C/EBPβ is required in HSPCs under stress conditions. (A) CD45.2⁺ WT or Cebpb^−/− whole BM cells (5 × 10⁵) and an equal number of WT whole BM cells (CD45.1⁺) were transplanted into lethally irradiated WT mice (CD45.1⁺). Chimerism of donor-derived (CD45.2⁺) cells among total white blood cells (WBCs), granulocytes, monocytes, and lymphocytes 1-6 months after the competitive BM transplantation was shown (n = 11 per group, representative of 3 independent experiments). (B) Chimerism of donor-derived cells among LT-HSCs (CD150⁺CD48⁻ KSL cells) and KSL (c-kit⁺Sca-1⁺lineage⁻) cells obtained from recipients 6 months after competitive BM transplantation (n = 5 per group, representative of 3 independent experiments). (C) Six months after the primary transplantation, 2 × 10⁶ chimeric BM cells harvested from the primary recipients were transplanted into lethally irradiated WT recipients (CD45.1⁺; secondary transplantation). Graphs show chimerism of donor-derived cells in the indicated PB fractions 1-4 months after secondary transplantation (n = 11 per group, representative of 2 independent experiments). (D) Chimerism of donor-derived cells among LT-HSCs (CD150⁺CD48⁻ KSL cells) and KSL cells in recipients 4 months after secondary transplantation (n = 5 per group, representative of 2 independent experiments). (E) CD45.2⁺ WT or Cebpb^−/− whole BM cells (5 × 10⁵) and an equal number of WT whole BM cells (CD45.1⁺) were transplanted into lethally irradiated WT mice (CD45.1⁺), and the recipients were treated with 5-FU monthly starting 1 month after transplantation. Graphs show chimerism of donor-derived cells in the indicated cellular fractions of PB (n = 10 per group, representative of 2 independent experiments). (F) Chimerism of donor-derived cells among LT-HSCs (CD150⁺CD48⁻ KSL cells) and KSL cells in recipients after 5 months of repetitive stress (n = 5 per group, representative of 2 independent experiments). Data are presented as means ± SD. *P < .05; **P < .01; and ***P < .001 (determined by the 2-tailed Student t test).