Ex vivo generation of human red blood cells: a new advance in stem cell engineering.

We describe a technological approach permitting the massive expansion of CD34(+) stem cells and their 100% conversion ex vivo into mature red blood cells (RBC). The protocol comprises three steps: a first step consisting of cell proliferation and induction of erythroid differentiation in a liquid medium without serum in the presence of growth factors (GF), a second based on a model reconstitution of the medullar microenvironment (ME) (human MSC or murine stromal cells) in the presence of GF, and a third in the presence of the ME alone, without any GF. This work highlights the impact of the ex vivo microenvironment on the terminal maturation of erythroid cells. A critical point is that the RBC generated in vitro have all the characteristics of functional native adult RBC. Moreover, this new concept of 'cultured RBC' (cRBC) is important for basic research into terminal erythropoiesis and has major clinical implications, especially in transfusion medicine. The three-step protocol can be adapted to use hematopoietic stem cells (HSC) from diverse sources: peripheral blood, bone marrow or cord blood.