Human alveolar macrophages have 15-lipoxygenase and generate 15(S)-hydroxy-5,8,11-cis-13-trans-eicosatetraenoic acid and lipoxins

Pro-resolving lipid mediators and therapeutic innovations in resolution of inflammation

This review summarizes findings presented at the 19th World Congress of Basic & Clinical Pharmacology 2023 (Glasgow, Scotland, July 3rd to 7th, 2023) from 8 speakers in the field of resolution of inflammation, resolution pharmacology and resolution biology. It is now accepted that the acute inflammatory response is protective to defend the host against infection or tissue injury. Acute inflammation is self-limited and programmed to be limited in space and time: this is achieved through endogenous resolution processes that ensure return to homeostasis. Resolution is brought about by agonist mediators that include specialized pro-resolving lipid mediators (SPMs) and pro-resolving proteins and peptides such as annexin A1 and angiotensin-(1-7), all acting to initiate anti-inflammatory and pro-resolving processes. If the inflammatory reaction remains unchecked through dysfunctional resolution mechanism, it can become chronic and contribute to a plethora of human diseases, including respiratory, cardiovascular, metabolic, allergic diseases, and arthritis. Herein, we discuss how non-resolving inflammation plays a role in the pathogenesis of these diseases. In addition to SPMs, we highlight the discovery, biosynthesis, biofunctions, and latest research updates on innovative therapeutics (including annexin-A1 peptide-mimetic RTP-026, small molecule FPR2 agonist BM-986235/LAR-1219, biased agonist for FPR1/FPR2 Cmpd17b, lipoxin mimetics AT-01-KG and AT-02-CT, melanocortin receptor agonist AP1189, gold nanoparticles, angiotensin-(1-7), and CD300a) that can promote resolution of inflammation directly or through modulation of SPMs production. Drug development strategies based on the biology of the resolution of inflammation can offer novel therapeutic means and/or add-on therapies for the treatment of chronic diseases.

Omega-3 long-chain polyunsaturated fatty acids and their bioactive lipids: A strategy to improve resistance to respiratory tract infectious diseases in the elderly?

Age-related changes in organ function, immune dysregulation, and the effects of senescence explain in large part the high prevalence of infections, including respiratory tract infections in older persons. Poor nutritional status in many older persons increases susceptibility to infection and worsens prognosis. Interestingly, there is an association between the amount of saturated fats in the diet and the rate of community-acquired pneumonia. Polyunsaturated fatty acids, particularly omega-3 long chain polyunsaturated fatty acids (ω-3 LC-PUFAs) including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have well-known anti-inflammatory, immunomodulatory, and antimicrobial effects, which may, in theory, be largely induced by PUFAs-derived lipids such as specialized pro-resolving mediators (SPMs). In adults, preliminary results of studies show that ω-3 LC-PUFAs supplementation can lead to SPM generation. SPMs have a crucial role in the resolution of inflammation, a factor relevant to survival from infection independent of the pathogen’s virulence. Moreover, the immune system of older adults appears to be more sensitive to ω-3 PUFAs. This review explores the effects of ω-3 LC-PUFAs, and PUFA bioactive lipid-derived SPMs in respiratory tract infections and the possible relevance of these data to infectious disease outcomes in the older population. The hypothesis that PUFAs have beneficial effects via SPM generation will need to be confirmed by animal experiments and patient-derived data.

Protective Effects of 7S,15R-Dihydroxy-16S,17S-Epoxy-Docosapentaenoic Acid (diHEP-DPA) against Blue Light-Induced Retinal Damages in A2E-Laden ARPE-19 Cells

The purpose of this study was to investigate the protective effects of 7S,15R-dihydroxy-16S,17S-epoxy-docosapentaenoic acid (diHEP-DPA) in retinal pigment epithelial (RPE) cell damage. ARPE-19 cells, a human RPE cell line, were cultured with diHEP-DPA and Bis-retinoid N-retinyl-N-retinylidene ethanolamine (A2E), followed by exposure to BL. Cell viability and cell death rates were determined. Western blotting was performed to determine changes in apoptotic factors, mitogen-activated protein kinase (MAPK) family proteins, inflammatory proteins, and oxidative and carbonyl stresses. The levels of pro-inflammatory cytokines in the culture medium supernatants were also measured. Exposure to A2E and BL increased the ARPE-19 cell death rate, which was alleviated by diHEP-DPA in a concentration-dependent manner. A2E and BL treatments induced apoptosis in ARPE-19 cells, which was also alleviated by diHEP-DPA. Analysis of the relationship with MAPK proteins revealed that the expression of p-JNK and p-P38 increased after A2E and BL treatments and decreased with exposure to diHEP-DPA in a concentration-dependent manner. DiHEP-DPA also affected the inflammatory response by suppressing the expression of inflammatory proteins and the production of pro-inflammatory cytokines. Furthermore, it was shown that diHEP-DPA regulated the proteins related to oxidative and carbonyl stresses. Taken together, our results provide evidence that diHEP-DPA can inhibit cell damage caused by A2E and BL exposure at the cellular level by controlling various pathways involved in apoptosis and inflammatory responses.

Lipoxygenases at the Intersection of Infection and Carcinogenesis

The persisting presence of opportunistic pathogens like Pseudomonas aeruginosa poses a significant threat to many immunocompromised cancer patients with pulmonary infections. This review highlights the complexity of interactions in the host's defensive eicosanoid signaling network and its hijacking by pathogenic bacteria to their own advantage. Human lipoxygenases (ALOXs) and their mouse counterparts are integral elements of the innate immune system, mostly operating in the pro-inflammatory mode. Taking into account the indispensable role of inflammation in carcinogenesis, lipoxygenases have counteracting roles in this process. In addition to describing the structure-function of lipoxygenases in this review, we discuss their roles in such critical processes as cancer cell signaling, metastases, death of cancer and immune cells through ferroptosis, as well as the roles of ALOXs in carcinogenesis promoted by pathogenic infections. Finally, we discuss perspectives of novel oncotherapeutic approaches to harness lipoxygenase signaling in tumors.

Immunomodulation of periodontitis with SPMs

Inflammation is a critical component in the pathophysiology of numerous disease processes, with most therapeutic modalities focusing on its inhibition in order to achieve treatment outcomes. The resolution of inflammation is a separate, distinct pathway that entails the reversal of the inflammatory process to a state of homoeostasis rather than selective inhibition of specific components of the inflammatory cascade. The discovery of specialized pro-resolving mediators (SPMs) resulted in a paradigm shift in our understanding of disease etiopathology. Periodontal disease, traditionally considered as one of microbial etiology, is now understood to be an inflammation-driven process associated with dysbiosis of the oral microbiome that may be modulated with SPMs to achieve therapeutic benefit.

Inflammation resolution and specialized pro-resolving lipid mediators in chronic rhinosinusitis

INTRODUCTION

In chronic rhinosinusitis (CRS), a complex pathophysiology results from varied pro-inflammatory stimuli but is consistently characterized by classic cellular, molecular, and microbial alterations. Normally, endogenous specialized pro-resolving mediators (SPM) actively promote resolution of inflammation through numerous pathways, including those involved in host antimicrobial defense. However, these pathways appear to be disrupted in CRS.

AREAS COVERED

This paper describes features of CRS in the context of chronic tissue inflammation, and potential mechanisms by which specialized pro-resolving mediators promote active resolution of tissue inflammation.

EXPERT OPINION

Temporal phases of resolution must be tightly regulated to successfully resolve inflammation in CRS while preserving tissue functions such as barrier maintenance and special sensory function. Dysregulation of SPM enzymatic pathways has been recently shown in CRS and is associated with disease phenotypes and microbial colonization patterns. Current research in animal models and in vitro human cell culture, as well as human dietary studies, demonstrate relevant changes in cell signaling with lipid mediator bioavailability. Further clinical research may provide insight into the therapeutic value of this approach in CRS.

