scholarly journals In vivo evidence for an instructive role of fms-like tyrosine kinase-3 (FLT3) ligand in hematopoietic development

Haematologica ◽  
2014 ◽  
Vol 99 (4) ◽  
pp. 638-646 ◽  
Author(s):  
P. Tsapogas ◽  
L. K. Swee ◽  
A. Nusser ◽  
N. Nuber ◽  
M. Kreuzaler ◽  
...  
Keyword(s):  
Tyrosine Kinase ◽  
Flt3 Ligand ◽  
Hematopoietic Development

Identification of Two Src Recruitment Sites in the Juxtamembrane Region of Flt3 with Opposing Effects on Flt3-Ligand-Induced Signaling.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 2289-2289
Author(s):  
Lars Ronnstrand ◽  
Elke Heiss ◽  
Christina Sundberg ◽  
Kristina Masson ◽  
Malin Pedersen ◽  
...  
Keyword(s):  
Signal Transduction ◽  
Tyrosine Kinase ◽  
Receptor Tyrosine Kinase ◽  
Src Family Kinases ◽  
Phosphorylation Sites ◽  
Flt3 Ligand ◽  
Erk Activation ◽  
Juxtamembrane Region ◽  
Signal Relay

Abstract Early signal relay steps upon ligand-binding to the receptor tyrosine kinase Flt3, i.e. sites of Flt3-autophosphorylation and subsequent docking partners, are mainly unresolved. Here we demonstrate for the first time identification of ligand-induced in vivo phosphorylation sites in Flt3. By immunoprecipitation of specific tryptic peptides contained in the juxtamembrane region of human Flt3 and subsequent radiosequencing we identified the tyrosine residues 572, 589, 591 and 599 as in vivo autophosphorylation sites. Focusing on Y589 and Y599, we examined Flt3-ligand-mediated responses in WT-Flt3, Y589F-Flt3 and Y599F-Flt3 expressing 32D cells. Compared to WT-Flt3-32D cells, 32D-Y589F-Flt3 showed upon ligand-stimulation enhanced Erk activation as well as proliferation/survival whereas 32D-Y599F-Flt3 cells displayed substantially diminished responses. Both pY589 and pY599 were identified as association sites for multiple signal relay molecules including Src family kinases. Consistently, 32D-Y589F-Flt3 and 32D-Y599F-Flt3 showed decreased FL-triggered Src activation, impaired phosphorylation of the adapter molecules Cbl and ShcA and deficient receptor ubiquitination and degradation. Interference with the Src-dependent negative regulation of Flt3 signaling may account for the enhanced mitogenic response of Y589F-Flt3. pY599 was additionally found to interact with the protein tyrosine phosphatase Shp2. As Y599F-Flt3-32D lacked ligand-induced Shp2 phosphorylation and since silencing of Shp2 in WT-Flt3-expressing cells mimicked the Y599F-Flt3-phenotype we hypothesize that recruitment of Shp2 to pY599 contributes to FL-mediated Erk activation and proliferation. To summarize, our work presents novel insights in Flt3-mediated signal transduction. We have identified the in vivo autophosphorylation sites of the juxtamembrane region of Flt3, revealed Src family kinases and Shp2 as binding partners of pY589 and/or pY599, respectively, as well as their potential impact on FL-mediated signaling in Flt3-32D cells. Future work will now focus on elucidation of additional and possibly novel interaction partners of the found phosphorylation sites by employing an unbiased proteomics approach. With this gained knowledge it will be of interest to see whether ITDs differing in the nature of the duplicated tyrosines also confer distinct signaling behavior. If so, these tyrosines might serve as a diagnostic marker and point towards a successful combinatorial therapy consisting of a receptor tyrosine kinase inhibitor and an inhibitor for the specifically affected signal transduction pathway.

scholarly journals Targeted Treatment of FLT3 -Overexpressing Acute Myeloid Leukemia with MiR-150 Nanoparticles Guided By Conjugated FLT3 Ligand Peptides

Blood ◽  
2015 ◽  
Vol 126 (23) ◽  
pp. 3784-3784
Author(s):  
Xi Jiang ◽  
Jason Bugno ◽  
Chao Hu ◽  
Yang Yang ◽  
Tobias Herold ◽  
...  
Keyword(s):  
Acute Myeloid Leukemia ◽  
Tyrosine Kinase ◽  
Median Survival ◽  
Myeloid Leukemia ◽  
Poor Prognosis ◽  
Prognostic Impact ◽  
Dependent Manner ◽  
Flt3 Ligand ◽  
Acute Myeloid

