IL‐5 receptor expression in lung fibroblasts: Potential role in airway remodeling in asthma

Tissue inflammation and airway structural changes, including sub-epithelial fibrosis are features of asthma pathogenesis. Whilst inflammatory cells, particularly CD4+ Th2 cells, innate lymphoid cells type 2 (ILC2), and eosinophils are usually responsive to regulation by corticosteroid therapy, such treatment is considerably less effective at modifying fibroblast activation and has little, if any, impact on airway remodeling.1 IL-5 is a key orchestrator of eosinophil-driven inflammation and is elevated in the blood and airways in asthma. It is essential for the maturation and activation of eosinophils as well as migration and survival.2-4 Recently, anti-IL-5 therapy has been shown to be effective in the management of severe asthma, both in clinical trials and in real-world use. Anti-IL-5 biologic therapy reduces blood and tissue eosinophils lower the annual rate of moderate or severe exacerbation, and reduces the need for oral corticosteroids and their dose.5 It is not known, however, if IL-5 has a direct role in airway remodeling. In an in vivo animal model of asthma, treatment with anti-IL-5 is reported to significantly reduce the sub-epithelial fibrosis that was produced by repeated allergen challenges.6 Further supportive of the relevance of IL-5 to airway remodeling, the genetic absence of IL-5 in a murine model has been shown to reduce allergen-induced lung eosinophilia and tissue remodeling in comparison to that in wild-type mice.7 In non-severe asthma, treatment with the anti-IL5 biologic, mepolizumab, has been shown to significantly reduce the immunoexpression of the extracellular matrix proteins, tenascin, lumican, and procollagen III in endobronchial biopsies.8 These findings all suggest an indirect or direct role for IL-5 in tissue remodeling. We, therefore, hypothesize that IL-5 could have a direct effect on bronchial fibroblasts which contribute further in remodeling. We have thus investigated the expression of the IL-5 receptor α-chain (IL-5Rα) in bronchial fibroblasts and tissue biopsies obtained from severe asthmatic and non-asthmatic subjects. It is well-known that eosinophils express IL-5 receptor α-chain9 and recently, it was reported that airway epithelial cells also express functional IL-5 receptor α-chain.10 There is, however, no information about IL-5 receptor α-chain fibroblast expression within the lower airways. Our results demonstrate that IL-5Rα transcript is expressed in lung fibroblasts of healthy individuals and those with asthma and that the expression of IL-5Rα is significantly greater in asthma than that evident in healthy counterparts (Figure 1A). To further validate the qPCR findings, immunoblotting analysis was undertaken (Figure 1B) and revealed that the expression of IL-5Rα protein in the fibroblasts is consistent with findings in Figure 1A. Figure 1B shows that asthmatic fibroblasts exhibit greater IL-5Rα protein expression compared to that in non-asthmatic fibroblasts. Cellular localization of IL-5Rα expression was evaluated by immunofluorescence, with vimentin utilized as fibroblast marker. The mean fluorescence intensity (MFI) expression of IL-5Rα was significantly higher in asthmatic fibroblasts compared to healthy cells (Figure 1C,D). We further evaluated the expression of IL-5Ra in human lung biopsies obtained from asthmatic patients (n = 3) with severe asthma compared to healthy control subjects (n = 3) by immunofluorescence analysis. Our results showed positive IL-5Ra expression among the two groups in the submucosal tissue. Co-localization study using vimentin as marker for fibroblasts confirmed that fibroblasts in tissue biopsies, where asthmatic fibroblasts exhibited increased expression in comparison to the healthy controls (Figure 1E,F). In eosinophils, IL-5 binds to its surface receptor and subsequently activates the Ras-ERK and PI3K-Akt signaling pathways among many others.11 We have thus examined the functional characteristics of the IL-5R in airway fibroblasts. Recombinant IL-5 (rIL-5) enhanced the proliferation of asthmatic fibroblasts significantly when compared to healthy cells (Figure 2A). Furthermore, there was a significant increase in ERK and Akt phosphorylation in asthmatic fibroblasts after rIL-5 exposure in comparison to the healthy controls (Figure 2B). Collectively, in this study, we highlighted the novel finding of IL-5α receptor expression in fibroblasts from the lower airways of healthy volunteers and those with asthma. This implies a potential pathological involvement of IL-5 in airway fibrosis and tissue remodeling in asthma. This finding thus extends the focus of IL-5 biology beyond eosinophils and raises the possibility that inhibiting IL-5 may prevent, decrease, or reverse airway remodeling in asthma. We would like to acknowledge the generous support of the Sharjah Institute for Medical Research (SIMR), University of Sharjah, UAE. We would like to acknowledge Mr. Abdalla Eltayeb for his contribution in staining the lung tissues. All authors agree to be accountable for all aspects of the work. This work was supported by the University of Sharjah Competitive grants, Ref. numbers: 1901090263 and 2001090282. GSK provided funding support for this study [Mepolizumab/214882 – ISS 10943]. GSK was provided the opportunity to review a preliminary version of this manuscript for factual accuracy, but the authors are solely responsible for final content and interpretation. Bassam Mahboub reports that he has received research funding from GlaxoSmithKline to support research related to the study. Appendix S1 Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.