Early lymphoid development and microenvironmental cues in B-cell acute lymphoblastic leukemia

The hematopoietic microenvironment: a network of niches for the development of all blood cell lineages

Blood cell formation (hematopoiesis) takes place mainly in the bone marrow, within the hematopoietic microenvironment, composed of a number of different cell types and their molecular products that together shape spatially organized and highly specialized microstructures called hematopoietic niches. From the earliest developmental stages and throughout the myeloid and lymphoid lineage differentiation pathways, hematopoietic niches play a crucial role in the preservation of cellular integrity and the regulation of proliferation and differentiation rates. Current evidence suggests that each blood cell lineage develops under specific, discrete niches that support committed progenitor and precursor cells and potentially cooperate with transcriptional programs determining the gradual lineage commitment and specification. This review aims to discuss recent advances on the cellular identity and structural organization of lymphoid, granulocytic, monocytic, megakaryocytic, and erythroid niches throughout the hematopoietic microenvironment and the mechanisms by which they interconnect and regulate viability, maintenance, maturation, and function of the developing blood cells.

Targeting mTOR in Acute Lymphoblastic Leukemia

Acute Lymphoblastic Leukemia (ALL) is an aggressive hematologic disorder and constitutes approximately 25% of cancer diagnoses among children and teenagers. Pediatric patients have a favourable prognosis, with 5-years overall survival rates near 90%, while adult ALL still correlates with poorer survival. However, during the past few decades, the therapeutic outcome of adult ALL was significantly ameliorated, mainly due to intensive pediatric-based protocols of chemotherapy. Mammalian (or mechanistic) target of rapamycin (mTOR) is a conserved serine/threonine kinase belonging to the phosphatidylinositol 3-kinase (PI3K)-related kinase family (PIKK) and resides in two distinct signalling complexes named mTORC1, involved in mRNA translation and protein synthesis and mTORC2 that controls cell survival and migration. Moreover, both complexes are remarkably involved in metabolism regulation. Growing evidence reports that mTOR dysregulation is related to metastatic potential, cell proliferation and angiogenesis and given that PI3K/Akt/mTOR network activation is often associated with poor prognosis and chemoresistance in ALL, there is a constant need to discover novel inhibitors for ALL treatment. Here, the current knowledge of mTOR signalling and the development of anti-mTOR compounds are documented, reporting the most relevant results from both preclinical and clinical studies in ALL that have contributed significantly into their efficacy or failure.

CRTAM+ NK cells endowed with suppressor properties arise in leukemic bone marrow

Due to their increasing rates of morbidity and mortality, childhood malignancies are considered a global health priority, with acute lymphoblastic leukemias (ALLs) showing the highest incidence worldwide. Control of malignant clone emergence and the subsequent normal-leukemic hematopoietic cell out-competition require antitumor monitoring mechanisms. Investigation of cancer surveillance innate cells may be critical to understand the mechanisms contributing in either disease progression or relapse, and to promote displacement of leukemic hematopoiesis by the normal counterpart. We report here that NK cell production is less and low hematopoietic progenitor numbers contribute to this defect. By investigating the expression of the activation molecule class I restricted T-cell associated molecule (CRTAM) along the hematopoietic lineage differentiation pathway, we have identified lymphoid precursor populations coexpressing CD34, CD56/CD3/CD19, and CRTAM as the earliest developmental stage where activation may take place in specialized niches that display the ligand nectin-like-2. Of note, bone marrow (BM) from patients with ALL revealed high contents of preactivated CD56^high NK cells expressing CRTAM and endowed with an exhaustion-like phenotype and the functional capability of producing IL-10 and TGF-β in vitro. Our findings suggest, for the first time, that the tumor microenvironment in ALL directly contribute to exhaustion of NK cell functions by the CRTAM/Necl-2 interaction, and that the potential regulatory role of exhausted-like NK cells may favor malignant progression at the expense of anti-tumor responses. Phenotypic and functional identity of this unique suppressor-like NK cell population within the leukemic BM would be of special interest for the pathobiology of ALL and development of targeting strategies.

