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Background: The success and limitations of current immunotherapies have pushed research toward the development of alternative approaches and the possibility to manipulate other cytotoxic immune cells such as natural killer (NK) cells. Here, we targeted an intracellular inhibiting protein 'cytokine inducible SH2-containing protein' (CISH) in NK cells to evaluate the impact on their functions and antitumor properties., Methods: To further understand CISH functions in NK cells, we developed a conditional Cish-deficient mouse model in NK cells ( Cish fl/fl Ncr1 Ki/+ ). NK cells cytokine expression, signaling and cytotoxicity has been evaluated in vitro. Using intravenous injection of B16F10 melanoma cell line and EO711 triple negative breast cancer cell line, metastasis evaluation was performed. Then, orthotopic implantation of breast tumors was performed and tumor growth was followed using bioluminescence. Infiltration and phenotype of NK cells in the tumor was evaluated. Finally, we targeted CISH in human NK-92 or primary NK cells, using a technology combining the CRISPR(i)-dCas9 tool with a new lentiviral pseudotype. We then tested human NK cells functions., Results: In Cish fl/fl Ncr1 Ki/+ mice, we detected no developmental or homeostatic difference in NK cells. Global gene expression of Cish fl/fl Ncr1 Ki/+ NK cells compared with Cish +/+ Ncr1 Ki/+ NK cells revealed upregulation of pathways and genes associated with NK cell cycling and activation. We show that CISH does not only regulate interleukin-15 (IL-15) signaling pathways but also natural cytotoxicity receptors (NCR) pathways, triggering CISH protein expression. Primed Cish fl/fl Ncr1 Ki/+ NK cells display increased activation upon NCR stimulation. Cish fl/fl Ncr1 Ki/+ NK cells display lower activation thresholds and Cish fl/fl Ncr1 Ki/+ mice are more resistant to tumor metastasis and to primary breast cancer growth. CISH deletion favors NK cell accumulation to the primary tumor, optimizes NK cell killing properties and decreases TIGIT immune checkpoint receptor expression, limiting NK cell exhaustion. Finally, using CRISPRi, we then targeted CISH in human NK-92 or primary NK cells. In human NK cells, CISH deletion also favors NCR signaling and antitumor functions., Conclusion: This study represents a crucial step in the mechanistic understanding and safety of Cish targeting to unleash NK cell antitumor function in solid tumors. Our results validate CISH as an emerging therapeutic target to enhance NK cell immunotherapy., Competing Interests: Competing interests: EVi is an employee of Innate Pharma and has ownership and stock options. DO is cofounder and shareholder of Imcheck Therapeutics, Emergence Therapeutics, and Alderaan Biotechnology. NDH is a founder and shareholder of oNKo-Innate. NDH receives research funding from Servier, Paranta Biosciences and Anaxis Pharma. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (© Author(s) (or their employer(s)) 2022. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)