Effects of flt3 ligand on acute myeloid and lymphocytic leukemic blast cells from children

A ligand for the flt3 tyrosine kinase receptor (flt3R) has recently been cloned. Forty-three cases of childhood acute myeloid leukemia (AML) and 27 cases of childhood acute lymphocytic leukemia (ALL) were examined by flow cytometric analysis for cell-surface flt3R and proliferative response in vitro to flt3 ligand (flt3L). Flt3R was commonly expressed on the cell surface of leukemic cells from all AML subclasses and B-ALL, but we did not detect cell-surface flt3R on T-ALL. Flt3L alone induced the proliferation of the monocytic AML-M5 cells and some erythroleukemic AML-M6 cells. Some isolated instances of weak proliferative responses were also noted in other AML subclasses. Interleukin-4 (IL-4) alone inhibited the proliferation of a group of AML-M5 cells and, when combined with flt3L, suppressed the proliferative effect of flt3L. In general, B-ALL and T-ALL cells failed to respond to flt3L alone or in the presence of combinations of IL-2, IL-3, or IL-7.

Effects of Flt3 ligand and interleukin-7 on in vitro growth of acute lymphoblastic leukemia cells

We investigated the effects of Flt3/Flk-2 ligand (FL) and interleukin-7 (IL-7) on DNA synthesis and proliferation of blast cells from patients with acute lymphoblastic leukemia (ALL). After 7 days of serum-free suspension culture of 19 samples, FL induced maximal DNA synthesis in two cases, whereas the combination of FL and IL-7 did so in another eight samples with a stimulation index (SI) >2. However, the number of viable cells after 7 days of liquid culture decreased in all but one sample. In this case of a pre-pre-B-ALL with a translocation t(4;11), FL induced dose-dependent proliferation (maximal 100 ng/mL) and cells stimulated with FL could be cultured for up to 4 weeks. A homogeneous population with 98% CD19-positive cells was detected before and after culture, and there was no evidence of nonleukemic cell proliferation as determined by immunophenotyping. The flt3 gene was transcribed in all seven cases studied by reverse-transcriptase polymerase chain reaction (RT-PCR). In the ALL cells responsive to FL, the expression of functional Flt3 receptors was confirmed by demonstrating FL-dependent tyrosine phosphorylation of Flt3. Furthermore, FL-dependent tyrosine phosphorylation of cellular proteins of estimated molecular weights of 70, 115, and 140 kD was detectable in these cells. These data demonstrate the functional heterogeneity of ALL samples and show that functional Flt3 receptors capable of mediating FL-dependent mitogenic signaling are expressed in a subset of ALL.

flk2/flt3 ligand is a potent cofactor for the growth of primitive B cell progenitors

We have investigated the role of flk2/flt3 ligand (FL) in B cell lymphopoiesis. The ability of FL to stimulate the growth of immature B cells was assessed using distinct populations: CD43lowB220+ pre-B cells, CD43+B220+ pro-B cells, and CD43+B220low progenitors. FL failed to affect the growth of the pro-B or pre-B cells whether used alone or in combination with stem cell factor (SCF) or IL-7. In striking contrast, FL was a potent cofactor for the CD43+B220low progenitor cells, interacting with either IL-7 and/or SCF to stimulate their growth. The combination of FL with IL-7 plus SCF stimulated maximum expansion of these cells, albeit, less than that stimulated in stromal cell cultures. When the CD43+B220low progenitors were divided based on expression of heat stable Ag (CD24) into a CD24- and a CD24+ subset, the FL-responsive cells were contained only within the CD24- subset. FL interacted with SCF or with IL-7 to stimulate their growth resulting in a 20- and 50-fold increase in cellularity, respectively. Since the CD24- subset was the most immature of the B cell populations studied, our data suggest that FL costimulates the expansion of very primitive B cell progenitors.

