Deciphering Macrophage Origin And Specification In Human Embryo At Single-Cell Resolution

Macrophages play an irreplaceable role in tissue homeostasis and inflammation.They are not only an important component of innate immunity,but also perform important tissue-specific functions.At the same time,they are also involved in the pathophysiological processes,such as tumor immunity,transplantation immunity and neurodegenerative diseases.For a long time,macrophages in adult mice have been considered to be derived from hematopoietic stem cells,but recent studies have found that most of the tissue-resident macrophages in mice are derived from primitive macrophages or pluripotent erythro-myeloid progenitor(EMP)of yolk sac at embryonic stage.Although the development of mammalian hematopoietic system is traditionally considered to be very conserved,there is no clear evidence for the presence of hematopoietic stem cell-independent macrophages in human,as in mice.Clarifying the relationship of macrophages of different origins in human body and the similarities and differences of their functions will be conducive to the development of new methods for the treatment of critical diseases.In recent years,single-cell sequencing technology has made great progress,especially suitable for studying hematopoietic system with high heterogeneity and cell population in early embryonic development.Therefore,we used single-cell transcriptome sequencing technology to accurately analyze CD45~+hematopoietic cells at different developmental stages and tissue sites in human embryos(within 8 weeks of gestation).After data dimensionality reduction,we identified and annotated 15 hematopoietic populations.After that,we deeply analyzed the transcriptomic characteristics and spatiotemporal dynamics of these hematopoietic populations.Our study found that,similar to the mice in which the tissue-resident macrophages were derived from primitive macrophages or EMP,the human embryonic tissue-resident macrophages also had two non-hematopoietic stem cell origins,namely,primitive macrophages and CD45~+CD34~+CD44~+human yolk sac-derived myeloid-biased progenitor(YSMP),where YSMP might be the counterpart of EMP in human embryos.Next,we focused on the development of microglia in the head during the embryonic stage.We found that the macrophages in the head showed significant heterogeneity at developmental stages,and the expression of such characteristic genes as CX3CR1,SALL1 and P2RY12 confirmed its continuous specialization process.Finally,after comparing the transcriptome data of macrophages at the embryonic,childhood and adult stages,we found that the specialization of tissue-resident macrophages in the head and liver had occurred at embryonic stage,while the specialization of resident macrophages in the skin and lung was mild or not.In conclusion,our study spanned multiple developmental stages of human embryos,covering multiple tissues and organs,mapped the development of hematopoietic cells at the early stage of human embryos,clarified the multiple origins of human embryo macrophages,defined the first population of hematopoietic progenitor cells(YSMP)with multi-lineage differentiation potential in human embryos,and analyzed the key molecular characteristics in the process of specialization of tissue-resident macrophages.In addition,our study has provided the most informative and reliable single-cell transcriptome data of human embryonic macrophages,which can provide reference and basis for the follow-up study of human macrophages.