Resolution of inflammation in oral diseases

The resolution of inflammation is an essential endogenous process that protects host tissues from an exaggerated chronic inflammatory response. Multiple interactions between host cells and resident oral microbiome regulate the protective functions that lead to inflammation in the oral cavity. Failure of appropriate regulation of inflammation can lead to chronic inflammatory diseases that result from an imbalance between pro-inflammatory and pro-resolution mediators. Thus, failure of the host to resolve inflammation can be considered an essential pathological mechanism for progression from the late stages of acute inflammation to a chronic inflammatory response. Specialized pro-resolving mediators (SPMs), which are essential polyunsaturated fatty acid (PUFA)-derived autacoid mediators, aid in regulating the endogenous inflammation resolving process by stimulating immune cell-mediated clearance of apoptotic polymorphonuclear neutrophils, cellular debris, and microbes, restricting further neutrophil tissue infiltration, and counter-regulating pro-inflammatory cytokine production. The SPM superfamily contains four specialized lipid mediator families: lipoxins, resolvins, protectins, and maresins that can activate resolution pathways. Understanding the crosstalk between resolution signals in the tissue response to injury has therapeutic application potential for preventing, maintaining, and regenerating chronically damaged tissues. Here, we discuss the fundamental concepts of resolution as an active biochemical process, novel concepts demonstrating the role of resolution mediators in tissue regeneration in periodontal and pulpal diseases, and future directions for therapeutic applications with particular emphasis on periodontal therapy.

The role of oxylipins in NSAID-exacerbated respiratory disease (N-ERD)

Nonsteroidal anti-inflammatory drug (NSAID)-exacerbated respiratory disease (N-ERD) is characterized by nasal polyp formation, adult-onset asthma, and hypersensitivity to all cyclooxygenase-1 (COX-1) inhibitors. Oxygenated lipids are collectively known as oxylipins and are polyunsaturated fatty acids (PUFA) oxidation products. The most extensively researched oxylipins being the eicosanoids formed from arachidonic acid (AA). There are four major classes of eicosanoids including leukotrienes, prostaglandins, thromboxanes, and lipoxins. In N-ERD, the underlying inflammatory process of the upper and lower respiratory systems begins and occurs independently of NSAID consumption and is due to the overproduction of cysteinyl leukotrienes. Leukotriene mediators all induce edema, bronchoconstriction, and airway mucous secretion. Thromboxane A2 is a potent bronchoconstrictor and induces endothelial adhesion molecule expression. Elevated Prostaglandin D2 metabolites lead to vasoconstriction, additionally impaired up-regulation of prostaglandin E2 leads to symptoms seen in N-ERD as it is essential for maintaining homeostasis of inflammatory responses in the airway and has bronchoprotective and anti-inflammatory effects. A characteristic feature of N-ERD is diminished lipoxin levels, this decreased capacity to form endogenous mediators with anti-inflammatory properties could facilitate local inflammatory response and expose bronchial smooth muscle to relatively unopposed actions of broncho-constricting substances. Treatment options, such as leukotriene modifying agents, aspirin desensitization, biologic agents and ESS, appear to influence eicosanoid pathways, however more studies need to be done to further understand the role of oxylipins. Besides AA-derived eicosanoids, other oxylipins may also pay a role but have not been sufficiently studied. Identifying pathogenic N-ERD mechanism is likely to define more effective treatment targets.

Pro- and anti-inflammatory bioactive lipids imbalance contributes to the pathobiology of autoimmune diseases

Autoimmune diseases are driven by T_H17 cells that secrete pro-inflammatory cytokines, especially IL-17. Under normal physiological conditions, autoreactive T cells are suppressed by TGF-β and IL-10 secreted by microglia and dendritic cells. When this balance is upset due to injury, infection and other causes, leukocyte recruitment and macrophage activation occurs resulting in secretion of pro-inflammatory IL-6, TNF-α, IL-17 and PGE2, LTs (leukotrienes) accompanied by a deficiency of anti-inflammatory LXA4, resolvins, protecting, and maresins. PGE2 facilitates T_H1 cell differentiation and promotes immune-mediated inflammation through T_H17 expansion. There is evidence to suggest that autoimmune diseases can be suppressed by anti-inflammatory bioactive lipids LXA4, resolvins, protecting, and maresins. These results imply that systemic and/or local application of LXA4, resolvins, protecting, and maresins and administration of their precursors AA/EPA/DHA could form a potential therapeutic approach in the prevention and treatment of autoimmune diseases.

Enteral nutrition ameliorates the symptoms of Crohn's disease in mice via activating special pro-resolving mediators through innate lymphoid cells

Crohn's disease activates the inflammatory reactions to induce intestinal disorders. Enteral nutrition (EN) could exert general immunomodulatory effects. Cecal ligation and perforation (CLP) surgery was utilized to establish Crohn's disease mice models. Survival analysis, hematoxylin-eosin staining, flow cytometry, ELISA, Western blot and liquid chromatography-tandem MS were applied. Baicalein was added to inhibit lipoxygenases. The survival rate was restored and inflammatory injury, exudate neutrophils in peritoneal lavage and serum levels of IL-6 and TNF-α were ameliorated by EN treatment as compared with CLP treatment. EN also increased ILC-3 content, 5/15-LOX level and RvD1-RvD5 in peritoneal lavage. Baicalein reversed all the detected effects of EN except ILC-3 content. EN could activate special pro-resolving mediators (SPMs) through ILCs to mitigate injuries of Crohn's disease.

Aspirin Actions in Treatment of NSAID-Exacerbated Respiratory Disease

Non-steroidal Anti-inflammatory drugs (NSAID)-exacerbated respiratory disease (N-ERD) is characterized by nasal polyposis, chronic rhinosinusitis, adult-onset asthma and hypersensitive reactions to cyclooxygenase-1 (COX-1) inhibitors. Among the available treatments for this disease, a combination of endoscopic sinus surgery followed by aspirin desensitization and aspirin maintenance therapy has been an effective approach. Studies have shown that long-term aspirin maintenance therapy can reduce the rate of nasal polyp recurrence in patients with N-ERD. However, the exact mechanism by which aspirin can both trigger and suppress airway disease in N-ERD remains poorly understood. In this review, we summarize current knowledge of aspirin effects in N-ERD, cardiovascular disease, and cancer, and consider potential mechanistic pathways accounting for the effects of aspirin in N-ERD.

Role of Apoptotic Cell Clearance in Pneumonia and Inflammatory Lung Disease

Pneumonia and inflammatory diseases of the pulmonary system such as chronic obstructive pulmonary disease and asthma continue to cause significant morbidity and mortality globally. While the etiology of these diseases is highly different, they share a number of similarities in the underlying inflammatory processes driving disease pathology. Multiple recent studies have identified failures in efferocytosis-the phagocytic clearance of apoptotic cells-as a common driver of inflammation and tissue destruction in these diseases. Effective efferocytosis has been shown to be important for resolving inflammatory diseases of the lung and the subsequent restoration of normal lung function, while many pneumonia-causing pathogens manipulate the efferocytic system to enhance their growth and avoid immunity. Moreover, some treatments used to manage these patients, such as inhaled corticosteroids for chronic obstructive pulmonary disease and the prevalent use of statins for cardiovascular disease, have been found to beneficially alter efferocytic activity in these patients. In this review, we provide an overview of the efferocytic process and its role in the pathophysiology and resolution of pneumonia and other inflammatory diseases of the lungs, and discuss the utility of existing and emerging therapies for modulating efferocytosis as potential treatments for these diseases.

