Targeting granulocyte apoptosis: mechanisms, models, and therapies

Exploring the impact of m6A modification on immune diseases: mechanisms and therapeutic implication

N6-methyladenosine (m6A) is a chemical modification of RNA and has become a widely discussed topic among scientific researchers in recent years. It is distributed in various organisms, including eukaryotes and bacteria. It has been found that m6A is composed of writers, erasers and readers and is involved in biological functions such as splicing, transport and translation of RNA. The balance of the human immune microenvironment is important for human health abnormalities. Increasing studies have found that m6A affects the development of immune diseases such as inflammatory enteritis and systemic lupus erythematosus (SLE) by participating in the homeostatic regulation of the immune microenvironment in vivo. In this manuscript, we introduce the composition, biological function, regulation of m6A in the immune microenvironment and its progression in various immune diseases, providing new targets and directions for the treatment of immune diseases in clinical practice.

Activating Macrophage Continual Efferocytosis via Microenvironment Biomimetic Short Fibers for Reversing Inflammation in Bone Repair

Efferocytosis-mediated inflammatory reversal plays a crucial role in bone repairing process. However, in refractory bone defects, the macrophage continual efferocytosis may be suppressed due to the disrupted microenvironment homeostasis, particularly the loss of apoptotic signals and overactivation of intracellular oxidative stress. In this study, a polydopamine-coated short fiber matrix containing biomimetic "apoptotic signals" to reconstruct the microenvironment and reactivate macrophage continual efferocytosis for inflammatory reversal and bone defect repair is presented. The "apoptotic signals" (AM/CeO2) are prepared using CeO2 nanoenzymes with apoptotic neutrophil membrane coating for macrophage recognition and oxidative stress regulation. Additionally, a short fiber "biomimetic matrix" is utilized for loading AM/CeO2 signals via abundant adhesion sites involving π-π stacking and hydrogen bonding interactions. Ultimately, the implantable apoptosis-mimetic nanoenzyme/short-fiber matrixes (PFS@AM/CeO2), integrating apoptotic signals and biomimetic matrixes, are constructed to facilitate inflammatory reversal and reestablish the pro-efferocytosis microenvironment. In vitro and in vivo data indicate that the microenvironment biomimetic short fibers can activate macrophage continual efferocytosis, leading to the suppression of overactivated inflammation. The enhanced repair of rat femoral defect further demonstrates the osteogenic potential of the pro-efferocytosis strategy. It is believed that the regulation of macrophage efferocytosis through microenvironment biomimetic materials can provide a new perspective for tissue repair.

Interaction between iron oxide nanoparticles (IONs) and primary human immune cells: An up-to-date review of the literature

Nanotechnology has been gaining more and more momentum lately and the potential use of nanomaterials such as nanoparticles (NPs) continues to grow in a variety of activity sectors. Among the NPs, iron oxide nanoparticles (IONs) have retained an increasing interest from the scientific community and industrials due to their superparamagnetic properties allowing their use in many fields, including medicine. However, some undesired effects of IONs and potential risk for human health are becoming increasingly reported in several studies. Although many in vivo studies reported that IONs induce immunotoxicity in different animal models, it is not clear how IONs can alter the biology of primary human immune cells. In this article, we will review the works that have been done regarding the interaction between IONs and primary immune cells. This review also outlines the importance of using primary immune cells in risk assessment of NPs as a reliable strategy for encouraging non-animal studies approaches, to determine risks that might affect the human immune system following different exposure scenarios. Taken all together, the reported observations help to get a more global picture on how IONs alter the human immune system especially the fact that inflammation, known to involve several immune cell types, is frequently reported as an undesired effect of IONs.

Functional responsiveness of in vitro-aged human neutrophils

Elimination of apoptotic neutrophils by macrophages is as a major step for the resolution of inflammation. However, the fate and the cellular functionality of neutrophils aged in the absence of macrophages are not well documented. Herein, freshly isolated human neutrophils were aged for several days in vitro and then stimulated with agonists for determining their cell responsiveness. In vitro-aged neutrophils were still able to generate reactive oxygen species after 48 h, exert phagocytosis after 72 h, and increase their adhesion onto a cell substratum after 48 h. These data demonstrate that a portion of neutrophils cultivated for several days in vitro are still able to exert biological functions. This opens the possibility that, during inflammation, neutrophils may still respond to agonists, a condition that is likely to occur in vivo when they are not efficiently eliminated by efferocytosis.

RNA modifications: importance in immune cell biology and related diseases

RNA modifications have become hot topics recently. By influencing RNA processes, including generation, transportation, function, and metabolization, they act as critical regulators of cell biology. The immune cell abnormality in human diseases is also a research focus and progressing rapidly these years. Studies have demonstrated that RNA modifications participate in the multiple biological processes of immune cells, including development, differentiation, activation, migration, and polarization, thereby modulating the immune responses and are involved in some immune related diseases. In this review, we present existing knowledge of the biological functions and underlying mechanisms of RNA modifications, including N⁶-methyladenosine (m⁶A), 5-methylcytosine (m⁵C), N¹-methyladenosine (m¹A), N⁷-methylguanosine (m⁷G), N⁴-acetylcytosine (ac⁴C), pseudouridine (Ψ), uridylation, and adenosine-to-inosine (A-to-I) RNA editing, and summarize their critical roles in immune cell biology. Via regulating the biological processes of immune cells, RNA modifications can participate in the pathogenesis of immune related diseases, such as cancers, infection, inflammatory and autoimmune diseases. We further highlight the challenges and future directions based on the existing knowledge. All in all, this review will provide helpful knowledge as well as novel ideas for the researchers in this area.

Impact of ultra-small silver nanoparticles of 2 nm (AgNP2) on neutrophil biology: AgNP2 alter the actin cytoskeleton and induce karyorrhexis by a mitogen-activated protein kinase-dependent mechanism in vitro and transitorily attract neutrophils in vivo

Silver (Ag) is known as an antibacterial agent and there is a growing interest to use silver nanoparticles (AgNPs) in a variety of medical applications and other sectors. Some studies reported that one of the undesired effects of AgNPs is inflammation and that these NPs can alter the biology of neutrophils. Since it is commonly accepted that the more NPs are small, the more toxic they are the aim of this study was to determine the impact of ultra-small silver nanoparticles of 2 nm (AgNP₂) on the biology of neutrophils, key player cells in inflammation. We report that AgNP₂ are potent neutrophil activators as they rapidly induce actin polymerization and dismantling the actin network. Although AgNP₂ are not necrotic for neutrophils and do not induce ROS production, kinetic studies reveal that AgNP₂ are rapid inducer of apoptosis. Pyknosis (mainly 1-2 large nuclear dots) was observed after only 1h of treatment followed by karyorrhexis (several small dots) and by a complete nuclear dissolution leading to anuclear neutrophils after 6h. These observations are not associated with the release of silver ions since treatment of neutrophils with 1-50 μg/ml AgNO₃ (as a source of Ag⁺) did not induce any apparent changes. AgNP₂ induce p38 and Erk-1/2 mitogen-activated protein kinase (MAPK) and although karyorrhexis was markedly reversed by MAPK inhibitors, the cell nuclei remain with a pyknotic-like phenotype but do not return to the characteristic polylobed nucleus. Using the murine air pouch model of inflammation AgNP₂ were found to induce a neutrophil influx. Our data indicate that AgNP₂ are potent neutrophil activators targeting the actin cytoskeleton and the mechanism involved for inducing apoptosis is rapid, complex, and partially includes MAPK pathways. Therefore, the ultra-small AgNP₂ are more potent than larger ones for inducing apoptosis and they can transitorily attract neutrophils in vivo.

