Macrophage Tumor Necrosis Factor-α Induces Epithelial Expression of Granulocyte–Macrophage Colony-stimulating Factor: Impact on Alveolar Epithelial Repair

RATIONALE: Resident alveolar macrophages have been attributed a crucial role in host defense toward pulmonary infection. Their contribution to alveolar repair processes, however, remains elusive. OBJECTIVES: We investigated whether activated resident alveolar macrophages contribute to alveolar epithelial repair on lipopolysaccharide (LPS) challenge in vitro and in vivo and analyzed the molecular interaction pathways involved. METHODS: We evaluated macrophage-epithelial cross-talk mediators for epithelial cell proliferation in an in vitro coculture system and an in vivo model of LPS-induced acute lung injury comparing wild-type, granulocyte-macrophage colony-stimulating factor (GM-CSF)-deficient (GM(-/-)), and human SPC-GM mice (GM(-/-) mice expressing an SPC-promotor-regulated GM-CSF transgene). MEASUREMENTS AND MAIN RESULTS: Using reverse transcription-polymerase chain reaction and ELISA we showed that LPS-activated alveolar macrophages stimulated alveolar epithelial cells (AEC) to express growth factors, particularly GM-CSF, in coculture. Antibody neutralization experiments revealed epithelial GM-CSF expression to be macrophage tumor necrosis factor (TNF)-alpha dependent. GM-CSF elicited proliferative signaling in AEC via autocrine stimulation. Notably, macrophage TNF-alpha induced epithelial proliferation in wild-type but not in GM-CSF-deficient AEC as shown by (3)H-thymidine incorporation and cell counting. Moreover, intraalveolar TNF-alpha neutralization impaired AEC proliferation in LPS-injured mice, as investigated by flow cytometric Ki-67 staining. Additionally, GM-CSF-deficient mice displayed reduced AEC proliferation and sustained alveolar barrier dysfunction on LPS treatment compared with wild-type mice. CONCLUSIONS: Collectively, these findings indicate that TNF-alpha released from activated resident alveolar macrophages induces epithelial GM-CSF expression, which in turn initiates AEC proliferation and contributes to restoring alveolar barrier function.