Macrophages and the maintenance of homeostasis

There have been many chapters written about macrophage polarization. These chapters generally focus on the role of macrophages in orchestrating immune responses by highlighting the T-cell-derived cytokines that shape these polarizing responses. This bias toward immunity is understandable, given the importance of macrophages to host defense. However, macrophages are ubiquitous and are involved in many different cellular processes, and describing them as immune cells is undoubtedly an oversimplification. It disregards their important roles in development, tissue remodeling, wound healing, angiogenesis, and metabolism, to name just a few processes. In this chapter, we propose that macrophages function as transducers in the body. According to Wikipedia, “A transducer is a device that converts energy from one form to another.” The word transducer is a term used to describe both the “sensor,” which can interpret a wide range of energy forms, and the “actuator,” which can switch voltages or currents to affect the environment. Macrophages are able to sense a seemingly endless variety of inputs from their environment and transduce these inputs into a variety of different response outcomes. Thus, rather than functioning as immune cells, they should be considered more broadly as cellular transducers that interpret microenvironmental changes and actuate vital tissue responses. In this chapter, we will describe some of the sensory stimuli that macrophages perceive and the responses they make to these stimuli to achieve their prime directive, which is the maintenance of homeostasis.

Roles of Specialized Pro-Resolving Lipid Mediators in Autophagy and Inflammation

Autophagy is a catabolic pathway that accounts for degradation and recycling of cellular components to extend cell survival under stress conditions. In addition to this prominent role, recent evidence indicates that autophagy is crucially involved in the regulation of the inflammatory response, a tightly controlled process aimed at clearing the inflammatory stimulus and restoring tissue homeostasis. To be efficient and beneficial to the host, inflammation should be controlled by a resolution program, since uncontrolled inflammation is the underlying cause of many pathologies. Resolution of inflammation is an active process mediated by a variety of mediators, including the so-called specialized pro-resolving lipid mediators (SPMs), a family of endogenous lipid autacoids known to regulate leukocyte infiltration and activities, and counterbalance cytokine production. Recently, regulation of autophagic mechanisms by these mediators has emerged, uncovering unappreciated connections between inflammation resolution and autophagy. Here, we summarize mechanisms of autophagy and resolution, focusing on the contribution of autophagy in sustaining paradigmatic examples of chronic inflammatory disorders. Then, we discuss the evidence that SPMs can restore dysregulated autophagy, hypothesizing that resolution of inflammation could represent an innovative approach to modulate autophagy and its impact on the inflammatory response.

The enzymology of human eicosanoid pathways: the lipoxygenase branches

Eicosanoids are short-lived derivatives of polyunsaturated fatty acids that serve as autocrine and paracrine signaling molecules. They are involved numerous biological processes of both the well state and disease states. A thorough understanding of the progression the disease state and homeostasis of the well state requires a complete evaluation of the systems involved. This review examines the enzymology for the enzymes involved in the production of eicosanoids along the lipoxygenase branches of the eicosanoid pathways with particular emphasis on those derived from arachidonic acid. The enzymatic parameters, protocols to measure them, and proposed catalytic mechanisms are presented in detail.

Long-term stimulation of toll-like receptor-2 and -4 upregulates 5-LO and 15-LO-2 expression thereby inducing a lipid mediator shift in human monocyte-derived macrophages

Macrophage polarization switches during the course of inflammation along with the lipid mediators released. We investigated the lipid mediator formation in human monocyte-derived macrophages during in vitro differentiation and pathogen stimulation. For this, peripheral blood monocytes were differentiated into M1 (CSF-2/IFNγ) or M2 (CSF-1/IL-4) macrophages followed by stimulation with the toll-like receptor (TLR) ligands zymosan (TLR-2), Poly(I:C) (TLR-3) or bacterial lipopolysaccharides (TLR-4) mimicking fungal, viral and bacterial infection, respectively. Expression of enzymes involved in lipid mediator formation such as 5- and 15-lipoxygenases (LO), the 5-LO activating protein and cyclooxygenase-2 (COX-2) was monitored on mRNA and protein level and lipid mediator formation was assessed. In addition, cytokine release was measured. In vitro differentiation of human peripheral blood monocytes to M1 and M2 macrophages considerably attenuated 5-LO activity. Furthermore, while TLR-2 and -4 stimulation of M1 macrophages primarily triggered pro-inflammatory cytokines and lipid mediators, persistent stimulation (16 h) of human M2 macrophages induced a coordinated upregulation of 5- and 15-LO-2 expression. This was accompanied by a marked increase in IL-10 and monohydroxylated 15-LO products in the conditioned media of the cells. After additional stimulation with Ca²⁺ ionophore combined with supplementation of arachidonic, eicosapentaenoic and docosahexaenoic acid these cells also released small amounts of SPM such as lipoxins and resolvins. From this we conclude that activation of TLR-2 or -4 triggers the biosynthesis of pro-inflammatory 5-LO and COX-2 derived lipid mediators in human monocyte-derived M1 macrophages while persistent stimulation of M2 macrophages induces a shift towards pro-resolving 15-LO derived oxylipins.

Therapeutic Potential of Lipoxin A4 in Chronic Inflammation: Focus on Cardiometabolic Disease

Several studies have shown that failure to resolve inflammation may contribute to the progression of many chronic inflammatory disorders. It has been suggested targeting the resolution of inflammation might be a novel therapeutic approach for chronic inflammatory diseases, including inflammatory bowel disease, diabetic complications, and cardiometabolic disease. Lipoxins [LXs] are a class of endogenously generated mediators that promote the resolution of inflammation. Biological actions of LXs include inhibition of neutrophil infiltration, promotion of macrophage polarization, increase of macrophage efferocytosis, and restoration of tissue homeostasis. Recently, several studies have demonstrated that LXs and synthetic analogues protect tissues from acute and chronic inflammation. The mechanism includes down-regulation of pro-inflammatory cytokines and chemokines (e.g., interleukin-1β and tumor necrosis factor-α), inhibition of the activation of the master pro-inflammatory pathway (e.g., nuclear factor κ-light-chain-enhancer of activated B cells pathway) and increased release of the pro-resolving cytokines (e.g., interleukin-10). Three generations of LXs analogues are well described in the literature, and more recently a fourth generation has been generated that appears to show enhanced potency. In this review, we will briefly discuss the potential therapeutic opportunity provided by lipoxin A₄ as a novel approach to treat chronic inflammatory disorders, focusing on cardiometabolic disease and the current drug development in this area.

Frontline Science: A reduction in DHA-derived mediators in male obesity contributes toward defects in select B cell subsets and circulating antibody

Obesity dysregulates B cell populations, which contributes toward poor immunological outcomes. We previously reported that differing B cell subsets are lowered in the bone marrow of obese male mice. Here, we focused on how lipid metabolites synthesized from docosahexaenoic acid (DHA) known as specialized pro-resolving lipid mediators (SPMs) influence specific B cell populations in obese male mice. Metabololipidomics revealed that splenic SPM precursors 14-hydroxydocosahexaenoic acid (14-HDHA), 17-hydroxydocosahexaenoic acid (17-HDHA), and downstream protectin DX (PDX) were decreased in obese male C57BL/6J mice. Simultaneous administration of these mediators to obese mice rescued major decrements in bone marrow B cells, modest impairments in the spleen, and circulating IgG2c, which is pro-inflammatory in obesity. In vitro studies with B cells, flow cytometry experiments with ALOX5^-/- mice, and lipidomic analyses revealed the lowering of 14-HDHA/17-HDHA/PDX and dysregulation of B cell populations in obesity was driven indirectly via B cell extrinsic mechanisms. Notably, the lowering of lipid mediators was associated with an increase in the abundance of n-6 polyunsaturated fatty acids, which have a high affinity for SPM-generating enzymes. Subsequent experiments revealed female obese mice generally maintained the levels of SPM precursors, B cell subsets, and antibody levels. Finally, obese human females had increased circulating plasma cells accompanied by ex vivo B cell TNFα and IL-10 secretion. Collectively, the data demonstrate that DHA-derived mediators of the SPM pathway control the number of B cell subsets and pro-inflammatory antibody levels in obese male but not female mice through a defect that is extrinsic to B cells.

