Conditional Loss of Ikkα in Sp7/osterix+ Cells Has No Effect on Bone, but Leads to Cell Autonomous, Age-related Loss of Peripheral Fat

NF-κB has been reported to both promote and inhibit bone formation. To further explore its role in osteolineage cells, we conditionally deleted IKKα, an upstream kinase required for non-canonical NF-κB activation, using Sp7/Osterix (Osx)-Cre. Surprisingly, we found no effect on either cancellous or cortical bone, even following mechanical loading. However, we noted that IKKα conditional knockout (cKO) mice began to lose body weight after 6 months of age with severe reductions in fat mass in geriatric animals. Low levels of recombination at the IKKα locus were detected in fat pads isolated from 15 month old cKO mice. To determine if these effects were mediated by unexpected deletion of IKKα in peripheral adipocytes, we looked for Osx-Cre-mediated recombination in fat using reporter mice, which showed increasing degrees of reporter activation in adipocytes with age up to 18 months. Since Osx-Cre-driven recombination in peripheral adipocytes increases over time, we conclude that loss of fat in aged cKO mice is most likely caused by progressive deficits of IKKα in adipocytes. To further explore the effect of IKKα loss on fat metabolism, we challenged mice with a high fat diet at 2 months of age, finding that cKO mice gained less weight and showed improved glucose metabolism, compared to littermate controls. Thus, Osx-Cre mediated recombination beyond bone, including within adipocytes, should be considered as a possible explanation for extraskeletal phenotypes, especially in aging and metabolic studies.

Canonical Signaling By TGF Family Members in Mesenchymal Stromal Cells Is Dispensable for Hematopoietic Niche Maintenance Under Basal and Stress Conditions

The bone marrow contains a complex population of stromal and hematopoietic cells that together generate a unique microenvironment, or niche, to support hematopoiesis. Mesenchymal stromal cells are an important component of the bone marrow hematopoietic niche and include CXCL12-abundant reticular (CAR) cells, adipocytes, osteolineage cells, and arteriolar pericytes, all of which have been implicated in hematopoietic stem/progenitor cell (HSPC) maintenance. There also is evidence that adaptive changes in bone marrow stromal cells contributes to recovery from myelosuppresive therapy and the development of certain hematopoietic malignancies. However, the signals that contribute to the development, maintenance, and stress response of bone marrow mesenchymal stromal cells are poorly understood. Here, we test the hypothesis that cytokines of the transforming growth factor superfamily, which include bone morphogenetic proteins (BMPs), growth differentiation factors (GDFs), and activins/inhibins, provide signals to mesenchymal stromal cells that contribute to basal and stress hematopoiesis responses. To test this hypothesis, we abrogated canonical TGF family signaling in mesenchymal stem/progenitor cells by deleting Smad4 using a doxycycline-repressible Osterix-Cre transgene (Osx-Cre), which targets all mesenchymal stromal cells in the bone marrow. We first performed lineage-tracing studies using Osx-Cre Smad4fl/fl Ai9 mice to show that activation of Osx-Cre at birth (by removal of doxycycline) results in the efficient targeting of bone marrow mesenchymal stromal cells. Moreover, we show that Smad4 mRNA expression is essentially undetectable in sorted mesenchymal stromal cells sorted from the bone marrow of these mice. Basal hematopoiesis and bone marrow stromal cells were analyzed in 6-8 week old Osx-Cre Smad4fl/fl mice. No alterations in the number or spatial organization of CAR cells, osteoblasts, or adipocytes was observed, and expression of key niche factors, including Scf, Cxcl12, and Spp1 was normal. Basal hematopoiesis, including the number of phenotypic HSCs in bone marrow and spleen, also was normal. Recent studies have shown that inhibition of activin signaling by treating with an activin receptor 2 alpha (ACVR2a) ligand trap stimulates erythropoiesis. Although ACVR2a signaling in erythroid progenitors contributes to this effect, two groups showed that inhibition of ACVR2a signaling in bone marrow stromal cells also stimulates erythropoiesis. Thus, we next characterized basal and stress erythropoiesis in Osx-Cre Smad4fl/fl mice. The frequency of phenotypic erythroid progenitors in bone marrow and spleen was similar to control mice. The stress erythropoiesis response was assessed after induction of acute hemolytic anemia by phenylhydrazine treatment. Both the magnitude of anemia and kinetics of erythroid recovery were similar to control mice. Myelosuppressive therapy induces marked alterations in the bone marrow microenvironment that includes an expansion of osteolineage cells and adipocytes, which have been linked to hematopoietic recovery. Thus, we next characterized stress hematopoiesis in Osx-Cre Smad4fl/fl mice in response to 5-fluorouracil (5-FU) treatment. Compared to control mice, the magnitude and duration of neutropenia following 5-FU were similar. Moreover, mouse survival after repeated weekly doses of 5-FU was comparable to control mice. HSPC mobilization by G-CSF is due, in large part, by downregulation of CXCL12 expression in bone marrow mesenchymal stromal cells. A prior study suggested that SMAD signaling negatively regulates CXCL12 expression in stromal cells. Consistent with this finding, we show that treatment of cultured bone marrow derived MSCs with TGF-b1 for 48 hours results in a significant (3.3-fold, P<0.0001) decrease in CXCL12 mRNA expression. Thus, in the final experiments, we characterized G-CSF induced HSPC mobilization in Osx-Cre, Smad4fl/fl or Osx-Cre, Tgfbr2fl/fl mice. HSPC mobilization, as quantified by CFU-C and Kit+ Sca+ lineage- (KSL) cell number in blood or spleen after 5 days of G-CSF treatment was comparable to control mice. Collectively, these data suggest the TGF family member signaling in mesenchymal stromal cells is dispensable for hematopoietic niche maintenance under basal and stress conditions. Disclosures No relevant conflicts of interest to declare.

