Long-term repopulating hematopoietic stem cells and “side population” in human steady state peripheral blood

This report brings the first experimental evidence for the presence of long-term (LT) repopulating hematopoietic stem cells (HSCs) and Side Population (SP) cells within human steady state peripheral blood CD34+ cells. Ex vivo culture, which reveals the LT-HSC, also increases short-term (ST) HSC engraftment capacity and SP cell number (as well as the SP subpopulations defined on the basis of CD38, CD90 and CD133 expression) which are very low in freshly isolated cells. Thus, ex vivo incubation either allows the expansion of the small fraction of HSCs or reveals “Scid Repopulating Cells - SRC” that are present in the initial CD34+ cell population but unable to engraft. In addition, among these CD34+ cells, we confirm the presence of committed progenitors at frequencies similar to those found in cord blood CD34+ cells. These cells, obtained from leukoreduction filters (LRFs) and rejected in the course of the preparation of red blood cell concentrates, are an abundant and reliable material for obtaining committed progenitors, short- and long-term HSCs of therapeutic interest, especially after the ex vivo expansion phase. Our results open a perspective to set up new therapeutic protocols using expanded LRFs-recovered CD34+ cells as a source of HSCs for autologous or allogeneic transplantation.