Excessive localized leukotriene B4 levels dictate poor skin host defense in diabetic mice

Prostaglandin E2/Leukotriene B4 balance and viral load in distinct clinical stages of COVID-19: A cross-sectional study

BACKGROUND

Prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) are eicosanoids involved in modulation of the antiviral immune response. Recent studies have identified increased levels of several eicosanoids in the plasma and bronchoalveolar lavage of patients with coronavirus disease (COVID-19). This study investigated correlations between plasma levels of PGE2 and LTB4 and clinical severity of COVID-19.

METHODS

This cross-sectional study involved non-infected (n = 10) individuals and COVID-19 patients classified as cured (n = 13), oligosymptomatic (n = 29), severe (n = 15) or deceased (n = 11). Levels of D-dimer a, known COVID-19 severity marker, PGE2 and LTB4 were measured by ELISAs and data were analysed with respect to viral load.

RESULTS

PGE2 plasma levels were decreased in COVID-19 patients compared to the non-infected group. Changes in PGE2 and LTB4 levels did not correlate with any particular clinical presentations of COVID-19. However, LTB4 was related to decreased SARS-CoV-2 burden in patients, suggesting that only LTB4 is associated with control of viral load.

CONCLUSIONS

Our data indicate that PGE2/LTB4 plasma levels are not associated with COVID-19 clinical severity. Hospitalized patients with COVID-19 are treated with corticosteroids, which may influence the observed eicosanoid imbalance. Additional analyses are required to fully understand the participation of PGE2 receptors in the pathophysiology of COVID-19.

Exploring Skin Wound Healing Models and the Impact of Natural Lipids on the Healing Process

Cutaneous wound healing is a complex biological process involving a series of well-coordinated events aimed at restoring skin integrity and function. Various experimental models have been developed to study the mechanisms underlying skin wound repair and to evaluate potential therapeutic interventions. This review explores the diverse array of skin wound healing models utilized in research, ranging from rodent excisional wounds to advanced tissue engineering constructs and microfluidic platforms. More importantly, the influence of lipids on the wound healing process is examined, emphasizing their role in enhancing barrier function restoration, modulating inflammation, promoting cell proliferation, and promoting remodeling. Lipids, such as phospholipids, sphingolipids, and ceramides, play crucial roles in membrane structure, cell signaling, and tissue repair. Understanding the interplay between lipids and the wound microenvironment provides valuable insights into the development of novel therapeutic strategies for promoting efficient wound healing and tissue regeneration. This review highlights the significance of investigating skin wound healing models and elucidating the intricate involvement of lipids in the healing process, offering potential avenues for improving clinical outcomes in wound management.

The role of neutrophils in diabetic ulcers and targeting therapeutic strategies

Diabetic ulcers (DUs) are a common complication of diabetes with high morbidity, poor prognosis, and a high socio-economic burden. The main pathological manifestations of DUs are chronic inflammation, impaired re-epithelialization, and impaired angiogenesis. During the inflammatory phase, neutrophils are one of the main DU cell types and act by releasing neutrophil extracellular traps (NETs), leading to poor healing in DUs. This review summarizes the role of neutrophils in the pathology and treatment of DUs, with a view to potential novel therapies and therapeutic targets.

Molecular regulation of neutrophil swarming in health and disease: Lessons from the phagocyte oxidase

Neutrophil swarming is a complex coordinated process in which neutrophils sensing pathogen or damage signals are rapidly recruited to sites of infections or injuries. This process involves cooperation between neutrophils where autocrine and paracrine positive-feedback loops, mediated by receptor/ligand pairs including lipid chemoattractants and chemokines, amplify localized recruitment of neutrophils. This review will provide an overview of key pathways involved in neutrophil swarming and then discuss the cell intrinsic and systemic mechanisms by which NADPH oxidase 2 (NOX2) regulates swarming, including modulation of calcium signaling, inflammatory mediators, and the mobilization and production of neutrophils. We will also discuss mechanisms by which altered neutrophil swarming in disease may contribute to deficient control of infections and/or exuberant inflammation. Deeper understanding of underlying mechanisms controlling neutrophil swarming and how neutrophil cooperative behavior can be perturbed in the setting of disease may help to guide development of tools for diagnosis and precision medicine.

