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The Gluthation peroxidase (GPX) enzymes are part of the protective system against lipid peroxydation that includes prevention of oxydation and reduction of already oxidized lipid through enzymatic reactions catalyzed by GSH. GPX4 is one of the five GPX able to incorporate selenium. It is also the only GPX able to directly reduce in the membrane the oxidized fatty acids and cholesterol. Recent reports identified GPX4 as the central inhibitor of ferroptosis, a process during which iron-induced peroxydation of membrane lipids causes a specific cell death that can be reverted by lipophilic antioxydants or by iron chelators. GPX4 has recently been involved during mice erythropoiesis: GPX4-/-mice present a hemolytic anemia and a high apoptotic rate in spleen erythroid progenitors. Although transcriptomics and proteomics found it expressed in human erythroid precursors, its role during human erythropoiesis has not been described. Using an in-vitroerythroid differentiation protocol from CD34+cells obtained from apheresis, we confirmed that GPX4 expression was induced at RNA and protein level during differentiation. RSL3, a specific GPX4 inhibitor, didn't affect early steps of erythropoiesis (i.eclonogenic potential and progenitor amplification) nor the early maturation of erythroid precursors (assessed by sequential CD49d/CD235/CD71 staining) but led to a significant decrease in the enucleation rate as assessed by Hoechst staining using flow cytometry (74%±9 DMSO versus 35%±6 RSL3, p Using Western Blot, we observed that RSL3 exposure induced a strong GPX4 depletion in erythroid progenitors while it didn't affect GPX1, another member of the GPX family expressed in erythroid cells. In order to confirm that enucleation defect was related to GPX4 knockdown, we used an Sh-RNA strategy that allowed a 62%±6 GPX4 decrease at RNA level and a 46%±5 at protein level. We observed a significant defect in terminal enucleation in the cells transduced with shGPX4-lentiviruses in comparison with sh-Scramble (59%±5 Sh-Scr versus 39%±6 Sh-Gpx4, p We investigated then whether GPX4-knock-down affected quantitatively or qualitatively the membrane lipid content, which was shown to be involved in the enucleation process. Addition of Cholesterol to RSL3 in the medium partially restored the enucleation rate. However, lipidomics failed to show any significant difference in the total membrane lipid content (and particularly in the cholesterol content) after RSL3 exposure in comparison to DMSO. Since cholesterol is particularly abundant in the lipid rafts, we investigated whether the lipid distribution was qualitatively altered within the cell membrane. We observed a disruption of membrane lipid rafts when cells were exposed to RSL3, as shown by a 60%±12 decrease in the mean cholera toxin fluorescence intensity. GPX4 presence in lipid rafts was confirmed using immunofluorescence showing their co-localization at cell surface in human primary erythroblasts. Since lipid rafts play a role in the contractile ring that separates pyrenocyte from reticulocyte, we evaluated the myosin-light chain phosphorylation using flow cytometry and Western Blot and found it drastically decreased in GPX4-knockdown conditions. In summary, we identified GPX4 as a new actor of human terminal erythroid differentiation, independently to its function in ferroptosis control. We described its interaction at cell surface with lipid rafts that are required for the assembly of the contractile ring and cytokinesis leading to the nucleus extrusion. Disclosures No relevant conflicts of interest to declare.

