IL2 Variant Circumvents ICOS+ Regulatory T-cell Expansion and Promotes NK Cell Activation

Clinical responses to high-dose IL2 therapy are limited due to selective expansion of CD4⁺CD25⁺Foxp3⁺ T-regulatory cells (Treg), especially ICOS⁺ Tregs, rather than natural killer (NK) cells and effector T cells. These ICOS⁺ Tregs are highly suppressive and constitutively express high levels of IL2Rα (CD25) and CD39. Here, we characterized the effect of a mutant form of IL2 (F42K), which preferentially binds to the lower affinity IL2Rβγ with reduced binding to CD25, on Tregs, effector NK cells, and T-cell subsets. Unlike wild-type (WT) IL2, F42K did not efficiently induce the expansion of highly suppressive ICOS⁺ Tregs in peripheral blood mononuclear cells (PBMC) from healthy controls and melanoma patients. Instead, it promoted the expansion of CD16⁺CD56⁺ NK cells and CD56^hiCD16^- NK cell subsets in both short- and long-term cultures, with enhanced Bcl-2 expression. Stimulation of PBMCs with F42K induced expression of more NK cell activation molecules, such as NKp30, NKp44, DNAM-1, NKG2D, 4-1BB/CD137, and Tim-3, than WT IL2. F42K induced greater upregulation of TRAIL, and NK-mediated cytolytic activity was increased against both autologous and HLA-mismatched melanoma cells compared with WT IL2. Gene expression analysis revealed distinct gene expression profiles stimulated by F42K, WT IL2, and IL15. F42K therapy in vivo also induced a dramatic reduction in the expansion of ICOS⁺ Tregs, promoted NK cell expansion, and inhibited melanoma tumor growth more efficiently than WT IL2 and more effectively than anti-CTLA-4. Our findings suggest that F42K could be a potential substitute for WT IL2 as a cytokine therapy for cancer. Cancer Immunol Res; 4(11); 983-94. ©2016 AACR.