Resolvin D4 stereoassignment and its novel actions in host protection and bacterial clearance

Lipid mediators in neutrophil biology: inflammation, resolution and beyond

PURPOSE OF REVIEW

Acute inflammation is the body's first defense in response to pathogens or injury. Failure to efficiently resolve the inflammatory insult can severely affect tissue homeostasis, leading to chronic inflammation. Neutrophils play a pivotal role in eradicating infectious pathogens, orchestrating the initiation and resolution of acute inflammation, and maintaining physiological functions. The resolution of inflammation is a highly orchestrated biochemical process, partially modulated by a novel class of endogenous lipid mediators known as specialized pro-resolving mediators (SPMs). SPMs mediate their potent bioactions via activating specific cell-surface G protein-coupled receptors (GPCR).

RECENT FINDINGS

This review focuses on recent advances in understanding the multifaceted functions of SPMs, detailing their roles in expediting neutrophil apoptosis, promoting clearance by macrophages, regulating their excessive infiltration at inflammation sites, orchestrating bone marrow deployment, also enhances neutrophil phagocytosis and tissue repair mechanisms under both physiological and pathological conditions. We also focus on the novel role of SPMs in regulating bone marrow neutrophil functions, differentiation, and highlight open questions about SPMs' functions in neutrophil heterogeneity.

SUMMARY

SPMs play a pivotal role in mitigating excessive neutrophil infiltration and hyperactivity within pathological milieus, notably in conditions such as sepsis, cardiovascular disease, ischemic events, and cancer. This significant function highlights SPMs as promising therapeutic agents in the management of both acute and chronic inflammatory disorders.

Is Lipid Metabolism of Value in Cancer Research and Treatment? Part II: Role of Specialized Pro-Resolving Mediators in Inflammation, Infections, and Cancer

Acute inflammation is the body's first defense in response to pathogens or injury that is partially governed by a novel genus of endogenous lipid mediators that orchestrate the resolution of inflammation, coined specialized pro-resolving mediators (SPMs). SPMs, derived from omega-3-polyunstaturated fatty acids (PUFAs), include the eicosapentaenoic acid-derived and docosahexaenoic acid-derived Resolvins, Protectins, and Maresins. Herein, we review their biosynthesis, structural characteristics, and therapeutic effectiveness in various diseases such as ischemia, viral infections, periodontitis, neuroinflammatory diseases, cystic fibrosis, lung inflammation, herpes virus, and cancer, especially focusing on therapeutic effectiveness in respiratory inflammation and ischemia-related injuries. Resolvins are sub-nanomolar potent agonists that accelerate the resolution of inflammation by reducing excessive neutrophil infiltration, stimulating macrophage functions including phagocytosis, efferocytosis, and tissue repair. In addition to regulating neutrophils and macrophages, Resolvins control dendritic cell migration and T cell responses, and they also reduce the pro-inflammatory cytokines, proliferation, and metastasis of cancer cells. Importantly, several lines of evidence have demonstrated that Resolvins reduce tumor progression in melanoma, oral squamous cell carcinoma, lung cancer, and liver cancer. In addition, Resolvins enhance tumor cell debris clearance by macrophages in the tumor's microenvironment. Resolvins, with their unique stereochemical structure, receptors, and biosynthetic pathways, provide a novel therapeutical approach to activating resolution mechanisms during cancer progression.

Resolvin D2 attenuates LPS-induced macrophage exhaustion

Early in sepsis, a hyperinflammatory response is dominant, but later, an immunosuppressive phase dominates, and the host is susceptible to opportunistic infections. Anti-inflammatory agents may accelerate the host into immunosuppression, and few agents can reverse immunosuppression without causing inflammation. Specialized pro-resolving mediators (SPMs) such as resolvin D2 (RvD2) have been reported to resolve inflammation without being immunosuppressive, but little work has been conducted to examine their effects on immunosuppression. To assess the effects of RvD2 on immunosuppression, we established a model of macrophage exhaustion using two lipopolysaccharide (LPS) treatments or hits. THP-1 monocyte-derived macrophages were first treated with RvD2 or vehicle for 1 h. One LPS hit increased NF-κB activity 11-fold and TNF-α release 60-fold compared to unstimulated macrophages. RvD2 decreased LPS-induced NF-κB activity and TNF-α production but increased bacterial clearance. Two LPS hits reduced macrophage bacterial clearance and decreased macrophage NF-κB activity (45%) and TNF-α release (75%) compared to one LPS hit, demonstrating exhaustion. RvD2 increased NF-κB activity, TNF-α release, and bacterial clearance following two LPS hits compared to controls. TLR2 inhibition abolished RvD2-mediated changes. In a mouse sepsis model, splenic macrophage response to exogenous LPS was reduced compared to controls and was restored by in vivo administration of RvD2, supporting the in vitro results. If RvD2 was added to monocytes before differentiation into macrophages, however, RvD2 reduced LPS responses and increased bacterial clearance following both one and two LPS hits. The results show that RvD2 attenuated macrophage suppression in vitro and in vivo and that this effect was macrophage-specific.

Resolution pharmacology and the treatment of infectious diseases

Inflammation is elicited by the host in response to microbes, and is believed to be essential for protection against infection. However, we have previously hypothesized that excessive or misplaced inflammation may be a major contributor to tissue dysfunction and death associated with viral and bacterial infections. The resolutive phase of inflammation is a necessary condition to achieve homeostasis after acute inflammation. It is possible that targeting inflammation resolution may be beneficial for the host during infection. In this review, we summarize the evidence demonstrating the expression, roles and effects of the best described pro-resolving molecules in the context of bacterial and viral infections. Pro-resolving molecules play a pivotal role in modulating a spectrum of pathways associated with tissue inflammation and damage during both viral and bacterial infections. These molecules offer a blend of anti-inflammatory, pro-resolving and sometimes anti-infective benefits, all the while circumventing the undesired and immune-suppressive unwanted effects associated with glucocorticoids. Whether these beneficial effects will translate into benefits to patients clearly deserve further investigation.

Immunosuppression in Sepsis: Biomarkers and Specialized Pro-Resolving Mediators

Severe infection can lead to sepsis. In sepsis, the host mounts an inappropriately large inflammatory response in an attempt to clear the invading pathogen. This sustained high level of inflammation may cause tissue injury and organ failure. Later in sepsis, a paradoxical immunosuppression occurs, where the host is unable to clear the preexisting infection and is susceptible to secondary infections. A major issue with sepsis treatment is that it is difficult for physicians to ascertain which stage of sepsis the patient is in. Sepsis treatment will depend on the patient's immune status across the spectrum of the disease, and these immune statuses are nearly polar opposites in the early and late stages of sepsis. Furthermore, there is no approved treatment that can resolve inflammation without contributing to immunosuppression within the host. Here, we review the major mechanisms of sepsis-induced immunosuppression and the biomarkers of the immunosuppressive phase of sepsis. We focused on reviewing three main mechanisms of immunosuppression in sepsis. These are lymphocyte apoptosis, monocyte/macrophage exhaustion, and increased migration of myeloid-derived suppressor cells (MDSCs). The biomarkers of septic immunosuppression that we discuss include increased MDSC production/migration and IL-10 levels, decreased lymphocyte counts and HLA-DR expression, and increased GPR18 expression. We also review the literature on the use of specialized pro-resolving mediators (SPMs) in different models of infection and/or sepsis, as these compounds have been reported to resolve inflammation without being immunosuppressive. To obtain the necessary information, we searched the PubMed database using the keywords sepsis, lymphocyte apoptosis, macrophage exhaustion, MDSCs, biomarkers, and SPMs.