Hyperglycemia Increases Severity of Staphylococcus aureus Osteomyelitis and Influences Bacterial Genes Required for Survival in Bone

Hyperglycemia, or elevated blood glucose, renders individuals more prone to developing severe Staphylococcus aureus infections. S. aureus is the most common etiological agent of musculoskeletal infection, which is a common manifestation of disease in hyperglycemic patients. However, the mechanisms by which S. aureus causes severe musculoskeletal infection during hyperglycemia are incompletely characterized. To examine the influence of hyperglycemia on S. aureus virulence during invasive infection, we used a murine model of osteomyelitis and induced hyperglycemia with streptozotocin. We discovered that hyperglycemic mice exhibited increased bacterial burdens in bone and enhanced dissemination compared to control mice. Furthermore, infected hyperglycemic mice sustained increased bone destruction relative to euglycemic controls, suggesting that hyperglycemia exacerbates infection-associated bone loss. To identify genes contributing to S. aureus pathogenesis during osteomyelitis in hyperglycemic animals relative to euglycemic controls, we used transposon sequencing (TnSeq). We identified 71 genes uniquely essential for S. aureus survival in osteomyelitis in hyperglycemic mice and another 61 mutants with compromised fitness. Among the genes essential for S. aureus survival in hyperglycemic mice was the gene encoding superoxide dismutase A (sodA), one of two S. aureus superoxide dismutases involved in detoxifying reactive oxygen species (ROS). We determined that a sodA mutant exhibits attenuated survival in vitro in high glucose and in vivo during osteomyelitis in hyperglycemic mice. SodA therefore plays an important role during growth in high glucose and promotes S. aureus survival in bone. Collectively, these studies demonstrate that hyperglycemia increases the severity of osteomyelitis and identify genes contributing to S. aureus survival during hyperglycemic infection.

Quantitative Assessment of Low-Dose Photodynamic Therapy Effects on Diabetic Wound Healing Using Raman Spectroscopy

One of challenges that faces diabetes is the wound healing process. The delayed diabetic wound healing is caused by a complicated molecular mechanism involving numerous physiological variables. Low-dose photodynamic therapy (LDPDT) provides excellent results in rejuvenation and wound healing. In this study, the LDPDT effect on diabetic wounds in mice was studied using two photosensitizers, 5-aminolevulinic acid and methylene blue, and two laser dose expositions of 1 J/cm² and 4 J/cm² by Raman spectroscopy (RS). The latter was used as a noninvasive method, providing specific information about tissue state based on the fundamental vibrational modes of its molecular components. RS allows high spatial resolution acquisition of biochemical and structural information through the generation of point spectra or spectral images. An approach to in vivo quantitative assessment of diabetic wound healing state was developed. This approach is based on an application of the principal component analysis combined with the Mahalanobis metrics to skin Raman spectra, in particular, intensities of the amide I and CH₂ bands.

Leukotrienes in Innate Immunity: Still Underappreciated after All These Years?

Leukotrienes (LTs) are lipid mediators derived from the 5-lipoxygenase pathway of arachidonate metabolism. Though best known for their role in asthma, they have broad actions that touch on virtually every aspect of mammalian biology. In a Brief Review published in the journal in 2005, we presented the existing evidence supporting a role for LTs in host defense. In this updated Brief Review, we focus on selected advances since then. We detail new insights into mechanisms and regulation of LT biosynthesis; the protective roles of LTs in the host response to diverse classes of pathogens, with an emphasis on viruses, including SARS-CoV-2; the phagocyte signal transduction mechanisms by which LTs exert their antimicrobial actions; the capacity for overexuberant LT production to promote tissue damage; and roles of LTs in the noninfectious immune-relevant conditions neuroinflammation and cancer.

