Expression of Flt3-ligand by the endothelial cell

FLT3L governs the development of partially overlapping hematopoietic lineages in humans and mice

FMS-related tyrosine kinase 3 ligand (FLT3L), encoded by FLT3LG, is a hematopoietic factor essential for the development of natural killer (NK) cells, B cells, and dendritic cells (DCs) in mice. We describe three humans homozygous for a loss-of-function FLT3LG variant with a history of various recurrent infections, including severe cutaneous warts. The patients' bone marrow (BM) was hypoplastic, with low levels of hematopoietic progenitors, particularly myeloid and B cell precursors. Counts of B cells, monocytes, and DCs were low in the patients' blood, whereas the other blood subsets, including NK cells, were affected only moderately, if at all. The patients had normal counts of Langerhans cells (LCs) and dermal macrophages in the skin but lacked dermal DCs. Thus, FLT3L is required for B cell and DC development in mice and humans. However, unlike its murine counterpart, human FLT3L is required for the development of monocytes but not NK cells.

Heterogeneity of Patient-Derived Acute Myeloid Leukemia Cells Subjected to SYK In Vitro Inhibition

Acute myeloid leukemia (AML) is an aggressive hematological malignancy with a dismal prognosis. The cytoplasmic spleen tyrosine kinase (SYK) is highly expressed by hematopoietic cells and has emerged as a potential therapeutic target. In this study, we evaluated the in vitro antileukemic effects of five SYK inhibitors, fostamatinib, entospletinib, cerdulatinib, TAK-659, and RO9021, in a consecutive AML patient cohort. All inhibitors demonstrated a concentration-dependent antiproliferative effect, although there was considerable heterogeneity among patients. For fostamatinib and TAK-659, the antiproliferative effects were significantly higher in FLT3 mutated patients compared to nonmutated patients. Fostamatinib, entospletinib, TAK-659, and RO9021 induced significant apoptosis in primary AML cells, although the proapoptotic effects of the SYK inhibitors were less pronounced than the antiproliferative effects. Finally, most of the SYK inhibitors caused a significant decrease in the release of cytokines and chemokines from primary AML cells, indicating a potent inhibitory effect on the release of these leukemic signaling molecules. We concluded that the SYK inhibitors had antileukemic effects in AML, although larger studies are strongly needed to identify which patient subsets will benefit most from such a treatment.

[GATA2 gene mutations: 3 cases]

INTRODUCTION

Heterozygous germline mutations of GATA2 gene (guanine-adenine-thymine-adenine binding protein 2) are hereditary mutations that can be pathogenic, sometimes occurring sporadically, responsible for a florid clinical-biological picture, sometimes serious and quickly leading to the death.

CASE REPORTS

We reported two women and one man with germline mutations in the GATA2 gene. The first patient, aged 19, initially presented with monocytopenia and chronic lymphedema of the four limbs, suggestive of Emberger syndrome. The second patient, 28-years-old, presented with a disseminated atypical mycobacterium (Mycobacterium kansasii) infection, raising suspicion of an immune deficiency such as MonoMAC syndrome (deficiency syndrome of dendritic cells, monocytes, B lymphocytes and NK cells). The last patient, 30-years-old, presented with pancytopenia, leading to the diagnosis of a family form of myelodysplastic syndromes and acute myeloid leukemia characterized by a mutation of the GATA2 gene.

CONCLUSIONS

Each case illustrates a typical clinical presentation of GATA2 deficiency, although the evolution of these syndromes ultimately reveals a complex, heterogeneous and intricate picture of hematological, dermatological, infectious, pulmonary, ENT or oncological symptoms. Mutations in the GATA2 gene remain a diagnostic and therapeutic challenge for the internist, and require multidisciplinary management given the florid picture that can be of interest to all specialties. The clinical spectrum of these GATA2 mutations as well as the latest management recommendations from the recent litterature and the "GATA2 club" are described in this article.

Integrins, anchors and signal transducers of hematopoietic stem cells during development and in adulthood

Hematopoietic stem cells (HSCs), the apex of the hierarchically organized blood cell production system, are generated in the yolk sac, aorta-gonad-mesonephros region and placenta of the developing embryo. To maintain life-long hematopoiesis, HSCs emigrate from their site of origin and seed in distinct microenvironments, called niches, of fetal liver and bone marrow where they receive supportive signals for self-renewal, expansion and production of hematopoietic progenitor cells (HPCs), which in turn orchestrate the production of the hematopoietic effector cells. The interactions of hematopoietic stem and progenitor cells (HSPCs) with niche components are to a large part mediated by the integrin superfamily of adhesion molecules. Here, we summarize the current knowledge regarding the functional properties of integrins and their activators, Talin-1 and Kindlin-3, for HSPC generation, function and fate decisions during development and in adulthood. In addition, we discuss integrin-mediated mechanosensing for HSC-niche interactions, ex vivo protocols aimed at expanding HSCs for therapeutic use, and recent approaches targeting the integrin-mediated adhesion in leukemia-inducing HSCs in their protecting, malignant niches.

Precision medicine in myeloid malignancies

Myeloid malignancies have always been at the forefront of an improved understanding of the molecular pathogenesis of cancer. In accordance, over the last years, basic research focusing on the aberrations underlying malignant transformation of myeloid cells has provided the basis for precision medicine approaches and subsequently has led to the development of powerful therapeutic strategies. In this review article, we will recapitulate what has happened since in the 1980s the use of all-trans retinoic acid (ATRA), as a first targeted cancer therapy, has changed one of the deadliest leukemia subtypes, acute promyelocytic leukemia (APL), into one that can be cured without classical chemotherapy today. Similarly, imatinib, the first molecularly designed cancer therapy, has revolutionized the management of chronic myeloid leukemia (CML). Thus, targeted treatment approaches have become the paradigm for myeloid malignancy, but many questions still remain unanswered, especially how identical mutations can be associated with different phenotypes. This might be linked to the impact of the cell of origin, gene-gene interactions, or the tumor microenvironment including the immune system. Continuous research in the field of myeloid neoplasia has started to unravel the molecular pathways that are not only crucial for initial treatment response, but also resistance of leukemia cells under therapy. Ongoing studies focusing on leukemia cell vulnerabilities do already point to novel (targetable) "Achilles heels" that can further improve myeloid cancer therapy.

Pharmacological impact of FLT3 mutations on receptor activity and responsiveness to tyrosine kinase inhibitors

Acute myelogenous leukaemia (AML) is an aggressive blood cancer characterized by the rapid proliferation of immature myeloid blast cells, resulting in a high mortality rate. The 5-year overall survival rate for AML patients is approximately 25%. Circa 35% of all patients carry a mutation in the FLT3 gene which have a poor prognosis. Targeting FLT3 receptor tyrosine kinase has become a treatment strategy in AML patients possessing FLT3 mutations. The most common mutations are internal tandem duplications (ITD) within exon 14 and a single nucleotide polymorphism (SNP) that leads to a point mutation in the D835 of the tyrosine kinase domain (TKD). Variations in the ITD sequence and the occurrence of other point mutations that lead to ligand-independent FLT3 receptor activation create difficulties in developing personalized therapeutic strategies to overcome observed mutation-driven drug resistance. Midostaurin and quizartinib are tyrosine kinase inhibitors (TKIs) with inhibitory efficacy against FLT3-ITD, but exhibit limited clinical impact. In this review, we focus on the structural aspects of the FLT3 receptor and correlate those mutations with receptor activation and the consequences for molecular and clinical responsiveness towards therapies targeting FLT3-ITD positive AML.

