Long-term Culture-initiating Cell Assays Research Articles

Transcriptomic Signature and Functional Abnormalities of Bone Marrow Mesenchymal Stromal Cells Mediate Disease Progression of Myelodysplastic/Myeloproliferative Neoplasms

Background Mesenchymal stromal cells (MSC) are vital components of the bone marrow (BM) microenvironment niche and regulate hematopoietic stem and progenitor cell (HSPC) activity. While HSPC harbor primary disease driver mutations, MSC in myelodysplastic syndrome and myeloproliferative neoplasm (MDS/MPN) have been shown to influence the emergence and selection of leukemic clones and disease progression. However, it remains unclear if there are specific functional characteristics or genetic signals of MSC that interact with HSPC at different stages of disease progression. We examined BM samples from MDS/MPN patients with high rate of leukemic transformation in less than 2 years (HR) versus those with stable disease for 5 years (SD), to investigate the phenotypic differentiation, functional characteristics, and transcriptomic profiles of MSC and the HSPC counterparts. Methods We obtained BM samples at diagnosis and follow-up from MDS/MPN patients including chronic myelomonocytic leukemia (CMML) treated at Singapore General Hospital. We studied 5 HR and 13 SD patient samples to compare against 5 healthy donor samples. MSC were derived in MesenCult-ACF media (StemCell Technologies), and phenotypic characterization was done after 1-2 passages. HSPC were subjected to co-culture expansion with MSC in MyeloCult-H5100 for 7 days, and long-term culture-initiating cell assays (LTC-IC) using MSC as feeder cells for 6 weeks. Total RNA extracted from MSC and HSPC were subjected to bulk and single-cell RNA sequencing (scRNA-Seq) (10X Genomics platform). Principal component analysis (PCA) was used for the uniform manifold approximation and projection (UMAP). Gene set enrichment analysis (GSEA) was performed on differentially expressed genes to examine canonical signaling pathways. Cellchat algorithm was applied to predict intercellular communication between MSC and HSPC using scRNA-Seq data. Results Baseline clinical variables for the international prognostic scoring system (IPSS) including age at diagnosis, myeloblast percentage and cytogenetics were not significantly different between HR and SD patients. Decreased osteogenic and chondrogenic differentiation, but relatively preserved adipogenic differentiation capacity was observed in all MDS/MPN MSC samples. A significantly lower yield of CD34+ CD38- primitive HSPC with myeloid skewing was observed in the co-culture experiments with MDS/MPN MSC, compared to healthy MSC. Pre-treatment of SD-MSC feeder with the hypomethylating agent 5-azacytidine improved the clonogenic potential of the corresponding co-cultured HSPC, but did not restore the HSPC colony yield with HR-MSC feeder. Unbiased PCA of bulk RNA-Seq data demonstrated greater clustering of HR-MSC, while SD and healthy MSC overlapped significantly. Genes associated with mesenchymal multipotency and differentiation (PODXL20, FOXQ122, SERPINA923) were generally downregulated in MDS/MPN MSC, likely attributed to promoter hypermethylation. Significant differential upregulation of CD74 and IL-6 expression was observed in HR-MSC, which suggested a uniquely enriched inflammatory signature. GSEA results showed that pathways associated with G2M cell cycle checkpoint, DNA repair, glycolysis, and response to azacytidine were suppressed in HR-MSC. UMAP on scRNA-Seq data of longitudinal MSC samples identified distinct clustering of HR-MSC at disease progression. Cell-cell interaction analysis predicted a strong communication between collagen-encoding genes (COL1A1 and COL1A2) in HR-MSC at diagnosis and CD44, a negative regulator of HSPC proliferation, and the interaction was further enhanced at disease progression (Figure 1). Ongoing analysis of downstream gene expression changes and secretome profile of HSPC in MDS/MPN MSC-based in-vivo scaffold models will be presented. Conclusion Together, our data point towards a degree of co-development and communication between MSC and HSPC during disease progression of MDS/MPN. Further characterization of the unique functional and transcriptomic signatures in MSC may help to optimize disease prognostication and identify novel therapeutic targets in MDS/MPN patients.

Role of Lama4 Expression in Bone Marrow Niche for the Progression and Treatment Response of Chronic Myeloid Leukemia

Although tyrosine kinase inhibitors (TKIs) have dramatically improved the treatment outcomes of chronic myeloid leukemia (CML), TKIs cannot eliminate CML-initiating stem cells (LSCs), leading to CML persistence and relapse post TKI-discontinuation. Thus, it is imperative to develop more effective therapeutic options to overcome the treatment challenges. Targeting the bone marrow (BM) niche has become one of the research focuses in drug discovery since increasing evidence suggests that BM niches protect CML LSCs. Laminin alpha4 chain (Lama4), a functional chain for several laminin isoforms, is widely expressed in bone marrow (BM). We have recently shown the critical impact of LAMA4 expression in the microenvironment for hematopoiesis regeneration including megakaryocyte maturation and acute myeloid leukemia progression in mice (Cai et al., Blood, 2022). Furthermore, we have by RNA sequencing shown downregulation of LAMA4 mRNA in freshly isolated BM mesenchymal stem cells (MSCs) from newly diagnosed patients with CML (Dolinska, Cai, Mansson, et al., Blood, 2023). However, the functional impact of the LAMA4 expression on CML LSCs remains to be investigated. We have here explored the role of LAMA4 in CML development and drug response by using both mouse models and primary patient BM samples. To first assess the in vivo impact of LAMA4 expression in BM niches on CML development, we transplanted BM cells (CD45.2) from Scl- tTA× TRE-BCR-ABL1 (BCR-ABL1) mice prior or following tetracycline withdrawal into irradiated Lama4+/+ and Lama4-/- CD45.1mice and analyzed CML progression and BCR-ABL1 LSC activity by flow cytometry and droplet digital PCR (ddPCR). Following transplantation of unfractionated BM cells from the BCR-ABL1 mice with CML induced by tetracycline withdrawal, Lama4-/- recipients showed significantly increased platelet counts in blood compared to Lama4+/+ recipients at 2-8 weeks post-transplantation (P=0.005 at week 2, P= 0.008, n=7-8, at week 8), which is in contrast to the impaired platelet recovery in non-transplanted Lama4-/- mice following sublethal irradiation. This suggests that Lama4 deficient microenvironment is favorable for platelet production from BCR-ABL1 CML cells. Consistent with this, megakaryocyte-erythrocyte progenitors (MEPs) derived from BCR-ABL1+ cells were significantly increased in Lama4 -/- BM at the endpoint (8 weeks, p=0.02, n=3). Megakaryocyte progenitors (MkPs) seemed to be increased in Lama4-/- recipient BM (P=0.05, n=3). Intriguingly, when LIN -SCA1 +KIT + CD150 + hematopoietic stem cells from BCR-ABL1 mice without prior tetracycline withdrawal were transplanted, the donor-derived MEPs were significantly reduced in Lama4-/- BM (P=0.02, n=4-6), which is accompanied with a trend of reduction in MkP but no changes in platelets. ddPCR analysis indicated that the ratio of BCR-ABL1 + cell fraction to total CD45.2 + cells (detected by FACS) seemed to be lower in Lama4-/- recipients (33.6%) than that in Lama4+/+ recipients (41.5%), raising a possibility that Lama4-/- niches is less suitable for maintenance of BCR-ABL1 + cells. Together, these data point to a possible differential regulation of CML stem and progenitor cells by the Lama4 expression. More work will be done to dissect the regulatory mechanisms through in vitro co-cultures, transplantation and confocal imaging. In parallel, the functional impact of LAMA4 on human CML LSC survival was evaluated by a revised long-term culture-initiating cell assay using CML patient derived MSCs. The preliminary result indicated that recombinant LAMA4 peptides appeared to inhibit CML patient BM-derived CD34 +CD38 - cell growth. In conclusion, LAMA4 is downregulated in BM MSCs and endothelial cells of CML patients at diagnosis. Lama4-/- microenvironment might act differentially in regulating BCR-ABL1 + hematopoietic stem and progenitor cell activity post-transplantation. However, more work is required to validate the finding and the effect of LAMA4 peptides on the megakaryocyte differentiation and maturation.

