Single-cell deciphering of human fetal innate lymphoid cell development

The functions of innate lymphoid cells (ILCs) in immune system are increasingly appreciated, whereas the early development of ILCs in human remains elusive. In this study, we sorted humanhematopoietic stem progenitor cells, lymphoid progenitors, presumed ILC progenitor/precursors and mature ILCs in the fetal hematopoietic, lymphoid and non-lymphoid tissues, from 8 to 12 PCW,for single-cell RNA-sequencing, followed by computational analysis and functional validation. We delineated the early phase of ILC development, from hematopoietic stem progenitor cells to multipotent lymphoid progenitors and to ILC progenitors, which mainly occurred in fetal liver and intestine. We further unveiled interleukin-3 receptor alpha (IL-3RA) as the surface marker for the lymphoid progenitors with T cell, B cell and ILC potentials. Notably, we determined the heterogeneity and tissue distribution of each ILC subpopulation, revealing the shared proliferating characteristics of the precursors of each ILC subtype. Additionally, a novel unconventional ILC2 subpopulation (CRTH2-CCR9+ ILC2) was identified in fetal thymus. Taken together, our study illuminates the precise cellular and molecular features underlying the stepwise formation of human fetal ILC hierarchy with remarkable spatiotemporal heterogeneity. Overall design: Here, we provided an unprecedented single-cell transcriptome atlas of stepwise ILC ontogeny in early human fetus using cells from fetal hematopoietic, lymphoid and non-lymphoid tissues spanning 8 to 12 post-conception weeks (PCWs).

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