IL-21 production from donor T cells is required for cGVHD pathology. B10.BR mice were transplanted with WT B6 BM and splenocytes from either WT B6 or B6 IL-21 KO mice. (A) Mice were evaluated for pulmonary function. (B) Lungs of mice were stained with trichrome, and the ratio of collagen stain to total stain was measured surrounding the bronchioles. (C) Lung sections were stained with FITC-conjugated anti-mouse Ig, and the area of Ig deposition was measured. (D) Whole spleen sections were stained with rhodamine-conjugated PNA, and the size of the PNA-positive sections was measured. Frequency of (E) Tfh and (F) GC B cells in the spleen of transplanted mice on day 60. Representative data from 3 experiments; n = 8 mice per group. *P < .05; **P < .01; ***P < .001.