Genome-wide identification of microRNA signatures associated with stem/progenitor cells in Philadelphia chromosome-positive acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is a malignant transformation with uncontrolled proliferation of lymphoid precursor cells within bone marrow including a dismal prognosis after relapse. Survival of a population of quiescent leukemia stem cells (LSCs, also termed leukemia-initiating cells (LICs)) after treatment is one of the relapse reasons in Ph⁺ ALL patient. MicroRNAs (miRNAs) are known as highly conserved 19-24 nucleotides non-protein-coding small RNAs that regulate the expression of human genes. miRNAs are often involved in the tuning of hematopoiesis. Therefore, the deregulation of miRNA expression and function in hematopoietic cells can cause cancer and promote its progression. This is the first comprehensive analysis of miRNA expression differences between CD34⁺CD38^- LSCs and CD34⁺CD38⁺ leukemic progenitors (LPs) from the same Ph⁺ B-ALL bone marrow samples using high-throughput sequencing technologies. We identified multiple differentially expressed miRNAs including hsa-miR-3143, hsa-miR-6503-3p, hsa-miR-744-3p, hsa-miR-1226-3p, hsa-miR-10a-5p, hsa-miR-4658 and hsa-miR-493-3p related to LSC and LP populations which have regulatory functions in stem-cell associated biological processes. The deregulation of these miRNAs could affect leukemogenesis, clonogenic and stemness capacities in these subpopulations of Ph⁺ B-ALL. Therefore, identification of these LSC associated miRNAs may improve the diagnosis and management of B-ALL. These findings may also lead to future strategies to eliminate the presence of resistant LSCs, either by induction of apoptosis or by sensitizing these cells to chemotherapy.

Bone marrow T helper cells with a Th1 phenotype induce activation and proliferation of leukemic cells in precursor B acute lymphoblastic leukemia patients

Precursor B cell acute lymphoblastic leukemia (BCP-ALL) constitutes the leading cause of cancer-related death in children. While chromosomal alterations contribute to BCP-ALL pathogenesis, they are insufficient for leukemia development. Epidemiological data and evidence from a mouse model suggest that immune responses to infections may trigger the emergence of leukemia, but the mechanisms remain unclear. Here, we show that T helper (Th) cells from bone marrow of pediatric BCP-ALL patients can be attracted and activated by autologous BCP-ALL cells. Bone-marrow Th cells supportively interacted with BCP-ALL cells, inducing upregulation of important surface molecules and BCP-ALL cell proliferation. These Th cells displayed a Th1-like phenotype and produced high levels of IFN-γ. IFN-γ was responsible for the upregulation of CD38 in BCP-ALL cells, a molecule which we found to be associated with early relapse, and accountable for the production of IP-10, a chemokine involved in BCP-ALL migration and drug resistance. Thus, our data provide mechanistic support for an involvement of Th cell immune responses in the propagation of BCP-ALL and suggest that BCP-ALL cell-supportive Th cells may serve as therapeutic target.

T cell acute lymphoblastic leukemia (T-ALL): New insights into the cellular origins and infiltration mechanisms common and unique among hematologic malignancies

T-cell acute lymphoblastic leukemia (T-ALL) accounts for 15% and 25% of total childhood and adult ALL cases, respectively. During T-ALL, patients are at risk of organ infiltration by leukemic T-cells. Infiltration is a major consequence of disease relapse and correlates with poor prognosis. Transendothelial migration of leukemic cells is required to exit the blood stream into target organs. While mechanisms of normal T-cell transmigration are well known, the mechanisms of leukemic T-cell extravasation remain elusive; but involvement of chemokines, integrins and Notch signaling play critical roles. Here, we summarize current knowledge about molecular mechanisms of leukemic T-cell infiltration with special emphasis on the newly identified subtype early T-cell-progenitor (ETP)-ALL. Furthermore, we compare the extravasation potential of T-ALL cells with that of other hematologic malignancies such as B-ALL and acute myeloid leukemia (AML).

