Many organs with a high cell turnover (for example, skin, intestine and blood) are composed of short-lived cells that require continuous replenishment by somatic stem cells1,2. Ageing results in the inability of these tissuesto maintain homeostasis and it is believed that somatic stem-cell ageing is one underlying cause of tissue attrition with age or age-related diseases. Ageing of haematopoietic stem cells (HSCs) is associated with impaired haematopoiesis in the elderly3–6. Despite a large amount of data describing the decline of HSC function on ageing, the molecular mechanisms of this process remain largely unknown, which precludes rational approaches to attenuate stem-cell ageing. Here we report an unexpected shift from canonical to non-canonical Wnt signalling in mice due to elevated expression of Wnt5a in aged HSCs, which causes stem-cell ageing. Wnt5a treatment of young HSCs induces ageing-associated stem-cell apolarity, reduction of regenerative capacity and an ageing-like myeloid–lymphoid differentiation skewing via activation of the small Rho GTPase Cdc42. Conversely, Wnt5a haploinsufficiency attenuates HSC ageing, whereas stem-cell-intrinsic reduction of Wnt5a expression results in functionally rejuvenated aged HSCs. Our data demonstrate a critical role for stem-cell-intrinsicnon-canonical Wnt5a signalling in HSC ageing.

Recruitment of human CD34+ progenitor cells toward vascular lesions and differentiation into vascular cells has been regarded as a critical initial step in atherosclerosis. Previously we found that adherent platelets represent potential mediators of progenitor cell homing besides their role in thrombus formation. On the other hand, foam cell formation represents a key process in atherosclerotic plaque formation. To investigate whether platelets are involved in progenitor cell recruitment and differentiation into endothelial cells and foam cells, we examined the interactions of platelets and CD34+ progenitor cells. Cocultivation experiments showed that human platelets recruit CD34+ progenitor cells via the specific adhesion receptors P-selectin/PSGL-1 and beta1- and beta2-integrins. Furthermore, platelets were found to induce differentiation of CD34+ progenitor cells into mature foam cells and endothelial cells. Platelet-induced foam cell generation could be prevented partially by HMG coenzyme A reductase inhibitors via reduction of matrix metalloproteinase-9 (MMP-9) secretion. Finally, agonists of peroxisome proliferator-activated receptor-alpha and -gamma attenuated platelet-induced foam cell generation and production of MMP-9. The present study describes a potentially important mechanism of platelet-induced foam cell formation and generation of endothelium in atherogenesis and atheroprogression. The understanding and modulation of these mechanisms may offer new treatment strategies for patients at high risk for atherosclerotic diseases.

Hematopoietic stem cells derived from human embryonic stem cells (hESCs) could provide a therapeutic alternative to bone marrow transplants, but the efficiency of currently available derivation protocols is low. In this study, we investigated whether coculture with monolayers of cells derived from mouse AGM and fetal liver, or with stromal cell lines derived from these tissues, can enhance hESC hematopoietic differentiation. We found that under such conditions hESC-derived differentiating cells formed early hematopoietic progenitors, with a peak at day 18-21 of differentiation that corresponded to the highest CD34 expression. These hESC-derived hematopoietic cells were capable of primary and secondary hematopoietic engraftment into immunocompromised mice at substantially higher levels than described previously. Transcriptional and functional analysis identified TGF-beta1 and TGF-beta3 as positive enhancers of hESC hematopoietic differentiation that can further stimulate this process when added to the culture. Overall, our findings represent significant progress toward the goal of deriving functional hematopoietic stem cells from hESCs.

Hematopoietic progenitor cells can be labeled with MR contrast agents and can be depicted with a standard 1.5-T MR imager.

The aorta-gonads-mesonephros (AGM) region autonomously generates the first adult repopulating hematopoietic stem cells (HSCs) in the mouse embryo. HSC activity is initially localized to the dorsal aorta and mesenchyme (AM) and vitelline and umbilical arteries. Thereafter, HSC activity is found in the urogenital ridges (UGs), yolk sac, and liver. As increasing numbers of HSCs are generated, it is thought that these sites provide supportive microenvironments in which HSCs are harbored until the bone marrow microenvironment is established. However, little is known about the supportive cells within these midgestational sites, and particularly which microenvironment is most supportive for HSC growth and maintenance. Thus, to better understand the cells and molecules involved in hematopoietic support in the midgestation embryo, more than 100 stromal cell lines and clones were established from these sites. Numerous stromal clones were found to maintain hematopoietic progenitors and HSCs to a similar degree as, or better than, previously described murine stromal lines. Both the AM and UG subregions of the AGM produced many supportive clones, with the most highly HSCsupportive clone being derived from the UGs. Interestingly, the liver at this stage yielded only few supportive stromal clones. These results strongly suggest that during midgestation, not only the AM but also the UG subregion provides a potent microenvironment for growth and maintenance of the first HSCs. IntroductionThroughout adult life, the hematopoietic hierarchy is derived from hematopoietic stem cells (HSCs) maintained in the supportive microenvironment of the bone marrow (BM). Functional blood cells arise through a series of differentiation steps first occurring within the HSCs and proceeding through a hierarchy of progenitor cell types with increasing lineage commitment. 1-3 Both maintenance and differentiation of HSCs are controlled by complex interactions with the stromal microenvironment, 4 which consists of several morphologically distinct cell types including myofibroblasts and macrophages.Efforts to examine the interactions between HSCs and the microenvironment have led to the establishment of in vitro culture systems with adherent cells from BM. 5,6 Further studies have demonstrated that BM and fetal liver stromal cells maintain hematopoietic progenitors, long-term culture-initiating cells, cobblestone area-forming cells, and HSCs (cells capable of permanently repopulating the entire hematopoietic system of irradiated adult recipients). 7-9 Numerous stromal cell lines of adult BM and fetal liver hematopoietic microenvironments have been cloned and characterized for the growth, maintenance, and differentiation of HSCs 8-10 (see also references in Remy-Martin et al 11 ). Generally, these studies show an initial decrease in HSC activity 12 followed by an expansion phase of immature hematopoietic progenitors. 13 Most notably, the AFT024 stromal clone, derived from mouse fetal liver at day 14.5 of gestation, shows the best continued maintenance of...

