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Chemotherapy and irradiation cause DNA damage to hematopoietic stem cells (HSCs), leading to HSC depletion and dysfunction and the risk of malignant transformation over time. Extrinsic regulation of HSC DNA repair is not well understood, and therapies to augment HSC DNA repair following myelosuppression remain undeveloped. We report that epidermal growth factor receptor (EGFR) regulates DNA repair in HSCs following irradiation via activation of the DNA-dependent protein kinase–catalytic subunit (DNA-PKcs) and nonhomologous end joining (NHEJ). We show that hematopoietic regeneration in vivo following total body irradiation is dependent upon EGFR-mediated repair of DNA damage via activation of DNA-PKcs. Conditional deletion of EGFR in hematopoietic stem and progenitor cells (HSPCs) significantly decreased DNA-PKcs activity following irradiation, causing increased HSC DNA damage and depressed HSC recovery over time. Systemic administration of epidermal growth factor (EGF) promoted HSC DNA repair and rapid hematologic recovery in chemotherapy-treated mice and had no effect on acute myeloid leukemia growth in vivo. Further, EGF treatment drove the recovery of human HSCs capable of multilineage in vivo repopulation following radiation injury. Whole-genome sequencing analysis revealed no increase in coding region mutations in HSPCs from EGF-treated mice, but increased intergenic copy number variant mutations were detected. These studies demonstrate that EGF promotes HSC DNA repair and hematopoietic regeneration in vivo via augmentation of NHEJ. EGF has therapeutic potential to promote human hematopoietic regeneration, and further studies are warranted to assess long-term hematopoietic effects.

Receptor type protein tyrosine phosphatase-sigma (PTPσ) is primarily expressed by adult neurons and regulates neural regeneration. We recently discovered that PTPσ is also expressed by hematopoietic stem cells (HSCs). Here, we describe small molecule inhibitors of PTPσ that promote HSC regeneration in vivo. Systemic administration of the PTPσ inhibitor, DJ001, or its analog, to irradiated mice promotes HSC regeneration, accelerates hematologic recovery, and improves survival. Similarly, DJ001 administration accelerates hematologic recovery in mice treated with 5-fluorouracil chemotherapy. DJ001 displays high specificity for PTPσ and antagonizes PTPσ via unique non-competitive, allosteric binding. Mechanistically, DJ001 suppresses radiation-induced HSC apoptosis via activation of the RhoGTPase, RAC1, and induction of BCL-XL. Furthermore, treatment of irradiated human HSCs with DJ001 promotes the regeneration of human HSCs capable of multilineage in vivo repopulation. These studies demonstrate the therapeutic potential of selective, small-molecule PTPσ inhibitors for human hematopoietic regeneration., Protein tyrosine phosphatase sigma (PTPσ) deficient haematopoietic stem cells (HSCs) demonstrate increased engraftment following transplantation. Here the authors identify a small molecule inhibitor of PTPσ that promotes murine and human haematopoietic stem cell regeneration via induction of the RAC pathway and BCL-XL.

The mechanisms that regulate hematopoietic stem cell (HSC) regeneration after myelosuppressive injury are not well understood. We identified epidermal growth factor (EGF) to be highly enriched in the bone marrow serum of mice bearing deletion of Bak and Bax in TIE2-expressing cells in Tie2Cre; Bak1(-/-); Bax(flox/-) mice. These mice showed radioprotection of the HSC pool and 100% survival after a lethal dose of total-body irradiation (TBI). Bone marrow HSCs from wild-type mice expressed functional EGF receptor (EGFR), and systemic administration of EGF promoted the recovery of the HSC pool in vivo and improved the survival of mice after TBI. Conversely, administration of erlotinib, an EGFR antagonist, decreased both HSC regeneration and the survival of mice after TBI. Mice with EGFR deficiency in VAV-expressing hematopoietic cells also had delayed recovery of bone marrow stem and progenitor cells after TBI. Mechanistically, EGF reduced radiation-induced apoptosis of HSCs and mediated this effect through repression of the proapoptotic protein PUMA. Our findings show that EGFR signaling regulates HSC regeneration after myelosuppressive injury.

