Augmentation of megakaryocytopoiesis within the hematopoietic microenvironment of human granulocyte colony-stimulating factor transgenic mice

Thrombocytopenia in association with splenomegaly during granulocyte-colony-stimulating factor treatment in mice is not caused by hypersplenism and is resolved spontaneously

BACKGROUND

Granulocyte-colony-stimulating factor (G-CSF) is used for hematopoietic progenitor cell (HPC) mobilization. Platelet (PLT) counts decrease during G-CSF administration. The mechanisms have not been determined, however. Because splenic pooling of PLTs caused thrombocytopenia in patients with splenomegaly and splenic enlargement was observed in G-CSF-treated donors, it was hypothesized that hypersplenism might cause G-CSF-induced thrombocytopenia.

STUDY DESIGN AND METHODS

Mice were treated with several concentrations of G-CSF, and PLT count was measured. Because transfused PLTs should be cleared rapidly from the blood stream under hypersplenic state, PLT life span was studied. To determine direct role of spleen on thrombocytopenia, G-CSF was given to splenectomized mice. Because PLT count did not decrease in G-CSF-expressing transgenic mice, G-CSF was given to mice for a longer period of time and PLT count was investigated.

RESULTS

PLT counts decreased while spleen weight increased in a dose-dependent manner by G-CSF treatment. No significant difference in PLT life span was found between G-CSF-treated and control mice. Histologic analysis showed no significant increase in PLT numbers trapped in either spleen or other tissue after PLT transfusion in G-CSF-treated mice. In splenectomized mice as well as in normal mice, G-CSF caused thrombocytopenia. When G-CSF was given to mice for a longer period of time, PLT counts decreased during the first 7 days and thereafter began to increase followed by returning to baseline on Day 15.

CONCLUSION

Thrombocytopenia coincided with splenomegaly during G-CSF treatment, but hypersplenism was not responsible for thrombocytopenia. G-CSF-induced thrombocytopenia was a transient event and improved spontaneously despite continual G-CSF treatment.

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.