Human long-term bone marrow cultures in aplastic anemia

Hematopoietic stem cell loss and hematopoietic failure in severe aplastic anemia is driven by macrophages and aberrant podoplanin expression

Severe aplastic anemia (SAA) results from profound hematopoietic stem cell loss. T cells and interferon gamma (IFNγ) have long been associated with SAA, yet the underlying mechanisms driving hematopoietic stem cell loss remain unknown. Using a mouse model of SAA, we demonstrate that IFNγ-dependent hematopoietic stem cell loss required macrophages. IFNγ was necessary for bone marrow macrophage persistence, despite loss of other myeloid cells and hematopoietic stem cells. Depleting macrophages or abrogating IFNγ signaling specifically in macrophages did not impair T-cell activation or IFNγ production in the bone marrow but rescued hematopoietic stem cells and reduced mortality. Thus, macrophages are not required for induction of IFNγ in SAA and rather act as sensors of IFNγ. Macrophage depletion rescued thrombocytopenia, increased bone marrow megakaryocytes, preserved platelet-primed stem cells, and increased the platelet-repopulating capacity of transplanted hematopoietic stem cells. In addition to the hematopoietic effects, SAA induced loss of non-hematopoietic stromal populations, including podoplanin-positive stromal cells. However, a subset of podoplanin-positive macrophages was increased during disease, and blockade of podoplanin in mice was sufficient to rescue disease. Our data further our understanding of disease pathogenesis, demonstrating a novel role for macrophages as sensors of IFNγ, thus illustrating an important role for the microenvironment in the pathogenesis of SAA.

Characterization of bone marrow mesenchymal stromal cells in aplastic anaemia

In aplastic anaemia (AA), haemopoietic activity is significantly reduced and generally attributed to failure of haemopoietic stem cells (HSC) within the bone marrow (BM). The regulation of haemopoiesis depends on the interaction between HSC and various cells of the BM microenvironment, including mesenchymal stromal cells (MSC). MSC involvement in the functional restriction of HSC in AA is largely unknown and therefore, the physical and functional properties of AA MSC were studied in vitro. MSC were characterized by their phenotype and ability to form adherent stromal layers. The functional properties of AA MSC were assessed through proliferative, clonogenic and cross-over culture assays. Results indicate that although AA MSC presented typical morphology and distinctive mesenchymal markers, stromal formation was reduced, with 50% of BM samples failing to produce adherent layers. Furthermore, their proliferative and clonogenic capacity was markedly decreased (P = 0·03 and P = 0·04 respectively) and the ability to sustain haemopoiesis was significantly reduced, as assessed by total cell proliferation (P = 0·032 and P = 0·019 at Week 5 and 6, respectively) and clonogenic potential of HSC (P = 0·02 at Week 6). It was concluded that the biological characteristics of AA MSC are different from those of control MSC and their in vitro haemopoiesis-supporting ability is significantly reduced.

Alteration in marrow stromal microenvironment and apoptosis mechanisms involved in aplastic anemia: an animal model to study the possible disease pathology

Aplastic anemia (AA) is a heterogeneous disorder of bone marrow failure syndrome. Suggested mechanisms include a primary stem cell deficiency or defect, a secondary stem cell defect due to abnormal regulation between cell death and differentiation, or a deficient microenvironment. In this study, we have tried to investigate the alterations in hematopoietic microenvironment and underlying mechanisms involved in such alterations in an animal model of drug induced AA. We presented the results of studying long term marrow culture, marrow ultra-structure, marrow adherent and hematopoietic progenitor cell colony formation, flowcytometric analysis of marrow stem and stromal progenitor populations and apoptosis mechanism involved in aplastic anemia. The AA marrow showed impairment in cellular proliferation and maturation and failed to generate a functional stromal microenvironment even after 19 days of culture. Ultra-structural analysis showed a degenerated and deformed marrow cellular association in AA. Colony forming units (CFUs) were also severely reduced in AA. Significantly decreased marrow stem and stromal progenitor population with subsequently increased expression levels of both the extracellular and intracellular apoptosis inducer markers in the AA marrow cells essentially pointed towards the defective hematopoiesis; moreover, a deficient and apoptotic microenvironment and the microenvironmental components might have played the important role in the possible pathogenesis of AA.

