Hematopoietic Stem/progenitor Cells In Bone Marrow Research Articles

The Effect of SIRT5 Deletion on Recovery of Hematopoietic Stem Cells after Injury in Mouse

To investigate the effect of deacylase Sirtuin 5 in the recovery of hematopoietic stem cells (HSCs) after treated by 5-FU in mouse. Flow cytometry was used to analyze the effect of SIRT5 deletion on the proportion of hematopoietic stem/progenitor cells (HSPCs) in bone marrow (BM), the proportion of T cells, B cells and myeloid cells (TBM) in peripheral blood (PB) and spleen, and the development of T cells in thymus. Mouse were treated with 5-FU to study the effect of SIRT5 deletion on the cell cycle, apoptosis and the proportion of HSPCs in BM. The effect of SIRT5 deletion on the proliferation of HSCs was analyzed by flow sorting in vitro. SIRT5 deletion did not affect the development of T cells in thymus and the proportion of TBM cells in PB and spleen compared with wild type mice. SIRT5 deletion increased proportion of HSPCs in BM. After 5-FU treatment, the proportion of HSCs in SIRT5 deletion mice was significant decreased (P < 0.05), the HSPC in SIRT5 deletion mice was activated from G0 to G1 phase (P < 0.05), and the proportion of early apoptosis increased (P < 0.05). By monoclonal culture in vitro, the ability of HSCs to form clones in SIRT5 deletion mice was decreased significantly (P < 0.05). SIRT5 deletion lead to a decreased the ability of HSCs to clone in vitro. SIRT5 deletion is not conducive to the recovery of HSPCs injury in mice under hematopoietic stress.

Dynamic Oscillation of Tetraspanin CD9 Orchestrates Engraftment and Mobilization of Human Hematopoietic Stem/Progenitor Cells

Background Trafficking of hematopoietic stem/progenitor cells (HSPCs) to and from their bone marrow (BM) niche is pivotal for engraftment and mobilization during clinical transplantation, where both processes are controlled by highly orchestrated molecular programs. We previously identified the tetraspanin CD9 as a downstream effector of the SDF-1/CXCR4 axis driving migration, adhesion and homing of human CD34+ HSPCs, but its consequential impact on stem cell engraftment and mobilization as well as the underlying mechanisms remains unknown. Methods Umbilical cord blood (UCB) CD34+ cells were exposed to a CD9 neutralizing antibody ALB6 prior to infusion into sublethally irradiated NOD/SCID mice, with overall engraftment and lineage output assessed by enumerating human CD45+ (pan-leukocytes), CD19+ (B-lymphoid), CD33+ (myeloid) and CD34+ (stem/progenitors) cells in the recipient BM at 8 weeks post-transplantation. CD9 expression on engrafted HSPCs was characterized by flow cytometry and compared to those before transplantation or after G-CSF mobilization. The oscillation pattern was then validated on HSPCs in pairwise BM, peripheral blood (PB) and G-CSF-mobilized peripheral blood (MPB) samples from healthy donors. Activity of integrin VLA-4 (an established binding partner of CD9) was evaluated on BM-residing and mobilized HSPCs by VCAM-1 binding assay. Results A short exposure of UCB CD34+ cells to the CD9 neutralizing antibody markedly inhibited their BM engraftment by 64.2% ( P=0.003), reflected by the general decline in human CD45+ cell population. There was no bias in lymphoid/myeloid lineage outputs, suggesting CD9-mediated engraftment is not restricted to specific subsets of hematopoietic precursors. CD9 expression was drastically elevated on engrafted CD34+ cells in the BM from 9.6% to 91.4% ( P&amp;lt;0.001) compared to pre-transplantation. In contrast, CD9 expression was significantly downregulated on BM CD34+ cells from 73.4% to 33.5% ( P&amp;lt;0.001) upon G-CSF mobilization, with the decline observed in both primitive stem cells (CD34+CD38-) and committed progenitors (CD34+CD38+), indicating that the oscillation of CD9 is highly associated with HSPC trafficking. Concordantly, pairwise analysis of steady-state HSPCs in BM and PB from healthy donors revealed higher CD9 level in BM-residing CD34+ cells than the PB-circulating portion (46.1% vs. 26.5%, P=0.004; n=13). In addition, a significantly higher CD9 expression was observed on BM CD34+ cells (47.7%; n=16) than those in MPB (32.3%, n=25; P=0.003). CD9 expression on MPB CD34+ cells was enhanced upon SDF-1 exposure ( P=0.006), revealing a dynamic regulation. In normal BM, CD9+ HSPCs exhibited elevated VLA-4 activities with preferential binding to soluble VCAM-1 compared to the CD9- counterpart (51.4% vs. 25.9%, P&amp;lt;0.001; n=10). Superiority in VCAM-1 binding capacity was also witnessed in BM-residing CD34+ cells compared to MPB (37.7% vs. 4.1%, P=0.014), implicating that the influence of CD9 on HSPC trafficking may due to the alteration of integrin activities and downstream adhesive properties. Conclusion This study describes the dynamic regulation of CD9 during entry and exit of HSPCs to and from their BM niche via modulation of integrin activities. The findings extend our understandings on HSPC trafficking that could potentially lead to new strategies to enhance clinical stem cell transplantation and mobilization.

Hematopoietic Stem/Progenitor Cell Intrinsic SAM Synthesis Is Required to Prevent p53 Pathway Activation By Maintaining DNA Stability

S-adenosylmethionine (SAM) is a principle methyl group donor to maintain epigenetic status. SAM is synthesized from methionine and ATP via the enzymatic activity of Mat2a (Methionine adenosyltransferase 2a). It has been reported that SAM reduction by methionine restriction or Mat2a inhibition suppresses the proliferation of leukemia cells to attract attentions as a potential therapeutic target for leukemia. To develop the therapy targeting methionine metabolism in leukemia, it is essential to elucidate the significance of methionine metabolism in non-leukemic hematopoiesis. However, still little is known about the importance of SAM synthesis in hematopoiesis, including hematopoietic stem/progenitor cells (HSPCs), in vivo. By using Mx1-Cre mediated Mat2a gene deletion in mice ( Mat2a fl/fl;Mx1-Cre mice), we investigated the roles of Mat2a in hematopoiesis in vivo. Reduction of Mat2a activity in Mat2a knockout (KO) mice in bone marrow (BM) was confirmed by ultra-high performance liquid chromatography tandem mass spectrometry analysis. Mat2a KO mice displayed severe pancytopenia in peripheral blood (PB). Mature cell fractions (Myeloid cells, B cells and T cells) and HSPCs (HSCs, MPP2s, MPP3s and MPP4s) in BM were also significantly decreased in Mat2a KO mice, suggesting that Mat2a is essential for the maintenance of hematopoiesis from HSPCs in vivo. To exclude the possible contributions of the effects induced by Mat2a KO in non-hematopoietic cells, we performed BM transplantation analyses in which hematopoietic cell specific KO or non-hematopoietic cell specific KO were achieved. These analyses clearly confirmed that HSPC intrinsic SAM synthesis is inevitable for the maintenance of hematopoiesis. To reveal the mechanistic insights of the regulatory roles for Mat2a in HSPCs, we performed RNA sequencing analysis ( Mat2a WT vs KO in HSPCs). GSEA (Gene Set Enrichment Analysis) using the Hallmark gene sets showed that p53 pathway was most highly enriched in Mat2a KO. In depth, genes downstream of p53 ( Bax, Apaf1, Bbc3, Casp3, Casp8, Fas, Cdkn1a etc.) were up-regulated by Mat2a KO, causing cell cycle arrest and apoptosis in HSPCs. On the other hand, gene expressions of the upstream of p53 ( Atr, Atm, Chek1, Chek2 etc.) were unchanged, suggesting that p53 activity was not increased at mRNA expression levels. In line with this, mRNA expression levels of Trp53 was unchanged, but total protein amount of p53 was increased in Mat2a KO HSPCs. Since DNA stability is known as a critical regulator of p53 pathway, we analyzed gH2AX levels as DNA damage marker and found that gH2AX levels were increased in Mat2a KO HSPCs. To be noted, expressions of genes involved in Fanc pathway ( Fanca, Fancc, Fancd2, Fance, Fancg, Fancl, Fanci etc), which is responsible for the DNA maintenance, were broadly down-regulated in Mat2a KO HSPCs at transcriptomic levels. Therefore, the Fanc pathway repression by SAM synthesis inhibition could cause DNA damage and p53 pathway dependent loss of maintenance in Mat2a KO HSPCs. To further understand the importance of SAM synthesis under stress hematopoiesis. We analyzed Mat2a expression changes in HSPCs by irradiation (5.5 Gy/mouse). Intriguingly, Mat2a expression was up-regulated by irradiation. We also found that dietary methionine restriction (MR) in mice prevents its hematopoietic recovery from irradiation stress. To be noted, long term MR (24 weeks) did not show any particular compared to control diet. Therefore, methionine metabolism possesses significant roles at least in the situation where acute hematopoietic recovery is required. Collectively, in this study, we found that cell intrinsic SAM synthesis is required for the maintenance (proliferation and survival) of HSPCs in vivo. SAM might be required for the prevention of DNA damage and p53 pathway activation by supporting Fanc pathway gene expressions. Methionine and/or SAM availability might possess critical roles for the regeneration of non-leukemic hematopoiesis. Further investigation about the significance of SAM synthesis in non-leukemic hematopoiesis is required for the proper establishment of SAM synthesis targeting therapy in cancers.

