Ligand for FLT3/FLK2 receptor tyrosine kinase regulates growth of haematopoietic stem cells and is encoded by variant RNAs

FLT-3 Ligand and Marrow Stroma-Derived Factors Promote CD3γ, CD3δ, CD3ζ, and RAG-2 Gene Expression in Primary Human CD34+LIN−DR− Marrow Progenitors

We hypothesize that early lymphoid commitment from primitive hematopoietic marrow progenitors is governed by signals from the marrow microenvironment leading to sequential induction of lineage-specific genes. Using expression of lymphoid genes as markers of differentiation, we characterize a highly purified population (>99.8% by double sorting) of primary human CD34+Lin−DR− progenitors. This population was then used to evaluate the effects of supplemental cytokines (interleukin-2 [IL-2], IL-3, IL-7, c-kit ligand), FLT-3 ligand (FL), and stroma-derived factors on lymphoid differentiation in vitro. CD3, RAG-1, Ikaros, CD10, and TdT transcripts were detected in the starting CD34+Lin−DR− population. By contrast, CD3γ, CD3δ, CD3ζ, and RAG-2 transcripts were not present in any samples tested. The presence of supplemental cytokines alone at culture initiation permitted stimulation of the expression of CD3ζ, but not of CD3γ or CD3δ. However, when FL and stroma-derived factors were added to cytokines, CD3 gene expression was induced in all samples. The predominant CD3 transcripts induced by optimal culture conditions were alternatively spliced isoforms lacking transmembrane sequences (CD3δ and CD3γ) and portions of the intracellular and extracellular domains (CD3γ). The combination of cytokines, FL, and stromal factors also provided a potent stimulus for RAG-2 gene expression. These findings show that FL in combination with stroma-derived factors provide important signals to promote early events required for lymphoid differentiation.

Epidermal Langerhans cell development and differentiation

Epidermal Langerhans cells (LC) play a critical role in host defense. Still we know rather little about the development and functional specialization of these bone marrow-derived dendritic cells (DC) located in the most peripheral ectodermal tissue of the mammalian organism. How LC develop from their primitive progenitors in bone marrow and to what extent LC are related in their development to other lineages of the hemopoietic system is still under debate. There are currently 3 major areas of debate: 1) which are the signals required for LC development and differentiation to occur, 2) what are the (molecular) characteristics of the intermediate stages of LC differentiation, and 3) how are LC related in their development and/or function to other cells of the hemopoietic system? A better understanding of LC development and answers to these questions can be expected from recently developed technologies which allow the in vitro generation of DC with the typical molecular, morphological and functional features of LC from purified CD34+ progenitor cells under defined serum-free culture conditions. TGF-beta 1 was found to be an absolute requirement for in vitro LC development under serum-free conditions upon stimulation with the classical DC growth and differentiation factors GM-CSF, TNF-alpha and SCF. The recently identified cytokine FLT3 ligand further dramatically enhanced in vitro LC development and even allowed efficient in vitro generation of LC colonies from serum-free single cell cultures of CD34+ hemopoietic progenitor cells.

FLT-3 Ligand and Marrow Stroma-Derived Factors Promote CD3γ, CD3δ, CD3ζ, and RAG-2 Gene Expression in Primary Human CD34+LIN−DR− Marrow Progenitors

We hypothesize that early lymphoid commitment from primitive hematopoietic marrow progenitors is governed by signals from the marrow microenvironment leading to sequential induction of lineage-specific genes. Using expression of lymphoid genes as markers of differentiation, we characterize a highly purified population (>99.8% by double sorting) of primary human CD34+Lin−DR− progenitors. This population was then used to evaluate the effects of supplemental cytokines (interleukin-2 [IL-2], IL-3, IL-7, c-kit ligand), FLT-3 ligand (FL), and stroma-derived factors on lymphoid differentiation in vitro. CD3, RAG-1, Ikaros, CD10, and TdT transcripts were detected in the starting CD34+Lin−DR− population. By contrast, CD3γ, CD3δ, CD3ζ, and RAG-2 transcripts were not present in any samples tested. The presence of supplemental cytokines alone at culture initiation permitted stimulation of the expression of CD3ζ, but not of CD3γ or CD3δ. However, when FL and stroma-derived factors were added to cytokines, CD3 gene expression was induced in all samples. The predominant CD3 transcripts induced by optimal culture conditions were alternatively spliced isoforms lacking transmembrane sequences (CD3δ and CD3γ) and portions of the intracellular and extracellular domains (CD3γ). The combination of cytokines, FL, and stromal factors also provided a potent stimulus for RAG-2 gene expression. These findings show that FL in combination with stroma-derived factors provide important signals to promote early events required for lymphoid differentiation.

FLT3: receptor and ligand. Biology and potential clinical application

Flt3 ligand (FL) is a recently identified cytokine having a central role in the proliferation, survival and differentiation of early murine and human hematopoietic precursor/stem cells. FL acts synergistically in vitro with a number of other hematopoietic growth factors such as IL-3, IL-6, IL-11, IL-12, KIT Ligand and GM-CSF. Recently, it has been shown the in vivo administration of FL results in a significant alteration of hematopoiesis in murine bone marrow (BM), spleen, peripheral blood, liver and lymph nodes. In addition, treatment with FL resulted in a significant accumulation of functionally active dendritic cells within murine lymphoid tissues. The possible applications of FL in dendritic cell-based immunotherapies are discussed.

Generation of a monoclonal antibody that induces apoptosis of hematopoietic cells

Here, we developed anti-murine myeloid cell (NFS-60 cell) monoclonal antibodies in order to characterize the further mechanism of cell-to-cell interaction between the stromal cells and the hematopoietic progenitor cells. The established antibody, designated mAb UNF, inhibited proliferation of NFS-60 cells and induced apoptosis. Incubation of NFS-60 cells with mAb UNF initiated the cell aggregation within 3 hours. This phenomena did not require newly synthesized proteins, since the pretreatment of 1 mM cycloheximide failed to prevent the agglutination. Fifty-eight percent of mouse hematopoietic stem cells, Lin-, c-Kit+, Sca-1+ bone marrow cells, were shown to express UNF antigen. The glycolipid prepared from NFS-60 cells was specifically immunostained by mAb UNF.

Thrombopoietin, kit Ligand, and flk2/flt3 Ligand Together Induce Increased Numbers of Primitive Hematopoietic Progenitors From Human CD34+Thy-1+Lin− Cells With Preserved Ability to Engraft SCID-hu Bone

CD34+Thy-1+Lin− cells are enriched for primitive hematopoietic progenitor cells (PHP), as defined by the cobblestone area-forming cell (CAFC) assay, and for bone marrow (BM) repopulating hematopoietic stem cells (HSC), as defined by the in vivo SCID-hu bone assay. We evaluated the effects of different cytokine combinations on BM-derived PKH26-labeled CD34+Thy-1+Lin− cells in 6-day stroma-free cultures. Nearly all (>95%) of the CD34+Thy-1+Lin− cells divided by day 6 when cultured in thrombopoietin (TPO), c-kit ligand (KL), and flk2/flt3 ligand (FL). The resulting CD34hiPKHlo (postdivision) cell population retained a high CAFC frequency, a mean 3.2-fold increase of CAFC numbers, as well as a capacity for in vivo marrow repopulation similar to freshly isolated CD34+Thy-1+Lin− cells. Initial cell division of the majority of cells occurred between day 2 and day 4, with minimal loss of CD34 and Thy-1 expression. In contrast, cultures containing interleukin-3 (IL-3), IL-6, and leukemia inhibitory factor contained a mean of 75% of undivided cells at day 6. These CD34hi PKHhi cells retained a high frequency of CAFC, whereas the small population of CD34hiPKHlo postdivision cells contained a decreased frequency of CAFC. These data suggest that use of a combination of TPO, KL, and FL for short-term culture of CD34+Thy-1+Lin− cells increases the number of postdivision PHP, measured as CAFC, while preserving the capacity for in vivo engraftment.

