Rob E. Ploemacher

Biological characteristics and prognosis of adult acute myeloid leukemia with internal tandem duplications in the Flt3 gene.

Internal tandem duplications of the Flt3 gene (Flt3/ITDs) are present in about 18% of all AML cases and are therefore one of the most frequent somatic gene mutations in AML. Little is known about the role of Flt3/ITDs in leukemogenesis or their clinical relevance. In this study we compared 18 samples with Flt3/ITDs and 63 AML samples without these mutations with respect to clinical prognosis, cytokine responsiveness, progenitor cell content and repopulation in the NOD/SCID mouse. We found that in patients with a mutation CR rates are reduced (P = 0.03) and relapse rates are increased (P = 0.01), indicating the prognostic importance of Flt3/ITDs. This is also emphasized by the finding that in patients under the age of 60 years, as well as in older patients the event-free survival was more unfavorable for the mutant patients (P = 0.003 and P = 0.03, respectively). At diagnosis Flt3/ITD and non-mutant AML bone marrow samples did not differ in their progenitor/stem cell frequencies. Cobblestone area forming cell (CAFC) subsets showed a similar frequency distribution in mutant and non-mutant samples. In 7-day liquid cultures, Flt3/ITD samples showed a reduced growth in response to a variety of myeloid growth factors. In contrast, Flt3/ITD samples displayed a higher ability to engraft the NOD/SCID bone marrow with leukemic cells. Together these data show that the Flt3/ITD represents an important diagnostic marker for patient prognosis, and that the presence of these mutations is associated with altered proliferative ability of progenitors in vivo and in vitro.

Relation between CXCR-4 expression, Flt3 mutations, and unfavorable prognosis of adult acute myeloid leukemia.

Recently it was shown that, analogous to normal hematopoietic cells, the level of CXC chemokine receptor 4 (CXCR-4) expression on acute myeloid leukemia (AML) cells correlates with stromal cell derived factor-1 alpha (SDF-1)-induced chemotaxis. As we speculated that an anomalous organ distribution of AML cells could affect cell survival and thus result in an altered fraction surviving chemotherapy, we examined a possible correlation between patient prognosis and CXCR-4 expression in AML patients. We found that patients with a high CXCR-4 expression in the CD34(+) subset had a significantly reduced survival and a higher probability of relapse, resulting in a median relapse-free survival (RFS) of only 8.3 months. CXCR-4 expression was significantly higher in fetal liver tyrosine kinase-3 (Flt3)/internal tandem duplication (ITD) AML than in Flt3/wild-type (wt) AML. Covariate analysis indicated that the prognostic significance of Flt3/ITDs with respect to RFS was no more apparent when analyzed in conjunction with the expression of CXCR-4 in the CD34(+) subset, suggesting that the poor prognosis of Flt3/ITD AML might be subordinate to the increased CXCR-4 expression. Using a granulocyte colony-stimulating factor receptor (G-CSF-R)-expressing 32D cell line, we observed that SDF-1/CXCR-4 interaction is required for the survival of myeloid differentiating cells, and it also induces a block in G-CSF-induced myeloid differentiation. These data suggest that the SDF-1/CXCR-4 axis may influence therapy responsiveness and defines unfavorable prognosis in AML.

GATA-2 plays two functionally distinct roles during the ontogeny of hematopoietic stem cells.

GATA-2 is an essential transcription factor in the hematopoietic system that is expressed in hematopoietic stem cells (HSCs) and progenitors. Complete deficiency of GATA-2 in the mouse leads to severe anemia and embryonic lethality. The role of GATA-2 and dosage effects of this transcription factor in HSC development within the embryo and adult are largely unexplored. Here we examined the effects of GATA-2 gene dosage on the generation and expansion of HSCs in several hematopoietic sites throughout mouse development. We show that a haploid dose of GATA-2 severely reduces production and expansion of HSCs specifically in the aorta-gonad-mesonephros region (which autonomously generates the first HSCs), whereas quantitative reduction of HSCs is minimal or unchanged in yolk sac, fetal liver, and adult bone marrow. However, HSCs in all these ontogenically distinct anatomical sites are qualitatively defective in serial or competitive transplantation assays. Also, cytotoxic drug-induced regeneration studies show a clear GATA-2 dose–related proliferation defect in adult bone marrow. Thus, GATA-2 plays at least two functionally distinct roles during ontogeny of HSCs: the production and expansion of HSCs in the aorta-gonad-mesonephros and the proliferation of HSCs in the adult bone marrow.

