In vivo administration of TNF suppresses cycling HSCs. (a) Experimental design. Age (8–12 wk old)- and sex-matched C57BL/6 WT mice received 150 mg/kg 5-FU or PBS 3 d before intravenous injections of 3 × 2 µg TNF or PBS. (b) BM cellularity in mice treated with PBS, PBS and TNF, 5-FU, or 5-FU and TNF. Mean (SD) values of three experiments, each with three individual mice in each treatment group, are shown. (c–e) At day 0, 1/50 of unfractionated BM cells (CD45.2⁺) in two femurs and two tibias from treated mice in each of the indicated groups were transplanted in competition with 10⁶ unfractionated WT BM (CD45.1⁺) cells into congenic lethally irradiated WT recipients (CD45.1⁺/2⁺). 16 wk after transplantation, PB reconstitution levels were analyzed for percentage of CD45.2⁺ contribution within the T cell (c), B cell (d), and myeloid blood cell (e) lineages. (f) Bars show calculated numbers (see Materials and methods) of total CRUs in PBS-, PBS + TNF–, 5-FU–, and 5-FU + TNF–treated donor BM cells based on PB levels of myeloid chimerism 16 wk after transplantation. (c–f) Results are mean (SD) values from three separate experiments, each with five to seven recipients/group.