Impaired clonogenic growth of myelodysplastic bone marrow progenitors in vitro is irrelevant to their apoptotic state

Bone Marrow Immune Microenvironment in Myelodysplastic Syndromes

The BM, the major hematopoietic organ in humans, consists of a pleiomorphic environment of cellular, extracellular, and bioactive compounds with continuous and complex interactions between them, leading to the formation of mature blood cells found in the peripheral circulation. Systemic and local inflammation in the BM elicit stress hematopoiesis and drive hematopoietic stem cells (HSCs) out of their quiescent state, as part of a protective pathophysiologic process. However, sustained chronic inflammation impairs HSC function, favors mutagenesis, and predisposes the development of hematologic malignancies, such as myelodysplastic syndromes (MDS). Apart from intrinsic cellular mechanisms, various extrinsic factors of the BM immune microenvironment (IME) emerge as potential determinants of disease initiation and evolution. In MDS, the IME is reprogrammed, initially to prevent the development, but ultimately to support and provide a survival advantage to the dysplastic clone. Specific cellular elements, such as myeloid-derived suppressor cells (MDSCs) are recruited to support and enhance clonal expansion. The immune-mediated inhibition of normal hematopoiesis contributes to peripheral cytopenias of MDS patients, while immunosuppression in late-stage MDS enables immune evasion and disease progression towards acute myeloid leukemia (AML). In this review, we aim to elucidate the role of the mediators of immune response in the initial pathogenesis of MDS and the evolution of the disease.

The orphan nuclear receptor EAR-2 (NR2F6) inhibits hematopoietic cell differentiation and induces myeloid dysplasia in vivo

Background

In patients with myelodysplastic syndrome (MDS), bone marrow cells have an increased predisposition to apoptosis, yet MDS cells outcompete normal bone marrow (BM)-- suggesting that factors regulating growth potential may be important in MDS. We previously identified v-Erb A related-2 (EAR-2, NR2F6) as a gene involved in control of growth ability.

Methods

Bone marrow obtained from C57BL/6 mice was transfected with a retrovirus containing EAR-2-IRES-GFP. Ex vivo transduced cells were flow sorted. In some experiments cells were cultured in vitro, in other experiments cells were injected into lethally irradiated recipients, along with non-transduced bone marrow cells. Short-hairpin RNA silencing EAR-2 was also introduced into bone marrow cells cultured ex vivo.

Results

Here, we show that EAR-2 inhibits maturation of normal BM in vitro and in vivo and that EAR-2 transplant chimeras demonstrate key features of MDS. Competitive repopulation of lethally irradiated murine hosts with EAR-2-transduced BM cells resulted in increased engraftment and increased colony formation in serial replating experiments. Recipients of EAR-2-transduced grafts had hypercellular BM, erythroid dysplasia, abnormal localization of immature precursors and increased blasts; secondary transplantation resulted in acute leukemia. Animals were cytopenic, having reduced numbers of erythrocytes, monocytes and granulocytes. Suspension culture confirmed that EAR-2 inhibits granulocytic and monocytic differentiation, while knockdown induced granulocytic differentiation. We observed a reduction in the number of BFU-E and CFU-GM colonies and the size of erythroid and myeloid colonies. Serial replating of transduced hematopoietic colonies revealed extended replating potential in EAR-2-overexpressing BM, while knockdown reduced re-plating ability. EAR-2 functions by recruitment of histone deacetylases, and inhibition of differentiation in 32D cells is dependent on the DNA binding domain.

Conclusions

This data suggest that NR2F6 inhibits maturation of normal BM in vitro and in vivo and that the NR2F6 transplant chimera system demonstrates key features of MDS, and could provide a mouse model for MDS.

Oxidative stress due to radiation in CD34(+) hematopoietic progenitor cells: protection by IGF-1

