Different subsets of primary chronic myeloid leukemia stem cells engraft immunodeficient mice and produce a model of the human disease.

Xenograft models of chronic phase human chronic myeloid leukemia (CML) have been difficult to develop because of the persistence of normal hematopoietic stem cells in most chronic phase CML patients and the lack of methods to selectively isolate the rarer CML stem cells. To circumvent this problem, we first identified nine patients' samples in which the long-term culture-initiating cells were predominantly leukemic and then transplanted cells from these samples into sublethally irradiated NOD/SCID and NOD/SCID-beta2microglobulin-/- mice. This resulted in the consistent and durable (>5 months) repopulation of both host genotypes with similar numbers of BCR-ABL+/Ph+ cells. The regenerated leukemic cells included an initial, transient population derived from CD34+CD38+ cells as well as more sustained populations derived from CD34+CD38- progenitors, indicative of a hierarchy of transplantable leukemic cells. Analysis of the phenotypes produced revealed a reduced output of B-lineage cells, enhanced myelopoiesis with excessive production of erythroid and megakaropoietic cells and the generation of primitive (CD34+) leukemic cells displaying an autocrine IL-3 and G-CSF phenotype, all characteristics of primary CML cells. These findings demonstrate the validity of this xenograft model of chronic phase human CML, which should enable future investigation of disease pathogenesis and new approaches to therapy.

High-efficiency retroviral transduction of mammalian cells on positively charged surfaces.

The efficiency of retroviruses as transducing agents has been appreciated for many years, particularly for hematopoietic cell targets for which alternative strategies applicable to adherent cells are not effective. Advances in vector design, pseudotyping, and infection conditions have eliminated the need to cocultivate the target cells with virus-producing cells. Nevertheless, improvements are still needed for many applications, including those with a therapeutic or clinical cell-tracking objective. In this study we show that more positively charged surfaces, including those designed for the culture of anchorage-dependent cells, allow measurable levels of adhesion by different pseudotypes of retroviruses, which can result in increased gene transfer efficiencies to a variety of target cells including normal primary human hematopoietic cells as well as human leukemic cell lines and rat and murine fibroblasts. In the experiments with primary human cells, equal aliquots of enriched CD34+ cord blood cells were first stimulated for 2 days with cytokines (Flt3 ligand, Steel factor, IL-3, IL-6, and G-CSF) and then exposed for 4 days to a green fluorescent protein (GFP)- and Neo(r)-encoding retrovirus produced in PG13 cells. Both the final yield (approximately 300% relative to initial numbers), and the proportion (approximately 60%) of transduced CD34+ cells, colony-forming cells, and long-term culture-initiating cells were the same for cells infected either in tissue culture dishes or in fibronectin-coated petri dishes. Similar proportions (approximately 10%) and absolute yields of GFP+ human cells were also found in multilineage engrafted NOD/SCID mice assessed 6 to 8 weeks after being transplanted with these two types of transduced, but unselected, cells. These findings suggest a new and simpler approach for achieving high gene transfer efficiencies to hematopoietic cells.

Quantitation of primitive and lineage-committed progenitors in mobilized peripheral blood for prediction of platelet recovery post autologous transplant.

Leukapheresis collections obtained following one of four mobilization regimens from 90 cancer patients were analyzed for their content of various progenitor cell types including erythroid and granulopoietic colony-forming cells in methylcellulose (total CFC), CFC-megakaryocyte (CFC-Mk), CFC detected after 10, 35 and 56 days in long-term culture (LTC), and total CD34+ cells. The number of each of these progenitor cell types collected from individual patients varied over 1000-fold. Nevertheless, within an individual leukapheresis, there was a significant correlation between the number of CD34+ cells and each progenitor type (except day 56 LTC CFC) suggesting that all of them are mobilized by a common mechanism. Patients who had previously received extensive chemotherapy and/or radiotherapy mobilized fewer of all these cell types than those who had not. For the 65 patients who proceeded to autologous transplantation, the median times to an absolute neutrophil count (ANC) of > or =0.5 x 109/l and the last platelet transfusion post transplant were 13 and 11 days, respectively, with 14 (22%) of patients having platelet recovery delayed beyond day 21. There was no significant difference between patients who had or had not received extensive chemo/radiotherapy or among the different mobilization regimens for time to neutrophil or platelet recovery or the number of platelet or red blood cell transfusions received post transplant. Threshold doses of the different cell types transplanted (per kg of patient weight) which predicted rapid platelet recovery were 2 x 106 CD34+ cells, 5 x 105 total CFC and 2.5 x 104CFC-Mk. Corresponding thresholds for progenitor activity measured in LTC could not be established. These results further support the view that standard mobilization regimens yield progenitor numbers that are, in most cases, nonlimiting for generating neutrophil and platelet recoveries within 2 to 3 weeks after myeloablative therapy. Assessment of the CD34+ cell and/or CFC content of leukapheresis collections may identify patients in whom platelet recovery is likely to be significantly delayed although CFC-Mk enumeration does not appear to offer any unique predictive advantage.