FLT-3 ligand mobilizes hematopoietic primitive and committed progenitor cells into blood in mice

We investigated the effects of the administration of FLT-3 ligand (FL) on mobilization of primitive and committed progenitor cells in mice. C57bl/6J mice were injected subcutaneously with FL once a day for 5 d at doses of 20, 100 and 200 microg/kg. After the collection of peripheral blood, we determined the number of white blood cells (WBCs) with the differential counts. The formation of colony-forming cells (CFCs) in peripheral blood, bone marrow and spleen was evaluated. Although the administration of FL, 20 microg/kg, did not stimulate leukocytosis, its administration at doses of 100 and 200 microg/kg increased the number of WBC up to 1.7- and 2.4-fold, respectively. Committed progenitor cells were mobilized into the peripheral blood dose-dependently and the number of CFCs was increased up to 5.5-fold by the administration of FL at 200 microg/kg on d 5. The number of CFCs in the bone marrow increased, but not dose-dependently. The number of CFCs in the spleen also increased up to 32-fold at a dose of 200 microg/kg FL. Mobilized peripheral blood mononuclear cells were transplanted into lethally irradiated mice and the number of CFU-S (d 12) was scored. A dose-dependent mobilization of CFU-S (d 12) into peripheral blood was also observed. These observations suggest that FL can mobilize hematopoietic primitive and committed progenitor cells into the peripheral blood of mice and those cells mobilized by FL may be applicable to peripheral blood stem cell transplantation.

Identification of clonogenic common lymphoid progenitors in mouse bone marrow

The existence of a common lymphoid progenitor that can only give rise to T cells, B cells, and natural killer (NK) cells remains controversial and constitutes an important gap in the hematopoietic lineage maps. Here, we report that the Lin(-)IL-7R(+)Thy-1(-)Sca-1loc-Kit(lo) population from adult mouse bone marrow possessed a rapid lymphoid-restricted (T, B, and NK) reconstitution capacity in vivo but completely lacked myeloid differentiation potential either in vivo or in vitro. A single Lin(-)IL-7R(+)Thy-1(-)Sca-1loc-Kit(lo) cell could generate at least both T and B cells. These data provide direct evidence for the existence of common lymphoid progenitors in sites of early hematopoiesis.

Early plasma membrane events occurring in ultraviolet-B-induced apoptosis

Whereas nonsolar ultraviolet C radiation primarily affects nuclei (i.e., where it is absorbed by nucleic acids) of eukaryotic cells, ultraviolet radiation of long (320-380 nm) wavelengths (ultraviolet A) and intermediate (290-320 nm) wavelengths (ultraviolet B) primarily affects lipid membranes. We have previously demonstrated that ultraviolet B irradiation alters the surface architecture of human B cells and impairs expression of an erythroid growth factor on their surface and on extracellular vesicles. Here, we examined the effects of ultraviolet B irradiation on the capacity of Chinese hamster ovary cells to undergo the process of exfoliation, and on the capacity of Chinese hamster ovary cells transfected with flt3/flk2 cDNA to express the cytokine flt3/flk2. Our results indicate that the rate of release of shed vesicles from untransfected Chinese hamster ovary cells is decreased after one to two h, at a time when there is electron microscopic evidence for retention of vesicles at the cell surface. These changes at the cell surface precede all other apparent morphological changes (including DNA condensation in the nucleus, swelling of the mitochondria and appearance of apoptotic bodies). Furthermore, plasma membranes and shed extracellular vesicles from ultraviolet B irradiated Chinese hamster ovary cells that have been transfected with flt3/flk2 cDNA fail to express the protein.

Flt3 Ligand Enhances the Yield of Primitive Cells After Ex Vivo Cultivation of CD34+ CD38dim Cells and CD34+ CD38dim CD33dim HLA-DR+ Cells

Flt3 ligand (FL) has been proposed as a possible modulator of early hematopoietic cell growth. The purpose of this study was to analyze the impact of FL on ex vivo expansion of hematopoietic cells obtained from adult donors. We sought to precisely identify hematopoietic populations responsive to FL and to quantitate the ability of FL to enhance the survival and/or proliferation of early hematopoietic precursors in a stroma-free culture system. Towards that end, four CD34+ subsets were isolated and their response to FL was characterized. In methylcellulose, FL significantly increased colony formation by CD34+ CD38dim cells but not CD34+ CD38+ cells. In suspension culture, the enhancement of cell expansion by FL was 10 times greater with the CD34+ CD38dim fraction than the CD34+ CD38+ fraction. FL stimulated the generation of colony-forming unit–granulocyte-macrophage (CFU-GM) from the CD34+CD38dim fraction by 14.5- ± 5.6-fold. To determine if CD34+ CD38dim cells responded uniformly to FL, the population was subdivided into a CD34+ CD38dim CD33dim HLA-DR+ (HLA-DR+) fraction and a CD34+ CD38dim CD33dim HLA-DRdim (HLA-DRdim) fraction. FL was far more effective at stimulating cell and progenitor growth from the HLA-DR+ fraction. To determine if FL enhanced or depleted the number of precommitted cells in expansion culture, CD34+ CD38dim and HLA-DR+ fractions were incubated in liquid culture and analyzed by flow cytometry. Inclusion of FL enhanced the absolute number of primitive CD34+ CD33dim cells and CD34+ HLA-DRdim cells after 5 to 12 days of cultivation. To confirm immunophenotypic data, the effect of FL on long-term culture-initiating cells (LTCIC) was determined. After 2 weeks of incubation of CD34+ CD38dim or HLA-DR+ cultures, LTCIC recoveries were significantly higher with FL in 5 of 6 trials (P < .05). For HLA-DR+ cells, LTCIC recoveries averaged 214% ± 87% of input with FL and 24% ± 16% without FL. In contrast, HLA-DRdim LTCIC could not be maintained in stroma-free culture. We conclude that less than 10% of CD34+ cells respond vigorously to FL and that those cells are contained within the HLA-DR+ fraction. FL stimulates the expansion of total cells, CD34+ cells, and CFU-GM and enhances the pool of early CD34+ CD33dim cells, CD34+ HLA-DRdim cells, and LTCIC. These data indicate that it is possible to expand hematopoietic progenitors from adult donors without losing precursors from the precommitted cell pool.

