Characterization and retroviral transduction of an early human lymphomyeloid precursor assayed in nonswitched long-term culture on murine stroma

Randomized phase IIb study of low-dose cytarabine and lintuzumab versus low-dose cytarabine and placebo in older adults with untreated acute myeloid leukemia

Improving outcomes in older adults with acute myeloid leukemia remains a formidable challenge. Lintuzumab (SGN-33; HuM195) is a humanized monoclonal antibody directed against CD33, which is expressed on the majority of myeloblasts in acute myeloid leukemia. The primary objective of this randomized, double-blinded, placebo-controlled trial was to determine whether addition of lintuzumab to low-dose cytarabine would increase overall survival in adults aged 60 years and over with untreated acute myeloid leukemia. Randomization was stratified by age, previous hematologic disorder, and performance status. All patients received cytarabine (20 mg subcutaneously twice daily) on Days 1-10 of each 28-day cycle. Patients received lintuzumab (600 mg) or placebo intravenously once weekly in Cycle 1 and once every other week in Cycles 2-12. A total of 211 patients (107 lintuzumab, 104 placebo) were randomized. Median age was 70 years (range 60-90). Survival was not significantly prolonged with lintuzumab treatment (hazard ratio 0.96; 95% confidence interval (CI) 0.72-1.28; P=0.7585). Median survival was similar between treatment arms (4.7 months lintuzumab vs. 5.1 months placebo) and in the subgroup of patients with high-risk cytogenetics (4.5 months). Infusion-related reactions, predominantly Grades 1-2, occurred more commonly in the lintuzumab arm (51% vs. 7% placebo); no other clinically significant difference in safety was noted. These results confirm that lintuzumab in combination with low-dose cytarabine did not prolong survival and that low-dose cytarabine remains a valid comparator for trials of non-intensive therapies in older patients with acute myeloid leukemia, regardless of cytogenetic profile.

Complete remissions observed in acute myeloid leukemia following prolonged exposure to lintuzumab: a phase 1 trial

A multi-institutional, phase 1 dose-escalation trial of lintuzumab (humanized anti-CD33 antibody; SGN-33, HuM195) was performed in patients with CD33-positive myeloid malignancies. In this study, higher doses than previously tested and prolonged duration of treatment for responding patients were evaluated. Over the dose range of 1.5-8 mg/kg/week, lintuzumab was well tolerated, and a maximum tolerated dose was not defined. The most common adverse event was transient chills with the initial lintuzumab infusion (39%). Responses were observed in 7 of 17 patients with acute myeloid leukemia: morphologic complete remission (n = 4), partial remission (n = 2), and morphologic leukemia-free state (n = 1). Of 14 patients with myelodysplastic syndrome or myeloproliferative diseases, 1 patient had major hematologic improvement and 9 patients had stable disease. In contrast to aggressive conventional chemotherapy, lintuzumab was administered in an ambulatory clinic setting with acceptable toxicity.

Inhibition of CXCR4 with the novel RCP168 peptide overcomes stroma-mediated chemoresistance in chronic and acute leukemias

The chemokine receptor CXCR4 mediates the migration of hematopoietic cells to the stroma-derived factor 1alpha (SDF-1alpha)-producing bone marrow microenvironment. Using peptide-based CXCR4 inhibitors derived from the chemokine viral macrophage inflammatory protein II, we tested the hypothesis that the inhibition of CXCR4 increases sensitivity to chemotherapy by interfering with stromal/leukemia cell interactions. First, leukemic cells expressing varying amounts of surface CXCR4 were examined for their chemotactic response to SDF-1alpha or stromal cells, alone or in the presence of different CXCR4 inhibitors. Results showed that the polypeptide RCP168 had the strongest antagonistic effect on the SDF-1alpha- or stromal cell-induced chemotaxis of leukemic cells. Furthermore, RCP168 blocked the binding of anti-CXCR4 monoclonal antibody 12G5 to surface CXCR4 in a concentration-dependent manner and inhibited SDF-1alpha-induced AKT and extracellular signal-regulated kinase phosphorylation. Finally, RCP168 significantly enhanced chemotherapy-induced apoptosis in stroma-cocultured Jurkat, primary chronic lymphocytic leukemia, and in a subset of acute myelogenous leukemia cells harboring Flt3 mutation. Equivalent results were obtained with the small-molecule CXCR4 inhibitor AMD3465. Our data therefore suggest that the SDF-1alpha/CXCR4 interaction contributes to the resistance of leukemia cells to chemotherapy-induced apoptosis. Disruption of these interactions by the peptide CXCR4 inhibitor RCP168 represents a novel strategy for targeting leukemic cells within the bone marrow microenvironment.

