Endogenous EPO induction by anemia is required for expansion of d3 BFU-E and CFU-E during recovery from 4 Gy TBI. (A) Endogenous plasma EPO levels, expressed in pg/mL, as determined by ELISA at 0 to 14 days post–4 Gy TBI. EPO levels increase 13-fold at 4 days post–4 Gy TBI and remain at high levels through 6 days after radiation. (B) Hematocrit levels post–4 Gy TBI ± transfusion of washed RBC at 2 days and 4 days after radiation. Transfusion of irradiated mice prevents radiation-induced anemia. (C) Plasma EPO levels post–4 Gy TBI ± transfusion. Transfusion blocks the induction of EPO normally seen at 4 days and 6 days after radiation. For all experiments, plasma EPO levels and HCT were performed in triplicate with 3 independent blood samples. For each EPO and HCT determination, plasma was isolated from a single terminally bled mouse. All mice were euthanized mid-morning. (D) Erythroid bone marrow progenitor expansion at 6 days after radiation ± transfusion, normalized per femur, and expressed as a percent of unirradiated control marrow. The transfusion-induced block of EPO induction abrogates d3 BFU-E and CFU-E expansion but has no effect on d7 BFU-E during erythroid recovery from 4 Gy TBI. Dotted lines represent unirradiated control levels. Error bars represent SEM of at least 3 experiments, and 3 or more independently assayed mice were used to determine each data point. Statistical analyses were performed using a 2-tailed Student t test (*P < .05; **P < .01; ***P < .001; significantly different from 4 Gy TBI untransfused mice at matched time points).