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

The potential of triplet combination therapies for patients with FLT3-ITD -mutated acute myeloid leukemia

INTRODUCTION

Acute myeloid leukemia (AML) encompasses a heterogeneous group of aggressive myeloid malignancies, where FMS-like tyrosine kinase 3 (FLT3) mutations are prevalent, accounting for approximately 25-30% of adult patients. The presence of this mutation is related to a dismal prognosis and high relapse rates. In the lasts years many FLT3 inhibitors have been developed.

AREAS COVERED

This review provides a comprehensive overview of FLT3mut AML, summarizing the state of art of current treatment and available data about combination strategies including an FLT3 inhibitor.

EXPERT OPINION

In addition, the review discusses the emergence of drug resistance and the need for a nuanced approaches in treating patients who are ineligible for or resistant to intensive chemotherapy. Specifically, it explores the historical context of FLT3 inhibitors (FLT3Is) and their impact on treatment outcomes, emphasizing the pivotal role of midostaurin, as well as gilteritinib and quizartinib, and providing detailed insights into ongoing trials exploring the safety and efficacy of novel triplet combinations involving FLT3Is in different AML settings.

Emerging DNA Methylome Targets in FLT3-ITD-Positive Acute Myeloid Leukemia: Combination Therapy with Clinically Approved FLT3 Inhibitors

OPINION STATEMENT

The internal tandem duplication (ITD) mutation of the FMS-like receptor tyrosine kinase 3 (FLT3-ITD) is the most common mutation observed in approximately 30% of acute myeloid leukemia (AML) patients. It represents poor prognosis due to continuous activation of downstream growth-promoting signaling pathways such as STAT5 and PI3K/AKT. Hence, FLT3 is considered an attractive druggable target; selective small FLT3 inhibitors (FLT3Is), such as midostaurin and quizartinib, have been clinically approved. However, patients possess generally poor remission rates and acquired resistance when FLT3I used alone. Various factors in patients could cause these adverse effects including altered epigenetic regulation, causing mainly abnormal gene expression patterns. Epigenetic modifications are required for hematopoietic stem cell (HSC) self-renewal and differentiation; however, critical driver mutations have been identified in genes controlling DNA methylation (such as DNMT3A, TET2, IDH1/2). These regulators cause leukemia pathogenesis and affect disease diagnosis and prognosis when they co-occur with FLT3-ITD mutation. Therefore, understanding the role of different epigenetic alterations in FLT3-ITD AML pathogenesis and how they modulate FLT3I's activity is important to rationalize combinational treatment approaches including FLT3Is and modulators of methylation regulators or pathways. Data from ongoing pre-clinical and clinical studies will further precisely define the potential use of epigenetic therapy together with FLT3Is especially after characterized patients' mutational status in terms of FLT3 and DNA methlome regulators.

Update on Small Molecule Targeted Therapies for Acute Myeloid Leukemia

OPINION STATEMENT

The search for effective therapies for the highly heterogenous disease acute myeloid leukemia (AML) has remained elusive. While cytotoxic therapies can induce complete remission and even, at times, long-term survival, this approach is associated with significant toxic effects to visceral organs and worsening of immune dysfunction and marrow suppression leading to death. Sophisticated molecular studies have revealed defects within the AML cell that can be exploited by utilizing small molecule agents to target these defects, often dubbed "target therapy." Several medications have already established new standards of care for many patients with AML, including FDA-approved agents that inhibitor IDH1, IDH2, FLT3, and BCL-2. Emerging small molecules hold additional to add to the armamentarium of AML treatment options including MCL-1 inhibitors, TP53 inhibitors, menin inhibitors, and E-selectin antagonists. Moreover, the increasing options also mean that future combinations of these agents need to be explored, including with cytotoxic drugs and other newer emerging strategies such as immunotherapies for AML. Recent investigations continue to show that overcoming many of the challenges of treating AML finally is on the horizon.

In Vitro Human Haematopoietic Stem Cell Expansion and Differentiation

The haematopoietic system plays an essential role in our health and survival. It is comprised of a range of mature blood and immune cell types, including oxygen-carrying erythrocytes, platelet-producing megakaryocytes and infection-fighting myeloid and lymphoid cells. Self-renewing multipotent haematopoietic stem cells (HSCs) and a range of intermediate haematopoietic progenitor cell types differentiate into these mature cell types to continuously support haematopoietic system homeostasis throughout life. This process of haematopoiesis is tightly regulated in vivo and primarily takes place in the bone marrow. Over the years, a range of in vitro culture systems have been developed, either to expand haematopoietic stem and progenitor cells or to differentiate them into the various haematopoietic lineages, based on the use of recombinant cytokines, co-culture systems and/or small molecules. These approaches provide important tractable models to study human haematopoiesis in vitro. Additionally, haematopoietic cell culture systems are being developed and clinical tested as a source of cell products for transplantation and transfusion medicine. This review discusses the in vitro culture protocols for human HSC expansion and differentiation, and summarises the key factors involved in these biological processes.

Activation of FMS-like tyrosine kinase 3 protects against isoprenaline-induced cardiac hypertrophy by improving autophagy and mitochondrial dynamics

FMS-like receptor tyrosine kinase 3 (Flt3) expression was reported to increase in the heart in response to pathological stress, but the role of Flt3 activation and its underlying mechanisms remain poorly elucidated. This study was designed to investigate the role of Flt3 activation in sympathetic hyperactivity-induced cardiac hypertrophy and its mechanisms through autophagy and mitochondrial dynamics. In vivo, cardiac hypertrophy was established by subcutaneous injection of isoprenaline (6 mg/kg·day) in C57BL/6 mice for 7 consecutive days. The Flt3-ligand intervention was launched 2 h prior to isoprenaline each day. In vitro, experiments of cardiomyocyte hypertrophy, autophagy, and mitochondrial dynamics were performed in neonatal rat cardiomyocytes (NRCMs). Our results revealed that the expression level of Flt3 protein was significantly increased in the hypertrophic myocardium provoked by isoprenaline administration. Flt3-ligand intervention alleviated isoprenaline-induced cardiac oxidative stress, hypertrophy, fibrosis, and contractile dysfunction. Isoprenaline stimulation impaired autophagic flux in hypertrophic mouse hearts, supported by the accumulation of LC3II and P62 proteins, while Flt3-ligand restored the impairment of autophagic flux. Flt3 activation normalized the imbalance of mitochondrial fission and fusion in the hearts of mice evoked by isoprenaline as evidenced by the neutralization of elevated mitochondrial fission markers and reduced mitochondrial fusion markers. In NRCMs, Flt3-ligand treatment attenuated isoprenaline-stimulated hypertrophy, which was abolished by a Flt3-specific blocker AC220. Activating Flt3 reversed isoprenaline-induced autophagosome accumulation and impairment of autophagic flux probably by enhancing SIRT1 expression and consequently TFEB nuclear translocation. Flt3 activation improved the imbalance of mitochondrial dynamics induced by isoprenaline in NRCMs through the SIRT1/P53 pathway. Activation of Flt3 mitigated ISO-stimulated hypertrophy probably involves the restoration of autophagic flux and balance of mitochondrial dynamics. Therefore, activation of Flt3 attenuates isoprenaline-induced cardiac hypertrophy in vivo and in vitro, the potential mechanism probably attributes to SIRT1/TFEB-mediated autophagy promotion and SIRT1/P53-mediated mitochondrial dynamics balance. These findings suggest that activation of Flt3 may be a novel target for protection against cardiac remodeling and heart failure during sympathetic hyperactivity.

