Primitive human hematopoietic progenitor cells express receptors for granulocyte-macrophage colony-stimulating factor

Most cytokines act only synergistically; in assays of primitive progenitor cell proliferation, and effects have usually been observed first after prol onged cell culture, Studies reporting that primitive progenitors lack recep tors for a number of cytokines. including granulocyte-macrophage colony sti mulating factor (GM-CSF), could indicate that several "synergistic" cytokin es primarily affect cells that have differentiated in vitro. Here, however, we show that freshly isolated primitive progenitor cells (CD33(hl)CD38(-)) express receptors for GM-CSF at levels 20%-30% of granulo-monocytic progen itors, Although GM-CSF had minimal effects on the survival or proliferation of primitive progenitors,when added alone, the cytokine enhanced stem cell factor (SCF) induced cell cycle entry in the first generation. The effect was not observed when cells were incubated sequentially with SCF and GM-CSF . The results suggest that the synergistic effects of GM-CSF are mediated d irectly on primitive progenitor cells and that the cytokine may be useful t o enhance cell cycle entry of hematopoietic stem cells. (C) 1999 internatio nal Society for Experimental Hematology, Published by Elsevier Science Inc.