The fate of IgE bound to rat basophilic leukemia cells. II. Endocytosis of IgE oligomers and effect on receptor turnover.

We have assessed the internalization of variously sized oligomers of IgE bound to rat basophilic leukemia (RBL) cells by measuring their accessibility to the extracellular environment, and by direct visualization of the radiolabeled ligands. We also followed the fate of the internalized ligands and their receptors, as well as the fate of the free receptor on cells internalizing oligomers. In contrast to monomeric IgE, surface-bound oligomeric IgE was internalized. Notably, dimers provided an effective signal for internalization, although larger oligomers seem to be internalized more efficiently. In our experiments, 48% of the cell-bound dimers and 67% of the trimers were eliminated from the cell surface in 180 min. One-half of the maximal internalization observed with dimers and trimers occurred in 25 and 11 min, respectively. Release of radioactivity into the supernatant followed internalization; the released radioactivity did not bind to fresh cells and was only partially TCA-precipitable. Radioactive ligands remaining associated with the cells were unchanged as judged by m.w; they also were shown to remain receptor-bound. During either internalization or release of substantial amounts of the originally cell-bound oligomers, there was no increase in IgE-binding activity. In contrast, there was a transient drop (25%) in the number of free surface receptors suggesting internalization of the free receptors together with the oligomer-occupied receptor. Cells that failed to release histamine (RBL-I) processed dimeric and trimeric IgE similarly to histamine-releasing (RBL-2H3) cells. We conclude that dimeric and trimeric IgE are internalized by RBL cells and later are released to the medium in a partially degraded form. The ligand-bound receptor seems to be internalized with the ligand, along with some free receptor, and does not appear to be reusable or to recycle rapidly to the cell surface.