Cyclooxygenase-2/prostaglandin D2/CRTH2 pathway mediates double-stranded RNA-induced enhancement of allergic airway inflammation

The Prostaglandin D2 Receptor CRTH2 Contributes to Airway Hyperresponsiveness during Airway Inflammation Induced by Sensitization without an Adjuvant in Mice

INTRODUCTION

Prostaglandin D2 (PGD2), which is produced mainly by Th2 cells and mast cells, promotes a type-2 immune response by activating Th2 cells, mast cells, eosinophils, and group 2 innate lymphoid cells (ILC2s) via its receptor, chemoattractant receptor-homologous molecules on Th2 cells (CRTH2). However, the role of CRTH2 in models of airway inflammation induced by sensitization without adjuvants, in which both IgE and mast cells may play major roles, remain unclear.

METHODS

Wild-type (WT) and CRTH2-knockout (KO) mice were sensitized with ovalbumin (OVA) without an adjuvant and then challenged intranasally with OVA. Airway inflammation was assessed based on airway hyperresponsiveness (AHR), lung histology, number of leukocytes, and levels of type-2 cytokines in the bronchoalveolar lavage fluid (BALF).

RESULTS

AHR was significantly reduced after OVA challenge in CRTH2 KO mice compared to WT mice. The number of eosinophils, levels of type-2 cytokines (IL-4, IL-5, and IL-13) in BALF, and IgE concentration in serum were decreased in CRTH2 KO mice compared to WT mice. However, lung histological changes were comparable between WT and CRTH2 KO mice.

CONCLUSION

CRTH2 is responsible for the development of asthma responses in a mouse model of airway inflammation that features prominent involvement of both IgE and mast cells.

P2X7 signaling influences the production of pro-resolving and pro-inflammatory lipid mediators in alveolar macrophages derived from individuals with asthma

Few new therapeutics exist to target airway inflammation in mild-to-moderate asthma. Alveolar macrophages regulate airway inflammation by producing proresolving eicosanoids. We hypothesized that stimulation of the purinergic receptor P2X7 in macrophages from individuals with asthma produces eicosanoids associated with airway inflammation and resolution, and that these responses are predicted, in part, by P2X7 pore function. Study subjects were recruited in an Institutional Review Board (IRB)-approved study. Alveolar macrophages were recovered from bronchoalveolar lavage fluid following bronchoscopy. Purinergic receptor classification was performed using flow cytometry and fluorescent cell assay. Macrophages were stimulated in vitro and eicosanoids were measured via ELISA or enzyme immunoassay (EIA) in the presence and absence of P2X7-specific agonist [2'(3')-O-(4-Benzoylbenzoyl)adenosine-5'-triphosphate tri(triethylammonium) salt (Bz-ATP)] and antagonist (AZD9056). Functional P2X7 pore status was confirmed in a live cell assay using P2X7-specific agonists and antagonists. Alveolar macrophages produced increased quantities of the oxylipins lipoxin A4 (LXA4), resolvin D1 (RvD1), and 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE) following stimulation with Bz-ATP compared with vehicle controls, responses that were attenuated in the presence of the P2X7-selective antagonist, AZD9056. LXA4 and RvD1 production was greatest at 1 h, whereas 15(S)-HETE was maximally produced 24 h. Prostaglandin E-2 and resolvin E1 were minimally produced by P2X7 activation, indicating differential signaling pathways involved in eicosanoid production in alveolar macrophages derived from individuals with asthma. The early production of the proresolving eicosanoids, LXA4 and resolvin D1, is regulated by P2X7, whereas generation of the proinflammatory eicosanoid, 15(S)-HETE, is only partially regulated through P2X7 signaling and reaches maximal production after the peak in proresolving eicosanoids.NEW & NOTEWORTHY Alveolar macrophages obtained from individuals with asthma produce soluble lipid mediators in response to P2X7 purinergic receptor signaling. Proinflammatory mediators may contribute to asthma exacerbations but proresolving mediators may help with resolution of asthma loss of control. These specialized proresolving lipid mediators may serve as future potential therapeutics for asthma exacerbation resolution and recovery.

Significance of Pulmonary Endothelial Injury and the Role of Cyclooxygenase-2 and Prostanoid Signaling

The endothelium plays a key role in the dynamic balance of hemodynamic, humoral and inflammatory processes in the human body. Its central importance and the resulting therapeutic concepts are the subject of ongoing research efforts and form the basis for the treatment of numerous diseases. The pulmonary endothelium is an essential component for the gas exchange in humans. Pulmonary endothelial dysfunction has serious consequences for the oxygenation and the gas exchange in humans with the potential of consecutive multiple organ failure. Therefore, in this review, the dysfunction of the pulmonary endothel due to viral, bacterial, and fungal infections, ventilator-related injury, and aspiration is presented in a medical context. Selected aspects of the interaction of endothelial cells with primarily alveolar macrophages are reviewed in more detail. Elucidation of underlying causes and mechanisms of damage and repair may lead to new therapeutic approaches. Specific emphasis is placed on the processes leading to the induction of cyclooxygenase-2 and downstream prostanoid-based signaling pathways associated with this enzyme.

Effect of CRTH2 antagonism on the response to experimental rhinovirus infection in asthma: a pilot randomised controlled trial

BACKGROUND AND AIMS

The chemoattractant receptor-homologous molecule expressed on T helper type 2 cells (CRTH2) antagonist timapiprant improved lung function and asthma control in a phase 2 study, with evidence suggesting reduced exacerbations. We aimed to assess whether timapiprant attenuated or prevented asthma exacerbations induced by experimental rhinovirus (RV) infection. We furthermore hypothesised that timapiprant would dampen RV-induced type 2 inflammation and consequently improve antiviral immune responses.

METHODS

Atopic patients with partially controlled asthma on maintenance inhaled corticosteroids were randomised to timapiprant (n=22) or placebo (n=22) and challenged with RV-A16 3 weeks later. The primary endpoint was the cumulative lower respiratory symptom score over the 14 days post infection. Upper respiratory symptoms, spirometry, airway hyperresponsiveness, exhaled nitric oxide, RV-A16 virus load and soluble mediators in upper and lower airways samples, and CRTH2 staining in bronchial biopsies were additionally assessed before and during RV-A16 infection.

RESULTS

Six subjects discontinued the study and eight were not infected; outcomes were assessed in 16 timapiprant-treated and 14 placebo-treated, successfully infected subjects. There were no differences between treatment groups in clinical exacerbation severity including cumulative lower respiratory symptom score day 0-14 (difference 3.0 (95% CI -29.0 to 17.0), p=0.78), virus load, antiviral immune responses, or RV-A16-induced airway inflammation other than in the bronchial biopsies, where CRTH2 staining was increased during RV-A16 infection in the placebo-treated but not the timapiprant-treated group. Timapiprant had a favourable safety profile, with no deaths, serious adverse events or drug-related withdrawals.

CONCLUSION

Timapiprant treatment had little impact on the clinicopathological changes induced by RV-A16 infection in partially controlled asthma.

Dengue virus co-opts innate type 2 pathways to escape early control of viral replication

Mast cell products and high levels of type 2 cytokines are associated with severe dengue disease. Group 2 innate lymphoid cells (ILC2) are type-2 cytokine-producing cells that are activated by epithelial cytokines and mast cell-derived lipid mediators. Through ex vivo RNAseq analysis, we observed that ILC2 are activated during acute dengue viral infection, and show an impaired type I-IFN signature in severe disease. We observed that circulating ILC2 are permissive for dengue virus infection in vivo and in vitro, particularly when activated through prostaglandin D2 (PGD2). ILC2 underwent productive dengue virus infection, which was inhibited through CRTH2 antagonism. Furthermore, exogenous IFN-β induced expression of type I-IFN responsive anti-viral genes by ILC2. PGD2 downregulated type I-IFN responsive gene and protein expression; and urinary prostaglandin D2 metabolite levels were elevated in severe dengue. Moreover, supernatants from activated ILC2 enhanced monocyte infection in a GM-CSF and mannan-dependent manner. Our results indicate that dengue virus co-opts an innate type 2 environment to escape early type I-IFN control and facilitate viral dissemination. PGD2 downregulates type I-IFN induced anti-viral responses in ILC2. CRTH2 antagonism may be a therapeutic strategy for dengue-associated disease.

