| Abstract: | Receptor-mediated endocytosis of IgG and immune complexes in macrophages is terminated with digestion of the ligand in lysosomes. However, there are controversial data on whether Fc receptors are degraded together with the ligand or recycled to the cell surface. In the present study, rat peritoneal macrophages were incubated at 4 degrees C with rat peroxidase-antiperoxidase (PAP) complex for 1 h, washed and warmed up to 37 degrees C for different time periods and reincubated with new PAP at 4 degrees C. In another series of experiments, the cells were preincubated with 50 nM monensin, then cooled to 4 degrees C and reincubated with PAP in the presence of monensin. The cells were fixed and processed for electron microscopy at different stages of the experiments. Quantitative data were obtained by measuring PAP-binding membrane lengths on electron micrographs (morphometry) and by determining surface-bound PAP with spectrophotometry. In macrophages which had bound PAP at 4 degrees C and were warmed up for 5 min, the PAP was cleared from the cell surface and was found in endosome-like structures. When reincubated with PAP at 4 degrees C, such cells again bound the ligand on the cell surface, mainly in labyrinthic invaginations of the plasma membrane (synonyms: lacunae, caveolar indentations). Macrophages which had been warmed up for longer periods (30 and 60 min) showed the bound ligand all along the plasma membrane. Treatment of cells with monensin did not affect internalization of PAP, however, it decreased the ligand binding ability of macrophages considerably. These findings led us to assume an Fc receptor replenishment from a cytoplasmic pool. |