Acute lymphoblastic leukemias (ALL) are characterized by a large number of genetic abnormalities of prognostic and theranostic interest. Their detection requires the use of several conventional and molecular cytogenetic techniques in order to establish the most exhaustive genetic profile: conventional karyotype, FISH, SNP-Array and RT-MLPA for detection of fusion transcripts. Optical genome mapping technique is based on analysis of ultra-high molecular weight DNA (UHMW: 150 kbp-2.5 Mbp). It allows an automatic, high resolution, genome-wide detection of structural anomalies, including balanced translocations and copy number anomalies, making this technology a promising tool in ALL. However, very few data are available in ALL so far. Our objective was to compare optical mapping (Bionano Genomics) to standard techniques performed routinely (combination of karyotype, FISH, SNP-array and RT-MLPA) in 10 selected B or T-ALL patients carrying at least one major genetic abnormality including t(10;14) TLX1-TRA, t(9;22) BCR-ABL1 (e1a2 and e14a2 transcripts), t(10;11) MLLT10-PICALM, t(11;19) KMT2A-MLLT1, typical hyperdiploid profile, iAMP21, SET-NUP214 fusion, various deletions involving CDKN2A/CDKN2B, IKZF1, PAX5, PTEN, ERG and FBXW7. Abnormalities on SNP-array were retained when allele fraction was > 10% and larger than 1Mbp or > 20 kbp and involving one of the 186 genes defined as potentially relevant in leukemogenesis. A total of 79 anomalies meeting these criteria in SNP-array, and/or found by other standard techniques were identified (45 deletions, 16 whole chromosome gains, 10 duplications or partial gains, and 8 translocations). Bone marrow DMSO pellets from the 10 patients, all treated at the CHU Amiens-Picardie, were used for this study. The UHMW DNA extraction technique provided molecules with an average N50 (≥ 150 kbp) of 286 kbp, and an average coverage of 339x. Direct imaging of the DNA molecules was performed on the Bionano Saphyr system and structural variants were automatically detected using the Rare Variant Pipeline. Abnormalities analysis was blindly performed by two independent operators on the Bionano Access software (v 1.5.2.), one cytogeneticist with no training in optical mapping (operator 1) and the other optical mapping technician without training in cytogenetics (operator 2). Among the 79 relevant anomalies, 72 (91.1%) were found by operator 1 and 71 (89.9%) by operator 2. All (8/8) translocations, 43/45 (95.6%) of deletions, 8/10 (80%) of gains were found by both operators whereas 13/16 (81.25%) and 12/16 (75%) of chromosomal gains were found by operator 1 and 2 respectively. Concerning the 8 discrepancies between optical mapping and standard techniques, 4 of them were filtering issues. Three were low allelic frequency large gains clearly visible in raw data but rejected by software's CNV filters, totally, or partially for a trisomy 3 that caused the discrepancy between the two observers. The fourth was a deletion of VPREB1 gene, erroneously eliminated by operators because included in recurrent deletion zones of IGL locus. The 4 others discrepancies were non detected anomalies. Two of them were copy number variations in Xp22.33 (pseudoautosomal region), a non-covered area with the optical mapping technology, one of which was a P2RY8-CRLF2 fusion. Another sub-clonal gain (trisomy 21) found by karyotype and FISH (quantified at 7% of all nuclei) but not by SNP-array, was missed, even in raw data. The last was a near-tetraploid subclone, found only on karyotype, that might be strictly tetraploid and by definition undetectable by quantitative techniques. Interestingly, new anomalies, not detected by standard techniques, were revealed, including a translocation t(11;14)(p13;q11.2) LMO2-TRA, a translocation with inversion t(7;8)(q34;q24.1) TRB-MYC and a translocation t(14;20)(q32.33;q13.13) IGH-CEBPB. At last, in 3 cases, unidentified complex rearrangements by cytogenetics could be resolved using optical mapping. Thus, optical mapping represents an innovative, time and cost effective alternative to the different cytogenetic techniques currently performed routinely for ALL genetic characterization. It enables identification of complex cytogenetic events, including those currently inaccessible to standard techniques such as chromotripsis or certain complex translocations. Disclosures Lee: Bionano Genomics, San Diego: Current Employment. Ferret:Bionano Genomics: Research Funding.

To the editor: Red cells of patients with hereditary spherocytosis (HS) have a decreased surface-to-volume ratio,[1][1] leading to their trapping and destruction during their passage through the splenic cords.[2][2][⇓][3]-[4][4] Therefore, surgical total splenectomy (TS) by removing the main site

Diamond Blackfan anemia (DBA) is a congenital disorder with erythroid (Ery) hypoplasia and tissue morphogenic abnormalities. Most DBA cases are caused by heterozygous null mutations in genes encoding ribosomal proteins. Understanding how haploinsufficiency of these ubiquitous proteins causes DBA is hampered by limited availability of tissues from affected patients. We generated induced pluripotent stem cells (iPSCs) from fibroblasts of DBA patients carrying mutations in RPS19 and RPL5. Compared with controls, DBA fibroblasts formed iPSCs inefficiently, although we obtained 1 stable clone from each fibroblast line. RPS19-mutated iPSCs exhibited defects in 40S (small) ribosomal subunit assembly and production of 18S ribosomal RNA (rRNA). Upon induced differentiation, the mutant clone exhibited globally impaired hematopoiesis, with the Ery lineage affected most profoundly. RPL5-mutated iPSCs exhibited defective 60S (large) ribosomal subunit assembly, accumulation of 12S pre-rRNA, and impaired erythropoiesis. In both mutant iPSC lines, genetic correction of ribosomal protein deficiency via complementary DNA transfer into the "safe harbor" AAVS1 locus alleviated abnormalities in ribosome biogenesis and hematopoiesis. Our studies show that pathological features of DBA are recapitulated by iPSCs, provide a renewable source of cells to model various tissue defects, and demonstrate proof of principle for genetic correction strategies in patient stem cells.