The role of osteolineage cells in regulating hematopoietic stem cell (HSC) regeneration following myelosuppression is not well understood. Here we show that deletion of the pro-apoptotic genes Bak and Bax in osterix (Osx, also known as Sp7 transcription factor 7)-expressing cells in mice promotes HSC regeneration and hematopoietic radioprotection following total body irradiation. These mice showed increased bone marrow (BM) levels of the protein dickkopf-1 (Dkk1), which was produced in Osx-expressing BM cells. Treatment of irradiated HSCs with Dkk1 in vitro increased the recovery of both long-term repopulating HSCs and progenitor cells, and systemic administration of Dkk1 to irradiated mice increased hematopoietic recovery and improved survival. Conversely, inducible deletion of one allele of Dkk1 in Osx-expressing cells in adult mice inhibited the recovery of BM stem and progenitor cells and of complete blood counts following irradiation. Dkk1 promoted hematopoietic regeneration via both direct effects on HSCs, in which treatment with Dkk1 decreased the levels of mitochondrial reactive oxygen species and suppressed senescence, and indirect effects on BM endothelial cells, in which treatment with Dkk1 induced epidermal growth factor (EGF) secretion. Accordingly, blockade of the EGF receptor partially abrogated Dkk1-mediated hematopoietic recovery. These data identify Dkk1 as a regulator of hematopoietic regeneration and demonstrate paracrine cross-talk between BM osteolineage cells and endothelial cells in regulating hematopoietic reconstitution following injury.

Hematopoietic stem cells (HSCs) are highly susceptible to ionizing radiation–mediated death via induction of ROS, DNA double-strand breaks, and apoptotic pathways. The development of therapeutics capable of mitigating ionizing radiation–induced hematopoietic toxicity could benefit both victims of acute radiation sickness and patients undergoing hematopoietic cell transplantation. Unfortunately, therapies capable of accelerating hematopoietic reconstitution following lethal radiation exposure have remained elusive. Here, we found that systemic administration of pleiotrophin (PTN), a protein that is secreted by BM-derived endothelial cells, substantially increased the survival of mice following radiation exposure and after myeloablative BM transplantation. In both models, PTN increased survival by accelerating the recovery of BM hematopoietic stem and progenitor cells in vivo. PTN treatment promoted HSC regeneration via activation of the RAS pathway in mice that expressed protein tyrosine phosphatase receptor-zeta (PTPRZ), whereas PTN treatment did not induce RAS signaling in PTPRZ-deficient mice, suggesting that PTN-mediated activation of RAS was dependent upon signaling through PTPRZ. PTN strongly inhibited HSC cycling following irradiation, whereas RAS inhibition abrogated PTN-mediated induction of HSC quiescence, blocked PTN-mediated recovery of hematopoietic stem and progenitor cells, and abolished PTN-mediated survival of irradiated mice. These studies demonstrate the therapeutic potential of PTN to improve survival after myeloablation and suggest that PTN-mediated hematopoietic regeneration occurs in a RAS-dependent manner.