Therapeutic Targeting of FLT3 in Acute Myeloid Leukemia: Current Status and Novel Approaches

FMS-like tyrosine kinase 3 (FLT3) is mutated in approximately 30% of acute myeloid leukemia (AML) patients. The presence of FLT3-ITD (internal tandem duplication, 20-25%) mutation and, to a lesser extent, FLT3-TKD (tyrosine kinase domain, 5-10%) mutation is associated with poorer diagnosis and therapy response since the leukemic cells become hyperproliferative and resistant to apoptosis after continuous activation of FLT3 signaling. Targeting FLT3 has been the focus of many pre-clinical and clinical studies. Hence, many small-molecule FLT3 inhibitors (FLT3is) have been developed, some of which are approved such as midostaurin and gilteritinib to be used in different clinical settings, either in combination with chemotherapy or alone. However, many questions regarding the best treatment strategy remain to be answered. On the other hand, various FLT3-dependent and -independent resistance mechanisms could be evolved during FLT3i therapy which limit their clinical impact. Therefore, identifying molecular mechanisms of resistance and developing novel strategies to overcome this obstacle is a current interest in the field. In this review, recent studies of approved FLT3i and knowledge about major resistance mechanisms of clinically approved FLT3i's will be discussed together with novel treatment approaches such as designing novel FLT3i and dual FLT3i and combination strategies including approved FLT3i plus small-molecule agents targeting altered molecules in the resistant cells to abrogate resistance. Moreover, how to choose an appropriate FLT3i for the patients will be summarized based on what is currently known from available clinical data. In addition, strategies beyond FLT3i's including immunotherapeutics, small-molecule FLT3 degraders, and flavonoids will be summarized to highlight potential alternatives in FLT3-mutated AML therapy.

Molecular Classification and Overcoming Therapy Resistance for Acute Myeloid Leukemia with Adverse Genetic Factors

The European LeukemiaNet (ELN) criteria define the adverse genetic factors of acute myeloid leukemia (AML). AML with adverse genetic factors uniformly shows resistance to standard chemotherapy and is associated with poor prognosis. Here, we focus on the biological background and real-world etiology of these adverse genetic factors and then describe a strategy to overcome the clinical disadvantages in terms of targeting pivotal molecular mechanisms. Different adverse genetic factors often rely on common pathways. KMT2A rearrangement, DEK-NUP214 fusion, and NPM1 mutation are associated with the upregulation of HOX genes. The dominant tyrosine kinase activity of the mutant FLT3 or BCR-ABL1 fusion proteins is transduced by the AKT-mTOR, MAPK-ERK, and STAT5 pathways. Concurrent mutations of ASXL1 and RUNX1 are associated with activated AKT. Both TP53 mutation and mis-expressed MECOM are related to impaired apoptosis. Clinical data suggest that adverse genetic factors can be found in at least one in eight AML patients and appear to accumulate in relapsed/refractory cases. TP53 mutation is associated with particularly poor prognosis. Molecular-targeted therapies focusing on specific genomic abnormalities, such as FLT3, KMT2A, and TP53, have been developed and have demonstrated promising results.

Target Therapy for Extramedullary Relapse of FLT3-ITD Acute Myeloid Leukemia: Emerging Data from the Field

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase family member. Mutations in FLT3, as well known, represent the most common genomic alteration in acute myeloid leukemia (AML), identified in approximately one-third of newly diagnosed adult patients. In recent years, this has represented an important therapeutic target. Drugs such as midostaurin, gilteritinib, and sorafenib, either alone in association with conventional chemotherapy, play a pivotal role in AML therapy with the mutated FLT3 gene. A current challenge lies in treating forms of AML with extramedullary localization. Here, we describe the general features of myeloid sarcoma and the ability of a targeted drug, i.e., gilteritinib, approved for relapsed or refractory disease, to induce remission of these extramedullary leukemic localizations in AML patients with FLT3 mutation, analyzing how in the literature, there is an important development of cases describing this promising potential for care.

Large-scale serum analysis identifies unique systemic biomarkers in psoriasis and hidradenitis suppurativa

BACKGROUND

Hidradenitis suppurativa (HS) is now recognized as a systemic inflammatory disease, sharing molecular similarities with psoriasis. Direct comparison of the systemic inflammation in HS with psoriasis is lacking.

OBJECTIVES

To evaluate the serum proteome of HS and psoriasis, and to identify biomarkers associated with disease severity.

METHODS

In this cross-sectional study, 1536 serum proteins were assessed using the Olink Explore (Proximity Extension Assay) high-throughput panel in patients with moderate-to-severe HS (n = 11), patients with psoriasis (n = 10) and age- and body mass index-matched healthy controls (n = 10).

RESULTS

HS displayed an overall greater dysregulation of circulating proteins, with 434 differentially expressed proteins (absolute fold change ≥ 1·2; P ≤ 0·05) in patients with HS vs. controls, 138 in patients with psoriasis vs. controls and 503 between patients with HS and patients with psoriasis. Interleukin (IL)-17A levels and T helper (Th)1/Th17 pathway enrichment were comparable between diseases, while HS presented greater tumour necrosis factor- and IL-1β-related signalling. The Th17-associated markers peptidase inhibitor 3 (PI3) and lipocalin 2 (LCN2) were able to differentiate psoriasis from HS accurately. Both diseases presented increases of atherosclerosis-related proteins. Robust correlations between clinical severity scores and immune and atherosclerosis-related proteins were observed across both diseases.

CONCLUSIONS

HS and psoriasis share significant Th1/Th17 enrichment and upregulation of atherosclerosis-related proteins. Despite the greater body surface area involved in psoriasis, HS presents a greater serum inflammatory burden.

A review of FLT3 inhibitors in acute myeloid leukemia

FLT3 mutations are the most common genetic aberrations found in acute myeloid leukemia (AML) and associated with poor prognosis. Since the discovery of FLT3 mutations and their prognostic implications, multiple FLT3-targeted molecules have been evaluated. Midostaurin is approved in the U.S. and Europe for newly diagnosed FLT3 mutated AML in combination with standard induction and consolidation chemotherapy based on data from the RATIFY study. Gilteritinib is approved for relapsed or refractory FLT3 mutated AML as monotherapy based on the ADMIRAL study. Although significant progress has been made in the treatment of AML with FLT3-targeting, many challenges remain. Several drug resistance mechanisms have been identified, including clonal selection, stromal protection, FLT3-associated mutations, and off-target mutations. The benefit of FLT3 inhibitor maintenance therapy, either post-chemotherapy or post-transplant, remains controversial, although several studies are ongoing.

Emerging Targeted Therapy for Specific Genomic Abnormalities in Acute Myeloid Leukemia

We describe recent updates of existing molecular-targeting agents and emerging novel gene-specific strategies. FLT3 and IDH inhibitors are being tested in combination with conventional chemotherapy for both medically fit patients and patients who are ineligible for intensive therapy. FLT3 inhibitors combined with non-cytotoxic agents, such as BCL-2 inhibitors, have potential therapeutic applicability. The menin-MLL complex pathway is an emerging therapeutic target. The pathway accounts for the leukemogenesis in AML with MLL-rearrangement, NPM1 mutation, and NUP98 fusion genes. Potent menin-MLL inhibitors have demonstrated promising anti-leukemic effects in preclinical studies. The downstream signaling molecule SYK represents an additional target. However, the TP53 mutation continues to remain a challenge. While the p53 stabilizer APR-246 in combination with azacitidine failed to show superiority compared to azacitidine monotherapy in a phase 3 trial, next-generation p53 stabilizers are now under development. Among a number of non-canonical approaches to TP53-mutated AML, the anti-CD47 antibody magrolimab in combination with azacitidine showed promising results in a phase 1b trial. Further, the efficacy was somewhat better in patients with the TP53 mutation. Although clinical evidence has not been accumulated sufficiently, targeting activating KIT mutations and RAS pathway-related molecules can be a future therapeutic strategy.

