Molecular Definition of Distinct Cytoskeletal Structures Involved in Complement- and Fc Receptor-mediated Phagocytosis in Macrophages

It has long been known from the results of ultrastructural studies that complement- and immunoglobulin G (IgG)-opsonized particles are phagocytosed differently by macrophages (Kaplan. G. 1977. Scand. J. Immunol. 6:797-807). Complement-opsonized particles sink into the cell, whereas IgG-coated particles are engulfed by lamellipodia, which project from the cell surface. The molecular basis for these differences is unknown. We used indirect immunofluorescence and confocal microscopy to examine how cytoskeletal proteins associate with phagosomes containing complement-opsonized zymosan (COZ) particles or IgG beads in phorbol-myristateacetate-treated peritoneal macrophages. During ingestion of COZ, punctate structures rich in F-actin, vinculin, alpha-actinin, paxillin, and phosphotyrosine-containing proteins are distributed over the phagosome surface. These foci are detected beneath bound COZ within 30 s of warming the cells to 37 degrees C, and their formation requires active protein kinase C. By contrast, during Fc receptor-mediated phagocytosis, all proteins examined were uniformly distributed on or near the phagosome surface. Moreover, ingestion of IgG beads was blocked by tyrosine kinase inhibitors, whereas phagocytosis of COZ was not. Thus, the signals required for particle ingestion, and the arrangement of cytoskeletal proteins on the phagosome surface, vary depending upon which phagocytic receptor is engaged. Moreover, complement receptor (CR)-mediated internalization required intact microtubules and was accompanied by the accumulation of vesicles beneath the forming phagosome, suggesting that membrane trafficking plays a key role in CR-mediated phagocytosis.