Lipid alterations in human blood-derived neutrophils lead to formation of neutrophil extracellular traps

Neutrophil extracellular traps in intracerebral hemorrhage: implications for pathogenesis and therapeutic targets

Intracerebral hemorrhage is a common neurological disease, and its pathological mechanism is complex. As the first recruited leukocyte subtype after intracerebral hemorrhage, neutrophils play an important role in tissue damage. In the past, it was considered that neutrophils performed their functions through phagocytosis, chemotaxis, and degranulation. In recent years, studies have found that neutrophils also have the function of secreting extracellular traps. Extracellular traps are fibrous structure composed of chromatin and granular proteins, which plays an important role in innate immunity. Studies have shown a large number of neutrophil extracellular traps in hematoma samples, plasma samples, and drainage samples after intracerebral hemorrhage. In this paper, we summarized the related mechanisms of neutrophil external traps and injury after intracerebral hemorrhage. Neutrophil extracellular traps are involved in the process of brain injury after intracerebral hemorrhage. The application of related inhibitors to inhibit the formation of neutrophil external traps or promote their dissolution can effectively alleviate the pathological damage caused by intracerebral hemorrhage.

Neutrophil extracellular traps contributing to atherosclerosis: From pathophysiology to clinical implications

Neutrophil extracellular traps (NETs) are network-like structures of chromatin filaments decorated by histones, granules, and cytoplasmic-derived proteins expelled by activated neutrophils under multiple pathogenic conditions. NETs not only capture pathogens in innate immunity but also respond to sterile inflammatory stimuli in atherosclerosis, such as lipoproteins and inflammatory cytokines. Atherosclerosis is a lipid-driven chronic inflammatory disease characterized by the accumulation and transformation of inflammatory cells, and smooth muscle cells in the intimal space. NETs-derived extracellular components possess toxic and proinflammatory properties leading to cellular dysfunction and tissue damage, which may establish a link among lipid metabolism, inflammatory immunity, and atherosclerosis. In this review, we discuss recent advances regarding the role of NETs engaged in the pathogenesis of atherosclerosis, particularly focusing on the interaction with lipids and inflammasomes, crosstalk with smooth muscle cells and inflammatory cells, and the association with aging. We also evaluate the current knowledge on the potential of NETs as biomarkers and therapeutic targets for atherosclerosis and its related diseases in clinical practice.

Role of phagocyte extracellular traps during Mycobacterium tuberculosis infections and tuberculosis disease processes

Mycobacterium tuberculosis (M.tb) infections remain one of the most significant causes of mortality worldwide. The current situation shows an emergence of new antibiotic-resistant strains making it difficult to control the tuberculosis (TB) disease. A large part of its success as a pathogen is due to its ability to persist for years or even decades without causing evident clinical manifestations. M.tb is highly successful in evading the host-defense by manipulating host-signalling pathways. Although macrophages are generally viewed as the key cell type involved in harboring M.tb, growing evidence shows that neutrophils also play a fundamental role. Both cells are known to act in multiple ways when encountering an invading pathogen, including phagocytosis, release of cytokines and chemokines, and oxidative burst. In addition, the formation of neutrophil extracellular traps (NETs) and macrophage extracellular traps (METs) has been described to contribute to M.tb infections. NETs/METs are extracellular DNA fibers with associated granule components, which are released upon activation of the cells by the pathogen or by pro-inflammatory mediators. On one hand, they can lead to a protective immune response by entrapment and killing of pathogens. However, on the other hand, they can also play a severe pathological role by inducing tissue damage. Extracellular traps (ETs) produced in the pulmonary alveoli can expand easily and expose tissue-damaging factors with detrimental effects. Since host-directed therapies offer a complementary strategy in TB, the knowledge of NET/MET formation is important for understanding potential protective versus detrimental pathways during innate immune signaling. In this review, we summarize the progress made in understanding the role of NETs/METs in the pathogenesis of TB.

The emerging role for metabolism in fueling neutrophilic inflammation

Neutrophils are a critical element of host defense and are rapidly recruited to inflammatory sites. Such sites are frequently limited in oxygen and/or nutrient availability, presenting a metabolic challenge for infiltrating cells. Long believed to be uniquely dependent on glycolysis, it is now clear that neutrophils possess far greater metabolic plasticity than previously thought, with the capacity to generate energy stores and utilize extracellular proteins to fuel central carbon metabolism and biosynthetic activity. Out-with cellular energetics, metabolic programs have also been implicated in the production of neutrophils and their progenitors in the bone marrow compartment, activation of neutrophil antimicrobial responses, inflammatory and cell survival signaling cascades, and training of the innate immune response. Thus, understanding the mechanisms by which metabolic processes sustain changes in neutrophil effector functions and how these are subverted in disease states provides exciting new avenues for the treatment of dysfunctional neutrophilic inflammation which are lacking in clinical practice to date.

