Beyond host defense and tissue injury: the emerging role of neutrophils in tissue repair.

Neutrophils, the most abundant immune cells in human blood, play a fundamental role in host defense against invading pathogens and tissue injury. Neutrophils carry potentially lethal weaponry to the affected site. Inadvertent and perpetual neutrophil activation could lead to nonresolving inflammation and tissue damage, a unifying mechanism of many common diseases. The prevailing view emphasizes the dichotomy of their function, host defense versus tissue damage. However, tissue injury may also persist during neutropenia, which is associated with disease severity and poor outcome. Numerous studies highlight neutrophil phenotypic heterogeneity and functional versatility, indicating that neutrophils play more complex roles than previously thought. Emerging evidence indicates that neutrophils actively orchestrate resolution of inflammation and tissue repair and facilitate return to homeostasis. Thus, neutrophils mobilize multiple mechanisms to limit the inflammatory reaction, assure debris removal, matrix remodeling, cytokine scavenging, macrophage reprogramming, and angiogenesis. In this review, we will summarize the homeostatic and tissue-reparative functions and mechanisms of neutrophils across organs. We will also discuss how the healing power of neutrophils might be harnessed to develop novel resolution and repair-promoting therapies while maintaining their defense functions.

C-reactive protein: a target for therapy to reduce inflammation.

C-reactive protein (CRP) is well-recognized as a sensitive biomarker of inflammation. Association of elevations in plasma/serum CRP level with disease state has received considerable attention, even though CRP is not a specific indicator of a single disease state. Circulating CRP levels have been monitored with a varying degree of success to gauge disease severity or to predict disease progression and outcome. Elevations in CRP level have been implicated as a useful marker to identify patients at risk for cardiovascular disease and certain cancers, and to guide therapy in a context-dependent manner. Since even strong associations do not establish causality, the pathogenic role of CRP has often been over-interpreted. CRP functions as an important modulator of host defense against bacterial infection, tissue injury and autoimmunity. CRP exists in conformationally distinct forms, which exhibit distinct functional properties and help explaining the diverse, often contradictory effects attributed to CRP. In particular, dissociation of native pentameric CRP into its subunits, monomeric CRP, unmasks "hidden" pro-inflammatory activities in pentameric CRP. Here, we review recent advances in CRP targeting strategies, therapeutic lowering of circulating CRP level and development of CRP antagonists, and a conformation change inhibitor in particular. We will also discuss their therapeutic potential in mitigating the deleterious actions attributed to CRP under various pathologies, including cardiovascular, pulmonary and autoimmune diseases and cancer.

Weathering the Storm: Harnessing the Resolution of Inflammation to Limit COVID-19 Pathogenesis.

The resolution of inflammation is a temporally and spatially coordinated process that in its innate manifestations, primarily involves neutrophils and macrophages. The shutdown of infection or injury-induced acute inflammation requires termination of neutrophil accumulation within the affected sites, neutrophil demise, and clearance by phagocytes (efferocytosis), such as tissue-resident and monocyte-derived macrophages. This must be followed by macrophage reprogramming from the inflammatory to reparative and consequently resolution-promoting phenotypes and the production of resolution-promoting lipid and protein mediators that limit responses in various cell types and promote tissue repair and return to homeostatic architecture and function. Recent studies suggest that these events, and macrophage reprogramming to pro-resolving phenotypes in particular, are not only important in the acute setting, but might be paramount in limiting chronic inflammation, autoimmunity, and various uncontrolled cytokine-driven pathologies. The SARS-CoV-2 (COVID-19) pandemic has caused a worldwide health and economic crisis. Severe COVID-19 cases that lead to high morbidity are tightly associated with an exuberant cytokine storm that seems to trigger shock-like pathologies, leading to vascular and multiorgan failures. In other cases, the cytokine storm can lead to diffuse alveolar damage that results in acute respiratory distress syndrome (ARDS) and lung failure. Here, we address recent advances on effectors in the resolution of inflammation and discuss how pro-resolution mechanisms with particular emphasis on macrophage reprogramming, might be harnessed to limit the universal COVID-19 health threat.

