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Population ageing and increase in cancer incidence may lead to a decreased availability of red blood cell units. Thus, finding an alternative source of red blood cells is a highly relevant challenge. The possibility to reproduce in vitro the human erythropoiesis opens a new era, particularly since the improvement in the culture systems allows to produce erythrocytes from induced-Pluripotent Stem Cells (iPSCs), or CD34

Mostly studied in murine models, Ikaros--a factor that positively or negatively controls gene transcription--was first described as essential to lymphopoiesis until its involvement in numerous hematopoietic lineages was documented. Indeed, Ikaros is also active in murine erythropoiesis and more precisely during fetal to adult globin switching. Recently, these observations were confirmed by our team in a human context. We here review some of the most important characteristics of Ikaros, and detail more precisely how defects of Ikaros activity either by gene inactivation or mutation in mice, or by forced expression of dominant negative isoforms in human precursor cells, modify the erythroid differentiation. An increased cell death, together with decreased cell proliferation, decreased expression of erythroid-specific genes including GATA1, and a delay in fetal to adult globin switching were observed. At the same time, myeloid differentiation was slightly favoured thus suggesting that Ikaros could be involved in the control of the myeloid/erythroid commitment.

Erythroid differentiation involves the transcription factor GATA-1 that positively regulates promoters of erythroid genes (including haemoglobin, glycophorin, erythropoietin receptor) and of erythropoietin. Terminal erythroid differentiation is characterized by major morphological changes that include chromatin condensation and cell size reduction. The morphological changes are partially similar at least to those observed during apoptosis. The production of red cells depends on the apoptosis rate of erythroid progenitors and precursors. Upon erythropoietin starvation or engagement of the death receptor Fas, caspases are activated in erythroid precursors and cleave GATA-1, thus inducing maturation arrest and apoptosis of immature erythroblasts. We have recently demonstrated that, upon erythropoietin stimulation, caspase-3 was also activated, an event required for human terminal erythroblast maturation. Proteins cleaved by caspases in erythroid cells undergoing terminal differentiation include Lamin B and Acinus, which are involved in chromatin condensation. In contrast, despite caspase-3 activation neither GATA-1 degradation nor apoptosis was observed. Thus, the fate of erythroid precursors is determined downstream of caspase activation by the pattern of cleaved targets. Therefore, there are some mechanisms underlying the selective protection of caspase-3 targets during erythropoiesis. This model in which caspases activation is required for differentiation may apply to other haematopoietic or non haematopoietic cellular systems which are described in this review.

In a 70 year old man with primary glomerulonephritis, severe anemia occurred after 4 years on hemodialysis and rHu-EPO. The usual mechanisms of EPO-resistance were excluded. A bone marrow sample showed red all aplasia. No circulating EPO could be detected; the serum inhibited the growth of erythroid precursors in bone marrow cultures. Immunoprecipitation identified an IgG anti-EPO, still active against deglycosylated EPO, i.e. directed against the peptidic matrix. Its high neutralising capacity and the absence of any immune abnormality rule out an auto-antibody. Anti-rHu EPO immunisation is a very rare occurrence, made severe by transfusion-dependence and the risk of hemosiderosis. An immuno-modulating treatment can therefore be justified.

Transferrin receptor is a key protein for the cellular uptake of transferrin iron. The highest number of transferrin receptors is on the surface of erythroblasts. The released iron is used for hemoglobinosynthesis. Regulation occurs at mRNA level depending on the intracellular iron concentration. The synthesis of ferritin and transferrin receptor are regulated in an opposite manner. Serum transferrin receptor is a truncated monomeric form of the cellular receptor. Most of the circulating receptors come from erythroid marrow precursors. Its level mirrors the total tissue receptor mass, it depends on the rate of erythropoiesis and on the iron status. Serum transferrin receptor is easily measured by Elisa methods but the lack of standardization triggers large differences in the results. Unlike ferritin, the concentration of serum transferrin receptors is unaffected in inflammatory diseases, infections, malignancies or cytolysis. In these conditions its measurement is particularly valuable for assessing an associated iron deficiency. It is a very useful tool for the diagnosis of different causes of anemia. In chronic renal failure serum transferrin receptor can predict whether patients will respond to rHu EPO therapy.

Amongst the various causes of anemia in systemic lupus erythematosus, isolated and acquired erythrocyte dysplasia is rare and most often part of global dysmyelopoiesis.The authors report a case of acquired erythrocyte dysplastic syndrome that occurred in a 34-year-old woman in whom previous diagnosis had evidenced systemic lupus erythematosus of rather benign course. Other causes of dysmyelopoiesis were ruled out. Myeloid stem cell cultures showed selective inhibition of erythroid cells growing, with no particular effect of the patient's serum. While a corticosteroid treatment with prednisone (1 mg/kg/d) did not show any efficacy upon anemia, the patient's pregnancy was followed by prolonged correction of hemoglobin, making possible the tapering of prednisone down to 10 mg/d.Acquired erythrocyte dysplastic syndrome remains a rare cause of anemia in systemic lupus erythematosus. This case report suggests an immunological phenomenon, but the mechanisms underlying both the appearance and long-lasting remission after pregnancy remain unexplained.

The CD 34 antigen is a glycoprotein found on the surface of hematopoietic stem cells and early committed progenitors. The CE 34 stem cells from 14 samples of bone marrow, cord blood or leucapheresis were isolated using a positive selection procedure involving an anti CD 34 biotinylated monoclonal antibody and an avidin immunoabsorption device. Results showed that in 60 percent of samples, the positively-selected fractions contained more than 70 percent CD 34 cells. Concentration in CFU-GM and BFU-E progenitors increased 15 and 26 fold respectively in the CE 34 enriched samples. Long-term culture of two samples demonstrated that nearly all of the most immature progenitors were recovered in the procedure. These preliminary results showed that the positive selection technique of CD 34 hematopoietic stem cells is now available for use in autologous or allogeneic hematopoietic stem cell transplantation.

Fifty cases of chronic pure red cell aplasia in adults (idiopathic in 32 cases, associated with chronic lymphocytic leukaemia in 18) were followed up after studying their erythroblastic precursors in vitro. Analysis of response to immunomodulators confirmed the existence, in patients with idiopathic chronic pure red cell aplasia, of a relationship between in vitro behaviour and obtention of a remission: patients with normal differentiation of erythroblasts consistently responded to treatments, whereas treatments were usually ineffective when the erythroblasts did not differentiate in vitro. When in vitro differentiation was below normal, responses to treatments were varied. In patients with chronic lymphocytic leukaemia, the erythroblastic precursors were normal in 15/18 cases, and response to immunomodulators was observed in 13/18 cases. A second objective of this study was to determine, on a large series, the most adequate immunomodulator treatment. In idiopathic chronic pure red cell aplasia the most efficient therapy was corticosteroids and cyclophosphamide administered separately or together. The antilymphocyte serum gave disappointing results. In pure red cell aplasia associated with leukaemia cyclophosphamide was the most frequently effective drug, with corticosteroids taking second rank; however, infections were frequent. The treatments to be considered when both corticosteroids and cyclophosphamide fail are discussed.

Rapid progress has occurred recently in characterizing the molecular structure of the glycoproteins controlling the growth, maturation and functional activities of hematopoietic cells. Several of these factors have been purified and cDNAs have been cloned, providing them in sufficient quantities for study and eventual clinical use. Both availability of these molecules and in vitro colony assays have revealed a large number of effects on erythroid differentiation. But, except erythropoietin (EPO), the physiological role of these hormones and their relative contributions to normal erythropoiesis remain to be specified. This paper attempts to review the most important data on this topic in 1990.