Hematopoietic Stem And Progenitor Cells Pool Research Articles

Myelodysplastic Syndromes Benefit from Iron-Restricted Bone Marrow Transplant in Preclinical Mouse Model

Background: Hematopoietic stem cell transplantation (HSCT) remains the only fully curative therapy for diseases such as b-thalassemia, sickle cell disease, MDS and AML and represents a relevant procedure for gene therapy .Tissue iron overload and non-transferrin bound iron (NTBI) have emerged as adverse prognostic factors for post-transplant survival. Recently, we showed that - in line with observations in transplanted individuals - preparatory chemotherapy conditioning with a combination of busulfan and fludarabine in wild-type mice resulted in elevated serum iron and transferrin (Tf) saturation and elicited NTBI formation. Tf saturation and NTBI levels increased proportionally with conditioning intensity. Interestingly, we found that pre-conditioning TMPRSS6 silencing through a GalNAc-conjugated siRNA - by increasing hepcidin levels - reduced the peri-transplant elevation of Tf saturation and limited the appearance of NTBI. These results proved that NTBI formation is induced by conditioning chemotherapy in the peri-trasplant period independently of the presence of pre-existing iron overload. Aims & Methods: Here we asked whether these observations hold true in conditions hallmarked by pre-existing iron overload and elevated NTBI and how iron restriction impacts on bone marrow transplant (BMT) outcome. To this aim, we applied the GalNAc TMPRSS6 siRNA as iron restriction strategy in a mouse model of myelodysplastic syndromes (MDS) and assessed its ability to reduce conditioning-elicited NTBI formation as well as its potential benefit on BMT. After TMPRSS6 siRNA administration, CD45.2 + MDS mice received conditioning chemotherapy and transplant of CD45.1 + wild-type bone marrow cells. Systemic iron parameters, engraftment and chimerism were then analyzed. Results: Importantly, conditioning exacerbated the elevated systemic iron levels in MDS mice, which was prevented by pre-conditioning TMPRSS6 silencing. Similar to our observation in wild-type mice, recipient CD45.2 + MDS mice showed a superior blood chimerism, resulting from increased bone marrow and spleen engraftment of CD45.1 + donor cells. Multilineage engraftment in bone marrow, spleen and peripheral blood was superior in iron-restricted recipient MDS mice compared to controls. Finally, donor chimerism of hematopoietic stem and progenitor cells (HSPCs) was higher in iron-restricted versus control MDS mice undergoing BMT. Conclusion: These findings demonstrate that pre-transplant conditioning profoundly alters iron homeostasis and restricting iron improves BMT outcome by increasing engraftment efficiency. Overall, these data suggest that conditioning-elicited NTBI is a critical mediator of peri-transplant toxicity and adversely impacts transplant outcome. Specifically, our observations prove that iron restriction improves donor HSPC engraftment in the recipient bone marrow, likely resulting in more efficient reconstitution of the HSPC pool and improved multilineage engraftment. Lastly, our results indicate that the peri-transplant application of novel iron restriction strategies aimed at reducing conditioning-induced NTBI - by positively affecting BMT outcome - is of potential benefit for patients undergoing BMT or gene therapy, presenting either with or without pre-transplant iron overload.

Clonal Hematopoiesis Is Common in Long-Term Survivors of Pediatric Hematopoietic Cell Transplantation, Including Umbilical Cord Blood Transplantation

Background 1. Approximately 20.000 to 200.000 hematopoietic stem and progenitor cells (HSPCs) contribute to steady-state adult hematopoiesis. During ageing, the overall diversity of the HSPC pool decreases, which is in part driven by the selective expansion of HSPC clones with driver mutations, a process called clonal hematopoiesis (CH). CH is an age-related condition that predisposes to hematologic malignancy, cardiovascular disease, and all-cause mortality. Hematopoietic stem cell transplantation (HCT) recipients are at increased risk of CH, but the mechanisms driving the emergence and expansion of CH clones are largely unknown. Furthermore, the prevalence of CH in pediatric HCT recipients, in whom the long-term consequences of CH are particularly relevant, remains to be defined. Aim To determine the prevalence and risk factors of CH in long-term survivors of HCT at pediatric age. Methods We included survivors of HCT at pediatric age, with a minimum survival of 5 years after HCT. CH was assessed by targeted, error-corrected sequencing of mutations in 27 cancer driver genes in whole blood, at a variant allele frequency detection threshold of 1%. HSC age was calculated by the sum of the age of the donor and the interval after HCT. In case the age of a donor was unknown (n=53), the median age for donors of the same donor relation were used (i.e., 12.6 years for sibling donors and 28.7 for matched unrelated donors). Clinical characteristics were compared using the Wilcoxon signed ranked test for continuous variables and Chi-squared or Fisher's exact test for categorical variables. Results Here, we report the interim results of the first 120 HCT survivors in this study. The median age of the survivors was 20.4 years (range 6.7-51.8), and the median interval after HCT was 11.8 years (range 5.2-38.2). Indications for HCT were hemato-oncology (n=95), bone marrow failure (n=22), or immune deficiency (n=3). The median calculated age of the HSCs was 31 years (range 5.2-61.8). CH was present in 13.3% of survivors (n=16). Mutated genes were DNMT3A (n=15), TET2 (n=4) and MPL (n=1), with three individuals having more than one driver mutation. The median variant allele frequency of the CH clones was 2.2%, but allele frequencies up to 30% were found. No differences were observed in recipient age or interval after HCT between individuals with or without CH (Figure 1A). However, the age of the HSCs was significantly higher in individuals with CH compared to those without (38.6 versus 26.3 years, p = 0.01, Figure 1B). Surprisingly, we detected CH in two recipients of umbilical cord blood grafts, at an interval of 11 and 15 years after HCT. Conclusion Clonal hematopoiesis is common in long-term survivors of pediatric HCT. After HCT, the prevalence of CH is related to the age of the transplanted HSCs, rather than the age of the survivor. The presence of CH in cord blood HSCs within two decades after HCT suggests that HCT-related factors may drive the de novo emergence and/or outgrowth of donor-derived CH. Ongoing research is aimed at investigating how factors such as stem cell source and post-transplant complications affect clonal expansion after HCT. Furthermore, we are investigating the prevalence of CH in an age-matched control cohort using the same detection pipeline, enabling direct comparison of the prevalence of CH in HCT survivors compared to the general population.

