SRP470798 PRJNA1037050 GSE247279 Ikaros and Aiolos directly regulate AP-1 transcriptional complexes and are essential for natural killer cell development [ATAC-seq] Ikaros family transcription factors regulate lymphocyte biology and are targets of the immunomodulatory imide drugs (IMiDs) for hematological malignancies. Ikaros (Ikzf1/IKZF1) is the most broadly expressed family member in lymphocytes, yet its role in innate lymphopoiesis is unknown. Here we used gene inactivation to reveal that NK cell-expression of Ikaros is essential for normal NK lymphopoiesis. Mechanistically, IKZF1 directly repressed Cish and Socs2, known negative regulators of IL-15R resulting in impaired IL-15 signaling in Ikzf1-null NK cells. Bcl2l11/BIM levels and apoptosis were increased in Ikzf1-null NK cells which in part accounted for the NK lymphopenia since both apoptosis and NK cell frequency were restored to normal levels when Ikzf1 and Bcl2l11 were co-deleted. Ikzf1-null NK cells presented extensive transcriptional alterations with a striking reduction in expression of genes encoding AP-1 transcriptional complexes (Fos/Jun members) and a compensatory increase in Ikzf2 and Ikzf3. Both IKZF1 and IKZF3 directly bound AP-1 family members and deletion of both Ikzf1 and Ikzf3 in NK cells resulted in a further reduction in Jun/Fos expression and a complete loss of peripheral NK cells in mice. Consistently, IKZF1 bound Jun/Fos genes at the same locations in activated B cells identifying a novel and conserved role for IKZF1 in Jun/Fos regulation. Collectively we show the Ikaros-family are novel regulators of apoptosis, cytokine responsiveness and AP-1 transcriptional activity required for NK cell development and function. Overall design: We generated NK cell conditional knockout mice for Ikzf1 and compared these to Cre negative, floxed control mice. Viable splenic NK (NK1.1+ NKp46+ CD3- TCRß- 7AAD-) cells were directly sorted from the spleens of mice prior to processing for ATAC-seq. GSE247279 parent_bioproject PRJNA1037047