Pediatric precursor B acute lymphoblastic leukemia: are T helper cells the missing link in the infectious etiology theory?

Precursor B acute lymphoblastic leukemia (BCP-ALL), the most common childhood malignancy, arises from an expansion of malignant B cell precursors in the bone marrow. Epidemiological studies suggest that infections or immune responses to infections may promote such an expansion and thus BCP-ALL development. Nevertheless, a specific pathogen responsible for this process has not been identified. BCP-ALL cells critically depend on interactions with the bone marrow microenvironment. The bone marrow is also home to memory T helper (Th) cells that have previously expanded during an immune response in the periphery. In secondary lymphoid organs, Th cells can interact with malignant cells of mature B cell origin, while such interactions between Th cells and malignant immature B cell in the bone marrow have not been described yet. Nevertheless, literature supports a model where Th cells—expanded during an infection in early childhood—migrate to the bone marrow and support BCP-ALL cells as they support normal B cells. Further research is required to mechanistically confirm this model and to elucidate the interaction pathways between leukemia cells and cells of the tumor microenvironment. As benefit, targeting these interactions could be included in current treatment regimens to increase therapeutic efficiency and to reduce relapses.

Pro-inflammatory-Related Loss of CXCL12 Niche Promotes Acute Lymphoblastic Leukemic Progression at the Expense of Normal Lymphopoiesis

Pediatric oncology, notably childhood acute lymphoblastic leukemia (ALL), is currently one of the health-leading concerns worldwide and a biomedical priority. Decreasing overall leukemia mortality in children requires a comprehensive understanding of its pathobiology. It is becoming clear that malignant cell-to-niche intercommunication and microenvironmental signals that control early cell fate decisions are critical for tumor progression. We show here that the mesenchymal stromal cell component of ALL bone marrow (BM) differ from its normal counterpart in a number of functional properties and may have a key role during leukemic development. A decreased proliferation potential, contrasting with the strong ability of producing pro-inflammatory cytokines and an aberrantly loss of CXCL12 and SCF, suggest that leukemic lymphoid niches in ALL BM are unique and may exclude normal hematopoiesis. Cell competence ex vivo assays within tridimensional coculture structures indicated a growth advantage of leukemic precursor cells and their niche remodeling ability by CXCL12 reduction, resulting in leukemic cell progression at the expense of normal niche-associated lymphopoiesis.

Integrated analysis of microRNA and transcription factor reveals important regulators and regulatory motifs in adult B-cell acute lymphoblastic leukemia

B-cell acute lymphoblastic leukemia (B‑ALL) is an aggressive hematological malignancy and a leading cause of cancer-related mortality in children and young adults. The molecular mechanisms involved in the regulation of its gene expression has yet to be fully elucidated. In the present study, we performed large scale expression profiling of microRNA (miRNA) and transcription factor (TF) by Illumina deep‑sequencing and TF array technology, respectively, and identified 291 differentially expressed miRNAs and 201 differentially expressed TFs in adult B‑ALL samples relative to their controls. After integrating expression profile data with computational prediction of miRNA and TF targets from different databases, we construct a comprehensive miRNA‑TF regulatory network specifically for adult B‑ALL. Network function analysis revealed 25 significantly enriched pathways, four pathways are well‑known to be involved in B‑ALL, such as PI3K‑Akt signaling pathway, Jak‑STAT signaling pathway, Ras signaling pathway and cell cycle pathway. By analyzing the network topology, we identified 28 hub miRNAs and 19 hub TFs in the network, and found nine potential B‑ALL regulators among these hub nodes. We also constructed a Jak‑STAT signaling sub‑network for B‑ALL. Based on the sub‑network analysis and literature survey, we proposed a cellular model to discuss MYC/miR‑15a‑5p/FLT3 feed-forward loop (FFL) with Jak‑STAT signaling pathway in B‑ALL. These findings enhance our understanding of this disease at the molecular level, as well as provide putative therapeutic targets for B-ALL.