Biology of Normal and Acute Myeloid Leukemia Stem Cells

The substantial understanding that has been gained over the past 5 decades of the biology of blood formation is largely due to the development of functional quantitative assays for cells at all stages of differentiation, from multipotential stem cells to mature cells. The majority of studies have involved the mouse because the ease with which repopulation studies can be carried out with this animal model allows the assay of complete lineage development from stem cells. In the past decade, advances in repopulation assays for human stem cells using xenotransplantation have greatly enhanced our understanding of human stem cell biology. Importantly, the xenotransplantation methodology has also been used to identify the cancer stem cell that initiates and sustains leukemic proliferation, providing key evidence for the cancer stem cell hypothesis. This hypothesis argues that cancer cells are functionally heterogeneous and hierarchically organized such that only specific cells are capable of sustaining tumor growth and continuously producing the cells that make up the bulk of the tumor. Recent studies have also brought into focus the importance of the intimate relationship between the stem cell (normal or leukemic) and its microenvironment. Coming into view are the molecular players involved in stem cell homing, migration, and adhesion, as well as the cellular components of the microenvironmental niche. Here we review recent studies that have begun, to elucidate the interplay between normal and leukemic human stem cells and their microenvironment.

Stromal cells prevent apoptosis of AML cells by up-regulation of anti-apoptotic proteins

The aim of this study was to study interactions between stromal bone marrow microenvironment and leukemic cells. We tested the hypothesis that stromal cells prevent apoptosis of AML cells by up-regulating anti-apoptotic proteins in leukemic blasts. In HL-60 and NB-4 cells, serum deprivation- and ara-C-induced apoptosis was diminished when cells were cocultured with murine MS-5 stromal cells (P < 0.02). This effect was reproduced with conditioned medium from MS-5 cells. Cocultivation with stromal cells induced Bcl-2 expression levels, both by PCR analysis and flow cytometry. In primary AML (n = 14), ara-C-induced apoptosis was significantly lower in cells cocultured with MS-5 cells than in controls (P < 0.001). This effect was partially preserved when leukemic cells were separated from stromal cells by a microporous insert (in 5/9 samples, P = 0.04). In addition, Bcl-2 levels were significantly higher in stroma-supported than in control CD34(+) AML cells (P < 0.01). Bcl-X(L) levels were higher in 5/7 samples grown on stromal layers. Of note, in AML patients resistant to induction chemotherapy (n = 6), Bcl-2 increased significantly after cultivation with stromal cells, but no such increase was noted in cells from chemotherapy-sensitive patients. In conclusion, MS-5 stromal cells prevented apoptosis in HL-60 cells and in primary AML blasts via modulation of Bcl-2 family proteins. The observed association of high Bcl-2 expression in stroma-supported AML blasts in vitro with resistance to chemotherapy in vivo suggests that the same mechanisms may be operational in vivo.

