Roles of neutrophils in the regulation of the extent of human inflammation through delivery of IL-1 and clearance of chemokines

Clinical, cellular and molecular effects of corticosteroids on the response to intradermal lipopolysaccharide administration in healthy volunteers

The intradermal lipopolysaccharide (LPS) challenge in healthy volunteers has proven to be a valuable tool to study local inflammation in vivo. In the current study the inhibitory effects of oral and topical corticosteroid treatment on intradermal LPS responses were evaluated to benchmark the challenge for future investigational drugs. Twenty‐four healthy male volunteers received a two‐and‐a‐half‐day twice daily (b.i.d.) pretreatment with topical clobetasol propionate 0.05% and six healthy volunteers received a two‐and‐a‐half‐day b.i.d. pretreatment with oral prednisolone at 0.25 mg/kg body weight per administration. Participants received one injection regimen of either 0, 2, or 4 intradermal LPS injections (5 ng LPS in 50 µL 0.9% sodium chloride solution). The LPS response was evaluated by noninvasive (perfusion, skin temperature, and erythema) and invasive assessments (cellular and cytokine responses) in suction blister exudate. Both corticosteroids significantly suppressed the clinical inflammatory response (erythema P = 0.0001 for clobetasol and P = 0.0016 for prednisolone; heat P = 0.0245 for clobetasol, perfusion P < 0.0001 for clobetasol and P = 0.0036 for prednisolone). Clobetasol also significantly reduced the number of monocytes subsets, dendritic cells, natural killer cells, and T cells in blister exudate. A similar effect was observed for prednisolone. No relevant corticosteroid effects were observed on the cytokine response to LPS. We successfully demonstrated that the anti‐inflammatory effects of corticosteroids can be detected using our intradermal LPS challenge model, validating it for evaluation of future investigational drugs, as an initial assessment of the anti‐inflammatory effects of such compounds in a minimally invasive manner.

Intradermal lipopolysaccharide challenge as an acute in vivo inflammatory model in healthy volunteers

Aims

Whereas intravenous administration of Toll‐like receptor 4 ligand lipopolysaccharide (LPS) to human volunteers is frequently used in clinical pharmacology studies, systemic use of LPS has practical limitations. We aimed to characterize the intradermal LPS response in healthy volunteers, and as such qualify the method as local inflammation model for clinical pharmacology studies.

Methods

Eighteen healthy male volunteers received 2 or 4 intradermal 5 ng LPS injections and 1 saline injection on the forearms. The LPS response was evaluated by noninvasive (perfusion, skin temperature and erythema) and invasive assessments (cellular and cytokine responses) in skin biopsy and blister exudate.

Results

LPS elicited a visible response and returned to baseline at 48 hours. Erythema, perfusion and temperature were statistically significant (P < .0001) over a 24‐hour time course compared to saline. The protein response was dominated by an acute interleukin (IL)‐6, IL‐8 and tumour necrosis factor response followed by IL‐1β, IL‐10 and interferon‐γ. The cellular response consisted of an acute neutrophil influx followed by different monocyte subsets and dendritic cells.

Discussion

Intradermal LPS administration in humans causes an acute, localized and transient inflammatory reaction that is well‐tolerated by healthy volunteers. This may be a valuable inflammation model for evaluating the pharmacological activity of anti‐inflammatory investigational compounds in proof of pharmacology studies.

Monocytes, macrophages, dendritic cells and neutrophils: an update on lifespan kinetics in health and disease

Phagocytes form a family of immune cells that play a crucial role in tissue maintenance and help orchestrate the immune response. This family of cells can be separated by their nuclear morphology into mononuclear and polymorphonuclear phagocytes. The generation of these cells in the bone marrow, to the blood and finally into tissues is a tightly regulated process. Ensuring the adequate production of these cells and their timely removal is key for both the initiation and resolution of inflammation. Insight into the kinetic profiles of innate myeloid cells during steady state and pathology will permit the rational development of therapies to boost the production of these cells in times of need or reduce them when detrimental.

