FcγR-Mediated Phagocytosis Stimulates Localized Pinocytosis in Human Neutrophils

Engulfment of IgG-coated particles by neutrophils and macrophages is an essential component of the innate immune response. This process, known as phagocytosis, is triggered by clustering of FcγR at sites where leukocytes make contact with the opsonized particles. We found that phagocytosis is accompanied by a burst of fluid phase pinocytosis, which is largely restricted to the immediate vicinity of the phagosomal cup. FcγR-induced pinocytosis preceded and appeared to be independent of phagosomal sealing. Accordingly, fluid phase uptake was accentuated by actin depolymerization, which precludes phagocytosis. Stimulation of pinocytosis required phosphatidylinositol 3-kinase activity and was eliminated when changes in the cytosolic free Ca2+ concentration were prevented. Because stimulation of FcγR also induces secretion, which is similarly calcium and phosphatidylinositol 3-kinase dependent, we studied the possible relationship between these events. Neutrophil fragments devoid of secretory granules (cytoplasts) were prepared by sedimentation through Ficoll gradients. Cytoplasts could perform FcγR-mediated phagocytosis, which was not accompanied by activation of pinocytosis. This observation suggests that granule exocytosis is required for stimulation of pinocytosis. Analysis of the cytosolic Ca2+ dependence of secretion and pinocytosis suggests that primary (lysosomal) granule exocytosis is the main determinant of pinocytosis during FcγR stimulation. Importantly, primary granules are secreted in a polarized fashion near forming phagosomes. Focal pinocytosis during particle engulfment may contribute to Ag processing and presentation and/or to retrieval of components of the secretory machinery. Alternatively, it may represent an early event in the remodeling of the phagosomal membrane, leading to phagosomal maturation.