scholarly journals ALL blasts drive primary mesenchymal stromal cells to increase asparagine availability during Asparaginase treatment

2021 ◽  
Author(s):  
Martina Chiu ◽  
Giuseppe Taurino ◽  
Erica Dander ◽  
Donatella Bardelli ◽  
Alessandra Fallati ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Amino Acid ◽  
Mesenchymal Stromal Cells ◽  
Stromal Cells ◽  
Lymphoblastic Leukemia ◽  
Leukemic Cell ◽  
Mesenchymal Cells ◽  
Leukemic Cells ◽  
Trade Off ◽  
Healthy Donors

Mechanisms underlying the resistance of Acute Lymphoblastic Leukemia (ALL) blasts to L-asparaginase are still incompletely known. Here we demonstrate that human primary bone marrow mesenchymal stromal cells (MSCs) successfully adapt to L-asparaginase and markedly protect leukemic blasts from the enzyme-dependent cytotoxicity through an amino acid trade-off. ALL blasts synthesize and secrete glutamine, thus increasing extracellular glutamine availability for stromal cells. In turn, MSCs use glutamine, either synthesized through Glutamine Synthetase (GS) or imported, to produce asparagine, which is then extruded to sustain asparagine-auxotroph leukemic cells. GS inhibition prevents mesenchymal cells adaptation to L-asparaginase, lowers glutamine secretion by ALL blasts, and markedly hinders the protection exerted by MSCs on leukemic cells. The pro-survival amino acid exchange is hindered by the inhibition or silencing of the asparagine efflux transporter SNAT5, which is induced in mesenchymal cells by ALL blasts. Consistently, primary MSCs from ALL patients express higher levels of SNAT5 (p < 0.05), secrete more asparagine (p < 0.05), and protect leukemic blasts (p < 0.05) better than MSCs isolated from healthy donors. In conclusion, ALL blasts arrange a pro-leukemic amino acid trade-off with bone marrow mesenchymal cells, which depends on GS and SNAT5 and promotes leukemic cell survival during L-asparaginase treatment.

Primary Acute Lymphoblastic Leukemia Cells Create a Drug-Resistant Niche By Tunneling Nanotube Signaling with Mesenchymal Stromal Cells

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 64-64
Author(s):  
Roel Polak ◽  
Bob De Rooij ◽  
Rob Pieters ◽  
Monique L. den Boer
Keyword(s):  
Bone Marrow ◽  
Flow Cytometry ◽  
Acute Lymphoblastic Leukemia ◽  
Stromal Cells ◽  
Lymphoblastic Leukemia ◽  
Leukemic Cell ◽  
Leukemic Cells ◽  
Dye Transfer ◽  
Tunneling Nanotubes ◽  
Insight Into

