Measurement of Respiratory Burst Products, Released or Retained, During Activation of Professional Phagocytes

Signaling by neutrophil G protein-coupled receptors that regulate the release of superoxide anions

In human peripheral blood, the neutrophil granulocytes (neutrophils) are the most abundant white blood cells. These professional phagocytes are rapidly recruited from the bloodstream to inflamed tissues by chemotactic factors that signal danger. Neutrophils, which express many receptors that are members of the large family of G protein-coupled receptors (GPCRs), are critical for the elimination of pathogens and inflammatory insults, as well as for the resolution of inflammation leading to tissue repair. Danger signaling molecular patterns such as the N-formylated peptides that are formed during bacterial and mitochondrial protein synthesis and recognized by formyl peptide receptors (FPRs) and free fatty acids recognized by free fatty acid receptors (FFARs) regulate neutrophil functions. Short peptides and short-chain fatty acids activate FPR1 and FFA2R, respectively, while longer peptides and fatty acids activate FPR2 and GPR84, respectively. The activation profiles of these receptors include the release of reactive oxygen species (ROS) generated by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Activation of the oxidase and the production of ROS are processes that are regulated by proinflammatory mediators, including tumor necrosis factor α and granulocyte/macrophage colony-stimulating factor. The receptors have signaling and functional similarities, although there are also important differences, not only between the two closely related neutrophil FPRs, but also between the FPRs and the FFARs. In neutrophils, these receptors never walk alone, and additional mechanistic insights into the regulation of the GPCRs and the novel regulatory mechanisms underlying the activation of NADPH oxidase advance our understanding of the role of receptor transactivation in the regulation of inflammatory reactions.

The Pseudomonas aeruginosa lectin LecB modulates intracellular reactive oxygen species production in human neutrophils

Pseudomonas aeruginosa is a Gram-negative bacterium and an opportunistic pathogen ubiquitously present throughout nature. LecB, a fucose-, and mannose-binding lectin, is a prominent virulence factor of P. aeruginosa, which can be expressed on the bacterial surface but also be secreted. However, the LecB interaction with human immune cells remains to be characterized. Neutrophils comprise the first line of defense against infections and their production of reactive oxygen species (ROS) and release of extracellular traps (NETs) are critical antimicrobial mechanisms. When profiling the neutrophil glycome we found several glycoconjugates on granule and plasma membranes that could potentially act as LecB receptors. In line with this, we here show that soluble LecB can activate primed neutrophils to produce high levels of intracellular ROS (icROS), an effect that was inhibited by methyl fucoside. On the other hand, soluble LecB inhibits P. aeruginosa-induced icROS production. In support of that, during phagocytosis of wild-type and LecB-deficient P. aeruginosa, bacteria with LecB induced less icROS production as compared with bacteria lacking the lectin. Hence, LecB can either induce or inhibit icROS production in neutrophils depending on the circumstances, demonstrating a novel and potential role for LecB as an immunomodulator of neutrophil functional responses.

Multifunctional PtCuTe Nanosheets with Strong ROS Scavenging and ROS-Independent Antibacterial Properties Promote Diabetic Wound Healing

Nanozymes, as one of the most efficient reactive oxygen species (ROS)-scavenging biomaterials, are receiving wide attention in promoting diabetic wound healing. Despite recent attempts at improving the catalytic efficiency of Pt-based nanozymes (e.g., PtCu, one of the best systems), they still display quite limited ROS scavenging capacity and ROS-dependent antibacterial effects on bacteria or immunocytes, which leads to uncontrolled and poor diabetic wound healing. Hence, a new class of multifunctional PtCuTe nanosheets with excellent catalytic, ROS-independent antibacterial, proangiogenic, anti-inflammatory, and immuno-modulatory properties for boosting the diabetic wound healing, is reported. The PtCuTe nanosheets show stronger ROS scavenging capacity and better antibacterial effects than PtCu. It is also revealed that the PtCuTe can enhance vascular tube formation, stimulate macrophage polarization toward the M2 phenotype and improve fibroblast mobility, outperforming conventional PtCu. Moreover, PtCuTe promotes crosstalk between different cell types to form a positive feedback loop. Consequently, PtCuTe stimulates a proregenerative environment with relevant cell populations to ensure normal tissue repair. Utilizing a diabetic mouse model, it is demonstrated that PtCuTe significantly facilitated the regeneration of highly vascularized skin, with the percentage of wound closure being over 90% on the 8th day, which is the best among the reported comparable multifunctional biomaterials.