Early Differentiation of Human CD11c+NK Cells with γδ T Cell Activation Properties Is Promoted by Dialyzable Leukocyte Extracts

Reconstitution of the hematopoietic system during immune responses and immunological and neoplastic diseases or upon transplantation depends on the emergent differentiation of hematopoietic stem/progenitor cells within the bone marrow. Although in the last decade the use of dialyzable leukocyte extracts (DLE) as supportive therapy in both infectious and malignant settings has increased, its activity on the earliest stages of human hematopoietic development remains poorly understood. Here, we have examined the ability of DLE to promote replenishment of functional lymphoid lineages from CD34⁺ cells. Our findings suggest that DLE increases their differentiation toward a conspicuous CD56⁺CD16⁺CD11c⁺ NK-like cell population endowed with properties such as IFNy production, tumor cell cytotoxicity, and the capability of inducing γδ T lymphocyte proliferation. Of note, long-term coculture controlled systems showed the bystander effect of DLE-stromal cells by providing NK progenitors with signals to overproduce this cell subset. Thus, by direct effect on progenitor cells and through activation and remodeling of the supporting hematopoietic microenvironment, DLE may contribute a robust innate immune response by promoting the emerging lymphopoiesis of functional CD11c⁺ NK cells in a partially TLR-related manner. Unraveling the identity and mechanisms of the involved DLE components may be fundamental to advance the NK cell-based therapy field.

Cyr61 participates in the pathogenesis of acute lymphoblastic leukemia by enhancing cellular survival via the AKT/NF-κB signaling pathway

Cyr61 (CCN1) is the product of a growth factor–inducible immediate early gene and is involved in cell adhesion, survival, proliferation, and differentiation. Cyr61 is overexpressed in human tumors and is involved in the development of tumors. However, the role that Cyr61 plays in acute lymphoblastic leukemia (ALL) cells remains undetermined. The aim of this study was to identify the role of Cyr61 in regulating ALL cell survival. Here, we found that the level of Cyr61 was increased in the plasma and bone marrow (BM) from ALL patients compared with samples from normal control patients. Furthermore, we observed that Cyr61 could effectively stimulate Jurkat (T ALL cell lines), Nalm-6 (B ALL cell lines), and primary ALL cell survival. Mechanistically, we showed that Cyr61 stimulated ALL cell survival via the AKT/NF-κB signaling pathways and the consequent up-regulation of Bcl-2. Taken together, our study is the first to reveal that Cyr61 is elevated in ALL and promotes cell survival through the AKT/NF-κB pathway by up-regulating Bcl-2. Our findings suggest that Cyr61 plays an important role in the pathogenesis of ALL.

Modeling the Pro-inflammatory Tumor Microenvironment in Acute Lymphoblastic Leukemia Predicts a Breakdown of Hematopoietic-Mesenchymal Communication Networks

Lineage fate decisions of hematopoietic cells depend on intrinsic factors and extrinsic signals provided by the bone marrow microenvironment, where they reside. Abnormalities in composition and function of hematopoietic niches have been proposed as key contributors of acute lymphoblastic leukemia (ALL) progression. Our previous experimental findings strongly suggest that pro-inflammatory cues contribute to mesenchymal niche abnormalities that result in maintenance of ALL precursor cells at the expense of normal hematopoiesis. Here, we propose a molecular regulatory network interconnecting the major communication pathways between hematopoietic stem and progenitor cells (HSPCs) and mesenchymal stromal cells (MSCs) within the BM. Dynamical analysis of the network as a Boolean model reveals two stationary states that can be interpreted as the intercellular contact status. Furthermore, simulations describe the molecular patterns observed during experimental proliferation and activation. Importantly, our model predicts instability in the CXCR4/CXCL12 and VLA4/VCAM1 interactions following microenvironmental perturbation due by temporal signaling from Toll like receptors (TLRs) ligation. Therefore, aberrant expression of NF-κB induced by intrinsic or extrinsic factors may contribute to create a tumor microenvironment where a negative feedback loop inhibiting CXCR4/CXCL12 and VLA4/VCAM1 cellular communication axes allows for the maintenance of malignant cells.