Abstract Background: Acute lymphoblastic leukemia (ALL) cells reside in the bone marrow microenvironment, which nurtures and protects cells from chemotherapeutic drugs. Bidirectional signaling between leukemic cells and their niche is shown to be essential for these processes. A major gap in our knowledge is the lack of insight into the functional mechanism regulating and controlling this crosstalk. Tunneling nanotubes (TNTs) have recently been reported as a novel mode of communication between eukaryotic cells, which can facilitate the transport of several types of cellular cargo. Objectives: This study aims to investigate the presence and functional importance of TNTs in the leukemic niche. Results: Here, we show for the first time that TNT signaling occurs between primary patient-derived leukemic cells and primary mesenchymal stromal cells using flow cytometry and time-lapse confocal microscopy. TNTs form within minutes after the start of co-culture and efficiently transfer lipophilic carbocyanine dye DiI from ALL cells towards MSCs. Dye transfer was significantly reduced when TNT signaling was inhibited using three independent experimental setups: actin inhibition, mechanical disruption through gentle shaking of cell cultures, and physical separation using a 3.0 µm pore-sized transwell system (4-fold, 5-fold and 35-fold reduction, respectively). In reciprocal experiments we also observed dye transfer via TNTs from MSCs to BCP-ALL cells, but the magnitude was strikingly less (175-fold, p ≤ 0.001). In addition, we show that leukemic cells use TNTs to modulate their microenvironment by directing non-malignant stromal cells to produce pro-survival cytokines. We observed patient-specific cytokine signatures after co-culture of primary ALL cells with different primary MSCs. For example, IP10/CXCL10 levels increased more than 1000-fold when patient ALL#1 cells were co-cultured with primary MSCs, but were undetectable in co-cultures from patient ALL#2 cells. Induction of all cytokines was dependent on TNT signaling, as TNT inhibition significantly lowered cytokine production (p ≤ 0.001). Importantly, we show that TNT signaling is functionally important for leukemic cell survival and stroma-driven drug resistance. Leukemic cell viability was assessed by flow cytometry after staining with Annexin V, Propidium Iodide and CD19. Primary leukemic cell survival significantly increased in 5-day co-cultures with primary MSCs compared to mono-culture (p ≤ 0.001). When TNT formation was prevented by shaking of these co-cultures or by transwell conditions, the cell viability reduced 3.5- and 3.6-fold, respectively (p ≤ 0.001). TNT inhibition similarly reduced the survival of primary ALL cells in co-culture with MSCs during prednisolone exposure (p ≤ 0.01). Co-culture with primary MSCs also induced resistance to prednisolone 2.5-fold compared to mono-culture in a proliferative setting, using the BCP-ALL cell line NALM6. Inhibition of TNT formation by shaking or transwell conditions significantly reduced this effect: cells in co-culture were only 1.4- and 1.1-fold more resistant compared to mono-culture, respectively (p ≤ 0.01). Conclusion: The presented study identifies TNT formation as a major regulator of interaction between ALL cells and their bone marrow niche, which facilitates signaling mainly from leukemic cells towards MSCs. This signaling drives the release of cytokines within the microenvironment. Disruption of tunneling nanotubes inhibits this release, diminishes the survival benefit that MSCs provide to primary ALL cells, and sensitizes ALL cells to the important anti-leukemic drug prednisolone. This observation gives insight into the pathogenesis of BCP-ALL and opens new avenues to develop more effective therapies that interfere with the leukemic niche. Disclosures No relevant conflicts of interest to declare.

TGF-β Signaling in Fetal Mesenchymal Progenitor Cells Plays an Essential Role in the Emergence of Bone Marrow Hematopoietic Niches

Blood ◽  
2018 ◽  
Vol 132 (Supplement 1) ◽  
pp. 3847-3847
Author(s):  
Grazia Abou Ezzi ◽  
Teerawit Supakorndej ◽  
Jingzhu Zhang ◽  
Joseph R. Krambs ◽  
Hamza Celik ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Progenitor Cells ◽  
Mesenchymal Stromal Cells ◽  
Stromal Cells ◽  
Essential Role ◽  
Suppressive Effect ◽  
Mesenchymal Cells ◽  
Lineage Specification ◽  
Mapk Activation ◽  
Hematopoietic Stem