Dendritic cell development in infection

The immune system protects from infections primarily by detecting and eliminating invading pathogens. This is predominantly mediated by innate immune cells like neutrophils, monocytes and dendritic cells (DCs) expressing specific receptors recognizing pathogen-associated molecular patterns. DC activation by pathogens leads to the initiation of antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. However, various pathogens have evolved immune evasion strategies to ensure their survival. In this review, we highlight recent findings on how various microorganisms or their structural features affect or modulate DC development and whether this has any consequences for a protective immune response.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

The FMS-like tyrosine kinase-3 ligand/lung dendritic cell axis contributes to regulation of pulmonary fibrosis

RATIONALE

Dendritic cells (DC) accumulate in the lungs of patients with idiopathic lung fibrosis, but their pathogenetic relevance is poorly defined.

OBJECTIVES

To assess the role of the FMS-like tyrosine kinase-3 ligand (Flt3L)-lung dendritic cell axis in lung fibrosis.

MEASUREMENTS AND MAIN RESULTS

We demonstrate in a model of adenoviral gene transfer of active TGF-β1 that established lung fibrosis was accompanied by elevated serum Flt3L levels and subsequent accumulation of CD11b^pos DC in the lungs of mice. Patients with idiopathic pulmonary fibrosis also demonstrated increased levels of Flt3L protein in serum and lung tissue and accumulation of lung DC in explant subpleural lung tissue specimen. Mice lacking Flt3L showed significantly reduced lung DC along with worsened lung fibrosis and reduced lung function relative to wild-type (WT) mice, which could be inhibited by administration of recombinant Flt3L. Moreover, therapeutic Flt3L increased numbers of CD11b^pos DC and improved lung fibrosis in WT mice exposed to AdTGF-β1. In this line, RNA-sequencing analysis of CD11b^pos DC revealed significantly enriched differentially expressed genes within extracellular matrix degrading enzyme and matrix metalloprotease gene clusters. In contrast, the CD103^pos DC subset did not appear to be involved in pulmonary fibrogenesis.

CONCLUSIONS

We show that Flt3L protein and numbers of lung DC are upregulated in mice and humans during pulmonary fibrogenesis, and increased mobilisation of lung CD11b^pos DC limits the severity of lung fibrosis in mice. The current study helps to inform the development of DC-based immunotherapy as a novel intervention against lung fibrosis in humans.

A natural killer-dendritic cell axis defines checkpoint therapy-responsive tumor microenvironments

Intratumoral stimulatory dendritic cells (SDCs) play an important role in stimulating cytotoxic T cells and driving immune responses against cancer. Understanding the mechanisms that regulate their abundance in the tumor microenvironment (TME) could unveil new therapeutic opportunities. We find that in human melanoma, SDC abundance is associated with intratumoral expression of the gene encoding the cytokine FLT3LG. FLT3LG is predominantly produced by lymphocytes, notably natural killer (NK) cells in mouse and human tumors. NK cells stably form conjugates with SDCs in the mouse TME, and genetic and cellular ablation of NK cells in mice demonstrates their importance in positively regulating SDC abundance in tumor through production of FLT3L. Although anti-PD-1 'checkpoint' immunotherapy for cancer largely targets T cells, we find that NK cell frequency correlates with protective SDCs in human cancers, with patient responsiveness to anti-PD-1 immunotherapy, and with increased overall survival. Our studies reveal that innate immune SDCs and NK cells cluster together as an excellent prognostic tool for T cell-directed immunotherapy and that these innate cells are necessary for enhanced T cell tumor responses, suggesting this axis as a target for new therapies.

Nlrp3-dependent IL-1β inhibits CD103+ dendritic cell differentiation in the gut

Inflammatory bowel disease (IBD) is associated with enhanced levels of the IL-1 family cytokines IL-1β and IL-18, which are activated by the Nlrp3 inflammasome. Here, we investigated the role of inflammasome-driven cytokine release on T cell polarization and DC differentiation in steady state and T cell transfer colitis. In vitro and in vivo data showed that IL-1β induces Th17 polarization and increases GM‑CSF production by T cells. Reduced IL-1β levels in Nlrp3-/- mice correlated with enhanced FLT3L levels and increased frequency of tolerogenic CD103+ DC. In the T cell transfer colitis model, Nlrp3 deficiency resulted in lower IL‑1β levels, reduced Th17 immunity, and less severe colitis. Unaltered IL-18 levels in both mouse strains pointed toward Nlrp3-independent processing. Importantly, cohousing revealed that the gut microbiome had no impact on the observed Nlrp3-/- phenotype. This study demonstrates that NLRP3 acts as a molecular switch of intestinal homeostasis by shifting local immune cells toward an inflammatory phenotype via IL-1β.

Serum Cytokine Profiling in Cats with Acute Idiopathic Cystitis

BACKGROUND

Feline idiopathic cystitis (FIC) is a common lower urinary tract disorder of domestic cats that resembles interstitial cystitis/painful bladder syndrome (IC/PBS) in humans. Diagnosis of FIC is based on clinical signs and exclusion of other disorders because of a lack of specific pathologic findings or other objective biomarkers. Cytokines are potential noninvasive biomarkers to define the presence, severity, and progression of disease, and response to treatment.

OBJECTIVES

The objective of this pilot study was to determine concentrations of selected cytokines in serum from healthy cats and cats with acute FIC.

ANIMALS

Serum samples from 13 healthy cats and from 12 cats with nonobstructive acute FIC were utilized.

METHODS

Multiplex analysis of 19 cytokines (CCL2, CCL5, CXCL1, CXCL12, CXCL8, Flt3L, GM-CSF, IFN-γ, IL-12 (p40), IL-13, IL-18, IL-1β, IL-2, IL-4, IL-6, PDGF-BB, SCF, sFas, and TNF-α) was performed with a commercially available feline-specific multiplex bead-based assay.

RESULTS

Mean serum concentrations of IL-12 (p40; P < 0.0001), CXCL12 (P = 0.002), IL-18 (P = 0.032), and Flt3L (P = 0.0024) were significantly increased in FIC cats compared to healthy cats. GM-CSF, IL-1b, IL-2, and PDGF-BB were undetectable or detected in an insufficient number of cats to allow meaningful comparisons.