Wounds under diabetic milieu: The role of immune cellar components and signaling pathways

A major challenge in the field of diabetic wound healing is to confirm the body's intrinsic mechanism that could sense the immune system damage promptly and protect the wound from non-healing. Accumulating literature indicates that macrophage, a contributor to prolonged inflammation occurring at the wound site, might play such a role in hindering wound healing. Likewise, other immune cell dysfunctions, such as persistent neutrophils and T cell infection, may also lead to persistent oxidative stress and inflammatory reaction during diabetic wound healing. In this article, we discuss recent advances in the immune cellular components in wounds under the diabetic milieu, and the role of key signaling mechanisms that compromise the function of immune cells leading to persistent wound non-healing.

Acute Inflammation in Tissue Healing

There are well-established links between acute inflammation and successful tissue repair across evolution. Innate immune reactions contribute significantly to pathogen clearance and activation of subsequent reparative events. A network of molecular and cellular regulators supports antimicrobial and tissue repair functions throughout the healing process. A delicate balance must be achieved between protection and the potential for collateral tissue damage associated with overt inflammation. In this review, we summarize the contributions of key cellular and molecular components to the acute inflammatory process and the effective and timely transition toward activation of tissue repair mechanisms. We further discuss how the disruption of inflammatory responses ultimately results in chronic non-healing injuries.

Iron Dyshomeostasis in COVID-19: Biomarkers Reveal a Functional Link to 5-Lipoxygenase Activation

Coronavirus disease 2019 (COVID-19) is characterized by a broad spectrum of clinical symptoms. After acute infection, some subjects develop a post-COVID-19 syndrome known as long-COVID. This study aims to recognize the molecular and functional mechanisms that occur in COVID-19 and long-COVID patients and identify useful biomarkers for the management of patients with COVID-19 and long-COVID. Here, we profiled the response to COVID-19 by performing a proteomic analysis of lymphocytes isolated from patients. We identified significant changes in proteins involved in iron metabolism using different biochemical analyses, considering ceruloplasmin (Cp), transferrin (Tf), hemopexin (HPX), lipocalin 2 (LCN2), and superoxide dismutase 1 (SOD1). Moreover, our results show an activation of 5-lipoxygenase (5-LOX) in COVID-19 and in long-COVID possibly through an iron-dependent post-translational mechanism. Furthermore, this work defines leukotriene B4 (LTB4) and lipocalin 2 (LCN2) as possible markers of COVID-19 and long-COVID and suggests novel opportunities for prevention and treatment.

Berberine Alleviates Insulin Resistance and Inflammation via Inhibiting the LTB4-BLT1 Axis

Background: Chronic low-grade inflammation is recognized as a key pathophysiological mechanism of insulin resistance. Leukotriene B4 (LTB4), a molecule derived from arachidonic acid, is a potent neutrophil chemoattractant. The excessive amount of LTB4 that is combined with its receptor BLT1 can cause chronic low-grade inflammation, aggravating insulin resistance. Berberine (BBR) has been shown to relieve insulin resistance due to its anti-inflammatory properties. However, it is not clear whether BBR could have any effects on the LTB4–BLT1 axis.

Methods: Using LTB4 to induce Raw264.7 and HepG2 cells, we investigated the effect of BBR on the LTB4–BLT1 axis in the progression of inflammation and insulin resistance.

Results: Upon exposure to LTB4, intracellular insulin resistance and inflammation increased in HepG2 cells, and chemotaxis and inflammation response increased in RAW264.7 cells. Interestingly, pretreatment with BBR partially blocked these changes. Our preliminary data show that BBR might act on BLT1, modulating the LTB4–BLT1 axis to alleviate insulin resistance and inflammation.

Conclusions: Our study demonstrated that BBR treatment could reduce intracellular insulin resistance and inflammation of hepatic cells, as well as chemotaxis of macrophages induced by LTB4. BBR might interact with BLT1 and alter the LTB4–BLT1 signaling pathway. This mechanism might be a novel anti-inflammatory and anti-diabetic function of BBR.