Larixol is not an inhibitor of Gαi containing G proteins and lacks effect on signaling mediated by human neutrophil expressed formyl peptide receptors

Neutrophils express several G protein-coupled receptors (GPCRs) connected to intracellular Gαi or Gαq containing G proteins for down-stream signaling. To dampen GPCR mediated inflammatory processes, several inhibitors targeting the receptors and/or their down-stream signals, have been developed. Potent and selective inhibitors for Gαq containing G proteins are available, but potent and specific inhibitors of Gαi containing G proteins are lacking. Recently, Larixol, a compound extracted from the root of Euphorbia formosana, was shown to abolish human neutrophil functions induced by N-formyl-methionyl-leucyl-phenylalanine (fMLF), an agonist recognized by formyl peptide receptor 1 (FPR1) which couple to Gαi containing G proteins. The inhibitory effect was suggested to be due to interference with/inhibition of signals transmitted by βγ complexes of the Gαi containing G proteins coupled to FPR1. In this study, we applied Larixol, obtained from two different commercial sources, to determine the receptor- and G protein- selectivity of this compound in human neutrophils. However, our data show that Larixol not only lacks inhibitory effect on neutrophil responses mediated through FPR1, but also on responses mediated through FPR2, a Gαi coupled GPCR closely related to FPR1. Furthermore, Larixol did not display any features as a selective inhibitor of neutrophil responses mediated through the Gαq coupled GPCRs for platelet activating factor and ATP. Hence, our results imply that the inhibitory effects described for the root extract of Euphorbia formosana are not mediated by Larixol and that the search for a selective inhibitor of G protein dependent signals generated by Gαi coupled neutrophil GPCRs must continue.

WITHDRAWN: Larixol is not an inhibitor of Gαi containing G proteins and lacks effect on signaling mediated by human neutrophil expressed formyl peptide receptors

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Severe chronic non-bacterial osteomyelitis in combination with total MPO deficiency and responsiveness to TNFα inhibition

We describe a female patient suffering from severe chronic non-bacterial osteomyelitis (CNO) with systemic inflammation and advanced malnutrition and complete deficiency of myeloperoxidase (MPO). CNO is a rare autoinflammatory bone disorder associated with dysregulation of the innate immune system. MPO deficiency is a genetic disorder with partial or complete absence of the phagocyte peroxidase MPO. MPO deficiency has no established clinical phenotype but reports indicate increased susceptibility to infection and chronic inflammation. The patient's symptoms began at 10 years of age with pain in the thighs, systemic inflammation and malnutrition. She was diagnosed with CNO at 14 years of age. Treatment with nonsteroidal anti-inflammatory drugs, corticosteroids, bisphosphonates or IL1-receptor antagonists (anakinra) did not relieve the symptoms. However, the patient responded instantly and recovered from her clinical symptoms when treated with TNFα blockade (adalimumab). Three years after treatment initiation adalimumab was withdrawn, resulting in rapid symptom recurrence. When reintroducing adalimumab, the patient promptly responded and went into remission. In addition to clinical and laboratory profiles, neutrophil functions (reactive oxygen species, ROS; neutrophil extracellular traps, NETs; degranulation; apoptosis; elastase activity) were investigated both in a highly inflammatory state (without treatment) and in remission (on treatment). At diagnosis, neither IL1β, IL6, nor TNFα was significantly elevated in serum, but since TNFα blockade terminated the inflammatory symptoms, the disease was likely TNFα-driven. All neutrophil parameters were normal both during treatment and treatment withdrawal, except for MPO-dependent intracellular ROS- and NET formation. The role of total MPO deficiency for disease etiology and severity is discussed.

