During mammalian ontogeny, haematopoietic stem cells (HSCs) translocate from the fetal liver to the bone marrow, where haematopoiesis occurs throughout adulthood. Unique features of bone that contribute to a microenvironmental niche for stem cells might include the known high concentration of calcium ions at the HSC-enriched endosteal surface. Cells respond to extracellular ionic calcium concentrations through the seven-transmembrane-spanning calcium-sensing receptor (CaR), which we identified as being expressed on HSCs. Here we show that, through the CaR, the simple ionic mineral content of the niche may dictate the preferential localization of adult mammalian haematopoiesis in bone. Antenatal mice deficient in CaR had primitive haematopoietic cells in the circulation and spleen, whereas few were found in bone marrow. CaR-/- HSCs from fetal liver were normal in number, in proliferative and differentiative function, and in migration and homing to the bone marrow. Yet they were highly defective in localizing anatomically to the endosteal niche, behaviour that correlated with defective adhesion to the extracellular matrix protein, collagen I. CaR has a function in retaining HSCs in close physical proximity to the endosteal surface and the regulatory niche components associated with it.

The specialized microenvironment or niche where stem cells reside provides regulatory input governing stem cell function. We tested the hypothesis that targeting the niche might improve stem cell-based therapies using three mouse models that are relevant to clinical uses of hematopoietic stem (HS) cells. We and others previously identified the osteoblast as a component of the adult HS cell niche and established that activation of the parathyroid hormone (PTH) receptor on osteoblasts increases stem cell number. Here we show that pharmacologic use of PTH increases the number of HS cells mobilized into the peripheral blood for stem cell harvests, protects stem cells from repeated exposure to cytotoxic chemotherapy and expands stem cells in transplant recipients. These data provide evidence that the niche may be an attractive target for drug-based stem cell therapeutics.

Biocompatible inorganic matrices have been used to enhance bone repair by integrating with endogenous bone architecture. Hypothesizing that a three-dimensional framework might support reconstruction of other tissues as well, we assessed the capacity of a tantalum-coated carbon matrix to support reconstitution of functioning thymic tissue. We engineered a thymic organoid by seeding matrices with murine thymic stroma. Co-culture of human bone marrow-derived hematopoietic progenitor cells within this xenogeneic environment generated mature functional T cells within 14 days. The proportionate T-cell yield from this system was highly reproducible, generating over 70% CD3+ T cells from either AC133+ or CD34+ progenitor cells. Cultured T cells expressed a high level of T-cell receptor excision circles (TREC), demonstrating de novo T lymphopoiesis, and function of fully mature T cells. This system not only facilitates analysis of the T-lymphopoietic potential of progenitor cell populations; it also permits ex vivo genesis of T cells for possible applications in treatment of immunodeficiency.

Hematopoietic stem/progenitor cells (HSPC) transition in location during development1 and circulate in mammals throughout life2, moving into and out of the bloodstream to engage bone marrow (BM) niches in sequential steps of homing, engraftment and retention3–5. We show here that HSPC engraftment of BM in fetal development is dependent upon the guanine nucleotide binding protein stimulatory alpha subunit (Gsα). Adult Gsα−/− HSPCs differentiate and undergo chemotaxis, but also do not home to or engraft in the BM in adult mice and demonstrate marked inability to engage the marrow microvasculature. If deleted after engraftment, Gsα did not lead to lack of retention in the marrow, rather cytokine-induced mobilization into the blood was impaired. Testing whether activation of Gsα affects HSPC, pharmacologic activators enhanced homing and engraftment in vivo. Gsα governs specific aspects of HSPC localization under physiologic conditions in vivo and may be pharmacologically targeted to improve transplantation efficiency.

T-lymphocyte depletion of bone marrow grafts compromises engraftment, suggesting a facilitating mechanism provided by the T cells that has been shown to associate with CD8 ؉ but not CD4 ؉ T cells. Explanations for this phenomenon have focused on immune targeting of residual host cells or cytokine production. We provide evidence for an alternative mechanism based on cooperative effects on cell motility. We observed that engraftment of CD34 ؉ cells in a

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, the y-axes of Figures 1e, 2c and 2d are labeled incorrectly. They should be labeled "% CD45.1 in PB." The error has been corrected for all versions of the article.

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