Inorganic Particles Induce Secretion of a Macrophage Homologue of Platelet-Derived Growth Factor in a Density- and Time-Dependent Manner in Vitro

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Abstract
The inhalation of inorganic dust can lead to the development of interstitial pulmonary fibrosis, characterized by the accumulation of fibroblasts and connective tissue matrix in the lung interstitium. The fibrosis causes alterations in the architecture of the lung parenchyma, resulting in abnormal gas exchange and hypoxemia. In a rat model of asbestos exposure, inhaled fibers are deposited on alveolar duct bifurcations, followed by an accumulation of alveolar macrophages at the sites of dust deposition. The alveolar macrophage is thought to be a major mediator of the pulmonary inflammatory response to inhaled dust. Platelet-derived growth factor (PDGF) is a cytokine that has potent chemotactic and mitogenic effects on mesenchymal cells, such as fibroblasts and smooth muscle cells. We studied the secretion of an alveolar macrophage-derived homologue of PDGF in response to carbonyl iron spheres or chrysotile asbestos fibers in vitro. We demonstrate here that rat alveolar macrophages attached to a plastic substrate produce 69 ± 79 picograms (pg) of PDGF per 10 million macrophages. This is similar to amounts recovered from human platelets. In contrast, macrophages exposed to iron spheres secrete 429 ± 177 pg of PDGF/106 macrophages after 24 h in culture. Exposure to asbestos fibers increased the PDGF production to 628 ± 213 pg/106 cells. PDGF secretion was influenced by the particles in a density- and time-dependent manner. We hypothesize that PDGF and other cytokines secreted by macrophages mediate the development of dust-induced lung disease.