Weathering the Storm: Harnessing the Resolution of Inflammation to Limit COVID-19 Pathogenesis

The resolution of inflammation is a temporally and spatially coordinated process that in its innate manifestations, primarily involves neutrophils and macrophages. The shutdown of infection or injury-induced acute inflammation requires termination of neutrophil accumulation within the affected sites, neutrophil demise, and clearance by phagocytes (efferocytosis), such as tissue-resident and monocyte-derived macrophages. This must be followed by macrophage reprogramming from the inflammatory to reparative and consequently resolution-promoting phenotypes and the production of resolution-promoting lipid and protein mediators that limit responses in various cell types and promote tissue repair and return to homeostatic architecture and function. Recent studies suggest that these events, and macrophage reprogramming to pro-resolving phenotypes in particular, are not only important in the acute setting, but might be paramount in limiting chronic inflammation, autoimmunity, and various uncontrolled cytokine-driven pathologies. The SARS-CoV-2 (COVID-19) pandemic has caused a worldwide health and economic crisis. Severe COVID-19 cases that lead to high morbidity are tightly associated with an exuberant cytokine storm that seems to trigger shock-like pathologies, leading to vascular and multiorgan failures. In other cases, the cytokine storm can lead to diffuse alveolar damage that results in acute respiratory distress syndrome (ARDS) and lung failure. Here, we address recent advances on effectors in the resolution of inflammation and discuss how pro-resolution mechanisms with particular emphasis on macrophage reprogramming, might be harnessed to limit the universal COVID-19 health threat.

Neutrophils Culture in Collagen Gel System

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

How Tim proteins differentially exploit membrane features to attain robust target sensitivity

Immune surveillance cells such as T cells and phagocytes utilize integral plasma membrane receptors to recognize surface signatures on triggered and activated cells such as those in apoptosis. One such family of plasma membrane sensors, the transmembrane immunoglobulin and mucin domain (Tim) proteins, specifically recognize phosphatidylserine (PS) but elicit distinct immunological responses. The molecular basis for the recognition of lipid signals on target cell surfaces is not well understood. Previous results suggest that basic side chains present at the membrane interface on the Tim proteins might facilitate association with additional anionic lipids including but not necessarily limited to PS. We, therefore, performed a comparative quantitative analysis of the binding of the murine Tim1, Tim3, and Tim4, to synthetic anionic phospholipid membranes under physiologically relevant conditions. X-ray reflectivity and vesicle binding studies were used to compare the water-soluble domain of Tim3 with results previously obtained for Tim1 and Tim4. Although a calcium link was essential for all three proteins, the three homologs differed in how they balance the hydrophobic and electrostatic interactions driving membrane association. The proteins also varied in their sensing of phospholipid chain unsaturation and showed different degrees of cooperativity in their dependence on bilayer PS concentration. Surprisingly, trace amounts of anionic phosphatidic acid greatly strengthened the bilayer association of Tim3 and Tim4, but not Tim1. A novel mathematical model provided values for the binding parameters and illuminated the complex interplay among ligands. In conclusion, our results provide a quantitative description of the contrasting selectivity used by three Tim proteins in the recognition of phospholipids presented on target cell surfaces. This paradigm is generally applicable to the analysis of the binding of peripheral proteins to target membranes through the heterotropic cooperative interactions of multiple ligands.

Targeting multiple cell death pathways extends the shelf life and preserves the function of human and mouse neutrophils for transfusion

Clinical outcomes from granulocyte transfusion (GTX) are disadvantaged by the short shelf life and compromised function of donor neutrophils. Spontaneous neutrophil death is heterogeneous and mediated by multiple pathways. Leveraging mechanistic knowledge and pharmacological screening, we identified a combined treatment, caspases-lysosomal membrane permeabilization-oxidant-necroptosis inhibition plus granulocyte colony-stimulating factor (CLON-G), which altered neutrophil fate by simultaneously targeting multiple cell death pathways. CLON-G prolonged human and mouse neutrophil half-life in vitro from less than 1 day to greater than 5 days. CLON-G-treated aged neutrophils had equivalent morphology and function to fresh neutrophils, with no impairment to critical effector functions including phagocytosis, bacterial killing, chemotaxis, and reactive oxygen species production. Transfusion with stored CLON-G-treated 3-day-old neutrophils enhanced host defenses, alleviated infection-induced tissue damage, and prolonged survival as effectively as transfusion with fresh neutrophils in a clinically relevant murine GTX model of neutropenia-related bacterial pneumonia and systemic candidiasis. Last, CLON-G treatment prolonged the shelf life and preserved the function of apheresis-collected human GTX products both ex vivo and in vivo in immunodeficient mice. Thus, CLON-G treatment represents an effective and applicable clinical procedure for the storage and application of neutrophils in transfusion medicine, providing a therapeutic strategy for improving GTX efficacy.

Del-1 enhances therapeutic efficacy of bacterial cancer immunotherapy by blocking recruitment of tumor-infiltrating neutrophils

BACKGROUND

Bacterial-mediated cancer immunotherapy (BCI) elicits a more robust initial immune response than conventional immunotherapy, but does not prevent tumor recurrence and metastasis. BCI is associated with recruitment of tumor-infiltrating neutrophils, which could suppress the therapeutic efficacy of this modality. Development endothelial locus 1 (Del-1), a potent inhibitor of neutrophil recruitment, antagonizes lymphocyte function-associated antigen-1 on the vascular endothelium. Here, we aimed to determine the effect of Del-1-secreting S.t△ppGpp on anti-tumor activity and tumor-infiltrating neutrophil recruitment in a mouse model of colon cancer.

METHODS

We investigated the anti-cancer activity of Del-1-secreting engineered Salmonella (△ppGpp S. Typhimurium) in the mice colon cancer models.

RESULTS

In the present study, we identified that Del-1-secreting engineered Salmonella had more potent anti-cancer activity compared with normal S.t△ppGpp without Del-1 secretion. We postulated that Del-1 expression increased M1 macrophage recruitment to tumors by decreasing tumor-infiltrating neutrophils. This approach could enhance the anti-cancer effects of S.t△ppGpp.

CONCLUSIONS

Collectively, the approach of using engineered bacteria that deliver Del-1 to block tumor-infiltrating neutrophil recruitment is a potential therapeutic approach.

Resolving inflammation by TAM receptor activation

The control of inflammation is strictly regulated to ensure the adequate intensity and duration of an inflammatory response, enabling the removal of the trigger factors and the restoration of the integrity of the tissues and their functions. This process is coordinated by anti-inflammatory and pro-resolving mediators that regulate the cellular and molecular events necessary to restore homeostasis, and defects in this control are associated with the development of chronic and autoimmune diseases. The TAM family of receptor tyrosine kinases-Tyro3, Axl, and MerTK-plays an essential role in efferocytosis, a key process for the resolution of inflammation. However, new studies have demonstrated that TAM receptor activation not only reduces the synthesis of pro-inflammatory mediators by different cell types in response to some stimuli but also stimulates the production of anti-inflammatory and pro-resolving molecules that control the inflammation. This review provides a comprehensive view of TAM receptor family members as important players in controlling inflammatory responses through anti-inflammatory and pro-resolving actions.

Assays of Eosinophil Apoptosis and Phagocytic Uptake

Eosinophil apoptosis (programmed cell death) plays an important role in several inflammatory and allergic conditions. Apoptosis triggers various mechanisms including activation of cysteine-aspartic proteases (caspases) and is characterized by morphological and biochemical changes. These include cellular condensation, nuclear fragmentation, increased mitochondrial permeability with loss of membrane potential, and exposure of phosphatidylserine on the cell membrane. A greater understanding of apoptotic mechanisms, subsequent phagocytosis (efferocytosis), and regulation of these processes is critical to understanding disease pathogenesis and development of potential novel therapeutic agents. Release of soluble factors and alterations to surface marker expression by eosinophils undergoing apoptosis aid them in signaling their presence to the immediate environment, and their subsequent recognition by phagocytic cells such as macrophages. Uptake of apoptotic cells usually suppresses inflammation by restricting proinflammatory responses and promoting anti-inflammatory and tissue repair responses. This, in turn, promotes resolution of inflammation. Defects in the apoptotic or efferocytosis mechanisms perpetuate inflammation, resulting in chronic inflammation and enhanced disease severity. This can be due to increased eosinophil life span or cell necrosis characterized by loss of cell membrane integrity and release of toxic intracellular mediators. In this chapter, we detail some of the key assays that are used to assess eosinophil apoptosis, as well as the intracellular signaling pathways involved and phagocytic clearance of these cells.