Regulatory Roles of Phospholipase A2 Enzymes and Bioactive Lipids in Mast Cell Biology

Lipids play fundamental roles in life as an essential component of cell membranes, as a major source of energy, as a body surface barrier, and as signaling molecules that transmit intracellular and intercellular signals. Lipid mediators, a group of bioactive lipids that mediates intercellular signals, are produced via specific biosynthetic enzymes and transmit signals via specific receptors. Mast cells, a tissue-resident immune cell population, produce several lipid mediators that contribute to exacerbation or amelioration of allergic responses and also non-allergic inflammation, host defense, cancer and fibrosis by controlling the functions of microenvironmental cells as well as mast cell themselves in paracrine and autocrine fashions. Additionally, several bioactive lipids produced by stromal cells regulate the differentiation, maturation and activation of neighboring mast cells. Many of the bioactive lipids are stored in membrane phospholipids as precursor forms and released spatiotemporally by phospholipase A₂ (PLA₂) enzymes. Through a series of studies employing gene targeting and lipidomics, several enzymes belonging to the PLA₂ superfamily have been demonstrated to participate in mast cell-related diseases by mobilizing unique bioactive lipids in multiple ways. In this review, we provide an overview of our current understanding of the regulatory roles of several PLA₂-driven lipid pathways in mast cell biology.

Effects of non-steroidal anti-inflammatory drugs and other eicosanoid pathway modifiers on antiviral and allergic responses: EAACI task force on eicosanoids consensus report in times of COVID-19

Non‐steroidal anti‐inflammatory drugs (NSAIDs) and other eicosanoid pathway modifiers are among the most ubiquitously used medications in the general population. Their broad anti‐inflammatory, antipyretic, and analgesic effects are applied against symptoms of respiratory infections, including SARS‐CoV‐2, as well as in other acute and chronic inflammatory diseases that often coexist with allergy and asthma. However, the current pandemic of COVID‐19 also revealed the gaps in our understanding of their mechanism of action, selectivity, and interactions not only during viral infections and inflammation, but also in asthma exacerbations, uncontrolled allergic inflammation, and NSAIDs‐exacerbated respiratory disease (NERD). In this context, the consensus report summarizes currently available knowledge, novel discoveries, and controversies regarding the use of NSAIDs in COVID‐19, and the role of NSAIDs in asthma and viral asthma exacerbations. We also describe here novel mechanisms of action of leukotriene receptor antagonists (LTRAs), outline how to predict responses to LTRA therapy and discuss a potential role of LTRA therapy in COVID‐19 treatment. Moreover, we discuss interactions of novel T2 biologicals and other eicosanoid pathway modifiers on the horizon, such as prostaglandin D2 antagonists and cannabinoids, with eicosanoid pathways, in context of viral infections and exacerbations of asthma and allergic diseases. Finally, we identify and summarize the major knowledge gaps and unmet needs in current eicosanoid research.

G-CSFR antagonism reduces mucosal injury and airways fibrosis in a virus-dependent model of severe asthma

BACKGROUND AND PURPOSE

Asthma is a chronic disease that displays heterogeneous clinical and molecular features. A phenotypic subset of late-onset severe asthmatics has debilitating fixed airflow obstruction, increased neutrophilic inflammation and a history of pneumonia. Influenza A virus (IAV) is an important viral cause of pneumonia and asthmatics are frequently hospitalised during IAV epidemics. This study aims to determine whether antagonising granulocyte colony stimulating factor receptor (G-CSFR) prevents pneumonia-associated severe asthma.

EXPERIMENTAL APPROACH

Mice were sensitised to house dust mite (HDM) to establish allergic airway inflammation and subsequently infected with IAV (HKx31/H3N2 subtype). A neutralising monoclonal antibody against G-CSFR was therapeutically administered.

KEY RESULTS

In IAV-infected mice with prior HDM sensitisation, a significant increase in airway fibrotic remodelling and airways hyper-reactivity was observed. A mixed granulocytic inflammatory profile consisting of neutrophils, macrophages and eosinophils was prominent and at a molecular level, G-CSF expression was significantly increased in HDMIAV-treated mice. Blockage of G-CSFR reduced neutrophilic inflammation in the bronchoalveolar and lungs by over 80% in HDMIAV-treated mice without altering viral clearance. Markers of NETosis (dsDNA and myeloperoxidase in bronchoalveolar), tissue injury (LDH activity in bronchoalveolar) and oedema (total bronchoalveolar-fluid protein) were also significantly reduced with anti-G-CSFR treatment. In addition, anti-G-CSFR antagonism significantly reduced bronchoalveolar gelatinase activity, active TFGβ lung levels, collagen lung expression, airways fibrosis and airways hyper-reactivity in HDMIAV-treated mice.

CONCLUSIONS AND IMPLICATIONS

We have shown that antagonising G-CSFR-dependent neutrophilic inflammation reduced pathological disruption of the mucosal barrier and airways fibrosis in an IAV-induced severe asthma model.

Ramatroban as a Novel Immunotherapy for COVID-19

SARS-CoV-2 virus suppresses host innate and adaptive immune responses, thereby allowing the virus to proliferate, and cause multiorgan failure, especially in the elderly. Respiratory viruses stimulate cyclooxygenase-2 (COX-2) to generate prostanoids including Prostaglandin D2 (PGD2) and thromboxane A2. Furthermore, PGD2 concentrations in the airways increase with aging. PGD2 action mediated via DP2 receptors suppresses both innate and adaptive immune responses, by inhibiting interferon-λ and stimulation of myeloid monocyte-derived suppressor cells respectively. PGD2 and thromboxane A2 actions via the TP receptors activate platelets leading to a prothrombotic state. Ramatroban, a small-molecule antagonist of DP2 and TP receptors, reverses viremia-associated proinflammatory, immunosuppressive5 and prothrombotic processes which are similar to those induced by SARS-Cov-2. Ramatroban, used for the treatment of allergic rhinitis in Japan for the past 20 years has an excellent safety profile. Therefore, Ramatroban merits investigation as a novel immunotherapy for the treatment of COVID-19 disease.

The impact of the prostaglandin D2 receptor 2 and its downstream effects on the pathophysiology of asthma

Current research suggests that the prostaglandin D₂ (PGD₂ ) receptor 2 (DP₂ ) is a principal regulator in the pathophysiology of asthma, because it stimulates and amplifies the inflammatory response in this condition. The DP₂ receptor can be activated by both allergic and nonallergic stimuli, leading to several pro-inflammatory events, including eosinophil activation and migration, release of the type 2 cytokines interleukin (IL)-4, IL-5 and IL-13 from T helper 2 (Th2) cells and innate lymphoid cells type 2 (ILCs), and increased airway smooth muscle mass via recruitment of mesenchymal progenitors to the airway smooth muscle bundle. Activation of the DP₂ receptor pathway has potential downstream effects on asthma pathophysiology, including on airway epithelial cells, mucus hypersecretion, and airway remodelling, and consequently might impact asthma symptoms and exacerbations. Given the broad distribution of DP₂ receptors on immune and structural cells involved in asthma, this receptor is being explored as a novel therapeutic target.

G Protein-Coupled Receptors in Asthma Therapy: Pharmacology and Drug Action

Asthma is a heterogeneous inflammatory disease of the airways that is associated with airway hyperresponsiveness and airflow limitation. Although asthma was once simply categorized as atopic or nonatopic, emerging analyses over the last few decades have revealed a variety of asthma endotypes that are attributed to numerous pathophysiological mechanisms. The classification of asthma by endotype is primarily routed in different profiles of airway inflammation that contribute to bronchoconstriction. Many asthma therapeutics target G protein-coupled receptors (GPCRs), which either enhance bronchodilation or prevent bronchoconstriction. Short-acting and long-acting β 2-agonists are widely used bronchodilators that signal through the activation of the β 2-adrenergic receptor. Short-acting and long-acting antagonists of muscarinic acetylcholine receptors are used to reduce bronchoconstriction by blocking the action of acetylcholine. Leukotriene antagonists that block the signaling of cysteinyl leukotriene receptor 1 are used as an add-on therapy to reduce bronchoconstriction and inflammation induced by cysteinyl leukotrienes. A number of GPCR-targeting asthma drug candidates are also in different stages of development. Among them, antagonists of prostaglandin D2 receptor 2 have advanced into phase III clinical trials. Others, including antagonists of the adenosine A2B receptor and the histamine H4 receptor, are in early stages of clinical investigation. In the past decade, significant research advancements in pharmacology, cell biology, structural biology, and molecular physiology have greatly deepened our understanding of the therapeutic roles of GPCRs in asthma and drug action on these GPCRs. This review summarizes our current understanding of GPCR signaling and pharmacology in the context of asthma treatment. SIGNIFICANCE STATEMENT: Although current treatment methods for asthma are effective for a majority of asthma patients, there are still a large number of patients with poorly controlled asthma who may experience asthma exacerbations. This review summarizes current asthma treatment methods and our understanding of signaling and pharmacology of G protein-coupled receptors (GPCRs) in asthma therapy, and discusses controversies regarding the use of GPCR drugs and new opportunities in developing GPCR-targeting therapeutics for the treatment of asthma.