An increase in the cytosolic concentration of free calcium ions activates the neutrophil NADPH-oxidase provided that the free fatty acid receptor 2 has been allosterically modulated

Signals generated by free fatty acid receptor 2 (FFA2R) can activate the neutrophil NADPH-oxidase without involvement of any orthosteric FFA2R agonist. The initiating signals may be generated by P2Y₂R, the receptor for ATP. An FFA2R specific allosteric modulator (PAM; Cmp58) was required for this response and used to investigate the mechanism by which signals generated by ATP/P2Y₂R activate an FFA2R dependent process. The P2Y₂R induced signal that together with the modulated FFA2R activates neutrophils, was generated downstream of the Gα_q containing G protein coupled to P2Y₂R. A rise in the cytosolic concentration of ionized calcium ([Ca²⁺]_i) was hypothesized to be the important signal. The hypothesis gained support from the finding that the modulator transferred the neutrophils to a Ca²⁺sensitive state. The rise in [Ca²⁺]_i induced by the Ca²⁺ specific ionophore ionomycin, activated the neutrophils provided that an allosteric modulator was bound to FFA2R. The activity of the superoxide generating NADPH-oxidase induced by ionomycin was rapidly terminated and the FFA2Rs could then no longer be activated by the FFA2R agonist propionate or by the signal generated by ATP/P2Y₂R. The non-responding state of FFA2R was, however, revoked by a cross-activating allosteric FFA2R modulator. The [Ca²⁺]_i mediated activation of neutrophils with their FFA2Rs allosterically modulated, represent a unique regulatory receptor crosstalk mechanism by which the activation potency of a G protein coupled receptor is controlled by a receptor-crosstalk signaling system operating from the cytosolic side of the plasma membrane.

Multiple Immune Defects in Two Patients with Novel DOCK2 Mutations Result in Recurrent Multiple Infection Including Live Attenuated Virus Vaccine

The dedicator of cytokinesis 2(DOCK2) protein, an atypical guanine nucleotide exchange factor (GEFs), is a member of the DOCKA protein subfamily. DOCK2 protein deficiency is characterized by early-onset lymphopenia, recurrent infections, and lymphocyte dysfunction, which was classified as combined immune deficiency with neutrophil abnormalities as well. The only cure is hematopoietic stem cell transplantation. Here, we report two patients harboring four novel DOCK2 mutations associated with recurrent infections including live attenuated vaccine-related infections. The patient's condition was partially alleviated by symptomatic treatment or intravenous immunoglobulin. We also confirmed defects in thymic T cell output and T cell proliferation, as well as aberrant skewing of T/B cell subset TCR-Vβ repertoires. In addition, we noted neutrophil defects, the weakening of actin polymerization, and BCR internalization under TCR/BCR activation. Finally, we found that the DOCK2 protein affected antibody affinity although with normal total serum immunoglobulin. The results reported herein expand the clinical phenotype, the pathogenic DOCK2 mutation database, and the immune characteristics of DOCK2-deficient patients.

AZ2158 is a more potent formyl peptide receptor 1 inhibitor than the commonly used peptide antagonists in abolishing neutrophil chemotaxis

Formyl peptide receptor 1 (FPR1), a G protein-coupled receptor expressed in phagocytes, recognizes short N-formylated peptides originating from proteins synthesized by bacteria and mitochondria. Such FPR1 agonists are important regulators of neutrophil functions and by that, determinants of inflammatory reactions. As FPR1 is implicated in promoting both pro-inflammatory and pro-resolving responses associated with inflammatory diseases, characterization of ligands that potently and selectively modulate FPR1 induced functions might be of high relevance. Accordingly, a number of FPR1 specific antagonists have been identified and shown to inhibit agonist binding or receptor down-stream signaling as well as neutrophil functions such as granule secretion and NADPH oxidase activity. The inhibitory effect on neutrophil chemotaxis induced by FPR1 agonists has generally not been part of basic antagonist characterization. In this study we show that the inhibitory effects on neutrophil chemotaxis of established FPR1 antagonists (i.e., cyclosporin H, BOC1 and BOC2) are limited. Our data demonstrate that the recently described small molecule AZ2158 is a potent and selective FPR1 antagonist in human neutrophils. In contrast to the already established FPR1 antagonists, AZ2158 also potently inhibits chemotaxis. Whereas the cyclosporin H inhibition was agonist selective, AZ2158 inhibited the FPR1 response induced by both a balanced and a biased FPR1 agonist equally well. In accordance with the species specificity described for many FPR1 ligands, AZ2158 was not recognized by the mouse orthologue of FPR1. Our data demonstrate that AZ2158 may serve as an excellent tool compound for further mechanistic studies of human FPR1 mediated activities.

