Kashiwakura, I., Inanami, O., Abe, Y., Takahashi, T. A. and Kuwabara, M. Different Radiosensitive Megakaryocytic Progenitor Cells Exist in Steady-State Human Peripheral Blood. Radiat. Res. 164, 10–16 (2005).

CD34 antigen is a novel marker for human hematopoietic stem/progenitor cells. In the present study, two cell fractions, CD34^low and CD34^high, were prepared from steady-state human peripheral blood on the basis of CD34 antigen expression. The colony-forming unit megakaryocytes (CFU-Meg) contained in each cell fraction were compared for X-radiation sensitivity and cytokine action. The content of CD34 CD45 cells in the CD34^low and CD34^high cell fractions was 74.8% and 88.8%, respectively, and the frequency of thrombopoietin (TPO)-supported CFU-Meg in the CD34^low cell fraction was 1.9 times higher than that in CD34^high. The CFU-Meg in CD34^high were more radiosensitive than those in CD34^low, indicating that steady-state human peripheral blood contains different types of CFU-Meg. However, no significant differences were observed between cell fractions in the radiation survival curves of CFU-Meg stimulated by TPO plus cytokines except granulocyte colony-stimulating factor (G-CSF). TPO plus interleukin 3 was the optimal combination for survival of both types of CFU-Meg after X irradiation. The present study also demonstrated that TPO plus G-CSF is able to increase the survival of irradiated CD34^low CFU-Meg. These results suggest that two megakaryocytic progenitor populations with different radiosensitivity and cytokine responses are found in steady-state human peripheral blood.