Receptor tyrosine kinases (RTKs), such as c-kit, Flt-3 and Tie2, regulate hematopoietic stem cell (HSC) proliferation, differentiation and maintenance. Substantially less is known regarding the function of receptor protein tyrosine phosphatases (PTPs) in regulating HSC fate. We recently discovered that receptor protein tyrosine phosphatase sigma (PTPσ) is highly expressed by murine and human HSCs. Interestingly, constitutive deletion of PTPσ caused a marked increase in HSC repopulating capacity as measured in primary and secondary competitive repopulation assays (J Clin Invest 2015;125:177-182). Further, negative selection of human cord blood (CB) HSCs for PTPσ surface expression (PTPσ - negative CB HSCs) conferred more than 10-fold increased human CB hematopoietic engraftment through 20 weeks in transplanted NSG mice. Additionally, PTPσ-deficient mice displayed significantly augmented recovery of phenotypic bone marrow (BM) HSCs and colony forming cells at day +10 following 550 cGy total body irradiation. Based on these observations, we hypothesized that PTPσ may function as a negative regulator of HSC self-renewal and regeneration. We sought to develop pharmacologic strategies to inhibit PTPσ function as a means to augment HSC functional capacity. Based on structure-activity-relationship analysis of PTPσ, we screened candidate small molecules for ability to modulate PTPσ function on BM HSCs. We identified a small molecule, 5483071 (Chembridge), as a candidate PTPσ inhibitor. In silico simulation indicated that 5483071 rigidly docked into the binding site of PTPσ intracellular domain through hydrogen bonding and electrostatic interactions. In a model of HSC injury and regeneration, we irradiated C57Bl6 mice with 700 cGy TBI and treated mice systemically with either 10 μcg of 5483071 or water subcutaneously every other day from day +1 to day +14. Irradiated mice treated with 5483071 displayed significantly increased 60 day survival compared to controls (P=0.0007). Irradiated mice treated with 5483071 showed accelerated recovery of BM SLAM+kit+sca-1+lin- HSCs (P=0.02), BM KSL cells (P=0.01), and colony forming cells (CFCs) (P=0.0003). In vitro culture of BM KSL cells with 5483071 significantly increased the levels of activated Rac1 (P=0.0004), which recapitulated the effects of PTPσ deletion on Rac1 activation in HSCs. Importantly, treatment of BM KSL cells from PTPσ -/- mice with 5483071 caused no change in Rac1 activation, suggesting that 5483071 acted specifically on PTPσ and was not mediating effects via inhibition of other phosphatases. Systemic administration of 5483071 to irradiated mice caused an increase in BM KSL cell cycling at 72 hours compared to irradiated control mice (p=0.02), while also decreasing BM KSL cell apoptosis at 24 hours after TBI (p=0.02). Subsequent to these findings, we have generated several new small molecule inhibitors of PTPσ with novel composition of matter and have taken a lead compound into pre-clinical studies for investigational new drug (IND) development. PTPσ represents a novel receptor tyrosine phosphatase that regulates HSC self-renewal and regeneration. Targeted inhibition of PTPσ has high therapeutic potential to promote hematopoietic regeneration in patients receiving myelosuppressive chemotherapy and/or radiotherapy or undergoing myeloablative hematopoietic cell transplantation. Disclosures No relevant conflicts of interest to declare.

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 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.

Elucidation of the mechanisms governing HSC regeneration has been impeded by difficulty in isolating HSCs early following genotoxic injury, such as total body irradiation (TBI). Using multiparametric flow cytometric cell sorting of BM ckit+sca-1+lin- cells coupled with gene expression analysis, we identified growth factor receptor-bound protein 10 (Grb10), a co-receptor which regulates Insulin Receptor/IGF-1 signaling, to be significantly overexpressed by BM KSL cells at the earliest detectable point of regeneration (day +10) following TBI (3.3-fold, p Grb10 is a member of the imprinted gene family which is predominately expressed in the stem cells of a variety of tissues, including embryonic stem cells, bone marrow, skin and muscle. Viral shRNA-mediated knockdown of Grb10 in BM KSL cells caused a significant decrease in KSL cells and colony forming cells (CFCs) detected in 7-day culture (p=0.03 and p=0.002, respectively). Furthermore, mice competitively transplanted with Grb10-deficient HSCs displayed 10-fold lower donor, multilineage hematopoietic cell engraftment than mice transplanted with Grb10-expressing HSCs (p=0.007 for %CD45.1+ donor cells). Secondary competitive repopulation assays confirmed a greater than 10-fold deficit in long-term repopulating capacity in Grb10-deficient KSL cells compared to Grb10-expressing KSL cells (p=0.006 for %CD45.1+ donor cells). In order to determine if Grb10 was necessary for HSC maintenance and normal hematopoiesis in vivo, we generated maternally-derived Grb10-deficient mice. Heterozygous 8 week old Grb10m/+ (1 mutant allele, 1 wild type allele) had 10-fold decreased Grb10 expression in BM lin-cells. BM CFCs and SLAM+ KSL cells were significantly decreased in Grb10m/+ mice compared to Grb10+/+ mice (p=0.006 and p=0.04, respectively). Competitive repopulation assays demonstrated significantly decreased donor hematopoietic cell repopulation in recipient mice transplanted with Grb10m/+ BM cells versus mice transplanted with Grb10+/+ BM cells (p=0.003 for %CD45.1+ donor cells). Mice transplanted with BM cells from homozygous Grb10-/- mice showed a similar decrease in donor-derived hematopoietic repopulation compared to mice transplanted with BM cells from Grb10+/+ mice (p=0.02 at 20 weeks post-transplantation). These results confirmed that Grb10 regulates HSC self-renewal capacity in vivo. To determine whether Grb10 regulates HSC regeneration after myelotoxic injury, we irradiated Grb10m/+ mice with 550cGy TBI, and monitored hematopoietic recovery over time in comparison to Grb10+/+ controls. Interestingly, Grb10m/+ mice displayed accelerated hematopoietic regeneration early following TBI. At day+10 after 550cGy, Grb10m/+ mice contained significantly increased numbers of BM SLAM+ KSL cells (p=0.04) and CFCs (p=0.009), compared to Grb10+/+ littermates. Similarly, mice transplanted with BM cells from irradiated, Grb10m/+ mice displayed 5-fold increased donor hematopoietic repopulation at 20 weeks post-transplantation compared to mice transplanted with BM cells from irradiated, Grb10+/+ mice (p=0.006). These data suggest that Grb10 deficiency accelerates hematopoietic recovery in the early period following myelosuppressive radiation injury. Mechanistically, Grb10-deficiency caused an increase in the percentage of BM KSL cells in G1 and G2/S/M phase of cell cycle compared to Grb10+/+ KSL cells (p=0.003). We also observed significantly increased levels of mTOR activation in Grb10m/+ BM KSL cells compared to Grb10+/+ BM KSL cells (p=0.001 for pS6, p=0.001 for pS6k and p=0.02 for p4EBP1). Furthermore, mTOR inhibition via siRNA-mTOR targeting rescued the defect in BM hematopoietic progenitor content (colony forming cells) in Grb10-deficient BM cells (p 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.