Expression and signal transduction of the FLT3 tyrosine kinase receptor

FLT3 is a receptor tyrosine kinase of 130-55 kDa expressed on normal bone marrow stem and early progenitor cells and on leukemic blasts from patients with acute leukemias. The FLT3 ligand, FL, is a new cytokine which acts on hematopoietic progenitors in synergy with other cytokines. FLT3 transduces FL-mediated signal through interaction with a number of cytoplasmic substrates.

FLT3/FLK-2 (STK-1) Ligand does not stimulate human megakaryopoiesis in vitro

It has not yet been determined if the FLT3/FLK-2 or STK-1 Ligand (STK-1L)/FLT3/FLK-2 or STK-1 receptor (STK-1R) axis has the ability to regulate human megakaryopoiesis in vitro. To address this question, we exposed normal human CD34+ marrow mononuclear cells to recombinant human STK-1L alone, or in combination with other growth factors. Colony-forming unit-megakaryocytic/thrombocytes (CFU-Meg) and BFU-E-derived colonies were then enumerated, and effects on colony size and maturation noted. As assessed by these parameters, STK-1L had no demonstrable effect on megakaryocyte colony formation. Similarly, suppressing STK-1R expression with oligodeoxynucleotides also had no influence on CFU-Meg-derived colony formation. To begin to derive a physiologic explanation for these findings, we examined freshly isolated normal human megakaryocytes for the presence of STK-1L and STK-1R mRNA. In contrast to a growing number of growth factors and growth factor receptors which appear to be expressed by megakaryocytes, normal mature human megakaryocytes express neither STK-1R or STK-1L mRNA. Accordingly, our results led us to hypothesize that if STK-1/STK-1L have any effects on megakaryocyte development in vitro, they are likely subtle and of uncertain physiologic significance.

CD34+ endothelial cell lines derived from murine yolk sac induce the proliferation and differentiation of yolk sac CD34+ hematopoietic progenitors

Embryonic hematopoiesis is initiated in part in the blood islands of the yolk sac. Previous confocal microscopic analysis has shown that the CD34 antigen, a mucin-like cell surface glycoprotein that is expressed by hematopoietic progenitors and all endothelial cells of the adult and embryo, is also found on a subset of luminal hematopoietic-like cells in the yolk sac blood islands as well as on the vascular endothelium lining these early hematopoietic locations. We show here that, as in all other hematopoietic sites thus far examined, immunoaffinity-purified CD34+ nonadherent cells from murine yolk sacs contain the vast majority of erythroid and myeloid progenitor cell colony forming activity. To examine the developmental interactions between these CD34+ hematopoietic progenitor cells of the yolk sac and the CD34+ yolk sac endothelium, we have immunaffinity-purified adherent endothelial cells from day 10.5 yolk sacs using CD34 antiserum and produced cell lines by transformation with a retrovirus expressing the polyoma middle T antigen. Analysis of these cell lines for CD34, von Willebrand's factor, FLK 1 and FLT 1 expression, and capillary growth in Matrigel indicates that they appear to be endothelial cells, consistent with their original phenotype in vivo. Coculture of yolk sac CD34+ hematopoietic cells on these endothelial cell lines results in up to a 60-fold increase in total hematopoietic cell number after approximately 8 days. Analysis of these expanded hematopoietic cells showed that the majority were of the monocyte/macrophage lineage. In addition, examination of the cultures showed the rapid formation of numerous cobblestone areas, a previously described morphologic entity thought to be representative of early pluripotential stem cells. Scrutiny of the ability of these endothelial cell lines to expand committed progenitor cells showed up to a sixfold increase in erythroid and myeloid colony-forming cells after 3 to 6 days in culture, consistent with the notion that these embryonic endothelial cells mediate the expansion of these precursor cells. Polymerase chain reaction analyses showed that most of the cell lines produce FLK-2/FLT-3 ligand, stem cell factor, macrophage colony-stimulating factor, leukemia-inhibitory factor, and interleukin-6 (IL-6), whereas there is a generally low or not measurable production of granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, IL-1, IL-3, transforming growth factor beta-1, erythropoietin, or thrombopoietin. The output of mature hematopoietic cells from these cocultures can be modified to include an erythroid population by the addition of exogenous erythropoietin. These data suggest that endothelial cell lines derived form the yolk sac provide an appropriate hematopoietic environment for the expansion and differentiation of yolk sac progenitor cells into at least the myeloid and erythroid lineages.