LTB4-Driven Inflammation and Increased Expression of ALOX5/ACE2 During Severe COVID-19 in Individuals With Diabetes

Diabetes is a known risk factor for severe coronavirus disease 2019 (COVID-19), the disease caused by the new coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, there is a lack of knowledge about the mechanisms involved in the evolution of COVID-19 in individuals with diabetes. We aimed to evaluate whether the chronic low-grade inflammation of diabetes could play a role in the development of severe COVID-19. We collected clinical data and blood samples of patients with and without diabetes hospitalized for COVID-19. Plasma samples were used to measure inflammatory mediators and peripheral blood mononuclear cells, for gene expression analysis of the SARS-CoV-2 main receptor system (ACE2/TMPRSS2), and for the main molecule of the leukotriene B₄ (LTB₄) pathway (ALOX5). We found that diabetes activates the LTB₄ pathway and that during COVID-19 it increases ACE2/TMPRSS2 as well as ALOX5 expression. Diabetes was also associated with COVID-19-related disorders, such as reduced oxygen saturation as measured by pulse oximetry/fraction of inspired oxygen (FiO₂) and arterial partial pressure of oxygen/FiO₂ levels, and increased disease duration. In addition, the expressions of ACE2 and ALOX5 are positively correlated, with increased expression in patients with diabetes and COVID-19 requiring intensive care assistance. We confirmed these molecular results at the protein level, where plasma LTB₄ is significantly increased in individuals with diabetes. In addition, IL-6 serum levels are increased only in individuals with diabetes requiring intensive care assistance. Together, these results indicate that LTB₄ and IL-6 systemic levels, as well as ACE2/ALOX5 blood expression, could be early markers of severe COVID-19 in individuals with diabetes.

Macrophage-Derived MicroRNA-21 Drives Overwhelming Glycolytic and Inflammatory Response during Sepsis via Repression of the PGE2/IL-10 Axis

Myeloid cells are critical for systemic inflammation, microbial control, and organ damage during sepsis. MicroRNAs are small noncoding RNAs that can dictate the outcome of sepsis. The role of myeloid-based expression of microRNA-21 (miR-21) in sepsis is inconclusive. In this study, we show that sepsis enhanced miR-21 expression in both peritoneal macrophages and neutrophils from septic C57BL/6J mice, and the deletion of miR-21 locus in myeloid cells (miR-21^Δmyel mice) enhanced animal survival, decreased bacterial growth, decreased systemic inflammation, and decreased organ damage. Resistance to sepsis was associated with a reduction of aerobic glycolysis and increased levels of the anti-inflammatory mediators PGE₂ and IL-10 in miR-21^Δmyel in vivo and in vitro. Using blocking Abs and pharmacological tools, we discovered that increased survival and decreased systemic inflammation in septic miR-21^Δmyel mice is dependent on PGE₂/IL-10-mediated inhibition of glycolysis. Together, these findings demonstrate that expression of miR-21 in myeloid cells orchestrates the balance between anti-inflammatory mediators and metabolic reprogramming that drives cytokine storm during sepsis.

Novel mechanisms of microbial crosstalk with skin innate immunity

Skin is an organ with a dynamic ecosystem that harbours pathogenic and commensal microbes, which constantly communicate amongst each other and with the host immune system. Evolutionarily, skin and its microbiota have evolved to remain in homeostasis. However, frequently this homeostatic relationship is disturbed by a variety of factors such as environmental stress, diet, genetic mutations, and the microbiome itself. Commensal microbes also play a major role in the maintenance of microbial homeostasis. In addition to their ability to limit pathogens, many skin commensals such as Staphylococcus epidermidis and Cutibacterium acnes have recently been implicated in disease pathogenesis either by directly modulating the host immune components or by supporting the expansion of other pathogenic microbes. Likewise, opportunistic skin pathogens such as Staphylococcus aureus and Staphylococcus lugdunensis are able to breach the skin and cause disease. Though much has been established about the microbiota's function in skin immunity, we are in a time where newer mechanistic insights rapidly redefine our understanding of the host/microbial interface in the skin. In this review, we provide a concise summary of recent advances in our understanding of the interplay between host defense strategies and the skin microbiota.

