Identification of polarized macrophage subsets in zebrafish

While the mammalian macrophage phenotypes have been intensively studied in vitro,the dynamic of their phenotypic polarization has never been investigated in live vertebrates. Weused the zebrafish as a live model to identify and trail macrophage subtypes. We generateda transgenic line whose macrophages expressingtumour necrosis factor alpha(tnfa), a key feature ofclassically activated (M1) macrophages, express fluorescent proteinsTg(mpeg1:mCherryF/tnfa:eGFP-F). Using 4D-confocal microscopy, we showed that both aseptic wounding andEscherichia coliinoculation triggered macrophage recruitment, some of which started to expresstnfa. RT-qPCR onFluorescence Activated Cell Sorting (FACS)-sorted tnfa+and tnfa−macrophages showed that they,respectively, expressed M1 and alternatively activated (M2) mammalian markers. Fate tracing oftnfa+macrophages during the time-course of inflammation demonstrated that pro-inflammatorymacrophages converted into M2-like phenotype during the resolution step. Our results reveal thediversity and plasticity of zebrafish macrophage subsets and underline the similarities withmammalian macrophages proposing a new system to study macrophage functional dynamic