The flt3/flk-2 ligand: receptor distribution and action on murine haemopoietic cell survival and proliferation

In this study the distribution and quantitation of the flt3/flk-2 receptor was examined on bone marrow cells and defined haemopoietic subpopulations. Undifferentiated cells expressed the greatest numbers of flt3/flk-2 receptors: 19% of primitive lin-kit+sca-1+ bone marrow cells and 16% of fetal liver lin-aa4.1+ cells exhibited over 15 000 receptors per cell as determined by binding of the radiolabeled cognate ligand (flt3/flk-2 ligand, FL). Moderate binding was demonstrated on early B lymphocyte subsets (4400 receptors per cell) and very low levels were detected on monocytes. Binding was not detected on promyelocytes, myelocytes, promonocytes, metamyelocytes, polymorphonuclear cells, eosinophils or nucleated erythroid cells. FL enhanced the survival of primitive lin kit+sca-1+ cells with an efficacy s with an efficacy equivalent to stem cell factor (SCF). FL stimulated predominantly blast and granulocyte-macrophage colony formation in cultures of bone marrow cells by both direct and indirect mechanisms. Marked synergistic effects of FL with combinations of colony stimulating factors (CSFs) or interleukin-6 occurred in the proliferation of primitive lin-kit+sca-1+ cells, but not lin-kit+sca-1- progenitor cells. Surprisingly, recloning experiments revealed that FL plus IL-3 increased the generation of progenitor cells by lin-kit a-1- cells compared with SCF plus IL-3. Thus FL functions as a factor with both direct and indirect stimulatory activities directed to the expansion, maintenance of clonogenic potential, and possibly limited self-renewal, of early haemopoietic cells.

Effect of flt3 ligand on the ex vivo expansion of human CD34+ hematopoietic progenitor cells

A ligand for the tyrosine kinase receptor flt3/flk-2, referred to here as flt3 ligand (flt3L), was recently cloned. The effect of flt3L on purified human CD34+ progenitor cells was examined. flt3 receptor (flt3R) was detected on the surface of human bone marrow cells that were enriched for CD34 expression. The effects of flt3L and the c-kit ligand Steel factor (SLF) on hematopoietic progenitors were compared in clonal colony assays. Both factors synergized with Pixy321 (interleukin-3 [IL-3]-granulocyte-macrophage colony-stimulating factor fusion protein) to induce granulocytic-monocytic (GM) and high proliferative potential (HPP) colonies and synergized with Pixy321 + erythropoietin (EPO) to induce multipotent granulocytic-erythroid-monocytic-megakaryocytic colonies. Although SLF had a potent effect on colony formation of erythroid restricted progenitor cells (burst-forming unit-erythroid), no effect by flt3L was observed. The addition of flt3L to irradiated long-term marrow cultures seeded with CD34+ cells augmented both total and progenitor cell production. Ex vivo expansion studies with isolated CD34+ bone marrow cells from normal donors showed that flt3L alone supported maintenance of both GM and HPP progenitors for 3 to 4 weeks in vitro. The addition of flt3L to a growth factor combination of IL-1 alpha + IL-3 + IL-6 + EPO resulted in a synergistic effect on progenitor cell expansion comparable to that observed with the addition of SLF to IL-1 alpha + IL-3 + IL-6 + EPO. These data show a function for flt3L in the regulation of both primitive multipotent and lineage-committed hematopoietic progenitor cells.

