Effect of ACK2 with low-dose irradiation treatment on marrow HSPC. (A) Schematic of treatment schedule using 2 mg ACK2 (IP) ± LD-IR at 300 cGy. Control mice received 0.5 cc saline on day 1. (B) Marrow HSCs (lin⁻ Sca-1⁺ KIT⁺ CD135⁻CD150⁺), HPCs (common myeloid progenitor lin⁻ Sca-1⁻KIT⁺ CD34^lowFcγR^low or, for the ACK2 treatment group at day 7, colony forming unit-granulocyte, monocyte) and cellularity at different times following treatment of F1 C57/BoyJ mice with ACK2, LD-IR, or ACK + LD-IR compared with control mice treated with saline. Data are either the number per 2 femurs or the percentage of the saline-treated cohort on the day of study; mean ± standard deviation (n = 3 to 4). *P < .05; **P < .01 versus control. (C) Relative LTRA was determined by a competitive repopulation assay. Marrow obtained at day 7 (Figure 1A) was studied from either control mice or mice conditioned with ACK2 (n = 4), LD-IR (n = 4), or ACK2 + LD-IR (n = 8). **P < .01.