Enhanced Levels and Enhanced Clonogenic Capacity of Blood Progenitor Cells Following Administration of Stem Cell Factor Plus Granulocyte Colony-Stimulating Factor to Humans

Administration of hematopoietic growth factors is being used increasingly to obtain populations of blood progenitor/stem cells (PBPC) for clinical transplantation. Here we examined the effect of combining stem cell factor (SCF ) and granulocyte colony-stimulating factor (G-CSF ) versus G-CSF alone in a randomized clinical study involving 62 women with early-stage breast cancer. In the first patient cohorts, escalating doses of SCF were administered for 7 days with concurrent G-CSF administration. At baseline, levels of progenitor cells in the bone marrow or blood were comparable in the different patient groups. As with administration of G-CSF alone, the combination of SCF plus G-CSF did not alter the wide variation in levels of PBPC observed between individuals and did not alter the selective nature of PBPC release, with preferential release of day-14 granulocyte-macrophage colony-stimulating factor (GM-CFC) versus day-7 GM-CFC. However, SCF acted to sustain the levels of PBPC after cessation of growth factor treatment; levels of PBPC were elevated 100-fold at later timepoints compared with G-CSF alone. In addition, the maximum levels of PBPC observed were increased approximately fivefold at day 5 of growth-factor administration. The increased levels of PBPC resulted in significantly increased levels of PBPC obtained by leukapheresis. In a subsequent patient cohort, 3-days pretreatment with SCF was introduced and followed by 7 days concurrent SCF plus G-CSF. The 3-days pretreatment with SCF resulted in an earlier wave of PBPC release in response to commencement of G-CSF. In addition, maximum PBPC levels in blood and PBPC yield in leukapheresis products were further increased. Unexpectedly however, SCF pretreatment resulted in progenitor cells with enhanced self-generation potential. Recloning assays documented the ability of approximately 30% of primary granulocyte-macrophage (GM) colonies from control cell populations to generate secondary GM colonies (n = 1,106 primary colonies examined). In contrast approximately 90% of GM colonies from PBPC after SCF pretreatment generated secondary clones and 65% generated secondary colonies. The action of SCF was not explicable in terms of altered SCF, GM-CSF, or G-CSF responsiveness, but SCF pretreatment was associated with maximum serum SCF levels at the time G-CSF was commenced. These results show that PBPC populations mobilized by different growth factor regimens can differ in their functional properties and caution against solely considering number of harvested progenitor cells without regard to their function.

Successful Reconstitution of Human Hematopoiesis in the SCID-hu Mouse by Genetically Modified, Highly Enriched Progenitors Isolated From Fetal Liver

Highly purified CD34++CD38−Lin− hematopoietic progenitors isolated from human fetal liver were infected with the murine retroviral vector, MFG nls-LacZ, which encodes a modified version of the Escherichia coli β-galactosidase gene. Progenitors that were cocultured with the packaging cell line could reconstitute human bone marrow or thymus implanted in SCID-hu mice. Expression of the β-galactosidase gene was observed in primitive and committed clonogenic progenitors, mature myeloid, B-lineage cells, and T-lineage cells for up to 4 months after injection into SCID-hu mice. Furthermore, hematopoietic reconstitution by genetically modified progenitor cells could be achieved by the injection of the cells generated from as few as 500 CD34++CD38−Lin− cells, suggesting efficient retroviral gene transfer into fetal liver progenitors.

Flt3 Ligand Synergizes With Granulocyte-Macrophage Colony-Stimulating Factor or Granulocyte Colony-Stimulating Factor to Mobilize Hematopoietic Progenitor Cells Into the Peripheral Blood of Mice

Peripheral blood progenitor cells (PBPC) are increasingly being used in the clinic as a replacement for bone marrow (BM) in the transplantation setting. We investigated the capacity of several different growth factors, including human flt3 ligand (FL), alone and in combination with granulocyte-macrophage colony-stimulating factor (GM-CSF ) or granulocyte colony-stimulating factor (G-CSF ), to mobilize colony forming cells (CFU) into the peripheral blood (PB) of mice. Mice were injected subcutaneously (SC) with growth factors daily for up to 10 days. Comparing the single agents, we found that FL alone was superior to GM-CSF or G-CSF in mobilizing CFU into the PB. FL synergized with both GM-CSF or G-CSF to mobilize more CFU, and in a shorter period of time, than did any single agent. Administration of FL plus G-CSF for 6 days resulted in a 1,423-fold and 2,717-fold increase of colony-forming unit–granulocyte-macrophage (CFU-GM) and colony-forming unit granulocyte, erythroid, monocyte, megakaryocyte (CFU-GEMM) in PB, respectively, when compared with control mice. We also followed the kinetics of CFU numerical changes in the BM of mice treated with growth factors. While GM-CSF and G-CSF alone had little effect on BM CFU over time, FL alone increased CFU-GM and CFU-GEMM threefold and fivefold, respectively. Addition of GM-CSF or G-CSF to FL did not increase CFU in BM over levels seen with FL alone. However, after the initial increase in BM CFU after FL plus G-CSF treatment for 3 days, BM CFU returned to control levels after 5 days treatment, and CFU-GM were significantly reduced (65%) after 7 days treatment, when compared with control mice. Finally, we found that transplantation of FL or FL plus G-CSF–mobilized PB cells protected lethally irradiated mice and resulted in long-term multilineage hematopoietic reconstitution.

The human PD-1 gene: complete cDNA, genomic organization, and developmentally regulated expression in B cell progenitors

We report the complete cDNA sequence and the genomic structure of the human PD-1 homologue. An analysis of the expression pattern of the human PD-1 gene (hPD-1) and the murine PD-1 gene (mPD-1) in developing bone marrow B-lineage cells was also undertaken. The full length hPD-1 cDNA is 2106 nucleotides long and encodes a predicted protein of 288 amino acid residues. The hPD-1 and mPD-1 genes share 70% homology at the nucleotide level and 60% homology at the amino acid level. Four potential sites for N-linked glycosylation are conserved, as are a stretch of amino acids between two cysteine residues resembling a V-set immunoglobulin domain, and another region containing a motif similar to an immunoreceptor tyrosine-based inhibitory motif. Isolation of the genomic locus of the hPD-1 gene reveals that the gene is composed of five exons located on human chromosome 2 at band q37. The 5' flanking region lacks TATA and CAAT cis-acting elements, but includes a number of potential transcription factor binding sites and a dominant transcription start site. The mPD-1 gene was preferentially expressed in pro-B cells from murine adult bone marrow. Although hPD-1 was not preferentially expressed in pro-B cells from human fetal bone marrow, treatment of isolated pro-B cells with interleukin-7 resulted in a dramatic increase in expression. These data suggest that PD-1 may play a role in B-cell differentiation during the pro-B cell stage.

flt3 Ligand in Cooperation With Transforming Growth Factor-β1 Potentiates In Vitro Development of Langerhans-Type Dendritic Cells and Allows Single-Cell Dendritic Cell Cluster Formation Under Serum-Free Conditions

Using a recently described serum-free culture system of purified human CD34+ progenitor cells, we show here a critical cooperation of flt3 ligand (FL) with transforming growth factor-β1 (TGF-β1) in the induction of in vitro dendritic cell/Langerhans cell (DC/LC) development. The addition of FL to serum-free cultures of CD34+ cells supplemented with TGF-β1, granulocyte-macrophage colony-stimulating factor, tumor necrosis factor α, and stem cell factor strongly increases both percentages (mean, 36% ± 5% v 64% ± 4%; P = .001) and total numbers (4.4- ± 0.8-fold) of CD1a+ dendritic cells. These in vitro-generated CD1a+ cells molecularly closely resemble a particular type of DC known as an epidermal Langerhans cell. Generation of DC under serum-free conditions was found to strictly require supplementation of culture medium with TGF-β1. Upon omission of TGF-β1, percentages of CD1a+ DC decreased (to mean, 10% ± 8%; P = .001) and, in turn, percentages of granulomonocytic cells (CD1a− cells that are lysozyme [LZ+]; myeloperoxidase [MPO+]; CD14+) increased approximately threefold (P < .05). Furthermore, in the absence of TGF-β1, FL consistently promotes generation of LZ+, MPO+, and CD14+ cells, but not of CD1a+ cells. Serum-free single-cell cultures set up under identical TGF-β1– and FL-supplemented culture conditions showed that high percentages of CD34+ cells (mean, 18% ± 2%; n = 4) give rise to day-10 DC colony formation. The majority of cells in these DC-containing colonies expressed the Langerhans cell/Birbeck granule specific marker molecule Lag. Without TGF-β1 supplementation, Lag+ colony formation is minimal and formation of monocyte/macrophage-containing colonies predominates. Total cloning efficiency in the absence and presence of TGF-β1 is virtually identical (mean, 41% ± 6% v 41% ± 4%). Thus, FL has the potential to strongly stimulate DC/LC generation, but has a strict requirement for TGF-β1 to show this costimulatory effect.

