Abstract 2864: A CLEC12A immunocytokine with target cell-restricted IL-15 activity shows a favorable toxicity profile and high potency in AML
Authors: Klimovich Boris , Anton Leonard , Lim Yangmi , Won Jonghwa , Chashchina Anna , Pflügler Martin , Zekri Latifa , Jung Gundram , Salih Helmut R. 
Published: June 2022

Abstract
IL-15, often referred to as the “anti-cancer cytokine”, potently stimulates proliferation and activation of NK and T cells, but unlike its close relative IL-2, does neither promote activation-induced death of lymphocytes nor activity of immunoinhibitory Tregs. So far, short half-life, poor accumulation at the tumor site and severe toxicity upon systemic application limit IL-15 efficacy in patients. Fusion of IL-15 to antibodies directed to tumor antigens (classical immunocytokines, ICs) improves on accumulation at the tumor site and pharmacokinetics. However, since the activity of the cytokine moiety within classical ICs does not depend on antigen binding, the application of clinically effective doses is still prevented by toxicity due to unspecific immune activation. To overcome this problem, we took advantage of the unique mechanism of action of IL-15 which stimulates IL-15Rβ/γ on NK and T cells as a membrane-bound complex with IL-15Rα on monocytes and DCs (trans-presentation). We used an Fc-optimized antibody directed to CLEC12A, a surface antigen abundantly expressed on AML cells and leukemic stem cells but not on healthy stem cells. This antibody termed 33C2-SDIE was fused to an IL-15E46K mutant with abolished binding to IL-15Rα, allowing to substitute physiological trans-presentation of IL-15 by binding of the construct to its tumor-expressed target. Antigen-specific binding of the resulting modified immunocytokine (MIC12) was confirmed using multiple CLEC12A-expressing cell lines and primary AML samples from patients. Functional analysis of activation, cytokine release, and target cell lysis demonstrated that MIC proteins, in contrast to classical IC, stimulate cytotoxic lymphocytes in a highly target cell-restricted manner, allowing for the desired reduction of unspecific immune activation. At the same time, significantly superior NK cell reactivity against AML cells as compared to even the Fc-optimized antibody was observed. Both, IL-15 signaling and engagement of Fc-receptors by the optimized Fc-domain were found to be essential for optimal activity of our MIC constructs. Importantly, only treatment with MIC12 was capable to induce NK cell proliferation, which is required to overcome unfavorable target to effector ratios that prevail in overt cancer disease. In summary, our novel CLEC12A-targeting immunocytokine allows for tumor-restricted stimulation of cytotoxic lymphocytes and reduced toxicity while displaying superior anti-leukemic activity and constitutes a promising compound for the treatment of AML.
Citation Format: Boris Klimovich, Leonard Anton, Yangmi Lim, Jonghwa Won, Anna Chashchina, Martin Pflügler, Latifa Zekri, Gundram Jung, Helmut R. Salih. A CLEC12A immunocytokine with target cell-restricted IL-15 activity shows a favorable toxicity profile and high potency in AML [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 2864.