Increased Th17 cells and IL-17A exist in patients with B cell acute lymphoblastic leukemia and promote proliferation and resistance to daunorubicin through activation of Akt signaling

BACKGROUND

Immune regulation is crucial for the pathogenesis of B-cell acute lymphoblastic leukemia (B-ALL). It has been reported that Th17 cells as a newly identified subset of CD4(+) T cells are involved in the pathogenesis of several hematological disorders. However, the role of Th17 cells in the pathophysiology of B-ALL is still unclear.

METHODS

The frequencies of T cells were determined by flow cytometry in the peripheral blood and bone marrow of 44 newly diagnosed B-ALL patients and 25 age-matched healthy donors. The cell viability and apoptosis were determined by CCK-8 assay and Annexin V staining, respectively. Western blot was applied to identify the level of Akt and Stat3 phosphorylation.

RESULTS

We assessed and observed a significantly increased frequency of Th17 cells and a drastically decreased frequency of Th1 cells in peripheral blood mononuclear cells and bone marrow mononuclear cells from newly diagnosed B-ALL patients compared with healthy donors. Furthermore, increased levels of Th17-related cytokines including IL-17, IL-21, IL-23, IL-1β, and IL-6 were presented in between blood and marrow in B-ALL patients. Both IL-17A and IL-21, two Th17-secreted cytokines, induced the proliferation of B-ALL cell line Nalm-6 and patient B-ALL cells isolated from B-ALL patients, herein either cytokine led to the phosphorylation of Akt and Stat3. Additionally, IL-17A promoted resistance to daunorubicin via activation of Akt signaling and the PI3K/Akt inhibitor LY294002 or perifosine almost completely rescued daunorubicin-induced cell death in B-ALL cells.

CONCLUSIONS

Our findings suggest that elevated Th17 cells secrete IL-17A by which promotes the proliferation and resistance to daunorubicin in B-ALL cells through activation of Akt signaling. Th17 cells may represent a novel target to improve B-ALL immunotherapy.

Can Peto's paradox be used as the null hypothesis to identify the role of evolution in natural resistance to cancer? A critical review

BACKGROUND

Carcinogenesis affects not only humans but almost all metazoan species. Understanding the rules driving the occurrence of cancers in the wild is currently expected to provide crucial insights into identifying how some species may have evolved efficient cancer resistance mechanisms. Recently the absence of correlation across species between cancer prevalence and body size (coined as Peto's paradox) has attracted a lot of attention. Indeed, the disparity between this null hypothesis, where every cell is assumed to have an identical probability to undergo malignant transformation, and empirical observations is particularly important to understand, due to the fact that it could facilitate the identification of animal species that are more resistant to carcinogenesis than expected. Moreover it would open up ways to identify the selective pressures that may be involved in cancer resistance. However, Peto's paradox relies on several questionable assumptions, complicating the interpretation of the divergence between expected and observed cancer incidences.

DISCUSSIONS

Here we review and challenge the different hypotheses on which this paradox relies on with the aim of identifying how this null hypothesis could be better estimated in order to provide a standard protocol to study the deviation between theoretical/theoretically predicted and observed cancer incidence. We show that due to the disproportion and restricted nature of available data on animal cancers, applying Peto's hypotheses at species level could result in erroneous conclusions, and actually assume the existence of a paradox. Instead of using species level comparisons, we propose an organ level approach to be a more accurate test of Peto's assumptions.

SUMMARY

The accuracy of Peto's paradox assumptions are rarely valid and/or quantifiable, suggesting the need to reconsider the use of Peto's paradox as a null hypothesis in identifying the influence of natural selection on cancer resistance mechanisms.