Reduction in Bone Marrow Insulin-like Growth Factor-1 Levels Enhances Long-Term Hematopoietic Stem Cell Engraftment Following Bone Marrow Transplantation

Efficient donor hematopoietic stem cell (HSC) engraftment is critical for successful bone marrow transplantation (BMT) and requires interactions between donor HSC and host-derived HSC niche elements. Due to disruption of perivascular stem cell niches induced by myeloablative BMT conditioning, initial engraftment of HSC following BMT occurs in niches located near the endosteum. We have previously shown that myeloablative BMT conditioning with total body irradiation (TBI) induces expansion of endosteal niche osteolineage cells in a time course correlating with increases in bone marrow (BM) levels of insulin-like growth factor (IGF)-1. We also have shown that inhibition of IGF-1 Receptor (IGF1R) signaling blocks osteolineage cell expansion after TBI, resulting in poor engraftment of long-term (LT)-HSC after BMT and suggesting a critical role for IGF-1/IGF1R interactions in regulating engraftment after BMT. We now present data in which we have defined the cellular sources of BM IGF-1 following TBI and have surprisingly discovered that reduction in BM IGF-1 levels promotes LT-HSC retention at baseline and donor LT-HSC engraftment following BMT. Using mice with floxed IGF-1 alleles (IGF1L/L) and with tissue-specific expression of Cre-recombinase, we generated models with deletion of IGF-1 derived from mature osteolineage cells (Col1A1CreIGF1Exc), hematopoietic cells (CD45CreIGF1Exc), or liver (AlbCreIGF1Exc). We additionally generated a conditional IGF-1 deletion model (MX1CreIGF1Exc) in which polyI:polyC (pI:pC) treatment disrupts IGF-1 production from all 3 of the above sources. By qPCR analysis, CD45CreIGF1Exc BM showed marked reduction (&gt;95%) in BM IGF-1 mRNA at baseline and following TBI, suggesting that hematopoietic cells produce most of the locally derived IGF-1 in BM. Unexpectedly however, neither deletion of osteoblast-derived (Col1A1CreIGF1Exc) nor hematopoietic cell-derived (CD45CreIGF1Exc) IGF-1 significantly reduced BM extracellular or intracellular IGF-1 protein levels at baseline, or extracellular BM IGF-1 levels following TBI. In contrast, elimination of liver-derived IGF-1 in AlbCreIGF1Exc mice not only reduced plasma IGF-1 levels by 80%, but surprisingly reduced BM extracellular IGF-1 protein levels by ≥80% both at baseline and 48 hours post-TBI, and reduced BM intracellular IGF-1 protein levels by &gt;80% and &gt;60% at baseline and post-TBI, respectively. These data suggest that most IGF-1 protein in BM during BMT is derived from liver. Conditional IGF-1 deletion in MX1CreIGF1Exc mice resulted in severe reduction of both BM IGF-1 mRNA and protein levels, combining effects seen in CD45CreIGF1Exc and AlbCreIGF1Exc mice. None of these models, however, showed deficits in TBI-induced endosteal cell expansion, suggesting that other ligands or perhaps only low levels of IGF-1 are required to induce IGFR1 signaling-dependent endosteal cell expansion. Unexpectedly, reduction of BM IGF-1 in ALBCreIGF1Exc and MX1CreIGF1Exc mice caused a 3-4 fold increase (% of total BM) in BM Lin-cKit+Sca1+CD48-CD150+ or Lin-cKit+Sca1+CD34-CD135- HSC. ALBCreIGF1Exc BM displayed enhanced multi-lineage long-term repopulating capacity in competitive transplant assays, proving that these cells are functional LT-HSC. Given this baseline LT-HSC enhancement, we asked if reduction in BM IGF-1 levels impacted capacity of ALBCreIGF1Exc BM niches to engraft donor HSC after BMT. Using competitive secondary transplantation assays to assess numbers of engrafted WT GFP+ LT-HSC 3 weeks after primary BMT into ALBCreIGF1Exc or control mice, we found that reduced IGF-1 in the ALBCreIGF1Exc BM microenvironment resulted in &gt;2.5 fold enhancement of donor LT-HSC engraftment. In an initial search of downstream mediators of this effect, we discovered that reduction of IGF-1 levels in MX1CreIGF1Exc BM results in a 2.5-fold increase in post-TBI CXCL12 mRNA expression, suggesting that reducing BM IGF-1 may enhance donor LT-HSC engraftment by promoting LT-HSC homing and maintenance within HSC niches. Taken together our data define a critical role for IGF-1/IGF1R interactions in regulating the efficiency of donor HSC engraftment following BMT. Therapeutic strategies to reduce BM IGF-1 may prove to be a valuable approach to improving engraftment following clinical BMT. Disclosures No relevant conflicts of interest to declare.

