Protein chlorination in neutrophil phagosomes and correlation with bacterial killing

Postantibiotic leukocyte enhancement-mediated reduction of intracellular bacteria by macrophages

INTRODUCTION

Potentiation of the bactericidal activities of leukocytes, including macrophages, upon antibacterial agent administration has been observed for several decades and is summarized as the postantibiotic leukocyte enhancement (PALE) theory. Antibiotics-induced bacterial sensitization to leukocytes is commonly recognized as the mechanism of PALE. However, the degree of sensitization drastically varies with antibiotic classes, and little is known about whether and how the potentiation of leukocytes contributes to PALE.

OBJECTIVES

In this study, we aim to develop a mechanistic understanding of PALE by investigating the immunoregulation of traditional antibiotics on macrophages.

METHODS

Interaction models between bacteria and macrophages were constructed to identify the effects of different antibiotics on the bactericidal activities of macrophages. Oxygen consumption rate, expression of oxidases, and antioxidants were then measured to evaluate the effects of fluoroquinolones (FQs) on the oxidative stress of macrophages. Furthermore, the modulation in endoplasmic reticulum stress and inflammation upon antibiotic treatment was detected to analyze the mechanisms. At last, the peritoneal infection model was utilized to verify the PALE in vivo.

RESULTS

Enrofloxacin significantly reduced the intracellular burden of diverse bacterial pathogens through promoting the accumulation of reactive oxygen species (ROS). The upregulated oxidative response accordingly reprograms the electron transport chain with decreased production of antioxidant enzymes to reduce internalized pathogens. Additionally, enrofloxacin modulated the expression and spatiotemporal localization of myeloperoxidase (MPO) to facilitate ROS accumulation to target invaded bacteria and downregulated inflammatory response to alleviate cellular injury.

CONCLUSION

Our findings demonstrate the crucial role of leukocytes in PALE, shedding light on the development of new host-directed antibacterial therapies and the design of rational dosage regimens.

In vitro cytotoxicity and antibacterial activity of hypochlorous acid antimicrobial agent

Background/purpose

Bacteria-associated oral diseases such as dental caries and periodontitis are widespread epidemics that cause oral pain and loss of function. The purpose of this study was to evaluate the in vitro cytotoxicity and antibacterial activity of different concentrations of hypochlorous acid (HOCl).

Materials and methods

Five different concentrations (100, 200, 300, 400, and 500 ppm) of HOCl were evaluated for their antimicrobial efficacy against Gram-negative (A. actinomycetcmcomitans and P. gingivalis) and Gram-positive bacteria (S. mutans and S. sanguinis) after treatment for 1 and 10 min. Sodium hypochlorite (NaOCl) and chlorhexidine (CHX) were used as positive controls. In addition, HOCl was examined for L929 cytotoxicity and RAW 264.7 growth.

Results

The bacteriostatic ratio of NaOCl was comparable to that of CHX and significantly (P < 0.05) higher than that of all HOCl solutions. Higher HOCl concentration had significantly (P < 0.05) higher antibacterial effect, and the bacteriostatic ratio of 10 min treatment was slightly higher than that of 1 min treatment. CHX and NaOCl seeded into L929 cells resulted in low cell viability with only 30-39%, much significantly (P < 0.05) lower than all HOCl groups (greater than 80%). All HOCl solutions met the recommendations of ISO 10993-5 and showed no cytotoxicity, although there was a concentration-dependent decrease in cell viability. All antimicrobial agents showed the same trend of response to RAW 264.7 as L929.

Conclusion

Within the limit of this study, 400 ppm HOCl disinfectant may be a potential antimicrobial candidate for mouthwash, endodontic irrigants, and periodontitis treatment.

Expression of RcrB confers resistance to hypochlorous acid in uropathogenic Escherichia coli

To eradicate bacterial pathogens, neutrophils are recruited to the sites of infection, where they engulf and kill microbes through the production of reactive oxygen and chlorine species (ROS/RCS). The most prominent RCS is the antimicrobial oxidant hypochlorous acid (HOCl), which rapidly reacts with various amino acid side chains, including those containing sulfur and primary/tertiary amines, causing significant macromolecular damage. Pathogens like uropathogenic Escherichia coli (UPEC), the primary causative agent of urinary tract infections, have developed sophisticated defense systems to protect themselves from HOCl. We recently identified the RcrR regulon as a novel HOCl defense strategy in UPEC. Expression of the rcrARB operon is controlled by the HOCl-sensing transcriptional repressor RcrR, which is oxidatively inactivated by HOCl resulting in the expression of its target genes, including rcrB. The rcrB gene encodes a hypothetical membrane protein, deletion of which substantially increases UPEC's susceptibility to HOCl. However, the mechanism behind protection by RcrB is unclear. In this study, we investigated whether (i) its mode of action requires additional help, (ii) rcrARB expression is induced by physiologically relevant oxidants other than HOCl, and (iii) expression of this defense system is limited to specific media and/or cultivation conditions. We provide evidence that RcrB expression is sufficient to protect E. coli from HOCl. Furthermore, RcrB expression is induced by and protects from several RCS but not from ROS. RcrB plays a protective role for RCS-stressed planktonic cells under various growth and cultivation conditions but appears to be irrelevant for UPEC's biofilm formation. IMPORTANCE Bacterial infections pose an increasing threat to human health, exacerbating the demand for alternative treatments. Uropathogenic Escherichia coli (UPEC), the most common etiological agent of urinary tract infections (UTIs), are confronted by neutrophilic attacks in the bladder, and must therefore be equipped with powerful defense systems to fend off the toxic effects of reactive chlorine species. How UPEC deal with the negative consequences of the oxidative burst in the neutrophil phagosome remains unclear. Our study sheds light on the requirements for the expression and protective effects of RcrB, which we recently identified as UPEC's most potent defense system toward hypochlorous acid (HOCl) stress and phagocytosis. Thus, this novel HOCl stress defense system could potentially serve as an attractive drug target to increase the body's own capacity to fight UTIs.

Effects of hypochlorous acid mouthwash on salivary bacteria including Staphylococcus aureus in patients with periodontal disease: a randomized controlled trial

BACKGROUND

The effects of a low concentration of hypochlorous acid (HOCl) mouthwash on salivary bacteria remained unclear. We aimed to evaluate the antibacterial effects of 100 ppm HOCl mouthwash on salivary bacteria, including Staphylococcus aureus (S. aureus), in patients with periodontal disease (PD).

