Prostaglandin E2 inhibits mast cell-dependent bronchoconstriction in human small airways through the E prostanoid subtype 2 receptor

Leukotriene E4 induces airflow obstruction and mast cell activation through the cysteinyl leukotriene type 1 receptor

BACKGROUND

Leukotriene (LT) E₄ is the final active metabolite among the cysteinyl leukotrienes (CysLTs). Animal studies have identified a distinct LTE₄ receptor, suggesting that current cysteinyl leukotriene type 1 (CysLT₁) receptor antagonists can provide incomplete inhibition of CysLT responses.

OBJECTIVE

We tested this hypothesis by assessing the influence of the CysLT₁ antagonist montelukast on responses induced by means of inhalation of LTE₄ in asthmatic patients.

METHODS

Fourteen patients with mild intermittent asthma and 2 patients with aspirin-exacerbated respiratory disease received 20 mg of montelukast twice daily and placebo for 5 to 7 days in a randomized, double-blind, crossover study (NCT01841164). The PD₂₀ value was determined at the end of each treatment period based on an increasing dose challenge. Measurements included lipid mediators in urine and sputum cells 4 hours after LTE₄ challenge.

RESULTS

Montelukast completely blocked LTE₄-induced bronchoconstriction. Despite tolerating an at least 10 times higher dose of LTE₄ after montelukast, there was no difference in the percentage of eosinophils in sputum. Urinary excretion of all major lipid mediators increased after LTE₄ inhalation. Montelukast blocked release of the mast cell product prostaglandin (PG) D₂, as well as release of PGF_2α and thromboxane (Tx) A₂, but not increased excretion of PGE₂ and its metabolites or isoprostanes.

CONCLUSION

LTE₄ induces airflow obstruction and mast cell activation through the CysLT₁ receptor.

Role of lipid mediators and control of lymphocyte responses in type 2 immunopathology

Type 2 immunopathology is a cardinal feature of allergic diseases and involves cooperation between adaptive immunity and innate effector responses. Virtually all cell types relevant to this pathology generate leukotriene and/or prostaglandin mediators that derive from arachidonic acid, express receptors for such mediators, or both. Recent studies highlight prominent functions for these mediators in communication between the innate and adaptive immune systems, as well as amplification or suppression of type 2 effector responses. This review focuses on recent advances and insights, and highlights existing and potential therapeutic applications of drugs that target these mediators or their receptors, with a special emphasis on their regulation of the innate and adaptive lymphocytes relevant to type 2 immunopathology.

Aspirin exacerbated respiratory disease: Current topics and trends

Aspirin-exacerbated respiratory disease is a chronic and treatment-resistant disease, characterized by the presence of eosinophilic rhinosinusitis, nasal polyposis, bronchial asthma, and nonsteroidal anti-inflammatory drugs hypersensitivity. Alterations in arachidonic acid metabolism may induce an imbalance between pro-inflammatory and anti-inflammatory substances, expressed as an overproduction of cysteinyl leukotrienes and an underproduction of prostaglandin E2. Although eosinophils play a key role, recent studies have shown the importance of other cells and molecules in the development of the disease like mast cells, basophils, lymphocytes, platelets, neutrophils, macrophages, epithelial respiratory cells, IL-33 and thymic stromal lymphopoietin, making each of them promissory diagnostic and treatment targets. In this review, we summarize the most important clinical aspects of the disease, including the current topics about diagnosis and treatment, like provocation challenges and aspirin desensitization. We also discuss recent findings in the pathogenesis of the disease, as well as future trends in diagnosis and treatment, including monoclonal antibodies and a low salicylate diet as a treatment option.