Pharmacokinetics and Changes in Lipid Mediator Profiling after Consumption of Specialized Pro-Resolving Lipid-Mediator-Enriched Marine Oil in Healthy Subjects

Omega-3 polyunsaturated fatty acids (PUFAs) play a vital role in human health, well-being, and the management of inflammatory diseases. Insufficient intake of omega-3 is linked to disease development. Specialized pro-resolving mediators (SPMs) are derived from omega-3 PUFAs and expedite the resolution of inflammation. They fall into categories known as resolvins, maresins, protectins, and lipoxins. The actions of SPMs in the resolution of inflammation involve restricting neutrophil infiltration, facilitating the removal of apoptotic cells and cellular debris, promoting efferocytosis and phagocytosis, counteracting the production of pro-inflammatory molecules like chemokines and cytokines, and encouraging a pro-resolving macrophage phenotype. This is an experimental pilot study in which ten healthy subjects were enrolled and received a single dose of 6 g of an oral SPM-enriched marine oil emulsion. Peripheral blood was collected at baseline, 3, 6, 9, 12, and 24 h post-administration. Temporal increases in plasma and serum SPM levels were found by using LC-MS/MS lipid profiling. Additionally, we characterized the temporal increases in omega-3 levels and established fundamental pharmacokinetics in both aforementioned matrices. These findings provide substantial evidence of the time-dependent elevation of SPMs, reinforcing the notion that oral supplementation with SPM-enriched products represents a valuable source of essential bioactive SPMs.

The therapeutic potential of resolvins in pulmonary diseases

Uncontrolled inflammation leads to nonspecific destruction and remodeling of tissues and can contribute to many human pathologies, including pulmonary diseases. Stimulation of inflammatory resolution is considered an important process that protects against the progression of chronic inflammatory diseases. Resolvins generated from essential omega-3 polyunsaturated fatty acids have been demonstrated to be signaling molecules in inflammation with important pro-resolving and anti-inflammatory capabilities. By binding to specific receptors, resolvins can modulate inflammatory processes such as neutrophil migration, macrophage phagocytosis and the presence of pro-inflammatory mediators to reduce inflammatory pathologies. The discovery of these pro-resolving mediators has led to a shift in drug research from suppressing pro-inflammatory molecules to investigating compounds that promote resolution to treat inflammation. The exploration of inflammatory resolution also provided the opportunity to further understand the pathophysiology of pulmonary diseases. Alterations of resolution are now linked to both the development and exacerbation of diseases such as asthma, chronic obstructive pulmonary disease, cystic fibrosis, acute respiratory distress syndrome, cancer and COVID-19. These findings have resulted in the rise of novel design and testing of innovative resolution-based therapeutics to treat diseases. Hence, this paper reviews the generation and mechanistic actions of resolvins and investigates their role and therapeutic potential in several pulmonary diseases that may benefit from resolution-based pharmaceuticals.

Specialized pro-resolving mediators enhance the clearance of dead cells

The failure to resolve inflammation underpins to several prevalent diseases, like atherosclerosis, and so identifying ways to boost resolution is unmet clinical needs. The resolution of inflammation is governed by several factors such as specialized pro-resolving mediators (SPMs) that counter-regulate pro-inflammatory pathways and promote tissue repair without compromising host defense. A major function of nearly all SPMs is to enhance the clearance of dead cells or efferocytosis. As such, phagocytes, such as macrophages, are essential cellular players in the resolution of inflammation because of their ability to rapidly and efficiently clear dead cells. This review highlights the role of SPMs in the clearance of apoptotic and necroptotic cells and offers insights into how targeting efferocytosis may provide new treatments for non-resolving diseases, like atherosclerosis.

Role of Resolvins in Inflammatory and Neuropathic Pain

Chronic pain is an unpleasant experience associated with actual or potential tissue damage. Inflammatory pain alerts the body to inflammation and promotes healing; however, unresolved inflammation can lead to chronic pain. Conversely, neuropathic pain, due to somatosensory damage, can be a disease in itself. However, inflammation plays a considerable role in the progression of both types of pain. Resolvins, derived from omega-3 fatty acids, actively suppress pro-inflammatory mediators and aid in the resolution of inflammation. Resolvins alleviate various inflammatory and neuropathic pain models by reducing hypersensitivity and regulating inflammatory cytokines and glial activation in the spinal cord and dorsal root ganglia. Thus, resolvins are a promising alternative for pain management with the potential to reduce the side effects associated with conventional medications. Continued research is crucial to unlock the therapeutic potential of resolvins and integrate them into effective clinical pain management strategies. This review aimed to evaluate the literature surrounding the resolvins in inflammatory and neuropathic pain.

Infectious neutrophil deployment is regulated by resolvin D4

Neutrophils reside in the bone marrow (BM), ready for deployment to sites of injury/infection, initiating inflammation and its resolution. Here, we report that distal infections signal to the BM via resolvins to regulate granulopoiesis and BM neutrophil deployment. Emergency granulopoiesis during peritonitis evoked changes in BM resolvin D1 (RvD1) and BM RvD4. We found that leukotriene B4 stimulates neutrophil deployment. RvD1 and RvD4 each limited neutrophilic infiltration to infections, and differently regulated BM myeloid populations: RvD1 increased reparative monocytes, and RvD4 regulated granulocytes. RvD4 disengaged emergency granulopoiesis, prevented excess BM neutrophil deployment, and acted on granulocyte progenitors. RvD4 also stimulated exudate neutrophil, monocyte, and macrophage phagocytosis, and enhanced bacterial clearance. This mediator accelerated both neutrophil apoptosis and clearance by macrophages, thus expediting the resolution phase of inflammation. RvD4 stimulated phosphorylation of ERK1/2 and STAT3 in human BM-aspirate-derived granulocytes. RvD4 in the 1 to 100 nM range stimulated whole-blood neutrophil phagocytosis of Escherichia coli. RvD4 increased BM macrophage efferocytosis of neutrophils. Together, these results demonstrate the novel functions of resolvins in granulopoiesis and neutrophil deployment, contributing to the resolution of infectious inflammation.

Stereochemistry and functions of the new cysteinyl-resolvin, 4S,5R-RCTR1, in efferocytosis and erythrophagocytosis of human senescent erythrocytes

Specialized pro-resolving lipid mediators play key functions in the resolution of the acute inflammatory response. Herein, we elucidate the stereochemical structure of the new 4S,5R-RCTR1, a cysteinyl-resolvin, recently uncovered in human leukocytes incubated with a 4S,5S-epoxy-resolvin intermediate, using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and ultra-violet (UV) spectrophotometry. With this approach, the physical properties of the new mediator prepared by total organic synthesis were matched to enzymatically produced biogenic material. In addition, we confirmed the potent biological actions of 4S,5R-RCTR1 with human M2-like macrophage phagocytosis of live bacteria, efferocytosis of apoptotic neutrophils, and erythrophagocytosis of senescent human red blood cells in a concentration-dependent manner from 0.1 to 10 nM. Taken together, these results establish the complete stereochemistry of 4S,5R-RCTR1 as 5R-glutathionyl-4S,17S-dihydroxy-6E,8E,10Z,13Z,15E,19Z-docosahexaenoic acid and give evidence of its novel bioactivities in human phagocyte responses. Moreover, they confirm and extend the stereoselective functions of the 4S,5R-RCTR1 with isolated human phagocytes of interest in the resolution of inflammation.

Validation of a Liquid-Liquid Extraction Method to Study the Temporal Production of D-Series Resolvins by Head Kidney Cells from Atlantic Salmon (Salmon salar) Exposed to Docosahexaenoic Acid

A simple and rapid method for the extraction of D-series resolvins (RvD1, RvD2, RvD3, RvD4, RvD5) released into Leibovitz's L-15 complete medium by head kidney cells from Atlantic salmon and the further determination of liquid chromatography triple quadrupole mass spectrometry is proposed. A three-level factorial design was proposed to select the optimal concentrations of internal standards that were used in the evaluation of the performance parameters, such as linear range (0.1-50 ng mL^-1), limits of detection and quantification (0.05 and 0.1 ng mL^-1, respectively), and recovery values ranging from 96.9 to 99.8%. The optimized method was used to determine the stimulated production of resolvins by head kidney cells exposed to docosahexaenoic acid, and the results indicated that it is possible that the production was controlled by circadian responses.