Primitive Sca-1 Positive Bone Marrow HSC in Mouse Model of Aplastic Anemia: A Comparative Study through Flowcytometric Analysis and Scanning Electron Microscopy

Self-renewing Hematopoietic Stem Cells (HSCs) are responsible for reconstitution of all blood cell lineages. Sca-1 is the “stem cell antigen” marker used to identify the primitive murine HSC population, the expression of which decreases upon differentiation to other mature cell types. Sca-1⁺ HSCs maintain the bone marrow stem cell pool throughout the life. Aplastic anemia is a disease considered to involve primary stem cell deficiency and is characterized by severe pancytopenia and a decline in healthy blood cell generation system. Studies conducted in our laboratory revealed that the primitive Sca-1⁺ BM-HSCs (bone marrow hematopoietic stem cell) are significantly affected in experimental Aplastic animals pretreated with chemotherapeutic drugs (Busulfan and Cyclophosphamide) and there is increased Caspase-3 activity with consecutive high Annexin-V positivity leading to premature apoptosis in the bone marrow hematopoietic stem cell population in Aplastic condition. The Sca-1^bright, that is, “more primitive” BM-HSC population was more affected than the “less primitive” BM-HSC Sca-1^dim population. The decreased cell population and the receptor expression were directly associated with an empty and deranged marrow microenvironment, which is evident from scanning electron microscopy (SEM). The above experimental evidences hint toward the manipulation of receptor expression for the benefit of cytotherapy by primitive stem cell population in Aplastic anemia cases.

Long-term bone marrow cultures in aplastic anaemia

The long-term bone marrow culture (LTBMC) system provides an in vitro physiological model for the study of stromal cell mediated haemopoiesis in patients with aplastic anaemia. The two aspects of haemopoiesis--stromal and stem cell function--can be analysed separately using a modification of LTBMC with cross-over studies. Patients with aplastic anaemia universally demonstrate defective stem cell function in terms of reduced or absent marrow repopulating ability, reflecting a deficiency of long-term culture initiating cells. Defects in stromal cell function, as assessed by the ability of aplastic anaemia stroma to support normal generation of haemopoietic progenitors, are not common, but may conceal an isolated deficiency of a particular growth factor in some patients due to the overlapping nature of haemopoietic growth factor activities. The stem cell abnormality in aplastic anaemia reflects a deficiency in cell numbers, as well as dysfunction in certain cases. An increased level of apoptosis in aplastic anaemia marrow CD34+ cells exists, and this correlates well with disease severity. LTBMC studies demonstrate that more of the haemopoietic cells are nonviable (apoptotic and dead) compared with normal controls, and this correlates with reduced colony (CFU-GM) generation. An increase in apoptosis among primitive haemopoietic cells may contribute to the stem cell defect in aplastic anaemia. Haemopoietic growth factors such as G-CSF, when given after immunosuppressive therapy such as antilymphocyte globulin and cyclosporin for aplastic anaemia, may act partly by reducing the increased level of apoptosis, resulting in improved stem cell survival.

Quiescent (5-fluorouracil-resistant) aplastic anemia hematopoietic cells in vitro

OBJECTIVE

Bone marrow from aplastic anemia (AA) patients shows reduced numbers in long-term culture (LTC)-initiating cell (LTC-IC) assays. The LTC-IC assay is based on assumptions of the culture kinetics of normal hematopoietic stem cells (HSC), which are not necessarily justified in a disease state. We therefore undertook a detailed examination of the kinetics of quiescent HSC from AA patients in LTC.

METHODS

Colony formation by quiescent HSC in LTC was tested by pretreating control (n=6) and AA bone marrow (n=7) with 5-fluorouracil. Secondly, we manipulated normal samples to inoculate cultures with proportions of CD34+ cells similar to those from AA samples. We obtained enough CD34+ cells to reconstitute one AA sample to "normal" levels.

RESULTS

Patient cells showed altered kinetics with rapid proliferation and premature termination of LTC. In vivo, decreased numbers of HSC may induce rapid proliferation and differentiation; a similar phenomenon could explain the observations in culture. We therefore manipulated normal samples to contain a proportion of CD34+ HSC similar to that in AA samples. Although absolute numbers of secondary colonies in LTC were reduced, the kinetics of culture were not altered. However, when AA CD34+ HSC were reconstituted to "normal" levels, the cultures still demonstrated early termination.

CONCLUSIONS

The kinetics of LTC are not affected by CD34+ HSC number. However, quiescent HSC derived from patients with AA have qualitative differences from normal cells, as reflected by distinct kinetics in long-term culture. This has implications for the interpretation of the LTC-IC assay with AA samples.

