Mucosal allergic sensitization to cockroach allergens is dependent on proteinase activity and proteinase-activated receptor-2 activation

German cockroach extract prevents IL-13-induced CCL26 expression in airway epithelial cells through IL-13 degradation

Inhaled aeroallergens can directly activate airway epithelial cells (AECs). Exposure to cockroach allergens is a strong risk factor for asthma. Cockroach allergens mediate some of their effects through their serine protease activity; protease activity is also a major contributor to allergenicity. The Th2 cytokine interleukin-13 (IL-13) induces upregulation of the eosinophil chemotactic factor CCL26. CCL26 induces eosinophil migration in allergic inflammation. In this work, we studied the effect of cockroach proteases on IL-13-induced effects. Immersed cultures of the human bronchial epithelial cell line BEAS-2B and air-liquid interface (ALI) cultures of primary normal human bronchial epithelial (NHBE) cells were stimulated with IL-13, Blattella Germanica cockroach extract (CE), or both. IL-13-induced genes were analyzed with qRT-PCR. IL-13 induced upregulation of CCL26, periostin, and IL-13Rα2 in bronchial epithelial cells which were decreased by CE. CE was heat-inactivated (HICE) or pre-incubated with protease inhibitors. HICE and CE preincubated with serine protease inhibitors did not prevent IL-13-induced CCL26 upregulation. CE-degraded IL-13 and specific cleavage sites were identified. CE also decreased IL-4-induced CCL26 upregulation and degraded IL-4. Other serine proteases such as bovine trypsin and house dust mite (HDM) serine proteases did not have the same effects on IL-13-induced CCL26. We conclude that CE serine proteases antagonize IL-13-induced effects in AECs, and this CE effect is mediated primarily through proteolytic cleavage of IL-13. IL-13 cleavage by cockroach serine proteases may modulate CCL26-mediated effects in allergic airway inflammation by interfering directly with the pro-inflammatory effects of IL-13 in vivo.

Protease allergens as initiators-regulators of allergic inflammation

Tremendous progress in the last few years has been made to explain how seemingly harmless environmental proteins from different origins can induce potent Th2-biased inflammatory responses. Convergent findings have shown the key roles of allergens displaying proteolytic activity in the initiation and progression of the allergic response. Through their propensity to activate IgE-independent inflammatory pathways, certain allergenic proteases are now considered as initiators for sensitization to themselves and to non-protease allergens. The protease allergens degrade junctional proteins of keratinocytes or airway epithelium to facilitate allergen delivery across the epithelial barrier and their subsequent uptake by antigen-presenting cells. Epithelial injuries mediated by these proteases together with their sensing by protease-activated receptors (PARs) elicit potent inflammatory responses resulting in the release of pro-Th2 cytokines (IL-6, IL-25, IL-1β, TSLP) and danger-associated molecular patterns (DAMPs; IL-33, ATP, uric acid). Recently, protease allergens were shown to cleave the protease sensor domain of IL-33 to produce a super-active form of the alarmin. At the same time, proteolytic cleavage of fibrinogen can trigger TLR4 signaling, and cleavage of various cell surface receptors further shape the Th2 polarization. Remarkably, the sensing of protease allergens by nociceptive neurons can represent a primary step in the development of the allergic response. The goal of this review is to highlight the multiple innate immune mechanisms triggered by protease allergens that converge to initiate the allergic response.

β-Arrestin-biased proteinase-activated receptor-2 antagonist C781 limits allergen-induced airway hyperresponsiveness and inflammation

BACKGROUND AND PURPOSE

Asthma is a heterogenous disease strongly associated with inflammation that has many different causes and triggers. Current asthma treatments target symptoms such as bronchoconstriction and airway inflammation. Despite recent advances in biological therapies, there remains a need for new classes of therapeutic agents with novel, upstream targets. The proteinase-activated receptor-2 (PAR2) has long been implicated in allergic airway inflammation and asthma and it remains an intriguing target for novel therapies. Here, we describe the actions of C781, a newly developed low MW PAR2 biased antagonist, in vitro and in vivo in the context of acute allergen exposure.

EXPERIMENTAL APPROACH

A human bronchial epithelial cell line expressing PAR2 (16HBE14o- cells) was used to evaluate the modulation in vitro, by C781, of physiological responses to PAR2 activation and downstream β-arrestin/MAPK and Gq/Ca2+ signalling. Acute Alternaria alternata sensitized and challenged mice were used to evaluate C781 as a prophylactically administered modulator of airway hyperresponsiveness, inflammation and mucus overproduction in vivo.

KEY RESULTS

C781 reduced in vitro physiological signalling in response to ligand and proteinase activation. C781 effectively antagonized β-arrestin/MAPK signalling without significant effect on Gq/Ca2+ signalling in vitro. Given prophylactically, C781 modulated airway hyperresponsiveness, airway inflammation and mucus overproduction of the small airways in an acute allergen-challenged mouse model.

CONCLUSION AND IMPLICATIONS

Our work demonstrates the first biased PAR2 antagonist for β-arrestin/MAPK signalling. C781 is efficacious as a prophylactic treatment for allergen-induced airway hyperresponsiveness and inflammation in mice. It exemplifies a key pharmacophore for PAR2 that can be optimized for clinical development.

Tryptase and Exogenous Trypsin: Mechanisms and Ophthalmic Applications

Ocular injuries caused by inflammation, surgery or accidents are subject to a physiological healing process that ultimately restores the structure and function of the damaged tissue. Tryptase and trypsin are essential component of this process and they play a role in promoting and reducing the inflammatory response of tissues, respectively. Following injury, tryptase is endogenously produced by mast cells and can exacerbate the inflammatory response both by stimulating neutrophil secretion, and through its agonist action on proteinase-activated receptor 2 (PAR2). In contrast, exogenously introduced trypsin promotes wound healing by attenuating inflammatory responses, reducing oedema and protecting against infection. Thus, trypsin may help resolve ocular inflammatory symptoms and promote faster recovery from acute tissue injury associated with ophthalmic diseases. This article describes the roles of tryptase and exogenous trypsin in affected tissues after onset of ocular injury, and the clinical applications of trypsin injection.

Innate Immunity, Epithelial Plasticity, and Remodeling in Asthma

Innate immune responses (IIR) of the epithelium play a critical role in the initiation and progression of asthma. The core of the IIR is an intracellular signaling pathway activated by pattern recognition receptors (PRRs) to limit the spread of infectious organisms. This chapter will focus on the epithelium as the major innate sentinel cell and its role in acute exacerbations (AEs). Although the pathways of how the IIR activates the NFκB transcription factor, triggering cytokine secretion, dendritic cell activation, and Th2 polarization are well-described, recent exciting work has developed mechanistic insights into how chronic activation of the IIR is linked to mucosal adaptive responses. These adaptations include changes in cell state, now called epithelial-mesenchymal plasticity (EMP). EMP is a coordinated, genomic response to airway injury disrupting epithelial barrier function, expanding the basal lamina, and producing airway remodeling. EMP is driven by activation of the unfolded protein response (UPR), a transcriptional response producing metabolic shunting of glucose through the hexosamine biosynthetic pathway (HBP) to protein N-glycosylation. NFκB signaling and UPR activation pathways potentiate each other in remodeling the basement membrane. Understanding of injury-repair process of epithelium provides new therapeutic targets for precision approaches to the treatment of asthma exacerbations and their sequelae.

