Bacterial DNA delays human eosinophil apoptosis

Eosinophilic Asthma: Pathophysiology and Therapeutic Horizons

Asthma is a prevalent chronic non-communicable disease, affecting approximately 300 million people worldwide. It is characterized by significant airway inflammation, hyperresponsiveness, obstruction, and remodeling. Eosinophilic asthma, a subtype of asthma, involves the accumulation of eosinophils in the airways. These eosinophils release mediators and cytokines, contributing to severe airway inflammation and tissue damage. Emerging evidence suggests that targeting eosinophils could reduce airway remodeling and slow the progression of asthma. To achieve this, it is essential to understand the immunopathology of asthma, identify specific eosinophil-associated biomarkers, and categorize patients more accurately based on the clinical characteristics (phenotypes) and underlying pathobiological mechanisms (endotypes). This review delves into the role of eosinophils in exacerbating severe asthma, exploring various phenotypes and endotypes, as well as biomarkers. It also examines the current and emerging biological agents that target eosinophils in eosinophilic asthma. By focusing on these aspects, both researchers and clinicians can advance the development of targeted therapies to combat eosinophilic pathology in severe asthma.

Chronic exposure to indoxacarb and pulmonary expression of toll-like receptor-9 in mice

AIM

Chronic exposure to indoxacarb and pulmonary expression of toll-like receptor 9 (TLR-9) in mice.

MATERIALS AND METHODS

In this study, healthy male Swiss albino mice (n=30) aging 8-10 weeks were used to evaluate TLR-9 expression in lungs of mice following indoxacarb exposure with and without lipopolysaccharide (LPS). Indoxacarb was administered orally dissolved in groundnut oil at 4 and 2 mg/kg/day for 90 days. On day 91, five animals from each group were challenged with LPS/normal saline solution at 80 µg/animal. The lung tissues were processed for real time and immunohistochemical studies.

RESULTS

LPS resulted increase in fold change m-RNA expression level of TLR-9 as compare to control, while indoxacarb (4 mg/kg) alone and in combination with LPS resulted 16.21-fold change and 29.4-fold change increase in expression of TLR-9 m-RNA, respectively, as compared to control. Similarly, indoxacarb (2 mg/kg) alone or in combination with LPS also altered TLR-9 expression. Further at protein level control group showed minimal expression of TLR-9 in lungs as compare to other groups, however, LPS group showed intense positive staining in bronchial epithelium as well as in alveolar septal cells. Indoxacarb at both doses individually showed strong immuno-positive reaction as compare to control, however when combined with LPS resulted intense staining in airway epithelium as compare to control.

CONCLUSION

Chronic oral administration of indoxacarb for 90 days (4 and 2 mg/kg) alters expression of TLR-9 at m-RNA and protein level and co-exposure with LPS exhibited synergistic effect.

Human eosinophils are direct targets to nanoparticles: Zinc oxide nanoparticles (ZnO) delay apoptosis and increase the production of the pro-inflammatory cytokines IL-1β and IL-8

Zinc oxide NPs (ZnO) have been recently proposed as novel candidates for the treatment of allergic inflammatory diseases. Paradoxically, recent data suggested that ZnO could cause eosinophilic airway inflammation in rodents. Despite the above observations, there are currently no studies reporting direct interaction between a given NP and human eosinophils themselves. In this study, freshly isolated human eosinophils were incubated with ZnO and several cellular functions were studied. We found that ZnO delay human eosinophil apoptosis, partially by inhibiting caspases and by preventing caspase-4 and Bcl-xL degradation. ZnO do not induce production of reactive oxygen species but increase de novo protein synthesis. In addition, ZnO were found to increase the production of the proinflammatory IL-1β and IL-8 cytokines. Using a pharmacological approach, we demonstrated that inhibition of caspase-1 reversed the ability of ZnO to induce IL-1β and IL-8 production, whereas inhibition of caspase-4 only reversed that of IL-8. Our results indicate the necessity of conducting studies to determine the potential of using NP as nanotherapies, particularly in diseases in which eosinophils may be involved. We conclude that, indeed, human eosinophils represent potential new direct targets to NPs, ZnO in the present case.

