Selective expansion of normal haemopoietic progenitors from chronic myelogenous leukaemia marrow

Molecular and functional analysis of the stem cell compartment of chronic myelogenous leukemia reveals the presence of a CD34- cell population with intrinsic resistance to imatinib

We show the molecular and functional characterization of a novel population of lineage-negative CD34-negative (Lin(-)CD34(-)) hematopoietic stem cells from chronic myelogenous leukemia (CML) patients at diagnosis. Molecular karyotyping and quantitative analysis of BCR-ABL transcript demonstrated that approximately one-third of CD34(-) cells are leukemic. CML Lin(-)CD34(-) cells showed kinetic quiescence and limited clonogenic capacity. However, stroma-dependent cultures induced CD34 expression on some cells and cell cycling, and increased clonogenic activity and expression of BCR-ABL transcript. Lin(-)CD34(-) cells showed hematopoietic cell engraftment rate in 2 immunodeficient mouse strains similar to Lin-CD34(+) cells, whereas endothelial cell engraftment was significantly higher. Gene expression profiling revealed the down-regulation of cell-cycle arrest genes and genes involved in antigen presentation and processing, while the expression of genes related to tumor progression, such as angiogenic factors, was strongly up-regulated compared with normal counterparts. Phenotypic analysis confirmed the significant down-regulation of HLA class I and II molecules in CML Lin(-)CD34(-) cells. Imatinib mesylate did not reduce fusion transcript levels, BCR-ABL kinase activity, and clonogenic efficiency of CML Lin(-)CD34(-) cells in vitro. Moreover, leukemic CD34(-) cells survived exposure to BCR-ABL inhibitors in vivo. Thus, we identified a novel CD34(-) leukemic stem cell subset in CML with peculiar molecular and functional characteristics.

Cell context-specific effects of the BCR-ABL oncogene monitored in hematopoietic progenitors

Acute BCR-ABL expression during in vitro hematopoietic development of embryonic stem (ES) cells causes expansion of multipotent and myeloid progenitors with a concomitant reduction in differentiation toward erythroblasts. Progenitor cell expansion is due to a rapid, cell autonomous, suppression of programmed cell death with an increase in expression of the antiapoptotic molecule BCL-X(L). Other antiapoptotic effectors, including AKT, STAT5, and BCL-2 are not up-regulated by BCR-ABL in this system. In addition, the proapoptotic p38 mitogen-activated protein kinase (MAPK) pathway is suppressed by BCR-ABL expression in ES-derived hematopoietic progenitors. Inhibition of p38 MAPK by the small molecule inhibitor SB203580 expanded ES-derived hematopoietic progenitors by an antiapoptotic mechanism and is sufficient to expand ES-derived hematopoietic progenitors to levels approaching 80% of that seen following BCR-ABL expression. In the cellular context of ES-derived hematopoietic progenitors, BCR-ABL expression expands cells by suppressing programmed cell death with a set of antiapoptotic pathways distinct from those previously reported in continuous cell line studies.

Stem cell factor and FLT3-ligand are strictly required to sustain the long-term expansion of primitive CD34+DR- dendritic cell precursors

We studied cytokine-driven differentiation of primitive human CD34(+)HLA-DR(-) cells to myeloid dendritic cells (DC). Hemopoietic cells were grown in long-term cultures in the presence of various combinations of early acting cytokines such as FLT3-ligand (FLT3-L) and stem cell factor (SCF) and the differentiating growth factors GM-CSF and TNF-alpha. Two weeks of incubation with GM-CSF and TNF-alpha generated fully functional DC. However, clonogenic assays demonstrated that CFU-DC did not survive beyond 1 wk in liquid culture regardless of whether FLT3-L and/or SCF were added. FLT3-L or SCF alone did not support DC maturation. However, the combination of the two early acting cytokines allowed a 100-fold expansion of CFU-DC for >1 month. Phenotypic analysis demonstrated the differentiation of CD34(+)DR(-) cells into CD34(-)CD33(+)DR(+)CD14(+) cells, which were intermediate progenitors capable of differentiating into functionally active DC upon further incubation with GM-CSF and TNF-alpha. As expected, GM-CSF and TNF-alpha generated DC from committed CD34(+)DR(+) cells. However, only SCF, with or without FLT3-L, induced the expansion of DC precursors for >4 wk, as documented by secondary clonogenic assays. This demonstrates that although GM-CSF and TNF-alpha do not require additional cytokines to generate DC from primitive human CD34(+)DR(-) progenitor cells, they do force terminal differentiation of DC precursors. Conversely, FLT3-L and SCF do not directly affect DC differentiation, but instead sustain the long-term expansion of CFU-DC, which can be induced to produce mature DC by GM-CSF and TNF-alpha.