Isolation and characterization of a monoclonal antibody binding to the extracellular domain of the flk-2 tyrosine kinase receptor

The flk-2 tyrosine kinase receptor is expressed on hematopoietic stem cells and on acute leukemias (AML and ALL). We have isolated a rat monoclonal antibody (71E1) that binds to this receptor with a relative affinity of 5 nM. The antibody immunoprecipitates both murine and human forms of flk-2 and can block receptor activation by its cognate ligand. In addition, 71E1 inhibits the in vitro proliferation of the murine leukemic cell line, M1, that expresses high levels of flk-2. These results suggest that 71E1 may have utility as both a reagent for elucidating the biological role of flk-2 in hematopoiesis and as an immunotherapeutic in the treatment of acute leukemias.

Molecular evolution of the genes encoding receptor tyrosine kinase with immunoglobulinlike domains

Receptor tyrosine kinases (RTK) with five, three, or seven immunoglobulinlike domains in their extracellular regions are classified as subclasses III, IV, and V, respectively. Conservation of the exon/intron structure of the downstream part of the human KIT, FMS, and FLT3 genes that encode RTK of subclass III together with the particular chromosomal localization of these genes suggests that RTKIII genes have evolved from a common ancestor by cis and trans duplications. To strengthen this model of evolution and to determine if it can be extended to RTKIV and V genes, we constructed a phylogenetic tree of RTKIII, IV, and V on the basis of a multiple alignment of their catalytic tyrosine kinase domain sequences and determined the exon/intron structure of PDGFRA (subclass III), FGFR4 (subclass IV), and FLT4 (subclass V) genes in their downstream part. Phylogenetic analyses with amino acid or nucleotide sequences both resulted in one most parsimonious tree. The phylogenetic trees obtained indicate that all three subclasses are well individuated and that RTKIII and RTKV are closer to each other than RTKIV. Furthermore, RTKIII and FLT4 (subclass V) genes possess the same exon/intron structure in their downstream part while the structure of the RTKIV genes is very similar to that of RTKIII and FLT4. Both approaches are in complete agreement and indicate that RTKIII, IV, and V genes most probably evolved from a common ancestor already "in pieces" by successive duplications involving entire genes.

Expression and physiologic significance of Kit ligand and stem cell tyrosine kinase-1 receptor ligand in normal human CD34+, c-Kit+ marrow cells

To determine the potential role of autocrine growth factor production in regulating primitive human hematopoietic cell development, we examined highly purified CD34+, c-Kit+ marrow mononuclear cells for expression of c-Kit ligand (KL) and stem cell tyrosine kinase 1 (stk1) ligand (STK1-L). Normal marrow mononuclear cells coexpressing CD34 and c-Kit were isolated by a combination of immunomagnetic bead isolation and fluorescence-activated cell sorting. Purified cells were then screened for expression of KL and stk1-L mRNA using a sensitive reverse transcription-polymerase chain reaction method. Using this approach, expression of both cytokine genes at the mRNA level was found in this highly enriched cell population. We then examined the functional significance of these mRNAs by inhibiting their expression with antisense (AS) oligodeoxynucleotides (ODN). In comparison to untreated or control ODN treated cells, inhibition of KL led to a 70% and 89% inhibition in burst-forming unit-erythroid (BFU-E) and colony-forming unit-Mix (CFU-Mix) colonies but had no significant effect on CFU-granulocyte-macrophage (CFU-GM) cloning efficiency. In contrast, inhibition of STK1-L alone had no effect on colony formation. However, when STK1-L AS ODN was combined with KL AS ODN, additive inhibition of CFU-GM and CFU-MIX but not of BFU-E colonies was observed. These findings, along with those of our previous studies showing inhibition of primitive hematopoietic cell growth with antisense ODN directed towards the stk1 receptor, suggest the possibility that both receptor/ligand axes regulate primitive hematopoietic cell growth via an autocrine growth loop.