Resolvins and cysteinyl-containing pro-resolving mediators activate resolution of infectious inflammation and tissue regeneration

This review is a synopsis of the main points from the opening presentation by the authors in the Resolution of Inflammation session at the 8th European Workshop on Lipid Mediators held at the Karolinska Institute, Stockholm, Sweden, June 29th, 2022. Specialized pro-resolving mediators (SPM) promote tissue regeneration, control infections and resolution of inflammation. These include resolvins, protectins, maresins and the newly identified conjugates in tissue regeneration (CTRs). We reported mechanisms of CTRs in activating primordial regeneration pathways in planaria using RNA-sequencing. Also, the 4S,5S-epoxy-resolvin intermediate in the biosynthesis of resolvin D3 and resolvin D4 was prepared by total organic synthesis. Human neutrophils convert this to resolvin D3 and resolvin D4, while human M2 macrophages transformed this labile epoxide intermediate to resolvin D4 and a novel cysteinyl-resolvin that is a potent isomer of RCTR1. The novel cysteinyl-resolvin significantly accelerates tissue regeneration with planaria and inhibits human granuloma formation.

Possibility of averting cytokine storm in SARS-COV 2 patients using specialized pro-resolving lipid mediators

Fatal "cytokine storms (CS)" observed in critically ill COVID-19 patients are consequences of dysregulated host immune system and over-exuberant inflammatory response. Acute respiratory distress syndrome (ARDS), multi-system organ failure, and eventual death are distinctive symptoms, attributed to higher morbidity and mortality rates among these patients. Consequent efforts to save critical COVID-19 patients via the usage of several novel therapeutic options are put in force. Strategically, drugs being used in such patients are dexamethasone, remdesivir, hydroxychloroquine, etc. along with the approved vaccines. Moreover, it is certain that activation of the resolution process is important for the prevention of chronic diseases. Until recently Inflammation resolution was considered a passive process, rather it's an active biochemical process that can be achieved by the use of specialized pro-resolving mediators (SPMs). These endogenous mediators are an array of atypical lipid metabolites that include Resolvins, lipoxins, maresins, protectins, considered as immunoresolvents, but their role in COVID-19 is ambiguous. Recent evidence from studies such as the randomized clinical trial, in which omega 3 fatty acid was used as supplement to resolve inflammation in COVID-19, suggests that direct supplementation of SPMs or the use of synthetic SPM mimetics (which are still being explored) could enhance the process of resolution by regulating the aberrant inflammatory process and can be useful in pain relief and tissue remodeling. Here we discussed the biosynthesis of SPMs, & their mechanistic pathways contributing to inflammation resolution along with sequence of events leading to CS in COVID-19, with a focus on therapeutic potential of SPMs.

Resolution Potential of Necrotic Cell Death Pathways

During tissue damage caused by infection or sterile inflammation, not only damage-associated molecular patterns (DAMPs), but also resolution-associated molecular patterns (RAMPs) can be activated. These dying cell-associated factors stimulate immune cells localized in the tissue environment and induce the production of inflammatory mediators or specialized proresolving mediators (SPMs). Within the current prospect of science, apoptotic cell death is considered the main initiator of resolution. However, more RAMPs are likely to be released during necrotic cell death than during apoptosis, similar to what has been observed for DAMPs. The inflammatory potential of many regulated forms of necrotic cell death modalities, such as pyroptosis, necroptosis, ferroptosis, netosis, and parthanatos, have been widely studied in necroinflammation, but their possible role in resolution is less considered. In this review, we aim to summarize the relationship between necrotic cell death and resolution, as well as present the current available data regarding the involvement of certain forms of regulated necrotic cell death in necroresolution.

Major structure-activity relationships of resolvins, protectins, maresins and their analogues

Resolvins, protectins and maresins are a series of polyunsaturated fatty acid-derived molecules which play important roles in the resolution of inflammation. They are termed specialized proresolving mediators and facilitate a return to homeostasis following an inflammatory response. These molecules are currently the focus of intensive investigation, primarily for their ability to suppress inflammation in chronic disease states. Researchers have employed different synthetic approaches to assess whether various structural modifications of these compounds could provide access to future therapeutics. This review summarizes the modifications made thus far and focuses on the key structure-activity relationships which have been uncovered for resolvins, protectins, maresins and their analogues.

Metabolic transformation of fat in obesity determines the inflammation resolving capacity of splenocardiac and cardiorenal networks in heart failure

All fats are not created equal, and despite the extensive literature, the effect of fat intake is the most debated question in obesity, cardiovascular, and cardiorenal research. Cellular and molecular mechanisms underlying cardiac dysfunction and consequent heart failure in the setting of obesity are not well understood. Our understanding of how fats are metabolically transformed after nonreperfused myocardial infarction (MI), in particular, is incomplete. Here, using male C57BL/6J mice (2 mo old), we determined the role of omega-6 fatty acids, provided as safflower oil (SO) for 12 wk, followed by supplementation with docosahexaenoic acid (DHA; n-3 fatty acids) for 8 wk before MI. With SO feeding, inflammation resolution was impaired. Specialized proresolving mediators (SPMs) increased in DHA-fed mice to reverse the effects of SO, whereas prostaglandins and thromboxane B2 were reduced in the spleen and amplified multiple resolving mechanisms in heart and kidney post-MI. DHA amplified the number of resolving macrophages and cardiac reparative pathways of the splenocardiac and cardiorenal networks in acute heart failure, with higher Treg cells in chronic heart failure and marked expression of Foxp3+ in the myocardium. Our findings indicate that surplus ingestion of SO intensified systemic, baseline, nonresolving inflammation, and DHA intake dominates splenocardiac resolving phase with the biosynthesis of SPMs and controlled cardiorenal inflammation in heart failure survivor mice.NEW & NOTEWORTHY Chronic and surplus dietary intake of safflower oil (SO) increased plasma creatinine dysregulated post-MI splenocardiac inflammation coincides with the dysfunctional cardiorenal network. In contrast, docosahexaenoic acid (DHA) increases post-MI survival in chronic heart failure. DHA transforms into specialized proresolving mediators (SPMs) and limited proinflammatory prostaglandins and thromboxanes following myocardial infarction (MI). DHA promotes Ly6Clow resolving macrophages and T regulatory cells (Foxp3+) in a splenocardiac manner post-MI.

Molecular Pharmacology of Inflammation Resolution in Atherosclerosis

Atherosclerosis is one of the most important problems of modern medicine as it is the leading cause of hospitalizations, disability, and mortality. The key role in the development and progression of atherosclerosis is the imbalance between the activation of inflammation in the vascular wall and the mechanisms of its control. The resolution of inflammation is the most important physiological mechanism that is impaired in atherosclerosis. The resolution of inflammation has complex, not fully known mechanisms, in which lipid mediators derived from polyunsaturated fatty acids (PUFAs) play an important role. Specialized pro-resolving mediators (SPMs) represent a group of substances that carry out inflammation resolution and may play an important role in the pathogenesis of atherosclerosis. SPMs include lipoxins, resolvins, maresins, and protectins, which are formed from PUFAs and regulate many processes related to the active resolution of inflammation. Given the physiological importance of these substances, studies examining the possibility of pharmacological effects on inflammation resolution are of interest.

The effect of polyunsaturated fatty acids on the severity and mortality of COVID patients: A systematic review

Background

Covid-19 mortality is largely associated with a severe increase in inflammatory cytokines and polyunsaturated fatty acids (PUFAs) play an important role in modulating immune pathways and inflammatory responses; so this study was done to evaluate the effect of polyunsaturated fatty acids on the prognosis of Covid-19 disease.

Methods and materials

A comprehensive search was conducted in PubMed, Scopus and Web of Science. For systematic identification, the search was performed based on the following keywords COVID-19, SARS-CoV-2, COVID, Coronavirus Disease 19, SARS COV- 2 Infection, SARS-CoV-2, COVID19, Coronavirus Disease, Fatty Acids, Omega-3, Omega-3 Fatty Acid, Omega-6, n 3 Fatty and Omega-9 in the mentioned databases, using OR, and AND. All searched articles were included in the study and retrieved, and End-Note X7 software was used to manage the studies.