Relationship between Cholesterol-Related Lipids and Severe Acute Pancreatitis: From Bench to Bedside

It is well known that hypercholesterolemia in the body has pro-inflammatory effects through the formation of inflammasomes and augmentation of TLR (Toll-like receptor) signaling, which gives rise to cardiovascular disease and neurodegenerative diseases. However, the interaction between cholesterol-related lipids and acute pancreatitis (AP) has not yet been summarized before. This hinders the consensus on the existence and clinical importance of cholesterol-associated AP. This review focuses on the possible interaction between AP and cholesterol-related lipids, which include total cholesterol, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C) and apolipoprotein (Apo) A1, from the bench to the bedside. With a higher serum level of total cholesterol, LDL-C is associated with the severity of AP, while the persistent inflammation of AP is allied with a decrease in serum levels of cholesterol-related lipids. Therefore, an interaction between cholesterol-related lipids and AP is postulated. Cholesterol-related lipids should be recommended as risk factors and early predictors for measuring the severity of AP. Cholesterol-lowering drugs may play a role in the treatment and prevention of AP with hypercholesterolemia.

Alterations to Sphingomyelin Metabolism Affect Hemostasis and Thrombosis

BACKGROUND

Our recent studies suggest that sphingomyelin levels in the plasma membrane influence TF (tissue factor) procoagulant activity. The current study was performed to investigate how alterations to sphingomyelin metabolic pathway would affect TF procoagulant activity and thereby affect hemostatic and thrombotic processes.

METHODS

Macrophages and endothelial cells were transfected with specific siRNAs or infected with adenoviral vectors to alter sphingomyelin levels in the membrane. TF activity was measured in factor X activation assay. Saphenous vein incision-induced bleeding and the inferior vena cava ligation-induced flow restriction mouse models were used to evaluate hemostasis and thrombosis, respectively.

RESULTS

Overexpression of SMS (sphingomyelin synthase) 1 or SMS2 in human monocyte-derived macrophages suppresses ATP-stimulated TF procoagulant activity, whereas silencing SMS1 or SMS2 increases the basal cell surface TF activity to the same level as of ATP-decrypted TF activity. Consistent with the concept that sphingomyelin metabolism influences TF procoagulant activity, silencing of acid sphingomyelinase or neutral sphingomyelinase 2 or 3 attenuates ATP-induced enhanced TF procoagulant activity in macrophages and endothelial cells. Niemann-Pick disease fibroblasts with a higher concentration of sphingomyelin exhibited lower TF activity compared with wild-type fibroblasts. In vivo studies revealed that LPS+ATP-induced TF activity and thrombin generation were attenuated in ASMase^-/- mice, while their levels were increased in SMS2^-/- mice. Further studies revealed that acid sphingomyelinase deficiency leads to impaired hemostasis, whereas SMS2 deficiency increases thrombotic risk.

CONCLUSIONS

Overall, our data indicate that alterations in sphingomyelin metabolism would influence TF procoagulant activity and affect hemostatic and thrombotic processes.

Formation of Neutrophil Extracellular Traps by Reduction of Cellular Cholesterol Is Independent of Oxygen and HIF-1α

Formation of neutrophil extracellular traps (NETs) is a two-faced innate host defense mechanism, which, on the one hand, can counteract microbial infections, but on the other hand, can contribute to massive detrimental effects on the host. Cholesterol depletion from the cellular membrane by Methyl-β-cyclodextrin (MβCD) is known as one of the processes initiating NET formation. Since neutrophils mainly act in an inflammatory environment with decreased, so-called hypoxic, oxygen conditions, we aimed to study the effect of oxygen and the oxygen stress regulator hypoxia-inducible factor (HIF)-1α on cholesterol-dependent NET formation. Thus, murine bone marrow-derived neutrophils from wild-type and HIF-knockout mice or human neutrophils were stimulated with MβCD under normoxic (21% O₂) compared to hypoxic (1% O₂) conditions, and the formation of NETs were studied by immunofluorescence microscopy. We found significantly induced NET formation after treatment with MβCD in murine neutrophils derived from wild-type as well as HIF-1α KO mice at both hypoxic (1% O₂) as well as normoxic (21% O₂) conditions. Similar observations were made in freshly isolated human neutrophils after stimulation with MβCD or statins, which block the HMG-CoA reductase as the key enzyme in the cholesterol metabolism. HPLC was used to confirm the reduction of cholesterol in treated neutrophils. In summary, we were able to show that NET formation via MβCD or statin-treatment is oxygen and HIF-1α independent.

