In vivo-transmigrated human neutrophils are resistant to antiapoptotic stimulation

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 Three years after treatment initiation adalimumab was withdrawn, resulting in rapid symptom recurrence.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.

NET Release of Long-Term Surviving Neutrophils

Background

Neutrophil extracellular traps (NETs)—as double-edged swords of innate immunity—are involved in numerous processes such as infection, inflammation and tissue repair. Research on neutrophil granulocytes is limited because of their short lifetime of only a few hours. Several attempts have been made to prolong the half-life of neutrophils using cytokines and bacterial products and have shown promising results. These long-term surviving neutrophils are reported to maintain phagocytic activity and cytokine release; however, little is known regarding their capability to release NETs.

Methods

We analysed the prolongation of neutrophil survival in vitro under various culture conditions using granulocyte colony-stimulating factor (G-CSF), lipopolysaccharide (LPS) or tumour necrosis factor alpha (TNF-α) by flow cytometry and a viability assay. Additionally, we assessed NET formation following stimulation with phorbol 12-myristate 13-acetate (PMA) by immunofluorescence staining, myeloperoxidase (MPO)-DNA sandwich-ELISA and fluorometric assays for cell-free DNA (cfDNA), neutrophil elastase (NE) and myeloperoxidase (MPO).

Results

Untreated neutrophils could form NETs after stimulation with PMA for up to 24 h. Incubation with LPS extended their ability to form NETs for up to 48 h. At 48 h, NET release of neutrophils cultured with LPS was significantly higher compared to that of untreated cells; however, no significantly different enzymatic activity of NE and MPO was observed. Similarly, incubation with G-CSF resulted in significantly higher NET release at 48 h compared to untreated cells. Furthermore, NETs showed significantly higher enzymatic activity of NE and MPO after incubation with G-CSF. Lastly, incubation with TNF-α had no influence on NET release compared to untreated cells although survival counts were altered by TNF-α.

Conclusions

G-CSF, LPS or TNF-α each at low concentrations lead to prolonged survival of cultured neutrophils, resulting in considerable differences in NET formation and composition. These results provide new information for the use of neutrophils in long-term experiments for NET formation and provide novel insights for neutrophil behaviour under inflammatory conditions.

CXCR2 Mediates Distinct Neutrophil Behavior in Brain Metastatic Breast Tumor

Brain metastasis is one of the main causes of mortality among breast cancer patients, but the origins and the mechanisms that drive this process remain poorly understood. Here, we report that the upregulation of certain CXCR2-associated ligands in the brain metastatic variants of the breast cancer cells (BrM) dynamically activate the corresponding CXCR2 receptors on the neutrophils, thereby resulting in the modulation of certain key functional neutrophil responses towards the BrM. Using established neutrophil-tumor biomimetic co-culture models, we show that the upregulation of CXCR2 increases the recruitment of Tumor-Associated Neutrophils (TANs) towards the BrM, to enable location-favored formation of Neutrophil Extracellular Traps (NETs). Inhibition of CXCR2 using small molecule antagonist AZD5069 reversed this behavior, limiting the neutrophil responses to the BrM and retarding the reciprocal tumor development. We further demonstrate that abrogation of NETs formation using Neutrophil Elastase Inhibitor (NEI) significantly decreases the influx of neutrophils towards BrM but not to their parental tumor, suggesting that CXCR2 activation could be used by the brain metastatic tumors as a mechanism to program the tumor-infiltrating TANs into a pro-NETotic state, so as to assume a unique spatial distribution that assists in the subsequent migration and invasion of the metastatic tumor cells. This new perspective indicates that CXCR2 is a critical target for suppressing neutrophilic inflammation in brain metastasis.