Mesenchymal Stromal Cells from Individuals with DNMT3A mut Clonal Hematopoiesis Instruct Young Healthy Hematopoietic Stem/Progenitor Cells Towards a Myeloid-Biased Aging Phenotype Resembling That of Patients with Clonal Cytopenia of Undetermined Significance (CCUS)

Clonal hematopoiesis of indeterminate potential (CHIP) describes the age-related acquisition of somatic mutations in hematopoietic stem/progenitor cells (HSPC) leading to clonal blood cell expansion and variable risk of progression to myeloid malignancy. DNMT3A is the most commonly mutated gene and while single mutations do not raise the likelihood of progression, high variant allele frequencies (VAF) and presence of co-mutations do. In myeloid diseases, the bone marrow (BM) microenvironment exhibits alterations promoting malignant transformation. However, it is unknown whether similar alterations exist in the pre-malignant niche. We set out to analyze whether human mesenchymal stromal cells (MSC) from CHIP individuals with a DNMT3A mutation ( DNMT3A mut) are altered and whether they are able to remodel the HSPC compartment in healthy hematopoiesis. From our published cohort of previously sequenced BM samples from hematologically healthy individuals undergoing hip replacement surgery, we selected samples with either an isolated DNMT3A mut at a low VAF (2.47-6.8%, n=4, median age 73.5y, termed low-risk CHIP), or DNMT3A mut samples with a high VAF (17.5-28.5%) and at least one co-mutation (n=5, median age 73y, termed high-risk CHIP) as well as 12 age-matched samples without any mutation (n=12, median age 72y). HSPC were isolated from 3 younger healthy donors (median age 50y). MSC were expanded by plastic adherence and subjected to multilevel characterization. Compared to age-matched healthy controls, MSC from DNMT3A mut BM samples (CHIP-MSC) exhibited higher proliferation rates, lower colony-forming-unit (CFU) fibroblast capacity, as well as reduced adipogenic and increased osteogenic differentiation potential. To understand how altered CHIP-MSC influence the hematopoietic compartment, younger healthy CD34+ HSPC were co-cultured with the different MSC subgroups for 42 hours in a low cytokine-containing medium, followed by bulk RNAseq on subsequently sorted cell populations (MSC: CD45-/CD90+; HSPC: CD45+/CD34+). We found 31 differentially expressed genes (DEG) (logFC±0.5, FDR&amp;lt;0.05) in CHIP-MSC compared to their age-matched counterparts. Functional analyses suggested an activation of communication towards HSPC via extracellular vesicles. Strikingly, differential gene expression of sorted young healthy HSPC after co-culture revealed 375 DEG (322 up-regulated, 53 down-regulated; logFC±0.5, FDR&amp;lt;0.05) in HSPC cultured with CHIP compared to non-CHIP age-matched MSC (Figure panel A). We then used gene set variation analysis (GSVA) to analyze HSPC compartment composition after co-culture. Based on enrichment scores, unsupervised clustering identified two distinctive profiles that correlated with HSPC cultured with either non-CHIP or high-risk CHIP-MSC, while those cultured with low-risk CHIP-MSC showed an intermediate phenotype (Figure panel B). Co-culture of HSPC with high-risk CHIP-MSC resulted in enrichment of hematopoietic stem cells (HSC) and multipotent progenitors (MPP), as well as myeloid skewing represented by higher scores for common-myeloid (CMP) and granulocyte-monocyte (GMP) progenitors. Conversely, in contrast to the control group, megakaryocyte-erythroid progenitors (MEP) were underrepresented. CFU capacity was increased in healthy HSPC cultured with CHIP-MSC and multiparameter flow cytometry analysis showed a trend towards increased CMP/GMP and decreased MEP numbers, consistent with the RNAseq data. Finally, we evaluated the BM HSPC compartment composition of a cohort of 49 CCUS patients with DNMT3A mut (VAF 1-49%, median age 76y) compared to 56 control BM samples using whole transcriptome sequencing data. Intriguingly, we observed the same profile with a myeloid shift in progenitor populations as in our healthy young HSPC exposed to high-risk CHIP-MSC. Taken together, our results indicate that MSC from individuals harboring a DNMT3Amutation are able to instruct young healthy HSPC and influence their fate, thereby promoting a differentiation bias that shifts the hematopoietic compartment toward that associated with an aged pre-malignant state, even in the absence of DNMT3A mut HSPC. These findings suggest that focusing on the interaction between MSC and HSPC is a promising therapeutic strategy to intercept disease progression. Functional analysis of identified deregulated pathways in DNMT3Amut CHIP is ongoing.

