New definition and methods for isolation of the earliest peripheral blood-derived hematopoietic stem cells.

Peripheral blood contains transplantable pluripotent hematopoietic progenitor cells, which are defined by the expression of certain surface antigens, such as CD34, and the absence of lineage-specific markers (lin-). Here we describe CD34- DR- adherent growing hematopoietic progenitor precursors which were separated and isolated from peripheral blood by interleukin 6(IL-6)-mediated plastic adherence. These peripheral blood mononuclear cells attach to the plastic surface of tissue culture flasks in the presence of human recombinant IL-6 and assume fibroblast-like morphology. These adherent cells are CD34- and HLA-DR-, and respond to various growth factors with the expression of surface antigens, changes in morphology and the expression of cell cycle-controlling kinases. Stem cell factor induces differentiation of the CD34- DR- adherent cells to become nonadherent and to differentiate into CD34+, and eventually HLA-DR+ cells, and produce colony-forming units (CFU) in semi-solid agar. While IL-6 conveys adherence of these cells, the addition of stem cell factor induces differentiation into nonadherent cells. Our observations suggest the possibility of isolating a true hematopoietic stem cell before the level of CD34 and HLA-DR expression.