Low Temperature Plasma Jet Affects Acute Skin Wounds in Diabetic Mice Through Reactive Components

As a common complication of diabetes, diabetic foot ulcers serious affect the life quality even lead to amputation if it's not properly treated. In this paper, we developed a Low Temperature Plasma Jet (LTPJ) system for treating diabetic foot ulcers on streptozotocin-induced diabetic mice. This system generates time-dependent reactive nitrogen and oxygen species (RNOS), which have temperature below 40°C. The wound area of normal mice was significantly reduced after LTPJ treatment. Histological and immunohistochemistry analysis showed faster deposition of collagen and more vessel formation both in plasma-treated normal and diabetic mice on Day 3. However, diabetic wounds showed poor collagen deposition and angiogenesis on Day 8, which might be the reason of slow wound healing. Reactive nitrogen species (RNS) that generated by LTPJ can promote endogenous nitric oxide (NO) production in diabetic wounds, thus promoting inflammation, stromal deposition, angiogenesis, cell proliferation and remodeling, while excess reactive oxygen species (ROS) will exacerbate oxidative stress in wound tissues of diabetic mice. In conclusion, LTPJ improved acute wound healing in normal mice, increased collagen deposition and angiogenesis in initial diabetic wound healing, but had no significant effect on diabetic wound healing rate.

GRK2 selectively attenuates the neutrophil NADPH-oxidase response triggered by β-arrestin recruiting GPR84 agonists

In order to avoid a prolonged pro-inflammatory neutrophil response, signaling downstream of an agonist-activated G protein-coupled receptor (GPCR) has to be rapidly terminated. Among the family of GPCR kinases (GRKs) that regulate receptor phosphorylation and signaling termination, GRK2, which is highly expressed by immune cells, plays an important role. The medium chain fatty acid receptor GPR84 as well as formyl peptide receptor 2 (FPR2), receptors expressed in neutrophils, play a key role in regulating inflammation. In this study, we investigated the effects of GRK2 inhibitors on neutrophil functions induced by GPR84 and FPR2 agonists. GRK2 was shown to be expressed in human neutrophils and analysis of subcellular fractions revealed a cytosolic localization. The GRK2 inhibitors enhanced and prolonged neutrophil production of reactive oxygen species (ROS) induced by GPR84- but not FPR2-agonists, suggesting a receptor selective function of GRK2. This suggestion was supported by β-arrestin recruitment data. The ROS production induced by a non β-arrestin recruiting GPR84 agonist was not affected by the GRK2 inhibitor. Termination of this β-arrestin independent response relied, similar to the response induced by FPR2 agonists, primarily on the actin cytoskeleton. In summary, we show that GPR84 utilizes GRK2 in concert with β-arrestin and actin cytoskeleton dependent processes to fine-tune the activity of the ROS generating NADPH-oxidase in neutrophils.

Structural Determinants in the Staphylococcus aureus-Derived Phenol-Soluble Modulin α2 Peptide Required for Neutrophil Formyl Peptide Receptor Activation

