Thrombocytopoiesis Is Not Regulated by Thrombopoietin (TPO) but Proplatelet Formation (PPF) Stimulating Factor.

TPO is believed to be the physiological humoral hematopoietic factor in megakaryopoiesis and thrombopoiesis. Some investigators reported the inverted relationship between platelet mass and serum TPO levels, resulting in the conclusion that the platelet mass may directly play a role of regulating the circulating TPO levels. In the previous ASH meeting, we reported that serum TPO levels were not elevated in Immune thrombocytopenic purpura (ITP) patients. In this study, we measured platelet count, the serum TPO level, % reticulated platelet (%RP) and megakaryocytes mass in patients with refractory anemia in myelodysplastic syndrome (MDS-RCMD). The mean plasma TPO level in 20 healthy persons was 57.3±15.2 pg/ ml, the percentage of RPs was 1.51±0.71%.

Plasma TPO levels were widely distributed (799.9±883.0 pg/ ml, n=35) in MDS-RCMD, while they were less than 131.0 pg/ml in ITP (48.5±37.0 pg/ ml, n=37). There were no significant relationship between platelet counts and plasma TPO levels in MDS-RCMD or ITP. These results indicated that plasma TPO level was not regulated by the platelet mass at least in thrombocytopenic patient with MDS-RCMD or ITP. We evaluated the thrombopoiesis in thrombocytopenic patients with MDS-RCMD or ITP. %RPs were 1.9±0.2 % in MDS-RCMD (n=34), indicating that increased thrombopoiesis was not observed in MDS-RCMD, in which serum TPO levels were increased. While %RPs were 6.6±4.9 % in ITP (n=45), indicating that thrombopoiesis was observed in ITP, in which serum TPO levels were not increased. These results indicated that thrombopoiesis was not regulated by plasma TPO levels but some thrombopoiesis stimulating factor(s). Therefore, we evaluated the thrombopoiesis stimulating activity in plasma of patients with MDS-RCMD or ITP as the indicator of PPF stimulating activity of murine megakaryocytes. PPF stimulating activity was measured as the ratio of PPF bearing megakaryocytes/ viable megakaryocytes after 48 hrs serum-free culture of murine megakaryocytes.

As the plasma have some inhibitory effects of PPF activiy, the HDL fraction, isolated from plasma, was used as the methods described previously in Ishida Y et al. (

The higher PPF stimulating activity was observed in patients with ITP (n=15) or MDS-RCMD (n=15) than that with healthy donors (n=10) {healthy donors 26.1±5.4 % versus ITP 38.0± 6.0% (p<0.01), MDS-RCMD 41.9±5.3%(p<0.01)}. There was a significant correlation between the PPF activity and platelet counts (R=0.68; p<0.01) in these thrombopenic patients. These data strongly suggest that thrombopoiesis may not be regulated by TPO but some PPF stimulating factor(s).