Lineage-specific growth factors can compensate for stem and progenitor cell deficiencies at the postprogenitor cell level: an analysis of doubly TPO- and G-CSF receptor–deficient mice

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Abstract
Multiple lines of evidence indicate that thrombopoietin (TPO) substantially impacts the number of hematopoietic stem cells and progenitors of all myeloid lineages. Nevertheless, tpoknock-out mice (T−) display thrombocytopenia only; blood erythroid and neutrophil levels are normal despite 60% to 85% reductions in stem and progenitor cells. The compensatory mechanism(s) for these deficiencies remains uncertain; lineage-specific cytokines such as erythropoietin or granulocyte colony-stimulating factor (G-CSF) have been postulated but never proven to be responsible. To directly test whether G-CSF can compensate for the myeloid progenitor cell reduction in the T−model of hematopoietic deficiency, T−and G-CSF–receptor knock-out(GR−) mice were crossed, and F1 animals bred to obtain doubly nullizygous mice(T−GR−). This experiment also allowed us to test the hypothesis that G-CSF contributes to the residual platelet production in T−mice. We found that T−GR−F2 mice displayed similar blood platelet levels as that seen inT−mice, indicating that G-CSF does not account for the residual megakaryopoiesis in T−mice. However, we also noted excessive perinatal mortality ofT−GR−animals, caused by infection due to a profound and significant decrease in marrow and peripheral blood neutrophils, far greater than that seen in eitherT−or GR−mice. These data indicate that in the additional absence of GR, T−mice cannot compensate for their 62% reduction in myeloid progenitors and become profoundly neutropenic, supporting the hypothesis that G-CSF can compensate for the myeloid effects of TPO deficiency by expanding the pool of cells between the granulocyte-macrophage colony-forming unit and mature neutrophil stages of granulopoiesis.