The activin A/follistatin axis in regulation of airway inflammation and remodelling using a murine model of chronic allergic asthma

Allergic asthma is a common chronic lung disease especially among children and adolescents but also adults. Asthma is characterised by chronic airway inflammation, airway hyperresponsiveness and airway remodelling. Airway remodelling encompasses progressive structural changes to the airway wall, including thickening of the sub-basement membrane, goblet cell hyperplasia/metaplasia, increased sub-epithelial smooth muscle mass and collagen deposition, and angiogenesis. Elucidation of the regulatory factors controlling initiation and progression of airway remodelling would point to effective treatments for prevention of asthma, currently lacking. Activin A, a member of the TGF-β superfamily, induces inflammation and there is accumulating evidence that it plays a role in airway remodelling. Follistatin, activin A’s natural antagonist, binds with high affinity to activin A, blocking the activin A receptor binding sites thus inhibiting activin A signalling and subsequent bioactivity. A model of chronic allergen challenge with features of airway inflammation and remodelling was established. BALB/c mice were systemically sensitised to ovalbumin (OVA) and intranasally challenged (3 times/week) with OVA for 10 weeks. Mice were observed to have persistent inflammation and airway remodelling features, including increased mucus producing goblet cells, and subepithelial smooth muscle and collagen deposition. Also, epithelial expression of activin A and follistatin was lost during OVA challenge with some recovery 2 weeks post-challenge. This model of chronic allergic airway disease was used to elucidate the role of activin A and follistatin during allergic airway disease. For use in my studies, purified recombinant FS288 was produced following generation of a FS288-expressing cell line and successful optimisation of a purification protocol. To investigate the role of activin A in airway inflammation and remodelling, mice were administered exogenous activin A in naive and acute OVA challenge settings, and activin A was over-expressed using activin A-adenovirus in the chronic challenge setting. Instillation of activin A (0.02/0.1/0.5 µg) into naive mice induced airway eosinophilia at day 1, but did not significantly alter airway remodelling. Activin A altered DC expression of F4/80 and co-stimulatory molecules (CD80 and CD86) in lymph nodes, and expression of CD11b, F4/80, CD40 and CD80 on lung DC and macrophages. Lung delivery of activin A into OVA-sensitised mice (0.02/0.1/0.5 µg) 45 min prior to acute challenge increased airway eosinophilia, Th2 cytokine production and goblet cell hyperplasia. In the activin A over-expression study, mice were sensitised and challenged with OVA and an activin A-adenovirus instilled into the lung at the start of the challenge phase. Activin A over-expression caused small, but mostly not significant, increases in airway inflammation and remodelling. Collectively the data suggest that activin A plays a role in immunoregulation of airway inflammation and remodelling during allergic airway disease with exacerbation of these changes on appropriate administration of exogenous activin A. Follistatin (FS288) was investigated as a potential therapeutic for chronic allergic airway disease. Mice were sensitised with OVA and challenged with OVA with or without concurrent FS288 (0.05/0.5/5 µg, i.n.) 3 times/week for 5 weeks. Treatment with FS288 during chronic allergen challenge inhibited goblet cell hyperplasia and sub-epithelial collagen deposition. FS288 treatment increased the CD4+ CD25+ Foxp3+ Treg population in lymph nodes at week 1 but not in the lungs and did not affect other immune cell populations in the lymph nodes and lungs. Further studies are required to fully elucidate the mechanisms for immunoregulation of chronic airway disease via the activin/follistatin axis. In summary, follistatin, through inhibition of activin A, attenuated airway remodelling in a mouse model of allergic asthma, highlighting its potential as a therapeutic for airway remodelling in asthma and other inflammatory lung diseases