The role of lipid-based signalling in wound healing and senescence

Lipid-based signalling modulates several cellular processes and intercellular communication during wound healing and tissue regeneration. Bioactive lipids include but are not limited to the diverse group of eicosanoids, phospholipids, and extracellular vesicles and mediate the attraction of immune cells, initiation of inflammatory responses, and their resolution. In aged individuals, wound healing and tissue regeneration are greatly impaired, resulting in a delayed healing process and non-healing wounds. Senescent cells accumulate with age in vivo, preferably at sites implicated in age-associated pathologies and their elimination was shown to alleviate many age-associated diseases and disorders. In contrast to these findings, the transient presence of senescent cells in the process of wound healing exerts beneficial effects and limits fibrosis. Hence, clearance of senescent cells during wound healing was repeatedly shown to delay wound closure in vivo. Recent findings established a dysregulated synthesis of eicosanoids, phospholipids and extracellular vesicles as part of the senescent phenotype. This intriguing connection between cellular senescence, lipid-based signalling, and the process of wound healing and tissue regeneration prompts us to compile the current knowledge in this review and propose future directions for investigation.

Biofilm-Innate Immune Interface: Contribution to Chronic Wound Formation

Delayed wound healing can cause significant issues for immobile and ageing individuals as well as those living with co-morbid conditions such as diabetes, cardiovascular disease, and cancer. These delays increase a patient’s risk for infection and, in severe cases, can result in the formation of chronic, non-healing ulcers (e.g., diabetic foot ulcers, surgical site infections, pressure ulcers and venous leg ulcers). Chronic wounds are very difficult and expensive to treat and there is an urgent need to develop more effective therapeutics that restore healing processes. Sustained innate immune activation and inflammation are common features observed across most chronic wound types. However, the factors driving this activation remain incompletely understood. Emerging evidence suggests that the composition and structure of the wound microbiome may play a central role in driving this dysregulated activation but the cellular and molecular mechanisms underlying these processes require further investigation. In this review, we will discuss the current literature on: 1) how bacterial populations and biofilms contribute to chronic wound formation, 2) the role of bacteria and biofilms in driving dysfunctional innate immune responses in chronic wounds, and 3) therapeutics currently available (or underdevelopment) that target bacteria-innate immune interactions to improve healing. We will also discuss potential issues in studying the complexity of immune-biofilm interactions in chronic wounds and explore future areas of investigation for the field.

SOCS-1 inhibition of type I interferon restrains Staphylococcus aureus skin host defense

The skin innate immune response to methicillin-resistant Staphylococcus aureus (MRSA) culminates in the formation of an abscess to prevent bacterial spread and tissue damage. Pathogen recognition receptors (PRRs) dictate the balance between microbial control and injury. Therefore, intracellular brakes are of fundamental importance to tune the appropriate host defense while inducing resolution. The intracellular inhibitor suppressor of cytokine signaling 1 (SOCS-1), a known JAK/STAT inhibitor, prevents the expression and actions of PRR adaptors and downstream effectors. Whether SOCS-1 is a molecular component of skin host defense remains to be determined. We hypothesized that SOCS-1 decreases type I interferon production and IFNAR-mediated antimicrobial effector functions, limiting the inflammatory response during skin infection. Our data show that MRSA skin infection enhances SOCS-1 expression, and both SOCS-1 inhibitor peptide-treated and myeloid-specific SOCS-1 deficient mice display decreased lesion size, bacterial loads, and increased abscess thickness when compared to wild-type mice treated with the scrambled peptide control. SOCS-1 deletion/inhibition increases phagocytosis and bacterial killing, dependent on nitric oxide release. SOCS-1 inhibition also increases the levels of type I and type II interferon levels in vivo. IFNAR deletion and antibody blockage abolished the beneficial effects of SOCS-1 inhibition in vivo. Notably, we unveiled that hyperglycemia triggers aberrant SOCS-1 expression that correlates with decreased overall IFN signatures in the infected skin. SOCS-1 inhibition restores skin host defense in the highly susceptible hyperglycemic mice. Overall, these data demonstrate a role for SOCS-1-mediated type I interferon actions in host defense and inflammation during MRSA skin infection.

