The evolutionarily conserved Notch signaling pathway has been shown to play a critical role in the development and patterning of a wide variety of organisms including worms, flies, and mice 1
. Instructive Notch signals are mediated by cell–cell interactions between Notch receptor and Notch ligands that control cell fate decisions. Although the definite relationship between Notch signaling and biological outcome is dependent on the specific cell type, growth factor environment, and species, it is clear that Notch activation is capable of inhibiting differentiation and progression of precursor populations to more developmentally restricted cells 23
. In addition to pluripotency, many of these precursors are capable of immense proliferative capacity, suggesting that Notch activation may be intricately related to the control of differentiation and proliferation of stem cells 4
. Although fundamental influences of Notch signaling in developmental processes may be pertinent to medical applications 5
, the specific role of ligand-induced Notch activation in primary human stem cells is currently unknown.
Notch receptors interact with a family of transmembrane proteins that serve as ligands containing a highly conserved Delta-Serrate-Lag-2 (DSL) domain in the extracellular region. Activation of Notch signaling is mediated through interactions between the DSL domain and specific epidermal growth factor (EGF) repeats located in the extracellular portion of the Notch receptor 6
. The Drosophila Notch ligand, Serrate, is a family member of Notch ligands cloned from invertebrates 78
. Jagged-1, the mammalian homologue of Serrate, has been shown to be a potent activator of the Notch signaling pathway in a variety of cell types, mediating signals via cellular interactions with adjacent Notch-expressing cells 9
. Here, using hematopoietic development as a well characterized model system for primary human cells, we investigated the potential role of Jagged-1–induced Notch signaling in humans 10
Ex vivo culture conditions originally optimized using surrogate in vivo models to detect human hematopoietic stem cell function in immunodeficient recipients have recently led to enhanced hematopoietic reconstitution and retroviral gene transfer of repopulating stem cells in clinical trials 1112
. However, although these ex vivo culture conditions provide a therapeutic benefit, the magnitude of the expansion and gene transfer efficiency is insufficient for the treatment of a wider variety of patient pathologies. Therefore, the success of ex vivo expansion conditions still awaits the identification of factors capable of inducing proliferation of rare stem cells. In this study, production of a soluble form of human (h)Jagged-1 provided a means to functionally evaluate the role of hJagged-1 in ex vivo cultures of highly purified human hematopoietic cells. Although addition of hJagged-1 had modest effects in enhancing cytokine-induced proliferation of primitive progenitors detected in vitro, intravenous transplantation of cultured cells into immunodeficient nonobese diabetic (NOD)/SCID mice revealed that hJagged-1 is capable of maintaining the survival and expanding human stem cells capable of pluripotent reconstituting capacity. Unlike hematopoietic cytokines, this unique function of hJagged-1 provides an opportunity for the optimization for clinical protocols aimed at ex vivo expansion and gene transfer of human stem cells.