flt3 Ligand in Cooperation With Transforming Growth Factor-β1 Potentiates In Vitro Development of Langerhans-Type Dendritic Cells and Allows Single-Cell Dendritic Cell Cluster Formation Under Serum-Free Conditions

Using a recently described serum-free culture system of purified human CD34+ progenitor cells, we show here a critical cooperation of flt3 ligand (FL) with transforming growth factor-β1 (TGF-β1) in the induction of in vitro dendritic cell/Langerhans cell (DC/LC) development. The addition of FL to serum-free cultures of CD34+ cells supplemented with TGF-β1, granulocyte-macrophage colony-stimulating factor, tumor necrosis factor α, and stem cell factor strongly increases both percentages (mean, 36% ± 5% v 64% ± 4%; P = .001) and total numbers (4.4- ± 0.8-fold) of CD1a+ dendritic cells. These in vitro-generated CD1a+ cells molecularly closely resemble a particular type of DC known as an epidermal Langerhans cell. Generation of DC under serum-free conditions was found to strictly require supplementation of culture medium with TGF-β1. Upon omission of TGF-β1, percentages of CD1a+ DC decreased (to mean, 10% ± 8%; P = .001) and, in turn, percentages of granulomonocytic cells (CD1a− cells that are lysozyme [LZ+]; myeloperoxidase [MPO+]; CD14+) increased approximately threefold (P < .05). Furthermore, in the absence of TGF-β1, FL consistently promotes generation of LZ+, MPO+, and CD14+ cells, but not of CD1a+ cells. Serum-free single-cell cultures set up under identical TGF-β1– and FL-supplemented culture conditions showed that high percentages of CD34+ cells (mean, 18% ± 2%; n = 4) give rise to day-10 DC colony formation. The majority of cells in these DC-containing colonies expressed the Langerhans cell/Birbeck granule specific marker molecule Lag. Without TGF-β1 supplementation, Lag+ colony formation is minimal and formation of monocyte/macrophage-containing colonies predominates. Total cloning efficiency in the absence and presence of TGF-β1 is virtually identical (mean, 41% ± 6% v 41% ± 4%). Thus, FL has the potential to strongly stimulate DC/LC generation, but has a strict requirement for TGF-β1 to show this costimulatory effect.

FLT3 ligand induces the generation of functionally active dendritic cells in mice

FLT3 ligand (FL) is a recently described hematopoietic growth factor that stimulates the proliferation and differentiation of hematopoietic progenitors. We have investigated the effect of FL on murine hematopoiesis and dendritic cell (DC) generation and accumulation in lymphoid tissues and liver in vivo and in vitro evaluating the morphologic, phenotypic, and functional characteristics of these DC. We have observed extramedullary hematopoiesis in the mouse spleen with all lineages of hematopoietic cells represented after the administration of FL. Injection of FL results in a time-dependent and reversible accumulation of DC in the spleen, bone marrow, lymph nodes, and liver. Both flow cytometry and immunohistochemistry revealed a significant accumulation of DC in these tissues. Results of mixed leukocyte reaction suggested that these cells, isolated from murine bone marrow or spleen, were active as antigen presenting cells. Furthermore, cultivation of splenic and marrow cells with GM-CSF and IL-4 gave rise to large numbers of functionally active mature DC. Thus, the results of this study suggest that FL is a promising growth factor that stimulates the generation of large number of DC and may be a useful cytokine for the immunotherapy of cancer.

In Vivo Effects of Flt3/Flk2 Ligand on Mobilization of Hematopoietic Progenitors in Primates and Potent Synergistic Enhancement With Granulocyte Colony-Stimulating Factor

The Flt3 receptor is expressed in primitive hematopoietic cells and its ligand exerts proliferative effects on these cells in vitro in synergy with other cytokines. To expand on the functional properties of Flt3 ligand (FL) in vivo we treated nonhuman primates with FL and tested its ability to mobilize stem/progenitor cells when given alone or in combination with granulocyte colony-stimulating factor (G-CSF ) treatment. FL alone (200 μg/kg/day) mobilizes progenitors with slow kinetics and with a peak effect at the end of 2 weeks of treatment. The spectrum of mobilized progenitors includes myeloid, lymphoid, megakaryocytic, and osteoclastogenic but a low proportion of burst-forming unit (BFU)e. Bone marrow (BM) studies before and during the treatment suggested that proliferative effects in BM may have preceded effects on peripheral blood mobilization. To assess the synergy of FL with G-CSF in mobilization of progenitors we used two schemes: one in which G-CSF was used for the last 5 days of a 12-day treatment with FL; the other in which both cytokines were given concurrently for 5 days only (FL, 200 μg/kg; G-CSF, 100 μg/kg). Both schemes yielded much higher progenitor mobilization levels (peak levels of colony-forming cells [CFSs] 41,000 to 95,000/mL blood) than observed with either FL (CFC 4,600 to 7,300/mL) or G-CSF (8,405 ± 3,024/mL) used alone at the same doses. Furthermore, there was a progressive and significant expansion of progenitors in vitro during 2 weeks in suspension cultures of mononuclear cells or of CD34+ cells only in the animal with the combined treatment. Likewise, substantial mobilization of osteoclastogenic progenitors was documented only with the combined treatment. Given the functional properties of FL, its synergistic mobilization with G-CSF, and its anticipated good tolerance (because of the absence of an effect on mast cell activation), a clinical use is projected for this cytokine in peripheral blood transplantation settings, as well as in experiments with ex vivo gene transfer.

In Vivo Effects of Flt3/Flk2 Ligand on Mobilization of Hematopoietic Progenitors in Primates and Potent Synergistic Enhancement With Granulocyte Colony-Stimulating Factor

The Flt3 receptor is expressed in primitive hematopoietic cells and its ligand exerts proliferative effects on these cells in vitro in synergy with other cytokines. To expand on the functional properties of Flt3 ligand (FL) in vivo we treated nonhuman primates with FL and tested its ability to mobilize stem/progenitor cells when given alone or in combination with granulocyte colony-stimulating factor (G-CSF ) treatment. FL alone (200 μg/kg/day) mobilizes progenitors with slow kinetics and with a peak effect at the end of 2 weeks of treatment. The spectrum of mobilized progenitors includes myeloid, lymphoid, megakaryocytic, and osteoclastogenic but a low proportion of burst-forming unit (BFU)e. Bone marrow (BM) studies before and during the treatment suggested that proliferative effects in BM may have preceded effects on peripheral blood mobilization. To assess the synergy of FL with G-CSF in mobilization of progenitors we used two schemes: one in which G-CSF was used for the last 5 days of a 12-day treatment with FL; the other in which both cytokines were given concurrently for 5 days only (FL, 200 μg/kg; G-CSF, 100 μg/kg). Both schemes yielded much higher progenitor mobilization levels (peak levels of colony-forming cells [CFSs] 41,000 to 95,000/mL blood) than observed with either FL (CFC 4,600 to 7,300/mL) or G-CSF (8,405 ± 3,024/mL) used alone at the same doses. Furthermore, there was a progressive and significant expansion of progenitors in vitro during 2 weeks in suspension cultures of mononuclear cells or of CD34+ cells only in the animal with the combined treatment. Likewise, substantial mobilization of osteoclastogenic progenitors was documented only with the combined treatment. Given the functional properties of FL, its synergistic mobilization with G-CSF, and its anticipated good tolerance (because of the absence of an effect on mast cell activation), a clinical use is projected for this cytokine in peripheral blood transplantation settings, as well as in experiments with ex vivo gene transfer.