SummaryTumor cell survival critically depends on heterotypic communication with benign cells in the microenvironment. Here, we describe a survival signaling pathway activated in stromal cells by contact to B cells from patients with chronic lymphocytic leukemia (CLL). The expression of protein kinase C (PKC)-βII and the subsequent activation of NF-κB in bone marrow stromal cells are prerequisites to support the survival of malignant B cells. PKC-β knockout mice are insusceptible to CLL transplantations, underscoring the in vivo significance of the PKC-βII-NF-κB signaling pathway in the tumor microenvironment. Upregulated stromal PKC-βII in biopsies from patients with CLL, acute lymphoblastic leukemia, and mantle cell lymphoma suggests that this pathway may commonly be activated in a variety of hematological malignancies.

The in vivo distribution of intravenously administered iron oxide-labeled hematopoietic progenitor cells can be monitored with 1.5-T MR imaging equipment.

The stages of human natural killer (NK) cell differentiation are not well established. Culturing CD34 ؉ progenitors with interleukin 7 (IL-7), IL-15, stem cell factor (SCF), FLT-3L, and murine fetal liver cell line (EL08.1D2), we identified 2 nonoverlapping subsets of differentiating CD56 ؉ cells based on CD117 and CD94 (CD117 high CD94 ؊ and CD117 low/؊ CD94 ؉ cells). Both populations expressed CD161 and NKp44, but differed with respect to NKp30, NKp46, NKG2A, NKG2C, NKG2D, CD8, CD16, and KIR. Only the CD117 low/؊ CD94 ؉ population displayed cytotoxicity and interferon-␥ production. Both populations arose from a single CD34 ؉ CD38 ؊ Lin ؊ cell and their percentages changed over time in a reciprocal fashion, with CD117 high CD94 ؊ cells predominating early and decreasing due to an increase of the CD117 low/؊ CD94 ؉ population. These 2 subsets represent distinct stages of NKcell differentiation, since purified CD117 high IntroductionNatural killer (NK) cells are CD56 ϩ CD3 Ϫ innate immune effector cells that recognize target cells that have undergone cellular stress, such as malignant transformation or viral infection. 1 Individual NK cells display a diverse repertoire of activating and inhibitory receptors, including the killer immunoglobulin-like receptors (KIRs), natural cytotoxicity receptors (NKp30, NKp44, and NKp46), and c-lectin receptors (NKG2A/ CD94, NKG2C/CD94, NKG2D, and CD161). The summation of activating signals, when not opposed by inhibitory signaling, leads to the release of granzymes and perforin and target cytolysis. 2 NK-cell activation also results in the elaboration of proinflammatory cytokines, including interferon ␥ (IFN-␥), 3,4 stimulating other compartments of the immune system.The stages of human NK-cell development are not well characterized. Since human NK cells can be differentiated from CD34 ϩ hematopoietic progenitor cells (HPCs) in vitro, such stages of differentiation may be elucidated. Early studies showed that interleukin 2 (IL-2) could induce the differentiation of NK cells from CD34 ϩ HPCs. 5,6 More recently, IL-15 has been found to be critical for NK-cell development since IL-15 Ϫ/Ϫ and IL-15R␣ Ϫ/Ϫ mice show a near-complete absence of NK cells. 7,8 Accordingly, culture of human HPCs with IL-15 gives rise to NK cells, 9 but other cytokines, such as IL-3, IL-7, stem cell factor (SCF), and FLT-3L increase the efficiency of in vitro NK-cell differentiation. [10][11][12] Both SCF and FLT-3L induce the expression of IL-15R␣ mRNA in developing progenitors, inducing IL-15 responsiveness. 11 CD34 ϩ cells cultured with cytokines and stromal cells give higher yields of NK cells that more accurately reflect peripheral blood NK cells with respect to KIR expression. [12][13][14][15] To prevent auto-aggression, NK cells express inhibitory receptors that recognize self major histocompatibility complex (MHC) class I. The current paradigm of NK-cell tolerance suggests that every NK cell must express at least one self MHC-specific inhibitory receptor, since this has been demonstrated at both t...