Rate and efficiency of complement-dependent phagocytosis in cytolytic and non-cytolytic inflammatory macrophages

An important strategy in defining the mechanisms by which macrophages (M∅) kill microorganisms and tumour cells has been to elucidate the structural and functional properties that are unique to cytolytic M∅. Previous studies have suggested that cytolytic and non-cytolytic inflammatory macrophages have similar levels of phagocytic activity. This issue was examined further by measuring the rate and efficiency of phagocytosis (ratio of the number of ingested particles to the number of particles initially attached to the M∅ surface) of concanavalin A-induced M∅ (Con A-M∅), a cytolytic inflammatory M∅, and thioglycolate medium-induced M∅ (TM-M∅), a non-cytolytic inflammatory M∅. These experiments were performed with freshly isolated M∅ populations since activated M∅ lost enhanced cytolytic and phagocytic activity with culture. Both cytolytic M∅ and non-cytolytic inflammatory M∅ were capable of ingesting complement-coated erythrocytes, E(IgM)C, when compared to resident M∅. However, Con A-M∅ had a three-fold greater rate of C3-dependent phagocytosis and an 18-fold greater efficiency of C3-dependent particle internalization than TM-M∅. In contrast, the rate and efficiency of Fc-mediated phagocytosis did not distinguish Con A-M∅ from TM-M∅, though both types of inflammatory cells ingested significantly more E(IgG) than resident M∅. Quantitative differences in C3-dependent phagocytic activity may occur as a result of higher concentrations of C3 receptors at sites of E(IgM)C attachment, thereby driving membrane pseudopod extension at a greater rate.