Pluripotent, cytokine-dependent, hematopoietic stem cells are immortalizedby constitutive Notch1 signaling

Hematopoietic stem cells give rise to progeny that either self-renew in an undifferentiated state or lose self-renewal capabilities and commit to lymp hoid or myeloid lineages. Here we evaluated whether hematopoietic stem cell self-renewal is affected by the Notch pathway. Notch signaling controls ce ll fate choices in both invertebrates and vertebrates(1-7) by inhibiting ce rtain differentiation pathways, thereby permitting cells to either differen tiate along an alternative pathway or to self-renew(1). Notch receptors are present in hematopoietic precursors and Notch signaling enhances the in vi tro generation of human and mouse hematopoietic precursors(8-15), determine s T- or B-cell lineage specification from a common lymphoid precursor(16,17 ) and promotes expansion of CD8(+) cells(18-20). Here, we demonstrate that constitutive Notch1 signaling in hematopoietic cells established immortaliz ed, cytokine-dependent cell lines that generated progeny with either lympho id or myeloid characteristics both in vitro and in vivo. These data support a role for Notch signaling in regulating hematopoietic stem cell self-rene wal. Furthermore, the establishment of clonal, pluripotent cell lines provi des the opportunity to assess mechanisms regulating stem cell commitment an d demonstrates a general method for immortalizing stem cell populations for further analysis.