Rapid isolation of cell-type-specific protein tyrosine kinases by degenerate polymerase chain reaction combined with differential hybridization technique

To identify protein tyrosine kinase (PTK) genes preferentially expressed in renal cell carcinoma cell line, we screened a PTK-cDNA-enriched library constructed from RNA of an renal cell carcinoma cell line with a PTK probe, each produced from renal cell carcinoma, gastric cancer or esophageal cancer cell lines by degenerate polymerase chain reaction. Two cDNA fragments of PTK genes, FRK and FLT-3, were isolated from the PTK-cDNA-enriched library of the renal cell carcinoma cell line by differential hybridization technique. The FRK cDNA clone represented 15.8% of the PTK-cDNA-enriched library from the renal cell carcinoma cell line, while the FLT-3 cDNA clone was 2.8% of the same library. Both of the two PTK genes were expressed preferentially in renal cell carcinoma cell lines. This method, described here, is useful for the rapid isolation of PTK cDNA fragments, including a low abundant cDNA, preferentially expressed in a specific cell line.

Trisomy 13 in a case of myelofibrosis with myeloid metaplasia with early blastic transformation

We describe a case of early myeloid blastic transformation in a 64-year-old man suffering from myelofibrosis with myeloid metaplasia. Both chronic and blastic phase cytogenetic analysis showed trisomy 13 to be the sole chromosome aberration. A potential role for this rare abnormality in determining such an unusually poor clinical outcome is discussed.

Multi-level effects of flt3 ligand on human hematopoiesis: expansion of putative stem cells and proliferation of granulomonocytic progenitors/monocytic precursors

We have evaluated the effects of the flt3 receptor ligand (FL) on hematopoietic progenitors/stem cells (HPCs/HSCs) stringently purified from adult peripheral blood and grown in different culture systems. In these experiments HPCs/HSCs were treated with FL +/- kit ligand (KL) +/- monocyte colony-stimulatory factor (M-CSF). In clonogenetic HPC culture supplemented with interleukin-3 (IL-3)/granulomonocyte-CSF (GM-CSF)/erythropoietin (Epo), FL potentiates colony-forming unit (CFU)-GM proliferation in terms of colony number and size, but exerts little effect on burst-forming units-erythroid (BFU-E) and CFU-granulocyte erythroid megakaryocyte macrophage (CFU-GEMM) growth, whereas KL enhances the proliferation of all HPC types; combined FL+KL +/- M-CSF treatment causes a striking shift of CFU-GM colonies from granulocytic to monocytic differentiation. In liquid suspension HPC culture, FL alone induces differentiation along the monocytic and to a minor extent the basophilic lineages, whereas M-CSF alone stimulates prevalent monocytic differentiation but little cell proliferation: combined M-CSF+FL treatment causes both proliferation and almost exclusive monocytic differentiation (97% monocytes in fetal calf serum-rich (FCS+) culture conditions, mean value). At primitive HPC level, FL potentiates the clonogenetic capacity of colony-forming units-blast (CFU-B) and high proliferative potential colony-forming cells (HPP-CFC) in primary and secondary culture; KL exerts a similar action, and additive effects are induced by FL combined with KL. More important, addition of FL alone causes a significant amplification of the number of long-term culture-initiating cells (LTC-ICs), ie, putative repopulating HSCs, whereas this effect is not induced by KL. The FL effects correlate with flt3 mRNA expression in HPCs differentiating throught the erythroid or GM pathway in liquid suspension culture: (1) flt3 mRNA is expressed in freshly purified, resting HPCs; after growth factor stimulus the message (2) is abruptly down-modulated in HPC erythroid differentiation, but (3) is sustainedly expressed through HPC GM differentiation and abolished in GM precursor maturation. This pattern contrasts with the gradual downmodulation of c-kit through both erythroid and GM HPC differentiation. The results indicate that FL exerts a stimulatory action on primitive HPCs, including a unique expanding effect on putative stem cells, whereas its distal proliferative/differentiative action is largely restricted to CFU-GM and monocytic precursors. The latter effect is potentiated by KL and M-CSF, thus suggesting that the structural similarities of FL, KL, M-CSF, and their tyrosine kinase receptors may mediate positive interactions of these growth factors son monocytic differentiation.

