Effects of an anti HLA-DR immunotoxin on leukaemia cells and hematopoietic progenitors

An anti HLA-class II immunotoxin has been prepared by coupling to ricin A-chain (RTA) and anti-DR-DP monoclonal antibody (2G5-MoAb). Protein synthesis inhibition assays showed that 2G5-RTA immunotoxin is: highly cytotoxic to B-cell lymphoid neoplastic cells, and variable so to ALL cells, while AML cell lines display a generally poor susceptibility. Toxicity of 2G5-RTA on normal hematopoietic cells (HPC) was found to be dose-dependent, and to increase significantly with the addition of NH₄Cl, used as an activating agent. After 4 h of incubation with 2G5-RTA (10⁻⁸M), without NH₄Cl, percentages of CFU-GM and CFU-GEMM (colony forming units-granulo-monocytes and -multipotent, respectively) progenitor cell recovery were in the order of 50% and 30% respectively. In the same treatment conditions, 2G5-RTA induced a 6 log kill on RAJI cells —measured by clonogenic assay. Finally, this study shows that anti-DR immunotoxins may represent an original, efficient, and relatively safe approach for bone marrow purging of DR positive malignant B-cell populations; while their clinical potential pertaining to ALL and AML remains uncertain.     

The effect of tumor necrosis factor on normal human hematopoietic progenitors

Tumor necrosis factor-alpha (TNF-alpha), a product of activated macrophages that is cytotoxic to tumor cells, could be used to purge tumor cells from bone marrow before autologous bone marrow transplantation for hematologic malignancies and/or solid tumors. To determine whether exposure to TNF-alpha would have an inhibitory effect on hematopoietic progenitors, we incubated normal human bone marrow with a wide range of concentrations of recombinant human TNF. In order to mimic the conditions that would be used in bone marrow purging, bone marrow cell suspensions were incubated with TNF in doses ranging from 500 to 100,000 U/ml for 24 hours, and were assayed for colony formation in agar. We noted a dose-dependent inhibition of total colony-forming units (CFU) at days 7 and 14, with 50% inhibition occurring at 60,000 U/ml of TNF. TNF exerted a differential effect on CFU so that colony formation by erythroid (CFU-E), multipotential (CFU-GEMM), and macrophage (CFU-M) progenitors was suppressed to a greater extent than that by granulocyte progenitors (CFU-G). However, even after preincubation with TNF at high doses such as 100,000 U/ml, the inhibitory effects of TNF could be abolished by washing cells before culturing. This study demonstrates that hematopoietic precursors survive treatment with TNF at doses that have been shown to be cytotoxic to tumor cells. Although TNF has a significant inhibitory effect on the growth of erythroid, multipotential, and macrophage progenitors in vitro, this effect depends on continuous exposure to TNF for more than 24 hours. Thus, TNF may be useful as a bone marrow purging agent against tumor cells, with relative sparing of normal marrow elements.     

The cytotoxicity of arsenic trioxide to normal hematopoietic progenitors and leukemic cells is dependent on their cell-cycle status

Arsenic trioxide (ATO) is a novel agent to treat acute promyelocytic leukemia (APL). ATO can degrade chimeric PML-RAR proteins and induce apoptosis in various cancer cells. However, its effects on primary hematopoietic CD34+ have not been examined. In this study, we compared the effects of ATO on HL60 leukemic cells and primary umbilical cord blood (UCB) CD34+ cells. HL60 cells and UCB CD34+ cells were cultured with different concentrations of ATO for up to three weeks and examined for changes of cell cycle. We found that ATO (< or = 5 microM) caused prolongation of G1/S and G2/M phase in a dose-dependent manner. The percentage of cells in G2/M increased significantly (from 8.6 to 53.8%). High-dose ATO (> or = 25 microM) caused non-specific cell death in HL60 cells without any changes in cell cycle. In contrast to HL60 cells, UCB CD34+ cells were more resistant to high-dose ATO and most ATO-resistant CD34+ cells remained in G0/G1 phase. Primary cells that were resistant to ATO were rich in CD34+ cells. We further show that the ATO resistance was not related to the expression of P-glycoprotein (MDR-1). Our results suggest that the resistance to ATO in primitive UCB CD34+ cells is most likely related to its cell-cycle status. These results could be useful to design treatments for non-APL malignancies and to enrich hematopoietic stem cells in clinically applicable settings.     

