Cell non-autonomous expansion of stem/progenitor cells and LSK bias toward megakaryopoiesis in Pf4-Cre–positive Jak2^f/f mice. (A) MKs, MKPs, and Pre-MegE are expanded in Pf4-Cre–positive (Pf4 Cre+) Jak2^f/f mice as assessed by flow cytometry according to Pronk et al²¹  (n = 3 to 12 mice per group and population). (B) Megakaryocytic expansion is highlighted by hematoxylin and eosin stained bone marrow. (C) Formation of MK colonies from 0.5 × 10⁵ whole bone marrow cells, 3000 sorted Pre-MegE or 3000 sorted MKP is increased in Pf4-Cre–positive Jak2^f/f mice. (D) Flow cytometry gating scheme for megakaryocyte progenitors according to Pronk et al.²¹  (E) LSK cells are expanded in Pf4-Cre–positive Jak2^f/f bone marrow analogous to Pre-MegE and MKP (n = 5 to 11 mice per group). (F) Formation of MK colonies from 3000 sorted LSK cells is increased in Pf4-Cre–positive Jak2^f/f mice. (G) Flow cytometry gating scheme for MK-primed LSK subsets based on c-Kit surface expression analogous to Shin et al.²⁶  (H) LSK CD150⁺CD48⁻ HSCs are skewed toward MK-primed HSCs (c-Kit^hi) in Pf4-Cre–positive Jak2^f/f mice. (I) Serum TPO is maintained at high steady state levels despite thrombocytosis in Pf4-Cre–positive Jak2^f/f mice, while serum TPO is reduced in thrombocythemic mice with the MPLW515L allele and intact Jak2²⁹  (n = 5 to 13 mice per group). (J) Surface Mpl expression in PLTs as shown by flow cytometry is slightly reduced (P > .05, n = 4 to 10 mice per group). (K) TPO uptake by PLTs of Pf4-Cre–positive Jak2^f/f mice, as reflected by TPO depletion of media incubated with purified PLTs, is compromised as compared with littermate controls. Results are representative of at least 2 independent experiments and shown as mean ± SEM. *P < .05; **P < .01; ***P < .001; ****P < .0001.