Results

Findings on the relationship between omega-3 and omega-6 fatty acids and the risk of Covid-19 are various, but omega-3 supplements have been found to be 12 to 21% effective in reducing the risk of Covid-19. Most studies emphasized the increasing severity of the disease and the need for mechanical ventilation and hospitalization due to polyunsaturated fatty acid deficiency. It is also demonstrated that omega-3 fatty acid deficiency increased mortality in patients with Covid-19. However, there is also a warning that in critical cases, elevated levels of fatty acids in patients' lungs and a cytokine storm are the main reasons for mortality in Covid-19 patients.

Conclusion

Polyunsaturated fatty acids can reduce the risk of covid-19 which could be considered as a preventative, inexpensive and safe method. However, the risk of taking high-dose omega-3 supplements before or during SARS-COV-2 infection needs to be investigated.

Resolvins, Protectins, and Maresins: DHA-Derived Specialized Pro-Resolving Mediators, Biosynthetic Pathways, Synthetic Approaches, and Their Role in Inflammation

Marine organisms are an important source of natural products with unique and diverse chemical structures that may hold the key for the development of novel drugs. Docosahexaenoic acid (DHA) is an omega-3 fatty acid marine natural product playing a crucial regulatory role in the resolution of inflammation and acting as a precursor for the biosynthesis of the anti-inflammatory specialized pro-resolving mediators (SPMs) resolvins, protectins, and maresins. These metabolites exert many beneficial actions including neuroprotection, anti-hypertension, or anti-tumorigenesis. As dysregulation of SPMs is associated with diseases of prolonged inflammation, the disclosure of their bioactivities may be correlated with anti-inflammatory and pro-resolving capabilities, offering new targets for drug design. The availability of these SPMs from natural resources is very low, but the evaluation of their pharmacological properties requires their access in larger amounts, as achieved by synthetic routes. In this report, the first review of the total organic syntheses carried out for resolvins, protectins, and maresins is presented. Recently, it was proposed that DHA-derived pro-resolving mediators play a key role in the treatment of COVID-19. In this work we also review the current evidence on the structures, biosynthesis, and functional and new-found roles of these novel lipid mediators of disease resolution.

E-series resolvin metabolome, biosynthesis and critical role of stereochemistry of specialized pro-resolving mediators (SPMs) in inflammation-resolution: Preparing SPMs for long COVID-19, human clinical trials, and targeted precision nutrition

The COVID-19 pandemic has raised international awareness of the importance of rigorous scientific evidence and the havoc caused by uncontrolled excessive inflammation. Here we consider the evidence on whether the specialized pro-resolving mediators (SPMs) are ready to meet this challenge as well as targeted metabololipidomics of the resolution-inflammation metabolomes. Specific stereochemical mechanisms in the biosynthesis of SPMs from omega-3 essential fatty acids give rise to unique local-acting lipid mediators. SPMs possess stereochemically defined potent bioactive structures that are high-affinity ligands for cognate G protein-coupled surface receptors that evoke the cellular responses required for efficient resolution of acute inflammation. The SPMs biosynthesized from the major omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are coined Resolvins (resolution phase interaction products; E series and D-series), Protectins and Maresins (macrophage mediators in resolving inflammation). Their biosynthesis and stereochemical assignments are established and confirmed (>1,441 resolvin publications in PubMed.gov) as well as their functional roles on innate immune cells and adaptive immune cells (both lymphocyte T-cell subsets and B-cells). The resolution of a protective acute inflammatory response is governed mainly by phagocytes that actively clear apoptotic cells, debris, blood clots and pathogens. These resolution phase functions of the acute inflammatory response are enhanced by SPMs, which together prepare the inflammatory loci for homeostasis and stimulate tissue regeneration via activating stem cells and the biosynthesis of novel cys-SPMs (e.g. MCTRs, PCTRs and RCTRs). These cys-SPMs also activate regeneration, are organ protective and stimulate resolution of local inflammation. Herein, we review the biosynthesis and functions of the E-series resolvins, namely resolvin E1 (the first n-3 resolvin identified), resolvin E2, resolvin E3 and resolvin E4 biosynthesized from their precursor eicosapentaenoic acid (EPA), and the critical role of total organic synthesis in confirming SPM complete stereochemistry, establishing their potent functions in resolution of inflammation, and novel structures. The physical properties of each biologically derived SPM, i.e., ultra-violet (UV) absorbance, chromatographic behavior, and tandem mass spectrometry (MS2) fragmentation, were matched to SPMs biosynthesized and prepared by stereospecific total organic synthesis. We briefly review this approach, also used with the endogenous D-series resolvins, protectins and maresins confirming their potent functions in resolution of inflammation, that paves the way for their rigorous evaluation in human tissues and clinical trials. The assignment of complete stereochemistry for each of the E and D series Resolvins, Protectins and Maresins was a critical and required step that enabled human clinical studies as in SPM profiling in COVID-19 infections and experimental animal disease models that also opened the promise of resolution physiology, resolution pharmacology and targeted precision nutrition as new areas for monitoring health and disease mechanisms.

Human leukocytes selectively convert 4S,5S-epoxy-resolvin to resolvin D3, resolvin D4, and a cys-resolvin isomer

Human phagocytes have key functions in the resolution of inflammation. Here, we assessed the role of the proposed 4S,5S-epoxy-resolvin intermediate in the biosynthesis of both resolvin D3 and resolvin D4. We found that human neutrophils converted this synthetic intermediate to resolvin D3 and resolvin D4. M2 macrophages transformed this labile epoxide intermediate to resolvin D4 and a previously unknown cysteinyl-resolvin isomer without appreciable amounts of resolvin D3. M2 macrophages play critical roles in the resolution of inflammation and in wound healing. Human M2 macrophages also converted leukotriene A₄ to lipoxins. The cysteinyl-resolvin isomer significantly accelerated tissue regeneration of surgically injured planaria. In a model of human granuloma formation, the cysteinyl-resolvin isomer significantly inhibited granuloma development by human peripheral blood leukocytes. Together, these results provide evidence for a human cell type-specific role of 4S,5S-epoxy-resolvin in the biosynthesis of resolvin D3 by neutrophils, resolvin D4 by both M2 macrophages and neutrophils, and a unique cysteinyl-resolvin isomer produced by M2 macrophages that carries potent biological activities in granuloma formation and tissue regeneration.

Specialized pro-resolving mediators: biosynthesis and biological role in bacterial infections

Acute inflammation caused by bacterial infections is an essential biological defence mechanism of the host in order to neutralize and clear the invaders and to return to homeostasis. Despite its protective function, inflammation may become persistent and uncontrolled, resulting in chronic diseases and tissue destruction as consequence of the unresolved inflammatory process. Therefore, spatiotemporal induction of endogenous inflammation resolution programs that govern bacterial clearance as well as tissue repair and regeneration, are of major importance in order to enable tissues to restore functions. Lipid mediators that are de-novo biosynthesized from polyunsaturated fatty acids (PUFAs) mainly by lipoxygenases and cyclooxygenases, critically regulate the initiation, the maintenance but also the resolution of infectious inflammation and tissue regeneration. The discovery of specialized pro-resolving mediators (SPMs) generated from omega-3 PUFAs stimulated intensive research in inflammation resolution, especially in infectious inflammation elicited by bacteria. SPMs are immunoresolvents that actively terminate inflammation by limiting neutrophil influx, stimulating phagocytosis, bacterial killing and clearance as well as efferocytosis of apoptotic neutrophils and cellular debris by macrophages. Moreover, SPMs prevent collateral tissue damage, promote tissue repair and regeneration and lower antibiotic requirement. Here, we review the biosynthesis of SPMs in bacterial infections and cover specific mechanisms of SPMs that govern the resolution of bacteria-initiated inflammation.

