Molecular mechanisms in the development of asthma
Immune system cells migrate to the lungs and display their functional properties to develop asthma.58 It was demonstrated that Th2 resident memory T cells and circulating memory Th2 traffic into the lung parenchyma and initiate a perivascular inflammation to promote eosinophil and CD4+ T cell recruitment. Th2 resident memory cells proliferate near airways and induce mucus metaplasia, AHR and airway eosinophil activation. Transcriptional analysis revealed that Th2 resident memory cells and circulating memory Th2 cells share a core Th2 gene signature, but also exhibit distinct transcriptional profiles (Figure 1).58
Braga et al.59 described the cellular landscape of airway lining at the single-cell level. This comprehensive analysis in asthma identified dominance of TH2 cells interacting with structural and inflammatory cells. The presented data open new perspective on lung biology and molecular mechanisms of asthma.59 A Th1/Th2 imbalance is commonly seen in allergic asthma and it is shifted back towards Th1 by Protein S. Protein S, an anticoagulant, anti-inflammatory and anti-apoptotic glycoprotein is associated with a reduction of AHR, lung tissue inflammatory cell infiltration, Th2 cytokines in the lung and IgE levels. Asayama et al. showed that it could inhibit allergic asthma by upregulating the type 1 cytokines TNF-α and IL-12 while downregulating IL-5+Th2 cells.60 A downregulation of Th2 cells is also achieved by intraperitoneal injection of cysteamine, along with IL17+ Th17 and IL13+IL17+ Th2/Th17 cells, thus effectively inhibiting AHR in an allergic mouse model upon retreatment with the allergen.61 Lu et al. reported high ILC2 levels in type 2 asthmatics, while non-type 2 asthmatics showed higher levels of Th17 cells and an inversed Th1/Th17 ratio.62The pathways of Th2-high and Th17-high inflammation were reciprocally regulated.
The type 2 immunity is mainly driven by IL-4 and IL-13 signaling which share the common receptor subunit interleukin-4 receptor alpha (IL-4Rα) (Figure 2). Withdrawal of IL-4Rα signaling prevents the development of AHR, eosinophilia, and goblet cell metaplasia in allergen-sensitized mice. However, the IL-4Rα-deficient mice does not develop type 17 immunity after allergic sensitization.63
The increased serum IL-33 levels in asthma patients has been linked to the activation of mast cells. To characterise the mechanisms of IL-33 contribution to asthma development, Ro et al.64 used knockdown or pharmacological inhibitors in bone marrow-derived mast cells and animal model. The study revealed that the ‘MyD88-5-/12-LO-BLT2-NF-κB’ cascade contributes to the IL-33 signaling to induce IL-13 synthesis in mast cells, which may represent a new therapeutic target for severe asthma.64
Lv et al. investigated the role and the mechanisms of IL-37 in type 2-mediated allergic lung inflammation in HDM-induced murine asthma model. They found IL-37 impairs HDM-induced asthma, most likely by preventing IL4/IL13-induced chemokine ligand (CCL)11 production from fibroblasts and airway smooth muscle cells through its direct effect on tracheobronchial epithelial cells.65
In a murine allergic asthma model, IL-4 receptor α blockade decreased serum IgE and IL-5 levels and increased the level of IgG1, IgG2a, IgG2b and IgG3 prior/during sensitization. Thus following the IL-4 receptor α blockage, an immediate immunoglobulin response is induced accompanying the suppression of type 2 cytokines with a potential long-lasting reduction in Th2-biased T-regulatory (Treg) cells.66
Chitotriosidase (chitinase 1, Chit1) has been known as regulator and stimulator in Th2 responses. Hong et. al. studied the possible mechanisms and its role in the pathogenesis of allergic asthma.67 Significantly elevated levels of Chit1 in the sputum of patients with childhood asthma were reported. Moreover, in the absence of Chit1 molecule, forkhead box P3 (Foxp3)+ Treg cell frequency decreased in the lungs of mice besides TGF-β1 levels, which suggests a protective role in asthmatic airway responses by regulating TGF-β secretion and Foxp3+ Treg cells.