Efficient and Specific In Vivo Genetic Engineering of Human Hematopoietic Stem Progenitor Cells without Selective Conditioning

Achieving in vivo genetic engineering of hematopoietic stem progenitor cells (HSPC) has the potential to transform treatment for hematological disorders. An ideal in vivo delivery platform should provide access to resting HSPC at low doses, in different in vivo compartments and with high specificity. We developed a multi-model strategy to assess efficiency and specificity of access to human HSPC, without reliance on conditioning to selectively enrich for gene-modified cells, and we achieved efficient genetic engineering of Lin-CD34+ cells and individual HSPC subtypes using highly potent lentiviral vectors (LV) and gene editing virus-like particles (VLP). We first conducted a high-resolution characterization of access to resting HSPC and individual FACS-sorted HSPC subtypes in vitro with different LV envelopes, including a detailed analysis of insertion site distribution in resting vs cytokine activated CD34+ cells. With a BaEVTR LV we achieved transduction of ~100% resting HSPC at MOI 66 (vs ~40% with a VSV-G LV at MOI 50,750) and ~20% of FACS-sorted hematopoietic stem cells (HSC) at MOI 32. This translated into 2-4% transduction of bone marrow (BM) huCD45+ cells when the same BaEVTR LV was dosed in NBSGW long-term humanized mice. However, such models carry a highly lymphoid-skewed human hematopoiesis and are not well suited for measuring access to all HSPC subtypes in all compartments. We therefore established 2 additional in vivo experimental scenarios. To measure LV access to HSPC in the peripheral blood (PB), we followed long-term NBSGW mice where BaEVTR LV was dosed intravenously (IV) immediately after HSPC infusion. In this model we achieved targeting of 23% early engrafting BM HSPC which was mirrored by a ~20% transduction in PB myeloid cells during the first 6 weeks, later stabilizing at 5-7% from week 8 up to week 16. To then measure access to HSPC in the BM niche, we characterized early human HSPC dynamics in the BM of NBSGW mice along 5 timepoints over the first 19 days after CD34+ cell infusion, with or without AMD3100/G-CSF mobilization. We then dosed BaEVTR LV at D7 post-humanization, a timepoint when we established that engrafted human cells have a more physiological lineage distribution, are few and remained confined in the BM even upon mobilization. In this model we demonstrated that a single BaEVTR LV dose injected IV can reach 5-6.4% of BM-resident HSPC. Based on these data, we developed a BaEVTR VLP carrying a CRISPR/Cas9 editor targeting the B2M locus. This VLP displayed 93% editing efficiency in vitro in resting HSPC and 38 to 61% in FACS-sorted HSC at MOI 16 and MOI 83 respectively. When dosed in vivo in the D7 post-humanization model, our VLP achieved B2M KO in 16% and 31% BM-resident Lin-CD34+ cells with 1 and 3 consecutive IV injections respectively, including 14% of phenotypic HSC. Importantly, a comparable level of in vivo editing of BM Lin-CD34+ cells (12%) was achieved upon equivalent pilot single VLP dosing in long-term humanized mice despite them carrying substantially higher numbers of human HSPC in the BM and a complete and stable PB human blood cell engraftment. Secondary transplantation experiments to confirm editing of long-term HSC are ongoing. We proved delivery of genetic payloads to resting HSPC in vitro and access to HSPC in different compartments in vivo with high efficiency. Adding a specific tropism for HSPC has the potential to enhance on-target bioavailability impacting effectiveness and safety of our delivery systems. To this aim, we are now testing LV and VLP carrying HSPC-targeted envelopes engineered with viral fusion proteins (retargetable fusogens). We will present comprehensivedata showing that a LV pseudotyped with a CD133-targeted fusogen combines in vitro and in vivo potency on par with BaEVTR LV with high specificity for CD133+ cells, as seen upon titrations on 8 cell lines and 10 primary human cell types as well as in spike-in experiments aimed at measuring CD133+ HSPC transduction when diluted in a mixture of CD133- human blood cells in vitro and in NBSGW mice. Furthermore, we will show that a CD117-targeted LV can transduce, in vivo, a cell population exclusively located in the BM and comprising as little as 0.3% of total huCD45+ cells with a 175-fold increased specificity as compared to envelopes with broad specificity. These results set the basis for generating a potent candidate for in vivo delivery of genetic payloads to HSPC with high efficiency and specificity.

Unraveling Pathophysiology and Hematopoiesis of Vexas Syndrome By Multi-Omics Analyses and Targeted Gene Editing

Background and Rationale VEXAS (vacuoles, E1 enzyme, X-linked, autoinflammatory, somatic) syndrome is a newly described, treatment-refractory, severe, highly prevalent disease (~1:4,000 males aged &amp;gt;50 years) caused by somatic mutations in the UBA1 (ubiquitin-activating enzyme 1) gene of hematopoietic stem/progenitor cells (HSPCs). VEXAS is hallmarked by vacuoles in myeloid/erythroid precursors and presents with systemic inflammation. About 50% patients develop hematologic malignancies, including myelodysplasia, contributing to the poor prognosis. The pathophysiology and impact of VEXAS syndrome on human hematopoiesis remain elusive. Moreover, the lack of suitable disease models hampers investigation of disease development and preclinical drug testing. To fill these gaps, here we: i) integrated multiparametric flow cytometry and multi-omics analyses on VEXAS patients to achieve comprehensive molecular and phenotypic characterization of hematopoiesis; and ii) successfully developed in vitro and in vivo models of VEXAS syndrome by cutting-edge targeted base editing strategies. Methods Multiparametric immunophenotypic analyses and single-cell transcriptomics were performed on peripheral blood and bone marrow (BM) cells from six VEXAS patients (p.Met41&amp;gt;Thr; p.Met41&amp;gt;Val; p.Met41&amp;gt;Leu; c.118-1 G&amp;gt;C) recruited by our Unit. Circulating monocytes were analyzed by whole RNA-sequencing (RNA-seq) and metabolomics. UBA1 mutations were introduced by gene editing technologies in myeloid cell lines, primary T cells and healthy human HSPCs. Engineered HSPCs were used to reconstitute the hematopoietic compartment of immunodeficient mice to generate a humanized VEXAS model. Variant allele frequency of UBA1 mutant cells was quantified by targeted sequencing in isolated hematopoietic lineages and HSPCs. Results i) Multiparametric immunophenotypic analyses of BM cells showed unbalanced composition of the HSPC compartment in VEXAS patients compared to age-matched healthy individuals, with lower abundance of stem cells, multipotent and lymphoid progenitors and expanded myeloid progenitors. This was paralleled by a significant increase of circulating myeloid-biased HSPCs and immature myeloid cells. Transcriptomic analyses of patient-derived peripheral monocytes showed sustained VEXAS-associated inflammatory signatures and heightened glycolysis, which were confirmed by metabolomic profiling. Single-cell transcriptomics on the BM hematopoietic compartment of VEXAS patients harboring different UBA1 mutations displayed genotype-dependent gene expression patterns. Single-cell RNA-seq of peripheral blood mononuclear cells from VEXAS patients is ongoing. ii) In vitro and in vivo models of VEXAS, generated by UBA1 gene editing, faithfully recapitulated patient hematopoiesis and pathophysiology . Editing efficiency approached 90% in wild-type myeloid cell lines, primary human T cells and HSPCs. While UBA1-mutant T cells were counterselected over time in culture, myeloid ones were enriched. Colony forming unit assays of UBA1-edited HSPCs revealed an exclusive myeloid output in vitro and a significant loss of the erythroid HSPC differentiation potential compared to mock-edited controls. Transplantation of edited HSPCs in immunodeficient mice resulted in a 100-fold reduction in circulating B cells, while NK and myeloid compartments were preserved. Human BM HSPCs were 5-fold lower than in control mice, largely myeloid-biased, and displayed abnormal vacuolar morphology. Concordantly, while myeloid cells and HSPCs were mainly (&amp;gt;80%) UBA1 mutated, the lymphoid compartment showed predominance of wild-type alleles. Strikingly, the distribution of UBA1-mutant cells across lineages in humanized mice mirrored the composition of our VEXAS patient cohort, where UBA1-mutant cells accounted for 90% of total myeloid cells and HSPCs and for less than 5% of lymphoid lineages. Conclusions Our findingsprovide evidence thatmutations in UBA1 drive accumulation of myeloid-biased HSPCs and immature myeloid precursors. Mutant lymphoid cells are negatively selected and their peripheral myeloid counterpart displays inflammatory activation. Gene editing-based in vitro and in vivo models phenocopy most hematopoietic features of the disease and hold promise to enable preclinical testing and validation of novel therapeutics to treat VEXAS syndrome.