Deletion of heterogeneous nuclear ribonucleoprotein F in renal tubules downregulates SGLT2 expression and attenuates hyperfiltration and kidney injury in a mouse model of diabetes.

AIMS/HYPOTHESIS: We previously reported that renal tubule-specific deletion of heterogeneous nuclear ribonucleoprotein F (Hnrnpf) results in upregulation of renal angiotensinogen (Agt) and downregulation of sodium-glucose co-transporter 2 (Sglt2) in HnrnpfRT knockout (KO) mice. Non-diabetic HnrnpfRT KO mice develop hypertension, renal interstitial fibrosis and glycosuria with no renoprotective effect from downregulated Sglt2 expression. Here, we investigated the effect of renal tubular Hnrnpf deletion on hyperfiltration and kidney injury in Akita mice, a model of type 1 diabetes. METHODS: Akita HnrnpfRT KO mice were generated through crossbreeding tubule-specific (Pax8)-Cre mice with Akita floxed-Hnrnpf mice on a C57BL/6 background. Male non-diabetic control (Ctrl), Akita, and Akita HnrnpfRT KO mice were studied up to the age of 24 weeks (n = 8/group). RESULTS: Akita mice exhibited elevated systolic blood pressure as compared with Ctrl mice, which was significantly higher in Akita HnrnpfRT KO mice than Akita mice. Compared with Akita mice, Akita HnrnpfRT KO mice had lower blood glucose levels with increased urinary glucose excretion. Akita mice developed kidney hypertrophy, glomerular hyperfiltration (increased glomerular filtration rate), glomerulomegaly, mesangial expansion, podocyte foot process effacement, thickened glomerular basement membranes, renal interstitial fibrosis and increased albuminuria. These abnormalities were attenuated in Akita HnrnpfRT KO mice. Treatment of Akita HnrnpfRT KO mice with a selective A1 adenosine receptor inhibitor resulted in an increase in glomerular filtration rate. Renal Agt expression was elevated in Akita mice and further increased in Akita HnrnpfRT KO mice. In contrast, Sglt2 expression was increased in Akita and decreased in Akita HnrnpfRT KO mice. CONCLUSIONS/INTERPRETATION: The renoprotective effect of Sglt2 downregulation overcomes the renal injurious effect of Agt when these opposing factors coexist under diabetic conditions, at least partly via the activation of tubuloglomerular feedback.

Leukocytes in Inflammation, Resolution of Inflammation, Autoimmune Diseases and Cancer.

Inflammation is a double-edged sword [...].

IFN-ß is a macrophage-derived effector cytokine facilitating the resolution of bacterial inflammation.

The uptake of apoptotic polymorphonuclear cells (PMN) by macrophages is critical for timely resolution of inflammation. High-burden uptake of apoptotic cells is associated with loss of phagocytosis in resolution phase macrophages. Here, using a transcriptomic analysis of macrophage subsets, we show that non-phagocytic resolution phase macrophages express a distinct IFN-ß-related gene signature in mice. We also report elevated levels of IFN-ß in peritoneal and broncho-alveolar exudates in mice during the resolution of peritonitis and pneumonia, respectively. Elimination of endogenous IFN-ß impairs, whereas treatment with exogenous IFN-ß enhances, bacterial clearance, PMN apoptosis, efferocytosis and macrophage reprogramming. STAT3 signalling in response to IFN-ß promotes apoptosis of human PMNs. Finally, uptake of apoptotic cells promotes loss of phagocytic capacity in macrophages alongside decreased surface expression of efferocytic receptors in vivo. Collectively, these results identify IFN-ß produced by resolution phase macrophages as an effector cytokine in resolving bacterial inflammation.

Targeting formyl peptide receptors to facilitate the resolution of inflammation.

