The Notch ligand, Jagged-1, plays an essential role in tissue formation during embryonic development of primitive organisms. However, little is known regarding the role of Jagged-1 in the regulation of tissue-specific stem cells or its function in humans. Here, we show that uncommitted human hematopoietic cells and cells that comprise the putative blood stem cell microenvironment express Jagged-1 and the Notch receptors. Addition of a soluble form of human Jagged-1 to cultures of purified primitive human blood cells had modest effects in augmenting cytokine-induced proliferation of progenitors. However, intravenous transplantation of cultured cells into immunodeficient mice revealed that human (h)Jagged-1 induces the survival and expansion of human stem cells capable of pluripotent repopulating capacity. Our findings demonstrate that hJagged-1 represents a novel growth factor of human stem cells, thereby providing an opportunity for the clinical utility of Notch ligands in the expansion of primitive cells capable of hematopoietic reconstitution.

The identification of molecules that regulate human hematopoietic stem cells has focused mainly on cytokines, of which very few are known to act directly on stem cells. Recent studies in lower organisms and the mouse have suggested that bone morphogenetic proteins (BMPs) may play a critical role in the specification of hematopoietic tissue from the mesodermal germ layer. Here we report that BMPs regulate the proliferation and differentiation of highly purified primitive human hematopoietic cells from adult and neonatal sources. Populations of rare CD34+CD38−Lin− stem cells were isolated from human hematopoietic tissue and were found to express the BMP type I receptors activin-like kinase (ALK)-3 and ALK-6, and their downstream transducers SMAD-1, -4, and -5. Treatment of isolated stem cell populations with soluble BMP-2, -4, and -7 induced dose-dependent changes in proliferation, clonogenicity, cell surface phenotype, and multilineage repopulation capacity after transplantation in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Similar to transforming growth factor β, treatment of purified cells with BMP-2 or -7 at high concentrations inhibited proliferation yet maintained the primitive CD34+CD38− phenotype and repopulation capacity. In contrast, low concentrations of BMP-4 induced proliferation and differentiation of CD34+ CD38−Lin− cells, whereas at higher concentrations BMP-4 extended the length of time that repopulation capacity could be maintained in ex vivo culture, indicating a direct effect on stem cell survival. The discovery that BMPs are capable of regulating repopulating cells provides a new pathway for controlling human stem cell development and a powerful model system for studying the biological mechanism of BMP action using primary human cells.

Human embryonic stem cells (hESCs) are envisioned to be a major source for cell-based therapies. Efforts to overcome rejection of hESCs include nuclear transfer and collection of hESC banks representing the broadest diversity of major histocompatability complex (MHC) polymorphorisms. Surprisingly, immune responses to hESCs have yet to be experimentally evaluated. Here, injection of hESCs into immune-competent mice was unable to induce an immune response. Undifferentiated and differentiated hESCs failed to stimulate proliferation of alloreactive primary human T cells and inhibited third-party allogeneic dendritic cell-mediated T-cell proliferation via cellular mechanisms independent of secreted factors. Upon secondary rechallenge, T cells cocultured with hESCs were still responsive to allogeneic stimulators but failed to proliferate upon re-exposure to hESCs. Our study demonstrates that hESCs possess unique immune-privileged characteristics and provides an unprecedented opportunity to further investigate the mechanisms of immune response to transplantation of hESCs that may avoid immune-mediated rejection. Stem