The fact that you can vaccinate a child at 5 years of age and find lymphoid B cells and antibodies specific for this vaccination 70 years later remains an immunologic enigma. It has never been determined how these long-lived memory B cells are maintained and whether they are protected by storage in a special niche. We report that, whereas blood and spleen compartments present similar frequencies of IgG+ cells, antismallpox memory B cells are specifically enriched in the spleen where they account for 0.24% of all IgG+ cells (ie, 10-20 million cells) more than 30 years after vaccination. They represent, in contrast, only 0.07% of circulating IgG+ B cells in blood (ie, 50-100 000 cells). An analysis of patients either splenectomized or rituximab-treated confirmed that the spleen is a major reservoir for long-lived memory B cells. No significant correlation was observed between the abundance of these cells in blood and serum titers of antivaccinia virus antibodies in this study, including in the contrasted cases of B cell– depleting treatments. Altogether, these data provide evidence that in humans, the two arms of B-cell memory—long-lived memory B cells and plasma cells—have specific anatomic distributions—spleen and bone marrow—and homeostatic regulation.

The biologic hallmark of polycythemia vera (PV) is the formation of endogenous erythroid colonies (EECs) with an erythropoietin-independent differentiation. Recently, it has been shown that an activating mutation of JAK2 (V617F) was at the origin of PV. In this work, we studied whether the STAT5/Bcl-xL pathway could be responsible for EEC formation. A constitutively active form of STAT5 was transduced into human erythroid progenitors and induced an erythropoietin-independent terminal differentiation and EEC formation. Furthermore, Bcl-xL overexpression in erythroid progenitors was also able to induce erythroid colonies despite the absence of erythropoietin. Conversely, siRNA-mediated STAT5 and Bcl-xL knock-down in human erythroid progenitors inhibited colony-forming unit-erythroid (CFU-E) formation in the presence of Epo. Altogether, these results demonstrate that a sustained level of the sole Bcl-xL is capable of giving rise to Epo-independent erythroid colony formation and suggest that, in PV patients, JAK2V617F may induce EEC via the STAT5/Bcl-xL pathway.

A defect in cell trafficking and chemotaxis plays an important role in the immune deficiency observed in Wiskott-Aldrich syndrome (WAS). In this report, we show that marrow cells from WAS protein (WASP)–deficient mice also have a defect in chemotaxis. Serial transplantation and competitive reconstitution experiments demonstrated that marrow cells, including hematopoietic progenitors and stem cells (HSCs), have decreased homing capacities that were associated with a defect in adhesion to collagen. During development, HSCs migrate from the liver to the marrow and the spleen, prompting us to ask if a defect in HSC homing during development may explain the skewed X-chromosome inactivation in WAS carriers. Preliminary evidence has shown that, in contrast to marrow progenitor cells, fetal liver progenitor cells from heterozygous females had a random X-chromosome inactivation. When fetal liver cells from WASP-carrier females were injected into irradiated recipients, a nonrandom inactivation of the X-chromosome was found at the level of hematopoietic progenitors and HSCs responsible for the short- and long-term hematopoietic reconstitution. Therefore, the mechanism of the skewed X-chromosomal inactivation observed in WAS carriers may be related to a migration defect of WASP-deficient HSCs.