METHODS

Patients with PD were randomized into mouthwash-only (MW, n = 26) and mouthwash with periodontal flosser (MWPF, n = 27) groups. Patients without PD were selected for the control group (n = 30). S. aureus culture and saliva samples (before and after the intervention) were collected for bacterial DNA extraction. A real-time polymerase chain reaction assay and serial dilutions of S. aureus culture and saliva samples were used to measure the salivary bacteria total count (SBTC) and confirm the antibacterial effects of the mouthwash using S. aureus.

RESULTS

No significant difference in demographic data was observed among the three groups. Before the intervention, the baseline SBTC of the MW and MWPF groups was significantly higher than that of the control group. After the mouthwash rinses, the SBTC data significantly changed in the MW and MWPF groups only (by 62.4% and 77.4%, respectively). After the base-2 log-transformation of the SBTC data, a similar trend was observed. Linear regression revealed that baseline SBTC and the MWPF intervention significantly affected SBTC reduction percentage by volume. After incubation with 10% (v/v) of mouthwash, the survival rates of 106 and 107 colony-forming units/mL of S. aureus were 0.51% ± 0.06% and 1.42% ± 0.37%, respectively.

CONCLUSIONS

These study results indicated that 100 ppm HOCl mouthwash treatment could effectively reduce SBTC in patients with PD and the abundance of S. aureus. It provides that the HOCl mouthwash can be an option for individuals to help control SBTC, especially in patients with PD.

TRIAL REGISTRATION

The study protocol was approved by the Institutional Review Board of Kaohsiung Medical University Hospital (KMUHIRB-F(I)-20200042) on 20/03/2020 and retrospectively registered at ClinicalTrial.gov (NCT05372835) on 13/05/2022.

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.

Expression of RcrB confers resistance to hypochlorous acid in uropathogenic Escherichia coli

To eradicate bacterial pathogens, neutrophils are recruited to the sites of infection, where they engulf and kill microbes through the production of reactive oxygen and chlorine species (ROS/RCS). The most prominent RCS is antimicrobial oxidant hypochlorous acid (HOCl), which rapidly reacts with various amino acids side chains, including those containing sulfur and primary/tertiary amines, causing significant macromolecular damage. Pathogens like uropathogenic Escherichia coli (UPEC), the primary causative agent of urinary tract infections (UTIs), have developed sophisticated defense systems to protect themselves from HOCl. We recently identified the RcrR regulon as a novel HOCl defense strategy in UPEC. The regulon is controlled by the HOCl-sensing transcriptional repressor RcrR, which is oxidatively inactivated by HOCl resulting in the expression of its target genes, including rcrB . rcrB encodes the putative membrane protein RcrB, deletion of which substantially increases UPEC's susceptibility to HOCl. However, many questions regarding RcrB's role remain open including whether (i) the protein's mode of action requires additional help, (ii) rcrARB expression is induced by physiologically relevant oxidants other than HOCl, and (iii) expression of this defense system is limited to specific media and/or cultivation conditions. Here, we provide evidence that RcrB expression is sufficient to E. coli 's protection from HOCl and induced by and protects from several RCS but not from ROS. RcrB plays a protective role for RCS-stressed planktonic cells under various growth and cultivation conditions but appears to be irrelevant for UPEC's biofilm formation.

IMPORTANCE

Bacterial infections pose an increasing threat to human health exacerbating the demand for alternative treatment options. UPEC, the most common etiological agent of urinary tract infections (UTIs), are confronted by neutrophilic attacks in the bladder, and must therefore be well equipped with powerful defense systems to fend off the toxic effects of RCS. How UPEC deal with the negative consequences of the oxidative burst in the neutrophil phagosome remains unclear. Our study sheds light on the requirements for the expression and protective effects of RcrB, which we recently identified as UPEC's most potent defense system towards HOCl-stress and phagocytosis. Thus, this novel HOCl-stress defense system could potentially serve as an attractive drug target to increase the body's own capacity to fight UTIs.

Superoxide: The enigmatic chemical chameleon in neutrophil biology

The burst of superoxide produced when neutrophils phagocytose bacteria is the defining biochemical feature of these abundant immune cells. But 50 years since this discovery, the vital role superoxide plays in host defense has yet to be defined. Superoxide is neither bactericidal nor is it just a source of hydrogen peroxide. This simple free radical does, however, have remarkable chemical dexterity. Depending on its environment and reaction partners, superoxide can act as an oxidant, a reductant, a nucleophile, or an enzyme substrate. We outline the evidence that inside phagosomes where neutrophils trap, kill, and digest bacteria, superoxide will react preferentially with the enzyme myeloperoxidase, not the bacterium. By acting as a cofactor, superoxide will sustain hypochlorous acid production by myeloperoxidase. As a substrate, superoxide may give rise to other forms of reactive oxygen. We contend that these interactions hold the key to understanding the precise role superoxide plays in neutrophil biology. State-of-the-art techniques in mass spectrometry, oxidant-specific fluorescent probes, and microscopy focused on individual phagosomes are needed to identify bactericidal mechanisms driven by superoxide. This work will undoubtably lead to fascinating discoveries in host defense and give a richer understanding of superoxide's varied biology.

Altered Mitochondrial Homeostasis during Systemic Lupus Erythematosus Impairs Neutrophil Extracellular Trap Formation Rendering Neutrophils Ineffective at Combating Staphylococcus aureus

Inflammation involves a delicate balance between pathogen clearance and limiting host tissue damage, and perturbations in this equilibrium promote disease. Patients suffering from autoimmune diseases, such as systemic lupus erythematosus (SLE), have higher levels of serum S100A9 protein and increased risk for infection. S100A9 is highly abundant within neutrophils and modulates antimicrobial activity in response to bacterial pathogens. We reasoned that increased serum S100A9 in SLE patients reflects accumulation of S100A9 protein in neutrophils and may indicate altered neutrophil function. In this study, we demonstrate elevated S100A9 protein within neutrophils from SLE patients, and MRL/lpr mice associates with lower mitochondrial superoxide, decreased suicidal neutrophil extracellular trap formation, and increased susceptibility to Staphylococcus aureus infection. Furthermore, increasing mitochondrial superoxide production restored the antibacterial activity of MRL/lpr neutrophils in response to S. aureus These results demonstrate that accumulation of intracellular S100A9 associates with impaired mitochondrial homeostasis, thereby rendering SLE neutrophils inherently less bactericidal.

