Donor CD4⁺CD25⁺ T cells increase frequency and total number of allogeneic CFUs in recipient spleens 7 days after BMT. B6-GFP BM-TCD (2 × 10⁶) was transplanted alone or with 0.11 × 10⁶ to 1 × 10⁶ B6 CD8^-/-CD4⁺CD25⁺ T cells into 7.0-Gy irradiated BALB/c recipients (n = 3/group). Seven days later, recipient spleens were analyzed for frequency of donor CFU-GMs (A) and total splenic donor CFU-GMs (B). Using this same model together with 1 × 10⁶ CD4⁺CD25⁺ T cells from B6 CD8^-/- donors, recipient spleens were analyzed 7 days later for total donor CFU-GMs (▦) and CFU-HPPs (▪) (C). Data represent mean CFU/spleen ± SEM of pooled splenocytes. Panel A indicates frequency; panels B and C, total. Results presented are from an individual experiment (representative of 3 independent experiments that simultaneously examined both CFU progenitors, and more than 20 experiments that examined CFU-IL-3 splenic activity). ^*P < .05; ^**P < .01; and ^***P < .001, respectively, compared with BM transplanted alone.