Lipid Mediators in Aspirin-Exacerbated Respiratory Disease

Aspirin-exacerbated respiratory disease (AERD) is a syndrome of severe asthma and rhinosinusitis with nasal polyposis with exacerbations of baseline eosinophil-driven and mast cell-driven inflammation after nonsteroidal antiinflammatory drug ingestion. Although the underlying pathophysiology is poorly understood, dysregulation of the cyclooxygenase and 5-lipoxygenase pathways of arachidonic acid metabolism is thought to be key. Central features of AERD pathogenesis are overproduction of proinflammatory and bronchoconstrictor cysteinyl leukotrienes and prostaglandin (PG) D₂ and inhibition of bronchoprotective and antiinflammatory PGE₂. Imbalance in the ratio of these lipid mediators likely leads to the increased eosinophilic and mast cell inflammatory responses in the respiratory tract.

Prostaglandin E2 suppresses human group 2 innate lymphoid cell function

Background

Group 2 innate lymphoid cells (ILC2s) are involved in the initial phase of type 2 inflammation and can amplify allergic immune responses by orchestrating other type 2 immune cells. Prostaglandin (PG) E₂ is a bioactive lipid that plays protective roles in the lung, particularly during allergic inflammation.

Objective

We set out to investigate how PGE₂ regulates human ILC2 function.

Methods

The effects of PGE₂ on human ILC2 proliferation and intracellular cytokine and transcription factor expression were assessed by means of flow cytometry. Cytokine production was measured by using ELISA, and real-time quantitative PCR was performed to detect PGE₂ receptor expression.

Results

PGE₂ inhibited GATA-3 expression, as well as production of the type 2 cytokines IL-5 and IL-13, from human tonsillar and blood ILC2s in response to stimulation with a combination of IL-25, IL-33, thymic stromal lymphopoietin, and IL-2. Furthermore, PGE₂ downregulated the expression of IL-2 receptor α (CD25). In line with this observation, PGE₂ decreased ILC2 proliferation. These effects were mediated by the combined action of E-type prostanoid receptor (EP) 2 and EP4 receptors, which were specifically expressed on ILC2s.

Conclusion

Our findings reveal that PGE₂ limits ILC2 activation and propose that selective EP2 and EP4 receptor agonists might serve as a promising therapeutic approach in treating allergic diseases by suppressing ILC2 function.

Evidence for autocrine and paracrine regulation of allergen-induced mast cell mediator release in the guinea pig airways

Mast cells play an essential role in immediate type hypersensitivity reactions and in chronic allergic diseases of the airways, including asthma. Mast cell mediator release can be modulated by locally released autacoids and circulating hormones, but surprisingly little is known about the autocrine effects of mediators released upon mast cell activation. We thus set out to characterize the autocrine and paracrine effects of mast cell mediators on mast cell activation in the guinea pig airways. By direct measures of histamine, cysteinyl-leukotriene and thromboxane release and with studies of allergen-evoked contractions of airway smooth muscle, we describe a complex interplay amongst these autacoids. Notably, we observed an autocrine effect of the cysteinyl-leukotrienes acting through cysLT₁ receptors on mast cell leukotriene release. We confirmed the results of previous studies demonstrating a marked enhancement of mast cell mediator release following cyclooxygenase inhibition, but we have extended these results by showing that COX-2 derived eicosanoids inhibit cysteinyl-leukotriene release and yet are without effect on histamine release. Given the prominent role of COX-1 inhibition in aspirin-sensitive asthma, these data implicate preformed mediators stored in granules as the initial drivers of these adverse reactions. Finally, we describe the paracrine signaling cascade leading to thromboxane synthesis in the guinea pig airways following allergen challenge, which occurs indirectly, secondary to cysLT₁ receptor activation on structural cells and/ or leukocytes within the airway wall, and a COX-2 dependent synthesis of the eicosanoid. The results highlight the importance of cell-cell and autocrine interactions in regulating allergic responses in the airways.

