Promotion of platelet production by hTPO cDNA injection.

Sir, thrombopoietin (TPO) is a regulator of megakaryocyte differentiation that stimulates platelet production and possesses great value in treatment of primary and secondary marrow failure. Previously, we observed that mice that received human TPO (hTPO) cDNA injection were resistant to the toxicity of cyclophosphamide. Here we report the activity of TPO cDNA injection in elevating platelet count. hTPO cDNA was separated from the total RNA of fetal liver by RT-PCR and subcloned in pcDNA3. It was packed with lipofectin (10 μg/μL, Gibco BRL) for injection. One-hundred and twenty Babl/c mice (20 g body weight, female) were injected with 60 μg of the reconstructed plasmid; another 40 non injected mice were used as controls. Platelets and leukocytes were counted directly. Paraffin sections of bone marrow were prepared to estimate megakaryocyte densities. Rabbit anti-hTPO serum was collected from rabbits immunized with truncated hTPO expressed in E. coli. The average platelet number of the hTPO cDNA-injected mice began to rise on the third day. A significant increase lasted for about 1 week. The average increase was about 184% that of age-matched controls, and the highest number was over 3 times greater than the control values. Thereafter, platelet counts fluctuated but for over 2 weeks they were still higher than controls (Figure 1). Leukocyte counts were not significantly affected except for the first 2 days. Megakaryocyte densities began to rise on the second day following gene injection; the