This cross-sectional study presents the positive association observed between Th2 biomarkers in circulating levels and demonstrated robust relationships between several biomarkers of T2 inflammation. In addition, it extended previous investigations regarding identifying the potential biomarkers predicting clinical remission in Th2-high asthma.
We found that when subjects were simply partitioned into asthma and control, no significance was observed in Th2 biomarkers between these two groups, whereas clinical measurements such as blood eosinophils (%) and eosinophils count were statistically significant. When we defined Th2 high asthma based on eosinophils count (≥ 140 cells/µl ) and total IgE (IgE > 100 IU/ml), the serum TSLP levels were observed higher in subjects experiencing acute exacerbation and persistent asthma compared with no asthma control subjects. In addition, our results also found an association between serum TSLP levels and clinical remission independent of confounding factors such as lung function measurements or allergy sensitization. However, the role of serum TSLP in identifying clinical remission of Th2-high asthma still needs further holistic investigations to reconfirm its specificity and sensitivity to be used as a prognostic biomarker in asthma.
4.1 Th2 biomarkers at baseline
Since asthma is a heterogeneous disease that can be divided into many subgroups, one of the widely accepted consensuses is that the research of it needs to evolve from phenotypes to endotypes [1]. Phenotypes are usually defined by clinical characteristics, allergen sensitization patterns, and physiologic lung function measurements which are readily observed or measured in clinical practice. Endotypes are a definition that is related to pathophysiological mechanisms underlying phenotype [19]. Thus, there is a growing urge to investigate asthma endotypes by connecting discernible and stable biomarkers with immunological mechanisms to make precision therapy decisions for asthma [20]. Th2-high asthma is a broad classification involving asthma endotypes based on type 2 (T2) inflammation pathobiology characterized by increased secretion of Th2 biomarkers such as IL-4, IL-5, IL-13, and IgE causing pathological changes, which can be summarized by a set of the cytokine-induced eosinophilic airway inflammation, epithelial dysfunction, and airway wall remodeling [21, 22]. And it is believed that Th2-high asthma account for only half of the people diagnosed with asthma [1, 23], which in a way supports our results that there was no considerable significance at the baseline levels of Th2 biomarkers between asthma and control when study population was not recruited specifically with Th2-high characteristics. When investigating the correlations between the biomarkers and clinical characteristics, there were no positive results observed between Th2 biomarkers and baseline blood eosinophil counts or total IgE except that IL-13 was positively correlated with blood eosinophils (%). Whereas, Corren, Pham, et al found that baseline levels of serum IL-5, IL-13, and Periostin correlated significantly with baseline blood eosinophil counts and FeNO levels in uncontrolled severe asthma subjects, which may account for the complex immune pathways in severe asthma [24, 25]. After all, this is a relatively representative study measuring baseline circulating levels of Th2 biomarkers, and the positive correlations among specific biomarkers on one side further support the molecular mechanism of Th2 biomarkers, on the other side it may provide insight into combined immune biologic interventions in personalized therapy.
4.2 Asthma remission, persistence and predictive role of Th2 biomarkers
Asthma remission is not emphasized in asthma treatment goals, unlike other chronic diseases, and it is widely accepted that clinical remission and complete remission have different criteria, and current research involving asthma remission is limited [26, 27].
Wang, Datta [13] demonstrated that baseline lung function can be used as a clinical prognostic indicator of asthma remission. And other independent predictors of asthma remission include less BHR, no nasal polyps, younger age, better asthma control, lower doses of ICS, and lower levels of blood neutrophils [28]. Actually, when comes to asthma remission, clinical features are mostly considered [29, 30]. However, with the prevalent use of biologics in asthma treatments, and the great development from phenotypes to endotypes in asthma pathophysiological mechanisms, identifying predicting biomarkers of asthma remission, especially in the Th2 high subtype may mean a step closer to clinical asthma remission evaluation. Thus, in this study, the study population was further divided into acute asthma attacks, persistent asthma, and clinical remission asthma among Th2-high subjects. The criteria of Th2-high and the definition of different phases were explained above. It is obvious that lung measurements including FEV1(% predicted), FEV1/FVC, %, FEF25-75, pred %, are statistically significant among groups which are in line with many research that lung measurements are sensitive clinical features evaluating asthma remission [13, 30, 31].When comparing circulating levels of Th2 biomarkers among groups, there was no marked significance except serum TSLP. However, Tan, Daniel J et al found that serum inflammatory cytokines including IL-4, and IL-5 have prognostic value in adults with spontaneous asthma remission. And there was solid evidence that Periostin was associated with asthma persistence, especially in severe uncontrolled asthma [32, 33]. In addition,Christianson, Goplen [34] elucidated that IL-13 is a crucial factor in the feedback and feed-forward interactions that contribute to the persistence of asthma in mouse models. TARC(thymus activation-regulated chemokine) which is expressed in T helper (Th2) cells, also known as CCL17, belongs to the CC chemokine family. TARC is believed to have a pivotal role in the pathogenesis of allergic diseases such as atopic dermatitis, bronchial asthma, and allergic rhinitis [35, 36]. What's more, the serial changes of plasma TARC concentrations in children with acute asthma attacks were monitored in a study which indicated that TARC is a potentially useful inflammatory maker to evaluate asthmatic exacerbation [37]. To sum up, these conflicts may due to the difference in study population and design, and it is crucial to emphasize the fact that the research on the levels of biomarkers associated with asthma remission and persistence is limited. Therefore, there is a pressing need to conduct more studies in this area to gain critical insights into this debilitating condition.