Highly pathogenic Staphylococcus aureus strains produce phenol-soluble modulins (PSMs), which are N-formylated peptides. Nanomolar concentrations of PSMα2 are recognized by formyl peptide receptor 2 (FPR2), but unlike the prototypic FPR2 agonist WKYMVM, PSMα2 is a biased signaling agonist. The truncated N-terminal PSMα2 variant, consisting of the five N-terminal residues, is no longer recognized by FPR2, showing that the C-terminal part of PSMα2 confers FPR2 selectivity, whereas the N-terminal part may interact with the FPR1 binding site. In the current study, a combined pharmacological and genetic approach involving primary human neutrophils and engineered FPR knock-in and knockout cells was used to gain molecular insights into FPR1 and FPR2 recognition of formyl peptides as well as the receptor downstream signaling induced by these peptides. In comparison with the full-length PSMα2, we show that the peptide in which the N-terminal part of PSMα2 was replaced by fMet-Ile-Phe-Leu (an FPR1-selective peptide agonist) potently activates both FPRs for production of superoxide anions and β-arrestin recruitment. A shortened analog of PSMα2 (PSMα2_1-12), lacking the nine C-terminal residues, activated both FPR1 and FPR2 to produce reactive oxygen species, whereas β-arrestin recruitment was only mediated through FPR1. However, a single amino acid replacement (Gly-2 to Ile-2) in PSMα2_1-12 was sufficient to alter FPR2 signaling to include β-arrestin recruitment, highlighting a key role of Gly-2 in conferring FPR2-biased signaling. In conclusion, we provide structural insights into FPR1 and FPR2 recognition as well as the signaling induced by interaction with formyl peptides derived from PSMα2, originating from S. aureus bacteria.

Chemiluminescence Detection in the Study of Free-Radical Reactions. Part 2. Luminescent Additives That Increase the Chemiluminescence Quantum Yield

The present review examines the use of chemiluminescence detection to evaluate the course of free radical reactions in biological model systems. The application of the method is analyzed by using luminescent additives that enhance the luminescence thanks to a triplet-singlet transfer of the electron excitation energy from radical reaction products and its emission in the form of light with a high quantum yield; these additives are called chemiluminescence enhancers or activators. Examples of these substances are provided; differences between the so-called chemical and physical enhancers are described; coumarin derivatives, as the most promising chemiluminescence enhancers for studying lipid peroxidation, are considered in detail. The main problems related to the use of coumarin derivatives are defined, and possible ways of solving these problems are presented. Intrinsic chemiluminescence and the mechanism of luminescence accompanying biomolecule peroxidation are discussed in the first part of the review.

Detection of Reactive Oxygen Species in Human Neutrophils Under Various Conditions of Exposure to Galectin

Reactive oxygen species (ROS) have been extensively studied in biology in the past years. This class of molecules can be derived from endogenous sources (e.g., phagocytic cells as neutrophils, eosinophils, monocytes, macrophages, and organelles as mitochondria and peroxisomes) and participate in physiological and pathological conditions. The beneficial and harmful effects of ROS depend on redox regulation, which establishes the balance between their production and the activity of antioxidant systems to prevent oxidative stress in vivo. Neutrophils are the immune effectors most well depicted with an intense oxidative burst in response to tissue inflammation. Several proteins and members of the galectin family are involved in this fine modulation of ROS production by neutrophils. Interestingly, studies have indicated that Galectin-1 (Gal-1) can up- or downregulate ROS production by neutrophils even when exposed to N-formyl-Met-Leu-Phe (fMLP) or Phorbol Myristate Acetate (PMA), both of which are potent neutrophil stimulants that trigger high levels of ROS production. Similarly, Galectin-3 (Gal-3) induces ROS in neutrophils from a sterile or nonsterile inflammatory environment, possibly creating a negative loop that could control ROS production. Besides, superoxide production is also induced by Galectin-8 (Gal-8) and Galectin-9 (Gal-9) in neutrophils but in a different manner. We describe herein the luminol and lucigenin-dependent chemiluminescence technique by using a luminometer as a method of assessment to measure ROS production by human neutrophils isolated and exposed to purified human recombinant Gal-1. The protocol described herein could be applied for the investigation of the role of other galectins in the modulation of ROS production by neutrophils.