The chemokine SDF-1, stromal cell-derived factor 1, attracts early stage B cell precursors via the chemokine receptor CXCR4

In the bone marrow, progenitor (pro-) and precursor (pre-) B cells depend on close contact with stromal cells for growth and maturation. Stromal cell-derived factor 1 (SDF-1), also known as pre-B cell growth-stimulating factor, is produced by bone marrow stromal cells and was reported to act together with interleukin-7 as co-mitogen for pre-B cells. SDF-1 was recently shown to be a chemokine which is chemotactic for different types of leukocytes and acts via the chemokine receptor CXCR4. Using sorted B220+ bone marrow cells and several B cell lines characteristic for different stages of B lymphopoiesis, we now show that SDF-1 is a potent attractant for pro- and pre-B cells, but is inactive on B cells at later stages of development. In early B cell precursors, SDF-1 induced intracellular Ca2+ mobilization and in vitro migration with a potency and efficacy similar to that observed for chemokines acting on blood leukocytes. These responses were mediated via CXCR4 as they could be inhibited by an antireceptor antibody. SDF-1 is the first chemokine shown to act on early-stage B cell precursors. Mice lacking SDF-1 die perinatally and show a severe deficiency in B lymphopoiesis. We propose that SDF-1 released from the stromal cells exerts its critical hematopoietic function by selectively attracting and confining early B cell precursors within the bone marrow microenvironment that provides the necessary factors for growth and differentiation.

Flt-3 Ligand Synergizes With Granulocyte Colony-Stimulating Factor to Increase Neutrophil Numbers and to Mobilize Peripheral Blood Stem Cells With Long-Term Repopulating Potential

Flt-3 ligand (FL) shares many features with stem cell factor (SCF), a widely documented cofactor for peripheral blood progenitor cell (PBPC) mobilization. We investigated the mobilization of PBPCs by FL in combination with granulocyte colony-stimulating factor (G-CSF). As a single agent, FL was a relatively modest mobilizer of PBPCs, resulting in 360 granulocyte/macrophage colony-forming cells (GM-CFCs)/mL blood (control, 155 GM-CFCs/mL blood) and no advantage in leukocyte recovery when these PBPCs were transplanted to irradiated recipient mice. G-CSF, on the other hand, mobilized over 20,000 GM-CFCs/mL blood, and the combination of G-CSF + FL resulted in over 100,000 GM-CFCs/mL blood. The combination of G-CSF + FL stimulated increased levels of monocytes and basophils in the peripheral blood. The performance of the mobilized PBPC product in irradiated hosts correlated with progenitor numbers resulting in long-term engraftment in association with accelerated short-term recovery of both leukocytes and platelets. These data demonstrate the potential of FL to synergize with G-CSF to mobilize PBPCs with both short- and long-term engraftment potential. The effect is similar to the synergistic interaction of G-CSF and SCF on PBPC mobilization. The use of FL as opposed to SCF may elicit a different spectrum of toxicities including lymphoid proliferation effects, in contrast to the mast cell degranulation effects of SCF. Clinical studies of FL are needed to evaluate its usefulness in man.

Striking augmentation of hematopoietic cell chimerism in noncytoablated allogeneic bone marrow recipients by FLT3 ligand and tacrolimus

The influence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and the recently identified hematopoietic stem-progenitor cell mobilizing factor flt3 ligand (FL) on donor leukocyte microchimerism in noncytodepleted recipients of allogeneic bone marrow (BM) was compared. B10 mice (H2b) given 50x10(6) allogeneic (B10.BR [H2k]) BM cells also received either GM-CSF (4 microg/day s.c.), FL (10 microg/day i.p.), or no cytokine, with or without concomitant tacrolimus (formerly FK506; 2 mg/kg) from day 0. Chimerism was quantitated in the spleen 7 days after transplantation by both polymerase chain reaction (donor DNA [major histocompatibility complex class II; I-E(k)]) and immunohistochemical (donor [I-E(k)+] cell) analyses. Whereas GM-CSF alone significantly augmented (fivefold) the level of donor DNA in recipients' spleens, FL alone caused a significant (60%) reduction. Donor DNA was increased 10-fold by tacrolimus alone, whereas coadministration of GM-CSF and tacrolimus resulted in a greater than additive effect (28-fold increase). A much more striking effect was observed with FL + tacrolimus (>125-fold increase in donor DNA compared with BM alone). These findings were reflected in the relative numbers of donor major histocompatibility complex class II+ cells (many resembling dendritic cells) detected in spleens, although quantitative differences among the groups were less pronounced. Evaluation of cytotoxic T lymphocyte generation by BM recipients' spleen cells revealed that FL alone augmented antidonor immunity and that this was reversed by tacrolimus. Thus, although FL may potentiate antidonor reactivity in nonimmunosuppressed, allogeneic BM recipients, it exhibits potent chimerism-enhancing activity when coadministered with recipient immunosuppressive therapy.

Chimerism after organ transplantation

Recent evidence suggests that passenger leukocytes migrate after organ transplantation and produce persistent chimerism, which is essential for sustained survival of the allograft. Here, we describe how this hematolymphopoietic chimerism provides an important framework for interpretation of post-transplant phenomena and for initiation of therapeutically oriented transplantation research.

Extensive Amplification and Self-Renewal of Human Primitive Hematopoietic Stem Cells From Cord Blood

The use of umbilical cord blood as a source of marrow repopulating cells for the treatment of pediatric malignancies has been established. Given the general availability, the ease of procurement, and progenitor content, cord blood is an attractive alternative to bone marrow or growth factor mobilized peripheral blood cells as a source of transplantable hematopoietic tissue. However, there is a major potential limitation to the widespread use of cord blood as a source of hematopoietic stem cells for marrow replacement and gene therapy. There may be enough hematopoietic stem cells to reconstitute children, but the ability to engraft an adult might require ex vivo manipulations. We describe an in vitro system in which the growth of cord blood CD34+ cells is sustained and greatly expanded for more than 6 months by the simple combination of two hematopoietic growth factors. Progenitors and cells belonging to all hematopoietic lineages are continuously and increasingly generated (the number of colony-forming unit–granulocyte-macrophage [CFU-GM] present at the end of 6 months of culture are well over 2,000,000-fold the CFU-GM present at the beginning of the culture). Very primitive hematopoietic progenitors, including long-term culture-initiating cells (LTC-ICs) and blast cell colony-forming units, are also greatly expanded (after 20 weeks of liquid culture, LTC-IC number is over 200,000-fold the initial number). The extremely prolonged maintenance and the massive expansion of these progenitors, which share many similarities with murine long-term repopulating cells, suggest that extensive renewal and little differentiation take place. This system might prove useful in diverse clinical settings involving treatment of grown-up children and adults with transplantation of normal or genetically manipulated hematopoietic stem cells.