Plasma Resolvin D2 to Leukotriene B4 Ratio Is Reduced in Diabetic Patients with Ischemic Stroke and Related to Prognosis

Background

Diabetes mellitus (DM) aggravates symptoms and prognosis of acute ischemic stroke (AIS), and inflammation plays an important role therein. Resolvin D2 (RvD2) is one of the specialized pro-resolving mediators (SPMs), while leukotriene B₄ (LTB₄) is a classic proinflammatory mediator. The ratio of RvD2 to LTB₄ is an index of pro-resolving/proinflammatory balance. We aim to explore the role of RvD2/LTB₄ ratio in ischemic stroke complicated with DM.

Methods

The plasma levels of RvD2 and LTB₄ were analyzed by enzyme immunoassay in stroke patients with DM (DM + AIS group) or without DM (nonDM+AIS group). Patients were followed up at 90 days after stroke onset, and modified Rankin Score (mRS) was assessed. The association of RvD2/LTB₄ ratio with stroke severity and prognosis was also analyzed.

Results

The plasma levels of RvD2 were positively correlated to LTB₄. The RvD2/LTB₄ ratio in DM + AIS group was lower than that in the nonDM+AIS group. No correlation was found between the RvD2/LTB₄ ratio and infarct size or NIHSS score. The RvD2/LTB₄ ratio at baseline was significantly lower in the poor prognosis group (mRS ≥ 3) than that in the good prognosis group (mRS ≤ 2).

Conclusions

Our study indicated that the balance between pro-resolving and proinflammatory mediators was impaired by diabetes in ischemic stroke. The RvD2/LTB₄ ratio may serve as a biomarker of prognosis for ischemic stroke.

Leukotriene B4 licenses inflammasome activation to enhance skin host defense

The initial production of inflammatory mediators dictates host defense as well as tissue injury. Inflammasome activation is a constituent of the inflammatory response by recognizing pathogen and host-derived products and eliciting the production of IL-1β and IL-18 in addition to inducing a type of inflammatory cell death termed "pyroptosis." Leukotriene B4 (LTB4) is a lipid mediator produced quickly (seconds to minutes) by phagocytes and induces chemotaxis, increases cytokine/chemokine production, and enhances antimicrobial effector functions. Whether LTB4 directly activates the inflammasome remains to be determined. Our data show that endogenously produced LTB4 is required for the expression of pro-IL-1β and enhances inflammasome assembly in vivo and in vitro. Furthermore, LTB4-mediated Bruton's tyrosine kinase (BTK) activation is required for inflammasome assembly in vivo as well for IL-1β-enhanced skin host defense. Together, these data unveil a new role for LTB4 in enhancing the expression and assembly of inflammasome components and suggest that while blocking LTB4 actions could be a promising therapeutic strategy to prevent inflammasome-mediated diseases, exogenous LTB4 can be used as an adjuvant to boost inflammasome-dependent host defense.