Unveiling the Biological Role of Peripheral Blood Human Circulating Hematopoietic Stem and Progenitor Cells

Although most hematopoietic stem/progenitor cells (HSPC) reside in the bone marrow (BM), few circulating HSPC (cHSPC) can be also found in the peripheral blood (PB) at steady state. The biological role and relationship with BM-resident counterpart in humans remain still not fully elucidated. In the present study, we phenotypically characterized cHSPC composition by applying multi-parametric flow cytometry on 110 PB and, as control, 48 BM samples of healthy donors (HD) of different age groups. These analyses were combined with cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) and ad hoc designed in vitro and in vivo assays to investigate the transcriptional and functional properties of steady-state cHSPC subpopulations with respect to BM counterpart. Moreover, to study circulating vs. resident HSPC differentiation potential in vivo in humans, we exploited integration site (IS) clonal tracking of cHSPC, BM HSPC and mature PB lineages isolated from patients treated with autologous HSPC-gene therapy (GT) at late follow-ups post-treatment (>2 years), once steady-state hematopoiesis is established. We observed that cHSPC show a progressive reduction in number during aging and a different composition than BM counterpart, with Multi Lymphoid Progenitors (MLP) displaying the highest PB circulation propensity. cHSPC were endowed with BM homing capability and multilineage differentiation potential both in vitro and in vivo, showing an early skewing toward erythroid differentiation after transplantation. Moreover, we detected a reduced long-term human cell engraftment in cHSPC- than BM HSPC-transplanted mice. This latter finding might be explained by the low primitive Hematopoietic Stem Cell (HSC) content and the transcriptional pre-activated state observed in steady-state PB HSC, as assessed in our single-cell RNA-sequencing analyses. Indeed, applying CITE-seq on PB- and BM-derived HSPC, we identified a unique transcriptional profile of both primitive and lineage-committed cHSPC subpopulations, characterized by lower replicative, metabolic and transcriptional activity, but increased differentiation-, adhesion- and immune response-priming than BM counterpart. Furthermore, we observed that PB phenotypic primitive HSC and Multipotent Progenitors (MPP) and Committed Myeloid Progenitors (CMP) were transcriptionally and functionally skewed toward erythroid lineages as compared to BM-resident counterpart. Finally, since upon gene correction each HSPC and its progeny became univocally marked by distinct IS, we exploited IS analyses from 5 patients treated with HSPC-GT at >2 years after GT to investigate the steady-state hematopoietic output of PB and BM HSPC in vivo in humans. In line with the enrichment of lymphoid transcriptional signature observed in trafficking HSPC, we found a higher IS sharing between PB mature lymphoid compartment and steady-state PB-derived HSPC. Moreover, a substantial fraction of trafficking HSPC displayed an enriched expression of the gene signature associated with thymus seeding progenitor type 1 (TSP1), a group of low-cycling immature lymphoid progenitors with thymus-emigrant properties. Although at lower level with respect to PB cells, we detected a TSP1 signature also in BM dataset, thus suggesting that few thymus-emigrant lymphoid progenitors originate in the BM and preferentially egress into the PB in order to seed lymphoid organs. Altogether, our findings indicate PB trafficking HSPC as a peculiar steady-state reservoir of low-cycling, pre-activated hematopoietic progenitors, which are endowed with BM homing and repopulation potential. The enrichment of phenotypic and functional erythroid-committed progenitors in cHSPC suggest a supportive role of these cells in sustaining extramedullary erythropoiesis. Furthermore, our results of IS analyses combined with the higher expression of TSP1 signature in PB vs. BM HSPC indicate the key function of steady-state trafficking HSPC in the seeding of the thymus, with the aim of locally differentiating into lymphoid progeny. Overall, our work represents one of the most comprehensive studies on cHSPC, unveiling fundamental insights on their biological role in humans.

Nociceptive Nerves Facilitate Hematopoietic Stem/Progenitor Cell Homing to Bone Marrow By Tuning Microcirculation

The homing process of hematopoietic stem/progenitor cells (HSPCs) to bone marrow (BM) is critical for engraftment after BM transplantation (BMT). Nociceptive nerves in BM reportedly regulate the mobilization of HSPCs from BM by directly supplying calcitonin gene-related peptide (CGRP) to HSPCs. However, it remains unclear whether nociceptive nerves control HSPC dynamics other than egress from BM, e.g., homing to BM. Here, we show that nociceptive nerves facilitate HSPCs homing to BM by controlling microcirculation. To assess the effects of CGRP on the BM microenvironment, we immunostained BM sections of mice and found that BM arteries, but not sinusoids, expressed CGRP receptors. We next employed intravital imaging system that visualized calvarial BM vessels and combined it with the transcalvarial administration (TCA), a method that enables manipulation of local BM microcirculation. TCA of CGRP receptor antagonist constricted BM arteries and decreased sinusoidal flux and shear stress, that impeded transendothelial migration (TEM) of transplanted HSPCs into BM, suggesting that CGRP released from nociceptive nerves supports local BM microcirculation and facilitates TEM of transplanted HSPCs into BM. In addition, systemic administration of CGRP receptor antagonist and pharmacological and genetic ablation of nociceptive nerves significantly decreased the homing efficiency of transplanted HSPCs to BM. Moreover, CGRP administration restored homing efficiency in nociceptive denervated-mice, and improved homing efficiency of wild-type mice. We next examined whether the improved homing efficiency of HSPCs with CGRP administration was mediated via arteries or via hematopoietic cells. CGRP is sensed by a heterodimeric CGRP receptor composed of calcitonin receptor-like receptor (CALCRL) and receptor activity modifying protein 1 (RAMP1), and hematopoietic cells reportedly express both CALCRL and RAMP1. We prepared Ramp1-/- mice to examine the homing efficiency of Ramp1-deficient hematopoietic cells. We found that the homing efficiency of hematopoietic cells from Ramp1-/- mice was not impaired upon transplantation into wild-type recipients. These results suggest that CGRP acts on the BM microenvironments, not on hematopoietic cells, to improve the homing efficiency. To determine the relationship between homing control by nociceptive nerves and survival after transplantation, we analyzed survival after BMT in mice in which nociceptive nerves were denervated. We found that pharmacological and genetic denervation of nociceptive nerves in recipient mice significantly reduced survival after BMT. Finally, to determine whether the intake of spicy food that activates nociceptive nerves enhances the efficiency of BMT, mice were fed with food containing capsaicin and subjected to homing experiments. HSPC homing efficiency to BM of mice fed with capsaicin diet was significantly improved compared to that of mice with normal diet. This study reveals a novel function of nociceptive nerves in regulating BM microcirculation and HSPC homing, and suggests that nociceptive nervous system could be a new therapeutic target for improving BMT efficiency.