CONCLUSIONS AND CLINICAL IMPORTANCE

We have identified increased serum concentrations of pro-inflammatory cytokines and chemokines CXCL12, IL-12, IL-18, and Flt3L in FIC-affected cats. These findings suggest potential candidates for noninvasive biomarkers for diagnosis, staging, and therapeutic outcome monitoring of affected cats and provide additional insight into the etiopathogenesis of FIC.

High levels of FLT3-ligand in bone marrow and peripheral blood of patients with advanced multiple myeloma

Introduction

Multiple myeloma (MM) is still incurable due to resistance against various therapies. Thus, the identification of biomarkers predicting progression is urgently needed. Here, we evaluated four biomarkers in bone marrow and peripheral blood of MM patients for their prognostic significance.

Materials & methods

Bone marrow- and peripheral blood plasma levels of FLT3-L, soluble TIE2, endostatin, and osteoactivin were determined in patients with monoclonal gammopathy of undetermined significance (MGUS, n = 14/n = 4), patients with newly diagnosed MM (NDMM, n = 42/n = 31) and patients with relapsed/refractory MM (RRMM, n = 27/n = 16) by sandwich ELISA.

Results

Median FLT3-L expression increased from MGUS (58.77 pg/ml in bone marrow; 80.40 pg/ml in peripheral blood) to NDMM (63.15 pg/ml in bone marrow; 85.05 pg/ml in peripheral blood) and was maximal in RRMM (122 pg/ml in bone marrow; 160.47 pg/ml in peripheral blood; NDMM vs. RRMM p<0.001). A cut-off value of FLT3-L >92 pg/ml in bone marrow and >121 pg/ml in peripheral blood was associated with relapse or refractoriness in MM patients. FLT3-L was found to be a high predictive marker for discrimination between NDMM and RRMM as well in bone marrow as in peripheral blood (AUC 0.75 in bone marrow; vs 0.84 in peripheral blood).

Conclusion

High levels of FLT3-L in bone marrow and peripheral blood of MM patients identify patients with progressive disease and are associated with relapse or refractoriness in MM patients. FLT3-L could be useful as a marker to identify RRMM patients and should be evaluated as target for future therapies.

The angiogenic factor Egfl7 alters thymogenesis by activating Flt3 signaling

Thymic regeneration is a crucial function that allows for the generation of mature T cells after myelosuppression like irradiation. However molecular drivers involved in this process remain undefined. Here, we report that the angiogenic factor, epidermal growth factor-like domain 7 (Egfl7), is expressed on steady state thymic endothelial cells (ECs) and further upregulated under stress like post-irradiation. Egfl7 overexpression increased intrathymic early thymic precursors (ETPs) and expanded thymic ECs. Mechanistically, we show that Egfl7 overexpression caused Flt3 upregulation in ETPs and thymic ECs, and increased Flt3 ligand plasma elevation in vivo. Selective Flt3 blockade prevented Egfl7-driven ETP expansion, and Egfl7-mediated thymic EC expansion in vivo. We propose that the angiogenic factor Egfl7 activates the Flt3/Flt3 ligand pathway and is a key molecular driver enforcing thymus progenitor generation and thereby directly linking endothelial cell biology to the production of T cell-based adaptive immunity.

Gene expression markers of Tumor Infiltrating Leukocytes

BACKGROUND

Assays of the abundance of immune cell populations in the tumor microenvironment promise to inform immune oncology research and the choice of immunotherapy for individual patients. We propose to measure the intratumoral abundance of various immune cell populations with gene expression. In contrast to IHC and flow cytometry, gene expression assays yield high information content from a clinically practical workflow. Previous studies of gene expression in purified immune cells have reported hundreds of genes showing enrichment in a single cell type, but the utility of these genes in tumor samples is unknown. We use co-expression patterns in large tumor gene expression datasets to evaluate previously reported candidate cell type marker genes lists, eliminate numerous false positives and identify a subset of high confidence marker genes.

METHODS

Using a novel statistical tool, we use co-expression patterns in 9986 samples from The Cancer Genome Atlas (TCGA) to evaluate previously reported cell type marker genes. We compare immune cell scores derived from these genes to measurements from flow cytometry and immunohistochemistry. We characterize the reproducibility of our cell scores in replicate runs of RNA extracted from FFPE tumor tissue.

RESULTS

We identify a list of 60 marker genes whose expression levels measure 14 immune cell populations. Cell type scores calculated from these genes are concordant with flow cytometry and IHC readings, show high reproducibility in replicate RNA samples from FFPE tissue and enable detailed analyses of the anti-tumor immune response in TCGA. In an immunotherapy dataset, they separate responders and non-responders early on therapy and provide an intricate picture of the effects of checkpoint inhibition. Most genes previously reported to be enriched in a single cell type have co-expression patterns inconsistent with cell type specificity.

CONCLUSIONS

Due to their concise gene set, computational simplicity and utility in tumor samples, these cell type gene signatures may be useful in future discovery research and clinical trials to understand how tumors and therapeutic intervention shape the immune response.

Decitabine and Sorafenib Therapy in FLT-3 ITD-Mutant Acute Myeloid Leukemia

BACKGROUND

Acute myeloid leukemia (AML) characterized by Feline McDonough Sarcoma-like tyrosine kinase-3 (FLT-3) internal tandem duplication (ITD) mutations have poor outcomes. Treatment options are limited, because these mutations confer resistance to conventional chemotherapy. FLT-3 inhibitors such as sorafenib have been studied as a single agent and in combination with conventional chemotherapy or azacytidine with fair responses.

PATIENTS AND METHODS

Here we describe our preclinical and clinical experience with the combination of the DNA hypomethylating agent, decitabine and sorafenib for the treatment of FLT-3 ITD-mutant AML.

RESULTS

In vitro treatment of the human FLT-3 ITD-mutant AML cell line, MV4-11, with both drugs significantly improved growth inhibition over single-agent therapy and resulted in synergistic antitumor effects (combination index < 1). A case series of 6 patients treated with off protocol combination of decitabine and sorafenib demonstrated overall responses in 5 patients (83%) with a median survival of 155 days. Four of the 5 patients (80%) with relapsed/refractory AML achieved complete responses with incomplete count recovery. The combination was also well tolerated.

CONCLUSION

Further investigation is warranted to confirm these responses.

Molecular and Cellular Mechanisms of Myelodysplastic Syndrome: Implications on Targeted Therapy

Myelodysplastic syndrome (MDS) is a group of heterogeneous clonal hematopoietic stem cell disorders characterized by cytopenia, ineffective hematopoiesis, and progression to secondary acute myeloid leukemia in high-risk cases. Conventional prognostication relies on clinicopathological parameters supplemented by cytogenetic information. However, recent studies have shown that genetic aberrations also have critical impacts on treatment outcome. Moreover, these genetic alterations may themselves be a target for treatment. The mutation landscape in MDS is shaped by gene aberrations involved in DNA methylation (TET2, DNMT3A, IDH1/2), histone modification (ASXL1, EZH2), the RNA splicing machinery (SF3B1, SRSF2, ZRSR2, U2AF1/2), transcription (RUNX1, TP53, BCOR, PHF6, NCOR, CEBPA, GATA2), tyrosine kinase receptor signaling (JAK2, MPL, FLT3, GNAS, KIT), RAS pathways (KRAS, NRAS, CBL, NF1, PTPN11), DNA repair (ATM, BRCC3, DLRE1C, FANCL), and cohesion complexes (STAG2, CTCF, SMC1A, RAD21). A detailed understanding of the pathogenetic mechanisms leading to transformation is critical for designing single-agent or combinatorial approaches in target therapy of MDS.

