Effects of MMP-12 on IL-13–induced TGF-β₁ bioactivity and tissue fibrosis. (a) Transgene^– and transgene⁺ CC10-rtTA-IL-13 mice were placed on dox water for 4 weeks. BAL fluids were then obtained and the levels of bioactive (striped bars) and total (black bars) TGF-β₁ were assessed using mink lung epithelial cells as described in Methods. *P < 0.05. (b) BAL fluids were obtained from 3-month-old CC10-IL-13 transgene^– and transgene⁺ mice, and the levels of bioactive and total TGF-β₁ were assessed using mink lung epithelial cells as described in Methods. (c) Trichrome stains are used to evaluate the subepithelial fibrosis in lungs from 3-month-old CC10-IL-13 transgene^– and transgene⁺ mice with wild-type and null MMP-12 loci.