The Kinetics and Extent of Engraftment of Chronic Myelogenous Leukemia Cells in Non-Obese Diabetic/Severe Combined Immunodeficiency Mice Reflect the Phase of the Donor’s Disease: An In Vivo Model of Chronic Myelogenous Leukemia Biology

Dose-dependent effects of Bcr-Abl in cell line models of different stages of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is caused by Bcr-Abl, an activated tyrosine kinase. The amounts of Bcr-Abl mRNA and protein in cells from patients in blast crisis (BC) are higher than in those chronic phase (CP), indicating that their expression rises with disease progression. In order to study this phenomenon on cells with the same genetic background, we transfected the 32D cell line with the BCR-ABL transgene and selected clones with graded expression of Bcr-Abl within the range found in cells from CP to BC. In vitro, we found that Bcr-Abl exerted dose-dependent effects upon growth factor dependence, clonogenicity and migration. However, the relationship between Bcr-Abl expression and cellular adhesion to fibronectin was more complex: rather than a direct positive correlation, clones that expressed low, but not high, levels of Bcr-Abl were less adhesive than growth factor stimulated, parental BCR-ABL-negative 32D cells. This finding parallels the situation with normal and CML-CP progenitors where the latter exhibit defective adhesion to fibronectin. Treatment with the tyrosine kinase inhibitor imatinib mesylate reversed the adhesion deficient phenotype of clones expressing low levels of Bcr-Abl. When injected subcutaneously into syngeneic mice, cell lines expressing high levels of Bcr-Abl rapidly induced tumors, whereas low-expressing clones led to tumor formation only after a prolonged latency. These findings suggest that the level of Bcr-Abl may be essential in determining the phenotype of the leukemic clone at different stages of the disease.

BCR gene expression blocks Bcr-Abl induced pathogenicity in a mouse model

It is well accepted that the Bcr-Abl oncoprotein encoded by the Philadelphia chromosome is responsible for causing chronic myelogenous leukemia (CML). We have previously demonstrated that expression of Bcr interferes with the oncogenic effects of Bcr-Abl. To examine the effects of increased Bcr expression on Bcr-Abl oncogenic effects in a more physiological system, we tested the leukemogenic potential of a clone of K562 cells (K6 K562) containing an inducible BCR gene in NOD/scid mice. In this clone, the BCR gene was placed under the control of a tetracycline (Tet) repression system with a cytomegalovirus (CMV) promoter. Induction of exogenous Bcr protein by removal of Tet from the culture medium caused a dramatic increase in Bcr serine kinase activity, yielding predominantly phosphoserine Bcr, despite the presence of Bcr-Abl in the kinase reaction mixture. Prior to induction, the endogenous Bcr was predominantly in the phosphotyrosine form because of phosphorylation by Bcr-Abl, which we previously have shown suppresses Bcr serine/threonine kinase activity. Injection of K6 K562 cells into NOD/scid mice under conditions where BCR expression was suppressed resulted in death or terminal illness in 100% of the mice within 35 days after injection. These mice had a severe wasting syndrome characterized by atrophy of bone marrow hematopoiesis, and/or neoplasia of liver, bone marrow and spleen. Neoplastic spleens from these mice usually contained b3a2 Bcr-Abl transcripts. In contrast, induction of BCR expression at the time of injection allowed 80% survival; these healthy mice had no detectable microscopic lesions in blood forming organs. This difference in survival was significant with P<0.0001. Of interest, mice that were fed Tet for 19 days to initiate the disease syndrome and then released from the BCR transcriptional block had a significantly better survival pattern than mice exposed to Tet throughout the entire period. Moreover, 30% of these mice (three mice) survived through day 50. We conclude from these findings that BCR gene expression strongly inhibits the oncogenic effects of Bcr-Abl in NOD/scid mice, yielding healthy mice in most cases.

Selective elimination of leukemic CD34+ progenitor cells by cytotoxic T lymphocytes specific for WT1

Hematologic malignancies such as acute and chronic myeloid leukemia are characterized by the malignant transformation of immature CD34+ progenitor cells. Transformation is associated with elevated expression of the Wilm's tumor gene encoded transcription factor (WT1). Here we demonstrate that WT1 can serve as a target for cytotoxic T lymphocytes (CTL) with exquisite specificity for leukemic progenitor cells. HLA-A0201– restricted CTL specific for WT1 kill leukemia cell lines and inhibit colony formation by transformed CD34+ progenitor cells isolated from patients with chronic myeloid leukemia (CML), whereas colony formation by normal CD34+ progenitor cells is unaffected. Thus, the tissue-specific transcription factor WT1 is an ideal target for CTL-mediated purging of leukemic progenitor cells in vitro and for antigen-specific therapy of leukemia and other WT1-expressing malignancies in vivo.