Abstract Acute myeloid leukemia (AML) is one of the most common and fatal forms of hematopoietic malignancies. With standard chemotherapies, only 30-50% of younger (aged <60) and 5-10% of older patients with AML survive longer than 5 years. Aberrancy of FMS-like tyrosine kinase 3 (FLT3) occurs in the majority cases of AML. Two major classes of constitutively activating mutations of FLT3, i.e. internal-tandem duplications (ITDs) and tyrosine kinase domain (TKD) point mutations are found in more than 30% of AML cases and usually predict poor prognosis. Overexpression of FLT3 has also been reported in more than 70% of AML cases with a variety of AML subtypes, e.g. MLL (Mixed Lineage Leukemia)-rearranged or FLT3 -ITD AML, and may be associated with poor survival in AML patients. Given the disappointing results with FLT3 tyrosine kinase inhibitors (TKIs) in clinical trials in the past decade, decreasing the overall abundance of FLT3 at the RNA and protein levels would be an alternative strategy to treat AMLs with FLT3 overexpression and/or FLT3 -ITD/TKD mutations. MicroRNAs (miRNA) are a class of small, non-coding RNAs that play important roles in post-transcriptional gene regulation. We recently reported that miR-150 functions as a pivotal tumor-suppressor gatekeeper in MLL-rearranged and other subtypes of AML, through targeting FLT3 and MYB directly, and the MYC/LIN28/HOXA9/MEIS1 pathway indirectly. Our data showed that MLL-fusion proteins up-regulate FLT3 level through inhibiting the maturation of miR-150. Therefore, our findings strongly suggest a significant clinical potential of restoration of miR-150 expression/function in treating FLT3 -overexpressing AML. In the present study, we first analyzed FLT3 expression patterns and prognostic impact in a large cohort of AML patients (n=562). We found that FLT3 is aberrantly highly expressed in FAB M1/M2/M5 AML or AML with t(11q23)/MLL -rearrangements, FLT3 -ITD or NPM1 mutations, and that increased expression of FLT3 is an independent predictor of poor prognosis in patients with FLT3 -overexpressing AML. To treat FLT3 -overexpressing AML, we developed a novel targeted nanoparticle system consisting of FLT3 ligand (FLT3L)-conjugated G7 poly(amidoamine) (PAMAM) dendriplexes encapsulating miR-150 oligos (see Figure 1A). In FLT3 -overexpressing cell lines, the uptake ratios of the G7-FLT3L dendrimers were much higher (50.3~97.1%) than the G7-histone 2B (H2B) control nanoparticles (4.3~33.2%). And the uptake only took minutes. By integrating the miR-150 oligo with G7-FLT3L dendrimers, we constructed the G7-FLT3L-miR-150 dendriplexes, which significantly reduced the viability and increased the apoptosis of MONOMAC-6 cells carrying t(9;11) in a dose-dependent manner. To increase the stability of miR-150 oligos, we incorporated a 2'-o -methyl (2'-O Me) modification into the miRNA oligos. Indeed, the G7-FLT3L nanoparticles carrying 2'-O Me modified miR-150 exhibited a more sustained inhibition on cell growth. In order to further investigate the in vivo therapeutic effects of the miR-150 nanoparticles, we used a MLL -rearranged leukemia model. We transplanted wild-type recipient mice with primary mouse leukemic cells bearing the MLL-AF9 fusion. After the onset of leukemia, the mice were treated with G7-Flt3L or G7-NH2 control nanoparticles complexed with 2'-O Me-modified miR-150 oligos. In these treated animals, G7-Flt3L-miR-150 nanoparticles tended to be enriched in the bone marrow. The G7-Flt3L-miR-150 nanoparticles showed the best therapeutic effect (with median survival of 86 days), as compared with the control group (Ctrl; PBS treated; with median survival of 54 days) or the G7-NH2-miR-150 treated group (with median survival of 63 days). Nanoparticles carrying miR-150 mutant oligos showed no anti-leukemia effect at all. Notably, the G7-Flt3L-miR-150 treatment almost completely blocked MLL-AF9 -induced leukemia in 20% of the mice (Fig. 1B). Furthermore, the G7-Flt3L-miR-150 nanoparticles showed a synergistic effect with JQ1, a small-molecule inhibitor of the MYC pathway, in treating AML in vivo (Fig. 1C). Collectively, we have developed a novel targeted therapeutic strategy to treat FLT3-overexpressing AML, such as MLL-rearranged leukemias, which are resistant to currently available therapies, with both high specificity and efficacy. Disclosures No relevant conflicts of interest to declare.