Efficient oncoretroviral transduction of extended long-term culture-initiating cells and NOD/SCID repopulating cells: enhanced reconstitution with gene-marked cells through an ex vivo expansion approach

Recent developments of surrogate assays for human hematopoietic stem cells (HSC) have facilitated efforts at improving HSC gene transfer efficiency. Through the use of xenograft transplantation models, such as nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice, successful oncoretroviral gene transfer to transplantable hematopoietic cells has been achieved. However, because of the low frequency and/or homing efficiency of SCID repopulating cells (SRC) in bone marrow (BM), studies have primarily focused on cord blood (CB). The recently developed extended (> 60 days) long-term culture-initiating cell (ELTC-IC) assay detects an infrequent and highly quiescent candidate stem cell population in BM as well as CB of the CD34(+)CD38(-) phenotype. Although these characteristics suggest that ELTC-IC and SRC might be closely related, attempts to oncoretrovirally transduce ELTC-IC have been unsuccessful. Here, recently developed conditions (high concentrations of SCF + FL + Tpo in serum-free medium) supporting expansion of BM CD34(+)CD38(-) 12 week ELTC-IC promoted efficient oncoretroviral transduction of BM and CB ELTC-IC. Although SRC can be transduced with oncoretroviral vectors, this is frequently associated with loss of reconstituting activity, posing a problem for development of clinical HSC gene therapy. However, previous attempts at expanding transduced HSC posttransduction resulted in compromised rather than improved gene marking. Utilizing conditions promoting cell divisions and transduction of ELTC-IC we show that although 5 days of ex vivo culture is sufficient to obtain maximum gene transfer efficiency to SRC, extension of the expansion period to 12 days significantly enhances multilineage reconstitution activity of transduced SRC, supporting the feasibility of improving gene marking through ex vivo expansion.

Successful transduction of human multipotent, lymphoid (T, B, NK) and myeloid, and transplantable CD34+CD38low cord blood cells using a murine oncoretroviral vector

Hematopoietic stem cells (HSC) are subject to great interest because of their medical importance and their biological properties. Therefore, the possibility of genetically modifying human HSC is a major concern in several inherited pathologies. In this study, we aimed to demonstrate that a murine oncoretroviral vector can transduce multipotential cord blood (CB) stem cells. Sorted CB CD34(+)CD38(low) cells were transduced with a Moloney-based MFG retroviral vector containing the coding sequence of the murine CD2 (mCD2). CD34(+)mCD2(+) cells were sorted by flow cytometry and cultured either in bulk or at one cell per well in culture conditions that allow differentiation along lymphoid (T, B, and NK) and myeloid (M) lineages. Phenotypic analysis of cells generated in culture showed that CD34(+)mCD2(+) cells could give rise to all lymphoid and myeloid progeny, indicating that the MFG/mCD2 vector had transduced progenitors of all tested lineages. Moreover, clonal cultures of 660 CD34(+)mCD2(+) cells showed that approximately 5% of these cells were able to generate both myeloid and lymphoid (B + NK) progenies; for 25% of them, this included the production of lymphoid T cells. We also demonstrate that transduced CD34(+)CD38(low) CB cells with lymphoid and myeloid potentials were capable of engraftment into the bone marrow (BM) of nonobese diabetic-severe combined immunodeficiency (NOD-SCID) mice during several months. These results show that MFG retroviral vectors can transduce multipotent (T, B, NK, M) human hematopoietic progenitors with in vivo repopulating activity.

Broad distribution of colony-forming cells with erythroid, myeloid, dendritic cell, and NK cell potential among CD34(++) fetal liver cells