Antimicrobial Activity of Neutrophils Against Mycobacteria

The mycobacterium genus contains a broad range of species, including the human pathogens M. tuberculosis and M. leprae. These bacteria are best known for their residence inside host cells. Neutrophils are frequently observed at sites of mycobacterial infection, but their role in clearance is not well understood. In this review, we discuss how neutrophils attempt to control mycobacterial infections, either through the ingestion of bacteria into intracellular phagosomes, or the release of neutrophil extracellular traps (NETs). Despite their powerful antimicrobial activity, including the production of reactive oxidants such as hypochlorous acid, neutrophils appear ineffective in killing pathogenic mycobacteria. We explore mycobacterial resistance mechanisms, and how thwarting neutrophil action exacerbates disease pathology. A better understanding of how mycobacteria protect themselves from neutrophils will aid the development of novel strategies that facilitate bacterial clearance and limit host tissue damage.

Mitochondrial calcium uniporter affects neutrophil bactericidal activity during Staphylococcus aureus infection

Neutrophils simultaneously restrict Staphylococcus aureus dissemination and facilitate bactericidal activity during infection through the formation of neutrophil extracellular traps (NETs). Neutrophils that produce higher levels of mitochondrial superoxide undergo enhanced terminal NET formation (suicidal NETosis) in response to S. aureus; however, mechanisms regulating mitochondrial homeostasis upstream of neutrophil antibacterial processes are not fully resolved. Here, we demonstrate that mitochondrial calcium uptake 1 (MICU1)-deficient (MICU1^-/-) neutrophils accumulate higher levels of calcium and iron within the mitochondria in a mitochondrial calcium uniporter (MCU)-dependent manner. Corresponding with increased ion flux through the MCU, mitochondrial superoxide production is elevated, thereby increasing the propensity for MICU1^-/- neutrophils to undergo suicidal NETosis rather than primary degranulation in response to S. aureus. Increased NET formation augments macrophage killing of bacterial pathogens. Similarly, MICU1^-/- neutrophils alone are not more antibacterial towards S. aureus, but rather enhanced suicidal NETosis by MICU1^-/- neutrophils facilitates increased bactericidal activity in the presence of macrophages. Similarly, mice with a deficiency in MICU1 restricted to cells expressing LysM exhibit lower bacterial burdens in the heart with increased survival during systemic S. aureus infection. Coinciding with the decrease in S. aureus burdens, MICU1^-/- neutrophils in the heart produced higher levels of mitochondrial superoxide and undergo enhanced suicidal NETosis. These results demonstrate that ion flux by the MCU affects the antibacterial function of neutrophils during S. aureus infection.

Neutrophil extracellular traps enhance macrophage killing of bacterial pathogens

Neutrophils and macrophages are critical to the innate immune response, but cooperative mechanisms used by these cells to combat extracellular pathogens are not well understood. This study reveals that S100A9-deficient neutrophils produce higher levels of mitochondrial superoxide in response to Staphylococcus aureus and, as a result, form neutrophil extracellular traps (suicidal NETosis). Increased suicidal NETosis does not improve neutrophil killing of S. aureus in isolation but augments macrophage killing. NET formation enhances antibacterial activity by increasing phagocytosis by macrophages and by transferring neutrophil-specific antimicrobial peptides to them. Similar results were observed in response to other phylogenetically distinct bacterial pathogens including Streptococcus pneumoniae and Pseudomonas aeruginosa, implicating this as an immune defense mechanism that broadly enhances antibacterial activity. These results demonstrate that achieving maximal bactericidal activity through NET formation requires macrophages and that accelerated and more robust suicidal NETosis makes neutrophils adept at increasing antibacterial activity, especially when A9 deficient.

Role of myeloperoxidase and oxidant formation in the extracellular environment in inflammation-induced tissue damage

The heme peroxidase family generates a battery of oxidants both for synthetic purposes, and in the innate immune defence against pathogens. Myeloperoxidase (MPO) is the most promiscuous family member, generating powerful oxidizing species including hypochlorous acid (HOCl). Whilst HOCl formation is important in pathogen removal, this species is also implicated in host tissue damage and multiple inflammatory diseases. Significant oxidant formation and damage occurs extracellularly as a result of MPO release via phagolysosomal leakage, cell lysis, extracellular trap formation, and inappropriate trafficking. MPO binds strongly to extracellular biomolecules including polyanionic glycosaminoglycans, proteoglycans, proteins, and DNA. This localizes MPO and subsequent damage, at least partly, to specific sites and species, including extracellular matrix (ECM) components and plasma proteins/lipoproteins. Biopolymer-bound MPO retains, or has enhanced, catalytic activity, though evidence is also available for non-catalytic effects. These interactions, particularly at cell surfaces and with the ECM/glycocalyx induce cellular dysfunction and altered gene expression. MPO binds with higher affinity to some damaged ECM components, rationalizing its accumulation at sites of inflammation. MPO-damaged biomolecules and fragments act as chemo-attractants and cell activators, and can modulate gene and protein expression in naïve cells, consistent with an increasing cycle of MPO adhesion, activity, damage, and altered cell function at sites of leukocyte infiltration and activation, with subsequent tissue damage and dysfunction. MPO levels are used clinically both diagnostically and prognostically, and there is increasing interest in strategies to prevent MPO-mediated damage; therapeutic aspects are not discussed as these have been reviewed elsewhere.

Mycobacterium smegmatis Resists the Bactericidal Activity of Hypochlorous Acid Produced in Neutrophil Phagosomes

Neutrophils are often the major leukocyte at sites of mycobacterial infection, yet little is known about their ability to kill mycobacteria. In this study we have investigated whether the potent antibacterial oxidant hypochlorous acid (HOCl) contributes to killing of Mycobacterium smegmatis when this bacterium is phagocytosed by human neutrophils. We found that M. smegmatis were ingested by neutrophils into intracellular phagosomes but were killed slowly. We measured a t _1/2 of 30 min for the survival of M. smegmatis inside neutrophils, which is 5 times longer than that reported for Staphylococcus aureus and 15 times longer than Escherichia coli Live-cell imaging indicated that neutrophils generated HOCl in phagosomes containing M. smegmatis; however, inhibition of HOCl production did not alter the rate of bacterial killing. Also, the doses of HOCl that are likely to be produced inside phagosomes failed to kill isolated bacteria. Lethal doses of reagent HOCl caused oxidation of mycothiol, the main low-m.w. thiol in this bacterium. In contrast, phagocytosed M. smegmatis maintained their original level of reduced mycothiol. Collectively, these findings suggest that M. smegmatis can cope with the HOCl that is produced inside neutrophil phagosomes. A mycothiol-deficient mutant was killed by neutrophils at the same rate as wild-type bacteria, indicating that mycothiol itself is not the main driver of M. smegmatis resistance. Understanding how M. smegmatis avoids killing by phagosomal HOCl could provide new opportunities to sensitize pathogenic mycobacteria to destruction by the innate immune system.

