Stat3 regulates developmental hematopoiesis and impacts myeloid cell function via canonical and non-canonical modalities.

Signal transducer and activator of transcription (STAT) 3 is extensively involved in the development, homeostasis and function of immune cells, with STAT3 disruption associated with human immune-related disorders. These roles have been assumed to be due to its canonical mode of action as an inducible transcription factor downstream of multiple cytokines, although alternative non-canonical functional modalities have also been described for STAT3. To further understand the roles of STAT3 gained from lineage-specific mouse knockouts, CRISPR/Cas9 was used to generate mutants of the conserved zebrafish Stat3 protein: a loss of function knockout (KO) mutant and a mutant lacking C-terminal sequences including the transactivation domain (ΔTAD). Analysis of the KO mutant identified conserved roles for Stat3 within hematopoietic stem cells impacting the development of all lineages throughout primitive and early definitive hematopoiesis, with altered immune cell populations in juveniles. The Stat3 KO mutant was unable to respond to lipopolysaccharide (LPS) or granulocyte colony-stimulating factor (G-CSF), and also exhibited significantly diminished neutrophil migration that correlated with abrogation of the Cxcl8b/Cxcr2 pathway. Many of these phenotypes were not shared by the Stat3 ΔTAD mutant. Indeed, only neutrophil and macrophage development were disrupted in these mutants with neutrophil migration actually increased, while responsiveness to LPS and G-CSF was maintained. This suggests that Stat3 participates in innate immune cell development and function through both canonical and non-canonical modalities, providing additional insights for relevant diseases.