Diphtheria toxin receptor-binding domain substitution with interleukin 6: genetic construction and interleukin 6 receptor-specific action of a diphtheria toxin-related interleukin 6 fusion protein

We have genetically replaced that portion of the diphtheriatoxin structural gene which encodes the native receptor-bindingdomain with a synthetic gene encoding the cytokine interleukin6 (IL-6/IFN-ß2/BSF-2). The resulting gene fusion encodesthe chimeric toxin DAB₃₈₉-IL-6. Following expression and purification,we demonstrate that DAB₃₈₉-IL-6 is selectively cytotoxic foreukaryotic cells bearing the interleukin 6 receptor. In addition,the cytotoxic action of DAB₃₈₉-IL-6 is shown to require bindingto the IL-6 receptor, internalization by receptor-mediated endocytosisand passage through an acidic compartment. Following the deliveryof the catalytically active fragment A to the cytosol of targetcells, cellular protein synthesis is inhibited by the ADP-ribosylationof elongation factor 2. While eukaryotic cells which are devoidof the IL-6 receptor are uniformly resistant to the action ofthis fusion toxin, the data presented suggest that a minimalnumber of IL-6 receptors may be necessary to mediate the internalizationof sufficient levels of DAB₃₈₉-IL-6 to result in the intoxicationof target cells.