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Human erythropoiesis is a complex multistep process that involves the differentiation of early erythroid progenitors to mature erythrocytes. Here we show that it is feasible to differentiate and mature human embryonic stem cells (hESCs) into functional oxygen-carrying erythrocytes on a large scale (1010-1011 cells/6-well plate hESCs). We also show for the first time that the oxygen equilibrium curves of the hESC-derived cells are comparable with normal red blood cells and respond to changes in pH and 2,3-diphosphoglyerate. Although these cells mainly expressed fetal and embryonic globins, they also possessed the capacity to express the adult β-globin chain on further maturation in vitro. Polymerase chain reaction and globin chain specific immunofluorescent analysis showed that the cells increased expression of β-globin (from 0% to > 16%) after in vitro culture. Importantly, the cells underwent multiple maturation events, including a progressive decrease in size, increase in glycophorin A expression, and chromatin and nuclear condensation. This process resulted in extrusion of the pycnotic nuclei in up to more than 60% of the cells generating red blood cells with a diameter of approximately 6 to 8 μm. The results show that it is feasible to differentiate and mature hESCs into functional oxygen-carrying erythrocytes on a large scale.

A cell culture system consisting of mouse S17 stromal cells supplemented with cytokines was developed for hematopoietic differentiation of rhesus monkey embryonic stem (ES) cells. The differentiated colonies that formed contained clusters of hematopoietic-like cells, as well as structures similar in appearance to embryonic blood islands. When this culture system was supplemented with bone morphogenetic protein 4 (BMP-4), the numbers of primary hematopoietic clusters increased by an average of 15 fold. The primary hematopoietic clusters containing clonogenic precursors (expandable hematopoietic clusters) increased by 18 fold. Immunofluorescence analysis showed that a substantial percentage of the hematopoietic-like cells were CD34+, with morphologic features of undifferentiated blast cells. Enrichment of the CD34+ cells was associated with enhanced stromal-dependent, cytokine-driven formation of cobblestone colonies on secondary plating. The hematopoietic identity of the precursors was further indicated by their expression of genes associated with hematopoietic differentiation, as well as morphologic assessments that showed erythroid and myeloid lineages among the progeny cells. In addition, reverse transcriptase–polymerase chain reaction analysis of BMP-4–treated rhesus monkey ES cells demonstrated an up-regulation of early-expressed genes responsible for embryonic hematopoiesis and angiogenesis during the first 7 days of culture. These observations suggest that embryonic mesoderm regulatory protein may mimic physiologic signals that are required for the onset of embryonic hematopoiesis and stem cell formation in rhesus monkey ES cells.

Gene expression patterns of CD34+CD38- cells derived from human embryonic stem cells (ESCs) were compared with those of cells isolated from adult human bone marrow (BM) using microarrays; 1692 and 1494 genes were expressed at levels at least 3-fold above background in cells from BM and ESCs, respectively. Of these, 494 showed similar levels of expression in cells from both sources, 791 genes were overexpressed in cells from BM (BM versus ESCs, at least 2-fold), and 803 genes were preferentially expressed in cells from ESCs (ESCs versus BM, at least 2-fold). The message of the flt-3 gene was markedly decreased in cells from ESCs, whereas there was substantial flt-3 expression in cells from BM. High levels of embryonic ϵ-globin expression were observed—but no adult β-globin message—in CD34+CD38- cells from ESCs, whereas high levels of β-globin expression—but no embryonic ϵ-globin message—could be detected in cells from BM. Furthermore, high levels of major histocompatibility complex (MHC) gene expression were demonstrated in cells from BM but very low levels of MHC message in corresponding cells from ESCs. These observations demonstrate that CD34+CD38- cells derived from ESCs correspond consistently to an early developmental stage at which the yolk sac and fetal liver are the primary sites of hematopoiesis.