Efficacy and Synergy of Small Molecule Inhibitors Targeting FLT3-ITD+ Acute Myeloid Leukemia

Constitutive activation of FLT3 by ITD mutations is one of the most common genetic aberrations in AML, present in ~1/3 of cases. Patients harboring FLT3-ITD display worse clinical outcomes. The integration and advancement of FLT3 TKI in AML treatment provided significant therapeutic improvement. However, due to the emergence of resistance mechanisms, FLT3-ITD+ AML remains a clinical challenge. We performed an unbiased drug screen to identify 18 compounds as particularly efficacious against FLT3-ITD+ AML. Among these, we characterized two investigational compounds, WS6 and ispinesib, and two approved drugs, ponatinib and cabozantinib, in depth. We found that WS6, although not yet investigated in oncology, shows a similar mechanism and potency as ponatinib and cabozantinib. Interestingly, ispinesib and cabozantinib prevent activation of AXL, a key driver and mechanism of drug resistance in FLT3-ITD+ AML patients. We further investigated synergies between the selected compounds and found that combination treatment with ispinesib and cabozantinib or ponatinib shows high synergy in FLT3-ITD+ AML cell lines and patient samples. Together, we suggest WS6, ispinesib, ponatinib and cabozantinib as novel options for targeting FLT3-ITD+ AML. Whether combinatorial tyrosine kinase and kinesin spindle blockade is effective in eradicating neoplastic (stem) cells in FLT3-ITD+ AML remains to be determined in clinical trials.

Immune Evasion Mechanism and AXL

Extensive interest in cancer immunotherapy is reported according to the clinical importance of CTLA-4 and (PD-1/PD-L1) [programmed death (PD) and programmed death-ligand (PD-L1)] in immune checkpoint therapies. AXL is a receptor tyrosine kinase expressed in different types of cancer and in relation to resistance against various anticancer therapeutics due to poor clinical prognosis. AXL and its ligand, i.e., growth arrest-specific 6 (GAS6) proteins, are expressed on many cancer cells, and the GAS6/AXL pathway is reported to promote cancer cell proliferation, survival, migration, invasion, angiogenesis, and immune evasion. AXL is an attractive and novel therapeutic target for impairing tumor progression from immune cell contracts in the tumor microenvironment. The GAS6/AXL pathway is also of interest immunologically because it targets fewer antitumor immune responses. In effect, several targeted therapies are selective and nonselective for AXL, which are in preclinical and clinical development in multiple cancer types. Therefore, this review focuses on the role of the GAS6/AXL signaling pathway in triggering the immunosuppressive tumor microenvironment as immune evasion. This includes regulating its composition and activating T-cell exclusion with the immune-suppressive activity of regulatory T cells, which is related to one of the hallmarks of cancer survival. Finally, this article discusses the GAS6/AXL signaling pathway in the context of several immune responses such as NK cell activation, apoptosis, and tumor-specific immunity, especially PD-1/PDL-1 signaling.

Dendritic cell Flt3 - regulation, roles and repercussions for immunotherapy

Dendritic cells (DCs) are essential for initiating immune responses. Depending on the environment, the type of DC and the way in which they interact with T cells, these immune responses can be beneficial or detrimental. DCs can be exploited as cellular vectors for vaccines against infection and cancer. The development and maintenance of DCs is dependent on the FMS-like tyrosine kinase 3 (Flt3)/Flt3 ligand (Flt3L) signaling cascade. Flt3 is also one of the most commonly mutated genes in acute myeloid leukemia and as such represents an attractive drug target. In this review, Flt3 is discussed with a particular focus on DCs. We detail the lifecycle of Flt3, from transcription to degradation, and interrogate recent studies as to how this pathway can be manipulated for immunotherapy, vaccination and treatment of autoimmune disease.

FMS-Related Tyrosine Kinase 3 Ligand Promotes Radioresistance in Esophageal Squamous Cell Carcinoma

Aim: The FMS-related tyrosine kinase 3 ligand (FL) has an important role in regulating FMS-related tyrosine kinase 3 (Flt-3) activity. Serum FL levels are markedly increased among patients with hematopoietic disease. However, its role in radiation treatment remains unclear. In this study, we investigated the effects of FL on radiotherapy for esophageal squamous cell carcinoma (ESCC).

Methods: KYSE150 and KYSE450 cells were stimulated with FL (200 ng/ml). mRNA expression was analyzed using qRT-PCR. Cell viability was checked using CCK-8 assay kits. Proliferation was determined using the EdU assay. Radiosensitivity was detected through a colony-forming assay. Flow cytometry was used to evaluate cell apoptosis. The number of γH2AX foci was verified using an immunofluorescence assay. The change in relative proteins was determined by western blot analysis. The growth of transplanted tumors was demonstrated in nude mice.

Results: Our results showed that FL increased the radiation resistance of ESCC cells by promoting clone formation, increasing EdU incorporation, enhancing DNA damage repair, and inhibiting apoptosis. Moreover, the Flt-3 receptor expression significantly increased in ESCC cells after radiation, which may have been an important factor in their radioresistance.

Conclusion: Our results suggest that FL increases the radioresistance of esophageal cancer cells and that FL-Flt-3 could be a potential target for enhancing radiosensitivity in ESCC.

Isatin-Hydrazones with Multiple Receptor Tyrosine Kinases (RTKs) Inhibitory Activity and In-Silico Binding Mechanism

Recently, we have reported a series of isatin hydrazone, two of them, namely, 3-((2,6-dichlorobenzylidene)hydrazono)indolin-2-one (1) and 3-((2-chloro-6-fluorobenzylidene)hydrazono)indolin-2-one (2) having potent cytotoxicity, showing cyclin-dependent kinases (CDK2) inhibitory activity and bearing recommended drug likeness properties. Since both compounds (1 and 2) showed inhibitory activity against CDK2, we assumed it would also have multiple receptor tyrosine kinases (RTKs) inhibitory activity. Considering those points, here, above-mentioned two isatin hydrazone 1 and 2 were synthesized using previously reported method for further investigation of their potency on RTKs (EGFR, VEGFR-2 and FLT-3) inhibitory activity. As expected, Compound 1 exhibited excellent inhibitory activity against epidermal growth factor receptor (EGFR, IC50 = 0.269 µM), vascular epidermal growth factor receptor 2 (VEGFR-2, IC50 = 0.232 µM) and FMS-like tyrosine kinase-3 (FLT-3, IC50 = 1.535 µM) tyrosine kinases. On the other hand, Compound 2 also exhibited excellent inhibitory activity against EGFR (IC50 = 0.369 µM), VEGFR-2 (IC50 = 0.266 µM) and FLT-3 (IC50 = 0.546 µM) tyrosine kinases. A molecular docking study with EGFR, VEGFR-2 and FLT-3 kinase suggested that both compounds act as type I ATP competitive inhibitors against EGFR and VEGFR-2, and type II ATP non-competitive inhibitors against FLT-3.

