Studies of differentiation mechanisms of hematopoietic stem cells using cytokine-receptor transgenic mice

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Granulocyte-macrophage colony stimulating factor (GM-CSF) stimulates proliferation and maturation of myeloid progenitor cells through cell surface receptors expressed on target cells. To analyze the relationship between receptor expression and differentiation of hemopoietic cells, transgenic mice (Tg mice) which express hGMR at all stages of hemopoitic cell development were generated using hGMR α and β cDNAs. hGM-CSF supported colony formation of various types including blast cell, erythroid (E), megakaryocyte (Meg), and mixed hematopoietic colonies. Administration of hGM-CSF to Tg mice resulted in increase of not only neutrophils, monocyte, eosinophils, NK cells but also erythrocytes in their peripheral blood. To clalify whether GM-CSFR signaling can compensate erythropoietin receptor (EPOR) signaling in erythropoiesis, we next generated double mutant mice, hGM-CSFR+/+ and EPOR-/-. Although double mutant mice were embryonic lethal, hGM-CSF induced a large number of erythroid colonies from day 14 fetal liver cells of double mutant mice. We also generated Tg mice that had ubiquitous expressions of wild type human G-CSFR, wild type murine G-CSFR. In methylcellulose colony assay of bone marrow and spleen cells G-CSF have an effect on promoting the proliferation and differentiation of not only granulocyte but also macrophage, megakaryocyte, mast cells, erythroid and more primitive hematopoietic progenitors in both serum-containing and -free culture. These results shows that when functional GM-CSFR or G-CSFR are present on the cell surface, both cytokines does not induce exclusive commitment to the neutrophil and macrophage lineages. In vitro and in vivo studies using the Tg mice indicated that cytokines such as hGM-CSF or hG-CSF supported the growth of various hematopoietic progenitors when the receptor was expressed, but did not alter their commitment program, thus favoring the "stochastic model" rather than the "deterministic model".