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

We studied the effect of human flt3/flk2 ligand (FL) on the proliferation and differentiation of purified CD34+ blood progenitors which express different levels of c-kit protein in clonal cell culture in comparison with that of stem cell factor (SCF). FL alone did not support significant colony formation. However, FL significantly enhanced neutrophil colony (CFU-G) formation in the presence of granulocyte-colony stimulating factor (G-CSF) by peripheral blood (PB)-derived CD34+c-kit- cells which contained a large number of CFU-G. In addition, FL could synergistically increase the number of CFU-G supported by a combination of interleukin (IL)-3 and G-CSF, as did SCF. As we reported previously, SCF showed a significant burst-promoting activity (BPA). In contrast, FL did not exhibit any BPA on PB-derived CD34+c-kithigh cells in which erythroid-burst (BFU-E) was highly enriched. However, FL could synergize with IL-3 or GM-CSF in support of erythrocyte-containing mixed (E-Mix) colony by PB-derived CD34+c-kithigh or low cells in the presence of Epo. Replating of E-Mix colonies derived from CD34+c-kithigh cells supported by IL-3+Epo+SCF yielded more secondary colonies than those supported by IL-3+Epo or IL-3+Epo+FL. When PB-derived CD34+c-kitlow cells which represent a more immature population than CD34+c-kithigh cells were used as the target, number of secondary colonies supported by IL-3+Epo, IL-3+Epo+SCF or IL-3+Epo+FL was comparable. However, the number of lineages expressed in the secondary culture was significantly larger in the primary culture containing IL-3+Epo+FL than in that containing IL-3+Epo. These results suggest that FL not only acts on neutrophilic progenitors, but also on more immature multipotential progenitors.

In Vitro Expansion of Hematopoietic Progenitors and Maintenance of Stem Cells: Comparison Between FLT3/FLK-2 Ligand and KIT Ligand

The effects of FLT3/FLK-2 ligand (FL) and KIT ligand (KL) on in vitro expansion of hematopoietic stem cells were studied using lineage-negative (Lin−)Sca-1–positive (Sca-1+) c-kit–positive (c-kit+) marrow cells from 5-fluorouracil (5-FU)–treated mice. As single agents, neither FL nor KL could effectively support the proliferation of enriched cells in suspension culture. However, in combination with interleukin-11 (IL-11), both FL and KL enhanced the production of nucleated cells and progenitors. The kinetics of stimulation by FL was different from that by KL in that the maximal expansion by FL of the nucleated cell and progenitor pools required a longer incubation than with KL. We then tested the reconstituting abilities of cells cultured for 1, 2, and 3 weeks by transplanting the expanded Ly5.1 cells together with “compromised” marrow cells into lethally irradiated Ly5.2 mice. Cells that had been expanded with either cytokine combination were able to maintain the reconstituting ability of the original cells. Only cells that had been incubated with KL and IL-11 for 21 days had less reconstituting ability than fresh marrow cells. These results indicate that there can be significant expansion of progenitors in vitro without compromising the reconstituting ability of stem cells. Addition of IL-3 to permissive cytokine combinations significantly reduced the ability of cultured cells to reconstitute the hematopoiesis of irradiated hosts. These observations should provide a basis for a rational approach to designing cytokine combinations for in vitro expansion of hematopoietic stem cells.

FLT3 Ligand Preserves the Ability of Human CD34+ Progenitors to Sustain Long-Term Hematopoiesis in Immune-Deficient Mice After Ex Vivo Retroviral-Mediated Transduction

Stromal support is required during retroviral-mediated transduction of human bone marrow-derived CD34+ cells to maintain the clonogenicity of the primitive progenitors. We hypothesized that the cytokine FLT3 ligand (FL) might be able to replace the maintenance role provided by the stroma. CD34+ progenitors from human bone marrow were transduced by the retroviral vector LN with the cytokines interleukin-3 (IL-3), IL-6, and stem cell factor (SCF ) present in all cultures. Transductions were performed with or without stromal support and with or without the inclusion of 100 U/mL FL. No significant increase in gene transfer into colony-forming cells was obtained by the addition of FL to the cultures. Transduction and survival of more primitive human hematopoietic cells was determined by growth in immune-deficient mice for 7 to 8 months. Human myeloid cells, T lymphocytes, and colony-forming progenitors were recovered from the marrow of mice that had received human cells transduced on stroma or in suspension culture with IL-3, IL-6, SCF, and FL, but not with IL-3, IL-6, and SCF alone. LN provirus was detected by polymerase chain reaction in the marrow recovered from 9 of 10 mice transplanted with human CD34+ cells transduced with stromal support, 5 of 11 mice that received human cells transduced in suspension culture with FL, but none of the 10 mice that received human cells transduced in suspension culture without FL. We conclude that FLT3 ligand, in conjunction with IL-3, IL-6, and SCF, preserves the generative capacity of primitive human hematopoietic cells during in vitro transductions in suspension culture.

Long-term cytokine production from engineered primary human stromal cells influences human hematopoiesis in an in vivo xenograft model

Human hematopoiesis can be supported in beige/nude/ XID (bnx) mice by coinjection of human bone marrow stromal cells engineered to secrete human interleukin 3 (HuIL-3). The major limitation is a total absence of human B cell development in the mice, which could be due to supraphysiological levels of HuIL-3 in the circulation. In an effort to obtain human B lymphoid, as well as T lymphoid and myeloid cell development in the mice, CD34+ cells were coinjected with human marrow stromal cells engineered to secrete human IL-2, IL-7, stem cell factor or FLT3 ligand, +/- IL-3. No single factor other than IL-3 supported sustained human hematopoiesis in the mice, although cytokines were expressed for four to six months post-transplantation. Production of both HuIL-3 and IL-7 in the mice supported extrathymic development of human T lymphocytes, but no B cells, myeloid cells, or clonogenic progenitors were detected. Human B cells were not produced from CD34+ cells in the bnx mice under any condition tested. Another limitation to the bnx/Hu system is a lack of maturation of human red blood cells, although BFU-E are maintained. Stromal cells secreting human erythropoietin and IL-3 were cotransplanted into mice with HuCD34+ cells and an increase in hematocrit from 40%-45% to 80%-85% resulted, with production of human and murine red blood cells. Unfortunately, all mice (n = 9) suffered strokes, displayed paralysis and died within three weeks. The bnx/Hu cotransplantation model provides an interesting system in which to study human hematopoietic cell differentiation under the influence of various cytokines.

