The Fas pathway is required for the therapeutic effect of anti-CD137 on cutaneous GVHD. (A,B) Anti-CD137 (200 μg per mouse) was administered 45 days after disease induction. Anti-FasL (200 μg per mouse) treatment occurred on days 43, 45, and 47. Forty days later, clinical scores for each mouse were evaluated and mice were killed for preparation of splenocytes (n = 6-7 for each group). (A) Changes of mean clinical scores. *P < .05 between the group of mice that received anti-CD137 and the group of mice that received anti-CD137 and anti-FasL. (B) Splenocytes were prepared and stimulated for 2 days in the presence of PMA and ionomycin. Culture supernatants were collected and levels of IL-5, IL-13, and IFN-γ were analyzed by ELISA. IFN-γ was undetectable. **P < .01 and **P < .001 between the group of mice that received anti-CD137 and the group of mice that received anti-CD137 and anti-FasL. (C) Anti-CD137 (200 μg per mouse) was administered 30 days after disease induction. Anti-FasL (200 μg per mouse) treatment occurred on days 28, 30, and 32. Splenocytes were prepared 5 days after treatment with anti-CD137 and triple-stained with anti-CD4, anti-Ly9.1, and annexin V. Ly9.1⁻CD4⁺ T cells were gated and analyzed for annexin V staining. *P < .05, between the group of mice that received anti-CD137 and the group of mice that received anti-CD137 and anti-FasL (n = 6-7 per group).