Hematopoietic stem cell (HSC) proliferation, differentiation and self-renewal are regulated by signaling through receptor tyrosine kinases (RTKs) such as c-kit, Flt-3 and Tie2. The functions of receptor protein tyrosine phosphatases (RPTPs) in counterbalancing RTK signaling in HSCs remain incompletely understood. Among 9 examined RPTPs, we found that PTP-sigma (PTPσ) was significantly overexpressed in mouse and human HSCs compared to more mature hematopoietic cells. PTPσ-/- mice displayed no difference in mature blood counts or phenotypic HSC frequency compared to PTPσ+/+ mice. However, competitive transplantation of BM cells from PTPσ-/- mice resulted in greater than 8-fold increased multilineage hematopoietic repopulation in primary and secondary recipient mice compared to mice transplanted with BM from PTPσ+/+ mice. While HSCs from PTPσ-/- mice displayed no differences in cell cycle status or homing capability compared to PTPσ+/+ mice, PTPσ-/- HSCs expressed significantly increased levels of activated Rac1, a RhoGTPase which regulates HSC engraftment capacity, compared to PTPσ+/+ BM cells. PTPσ-/- BM cells also displayed significantly increased transendothelial migration capacity and cobblestone area forming cells (CAFCs), consistent with increased Rac1 activation. Furthermore, Rac inhibition abrogated the increased migration capacity of PTPσ-/- BM cells, suggesting that the augmented engraftment capacity of PTPσ-/- BM cells was mediated via Rac activation. Translationally, we demonstrate that negative selection of human cord blood CD34+CD38-CD45RA-Lin- cells for PTPσ expression yielded a 15-fold enrichment for human long-term repopulating HSCs compared to CD34+CD38-CD45RA-Lin- cells or CD34+CD38-CD45RA-lin-PTPσ+ cells. These data suggest that PTPσ regulates HSC repopulating capacity via inhibition of Rac1 and that selection for human PTPσ - negative HSCs is a translatable strategy to significantly enrich human HSCs for transplantation. Disclosures No relevant conflicts of interest to declare.

