Abstract Interleukin 2 (IL-2) is a pivotal immune agonist for tumor immunotherapy that has produced impressive clinical efficacy in a number of cancer types. Nevertheless, its pleiotropic effect has led to prohibitory side effects and immune restraint. In contrast, the PD-1 blockade-based cancer immunotherapy has a good safety profile by targeting and sustaining the activity of tumor-antigen specific T cells within cancer tissues. To take full advantage of complementary antitumor activity of PD-1 monoclonal antibody (mAb) and IL-2 as well as supreme intratumoral T-cell-targeting property of PD-1, a bifunctional fusion protein composed of PD-1 mAb and IL-2 mutein (αPD-1-IL-2c) is designed. IL-2c is a chimeric molecule that contains a fragment from IL-15 and does not bind to IL-2 receptor alpha (IL-2Rα). αPD-1-IL-2c stimulated greater pSTAT5 activation and proliferation in PD-1+ T cells than PD-1- T cells. αPD-1-IL-2c also induced a much stronger anti-tumor immune response compared to PD-1 mAb, IL-2c, or PD-1 mAb in combination with IL-2c in multiple syngeneic models such as B16F10, CT26, and MC38. In CT26 and MC38 models, αPD-1-IL-2c achieved complete response starting from 0.3 mg/kg dosing. In B16F10 model, αPD-1-IL-2c showed greater than 70% tumor growth inhibition (TGI). Global gene expression profiling studies showed that PD-1-IL-2c treatment significantly elevated expression levels of Cd3d, Cd3e, Cd8a, Il2rα, Cxcr3, Cxcr6, Zap70, Lck, and Pdcd1 in tumor tissues compared to treatment with PD-1 mAbs or IL-2c, indicating a specific expansion and activation of T cell in the tumor microenvironment (TME). The safety profile of αPD-1-IL-2c was assessed and compared to WT IL-2 and a leading “No-α” αPD-1-IL-2 fusion protein (αPD-1-IL-2x). WT IL-2 and αPD-1-IL-2x induced significant body weight loss and severe splenomegaly when dosed at 1.25 mg/kg. In contrast, αPD-1-IL-2c dosed at 10 mg/kg did not cause appreciable body weight loss and only led to minimal splenomegaly. In summary, we have developed a safe and highly potent αPD-1-IL-2 mutein that achieves full PD-1 blockade and TME-specific T cell activation and expansion without appreciable toxicity. Citation Format: Fan Ye, Jianing Huang, Zoey Huang, Sandra Chen, Eric Liao, Ella Li, Allison Quach, Jenny Jiang, Hanna Lin, Michael Hua, Savannah Yung, Lili Cheng, Ziyang Zhong. A safe and highly potent anti-PD-1-IL-2c fusion that decouples efficacy and toxicity of IL-2 therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB218.