The Flt3L/Flt3 Axis in Dendritic Cell Biology and Cancer Immunotherapy

Dendritic cells (DCs) prime anti-tumor T cell responses in tumor-draining lymph nodes and can restimulate T effector responses in the tumor site. Thus, in addition to unleashing T cell effector activity, current immunotherapies should be directed to boost DC function. Herein, we review the potential function of Flt3L as a tool for cancer immunotherapy. Flt3L is a growth factor that acts in Flt3-expressing multipotent progenitors and common lymphoid progenitors. Despite the broad expression of Flt3 in the hematopoietic progenitors, the main effect of the Flt3/Flt3L axis, revealed by the characterization of mice deficient in these genes, is the generation of conventional DCs (cDCs) and plasmacytoid DCs (pDCs). However, Flt3 signaling through PI3K and mTOR may also affect the function of mature DCs. We recapitulate the use of Flt3L in preclinical studies either as a single agent or in combination with other cancer therapies. We also analyze the use of Flt3L in clinical trials. The strong correlation between type 1 cDC (cDC1) infiltration of human cancers with overall survival in many cancer types suggests the potential use of Flt3L to boost expansion of this DC subset. However, this may need the combination of Flt3L with other immunomodulatory agents to boost cancer immunotherapy.

Megakaryocytic Expansion in Gilteritinib-Treated Acute Myeloid Leukemia Patients Is Associated With AXL Inhibition

Numerous recurrent genetic mutations are known to occur in acute myeloid leukemia (AML). Among these common mutations, Fms-like tyrosine kinase 3 remains as one of the most frequently mutated genes in AML. We observed apparent marrow expansion of megakaryocytes in three out of six patients with Flt3-mutated AML following treatment with a recently FDA-approved Flt3 inhibitor, gilteritinib which possesses activity against internal tandem duplication and tyrosine kinase domain Flt3 mutations and also inhibits tyrosine kinase AXL. To assess whether biopsy findings can be attributed to promotion of megakaryocytic (Mk) differentiation with gilteritinib, we devised a cellular assay by overexpressing double mutated Flt3-ITD^Y591F/Y919F in chronic myeloid leukemia cell line K562 to study Mk differentiation in the presence of Flt3 and AXL inhibitors with non-mutually exclusive mechanisms. These experiments demonstrated the lack of direct effect Flt3 inhibitors gilteritinib and quizartinib on megakaryocytic differentiation at either transcriptional or phenotypic levels, and highlighted antileukemic effects of AXL receptor tyrosine kinase inhibitor and its potential role in megakaryocytic development.

Towards Mimicking the Fetal Liver Niche: The Influence of Elasticity and Oxygen Tension on Hematopoietic Stem/Progenitor Cells Cultured in 3D Fibrin Hydrogels

Hematopoietic stem/progenitor cells (HSPCs) are responsible for the generation of blood cells throughout life. It is believed that, in addition to soluble cytokines and niche cells, biophysical cues like elasticity and oxygen tension are responsible for the orchestration of stem cell fate. Although several studies have examined the effects of bone marrow (BM) niche elasticity on HSPC behavior, no study has yet investigated the effects of the elasticity of other niche sites like the fetal liver (FL), where HSPCs expand more extensively. In this study, we evaluated the effect of matrix stiffness values similar to those of the FL on BM-derived HSPC expansion. We first characterized the elastic modulus of murine FL tissue at embryonic day E14.5. Fibrin hydrogels with similar stiffness values as the FL (soft hydrogels) were compared with stiffer fibrin hydrogels (hard hydrogels) and with suspension culture. We evaluated the expansion of total nucleated cells (TNCs), Lin⁻/cKit⁺ cells, HSPCs (Lin⁻/Sca⁺/cKit⁺ (LSK) cells), and hematopoietic stem cells (HSCs: LSK- Signaling Lymphocyte Activated Molecule (LSK-SLAM) cells) when cultured in 5% O₂ (hypoxia) or in normoxia. After 10 days, there was a significant expansion of TNCs and LSK cells in all culture conditions at both levels of oxygen tension. LSK cells expanded more in suspension culture than in both fibrin hydrogels, whereas TNCs expanded more in suspension culture and in soft hydrogels than in hard hydrogels, particularly in normoxia. The number of LSK-SLAM cells was maintained in suspension culture and in the soft hydrogels but not in the hard hydrogels. Our results indicate that both suspension culture and fibrin hydrogels allow for the expansion of HSPCs and more differentiated progeny whereas stiff environments may compromise LSK-SLAM cell expansion. This suggests that further research using softer hydrogels with stiffness values closer to the FL niche is warranted.

Mechanisms Underlying Resistance to FLT3 Inhibitors in Acute Myeloid Leukemia

FLT3-ITD and FLT3-TKD mutations were observed in approximately 20 and 10% of acute myeloid leukemia (AML) cases, respectively. FLT3 inhibitors such as midostaurin, gilteritinib and quizartinib show excellent response rates in patients with FLT3-mutated AML, but its duration of response may not be sufficient yet. The majority of cases gain secondary resistance either by on-target and off-target abnormalities. On-target mutations (i.e., FLT3-TKD) such as D835Y keep the TK domain in its active form, abrogating pharmacodynamics of type II FLT3 inhibitors (e.g., midostaurin and quizartinib). Second generation type I inhibitors such as gilteritinib are consistently active against FLT3-TKD as well as FLT3-ITD. However, a “gatekeeper” mutation F691L shows universal resistance to all currently available FLT3 inhibitors. Off-target abnormalities are consisted with a variety of somatic mutations such as NRAS, AXL and PIM1 that bypass or reinforce FLT3 signaling. Off-target mutations can occur just in the primary FLT3-mutated clone or be gained by the evolution of other clones. A small number of cases show primary resistance by an FL-dependent, FGF2-dependent, and stromal CYP3A4-mediated manner. To overcome these mechanisms, the development of novel agents such as covalently-coupling FLT3 inhibitor FF-10101 and the investigation of combination therapy with different class agents are now ongoing. Along with novel agents, gene sequencing may improve clinical approaches by detecting additional targetable mutations and determining individual patterns of clonal evolution.

Dasatinib overcomes stroma-based resistance to the FLT3 inhibitor quizartinib using multiple mechanisms

FLT3-ITD mutations occur in 20–30% of AML patients and are associated with aggressive disease. Patients with relapsed FLT3-mutated disease respond well to 2^nd generation FLT3 TKIs but inevitably relapse within a short timeframe. In this setting, until overt relapse occurs, the bone marrow microenvironment facilitates leukemia cell survival despite continued on-target inhibition. We demonstrate that human bone marrow derived conditioned medium (CM) protects FLT3-ITD⁺ AML cells from the 2^nd generation FLT3 TKI quizartinib and activates STAT3 and STAT5 in leukemia cells. Extrinsic activation of STAT5 by CM is the primary mediator of leukemia cell resistance to FLT3 inhibition. Combination treatment with quizartinib and dasatinib abolishes STAT5 activation and significantly reduces the IC₅₀ of quizartinib in FLT3-ITD⁺ AML cells cultured in CM. We demonstrate that CM protects FLT3-ITD⁺ AML cells from the inhibitory effects of quizartinib on glycolysis and that this is partially reversed by treating cells with the combination of quizartinib and dasatinib. Using a doxycycline-inducible STAT5 knockdown in the FLT3-ITD⁺ MOLM-13 cell line, we show that dasatinib-mediated suppression of leukemia cell glycolytic activity is STAT5-independent and provide a preclinical rationale for combination treatment with quizartinib and dasatinib in FLT3-ITD⁺ AML.

