Cas9<sup>+</sup> conditionally immortalized neutrophil progenitors as a tool for genome-wide CRISPR screening for neutrophil differentiation and function

Neutrophils are short-lived cells of the innate immune system that play numerous roles in defense against infection, regulation of immune responses, tissue damage and repair, autoimmunity, and other non-communicable diseases. Understanding neutrophil function at a mechanistic level has been hampered by the difficulty of working with primary neutrophils, which die rapidly upon isolation, and the relative paucity of neutrophil cell lines. Here, we report the creation of a Cas9 +ER-Hoxb8 neutrophil progenitor cell line that enables both forward and reverse genetic analysis of neutrophils. By editing progenitors via transduction with sgRNAs and then withdrawing estrogen, Cas9-edited neutrophils are produced with high efficiency. Importantly, neutrophil differentiation of edited progenitors occurs both in vitro in cell culture and when transferred into murine recipients. To demonstrate the utility of Cas9 +ER-Hoxb8 progenitors for forward genetics, we performed a pooled CRISPR screen to identify factors required for survival during neutrophil differentiation. This screen identified hundreds of genes, including <i>Cebpe</i>, a transcription factor known to be required for neutrophil differentiation from pre-neutrophils to immature neutrophils. Using this progenitor cell line, we confirmed that <i>Cebpe</i> is required for neutrophil differentiation in vivo, validating the utility of this line for studying in vivo phenotypes. The screen also identified all components of the WASH complex as being required for neutrophil differentiation, extending its known role in hematopoietic stem cell differentiation to later stages of neutrophil development. Taken together, this resource enables the analysis of the role of neutrophils in numerous disease states using genetics for the first time.