Human lipopolysaccharide models provide mechanistic and therapeutic insights into systemic and pulmonary inflammation

Inflammation is a key feature in the pathogenesis of sepsis and acute respiratory distress syndrome (ARDS). Sepsis and ARDS continue to be associated with high mortality. A key contributory factor is the rudimentary understanding of the early events in pulmonary and systemic inflammation in humans, which are difficult to study in clinical practice, as they precede the patient's presentation to medical services. Lipopolysaccharide (LPS), a constituent of the outer membrane of Gram-negative bacteria, is a trigger of inflammation and the dysregulated host response in sepsis. Human LPS models deliver a small quantity of LPS to healthy volunteers, triggering an inflammatory response and providing a window to study early inflammation in humans. This allows biological/mechanistic insights to be made and new therapeutic strategies to be tested in a controlled, reproducible environment from a defined point in time. We review the use of human LPS models, focussing on the underlying mechanistic insights that have been gained by studying the response to intravenous and pulmonary LPS challenge. We discuss variables that may influence the response to LPS before considering factors that should be considered when designing future human LPS studies.

Neutrophil GM-CSF receptor dynamics in acute lung injury

GM‐CSF is important in regulating acute, persistent neutrophilic inflammation in certain settings, including lung injury. Ligand binding induces rapid internalization of the GM‐CSF receptor (GM‐CSFRα) complex, a process essential for signaling. Whereas GM‐CSF controls many aspects of neutrophil biology, regulation of GM‐CSFRα expression is poorly understood, particularly the role of GM‐CSFRα in ligand clearance and whether signaling is sustained despite major down‐regulation of GM‐CSFRα surface expression. We established a quantitative assay of GM‐CSFRα surface expression and used this, together with selective anti‐GM‐CSFR antibodies, to define GM‐CSFRα kinetics in human neutrophils, and in murine blood and alveolar neutrophils in a lung injury model. Despite rapid sustained ligand‐induced GM‐CSFRα loss from the neutrophil surface, which persisted even following ligand removal, pro‐survival effects of GM‐CSF required ongoing ligand‐receptor interaction. Neutrophils recruited to the lungs following LPS challenge showed initially high mGM‐CSFRα expression, which along with mGM‐CSFRβ declined over 24 hr; this was associated with a transient increase in bronchoalveolar lavage fluid (BALF) mGM‐CSF concentration. Treating mice in an LPS challenge model with CAM‐3003, an anti‐mGM‐CSFRα mAb, inhibited inflammatory cell influx into the lung and maintained the level of BALF mGM‐CSF. Consistent with neutrophil consumption of GM‐CSF, human neutrophils depleted exogenous GM‐CSF, independent of protease activity. These data show that loss of membrane GM‐CSFRα following GM‐CSF exposure does not preclude sustained GM‐CSF/GM‐CSFRα signaling and that this receptor plays a key role in ligand clearance. Hence neutrophilic activation via GM‐CSFR may play an important role in neutrophilic lung inflammation even in the absence of high GM‐CSF levels or GM‐CSFRα expression.

Candidate Genes as Biomarkers in Lipopolysaccharide-Induced Acute Respiratory Distress Syndrome Based on mRNA Expression Profile by Next-Generation RNA-Seq Analysis

Up until now, the regulation mechanism at the level of gene during lipopolysaccharide- (LPS-) induced acute respiratory distress syndrome (ARDS) remains unclear. The discovery of differentially expressed genes (DEGs) between LPS-induced ARDS rats and normal rats by next-generation RNA sequencing analysis is of particular interest for the current study. These DEGs may help clinical diagnosis of ARDS and facilitate the selection of the optimal treatment strategy. Randomly, 20 rats were equally divided into 2 groups, the control group and the LPS group. Three rats from each group were selected at random for RNA sequencing analysis. Sequence reads were obtained from Illumina HiSeq4000 and mapped onto the rat reference genome RN6 using Hisat2. We identified 5244 DEGs (Fold_Change > 1.5, and P < 0.05) in the lung tissues from LPS-treated rats compared with normal rats, including 1413 upregulated and 3831 downregulated expressed genes. Lots of chemokine family members were among the most upregulated genes in LPS group. Gene ontology (GO) analysis revealed that almost all of the most enriched and meaningful biological process terms were mainly involved in the functions like immune-inflammation response and the pathways like cytokine-cytokine receptor interaction. We also found that, as for GO molecular function terms, the enriched terms were mainly related to chemokines and cytokines. DEGs with fold change over 100 were verified by quantitative real-time polymerase chain reaction and reanalyzed by gene-gene coexpression network, and the results elucidated central roles of chemokines in LPS-induced ARDS. Our results revealed some new biomarkers for uncovering mechanisms and processes of ARDS.