FLT3 mutations in acute myeloid leukemia: Therapeutic paradigm beyond inhibitor development

FMS-like tyrosine kinase 3 (FLT3) is a type III receptor tyrosine kinase that plays an important role in hematopoietic cell survival, proliferation and differentiation. The most clinically important point is that mutation of the FLT3 gene is the most frequent genetic alteration and a poor prognostic factor in acute myeloid leukemia (AML) patients. There are two major types of FLT3 mutations: internal tandem duplication mutations in the juxtamembrane domain (FLT3-ITD) and point mutations or deletion in the tyrosine kinase domain (FLT3-TKD). Both mutant FLT3 molecules are activated through ligand-independent dimerization and trans-phosphorylation. Mutant FLT3 induces the activation of multiple intracellular signaling pathways, mainly STAT5, MAPK and AKT signals, leading to cell proliferation and anti-apoptosis. Because high-dose chemotherapy and allogeneic hematopoietic stem cell transplantation cannot sufficiently improve the prognosis, clinical development of FLT3 kinase inhibitors expected. Although several FLT3 inhibitors have been developed, it takes more than 20 years from the first identification of FLT3 mutations until FLT3 inhibitors become clinically available for AML patients with FLT3 mutations. To date, three FLT3 inhibitors have been clinically approved as monotherapy or combination therapy with conventional chemotherapeutic agents in Japan and/or Europe and United states. However, several mechanisms of resistance to FLT3 inhibitors have already become apparent during their clinical trials. The resistance mechanisms are complex and emerging resistant clones are heterogenous. Further basic and clinical studies are required to establish the best therapeutic strategy for AML patients with FLT3 mutations.

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.

Aptamer-based search for correlates of plasma and serum water T2: implications for early metabolic dysregulation and metabolic syndrome

Background

Metabolic syndrome is a cluster of abnormalities that increases the risk for type 2 diabetes and atherosclerosis. Plasma and serum water T₂ from benchtop nuclear magnetic resonance relaxometry are early, global and practical biomarkers for metabolic syndrome and its underlying abnormalities. In a prior study, water T₂ was analyzed against ~ 130 strategically selected proteins and metabolites to identify associations with insulin resistance, inflammation and dyslipidemia. In the current study, the analysis was broadened ten-fold using a modified aptamer (SOMAmer) library, enabling an unbiased search for new proteins correlated with water T₂ and thus, metabolic health.

Methods

Water T₂ measurements were recorded using fasting plasma and serum from non-diabetic human subjects. In parallel, plasma samples were analyzed using a SOMAscan assay that employed modified DNA aptamers to determine the relative concentrations of 1310 proteins. A multi-step statistical analysis was performed to identify the biomarkers most predictive of water T₂. The steps included Spearman rank correlation, followed by principal components analysis with variable clustering, random forests for biomarker selection, and regression trees for biomarker ranking.

Results

The multi-step analysis unveiled five new proteins most predictive of water T₂: hepatocyte growth factor, receptor tyrosine kinase FLT3, bone sialoprotein 2, glucokinase regulatory protein and endothelial cell-specific molecule 1. Three of the five strongest predictors of water T₂ have been previously implicated in cardiometabolic diseases. Hepatocyte growth factor has been associated with incident type 2 diabetes, and endothelial cell specific molecule 1, with atherosclerosis in subjects with diabetes. Glucokinase regulatory protein plays a critical role in hepatic glucose uptake and metabolism and is a drug target for type 2 diabetes. By contrast, receptor tyrosine kinase FLT3 and bone sialoprotein 2 have not been previously associated with metabolic conditions. In addition to the five most predictive biomarkers, the analysis unveiled other strong correlates of water T₂ that would not have been identified in a hypothesis-driven biomarker search.

Conclusions

The identification of new proteins associated with water T₂ demonstrates the value of this approach to biomarker discovery. It provides new insights into the metabolic significance of water T₂ and the pathophysiology of metabolic syndrome.

Electronic supplementary material

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Inhibition of neuronal FLT3 receptor tyrosine kinase alleviates peripheral neuropathic pain in mice

Peripheral neuropathic pain (PNP) is a debilitating and intractable chronic disease, for which sensitization of somatosensory neurons present in dorsal root ganglia that project to the dorsal spinal cord is a key physiopathological process. Here, we show that hematopoietic cells present at the nerve injury site express the cytokine FL, the ligand of fms-like tyrosine kinase 3 receptor (FLT3). FLT3 activation by intra-sciatic nerve injection of FL is sufficient to produce pain hypersensitivity, activate PNP-associated gene expression and generate short-term and long-term sensitization of sensory neurons. Nerve injury-induced PNP symptoms and associated-molecular changes were strongly altered in Flt3-deficient mice or reversed after neuronal FLT3 downregulation in wild-type mice. A first-in-class FLT3 negative allosteric modulator, discovered by structure-based in silico screening, strongly reduced nerve injury-induced sensory hypersensitivity, but had no effect on nociception in non-injured animals. Collectively, our data suggest a new and specific therapeutic approach for PNP.

Sensitisation of dorsal root ganglia neurons contributes to neuropathic pain. Here the authors demonstrate the cytokine FL contributes to sensitisation of DRGs via its receptor FLT3 expressed on neurons, and identify a novel FLT3 inhibitor that attenuates neuropathic pain in mice.

Midostaurin: A Multiple Tyrosine Kinases Inhibitor in Acute Myeloid Leukemia and Systemic Mastocytosis

Midostaurin (PKC412, Rydapt^®) is an oral multiple tyrosine kinase inhibitor. Main targets are the kinase domain receptor, vascular endothelial-, platelet derived-, and fibroblast growth factor receptor, stem cell factor receptor c-KIT, as well as mutated and wild-type FLT3 kinases. Midostaurin was approved by the Food and Drug Administration (FDA) and the European Medical Agency (EMA) for acute myeloid leukemia with activating FLT3 mutations in combination with intensive induction and consolidation therapy as well as aggressive systemic mastocytosis (ASM), systemic mastocytosis with associated hematological neoplasm (SM-AHN) or mast cell leukemia (MCL). Several clinical trials are active or are planned to further investigate the role of midostaurin in myeloid malignancies and mastocytosis.

Expression of GADS enhances FLT3-induced mitogenic signaling

GADS is a member of a family of SH2 and SH3 domain-containing adaptors that functions in tyrosine kinase-mediated signaling cascades. Its expression is largely restricted to hematopoietic tissues and cell lines. Therefore, GADS is mainly involved in leukocyte-specific protein tyrosine kinase signaling. GADS is known to interact with tyrosine-phosphorylated SHC, BCR-ABL and KIT. The SH2 domain of GADS has a similar binding specificity to that of GRB2 but its SH3 domain displays a different binding specificity, and thus it is involved in other downstream signaling pathways than GRB2. In the present study, we examined the role of GADS in FLT3 signaling. FLT3 is a type III receptor tyrosine kinase, which is mutated in more than 30% of acute myeloid leukemia (AML) and the most common mutations is the internal tandem duplication (ITD) mutations. We observed that expression of GADS enhanced oncogenic FLT3-ITD-induced cell proliferation and colony formation in vitro. In a mouse xenograft model, GADS accelerated FLT3-ITD-dependent tumor formation. Furthermore, expression of GADS induced a transcriptional program leading to upregulation of MYC and mTORC1 target genes. GADS localizes to the cell membrane and strongly binds to ligand-stimulated wild-type FLT3 or is constitutively associated with the oncogenic mutant FLT3-ITD. We mapped the binding sites in FLT3 to pY955 and pY969 which overlaps with the GRB2 binding sites. Expression of GADS enhanced FLT3-mediated phosphorylation of AKT, ERK1/2, p38 and STAT5. Taken together, our data suggests that GADS is an important downstream component of FLT3 signaling and expression of GADS potentiates FLT3-mediated mitogenic signaling.