Induction of the reactive chlorine-responsive transcription factor RclR in Escherichia coli following ingestion by neutrophils

Neutrophils generate hypochlorous acid (HOCl) and related reactive chlorine species as part of their defence against invading microorganisms. In isolation, bacteria respond to reactive chlorine species by upregulating responses that provide defence against oxidative challenge. Key questions are whether these responses are induced when bacteria are phagocytosed by neutrophils, and whether this provides them with a survival advantage. We investigated RclR, a transcriptional activator of the rclABC operon in Escherichia coli that has been shown to be specifically activated by reactive chlorine species. We first measured induction by individual reactive chlorine species, and showed that HOCl itself activates the response, as do chloramines (products of HOCl reacting with amines) provided they are cell permeable. Strong RclR activation was seen in E. coli following phagocytosis by neutrophils, beginning within 5 min and persisting for 40 min. RclR activation was suppressed by inhibitors of NOX2 and myeloperoxidase, providing strong evidence that it was due to HOCl production in the phagosome. RclR activation demonstrates that HOCl, or a derived chloramine, enters phagocytosed bacteria in sufficient amount to induce this response. Although RclR was induced in wild-type bacteria following phagocytosis, we detected no greater sensitivity to neutrophil killing of mutants lacking genes in the rclABC operon.

Azithromycin potentiates avian IgY effect against Pseudomonas aeruginosa in a murine pulmonary infection model

Cystic fibrosis (CF) patients are at risk of acquiring chronic Pseudomonas aeruginosa lung infections. The biofilm mode of growth of P. aeruginosa induces tolerance to antibiotics and the host response; accordingly, treatment failure occurs. Supplemental azithromycin has proven beneficial in CF owing to potential immunomodulatory mechanisms. Clinical studies have demonstrated a reduction in exacerbations in CF patients by avian IgY anti-Pseudomonas immunotherapy. We hypothesise that azithromycin pre-treatment could potentiate the observed anti-Pseudomonas effect of IgY opsonisation in vivo. Evaluation of phagocytic cell capacity was performed using in vitro exposure of azithromycin pre-treated human polymorphonuclear neutrophils to IgY opsonised P. aeruginosa PAO3. A murine lung infection model using nasal planktonic P. aeruginosa inoculation and successive evaluation 24 h post-infection was used to determine lung bacteriology and subsequent pulmonary inflammation. Combined azithromycin treatment and IgY opsonisation significantly increased bacterial killing compared with the two single-treated groups and controls. In vivo, significantly increased bacterial pulmonary elimination was revealed by combining azithromycin and IgY. A reduction in the inflammatory markers mobiliser granulocyte colony-stimulating factor (G-CSF), macrophage inflammatory protein 2 (MIP-2) and interleukin 1 beta (IL-1β) paralleled this effect. Combination of azithromycin and anti-Pseudomonas IgY potentiated the killing and pulmonary elimination of P. aeruginosa in vitro and in vivo. The augmented effect of combinatory treatment with azithromycin and IgY constitutes a potential clinical application for improving anti-Pseudomonas strategies.

The In Vivo Toxicity and Antimicrobial Properties for Electrolyzed Oxidizing (EO) Water-Based Mouthwashes

The objective of this study was to verify the feasibility of electrolyzed oxidizing (EO) water as a mouthwash through the evaluation of its in vivo toxicity by embryonic zebrafish and antimicrobial efficacy against Streptococcus mutans (S. mutans).

METHODOLOGY

Each 1.5-3.0 g of sodium chloride (NaCl), sodium bromide (NaBr), or calcium chloride (CaCl2) were added into an electrolyzer with 300 mL of DD water to produce electrolyzed oxidizing (EO) water. A zebrafish embryo assay was used to evaluate acute toxicity of specimens. Antimicrobial property was conducted with 100 μL microbial count of 1 × 108 cfu/mL S. mutans to blend with each 10 mL specimen of chlorhexidine (CHX) gluconate or hypochlorous acid (HOCl) for various time points. The concentration of viable microorganisms was assessed according to individually standardized inoculum by a plate-count method.

RESULTS

Among the EO water produced from NaCl, NaBr, and CaCl2, the EO water from NaCl showed a relatively low mortality rate of zebrafish embryos and was chosen for a detailed investigation. The mortality rates for the groups treated with EO water containing 0.0125% and 0.0250% HOCl were not statically different from those of a negative control, however the mortality rate was 66.7 ± 26.2% in 0.2% CHX gluconate for the same treatment time of 0.5 min. All of the HOCl or 2.0% CHX gluconate groups showed >99.9% antimicrobial effectiveness against S. mutans; while the 0.2% CHX gluconate group showed a bacterial reduction rate of 87.5% and 97.1% for treatment times of 0.5 min and 1.0 min, respectively.

CONCLUSIONS

Except for the 0.2% CHX gluconate, all the HOCl specimens and 2.0% CHX gluconate revealed similar antimicrobial properties (>99.9%) against S. mutans. The EO water comprised of both 0.0125% and 0.0250% HOCl showed >99.9% antimicrobial efficacy but with little in vivo toxicity, illuminating the possibility as an alternative mouthwash for dental and oral care.

Bacterial Defense Systems against the Neutrophilic Oxidant Hypochlorous Acid

Neutrophils kill invading microbes and therefore represent the first line of defense of the innate immune response. Activated neutrophils assemble NADPH oxidase to convert substantial amounts of molecular oxygen into superoxide, which, after dismutation into peroxide, serves as the substrate for the generation of the potent antimicrobial hypochlorous acid (HOCl) in the phagosomal space. In this minireview, we explore the most recent insights into physiological consequences of HOCl stress. Not surprisingly, Gram-negative bacteria have evolved diverse posttranslational defense mechanisms to protect their proteins, the main targets of HOCl, from HOCl-mediated damage. We discuss the idea that oxidation of conserved cysteine residues and partial unfolding of its structure convert the heat shock protein Hsp33 into a highly active chaperone holdase that binds unfolded proteins and prevents their aggregation. We examine two novel members of the Escherichia coli chaperone holdase family, RidA and CnoX, whose thiol-independent activation mechanism differs from that of Hsp33 and requires N-chlorination of positively charged amino acids during HOCl exposure. Furthermore, we summarize the latest findings with respect to another bacterial defense strategy employed in response to HOCl stress, which involves the accumulation of the universally conserved biopolymer inorganic polyphosphate. We then discuss sophisticated adaptive strategies that bacteria have developed to enhance their survival during HOCl stress. Understanding bacterial defense and survival strategies against one of the most powerful neutrophilic oxidants may provide novel insights into treatment options that potentially compromise the ability of pathogens to resist HOCl stress and therefore may increase the efficacy of the innate immune response.