The key role of proinflammatory cytokines, matrix proteins, RANKL/OPG and Wnt/β-catenin in bone healing of hip arthroplasty patients

INTRODUCTION

We still lack understanding of why some implants fail while most remain stable after decades of use. Proinflammatory cytokines, matrix proteins and bone regulating cytokines of the RANKL/OPG (receptor activator of nuclear factor kappa B ligand/osteoprotegerin) and Wnt/β-catenin pathways are mandatory for normal bone repair but their spatial and temporal role in the healing of primary total hip arthroplasties (THA) has not been previously shown.

MATERIALS AND METHODS

Twenty-four osteoarthritis patients with one-sided well-fixed primary THA were prospectively monitored during 18years (18Y) with repeated blood samples, clinical variables and radiographs. Eighty-one healthy donors divided in three age- and gender-matched groups and twenty osteoarthritis patients awaiting THA and serving as control of the validity of stored plasma in THA patients, were included. Plasma was analyzed for C-reactive protein (CRP), interleukin (IL)-6, IL-8, IL-1β, tumor necrosis factor (TNF)-α, osteopontin (OPN), secreted protein acidic and rich in cysteine (SPARC/osteonectin), osteocalcin (OC), bone specific alkaline phosphatase (BALP), N-terminal propeptide of collagen type I (P1NP), RANKL, OPG, the Wnt agonistic ligands (Wnt)-1 and Wnt-3a, and the Wnt antagonists sclerostin, Dickkopf (Dkk)-1, Dkk-3, Dkk-4, secreted frizzled related protein (sFRP)-1, sFRP-3 and Wnt inhibitory factor-1 (Wif-1).

RESULTS

Inflammatory mediators in arthroplasty patients (CRP, IL-6, OPN) increased significantly on day one after surgery vs preoperative value (PR) and healthy subjects and returned to baseline at 6W. TNF-α did not change relative preoperative level or healthy subjects. SPARC and OC increased in a biphasic fashion with the primary phase beginning shortly after surgery and lasting 3M (SPARC) and 2Y (OC) while the secondary phase peaked at 1Y (SPARC) and 13Y (OC), with both returning to basal level at 15Y. BALP peaked at 3M after surgery with a return to basal level at 2Y followed by a continuous increase from 5Y until 18Y. P1NP increased immediately after surgery and returned to basal level at 6W followed by a new peak at 10Y returning to basal at 13Y. IL-8 and IL-1β peaked at 5Y post-THA and returned to basal level at 10Y. RANKL/OPG and Wnt/β-catenin remained at preoperative levels until 5Y post-THA when a sustained increase in OPG level, paralleled by a sustained decrease in sclerostin, started and lasted until 18Y. Despite a strong increase by RANKL at 13Y, the OPG/RANKL-ratio remained high between 5Y and 18Y. Dkk-1 and sFRP-1 remained at basal level until 5Y followed by a peak at 7Y and a return to basal level at 15Y. Similarly, RANKL increased after 5Y, peaked at 13Y and returned to basal levels at 18Y, thus coinciding with Wnt-1. In contrast, Wnt3a, Dkk-3, Dkk-4, sFRP-3 and Wif-1 did not differ from preoperative levels or healthy subjects during the course of the follow-up.