The neutrophil subset defined by CD177 expression is preferentially recruited to gingival crevicular fluid in periodontitis

In recent years, the concept of distinct subpopulations of human neutrophils has attracted much attention. One bona fide subset marker, exclusively expressed by a proportion of circulating neutrophils in a given individual, and therefore dividing neutrophils in two distinct subpopulations, is the glycoprotein CD177. CD177 is expressed on the plasma and granule membranes of 0-100% of circulating neutrophils depending on the donor. Several in vitro studies have linked CD177 to neutrophil transmigration, yet very few have looked at the role of CD177 for tissue recruitment in vivo. We investigate whether the CD177⁺ and CD177^- neutrophil subsets differ in their propensity to migrate to both aseptic- and microbe-triggered inflamed human tissues. Microbe-triggered neutrophil migration was evaluated in samples of gingival crevicular fluid (GCF) from patients with periodontitis, whereas neutrophil migration to aseptic inflammation was evaluated in synovial fluid from patients with inflammatory arthritis, as well as in exudate from experimental skin chambers applied on healthy donors. We found that the proportion of CD177⁺ neutrophils was significantly higher in GCF from patients with periodontitis, as compared to blood from the same individuals. Such accumulation of CD177⁺ neutrophils was not seen in the two models of aseptic inflammation. Moreover, the proportion of CD177⁺ neutrophils in circulation was significantly higher in the periodontitis patient group, as compared to healthy donors. Our data indicate that the CD177⁺ neutrophil subset is preferentially recruited to the gingival crevice of periodontitis patients, and may imply that this subtype is of particular importance for situations of microbe-driven inflammation.

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.

Downregulation of HLA Class I Renders Inflammatory Neutrophils More Susceptible to NK Cell-Induced Apoptosis

Neutrophils are potent effector cells and contain a battery of harmful substances and degrading enzymes. A silent neutrophil death, i.e., apoptosis, is therefore of importance to avoid damage to the surrounding tissue and to enable termination of the acute inflammatory process. There is a pile of evidence supporting the role for pro-inflammatory cytokines in extending the life-span of neutrophils, but relatively few studies have been devoted to mechanisms actively driving apoptosis induction in neutrophils. We have previously demonstrated that natural killer (NK) cells can promote apoptosis in healthy neutrophils. In this study, we set out to investigate how neutrophil sensitivity to NK cell-mediated cytotoxicity is regulated under inflammatory conditions. Using in vitro-activated neutrophils and a human skin chamber model that allowed collection of in vivo-transmigrated neutrophils, we performed a comprehensive characterization of neutrophil expression of ligands to NK cell receptors. These studies revealed a dramatic downregulation of HLA class I molecules in inflammatory neutrophils, which was associated with an enhanced susceptibility to NK cell cytotoxicity. Collectively, our data shed light on the complex regulation of interactions between NK cells and neutrophils during an inflammatory response and provide further support for a role of NK cells in the resolution phase of inflammation.

Neutrophils at work

In this Review we discuss data demonstrating recently recognized aspects of neutrophil homeostasis in the steady state, granulopoiesis in 'emergency' conditions and interactions of neutrophils with the adaptive immune system. We explore in vivo observations of the recruitment of neutrophils from blood to tissues in models of blood-borne infections versus bacterial invasion through epithelial linings. We examine data on novel aspects of the activation of NADPH oxidase and the heterogeneity of phagosomes and, finally, consider the importance of two neutrophil-derived biological agents: neutrophil extracellular traps and ectosomes.

Signal regulatory protein alpha is present in several neutrophil granule populations and is rapidly mobilized to the cell surface to negatively fine-tune neutrophil accumulation in inflammation

Signal regulatory protein alpha (SIRPα) is a cell surface glycoprotein with inhibitory functions, which may regulate neutrophil transmigration. SIRPα is mobilized to the neutrophil surface from specific granules, gelatinase granules, and secretory vesicles following inflammatory activation in vitro and in vivo. The lack of SIRPα signaling and the ability to upregulate SIRPα to the cell surface promote neutrophil accumulation during inflammation in vivo.

Collection of in vivo transmigrated neutrophils from human skin

A wealth of knowledge on the life and death of human neutrophils has been obtained by the in vitro study of isolated cells derived from peripheral blood. However, neutrophils are of main importance, physiologically as well as pathologically, after they have left circulation and transmigrated to extravascular tissues. The journey from blood to tissue is complex and eventful, and tissue neutrophils are in many aspects distinct from the cells left in circulation. Here we describe how to obtain human tissue neutrophils in a controlled experimental setting from aseptic skin lesions created by the application of negative pressure. One protocol enables the direct analysis of the blister content, infiltrating leukocytes as well as exudate fluid, and is a simple method to follow multiple parameters of aseptic inflammation in vivo. Also described is the skin chamber technique, a method based on denuded skin blisters which are subsequently covered by collection chambers filled with autologous serum. Although slightly more artificial as compared to analysis of the blister content directly, the cellular yield of this skin chamber method is sufficient to perform a large number of functional analyses of in vivo transmigrated cells.