An Amide Alkaloid Isolated from Ephedra sinica Ameliorates OVA-Induced Allergic Asthma by Inhibiting Mast Cell Activation and Dendritic Cell Maturation

Asthma, which is a chronic inflammatory disease of the airways, is usually caused by allergens in which various structures and immune cells are involved. Ephedra sinica, the most commonly used Chinese medicine, has significant clinical effects on asthma, but its components are complex and the mechanism of action has not been fully elucidated. Among its components, we identified an amide alkaloid (EB-A) and investigated its anti-asthmatic activity and the underlying mechanisms. In this study, we replicated an OVA-sensitized/challenged allergic asthma mouse model, and divided the mice into a model (OVA) group, positive drug (Y, 0.5 mg/kg/day) group, and EB-A treatment with low (Low, 10 mg/kg/day) and high dose (High, 20 mg/kg/day) groups. Asthma-related features were analyzed through the airway hyperresponsiveness (AHR), cough and wheeze indexes, allergen-specific IgE, prostaglandin D2 (PDG2), and lung histology in mice. The levels of apoptosis and reactive oxygen species (ROS) in the primary lung cells, cytokines in the serum and broncho-alveolar lavage fluid (BALF), and proteinase-activated receptor-2 (PAR2) pathway activation in the lung tissue were measured to evaluate the inflammatory injury and lung epithelial barrier damage in the mice. Dendritic cell (DC) maturation and mast cell (MC) activation were verified in vitro and in vivo. Furthermore, the effect of a PAR2 activation in lung epithelial cells on the maturation of DCs was evaluated by the co-culture system of (human bronchial epithelial cell lines) 16HBE and bone marrow-derived dendritic cells (BMDCs). The results showed that EB-A inhibited the typical asthmatic phenotypes, as well as lung injury and inflammation, MC activation and degranulation, and DC maturation in the OVA-sensitized/challenged BALB/c mice. In addition, EB-A inhibited the expression of PAR2 in the lung epithelial cells and significantly interfered with the maturation of DCs after inhibiting PAR2. Taken together, our study firstly demonstrated that EB-A could ameliorate OVA-induced allergic asthma by inhibiting MC activation and DC maturation, and the molecular mechanism of EB-A's anti-asthmatic activity might be mediated by inhibiting PAR2. Our data provide a molecular justification for the use of EB-A in the treatment of allergic asthma.

Novel inhibitors and activity-based probes targeting serine proteases

Serine proteases play varied and manifold roles in important biological, physiological, and pathological processes. These include viral, bacterial, and parasitic infection, allergic sensitization, tumor invasion, and metastasis. The use of activity-based profiling has been foundational in pinpointing the precise roles of serine proteases across this myriad of processes. A broad range of serine protease-targeted activity-based probe (ABP) chemotypes have been developed and we have recently introduced biotinylated and "clickable" peptides containing P1 N-alkyl glycine arginine N-hydroxy succinimidyl (NHS) carbamates as ABPs for detection/profiling of trypsin-like serine proteases. This present study provides synthetic details for the preparation of additional examples of this ABP chemotype, which function as potent irreversible inhibitors of their respective target serine protease. We describe their use for the activity-based profiling of a broad range of serine proteases including trypsin, the trypsin-like protease plasmin, chymotrypsin, cathepsin G, and neutrophil elastase (NE), including the profiling of the latter protease in clinical samples obtained from patients with cystic fibrosis.

Protease-activated receptor-2: Role in asthma pathogenesis and utility as a biomarker of disease severity

PAR₂, a receptor activated by serine proteases, has primarily pro-inflammatory roles in the airways and may play a role in asthma pathogenesis. PAR₂ exerts its effects in the lungs through activation of a variety of airway cells, but also activation of circulating immune cells. There is evidence that PAR₂ expression increases in asthma and other inflammatory diseases, although the regulation of PAR₂ expression is not fully understood. Here we review the available literature on the potential role of PAR₂ in asthma pathogenesis and propose a model of PAR₂-mediated development of allergic sensitization. We also propose, based on our previous work, that PAR₂ expression on peripheral blood monocyte subsets has the potential to serve as a biomarker of asthma severity and/or control.

Novel Inhibitors and Activity-Based Probes Targeting Trypsin-Like Serine Proteases

The trypsin-like proteases (TLPs) play widespread and diverse roles, in a host of physiological and pathological processes including clot dissolution, extracellular matrix remodelling, infection, angiogenesis, wound healing and tumour invasion/metastasis. Moreover, these enzymes are involved in the disruption of normal lung function in a range of respiratory diseases including allergic asthma where several allergenic proteases have been identified. Here, we report the synthesis of a series of peptide derivatives containing an N-alkyl glycine analogue of arginine, bearing differing electrophilic leaving groups (carbamate and triazole urea), and demonstrate their function as potent, irreversible inhibitors of trypsin and TLPs, to include activities from cockroach extract. As such, these inhibitors are suitable for use as activity probes (APs) in activity-based profiling (ABP) applications.

Proteinase-activated receptor-2 antagonist C391 inhibits Alternaria-induced airway epithelial signalling and asthma indicators in acute exposure mouse models

BACKGROUND AND PURPOSE

Despite the availability of a variety of treatment options, many asthma patients have poorly controlled disease with frequent exacerbations. Proteinase-activated receptor-2 (PAR2) has been identified in preclinical animal models as important to asthma initiation and progression following allergen exposure. Proteinase activation of PAR2 raises intracellular Ca²⁺ , inducing MAPK and β-arrestin signalling in the airway, leading to inflammatory and protective effects. We have developed C391, a potent PAR2 antagonist effective in blocking peptidomimetic- and trypsin-induced PAR2 signalling in vitro as well as reducing inflammatory PAR2-associated pain in vivo. We hypothesized that PAR2 antagonism by C391 would attenuate allergen-induced acutely expressed asthma indicators in murine models.

EXPERIMENTAL APPROACH

We evaluated the ability of C391 to alter Alternaria alternata-induced PAR2 signalling pathways in vitro using a human airway epithelial cell line that naturally expresses PAR2 (16HBE14o-) and a transfected embryonic cell line (HEK 293). We next evaluated the ability for C391 to reduce A. alternata-induced acutely expressed asthma indicators in vivo in two murine strains.

KEY RESULTS

C391 blocked A. alternata-induced, PAR2-dependent Ca²⁺ and MAPK signalling in 16HBE14o- cells, as well as β-arrestin recruitment in HEK 293 cells. C391 effectively attenuated A. alternata-induced inflammation, mucus production, mucus cell hyperplasia and airway hyperresponsiveness in acute allergen-challenged murine models.

CONCLUSIONS AND IMPLICATIONS

To our best knowledge, this is the first demonstration of pharmacological intervention of PAR2 to reduce allergen-induced asthma indicators in vivo. These data support further development of PAR2 antagonists as potential first-in-class allergic asthma drugs.

The Hexosamine Biosynthetic Pathway Links Innate Inflammation With Epithelial-Mesenchymal Plasticity in Airway Remodeling

Disruption of the lower airway epithelial barrier plays a major role in the initiation and progression of chronic lung disease. Here, repetitive environmental insults produced by viral and allergens triggers metabolic adaptations, epithelial-mesenchymal plasticity (EMP) and airway remodeling. Epithelial plasticity disrupts epithelial barrier function, stimulates release of fibroblastic growth factors, and remodels the extracellular matrix (ECM). This review will focus on recent work demonstrating how the hexosamine biosynthetic pathway (HBP) links innate inflammation to airway remodeling. The HBP is a core metabolic pathway of the unfolded protein response (UPR) responsible for protein N-glycosylation, relief of proteotoxic stress and secretion of ECM modifiers. We will overview findings that the IκB kinase (IKK)-NFκB pathway directly activates expression of the SNAI-ZEB1 mesenchymal transcription factor module through regulation of the Bromodomain Containing Protein 4 (BRD4) chromatin modifier. BRD4 mediates transcriptional elongation of SNAI1-ZEB as well as enhancing chromatin accessibility and transcription of fibroblast growth factors, ECM and matrix metalloproteinases (MMPs). In addition, recent exciting findings that IKK cross-talks with the UPR by controlling phosphorylation and nuclear translocation of the autoregulatory XBP1s transcription factor are presented. HBP is required for N glycosylation and secretion of ECM components that play an important signaling role in airway remodeling. This interplay between innate inflammation, metabolic reprogramming and lower airway plasticity expands a population of subepithelial myofibroblasts by secreting fibroblastic growth factors, producing changes in ECM tensile strength, and fibroblast stimulation by MMP binding. Through these actions on myofibroblasts, EMP in lower airway cells produces expansion of the lamina reticularis and promotes airway remodeling. In this manner, metabolic reprogramming by the HBP mediates environmental insult-induced inflammation with remodeling in chronic airway diseases.

