GROWTH-FACTOR REQUIREMENTS FOR SURVIVAL IN G0 AND ENTRY INTO THE CELL-CYCLE OF PRIMITIVE HUMAN HEMATOPOIETIC PROGENITORS

In this study we have isolated populations of dormant human hemopoietic progenitors by two different approaches. First CD34+ cells isolated by panning were further separated on the basis of absence of HLA-DR expression by using fluorescence-activated cell sorting. Second, CD34+ HLA-DR- cells were isolated by nonadherence to soybean agglutinin, negative immunomagnetic bead selection with lineage-specific antibodies, and two-color cell sorting. Progenitors in either cell population were unable to form colonies in the presence of interleukin (IL)-3 alone but yielded a substantial number of colonies, including multilineage colonies, in the presence of combinations of IL-3 and IL-6. Similarly, IL-3 plus any one of the other synergistic factors, including granulocyte colony-stimulating factor, IL-11, leukemia inhibitory factor, and steel factor, effectively supported colony formation from CD34+ HLA-DR- progenitors. Sequential observation of colony formation from single CD34+ HLA-DR- cells provided definitive evidence that the synergistic factors trigger cell divisions of dormant cells. Studies with delayed addition of factors to the cultures provided evidence that this population of cells also requires IL-3 or granulocyte/macrophage colony-stimulating factor (GM-CSF) to survive even while dormant. In contrast, none of the synergistic factors were able to replace IL-3 or GM-CSF in this function. These findings confirm and extend the model that multiple factors with overlapping functions operate both independently and in combination to regulate early stages of hemopoiesis.