Gata2-regulated Gfi1b expression controls endothelial programming during endothelial-to-hematopoietic transition.

The first hematopoietic stem cells (HSCs) are formed through endothelial-to-hematopoietic transition (EHT) during embryonic development. The transcription factor GATA2 is a crucial regulator of EHT and HSC function throughout life. Because patients with GATA2 haploinsufficiency have inborn mutations, prenatal defects are likely to influence disease development. In mice, Gata2 haploinsufficiency (Gata2+/-) reduces the number and functionality of embryonic hematopoietic stem and progenitor cells (HSPCs) generated through EHT. However, the embryonic HSPC pool is heterogeneous and the mechanisms underlying this defect in Gata2+/- embryos remain unclear. Here, we investigated whether Gata2 haploinsufficiency selectively affects a cellular subset undergoing EHT. We showed that Gata2+/- HSPCs initiate, but cannot fully activate, hematopoietic programming during EHT. In addition, due to the reduced activity of the endothelial repressor Gfi1b, Gata2+/- HSPCs cannot repress endothelial identity to complete maturation. Finally, we showed that hematopoietic-specific induction of gfi1b could restore HSC production in gata2b-null (gata2b-/-) zebrafish embryos. This study illustrates the pivotal role of Gata2 in the regulation of the transcriptional network governing HSPC identity throughout the EHT.

Bnip3lb Regulated Mitophagy Maintains the Embryonic Pool of Hematopoietic Stem Cells By Protecting Them from ROS Induced Apoptosis

We have previously shown that Reactive Oxygen Species (ROS) promote Hematopoietic Stem and Progenitor Cell (HSPC) specification in the dorsal aorta, however, in the adult marrow it is established that HSPCs must maintain low levels of ROS to protect the stem cell pool against excessive cycling, differentiation, loss of stemness and apoptosis. We aim to elucidate the developmental timing and the mechanism by which HSCs transition from their initial dependence on ROS, to their precise regulation of it in the adult marrow. Since adult HSPCs regulate ROS by maintaining high levels of mitophagy, a process in which damaged mitochondria, which disproportionately generate ROS, are removed from the cell and recycled, we sought to define the initiation, regulation and impact of mitophagy in embryonic HSPC development. Our recent data shows that oxidative stress limits HSPC numbers as early as their colonization of the secondary hematopoietic niche, the Caudal Hematopoietic Tissue (CHT). This is the zebrafish analog of the mammalian fetal liver in that it is the tissue to which the HSPCs migrate after emerging from the hemogenic endothelium of the dorsal aorta. Once in the CHT they mature and proliferate before migrating again to their terminal niche in the marrow. In contrast to published data showing that ROS promotes HSPC specification in the dorsal aorta, when ROS levels were increased in the embryo using a morpholino against PRDX1 we saw a decrease in runx1/c-myb expression in the CHT. Conversely, when ROS was decreased using the antioxidant N-acetyl cystine (NAC), there was an increase in HSPC marker expression. By live imaging of zebrafish embryos expressing a recently developed fluorescent transgenic mitophagy-reporter, Tg(ubi:mitoGR), we observed that mitophagy is low in the hemogenic endothelial tissue which gives rise to HSPCs, but is upregulated as HSPCs emerge and begin to migrate to the CHT, and is occurring at high levels at the developmental stage when ROS begins to become damaging to the HSPC population. Our scRNAseq analysis and in situ-hybridization experiments allowed us to examine which genes and pathways were regulating the induction of mitophagy in these cells. These data show that the increase in mitophagy induction in the HSPC population at this timepoint coincides precisely with the initialization of expression of the bnip3lb mitophagy receptor gene. Fluorescent whole-mount in situ hybridization experiments demonstrated that bnip3lb is co-expressed with runx1 (a HSPC marker), in the CHT (figure 1), and, by examining our Tg(ubi:mitoGR) mitophagy reporter line after morpholino knock down of bnip3lb we confirmed that it is this gene which is driving mitophagy in HSPCs at this developmental stage. NIX, the murine homolog of bnip3lb, is responsible for developmentally regulated mitophagy in reticulocytes, lens fibers and macrophages, but has, thus far, had no reported role in maintaining HSPCs. Functionally, mitophagy appears to be essential for the survival of the HSPC population in the CHT. Reducing mitophagy via morpholino directed knockdown of bnip3lb has no effect on the initial specification of HSPCs in the dorsal aorta, but significantly decreases HSPC marker expression in the CHT, as measured by runx1/c-myb in-situ hybridization, and quantified by CD41:GFP HSPC reporter expression. In contrast upregulating mitophagy, which can be done via a novel heat-shock driven ΔOTC transgenic line or via small molecule mitophagy induction, elevates HSPC numbers in the CHT. Mechanistically, we found that bnip3lb knockdown increases detection of CellROX, a fluorescent dye which reports ROS levels, and increases the number of acridine orange+ apoptotic runx1+ HSPCs. Further, reducing ROS with antioxidant NAC reduces apoptosis and rescues the HSPC population (figure 2). We therefore propose that developmentally programmed mitophagy directed by bnip3lb is responsible for protecting proliferative embryonic HSPCs from the harmful effects of oxidative stress while the HSPC pool is expanding to seed lifelong hematopoiesis. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Blockade of Ceramide-Dependent Extracellular Vesicle Secretion Improves Hematopoietic Stem Cell Transplantation