Proresolving Lipid Mediators and Receptors in Stem Cell Biology: Concise Review

Accumulating evidence indicates that stem cells (SCs) possess immunomodulatory, anti‐inflammatory, and prohealing properties. The mechanisms underlying these functions are being investigated with the final goal to set a solid background for the clinical use of SCs and/or their derivatives. Specialized proresolving lipid mediators (SPMs) are small lipids formed by the enzymatic metabolism of polyunsaturated fatty acids. They represent a leading class of molecules that actively and timely regulate the resolution of inflammation and promote tissue/organ repair. SC formation of these mediators as well as expression of their receptors has been recently reported, suggesting that SPMs may be involved in the immunomodulatory, proresolving functions of SCs. In the present review, we summarize the current knowledge on SPMs in SCs, focusing on biosynthetic pathways, receptors, and bioactions, with the intent to provide an integrated view of SPM impact on SC biology. stem cells translational medicine2019;8:992–998

Roles, Actions, and Therapeutic Potential of Specialized Pro-resolving Lipid Mediators for the Treatment of Inflammation in Cystic Fibrosis

Non-resolving inflammation is the main mechanism of morbidity and mortality among patients suffering from cystic fibrosis (CF), the most common life-threatening human genetic disease. Resolution of inflammation is an active process timely controlled by endogenous specialized pro-resolving lipid mediators (SPMs) produced locally in inflammatory loci to restrain this innate response, prevent further damages to the host, and permit return to homeostasis. Lipoxins, resolvins, protectins, and maresins are SPM derived from polyunsaturated fatty acids that limit excessive leukocyte infiltration and pro-inflammatory signals, stimulate innate microbial killing, and enhance resolution. Their unique chemical structures, receptors, and bioactions are being elucidated. Accruing data indicate that SPMs carry protective functions against unrelenting inflammation and infections in preclinical models and human CF systems. Here, we reviewed their roles and actions in controlling resolution of inflammation, evidence for their impairment in CF, and proofs of principle for their exploitation as innovative, non-immunosuppressive drugs to address inflammation and infections in CF.

Specialized Proresolving Mediators in Innate and Adaptive Immune Responses in Airway Diseases

Airborne pathogens and environmental stimuli evoke immune responses in the lung. It is critical to health that these responses be controlled to prevent tissue damage and the compromise of organ function. Resolution of inflammation is a dynamic process that is coordinated by biochemical and cellular mechanisms. Recently, specialized proresolving mediators (SPMs) have been identified in resolution exudates. These molecules orchestrate anti-inflammatory and proresolving actions that are cell type specific. In this review, we highlight SPM biosynthesis, the influence of SPMs on the innate and adaptive immune responses in the lung, as well as recent insights from SPMs on inflammatory disease pathophysiology. Uncovering these mediators and cellular mechanisms for resolution is providing new windows into physiology and disease pathogenesis.

Immunoresolvents in asthma and allergic diseases: Review and update

Asthma and allergic diseases are inflammatory conditions developed by excessive reaction of the immune system against normally harmless environmental substances. Although acute inflammation is necessary to eradicate the damaging agents, shifting to chronic inflammation can be potentially detrimental. Essential fatty-acids-derived immunoresolvents, namely, lipoxins, resolvins, protectins, and maresins, are anti-inflammatory compounds that are believed to have protective and beneficial effects in inflammatory disorders, including asthma and allergies. Accordingly, impaired biosynthesis and defective production of immunoresolvents could be involved in the development of chronic inflammation. In this review, recent evidence on the anti-inflam]matory effects of immunoresolvents, their enzymatic biosynthesis routes, as well as their receptors are discussed.

Eicosanoid Control Over Antigen Presenting Cells in Asthma

Asthma is a common lung disease affecting 300 million people worldwide. Allergic asthma is recognized as a prototypical Th2 disorder, orchestrated by an aberrant adaptive CD4+ T helper (Th2/Th17) cell immune response against airborne allergens, that leads to eosinophilic inflammation, reversible bronchoconstriction, and mucus overproduction. Other forms of asthma are controlled by an eosinophil-rich innate ILC2 response driven by epithelial damage, whereas in some patients with more neutrophilia, the disease is driven by Th17 cells. Dendritic cells (DCs) and macrophages are crucial regulators of type 2 immunity in asthma. Numerous lipid mediators including the eicosanoids prostaglandins and leukotrienes influence key functions of these cells, leading to either pro- or anti-inflammatory effects on disease outcome. In this review, we will discuss how eicosanoids affect the functions of DCs and macrophages in the asthmatic lung and how this leads to aberrant T cell differentiation that causes disease.

Emerging cellular functions of the lipid metabolizing enzyme 15-Lipoxygenase-1

The oxygenation of polyunsaturated fatty acids such as arachidonic and linoleic acid through lipoxygenases (LOXs) and cyclooxygenases (COXs) leads to the production of bioactive lipids that are important both in the induction of acute inflammation and its resolution. Amongst the several isoforms of LOX that are expressed in mammals, 15-LOX-1 was shown to be important both in the context of inflammation, being expressed in cells of the immune system, and in epithelial cells where the enzyme has been shown to crosstalk with a number of important signalling pathways. This review looks into the latest developments in understanding the role of 15-LOX-1 in different disease states with emphasis on the emerging role of the enzyme in the tumour microenvironment as well as a newly re-discovered form of cell death called ferroptosis. We also discuss future perspectives on the feasibility of use of this protein as a target for therapeutic interventions.

Lipoxin A4 attenuates LPS-induced acute lung injury via activation of the ACE2-Ang-(1-7)-Mas axis

Previous studies have reported that lipoxin A4 (LXA4) and the angiotensin I-converting enzyme 2 (ACE2), angiotensin-(1-7) [Ang-(1-7)], and its receptor Mas [ACE2-Ang-(1-7)-Mas] axis play important protective roles in acute lung injury (ALI). However, there is still no direct evidence of LXA4-mediated protection via the ACE2-Ang-(1-7)-Mas axis during ALI. This work was performed using an LPS-induced ALI mouse model and the data indicated the following. First, the animal model was established successfully and LXA4 ameliorated LPS-induced ALI. Second, LXA4 could increase the concentration and activity of ACE2 and the levels of Ang-(1-7) and Mas markedly. Third, LXA4 decreased the levels of TNF-α, IL-1β, and reactive oxygen species while increasing IL-10 levels. Fourth, LXA4 inhibited the activation of the NF-κB signal pathway and repressed the degradation of inhibitor of NF-κB, the phosphorylation of NF-κB, and the translocation of NF-κB. Finally, and more importantly, BOC-2 (LXA4 receptor inhibitor), MLN-4760 (ACE2 inhibitor), and A779 (Mas receptor antagonist) were found to reverse all of the effects of LXA4. Our data provide evidence that LXA4 protects the lung from ALI through regulation of the ACE2-Ang-(1-7)-Mas axis.

Identification of differentially expressed genes in childhood asthma

Asthma has been the most common chronic disease in children that places a major burden for affected people and their families.An integrated analysis of microarrays studies was performed to identify differentially expressed genes (DEGs) in childhood asthma compared with normal control. We also obtained the differentially methylated genes (DMGs) in childhood asthma according to GEO. The genes that were both differentially expressed and differentially methylated were identified. Functional annotation and protein-protein interaction network construction were performed to interpret biological functions of DEGs. We performed q-RT-PCR to verify the expression of selected DEGs.One DNA methylation and 3 gene expression datasets were obtained. Four hundred forty-one DEGs and 1209 DMGs in childhood asthma were identified. Among which, 16 genes were both differentially expressed and differentially methylated in childhood asthma. Natural killer cell mediated cytotoxicity pathway, Jak-STAT signaling pathway, and Wnt signaling pathway were 3 significantly enriched pathways in childhood asthma according to our KEGG enrichment analysis. The PPI network of top 20 up- and downregulated DEGs consisted of 822 nodes and 904 edges and 2 hub proteins (UBQLN4 and MID2) were identified. The expression of 8 DEGs (GZMB, FGFBP2, CLC, TBX21, ALOX15, IL12RB2, UBQLN4) was verified by qRT-PCR and only the expression of GZMB and FGFBP2 was inconsistent with our integrated analysis.Our finding was helpful to elucidate the underlying mechanism of childhood asthma and develop new potential diagnostic biomarker and provide clues for drug design.