CONCLUSION

The primary peak of proinflammatory cytokines involved in the initiation of bone healing after trauma is in line with previous results. The primary phase of increased matrix proteins, P1NP and BALP paralleled by RANKL, OPG and Wnt/β-catenin remaining at preoperative level until 5Y, support a strong formation of mineralized matrix and to a lesser degree bone during this phase. The secondary proinflammatory peak at 5Y is likely a trigger of coupled bone remodeling and neosynthesis as it is followed by increased levels of the bone anabolic turnover marker, BALP, and mediators of the RANKL/OPG and Wnt/β-catenin pathways. A continuous increase by OPG level and the bone turnover marker, BALP, lasting from 5Y until 18Y and paralleled by a similar decrease in sclerostin level support their being key regulators of bone anabolism, whereas the transient and opposed activities of RANKL, Wnt-1, Dkk-1 and sFRP-1 serve as fine tuning tools during the coupled remodeling phase.

NR4A orphan nuclear receptor family members, NR4A2 and NR4A3, regulate neutrophil number and survival

The lifespan of neutrophils is plastic and highly responsive to factors that regulate cellular survival. Defects in neutrophil number and survival are common to both hematologic disorders and chronic inflammatory diseases. At sites of inflammation, neutrophils respond to multiple signals that activate protein kinase A (PKA) signaling, which positively regulates neutrophil survival. The aim of this study was to define transcriptional responses to PKA activation and to delineate the roles of these factors in neutrophil function and survival. In human neutrophil gene array studies, we show that PKA activation upregulates a significant number of apoptosis-related genes, the most highly regulated of these being NR4A2 and NR4A3 Direct PKA activation by the site-selective PKA agonist pair N6/8-AHA (8-AHA-cAMP and N6-MB-cAMP) and treatment with endogenous activators of PKA, including adenosine and prostaglandin E2, results in a profound delay of neutrophil apoptosis and concomitant upregulation of NR4A2/3 in a PKA-dependent manner. NR4A3 expression is also increased at sites of neutrophilic inflammation in a human model of intradermal inflammation. PKA activation also promotes survival of murine neutrophil progenitor cells, and small interfering RNA to NR4A2 decreases neutrophil production in this model. Antisense knockdown of NR4A2 and NR4A3 homologs in zebrafish larvae significantly reduces the absolute neutrophil number without affecting cellular migration. In summary, we show that NR4A2 and NR4A3 are components of a downstream transcriptional response to PKA activation in the neutrophil, and that they positively regulate neutrophil survival and homeostasis.

Intravenous Endotoxin Challenge in Healthy Humans: An Experimental Platform to Investigate and Modulate Systemic Inflammation

Activation of inflammatory pathways represents a central mechanism in multiple disease states both acute and chronic. Triggered via either pathogen or tissue damage-associated molecular motifs, common biochemical pathways lead to conserved yet variable physiological and immunological alterations. Dissection and delineation of the determinants and mechanisms underlying phenotypic variance in response is expected to yield novel therapeutic advances.

Intravenous (IV) administration of endotoxin (gram-negative bacterial lipopolysaccharide), a specific Toll-like receptor 4 agonist, represents an in vivo model of systemic inflammation in man. National Institutes for Health Clinical Center Reference Endotoxin (CCRE, Escherichia coli O:113:H10:K negative) is employed to reliably and reproducibly generate vascular, hematological, endocrine, immunological and organ-specific functional effects that parallel, to varying degrees, those seen in the early stages of pathological states. Alteration of dose (0.06 - 4 ng/kg) and time-scale of exposure (bolus vs. infusion) allows replication of either acute or chronic inflammation and a range of severity to be elicited, with higher doses (2 - 4 ng/kg) frequently being used to create a 'sepsis-like' state. Established and novel medicinal compounds may additionally be administered prior to or post endotoxin exposure to appreciate their effect on the inflammatory cascade. Despite limitations in scope and generalizability, human IV endotoxin challenge offers a unique platform to gain mechanistic insights into inducible physiological responses and inflammatory pathways. Rationally employed it may aid translation of this knowledge into therapeutic innovations.