Hematopoietic stem cell (HSC) transplantation serves as a curative therapy for numerous benign and malignant hematopoietic diseases, and as a platform for HSC gene therapy. A main constraint is the limited quantity of HSCs collected from patients or donors and the inherent risk of graft failure, or oligocolonal reconstitution, especially after ex vivo manipulation. Current efforts to mitigate these limitations have predominantly focused on the discovery of extrinsic factors to maintain or expand hematopoietic stem and progenitor cells (HSPCs), whereas the role of the HSPC secretome in modulating niche function and improving occupancy have not been extensively studied. Recently, classes of nanometer-sized extracellular vesicles (EVs) have been shown to regulate tissue function by providing potent proangiogenic signals and remodeling cellular microenvironments. EV secretion additionally regulates homeostasis and stemness properties of HSPCs; however, in-depth studies into HSPC EV biogenesis and the impact on homeostatic and regenerative potency have been hindered by numerous technical challenges. Here, we utilize an optimized ex vivo expansion protocol for human and murine HSPCs to examine the impact of different EV biogenesis pathways on vesicle secretion. Our studies rapidly focused on ceramide-dependent vesicle secretion; blockade of ceramide biosynthesis with a widely used neutral sphingomyelinase inhibitor, GW4869 significantly reduced HSPC EV secretion. Treatment of HSPCs with GW4869 further inhibited mTOR signaling, altered autophagic flux, and decreased cell cycle entry. Increased ratios of lymphoid (MPP4):myeloid (MPP2/3) primed progenitors accumulated in culture. Upon competitive transplantation into myeloablated recipients, GW4869-exposed murine HSPCs demonstrated superior long-term engraftment potential (>8 steady-state fold change, p < 0.001), independent of bone marrow or splenic homing. Ceramide inhibition did not significantly alter long-term lineage output from transplanted HSPCs. Confirmatory genetic knockout (KO) of neutral sphingomyelinase (nSMase2) in murine and human HSPCs using Cas9 ribonucleoprotein complex delivery similarly demonstrated enhanced colony forming potential. To understand the impact of ceramide inhibition on vesicle cargo, comparative mass spectrometry was performed on HSPC nSMase2 KO EVs (EVnSmase2-KO). Nearly 900 proteins were depleted (fold change <2) in EVnSmase2-KO; these underrepresented proteins were noted to be involved in myeloid differentiation and activation, immune response, and exocytosis, suggesting vesicle modulation of these processes. These results suggest that ceramide inhibition not only leads to a reduction in EV release, but also impacts selective trafficking of EV cargo, ultimately altering HSPC function. While other EV biogenesis pathways have been demonstrated to promote HSC-self renewal, these findings implicate ceramide-dependent EV secretion as a novel negative regulator of HSPC repopulation. Transient disruption of ceramide-dependent EV secretion results in a more quiescent and less metabolically active HSPC pool, and additionally prevents bias toward a myeloid-primed progenitor pool seen with ex vivo expansion. These effects culminate in dramatically improved repopulation potential of HSPCs. Furthermore, unlike other small molecular inhibitors proposed to enhance engraftment of HSPCs, including rapamycin, no significant hinderance of ex vivo expansion was seen with GW4869 treatment. We therefore propose broadened exploration of ceramide inhibition for ex vivo maintenance of HSPCs prior to transplantation.

Murine fetal bone marrow does not support functional hematopoietic stem and progenitor cells until birth

While adult bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs) and their extrinsic regulation is well studied, little is known about the composition, function, and extrinsic regulation of the first HSPCs to enter the BM during development. Here, we functionally interrogate murine BM HSPCs from E15.5 through P0. Our work reveals that fetal BM HSPCs are present by E15.5, but distinct from the HSPC pool seen in fetal liver, both phenotypically and functionally, until near birth. We also generate a transcriptional atlas of perinatal BM HSPCs and the BM niche in mice across ontogeny, revealing that fetal BM lacks HSPCs with robust intrinsic stem cell programs, as well as niche cells supportive of HSPCs. In contrast, stem cell programs are preserved in neonatal BM HSPCs, which reside in a niche expressing HSC supportive factors distinct from those seen in adults. Collectively, our results provide important insights into the factors shaping hematopoiesis during this understudied window of hematopoietic development.

Csf1rb regulates definitive hematopoiesis in zebrafish.

In vertebrates, hematopoietic stem and progenitor cells (HSPCs) are capable of self-renewal and continuously replenishing all mature blood lineages throughout life. However, the molecular signaling regulating the maintenance and expansion of HSPCs remains incompletely understood. Colony-stimulating factor 1 receptor (CSF1R) is believed to be the primary regulator for the myeloid lineage but not HSPC development. Here, we show a surprising role of Csf1rb, a zebrafish homolog of mammalian CSF1R, in preserving the HSPC pool by maintaining the proliferation of HSPCs. Deficiency of csf1rb leads to a reduction in both HSPCs and their differentiated progenies, including myeloid, lymphoid and erythroid cells at early developmental stages. Likewise, the absence of csf1rb conferred similar defects upon HSPCs and leukocytes in adulthood. Furthermore, adult hematopoietic cells from csf1rb mutants failed to repopulate immunodeficient zebrafish. Interestingly, loss-of-function and gain-of-function assays suggested that the canonical ligands for Csf1r in zebrafish, including Csf1a, Csf1b and Il34, were unlikely to be ligands of Csf1rb. Thus, our data indicate a previously unappreciated role of Csf1r in maintaining HSPCs, independently of known ligands.

