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 wildtype but not in GM-CSF-deficient AEC as shown by [H-3]-thymidine incorporation and cell counting. Moreover, intraalveolar TNF-alpha neutralization impaired AEC proliferation in LIPS-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 LIPS treatment compared with wildtype mice. Conclusions: Collectively, these findings indicate that TNF-alpha released from activated resident alveolar macrophages induces epithelial GMCSF expression, which in turn initiates AEC proliferation and contributes to restoring alveolar barrier function.