High-dose radiation exposure, usually caused by radiological accidents or nuclear disasters, can cause lead to acute radiation syndrome (ARS), which is a life-threatening condition. The International Atomic Energy Agency recommends the use of a combination of various cytokines or bone marrow (BM) transplant for treating high-dose exposure, depending on the degree of radiation exposure and ARS. Therefore, drugs that respond quickly, which can be used to treat a large number of radiation-associated casualties represent a good treatment option. However, certain hematopoietic cytokines, such as the granulocyte macrophage-colony stimulating factor and interleukin-3, which are effective against hematopoietic dysfunction and have been used in previous accidents, are not approved as pharmaceutical drugs in Japan. Although granulocyte-colony stimulating factor is only approved domestically for the treatment of ARS, it has been found that a single dose is insufficient to improve the survival rate of individuals exposed to lethal ionizing radiation. We have previously demonstrated that an effective protocol of drug therapy, based on approved pharmaceutical drugs in Japan, is the administration of romiplostim (RP), a thrombopoietin-receptor myeloproliferative leukemia virus proto-oncogene (c-mpl) agonist, at a dose of 50 μg/kg body weight/day for 3 consecutive days. This regimen achieved 100% survival at 30-days in 8-weekold female C57BL/6JJcl mice when administered immediately following exposure to a lethal dose (7 Gy) of 137Cs γ-rays. In the present study, we assessed the mechanisms by which RP ameliorated radiation-induced hematopoietic damage in mice after lethal total body γ-irradiation (TBI) by focusing on the recovery and regeneration of BM and spleen. Mice were divided into 4 groups: control group, RP group, TBI group, and TBI + RP group. On days 4, 10, 18, and 30 after TBI, morphological and histological changes of BM and spleen and antigen expression changes of cells isolated from both tissues were assessed. While the body weight of animals in the TBI group decreased significantly by day 18 compared to controls, that of mice receiving RP remained comparable. Similarly, on day 18, leukocyte, platelet, erythrocyte, and hemoglobin levels were the lowest in TBI mice. Conversely, these values markedly recovered in the TBI + RP group. Splenic atrophy was assessed by weighing the spleen after TBI, and it was observed that while the spleen contracted significantly following TBI, mice in the RP + TBI group showed a 3 to 4-fold increase in spleen weight and in cell numbers on day 18. Endogenous colonies, which are indicators of hematopoiesis, were also observed in the spleen after day 10. In addition, TBI induced severe myelosuppression that resulted in the loss of hematopoietic function. On day 18 after irradiation, while the BM exhibited obvious myeloablation, that of the RP + TBI treated mice was comparable to controls. Further, the number of BM cells in mice RP + TBI treated mice continued to increase till 30 days after TBI but did not attain pre-TBI levels. Additionally, analysis of cell-surface antigen expression in BM and spleen cells showed that, compared to the TBI group, RP administration promoted the recovery of hematopoietic stem cells, multipotent progenitor cells, and common lymphocyte precursor cells. In particular, a dramatic increase in the number of common myelo-erythroid progenitors were observed in the spleen on day 18 in the TBI + RP treated mice, accompanied with the disappearance of splenic cyst structures and acceleration of megakaryocyte hematopoiesis. These results suggest that thrombopoietin-receptor agonist RP promotes the recovery of early hematopoiesis and hematopoietic environment in both BM and spleen of lethal TBI mice, especially the promoting megakaryocytopoiesis in the spleen. As a result, treating lethally-irradiated mice with RP results in 100% survival.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.