New treatments for asthma: From the pathogenic role of prostaglandin D2 to the therapeutic effects of fevipiprant

Prostaglandin D₂ (PGD₂) is a pleiotropic mediator, significantly involved in the pathogenesis of type 2 (T2) asthma because of its biologic actions exerted on both immune/inflammatory and airway structural cells. In particular, the pro-inflammatory and pro-remodelling effects of PGD₂ are mainly mediated by stimulation of chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2). This receptor is the target of the oral competitive antagonist fevipiprant, which on the basis of recent phase II studies is emerging as a potential very promising anti-asthma drug. Indeed, fevipiprant appears to be safe and effective, especially in consideration of its ability to inhibit eosinophilic bronchial inflammation and improve forced expiratory volume in one second (FEV₁). Further ongoing phase III trials will definitely clarify if fevipiprant can prospectively become a valid option for an efficacious add-on treatment of moderate-to-severe T2-high asthma.

Effect Of Dual sEH/COX-2 Inhibition on Allergen-Induced Airway Inflammation

Arachidonic acid metabolites resulting from the cyclooxygenase (COX), lipoxygenase, and cytochrome P450 oxidase enzymatic pathways play pro- and anti-inflammatory roles in allergic airway inflammation (AAI) and asthma. Expression of COX-2 and soluble epoxide hydrolase (sEH) are elevated in allergic airways and their enzymatic products (e.g., prostaglandins and diols of epoxyeicosatrienoic acids, respectively) have been shown to participate in the pathogenesis of AAI. Here, we evaluated the outcome of inhibiting the COX-2 and sEH enzymatic pathways with a novel dual inhibitor, PTUPB, in A. alternata-induced AAI. Allergen-challenged mice were administered with 10 or 30 mg/kg of PTUPB, celecoxib (selective COX-2 inhibitor), t-TUCB (selective sEH inhibitor) or vehicle daily by gavage and evaluated for various features of AAI. PTUPB and t-TUCB at 30 mg/kg, but not celecoxib, inhibited eosinophilic infiltration and significantly increased levels of anti-inflammatory EETs in the lung tissue of allergen-challenged mice. t-TUCB significantly inhibited allergen-induced IL-4 and IL-13, while a less pronounced reduction was noted with PTUPB and celecoxib. Additionally, t-TUCB markedly inhibited eotaxin-2, an eosinophil-specific chemokine, which was only marginally reduced by PTUPB and remained elevated in celecoxib-treated mice. PTUPB or t-TUCB administration reversed allergen-induced reduction in levels of various lipid mediators in the lungs, with only a minimal effect noted with celecoxib. Despite the anti-inflammatory effects, PTUPB or t-TUCB did not reduce allergen-induced airway hyperresponsiveness (AHR). However, development of structural changes in the allergic airways, such as mucus hypersecretion and smooth muscle hypertrophy, was significantly inhibited by both inhibitors. Celecoxib, on the other hand, inhibited only airway smooth muscle hypertrophy, but not mucus hypersecretion. In conclusion, dual inhibition of COX-2 and sEH offers no additional advantage relative to sEH inhibition alone in attenuating various features associated with A. alternata-induced AAI, while COX-2 inhibition exerts only moderate or no effect on several of these features. Dual sEH/COX-2 inhibition may be useful in treating conditions where eosinophilic inflammation co-exists with pain-associated inflammation.

Immune Sensing of Aeroallergen-Associated Double-Stranded RNA Triggers an IFN Response and Modulates Type 2 Lung Inflammation

The innate immune sensing of allergens or allergen-associated components regulate the development of type 2 inflammatory responses. However, the underlying molecular basis by which allergens or allergen-associated components are detected by innate immune receptors remains elusive. In this study, we report that the most common aeroallergen, house dust mite (HDM), harbors a dsRNA species (HDM-dsRNA) that can activate TLR3-mediated IFN responses and counteract the development of an uncontrolled type 2 immune response. We demonstrate that the mouse strains defective in the dsRNA-sensing pathways show aggravated type 2 inflammation defined by severe eosinophilia, elevated level of type 2 cytokines, and mucus overproduction in a model of allergic lung inflammation. The inability to sense HDM-dsRNA resulted in significant increases in airway hyperreactivity. We further show that the administration of the purified HDM-dsRNA at a low dose is sufficient to induce an immune response to prevent the onset of a severe type 2 lung inflammation. Collectively, these results unveil a new role for the HDM-dsRNA/TLR3-signaling axis in the modulation of a type 2 lung inflammation in mice.

The prostaglandin D2 receptor 2 pathway in asthma: a key player in airway inflammation

Asthma is characterised by chronic airway inflammation, airway obstruction and hyper-responsiveness. The inflammatory cascade in asthma comprises a complex interplay of genetic factors, the airway epithelium, and dysregulation of the immune response.Prostaglandin D2 (PGD2) is a lipid mediator, predominantly released from mast cells, but also by other immune cells such as TH2 cells and dendritic cells, which plays a significant role in the pathophysiology of asthma. PGD2 mainly exerts its biological functions via two G-protein-coupled receptors, the PGD2 receptor 1 (DP1) and 2 (DP2). The DP2 receptor is mainly expressed by the key cells involved in type 2 immune responses, including TH2 cells, type 2 innate lymphoid cells and eosinophils. The DP2 receptor pathway is a novel and important therapeutic target for asthma, because increased PGD2 production induces significant inflammatory cell chemotaxis and degranulation via its interaction with the DP2 receptor. This interaction has serious consequences in the pulmonary milieu, including the release of pro-inflammatory cytokines and harmful cationic proteases, leading to tissue remodelling, mucus production, structural damage, and compromised lung function. This review will discuss the importance of the DP2 receptor pathway and the current understanding of its role in asthma.

Eicosanoid Control Over Antigen Presenting Cells in Asthma

Asthma is a common lung disease affecting 300 million people worldwide. Allergic asthma is recognized as a prototypical Th2 disorder, orchestrated by an aberrant adaptive CD4+ T helper (Th2/Th17) cell immune response against airborne allergens, that leads to eosinophilic inflammation, reversible bronchoconstriction, and mucus overproduction. Other forms of asthma are controlled by an eosinophil-rich innate ILC2 response driven by epithelial damage, whereas in some patients with more neutrophilia, the disease is driven by Th17 cells. Dendritic cells (DCs) and macrophages are crucial regulators of type 2 immunity in asthma. Numerous lipid mediators including the eicosanoids prostaglandins and leukotrienes influence key functions of these cells, leading to either pro- or anti-inflammatory effects on disease outcome. In this review, we will discuss how eicosanoids affect the functions of DCs and macrophages in the asthmatic lung and how this leads to aberrant T cell differentiation that causes disease.

Influenza in Asthmatics: For Better or for Worse?