Neurotransmitter System-Targeting Drugs Antagonize Growth of the Q Fever Agent, Coxiella burnetii, in Human Cells

Coxiella burnetii is a highly infectious, intracellular, Gram-negative bacterial pathogen that causes human Q fever, an acute flu-like illness that can progress to chronic endocarditis. C. burnetii is transmitted to humans via aerosols and has long been considered a potential biological warfare agent. Although antibiotics, such as doxycycline, effectively treat acute Q fever, a recently identified antibiotic-resistant strain demonstrates the ability of C. burnetii to resist traditional antimicrobials, and chronic disease is extremely difficult to treat with current options. These findings highlight the need for new Q fever therapeutics, and repurposed drugs that target eukaryotic functions to prevent bacterial replication are of increasing interest in infectious disease. To identify this class of anti-C. burnetii therapeutics, we screened a library of 727 FDA-approved or late-stage clinical trial compounds using a human macrophage-like cell model of infection. Eighty-eight compounds inhibited bacterial replication, including known antibiotics, antipsychotic or antidepressant treatments, antihistamines, and several additional compounds used to treat a variety of conditions. The majority of identified anti-C. burnetii compounds target host neurotransmitter system components. Serotoninergic, dopaminergic, and adrenergic components are among the most highly represented targets and potentially regulate macrophage activation, cytokine production, and autophagy. Overall, our screen identified multiple host-directed compounds that can be pursued for potential use as anti-C. burnetii drugs.

IMPORTANCECoxiella burnetii causes the debilitating disease Q fever in humans. This infection is difficult to treat with current antibiotics and can progress to long-term, potentially fatal infection in immunocompromised individuals or when treatment is delayed. Here, we identified many new potential treatment options in the form of drugs that are either FDA approved or have been used in late-stage clinical trials and target human neurotransmitter systems. These compounds are poised for future characterization as nontraditional anti-C. burnetii therapies.

Polyunsaturated fatty acids, specialized pro-resolving mediators, and targeting inflammation resolution in the age of precision nutrition

Chronic inflammation contributes toward the pathogenesis of numerous diseases including, but not limited to, obesity, autoimmunity, cardiovascular diseases, and cancers. The discovery of specialized pro-resolving mediators (SPMs), which are critical for resolving inflammation, has commenced investigation into targeting pathways of inflammation resolution to improve physiological outcomes. SPMs are predominately synthesized from the n-3 polyunsaturated fatty acids (PUFA) eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids. Therefore, one viable strategy to promote inflammation resolution would be to increase dietary intake of EPA/DHA, which are deficient in select populations. However, there are inconsistencies between the use of EPA/DHA as dietary or pharmacological supplements and improved inflammatory status. Herein, we review the literature on the relationship between the high n-6/n-3 PUFA ratio, downstream SPM biosynthesis, and inflammatory endpoints. We highlight key studies that have investigated how dietary intake of EPA/DHA increase tissue SPMs and their effects on inflammation. We also discuss the biochemical pathways by which EPA/DHA drive SPM biosynthesis and underscore mechanistic gaps in knowledge about these pathways which include a neglect for host genetics/ethnic differences in SPM metabolism, sexual dimorphism in SPM levels, and potential competition from select dietary n-6 PUFAs for enzymes of SPM synthesis. Altogether, establishing how dietary PUFAs control SPM biosynthesis in a genetic- and sex-dependent manner will drive new precision nutrition studies with EPA/DHA to prevent chronic inflammation in select populations.

Pro-Resolving Ligands Orchestrate Phagocytosis

The resolution of inflammation is a tissue protective program that is governed by several factors including specialized pro-resolving mediators (SPMs), proteins, gasses and nucleotides. Pro-resolving mediators activate counterregulatory programs to quell inflammation and promote tissue repair in a manner that does not compromise host defense. Phagocytes like neutrophils and macrophages play key roles in the resolution of inflammation because of their ability to remove debris, microbes and dead cells through processes including phagocytosis and efferocytosis. Emerging evidence suggests that failed resolution of inflammation and defective phagocytosis or efferocytosis underpins several prevalent human diseases. Therefore, understanding factors and mechanisms associated with enhancing these processes is a critical need. SPMs enhance phagocytosis and efferocytosis and this review will highlight mechanisms associated with their actions.

SARS-CoV-2 journey to the brain with a focus on potential role of docosahexaenoic acid bioactive lipid mediators

Coronavirus Disease 2019 or COVID-19 have infected till day 82,579,768 confirmed cases including 1,818,849 deaths, reported by World Health Organization WHO. COVID-19, originated by Severe Acute respiratory syndrome Coronavirus 2 (SARS-CoV-2), contributes to respiratory distress in addition to neurological symptoms in some patients. In the current review, we focused on the neurological complications associated with COVID-19. We discussed different pathways followed by RNA-virus, especially Flaviviridae family in the brain and passage through the Blood-Brain-Barrier BBB. Then, we explored SARS-CoV-2 mechanisms responsible of neuroinvasion and BBB disruption as well as the immunopathogenesis of SARS-CoV-2 in the central nervous system CNS. Since SARS-CoV-2 is an enveloped virus, enclosed in a lipid bilayer and that lipids are essential cell components playing numerous biological roles in viral infection and replication, we investigated the lipid metabolism remodeling upon coronavirus replication. We also highlighted the anti-inflammatory and neuroprotective potential of an omega-3 polyunsaturated fatty acid, docosahexaenoic acid DHA, as well as several bioactive lipid mediators. Altogether, our data allow better understanding of SARS-CoV-2 neuroinvasion and could assist in drug targeting to decline the burden of short-term and long-term neurological manifestations of SARS-CoV-2.

Long-Chain Polyunsaturated Fatty Acids (LCPUFAs) and the Developing Immune System: A Narrative Review

The immune system is complex: it involves many cell types and numerous chemical mediators. An immature immune response increases susceptibility to infection, whilst imbalances amongst immune components leading to loss of tolerance can result in immune-mediated diseases including food allergies. Babies are born with an immature immune response. The immune system develops in early life and breast feeding promotes immune maturation and protects against infections and may protect against allergies. The long-chain polyunsaturated fatty acids (LCPUFAs) arachidonic acid (AA) and docosahexaenoic acid (DHA) are considered to be important components of breast milk. AA, eicosapentaenoic acid (EPA) and DHA are also present in the membranes of cells of the immune system and act through multiple interacting mechanisms to influence immune function. The effects of AA and of mediators derived from AA are often different from the effects of the n-3 LCPUFAs (i.e., EPA and DHA) and of mediators derived from them. Studies of supplemental n-3 LCPUFAs in pregnant women show some effects on cord blood immune cells and their responses. These studies also demonstrate reduced sensitisation of infants to egg, reduced risk and severity of atopic dermatitis in the first year of life, and reduced persistent wheeze and asthma at ages 3 to 5 years, especially in children of mothers with low habitual intake of n-3 LCPUFAs. Immune markers in preterm and term infants fed formula with AA and DHA were similar to those in infants fed human milk, whereas those in infants fed formula without LCPUFAs were not. Infants who received formula plus LCPUFAs (both AA and DHA) showed a reduced risk of allergic disease and respiratory illness than infants who received standard formula. Studies in which infants received n-3 LCPUFAs report immune differences from controls that suggest better immune maturation and they show lower risk of allergic disease and respiratory illness over the first years of life. Taken together, these findings suggest that LCPUFAs play a role in immune development that is of clinical significance, particularly with regard to allergic sensitisation and allergic manifestations including wheeze and asthma.

Role of specialized pro-resolving lipid mediators and their receptors in virus infection: a promising therapeutic strategy for SARS-CoV-2 cytokine storm

Unexpected viral infections outbreaks, significantly affect human health, leading to increased mortality and life disruption. Among them is the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which emerged as a deadly pandemic, calling for intense research efforts on its pathogenicity mechanism and development of therapeutic strategies. In the SARS-CoV-2 cytokine storm, systemic inflammation has been associated with severe illness and mortality. Recent studies have demonstrated special pro-resolving lipids mediators (SPMs) lipoxins, resolvins, maresins, and protectins as potential therapeutic options for abnormal viral-triggered inflammation. Pro-resolving lipids mediators have shown great promise for the treatment of Herpes simplex virus, respiratory syncytial virus, human immunodeficiency virus, and hepatitis C virus. Based on this, studies are being conducted on their therapeutic effects in SARS-CoV-2 infection. In this review, we discussed SPMs and reviewed evidence from recent studies on SPMs as therapeutic options for viral infections, including SARS-CoV2. Based on our analysis of the previous study, we argue that SPMs are a potential treatment for SARS-CoV-2 infection and other viral infections. We expect further research on how SPMs modulate viral-triggered inflammation through G-protein-coupled receptors (GPCRs), and chemical stability and druggability of SPMs.

