Diverse origins and transcriptional profiles of macrophages in a spinal lesion in zebrafish

The injury responses of tissue-resident macrophages in the CNS (microglia) and blood-derived macrophages (BDMs) play key roles in successful regeneration of the zebrafish spinal cord, but the origins and dynamic behaviours of these immune cells are not well characterized. Here, we find that microglia, labelled by the p2ry12:GFP reporter gene, migrate long-distance through neural tissue from the brain to the spinal lesion site, while BDMs, labelled by the mpeg1:mCherry reporter gene, migrate mainly from the caudal hematopoietic tissue to the lesion and back. Half of p2ry12:GFP-positive microglia co-express mpeg1:mCherry, while mpeg1:mCherry-positive BDMs are mostly p2ry12:GFP-negative. However, a BDM sub-population starts to express p2ry12:GFP in the lesion. This indicates heterogeneous and dynamic gene expression in macrophage populations. Gene expression profiling reveals several microglia-like and BDM-like clusters in the lesion with gene expression profiles related to proliferation, phagocytosis, pro- and anti-inflammatory phenotypes and distinct expression of regeneration-relevant genes. The most abundant cell cluster are densely-packed microglia-like cells in the lesion core, which express the novel marker g0s2, as well as phagocytosis-related genes. Hence, regenerative success of the zebrafish spinal cord is linked to a heterogeneous and dynamic response of microglia and BDM subpopulations.