Role of Secretoglobin+ (club cell) NFκB/RelA-TGFβ signaling in aero-allergen-induced epithelial plasticity and subepithelial myofibroblast transdifferentiation

Repetitive aeroallergen exposure is linked to sensitization and airway remodeling through incompletely understood mechanisms. In this study, we examine the dynamic mucosal response to cat dander extract (CDE), a ubiquitous aero-allergen linked to remodeling, sensitization and asthma. We find that daily exposure of CDE in naïve C57BL/6 mice activates innate neutrophilic inflammation followed by transition to a lymphocytic response associated with waves of mucosal transforming growth factor (TGF) isoform expression. In parallel, enhanced bronchiolar Smad3 expression and accumulation of phospho-SMAD3 was observed, indicating paracrine activation of canonical TGFβR signaling. CDE exposure similarly triggered epithelial cell plasticity, associated with expression of mesenchymal regulatory factors (Snai1 and Zeb1), reduction of epithelial markers (Cdh1) and activation of the NFκB/RelA transcriptional activator. To determine whether NFκB functionally mediates CDE-induced growth factor response, mice were stimulated with CDE in the absence or presence of a selective IKK inhibitor. IKK inhibition substantially reduced the level of CDE-induced TGFβ1 expression, pSMAD3 accumulation, Snai1 and Zeb1 expression. Activation of epithelial plasticity was demonstrated by flow cytometry in whole lung homogenates, where CDE induces accumulation of SMA⁺Epcam⁺ population. Club cells are important sources of cytokine and growth factor production. To determine whether Club cell innate signaling through NFκB/RelA mediated CDE induced TGFβ signaling, we depleted RelA in Secretoglobin (Scgb1a1)-expressing bronchiolar cells. Immunofluorescence-optical clearing light sheet microscopy showed a punctate distribution of Scgb1a1 progenitors throughout the small airway. We found that RelA depletion in Secretoglobin⁺ cells results in inhibition of the mucosal TGFβ response, blockade of EMT and reduced subepithelial myofibroblast expansion. We conclude that the Secretoglobin—derived bronchiolar cell is central to coordinating the innate response required for mucosal TGFβ1 response, EMT and myofibroblast expansion. These data have important mechanistic implications for how aero-allergens trigger mucosal injury response and remodeling in the small airway.

The development of proteinase-activated receptor-2 modulators and the challenges involved

Protease-activated receptor-2 (PAR2) has been extensively studied since its discovery in the mid-1990. Despite the advances in understanding PAR2 pharmacology, it has taken almost 25 years for the first inhibitor to reach clinical trials, and so far, no PAR2 antagonist has been approved for human use. Research has employed classical approaches to develop a wide array of PAR2 agonists and antagonists, consisting of peptides, peptoids and antibodies to name a few, with a surge in patent applications over this period. Recent breakthroughs in PAR2 structure determination has provided a unique insight into proposed PAR2 ligand binding sites. Publication of the first crystal structures of PAR2 resolved in complex with two novel non-peptide small molecule antagonists (AZ8838 and AZ3451) revealed two distinct binding pockets, originally presumed to be allosteric sites, with a PAR2 antibody (Fab3949) used to block tethered ligand engagement with the peptide-binding domain of the receptor. Further studies have proposed orthosteric site occupancy for AZ8838 as a competitive antagonist. One company has taken the first PAR2 antibody (MEDI0618) into phase I clinical trial (NCT04198558). While this first-in-human trial is at the early stages of the assessment of safety, other research into the structural characterisation of PAR2 is still ongoing in an attempt to identify new ways to target receptor activity. This review will focus on the development of novel PAR2 modulators developed to date, with an emphasis placed upon the advances made in the pharmacological targeting of PAR2 activity as a strategy to limit chronic inflammatory disease.

PAR-2-activated secretion by airway gland serous cells: role for CFTR and inhibition by Pseudomonas aeruginosa

Airway submucosal gland serous cells are important sites of fluid secretion in conducting airways. Serous cells also express the cystic fibrosis (CF) transmembrane conductance regulator (CFTR). Protease-activated receptor 2 (PAR-2) is a G protein-coupled receptor that activates secretion from intact airway glands. We tested if and how human nasal serous cells secrete fluid in response to PAR-2 stimulation using Ca²⁺ imaging and simultaneous differential interference contrast imaging to track isosmotic cell shrinking and swelling reflecting activation of solute efflux and influx pathways, respectively. During stimulation of PAR-2, serous cells exhibited dose-dependent increases in intracellular Ca²⁺. At stimulation levels >EC₅₀ for Ca²⁺, serous cells simultaneously shrank ∼20% over ∼90 s due to KCl efflux reflecting Ca²⁺-activated Cl^- channel (CaCC, likely TMEM16A)-dependent secretion. At lower levels of PAR-2 stimulation (<EC₅₀ for Ca²⁺), shrinkage was not evident due to failure to activate CaCC. Low levels of cAMP-elevating VIP receptor (VIPR) stimulation, also insufficient to activate secretion alone, synergized with low-level PAR-2 stimulation to elicit fluid secretion dependent on both cAMP and Ca²⁺ to activate CFTR and K⁺ channels, respectively. Polarized cultures of primary serous cells also exhibited synergistic fluid secretion. Pre-exposure to Pseudomonas aeruginosa conditioned media inhibited PAR-2 activation by proteases but not peptide agonists in primary nasal serous cells, Calu-3 bronchial cells, and primary nasal ciliated cells. Disruption of synergistic CFTR-dependent PAR-2/VIPR secretion may contribute to reduced airway surface liquid in CF. Further disruption of the CFTR-independent component of PAR-2-activated secretion by P. aeruginosa may also be important to CF pathophysiology.

Role of Environmental Adjuvants in Asthma Development

PURPOSE OF REVIEW

This review summarizes recent progress in our understanding how environmental adjuvants promote the development of asthma.

RECENT FINDINGS

Asthma is a heterogeneous set of lung pathologies with overlapping features. Human studies and animal models suggest that exposure to different environmental adjuvants activate distinct immune pathways, which in turn give rise to distinct forms, or endotypes, of allergic asthma. Depending on their concentrations, inhaled TLR ligands can activate either type 2 inflammation, or Th17 differentiation, along with regulatory responses that function to attenuate inflammation. By contrast, a different category of environmental adjuvants, proteases, activate distinct immune pathways and prime predominantly type 2 immune responses. Asthma is not a single disease, but rather a group of pathologies with overlapping features. Different endotypes of asthma likely arise from perturbations of distinct immunologic pathways during allergic sensitization.

Airway Mycosis and the Regulation of Type 2 Immunity

Filamentous fungi of the Aspergillus genus and others have long been linked to the induction of type 2 immunity that underlies IgE-mediated hypersensitivity responses. This unique immune response is characterized by the production of the allergy-associated T helper cell type 2 (Th2) and Th17 cytokines interleukin 4 (IL-4), IL-13, and IL-17 that drive IgE, eosinophilia, airway hyperresponsiveness and other manifestations of asthma. Proteinases secreted by filamentous fungi promote type 2 immunity, but the mechanism by which this occurs has long remained obscure. Through detailed biochemical analysis of household dust, microbiological dissection of human airway secretions, and extensive modeling in mice, our laboratory has assembled a detailed mechanistic description of how type 2 immunity evolves after exposure to fungi. In this review we summarize three key discoveries: (1) fungal proteinases drive the type 2 immune response; (2) the relationship between fungi, proteinases, and type 2 immunity is explained by airway mycosis, a form of non-invasive fungal infection of the airway lumen; and (3) the innate component of proteinase-driven type 2 immunity is mediated by cleavage of the clotting protein fibrinogen. Despite these advances, additional work is required to understand how Th2 and Th17 responses evolve and the role that non-filamentous fungi potentially play in allergic diseases.

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.

The airway epithelium in asthma

Asthma is a genetically and phenotypically complex disease that has a major impact on global health. Signs and symptoms of asthma are caused by the obstruction of airflow through the airways. The epithelium that lines the airways plays a major role in maintaining airway patency and in host defense. The epithelium initiates responses to inhaled or aspirated substances, including allergens, viruses, and bacteria, and epithelial-derived cytokines are important in the recruitment and activation of immune cells in the airway. Changes in the structure and function of the airway epithelium are a prominent feature of asthma. Approximately half of individuals with asthma have evidence of active type 2 immune responses in the airway. In these individuals, epithelial cytokines promote type 2 responses, and responses to type 2 cytokines result in increased epithelial mucus production and other effects that cause airway obstruction. Recent work also implicates other epithelial responses, including interleukin-17, interferon and ER stress responses, that may contribute to asthma pathogenesis and provide new targets for therapy.