The formyl peptide receptors (FPRs) are G protein coupled receptors that recognize a broad range of structurally distinct pathogen and danger-associated molecular patterns and mediate host defense to infection and tissue injury. It became evident that the cellular distribution and biological functions of FPRs extend beyond myeloid cells and governing their activation and trafficking. In recent years, significant progress has been made to position FPRs at check points that control the resolution of inflammation, tissue repair and return to homeostasis. Accumulating data indicate a role for FPRs in an ever-increasing range of human diseases, including atherosclerosis, chronic obstructive pulmonary disease, asthma, autoimmune diseases and cancer, in which dysregulated or defective resolution are increasingly recognized as critical component of the pathogenesis. This review summarizes recent advances on how FPRs recognize distinct ligands and integrate opposing cues to govern various responses and will discuss how this knowledge could be harnessed for developing novel therapeutic strategies to counter inflammation that underlies many human diseases.

Mild acidosis delays neutrophil apoptosis via multiple signaling pathways and acts in concert with inflammatory mediators.

Accumulating evidence indicates development of local extracellular acidosis in inflamed tissues in response to infection and tissue injury. Activation of infiltrating neutrophils contributes to a transient decrease in pH, which, in turn, triggers innate immunity. In this study, we investigated the impact of extracellular acidosis on neutrophil apoptosis, a critical determinant of the outcome of the inflammatory response and analyzed the underlying signaling pathways. Culture of human isolated neutrophils in mildly acidotic conditions (pH 6.5-7.0) resulted in activation of NF-κB; intracellular accumulation of cAMP; and phosphorylation of Akt, ERK, and p38 MAPK; and preservation of Mcl-1 expression. Consequently, extracellular acidosis prevented disruption of mitochondrial transmembrane potential and translocation of cytochrome c and apoptosis-inducing factor from the mitochondria to cytoplasm and nuclei, respectively and inhibited caspase-3 activity. Pharmacological inhibition of ERK, PI3K, NF-κB, or PKA partially reversed survival cues by extracellular acidosis and redirected neutrophils to apoptosis. Conversely, dibutyryl cAMP (100-500 µM) delayed apoptosis of neutrophils cultured at pH 7.4. Extracellular acidosis-generated survival cues were additive to the potent prosurvival signals from bacterial DNA, LPS, modified C-reactive protein, and serum amyloid A. Acidosis increased CpG DNA uptake by neutrophils and augmented phosphorylation of ERK and Akt, leading to preservation of Mcl-1 expression. Our results identified extracellular acidosis as a survival signal for neutrophils by suppressing the constitutive apoptotic machinery and suggest that transient decreases in local pH can enhance neutrophil responses to inflammatory stimuli, thereby contributing to amplification or prolongation of the inflammatory response.

Heterogeneous Nuclear Ribonucleoprotein F Stimulates Sirtuin-1 Gene Expression and Attenuates Nephropathy Progression in Diabetic Mice.

We investigated the mechanism of heterogeneous nuclear ribonucleoprotein F (hnRNP F) renoprotective action in a type 2 diabetes (T2D) mouse model (db/db). Immortalized rat renal proximal tubular cells (IRPTCs) and kidneys from humans with T2D were also studied. The db/db mice developed hyperglycemia, oxidative stress, and nephropathy at age 20 weeks compared with their db/m littermates. These abnormalities, with the exception of hyperglycemia, were attenuated in db/dbhnRNP F-transgenic (Tg) mice specifically overexpressing hnRNP F in their RPTCs. Sirtuin-1, Foxo3α, and catalase expression were significantly decreased in RPTCs from db/db mice and normalized in db/dbhnRNP F-Tg mice. In vitro, hnRNP F overexpression stimulated Sirtuin-1 and Foxo3α with downregulation of acetylated p53 expression and prevented downregulation of Sirtuin-1 and Foxo3α expression in IRPTCs by high glucose plus palmitate. Transfection of Sirtuin-1 small interfering RNA prevented hnRNP F stimulation of Foxo3α and downregulation of acetylated p53 expression. hnRNP F stimulated Sirtuin-1 transcription via hnRNP F-responsive element in the Sirtuin-1 promoter. Human T2D kidneys exhibited more RPTC apoptosis and lower expression of hnRNP F, SIRTUIN-1, and FOXO3α than nondiabetic kidneys. Our results demonstrate that hnRNP F protects kidneys against oxidative stress and nephropathy via stimulation of Sirtuin-1 expression and signaling in diabetes.