Hematopoeitic stem and progenitor cells (HSPC) dynamically switch between a quiescent, non-motile mode in the bone marrow (BM), to an active state, in which they proliferate, differentiate and egress to the circulation. Injection of the coagulation protease thrombin induced rapid HSPC mobilization to the blood via activation of its major receptor, protease activated receptor 1 (PAR1) on BM hematopoietic and stromal cells. We hypothesized that coagulation factors control stem cells fate in the BM. We examined if thrombin is generated in the murine BM and found by immunohistochemistry prothrombin associated with bone lining osteoblasts in the endosteum region. These cells also highly express osteopontin which induces stem cell quiescence and retention. Cleavage of osteopontin by thrombin or by osteoclast derived cathepsin K induces stem cell mobilization. In addition, a unique structure of multinucleated CD45+ cell clusters in the trabecular-rich area of the murine femoral metaphysis express the cell surface receptor Tissue Factor (TF), a potent initiator of the coagulation cascade leading to thrombin generation. These clusters were found adjacent to multinucleated TRAP (tatrate resistant acid phosphate) positive active osteoclasts. In vitro, we found that immature osteoclasts expressed TF in cell fusion areas, suggesting that osteoclast maturation also activates the coagulation thrombin/PAR1 axis, thus mediating HSPC recruitment to the circulation. Supporting this notion, bleeding which prompts a hemostatic response and thrombin production, is a strong inducer of osteoclasts activation and HSPC mobilization. In addition, injection of bacterial lipopolysaccharides (LPS) is known to activate osteoclasts and induce HSPC mobilization (Kollet et al Nat Med 06). We found that LPS injection upregulated TF expression by CD45+ myeloid cells in the murine BM. LPS treatment provoked massive HSPC mobilization, which was attenuated by PAR1 inhibition. To further address the role of thrombin in stem cell maintenance, we targeted prothrombin in vivo by applying Antisense Oligonucleotides (ASO) knockdown technology, previously shown to induce a dose- and time-dependent up to 90% reduction of prothrombin mRNA levels in the murine liver (Monia et al Blood 2010). Prothrombin depletion altered the BM niche microenvironment by expanding the mesenchymal stem and progenitor (MSPC) population and the long-term repopulating CD34-/ROSlow/LSK HSPC population in the BM. In untreated mice, TF was also expressed by a small MSPC population, suggesting that the bone stomal compartment may also contribute to the regulation of HSPC mobilization upon demand. To further asses the role of thrombin generation in HSPC development, we examined the involvement of the endothelial cell receptor Thrombomodulin (TM) that is pivotal for the anticoagulant pathway which mediates activation of protein C. TM protein is expressed by BM small blood vessels resembling sinusoids and by neighboring MSPC. By immunohistochemistry, we also detected activated protein C on the same blood vessels. A mouse model with a mutation in the TM gene (TMPro/Pro) is characterized by reduced capacity for activated protein C generation which in turn increases thrombin levels in these mice. We found increased circulating hematopoietic stem cells in TMPro/Pro mice, suggesting that chronically increased basal levels of thrombin generation can promote HSC egress. Conversely, short term (5 day) intermittent treatment of mice with low dose thrombin that mainly causes activated protein C formation in vivo, display higher levels of CD34-/ROSlow/LSK and EPCR+LSK stem cells in the BM, indicating additional roles for the anticoagulant pathway in BM stem cell pool maintenance. In summary, our results provide evidence that the activator of the coagulation cascade, TF, and coagulation factors Thrombin and activated protein C are present in the BM and regulate and integrate functions of hematopoietic stem and progenitor cells and BM stromal progenitor cells. Disclosures: Crosby: Isis pharmaceuticals: the ASO for prothrombin was obtained from Isis pharmaceuticals Other. Monia:Isis pharmaceuticals: the ASO for prothrombin was obtained from Isis pharmaceuticals Other.