Evaluation at single cell level of residual Philadelphia negative hemopoietic stem cells in chronic phase CML patients

In chronic myeloid leukemia, accurate determination of Ph(-) Hemopoietic stem cells (HSC) in peripheral blood (PB), bone marrow (BM) and leukapheresis products is important for the selection of patients for whom mobilization, collection, and autografting of Ph(-) HSC are envisaged. To this effect, the BCR/ABL fusion was assessed at the single cell level in 25 sets of PB and BM samples using dual-color I-FISH in immunophenotyped CD34(+) cells and RT-PCR of individual CFU-GM colonies. In 15 cases found to be 100% Ph(+), the respective BCR/ABL gene was absent in 30% of CD34(+) cells, while the respective transcripts could not be identified in 17% of CFU-GM. The mean percentage of BCR/ABL(-) CD34(+) cells and CFU-GM cells was higher (38% and 29%, respectively) in untreated patients than in treated patients (24% and 7%, respectively). In eight cases with cytogenetic response (CgR), the percentage of Ph(-) metaphases correlated with the level of BCR/ABL(-) colonies in BM and PB and with the proportion of BCR/ABL(-) CD34(+) cells in the BM. Immunophenotyping and FISH was fast, easy, always informative, and quantitative for the BCR/ABL(-) CD34(+) cells. Our results show that (a) at early diagnosis a high frequency of BCR/ABL(-) HSC circulate in the PB and that Ph(-) hematopoiesis is not completely suppressed; (b) although normal clonogenic cells decline rapidly within a few months after diagnosis, appreciable numbers of normal CD34(+) cells survive in chronic phase, especially in patients with CgR.

In vitro anti-tumour activity of anti-CD80 and anti-CD86 immunotoxins containing type 1 ribosome-inactivating proteins

Immunotoxins specific for the CD80 and CD86 antigens were prepared by linking three type 1 ribosome-inactivating proteins (RIPs), namely bouganin, gelonin and saporin-S6, to the monoclonal antibodies M24 (anti-CD80) and 1G10 (anti-CD86). These immunotoxins showed a specific cytotoxicity for the CD80/CD86-expressing cell lines Raji and L428. The immunotoxins inhibited protein synthesis by target cells with IC50s (concentration causing 50% inhibition) ranging from 0.25 to 192 pmol/l as RIPs. The anti-CD80 immunotoxins appeared 1-2 log more toxic for target cells than the anti-CD86 ones. Immunotoxins containing saporin and bouganin induced apoptosis of target cells. The toxicity for bone marrow haemopoietic progenitors of these conjugates was also evaluated. Bouganin and related immunotoxins at concentrations up to 100 nmol/l did not significantly affect the recovery of committed progenitors or of more primitive cells. The saporin-containing immunotoxins at concentrations >/= 1 nmol/l showed some toxicity on colony-forming unit cells (CFU-C). The expression of the CD80 and CD86 molecules is prevalently restricted to antigen-presenting cells and is also strong on Hodgkin and Reed-Sternberg cells in Hodgkin's disease. Present results suggest that immunotoxins targeting type 1 ribosome-inactivating proteins to these antigens could be considered and further studied for the therapy of Hodgkin's disease or other CD80/CD86-expressing tumours.

Transforming growth factor beta3 inhibits chronic myelogenous leukemia hematopoiesis by inducing Fas-independent apoptosis

OBJECTIVE

Transforming growth factor beta3 (TGF-beta3) is a potent suppressor of human hematopoietic progenitor cells. In this article, we compare the activity of TGF-beta3 on highly purified CD34+ cells and more immature CD34-DR(-) cells from chronic myelogenous leukemia (CML) patients in chronic phase and normal donors.

MATERIALS AND METHODS

Primitive hematopoietic progenitors were stimulated in liquid cultures and clonogenic assays by early-acting growth factors such as stem cell factor (SCF) and interleukin 11 (IL-11) and the intermediate-late-acting stimulating factors IL-3, granulocyte-macrophage colony-stimulating factor, and erythropoietin. Molecular analysis of bcr/abl mRNA was performed on single CML colonies by nested reverse transcriptase polymerase chain reaction. Moreover, cell cycle analysis and assessment of apoptosis of normal and leukemic CD34+ cells were performed by propidium iodide (PI) alone and simultaneous staining with annexin V and PI, respectively.

RESULTS

The colony-forming efficiency of CML CD34+ cells was generally inhibited by more than 90% regardless of whether the colony-stimulating factors were used alone or combined. When compared to normal CD34+ cells, leukemic cells were significantly more suppressed in 6 of 8 culture conditions. The inhibitory effect of TGF-beta3 on CD34+ cells was exerted within the first 24 hours of incubation as demonstrated by short-term preincubation followed by IL-3-and SCF-stimulated colony assays. Evaluation of bcr/abl transcript on residual CML colonies incubated with TGF-beta3 demonstrated a small subset of neoplastic CD34+ cells unresponsive to the inhibitory effect of the study cytokine. TGF-beta3 demonstrated a greater inhibitory activity on primitive CD34+DR cells than on more mature CD34+ cells. Again, CML CD34+DR(-) cells were significantly more inhibited by TGF-beta3 than their normal counterparts in 3 of 8 culture conditions. Kinetic analysis performed on CD34+ cells showed that TGF-beta induces cell cycle arrest in G(1) phase. However, this mechanism of action is shared by normal and leukemic cells. Conversely, TGF-beta3 preferentially triggered the programmed cell death of CML CD34-cells without increasing the proportion of leukemic cells coexpressing CD95 (Fas receptor), and this effect was not reversed by functional blockade of Fas receptor. Conclusion. We demonstrate that TGF-beta3 exerts a potent suppressive effect on CML cells that is partly mediated by Fas-independent apoptosis.

