D Metcalf

Mice lacking granulocyte colony-stimulating factor (G-CSF) were generated by targeted disruption of the G-CSF gene in embryonal stem cells. G-CSF-deficient mice (genotype G-CSF-/-) are viable, fertile, and superficially healthy, but have a chronic neutropenia. Peripheral blood neutrophil levels were 20% to 30% of wild-type mice (genotype G-CSF+/+) and mice heterozygous for the null mutation had intermediate neutrophil levels, suggesting a gene-dosage effect. In the marrow of G-CSF-/- mice, granulopoietic precursor cells were reduced by 50% and there were reduced levels of granulocyte, macrophage, and blast progenitor cells. Despite G-CSF deficiency, mature neutrophils were still present in the blood and marrow, indicating that other factors can support neutrophil production in vivo. G-CSF-/- mice had reduced numbers of neutrophils available for rapid mobilization into the circulation by a single dose of G-CSF. G-CSF administration reversed the granulopoietic defect of G-CSF-/- mice. One day of G-CSF administration to G-CSF-/- mice elevated circulating neutrophil levels to normal, and after 4 days of G-CSF administration, G-CSF+/+ and G-CSF-/- marrows were morphologically indistinguishable. G-CSF-/- mice had a markedly impaired ability to control infection with Listeria monocytogenes, with diminished neutrophil and delayed monocyte increases in the blood and reduced infection-driven granulopoiesis. Collectively, these observations indicate that G-CSF is indispensible for maintaining the normal quantitative balance of neutrophil production during "steady-state" granulopoiesis in vivo and also implicate G-CSF in "emergency" granulopoiesis during infections.

Mice homozygous for a disrupted granulocyte/macrophage colony-stimulating factor (GM-CSF) gene develop normally and show no major perturbation of hematopoiesis up to 12 weeks of age. While most GM-CSF-deficient mice are superficially healthy and fertile, all develop abnormal lungs. There is extensive peribronchovascular infiltration with lymphocytes, predominantly B cells. Alveoli contain granular eosinophilic material and lamellar bodies, indicative of surfactant accumulation. There are numerous large intraalveolar phagocytic macrophages. Some mice have subclinical lung infections involving bacterial or fungal organisms, occasionally with focal areas of acute purulent inflammation or lobar pneumonia. Some features of this pathology resemble the human disorder alveolar proteinosis. These observations indicate that GM-CSF is not essential for the maintenance of normal levels of the major types of mature hematopoietic cells and their precursors in blood, marrow, and spleen. However, they implicate GM-CSF as essential for normal pulmonary physiology and resistance to local infection.

The c-rel proto-oncogene, which is expressed predominantly in hemopoietic cells encodes a subunit of the NF-kappa B-like family of transcription factors. In mice with an inactivated c-rel gene, whereas development of cells from all hemopoietic lineages appeared normal, humoral immunity was impaired and mature B and T cells were found to be unresponsive to most mitogenic stimuli. Phorbol ester and calcium ionophore costimulation, in contrast to certain membrane receptor-mediated signals, overcame the T cell-proliferative defect, demonstrating that T cell proliferation occurs by Rel-dependent and -independent mechanisms. The ability of exogenous interleukin-2 to restore T Cell, but not B cell, proliferation indicates that Rel regulates the expression of different genes in B and T cells that are crucial for cell division and immune function.

Mice with a null mutation in the thrombopoietin (TPO) receptor c-Mpl were generated by gene targeting. c-mpl-deficient mice developed normally but were deficient in megakaryocytes and severely thrombocytopenic. The hematocrit and numbers of mature circulating leukocytes were normal in mpl-/- mice, as was the distribution of morphologically identifiable precursors in hematopoietic tissues. Bone marrow and spleen cells of adult mpl-/- mice lacked specific binding sites for TPO, were unresponsive to TPO in culture, and displayed a marked deficiency in progenitor cells with megakaryocytic potential. Significantly, total hematopoietic progenitor cell numbers were also reduced in mpl-/- mice including multipotential, blast cell, and committed progenitors of multiple lineages. The megakaryocyte deficiency was evident as early as 14 days of gestation in mpl-deficient mice, although reductions in progenitor cell numbers arose only later in development. The data suggest that the critical function of c-Mpl signalling in megakaryocytopoiesis is in maintenance of mature megakaryocyte numbers through control of progenitor cell proliferation and maturation. Moreover, our results also imply an important role for TPO and c-Mpl in the production of primitive pluripotent progenitor cells as well as progenitor cells committed to nonmegakaryocytic lineages.