Similar immune mechanisms control experimental airway eosinophilia elicited by different allergens and treatment protocols

Background

Mouse models have been extremely valuable in identifying the fundamental mechanisms of airway inflammation that underlie human allergic asthma. Several models are commonly used, employing different methods and routes of sensitisation, and allergens of varying clinical relevance. Although all models elicit similar hallmarks of allergic airway inflammation, including airway eosinophilia, goblet cell hyperplasia and cellular infiltration in lung, it is not established whether they do so by involving the same mechanisms.

Results

We compared the impact of inactivation of various innate or adaptive immune genes, as well as sex, in different models of allergic airway inflammation in mice of C57BL/6 background. Chicken ovalbumin (OVA) and house dust mite (HDM) were used as allergens in settings of single or multiple intranasal (i.n.) challenges, after sensitisation in adjuvant or in adjuvant-free conditions. Eosinophil numbers in the broncho-alveolar lavage and lung histopathology were assessed in each model. We found that Major Histocompatibility Complex Class II (MHCII) deficiency and lack of conventional CD4+ T cells had the most profound effect, essentially ablating airway eosinophilia and goblet cell hyperplasia in all models. In contrast, Thymic stromal lymphopoietin receptor (TSLPR) deficiency greatly reduced eosinophilia but had a variable effect on goblet cells. CD1d deficiency and lack of Natural Killer T (NKT) cells moderately impaired inflammation in OVA models but not HDM, whereas sex affected the response to HDM but not OVA. Lastly, defective Toll-like receptor (TLR)4 expression had only a relatively modest overall impact on inflammation.

Conclusion

All the models studied were comparably dependent on adaptive CD4+ T cell responses and TSLP. In contrast, sex, NKT cells and TLR4 appeared to play subtler and more variable roles that were dependent on the type of allergen and mode of immunization and challenge. These results are consistent with clinical data suggesting a key role of CD4+ T cells and TSLP in patients with allergic asthma.

Electronic supplementary material

The online version of this article (10.1186/s12865-019-0295-y) contains supplementary material, which is available to authorized users.

IL-33, IL-25 and TSLP contribute to development of fungal-associated protease-induced innate-type airway inflammation

Certain proteases derived from house dust mites and plants are considered to trigger initiation of allergic airway inflammation by disrupting tight junctions between epithelial cells. It is known that inhalation of proteases such as house dust mite-derived Der p1 and/or papaya-derived papain caused airway eosinophilia in naïve mice and even in Rag-deficient mice that lack acquired immune cells such as T, B and NKT cells. In contrast, little is known regarding the possible involvement of proteases derived from Aspergillus species (fungal-associated proteases; FAP), which are ubiquitous saprophytic fungi in the environment, in the development of allergic airway eosinophilia. Here, we found that inhalation of FAP by naïve mice led to airway eosinophilia that was dependent on protease-activated receptor-2 (PAR2), but not TLR2 and TLR4. Those findings suggest that the protease activity of FAP, but not endotoxins in FAP, are important in the setting. In addition, development of that eosinophilia was mediated by innate immune cells (ILCs) such as innate lymphoid cells, but not by acquired immune cells such as T, B and NKT cells. Whereas IL-33, IL-25 and thymic stromal lymphopoietin (TSLP) are involved in induction of FAP-induced ILC-mediated airway eosinophilia, IL-33-rather than IL-25 and/or TSLP-was critical for the eosinophilia in our model. Our findings improve our understanding of the molecular mechanisms involved in induction of airway inflammation by FAP.

Protease-activated receptor-2 signaling through β-arrestin-2 mediates Alternaria alkaline serine protease-induced airway inflammation

Alternaria alternata is a fungal allergen associated with severe asthma and asthma exacerbations. Similarly to other asthma-associated allergens, Alternaria secretes a serine-like trypsin protease(s) that is thought to act through the G protein-coupled receptor protease-activated receptor-2 (PAR2) to induce asthma symptoms. However, specific mechanisms underlying Alternaria-induced PAR2 activation and signaling remain ill-defined. We sought to determine whether Alternaria-induced PAR2 signaling contributed to asthma symptoms via a PAR2/β-arrestin signaling axis, identify the protease activity responsible for PAR2 signaling, and determine whether protease activity was sufficient for Alternaria-induced asthma symptoms in animal models. We initially used in vitro models to demonstrate Alternaria-induced PAR2/β-arrestin-2 signaling. Alternaria filtrates were then used to sensitize and challenge wild-type, PAR2-/- and β-arrestin-2-/- mice in vivo. Intranasal administration of Alternaria filtrate resulted in a protease-dependent increase of airway inflammation and mucin production in wild-type but not PAR2-/- or β-arrestin-2-/- mice. Protease was isolated from Alternaria preparations, and select in vitro and in vivo experiments were repeated to evaluate sufficiency of the isolated Alternaria protease to induce asthma phenotype. Administration of a single isolated serine protease from Alternaria, Alternaria alkaline serine protease (AASP), was sufficient to fully activate PAR2 signaling and induce β-arrestin-2-/--dependent eosinophil and lymphocyte recruitment in vivo. In conclusion, Alternaria filtrates induce airway inflammation and mucus hyperplasia largely via AASP using the PAR2/β-arrestin signaling axis. Thus, β-arrestin-biased PAR2 antagonists represent novel therapeutic targets for treating aeroallergen-induced asthma.

Mucosal bromodomain-containing protein 4 mediates aeroallergen-induced inflammation and remodeling

BACKGROUND

Frequent exacerbations of allergic asthma lead to airway remodeling and a decrease in pulmonary function, producing morbidity. Cat dander is an aeroallergen associated with asthma risk.

OBJECTIVE

We sought to elucidate the mechanism of cat dander-induced inflammation-remodeling.

METHODS

We identified remodeling in mucosal samples from allergic asthma by using quantitative RT-PCR. We developed a model of aeroallergen-induced experimental asthma using repetitive cat dander extract exposure. We measured airway inflammation using immunofluorescence, leukocyte recruitment, and quantitative RT-PCR. Airway remodeling was measured by using histology, collagen content, myofibroblast numbers, and selected reaction monitoring. Inducible nuclear factor κB (NF-κB)-BRD4 interaction was measured by using a proximity ligation assay in situ.

RESULTS

Enhanced mesenchymal signatures are observed in bronchial biopsy specimens from patients with allergic asthma. Cat dander induces innate inflammation through NF-κB signaling, followed by production of a profibrogenic mesenchymal transition in primary human small airway epithelial cells. The IκB kinase-NF-κB signaling pathway is required for mucosal inflammation-coupled airway remodeling and myofibroblast expansion in the mouse model of aeroallergen exposure. Cat dander induces NF-κB/RelA to complex with and activate BRD4, resulting in modifying the chromatin environment of inflammatory and fibrogenic genes through its atypical histone acetyltransferase activity. A novel small-molecule BRD4 inhibitor (ZL0454) disrupts BRD4 binding to the NF-κB-RNA polymerase II complex and inhibits its histone acetyltransferase activity. ZL0454 prevents epithelial mesenchymal transition, myofibroblast expansion, IgE sensitization, and fibrosis in airways of naive mice exposed to cat dander.

CONCLUSIONS

NF-κB-inducible BRD4 activity mediates cat dander-induced inflammation and remodeling. Therapeutic modulation of the NF-κB-BRD4 pathway affects allergen-induced inflammation, epithelial cell-state changes, extracellular matrix production, and expansion of the subepithelial myofibroblast population.