Effects of anti-transferrin receptor antibodies on growth of normal and malignant myeloid cells

The effects of three monoclonal antibodies (B3/25, 43/31, and 42/6) reactive with human transferrin (Tf) receptors on growth of normal and malignant myeloid cells were examined using in vitro culture techniques. When added directly to cultures, all three antibodies caused dose-dependent inhibition of normal granulocyte/macrophage progenitor (CFU-GM) growth. Monoclonal antibody 42/6 was by far the most potent of the three, with an ID50 of less than 5 micrograms/ml. Identical effects were seen on CFU-GM from three patients with chronic myelogenous leukemia. Growth of colonies from two myeloid leukemia cells lines (KG-I, HL60) was also inhibited by all three antibodies, and these cells were generally more sensitive than normal CFU-GM. Blast colony-forming cells from three patients with acute non-lymphocytic leukemia were relatively resistant to the antibodies, and CFU-GM from a patient with myeloid metaplasia were resistant (ID50 greater than 50 micrograms/ml) to 42/6. In liquid culture, growth of the leukemic cell lines was inhibited by saturating concentrations of the three antibodies, although in both liquid and colony culture recovery was seen even after exposure to antibody for periods of up to 72 h. Analysis of the cell-cycle status of these cells showed that the antibodies did not cause accumulation of cells in any particular phase of the cell cycle. Addition to cultures of large quantities of human Tf failed to reverse the inhibitory effects of the antibodies. Competitive binding studies on the leukemia cell lines showed that only 42/6 inhibited binding of Tf to its receptor, although all three antibodies inhibited cell growth. Addition of Fe chelate (as ferric nitriloacetic acid, FeNTA) failed to reverse the inhibitory effects of the antibodies on CFU-GM and HL60 cells, but had variable effects on KG-I cell growth. FeNTA fully reversed inhibitory effects of 42/6 on KG-I cells. We conclude that monoclonal antibodies to Tf receptors can inhibit growth of both normal and malignant myeloid cells. Overall, no selectivity for malignant vs normal cells is apparent, although malignant cells from one individual were more sensitive to colony inhibition by 43/31 monoclonal antibody than normal CFU-GM.     

Telomeres and telomerase in normal and leukemic hematopoietic cells

Telomere length and telomerase have an important role in normal and malignant hematopoiesis. Telomere erosion can lead to chromosome end fusion and thereby contribute to genomic instability during tumorigenesis. Thus, like complex chromosomal aberrations, telomere length may be a prognostic factor in hematopoietic malignancies. A paper by Sieglova et al. in this issue of Leukemia Research reports on the prognostic impact of telomere shortening in bone marrow (BM) and peripheral blood (PB) specimens of myelodysplastic syndrome (MDS) and MDS converted-AML patients (pts). Their results underline the importance to study telomere biology together with cytogenetics, genomic and proteomic profiling as prognostic factors, in order to improve risk-adapted therapy of MDS and AML pts.     

Comparative cytotoxicity of various drug combinations for human leukemic cells and normal hematopoietic precursors