Discovery of Novel Antiangiogenic Marine Natural Product Scaffolds

Marine natural products (MNPs) are recognized for their structural complexity, diversity, and novelty. The vast majority of MNPs are pharmacologically relevant through their ability to modulate macromolecular targets underlying human diseases. Angiogenesis is a fundamental process in cancer progression and metastasis. Targeting angiogenesis through selective modulation of linked protein kinases is a valid strategy to discover novel effective tumor growth and metastasis inhibitors. An in-house marine natural products mini-library, which comprises diverse MNP entities, was submitted to the Lilly’s Open Innovation Drug Discovery platform. Accepted structures were subjected to in vitro screening to discover mechanistically novel angiogenesis inhibitors. Active hits were subjected to additional angiogenesis-targeted kinase profiling. Some natural and semisynthetic MNPs, including multiple members of the macrolide latrunculins, the macrocyclic oxaquinolizidine alkaloid araguspongine C, and the sesquiterpene quinone puupehenone, showed promising results in primary and secondary angiogenesis screening modules. These hits inhibited vascular endothelial growth factor (VEGF)-mediated endothelial tube-like formation, with minimal cytotoxicity at relevant doses. Secondary kinase profiling identified six target protein kinases, all involved in angiogenesis signaling pathways. Molecular modeling and docking experiments aided the understanding of molecular binding interactions, identification of pharmacophoric epitopes, and deriving structure-activity relationships of active hits. Marine natural products are prolific resources for the discovery of chemically and mechanistically unique selective antiangiogenic scaffolds.

Molecularly targeted therapy in acute myeloid leukemia

Acute myeloid leukemia (AML) is molecularly heterogeneous. Formerly categorized cytogenetically and molecularly, AML may be classified by genomic and epigenomic analyses. These genetic lesions provide therapeutic targets. Genes targeted currently include mutated FLT3, NPM1 and KIT with drugs entering Phase III trials. Complete remission can be achieved in relapsed/refractory AML, albeit mostly transient. Mutated epigenetic modifiers, including DNMT3A, IDH1/2 and TET2, can be targeted by small molecule inhibitors, hypomethylating agents and histone deacetylase inhibitors. Other agents include cellular signaling pathway inhibitors and monoclonal antibodies against myeloid-associated antigens. Combinatorial strategies appear logical, mostly involving smaller molecular inhibitors partnering with hypomethylating agents. Currently limited to relapsed/refractory AML, targeted therapies are increasingly tested in frontline treatment with or without standard chemotherapy.

Innate immune regulation by STAT-mediated transcriptional mechanisms

The term innate immunity typically refers to a quick but non-specific host defense response against invading pathogens. The innate immune system comprises particular immune cell populations, epithelial barriers, and numerous secretory mediators including cytokines, chemokines, and defense peptides. Innate immune cells are also now recognized to play important contributing roles in cancer and pathological inflammatory conditions. Innate immunity relies on rapid signal transduction elicited upon pathogen recognition via pattern recognition receptors (PRRs) and cell:cell communication conducted by soluble mediators, including cytokines. A majority of cytokines involved in innate immune signaling use a molecular cascade encompassing receptor-associated Jak protein tyrosine kinases and STAT (signal transducer and activator of transcription) transcriptional regulators. Here, we focus on roles for STAT proteins in three major innate immune subsets: neutrophils, macrophages, and dendritic cells (DCs). While knowledge in this area is only now emerging, understanding the molecular regulation of these cell types is necessary for developing new approaches to treat human disorders such as inflammatory conditions, autoimmunity, and cancer.

Targeting FLT3 to treat leukemia

INTRODUCTION

Approximately 23% of acute myeloid leukemia (AML) patients younger than 60 years of age carry a mutation in the transmembrane domain of the FMS-like tyrosine kinase-3 (FLT3) gene (FLT3/internal tandem duplications [ITD]). In normal karyotype AML, the presence of a FLT3/ITD mutation is associated with poor prognosis, as mirrored by a high risk of relapse even after allogeneic stem cell transplantation. The poor prognostic impact along with the observation that FLT3 is frequently overexpressed in the majority of AML cases has formed the platform for the development of FLT3-targeted strategies. To date, several FLT3 kinase inhibitors have been investigated in preclinical and clinical studies. However, as of yet, none of the studied FLT3 inhibitors has received FDA approval for routine clinical use in AML. This is in part due to the 'off target' effects observed with most inhibitors when administered at concentrations needed to achieve sustained levels of FLT3 inhibition, which are required to exhibit substantial cytotoxic effects against leukemic blasts. Furthermore, the development of resistance mutations has emerged as a clinical issue posing a threat to successful FLT3 inhibitor therapy.

AREAS COVERED

In this review, the authors provide a brief summary of FLT3 inhibitors investigated thus far, and discuss current treatment approaches and strategies how to best incorporate FLT3 tyrosine kinase inhibitors (TKIs) into therapy.

EXPERT OPINION

The combination of a FLT3 inhibitor with conventional chemotherapeutic regimens, epigenetic modifiers or inhibitors of FLT3 downstream and collateral effectors has emerged as a promising strategy to improve treatment outcome. The future of a tailored, molecular-based treatment approach for FLT3-mutated AML demands novel clinical trial concepts based on harmonized and aligned research goals between clinical and research centers and industry.

A phase II study of the oral VEGF receptor tyrosine kinase inhibitor vatalanib (PTK787/ZK222584) in myelodysplastic syndrome: Cancer and Leukemia Group B study 10105 (Alliance)

BACKGROUND

Angiogenesis is implicated in the pathophysiology and progression of myelodysplastic syndromes (MDS). Vatalanib (PTK787/ZK222584; Novartis and Schering AG) inhibits receptor tyrosine kinases of vascular endothelial growth factor, platelet derived growth factor and c-Kit. We examined whether vatalanib induces hematological responses in MDS and/or delays progression to acute myeloid leukemia (AML) or death.

METHODS

Two cohorts were studied. Vatalanib 1250 mg orally was given once daily (cohort 1) or 750-1250 mg once daily in an intra-patient dose escalating schedule (cohort 2) in 28-day cycles to 155 patients with MDS; 142 patients were evaluable for response and 153 for toxicity.

