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Articles |
Receptor–mediated Phagocytosis

Department of Cell Biology, Neurobiology, and Anatomy, The Ohio State University College of Medicine, Columbus, Ohio 43210
Receptors for the Fc portion of immunoglobulin (Ig)G (Fc
R) mediate phagocytosis of IgG-opsonized particles by a process that can be divided into four major steps: receptor–ligand binding, pseudopod extension, internalization, and lysosomal fusion. We have expressed single classes of Fc
R in COS fibroblasts to examine the structural determinants necessary to complete the four steps of phagocytosis. Using phase contrast, fluorescence, confocal, and electron microscopy we have demonstrated that Fc
R-expressing COS cells can phagocytose in a manner similar to that of professional phagocytes. We have further analyzed the capacity of the three classes of Fc
R to phagocytose, placing special emphasis on the Fc
RIA–
chain complex, which allowed us to examine independently the roles of the ligand-binding unit (Fc
RIA) and the signaling unit (
chain). We found that receptor complexes containing a conserved tyrosine activation motif (ITAM), as found in the cytoplasmic domain of Fc
RIIA and in the
chain associated with Fc
RIA and Fc
RIIIA, readily internalized target particles. In contrast, Fc
RIA alone, having no ITAM, was unable to internalize target particles efficiently, but did mediate pseudopod extension. Cotransfection of
chain with Fc
RIA restored the ability of the receptor to internalize target particles. A mutant Fc
RIA in which the cytoplasmic domain had been deleted was also capable of mediating pseudopod extension, showing that neither the
chain nor the cytoplasmic domain of Fc
RIA were required for this step. Cytochalasin D, an inhibitor of actin polymerization, blocked particle internalization by all Fc
R, but did not block pseudopod extension. Staining the Fc
RIA COS cells for F-actin and for tyrosine phosphoproteins, we found that actin did not polymerize during Fc
RIA-mediated pseudopod extension, nor were tyrosine kinases activated. Our data suggest that pseudopod extension and internalization are functionally distinct steps mediated through different pathways.
1 Abbreviations used in this paper: DAB, diaminobenzidine; Fc
R, Fc receptor for IgG; HRP, horseradish peroxidase; ITAM, immunoreceptor tyrosine activation motif; PI, phagocytic index.
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