Specificity and Heterogeneity of Trained Immunity in Hematopoietic Stem and Progenitor Cells

When innate immune cells and hematopoietic stem and progenitor cells (HSPCs) are exposed to pathogenic agents, they develop heightened responses to subsequent infection through a process called trained immunity. After exposure to a pathogen, bone marrow derived macrophages (BMDMs) generated from trained HSPCs are capable of enhanced pathogen clearance and cytokine production, while exhibiting persistent metabolic rewiring. Because some models of HSPC trained immunity are dependent on interferon gamma (IFNγ) signaling, and our lab has described extensive changes in HSC self-renewal and differentiation upon IFNγ exposure, we hypothesized that persistent IFNγ signaling induced by chronic infection results in reprogramming of HSPCs, causing improved non-specific immunity. To test our hypothesis, we generated a chimeric mouse model of trained immunity by transplanting control or M. avium-exposed HSPCs into naïve recipient mice. Mice that received M. avium trained HSPCs had decreased bacterial load, less splenomegaly, and fewer granulomas upon subsequent M. avium infection, indicating improved immunity. Furthermore, BMDMs generated from mice trained with a single dose of recombinant IFNγ (rIFNγ) exhibited increased pathogen clearance and metabolic profiles ex vivo. To test if rIFNγ training was sufficient to induce HSPC trained immunity in vivo, we isolated HSPCs from rIFNγ-exposed mice and challenged the recipients 4 months later. These in vivo experiments demonstrated that rIFNγ training was insufficient to induce substantial protection against subsequent M. avium challenge, but still induced BMDM metabolic rewiring four months post HSPC training. Collectively, our studies indicate that there are degrees of training that occur upon IFNγ exposure, likely related to the concentration and duration of the primary stimulus.To assess the specificity of cross protection of HSPC trained immunity, we utilized our chimeric mouse model and tested two different training and infection pathogens: M. avium and influenza. When we challenged M. avium-trained HSPC recipients with influenza, we found that although there was mildly decreased lung histopathology and increased production of IFNγ and TNFα, mice succumbed to infection like untrained controls. When we swapped the order of pathogens, we observed that mice receiving influenza-trained HSPCs produced BMDMs with increased killing capability and systemically higher IL-6 and RANTES levels, but these features were insufficient to significantly reduce bacterial CFU counts upon M. avium challenge. These transplant experiments indicate that trained immunity encoded in HSPCs is pathogen specific.To dissect the mechanism of M. avium-induced trained immunity in HSPCs, we performed RNAseq analysis on M. avium-trained HSPCs post-transplant and cross referenced it with RNAseq and WGBS data on primary M. avium-exposed HSPCs. These studies showed consistent differences in cellular signaling, metabolism, immunity, and antigen processing and presentation in the trained HSPCs, indicating that epigenetic and transcriptional reprogramming induced by M. avium exposure is durable following transplant and secondary challenge.To ascertain whether transcriptional changes are homogeneous throughout the HSC compartment, we completed scRNA-seq on naïve and M. avium-exposed hematopoietic cells. We found that genes upregulated upon M. avium exposure in HSCs, including Batf2 and Cxcl9, were induced in a subset of HSPCs, indicating that there is a heterogeneous response to training within the HSPC pool. Strikingly, the trained immunity signature was maintained in neutrophils and macrophages but lost in mature B cells, indicating specific propagation of genetic signatures induced by training in certain lineages. Finally, we found an emerging population of HSCs with B cell gene signatures upon M. avium exposure. Emergence of this HSPC subpopulation may suggest the development of a cell that acts as a direct intermediate between HSC and B cells following training.Our work shows that trained immunity induced by M. avium and persistent IFNγ signaling is pathogen-specific and heterogeneous among primitive HSPCs. Emergence of specific responder cell populations within the HSPC pool may be responsible for enhanced protection against specific infection stimuli, whereas the presence of non-responders may insure long term health of the HSPC pool. DisclosuresNo relevant conflicts of interest to declare.

P300 suppresses the transition of myelodysplastic syndromes to acute myeloid leukemia.

Myelodysplastic syndromes (MDS) are hematopoietic stem and progenitor cell (HSPC) malignancies characterized by ineffective hematopoiesis and an increased risk of leukemia transformation. Epigenetic regulators are recurrently mutated in MDS, directly implicating epigenetic dysregulation in MDS pathogenesis. Here, we identified a tumor suppressor role of the acetyltransferase p300 in clinically relevant MDS models driven by mutations in the epigenetic regulators TET2, ASXL1, and SRSF2. The loss of p300 enhanced the proliferation and self-renewal capacity of Tet2-deficient HSPCs, resulting in an increased HSPC pool and leukemogenicity in primary and transplantation mouse models. Mechanistically, the loss of p300 in Tet2-deficient HSPCs altered enhancer accessibility and the expression of genes associated with differentiation, proliferation, and leukemia development. Particularly, p300 loss led to an increased expression of Myb, and the depletion of Myb attenuated the proliferation of HSPCs and improved the survival of leukemia-bearing mice. Additionally, we show that chemical inhibition of p300 acetyltransferase activity phenocopied Ep300 deletion in Tet2-deficient HSPCs, whereas activation of p300 activity with a small molecule impaired the self-renewal and leukemogenicity of Tet2-deficient cells. This suggests a potential therapeutic application of p300 activators in the treatment of MDS with TET2 inactivating mutations.

