Abbreviations: CCSP, clara cell secretory protein; Sec14l3, Sec14-like protein 3; YKL-40, chitinase-3-like protein 1; ACO, asthma-COPD overlap; ICS, inhaled corticosteroid

Epithelium-derived biomarkers ：

Several studies have reported the role of potential epithelium-derived biomarkers in asthma due, in part, to the relative ease of obtaining samples from the respiratory epithelium. Prominent among these are periostin, ezrin, fibrinogen, CCSP, IL-33, TSLP, claudin 4, claudin 18, MMP-9 and Sec14l3.

Epithelium- and tissue-associated biomarkers (biomarkers of mixed origin) ：

In this section we focus on a few key current and emerging potential asthma biomarkers but others including osteopontin, YKL40 and IL-25 have also been postulated as important.

4.1. Periostin is an extracellular matrix protein induced by IL-4 and IL-13 in AECs and lung fibroblasts. It is a key molecule connecting T2 airway inflammation and airway remodeling, and is related to T2_high eosinophilic asthma [53]. Mouse models suggest a role of periostin in subepithelial fibrosis, eosinophil recruitment, and mucus production from goblet cells [54]. In childhood asthma, the level of periostin was significantly higher than that of the healthy control groups [55]. Serum periostin levels in 2-year-old children are 2-3 fold higher than previously observed adult levels and can predict asthma at aged 6. Previous data in adults found elevated levels of periostin in the serum of asthmatics and that levels were associated with fixed and more severe airflow obstruction [56] and greater decline in lung function [57]. Thus, periostin was reported to be a systemic and promising biomarker of T2, IL-13-driven, corticosteroid-responsive asthma. Furthermore, serum periostin levels were stable during disease progression in adults with asthma and did not show a seasonal variation [58]. In children between 4 and 11 years of age, serum periostin was the best predictor of airway eosinophilia compared with FeNO, blood eosinophil counts and serum IgE [59]. Its movement from inflamed tissues to the systemic circulation further supported its use as a biomarker for T2-high asthma.

4.2. Club cell secretory protein 16 (CCSP16) is produced predominantly by club cells and non-ciliated epithelial cells in the distal airways and is readily detectable in the peripheral circulation [65]. Mounting evidence suggests that this protein is critical in mediating anti-inflammatory and anti-oxidant functions within the lung and, by virtue of these activities, may protect against development of obstructive lung diseases [66].

4.3. Ezrin is a membrane-associated cytoskeleton protein that plays a role in maintaining cell morphology and adhesion between cells and protects AEC barrier function. We have proposed that the downregulation of ezrin indicates AEC injury in asthma and may be a potential marker for monitoring the severity of disease. This concept is based upon the functional effect of ezrin on AEC barrier function and the high degree of correlation between decreased ezrin levels in several asthma biosamples, including EBCs and serum in humans and BAL in mice, and decreased lung function [70]. Furthermore, serum ezrin levels negatively correlated with serum periostin and IL-13 levels. Although exosome secretion from AECs was suggested as a mechanism by which ezrin localizes in EBC, BAL and serum, further work is needed to confirm this [71]. In contrast, acute bronchial challenge of patients with steroid-naïve mild allergic asthma with Dermatophagoides pteronyssinus resulted in enhanced serum levels of ezrin and IL-13 after 24 hours [72]. The authors suggested that acute asthma attacks result in heightened release of biologically active substances such as ezrin from damaged AECs, which initiates an IL-13-driven immune cascade that results in further increases in ezrin levels [72]. Further studies are required to look at temporal changes in ezrin levels in various biosamples and the impact of natural asthma exacerbations.

4.4. Chitinase-3-like protein 1 (CHI3L1), also known as YKL40, is expressed and secreted by various cells such as epithelial cells, macrophages, neutrophils, and smooth muscle cells. It is significantly increased in asthma patients and its expression is closely related to asthma severity and airway remodeling. YKL40 may promote the airway remodeling of asthma by activating FAK and MAPK signaling pathways, inducing epithelial mesenchymal transition (EMT) and subepithelial fibrosis [73]. Tang and colleagues [74] showed that serum YKL40 levels of Chinese patients with asthma were increased and correlated with the number of exacerbations. Serum YKL40 levels are correlated with total IgE, blood eosinophils and inversely with lung function and could predict the longitudinal decline of lung function in response to cigarette smoke exposure [75]. Two distinct asthma phenotypes were identified with high YKL40 levels, which were associated with non-T2 inflammatory pathways, one with irreversible airway obstruction disease and another with severe exacerbations [76]. Hence, the YKL40 clusters are potentially useful for identification of severe or exacerbation-prone asthma in non-T2 patients. In addition, YKL40 may also be a blood-based biomarker in neutrophilic asthma [77] as serum levels correlate with sputum neutrophils [26]. YKL40 has also been used to distinguish asthma from chronic obstructive pulmonary disease (COPD) and healthy controls [78], as well as between patients with asthma-COPD overlap (ACO) and COPD [79]. YKL40 needs to be evaluated in a larger asthma population to prove the its role in assessing asthma outcomes and risks.