Characterization of the bone marrow niche in patients with chronic myeloid leukemia identifies CXCL14 as a new therapeutic option

AbstractAlthough tyrosine kinase inhibitors (TKIs) are effective in treating chronic myeloid leukemia (CML), they often fail to eradicate the leukemia-initiating stem cells (LSCs), causing disease persistence and relapse. Evidence indicates that LSC persistence may be because of bone marrow (BM) niche protection; however, little is known about the underlying mechanisms. Herein, we molecularly and functionally characterize BM niches in patients with CML at diagnosis and reveal the altered niche composition and function in these patients. Long-term culture initiating cell assay showed that the mesenchymal stem cells from patients with CML displayed an enhanced supporting capacity for normal and CML BM CD34+CD38– cells. Molecularly, RNA sequencing detected dysregulated cytokine and growth factor expression in the BM cellular niches of patients with CML. Among them, CXCL14 was lost in the BM cellular niches in contrast to its expression in healthy BM. Restoring CXCL14 significantly inhibited CML LSC maintenance and enhanced their response to imatinib in vitro, and CML engraftment in vivo in NSG-SGM3 mice. Importantly, CXCL14 treatment dramatically inhibited CML engraftment in patient-derived xenografted NSG-SGM3 mice, even to a greater degree than imatinib, and this inhibition persisted in patients with suboptimal TKI response. Mechanistically, CXCL14 upregulated inflammatory cytokine signaling but downregulated mTOR signaling and oxidative phosphorylation in CML LSCs. Together, we have discovered a suppressive role of CXCL14 in CML LSC growth. CXCL14 might offer a treatment option targeting CML LSCs.

BCR::ABL1 -Dependent Promotion of Hematopoietic Stem Cells (HSC) and Progenitors in an Inducible Embryonic Stem Cell (ESC) Model

Our current understanding of the effect of BCR::ABL1 oncogene in leukemia development largely relies on studies using in vitro and in vivo leukemogenesis models. Among them, in vivo models using retrovirus-mediated BCR-ABL gene transfer in murine embryonic stem cell (ESC)-derived hematopoietic cells suggested that the enforced expression of BCR::ABL1 was sufficient to confer them an in vivo long-term repopulation potential. However, in these models, the unique BCR::ABL1 dependence of the generation of this potential has not been shown. Our study aimed at getting a better understanding of the early events regulated by BCR::ABL1 during the course of ESC differentiation without retroviral-mediated gene transfer. The elucidation of these early events could also be of major interest in therapy as generation of bona fide HSCs from normal ESC or iPSC remains a major challenge. To this end, we engineered a unique Tet-ON inducible system in which BCR::ABL1(p210) expression level can be tightly controlled using doxycycline (Dox). As compared to previously reported ESC-derived models, our system contains a single copy of BCR::ABL1 inserted into the murine Col1a1 locus under the control of Tetracycline operator, is sensitive to Dox, leakproof and reversible. The first striking results was the timing of Dox-inducible BCR-ABL expression during the course of ESC differentiation, impacting both the frequency and emergence of hematopoietic progenitors. Indeed, the expression of BCR::ABL1 at day+5 favors significantly the emergence and maintenance of immature hematopoietic progenitors. Importantly, these cells can self-renew only in the presence of Dox. Several hematopoietic stem-like cell lines were established and two of them (HSCL-A and HSCL-C) have been explored in detail. A major BCR-ABL-dependent proliferative ability was documented in both (Figure 1D). Both expressed Lin-Sca-1+c-Kit+CD150+CD48- and eSLAM, corresponding to phenotypically identified adult mouse HSCs. Importantly, these cells exhibited a normal myeloid and erythroid differentiation capacity in CFC assays, giving rise also to CFU-GEMM. We then evaluated the ability of HSCL to self-renew using long-term culture-initiating cell (LTC-IC) assays. To this purpose, EPCR+CD45+/loCD48-CD150+ (ESLAM) cells were sorted at a single cell level and co-cultured on a supportive AFT024 feeder layers for 4 weeks. LTC-IC-derived progenitors were then assessed using CFC assays. The frequency of ESLAM LTC-ICs was 0.53 from adult bone marrow and 0.52 from HSCL lines demonstrating their self-renewal ability. To better characterize HSCL cell lines, we sorted LSK cell population from HSCL#A, HSCL#C, E14.5 fetal liver and murine adult bone marrow and analyzed their transcriptome signatures by RNA-Seq. Unsupervised principal component analysis with the 1333 genes exhibiting the highest variability revealed that HSCL cell lines clustered together and exhibited distinct molecular signature from fetal and adult LSK. This stratification was validated by clustering with the 50 best variable genes. Among them, we found 17 genes highly expressed in HSCL cell lines associated with HSC quiescence (such as NupR1). GSEA analyses confirmed an enrichment of several cell cycle genes as well as genes involved in unfolded protein response (UPR) and autophagy (p <0.001). Thus, we report here for the first time, a unique BCR-ABL inducible ESC-derived hematopoiesis model which demonstrates the ability of BCR-ABL expression to induce a proliferative signal for extended periods of time in highly primitive hematopoietic cells. This novel tool can be used to dissect CML ontogeny both in vitro and in vivo and perform drug screening at the most primitive stem cell level. It can also be of major interest to uncover molecular events and signaling pathways activated during normal ESC-derived hematopoiesis. Such knowledge will open up new avenues to differentiate human iPSC towards bona fide HSCs for potential clinical applications. Figure 1. Establishment of ES cell-derived hematopoietic cell lines expressing BCR::ABLp210.(A) Representation of the timeline of ES cell differentiation protocol and doxycycline induction. (B) HSCL#A cell morphology. (C) MGG staining of HSCL#A cells. (D) Proliferation curves of HSCL#A and HSCL#C cell lines over 40 days in culture. (E)Kinetics of the effect of doxycycline removal on HSCL#A cells cultured on OP9 cells for 4 consecutive days. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Engraftment characterization of risk-stratified AML in NSGS mice