Abstract Hematopoietic stem/progenitor cells (HSPC) reside in a unique microenvironment within the bone marrow called the bone marrow hematopoietic niche. Mesenchymal stromal cells, including CXCL12-abundant reticular (CAR) cells, osteoblasts, arteriolar pericytes, and adipocytes are all important components of the niche. The development and maintenance of mesenchymal stromal cells in the bone marrow is not well characterized. A prior study suggested that these stromal cells are derived from two distinct types of mesenchymal stem/progenitor cells (MSPCs). Primitive MSPCs are present in fetal bone and are responsible for osteoblasts, CAR cells, and adipocytes through approximately 3 weeks after birth, and definitive MPSCs are present at birth and generate bone marrow mesenchymal stromal cells in adult mice. In this study, we abrogated transforming growth factor-b (TGF-β) signaling in MSPCs by deleting Tgfbr2in mesenchymal cells using a doxycycline-repressible Sp7(osterix)-Cre transgene (Osx-Cre).We previously reported that loss of TGF-βsignaling during fetal development results in a marked expansion of CAR cells and adipocytes in the bone marrow, while osteoblasts are significantly reduced. These stromal alterations are associated with significant defects in hematopoiesis, including a shift from lymphopoiesis to myelopoiesis. However, hematopoietic stem cell function is preserved. Interestingly, TGF-βsignaling is dispensable for the maintenance of mesenchymal cells in the bone marrow after birth under steady state conditions. These data show that TGF-βplays an essential role in the lineage specification of fetal but not definitive MSPCs and is required for the establishment of normal hematopoietic niches in fetal and perinatal bone marrow. Canonical TGF-bsignaling is dependent on SMAD4. To investigate whether MSPC lineage specification by TGF-bis dependent on SMAD4, we generated Osx-Cre Smad4Δ/Δmice. Osx-Cre Smad4Δ/Δmice are runted to a similar degree as Osx-CreTgfbr2Δ/Δmice secondary to a loss of mature osteoblasts. However, the magnitude of the increase in bone marrow adiposity is significantly reduced in Osx-Cre Smad4∆/∆mice compared to Osx-Cre, Tgfbr2Δ/Δmice. These data suggested that non-canonical signaling contributes to the suppressive effect of TGF-b on adipogenesis. To test this hypothesis, we generated cultures of mesenchymal stromal cells from wildtype neonatal bone marrow. As expected, in wildtype cultures, the addition of TGF-bpotently suppressed adipocyte formation. To assess the role of MAPK activation on the suppression of adipogenesis by TGF-b, we pharmacologically inhibited MAPK activation. Inhibition of MAPK alone did not suppress adipocyte formation. However, it completely blocked the suppressive effect of TGF-bon adipogenesis. Prior studies showed that phosphorylation of serine 82 of PPARgby MAPK decreases its transcriptional activity. Since PPARgis a master regulator of adipogenesis, we assessed the ability of TGF-b to induce PPARgphosphorylation. Indeed, the addition of TGF-b to the MSPC cultures resulted in reproducible PPARgphosphorylation. These data suggest that TGF-b suppresses adipocyte specification of MSPCs, in part, in a MAPK-dependent fashion through phosphorylation of PPARg. In summary, our data suggest that TGF-b plays a key role in the lineage specification of fetal MSPCs during development and is required for the proper development of fetal hematopoietic niches in the bone marrow. The contribution of TGF-b signaling in MSPCs to the stromal and hematopoietic response to different stressors is an active area of investigation. Disclosures No relevant conflicts of interest to declare.

Lenalidomide Modulates the Phenotype and Function of Human Mesenchymal Stromal Cells From Healthy Donors and MDS Patients.

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 3816-3816
Author(s):  
Manja Wobus ◽  
Gwendolin Dünnebier ◽  
Silvia Feldmann ◽  
Gerhard Ehninger ◽  
Martin Bornhauser ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Osteogenic Differentiation ◽  
Clinical Efficacy ◽  
Mesenchymal Stromal Cells ◽  
Stromal Cells ◽  
Morphological Changes ◽  
Adipogenic Differentiation ◽  
Healthy Controls ◽  
Healthy Donors

Abstract Abstract 3816 Poster Board III-752 Introduction Recent studies in patients with MDS have clearly demonstrated the clinical efficacy of lenalidomide. However, its exact mechanisms of action have not been elucidated yet. Myelosuppression is the most common adverse event and seems to be dependent on dose as well MDS subtype, being rather infrequent in patients other than del5q. The aim of this study was to investigate whether lenalidomide affects the bone marrow microenvironment. Therefore, we analyzed in-vitro characteristics of isolated mesenchymal stromal cells (MSCs) from MDS patients and from healthy controls. Methods Bone marrow samples were collected from healthy donors (n=5) and patients with MDS (del5q MDS n=3, RA n=2, RAEB1/2 n=3). MSCs were isolated according to the standard adhesion protocol and cultured in the presence or absence of lenalidomide. Results Lenalidomide treatment of MSCs caused no morphological changes but proliferation was slightly increased. Typical surface molecules as CD73, CD90, CD105 and CD166 were expressed in MSCs from MDS patients at comparable levels to healthy controls. Lenalidomide treatment caused an upregulation of CD29 by 17.8 ± 4.4% and of CD73 by 24 ± 5.7% (mean fluorescence intensity). Investigating the cytokine production, we found lower IL-8 mRNA and protein levels in MSCs from MDS patients (mean in MDS MSC: 138.1 pg/ml vs. mean in healthy MSC: 1177 pg/ml). Interestingly, the IL-8 production can be increased by approximately 40% under lenalidomide treatment. MDS MSCs retained the capacity for adipogenic and osteogenic differentiation as well as their supportive function towards hematopoietic cells in long term culture-initiating assays (LTC-IC). However, the LTC-IC frequency was lower on MSC which had been preincubated with lenalidomide compared to controls. Lenalidomide also slightly accelerated osteogenic differentiation because mineralization started as early as on day 5 with lenalidomide whereas in the control cells first calcium deposits were visible after 7 days. Other samples showed augmented lipid vacuoles after adipogenic differentiation under lenalidomide treatment. Conclusion In conclusion, lenalidomide modulates the phenotype of MSC and leads to an increase of their IL-8 secretion by a yet unknown mechanism. Whether these in-vitro effects are associated with the clinical efficacy of this compound in patients with MDS remains to be investigated. Disclosures: Platzbecker: Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