Hematopoietic stem and progenitor cells (HSPC) are mostly retained in a quiescent, non-motile mode in the bone marrow (BM), shifting to a cycling, differentiating and migratory state on demand. How HSC replenish the blood with new mature leukocytes on a daily basis while maintaining a constant pool of primitive cells in the BM throughout life is not clear. Recently, we reported that the bioactive lipid Sphingosine 1-Phosphate (S1P) regulates HSPC mobilization via ROS signaling and CXCL12 secretion (Golan et al, Blood 2012). We hypothesize that S1P influences the daily circadian egress of HSPC and their proliferation. We report that S1P levels in the blood are increased following initiation of light at the peak of HSPC egress and are reduced towards the termination of light when circulating HSPC reach a nadir. Interestingly, mice with constitutively low S1P plasma levels due to lack of one of the enzymes that generates S1P (Sphingosine kinase 1), do not exhibit fluctuations of HSPC levels in the blood between day and night. We report that HSPC numbers in the BM are also regulated in a circadian manner. Unexpectedly, we found two different daily peaks: one in the morning, following initiation of light, which is accompanied by increased HSPC egress and the other at night after darkness, which is associated with reduced HSPC egress. In both peaks HSPC begin to cycle and differentiate via up-regulation of reactive oxygen species (ROS) however, the night peak had lower ROS levels. Concomitant with the peak of primitive stem and progenitor cells, we also observed (to a larger extent in the night peak), expansion of a rare activated macrophage/monocyte αSMA/Mac-1 population. This population maintains HSPC in a primitive state via COX2/PGE2 signaling that reduces ROS levels and increases BM stromal CXCL12 surface expression (Ludin et al, Nat. Imm. 2012). We identified two different BM peaks in HSPC levels that are regulated by the nervous system via circadian changes in ROS levels. Augmented ROS levels induce HSPC proliferation, differentiation and motility, which take place in the morning peak; however, they need to be restored to normal levels in order to prevent BM HSPC exhaustion. In the night peak, HSPC proliferate with less differentiation and egress, and activated macrophage/monocyte αSMA/Mac-1 cells are increased to restore ROS levels and activate CXCL12/CXCR4 interactions to maintain a HSPC primitive phenotype. Additionally, S1P also regulates HSPC proliferation, thus mice with low S1P levels share reduced hematopoietic progenitor cells in the BM. Interestingly S1P is required more for the HSPC night peak since in mice with low S1P levels, HSPC peak normally during day time but not at darkness. We suggest that the first peak is initiated via elevation of ROS by norepinephrine that is augmented in the BM following light-driven cues from the brain (Mendez-Ferrer at al, Nature 2008). The morning elevated ROS signal induces a decrease in BM CXCL12 levels and up-regulated MMP-9 activity, leading to HSC proliferation, as well as their detachment from their BM microenvironment, resulting in enhanced egress. Importantly, ROS inhibition by N-acetyl cysteine (NAC) reduced the morning HSPC peak. Since norepinephrine is an inhibitor of TNFα, upon light termination norepinephrine levels decrease and TNFα levels are up-regulated. TNFα induces activation of S1P in the BM, leading to the darkness peak in HSPC levels. S1P was previously shown also to induce PGE2 signaling, essential for HSPC maintenance by the rare activated αSMA/Mac-1 population. Indeed, in mice with low S1P levels, we could not detect a peak in COX2 levels in these BM cells during darkness. We conclude that S1P not only induces HSPC proliferation via augmentation of ROS levels, but also activates PGE2/COX2 signaling in αSMA/Mac-1 population to restore ROS levels and prevent HSPC differentiation and egress during the night peak. We hypothesize that the morning HSPC peak, involves proliferation, differentiation and egress, to allow HSPC to replenish the blood circulation with new cells. In contrast, the second HSPC night peak induces proliferation with reduced differentiation and egress, allowing the renewal of the BM HSPC pool. In summary, we identified two daily circadian peaks in HSPC BM levels that are regulated via light/dark cues and concomitantly allow HSPC replenishment of the blood and immune system, as well as maintenance of the HSPC constant pool in the BM. Disclosures: No relevant conflicts of interest to declare.