Biology of flt3 ligand and receptor

The flt3 ligand is a member of a small family of growth factors that stimulate the proliferation of hematopoietic cells; other members of this family include Steel factor (also known as mast cell growth factor, stem cell factor, and kit ligand) and colony stimulating factor 1. These proteins function by binding to and activating unique tyrosine kinase receptors. Expression of the flt3 receptor is primarily restricted among hematopoietic cells to the most primitive progenitor cells. The flt3 ligand is similar to Steel factor in that both proteins stimulate the proliferation of early progenitor or stem cells. Neither of these factors has much proliferative activity on its own, but each factor can synergize with a wide range of other colony stimulating factors and interleukins (ILs) to stimulate proliferation. One major difference between the two factors appears to be their effect on mast cells, which Steel factor stimulates but flt3 ligand does not. Although flt3 ligand and Steel factor each act on early hematopoietic cells, differences in their activities suggest that they are not redundant and both are required for normal hematopoiesis. There are a number of clinical settings in which the flt3 ligand may potentially prove quite useful.

Expression of FLT3 receptor and FLT3-ligand in human leukemia-lymphoma cell lines

The FLT3 gene encodes a receptor tyrosine kinase that is closely related to two well-known receptors, KIT and FMS, that regulate with their respective ligands, stem cell factor (SCF) and macrophage colony-stimulating factor (M-CSF), proliferation and differentiation of hematopoietic cells. The ligand for FLT3, FL, is active in both soluble and membrane-bound forms. We examined expression of FL and FLT3 mRNA in a panel of some 110 continuous human leukemia-lymphoma cell lines from all major hematopoietic cell lineages by Northern blot analysis. FLT3 mRNA is expressed primarily in pre-B cell lines, myeloid and monocytic cell lines whereas FL mRNA was detected in most cell lines from all cell lineages. Analysis of FLT3 receptor protein expression examined with a specific anti-FLT3 monoclonal antibody and flow cytometry in 17 cell lines confirmed the results obtained at the mRNA level. Forty of 110 cell lines displayed both receptor and ligand mRNA suggesting a possible autocrine or intracrine stimulation. In normal hematopoietic cells expression of FLT3 was reported to be associated with CD34 positivity, a cell surface marker of immature and precursor cells. No correlation between FLT3 and CD34 expression was found in the cell lines analyzed. These studies served to illustrate further the importance of the FL-FLT3 ligand-receptor system in the regulation of hematopoietic cells.

Expression of the flt3 receptor and its ligand on hematopoietic cells

Expression of the flt3 tyrosine kinase receptor and its ligand were examined on various murine and human hematopoietic cell lines. Surface expression of flt3 receptor and flt3 ligand were detected by flow cytometry using biotinylated human flt3 ligand or biotinylated soluble human flt3 receptor Fc fusion protein (flt3R-Fc), respectively. Flt3 receptor and ligand expression were also examined by Northern blot analysis. Flt3 receptor was expressed on the surface of only two of nine murine cell lines and nine of 15 human cell lines, with positive cells representing the B cell, early myeloid, and monocytic lineages. Staining for surface expression of the flt3 ligand revealed that seven of nine murine cell lines and nine of 15 human cell lines screened were positive by flow cytometry. All murine and human cell lines assayed were positive for flt3 ligand RNA expression by Northern blot analysis, but not all cell lines expressing flt3 ligand mRNA had detectable surface expression. Cells expressing the flt3 ligand were of the myeloid, B cell and T cell lineages at various stages of differentiation. Only the OCI-AML-5, NALM-6, and AML-193 cell lines coexpressed both surface flt3 receptor and ligand. The myeloid leukemic M1 cell terminally differentiate into macrophage-like cells under the influence of leukemia inhibitory factor (LIF). We found that LIF-stimulated M1 cells down-regulated surface expression and mRNA levels of the flt3 receptor, but up-regulated expression of the flt3 ligand. Although we could demonstrate that the flt3 receptor was functional in the M1 cell line, flt3 ligand could not induce the M1 cells to differentiate.

