IgE-immunotoxins. I. IgE-intact ricin

An IgE immunotoxin consisting of rat IgE myeloma protein, IR 162, conjugated via the heterobifunctional linking agent N-succinimidyl-3-(2-pyridyldithio)propionate to intact ricin was synthesized and evaluated. The capacity of this IgE-immunotoxin to bind to rat basophilic leukemia cells (RBL cells) and to inhibit RBL cell incorporation of [3H]leucine was assessed. The IgE-intact ricin conjugate sensitized RBL cells for histamine release after treatment with anti-IgE with a time-course of sensitization and dose-response equivalent to native IgE. Intact ricin and IgE-intact ricin were both cytotoxic to RBL cells as assessed by [3H]leucine incorporation. Lactose (50 mM) competed with intact ricin binding and toxicity such that more than 100 ng/ml ricin (8 times its IC50 in the absence of lactose) was required for ricin to kill RBL cells in the presence of lactose. Lactose (50 mM) was not able to fully inhibit 1-100 ng/ml IgE-ricin immunotoxin killing of RBL cells. Saturation of RBL cell IgE receptors by preincubation with IgE totally inhibited IgE-intact ricin-induced toxicity, in the presence of lactose, indicating that toxicity required IgE Fc receptor binding.