Medicine. The blood stem cell Holy Grail?

Coinhibition of activated p38 MAPKα and mTORC1 potentiates stemness maintenance of HSCs from SR1-expanded human cord blood CD34+ cells via inhibition of senescence

The stemness of ex vivo expanded hematopoietic stem cells (HSCs) is usually compromised by current methods. To explore the failure mechanism of stemness maintenance of human HSCs, which were expanded from human umbilical cord blood (hUCB) CD34⁺ cells, by differentiation inhibitor Stem Regenin 1 (SR1), an antagonist of aryl hydrocarbon receptor, we investigated the activity of p38 mitogen‐activated protein kinase α (p38 MAPKα, p38α) and mammalian target of rapamycin complex 1 (mTORC1), and their effect on SR1‐expanded hUCB CD34⁺ cells. Our results showed that cellular senescence occurred in the SR1‐expanded hUCB CD34⁺ cells in which p38α and mTORC1 were successively activated. Furthermore, their coinhibition resulted in a further decrease in hUCB CD34⁺ cell senescence without an effect on apoptosis, promoted the maintenance of expanded phenotypic HSCs without differentiation inhibition, increased the hematopoietic reconstitution ability of multiple lineages, and potentiated the long‐term self‐renewal capability of HSCs from SR1‐expanded hUCB CD34⁺ cells in NOD/Shi‐scid/IL‐2Rγ^null mice. Our mechanistic study revealed that senescence inhibition by our strategy was mainly attributed to downregulation of the splicesome, proteasome formation, and pyrimidine metabolism signaling pathways. These results suggest that coinhibition of activated p38α and mTORC1 potentiates stemness maintenance of HSCs from SR1‐expanded hUCB CD34⁺ cells via senescence inhibition. Thus, we established a new strategy to maintain the stemness of ex vivo differentiation inhibitor‐expanded human HSCs via coinhibition of multiple independent senescence initiating signal pathways. This senescence inhibition‐induced stemness maintenance of ex vivo expanded HSCs could also have an important role in other HSC expansion systems.

Engineering cord blood to improve engraftment after cord blood transplant

Umbilical cord blood transplant (CBT) has traditionally been associated with slower engraftment of neutrophils, delayed immune reconstitution and consequently higher risk of infections as compared with peripheral blood progenitor cell (PBPC) or bone marrow (BM) transplants. This is primarily due to low numbers of total nucleated cells (TNCs) and the naive nature of CB immune cells. The use of double unit CB transplant (DCBT) increases the total cell dose in the graft, but it still does not produce as rapid engraftment as seen with PBPC or even BM transplants. Herein, we discuss strategies to improve engraftment after CBT. We describe methods of (I) expansion of CB graft ex vivo to increase the total cell dose; and (II) enhancement of BM homing capability of CB progenitor cells; (III) ex vivo expansion of CB derived T cells for improving T cell function against viruses, tumors and protection from graft versus host disease (GVHD). With these novel approaches, engraftment after CBT is now reaching levels comparable to that of other graft types.

Platelets from mice lacking the aryl hydrocarbon receptor exhibit defective collagen-dependent signaling

BACKGROUND

We previously identified aryl hydrocarbon receptor (AHR) as a novel regulator of megakaryocytic differentiation and polyploidization and reported that AHR-null mice have approximately 15% fewer platelets than do wild-type mice, yet they exhibit a dramatic, unexplained bleeding phenotype.

OBJECTIVES

The current work tests our hypothesis that AHR-null platelets are functionally deficient, contributing to the previously reported (yet unexplained) bleeding phenotype present in AHR-null mice.

METHODS

AHR-null bone marrow was ex vivo differentiated with thrombopoietin with or without AHR ligands or AHR inhibitors and analyzed for degree of megakaryopoiesis and polyploidization. Platelet function of AHR-null mice was assessed with aggregation and spreading assays. Platelet signaling was examined using Western analysis and Rac activity assays.

