Mice lacking myeloperoxidase are more susceptible to experimental autoimmune encephalomyelitis

MPO and its role in cancer, cardiovascular and neurological disorders: An update

Myeloperoxidase (MPO) is an enzyme that contains a heme group, found mostly in neutrophils and in small amounts in monocytes and plays a major role in their anti-microbial activity. However, excessive levels of MPO have been linked to various disorders and identified as a major cause of tissue destruction. Inhibiting its activity can reduce the severity and extent of tissue damage. Over activity of MPO during chronic inflammation has been shown to be involved in tumorigenesis by inducing a hyper-mutagenic environment through oxidant interaction with DNA, causing DNA modification. Vascular endothelium is one of the most important targets of MPO and high levels have been associated with increased rates of cardiomyopathy, ischemic stroke, heart failure, myocardial infarction, and atrial fibrillation. Therefore, it may be considered a therapeutic target in the treatment of cardiovascular disorders. MPO also participates in the pathogenesis of neurodegenerative diseases. For example, an increase in MPO levels has been observed in the brain tissue of patients with Alzheimer's, Multiple sclerosis (MS), and Parkinson's diseases. In Alzheimer's disease, active MPO is mostly found in the location of beta amyloids and microglia. Therefore, targeting MPO may be a potential treatment and prevention strategy for neurological disorders. This review will discuss MPO's physiological and pathological role in cancer, cardiovascular, and neurological disorders.

Myeloperoxidase-deficient mice exposed to Zymosan exhibit severe neutrophilia and anemia with enhanced granulopoiesis and reduced erythropoiesis, accompanied by pulmonary inflammation

We previously reported that myeloperoxidase-deficient (MPO-/-) mice develop more severe neutrophil-rich lung inflammation than wild-type mice following intranasal Zymosan administration. Interestingly, we found that these mutant mice with severe lung inflammation also displayed pronounced neutrophilia and anemia, characterized by increased granulopoiesis and decreased erythropoiesis in the bone marrow, compared to wild-type mice. This condition was associated with higher concentrations of granulocyte-colony stimulating factor (G-CSF) in both the lungs and serum, a factor known to enhance granulopoiesis. Neutrophils accumulating in the lungs of MPO-/- mice produced greater amounts of G-CSF than those in wild-type mice, indicating that they are a significant source of G-CSF. In vitro experiments using signal transduction inhibitors and Western blot analysis revealed that MPO-/- neutrophils express higher levels of G-CSF mRNA in response to Zymosan, attributed to the upregulation of the IκB kinase/nuclear factor (NF)-κB pathway and the extracellular-signal-regulated kinase/NF-κB pathway. These findings highlight MPO as a critical regulator of granulopoiesis and erythropoiesis in inflamed tissues.

Unravelling monocyte functions: from the guardians of health to the regulators of disease

Monocytes are a key component of the innate immune system. They undergo intricate developmental processes within the bone marrow, leading to diverse monocyte subsets in the circulation. In a state of healthy homeostasis, monocytes are continuously released into the bloodstream, destined to repopulate specific tissue-resident macrophage pools where they fulfil tissue-specific functions. However, under pathological conditions monocytes adopt various phenotypes to resolve inflammation and return to a healthy physiological state. This review explores the nuanced developmental pathways and functional roles that monocytes perform, shedding light on their significance in both physiological and pathological contexts.

Myeloperoxidase Gene Deletion Causes Drastic Microbiome Shifts in Mice and Does Not Mitigate Dextran Sodium Sulphate-Induced Colitis

Neutrophil-myeloperoxidase (MPO) is a heme-containing peroxidase which produces excess amounts of hypochlorous acid during inflammation. While pharmacological MPO inhibition mitigates all indices of experimental colitis, no studies have corroborated the role of MPO using knockout (KO) models. Therefore, we investigated MPO deficient mice in a murine model of colitis. Wild type (Wt) and MPO-deficient mice were treated with dextran sodium sulphate (DSS) in a chronic model of experimental colitis with three acute cycles of DSS-induced colitis over 63 days, emulating IBD relapse and remission cycles. Mice were immunologically profiled at the gut muscoa and the faecal microbiome was assessed via 16S rRNA amplicon sequencing. Contrary to previous pharmacological antagonist studies targeting MPO, MPO-deficient mice showed no protection from experimental colitis during cyclical DSS-challenge. We are the first to report drastic faecal microbiota shifts in MPO-deficient mice, showing a significantly different microbiome profile on Day 1 of treatment, with a similar shift and distinction on Day 29 (half-way point), via qualitative and quantitative descriptions of phylogenetic distances. Herein, we provide the first evidence of substantial microbiome shifts in MPO-deficiency, which may influence disease progression. Our findings have significant implications for the utility of MPO-KO mice in investigating disease models.

The Roles of Neutrophil-Derived Myeloperoxidase (MPO) in Diseases: The New Progress

Myeloperoxidase (MPO) is a heme-containing peroxidase, mainly expressed in neutrophils and, to a lesser extent, in monocytes. MPO is known to have a broad bactericidal ability via catalyzing the reaction of Cl- with H2O2 to produce a strong oxidant, hypochlorous acid (HOCl). However, the overproduction of MPO-derived oxidants has drawn attention to its detrimental role, especially in diseases characterized by acute or chronic inflammation. Broadly speaking, MPO and its derived oxidants are involved in the pathological processes of diseases mainly through the oxidation of biomolecules, which promotes inflammation and oxidative stress. Meanwhile, some researchers found that MPO deficiency or using MPO inhibitors could attenuate inflammation and tissue injuries. Taken together, MPO might be a promising target for both prognostic and therapeutic interventions. Therefore, understanding the role of MPO in the progress of various diseases is of great value. This review provides a comprehensive analysis of the diverse roles of MPO in the progression of several diseases, including cardiovascular diseases (CVDs), neurodegenerative diseases, cancers, renal diseases, and lung diseases (including COVID-19). This information serves as a valuable reference for subsequent mechanistic research and drug development.

Hypothiocyanous Acid Disrupts the Barrier Function of Brain Endothelial Cells

Inflammation is a common feature of neurological diseases. During neuroinflammation, neutrophils are recruited to the brain vasculature, where myeloperoxidase can produce hypochlorous acid and the less well-studied oxidant hypothiocyanous acid (HOSCN). In this study, we exposed primary brain endothelial cells (BECs) to HOSCN and observed a rapid loss of transendothelial electrical resistance (TEER) at sublethal concentrations. Decreased barrier function was associated with a loss of tight junctions at cellular contacts and a concomitant loss of dynamic microtubules. Both tight junction and cytoskeletal disruptions were visible within 30 min of exposure, whereas significant loss of TEER took more than 1 h. The removal of the HOSCN after 30 min prevented subsequent barrier dysfunction. These results indicate that BECs are sensitive to HOSCN, resulting in the eventual loss of barrier function. We hypothesise that this mechanism may be relevant in neutrophil transmigration, with HOSCN facilitating blood–brain barrier opening at the sites of egress. Furthermore, this mechanism may be a way through which neutrophils, residing in the vasculature, can influence neuroinflammation in diseases.

