Phagocytes engulf microbes by enveloping them in a patch of membrane and internalizing them in phagosomes, which fuse with endosomes and lysosomes to form the phagolysosomes in which the internalized pathogens are destroyed. Trivedi et al. exposed mouse macrophages to latex beads to investigate mechanisms through which immunoglobulin G (IgG) class antibodies, which stimulate phagocytosis, might promote the latter part of this process. When macrophages incubated with beads coated with either bovine serum albumin or IgG at 15oC (allowing bead engulfment but not fusion of phagosomes with lysosomes) were warmed to 37oC, the association of IgG-coated beads with fluorescently labeled phagolysosomes was faster than that of the albumin-coated beads. A cytosol-dependent cell-free scintillation proximity assay, in which phagosomes containing scintillant beads were incubated with tritium-labeled lysosomes, revealed that cytosol from phagocytes incubated with IgG beads was more effective at promoting phagosome-lysosome interactions than cytosol from cells that had not been incubated with IgG beads. Similarly, cytosol from CHO cells transfected with human Fcγ receptor IIa (making them phagocytic) and incubated with IgG beads promoted phagosome-lysosome interactions more effectively than that from cells unexposed to IgG beads, an effect enhanced by transfection of the cells with protein kinase C (PKC)-α. Moreover, inhibition of PKC abolished the stimulatory effect of IgG in the in vitro assay and blocked IgG-dependent stimulation of phagosome maturation in cells. Further pharmacological analysis indicated that IgG stimulated the actin-dependent tethering or docking (or both) of phagosomes and lysosomes. Thus, facilitation of phagosome-lysosome attachment by way of PKC appears to be one mechanism whereby IgG signaling stimulates phagocytosis.
V. Trivedi, S. C. Zhang, A. B. Castoreno, W. Stockinger, E. C. Shieh, J. M. Vyas, E.-M. Frickel, A. Nohturfft, Immunoglobulin G signaling activates lysosome/phagosome docking. Proc. Natl. Acad. Sci. U.S.A. 103, 18226-18231 (2006). [Abstract] [Full Text]