Reduced hematopoietic reserves in DNA interstrand crosslink repair- deficient Ercc1-/- mice.

The ERCC1‐XPF heterodimer is a structure‐specific endonuclease involved in both nucleotide excision repair and interstrand crosslink repair. Mice carrying a genetic defect in Ercc1 display symptoms suggestive of a progressive, segmental progeria, indicating that disruption of one or both of these DNA damage repair pathways accelerates aging. In the hematopoietic system, there are defined age‐associated changes for which the cause is unknown. To determine if DNA repair is critical to prolonged hematopoietic function, hematopoiesis in Ercc1−/− mice was compared to that in young and old wild‐type mice. Ercc1−/− mice (3‐week‐old) exhibited multilineage cytopenia and fatty replacement of bone marrow, similar to old wild‐type mice. In addition, the proliferative reserves of hematopoietic progenitors and stress erythropoiesis were significantly reduced in Ercc1−/− mice compared to age‐matched controls. These features were not seen in nucleotide excision repair‐deficient Xpa−/− mice, but are characteristic of Fanconi anemia, a human cancer syndrome caused by defects in interstrand crosslink repair. These data support the hypothesis that spontaneous interstrand crosslink damage contributes to the functional decline of the hematopoietic system associated with aging.

MOBILIZATION OF HAEMOPOIETIC STEM CELLS (CFU) INTO THE PERIPHERAL BLOOD OF THE MOUSE; EFFECTS OF ENDOTOXIN AND OTHER COMPOUNDS

Factors affecting the circulation of haemopoietic stem cells (CFU) in the peripheral blood of mice were investigated. I.v. injection of sublethal doses of endotoxin, trypsin and proteinase appeared to raise the number of CFU per ml blood from about 30–40 to about 300–400 or more within 10 min. The effect was smaller when smaller doses of the substances were injected. After this initial rise the number of circulating cells returned to normal in a few hours. Following endotoxin there was a second rise which started 2–3 days after injection and attained a peak on the 6th–7th day. The first rise is explained as a mobilization of stem cells from their normal microenvironments into the blood stream; the second rise is considered to reflect proliferation of CFUs in the haemopoietic tissues. The spleen seems to be acting as an organ capturing CFUs from the blood and not as a source adding stem cells to the blood.

GRK6 deficiency is associated with enhanced CXCR4-mediated neutrophil chemotaxis in vitro and impaired responsiveness to G-CSF in vivo.

The stromal cell‐derived factor‐1 (SDF‐1)/CXC chemokine receptor 4 (CXCR4) signaling pathway is thought to play an important role in the induction of neutrophil mobilization from the bone marrow in response to granulocyte‐colony stimulating factor (G‐CSF) treatment. CXCR4 belongs to the family of G protein‐coupled receptors. Multiple members of this receptor family are desensitized by agonist‐induced G protein‐coupled receptor kinase (GRK)‐mediated phosphorylation. Here, we demonstrate that in vitro SDF‐1‐induced chemotaxis of bone marrow‐derived neutrophils from GRK6‐deficient mice is significantly enhanced and that desensitization of the calcium response to SDF‐1 is impaired in GRK6−/− neutrophils. CXCR4 activation by SDF‐1 provides a key retention signal for hematopoietic cells in the bone marrow. It is interesting that we observed that in the absence of GRK6, the G‐CSF‐induced increase in circulating neutrophils is profoundly impaired. Three days after injection of pegylated‐G‐CSF, significantly lower numbers of circulating neutrophils were observed in GRK6−/− as compared with wild‐type (WT) mice. In addition, early/acute neutrophil mobilization in response to G‐CSF (3 h after treatment) was also impaired in GRK6−/− mice. However, blood neutrophil levels in untreated GRK6−/− and WT mice were not different. Moreover, the percentage of neutrophils in the bone marrow after G‐CSF treatment was increased to the same extent in WT and GRK6−/− mice, indicating that neutrophil production is normal in the absence of GRK6. However, the increased chemotactic sensitivity of GRK6−/− neutrophils to SDF‐1 was retained after G‐CSF treatment. In view of these data, we suggest that the impaired G‐CSF‐induced neutrophil mobilization in the absence of GRK6 may be a result of enhanced CXCR4‐mediated retention of PMN in the bone marrow.

