| Abstract: | To characterize the internalization and degradation of model immune complexes in murine macrophages, the endocytosis of well-defined radiolabeled IgG dimers and heavy oligomers (5 to 7 IgG molecules per complex), which were covalently cross-linked at the antigen-combining site, was studied. Of those heavy oligomers which were bound to the cell at 4 degrees C, 50 to 60% (400,000 molecules of IgG) were internalized within 30 min at 37 degrees C and, subsequently, were completely degraded over a period of 3 hr. Low pH had little effect on the dissociation of the oligomer from its receptor. The degradation of oligomers was markedly inhibited when macrophages were treated with monensin, a proton ionophore which raises organelle pH. Because this treatment did not prevent the delivery of oligomer into the lysosome, the transport of a soluble complex of IgG from the cell surface to the lysosome was not a pH-dependent event. On the other hand, 25 to 30% (50,000 molecules) of those dimers capable of binding to the cell entered the macrophage, but only 5000 molecules were degraded. When macrophages were studied by using density gradient centrifugation, within 15 min, heavy oligomers were found in a vesicle which sedimented at a density between that of the plasma membrane and lysosome. The density of this vesicle was similar to that of endosomes studied in other receptor-ligand systems. Heavy oligomers were within lysosomes shortly thereafter. Incubation of cells at 18 degrees C prevented the appearance of heavy oligomer within the lysosomes and resulted in the concentration of oligomers within an intracellular compartment of a density slightly heavier than that of plasma membrane. At 37 degrees C, dimers sedimented in a similar region of the gradient. But unlike heavy oligomers, dimers never entered lysosomes. These data suggest that the degree of Fc receptor clustering induced by oligomers of IgG influenced the intracellular fate of the ligand. |
