IL2 FUSED TO LECTIN-DEFICIENT RICIN IS TOXIC TO HUMAN LEUKEMIA-CELLS EXPRESSING THE IL2 RECEPTOR

Interleukin-2 (IL2) fused to ricin B chain (RTB) with modifications of amino acid residues in each of three galactose binding subdomains (1 alpha, 1 beta and 2 gamma) was expressed in insect cells, purified by immunoaffinity chromatography and reassociated with ricin A chain (RTA ). The fusion toxin-bound human leukemic cells with IL2 receptors and the binding was competed with IL2 but not asialofetuin. In contrast, b inding was not observed with receptor negative human cell lines, and t he fusion molecule very weakly bound asialofetuin (K-d=10(-6)M), indic ating lectin-deficient RTB. The IL2-lectin-deficient RTB-RTA intoxicat ed IL2 receptor bearing cells as well as ricin or IL2-wild-type RTB-RT A. While ricin and IL2-wild-type RTB-RTA were equally toxic to recepto r negative cell lines, the IL2-lectin-deficient RTB-RTA was two-two an d one half logs less cytotoxic to these cell lines. The sensitivity of receptor-positive cells to the lectin-deficient fusion protein sugges ts that high avidity intracellular galactose binding may not be requir ed for ricin intoxication, at least in the case of IL2 receptor-target ed molecules. Furthermore, the potent selective cytotoxicity of the fu sion protein suggests that the IL2-lectin-deficient RTB-RTA and simila r ricin fusion molecules directed against other leukemic cell surface receptors provide a novel class of fusion toxins for therapy of human leukemias.