Lipid Mediators in Critically Ill Patients: A Step Towards Precision Medicine

A dysregulated response to systemic inflammation is a common pathophysiological feature of most conditions encountered in the intensive care unit (ICU). Recent evidence indicates that a dysregulated inflammatory response is involved in the pathogenesis of various ICU-related disorders associated with high mortality, including sepsis, acute respiratory distress syndrome, cerebral and myocardial ischemia, and acute kidney injury. Moreover, persistent or non-resolving inflammation may lead to the syndrome of persistent critical illness, characterized by acquired immunosuppression, catabolism and poor long-term functional outcomes. Despite decades of research, management of many disorders in the ICU is mostly supportive, and current therapeutic strategies often do not take into account the heterogeneity of the patient population, underlying chronic conditions, nor the individual state of the immune response. Fatty acid-derived lipid mediators are recognized as key players in the generation and resolution of inflammation, and their signature provides specific information on patients' inflammatory status and immune response. Lipidomics is increasingly recognized as a powerful tool to assess lipid metabolism and the interaction between metabolic changes and the immune system via profiling lipid mediators in clinical studies. Within the concept of precision medicine, understanding and characterizing the individual immune response may allow for better stratification of critically ill patients as well as identification of diagnostic and prognostic biomarkers. In this review, we provide an overview of the role of fatty acid-derived lipid mediators as endogenous regulators of the inflammatory, anti-inflammatory and pro-resolving response and future directions for use of clinical lipidomics to identify lipid mediators as diagnostic and prognostic markers in critical illness.

Specialized pro-resolving mediator network: an update on production and actions

Today, persistent and uncontrolled inflammation is appreciated to play a pivotal role in many diseases, such as cardiovascular diseases, neurodegenerative diseases, metabolic syndrome and many other diseases of public health concern (e.g. Coronavirus Disease 2019 (COVID-19) and periodontal disease). The ideal response to initial challenge in humans is a self-limited inflammatory response leading to complete resolution. The resolution phase is now widely recognized as a biosynthetically active process, governed by a superfamily of endogenous chemical mediators that stimulate resolution of inflammatory responses, namely specialized proresolving mediators (SPMs). Because resolution is the natural ideal response, the SPMs have gained attention. SPMs are mediators that include ω-6 arachidonic acid-derived lipoxins, ω-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)-derived resolvins, protectins and maresins, cysteinyl-SPMs, as well as n-3 docosapentaenoic acid (DPA)-derived SPMs. These novel immunoresolvents, their biosynthetic pathways and receptors have proven to promote resolution of inflammation, clearance of microbes, reduce pain and promote tissue regeneration via specific cellular and molecular mechanisms. As of 17 August, 2020, PubMed.gov reported >1170 publications for resolvins, confirming their potent protective actions from many laboratories worldwide. Since this field is rapidly expanding, we provide a short update of advances within 2-3 years from human and preclinical animal studies, together with the structural-functional elucidation of SPMs and identification of novel SPM receptors. These new discoveries indicate that SPMs, their pathways and receptors could provide a basis for new approaches for treating inflammation-associated diseases and for stimulating tissue regeneration via resolution pharmacology and precision nutrition.

Eicosapentaenoic and docosahexaenoic acid derived specialised pro-resolving mediators: Concentrations in humans and the effects of age, sex, disease and increased omega-3 fatty acid intake

Although inflammation has a physiological role, unrestrained inflammation can be detrimental, causing tissue damage and disease. Under normal circumstances inflammation is self-limiting with induction of active resolution processes. Central to these is the generation of specialised pro-resolving lipid mediators (SPMs) from eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These include resolvins, protectins and maresins whose activities have been well described in cell and animal models. A number of SPMs have been reported in plasma or serum in infants, children, healthy adults and individuals with various diseases, as well as in human sputum, saliva, tears, breast milk, urine, synovial fluid and cerebrospinal fluid and in human adipose tissue, skeletal muscle, hippocampus, skin, placenta, lymphoid tissues and atherosclerotic plaques. Differences in SPM concentrations have been reported between health and disease, as would be expected. However, sometimes SPM concentrations are lower in disease and sometimes they are higher. Human studies report that plasma or serum concentrations of some SPMs can be increased by increasing intake of EPA and DHA. However, the relationship of specific intakes of EPA and DHA to enhancement in the appearance of specific SPMs is not clear and needs a more thorough investigation. This is important because of the potential for EPA and DHA to be used more effectively in prevention and treatment of inflammatory conditions. If generation of SPMs represents an important mechanism of action of EPA and DHA, then more needs to be known about the most effective strategies by which EPA and DHA can increase SPM concentrations.

The Role of Endogenous Eicosapentaenoic Acid and Docosahexaenoic Acid-Derived Resolvins in Systemic Sclerosis

Resolvins, the member of specialized pro-resolving mediators, are produced from omega-3 polyunsaturated fatty acids as a response to an acute inflammatory process in that termination and resolution of inflammation. In the acute inflammation, these lipid mediators limit polymorphonuclear cells infiltration, proinflammatory cytokine production; promote efferocytosis, and regulate several cell types being important roles in innate and adaptive immunity. Any dysregulation or defect of the resolution phase result in prolonged, persistent inflammation and eventually fibrosis. Resolvins are implicated in the development of various chronic autoimmune diseases. Systemic sclerosis (SSc) is a very complicated, chronic autoimmune disorder proceeding with vasculopathy, inflammation, and fibrosis. Dysregulation of innate and adaptive immunity is another important contributing factor in the pathogenesis of SSc. In this review, we will focus on the different roles of this new family of lipid mediators, characterized by the ability to prevent the spread of inflammation and its chronicity in various ways and how they can control the development of fibrotic diseases like SSc.

Periodontitis: a host mediated disruption of microbial homeostasis

PURPOSE OF REVIEW

In a prolific scientific career, Dr. Robert J. Genco dedicated himself to enriching our understanding of the pathogenesis of periodontitis. During a period of time in the 1970s and 1980s, when periodontitis was considered a classic infectious disease, Bob had the foresight to investigate and characterize the immune/inflammatory response in periodontitis, particularly Juvenile Periodontitis. His leadership in this area brought to the fore our appreciation of host-microbiome interactions that many years later (2008) culminated in the realization that periodontitis is a fundamental inflammatory disease. In this review, the question of how the host regulates the inflammatory response will be addressed in the context of how more recently-discovered pathways of resolution of inflammation play a role in disease pathogenesis.

RECENT FINDINGS

The host inflammatory response to commensal organisms creates excess inflammation in susceptible individuals and likely drives the dysbiosis of the oral microbiome observed in people with Periodontitis. In periodontal health, the oral microbiome is in balance with the host response. It is the loss of this symbiotic relationship with excess inflammation and microbiome dysbiosis that characterizes progressive disease. In recent years, the role of mediators of resolution of inflammation in the loss of balance and their potential use as therapeutics to restore homeostasis has extended our knowledge of how the host drives immune responses to affect oral dysbiosis.

SUMMARY

Dr. Genco provided the foundation for our ever-emerging understanding host-microbial interactions. The discovery of inflammation resolution pathways has furthered our knowledge in periodontal homeostasis. More studies are needed to understand how the host regulates the microbiome to fulfill the ultimate goal of more efficient therapeutics for periodontitis and related inflammatory diseases.

A Modular, Enantioselective Synthesis of Resolvins D3, E1, and Hybrids

Resolvins D3 and E1 are important signaling molecules in the resolution of inflammation. Here, we report a convergent and flexible strategy to prepare these natural products using Hiyama-Denmark coupling of five- and six-membered cyclic alkenylsiloxanes to connect three resolvin fragments, and control the stereochemistry of the natural product (Z)-alkenes. The modular nature of this approach enables the synthesis of novel resolvin hybrids, opening up opportunities for more-extensive investigations of resolvin biology.