Host Defense Peptides LL-37 and Lactoferrin Trigger ET Release from Blood-Derived Circulating Monocytes

Neutrophils are commonly regarded as the first line of immune response during infection or in tissue injury-induced inflammation. The rapid influx of these cells results in the release of host defense proteins (HDPs) or formation of neutrophil extracellular traps (NETs). As a second wave during inflammation or infection, circulating monocytes arrive at the site. Earlier studies showed that HDPs LL-37 and Lactoferrin (LTF) activate monocytes while neutrophil elastase facilitates the formation of extracellular traps (ETs) in monocytes. However, the knowledge about the impact of HDPs on monocytes remains sparse. In the present study, we investigated the effect of LL-37 and LTF on blood-derived CD14⁺ monocytes. Both HDPs triggered a significant release of TNFα, nucleosomes, and monocyte ETs. Microscopic analysis indicated that ET formation by LL-37 depends on storage-operated calcium entry (SOCE), mitogen-activated protein kinase (MAPK), and ERK1/2, whereas the LTF-mediated ET release is not affected by any of the here used inhibitors. Quantitative proteomics mass spectrometry analysis of the neutrophil granular content (NGC) revealed a high abundance of Lactoferrin. The stimulation of CD14⁺ monocytes with NGC resulted in a significant secretion of TNFα and nucleosomes, and the formation of monocyte ETs. The findings of this study provide new insight into the complex interaction of HDPs, neutrophils, and monocytes during inflammation.

Neutrophils Culture in Collagen Gel System

Neutrophils (Neu) migrate rapidly to damaged tissue and play critical roles in host defense and tissue homeostasis, including the intestinal epithelia injuries and immune responses. Although their important roles in these diseases, they are challenging to study due to their short life span and the inability to cryopreserve or expand them in vitro. Moreover, the standard cell culturing on plastic plates (two-dimensional (2D) cultures) does not represent the actual microenvironment where cells reside in tissues. In this study, we developed a new three-dimensional (3D) culture system for human and mouse peripheral blood Neu, which is made of hydrogel. The Neu showed much better cell integrity and less cell debris in the 3D culture system compared to that in 2D culture system. Moreover, the 3D culture system was more suitable for the observation of neutrophil extracellular traps (NETs) stimulated by the classical stimulation phorbol ester (PMA), and other damage associated molecular patterns (DAMPs) such as Lipopolysaccharide (LPS)/ATP, interleukin-1 β (IL-1β) and tumor necrosis factor α (TNFα) than the 2D culture system. Moreover, NETs phenomenon in 3D culture system is similar to that in vivo. In addition, the 3D culture system was evaluated to co-culturing Neu and other parenchymal cells, such as colon mucosal epithelial cell lines. In conclusion, the 3D culture system could maintain better properties of Neu than that in 2D culture system and it may reduce the gap between in vitro an in vivo experimentations.

Macrophage and Neutrophil Interactions in the Pancreatic Tumor Microenvironment Drive the Pathogenesis of Pancreatic Cancer

Simple Summary

The survival rates for patients with pancreatic adenocarcinoma are very low. This dismal prognosis is due in part to late detection and early development of metastases, and successful treatments for pancreatic adenocarcinoma are also lacking. One potential method of treatment is immunotherapy, which has been successfully implemented in several cancers. Despite success in other cancer types, there has been little progress in pancreatic adenocarcinoma. To understand these shortcomings, we explore the roles of macrophages and neutrophils, two prominent immune cell types in the pancreatic tumor environment. In this review, we discuss how macrophages and neutrophils lead to the harsh environment that is unique to pancreatic adenocarcinoma. We further explore how these immune cells can impact standard of care therapies and decrease their effectiveness. Macrophages and neutrophils could ultimately be targeted to improve outcomes for patients with pancreatic adenocarcinoma.

Abstract

Despite modest improvements in survival in recent years, pancreatic adenocarcinoma remains a deadly disease with a 5-year survival rate of only 9%. These poor outcomes are driven by failure of early detection, treatment resistance, and propensity for early metastatic spread. Uncovering innovative therapeutic modalities to target the resistance mechanisms that make pancreatic cancer largely incurable are urgently needed. In this review, we discuss the immune composition of pancreatic tumors, including the counterintuitive fact that there is a significant inflammatory immune infiltrate in pancreatic cancer yet anti-tumor mechanisms are subverted and immune behaviors are suppressed. Here, we emphasize how immune cell interactions generate tumor progression and treatment resistance. We narrow in on tumor macrophage (TAM) spatial arrangement, polarity/function, recruitment, and origin to introduce a concept where interactions with tumor neutrophils (TAN) perpetuate the microenvironment. The sequelae of macrophage and neutrophil activities contributes to tumor remodeling, fibrosis, hypoxia, and progression. We also discuss immune mechanisms driving resistance to standard of care modalities. Finally, we describe a cadre of treatment targets, including those intended to overcome TAM and TAN recruitment and function, to circumvent barriers presented by immune infiltration in pancreatic adenocarcinoma.