Total splenectomy is recommended in symptomatic cases of hereditary spherocytosis (HS) to reduce the severity of anemia but exposes patients to long-term infectious or thrombotic complications. Alternative strategies such as subtotal splenectomy (STS) have been developed, principally for children under the age of 6 with severe HS, who are not eligible to total splenectomy because of the high infectious risk and for older patients with mild HS complaining of chronic discomfort. Since our original report, several groups have shown that STS reduced the hemolytic rate, increased the red cell lifespan while maintaining an efficient splenic phagocytic function but the extent of follow-up has been limited. In order to define the long-term benefits of STS, we report here an update of our series that includes 90 patients who underwent STS at the Bicêtre hospital between 1985 and 2013, with a median-follow-up of 9.3 years (range 1-23 years). Two groups were defined on the basis of the disease phenotype: Group A included 42 patients for whom STS was performed because of severe/intermediate HS (transfusion requirement or hemoglobin (Hb) level In summary, our results show that in the long-term, STS resulted in a decrease in the hemolytic rate in HS. In children under the age of six years with severe HS, STS decreased the transfusion rate and increased the hemoglobin to a level compatible with normal growth while retaining splenic function. However, half of these patients will require total splenectomy, but at an age where it will be much safer. In milder HS, STS removed discomfort related to the high hemolytic rate whithout exposing the patients to the risks of a total splenectomy. Disclosures No relevant conflicts of interest to declare.

DBA is an inherited bone marrow failure syndrome that usually presents in the first year of life with red cell aplasia and variable developmental abnormalities. Most affected patients have heterozygous loss of function mutations in one of 11 genes encoding small or large subunit ribosomal proteins (RPs). It is not known how RP haploinsufficiency causes failed erythropoiesis. The penetrance of DBA varies widely, even between members of the same family. This is presumably due to modifying factors but the nature of these remains enigmatic. We have previously derived induced pluripotent stem cells (iPSCs) from fibroblasts of a DBA patient with a RPS19 mutation and the mutant line showed abnormal ribosome biogenesis and altered erythropoiesis, thus recapitulating the DBA phenotype. To investigate the mechanisms underlying variable penetrance in DBA, we generated induced pluripotent stem cells (iPSCs) from members of a family with DBA caused by a nonsense mutation in RPS19 (p.Q126X). We analyzed iPSCs from two DBA patients and an asymptomatic individual, all of whom carry the RPS19 mutation. iPSC lines from the DBA affected individuals showed faulty 40S (small) ribosomal subunit assembly, reduced processing of 18S rRNA, and impaired hematopoiesis, with about 2.5 fold decreased expansion of mutant erythroblasts compared to those derived from iPSCs from normal individuals or the asymptomatic carrier. In contrast, iPSCs from the asymptomatic carrier showed milder defects in ribosome biogenesis and hematopoiesis. Genetic correction of ribosomal protein deficiency via cDNA transfer into the “safe harbor” AAVS1 locus alleviated abnormalities in ribosome biogenesis and hematopoiesis in the DBA iPSCs. Using an Affymetrix human exon array to investigate the pathways that are affected in these iPSCs, we observed striking over-activation of the TGF beta pathway in iPSCs of the DBA affected individuals compared to the normal control lines. For example, TGF beta downstream genes, such as DKK1, BAMBI, IGFBP5, COL3A1, COLA1A1, SERPINE1, TGFBI and RGS5 were significantly increased in the DBA iPSCs compared to wild type control cells (Table 1Table 1TGF beta signaling that are upregulated in DBA iPSCs with RPS19 (p.Q126X) mutation.GENERPS19+/p.Q126XpCommentsFN13.6770.002Initiation and progression of matrix assemblyRGS52.0310.002Regulator of vascular remodelingTGFBI4.4410.000Interacts with Extracellular Matrix (ECM)IGFBP55.1720.001Regulator of IGF, interacts with ECM, regulator cell growth/differentiation/ apoptosisBAMBI1.7540.017TGF beta negative regulatorSERPINE12.620.001Inhibitor of fibrinolysis; Inhibits malignant cell invasionCOL3A11.9890.007Component of ECMCOL1A12.1320.001Component of ECMTGFB11.3710.025Regulates cell proliferation, differentiation and apoptosisDKK15.3810.001Negative regulator of Wnt signaling). We conclude that the variable penetrance of the RPS19 mutation in this family is due to modulation of its effect on ribosome biogenesis and that TGF beta signaling may be important in DBA pathogenesis. Whether the genetic factors responsible for the variable penetrance are inherited or acquired remains an important question. Disclosures: No relevant conflicts of interest to declare.

