Role of Lung Phagocytes in the Clearance of Particles by the Mucociliary Apparatus

Abstract

Pulmonary alveolar macrophages are the cell type predominately responsible for physical clearance of inhaled particles from the alveoli. However inhalation of large numbers of particles or chronic exposures induce inflammation, characterized by an accumulation of neutrophils in the lung. Because of their phagocytic capabilities, neutrophils could also be important in clearing particles from the lung. This study was designed to compare the roles of macrophages and neutrophils in the clearance of particles up the' mucociliary escalator from sites of inflammation in the lung. Lung lobes of three beagle dogs were instilled with 3.9– or 0.9–μm fluorescent polystyrene microspheres. The trachea of each dog was lavaged at 2 h, and again at 1, 3, 7, 14, 21, 35, and 49 days after instillation, and the numbers of free and cell-associated particles were determined. At 2 h, only about 12% of the microspheres in lavage fluid were associated with phagocytes. By 1 day after instillation, and for the remainder of the experiment, about 70% of the microspheres in the lavage fluids were associated with phagocytes. During the first 7 days after instillation, most of the microspheres in the tracheas were associated with neutrophils; at 14 days and later, most of the micro-spheres were associated with macrophages. Over the 49-day period of this experiment, approximately 14% of he microspheres found in tracheal lavage fluids were associated with neutrophils, and approximately 57% were associated with macrophages. The two different microsphere sizes were cleared from the lung at similar rates. Our results confirmed the role of macrophages in particle clearance and suggested that neutrophils, when present in the alveoli, may also play an important role in lung clearance.