Low doses of JAK2 chemical inhibitors induce a paradoxical increase in MKs production both in vitro and in vivo. (A) The effect of AZD1480, a JAK2 chemical inhibitor, at different concentrations on cellular proliferation of CD34⁺ cells from cord blood cultured in MSS with TPO at 10 ng/mL. Cells were counted at D8. (B) CD34⁺ cells were cultured with TPO for 6 days. After a 12-hour cytokine starvation, these cells were restimulated with 10 ng/mL TPO with or without AZD1480 pretreatment, at 0.2 μM during 2 hours. We determined the efficacy of AZD1480 on cell signaling by immunoblotting. (C) CD34⁺ cells from cytapheresis were cultured in MSS with TPO, with or without AZD1480 at a concentration of 0.2 μM. Cellular proliferation was evaluated until D13 and CD41 and CD42 expression was determined by flow cytometry at D8 (D). (E) C57BL/6 mice were treated with different doses of ruxolitinib, a JAK2 inhibitor used in clinics (20 and 60 mg/kg of body weight per day) or vehicule (methocellulose 0.5%, tween 80 0.05%) by oral gavage, twice daily. Blood samples were analyzed after 5 days of treatment. (F) Platelet counts for C57BL/6 mice treated during 5 days with vehicule (n = 9), ruxolitinib at 20 mpk (n = 12), or 60 mpk (n = 9). Error bars represent mean ± standard deviation of 3 (A,F) or 2 (C) independent experiments. *P < .05; **P < .01.