Hematopoietic stem and progenitor cells (HSPC) are mostly retained in the bone marrow (BM) in a ROSlow quiescent mode via adhesion interactions with stromal cells, while mature blood cells are generated on demand via ROShigh stem cell proliferation and differentiation. The metabolic state of BM stromal cells and their cross-talk with quiescent and cycling HSPC is poorly understood. The bioactive lipid Sphingosine 1-Phosphate (S1P) is an important inducer of ROS in HSPC, increasing their motility (Golan et al, Blood 2012). We report that by increasing S1P levels in the murine BM, we also increased HSPC levels via augmented ROS production and cell cycling. Unexpectedly, we discovered an opposite effect of S1P on BM stromal cells, which contained less mesenchymal stem and progenitor cells (MSPCs) and also lower ROS levels. To further test the opposite metabolic relationship between BM HSPC and stromal cells, we utilized 3 repeated (once every 24h) G-CSF stimulations, which induce HSPC proliferation without mobilization. As expected, we found increased HSPC levels in the BM accompanied by augmented ROS levels. Importantly, we found again the opposite effect between HSPC and BM stromal cells, i.e. reduced MSPC levels that were less cycling and contained lower levels of ROS. These results suggest an inverse metabolic state between BM HSPC and their stromal microenvironment. While HSPC utilize oxidative phosphorylation and ROS generation during proliferation and differentiation, their stromal microenvironment share a low metabolic state, most probably due to anaerobic glycolysis. Mimicking alarm situations by clinical G-CSF treatment blocks bone remodeling to allow HSPC development and mature blood replenishment. As expected, mice with reduced S1P levels due to lack of one of its biosynthetic enzymes (Sphingosine kinase 1), exhibit lower levels of BM HSPCs accompanied by reduced ROS levels. More importantly, these mice share an augmented population of undifferentiated, primitive ROSlow, quiescent non-motile HSCs identified as EPCR+/LSK cells, which results in higher levels of long-term HSC repopulation in transplanted wild type mice (with normal S1P levels). Concomitantly, S1P low mice have BM MSPCs with increased ROS levels and higher colony-forming unit fibroblasts (CFU-F) potential in vitro. We conclude that when HSPC are maintained quiescent due to a low energy metabolic state of anaerobic glycolysis, the BM stromal microenvironment utilizes a high energetic metabolism by oxidative phosphorylation and ROS generation. Our data suggests that the dynamic, inverse metabolic state between HSPC and the stromal microenvironment is induced by energy transfer between the two populations. HSPC can eliminate excess ROS post 5-FU treatment, by transferring ROS to BM stromal cells via connexin43 (Cx43) gap-junctions (Taniguchi et al, PNAS 2012). In agreement, we found that mice treated with a broad inhibitor of connexin gap-junctions (CBX), induced BM HSPC proliferation via ROS elevation, concomitant with down regulation of ROS in MSPCs. Interestingly, in mice with low S1P levels, CBX inhibition of connexins did not elevate HSPC ROS levels or their numbers, implying that cell contact blockage is essential, but not sufficient to induce proliferation. To induce HSPC proliferation and differentiation, an activator of ROS, such as S1P, is also needed to initiate cell cycle entry and blockage of Cx43, further inducing CXCL12 secretion to the circulation and HSPC migration. Our results imply that once ROS reaches a certain level in HSPC, it is scavenged by the stromal microenvironment to prevent exhaustion of the hematopoietic stem cell pool and in parallel to initiate new bone development and remodeling. Altogether, we have discovered a dynamic, opposite metabolic state between BM HSPCs and their supporting stromal microenvironment during quiescence, proliferation and differentiation of the two populations, which are both ROS regulated on a one population at a time basis. Thus, either blood cell production or bone development takes place at the expense of the other. This metabolic seesaw is regulated via ROS transfer from HSCs to their supportive microenvironment via Cx43 gap-junctions, which also regulates surface CXCL12 expression that is essential for stem cell quiescence. Disclosures: No relevant conflicts of interest to declare.