The Bone Marrow Niche of Patients with Acute Lymphoblastic Leukemia Produces No Increased Asparagine Levels In Vivo That May Lead to Clinical Asparaginase Resistance

Blood ◽  
2011 ◽  
Vol 118 (21) ◽  
pp. 1505-1505
Author(s):  
Wing H. Tong ◽  
Rob Pieters ◽  
Wim C.J. Hop ◽  
Claudia Lanvers-Kaminsky ◽  
Joachim Boos ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Aspartic Acid ◽  
Peripheral Blood ◽  
Lymphoblastic Leukemia ◽  
Mesenchymal Cells ◽  
Leukemic Cells ◽  
Significant Difference ◽  
Trough Levels

Abstract Abstract 1505 Asparaginase is an essential component of combination chemotherapy of acute lymphoblastic leukemia (ALL). Asparaginase breaks down asparagine into aspartic acid and ammonia. Because asparagine is necessary for protein synthesis, its depletion leads to cell death. Recently, it has been suggested that mesenchymal cells in the bone marrow may produce asparagine and form ‘protective niches’ for leukemic cells. In vitro, this led to high levels of asparagine and asparaginase resistance of the ALL cells (Iwamoto et al. (J Clin Invest. 2007)). However, it is unknown if this holds true for the clinical in vivo situation. The aim of our study is to analyse whether mesenchymal cells or other cells in the bone marrow indeed produce significant amounts of asparagine in vivo that may lead to clinical asparaginase resistance. Ten de novo ALL patients were enrolled in this study. All children received induction chemotherapy according to protocol 1-A and 1-B of the Dutch Childhood Oncology Group (DCOG) ALL-10 protocol. Asparaginase levels and amino acid levels (asparagine, aspartic acid, glutamine and glutamic acid) were measured in bone marrow (BM) and peripheral blood at diagnosis (day 1), days 15, 33 and 79. On days that asparaginase was administered (days 15 and 33) it was ensured that study material was obtained before the E-coli L-asparaginase infusions. Changes over time of asparaginase trough levels in BM and peripheral blood were evaluated using Mixed models ANOVA. The amino acids levels in 0.5 ml BM, 3 ml BM and peripheral blood at days 15 and 33 were also compared using Mixed models ANOVA. All these analyses were done after log transformation of measured values to get approximate normal distributions. A two-sided p-value < 0.05 was considered statistically significant. The asparaginase levels were all below detection limit (< 5 IU/L) in BM and peripheral blood at days 1 and 79. In both compartments, the median asparaginase trough levels were not significantly different at days 15 and 33. At diagnosis, no significant difference in asparagine level between 3 ml BM and peripheral blood was found (median: 44.5 μM (range 20.6–59.6 μM) and 43.9 μM (range 18.4 –58.5 μM), respectively). However, the median level of aspartic acid at diagnosis in 3 ml BM (19.2 μM; range 6.2–52.6 μM) was significantly higher as compared to median level of peripheral blood (5.7 μM; range 2.4–10.1 μM) (p=0.002). The aspartic acid levels were also higher in BM compared to peripheral blood at days 15 and 33 (both p=0.001) and at day 79 (p=0.002). Aspartic acid levels were significantly higher in 0.5 ml versus 3 ml BM (p=0.001) and this difference was also found when comparing 0.5 ml BM versus peripheral blood (p<0.001) suggesting dilution with peripheral blood when taking higher volumes of ‘bone marrow’. Asparagine levels were all below the lower limit of quantification (LLQ < 0.2 μM) in both BM and blood during asparaginase treatment at days 15 and 33. At day 79, no significant difference in asparagine levels between BM (37.7 μM; range 33.4–50.3 μM) and peripheral blood (38.9 μM; range 25.7 –51.3 μM) was seen. During the time course of asparaginase infusions, the glutamine and glutamic acid levels did not change significantly. In conclusion, we demonstrate higher aspartic acid levels in bone marrow compared to peripheral blood. The higher aspartic acid levels are detected at diagnosis, during asparaginase therapy at days 15 and 33, and also at day 79 at complete remission, showing that these do not originate from leukemic cells nor from asparagine breakdown by asparaginase but from cells in the microenvironment of the bone marrow. However, there is no increased asparagine synthesis in vivo in the bone marrow of ALL patients. Therefore, increased asparagine synthesis by mesenchymal cells may be of relevance for resistance to asparaginase of leukemic cells in vitro but not in vivo. Disclosures: No relevant conflicts of interest to declare.