The effects of Flk-2/flt3 ligand as compared with c-kit ligand on short-term and long-term proliferation of CD34+ hematopoietic progenitors elicited from human fetal liver, umbilical cord blood, bone marrow, and mobilized peripheral blood

The Flk-2/flt3 ligand (FL) was evaluated and compared with c-kit ligand (KL) for its in vitro proliferative effects on CD34+ cells from human fetal liver, umbilical cord blood, bone marrow, and mobilized peripheral blood. Using a 7-day liquid culture system, FL in combination with interleukin-3 (IL-3), interleukin-6 (IL-6), and granulocyte colony-stimulating factor (G-CSF) was comparable with KL in combination with IL-3, IL-6, and G-CSF for the expansion of hematopoietic progenitors. When FL-containing cultures were assayed after 21 or 28 days, a greater number of progenitors were generated as compared with KL-containing cultures. Using bone marrow microvascular endothelial cells as support stroma, cultures supplemented with FL generated a greater number of progenitors in both the nonadherent and adherent layers at day 35. These data suggest that FL ligand, in combination with other cytokines, can be used for short-term ex vivo expansion of hematopoietic progenitors and facilitates the preservation and possible expansion of primitive cells capable of long-term generation of progenitors.

flt-3 ligand is more potent than c-kit ligand for the synergistic stimulation of ex vivo hematopoietic cell expansion

The c-kit and flt-3 tyrosine kinase receptors are expressed on primitive hematopoietic cells, and ligands for both receptors have been cloned. In this study, the effects of c-kit ligand (KL) and flt-3 ligand (FL) were compared in the presence of IL-3, GM-CSF, and erythropoietin (3/GM/EPO), using frequent medium exchange cultures of human bone marrow mononuclear cells (BMMNC) and CD34-enriched cells. In MNC cultures, KL increased cell output by 1.7-fold (p < 10(-4), n = 13) and CFU-GM output by 2.4-fold (p < 10(-3)) as compared with control cultures containing only 3/GM/EPO. Analogously, FL increased cell output by 1.3-fold (p < 10(-3)) and CFU-GM output by 4.4-fold (p < 10(-6)). Therefore, FL was more potent on CFU-GM output than KL, but neither altered the lineage composition (granulocyte, monocyte, macrophage) of the colonies produced. Direct addition of KL or FL to colony assays resulted in only a 1.2-fold increase in CFU-GM outgrowth, suggesting that the effects on increased CFU-GM output were at the preprogenitor stage. In CD34-enriched cell cultures, the effects of KL and FL on CFU-GM output were similar (9-fold above control). Nevertheless, MNC cultures (containing an equivalent number of CD34+lin- cells) always generated more cells (2-fold to 4-fold) and CFU-GM (3-fold to 6-fold) than did parallel cultures of CD34-enriched cells. The greater effect of FL (over KL) in MNC cultures was probably due to synergy with endogenously produced growth factors that were absent in CD34-enriched cell cultures. FL-containing cultures (+/-KL) generated cells that formed larger colonies, and these cells had more proliferative potential on replating into secondary and tertiary cultures. Furthermore, FL increased the output of LTC-IC by 2.1-fold (p < 0.01) and CD34+lin- cells by 6-fold (p < 0.05) as compared with 3/GM/EPO cultures. In contrast, KL did not affect the output of LTC-IC and only slightly increased CD34+lin- cell output (by 1.4-fold). Erythrocytes were increased by KL (2.8-fold) and decreased by FL (0.6-fold), whereas granulocytes and monocytes were increased by both KL (1.4-fold) and FL (2.0-fold). When used together, KL and FL were completely additive with respect to cell, CFU-GM, and LTC-IC output, as well as lineage composition. The results indicate that FL is a more potent synergistic growth factor than KL for MNC expansion and that KL and FL act in an independent, direct, additive manner.

Ex vivo expansion of haemopoietic progenitor cells

Over the last few years, techniques have become available that allow the extensive proliferation of haemopoietic progenitor cells in ex vivo culture systems. The most commonly used method involves a simple liquid suspension culture system supplemented with a range of cytokines. Alternatively, more complex systems have been devised in which the formation of a stromal layer is required. Large increases in total cell numbers and committed progenitor cells can be readily obtained and, with some techniques, significant expansion of primitive haemopoietic cells has been demonstrated. Although these strategies have several potential applications, few clinical studies have been performed. It has been shown that infusion of ex vivo cultured cells is well tolerated with no associated toxicity. However, it is still unclear whether these culture systems sustain sufficient numbers of long-term repopulating cells to secure durable engraftment following myeloablative therapy. In gene therapy studies, ex vivo expansion of stem cells should improve the efficiency of gene transduction to enable the production of genetically modified cells that are capable of expressing the gene of interest for extended periods of time.

Computational simulations of stem-cell factor/c-kit receptor interaction

Stem-cell factor (SCF) is a noncovalent homodimeric cytokine that exhibits profound biological function in the early stages of hematopoiesis by binding to a cell surface tyrosine kinase receptor that is encoded by the c-Kit proto-oncogene. The results obtained from a combined implementation of homology-based molecular modeling and computational simulations in the study of species-specific SCF/ c-Kit interactions are reported. The structural models of the human and rat SCF ligands are based on the close structural similarity to the cytokine M-CSF, whose C alpha structure has recently become available. The constant domains of the human Fc fragment are used as a template for the ligand binding domains of the c-Kit receptor. The factors responsible for the stabilization of the SCF quaternary structure and the molecular determinants for ligand recognition and ligand specificity have been identified by assessing the conformational, topographical, and dynamic features of the isolated ligands and of the ligand-receptor complexes.

Cytokines at the research-clinical interface: potential applications

Developments in the characterization of growth factors and the recognition of their potential for clinical use has advanced through a number of stages. The development of clonogenic haemopoietic colony assays in the 1960s led to the discovery of colony-stimulating activity in the conditioned medium produced by certain cell lines. This activity was then purified and the colony-stimulating factors were identified. With rapid progress in molecular biology techniques in the 1980s, many further growth factors were cloned and produced on an industrial scale. Although erythropoietin, interferons, G-CSF, GM-CSF and IL-2 were all introduced into clinical practice as single agents, cytokines have more recently been investigated for use either in combination, or sequentially. Clinical trials are currently in progress to examine the optimum combinations and timing of administration. Current clinical applications include optimization of methods for mobilization of peripheral blood progenitor cells and amelioration of cytopenias following chemotherapy and bone-marrow transplantation. In the future, cytokines will be employed to expand stem and progenitor cells ex vivo, to improve gene transduction strategies, possibly to protect the gastrointestinal epithelium and as immunomodulators, both in vivo and in vitro. This review will focus on recently characterized growth factors including c-kit ligand/stem cell factor, flt3 ligand, c-mpl ligand/thrombopoietin and interleukins-11, 4, 7, 10, 12 and 13.

Phosphatidylinositol-3' kinase is not required for mitogenesis or internalization of the Flt3/Flk2 receptor tyrosine kinase

Flt3/Flk2 is a receptor tyrosine kinase that is expressed on early hematopoietic progenitor cells. Flt3/Flk2 belongs to a family of receptors, including Kit and colony-stimulating factor-1R, which support growth and differentiation within the hematopoietic system. The Flt3/Flk2 ligand, in combination with other growth factors, stimulates the proliferation of hematopoietic progenitors of both lymphoid and myeloid lineages in vitro. We report that phosphatidylinositol 3'-kinase (PI3K) binds to a unique site in the carboxy tail of murine Flt3/Flk2. In distinction to Kit and colony-stimulating factor-1R, mutant receptors unable to couple to PI3K and expressed in rodent fibroblasts or in the interleukin 3-dependent cell line Ba/F3 provide a mitogenic signal comparable to wild-type receptors. Flt3/Flk2 receptors that do not bind to PI3K also normally down-regulate, a function ascribed to PI3K in the context of other receptor systems. These data point to the existence of other unidentified pathways that, alone or in combination with PI3K, transduce these cellular responses following the activation of Flt3/Flk2.

