A yolk sac-derived hepatic γδ T cell subpopulation develops in a hematopoietic stem cell- and thymus-independent manner

Concurrently with and prior to the emergence of hematopoietic stem cells within the intraembryonic aorta, various types of hematopoietic progenitors are believed to arise in the extraembryonic yolk sac through a process of endothelial-to-hematopoietic transition. However, the persistent lack of genetic lineage tracing models specifically targeted to the extraembryonic yolk sac has left the precise embryonic origins of various hematopoietic lineages unclear. Furthermore, it remains entirely unknown whether and to what extent hematopoietic lineages of distinct origins differ in their functional characteristics. γδ T cells are a highly heterogeneous T cell lineage with diverse immunological functions in tissue surveillance, barrier maintenance, and homeostasis. However, the developmental origins underlying this heterogeneity, particularly the existence and physiological relevance of extrathymic pathways, remain incompletely defined.

Through computational screening, we identified Stab2 as a yolk sac endothelial cell-specific marker at the blood forming stages in mouse embryos, therefore generated a corresponding inducible genetic lineage tracing mouse model and demonstrated its ability to specifically label yolk sac–derived hematopoietic cells in vivo. Importantly, we detected the contribution of the yolk sac endothelial cells labeled at embryonic day 9.5 to a population of Vγ1 γδ T cells in the adult liver but hardly to any hematopoietic stem and progenitor cells in the thymus and bone marrow. Single-cell transcriptomic profiling combined with functional evaluation revealed that these yolk sac-derived hepatic γδ T cells preferentially enrich IFN-γ–producing γδ T1 cells and overrepresent characteristics associated with tissue homeostasis and repair. 

Collectively, this study established an unprecedented genetic lineage tracing mouse model capable of labeling hematopoietic cells of extraembryonic rather than intraembryonic origin irrespective of time window, provided direct in vivo evidence for the existence of a bone marrow hematopoietic stem cell-independent and thymus-independent developmental pathway of liver γδ T cells originating from yolk sac endothelial cells, and suggested that different origins and developmental paths may confer distinct functional properties to γδ T cells.