Prevalence and Growth Characteristics of Malignant Stem Cells in B-Lineage Acute Lymphoblastic Leukemia

Notch signaling in acute lymphoblastic leukemia: any role for stromal microenvironment?

Notch signaling pathway regulates many different events of embryonic and adult development; among them, Notch plays an essential role in the onset of hematopoietic stem cells and influences multiple maturation steps of developing lymphoid and myeloid cells. Deregulation of Notch signaling determines several human disorders, including cancer. In the last decade it became evident that Notch signaling plays pivotal roles in the onset and development of T- and B-cell acute lymphoblastic leukemia by regulating the intracellular molecular pathways involved in leukemia cell survival and proliferation. On the other hand, bone marrow stromal cells are equally necessary for leukemia cell survival by preventing blast cell apoptosis and favoring their reciprocal interactions and cross-talk with bone marrow microenvironment. Quite surprisingly, the link between Notch signaling pathway and bone marrow stromal cells in acute lymphoblastic leukemia has been pointed out only recently. In fact, bone marrow stromal cells express Notch receptors and ligands, through which they can interact with and influence normal and leukemia T- and B-cell survival. Here, the data concerning the development of T- and B-cell acute lymphoblastic leukemia has been critically reviewed in light of the most recent findings on Notch signaling in stromal microenvironment.

Notch-3 and Notch-4 signaling rescue from apoptosis human B-ALL cells in contact with human bone marrow-derived mesenchymal stromal cells

Although many literature data are available on the role of Notch signaling in T-cell acute lymphoblastic leukemia (ALL) biology, the importance of this molecular pathway in the development of B-lineage ALL (B-ALL) cells in the BM microenvironment is unknown so far. In this study, we used anti-Notch molecules neutralizing Abs and γ-secretase inhibitor (GSI) XII to investigate the role of the Notch signaling pathway in the promotion of human B-ALL cell survival in presence of stromal cell support. The treatment with combinations of anti-Notch molecule neutralizing Abs resulted in the decrease of B-ALL cell survival, either cultured alone or cocultured in presence of stromal cells from normal donors and B-ALL patients. Interestingly, the inhibition of Notch-3 and -4 or Jagged-1/-2 and DLL-1 resulted in a dramatic increase of apoptotic B-ALL cells by 3 days, similar to what is obtained by blocking all Notch signaling with the GSI XII. Our data suggest that the stromal cell-mediated antiapoptotic effect on B- ALL cells is mediated by Notch-3 and -4 or Jagged-1/-2 and DLL-1 in a synergistic manner.

CD38-Mediated Signaling Events in Murine Pro-B Cells Expressing Human CD38 With or Without Its Cytoplasmic Domain

To elucidate the signaling mechanism of CD38 (a transmembrane molecule highly expressed in immature hemopoietic cells), we transfected Ba/F3 murine pro-B cells with a cDNA encoding human CD38. CD38 ligation with anti-CD38 Abs caused rapid, transient, dose-dependent tyrosine phosphorylation of several proteins, including the tyrosine kinase TEC and the adaptor molecule CBL, and association of tyrosine-phosphorylated proteins with phosphatidylinositol 3-kinase p85. Exposure to anti-CD38 Abs or their F(ab′)2 and Fab also induced tight aggregation of CD38-transfected Ba/F3 cells, which appeared to be Ca2+ and Mg2+ independent and did not involved LFA-1. Aggregation was abrogated by addition of the tyrosine kinase inhibitor herbimycin A and was delayed by the phosphatidylinositol 3-kinase inhibitor wortmannin, suggesting a link between biochemical events and cellular effects induced by CD38. Cell aggregation was accompanied by a decrease in cell recovery. After 3 days of culture on bone marrow-derived stroma, the mean (±SD) cell recovery in the presence of anti-CD38 (T16) was 10.5 ± 9.2% (n = 7) of that in parallel cultures with an isotype-matched nonreactive Ab. Finally, CD38 ligation in Ba/F3 cells expressing a mutant human CD38 lacking the cytoplasmic domain induced tyrosine phosphorylation with intensity and kinetics similar to those seen with the entire protein. It also induced cell aggregation and decreased cell recovery. We conclude that CD38 triggers remarkably similar signaling pathways in human and murine immature B cells. This signaling is independent of the CD38 cytoplasmic domain, suggesting the existence of accessory transmembrane molecules associated with CD38.

