The Role of Particle Inhalation in Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is currently defined as a progressive fibrosing interstitial lung disease (ILD) associated with a histopathologic and radiologic pattern of usual interstitial pneumonia (UIP). The relationship between IPF and particles is described, and a pathogenesis for the disease is proposed based on an association with these exposures. In clinical studies and epidemiological investigations, the majority of IPF diagnoses are associated with particle exposures. Cigarette smoking presents the greatest particle challenge in any society, and a relationship with IPF has repeatedly been demonstrated. Environmental exposures to particles other than cigarette smoking, including biomass fuel smoke and ambient air pollution, as well as numerous occupational particle exposures, have also been associated with IPF. The pathogenesis of the disease includes a complexation and sequestration of cell iron at the particle surface, which results in a functional cell deficiency of the requisite metal. In response to the insufficiency of metal in cells, there is the synthesis of biopolymers, including exopolysaccharides (e.g., hyaluronic acid), which accumulate in the extracellular matrix. These biopolymers complex iron and, following depolymerization, facilitate the delivery of the metal intracellularly via receptor-mediated uptake. This process reverses the functional iron deficiency introduced by the particle. Pulmonary fibrosis after particle exposure reflects a response to the modification of a functional intracellular iron deficiency in the lower respiratory tract. The temporal and spatial heterogeneity of IPF results from a dose-response with retained particles and reversibility of the fibrosis.