TGFB1 and infection: Given that IAV-infected IL-6−/− mice produced higher levels of TGF-β in the BAL fluid (Fig. 1d) and their lung fibroblasts displayed higher migratory capability in vitro (Fig. 3e) compared with their WT counterparts, we further examined TGF-β levels in the supernatants of IL-6−/− and WT fibroblasts with or without infection with IAV.