S138 A DISTINCT WAVE OF HSC‐DERIVED T CELL‐ILC‐RESTRICTED PROGENITORS INITIATES THE EMBRYONIC THYMUS
Authors: Elsaid R. , Yang J. , Iturri L. , Rodewald H.‐R. , Perdiguero E.G. , Cumano A. 
Published: June 2019

Background:The emergence of definitive hematopoietic stem cells (HSCs) occurs around E9.5 in the aorta‐gonad‐mesonephros (AGM) cells, then colonize the fetal liver (FL) around E10.5 and starting at E16 migrate to the bone marrow (BM) niche. HSCs are distinguished from all other progenitors by their ability to long‐term repopulate all hematopoietic lineages when transplanted into irradiated recipient mice. The fetal thymus is colonized by two successive waves of hematopoietic progenitors. Thymic settling progenitors (TSP) of the first and second wave represent cells at different stages of differentiation, while cells in the first wave resemble common lymphoid progenitors (CLP) lacking any detectable myeloid or B cell potential and gives rise to skin‐resident Vg5+ T cells, TSPs from the second wave phenotypically resemble lympho‐myeloid primed progenitors (LMPP) and restrict their differentiation potential to the T lineage only in the thymus. However, the origin of the first wave of TSP and their differentiation potential into innate lymphoid cells (ILCs) are not directly assessed. The view that early thymic progenitors branch off from HSCs‐dependent progenitors has been recently challenged. Evidence of the heterogeneity and layered organization of the hematopoietic system is leading to a common speculation that the first TSPs may be derived of HSCs‐independent progenitors. Consistent with this concept of HSCs dependency and independency, numerous studies have reported detection of lymphoid potential and putative origins of lymphoid lineages before HSCs.Aims:Our study aims at unveiling the origin of the first lymphoid progenitors and the cellular pathways required for the development of lymphocytes during embryogenesis.Methods:In this work we combined in vivo constitutive and inducible lineage tracing models with in vitro lineage potential assays to conclusively show that the first TSPs are HSCs‐derived T‐ILCs restricted progenitors, whereas during hematopoiesis lymphoid‐associated gene expression in certain subsets does not equate with differentiation potential.Results:Here we show that Il7rα expression starts in yolk sac (YS)‐derived progenitors that are devoid of lymphoid potential. YS‐derived Il7rα

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progenitors are multi‐lineage and contribute to erythrocytes, megakaryocytes, mast cells and tissue resident macrophages.
Il7rα is also expressed in multipotent and myeloid progenitors in the fetal liver (FL), unlike in adult bone marrow (BM) where it is expressed exclusively in lymphoid progenitors. We found that in the FL, lymphoid lineage choice starts at the multipotent stage early before lineage restriction, a unique feature of embryonic but not adult lymphopoiesis. Moreover, we found the first TSPs and Vg5+ dendritic epithelial T cells (DETC) originate exclusively from HSCs‐derived progenitors. In addition, we show that the first TSPs give rise to all types of innate lymphoid cells (ILCs) both in vivo and in vitro.Summary/Conclusion:Our work provides temporal and spatial analysis of potential lymphoid origin(s) during embryogenesis, and how TSPs contribute to thymic innate lymphoid cells and thymic homeostasis, thus settling the controversy over the origin and the differentiation potential of the first TSPs.