Binding and pinocytosis of polyvalent IgG-containing immune complexes by mouse macrophages leads to the selective removal of Fc receptors (FcR) from the cell surface and to the rapid delivery of receptor and ligand to lysosomes, where both are degraded (I. Mellman and H. Plutner, 1984, Journal of Cell Biology, 98:1170-1177). In this paper, we have studied the internalization of FcR tagged with a monovalent probe that, unlike IgG-complexes, cannot cross-link adjacent receptors. We have used an Fab fragment of high affinity anti-FcR monoclonal antibody whose binding was completely sensitive to low pH (4.0) at 4 degrees C. Thus, surface-bound (acid-releasable) and intracellular (acid-resistant) 125I-Fab could be readily distinguished. Incubation of J774 macrophages with 125I-Fab at 37 degrees C did not lead to the accumulation of large amounts of the antibody in the acid-resistant compartment. After 3 h, only 20% of the total cell-associated radiolabel was intracellular. The internalized 125I-Fab was also shown by Percoll gradient centrifugation to be associated primarily with low density endosomes, as opposed to lysosomes. Significantly, most of the labeled antibody returned rapidly to the plasma membrane, still bound to FcR. This recycling was complete within 10 min, was unaffected by NH4Cl, and was only slightly inhibited by the Na+-H+ ionophore monensin. These results indicate that monovalent Fab-FcR complexes are internalized, delivered to endosomes, and rapidly returned to the cell surface. Since the internalization of polyvalent IgG-complexes removed the FcR from this recycling pathway and caused its transport to lysosomes, we suggest that the state of receptor aggregation in the endosome membrane helps determine its intracellular fate.

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