Midostaurin treatment in FLT3-mutated acute myeloid leukemia and systemic mastocytosis

INTRODUCTION

A number of tyrosine kinase inhibitors (TKIs) have been developed that inhibit the constitutively activated kinase activity caused by activating tyrosine kinase mutations, such as FLT3 or KIT, thus interrupting signaling pathways. Currently, midostaurin is the only approved TKI as monotherapy for aggressive systemic mastocytosis (SM), SM with associated hematological neoplasm, or mast cell leukemia displaying a KIT mutation as well as in combination with standard intensive chemotherapy for adult patients with newly diagnosed FLT3-mutated acute myeloid leukemia (AML). Areas covered: We provide a concise review of the pharmacology, tolerability and clinical efficacy of midostaurin and emerging new treatment options for ASM and FLT3-mutated AML. Expert commentary: Currently, midostaurin is the only approved TKI in aggressive SM, SM with associated hematological neoplasm, or mast cell leukemia inducing responses including complete remissions. With regard to AML, midostaurin is the first drug to receive regulatory approval in this indication in the molecularly defined subgroup of AML with FLT3 mutations. By introduction of this new standard in AML with FLT3 mutations, the bare has been raised for future approvals of next generation FLT3 inhibitors which will be based increasingly on head to head comparisons with midostaurin.

The impact of FLT3 mutations on treatment response and survival in Chinese de novo AML patients

OBJECTIVE

Two distinct forms of FMS-like tyrosine kinase 3 (FLT3) mutations, internal tandem duplication (ITD) in the juxtamembrane domain and point mutation within the activation loop of the tyrosine kinase domain (TKD), have been identified in considerable number of patients with AML. This study was aimed to analyze the impacts of these mutations on clinical outcomes, and assess the efficacy of different therapeutic regimens (allo-HSCT, sorafenib, or conventional chemotherapy) for AML patients with FLT3 mutations after the standard induction therapy.

MATERIALS AND METHODS

We analyzed DNA samples from 158 consecutive de novo AML patients (18-60 years, excluding APL) with FLT3 mutations between July 2010 and October 2015.

RESULTS

We found that AML patients with FLT3-TKD mutations have more favorable clinical outcomes than those with FLT3-ITD mutations. We also found that allo-HSCT therapy subgroup achieved longer OS and RFS than non-allo-HSCT therapy subgroup for FLT3-ITD positive patients (p < 0.001, p = 0.071). However, compared with the clinical outcomes in non-primary refractory patients, sorafenib did not show an obvious beneficial effect for the primary refractory patients. Further study on a large scale is still recommended.

CONCLUSIONS

FLT3-TKD-mutated AML patients have more favorable clinical outcomes than those with FLT3-ITD mutations. Allo-HSCT therapy subgroup achieved longer OS and RFS than non-allo-HSCT therapy subgroup for FLT3-ITD positive patients. Compared with the clinical outcomes in non-primary refractory patients, sorafenib did not show an obvious beneficial effect for the primary refractory patients.

Selective Expression of Flt3 within the Mouse Hematopoietic Stem Cell Compartment

The fms-like tyrosine kinase 3 (Flt3) is a cell surface receptor that is expressed by various hematopoietic progenitor cells (HPC) and Flt3-activating mutations are commonly present in acute myeloid and lymphoid leukemias. These findings underscore the importance of Flt3 to steady-state and malignant hematopoiesis. In this study, the expression of Flt3 protein and Flt3 mRNA by single cells within the hematopoietic stem cell (HSC) and HPC bone marrow compartments of C57/BL6 mice was investigated using flow cytometry and the quantitative reverse transcription polymerase chain reaction. Flt3 was heterogeneously expressed by almost all of the populations studied, including long-term reconstituting HSC and short-term reconstituting HSC. The erythropoietin receptor (EpoR) and macrophage colony-stimulating factor receptor (M-CSFR) were also found to be heterogeneously expressed within the multipotent cell compartments. Co-expression of the mRNAs encoding Flt3 and EpoR rarely occurred within these compartments. Expression of both Flt3 and M-CSFR protein at the surface of single cells was more commonly observed. These results emphasize the heterogeneous nature of HSC and HPC and the new sub-populations identified are important to understanding the origin and heterogeneity of the acute myeloid leukemias.

Detection of FLT3/TKD and IDH1 Mutations in Pakistani Acute Myeloid Leukemia Patients by Denaturing HPLC

Acute myeloid leukemia (AML) is characterized by an increase in the number of myeloid cells in the marrow and an arrest in their maturation. Various genetic mutations are associated with AML. FMS-like tyrosine kinase 3 (FLT3), a member of the class III receptor tyrosine kinase family, plays an important role in stem cell survival, and the development of dendritic and natural killer cells. FLT3/TKD mutations are generally missense mutations or in-frame alterations of residues D835 and I836 within the activation loop of the FLT3 protein. D835 mutations have been reported to occur in ≈ 7% of AML patients. Mutations have also been reported in exon 4 of isocitrate dehydrogenase 1 (IDH1) in ≈9% of AML patients. Mutations in FLT3/TKD and IDH1 genes were studied in AML patients from Pakistan and correlated with the laboratory findings. FLT3/TKD mutations were found in 7%, while IDH1 mutations were found in 10% Pakistani AML patients. Neither of these mutations was significantly correlated with age and sex, although the incidence of these mutations was higher in female patients. These mutations were found to be positively associated with each other. IDH1 mutations were positively associated with FAB type M1 and negatively associated with FAB type M2. In conclusion, the overall incidence of all these mutations in Pakistani patients was within the globally reported ranges. J. Cell. Biochem. 118: 1174-1181, 2017. © 2016 Wiley Periodicals, Inc.

Peripheral FLT-3 ligand levels as a pathobiological parameter duringthe clinical course of acute myeloid leukemia

BACKGROUND/AIM

FLT-3 ligand is a growth factor affecting the hematopoietic lineage. The aim of this study was to evaluate the variability of peripheral FLT-3 ligand during the clinical course of acute myeloid leukemia (AML) patients.

MATERIALS AND METHODS

Twenty-four patients were enrolled in this study in order to assess alterations in the circulating levels of FLT-3 ligand during the clinical course of AML.

RESULTS

We studied the association in the diagnostic period between the FLT-3 ligand and peripheral blood cells together with serum electrolytes. FLT-3 ligand levels (pg/mL) during the aplastic period due to remission induction and consolidation were higher than the levels at initial diagnosis. On the other hand, the diagnostic and remission induction values of leukocytes and FLT-3 ligand showed an inverse association. These results indicate to us that higher white cell counts are associated with lower FLT-3 ligand levels. We also found a reversed association between FLT-3 ligand and serum lactate dehydrogenase level. However, there was no association between FLT-3 ligand and other serum electrolyte levels. We also found higher FLT-3 ligand levels in male patients.