Targeted disruption of the flk2/flt3 gene leads to deficiencies in primitive hematopoietic progenitors

The flk2 receptor tyrosine kinase has been implicated in hematopoietic development. Mice deficient in flk2 were generated. Mutants developed into healthy adults with normal mature hematopoietic populations. However, they possessed specific deficiencies in primitive B lymphoid progenitors. Bone marrow transplantation experiments revealed a further deficiency in T cell and myeloid reconstitution by mutant stem cells. Mice deficient for both c-kit and flk2 exhibited a more severe phenotype characterized by large overall decreases in hematopoietic cell numbers, further reductions in the relative frequencies of lymphoid progenitors, and a postnatal lethality. Taken together, the data suggest that flk2 plays a role both in multipotent stem cells and in lymphoid differentiation.

Protein kinases in human breast cancer

The family of protein kinases includes many oncogenes and growth factor receptors, many of which have been linked to the pathogenesis and progression of cancer. Protein tyrosine kinases such as HER-2/c-erbB-2 and the epidermal growth factor receptor (EGFR) have been linked specifically to breast cancer, and perturbations of HER-2 affect response to chemotherapy. We have reviewed the biology of protein kinases in human breast cancer, as well as their translational applications to breast cancer patients. We have studied the spectrum of protein kinases expressed in human breast cancer cells and have identified four protein kinases with potentially important functions in breast cancer: rak (src-related), TK5 (which we now designate JAK3), the focal adhesion kinase (FAK), and STK1 (human M015/CAK). We describe the potential significance of these genes in breast cancer, as well as our methodology for identifying and characterizing novel genes in breast cancer.

Expression of Flt3 tyrosine kinase receptor gene in mouse hematopoietic and nervous tissues

The Flt3 gene encodes a tyrosine kinase receptor highly related to the Kit and Fms gene products. We have studied the expression of Flt3 by using in situ hybridization of mouse tissue sections. The results show that Flt3 RNAs are present in certain regions of lymphohematopoietic organs, placenta and nervous system. Flt3 is expressed in the medullary area of fetal and newborn thymus, in the paracortical regions of lymph nodes and in the red pulp of spleen. In placenta, labyrinthine trophoblasts express Flt3. Finally, Flt3 RNAs are found in several regions of the brain and in cerebellar Purkinje cells. Western-blot analysis showed that the FLT3 protein is present in the tissues positive for Flt3 RNA expression. Our observations allow for a comparison with the distribution of the Kit gene and analysis of a possible redundancy between KIT and FLT3 receptors.

The flt3 ligand supports proliferation of lymphohematopoietic progenitors and early B-lymphoid progenitors

We have examined the effects of the murine ligand (FL) for the flt3/flk2 tyrosine kinase receptor on the proliferation of murine lymphohematopoietic progenitors as well as committed myeloid and B-cell progenitors. In the presence of erythropoietin, FL alone supported scant colony formation from enriched marrow cells of normal mice. However, when it was combined with interleukin-3 (IL-3), steel factor (SF), or IL-11, FL significantly enhanced colony formation. When tested on enriched marrow cells from 5-fluorouracil (5-FU)-treated mice, FL neither enhanced IL-3-dependent colony formation nor synergized with SF in support of colony formation. However, FL synergized with IL-6, IL-11, or granulocyte-colony stimulating factor (G-CSF) in support of formation of various types of colonies, including multilineage colonies. Approximately 30% of these colonies yielded pre-B-cell colonies when replated in secondary cultures containing SF and IL-7, indicating that 2-cytokine combinations, including FL and IL-6, IL-11, or G-CSF can support the proliferation of primitive lymphohematopoietic progenitors. FL, by itself and in synergy with IL-7 or SF, supported the proliferation of B-cell progenitors. These results show that FL has a wide range of activities in early hematopoiesis and B lymphopoiesis.

