MLR by T cells or APCs from IL-2Rβ^−/− mice that were adoptively transferred with allogeneic Treg cells. (A) MLR responses by WT and recipient-derived T cells to allogeneic APCs. The source of the responding T cells is listed below the x-axis. Unfractionated spleen cells (2 × 10⁵) isolated from WT BALB/c and BALB/c→BALB/c^−/− mice or spleen cells depleted of H-2^b+ donor cells (2 × 10⁵) isolated from B6→BALB/c^−/− mice were cultured with BALB/c (left), B6 (center), or C3H (right) APCs (2 × 10⁵) for 96 hours. (B) MLR response of WT B6 T cells to APC derived from the spleen of indicated mice as listed below the x-axis. ³H-thymidine was added in the last 6 hours of culture. Data represent the mean ± SEM of 2 to 3 independent experiments with the use of mice that were either tolerant skin graft recipients (B6→BALB/c^−/−) or mice that rejected both donor skin (BALB/c^+/+ and BALB/c→BALB/c^−/−).