The decomposition of N-chloro amino acids of essential branched-chain amino acids: Kinetics and mechanism

The formation of N-chloro-amino acids is of outmost importance in water treatment technologies and also in vivo processes. These compounds are considered as secondary disinfectants and play important role in the defense mechanism against invading pathogens in biological systems. Adversary effects, such as apoptosis or necrosis are also associated with these compounds and the intermediates and final products formed during their decomposition. In the present study, the decomposition kinetics of the N-chloro derivatives of branched chain amino acids (BCAAs) - leucine, isoleucine, valine - were studied. On the basis of spectrophotometric measurements, it was confirmed that the decomposition proceeds via a spontaneous and an OH^- assisted path in each case: k_obs = k + k_OH[OH^-]. ¹H, ¹³C NMR and MS experiments were also performed to identify the products and to monitor the progress of the reactions. It was established that the pH independent and the [OH^-] dependent paths lead to the formation of the same aldehyde in each system (isovaleraldehyde, 2-methyl-butyraldehyde, and isobutyraldehyde) as a primary product. Under alkaline conditions, a portion of the aldehydes are converted into the corresponding Schiff-bases by the excess amino acid in a reversible process. A common mechanism was proposed for these reactions which postulates the formation of imines and hemiaminals as reactive intermediates.

Measurement of Respiratory Burst Products, Released or Retained, During Activation of Professional Phagocytes

Activation of professional phagocytes, potent microbial killers of our innate immune system, is associated with an increased cellular consumption of molecular oxygen (O₂). The O₂ molecules consumed are reduced by electrons delivered by a membrane localized NADPH-oxidase that initially generate one- and two electron reduced superoxide anions (O₂^-) and hydrogen peroxide (H₂O₂), respectively. These oxidants can then be processed into other highly reactive oxygen species (ROS) that can kill microbes, but that may also cause tissue destruction and drive other immune cells into apoptosis. The development of basic techniques to measure and quantify ROS generation by phagocytes is of great importance, and a large number of methods have been used for this purpose. A selection of methods (including chemiluminescence amplified by luminol or isoluminol, absorbance change following reduction of cytochrome c, and fluorescence increase upon oxidation of PHPA) are described in detail in this chapter with special emphasis on how to distinguish between ROS that are released extracellularly, and those that are retained within intracellular organelles. These techniques can be valuable tools in research spanning from basic phagocyte biology to diagnosis of diseases linked to the NADPH-oxidase and more clinically oriented research on innate immune mechanisms and inflammation.

Origin and pathophysiology of protein carbonylation, nitration and chlorination in age-related brain diseases and aging

Non-enzymatic protein modifications occur inevitably in all living systems. Products of such modifications accumulate during aging of cells and organisms and may contribute to their age-related functional deterioration. This review presents the formation of irreversible protein modifications such as carbonylation, nitration and chlorination, modifications by 4-hydroxynonenal, removal of modified proteins and accumulation of these protein modifications during aging of humans and model organisms, and their enhanced accumulation in age-related brain diseases.

Exposure of Pseudomonas aeruginosa to bactericidal hypochlorous acid during neutrophil phagocytosis is compromised in cystic fibrosis

Myeloperoxidase is a major neutrophil antimicrobial protein, but its role in immunity is often overlooked because individuals deficient in this enzyme are usually in good health. Within neutrophil phagosomes, myeloperoxidase uses superoxide generated by the NADPH oxidase to oxidize chloride to the potent bactericidal oxidant hypochlorous acid (HOCl). In this study, using phagocytosis assays and LC-MS analyses, we monitored GSH oxidation in Pseudomonas aeruginosa to gauge their exposure to HOCl inside phagosomes. Doses of reagent HOCl that killed most of the bacteria oxidized half the cells' GSH, producing mainly glutathione disulfide (GSSG) and other low-molecular-weight disulfides. Glutathione sulfonamide (GSA), a HOCl-specific product, was also formed. When neutrophils phagocytosed P. aeruginosa, half of the bacterial GSH was lost. Bacterial GSA production indicated that HOCl had reacted with the bacterial cells, oxidized their GSH, and was sufficient to be solely responsible for bacterial killing. Inhibition of NADPH oxidase and myeloperoxidase lowered GSA formation in the bacterial cells, but the bacteria were still killed, presumably by compensatory nonoxidative mechanisms. Of note, bacterial GSA formation in neutrophils from patients with cystic fibrosis (CF) was normal during early phagocytosis, but it was diminished at later time points, which was mirrored by a small decrease in bacterial killing. In conclusion, myeloperoxidase generates sufficient HOCl within neutrophil phagosomes to kill ingested bacteria. The unusual kinetics of phagosomal HOCl production in CF neutrophils confirm a role for the cystic fibrosis transmembrane conductance regulator in maintaining HOCl production in neutrophil phagosomes.

Heterogeneity of hypochlorous acid production in individual neutrophil phagosomes revealed by a rhodamine-based probe

The rhodamine-based probe R19-S has been shown to react with hypochlorous acid (HOCl) to yield fluorescent R19, but not with some other oxidants including hydrogen peroxide. Here, we further examined the specificity of R19-S and used it for real-time monitoring of HOCl production in neutrophil phagosomes. We show that it also reacts rapidly with hypobromous acid, bromamines, and hypoiodous acid, indicating that R19-S responds to these reactive halogen species as well as HOCl. Hypothiocyanous acid and taurine chloramine were unreactive, however, and ammonia chloramine and dichloramine reacted only very slowly. MS analyses revealed additional products from the reaction of HOCl with R19-S, including a chlorinated species as a minor product. Of note, phagocytosis of opsonized zymosan or Staphylococcus aureus by neutrophils was accompanied by an increase in R19 fluorescence. This increase depended on NADPH oxidase and myeloperoxidase activities, and detection of chlorinated R19-S confirmed its specificity for HOCl. Using live-cell imaging to track individual phagosomes in single neutrophils, we observed considerable heterogeneity among the phagosomes in the time from ingestion of a zymosan particle to when fluorescence was first detected, ranging from 1 to >30 min. However, once initiated, the subsequent fluorescence increase was uniform, reaching a similar maximum in ∼10 min. Our results confirm the utility of R19-S for detecting HOCl in real-time and provide definitive evidence that isolated neutrophils produce HOCl in phagosomes. The intriguing variability in the onset of HOCl production among phagosomes identified here could influence the way they kill ingested bacteria.

