Treatment with a combination of trametinib and taselisib inhibits the development of pulmonary GVHD in mice. B10.BR mice were given CY/TBI and were transplanted with bone marrow cells (BM) only, or bone marrow cells plus splenocytes (BM + S) from C57BL/6 mice. Trametinib, taselisib, cobimetinib, and trametinib plus taselisib were administered. (A) Survival curves of recipient mice are shown. Vehicle or combination therapies, n = 12 per group; others, n = 11 per group. Transplantations were performed twice, independently. (B) Pathologic images around bronchioles of lung specimens stained with H&E. (C) The proportions of bronchiolitis in all bronchioles in mice receiving BM/BM + S and treated with taselisib, cobimetinib, or taselisib + trametinib (n = 5 per group). Bars, mean values; error range, standard error of the mean. (D) Immunohistochemically stained images of murine bronchioles. Upper, CD4; lower, B220. (E) Numbers of CD4⁺ and B220⁺ cells in the peribronchial areas (n = 5 per group). Bars, mean values; error range, standard error of the mean. The log-rank test was used to compare survival between 2 groups, and a two-tailed unpaired t test was used for comparisons between 2 groups of continuous variables. ∗P < .05, ∗∗P < .01, Cobi, cobimetinib; N.S., not significant; Tase, taselisib; Tra, trametinib.