Interleukin 2 (IL-2) Variants Engineered for Increased IL-2 Receptor α-Subunit Affinity Exhibit Increased Potency Arising from a Cell Surface Ligand Reservoir Effect

Proliferation of activated T cells and CD56 bright natural killer (Cytokine Growth Factor Rev 13:169 –183, 1995) cells caused by interleukin-2 (IL-2) has been exploited in IL-2-based therapies for the treatment of metastatic renal cell carcinoma and melanoma (J Clin Oncol 13:688 – 696, 1995; J Clin Oncol 17: 2105–2116, 1999). In this study, we demonstrate the potentially improved therapeutic value of IL-2 variants engineered to gain 15- to 30-fold increased affinity for the IL-2 receptor -subunit (IL-2R). A novel pulsed bioassay was used to more closely approximate the rapid systemic clearance pharmacokinetics of cytokines such as IL-2, compared with conventional static bioassays. In this assay, mutants with increased affinity for IL-2R exhibit significantly increased activity for T-cell proliferation, whereas static bioassays not only fail to reveal the increased activity resulting from enhanced IL-2R affinity (false negatives), but also suggest improved activity for another mutant without enhanced activity in the pulsed assay (false positive). Our studies on the mechanism leading to increased activity of IL-2 mutants with increased IL-2R affinity suggest that cellsurface IL-2R acts as a ligand reservoir for the IL-2 mutants. This leads to increased cell-surface persistence of the IL-2 mutants with increased IL-2R affinity in cell-surface ligand reservoirs and consequently increased integrated growth signal. Furthermore, a mathematical model predicts increased persistence of cell surface-bound IL-2 in vivo for enhanced IL-2R-binding IL-2 mutants, suggesting potentially improved therapeutic value of allowing cellular capture of ligands in persistent cell-surface reservoirs. Finally, our findings emphasize the critical choice of appropriate bioassays to evaluate engineered proteins and other drugs.