RESULTS

The median age was 70.5 years; 51 % had low risk (International Prognostic Scoring System {IPSS} Low/Intermediate-1) and 32 % had high risk (IPSS Intermediate-2/High) MDS. Hematological improvement was achieved in 7/142 (5 %) patients; all 7 were among the 47 patients able to remain on vatalanib for at least 3 months (hematological improvement achieved in 15 % of these 47 patients). For patients with low risk and high risk MDS, respectively, median progression-free survivals were 15 and 6 months, median times to transformation to AML were 28 and 6 months, and median overall survivals were 36 and 10 months. The most frequent non-hematological adverse events grade ≥ 2 were fatigue, nausea or vomiting, dizziness, anorexia, ataxia, diarrhea, and pain. Two deaths (one intra-cerebral hemorrhage and one sudden death) were possibly related to vatalanib.

CONCLUSIONS

Vatalanib induces improvement in blood counts in a small proportion of MDS patients. Clinical applicability is limited by side effects.

Instructive cytokine signals in dendritic cell lineage commitment

Clarifying the signals that lead to dendritic cell (DC) development and identifying cellular intermediates on their way to DC differentiation are essential steps to understand the dynamic regulation of number, localization, and functionality of these cells. In the past decade, much knowledge on cytokines, transcription factors, and successive progenitors involved in steady-state and demand-adapted DC development was gained. From the stage of multipotent progenitors, DCs are generated from Flt3(+) intermediates, irrespective of lymphoid or myeloid commitment, making fms-related tyrosine kinase 3 ligand one of the major regulators for DC development. Additional key cytokines involved are granulocyte-macrophage colony-stimulating factor (GM-CSF) and M-CSF, with each being essential for particular DC subsets and leading to specific activation of downstream transcription factors. In this review, we seek to draw an integrative view on how instructive cytokine signals acting on intermediate progenitors might lead to the generation of specific DC subsets in steady-state and during inflammation. We hypothesize that the lineage potential of a progenitor might be determined by the set of cytokine receptors expressed that make it responsive to further receive lineage instructive signals. Commitment to a certain lineage might consequently occur when lineage-relevant cytokine receptors are further upregulated and others for alternative lineages are lost. Along this line, we emphasize the role that diverse microenvironments have in influencing the generation of DC subsets with specific functions throughout the body.

Kinetics of plasma FLT3 ligand concentration in hematopoietic stem cell transplanted patients

The present study aimed to follow-up variations in plasma Flt3 ligand (FL) concentration after hematopoietic stem cell transplantation and to compare the influence of conditioning regimens on variations in FL concentration. Ten patients undergoing a conditioning regimen, including BEAM, cyclophosphamide (Cy) + total body irradiation or Cy + anti-thymocyte globulins (ATG), which was then followed by hematopoietic stem cell transplantation, were studied. Plasma FL concentrations, white blood cell (WBC) expression of both FL mRNA and the membrane-bound form of FL were carried out at different times post-treatment. The results indicated that plasma FL concentration increased rapidly after the conditioning regimen in all patients, in correlation with the decrease in number of WBCs. The area under the curve of FL according to time was directly correlated with the duration of pancytopenia, except when ATG was included in the conditioning regimen. Although the number of patients was limited in this study, the comparison of ATG-treated patients and other patients suggests that plasma FL concentration is regulated by a complex mechanism partly involving circulating blood cells.

Cytokine treatment improves parenchymal and vascular damage of salivary glands after irradiation

PURPOSE

During radiotherapy for head and neck cancer, co-irradiation (IR) of salivary glands results in acute and often lifelong hyposalivation. Recently, we showed that bone marrow-derived cells (BMC) can partially facilitate postradiation regeneration of the mouse submandibular gland. In this study, we investigate whether optimized mobilization of BMCs can further facilitate regeneration of radiation-damaged salivary glands.

EXPERIMENTAL DESIGN

Salivary glands of mice reconstituted with eGFP+ bone marrow cells were irradiated with a single dose of 15 Gy. One month later, BMCs were mobilized using granulocyte colony-stimulating factor (G-CSF) or the combination of FMS-like tyrosine kinase-3 ligand, stem cell factor, and G-CSF (termed F/S/G) as mobilizing agents. Salivary gland function and morphology were evaluated at 90 days post-IR by measuring the saliva flow rate, the number of acinar cells, and the functionality of the vasculature.

RESULTS

Compared with G-CSF alone, the combined F/S/G treatment mobilized a 10-fold higher number and different types of BMCs to the bloodstream and increased the number of eGFP+ cells in the irradiated submandibular gland from 49% to 65%. Both treatments reduced radiation-induced hyposalivation from almost nothing in the untreated group to approximately 20% of normal amount. Surprisingly, however, F/S/G treatment resulted in significant less damage to submandibular blood vessels and induced BMC-derived neovascularization.

CONCLUSIONS

Post-IR F/S/G treatment facilitates regeneration of the submandibular gland and ameliorates vascular damage. The latter is partly due to BMCs differentiating in vascular cells but is likely to also result from direct stimulation of existing blood vessel cells.

Ligand-induced Flt3-downregulation modulates cell death associated proteins and enhances chemosensitivity to idarubicin in THP-1 acute myeloid leukemia cells

Sustained ligand stimulation of the receptor tyrosine kinase Flt3 resulted in its downregulation and a refractory signaling phase in primary acute myeloid leukemia (AML) cells and in the AML cell line THP-1. Stable isotope amino acid labeling in cell culture and mass spectrometry were used to compare protein expression patterns in THP-1 before and after Flt3-downregulation. 375 distinct proteins were identified where ATP-dependent RNA helicase DDX3, HNRPU, Matrin-3, Importin-7 and Bax were among the 25 most upregulated proteins and Hausp/UBP7, UBE2N and ERp29 among the 17 most downregulated. THP-1 cells with receptor downregulation were sensitized to idarubicin-induced apoptosis but not cytarabine. We hypothesize that FL-induced receptor modulation may chemosensitize selected AML subsets.

Dendritic cells are abundant in non-lesional gray matter in multiple sclerosis

We have analyzed the localization of dendritic cells (DCs) in non-lesional gray matter (NLGM) in comparison to non-lesional white matter (NLWM) and acute or chronic active multiple sclerosis (MS) lesions. Immunohistochemistry was performed on cryostat sections for DCs markers (CD209, CD205, CD83) and other markers for inflammatory cells (CD68, CD8, CD4, CD3, CCR7, CCR5). We found cells expressing CD209 and containing myelin basic protein in both perivascular and parenchymal areas of NLGM. Our findings showing the expression of CD209(+) cells in NLGM parenchymal areas are surprising relative to the previous literature which reported the presence of CD209(+) DCs only in MS plaque perivascular areas. Although less numerous than CD209(+) cells, NLGM cells expressing mature DCs marker CD205 were consistently detected in perivascular cuffs of most lesions. In double labeling experiments, some but not all of the CD209(+) cells also expressed CD68 and CCR5. We also found CD209(+) cells in close contact with CD3(+) lymphocytes suggesting that DCs might contribute to the local activation of pathogenic T cells in the NLGM. Since injury to the NLGM is one of the key factors associated with disability accumulation, targeting DCs may represent a possible new therapeutic approach in MS to prevent disease progression.