Expression profile analysis reveals hub genes that are associated with immune system dysregulation in primary myelofibrosis

OBJECTION

Primary myelofibrosis (PMF) is a familiar chronic myeloproliferative disease with an unfavorable prognosis. The effect of infection on the prognosis of patients with PMF is crucial. Immune system dysregulation plays a central role in the pathophysiology of PMF. To date, very little research has been conducted on the molecular mechanism of immune compromise in patients with PMF.

METHODS

To explore potential candidate genes, microarray datasets GSE61629 and 26049 were obtained from the Gene Expression Omnibus (GEO) database. The differentially expressed genes (DEGs) between PMF patients and normal individuals were evaluated, gene function was measured and a series of hub genes were identified. Several significant immune cells were selected via cell type enrichment analysis. The correlation between hub genes and significant immune cells was determined.

RESULTS

A total of 282 DEGs were found, involving 217 upregulated genes and 65 downregulated genes. Several immune cells were found to be reduced in PMF, such as CD4⁺ T cells, CD4⁺ Tems, CD4⁺ memory T cells. Gene Ontology (GO) enrichment analysis of DEGs reflected that most biological processes were associated with immune processes. Six hub genes, namely, HP, MPO, MMP9, EPB42, SLC4A1, and ALAS2, were identified, and correlation analysis revealed that these hub genes have a negative correlation with immune cell abundance.

CONCLUSIONS

Taken together, the gene expression profile of whole blood cells in PMF patients indicated a battery of immune events, and the DEGs and hub genes might contribute to immune system dysregulation.

Released Myeloperoxidase Attenuates Neutrophil Migration and Accumulation in Inflamed Tissue

Neutrophil (PMN) recruitment to sites of insult is critical for host defense, however excessive PMN activity and tissue accumulation can lead to exacerbated inflammation and injury. Myeloperoxidase (MPO) is a PMN azurophilic granule enzyme, which together with H₂O₂, forms a powerful antimicrobial system designed to kill ingested bacteria. Intriguingly, in addition to intracellular killing of invading microorganisms and extracellular tissue damage due generation of ROS, soluble MPO has been directly implicated in modulating cellular responses and tissue homeostasis. In the current work, we used several models of inflammation, murine and human PMNs and state-of-the-art intravital microscopy to examine the effect of MPO on PMN migration and tissue accumulation. We found that in the absence of functional MPO (MPO knockout, KO mice) inflammatory PMN tissue accumulation was significantly enhanced. We determined that the elevated numbers of PMNs in MPO knockout mice was not due to enhanced viability, but due to increased migratory ability. Acute PMN migration in models of zymosan-induced peritonitis or ligated intestinal loops induced by intraluminal administration of PMN-chemokine CXCL1 was increased over 2-fold in MPO KO compared to wild type (WT) mice. Using real-time intravital imaging of inflamed mouse cremaster muscle and ex vivo PMN co-culture with inflamed endothelial cells (ECs) we demonstrate that elevated migration of MPO KO mice was due to enhanced adhesive interactions. In contrast, addition of soluble recombinant MPO both in vivo and ex vivo diminished PMN adhesion and migration. Although MPO has been previously suggested to bind CD11b, we found no significant difference in CD11b expression in either resting or activated PMNs and further showed that the MPO binding to the PMN surface is not specific to CD11b. As such, our data identify MPO as a novel regulator of PMN trafficking in inflammation.

The Dual Role of Myeloperoxidase in Immune Response

The heme protein myeloperoxidase (MPO) is a major constituent of neutrophils. As a key mediator of the innate immune system, neutrophils are rapidly recruited to inflammatory sites, where they recognize, phagocytose, and inactivate foreign microorganisms. In the newly formed phagosomes, MPO is involved in the creation and maintenance of an alkaline milieu, which is optimal in combatting microbes. Myeloperoxidase is also a key component in neutrophil extracellular traps. These helpful properties are contrasted by the release of MPO and other neutrophil constituents from necrotic cells or as a result of frustrated phagocytosis. Although MPO is inactivated by the plasma protein ceruloplasmin, it can interact with negatively charged components of serum and the extracellular matrix. In cardiovascular diseases and many other disease scenarios, active MPO and MPO-modified targets are present in atherosclerotic lesions and other disease-specific locations. This implies an involvement of neutrophils, MPO, and other neutrophil products in pathogenesis mechanisms. This review critically reflects on the beneficial and harmful functions of MPO against the background of immune response.

On the Clinical Pharmacology of Reactive Oxygen Species

Reactive oxygen species (ROS) have been correlated with almost every human disease. Yet clinical exploitation of these hypotheses by pharmacological modulation of ROS has been scarce to nonexistent. Are ROS, thus, irrelevant for disease? No. One key misconception in the ROS field has been its consideration as a rather detrimental metabolic by-product of cell metabolism, and thus, any approach eliminating ROS to a certain tolerable level would be beneficial. We now know, instead, that ROS at every concentration, low or high, can serve many essential signaling and metabolic functions. This likely explains why systemic, nonspecific antioxidants have failed in the clinic, often with neutral and sometimes even detrimental outcomes. Recently, drug development has focused, instead, on identifying and selectively modulating ROS enzymatic sources that in a given constellation cause disease while leaving ROS physiologic signaling and metabolic functions intact. As sources, the family of NADPH oxidases stands out as the only enzyme family solely dedicated to ROS formation. Selectively targeting disease-relevant ROS-related proteins is already quite advanced, as evidenced by several phase II/III clinical trials and the first drugs having passed registration. The ROS field is expanding by including target enzymes and maturing to resemble more and more modern, big data-enhanced drug discovery and development, including network pharmacology. By defining a disease based on a distinct mechanism, in this case ROS dysregulation, and not by a symptom or phenotype anymore, ROS pharmacology is leaping forward from a clinical underperformer to a proof of concept within the new era of mechanism-based precision medicine. SIGNIFICANCE STATEMENT: Despite being correlated to almost every human disease, nearly no ROS modulator has been translated to the clinics yet. Here, we move far beyond the old-fashioned misconception of ROS as detrimental metabolic by-products and suggest 1) novel pharmacological targeting focused on selective modulation of ROS enzymatic sources, 2) mechanism-based redefinition of diseases, and 3) network pharmacology within the ROS field, altogether toward the new era of ROS pharmacology in precision medicine.

Mouse Models and Tools for the in vivo Study of Neutrophils

Neutrophils are the most abundant leukocytes in human blood and critical actors of the immune system. Many neutrophil functions and facets of their activity in vivo were revealed by studying genetically modified mice or by tracking fluorescent neutrophils in animals using imaging approaches. Assessing the roles of neutrophils can be challenging, especially when exact molecular pathways are questioned or disease states are interrogated that alter normal neutrophil homeostasis. This review discusses the main in vivo models for the study of neutrophils, their advantages and limitations. The side-by-side comparison underlines the necessity to carefully choose the right model(s) to answer a given scientific question, and exhibit caveats that need to be taken into account when designing experimental procedures. Collectively, this review suggests that at least two models should be employed to legitimately conclude on neutrophil functions.

