Rescue of thrombocytopenia in Mpl^−/− mice after Mpl gene therapy. Mpl^−/− mice were transplanted with Lin⁻Mpl^−/− BM cells transduced with SIN lentiviral vectors expressing Mpl from lineage-specific promoters as indicated, or eGFP from the PGK promoter (GFP). Lin⁻ BM from C57Bl/6J was transplanted as positive control (wt). (A) Peripheral blood thrombocyte counts of transplanted Mpl^−/− mice were monitored every 6 weeks in 3 independent experiments up to 25 or 31 weeks, respectively. Hatched lines represent the window of therapeutic success (> 2-fold more than mean eGFP platelet counts and lower as 1500 × 10⁶/mL). (B) Thrombopoietin plasma levels as determined by ELISA from plasma taken on final analysis (25 or 31 weeks). Untreated Mpl^−/− mice (Mpl^−/− ref) show 5- to 6-fold increased Thpo levels compared with untreated C57Bl/6 (C57Bl/6 ref) mice (GPIbaP vs GFP, P < .0001; hMplP vs GFP, P = .11; GPIbaP vs WT, P = .11; WT vs GFP, P < .0001; mean ± SEM; Student t test, 2-tailed, unpaired). (C) Representative histologic analysis of the BM from a control Mpl^−/− mouse, which expressed eGFP by the PGK promoter (i) and a mouse that expressed Mpl by the GPIba promoter (ii). Advanced megakaryocyte maturation was found in treated mice compared with micro-megakaryocytes (arrows) in GFP control animals. Hematoxylin and eosin staining; original magnification, ×200.