Angiotensin-(1-7) prevents systemic hypertension, attenuates oxidative stress and tubulointerstitial fibrosis, and normalizes renal angiotensin-converting enzyme 2 and Mas receptor expression in diabetic mice.

We investigated the relationship between Ang-(1-7) [angiotensin-(1-7)] action, sHTN (systolic hypertension), oxidative stress, kidney injury, ACE2 (angiotensin-converting enzyme-2) and MasR [Ang-(1-7) receptor] expression in Type 1 diabetic Akita mice. Ang-(1-7) was administered daily [500 µg/kg of BW (body weight) per day, subcutaneously] to male Akita mice from 14 weeks of age with or without co-administration of an antagonist of the MasR, A779 (10 mg/kg of BW per day). The animals were killed at 20 weeks of age. Age-matched WT (wild-type) mice served as controls. Ang-(1-7) administration prevented sHTN and attenuated kidney injury (reduced urinary albumin/creatinine ratio, glomerular hyperfiltration, renal hypertrophy and fibrosis, and tubular apoptosis) without affecting blood glucose levels in Akita mice. Ang-(1-7) also attenuated renal oxidative stress and the expression of oxidative stress-inducible proteins (NADPH oxidase 4, nuclear factor erythroid 2-related factor 2, haem oxygenase 1), pro-hypertensive proteins (angiotensinogen, angiotensin-converting enzyme, sodium/hydrogen exchanger 3) and profibrotic proteins (transforming growth factor-ß1 and collagen IV), and increased the expression of anti-hypertensive proteins (ACE2 and MasR) in Akita mouse kidneys. These effects were reversed by A779. Our data suggest that Ang-(1-7) plays a protective role in sHTN and RPTC (renal proximal tubular cell) injury in diabetes, at least in part, through decreasing renal oxidative stress-mediated signalling and normalizing ACE2 and MasR expression.

Toll-like receptor 9 signaling delays neutrophil apoptosis by increasing transcription of Mcl-1.

Neutrophils detect bacterial constituents, including bacterial DNA (CpG DNA), which elicits innate immunity and prolongs the functional life span of neutrophils through suppression of apoptosis. Both the anti-apoptotic protein Mcl-1 and activation of NF-κB have been implicated in neutrophil survival, but there is no evidence that these are linked in neutrophils. We hypothesized that CpG DNA could simultaneously activate these pathways. High purity CpG DNA (0.4-3.2 µg/ml) extended the life span of human neutrophils in vitro by delaying apoptosis through altering the rate of Mcl-1 turnover. CpG DNA slightly decreased Mcl-1 protein level in the presence of cyclohexmide and the proteasome inhibitor MG132 had little effect on Mcl-1 expression in CpG DNA-treated neutrophils. In contrast, CpG DNA evoked rapid increases in DNA binding by NF-κB/p65 and Mcl-1 mRNA. NF-κB inhibitors and the telomere-derived TLR9 inhibitory oligonucleotide 5'-TTT AGG GTT AGG GTT AGG G-3' markedly reduced Mcl-1 protein levels and subsequently abrogated suppression of apoptosis by CpG DNA. Furthermore, CpG DNA attenuated the decreases in Mcl-1 in both cell lysate and nucleus of neutrophils undergoing spontaneous apoptosis and increased Mcl-1 translocation to the mitochondria, leading to preservation of mitochondrial transmembrane potential. These results demonstrate that CpG DNA through toll-like receptor 9 links two survival signaling pathways by delaying apoptosis through induction of NF-κB-mediated Mcl-1 gene transcription and promoting Mcl-1 translocation to the mitochondria.

Bcl-2-modifying factor induces renal proximal tubular cell apoptosis in diabetic mice.

