Jak2 loss in PLTs and MKs of Pf4-Cre–positive Jak2^f/f mice results in a thrombocytosis phenotype. (A) Jak2 mRNA expression is abolished in PLTs and MKs, and reduced in MKPs (mean reduction 33%, P = .029) of Pf4-Cre–positive (Pf4 Cre+) Jak2^f/f mice, whereas it is preserved in Pre-MegE and LSK cells, consistent with Jak2 loss in late megakaryo-/thrombopoiesis. Mpl mRNA expression is unchanged in MKs of Pf4-Cre–positive Jak2^f/f mice (n = 3 to 6 mice per group and population). (B) Jak2 protein is absent in PLTs of Pf4-Cre–positive Jak2^f/f mice and intact in Pf4-Cre–negative Jak2^f/f littermate controls, while the TPO receptor Mpl is expressed. (C) Mpl protein expression in PLTs, as assessed by densitometry of Western bands is slightly reduced (P > .05). (D) PLTs devoid of Jak2 in Pf4-Cre–positive Jak2^f/f mice are not reactive to stimulation with TPO (100 ng/ml) with inactive Stat3 and Stat5, whereas Jak signaling is activated in Pf4-Cre–negative Jak2^f/f littermate controls. Tyk2 remains inactive in the absence of Jak2. (E) Elevated PLTs with unaffected survival and volume in Pf4-Cre–positive Jak2^f/f mice, as compared with littermate controls demonstrate the dispensability of PLT Jak2 signaling for the production of PLTs (n = 5 to 17 mice per group and time-point). (F) WBC count is slightly elevated in Pf4-Cre–positive Jak2^f/f mice. (G) Elevated WBCs are due to a proportionate expansion of all leukocyte subsets with a subtle relative disadvantage of lymphocytes in Pf4-Cre–positive Jak2^f/f mice. Results are representative of at least 2 independent experiments and shown as mean ± SEM. *P < .05; **P < .01; ****P < .0001.