The FLT3 ligand potently and directly stimulates the growth and expansion of primitive murine bone marrow progenitor cells in vitro: synergistic interactions with interleukin (IL) 11, IL-12, and other hematopoietic growth factors

The recently cloned murine flt3 ligand (FL) was studied for its ability to stimulate the growth of primitive (Lin-Sca-1+) and more committed (Lin-Sca-1-) murine bone marrow progenitor cells, alone and in combination with other hematopoietic growth factors (HGFs). Whereas FL was a weak proliferative stimulator alone, it potently synergized with a number of other HGFs, including all four colony-stimulating factor (CSF), interleukin (IL) 6, IL-11, IL-12, and stem cell factor (SCF), to promote the colony formation of Lin-Sca-1+, but not Lin-Sca-1- or erythroid progenitor cells. The synergistic activity of FL was concentration dependent, with maximum stimulation occurring at 250 ng/ml, and was observed when cells were plated at a concentration of one cell per culture, suggesting that its effects are directly mediated. 2 wk of treatment with FL in combination with IL-3 or SCF resulted in the production of a high proportion of mature myeloid cells (granulocytes and macrophages), whereas the combination of FL with G-CSF, IL-11, or IL-12 resulted predominantly in the formation of cells with an immature blast cell appearance. Accordingly, FL in combination with G-CSF or IL-11 expanded the number of progenitors more than 40-fold after 2 wk incubation. Thus, FL emerges as a potent synergistic HGF, that in combination with numerous other HGFs, can directly stimulate the proliferation, myeloid differentiation, and expansion of primitive hematopoietic progenitor cells.

Identification and characterization of a functional murine FLT3 isoform produced by exon skipping

The FLT3 gene encodes an hematopoietic receptor related to the receptors for colony-stimulating factor 1, FMS, and for Steel factor, KIT. The extracellular part of these molecules is exclusively composed of five immunoglobulin (Ig)-like domains, designated 1 to 5, from the amino terminus to the carboxyl terminus of the extracellular region. We have isolated a unique murine FLT3 cDNA that codes for a variant isoform of FLT3, devoid of the fifth Ig-like domain, by comparison with the prototypic form. The corresponding mRNA is the result of a splicing event that leads to the elimination of two coding exons. mRNA coding for this variant was detected in almost all the tissues expressing the mRNA coding for the prototypic molecule, although at a lower level. Ligand-induced tyrosine phosphorylation of the two isoforms was equivalent in COS-1 transfected cells, indicating that the fifth Ig-like domain is not strictly necessary for either ligand-binding or kinase activation.

FLK-2/FLT-3 ligand regulates the growth of early myeloid progenitors isolated from human fetal liver

The effects of the recently identified FLK-2/FLT-3 ligand (FL) on the growth of purified human fetal liver progenitors were investigated under serum-deprived culture conditions. FL alone was found to stimulate modest proliferation in short-term cultures of CD34++ CD38+ lineage (Lin)- light-density fetal liver (LDFL) cells and the more primitive CD34++ CD38- Lin- LDFL cells. However, the low levels of growth induced by FL were insufficient for colony formation in clonal cultures. Synergism between FL and either granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) or KIT ligand (KL) was observed in promoting the growth of high-proliferative potential (HPP) colony-forming cells (CF) and/or low-proliferative potential (LPP)-CFC in cultures of CD34++ CD38+ Lin- and CD34++ CD38- Lin- LDFL-cells. FL, alone or in combination with other cytokines, was not found to affect the growth of CD34+ Lin- LDFL cells, the most mature subpopulation of fetal liver progenitors investigated. The growth of the most primitive subset of progenitors studied, CD34++ CD38- Lin- LDFL cells, required the interactions of at least two cytokines, because only very low levels of growth were observed in response to either FL, GM-CSF, IL-3 or KL alone. However, the results of delayed cytokine-addition experiments suggested that individually these cytokines did promote the survival of this early population of progenitors. Although two-factor combinations of FL, KL, and GM-CSF were observed to promote the growth of early progenitors in a synergistic manner, neither of these factors was found to make fetal liver progenitors more responsive to suboptimal concentrations of a second cytokine. Only myeloid cells were recovered from liquid cultures of CD34++ CD38- Lin- LDFL cells grown in the presence of combinations of FL, KL, and GM-CSF. These results indicate that FL is part of a network of growth factors that regulate the growth and survival of early hematopoietic progenitors.