Reduced TGF-beta1 in patients with aplastic anaemia in vivo and in vitro

Transforming growth factor beta (TGF-beta) 1 is a ubiquitous bifunctional cytokine implicated in the regulation of haemopoietic stem cells and bone marrow stromal cells. We analysed sera from 63 patients with aplastic anaemia and describe a significant reduction of TGF-beta1 that was directly related to their treatment status. Untreated patients (n = 35), patients who did not respond (n = 15) and those with a partial response (n = 23) to treatment had significantly lower TGF-beta1 than the normal control group (n = 55), P < 0.0001, P < 0.0001 and P = 0.002 respectively. Patients in complete remission (n = 15) exhibited TGF-beta1 serum levels comparable to the control group. In addition, there was a correlation (r = 0.83, P < 0.0001) between serum TGF-beta1 and platelet count at time of sample. We have demonstrated that the primary source of TGF-beta1 in peripheral blood mononuclear cell (PBMC) cultures was not CD3-positive cells. These data indicate aplastic anaemia is associated with a decreased TGF-beta1 expression in peripheral blood circulation, which may be a direct consequence of thrombocytopenia. In vitro stromal layers grown from aplastic patient bone marrow (n = 14) produced significantly lower levels of TGF-beta1 (P = 0.02) when compared to normal stroma (n = 15). In the aplastic anaemia bone marrow compartment we postulate that accessory cells down-regulate TGF-beta1 expression to allow stem cell cycling to counteract hypoplasia. As TGF-beta1 is important in the regulation of haemopoiesis, dysregulation of this cytokine in combination with previously described abnormal cytokine expression may contribute significantly to the pathophysiology of aplastic anaemia by exacerbating primary stem cell defects.

GATA-1 Regulates Growth and Differentiation of Definitive Erythroid Lineage Cells During In Vitro ES Cell Differentiation

Although the importance of GATA-1 in both primitive and definitive hematopoietic lineages has been shown in vivo, the precise roles played by GATA-1 during definitive hematopoiesis have not yet been clarified. In vitro differentiation of embryonic stem (ES) cells using OP9 stroma cells can generate primitive and definitive hematopoietic cells separately, and we have introduced a method that separates hematopoietic progenitors and differentiated cells produced in this system. Closer examination showed that the expression of erythroid transcription factors in this system is regulated in a differentiation stage-specific manner. Therefore, we examined differentiation of GATA-1 promoter-disrupted (GATA-1.05) ES cells using this system. Because the GATA-1.05 mice die by 12.5 embryonic days due to the lack of primitive hematopoiesis, the in vitro analysis is an important approach to elucidate the roles of GATA-1 in definitive hematopoiesis. Consistent with the in vivo observation, differentiation of GATA-1.05 mutant ES cells along both primitive and definitive lineages was arrested in this ES cell culture system. Although the maturation-arrested primitive lineage cells did not express detectable amounts of ɛy-globin mRNA, the blastlike cells accumulated in the definitive stage showed β-globin mRNA expression at approximately 70% of the wild type. Importantly, the TER119 antigen was expressed and porphyrin was accumulated in the definitive cells, although the levels of both were reduced to approximately 10%, indicating that maturation of definitive erythroid cells is arrested by the lack of GATA-1 with different timing from that of the primitive erythroid cells. We also found that the hematopoietic progenitor fraction of GATA-1.05 cells contains more colony-forming activity, termed CFU-OP9. These results suggest that theGATA-1.05 mutation resulted in proliferation of proerythroblasts in the definitive lineage.

GATA-1 Regulates Growth and Differentiation of Definitive Erythroid Lineage Cells During In Vitro ES Cell Differentiation

Although the importance of GATA-1 in both primitive and definitive hematopoietic lineages has been shown in vivo, the precise roles played by GATA-1 during definitive hematopoiesis have not yet been clarified. In vitro differentiation of embryonic stem (ES) cells using OP9 stroma cells can generate primitive and definitive hematopoietic cells separately, and we have introduced a method that separates hematopoietic progenitors and differentiated cells produced in this system. Closer examination showed that the expression of erythroid transcription factors in this system is regulated in a differentiation stage-specific manner. Therefore, we examined differentiation of GATA-1 promoter-disrupted (GATA-1.05) ES cells using this system. Because the GATA-1.05 mice die by 12.5 embryonic days due to the lack of primitive hematopoiesis, the in vitro analysis is an important approach to elucidate the roles of GATA-1 in definitive hematopoiesis. Consistent with the in vivo observation, differentiation of GATA-1.05 mutant ES cells along both primitive and definitive lineages was arrested in this ES cell culture system. Although the maturation-arrested primitive lineage cells did not express detectable amounts of ɛy-globin mRNA, the blastlike cells accumulated in the definitive stage showed β-globin mRNA expression at approximately 70% of the wild type. Importantly, the TER119 antigen was expressed and porphyrin was accumulated in the definitive cells, although the levels of both were reduced to approximately 10%, indicating that maturation of definitive erythroid cells is arrested by the lack of GATA-1 with different timing from that of the primitive erythroid cells. We also found that the hematopoietic progenitor fraction of GATA-1.05 cells contains more colony-forming activity, termed CFU-OP9. These results suggest that theGATA-1.05 mutation resulted in proliferation of proerythroblasts in the definitive lineage.