The mechanisms which regulate HSC regeneration following stress or injury remain poorly understood. Precise study of HSCs during regeneration has been impeded by the rarity of the HSC population and depletion of phenotypic HSCs early following genotoxic stresses, such as total body irradiation (TBI). We isolated bone marrow (BM) ckit+sca-1+lin- (KSL) cells, which are enriched for HSCs, from adult C57Bl6 mice before and at several time points following TBI, as a means to map the dynamic molecular response of HSC regeneration. Following 550cGy TBI, BM KSL cells were depleted by 7 days post-TBI, whereas KSL cell recovery was evident at day+14. We isolated BM KSL cells and myeloid progenitor cells (c-kit+sca-1-lin- cells) at day +14 and compared the gene expression profile of regenerating HSCs versus steady state HSCs (non-irradiated) and committed progenitor cells. We identified growth factor receptor-bound protein 10 (Grb10), a co-receptor which regulates Insulin Receptor/IGF-1 signaling, to be significantly overexpressed in regenerating BM KSL cells compared to non-irradiated KSL cells (3.3 fold, p Grb10 is a member of the family of imprinted genes which are predominately expressed in numerous stem cell populations, including embryonic stem cells, skin and muscle stem cells. Viral shRNA-mediated knockdown of Grb10 in BM KSL cells caused a significant decrease in KSL cells and colony forming cells (CFCs) in detected in 7 day culture (p=0.03 and p=0.002, respectively). Furthermore, mice which were competitively transplanted with Grb10-deficient KSL cells had 10-fold decreased donor, multilineage hematopoietic cell engraftment than mice transplanted with Grb10-expressing HSCs (p=0.007 for %CD45.1+ donor cells). Secondary competitive repopulation assays confirmed > 10-fold deficit in long-term repopulating capacity in Grb10 deficient KSL cells compared to Grb10 expressing KSL cells (p=0.006 for %CD45.1+ donor cells in secondary mice). In order to examine the effect of Grb10-deficiency on HSC fate and hematopoiesis in vivo, we generated maternally-derived Grb10-deficient mice. Heterozygous 8-week old Grb10m/+ (1 mutant allele, 1 wild type allele) were found to have 10-fold decreased Grb10 expression in BM lin- cells and had normal range complete blood counts. However, BM CFCs were significantly decreased in Grb10m/+ mice compared to Grb10+/+ mice (p=0.006) and competitive repopulation assays demonstrated significantly decreased donor hematopoietic cell repopulation in recipient mice transplanted with Grb10m/+ BM cells versus mice transplanted with Grb10+/+ BM cells (1/14, 7% vs. 5/14, 38% of mice with > 0.1% donor CD45.2+ cells). These results suggest that Grb10 regulates HSC self-renewal in vitro and in vivo. Mechanistically, Grb10m/+ mice displayed no alterations in the cell cycle status or frequency of apoptotic cells within BM HSCs compared to Grb10+/+ mice. However, single cytokine functional screening suggested that Grb10 regulates SCF-mediated proliferation of HSCs. Grb10m/+ BM KSL cells generated significantly less CFCs in culture in response to SCF treatment compared to Grb10+/+ KSL cells (p=0.008). Commensurate with this, SCF-mediated activation of mTOR was significantly increased in Grb10m/+ KSL cells compared to that observed in Grb10+/+ KSL cells (p=0.006). These data suggest that cytokine-mediated induction of mTOR signaling, which has been shown to deplete functional HSCs, is antagonized by Grb10, and that Grb10 is necessary to block cytokine-mediated HSC differentiation in vitro and in vivo. Grb10 represents a novel regulator of HSC fate determination and a new mechanistic target to facilitate HSC self-renewal. Studies are underway to determine whether Grb10 is also necessary for HSC regeneration after TBI. Disclosures No relevant conflicts of interest to declare.

Discovery of the mechanisms through which the bone marrow microenvironment stimulates hematopoietic regeneration following myelosuppression could lead to therapies to accelerate hematopoietic reconstitution in patients receiving chemotherapy, total body irradiation and stem cell transplantation. We have previously shown that treatment with pleiotrophin (PTN), a heparin-binding growth factor which is secreted by BM endothelial cells (ECs), causes a 10-fold expansion of murine long term-HSCs in culture (Himburg et al. Nat Med 2010). More recently, we demonstrated that PTN-deficient mice have a >10-fold deficit in LT-HSCs and hematopoietic regenerative capacity compared to PTN+/+ mice, suggesting an important role for PTN in maintaining the HSC pool in vivo (Himburg et al. Cell Reports 2012). In keeping with this, 100% of PTN-deficient mice died prior to day +30 following 700 cGy total body irradiation (TBI) compared to 30% mortality in irradiated, PTN+/+ mice (P Disclosures: No relevant conflicts of interest to declare.

