A transient wave of erythroid progenitors circulates in the bloodstream at 6 to 8 days post–4 Gy TBI and selectively engrafts spleen. (A) Erythroid progenitor kinetics in the peripheral blood post–4 Gy TBI. Erythroid progenitors (predominantly d7 BFU-E) are completely lost from the bloodstream after irradiation. A transient wave of erythroid progenitors emerges into the bloodstream between 6 and 8 days after radiation. (B) Erythroid precursor kinetics in the peripheral blood post–4 Gy TBI. Erythroid precursors are also lost after irradiation and transiently emerge into the bloodstream at 6 to 9 days after radiation. Erythroid progenitors and precursors are expressed as the total number of each subpopulation per 300 μL whole blood. (C) Median fluorescence intensity of surface α₄-integrin levels on BFU-E and CFU-E obtained from the bone marrow (BM; blue) and circulating blood (red) of mice at 6 days post–4 Gy TBI. α₄-integrin levels are significantly decreased on both BFU-E and CFU-E in the bloodstream compared with bone marrow erythroid progenitors at 6 days after radiation. (D) Median fluorescence intensity of surface α₅-integrin levels on BFU-E and CFU-E obtained from the bone marrow (BM; blue) and circulating blood (red) of mice at 6 days post–4 Gy TBI. Error bars represent SEM of 3 or more independently assayed mice for each data point. Statistical analyses were performed using a 2-tailed Student t test (**P < .01; ***P < .001; significantly different from 6 day post-TBI BM progenitors at matched time points). (E) Flow cytometric analysis of erythroid progenitor engraftment in irradiated recipient BM and spleen at 12 hours after intravenous injection of Sca1⁻CD16/32⁻ lineage-depleted donor UBC-GFP hematopoietic progenitors from BM and peripheral blood (PB) isolated at 6 days post–4 Gy TBI. Progenitors from bone marrow of UBC-GFP mice at 6 days post–4 Gy TBI can successfully engraft and mature into the bone marrow and spleen of 6.5 day post-TBI recipient mice (center column). In contrast, progenitors isolated from the peripheral blood of UBC-GFP at 6 days post–4 Gy TBI selectively engraft the spleen of recipient mice (right column). GFP⁻ Ter119⁺ recipient BM and spleen cells were used to gate subpopulations of maturing erythroblasts using CD71 and forward scatter characteristics¹⁸  (left column), with R1 representing the most immature and R5 the most mature erythroid subpopulations. Data are shown from 2 independent experiments (shown in black and red), with numbers of GFP⁺ Ter119⁺ donor cells present in recipient marrow and spleen per 2 × 10⁶ cells analyzed.