MULTILEVEL EFFECTS OF FLT3 LIGAND ON HUMAN HEMATOPOIESIS - EXPANSION OF PUTATIVE STEM-CELLS AND PROLIFERATION OF GRANULOMONOCYTIC PROGENITORS MONOCYTIC PRECURSORS

We have evaluated the effects of the flt3 receptor ligand (FL) on hema topoietic progenitors/stem cells (HPCs/HSCs) stringently purified from adult peripheral blood and grown in different culture systems. In the se experiments HPCs/HSCs were treated with FL +/- kit ligand (KL) +/- monocyte colony-stimulatory factor (M-CSF). In clonogenetic HPC cultur e supplemented with interleukin-3 (IL-B)/granulomonocyte-CSF (GM-CSF)/ erythropoietin (Epo), FL potentiates colony-forming unit (CFU)-GM prol iferation in terms of colony number and size, but exerts little effect on burst-forming units-erythroid (BFU-E) and CFU-granulocyte erythroi d megakaryocyte macrophage (CFU-GEMM) growth, whereas KL enhances the proliferation of all HPC types; combined FL + KL +/- M-CSF treatment c auses a striking shift of CFU-GM colonies from granulocytic to monocyt ic differentiation, In liquid suspension HPC culture, FL alone induces differentiation along the monocytic and to a minor extent the basophi lic lineages, whereas M-CSF alone stimulates prevalent monocytic diffe rentiation but little cell proliferation: combined M-CSF + FL treatmen t causes both proliferation and almost exclusive monocytic differentia tion (97% monocytes in fetal calf serum-rich (FCS+) culture conditions , mean value). At primitive HPC level, FL potentiates the clonogenetic capacity of colony-forming units-blast (CFU-B) and high proliferative potential colony-forming cells (HPP-CFC) in primary and secondary cul ture; KL exerts a similar action, and additive effects are induced by FL combined with KL, More important, addition of FL alone causes a sig nificant amplification of the number of long-term culture-initiating c ells (LTC-ICs), ie, putative repopulating HSCs, whereas this effect is not induced by KL. The FL effects correlate with flt3 mRNA expression in HPCs differentiating through the erythroid or GM pathway in liquid suspension culture: (1) flt3 mRNA is expressed in freshly purified, r esting HPCs; after growth factor stimulus the message (2) is abruptly down-modulated in HPC erythroid differentiation, but (3) is sustainedl y expressed through HPC GM differentiation and abolished in GM precurs or maturation. This pattern contrasts with the gradual downmodulation of c-kit through both erythroid and GM HPC differentiation. The result s indicate that FL exerts a stimulatory action on primitive HPCs, incl uding a unique expanding effect on putative stem cells, whereas its di stal proliferative/differentiative action is largely restricted to CFU -GM and monocytic precursors. The latter effect is potentiated by KL a nd WI-CSF, thus suggesting that the structural similarities of FL, KL, M-CSF, and their tyrosine kinase receptors may mediate positive inter actions of these growth factors on monocytic differentiation. (C) 1995 by The American Society of Hematology.