Oxidative stress, autophagy and airway ion transport

Mucociliary clearance is critically important in protecting the airways from infection and from the harmful effects of smoke and various inspired substances known to induce oxidative stress and persistent inflammation. An essential feature of the clearance mechanism involves regulation of the periciliary liquid layer on the surface of the airway epithelium, which is necessary for normal ciliary beating and maintenance of mucus hydration. The underlying ion transport processes associated with airway surface hydration include epithelial Na⁺ channel-dependent Na⁺ absorption occurring in parallel with CFTR and Ca²⁺-activated Cl^- channel-dependent anion secretion, which are coordinately regulated to control the depth of the periciliary liquid layer. Oxidative stress is known to cause both acute and chronic effects on airway ion transport function, and an increasing number of studies in the past few years have identified an important role for autophagy as part of the physiological response to the damaging effects of oxidation. In this review, recent studies addressing the influence of oxidative stress and autophagy on airway ion transport pathways, along with results showing the potential of autophagy modulators in restoring the function of ion channels involved in transepithelial electrolyte transport necessary for effective mucociliary clearance, are presented.

A bronchoprotective role for Rgs2 in a murine model of lipopolysaccharide-induced airways inflammation

Background

Asthma exacerbations are associated with the recruitment of neutrophils to the lungs. These cells release proteases and mediators, many of which act at G protein-coupled receptors (GPCRs) that couple via Gq to promote bronchoconstriction and inflammation. Common asthma therapeutics up-regulate expression of the regulator of G protein signalling (RGS), RGS2. As RGS2 reduces signaling from Gq-coupled GPCRs, we have defined role(s) for this GTPase-activating protein in an acute neutrophilic model of lung inflammation.

Methods

Wild type and Rgs2^−/− C57Bl6 mice were exposed to nebulized lipopolysaccharide (LPS). Lung function (respiratory system resistance and compliance) was measured using a SCIREQ flexivent small animal ventilator. Lung inflammation was assessed by histochemistry, cell counting and by cytokine and chemokine expression in bronchoalveolar lavage (BAL) fluid.

Results

Lipopolysaccharide inhalation induced transient airways hyperreactivity (AHR) and neutrophilic lung inflammation. While AHR and inflammation was greatest 3 h post-LPS exposure, BAL neutrophils persisted for 24 h. At 3 h post-LPS inhalation, multiple inflammatory cytokines (CSF2, CSF3, IL6, TNF) and chemokines (CCL3, CCL4, CXCL1, CXCL2) were highly expressed in the BAL fluid, prior to declining by 24 h. Compared to wild type counterparts, Rgs2^−/− mice developed significantly greater airflow resistance in response to inhaled methacholine (MCh) at 3 h post-LPS exposure. At 24 h post-LPS exposure, when lung function was recovering in the wild type animals, MCh-induced resistance was increased, and compliance decreased, in Rgs2^−/− mice. Thus, Rgs2^−/− mice show AHR and stiffer lungs 24 h post-LPS exposure. Histological markers of inflammation, total and differential cell counts, and major cytokine and chemokine expression in BAL fluid were similar between wild type and Rgs2^−/− mice. However, 3 and 24 h post-LPS exposure, IL12B expression was significantly elevated in BAL fluid from Rgs2^−/− mice compared to wild type animals.

Conclusions

While Rgs2 is bronchoprotective in acute neutrophilic inflammation, no clear anti-inflammatory effect was apparent. Nevertheless, elevated IL12B expression in Rgs2^−/− animals raises the possibility that RGS2 could dampen Th1 responses. These findings indicate that up-regulation of RGS2, as occurs in response to inhaled corticosteroids and long-acting β₂-adrenoceptor agonists, may be beneficial in acute neutrophilic exacerbations of airway disease, including asthma.

Electronic supplementary material

The online version of this article (10.1186/s13223-018-0266-5) contains supplementary material, which is available to authorized users.

Plasticizer, di(2-ethylhexyl)phthalate (DEHP) enhances cockroach allergen extract-driven airway inflammation by enhancing pulmonary Th2 as well as Th17 immune responses in mice

In recent decades, there has been a gradual increase in the prevalence of asthma. Various factors including environmental pollutants have contributed to this phenomenon. Plasticizer, di(2-ethylhexyl)phthalate (DEHP) is one of the commonest environmental pollutants due to its association with plastic products. DEHP gets released from plastic products easily leading to respiratory exposure in humans. As a consequence, DEHP is associated with allergic asthma in humans and animals. DEHP is reported to act as an adjuvant in ovalbumin-induced mouse models of asthma at high doses. However, these studies mostly looked into the role of DEHP on Th2 cytokines/eosinophilic inflammation without investigating the role of airway epithelial cells (AECs)/dendritic cells (DCs)/Th17 cells. Its adjuvant activity with natural allergens such as cockroach allergens at tolerable daily intake needs to be explored. Cockroach allergens and DEHP may be inhaled together due to their coexistence in work place as well as household environments. Therefore, effect of DEHP was assessed in cockroach allergens extract (CE)-induced mouse model of asthma. Airway inflammation, histopathology, mucus secretion, and immune responses related to Th2/Th17/DCs and AECs were assessed in mice with DEHP exposure alone and in combination with CE. Our study shows that DEHP converts CE-induced eosinophilic inflammation into mixed granulocytic inflammation by promoting Th2 as well as Th17 immune responses. This was probably due to downregulation of E-cadherin in AECs, and enhancement of costimulatory molecules (MHCII/CD86/CD40)/pro-inflammatory cytokines (IL-6/MCP-1) in DCs by DEHP. This suggests that DEHP facilitates development of mixed granulocytic airway inflammation in the presence of a natural allergen.

Endothelial cells in the innate response to allergens and initiation of atopic asthma

Protease-activated receptor 2 (PAR-2), an airway epithelial pattern recognition receptor (PRR), participates in the genesis of house dust mite-induced (HDM-induced) asthma. Here, we hypothesized that lung endothelial cells and proangiogenic hematopoietic progenitor cells (PACs) that express high levels of PAR-2 contribute to the initiation of atopic asthma. HDM extract (HDME) protease allergens were found deep in the airway mucosa and breaching the endothelial barrier. Lung endothelial cells and PACs released the Th2-promoting cytokines IL-1α and GM-CSF in response to HDME, and the endothelium had PAC-derived VEGF-C-dependent blood vessel sprouting. Blockade of the angiogenic response by inhibition of VEGF-C signaling lessened the development of inflammation and airway remodeling in the HDM model. Reconstitution of the bone marrow in WT mice with PAR-2-deficient bone marrow also reduced airway inflammation and remodeling. Adoptive transfer of PACs that had been exposed to HDME induced angiogenesis and Th2 inflammation with remodeling similar to that induced by allergen challenge. Our findings identify that lung endothelium and PACs in the airway sense allergen and elicit an angiogenic response that is central to the innate nonimmune origins of Th2 inflammation.

Inhibition of BET bromodomains restores corticosteroid responsiveness in a mixed granulocytic mouse model of asthma

Asthma is a heterogeneous disease characterized by different endotypes/phenotypes. Th2/Th17 driven mixed granulocytic asthma is one of them and shows resistance to corticosteroid therapy. Bromodomain and extra-terminal (BET) proteins are required for differentiation of Th17 cells which play a pivotal role in neutrophilic inflammation. Therefore, we sought to characterize the differential effects of BET inhibitor versus corticosteroids, and their potential synergism in cockroach allergen extract (CE)-induced mixed granulocytic (eosinophilic and neutrophilic) mouse model of asthma having Th2/Th17 endotype. Effects of BET inhibitor, (+)JQ-1 alone and in combination with dexamethasone (Dexa) were assessed on airway inflammation as well as Th2/Th17 related airway immune responses in CE-induced mixed granulocytic asthma. Markers of steroid resistance [histone deacetylase 2 (HDAC2), and oxidative stress] were also assessed in the lungs of mice and primary human bronchial epithelial cells (HBECs). BET inhibitor, (+)JQ-1 abolished Th17 driven neutrophilic inflammation in CE-induced mixed granulocytic asthma. Dexa had limited effect on overall airway inflammation despite having significant reductions in Th2 driven immune responses. However, combination of (+)JQ-1 with Dexa completely blocked both Th2 and /Th17 driven immune responses in the lung which led to significant reductions in eosinophils, neutrophils, and mucin secretion. (+)JQ-1 also reversed CE- and IL-17A-induced decrease in HDAC2 expression in murine and human airway epithelial cells respectively.