Dickkopf 1 (Dkk1) Regulates Hematopoietic Stem Cell Regeneration

Bone marrow endothelial cells (BM ECs) have been shown to regulate HSC regeneration following myelosuppression. The role of osteolineage cells in regulating HSC regeneration remains less well understood. Here, we show that deletion of the pro-apoptotic genes, Bak and Bax, in osterix (Osx)-expressing osteoprogenitor cells promoted HSC regeneration and hematopoietic radioprotection of mice following total body irradiation (TBI). We identified Dkk1 to be enriched in the BM of radioprotected OsxCre;Bak1-/-;BaxFL/- mice and found that Bak /Bax-deficient osteolineage cells expressed increased levels of Dkk1 compared to Bax-expressing osteolineage cells (p=0.003). Treatment of irradiated BM HSCs with DKK1 in vitro significantly increased the recovery of phenotypic HSCs (p=0.0002), colony forming cells (CFCs)(p=0.003) and long-term repopulating HSCs compared to control cultures (p=0.009). Systemic administration of Dkk1 to lethally irradiated C57Bl6 mice accelerated the recovery of mature blood counts (p=0.008), BM HSCs (p=0.008) and progenitor cells (p=0.007). Furthermore, survival after lethal irradiation was markedly increased in Dkk1 treated mice (93%) compared to saline controls (27%; p=0.0004). Conversely, systemic administration of anti-Dkk1 antibody significantly delayed recovery of BM HSCs (p=0.002), peripheral white blood cells (p=0.0004), neutrophils (p<0.0001) and lymphocytes (p=0.002) in irradiated mice compared to irradiated, control mice. Dkk1 promoted HSC regeneration via suppression of reactive oxygen species (ROS) and inhibition of caspase activation in HSCs following irradiation. Dkk1 represents a novel, osteoprogenitor cell-derived paracrine factor which is necessary for normal hematopoietic regeneration following irradiation and can be therapeutically delivered to accelerate hematopoietic reconstitution. Disclosures Himburg: Duke University: Patents & Royalties: Patent Application for use of Pleiotrophin as a hematopoietic stem cell growth factor. Chute:C2 Regenerate: Equity Ownership; Duke University: Patents & Royalties: Application to use PTN as growth factor as hematopoietic stem cell growth factor.

Heterogeneity in the Making of Blood

It is increasingly clear that the bone marrow is comprised of a heterogeneous complex of niches for hematopoietic cells, some for stem cells in the perivascular space and some for progenitors. We have used two approaches to define the role of specific cells in the marrow. First, single cell selection and characterization based on in vivo proximity to HSPC. This method has defined a subset of endosteal lining cells that can be immunophenotypically defined and isolated and reveals IL-18 as a regulator of hematopoietic progenitor quiescence. Second, candidate cell depletion that revealed mature osteolineage cells expressing osteocalcin as regulating the production of thymic emigrants through the expression of Dll4. Deletion of these cells reduces the number and function of T-biased lymphoid progenitors in the marrow space as well as thymic populations and mature T cells in the blood. These data suggest that specific niche subsets can be defined and through them, novel molecular regulators of HSPC function. The bone marrow niche is a heterogeneous composite of distinctive niches. Disclosures No relevant conflicts of interest to declare.

