TPO links G-CSF to megakaryocytes. (A) Time course of KC production by purified mouse BM megakaryocytes cultured in vitro ± TPO (TPO, mean ± SEM of 3 separate experiments per treatment). (B) Time course of IL-8 production by the human megakaryocyte cell line MEG-01 after culture ± TPO (mean ± SEM of 3 separate experiments per treatment). (C) Expression of TPO on BM cells 2 hours after injection of mice with either control PBS or G-CSF (images are representative for data obtained in 2 independent experiments with 3 mice per treatment group). Scale bar = 20 μm. (D) The effect on PMN mobilization at 2 hours after a single intravenous injection of 1 μg of TPO compared with a control injection of PBS. Data represent 1 experiment with 4-5 mice per group. The horizontal bar indicates the mean value. (E) Neutrophil mobilization 2 hours after a single injection of G-CSF in littermate versus Mpl^−/− animals. Data represent one experiment with 5-7 mice per group. The horizontal bar indicates the mean value.