Hyperdiploid Acute Lymphoblastic Leukemia With 51 to 65 Chromosomes: A Distinct Biological Entity With a Marked Propensity to Undergo Apoptosis

To determine the cellular basis for the excellent clinical outcome of hyperdiploid acute lymphoblastic leukemia (ALL), defined by a modal chromosome number of 51 to 65, we assessed the growth potential of leukemic cells from 129 children with newly diagnosed ALL. Flow cytometric analysis was used to compare leukemic cell recoveries at the beginning and at the end of 7-day cultures on allogeneic bone marrow–derived stromal layers. The median percentage of cell recovery after culture was 91% (range, <1% to 550%). Among the 25 hyperdiploid cases, only two had cell recoveries above the median value, compared with 63 of 104 cases with different ploidies (P< .001); 21 had recoveries within the first quartile, in contrast to only 12 of the 104 other cases. Cell recoveries in the 16 cases with duplications of chromosomes 4 and 10, a feature previously associated with a superior outcome, were all within the first quartile. Flow cytometric studies indicated that rapid induction of apoptosis was the underlying cause of low cell recoveries in cases with hyperdiploidy. The demise of hyperdiploid cells on stroma was not due to failure to adhere with stromal elements (as shown by electron microscopy) or to deficiencies of interleukin-1 (IL-1), IL-2, IL-3, IL-4, IL-6, IL-7, IL-11, stem-cell factor, interferon- (IFN-), tumor necrosis factor- (TNF-), or to combinations of these cytokines. Inactivation of IL-4, IFN- and TNF-, which if secreted by stromal layers could be toxic to ALL cells, failed to improve the survival of hyperdiploid blasts. We conclude that leukemic cells bearing 51 to 65 chromosomes have a marked propensity to undergo apoptosis. The stringent survival requirements of these cells, together with their potentially higher sensitivity to antileukemic drugs, may well account for the high cure rates achieved in patients with this form of ALL.

Gene Delivery to Human B-Precursor Acute Lymphoblastic Leukemia Cells

Autologous leukemia cells engineered to express immune-stimulating molecules may be used to elicit antileukemia immune responses. Gene delivery to human B-precursor acute lymphoblastic leukemia (ALL) cells was investigated using the enhanced green fluorescent protein (EGFP) as a reporter gene, measured by flow cytometry. Transfection of the Nalm-6 and Reh B-precursor ALL leukemia cell lines with an expression plasmid was investigated using lipofection, electroporation, and a polycationic compound. Only the liposomal compound Cellfectin showed significant gene transfer (3.9% to 12% for Nalm-6 cells and 3.1% to 5% for Reh cells). Transduction with gibbon-ape leukemia virus pseudotyped Moloney murine leukemia virus (MoMuLV)-based retrovirus vectors was investigated in various settings. Cocultivation of ALL cell lines with packaging cell lines showed the highest transduction efficiency for retroviral gene transfer (40.1% to 87.5% for Nalm-6 cells and 0.3% to 9% for Reh cells), followed by transduction with viral supernatant on the recombinant fibronectin fragment CH-296 (13% to 35.5% for Nalm-6 cells and 0.4% to 6% Reh cells), transduction on human bone marrow stroma monolayers (3.2% to 13.3% for Nalm-6 cells and 0% to 0.2% Reh cells), and in suspension with protamine sulfate (0.7% to 3.1% for Nalm-6 cells and 0% for Reh cells). Transduction of both Nalm-6 and Reh cells with human immunodeficiency virus–type 1 (HIV-1)–based lentiviral vectors pseudotyped with the vesicular stomatitis virus-G envelope produced the best gene transfer efficiency, transducing greater than 90% of both cell lines. Gene delivery into primary human B-precursor ALL cells from patients was then investigated using MoMuLV-based retrovirus vectors and HIV-1–based lentivirus vectors. Both vectors transduced the primary B-precursor ALL cells with high efficiencies. These studies may be applied for investigating gene delivery into primary human B-precursor ALL cells to be used for immunotherapy.

