Interleukin-2 mutants with enhanced alpha-receptor subunit binding affinity

Stimulation of T-cells by IL-2 has been exploited for treatment of metastatic renal carcinoma and melanoma. However, a narrow therapeutic window delimited by negligible stimulation of T-cells at low picomolar concentrations and undesirable stimulation of NK cells at nanomolar concentrations hampers IL-2-based therapies. We hypothesized that increasing the affinity of IL-2 for IL-2Ralpha may create a class of IL-2 mutants with increased biological potency as compared with wild-type IL-2. Towards this end, we have screened libraries of mutated IL-2 displayed on the surface of yeast and isolated mutants with a 15-30-fold improved affinity for the IL-2Ralpha subunit. These mutants do not exhibit appreciably altered bioactivity at 0.5-5 pM in steady-state bioassays, concentrations well below the IL-2Ralpha equilibrium binding constant for both the mutant and wild-type IL-2. A mutant was serendipitously identified that exhibited somewhat improved potency, perhaps via altered endocytic trafficking mechanisms described previously.