Asthma and influenza are two pathologic conditions of the respiratory tract that affect millions worldwide. Influenza virus of the 2009 pandemic was highly transmissible and caused severe respiratory disease in young and middle-aged individuals. Asthma was discovered to be an underlying co-morbidity that led to hospitalizations during this influenza pandemic albeit with less severe outcomes. However, animal studies that investigated the relationship between allergic inflammation and pandemic (p)H1N1 infection, showed that while characteristics of allergic airways disease were exacerbated by this virus, governing immune responses that cause exacerbations may actually protect the host from severe outcomes associated with influenza. To better understand the relationship between asthma and severe influenza during the last pandemic, we conducted a systematic literature review of reports on hospitalized patients with asthma as a co-morbid condition during the pH1N1 season. Herein, we report that numerous other underlying conditions, such as cardiovascular, neurologic, and metabolic diseases may have been underplayed as major drivers of severe influenza during the 2009 pandemic. This review synopses, (1) asthma and influenza independently, (2) epidemiologic data surrounding asthma during the 2009 influenza pandemic, and (3) recent advances in our understanding of allergic host–pathogen interactions in the context of allergic airways disease and influenza in mouse models. Our goal is to showcase possible immunological benefits of allergic airways inflammation as countermeasures for influenza virus infections as a learning tool to discover novel pathways that can enhance our ability to hinder influenza virus replication and host pathology induced thereof.

Activation of Th2 cells downregulates CRTh2 through an NFAT1 mediated mechanism

CRTh2 (encoded by PTGDR2) is a G-protein coupled receptor expressed by Th2 cells as well as eosinophils, basophils and innate lymphoid cells (ILC)2s. Activation of CRTh2, by its ligand prostaglandin (PG)D2, mediates production of type 2 cytokines (IL-4, IL-5 and IL-13), chemotaxis and inhibition of apoptosis. As such, the PGD2-CRTh2 pathway is considered important to the development and maintenance of allergic inflammation. Expression of CRTh2 is mediated by the transcription factor GATA3 during Th2 cell differentiation and within ILC2s. Other than this, relatively little is known regarding the cellular and molecular mechanisms regulating expression of CRTh2. Here, we show using primary human Th2 cells that activation (24hrs) through TCR crosslinking (αCD3/αCD28) reduced expression of both mRNA and surface levels of CRTh2 assessed by flow cytometry and qRT-PCR. This effect took more than 4 hours and expression was recovered following removal of activation. EMSA analysis revealed that GATA3 and NFAT1 can bind independently to overlapping sites within a CRTh2 promoter probe. NFAT1 over-expression resulted in loss of GATA3-mediated CRTh2 promoter activity, while inhibition of NFAT using a peptide inhibitor (VIVIT) coincided with recovery of CRTh2 expression. Collectively these data indicate that expression of CRTh2 is regulated through the competitive action of GATA3 and NFAT1. Though prolonged activation led to NFAT1-mediated downregulation, CRTh2 was re-expressed when stimulus was removed suggesting this is a dynamic mechanism and may play a role in PGD2-CRTh2 mediated allergic inflammation.

Targeting the PGD2/CRTH2/DP1 Signaling Pathway in Asthma and Allergic Disease: Current Status and Future Perspectives

Prostaglandin D₂ (PGD₂) released by degranulating mast cells is believed to play a key role in orchestrating mechanisms of inflammation in allergies and asthma. The biological effects of PGD₂ are mediated by D-prostanoid (DP1), CRTH2 (DP2), and thromboxane prostanoid (TP) receptors. The CRTH2 receptor is involved in induction of migration and activation of T helper type 2 (Th₂) lymphocytes, eosinophils, and basophils; up-regulation of adhesion molecules; and promotion of pro-inflammatory Th₂-type cytokines (interleukin [IL]-4, 5, 13), whereas the DP receptor is associated with relaxation of smooth muscles, vasodilation, inhibition of cell migration, and apoptosis of eosinophils. A number of CRTH2/PGD₂ receptor antagonists have been investigated in asthma and allergic diseases. The CRTH2 antagonist (OC000459) or dual CRTH2 and TP receptor antagonist (ramatroban) were effective in reducing eosinophilia, nasal mucosal swelling, and clinical symptoms of allergic rhinitis, with the latter drug registered for clinical use in this indication. OC000459 and setipiprant reduced the late but not early phase of response in an allergen challenge in atopic asthmatics. In persistent asthma, some molecules induced limited improvement in lung function, quality of life, and asthma symptoms (OC000459, BI671800), but in other trials with AMG 853 and AZ1981 these findings were not confirmed. The clear discrepancy between animal studies and clinical efficacy of CRTH2 antagonism in allergic rhinitis, and lack of efficacy in a general cohort of asthmatics, highlight the issue of patient phenotyping. There is no doubt that the PGD₂/CATH2/DP1 pathway plays a key role in allergic inflammation and further studies with selective or combined antagonisms in well defined cohorts of patients are needed.

Biological therapies for eosinophilic gastrointestinal diseases

The scientific basis and the clinical application of mAb therapies that target specific immunologic pathways for eosinophilic gastrointestinal diseases are areas of active interest. There is a growing recognition of a subset of patients with eosinophilic esophagitis whose disease does not respond well to topical steroids or elimination diets. In addition, long-term use of corticosteroids presents possible risks that are currently being evaluated. Systemic therapy with a biologic agent offers potential advantages as a global approach that could limit the need for multiple, locally active medical therapies and allergen avoidance. The identification of novel biologic strategies is ongoing, and the recent validation of instruments and outcome measures to assess disease activity has proved essential in demonstrating efficacy. Studies using biologics that target IL-13 pathways in the treatment of eosinophilic esophagitis have demonstrated substantial promise.

Elevated levels of circulating CD4(+) CRTh2(+) T cells characterize severe asthma

BACKGROUND

Chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTh2) is a receptor for PGD2 and expressed by T cells, eosinophils, basophils, and ILC2 cells. CRTh2 expression by CD4(+) T cells identifies the Th2 subset, and these cells have been characterized as allergen-specific central memory Th2 cells. Recently, activation of the PGD2 -CRTh2 pathway in the lungs was associated with severe asthma.

OBJECTIVE

To assess circulating levels of Th2 cells and related mediators in severe asthma and those who experience asthma exacerbations.

METHODS

Peripheral blood cells expressing CRTh2 were characterized by flow cytometry and qRT-PCR. Serum IL-13 and PGD2 were measured by ELISA and compared with asthma severity and tendency to exacerbate.

RESULTS

Severe asthmatics had more circulating CD4(+) CRTh2(+) T cells, CRTh2 and GATA3 mRNA, and a higher level of serum IL-13 compared to mild/moderate asthmatics. The proportion of CD4(+) CRTh2(+) T cells was associated with lower lung function and was highest in severe asthmatics that exacerbated in the last year. Circulating CD4(+) CRTh2(+) T cells, unlike eosinophils, were positively correlated with inhaled steroid dose.

CONCLUSIONS AND CLINICAL RELEVANCE

Elevated levels of circulating CD4(+) CRTh2(+) T cells are a feature of severe asthma, despite high-dose corticosteroids. Tracking the systemic level of these cells may help identify type 2 severe asthmatics at risk of exacerbation.

Pulmonary C-fiber degeneration downregulates IFN-γ receptor 1 via IFN-α induction to attenuate RSV-induced airway hyperresponsiveness

Respiratory syncytial virus (RSV) is a leading cause of respiratory infection in infants. Unfortunately, no effective vaccine or treatment against RSV is currently available. Pulmonary C-fibers (PCFs) are critical for regulating pulmonary inflammation and airway hyperresponsiveness (AHR). We previously reported that IFN-γ partially mediated RSV-induced airway disorders. In this study, we found that PCF degeneration alleviated RSV-induced airway inflammation, especially AHR by downregulating IFN-γ receptor 1 (IFNGR1), but had no effect on IFN-γ induction. In contrast, PCF degeneration actually increased IFN-α/β levels, as were the levels of STAT1 and phosphorylated STAT1 (pSTAT1). Exogenous IFN-α treatment induced STAT1 activation and downregulated IFNGR1 expression. These results suggest that PCFs affect IFNGR1 expression by inducing IFN-α to regulate IFN-γ-mediated airway inflammation and AHR. Thus, targeting PCFs activation may help control RSV-induced airway disorders, especially AHR, even with the presence of inflammation.

