Growth factors and hematopoietic stem cells

Bone Marrow and Peripheral Blood C-kit Ligand Concentrations in Patients with Thrombocytosis and Thrombocytopenia

C-kit ligand (stem cell factor, SCF) is a hematopoietic growth factor with diverse effects. It has stimulatory effects on megakaryocytopoiesis acting in synergism with interleukin-3 (IL-3), thrombopoietin (TPO) and granulocyte-macrophage colony stimulating factor (GM-CSF). The relationship between SCF and megakaryocytopoiesis, especially the correlations between blood and bone marrow SCF levels have been not clearly established in the literature. We therefore, investigated peripheral and bone marrow SCF levels in patients with thrombocytosis and thrombocytopenia. Subjects were divided into three groups: those with (i) thrombocytopenia, (ii) thrombocytosis and (iii) healthy adults as controls. When the three groups were compared, the mean peripheral blood SCF level of the thrombocytosis group (2149±197) was significantly higher than the thrombocytopenia (1586±178) and normal control groups (1371±68; p<0.05) and the bone marrow SCF level was higher (2694±267) than the thrombocytopenia group (1700±182; p<0.05). In the correlation analysis, considering all the groups together the bone marrow and peripheral blood SCF concentrations were positively and significantly correlated (p<0.01; r=0.93). Correlations between platelet number and both bone marrow SCF concentration (p<0.01; r=0.51) and peripheral blood concentrations (p<0.01; r=0.40) were also shown. Our results indicate that SCF is operative in the pathological megakaryopoiesis of clonal origin and reactive thrombocytosis both in the local bone marrow microenvironment and the peripheral circulating blood. We feel that further studies on the platelet-SCF relationship and SCF levels in different disease states are required.

Growth factor receptor profile of CD34 cells in normal bone marrow, cord blood and mobilized peripheral blood

Growth factors regulate the proliferation and differentiation of hemopoietic cells. Their effect on hemopoietic precursors differs according to the ontogenic source of the cells. Cord blood and mobilized blood CD34(+) cells have a higher sensitivity for growth factors than bone marrow CD34(+) cells. This could be due to a higher expression of growth factor receptors. Therefore, we examined the expression of receptors for stem cell factor (SCF), interleukin-6 (IL-6), IL-3, granulocyte colony-stimulating factor (G-CSF) and IL-7 on the CD34(+) cells of cord blood, mobilized peripheral blood and bone marrow. The receptors were detected with monoclonal antibodies and flow cytometry. The majority of the CD34(+) cells in bone marrow clearly expressed SCFR; they showed a moderate positivity for IL-3Ralpha and a weak staining for G-CSFR and IL-6 Ralpha. Less than 10% of the cells were IL-7R positive. Cord blood CD34(+) cells showed a higher expression of SCFR and a lower positivity for G-CSFR and IL-6Ralpha. Mobilized blood CD34(+) cells showed a lower expression of SCFR and G-CSFR, and a higher positivity for IL-3Ralpha. This was not solely due to the presence of more myeloid precursors in mobilized blood, as the growth factor receptor profile did not correspond to that of early or late myeloid CD34(+) precursors in normal bone marrow. Changes induced by the mobilization procedure occurred as well. In conclusion, the higher sensitivity for growth factors of hemopoietic precursors in cord blood and mobilized blood cannot be explained by a general increase of the growth factor receptor expression on the CD34(+) cells.

Mechanism, detection and clinical significance of the reciprocal translocation t(12;21)(p12;q22) in the children suffering from acute lymphoblastic leukaemia

The t(12;21)(p12;q22) is the most frequent chromosomal rearrangement observed in acute lymphoblastic leukaemia (ALL) and is associated with favourable prognosis and good response to initial treatment. The translocation-Ets-leukaemia (TEL) and AML1 genes are very often involved in chromosomal translocations in haematopoietic malignancies. This review presents the structure, roles of TEL and AML1 genes, and their proteins in haematopoiesis and in leukaemiogenesis as well. Aspects such as: the mechanism of translocation t(12;21)(p12;q22), function of TEL/AML1 fusion gene and chimeric protein, clinical significance of this abnormality and methods allowing to detect this translocation and its transcript are also discussed in this paper.

Growth factor receptor profile of CD34+ cells in AML and B-lineage ALL and in their normal bone marrow counterparts

Leukaemic cells show a low clonogenic activity and a heterogeneous proliferative response to growth factors. We investigated whether this could be due to an altered expression of growth factor receptors on the leukaemic precursors. Receptors for G-CSF, stem cell factor (SCF), IL-3, IL-6 and IL-7 were detected on CD34+ cells in AML and B-lineage ALL with monoclonal antibodies and flow cytometry. The expression was compared with that on myeloid and B-lymphoid CD34+ cells in normal bone marrow. Leukaemic CD34+ cells expressed the same receptors as their normal counterparts. AML and B-lineage ALL could be distinguished by the growth factor receptor profile of their CD34+ cells. SCFR, G-CSFR and IL-6Ralpha were found in AML, IL-7R in B-lineage ALL and IL-3Ralpha in both. IL-3Ralpha was upregulated in AML and B-lineage ALL CD34+ cells, while samples with low or high expression were present for the other receptors. This variable expression could correlate with the heterogeneous response of leukaemic cells to growth factors. Functional studies on isolated CD34+ cells are needed to investigate this further.

