Naive B6 Tregs enhance engraftment of HSCs in a cell-dose–dependent manner. CD8⁻/CD4⁺/CD25^bright Tregs were sorted from naive B6 spleens. Purified naive B6 Tregs + B6 HSCs were transplanted into NOD recipients conditioned with 950 cGy of TBI. (A) Splenocytes were stained (CD8⁻/CD4⁺/CD25^bright) and gated for CD8⁻ (top middle panel) and CD4⁺/CD25⁻, CD4⁺/CD25^dim or CD4⁺/CD25^bright (bottom middle panel). The level of FoxP3 expression in these cell fractions was analyzed (right panel). (B) Percent engraftment in NOD recipients given 10 000 B6 HSCs + 50 000, 100 000, or 200 000 Tregs from spleens of naive B6 mice. (C) Percentage of donor chimerism in engrafted NOD recipients. (D) Survival of NOD recipients conditioned 950 cGy of TBI and given 10 000 B6 HSCs + 50 000 B6 Tregs (●), 100 000 B6 Tregs (□), or 200 000 B6 Tregs (▲). Results are from 3 separate transplant experiments.