Radiation exerts direct as well as indirect effects on DNA through the generation of reactive oxygen species (ROS). Irradiated hematopoietic progenitor cells (HPCs) experience DNA strand breaks, favoring genetic instability, due to ROS generation. Our aim was to study the effect of a range of radiation doses in HPCs and the possible protective mechanisms activated by insulin-like growth factor-1 (IGF-1). ROS generation was evaluated, in the presence or absence of IGF-1 in liquid cultures of human HPCs-CD34(+) irradiated with 1-, 2- and 5-Gy X-rays, using a flow cytometry assay. Manganese superoxide dismutase (MnSOD) expression was studied by western blot analysis and visualized by an immunofluorescence assay. Apoptosis was estimated using the following assays: Annexin-V assay, DNA degradation assay, BCL-2/BAX mRNA and protein levels and caspase-9 protein immunofluorescence visualization. Viability and clonogenic potential were studied in irradiated HPCs. The generation of superoxide anion radicals at an early and a late time point was increased, while the hydrogen peroxide generation at a late time point was stable. IGF-1 presence further enhanced the radiation-induced increase of MnSOD at 24 h post irradiation. IGF-1 inhibited the mitochondria-mediated pathway of apoptosis by regulating the m-RNA and protein expression of BAX, BCL-2 and the BCL-2/BAX ratio and by decreasing caspase-9 protein expression. IGF-1 presence in culture media of irradiated cells restored the clonogenic capacity and the viability of HPCs as well. In conclusion, IGF-1 protects HPCs-CD34(+) from radiation effects, by eliminating the oxidative microenvironment through the enhancement of MnSOD activation and by regulating the mitochondria-mediated pathway of apoptosis.

Hypocellular myelodysplasia

Myelodysplasia must be considered in the differential diagnosis of patients who have bone marrow failure, but bone marrow cellularity per se may not substantially affect either response to therapy or prognosis. It is unclear whether the primary pathophysiologic defect differs between hyper- and hypoplastic patients who have myelodysplasia. Cellularity does not seem to affect response to immunosuppressive therapy significantly and does not seem to be the major factor affecting improvements in response to lenalidomide, stem cell transplantation, or hematopoietic growth factors.

CD34 cells from patients with trisomy 8 myelodysplastic syndrome (MDS) express early apoptotic markers but avoid programmed cell death by up-regulation of antiapoptotic proteins

CD34 cells from patients with trisomy 8 myelodysplastic syndrome (MDS) are distinguished from other MDS cells and from normal hematopoietic cells by their pronounced expression of apoptotic markers. Paradoxically, trisomy 8 clones can persist in patients with bone marrow failure and expand following immunosuppression. We previously demonstrated up-regulation of c-myc and CD1 by microarray analysis. Here, we confirmed these findings by real-time polymerase chain reaction (PCR), demonstrated up-regulation of survivin, c-myc, and CD1 protein expression, and documented comparable colony formation by annexin(+) trisomy 8(-) CD34(+) and annexin(-) CD34 cells. There were low levels of DNA degradation in annexin(+) trisomy 8 CD34 cells, which were comparable with annexin(-) CD34 cells. Trisomy 8 cells were resistant to apoptosis induced by gamma irradiation. Knock-down of survivin by siRNA resulted in preferential loss of trisomy 8 cells. These results suggest that trisomy 8 cells undergo incomplete apoptosis and are nonetheless capable of colony formation and growth.

Impaired generation of bone marrow CD34-derived dendritic cells with low peripheral blood subsets in patients with myelodysplastic syndrome

Myelodysplastic syndrome (MDS) is a stem cell disorder characterized by ineffective haematopoiesis and blood cytopenias. The present study investigated the potential of bone marrow CD34(+) progenitors in MDS patients to proliferate and differentiate into dendritic cells (DCs) in a cytokine-supplemented liquid culture system and analysed the status of blood DC subsets in these patients. CD34(+) progenitors had low potential to generate DCs in vitro, as the number of DCs obtained from one CD34(+) cell was significantly lower compared with controls (median value 0.2 vs. 4, P = 0.003). In patients, the survival and proliferation of CD34(+) cells in culture was not correlated to the degree of apoptosis. Phenotypically and functionally CD34(+)-derived DCs were similar in MDS patients and normal subjects. The percentage of both circulating DC subsets in patients was extremely diminished compared with controls (myeloid DC: 0.10 +/- 0.10% vs. 0.35 +/- 0.13%, P < 0.001; plasmacytoid DC: 0.11 +/- 0.10% vs. 0.37 +/- 0.14%, P < 0.001). In cases with the 5q deletion both CD34-derived DCs and blood DCs harboured the cytogenetic abnormality. Our results indicate that, in MDS, the production of DCs is affected by the neoplastic process resulting in ineffective 'dendritopoiesis' with low blood DC precursor numbers. This quantitative DC defect probably contributes to the poor immune response against infectious agents and to the escape of the malignant clone from immune recognition with disease progression towards acute leukaemia.