Fc-mediated endocytosis by human neutrophils. Ultrastructural studies.

Fc Receptors (FcR) mediate the binding and uptake by polymorphonuclear leukocytes (PMN) of antibody-coated particles and soluble immune complexes. We have studied Fc-mediated endocytosis by PMN ultrastructurally using a gold-conjugated monoclonal antibody (3G8) to block or to mark the location of FcR. Phagocytosis of antibody-coated erythrocytes (EIgG) was initiated rapidly after binding to discrete foci on the PMN plasma membrane. After the phagocytosis of EIgG, we examined the distribution of FcR remaining on the PMN plasma membrane. 3G8-Colloidal gold continued to bind to PMN after ingestion of up to three EIgG, demonstrating that all PMN FcR are not utilized during a brief phagocytic event. The endocytosis of soluble immune complexes was examined by labeling plasma membrane-bound rabbit immune complexes with goat anti-rabbit IgG conjugated to colloidal gold. Gold was found in clusters randomly distributed over the plasma membrane at 4 degrees C. When cells were warmed to 37 degrees C, numerous endocytic vesicles were observed as early as 2.5 minutes after warming. After 30 minutes at 37 degrees C, large vesicles, 1 micron in diameter, were found to contain 20 to 30 gold particles. The endocytosis of 3G8 was also examined using colloidal gold. After binding of 3G8-gold at 4 degrees C, clusters of large vesicles, up to 2 micron in diameter, were rapidly formed at 37 degrees C.