The Dynamics of Circulating Heparin-Binding Protein: Implications for Its Use as a Biomarker

Heparin-binding protein (HBP) is a promising biomarker for the development and severity of sepsis. To guide its use, it is important to understand the factors that could lead to false-positive or negative results, such as inappropriate release and inadequate clearance of HBP. HBP is presumably released only by neutrophils, and the organs responsible for its elimination are unknown. In this study, we aimed to determine whether non-neutrophil cells can be a source of circulating HBP and which organs are responsible for its removal. We found that in two cohorts of neutropenic patients, 12% and 19% of patients in each cohort, respectively, had detectable plasma HBP levels. In vitro, three leukemia-derived monocytic cell lines and healthy CD14+ monocytes constitutively released detectable levels of HBP. When HBP was injected intravenously in rats, we found that plasma levels of HBP decreased rapidly, with a distribution half-life below 10 min and an elimination half-life of 1-2 h. We measured HBP levels in the liver, spleen, kidneys, lungs, and urine using both ELISA and immunofluorescence quantitation, and found that the majority of HBP was present in the liver, and a small amount was present in the spleen. Immunofluorescence imaging indicated that HBP is associated mainly with hepatocytes in the liver and monocytes/macrophages in the spleen. The impact of hematologic malignancies and liver diseases on plasma HBP levels should be explored further in clinical studies.

Contemporary Lifestyle and Neutrophil Extracellular Traps: An Emerging Link in Atherosclerosis Disease

Neutrophil extracellular traps (NETs) are networks of extracellular genetic material decorated with proteins of nuclear, granular and cytosolic origin that activated neutrophils expel under pathogenic inflammatory conditions. NETs are part of the host's innate immune defense system against invading pathogens. Interestingly, these extracellular structures can also be released in response to sterile inflammatory stimuli (e.g., shear stress, lipidic molecules, pro-thrombotic factors, aggregated platelets, or pro-inflammatory cytokines), as in atherosclerosis disease. Indeed, NETs have been identified in the intimal surface of diseased arteries under cardiovascular disease conditions, where they sustain inflammation via NET-mediated cell-adhesion mechanisms and promote cellular dysfunction and tissue damage via NET-associated cytotoxicity. This review will focus on (1) the active role of neutrophils and NETs as underestimated players of the inflammatory process during atherogenesis and lesion progression; (2) how these extracellular structures communicate with the main cell types present in the atherosclerotic lesion in the arterial wall; and (3) how these neutrophil effector functions interplay with lifestyle-derived risk factors such as an unbalanced diet, physical inactivity, smoking or lack of sleep quality, which represent major elements in the development of cardiovascular disease.

Cholesterol metabolism: a new molecular switch to control inflammation

The immune system protects the body against harm by inducing inflammation. During the immune response, cells of the immune system get activated, divided and differentiated in order to eliminate the danger signal. This process relies on the metabolic reprogramming of both catabolic and anabolic pathways not only to produce energy in the form of ATP but also to generate metabolites that exert key functions in controlling the response. Equally important to mounting an appropriate effector response is the process of immune resolution, as uncontrolled inflammation is implicated in the pathogenesis of many human diseases, including allergy, chronic inflammation and cancer. In this review, we aim to introduce the reader to the field of cholesterol immunometabolism and discuss how both metabolites arising from the pathway and cholesterol homeostasis are able to impact innate and adaptive immune cells, staging cholesterol homeostasis at the centre of an adequate immune response. We also review evidence that demonstrates the clear impact that cholesterol metabolism has in both the induction and the resolution of the inflammatory response. Finally, we propose that emerging data in this field not only increase our understanding of immunometabolism but also provide new tools for monitoring and intervening in human diseases, where controlling and/or modifying inflammation is desirable.

The vitals of NETs

Neutrophils produce neutrophil extracellular traps (NETs) by expelling their extracellular chromatin embedded with citrullinated histone H3, myeloperoxidase, and other intracellular molecules. Since their discovery in 2004, numerous articles have demonstrated the mechanism of NET formation and their function in innate immunity and inflammation. NET components often play an antimicrobial role, but excessive NETs are deleterious and can cause inflammation and tissue damage. This review highlights recent advancements in the identification of novel pathways and mechanisms of NET formation. We also focus on the specific damaging impact of NETs in individual organs. We then discuss the progress and limitations of various NET detection assays. Collectively, these vital aspects of NETs significantly improve our understanding of the pathobiology of NETs and future diagnostics and therapeutic tools for examining and modulating NETs in inflammatory diseases.