CONCLUSION

Our study demonstrates the inverse proliferative action of FLT-3 ligand on the early myeloid lineage. In addition, this study showed us that FLT-3 receptor inhibition during chemotherapy-induced aplasia causes a compensative ligand overexpression.

Infusion of CD3/CD28 costimulated umbilical cord blood T cells at the time of single umbilical cord blood transplantation may enhance engraftment

Limited cell numbers in umbilical cord blood (UCB) grafts present a major impediment to favorable outcomes in adult transplantation, largely related to delayed or failed engraftment. The advent of UCB transplantation (UCBT) using two grafts successfully circumvents this obstacle, despite the engraftment of only one unit. Preclinical models suggested that the addition of UCB T cells at the time of transplant can enhance engraftment. We tested whether ex vivo activation by CD3/CD28 costimulation and expansion of T cells from a single UCB graft would be safe and feasible in adults with advanced hematologic malignancies, with an overall objective of optimizing engraftment in single unit UCBT. In this phase 1 study, recipients of single UCB units were eligible if the unit was stored in two adequate fractions. Dose limiting toxicity was defined as grade 3 or grade 4 GVHD within 90 days of UCBT. Four patients underwent UCBT; all were treated at the first dose level (10(5) cells/kg). At the 10(5) cells/kg dose level two subjects experienced grade 3 intestinal GVHD, thus meeting stopping criteria. For three subjects, neutrophil engraftment was early (12, 17, and 20 days), while one subject experienced primary graft failure. We observed early donor T cell trafficking and found that expanded T cells produced supraphysiologic levels of cytokines relevant to engraftment and to lymphoid differentiation and function. Taken together, these preliminary data suggest rapid engraftment in recipients of a single UCBT combined with relatively low doses of activated T cells, though potentially complicated by severe GVHD.

A Closer Look at Schlemm's Canal Cell Physiology: Implications for Biomimetics

Among ocular pathologies, glaucoma is the second leading cause of progressive vision loss, expected to affect 80 million people worldwide by 2020. A primary cause of glaucoma appears to be damage to the conventional outflow tract. Conventional outflow tissues, a composite of the trabecular meshwork and the Schlemm's canal, regulate and maintain homeostatic responses to intraocular pressure. In glaucoma, filtration of aqueous humor into the Schlemm's canal is hindered, leading to an increase in intraocular pressure and subsequent damage to the optic nerve, with progressive vision loss. The Schlemm's canal encompasses a unique endothelium. Recent advances in culturing and manipulating Schlemm's canal cells have elucidated several aspects of their physiology, including ultrastructure, cell-specific marker expression, and biomechanical properties. This review highlights these advances and discusses implications for engineering a 3D, biomimetic, in vitro model of the Schlemm's canal endothelium to further advance glaucoma research, including drug testing and gene therapy screening.

Fish Oil-Rich Diet Promotes Hematopoiesis and Alters Hematopoietic Niche

The self-renewal and differentiation of hematopoietic stem cells (HSCs) in bone marrow are essential to replenish all blood cell types, but how this process is influenced by diet remains largely unclear. Here we show that a diet rich in fish oils promotes self-renewal of HSCs and extramedullary hematopoiesis. Chronic intake of a fish oil-rich diet increases the abundance of HSCs, alters the hematopoietic microenvironment, and, intriguingly, induces the expression of matrix metalloproteinase 12 (MMP12) in the bone marrow. Pointing to a direct effect of fish oil on MMP12 expression, omega-3 polyunsaturated fatty acids induce the expression of MMP12 in a dose-dependent manner in bone marrow cells. Importantly, down-regulation of MMP12 activity using an MMP12-specific inhibitor attenuates diet-induced myelopoiesis in both bone marrow and spleen. Thus, a fish oil-rich diet promotes hematopoiesis in the bone marrow and spleen, in part via the activity of MMP12. Taken together, these data provide new insights into diet-mediated regulation of hematopoiesis.

Robust expansion of dendritic cells in vivo by hydrodynamic FLT3L-FC gene transfer

Due to low numbers of endogenous dendritic cells (DCs) in vivo, exogenous DC-poietin Fms-like tyrosine kinase 3-ligand (FLT3L) is routinely used to generate DC for subsequent studies. We engineered a novel FLT3L-FC DNA construct that, when combined with hydrodynamic gene transfer (HDT), induced robust DC expansion in mice. DC generated in vivo by FLT3L-FC HDT produced cytokines in response to stimulation by an array of TLR agonists and promoted T cell proliferation. The FLT3L-FC protein produced in vivo spontaneously homodimerized to enable effective FLT signaling and the FC-domain enhanced its plasma half-life, providing an improved reagent and method to boost DC numbers.

The Biology and Targeting of FLT3 in Pediatric Leukemia

Despite remarkable improvement in treatment outcomes in pediatric leukemia over the past several decades, the prognosis for high-risk groups of acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL), as well as for relapsed leukemia, remains poor. Intensification of chemotherapy regimens for those at highest risk has improved success rates, but at the cost of significantly increased morbidity and long-term adverse effects. With the success of imatinib in Philadelphia-chromosome-positive leukemia and all-trans retinoic acid in acute promyelocytic leukemia, the quest to find additional molecularly targeted therapies has generated much excitement over recent years. Another such possible target in pediatric acute leukemia is FMS-like tyrosine kinase 3 (FLT3). FLT3 aberrations are among the most frequently identified transforming events in AML, and have significant clinical implications in both high-risk pediatric AML and in certain high-risk groups of pediatric ALL. Therefore, the successful targeting of FLT3 has tremendous potential to improve outcomes in these subsets of patients. This article will give an overview of the molecular function and signaling of the FLT3 receptor, as well as its pathogenic role in leukemia. We review the discovery of targeting FLT3, discuss currently available FLT3 inhibitors in pediatric leukemia and results of clinical trials to date, and finally, consider the future promise and challenges of FLT3 inhibitor therapy.

Influence of relative NK-DC abundance on placentation and its relation to epigenetic programming in the offspring

Normal placentation relies on an efficient maternal adaptation to pregnancy. Within the decidua, natural killer (NK) cells and dendritic cells (DC) have a critical role in modulating angiogenesis and decidualization associated with pregnancy. However, the contribution of these immune cells to the placentation process and subsequently fetal development remains largely elusive. Using two different mouse models, we here show that optimal placentation and fetal development is sensitive to disturbances in NK cell relative abundance at the fetal–maternal interface. Depletion of NK cells during early gestation compromises the placentation process by causing alteration in placental function and structure. Embryos derived from NK-depleted dams suffer from intrauterine growth restriction (IUGR), a phenomenon that continued to be evident in the offspring on post-natal day 4. Further, we demonstrate that IUGR was accompanied by an overall reduction of global DNA methylation levels and epigenetic changes in the methylation of specific hepatic gene promoters. Thus, temporary changes within the NK cell pool during early gestation influence placental development and function, subsequently affecting hepatic gene methylation and fetal metabolism.

Recent advances and novel agents for FLT3 mutated acute myeloid leukemia

Acute myeloid leukemia (AML) is a devastating hematologic malignancy that affects both older adults as well as children. Treatments available for AML largely depend on cytotoxic agents and often the only curative option is an allogeneic bone marrow transplant, an option limited to young persons and associated with high morbidity and mortality. There is an urgent need for the identification of new myeloid targets and an understanding of the key genetic mutations involved in disease progression and prognosis. One such mutation is the internal tandem duplication (ITD) in the FMS-like tyrosine kinase receptor-3 (FLT3) gene which confers an inferior outcome that is attributed to a higher relapse rate. In this review, we evaluate the FLT3-ITD mutation and discuss the recent data regarding emerging approaches using FLT3 inhibitors for the treatment of AML.