Neutrophils require SKAP2 for reactive oxygen species production following C-type lectin and Candida stimulation

Signaling cascades converting the recognition of pathogens to efficient inflammatory responses by neutrophils are critical for host survival. SKAP2, an adaptor protein, is required for reactive oxygen species (ROS) generation following neutrophil stimulation by integrins, formyl peptide receptors, and for host defense against the Gram-negative bacterial pathogens, Klebsiella pneumoniae and Yersinia pseudotuberculosis. Using neutrophils from murine HoxB8-immortalized progenitors, we show that SKAP2 in neutrophils is crucial for maximal ROS response to purified C-type lectin receptor agonists and to the fungal pathogens, Candida glabrata and Candida albicans, and for robust killing of C. glabrata. Inside-out signaling to integrin and Syk phosphorylation occurred independently of SKAP2 after Candida infection. However, Pyk2, ERK1/2, and p38 phosphorylation were significantly reduced after infection with C. glabrata and K. pneumoniae in Skap2-/- neutrophils. These data demonstrate the importance of SKAP2 in ROS generation and host defense beyond antibacterial immunity to include CLRs and Candida species.

The Two Formyl Peptide Receptors Differently Regulate GPR84-Mediated Neutrophil NADPH Oxidase Activity

Neutrophils express the two formyl peptide receptors (FPR1 and FPR2) and the medium-chain fatty acid receptor GPR84. The FPRs are known to define a hierarchy among neutrophil G protein-coupled receptors (GPCRs), that is, the activated FPRs can either suppress or amplify GPCR responses. In this study, we investigated the position of GPR84 in the FPR-defined hierarchy regarding the activation of neutrophil nicotine adenine dinucleotide phosphate (NADPH) oxidase, an enzyme system designed to generate reactive oxygen species (ROS), which are important regulators in cell signaling and immune regulation. When resting neutrophils were activated by GPR84 agonists, a modest ROS release was induced. However, vast amounts of ROS were induced by these GPR84 agonists in FPR2-desensitized neutrophils, and the response was inhibited not only by a GPR84-specific antagonist but also by an FPR2-specific antagonist. This suggests that the amplified GPR84 agonist response is achieved through a reactivation of desensitized FPR2s. In addition, the GPR84-mediated FPR2 reactivation was independent of β-arrestin recruitment and sensitive to a protein phosphatase inhibitor. In contrast to FPR2-desensitized cells, FPR1 desensitization primarily resulted in a suppressed GPR84 agonist-induced ROS response, indicating a receptor hierarchical desensitization of GPR84 by FPR1-generated signals. In summary, our data show that the two FPRs in human neutrophils control the NADPH oxidase activity with concomitant ROS production by communicating with GPR84 through different mechanisms. While FPR1 desensitizes GPR84 and by that suppresses the release of ROS induced by GPR84 agonists, amplified ROS release is achieved by GPR84 agonists through reactivation of the desensitized FPR2.

Dynamic Function and Composition Shift in Circulating Innate Immune Cells in Hibernating Garden Dormice

Hibernation is characterized by successive torpor bouts during which metabolic rate is down-regulated to 2-4% of euthermic levels along with core body temperatures (T b ) ranging between 0 and 10°C. One characteristic of the torpid state, which is periodically interrupted by a few hours of euthermic phases or arousals during hibernation, resides in an overall impairment of the immune system. The most striking change during torpor is the reduction of circulating white blood cells up to 90%, while their numbers rise to near summer euthermic level upon rewarming. However, potential changes in responsiveness and function of neutrophil granulocytes, accounting for the primary cellular innate immune defense, are unknown. Here we present the first data on shifts in oxidative burst capacity, i.e., the ability to produce reactive oxygen species (ROS), of neutrophils during hibernation. Using a chemiluminescence assay, we measured real-time ROS production in whole blood of hibernating garden dormice (Eliomys quercinus) in early or late torpor, and upon arousals. Accounting for changes in neutrophil numbers along the torpor-arousal cycle, we found significant differences, between torpid and euthermic states, in the neutrophil oxidative burst capacity (NOC), with shallow cell responses during torpor and a highly significant increase by up to 30-fold during arousals. Further, we observed a significant reduction of NOC from aroused animals with euthermic T b of 36.95 ± 0.37°C, when tested at 6°C, whereas no change occurred in NOC from torpid individuals reaching constant T b of 4.67 ± 0.42°C, when measured at 35°C. This dynamic indicates that the reduction in NOC during torpor may be temperature-compensated. These results linked to the understanding of immune function during the torpor-arousal cycle might have clinical relevance in the context of therapeutic hypothermia and reperfusion injury.