The development of suitable methods for purging the malignant cells contaminating the bone marrow of patients with cancer may offer a better chance of success for autologous bone marrow transplantation. In this paper, we further describe our efforts at purging acute myelogenous leukemia cells. HL-60, a promyelocytic leukemia cell line, was used as a model. 4-Hydroperoxycyclophosphamide (4-HC), VP-16-213 (VP-16), and Adriamycin were used alone or in combination to develop the best method to purge HL-60 cells. The cytotoxicity of 29.2 micrograms/ml (100 microM) of 4-HC was 99.8 +/- 0.12% (SD) on HL-60 cells and 82.5% on colony forming units-granulocyte, macrophage. Ninety-nine % of HL-60 cells and 72.7% of colony forming units-granulocyte, macrophage were inhibited by VP-16 at a concentration of 25 micrograms/ml (42.5 microM). The cytotoxicity of 1.5 micrograms/ml (2.76 microM) of Adriamycin on HL-60 cells was 98.6 +/- 0.8% and inhibited colony forming units-granulocyte, macrophage by 50.8%. The cytotoxicity and interactions of any two drug combinations at different combination ratios and the different effect levels were quantitatively determined by median effect plot and the multiple drug effect equation (T-C. Chou and P. Talalay. Adv. Enzyme Regul. 22: 27-55, 1984). The combination of 4-HC and VP-16 at a 4-HC:VP-16 drug ratio of 1:0.342 was found to be the best for selective toxicity towards HL-60 cells and was superior to the 4-HC-Adriamycin or VP-16 Adriamycin combination for usefulness in purging bone marrow.     

The effect of the leukemic cell line HL60 and acute myeloblastic leukemic cells before and after induction of differentiation on normal pluripotent hematopoietic progenitors (CFU-GEMM)

A leukemic cell line (HL60) and acute myeloblastic leukemia (AML) cells from six patients were co-cultured with normal marrow cells to assess their effects on growth of normal CFU-GEMM. The effects of the following inducers: 12-0-tetradecanoyl-phorbol-13-acetate (TPA), retinoic acid (RA), dimethylsulphoxide (DMSO), 1-25 (OH) D3 (Vitamin D3) and PHA-LCM on both the HL60 and AML cells, were studied. Inhibition of growth of normal CFU-GEMM was observed in the co-cultures in the presence of 1 X 10(4) HL60 or AML leukemic cells/ml. This inhibition was reversed by pretreating the HL60 line with vitamin D3, TPA and RA. No effect on growth of CFU-GEMM was noted when DMSO and PHA-LCM were used. AML cells were morphologically induced to differentiate by TPA or RA in all six cases. In three cases, reversal of inhibition of growth of normal pluripotent hemopoietic progenitors occurred and in three the inhibition of growth persisted. Regulation of inhibition by different inducers did not seem to correlate in all cases with morphological differentiation.     

Cytotoxic and antiproliferative effects of heptaacetyltiliroside on human leukemic cell lines

The peracetylated derivative of kaempferol-3-O-beta-D-(6'-E-p-coumaroyl) glycopyranoside (tiliroside) (1a) was tested for its cytotoxic and cytostatic activity against several human leukemic cell lines. The significant cytotoxic activity of this derivative, prompted to an additional examination on some of the cell lines used. The effect on the uptake of [3H]thymidine as a marker of DNA synthesis and on the cell proliferation, was investigated as well as the morphology of the cells and the kind of death induced, using the Wright-Giemsa dye and horizontal agarose-gel electrophoresis. Flow cytometric experiments of 1a on some leukemic cell lines was also performed. Compound 1a showed a significant antiproliferative effect as soon as 1 h of continuous incubation at all cell lines tested. Cells were killed, through the process of apoptosis and the appearance of the apoptotic signs was time and dose-dependent, while from the flow cytometric experiments, a synchronisation (through a delay probably in the G(0/1) phase) of the cells seems to take place.     

Effects of desferrioxamine on normal and leukemic human hematopoietic cell growth: in vitro and in vivo studies