scholarly journals Role of Tyrosine Kinase in Serotonin-Induced Constriction of the Basilar Artery In Vivo

Stroke ◽  
1998 ◽  
Vol 29 (2) ◽  
pp. 494-498 ◽  
Author(s):  
Takanari Kitazono ◽  
Setsuro Ibayashi ◽  
Tetsuhiko Nagao ◽  
Tomoko Kagiyama ◽  
Jiro Kitayama ◽  
...  
Keyword(s):  
Tyrosine Kinase ◽  
Basilar Artery

scholarly journals Delineating the role of cooperativity in the design of potent PROTACs for BTK

2018 ◽  
Vol 115 (31) ◽  
pp. E7285-E7292 ◽  
Author(s):  
Adelajda Zorba ◽  
Chuong Nguyen ◽  
Yingrong Xu ◽  
Jeremy Starr ◽  
Kris Borzilleri ◽  
...  
Keyword(s):  
Tyrosine Kinase ◽  
E3 Ligase ◽  
Nonreceptor Tyrosine Kinase ◽  
Binary And Ternary Complexes ◽  
B Cell Maturation ◽  
Exciting Opportunity ◽  
Subsequent Degradation

Proteolysis targeting chimeras (PROTACs) are heterobifunctional small molecules that simultaneously bind to a target protein and an E3 ligase, thereby leading to ubiquitination and subsequent degradation of the target. They present an exciting opportunity to modulate proteins in a manner independent of enzymatic or signaling activity. As such, they have recently emerged as an attractive mechanism to explore previously “undruggable” targets. Despite this interest, fundamental questions remain regarding the parameters most critical for achieving potency and selectivity. Here we employ a series of biochemical and cellular techniques to investigate requirements for efficient knockdown of Bruton’s tyrosine kinase (BTK), a nonreceptor tyrosine kinase essential for B cell maturation. Members of an 11-compound PROTAC library were investigated for their ability to form binary and ternary complexes with BTK and cereblon (CRBN, an E3 ligase component). Results were extended to measure effects on BTK–CRBN cooperative interactions as well as in vitro and in vivo BTK degradation. Our data show that alleviation of steric clashes between BTK and CRBN by modulating PROTAC linker length within this chemical series allows potent BTK degradation in the absence of thermodynamic cooperativity.

A CD44v6 peptide reveals a role of CD44 in VEGFR-2 signaling and angiogenesis

Blood ◽  
2009 ◽  
Vol 114 (25) ◽  
pp. 5236-5244 ◽  
Author(s):  
Martina Tremmel ◽  
Alexandra Matzke ◽  
Imke Albrecht ◽  
Anna M. Laib ◽  
Vivienne Olaku ◽  
...  
Keyword(s):  
Tyrosine Kinase ◽  
Receptor Tyrosine Kinase ◽  
Surface Protein ◽  
Receptor Activation ◽  
Tubule Formation ◽  
Kinase C ◽  
Promising Target ◽  
Hepatocyte Growth

Abstract A specific splice variant of the CD44 cell- surface protein family, CD44v6, has been shown to act as a coreceptor for the receptor tyrosine kinase c-Met on epithelial cells. Here we show that also on endothelial cells (ECs), the activity of c-Met is dependent on CD44v6. Furthermore, another receptor tyrosine kinase, VEGFR-2, is also regulated by CD44v6. The CD44v6 ectodomain and a small peptide mimicking a specific extracellular motif of CD44v6 or a CD44v6-specific antibody prevent CD44v6-mediated receptor activation. This indicates that the extracellular part of CD44v6 is required for interaction with c-Met or VEGFR-2. In the cytoplasm, signaling by activated c-Met and VEGFR-2 requires association of the CD44 carboxy-terminus with ezrin that couples CD44v6 to the cytoskeleton. CD44v6 controls EC migration, sprouting, and tubule formation induced by hepatocyte growth factor (HGF) or VEGF-A. In vivo the development of blood vessels from grafted EC spheroids and angiogenesis in tumors is impaired by CD44v6 blocking reagents, suggesting that the coreceptor function of CD44v6 for c-Met and VEGFR-2 is a promising target to block angiogenesis in pathologic conditions.

