Recipient-derived IL-10 attenuates the severity of acute GVHD following allogeneic BMT. (A) Purified CFSE-labeled Balb/c T cells were transplanted into lethally irradiated wild-type and IL-10^–/– recipient mice. Five days later, proliferation of splenic donor T cells (H-2^d+) was determined by CFSE intensity, and representative examples are shown. (B) ModFit analysis of proliferation index (via CFSE intensity) and donor T-cell expansion in IL-10^–/– (□, n = 8) and wild-type (▪, n = 8) allogeneic recipients. Results are mean ± SE of individual animals from 2 replicate experiments. *P < .05 and **P < .01 versus wild-type. (C) Survival by Kaplan-Meier analysis. Bone marrow and purified splenic T cells from Balb/c (allogeneic) donors were transplanted into lethally (900 cGy) irradiated IL-10^–/– (n = 12) or wild-type (n = 12) recipient B6 mice. B6 (syngeneic) bone marrow was transplanted into IL-10^–/– B6 mice (n = 9) as non-GVHD controls. **P < .01, IL-10^–/– versus wild-type allogeneic recipients. Data combined from 2 experiments. (D) Clinical scores as described in “Assessment of GVHD.” **P < .01 and *P < .05, IL-10^–/– versus wild-type allogeneic recipients.