Crucial role of FLT3 ligand in immune reconstitution after bone marrow transplantation and high-dose chemotherapy

Almost 5 decades after the first clinical transplantations, delayed immune reconstitution remains a considerable hurdle in bone marrow transplantation, and the mechanisms regulating immune reconstitution after transplantation remain to be established. Whereas adult fms-like tyrosine kinase 3 ligand-deficient (FL(-/-)) mice have reduced numbers of early B- and T-cell progenitors, they sustain close to normal levels of mature B and T cells. Herein, we demonstrate that adult bone marrow cells fail to reconstitute B-cell progenitors and conventional B cells in lethally irradiated FL(-/-) recipients, which also display delayed kinetics of T-cell reconstitution. Similarly, FL is essential for B-cell regeneration after chemotherapy-induced myeloablation. In contrast, fetal progenitors reconstitute B lymphopoiesis in FL(-/-) mice, albeit at reduced levels. A critical role of FL in adult B lymphopoiesis is further substantiated by an age-progressive decline in peripheral conventional B cells in FL(-/-) mice, whereas fetally and early postnatally derived B1 and marginal zone B cells are sustained in a FL-independent manner. Thus, FL plays a crucial role in sustaining conventional B lymphopoiesis in adult mice and, as a consequence, our findings implicate a critical role of FL in promoting immune reconstitution after myeloablation and bone marrow transplantation.

Dendritic cell transmigration through brain microvessel endothelium is regulated by MIP-1alpha chemokine and matrix metalloproteinases

Dendritic cells (DCs) accumulate in the CNS during inflammatory diseases, but the exact mechanism regulating their traffic into the CNS remains to be defined. We now report that MIP-1alpha increases the transmigration of bone marrow-derived, GFP-labeled DCs across brain microvessel endothelial cell monolayers. Furthermore, occludin, an important element of endothelial tight junctions, is reorganized when DCs migrate across brain capillary endothelial cell monolayers without causing significant changes in the barrier integrity as measured by transendothelial electrical resistance. We show that DCs produce matrix metalloproteinases (MMP) -2 and -9 and GM6001, an MMP inhibitor, decreases both baseline and MIP-1alpha-induced DC transmigration. These observations suggest that DC transmigration across brain endothelial cell monolayers is partly MMP dependent. The migrated DCs express higher levels of CD40, CD80, and CD86 costimulatory molecules and induce T cell proliferation, indicating that the transmigration of DCs across brain endothelial cell monolayers contributes to the maintenance of DC Ag-presenting function. The MMP dependence of DC migration across brain endothelial cell monolayers raises the possibility that MMP blockers may decrease the initiation of T cell recruitment and neuroinflammation in the CNS.

Converging pathways in leukemogenesis and stem cell self-renewal

Studies over the last 40 years have led to an understanding of the hierarchical organization of the hematopoietic system and the role of the pluripotential hematopoietic stem cell. Earlier recognition of the importance of bone marrow hematopoietic microenvironments has evolved into the recognition of specific niches that regulate stem cell pool size, proliferative status, mobilization, and differentiation. The discovery of the role of multiple hematopoietic growth factors and their receptors in the orchestration of stem cell self-renewal and differentiation has been followed by recognition of the importance of the Notch and Wnt pathways. The homeobox family of transcription factors serve as master regulators of development and are increasingly found to be critical regulators of hematopoiesis. In parallel with this understanding of normal hematopoiesis has come a recognition that stem cell dysregulation at various levels is involved in leukemogenesis. Furthermore, the progression from chronic leukemia or myelodysplasia to acute leukemia involves accumulation of at least two mutational events that lead to enhancement of stem cell proliferation, or acquisition of stem cell behavior by a progenitor cell, coupled with maturation inhibition. Translocations resulting in development of oncogenic fusion genes are found in AML and the transforming potential of two of these, AML1-ETO and NUP98-HOXA9, will be discussed. Secondary, constitutively activating mutations of the Flt3 and c-kit receptors and of K- and N-ras are found with high frequency in AML, and the transforming potential of mutated FLT3 and the role of STAT5A activation in human stem cell transformation will be reviewed.

Radiation-induced increase in plasma Flt3 ligand concentration in mice: evidence for the implication of several cell types

Circulating T lymphocytes were proposed as the main producer of Flt3 ligand. However, during aplasia, there is a drastic reduction in the number of T lymphocytes, while plasma Flt3 ligand concentration is increased. This contradiction prompted us to compare variations in plasma Flt3 ligand during radiation-induced aplasia in BALB/c mice and in T-lymphocyte-deficient NOD-SCID mice to delineate the role of T lymphocytes in the increase in Flt3 ligand concentration. The results showed that plasma Flt3 ligand concentration was increased similarly in the two strains of mice, and that Flt3 ligand concentration was negatively correlated to the number of residual hematopoietic progenitors. Moreover, the Flt3 ligand mRNA expression and Flt3 ligand protein concentration were similar in the two strains of mice in all organs tested, i.e. thymus, spleen, bone marrow, liver, brain and blood cells. These results confirm that Flt3 ligand concentration in the blood is a reflection of bone marrow function and that T lymphocytes are not the main regulator of Flt3 ligand variations during aplasia.

The effects and the mechanism of YSEC-CM on the growth of yolk sac hematopoietic stem/progenitor cells

To explore the distinct background of bone marrow and embryonic yolk sac hematopoiesis performance, serum free murine yolk sac endothelial cell and bone marrow endothelial cell conditioned medium were compared for their effects on the development profiles of yolk sac hematopoietic stem/progenitor cells. The mRNAs expression techniques were applied to understand the cytokine and receptor genes expression and the possible mechanisms. The results suggested that the differential gene expressions were existed between the hematopoietic cells of yolk sac and bone marrow and between the microenvironment of yolk sac and bone marrow.

Plasma Flt-3 ligand concentration correlated with radiation-induced bone marrow damage during local fractionated radiotherapy

PURPOSE

To determine whether variations in the plasma Flt-3 ligand (FL) concentration after radiotherapy (RT) may serve as a biomarker for radiation-induced bone marrow damage.

METHODS AND MATERIALS

Twenty-seven patients were followed during RT. The irradiated bone marrow volume was determined. The blood cell counts and plasma FL concentrations were evaluated before and after RT. The expression of membrane-bound FL and mRNA expression were also defined in circulating blood cells.

RESULTS

We found a negative correlation between the plasma FL concentration and the number of circulating white blood cells and platelets during RT. Moreover, the overall amount of FL in the blood of patients during RT correlated directly with both the cumulated radiation dose and the proportion of irradiated bone marrow.

CONCLUSIONS

We demonstrated that the variations in plasma FL concentration directly reflect the radiation-induced bone marrow damage during fractionated local RT. We suggest a possible use for FL monitoring as a means to predict the occurrence of Grade 3-4 leukopenia or thrombocytopenia during the course of RT.

