Taken together, these data indicate that, although increased AM recruitment to the lungs of HET mice is not TGF-β or IL-6 dependent, AMs are necessary for protection from development of ALI in HETs because they release large amounts of IL-6 at 6 d.p.i. in response to TGF-β produced at 2 d.p.i. However, because bone marrow transfer experiments show a role for stromal cells in protection from ALI in HETs, we cannot exclude an additional role for alveolar epithelial cells in IL-6 production. Here, IL6 is linked to acute respiratory distress syndrome.