Helios represses megakaryocyte priming in hematopoietic stem and progenitor cells.

Our understanding of cell fate decisions in hematopoietic stem cells is incomplete. Here, we show that the transcription factor Helios is highly expressed in murine hematopoietic stem and progenitor cells (HSPCs), where it is required to suppress the separation of the platelet/megakaryocyte lineage from the HSPC pool. Helios acts mainly in quiescent cells, where it directly represses the megakaryocyte gene expression program in cells as early as the stem cell stage. Helios binding promotes chromatin compaction, notably at the regulatory regions of platelet-specific genes recognized by the Gata2 and Runx1 transcriptional activators, implicated in megakaryocyte priming. Helios null HSPCs are biased toward the megakaryocyte lineage at the expense of the lymphoid and partially resemble cells of aging animals. We propose that Helios acts as a guardian of HSPC pluripotency by continuously repressing the megakaryocyte fate, which in turn allows downstream lymphoid priming to take place. These results highlight the importance of negative and positive priming events in lineage commitment.

FDA-Approved Drug Screening for Compounds That Facilitate Hematopoietic Stem and Progenitor Cells (HSPCs) Expansion in Zebrafish.

Hematopoietic stem cells (HSCs) are a specialized subset of cells with self-renewal and multilineage differentiation potency, which are essential for their function in bone marrow or umbilical cord blood transplantation to treat blood disorders. Expanding the hematopoietic stem and progenitor cells (HSPCs) ex vivo is essential to understand the HSPCs-based therapies potency. Here, we established a screening system in zebrafish by adopting an FDA-approved drug library to identify candidates that could facilitate HSPC expansion. To date, we have screened 171 drugs of 7 categories, including antibacterial, antineoplastic, glucocorticoid, NSAIDS, vitamins, antidepressant, and antipsychotic drugs. We found 21 drugs that contributed to HSPCs expansion, 32 drugs’ administration caused HSPCs diminishment and 118 drugs’ treatment elicited no effect on HSPCs amplification. Among these drugs, we further investigated the vitamin drugs ergocalciferol and panthenol, taking advantage of their acceptability, limited side-effects, and easy delivery. These two drugs, in particular, efficiently expanded the HSPCs pool in a dose-dependent manner. Their application even mitigated the compromised hematopoiesis in an ikzf1−/− mutant. Taken together, our study implied that the larval zebrafish is a suitable model for drug repurposing of effective molecules (especially those already approved for clinical use) that can facilitate HSPCs expansion.

Abrogated GADD45b-Mediated Integrity Control of Hematopoietic Stem Cells upon ER Stress and DNA Damage Triggers Lymphopoiesis over Granulopoiesis in Congenital Neutropenia

In patients with severe congenital neutropenia (CN) with maturation arrest of bone marrow granulopoiesis, hematopoietic and myeloid progenitor cells exhibit elevated endoplasmic reticulum (ER) stress and unfolded protein response (UPR) due to inherited mutations in ELANE (neutrophil elastase, NE) or HAX1 (HCLS1-associated protein X-1). UPR and ER stress may induce genetic instability in hematopoietic stem and progenitor cells (HSPCs). Indeed, we recently reported severe changes in cell cycle control and DNA repair pathways (e.g. CHEK1 and BRCA1) in HSPCs of CN patients, as compared to G-CSF treated healthy individuals. We further studied the molecular mechanism of the hyper-sensitivity of CN HSPC to stress conditions. We found that expression of Growth Arrest and DNA-Damage-inducible, beta (GADD45b), which functions as stress sensor regulating cell cycle, survival and apoptosis in response to ER stress or DNA damage, was markedly reduced in myeloid progenitors of CN patients, as compared to G-CSF-treated healthy individuals. GADD45b is essential in stress-induced murine hematopoiesis and myeloid differentiation is severely compromised in gadd45b knockout mice (Gupta S. et al., 2006). Less is known about the functions of GADD45b in the DNA damage response and myeloid differentiation of human HSPCs. To study GADD45b functions in human hematopoiesis, we generated human induced pluripotent stem cells (iPSC) of healthy individuals haplo-insufficient for GADD45b using CRISPR/Cas9-mediated knockout of GADD45b . Human iPSCs were transfected with GADD45b -specific CRISPR/Cas9-gRNA RNP and gene-edited clones were selected for further study. We could not detect any iPSCs clones with frameshift homozygote or compound heterozygote mutations in exon 1 of GADD45b leading to the complete knockout of GADD45b . Most probably GADD45b-deficient iPSCs are lethal. GADD45b+/- iPSCs also presented a profound phenotype. While these cells retained pluripotency, we detected elevated γH2AX levels in GADD45b+/- iPSCs upon UV light exposure, as compared to GADD45b-WT cells. In line with up-regulated γH2AX levels, we measured strongly induced DNA damage, as quantified by Lord-Q DNA damage assay. We further analyzed myeloid differentiation of GADD45b+/- iPSCs using embryoid body (EB)-based method and found that GADD45b+/- iPSCs cells completely lost the capacity to differentiate into hematopoietic cells. We were able to make EBs of GADD45b+/- cells, these EBs survived and showed no signs of degradation or cell death, but they produced no floating (hematopoietic) cells. To study the effects of GADD45b inhibition in hematopoietic cells we transfected bone marrow CD34+ cells with GADD45b -specific CRISPR/Cas9-gRNA RNP. Similar to iPSCs, we detected markedly elevated UV-induced DNA damage and diminished G-CSF-mediated myeloid differentiation of GADD45b-edited CD34+cells in colony-forming units (CFU) assay. These data strongly support the essential role of GADD45b in hematopoietic and myeloid differentiation. Diminished GADD45b expression in CN HSPCs may increase susceptibility to stress and may contribute to malignant transformation.In order to prevent the appearance and clonal outgrowth of stressed HSPCs with inherited disease-causing mutations in combination with acquired oncogenic mutations, intrinsic DNA damage must be tightly controlled. Failures in the control mechanisms may affect the fate of HSPCs as for instance altered proliferation and wrongly biased differentiation. Therefore, we further evaluated the composition of the HSPC pool in GADD45b-deficient bone marrow cells of CN patients (CN- ELANE n=5, CN- HAX1 n=5), in comparison to healthy individuals treated (n=5) or not (n =7) with G-CSF using multicolor FACS panel of J. Dick. Treatment of healthy individuals with G-CSF induces increase of common myeloid progenitors (CMP, 7-fold) and decrease of multi-lymphoid progenitors (MLP, 10-fold). At the same time, the hematopoietic differentiation in CN patients is substantially shifted from CMP (14-fold decrease) to MLP (36-fold increase) lineage with dramatically reduced numbers of GMPs (3-fold) and increased numbers of B/NK progenitors (5-fold). These data suggest that deregulated integrity control of HSPC pool due to elevated UPR and ER stress and reduced GADD45b expression in hematopoietic cells of CN patients may change composition of HSPC pool and G-CSF response. DisclosuresNo relevant conflicts of interest to declare.