The pathophysiology of SARS-CoV-2: A suggested model and therapeutic approach

In this paper, a model is proposed of the pathophysiological processes of COVID-19 starting from the infection of human type II alveolar epithelial cells (pneumocytes) by SARS-CoV-2 and culminating in the development of ARDS. The innate immune response to infection of type II alveolar epithelial cells leads both to their death by apoptosis and pyroptosis and to alveolar macrophage activation. Activated macrophages secrete proinflammatory cytokines and chemokines and tend to polarise into the inflammatory M1 phenotype. These changes are associated with activation of vascular endothelial cells and thence the recruitment of highly toxic neutrophils and inflammatory activated platelets into the alveolar space. Activated vascular endothelial cells become a source of proinflammatory cytokines and reactive oxygen species (ROS) and contribute to the development of coagulopathy, systemic sepsis, a cytokine storm and ARDS. Pulmonary activated platelets are also an important source of proinflammatory cytokines and ROS, as well as exacerbating pulmonary neutrophil-mediated inflammatory responses and contributing to systemic sepsis by binding to neutrophils to form platelet-neutrophil complexes (PNCs). PNC formation increases neutrophil recruitment, activation priming and extraversion of these immune cells into inflamed pulmonary tissue, thereby contributing to ARDS. Sequestered PNCs cause the development of a procoagulant and proinflammatory environment. The contribution to ARDS of increased extracellular histone levels, circulating mitochondrial DNA, the chromatin protein HMGB1, decreased neutrophil apoptosis, impaired macrophage efferocytosis, the cytokine storm, the toll-like receptor radical cycle, pyroptosis, necroinflammation, lymphopenia and a high Th17 to regulatory T lymphocyte ratio are detailed.

A fluorogenic cyclic peptide for imaging and quantification of drug-induced apoptosis

Programmed cell death or apoptosis is a central biological process that is dysregulated in many diseases, including inflammatory conditions and cancer. The detection and quantification of apoptotic cells in vivo is hampered by the need for fixatives or washing steps for non-fluorogenic reagents, and by the low levels of free calcium in diseased tissues that restrict the use of annexins. In this manuscript, we report the rational design of a highly stable fluorogenic peptide (termed Apo-15) that selectively stains apoptotic cells in vitro and in vivo in a calcium-independent manner and under wash-free conditions. Furthermore, using a combination of chemical and biophysical methods, we identify phosphatidylserine as a molecular target of Apo-15. We demonstrate that Apo-15 can be used for the quantification and imaging of drug-induced apoptosis in preclinical mouse models, thus creating opportunities for assessing the in vivo efficacy of anti-inflammatory and anti-cancer therapeutics.

Programmed cell death or apoptosis is an essential biological process that is impaired in some diseases and can be used to assess the effectiveness of drugs. Here the authors design Apo-15 as a fluorogenic peptide for the detection and real-time imaging of apoptotic cells.

Chlorogenic acid-enriched extract of Ilex kudingcha C.J. Tseng tea inhibits neutrophil recruitment in injured zebrafish by promoting reverse migration via the focal adhesion pathway

Neutrophil-regulated inflammation plays crucial roles in tissue damage and repair. Dysregulation of the neutrophil response system can contribute to diseases such as cancer. Clearance of excessive neutrophils at the site of inflammation by reverse migration provides a promising strategy to mitigate the negative effects. Chlorogenic acid treatment of injured zebrafish embryos showed low-developmental toxicity. Using a transgenic zebrafish Tg (mpx: egfp) model, chlorogenic acid-enriched kudingcha extract promoted neutrophil reverse migration via phosphorylation of ERK and AKT. Using i-TRAQ analysis, differentially expressed proteins involved in focal adhesion were identified, such as: Cdc42, SRC, MLC, ITGA, and Calpain. In support of this, ERK and AKT proteins are involved in the focal adhesion pathway. Real time qPCR determined that CGA downregulates genes associated with cancer metastasis, such as: HSPA5, YWHAZ, RP17, and ITGAV. Together, these results suggest that CGA-enriched Kudingcha extract may have potential as an anticancer or anti-inflammatory therapeutic agent. PRACTICAL APPLICATIONS: Ilex kudingcha C.J Tseng, commonly referred to as the large-leaved kudingcha, is a tea variety naturally rich in chlorogenic acid. Chlorogenic acid, the ester of caffeic and quinic acids, has antioxidant, antibacterial, anticancer, and anti-inflammatory, activities. Kudingcha has several known biological functions, including: anticancer, anti-inflammatory, antidiabetic, and hypolipidemic effects. Treatment with kudingcha extract reduces the recruitment of neutrophils, potentially by inhibiting focal adhesion, and activation of cancer metastasis-related genes. Importantly, kudingcha extract could be used as an alternative nutritional supplement for anticancer or anti-inflammation via its ability to suppress neutrophil recruitment.

Granulocyte-targeted therapies for airway diseases

The average respiration rate for an adult is 12-20 breaths per minute, which constantly exposes the lungs to allergens and harmful particles. As a result, respiratory diseases, which includes asthma, chronic obstructive pulmonary disease (COPD) and acute lower respiratory tract infections (LTRI), are a major cause of death worldwide. Although asthma, COPD and LTRI are distinctly different diseases with separate mechanisms of disease progression, they do share a common feature - airway inflammation with intense recruitment and activation of granulocytes and mast cells. Neutrophils, eosinophils, basophils, and mast cells are crucial players in host defense against pathogens and maintenance of lung homeostasis. Upon contact with harmful particles, part of the pulmonary defense mechanism is to recruit these cells into the airways. Despite their protective nature, overactivation or accumulation of granulocytes and mast cells in the lungs results in unwanted chronic airway inflammation and damage. As such, understanding the bright and the dark side of these leukocytes in lung physiology paves the way for the development of therapies targeting this important mechanism of disease. Here we discuss the role of granulocytes in respiratory diseases and summarize therapeutic strategies focused on granulocyte recruitment and activation in the lungs.

New insights into the novel anti-inflammatory mode of action of glucocorticoids

Inflammation is a physiological intrinsic host response to injury meant for removal of noxious stimuli and maintenance of homeostasis. It is a defensive body mechanism that involves immune cells, blood vessels and molecular mediators of inflammation. Glucocorticoids (GCs) are steroidal hormones responsible for regulation of homeostatic and metabolic functions of body. Synthetic GCs are the most useful anti-inflammatory drugs used for the treatment of chronic inflammatory diseases such as asthma, chronic obstructive pulmonary disease (COPD), allergies, multiple sclerosis, tendinitis, lupus, atopic dermatitis, ulcerative colitis, rheumatoid arthritis and osteoarthritis whereas, the long term use of GCs are associated with many side effects. The anti-inflammatory and immunosuppressive (desired) effects of GCs are usually mediated by transrepression mechanism whereas; the metabolic and toxic (undesired) effects are usually manifested by transactivation mechanism. Though GCs are most potent anti-inflammatory and immunosuppressive drugs, the common problem associated with their use is GC resistance. Several research studies are rising to comprehend these mechanisms, which would be helpful in improving the GC resistance in asthma and COPD patients. This review aims to focus on identification of new drug targets in inflammation which will be helpful in the resolution of inflammation. The ample understanding of GC mechanisms of action helps in the development of novel anti-inflammatory drugs for the treatment of inflammatory and autoimmune disease with reduced side effects and minimal toxicity.