Stem cell factor and chronic myeloid leukemia CD34+ cells

Normal hematopoiesis is a tightly regulated process involving a balance between signals that stimulate and those that inhibit the proliferation and differentiation of hematopoietic progenitors. In chronic myeloid leukemia (CML) there is a perturbation of these controlling elements, resulting in overgrowth of leukemic cells in the bone marrow and spleen. In part, the proliferation of CML CD34+ cells may result from an abnormal response to the cytokine Stem Cell Factor (SCF). SCF induced proliferation and adhesion to the extracellular matrix via fibronectin are not coupled in CML as they are in normal cells and this may contribute to the accumulation of leukemic progenitors. We have previously shown that CD34+ CML cells and the more primitive CD34+ CD38- CML cells do not require the addition of synergistic cytokines to cultures, but are capable of proliferation in SCF alone, and that leukemic CFU-GM are selectively supported in these cultures. In the presence of other cytokines the response of CML cells to SCF is no greater than that of cells from normal donors, suggesting that the leukemic cells are not more sensitive to SCF, but that accessory pathways are already activated in these cells. Cells from patients with myeloproliferative disorders show variable proliferative response to SCF as the sole mitogenic stimulus, suggesting that expression of bcr-abl is essential for proliferation in this cytokine. Further studies to identify the key determinants of the abnormal response to SCF in CML may lead to a better understanding of the proliferative abnormality that underlies CML.

Selection and transplantation of autologous CD34+ B-lineage negative cells in advanced-phase multiple myeloma patients: a pilot study

The feasibility of sequential positive and negative selection of mobilized CD34+ B-lineage negative cells to achieve tumour-free autografts in multiple myeloma (MM) patients was evaluated. Peripheral blood stem cells (PBSC) of 14 patients with advanced disease were mobilized. CD34+ cells were enriched in 12 of the patients by the avidin-biotin immunoabsorption technique. Subsequently, CD10+, CD19+, CD20+ and CD56+ cells (B-lin cells) were removed by immunomagnetic depletion. Minimal residual disease (MRD) was detected by flow cytometry and PCR-based molecular analysis of the patient specific IgH complementary-determining region III (CDRIII). Positive selection of stem cells produced a median recovery of 54.7% of the initial content of CD34+ cells (median purity 71.9%). Negative depletion of B-lineage cells reduced the number of CD34+ cells to 33.3% of the baseline value (median purity 72.7%). However, long-term culture assays showed the recovery of >60% of primitive haemopoietic progenitor cells after depletion of the B-lineage-positive cells. All evaluable patients had detectable disease in PBSC collections. The first step of positive selection of CD34+ cells resulted in >2 logs of tumour cell purging. However, molecular assessment showed the persistence of the disease in 6/7 cases. Immunofluorescence analysis demonstrated 1 additional log of B-cell purging by negative depletion. More importantly, molecular evaluation of IgH CDRIII region showed the disappearance of myeloma cells in 6/7 patients. 12 patients received a median of 3.9 x 106 CD34+ B-lin- cells/kg after conditioning with high-dose melphalan and showed a rapid reconstitution of haemopoiesis. These results were similar to two similar cohorts of patients who received either unmanipulated PBSC or positively selected CD34+ cells after the same conditioning regimen. Severe extrahaematological toxicity was limited to mucositis; no late infections were observed. We concluded that autotransplantation of purified CD34+ B-lin- cells was associated with a rapid and sustained recovery of haemopoiesis and low peritransplant morbidity. Sequential positive and negative enrichment of stem cells reduced tumour cell contamination in B-cell malignancies below the lower limit of detection of molecular analysis.

Chronic myelogenous leukemia

Over the past year, new information has been reported on the biology and treatment of chronic myelogenous leukemia (CML). Chronic myelogenous leukemia is characterized by the breakpoint cluster region (BCR-ABL) chimeric gene, the product of which is p210BCR-ABL, a tyrosine kinase that gives hematopoietic cells the characteristics of excessive proliferation, resistance to physiologic apoptotic signals, and resistance to chemotherapy. Recently, investigators have attempted to 1) elucidate the mechanisms by which the BCR-ABL gene and its product initiate and maintain the malignant phenotype, 2) improve the use of the BCR-ABL gene as a diagnostic marker of disease, and 3) inhibit the expression of this gene as a therapeutic maneuver. Other investigators have tried to explain interferon's mechanism of action in the treatment of CML and to improve the safety and applicability of stem cell transplantation (SCT) as a therapy for CML.