Overlapping Definitive Progenitor Waves Divide and Conquer to Build a Layered Hematopoietic System

Abstract

Adult innate immune cells are part of a layered hematopoietic system constructed from definitive hematopoietic stem and progenitor cells (HSPC) with diverse origins during development. One source of HSPC are fetal hematopoietic stem cells (HSC) that provide long-term reconstitution throughout life. However, the extent to which HSC produce mature cells in utero is only recently being uncovered. This is in part due to the added complexity of an overlapping wave of definitive progenitors that derive from yolk sac erythro-myeloid progenitors (EMP). HSC and EMP are generated from spatiotemporally distinct hemogenic endothelia, yet they both migrate to the fetal liver niche where they co-habitate and are presumed to reach their full potential. Delineation of the respective HSC and EMP pathways towards developmental immune cell differentiation has been confounded by challenges in ontogeny-specific cell labeling. In this study, in vivo inducible pulse chase labeling revealed that HSC contribute little to fetal myelopoiesis and that EMP are the predominant source of mature myeloid cells until birth. This is similar to what has been reported for the erythroid branch of hematopoiesis thereby establishing a developmentally-restricted privilege for erythro-myeloid differentiation from EMP compared to HSC. Tracing the origins of mature cells to the progenitor level by immunophenotyping and single cell RNA sequencing uncovered a dichotomy in the allocation of fetal liver EMP and HSC to myeloid progenitor subsets, both in timing and lineage bias. This has exposed an uncoupling between developmental granulopoiesis and monopoiesis from EMP and HSC pathways, and provides a framework for future studies of HSC-dependent and -independent hematopoiesis.HIGHLIGHTSEMP-to-HSC switch in fetal liver myelopoiesis occurs late in gestationEMP are efficient at producing early transit amplifying erythroid and myeloid intermediatesscRNA-seq reveals three trajectories of EMP myelopoiesisMyeloid lineage commitment during development is cell type and ontogeny specific

A Refined Single Cell Landscape of Haematopoiesis in the Mouse Foetal Liver

“…Defining the mouse embryonic liver environment at single-cell resolution using scRNA-seq informs whether and how the mouse provides a suitable model to understand the molecular bases of human liver development, function and disease, including congenital immunodeficiencies, anaemia and also childhood leukaemia [ 26 ]. Here we addressed limitations in prior studies that either performed a preselection step to enrich the dataset only for specific cell types [ 28 , 31 ] or included only a small number of all foetal liver cell types [ 27 , 29 ] and extended the analysis of a study that obtained the transcriptomes of a larger number of liver cells but analysed mostly hepatocyte development [ 30 ].…”

“…Towards this aim, several single cell (sc) RNA-seq datasets have been recently generated, which sought to identify mouse foetal liver cell types via their transcriptomic signature and compared gene expression patterns at single cell level across the cell types in the foetal liver (e.g., [ 27 , 28 , 29 , 30 ]. However, the haematopoiesis studies of the mouse foetal liver using scRNA-seq to date focussed on the analysis of selected cell subsets, isolated by genetic lineage-tracing [ 31 ] or surface phenotyping [ 28 ]. Other scRNA-seq datasets were generated without isolation bias, but from only a small number of foetal liver cells [ 27 , 29 ], which is suboptimal for deep phenotyping, or they were comprised of larger cell numbers but analysed mostly hepatocyte development [ 30 ].…”

A refined single cell landscape of haematopoiesis in the mouse foetal liver

“…Defining the mouse embryonic liver environment at single-cell resolution using scRNA-seq will inform whether and how the mouse provides a suitable model to understand the molecular bases of human liver development, function and disease, including congenital immunodeficiencies, anaemia and also childhood leukaemia (Cazzola et al, 2020). Here we addressed limitations in prior studies that either performed a preselection step to enrich the dataset only for specific cell types (Freyer et al, 2020; Gao et al, 2022) or included only a small number of all foetal liver cell types (Dong et al, 2018; Su et al, 2017) and extended the analysis of a study that obtained the transcriptomes of a larger number of liver cells but analysed mostly hepatocyte development (Wang et al, 2020).…”

“…Towards this aim, several single cell (sc) RNA-seq datasets have been recently generated, which sought to identify foetal liver cell types via their transcriptomic signature and compared gene expression patterns at single cell level across the cell types in the foetal liver (e.g., (Dong et al, 2018; Gao et al, 2022; Su et al, 2017; Wang et al, 2020)). However, the haematopoiesis studies using foetal liver scRNA-seq to date focussed on the analysis of selected cell subsets, isolated by genetic lineage-tracing (Freyer et al, 2020) or surface phenotyping (Gao et al, 2022). Other scRNA-seq datasets were generated without isolation bias, but from only a small number of foetal liver cells (Dong et al, 2018; Su et al, 2017), which is suboptimal for deep phenotyping, or they were comprised of larger cell numbers but analysed mostly hepatocyte development (Wang et al, 2020).…”

Robust temporal map of human in vitro myelopoiesis using single-cell genomics

“…During the second trimester of development, HSC migrate to the bone marrow, the only remaining site of hematopoiesis in adulthood 10 . In mice, myeloid progenitors derived from the yolk-sac (YS) generate a wide range of myeloid cells, including monocytes and neutrophils, and are thought to be the main contributors to this lineage during fetal development 11 . In humans, we have limited knowledge about how the YS myeloid progenitors expand, proliferate and differentiate, and a poor understanding of the regulatory mechanisms involved.…”