Identification of variables determining the engraftment potential of human acute myeloid leukemia in the immunodeficient NOD/SCID human chimera model

Among a variety of immunodeficient mouse strains the nonobese diabetic (NOD)/LtSz scid/scid strain appears to be most useful in allowing the engraftment of human AML. However, the large variability in ability to engraft and the levels of engraftment reached have not been explained. To address these issues we have investigated the NOD/SCID repopulating ability of 27 newly diagnosed AML samples. Patients were selected for the absence of internal tandem duplications in the Flt3 gene as we previously reported this mutation to be associated with an enhanced engraftment potential in this model. We observed that secondary AML (n = 6) had a significantly increased level of engraftment when compared to primary AML (n = 21, median levels 73.3% for secondary AML vs 8.94% for primary AML, P = 0.01). Within the primary AML, a significantly higher engraftment was observed in the FAB class M0 than in FAB classes M2, M4 and M5. Within primary AML, samples of patients who failed to respond to the initial therapy gave rise to a higher level of engraftment than samples of patients who did respond to therapy. A similar observation of an increased engraftment correlating with a poorer patient prognosis could be made when applying cytogenetic risk stratification. However, within the primary AML the most important clinical parameter correlating with the level of engraftment appeared to be the patients WBC count at diagnosis (P = 0.0000). Covariate analysis with the WBC count as a covariate could also fully explain the differences observed in the cytogenetic risk groups, or on the basis of the initial therapy response. Although large differences could be observed, the ability to engraft the NOD/SCID mice was not linked to either the autonomous or cytokine-induced proliferation in vitro. As the leukemic cobblestone area-forming cell frequencies also revealed no correlation with repopulation in the NOD/SCID model, we consider it very likely that the level of engraftment reflects the in vivoproliferative ability of the AML samples assayed rather than the number of leukemia-initiating cells infused into the NOD/SCID mice. Phenotypic analysis based on the expression of CD33, CD34 and CD38 before and after passage in NOD/SCID showed that in 10 out of 16 samples investigated phenotypes were different. In summary, in addition to the Flt3 internal tandem duplications we have identified a series of clinical parameters that determine the NOD/SCID repopulating ability of AML samples, whilst our data strongly suggest that AML in NOD/SCID does not reflect the leukemic process in the patient.

Transplantation of bone marrow fibroblastoid stromal cells in mice via the intravenous route

Summary. Kinetics of fibroblastic colony‐forming cells (CFU‐F) were studied in mouse bone marrow after lethal total body irradiation and intravenous bone marrow transplantation. After an initial decrease, CFU‐F numbers recovered, and plateaued 5 weeks post‐treatment at 10% of normal values.

Murine Haemopoietic Stem Cells with Long-term Engraftment and Marrow Repopulating Ability are More Resistant to Gamma-radiation than are Spleen Colony Forming Cells