New Advances in Targeting the Resolution of Inflammation: Implications for Specialized Pro-Resolving Mediator GPCR Drug Discovery

Chronic inflammation is a component of numerous diseases including autoimmune, metabolic, neurodegenerative, and cancer. The discovery and characterization of specialized pro-resolving mediators (SPMs) critical to the resolution of inflammation, and their cognate G protein-coupled receptors (GPCRs) has led to a significant increase in the understanding of this physiological process. Approximately 20 ligands, including lipoxins, resolvins, maresins, and protectins, and 6 receptors (FPR2/ALX, GPR32, GPR18, chemerin₁, BLT₁, and GPR37) have been identified highlighting the complex and multilayered nature of resolution. Therapeutic efforts in targeting these receptors have proved challenging, with very few ligands apparently progressing through to preclinical or clinical development. To date, some knowledge gaps remain in the understanding of how the activation of these receptors, and their downstream signaling, results in efficient resolution via apoptosis, phagocytosis, and efferocytosis of polymorphonuclear leukocytes (mainly neutrophils) and macrophages. SPMs bind and activate multiple receptors (ligand poly-pharmacology), while most receptors are activated by multiple ligands (receptor pleiotropy). In addition, allosteric binding sites have been identified signifying the capacity of more than one ligand to bind simultaneously. These fundamental characteristics of SPM receptors enable alternative targeting strategies to be considered, including biased signaling and allosteric modulation. This review describes those ligands and receptors involved in the resolution of inflammation, and highlights the most recent clinical trial results. Furthermore, we describe alternative mechanisms by which these SPM receptors could be targeted, paving the way for the identification of new therapeutics, perhaps with greater efficacy and fidelity.

Mycobacterium tuberculosis Lipoarabinomannan Activates Human Neutrophils via a TLR2/1 Mechanism Distinct from Pam3CSK4

Neutrophils, polymorphonuclear (PMN) leukocytes, play an important role in the early innate immune response to Mycobacterium tuberculosis infection in the lung. Interactions between PMN and mycobacterial lipids impact the activation state of these migrated cells with consequences for the surrounding tissue in terms of resolution versus ongoing inflammation. We hypothesized that lipoarabinomannan from M. tuberculosis (Mtb LAM) would prime human PMN in a TLR2-dependent manner and investigated this with specific comparison with the purified synthetic TLR2 agonists, Pam₃CSK₄ and FSL-1. In contrast to Pam₃CSK₄ and FSL-1, we found Mtb LAM did not induce any of the classical PMN priming phenotypes, including enhancement of NADPH oxidase activity, shedding of l-selectin, or mobilization of CD11b. However, exposure of PMN to Mtb LAM did elicit pro- and anti-inflammatory cytokine production and release in a TLR2/1-dependent manner, using the TLR1 single-nucleotide polymorphism rs5743618 (1805G/T) as a marker for TLR2/1 specificity. Moreover, Mtb LAM did not elicit p38 MAPK phosphorylation or endocytosis, although these processes occurred with Pam₃CSK₄ stimulation, and were necessary for the early priming events to occur. Interestingly, Mtb LAM did not abrogate priming responses elicited by Pam₃CSK₄ Notably, subfractionation of light membranes from Pam₃CSK₄ versus Mtb LAM-stimulated cells demonstrated differential patterns of exocytosis. In summary, Mtb LAM activates PMN via TLR2/1, resulting in the production of cytokines but does not elicit early PMN priming responses, as seen with Pam₃CSK₄ We speculate that the inability of Mtb LAM to prime PMN may be due to differential localization of TLR2/1 signaling.

Assessment of Neutrophil Apoptosis

The process of neutrophil apoptosis has an important role in the resolution of acute inflammation. Apoptotic cell death is characterized by a coordinated sequence of cellular alterations that serve to uncouple neutrophil effector functions whilst maintaining plasma membrane integrity. In this way the release on neutrophil intracellular contents, including proteases, glycosidases, and reactive oxygen species, is limited during apoptosis. In addition, plasma membrane alterations associated with neutrophil apoptosis provide molecular cues that enable recognition by phagocytic cells, including macrophages. The recognition and uptake of apoptotic neutrophils by macrophages dampens proinflammatory responses to pathogen- or damage-associated molecular patterns and triggers release of proresolution mediators, that further promote resolution of inflammation. The key cellular and molecular events that act to control neutrophil apoptosis and subsequent macrophage phagocytosis have been characterized by in vitro studies, unveiling potential therapeutic targets for the manipulation of these regulatory pathways. In this chapter, we outline some of the key assays that are used to assess neutrophil apoptosis in vitro, together with methods to assess activation of the apoptotic machinery and phagocytic clearance of apoptotic neutrophils.

β-glucan-dependent shuttling of conidia from neutrophils to macrophages occurs during fungal infection establishment

The initial host response to fungal pathogen invasion is critical to infection establishment and outcome. However, the diversity of leukocyte–pathogen interactions is only recently being appreciated. We describe a new form of interleukocyte conidial exchange called “shuttling.” In Talaromyces marneffei and Aspergillus fumigatus zebrafish in vivo infections, live imaging demonstrated conidia initially phagocytosed by neutrophils were transferred to macrophages. Shuttling is unidirectional, not a chance event, and involves alterations of phagocyte mobility, intercellular tethering, and phagosome transfer. Shuttling kinetics were fungal-species–specific, implicating a fungal determinant. β-glucan serves as a fungal-derived signal sufficient for shuttling. Murine phagocytes also shuttled in vitro. The impact of shuttling for microbiological outcomes of in vivo infections is difficult to specifically assess experimentally, but for these two pathogens, shuttling augments initial conidial redistribution away from fungicidal neutrophils into the favorable macrophage intracellular niche. Shuttling is a frequent host–pathogen interaction contributing to fungal infection establishment patterns.

Imaging of the behaviour of white blood cells in living zebrafish embryos infected with fungi reveals “shuttling,” a specific and previously undescribed form of microorganism exchange between neutrophils and macrophages.

Inhibition of the sphingosine-1-phosphate pathway promotes the resolution of neutrophilic inflammation

Sphingosine-1-phosphate (S1P) is an important sphingolipid derived from plasma membrane and has a known role in productive phase of inflammation, but its role in neutrophil survival and resolution phase of inflammation is unknown. Here, we investigated the effects of inhibition of S1P receptors and the blockade of S1P synthesis in BALB/c mice and human neutrophils. S1P and S1PR1-3 receptors expression were increased in cells from the pleural cavity stimulated with LPS. Using different antagonists of S1PRs and inhibitors of different steps of the metabolic pathway of S1P production, we show that S1P and its receptors are involved in regulating neutrophil survival and resolution of inflammation in the pleural cavity. Given the role of the S1P-S1PR axis in resolution of inflammation, we sought to identify whether blockade at different levels of the sphingosine-1-phosphate synthesis pathway could affect neutrophil survival in vitro. Inhibitors of the S1P pathway were also able to induce human neutrophil apoptosis. In addition, blockade of S1P synthesis or its receptor facilitated the efferocytosis of apoptotic neutrophil. Taken together, our data demonstrate a fundamental role for S1P in regulating the outcome of inflammatory responses, and position S1P-S1PR axis as a potential target for treatment of neutrophilic inflammation.