Isolation of a Highly Quiescent Subpopulation of Primitive Leukemic Cells in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is characterized by an increased proliferative activity of the leukemic progenitors that produce an elevated number of mature granulocytes. Nevertheless, cell cycle-active agents, even in very high doses, are alone unable to eradicate the leukemic clone, suggesting the presence of a rare subset of quiescent leukemic stem cells. To isolate such cells, we first used Hoechst 33342 and Pyronin Y staining to obtain viable G0 and G1/S/G2/M fractions of CD34+cells by fluorescence-activated cell sorting (FACS) from 6 chronic-phase CML patients’ samples and confirmed the quiescent and cycling status of the 2 fractions by demonstration of expected patterns of Ki-67 and D cyclin expression. Leukemic (Ph+/BCR-ABL+) cells with in vitro progenitor activity and capable of engrafting immunodeficient mice were identified in the directly isolated G0 cells. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed that many leukemic CD34+ G0cells also expressed BCR-ABL mRNA. CD34+ from 8 CML patients were also labeled with carboxyfluorescein diacetate succinimidyl diester (CFSE) before being cultured (with and without added growth factors) to allow viable cells that had remained quiescent (ie, CFSE+) after 4 days to be retrieved by FACS. Leukemic progenitors were again detected in all quiescent populations isolated by this second strategy, including those exposed to a combination of flt3-ligand, Steel factor, interleukin-3, interleukin-6, and granulocyte colony-stimulating factor. These findings provide the first direct and definitive evidence of a deeply but reversibly quiescent subpopulation of leukemic cells in patients with CML with both in vitro and in vivo stem cell properties.

Effects of the Tyrosine Kinase Inhibitor AG957 and an Anti-Fas Receptor Antibody on CD34+ Chronic Myelogenous Leukemia Progenitor Cells

The hallmark of chronic myelogenous leukemia (CML) is the Philadelphia (Ph) chromosome that fuses genetic sequences of the BCR gene on chromosome 22 with c-ABL sequences translocated from chromosome 9. BCR/ABL fusion proteins have a dysregulated protein tyrosine kinase (PTK) activity exerting a key role in malignant transformation. Targeting the tyrosine kinase activity of BCR/ABL or using agents capable of triggering apoptosis might represent attractive therapeutic approaches for ex vivo purging. AG957, a member of the tyrphostin compounds, exerts a selective inhibition of p210BCR/ABLtyrosine phosphorylation. We report here that preincubation of CML or normal CD34+ cells with graded concentration of AG957 (1 to 100 μmol/L) resulted in a statistically significant, dose-dependent suppression of colony growth from multipotent, erythroid, and granulocyte-macrophage progenitors as well as the more primitive long-term culture-initiating cells (LTC-IC). However, AG957 doses causing 50% inhibition (ID50) of CML and normal progenitors were significantly different for multilineage colony-forming units (CFU-Mix; 12 v 64 μmol/L; P = .008), burst-forming unit-erythroid (BFU-E; 29 v 89 μmol/L;P = .004), colony-forming unit–granulocyte-macrophage (CFU-GM; 34 v 85 μmol/L; P = .004), and LTC-IC (43 v 181 μmol/L; P = .004). In 5 of 10 patients, analysis of BCR/ABL mRNA on single progenitors by reverse transcription-polymerase chain reaction showed that AG957 at 50 μmol/L significantly reduced the mean (±SD) percentage of BCR/ABL-positive progenitors (92% ± 10% v 33 ± 5%;P = .001). Because AG957 treatment resulted in significantly higher percentages of apoptotic cells (30% v9%) in the BCR/ABL-transfected 32DLG7 cells as compared with 32D-T2/93 cells (BCR/ABL-negative), we investigated the combined effects of AG957 with the anti-Fas receptor (Fas-R) monoclonal antibody CH11 that triggers apoptosis. As compared with AG957 alone, the sequential treatment of CML CD34+ cells with AG957 (1 μmol/L) and CH11 (1 μg/mL) increased CFU-Mix, BFU-E, and CFU-GM growth inhibition by 1.6-fold, 3-fold, and 4-fold, respectively. In contrast, the treatment of normal CD34+ cells with AG957 and CH11 failed to enhance AG957-induced colony growth inhibition. We conclude that (1) AG957 inhibits in a dose-dependent manner CML CD34-derived colony formation by both primitive LTC-IC as well as committed CFU-Mix, BFU-E, and CFU-GM; (2) this growth inhibition is associated with the selection of a substantial amount of BCR/ABL-negative progenitors; and (3) the antiproliferative effect of AG957 is dramatically increased by combining this compound with the anti–Fas-R antibody CH11. These data may have significant therapeutic applications.