Proteinase-activated receptor-2 blockade inhibits changes seen in a chronic murine asthma model

BACKGROUND

Proteinase-Activated Receptor-2 (PAR₂ ) is a G protein-coupled receptor activated by serine proteinases. We have shown that PAR₂ activation in the airways is involved in the development of allergic inflammation and airway hyperresponsiveness (AHR) in acute murine models. We hypothesized that functional inhibition of PAR₂ prevents allergic inflammation, AHR and airway remodeling in chronic allergic airway inflammation models.

MATERIAL AND METHODS

We developed and used a 12 week model of cockroach extract (CE)-mediated AHR, airway inflammation and remodeling in BALB/c mice.

RESULTS

Mice sensitized and challenged with CE for 12 weeks exhibit AHR, increased numbers of eosinophils in bronchoalveolar lavage (BAL) and increased collagen content in the lung tissue compared to saline controls. Administration of an anti-PAR₂ antibody, SAM-11, after the initial development of airway inflammation significantly inhibited all these parameters.

CONCLUSIONS

Our data demonstrate that PAR₂ signaling plays a key role in CE-induced AHR and airway inflammation/remodeling in long term models of allergic airway inflammation. Targeting PAR₂ activation may be a successful therapeutic strategy for allergic asthma.

Developments in the field of allergy mechanisms in 2015 through the eyes of Clinical & Experimental Allergy

In the first of two papers we described the development in the field of allergy mechanisms as described by Clinical and Experimental Allergy in 2015. Experimental models of allergic disease, basic mechanisms, clinical mechanisms and allergens are all covered. A second paper will cover clinical aspects.

Modified High-Molecular-Weight Hyaluronan Promotes Allergen-Specific Immune Tolerance

The extracellular matrix in asthmatic lungs contains abundant low-molecular-weight hyaluronan, and this is known to promote antigen presentation and allergic responses. Conversely, high-molecular-weight hyaluronan (HMW-HA), typical of uninflamed tissues, is known to suppress inflammation. We investigated whether HMW-HA can be adapted to promote tolerance to airway allergens. HMW-HA was thiolated to prevent its catabolism and was tethered to allergens via thiol linkages. This platform, which we call "XHA," delivers antigenic payloads in the context of antiinflammatory costimulation. Allergen/XHA was administered intranasally to mice that had been sensitized previously to these allergens. XHA prevents allergic airway inflammation in mice sensitized previously to either ovalbumin or cockroach proteins. Allergen/XHA treatment reduced inflammatory cell counts, airway hyperresponsiveness, allergen-specific IgE, and T helper type 2 cell cytokine production in comparison with allergen alone. These effects were allergen specific and IL-10 dependent. They were durable for weeks after the last challenge, providing a substantial advantage over the current desensitization protocols. Mechanistically, XHA promoted CD44-dependent inhibition of nuclear factor-κB signaling, diminished dendritic cell maturation, and reduced the induction of allergen-specific CD4 T-helper responses. XHA and other potential strategies that target CD44 are promising alternatives for the treatment of asthma and allergic sinusitis.

GM-CSF produced by the airway epithelium is required for sensitization to cockroach allergen

Airway epithelial cells are among the first to encounter inhaled allergens and can initiate allergic responses by producing pro-Th2 innate cytokines. In this study, we investigated the role of epithelial-derived cytokines in sensitization to a clinically relevant allergen, cockroach allergen (CRA). Among the epithelial-derived cytokines, granulocyte macrophage colony-stimulating factor (GM-CSF) had a central role in the initiation of Th2 allergic responses to CRA. We show that initial exposure to CRA directly activated airway epithelial cells through a TLR4-MyD88-dependent pathway and MyD88 signaling in epithelial cells induced upregulation of GM-CSF during sensitization. Epithelial-derived GM-CSF was required for allergic sensitization and selectively restored Th2 responses in the absence of MyD88. Thus, we demonstrate that epithelial-derived GM-CSF is a critical early signal during allergic sensitization to CRA.

Cockroach allergen serine proteinases: Isolation, sequencing and signalling via proteinase-activated receptor-2

BACKGROUND

Allergy to the German cockroach (Blattella germanica) is a significant asthma risk factor for inner-city communities. Cockroach, like other allergens, contains trypsin-like enzyme activity that contributes to allergenicity and airway inflammation by activating proteinase-activated receptors (PARs). To date, the enzymes responsible for the proteolytic activity of German cockroach allergen have not been characterized.

OBJECTIVES

We aimed to identify, isolate and characterize the trypsin-like proteinases in German cockroach allergen extracts used for clinical skin tests. For each enzyme, we sought to determine (1) its substrate and inhibitor enzyme kinetics (Km and IC50), (2) its amino acid sequence and (3) its ability to activate calcium signalling and/or ERK1/2 phosphorylation via PAR2.

METHODS

Using a trypsin-specific activity-based probe, we detected three distinct enzymes that were isolated using ion-exchange chromatography. Each enzyme was sequenced by mass spectometery (deconvoluted with an expressed sequence tag library), evaluated kinetically for its substrate/inhibitor profile and assessed for its ability to activate PAR2 signalling.

FINDINGS

Each of the three serine proteinase activity-based probe-labelled enzymes isolated was biochemically distinct, with different enzyme kinetic profiles and primary amino acid sequences. The three enzymes showed a 57%-71% sequence identity with a proteinase previously cloned from the American cockroach (Per a 10). Each enzyme was found to activate both Ca⁺⁺ and MAPK signalling via PAR2.

CONCLUSIONS AND RELEVANCE

We have identified three different serine proteinases from the German cockroach that may, via PAR2 activation, play different roles for allergen sensitization in vivo and may represent attractive therapeutic targets for asthma.

Protective Roles for RGS2 in a Mouse Model of House Dust Mite-Induced Airway Inflammation

The GTPase-accelerating protein, regulator of G-protein signalling 2 (RGS2) reduces signalling from G-protein-coupled receptors (GPCRs) that signal via Gαq. In humans, RGS2 expression is up-regulated by inhaled corticosteroids (ICSs) and long-acting β2-adrenoceptor agonists (LABAs) such that synergy is produced in combination. This may contribute to the superior clinical efficacy of ICS/LABA therapy in asthma relative to ICS alone. In a murine model of house dust mite (HDM)-induced airways inflammation, three weeks of intranasal HDM (25 μg, 3×/week) reduced lung function and induced granulocytic airways inflammation. Compared to wild type animals, Rgs2-/- mice showed airways hyperresponsiveness (increased airways resistance and reduced compliance). While HDM increased pulmonary inflammation observed on hematoxylin and eosin-stained sections, there was no difference between wild type and Rgs2-/- animals. HDM-induced mucus hypersecretion was also unaffected by RGS2 deficiency. However, inflammatory cell counts in the bronchoalveolar lavage fluid of Rgs2-/- animals were significantly increased (57%) compared to wild type animals and this correlated with increased granulocyte (neutrophil and eosinophil) numbers. Likewise, cytokine and chemokine (IL4, IL17, IL5, LIF, IL6, CSF3, CXCLl, CXCL10 and CXCL11) release was increased by HDM exposure. Compared to wild type, Rgs2-/- animals showed a trend towards increased expression for many cytokines/chemokines, with CCL3, CCL11, CXCL9 and CXCL10 being significantly enhanced. As RGS2 expression was unaffected by HDM exposure, these data indicate that RGS2 exerts tonic bronchoprotection in HDM-induced airways inflammation. Modest anti-inflammatory and anti-remodelling roles for RGS2 are also suggested. If translatable to humans, therapies that maximize RGS2 expression may prove advantageous.

A review on emerging frontiers of house dust mite and cockroach allergy research

Currently, mankind is afflicted with diversified health issues, allergies being a common, yet little understood malady. Allergies, the outcome of a baffled immune system encompasses myriad allergens and causes an array of health consequences, ranging from transient to recurrent and mild to fatal. Indoor allergy is a serious hypersensitivity in genetically-predisposed people, triggered by ingestion, inhalation or mere contact of allergens, of which mite and cockroaches are one of the most-represented constituents. Arduous to eliminate, these aeroallergens pose constant health challenges, mostly manifested as respiratory and dermatological inflammations, leading to further aggravations if unrestrained. Recent times have seen an unprecedented endeavour to understand the conformation of these allergens, their immune manipulative ploys and other underlying causes of pathogenesis, most importantly therapies. Yet a large section of vulnerable people is ignorant of these innocuous-looking immune irritants, prevailing around them, and continues to suffer. This review aims to expedite this field by a concise, informative account of seminal findings in the past few years, with particular emphasis on leading frontiers like genome-wide association studies (GWAS), epitope mapping, metabolomics etc. Drawbacks linked to current approaches and solutions to overcome them have been proposed.