Mast cell-dependent IL-33/ST2 signaling is protective against the development of airway hyperresponsiveness in a house dust mite mouse model of asthma

Interleukin-33 (IL-33) and its receptor ST2 have been influentially associated with the pathophysiology of asthma. Due to the divergent roles of IL-33 in regulating mast cell functions, there is a need to further characterize IL-33/ST2-dependent mast cell responses and their significance in the context of asthma. This study aimed to investigate how IL-33/ST2-dependent mast cell responses contribute to the development of airway hyperresponsiveness (AHR) and airway inflammation in a mouse model of house dust mite (HDM)-induced asthma. Mast cell-deficient C57BL/6-Kit^W-sh (Wsh) mice engrafted with either wild-type (Wsh + MC-WT) or ST2-deficient bone marrow-derived mast cells (Wsh + MC-ST2KO) were exposed to HDM delivered intranasally. An exacerbated development of AHR in response to HDM was seen in Wsh + MC-ST2KO compared with Wsh + MC-WT mice. The contribution of this IL-33/ST2-dependent mast cell response to AHR seems to reside within the smaller airways in the peripheral parts of the lung, as suggested by the isolated yet marked effect on tissue resistance. Considering the absence of a parallel increase in cellular inflammation in bronchoalveolar lavage fluid (BALF) and lung, the aggravated AHR in Wsh + MC-ST2KO mice seems to be independent of cellular inflammation. We observed an association between the elevated AHR and reduced PGE₂ levels in BALF_. Due to the protective properties of PGE₂ in airway responses, it is conceivable that IL-33/ST2-dependent mast cell induction of PGE₂ could be responsible for the dampening effect on AHR. In conclusion, we reveal that IL-33/ST2-dependent mast cell responses can have a protective, rather than causative role, in the development of AHR.

Identification of Four Novel Loci in Asthma in European American and African American Populations

RATIONALE

Despite significant advances in knowledge of the genetic architecture of asthma, specific contributors to the variability in the burden between populations remain uncovered.

OBJECTIVES

To identify additional genetic susceptibility factors of asthma in European American and African American populations.

METHODS

A phenotyping algorithm mining electronic medical records was developed and validated to recruit cases with asthma and control subjects from the Electronic Medical Records and Genomics network. Genome-wide association analyses were performed in pediatric and adult asthma cases and control subjects with European American and African American ancestry followed by metaanalysis. Nominally significant results were reanalyzed conditioning on allergy status.

MEASUREMENTS AND MAIN RESULTS

The validation of the algorithm yielded an average of 95.8% positive predictive values for both cases and control subjects. The algorithm accrued 21,644 subjects (65.83% European American and 34.17% African American). We identified four novel population-specific associations with asthma after metaanalyses: loci 6p21.31, 9p21.2, and 10q21.3 in the European American population, and the PTGES gene in African Americans. TEK at 9p21.2, which encodes TIE2, has been shown to be involved in remodeling the airway wall in asthma, and the association remained significant after conditioning by allergy. PTGES, which encodes the prostaglandin E synthase, has also been linked to asthma, where deficient prostaglandin E₂ synthesis has been associated with airway remodeling.

CONCLUSIONS

This study adds to understanding of the genetic architecture of asthma in European Americans and African Americans and reinforces the need to study populations of diverse ethnic backgrounds to identify shared and unique genetic predictors of asthma.

Prostaglandin E2 promotes human CD34+ cells homing through EP2 and EP4 in vitro

Recently, certain studies have demonstrated in vitro that prostaglandin E2 (PGE2) promotes human cluster of differentiation (CD)34⁺ cell homing. However, the sub-type receptors activated by PGE2 are unknown, as the PGE2 receptor EP1-4 subtypes (EP1-4) are expressed on the membrane of human CD34⁺ cells. Based on the above, the present study aimed to screen the receptor subtype activity by PGE2 to promote human CD34⁺ cell homing. It was observed that human CD34⁺ cells expressed the four PGE2 sub-receptors, particularly EP2 and 4. PGE2 increased EP2 and 4 mRNA expression significantly, while EP1 and 3 mRNA exhibited no significant alteration. PGE2, EP2 agonist (EP2A), and EP4A upregulated C-X-C chemokine receptor 4 mRNA and protein expression in human CD34⁺ cells, and promoted stromal cell-derived factor 1α (SDF-1α) expression in bone marrow mesenchymal stem cells (BMMSCs). These phenomena were inhibited by the associated receptor antagonists. PGE2, EP2A, and EP4A facilitated human CD34⁺ cell migration towards SDF-1α and BMMSCs. The results of the present study suggested that PGE2 promoted human CD34⁺ cell homing through EP2 and 4 receptors in vitro.