Abstract 4033 Poster Board III-969 G6PD deficiency is the more common human enzyme defect, leading typically to an acute intravascular hemolysis occurring when red cells are exposed to an oxidative stress. However, in rare patients, very low enzymatic level induces the class I G6PD deficiency according to the WHO classification, i.e. a chronic non-spherocytic hemolytic anemia. These cases are all sporadic, occur worldwide, and almost all arise from de novo independent mutations. These mutations are found at the genomic level in both hematopoietic and non hematopoietic cells and occur recurrently in “hot spots”: most of them are located in the exon 10 of the G6PD gene, implicated in the dimer formation and the stability of the active enzyme. No null or frameshift mutations have been reported yet, probably because such mutations would be lethal; indeed, a minimal residual G6PD activity is essential during embryogenesis. We report here the case of 65 years-old Caucasian man referred in 1993 for hemolytic anemia. No personal or familial history of anemia was noted. Nine and five years before, the hemoglobin (Hb) level and the mean corpuscular volume (MCV) were normal. At diagnosis, the anemia was moderate (Hb: 11.9 g/dL), macrocytic (MCV: 104 fL) and regenerative (reticulocyte count: 550G/L) with hemolytic features. Platelets and leucocytes counts were normal. No clinical or ultrasound spleen enlargement was noted. The screening tests ruled out all the classical causes of acquired hemolytic anemia. Red cells half life after Cr51 labelling was shortened with autologous (5 days) but not with allogenous red cells confirming the corpuscular mechanism of the hemolysis. Dosage of erythocyte G6PD activity revealed a very low level (from 0.6 to 1.5 UI/g of Hb, normal range 5.3-7.9). Pyruvate kinase, pyrimidine 5'-nucleotidase and hexokinase enzymatic activities were increased in agreement with the reticulocytosis. With a follow-up of 16 years, evolution was marked by a progressive worsening of the anemia, requiring 8 to 16 packed red cell transfusions per year in parallel with an iron chelation and folic acid therapy. Concomittantly, the macrocytosis increased up to 144 fL. At the molecular level, sequencing of the genomic DNA of the G6PD gene revealed presence of a single nucleotide mutation that altered the IVS 10 nucleotide 1 G>A from donor consensus sequence, leading to an impaired splicing between exon 10 and exon 11. The missplicing creates a premature termination codon giving theorically a truncated protein (464 versus 514 amino-acid). This severe genotype was discordant with the normal hemoglobin level five years before the diagnosis. Moreover, such a mutation at the germinal level would be expected to be lethal; therefore, we hypothesized that it arised at the somatic level in an hematopoietic stem cell; we measured the G6PD activity in hematopoietic and non hematopoietic cells and observed that it was decreased in red cells, polymorphonuclear cells and lymphocytes from blood, but was normal in skin fibroblasts; molecular analysis confirmed that the mutation was present in blood mononuclear cells but was absent in the fibroblasts. Bone marrow CD34+ cells were then sorted and plated at one cell per dish in the presence of a cocktail of cytokines including EPO and the different clones were harvested at day 10 for genomic analysis of the G6PD gene. Out of the 44 tested clones, we found that most of them carried the mutation (61%), but we were still able to detect a few unmutated clones (7%), reflecting the persistence of a minor polyclonal unmutated hematopoiesis. Surprisingly, we detect in the rest of the clones both the normal and mutated genotypes; at this time, we have no clear explanation to this finding. Acquired modifications of structure or expression of genes implicated in red cell physiology have been long recognized, occurring principally in myeloid malignancies such as myelodysplastic syndromes (MDS). This includes genes implicated in the red cell membrane skeleton or in haemoglobin synthesis or structure. However, our patient presented no MDS features on repeated bone marrow examination and on cytogenetic analysis. Regardless to all our data, we concluded that our patient present the first case ever reported of chronic non spherocytic anemia related to an acquired somatic mutation of the G6PD gene at the level of a hematopoietic pluripotent cell. Disclosures: No relevant conflicts of interest to declare.