Phenotypic distinction and functional characterization of pro-B cells in adult mouse bone marrow

A lymphoid-committed progenitor population was isolated from mouse bone marrow based on the cell surface phenotype Thy-1.1(neg)Sca-1(pos)c-Kit(low)Lin(neg). These cells were CD43(pos)CD24(pos) on isolation and proliferated in response to the cytokine combination of steel factor, IL-7, and Flt3 ligand. Lymphoid-committed progenitors could be segregated into more primitive and more differentiated subsets based on expression of AA4.1. The more differentiated subset generated only B lymphoid cells in 92% of total colonies assayed, lacked T lineage potential, and expressed Pax5. These studies have therefore defined and isolated a B lymphoid-committed progenitor population at a developmental stage corresponding to the initial expression of CD45R.

Preincubation with endothelial cell monolayers increases gene transfer efficiency into human bone marrow CD34(+)CD38(-) progenitor cells

Retroviral gene transfer studies targeting bone marrow CD34(+)CD38(-) stem cells have been disappointing because of the rarity of these cells, their G(0) cell cycle status, and their low or absent expression of surface retroviral receptors. In this study, we examined whether preincubation of bone marrow CD34(+)CD38(-) stem cells with a hematopoietically supportive porcine microvascular endothelial cell line (PMVECs) could impact the cell cycle status and expression of retroviral receptors in pluripotent CD34+CD38- cells and the efficiency of gene transfer into these primitive target cells. PMVEC coculture supplemented with GM-CSF + IL-3 + IL-6 + SCF + Flt-3 ligand induced >93% of the CD34(+)CD38(-) population to enter the G(1) or G(2)/S/M phase while increasing this population from 1.4% on day 0 to 6.5% of the total population by day 5. Liquid cultures supplemented with the identical cytokines induced 73% of the CD34(+)CD38(-) population into cell cycle but did not maintain cells with the CD34(+)CD38(-) phenotype over time. We found no significant increase in the levels of AmphoR or GaLVR mRNA in PMVEC-expanded CD34(+)CD38(-) cells after coculture. Despite this, the efficiency of gene transfer using either amphotropic vector (PA317) or GaLV vector (PG13) was significantly greater in PMVEC-expanded CD34(+)CD38(-) cells (11.4 +/- 5.6 and 10.9 +/- 5.2%, respectively) than in either steady state bone marrow CD34(+)CD38(-) cells (0.6 +/- 1.7 and 0.2 +/- 0.6%, respectively; p < 0.01 and p < 0.01) or liquid culture-expanded CD34(+)CD38(-) cells (1.4 +/- 3.5 and 0.0%, respectively; p < 0.01 and p < 0.01). Since PMVEC coculture induces a high level of cell cycling in human bone marrow CD34(+)CD38(-) cells and expands hematopoietic cells capable of in vivo repopulation, this system offers potential advantages for application in clinical gene therapy protocols.

Immunobiology of allogeneic peripheral blood mononuclear cells mobilized with granulocyte-colony stimulating factor

The use of mobilized peripheral blood (PB) stem cells for autologous transplantation initially generated much enthusiasm because of enhanced engraftment in comparison to marrow stem cells and avoidance of general anesthesia for the donor. Its application to the allogeneic setting seemed inevitable. For obvious ethical reasons, allogeneic donors are mobilized with cytokines only, mainly granulocyte colony-stimulating factor (G-CSF). Results from preliminary studies suggest that in comparison to standard bone marrow transplants, outcomes such as engraftment, host-versus-graft reaction, graft-versus-host disease, graft-versus-leukemia and immunological reconstitution may be different. Surprisingly, G-CSF, previously recognized as a late acting lineage-specific factor for neutrophil production, not only disrupts homeostasis between stem cells and their microenvironment, but also induces significant quantitative and qualitative changes in the accessory cell compartment, affecting lymphocytes, monocytes, natural killer, dendritic, and stromal cells. Furthermore, mobilization of huge numbers of non-professional antigen presenting cells (CD34+ stem cells) amplifies the tolerizing potential of PB stem cell grafts. Thus, G-CSF mobilization provides PB transplants with different immunobiologic properties in comparison to standard bone marrow grafts. Whether these immunobiologic differences will lead to better transplant outcomes remains to be shown through much awaited results of large randomized clinical trials.

The TEL/ETV6 gene is required specifically for hematopoiesis in the bone marrow

The TEL (translocation-Ets-leukemia or ETV6) locus, which encodes an Ets family transcription factor, is frequently rearranged in human leukemias of myeloid or lymphoid origins. By gene targeting in mice, we previously showed that TEL-/- mice are embryonic lethal because of a yolk sac angiogenic defect. TEL also appears essential for the survival of selected neural and mesenchymal populations within the embryo proper. Here, we have generated mouse chimeras with TEL-/- ES cells to examine a possible requirement in adult hematopoiesis. Although not required for the intrinsic proliferation and/or differentiation of adult-type hematopoietic lineages in the yolk sac and fetal liver, TEL function is essential for the establishment of hematopoiesis of all lineages in the bone marrow. This defect is manifest within the first week of postnatal life. Our data pinpoint a critical role for TEL in the normal transition of hematopoietic activity from fetal liver to bone marrow. This might reflect an inability of TEL-/- hematopoietic stem cells or progenitors to migrate or home to the bone marrow or, more likely, the failure of these cells to respond appropriately and/or survive within the bone marrow microenvironment. These data establish TEL as the first transcription factor required specifically for hematopoiesis within the bone marrow, as opposed to other sites of hematopoietic activity during development.