The protection from GVHD afforded by peg-G-CSF is dependent on IL-10 production from the donor T cell. All donors were pretreated with a single dose of 12 μg peg-G-CSF at day -6. T-cell-depleted (TCD) splenocytes from wild-type or IL-10^-/- donors plus purified CD3⁺ T cells from wild-type or IL-10^-/- B6 donors were combined as indicated and injected into lethally irradiated B6D2F1 recipients (wild-type TCD spleen only, n = 6; wild-type T cells plus wild-type or IL-10^-/- spleen, n = 15; IL-10^-/- T cells plus wild-type or IL-10^-/- spleen, n = 13). (A) Survival was determined by Kaplan-Meier analysis. P < .001, wild-type TCD spleen + wild-type T cells versus wild-type TCD spleen + IL10^-/- T cells; P < .0001, IL10^-/- TCD spleen + wild-type T cells versus IL10^-/- spleen + IL10^-/- T cells; (B) GVHD clinical scores were determined as a measure of GVHD severity in surviving animals. ^*P < .05, wild-type TCD spleen + wild-type T cells versus wild-type TCD spleen + IL10^-/- T cells. Data are presented as mean ± standard deviation.