How Long Does a Neutrophil Live?-The Effect of 24 h Whole Blood Storage on Neutrophil Functions in Pigs

Neutrophils are important effector cells of the innate immune system, traditionally regarded to have a short life span. The goal of this study was to evaluate the effect of the whole blood storage on neutrophil functions, e.g., viability, antimicrobial effect, neutrophil extracellular trap (NET) formation and phagocytosis. Therefore, fresh porcine whole blood was compared to whole blood stored for 24 h in the dark at room temperature. Different cell parameters in whole blood and in isolated neutrophils were analyzed. The following parameters were analyzed: cell count, band and segmented neutrophil count, viability, cholesterol content, release of free DNA as a marker for cell death, phagocytic activity in whole blood and in isolated neutrophils, the transmigration rate of neutrophils to IL8 stimulus, the production of reactive oxygen species (ROS), and the formation of NETs. It was observed that the number of isolated neutrophils decreased over time, indicating cell death occurs during 24 h of blood storage. However, the surviving neutrophils isolated from stored blood reacted comparably or even showed enhanced antimicrobial activity in the case of phagocytosis of Streptococcus (S.) suis, ROS production, and transmigration. The slightly altered cholesterol level of the harvested neutrophils in stored blood when compared to fresh blood partially explains some of the detected differences.

Extracellular Traps: An Ancient Weapon of Multiple Kingdoms

The discovery, in 2004, of extracellular traps released by neutrophils has extended our understanding of the mode of action of various innate immune cells. This fascinating discovery demonstrated the extracellular trapping and killing of various pathogens by neutrophils. During the last decade, evidence has accumulated showing that extracellular traps play a crucial role in the defence mechanisms of various cell types present in vertebrates, invertebrates, and plants. The aim of this review is to summarise the relevant literature on the evolutionary history of extracellular traps used as a weapon in various kingdoms of life.

Influence of Oxygen on Function and Cholesterol Composition of Murine Bone Marrow-Derived Neutrophils

During inflammation and infection, invading pathogens as well as infiltrating neutrophils locally consume oxygen and reduce the present oxygen level. Since oxygen is an elementary component of the microenvironment required for cell activity and alters multiple cellular functions, it is important to study neutrophil functionality and phenotype at characteristic pathophysiological oxygen levels that reflect the hypoxic phenotype during infection and inflammation. Here, we describe methods to study murine neutrophils under hypoxic compared to normoxic conditions, including analysis of cholesterol content as a key lipid involved in biological functions.

Nano-bio interactions: a neutrophil-centric view

Neutrophils are key components of the innate arm of the immune system and represent the frontline of host defense against intruding pathogens. However, neutrophils can also cause damage to the host. Nanomaterials are being developed for a multitude of different purposes and these minute materials may find their way into the body through deliberate or inadvertent exposure; understanding nanomaterial interactions with the immune system is therefore of critical importance. However, whereas numerous studies have focused on macrophages, less attention is devoted to nanomaterial interactions with neutrophils, the most abundant leukocytes in the blood. We discuss the impact of engineered nanomaterials on neutrophils and how neutrophils, in turn, may digest certain carbon-based materials such as carbon nanotubes and graphene oxide. We also discuss the role of the corona of proteins adsorbed onto the surface of nanomaterials and whether nanomaterials are sensed as pathogens by cells of the immune system.

Excessive Neutrophil Extracellular Trap Formation Aggravates Acute Myocardial Infarction Injury in Apolipoprotein E Deficiency Mice via the ROS-Dependent Pathway

Genetically human apolipoprotein E (APOE) ε32 is associated with a decreased risk of ischemic heart disease. ApoE deficiency in mice impairs infarct healing after myocardial infarction (MI). After the ischemic injury, a large number of neutrophils are firstly recruited into the infarct zone and then degrade dead material and promote reparative phase transformation. The role of ApoE in inflammation response in the early stage of MI remains largely unclear. In this study, we investigated the effect of ApoE deficiency on neutrophils' function and myocardial injury after myocardial infarction. By left coronary artery ligation in ApoE ^-/- and wild-type (WT) mice, we observed increased infarct size and neutrophil infiltration in ApoE ^-/- mice. Within the infarct zone, more neutrophil extracellular traps (NETs) were observed in ApoE ^-/- mice, while increased ex vivo NET formation was detected in ApoE ^-/- mouse-derived neutrophils through the NADPH oxidase-ROS-dependent pathway. Suppressing overproduced NETs reduced myocardial injury in ApoE ^-/- mice after ligation. In general, our findings reveal a critical role of apolipoprotein E in regulating Ly6G⁺ neutrophil activation and NET formation, resulting in limiting myocardial injury after myocardial infarction. In such a process, apolipoprotein E regulates NET formation via the ROS-MAPK-MSK1 pathway.