Psoriatic inflammation enhances allergic airway inflammation through IL-23/STAT3 signaling in a murine model

Psoriasis is an autoimmune inflammatory skin disease characterized by activated IL-23/STAT3/Th17 axis. Recently psoriatic inflammation has been shown to be associated with asthma. However, no study has previously explored how psoriatic inflammation affects airway inflammation. Therefore, this study investigated the effect of imiquimod (IMQ)-induced psoriatic inflammation on cockroach extract (CE)-induced airway inflammation in murine models. Mice were subjected to topical and intranasal administration of IMQ and CE to develop psoriatic and airway inflammation respectively. Various analyses in lung/spleen related to inflammation, Th17/Th2/Th1 cell immune responses, and their signature cytokines/transcription factors were carried out. Psoriatic inflammation in allergic mice was associated with increased airway inflammation with concurrent increase in Th2/Th17 cells/signature cytokines/transcription factors. Splenic CD4+ T and CD11c+ dendritic cells in psoriatic mice had increased STAT3/RORC and IL-23 mRNA expression respectively. This led us to explore the effect of systemic IL-23/STAT3 signaling on airway inflammation. Topical application of STA-21, a small molecule STAT3 inhibitor significantly reduced airway inflammation in allergic mice having psoriatic inflammation. On the other hand, adoptive transfer of IL-23-treated splenic CD4+ T cells from allergic mice into naive recipient mice produced mixed neutrophilic/eosinophilic airway inflammation similar to allergic mice with psoriatic inflammation. Our data suggest that systemic IL-23/STAT3 axis is responsible for enhanced airway inflammation during psoriasis. The current study also suggests that only anti-asthma therapy may not be sufficient to alleviate airway inflammatory burden in asthmatics with psoriasis.

Proteinases, Their Extracellular Targets, and Inflammatory Signaling

Given that over 2% of the human genome codes for proteolytic enzymes and their inhibitors, it is not surprising that proteinases serve many physiologic-pathophysiological roles. In this context, we provide an overview of proteolytic mechanisms regulating inflammation, with a focus on cell signaling stimulated by the generation of inflammatory peptides; activation of the proteinase-activated receptor (PAR) family of G protein-coupled receptors (GPCR), with a mechanism in common with adhesion-triggered GPCRs (ADGRs); and by proteolytic ion channel regulation. These mechanisms are considered in the much wider context that proteolytic mechanisms serve, including the processing of growth factors and their receptors, the regulation of matrix-integrin signaling, and the generation and release of membrane-tethered receptor ligands. These signaling mechanisms are relevant for inflammatory, neurodegenerative, and cardiovascular diseases as well as for cancer. We propose that the inflammation-triggering proteinases and their proteolytically generated substrates represent attractive therapeutic targets and we discuss appropriate targeting strategies.

Functional inhibition of PAR2 alleviates allergen-induced airway hyperresponsiveness and inflammation

BACKGROUND

Proteinase-activated receptor 2 (PAR2 ) is a G protein-coupled receptor activated by trypsin-like serine proteinases. PAR2 activation has been associated with inflammation including allergic airway inflammation. We have also shown that PAR2 activation in the airways leads to allergic sensitization. The exact contribution of PAR2 in the development of eosinophilic inflammation and airway hyperresponsiveness (AHR) in sensitized individuals is not clear.

OBJECTIVE

To investigate whether functional inhibition of PAR2 during allergen challenge of allergic mice would inhibit allergen-induced AHR and inflammation in mouse models of asthma.

METHODS

Mice were sensitized and challenged with ovalbumin (OVA) or cockroach extract (CE). To investigate the role of PAR2 in the development of AHR and airway inflammation, we administered blocking anti-PAR2 antibodies, or a cell permeable peptide inhibitor of PAR2 signalling, pepducin, i.n. before allergen challenges and then assessed AHR and airway inflammation.

RESULTS

Administration of anti-PAR2 antibodies significantly inhibited OVA- and CE-induced AHR and airway inflammation. In particular, two anti-PAR2 antibodies, the monoclonal SAM-11 and polyclonal B5, inhibited AHR, airway eosinophilia, the increase of cytokines in the lung tissue and antigen-specific T cell proliferation, but had no effect on antigen-specific IgG and IgE levels. Pepducin was also effective in inhibiting AHR and airway inflammation in an OVA model of allergic airway inflammation.

CONCLUSIONS AND CLINICAL RELEVANCE

Functional blockade of PAR2 in the airways during allergen challenge improves allergen-induced AHR and inflammation in mice. Therefore, topical PAR2 blockade in the airways, through anti-PAR2 antibodies or molecules that interrupt PAR2 signalling, has the potential to be used as a therapeutic option in allergic asthma.

Challenges and Opportunities in Protease-Activated Receptor Drug Development

Protease-activated receptors (PARs) are a unique class of G protein-coupled receptors (GPCRs) that transduce cellular responses to extracellular proteases. PARs have important functions in the vasculature, inflammation, and cancer and are important drug targets. A unique feature of PARs is their irreversible proteolytic mechanism of activation that results in the generation of a tethered ligand that cannot diffuse away. Despite the fact that GPCRs have proved to be the most successful class of druggable targets, the development of agents that target PARs specifically has been challenging. As a consequence, researchers have taken a remarkable diversity of approaches to develop pharmacological entities that modulate PAR function. Here, we present an overview of the diversity of therapeutic agents that have been developed against PARs. We further discuss PAR biased signaling and the influence of receptor compartmentalization, posttranslational modifications, and dimerization, which are important considerations for drug development.

Allergens stimulate store-operated calcium entry and cytokine production in airway epithelial cells

Aberrant immune responses to environmental allergens including insect allergens from house dust mites and cockroaches contribute to allergic inflammatory diseases such as asthma in susceptible individuals. Airway epithelial cells (AECs) play a critical role in this process by sensing the proteolytic activity of allergens via protease-activated receptors (PAR2) to initiate inflammatory and immune responses in the airway. Elevation of cytosolic Ca²⁺ is an important signaling event in this process, yet the fundamental mechanism by which allergens induce Ca²⁺ elevations in AECs remains poorly understood. Here we find that extracts from dust mite and cockroach induce sustained Ca²⁺ elevations in AECs through the activation of Ca²⁺ release-activated Ca²⁺ (CRAC) channels encoded by Orai1 and STIM1. CRAC channel activation occurs, at least in part, through allergen mediated stimulation of PAR2 receptors. The ensuing Ca²⁺ entry then activates NFAT/calcineurin signaling to induce transcriptional production of the proinflammatory cytokines IL-6 and IL-8. These findings highlight a key role for CRAC channels as regulators of allergen induced inflammatory responses in the airway.

Bronchoprotection and bronchorelaxation in asthma: New targets, and new ways to target the old ones

Despite over 50years of inhaled beta-agonists and corticosteroids as the default management or rescue drugs for asthma, recent research suggests that new therapeutic options are likely to emerge. This belief stems from both an improved understanding of what causes and regulates airway smooth muscle (ASM) contraction, and the identification of new targets whose inhibition or activation can relax ASM. In this review we discuss the recent findings that provide new insight into ASM contractile regulation, a revolution in pharmacology that identifies new ways to "tune" G protein-coupled receptors to improve therapeutic efficacy, and the discovery of several novel targets/approaches capable of effecting bronchoprotection or bronchodilation.