Successful treatment of aplastic anemia with G-CSF and high dose erythropoietin

We report the successful treatment of pancytopenia with G-CSF and high dose erythropoietin (Epo) in an elderly patient diagnosed with aplastic anemia (AA). Furthermore this effect is dose dependent for Epo in vivo. Detection of apoptosis by gel electrophoresis shows that high dose Epo protects bone marrow mononuclear cells from spontaneous apoptosis in vitro. These findings may explain some of the mechanisms of aplastic anemia.

Progressive Telomere Shortening in Aplastic Anemia

Improved survival in aplastic anemia (AA) has shown a high incidence of late clonal marrow disorders. To investigate whether accelerated senescence of hematopoietic stem cells might underlie the pathophysiology of myelodysplasia (MDS) or paroxysmal nocturnal hemoglobinuria (PNH) occurring as a late complication of AA, we studied mean telomere length (TRF) in peripheral blood leukocytes from 79 patients with AA, Fanconi anemia, or PNH in comparison with normal controls. TRF lengths in the patient group were significantly shorter for age than normals (P < .0001). Telomere shortening was apparent in both granulocyte and mononuclear cell fractions, suggesting loss at the level of the hematopoietic stem cell. In patients with acquired AA with persistent cytopenias (n = 40), there was significant correlation between telomere loss and disease duration (r = −.685; P < .0001), equivalent to progressive telomere erosion at 216 bp/yr, in addition to the normal age-related loss. In patients who had achieved normal full blood counts (n = 20), the rate of telomere loss had apparently stabilised. There was no apparent association between telomere loss and secondary PNH (n = 13). However, of the 5 patients in the study with TRF less than 5.0 kb, 3 had acquired cytogenetic abnormalities, suggesting that telomere erosion may be relevant to the pathogenesis of MDS in aplastic anemia.

Progressive Telomere Shortening in Aplastic Anemia

Improved survival in aplastic anemia (AA) has shown a high incidence of late clonal marrow disorders. To investigate whether accelerated senescence of hematopoietic stem cells might underlie the pathophysiology of myelodysplasia (MDS) or paroxysmal nocturnal hemoglobinuria (PNH) occurring as a late complication of AA, we studied mean telomere length (TRF) in peripheral blood leukocytes from 79 patients with AA, Fanconi anemia, or PNH in comparison with normal controls. TRF lengths in the patient group were significantly shorter for age than normals (P < .0001). Telomere shortening was apparent in both granulocyte and mononuclear cell fractions, suggesting loss at the level of the hematopoietic stem cell. In patients with acquired AA with persistent cytopenias (n = 40), there was significant correlation between telomere loss and disease duration (r = −.685; P < .0001), equivalent to progressive telomere erosion at 216 bp/yr, in addition to the normal age-related loss. In patients who had achieved normal full blood counts (n = 20), the rate of telomere loss had apparently stabilised. There was no apparent association between telomere loss and secondary PNH (n = 13). However, of the 5 patients in the study with TRF less than 5.0 kb, 3 had acquired cytogenetic abnormalities, suggesting that telomere erosion may be relevant to the pathogenesis of MDS in aplastic anemia.

Haemopoietic growth factor production by normal and aplastic anaemia stroma in long-term bone marrow culture