Chronic Lymphocytic Leukemia-Exosomes Switch Endothelial and Mesenchymal Stromal Cells into Cancer-Associated Fibroblasts to Sustain Leukemic Cell Survival

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 2927-2927 ◽  
Author(s):  
Jerome Paggetti ◽  
Franziska Haderk ◽  
Martina Seiffert ◽  
Bassam Janji ◽  
Yeoun Jin Kim ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Chronic Lymphocytic Leukemia ◽  
Cell Survival ◽  
B Lymphocytes ◽  
Stromal Cells ◽  
Leukemic Cell ◽  
Lymphoid Organs ◽  
Lymphocytic Leukemia ◽  
Leukemic Cells

Abstract Chronic lymphocytic leukemia (CLL), the most common hematologic malignancy in Western countries, is mostly affecting the elderly over 65 year-old. CLL is characterized by the accumulation of mature but non-functional B lymphocytes of clonal origin in the blood and the primary lymphoid organs. CLL was previously considered as a relatively static disease resulting from the accumulation of apoptosis-resistant but quiescent B lymphocytes. However, recent studies using heavy water labeling indicated that CLL is in fact a very dynamic disease with alternation of proliferation phases and peripheral circulation. A focus on the trafficking of CLL cells in vivo has shown that leukemic cells circulate between the blood and the lymphoid organs but have a preference for the bone marrow. Recent next-generation sequencing of CLL cells indicated the presence of different genetic subclones. This intraclonal heterogeneity observed in CLL subpopulations may be in part determined by the interactions that leukemic cells entertain with their microenvironment when B cells migrate into the lymph nodes and the bone marrow. Indeed, tumor-stroma interactions are not only providing signals necessary for leukemic cells survival but may also influence the clonal architecture and evolution. One of these interactions involves CLL-derived exosomes. Here, we show that CLL-exosomes efficiently transfer nucleic acids, including functional microRNAs, and proteins, including MHC-Class II molecules and B-cell specific proteins, to bone marrow mesenchymal stem cells and endothelial cells. CLL-exosomes also activate signaling pathways, including PI3K and NF-κB pathways, in these stromal cells. As a consequence, gene expression is strongly modified indicating a switch towards a cancer-associated fibroblast phenotype. Functionally, exosome-stimulated stromal cells show a striking actin cytoskeleton remodeling characterized by the formation of stress fibers, and enhanced proliferation, motility and angiogenic properties. We also identified several proteins synthesized and secreted by stromal cells that promote leukemic cell adhesion and survival ex vivo. To confirm the involvement of CLL-exosomes in CLL pathology in vivo, MEC-1-eGFP cells were subcutaneously injected into immunocompromised NSG mice together with CLL-exosomes. We observed a significant increase in tumor size and a reduction in survival of exosome-treated animals. Flow cytometry analysis of selected organs indicated an enrichment in leukemic cells in the kidney, providing a potential explanation to the renal failures observed in CLL patients. In conclusion, the communication between CLL cells and stromal cells may be a critical factor influencing CLL progression by promoting leukemic cell survival. This study demonstrates the crucial role of exosomes as mediators of the communication between leukemic cells and their microenvironment. Exosomes could thus represent a suitable target for therapeutic intervention in CLL. Disclosures No relevant conflicts of interest to declare.

scholarly journals Human extramedullary bone marrow in mice: a novel in vivo model of genetically controlled hematopoietic microenvironment

