Blockade of BCL-2 proteins efficiently induces apoptosis in progenitor cells of high-risk myelodysplastic syndromes patients.

Deregulated apoptosis is an identifying feature of myelodysplastic syndromes (MDS). Whereas apoptosis is increased in the bone marrow (BM) of low-risk MDS patients, progression to high-risk MDS correlates with an acquired resistance to apoptosis and an aberrant expression of BCL-2 proteins. To overcome the acquired apoptotic resistance in high-risk MDS, we investigated the induction of apoptosis by inhibition of pro-survival BCL-2 proteins using the BCL-2/-XL/-W inhibitor ABT-737 or the BCL-2-selective inhibitor ABT-199. We characterized a cohort of 124 primary human BM samples from MDS/secondary acute myeloid leukemia (sAML) patients and 57 healthy, age-matched controls. Inhibition of anti-apoptotic BCL-2 proteins was specifically toxic for BM cells from high-risk MDS and sAML patients, whereas low-risk MDS or healthy controls remained unaffected. Notably, ABT-737 or ABT-199 treatment was capable of targeting the MDS stem/progenitor compartment in high-risk MDS/sAML samples as shown by the reduction in CD34(+) cells and the decreased colony-forming capacity. Elevated expression of MCL-1 conveyed resistance against both compounds. Protection by stromal cells only partially inhibited induction of apoptosis. Collectively, our data show that the apoptotic resistance observed in high-risk MDS/sAML cells can be overcome by the ABT-737 or ABT-199 treatment and implies that BH3 mimetics might delay disease progression in higher-risk MDS or sAML patients.

Murine stromal cells producing hyper-interleukin-6 and Flt3 ligand support expansion of early human hematopoietic progenitor cells without need of exogenous growth factors.

Expansion of primitive hematopoietic progenitor cells (HPC) is a major challenge in stem cell biology. Stimulation by growth factors (GF) is essential for proliferation of HPC, while the role of stromal cell coculture for maintenance of progenitor/stem cell potential is unclear. We evaluated the potential of a murine stromal cell layer providing hematopoietic GF to support expansion of human CD34(+) cells. Murine MS-5 cells were transfected with the cDNA encoding huFlt3 ligand and the interleukin6/sinterleukin-6R fusion protein hyper-IL-6. Expansion of CFC and week6 CAFC was at least as efficient in transfected clones compared to control cocultures supported with exogenous GF. Cell numbers reached 17.5- to 62.3- (day 14) and 17.4- to 92.4-fold (day 21) of input cells. Expansion of CFU-GM/Mix was 4.0- to 12.8-fold (day 14) and 4.9- to 11.7-fold (day 21). Primitive week6 CAFC were expanded up to 6.5-fold (day 14) and 6.2-fold (day 21) without exogenous GF. When direct contact of HPC and stromal cells was inhibited, a loss of CFC and much more of CAFC potential was observed with unaffected overall cell proliferation. Here, we show the generation of GF producing murine stromal cells which efficiently support early hematopoiesis without exogenous GF. Direct stromal cell-HPC contact is advantageous for maintenance of differentiation potential.

Effects of imatinib on bone marrow engraftment in syngeneic mice.

Chronic myeloid leukemia (CML) and a subset of acute lymphoblastic leukemias arise from the genetic reciprocal translocation t(9;22), forming the BCR-ABL fusion gene. These lead to the expression of the constitutively active tyrosine kinase BCR-ABL, which is the causative oncogene for these leukemias. Allogeneic bone marrow transplantation (BMT) or stem cell transplantation (SCT) is currently considered the only curative treatment for chronic myeloid leukemia (CML). Recently, the selective tyrosine kinase inhibitor imatinib mesylate (Glivec, formerly STI-571) has been shown to induce durable hematologic and major cytogenetic responses in a high percentage of patients with chronic phase CML. In patients with advanced disease remissions are transient and most patients relapse despite continued imatinib treatment. Some of these patients go on to receive allogeneic BMT or SCT, during which administration of imatinib is usually discontinued as it is believed to interfere with bone marrow engraftment. In this study, we examined the effect of imatinib on hematopoietic engraftment in a syngeneic mouse model. We found that imatinib has no significant influence on hematopoietic recovery in lethally irradiated mice in vivo. Thus, our results suggest that continued administration of imatinib in the course of BMT or SCT may be a feasible therapeutic regimen.

gp130-stimulating designer cytokine Hyper-interleukin-6 synergizes with murine stroma for long-term survival of primitive human hematopoietic progenitor cells.