The iron chelator desferrioxamine (DFO) has been previously shown to be an S-phase inhibitor of cell proliferation. To investigate its potential as an antileukemic drug, we first studied the effects of DFO on the in vitro growth of normal human hematopoietic progenitors (CFU-GM and BFU-E) and clonogenic cells from human leukemic cell lines. Then we evaluated the effects of DFO on progression of leukemia refractory to conventional therapy in two individuals. Micromolar concentrations of DFO determined a dose-dependent inhibition of normal progenitor growth, with inhibitory dose 50% (ID50) for CFU-GM and BFU-E being 6.7 and 5.5 microM/liter, respectively. Marked inhibitory effects were observed on clonogenic cells from HL-60 (ID50 = 1.4 microM/liter) and U-937 (ID50 = 3.6 microM/liter) human leukemic cell lines grown in semisolid medium. When DFO was given intravenously to a patient with lymphoid blast crisis of chronic myelogenous leukemia, a marked reduction in circulating blast count was observed. On the contrary, no in vivo effect was observed in a patient with acute nonlymphocytic leukemia having transfusional iron overload. We conclude that: (a) DFO is an inhibitor of both normal and leukemic myeloid cell proliferation in vitro; (b) our limited in vivo observations and a previous case study suggest that intravenous administration of DFO to patients with normal to low plasma iron may result in leukemic cytoreduction in vivo.     

Enhancement of ricin A chain immunotoxin activity by perhexiline on established and fresh leukemic cells

In the perspective of increasing the clinical potential of ricin A chain immunotoxins (RTA-ITs), perhexiline (Pex) and four structural analogues (Pex 2, Pex 3, Pex 7, and Pex 11) were evaluated for their ability to enhance RTA-IT activity in vitro. Only perhexiline significantly enhanced the cytotoxic activity of anti-CD5 RTA-ITs, T101 and T101-F(ab')2, on CEM III cell line (30- to 2000-fold), and of anti-HLA-DR RTA-IT, HNC-241, on both RAJI cell line (greater than 100-fold) and two immortalized cell lines originating from patients suffering from B-cell chronic lymphocytic leukemia, EHEB and FS2 D5 (10-fold). On 16 consecutive fresh B-cell chronic lymphocytic leukemia cell samples, significant T101-F(ab')2 RTA-IT and HNC-241 RTA-IT enhancement was observed with perhexiline which was comparable to that of NH4Cl and monensin. Perhexiline almost completely blocked RTA-IT intracellular degradation and profoundly modified its routing. These observations were linked to perhexiline-induced lipidosis via inhibition of sphingomyelinase activity. In conclusion, since the concentrations used are relevant with the pharmacokinetics of this agent, perhexiline appears to be a promising agent for in vivo enhancement of ricin A chain immunotoxins.     

Escape mechanisms of human leukemic cells to long-term immunotoxin treatment in an in vitro experimental model

In kinetic assays, an anti-CD5-ricin A chain (ST.1 -RTA) immunoconjugate (immunotoxins, IT) specifically inhibited up to 40% the protein synthesis of Jurkat target cells within the first 40 hr. Longer exposures of leukemia cells to ST.I-RTA resulted in a progressively higher number of target cells escaping IT treatment and becoming resistant to further treatment with ST.I-RTA even in the presence of the RTA-IT enhancer monensin. Resistant Jurkat cells proliferated at the same rate as control untreated cells, and were as sensitive as control cells to a transferrin-RTA IT, indicating that the ST.I-RTA-resistant tumor-cell population did not become insensitive to the enzymatic activity of RTA. Binding studies revealed that the anti-CD5 IT treatment induced a transient modulation of CD5 antigens but not of the functionally related CD3 antigens. The CD5 antigens were re-expressed at the cell surface following removal of the IT molecules from the culture medium with 1.1% of the total CD5 Ag being re-expressed per hr. When our experimental data on the kinetics of cell intoxication by the IT were corrected for the proliferative potential of the resistant and of the sensitive tumor-cell populations, it appeared that the effect of ST.I-RTA treatment on Jurkat cells was only to delay cell growth for a limited time period (20 hr) without reducing effectively the tumor-cell burden. Our results may have implications for the long-term treatment of target tumor cells with IT. © 1995 Wiley-Liss, Inc.     

