Rationale Asthmatic airway remodelling is characterized by myofibroblast hyperplasia and subbasement membrane collagen deposition. We hypothesized that cytokines and growth factors implicated in asthmatic airway remodelling are increased in bronchoalveolar lavage (BAL) fluid of asthmatics after segmental allergen challenge (SAC), and that these growth factors and cytokines increase α-smooth muscle actin (α-SMA) and collagen III synthesis by human lung fibroblasts (HLFs).
Methods Transforming growth factor (TGF)-β1, TGF-β2, IL-4 and IL-13 levels were measured in BAL fluid from 10 asthmatics and 9 non-asthmatic controls at baseline and then 1 day, 1 week and 2 weeks after SAC. Confluent cultures of HLFs were stimulated by exogenous addition of TGF-β1, TGF-β2, IL-4 or IL-13 (concentration range 0.01–10 ng/mL) over 48 h. Collagen III was measured in culture supernates and α-SMA in cell lysates by Western blot.
Results At baseline, there was no difference in BAL fluid concentrations of TGF-β1, IL-4 and IL-13 between asthmatics and controls; however, non-asthmatics had higher concentrations of total TGF-β2. In asthmatics, BAL fluid concentrations of all four factors increased significantly 1 day after SAC. TGF-β1, TGF-β2 and IL-13 concentrations returned to baseline by 1 week after SAC, but BAL fluid IL-4 concentration remained elevated for at least 2 weeks. TGF-β1, TGF-β2 and IL-4 significantly increased α-SMA in fibroblasts, but only IL-4 caused corresponding increases in collagen III synthesis. IL-13 had no direct effects on collagen III synthesis and α-SMA expression.
Conclusions Because IL-4 caused a dose-dependent increase in α-SMA and collagen III synthesis, it may be an important cytokine mediating asthmatic airway remodelling. TGF-β1 and TGF-β2 may also play a role in airway remodelling by stimulating phenotypic change of fibroblasts to myofibroblasts. Additionally, collagen III synthesis appears to be independent of myofibroblast phenotype and is apparently regulated by different growth factors and cytokines.
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