Niche Regulation of Residual Hematopoietic Stem Cell Protein Synthesis By Extracellular Vesicle (EV) Trafficking in the Acute Myeloid Leukemia (AML) Microenvironment

Work by several groups has shown that the presence of Acute Myeloid Leukemia (AML) deregulates critical functions of the bone marrow microenvironment, including hematopoiesis, in part through indirect stroma-secreted factors. We recently reported that Extracellular Vesicles (EV) released from AML blasts directly suppress the clonogenicity of hematopoietic progenitors via the action of miRNA-150 and miR-155 on cMyb, a transcription factor highly expressed in differentiating progenitors. The fate of long-term hematopoietic stem cells (LT- HSC) in the AML niche has not been widely studied. Using a xenograft model of human AML (Molm-14, U937 and HL-60 cell lines), we first ascertained the in vivo uptake of AML-EVs and found different internalization efficiencies among hematopoietic stem and progenitor cell (HSPC) populations, including LT-HSC (CD150+CD48−KSL) by imaging and flow cytometry. Live-cell microscopy studies of LT-HSCs revealed AML-EV binding and uptake within minutes following in vitro exposure. To determine the functional implications of EV trafficking, we next evaluated the relative frequency of the hematopoietic populations in low chimerism xenografts and found a consistent increase in the LT-HSC population, but not multi-potent progenitors, suggestingthe proportional accumulation of LT-HSC population caused by mobilization and lack of expansion and differentiation of other hematopoietic cells. To gain insight into how AML-EVs may regulate cell fate and function following internalization, we performed a proteomic profiling of HSPC cells exposed to EVs from two cell lines representing different AML subtypes. Bioinformatics analysis found significant enrichment for the ribosomal biogenesis pathways and negative cell cycle regulators in the EV-exposed cells. Because LT-HSC have been shown to be particularly susceptible to impairment of ribosome biogenesis, we wished to determine if AML-EVs modulate the LT-HSC fate by suppressing protein synthesis. We therefore injected AML-EVs into the femurs of both immuno-competent and immuno-compromised mice and evaluated the cellular protein synthesis by quantifying O-Propargyl-Puromycin (OPP) incorporation. Results show that AML-EVs significantly suppress the protein synthesis in the LT-HSC, but not in the HSPC pool population. The mTOR signaling pathway is known to be greatly involved in ribosome biogenesis via phosphorylation of the ribosomal protein S6 (S6RP) which in turn regulates several repressors of rRNA and ribosomal proteins. Consistent with the notion of translational suppression, we found that AML-EVs reduced the phosphorylation of S6RP in LT-HSC. In correlation with the observation that ribosomal stress induction via suppression of ribosomal proteins we found upregulation of P53 and its transcriptional target p21, as well as a gain in quiescence (G0) in the LT-HSC population. To address the functional fate of deregulated protein synthesis and quiescence, we transplanted KSL cells purified from male mice engrafted with AML cells into irradiated female recipient mice and measured the donor chimerism via qPCR for Sex-determining Region Y (SRY). Early repopulation capacities between cohorts of mice transplanted with KSL from AML xenograft versus those from control non-engrafted mice are similar (with long-term evaluation ongoing) in the host non-leukemic microenvironment. This suggesting that hematopoietic impairment by AML-EVs is reversible upon transplantation to a different host environment. Collectively, our results provide the first evidence that AML-EV impair LT-HSC via suppressing of ribosomal biogenesis. Understanding the regulation of LT-HSC by AML-EV will help reduce the morbidity and mortality from hematopoietic suppression in patients with leukemia. DisclosuresNo relevant conflicts of interest to declare.

Ddx41 loss R-loops in cGAS to fuel inflammatory HSPC production

In this issue of Developmental Cell, Weinreb etal. reveal that loss of the DEAD-box helicase Ddx41 unexpectedly triggers an R-loop-mediated sterile inflammatory cascade which drives HSPC production during embryonic development. Human studies suggest mechanistic conservation for inflammation in DDX41-associated hematologic disease, uncovering a potential route for future therapeutic intervention.

