Lipopolysaccharide-induced stimulation of alveolar macrophage opsonin-independent phagocytosis

Alveolar macrophage (AM) opsonin-independent phagocytosis may be an important process by which bacteria are cleared from the airspaces. Although it has been shown that clearance of bacteria from the lung is altered in association with endotoxemia, the effect of endotoxemia on opsonin-independent phagocytosis is unknown. Because alterations in AM opsonin-independent phagocytosis could potentially influence bacterial clearance in the lung, we studied the effects of the intravenous injection of Escherichia coli lipopolysaccharide (LPS) into rats on AM opsonin-independent phagocytosis of latex particles. AM obtained by bronchoalveolar lavage (BAL) 2 or 4 hr after injection of LPS demonstrated phagocytosis comparable to AM from control animals. In contrast, AM obtained 12 hr after injection of LPS demonstrated a nearly threefold increase in phagocytosis. Depletion of serum complement with Naja naja cobra venom factor (CVF) did not alter LPS-induced augmentation of phagocytosis. Furthermore, phagocytosis of AM obtained by BAL 2 or 12 hr after activation of complement by injection of CVF was not significantly different from that of AM from control animals, indicating that complement activation alone was not sufficient to stimulate opsonin-independent phagocytosis. These observations suggest that opsonin-independent phagocytosis may represent an important route of bacterial uptake by AM following endotoxemia, and that LPS-induced stimulation of AM opsonin-independent phagocytosis may occur independently of activation of complement by LPS.