Defective marrow stroma, or microenvironment, have been proposed as one of several mechanisms to account for bone marrow failure in aplastic anaemia (AA). This could involve defects in positive- or negative-acting haemopoietic regulator expression by AA stroma, or alteration of normal stroma-stem cell interactions. We have used a sensitive bioassay to investigate production of granulocyte-colony stimulating factor (G-CSF), granulocyte-macrophage-colony stimulating factor (GM-CSF), interleukin (IL)-3, IL-6 and stem cell growth factor (SCF), by normal and AA stroma in long-term bone marrow culture (LTBMC). LTBMC were grown to confluence, irradiated and harvested to yield a single cell suspension. These cells were cocultured with normal target bone marrow mononuclear cells (BMMC), or CD34+ cells, in clonogenic assays, in the absence of exogenous cytokines. Cytokines responsible for the colony-stimulating activity (CSA) and burst-promoting activity (BPA) produced by stromal cells were identified by neutralizing antibodies to specific cytokines. All normal stroma populations produced G-CSF and GM-CSF, 93% produced IL-3, 80% produced IL-6, and 70% produced SCF. Similarly, all AA stroma produced G-CSF and GM-CSF, and 71% produced SCF. In contrast, only 71% of AA stroma produced IL-3 and 36% produced IL-6. Target cell stimulation was not dependent on direct stroma-target cell contact, suggesting production of soluble cytokines. However, although both IL-6 and G-CSF were detected in LTBMC supernatants by enzyme-linked immunoassay (ELISA), IL-3 and GM-CSF were undetectable, perhaps indicating low-level local production of these factors.

Haemopoietic progenitor cells are reduced in aplastic anaemia

We investigated the frequencies of early populations of progenitors in aplastic anaemia (AA) bone marrow, from patients with a range of disease severity, compared with normal. Double-colour immunofluorescent staining for CD34 and CD33 was carried out on bone marrow mononuclear cells (BMMC) and analysed using fluorescence activated cell sorting (FACS). AA CD34+ cells were reduced by 68% compared to normal. In addition, AA CD33+ cells and the three progenitor subsets (CD34+/CD33-, CD34+/CD33+ and CD34-/CD33+) were reduced by 44-80%. Our data lend further support for an early stem cell deficiency in AA.

In vitro revelations of aplastic anemia

Aplastic anemia (AA) is a most difficult disease to study in vitro. By the time the disease presents, the marrow is already hypocellular and the peripheral blood shows pancytopenia, leaving little material remaining for study. However, an understanding of its pathogenesis could provide insight into the control of normal hemopoiesis since AA is an in vivo manifestation of failure of normal hemopoiesis and may provide a way of examining stromal cell-stem cell relationships. Recent interest in the pathogenesis of AA has resulted from a) new laboratory techniques, such as stem cell purification used with modifications of the long-term bone marrow culture system and analysis of stem cells at the molecular level with X-linked DNA probes, and b) the availability of recombinant human hemopoietic growth factors (HGF) in large quantities. Consequently, analyses of the function of some of the individual components of stromal cell mediated hemopoiesis in AA patients have been performed. This has been paralleled, and in some instances preceded, by clinical trials of HGF in patients with AA.

The hematopoietic microenvironment

Hematopoietic microenvironment is comprised of an admixture of several adherent cell types including fibroblasts, reticular adventitial cells, and marcophages. The biologic interaction of these cells with the most primitive hematopoietic progenitor cells capable of reconstituting all hematopoietic lineages within an irradiated host, as well as differentiated progenitor cells and cells of each committed lineage, has been the subject of intense investigation. Transplantation of the hematopoietic microenvironment has recently been demonstrated and this technique has been used to partially correct the microenvironmental defect in the Sl/Sld mouse. The molecular mechanism of cell surface interaction between stromal and hematopoietic stem cells is being elucidated by molecular transfection techniques in which genes for specific receptors are introduced into hematopoietic stem cell lines and then demonstrated to adhere and proliferate in contact with stromal cells expressing transfected recombinant ligands. This model has been demonstrated with the EGF receptor bearing 32D cl 3 stem cells bound to TGF alpha-producing stromal cells. Extracellular matrix components of the adherent cell layer, the binding of hematopoietic growth factor interaction with matrix components, as well as the positive and negative feedback regulatory role of hematopoietic stem cells bound to the microenvironment, represents the focus of current investigation.

Bone marrow stromal elements in murine leukemia: decreased CSF-producing fibroblasts and normal IL-1 expression by macrophages

A study of bone marrow stromal elements in murine acute myeloid leukemia (AML) was carried out. Our previous studies had indicated marrow stromal deficiency in murine AML. In the current investigation, separate stromal cells were cultured and the results obtained have shown that, while marrow stromal macrophages are normal in leukemia and express adequate amounts of IL-1, the fibroblasts are markedly reduced. However, if sufficient fibroblasts are pooled in vitro, they produce adequate amounts of CSF. Test of TNF alpha in leukemic cells CM, as possible cause of marrow stromal inhibition in leukemia, had not disclosed this cytokine. Further, it was observed that total body lethal irradiation of leukemic mice aggravates the stromal deficiency, confirming results of our previous investigations. It is concluded that bone marrow stromal deficiency in murine AML is due to decreased fibroblasts and, implicitly, reduced CSF production.