The role of endosomal toll-like receptors in asthma

Asthma is a heterogeneous inflammatory disease caused by association of genetic and environmental factors and its incidence has significantly increased over the latest years. The clinical manifestations of asthma are the result of airway hyper-reactivity to a variety of triggers such as aeroallergens, viral and bacterial components. Toll-like receptors (TLRs) are pathogen associated molecular pattern receptors, which are also expressed in the lung tissue as well as in several cells of the innate and adaptive immune system. Ligation of TLRs results in alterations in the expression of several inflammatory and anti-inflammatory mediators, which are known to be involved in the pathogenesis of asthma. The endosomal TLRs have been shown to be associated with the induction of asthmatic inflammation (TLR3), and with disease exacerbations (TLR7, TLR8 and TLR9). Targeting these receptors seems to be an effective choice for suppressing airway inflammation, eosinophilia and airway hyperresponsiveness in asthmatic patients. In this review we provide information regarding endosomal TLRs and their role in the pathogenesis of asthma as well as their potential use as targets for the development of novel treatments for the therapy of asthma.

Investigational prostaglandin D2 receptor antagonists for airway inflammation

INTRODUCTION

By activating DP1 and DP2 receptors on immune and non-immune cells, prostaglandin D2 (PGD2), a major metabolic product of cyclo-oxygenase pathway released after IgE-mediated mast cell activation, has pro-inflammatory effects, which are relevant to the pathophysiology of allergic airway disease. At least 15 selective, orally active, DP2 receptor antagonists and one DP1 receptor antagonist (asapiprant) are under development for asthma and/or allergic rhinitis.

AREAS COVERED

In this review, the authors cover the pharmacology of PGD2 and PGD2 receptor antagonists and look at the preclinical, phase I and phase II studies with selective DP1 and DP2 receptor antagonists.

EXPERT OPINION

Future research should aim to develop once daily compounds and increase the drug clinical potency which, apart from OC000459 and ADC-3680, seems to be relatively low. Further research and development of DP2 receptor antagonists is warranted, particularly in patients with severe uncontrolled asthma, whose management is a top priority. Pediatric studies, which are not available, are required for assessing the efficacy and safety of this novel drug class in children with asthma and allergic rhinitis. Studies on the efficacy of DP2 receptor antagonists in various asthma phenotypes including: smokers, obese subjects, early vs late asthma onset, fixed vs reversible airflow limitation, are required for establishing their pharmacotherapeutic role.

Neural dysfunction following respiratory viral infection as a cause of chronic cough hypersensitivity

Respiratory viral infections are a common cause of acute coughing, an irritating symptom for the patient and an important mechanism of transmission for the virus. Although poorly described, the inflammatory consequences of infection likely induce coughing by chemical (inflammatory mediator) or mechanical (mucous) activation of the cough-evoking sensory nerves that innervate the airway wall. For some individuals, acute cough can evolve into a chronic condition, in which cough and aberrant airway sensations long outlast the initial viral infection. This suggests that some viruses have the capacity to induce persistent plasticity in the neural pathways mediating cough. In this brief review we present the clinical evidence of acute and chronic neural dysfunction following viral respiratory tract infections and explore possible mechanisms by which the nervous system may undergo activation, sensitization and plasticity.

MicroRNA-26a/-26b-COX-2-MIP-2 Loop Regulates Allergic Inflammation and Allergic Inflammation-promoted Enhanced Tumorigenic and Metastatic Potential of Cancer Cells

Cyclooxgenase-2 (COX-2) knock-out mouse experiments showed that COX-2 was necessary for in vivo allergic inflammation, such as passive cutaneous anaphylaxis, passive systemic anaphylaxis, and triphasic cutaneous allergic reaction. TargetScan analysis predicted COX-2 as a target of miR-26a and miR-26b. miR-26a/-26b decreased luciferase activity associated with COX-2-3'-UTR. miR-26a/-26b exerted negative effects on the features of in vitro and in vivo allergic inflammation by targeting COX-2. ChIP assays showed the binding of HDAC3 and SNAIL, but not COX-2, to the promoter sequences of miR-26a and miR-26b. Cytokine array analysis showed that the induction of chemokines, such as MIP-2, in the mouse passive systemic anaphylaxis model occurred in a COX-2-dependent manner. ChIP assays showed the binding of HDAC3 and COX-2 to the promoter sequences of MIP-2. In vitro and in vivo allergic inflammation was accompanied by the increased expression of MIP-2. miR-26a/-26b negatively regulated the expression of MIP-2. Allergic inflammation enhanced the tumorigenic and metastatic potential of cancer cells and induced positive feedback involving cancer cells and stromal cells, such as mast cells, macrophages, and endothelial cells. miR-26a mimic and miR-26b mimic negatively regulated the positive feedback between cancer cells and stromal cells and the positive feedback among stromal cells. miR-26a/-26b negatively regulated the enhanced tumorigenic potential by allergic inflammation. COX-2 was necessary for the enhanced metastatic potential of cancer cells by allergic inflammation. Taken together, our results indicate that the miR26a/-26b-COX-2-MIP-2 loop regulates allergic inflammation and the feedback relationship between allergic inflammation and the enhanced tumorigenic and metastatic potential.

A new era of targeting the ancient gatekeepers of the immune system: toll-like agonists in the treatment of allergic rhinitis and asthma

Toll-like receptors (TLR) belong to a large family of pattern recognition receptors known as the ancient 'gatekeepers' of the immune system. TLRs are located at the first line of defense against invading pathogens as well as aeroallergens, making them interesting targets to modulate the natural history of respiratory allergy. Agonists of TLRs have been widely employed in therapeutic or prophylactic preparations useful for asthma/allergic rhinitis (AR) patients. MPL® (a TLR4 agonist) and the CpG oligodeoxynucleotide of 1018 ISS, a TLR9 agonist, show strong immunogenicity effects that make them appropriate adjuvants for allergy vaccines. Targeting the TLRs can enhance the efficacy of specific allergen immunotherapy, currently the only available 'curative' treatment for respiratory allergies. In addition, intranasal administration of AZD8848 (a TLR7 agonist) and VTX-1463 (a TLR8 agonist) as stand-alone therapeutics have revealed efficacy in the relief of the symptoms of AR patients. No anaphylaxis has been so far reported with such compounds targeting TLRs, with the most common adverse effects being transient and local irritation (e.g. redness, swelling and pruritus). Many other compounds that target TLRs have been found to suppress airway inflammation, eosinophilia and airway hyper-responsiveness in various animal models of allergic inflammation. Indeed, in the future a wide variability of TLR agonists and even antagonists that exhibit anti-asthma/AR effects are likely to emerge.

Role of cyclooxygenase-2 in exacerbation of allergen-induced airway remodeling by multiwalled carbon nanotubes

The emergence of nanotechnology has produced a multitude of engineered nanomaterials such as carbon nanotubes (CNTs), and concerns have been raised about their effects on human health, especially for susceptible populations such as individuals with asthma. Multiwalled CNTs (MWCNTs) have been shown to exacerbate ovalbumin (OVA)-induced airway remodeling in mice. Moreover, cyclooxygenase-2 (COX-2) has been described as a protective factor in asthma. We postulated that COX-2-deficient (COX-2(-/-)) mice would be susceptible to MWCNT-induced exacerbations of allergen-induced airway remodeling, including airway inflammation, fibrosis, and mucus-cell metaplasia (i.e., the formation of goblet cells). Wild-type (WT) or COX-2(-/-) mice were sensitized to OVA to induce allergic airway inflammation before a single dose of MWCNTs (4 mg/kg) delivered to the lungs by oropharyngeal aspiration. MWCNTs significantly increased OVA-induced lung inflammation and mucus-cell metaplasia in COX-2(-/-) mice compared with WT mice. However, airway fibrosis after exposure to allergen and MWCNTs was no different between WT and COX-2(-/-) mice. Concentrations of certain prostanoids (prostaglandin D2 and thromboxane B2) were enhanced by OVA or MWCNTs in COX-2(-/-) mice. No differences in COX-1 mRNA concentrations were evident between WT and COX-2(-/-) mice treated with OVA and MWCNTs. Interestingly, MWCNTs significantly enhanced allergen-induced cytokines involved in Th2 (IL-13 and IL-5), Th1 (CXCL10), and Th17 (IL-17A) inflammatory responses in COX-2(-/-) mice, but not in WT mice. We conclude that exacerbations of allergen-induced airway inflammation and mucus-cell metaplasia by MWCNTs are enhanced by deficiencies in COX-2, and are associated with the activation of a mixed Th1/Th2/Th17 immune response.