AbstractAcute myeloid leukemia (AML) is the most common acute leukemia in adults. Disease heterogeneity is well documented, and patient stratification determines treatment decisions. Patient-derived xenografts (PDXs) from risk-stratified AML are crucial for studying AML biology and testing novel therapeutics. Despite recent advances in PDX modeling of AML, reproducible engraftment of human AML is primarily limited to high-risk (HR) cases, with inconsistent or very protracted engraftment observed for favorable-risk (FR) and intermediate-risk (IR) patients. We used NSGS mice to characterize the engraftment robustness/kinetics of 28 AML patient samples grouped according to molecular/cytogenetic classification and assessed whether the orthotopic coadministration of patient-matched bone marrow mesenchymal stromal cells (BM MSCs) improves AML engraftment. PDX event-free survival correlated well with the predictable prognosis of risk-stratified AML patients. The majority (85-94%) of the mice were engrafted in bone marrow (BM) independently of the risk group, although HR AML patients showed engraftment levels that were significantly superior to those of FR or IR AML patients. Importantly, the engraftment levels observed in NSGS mice by week 6 remained stable over time. Serial transplantation and long-term culture-initiating cell (LTC-IC) assays revealed long-term engraftment limited to HR AML patients, fitter leukemia-initiating cells (LICs) in HR AML samples, and the presence of AML LICs in the CD34− leukemic fraction, regardless of the risk group. Finally, orthotopic coadministration of patient-matched BM MSCs and AML cells was dispensable for BM engraftment levels but favored peripheralization of engrafted AML cells. This comprehensive characterization of human AML engraftment in NSGS mice offers a valuable platform for in vivo testing of targeted therapies in risk-stratified AML patient samples.

Molecular and Functional Characterization of Disease-Propagating Stem Cells in Juvenile Myelomonocytic Leukemia

Juvenile Myelomonocytic Leukemia (JMML) is a rare, poor prognosis childhood leukemia characterized by aberrant proliferation of dysplastic myeloid cells and germline or somatic RAS-activating mutations. The only curative treatment is hematopoietic stem cell (HSC) transplantation, however, post-transplant relapse is frequent (~35%) and long-term disease-free survival is ~64%. Approximately half of patients show evidence of clonal evolution with acquisition of additional abnormalities, e.g. mutations of epigenetic/spliceosome components which confer a particularly adverse prognosis. The cellular hierarchy in JMML is poorly characterized with little known about the identity of leukemia stem cells (LSCs), a crucial step towards understanding disease evolution and propagation of relapse.We therefore set out to functionally and molecularly characterize LSCs in a cohort of 16 JMML patients enrolled in the UK NIHR Paediatric MDS/JMML study with samples at diagnosis, post-transplant and relapse. All patients underwent genetic analysis using a novel targeted next generation sequencing (NGS) panel confirming typical RAS pathway mutations in all patients and additional mutations in epigenetic/spliceosome genes in 44% of the patients. Based on adult MDS/MPN, we hypothesized that the CD34+ HSC and progenitor cell compartment (HSPC) would be preserved in JMML and that JMML-LSCs may reside within the phenotypic HSC compartment (Lin-CD34+CD38-CD90+CD45RA-). However, FACS analysis revealed disruption of HSPCs with a 2 fold reduction in phenotypic HSCs (Lin-CD34+CD38-CD90+CD45RA-) and an aberrant Lin-CD34+CD38-CD90+CD45RA+ (90+/45RA+) population not present in normal pediatric bone marrow (BM). In single cell and bulk in vitro assays, JMML-HSPCs had myeloid-biased lineage output with reduced and dysplastic erythroid and severely impaired megakaryocyte and lymphoid potential. Using a novel, integrated single cell genotyping technique combining HSPC index-sorting with clonogenic assays we backtracked patient-specific mutations to HSPC populations, establishing that: 1. all somatic mutations could be backtracked to the phenotypic HSC compartment (492 colonies genotyped for 9 different mutations in 4 patients); and 2. clonal evolution events (eg ASXL1 mutation) in JMML occurred within the HSC compartment with RAS pathway mutations as a “first hit”. Moreover, a PTPN11 mutation could be traced back to cells within the HSPC compartment of a post-BMT patient prior to molecular evidence of overt disease relapse in whole BM.We next assessed the stem cell capacity of FACS-purified phenotypic HSPCs in JMML by in vitro long-term culture initiating cell (LTC-IC) assays and in vivo xenotransplantation in NSG mice. LTC-IC assays consistently demonstrated self-renewal capacity in HSCs together with variable LTC potential in the novel 90+45RA+ population. Similarly, transplantation of purified JMML HSCs resulted in higher engraftment at 5, 10, and 16 weeks than 90+45RA+ or GMP populations (16 weeks post BMT: HSC = 91.7% human CD45; 90+/45RA+ = 0.14%; GMP = 0.2%, p < 0.0005 for HSC vs GMP, n = 3). Importantly, mice transplanted with HSCs, but not other HSPC populations, developed myeloid leukemia with 67% mortality (p < 0.01, n = 6). Secondary transplantation of HSCs confirmed LT potential of JMML HSCs.Single cell RNA-seq on Lin-CD34+ HSPCs using the Chromium 10X platform (n > 10,000 single cells) showed co-clustering of JMML and normal cells transcriptionally resembling myeloid progenitors expressing MPO and MS4A3, while a distinct cluster of JMML HSPCs uniquely co-expressed fetal genes (e.g. HMGA2), stem cell transcription factors (e.g. GATA2) and myeloid leukemia-associated genes (e.g HEMGN, HOPX, MEF2C). RNA-seq of FACS-purified JMML HSCs also showed upregulation of stem cell and fetal genes (HLF, MEIS1, ABCB1, MPL, and HMGA2) as well as candidate therapeutic targets e.g. MTOR, PIM1, SLC2A1, and CD96 compared to normal HSCs. We confirmed aberrant expression of CD96 in JMML HSCs by flow cytometry, and experiments inhibiting therapeutic targets in vitro and in vivo are underway. In conclusion, we show that JMML HSPCs are organized as a hierarchy, with counterparts of phenotypic HSCs representing disease propagating LSCs. Single cell molecular profiling of JMML HSCs has identified a number of novel potential therapeutic candidates for future clinical development. Disclosuresde la Fuente:bluebird bio: Consultancy; HCA: Consultancy; Jazz Pharmaceuticals: Honoraria. Mead:BMS: Honoraria; Pfizer: Honoraria; Novartis: Honoraria, Research Funding, Speakers Bureau.

