Human FLT3 ligand acts on myeloid as well as multipotential progenitors derived from purified CD34+ blood progenitors expressing different levels of c-kit protein

Macrophages Derived From Human Induced Pluripotent Stem Cells: The Diversity of Protocols, Future Prospects, and Outstanding Questions

Macrophages (Mφ) derived from induced pluripotent stem cells (iMphs) represent a novel and promising model for studying human Mφ function and differentiation and developing new therapeutic strategies based on or oriented at Mφs. iMphs have several advantages over the traditionally used human Mφ models, such as immortalized cell lines and monocyte-derived Mφs. The advantages include the possibility of obtaining genetically identical and editable cells in a potentially scalable way. Various applications of iMphs are being developed, and their number is rapidly growing. However, the protocols of iMph differentiation that are currently used vary substantially, which may lead to differences in iMph differentiation trajectories and properties. Standardization of the protocols and identification of minimum required conditions that would allow obtaining iMphs in a large-scale, inexpensive, and clinically suitable mode are needed for future iMph applications. As a first step in this direction, the current review discusses the fundamental basis for the generation of human iMphs, performs a detailed analysis of the generalities and the differences between iMph differentiation protocols currently employed, and discusses the prospects of iMph applications.

Myeloid-derived suppressor cells: their role in the pathophysiology of hematologic malignancies and potential as therapeutic targets

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells at various stages of differentiation/maturation that have a role in cancer induction and progression. They function as vasculogenic and immunosuppressive cells, utilizing multiple mechanisms to block both innate and adaptive anti-tumor immunity. Recently, their mechanism of action and clinical importance have been defined, and the cross-talk between myeloid cells and cancer cells has been shown to contribute to tumor induction, progression, metastasis and tolerance. In this review, we focus on the role of MDSCs in hematologic malignancies and the therapeutic approaches targeting MDSCs that are currently in clinical studies.

Identification of long-term repopulating potential of human cord blood-derived CD34-flt3- severe combined immunodeficiency-repopulating cells by intra-bone marrow injection

Recently, we have identified human cord blood (CB)-derived CD34-negative (CD34(-)) severe combined immunodeficiency (SCID)-repopulating cells (SRCs) using the intra-bone marrow injection (IBMI) method (Blood 2003;101:2924). In contrast to murine CD34(-) Kit(+)Sca-1(+)Lineage(-) (KSL) cells, human CB-derived Lin(-)CD34(-) cells did not express detectable levels of c-kit by flow cytometry. In this study, we have investigated the function of flt3 in our identified human CB-derived CD34(-) SRCs. Both CD34(+)flt3(+/-) cells showed SRC activity. In the CD34(-) cell fraction, only CD34(-)flt3(-) cells showed distinct SRC activity by IBMI. Although CD34(+)flt3(+) cells showed a rather weak secondary repopulating activity, CD34(+)flt3(-) cells repopulated many more secondary recipient mice. However, CD34(-)flt3(-) cells repopulated all of the secondary recipients, and the repopulating rate was much higher. Next, we cocultured CD34(-)flt3(-) cells with the murine stromal cell line HESS-5. After 1 week, significant numbers of CD34(+)flt3(+/-) cells were generated, and they showed distinct SRC activity. These results indicated that CB-derived CD34(-)flt3(-) cells produced CD34(+)flt3(-) as well as CD34(+)flt3(+) SRCs in vitro. The present study has demonstrated for the first time that CB-derived CD34(-) SRCs, like murine CD34(-) KSL cells, do not express flt3. On the basis of these data, we propose that the immunophenotype of very primitive long-term repopulating human hematopoietic stem cells is Lin(-)CD34(-)c-kit(-)flt3(-). Disclosure of potential conflicts of interest is found at the end of this article.

After chronic myelogenous leukemia: tyrosine kinase inhibitors in other hematologic malignancies

Tyrosine kinases phosphorylate proteins on tyrosine residues, producing a biologic signal that influences many aspects of cellular function including cell growth, proliferation, differentiation, and death. Constitutive or unregulated activity through mutation or overexpression of these enzymes is a common pathologic feature in many acute and chronic leukemias. Inhibition of tyrosine kinases represents a strategy to disrupt signaling pathways that promote neoplastic growth and survival in hematologic malignancies and likely in other neoplasias as well. This review focuses on tyrosine kinases that have been implicated in the pathogenesis of hematologic diseases other than chronic myelogenous leukemia and discusses the evidence for the use of small molecules to target these kinases.

Durable responses to thalidomide-based drug therapy for myelofibrosis with myeloid metaplasia

OBJECTIVE

To present the results of a long-term analysis of 2 sequential phase 2 trials of thalidomide (alone or in combination) for palliation of myelofibrosis with myeloid metaplasia (MMM).