Comparison Between K3EDTA and Lithium Heparin as Anticoagulant to Isolate Bovine Granulocytes From Blood

Granulocytes play a key role in the defense against invading pathogens. To study granulocyte functions, the isolation of a pure and active cell population from fresh blood is required. Anticoagulants and red blood cells (RBCs) lysis used in the isolation procedure may influence cell harvest, cell marker expression, and pre-activation of cells. In this study, the influence of the anticoagulants K₃EDTA or lithium heparin and the effect of different RBCs lysis methods on bovine granulocyte population from fresh blood of healthy cows after density gradient centrifugation were investigated. Venous blood from healthy cows was collected in K₃EDTA and lithium heparin tubes. Density gradient centrifugation to separate granulocytes from other cells was conducted using Biocoll. Then, RBCs were lysed with hypotonic water or 0.2% sodium chloride (NaCl). Immediately after isolation, harvest, viability, size, granularity, purity, and CD11b expression as a marker for granulocytes was analyzed by flow cytometry. In addition, as a marker for activation and reactivity of the granulocytes, we stimulated cells with phorbol-myristate-acetate to evaluate the release of reactive oxygen species. Furthermore, extracellular trap (ET) formation was investigated by confocal immunofluorescence microscopy in untreated control cells and cells treated with the cholesterol-depleting agent methyl-β-cyclodextrin. We did not find a significant difference in percentage of dead cells when comparing the two anticoagulants or the different RBCs lysis methods. However, the percentage of granulocytes in the harvested population was significantly less using lithium heparin blood as anticoagulant compared to K₃EDTA. The granulocytes harvested from lithium heparin blood and water lysis exhibited higher clumping and pre-activation of unstimulated control cells as indicated by isolation of doublet cells, increased CD11b expression, and increased oxidative burst and higher amount of ET-releasing cells. Furthermore, the combination of K₃EDTA as anticoagulant and NaCl as RBCs lysis method revealed the lowest variability and highest difference between untreated and methyl-β-cyclodextrin-treated cells when quantifying ET formation. In conclusion, density gradient centrifugation of K₃EDTA blood resulted in higher purity of bovine granulocytes compared to lithium heparin blood. In contrast to water lysis, NaCl lysis method is recommended to avoid pre-activation of cells which may occur during hypotonic water lysis.

Turning the Spotlight on Lipids in Non-Apoptotic Cell Death

Although apoptosis has long dominated the spotlight, studies in the past two decades have expanded the repertoire of programmed cell death (PCD). Several forms of non-apoptotic regulated cell death have been identified, with important links to organismal homeostasis and different disease pathologies. Necroptosis, ferroptosis, pyroptosis, and NETosis are the major forms of PCD that have attracted attention. Clear biochemical distinctions differentiate these forms of non-apoptotic PCD at the protein and membrane levels. For instance, pore formation at the plasma membrane is a hallmark of necroptosis and pyroptosis; however, different proteins facilitate pore formation in these processes. Here, we will highlight the role of lipids in different forms of non-apoptotic PCD. In particular, we discuss how lipids can trigger or facilitate the membrane-related changes that result in cell death. We also highlight the use of small molecules in elucidating the mechanisms of non-apoptotic PCD and the potential of lipid biosynthetic pathways to perturb these processes for therapeutic applications as a future avenue of research.

The Role of Lipid Metabolism in T Lymphocyte Differentiation and Survival

The differentiation and effector functions of both the innate and adaptive immune system are inextricably linked to cellular metabolism. The features of metabolism which affect both arms of the immune system include metabolic substrate availability, expression of enzymes, transport proteins, and transcription factors which control catabolism of these substrates, and the ability to perform anabolic metabolism. The control of lipid metabolism is central to the appropriate differentiation and functions of T lymphocytes, and ultimately to the maintenance of immune tolerance. This review will focus on the role of fatty acid (FA) metabolism in T cell differentiation, effector function, and survival. FAs are important sources of cellular energy, stored as triglycerides. They are also used as precursors to produce complex lipids such as cholesterol and membrane phospholipids. FA residues also become incorporated into hormones and signaling moieties. FAs signal via nuclear receptors and their channeling, between storage as triacyl glycerides or oxidation as fuel, may play a role in survival or death of the cell. In recent years, progress in the field of immunometabolism has highlighted diverse roles for FA metabolism in CD4 and CD8 T cell differentiation and function. This review will firstly describe the sensing and modulation of the environmental FAs and lipid intracellular signaling and will then explore the key role of lipid metabolism in regulating the balance between potentially damaging pro-inflammatory and anti-inflammatory regulatory responses. Finally the complex role of extracellular FAs in determining cell survival will be discussed.