Frizzled-6 promotes hematopoietic stem/progenitor cell mobilization and survival during LPS-induced emergency myelopoiesis

SummaryEmergency hematopoiesis involves the activation of bone marrow hematopoietic stem/progenitor cells (HSPCs) in response to systemic inflammation by a combination of cell-autonomous and stroma-dependent signals and leads to their release from bone marrow and migration to periphery. We have previously shown that FZD6 plays a pivotal role in regulating HSPC expansion and long-term maintenance. Now we sought to better understand the underlying mechanisms. Using lipopolysaccharide (LPS)-induced emergency granulopoiesis as a model, we show that failed expansion was intrinsic to FZD6-deficient HSPCs but also required a FZD6-deficient environment. FZD6-deficient HSPCs became more strongly activated, but their mobilization to peripheral blood was impaired and they were more susceptible to inflammatory cell death, leading to enhanced release of pro-inflammatory cytokines in the marrow. These studies indicate that FZD6 has a protective effect in the bone marrow to prevent an overactive inflammatory response and further suggest that mobilization improves HSPC survival during bone marrow inflammation.

Efficient expansion of rare human circulating hematopoietic stem/progenitor cells in steady-state blood using a polypeptide-forming 3D culture

Although widely applied in treating hematopoietic malignancies, transplantation of hematopoietic stem/progenitor cells (HSPCs) is impeded by HSPC shortage. Whether circulating HSPCs (cHSPCs) in steady-state blood could be used as an alternative source remains largely elusive. Here we develop a three-dimensional culture system (3DCS) including arginine, glycine, aspartate, and a series of factors. Fourteen-day culture of peripheral blood mononuclear cells (PBMNCs) in 3DCS led to 125- and 70-fold increase of the frequency and number of CD34+ cells. Further, 3DCS-expanded cHSPCs exhibited the similar reconstitution rate compared to CD34+ HSPCs in bone marrow. Mechanistically, 3DCS fabricated an immunomodulatory niche, secreting cytokines as TNF to support cHSPC survival and proliferation. Finally, 3DCS could also promote the expansion of cHSPCs in patients who failed in HSPC mobilization. Our 3DCS successfully expands rare cHSPCs, providing an alternative source for the HSPC therapy, particularly for the patients/donors who have failed in HSPC mobilization.

Persistent Cutaneous Leishmania major Infection Promotes Infection-Adapted Myelopoiesis.

Hematopoietic stem/progenitor cells (HSPC) are responsible for the generation of most immune cells throughout the lifespan of the organism. Inflammation can activate bone marrow HSPCs, leading to enhanced myelopoiesis to replace cells, such as neutrophils, which are attracted to inflamed tissues. We have previously shown that HSPC activation promotes parasite persistence and expansion in experimental visceral leishmaniasis through the increased production of permissive monocytes. However, it is not clear if the presence of the parasite in the bone marrow was required for infection-adapted myelopoiesis. We therefore hypothesized that persistent forms of Leishmania major (cutaneous leishmaniasis) could also activate HSPCs and myeloid precursors in the C57Bl/6 mouse model of intradermal infection in the ear. The accrued influx of myeloid cells to the lesion site corresponded to an increase in myeloid-biased HSPCs in the bone marrow and spleen in mice infected with a persistent strain of L. major, together with an increase in monocytes and monocyte-derived myeloid cells in the spleen. Analysis of the bone marrow cytokine and chemokine environment revealed an attenuated type I and type II interferon response in the mice infected with the persistent strain compared to the self-healing strain, while both strains induced a rapid upregulation of myelopoietic cytokines, such as IL-1β and GM-CSF. These results demonstrate that an active infection in the bone marrow is not necessary for the induction of infection-adapted myelopoiesis, and underline the importance of considering alterations to the bone marrow output when analyzing in vivo host-pathogen interactions.

Diaphanous-Related Formin Protein mDia2 Regulates Hematopoietic Stem and Progenitor Cell Functions through Srf-Integrins Pathway

Distinct mature cell types in mammalian blood system are derived from the Hematopoietic Stem/Progenitor cells (HSPCs), which are multipotent progenitor cells with self-renewing and engraftment ability. Various intrinsic and extrinsic molecules modulate HSPCs' fates to either undergo self-renewal or differentiation. The Diaphanous-related formin protein mDia2, is an effector of Rho GTPases. Previously, we reported that hematopoietic-specific mDia2 knock out mice developed anemia due to ineffective erythropoiesis caused by bi-nucleated erythroblasts. However, how mDia2 is involved in HSPC functions are unknown. Here we show that mDia2 expression is enriched in HSPCs. Although mDia2 was largely dispensable for HSPCs' function at steady-state, hematopoietic-specific inactivation of mDia2 severely impaired the competitive repopulation capacity of HSPCs. After primary bone marrow transplantation, the absolute number of bone marrow HSPCs from mDia2 knockout (KO) transplants was significantly decreased. mDia2 deficient LSK cells also showed skewed differentiation with increased percentage of more differentiated multi-potent progenitor cells (MPP), which was further supported by loss of stem cell quiescence. Under serial transplantation, mice transplanted with mDia2 knockout (KO) bone marrow showed rapid lethality. Importantly, these defects can be rescued by the constitutively active mDia2 mutant mDia2-ΔDAD. Furthermore, mDia2 KO HSPCs showed a marked reduction in the motility ability from bone marrow to the peripheral blood upon G-CSF stimulation. Loss of mDia2 also severely compromised the recovery of the mice from 5-FU-induced myelosuppression.mDia2 functions to nucleate linear actin filaments. We found that loss of mDia2 expression in HSPCs reduced the F-actin levels. This further attenuated the adhesion of HSPCs that was confirmed by adhesion assay and decreased protrusion formation in vitro . We demonstrated that the defect in F-actin formation caused by mDia2 deficiency repressed the transcriptional activity of serum response factor (SRF), which caused down-regulation of SRF target genes such as Acta2, Actb and SRF itself. Ectopic expression of SRF rescued HSPC phenotypes with loss of mDia2. We further identified that the expression levels of several integrins, including Itga2, Itgam (encodes CD11b) , Itagal (encodes CD11a)and Itgb2 (encodes CD18) , were significantly attenuated in mDia2 KO c-Kit+ HSPCs. Chromatin immunoprecipitation (ChIP) sequencing data further revealed intronic SRF binding sites in the genomic loci of Itgam and Itgb2, therefore suggesting Itgam and Itgb2 as new downstream targets of SRF pathway. Our data revealed a novel mDia2-SRF-integrin axis that regulates HSPCs adhesion and engraftment. DisclosuresNo relevant conflicts of interest to declare.

Glucose oxidase induces mobilization of long-term repopulating hematopoietic cells in mice.

Hematopoietic stem progenitor cells (HSPCs) mobilized to peripheral blood, rather than those remaining in the bone marrow (BM), are commonly used as stem cell source in the clinic. As reactive oxygen species (ROS) are suggested as mediator of HSPC mobilization, we examined the impacts of glucose oxidase (GO) on peripheral mobilization of BM HSPCs and the associated mechanisms. Intravenous injection of GO induced HSPC mobilization even by single treatment, and the GO‐mobilized cells maintained their long‐term reconstituting and differentiating potentials in conditioned recipients. GO‐injected mice lived a normal life without adverse effects such as stem cell senescence, hematopoietic disorders, and blood parameter alteration. The mobilization effect of GO was even evident in animal models showing poor mobilization, such as old, 5‐fluorouracil‐treated, or alendronate‐treated mice. Importantly, combined injection of GO with granulocyte colony‐stimulating factor (G‐CSF) and/or AMD3100 enhanced more greatly HSPC mobilization than did G‐CSF, AMD3100, or both. The GO‐stimulated HSPC mobilization was almost completely attenuated by n‐acetyl‐L‐cysteine treatment. Collectively, our results not only highlight the potential role of GO in HSPC mobilization via ROS signaling, but also provide a GO‐based new strategy to improve HSPC mobilization in poorly mobilizing allogeneic or autologous donors via combination with G‐CSF and/or AMD3100.