RESULTS

AHR ligands differentiate murine bone marrow-derived progenitors into polyploid megakaryocytes in the absence of thrombopoietin, and AHR inhibitors block thrombopoietin-induced megakaryocytic differentiation. Despite their responsiveness toward thrombin, AHR-null platelets demonstrate decreased aggregation and spreading in response to collagen compared with wild-type platelets. AHR-null platelets bind fibrinogen after stimulation with thrombin or AYPGKF and aggregate in response to AYPGKF and adenosine diphosphate. Mechanistically, AHR absence led to down-regulation of Vav1 and Vav3, altered phospholipase Cγ2 phosphorylation, decreased Rac1 activation, and reduced platelet activation in response to collagen.

CONCLUSIONS

These results are consistent with a role for AHR in platelet function, especially as it relates to platelet aggregation and spreading in response to collagen. Our work suggests AHR is a critical component of the physiologic response that platelets undergo in response to collagen and may provide novel treatment options for patients with bleeding disorders.

New relationships of human hematopoietic lineages facilitate detection of multipotent hematopoietic stem and progenitor cells

Three important goals of hematopoietic stem cell research are to understand of how hematopoietic stem cells (HSCs) self-renew, how lineage commitment takes place, and how HSCs can be expanded ex vivo. Research in this area requires a reliable model of hematopoiesis. Performing detailed functional analyses of human hematopoietic progenitor subsets, we recently gained evidence for new hematopoietic lineage relationships. (1) According to our data, neutrophils belong to the same branch of the hematopoietic tree as lymphocytes. In contrast, eosinophils and basophils derive from another branch, the erythro-myeloid branch. Here, after introducing the newly proposed hematopoietic model, we discuss its consequences for the identification and expansion of human multipotent progenitors and suggest a fast and reliable method to screen for multipotent hematopoietic cells in vitro.

Haematopoietic stem cell niches: new insights inspire new questions

Haematopoietic stem cell (HSC) niches provide an environment essential for life-long HSC function. Intense investigation of HSC niches both feed off and drive technology development to increase our capability to assay functionally defined cells with high resolution. A major driving force behind the desire to understand the basic biology of HSC niches is the clear implications for clinical therapies. Here, with particular emphasis on cell type-specific deletion of SCL and CXCL12, we focus on unresolved issues on HSC niches, framed around some very recent advances and novel discoveries on the extrinsic regulation of HSC maintenance. We also provide ideas for possible paths forward, some of which are clearly within reach while others will require both novel tools and vision.

Inhibition of p38 MAPK activity promotes ex vivo expansion of human cord blood hematopoietic stem cells

Ex vivo expansion of hematopoietic stem cells (HSCs) depends on HSC self-renewing proliferation and functional maintenance, which can be negatively affected by HSC differentiation, apoptosis, and senescence. Therefore, inhibition of HSC senescence may promote HSC expansion. To test this hypothesis, we examined the effect of inhibition of p38 mitogen-activated protein kinase (p38) on the expansion of human umbilical cord blood (hUCB) CD133(+) cells because activation of p38 has been implicated in the induction of HSC senescence under various physiological and pathological conditions. Our results showed that ex vivo expansion of hUCB CD133(+) cells activated p38, which was abrogated by the p38 specific inhibitor SB203580 (SB). Inhibition of p38 activity with SB promoted the expansion of CD133(+) cells and CD133(+)CD38(-) cells. In addition, hUCB CD133(+) cells expanded in the presence of SB for 7 days showed about threefold increase in the clonogenic function of HSCs and engraftment in non-obese diabetic/severe combined immunodeficient mice after transplantation compared to the input cells. In contrast, the cells expanded without SB exhibited a significant reduction in these HSC functions. The enhancement of ex vivo expansion of hUCB HSCs is primarily attributable to SB-mediated inhibition of HSC senescence. In addition, inhibition of HSC apoptosis and upregulation of CXCR4 may also contribute to the enhancement. However, p38 inhibition had no significant effect on HSC differentiation and proliferation. These findings suggest that inhibition of p38 activation may represent a novel strategy to promote ex vivo expansion of hUCB HSCs.