Abstract 404 Several signaling pathways have been elucidated which regulate hematopoietic stem cell self-renewal, including the Notch, Wnt, HOX and BMP signaling pathways. However, several of these pathways (e.g. Notch, Wnt) may not be necessary for maintenance of HSCs in vivo. We recently demonstrated that treatment of murine and human HSCs with the heparin binding growth factor, pleiotrophin (PTN), was sufficient to induce self-renewal of murine and human HSCs in culture (Himburg, Nat Med, 2010). In order to determine if PTN signaling is necessary for HSC self renewal and normal hematopoiesis in vivo, we examined the bone marrow HSC content and hematopoietic profile of mice bearing a constitutive deletion of PTN (PTN−/− mice) as well as mice bearing constitutive deletion of the PTN receptor, receptor protein tyrosine phosphatase β/ζ (RPTPβ/ζ) (courtesy of Dr. Gonzalo Herradon, Spain and Dr. Sheila Harroch, L'Institut Pasteur, Paris, FR). PTN−/− mice demonstrated no significant differences in total bone marrow (BM) cells or BM colony forming cells (CFCs) but had significantly decreased bone marrow CD34(-)c-kit(+)sca-1(+)lin(-) (34-KSL) cells compared to littermate controls which retained PTN (PTN+/+) mice (0.007% vs. 0.02%, p=0.03). Consistent with this phenotype, PTN−/− mice also contained 2–fold decreased CFU-S12 compared to control PTN+/+ mice (p= 0.003). PTN−/− mice also demonstrated an 11-fold reduction in long-term repopulating HSC content compared to PTN+/+ mice as measured via competitive repopulating assay (12 week CRU frequency: 1 in 6 cells vs. 1 in 66 cells). Taken together, these data demonstrate that PTN signaling is necessary for maintenance of the BM HSC pool in vivo. Since PTN is known to antagonize the phosphatase activity of RPTPβ/ζ, we hypothesized that deletion of RPTPβ/ζ would increase BM HSC self-renewal and result in expansion of the BM HSC pool in vivo. Consistent with this hypothesis, RPTPβ/ζ−/− mice displayed a 1.3-fold increase in total BM cells (p= 0.04), 1.8-fold increase in BM 34-KSL cells (p=0.03), 1.6-fold increase in BM CFCs (p= 0.002) and 1.6–fold increase in BM CFU-S (p< 0.0001). RPTPβ/ζ−/− mice also demonstrated 1.4–fold higher long-term repopulating capacity (12 weeks) following competitive repopulating assay compared to RPTPβ/ζ+/+ mice (Donor CD45.1+ cell engraftment: 4.2% vs. 1.5%). Interestingly, RPTPβ/ζ −/− mice had significantly increased PB white blood cell counts, hemoglobin and platelet counts compared to RPTPβ/ζ+/+ mice coupled with splenomegaly. The RPTPβ/ζ−/− mice also had significantly increased BM vascular density (via quantitative mouse endothelial cell antigen staining) compared to RPTPβ/ζ+/+ mice, suggesting that PTN/RPTPβ/ζ signaling may augment the HSC pool size directly and also indirectly via activation of the BM vascular niche. These results demonstrate that PTN signaling is necessary and sufficient for induction of HSC self-renewal in vivo. Disclosures: No relevant conflicts of interest to declare.