4.3 Association between clinical asthma remission and serum TSLP
Thymic stromal lymphopoietin (TSLP), part of the IL-2 cytokine family, was discovered as a growth factor for lymphocyte progenitors and is now recognized as a protein primarily released from epithelial cells and has a sensitive response to irritating stimuli [38, 39]. and research involving mice models to human subjects and the genome-wide association had demonstrated that TSLP is associated with asthma [40–42]. Indeed, TSLP regulates dendritic cells which are considered one of the central responders of Th2 immunity [43]. What’s more, with the approval of TSLP-targeted antibody for severe asthma, it is reported that TSLP plays a key role in allergen-induced airway responses and persistent airway inflammation in patients with allergic asthma [42]. However, whether it can be a predictor to evaluate the clinical remission of Th2-high asthma is still understudied. There was research based on sputum type 2 makers and found that the sputum TSLP levels were higher at baseline in the remission group compared with the non-remission group and TSLP was not listed as a potential predictor of remission by univariate regression analysis [44]. On the contrary, our results found that serum TSLP has a significant difference among acute asthma groups, asthma persistence groups, and clinical remission group, and multivariable regression also revealed that serum TSLP levels were associated with clinical remission in Th2 high asthma children. The fundamental role of TSLP in asthma pathogenesis was underscored by the fact that the latest clinical trials have reconfirmed that treatment with Tezepelumab, which blocks TSLP, can lead to sustained and significant reductions in asthma exacerbations in individuals with severe, uncontrolled asthma [45]. A multi-center prospective observational study suggests that TSLP may play a pathogenic role across different asthma phenotypes [46].
4.4 The expression of TSLP in BALF, serum, and lung tissue of different stages of asthmatic mice.
We further conducted an animal model of allergic asthma which shows the levels of TSLP vary in acute, chronic, and remission asthma mouse models. Our findings indicate that the TSLP levels are remarkably high in chronic asthma mouse models among the lung tissue, BALF, and serum. Furthermore, both in the serum and lung tissue samples, we have also observed that the decrease in TSLP levels may indicate the remission of asthma. Numerous murine research have demonstrated that TSLP plays an important role in the development of asthma [47, 48]. The anti-TSLP method or TSLP-R deficiency can effectively attenuate AHR and TH2 airway inflammation [49, 50]. Notably, an interesting research also revealed that mice received TSLP receptor-deficient CD4 + T cells had alleviated pathogenic TH2 cell responses, and asthmatic children showed an increase in the production of TH2 cells, promoted by TSLP and IL-4, which resulted in higher amounts of IL-5 and IL-13 in their peripheral blood, compared to healthy controls [51]. Although it is commonly believed that TSLP plays a crucial role in the pathogenesis of asthma, there is still a lack of research on whether TSLP can be utilized as a biomarker for indicating asthma remission or persistence. We believe that this possibility has not been discussed enough, and therefore we are interested in exploring this topic further.
4.5 Conclusion
We report for the first time that serum TSLP may be used as one of the biomarkers to evaluate the remission of asthma along with other measurements such as lung function to improve the accuracy sensitivity and specificity and to testify our findings in acute chronic and remission asthma mouse models.
However, despite the positive likelihood of evaluating remission shown in ROC curves analysis, its diagnostic role and prognostic role in identifying clinical asthma remission still need further exploration.
We aim to expand our understanding of TSLP and its clinical significance while advancing our current research.
4.6 limitation and strength
The limitations of this study lie in that subjects and the Th2 biomarkers should have been be detected in a larger cohort of multiple centers and the definition of asthma remission is still under-refined. Also, the steadiness of the circulating biomarkers should be validated in the longitudinal study. Furthermore, investigation related to other biomarkers of different pathways is expected to be conducted to form a more holistic understanding of asthma remission. It is the first research to investigate the role of circulating levels of Th2 biomarkers as predictors of remission in a cohort of asthma subjects though it is a preliminary and retrospective study.