The other myeloperoxidase: Emerging functions

Myeloperoxidase (MPO) is a member of the mammalian peroxidase family. It is mainly expressed in neutrophils, monocytes and macrophages. As a catalyzer of reactive oxidative species and radical species formation, it contributes to neutrophil bactericidal activity. Nevertheless MPO invalidation does not seem to have major health consequences in affected individuals. This suggests that MPO might have alternative functions supporting its conservation during evolution. We will review the available data supporting these non-canonical functions in terms of tissue specific expression, function and enzymatic activity. Thus, we discuss its cell type specific expression. We review in between others its roles in angiogenesis, endothelial (dys-) function, immune reaction, and inflammation. We summarize its pathological actions in clinical conditions such as cardiovascular disease and cancer.

Myeloperoxidase and Neurological Disorder: A Crosstalk

Myeloperoxidase (MPO) is a protein present in azurophilic granules, macrophages, and neutrophils that are released into extracellular fluid (ECF) during inflammation. MPO releases hypochlorous acid (HOCl) and other chlorinated species. It is derived from hydrogen peroxide (H₂O₂) showing response during inflammatory conditions and plays a role in the immune defense against pathogens. MPO may show unwanted effects by indirectly increasing the formation of reactive nitrogen species (RNS), reactive oxygen species (ROS), and tumor necrosis factor alpha (TNF-α) leading to inflammation and oxidative stress. As neuroinflammation is one of the inevitable biological components among most of neurological disorders, MPO and its receptor may be explored as candidates for future clinical interventions. The purpose of this review is to provide an overview of the pathophysiological characteristics of MPO and further explore the possibilities to target it for clinical use. Targeting MPO is promising and may open an avenue to act as a biomarker for diagnosis with defined risk stratification in patients with various neurological disorders.

Microglial Phenotypes and Functions in Multiple Sclerosis

Microglia are the resident immune cells that constantly survey the central nervous system. They can adapt to their environment and respond to injury or insult by altering their morphology, phenotype, and functions. It has long been debated whether microglial activation is detrimental or beneficial in multiple sclerosis (MS). Recently, the two opposing yet connected roles of microglial activation have been described with the aid of novel microglial markers, RNA profiling, and in vivo models. In this review, microglial phenotypes and functions in the context of MS will be discussed with evidence from both human pathological studies, in vitro and in vivo models. Microglial functional diversity-phagocytosis, antigen presentation, immunomodulation, support, and repair-will also be examined in detail. In addition, this review discusses the emerging evidence for microglia-related targets as biomarkers and therapeutic targets for MS.

Myeloperoxidase: Its role for host defense, inflammation, and neutrophil function

Myeloperoxidase (MPO) is a heme-containing peroxidase expressed mainly in neutrophils and to a lesser degree in monocytes. In the presence of hydrogen peroxide and halides, MPO catalyzes the formation of reactive oxygen intermediates, including hypochlorous acid (HOCl). The MPO/HOCl system plays an important role in microbial killing by neutrophils. In addition, MPO has been demonstrated to be a local mediator of tissue damage and the resulting inflammation in various inflammatory diseases. These findings have implicated MPO as an important therapeutic target in the treatment of inflammatory conditions. In contrast to its injurious effects at sites of inflammation, recent studies using animal models of various inflammatory diseases have demonstrated that MPO deficiency results in the exaggeration of inflammatory response, and that it affects neutrophil functions including cytokine production. Given these diverse effects, a growing interest has emerged in the role of this well-studied enzyme in health and disease.

Myeloperoxidase Inhibition Increases Neurogenesis after Ischemic Stroke

The relationship between inflammation and neurogenesis in stroke is currently not well understood. Focal ischemia enhances cell proliferation and neurogenesis in the neurogenic regions, including the subventricular zone (SVZ), dentate gyrus, as well as the non-neurogenic striatum, and cortex in the ischemic hemisphere. Myeloperoxidase (MPO) is a potent oxidizing enzyme secreted during inflammation by activated leukocytes, and its enzymatic activity is highly elevated after stroke. In this study, we investigated whether the inhibition of MPO activity by a specific irreversible inhibitor, 4-aminobenzoic acid hydrazide (ABAH) (MPO-/- mice) can increase neurogenesis after transient middle cerebral artery occlusion in mice. ABAH administration increased the number of proliferating bromodeoxyuridine (BrdU)-positive cells expressing markers for neural stems cells, astrocytes, neuroprogenitor cells (Nestin), and neuroblasts (doublecortin) in the ischemic SVZ, anterior SVZ, striatum, and cortex. MPO inhibition also increased levels of brain-derived neurotrophic factor, phosphorylation of cAMP response element-binding protein (Ser133), acetylated H3, and NeuN to promote neurogenesis in the ischemic SVZ. ABAH treatment also increased chemokine CXC receptor 4 expression in the ischemic SVZ. MPO-deficient mice treated with vehicle or ABAH both showed similar effects on the number of BrdU+ cells in the ischemic hemisphere, demonstrating that ABAH is specific to MPO. Taken together, our results underscore a detrimental role of MPO activity to postischemia neurogenesis and that a strategy to inhibit MPO activity can increase cell proliferation and improve neurogenesis after ischemic stroke.

Genes differentially expressed by methylprednisolone in vivo in CD4 T lymphocytes from multiple sclerosis patients: potential biomarkers

Intravenous methylprednisolone (IVMP) is the gold standard treatment in acute relapses of multiple sclerosis. Knowing the response to IVMP in advance could facilitate earlier selection of patients for subsequent courses of therapy. However, molecular mechanisms and changes in gene expression induced by methylprednisolone remain unknown. The aim of the study was to identify in vivo differentially expressed genes in relapsing-remitting multiple sclerosis patients after 3-6 days of treatment with IVMP. For this purpose, whole-genome transcription profiling of CD4+ T lymphocytes was performed before and after treatment with IVMP in 8 relapsing-remitting multiple sclerosis patients during relapse using Human GE 4x44K v2 microarrays. Differentially expressed genes were identified using a paired t test on GeneSpring v13.0 software. A P-value <0.001 and a twofold change were considered significant. Microarray data were confirmed using real-time PCR. Microarray revealed changes in gene expression: four genes were downregulated (B3GNT3, ZNF683, IFNG and TNF) and seven upregulated (DEFA4, CTSG, DEFA8P, AZU1, MPO, ELANE and PRTN3). Pathway analysis revealed the transforming growth factor-β signaling pathway to be affected. Comparison with previously published data on in vitro methylprednisolone-regulated genes showed that SMAD7, TNF and CHI3L1 were also downregulated in vivo in relapsing-remitting multiple sclerosis patients. In summary, we performed the first in vivo transcriptome analysis in CD4+ T lymphocytes before and after the treatment with IVMP in patients with multiple sclerosis. Identification of differentially expressed genes in patients receiving IVMP could improve our understanding of the molecular mechanisms underlying the therapeutic effects of IVMP and highlight potential biomarkers of the response to IVMP.

