The protective IL-10 producing donor T cell has regulatory function. Lethally irradiated B6D2F1 recipients received splenocytes from wild-type B6 donors plus additional purified T cells from control, G-CSF, or peg-G-CSF pretreated donors as shown (syngeneic control, n = 3; allogeneic control, n = 5; allogeneic control + wild-type control T cells, n = 9, ○; allogeneic control + wild-type G-CSF T cells, n = 10; allogeneic control + wild-type peg-G-CSF T cells, n = 14; allogeneic control + IL-10^-/- peg-G-CSF T cells, n = 13). Survival was determined by Kaplan-Meier analysis. P < .001, allogeneic control + wild-type peg-G-CSF T cells versus all other groups. Data were combined from 2 identical experiments.