Fc receptor-mediated endocytosis of monomeric IgG1 by human mononuclear phagocytes was evaluated under conditions where aggregated IgG and insulin readily undergo receptor-mediated internalization. U937 cells or normal human peripheral blood monocytes were incubated at 37 degrees C in the absence of free radioligand after having first bound 125I-IgG1 at 0 degrees C. To determine the amount of cell-associated IgG1 internalized after varying periods of 37 degrees C incubation, surface-bound IgG1 was removed by sequential exposure of cells at 0 degrees C to a nonspecific proteinase for 1 h and to acetic acid at pH 3.2 for 3 min. The failure to develop a proteinase- and acid-resistant fraction, similar to that seen over time at 37 degrees C in parallel experiments with 125I-insulin and 125I-aggregated IgG, and the lack of degradation of the IgG1 released into the medium from the same cells over time show that these cells do not endocytose and degrade monomeric IgG by an Fc receptor-specific mechanism and suggest that constitutive recycling without degradation is unlikely to be occurring. These data fulfill one prediction of the hypothesis that receptor-receptor interaction triggers Fc receptor-mediated endocytosis.

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