FLT3: ITDoes matter in leukemia

FMS-like tyrosine kinase-3 (FLT3), a receptor tyrosine kinase, is important for the development of the hematopoietic and immune systems. Activating mutations of FLT3 are now recognized as the most common molecular abnormality in acute myeloid leukemia, and FLT3 mutations may play a role in other hematologic malignancies as well. The poor prognosis of patients harboring these mutations renders FLT3 an obvious target of therapy. This review summarizes the data on the molecular biology and clinical impact of FLT3 mutations, as well as the therapeutic potential of several small-molecule FLT3 inhibitors currently in development.

Flt3-ligand induces adhesion of haematopoietic progenitor cells via a very late antigen (VLA)-4- and VLA-5-dependent mechanism

The adhesion of haematopoietic progenitor cells (HPC) to the bone marrow microenvironment is a process regulated by cytokines. In this study, we have shown that flt3-ligand (FL), a growth factor that controls early haematopoiesis, regulated the function and expression of the beta-1 integrins, very late antigen (VLA)-4 and VLA-5 on HPC. The modulation of the adhesiveness of HPC by FL was studied by adhesion assays on umbilical vein endothelial cells (HUVEC). Stimulation by FL induced two peaks of increased adhesiveness of HPC. The first peak was at around 30 min and was mechanistically related to an activation of the beta-1 integrins, mainly VLA-4 and VLA-5. The second peak was at around 12 h and was related to increased expression of VLA-4 and VLA-5. The control of HPC adhesiveness by FL is a previously unreported property of FL that may be important for the homing and the retention of flt3-expressing HPC within the bone marrow microenvironment.

Inhibition of the transforming activity of FLT3 internal tandem duplication mutants from AML patients by a tyrosine kinase inhibitor

FLT3 is a receptor tyrosine kinase that may play a role in a significant proportion of leukemias. In addition to being aberrantly expressed in acute leukemias, activating mutations of the FLT3 gene have been found in patients with AML, myelodysplastic syndrome (MDS) and more rarely, ALL. Internal tandem duplications (ITDs) of the FLT3 gene have been detected in 17-34% of patients with AML and portend a poor prognosis for these patients. FLT3 receptors containing ITD mutations (FLT3/ITDs) are constitutively activated in the absence of FLT3 ligand (FL) stimulation leading to the activation of downstream signaling proteins, including ERK and STAT 5. FLT3 activity, therefore, is a logical target for therapeutic intervention. AG1296 is a tyrosine kinase inhibitor of the tyrphostin class that shows inhibitory activity for wild-type FLT3, in addition to the PDGF and c-KIT receptors. We examined the inhibitory effects of AG1296 on FLT3/ITDs isolated from AML patients in the IL-3-dependent cell line, Ba/F3, as well as in primary leukemia samples from AML patients. Immunoprecipitation and immunoblotting analyses demonstrated that FLT3/ITDs were constitutively phosphorylated in the absence of FL. The auto-phosphorylation of FLT3/ITDs was inhibited by AG1296 with an IC(50) of approximately 1 microM. FLT3/ITDs were associated with constitutive phosphorylation of ERK, STAT 5A, STAT 5B, CBL, VAV and SHP2 in Ba/F3 cells. The phosphorylation of these downstream signaling molecules was suppressed in a dose-responsive fashion by AG1296. AG1296 inhibited IL-3 independent growth and induced apoptosis in Ba/F3 cells transformed by FLT3/ITDs. AG1296 also inhibited FLT3 auto-phosphorylation, and induced a cytotoxic effect, in primary AML cells. These findings suggest that inhibiting the activity of FLT3 may have a therapeutic value in some leukemias expressing FLT3/ITDs.

Serum Flt3 ligand variation as a predictive indicator of hematopoietic stem cell mobilization

Flt3 ligand (FL) is a good indicator of bone marrow (BM) cellularity, having a reciprocal relationship with white blood cell (WBC) count in aplastic anemia and chemotherapy-induced neutropenia. In this report, serum FL level was measured by enzyme-linked immunosorbent assay (ELISA), in 16 consecutive patients undergoing autologous peripheral stem cell transplantation, with an average of 12 selected levels for each patient based on major changes of WBC counts at different procedure stations. We found a significant increase of serum FL level at the WBC nadir after mobilization chemotherapy and a more dramatic increase at the WBC nadir post transplantation, consistent with a more profound BM aplasia after myeloablative chemotherapy as compared to high-dose cyclophosphamide used for mobilization. Hence, we reproduced the reciprocal relationship between serum FL and BM cellularity. A direct correlation between the increase of FL level after mobilization chemotherapy and the length of mobilization was also established, which may help physicians, at the individual patient level, to predict the time of stem cell collection. Finally, we showed a direct correlation between the peripheral CD34+ count at the time of stem cell collection and the peak FL level after transplantation, which can reflect BM stromal cell function. Our results suggest that variation of serum FL level may be used as predictive indicator of hematopoietic stem cell (HSC) mobilization.

Angiogenesis in hematologic malignancies and its clinical implications

Angiogenesis is defined as a neoformation of blood vessels of capillary origin. Hematopoiesis is closely linked with angiogenesis, for they share a common ancestor, the hemangioblast. Although it is well established that growth in solid tumors is dependent on angiogenesis, its role in hematologic malignancies has not yet been clarified. In this review, the direct evidence, ie, increased microvessel density, and the indirect evidence, ie, elevated level of angiogenic factors or overexpression of messenger RNA or protein of angiogenic factors, for and against the role of angiogenesis in the development and progression of hematologic malignancies are presented.

FLT3 ligand can substitute for macrophage colony-stimulating factor in support of osteoclast differentiation and function

Although bone resorption and osteoclast numbers are reduced in osteopetrotic (op/op) mice, osteoclasts are nevertheless present and functional, despite the absence of macrophage colony-stimulating factor (M-CSF). This suggests that alternative factors can partly compensate for the crucial actions of M-CSF in osteoclast induction. It was found that when nonadherent bone marrow cells were incubated in RANKL with Flt3 ligand (FL) without exogenous M-CSF, tartrate-resistance acid phosphatase (TRAP)-positive cells were formed, and bone resorption occurred. Without FL, only macrophagelike TRAP-negative cells were present. Granulocyte-macrophage CSF, stem cell factor, interleukin-3, and vascular endothelial growth factor could not similarly replace the need for M-CSF. TRAP-positive cell induction in FL was not due to synergy with M-CSF produced by the bone marrow cells themselves because FL also enabled their formation from the hemopoietic cells of op/op mice, which lack any M-CSF. FL appeared to substitute for M-CSF by supporting the differentiation of adherent cells that express mRNA for RANK and responsiveness to RANKL. To determine whether FL can account for the compensation for M-CSF deficiency that occurs in vivo, FL signaling was blockaded in op/op mice by the injection of soluble recombinant Flt3. It was found that the soluble receptor induced a substantial decrease in osteoclast number, strongly suggesting that FL is responsible for the partial compensation for M-CSF deficiency that occurs in these mice.