Unbalanced production of LTB4/PGE2 driven by diabetes increases susceptibility to cutaneous leishmaniasis

Poorly controlled diabetes mellitus leads to several comorbidities, including susceptibility to infections. Hyperglycemia increases phagocyte responsiveness, however immune cells from people with diabetes show inadequate antimicrobial functions. We and others have shown that aberrant production of leukotriene B4 (LTB4) is detrimental to host defense in models of bacterial infection. Here, we will unveil the consequences of high glucose in the outcome of Leishmania braziliensis skin infection in people with diabetes and determine the role of LTB4 in human phagocytes. We show that diabetes leads to higher systemic levels of LTB4, IL-6 and TNF-α in cutaneous leishmaniasis. Only LTB4 correlated with blood glucose levels and healing time in diabetes comorbidity. Skin lesions of people with leishmaniasis and diabetes exhibit increased neutrophil and amastigote numbers. Monocyte-derived macrophages from these individuals showed higher L. braziliensis loads, reduced production of Reactive Oxygen Species and unbalanced LTB4/PGE2 ratio. Our data reveal a systemic inflammation driven by diabetes comorbidity in opposition to a local reduced capacity to resolve L. braziliensis infection and a worse disease outcome.

Eicosanoids in Skin Wound Healing

Wound healing is an important process in the human body to protect against external threats. A dysregulation at any stage of the wound healing process may result in the development of various intractable ulcers or excessive scar formation. Numerous factors such as growth factors, cytokines, and chemokines are involved in this process and play vital roles in tissue repair. Moreover, recent studies have demonstrated that lipid mediators derived from membrane fatty acids are also involved in the process of wound healing. Among these lipid mediators, we focus on eicosanoids such as prostaglandins, thromboxane, leukotrienes, and specialized pro-resolving mediators, which are produced during wound healing processes and play versatile roles in the process. This review article highlights the roles of eicosanoids on skin wound healing, especially focusing on the biosynthetic pathways and biological functions, i.e., inflammation, proliferation, migration, angiogenesis, remodeling, and scarring.

Epigenetic regulation of the PGE2 pathway modulates macrophage phenotype in normal and pathologic wound repair

Macrophages are a primary immune cell involved in inflammation, and their cell plasticity allows for transition from an inflammatory to a reparative phenotype and is critical for normal tissue repair following injury. Evidence suggests that epigenetic alterations play a critical role in establishing macrophage phenotype and function during normal and pathologic wound repair. Here, we find in human and murine wound macrophages that cyclooxygenase 2/prostaglandin E2 (COX-2/PGE2) is elevated in diabetes and regulates downstream macrophage-mediated inflammation and host defense. Using single-cell RNA sequencing of human wound tissue, we identify increased NF-κB-mediated inflammation in diabetic wounds and show increased COX-2/PGE2 in diabetic macrophages. Further, we identify that COX-2/PGE2 production in wound macrophages requires epigenetic regulation of 2 key enzymes in the cytosolic phospholipase A2/COX-2/PGE2 (cPLA2/COX-2/PGE2) pathway. We demonstrate that TGF-β-induced miRNA29b increases COX-2/PGE2 production via inhibition of DNA methyltransferase 3b-mediated hypermethylation of the Cox-2 promoter. Further, we find mixed-lineage leukemia 1 (MLL1) upregulates cPLA2 expression and drives COX-2/PGE2. Inhibition of the COX-2/PGE2 pathway genetically (Cox2fl/fl Lyz2Cre+) or with a macrophage-specific nanotherapy targeting COX-2 in tissue macrophages reverses the inflammatory macrophage phenotype and improves diabetic tissue repair. Our results indicate the epigenetically regulated PGE2 pathway controls wound macrophage function, and cell-targeted manipulation of this pathway is feasible to improve diabetic wound repair.

The role of the LTB4-BLT1 axis in health and disease

Leukotriene B4 (LTB4) is a major type of lipid mediator that is rapidly generated from arachidonic acid through sequential action of 5-lipoxygenase (5-LO), 5-lipoxygenase-activating protein (FLAP) and LTA4 hydrolase (LTA4H) in response to various stimuli. LTB4 is well known to be a chemoattractant for leukocytes, particularly neutrophils, via interaction with its high-affinity receptor BLT1. Extensive attention has been paid to the role of the LTB4-BLT1 axis in acute and chronic inflammatory diseases, such as infectious diseases, allergy, autoimmune diseases, and metabolic disease via mediating recruitment and/or activation of different types of inflammatory cells depending on different stages or the nature of inflammatory response. Recent studies also demonstrated that LTB4 acts on non-immune cells via BLT1 to initiate and/or amplify pathological inflammation in various tissues. In addition, emerging evidence reveals a complex role of the LTB4-BLT1 axis in cancer, either tumor-inhibitory or tumor-promoting, depending on the different target cells. In this review, we summarize both established understanding and the most recent progress in our knowledge about the LTB4-BLT1 axis in host defense, inflammatory diseases and cancer.