Lipid Mediators of Allergic Disease: Pathways, Treatments, and Emerging Therapeutic Targets

Bioactive lipids are critical regulators of inflammation. Over the last 75 years, these diverse compounds have emerged as clinically-relevant mediators of allergic disease pathophysiology. Animal and human studies have demonstrated the importance of lipid mediators in the development of asthma, allergic rhinitis, urticaria, anaphylaxis, atopic dermatitis, and food allergy. Lipids are critical participants in cell signaling events which influence key physiologic (bronchoconstriction) and immune phenomena (degranulation, chemotaxis, sensitization). Lipid-mediated cellular mechanisms including: (1) formation of structural support platforms (lipid rafts) for receptor signaling complexes, (2) activation of a diverse family of G-protein coupled receptors, and (3) mediating intracellular signaling cascades by acting as second messengers. Here, we review four classes of bioactive lipids (platelet activating factor, the leukotrienes, the prostanoids, and the sphingolipids) with special emphasis on lipid synthesis pathways and signaling, atopic disease pathology, and the ongoing development of atopy treatments targeting lipid mediator pathways.

Assessment of in vivo mast cell reactivity in patients with systemic mastocytosis

BACKGROUND

Patients with systemic mastocytosis (SM) have clinical signs of mast cell (MC) activation and increased levels of MC mediators. It is unclear whether the increased mediator levels are caused by increased numbers of tissue MCs, or whether these cells in affected individuals have a hyperactive phenotype.

OBJECTIVE

To determine reactivity of the skin and the airways to directly acting mediators and indirectly acting mast cell secretagogues in subjects with SM.

METHODS

Skin reactivity to morphine and histamine, and airway responsiveness to mannitol and methacholine, was assessed in 15 patients with SM, 11 patients with allergic asthma (A) and 13 healthy controls (HC). Serum tryptase and urinary metabolites of the MC mediators histamine and prostaglandin D₂ were measured, as well as ex vivo basophil histamine release.

RESULTS

Mast cell mediators in the blood and urine were significantly higher in patients with SM than in HC and A controls. Responsiveness to local activation of skin MCs (by morphine) and airway MCs (by mannitol) was similar in SM and HC groups. Likewise, end-organ responsiveness in the skin to histamine, and in the airways to methacholine, was similar in all three subject groups. There was no evidence of increased basophil reactivity in SM patients.

CONCLUSIONS AND CLINICAL RELEVANCE

Mast cells in the skin and airways of subjects with SM do not exhibit hyper-reactivity towards the MC-activating stimuli morphine and mannitol, respectively. Therefore, the highly elevated baseline levels of MC mediators in SM are most likely due to increased MC numbers, rather than altered MC responsiveness. The underlying mechanisms could involve leakage of MC mediators, or dysfunctions in mediator synthesis, storage and release. One clinical implication of our study is that there is no contraindication to perform skin tests using morphine in subjects with mastocytosis.

The EP1/EP3 receptor agonist 17-pt-PGE2 acts as an EP4 receptor agonist on endothelial barrier function and in a model of LPS-induced pulmonary inflammation

Endothelial dysfunction is a hallmark of inflammatory conditions. We recently demonstrated that prostaglandin (PG)E₂ enhances the resistance of pulmonary endothelium in vitro and counteracts lipopolysaccharide (LPS)-induced pulmonary inflammation in vivo via EP4 receptors. The aim of this study was to investigate the role of the EP1/EP3 receptor agonist 17-phenyl-trinor-(pt)-PGE₂ on acute lung inflammation in a mouse model. In LPS-induced pulmonary inflammation in mice, 17-pt-PGE₂ reduced neutrophil infiltration and inhibited vascular leakage. These effects were unaltered by an EP1 antagonist, but reversed by EP4 receptor antagonists. 17-pt-PGE₂ increased the resistance of pulmonary microvascular endothelial cells and prevented thrombin-induced disruption of endothelial junctions. Again, these effects were not mediated via EP1 or EP3 but through activation of the EP4 receptor, as demonstrated by the lack of effect of more selective EP1 and EP3 receptor agonists, prevention of these effects by EP4 antagonists and EP4 receptor knock-down by siRNA. In contrast, the aggregation enhancing effect of 17-pt-PGE₂ in human platelets was mediated via EP3 receptors. Our results demonstrate that 17-pt-PGE₂ enhances the endothelial barrier in vitro on pulmonary microvascular endothelial cells, and accordingly ameliorates the recruitment of neutrophils, via EP4 receptors in vivo. This suggests a beneficial effect of 17-pt-PGE₂ on pulmonary inflammatory diseases.

