Apoptosis in human alveolar macrophages is induced by endotoxin and is modulated by cytokines.

A central factor in the pathogenesis of inflammatory and fibrotic lung disease (adult respiratory distress syndrome, sarcoidosis, idiopathic pulmonary fibrosis) is the locally elevated number of alveolar macrophages (AM). An elevation in the production rate of AM, chemoattraction and differentiation of monocytes, or a diminution in the death rate might be underlying mechanisms. The aim of the present study was to investigate the modulatory role of endotoxin and cytokines on the death rate of human AM. Lipopolysaccharide (LPS) treatment resulted in a 4-fold increase (7.6 to 30.2%) of AM death. AM death was apoptotic as assessed by in situ DNA end labeling (ISDE), transmission electron microscopy, DNA gel electrophoresis, fluorometry of fragmented DNA, and an ELISA specific for histone-associated DNA fragments. Among the different bacterial cell wall components tested, LPS was the only inducer of apoptosis in human AM. None of the tested cytokines (interleukin-1 beta [IL-1 beta], IL-4, IL-6, IL-10, tumor necrosis factor-alpha [TNF-alpha], transforming growth factor-beta 2 [TGF-beta 2], interferon-gamma [IFN-gamma], macrophage colony-stimulating factor [M-CSF], granulocyte colony-stimulating factor [G-CSF], and granulocyte-macrophage colony-stimulating factor [GM-CSF]) was capable of enhancing the spontaneous rate of apoptosis. However, LPS-induced apoptosis was significantly enhanced by the macrophage-activating cytokine IFN-gamma, and reduced by the macrophage-deactivating cytokines IL-4, IL-10, and TGF-beta.