Protein engineering of diphtheria-toxin-related interleukin-2 fusion toxins to increase cytotoxic potency for high-affinity IL-2-receptor-bearing target cells

We have used site-directed insertion and point mutagenesis in an attempt to increase the cytotoxic potency and receptor-binding affinity of the diphtheria-toxin-related interleukin-2 (IL-2) fusion toxins. Previous studies have demonstrated that both the DAB486-IL-Z and DAB389-IL-2 forms of the fusion toxin consist of three functional domains: the N-tenninal fragment-A-assodated ADP-ribosyltransferase, the hydrophobk-membrane-associating domains, and the C-terminal receptor-binding domain of human IL-2. By insertion mutagenesis we have increased the apparent flexibility of the polypeptide chain between the membraneassociating domains and the receptor-binding domain of this fusion toxin. In comparison to DAB486-IL-2, the cytotoxic potency of the insertion mutants was increased by ∼17-fold for high-affinity IL-2-receptor-bearing cell lines in vitro. Moreover, competitive displacement experiments using [125I]rIL-2 demonstrate that the increase in cytotoxic potency correlates with an increase in receptor-binding affinity for both the high and intermediate forms of the IL-2 receptor.