Abstract PO007: Role of PI3K pathway in reprogramming the tumor niche of angiosarcoma

Abstract Angiosarcoma (AS) is a highly aggressive, rare soft-tissue sarcoma that forms malignant blood vessels. Approximately half of patients have metastatic or unresectable disease with a median overall survival of less than 6 months, and tumor-related mortality is high. Inflamed tissue is frequently seen in angiosarcomas, and immune cells are key components that contribute to the landscape of the tumor microenvironment. Hemangiosarcoma (HSA) occurs commonly in companion dogs, and it shares clinical and morphological features with human AS. Recurrent mutations in TP53 and genes involved in PI3K pathway such as PIK3CA, and PIK3R1 were observed in both human AS and canine HSA, and the mutational landscape was associated with distinct molecular tumor subtypes; in particular, differing immunophenotypes. Recently, we have shown that human AS and canine HSA establish convergent transcriptional programs driven by angiogenic fusion genes and TP53 mutation. However, these vascular tumors are genomically complex, and molecular mechanisms that establish the tumor microenvironment are incompletely known. In this study, we analyzed RNA-seq transcriptomic data generated from human AS tissues (n=13) and canine HSA tissues (n=76) and HSA cell lines (n=11). The data showed that the tumors of both species enriched gene signatures associated with maintenance of hematopoietic stem cells. Then, we found that canine HSA cells supported expansion and differentiation of human CD34+ umbilical cord blood cells using long-term culture-initiating cell and colony-forming unit assays. We also developed xenograft models of canine HSA that facilitate myelopoiesis and macrophage infiltration in the tumor tissue. Our data showed that gene signatures of monocytes and naïve macrophages were detectable in canine HSA cells. In human ASs, macrophage-associated genes, in particular M2 macrophage signatures were significantly enriched, compared to normal tissues. Collectively, our data suggest that human AS and canine HSA have cell-autonomous capacity to govern hematopoietic progenitors and immune cells, potentially establishing the tumor immune niche. We engineered HSA cells to induce homozygous H1047R mutations in PIK3CA gene using CRISPR-Cas9 system. Our ongoing work is to determine if regulating PI3K pathway in the tumor cells contributes to the molecular programs that create the tumor niche of the vascular malignancy. Citation Format: Sophia Wenthe, Ashley J. Schulte, Mathew G. Angelos, Dan S. Kaufman, Jaime F. Modiano, Jong Hyuk Kim. Role of PI3K pathway in reprogramming the tumor niche of angiosarcoma [abstract]. In: Proceedings of the AACR Virtual Special Conference on the Evolving Tumor Microenvironment in Cancer Progression: Mechanisms and Emerging Therapeutic Opportunities; in association with the Tumor Microenvironment (TME) Working Group; 2021 Jan 11-12. Philadelphia (PA): AACR; Cancer Res 2021;81(5 Suppl):Abstract nr PO007.

Neuroimmune/Hematopoietic Axis with Distinct Regulation by the High-Mobility Group Box 1 in Association with Tachykinin Peptides.

Hematopoiesis is tightly regulated by the bone marrow (BM) niche. The niche is robust, allowing for the return of hematopoietic homeostasis after insults such as infection. Hematopoiesis is partly regulated by soluble factors, such as neuropeptides, substance P (SP), and neurokinin A (NK-A), which mediate hematopoietic stimulation and inhibition, respectively. SP and NK-A are derived from the Tac1 gene that is alternately spliced into four variants. The hematopoietic effects of SP and NK-A are mostly mediated via BM stroma. Array analyses with 2400 genes indicated distinct changes in SP-stimulated BM stroma. Computational analyses indicated networks of genes with hematopoietic regulation. Included among these networks is the high-mobility group box 1 gene (HMGB1), a nonhistone chromatin-associated protein. Validation studies indicated that NK-A could reverse SP-mediated HMGB1 decrease. Long-term culture-initiating cell assay, with or without NK-A receptor antagonist (NK2), showed a suppressive effect of HMGB1 on hematopoietic progenitors and increase in long-term culture-initiating cell assay cells (primitive hematopoietic cells). These effects occurred partly through NK-A. NSG mice with human hematopoietic system injected with the HMGB1 antagonist glycyrrhizin verified the in vitro effects of HMGB1. Although the effects on myeloid lineage were suppressed, the results suggested a more complex effect on the lymphoid lineage. Clonogenic assay for CFU- granulocyte-monocyte suggested that HMGB1 may be required to prevent hematopoietic stem cell exhaustion to ensure immune homeostasis. In summary, this study showed how HMGB1 is linked to SP and NK-A to protect the most primitive hematopoietic cell and also to maintain immune/hematopoietic homeostasis.

A Role for TNF-α in Chronic Lymphocytic Leukemia Bone Marrow Hematopoietic Dysfunction

The current paradigms of known peripheral immune abnormalities in B-Chronic Lymphocytic Leukemia (CLL) are not able to consistently explain patient complications and it is difficult to correct a given CLL patient's immune status. Here, we expand on our initial report demonstrating bone marrow (BM) hematopoietic dysfunction in untreated CLL patients (Manso et al., Leukemia volume 33, pages 638-652, 2019). CLL patient BM had significantly reduced frequencies and short-term functional capacity of hematopoietic stem and progenitor cells (HSPCs). Additionally, the remaining progenitors exhibited increased protein levels of the key hematopoietic transcriptional regulators GATA-2 and PU.1. We further evaluated the frequency and function of myeloid stem cells from controls and untreated CLL patients by limiting dilution analysis in long-term culture-initiating cell (LTC-IC) assays. Over the 5 week duration of the assay, we observed delayed and partial recovery of myelopoiesis from CLL-derived HSPCs (Figure 1A). These data suggest that removal of HSPCs from the CLL leukemic microenvironment partially recovers their ability to sustain myelopoiesis. A known inflammatory mediator and hematopoiesis-modulating cytokine that is constitutively produced by CLL cells, TNF-α, induced increased expression of GATA-2 and PU.1 in specific HSPC subsets and reduced formation of short-term colony forming units in vitro. Addition of TNF-α to LTC-IC assays resulted in a striking ablation of myelopoiesis in a dose-dependent manner, partially reproducing the ex vivo results (Figure 1B). To further assess the direct impact of CLL cells on HSPC biology, isolated HSPCs from controls were exposed in vitro to leukemic CLL cells. The co-culture induced overexpression of GATA-2 and PU.1 in distinct HSPC populations, recapitulating our ex vivo findings (Figure 1C-D). When cell-cell contact was inhibited by use of Transwell inserts, an intermediate increase in GATA-2 and PU.1 was observed, highlighting the contributions of both soluble mediators and cell-cell contact to HSPC alterations. In both direct and Transwell co-culture conditions, overexpression of GATA-2 and PU.1 was reversed when TNF-α was neutralized (Figure 1E-F). Taken together, these findings indicate a significant role for CLL-derived TNF-α in HSPC modulation and expand our previous observations of BM dysfunction in untreated CLL patients. This data offers new molecular insight into the contribution of the leukemic microenvironment to altered hematopoiesis, contributing to immunodeficiency in CLL, and identifies TNF-α as a potential therapeutic target for correction of hematopoiesis in CLL disease. Disclosures Ding: Merck: Research Funding; DTRM Biopharma: Research Funding. Parikh:AstraZeneca: Honoraria, Research Funding; MorphoSys: Research Funding; AbbVie: Honoraria, Research Funding; Acerta Pharma: Research Funding; Pharmacyclics: Honoraria, Research Funding; Janssen: Research Funding; Ascentage Pharma: Research Funding; Genentech: Honoraria. Novak:Celgene Coorperation: Research Funding. Kay:Agios: Other: DSMB; Celgene: Other: Data Safety Monitoring Board; Infinity Pharmaceuticals: Other: DSMB; MorphoSys: Other: Data Safety Monitoring Board.

