Fc- and complement-receptor activation stimulates cell cycle progression of macrophage cells from G₁ to S¹

Phagocytosis of microorganisms by macrophages is an important host defense mechanism. While studying the phagocytosis of the human pathogenic fungus Cryptococcus neoformans, we noted that macrophage-like J774 cells with ingested fungal cells had frequent mitotic figures. By analyzing the relative proportion of phagocytic cells as a function of cell cycle phase, we observed an increase in S phase cells after Fc-mediated phagocytosis of polystyrene beads, live or heat-killed C. neoformans. This result was confirmed by increased nuclear BrdU incorporation after Fc-mediated phagocytosis. The induced progression to S phase was observed after both Fc- and complement-mediated phagocytosis of live yeasts. Fc-mediated stimulation of cell division did not require ingestion, because it could be triggered by incubating cells in IgG1-coated plates. Phagocytosis-mediated stimulation of replication was confirmed in vitro using primary bone marrow macrophages and in vivo for peritoneal macrophages. We conclude that phagocytosis of microbes or inert particles can stimulate macrophages to enter S phase and commence cell division. This observation suggests a potential mechanism for increasing the number of effector cells after microbial ingestion, but can also promote the spread of infection.