Association of apolipoprotein E and myeloperoxidase genotypes to clinical course of familial and sporadic multiple sclerosis

The importance of apolipoprotein E (ApoE) and myeloperoxidase (MPO) genotypes in the clinical characteristics of multiple sclerosis (MS) has been recently emphasized. In a large group of Polish patients we have tested the hypothesis that polymorphism in ApoE and MPO genes may influence the course of the disease. G enotypes were determined in 117 MS patients (74 females and 43 males; 99 sporadic and 18 familial cases) with mean EDSS of 3.6, mean age of 44.1 years, mean duration of the disease 12.8 years and mean onset of MS at 31.2 years, and in 100 healthy controls. The relationship between ApoE and MPO genes’ polymorphism and the MS activity as well as the defect of remyelination (diffuse demyelination) and brain atrophy on MRI were analysed. The ApoE o4 allele was not related to the disease course or the ApoE o2 to the intensity of demyelination on MRI. The genotype MPO G/G was found in all familial MS and in 57% (56/99) of sporadic cases. This genotype was also related to more pronounced brain atrophy on MRI. The MPO G/G subpopulation was characterized by a significantly higher proportion of patients with secondary progressive MS (PB- 0.05) and by a higher value of EDSS. A ccording to our results the MPO G allele is frequently found (in 96% of cases) among Polish patients with MS. More severe nervous tissue damage in the MPO G/G form can be explained by the mechanism of accelerated oxidative stress. It seems that MPO G/G genotype may be one of the genetic factors influencing the progression rate of disability in MS patients.

New insights into thiocyanate oxidation by human myeloperoxidase

Human myeloperoxidase (MPO) uses chloride and thiocyanate as physiological substrates at neutral pH. Oxidation of thiocyanate to hypothiocyanite mediated by the redox intermediate Compound I rapidly restores the ferric state of MPO. At low thiocyanate concentration and in the presence of hydrogen peroxide the observed reaction sequence is Compound I→ferric MPO→Compound II→MPO-cyanide complex, whereas at high thiocyanate concentrations and in the absence of H₂O₂ the only observed transition is Compound I→ferric MPO. The reaction of ferric MPO with hypothiocyanite directly forms the MPO-cyanide complex, whereas a transient product derived from the reaction between hypothiocyanite and hydrogen peroxide is demonstrated to mediate the conversion of ferric MPO to Compound II. Mechanisms for those reactions are discussed and proposed.

Lung Neutrophilia in Myeloperoxidase Deficient Mice during the Course of Acute Pulmonary Inflammation

Systemic inflammation accompanying diseases such as sepsis affects primarily lungs and induces their failure. This remains the most common cause of sepsis induced mortality. While neutrophils play a key role in pulmonary failure, the mechanisms remain incompletely characterized. We report that myeloperoxidase (MPO), abundant enzyme in neutrophil granules, modulates the course of acute pulmonary inflammatory responses induced by intranasal application of lipopolysaccharide. MPO deficient mice had significantly increased numbers of airway infiltrated neutrophils compared to wild-type mice during the whole course of lung inflammation. This was accompanied by higher levels of RANTES in bronchoalveolar lavage fluid from the MPO deficient mice. Other markers of lung injury and inflammation, which contribute to recruitment of neutrophils into the inflamed lungs, including total protein and other selected proinflammatory cytokines did not significantly differ in bronchoalveolar lavage fluid from the wild-type and the MPO deficient mice. Interestingly, MPO deficient neutrophils revealed a decreased rate of cell death characterized by phosphatidylserine surface expression. Collectively, the importance of MPO in regulation of pulmonary inflammation, independent of its putative microbicidal functions, can be potentially linked to MPO ability to modulate the life span of neutrophils and to affect accumulation of chemotactic factors at the inflammatory site.

Neutrophil-Mediated Regulation of Innate and Adaptive Immunity: The Role of Myeloperoxidase

Neutrophils are no longer seen as leukocytes with a sole function of being the essential first responders in the removal of pathogens at sites of infection. Being armed with numerous pro- and anti-inflammatory mediators, these phagocytes can also contribute to the development of various autoimmune diseases and can positively or negatively regulate the generation of adaptive immune responses. In this review, we will discuss how myeloperoxidase, the most abundant neutrophil granule protein, plays a key role in the various functions of neutrophils in innate and adaptive immunity.

Inhibition of myeloperoxidase at the peak of experimental autoimmune encephalomyelitis restores blood-brain barrier integrity and ameliorates disease severity

Oxidative stress is thought to contribute to disease pathogenesis in the central nervous system (CNS) disease multiple sclerosis (MS). Myeloperoxidase (MPO), a potent peroxidase that generates toxic radicals and oxidants, is increased in the CNS during MS. However, the exact mechanism whereby MPO drives MS pathology is not known. We addressed this question by inhibiting MPO in mice with experimental autoimmune encephalomyelitis (EAE) using our non-toxic MPO inhibitor N-acetyl lysyltyrosylcysteine amide (KYC). We found that therapeutic administration of KYC for 5 days starting at the peak of disease significantly attenuated EAE disease severity, reduced myeloid cell numbers and permeability of the blood-brain barrier. These data indicate that inhibition of MPO by KYC restores blood-brain barrier integrity thereby limiting migration of myeloid cells into the CNS that drive EAE pathogenesis. In addition, these observations indicate that KYC may be an effective therapeutic agent for the treatment of MS. We propose that during experimental autoimmune encephalomyelitis (EAE) onset macrophages and neutrophils migrate into the CNS and upon activation release myeloperoxidase (MPO) that promotes disruption of the blood-brain barrier (BBB) and disease progression. KYC restores BBB function by inhibiting MPO activity and in so doing ameliorates disease progression.

Pharmacology and Clinical Drug Candidates in Redox Medicine

SIGNIFICANCE

Oxidative stress is suggested to be a disease mechanism common to a wide range of disorders affecting human health. However, so far, the pharmacotherapeutic exploitation of this, for example, based on chemical scavenging of pro-oxidant molecules, has been unsuccessful.

RECENT ADVANCES

An alternative emerging approach is to target the enzymatic sources of disease-relevant oxidative stress. Several such enzymes and isoforms have been identified and linked to different pathologies. For some targets, the respective pharmacology is quite advanced, that is, up to late-stage clinical development or even on the market; for others, drugs are already in clinical use, although not for indications based on oxidative stress, and repurposing seems to be a viable option.

CRITICAL ISSUES

For all other targets, reliable preclinical validation and drug ability are key factors for any translation into the clinic. In this study, specific pharmacological agents with optimal pharmacokinetic profiles are still lacking. Moreover, these enzymes also serve largely unknown physiological functions and their inhibition may lead to unwanted side effects.

FUTURE DIRECTIONS

The current promising data based on new targets, drugs, and drug repurposing are mainly a result of academic efforts. With the availability of optimized compounds and coordinated efforts from academia and industry scientists, unambiguous validation and translation into proof-of-principle studies seem achievable in the very near future, possibly leading towards a new era of redox medicine.

Inhibition of NADPH oxidase activation reduces EAE-induced white matter damage in mice

Background

To evaluate the role of NADPH oxidase-mediated reactive oxygen species (ROS) production in multiple sclerosis pathogenesis, we examined the effects of apocynin, an NADPH oxidase assembly inhibitor, on experimental autoimmune encephalomyelitis (EAE).