Blood ◽  
2012 ◽  
Vol 119 (21) ◽  
pp. 4971-4980 ◽  
Author(s):  
Ye Chen ◽  
Rodrigo Jacamo ◽  
Yue-xi Shi ◽  
Rui-yu Wang ◽  
Venkata Lokesh Battula ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Mesenchymal Stromal Cells ◽  
Stromal Cells ◽  
Critical Role ◽  
Hematopoietic Cells ◽  
Leukemic Cells ◽  
In Vivo Model ◽  
Hematopoietic Microenvironment ◽  
Cell Engraftment

Abstract The interactions between hematopoietic cells and the bone marrow (BM) microenvironment play a critical role in normal and malignant hematopoiesis and drug resistance. These interactions within the BM niche are unique and could be important for developing new therapies. Here, we describe the development of extramedullary bone and bone marrow using human mesenchymal stromal cells and endothelial colony-forming cells implanted subcutaneously into immunodeficient mice. We demonstrate the engraftment of human normal and leukemic cells engraft into the human extramedullary bone marrow. When normal hematopoietic cells are engrafted into the model, only discrete areas of the BM are hypoxic, whereas leukemia engraftment results in widespread severe hypoxia, just as recently reported by us in human leukemias. Importantly, the hematopoietic cell engraftment could be altered by genetical manipulation of the bone marrow microenvironment: Extramedullary bone marrow in which hypoxia-inducible factor 1α was knocked down in mesenchymal stromal cells by lentiviral transfer of short hairpin RNA showed significant reduction (50% ± 6%; P = .0006) in human leukemic cell engraftment. These results highlight the potential of a novel in vivo model of human BM microenvironment that can be genetically modified. The model could be useful for the study of leukemia biology and for the development of novel therapeutic modalities aimed at modifying the hematopoietic microenvironment.

Cell-cycle phases and genetic profile of bone marrow-derived mesenchymal stromal cells expanded in vitro from healthy donors

2011 ◽  
Vol 112 (7) ◽  
pp. 1817-1821 ◽  
Author(s):  
Valentina Achille ◽  
Melissa Mantelli ◽  
Giulia Arrigo ◽  
Francesca Novara ◽  
Maria Antonietta Avanzini ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Bone Marrow ◽  
Mesenchymal Stromal Cells ◽  
Stromal Cells ◽  
Genetic Profile ◽  
Healthy Donors ◽  
Cell Cycle Phases

scholarly journals Adhesion-dependent survival of normal and leukemic human B lymphoblasts on bone marrow stromal cells

Blood ◽  
1994 ◽  
Vol 83 (3) ◽  
pp. 758-766 ◽  
Author(s):  
A Manabe ◽  
KG Murti ◽  
E Coustan-Smith ◽  
M Kumagai ◽  
FG Behm ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Stromal Cells ◽  
Lymphoblastic Leukemia ◽  
Leukemic Cells ◽  
Cell Recovery ◽  
Intimate Contact ◽  
Late Antigen ◽  
Marrow Stroma ◽  
B Lineage ◽  
Student’S T

We investigated the requirement for intimate contact between bone marrow stroma and B lymphoblasts from normal donors and children with leukemia. By scanning electron microscopy, both normal and leukemic cells seeded onto stroma were surrounded by folds of stromal cells or were linked to the stroma by fine tendrils and uropods. Separation of normal B progenitors from stroma by use of microporous membranes led to significantly lower cell recoveries compared with results when contact was unimpeded. For instance, 22.5% +/- 1.8% (mean +/-SEM) of CD19+, CD34+ cells (most immature subset) were recovered after a 7-day culture directly on stroma, compared with 5.2%+/-0.7% after growth on membranes (P < .001 by Student's t test). In 6 of 11 cases of B-lineage acute lymphoblastic leukemia, separation of progenitors from stroma resulted in apoptosis and a greater than 60% reduction in cell recovery. In the remaining 5 cases, however, this effect was much less pronounced, with reductions in cell recoveries ranging from 48.5% to less than 1% (median, 39.0%) of control values. Inhibition of very late antigen-4, a surface molecule critical for adhesion of B lymphoblasts to stroma, was associated with a greater loss of normal CD34+ B progenitors compared with that for equivalent leukemic cells. These results establish direct contact with stroma as a survival requirement of normal B lymphoblasts and show marked heterogeneity in stromal dependency among B-lineage leukemic cells.

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