Augmentation of rat alveolar macrophage migration by surfactant protein

Since the alveolar macrophage (AM) resides in an environment made unique by the presence of pulmonary surfactant, we studied the effect of constituents of pulmonary surfactant on the migration of these cells in vitro. Whole pulmonary surfactant, surfactant phospholipids, and a delipidated preparation of surfactant consisting mainly of protein were tested for their effect on AM migration. Migration was measured in multiwell chemotaxis chambers with endotoxin-activated rat serum (EARS) as the stimulus. The delipidated surfactant material (DSM) markedly augmented the migration of rat AM. Enhancement was seen only when migration was stimulated with EARS. Augmentation of stimulated migration occurred over a wide range of protein concentrations in DSM (3.5 to 56 micrograms/ml) in a dose-dependent fashion. Rat AM that were preincubated with DSM and then tested in the chemotaxis assay without DSM showed increased migration towards EARS when compared with AM preincubated in buffer alone. The enhancing effect of DSM on AM migration was significantly diminished by heating the DSM and by enzymatic digestion with trypsin, suggesting that the active constituent in DSM was protein. Thus, it appears that proteins isolated from pulmonary surfactant augment the stimulated migration of AM via an interaction between the protein and the macrophage. Although the identity of these proteins remains to be elucidated, it is unlikely they include albumin since purified rat serum albumin in concentrations comparable to that of protein in DSM did not enhance the stimulated migration of AM. These results represent further confirmation that constituents of surfactant modulate the clearance function of AM in vitro. Thus, it is likely that surfactant plays a role in lung defense mediated by AM.