Methods

EAE was induced by immunization with myelin oligodendrocyte glycoprotein (MOG (35-55)) in C57BL/6 female mice. Three weeks after initial immunization, the mice were analyzed for demyelination, immune cell infiltration, and ROS production. Apocynin (30 mg/kg) was given orally once daily for the entire experimental course or after the typical onset of clinical symptom (15 days after first MOG injection).

Results

Clinical signs of EAE first appeared on day 11 and reached a peak level on day 19 after the initial immunization. The daily clinical symptoms of EAE mice were profoundly reduced by apocynin. The apocynin-mediated inhibition of the clinical course of EAE was accompanied by suppression of demyelination, reduced infiltration by encephalitogenic immune cells including CD4, CD8, CD20, and F4/80-positive cells. Apocynin reduced MOG-induced pro-inflammatory cytokines in cultured microglia. Apocynin also remarkably inhibited EAE-associated ROS production and blood–brain barrier (BBB) disruption. Furthermore, the present study found that post-treatment with apocynin also reduced the clinical course of EAE and spinal cord demyelination.

Conclusions

These results demonstrate that apocynin inhibits the clinical features and neuropathological changes associated with EAE. Therefore, the present study suggests that inhibition of NADPH oxidase activation by apocynin may have a high therapeutic potential for treatment of multiple sclerosis pathogenesis.

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.

Myeloperoxidase deficiency ameliorates progression of chronic kidney disease in mice

Myeloperoxidase (MPO) is an enzyme expressed in neutrophils and monocytes/macrophages. Beside its well-defined role in innate immune defence, it may also be responsible for tissue damage. To identify the role of MPO in the progression of chronic kidney disease (CKD), we investigated CKD in a model of renal ablation in MPO knockout and wild-type mice. CKD was induced by 5/6 nephrectomy. Mice were followed for 10 wk to evaluate the impact of MPO deficiency on renal morbidity. Renal ablation induced CKD in wild-type mice with increased plasma levels of MPO compared with controls. No difference was found between MPO-deficient and wild-type mice regarding albuminuria 1 wk after renal ablation, indicating similar acute responses to renal ablation. Over the next 10 wk, however, MPO-deficient mice developed significantly less albuminuria and glomerular injury than wild-type mice. This was accompanied by a significantly lower renal mRNA expression of the fibrosis marker genes plasminogen activator inhibitor-I, collagen type III, and collagen type IV as well as matrix metalloproteinase-2 and matrix metalloproteinase-9. MPO-deficient mice also developed less renal inflammation after renal ablation, as indicated by a lower infiltration of CD3-positive T cells and F4/80-positive monocytes/macrophages compared with wild-type mice. In vitro chemotaxis of monocyte/macrophages isolated from MPO-deficient mice was impaired compared with wild-type mice. No significant differences were observed for mortality and blood pressure after renal ablation. In conclusion, these results demonstrate that MPO deficiency ameliorates renal injury in the renal ablation model of CKD in mice.

Endogenous myeloperoxidase is a mediator of joint inflammation and damage in experimental arthritis

OBJECTIVE

Myeloperoxidase (MPO) is implicated as a local mediator of tissue damage when released extracellularly in many chronic inflammatory diseases. The purpose of this study was to explore the role of endogenous MPO in experimental rheumatoid arthritis (RA).

METHODS

K/BxN serum-transfer arthritis was induced in C57BL/6 wild-type (WT) and MPO knockout (MPO(-/-) ) mice, and disease development was assessed. MPO activity was measured in joint tissues from mice with or without K/BxN arthritis. Collagen-induced arthritis (CIA) was induced in WT and MPO(-/-) mice, and disease development and immune responses were examined. MPO expression was assessed in synovial biopsy samples from patients with active RA, and the effect of MPO on synovial fibroblasts was tested in vitro.

RESULTS

MPO was up-regulated in the joints of mice with K/BxN arthritis, and MPO deficiency attenuated the severity of the disease without affecting circulating cytokine levels. In CIA, MPO(-/-) mice had enhanced CD4+ T cell responses and reduced frequency of regulatory T cells in the lymph nodes and spleen, as well as augmented interleukin-17A and diminished interferon-γ secretion by collagen-stimulated splenocytes, without an effect on circulating anticollagen antibody levels. Despite enhanced adaptive immunity in secondary lymphoid organs, CIA development was attenuated in MPO(-/-) mice. Intracellular and extracellular MPO was detected in the synovium of patients with active RA, and human MPO enhanced the proliferation and decreased the apoptosis of synovial fibroblasts in vitro.

CONCLUSION

MPO contributes to the development of arthritis despite suppressing adaptive immunity in secondary lymphoid organs. This suggests distinct effects of local MPO on arthritogenic effector responses.

Low-density lipoprotein modified by myeloperoxidase in inflammatory pathways and clinical studies

Oxidation of low-density lipoprotein (LDL) has a key role in atherogenesis. Among the different models of oxidation that have been studied, the one using myeloperoxidase (MPO) is thought to be more physiopathologically relevant. Apolipoprotein B-100 is the unique protein of LDL and is the major target of MPO. Furthermore, MPO rapidly adsorbs at the surface of LDL, promoting oxidation of amino acid residues and formation of oxidized lipoproteins that are commonly named Mox-LDL. The latter is not recognized by the LDL receptor and is accumulated by macrophages. In the context of atherogenesis, Mox-LDL accumulates in macrophages leading to foam cell formation. Furthermore, Mox-LDL seems to have specific effects and triggers inflammation. Indeed, those oxidized lipoproteins activate endothelial cells and monocytes/macrophages and induce proinflammatory molecules such as TNFα and IL-8. Mox-LDL may also inhibit fibrinolysis mediated via endothelial cells and consecutively increase the risk of thrombus formation. Finally, Mox-LDL has been involved in the physiopathology of several diseases linked to atherosclerosis such as kidney failure and consequent hemodialysis therapy, erectile dysfunction, and sleep restriction. All these issues show that the investigations of MPO-dependent LDL oxidation are of importance to better understand the inflammatory context of atherosclerosis.

Myeloperoxidase (MPO)-specific CD4+ T cells contribute to MPO-anti-neutrophil cytoplasmic antibody (ANCA) associated glomerulonephritis

Autoimmunity to the neutrophil enzyme myeloperoxidase (MPO) is an important cause of rapidly progressive glomerulonephritis, but the relative roles of MPO-specific anti-neutrophil cytoplasmic antibodies (MPO-ANCA) and autoreactive effector MPO-specific CD4(+) T cells are unclear. We confirmed that passive transfer of murine MPO-ANCA to agammaglobulinemic μMT mice immunized with OVA induces glomerular injury with capillary wall thickening, fibrinoid necrosis, mesangial cell proliferation, and periglomerular cell infiltration. Preimmunization of μMT mice with MPO induced MPO-specific CD4(+) T cells and significantly enhanced renal injury after MPO-ANCA transfer. CD4(+) T cell depletion prevented this augmentation of injury, confirming the importance of effector T cells in the development of MPO-ANCA associated glomerulonephritis. Therefore, MPO-ANCA can induce glomerulonephritis through both direct humoral mechanisms (recruitment of neutrophils and deposition of MPO) and indirectly by initiating MPO deposition in glomeruli, thereby directing effector CD4(+) T cell mediated injury. To confirm and support this data, we transferred T cells from MPO-immunized Mpo(-/-)μMT mice into Rag1(-/-) mice (control mice received ovalbumin specific T cells) and triggered injury by passive MPO-ANCA. Renal injury was significantly greater in mice transferred with T cells from MPO-immunized mice. These current studies demonstrate that MPO-ANCA induces injury via both humoral and cell mediated immune mechanisms.

