Recycling of receptor-bound IgE by rat basophilic leukemia cells.

Rat basophilic leukemia (RBL) cells have receptors for immunoglobulin E (IgE). We previously showed that unlike some other ligands, the binding of monomeric rat or mouse IgE to RBL cells does not induce endocytosis. However, aggregation of the cell-bound, monomeric mouse IgE anti-dinitrophenyl (DNP) with DNP-protein conjugates leads to endocytosis of the aggregated mouse IgE and to the co-endocytosis of some unaggregated monomeric rat IgE. In this study we analyzed and compared the fate of co-endocytosed and endocytosed IgE. We found that co-endocytosed rat IgE recycled back to the cell surface within 3 to 4 hr. In contrast, endocytosed, immunochemically cross-linked, receptor-bound mouse IgE anti-DNP was partially degraded and was released into the medium, with no observable recycling of receptors, by 3 hr. However, addition of the hapten, DNP-lysine, resulted in rapid recycling (t1/2 10 min) of the endocytosed receptor-bound IgE to the plasma membrane and blocked additional endocytosis. Recycling of the endocytosed mouse IgE was more pronounced when the hapten was added to cells within 20 min of the initiation of endocytosis. When the hapten was added to the cells at later times (60 to 180 min), progressively less IgE recycled to the surface. This may reflect shuttling of the internalized IgE from a 'prelysosomal' to a 'lysosomal' compartment. Thus we provide evidence for recycling of monomeric IgE receptor complexes, sorting between cross-linked and non-cross-linked IgE receptor complexes, the freeing of receptor-bound monomeric IgE from the endocytosed immune-complexed IgE, and the apparent dependence of the recycling efficiency upon intracellular localization.