Osteocyte-Mediated Parathyroid Hormone (PTH) Signaling Regulates Hematopoietic Stem Cells Under Physiologic and Continuous PTH Exposure

The bone marrow microenvironment, including osteolineage cells, regulates hematopoietic stem cell (HSC) fate choices. Intermittent pharmacologic treatment of mice with parathyroid hormone, PTH (1-34), indirectly increases HSCs through their niche, as HSCs do not express the PTH receptor (PTH1R). Osteocytes, the most abundant osteolineage cells in bone, are a critical target of the skeletal actions of PTH and coordinate multiple cell types that are components of the HSC niche including osteoblasts, osteoclasts and resident macrophages. While osteocytes express the PTH1R, the role of osteocytes in HSC regulation is unclear. Therefore, we studied the role of osteocyte-mediated PTH regulation of HSCs, using cre recombinase driven by the 8kb-DMP1 promoter to conditionally delete PTH1R in osteocytes (OCyPTHRko mice). OCyPTHRko mice were viable, fertile, did not exhibit any significant skeletal defect as juveniles or at 6 months of age, had no significant difference in serum PTH levels, and had no significant difference in osteoblastic or mesenchymal stem cell numbers compared to WT mice. In juvenile OCyPTH1Rko mice there was a decrease in long-term HSCs as measured by flow cytometric analysis (0.0029 ± 0.00028 vs. 0.0021 ± 0.00021 % of cells, WT vs. OCyPTH1Rko p≤0.05 N≥19 mice/group). OCyPTH1Rko mice had 4 fold lower long-term engraftment capacity as measured by secondary competitive transplantation over 16 weeks (WT vs. OCyPTH1Rko donors, 2-way ANOVA p≤0.001, N≥10 mice/group) that was evident in all hematopoietic lineages. Short-term engraftment however was increased in OCyPTH1Rko mice as measured by primary competitive transplantation (WT vs. OCyPTH1Rko donors, 2-way ANOVA p≤0.01, N≥9 mice/group). These data demonstrate that physiologic PTH signaling in osteocytes regulates the balance of long-term and short-term HSC potential in juvenile, growing mice. Adult OCyPTH1Rko mice also had 5 fold lower long-term engraftment as measured by secondary competitive transplantation over 16 weeks (WT vs. OCyPTH1Rko donors, 2-way ANOVA p≤0.001, N≥15 mice/group). These findings demonstrate a previously unrecognized physiologic role of PTH signaling in HSC regulation. Having demonstrated a role for PTH signaling in HSC homeostasis, we investigated if sustained PTH elevations (as are found in vitamin D deficiency and in hyperparathyroidism) alter HSC function. Therefore, we utilized a murine model of secondary hyperparathyroidism caused by a low calcium (LCa) diet. In juvenile mice placed on the LCa diet immediately upon weaning, serum PTH levels were significantly elevated. Fourteen days on the LCa diet caused a significant reduction in long-term engraftment potential as measured by secondary competitive transplants over 22 weeks (Normal vs. LCa diet donors, 2-way ANOVA p≤0.001, N≥20 mice/group), while there was no decrease in HSCs when adult mice were placed on the LCa diet. These data suggest that sustained PTH signaling decreases microenvironmental support for HSCs in juvenile mice. We utilized the OCyPTHRko mice to study the role of osteocytes in hyperparathyroidism-induced loss of functional HSCs. In juvenile mice the lack of PTH signaling in osteocytes rescued the long-term engraftment defects, suggesting that PTH signaling in osteocytes mediates the loss of long-term HSC support caused by the LCa diet. In further support of a deleterious effect mediated by the PTH1R in osteocytes in the setting of continuous PTH, adult OCyPTH1Rko mice placed on LCa diet had superior long term HSC function. Our findings demonstrate a physiologic role for PTH in HSC regulation and identify osteocytes as a critical constituent of the HSC niche that, either directly or indirectly, contribute to maintenance of the long-term repopulating HSC pool. In addition, we show that continuous exposure to elevated levels of PTH in a model of secondary hyperparathyroidism leads to osteocyte-mediated loss of long-term engraftment potential of HSCs in juvenile mice. We speculate that removing the effect of continuous PTH from osteocytes uncovers additional HSC-supportive effects of continuous PTH, mediated by non-osteocyte HSC niche cellular populations. Together these data establish PTH as a critical regulatory signal in the HSC niche, and show that the relative contributions of niche populations to HSC regulation are modulated by age. Disclosures Calvi: Fate Therapeutics: Patents & Royalties.