NADPH oxidases and ROS signaling in the gastrointestinal tract

Reactive oxygen species (ROS), initially categorized as toxic by-products of aerobic metabolism, have often been called a double-edged sword. ROS are considered indispensable when host defense and redox signaling is concerned and a threat in inflammatory or degenerative diseases. This generalization does not take in account the diversity of oxygen metabolites being generated, their physicochemical characteristics and their production by distinct enzymes in space and time. NOX/DUOX NADPH oxidases are the only enzymes solely dedicated to ROS production and the prime ROS producer for intracellular and intercellular communication due to their widespread expression and intricate regulation. Here we discuss new insights of how NADPH oxidases act via ROS as multifaceted regulators of the intestinal barrier in homeostasis, infectious disease and intestinal inflammation. A closer look at monogenic VEOIBD and commensals as ROS source supports the view of H₂O₂ as key beneficial messenger in the barrier ecosystem.

Melatonin prevents hypochlorous acid-mediated cyanocobalamin destruction and cyanogen chloride generation

Hypochlorous acid (HOCl) is a potent cytotoxic oxidant generated by the enzyme myeloperoxidase (MPO) in the presence of hydrogen peroxide (H₂ O₂ ) and chloride (Cl^- ). Elevated levels of HOCl play an important role in various pathological conditions through oxidative modification of several biomolecules. Recently, we have highlighted the ability of HOCl to mediate the destruction of the metal-ion derivatives of tetrapyrrole macrocyclic rings such as hemoproteins and vitamin B₁₂ (VB₁₂ ) derivatives. Destruction of cyanocobalamin, a common pharmacological form of VB₁₂ mediated by HOCl, results in the generation of toxic molecular products such as chlorinated derivatives, corrin ring cleavage products, the toxic blood agents cyanide (CN^- ) and cyanogen chloride (CNCl), and redox-active free cobalt. Here, we show that melatonin prevents HOCl-mediated cyanocobalamin destruction, using a combination of UV-Vis spectrophotometry, high-performance liquid chromatography analysis, and colorimetric CNCl assay. Identification of several melatonin oxidation products suggests that the protective role of melatonin against HOCl-mediated cyanocobalamin destruction and subsequent CNCl generation is at the expense of melatonin oxidation. Collectively, this work highlights that, in addition to acting as an antioxidant and as a MPO inhibitor, melatonin can also prevent VB₁₂ deficiency in inflammatory conditions such as cardiovascular and neurodegenerative diseases, among many others.

Interactions of staphyloxanthin and enterobactin with myeloperoxidase and reactive chlorine species

When neutrophils engulf bacteria, myeloperoxidase converts hydrogen peroxide to hypochlorous acid, which is toxic to all micro-organisms. It has been suggested that some pathogens have virulence factors that target myeloperoxidase to dampen the oxidative reactions of neutrophils. These virulence factors include staphyloxanthin, the golden pigment of Staphylococcus aureus, and enterobactin - a siderophore released by gram-negative bacteria. We investigated the potential of staphyloxanthin and enterobactin to shield bacteria from hypochlorous acid and related chloramines. Clinical strains of S. aureus with high levels of staphyloxanthin and related carotenoids were in general more resistant to low doses of hypochlorous acid than non-pigmented bacteria. But some non-pigmented strains were also resistant to the oxidant. Doses of reactive chlorine species that killed bacteria also bleached their carotenoids. Hypochlorous acid, NH₂Cl, and NHCl₂ bleached purified staphyloxanthin. When S. aureus were phagocytosed by neutrophils there was no discernible loss of staphyloxanthin. These data suggest that staphyloxanthin is capable of protecting bacteria from low doses of reactive chlorine species formed inside phagosomes. Enterobactin was not an inhibitor of myeloperoxidase. We conclude that staphyloxanthin may protect some bacterial strains against oxidative killing by neutrophils, but enterobactin will not inhibit the production of hypochlorous acid.

Kinetics and mechanism of the chromium(vi) catalyzed decomposition of hypochlorous acid at elevated temperature and high ionic strength

The decomposition of hypochlorous acid was studied under industrially relevant conditions (6.0 M NaClO₃, 80 °C). Chromium(vi) catalyzes the decomposition and the catalytically active form is CrO₄²⁻. A detailed kinetic model is proposed for the process.

Neutrophil granule proteins generate bactericidal ammonia chloramine on reaction with hydrogen peroxide

The neutrophil enzyme, myeloperoxidase, by converting hydrogen peroxide (H₂O₂) and chloride to hypochlorous acid (HOCl), provides important defense against ingested micro-organisms. However, there is debate about how efficiently HOCl is produced within the phagosome and whether its reactions with phagosomal constituents influence the killing mechanism. The phagosome is a small space surrounding the ingested organism, into which superoxide, H₂O₂ and high concentrations of proteins from cytoplasmic granules are released. Previous studies imply that HOCl is produced in the phagosome, but a large proportion should react with proteins before reaching the microbe. To mimic these conditions, we subjected neutrophil granule extract to sequential doses of H₂O₂. Myeloperoxidase in the extract converted all the H₂O₂ to HOCl, which reacted with the granule proteins. 3-Chlorotyrosine, protein carbonyls and large amounts of chloramines were produced. At higher doses of H₂O₂, the extract developed potent bactericidal activity against Staphylococcus aureus. This activity was due to ammonia monochloramine, formed as a secondary product from protein chloramines and dichloramines. Isolated myeloperoxidase and elastase also became bactericidal when modified with HOCl and antibacterial activity was seen with a range of species. Comparison of levels of protein modification in the extract and in phagosomes implies that a relatively low proportion of phagosomal H₂O₂ would be converted to HOCl, but there should be sufficient for substantial protein chloramine formation and some breakdown to ammonia monochloramine. It is possible that HOCl could kill ingested bacteria by an indirect mechanism involving protein oxidation and monochloramine formation.

Chloride flux in phagocytes

Phagocytes, such as neutrophils and macrophages, engulf microbes into phagosomes and launch chemical attacks to kill and degrade them. Such a critical innate immune function necessitates ion participation. Chloride, the most abundant anion in the human body, is an indispensable constituent of the myeloperoxidase (MPO)-H2 O2 -halide system that produces the potent microbicide hypochlorous acid (HOCl). It also serves as a balancing ion to set membrane potentials, optimize cytosolic and phagosomal pH, and regulate phagosomal enzymatic activities. Deficient supply of this anion to or defective attainment of this anion by phagocytes is linked to innate immune defects. However, how phagocytes acquire chloride from their residing environment especially when they are deployed to epithelium-lined lumens, and how chloride is intracellularly transported to phagosomes remain largely unknown. This review article will provide an overview of chloride protein carriers, potential mechanisms for phagocytic chloride preservation and acquisition, intracellular chloride supply to phagosomes for oxidant production, and methods to measure chloride levels in phagocytes and their phagosomes.