Towards targeting resolution pathways of airway inflammation in asthma

Asthma is a chronic disorder characterized by persistent inflammation of the airways with mucosal infiltration of eosinophils, T lymphocytes, and mast cells, and release of proinflammatory cytokines and lipid mediators. The natural resolution of airway inflammation is now recognized as an active host response, with highly coordinated cellular events under the control of endogenous pro-resolving mediators that enable the restoration of tissue homeostasis. Lead members of proresolving mediators are enzymatically derived from essential polyunsaturated fatty acids, including arachidonic acid-derived lipoxins, eicosapentaenoic acid-derived E-series resolvins, and docosahexaenoic acid-derived D-series resolvins, protectins, and maresins. Functionally, these specialized pro-resolving mediators can limit further leukocyte recruitment, induce granulocyte apoptosis, and enhance efferocytosis by macrophages. They can also switch macrophages from classical to alternatively activated cells, promote the return of non-apoptotic cells to lymphatics and blood vessels, and help initiate tissue repair and healing. In this review, we highlight cellular and molecular mechanisms for successful resolution of inflammation, and describe the main specialized pro-resolving mediators that drive these processes. Furthermore, we report recent data suggesting that the pathobiology of severe asthma may result in part from impaired resolution of airway inflammation, including defects in the biosynthesis of these specialized pro-resolving mediators. Finally, we discuss resolution-based therapeutic perspectives.

NLRP3 Inflammasome Deficiency Protects against Microbial Sepsis via Increased Lipoxin B4 Synthesis

RATIONALE

Sepsis, a life-threatening organ dysfunction caused by a dysregulated host response to infection, is a major public health concern with high mortality and morbidity. Although inflammatory responses triggered by infection are crucial for host defense against invading microbes, the excessive inflammation often causes tissue damage leading to organ dysfunction. Resolution of inflammation, an active immune process mediated by endogenous lipid mediators (LMs), is important to maintain host homeostasis.

OBJECTIVES

We sought to determine the role of the nucleotide-binding domain, leucine-rich repeat-containing receptor, pyrin domain-containing-3 (NLRP3) inflammasome in polymicrobial sepsis and regulation of LM biosynthesis.

METHODS

We performed cecal ligation and puncture (CLP) using mice lacking NLRP3 inflammasome-associated molecules to assess mortality. Inflammation was evaluated by using biologic fluids including plasma, bronchoalveolar, and peritoneal lavage fluid. Local acting LMs in peritoneal lavage fluid from polymicrobacterial septic mice were assessed by mass spectrometry-based metabololipidomics.

MEASUREMENTS AND MAIN RESULTS

Genetic deficiency of NLRP3 inhibited inflammatory responses and enhanced survival of CLP-induced septic mice. NLRP3 deficiency reduced proinflammatory LMs and increased proresolving LM, lipoxin B₄ (LXB₄) in septic mice, and in macrophages stimulated with LPS and ATP. Activation of the NLRP3 inflammasome induced caspase-7 cleavage and pyroptosis. Caspase-7 deficiency similarly reduced inflammation and mortality in CLP-induced sepsis, and increased LXB₄ production in vivo and in vitro. Exogenous application of LXB₄ reduced inflammation, pyroptosis, and mortality of mice after CLP.

CONCLUSIONS

Genetic deficiency of NLRP3 promoted resolution of inflammation in polymicrobial sepsis by relieving caspase-7-dependent repression of LXB₄ biosynthesis, and increased survival potentially via LXB₄ production and inhibition of proinflammatory cytokines.

The expansive role of oxylipins on platelet biology

In mammals, three major oxygenases, cyclooxygenases (COXs), lipoxygenases (LOXs), and cytochrome P450 (CYP450), generate an assortment of unique lipid mediators (oxylipins) from polyunsaturated fatty acids (PUFAs) which exhibit pro- or anti-thrombotic activity. Over the years, novel oxylipins generated from the interplay of theoxygenase activity in various cells, such as the specialized pro-resolving mediators (SPMs), have been identified and investigated in inflammatory disease models. Although platelets have been implicated in inflammation, the role and mechanism of these SPMs produced from immune cells on platelet function are still unclear. This review highlights the burgeoning classes of oxylipins that have been found to regulate platelet function; however, their mechanism of action still remains to be elucidated.

Aspirin-triggered resolvin D1 is produced during self-resolving gram-negative bacterial pneumonia and regulates host immune responses for the resolution of lung inflammation

Bacterial pneumonia is a leading cause of morbidity and mortality worldwide. Host responses to contain infection and mitigate pathogen-mediated lung inflammation are critical for pneumonia resolution. Aspirin-triggered resolvin D1 (AT-RvD1; 7S,8R,17R-trihydroxy-4Z,9E,11E,13Z,15E,19Z-docosahexaenoic acid) is a lipid mediator (LM) that displays organ-protective actions in sterile lung inflammation, and regulates pathogen-initiated cellular responses. Here, in a self-resolving murine model of Escherichia coli pneumonia, LM metabololipidomics performed on lungs obtained at baseline, 24, and 72 h after infection uncovered temporal regulation of endogenous AT-RvD1 production. Early treatment with exogenous AT-RvD1 (1 h post infection) enhanced clearance of E. coli and Pseudomonas aeruginosa in vivo, and lung macrophage phagocytosis of fluorescent bacterial particles ex vivo. Characterization of macrophage subsets in the alveolar compartment during pneumonia identified efferocytosis by infiltrating macrophages (CD11b(Hi) CD11c(Low)) and exudative macrophages (CD11b(Hi) CD11c(Hi)). AT-RvD1 increased efferocytosis by these cells ex vivo, and accelerated neutrophil clearance during pneumonia in vivo. These anti-bacterial and pro-resolving actions of AT-RvD1 were additive to antibiotic therapy. Taken together, these findings suggest that the pro-resolving actions of AT-RvD1 during pneumonia represent a novel host-directed therapeutic strategy to complement the current antibiotic-centered approach for combatting infections.

Structural and functional basis of phospholipid oxygenase activity of bacterial lipoxygenase from Pseudomonas aeruginosa

Pseudomonas aeruginosa expresses a secreted LOX-isoform (PA-LOX, LoxA) capable of oxidizing polyenoic fatty acids to hydroperoxy derivatives. Here we report high-level expression of this enzyme in E. coli and its structural and functional characterization. Recombinant PA-LOX oxygenates polyenoic fatty acids including eicosapentaenoic acid and docosahexaenoic acid to the corresponding (n-6)S-hydroperoxy derivatives. This reaction involves abstraction of the proS-hydrogen from the n-8 bisallylic methylene. PA-LOX lacks major leukotriene synthase activity but converts 5S-HETE and 5S,6R/S-DiHETE to anti-inflammatory and pro-resolving lipoxins. It also exhibits phospholipid oxygenase activity as indicated by the formation of a specific pattern of oxygenation products from different phospholipid subspecies. Multiple mutagenesis studies revealed that PA-LOX does not follow classical concepts explaining the reaction specificity of mammalian LOXs. The crystal structure of PA-LOX was solved with resolutions of up to 1.48Å and its polypeptide chain is folded as single domain. The substrate-binding pocket consists of two fatty acid binding subcavities and lobby. Subcavity-1 contains the catalytic non-heme iron. A phosphatidylethanolamine molecule occupies the substrate-binding pocket and its sn1 fatty acid is located close to the catalytic non-heme iron. His377, His382, His555, Asn559 and the C-terminal Ile685 function as direct iron ligands and a water molecule (hydroxyl) completes the octahedral ligand sphere. Although the biological relevance of PA-LOX is still unknown its functional characteristics (lipoxin synthase activity) implicate this enzyme in a bacterial evasion strategy aimed at downregulating the hosts' immune system.