The generation of erythroid, myeloid, and lymphoid cells from human fetal liver progenitors was studied in colony-forming cell (CFC) assays. CD38(-) and CD38(+) progenitors that expressed high levels of CD34 were grown in serum-deprived medium supplemented with kit ligand, flk2/flt3 ligand, GM-CSF, c-mpl ligand, erythropoietin, and IL-15. The resulting colonies were individually analyzed by flow cytometry. CD56(+) NK cells were detected in 21.9 and 9.9% of colonies grown from CD38(-) and CD38(+) progenitors, respectively. NK cells were detected in mostly large CD14(+)/CD15(+) myeloid colonies that also, in some cases, contained red cells. NK cells were rarely detected in erythroid colonies, suggesting an early split between the erythroid and the NK cell lineages. CD1a(+) dendritic cells were also present in three-quarters of the colonies grown from CD38(-) and CD38(+) progenitors. Multilineage colonies containing erythrocytes, myeloid cells, and NK cells were present in 13.7 and 2.7% of colonies grown from CD38(-) and CD38(+) progenitors, respectively. High proliferative-potential CFCs that generated multilineage colonies were also detected among both populations of progenitors. The total number of high proliferative-potential CFCs with erythroid, myeloid, and NK cell potential was estimated to be 2-fold higher in the CD38(+) fraction compared with the CD38(-) fraction because of the higher frequency of CD38(+) cells among CD34(++) cells. The broad distribution of multipotent CFCs among CD38(-) and CD38(+) progenitors suggests that the segregation of the erythroid, myeloid, and lymphoid lineages may not always be an early event in hemopoiesis. Alternatively, some stem cells may be present among CD38(+) cells.

Stromal support augments extended long-term ex vivo expansion of hemopoietic progenitor cells

Current technology to numerically expand hemopoietic stem/progenitor cells (HSPC) ex vivo within 1 to 2 weeks is insufficient to warrant significant gain in reconstitution time following their transplantation. In order to more stringently test the parameters affecting HSPC expansion, we followed ex vivo cultures of CD34+-selected umbilical cord blood (UCB) HSPC for up to 10 weeks and investigated the effects of stromal support and cytokine addition. The cytokine combinations included FL + TPO, FL + TPO plus SCF and/or IL6, or SCF + IL6. To identify the HSPC in uncultured and cultured material, we determined the number of colony-forming cells (CFC), cobblestone area forming cells (CAFC), the NOD/SCID repopulating ability (SRA), and CD34+ subsets by phenotyping. The highest fold-increase obtained for CD34+ and CD34+ CD38- cell numbers was, respectively, 1197 and 30,937 for stroma-free and 4066 and 117,235 for stroma-supported cultures. In general, CFC generation increased weekly in FL + TPO containing groups up to week 5 with a 28- to 195-fold expansion whereafter the weekly CFC output stabilized. Stroma support enhanced the expansion of CAFC week 6 maximally 11-fold to 89-fold with FL + TPO + IL6. Cultures stimulated with at least FL + TPO gave an estimated 10- to 14-fold expansion of the ability of CD34+ UCB cells to multilineage engraft the BM of sublethally irradiated NOD/SCID mice at 2 weeks of stroma-free and stroma-supported cultures, while at week 5 and later the estimated SRA decreased to low or undetectable levels in all groups. Our results show that stroma and FL + TPO but also inclusion of bovine serum albumin, greatly increase the long-term generation of HSPC as measured by in vitro assays and is indispensable for long-term expansion of CD34+ CD38- CXCR4+ cells. However, the different surrogate methods to quantify the HSPC (CD34+ CD38-, CFC, CAFC week 6 and SRA) show increasing incongruency with increasing culture time, while especially the phenotypic analysis and the CFC generation greatly overestimate the CAFC and SRA expansion in 10-week cultures.

Identification of a novel, human multilymphoid progenitor in cord blood

The earliest stages of lymphoid commitment from human pluripotent hematopoietic stem cells have not been defined. A clonogenic subpopulation of CD34(+)CD38(-) cord blood cells were identified that expressed high levels of the CD7 antigen and possessed only lymphoid potential. CD34(+)CD38(-)CD7(+) (CD7(+)) cells uniformly coexpressed CD45RA and HLA-DR; c-kit and Thy-1 expression was absent to low. Clonal analysis demonstrated that single CD7(+) cells could generate B cells, natural killer cells, and dendritic cells but were devoid of myeloid or erythroid potential. In contrast, control CD34(+)CD38(-)CD7(-) (CD7(-)) cells generated both lymphoid and myelo-erythroid cells. The lymphoid potential (generation of lymphoid progeny in bulk and single cell cultures) of CD7(+) cells was equivalent to that of the pluripotent CD7(-) cells. RNA expression studies showed that CD7(+) cells expressed PU.1 and GATA-3, but did not express Pax-5, terminal deoxynucleotide transferase, or CD3epsilon. In contrast to the previously described murine common lymphoid progenitor, the alpha chain of the receptor for interleukin-7 was not detected by fluorescence-activated cell sorting analysis or RNA polymerase chain reaction in CD7(+) cells. These studies identify a clonogenic lymphoid progenitor with both B-cell and natural killer cell lineage potential with a molecular profile that suggests a developmental stage more primitive than previously identified lymphoid progenitors. The CD7(+) phenotype distinguishes primitive human lymphoid progenitors from pluripotent stem cells, thus allowing the study of regulation of early human lymphopoiesis and providing an alternative to pluripotent stem cells for genetic manipulation and transplantation. (Blood. 2001;97:3683-3690)

