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

Multiple lines of evidence indicate that thrombopoietin (TPO) substantially impacts the number of hematopoietic stem cells and progenitors of all myeloid lineages. Nevertheless, tpo knock-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 in T^- mice, indicating that G-CSF does not account for the residual megakaryopoiesis in T^- mice. However, we also noted excessive perinatal mortality of T^-GR^- animals, caused by infection due to a profound and significant decrease in marrow and peripheral blood neutrophils, far greater than that seen in either T^- 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.