A neutrophilic inflammatory response or Th17 response is classified as non‐type 2 asthma. The potential mechanisms of non‐type 2 asthma have been well-described in a comprehensive review article.68 Th17 cells are shown to play a key role in allergic asthma by the secretion of inflammatory cytokines, including IL-17A, IL-17F, IL-21, and IL-23. Worth et al.successfully demonstrated that genetic variants in IL17F, L17A, and IL-23 signaling genes (IL23A, IL23R, IL12B) are associated with asthma. Their results also confirmed that IgE levels could influence the Th17-related gene expression.69 Moreover, the IL-17 levels and leukotriene (LT)B4 were shown to increase with disease severity in serum and sputum. Ro et al. investigated the role of LTB4 receptors, BLT1 and BLT2, in a neutrophil‐dominant pulmonary inflammation murine model. BLT1 and BLT2 were proven as important in asthma development, and IL‐17 was identified as a key cytokine synthesized through the BLT1/2‐cascade.70 In humans, neutrophils released from the bone marrow express low levels of CD16 and high levels of CD62L. This CD16dimCD62Lhigh subset is considered representing the immature neutrophil, whereas the CD16highCD62Ldim subset is thought to be mature and induced systemic inflammation.71 Ekstedt et al.recently found that this novel neutrophil subset, CD16highCD62Ldim, is increased in the blood following an inhaled allergen bronchial challenge bronchoprovocation test. They proposed that increasing neutrophil subgroups in allergic asthma might offer new opportunities in advancing allergic asthma research (Figure 3).71
‘Eat me’ and ‘Don’t eat me’ signals are an integral part of phagoptosis and thus, in neutrophils, they are important for the resolution of pulmonary inflammation as prolonged survival of airway neutrophils is directly related to asthma severity.72 In allergic donors, neutrophils showed an upregulation of CD47 (‘don’t eat me’) and a simultaneous downregulation of CD36 and CD43 (‘eat me’) compared to healthy controls (Figure 3).73 Additionally, less mRNA for CCL4 and CCL20 (homing cytokines for macrophages) was found in airway neutrophils of asthmatic donors than in healthy controls.
A study performed by Guan et al.74 has reported that reduced monocytic myeloid-derived suppressor cells (M-MDSC) may result in abnormal T responses with the increase in Th2 and Th17 cells and decrease in Treg cells in asthma patients. These results suggest a new immune regulatory mechanism in the pathogenesis of asthma open for further research.74 To define whether there is a deficiency in Breg subsets, Wirz et al.75 compared the percentages of IL-10-producing Breg subsets in peripheral blood from patients with asthma and AR. They demonstrated that there is no difference in numbers of Bregs in the patients when compared to healthy controls.75
Current research suggests that several receptors involving in asthma pathophysiology, including PGD2 receptor 2 (DP2 or CRTH2), as well as, colony‐stimulating factor 1 receptor (CSF1R). The DP2 receptor is an essential regulator in allergic asthma because it can be activated by both allergic and nonallergic stimuli.76 The activation of the DP2 receptor pathway increases both the airway smooth muscle mass and vascularization in airway walls, resulting in downstream effects on asthma development. Interestingly, airway epithelial cells secrete CSF1 into the alveolar space in response to aeroallergen. Moon et al. demonstrated that inhibition of the CSF1‐CSF1R signaling pathway could suppress sensitization to aeroallergens and subsequent allergic lung inflammation in mice with chronic asthma. They conclude that inhibition of CSF1R is a potential new target for the medical treatment of allergic asthma.77
Kim et al78 showed that the ceramide/sphingosine-1-phosphate ratio can discriminate between two different asthma endotypes. Sphingosine-1-phosphate (S1P) was found to positively correlate with the percentage of platelet adherent eosinophils, indicating eosinophilic inflammation. The percentage of CD66+ activated neutrophils positively correlates with C16:0 ceramide levels, indicating neutrophilic inflammation. An upregulation of ceramide-mediated pro-apoptotic signals were found in patients with higher CD66+ neutrophils. For both eosinophilic and neutrophilic pathways, genetic SNPs were found. Increased ceramide levels may also contribute to the development of obese asthma. Choi et al.79 found multiple ceramides (C16:0, C18:0 and C20:0) to accumulate in obese mice as a result of a high fat diet, inducing AHR and inflammation. Increased expression of ceramide synthase (CerS)1 and CerS6 were found in the lungs, and CerS6 was identified as a potential future therapeutic target for obese asthma.
