Prolonged IFN treatment induces depletion of quiescent HSCs via continuous induction of CD41^hi HSC subset. (A-B) Percentages of CD41^hi and Epcr^hi HSCs in the BM after 24 hours of saline or pIpC treatment in mice with the indicated genotypes (2-way analysis of variance [ANOVA]). (C) Competitive BM transplantation and pegIFN-α treatment regimen, with hemoglobin levels and platelet counts in the PB of pegIFN-α– or vehicle-treated recipient mice and spleen weight in the recipient mice at 22 weeks after the treatment (n = 5 mice per group). (D) Analysis of HSC subpopulations after 22 weeks of treatment in BM and spleen of pegIFN-α– or vehicle-treated, transplant-recipient mice. Gating on GFP⁺ cells enabled determination of the CD41^hi/lo ratios selectively in JAK2-mutant (VF) vs GFP⁻ HSCs. Shown are the frequencies (top) and percentages (bottom) of HSC subsets (n = 5 mice per genotype). Statistical significance between pegIFN-α– and vehicle-treated mice was derived by 2-way ANOVA. (E) Frequencies and percentages of CD41^hi and CD41^lo HSCs in PBMCs of pegIFN-α–treated (n = 13) and control patients with MPN receiving best available therapy (BAT; n = 33). Unpaired Student t test with Welch’s correction. All data are means ± standard error of the mean. *P < .05; **P < .01; ***P < .001.