Embigin Regulates HSPC Homing and Quiescence and Acts As a Cell Surface Marker for a Niche Factor-Enriched Subset of Osteolineage Cells

Background. Discovery of niche-derived HSPC regulators is critical for further development of novel therapeutic approaches to promote HSPC regeneration. We have previously reported a proximity-based approach to the study of the bone marrow niche, which is based on the transcriptional comparison of osteolineage cells (OLCs) located close (proximal OLCs) or further away from transplanted HSPC (Blood 2014;124: 773), which lead to identification of IL18 as a quiescence regulator of early progenitors. We now report the results of functional validation for another molecule - Embigin - identified through this strategy as a hematopoietic regulator. Results. Embigin is a cell adhesion molecule with poorly characterized function. Our analysis showed that Embigin expression in proximal OLCs was significantly higher. Since the genetic tools for this molecule do not exist, we used a neutralizing antibody against Embigin to investigate the effect of Embigin blockade on primitive hematopoietic cells. We found that injection of anti-Embigin resulted in mobilization of myeloid progenitors and colony-forming cells (CFC) into the blood. In contrast, Embigin blockade was associated with a homing defect when LKS cells [known to express Embigin] were either pre-incubated with anti-Embigin antibody or injected into anti-Embigin pretreated animals, overall suggesting that Embigin regulates retention and localization of primitive hematopoietic cells in the bone marrow. Moreover, animals treated with anti-Embigin antibody had a higher frequency and proliferative activity of primitive hematopoietic cells, as demonstrated by cell cycle and BrdU incorporation studies and an increased CFC, consistent with Embigin-mediated localization also affecting HSPC cell cycling. Bone marrow from anti-Embigin treated mice reconstituted poorly when competitively transplanted with untreated animal marrow into irradiated recipients, likely due to the impaired homing and increased cell cycling. Finally, pre-treating irradiated recipients with anti-Embigin resulted in increased proliferation of transplanted WT LKS cells. Collectively, these data are consistent with Embigin serving as a regulator of HSPC localization and quiescence. Given a functional role of Embigin in the bone marrow niche and an overlapping pattern of expression with VCAM1, a known niche-derived HSPC regulator, in proximal OLCs we explored the use of Embigin in conjunction with VCAM1 as cell surface markers for prospective isolation of niche factor-enriched OLC subset by flow cytometry. Using this strategy, we were able to purify a rare OLC subset of CD45- Ter119-V CAM-1+E mbigin+ cells (termed VE cells), which were enriched for niche factor expression as compared to their non-VE counterparts. VE cells were transcriptionally distinct from other, previously defined niche subsets such as nestin-GFPdim mesenchymal stem cells, nestin-GFPbright pericytes and N-cadherin-positive osteoblastic cells. In particular, VE cells expressed higher levels of most niche factors than nestin-GFPbright cells - a cell population recently characterized as regulating HSPC quiescence. Interestingly, the expression profile of VE cells from animals transplanted with LT-HSCs and those which were irradiated but injected with saline alone demonstrated upregulation of cell adhesion molecules in the LT-HSC-injected group, suggesting that VE cells are involved in bidirectional communication within the niche. To investigate a functional role of VE cells, we performed co-culture experiments in which we compared the effect of VE and non-VE cells on HSPC proliferation and engraftment. We found that in the presence of VE cells, HSPCs proliferated at slower rate and generated a lower hematopoietic colony number, consistent with quiescence-inducing effect. Upon transplantation, VE-cultured HSPCs generated a higher level of chimerism compared to those cultured on non-VE layer, indicating a superior ability of VE cells to support engraftment and reconstitution properties of HSPC during the in vitro culture. Conclusion. Our work defines Embgin as a previously unrecognized hematopoietic regulator and a cell surface marker for a niche factor-enriched subset of the osteolineage cells which regulates HSPC quiescence. Pharmacological blockade of Embigin signaling may serve as a potential therapeutic tool to enhance hematopoietic regeneration. Disclosures No relevant conflicts of interest to declare.

