ATAC-Sequencing of human fetal PreProB and ProB progenitors.

Human lymphopoiesis is a dynamic life-long process that starts in utero at 6 weeks gestation; however developmental pathways defining fetal lymphopoiesis are largely unexplored. While the first committed B-progenitor in normal human bone marrow (BM) is a CD10+ve ProB-progenitor, the identification of a CD10-ve B-progenitor (Pre/ProB progenitor) in cord blood suggests there may be a second B-lymphoid development program in fetal life. Here, we provide a comprehensive analysis of the fetal B-cell developmental hierarchy and report the presence of both PreProB- and ProB-progenitors in fetal tissues and describe their ontogeny, molecular and functional characteristics. PreProB- and ProB-progenitors appear early in the first trimester in fetal liver, followed by a sustained second wave of B-progenitor development in fetal BM, where together they form >40% of the total HSC/progenitor pool. Unexpectedly, one-third of the B-progenitors (13±1% of lin-CD34+ cells) are CD10-ve PreProB-progenitors while, by contrast, PreProB-progenitors are virtually undetectable in adult BM. Single-cell transcriptomics and functional assays place PreProB- upstream of ProB-progenitors, identifying them as the first B-lymphoid restricted progenitor in human fetal life. Fetal BM PreProB- and ProB-progenitors both give rise solely to B-lineage cells yet they are transcriptionally distinct. PreProB- unlike ProB-progenitors, continue to express a number of key myeloid and stem cell genes and share some transcriptomic signatures with CD10-ve B-progenitor infant acute lymphoblastic leukemia blast cells. Our data identify PreProB-progenitors as the earliest B-lymphoid restricted progenitor in human fetal life, and suggest that this fetal-restricted committed B-progenitor might provide a permissive cellular context for prenatal B-progenitor leukemia initiation. Overall design: Comparison of chromatin accesibility between two committed B progenitors. Three biological replicates are included in addition to one positive control (Lin+ MNC)