Single-cell RNA-seq reveals endoimmune cells in zebrafish

Endothelial cells (ECs) constitute a monolayer that covers the interior surface of blood vessels and participates in various processes. Although vascular ECs share certain common properties, they differ in both structure and function. So far, the transcriptome profile and heterogeneity of the full repertoire of ECs in vertebrates remain poorly understood. The relatively small size of zebrafish embryos and larvae allows a feasible analysis of the broad spectrum of ECs within every tissue and organ of a whole organism. ECs have been suggested to be conditional innate immune cells. Whether ECs possess the comparable capacity of involvement in immune response is so far undetermined. Currently, through single-cell RNA sequencing analysis of total ECs of zebrafish we identified a fraction of endothelial cells expressing the marker genes of innate immune cells, named "endoimmune cells". We found the percentage of these cells gradually increased along with the embryonic development. Then, we observed the patrolling mCherry+ cells displayed the morphology alike to the macrophages and neutrophils. Furthermore, we revealed that some of the kdrl:ras-mCherry ECs were labelled with coro1a:EGFP as well. In addition, we demonstrated that the mCherry+ EC from intersegmental vessel could gradually present the expression of GFP in Tg(kdrl:ras-mCherry::coro1a:GFP) line, suggesting the endoimmune cells are derived from ECs. Importantly, we showed the endoimmune cells are responsive to the inflammation in zebrafish. Taken together, these data suggested the existence of endoimmune cells, a novel type of subpopulations of ECs. It will provide novel insights for understanding endothelial roles in both normal physiological function and human diseases and enable endoimmune cells-based target therapies.