Resolvin D4 attenuates the severity of pathological thrombosis in mice

Deep vein thrombosis (DVT) is a common cardiovascular disease with a major effect on quality of life, and safe and effective therapeutic measures to efficiently reduce existent thrombus burden are scarce. Using a comprehensive targeted liquid chromatography-tandem mass spectrometry-based metabololipidomics approach, we established temporal clusters of endogenously biosynthesized specialized proresolving mediators (SPMs) and proinflammatory and prothrombotic lipid mediators during DVT progression in mice. Administration of resolvin D4 (RvD4), an SPM that was enriched at the natural onset of thrombus resolution, significantly reduced thrombus burden, with significantly less neutrophil infiltration and more proresolving monocytes in the thrombus, as well as an increased number of cells in an early apoptosis state. Moreover, RvD4 promoted the biosynthesis of other D-series resolvins involved in facilitating resolution of inflammation. Neutrophils from RvD4-treated mice were less susceptible to an ionomycin-induced release of neutrophil extracellular traps (NETs), a meshwork of decondensed chromatin lined with histones and neutrophil proteins critical for DVT development. These results suggest that delivery of SPMs, specifically RvD4, modulates the severity of thrombo-inflammatory disease in vivo and improves thrombus resolution.

New Advances in Targeting the Resolution of Inflammation: Implications for Specialized Pro-Resolving Mediator GPCR Drug Discovery

Chronic inflammation is a component of numerous diseases including autoimmune, metabolic, neurodegenerative, and cancer. The discovery and characterization of specialized pro-resolving mediators (SPMs) critical to the resolution of inflammation, and their cognate G protein-coupled receptors (GPCRs) has led to a significant increase in the understanding of this physiological process. Approximately 20 ligands, including lipoxins, resolvins, maresins, and protectins, and 6 receptors (FPR2/ALX, GPR32, GPR18, chemerin₁, BLT₁, and GPR37) have been identified highlighting the complex and multilayered nature of resolution. Therapeutic efforts in targeting these receptors have proved challenging, with very few ligands apparently progressing through to preclinical or clinical development. To date, some knowledge gaps remain in the understanding of how the activation of these receptors, and their downstream signaling, results in efficient resolution via apoptosis, phagocytosis, and efferocytosis of polymorphonuclear leukocytes (mainly neutrophils) and macrophages. SPMs bind and activate multiple receptors (ligand poly-pharmacology), while most receptors are activated by multiple ligands (receptor pleiotropy). In addition, allosteric binding sites have been identified signifying the capacity of more than one ligand to bind simultaneously. These fundamental characteristics of SPM receptors enable alternative targeting strategies to be considered, including biased signaling and allosteric modulation. This review describes those ligands and receptors involved in the resolution of inflammation, and highlights the most recent clinical trial results. Furthermore, we describe alternative mechanisms by which these SPM receptors could be targeted, paving the way for the identification of new therapeutics, perhaps with greater efficacy and fidelity.

N-3 Polyunsaturated Fatty Acids and the Resolution of Neuroinflammation

In the past few decades, as a result of their anti-inflammatory properties, n-3 long chain polyunsaturated fatty acids (n-3 LC-PUFAs), have gained greater importance in the regulation of inflammation, especially in the central nervous system (in this case known as neuroinflammation). If sustained, neuroinflammation is a common denominator of neurological disorders, including Alzheimer’s disease and major depression, and of aging. Hence, limiting neuroinflammation is a real strategy for neuroinflammatory disease therapy and treatment. Recent data show that n-3 LC-PUFAs exert anti-inflammatory properties in part through the synthesis of specialized pro-resolving mediators (SPMs) such as resolvins, maresins and protectins. These SPMs are crucially involved in the resolution of inflammation. They could be good candidates to resolve brain inflammation and to contribute to neuroprotective functions and could lead to novel therapeutics for brain inflammatory diseases. This review presents an overview 1) of brain n-3 LC-PUFAs as precursors of SPMs with an emphasis on the effect of n-3 PUFAs on neuroinflammation, 2) of the formation and action of SPMs in the brain and their biological roles, and the possible regulation of their synthesis by environmental factors such as inflammation and nutrition and, in particular, PUFA consumption.

Novel mediators and mechanisms in the resolution of infectious inflammation: evidence for vagus regulation

Excessive chronic inflammation is linked to many diseases and considered a stress factor in humans (Robbins Pathologic Basis of Disease. Philadelphia: W.B. Saunders Co., 1999, Proc Natl Acad Sci USA, 2008, 105: 17949, Immunity, 44, 2016, 44: 463, N Engl J Med, 2011, 364: 656). Today, the resolution of inflammation is widely recognized as a cellular biochemically active process involving biosynthesis of a novel superfamily of endogenous chemical signals coined specialized pro-resolving mediators (SPMs; Nature, 2014, 510:92). Herein, we review recent evidence, indicating a role for the vagus nerve and vagotomy in the regulation of lipid mediators. Vagotomy reduces pro-resolving mediators, including the lipoxins, resolvins, protectins and maresins, delaying resolution in mouse peritonitis. Vagotomy also delays resolution of Escherichia coli infection in mice. Specifically, right vagus regulates peritoneal Group 3 innate lymphoid cell (ILC-3) number and peritoneal macrophage responses with lipid mediator profile signatures with elevated pro-inflammatory eicosanoids and reduced resolvins, including the novel protective immunoresolvent agonist protectin conjugate in tissue regeneration1 (PCTR1). Acetylcholine upregulates PCTR biosynthesis, and administration of PCTR1 to vagotomized mice restores tissue resolution and host responses to E. coli infections. Results obtained with human vagus ex vivo indicate that vagus can produce both pro-inflammatory eicosanoids, such as prostaglandins and leukotrienes, as well as the SPM. Electrical stimulation of human vagus in vitro reduces both prostaglandins and leukotrienes and enhances resolvins and the other SPM. These results elucidate a host protective mechanism mediated by vagus stimulation of SPM that includes resolvins and PCTR1 to regulate myeloid antimicrobial functions and resolution of infection. Moreover, they define a new pro-resolution of inflammation reflex operative in mice and human tissue that involves a vagus SPM circuit.

Activation of Resolution Pathways to Prevent and Fight Chronic Inflammation: Lessons From Asthma and Inflammatory Bowel Disease

Formerly considered as a passive process, the resolution of acute inflammation is now recognized as an active host response, with a cascade of coordinated cellular and molecular events that promotes termination of the inflammatory response and initiates tissue repair and healing. In a state of immune fitness, the resolution of inflammation is contained in time and space enabling the restoration of tissue homeostasis. There is increasing evidence that poor and/or inappropriate resolution of inflammation participates in the pathogenesis of chronic inflammatory diseases, extending in time the actions of pro-inflammatory mechanisms, and responsible in the long run for excessive tissue damage and pathology. In this review, we will focus on how resolution can be the target for therapy in "Th1/Th17 cell-driven" immune diseases and "Th2 cell-driven" immune diseases, with inflammatory bowel diseases (IBD) and asthma, as relevant examples. We describe the main cells and mediators stimulating the resolution of inflammation and discuss how pharmacological and dietary interventions but also life style factors, physical and psychological conditions, might influence the resolution phase. A better understanding of the impact of endogenous and exogenous factors on the resolution of inflammation might open a whole area in the development of personalized therapies in non-resolving chronic inflammatory diseases.

