Clonal analysis of fetal hematopoietic stem/progenitor cell subsets reveals how post-transplantation capabilities are distributed

It has been proposed that adult haematopoiesis is sustained by multipotent progenitor (MPP) clones that are specified during development. From an immunophenotypic perspective, it is known that hematopoietic stem cell (HSC) and MPPs are present in the fetal liver yet our understanding of how fetal MPPs functionally compare to those in the adult bone marrow is incomplete. Using acute-term transplantations, we found that at a population-level fetal immunophenotypic MPP classes exhibited similar lineage biases as adult cells, albeit with some difference in lymphoid output. Clonal assessment of fetal MPPs engraftment revealed that lineage biases largely resulted from differences in the pattern of single- or bi-lineage differentiation. Immunophenotypic long-term (LT)- and short-term (ST)-HSCs in the fetal liver were distinguished from MPPs according to propensity for clonal multi-lineage differentiation. We also discovered that a large cohort of long-term repopulating units (LT-RU) were within the immunophenotypic ST-HSC population, a significant portion of these were labelled using Flt3-cre. This finding has two implications: (1) use of the CD150+ LT-HSC immunophenotype alone will systematically underestimate the size and diversity of the fetal LT-RU pool; and, (2), given fetal LT-RUs with a ST-HSC immunophenotype have the functional attributes required to persist into adulthood.