The NADPH Oxidase and Microbial Killing by Neutrophils, With a Particular Emphasis on the Proposed Antimicrobial Role of Myeloperoxidase within the Phagocytic Vacuole

This review is devoted to a consideration of the way in which the NADPH oxidase of neutrophils, NOX2, functions to enable the efficient killing of bacteria and fungi. It includes a critical examination of the current dogma that its primary purpose is the generation of hydrogen peroxide as substrate for myeloperoxidase-catalyzed generation of hypochlorite. Instead, it is demonstrated that NADPH oxidase functions to optimize the ionic and pH conditions within the vacuole for the solubilization and optimal activity of the proteins released into this compartment from the cytoplasmic granules, which kill and digest the microbes. The general role of other NOX systems as electrochemical generators to alter the pH and ionic composition in compartments on either side of a membrane in plants and animals will also be examined.

Epic Immune Battles of History: Neutrophils vs. Staphylococcus aureus

Neutrophils are the most abundant leukocytes in human blood and the first line of defense after bacteria have breached the epithelial barriers. After migration to a site of infection, neutrophils engage and expose invading microorganisms to antimicrobial peptides and proteins, as well as reactive oxygen species, as part of their bactericidal arsenal. Ideally, neutrophils ingest bacteria to prevent damage to surrounding cells and tissues, kill invading microorganisms with antimicrobial mechanisms, undergo programmed cell death to minimize inflammation, and are cleared away by macrophages. Staphylococcus aureus (S. aureus) is a prevalent Gram-positive bacterium that is a common commensal and causes a wide range of diseases from skin infections to endocarditis. Since its discovery, S. aureus has been a formidable neutrophil foe that has challenged the efficacy of this professional assassin. Indeed, proper clearance of S. aureus by neutrophils is essential to positive infection outcome, and S. aureus has developed mechanisms to evade neutrophil killing. Herein, we will review mechanisms used by S. aureus to modulate and evade neutrophil bactericidal mechanisms including priming, activation, chemotaxis, production of reactive oxygen species, and resolution of infection. We will also highlight how S. aureus uses sensory/regulatory systems to tailor production of virulence factors specifically to the triggering signal, e.g., neutrophils and defensins. To conclude, we will provide an overview of therapeutic approaches that may potentially enhance neutrophil antimicrobial functions.

The LRRC8A Mediated "Swell Activated" Chloride Conductance Is Dispensable for Vacuolar Homeostasis in Neutrophils

The dialysis of human and mouse neutrophils in patch clamp experiments in the conventional whole-cell mode induces the emergence of a chloride (Cl^-) current that appeared to be primarily regulated by cytoplasmic ionic strength. The characteristics of this current resembled that of the classical, and ubiquitous volume-sensitive outwardly rectifying Cl^- current: strong outward rectification, selectivity sequence of the Eisenman1 type, insensitivity to external pH and strong inhibition by tamoxifen, DCPIB and WW781. We show that this current is essentially supported by the leucine rich repeat containing 8 A (LRRC8A); the naturally occurring LRRC8A truncation mutant in ebo/ebo mice drastically reduced Cl^- conductance in neutrophils. Remarkably, the residual component presents a distinct pharmacology, but appears equally potentiated by reduced ionic strength. We have investigated the role of the LRRC8A-supported current in the ionic homeostasis of the phagosomal compartment. The vacuolar pH, measured using SNARF-1 labeled Candida albicans, normally rises because of NADPH oxidase activity, and this elevation is blocked by certain Cl^- channel inhibitors. However, the pH rise remains intact in neutrophils from the ebo/ebo mice which also demonstrate preserved phagocytic and respiratory burst capacities and normal-sized vacuoles. Thus, the LRRC8A-dependent conductance of neutrophils largely accounts for their “swell activated” Cl^- current, but is not required for homeostasis of the phagosomal killing compartment.

Myeloperoxidase targets oxidative host attacks to Salmonella and prevents collateral tissue damage

Host control of infections crucially depends on the capability to kill pathogens with reactive oxygen species (ROS). However, these toxic molecules can also readily damage host components and cause severe immunopathology. Here, we show that neutrophils use their most abundant granule protein, myeloperoxidase, to target ROS specifically to pathogens while minimizing collateral tissue damage. A computational model predicted that myeloperoxidase efficiently scavenges diffusible H₂O₂ at the surface of phagosomal Salmonella and converts it into highly reactive HOCl (bleach), which rapidly damages biomolecules within a radius of less than 0.1 μm. Myeloperoxidase-deficient neutrophils were predicted to accumulate large quantities of H₂O₂ that still effectively kill Salmonella, but most H₂O₂ would leak from the phagosome. Salmonella stimulation of neutrophils from normal and myeloperoxidase-deficient human donors experimentally confirmed an inverse relationship between myeloperoxidase activity and extracellular H₂O₂ release. Myeloperoxidase-deficient mice infected with Salmonella had elevated hydrogen peroxide tissue levels and exacerbated oxidative damage of host lipids and DNA, despite almost normal Salmonella control. These data show that myeloperoxidase has a major function in mitigating collateral tissue damage during antimicrobial oxidative bursts, by converting diffusible long-lived H₂O₂ into highly reactive, microbicidal and locally confined HOCl at pathogen surfaces.

Hypoxic regulation of neutrophil function and consequences for Staphylococcus aureus infection

Staphylococcal infection and neutrophilic inflammation can act in concert to establish a profoundly hypoxic environment. In this review we summarise how neutrophils and Staphylococcus aureus are adapted to function under hypoxic conditions, with a particular focus on the impaired ability of hypoxic neutrophils to effect Staphylococcus aureus killing.

Neutrophil-generated oxidative stress and protein damage in Staphylococcus aureus

Staphylococcus aureus is a ubiquitous, versatile and dangerous pathogen. It colonizes over 30% of the human population, and is one of the leading causes of death by an infectious agent. During S. aureus colonization and invasion, leukocytes are recruited to the site of infection. To combat S. aureus, leukocytes generate an arsenal of reactive species including superoxide, hydrogen peroxide, nitric oxide and hypohalous acids that modify and inactivate cellular macromolecules, resulting in growth defects or death. When S. aureus colonization cannot be cleared by the immune system, antibiotic treatment is necessary and can be effective. Yet, this organism quickly gains resistance to each new antibiotic it encounters. Therefore, it is in the interest of human health to acquire a deeper understanding of how S. aureus evades killing by the immune system. Advances in this field will have implications for the design of future S. aureus treatments that complement and assist the host immune response. In that regard, this review focuses on how S. aureus avoids host-generated oxidative stress, and discusses the mechanisms used by S. aureus to survive oxidative damage including antioxidants, direct repair of damaged proteins, sensing oxidant stress and transcriptional changes. This review will elucidate areas for studies to identify and validate future antimicrobial targets.

