The ETS family transcription factor PU.1 is necessary for the maintenance of fetal liver hematopoietic stem cells

PU.1 is a member of the ETS family of transcription factors and is required for the development of multiple hematopoietic lineages. PU.1(-/-) mice die from hematopoietic failure at about embryonic day 18.5 (e18.5) and show a complete absence of B cells, mature T cells, and macrophages. This phenotype suggests that PU.1 may function at the level of the hematopoietic stem cell (HSC) or a multilineage progenitor. To investigate the role of PU.1 in the regulation of HSCs, PU.1(-/-) embryos were analyzed at various stages of embryonic development. The absolute number and frequency of HSCs were determined by flow cytometric analysis of c-Kit(+)Thy-1.1(lo)Lin(-)Sca-1(+) (KTLS) cells. We found that KTLS cells were absent or severely reduced in PU.1(-/-) fetal liver from e12.5 to e15.5. Progenitor cells with a c-Kit(+)Lin(-)AA4.1(+) and c-Kit(+)Lin(-)CD34(+) phenotype were also severely reduced. In addition, PU.1(-/-) fetal liver at e14.5 lacked common myeloid progenitors (CMPs) and granulocyte-macrophage progenitors (GMPs) but retained megakaryocyte-erythroid progenitors (MEPs). Consistent with the loss of HSC activity, a 10-fold reduction in erythroid progenitors (mature erythroid burst-forming units [BFU-Es]) was observed between e14.5 and e16.5. These data suggest that PU.1 plays an important role in the maintenance or expansion of HSC number in murine fetal liver. (C) 2004 by The American Society of Hematology.