The therapeutic potential of interleukin-2 (IL-2) in cancer treatment is limited by toxicity challenges, partly due to an unfavorable pharmacokinetic profile and unintended activation of regulatory T (T _reg ) cells alongside the desired activation of CD8 ⁺ effector T cells. To selectively stimulate CD8 ⁺ T cells over T _reg cells, we engineered an IL-2 variant (IL-2var) with a dual-tuned affinity profile that features reduced binding to IL-2Rα (CD25) and enhanced binding to IL-2Rβ (CD122). To optimize its pharmacokinetics and facilitate tumor enrichment, the variant is fused to albumin and delivered as an mRNA encapsulated in a lipid nanoparticle (Alb–IL-2var RNA-LNP), enabling sustained systemic exposure from hepatic production upon intravenous administration. We show that Alb–IL-2var has a favorable pharmacokinetic profile and is tolerated at biologically active doses in immunocompetent mice and cynomolgus monkeys. Selective enhancement of CD8 ⁺ T cell responses over T _reg cells is demonstrated in vitro in human peripheral blood mononuclear cells and in vivo in mice and cynomolgus monkeys. When combined with an mRNA cancer vaccine in syngeneic subcutaneous mouse tumor models, Alb–IL-2var RNA-LNP stimulates the expansion of tumor-infiltrating and circulating tumor antigen–specific CD8 ⁺ T cells, but not T _reg cells. In advanced and cold syngeneic tumor models, it enhances the efficacy of radiotherapy, checkpoint inhibitors, and cancer vaccines. Combination with checkpoint inhibitors and vaccine induces profound proinflammatory conversion of cold tumors. These preclinical results validate a rational design approach to overcome the limitations of IL-2 therapy and support the clinical evaluation of Alb–IL-2var RNA-LNP for solid cancers.

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