Stable colony-stimulating factor 1 fusion protein treatment increases hematopoietic stem cell pool and enhances their mobilisation in mice

BackgroundPrior chemotherapy and/or underlying morbidity commonly leads to poor mobilisation of hematopoietic stem cells (HSC) for transplantation in cancer patients. Increasing the number of available HSC prior to mobilisation is a potential strategy to overcome this deficiency. Resident bone marrow (BM) macrophages are essential for maintenance of niches that support HSC and enable engraftment in transplant recipients. Here we examined potential of donor treatment with modified recombinant colony-stimulating factor 1 (CSF1) to influence the HSC niche and expand the HSC pool for autologous transplantation.MethodsWe administered an acute treatment regimen of CSF1 Fc fusion protein (CSF1-Fc, daily injection for 4 consecutive days) to naive C57Bl/6 mice. Treatment impacts on macrophage and HSC number, HSC function and overall hematopoiesis were assessed at both the predicted peak drug action and during post-treatment recovery. A serial treatment strategy using CSF1-Fc followed by granulocyte colony-stimulating factor (G-CSF) was used to interrogate HSC mobilisation impacts. Outcomes were assessed by in situ imaging and ex vivo standard and imaging flow cytometry with functional validation by colony formation and competitive transplantation assay.ResultsCSF1-Fc treatment caused a transient expansion of monocyte-macrophage cells within BM and spleen at the expense of BM B lymphopoiesis and hematopoietic stem and progenitor cell (HSPC) homeostasis. During the recovery phase after cessation of CSF1-Fc treatment, normalisation of hematopoiesis was accompanied by an increase in the total available HSPC pool. Multiple approaches confirmed that CD48−CD150+ HSC do not express the CSF1 receptor, ruling out direct action of CSF1-Fc on these cells. In the spleen, increased HSC was associated with expression of the BM HSC niche macrophage marker CD169 in red pulp macrophages, suggesting elevated spleen engraftment with CD48−CD150+ HSC was secondary to CSF1-Fc macrophage impacts. Competitive transplant assays demonstrated that pre-treatment of donors with CSF1-Fc increased the number and reconstitution potential of HSPC in blood following a HSC mobilising regimen of G-CSF treatment.ConclusionThese results indicate that CSF1-Fc conditioning could represent a therapeutic strategy to overcome poor HSC mobilisation and subsequently improve HSC transplantation outcomes.

Intact in situ Preparation of the Drosophila melanogaster Lymph Gland for a Comprehensive Analysis of Larval Hematopoiesis.

Blood cells have a limited lifespan and are replenished by a small number of hematopoietic stem and progenitor cells (HSPCs). Adult vertebrate hematopoiesis occurs in the bone marrow, liver, and spleen, rendering a comprehensive analysis of the entire HSPC pool nearly impossible. The Drosophila blood system is well studied and has developmental, molecular, and functional parallels with that of vertebrates. Unlike vertebrates, post-embryonic hematopoiesis in Drosophila is essentially restricted to the larval lymph gland (LG), a multi-lobed organ that flanks the dorsal vessel. Because the anterior-most or primary lobes of the LG are easy to dissect out, their cellular and molecular characteristics have been studied in considerable detail. The 2-3 pairs of posterior lobes are more delicate and fragile and have largely been ignored. However, posterior lobes harbor a significant blood progenitor pool, and several hematopoietic mutants show differences in phenotype between the anterior and posterior lobes. Hence, a comprehensive analysis of the LG is important for a thorough understanding of Drosophila hematopoiesis. Most studies focus on isolating the primary lobes by methods that generally dislodge and damage other lobes. To obtain preparations of the whole LG, including intact posterior lobes, here we provide a detailed protocol for larval fillet dissection. This allows accessing and analyzing complete LG lobes, along with dorsal vessel and pericardial cells. We demonstrate that tissue architecture and integrity is maintained and provide methods for quantitative analysis. This protocol can be used to quickly and effectively isolate complete LGs from first instar larval to pupal stages and can be implemented with ease.

Ultrastructure of the Adult Hematopoietic Stem Cell Niche Defined Using Correlative Light and Electron Microscopy

Hematopoietic stem and progenitor cells (HSPCs) that originate from the hemogenic endothelium in the dorsal aorta finally home and engraft within the adult niche, the fetal bone marrow. Within the bone marrow, HSPCs are retained in a complex microenvironment surrounded by niche cells such as megakaryocytes, peripheral nerves, endothelial and mesenchymal stromal cells. While it is known that niche cells secrete hematopoietic factors that are essential for HSPCs to self-renew and repopulate the blood lineages, the ultrastructure of this niche is not well defined as current imaging technology does not allow direct visualization of the fetal bone marrow niche. Zebrafish share a similar hematopoietic ontogeny to mammals, and because the embryos are transparent, intrinsic HSPC interactions with the niche can be directly visualized. To precisely locate rare HSPCs within the large dense kidney marrow, the presumptive adult niche, we genetically tagged endogenous HSPCs to track them live using lightsheet microscopy, followed by high-resolution serial block-face scanning electron microscopy (SBEM) (XY = 10.8 nm/pixel, Z = 70 nm/pixel). Using this technique, we could visually track single mCherry+ HSPCs, then confirm their exact location in the SBEM dataset with high contrast APEX2 peroxidase label. We found HSPC clusters within vessel lumens, as well as single HSPC in a novel perivascular niche. In this perivascular site, a single HSPC simultaneously contacted one mesenchymal stromal cell, multiple endothelial cells, a glial-like cell, and other hematopoietic cells. After extensive tracing of the glial-like cells within the 3D SBEM data, we identified that these cells extended as a long chain and formed contact with HSPCs. Our search for candidate structures in the kidney marrow region suggested that the glial-like cells could be dopamine beta-hydroxylase positive (dbh+) cells. Through fluorescence imaging, we observed cytoplasmic projections extending from dbh+ cells into Runx+ HSPC clusters. To confirm the identity of these glial-like cells, we performed fluorescence imaging of dbh+ transgenic larvae followed by SBEM. By correlating the two datasets, we identified dbh+ glial-like cell in contact with a single HSPC in the niche. Furthermore, treatment with a neurotoxin, 6-hydroxydopamine, led to a significant reduction in HSPC numbers within the niche highlighting the biological significance of dbh+ cells. In summary, through this approach, we could identify a previously unknown cell type within the zebrafish adult hematopoietic niche that is pivotal in maintaining the HSPC pool. Our finding validates the importance of having multiple niche cells that concurrently regulate HSPCs within their niche. Importantly, our approach can be applied to characterize the ultrastructure of rare cells, such as other stem cells, and their niche by using multiple imaging modalities and can also lead to the identification of previously uncharacterized cell types. Further, we can now identify novel intercellular structures that form between an unperturbed HSPC in its endogenous perivascular niche. Disclosures No relevant conflicts of interest to declare.