PATIENTS AND METHODS

We analyzed (March 1999 to August 2003) initial and long-term outcomes from 36 patients with symptomatic MMM who had enrolled in either our thalidomide single-agent trial (n=15) or our trial of low-dose thalidomide (50 mg/d) combined with prednisone (n=21).

RESULTS

Among the 36 study patients, 20 (56%) showed some improvement in their clinical course. Response rates for specific end points included improvements in anemia (15 of 36 [42%]), thrombocytopenia (10 of 13 [77%]), or splenomegaly (5 of 30 [17%]). The combination of low-dose thalidomide and prednisone, as opposed to single-agent thalidomide, was better tolerated and more efficacious. After a median follow-up of 25 months (range, 20-56 months), 10 of 36 patients (28%) showed an ongoing response, including 8 patients in whom protocol treatment has been discontinued for a median of 21 months (range, 16-31 months). Durable treatment responses were documented for only anemia and thrombocytopenia. Treatment response was not affected by the baseline status of bone marrow fibrosis, angiogenesis, osteosclerosis, cytogenetics, or circulating myeloid progenitor (CD34) cell count. Unusual drug effects, all reversible, included leukocytosis (8 patients) and/or thrombocytosis (6 patients).

CONCLUSIONS

Thalidomide (alone or combined with prednisone) is an effective first-line treatment of symptomatic anemia or thrombocytopenia in MMM. Thalidomide-based therapy has the potential to produce durable responses in MMM-associated cytopenias, even after discontinuation of the drug.

The roles of FLT3 in hematopoiesis and leukemia

FLT3 is a receptor tyrosine kinase expressed by immature hematopoietic cells and is important for the normal development of stem cells and the immune system. The ligand for FLT3 is expressed by marrow stromal cells and other cells and synergizes with other growth factors to stimulate proliferation of stem cells, progenitor cells, dendritic cells, and natural killer cells. Mutations of FLT3 have been detected in about 30% of patients with acute myelogenous leukemia and a small number of patients with acute lymphocytic leukemia or myelodysplastic syndrome. Patients with FLT3 mutations tend to have a poor prognosis. The mutations most often involve small tandem duplications of amino acids within the juxtamembrane domain of the receptor and result in constitutive tyrosine kinase activity. Expression of a mutant FLT3 receptor in murine marrow cells results in a lethal myeloproliferative syndrome and preliminary studies suggest that mutant FLT3 cooperates with other leukemia oncogenes to confer a more aggressive phenotype. Taken together, these results suggest that FLT3 is an attractive therapeutic target for kinase inhibitors or other approaches for patients with mutations of this gene.

Flt3 ligand augmentation of T cell mitogenesis and expansion of type 1 effector/memory T cells

Herein we report mechanisms whereby Flt3 ligand (FL) augments steady state T cell activity in addition to the expansion of dendritic cells (DCs). We demonstrate that in vivo administration of FL increases the frequency and absolute number of effector/memory T cells and preferentially expands T cells that express a type-1 cytokine phenotype. In addition, FL enhances T cell proliferative responses to Concanavalin A that directly correlated with increased frequencies in effector/memory T cells and expansion of lymphoid-derived (type 1) DCs (DC1s). Together, these data demonstrate that mechanisms of FL-induced T cell regulation include not only the expansion of DC subsets, but also the preferential expansion of type 1 -effector/memory T cell populations, and suggest multiple mechanisms of action for FL as a vaccine adjuvant and as a therapeutic modality.

Early progenitor cells from human mobilized peripheral blood express low levels of the flt3 receptor, but exhibit various biological responses to flt3-L

The biological effects of flt3-L, and the expression of its tyrosine kinase receptor (flt3, CD135) were investigated on the immature subsets of human circulating peripheral blood progenitors obtained from cancer patients or normal volunteer donors, after mobilization with rhG-CSF or chemotherapy. flt3 was expressed at low levels, and its expression increased concomitantly with expression of CD38 within the CD34+ cell population. Despite this low-level expression, flt3-L exerted synergistic effects with a combination of c-kit ligand, IL-3, IL-6, GM-CSF and G-CSF, mainly to induce proliferation of CD34+/CD38- cells. In addition, flt3-L increased the detection of HPP-CFC, both immediately after cell selection, and after 7 and 14 d of cultures. We conclude that flt3-L is active on circulating early mobilized haemopoietic progenitors, despite the low- level expression of its receptor.

