Stat3 expression in T cells is necessary for the development of cGVHD. B10.BR mice were transplanted with B6 WT BM and either WT or Stat3-deficient T cells. (A) Day 60 pulmonary function tests. (B) Pathology scores from lung, liver, spleen, and colon on mice killed on day 60. (C) Frequency of germinal centers in spleens of mice killed on day 60. (D) Frequency of T follicular helper cells on day 60. *P < .05; **P < .01; ***P < .001; ****P < .0001. Error bars represent SEM; data from 2 separate experiments with n = 8 per group.