The radiation sensitivity of various subsets in the haemopoietic stem cell hierarchy was defined using a limiting dilution type long-term bone marrow culture technique that was previously shown to allow quantification of cells with spleen colony-forming potential (day-12 CFU-S) and in vivo marrow repopulating ability (MRA). Primitive stem cells that generate new in vitro clonable colony-forming cells (CFU-C) in the irradiated marrow (MRA) and have long-term repopulation ability (LTRA) in vitro (cobblestone area forming cell, CAFC day-28) had D0 values of 1.25 and 1.38 Gy, respectively. A lower D0 was found for the less primitive CFU-S day-12, CAFC day-12 and cells with erythroid repopulating ability (0.91, 1.08 and 0.97 Gy, respectively). CFU-S day-7 were the most radiosensitive (D0 equalling 0.79 Gy), while CFU-C and CAFC day-5 were relatively resistant to irradiation (D0 1.33 and 1.77 Gy). Split-dose irradiation with a 6 h interval gave dose sparing for stem cells with MRA and even more with in vitro LTRA, less for CFU-S day-12 and CAFC day-10 and none for CFU-S day-7. The cell survival data of the specified stem cell populations were compared with the ability of a fixed number of B6-Gpi-1a donor bone marrow cells to provide for short- and long-term engraftment in single- and split-dose irradiated congenic B6-Gpi-1b mice. Serial blood glucose phosphate isomerase (Gpi) phenotyping showed less chimerism in the split as compared to the single radiation dose groups beyond 4 weeks after transplant. Radiation dose-response curves corresponding to stable chimerism at 12 weeks for single and fractionated doses revealed appreciable split-dose recovery (D2-D1) in the order of 2 Gy. This was comparable to D2-D1 estimates for MRA and late-developing CAFC (1.27 and 1.43 Gy, respectively), but differed from the poor dose recovery in cells corresponding to the committed CFU-S day-7/12 and CAFC day-10 population (0.14-0.33 Gy). These data are together consistent with differential radiosensitivity and repair in the haemopoietic stem cell hierarchy, and provide a cellular basis for explaining the dose-sparing effect of fractionated total-body irradiation conditioning on long-term host marrow repopulation.

Stroma-conditioned media improve expansion of human primitive hematopoietic stem cells and progenitor cells

It has been reported that stroma-dependent cultures support proliferation of hematopoietic stem cells (HSC). In order to investigate the effect of soluble stromal factors, we developed short-term serum-low liquid cultures in which the effect of stroma-conditioned media (SCM) from the murine FBMD-1, and human L87/4 and L88/5 cell lines was studied on the maintenance and expansion of various human HSC subsets in CD34-positive selected mobilized peripheral blood stem cells (PBSC) from autologous transplants of lymphoma and multiple myeloma patients. The human cobblestone area forming cell (CAFC) assay was employed to determine the frequencies of both the CAFC weeks 2 to 4 as tentative indicators of progenitor and transiently repopulating HSC, and the more primitive CAFC weeks 6 to 8 as indicators of long-term repopulating HSC. In 7-day liquid cultures containing interleukin-3 (IL-3), stem cell factor (SCF) and IL-6, we recovered 3.0-fold more colony-forming cells (CFC) and 1.7- to 1.9-fold more CAFC weeks 2 and 4. The absolute number of primitive CAFC weeks 6 and 8 were only maintained (1.1- to 1.4-fold) in these liquid cultures. This modest expansion was significantly improved by the addition of SCM from the FBMD-1, L87/4 or L88/5 cell lines. Output CFC numbers were 6.8-, 5.8- and 9.9-fold higher, respectively, than the input values, while absolute CAFC week 2 to 4 numbers were 4.5-, 10.2- and 10.2-fold expanded, respectively. The addition of SCM also improved expansion of the more primitive CAFC week 6 to 8 stem cell subsets by 2.2-, 4.5- and 4.9-fold, respectively. The addition of granulocyte colony-stimulating factor (G-CSF), granulocyte–macrophage-CSF (GM-CSF), IL-1β, IL-11 or macrophage inflammatory protein-1α to cultures containing IL-3, SCF and IL-6 could not explain the SCM effect and in all these combinations SCM addition further increased the recovery of HSC subsets. Similarly, addition of anti-cytokine antibodies (ie α-G-CSF, α-GM-CSF, α-IL-11, α -leukemia inhibitory factor) to liquid cultures containing IL-3, SCF, IL-6 and SCM could not neutralize the SCM effect. These data indicate that SCM significantly enhances expansion of primitive HSC and progenitor cells from CD34-selected PBSC in 7-day cultures and in synergistic combination with multiple cytokines at optimal concentrations. As a result, SCM is a useful component of short-term liquid culture procedures for clinical expansion or manipulation of primitive HSC.