TNFAIP3 Plays a Role in Aging of the Hematopoietic System.

Hematopoietic stem and progenitor cells (HSPC) experience a functional decline in response to chronic inflammation or aging. Haploinsufficiency of A20, or TNFAIP3, an innate immune regulator, is associated with a variety of autoimmune, inflammatory, and hematologic malignancies. Based on a prior analysis of epigenomic and transcriptomic changes during normal human aging, we find that the expression of A20 is significantly reduced in aged HSPC as compared to young HSPC. Here, we show that the partial reduction of A20 expression in young HSPC results in characteristic features of aging. Specifically, heterozygous deletion of A20 in hematopoietic cells resulted in expansion of the HSPC pool, reduced HSPC fitness, and myeloid-biased hematopoiesis. These findings suggest that altered expression of A20 in HSPC contributes to an aging-like phenotype, and that there may be a common underlying mechanism for diminished HSPC function between inflammatory states and aging.

1015 – HUMAN HAEMATOPOIETIC STEM AND PROGENITOR CELL LANDSCAPES: LOCATION MATTERS

In adults, most hematopoietic stem and progenitor cells (HSPCs) reside within the bone marrow (BM), giving rise to all mature blood cells. Yet at any given time, a small proportion of HSPCs circulates in peripheral blood (PB), and under severe stress and disease, the spleen can significantly contribute to blood production. However, the cellular, molecular and functional composition of circulating and extramedullary HSPC pools remains unexplored. Here I will discuss the single cell characterisation of the adult human HSPC pool found in spleen and non-mobilised PB, comparing and contrasting it to BM. Using matched and unmatched samples from deceased and living donors, we profiled more than 50,000 single CD34+ HSPCs by scRNA-seq and 3,900 single phenotypic haematopoietic stem cells / multipotent progenitors (HSC/MPPs) in functional assays. In BM, we find a topography of the hematopoietic hierarchy that supports continuous HSPC proliferation and blood production. In contrast, the cellular configuration in extramedullary tissues is positioned for lineage-primed demand-adapted haematopoiesis, including a molecularly distinct subset of HSC/MPPs not found in BM. We also find steady-state non-mobilised PB is dominated by quiescent HSC-like cells and non-proliferative early progenitors functionally restricted to erythrocyte and megakaryocyte production. PB HSC/MPPs thus sustain a unique differentiation potential and configuration in healthy conditions, but which become imbalanced with age and in haematological conditions. Overall these data identify extramedullary cellular reservoirs for demand-adapted haematopoiesis and provide a framework of clinical relevance.

2025 - RHEUMATOID ARTHRITIS INCREASES SYMPATHETIC NERVOUS SYSTEM DRIVEN HEMATOPOIETIC STEM CELL MOBILISATION AND ATHEROGENESIS

Rheumatoid arthritis (RA) patients have an increased risk for cardiovascular disease (CVD), independent of traditional risk factors such as plasma cholesterol levels. Interestingly, RA patients have elevated activity of the sympathetic nervous system (SNS), which has been known to play an important role in producing and releasing hematopoietic stem and progenitor cells (HSPCs) to produce inflammatory monocytes, the major contributor to atherosclerotic-CVD. Here we investigated whether increased atherosclerotic progression in mice with RA was due to SNS overactivity. We administered 8wk old C57/B6 mice with KBxN serum to elicit RA and found that RA mice had elevated blood HSPCs and myeloid (monocytes/neutrophils) levels. Further investigation of the bone marrow (BM) revealed an expansion of the HSPC pool, with a strong skewing towards myeloid progenitors, explaining the elevation in blood myeloid populations. Sympathetic activity levels, indicated by tyrosine staining, was also increased in the BM of RA mice. Administering RA mice with propanolol (b1/b2 receptor antagonist) and SR59230A (b3 receptor antagonist) to block SNS activity, led to a reduction in blood HSPC and myeloid populations. To explore the role of RA in atherogenesis we fed Ldlr-/- mice a high fat/high cholesterol diet for 14wks to stimulate lesion development. Mice were then switched to a normal chow diet for 3 weeks to allow lesion regression. Mice were injected with KBxN and treated with or without b-blockers. Mice injected with KBxN had impaired lesion regression which was prevented with b-blockers, likely due to a reduction in HSPC mobilisation and myeloid expansion. Therefore, targeting the SNS in patients with RA with b-blockers may reduce the risk for CVD.