The double-edged role of 12/15-lipoxygenase during inflammation and immunity

12/15-Lipoxygenase (12/15-LOX) mediates the enzymatic oxidation of polyunsaturated fatty acids, thereby contributing to the generation of various bioactive lipid mediators. Although 12/15-LOX has been implicated in the pathogenesis of multiple chronic inflammatory diseases, its physiologic functions seem to include potent immune modulatory properties that physiologically contribute to the resolution of inflammation and the clearance of inflammation-associated tissue damage. This review aims to give a comprehensive overview about our current knowledge on the role of this enzyme during the regulation of inflammation and immunity. This article is part of a Special Issue entitled: Lipid modification and lipid peroxidation products in innate immunity and inflammation edited by Christoph J. Binder.

Biochemical and Cellular Characterization and Inhibitor Discovery of Pseudomonas aeruginosa 15-Lipoxygenase

Pseudomonas aeruginosa is an opportunistic pathogen that can cause nosocomial and chronic infections in immunocompromised patients. P. aeruginosa secretes a lipoxygenase, LoxA, but the biological role of this enzyme is currently unknown. LoxA is poorly similar in sequence to both soybean LOX-1 (s15-LOX-1) and human 15-LOX-1 (37 and 39%, respectively) yet has kinetics comparably fast versus those of s15-LOX-1 (at pH 6.5, Kcat = 181 ± 6 s(-1) and Kcat/KM = 16 ± 2 μM(-1) s(-1)). LoxA is capable of efficiently catalyzing the peroxidation of a broad range of free fatty acid (FA) substrates (e.g., AA and LA) with high positional specificity, indicating a 15-LOX. Its mechanism includes hydrogen atom abstraction [a kinetic isotope effect (KIE) of >30], yet LoxA is a poor catalyst against phosphoester FAs, suggesting that LoxA is not involved in membrane decomposition. LoxA also does not react with 5- or 15-HETEs, indicating poor involvement in lipoxin production. A LOX high-throughput screen of the LOPAC library yielded a variety of low-micromolar inhibitors; however, none selectively targeted LoxA over the human LOX isozymes. With respect to cellular activity, the level of LoxA expression is increased when P. aeruginosa undergoes the transition to a biofilm mode of growth, but LoxA is not required for biofilm growth on abiotic surfaces. However, LoxA does appear to be required for biofilm growth in association with the host airway epithelium, suggesting a role for LoxA in mediating bacterium-host interactions during colonization.

Strict Regiospecificity of Human Epithelial 15-Lipoxygenase-2 Delineates Its Transcellular Synthesis Potential

Lipoxins are an important class of lipid mediators that induce the resolution of inflammation and arise from transcellular exchange of arachidonic acid (AA)-derived lipoxygenase products. Human epithelial 15-lipoxygenase-2 (h15-LOX-2), the major lipoxygenase in macrophages, has exhibited strict regiospecificity, catalyzing only the hydroperoxidation of carbon 15 of AA. To determine the catalytic potential of h15-LOX-2 in transcellular synthesis events, we reacted it with the three lipoxygenase-derived monohydroperoxy-eicosatetraenoic acids (HPETE) in humans: 5-HPETE, 12-HPETE, and 15-HPETE. Only 5-HPETE was a substrate for h15-LOX-2, and the steady-state catalytic efficiency (kcat/Km) of this reaction was 31% of the kcat/Km of AA. The only major product of h15-LOX-2's reaction with 5-HPETE was the proposed lipoxin intermediate, 5,15-dihydroperoxy-eicosatetraenoic acid (5,15-diHPETE). However, h15-LOX-2 did not react further with 5,15-diHPETE to produce lipoxins. This result is consistent with the specificity of h15-LOX-2 despite the increased reactivity of 5,15-diHPETE. Density functional theory calculations determined that the radical, after abstracting the C10 hydrogen atom from 5,15-diHPETE, had an energy 5.4 kJ/mol lower than that of the same radical generated from AA, demonstrating the facility of 5,15-diHPETE to form lipoxins. Interestingly, h15-LOX-2 does react with 5S,6R-diHETE, forming LipoxinA4, indicating the gemdiol does not prohibit h15-LOX-2 reactivity. Taken together, these results demonstrate the strict regiospecificity of h15-LOX-2 that circumscribes its role in transcellular synthesis.

Lipoxin Signaling in Murine Lung Host Responses to Cryptococcus neoformans Infection

Lipoxins (LX) are proresolving mediators that augment host defense against bacterial infection. Here, we investigated roles for LX in lung clearance of the fungal pathogen Cryptococcus neoformans (Cne). After intranasal inoculation of 5,000 CFU Cne, C57BL/6 and C.B-17 mice exhibited strain-dependent differences in Cne clearance, immunologic responses, and lipoxin A4 (LXA4) formation and receptor (ALX/FPR2) expression. Compared with C.B-17 mice, C57BL/6 lungs had increased and persistent Cne infection 14 days after inoculation, increased eosinophils, and distinct profiles of inflammatory cytokines. Relative to C.B-17 mice, bronchoalveolar lavage fluid levels of LXA4 were increased before and after infection in C57BL/6. The kinetics for 15-epi-LXA4 production were similar in both strains. Lung basal expression of the LX biosynthetic enzyme Alox12/15 (12/15-lipoxygenase) was increased in C57BL/6 mice and further increased after Cne infection. In contrast, lung basal expression of the LXA4 receptor Alx/Fpr2 was higher in C.B-17 relative to C57BL/6 mice, and after Cne infection, Alx/Fpr2 expression was significantly increased in only C.B-17 mice. Heat-killed Cne initiated lung cell generation of IFN-γ and IL-17 and was further increased in C.B-17 mice by 15-epi-LXA4. A trend toward reduced Cne clearance and IFN-γ production was observed upon in vivo administration of an ALX/FPR2 antagonist. Together, these findings provide the first evidence that alterations in cellular immunity against Cne are associated with differences in LXA4 production and receptor expression, suggesting an important role for ALX/FPR2 signaling in the regulation of pathogen-mediated inflammation and antifungal lung host defense.

Key mechanisms governing resolution of lung inflammation

Innate immunity normally provides excellent defence against invading microorganisms. Acute inflammation is a form of innate immune defence and represents one of the primary responses to injury, infection and irritation, largely mediated by granulocyte effector cells such as neutrophils and eosinophils. Failure to remove an inflammatory stimulus (often resulting in failed resolution of inflammation) can lead to chronic inflammation resulting in tissue injury caused by high numbers of infiltrating activated granulocytes. Successful resolution of inflammation is dependent upon the removal of these cells. Under normal physiological conditions, apoptosis (programmed cell death) precedes phagocytic recognition and clearance of these cells by, for example, macrophages, dendritic and epithelial cells (a process known as efferocytosis). Inflammation contributes to immune defence within the respiratory mucosa (responsible for gas exchange) because lung epithelia are continuously exposed to a multiplicity of airborne pathogens, allergens and foreign particles. Failure to resolve inflammation within the respiratory mucosa is a major contributor of numerous lung diseases. This review will summarise the major mechanisms regulating lung inflammation, including key cellular interplays such as apoptotic cell clearance by alveolar macrophages and macrophage/neutrophil/epithelial cell interactions. The different acute and chronic inflammatory disease states caused by dysregulated/impaired resolution of lung inflammation will be discussed. Furthermore, the resolution of lung inflammation during neutrophil/eosinophil-dominant lung injury or enhanced resolution driven via pharmacological manipulation will also be considered.