Prostaglandin D2 activates group 2 innate lymphoid cells through chemoattractant receptor-homologous molecule expressed on TH2 cells

Background

Activation of the group 2 innate lymphoid cell (ILC2) population leads to production of the classical type 2 cytokines, thus promoting type 2 immunity. Chemoattractant receptor-homologous molecule expressed on T_H2 cells (CRTH2), a receptor for prostaglandin D₂ (PGD₂), is expressed by human ILC2s. However, the function of CRTH2 in these cells is unclear.

Objectives

We sought to determine the role of PGD₂ and CRTH2 in human ILC2s and compare it with that of the established ILC2 activators IL-25 and IL-33.

Methods

The effects of PGD₂, IL-25, and IL-33 on the cell migration, cytokine production, gene regulation, and receptor expression of ILC2s were measured with chemotaxis, ELISA, Luminex, flow cytometry, quantitative RT-PCR, and QuantiGene assays. The effects of PGD₂ under physiologic conditions were evaluated by using the supernatant from activated mast cells.

Results

PGD₂ binding to CRTH2 induced ILC2 migration and production of type 2 cytokines and many other cytokines. ILC2 activation through CRTH2 also upregulated the expression of IL-33 and IL-25 receptor subunits (ST2 and IL-17RA). The effects of PGD₂ on ILC2s could be mimicked by the supernatant from activated human mast cells and inhibited by a CRTH2 antagonist.

Conclusions

PGD₂ is an important and potent activator of ILC2s through CRTH2 mediating strong proallergic inflammatory responses. Through IgE-mediated mast cell degranulation, these innate cells can also contribute to adaptive type 2 immunity; thus CRTH2 bridges the innate and adaptive pathways in human ILC2s.

Viral infections in asthma and COPD

Airway viral infections are associated with the pathogenesis of asthma and COPD. It has been argued that respiratory syncytial virus (RSV) infection in infancy is a probable causal factor in the development of pediatric asthma. RSV infections tend to induce Th2-biased immune responses in the host airways. RSV infection, atopy, and low pulmonary function in neonates may work synergistically toward the development of pediatric asthma. Human rhinovirus (HRV) is a representative virus associated with the exacerbation of asthma in both children and adults. Viral infections trigger innate immune responses including granulocytic inflammation and worsen the underlying inflammation due to asthma and COPD. The innate immune responses involve type-I and -III interferon (IFN) production, which plays an important role in anti-viral responses, and the airway epithelia of asthmatics reportedly exhibit defects in the virus-induced IFN responses, which renders these individuals more susceptible to viral infection. A similarly impaired IFN response is seen in COPD, and several investigators propose that latent adenoviral infection may be involved in COPD development. Persistent RSV infections were detected in a sub-population of patients with COPD and were associated with the accelerated decline of lung function. The virus-induced upregulation of co-inhibitory molecules in the airway epithelium partly accounts for the persistent infections. Experimental animal models for virus-asthma/COPD interactions have shed light on the underlying immune mechanisms and are expected to help develop novel approaches to treat respiratory diseases.

Anti-inflammatory activity of edible brown alga Saccharina japonica and its constituents pheophorbide a and pheophytin a in LPS-stimulated RAW 264.7 macrophage cells

Anti-inflammatory activity of Saccharina japonica and its active components was evaluated via in vitro inhibitory activities against lipopolysaccharide (LPS)-induced nitric oxide (NO) production, inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) expression in RAW 264.7 murine macrophage cells. Since the methanolic extract of S. japonica showed strong anti-inflammatory activity, it was fractionated with several solvents. Among the fractions, the ethyl acetate fraction demonstrated the highest inhibition of LPS-induced NO production (IC50=25.32μg/mL), followed by the CH2Cl2 fraction (IC50=75.86μg/mL). Considering the yield and anti-inflammatory potential together, the CH2Cl2 fraction was selected for chromatographic separation to yield two active porphyrin derivatives, pheophorbide a and pheophytin a, together with an inactive fucoxanthin. In contrast to fucoxanthin, pheophorbide a and pheophytin a showed dose-dependent inhibition against LPS-induced NO production at nontoxic concentrations in RAW 264.7 cells. Both compounds also suppressed the expression of iNOS proteins, while they did not inhibit the COX-2 expression in LPS-stimulated macrophages. These results indicate that pheophorbide a and pheophytin a are two important candidates of S. japonica as anti-inflammatory agents which can inhibit the production of NO via inhibition of iNOS protein expression. Thus, these compounds hold great promise for use in the treatment of various inflammatory diseases.

Mast cells contribute to double-stranded RNA-induced augmentation of airway eosinophilia in a murine model of asthma

BACKGROUND

Clinical studies showed the contribution of viral infection to the development of asthma. Although mast cells have multiple roles in the pathogenesis of allergic asthma, their role of in the virus-associated pathogenesis of asthma remains unknown. Most respiratory viruses generate double-stranded (ds) RNA during their replication. dsRNA provokes innate immune responses. We recently showed that an administration of polyinocinic polycytidilic acid (poly IC), a mimetic of viral dsRNA, during allergen sensitization augments airway eosinophilia and hyperresponsiveness in mice via enhanced production of IL-13.

METHODS

The effect of poly IC on allergen-induced airway eosinophilia was investigated for mast cell-conserved Kit+/+ mice and -deficient KitW/KitW-v mice. The outcome of mast cell reconstitution was further investigated.

RESULTS

Airway eosinophilia and IL-13 production were augmented by poly IC in Kit+/+ mice but not in KitW/KitW-v mice. When KitW/KitW-v mice were reconstituted with bone marrow-derived mast cells (BMMCs), the augmentation was restored. The augmentation was not induced in the mice systemically deficient for TIR domain-containing adaptor-inducing IFN-β (TRIF) or interferon regulatory factor (IRF)-3, both mediate dsRNA-triggered innate immune responses. The augmentation was, however, restored in KitW/KitW-v mice reconstituted with TRIF-deficient or IRF-3-deficient BMMCs. Although leukotriene B4 and prostaglandin D2 are major lipid mediators released from activated mast cells, no their contribution was shown to the dsRNA-induced augmentation of airway eosinophilia.

CONCLUSIONS

We conclude that mast cells contribute to dsRNA-induced augmentation of allergic airway inflammation without requiring direct activation of mast cells with dsRNA or involvement of leukotriene B4 or prostaglandin D2.

Prostaglandin D2 inhibits wound-induced hair follicle neogenesis through the receptor, Gpr44

Prostaglandins (PGs) are key inflammatory mediators involved in wound healing and regulating hair growth; however, their role in skin regeneration after injury is unknown. Using wound-induced hair follicle neogenesis (WIHN) as a marker of skin regeneration, we hypothesized that PGD₂ decreases follicle neogenesis. PGE₂ and PGD₂ were elevated early and late respectively during wound healing. The levels of WIHN, lipocalin-type prostaglandin D₂ synthase (Ptgds) and its product PGD₂ each varied significantly among background strains of mice after wounding and all correlated such that the highest Ptgds and PGD₂ levels were associated with the lowest amount of regeneration. Additionally, an alternatively spliced transcript variant of Ptgds missing exon 3 correlated with high regeneration in mice. Exogenous application of PGD₂ decreased WIHN in wild type mice and PGD₂ receptor Gpr44 null mice showed increased WIHN compared to strain-matched control mice. Furthermore, Gpr44 null mice were resistant to PGD₂-induced inhibition of follicle neogenesis. In all, these findings demonstrate that PGD₂ inhibits hair follicle regeneration through the Gpr44 receptor and imply that inhibition of PGD₂ production or Gpr44 signaling will promote skin regeneration.

CRTH2 Antagonists

Prostaglandin D2 (PGD2) plays a key role in many of the physiological markings of allergic inflammation including vasodilation, bronchoconstriction, vascular permeability and lymphocyte recruitment. The action of this molecule is elicited through its two primary receptors, DP and CRTH2. Activation of CRTH2 leads to lymphocyte chemotaxis, potentiation of histamine release from basophils, production of inflammatory cytokines (IL-4, IL-5 and IL-13) by Th2 cells, eosinophil degranulation and prevention of Th2 cell apoptosis. As such, antagonism of CRTH2 has been reported to ameliorate the symptoms associated with various allergen challenge animal models including murine antigen induced lung inflammation, murine cigarette smoke induced lung inflammation, murine allergic rhinitis, guinea pig PGD2-induced airflow obstruction, guinea pig airway hyper-responsiveness, sheep airway hyper-responsiveness and murine contact hypersensitivity. CRTH2 antagonists fall into four broad categories: tricyclic ramatroban analogues, indole acetic acids, phenyl/phenoxy acetic acids and non-acid-containing tetrahydroquinolines. Numerous CRTH2 antagonists have been advanced into the clinic and early reports from two Phase II trials suggest promising activity in the alleviation of atopic symptoms.