Bone Marrow Cells in Acute Lymphoblastic Leukemia Create a Proinflammatory Microenvironment Influencing Normal Hematopoietic Differentiation Fates

B-cell acute lymphoblastic leukemia (B-ALL) is a serious public health problem in the pediatric population worldwide, contributing to 85% of deaths from childhood cancers. Understanding the biology of the disease is crucial for its clinical management and the development of therapeutic strategies. In line with that observed in other malignancies, chronic inflammation may contribute to a tumor microenvironment resulting in the damage of normal processes, concomitant to development and maintenance of neoplastic cells. We report here that hematopoietic cells from bone marrow B-ALL have the ability to produce proinflammatory and growth factors, including TNFα, IL-1β, IL-12, and GM-CSF that stimulate proliferation and differentiation of normal stem and progenitor cells. Our findings suggest an apparently distinct CD13(+)CD33(+) population of leukemic cells contributing to a proinflammatory microenvironment that may be detrimental to long-term normal hematopoiesis within B-ALL bone marrow.

Interleukin-18 produced by bone marrow-derived stromal cells supports T-cell acute leukaemia progression

Development of novel therapies is critical for T-cell acute leukaemia (T-ALL). Here, we investigated the effect of inhibiting the MAPK/MEK/ERK pathway on T-ALL cell growth. Unexpectedly, MEK inhibitors (MEKi) enhanced growth of 70% of human T-ALL cell samples cultured on stromal cells independently of NOTCH activation and maintained their ability to propagate in vivo. Similar results were obtained when T-ALL cells were cultured with ERK1/2-knockdown stromal cells or with conditioned medium from MEKi-treated stromal cells. Microarray analysis identified interleukin 18 (IL-18) as transcriptionally up-regulated in MEKi-treated MS5 cells. Recombinant IL-18 promoted T-ALL growth in vitro, whereas the loss of function of IL-18 receptor in T-ALL blast cells decreased blast proliferation in vitro and in NSG mice. The NFKB pathway that is downstream to IL-18R was activated by IL-18 in blast cells. IL-18 circulating levels were increased in T-ALL-xenografted mice and also in T-ALL patients in comparison with controls. This study uncovers a novel role of the pro-inflammatory cytokine IL-18 and outlines the microenvironment involvement in human T-ALL development.

Acute promyelocytic leukemia relapsing into acute myeloid leukemia-M2 with normal cytogenetics

The use of all trans-retinoic acid (ATRA) and combination chemotherapy has made acute promyelocytic leukemia (APL) a potentially curable leukemia. Late sequelae of the treatment of APL have therefore become an important consideration in the overall treatment strategy. We report a patient with APL who achieved complete clinical and molecular remission after treatment with daunorubicin and ATRA. Three years later, she developed acute myeloid leukemia (AML), M2 subtype without any evidence of relapse of the APL clone. Karyotypic analysis showed a normal female karyotype.

Midkine as a regulator of B cell survival in health and disease

In healthy individuals, the pool of peripheral lymphocytes is constant in size. The control of lymphoid homeostasis is the result of a very fine balance between lymphocyte production, survival and proliferation. Survival factors have been shown to play a critical role in maintaining the correct size of lymphocyte populations. Midkine, a heparin-binding cytokine was recently shown to be involved in cell proliferation, differentiation and apoptosis in various cell types including normal and malignant B cells. This review focuses on the role of midkine in the regulation of peripheral B cell survival in health and disease.

LINKED ARTICLES

This article is part of a themed section on Midkine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-4.