Cord blood neutrophils display a galectin-3 responsive phenotype accentuated by vaginal delivery

BACKGROUND

Term neonates are at increased risk of infections due to undeveloped immune mechanisms, and proper neutrophil function is important for perinatal immune defence. Galectin-3, an endogenous β-galactoside-binding lectin, is emerging as an inflammatory mediator and we have previously shown that primed/activated, but not resting, adult neutrophils respond to this lectin by production of reactive oxygen species (ROS). We investigated if galectin-3 is of importance in perinatal immune defence, focusing on plasma levels and neutrophil responsiveness.

METHODS

Neutrophils were isolated from peripheral blood of healthy adults and cord blood (CB) after elective Caesarean section (CSCB) and vaginal delivery (VDCB). ROS production was measured by chemiluminescence, L-selectin expression by flow cytometry, and interleukin-8 (IL-8) and galectin-3 concentrations by ELISA. Statistical evaluations were performed using the Mann-Whitney test.

RESULTS

In response to galectin-3, CSCB neutrophils showed a small but clear ROS production not evident in adult cells, signifying that neonatal neutrophils exist in a primed state. IL-8 production was elevated in CSCB cells while L-selectin exposure was equal to adult cells. Comparing CSCB to VDCB neutrophils, the latter showed an extensive galectin-3 responsiveness, indicating that the degree of priming is dependent on mode of delivery. VDCB neutrophils were increasingly prone to shed L-selectin, while the amount of IL-8 was similar to CSCB cells. The endogenous galectin-3 levels were higher in neonatal as compared to adult plasma, unaffected by mode of delivery.

CONCLUSIONS

Neutrophils enter a pre-primed state already in the fetus. Upon exposure to the inflammatory stimuli that are associated with labor, the neutrophils develop a reactive phenotype with extensive priming features.

The human neutrophil subsets defined by the presence or absence of OLFM4 both transmigrate into tissue in vivo and give rise to distinct NETs in vitro

Neutrophil heterogeneity was described decades ago, but it could not be elucidated at the time whether the existence of different neutrophil subsets had any biological relevance. It has been corroborated in recent years that neutrophil subsets, defined by differential expression of various markers, are indeed present in human blood, calling for renewed attention to this question. The expression of the granule protein olfactomedin 4 (OLFM4) has been suggested to define two such neutrophil subsets. We confirm the simultaneous presence of one OLFM4-positive and one OLFM4-negative neutrophil subpopulation as well as the localization of the protein to specific granules. In vitro, these neutrophil subsets displayed equal tendency to undergo apoptosis and phagocytose bacteria. In addition, the subpopulations were recruited equally to inflammatory sites in vivo, and this was true both in an experimental model of acute inflammation and in naturally occurring pathological joint inflammation. In line with its subcellular localization, only limited OLFM4 release was seen upon in vivo transmigration, and release through conventional degranulation required strong secretagogues. However, extracellular release of OLFM4 could be achieved upon formation of neutrophil extracellular traps (NETs) where it was detected only in a subset of the NETs. Although we were unable to demonstrate any functional differences between the OLFM4-defined subsets, our data show that different neutrophil subsets are present in inflamed tissue in vivo. Furthermore, we demonstrate NETs characterized by different markers for the first time, and our results open up for functions of OLFM4 itself in the extracellular space through exposure in NETs.