CRTH2 is a critical regulator of neutrophil migration and resistance to polymicrobial sepsis

Although arachidonic acid cascade has been shown to be involved in sepsis, little is known about the role of PGD(2) and its newly found receptor, chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2), on the septic response. Severe sepsis is associated with the failure of neutrophil migration. To investigate whether CRTH2 influences neutrophil recruitment and the lethality during sepsis, sepsis was induced by cecal ligation and puncture (CLP) surgery in mice. CRTH2 knockout (CRTH2(-/-)) mice were highly resistant to CLP-induced sepsis, which was associated with lower bacterial load and lower production of TNF-α, IL-6, and CCL3. IL-10, an anti-inflammatory cytokine, was higher in CRTH2(-/-) mice, blunting CLP-induced lethality in CRTH2(-/-) mice. Neutrophil accumulation in the peritoneum was more pronounced after CLP in CRTH2(-/-) mice, which was associated with higher CXCR2 levels in circulating neutrophils. Furthermore, sepsis caused a decrease in the level of acetylation of histone H3, an activation mark, at the CXCR2 promoter in wild-type neutrophils, suggesting that CXCR2 expression levels are epigenetically regulated. Finally, both pharmacological depletion of neutrophils and inhibition of CXCR2 abrogated the survival benefit in CRTH2(-/-) mice. These results demonstrate that genetic ablation of CRTH2 improved impaired neutrophil migration and survival during severe sepsis, which was mechanistically associated with epigenetic-mediated CXCR2 expression. Thus, CRTH2 is a potential therapeutic target for polymicrobial sepsis.

Prostaglandins and chronic inflammation

Chronic inflammation is the basis of various chronic illnesses including cancer and vascular diseases. However, much has yet to be learned how inflammation becomes chronic. Prostaglandins (PGs) are well established as mediators of acute inflammation, and recent studies in experimental animals have provided evidence that they also function in transition to and maintenance of chronic inflammation. One role PGs play in such processes is amplification of cytokine signaling. As such, PGs can facilitate acquired immunity and induce long-lasting immune inflammation. PGs also contribute to chronic inflammation by making a positive feedback loop and/or by inducing chemokines and recruiting inflammatory cells to alternate active cell populations at affected sites. PGs also contribute to tissue remodeling as seen in angiogenesis and fibrosis. Although such roles of PGs should be verified in human diseases, these findings suggest that PG signaling is a promising therapeutic target of chronic inflammatory diseases.

PGH1, the precursor for the anti-inflammatory prostaglandins of the 1-series, is a potent activator of the pro-inflammatory receptor CRTH2/DP2

Prostaglandin H₁ (PGH₁) is the cyclo-oxygenase metabolite of dihomo-γ-linolenic acid (DGLA) and the precursor for the 1-series of prostaglandins which are often viewed as “anti-inflammatory”. Herein we present evidence that PGH₁ is a potent activator of the pro-inflammatory PGD₂ receptor CRTH2, an attractive therapeutic target to treat allergic diseases such as asthma and atopic dermatitis. Non-invasive, real time dynamic mass redistribution analysis of living human CRTH2 transfectants and Ca²⁺ flux studies reveal that PGH₁ activates CRTH2 as PGH₂, PGD₂ or PGD₁ do. The PGH₁ precursor DGLA and the other PGH₁ metabolites did not display such effect. PGH₁ specifically internalizes CRTH2 in stable CRTH2 transfectants as assessed by antibody feeding assays. Physiological relevance of CRTH2 ligation by PGH₁ is demonstrated in several primary human hematopoietic lineages, which endogenously express CRTH2: PGH₁ mediates migration of and Ca²⁺ flux in Th2 lymphocytes, shape change of eosinophils, and their adhesion to human pulmonary microvascular endothelial cells under physiological flow conditions. All these effects are abrogated in the presence of the CRTH2 specific antagonist TM30089. Together, our results identify PGH₁ as an important lipid intermediate and novel CRTH2 agonist which may trigger CRTH2 activation in vivo in the absence of functional prostaglandin D synthase.

Synthetic double-stranded RNA enhances airway inflammation and remodelling in a rat model of asthma

Respiratory viral infections are frequently associated with exacerbations of asthma. Double-stranded RNA (dsRNA) produced during viral infections may be one of the stimuli for exacerbation. We aimed to assess the potential effect of dsRNA on certain aspects of chronic asthma through the administration of polyinosine-polycytidylic acid (poly I:C), synthetic dsRNA, to a rat model of asthma. Brown Norway rats were sensitized to ovalbumin and challenged three times to evoke airway remodelling. The effect of poly I:C on the ovalbumin-induced airway inflammation and structural changes was assessed from bronchoalveolar lavage fluid and histological findings. The expression of cytokines and chemokines was evaluated by real-time quantitative reverse transcription PCR and ELISA. Ovalbumin-challenged animals showed an increased number of total cells and eosinophils in bronchoalveolar lavage fluid compared with PBS-challenged controls. Ovalbumin-challenged animals treated with poly I:C showed an increased number of total cells and neutrophils in bronchoalveolar lavage fluid compared with those without poly I:C treatment. Ovalbumin-challenged animals showed goblet cell hyperplasia, increased airway smooth muscle mass, and proliferation of both airway epithelial cells and airway smooth muscle cells. Treatment with poly I:C enhanced these structural changes. Among the cytokines and chemokines examined, the expression of interleukins 12 and 17 and of transforming growth factor-β(1) in ovalbumin-challenged animals treated with poly I:C was significantly increased compared with those of the other groups. Double-stranded RNA enhanced airway inflammation and remodelling in a rat model of bronchial asthma. These observations suggest that viral infections may promote airway remodelling.

DP2 (CRTh2) antagonism reduces ocular inflammation induced by allergen challenge and respiratory syncytial virus

BACKGROUND

Allergic conjunctivitis is characterized by itchy, watery and swollen eyes which occur in response to exposure to seasonal or environmental allergens. The early phase reaction of allergic conjunctivitis is primarily mediated by mast cell degranulation while the late phase reaction is driven by Th2 cells and eosinophils. Prostaglandin D(2) (PGD(2)), released from mast cells, is present in allergic conjunctival tears and may elicit classical allergic responses via interaction with the high-affinity DP2 receptor (chemoattractant receptor-homologous molecule expressed on Th2 cells, CRTh2). Furthermore, antagonism of this receptor is well known to inhibit eosinophil chemotaxis, basophil activation and Th2 cytokine production. PGD(2), therefore, may be involved in both early and late phase reactions in response to allergen challenge.

METHODS

Thus, we explored whether our novel and selective DP2 antagonist AM156 would be efficacious in animal models of allergic conjunctivitis. Furthermore, as respiratory syncytial virus (RSV) has been implicated in the pathogenesis of allergic conjunctivitis, we examined the effects of DP2 antagonism in a murine model of RSV ocular infection.

RESULTS

Utilizing a guinea pig ovalbumin model and a murine ragweed model we demonstrated that AM156 reduces redness, discharge and swelling in response to allergen challenge. These effects were equal to or greater than those of current clinical treatment options for allergic conjunctivitis including topical corticosteroids and a dual-mechanism antihistamine and decongestant. AM156 significantly reduced RSV-induced ocular inflammation and IL-4 production.

CONCLUSION

These results suggest that a topical DP2 antagonist such as AM156 may represent a novel therapeutic for allergic conjunctivitis.