Selective COX-2 Inhibition Exerts No Negative Effects on Peripheral Blood Lymphocytes in Allergic Asthmatics

BACKGROUND

Selective inhibition of cyclooxygenase-2 (COX-2) reduces the production of prostaglandin E2 (PGE2), which can have both pro- and anti-inflammatory effects on allergic inflammation. Moreover, in vitro PGE2 has been shown to affect inflammation through the modulation of lymphocyte responses.

METHODS

Sixteen subjects with mild allergic asthma were recruited to a two-period cross-over study: one treatment period with the selective COX-2 inhibitor etoricoxib and one without. Each treatment period ended with an airway challenge with the patient's relevant allergen. Antigen-specific proliferation with the major cat allergen, Fel d 1, was analysed in PBMCs. CD4+ T cells were phenotyped using flow cytometry, and mRNA expression of FOXP3 in anti-CD3-stimulated CD4+ cells were analysed.

RESULTS

No significant impact of in vivo inhibition of COX-2 was detected on the proportion of Th1, Th2, or Treg cells in peripheral blood. Likewise, the treatment had minor effects on the stimulated expression of FOXP3 mRNA in CD4+ T cells. Proliferation of PBMCs to the major cat allergen Fel d 1 was slightly reduced by etoricoxib treatment in cat-allergic patients.

CONCLUSIONS

Short-term treatment with the COX-2 inhibitor etoricoxib had a minor impact on T-cell responses, supporting its safe use also in subjects exposed to triggers of lymphocyte activation.

Association between thromboxane A2 receptor polymorphisms and asthma risk: A meta-analysis

OBJECTIVE

To determine whether there is an association between thromboxane A2 receptor (TBXA2R) gene polymorphisms (+924C/T and +795C/T) and asthma risk by conducting a meta-analysis.

DATA SOURCES

Pubmed, Embase, Chinese National Knowledge Infrastructure (CNKI) and Wanfang database were searched (updated May 1, 2015).

STUDY SELECTIONS

Articles evaluating the association between TBXA2R gene polymorphisms and asthma risk were selected.

RESULTS

A total of 7 studies on +924C/T polymorphism and 6 studies on +795C/T polymorphism were included in this meta-analysis. There was a significant association between TBXA2R +924C/T polymorphism and asthma risk in the recessive model (OR = 1.33, 95% CI = 1.01-1.75, P = 0.045). No significant association between +795C/T polymorphism and asthma risk in the overall population was demonstrated. In subgroup analyzes, significant association was observed in atopic asthma risk in the recessive model (OR = 1.43, 95% CI = 1.01-2.01, P = 0.043), but no significant association was found between TBXA2R +924C/T polymorphism and asthma risk in Asians (OR = 1.14, 95% CI = 0.80-1.63, P = 0.457). TBXA2R +795C/T polymorphism was associated with aspirin-intolerant asthma (AIA) risk when stratified by asthma subphenotype in the allelic model (OR = 1.30, 95% CI = 1.05-1.60, P = 0.014) and dominant model (OR = 1.50, 95% CI = 1.11-2.03, P = 0.008).

CONCLUSION

Our results suggested that TBXA2R +924C/T polymorphism is associated with asthma risk, and +795C/T polymorphism may be a risk factor for AIA. Larger-scale and well-designed studies are required to validate the association identified in the current meta-analysis.