Proliferation in Monocyte-Derived Dendritic Cell Cultures Is Caused by Progenitor Cells Capable of Myeloid Differentiation

Dendritic cells (DC) can be generated by culture of adherent peripheral blood (PB) cells in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). There is controversy as to whether these DC arise from proliferating precursors or simply from differentiation of monocytes. DC were generated from myeloid-enriched PB non-T cells or sorted monocytes. DC generated from either population functioned as potent antigen-presenting cells. Uptake of [3H]-thymidine was observed in DC cultured from myeloid-enriched non-T cells. Addition of lipopolysaccharide or tumor necrosis factor- led to maturation of the DC, but did not inhibit proliferation. Ki67+ cells were observed in cytospins of these DC, and by double staining were CD3−CD19−CD11c−CD40−and myeloperoxidase+, suggesting that they were myeloid progenitor cells. Analysis of the starting population by flow cytometry demonstrated small numbers of CD34+CD33−CD14− progenitor cells, and numerous granulocyte-macrophage colony-forming units were generated in standard assays. Thus, production of DC in vitro from adherent PB cells also enriches for progenitor cells that are capable of proliferation after exposure to GM-CSF. Of clinical importance, the yield of DC derived in the presence of GM-CSF and IL-4 cannot be expanded beyond the number of starting monocytes.

© 1998 by The American Society of Hematology.

Proliferation in Monocyte-Derived Dendritic Cell Cultures Is Caused by Progenitor Cells Capable of Myeloid Differentiation

Dendritic cells (DC) can be generated by culture of adherent peripheral blood (PB) cells in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). There is controversy as to whether these DC arise from proliferating precursors or simply from differentiation of monocytes. DC were generated from myeloid-enriched PB non-T cells or sorted monocytes. DC generated from either population functioned as potent antigen-presenting cells. Uptake of [3H]-thymidine was observed in DC cultured from myeloid-enriched non-T cells. Addition of lipopolysaccharide or tumor necrosis factor- led to maturation of the DC, but did not inhibit proliferation. Ki67+ cells were observed in cytospins of these DC, and by double staining were CD3−CD19−CD11c−CD40−and myeloperoxidase+, suggesting that they were myeloid progenitor cells. Analysis of the starting population by flow cytometry demonstrated small numbers of CD34+CD33−CD14− progenitor cells, and numerous granulocyte-macrophage colony-forming units were generated in standard assays. Thus, production of DC in vitro from adherent PB cells also enriches for progenitor cells that are capable of proliferation after exposure to GM-CSF. Of clinical importance, the yield of DC derived in the presence of GM-CSF and IL-4 cannot be expanded beyond the number of starting monocytes.

© 1998 by The American Society of Hematology.

Haematopoietic action of flt3 ligand on cord blood-derived CD34-positive cells expressing different levels of flt3 or c-kit tyrosine kinase receptor: comparison with stem cell factor

We compared the effect of human flt3 ligand (FL) and stem cell factor (SCF) on cord blood (CB)-derived CD34+ cells expressing different levels of flt3 or c-kit tyrosine kinase (TK) receptor in clonal cell culture. The c-kit receptor was expressed by 58.5+/-16.7% of CB CD34+ cells (n=19), in which c-kit(high), c-kit(low) and c-kit cell populations could be identified. In contrast, the flt3 receptor (FR) was weakly expressed on 58.6+/-8.3% (n=9) of CB CD34+ cells. FL+erythropoietin (Epo) failed to support erythroid burst (BFU-E) formation by any subpopulation of CD34+ cells. However, SCF + Epo supported BFU-E and erythrocyte-containing mixed (CFU-mix) colony formation from all subpopulations. Interestingly, FL markedly augmented CFU-mix colony formation supported by interleukin (IL)-3 + Epo when CD34+c-kit(low) or CD34+FR+ cells were used as the target. On the other hand, SCF significantly enhanced CFU-mix colony formation supported by IL-3 + Epo when CD34+c-kit(high) or low and CD34+FR+ cells were used. The replating potential of CFU-mix supported by IL-3 + Epo+ FL was greater when CD34+c-kit(low) or CD34+FR+ cells were used. When the CD34+c-kit(low) cells were used, the number of lineages expressed in secondary cultures of CFU-mix colonies derived from primary cultures containing IL-3 + Epo + FL or SCF was significantly larger than when the primary cultures contained IL-3 + Epo. Furthermore, the number of long-term culture-initiating cells found in CD34+FR+ cells was larger than that in FR cells. CB-derived CD34+c-kit(low) cells represent a less mature population than c-kit(high) cells, as reported previously. Therefore, these results indicate that both FL and SCF can act on primitive multipotential progenitors. However, it is still uncertain whether CB-derived CD34+FR+ cells are less mature than CD34+FR- cells.