Phenotypes, Risk Factors, and Mechanisms of Adult-Onset Asthma

Asthma is a heterogeneous disease with many phenotypes, and age at disease onset is an important factor in separating the phenotypes. Genetic factors, atopy, and early respiratory tract infections are well-recognized factors predisposing to childhood-onset asthma. Adult-onset asthma is more often associated with obesity, smoking, depression, or other life-style or environmental factors, even though genetic factors and respiratory tract infections may also play a role in adult-onset disease. Adult-onset asthma is characterized by absence of atopy and is often severe requiring treatment with high dose of inhaled and/or oral steroids. Variety of risk factors and nonatopic nature of adult-onset disease suggest that variety of mechanisms is involved in the disease pathogenesis and that these mechanisms differ from the pathobiology of childhood-onset asthma with prevailing Th2 airway inflammation. Recognition of the mechanisms and mediators that drive the adult-onset disease helps to develop novel strategies for the treatment. The aim of this review was to summarize the current knowledge on the pathogenesis of adult-onset asthma and to concentrate on the mechanisms and mediators involved in establishing adult-onset asthma in response to specific risk factors. We also discuss the involvement of these mechanisms in the currently recognized phenotypes of adult-onset asthma.

Determinants of eosinophil survival and apoptotic cell death

Eosinophils (Eos) are potent inflammatory cells and abundantly present in the sputum and lung of patients with allergic asthma. During both transit to and residence in the lung, Eos contact prosurvival cytokines, particularly IL-3, IL-5 and GM-CSF, that attenuate cell death. Cytokine signaling modulates the expression and function of a number of intracellular pro- and anti-apoptotic molecules. Both intrinsic mitochondrial and extrinsic receptor-mediated pathways are affected. This article discusses the fundamental role of the extracellular and intracellular molecules that initiate and control survival decisions by human Eos and highlights the role of the cis-trans isomerase, Pin1 in controlling these processes.

Eosinophils regulate peripheral B cell numbers in both mice and humans

The view of eosinophils (Eos) as solely effector cells involved in host parasite defense and in the pathophysiology of allergic diseases has been challenged in recent years. In fact, there is a growing realization that these cells interact with other components of innate and adaptive immunity. For example, mouse Eos were recently demonstrated to promote plasma cell retention in the bone marrow. However, it remains unknown whether Eos influence the biology of normal B lymphocytes. In this study, we specifically assessed the effect of Eos on B cell survival, proliferation, and Ig secretion. Our data first revealed that the genetic deletion of Eos from NJ1638 IL-5 transgenic hypereosinophilic mice (previously shown to display profound B cell expansion) resulted in the near abolishment of the B cell lymphocytosis. In vitro studies using human tissues demonstrated Eos' proximity to B cell follicles and their ability to promote B cell survival, proliferation, and Ig secretion via a contact-independent mechanism. Additionally, this ability of Eos to enhance B cell responsiveness was observed in both T-independent and T-dependent B cell activation and appears to be independent of the activation state of Eos. Finally, a retrospective clinical study of hypereosinophilic patients revealed a direct correlation between peripheral blood eosinophil levels and B cell numbers. Taken together, our study identifies a novel role for Eos in the regulation of humoral immunity via their impact on B cell homeostasis and proliferation upon activation.