Regulatory roles of the ligand for Flk2/Flt3 tyrosine kinase receptor on human hematopoiesis

The biological activities of the ligand for the Flk2/Flt3 receptor tyrosine kinase (FL) on human hematopoietic cells are reviewed. In in vitro studies, FL shows relatively few effects by itself on the proliferation and differentiation of hematopoietic cells, but exhibits a potent costimulatory activity in enhancing the proliferation of progenitor cells of multiple lineages. FL promotes the growth of clonogenic myeloid progenitor cells in the presence of other cytokines known to be active on myeloid progenitors, including GM-CSF, interleukin 3 (IL-3), kit ligand (KL), M-CSF and G-CSF. In addition, FL synergizes with IL-7 in inducing the proliferation of pro-B cells, whereas FL has little effect on the growth of clonogenic erythroid progenitors. Furthermore, FL induces the in vitro expansion of the high proliferative potential colony-forming cells (HPP-CFC) and stimulates the proliferation of long-term culture-initiating cells (LTC-IC), suggesting an activity on the proliferation of putative stem cells. Thus, FL plays important roles in regulating the proliferation of hematopoietic progenitor cells and, therefore, may have therapeutic applications.

Flt3 ligand supports the differentiation of early B cell progenitors in the presence of interleukin-11 and interleukin-7

B cell development is influenced by interactions between B cell progenitors and stromal cells. The precise mechanisms by which these interactions regulate B cell differentiation are currently unknown. Flt3 ligand (FL) is a growth factor which stimulates the proliferation of stem cells and early progenitors. Mice deficient for the FLT3 receptor exhibit severe reductions in early B lymphoid progenitors. We have previously described a clonal assay in vitro which allows us to follow the entire B cell differentiation pathway from uncommitted progenitors to mature, immunoglobulin-secreting plasma cells. The growth factor combination of interleukin (IL)-11, mast cell growth factor (MGF) and IL-7 was shown to maintain the differentiation of these hematopoietic precursors into B cell progenitors capable of giving rise to functionally mature B cells in secondary cultures. Here, we show that FL in combination with IL-11 and IL-7 is sufficient to support the differentiation of uncommitted progenitors from day 10 yolk sac (AA4.1+) or day 12 fetal liver (AA4.1+ B220- Mac-1- Sca-1+) into the B lineage. The frequency of B cell progenitors obtained in these conditions was similar, if not better, than the frequency of B cell precursors that arose when cultured in IL-11+MGF+IL-7. Furthermore, the growth factor combination of IL-11+FL+ IL-7 was able to maintain the potential of bipotent precursors giving rise to both the B and myeloid lineages in secondary cultures. We also show that FL synergizes with IL-7 in the proliferation of committed B220+ pro-B cells and may contribute to the maintenance of an earlier pro-B cell population. Together, these results show that FL is important in supporting the differentiation and proliferation of early B cell progenitors in vitro.

Self-renewal of primitive human hematopoietic cells (long-term-culture-initiating cells) in vitro and their expansion in defined medium

A major goal of experimental and clinical hematology is the identification of mechanisms and conditions that support the expansion of transplantable hematopoietic stem cells. In normal marrow, such cells appear to be identical to (or represent a subset of) a population referred to as long-term-culture-initiating cells (LTC-ICs) so-named because of their ability to produce colony-forming cell (CFC) progeny for > or = 5 weeks when cocultured with stromal fibroblasts. Some expansion of LTC-ICs in vitro has recently been described, but identification of the factors required and whether LTC-IC self-renewal divisions are involved have remained unresolved issues. To address these issues, we examined the maintenance and/or generation of LTC-ICs from single CD34+ CD38- cells cultured for variable periods under different culture conditions. Analysis of the progeny obtained from cultures containing a feeder layer of murine fibroblasts engineered to produce steel factor, interleukin (IL)-3, and granulocyte colony-stimulating factor showed that approximately 20% of the input LTC-ICs (representing approximately 2% of the original CD34+ CD38- cells) executed self-renewal divisions within a 6-week period. Incubation of the same CD34+ CD38- starting populations as single cells in a defined (serum free) liquid medium supplemented with Flt-3 ligand, steel factor, IL-3, IL-6, granulocyte colony-stimulating factor, and nerve growth factor resulted in the proliferation of initial cells to produce clones of from 4 to 1000 cells within 10 days, approximately 40% of which included > or = 1 LTC-IC. In contrast, in similar cultures containing methylcellulose, input LTC-ICs appeared to persist but not divide. Overall the LTC-IC expansion in the liquid cultures was 30-fold in the first 10 days and 50-fold by the end of another 1-3 weeks. Documentation of human LTC-IC self-renewal in vitro and identification of defined conditions that permit their extensive and rapid amplification should facilitate analysis of the molecular mechanisms underlying these processes and their exploitation for a variety of therapeutic applications.

Generation of human T lymphocytes from bone marrow CD34+ cells in vitro

Analysis of the events that regulate development of red blood cells or granulocytes has led to therapies altering clinical conditions associated with anemia or neutropenia. The development of therapeutic approaches to target conditions associated with lymphopenia, such as AIDS, has been thwarted by limited techniques for studying T-lymphocyte development. We describe an in vitro system in which human bone marrow CD34+ cells proliferate, acquire the expression of the lymphoid-specific RAG-2 gene and a broad repertoire of rearranged T-cell receptor genes, develop the ability to produce T cell-specific interleukin-2 and achieve a range of T-cell immunophenotypes. The cells also become susceptible to infection with the T-lymphotropic strain of human immunodeficiency virus-1, HIV-1IIIB. This culture system induces human T lymphopoiesis and may permit further analysis of the events regulating human T-lineage differentiation. It provides a preclinical model for screening stem cell gene therapies directed toward AIDS.

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.

The effect of human flt-3 ligand on committed progenitor cell production from normal, aplastic anaemia and Diamond-Blackfan anaemia bone marrow

We investigated the effect of the human ligand for flt-3 (FL) on the committed progenitor colony formation of normal bone marrow (BM) (n = 9) and BM from four aplastic anaemia (AA) and three Diamond-Blackfan anaemia (DBA) patients. Methylcellulose committed progenitor cell assays were carried out using FL alone and in combinations with granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) and c-kit ligand (KL). FL alone had a limited, though significant, effect on the production of granulocyte-macrophage colony-forming unit (CFU-GM) colonies from normal BM and showed an additive effect with IL-3 and GM-CSF separately, but not in combination. FL did not increase the stimulation of KL and did not have an effect on the production of erythroid progenitor colonies. FL had no effect on the AA and DBA BMs studied.

Human T, B, natural killer, and dendritic cells arise from a common bone marrow progenitor cell subset

The early stages of lymphoid cell formation were studied by testing the differentiative potential of phenotypically defined subsets of CD34+ bone marrow cells. A subpopulation of CD34+ Lin- CD45RA+ cells expressing CD10 was isolated by flow cytometry. Such cells are CD38+, HLA-DR+, do not express significant levels of Thy-1 and c-kit, lack erythroid, myeloid, megakaryocytic potential, and give rise only to lymphoid T, B, natural killer (NK), and dendritic cells (DC) in kinetics and titration experiments. Limiting dilution analysis demonstrates the existence of multipotential B/NK/DC progenitor clones in the CD34hi Lin-CD10+ adult bone marrow cell population. Thus, nonprimitive progenitors for lymphoid cells and for DCs can be distinct from those of myeloid, megakaryocytic, and erythroid cells, implying that the DC lineage is developmentally more closely related to the lymphoid lineage than to the myeloid lineage. This study provides new insights into the organization and development of the human lympho-hematopoietic system.

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.