Inflammation Resolution and the Induction of Granulocyte Apoptosis by Cyclin-Dependent Kinase Inhibitor Drugs

Inflammation is a necessary dynamic tissue response to injury or infection and it's resolution is essential to return tissue homeostasis and function. Defective or dysregulated inflammation resolution contributes significantly to the pathogenesis of many, often common and challenging to treat human conditions. The transition of inflammation to resolution is an active process, involving the clearance of inflammatory cells (granulocytes), a change of mediators and their receptors, and prevention of further inflammatory cell infiltration. This review focuses on the use of cyclin dependent kinase inhibitor drugs to pharmacologically target this inflammatory resolution switch, specifically through inducing granulocyte apoptosis and phagocytic clearance of apoptotic cells (efferocytosis). The key processes and pathways required for granulocyte apoptosis, recruitment of phagocytes and mechanisms of engulfment are discussed along with the cumulating evidence for cyclin dependent kinase inhibitor drugs as pro-resolution therapeutics.

Neutrophils in the initiation and resolution of acute pulmonary inflammation: understanding biological function and therapeutic potential

Acute respiratory distress syndrome (ARDS) is the often fatal sequelae of a broad range of precipitating conditions. Despite decades of intensive research and clinical trials there remain no therapies in routine clinical practice that target the dysregulated and overwhelming inflammatory response that characterises ARDS. Neutrophils play a central role in the initiation, propagation and resolution of this complex inflammatory environment by migrating into the lung and executing a variety of pro-inflammatory functions. These include degranulation with liberation of bactericidal proteins, release of cytokines and reactive oxygen species as well as production of neutrophil extracellular traps. Although these functions are advantageous in clearing bacterial infection, the consequence of associated tissue damage, the contribution to worsening acute inflammation and prolonged neutrophil lifespan at sites of inflammation are deleterious. In this review, the importance of the neutrophil will be considered, together with discussion of recent advances in understanding neutrophil function and the factors that influence them throughout the phases of inflammation in ARDS. From a better understanding of neutrophils in this context, potential therapeutic targets are identified and discussed. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Immune complex-induced neutrophil functions: A focus on cell death

Neutrophils are amongst the first cells to be recruited to sites of infection or trauma. Neutrophil functional responsiveness is tightly regulated by many agents including immune complexes. These immune cells can generate reactive oxygen species and degranulate to release abundant cytotoxic products, making them efficient at killing invading microorganisms. If neutrophil function is dysregulated, however, these cells have the potential to cause unwanted host tissue damage as exemplified by pathological and chronic inflammatory conditions. In physiological inflammation, once the initial insult has efficiently been dealt with, neutrophils are thought to leave the tissues or undergo programmed cells death, especially apoptosis. Apoptotic neutrophils are then rapidly removed by other phagocytes, primarily macrophages, by mechanisms that do not elicit a pro-inflammatory response. In this review, we discuss the interesting observations and consequences that immune complexes have on neutrophil cell death processes such as apoptosis.

Immunological and hematological effects of IL-5(Rα)-targeted therapy: An overview

IL‐5 is an important cytokine for priming and survival of mature eosinophils and for proliferation and maturation of their progenitors. Hence, IL‐5(Rα) targeting will be increasingly used in diseases where eosinophils are the key immune effector cells such as eosinophilic asthma (EA), hypereosinophilic syndrome (HES), eosinophilic esophagitis (EE), and eosinophilic granulomatosis with polyangiitis (EGPA). Therefore, several neutralizing monoclonal antibodies directed against IL‐5 (mepolizumab and reslizumab) and its receptor IL‐5Rα (benralizumab) have found or will find their way to the clinic. While the clinical effect of these drugs has been extensively investigated and reviewed, the understanding of the underlying immunological and hematological mechanisms remains less clear. This review will discuss the translational outcomes of treatment with these monoclonal antibodies in humans to shed light on the mechanisms underlying the main immunological and hematological findings from these clinical trials in humans.

Loss of BID Delays FASL-Induced Cell Death of Mouse Neutrophils and Aggravates DSS-Induced Weight Loss

Neutrophils are key players in the early defense against invading pathogens. Due to their potent effector functions, programmed cell death of activated neutrophils has to be tightly controlled; however, its underlying mechanisms remain unclear. Fas ligand (FASL/CD95L) has been shown to induce neutrophil apoptosis, which is accelerated by the processing of the BH3-only protein BH3 interacting domain death agonist (BID) to trigger mitochondrial apoptotic events, and been attributed a regulatory role during viral and bacterial infections. Here, we show that, in accordance with previous works, mouse neutrophils underwent caspase-dependent apoptosis in response to FASL, and that this cell death was significantly delayed upon loss of BID. However, pan-caspase inhibition failed to protect mouse neutrophils from FASL-induced apoptosis and caused a switch to RIPK3-dependent necroptotic cell death. Intriguingly, such a switch was less evident in the absence of BID, particularly under inflammatory conditions. Delayed neutrophil apoptosis has been implicated in several auto-inflammatory diseases, including inflammatory bowel disease. We show that neutrophil and macrophage driven acute dextran sulfate sodium (DSS) induced colitis was slightly more aggravated in BID-deficient mice, based on significantly increased weight loss compared to wild-type controls. Taken together, our data support a central role for FASL > FAS and BID in mouse neutrophil cell death and further underline the anti-inflammatory role of BID.

Aged polymorphonuclear leukocytes cause fibrotic interstitial lung disease in the absence of regulation by B cells

Pulmonary immunity requires tight regulation, as interstitial inflammation can compromise gas exchange and lead to respiratory failure. Here we found a greater number of aged CD11b^hiL-selectin^loCXCR4⁺ polymorphonuclear leukocytes (PMNs) in lung vasculature than in the peripheral circulation. Using pulmonary intravital microscopy, we observed lung PMNs physically interacting with B cells via β₂ integrins; this initiated neutrophil apoptosis, which led to macrophage-mediated clearance. Genetic deletion of B cells led to the accumulation of aged PMNs in the lungs without systemic inflammation, which caused pathological fibrotic interstitial lung disease that was attenuated by the adoptive transfer of B cells or depletion of PMNs. Thus, the lungs are an intermediary niche in the PMN lifecycle wherein aged PMNs are regulated by B cells, which restrains their potential to cause pulmonary pathology.

Tamoxifen induces apoptotic neutrophil efferocytosis in horses

Macrophages and neutrophils are important cellular components in the process of acute inflammation and its subsequent resolution, and evidence increasingly suggests that they play important functions during the resolution of chronic, adaptive inflammatory processes. Exacerbated neutrophil activity can be harmful to surrounding tissues; this is important in a range of diseases, including allergic asthma and chronic obstructive pulmonary disease in humans, and equine asthma (also known as recurrent airway obstruction (RAO). Tamoxifen (TX) is a non-steroidal estrogen receptor modulator with effects on cell growth and survival. Previous studies showed that TX treatment in horses with induced acute pulmonary inflammation promoted early apoptosis of blood and BALF neutrophils, reduction of BALF neutrophils, and improvement in animals' clinical status. The aim of this study was to describe if TX induces in vitro efferocytosis of neutrophils by alveolar macrophages. Efferocytosis assay, myeloperoxidase (MPO) detection and translocation phosphatidylserine (PS) were performed on neutrophils isolated from peripheral blood samples from five healthy horses. In in vitro samples from heathy horses, TX treatment increases the phenomenon of efferocytosis of peripheral neutrophils by alveolar macrophages. Similar increases in supernatant MPO concentration and PS translocation were observed in TX-treated neutrophils, compared to control cells. In conclusion, these results confirm that tamoxifen has a direct effect on equine peripheral blood neutrophils, through stimulation of the engulfment of apoptotic neutrophils by alveolar macrophages.