The combined effects of hierarchical scaffolds and mechanical stimuli on ex vivo expansion of haematopoietic stem/progenitor cells

We describe the ex vivo expansion of haematopoietic stem/progenitor cells (HSPCs) with consideration of their eventual in-vivo niche. We firstly fabricated hierarchically structured scaffolds (lattices derived via three-dimensional plotting combined with electrospun submicron fibers coated with vitronectin to increase cell affinity). We also applied intermittent hydrostatic pressure (IHP) to mimic the physical environment of the in vivo niche. In the absence of mechanical stimuli, the cell phenotype (CD34+, CD34+CD38–) remained excellent in the vitronectin-treated group. Two IHP regimens were tested; optimally, cells were pressurized (20 kPa) for 2 min and then rested for 13 min. On day 7 of culture, the total cell number had increased 21.2-fold and that of CD34+ cells 10.94-fold. CD34+ and CD34+CD38– cells constituted 44.50 and 44.07% of total cells, respectively. Colony-forming counts and the long-term culture-initiating cell assay showed that clonogenic potential was greatly improved under our experimental conditions. Scaffolds with hierarchical structures were valuable in this context. Furthermore, ex vivo expansion of HSPCs was improved by physical stimulation.

Targeting NAD+ Modulating Enzymes to Improve HSC Function in Aging and Bone Marrow Failure

Human hematopoietic system produces various types of differentiated and short-lived cells with specialized functions, which require continuous replenishment through the function of hematopoietic stem cells (HSC). HSC failure is a common distal endpoint of various pathogenic mechanisms in almost all bone marrow failure (BMF) syndromes and associated diseases. Hematopoietic growth factor cocktails (HGF) used in expanding bone marrow cells e.g., to increase cellularity of the HSC grafts, lead to differentiation and decreased HSC count. Theoretically, when used in vivo, they may act on progenitors rather than HSC and lead to stimulation of clonal outgrowth. Our current ability to stimulate HSC self-renewal to provide reconstitution of long-term hematopoiesis is limited.Nicotinamide adenine dinucleotide (NAD+) serves as an essential cofactor and substrate for a number of critical cellular processes. NAD+ depletion may occur in response to DNA damage due to free radical/ionizing radiation attack, resulting in significant activation of NAD+ consuming PARPs. Because of their long lifespan, maintenance of the genomic integrity of HSCs by efficient and accurate DNA repair to reduce the risk of BMF and cellular transformation is essential. NAD+ is also required for the maintenance of sirtuins activity, important class III HDAC essential for the prevention of senescence. Aging or chronic immune activation and inflammatory cytokine production result in upmodulation of NAD+ degrading enzyme CD38 that rapidly depletes cellular and extracellular levels of NAD+. Various lines of evidence suggest that regulation of CD38 NADase activity is essential for maintenance of physiologic NAD+ levels. Enhancing NAD+ level can profoundly reduce oxidative cell damage in catabolic tissue, including blood. Consequently, promotion of intracellular NAD+ by preventing NAD+ catabolism represents a promising therapeutic strategy for degenerative diseases in general, and BMF and associated diseases in particular. Therefore; CD38, a major NAD+ degrading enzyme, can be an excellent therapeutic target to increase the cellular levels of NAD+ and consequently improve the function of HSC. Here we report the development of inhibitors of CD38 NADase activity that extends the self-renewal and proliferative life span of HSC.We used structure-guided virtual screening followed by docking simulation to develop CD38 inhibitors. The compounds were synthesized using rational chemical synthesis and characterized by high-resolution mass spectroscopy and C13 & H1NMR. HPLC based assays were performed to assess the ability of compounds to inhibit NAD+ degradation by recombinant CD38. Using an iterative approach of synthesis characterization and activity, we selected the most potent compound, designated as ccf1172, for further studies. Docking simulations, surface plasmon resonance, and HPLC based assays demonstrate that ccf1172 binds (KD=12 nM) and inhibits CD38 (IC50=10 nM) (Fig.1B, C&D). To further characterize the ability of ccf1172, colony forming assays (CFU-A) and long-term culture-initiating cell assays (LTCIC-A) were performed with cord blood, human and murine bone marrows. No GF-like activity was observed, but in combination with GF mix ccf1172 increased the number of erythroid and myeloid colonies (n=9) in dose-dependent manner with a maximal effect seen at 100 nM in a serial replating assay. Significant extension of proliferative life span of hematopoietic progenitors (n=5) were observed (Fig 1E). When we studied the ability of CD38 inhibitor to expand LTCICs in stromal cultures (n=3) as best in vitro surrogates of HSC, ccf1172 increased LTCIC numbers 2.6-fold at 10 nM. The effect did not require the presence of accessory cells as ccf1172 treatment resulted in ~2-fold increase in CD34+Lin-/CD45+ cells in stem cell culture media supplemented with growth factors over a period of 25 days (Fig 1F). The CD38 inhibitor demonstrated cytotoxic effects on nine different leukemic cell lines with IC 50 ranging from 1 to 5 µM while no effect was observed on normal bone marrow.Here, we demonstrate that CD38 inhibition may be a potential therapeutic principle for ex vivo and in vivo expansion of HSC. Decreasing levels of NAD+ have been linked to aging and stem cell dysfunction, as a key aspect of various BMF syndromes. The strategy of CD38 inhibition to preserve NAD+ is innovative and relevant therapeutic strategy. [Display omitted] DisclosuresSaunthararajah:Novo Nordisk, A/S: Patents & Royalties; EpiDestiny, LLC: Patents & Royalties. Maciejewski:Apellis Pharmaceuticals: Consultancy; Ra Pharmaceuticals, Inc: Consultancy; Ra Pharmaceuticals, Inc: Consultancy; Alexion Pharmaceuticals, Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Alexion Pharmaceuticals, Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Apellis Pharmaceuticals: Consultancy.

