Hhex promotes NK cell homeostasis by repressing BIM-dependent apoptosis

Hhex encodes a homeobox transcriptional regulator that plays a key role in embryonic development and hematopoiesis. Hhex is highly expressed in NK cells and germline deletion of Hhex results in significant defects in lymphoid development, including NK cells. However, whether Hhex is intrinsically required throughout NK cell development or for NK cell function remains unknown. To investigate this, we generated mice that specifically lack Hhex in NK cells. Hhex was intrinsically required for NK cell homeostasis, while NK cell differentiation, IL-15 responsiveness and cytotoxic function were largely normal in the absence of Hhex. Unexpectedly, increased IL-15 availability failed to rescue Hhex-deficient NK cell homeostasis, suggesting that Hhex regulates developmental pathways extrinsic to those dependent on IL-15. Gene expression and functional genetic approaches revealed that Hhex promoted NK cell survival by repressing BIM expression, a key apoptotic mediator in NK cells. This study identifies Hhex as a novel transcription factor essential for NK cell biology. 4 biological repliciates of Hhex-deficient and 3 biological replicates of Cre control splenic NK cells (viable C45+CD3-Nk1.1+NKp46+CD49b+) were isolated ex vivo by sorting and subjected to mRNA extraction, library preparation and sequencing on an Illumina NextSeq 500 sequencer