This study investigated the mechanisms underlying tubular apoptosis in diabetes by identifying proapoptotic genes that are differentially upregulated by reactive oxygen species in renal proximal tubular cells (RPTCs) in models of diabetes. Total RNAs isolated from renal proximal tubules (RPTs) of 20-week-old heterozygous db/m+, db/db, and db/db catalase (CAT)-transgenic (Tg) mice were used for DNA chip microarray analysis. Real-time quantitative PCR assays, immunohistochemistry, and mice rendered diabetic with streptozotocin were used to validate the proapoptotic gene expression in RPTs. Cultured rat RPTCs were used to confirm the apoptotic activity and regulation of proapoptotic gene expression. Additionally, studies in kidney tissues from patients with and without diabetes were used to confirm enhanced proapoptotic gene expression in RPTs. Bcl-2-modifying factor (Bmf) was differentially upregulated (P<0.01) in RPTs of db/db mice compared with db/m+ and db/db CAT-Tg mice and in RPTs of streptozotocin-induced diabetic mice in which insulin reversed this finding. In vitro, Bmf cDNA overexpression in rat RPTCs coimmunoprecipated with Bcl-2, enhanced caspase-3 activity, and promoted apoptosis. High glucose (25 mmol/L) induced Bmf mRNA expression in RPTCs, whereas rotenone, catalase, diphenylene iodinium, and apocynin decreased it. Knockdown of Bmf with small interfering RNA reduced high glucose-induced apoptosis in RPTCs. More important, enhanced Bmf expression was detected in RPTs of kidneys from patients with diabetes. These data demonstrate differential upregulation of Bmf in diabetic RPTs and suggest a potential role for Bmf in regulating RPTC apoptosis and tubular atrophy in diabetes.

Myeloid nuclear differentiation antigen, neutrophil apoptosis and sepsis.

Sepsis and septic shock are characterized by prolonged inflammation and delayed resolution, which are associated with suppression of neutrophil apoptosis. The role of the intrinsic apoptotic pathway and intracellular factors in regulation of neutrophil apoptosis remain incompletely understood. We previously reported that the nuclear factor MNDA (myeloid nuclear differentiation antigen) is fundamental to execution of the constitutive neutrophil death program. During neutrophil apoptosis MNDA is cleaved by caspases and relocated to the cytoplasm. However, when challenged with known mediators of sepsis, human neutrophils of healthy donors or neutrophils from patients with sepsis exhibited impaired MNDA relocation/cleavage parallel with myeloid cell leukemia-1 (MCL-1) accumulation and suppression of apoptosis. MNDA knockdown in a model cell line indicated that upon induction of apoptosis, MNDA promotes proteasomal degradation of MCL-1, thereby aggravating mitochondrial dysfunction. Thus, MNDA is central to a novel nucleus-mitochondrion circuit that promotes progression of apoptosis. Disruption of this circuit contributes to neutrophil longevity, thereby identifying MNDA as a potential therapeutic target in sepsis and other inflammatory pathologies.

Regulation of neutrophil survival/apoptosis by Mcl-1.

Neutrophil granulocytes have the shortest lifespan among leukocytes in the circulation and die via apoptosis. At sites of infection or tissue injury, prolongation of neutrophil lifespan is critical for effective host defense. Apoptosis of inflammatory neutrophils and their clearance are critical control points for termination of the inflammatory response. Evasion of neutrophil apoptosis aggravates local injury and leads to persistent tissue damage. The short-lived prosurvival Bcl-2 family protein, Mcl-1 (myeloid cell leukemia-1), is instrumental in controlling apoptosis and consequently neutrophil lifespan in response to rapidly changing environmental cues during inflammation. This paper will focus on multiple levels of control of Mcl-1 expression and function and will discuss targeting Mcl-1 as a potential therapeutic strategy to enhance the resolution of inflammation through accelerating neutrophil apoptosis.

A redox switch in C-reactive protein modulates activation of endothelial cells.