PGE2-treated macrophages inhibit development of allergic lung inflammation in mice

In healthy lungs, many macrophages are characterized by IL-10 production, and few are characterized by expression of IFN regulatory factor 5 (formerly M1) or YM1 and/or CD206 (formerly M2), whereas in asthma, this balance shifts toward few producing IL-10 and many expressing IFN regulatory factor 5 or YM1/CD206. In this study, we tested whether redressing the balance by reinstating IL-10 production could prevent house dust mite-induced allergic lung inflammation. PGE2 was found to be the best inducer of IL-10 in macrophages in vitro. Mice were then sensitized and challenged to house dust mites during a 2 wk protocol while treated with PGE2 in different ways. Lung inflammation was assessed 3 d after the last house dust mite challenge. House dust mite-exposed mice treated with free PGE2 had fewer infiltrating eosinophils in lungs and lower YM1 serum levels than vehicle-treated mice. Macrophage-specific delivery of PGE2 did not affect lung inflammation. Adoptive transfer of PGE2-treated macrophages led to fewer infiltrating eosinophils, macrophages, (activated) CD4(+), and regulatory T lymphocytes in lungs. Our study shows that the redirection of macrophage polarization by using PGE2 inhibits development of allergic lung inflammation. This beneficial effect of macrophage repolarization is a novel avenue to explore for therapeutic purposes.

Prostanoids in Asthma and COPD: Actions, Dysregulation, and Therapeutic Opportunities

Pathophysiologic gaps in the actions of currently available treatments for asthma and COPD include neutrophilic inflammation, airway remodeling, and alveolar destruction. All of these processes can be modulated by cyclic adenosine monophosphate-elevating prostaglandins E2 and I2 (also known as prostacyclin). These prostanoids have long been known to elicit bronchodilation and to protect against bronchoconstriction provoked by a variety of stimuli. Much less well known is their capacity to inhibit inflammatory responses involving activation of lymphocytes, eosinophils, and neutrophils, as well as to attenuate epithelial injury and mesenchymal cell activation. This profile of actions identifies prostanoids as attractive candidates for exogenous administration in asthma. By contrast, excessive prostanoid production and signaling might contribute to both the increased susceptibility to infections that drive COPD exacerbations and the inadequate alveolar repair that characterizes emphysema. Inhibition of endogenous prostanoid synthesis or signaling, thus, has therapeutic potential for these types of patients. By virtue of their pleiotropic capacity to modulate numerous pathophysiologic processes relevant to the expression and natural history of airway diseases, prostanoids emerge as attractive targets for therapeutic manipulation.

Aspirin-Exacerbated Respiratory Disease Involves a Cysteinyl Leukotriene-Driven IL-33-Mediated Mast Cell Activation Pathway

Aspirin-exacerbated respiratory disease (AERD), a severe eosinophilic inflammatory disorder of the airways, involves overproduction of cysteinyl leukotrienes (cysLTs), activation of airway mast cells (MCs), and bronchoconstriction in response to nonselective cyclooxygenase inhibitors that deplete homeostatic PGE2. The mechanistic basis for MC activation in this disorder is unknown. We now demonstrate that patients with AERD have markedly increased epithelial expression of the alarmin-like cytokine IL-33 in nasal polyps, as compared with polyps from aspirin-tolerant control subjects. The murine model of AERD, generated by dust mite priming of mice lacking microsomal PGE2 synthase (ptges(-/-) mice), shows a similar upregulation of IL-33 protein in the airway epithelium, along with marked eosinophilic bronchovascular inflammation. Deletion of leukotriene C4 synthase, the terminal enzyme needed to generate cysLTs, eliminates the increased IL-33 content of the ptges(-/-) lungs and sharply reduces pulmonary eosinophilia and basal secretion of MC products. Challenges of dust mite-primed ptges(-/-) mice with lysine aspirin induce IL-33-dependent MC activation and bronchoconstriction. Thus, IL-33 is a component of a cysLT-driven innate type 2 immune response that drives pathogenic MC activation and contributes substantially to AERD pathogenesis.