BAFF, a member of the TNF family, has been shown to be implicated in B cell maturation and survival through an interaction with its three receptors, BCMA, TACI and BAFF-R. Recent data revealed that it also plays a role in B cell malignancies. For exemple, an autocrine activation of the BAFF receptors could participate to oncogenesis during multiple myeloma or chronic lymphocytic leukemia. BAFF-R, the sole BAFF specific receptor, is expressed in a subset of Diffuse Large B Cell Lymphomas (DLBCL). Since BAFF-R activation in B cells leads to cell proliferation and survival, expression of this receptor in DLBCL could correlate with more aggressive lymphomas. DLBCL can be divided into prognostically important subgroups with germinal center B cell-like (GCB), activated B cell-like (ABC) and type 3 gene expression profiles using a cDNA microarray. The germinal center origin of malignant B cells in DLBCL can also be characterized by immunohistochemistry (IHC) according to CD10, Bcl6 CD138 and MUM1 expression. In this work, we investigated whether expression of BAFF-R in DLBCL correlate or not with the GCB vs non GCB phenotype of lymphomatous cells. Lymph nodes biopsies from 23 DLBCL (among whom 3 post-transplantation lymphomas and 1 AIDS-related DLBCL) were analyzed. In a first step, we characterized by IHC the precise phenotype of each DLBCL: CD10+/Bcl6+ DLBCL were classified into the germinal center (GCB) group (n=9), whereas CD10-/BCL6-/MUM1+ lymphomas defined the non germinal center (NGCB) one (n=14). Each DLBCL was then analyzed for BAFF-R expression by IHC. In agreement with a recent report, most of the DLBCL were found to be negative for BAFF-R expression (16 out of 23). Interestingly, 4 of the 9 GCB DLBCL were found to be BAFF-R positive (44%), whereas only 3 out of 14 (21%) in the NGCB group. Although the difference did not reach statistical significance, we concluded from this observation that a consequent percentage of GCB DLBCL express the specific receptor BAFF-R. Thus, we actually analyze the clinical and biological characteristics of BAFF-R positive DLBCL and investigate whether expression of BAFF-R could represent a bad prognosis factor in term of clinical outcome and response to treatment.

In contrast with secondary erythrocytosis, progenitor cells from polycythemia vera (PV) patients can undergo in vitro erythroid differentiation despite absence of erythropoietin (EPO) and presence of such endogenous erythroid colonies (EEC) is routinely used as a diagnostic assay. Recent focus on the JAK2 mutation V617F in PV patients argue for a direct implication of JAK2 dependent signaling pathways in EEC formation. Because STAT5 is the principal target of JAK2 in erythroid cells, we investigated whether EEC formation was only dependent on STAT5 activation or required other signaling pathways that would be activated by JAK2. For this purpose, we transduced a retroviral vector coding for a constitutively active form of STAT5 (MIGR-STAT5CA) in UT7 cells, a leukemic cell line with erythroid properties. We observed in cells transduced with the MIGR-STAT5CA vector a spontaneous induction of erythroid differentiation in comparison with cells infected with the empty vector MIGR, as assessed by GPA staining. We next investigated effects of STAT5CA on erythroid differentiation of human primary progenitors. Purified CD34+ cells obtained from peripheral blood (PB) of patients treated with G-CSF were transduced with the STA5CA vector, the CD36+/GPA− erythroid progenitor cells were sorted and cultured in presence of SCF alone. When expressing STAT5CA, they both proliferate and undergo erythroid terminal differentiation despite the absence of EPO. We concluded that a phosphorylated form of STAT5 was sufficient to support in vitro erythroid differentiation of human primary cells. Because STAT5 has been shown to play a crucial role in erythropoiesis via induction of the antiapoptotic protein Bcl-xL, we next investigated whether effects of STAT5CA on erythroid maturation was dependent on Bcl-xL induction. Tansduction of human CD36+/GPA− cells with a retrovirus containing the coding sequence of human Bcl-xL progenitors allowed survival, proliferation and GPA acquisition despite the absence of EPO. We next investigated whether STAT5CA or Bcl-xL overexpression in normal primary cells could reproduce the malignant phenotype observed in PV patients, i.e. induction of EEC formation. CD36+/GPA− transduced with either the STAT5 CA or the Bcl-XL vectors were plated in methylcellulose in the absence of EPO. Bcl-xL as well as STAT5CA vectors could both induce endogenous erythroid colony formation. Regardless to these results, we hypothesized that the EEC formation observed in myeloproliferative disorders could be at least partially due to the JAK2 dependent activation of the STAT5/Bcl-XL pathway. Thus, both constitutive activation of STAT5 and Bcl-xL overexpression could substitute to EPO to induce terminal differentiation of human primary erythroid progenitors.