Reactive Oxygen Species and Neutrophil Function

Neutrophils are essential for killing bacteria and other microorganisms, and they also have a significant role in regulating the inflammatory response. Stimulated neutrophils activate their NADPH oxidase (NOX2) to generate large amounts of superoxide, which acts as a precursor of hydrogen peroxide and other reactive oxygen species that are generated by their heme enzyme myeloperoxidase. When neutrophils engulf bacteria they enclose them in small vesicles (phagosomes) into which superoxide is released by activated NOX2 on the internalized neutrophil membrane. The superoxide dismutates to hydrogen peroxide, which is used by myeloperoxidase to generate other oxidants, including the highly microbicidal species hypochlorous acid. NOX activation occurs at other sites in the cell, where it is considered to have a regulatory function. Neutrophils also release oxidants, which can modify extracellular targets and affect the function of neighboring cells. We discuss the identity and chemical properties of the specific oxidants produced by neutrophils in different situations, and what is known about oxidative mechanisms of microbial killing, inflammatory tissue damage, and signaling.

Viability and Effects on Bacterial Proteins by Oral Rinses with Hypochlorous Acid as Active Ingredient

Abstract: This study investigated the effect of hypochlorous acid (HOCl) rinses and chlorhexidine (CHX) on the bacterial viability of S. mutans, A. israelii, P. gingivalis, A. actinomycetemcomitans, E. corrodens, C. rectus, K. oxytoca, K. pneumoniae and E. cloacae. The percentage of live bacteria was tested by fluorescence method using Live/Dead kit(r) and BacLight (Molecular Probes(r)) and compared between groups by the Kruskal-Wallis and U Mann-Whitney tests with Bonferroni correction (p value<0.012). The effect of HOCl and CHX on total proteins of P. gingivalis and S. mutans was determined by SDS-PAGE. CHX showed a higher efficacy than HOCl against S. mutans, A. israelii, E. corrodens and E. cloacae (p<0.001) while HOCl was more effective than CHX against P. gingivalis, A. actinomycetemcomitans, C. rectus and K. oxytoca (p=0.001). CHX and HOCl had similar efficacy against K. pneumoniae. Proteins of P. gingivalis and S. mutans were affected similarly by HOCl and CHX. HOCl reduced the bacterial viability especially in periodontopathic bacteria, which may support its use in the control of subgingival biofilm in periodontal patients.

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.

Imidazole catalyzes chlorination by unreactive primary chloramines

Hypochlorous acid and simple chloramines (RNHCl) are stable biologically derived chlorinating agents. In general, the chlorination potential of HOCl is much greater than that of RNHCl, allowing it to oxidize or chlorinate a much wider variety of reaction partners. However, in this study we demonstrate by kinetic analysis that the reactivity of RNHCl can be dramatically promoted by imidazole and histidyl model compounds via intermediary formation of the corresponding imidazole chloramines. Two biologically relevant reactions were investigated--loss of imidazole-catalyzed chlorinating capacity and phenolic ring chlorination using fluorescein and the tyrosine analog, 4-hydroxyphenylacetic acid (HPA). HOCl reacted stoichiometrically with imidazole, N-acetylhistidine (NAH), or imidazoleacetic acid to generate the corresponding imidazole chloramines which subsequently decomposed. Chloramine (NH2Cl) also underwent a markedly accelerated loss in chlorinating capacity when NAH was present, although in this case N-α-acetylhistidine chloramine (NAHCl) did not accumulate, indicating that the catalytic intermediate must be highly reactive. Mixing HOCl with 1-methylimidazole (MeIm) led to very rapid loss in chlorinating capacity via formation of a highly reactive chlorinium ion (MeImCl(+)) intermediate; this behavior suggests that the reactive forms of the analogous imidazole chloramines are their conjugate acids, e.g., the imidazolechlorinium ion (HImCl(+)). HOCl-generated imidazole chloramine (ImCl) reacted rapidly with fluorescein in a specific acid-catalyzed second-order reaction to give 3'-monochloro and 3',5'-dichloro products. Equilibrium constants for the transchlorination reactions HOCl + HIm = H2O + ImCl and NH2Cl + HIm = NH3 + ImCl were estimated from the dependence of the rate constants on [HIm]/[HOCl] and literature data. Acid catalysis again suggests that the actual chlorinating agent is HImCl(+); consistent with this interpretation, MeIm markedly catalyzed fluorescein chlorination by HOCl. Time-dependent imidazole-catalyzed HPA chlorination by NH2Cl was also demonstrated by product analyses. Quantitative assessment of the data suggests that physiological levels of histidyl groups will react with primary chloramines to generate a flux of imidazole chloramine sufficient to catalyze biological chlorination via HImCl(+), particularly in environments that generate high concentrations of HOCl such as the neutrophil phagosome.

Protein quality control under oxidative stress conditions

Accumulation of reactive oxygen and chlorine species (RO/CS) is generally regarded to be a toxic and highly undesirable event, which serves as contributing factor in aging and many age-related diseases. However, it is also put to excellent use during host defense, when high levels of RO/CS are produced to kill invading microorganisms and regulate bacterial colonization. Biochemical and cell biological studies of how bacteria and other microorganisms deal with RO/CS have now provided important new insights into the physiological consequences of oxidative stress, the major targets that need protection, and the cellular strategies employed by organisms to mitigate the damage. This review examines the redox-regulated mechanisms by which cells maintain a functional proteome during oxidative stress. We will discuss the well-characterized redox-regulated chaperone Hsp33, and we will review recent discoveries demonstrating that oxidative stress-specific activation of chaperone function is a much more widespread phenomenon than previously anticipated. New members of this group include the cytosolic ATPase Get3 in yeast, the Escherichia coli protein RidA, and the mammalian protein α2-macroglobulin. We will conclude our review with recent evidence showing that inorganic polyphosphate (polyP), whose accumulation significantly increases bacterial oxidative stress resistance, works by a protein-like chaperone mechanism. Understanding the relationship between oxidative and proteotoxic stresses will improve our understanding of both host-microbe interactions and how mammalian cells combat the damaging side effects of uncontrolled RO/CS production, a hallmark of inflammation.