Interaction of a CD44+ head and neck squamous cell carcinoma cell line with a stromal cell-derived factor-1-expressing supportive niche: An in vitro model

The cancer stem cell (CSC) theory implies that CSCs are surrounded by supportive stromal cells, which are known as the CSC niche. Stromal cell-derived factor-1 (SDF-1) shows a multitude of functional effects in head and neck squamous cell carcinoma (HNSCC) cells, including migration and polarization. Therefore, the SDF-1-CXCR4 axis may be involved in the pathophysiology of the progression, recurrence and metastasis of malignant diseases of the head and neck. In the present study, the CD44⁺ HNSCC UM-SCC-11A cell line was used as a model for CSCs. The interaction between the UM-SCC-11A cells and the supportive microenvironmental cells, including fibrocytes, human umbilical vein endothelial cells (HUVECs) and human microvascular vein endothelial cells (HMVECs) was evaluated. All the cell types that were tested were shown to secrete different concentrations of SDF-1 into the surrounding culture medium [mean (m)_fibro, 1243.3±156.2 pg/ml; m_HMVEC, 1061.4±23.2 pg/ml; m_HUVEC, 849.6±110.9 pg/ml]. The migration of the UM-SCC-11A cells towards the supportive cells was increased by a higher supply of SDF-1 (contr_fibro, 315.23±61.55 μm; m_fibro, 477.73±143.7 μm; P_fibro=0.003; contr_HMVEC, 123.41±66.68 μm; m_HMVEC, 249.04±111.95 μm; P_HMVEC=0.004; contr_HUVEC, 189.7±93.26 μm; m_HUVEC, 260.82±161.58 μm). The amount of the UM-SCC-11A cells that migrated towards the differentiated fibrocytes was significantly higher than that which migrated towards the HMVECs or HUVECs (P_fibro/HMVEC=2.12E-11; P_fibro/HUVEC=2.28E-5). Cell-cell interaction by podia formation of the UM-SCC-11A cells was observed in all the supportive cell types that were tested. Broadly based cell-cell contacts were observed. By contrast, digitiform podia formations presented by the UM-SCC-11A cells were determined using fluorescence microscopy. The SDF-1-CXCR4 axis is postulated to be a crucial pathway in the interaction between CSCs and their surrounding supportive cells. Understanding the cell-cell interactions in the CSC niche using in vitro models may aid in gaining further insight into these mechanisms and finding new strategies of therapy in this field.

Lymphoid progenitor cells from childhood acute lymphoblastic leukemia are functionally deficient and express high levels of the transcriptional repressor Gfi-1

Acute lymphoblastic leukemia (ALL) is the most frequent malignancy of childhood. Substantial progress on understanding the cell hierarchy within ALL bone marrow (BM) has been recorded in the last few years, suggesting that both primitive cell fractions and committed lymphoid blasts with immature stem cell-like properties contain leukemia-initiating cells. Nevertheless, the biology of the early progenitors that initiate the lymphoid program remains elusive. The aim of the present study was to investigate the ability of lymphoid progenitors from B-cell precursor ALL BM to proliferate and undergo multilineage differentiation. By phenotype analyses, in vitro proliferation assays, and controlled culture systems, the lymphoid differentiation potentials were evaluated in BM primitive populations from B-cell precursor ALL pediatric patients. When compared to their normal counterparts, functional stem and progenitor cell contents were substantially reduced in ALL BM. Moreover, neither B nor NK or dendritic lymphoid-cell populations developed recurrently from highly purified ALL-lymphoid progenitors, and their proliferation and cell cycle status revealed limited proliferative capacity. Interestingly, a number of quiescence-associated transcription factors were elevated, including the transcriptional repressor Gfi-1, which was highly expressed in primitive CD34⁺ cells. Together, our findings reveal major functional defects in the primitive hematopoietic component of ALL BM. A possible contribution of high levels of Gfi-1 expression in the regulation of the stem/progenitor cell biology is suggested.