Abstract 3859 Hematopoietic stem cell (HSC) regeneration is influenced by specialized bone marrow (BM) microenvironments, but the mechanisms that drive HSC regeneration remain incompletely defined. We have recently reported that deletion of the pro-apoptotic proteins, Bak and Bax, in Tie2+ bone marrow endothelial cells (BM ECs)(Tie2Cre;Bak-/-;BaxFl/- mice) caused a significant protection of the BM HSC pool and the BM sinusoidal vasculature in mice following high dose total body irradiation (TBI). We also confirmed that this protection of the BM HSC pool was caused by protection of BM Tie2+ ECs via generation of chimeric mice (Tie2Cre;Bak-/-;BaxFl/- BM; wild type BM ECs) which contained 4.8-fold less BM long-term repopulating HSCs compared to mice bearing deletion of Bak and Bax in both BM HSCs and BM ECs. In order to determine the mechanism through which Tie2+ BM ECs regulate HSC regeneration, we generated primary BM EC lines from Tie2Cre;Bak-/-;BaxFl/- mice and Tie2Cre;Bak-/-;BaxFl/+ control mice. We then compared the capacity for Bak/Bax -/- BM ECs to support BM HSC regeneration in vitro compared to Bak/Bax +/− BM ECs. BM c-kit+sca-1+lin- (KSL) stem/progenitor cells were irradiated with 300 cGy and then placed in 7 day culture with Bak/Bax -/- BM ECs or Bak/Bax +/− BM ECs. Culture with Bak/Bax -/- BM ECs did not yield a significant increase in total viable cells, but yielded 2000-fold increased number of BM KSL cells (p < 0.05, n=3) compared to cultures with Bak/Bax +/− ECs. This significant expansion of phenotypic BM stem/progenitor cells corresponded to a 4-fold increase in CFU-S12 cells in the Bak/Bax -/- EC cultures vs. Bak/Bax +/− EC cultures (p=0.01, n=5). We subsequently compared the level of expression of several microenvironmental ligands which are putatively involved in regulating hematopoiesis. We found that BM ECs from Tie2Cre;Bak-/-;BaxFl/- mice had 37-fold lower expression of stromal-derived factor-1 (SDF-1, CXCL12) compared to BM ECs from Tie2Cre;Bak-/-;BaxFl/+ mice. Moreover, 7 days after TBI, Tie2Cre;Bak-/-;BaxFl/- mice had a 41-fold increase in total viable BM cell counts and had a persistently lower SDF-1 expression on BM ECs (2.7-fold) compared to Tie2Cre;Bak-/-;BaxFl/+ mice (p=0.003). Therefore, we hypothesized that inhibition of SDF-1 signaling might facilitate hematopoietic regeneration following injury. Interestingly, the addition of a blocking anti-SDF1 antibody to cultures of irradiated BM KSL cells with Bak/Bax -/- ECs caused a 50% increase in total cell recovery (p Disclosures: No relevant conflicts of interest to declare.

Abstract 3734 We have recently demonstrated that targeted deletion of the pro-apoptotic genes, Bak and Bax, in Tie2+ bone marrow endothelial cells (BM ECs) causes the protection of the BM sinusoidal vasculature, and the BM hematopoietic stem cell (BM HSC) pool following high dose total body irradiation (TBI). Since Tie2 is expressed on BM ECs and a small subset of quiescent BM HSCs, we developed a complementary model utilizing VE-cadherin Cre mice to more specifically confirm the function of BM ECs in regulating hematopoietic reconstitution in vivo. VE-cadherinCre;Bak−/−;BaxFl/− mice, which bear deletion of Bak and Bax in VE-cadherin+ ECs, and VE-cadherinCre;Bak−/−;BaxFl/+ control mice were generated and we compared the hematopoietic responses of these animals to high dose TBI. At steady state, these mice showed no difference in total BM cells, bone marrow ckit+sca+lineage- (KSL) progenitor cells, BM colony forming cells (CFCs), or BM colony forming unit spleen day 12 (CFU-S12) counts. At 2 hours after exposure to 500cGy TBI, the mice also did not demonstrate any significant differences in total BM cells, BM KSL cells, CFCs, or CFU-S12. However, 7 days after exposure to 500cGy TBI, VE-cadherin;Bak−/−;BaxFl/− mice displayed a 2-fold increase in total BM cells (p=0.006), a 3-fold increase in BM CFCs (p=0.0009), and 4-fold increase in BM CFU-S12 (p=0.079) compared to VE-cadherinCre;Bak−/−;BaxFl/+ control mice. Microscopic examination confirmed severe hypocellularity and corruption of the BM sinusoidal vasculature at day +7 post-TBI in VE-cadherinCre;Bak−/−;BaxFl/+ mice, whereas VE-cadherinCre;Bak−/−;BaxFl/− mice displayed nearly normal cellularity and preserved BM sinusoidal vessels. Taken together, these data demonstrate that VE-cadherin+ BM ECs regulate hematopoietic reconstitution in vivo and that targeted therapies aimed at augmentation of BM EC function can accelerate hematopoietic regeneration in vivo. Currently, we are comparing the cytokine production and gene expression profiles of VE-cadherinCre;Bak−/−;BaxFl/− BM ECs versus VE-cadherinCre;Bak−/−;BaxFl/+ BM ECs to identify candidate BM EC-derived proteins which are responsible for the protection of the BM stem/progenitor cell pool from radiation injury. VE-cadherin+ BM ECs represent an attractive mechanistic target for the identification of signaling pathways that regulate hematopoietic regeneration following injury. Disclosures: Chao: Genzyme: Research Funding.