Sca1⁺ progenitor cells isolated from Tr/Tr tumors delay F-MuLV–induced erythroleukemogenesis. (A) Flow cytometric analysis of stem/progenitor markers in the spleens of uninfected (control) and erythroleukemic (F-MuLV) H/H and Tr/Tr mice. Spleen cells from control or tumorigenic spleens were stained with anti-cKit and anti-Sca1 antibodies. Survival analyses of CB17-SCID mice injected with (B) cKit⁺ (10⁵) cells isolated from the spleens of F-MuLV–infected H/H (H/H-cKit⁺, n = 6) and Tr/Tr (Tr/Tr-cKit⁺, n = 6) mice by flow cytometry using anti-cKit and anti-Sca1 antibodies (“Methods”); (C) (10⁶) erythroleukemic HB60-5 (n = 4), CB3 (n = 6), H/H-derived H9 (n = 6), or Tr/Tr-derived T5 (n = 5) and T6 (n = 6) cells. (D) Sca1⁺ cells isolated from Tr/Tr tumors (Tr/Tr-Sca1⁺, n = 8), cKit⁺ cells isolated from Tr/Tr tumors (Tr/Tr-cKit⁺, n = 6), and equal numbers of Tr/Tr-cKit⁺ and Tr/Tr-Sca1⁺ cells (n = 6). (E) Kinetics of tumor development in CB17-SCID mice injected with 10⁶ CB3 cells (n = 8), T6 cells (n = 6), and equal numbers of CB3 + T6 cells (n = 5) or equal numbers of CB3 + T6 cells pretreated for 1 hour before injection with 10 μg/mL mitomycin C (CB3 + T6-MMC) (n = 3). Significant differences between survival curves of Tr/Tr-cKit⁺ and Tr/Tr-Sca1⁺ transplanted SCID mice were calculated according to the Kaplan-Meier method using a Mann-Whitney U test with 2-sided P values less than .05.