Concepts of GPCR-controlled navigation in the immune system

G‐protein–coupled receptor (GPCR) signaling is essential for the spatiotemporal control of leukocyte dynamics during immune responses. For efficient navigation through mammalian tissues, most leukocyte types express more than one GPCR on their surface and sense a wide range of chemokines and chemoattractants, leading to basic forms of leukocyte movement (chemokinesis, haptokinesis, chemotaxis, haptotaxis, and chemorepulsion). How leukocytes integrate multiple GPCR signals and make directional decisions in lymphoid and inflamed tissues is still subject of intense research. Many of our concepts on GPCR‐controlled leukocyte navigation in the presence of multiple GPCR signals derive from in vitro chemotaxis studies and lower vertebrates. In this review, we refer to these concepts and critically contemplate their relevance for the directional movement of several leukocyte subsets (neutrophils, T cells, and dendritic cells) in the complexity of mouse tissues. We discuss how leukocyte navigation can be regulated at the level of only a single GPCR (surface expression, competitive antagonism, oligomerization, homologous desensitization, and receptor internalization) or multiple GPCRs (synergy, hierarchical and non‐hierarchical competition, sequential signaling, heterologous desensitization, and agonist scavenging). In particular, we will highlight recent advances in understanding GPCR‐controlled leukocyte navigation by intravital microscopy of immune cells in mice.

The Dynamics of the Skin's Immune System

The skin is a complex organ that has devised numerous strategies, such as physical, chemical, and microbiological barriers, to protect the host from external insults. In addition, the skin contains an intricate network of immune cells resident to the tissue, crucial for host defense as well as tissue homeostasis. In the event of an insult, the skin-resident immune cells are crucial not only for prevention of infection but also for tissue reconstruction. Deregulation of immune responses often leads to impaired healing and poor tissue restoration and function. In this review, we will discuss the defensive components of the skin and focus on the function of skin-resident immune cells in homeostasis and their role in wound healing.

Eicosanoids in tissue repair

Trauma or infection can result in tissue damage, which needs to be repaired in a well-orchestrated manner to restore tissue function and homeostasis. Lipid mediators derived from arachidonic acid (termed eicosanoids) play central and versatile roles in the regulation of tissue repair. Here, I summarize the current state-of the-art regarding the functional activities of eicosanoids in tissue repair responses during homeostasis and disease. I also describe how eicosanoids are produced during tissue damage and repair in a time-, cell- and tissue-dependent fashion. In particular, recent insights into the roles of eicosanoids in epithelial barrier repair are reviewed. Furthermore, the distinct roles of different eicosanoids in settings of pathological tissue repair such as chronic wounds, scarring or fibrosis are discussed. Finally, an outlook is provided on how eicosanoids may be targeted by future therapeutic strategies to achieve physiological tissue repair and prevent scarring and loss of tissue function in various disease contexts.

Arachidonic Acid Metabolites in Cardiovascular and Metabolic Diseases

Lipid and immune pathways are crucial in the pathophysiology of metabolic and cardiovascular disease. Arachidonic acid (AA) and its derivatives link nutrient metabolism to immunity and inflammation, thus holding a key role in the emergence and progression of frequent diseases such as obesity, diabetes, non-alcoholic fatty liver disease, and cardiovascular disease. We herein present a synopsis of AA metabolism in human health, tissue homeostasis, and immunity, and explore the role of the AA metabolome in diverse pathophysiological conditions and diseases.