Differential requirement for irf 8 in formation of embryonic and adult macrophages in zebrafish
Interferon regulatory factor 8 (Irf8) is critical for mammalian macrophage development and innate immunity, but its role in teleost myelopoiesis remains incompletely understood. Specifically, genetic tools to analyze the role of irf 8 in zebrafish macrophage development at larval and adult stages are lacking. In this study, we generated irf 8 null mutants in zebrafish using TALEN-mediated targeting. Our analysis defines different requirements for irf 8 at different stages. irf 8 is required for formation of all macrophages during primitive and transient definitive hematopoiesis, but not during adult-phase definitive hematopoiesis starting at 5-6 days postfertilization. At early stages, irf 8 mutants have excess neutrophils and excess cell death in pu.1-expressing myeloid cells. Macrophage fates were recovered in irf 8 mutants after wildtype irf 8 expression in neutrophil and macrophage lineages, suggesting that irf 8 regulates macrophage specification and survival. In juvenile irf 8 mutant fish, mature macrophages are present, but at numbers significantly reduced compared to wildtype, indicating an ongoing requirement for irf 8 after embryogenesis. As development progresses, tissue macrophages become apparent in zebrafish irf 8 mutants, with the possible exception of microglia. Our study defines distinct requirement for irf 8 in myelopoiesis before and after transition to the adult hematopoietic system.