Acquisition of peroxidase activity by rat alveolar macrophages during pulmonary inflammation

The authors investigated the ability of rat alveolar macrophages to acquire peroxidase activity in the course of pulmonary inflammation. Granulomatous pulmonary inflammation was induced in bacille Calmette-Guérin (BCG)-immunized rats by intravenous injection of BCG in mineral oil. In contrast to normal alveolar macrophages, which are peroxidase-negative, alveolar macrophages lavaged from the BCG-treated rats showed significant peroxidase activity in large cytoplasmic inclusions compatible with internalized exogenous material. Alveolar macrophage uptake of intact peroxidase-positive neutrophils was also observed. Maximal numbers of peroxidase-positive alveolar macrophages were observed after the initial influx of neutrophils into the lungs, and peroxidase activity could be demonstrated in cell-free lavage fluid during the acute phase of lung injury. Normal alveolar macrophages acquired peroxidase activity after incubation with peritoneal exudate neutrophils, with purified soluble human myeloperoxidase, and with opsonized erythrocytes. It is concluded that alveolar macrophages acquire peroxidase activity from multiple sources during pulmonary inflammation. Internalization of peroxidase by the alveolar macrophage may serve to clear a potentially toxic enzyme(s) from the alveolar space and contribute to the resolution of pulmonary inflammation.