Mechanical stress-induced mast cell degranulation activates TGF-β1 signalling pathway in pulmonary fibrosis

Background: The role of mast cells accumulating in idiopathic pulmonary fibrosis (IPF) lungs is unknown.

Objectives: We investigated the effect of fibrotic extracellular matrix (ECM) on mast cells in experimental and human pulmonary fibrosis.

Results: In IPF lungs, mast cell numbers were increased and correlated with disease severity (control vs 60%<FVC<90%, mean difference=-222.7, 95% CI -386.3 to -59.2, p=0.004; control vs FVC<60%, mean difference=-301.7, 95% CI of difference -474.1 to -129.34, p=0.0001; FVC>90% vs 60%<FVC<90%, mean difference=-189.6, 95% CI of difference -353.1 to -26.03, p=0.017; FVC>90% vs FVC<60%, mean difference=-268.6, 95% CI of difference -441.0 to -96.17, p=0.0007). Plasma tryptase levels were increased in IPF and negatively correlated with FVC (control vs FVC<60%, mean difference=-17.12, 95% CI of difference -30.02 to -4.22, p=0.006: correlation curves R=-0.045, p=0.025). In a transforming growth factor (TGF)-β1-induced pulmonary fibrosis model, chymase-positive and tryptase-positive mast cells accumulated in fibrotic lung. Lung tissue was decellularised and reseeded with bone marrow or peritoneum-derived mast cells; cells on fibrotic ECM released more TGF-β1 compared with normal ECM (active TGF-β1: bone marrow-derived mast cell (BMMC)-DL vs BMMC-TGF-β1 p=0.0005, peritoneal mast cell (PMC)-DL vs PMC-TGF-β1 p=0.0003, total TGF-β1: BMMC-DL vs BMMC-TGF-β1 p=0.013, PMC-DL vs PMC-TGF-β1 p=0.001). Mechanical stretch of lungs caused mast cell degranulation; mast cell stabilisers inhibited degranulation (histamine: cont vs doxantrazole p=0.004, β-hexosaminidase: cont vs doxantrazole, mean difference=1.007, 95% CI of difference 0.2700 to 1.744, p=0.007) and TGF-β1 activation (pSmad2/Smad2: cont vs dox p=0.006). Cromoglycate attenuated pulmonary fibrosis in rats (collagen: phosphate-buffered saline (PBS) vs cromoglycate p=0.036, fibrotic area: PBS vs cromoglycate p=0.031).

Conclusion: This study suggests that mast cells may contribute to the progression of pulmonary fibrosis.