Distinct classes of human stem cells that differ in proliferative and self-renewal potential

The composition of the human hematopoietic stem cell compartment is poorly understood due to the absence of experimental tools with which to characterize the developmental program of individual stem cells. We report here that human stem cells differ markedly in their repopulation capacity and self-renewal potential, as determined using nonobese diabetic-severe combined immunodeficiency (NOD-SCID) mice transplanted with retrovirally transduced cord blood stem cells, called SCID-repopulating cells (SRCs). Clonal stem cell analysis based on the identification of unique retroviral integration sites within serial bone marrow aspirates showed that repopulation was generally oligoclonal with extensive variability in the lifespan and proliferative capacity of individual SRCs. Most clones contributed to human cell engraftment for several weeks after transplantation and then disappeared but others appeared later and persisted. Further evidence for stem cell heterogeneity was found in the secondary transplantation capacity of SRCs. These data point to the existence of different classes of human stem cells with variable self-renewal potential and short- or long-term repopulating capacity.

Loss of marrow reserve from dose-intensified chemotherapy results in impaired hematopoietic reconstitution after autologous transplantation: CD34(+), CD34(+)38(-), and week-6 CAFC assays predict poor engraftment

Autologous hematopoietic stem cell transplantation (HSCT) is an increasingly successful modality for treating a variety of malignant disorders in the clinic. Experimental and clinical data suggest that prior exposure to cytotoxic agents that damage primitive stem cells results in impaired hematopoiesis after autologous HSCT. To further investigate the ability to predict for impaired hematopoiesis, we measured different stem/progenitor cell populations transplanted and time to engraftment. Patients with previously untreated, advanced-stage follicular lymphoma were treated in sequential prospective protocols with 6-8 cycles of standard-dose (SD) cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP), or four cycles of a higher-dose (HD) CHOP and granulocyte colony-stimulating factor, to induce remission prior to high-dose cyclophosphamide, total body irradiation, and autologous bone marrow transplantation (ABMT). Cryopreserved marrow samples obtained prior to ABMT were assayed for CD34(+), CD34(+)38(-), and cobblestone area-forming cell (CAFC) frequencies. Despite receiving similar numbers of nucleated cells at ABMT, HD-CHOP patients took significantly longer to attain platelet engraftment than the SD-CHOP patients. Marrow from the HD-CHOP patients contained significantly lower CD34(+), CD34(+)38(-), and week 6-8 CAFC frequencies than marrow from SD-CHOP-treated patients. Time to platelet engraftment was plotted against progenitor/stem cell numbers transplanted for each patient and threshold values were developed for all three stem/progenitor cell populations. These values were 0.5 x 10(6) CD34(+) cells/kg, 0.14 x 10(6) CD34(+)38(-) cells/kg, and 9500 week-6 CAFC/kg transplanted. Approximately 50% of patients received marrow progenitor/stem cell numbers above the threshold values and all engrafted without delay. However, transplantation of stem/progenitor cell numbers below threshold values did not uniformly predict for delayed platelet engraftment. These data provide further evidence for the association of low marrow reserve at ABMT, low numbers of stem/progenitor cells transplanted, and delayed hematopoietic recovery. However, there remains a group of patients who have rapid platelet engraftment after ABMT despite low numbers of progenitor/stem cells transplanted. These data suggest the presence of a crucial stem cell population not represented by the stem/progenitor cell populations studied in these experiments.