Tumour necrosis factor-α regulates human eosinophil apoptosis via ligation of TNF-receptor 1 and balance between NF-κB and AP-1

Eosinophils play a central role in asthma. The present study was performed to investigate the effect of tumour necrosis factor-α (TNF-α) on longevity of isolated human eosinophils. In contrast to Fas, TNF-α inhibited eosinophil apoptosis as evidenced by a combination of flow cytometry, DNA fragmentation assay and morphological analyses. The effect of TNF-α on eosinophil apoptosis was reversed by a TNF-α neutralising antibody. The anti-apoptotic effect of TNF-α was not due to autocrine release of known survival-prolonging cytokines interleukins 3 and 5 or granulocyte-macrophage-colony-stimulating factor as their neutralisation did not affect the effect of TNF-α. The anti-apoptotic signal was mediated mainly by the TNF-receptor 1. TNF-α induced phosphorylation and degradation of IκB and an increase in NF-κB DNA-binding activity. The survival-prolonging effect of TNF-α was reversed by inhibitors of NF-κB pyrrolidinedithiocarbamate and gliotoxin and by an inhibitor of IκB kinase, BMS-345541. TNF-α induced also an increase in AP-1 DNA-binding activity and the antiapoptotic effect of TNF-α was potentiated by inhibitors of AP-1, SR 11302 and tanshinone IIA and by an inhibitor of c-jun-N-terminal kinase, SP600125, which is an upstream kinase activating AP-1. Our results thus suggest that TNF-α delays human eosinophil apoptosis via TNF-receptor 1 and the resulting changes in longevity depend on yin-yang balance between activation of NF-κB and AP-1.

Regulation of spontaneous eosinophil apoptosis-a neglected area of importance

Asthma is characterized by the accumulation of eosinophils in the airways in most phenotypes. Eosinophils are inflammatory cells that require an external survival-prolonging stimulus such as granulocyte macrophage-colony-stimulating factor (GM-CSF), interleukin (IL)-5, or IL-3 for survival. In their absence, eosinophils are programmed to die by spontaneous apoptosis in a few days. Eosinophil apoptosis can be accelerated by Fas ligation or by pharmacological agents such as glucocorticoids. Evidence exists for the relevance of these survival-prolonging and pro-apoptotic agents in the regulation of eosinophilic inflammation in inflamed airways. Much less is known about the physiological significance and mechanisms of spontaneous eosinophil apoptosis even though it forms the basis of regulation of eosinophil longevity by pathophysiological factors and pharmacological agents. This review concentrates on discussing the mechanisms of spontaneous eosinophil apoptosis compared to those of glucocorticoid- and Fas-induced apoptosis. We aim to answer the question whether the external apoptotic stimuli only augment the ongoing pathway of spontaneous apoptosis or truly activate a specific pathway.

Eosinophil intracellular signalling: apoptosis

Eosinophil apoptosis is considered critical for the resolution of eosinophilic inflammation in the airways of asthmatics. Apoptosis can be mediated by an extrinsic receptor-activated pathway or alternatively by an intrinsic pathway via distortion of mitochondrial function. Both of these pathways lead to activation of the caspase cascade resulting in degradation of cellular components. We describe here two methods to explore intracellular mechanisms mediating eosinophil apoptosis. Eosinophil staining by fluorescent probe JC-1 followed by flow cytometric analysis is a reliable method for determination of the state of mitochondrial membrane potential (∆Ψm). Lost ∆Ψm indicates distorted mitochondrial function and apoptosis. We also describe a method to explore the activation of effector caspase-6 by assessing degradation of its substrate lamin A/C by immunoblotting.