Bone Marrow Hematopoietic Dysfunction in Untreated Chronic Lymphocytic Leukemia Is Partially Mediated By Exposure to Constituents of the Leukemic Microenvironment

Peripheral immune dysfunction in B-Chronic Lymphocytic Leukemia (CLL) is well-studied and likely relates to the incidence of serious recurrent infections and second malignancies that plague CLL patients. However, the current paradigms of known immune abnormalities are not able to consistently explain these complications and it is not easy to correct CLL patient immune status. Here, we expand on our preliminary reports that demonstrate bone marrow (BM) hematopoietic dysfunction in early and late stage untreated CLL patients. We found reduced short-term functional capacity of hematopoietic progenitors in BM using colony forming unit assays (Figure 1A-C) and flow cytometry revealed significant reductions in frequencies of hematopoietic stem and progenitor cell (HSPC) populations (exemplified by Lin-CD34+ HSPCs, Figure 1D). We further report that protein levels of the transcriptional regulators HIF-1α, GATA-1, PU.1, and GATA-2 are overexpressed in distinct HSPC subsets from CLL patient BM, providing molecular insight into the basis of HSPC dysfunction. Interestingly, sustained myelopoiesis, evaluated by limiting dilution analysis in long-term culture-initiating cell (LTC-IC) assays maintained for five weeks, revealed no difference between healthy controls and CLL patients. These new data indicate that when HSPCs are removed from the leukemic microenvironment for ample in vitro culture time, they recover the ability to sustain myelopoiesis. To further assess the impact of the CLL microenvironment on HSPC biology, isolated HSPCs (CD34+ BM cells) from healthy controls were exposed in vitro to known leukemic microenvironment constituents. Exposure to TNFα, a cytokine constitutively produced by CLL B cells, resulted in rapid increases in PU.1 and GATA-2 proteins (Figure 2A-D). Similarly, addition of TNFα to the LTC-IC assay resulted in a striking ablation of myelopoiesis, even at the highest input cell concentration. Further, overexpression of PU.1 and GATA-2 were observed in HSPCs following co-culture with CLL B cells, a result that was not recapitulated when cells were exposed to IL-10, another cytokine constitutively produced by CLL B cells. These findings indicate specific components of the leukemic microenvironment are involved in HSPC modulation. Together, these findings expand on our previous observations of BM hematopoietic dysfunction in untreated CLL patients and offer new molecular insights into the contribution of the leukemic microenvironment on immunodeficiency in CLL. [Display omitted] DisclosuresDing:Merck: Research Funding. Parikh:Pharmacyclics: Honoraria, Research Funding; MorphoSys: Research Funding; Janssen: Research Funding; Abbvie: Honoraria, Research Funding; Gilead: Honoraria; AstraZeneca: Honoraria, Research Funding. Kay:Morpho-sys: Membership on an entity's Board of Directors or advisory committees; Agios Pharm: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Acerta: Research Funding; Infinity Pharm: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees; Tolero Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Cytomx Therapeutics: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Pharmacyclics: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Bone mesenchymal stromal cells exhibit functional inhibition but no chromosomal aberrations in chronic myelogenous leukemia.

Chronic myelogenous leukemia (CML) is a myeloproliferative neoplasia characterized by the presence of the Philadelphia (Ph) chromosome in hematopoietic cells (HCs). As one of the most important components of the bone marrow microenvironment (BMM), bone mesenchymal stromal cells (BMSCs) are critical in the development of leukemia and essential in the regulation of hematopoiesis. However, little is known regarding the alterations of BMSCs in CML. The current study performed Cell Counting Kit-8 and colony-forming unit fibroblast assays to evaluate the proliferative ability of BMSCs. The percentage of senescent BMSCs was evaluated by a senescence-associated β-galactosidase staining assay. Subsequently, a long-term culture-initiating cell assay was designed to explore the HC-supporting capacity of the BMSCs. Furthermore, cytogenetics were detected by conventional cytogenetic analysis and fluorescence in situ hybridization analysis. The current results revealed that CML-BMSCs exhibited decreased cell proliferation and impaired HC-support capacity, as well as increased susceptibility to senescence. No chromosomal aberrations, including the absence of the Ph chromosome, were noted in all CML-BMSCs. In conclusion, the current study demonstrated functional inhibition of CML-BMSCs; however, no signs of chromosomal aberrations were observed, thereby providing insight into the changes occurring in the CML-BMM.

Loss of EfnB1 in the osteogenic lineage compromises their capacity to support hematopoietic stem/progenitor cell maintenance

The bone marrow stromal microenvironment contributes to the maintenance and function of hematopoietic stem/progenitor cells (HSPCs). The Eph receptor tyrosine kinase family members have been implicated in bone homeostasis and stromal support of HSPCs. The present study examined the influence of EfnB1-expressing osteogenic lineage on HSPC function. Mice with conditional deletion of EfnB1 in the osteogenic lineage (EfnB1OB-/-), driven by the Osterix promoter, exhibited a reduced prevalence of osteogenic progenitors and osteoblasts, correlating to lower numbers of HSPCs compared with Osx:Cre mice. Long-term culture-initiating cell (LTC-IC) assays confirmed that the loss of EfnB1 within bone cells hindered HSPC function, with a significant reduction in colony formation in EfnB1OB-/- mice compared with Osx:Cre mice. Human studies confirmed that activation of EPHB2 on CD34+ HSPCs via EFNB1-Fc stimulation enhanced myeloid/erythroid colony formation, whereas functional blocking of either EPHB1 or EPHB2 inhibited the maintenance of LTC-ICs. Moreover, EFNB1 reverse signaling in human and mouse stromal cells was found to be required for the activation of the HSPC-promoting factor CXCL12. Collectively, the results of this study confirm that EfnB1 contributes to the stromal support of HSPC function and maintenance and may be an important factor in regulating the HSPC niche.

Characterization of Imatinib Resistant CML Leukemic Stem/Initiating Cells and Their Sensitivity to CBP/Catenin Antagonists.

The development of the tyrosine kinase inhibitor Imatinib (IM) represents a milestone in CML (Chronic Myeloid Leukemia) treatment. However, it is not curative and patients develop IM resistance. IM resistance has been previously correlated with the emergence of drug-resistant LIC/LSC (Leukemia Initiating Cell/Leukemia Stem Cell) and increased nuclear catenin levels and enhanced Wnt signaling. It has been demonstrated previously that drug resistant CML LIC/LSC can be safely eliminated both in vitro and in vivo via disruption of the CBP/catenin interaction, utilizing the highly biochemically selective small molecule CBP/catenin antagonist ICG- 001. Here, we utilized an in vitro IM selection of primary CML patients' samples to identify drug-resistant LIC/LSC populations. In this report, we characterized the drug-resistant CML LIC/LSC population using FACS, Smartchip qPCR and colony assays to analyze cell surface markers, transcriptomics and function. As opposed to previous characterization of the CML leukemic stem cell population as being either CD34+CD38- or CD34+CD38+, the in vitro selected Imatinib resistant (IM-R) CML LSC population was consistently CD34-CD38-. In Long-Term Culture Initiating Cell assay (LTC-IC, a surrogate assay for long term repopulating stem cells), our results suggest that the CBP/catenin antagonist ICG- 001 sensitizes LIC/LSC to IM treatment by forced differentiative elimination of the CML LIC/LSC population. In vitro selected IM resistant cells are negative for both CD34 and CD38 by FACS analysis. These cells acquire CD34/CD38 expression after co-culture with stromal cells. CBP/catenin antagonist ICG-001 facilitates IM function in eliminating these cells.