Efficient Expansion of Human Rare Circulating Hematopoietic Stem/Progenitor Cells in Steady-State Blood Using a Polypeptide-Forming 3D Culture

Although widely applied in treating hematopoietic malignancies, transplantation of hematopoietic stem/progenitor cells (HSPCs) is impeded by HSPC shortage. Whether circulating HSPCs (cHSPCs) in steady-state blood could be used as an alternative source remains largely elusive. Here we develop a three-dimensional culture system (3DCS) including arginine, glycine, aspartate, and a series of factors. Culture of peripheral blood mononuclear cells (PBMNCs) in 3DCS led to 125- and 70-fold increase of the frequency and number of CD34+ cells. Further, 3DCS-expanded cHSPCs exhibited 1.65-fold increase in the reconstitution rate compared to CD34+ HSPCs in bone marrow. Mechanistically, 3DCS fabricated an immunomodulatory niche, secreting cytokines as TNF-α to support cHSPC survival and proliferation. Finally, 3DCS could also promote the expansion of cHSPCs in patients who failed in HSPC mobilization. Our 3DCS successfully expands rare cHSPCs, providing an alternative source for the HSPC therapy, particularly for the patients/donors who have failed in HSPC mobilization.

Bone Marrow-Derived Mesenchymal Stromal Cells: A Novel Target to Optimize Hematopoietic Stem Cell Transplantation Protocols in Hematological Malignancies and Rare Genetic Disorders.

Mesenchymal stromal cells (MSCs) are crucial elements in the bone marrow (BM) niche where they provide physical support and secrete soluble factors to control and maintain hematopoietic stem progenitor cells (HSPCs). Given their role in the BM niche and HSPC support, MSCs have been employed in the clinical setting to expand ex-vivo HSPCs, as well as to facilitate HSPC engraftment in vivo. Specific alterations in the mesenchymal compartment have been described in hematological malignancies, as well as in rare genetic disorders, diseases that are amenable to allogeneic hematopoietic stem cell transplantation (HSCT), and ex-vivo HSPC-gene therapy (HSC-GT). Dissecting the in vivo function of human MSCs and studying their biological and functional properties in these diseases is a critical requirement to optimize transplantation outcomes. In this review, the role of MSCs in the orchestration of the BM niche will be revised, and alterations in the mesenchymal compartment in specific disorders will be discussed, focusing on the need to correct and restore a proper microenvironment to ameliorate transplantation procedures, and more in general disease outcomes.

Abstract 100: Mas Receptor Expression in Bone Marrow Stem/Progenitor Cells is Both Required and Sufficient for the Mobilization of Cells in Response to Ischemic Injury

Bone marrow-derived hematopoietic stem/progenitor cells (HSPCs) are mobilized into the circulation in response to ischemic injury and stimulate vascular regeneration. Angiotensin-(1-7) (Ang-(1-7)) activates vasoprotective functions in progenitor cells by activating Mas receptor (MasR). We have shown that mobilization of progenitor cells in response to ischemic injury is impaired in MasR -/- mice (KO) and that blood flow recovery is reduced. This study tested if MasR expression in the bone marrow-resident HSPCs is sufficient to rescue mobilization in MasR -/- mice. Bone marrow chimeric mice were made by lethal irradiation of wild type (WT) or MasR -/- mice followed by transplantation with bone marrow derived from either WT or KO mice. Circulating lineage-negative Sca-1 + and cKit + (LSK) cells were enumerated in basal or in response to the treatment with plerixafor (5 mg/Kg, ip) or G-CSF (125 mg/Kg, sc, twice-a-day, four days). Hind-limb ischemia (HLI) was induced by femoral artery ligation and the circulating LSK cells were monitored over time. Either WT or MasR -/- mice transplanted with MasR -/- bone marrow have shown 100% mortality following irradiation (n=6). Circulating LSK cells are lower in MasR -/- mice (14±5 cells/mL of blood) compared to WT (39±7/mL, P&lt;0.01, n=6). On the other hand, circulating cells were comparable in WT→MasR -/- (20±3/mL blood) and WT→WT chimeras (31±6/mL, n=6). Mobilization of LSK cells by plerixafor (5 mg/Kg, ip) or G-CSF (125 mg/Kg, sc., twice-a-day, four days) are lower in MasR -/- mice (4- and 2-fold, respectively, compared to WT, P&lt;0.05, n=6). In contrast, WT→MasR -/- or WT→WT chimeras responded similarly to either of the mobilizers, and responses were comparable to that observed in WT mice. Mobilization of LSK cells in response to HLI is attenuated to a maximum of 5-fold on day-5 post-HLI in MasR -/- mice (vs 10-fold in WT on day-3 post-HLI, P&lt;0.05 n=5). This response was rescued to a larger extent in WT→MasR -/- chimeras (7-fold, compared to 9-fold in WT→WT chimeras, P&lt;0.05, n=5). These studies infer that MasR -/- HSPCs cannot rescue WT or MasR -/- mice following lethal irradiation, and that MasR expression in the bone marrow HSPCs is sufficient and required for mobilization of cells in response to mobilizers or to an ischemic injury.

Epidermal Growth Factor Receptor - Mediated DNA Repair Regulates Hematopoietic Stem Cell Regeneration

Hematopoietic myelosuppression is the principal dose-limiting toxicity of curative chemotherapy and radiotherapy in patients with cancer. Both chemotherapy and radiotherapy deplete hematopoietic stem cells (HSCs) via induction of DNA damage. DNA damage contributes to both HSC dysfunction and risk for malignant transformation over time. Extrinsic signals capable of promoting DNA repair in HSCs following injury can potentially improve HSC function and may decrease risk for dysplasia and leukemia over time. Here, we show that treatment with epidermal growth factor (EGF) decreases DNA damage in murine HSCs following irradiation via activation of the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and nonhomologous end joining (NHEJ) repair. Specifically, we observed that in vitro treatment of irradiated bone marrow (BM) ckit+sca-1+lin- (KSL) hematopoietic stem/progenitor cells (HSPCs) with 100 ng/ml EGF decreased HSPC DNA damage at 1 hour as measured by gamma-H2AX foci and Comet assay (p<0.0001, p<0.0001). Further, EGF treatment increased phosphorylation and nuclear localization of DNA-PKcs, which is integral to NHEJ repair, and increased phosphorylation of Akt in irradiated HSPCs. Inhibition of Akt or DNA-PKcs completely abrogated EGF - mediated DNA repair and EGF - mediated recovery of HSPCs following irradiation in vitro. In irradiated (500 cGy) C57BL/6 mice, subcutaneous administration of 10 μg/day of EGF significantly increased recovery of peripheral blood white blood cells and lymphocytes (p=0.03, p=0.04), as well as BM SLAM+KSL cells (HSCs) at day +14 (p=0.0004). Treatment with the DNA-PKcs inhibitor, NU7441, abrogated EGF - mediated recovery of peripheral blood WBCs in irradiated mice (p=0.007), suggesting that EGF - mediated mitigation of radiation injury in vivo was dependent on DNA-PKcs - mediated NHEJ repair. Importantly, EGF treatment of irradiated mice also substantially increased the survival of irradiated mice compared to irradiated control mice (p=0.009) and caused the recovery of long-term HSCs capable of 20 week competitive repopulation in congenic recipient mice (p=0.01, 20 weeks).Conversely, doxycycline inducible, cell - specific suppression of EGFR in hematopoietic cells using SCL-tTA;EGFR-DN mice caused a significant depressed regeneration of BM SLAM+KSL cells and KSL cells at day +10 after irradiation (p=0.005, p=0.02, respectively). SCL-tTA;EGFR-DN mice also displayed significantly decreased survival at day +30 compared to EGFR wild type mice (23.1% survival vs. 58.3% survival, p=0.04). Consistent with our hypothesis, BM HSPCs from EGFR-DN mice displayed increased DNA damage in response to total body irradiation, as measured by gamma H2AX foci and the Comet assay (p<0.0001, p=0.04). Coordinately, BM HSPCs from EGFR-DN mice displayed decreased phosphorylation of Akt and DNA-PKcs in response to irradiation in vitro (p<0.0001, p=0.004).These studies suggest that EGFR - mediated DNA repair is necessary for HSC and progenitor cell recovery following irradiation and that EGF treatment accelerates hematopoietic regeneration via augmentation of NHEJ repair in HSCs. EGF treatment following high dose chemotherapy and/or radiotherapy may increase HSC function and lessen near - term hematologic toxicities and morbidities. DisclosuresNo relevant conflicts of interest to declare.

