Pluripotent, cytokine-dependent, hematopoietic stem cells are immortalized by 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 lymphoid or myeloid lineages. Here we evaluated whether hematopoietic stem cell self-renewal is affected by the Notch pathway. Notch signaling controls cell fate choices in both invertebrates and vertebrates(1-7) by inhibiting certain differentiation pathways, thereby permitting cells to either differentiate along an alternative pathway or to self-renew(1). Notch receptors are present in hematopoietic precursors and Notch signaling enhances the in vitro generation of human and mouse hematopoietic precursors(8-15), determines 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 immortalized, cytokine-dependent cell lines that generated progeny with either lymphoid or myeloid characteristics both in vitro and in vivo. These data support a role for Notch signaling in regulating hematopoietic stem cell self-renewal. Furthermore, the establishment of clonal, pluripotent cell lines provides the opportunity to assess mechanisms regulating stem cell commitment and demonstrates a general method for immortalizing stem cell populations for further analysis.