Neutrophil myeloperoxidase regulates T-cell-driven tissue inflammation in mice by inhibiting dendritic cell function

Myeloperoxidase (MPO) is important in intracellular microbial killing by neutrophils but extracellularly causes tissue damage. Its role in adaptive immunity and T-cell-mediated diseases is poorly understood. Here, T-cell responses in lymph nodes (LNs) were enhanced by MPO deletion or in vivo inhibition, causing enhanced skin delayed-type hypersensitivity and antigen (Ag)-induced arthritis. Responses of adoptively transferred OT-II T cells were greater in MPO-deficient than wild-type (WT) recipients. MPO, deposited by neutrophils in LNs after Ag injection, interacted with dendritic cells (DCs) in vivo. Culture of murine or human DCs with purified MPO or neutrophil supernatant showed that enzymatically dependent MPO-mediated inhibition of DC activation occurs via MPO-generated reactive intermediates and involves DC Mac-1. Transfer of DCs cultured with WT, but not MPO-deficient, neutrophil supernatant attenuated Ag-specific immunity in vivo. MPO deficiency or in vivo inhibition increased DC activation in LNs after immunization. Studies with DQ-ovalbumin showed that MPO inhibits Ag uptake/processing by DCs. In vivo DC transfer and in vitro studies showed that MPO inhibits DC migration to LNs by reducing their expression of CCR7. Therefore, MPO, via its catalytic activity, inhibits the generation of adaptive immunity by suppressing DC activation, Ag uptake/processing, and migration to LNs to limit pathological tissue inflammation.

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.

Demyelinating diseases: myeloperoxidase as an imaging biomarker and therapeutic target

PURPOSE

To evaluate myeloperoxidase (MPO) as a newer therapeutic target and bis-5-hydroxytryptamide-diethylenetriaminepentaacetate-gadolinium (Gd) (MPO-Gd) as an imaging biomarker for demyelinating diseases such as multiple sclerosis (MS) by using experimental autoimmune encephalomyelitis (EAE), a murine model of MS.

MATERIALS AND METHODS

Animal experiments were approved by the institutional animal care committee. EAE was induced in SJL mice by using proteolipid protein (PLP), and mice were treated with either 4-aminobenzoic acid hydrazide (ABAH), 40 mg/kg injected intraperitoneally, an irreversible inhibitor of MPO, or saline as control, and followed up to day 40 after induction. In another group of SJL mice, induction was performed without PLP as shams. The mice were imaged by using MPO-Gd to track changes in MPO activity noninvasively. Imaging results were corroborated by enzymatic assays, flow cytometry, and histopathologic analyses. Significance was computed by using the t test or Mann-Whitney U test.

RESULTS

There was a 2.5-fold increase in myeloid cell infiltration in the brain (P = .026), with a concomitant increase in brain MPO level (P = .0087). Inhibiting MPO activity with ABAH resulted in decrease in MPO-Gd-positive lesion volume (P = .012), number (P = .009), and enhancement intensity (P = .03) at MR imaging, reflecting lower local MPO activity (P = .03), compared with controls. MPO inhibition was accompanied by decreased demyelination (P = .01) and lower inflammatory cell recruitment in the brain (P < .0001), suggesting a central MPO role in inflammatory demyelination. Clinically, MPO inhibition significantly reduced the severity of clinical symptoms (P = .0001) and improved survival (P = .0051) in mice with EAE.

CONCLUSION

MPO may be a key mediator of myeloid inflammation and tissue damage in EAE. Therefore, MPO could represent a promising therapeutic target, as well as an imaging biomarker, for demyelinating diseases and potentially for other diseases in which MPO is implicated.

Neutrophil myeloperoxidase: soldier and statesman

Myeloperoxidase (MPO) is a major protein constituent of the primary granules of vertebrate neutrophils. It catalyses the hydrogen peroxide-mediated oxidation of halide ions to hypohalous acids, especially HOCl. These reactive oxygen species can participate in a variety of secondary reactions, leading to modifications of amino acids and many types of biological macromolecules. The classic paradigm views MPO as a component of the phagocyte oxygen-dependent intracellular microbicidal system, and thus an important arm of the effector phase of innate immune responses. However, the limited immunodeficiency associated with lack of MPO in mouse and human models has challenged this paradigm. In this review we examine more recent information on the interaction between MPO, its bioreactive reaction products, and targets within the inflammatory microenvironment. We propose that two assumptions of the current model may require revisiting. First, many important targets of MPO modification are extracellular, rather than present only within the phagolysosome, such as various components of neutrophil extracellular traps. Second, we suggest that the pro-inflammatory pathological role of MPO may be a particular feature of chronic inflammation. In the physiological setting of acute neutrophil-mediated inflammation MPO may also form part of a negative feedback loop which down-regulates inflammation, limits tissue damage, and facilitates the switch from innate to adaptive immunity. This different perspective on this well-studied enzyme may usefully inform further research into its function in health and disease.

Discovery and validation of serum protein changes in type 1 diabetes patients using high throughput two dimensional liquid chromatography-mass spectrometry and immunoassays

Type 1 diabetes (T1D) is expected to cause significant changes in the serum proteome; however, few studies have systematically assessed the proteomic profile change associated with the disease. In this study, a semiquantitative spectral counting-based two dimensional liquid chromatography mass spectrometry platform was used to analyze serum samples from T1D patients and controls. In this discovery phase, significant differences were found for 21 serum proteins implicated in inflammation, oxidation, metabolic regulation, and autoimmunity. To assess the validity of these findings, six candidate proteins including adiponectin, insulin-like growth factor binding protein 2, serum amyloid protein A, C-reactive protein, myeloperoxidase, and transforming growth factor beta induced were selected for subsequent immune assays for 1139 T1D patients and 848 controls. A series of statistical analyses using cases and controls matched for age, sex, and genetic risk confirmed that T1D patients have significantly higher serum levels for four of the six proteins: adiponectin (odds ratio (OR) = 1.95, p = 10(-27)), insulin-like growth factor binding protein 2 (OR = 2.02, p < 10(-20)), C-reactive protein (OR = 1.13, p = 0.007), serum amyloid protein A (OR = 1.51, p < 10(-16)); whereas the serum levels were significantly lower in patients than controls for the two other proteins: transforming growth factor beta induced (OR = 0.74, p < 10(-5)) and myeloperoxidase (OR = 0.51, p < 10(-41)). Compared with subjects in the bottom quartile, subjects in the top quartile for adiponectin (OR = 6.29, p < 10(-37)), insulin-like growth factor binding protein 2 (OR = 7.95, p < 10(-46)), C-reactive protein (OR = 1.38, p = 0.025), serum amyloid protein A (OR = 3.36, p < 10(-16)) had the highest risk of T1D, whereas subjects in the top quartile of transforming growth factor beta induced (OR = 0.41, p < 10(-11)) and myeloperoxidase (OR = 0.10, p < 10(-43)) had the lowest risk of T1D. These findings provided valuable information on the proteomic changes in the sera of T1D patients.