Subepithelial Accumulation of Versican in a Cockroach Antigen-Induced Murine Model of Allergic Asthma

The extracellular matrix (ECM) is an important contributor to the asthmatic phenotype. Recent studies investigating airway inflammation have demonstrated an association between hyaluronan (HA) accumulation and inflammatory cell infiltration of the airways. The ECM proteoglycan versican interacts with HA and is important in the recruitment and activation of leukocytes during inflammation. We investigated the role of versican in the pathogenesis of asthmatic airway inflammation. Using cockroach antigen (CRA)-sensitized murine models of allergic asthma, we demonstrate increased subepithelial versican in the airways of CRA-treated mice that parallels subepithelial increases in HA and leukocyte infiltration. During the acute phase, CRA-treated mice displayed increased gene expression of the four major versican isoforms, as well as increased expression of HA synthases. Furthermore, in a murine model that examines both acute and chronic CRA exposure, versican staining peaked 8 days following CRA challenge and preceded subepithelial leukocyte infiltration. We also assessed versican and HA expression in differentiated primary human airway epithelial cells from asthmatic and healthy children. Increases in the expression of versican isoforms and HA synthases in these epithelial cells were similar to those of the murine model. These data indicate an important role for versican in the establishment of airway inflammation in asthma.

Airway responses towards allergens - from the airway epithelium to T cells

The prevalence of allergic diseases such as allergic rhinitis is increasing, affecting up to 30% of the human population worldwide. Allergic sensitization arises from complex interactions between environmental exposures and genetic susceptibility, resulting in inflammatory T helper 2 (Th2) cell-derived immune responses towards environmental allergens. Emerging evidence now suggests that an epithelial dysfunction, coupled with inherent properties of environmental allergens, can be responsible for the inflammatory responses towards allergens. Several epithelial-derived cytokines, such as thymic stromal lymphopoietin (TSLP), IL-25 and IL-33, influence tissue-resident dendritic cells (DCs) as well as Th2 effector cells. Exposure to environmental allergens does not elicit Th2 inflammatory responses or any clinical symptoms in nonatopic individuals, and recent findings suggest that a nondamaged, healthy epithelium lowers the DCs' ability to induce inflammatory T-cell responses towards allergens. The purpose of this review was to summarize the current knowledge on which signals from the airway epithelium, from first contact with inhaled allergens all the way to the ensuing Th2-cell responses, influence the pathology of allergic diseases.

Increased Protease-Activated Receptor-2 (PAR-2) Expression on CD14++CD16+ Peripheral Blood Monocytes of Patients with Severe Asthma

BACKGROUND

Protease-Activated Receptor-2 (PAR-2), a G protein coupled receptor activated by serine proteases, is widely expressed in humans and is involved in inflammation. PAR-2 activation in the airways plays an important role in the development of allergic airway inflammation. PAR-2 expression is known to be upregulated in the epithelium of asthmatic subjects, but its expression on immune and inflammatory cells in patients with asthma has not been studied.

METHODS

We recruited 12 severe and 24 mild/moderate asthmatics from the University of Alberta Hospital Asthma Clinics and collected baseline demographic information, medication use and parameters of asthma severity. PAR-2 expression on blood inflammatory cells was analyzed by flow cytometry.

RESULTS

Subjects with severe asthma had higher PAR-2 expression on CD14++CD16+ monocytes (intermediate monocytes) and also higher percentage of CD14++CD16+PAR-2+ monocytes (intermediate monocytes expressing PAR-2) in blood compared to subjects with mild/moderate asthma. Receiver operating characteristics (ROC) curve analysis showed that the percent of CD14++CD16+PAR-2+ in peripheral blood was able to discriminate between patients with severe and those with mild/moderate asthma with high sensitivity and specificity. In addition, among the whole populations, subjects with a history of asthma exacerbations over the last year had higher percent of CD14++CD16+ PAR-2+ cells in peripheral blood compared to subjects without exacerbations.

CONCLUSIONS

PAR-2 expression is increased on CD14++CD16+ monocytes in the peripheral blood of subjects with severe asthma and may be a biomarker of asthma severity. Our data suggest that PAR-2 -mediated activation of CD14++CD16+ monocytes may play a role in the pathogenesis of severe asthma.

Proteinase activated receptor-2-mediated dual oxidase-2 up-regulation is involved in enhanced airway reactivity and inflammation in a mouse model of allergic asthma

Airway epithelial cells (AECs) express a variety of receptors, which sense danger signals from various aeroallergens/pathogens being inhaled constantly. Proteinase-activated receptor 2 (PAR-2) is one such receptor and is activated by cockroach allergens, which have intrinsic serine proteinase activity. Recently, dual oxidases (DUOX), especially DUOX-2, have been shown to be involved in airway inflammation in response to Toll-like receptor activation. However, the association between PAR-2 and DUOX-2 has not been explored in airways of allergic mice. Therefore, this study investigated the contribution of DUOX-2/reactive oxygen species (ROS) signalling in airway reactivity and inflammation after PAR-2 activation. Mice were sensitized intraperitoneally with intact cockroach allergen extract (CE) in the presence of aluminium hydroxide followed by intranasal challenge with CE. Mice were then assessed for airway reactivity, inflammation, oxidative stress (DUOX-2, ROS, inducible nitric oxide synthase, nitrite, nitrotyrosine and protein carbonyls) and apoptosis (Bax, Bcl-2, caspase-3). Challenge with CE led to up-regulation of DUOX-2 and ROS in AECs with concomitant increases in airway reactivity/inflammation and parameters of oxidative stress, and apoptosis. All of these changes were significantly inhibited by intranasal administration of ENMD-1068, a small molecule antagonist of PAR-2 in allergic mice. Administration of diphenyliodonium to allergic mice also led to improvement of allergic airway responses via inhibition of the DUOX-2/ROS pathway; however, these effects were less pronounced than PAR-2 antagonism. The current study suggests that PAR-2 activation leads to up-regulation of the DUOX-2/ROS pathway in AECs, which is involved in regulation of airway reactivity and inflammation via oxidative stress and apoptosis.

Allergens and the airway epithelium response: gateway to allergic sensitization

Allergic sensitization to inhaled antigens is common but poorly understood. Although lung epithelial cells were initially merely regarded as a passive barrier impeding allergen penetrance, we now realize that they recognize allergens through expression of pattern recognition receptors and mount an innate immune response driven by activation of nuclear factor κB. On allergen recognition, epithelial cells release cytokines, such as IL-1, IL-25, IL-33, thymic stromal lymphopoietin, and GM-CSF, and endogenous danger signals, such as high-mobility group box 1, uric acid, and ATP, that activate the dendritic cell network and other innate immune cells, such as basophils and type 2 innate lymphoid cells. Different allergens stimulate different aspects of this general scheme, and common environmental risk factors for sensitization, such as cigarette smoke and diesel particle exposure, do so as well. All of this is influenced by genetic polymorphisms affecting epithelial pattern recognition, barrier function, and cytokine production. Therefore, epithelial cells are crucial in determining the outcome of allergen inhalation.

Airway epithelium interactions with aeroallergens: role of secreted cytokines and chemokines in innate immunity

Airway epithelial cells are the first line of defense against the constituents of the inhaled air, which include allergens, pathogens, pollutants, and toxic compounds. The epithelium not only prevents the penetration of these foreign substances into the interstitium, but also senses their presence and informs the organism’s immune system of the impending assault. The epithelium accomplishes the latter through the release of inflammatory cytokines and chemokines that recruit and activate innate immune cells at the site of assault. These epithelial responses aim to eliminate the inhaled foreign substances and minimize their detrimental effects to the organism. Quite frequently, however, the innate immune responses of the epithelium to inhaled substances lead to chronic and high level release of pro-inflammatory mediators that may mediate the lung pathology seen in asthma. The interactions of airway epithelial cells with allergens will be discussed with particular focus on interactions-mediated epithelial release of cytokines and chemokines and their role in the immune response. As pollutants are other major constituents of inhaled air, we will also discuss how pollutants may alter the responses of airway epithelial cells to allergens.

The role of proteases in pain

Proteinase-activated receptors (PARs) are a family of G protein-coupled receptor that are activated by extracellular cleavage of the receptor in the N-terminal domain. This slicing of the receptor exposes a tethered ligand which binds to a specific docking point on the receptor surface to initiate intracellular signalling. PARs are expressed by numerous tissues in the body, and they are involved in various physiological and pathological processes such as food digestion, tissue remodelling and blood coagulation. This chapter will summarise how serine proteinases activate PARs leading to the development of pain in several chronic pain conditions. The potential of PARs as a drug target for pain relief is also discussed.