Human CD34+ CD133+ hematopoietic stem cells cultured with growth factors including Angptl5 efficiently engraft adult NOD-SCID Il2rγ-/- (NSG) mice

Increasing demand for human hematopoietic stem cells (HSCs) in clinical and research applications necessitates expansion of HSCs in vitro. Before these cells can be used they must be carefully evaluated to assess their stem cell activity. Here, we expanded cord blood CD34⁺ CD133⁺ cells in a defined medium containing angiopoietin like 5 and insulin-like growth factor binding protein 2 and evaluated the cells for stem cell activity in NOD-SCID Il2rg^−/− (NSG) mice by multi-lineage engraftment, long term reconstitution, limiting dilution and serial reconstitution. The phenotype of expanded cells was characterized by flow cytometry during the course of expansion and following engraftment in mice. We show that the SCID repopulating activity resides in the CD34⁺ CD133⁺ fraction of expanded cells and that CD34⁺ CD133⁺ cell number correlates with SCID repopulating activity before and after culture. The expanded cells mediate long-term hematopoiesis and serial reconstitution in NSG mice. Furthermore, they efficiently reconstitute not only neonate but also adult NSG recipients, generating human blood cell populations similar to those reported in mice reconstituted with uncultured human HSCs. These findings suggest an expansion of long term HSCs in our culture and show that expression of CD34 and CD133 serves as a marker for HSC activity in human cord blood cell cultures. The ability to expand human HSCs in vitro should facilitate clinical use of HSCs and large-scale construction of humanized mice from the same donor for research applications.

Serum after autologous transplantation stimulates proliferation and expansion of human hematopoietic progenitor cells

Regeneration after hematopoietic stem cell transplantation (HSCT) depends on enormous activation of the stem cell pool. So far, it is hardly understood how these cells are recruited into proliferation and self-renewal. In this study, we have addressed the question if systemically released factors are involved in activation of hematopoietic stem and progenitor cells (HPC) after autologous HSCT. Serum was taken from patients before chemotherapy, during neutropenia and after hematopoietic recovery. Subsequently, it was used as supplement for in vitro culture of CD34⁺ cord blood HPC. Serum taken under hematopoietic stress (4 to 11 days after HSCT) significantly enhanced proliferation, maintained primitive immunophenotype (CD34⁺, CD133⁺, CD45⁻) for more cell divisions and increased colony forming units (CFU) as well as the number of cobblestone area-forming cells (CAFC). The stimulatory effect decays to normal levels after hematopoietic recovery (more than 2 weeks after HSCT). Chemokine profiling revealed a decline of several growth-factors during neutropenia, including platelet-derived growth factors PDGF-AA, PDGF-AB and PDGF-BB, whereas expression of monocyte chemotactic protein-1 (MCP-1) increased. These results demonstrate that systemically released factors play an important role for stimulation of hematopoietic regeneration after autologous HSCT. This feedback mechanism opens new perspectives for in vivo stimulation of the stem cell pool.

Synergistic effects of growth factors and mesenchymal stromal cells for expansion of hematopoietic stem and progenitor cells

OBJECTIVE

The number of hematopoietic stem and progenitor cells (HPCs) per cord blood unit is limited, and this can result in delayed engraftment or graft failure. In vitro expansion of HPCs provides a perspective to overcome these limitations. Cytokines as well as mesenchymal stromal cells (MSCs) have been shown to support HPCs ex vivo expansion, but a systematic analysis of their interplay remains elusive.

MATERIALS AND METHODS

Twenty different combinations of growth factors (stem cell factor [SCF], thrombopoietin [TPO], fibroblast growth factor-1 [FGF-1], angiopoietin-like 5, and insulin-like growth factor-binding protein 2), either with or without MSC coculture were systematically compared for their ability to support HPC expansion. CD34(+) cells were stained with carboxyfluorescein diacetate N-succinimidyl ester to monitor cell division history in conjunction with immunophenotype. Colony-forming unit frequencies and hematopoietic reconstitution of nonobese diabetic severe combined immunodeficient mice were also assessed.

RESULTS

Proliferation of HPCs was stimulated by coculture with MSCs. This was further enhanced in combination with SCF, TPO, and FGF-1. Moreover, these conditions maintained expression of primitive surface markers for more than four cell divisions. Colony-forming unit-initiating cells were not expanded without stromal support, whereas an eightfold increase was reached by simultaneous cytokine-treatment and MSC coculture. Importantly, in comparison to expansion without stromal support, coculture with MSCs significantly enhanced hematopoietic chimerism in a murine transplantation model.

CONCLUSIONS

The supportive effect of MSCs on hematopoiesis can be significantly increased by addition of specific recombinant growth factors; especially in combination with SCF, TPO, and FGF-1.