Nanosensor dosimetry of mouse blood proteins after exposure to ionizing radiation

Giant magnetoresistive (GMR) nanosensors provide a novel approach for measuring protein concentrations in blood for medical diagnosis. Using an in vivo mouse radiation model, we developed protocols for measuring Flt3 ligand (Flt3lg) and serum amyloid A1 (Saa1) in small amounts of blood collected during the first week after X-ray exposures of sham, 0.1, 1, 2, 3, or 6 Gy. Flt3lg concentrations showed excellent dose discrimination at ≥ 1 Gy in the time window of 1 to 7 days after exposure except 1 Gy at day 7. Saa1 dose response was limited to the first two days after exposure. A multiplex assay with both proteins showed improved dose classification accuracy. Our magneto-nanosensor assay demonstrates the dose and time responses, low-dose sensitivity, small volume requirements, and rapid speed that have important advantages in radiation triage biodosimetry.

Stem cells in a three-dimensional scaffold environment

Stem cells have emerged as important players in the generation and maintenance of many tissues. However, the accurate in vitro simulation of the native stem cell niche remains difficult due at least in part to the lack of a comprehensive definition of the critical factors of the stem cell niche based on in vivo models. Three-dimensional (3D) cell culture systems have allowed the development of useful models for investigating stem cell physiology particularly with respect to their ability to sense and generate mechanical force in response to their surrounding environment. We review the use of 3D culture systems for stem cell culture and discuss the relationship between stem cells and 3D growth matrices including the roles of the extracellular matrix, scaffolds, soluble factors, cell-cell interactions and shear stress effects within this environment. We also discuss the potential for novel methods that mimic the native stem cell niche in vitro as well as the current associated challenges.

The evolution of cellular deficiency in GATA2 mutation

Constitutive heterozygous GATA2 mutation is associated with deafness, lymphedema, mononuclear cytopenias, infection, myelodysplasia (MDS), and acute myeloid leukemia. In this study, we describe a cross-sectional analysis of 24 patients and 6 relatives with 14 different frameshift or substitution mutations of GATA2. A pattern of dendritic cell, monocyte, B, and natural killer (NK) lymphoid deficiency (DCML deficiency) with elevated Fms-like tyrosine kinase 3 ligand (Flt3L) was observed in all 20 patients phenotyped, including patients with Emberger syndrome, monocytopenia with Mycobacterium avium complex (MonoMAC), and MDS. Four unaffected relatives had a normal phenotype indicating that cellular deficiency may evolve over time or is incompletely penetrant, while 2 developed subclinical cytopenias or elevated Flt3L. Patients with GATA2 mutation maintained higher hemoglobin, neutrophils, and platelets and were younger than controls with acquired MDS and wild-type GATA2. Frameshift mutations were associated with earlier age of clinical presentation than substitution mutations. Elevated Flt3L, loss of bone marrow progenitors, and clonal myelopoiesis were early signs of disease evolution. Clinical progression was associated with increasingly elevated Flt3L, depletion of transitional B cells, CD56(bright) NK cells, naïve T cells, and accumulation of terminally differentiated NK and CD8(+) memory T cells. These studies provide a framework for clinical and laboratory monitoring of patients with GATA2 mutation and may inform therapeutic decision-making.

Axl/Gas6 pathway positively regulates FLT3 activation in human natural killer cell development

Activation of the fibromyalgia syndrome-like tyrosine kinase 3 (FLT3) by its ligand, FLT3 ligand (FL), strongly augments the development of natural killer (NK) cells from human CD34⁺ hematopoietic progenitor cells (HPCs) in the presence of IL-15, compared with NK-cell development in the presence of IL-15 alone. In this study, we observed that blocking the receptor tyrosine kinase Axl/Gas6 pathway with a soluble Axl-IgG1 Fc fusion protein (Axl-Fc) in the presence of FL significantly diminished the absolute number of CD3⁻ CD56⁺ NK cells derived from human CD34⁺ HPCs. Axl-Fc reduced the expression levels of the IL-2/15 receptor β chain (CD122) and γ chain (CD132) induced by activation of FLT3 and consequently reduced the frequency of NK precursor cells responding to IL-15. Furthermore, Axl-Fc diminished FL-induced FLT3 phosphorylation and impeded the physical interaction between Axl and FLT3 in CD34⁺ HPCs. Collectively, our data suggest that the Axl/Gas6 pathway contributes to normal human NK-cell development at least in part via its positive regulatory effect on FLT3 signaling in CD34⁺ HPCs.

Dendritic cell homeostasis is maintained by nonhematopoietic and T-cell-produced Flt3-ligand in steady state and during immune responses

Lymphoid-tissue dendritic cells (DCs) are short-lived and need to be continuously replenished from bone marrow-derived DC progenitor cells. Fms-related tyrosine kinase 3 is expressed during cellular development from hematopoietic progenitors to lymphoid-tissue DCs. Fms-related tyrosine kinase 3 ligand (Flt3L) is an essential, nonredundant cytokine for DC progenitor to lymphoid tissue DC differentiation and maintenance. However, which cells contribute to Flt3L production and how Flt3L cytokine levels are regulated in steady state and during immune reactions remains to be determined. Here we demonstrate that besides nonhematopoietic cells, WT T cells produce Flt3L and contribute to the generation of both classical DCs (cDCs) and plasmacytoid DCs in Flt3L(-/-) mice. Upon stimulation in vitro, CD4(+) T cells produce more Flt3L than CD8(+) T cells. Moreover, in vivo stimulation of naïve OT-II CD4(+) T cells with OVA leads to increase of pre-cDCs and cDCs in draining lymph nodes of Flt3L(-/-) mice in a partially Flt3L-dependent manner. Thus, Flt3L-mediated lymphoid tissue DC homeostasis is regulated by steady-state T cells as well as by proliferative T cells, fostering local development of lymphoid organ resident DCs.

Noncanonical NF-κB signaling regulates hematopoietic stem cell self-renewal and microenvironment interactions

RelB and nuclear factor κB (NF-κB2) are the main effectors of NF-κB noncanonical signaling and play critical roles in many physiological processes. However, their role in hematopoietic stem/progenitor cell (HSPC) maintenance has not been characterized. To investigate this, we generated RelB/NF-κB2 double-knockout (dKO) mice and found that dKO HSPCs have profoundly impaired engraftment and self-renewal activity after transplantation into wild-type recipients. Transplantation of wild-type bone marrow cells into dKO mice to assess the role of the dKO microenvironment showed that wild-type HSPCs cycled more rapidly, were more abundant, and had developmental aberrancies: increased myeloid and decreased lymphoid lineages, similar to dKO HSPCs. Notably, when these wild-type cells were returned to normal hosts, these phenotypic changes were reversed, indicating a potent but transient phenotype conferred by the dKO microenvironment. However, dKO bone marrow stromal cell numbers were reduced, and bone-lining niche cells supported less HSPC expansion than controls. Furthermore, increased dKO HSPC proliferation was associated with impaired expression of niche adhesion molecules by bone-lining cells and increased inflammatory cytokine expression by bone marrow cells. Thus, RelB/NF-κB2 signaling positively and intrinsically regulates HSPC self-renewal and maintains stromal/osteoblastic niches and negatively and extrinsically regulates HSPC expansion and lineage commitment through the marrow microenvironment.