Phagocytosis of surfactant by alveolar macrophages in vitro

Because of their localization and function as phagocytes, alveolar macrophages could take part in the catabolism of surfactant, including surfactants used for treatment. Conditions of the ingestion of the surfactant preparation AWD 56-02 by alveolar macrophages in vitro are described in this paper. The surfactant was labeled with rhodaminyl phosphatidylethanolamine and incubated with alveolar macrophages lavaged from rat lungs. Membrane binding and phagocytosis were evaluated by fluorescence microscopy and quantified fluorimetrically after extraction of the dye. The surfactant was phagocytosed only in the presence of rat serum. The opsonins in the serum are related to complement and Fc receptors as demonstrated by the heat lability of the factor and the inhibition by aggregated gamma-globulins. Furthermore, the phagocytosis depends on calcium and on a factor in the surfactant preparation. Experiments with inhibitors and competition with unlabeled surfactant show that the phagocytosis is a specific and energy-dependent process. Catabolism by alveolar macrophages might be an important step in the metabolism of surfactant, especially when administered in pathological conditions characterized by the presence of serum in the airspaces.