Non-canonical PI3K-Cdc42-Pak-Mek-Erk Signaling Promotes Immune-Complex-Induced Apoptosis in Human Neutrophils

Neutrophils are peripheral blood leukocytes that represent the first line of immune cell defense against bacterial and fungal infections but are also crucial players in the generation of the inflammatory response. Many neutrophil cell surface receptors regulate important cellular processes via activation of agonist-activated PI3Ks. We show here that activation of human neutrophils with insoluble immune complexes drives a previously uncharacterized, PI3K-dependent, non-canonical, pro-apoptotic signaling pathway, FcγR-PI3Kβ/δ-Cdc42-Pak-Mek-Erk. This is a rare demonstration of Ras/Raf-independent activation of Erk and of PI3K-mediated activation of Cdc42. In addition, comparative analysis of immune-complex- and fMLF-induced signaling uncovers key differences in pathways used by human and murine neutrophils. The non-canonical pathway we identify in this study may be important for the resolution of inflammation in chronic inflammatory diseases that rely on immune-complex-driven neutrophil activation.

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Immune-complex-activated human neutrophils use PI3Kβ/δ-Cdc42-Pak-Mek-Erk signaling

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Immune-complex-induced non-canonical neutrophil signaling is pro-apoptotic

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Other immune-complex-induced neutrophil functions depend on alternative PI3K effectors

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Immune-complex-induced PI3K signaling is not conserved between humans and mice

Platelet-derived PF4 reduces neutrophil apoptosis following arterial occlusion

Note: The editorial process for this manuscript was fully handled by Gregory Y. H. Lip, Editor in Chief.

Resolution of Cochlear Inflammation: Novel Target for Preventing or Ameliorating Drug-, Noise- and Age-related Hearing Loss

A significant number of studies support the idea that inflammatory responses are intimately associated with drug-, noise- and age-related hearing loss (DRHL, NRHL and ARHL). Consequently, several clinical strategies aimed at reducing auditory dysfunction by preventing inflammation are currently under intense scrutiny. Inflammation, however, is a normal adaptive response aimed at restoring tissue functionality and homeostasis after infection, tissue injury and even stress under sterile conditions, and suppressing it could have unintended negative consequences. Therefore, an appropriate approach to prevent or ameliorate DRHL, NRHL and ARHL should involve improving the resolution of the inflammatory process in the cochlea rather than inhibiting this phenomenon. The resolution of inflammation is not a passive response but rather an active, highly controlled and coordinated process. Inflammation by itself produces specialized pro-resolving mediators with critical functions, including essential fatty acid derivatives (lipoxins, resolvins, protectins and maresins), proteins and peptides such as annexin A1 and galectins, purines (adenosine), gaseous mediators (NO, H₂S and CO), as well as neuromodulators like acetylcholine and netrin-1. In this review article, we describe recent advances in the understanding of the resolution phase of inflammation and highlight therapeutic strategies that might be useful in preventing inflammation-induced cochlear damage. In particular, we emphasize beneficial approaches that have been tested in pre-clinical models of inflammatory responses induced by recognized ototoxic drugs such as cisplatin and aminoglycoside antibiotics. Since these studies suggest that improving the resolution process could be useful for the prevention of inflammation-associated diseases in humans, we discuss the potential application of similar strategies to prevent or mitigate DRHL, NRHL and ARHL.

Slow release of etoposide from dextran conjugation shifts etoposide activity from cytotoxicity to differentiation: A promising tool for dosage control in anticancer metronomic therapy

Drug conjugation, improving drug stability, solubility and body permanence, allows achieving impressive results in tumor control. Here, we show that conjugation may provide a straightforward method to administer drugs by the emerging anticancer metronomic approach, presently consisting of low, repeated doses of cytotoxic drugs used in traditional chemotherapy, thus reducing toxicity without reducing efficiency; however, low dose maintenance in tumor sites is difficult. We show that conjugating the antitumor drug etoposide to dextran via pH-sensitive bond produces slow releasing, apoptosis-proficient conjugates rapidly internalized into acidic lysosomes; importantly, release of active etoposide requires cell internalization and acidic pH. Conjugation, without impairing etoposide-induced complete elimination of tumor cells, shifted the mode of apoptosis from cytotoxicity- to differentiation-related; interestingly, high conjugate doses acted as low doses of free etoposide, thus mimicking the effect of metronomic therapy. This indicates slow release as a promising novel strategy for stabilizing low drug levels in metronomic regimens.

Development and Identification of a Novel Subpopulation of Human Neutrophil-derived Giant Phagocytes In Vitro

Neutrophils (PMN) are best known for their phagocytic functions against invading pathogens and microorganisms. They have the shortest half-life amongst leukocytes and in their non-activated state are constitutively committed to apoptosis. When recruited to inflammatory sites to resolve inflammation, they produce an array of cytotoxic molecules with potent antimicrobial killing. Yet, when these powerful cytotoxic molecules are released in an uncontrolled manner they can damage surrounding tissues. In recent years however, neutrophil versatility is increasingly evidenced, by demonstrating plasticity and immunoregulatory functions. We have recently identified a new neutrophil-derived subpopulation, which develops spontaneously in standard culture conditions without the addition of cytokines/growth factors such as granulocyte colony-stimulating factor (GM-CSF)/interleukin (IL)-4. Their phagocytic abilities of neutrophil remnants largely contribute to increase their size immensely; therefore they were termed giant phagocytes (Gϕ). Unlike neutrophils, Gϕ are long lived in culture. They express the cluster of differentiation (CD) neutrophil markers CD66b/CD63/CD15/CD11b/myeloperoxidase (MPO)/neutrophil elastase (NE), and are devoid of the monocytic lineage markers CD14/CD16/CD163 and the dendritic CD1c/CD141 markers. They also take-up latex and zymosan, and respond by oxidative burst to stimulation with opsonized-zymosan and PMA. Gϕ also express the scavenger receptors CD68/CD36, and unlike neutrophils, internalize oxidized-low density lipoprotein (oxLDL). Moreover, unlike fresh neutrophils, or cultured monocytes, they respond to oxLDL uptake by increased reactive oxygen species (ROS) production. Additionally, these phagocytes contain microtubule-associated protein-1 light chain 3B (LC3B) coated vacuoles, indicating the activation of autophagy. Using specific inhibitors it is evident that both phagocytosis and autophagy are prerequisites for their development and likely NADPH oxidase dependent ROS. We describe here a method for the preparation of this new subpopulation of long-lived, neutrophil-derived phagocytic cells in culture, their identification and their currently known characteristics. This protocol is essential for obtaining and characterizing Gϕ in order to further investigate their significance and functions.

Genetic and pharmacological inhibition of CDK9 drives neutrophil apoptosis to resolve inflammation in zebrafish in vivo

Neutrophilic inflammation is tightly regulated and subsequently resolves to limit tissue damage and promote repair. When the timely resolution of inflammation is dysregulated, tissue damage and disease results. One key control mechanism is neutrophil apoptosis, followed by apoptotic cell clearance by phagocytes such as macrophages. Cyclin-dependent kinase (CDK) inhibitor drugs induce neutrophil apoptosis in vitro and promote resolution of inflammation in rodent models. Here we present the first in vivo evidence, using pharmacological and genetic approaches, that CDK9 is involved in the resolution of neutrophil-dependent inflammation. Using live cell imaging in zebrafish with labelled neutrophils and macrophages, we show that pharmacological inhibition, morpholino-mediated knockdown and CRISPR/cas9-mediated knockout of CDK9 enhances inflammation resolution by reducing neutrophil numbers via induction of apoptosis after tailfin injury. Importantly, knockdown of the negative regulator La-related protein 7 (LaRP7) increased neutrophilic inflammation. Our data show that CDK9 is a possible target for controlling resolution of inflammation.

