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Background Interleukin-2 (IL-2) is a well-established immune-oncology agent that activates CD8 + T cells. However, its therapeutic efficacy is limited due to its short half-life and IL-2 receptor alpha (IL-2Ra) capability. Moreover, the activation of CD8+ T cells by IL-2 is especially suppressed in the tumor microenvironment (TME) due to the acidic condition. Therefore, long-acting IL-2, which reduces IL-2Ra capability and enhances the Cytotoxic T lymphocyte (CTL) activity of CD8+ T cells is desired to enhance the therapeutic efficacy of IL-2. Here, we developed IL-2 mutein conjugated by releasable PEG that improves the pharmacokinetic profile and selectively activates CD8+ T cells residing in lymphoid organs and tumor microenvironment without inducing significant serious side effects.Methods IL-2Ra capabilities of human IL-2 (hIL-2) and IL-2 mutein were evaluated by ELISA. Cell proliferation ability was assessed using murine T cell line CTLL-2 and human T cell line TPA-Mat cultured at pH 7.4 and 7.2. Cell killing ability was evaluated using CD8a + T cells from the spleens of OTI mice were isolated, MC38OVA-NLuc target cells. CD8a+ T cells were stimulated at pH7.4 and 7.2 for 4 days. When stimulated, serial dilutions of IL-2 mutein conjugated by releasable PEG or non-releasable PEG were added. After this, co-culture was conducted for 1day. PK study and Cell distribution analysis were performed using BALB/c mice.Results The IL-2Ra capability of IL-2 mutein was dramatically decreased compared to that of hIL-2. Cell proliferation ability of IL-2 mutein conjugated by releasable PEG was 25 times higher than that of IL-2 mutein conjugated by non-releasable PEG. CD8a + T cells treated with IL-2 mutein conjugated by releasable PEG showed higher CTL activity than those conjugated by non-releasable PEG. Releasable PEG conjugation greatly improves pharmacokinetic profile of mutein IL-2. IL-2 mutein conjugated by releasable PEG selectively activates CD8+ T cells residing in lymphoid organs and tumor microenvironment without inducing significant serious side effects.Conclusions IL-2 mutein conjugated by releasable PEG improved the pharmacokinetic profile and selectively activates CD8 + T cells residing in lymphoid organs and tumor microenvironment without inducing significant serious side effects. These findings suggest its potential for the treatment of solid tumors in humans.