Control of local immunity by airway epithelial cells

The lung is ventilated by thousand liters of air per day. Inevitably, the respiratory system comes into contact with airborne microbial compounds, most of them harmless contaminants. Airway epithelial cells are known to have innate sensor functions, thus being able to detect microbial danger. To avoid chronic inflammation, the pulmonary system has developed specific means to control local immune responses. Even though airway epithelial cells can act as proinflammatory promoters, we propose that under homeostatic conditions airway epithelial cells are important modulators of immune responses in the lung. In this review, we discuss epithelial cell regulatory functions that control reactivity of professional immune cells within the microenvironment of the airways and how these mechanisms are altered in pulmonary diseases. Regulation by epithelial cells can be divided into two mechanisms: (1) mediators regulate epithelial cells' innate sensitivity in cis and (2) factors are produced that limit reactivity of immune cells in trans.

The Protectin PCTR1 Is Produced by Human M2 Macrophages and Enhances Resolution of Infectious Inflammation

Inflammation and its natural resolution are host-protective responses triggered by infection or injury. The resolution phase of inflammation is regulated by enzymatically produced specialized pro-resolving mediators. We recently identified a new class of peptide-conjugated specialized pro-resolving mediators that carry potent tissue regenerative actions that belong to the protectin family and are coined protectin conjugates in tissue regeneration (PCTR). Herein, with the use of microbial-induced peritonitis in mice and liquid chromatography-tandem mass spectrometry-based lipid mediator metabololipidomics, we found that PCTR1 is temporally regulated during self-resolving infection. When administered at peak of inflammation, PCTR1 enhanced macrophage recruitment and phagocytosis of Escherichia coli, decreased polymorphonuclear leukocyte infiltration, and counter-regulated inflammation-initiating lipid mediators, including prostaglandins. In addition, biologically produced PCTR1 promoted human monocyte and macrophage migration in a dose-dependent manner (0.001 to 10.0 nmol/L). We prepared PCTR1 via organic synthesis and confirmed that synthetic PCTR1 increased macrophage and monocyte migration, enhanced macrophage efferocytosis, and accelerated tissue regeneration in planaria. With human macrophage subsets, PCTR1 levels were significantly higher in M2 macrophages than in M1 phenotype, along with members of the resolvin conjugates in tissue regeneration and maresin conjugate families. In contrast, M1 macrophages gave higher levels of cysteinyl leukotrienes. Together, these results demonstrate that PCTR1 is a potent monocyte/macrophage agonist, regulating key anti-inflammatory and pro-resolving processes during bacterial infection.

Specialized pro-resolving mediators: endogenous regulators of infection and inflammation

The immune response comprises not only pro-inflammatory and anti-inflammatory pathways but also pro-resolution mechanisms that serve to balance the need of the host to target microbial pathogens while preventing excess inflammation and bystander tissue damage.

Specialized pro-resolving mediators (SPMs) are enzymatically derived from essential fatty acids to serve as a novel class of immunoresolvents that limit acute responses and orchestrate the clearance of tissue pathogens, dying cells and debris from the battlefield of infectious inflammation.

SPMs are composed of lipoxins, E-series and D-series resolvins, protectins and maresins. Individual members of the SPM family serve as agonists at cognate receptors to induce cell-type specific responses.

Important regulatory roles for SPMs have been uncovered in host responses to several microorganisms, including bacterial, viral, fungal and parasitic pathogens.

SPMs also promote the resolution of non-infectious inflammation and tissue injury. Defects in host SPM pathways contribute to the development of chronic inflammatory diseases.

With the capacity to enhance host defence and modulate inflammation, SPMs represent a promising translational approach to enlist host resolution programmes for the treatment of infection and excess inflammation.

Here, the authors detail our current understanding of specialized pro-resolving mediators (SPMs), a family of endogenous mediators that have important roles in promoting the resolution of inflammation. With a focus on the lungs, they discuss the contribution of SPMs to infectious and chronic inflammatory diseases and their emerging therapeutic potential.

Specialized pro-resolving mediators (SPMs) are enzymatically derived from essential fatty acids and have important roles in orchestrating the resolution of tissue inflammation — that is, catabasis. Host responses to tissue infection elicit acute inflammation in an attempt to control invading pathogens. SPMs are lipid mediators that are part of a larger family of pro-resolving molecules, which includes proteins and gases, that together restrain inflammation and resolve the infection. These immunoresolvents are distinct from immunosuppressive molecules as they not only dampen inflammation but also promote host defence. Here, we focus primarily on SPMs and their roles in lung infection and inflammation to illustrate the potent actions these mediators play in restoring tissue homeostasis after an infection.

Innate immunity is a key factor for the resolution of inflammation in asthma

The resolution of inflammation is an integral and natural part of the physiological response to tissue injury, infection and allergens or other noxious stimuli. Resolution is now recognised as an active process with highly regulated cellular and biochemical events. Recent discoveries have highlighted that innate inflammatory cells have bimodal effector functions during the inflammatory response, including active roles during the resolution process. Several mediators displaying potent pro-resolving actions have recently been uncovered. Lipoxin A4, the lead member of this new class of pro-resolving mediators, has anti-inflammatory actions on type 2 innate lymphoid cells and pro-resolving actions through natural killer cells in asthma immunobiology. Eosinophils are also able to control crucial aspects of resolution through the generation of pro-resolving mediators. Uncontrolled asthma has been associated with a defect in the generation of specialised pro-resolving mediators, including lipoxin A4 and protectin D1. Thus, bioactive stable analogue mimetics of these mediators that can harness endogenous resolution mechanisms for inflammation may offer new therapeutic strategies for asthma and airway inflammation associated diseases.

Structural and functional biology of arachidonic acid 15-lipoxygenase-1 (ALOX15)

Lipoxygenases (LOX) form a family of lipid peroxidizing enzymes, which have been implicated in a number of physiological processes and in the pathogenesis of inflammatory, hyperproliferative and neurodegenerative diseases. They occur in two of the three domains of terrestrial life (bacteria, eucarya) and the human genome involves six functional LOX genes, which encode for six different LOX isoforms. One of these isoforms is ALOX15, which has first been described in rabbits in 1974 as enzyme capable of oxidizing membrane phospholipids during the maturational breakdown of mitochondria in immature red blood cells. During the following decades ALOX15 has extensively been characterized and its biological functions have been studied in a number of cellular in vitro systems as well as in various whole animal disease models. This review is aimed at summarizing the current knowledge on the protein-chemical, molecular biological and enzymatic properties of ALOX15 in various species (human, mouse, rabbit, rat) as well as its implication in cellular physiology and in the pathogenesis of various diseases.

Lipoxin and resolvin biosynthesis is dependent on 5-lipoxygenase activating protein

Resolution of acute inflammation is an active process coordinated by proresolving lipid mediators (SPMs) such as lipoxins (LXs) and resolvins (Rvs), which are formed by the concerted action of 2 lipoxygenases (LOs). Because the exact molecular mechanisms of SPM biosynthesis are not completely understood, we aimed to investigate LX and D-type Rv formation in human leukocytes and HEK293T cells overexpressing leukotriene (LT) pathway enzymes. Activity assays in precursor (15-hydroxyeicosatetraenoic acids, 17-HDoHE)-treated granulocytes [polymorphonuclear leukocytes (PMNLs)] showed a strict dependence of LXA4/RvD1 biosynthesis on cell integrity, and incubation with recombinant human 5-LO did not lead to LX or Rv formation. Pharmacologic inhibition of 5-LO activating protein (FLAP) by MK-886 inhibited LXA4/RvD1 biosynthesis in precursor-treated PMNLs (drug concentration causing 50% inhibition ∼ 0.3/0.2 µM), as did knockdown of the enzyme in MM6 cells, and precursor-treated HEK293T overexpressing 5-LO produced high amounts of LXA4 only in the presence of FLAP. In addition, inhibition of cytosolic phospholipase A2α (cPLA2α) interfered with LXA4/RvD1 formation from exogenous precursors in PMNLs. Furthermore, inhibition of the LT synthases LTA4 hydrolase and LTC4 synthase in PMNL/platelet coincubations augmented LXA4 levels. These findings show that several enzymes known to be involved in the biosynthesis of proinflammatory LTs, such as FLAP and cPLA2α, also contribute to LX and Rv formation.

