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 T2high 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.
Serum periostin was used as a biomarker in phase 2 and 3 clinical studies of the anti-IL-13 antibody lebrikizumab [60]. However, recent evidence indicates little selectivity of serum periostin for T2 asthma and it is not surprising, therefore, that phase 3 studies of lebrikizumab were not considered effective [61]. In contrast, high sputum periostin reflects T2high asthma [62] whereashigh serum periostin is now considered indicative of omalizumab efficacy in asthma [63]. While periostin may have prognostic, predictive, and pharmacodynamic properties, the inconsistency of results, serum levels that change with age, and increased expression in other diseases limit its clinical applicability and affect its utility as an independent biomarker [64].
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].
CCSP16 is considered to be both a sign of the loss in airway epithelial barrier integrity and a common participant in the anti-inflammatory response. Low levels of CCSP16 in the serum are associated with decreased lung function in childhood, accelerated decline in lung function in adulthood and restricted airflow [67]. While studies have shown that CCSP16 and surfactant protein D (SPD) in sputum and BAL were significantly higher in patients with severe asthma compared to mild-moderate and healthy controls [68] serum CCSP16 levels were reduced in asthmatics. BAL levels of CCSP16 correlated with epithelial detachment suggesting its possible role of in the remodeling process. Zhai and colleagues [66] using human data from a birth cohort, suggested that low circulating CCSP16 levels were not only a biomarker of airway pathology but may be implicated in the pathophysiology of the progressive airway damage that characterize obstructive lung diseases. Moreover, urinary CCSP16 may be a useful tool or biomarker for studying asthma and the integrity of the alveolar epithelium in children with lung injury [69].Emerging biomarkers
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.