Abstract
Although Fc receptor-mediated phagocytosis is accompanied by a variety of transmembrane signaling events, not all signaling events are required for particle ingestion. For example, Fc receptor-mediated phagocytosis in mouse inflammatory macrophages (Di Virgilio, F., B. C. Meyer, S. Greenberg, and S. C. Silverstein. 1988. J. Cell Biol. 106:657; Greenberg, S., J. El Khoury, F. Di Virgilio, and S. C. Silverstein. 1991. J. Cell Biol. 113:757) and neutrophils (Della Bianca, V., M. Grzeskowiak, and F. Rossi. 1990. J. Immunol. 144:1411) occurs in the absence of cytosolic calcium transients. We sought to identify transmembrane signaling events that are essential for phagocytosis. Here we show that tyrosine phosphorylation is an early event after Fc receptor ligation in mouse inflammatory macrophages, and that the formation of tyrosine phosphoproteins coincides temporally with the appearance of F-actin beneath phagocytic cups. The distribution of tyrosine phosphoproteins that accumulated beneath phagocytic cups was punctate and corresponded to areas of high ligand density on the surface of the antibody-coated red blood cells, which provided the phagocytic stimulus. A tyrosine kinase inhibitor, genistein, but not several inhibitors of protein kinase C, blocked the appearance of tyrosine phosphoproteins as assessed by immunofluorescence, the focal accumulation of F-actin beneath immunoglobulin G-opsonized particles, and the ingestion of these particles as well. We suggest that tyrosine phosphorylation is a critical signaling event that underlies Fc receptor-mediated phagocytosis in mouse macrophages, and is necessary for the engulfment per se.
Highlights
Thio-macrophages were incubated with IgG-RBC at 4~ a temperature that allows binding but not ingestion of these red cells, and were fixed and stained for F-actin and for tyrosine phosphoproteins
There were no focal accumulations of F-actin or tyrosine phosphoproteins in the submembranous region beneath the particles
After 30 s at 37~ there were focal increases in the appearance of F-actin and PY-20 staining beneath some adherent IgG-RBC, at this early time point most of the attached erythrocytes were not associated with detectable increases in F-actin or PY-20 staining in the macrophages
Summary
Fc receptor-mediated phagocytosis is accompaniedby a variety of transmembrane signaling events, not all signaling events are required for particle ingestion. We suggest that tyrosine phosphorylation is a critical signaling event that underlies Fc receptor-mediated phagocytosis in mouse macrophages, and is necessary for the engulfment per se. Recent studies suggest that increased tyrosine phosphorylation accompanies activation of Fc receptors in neutrophils [2] and platelets [3], and is required for Fc receptor-mediated cytotoxicity in NK cells [4]. It is not known, whether tyrosine phosphorylation is necessary for any of the physiological functions mediated by Fc receptors in phagocytic leukocytes. But not inhibitors of protein kinase C, inhibits both the focal accumulation of tyrosine phosphoproteins and F-actin beneath sites of attachment of IgG-coated SRBC to mouse macrophages, and reversibly blocks the ingestion process
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