Effects of the differentiating agent hexamethylene bisacetamide on normal and myelodysplastic hematopoietic progenitors

Hexamethylene bisacetamide (HMBA; NSC 95580) is a potent polar-planar differentiating agent of leukemia and solid tumor cell lines in vitro at clinically achievable concentrations. HMBA is currently being studied in patients with myelodysplastic syndrome. Previous phase I trials have demonstrated that HMBA produces hematologic toxicity in morphologically normal bone marrows of patients with solid tumors. Because of concern that HMBA may produce more severe myelotoxicity in patients with myelodysplastic syndrome since these patients have limited hematopoietic reserves, we studied the effects of HMBA on myelodysplastic and normal hematopoietic progenitors in vitro. HMBA concentrations that are optimal for differentiation in vitro (2 to 5 mmol/L) and HMBA concentrations that are being achieved in clinical trials (1 to 2 mmol/L) inhibited the growth of granulocyte-macrophage colony-forming units and erythroid burst-forming units from all 15 patients with myelodysplastic syndrome and all 4 normal subjects, HMBA did not induce proliferation of myelodysplastic or normal progenitors at any concentration; rather, it produced nearly identical inhibition of normal and myelodysplastic hematopoietic progenitors. HMBA also produced quantitatively similar inhibition of clonogenic leukemic growth of two myeloid leukemia cell lines. For a differentiating agent to be effective, it will likely have to either produce both differentiation and proliferation of abnormal hematopoietic progenitors or show selective inhibitory effects on abnormal as compared with normal progenitors. Although the mechanisms responsible for the antiproliferative effects of HMBA cannot be determined from this study, similar inhibitory effects of HMBA on normal and abnormal hematopoietic progenitors suggest that HMBA may be of limited utility in producing and sustaining elevations of peripheral blood cell counts in patients with myelodysplastic syndrome.     

Methylation analysis of the human multidrug resistance 1 gene in normal and leukemic hematopoietic cells.

Expression of the human multidrug resistance 1 gene (MDR1), which encodes the P-glycoprotein transmembrane efflux pump, has been associated with treatment failure of some leukemias, primarily acute myeloid leukemia (AML). To elucidate the epigenetic events associated with overexpression of MDR1 in AML, we analyzed the methylation status of a 2000 bp region within the MDR1 locus using a bisulphite genomic sequencing technique. A CpG-rich domain, approximately 1 kb in size, encompasses the promoter region, exon I, and intron I. This domain was found to be relatively unmethylated in five out of six primary and cultured human hematopoietic cells, as well as five out of six AML patient samples, independent of the MDR1 phenotype. The data suggest that the methylation status of the CpG-rich domain does not act as a 'switch' to regulate expression of the MDR1 gene. In addition, we found an upstream Alu repeat sequence to be unmethylated in three out of five cultured hematopoietic cell lines, both MDR1 expressing and non-expressing. However, analysis of primary CD8-positive T cells and AML patient samples revealed dense methylation of this region which is consistent with methylation of Alu repeat sequences observed in somatic tissues.     

Intracellular uptake and cytotoxic effect in vitro of doxorubicin and epirubicin in human leukemic and normal hematopoietic cells

Summary Leukemic cells from patients presenting with acute nonlymphoblastic leukemia and normal hematopoietic bone marrow cells from healthy donors for allogeneic bone marrow transplantation were incubated for 3 h with doxorubicin and epirubicin at different concentrations. The intracellular uptake at the end of the incubation was determined by photofluorometry in leukemic cells from 15 patients and in normal cells from 9 donors for bone marrow transplantation. Cytotoxicity in vitro against granulocyte/macrophage colony-forming units (CFU-GM) was determined in normal cells from 7 donors, and in vitro toxicity against leukemic cells was determined by a clonogenic technique in cells from 6 patients and by vital dye staining (DiSC) following 4 days' culture in cells from 15 patients. Epirubicin was significantly less toxic than doxorubicin to normal hematopoetic cells (72%±20% survival of cells for epirubicin vs 45%±13% for doxorubicin at a concentration of 0.2 m;P0.005). As analyzed by the DiSC assay, 0.2 m epirubicin was slightly more toxic to leukemic cells than was the same concentration of doxorubicin (47% vs 61% survival,P0.01), but the clonogenic assay revealed no difference in toxicity to leukemic cells. At a concentration of 0.2 m, the mean intracellular uptake of epirubicin in leukemic cells was 0.43±0.26 nmol/mg protein as compared with 0.33±0.14 nmol/mg protein for doxorubicin (not significant). In normal cells, the uptake of epirubicin at a concentration of 0.2 m was 0.47±0.25 nmol/mg protein as compared with 0.31±0.21 nmol/mg protein for doxorubicin (not significant). The reduced myelotoxicity observed in vitro together with the retained toxicity to leukemic cells indicates that the therapeutic index of epirubicin is better than that of doxorubicin.     