Inhibition of FLT3-mediated transformation by use of a tyrosine kinase inhibitor

FLT3 is a member of the type III receptor tyrosine kinase (RTK) family. These receptors all contain an intrinsic tyrosine kinase domain that is critical to signaling. Aberrant expression of the FLT3 gene has been documented in both adult and childhood leukemias including AML, ALL and CML. In addition, 17-27% of pediatric and adult patients with AML have small internal tandem duplication mutations in FLT3. Patients expressing the mutant form of the receptor have been shown to have a decreased chance for cure. Our previous study, using a constitutively activated FLT3, demonstrated transformation of Ba/F3 cells and leukemic development in an animal model. Thus, there is accumulating evidence for a role for FLT3 in human leukemias. This has prompted us to search for inhibitors of FLT3 as a possible therapeutic approach in these patients. AG1296 is a compound of the tyrphostin class that is known to selectively inhibit the tyrosine kinase activity of the PDGF and KIT receptors. Since FLT3 is a close relative of KIT, we wanted to test the possible inhibitory activity of AG1296 on FLT3. In transfected Ba/F3 cells, AG1296 selectively and potently inhibited autophosphorylation of FL-stimulated wild-type and constitutively activated FLT3. Treatment by AG1296 abolished IL-3-independent proliferation of Ba/F3 cells expressing the constitutively activated FLT3 and thus, reversed the transformation mediated by activated FLT3. Inhibition of FLT3 activity by AG1296 in cells transformed by activated FLT3 resulted in apoptotic cell death, with no deleterious effect on their parental counterparts. Addition of IL-3 rescued the growth of cells expressing activated FLT3 in the presence of AG1296. This demonstrates that the inhibition is specific to the FLT3 pathway in that it leaves the kinases of the IL-3 pathway and other kinases further downstream involved in proliferation intact. Several proteins phosphorylated by the activated FLT3 signaling pathway, including STAT 5A, STAT 5B and CBL, were no longer phosphorylated when these cells were treated with AG1296. The activity against FLT3 suggests a potential therapeutic application for AG1296 or similar drugs in the treatment of leukemias involving deregulated FLT3 tyrosine kinase activity and as a tool for studying the biology of FLT3.

Current treatment practice in immunosuppression

New drugs have recently been added that may eventually replace the two-decade dominance of cyclosporin in solid organ transplantation. This cornerstone of immunosuppression was introduced by Borel [1] and Calne [2] in the mid-70s. In 1989, Starzl et al., after 2 years of preclinical experimentation, introduced tacrolimus (originally designated as FK506 by the Fujisawa Pharmaceutical Company of Japan) as a potent immunosuppressant for liver transplants [3]. Also, in recent years, a variety of novel purine and pyrimidine biosynthesis inhibitors have been tested for transplantation therapy. The leading agent which appears to be replacing the 35-year position occupied by azathioprine is the semi-synthetic morpholinoethyl ester of mycophenolic acid (MPA), mycophenolate mofetil (MMF), introduced by Allison [4] and Sollinger [5], and developed by the Syntex Corporation (now Roche Pharmaceuticals). Others, affecting different intra- or intercellular messages amplifying immunity, are in the pipeline.

Constitutive activation of FLT3 stimulates multiple intracellular signal transducers and results in transformation

Aberrant expression of FLT3 has been found in most cases of B-lineage ALL and AML, and subsets of T cell ALL, CML in blast crisis and CLL. In 20% of patients with AML the receptor has small internal tandem duplications of the juxtamembrane region which appear to contitutively activate the receptor. To investigate whether FLT3 activation could play a role in leukemia, we generated a constitutively activated FLT3 by fusing its cytoplasmic domain to the helix-loop-helix domain of TEL in analogy to the fusion that occurs with TEL-PDGFR in CMML. In vitro translation assays demonstrated oligomerization and intrinsic tyrosine kinase activity of the TEL-FLT3 chimeric receptor. Constitutively activated TEL-FLT3 conferred IL-3 independence and long-term proliferation to transfected Ba/F3 cells. Immunoblot analyses showed that JAK 2, STAT 3, STAT 5a, STAT 5b and CBL were tyrosine-phosphorylated in TEL-FLT3 expressing Ba/F3 cells in the absence of IL-3. These data suggest a possible role for the JAK/STAT pathway in FLT3 signaling. Transplantation of TEL-FLT3 expressing Ba/F3 cells into syngeneic mice caused mortality in all mice by 3 weeks after injection. Histopathologic analysis demonstrated a massive infiltration of mononuclear cells in the liver, spleen and bone marrow. The mimicking of naturally occurring TEL fusions provides an approach to assess aspects of the biology of activated FLT3, or other receptor-type tyrosine kinases (RTKs) in leukemic transformation.

The thin red line: angiogenesis in normal and malignant hematopoiesis

This review describes the current knowledge about cell subsets involved in vasculogenesis (i.e., differentiation of endothelial cells from mesodermal precursors) and angiogenesis (i.e., blood vessel generation from pre-existing vessels), together with recent findings about angiogenesis and antiangiogenic therapies in hematopoietic malignancies such as leukemia, lymphoma, myeloma, and myelodysplastic syndromes.

CD40-ligand stimulates myelopoiesis by regulating flt3-ligand and thrombopoietin production in bone marrow stromal cells

CD40 ligand (CD40L)/CD40 interactions play a central role in T-cell–dependent B-cell activation as previously shown by in vitro studies, the phenotype of CD40L knockout mice and the defective expression of CD40L in patients who have X-linked immunodeficiency with hyper-IgM. The distribution of CD40 in cells other than of myeloid and lymphoid lineages has suggested additional functions for this receptor/ligand couple. Here we show that CD40L stimulates myelopoiesis with a noticeable effect on megakaryocytopoiesis in cocultures of hematopoietic progenitor cells and bone marrow stromal cells. These results suggest a mechanism by which T-cell or platelet-associated or soluble CD40L may regulate myelopoiesis.

CD40-ligand stimulates myelopoiesis by regulating flt3-ligand and thrombopoietin production in bone marrow stromal cells

CD40 ligand (CD40L)/CD40 interactions play a central role in T-cell–dependent B-cell activation as previously shown by in vitro studies, the phenotype of CD40L knockout mice and the defective expression of CD40L in patients who have X-linked immunodeficiency with hyper-IgM. The distribution of CD40 in cells other than of myeloid and lymphoid lineages has suggested additional functions for this receptor/ligand couple. Here we show that CD40L stimulates myelopoiesis with a noticeable effect on megakaryocytopoiesis in cocultures of hematopoietic progenitor cells and bone marrow stromal cells. These results suggest a mechanism by which T-cell or platelet-associated or soluble CD40L may regulate myelopoiesis.