Cyclosporin A Induces Apoptosis in Childhood Acute Lymphoblastic Leukemia Cells

In an effort to identify novel antileukemic agents that can bypass the mechanisms of multidrug resistance, we found that cyclosporin A ([CyA] 5 μmol/L) produced a median cell kill of 69% (range, 47% to 85%) in seven B-lineage acute lymphoblastic leukemia (ALL) cell lines (OP-1, SUP-B15, KOPN-55bi, RS4;11, NALM6, REH, and 380) and three T-lineage ALL cell lines (MOLT4, CCRF-CEM, and CEM-C7) after 4 days of culture. At 10 μmol/L, median CyA toxicity was 99% (range, 88% to >99%). CyA was equally toxic to both a multidrug-resistant cell line, CEM-VLB100, which overexpresses gp-170 P-glycoprotein, and one resistant to topoisomerase II inhibitors, CEM-VM1-5, which has a mutation in the topoisomerase II gene. CyA was also toxic to primary leukemic cells maintained in stroma-based culture, a system that substantially prolongs in vitro cell survival. Against lymphoblasts from 21 patients with B-lineage ALL, the compound (at 5 μmol/L) reduced the leukemic cell number by a median of 87% (range, 27% to >99%) compared with results for parallel control cultures lacking CyA. Seven of these samples were from cases with unfavorable genetic features (eg, Philadelphia-chromosome orMLL gene rearrangements); three were obtained at relapse. Against T lymphoblasts (from six patients), the median reduction in cell number was 79% (range, 30% to >99%). At 10 μmol/L, the cell kill exceeded 97% in all cases studied. The mechanism of CyA cytotoxicity was found to be the activation of apoptosis, which was suppressed by phorbol myristate acetate but not by inhibitors of ceramide-mediated apoptosis, phosphatidyl inositol-3 kinase activity, or tyrosine kinase activity. These findings demonstrate high levels of CyA-induced toxicity against ALL cells at concentrations achievable in vivo, thus providing a strong rationale for clinical testing of this agent in patients with ALL.

Cyclosporin A Induces Apoptosis in Childhood Acute Lymphoblastic Leukemia Cells

In an effort to identify novel antileukemic agents that can bypass the mechanisms of multidrug resistance, we found that cyclosporin A ([CyA] 5 μmol/L) produced a median cell kill of 69% (range, 47% to 85%) in seven B-lineage acute lymphoblastic leukemia (ALL) cell lines (OP-1, SUP-B15, KOPN-55bi, RS4;11, NALM6, REH, and 380) and three T-lineage ALL cell lines (MOLT4, CCRF-CEM, and CEM-C7) after 4 days of culture. At 10 μmol/L, median CyA toxicity was 99% (range, 88% to >99%). CyA was equally toxic to both a multidrug-resistant cell line, CEM-VLB100, which overexpresses gp-170 P-glycoprotein, and one resistant to topoisomerase II inhibitors, CEM-VM1-5, which has a mutation in the topoisomerase II gene. CyA was also toxic to primary leukemic cells maintained in stroma-based culture, a system that substantially prolongs in vitro cell survival. Against lymphoblasts from 21 patients with B-lineage ALL, the compound (at 5 μmol/L) reduced the leukemic cell number by a median of 87% (range, 27% to >99%) compared with results for parallel control cultures lacking CyA. Seven of these samples were from cases with unfavorable genetic features (eg, Philadelphia-chromosome orMLL gene rearrangements); three were obtained at relapse. Against T lymphoblasts (from six patients), the median reduction in cell number was 79% (range, 30% to >99%). At 10 μmol/L, the cell kill exceeded 97% in all cases studied. The mechanism of CyA cytotoxicity was found to be the activation of apoptosis, which was suppressed by phorbol myristate acetate but not by inhibitors of ceramide-mediated apoptosis, phosphatidyl inositol-3 kinase activity, or tyrosine kinase activity. These findings demonstrate high levels of CyA-induced toxicity against ALL cells at concentrations achievable in vivo, thus providing a strong rationale for clinical testing of this agent in patients with ALL.