Key Nodes In FLT3 Dependent Signaling Determine Growth and Survival Of Childhood Acute Lymphoblastic Leukemia

Blood ◽  
2013 ◽  
Vol 122 (21) ◽  
pp. 2508-2508
Author(s):  
Kevin Dierck ◽  
Ina Siekmann ◽  
Sebastian Prall ◽  
Florian Beck ◽  
Irmela Jeremias ◽  
...  
Keyword(s):  
Signal Transduction ◽  
Cell Proliferation ◽  
Acute Lymphoblastic Leukemia ◽  
Rna Interference ◽  
Tyrosine Kinase ◽  
Lymphoblastic Leukemia ◽  
Signaling Networks ◽  
Rtk Signaling

Abstract The balanced regulation of complex signaling networks plays an important role in cell proliferation, survival and apoptosis. Receptor tyrosine kinase (RTK) dependent signal transduction has been implicated in the pathogenesis of many malignancies including acute lymphoblastic leukemia (ALL) of childhood. Moreover, deregulated RTK activity was observed following targeted inhibition of constitutively active kinases or kinase-dependent pathways in a variety of malignancies, which conferred pharmacological resistance. In ALL, fms-like-tyrosine kinase 3 (FLT3) and platelet-derived growth factor receptor beta (PDGFRβ) are targeted for mutation (Roberts et al., 2012). However, in primary ALL the RTK dependent signaling state is poorly defined and the occurrence of relapse in the context of genotype-directed monotherapy regimens targeting RTKs, such as FLT3, underlines the need for an activity-based approach to RTK signaling in leukemia comprising the identification of critical downstream target proteins. To select for driver RTKs in ALL, we combined the analyses of RTK expression in primary ALL (n=102), ALL cell lines and normal hematopoietic cells with the characterization of ligand dependent cell proliferation as well as shRNA mediated RTK repression in vitro and in vivo. We observed aberrant RTK expression patterns in ALL cells compared to normal lymphoid progenitor and stem cell populations as well as mature T- and B-lymphocytes. RNA interference mediated repression of growth promoting RTKs FLT3 and PDGFRβ in primary ALL led to a loss or reduction of the affected cell population in vivo. To identify critical signaling nodes we performed a phosphoproteomic characterization by iTRAQ (isobaric tags for relative and absolute quantification)-based mass spectrometry of the signal transduction of selected driver RTKs in the corresponding primary ALL samples. Primary ALL cells were propagated in NSG mice after xenotransplantation, and regulated phosphoproteins were identified after ligand stimulation. Our network-directed approach to RTK signaling in ALL thus allowed for the identification of downstream signaling nodes implicated in aberrant RTK activity. We identified a total of 2241 phosphoproteins and observed a striking diversity of RTK driven signaling processes in primary ALL exhibiting only a marginal overlap between phosphoregulated proteins which illustrates the inter-individual heterogeneity and the challenge for non-combinatorial therapies. Despite a predominant receptor and cell type specific composition of potentiated signaling networks our phosphoproteomic analyses identified p21-activated protein kinase PAK2 as a novel key nodal point in FLT3 dependent signaling in ALL. The importance of PAK protein family members in the regulation of cell proliferation and survival and the emerging role of PAK proteins in the pathogenesis of a broad range of tumors suggests a hitherto unanticipated function in the malignant transformation of ALL and the signal transduction of FLT3 (Ye and Field, 2012). As a kinase PAK2 represents a druggable target and may be suited for combinatorial intervention strategies targeting FLT3 signaling in order to induce synthetic lethality. Inhibition of group I PAK kinases (PAK1, 2, 3) using the allosteric inhibitor IPA-3 and RNA interference mediated repression of PAK2 led to the loss of ALL cells due to an impaired cell proliferation and an increased apoptosis. Notably, PAK2 depleted ALL cells showed an elevated sensitivity towards pharmacological FLT3 inhibition which underlines the potential role of PAK2 as a novel target in ALL and the need for novel small molecule PAK inhibitors with higher specificity and improved applicability in vivo. Disclosures: No relevant conflicts of interest to declare.