C-reactive protein (CRP) has been implicated in the regulation of inflammation underlying coronary artery disease; however, little is known about the molecular mechanisms responsible for the expression of its pro- or anti-inflammatory activities. Here, we have identified the intrasubunit disulfide bond as a conserved switch that controls the structure and functions of CRP. Conformational rearrangement in human pentameric CRP to monomeric CRP (mCRP) is the prerequisite for this switch to be activated by reducing agents, including thioredoxin. Immunohistochemical analysis revealed 36-79% colocalization of thioredoxin and mCRP in human advanced coronary atherosclerotic lesions. Nonreduced mCRP was largely inert in activating human coronary artery endothelial cells (HCAECs), whereas reduced or cysteine-mutated mCRP evoked marked release of IL-8 and monocyte chemoattractant protein-1 from HCAECs, with ~50% increase at a concentration of 1 µg/ml. Reduced mCRP was ~4 to 40-fold more potent than mCRP in up-regulating adhesion molecule expression, promoting U937 monocyte adhesion to HCAECs, and inducing cytokine release from rabbit arteries ex vivo and in mice. These actions were primarily due to unlocking the lipid raft interaction motif. Therefore, expression of proinflammatory properties of CRP on endothelial cells requires sequential conformational changes, i.e., loss of pentameric symmetry followed by reduction of the intrasubunit disulfide bond.

Role for myeloid nuclear differentiation antigen in the regulation of neutrophil apoptosis during sepsis.

RATIONALE: Suppressed neutrophil apoptosis, a hallmark of sepsis, perpetuates inflammation and delays resolution. Myeloid nuclear differentiation antigen (MNDA) is expressed only in myeloid cells and has been implicated in cell differentiation; however, its function in mature neutrophils is not known. OBJECTIVES: We studied whether MNDA could contribute to regulation of apoptosis of neutrophils from healthy subjects and patients with sepsis, and investigated the impact of MNDA knockdown on apoptosis. METHODS: Human neutrophils were challenged with mediators of sepsis and neutrophils from patients with sepsis were cultured to investigate cleavage and cytoplasmic accumulation of MNDA. MNDA was knocked down in myeloid HL-60 cells to investigate development of apoptosis. MEASUREMENTS AND MAIN RESULTS: During constitutive apoptosis of human neutrophils, MNDA is cleaved by caspases and accumulated in the cytoplasm, where it promotes degradation of the antiapoptotic protein Mcl-1, thereby accelerating collapse of mitochondrial transmembrane potential. Culture of neutrophils with LPS, bacterial DNA, or platelet-activating factor prevented MNDA cleavage and cytoplasmic accumulation. MNDA knockdown with short hairpin RNA markedly attenuated Mcl-1 turnover and conferred resistance to stress-induced apoptosis in HL-60 cells. Neutrophils from patients with severe sepsis exhibited markedly suppressed apoptosis that was associated with impaired cytoplasmic MNDA accumulation, preservation of Mcl-1 expression, and mitochondrial transmembrane potential. Culture of neutrophils of healthy subjects with septic plasma delayed apoptosis and cytoplasmic MNDA accumulation. CONCLUSIONS: These results indicate that cytoplasmic accumulation of MNDA facilitates progression of apoptosis and suggest that impaired cytoplasmic MNDA accumulation contributes to delayed neutrophil apoptosis in patients with severe sepsis.

Lipid mediator interplay: resolvin D1 attenuates inflammation evoked by glutathione-conjugated lipid peroxidation products.

Non-enzymatic oxidation of cellular lipids, one of the characteristic features of inflammation, leads to formation of highly reactive and toxic alpha,beta-unsaturated aldehydes, such as 4-hydroxy-trans-2-nonenal (HNE). Conjugation of HNE with reduced glutathione (GS-HNE) is widely believed to represent a form of detoxification. The study by Spite et al. in the current issue of the British Journal of Pharmacology shows that glutathiolation of HNE confers potent pro-inflammatory properties on this alpha,beta-unsaturated aldehyde. They find that GS-HNE directly activates human neutrophil granulocytes in vitro and evokes peritonitis in mice. Pre-treatment with resolvin D1, which is derived from omega-3 fatty acids, markedly attenuated the peritoneal leukocyte accumulation and production of prostaglandins and leukotrienes induced by GS-HNE. Their findings have profound implications for the analysis of inflammation in describing the generation of a novel class of pro-inflammatory mediators, through glutathione-dependent metabolism of lipid-peroxidation products, and emphasize the therapeutic potential of resolvin D1 in inflammatory diseases.