Resolvin D5 Inhibits Neuropathic and Inflammatory Pain in Male But Not Female Mice: Distinct Actions of D-Series Resolvins in Chemotherapy-Induced Peripheral Neuropathy

Earlier studies have demonstrated that essential fatty acid-derived specialized pro-resolving mediators (SPMs) promote the resolution of inflammation and pain. However, the potential analgesic actions of SPMs in chemotherapy-induced peripheral neuropathy (CIPN) are not known. Recent results also showed sex dimorphism in immune cell signaling in neuropathic pain. Here, we evaluated the analgesic actions of D-series resolvins (RvD1, RvD2, RvD3, RvD4, and RvD5) on a CIPN in male and female mice. Paclitaxel (PTX, 2 mg/kg), given on days 0, 2, 4, and 6, produced robust mechanical allodynia in both sexes at 2 weeks. Intrathecal injection of RvD1 and RvD2 (100 ng, i.t.) at 2 weeks reversed PTX-induced mechanical allodynia in both sexes, whereas RvD3 and RvD4 (100 ng, i.t.) had no apparent effects on either sex. Interestingly, RvD5 (100 ng, i.t.) only reduced mechanical allodynia in male mice but not in female mice. Notably, PTX-induced mechanical allodynia was fully developed in Trpv1 or Trpa1 knockout mice, showing no sex differences. Also, intrathecal RvD5 reduced mechanical allodynia in male mice lacking Trpv1 or Trpa1, whereas female mice with Trpv1 or Trpa1 deficiency had no response to RvD5. Finally, RvD5-induced male-specific analgesia was also confirmed in an inflammatory pain condition. Formalin-induced second phase pain (licking and flinching) was reduced by intrathecal RvD5 in male but not female mice. These findings identified RvD5 as the first SPM that shows sex dimorphism in pain regulation. Moreover, these results suggest that specific resolvins may be used to treat CIPN, a rising health concern in cancer survivors.

Preoperative stimulation of resolution and inflammation blockade eradicates micrometastases

Cancer therapy is a double-edged sword, as surgery and chemotherapy can induce an inflammatory/immunosuppressive injury response that promotes dormancy escape and tumor recurrence. We hypothesized that these events could be altered by early blockade of the inflammatory cascade and/or by accelerating the resolution of inflammation. Preoperative, but not postoperative, administration of the nonsteroidal antiinflammatory drug ketorolac and/or resolvins, a family of specialized proresolving autacoid mediators, eliminated micrometastases in multiple tumor-resection models, resulting in long-term survival. Ketorolac unleashed anticancer T cell immunity that was augmented by immune checkpoint blockade, negated by adjuvant chemotherapy, and dependent on inhibition of the COX-1/thromboxane A2 (TXA2) pathway. Preoperative stimulation of inflammation resolution via resolvins (RvD2, RvD3, and RvD4) inhibited metastases and induced T cell responses. Ketorolac and resolvins exhibited synergistic antitumor activity and prevented surgery- or chemotherapy-induced dormancy escape. Thus, simultaneously blocking the ensuing proinflammatory response and activating endogenous resolution programs before surgery may eliminate micrometastases and reduce tumor recurrence.

Lipid metabolism in inflammation-related diseases

There are thousands of lipid species existing in cells, which belong to eight different categories. Lipids are the essential building blocks of cells. Recent studies have started to unveil the important functions of lipids in regulating cell metabolism. However, we are still at a very early stage in fully understanding the physiological and pathological functions of lipids. The application of lipidomics for studying lipid metabolism can provide a direct readout of the cellular status and broadens our understanding of the mechanisms that underpin metabolic disease states. This review provides an introduction to lipid metabolism and its role in modulating homeostasis and immunity. We also describe representative applications of lipidomics for studying lipid metabolism in inflammation-related diseases.

Omega-3 fatty acid-derived mediators that control inflammation and tissue homeostasis

Omega-3 polyunsaturated fatty acids (PUFAs), including eicosapentaenoic acid, docosapentaenoic acid and docosahexaenoic acid, display a wide range of beneficial effects in humans and animals. Many of the biological functions of PUFAs are mediated via bioactive metabolites produced by fatty acid oxygenases such as cyclooxygenases, lipoxygenases and cytochrome P450 monooxygenases. Liquid chromatography–tandem mass spectrometry-based mediator lipidomics revealed a series of novel bioactive lipid mediators derived from omega-3 PUFAs. Here, we describe recent advances on omega-3 PUFA-derived mediators, mainly focusing on their enzymatic oxygenation pathway, and their biological functions in controlling inflammation and tissue homeostasis.

Synthesis of resolvin E3 via palladium-catalyzed addition of AcOH to vinyl epoxy alcohols

(18R)- and (18S)-resolvins E3 were synthesized via the Pd-catalyzed addition of AcOH to the syn and anti isomers of vinyl epoxy alcohols.

Identification of specialized pro-resolving mediator clusters from healthy adults after intravenous low-dose endotoxin and omega-3 supplementation: a methodological validation

Specialized pro-resolving mediator(s) (SPMs) are produced from the endogenous ω-3 polyunsaturated fatty acids (PUFA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and accelerate resolution of acute inflammation. We identified specific clusters of SPM in human plasma and serum using LC-MS/MS based lipid mediator (LM) metabololipidomics in two separate laboratories for inter-laboratory validation. The human plasma cluster consisted of resolvin (Rv)E1, RvD1, lipoxin (LX)B₄, 18-HEPE, and 17-HDHA, and the human serum cluster consisted of RvE1, RvD1, AT-LXA₄, 18-HEPE, and 17-HDHA. Human plasma and serum SPM clusters were increased after ω-3 supplementation (triglyceride dietary supplements or prescription ethyl esters) and low dose intravenous lipopolysaccharide (LPS) challenge. These results were corroborated by parallel determinations with the same coded samples in a second, separate laboratory using essentially identical metabololipidomic operational parameters. In these healthy subjects, two ω-3 supplementation protocols (Study A and Study B) temporally increased the SPM cluster throughout the endotoxin-challenge time course. Study A and Study B were randomized and Study B also had a crossover design with placebo and endotoxin challenge. Endotoxin challenge temporally regulated lipid mediator production in human serum, where pro-inflammatory eicosanoid (prostaglandins and thromboxane) concentrations peaked by 8 hours post-endotoxin and SPMs such as resolvins and lipoxins initially decreased by 2 h and were then elevated at 24 hours. In healthy adults given ω-3 supplementation, the plasma concentration of the SPM cluster (RvE1, RvD1, LXB₄, 18-HEPE, and 17-HDHA) peaked at two hours post endotoxin challenge. These results from two separate laboratories with the same samples provide evidence for temporal production of specific pro-resolving mediators with ω-3 supplementation that together support the role of SPM in vivo in inflammation-resolution in humans.

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.

The roles of special proresolving mediators in pain relief

The resolution of acute inflammation, once thought to be a passive process, is now recognized as an active one. The productions of endogenous special proresolving mediators (SPMs) are involved in this process. SPMs, including lipoxins, resolvins, protectins, and maresins, are endogenous lipid mediators generated from ω-6 arachidonic acid or ω-3 poly-unsaturated fatty acids during the resolution phase of acute inflammation. They have potent anti-inflammatory and proresolving actions in various inflammatory disorders. Due to the potent proresolving and anti-inflammatory effects, SPMs are also used for pain relief. This review focuses on the mechanisms by which SPMs act on their respective G-protein-coupled receptors in immune cells and nerve cells to normalize pain via regulating inflammatory mediators, transient receptor potential ion channels, and central sensitization. SPMs may offer novel therapeutic approaches for preventing and treating pain conditions associated with inflammation.

Can Specialized Pro-resolving Mediators Deliver Benefit Originally Expected from Fish Oil?

PURPOSE OF THE REVIEW

Fish oil (FO) supplementation has historically been used by individuals suffering from cardiovascular disease and other inflammatory processes. However, a meta-analysis of several large randomized control trials (RCTs) suggested FO conferred no benefit in reducing cardiovascular risk. Skeptics surmised that the lack of benefit was related to FO dose or drug interactions; therefore, the widely accepted practice of FO consumption was brought into question.

RECENT FINDINGS

Thereafter, Serhan et al. identified specialized pro-resolving mediators (SPMs) to be one of the bioactive components and mechanisms of action of FO. SPMs are thought to enhance resolution of inflammation, as opposed to classic anti-inflammatory agents which inhibit inflammatory pathways. Numerous diseases, including persistent Inflammation, immunosuppression, and catabolic syndrome (PICS), are rooted in a burden of chronic inflammation. SPMs are gaining traction as potential therapeutic agents used to resolve inflammation in cardiovascular disorders, inflammatory bowel disease, sepsis, pancreatitis, and acute respiratory distress syndrome (ARDS). This narrative reviews the history of FO and the various studies that made the health benefits of FO inconclusive, as well as an overview of SPMs and their use in specific disease states.