Eosinophil apoptosis as a therapeutic target in allergic asthma

Asthma is a chronic inflammatory disease of the airways manifesting in many different phenotypes. Allergic asthma, comprising approximately half of patients with asthma, is characterized by the accumulation of eosinophils into the lungs. Eosinophils release factors that damage the surrounding cells and participate in the maintenance and exacerbation of inflammation. In the absence of any inflammatory survival-prolonging factors, eosinophils die by apoptosis in few days but in inflamed airways, eosinophil survival is thought to be prolonged due to the surrounding pro-inflammatory factors such as IL-5, IL-3 and GM-CSF. Resolution of eosinophilic inflammation is an important goal in the treatment of allergic asthma. Apoptosis is a physiological and non-inflammatory way to eliminate these harmful cells, and development of drugs targeting eosinophil apoptosis is one possible strategy for the therapy of allergic asthma. Importance of this strategy is supported by the fact that promotion of eosinophil apoptosis is a property of many anti-asthmatic agents such as glucocorticoids, the current main anti-inflammatory therapy of asthma, theophylline and leukotriene modifiers. β2 agonists have been shown to modulate eosinophil longevity by increasing survival. Also, anti-IL-5 antibody mesolizumab has shown efficacy in reducing asthma exacerbations in patients with severe eosinophilic asthma. Many potential future anti-asthmatic agents, such as Siglec-8 activating antibody and novel humanized anti-IL-5 antibody MEDI-563, have the property of inducing eosinophil apoptosis. This MiniReview aims to present eosinophil apoptosis as a therapeutic target in the treatment of allergic asthma. We summarize the effects and mechanisms of current and potential future anti-asthmatic drugs on eosinophil apoptosis and additionally, discuss the potential factors that promote eosinophil longevity in the lungs.

Living and dying for inflammation: neutrophils, eosinophils, basophils

Neutrophils, eosinophils, and basophils play essential roles during microbe-induced and sterile inflammation. The severity of such inflammatory processes is controlled, at least in part, by factors that regulate cell death and survival of granulocytes. In recent years, major progress has been made in understanding the molecular mechanisms of granulocyte cell death and in identifying novel damage- and pathogen-associated molecular patterns as well as regulatory cytokines impacting granulocyte viability. Furthermore, an increased interest in innate immunity has boosted our overall understanding of granulocyte biology. In this review, we describe and compare factors and mechanisms regulating neutrophil, eosinophil, and basophil lifespan. Because dysregulation of death pathways in granulocytes can contribute to inflammation-associated immunopathology, targeting granulocyte lifespan could be therapeutically promising.

Expression of Toll-like receptor 9 in mouse and human lungs

Toll-like receptors (TLR) recognize conserved molecular motifs of microorganisms, and constitute an important part of the innate immune system. Numerous studies have shown the importance of these receptors, including TLR9, in establishing effective immune responses to a broad range of infections, and in disorders such as COPD. TLR9 detects unmethylated DNA and is expressed in a wide range of immune cells in mice and humans, as well as other species. Most TLR9 expression studies have been done on cultured or isolated cells, but none that we know of on intact lung. Because cell-specific expression of TLR9 is important to understand its precise role in lung physiology, we tested mouse and human lung tissues for expression of TLR9 mRNA and protein with in situ hybridization and immunohistochemistry, respectively. We found TLR9 mRNA and protein expression in bronchial epithelium, vascular endothelium, alveolar septal cells and alveolar macrophages in both species. Immuno-electron microscopy delineated TLR9 expression in plasma membrane, cytoplasm and the nucleus of various lung cells. Lungs from human cases of COPD had significantly increased numbers of TLR9-positive cells. These are the first data showing TLR9 mRNA and protein expression in intact human and mouse lungs. The data may be useful for clarifying the role of TLR9 in the contributions of specific cells to lung physiology.

The fluorescein-derived dye aminophenyl fluorescein is a suitable tool to detect hypobromous acid (HOBr)-producing activity in eosinophils

The specific detection of peroxidase activity in human granulocytes is essential to elucidate their role in innate immune responses, immune regulation, and inflammatory diseases. The halogenating activity of myeloperoxidase in neutrophils can be determined by the novel fluorescent probe aminophenyl fluorescein (APF). Thereby non-fluorescent APF is oxidized by HOCl to form fluorescein. We successfully verified that APF equally detects the hypobromous acid (HOBr)-producing activity of eosinophil granulocytes. This was revealed by three different approaches. First, we investigated the conversion of non-fluorescent APF into fluorescein by HOCl and HOBr by means of fluorescence and mass spectrometry approaches. Thereby comparable chemical mechanisms were observed for both acids. Furthermore in vitro kinetic studies were used to detect the halogenating activity of myeloperoxidase and eosinophil peroxidase by using APF. Here the dye well reflected the different substrate specificities of myeloperoxidase and eosinophil peroxidase regarding chloride and bromide. Finally, peroxidase activities were successfully detected in phorbol ester-stimulated neutrophils and eosinophils using flow cytometry. Thereby inhibitory studies confirmed the peroxidase-dependent oxidation of APF. To sum up, APF is a promising tool for further evaluation of the halogenating activity of peroxidases in both neutrophils and eosinophils.