Role of RasGRP3 in EPO/EPOR Signaling and Transmigration of Human Hematopoietic CD34+ Cells

Introduction: Umbilical cord blood (UCB) is a salient source of primitive hematopoietic stem progenitor cells (HSPCs) for bone marrow (BM) reconstitution in patients with hematologic and non-hematologic malignancies. However, a relatively low number of HSPCs in UCB units and poor BM homing efficiency greatly hinders the clinical application of UCB CD34+ cells for transplantation. To overcome these hurdles, we developed two independent strategies that increase CD34+ cell numbers and improve BM homing efficiency of UCB HSPCs. First, we expanded UCB HSPCs by culturing them in decellularized Wharton's jelly matrix (DWJM), a biometric scaffold mimicking the 3-dimenstional (3D) microenvironment of BM. Second, we enhanced the in vitro transmigration and in vivo BM homing efficiency of UCB CD34+ cells by blocking EPO/EPOR signaling. Both approaches enhance UCB CD34+ cell migration toward stromal cell-derived factor 1 (SDF1). In this study, we employed RNA-Seq and RT-PCR approaches to analyze UCB HSPCs treated with EPO and co-cultured with DWJM, aiming to identify molecules that regulate UCB HSPC transmigration via EPO/EPOR signaling.Methods: CD34+ cells from highly enriched UCB units (>90% purity) were treated with EPO for 24 hours and separately co-cultured with DWJM for 1 week. UCB CD34+ cells were collected and subjected to RNA-Seq and real-time PCR (RT-PCR) analyses. In vitro transmigration toward SDF-1 was assessed by transwell assay. To assess the involvement of RasGRP3 in UCB CD34+ cell mobility, cells were treated with 100 nM ingenol-3-angelate (I3A), a diacylglycerol (DAG) analog that specifically targets RAS Guanyl Releasing-Protein 3 (RasGRP3), for 16 hours followed by transwell assay. Anti-EPOR antibody-treated or EPO-treated cells were used as controls. In addition, RasGRP3 gene expression was examined in CD34+ cells from peripheral blood (PB) and BM samples collected from the same donor, and compared to RasGRP3 expression in UCB CD34+ cells. Unpaired, 2-tailed t-test was used to analyze results.Results: RasGRP3 was identified by RNA-Seq from the two independent approaches, EPO treatment and DWJM co-culture. EPO downregulated and DWJM upregulated RasGRP3 gene expression in UCB CD34+ cells. RasGRP3 expression was confirmed by qPCR. UCB CD34+ cells that migrated to the bottom chamber of the transwell assays, a population that has a higher mobility, showed an elevated RasGRP3 gene expression and a decreased EPOR cell surface expression. Activation of RasGRP3 by DAG analog I3A induced a significant increase in RasGRP3 gene expression (control: I3A treatment = 1: 202 ± 58, p=0.00012) that was associated with an enhanced transmigration capability (control: I3A = 41%+/-5: 54%+/- 3, p=0.032). Knocking-down of RasGRP3 in K562 cells, a known EPOR expressing cell line, impaired the transmigration capability of K562. CD34+ cells in peripheral blood (PB) showed a higher level of RasGRP3 gene expression compared to CD34+ cells in BM samples from the same healthy donors. RasGRP3 expression in PB CD34+ cells was significantly higher than BM and UCB CD34+ cells (qPCR signals relative to BM, BM: PB: UCB = 1: 431±65: 21±8, p=0.0012, 0.0023, and <0.0001 for BM vs. PB, BM vs. UCB and PB vs. UCB, respectively).Conclusions: By employing transwell assays, flow cytometry and molecular analyses, we demonstrate for the first time that RasGRP3, a protein responsible for GDP/GTP exchange of Ras, regulates the transmigration ability of human CD34+ cells. In addition, our findings connect RasGRP3 expression to the EPOR-mediated signaling pathway in CD34+ cells. A significantly higher level of RasGRP3 expression in PB CD34+ cells than its counterparts in BM might provide an explanation for why PB HSPCs show relatively faster BM engraftment than BM HSPCs during transplantation. Ongoing follow-up studies will elucidate the molecular mechanism(s) underlying EPOR signaling, which holds clinical potential to improve the BM homing deficiency of UCB CD34+ cells via modulating EPOR and RasGRP3 expression (Figure 1). [Display omitted] DisclosuresLiesveld:Onconova: Other: DSMB; Abbvie: Honoraria. Aljitawi:Medpace: Consultancy; The University of Rochester Medical Center: Patents & Royalties: Pending patent related to decellularized Wharton's jelly matrix.

Single-Nucleotide Human Disease Mutation Inactivates a Blood-Regenerative Enhancer