Fever-range hyperthermia improves the anti-apoptotic effect induced by low pH on human neutrophils promoting a proangiogenic profile

Neutrophils have the shortest lifespan among leukocytes and usually die via apoptosis, limiting their deleterious potential. However, this tightly regulated cell death program can be modulated by pathogen-associated molecular patterns (PAMPs), danger-associated molecular pattern (DAMPs), and inflammatory cytokines. We have previously reported that low pH, a hallmark of inflammatory processes and solid tumors, moderately delays neutrophil apoptosis. Here we show that fever-range hyperthermia accelerates the rate of neutrophil apoptosis at neutral pH but markedly increases neutrophil survival induced by low pH. Interestingly, an opposite effect was observed in lymphocytes; hyperthermia plus low pH prevents lymphocyte activation and promotes the death of lymphocytes and lymphoid cell lines. Analysis of the mechanisms through which hyperthermia plus low pH increased neutrophil survival revealed that hyperthermia further decreases cytosolic pH induced by extracellular acidosis. The fact that two Na⁺/H⁺ exchanger inhibitors, 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and amiloride, reproduced the effects induced by hyperthermia suggested that it prolongs neutrophil survival by inhibiting the Na⁺/H⁺ antiporter. The neutrophil anti-apoptotic effect induced by PAMPs, DAMPs, and inflammatory cytokines usually leads to the preservation of the major neutrophil effector functions such as phagocytosis and reactive oxygen species (ROS) production. In contrast, our data revealed that the anti-apoptotic effect induced by low pH and hyperthermia induced a functional profile characterized by a low phagocytic activity, an impairment in ROS production and a high ability to suppress T-cell activation and to produce the angiogenic factors VEGF, IL-8, and the matrix metallopeptidase 9 (MMP-9). These results suggest that acting together fever and local acidosis might drive the differentiation of neutrophils into a profile able to promote both cancer progression and tissue repair during the late phase of inflammation, two processes that are strongly dependent on the local production of angiogenic factors by infiltrating immune cells.

Immune tolerance to an intestine-adapted bacteria, Chryseobacterium sp., injected into the hemocoel of Protaetia brevitarsis seulensis

To explore the interaction of gut microbes and the host immune system, bacteria were isolated from the gut of Protaetia brevitarsis seulensis larvae. Chryseobacterium sp., Bacillus subtilis, Arthrobacter arilaitensis, Bacillus amyloliquefaciens, Bacillus megaterium, and Lysinibacillus xylanilyticus were cultured in vitro, identified, and injected in the hemocoel of P. brevitarsis seulensis larvae, respectively. There were no significant changes in phagocytosis-associated lysosomal formation or pathogen-related autophagosome in immune cells (granulocytes) from Chryseobacterium sp.-challenged larvae. Next, we examined changes in the transcription of innate immune genes such as peptidoglycan recognition proteins and antimicrobial peptides following infection with Chryseobacterium sp. PGRP-1 and -2 transcripts, which may be associated with melanization generated by prophenoloxidase (PPO), were either highly or moderately expressed at 24 h post-infection with Chryseobacterium sp. However, PGRP-SC2 transcripts, which code for bactericidal amidases, were expressed at low levels. With respect to antimicrobial peptides, only coleoptericin was moderately expressed in Chryseobacterium sp.-challenged larvae, suggesting maintenance of an optimum number of Chryseobacterium sp. All examined genes were expressed at significantly higher levels in larvae challenged with a pathogenic bacterium. Our data demonstrated that gut-inhabiting bacteria, the Chryseobacterium sp., induced a weaker immune response than other pathogenic bacteria, E. coli K12.

Key mechanisms governing resolution of lung inflammation

Innate immunity normally provides excellent defence against invading microorganisms. Acute inflammation is a form of innate immune defence and represents one of the primary responses to injury, infection and irritation, largely mediated by granulocyte effector cells such as neutrophils and eosinophils. Failure to remove an inflammatory stimulus (often resulting in failed resolution of inflammation) can lead to chronic inflammation resulting in tissue injury caused by high numbers of infiltrating activated granulocytes. Successful resolution of inflammation is dependent upon the removal of these cells. Under normal physiological conditions, apoptosis (programmed cell death) precedes phagocytic recognition and clearance of these cells by, for example, macrophages, dendritic and epithelial cells (a process known as efferocytosis). Inflammation contributes to immune defence within the respiratory mucosa (responsible for gas exchange) because lung epithelia are continuously exposed to a multiplicity of airborne pathogens, allergens and foreign particles. Failure to resolve inflammation within the respiratory mucosa is a major contributor of numerous lung diseases. This review will summarise the major mechanisms regulating lung inflammation, including key cellular interplays such as apoptotic cell clearance by alveolar macrophages and macrophage/neutrophil/epithelial cell interactions. The different acute and chronic inflammatory disease states caused by dysregulated/impaired resolution of lung inflammation will be discussed. Furthermore, the resolution of lung inflammation during neutrophil/eosinophil-dominant lung injury or enhanced resolution driven via pharmacological manipulation will also be considered.

Induction of human IL-10-producing neutrophils by LPS-stimulated Treg cells and IL-10

Recent evidence has revealed an unsuspected suppressive role played by neutrophils during microbial infections. An especially intriguing aspect of this role is the ability of neutrophils to produce interleukin (IL)-10 following interaction with lipopolysaccharide (LPS)-stimulated regulatory T (Treg) cells. The present study demonstrates that generation of IL-10 in neutrophils induced by LPS-stimulated Treg cells required direct cell-cell contact. This effect was dependent on the binding of CD11b and intercellular adhesion molecule 1. Neither stimulation of neutrophils with LPS nor their culture with unstimulated Treg cells, CD3/CD28 monoclonal antibodies-stimulated Treg cells, or T conventional cells affected intracellular IL-10 expression. IL-10-positive neutrophils were also induced by exogenous IL-10, providing an example of a positive feedback loop. Both LPS-stimulated Treg cells and exogenous IL-10 exclusively promoted posttranslational modifications of histones, H3K4me3 and H3Ac Lys4, that activate IL-10 genomic locus in neutrophils, while the promoter of IL-10 gene was inactive in resting, LPS-stimulated neutrophils, following blocking of direct interaction with LPS-stimulated Treg cells or in LPS-preactivated neutrophils incubated with LPS-stimulated Treg cells. We additionally confirmed the presence of IL-10-producing neutrophils in vivo in patients with periodontal abscess induced by Gram-negative bacteria, as opposed to neutrophils isolated from the site of aseptic inflammation in patients with neuromyelitis optica.

Prolonged cultures of unstimulated human neutrophils lead to the apparition and persistence of rest-in-plate structures (RIPs) recognized by professional phagocytes in vitro and in vivo

Polymorphonuclear neutrophil cells (PMNs) are known to spontaneously undergo apoptosis and then eliminated by professional phagocytes to prevent inflammation, a process called efferocytosis. However, when efferocytosis is impaired, PMNs will fall into secondary necrosis. Whether this state can persist for a certain period of time is unclear, since most of the studies investigating secondary necrosis are performed within 24h following induction by a proapoptotic agent. In this study, freshly isolated human PMNs were incubated without addition of exogenous agents in order to force them to undergo apoptosis and then secondary necrosis, an ideal experimental condition to study the behavior of secondary necrotic PMNs in absence of efferocytosis. By monitoring PMN cell morphology over time, we observed that an increasing proportion of cells harbored a ghost-like phenotype. Because these cellular remnants persist in plates for several days, we introduce here the terminology RIPs for 'rest-in-plate' structure. Heating of freshly isolated PMNs for 5min did not lead to the apparition of RIPs over time. In vivo administration of 7-days old RIPs in the murine air pouch model induced a slight inflammation resorbed within 24h. PKH26-stained RIPs were found to be ingested by professional phagocytes in vitro and in vivo in the murine air pouch and peritonitis models. Therefore, aged-PMNs have the potential to become RIPs in absence of efficient efferocytosis. Fortunately RIPs are recognized by professional phagocytes and, therefore, the concept of resolution of inflammation based on elimination of apoptotic and secondary necrotic PMNs could also be applied to RIPs.