Novel translational model of resolving inflammation triggered by UV-killed E. coli

Whilst numerous studies investigating the aetiology of inflammatory diseases have been performed in rodents, the applicability of these data to human pathophysiology is frequently debated. Regardless of the strengths and weaknesses of rodent models in biomedical research, there is a need to develop models of experimental inflammation in humans. Here, we describe a self‐resolving acute inflammatory response triggered by the intradermal injection of UV‐killed Escherichia coli into the forearm of healthy volunteers. Cells and exudates were harvested from onset to resolution by applying negative pressure over the inflamed site. Onset was characterized by high blood flow, neutrophilia and peak levels of pro‐inflammatory cytokines, whilst resolution showed a decline in blood blow, reduction in neutrophils, increase in monocytes/macrophages and waning of classic pro‐inflammatory cytokine levels. An anti‐inflammatory effect, defined as suppression of onset phase events, was demonstrated by administering naproxen, a conventional non‐steroidal anti‐inflammatory drug. In summary, this model of resolving acute inflammation is minimally invasive, highly tractable and allows simultaneous investigation of the vascular response, cellular trafficking and chemical mediator profile of onset and resolution phases of acute inflammation in humans. It can serve as a translational platform to provide mechanistic insights and to test the clinical efficacy of novel anti‐inflammatory and pro‐resolving drugs, and also as a tool in patients to explore inherent defects in resolution pathways.

Resolution of inflammation: a new therapeutic frontier

Dysregulated inflammation is a central pathological process in diverse disease states. Traditionally, therapeutic approaches have sought to modulate the pro- or anti-inflammatory limbs of inflammation, with mixed success. However, insight into the pathways by which inflammation is resolved has highlighted novel opportunities to pharmacologically manipulate these processes - a strategy that might represent a complementary (and perhaps even superior) therapeutic approach. This Review discusses the state of the art in the biology of resolution of inflammation, highlighting the opportunities and challenges for translational research in this field.

Fenofibrate Attenuates Neutrophilic Inflammation in Airway Epithelia: Potential Drug Repurposing for Cystic Fibrosis

A hallmark of cystic fibrosis (CF) lung disease is neutrophilic airway inflammation. Elevated neutrophil counts have been associated with decreased forced expiratory volume in 1 second and poor clinical measures in patients with CF. Interleukin 8 (IL-8), epithelial neutrophil activating protein 78 (ENA-78), tumor necrosis factor alpha (TNF-α), granulocyte macrophage colony-stimulating factor (GM-CSF), and granulocyte colony-stimulating factor (G-CSF) contribute to neutrophil activation and disease pathogenesis in the airways of patients with CF. Drugs that modify the production of these chemokines in the airways could potentially benefit CF patients. Thus, we determined the effects of fenofibrate on their production in cell populations obtained from the airways. Human small airway epithelial cells and CF bronchial epithelial cells were treated with IL-1β to induce inflammation. We cotreated the cells with fenofibrate at concentrations ranging from 10 to 50 μM to determine if this drug could attenuate the inflammation. IL-8, ENA-78, TNF-α, GM-CSF, and G-CSF production were measured from the cell culture supernates by ELISA. ANOVA statistical testing was conducted using SPSS 17.0. IL-1β increased the production of each of the chemokines by several fold. Fenofibrate reduced IL-1β induced production of each of these neutrophilic chemokines at the concentrations used. IL-1β increases the production of neutrophilic chemokines in airway epithelial cells. Cotreatment with fenofibrate blunts these processes. Fenofibrate should be explored as a therapeutic option to modulate the abundant neutrophilic inflammation observed in CF.

Digital gene expression analysis of transcriptomes in lipopolysaccharide-induced acute respiratory distress syndrome

BACKGROUND

The mortality from acute respiratory distress syndrome (ARDS) is high, and its exact pathogenesis remains unclear, which forms a major obstacle for prevention and treatment of this disease. In the present study, we used digital gene expression (DGE) to detect the differentially expressed genes of the lung at 4h after lipopolysaccharide (LPS) exposure in a mouse model.