Modulation of Interaction of Human Osteoprogenitor Cells with Hematopoietic Stem and Progenitor Cells

Osteoprogenitor cells (OPCs) are marrow microenvironmental cells known to modulate hematopoietic stem and progenitor cells (HSPCs). Specifically, OPCs regulate HSPCs in response to Parathyroid hormone (PTH) treatment in murine models. However, the role of OPCs in human HSPC regulation and whether human OPCs can be manipulated is poorly understood. Niche stimulation is an appealing strategy to aid in the treatment of hematopoietic dysfunction. Myelodysplastic syndromes (MDS) are clonal disorders with ineffective hematopoiesis resulting in cytopenias and risk of transformation to acute leukemia (AML). In mouse models, disruption of the osteolineage cells can contribute to initiation of ineffective hematopoiesis with phenotypic features of MDS. Our long term goal is to utilize microenvironmental stimulation as a therapeutic tool to improve hematopoietic disorders. We hypothesized that human cells isolated from the marrow fraction containing spicules harbor HSPC supportive cells, which can be manipulated to improve HSPC support. Moreover we hypothesized that OPC number and function is impaired by dysplasia-initiated microenvironmental disruption as a potential mechanism for reduced support of HSPCs and ineffective hematopoiesis. Our objective was to isolate human bone marrow spicule associated cells (SACs) and define their ability to support HSPCs, determine the impact of PTH treatment of SAC/HSPCs interactions and characterize dysplasia-induced osteolineage changes in human MDS and AML bone marrow. To achieve this objective, we used normal as well as MDS/AML patient-derived OPCs using a mouse-human co-culture system. Human bone marrow SACs isolated by collagenase digestion were either used for co-culture, analyzed with flow cytometry or cultured in mineralization media in limited dilutions. To assess the potential impact of PTH on human OPC interaction with HSPCs, we developed a 7 day co-culture of human bone marrow SACs treated with either vehicle or PTH, with mouse Lineage- Sca1+ c-Kit+ (LSK) hematopoietic progenitor cells. At the end of the co-culture, all cells present were used for competitive transplantation. Transplant experiments demonstrated that PTH treatment of the human bone marrow SACs leads to improved function of the co-cultured LSK cells as demonstrated by significantly improved engraftment of the LSK cells after transplant into irradiated C57/bl6 recipient mice when sampled at pre-specified time points over a 20-week period (N=12, 2-way ANOVA; p < 0.05). Flow cytometry analysis showed that mature (Lin- CD31- CD146+ CD105-) and immature osteolineage (Lin- CD31- CD146+ CD105+) cells were present in SACs and more abundant compared to within BMMCs (1% vs 0.1% and 0.24% vs 0.12% for the same patient). Notably, the putative HSC-supportive MSC pool was increased in SACs vs BMMCs (0.052% vs 0.019%). The presence of OPCs was functionally confirmed using colony forming unit osteoblasts (CFU-OBs). CFU-OB frequency was calculated using L-Calc TM (StemCell technologies). Among normal donors the frequency of CFU-OBs was low in marrow donors >50 years old compared to <50 years old donors (2.240e-005 ± 3.300e-006 N=2 vs. 0.0001146 ± 4.163e-005 N=9). We identified non-statistically significant decrease in the frequency of CFU-OBs in bone marrow SACs from MDS patients compared to normal donors (1.090e-005 ± 1.400e-006 N=2 vs. 5.024e-005 ± 1.277e-005 N=8; p= 0.179); and similar decrease in frequency of osteoprogenitor cells in the bone marrow aspirates from AML patients compared to normal donors (2.303e-005 ± 9.371e-006 N=3 vs. 5.024e-005 ± 1.277e-005 N=8; p= 0.251). These data support our hypothesis that OPCs in patients with MDS and AML are negatively impacted compared to normal bone marrow. These data demonstrate that human SACs contain HSPC-supportive cells which can be stimulated to improve HSPC function. Human SACs comprise MSCs and osteolineage cells including osteoprogenitor cells. Aging decreases OPC pools in SACs. Our data in our small sample also suggest that dysplastic bone marrow microenvironment may negatively impact OPCs, which may in turn decrease OPC support of HPSCs. PTH treatment in our in-vitro model shows the potential to improve the interaction between the OPCs and HSPCs, resulting in amelioration of HSC function. Together these data suggest a strategy where targeting the MDS microenvironment may add to the currently available treatment modalities. Disclosures Calvi: Fate Therapeutics: Patents & Royalties.