Dual functionality of myeloperoxidase in rotenone-exposed brain-resident immune cells

Rotenone exposure has emerged as an environmental risk factor for inflammation-associated neurodegenerative diseases. However, the underlying mechanisms responsible for the harmful effects of rotenone in the brain remain poorly understood. Herein, we report that myeloperoxidase (MPO) may have a potential regulatory role in rotenone-exposed brain-resident immune cells. We show that microglia, unlike neurons, do not undergo death; instead, they exhibit distinctive activated properties under rotenone-exposed conditions. Once activated by rotenone, microglia show increased production of reactive oxygen species, particularly HOCl. Notably, MPO, an HOCl-producing enzyme that is undetectable under normal conditions, is significantly increased after exposure to rotenone. MPO-exposed glial cells also display characteristics of activated cells, producing proinflammatory cytokines and increasing their phagocytic activity. Interestingly, our studies with MPO inhibitors and MPO-knockout mice reveal that MPO deficiency potentiates, rather than inhibits, the rotenone-induced activated state of glia and promotes glial cell death. Furthermore, rotenone-triggered neuronal injury was more apparent in co-cultures with glial cells from Mpo(-/-) mice than in those from wild-type mice. Collectively, our data provide evidence that MPO has dual functionality under rotenone-exposed conditions, playing a critical regulatory role in modulating pathological and protective events in the brain.

Modulation of arachidonic and linoleic acid metabolites in myeloperoxidase-deficient mice during acute inflammation

Acute inflammation is a common feature of many life-threatening pathologies, including septic shock. One hallmark of acute inflammation is the peroxidation of polyunsaturated fatty acids forming bioactive products that regulate inflammation. Myeloperoxidase (MPO) is an abundant phagocyte-derived hemoprotein released during phagocyte activation. Here, we investigated the role of MPO in modulating biologically active arachidonic acid (AA) and linoleic acid (LA) metabolites during acute inflammation. Wild-type and MPO-knockout (KO) mice were exposed to intraperitoneally injected endotoxin for 24 h, and plasma LA and AA oxidation products were comprehensively analyzed using a liquid chromatography-mass spectrometry method. Compared to wild-type mice, MPO-KO mice had significantly lower plasma levels of LA epoxides and corresponding LA- and AA-derived fatty acid diols. AA and LA hydroxy intermediates (hydroxyeicosatetraenoic and hydroxyoctadecadienoic acids) were also significantly lower in MPO-KO mice. Conversely, MPO-deficient mice had significantly higher plasma levels of cysteinyl-leukotrienes with well-known proinflammatory properties. In vitro experiments revealed significantly lower amounts of AA and LA epoxides, LA- and AA-derived fatty acid diols, and AA and LA hydroxy intermediates in stimulated polymorphonuclear neutrophils isolated from MPO-KO mice. Our results demonstrate that MPO modulates the balance of pro- and anti-inflammatory lipid mediators during acute inflammation and, in this way, may control acute inflammatory diseases.

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.

Systems-based medicine approaches to understand and treat complex diseases. The example of multiple sclerosis

Systems medicine is an emerging concept that acknowledges the complexity of a multitude of non-linear interactions among molecular and physiological variables. Under this new paradigm, rather than a collection of symptoms, diseases are seen as the product of deviations from a robust steady state compatible with life. This concept requires the incorporation of mathematics and physics to the more classical arsenal of physiology and molecular biology with which physicians are trained today. This review explores the diverse types of information that can be accumulated towards the understanding of multiple sclerosis (MS), a complex autoimmune disease that targets the central nervous system (CNS). The challenge of data integration and modeling of dynamical systems is discussed in the context of disease susceptibility and response to treatment. A theoretical framework that supports the use of combination therapy is also presented.

The genetics of clinical outcome in multiple sclerosis

Multiple sclerosis (MS) is a common inflammatory disease of the central nervous system (CNS), the clinical course of which varies considerably between patients. Genetic complexity and interactions with as yet unknown environmental factors have hindered researchers from fully elucidating the aetiology of the disease. In addition to influencing disease susceptibility, epidemiological evidence suggests that genetic factors may affect phenotypic expression of the disease. Genes that affect clinical outcome may be more effective therapeutic targets than those which determine susceptibility. We present in this review a comprehensive survey of the genes (both MHC- and non-MHC-related) that have been investigated for their role in disease outcome in MS. Recent studies implicating the role of the genotype and epistatic interactions in the MHC in determining outcome are highlighted.

Myeloperoxidase binds to non-vital spermatozoa on phosphatidylserine epitopes

The heme protein myeloperoxidase is released from stimulated polymorphonuclear leukocytes, a cell species found in increasing amounts in the male and female genital tract of patients with genital tract inflammations. Myeloperoxidase binds only to a fraction of freshly prepared human spermatozoa. The number of spermatozoa able to bind myeloperoxidase raised considerably in samples containing pre-damaged cells or in acrosome-reacted samples. In addition, myeloperoxidase released from zymosan-stimulated polymorphonuclear leukocytes was also able to bind to pre-damaged spermatozoa. The ability of spermatozoa to bind myeloperoxidase coincided with the binding of annexin V to externalized phosphatidylserine epitopes indicating the loss of plasma membrane integrity and with the incorporation of ethidium homodimer I. Myeloperoxidase did not interact with intact spermatozoa. Annexin V and myeloperoxidase bind to the same binding sites as verified by double fluorescence techniques, flowcytometry analyses as well as competition experiments. We demonstrated also that myeloperoxidase is eluted together with pure phosphatidylserine liposomes or liposomes composed of phosphatidylserine and phosphatidylcholine in gel filtration, but not with pure phosphatidylcholine liposomes. In conclusion, myeloperoxidase interacts with apoptotic spermatozoa via binding to externalized phosphatidylserine indicating a yet unknown role of this protein in recognition and removal of apoptotic cells during inflammation.

Myeloperoxidase-targeted imaging of active inflammatory lesions in murine experimental autoimmune encephalomyelitis

Inflammatory demyelinating plaques are the pathologic hallmark of active multiple sclerosis and often precede clinical manifestations. Non-invasive early detection of active plaques would thus be crucial in establishing pre-symptomatic diagnosis and could lead to early preventive treatment strategies. Using murine experimental autoimmune encephalomyelitis as a model of multiple sclerosis, we demonstrate that a prototype paramagnetic myeloperoxidase (MPO) sensor can detect and confirm more, smaller, and earlier active inflammatory lesions in living mice by in vivo MRI. We show that MPO expression corresponded with areas of inflammatory cell infiltration and demyelination, and higher MPO activity as detected by MPO imaging, biochemical assays, and histopathological analyses correlated with increased clinical disease severity. Our findings present a potential new translational approach for specific non-invasive inflammatory plaque imaging. This approach could be used in longitudinal studies to identify active demyelinating plaques as well as to more accurately track disease course following treatment in clinical trials.