METHODS

Mice were treated with LPS or control saline by intratracheal instillation for 4h, and their lung tissues were collected for DGE analysis. We used a false discovery rate ≤0.001 and an absolute value of the log2 ratio≥1 as the thresholds for judging the significance of any difference in gene expression between the two members of each pair of mice.

RESULTS

We obtained 3,387,842 clean tags (i.e., after filtering to remove potentially erroneous tags) and about 84,513 corresponding distinct clean tags (i.e., types of tag). Approximately 91.20% of the clean tags could be mapped, and 82.71% could be uniquely mapped, to the reference tags, and 3.82% were unknown tags. At least 2200 differentially expressed genes were identified and analyzed for enrichment of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway. Twenty genes with the greatest difference in expression levels between the two members of every pair of mice were chosen. The majority of these genes are involved in signaling transduction, molecular adhesion, and metabolic pathways.

CONCLUSIONS

Using the powerful technology of DGE, we present, to our knowledge, the first in-depth transcriptomic analysis of mouse lungs after LPS exposure. We found some differentially expressed genes that might play important roles in the pathogenesis of ARDS.

Role of brain transmigrating neutrophils in depression-like behavior during systemic infection

Peripheral inflammation induces transmigration of interleukin (IL)-1β-expressing neutrophils to the brain. We investigated the possibility that this presents a new route of immune-to-brain communication by assessing their role in sickness behaviors relevant for mood disorders. Mice treated with lipopolysaccharide (LPS) developed despair-like behavior, and administration of an anti-polymorphonuclear antibody abolished LPS-induced despair-like and asocial behaviors, which correlated with the levels of IL-1β expression in the brain. These behavioral changes were directly mediated by the energy-regulating hormone, leptin. Increasing the concentration of endogenous leptin during obesity exacerbated, whereas its neutralization using a specific antiserum attenuated sickness behaviors and importantly the neutrophil transmigrating process. Our results indicate a role for peripheral neutrophils in conveying inflammatory signals to the brain, which appears to be dependent on the energy status of the organism. This constitutes a novel mechanism of immune-to-brain communication relevant to mood disorders.

Human polymorphonuclear leukocytes produce cytokines in response to Leishmania major promastigotes

Polymorphonuclear leukocytes (PMN) release cytokines that may influence the development of the subsequent adaptive immune response. Little is known about cytokines produced by human PMN in response to Leishmania (L.). In this study, mRNA expression of Interleukin (IL)-12p40, IL-12p35, Interferon (IFN)-γ, transforming growth factor (TGF)-β, IL-1, and IL-4 in PMN of volunteers stimulated with L. major promastigotes has been investigated by real-time PCR and the results were confirmed by flow cytometer. The results showed that L. major did not induce mRNA expression of IL12p40, IL12p35, IFN-γ, and TGF-β in PMN, while IL-1 and IL-4 mRNA were induced. Flow cytometry results confirmed no IFN-γ production by PMN with or without stimulation. IL-12p70 was present in untreated and L. major-treated PMN, and these cells release IL-12 following incubation with L. major. Significant amount of IL-1 even without treatment with promastigotes was detected in PMN. Moreover, the proportion of PMN, which produce IL-1 in response to L. major, was increased compared with the percent of unstimulated IL-1-producing PMN. The results showed the accumulation of small amounts of IL-4 in PMN after stimulation. In conclusion, our results indicate that IL-12 and IL-1 are pre-stored in human PMN, nor L. major induces IL-1 and IL-4, but not IL-12, IFN-γ, nor TGF-β expression in these cells.