Analysis of two major intracellular phospholipases A(2) (PLA(2)) in mast cells reveals crucial contribution of cytosolic PLA(2)α, not Ca(2+)-independent PLA(2)β, to lipid mobilization in proximal mast cells and distal fibroblasts

Mast cells release a variety of mediators, including arachidonic acid (AA) metabolites, to regulate allergy, inflammation, and host defense, and their differentiation and maturation within extravascular microenvironments depend on the stromal cytokine stem cell factor. Mouse mast cells express two major intracellular phospholipases A(2) (PLA(2)s), namely group IVA cytosolic PLA(2) (cPLA(2)α) and group VIA Ca(2+)-independent PLA(2) (iPLA(2)β), and the role of cPLA(2)α in eicosanoid synthesis by mast cells has been well documented. Lipidomic analyses of mouse bone marrow-derived mast cells (BMMCs) lacking cPLA(2)α (Pla2g4a(-/-)) or iPLA(2)β (Pla2g6(-/-)) revealed that phospholipids with AA were selectively hydrolyzed by cPLA(2)α, not by iPLA(2)β, during FcεRI-mediated activation and even during fibroblast-dependent maturation. Neither FcεRI-dependent effector functions nor maturation-driven phospholipid remodeling was impaired in Pla2g6(-/-) BMMCs. Although BMMCs did not produce prostaglandin E(2) (PGE(2)), the AA released by cPLA(2)α from BMMCs during maturation was converted to PGE(2) by microsomal PGE synthase-1 (mPGES-1) in cocultured fibroblasts, and accordingly, Pla2g4a(-/-) BMMCs promoted microenvironmental PGE(2) synthesis less efficiently than wild-type BMMCs both in vitro and in vivo. Mice deficient in mPGES-1 (Ptges(-/-)) had an augmented local anaphylactic response. These results suggest that cPLA(2)α in mast cells is functionally coupled, through the AA transfer mechanism, with stromal mPGES-1 to provide anti-anaphylactic PGE(2). Although iPLA(2)β is partially responsible for PGE(2) production by macrophages and dendritic cells, it is dispensable for mast cell maturation and function.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Prostaglandin D2 regulates CD4+ memory T cell trafficking across blood vascular endothelium and primes these cells for clearance across lymphatic endothelium

Memory lymphocytes support inflammatory and immune responses. To do this, they enter tissue via blood vascular endothelial cells (BVEC) and leave tissue via lymphatic vascular endothelial cells (LVEC). In this study, we describe a hierarchy of signals, including novel regulatory steps, which direct the sequential migration of human T cells across the blood and the lymphatic EC. Cytokine-stimulated (TNF and IFN) human BVEC preferentially recruited memory T cells from purified PBL. Lymphocyte recruitment from flow could be blocked using a function-neutralizing Ab against CXCR3. However, a receptor antagonist directed against the PGD(2) receptor DP2 (formerly chemoattractant receptor-homologous molecule expressed on Th2 cells) inhibited transendothelial migration, demonstrating that the sequential delivery of the chemokine and prostanoid signals was required for efficient lymphocyte recruitment. CD4(+) T cells recruited by BVEC migrated with significantly greater efficiency across a second barrier of human LVEC, an effect reproduced by the addition of exogenous PGD(2) to nonmigrated cells. Migration across BVEC or exogenous PGD(2) modified the function, but not the expression, of CCR7, so that chemotaxis toward CCL21 was significantly enhanced. Thus, chemokines may not regulate all stages of lymphocyte migration during inflammation, and paradigms describing their trafficking may need to account for the role of PGD(2).

Protective role of 14-deoxy-11,12-didehydroandrographolide, a noncytotoxic analogue of andrographolide, in allergic airway inflammation

Our group recently reported novel anti-inflammatory effects of andrographolide (2), a bioactive molecule isolated from Andrographis paniculata, in a mouse asthma model. However, 2 has been shown to possess cytotoxic activity. 14-Deoxy-11,12-didehydroandrographolide (1) is an analogue of 2 that can be isolated from A. paniculata. We hypothesized that 1 retains the anti-inflammatory effects for asthma but is devoid of cytotoxicity. In contrast to 2, 1 did not elicit any cytotoxic activity in A549 and BEAS-2B human lung epithelial cells and rat basophilic leukemia (RBL)-2H3 cells using a MTS assay. Compound 1 dose-dependently inhibited ovalbumin (OVA)-induced increases in total and eosinophil counts, IL-4, IL-5, and IL-13 levels in lavage fluid, and serum OVA-specific IgE level in a mouse asthma model. Compound 1 attenuated OVA-induced airway eosinophilia, mucus production, mast cell degranulation, pro-inflammatory biomarker expression in lung tissues, and airway hyper-responsiveness. This substance also blocked p65 nuclear translocation and DNA-binding activity in the OVA-challenged lung and in TNF-α-stimulated human lung epithelial cells. The present findings reveal for the first time that 1 retains the anti-inflammatory activities of 2 for asthma probably through the inhibition of NF-κB. 14-Deoxy-11,12-didehydroandrographolide (1) may be considered as a safer analogue of 2 for the potential treatment of asthma.

Viral stimuli trigger exaggerated thymic stromal lymphopoietin expression by chronic obstructive pulmonary disease epithelium: role of endosomal TLR3 and cytosolic RIG-I-like helicases

BACKGROUND

Rhinovirus (RV)-induced chronic obstructive pulmonary disease (COPD) exacerbations exhibit TH(2)-like inflammation. We hypothesized that RV-infected bronchial epithelial cells (BEC) overproduce TH(2)-switching hub cytokine, thymic stromal lymphopoietin (TSLP) in COPD.

METHODS

Primary BEC from healthy (HBEC) and from COPD donors (COPD-BEC) were grown in 12-well plates, infected with RV16 (0.5-5 MOI) or stimulated with agonists for either toll-like receptor (TLR) 3 (dsRNA, 0.1-10 μg/ml) or RIG-I-like helicases (dsRNA-LyoVec, 0.1-10 μg/ml). Cytokine mRNA and protein were determined (RTqPCR; ELISA).

RESULTS

dsRNA dose-dependently evoked cytokine gene overproduction of TSLP, CXCL8 and TNF-α in COPD-BEC compared to HBEC. This was confirmed using RV16 infection. IFN-β induction did not differ between COPD-BEC and HBEC. Endosomal TLR3 inhibition by chloroquine dose-dependently inhibited dsRNA-induced TSLP generation and reduced generation of CXCL8, TNF-α, and IFN-β. Stimulation of cytosolic viral sensors (RIG-I-like helicases) with dsRNA-LyoVec increased production of CXCL8, TNF-α, and IFN-β, but not TSLP.

CONCLUSIONS

Endosomal TLR3-stimulation, by dsRNA or RV16, induces overproduction of TSLP in COPD-BEC. dsRNA- and RV-induced overproduction of TNF-α and CXCL8 involves endosomal TLR3 and cytosolic RIG-I-like helicases and so does the generation of IFN-β in COPD-BEC. RV16 and dsRNA-induced epithelial TSLP may contribute to pathogenic effects at exacerbations and development of COPD.

Role of prostaglandin D2 receptor CRTH2 in sustained eosinophil accumulation in the airways of mice with chronic asthma

The prostaglandin D(2) (PGD(2))/CRTH2 pathway is important for eosinophil trafficking in vitro; however, genetic deficiency of CRTH2 does not suppress in vivo eosinophilic airway inflammation in acute models of asthma, and the role of CRTH2 in the pathogenesis of asthma is still ambiguous. Therefore, in the present study we explored whether the PGD(2)/CRTH2 pathway could affect the phenotypes of chronic asthma. Either CRTH2-deficient (CRTH2-/-) or wild-type mice were sensitized and exposed to ovalbumin (OVA) for 3 days (acute model) or 6 weeks (chronic model). While the magnitude of the acute eosinophilic inflammation was equivalent between CRTH2-/- and wild-type mice, the number of inflammatory cells and eosinophils in bronchoalveolar lavage fluid after chronic OVA exposure was significantly reduced in CRTH2-/- mice (18.0 ± 2.6 × 10(4) cells and 2.0 ± 0.5 × 10(4) cells) compared to wild-type mice (27.9 ± 2.5 × 10(4) cells and 6.8 ± 1.1 × 10(4) cells, p < 0.001). On the contrary, no difference was observed between CRTH2-/- and wild-type mice in terms of airway hyperresponsiveness or remodeling (goblet cell hyperplasia) in the chronic model of asthma. In conclusion, CRTH2 that mediates PGD(2) activity is essential for sustained eosinophilic inflammation in the airways, and its antagonists could exert an anti-inflammatory effect in chronic asthma.