Oral neutrophil transcriptome changes result in a pro-survival phenotype in periodontal diseases

Background

Periodontal diseases are inflammatory processes that occur following the influx of neutrophils into the periodontal tissues in response to the subgingival bacterial biofilm. Current literature suggests that while neutrophils are protective and prevent bacterial infections, they also appear to contribute to damage of the periodontal tissues. In the present study we compare the gene expression profile changes in neutrophils as they migrate from the circulation into the oral tissues in patients with chronic periodontits and matched healthy subjects. We hypothesized that oral neutrophils in periodontal disease patients will display a disease specific transcriptome that differs from the oral neutrophil of healthy subjects.

Methods

Venous blood and oral rinse samples were obtained from healthy subjects and chronic periodontitis patients for neutrophil isolation. mRNA was isolated from the neutrophils, and gene expression microarray analysis was completed. Results were confirmed for specific genes of interest by qRT-PCR and Western Blot analysis.

Results and Discussion

Chronic periodontitis patients presented with increased recruitment of neutrophils to the oral cavity. Gene expression analysis revealed differences in the expression levels of genes from several biological pathways. Using hierarchical clustering analysis, we found that the apoptosis network was significantly altered in patients with chronic inflammation in the oral cavity, with up-regulation of pro-survival members of the Bcl-2 family and down-regulation of pro-apoptosis members in the same compartment. Additional functional analysis confirmed that the percentages of viable neutrophils are significantly increased in the oral cavity of chronic periodontitis patients.

Conclusions

Oral neutrophils from patients with periodontal disease displayed an altered transcriptome following migration into the oral tissues. This resulted in a pro-survival neutrophil phenotype in chronic periodontitis patients when compared with healthy subjects, resulting in a longer-lived neutrophil. This is likely to impact the severity and length of the inflammatory response in this oral disease.

Another look at the life of a neutrophil

Neutrophils are considered as the privates of the innate immune system. They are born in the bone marrow, migrate to the tissues where they kill putative intruders. After their job they are quickly removed from the battlefield by macrophages. This view of a predetermined pathway fitted nicely in their short lifespan of 5 h. However, recent studies indicated that their lifespan was in the order of several days. Recently, it became clear that neutrophils have functions beyond killing of pathogens. The reported half-life of 5 h is hardly compatible with those functions. Moreover, the organism actively invests in rescuing primed neutrophils from clearance by the body. It appears that their half-life is highly dependent on the method used to measure their life span. Here, we discuss the literature and show that neutrophils compartmentalize which could explain partially the differences reported for their lifespan. Moreover, the methodology to label neutrophils ex-vivo could have similar deteriorating effects on their lifespan as found for transfused red blood cells.

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.

Sputum neutrophils in cystic fibrosis patients display a reduced respiratory burst

BACKGROUND

Few data exist on the functional activity of airway neutrophils in the milieu of the cystic fibrosis (CF) lung. We assessed reactive oxygen species (ROS) production by sputum neutrophils and the relationship to neutrophil viability. Identical assessments were made on peripheral blood neutrophils from CF patients.

METHODS

ROS production in sputum neutrophils was assessed in 31 CF patients at varying phases of clinical disease using flow cytometry. Twenty patients provided blood samples (including 16 who also provided a matched sputum sample). Neutrophil viability was determined using dual annexin V (apoptosis) and propidium iodide (necrosis) staining. Comparative peripheral blood data were obtained from 7 healthy controls.

RESULTS

ROS production was reduced in sputum compared to blood neutrophils and they demonstrated a higher level of necrosis. Subpopulations of neutrophils with different ROS production capacity were apparent in peripheral blood. Lung function was positively associated with both the proportion of blood neutrophils demonstrating increased ROS production and the proportion of apoptotic sputum neutrophils.

CONCLUSIONS

CF airway neutrophils display functional exhaustion. Healthier lungs in CF appear to be associated with subpopulations of blood neutrophils with increased oxidative burst capacity and evidence for increased neutrophil apoptosis within the airway.

