Anti-IL-21 mAb reverses BOS. cGVHD was established in B10.BR mice transplanted with WT B6 BM and WT B6 T cells, and mice were treated with either irrelevant mouse IgG1 or anti-IL-21. (A) PFTs from mice treated with either neutralizing anti-IL-21 mAb or irrelevant mAb. Splenocytes were isolated and analyzed for (B) GC B cells or (C) Tfh cells. (D) Ig and (E) ratio of trichrome-positive to total area surrounding bronchioles in trichrome stain. Representative data from 3 experiments; n = 8. *P < .05; **P < .01; ***P < .001.