Introduction Macrophages play an important role in the immune response to injury. However, a broader role in repair and regeneration has been reported. While macrophage ablation impairs epithelialisation and reduces scar formation in skin repair model in mammals, it impairs the regeneration of the limb in axolotl and of the fin in adult zebrafish. This discrepancy is likely due to the high plasticity of macrophages, which can adopt various phenotypes and functions. Here we used the zebrafish larva to study the role of pro-inflammatory, M1-like macrophages during caudal fin regeneration.
Materials, methods and results Using 4D confocal microscopy and double transgenic larvae to visualise macrophages and TNF-α producer cells, we showed that initially most recruited macrophages expressed TNF-α (referred as M1-like). However TNF-α expression was transient and decreased from 24 h post amputation, due to the recruitment of a second wave of non-inflammatory macrophages (referred as M2-like) and to a local phenotypic conversion of macrophages. We demonstrated that TNF-α expressing macrophages were crucial for fin regeneration as early depletion of macrophages blocked blastema formation and regeneration process, while the depletion of late M2-like macrophages did not. We then turned our attention to the trophic control exerted by M1-like macrophages on the regeneration process and investigated the role of TNF-α signalling. Using a combination of morpholino knock down strategy and parabiosis experiments, we showed that TNFR1 played a necessary and direct role in the induction of fin regeneration by expanding blastemal cell.
Conclusion In summary the present data strongly suggest that TNF-α producing macrophages stimulate blastemal cell proliferation through the TNFR1 activation, revealing a new insight into the mechanism of caudal fin regeneration.