Cytotoxic activity of immunotoxin SS1P is modulated by TACE-dependent mesothelin shedding

Mesothelin is a cell-surface tumor-associated antigen expressed in several human cancers. The limited expression of mesothelin on normal tissues and its high expression in many cancers make it an attractive candidate for targeted therapies using monoclonal antibodies, immunoconjugates, and immunotoxins. Mesothelin is actively shed from the cell surface and is present in the serum of patients with malignant mesothelioma, which could negatively affect the response to these therapies. We have found that mesothelin sheddase activity is mediated by a TNF-α converting enzyme (TACE), a member of the matrix metalloproteinase/a disintegrin and metalloprotease family. We showed that EGF and TIMP-3 act through TACE as endogenous regulators of mesothelin shedding. We also found that reducing shedding significantly improved the in vitro cytotoxicity of immunotoxin SS1P, which targets mesothelin and is currently in clinical trials for the treatment of patients with mesothelioma and lung cancer. Our findings provide a mechanistic understanding of mesothelin shedding and could help improve mesothelin-based targeted therapies.     

Direct evidence for cooperating genetic events in the leukemic transformation of normal human hematopoietic cells.

Although genetic abnormalities associated with hematological malignancies are readily identified, the natural history of human leukemia cannot be observed because initiating and subsequent transforming events occur before clinical presentation. Furthermore, it has not been possible to study leukemogenesis in vitro as normal human cells do not spontaneously transform. Thus, the nature and sequence of genetic changes required to convert human hematopoietic cells into leukemia cells have never been directly examined. We have developed a system where the first step in the leukemogenic process is an engineered disruption of differentiation and self-renewal due to expression of the TLS-ERG oncogene, followed in some cases by overexpression of hTERT. In two of 13 experiments, transduced cells underwent step-wise transformation and immortalization through spontaneous acquisition of additional changes. The acquired karyotypic abnormalities and alterations including upregulation of Bmi-1 and telomerase all occur in acute myeloid leukemia (AML), establishing the relevance of this system. One resultant cell line studied in depth exhibits cellular properties characteristic of AML, notably a hierarchical organization initiated by leukemic stem cells that differentiate abnormally. These findings provide direct evidence for multiple cooperating events in human leukemogenesis, and provide a foundation for studying the genetic changes that occur during leukemic initiation and progression.     

Partial characterization of protein tyrosine kinase activity in normal and leukemic human myeloid cells

We have examined the expression of the protein tyrosine kinase (PTK) encoding oncogenes fes and abl in normal and malignant human myeloid cells in immunoblotting experiments. fes was markedly present in all cytosolic and most membrane fractions of normal and malignant cells. abl was only visible in normal cells, and occurred mostly in the cytosolic fractions. Molecular weights of identified proteins were different from the known products of fes and abl, possibly by alternative splicing at the mRNA level or by proteolysis. PTKs in myeloid cells were further purified by fast liquid protein chromatography (FPLC). PTK-activities of column fractions were assayed using a solid-phase non-radioactive dot-blot assay. Cytosolic and membrane fractions showed a FPLC pattern with a constant as well as a variable part in both normal and malignant cells, possibly indicative for PTKs with specialized functions in normal cell growth and transformation. Partial characterization of PTKs from different eluted peaks of AML-M4 blast cells demonstrated that PTKs from these peaks are kinetically distinct from each other.