OBJECTIVE: Experimental strategies for ex vivo expansion of human hematopoietic stem/progenitor cells involve the use of exogenous cytokines as well as direct interaction with stromal elements. We examined the use of the interleukin-6/soluble interleukin-6 receptor (IL-6/sIL-6R) fusion protein Hyper-IL-6 (H-IL-6), which interacts directly with gp130, in conjunction with stromal support for expansion of human progenitors. MATERIALS AND METHODS: Peripheral blood CD34+ cells were cultured on the murine stromal cell line FBMD1 or in suspension for up to 28 days with different cytokines. Cells were evaluated at various time points for phenotype, proliferative and clonogenic capacity, and long-term hematopoietic activity. RESULTS: The combination of Flt3 ligand and H-IL-6 was markedly more effective than Flt3 ligand and IL-6/sIL-6R for expansion of CD34+ cells in suspension culture and on FBMD1. Addition of kit ligand but not thrombopoietin to Flt3 ligand and H-IL-6 significantly augmented proliferation and enhanced colony formation three-fold. However, long-term cobblestone area-forming cell assays indicated that although multipotent progenitors were maintained up to 21 days on FBMD1 in the presence of Flt3 ligand alone and were amplified three-fold by addition of H-IL-6, CD34+ cells cultured in the absence of stromal support rapidly lost their cobblestone area-forming cell potential. Immunophenotyping revealed that stromal support prevented up-regulation of IL-6R on CD34+ cells, which was induced within 3 days in stroma-free cultures and was enhanced in the presence of kit ligand. Delayed addition of H-IL-6 to the cultures resulted in reduced proliferation and colony-forming unit potential. CONCLUSION: H-IL-6 synergizes with stromal elements to effectively enhance proliferation and maintenance of primitive hematopoietic progenitors under prolonged ex vivo culture conditions.

Flt3high and Flt3low CD34+ progenitor cells isolated from human bone marrow are functionally distinct.

We generated monoclonal antibodies against the human Flt3 receptor and used them to study the characteristics of normal human bone marrow cells resolved based on Flt3 expression. Human CD34+ or CD34+lin- marrow cells were sorted into two populations: cells expressing high levels of Flt3 receptor (Flt3high) and cells with little or no expression of Flt3 receptor (Flt3low). Flt3 receptor was detected on a subset of CD34+CD38- marrow cells, as well as on CD34+CD19+ B lymphoid progenitors and CD34+CD14+CD64+ monocytic precursors. Flt3 receptor was also present on more mature CD34-CD14+ monocytes. In colony-forming assays, Flt3high cells gave rise mainly to colony-forming unit-granulocyte-macrophage (CFU-GM) colonies, whereas Flt3low cells produced mostly burst-forming unit-erythroid colonies. There was no difference in the number of multilineage CFU-Mix colonies between the two cell fractions. Cell cycle analysis showed that a large number of the Flt3low cells were in the G0 phase of the cell cycle, whereas Flt3high cells were predominantly in G1. Cell numbers in the suspension cultures initiated with Flt3high cells were maintained in the presence of Flt3 ligand (FL) alone, and increased in response to FL plus kit ligand (KL). In contrast, cell numbers in the suspension cultures started with Flt3low cells did not increase in the presence of FL, or FL plus KL. Upregulation of Flt3 receptor on Flt3low cells was not detected during suspension culture. CD14+ monocytes were the major cell type generated from CD34+lin-Flt3high cells in liquid suspension culture, whereas cells generated from CD34+lin-Flt3low cells were mainly CD71+GlycA+ erythroid cells. These results show clear functional differences between CD34+Flt3high and CD34+Flt3low cells and may have implications concerning the in vitro expansion of human hematopoietic progenitor cells.

Human herpes virus 8 interleukin-6 homologue triggers gp130 on neuronal and hematopoietic cells.

Human herpes virus-8 (HHV8) encodes a cytokine named viral interleukin-6 (vIL-6) that shares 25% amino-acid identity with its human homologue. Human IL-6 is known to be a growth and differentiation factor of lymphatic cells and plays a potential role in the pathophysiology of various lymphoproliferative diseases. vIL-6 is expressed in HHV8-associated-diseases including Kaposi's sarcoma, Body-cavity-based-lymphoma and Castleman's disease, suggesting a pathogenetic involvement in the malignant growth of B-cell associated diseases and other malignant tumours. We expressed vIL-6 in Escherichia coli as a fusion protein with recombinant periplasmic maltose binding protein. After cleavage from the maltose binding protein moiety and purification, vIL-6 was shown to be correctly folded using circular dichroism spectroscopy. A rabbit antiserum was raised against the recombinant vIL-6 protein. vIL-6 turned out to be active on cells that expressed gp130 but no IL-6 receptor (IL-6-R) suggesting that, in contrast to human IL-6, vIL-6 stimulated gp130 directly. Accordingly, vIL-6 activity could be inhibited by a soluble gp130 Fc Fusion protein. vIL-6 was shown to induce neuronal differentiation of rat pheochromocytoma cells and to stimulate colony formation of human hematopoietic progenitor cells. Thus, vIL-6 exhibits biologic activity that has only been observed for the IL-6/soluble IL-6-R complex but not for IL-6 alone. These properties are important for the evaluation of the pathophysiological potential of vIL-6.