Localization and functionality of the inflammasome in neutrophils

Neutrophils represent the major fraction of circulating immune cells and are rapidly recruited to sites of infection and inflammation. The inflammasome is a multiprotein complex that regulates the generation of IL-1 family proteins. The precise subcellular localization and functionality of the inflammasome in human neutrophils are poorly defined. Here we demonstrate that highly purified human neutrophils express key components of the NOD-like receptor family, pyrin domain containing 3 (NLRP3), and absent in melanoma 2 (AIM2) inflammasomes, particularly apoptosis-associated speck-like protein containing a CARD (ASC), AIM2, and caspase-1. Subcellular fractionation and microscopic analyses further showed that inflammasome components were localized in the cytoplasm and also noncanonically in secretory vesicle and tertiary granule compartments. Whereas IL-1β and IL-18 were expressed at the mRNA level and released as protein, highly purified neutrophils neither expressed nor released IL-1α at baseline or upon stimulation. Upon inflammasome activation, highly purified neutrophils released substantially lower levels of IL-1β protein compared with partially purified neutrophils. Serine proteases and caspases were differentially involved in IL-1β release, depending on the stimulus. Spontaneous activation of the NLRP3 inflammasome in neutrophils in vivo affected IL-1β, but not IL-18 release. In summary, these studies show that human neutrophils express key components of the inflammasome machinery in distinct intracellular compartments and release IL-1β and IL-18, but not IL-1α or IL-33 protein. Targeting the neutrophil inflammasome may represent a future therapeutic strategy to modulate neutrophilic inflammatory diseases, such as cystic fibrosis, rheumatoid arthritis, or sepsis.

Zebrafish tissue injury causes upregulation of interleukin-1 and caspase-dependent amplification of the inflammatory response

Interleukin-1 (IL-1), the 'gatekeeper' of inflammation, is the apical cytokine in a signalling cascade that drives the early response to injury or infection. Expression, processing and secretion of IL-1 are tightly controlled, and dysregulated IL-1 signalling has been implicated in a number of pathologies ranging from atherosclerosis to complications of infection. Our understanding of these processes comes from in vitro monocytic cell culture models as lines or primary isolates, in which a range and spectra of IL-1 secretion mechanisms have been described. We therefore investigated whether zebrafish embryos provide a suitable in vivo model for studying IL-1-mediated inflammation. Structurally, zebrafish IL-1β shares a β-sheet-rich trefoil structure with its human counterpart. Functionally, leukocyte expression of IL-1β was detectable only following injury, which activated leukocytes throughout zebrafish embryos. Migration of macrophages and neutrophils was attenuated by inhibitors of either caspase-1 or P2X7, which similarly inhibited the activation of NF-κB at the site of injury. Zebrafish offer a new and versatile model to study the IL-1β pathway in inflammatory disease and should offer unique insights into IL-1 biology in vivo.

Regulation of neutrophil apoptosis differs after in vivo transmigration to skin chambers and synovial fluid: a role for inflammasome-dependent interleukin-1β release

Short-lived neutrophils are major players in inflammation, arriving early to infected and/or injured tissues. After performed duty, neutrophils are programmed to die by apoptosis and are thereafter rapidly cleared by other phagocytes. In vitro, modulation of the apoptotic process has been thoroughly investigated in neutrophils isolated from peripheral blood, but less is known about the regulation of this process in neutrophils derived from extravascular tissues. We recently demonstrated that neutrophils having transmigrated in vivo, obtained from experimental skin chambers of healthy human subjects, are resistant to the death-delaying signals induced by a range of antiapoptotic stimuli. In the current study, we show that skin chamber neutrophils spontaneously secrete high levels of antiapoptotic interleukin (IL)-1β which delays neutrophil apoptosis. Contrary to skin chamber fluid, synovial fluid from patients with rheumatic arthritis contained only moderate levels of IL-1β, and neutrophils taken from this site were fully responsive to antiapoptotic stimulation during in vitro culture. Our data demonstrate that resistance to antiapoptotic stimulation is not a general feature of tissue neutrophils and imply that autocrine IL-1β signaling could be an important factor in determining how life and death of neutrophils is regulated in inflamed tissues.

Editorial: Gazing forward while looking back

Discussion of how transcriptional responses of neutrophils contribute to the resolution of inflammation, and direct studies of human innate immune responses.