How human neutrophils kill and degrade microbes: an integrated view

Neutrophils constitute the dominant cell in the circulation that mediates the earliest innate immune human responses to infection. The morbidity and mortality from infection rise dramatically in patients with quantitative or qualitative neutrophil defects, providing clinical confirmation of the important role of normal neutrophils for human health. Neutrophil-dependent anti-microbial activity against ingested microbes represents the collaboration of multiple agents, including those prefabricated during granulocyte development in the bone marrow and those generated de novo following neutrophil activation. Furthermore, neutrophils cooperate with extracellular agents as well as other immune cells to optimally kill and degrade invading microbes. This brief review focuses attention on two examples of the integrated nature of neutrophil-mediated anti-microbial action within the phagosome. The importance and complexity of myeloperoxidase-mediated events illustrate a collaboration of anti-microbial responses that are endogenous to the neutrophil, whereas the synergy between the phagocyte NADPH (nicotinamide adenine dinucleotide phosphate) oxidase and plasma-derived group IIA phospholipase A(2) exemplifies the collective effects of the neutrophil with an exogenous factor to achieve degradation of ingested staphylococci.

Endogenous myeloperoxidase promotes neutrophil-mediated renal injury, but attenuates T cell immunity inducing crescentic glomerulonephritis

Myeloperoxidase (MPO) is an enzyme that is found in neutrophils and monocytes/macrophages. Intracellularly, it plays a major role in microbial killing, but extracellularly, it may cause host tissue damage. The role of endogenous MPO was studied during neutrophil-mediated (heterologous) and T helper 1 (Th1)/macrophage-mediated (autologous) phases of crescentic glomerulonephritis. Glomerulonephritis was induced in C57BL/6 wild-type (WT) and MPO-deficient (MPO(-/-)) mice by intravenous injection of sheep anti-mouse glomerular basement membrane globulin. MPO activity was increased in kidneys of WT mice during both the heterologous and autologous phases of glomerulonephritis. During the heterologous phase of glomerulonephritis, proteinuria was decreased, whereas glomerular neutrophil accumulation and P-selectin expression were enhanced in MPO(-/-) mice. In the autologous, crescentic phase of glomerulonephritis, MPO(-/-) mice had increased accumulation of CD4(+) cells and macrophages in glomeruli compared with WT mice. However, no difference in renal injury (crescent formation, proteinuria, and serum creatinine levels) was observed. Neutrophils and macrophages from MPO(-/-) mice exhibited reduced production of reactive oxygen species. Assessment of systemic immunity to sheep globulin showed that MPO(-/-) mice had increased splenic CD4(+) cell proliferation, cytokine production, and dermal delayed-type hypersensitivity, as well as enhanced levels of circulating IgG, IgG1, and IgG3. MPO(-/-) mice also had an augmented Th1:Th2 ratio compared with WT mice (IFN-gamma:IL-4 and IgG3:IgG1 ratios). These results suggest that endogenous MPO locally contributes to glomerular damage during neutrophil-mediated glomerulonephritis, whereas it attenuates initiation of the adaptive immune response inducing crescentic, autologous-phase glomerulonephritis by suppressing T cell proliferation, cytokine production, and Th1:Th2 ratio.

The human myeloperoxidase gene is regulated by LXR and PPARalpha ligands

Myeloperoxidase (MPO) is an oxidant-generating enzyme expressed in macrophages and implicated in atherosclerosis and cholesterol homeostasis. LXRalpha and PPARalpha regulate genes involved in cholesterol metabolism and the inflammatory response in macrophages. Here, we examine the effect of LXR and PPARalpha ligands on MPO expression. LXR and PPARalpha, as heterodimers with RXR, are shown to bind overlapping sites in an Alu receptor response element (AluRRE) in the MPO promoter. The LXR ligand T0901317 suppresses MPO mRNA expression in primary human macrophages, and in bone marrow cells and macrophages from huMPO transgenic mice. The PPARalpha ligand GW9578 downregulates MPO expression in GMCSF-macrophages, while upregulating in MCSF-macrophages. In contrast, the mouse MPO gene, which lacks the primate-specific AluRRE, is not regulated by LXR or PPARalpha ligands. These findings identify human MPO as a novel LXR and PPARalpha target gene, consistent with the role of these receptors in regulation of proinflammatory genes in macrophages.

Inducible nitric oxide synthase expression is inhibited by myeloperoxidase

Nitric oxide (NO) plays key roles in vasodilation and host defense, yet the overproduction of NO by inducible nitric oxide synthase (iNOS) at inflammatory sites can also be pathogenic. Here, we investigate the role of MPO in modulating the induction of iNOS by IFNgamma/LPS (IL). In monocyte-macrophages (Mvarphi) treated with IL, MPO gene expression was found to be downregulated as iNOS was upregulated. In Mvarphi from MPO-knockout (KO) mice, the induction of iNOS by IL was earlier and higher than in MPO-positive cells, suggesting MPO is inhibitory. Consistent with that interpretation, the addition of purified MPO enzyme to cultured macrophages inhibited iNOS induction by IL. In addition, an inhibitor of MPO enzyme, 4-aminobenzohydrazide, enhanced iNOS induction in MPO-positive cells, but not in MPO-KO cells. Similarly, taurine, a scavenger of MPO-generated HOCl, enhanced iNOS induction in MPO-positive cells, but not in MPO-KO cells. MPO affects an early event, suppressing iNOS induction when added within 2h of IL, but not when added several hours after IL. The suppression by MPO was alleviated by NO donor, sodium nitroprusside, suggesting the suppression results from scavenging of NO by MPO. This interpretation is consistent with earlier reports that MPO consumes NO, and that low levels of NO donor augment induction of iNOS by IFNgamma/LPS. The implication of these findings is that MPO acts as gatekeeper, suppressing the deleterious induction of iNOS at inflammatory sites by illegitimate signals. The combined signaling of IFNgamma/LPS overrides the gatekeeper function by suppressing MPO gene expression.

Statins downregulate myeloperoxidase gene expression in macrophages

Statins, inhibitors of HMG-CoA reductase, have pleiotropic benefits independent of cholesterol levels, including anti-oxidant and anti-inflammatory effects. Here, we investigate the effect of statins on myeloperoxidase (MPO) expression. MPO, expressed in foam cell macrophages, was recently shown to oxidize the ApoA-1 component of HDL, impairing ABCA-1 mediated cholesterol efflux. High levels of serum MPO correlate with increased risk of CAD events. Findings here show that statins strongly inhibit MPO mRNA expression in human and murine monocyte-macrophages. Suppression was reversed by downstream intermediates of HMG-CoA reductase, mevalonate, and geranylgeranylpyrophosphate, but not farnesylpyrophosphate. An inhibitor of geranylgeranyltransferase, GGTI-286, mimics the effects of statins, indicating geranylgeranylation is key to MPO expression. Reduction of MPO mRNA levels was observed in vivo in leukocytes from statin-fed mice, correlating with reductions in MPO protein and enzyme activity. These findings suggest that the pleiotropic protections afforded by statins may be due in part to suppression of MPO expression.