Role of interleukin-6 in regulation of immune responses to remodeling after myocardial infarction

Myocardial remodeling following myocardial infarction (MI) is emerging as key causes of chronic infarct mortality. Interleukin-6 is a classic pro-inflammatory cytokine needed to mount an effective immune response. It seems that interleukin-6 acts as an important role in the dynamic and superbly orchestrated process of innate immunity after MI. Interleukin-6 timely suppresses of innate immune signals to prevent the catastrophic consequences of uncontrolled inflammation on cardiac geometry and function, and thus tunes myocardial remodeling. A comprehensive understanding of biological processes of interleukin-6 in innate immunity leading to inflammatory response and disease-related ventricular remodeling is helpful to find the solution of chronic heart failure. To accomplish this, we reviewed the articles of interleukin-6 regard to inflammation, innate immunity, and cardiac remodeling. This review focuses on the role of interleukin-6 that dominates cell-mediated immunity, especially on neutrophils, monocytes, macrophages, and fibroblasts. In addition, we will also briefly discuss other inflammatory cytokines involved in this process within the paper.

Transcription Factor Repertoire of Homeostatic Eosinophilopoiesis

The production of mature eosinophils (Eos) is a tightly orchestrated process with the aim to sustain normal Eos levels in tissues while also maintaining low numbers of these complex and sensitive cells in the blood. To identify regulators of homeostatic eosinophilopoiesis in mice, we took a global approach to identify genome-wide transcriptome and epigenome changes that occur during homeostasis at critical developmental stages, including Eos-lineage commitment and lineage maturation. Our analyses revealed a markedly greater number of transcriptome alterations associated with Eos maturation (1199 genes) than with Eos-lineage commitment (490 genes), highlighting the greater transcriptional investment necessary for differentiation. Eos-lineage-committed progenitors (EoPs) were noted to express high levels of granule proteins and contain granules with an ultrastructure distinct from that of mature resting Eos. Our analyses also delineated a 976-gene Eos-lineage transcriptome that included a repertoire of 56 transcription factors, many of which have never previously been associated with Eos. EoPs and Eos, but not granulocyte-monocyte progenitors or neutrophils, expressed Helios and Aiolos, members of the Ikaros family of transcription factors, which regulate gene expression via modulation of chromatin structure and DNA accessibility. Epigenetic studies revealed a distinct distribution of active chromatin marks between genes induced with lineage commitment and genes induced with cell maturation during Eos development. In addition, Aiolos and Helios binding sites were significantly enriched in genes expressed by EoPs and Eos with active chromatin, highlighting a potential novel role for Helios and Aiolos in regulating gene expression during Eos development.

Unconventional apoptosis of polymorphonuclear neutrophils (PMN): staurosporine delays exposure of phosphatidylserine and prevents phagocytosis by MΦ-2 macrophages of PMN

Apoptosis of polymorphonuclear neutrophils (PMN) and subsequent 'silent' removal represents an important check-point for the resolution of inflammation. Failure in PMN clearance resulting in secondary necrosis-driven tissue damage has been implicated in conditions of chronic inflammation and autoimmunity. Apoptotic PMN undergo profound biophysical changes that warrant their efficient recognition and uptake by phagocytes before fading to secondary necrosis. In this study, we demonstrate that staurosporine (STS), a non-selective but potent inhibitor of cyclin-dependent kinase and protein kinase C, exerts a drastic impact on PMN apoptosis. PMN treated with STS underwent an unconventional form of cell death characterized by a delayed exposure of aminophospholipids, including phosphatidylserine (PS) and phosphatidylethanolamine and an increased exposure of neo-glycans. STS caused an impaired cellular fragmentation and accelerated DNA fragmentation. Phagocytosis of STS-treated PMN lacking PS on their surfaces was decreased significantly, which highlights the importance of PS for the clearance of apoptotic PMN. Specific opsonization with immune complexes completely restored phagocytosis of STS-treated PMN, demonstrating the efficiency of back-up clearance pathways in the absence of PS exposure.

FPR2/ALXR agonists and the resolution of inflammation

The resolution of inflammation (RoI), once believed to be a passive process, has lately been shown to be an active and delicately orchestrated process. During the resolution phase of acute inflammation, novel mediators, including lipoxins and resolvins, which are members of the specialized pro-resolving mediators of inflammation, are produced. FPR2/ALXR, a receptor modulated by some of these lipids as well as by peptides (e.g., annexin A1), has been shown to be one of the receptors involved in the RoI. The aim of this perspective is to present the concept of the RoI from a medicinal chemistry point of view and to highlight the effort of the research community to discover and develop anti-inflammatory/pro-resolution small molecules to orchestrate inflammation by activation of FPR2/ALXR.

Sex differences in inflammatory and apoptotic signaling molecules in normal and diseased human gingiva

BACKGROUND

The purpose of this study is to determine whether sex dimorphism exists in the expression of inflammatory and apoptotic mediators in gingiva obtained from normal and diseased sites of periodontal disease.

METHODS

Gingival papillae were obtained from individuals (56 males and 62 females) who required extraction of adjacent teeth. Gingival samples were grouped by adjacent sulcus depth: 1 to 3 mm (normal), 3 mm with bleeding on probing (slight disease), 3 to 6 mm (moderate disease), and >6 mm (severe disease). The tissue concentrations of cysteine-requiring aspartate-directed protease 3 (caspase-3), interleukin-2, tumor necrosis factor-related apoptosis-inducing ligand, Fas ligand, p38α mitogen-activated protein kinase, extracellular signal-related kinase 1/2, and survivin were determined by enzyme-linked immunosorbent assay. These mediator concentrations, age of donor, sex of donor, and gingival sulcular depth were the outcome variables. Data were compared by factorial analysis of variance, post hoc Tukey, and Pearson correlation test. P <0.05 was used to indicate significant differences among the outcome variables.

RESULTS

The mean gingival sulcular depth was significantly greater in male than in female groups (P <0.05). The majority of the tested mediators were significantly correlated with both sex and sulcular depth and with caspase-3 (P <0.05). The concentration of caspase-3 in female gingiva at all diseased sites was significantly greater than in gingiva derived from male sites (P <0.05).

CONCLUSIONS

These data suggest sex dimorphism in the presence of gingival apoptosis at sites of periodontal disease, with females having the highest incidence of apoptosis. Because apoptosis clears inflammatory cells and promotes healing, this phenomenon could provide a mechanism for sex dimorphism for the incidence of periodontal disease.

Characterization of the hemocytes in Larvae of Protaetia brevitarsis seulensis: involvement of granulocyte-mediated phagocytosis

Hemocytes are key players in the immune response against pathogens in insects. However, the hemocyte types and their functions in the white-spotted flower chafers, Protaetia brevitarsis seulensis (Kolbe), are not known. In this study, we used various microscopes, molecular probes, and flow cytometric analyses to characterize the hemocytes in P. brevitarsis seulensis. The circulating hemocytes were classified based on their size, morphology, and dye-staining properties into six types, including granulocytes, plasmatocytes, oenocytoids, spherulocytes, prohemocytes, and adipohemocytes. The percentages of circulating hemocyte types were as follows: 13% granulocytes, 20% plasmatocytes, 1% oenocytoids, 5% spherulocytes, 17% prohemocytes, and 44% adipohemocytes. Next, we identified the professional phagocytes, granulocytes, which mediate encapsulation and phagocytosis of pathogens. The granulocytes were immunologically or morphologically activated and phagocytosed potentially hazardous substances in vivo. In addition, we showed that the phagocytosis by granulocytes is associated with autophagy, and that the activation of autophagy could be an efficient way to eliminate pathogens in this system. We also observed a high accumulation of autophagic vacuoles in activated granulocytes, which altered their shape and led to autophagic cell death. Finally, the granulocytes underwent mitotic division thus maintaining their number in vivo.