Hypoxia Modulates the Response of Mast Cells to Staphylococcus aureus Infection

To study the antimicrobial function of immune cells ex vivo, cells are commonly cultivated under atmospheric oxygen concentrations (20–21%; normoxia), although the physiological oxygen conditions in vivo are significantly lower in most tissues. Especially during an acute infection, oxygen concentration locally decreases to hypoxic levels around or below 1%. The goal of this study was to investigate the effect of hypoxia on the activity of mast cells (MCs). MCs were cultivated for 3 or 24 h at 1% O₂ in a hypoxia glove box and co-incubated with heat-inactivated Staphylococcus aureus. When incubating the cells for 24 h under hypoxia, the transcriptional regulator hypoxia-inducible factor 1α (HIF-1α) was stabilized and resulted in increased extracellular trap formation and decreased phagocytosis. Interestingly, while phagocytosis of fluorescent S. aureus bioparticles as well as the release of extracellular traps remained unaffected at 3 h hypoxia, the secretion of the prestored mediator histamine was increased under hypoxia alone. In contrast, the release of TNF-α was generally reduced at 3 h hypoxia. Microarray transcriptome analysis revealed 13 genes that were significantly downregulated in MCs comparing 3 h hypoxia versus normoxia. One interesting candidate is sec24, a member of the pre-budding complex of coat protein complex II (COPII), which is responsible for the anterograde transport of proteins from the ER to the Golgi apparatus. These data lead to the suggestion that de novo synthesized proteins including crucial factors, which are involved in the response to an acute infection like TNF-α, may eventually be retained in the ER under hypoxia. Importantly, the expression of HIF-1α was not altered at 3 h. Thus, our data exhibit a HIF-1α-independent reaction of MCs to short-term hypoxia. We hypothesize that MCs respond to short-term low oxygen levels in a HIF-1α-independent manner by downregulating the release of proinflammatory cytokines like TNF-α, thereby avoiding uncontrolled degranulation, which could lead to excessive inflammation and severe tissue damage.

Methods to Study Lipid Alterations in Neutrophils and the Subsequent Formation of Neutrophil Extracellular Traps

Lipid analysis performed by high performance thin layer chromatography (HPTLC) is a relatively simple, cost-effective method of analyzing a broad range of lipids. The function of lipids (e.g., in host-pathogen interactions or host entry) has been reported to play a crucial role in cellular processes. Here, we show a method to determine lipid composition, with a focus on the cholesterol level of primary blood-derived neutrophils, by HPTLC in comparison to high performance liquid chromatography (HPLC). The aim was to investigate the role of lipid/cholesterol alterations in the formation of neutrophil extracellular traps (NETs). NET release is known as a host defense mechanism to prevent pathogens from spreading within the host. Therefore, blood-derived human neutrophils were treated with methyl-β-cyclodextrin (MβCD) to induce lipid alterations in the cells. Using HPTLC and HPLC, we have shown that MβCD treatment of the cells leads to lipid alterations associated with a significant reduction in the cholesterol content of the cell. At the same time, MβCD treatment of the neutrophils led to the formation of NETs, as shown by immunofluorescence microscopy. In summary, here we present a detailed method to study lipid alterations in neutrophils and the formation of NETs.

Formation of Neutrophil Extracellular Traps under Low Oxygen Level

Since their discovery, neutrophil extracellular traps (NETs) have been characterized as a fundamental host innate immune defense mechanism. Conversely, excessive NET-release may have a variety of detrimental consequences for the host. A fine balance between NET formation and elimination is necessary to sustain a protective effect during an infectious challenge. Our own recently published data revealed that stabilization of hypoxia-inducible factor 1α (HIF-1α) by the iron chelating HIF-1α-agonist desferoxamine or AKB-4924 enhanced the release of phagocyte extracellular traps. Since HIF-1α is a global regulator of the cellular response to low oxygen, we hypothesized that NET formation may be similarly increased under low oxygen conditions. Hypoxia occurs in tissues during infection or inflammation, mostly due to overconsumption of oxygen by pathogens and recruited immune cells. Therefore, experiments were performed to characterize the formation of NETs under hypoxic oxygen conditions compared to normoxia. Human blood-derived neutrophils were isolated and incubated under normoxic (21%) oxygen level and compared to hypoxic (1%) conditions. Dissolved oxygen levels were monitored in the primary cell culture using a Fibox4-PSt3 measurement system. The formation of NETs was quantified by fluorescence microscopy in response to the known NET-inducer phorbol 12-myristate 13-acetate (PMA) or Staphylococcus (S.) aureus wild-type and a nuclease-deficient mutant. In contrast to our hypothesis, spontaneous NET formation of neutrophils incubated under hypoxia was distinctly reduced compared to control neutrophils incubated under normoxia. Furthermore, neutrophils incubated under hypoxia showed significantly reduced formation of NETs in response to PMA. Gene expression analysis revealed that mRNA level of hif-1α as well as hif-1α target genes was not altered. However, in good correlation to the decreased NET formation under hypoxia, the cholesterol content of the neutrophils was significantly increased under hypoxia. Interestingly, NET formation in response to viable S. aureus wild-type or nuclease-deficient strain was retained under hypoxia. Our results lead to the conclusion that hypoxia is not the ideal tool to analyze HIF-1α in neutrophils. However, the data clearly suggest that neutrophils react differently under hypoxia compared to normoxia and thereby highlight the importance of the usage of physiological relevant oxygen level when studying neutrophil functions.

