ICOS or CD40L costimulatory pathway blockade is sufficient for reducing GCs and preventing BOS. cGVHD was established in B10.BR mice transplanted with WT B6 BM and WT B6 T cells, and mice were treated with either anti-ICOS, anti-CD40L, or irrelevant rat antibody from day 28 to day 56. (A) PFT. (B) GC size was analyzed in situ by measuring the PNA-positive sections in transplanted spleens from day 56. Splenocytes were harvested from transplanted mice on day 56, made into a single-cell suspension, and analyzed for (C) Tfh cells or (D) GC B cells. (E) Amount of Ig deposited in the lungs of transplanted mice on day 56 with (F) representative images. (G) Quantified trichrome present in the lungs of mice on day 56 with (H) representative images. Representative data from 2 experiments; n = 8. *P < .05; **P < .01; ***P < .001.