15-epi-lipoxin A4 inhibits myeloperoxidase signaling and enhances resolution of acute lung injury.

RATIONALE: Apoptosis is essential for removal of neutrophils from inflamed tissues and efficient resolution of inflammation. Myeloperoxidase (MPO), abundantly expressed in neutrophils, not only generates cytotoxic oxidants but also signals through the beta(2) integrin Mac-1 to rescue neutrophils from constitutive apoptosis, thereby prolonging inflammation. OBJECTIVES: Because aspirin-triggered 15-epi-lipoxin A(4) (15-epi-LXA(4)) modulates Mac-1 expression, we investigated the impact of 15-epi-LXA(4) on MPO suppression of neutrophil apoptosis and MPO-mediated neutrophil-dependent acute lung injury. METHODS: Human neutrophils were cultured with MPO with or without 15-epi-LXA(4) to investigate development of apoptosis. Acute lung injury was produced by intratracheal injection of carrageenan plus MPO or intraperitoneal injection of live Escherichia coli in mice, and the animals were treated with 15-epi-LXA(4) at the peak of inflammation. MEASUREMENTS AND MAIN RESULTS: 15-Epi-LXA(4) through down-regulation of Mac-1 expression promoted apoptosis of human neutrophils by attenuating MPO-induced activation of extracellular signal-regulated kinase and Akt-mediated phosphorylation of Bad and by reducing expression of the antiapoptotic protein Mcl-1, thereby aggravating mitochondrial dysfunction. The proapoptotic effect of 15-epi-LXA(4) was dominant over MPO-mediated effects even when it was added at 4 hours post MPO. In mice, treatment with 15-epi-LXA(4) accelerated the resolution of established carrageenan plus MPO-evoked as well as E. coli-induced neutrophil-dependent pulmonary inflammation through redirecting neutrophils to caspase-mediated cell death and facilitating their removal by macrophages. CONCLUSIONS: These results demonstrate that aspirin-triggered 15-epi-LXA(4) enhances resolution of inflammation by overriding the powerful antiapoptosis signal from MPO, thereby demonstrating a hitherto unrecognized mechanism by which aspirin promotes resolution of inflammation.

Myeloperoxidase delays neutrophil apoptosis through CD11b/CD18 integrins and prolongs inflammation.

Polymorphonuclear neutrophil granulocytes have a central role in innate immunity and their programmed cell death and removal are critical for efficient resolution of acute inflammation. Myeloperoxidase (MPO), a heme protein abundantly expressed in neutrophils, is generally associated with killing of bacteria and oxidative tissue injury. Because MPO also binds to neutrophils, we investigated whether MPO could affect the lifespan of neutrophils. Here, we report that MPO independent of its catalytic activity through signaling via the adhesion molecule CD11b/CD18 rescued human neutrophils from constitutive apoptosis and prolonged their life span. MPO evoked a transient concurrent activation of extracellular signal-regulated kinase and Akt, leading to phosphorylation of Bad at both Ser112 and Ser136, prevention of mitochondrial dysfunction, and subsequent activation of caspase-3. Consistently, pharmacological inhibition of extracellular signal-regulated kinase, Akt, or caspase-3 reversed the antiapoptosis action of MPO. Acute increases in plasma MPO delayed murine neutrophil apoptosis assayed ex vivo. In a mouse model of self-resolving inflammation, MPO also prolonged the duration of carrageenan-induced acute lung injury, as evidenced by enhanced alveolar permeability and accumulation of neutrophils parallel with suppression of neutrophil apoptosis. Our results indicate that MPO functions as a survival signal for neutrophils and thereby contribute to prolongation of inflammation.