Expansion of Red Blood Cells for Transfusion

Abstract SCI-46

Blood transfusion, the earliest form of cell replacement therapy, has become indispensable for modern medicine making the safety and adequacy of the blood supply a national priority. The US blood supply is adequate overall because in 2006 the number of blood units collected exceed by 7.8% the number of those transfused. However, issues surrounding blood transfusion, such as sporadic shortages and potential adverse events to recipients (related to changes in red cell physiology during storage and alloimmunization in chronically transfused patients) prompted past and current efforts to develop alternative transfusion products. Recently, the culture conditions to generate erythroid cells have greatly improved making the production of a transfusion product ex-vivo a theoretically possible, although expensive, proposition. This recognition is inspiring several investigators to develop production processes for ex-vivo generation of red cell transfusion products. A proof-of-concept demonstrating that ex-vivo generated red cells protect mice from experimentally induced lethal anemia has been obtained. Alternative sources of stem cells which include human embryonic stem cells (hESC) and induced pluripotency stem cells (iPS), are being explored. Since red cells do not have a nucleus, safety considerations suggest that they may represent the first cell therapy product to be generated from hESC and iPS. In addition, discarded hematopoietic stem cells present in adult and cord blood donations may theoretically generate numbers of red cells ex-vivo sufficient for transfusion. Affordable clinical grade humanized culture media have also been developed. Possible differences in immunological and biological properties of erythroid cells from different sources are under investigation. These differences include size, levels of activity of glycolytic enzymes and carbonic anhydrase, expression of different isozymes, hemoglobin and antigenic profiles (HLA class II antigens). This last aspect is particularly important because ex-vivo expanded red cells pose the same risk for infection and incompatibility as any transfusion product but pose unique antigenic risks. Since expression of blood group antigens is susceptible to post-transcriptional modifications, the ex-vivo expansion process itself may induce antigenic variability. Therefore, even cells generated from completely matched stem cell sources may induce auto-immunity and/or appear incompatible. Regarding the identity of ex-vivo generated red cell transfusion products, a conservative approach would be to define them as “enucleated red cells”. In principle, however, ex-vivo generated erythroblasts may also serve as transfusion product. Since they undergo 4–64 further divisions and reduce iron overload, they may represent a more potent transfusion product for patients that require chronic transfusion. The clinical use of these cells, however, may involve development of specific procedures to facilitate their homing/maturation in the erythroid niches of the recipients. In summary, on the basis of these cost, logistic and safety considerations we hypothesize that the clinical application of ex-vivo expanded erythroblasts will involve in sequence, drug discovery for personalized therapy, systemic drug delivery, genotypically matched transfusion for alloimmunized patients and then transfusion in the general population.