Effect of IFN-α on hematopoietic progenitors and fibrocytes in Jak2V617F and Jak2V617F;Tyk2^−/− mice. (A) Number of hematopoietic progenitors, assessed by colony-forming assay. (Left) CFU-GM. (Right) CFU-Mk. BM cells (2 × 10⁴ per well) from WT, Tyk2^−/−, Jak2V617F, and Jak2V617F;Tyk2^−/− mice were plated in methylcellulose containing erythropoietin, interleukin-3 (IL-3), stem cell factor, and IL-6 for CFU-GM, and containing thrombopoietin, IL-3, IL-6, and IL-11 for CFU-Mk in the presence or absence of ropeg-IFN-α (0.1 and 0.5 μg/mL). The numbers of CFU-GMs and CFU-Mks were quantified after 7 days of culture. Each experiment was performed in duplicate, and 3 independent experiments were performed. Data are presented as mean ± standard error (SE) of 3 experiments. P values were calculated by the Tukey test after 1-way ANOVA; ∗P < .05; ∗∗P < .01. (B) Number of fibrocytes. BM cells (2 × 10⁵ per well) from each genotype mouse were cultured for 7 days under conditions that promote the differentiation of fibrocytes. After a medium change, ropeg-IFN-α or PBS was added, followed by a further 5 days of culture. The number of spindle-shaped cells was counted using a phase-contrast microscope. Each culture was performed in triplicate, and 3 independent experiments were performed. Data are presented as mean ± SE. IC50 was measured using a 4-parameter log-logistic model. n.s., not significant.