Flt3-Ligand Plays a Critical Role in Development and Function of CD8+/TCR− Facilitating Cells in Bone Marrow,

Blood ◽  
2011 ◽  
Vol 118 (21) ◽  
pp. 4003-4003
Author(s):  
Yiming Huang ◽  
Thomas Miller ◽  
Hong Xu ◽  
Yujie Wen ◽  
Suzanne T Ildstad
Keyword(s):  
Bone Marrow ◽  
Critical Role ◽  
Flt3 Ligand ◽  
Wild Type ◽  
Graft Versus Host ◽  
Equity Ownership ◽  
Total Body ◽  
And Function

Abstract Abstract 4003 Graft facilitating cells (FC) are a CD8+/TCR− bone marrow subpopulation that enhance engraftment of purified hematopoietic cells (HSC) in allogeneic mouse recipients without causing graft-versus-host disease. They also enhance engraftment of suboptimal numbers of syngeneic HSC. FC induce antigen-specific CD4+/CD25+/FoxP3+ regulatory T cells in vivo. The major subpopulation in FC is resembles plasmacytoid precursor dendritic cells (p-preDC) both phenotypically and functionally. Treatment of mice with Flt3 ligand (FL) results in a significant increase in FC in peripheral blood (PB) and FL-expanded-PB FC enhanced HSC engraftment. In this study, we evaluated the role of FL in FC development using FL-KO mice. We first compared FC from FL-KO B6 mice with FC from B6 mice to evaluate the FC total cellular composition. The number of FC was significantly decreased in FL-KO mice compared to wild type controls (P = 0.0003). The number of p-preDC FC was also significantly decreased (P = 0.0001), suggesting that FL is important in the development of p-preDC FC. Next, we tested whether FL-KO FC facilitate engraftment of HSC in allogeneic recipients. FC were sorted from FL-KO B6 mice and HSC (C-Kit+/Sca-1+/Lin−) were sorted from B6 mice. 10,000 B6 HSC plus 30,000 FL-KO FC were transplanted into NOD recipients conditioned with 950 cGy of total body irradiation. Controls received 10,000 B6 HSC with or without 30,000 B6 FC. Only 36% (5 of 14) NOD recipients of B6 HSC alone engrafted and two mice survived up to 160 days (Figure). Sixty-three percent (5 of 8) of recipients transplanted with B6 HSC + FL-KO B6 FC engrafted and only one mouse survived up to 160 days. Seventy-five percent (9 of 12) recipients of B6 HSC + B6 FC engrafted and seven of the mice survived more than 160 days. The level of donor chimerism in recipients of B6 HSC + B6 FC (57% ± 10%) was significantly higher than recipients of B6 HSC + FL-KO B6 FC (14% ± 3%; P = 0.003) or B6 HSC alone (22% ± 6%; P = 0.005). These data demonstrate that FL-KO FC fail to facilitate durable allogeneic HSC engraftment, suggesting that flt3-ligand plays a critical role in development of functional FC. Disclosures: Ildstad: Regenerex, LLC: Equity Ownership.

Proline-Rich Tyrosine Kinase (Pyk2) Promotes Tumor Progression In Multiple Myeloma Through Modulation Of Wnt/β-Catenin Signaling Pathway

Blood ◽  
2013 ◽  
Vol 122 (21) ◽  
pp. 3094-3094
Author(s):  
Yu Zhang ◽  
Yong Zhang ◽  
Yuji Mishima ◽  
Michele Moschetta ◽  
Wenjing Zhang ◽  
...  
Keyword(s):  
Multiple Myeloma ◽  
Tyrosine Kinase ◽  
Tumor Growth ◽  
Dna Synthesis ◽  
Cell Viability ◽  
Cyclin D1 ◽  
Research Funding ◽  
Advisory Board