EBF1 As a Novel Regulator of Bone Marrow Mesenchymal Stromal Progenitors

Hematopoietic stem and progenitor cells (HSPC) are in daily demand worldwide because of their ability to replenish entire blood system. However, the in vitro expansion of HSPC is still a major challenge since the cues from bone marrow microenvironment remain largely elusive. Signals coming from the bone marrow niche, and specifically mesenchymal stem and progenitor cells (MSPC), orchestrate maintenance, trafficking and stage specific differentiation of HSPCs. Although, it is generally accepted that MSPCs are essential for hematopoietic homeostasis and generating multiple types of stromal cells, the exact transcriptional networks regulating MSPCs are not well established. Early B-cell factor 1 (Ebf1) has been discovered as lineage-specific transcription factor governing B lymphopoiesis. Additionally, it has been shown to play important role in differentiation of adipocytes, which are a niche component supporting hematopoietic regeneration. Thus, in this study we seek to examine if Ebf1 has an alternative function in non-hematopoietic compartment of bone marrow, specifically in mesenchymal stromal cells that maintain proper hematopoiesis.Here, we identified Ebf1 as new transcription regulator of MSPCs activity. Mesenchymal progenitors isolated from Ebf1-/- mice show diminished capacity to form fibroblasticcolonies (CFU-F) indicating reduced self-renewal. Moreover, cells expanded from these colonies display impaired in vitro differentiation towards osteoblasts, chondrocytes and adipocytes. In order to test how this defective MSPCs influence maintenance of HSPCs, we performed long-term culture-initiating cell assay (LTC-IC). After 5 weeks of co-culture of Ebf1-deficient stromal cells with wild type HSPCs we could observe significantly decreased number of cobblestone and CFU colonies formed by primitive HSPCs, in comparison to co-cultures with control stromal cells. Furthermore, in vivo adoptive transfers of wild type HSPCs to Ebf1+/- recipient mice showed a decrease in the absolute numbers of HSPCs in primary recipients and reduced donor chimerism within the HSCP compartment in competitive secondary transplant experiments. Additionally, Prx1-Cre-mediated deletion of Ebf1 specifically in MSPCs of mice leads to reduced frequency and numbers of HSPCs and myeloid cells in the bone marrow. These results confirm that mesenchymal stromal cells lacking Ebf1 render insufficient support for HSPCs to sustain proper hematopoiesis. Interestingly, we also observed a reduced ability of HSPCs sorted from Prx1CreEbf1fl/fl mice to form colonies in methylcellulose, suggesting not only impaired maintenance but also hindered function of these cells. Moreover, HSPCs exposed to Ebf1-deficient niche exhibit changes in chromatin accessibility with reduced occupancy of AP-1, ETS, Runx and IRF motifs, which is consistent with decreased myeloid output seen in Prx1CreEbf1fl/fl mice. These results support the hypothesis that defective niche can cause epigenetic reprograming of HSPCs. Finally, single cell and bulk transcriptome analysis of MSPCs lacking Ebf1 revealed differences in the niche composition and decreased expression of lineage-instructive signals for myeloid cells. Thus, our study establishes Ebf1 as a novel regulator of MSPCs playing a crucial role in the maintenance and differentiation of HSPCs. DisclosuresNo relevant conflicts of interest to declare.

Leukotriene signaling via ALOX5 and cysteinyl leukotriene receptor 1 is dispensable for in vitro growth of CD34+CD38− stem and progenitor cells in chronic myeloid leukemia

Tyrosine kinase inhibitors targeting the BCR-ABL oncoprotein in chronic myeloid leukemia (CML) are remarkably effective inducing deep molecular remission in most patients. However, they are less effective to eradicate the leukemic stem cells (LSC), resulting in disease persistence. Therefore, there is great need to develop novel therapeutic strategies to specifically target the LSC. In an experimental mouse CML model system, the leukotriene pathway, and specifically, the expression ALOX5, encoding 5-lipoxygenase (5-LO), has been reported as a critical regulator of the LSC. Based on these results, the 5-LO inhibitor zileuton has been introduced in clinical trials as a therapeutic option to target the LSC although its effect on primary human CML LSC has not been studied. We have here by using multiplex single cell PCR analyzed the expression of the mediators of the leukotriene pathway in bone marrow (BM) BCR-ABL+CD34+CD38− cells at diagnosis, and found low or undetectable expression of ALOX5. In line with this, zileuton did not exert significant overall growth inhibition in the long-term culture-initiating cell (LTC-IC) and colony (CFU-C) assays of BM CD34+CD38− cells from 7 CML patients. The majority of the single leukemic BCR-ABL+CD34+CD38− cells expressed cysteinyl leukotriene receptors CYSLT1 and CYSLT2. However, montelukast, an inhibitor of CYSLT1, also failed to significantly suppress CFU-C and LTC-IC growth. These findings indicate that targeting ALOX5 or CYSLT1 signaling with leukotriene antagonists, introduced into the clinical practice primarily as prophylaxis and treatment for asthma, may not be a promising pharmacological strategy to eradicate persisting LSC in CML patients.

Phenotype and Function of Human Gut Hematopoietic Stem Cells and Progenitors

Lin−CD45+CD34+ cells have been reported in human small intestine and liver, but further phenotypic or functional analysis was not described[1][2]. We utilized flow cytometry and cytometry by time of flight (CyTOF) to study the composition and phenotype of human gut hematopoietic stem cells (HSCs) and hematopoietic progenitors (HPs) from deceased donors. Colony-forming-cell (CFC) assays, long-term-culture-initiating-cell (LTC-IC) assays and humanized mouse models were established to further evaluate the differentiation potential of gut HSCs/HPs. HSCs and multiple types of HPs, including multipotent progenitors (MPP), lymphoid-primed multipotent progenitors (LMPP), common lymphoid progenitors (CLP), myeloid progenitors (MP), and unclassified progenitors co-expressing CD56, were detected in human intestinal mucosa and mesenteric lymph nodes (MLNs). Ileum lamina propria lymphocytes (LPL) included significantly higher percentages of HSCs than intraepithelial lymphocytes (IEL). Both IEL and LPL showed significantly lower percentages of CLP compared to MLNs. From jejunum or ileum specimens of ITx patients collected from their stoma revision/closure (POD47- 1544), variable levels of Lin−CD45+CD34+ cells were detected (Figure 1A). However, when gating on HSCs, donor cells (>90%) predominated over recipient cells in four patients with chimerism, regardless of their ages or early clinical outcomes (Figure 1B-C). Lin−CD45+CD34+ cells from ileum LP and bone marrow (BM) demonstrated similar properties in viSNE plots from CyTOF analysis (Figure 2), suggesting similar functional phenotypes. Both ileum LP and BM Lin−CD45+CD34+ cells contained subpopulations expressing the thymus-homing marker CCR9, and T/NK lineage polarized progenitors coexpressing CD45RA and CD7. Additionally, CFC assays showed that both ileum LP and BM Lin−CD34-enriched cells can form myeloid/erythroid colonies, although ileum LP CD34 cells were less efficient than BM CD34 cells. LTC-IC assay using single cell sorted ileum LP HSC co-culture with murine feeder cells suggested myeloid/lymphoid differentiation potential of human gut HSCs. Experiments using human gut Lin−CD45+CD34+ cells to reconstitute humanized mice are ongoing.FIGURE 1: (A) Lin−CD45+CD34+ cells were detected in human intestinal mucosa long-term post-Tx (POD47-1544) (B) Donor-derived hematopoietic stemcells (HSCs) and/or multiple types of hematopoietic progenitors (HPs)were detected in transplanted small bowel specimens. (C) Composition of HSCs/HPs when gating on donor (upper panel) or recipient (lower panel) Lin−CD45+CD34+ cells.FIGURE 2: Lin−CD45+CD34+ cells from bone marrow (BM) and ileum lamina propria (LP) located in the same region of the viSNE plot from CyTOF analysis of individual organ donor, suggesting the similarity of their phenotype and origin.Taken together, our data suggest that human gut HSCs/HPs may be functionally similar to BM HSCs/HPs that have thymus-homing ability and can give rise to multiple lineages, which likely contribute to the long-persistent mixed chimerism post-ITx. Research reported in this publication was performed in the Columbia Center for Translational Immunology (CCTI) Flow Cytometry Core, supported in part by the Office of the Director, National Institutes of Health under awards S10RR027050 and S10OD020056. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. CyTOF study was performed in collaboration with the Human Immune Monitoring Core (HIMC) of Icahn School of Medicine at Mount Sinai.

