Dysregulated Mpl-Ligand production by hemopoietic cellsinduces a fatal myeloproliferative syndrome in mice

Abstract

To evaluate the effects of long-term high-dose exposure to Mpl-ligand also called thrombopoietin (TPO), C57BL/6J murine marrow cells were infected with a retrovirus carrying the murine TPO gene. Mice were treated 4 days by 5-FU and marrow cells were then infected by coculture using a MPZen vector containing the murine TPO cDNA. Non adherent marrow cells were transplanted into lethally irradiated recipients. A majority of hematopoietic cells in the marrow, spleen, thymus and blood was transduced by the retroviral vector, one and three months after reconstitution. Plasma TPO activity in transplanted mice was extremely high (104 U/ml). A disease with two distinct steps was observed. During the two first months after transplantation, platelet (plt) and white blood cell (WBC) counts increased 4- and 10-fold, respectively. Abnormal platelet size and granules were observed. Spleen weight increased 4-fold and marrow cellularity decreased 5-fold. Histology revealed hyperplasia of the megakaryocytic and myeloid cells. Total numbers of CFU-MK and CFU-GM increased. In contrast, the hematocrit progressively fell accompanied by a decrease in the erythroblasts and CFU-E numbers. Beginning two months after transplantation, plt and WBC numbers also declined. Thrombocytopenia was noted 5 months after transplantation. The Hcts continued to decrease. Few cells were isolated from the marrow cavities and spleens. Histology revealed fibrosis of the marrow and spleen and significant osteosclerosis of the marrow. An extramedullary hematopoiesis was observed in numerous organs such as the liver or the kidney. Total numbers of progenitors were very low in hematopoietic organs. Mice died 7 months after transplantation with severe pancytopenia. Two early deaths were observed with a marked increase in blast cells. This disorder was transplantable into secondary recipients who developed an attenuated form of the disease similar to the one previously described [Yan et al (1995) Blood 86: 4025]. In conclusion, dysregulated TPO production by hemopoietic cells in mice results in a fatal myeloproliferative disease which mimics the clinical evolution of idiopathic myelofibrosis observed in man.