Fish and mammalian phagocytes differentially regulate pro-inflammatory and homeostatic responses in vivo

Phagocytosis is a cellular mechanism that is important to the early induction of antimicrobial responses and the regulation of adaptive immunity. At an inflammatory site, phagocytes serve as central regulators for both pro-inflammatory and homeostatic anti-inflammatory processes. However, it remains unclear if this is a recent evolutionary development or whether the capacity to balance between these two seemingly contradictory processes is a feature already displayed in lower vertebrates. In this study, we used murine (C57BL/6) and teleost fish (C. auratus) in vitro and in vivo models to assess the evolutionary conservation of this dichotomy at a site of inflammation. At the level of the macrophage, we found that teleost fish already displayed divergent pro-inflammatory and homeostatic responses following internalization of zymosan or apoptotic bodies, respectively, and that these were consistent with those of mice. However, fish and mice displayed significant differences in vivo with regards to the level of responsiveness to zymosan and apoptotic bodies, the identity of infiltrating leukocytes, their rate of infiltration, and the kinetics and strength of resulting antimicrobial responses. Unlike macrophages, significant differences were identified between teleost and murine neutrophilic responses. We report for the first time that activated murine, but not teleost neutrophils, possess the capacity to internalize apoptotic bodies. This internalization translates into reduction of neutrophil ROS production. This may play an important part in the recently identified anti-inflammatory activity that mammalian neutrophils display during the resolution phase of inflammation. Our observations are consistent with continued honing of inflammatory control mechanisms from fish to mammals, and provide added insights into the evolutionary path that has resulted in the integrated, multilayered responses that are characteristic of higher vertebrates.

Proinflammatory macrophages enhance the regenerative capacity of human myoblasts by modifying their kinetics of proliferation and differentiation

Macrophages have been shown to be essential for muscle repair by delivering trophic cues to growing skeletal muscle precursors and young fibers. Here, we investigated whether human macrophages, either proinflammatory or anti-inflammatory, coinjected with human myoblasts into regenerating muscle of Rag2(-/-) γC(-/-) immunodeficient mice, could modify in vivo the kinetics of proliferation and differentiation of the transplanted human myogenic precursors. Our results clearly show that proinflammatory macrophages improve in vivo the participation of injected myoblasts to host muscle regeneration, extending the window of proliferation, increasing migration, and delaying differentiation. Interestingly, immunostaining of transplanted proinflammatory macrophages at different time points strongly suggests that these cells are able to switch to an anti-inflammatory phenotype in vivo, which then may stimulate differentiation during muscle regeneration. Conceptually, our data provide for the first time in vivo evidence strongly suggesting that proinflammatory macrophages play a supportive role in the regulation of myoblast behavior after transplantation into preinjured muscle, and could thus potentially optimize transplantation of myogenic progenitors in the context of cell therapy.

A non-peptide receptor inhibitor with selectivity for one of the neutrophil formyl peptide receptors, FPR 1

The neutrophil formyl peptide receptors (FPR1 and FPR2) are members of the G-protein coupled receptor family. The signals generated by occupied FPRs are both pro-inflammatory and anti-inflammatory. Accordingly, these receptors have become a therapeutic target for the development of novel drugs that may be used to reduce injuries in inflammatory diseases including asthma, rheumatoid arthritis, Alzheimer's disease and cardiovascular diseases. To support the basis for a future pharmacological characterization, we have identified a small molecular non-peptide inhibitor with selectivity for FPR1. We used the FPR1 and FPR2 specific ligands fMLF and WKYMVM, respectively, and an earlier described ratio technique, to determine inhibitory activity combined with selectivity. We show that the compound 3,5-dichloro-N-(2-chloro-5-methyl-phenyl)-2-hydroxy-benzamide (BVT173187) fulfills the criteria for an FPR1 inhibitor selective for FPR1 over FPR2, and it inhibits the same functional repertoire in neutrophils as earlier described peptide antagonists. Accordingly, the new inhibitor reduced neutrophil activation with FPR1 agonists, leading to mobilization of adhesion molecules (CR3) and the generation of superoxide anion from the neutrophil NADPH-oxidase. The effects of a number of structural analogs were determined but these were either without activity or less active/specific than BVT173187. The potency of the new inhibitor for reduction of FPR1 activity was the same as that of the earlier described FPR1 antagonist cyclosporine H, but signaling through the C5aR and CXCR (recognizing IL8) was also affected by BVT173187.

Editorial: will the real neutrophil please stand up?

Discussion on exudate neutrophils as a more accurate model of the “working” functional in vivo neutrophil than their circulating progenitors.