Ex vivo expansion of normal and chronic myeloid leukemic stem cells without functional alteration using a NUP98HOXA10homeodomain fusion gene.

HOX genes have been implicated as regulators of normal and leukemic stem cell functionality, but the extent to which these activities are linked is poorly understood. Previous studies revealed that transduction of primitive mouse hematopoietic cells with a NUP98HOXA10homeodomain (NA10HD) fusion gene enables a subsequent rapid and marked expansion in vitro of hematopoietic stem cell numbers without causing their transformation or deregulated expansion in vivo. To determine whether forced expression of NA10HD in primitive human cells would have a similar effect, we compared the number of long-term culture-initiating cells (LTC-ICs) present in cultures of lenti-NA10HD versus control virus-transduced CD34(+) cells originally isolated from human cord blood and chronic phase (CP) chronic myeloid leukemia (CML) patients. We found that NA10HD greatly increases outputs of both normal and Ph(+)/BCR-ABL(+) LTC-ICs, and this effect is particularly pronounced in cultures containing growth factor-producing feeders. Interestingly, NA10HD did not affect the initial cell cycle kinetics of the transduced cells nor their subsequent differentiation. Moreover, immunodeficient mice repopulated with NA10HD-transduced CP-CML cells for more than 8 months showed no evidence of altered behavior. Thus, NA10HD provides a novel tool to enhance both normal and CP-CML stem cell expansion in vitro, without apparently altering other properties.

Unique differentiation programs of human fetal liver stem cells shown both in vitro and in vivo in NOD/SCID mice.

Comparative measurements of different types of hematopoietic progenitors present in human fetal liver, cord blood, and adult marrow showed a large (up to 250-fold), stage-specific, but lineage-unrestricted, amplification of the colony-forming cell (CFC) compartment in the fetal liver, with a higher ratio of all types of CFC to long-term culture-initiating cells (LTC-IC) and a lower ratio of total (mature) cells to CFC. Human fetal liver LTC-IC were also found to produce more CFC in LTC than cord blood or adult marrow LTC-IC, and more of the fetal liver LTC-IC-derived CFC were erythroid. Human fetal liver cells regenerated human multilineage hematopoiesis in NOD/SCID mice with the same kinetics as human cord blood and adult marrow cells, but sustained a high level of terminal erythropoiesis not seen in adult marrow-engrafted mice unless exogenous human erythropoietin (Epo) was injected. This may be due to a demonstrated 10-fold lower activity of murine versus human Epo on human cells, sufficient to distinguish between a differential Epo sensitivity of fetal and adult erythroid precursors. Examination of human LTC-IC, CFC, and erythroblasts generated either in NOD/SCID mice and/or in LTC showed the types of cells and hemoglobins produced also to reflect their ontological origin, regardless of the environment in which the erythroid precursors were generated. We suggest that ontogeny may affect the behavior of cells at many stages of hematopoietic cell differentiation through key changes in shared signaling pathways.

Quantitation and characterization of human megakaryocyte colony-forming cells using a standardized serum-free agarose assay.

Human progenitors of the megakaryocyte (Mk) lineage were detected by their ability to generate colonies-containing from 3 to > 100 Mk, detectable as glycoprotein IIb/IIIa+ cells in APAAP-stained whole mount agarose cultures. Optimal growth conditions were achieved through the use of a defined serum substitute and a suitable cocktail of recombinant cytokines. Under these culture conditions, the smallest Mk-containing colonies (CFC-Mk) were detectable within a week followed by colonies containing larger numbers of Mk over the ensuing 2 weeks. The total number of CFC-Mk at 18-21 d was linearly related to the number of cells plated. Variation in the cytokines added showed that thrombopoietin (TPO) or IL-3 alone would support the formation of large numbers of CFC-Mk. However, optimal yields of colonies containing cells of both Mk and non-Mk lineages required the addition of other growth factors, of which a combination of IL-3, IL-6, GM-CSF and Steel factor (SF) +/- TPO was the best of those tested. The further addition of erythropoietin to this combination reduced the number of large "pure' Mk colonies seen and in their place a corresponding number of mixed erythroid-Mk colonies became detectable. Flt3-ligand alone was unable to support the growth of CFC-Mk nor did it enhance their growth when combined with other factors. Plating of FACS-sorted sub-populations of CD34+ marrow cells in both serum-free agarose and methylcellulose assays demonstrated that most CFC-Mk are generated from CD34+ cells that are CD45RA- and CD71+, approximately half of which are CD41+. Thus, CFC-Mk are more similar to primitive clonogenic erythroid progenitors than to their granulopoietic counterparts in their expression of CD34, CD45RA and CD71. Taken together, these findings support the concept that some erythroid and Mk progenitors may share a common developmental pathway. The availability of sensitive and reproducible procedures for isolating and detecting human Mk progenitors should facilitate future investigations of their biology and role in a variety of haematological conditions.