Functional selective FPR1 signaling in favor of an activation of the neutrophil superoxide generating NOX2 complex

The formyl peptide receptors FPR1 and FPR2 are abundantly expressed by neutrophils, in which they regulate proinflammatory tissue recruitment of inflammatory cells, the production of reactive oxygen species (ROS), and resolution of inflammatory reactions. The unique dual functionality of the FPRs makes them attractive targets to develop FPR‐based therapeutics as novel anti‐inflammatory treatments. The small compound RE‐04‐001 has earlier been identified as an inducer of ROS in differentiated HL60 cells but the precise target and the mechanism of action of the compound was has until now not been elucidated. In this study, we reveal that RE‐04‐001 specifically targets and activates FPR1, and the concentrations needed to activate the neutrophil NADPH‐oxidase was very low (EC₅₀ ∼1 nM). RE‐04‐001 was also found to be a neutrophil chemoattractant, but when compared to the prototype FPR1 agonist N‐formyl‐Met‐Leu‐Phe (fMLF), the concentrations required were comparably high, suggesting that signaling downstream of the RE‐04‐001‐activated‐FPR1 is functionally selective. In addition, the RE‐04‐001‐induced response was strongly biased toward the PLC‐PIP₂‐Ca²⁺ pathway and ERK1/2 activation but away from β‐arrestin recruitment. Compared to the peptide agonist fMLF, RE‐04‐001 is more resistant to inactivation by the MPO‐H₂O₂‐halide system. In summary, this study describes RE‐04‐001 as a novel small molecule agonist specific for FPR1, which displays a biased signaling profile that leads to a functional selective activating of human neutrophils. RE‐04‐001 is, therefore, a useful tool, not only for further mechanistic studies of the regulatory role of FPR1 in inflammation in vitro and in vivo, but also for developing FPR1‐specific drug therapeutics.

Data showing effects of a PI3K-δ inhibitor on neutrophil superoxide production during FPR2 activation and reactivation

Reactive oxygen species (ROS) generated by the NADPH oxidase are crucial for antimicrobial host defense and also play a role in the regulation of inflammatory processes. Signals generated by formyl peptide receptor 2 (FPR2) activate the neutrophil ROS generating NADPH oxidase; such signals are mediated when the receptors bind an activating agonist, as well as when agonist desensitized FPR2 are reactivated by the receptor for platelet-activating factor (PAF). We present data on the effects of Idelalisib, a specific inhibitor for the PI3Kδ isoform, on ROS production during FPR2 activation and reactivation by PAF, respectively. Neutrophils were isolated from peripheral blood of healthy adults obtained from the blood bank at Sahlgrenska University Hospital and ROS release was measured using isoluminol-amplified chemiluminescence.

In Vivo Transmigrated Human Neutrophils Are Highly Primed for Intracellular Radical Production Induced by Monosodium Urate Crystals

Gout is an inflammatory disease caused by monosodium urate (MSU) crystals. The role of neutrophils in gout is less clear, although several studies have shown neutrophil extracellular trap (NET) formation in acutely inflamed joints of gout patients. MSU crystals are known to induce the production of reactive oxygen species (ROS) and NET formation in neutrophils isolated from blood, but there is inconclusive knowledge on the localization of ROS production as well as whether the ROS are required for NET formation. In this report we demonstrate that MSU crystals activate human neutrophils to produce ROS exclusively in intracellular compartments. Additionally, in vivo transmigrated neutrophils derived from experimental skin chambers displayed markedly increased ROS production as compared to resting blood neutrophils. We also confirmed that MSU stimulation potently induced NET formation, but this response was not primed in in vivo transmigrated neutrophils. In line with this we found that MSU-triggered NET formation was independent of ROS production and proceeded normally in neutrophils from patients with dysfunctional respiratory burst (chronic granulomatous disease (CGD) and complete myeloperoxidase (MPO) deficiency). Our data indicate that in vivo transmigrated neutrophils are markedly primed for oxidative responses to MSU crystals and that MSU triggered NET formation is independent of ROS production.