WT and Tsp-1–null mice display comparable TPO-induced myeloproliferative syndrome leading to similar TGF-β1 overproduction and activation. TPO level (A), platelet number (B), and leukocyte number (C) are shown in unmanipulated WT and Tsp-1–null control mice and in WT and Tsp-1–null TPO^high mice during the 3 months of follow-up after engraftment. Results are the mean of 6 to 15 animals per experimental group. Results of statistical analysis with the Wilcoxon test are as follows: Tsp-1–null control mice versus WT control mice and Tsp-1–null TPO^high mice versus WT TPO^high mice: *P < .05. Total TGF-β1 (active + latent forms) levels were quantified in plasma (D) and extracellular fluids (E,G) using an enzyme-linked immunosorbent assay after acidification of samples. Active TGF-β1 levels in extracellular fluids (F,H) were determined without acidification. Total TGF-β1 plasma levels (D) are shown in WT and Tsp-1–null control mice and in WT and Tsp-1–null TPO^high mice during the 3 months of follow-up after engraftment. No spontaneously active TGF-β1 was detected before acidification of the plasma samples. Total (E) and active TGF-β1 (F) levels in extracellular fluids of BM as well as total (G) and active TGF-β1 (H) levels in extracellular fluids of spleen are shown in WT and Tsp-1–null control mice and in WT and Tsp-1–null TPO^high mice at times of death (8 and 12 weeks after engraftment). Note that the media supplemented with 10% fetal calf serum used to prepare extracellular fluids of BM and spleen contains less than 1.5 ng/mL TGF-β1 and no active form. Results in plasma and in extracellular fluids are presented as the mean of 6 to 15 and of 5 animals per experimental group, respectively. Results of statistical analysis with the Wilcoxon test are as follows: WT control mice versus Tsp-1–null control mice and WT TPO^high mice versus Tsp-1–null TPO^high mice: *P < .05.