15-Lipoxygenases regulate the production of chemokines in human lung macrophages

BACKGROUND AND PURPOSE

15-Lipoxygenase (15-LOX) activity is associated with inflammation and immune regulation. The objectives of the present study were to investigate the expression of 15-LOX-1 and 15-LOX-2 and evaluate the enzymes' roles in the polarization of human lung macrophages (LMs) in response to LPS and Th2 cytokines (IL-4/-13).

EXPERIMENTAL APPROACH

LMs were isolated from patients undergoing surgery for carcinoma. The cells were cultured with a 15-LOX inhibitor (PD146176 or ML351), a COX inhibitor (indomethacin), a 5-LOX inhibitor (MK886) or vehicle and then stimulated with LPS (10 ng · mL(-1)), IL-4 (10 ng · mL(-1)) or IL-13 (50 ng · mL(-1)) for 24 h. Levels of ALOX15 (15-LOX-1) and ALOX15B (15-LOX-2) transcripts were determined by real-time quantitative PCR. Immunoassays were used to measure levels of LPS-induced cytokines (TNF-α, CCL2, CCL3, CCL4, CXCL1, CXCL8 and CXCL10) and Th2 cytokine-induced chemokines (CCL13, CCL18 and CCL22) in the culture supernatant.

KEY RESULTS

Stimulation of LMs with LPS was associated with increased expression of ALOX15B, whereas stimulation with IL-4/IL-13 induced the expression of ALOX15. PD146176 and ML351 (10 μM) reduced the release of the chemokines induced by LPS and Th2 cytokines. The effects of these 15-LOX inhibitors were maintained in the presence of indomethacin and MK886. Furthermore, indomethacin revealed the inhibitory effect of PD146176 on TNF-α release.

CONCLUSIONS AND IMPLICATIONS

Inhibition of the 15-LOX pathways is involved in the down-regulation of the in vitro production of chemokines in LMs. Our results suggest that the 15-LOX pathways have a role in the pathogenesis of inflammatory lung disorders and may thus constitute a potential drug target.

Lipoxin B₄ promotes the resolution of allergic inflammation in the upper and lower airways of mice

Chronic mucosal inflammation is the hallmark of important and common airway diseases, such as allergic rhinitis (AR) and asthma. Lipoxin A4 (LXA4) is an endogenous pro-resolving mediator for mucosal inflammation that decreases allergic and asthmatic responses. Lipoxin B4 (LXB4) is a structurally distinct member of the lipoxin family that signals in a manner distinct from LXA4. LXB4 is generated by mucosal tissues, but its actions in allergic inflammation are unknown. Here, we used murine models of AR and asthma to investigate LXB4's activity in mucosal inflammation. In the upper airway, LXB4 significantly decreased nasal mucosal leukocytes and degranulation of mast cells (MCs) and eosinophils. In the lower airway, LXB4 significantly decreased airway inflammation, mucus metaplasia, and hyper-responsiveness. Inhibition of MC degranulation in vivo by LXB4 was more potent than dexamethasone, and these agents displayed unique profiles for cytokine regulation; however, their overall anti-inflammatory actions were comparable. LXB4 decreased eotaxin-dependent eosinophil chemotaxis, IgE-mediated MC degranulation, and expression of type 2 cytokine receptors. Together, these findings indicate that LXB4 carries cell type selective and mucosal protective actions that broaden the lipoxin family's therapeutic potential for upper and lower airway catabasis.

Human Mesenchymal Stem (Stromal) Cells Promote the Resolution of Acute Lung Injury in Part through Lipoxin A4

Previous studies demonstrated that bone marrow-derived mesenchymal stem (stromal) cells (MSCs) reduce the severity of acute lung injury in animal models and in an ex vivo perfused human lung model. However, the mechanisms by which MSCs reduce lung injury are not well understood. In the present study, we tested the hypothesis that human MSCs promote the resolution of acute lung injury in part through the effects of a specialized proresolving mediator lipoxin A4 (LXA4). Human alveolar epithelial type II cells and MSCs expressed biosynthetic enzymes and receptors for LXA4. Coculture of human MSCs with alveolar epithelial type II cells in the presence of cytomix significantly increased the production of LXA4 by 117%. The adoptive transfer of MSCs after the onset of LPS-induced acute lung injury (ALI) in mice led to improved survival (48 h), and blocking the LXA4 receptor with WRW4, a LXA4 receptor antagonist, significantly reversed the protective effect of MSCs on both survival and the accumulation of pulmonary edema. LXA4 alone improved survival in mice, and it also significantly decreased the production of TNF-α and MIP-2 in bronchoalveolar lavage fluid. In summary, these experiments demonstrated two novel findings: human MSCs promote the resolution of lung injury in mice in part through the proresolving lipid mediator LXA4, and LXA4 itself should be considered as a therapeutic for acute respiratory distress syndrome.

Physiological impact of abnormal lipoxin A₄ production on cystic fibrosis airway epithelium and therapeutic potential

Lipoxin A4 has been described as a major signal for the resolution of inflammation and is abnormally produced in the lungs of patients with cystic fibrosis (CF). In CF, the loss of chloride transport caused by the mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel gene results in dehydration, mucus plugging, and reduction of the airway surface liquid layer (ASL) height which favour chronic lung infection and neutrophil based inflammation leading to progressive lung destruction and early death of people with CF. This review highlights the unique ability of LXA4 to restore airway surface hydration, to stimulate airway epithelial repair, and to antagonise the proinflammatory program of the CF airway, circumventing some of the most difficult aspects of CF pathophysiology. The report points out novel aspects of the cellular mechanism involved in the physiological response to LXA4, including release of ATP from airway epithelial cell via pannexin channel and subsequent activation of and P2Y11 purinoreceptor. Therefore, inadequate endogenous LXA4 biosynthesis reported in CF exacerbates the ion transport abnormality and defective mucociliary clearance, in addition to impairing the resolution of inflammation, thus amplifying the vicious circle of airway dehydration, chronic infection, and inflammation.

Protective effects of n-6 fatty acids-enriched diet on intestinal ischaemia/reperfusion injury involve lipoxin A4 and its receptor

BACKGROUND AND PURPOSE

Long-term intake of dietary fatty acids is known to predispose to chronic inflammation, but their effects on acute intestinal ischaemia/reperfusion (I/R) injury is unknown. The aim of this study was to determine the consequences of a diet rich in n-3 or n-6 polyunsaturated fatty acids (PUFA) on intestinal I/R-induced damage.

EXPERIMENTAL APPROACH

Mice were fed three different isocaloric diets: a balanced diet used as a control and two different PUFA-enriched diets, providing either high levels of n-3 or of n-6 PUFA. Intestinal injury was evaluated after intestinal I/R. PUFA metabolites were quantitated in intestinal tissues by LC-MS/MS.

KEY RESULTS

In control diet-fed mice, intestinal I/R caused inflammation and increased COX and lipoxygenase-derived metabolites compared with sham-operated animals. Lipoxin A4 (LxA4 ) was significantly and selectively increased after ischaemia. Animals fed a high n-3 diet did not display a different inflammatory profile following intestinal I/R compared with control diet-fed animals. In contrast, intestinal inflammation was decreased in the I/R group fed with high n-6 diet and level of LxA4 was increased post-ischaemia compared with control diet-fed mice. Blockade of the LxA4 receptor (Fpr2), prevented the anti-inflammatory effects associated with the n-6 rich diet.

CONCLUSIONS AND IMPLICATIONS

This study indicates that high levels of dietary n-6, but not n-3, PUFAs provides significant protection against intestinal I/R-induced damage and demonstrates that the endogenous production of LxA4 can be influenced by diet.