Expansion of human cord blood CD34+CD38−cells in ex vivo culture during retroviral transduction without a corresponding increase in SCID repopulating cell (SRC) frequency: dissociation of SRC phenotype and function

Current procedures for the genetic manipulation of hematopoietic stem cells are relatively inefficient due, in part, to a poor understanding of the conditions for ex vivo maintenance or expansion of stem cells. We report improvements in the retroviral transduction of human stem cells based on the SCID-repopulating cell (SRC) assay and analysis of Lin− CD34+CD38−cells as a surrogate measure of stem cell function. Based on our earlier study of the conditions required for ex vivo expansion of Lin−CD34+ CD38− cells and SRC, CD34+–enriched lineage–depleted umbilical cord blood cells were cultured for 2 to 6 days on fibronectin fragment in MGIN (MSCV-EGFP-Neo) retroviral supernatant (containing 1.5% fetal bovine serum) and IL-6, SCF, Flt-3 ligand, and G-CSF. Both CD34+CD38− cells (20.8%) and CFC (26.3%) were efficiently marked. When the bone marrow of engrafted NOD/SCID mice was examined, 75% (12/16) contained multilineage (myeloid and B lymphoid) EGFP+ human cells composing as much as 59% of the graft. Half of these mice received a limiting dose of SRC, suggesting that the marked cells were derived from a single transduced SRC. Surprisingly, these culture conditions produced a large expansion (166-fold) of cells with the CD34+CD38− phenotype (n = 20). However, there was no increase in SRC numbers, indicating dissociation between the CD34+CD38− phenotype and SRC function. The underlying mechanism involved apparent downregulation of CD38 expression within a population of cultured CD34+CD38+ cells that no longer contained any SRC function. These results suggest that the relationship between stem cell function and cell surface phenotype may not be reliable for cultured cells. (Blood. 2000;95:102-110)

Expansion of human cord blood CD34+CD38−cells in ex vivo culture during retroviral transduction without a corresponding increase in SCID repopulating cell (SRC) frequency: dissociation of SRC phenotype and function

Current procedures for the genetic manipulation of hematopoietic stem cells are relatively inefficient due, in part, to a poor understanding of the conditions for ex vivo maintenance or expansion of stem cells. We report improvements in the retroviral transduction of human stem cells based on the SCID-repopulating cell (SRC) assay and analysis of Lin− CD34+CD38−cells as a surrogate measure of stem cell function. Based on our earlier study of the conditions required for ex vivo expansion of Lin−CD34+ CD38− cells and SRC, CD34+–enriched lineage–depleted umbilical cord blood cells were cultured for 2 to 6 days on fibronectin fragment in MGIN (MSCV-EGFP-Neo) retroviral supernatant (containing 1.5% fetal bovine serum) and IL-6, SCF, Flt-3 ligand, and G-CSF. Both CD34+CD38− cells (20.8%) and CFC (26.3%) were efficiently marked. When the bone marrow of engrafted NOD/SCID mice was examined, 75% (12/16) contained multilineage (myeloid and B lymphoid) EGFP+ human cells composing as much as 59% of the graft. Half of these mice received a limiting dose of SRC, suggesting that the marked cells were derived from a single transduced SRC. Surprisingly, these culture conditions produced a large expansion (166-fold) of cells with the CD34+CD38− phenotype (n = 20). However, there was no increase in SRC numbers, indicating dissociation between the CD34+CD38− phenotype and SRC function. The underlying mechanism involved apparent downregulation of CD38 expression within a population of cultured CD34+CD38+ cells that no longer contained any SRC function. These results suggest that the relationship between stem cell function and cell surface phenotype may not be reliable for cultured cells. (Blood. 2000;95:102-110)