Abstract 2540 Poster Board II-517 Platelets collected from donors have very limited shelf life and are increasingly needed for transfusions. In contrast to donor dependent cord blood or bone marrow CD34+ stem cells, hESCs are a promising alternative source for continuous in vitro production of platelets under controlled conditions. Current procedures for in vitro generation of megakaryocytes/platelets from hESCs are not efficient and require undefined animal stromal cells. We have developed a novel system to generate megakaryocytes (MKs) from human ES cells under serum and stromal-free conditions. In the current system, hESCs are directed towards megakaryocytes through distinct steps including embryoid body formation and hemangioblast development (Lu et al, Nature Methods, 4:501–509, 2007). A transient bi-potential cell population expressing both CD41a and CD235a markers has been identified at the end of hemangioblast culture. These cells are capable of generating both MKs and erythroid cells as demonstrated by FACS sorting and CFU assays. TPO, SCF and IL-11 are used to further direct MK differentiation of hemangioblasts derived from human ES cells in suspension culture. Currently, up to 2.5×107 MKs (CD41a+) can be generated from 1×106 hESCs, which is approximately 10 times more efficient than recently reported methods (Takayama et al Blood, 111(11):5298–5306, 2008). Without further purification, >90% of live cells from the suspension cultures are CD41a+ and the majority of these cells are also CD42b+ (>70%). These in vitro derived MK cells have morphological characteristics of mature, polyploid MKs as shown by Giemsa staining and immunofluorescent staining of vWF in cytoplasmic granules. Importantly, proplatelet forming cells are constantly observed at the late stage of MK culture indicating that MKs generated in this system are able to undergo terminal differentiation under feeder-free conditions. Platelet-like particles are also detected in culture media by FACS. When plated on OP9 cells, these MKs generate functional platelets that are responsive to thrombin stimulation. In summary, we have established a novel system for the generation of platelet-producing MKs from human ES cells that is suitable for scale up and future preclinical and clinical studies. Disclosures: Li: Stem Cell & Regenerative Medicine International: Employment. Lu:Stem Cell & Regenerative Medicine International: Employment. Feng:Stem Cell & Regenerative Medicine International: Employment. Lanza:Stem Cell & Regenerative Medicine International/Advanced Cell Technology, Inc: Employment.

Abstract 1502 Poster Board I-525 Studies with human and mouse embryonic stem cells (ESCs) have shown that a common precursor to both vascular (endothelial and smooth muscle cells) and hematopoietic cell lineages called the hemangioblast can be produced from ESC-derived embryoid bodies in culture. We have developed a simple strategy to efficiently and reproducibly generate hemangioblasts from multiple hESC lines under serum- and stromal-free conditions, which will be important for their productive use in regenerative medicine. Previous work has shown that hESC-derived hemangioblasts can effectively differentiate into erythroid and myeloid lineages, but their ability to produce lymphoid lineage cells, including those with immunotherapeutic potential, is relatively unknown. Natural killer (NK) cells, which are part of the innate immune system, provide rapid, non-specific responses against viral infection and are involved in tumor cell detection and elimination. Interplay between various activating and inhibitory signals control the three main functions of NK cells, which are cytokine release, natural cytotoxicity, and antibody-dependent cellular cytotoxicity. Using hemangioblasts generated from both H7 and HuES-3 hESC lines, we have been able to produce mature CD56low/−CD16+ NK cells and found that their production does not require the use of stromal feeder layers. The differentiation procedure involves an initial 4 day culture to generate embryoid bodies, followed by a 12-14 day culture in methylcellulose supplemented with a set of cytokines and growth factors for the production and expansion of a hemangioblastic population. An additional 14-17 days in liquid culture plus human serum and a cocktail of cytokines allows for the differentiation of NK cells as assessed by flow cytometry. A non-radioactive cytotoxicity assay similar to the 51Cr release assay shows that these hemangioblast-derived NK cells harbor natural cytotoxicity function as they are able to effectively induce apoptosis in target K562 erythroblastic leukemia cells after a standard 4 hr co-culture. Using hemangioblasts as an intermediary cell source may enhance the capability and/or efficiency of hESCs to differentiate in vitro and importantly, allow for the development of feeder-free systems for the production of cells with immunotherapeutic potential. Disclosures: Kimbrel: Stem Cell & Regenerative Medicine International: Employment. Lu: Stem Cell & Regenerative Medicine International: Employment. Lanza: Stem Cell & Regenerative Medicine International/Advanced Cell Technology: Employment.