Background and objectives Zebrafish have emerged as a powerful model system to study leukocyte recruitment and inflammation. In human and mouse, several subsets of the macrophage lineage have been described. The two extremes in the range of macrophage function are characterised by the pro-inflammatory (M1) and the anti-inflammatory (M2) phenotypes. M1 macrophages, efficient producers of inflammatory cytokines such as TNF-α, exert cytotoxic functions, whereas M2 are anti-inflammatory or associated with tissue remodelling. In zebrafish embryos, primitive macrophages have been identified, however, although the existence of different macrophage subtypes has been suggested they are not fully characterised.
Materials, Methods and results To assess the phenotype of activated macrophages in zebrafish, we generated a double transgenic line to visualise TNF-α expressing macrophages. Using 4D-confocal microscopy, we showed that both wound induced inflammation and E. coli infection triggered the recruitment of macrophages, which started to express TNF-α few hours following stimulation. These results are the first showing in real time the mobilisation of pro-inflammatory macrophages (referred as M1-like) at the inflammation site. To further characterise macrophage subsets at the molecular level, we sorted by FACS TNF-α+ and TNF-α- macrophages from transgenic larvae to enrich in M1- or M2-like macrophages. RT-qPCR analysis revealed that TNF-α+ and TNF-α- macrophages express markers of human M1 and M2 subsets, respectively.
Conclusion Our data suggests a high conservation in the phenotypes of macrophages in vertebrates. We thus propose that these findings combined with the specific advantages of the larval zebrafish model system – transparency, genetic tractability – should facilitate the study of inflammation, infection, and leukocyte biology.