EphA5 and EphA7 forward signaling enhances human hematopoietic stem and progenitor cell maintenance, migration, and adhesion via Rac1 activation.

The proliferation, differentiation, adhesion, and migration of hematopoietic stem and progenitor cells (HSPCs) are dependent upon bone marrow stromal cells (BMSCs). In this study, we found that human primitive HSPCs (CD34+CD38-), but not lineage-committed hematopoietic cell populations, express the tyrosine kinase receptors EphA5 and EphA7. Moreover, we found that the ephrinA5 ligand, the high-affinity binding partner of EphA5 and EphA7, is highly expressed by primary human BMSCs. Previous studies have reported that interactions between EphA and ephrinA play important roles in hematopoietic cell trafficking; however, their role in BMSC support of hematopoiesis had not been described previously. Herein, we show that stimulating EphA5 and/or EphA7 forward signaling in HSPCs using soluble ephrinA5-Fc molecules promoted human HSPC-derived colony formation significantly and was associated with increased expression of granulocyte macrophage colony-stimulating factor receptor on HSPCs. Studies using functional blocking peptides to EphA5/7 found that disruption of EphA5/ephrinA5 and/or EphA7/ephrinA5 interactions inhibited HSPC function in BMSC-dependent long-term culture-initiating cell assays. Furthermore, the adhesion and migration of HSPCs was increased significantly in the presence of ephrinA5-Fc molecules compared with human immunoglobulin G-treated controls. Conversely, blocking EphA5 activation led to a reduction of HSPC adhesion, whereas inhibiting EphA5 and/or EphA7 activation hindered HSPC migration. Analysis of HSPC cultured in the presence of ephrinA5-Fc showed that EphA forward signaling stimulated Rac1 gene and protein expression and the Rac1 target molecule WAVE1. Moreover, a significant reduction of ephrinA5-mediated HSPC adhesion and migration was observed in the presence of Rac1 inhibitor. These findings suggest that interactions between EphA and ephrinA5 are important in maintaining the HSPC niche mediated in part by activation of Rac1 signaling.

Therapy Resistant CML Stem Cells Are Dependent on Mitochondrial Oxidative Metabolism for Their Survival

We and others have shown that tyrosine kinase inhibitors (TKIs), such as imatinib, fail to eliminate primitive chronic myeloid leukaemia (CML) stem cells (LSCs), suggesting that combination of TKIs with other targeted agents will be required to eradicate the LSC-pool. Therefore, identification of targetable pathways that selectively maintain CML LSC survival is critical.Metabolic reprogramming is a core feature of cancer cells making them susceptible to manipulation in a selective manner. Indeed, in recent years numerous studies have shown that targeting abnormal aspects of metabolism can be of therapeutic value. The aim of this study was to identify and target metabolic dependencies in CML LSCs using stem cell-enriched (CD34+) primary cells isolated from CML patients and healthy donors.Although CML represents a highly suitable model for cancer stem cell studies, investigation of LSCs metabolism has so far been restricted by technical limitations. Therefore, we have applied improved protocols for metabolomics using liquid chromatography-mass spectrometry (LC-MS) and functional assays.Initially we cultured leukaemic cells in the presence of uniformly-labelled glucose with stable (heavy) 13C isotope (13C6-glucose) and compared isotopic enrichment in CD34+ versus CD34- cells isolated from the same CML patient. Our results showed that CD34+ cells contained an increased proportion of isotopologues with 2 or more labelled carbons in tricarboxylic acid (TCA) cycle metabolites (such as citrate, α-ketoglutarate and malate) when compared with CD34- cells, indicating an increase in flux of glucose through the TCA cycle in more primitive CML cells. In contrast, CML CD34+ cells contained reduced levels of glucose-derived lactate when compared with patient-matched CD34- cells, suggesting that more primitive CML cells utilise mitochondrial metabolism rather than glycolysis to supply their energy demands. In line with this, CML CD34+ cells displayed more than a two-fold increase in their mitochondrial oxygen consumption rates (OCR) when compared with CD34- cells (p≤0.05), confirming that mitochondrial metabolism is enhanced in stem cell-enriched CML cells.Next we traced 13C6-glucose in CD34+ cells from healthy donors and compared this with isotopic enrichment in CML CD34+ cells. This revealed that 13C enrichment in TCA cycle metabolites was significantly higher in CML CD34+ cells when compared with their normal counterparts. This correlated with a significant increase in mitochondrial respiration (p≤0.001) and mitochondrial membrane potential in primitive CML cells, including CD34+38- cells (p≤0.001), suggesting that CML LSCs may be selectively sensitive to inhibition of mitochondrial metabolism.Of clinical significance, we show that the antibiotic tigecycline, an inhibitor of mitochondrial translation, reduced this aberrant oxidative metabolism and selectively induced death in primitive CML cells at a clinically achievable concentration. More precisely, in vitro treatment with tigecycline as a single agent, decreased the number of CML CD34+ cell-derived colonies in comparison with untreated conditions (p≤0.001), and combining tigecycline with imatinib resulted in a 50% decrease in colony number when compared to tigecycline or imatinib alone (p≤0.01). Importantly, this drug combination had no effect on colony formation potential of CD34+ cells derived from healthy donors. Moreover, we show that tigecycline alone, or in combination with imatinib, reduced the number of colonies in a long-term culture-initiating cell assay (p≤0.001) while imatinib, as a single agent, did not have any significant effect.To examine the effect on transplantable CML LSCs in vivo, human CML CD34+ cells were injected into irradiated NSG mice. Following confirmation of engraftment mice were treated with imatinib, tigecycline alone and in combination with imatinib. Remarkably, 4-week combination treatment with tigecycline and imatinib led to near complete elimination of CML LSCs measured by the number of human CD45+ and CD34+38- cells in the bone marrow.We conclude that CML LSC are dependent on oxidative phosphorylation for their survival and tigecycline-mediated inhibition of mitochondrial metabolism, combined with TKI treatment, shows potential as a novel therapeutic strategy to selectively target these cells to enhance cure rates. DisclosuresHolyoake:Bristol Myers Squib: Honoraria, Research Funding; Novartis: Honoraria, Research Funding.