Pattern-recognition receptors in human eosinophils

The pattern-recognition receptor (PRR) family includes Toll-like receptors (TLRs), nucleotide-binding oligomerization domain (NOD) -like receptors (NLRs), RIG-I-like receptors (RLRs), C-type lectin receptors (CLRs) and the receptor for advanced glycation end products (RAGE). They recognize various microbial signatures or host-derived danger signals and trigger an immune response. Eosinophils are multifunctional leucocytes involved in the pathogenesis of several inflammatory processes, including parasitic helminth infection, allergic diseases, tissue injury and tumour immunity. Human eosinophils express several PRRs, including TLR1-5, TLR7, TLR9, NOD1, NOD2, Dectin-1 and RAGE. Receptor stimulation induces survival, oxidative burst, activation of the adhesion system and release of cytokines (interleukin-1β, interleukin-6, tumour necrosis factor-α and granulocyte-macrophage colony-stimulating factor), chemokines (interleukin-8 and growth-related oncogene-α) and cytotoxic granule proteins (eosinophil cationic protein, eosinophil-derived neurotoxin, eosinophil peroxidase and major basic protein). It is also evident that eosinophils play an immunomodulatory role by interacting with surrounding cells. The presence of a broad range of PRRs in eosinophils indicates that they are not only involved in defence against parasitic helminths, but also against bacteria, viruses and fungi. From a clinical perspective, eosinophilic PRRs seem to be involved in both allergic and malignant diseases by causing exacerbations and affecting tumour growth, respectively.

Salbutamol delays human eosinophil apoptosis via a cAMP-dependent mechanism

Eosinophils play a major role in asthma. One described mechanism leading to the impaired clearance of these cells from the lung is the delay in their programmed cell death (apoptosis). β(2)-Adrenoceptor agonists have been shown to prolong survival and delay apoptosis of eosinophils. The aim of the present study was to evaluate the mechanisms, especially the role of cAMP pathway, in the prolongation of human eosinophil survival by a selective β(2)-agonist salbutamol. Isolated human peripheral blood eosinophils were cultured in the absence or presence of a β(2)-agonist salbutamol and the indicated antagonists/inhibitors under sterile conditions. Apoptosis was measured by using the relative DNA fragmentation assay and Annexin-V binding. Salbutamol prolonged survival of human eosinophils and it was inhibited by a β-receptor antagonist propranolol and mimicked by cell-permeant cAMP analogues dibutyryl- and 8-bromo-cAMP. Pharmacological inhibitors of adenylyl cyclase (SQ-22,536) and protein kinase A (Rp-8-CPT-cAMPS) antagonized the effects of salbutamol. The survival-prolonging action of salbutamol was potentiated by a phosphodiesterase inhibitor rolipram (EC(50) for the salbutamol effect was 13.6 ± 4.0 and 8.1 ± 3.1 nM in the absence and presence of rolipram, respectively; p=0.0142, n=10). In contrast, inhibition of Ca(2+)-activated K(+)-channels by apamin, charybdotoxin, iberiotoxin or paxilline did not affect the ability of salbutamol to prolong eosinophil survival. Taken together, the present results suggest that salbutamol at clinically relevant concentrations decreases apoptosis in human eosinophils by activating the cannonical β(2)-receptor-adenylyl cyclase-cAMP-protein kinase A pathway.