TLR stimulation of bone marrow lymphoid precursors from childhood acute leukemia modifies their differentiation potentials

Acute leukemias are the most frequent childhood malignancies worldwide and remain a leading cause of morbidity and mortality of relapsed patients. While remarkable progress has been made in characterizing genetic aberrations that may control these hematological disorders, it has also become clear that abnormalities in the bone marrow microenvironment might hit precursor cells and contribute to disease. However, responses of leukemic precursor cells to inflammatory conditions or microbial components upon infection are yet unexplored. Our previous work and increasing evidence indicate that Toll-like receptors (TLRs) in the earliest stages of lymphoid development in mice and humans provide an important mechanism for producing cells of the innate immune system. Using highly controlled co-culture systems, we now show that lymphoid precursors from leukemic bone marrow express TLRs and respond to their ligation by changing cell differentiation patterns. While no apparent contribution of TLR signals to tumor progression was recorded for any of the investigated diseases, the replenishment of innate cells was consistently promoted upon in vitro TLR exposure, suggesting that early recognition of pathogen-associated molecules might be implicated in the regulation of hematopoietic cell fate decisions in childhood acute leukemia.

Functional effects of SDF-1α on a CD44(+) CXCR4(+) squamous cell carcinoma cell line as a model for interactions in the cancer stem cell niche

Stromal cell-derived factor-1α (SDF-1α), also known as CXCL12, has variable effects on a plurality of cells. It is known to have selective effects on cell migration, morphology, survival and cell homing. As such the SDF-1-CXCR4 axis is postulated to be a crucial key pathway in the interaction between (cancer) stem cells and their surrounding supportive cells, the so-called (cancer) stem cell niche. We evaluated the expression of CD44 as a cancer stem cell (CSC) marker and the expression of CXCR4 in the head and neck squamous cell carcinoma (HNSCC) cell line UM-SCC 11A. In addition, we monitored proliferation, formation of podia and migration of UM-SCC 11A cells under the influence of SDF-1α. Whereas SDF-1α induced the formation of podia of CD44(+) CXCR4(+) UM-SCC 11A cells in a dose-dependent manner and the maximum number of cells exhibiting the formation of podia was observed under the influence of 10 ng/ml SDF-1α (P=5.3x10(-6)), the highest number of migrating cells was noted using a concentration of 100 ng/ml (P=0.027). Proliferation and survival were not affected by SDF-1α. We showed that UM-SCC 11A cells could be a target for SDF-1α by CXCR4 expression and these cells also showed characteristics of HNSCC CSCs via CD44 expression. We demonstrated that SDF-1α is a chemoattractant for UM-SCC 11A cells, and a maximum directed migration was achieved under the influence of 100 ng/ml SDF-1α. Changes in cell morphology by presenting filopodia or a prominent uropod were noted following treatment of 10 ng/ml SDF-1α. The SDF-CXCR4 axis may play a crucial role in the interaction between CSCs and their supportive cells in the CSC niche. Understanding these interactions may help to gain further insight into the pathophysiology of the progression and recurrence of malignant diseases and thus help to develop novel strategies for therapy.

Lineage switching in acute leukemias: a consequence of stem cell plasticity?

Acute leukemias are the most common cancer in childhood and characterized by the uncontrolled production of hematopoietic precursor cells of the lymphoid or myeloid series within the bone marrow. Even when a relatively high efficiency of therapeutic agents has increased the overall survival rates in the last years, factors such as cell lineage switching and the rise of mixed lineages at relapses often change the prognosis of the illness. During lineage switching, conversions from lymphoblastic leukemia to myeloid leukemia, or vice versa, are recorded. The central mechanisms involved in these phenomena remain undefined, but recent studies suggest that lineage commitment of plastic hematopoietic progenitors may be multidirectional and reversible upon specific signals provided by both intrinsic and environmental cues. In this paper, we focus on the current knowledge about cell heterogeneity and the lineage switch resulting from leukemic cells plasticity. A number of hypothetical mechanisms that may inspire changes in cell fate decisions are highlighted. Understanding the plasticity of leukemia initiating cells might be fundamental to unravel the pathogenesis of lineage switch in acute leukemias and will illuminate the importance of a flexible hematopoietic development.