Abstract Background Proline-rich tyrosine kinase (Pyk2) is a non-receptor tyrosine kinase which belongs to the focal adhesion kinase (FAK) family. It is known to facilitate TNF-α induced EMT process in solid tumors, but this has not been investigated in the field of hematologic malignancies. We therefore dissected the role of PyK2 in multiple myeloma (MM) by looking at its ability to modulate MM biology both in vitro and in vivo. Methods Lentiviral packaged small hairpin RNA (shRNA), overexpression plasmid, related scramble probe and empty vector were introduced into MM1.S (GFP+/Luc+) cell line, to generate stable Pyk2 K.D. (#A2 and #A4), Pyk2 K.I., and control cells, respectively. The efficiency of knock-down or knock-in was validated by qPCR and immunoblotting. Cell viability and cell proliferation were detected by using CellTiter-Glo® luminescent assay and thymidine uptake, respectively. Gain- and loss-of fucntion studies were also performed on MM cells in presence of primary bone marrow stromal cells isolated from MM patients (MM-BMSCs). Adhesion of Pyk2 stable cells to fibronectin was measured by using a ECM cell adhesion assay kit. The synergistic effects of Pyk2 with Bortezomib was determined through calculating the DNA synthesis of Pyk2 K.D. cells treated with Bortezomib (2.5-5µM), using Calcusyn software and Chou-Talalay method. Pyk2 K.D. stable cells were intravenously injected into SCID-Biege mice to generate xenograft model. In vivo tumor growth was observed by Bioluminescent Imaging. Pyk2 -dependent-modulation of Wnt/β-catenin pathway signaling was indentified by using immunobloting. Results Knockdown of Pyk2 in MM cells significantly repressed cell viability and proliferation, as well as their adhesive ability to BMSCs, compared to scramble control cells. Moreover, Pyk2 knockdown induced de-adhesion of MM cells from BMSCs thus inducing chemosensitivity of tumor cells to Bortezomib. We next corroborated our findings by studying Pyk2 knock-in MM cells, and showed that stably upregulated Pyk2 expression promoted MM cell growth as measured by either ATP quantitation or DNA synthesis. Upregulation of Pyk2 expression also stablized the adhesion of MM cells to BMSCs, leading to a drug-resistance of MM cells to Bortezomib, compared with vector control cells. Pyk2 related tumor growth was further validated by establishing a xenograft mouse model. By using bioluminescence imaging, we found a significantly lower tumor burden in mice injected with Pyk2 K.D. cells, compared to mice controls (injected with scramble cells). We next dissected the effect of Pyk2 in modulation of cellular signaling in MM cells by using immunoblotting, and demonstrated that Pyk2 played an important role in regulating β-catenin signaling. Indeed, knockdown of Pyk2 induced GSK3β-phosphorylation, leading to increased β-catening-phosphorylation, thus resulting in β-catenin degradation and inhibited translocation to nucleus. Importantly, Pyk2 K.D. cells presented with reduced expression of c-myc and cyclin D1 at protein level. Conversely, Pyk2 overexpression enhanced β-catenin expression together with c-myc and cyclin D1 up-regulation, thus confirming the role of Pyk2 in modulating Wnt/β-catenin signaling activity in MM. Conclusion These findings indicate that Pyk2 exhibits pro-oncogenic properties in MM through modulation of Wnt/β-catenin signaling. Therefore, Pyk2 represents a novel therapeutic target in MM. Disclosures: Ghobrial: Sanofi: Research Funding; Noxxon: Research Funding; BMS: Advisory board, Advisory board Other, Research Funding; Onyx: Advisoryboard Other.

Mice lacking flt3 ligand have deficient hematopoiesis affecting hematopoietic progenitor cells, dendritic cells, and natural killer cells

Blood ◽  
2000 ◽  
Vol 95 (11) ◽  
pp. 3489-3497 ◽  
Author(s):  
Hilary J. McKenna ◽  
Kim L. Stocking ◽  
Robert E. Miller ◽  
Kenneth Brasel ◽  
Thibaut De Smedt ◽  
...  
Keyword(s):  
Dendritic Cells ◽  
Tyrosine Kinase ◽  
Nk Cells ◽  
Natural Killer ◽  
Fetal Liver ◽  
Flt3 Ligand ◽  
Hematopoietic Progenitors ◽  
Hematopoietic Progenitor ◽  
Cell Mobilization

Abstract The ligand for the receptor tyrosine kinase fms-like tyrosine kinase 3 (flt3), also referred to as fetal liver kinase-2 (flk-2), has an important role in hematopoiesis. The flt3 ligand (flt3L) is a growth factor for hematopoietic progenitors and induces hematopoietic progenitor and stem cell mobilization in vivo. In addition, when mice are treated with flt3L immature B cells, natural killer (NK) cells and dendritic cells (DC) are expanded in vivo. To further elucidate the role of flt3L in hematopoiesis, mice lacking flt3L (flt3L−/−) were generated by targeted gene disruption. Leukocyte cellularity was reduced in the bone marrow, peripheral blood, lymph nodes (LN), and spleen. Thymic cellularity, blood hematocrit, and platelet numbers were not affected. Significantly reduced numbers of myeloid and B-lymphoid progenitors were noted in the BM of flt3L−/− mice. In addition a marked deficiency of NK cells in the spleen was noted. DC numbers were also reduced in the spleen, LN, and thymus. Both myeloid-related (CD11c++ CD8−) and lymphoid-related (CD11c++ CD8+) DC numbers were affected. We conclude that flt3L has an important role in the expansion of early hematopoietic progenitors and in the generation of mature peripheral leukocytes.

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