Ineffectiveness of primitive HO-1^+/− cells in radioprotection of lethally irradiated recipients and in preservation of their adoptive reserve. (A) Kaplan-Meier plots showing a defect of HO-1^+/− BM cells in providing radioprotection. BM cells (2 × 10⁵) from either HO-1^+/− or HO-1^+/+ mice were transferred into lethally irradiated recipients (n = 30) and survival rates were compared. (B,C) Assessment of short-term hematopoiesis from limited number of HO-1^+/− BM cells. BM (2 × 10⁵) cells from either luc⁺HO-1^+/− or luc⁺HO-1^+/+ mice were transferred together with 2 × 10⁶ unlabeled BM cells into lethally irradiated recipients and overall hematopoiesis was visualized over time by BLI. (B) Representative BLI images of the recipients at day 22 displayed at the same scale. (C) Kinetics of hematopoietic reconstitution from luc⁺ BM cells. (D,E) Frequency of HO-1^+/− Lin⁻Sca-1 + and LSK populations in hematopoietic cells recovered from recipients of 10⁷ nucleated BM cells from either HO-1^+/− or HO-1^+/+ mice. CFSE-labeled nucleated BM cells were transferred into lethally irradiated recipients. Forty-eight hours after transfer, cells were recovered from BM of the recipients, pooled (3 mice/group), and costained by antibodies to lineage markers and Sca-1. The frequency of CFSE⁺ Lin⁻ Sca-1⁺ cells was analyzed between groups. Values are means of 2 experiments (D). In related experiments, a decreased frequency of the LSK population in hematopoietic tissues of recipients was observed. Because the levels of c-Kit expression are lower after transplantation and return to normal by approximately 2 weeks²¹  (A.J.W., unpublished observations), it enabled us to analyze LSK population in cells recovered from recipients. BM cells (2 × 10⁶) from GFP⁺HO-1^+/− or GFP⁺HO-1^+/+ mice were transplanted into lethally irradiated recipients (n = 3) and donor-derived cells were then recovered for assessment of either donor-derived (GFP⁺) LSK population at day 20 after transfer (E).