Coding and regulatory GATA2 mutations that deregulate protein expression and/or function cause MDS/AML (McReynolds et al., 2018). In the mouse, decreased GATA-2 expression impairs hematopoietic stem/progenitor cell (HSPC) genesis and function. While prior studies demonstrated the requirement for Gata2 +9.5 and -77 enhancers to regulate GATA-2 and HSPC transitions in vivo (Gao et al., 2013; Johnson et al., 2012; Johnson et al., 2015; Mehta et al., 2017; Sanalkumar et al., 2014), nothing is known about the contribution of the individual motifs within these enhancers for GATA-2 expression, hematopoiesis and hematopoietic regeneration.To elucidate mechanisms conferring +9.5 enhancer activity that regulate HSPC genesis and function, we used CRISPR/Cas9 technology to generate cis-element-mutant mouse strains. The first strain recapitulates human disease mutations and involves corruption of the E-box and a C>T transition within an Ets site (+9.5(E-box;Ets)), while retaining a GATA motif. The second strain contains the Ets 1017+572C>T transition (+9.5(Ets)), representing the most frequent noncoding GATA2 mutation in human patients (Ganapathi et al., 2015; Hsu et al., 2013). We compared the phenotypes of these strains with our previously generated +9.5(E-box;GATA)-/- strain (+9.5-/-) (Gao et al., 2013; Johnson et al., 2012) to ascertain how the +9.5 regulates hematopoietic processes. +9.5-/- embryos have nearly complete ablation of HSC emergence and die by E14. +9.5(E-box;Ets-/- embryos recapitulate +9.5 defects, indicating that neither the GATA nor Ets motifs suffice for +9.5 developmental functions. In contrast, +9.5(Ets)-/- mutant animals are born at normal Mendelian ratios - the sole example of a +9.5 mutation in the mouse that is not embryonic lethal. Mutant E15.5 embryos have minor decreases in fetal liver cell number (20%, P=0.023), no hemorrhaging or edema, and Gata2 mRNA is reduced 25-50% (P=0.006).+9.5(Ets)-/- adult mice exhibited normal steady-state hematopoiesis. To test if stress-dependent hematopoietic regeneration revealed Ets motif activity, 5-fluorouracil (5FU) was used to kill cycling cells and promote quiescent HSC proliferation. Two doses of 5FU (250 mg/kg) were administered at an interval of eleven days to stimulate maximal HSPC expansion (Xu et al., 2017). +9.5(Ets)-/- mice had a significantly reduced (P<0.0001) median survival of 14.5 days, versus 22 and 21 days for wild type (WT) and +9.5(Ets)+/-, respectively. We hypothesized that the hypersensitivity of +9.5(Ets)-/- mice to myeloablative stress was due to impaired HSC mobilization and/or recovery. To identify cell populations altered in +9.5(Ets)-/- bone marrow, we quantified immunophenotypic HSPCs in untreated and treated (9 and 11 days post-5FU) mice. In untreated mice, +9.5(Ets)-/- and WT HSC (Lin−Sca1+Kit+CD48-CD150+) numbers were similar. MPPs (Lin-Sca1+Kit+CD48-CD150-) were 9-fold lower in mutants. Nine and eleven days post-5FU treatment, HSCs increased 17- and 10-fold in WT (P<0.001 and P=0.003, respectively), but not +9.5(Ets)-/-, mice. To assess HSPC function, bone marrow was competitively transplanted into lethally-irradiated mice. Four months post-transplant, +9.5(Ets)-/- marrow was defective in multi-lineage repopulation (3-fold decrease in donor-derived cells from peripheral blood (P=0.0005), with significant reductions of HSPCs in bone marrow). These results demonstrate that the Ets motif confers Gata2 function as a key step in HSC regeneration.In summary, multiple cis-elements confer the developmental function of an essential enhancer, while a single cis-element mutated in disease confers regeneration in response to injury. Our results establish a paradigm in which the cis-element predisposition mutation, combined with a secondary insult, underlies pathogenesis and illustrates the power of micro-dissecting enhancers to unveil physiological and pathological mechanisms. Studies are underway to rigorously analyze the functional consequences of diverse secondary insults to identify conditions that cause bone marrow failure and hematologic malignancy. Comparing and contrasting developmental and regenerative genetic networks, including those operational in single cells from our unique models, will reveal broad principles and strategies to manipulate normal and malignant hematopoiesis, for example in the context of GATA-2-dependent pathologies. DisclosuresNo relevant conflicts of interest to declare.

Niche Heterogeneity Impacts Evolution of Myeloproliferative Neoplasms Driven By the Same Oncogenic Pathway

Different MPNs have distinct rates of malignant transformation (PMF>PV>ET). Although PV and ET can arise from hematopoietic stem/progenitor cells (HSPCs) with similarly activated JAK-STAT oncogenic pathway, the transformation rate into secondary myelofibrosis and leukemia is higher for PV than for ET. However, the underlying reasons are not fully clear. Whereas in some cases secondary mutations might cause transformation, it remains unclear whether distinct bone marrow (BM) microenvironments can influence the progression of MPNs or any preleukemic disorder. Previous studies have suggested that normal BM niches close to bone (endosteal) promote HSPC quiescence, whereas non-endosteal vessels permit transmigration of activated HSPCs. We hypothesized that PV and ET HSPCs might expand in different BM niches, which could thereby influence disease progression.To address this question, we performed combined two-photon and confocal real-time intravital microscopy in the skull BM of WT mice transplanted with HSPCs from WT mice or MPN mice carrying the same driver mutation (JAK2V617F) but showing phenotypically distinct diseases (ET or PV). ET HSPCs resembled WT HSPCs in their homing and engraftment near bone (22±15 and 26±13 μm, respectively) 3 days after i.v. injection into lethally-irradiated WT mice. In contrast, PV HSPCs located significantly further (31±21 μm) from bone, which was independently confirmed in a distinct PV model. The different homing of ET and PV HSPCs was similarly observed in non-irradiated WT recipients and was therefore independent of myeloablative conditioning. Following engraftment, ET and PV HSPCs preferentially expanded in endosteal and non-endosteal locations, respectively, over 7 months' follow up. Importantly, the asymmetric expansion of HSPCs in ET and PV was confirmed in human BM trephines. Human CD34+ HSPCs were significantly closer to bone in ET than in PV patients (86±2 vs. 109±6 μm). Together, these results suggest that mutated HSPCs preferentially occupy distinct BM niches in ET and PV.Murine ET and PV HSPCs also differed in their dynamic interactions with the microenvironment. ET HSPCs migrated significantly faster (1 μm/min) than PV HSPCs (0.8 μm/min) and covered longer tracks after 1 h (57±2 and 35±1 μm, respectively). Moreover, ET HSPCs (but not PV HSPCs) migrated faster when closer to bone, suggesting an exploratory strategy of ET HSPCs to find endosteal niches. Separation of endosteal and non-endosteal BM fractions revealed increased abundance of integrin β3+ HSPCs in the endosteal BM of ET mice carrying JAK2V617F or CALRdel52/del52 mutations, but not in different PV models (despite generally sharing oncogenic JAK-STAT activation). Furthermore, competitive gravity adhesion assay and interference reflection microscopy showed that ET HSPCs are more adhesive to ECM substrates of integrin β3, suggesting that integrin β3 might trigger endosteal lodging of ET HSPCs.Asymmetric HSPC expansion caused differential microenvironmental remodeling possibly explaining differences in the pathophysiology and secondary outcomes of ET and PV. Non-endosteal sinusoids were dilated in different PV (but not ET) models, whereas CD31hiSca1hi arterioles and aberrant bone-forming integrin β1+ blood vessels increased only in ET mice. Similarly, sinusoidal vessels showed increased coverage by the integrin β1 ligand laminin α4 only in ET mice. Consequently, increased bone (μCT), osteoblasts and osteoclasts were found in ET but not PV mice. Increased expression of vascular-derived bone-forming factors (such as Bmp1 and Dll4) downstream of endothelial laminin α4/integrin β1 signaling might trigger osteosclerosis in ET mice.Finally, we tested whether HSPC location might directly impact ET progression. In a separate study we found that β3-adrenergic receptor (AR) signaling regulates Cxcl12-dependent BM HSPC localization (ASH abstract ID 116015). Therefore, we transplanted WT or ET donor BM cells into WT and β3-AR KO mice. Measurement of peripheral blood counts over 28 weeks showed that the microenvironment lacking β3-AR significantly worsened thrombocytosis and leukocytosis in ET, which correlated with redistribution of HSPCs and their progeny away from bone towards central BM.Altogether, these results suggest that differential interactions with the microenvironment might impact disease progression in MPNs and possibly in other preleukemic disorders. DisclosuresMead:Cell Therapeutics: Consultancy; Bristol-Myers Squibb: Consultancy; ARIAD: Consultancy; Novartis: Consultancy, Honoraria, Research Funding, Speakers Bureau; Celgene: Research Funding; Elstar: Research Funding; Evotek: Research Funding.