NET formation can occur independently of RIPK3 and MLKL signaling

The importance of neutrophil extracellular traps (NETs) in innate immunity is well established but the molecular mechanisms responsible for their formation are still a matter of scientific dispute. Here, we aim to characterize a possible role of the receptor‐interacting protein kinase 3 (RIPK3) and the mixed lineage kinase domain‐like (MLKL) signaling pathway, which are known to cause necroptosis, in NET formation. Using genetic and pharmacological approaches, we investigated whether this programmed form of necrosis is a prerequisite for NET formation. NETs have been defined as extracellular DNA scaffolds associated with the neutrophil granule protein elastase that are capable of killing bacteria. Neither Ripk3‐deficient mouse neutrophils nor human neutrophils in which MLKL had been pharmacologically inactivated, exhibited abnormalities in NET formation upon physiological activation or exposure to low concentrations of PMA. These data indicate that NET formation occurs independently of both RIPK3 and MLKL signaling.

Sphingolipids as Regulators of the Phagocytic Response to Fungal Infections

Fungal infections pose a significant risk for the increasing population of individuals who are immunocompromised. Phagocytes play an important role in immune defense against fungal pathogens, but the interactions between host and fungi are still not well understood. Sphingolipids have been shown to play an important role in many cell functions, including the function of phagocytes. In this review, we discuss major findings that relate to the importance of sphingolipids in macrophage and neutrophil function and the role of macrophages and neutrophils in the most common types of fungal infections, as well as studies that have linked these three concepts to show the importance of sphingolipid signaling in immune response to fungal infections.

Cholesterol-rich lipid rafts play an important role in the Cyprinid herpesvirus 3 replication cycle

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Sequestration of cholesterol from the cell membrane inhibits CyHV-3 entry.

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CyHV-3 egress requires cholesterol.

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Lipid composition of the CyHV-3 envelope is similar to that of CCB lipid rafts.

The Cyprinus herpesvirus 3 (CyHV-3) is a member of the new Alloherpesviridae virus family in the Herpesvirales order. CyHV-3 has been implicated in a large number of disease outbreaks in carp populations causing up to 100% mortality. The aim of this study was to investigate the requirement of cholesterol-rich lipid rafts in CyHV-3 entry and replication in carp cells. Plasma membrane cholesterol was depleted from common carp brain (CCB) cells with methyl-β-cyclodextrin (MβCD). Treated and non-treated cells were infected with CyHV-3 and virus binding and infection parameters were assessed using RT-qPCR, immunocytochemistry and virus titration. The effect of cholesterol reduction severely stunted virus entry in vitro, however after cholesterol replenishment virus entry and subsequent replication rates were similar to the control infection. Furthermore, cholesterol depletion did not significantly influence virus binding and the subsequent post-entry replication stage, however had an impact on virus egress. Comparative analysis of the lipid compositions of CyHV-3 and CCB membrane fractions revealed strong similarities between the lipid composition of the CyHV-3 and CCB lipid rafts. The results presented here show that cholesterol-rich lipid rafts are important for the CyHV-3 replication cycle especially during entry and egress.

Cholesterol: A modulator of the phagocyte NADPH oxidase activity - A cell-free study

The NADPH oxidase Nox2, a multi-subunit enzyme complex comprising membrane and cytosolic proteins, catalyzes a very intense production of superoxide ions O₂^•−, which are transformed into other reactive oxygen species (ROS). In vitro, it has to be activated by addition of amphiphiles like arachidonic acid (AA). It has been shown that the membrane part of phagocyte NADPH oxidase is present in lipid rafts rich in cholesterol. Cholesterol plays a significant role in the development of cardio-vascular diseases that are always accompanied by oxidative stress. Our aim was to investigate the influence of cholesterol on the activation process of NADPH oxidase. Our results clearly show that, in a cell-free system, cholesterol is not an efficient activator of NADPH oxidase like arachidonic acid (AA), however it triggers a basal low superoxide production at concentrations similar to what found in neutrophile. A higher concentration, if present during the assembly process of the enzyme, has an inhibitory role on the production of O₂^•−. Added cholesterol acts on both cytosolic and membrane components, leading to imperfect assembly and decreasing the affinity of cytosolic subunits to the membrane ones. Added to the cytosolic proteins, it retains their conformations but still allows some conformational change induced by AA addition, indispensable to activation of NADPH oxidase.

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Natural cholesterol is important for the NADPH oxidase function.

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Added cholesterol alone activates slightly the NADPH oxidase.

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Cholesterol addition lowers the AA dependent activity of NADPH oxidase.

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Added cholesterol acts on both cytosolic and membrane components.