Felix and Kuschnir80 pointed out that Arginase inhibitors could act beneficial for obese asthma patients by upregulating the L-Arginine/asymmetric dimethylarginine ratio as an addition to a previous article by Meurs et al.81 The reply by Meurs et al.82 agreed and suggested an even more direct link in pointing to the previously known increased arginase expression and activity in obese asthma patients. However, the mechanism behind this is still unknown, opening up avenues for future research. One of the characteristics of asthmatic lung is airway remodeling. Whether airway inflammation and remodeling in asthmatics can be related to persistent airflow obstruction was evaluated in a recent study that showed in asthmatics with persistent airflow obstruction increased airway smooth muscle area, decreased periostin and transforming growth factor beta (TGF-β) and chymase positive cells compared to patient with non-persistent obstruction.83 Asaduzzaman et al.84 studied cockroach-induced chronic murine asthma models by using a specific inhibitor of proteinase-activated receptor-2 (PAR2) to identify a role of PAR2signalling on AHR and airway inflammation/remodelling. They showed that administration of an anti-PAR2 antibody significantly inhibited AHR, inflammation and collagen accumulation in the lung tissue. The authors suggest that PAR2 blockade may be a successful therapeutic strategy for human allergic airway diseases.84
A recent study has introduced an in vivo molecular platform to elucidate both disease mechanisms and therapeutic targets of virus associated and non-virus-associated asthma exacerbations. A group of exacerbation-related modules included SMAD3 signaling, epidermal growth factor receptor signaling, extracellular matrix production, mucus hypersecretion, and eosinophil activation.85  Another study in infants suffering from rhinovirus bronchiolitis showed that concentrations of IL‐4, IL‐5, IL‐13, and TSLP, were correlated with a higher risk of asthma onset in childhood.86 In a study with 5-year-old children demonstrated that the development of atopic asthma in children with early rhinovirus-induced wheezing was associated with differentially methylated genomic regions. The strongest methylation changes were observed in the SMAD3 gene promoter region at chr 15q22.33 and in introns of the DDO/METTL24  genes at 6q21.87
Respiratory syncytial virus (RSV) infection affects a large number of the population in the early years of their life and is associated with an increased asthma risk. Schuler et al. show that uric acid and IL-1β play a role in the mechanism, and their inhibition can prevent the development of asthma after neonatal RSV infection, thus being a possible therapeutic target.88 A prospective study suggested infants suffering from bronchiolitis at less than 6 months of age, have a twice higher risk of doctor-diagnosed asthma after follow-up for 11-13 years compared to general Finnish population, and that the RSV was the main reason for bronchiolitis in these infants.89 The EAACI Influenza in Asthma Task Force performed a scoping review about the influenza burden, prevention, and treatment in asthma. In this review, vaccination conferred a degree of protection against influenza illness and asthma-related morbidity to children with asthma, but not to adults with asthma. Although influenza vaccines appeared to be safe for asthmatic patients, there is a lack of data regarding efficacy in adults.90
A few studies have been performed to reveal the effects of medical treatments with ILCs in allergic airway diseases so far. The mechanisms in action of glucocorticoid therapy on ILCs were assessed in a prospective study by Yu et al.91 Their data showed the administration of glucocorticoids regulates ILC2s via MEK/JAK-STAT signalization pathways in asthma patients.
To test the role of MUC1 membrane mucin in uncontrolled severe asthma, the recent study analysed the association of MUC1-CT (cytoplasmic tail in the C-terminal subunit) with corticosteroid efficacy in in vitro and in vivo models. The results suggested that MUC1-CT has an important role in the modulation of the anti-inflammatory effects of corticosteroids and may be a promising new approach for the treatment of asthma.92
MicroRNAs (miRNA) are secreted in extracellular vesicles and regulate signaling pathways by being transferred between cells. Recently, Bartel et al. characterized the miRNA secretion in extracellular vesicles from normal bronchial epithelial cells treated with IL‐13 to induce an asthma‐like epithelium. They observed that miR‐34a, miR‐92b, and miR‐210 were involved in the early development of a Th2 response in the airways and asthma93.
Suojalehto et al. examined the protein expressions from nasal brush samples from work-related asthma patients and healthy controls using the proteomic approach. Work-related asthma patients often are exposed to welding fumes, aerosols composed of hazardous metals and gases. The nasal brush samples are a relatively non-invasive specimen containing proteins secreted from epithelial and inflammatory cells. Their results indicated that the nasal epithelial proteome of the work-related asthma patients is highly enriched in processes related to inflammatory and calcium signalling, free radical scavenging and oxidative stress response, and metabolism.94
Uwadiae et al.95 studied the role of T follicular helper (TFH) cells of allergen airway disease in a mouse model of allergic asthma. They found that TFH cells accumulate beside the germinal center B cells with constant allergen exposure. Furthermore, blocking the inducible costimulatory (ICOS) signaling disrupted the TFH cell response; however, it did not have an impact on the differentiating of other CD4+ T-cell subsets. Based on these observations, the authors suggest that TFH cells have critical roles in the regulation and the ICOS/ICOS-L pathway can be a novel therapeutic target in allergic airway disease.95