Transforming growth factor-β1 is a potent inhibitor of secretory leukoprotease inhibitor expression in a bronchial epithelial cell line

Obliterative bronchiolitis (OB) is the major long-term complication following lung and heart-lung transplantation. In bronchoalveolar lavage fluid samples obtained from patients suffering from OB, a marked increase in the number of neutrophils and elevated expression of transforming growth factor (TGF)-beta1 had been found. The goal of the study was to evaluate whether TGF-beta1 is capable of interfering with the expression of the secretory leukoprotease inhibitor (SLPI), the dominating defence of the conducting airways against neutrophil elastase (NE). The authors analysed the effects of TGF-beta1 on gene expression and protein release of SLPI by cultured human bronchial epithelial (BEAS-2B) cells. SLPI protein levels in the supernatants were quantified with a specific enzyme-linked immunosorbent assay; SLPI messenger ribonucleic acid (mRNA) levels were measured by reverse transcriptase polymerase chain reaction. Incubation with TGF-beta1 induced a marked decrease in SLPI protein levels (1 ng x mL(-1) TGF-beta1: stimulation index (SI; protein: relation to SLPI protein release of resting cells)=0.56; 10 ng x mL(-1) TGF-beta1: SI=0.48; 50 ng x mL(-1) TGF-beta1: SI=0.37, p<0.01 each) and mRNA expression (1 ng x mL(-1) TGF-beta1: SI (SI mRNA: relation to SLPI mRNA expression of resting cells)=0.46; 10 ng x mL(-1) TGF-beta1: SI=0.31; 50 ng x mL(-1) TGF-beta1: SI=0.18, p<0.01 each) in a dose dependent fashion. Simultaneous incubation of BEAS-2B cells with TGF-beta1 and NE also caused a significant reduction in SLPI synthesis (10 ng x mL(-1) TGF-beta1 + 7.5 U x mL(-1) NE: mRNA SI=0.61, p<0.05; protein SI=0.65, p<0.05; 50 ng x mL(-1) TGF-beta1 + 7.5 U x mL(-1) NE: mRNASI=0.52, p<0.05; protein SI=0.58, p<0.05; 10 ng x mL(-1) TGF-beta1: mRNA SI=0.33, p<0.01; protein SI=0.38, p<0.01). In conclusion, the data suggest that the coincidence of neutrophilia and upregulation of transforming growth factor-beta1 in obliterative bronchiolitis may lead to uninhibited neutrophil elastase activity by downregulation of secretory leukoprotease inhibitor, with the consequence of ongoing injury to the epithelium.